Microstructure and morphology evolution in chemical solution

Eur. Phys. J. Appl. Phys. 24, 13–20 (2003) DOI: 10.1051/epjap:2003063

THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS

Microstructure and morphology evolution in chemical solution deposited PbSe films on GaAs(100) M. Shandalov and Y. Golana Department of Materials Engineering, Ben-Gurion University of the Negev Beer-Sheva 84105, Israel Received: 28 January 2003 / Received in final form: 30 May 2003 / Accepted: 17 June 2003 c EDP Sciences Published online: 3 September 2003 – Abstract. We have studied the microstructure and morphology evolution in PbSe films chemically deposited on GaAs(100) substrates. The films consisted of a single phase of nanocrystalline rocksalt PbSe. The deposition temperature was found to be an important parameter which strongly influences the film morphology. A gradual transition to strong (111) texture was obtained with increasing deposition temperature, accompanied by a significant increase in crystallite size. Transmission electron microscopy (TEM) cross-sections showed two distinct regions. A layer of small, rounded crystals near the GaAs/PbSe interface above which a second region composed of columnar, 111 oriented crystallites was observed. High resolution TEM and Fourier analysis showed that the first layer of crystallites are in epitaxial registry with the GaAs substrate, in spite of the large (8%) lattice mismatch and the presence of a thin, amorphous interfacial layer. PACS. 68.55.Jk Structure and morphology; thickness; crystalline orientation and texture – 81.07.Bc Nanocrystalline materials – 81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)

1 Introduction In recent years, there has been increased interest in the scientific and technological aspects of nanometer-sized semiconductors. These particles exhibit unique chemical and physical properties, differing substantially from those of the corresponding bulk solids. Interest has been mainly focused on the synthesis of II-VI and III-V semiconductor nanoparticles, which show significant quantum confinement effects [for recent reviews, see [1–4]]. The IV-VI semiconductor lead selenide (PbSe) is useful for infrared detector applications, as well as in other photonic applications since the lowest exciton energy transition occurs at technologically important wavelengths in the 1–2 µm range [5,6]. There is particular interest in nanocrystalline PbSe, mainly due to the exceptionally large exciton Bohr radius (aB ) in this material (46 nm). The Bohr radius is about equally split between the electron and hole carriers (ae , ah = 23 nm), therefore the electronic spectrum of PbSe is simple compared to spectra of most II-VI and IIIV semiconductors. Hence, significant quantum size effects are readily observed in relatively large PbSe nanoparticles that do not have a large surface to volume ratio [7]. Chemical bath deposition from solution offers a simple and cost-effective route for the fabrication of high quality semiconductor films, without the need for high deposition a

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temperatures, stringent vacuum or plasma generators [8]. Rather, the deposition system includes a thermostatic bath in which we place a beaker containing the substrates immersed within the desired deposition solution. Variation of the deposition temperature can allow control of the deposition reaction rates, solubility and convection and thus affect structural and optical film properties. A number of studies have focused on the chemical bath deposition of PbSe films using selenium sources based on selenourea [9] and selenosulphate [10–18]. Gorer and Hodes investigated the influence of different complexing agents: trisodium citrate, potassium nitrilotriacetate and potassium hydroxide on PbSe deposited on glass and gold substrates [10, 11]. Sodium selenosulfate was used as a source for Se ions. Various concentration ratios of complex-to-Pb and pH values of solution were used, and deposition temperatures were varied from 0 ◦ C to 80 ◦ C. Spherical, cubic and hexagonal crystal shapes were reported. In some cases, amorphous matrix surrounding individual crystallites appeared in early stages of the nanocrystalline film growth. These authors investigated the deposition mechanism at various complex-to-Pb ratios and its influence on crystallite sizes. Films deposited from KOH solution at 0 ◦ C with low complex-to-Pb ratio were composed of rocksalt PbSe crystallites with a typical size of 4 nm which were embedded in an amorphous PbSe matrix. There was a large difference between the crystallite size of films

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The European Physical Journal Applied Physics

deposited at 0 ◦ C and 60 ◦ C (4 and 12 nm, respectively). No preferred crystallographic orientation was found. Post deposition annealing of films deposited from trisodium citrate solution resulted in an increase in crystallite size from 3.5–4.0 nm to 25–35 nm and disappearance of the amorphous matrix. X-ray diffraction (XRD) showed measurable amounts of other phases (Pb3 (CO3 )2 (OH)2 , Pb5 O4 SO4 ) present in the annealed films. Absorption spectra of deposited films at various complex-to-Pb ratios and deposition temperatures showed a blue shift with decreasing crystallite size [11]. Grozdanov et al. reported a simple bath deposition technique for PbSe films using a solution containing lead nitrate, sodium selenosulfate, sodium hydroxide, and acetic acid [12]. The films were deposited on glass and polyester substrates. Final thickness was ca. 300 nm for films deposited at 25 ◦ C for 3 h. The as-deposited films were specularly reflective. At 60 ◦ C, a final thickness of approximately 400 nm was reached within 30 min. XRD showed reflections corresponding to polycrystalline PbSe, except for one peak which remained unidentified. Annealing reduced the amount of the amorphous phase obtained along with the polycrystalline material [12]. Pramanik et al. reported a solution growth technique for the deposition of PbSe thin films on glass substrates at 30 ◦ C and 75 ◦ C using lead acetate, sodium selenosulfate, sodium hydroxide and triethanolamine as complexing agent [13]. XRD indicated the presence of polycrystalline PbSe films, while scanning electron microscopy (SEM) micrographs showed smooth films with clusters of overgrown material on the surface. For films prepared at 75 ◦ C the number of clusters was found to be larger than for those prepared at 30 ◦ C. The resulting films were found to be p-type semiconductors [13]. Amorphous PbSe films were obtained in a selenosulphate bath at room temperature in the presence of thiosulfate ions [14]. Upon annealing at 350 ◦ C for one minute, clear Bragg peaks were obtained, indicating transformation into nanocrystalline films with a grain size of 13 nm. While we are not aware of any reports on epitaxial PbSe films deposited using chemical solution deposition, the sulfide analog of PbSe, PbS, was epitaxially grown using chemical bath deposition on various faces of Ge [19,20] wurtzite CdS [21], and on InP (100) [20]. Growth of PbSe on GaAs can potentially allow the integration of PbSe active semiconductor layers with GaAsbased electronics. In this work, we have studied the microstructure and morphology evolution in nanocrystalline PbSe films chemically deposited on GaAs(100). Our goal was to investigate the effect of the deposition temperature on the deposition rate and on the morphology and structural evolution of the films, with emphasis on the crystallographic orientation and registry with the GaAs(100) substrate.

2 Experimental 2.1 Materials and chemicals Sodium sulfite (Aldrich, analytical 99.95%), selenium powder (Aldrich, 100 mesh, analytical 95+%), lead ac-

etate trihydrate (Aldrich, analytical 99+%) and potassium hydroxide (Frutarom, analytical) were used without further purification. Single crystal GaAs(100) substrates (Wafer Technology Ltd., UK, epi-polished, Si doped, ±0.1◦ miscut). The films were deposited from a solution with a final composition of 60 mM Pb(CH3 COO)2 3H2 O, 50 mM Na2 SeSO3 and 0.6M KOH (complexing agent for Pb) at final pH > 13. Stock solution of Na2 SeSO3 (0.2M) was prepared with excess of sodium sulfite (0.5M), which was mixed with selenium powder in distilled water and stirred at 90 ◦ C for 1 h. The Na2 SeSO3 solution was filtered in order to remove non-reacted selenium powder. Prior to deposition, the solution contained in a Pyrex beaker was purged with pure N2 for 60 min in order to reduce levels of dissolved O2 and CO2 , and placed in the dark in a thermostatic bath to reach the desired temperature. Growth of PbSe films on GaAs was carried out at −2, 0, 5, 10, 15 and 20 ◦ C for periods of 3 and 72 h. Single crystal GaAs(100) wafer substrates were cleaved into 1×3 cm2 rectangles and cleaned with distilled water, then with analytical ethanol and dried. Films were deposited on the bottom face of the substrates in order to prevent large particles from adhering to the growing film. Therefore, the substrates were placed epi-side down in the solution, mounted on a custom-designed teflon stage at an angle of ∼70◦ with respect to the air-solution interface. 2.2 Deposition mechanism Upon shining a green HeNe laser beam through the deposition solution, complete scattering of the light was observed. This is indicative of the cluster mechanism, in which film growth occurs by adsorption and coagulation of colloidal particles onto the substrate [10]. Low hydroxide complex-to-Pb ratios allowed the formation of Pbhydroxide1 clusters in solution (Eq. (1)), which gradually react with selenide ions (HSe− ) (Eq. (2)) formed by slow hydrolysis of selenosulfate to form PbSe (Eq. (3)): 2OH− + Pb2+ Pb(OH)2 2SeSO2− 3

−

+ H2 O → HSe + −

SeS2 O2− 6 −

(1) −

+ OH

Pb(OH)2 + HSe → PbSe + OH + H2 O.

(2) (3)

2.3 X-Ray diffraction (XRD) Phase identification of the PbSe films deposited for 72 h was carried out by XRD. A Rigaku Model 2000 diffractometer operating in the θ/2θ geometry usA was used at 40 kV ing Cu Kα radiation, λ = 1.5405 ˚ and 30 mA. Scans were run in a 2θ range of 20–57◦ in 0.02◦ steps with intervals of 1◦ /min. Films deposited for shorter deposition times did not give sufficient signal for XRD characterization. After correcting for instrumental broadening, the coherence length was estimated 1 Note that the simple Pb(OH)2 species is not known to exist and this hydroxide is probably one of the forms of hydrated lead oxide.

M. Shandalov and Y. Golan: Microstructure of chemically deposited PbSe films on GaAs(100)

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from the X-ray peak widths at half maximum using the Scherrer formula (4), with K values of K100 = 1.0 and K111 = 1.1 used according to the crystal geometry [22]. D=

Kλ · β cos θ

(4)

2.4 Atomic force microscopy (AFM) AFM was carried out in ambient conditions using a Thermomicroscopes CP Research Instrument operating in intermittent contact mode with a scan rate of 1 Hz. The typical PbSe crystallite size was estimated from large numbers of topography images using the Thermomicroscopes Pro Scan Image Processing software. 2.5 Scanning electron microscopy (SEM) The morphology of the films was observed using a JEOL JSM-5600 SEM after coating with a thin Au layer (few tens of nm coating thickness). Acceleration voltages ranged from 5 to 25 kV.

Fig. 1. X-ray diffraction pattern of samples grown in a temperature range of −2 to 20 ◦ C. The Miller indices of the resulting reflections are indicated with arrows.

2.6 Transmission electron microscopy (TEM) Cross-sections of samples deposited for 72 h at 0, 10 and 20 ◦ C on GaAs(100) were prepared by cutting the sample into slices normal to the interface and gluing together face-to-face using M-Bond 610 adhesive (Allied HighTech Ltd.). The sample was polished with a precision smallangle tripod holder on a series of diamond polishing papers (Allied HighTech Ltd.) until a thin wedge was formed. The sample was glued to a Cu slot grid (slot dimensions were 1 × 2 mm2 ) and final thinning was done by Ar ion milling using a GATAN model 691 Precision Ion Polishing System. TEM and high resolution TEM (HRTEM) were carried out using a JEOL 2010 instrument operating at 200 keV. Film thickness was measured from the TEM cross-sections. Selected area electron diffraction patterns were obtained from both PbSe film and GaAs substrate. The Gatan Digital Micrograph 3 software was used for fast Fourier transform (FFT) analysis of HRTEM lattice images of the PbSe/GaAs interface.

3 Results X-ray diffraction spectra films deposited at −2, 0, 5, 10, 15 and 20 ◦ C are shown in Figure 1. The data indicates the presence of single-phase nanocrystalline PbSe films, with all reflections corresponding to rocksalt PbSe (JCPDS powder diffraction file #6-354). A gradual transition to a strong 111 texture was observed with increasing temperature, as shown by the intensity ratio of the (111):(200) Bragg reflections plotted vs. the deposition temperature in Figure 2. The horizontal line represents the

Fig. 2. (111)/(200) X-ray diffraction intensity ratio vs. temperature.

intensity ratio (111):(200) according to the powder diffraction file. Despite the similar ratio observed at low deposition temperatures (−2 ◦ C–0 ◦ C), the very weak intensity of the (220) peak (70% theoretical intensity) indicates some preferred crystallographic texture. As the deposition temperature is increased, the 111 orientation gradually dominates and the (111):(200) intensity ratio increases to large values indicating a very strong 111 texture. Interestingly, an exponential variation of the (111):(200) intensity ratio with temperature is observed (note the logarithmic y-axis in Fig. 2). The coherence length of the X-ray peak widths at half maximum was calculated using the Scherrer formula, providing an estimate for the average crystallite size in these crystallographic directions. Figure 3 shows the coherence lengths, together with the typical sizes of the surface crystallites as estimated from AFM, plotted on a logarithmic axis vs. the deposition temperature. The crystallite size strongly increases with temperature, with the coherence length in the 111 direction

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Fig. 3. Average crystal sizes estimated from AFM (squares) and coherence length estimated from the FWHM X-ray broadening (triangles) plotted vs. temperature.

clearly larger than coherence length obtained for the 100 direction. The surface crystallite diameters measured by AFM are of the same order as calculated from XRD in the two directions 111 and 100, and are somewhat lower than the bulk coherence length values. AFM topography 1 × 1 µm2 images of films deposited for 3 h at deposition temperatures of −2, 5 and 20 ◦ C are shown in Figures 4a, 4b and 4c respectively. The scans show the compact, continuous nature of the film surface, which consists of roughly spherical crystallites with a characteristic diameter for each deposition temperature. The dependence of the typical crystallite size determined from the AFM images on the deposition temperature for films deposited for 3 h and 72 h is seen in Figure 3. Films deposited for longer time (72 h) clearly had larger crystallite sizes which increased exponentially with temperature. Secondary electron SEM images of the samples deposited for 3 h at 0 and 20 ◦ C are shown in Figures 5a and 5b, respectively. The image of the 0 ◦ C sample shows a smooth film with some particle aggregates seen on the compact layer. The image of the 20 ◦ C sample shows a characteristic faceted surface with larger, triangular pyramidal features, which are typical of a 111 orientation of these crystals with respect to the surface. Note that samples deposited at 0 ◦ C have a reflective appearance, as opposed to the matte appearance of the samples deposited at 20 ◦ C, in agreement with the SEM results. Cross-sectional TEM was carried out in order to monitor the evolution of the film morphology. The bright field (BF) TEM images shown in Figures 6a and 6b demonstrate the microstructure of the films deposited for 72 h at 0 and 20 ◦ C, respectively. The film deposited at 0 ◦ C showed rounded PbSe crystallites, with a typical crystallite size of 25 nm and a thickness of 175 nm (Fig. 6a). The films deposited at 20 ◦ C were considerably thicker, 1.1 µm, as shown in Figure 6b. The image clearly shows two distinct regions in these films. Near the GaAs/PbSe interface there is a layer about 300 nm thick of small rounded crystallites with a typical size of 25 nm. Above this layer, a region composed of long columnar crystals

Fig. 4. Height mode AFM images showing the surface topography of PbSe films grown at (a) −2 ◦ C, (b) 5 ◦ C and (c) 20 ◦ C.

is observed. Selected area electron diffraction confirmed the 111 orientation of the columnar crystals (Fig. 7a). Moiré fringes present in the high magnification BF image of a region composed of columnar crystals (Fig. 7b) suggest a small rotational misalignment between the two overlapping crystals. Figures 7c and 7d show a BF image and the corresponding dark field (DF) image of the same area of columnar crystals, respectively. The variations in crystal brightness seen in the DF image suggest the existence of smaller domains within the columnar crystals, in agreement with the coherence lengths obtained from XRD for these samples. HRTEM of a PbSe film deposited at 10 ◦ C is shown in Figure 8a, providing a magnified view of the film/substrate interface region. A very thin layer (about 3–5 ˚ A) of amorphous material is present between the GaAs substrate and polycrystalline PbSe film. While the chemical composition of this layer is still unclear, it is interesting to note that FFT performed on HRTEM images of a number of PbSe crystallites situated at the interface suggests a well-defined orientation relationship between the monocrystalline GaAs substrate


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Fig. 5. Secondary electron SEM images of PbSe films grown at (a) 0 ◦ C and (b) 20 ◦ C.

and PbSe crystallites, in which (100)PbSe(100)GaAs and 111PbSe111GaAs with a slight misalignment of 1–2◦ or less. This observation was confirmed also for samples deposited at 0 ◦ C and 20 ◦ C. This orientation relationship was observed only for the first layer of crystallites that were in immediate contact with the interfacial layer.

4 Discussion Chemically deposited PbSe films were characterized by several complimentary structural techniques including XRD, AFM, SEM, TEM and HRTEM. According to XRD and electron diffraction data, single-phase polycrystalline rocksalt PbSe films were obtained with no evidence for other crystal phases reported by others such as basic lead carbonate [10]. TEM did not show any evidence for the presence of an amorphous PbSe phase besides for an ultrathin amorphous interfacial layer, in contrast to reports on the presence of appreciable amounts of amorphous material [10–12,14]. The deposition temperature was found to be an important parameter that strongly influences the film morphology. AFM showed that the typical crystallite size observed on the surface increased exponentially with temperature (Fig. 3). Similarly, XRD showed that the average crystallite size of the films strongly increased with temperature. The rate of the deposition reaction increases with temperature and the crystallites grow faster reaching larger sizes. AFM images gave topography information for the crystallites present at the film surface. Due to tip broadening effects, samples deposited at low temperatures might have even smaller typical crystallite size than presented in the results. The surprisingly smaller typical size obtained from

Fig. 6. Cross-sectional bright field TEM images of PbSe on GaAs deposited at (a) 0 ◦ C and (b) 20 ◦ C.

AFM compared to the coherence length values calculated from XRD can be understood keeping in mind that the AFM is sensitive only to the surface crystallites while XRD averages the entire population in the bulk. In addition, a gradual transition to strong (111) texture was obtained with increasing deposition temperature (Fig. 2). At low temperatures, relatively thin films with rounded, nanosized crystallites were obtained. At a deposition temperature of 0 ◦ C, 25 nm sized PbSe crystallites were obtained, compared to 4 nm sized crystallites obtained on glass substrates by Gorer and Hodes using the same deposition conditions [10,11]. In their work, no preferred orientation of the films was noted. Cross-sectional TEM showed that films deposited at low temperatures (−2 ◦ C to +10 ◦ C) were composed of spherical crystallites (Fig. 6a), while at 20 ◦ C, a layer of larger, ca. 800 nm long 111 oriented columnar crystals was observed on top of a 300 nm thick layer of rounded crystallites (see regions A and B marked in Fig. 6b). In the 20 ◦ C films, the first layer of

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Fig. 7. Cross-sectional TEM of PbSe on GaAs(100) grown at 20 ◦ C for 72 h. (a) Selected area electron diffraction from a single columnar crystal. Diameter of selected area was 145 nm. (b) High magnification bright-field image showing Moiré fringes from overlapping crystals. (c) Bright-field image showing the columnar crystal morphology. (d) Dark-field image corresponding to (c).

Fig. 8. (a) High-resolution TEM lattice image of a cross-section sample deposited at 10 ◦ C. (b, c) Fast Fourier Transform (FFT) obtained from marked regions.


crystallites adjacent to the PbSe/GaAs interface showed a typical size of 25 nm. TEM BF and DF imaging (Figs. 7c and 7d) confirmed that the columnar crystals each consisted of smaller domains, in agreement with the smaller coherence lengths obtained from XRD. At higher deposition temperatures, the relative intensity of the XRD (100) reflection gradually decreases due to the developing layer of large columnar crystals above the layer of small round crystallites. TEM showed that these columnar crystals were oriented in the 111 direction (Fig. 7c), leading to the enhancement of the (111) texture in the films. HRTEM and FFT analysis showed that the first layer of crystallites is in epitaxial registry with the GaAs substrate, in spite of the large (8%) interfacial lattice mismatch and the presence of a thin amorphous layer at the interface. The thin amorphous layer at the PbSe/GaAs interface might be either an amorphous phase of PbSe or a native oxide on the GaAs substrate. Note that the presence of a thin PbSe wetting layer would imply a StranskiKrastanov type growth mode. However, due to the extreme difficulty in chemical analysis of ultrathin amorphous films, so far we have not been able to identify the chemical nature of this layer. The crystallites at the interface showed an epitaxial relationship with the substrate in which (100)PbSe(100)GaAs and 111PbSe111GaAs , that was established by FFT analysis of HRTEM images (Fig. 8). This may be explained by possible existence of epitaxial short-range order within the amorphous interfacial layer [23]. In order to assess the dependence of the deposition rate on temperature, log rate (thickness/deposition time) values were plotted vs. 1/T (K) (not shown). The plot showed a reasonably straight line, indicating an Arrhenius (activation controlled) behavior of the deposition rate with temperature. This suggests that the temperature-dependent morphological changes in the films observed in this work occur in fact as a function of sample thickness. This is reflected also in the surface morphology differences seen by SEM for films deposited at low vs. high temperatures (Fig. 5). Films deposited at low temperatures show a smooth, reflective appearance while at high temperatures the pyramidal edges of the columnar crystals are seen at the surface by SEM as well as by TEM (Fig. 6b). These observations simply reflect the slower deposition rates at lower temperatures, which results in nanocrystalline films in which only rounded crystallites are present. On the other hand, higher deposition rates at higher temperatures resulted in films with two types of grains, small round crystallites at the interface and large columnar crystals above them. Small crystallites create a smooth film surface and are not visible in SEM (Fig. 5a). The faceted surface of the large columnar crystal edges is clearly seen in SEM (Fig. 5b). Variation of the deposition temperature, as well as control of the deposition time, allowed to determine the thickness of the PbSe films and thus to obtain the corresponding film morphology. The complete picture of the film morphology is seen in the thickest film deposited. First, a few layers of nanosized crystallites nucleate on the substrate, followed by larger, columnar crystals that develop with film thickness. The first layer of crystallites

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shows epitaxial registry with the GaAs substrate which disappears in the subsequent layers of crystallites that are not in direct contact with the substrate. Such behavior is commonly seen for epitaxial nanocrystals [24]. On the other hand, the columnar crystals show a prominent 111 orientation which contributes to the increasing (111) texture measured by XRD with increasing thickness. Further work showed that on GaAs(100) substrates, the deposition rate of chemically deposited PbSe films was significantly higher compared with PbSe grown on Si(100) under similar conditions, and the resulting films were more compact and uniform (to be published). Improved wetting of the GaAs substrate by PbSe seems to be a key factor in this difference which is currently under focused investigation. We wish to thank Professor G. Hodes, the Weizmann Institute of Science, for stimulating discussions. We thank R. Golan for expert assistance in atomic force microscopy and Dr. V. Ezersky for expert assistance in transmission electron microscopy.

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