crystals Review
Brief Review of Epitaxy and Emission Properties of GaSb and Related Semiconductors Shouzhu Niu 1 and Rui Chen 2 1
2
*
ID
, Zhipeng Wei 1, *, Xuan Fang 1 , Dengkui Wang 1 , Xinwei Wang 1 , Xian Gao 1
ID
State Key Laboratory of High Power Semiconductor Lasers, School of Science, Changchun University of Science and Technology, 7089 Wei-Xing Road, Changchun 130022, China;
[email protected] (S.N.);
[email protected] (X.F.);
[email protected] (D.W.);
[email protected] (X.W.);
[email protected] (X.G.) Department of Electrical and Electronic Engineering, South University of Science and Technology of China, Shenzhen 518055, China;
[email protected] Correspondence:
[email protected]; Tel.: +86-158-4309-5977
Academic Editor: Paul J. Simmonds Received: 31 August 2017; Accepted: 25 October 2017; Published: 2 November 2017
Abstract: Groups III–V semiconductors have received a great deal of attention because of their potential advantages for use in optoelectronic and electronic applications. Gallium antimonide (GaSb) and GaSb-related semiconductors, which exhibit high carrier mobility and a narrow band gap (0.725 eV at 300 K), have been recognized as suitable candidates for high-performance optoelectronics in the mid-infrared range. However, the performances of the resulting devices are strongly dependent on the structural and emission properties of the materials. Enhancement of the crystal quality, adjustment of the alloy components, and improvement of the emission properties have therefore become the focus of research efforts toward GaSb semiconductors. Molecular beam epitaxy (MBE) is suitable for the large-scale production of GaSb, especially for high crystal quality and beneficial optical properties. We review the recent progress in the epitaxy of GaSb materials, including films and nanostructures composed of GaSb-related alloys and compounds. The emission properties of these materials and their relationships to the alloy components and material structures are also discussed. Specific examples are included to provide insight on the common general physical and optical properties and parameters involved in the synergistic epitaxy processes. In addition, the further directions for the epitaxy of GaSb materials are forecasted. Keywords: gallium antimonide; alloys and compounds; nanostructures; photoluminescence; molecular beam epitaxy
1. Introduction The mid-infrared (MIR) spectral region is of great research interest because the practical realization of optoelectronic devices that operate in the 2–5 µm wavelength region would bring potential applications in a wide range of areas, including optical gas sensing, environmental monitoring, free-space optical communications, infrared countermeasures, and thermal imaging [1–3]. Research into MIR semiconductor devices has thus become a focus of research attention worldwide. Groups III–V semiconductors are the most promising candidate infrared materials for use in lasers and detectors, owing to their high absorption coefficients, high carrier mobilities and widely tunable band gaps [1,4]. Among these materials, gallium antimonide (GaSb)-based alloys and compounds offer a wide range of electronic band gaps, band gap offsets and electronic barriers along with extremely high electron mobility; these materials would thus enable a variety of devices that are much faster than the equivalent InP- and GaAs-based devices and infrared light sources, and would Crystals 2017, 7, 337; doi:10.3390/cryst7110337
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facilitate lower power consumption [5,6]. Therefore, GaSb materials in the forms of epitaxial layers, multi-element alloys, quantum wells, superlattices and low-dimensional nanostructures have been attracting considerable attention. Additionally, based on the GaSb materials described above, a variety of advanced optoelectronic devices, including laser diodes, detectors, and transistors, have been realized [1–4]. Therefore, considerable effort has been devoted to the growth of high-quality GaSb-based semiconductor materials. To date, the growth of GaSb-based semiconductors has mainly been dependent on liquid phase epitaxy (LPE), metal-organic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE). MBE is preferred as a high-efficiency epitaxial growth technique that is used to manufacture light-emitting diodes, lasers, and detectors for MIR waveband operation by varying the material components to adjust their energy bands [1,5,7]. By tuning of the growth parameters, a variety of complex quantum structures with high surface and interface quality can be realized. Problems that occur on the GaSb surface during reactive processing, such as high surface state densities, surface Fermi level pinning and a residual native oxide layer, have affected the optical properties, including their photoresponse range and luminescence intensities, of GaSb-based devices. Therefore, procedures that involve the removal of native oxides and the fabrication of passivation coatings are necessary to overcome such problems [8]. Improvements in the surface properties of GaSb materials have led to excellent optoelectronic device performances. As the research into optoelectronic devices has advanced, spectroscopic techniques have become increasingly important because they are highly efficient and rapid modern analysis methods. These techniques can effectively determine the photophysical properties of the materials and reveal their excited-state processes to determine their potential use in optoelectronic devices. Further development of spectroscopic techniques will therefore be helpful in the design and improvement of the next generation of optoelectronic devices. Additionally, two-dimensional (2D) materials offer further promise for the development of a new range of fundamental optoelectronic materials owing to their high crystal quality features and rich photophysical properties that will provide new material options for next generation optoelectronic devices [2,8–10]. Therefore, owing to the advantages of 2D materials, further research should be directed to the epitaxy and optical properties of GaSb materials. In this paper, we briefly review the literature for the recent progress in the growth and the optical properties of GaSb and GaSb-related materials, and describe the evolution and the current status of epitaxial growth processes and the optical properties of GaSb and related semiconductors. Section 2 provides a brief introduction to some of the epitaxial GaSb materials that have already been grown by the MBE method, Section 3 covers the luminescence mechanism and the spectral characteristics of these materials, along with the factors that influence their optical properties. Section 4 is devoted to the effects of low-dimensional nanostructures on the luminescence properties of these materials. Finally, we discuss GaSb materials fabricated by the MBE method that exhibit unusual optical properties and provide a foundation for the application of these materials in optoelectronic devices. 2. Epitaxy of GaSb Materials In this review, GaSb and related semiconductors refers to the groups III–V semiconductors that contain the Sb element, including GaAsSb, InAsSb, InGaAsSb and AlGaAsSb. Among these materials, GaSb has a prominent position among the III–V compound semiconductors. GaSb has a lattice constant of 6.0959 Å that mediates between ternary and quaternary III–V compound semiconducting materials (as shown in Figure 1), which enables perfect lattice matching of GaSb to be achieved with many III–V phosphonium-type materials, which is then favorable for high-quality antimony material growth. Additionally, GaSb has a zinc blende (ZB) structure similar to that of GaAs, and its band gap is 0.822 eV at 0 K and 0.725 eV at 300 K [11–13].
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Figure 1. Band gap as a function of lattice constant for III–V compounds and their ternary and
Figure 1. Band gap as a function of lattice constant for III–V compounds and their ternary and quaternary alloys. Reproduced with permission from [11], copyright AIP Publishing LLC (2009). quaternary alloys. Reproduced with permission from [11], copyright AIP Publishing LLC (2009). Figure 1. Band gap as(LPE) a function lattice constant for III–Vused compounds their ternary and of Liquid phase epitaxy was ofthe earliest method in theandextended growth quaternary alloys. Reproduced with permission fromon [11], copyright AIP Publishing LLC techniques. (2009). GaSb-based materials, and there are numerous reports the growth of GaSb by LPE In Liquid phase epitaxy (LPE) was the earliest method used in the extended growth of GaSb-based the early studies of the surface morphology and the electrical and optical properties of GaSb layers, materials, and there are numerous reports on the growth of GaSb by the LPEextended techniques. In of the early Liquid phase epitaxy (LPE) was the earliest method used in growth it was observed that the growth temperature range of 500–550 °C for the Ga melt is the optimal materials, and there are numerous reports and on theoptical growthproperties of GaSb by LPE techniques. In it was studies GaSb-based of the surface morphology and the electrical of GaSb layers, range to produce high-quality layers with excellent surface◦ morphologies [14]. While epilayers thethat earlythe studies of thetemperature surface morphology electricalCand properties GaSb layers, range observed growth rangeand of the 500–550 foroptical the Ga melt isofthe optimal grownit at low temperatures lowtemperature native defect concentrations, have was observed that thehave growth range of 500–550 °Cthey for the Gapoor melt morphologies is the optimal to produce high-quality layers withapplications excellent surface morphologies [14]. While epilayers grown at and thus for device [15]. The typical surface morphologies of GaSb rangeare to unsuitable produce high-quality layers with excellent surface morphologies [14]. While epilayers low temperatures have low native defect concentrations, they have poor morphologies and epilayers that grown at 550have °C from a 660 °C from an Sb-rich melt arehave as shown in Figure 2.thus are grown at were low temperatures low native defect concentrations, they poor morphologies unsuitable for device applications [15]. The typical surface morphologies of GaSb epilayers that were CVD and growth GaSb has beenforreported by Jakowetzet et al. MOCVD growth of GaSb has also thusofare unsuitable device applications [15]. Theand typical surface morphologies of GaSb ◦ ◦ been at performed [16,17]. grown 550 Cthat from a 660 C at from an from Sb-rich melt are as in Figure CVDingrowth epilayers were grown 550 °C a 660 °C from anshown Sb-rich melt are as 2. shown Figure 2.of GaSb
growthby of Jakowetzet GaSb has been Jakowetzet et al. and MOCVD of GaSb has also [16,17]. has beenCVD reported etreported al. and by MOCVD growth of GaSb hasgrowth also been performed been performed [16,17].
Figure 2. Typical surface morphologies of GaSb epilayers (a) grown on a (100) substrate at 550 °C from a Ga-rich melt and (b) on a 7° off-axis (111) substrate at 660 °C from an Sb-rich melt. Figure 2. Typical surface morphologies of GaSb epilayers (a) grown on a (100) substrate at 550 °C Reproduced withsurface permission from [15], copyright Springer (1995). Figure 2. Typical morphologies of GaSb epilayers (a) grown on a (100) substrate at 550 ◦ C from from a Ga-rich melt and (b) on a 7° off-axis (111) substrate at 660 °C from an Sb-rich melt.
◦ C from an Sb-rich melt. Reproduced with a Ga-rich melt and with (b) on a 7◦ off-axis substrate at 660 (1995). Reproduced permission from (111) [15], copyright Springer At present, antimony is mainly grown using molecular beam epitaxy (MBE) technology, which permission from [15], copyright Springer (1995). is based on a nonthermodynamic equilibrium. In recent years, following development the At present, antimony is mainly grown using molecular beam epitaxy the (MBE) technology, of which nonthermodynamic equilibrium, MBE has been Inwidely used following to fabricate variety of ofboth is based on a nonthermodynamic equilibrium. recent years, the adevelopment the At nonthermodynamic present, antimony isand mainly grown using molecular beam epitaxy (MBE) technology, high-purity single crystalsequilibrium, ultra-fine structures andwidely has thus greatly enhanced evolution ofwhich is MBE has been used to fabricate a the variety of both based on ainto nonthermodynamic In MBE recent following development research antimony-based and devices. ayears, technique in enhanced whichthe single or complex high-purity single crystalsmaterials andequilibrium. ultra-fine structures andishas thus greatly the evolution of of the research of intoanantimony-based materials devices. is ato technique single complex nonthermodynamic equilibrium, hasand been widely used fabricate aheated variety of or both high-purity components atomic beamMBE or molecular beam MBE are propelled to ina which substrate, are components ofand anare atomic beamon orthat molecular beam areultra-high propelled tothe a heated substrate, are absorbed, migrate, deposited substrate under vacuum conditions single crystals and ultra-fine structures and has thus greatly enhanced evolution of[18]. research into absorbed, migrate, and are deposited onepitaxy that substrate under ultra-high vacuum conditions [18].and To improve the crystal quality of the film, a buffer layer between the epitaxy film antimony-based materials and devices. MBE is a technique in which single or complex components of Tocommonly improve the crystal quality of the epitaxy film, ainbuffer layer between the epitaxy filmGaSb and is used, owing to the large difference lattice constants. AlSb and an substrate atomic beam orcommonly molecular beam are propelled to a heated substrate, are InAs, absorbed, migrate, and are to the large difference in lattice constants. InAs,beAlSb and GaSb can besubstrate used as isbuffer layers.used, Nohowing et al. reported that high quality GaSb films could realized on a deposited on that under ultra-high vacuum conditions [18]. films could be realized on a be substrate used substrate as buffer layers. Noh et al.and reported that high GaSb GaAscan (001) at low temperature, the strain reliefquality and structural properties of the GaSb To GaAs improve crystal quality of the epitaxy film, relief a buffer between theofepitaxy film and (001)the substrate at low temperature, and the strain and layer structural properties the GaSb films with different buffer layers were investigated [6]. Their results show that the insertion of an films with different buffer layers to were investigated [6]. Their results show that theInAs, insertion of and an GaSb substrate is commonly used, owing the large difference in lattice constants. AlSb AlSb or GaSb buffer layer is very useful for improving the quality of the GaSb grown on GaAs AlSb as or GaSb buffer layer is very useful for improving the quality of the GaSb grown on GaAs cansubstrates be used [6].buffer layers. Noh et al. reported that high quality GaSb films could be realized on a
substrates [6].
GaAs (001) substrate at low temperature, and the strain relief and structural properties of the GaSb films with different buffer layers were investigated [6]. Their results show that the insertion of an AlSb or GaSb buffer layer is very useful for improving the quality of the GaSb grown on GaAs substrates [6].
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As an important part of GaSb materials, multi-component alloys have been intensively investigated. By tuningpart the of components in the alloys, the band gap and have latticebeen constant can be As an important GaSb materials, multi-component alloys intensively adjusted over By a wide range. example, by Wieder Clawson determined investigated. tuning theFor components intheoretical the alloys,calculations, the band gap andand lattice constant can be an expression for the band gap of InAs Sb materials with composition x at temperature T, x x theoretical calculations, Wieder and Clawson adjusted over a wide range. For example,1−by as follows [19]: determined an expression for the band gap of InAsxSb1−x materials with composition x at
temperature T, as follows [19]: 3.4 × 10−4 T2 2 −4 Eg ( x, T) = 0.441 − xT(1 − x ) (1) -4 2 − 0.876x + 0.70x + 3.4 × 10 3.4×10 210 + T T -4 2 (1) Eg (x,T)=0.441-0.876x+0.70x +3.4×10 xT(1-x) 210+T Among the ternary alloys [20], GaAsx Sb1−x is important owing to the advantage that its Among can thebe ternary alloysover [20], xSb1−x is important owing to the advantage that its wavelength modulated theGaAs range from 0.8 µm to 1.7 µm, and such a wavelength band wavelength can be modulated over the range from 0.8 µm 1.7 µm, and suchprospects a wavelength band would enable the production of optoelectronic devices withtobroad application compared would enable the production of optoelectronic devices with broad application prospects compared with those of InP- and GaAs-based devices. Chou et al. reported on the effects of substrate tilting with those of InPand GaAs-based Chou et alloys al. reported on the effectslattice-matched of substrate tilting on on the material properties of MBEdevices. grown GaAsSb that were closely to InP the material properties MBE grown GaAsSb alloysgrown that on were closely lattice-matched to InP substrates. GaAsSb alloysofwith a thickness of 1 µm were InP(100) and off-axis InP substrates substrates. GaAsSb alloys with a thickness of 1 µm were grown on InP(100) and off-axis InP ◦ ◦ ◦ (2 , 3 and 4 off-axis) and were characterized by XRD, as shown in Figure 3. Their results are expected substrates (2°, 3° and 4° off-axis) and were characterized by XRD, as shown in Figure 3. Their results to be applicable to devices that incorporate MBE grown GaAsSb in their active layer [21]. However, are expected be applicable to cause devices that incorporate MBEgrowth, grown GaAsSb their active layer [21]. using Sb as ato surfactant would a difficulty in epitaxial such as in component distribution However, using Sb as a surfactant would cause a difficulty in epitaxial growth, such as component aggregation. Therefore, the regulation of alloy composition is very important. For this purpose, distribution aggregation. Therefore, the regulation of alloy composition is very important. For this using a mixed beam, Wei’s group reported the carrier dynamics in GaAsSb ternary alloys grown by purpose, using a mixed beam, Wei’s group reported the carrier dynamics in GaAsSb ternary alloys MBE. The growth conditions and Sb component of the GaAsSb alloy samples were as those shown grown The results growthindicated conditionsthat andthe Sblocalized component of the were as of those in Tableby1 MBE. [22]. The states areGaAsSb related alloy to thesamples Sb component the shown in Table 1 [22]. The results indicated that the localized states are related to the Sb component GaAsSb alloy; however, this component also leads to poor crystal quality in the material, and the of the GaAsSb however, thislimited component leads to poor crystal quality in the material, and applications of alloy; GaAsSb alloys are owingalso to the deterioration in quality. the applications of GaAsSb alloys are limited owing to the deterioration in quality.
Figure -ray diffraction onon InP (100) and 2°,23° and 4° ◦, 3 ◦ and Figure 3. 3. X X-ray diffraction(XRD) (XRD)ofof11µm-thick µm-thickGaAsSb GaAsSbepi-grown epi-grownlayers layers InP (100) and off-axis InP (100) substrates. Reproduced with permission from [21], copyright AIP Publishing LLC ◦ 4 off-axis InP (100) substrates. Reproduced with permission from [21], copyright AIP Publishing (2013). LLC (2013). Table 1. Growth Table 1. Growth conditions conditions of of the the GaAsSb GaAsSb alloy alloy samples. samples. Reproduced Reproducedwith with permission permission from from [22], [22], copyright Nature Publishing Group (2016). copyright Nature Publishing Group (2016).
Sample Growth Temperature As/Sb Beam Ratio Sample Growth Temperature As/Sb Beam Ratio 1 28:1 1 28:1 2 600 °C ◦ 16:1 2 600 C 16:1 3 7:1 3
7:1
Sb Component 6% 6% 8% 8% 9%
Sb Component
9%
InGaAsSb, which is lattice-matched to GaSb, has been studied in particular depth, in part because it is an important active region constituent of diode lasers emitting in the 2–3 µm range [23]. Early work on the direct band gap in GaSb-rich GaInAsSb has been summarized by Karouta et al.,
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InGaAsSb, which is lattice-matched to GaSb, has been studied in particular depth, in part Crystals 2017, 337important active region constituent of diode lasers emitting in the 2–3 µm range5[23]. of 21 because it is7,an Early work on the direct band gap in GaSb-rich GaInAsSb has been summarized by Karouta et al., whoproposed proposeda bowing a bowing parameter In report, a recent report, an layer absorption layer who parameter of 0.6ofeV0.6 [24].eVIn [24]. a recent an absorption composition composition of In 0.28Ga0.72As0.25Sb0.75 allowed for lattice matching to GaSb and demonstrated cut-off of In0.28 Ga0.72 As0.25 Sb0.75 allowed for lattice matching to GaSb and demonstrated cut-off wavelengths wavelengths at at 250room K and 3.0 µm [25]. at room [25]. Figure 4 shows the of 2.9 µm at 250ofK 2.9 andµm 3.0 µm temperature Figuretemperature 4 shows the photoluminescence results photoluminescence results recorded between 4 and 300 K. Additionally, short-wave infrared barriode recorded between 4 and 300 K. Additionally, short-wave infrared barriode detectors can be realized. detectors can be realized. Crystals 2017, 7, 337
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who proposed a bowing parameter of 0.6 eV [24]. In a recent report, an absorption layer composition of In0.28Ga0.72As0.25Sb0.75 allowed for lattice matching to GaSb and demonstrated cut-off wavelengths of 2.9 µm at 250 K and 3.0 µm at room temperature [25]. Figure 4 shows the photoluminescence results recorded between 4 and 300 K. Additionally, short-wave infrared barriode detectors can be realized.
Figure Figure4.4.(a) (a)Photoluminescence Photoluminescence results results measured measured between between 44 and and 300 300KKand and(b) (b)normalized normalizedspectral spectral response 190190 andand 300 K. Reproduced with permission from [25],from copyright Publishing responsedata databetween between 300 K. Reproduced with permission [25], AIP copyright AIP LLC (2015). LLC (2015) Publishing Figure 4. (a) Photoluminescence results measured between 4 and 300 K and (b) normalized spectral data between 190 and 300 K. Reproduced with permission as from AIP In aa similar similarresponse way AlGaAsSb isisalso important, it [25], isit lattice-matched to GaSb and In way to toInGaAsSb, InGaAsSb, AlGaAsSb also important, as iscopyright lattice-matched to GaSb Publishing LLC (2015) forms a natural barrier and cladding material for use in mid-infrared semiconductor lasers. and forms a natural barrier and cladding material for use in mid-infrared semiconductor lasers. The relationships relationships for the the direct and AlGaAsSb indirectisenergy energy gapsaswere were calculatedto by by Adachi Adachi and the the In a for similar waydirect to InGaAsSb, also important, it is lattice-matched GaSb and The and indirect gaps calculated and forms a natural and cladding for et mid-infrared semiconductor lasers. experimental results werebarrier summarized bymaterial Ait-Kaci al.in[26,27]. [26,27]. An accurate accurate determination determination of the the experimental results were summarized by Ait-Kaci etuse al. An of The relationships for the direct and indirect energy gaps were calculated by Adachi and the chemical composition composition of AlGaAsSb AlGaAsSb layersbyis isAit-Kaci difficult. FigureAn55accurate showsdetermination the band-gap band-gap energies as a chemical of layers difficult. Figure shows the experimental results were summarized et al. [26,27]. of theenergies as a functionof ofthe the x-composition parameter forAl Al xGa Ga11−x As ySb 1−y shows when it is GaSb. chemical composition of AlGaAsSb layers is xdifficult. Figure the band-gap energies as a to to function x-composition parameter for it lattice-matched is lattice-matched GaSb. y Sb51− −x As y when
function of the x-composition parameter for AlxGa1−xAsySb1−y when it is lattice-matched to GaSb.
Figure 5. Band-gap energies as a function of the x-composition parameter for AlxGa1−xAsySb1−y when
Figure 5. Band-gap energies as a function of the x-composition parameter for Alx Ga1−xLLC Asy Sb1−y when lattice matched to GaSb. Reproduced with permission from [26], copyright AIP Publishing lattice matched to GaSb. Reproduced with permission from [26], copyright AIP Publishing LLC (1987). (1987) In a recent study, Jasik et al. reported on their investigation of the composition of MBE-grown
In a recent study,layers Jasik reported on their investigation of theBased composition Figure 5. Band-gap energies ascontext a function of the x-composition parameter for Al Asof ySbMBE-grown 1−y when AlGaAsSb in et theal. of unintentional arsenic incorporation. onxGa the1−xresults obtained, atoprocedure forofthe determination ofarsenic the chemical composition ofBased AlGaAsSb materials lattice layers matched GaSb. Reproduced with permission from [26], copyright AIP Publishing LLC AlGaAsSb in the context unintentional incorporation. on the results obtained, proposed. Table 2 lists the chemical compositions of multicomponent layers [28]. (1987) was a procedure for the determination of the chemical composition of AlGaAsSb materials was proposed. Table 2 lists the chemical compositions of multicomponent layers [28]. In a recent study, Jasik et al. reported on their investigation of the composition of MBE-grown AlGaAsSb layers in the context of unintentional arsenic incorporation. Based on the results obtained, a procedure for the determination of the chemical composition of AlGaAsSb materials was proposed. Table 2 lists the chemical compositions of multicomponent layers [28].
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Table 2. Chemical compositions of multicomponent layers. (Reproduced with permission from Jasik et al., Journal of Applied Physics; published by the American Institute of Physics, 2011 [28]).
Sample
Relaxation Degree (%)
#A01T #A02T #A03T #A01Q #A02Q #A03Q
45 10 44 0 10 0
Stoichiometry Coefficient xy 0.430 0.355 0.480 0.430 0.355 0.480
– – – 0.0437 0.0045 0.0319
Lattice Constant (Å)
Lattice Mismatch (ppm), Relaxed
Lattice Mismatch (ppm),(sinθs /sinθL )−1
6.11255 6.10956 6.11455 6.09277 6.10753 6.09997
2814 2324 3142 −431 1989 750
2837 4267 4782 −820 3664 1463
3. Optical and Emission Properties of GaSb Materials Optical properties have also been intensely investigated as a standard for evaluating material quality and device performance. At present, GaSb materials are believed to have potential for optoelectronic applications. To exploit the full potential of these materials for structural design, a clear understanding of the properties of the material system, e.g., the band parameters and carrier dynamics, is required. Photoluminescence (PL) is a type of spontaneous emission in which light is emitted from a material. The peak energy and intensity of PL are mostly dependent on the regions in the materials that are excited. Basically, PL measurement is a useful tool for the characterization of material parameters and is helpful for understanding the dynamic carrier processes in materials [29]. For example, PL measurements are advantageous in studies of optical emission processes, material composition, and impurity content. To give an overview of the optical properties of GaSb materials, we will first review the previous work in this field over recent decades. 3.1. GaSb Emission Properties Undoped GaSb is a direct band gap semiconductor with a gap of approximately 0.8 eV, which is always p-type. In 1997, Dutta et al. presented a comprehensive review of GaSb growth technology, including the resulting structural, electronic and transport properties, along with a few optical properties [11]. In this section, we will provide a brief review of the PL properties of GaSb materials. The luminescence properties of undoped p-GaSb were studied as early as 1972 by Jakowetz et al., who observed that a doubly-ionizable acceptor affected the concentration of holes in undoped GaSb [16]. Figure 6 shows the experimental PL data for undoped GaSb. The peak emission at 810 meV was attributed to a free exciton, and the emission line at 795.5 meV was identified as arising from to a bound exciton [30]. The lines A, B and C were interpreted as transitions from the conduction band to the three acceptor levels. The acceptor ionization energies denoted by EA, EB and EC were 34.5, 55 and 102 meV, respectively [16,31–33]. The energy positions and corresponding interpretations are listed in Table 3. Jakowetz et al. concluded that the simplest possible model for the acceptor is that of an antistructure defect, that is, as a Ga atom on an Sb site. However, they also admitted that it may possible for more complex defect structures in undoped GaSb to exist. According to the work of Lee et al., unintentionally doped GaSb can display approximately 20 transitions in the 680–810 meV range in the low temperature PL spectra. While a few of these transitions have clear associations with specific defects, the PL measurements offer great advantages for further research. The PL properties can be very different when using a specific growth method or specific growth conditions. For samples grown using MBE technology, better crystallinity is obtained when using a growth temperature range of 500–550 ◦ C and an Sb4 :Ga flux ratio of more than 1.5 [34]. In another study, buffer layers were used to reduce the defect concentrations [35]. Clearly, high-quality MBE-grown GaSb, which demonstrates higher carrier mobility, can be obtained by the optimization of the growth conditions [33–36]. More importantly, PL measurements can serve to determine the transitions that occur in good quality GaSb materials [33]. First, the free exciton (FE) transition is an indication of high optical quality. However, several bound excitonic transitions (BE1 –BE4 ) are generally
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observed and tend to dominate the emissions from MBE-grown samples. These acceptor-related transitions are listed in Table 4. Additionally, the transition energy may be caused by a concentration of unintentionally incorporated donors, strain effects, or competing free-to-bound and donor-acceptor Crystals 2017, 7, 337 7 of 21 transitions. More recently, researchers found that the free electron to hole bound to unidentified acceptor-related transitions are listed in Table 4. Additionally, the transition energy may be caused acceptor transition dominate recombination after Tedonors, dopingstrain of GaSb, by acould concentration of unintentionally incorporated effects, particularly or competing at high doping free-to-bound and PL donor-acceptor transitions. More researchers the free scientific and levels [36]. Since then, the properties caused by recently, doping effects found havethat attracted electron to hole bound to unidentified acceptor transition could dominate recombination after Te technological interest motivated by optoelectronic device design considerations [37]. As the growth doping of GaSb, particularly at high doping levels [36]. Since then, the PL properties caused by techniques matured, the next wasand to fabricate and quaternary alloys to meet the doping effects have challenge attracted scientific technologicalternary interest motivated by optoelectronic design considerations [37]. As the growth techniques matured, the next challenge was to requirements ofdevice heterostructure fabrication. fabricate ternary and quaternary alloys to meet the requirements of heterostructure fabrication.
Figure 6. Experimental photoluminescence (PL) data for undoped GaSb. (a) PL spectrum of GaSb Figure 6. Experimental photoluminescence (PL) data for undoped GaSb. (a) PL spectrum of GaSb sample W4, grown from a stoichiometric melt. p300 = 1.8 × 1017 cm−8, T = 2 K; (b) PL spectrum of GaSb 17 −8 , T = 2 K; (b) PL spectrum of sample W4, grown from a stoichiometric p300 sample W7, grown from an antimonymelt. rich melt. p300== 1.8 3.5 × × 101610 cm−3, cm T = 2 K; Reproduced with permission from [16], copyright Wiley-VCH (1972). GaSb sample W7, grown from an antimony rich melt. p300 = 3.5 × 1016 cm−3 , T = 2 K; Reproduced with permissionTable from copyright (1972). 3. [16], Energetic positions Wiley-VCH and corresponding interpretations for GaSb. Reproduced with permission from Jakowetz et al., Physica Status Solidi; published by the Weinheim, Germany: Wiley-VCH, 1972 [16]. Table 3. Energetic positions and corresponding interpretations for GaSb. Reproduced with permission line (mev) from Jakowetz et al., PhysicahυStatus Solidi; published by the Weinheim, Germany: Wiley-VCH, FE 810 free exciton 1972 [16]. BE 795.5 bound exciton A
Line
777.5
hυ (mev)
FE
BE-LO
BE
B
A BE-LO
810 766
795.5 757
A-LO777.5748.5 B-LO
766
728
band-acceptor transition EA = 34.5 meV phonon replica of line BE (hυ)Phexciton = 29.5 meV (LO)Γ-phonon: free band-acceptor transition bound exciton EB = 55 meV phonon replica of linetransition A band-acceptor Ph = 29 meV (LO)Γ-phonon: EA(hυ) = 34.5 meV phonon replica of line B replica of line BE phonon: (hυ) Ph = 29 meV (LO)Γ-phonon
(LO)Γ- phonon: (hυ)Ph = 29.5 meV
B
757
band-acceptor transition EB = 55 meV
A-LO
748.5
phonon replica of line A (LO)Γ- phonon: (hυ)Ph = 29 meV
B-LO
728
phonon replica of line B (LO)Γ- phonon: (hυ)Ph = 29 meV
C
710
band-acceptor transition Ec = 102 meV second ionization step of the acceptor involved in line A
D
803
band-acceptor transition? ED = 9 meV
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Table 4. Acceptor-related transitions of GaSb. Reproduced with permission from Lee et al., Journal of Applied Physics; published by the American Institute of Physics, 1986 [33]. Energy (meV)
FWHM (meV)
Notation
806 802 800–790 785 775 757 746 725 722 717 710
3 2 Unresolved 10–15 5 Unresolved 7–8 10 Unresolved 2–3 5
BE1 BE2 or BE3 BE4 and other acceptor related transitions Unidentified acceptor U1
ALO Phonon replica U2 U3 U4
3.2. GaSb Alloy Emission Properties From the previous section, owing to band tailoring properties of GaAsSb being important for devices with hetero-epitaxial structural designs, and particularly edge-emitting lasers [35,38] and vertical cavity surface-emitting lasers (VCSELs), many research efforts have been made in epitaxy growth of GaSb materials. In recent decades, MBE growth of GaAs1−x Sbx over the entire composition range has been reported [39,40], and ternary compounds have been estimated using Vegard’s law [41]. The relationship between the composition and the band gap energy can be expressed as [42] Eg = 1.43 − 1.9x + 1.2x2
(2)
However, with the component distribution aggregation effect, the optical properties would exhibit a distinct phenomenon, such as emission related to localized states [22]. Unfortunately, there have only been a few reports on the optical properties of GaAsSb materials [43–50]. In addition, GaAsSb has been recognized as an important material for use in high-performance optoelectronic devices, and its optical properties are thus significant for the improvement of the performance of these devices. Basic knowledge of the properties of GaAsSb is thus important for further applications of the material. Specifically, the incorporation of small fractions of antimony (Sb) in GaAsSb materials leads to the reduction of the band gap and introduces some localized states that are caused by fluctuations in the Sb content. The GaAsx Sb1−x transition process is an important advantage of the material when used in potential band engineering applications, particularly in the research on GaAsx Sb1−x grown by MBE technology. Hsu et al. presented an optical study of low-Sb-component GaAsx Sb1−x layers and identified the conduction to heavy-hole (HH) band and conduction to light-hole (LH) band transitions that were determined to originate from strain-induced valence band splitting by comparing the relative intensities of the photoreflectance (PR) and piezoreflectance (PzR) spectra [48]. The splitting behavior of the valence band relative to the Sb content is shown in Figure 7. Qiu et al. studied the optical spin polarization properties of GaAs0.94 Sb0.06 to demonstrate how Sb incorporation modified the spin properties of GaAs [49]. Gao et al. focused on the carrier dynamics of GaAsx Sb1−x epilayers with low Sb content [22]. They discovered the phenomenon of carrier localization, and the degree of localization showed a linear relationship with increasing Sb content. These localized states were related to the Sb component in GaAsx Sb1−x , and could lead to optical emission degeneration (as shown in Figure 8.).
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Figure 7. Valence band splitting behavior with respect to the Sb content of GaAsx Sb1−x . PR spectra Figure 7. Valence band splitting behavior with respect to the Sb content of GaAsxSb1−x. PR spectra Figure 7. Valence band behavior with content GaAs 1−x. PR (dotted curves) of GaAs Sbx samples for (a) Sb respect = 5.9%,to(c)the SbSb = 8.4% andof(e) Sb x=Sb9.7% at spectra 15, 77, 150, 1−xsplitting (dotted curves) of GaAs1−x Sbx for (a) Sb 5.9%,(c)Sb (c)Sb= =8.4% 8.4%and and Sb = 9.7% at 77, 15, 77, curves) GaAs1−x Sbxsamples samplesfits forto (a)FDLL; Sb == 5.9%, (e)(e) Sbthe = 9.7% at 15, and (dotted 300 K. The full of lines are least-squares Temperature variations of experimental PR 150,150, andand 300300 K. K. The full lines are least-squares fits to FDLL; Temperature variations of the The full lineswith are representative least-squares fits to bars FDLL;forTemperature of = the values for HH and LH transition error GaAs1−x Sbxvariations with (b) Sb 5.9%, experimental PRPR values forfor HH and LH with representative representativeerror error bars for GaAs 1−x x with experimental HH LHtransition transition bars forThe GaAs 1−xSb x Sb with (d) Sb = 8.4%, and (f)values Sb = 9.7% asand open-circles and with open-diamonds, respectively. obtained values (b) Sb = 5.9%, (d) Sb = 8.4%, and (f) Sb = 9.7% as open-circles and open-diamonds, respectively. (b)transition Sb = 5.9%,energies (d) Sb = are 8.4%, and (f) Sb 9.7% as open-circles and with open-diamonds, The The of the indicated by =the arrows. Reproduced permission respectively. from [48], copyright obtained values of the transitionenergies energiesare are indicated indicated by with permission obtained of the transition bythe thearrows. arrows.Reproduced Reproduced with permission Elsevier S.values A. (2010). copyright Elsevier (2010). fromfrom [48],[48], copyright Elsevier S.S.A.A.(2010).
Figure 8. (a) PL spectra of GaAsxSb1−x alloy samples at 10 K; the inset shows the PL spectrum of GaAs substrate at 10 K, (b)x Sb temperature-dependent PL characteristics of GaAsSb, and (c) (i) Figure 8. (a) PL spectra of GaAs 1−x alloy samples at 10 K; the inset shows the PL spectrum of GaAs Figure 8. (a) PL spectra of GaAs xSb1−x alloy samples at 10 K; the inset shows the PL spectrum of temperature dependent peak position and (ii) dependent FWHM samples. substrate at 10 K, (b) temperature-dependent PL temperature characteristics of GaAsSb, and of (c)the (i) temperature GaAs substratewith at 10 K, (b) from temperature-dependent PL characteristics of GaAsSb, and (c) (i) Reproduced permission [22], copyright Nature Publishing Group (2016). dependent peak position and (ii) temperature dependent FWHM of the samples. Reproduced with temperature dependent peak position and (ii) temperature dependent FWHM of the samples. permission from [22], copyright Nature Publishing Group (2016). Reproduced with permission from [22], copyright Nature Publishing Group (2016).
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The localized states may be generated by the composition fluctuation, which should therefore The localized states may generated by theofcomposition which should treatment therefore be suppressed to optimize thebe optical properties the material.fluctuation, However, post annealing be to optimize the optical the properties of the However,quality post annealing treatment hassuppressed been commonly used to improve structural andmaterial. optical properties of semiconductors, has been commonly used to improve the structural and optical properties quality of and can be used to reduce the localized states occurring in GaAsx Sb1−x layers. It should be noted semiconductors, andofcan be used to reduce the localized statesformation occurringinside in GaAs 1−x layers. that the formation localized states was related to Sb cluster thexSb GaAs x Sb1−x Italloy, should be noted that the formation of localized states was related to Sb cluster formation inside which was influenced by the rapid thermal annealing (RTA) temperature. Figure 9 shows the the GaAs xSb1−x alloy, which was influenced by the rapid thermal annealing (RTA) temperature. effects of the RTA temperature on the emission properties [50]. During the RTA process, a significant Figure shows theowing effectsto ofthe thereduction RTA temperature on the emission propertiesThe [50].distribution During theof RTA change9 in the PL, of localized states, was observed. the process, a significant change in the PL, owing to the reduction of localized states, was observed. The composition in GaAsx Sb1−x resulting from the annealing can cause a slight red-shift of the maximum distribution of the peak. composition in GaAsxthe Sb1−x resulting the annealing canand cause a slight photoluminescence More important, RTA process from can induce Sb migration a fluctuation red-shift of the maximum photoluminescence peak. More important, the RTA in alloy composition; therefore, the optimal RTA temperature should be used to process improvecan theinduce optical Sb migration and a fluctuation in alloy composition; therefore, the optimal RTA temperature should properties of the GaAsx Sb1−x alloy material. be used to improve the optical properties of the GaAsxSb1−x alloy material.
Figure 9. 9. Effects Effectsofofrapid rapidthermal thermalannealing annealing(RTA) (RTA) temperature temperature on on the the emission emission properties properties of of Figure GaAs Sb x . Reproduced with permission from Gao et al., Optical Materials Express; published . Reproduced with permission from Gao et al., Optical Materials Express; published by GaAsxxSb1−x1− by the Optical Society of America, 2017 [50]. the Optical Society of America, 2017 [50].
3.3.Effects EffectsofofSurface SurfaceStates Stateson onGaSb GaSbMaterials Materials 3.3. The superficial characteristics can can affect the performance of semiconductor devices. The superficial and andinterfacial interfacial characteristics affect the performance of semiconductor The surfaces GaSb materials oxidizecan rapidly in the atmosphere of their highly of reactive devices. The of surfaces of GaSb can materials oxidize rapidly in thebecause atmosphere because their chemical properties, and the performance of devices based on these materials is largely dependent highly reactive chemical properties, and the performance of devices based on these materials is on the properties or interfaces. The development GaSb-based devices could thus largely dependent of onthe thesurfaces properties of the surfaces or interfaces. of The development of GaSb-based easily be hampered by the surface properties of the material. Generally, these surface states can lead to devices could thus easily be hampered by the surface properties of the material. Generally, these the poor photoelectric efficiency of optoelectronic devices. Specifically, an oxide layer forms during surface states can lead to the poor photoelectric efficiency of optoelectronic devices. Specifically, an the oxidation process and the an undesirable conduction path arises at the oxide/GaSb-based material oxide layer forms during oxidation process and an undesirable conduction path arises at the interface because of the elemental Sb content [51]. Surface states are induced by these native oxides oxide/GaSb-based material interface because of the elemental Sb content [51]. Surface states are and can cause high native surfaceoxides recombination and surface large leakage currents [52–55]. Withand the intent induced by these and canvelocities cause high recombination velocities large to improve these surface chemical treatments have been used to various reduce the surface leakage currents [52–55].properties, With the various intent to improve these surface properties, chemical states in GaSb materials and thus improve properties materials. In particular, sulfuration treatments have been used to reduce thethe surface statesofinthe GaSb materials and thusthe improve the technique (a treatment a sulfur-containing solution) is showing potential to improve properties ofsurface the materials. Inusing particular, the sulfuration technique (a surface treatment usingthe a density of surface states for surface passivation of III–V compounds. A number of works have focused sulfur-containing solution) is showing potential to improve the density of surface states for surface on the use of of III–V models to describeAthe sulfurofpassivation of the widely passivation compounds. number works haveprocess. focusedOne on the use most of models to accepted describe models can be expressed usingOne the following form [54]: accepted models can be expressed using the the sulfur passivation process. of the most widely following form [54]: A I I I BV + HS− + H2 O → A I I I , Sy + BV , Sy + OH − + H2 ↑ + + →( ) , X + ( ) , , X+ + ↑
(3) (3)
The sulfur passivation process can be described using the schematic diagram shown in Figure 10. The first stage involves breaking the bonds between the AIII and BV− atoms (as shown in Figure 10a).
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The sulfur passivation process can be described using the schematic diagram shown in Figure 10. Crystals 2017, 7, 337 of 21 The first stage involves breaking the bonds between the AIII and BV− atoms (as shown in Figure1110a). III V Next, the chemical bonds are released to enable the formation of S-A and/or S-B (as shown III and/or S-BV (as shown in Next, the 10b). chemical bonds are released enable the between formationsulfur of S-A in Figure Then, chemical bondstoare formed and the other atoms on the Figure 10b). Then, chemical bonds are formed between sulfur and the examples other atoms on the semiconductor surface (as shown in Figure 10c,d). We therefore present some to introduce semiconductor surface (as shown in Figure 10c,d). We therefore present some examples to the wet method used for the sulfur passivation process. introduce the wet method used for the sulfur passivation process.
Figure 10. Schematic showing the chemical bonds between sulfur and other atoms formed at the Figure 10. Schematic showing the chemical bonds between sulfur and other atoms formed at the semiconductor surface. Reproduced with permission from [8], copyright NORTH-HOLLAND (2013). semiconductor surface. Reproduced with permission from [8], copyright NORTH-HOLLAND (2013).
Wet methods of sulfur passivation based on the use of sulfur solutions have been proved to effectively reduce the surfacepassivation states. For based example, and Na2 Ssolutions solutionshave are usually used to Wet methods of sulfur on (NH the use sulfur been proved to 4 )2 Sof passivate surfaces [8]. Figure 11For presents the passivation intensities effectivelyGaSb reduce the surface states. example, (NH4)2S andtime-dependent Na2S solutions emission are usually used to of GaSb samples. The PL[8]. results indicate that the emission intensity of the passivated GaSb sample passivate GaSb surfaces Figure 11 presents the passivation time-dependent emission intensities could be samples. raised to The be 15 higher thanthat thatthe of the as-grown samples. From low temperature PL of GaSb PLtimes results indicate emission intensity of the passivated GaSb sample measurements, reason for higher the enhanced emission intensity samples. was determined to temperature originate from could be raised the to be 15 times than that of the as-grown From low PL the enhancement ofreason free exciton wet method of was sulfur passivation GaSb-based measurements, the for theemission. enhancedThis emission intensity determined to of originate from materials is thus demonstrated to beemission. promisingThis for the efficient light-emitting materials, the enhancement of free exciton wetdevelopment method of of sulfur passivation of GaSb-based which means the application of GaSb materials in potential is feasible [55]. materials is thus demonstrated to be promising for the optoelectronic development device of efficient light-emitting materials, which means the application of GaSb materials in potential optoelectronic device is feasible [55].
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Figure dependence of the emission intensity of theofGaSb samples, (a,b) shows PL Figure11. 11.Passivation Passivationtime time dependence of the emission intensity the GaSb samples, (a,b) the shows spectra the untreated and the passivated GaSb in (NH4 )2GaSb S solution for different passivation times, 4)2S solution for different the PLofspectra of the GaSb untreated GaSb and the passivated in (NH (c) shows the low temperature spectra GaSb and passivated GaSb, logarithmic PL spectrum of passivation times, (c) showsPLthe low oftemperature PL spectra of (d) GaSb and passivated GaSb, GaSb sample at 10 K. Reproduced with permission from [8], copyright NORTH-HOLLAND (2013). (d) logarithmic PL spectrum of GaSb sample at 10 K. Reproduced with permission from [8],
copyright NORTH-HOLLAND (2013).
4. Low-Dimensional Nanostructures of GaSb Materials 4. Low-Dimensional Nanostructures of GaSb Materials The nanowire (NW) with its one-dimensional (1D) structure is the smallest structure that can be used for efficient(NW) transport electrons and optical(1D) excitation; NWs are smallest thus expected to bethat critical Thethe nanowire withofits one-dimensional structure is the structure can to the functionality and integration of nanoscale devices [56–59]. Among these structures, because be used for the efficient transport electrons and optical excitation; NWs are thus expected to of be their outstanding potential for useintegration in the fabrication of high-performance nano-optoelectronic devices, critical to the functionality and of nanoscale devices [56–59]. Among these structures, III–V semiconductor NWs have attracted research attention. For applications, because of their outstanding potentialconsiderable for use in the fabrication of laser high-performance the NW provides two of the required components for lasing, that is, the gain medium and cavity, nano-optoelectronic devices, III–V semiconductor NWs have attracted considerablethe research and thus isFor a suitable material forthe useNW in photonic [60]. Recently, lasing has been attention. laser applications, provideslaser two applications of the required components for lasing, that realized at room temperature in core-shell GaAs/AlGaAs NWs [61]. Simultaneously, the laser NW is, the gain medium and the cavity, and thus is a suitable material for use inbecause photonic has the obvious[60]. virtues of light absorption carrier collection, has also been the focus research applications Recently, lasing hasandbeen realized at itroom temperature in ofcore-shell for the development the next generation of photovoltaic et al.virtues reported a GaAs/AlGaAs NWsof[61]. Simultaneously, because the materials. NW has Krogstrup the obvious ofthat light solar cell made from NWs can even break the Shockley-Queisser limit [62]. In addition, III–V NWs absorption and carrier collection, it has also been the focus of research for the development of the have widelyofstudied for usematerials. in detectors, single-photon sourcesthat andatransistors [63–65]. next been generation photovoltaic Krogstrup et al. reported solar cell made from NWs As a result the narrow bang gap limit and high mobilities, GaSb have NWsbeen can offer significant can even break of the Shockley-Queisser [62].electron In addition, III–V NWs widely studied advantages in the mid-infrared range, especially for ~2 µm. Consequently, there has been considerable for use in detectors, single-photon sources and transistors [63–65]. interest intothe GaSb NWs for nanodevice [66–69]. Yang et al. reported growth Asinaresearch result of narrow bang gap andapplications high electron mobilities, GaSb NWsthe can offer of GaSb nanowires on Si in substrates using CVD. However, these GaSb coil-like andthere became significant advantages the mid-infrared range, especially for ~2NWs µm.were Consequently, has intertwined, which made device fabrication [70].for A. nanodevice H. Chin used GaSb subwavelength wires been considerable interest in research into difficult GaSb NWs applications [66–69]. Yang et with a cross-sectional dimension of 700–1500 and lengths of 10–70 to lase atthese approximately al. reported the growth of GaSb nanowires onnm Si substrates using CVD.µm However, GaSb NWs 1550 as shown Figure intertwined, 12 [71]. Additionally, the higher concentrations meanA.that werenm, coil-like and in became which made devicecarrier fabrication difficult [70]. H. these Chin NWs large numbers native crystal defects.dimension Clearly, there is a neednm for and a more robust used also GaSbcontain subwavelength wiresofwith a cross-sectional of 700–1500 lengths of method for to thelase preparation of GaSb NWs. 10–70 µm at approximately 1550 nm, as shown in Figure 12 [71]. Additionally, the higher carrier concentrations mean that these NWs also contain large numbers of native crystal defects. Clearly, there is a need for a more robust method for the preparation of GaSb NWs.
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Figure 12. 12. GaSb GaSb semiconductor semiconductor subwavelength wires (SWW) NIR lasing, (a) PL spectra Figure spectra below below (black) (black) and well above threshold (gray), (b) Peak PL height at the lasing line wavelength of 1553 nm (black, up and well above threshold (gray), (b) Peak PL height at the lasing line wavelength of 1553 nm (black, triangles) and and PL width (gray,(gray, down down triangles) vs laservs fluence. Reproduced with permission from [71], up triangles) PL width triangles) laser fluence. Reproduced with permission Figure 12. GaSb semiconductor subwavelength wires (SWW) NIR lasing, (a) PL spectra below (black) copyright AIP Publishing LLC (2006). from [71], copyright AIP Publishing LLC (2006).
and well above threshold (gray), (b) Peak PL height at the lasing line wavelength of 1553 nm (black, up triangles) and PL width (gray, down triangles) vs laser fluence. Reproduced with permission MBE isfrom feasible method to grow GaSb NWs which has used beentoused to produce a number of [71], copyright AIP LLC (2006). MBE is aafeasible method toPublishing grow GaSb NWs which has been produce a number of different
different III–V NWs, including GaN, GaAs, InAsstructures and InSb [72–74]. structures [72–74]. MBE, the vapor– III–V NWs, including GaN, GaAs, InAs and InSb In MBE, theInvapor–liquid–solid MBE is a feasible method to grow GaSb NWs which has been used to produce a number of liquid–solid (VLS) mechanism is the most extensively used growth mechanism. This mechanism is (VLS) mechanism is the most extensively used growth mechanism. This mechanism is dependent on the different III–V NWs, including GaN, GaAs, InAs and InSb structures [72–74]. In MBE, the vapor– dependent on the(VLS) liquid metal catalyst. During thegroup growth process, group IIIare and group V liquid metal catalyst. During the growth process, the III and groupthe V elements dissolved in liquid–solid mechanism is the most extensively used growth mechanism. This mechanism is elements are dissolved in the catalyst and the catalyst-substrate interface reaches supersaturation. the catalyst and the interface reaches supersaturated elements dependent oncatalyst-substrate the liquid metal catalyst. During the supersaturation. growth process, theMore group III and group V More supersaturated elements arecatalyst precipitated out and this to the NW growth. is elements are in leads the thegrowth. catalyst-substrate interface supersaturation. are precipitated outdissolved and this to theand NW Thisleads process is reaches influenced byThis the process substrate, More supersaturated elements are precipitated out and this leads to the NW growth. This process is influenced the substrate, the growth temperature, V/III flux ratio These and the flux the growth by temperature, the catalyst, the V/III flux ratiothe andcatalyst, the flux the types used [75–78]. factors influenced by These the substrate, the growth temperature, the catalyst, dimensions the V/III flux and ratio the and crystal the flux quality types used [75–78]. factors thus jointly affect the physical thus jointly affect the physical dimensions and the crystal quality of the resulting NWs. It should be types used [75–78]. These factors thus jointly affect the physical dimensions and the crystal quality of the that resulting NWs. It should be noted thatstructures: the NWs the show two typical crystal structures: the zinc noted theresulting NWs show typical and wurtzite of the NWs.two It should be crystal noted that the NWs showzinc two blend typical (ZB) crystal structures: the(WZ) zinc crystal blend (ZB) and wurtzite (WZ) crystal structures (as shown Figure 13), which are difficult to structures (as shown Figure 13), which are difficult to achieve in thin film growth [79]. Therefore, blend (ZB) and wurtzite (WZ) crystal structures (as shown Figure 13), which are difficult to achieveachieve in thin film [79]. Therefore, structure control technology fortheNW fabrication has in thingrowth film growth [79]. Therefore, crystal structure control technology for of NW fabrication has crystal structure control technology forcrystal NW fabrication has become a focus current research. focus the current research. becomebecome a focusa of theofcurrent research.
Figure 13. HRTEM image of the GaAsSb/GaAs/GaAsSb heterostructurewith withthe thezinc zincblend blendand andwurtzite Figure 13. HRTEM image of the GaAsSb/GaAs/GaAsSb heterostructure wurtzite crystal structures. Reproduced with permission from [79], copyright AIP Publishing LLC crystal structures. Reproduced with permission from [79], copyright AIP Publishing LLC (2010). (2010).
Figure 13. HRTEM image of the GaAsSb/GaAs/GaAsSb heterostructure with the zinc blend and wurtzite crystal structures. Reproduced with permission from [79], copyright AIP Publishing LLC (2010).
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While MBE is widely used to grow III–V NWs, many problems remain to be solved for the While MBE is widely used to grow NWs, many problemsNWs remain to be solved for the growth of GaSb-based NWs. However, theIII–V fabrication of GaSb-based is thus a core challenge. growthcompared of GaSb-based NWs.group However, the fabrication of GaSb-based NWs isofthus core challenge. When with other V elements, the saturated vapor pressure Sb isa higher and this When compared with other group V elements, the saturated vapor pressure of Sb is higher makes the growth of these NWs much more difficult. In the III–Sb NW formation process,and thethis Sb makes the growth of these much difficult. the III–Sb [80]. NW All formation process, thethat Sb suppresses Ga diffusion andNWs reduces the more contact angle ofInthe catalyst these factors mean suppresses Ga diffusion and reduces the contact angle of the catalyst [80]. All these factors mean GaSb can easily form thin films but NW formation is much more difficult. Recently, MBE growth of that GaSb can easily form filmsonbut NW stem formation is much difficult. into Recently, MBE Ga-catalyzed GaSb NWs wasthin realized a GaAs (as shown Figuremore 14). Research this growth growth GaSb NWsinvestigation was realizedison a GaAs[81]. stem (as shown Figure 14). Research method of hasGa-catalyzed just started and further required into this growth method has just started and further investigation is required [81].
Figure 14. (a) GaSb nanowire nanowire Figure 14. (a) A A typical typical SEM SEM image image of of the the GaSb GaSb section section (b) (b) High High resolution resolution TEM TEM of of aa GaSb (NW). The inset is the SAED image from the same GaSb NW. (c) TEM image of a GaSb section (NW). The inset is the SAED image from the same GaSb NW. (c) TEM image of a GaSb section showing showing the directions of EDX line scans. (d,e) are the corresponding EDX line scans along the the directions of EDX line scans. (d,e) are the corresponding EDX line scans along the axialaxial and and cross-axial directions, respectively. Reproduced with permission fromcopyright [81], copyright RSC Pub cross-axial directions, respectively. Reproduced with permission from [81], RSC Pub (2016). (2016).
To control the photoelectric characteristics of NWs, other III–V elements have been introduced To control the photoelectric characteristics of NWs, other III–V elements have been introduced into GaSb NWs to form ternary nanowires, such as GaAsSb, InAsSb, GaAsP, and GaInSb NWs [82–84]. into GaSb NWs to form ternary nanowires, such as GaAsSb, InAsSb, GaAsP, and GaInSb NWs Among these structures, the GaAsSb NWs are one of the most frequently researched structures. GaAsSb [82–84]. Among these structures, the GaAsSb NWs are one of the most frequently researched NWs fabricated by MBE can be divided into two types: those grown on Si(111) substrates and those structures. GaAsSb NWs fabricated by MBE can be divided into two types: those grown on Si(111) grown on GaAs nanowire stems. The GaAsSb NWs have been realized with Sb components in the 0–0.44 substrates and those grown on GaAs nanowire stems. The GaAsSb NWs have been realized with Sb range [85–87]. This makes the band gap of GaAsSb NWs easier to be tuned. By growth of the AlGaAs components in the 0–0.44 range [85–87]. This makes the band gap of GaAsSb NWs easier to be passive layer, Dingding Ren et al. realized for the first time the wavelength tunability of the optical tuned. By growth of the AlGaAs passive layer, Dingding Ren et al. realized for the first time the emission from self-catalyzed GaAsSb nanowire arrays, and a tuning range from 1.219 to 1.422 eV [80]. wavelength tunability of the optical emission from self-catalyzed GaAsSb nanowire arrays, and a Recently, by tuning of the background As, Li et al. realized near-full composition range GaAsSb tuning range from 1.219–1.422 eV [80]. Recently, by tuning of the background As, Li et al. realized NWs (GaAs1−x Sbx nanowires with x ranging from 0.60 to 0.93), and the emission wavelength of these near-full composition range GaAsSb NWs (GaAs1−xSbx nanowires with x ranging from 0.60 to 0.93), GaAsSb NWs was tunable from 844 nm to 1760 nm [5]. Moreover, Dheeraj et al. demonstrated for GaAs and the emission wavelength of these GaAsSb NWs was tunable from 844 nm to 1760 nm [5]. NWs with GaAsSb inserted, the band alignment was a staggered type II (as shown Figure 15) [88]. Moreover, Dheeraj et al. demonstrated for GaAs NWs with GaAsSb inserted, the band alignment was The electrons and holes were confined in the GaAs layers and GaAsSb layers separately. The emission a staggered type II (as shown Figure 15) [88]. The electrons and holes were confined in the GaAs wavelength changed when the band offset of GaAs/GaAsSb was changed. This provides a new way layers and GaAsSb layers separately. The emission wavelength changed when the band offset of to control the emission wavelength of NWs. GaAs/GaAsSb was changed. This provides a new way to control the emission wavelength of NWs.
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Figure 15. (a) TEM images of the GaAs/GaAsSb NWs and and (b) (b) low low temperature temperature PL spectrum of a single GaAs/GaAsSb NWs GaAs/GaAsSb NW. permission from [88],[88], copyright American Chemical SocietySociety (2010). NW.Reproduced Reproducedwith with permission from copyright American Chemical (2010).
The crystal structure is another focus of research in this field. Ren et al. demonstrated Sb-induced The structure is another focus of research in this field. Ren et al. the demonstrated effects on crystal the crystal structures of self-catalyzed GaAsSb NWs [80]. By increasing Sb content, Sb-induced effects on the crystal structures of self-catalyzed GaAsSb NWs [80]. By increasing the Sb a WZ phase GaAsSb NW can be changed into a ZB GaAsSb NW (as shown Figure 16). Conesa-Boj content, a WZ phase GaAsSb NW can be changed into a ZB GaAsSb NW (as shown Figure 16). et al. demonstrated large area vertical GaAsSb nanowire arrays [89]. The nanowires exhibited a pure Conesa-Boj al. demonstrated vertical GaAsSb nanowire arrays [89]. The provides nanowires zinc blende et crystal structure and large were area entirely twin-free down to the first bilayer, which a exhibited a pure zinc blende crystal structure and were entirely twin-free down to the first bilayer, foundation for the production of a high-quality NWs device. which provides a foundation for the production of a high-quality NWs device.
(a)
(b)
Figure 16. (a) SEM images of GaAsSb NW arrays and (b) WZ-ZB crystal phase diagram for Figure 16. (a) SEM images of GaAsSb NW arrays and (b) WZ-ZB crystal phase diagram for catalyst-assisted NW growth. The solid lines show the WZ-ZB nanowire crystal catalyst-assisted NW growth. The solid lines show the WZ-ZB nanowire crystal phase-equilibriumlines phase-equilibriumlines respect arrows to η and θ. The labeled arrows how thecorrelate crystal phase with respect to η and θ. with The labeled represent how the crystalrepresent phase diagrams with diagrams correlate with different crystal phase control strategies. Reproduced with permission from different crystal phase control strategies. Reproduced with permission from [80], copyright American [80], copyright American Chemical Society (2016). Chemical Society (2016).
When compared with GaAs and InAs NWs, the research into GaSb and related alloy nanowires is When compared with GaAs and InAs NWs, the research into GaSb and related alloy nanowires still insufficient. Current studies are focused on the preparation of the NWs, and there are many is still insufficient. Current studies are focused on the preparation of the NWs, and there are many challenges to be overcome. Therefore, more research is needed on antimony-based NW growth and challenges to be overcome. Therefore, more research is needed on antimony-based NW growth characteristics. and characteristics. 5. Conclusions Over the past few decades, many III–V semiconductor materials and devices have been grown using molecular beam epitaxy (MBE) methods [90–93]. The properties of antimonide materials have
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5. Conclusions Over the past few decades, many III–V semiconductor materials and devices have been grown using molecular beam epitaxy (MBE) methods [90–93]. The properties of antimonide materials have been studied intensively. Infrared lasers and detectors based on MBE-grown materials have been widely investigated, and their performances have been adequate for their intended applications [94,95]. The variable temperature PL properties of antimonide films and nanostructures have also been reported, and the origins of the PL peaks have been determined. The PL of antimonide alloys is usually dominated by surface states, bound carrier recombination and vacancy defect-related emissions [96,97]. Surface passivation can improve the optical properties of these materials because it eliminates surface dangling bonds and oxides [8]. Low-dimensional antimonide alloy structures were then grown as the MBE technology matured. The controllable growth of GaSb-based NWs has already been realized by some research groups [98]. The optical properties of NWs are similar to those of thin films, but the surface states in NWs are comparatively enormous; this can be attributed to their high surface-to-volume ratios. Establishing the relationship between the growth conditions and the optical properties of these structures is highly significant for the investigations of low-dimensional structures. On this basis, the research efforts into one-dimensional semiconductor devices remain ongoing and are still in their initial stages. While GaSb-based ternary and quaternary alloys have also been grown by MBE, other multicomponent alloys, such as quinary alloys, must be studied for practical applications. Component segregation, aggregation and diffusion processes are more likely to occur in quinary alloys; this has great value for component control research. For one-dimensional GaSb materials, it is also necessary to control the component ratio. While the optical properties of alloy NWs have been reported by several groups, a further comprehensive study of the relationships among the surface states, the defects and the emission spectra is required. Additionally, improvement of the optical properties of these materials is also an important research direction. New methods should be developed in addition to surface passivation. Meanwhile, besides the above-mentioned research direction, with the lattice and coefficient of thermal expansion ability, GaSb is a desirable substrate candidate for epitaxial growth of mid-wave infrared (MWIR) materials and devices, such as HgCdTe on GaSb. The results indicate that the lattice misorientation/misfit dislocations in HgCdTe could be greatly suppressed by using a GaSb substrate [99–101]. Therefore, it is widely considered that MBE growth of HgCdTe on GaSb substrates should be important for application in next generation infrared detectors. As we know, two-dimensional or layered materials may also offer remarkable optical performances, showing potential applications in a range of fields including hydrogen evolution, transistors and optoelectronic devices [102,103]. To date, two-dimensional GaN has been grown by a migration-enhanced encapsulated growth technique that uses epitaxial graphene [104]. Additionally, As- and Sb-based two-dimensional materials, called arsenene and antimonene, respectively, have been synthesized [105]. Although 2D materials based on GaSb have not been realized, many experts have already carried out the theoretical research of GaSb two-dimensional structures [106–108]. Therefore, GaSb 2D materials should also be realized in the future and represent another important research direction. While there have been no reports of two-dimensional GaSb structures to date, major research efforts should be made in this area. Acknowledgments: We would like to thank the National Natural Science Foundation of China (61404009, 61474010, 61574022, 61504012, 61674021, 11404219, 11404161, 11574130, 11674038), the Foundation of State Key Laboratory of High Power Semiconductor Lasers, the Developing Project of Science and Technology of Jilin Province (20160519007JH, 20160520117JH, 20160101255JC, 20160204074GX, 20170520117JH). R. C. acknowledges the national 1000 plan for young talents and Shenzhen Science and Technology Innovation Committee (Projects Nos. JCYJ20150630162649956, JCYJ20150930160634263, and KQTD2015071710313656). We thank Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.
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Author Contributions: Shouzhu Niu wrote the paper; Zhipeng Wei and Rui Chen led the discussion and summarized the literature; Xuan Fang and Dengkui Wang contributed the epitaxy section of the paper; Xinwei Wang and Xian Gao contributed the PL section of the paper and helped to summarize the literature. Conflicts of Interest: The authors declare no conflict of interest.
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