ISSN 10637850, Technical Physics Letters, 2015, Vol. 41, No. 2, pp. 164–167. © Pleiades Publishing, Ltd., 2015. Original Russian Text © R.K. Savkina, A.B. Smirnov, 2015, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2015, Vol. 41, No. 4, pp. 15–23.
UltrasoundInduced Nitride Formation on the Surface of SingleCrystalline GaAs in Cryogenic Fluid R. K. Savkina* and A. B. Smirnov V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 03028 Kyiv, Ukraine *email:
[email protected] Received June 26, 2014
Abstract—We have developed and successfully used a new method for structuring semiconductor surfaces that is based on the phenomenon of cavitation excited by focused ultrasound in a liquid medium. In this work, the cavitation impact of ultrasound at a frequency of ~1 MHz and a power density of ~15 W/cm2 on the sur face of singlecrystalline (001) GaAs in liquid nitrogen led to the formation of a submicronsized relief of rip pled and concentric structures with a height of up to 300 nm. Data of Raman spectroscopy and energydis persive Xray spectroscopy showed the formation of GaAs1 – xNx surface compound with a nitrogen content of 5–7%. DOI: 10.1134/S1063785015020248
Acoustic cavitation in a viscous medium provides an effective means of ultrasound energy cumulation that is widely used in chemistry [1], medicine [2], food industry [3], and cleaning technologies [4]. Theoreti cally, the local energy density in a cavitating liquid, as expressed in terms of temperature and pressure, is almost unlimited. In experiment, reported tempera ture and pressure values amount up to ~5000 K and ~1000 bar, respectively [1]. In addition, cavitation is accompanied by phenomena such as sonolumines cence, liquid injection out of the region of collapsing bubbles at a velocity above 330 m/s, and highpower shock wave generation. The stability and simplicity of reproduction, in combination with extremely high energy characteristics, account for the increasing interest in studying and using cavitation. In particular, this interest is devoted to the field of sonochemical synthesis of nanostructured materials [1, 5–7]. Beginning with the first experiments of W.T. Rich ards and A.L. Loomis on the isolation of molecular iodine from potassium iodide solutions, ultrasound has been widely used for initiating chemical reactions with a high energy threshold in liquid precursors [5]. The use of ultrasound in chemical synthesis allows one to accelerate reactions and carry out processes under less severe conditions, reduces the number of stages as compared to that in usual syntheses, and makes alter native reaction pathways possible. At the same time, the joint action of factors accompanying the phenom enon of cavitation allows it to be used for structuring solid surfaces and even forming new surface phases. For example, it has been found that ultrasound excited cavitation produces effective hydrogena tion/deuteration and N+ and Ar+ ion implantation in metal (Pd, Ag, Ta, Pt, Au) powders suspended in usual
(H2O) and deuterated (D2O) water [8]. A combination of the ultrasonic cavitation effect and microwave (2.45 GHz) irradiation of ndodecane (C12H26) leads to deposition of a diamondlike film onto a silicon substrate [9]. Khachatryan et al. [10] reported on the sonochemical synthesis of diamond microcrystals from a suspension of powdered graphite in CnHmOx. Previously, we have established [11, 12] that the treatment of singlecrystalline semiconductors (Si, GaAs) in cavitating liquid nitrogen leads to the forma tion of nanostructures and new compounds on the sample surface. It was established that the ultrasonic cavitation energy is sufficient to produce the catalytic dissociation of nitrogen molecules, atoms of which are then incorporated into a semiconductor matrix with the formation of chemical bonds [11]. In the present work, singlecrystalline GaAs was exposed in a cavitating cryogenic fluid so as to perform controlled synthesis of GaAs1 – xNx compound on the sample surface. The development of a new method of GaAs surface nitriding based on the cumulation of ultrasonic cavitation energy, together with wellknown chemical and plasma technologies [13], is a topical task that is related to the importance of GaN passivat ing coatings on GaAs. It should also be noted that (III–V)1 – xNx solid solutions, in which bandgap varia tion in the energy interval covering the solar radiation spectrum is controlled by small changes in the nitro gen content (x), are valuable potential materials for multispectral photodetectors and highefficiency solar cells [14]. Ultrasonic treatment (UST) of GaAs samples was carried out in a special lowtemperature reactor [11] that was designed so as to introduce ultrasound into the cryogenic fluid. The initial sound intensity did not
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165 (a)
0.96 eV
Vph, a.u.
1.44 eV
1.12 eV 40 μm (b) 1 λ, μm
2
Fig. 1. Typical photoemf spectrum of (001)GaAs sample treated for 30 min in cavitating liquid nitrogen at a focused ultrasound power density of 15 W/cm2.
exceed 1 W/cm2. The radiation power density was increased by a focusing system with a gain factor of ~60. Taking acoustic losses into account, the ultra sound power density in the focus was ~15 W/cm2. The samples of singlecrystalline semiinsulating (001)GaAs (with resistivity ρ > 107 Ω cm) were placed directly into liquid nitrogen in the region of focusing for a period of time not exceeding 1 h. GaAs samples in the initial state and after the UST were characterized by measuring their photoemf spectra. These measurements were performed using a lockin detection scheme with modulation at 300 Hz at low level of homogeneous excitation by a mono chromatic light in a wavelength range of λ = 0.5– 2 μm. It was established that the illumination of sam ples in the initial state did not produce any photoemf response. The UST in cavitating liquid nitrogen led to the appearance of photoemf in a broad spectral range from 0.5 to 2 μm (Fig. 1). In order to determine the intensities and positions of spectral components, we have used a deconvolution procedure with fitting to Lorentz functions. The photoemf spectra had com plicated shapes that could be described by a sum of three Lorentzian components. The energy positions of components were 1.44 eV (peak A), 1.12 eV (peak B), and 0.96 eV (peak C). Additional investigations of the morphology and elemental composition of the surface of GaAs samples before and after the UST and the data of Raman spec troscopy showed the following. Microphotographies of the surface of samples after the UST (Fig. 2) clearly reveal characteristic regions formed under the action of the cavitating fluid. The cavitationinduced changes depended both on the UST duration and on the power and frequency of ultrasound. Processing at TECHNICAL PHYSICS LETTERS
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40 μm Fig. 2. Microphotographies (NV2E microscope, Carl Zeiss Jena) of the surface of GaAs samples after the UST at a focal power density of 15 W/cm2 for (a) 30 min and (b) 60 min.
1–3 MHz for a minimum time (5–10 min) led to the appearance of small, chaotically arranged depressions on the sample surface. An increase in the UST dura tion to 1 h (for the same frequency and maximum intensity of ultrasound) led to the formation of a sub micronsized relief of rippled and concentric struc tures with small rounded bumps. Separate concentric structures have lateral dimensions within 5–10 μm. The results of surface reconstruction by the atomic force microscopy (AFM) on a Digital Instruments NanoScope IIIA showed that the structured surface occurs significantly below the initial surface level. The elemental composition of samples treated in the cavitating fluid was studied by the method of energydispersive Xray (EDX) spectroscopy on a JSM6490 instrument. The EDX spectroscopy data confirmed the presence of 5–7% nitrogen in the whole structured layer and up to 14% in some local regions. The results of Raman spectroscopy measurements are presented in the table as the vibrational frequencies of GaAs crystals in the initial state and after the UST in cavitating liquid nitrogen. It was found that the spectrum of GaAs in the initial state had a single mode character. The line at ~292 cm–1 corresponds to the mode that is allowed in the given geometry and
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SAVKINA, SMIRNOV GaN
Eg (x), eV
3
2 GaAs 1
0
0.2
0.4
0.6
0.8
1.0
1− x Fig. 3. Dependence of bandgap width Eg on the composi tion of GaAs1 – xNx compound calculated using the for GaAs
mula Eg(x) = x E g GaAs
Eg
GaN
= 1.44 eV, E g
GaN
+ (1 – x) E g
– bx(1 – x) [16] for
= 3.2 eV, b = 7.5 eV.
corresponds to a longitudinal optical phonon (LO1). The Raman spectra of GaAs upon the UST in liquid nitrogen were reported and analyzed previously [11]. Here, we only briefly present the main results, accord ing to which the spectra of these samples have a multi mode character that is related to the incorporation of nitrogen into the GaAs matrix lattice and the forma tion of an ordered system of microscopic clusters (GaN)m(GaAs)n (m = n = 1), which results in defor mation of the matrix. This leads to violation of the selection rules that is accompanied by the appearance of initially forbidden modes and additional “folded” phonon modes, which are related to the ordering of nitrogen atoms during the formation of a solid solu tion, especially when the concentration of nitrogen exceeds its solubility level (~1%). The incorporation Vibrational frequencies of (001) GaAs crystals in the initial state and after the UST in cavitating liquid nitrogen Initial (001)GaAs sample 295 cm–1 (LO1)
(001)GaAs upon UST (3 MHz, 15 W/cm2, 50 min) 281 cm–1 (LO1, Ga–As) 267 cm–1 (TO1, Ga–As) 276 cm–1 (LOf, folded mode) 255 cm–1 (LOc, confined mode) 426 cm–1 (Ga–N) 475 cm–1 (Ga–N)
Raman spectra were measured in the backscattering geometry at room temperature with Ar+ laser excitation at λ = 514.5 nm on a doublegrating spectrometer at a spectral resolution of 2 cm–1 and signal detection in the photon count regime.
of nitrogen atoms into the GaAs lattice is directly con firmed by the appearance of vibrational modes corre sponding to Ga–N bonds (see table). On the basis of an analysis of the results presented above, it is possible to explain features of the photo emf spectra observed for the samples ulrtrasonically treated in liquid nitrogen. As is known, the emf that arises in a semiconductor as a result of the absorption of electromagnetic radiation is due to the spatial sepa ration of generated charge carriers. Since the samples were exposed to a homogeneous light, the diffusion component of the photoemf can be ignored. The nearcontact regions are also not involved in the photoemf formation. Evidently, the measured photo emf signal is related to inhomogeneity caused by the treatment of the semiconductor in the cavitating fluid. The electric field that appears in the region of inhomo geneity (in this case, inhomogeneous chemical com position) accelerates the photogenerated charge carri ers, which leads to the spatial separation of carriers possessing opposite signs. In addition, it can be sug gested that the photoeffect is additionally increased due to a significant (by orders of magnitude) differ ence between the mobilities and diffusion coefficients of electrons and holes in GaAs and in solid solution of component nitrides. Concerning the photoemf spectrum (Fig. 1), it should also be noted that the position of peak A (1.44 eV) is close to the bandgap width of GaAs at T = 300 K [15]. The photoresponse at longer wavelengths (1–2 μm) is probably related to the aforementioned incorporation of nitrogen atoms into the GaAs lattice. As is known, the introduction of nitrogen into GaAs leads to signif icant variations in the bandgap width Eg of GaAs1 – xNx compound (see, e.g., [16]). Moreover, the dependence of Eg on composition x exhibits an anomalous charac ter: as nitrogen content x increases, Eg decreases and passes through a minimum of ~0.3 eV at x ~ 40% (Fig. 3). Let us assume that peaks B and C (in the region of 1–2 μm) in the photoemf spectrum (Fig. 1) are related to the absorption in GaAs1 – xNx. We can then find the corresponding x values using an empiri cal formula for Eg(x) (see the caption to Fig. 3) [16]. These estimates are close to the values determined by EDX spectroscopy. For correct determination of x from photoemf spectra, one should take into account that GaAs1 – xNx solid solution usually occurs in a somewhat extended state relative to the GaAs lattice, which must shift the bandgap width of GaAs1 – xNx toward lower values [17]. The results of our investigation lead to the follow ing conclusions. It is established that GaAs1 – xNx solid solution is formed on the surface of singlecrystalline GaAs structured as a result of UST in liquid nitrogen. The content of nitrogen in these structured surface layers in on a level of 5–7% and reaches up to 14% in some local regions. Spatial variation of the nitrogen distribution leads to the appearance of a barrier photo emf in the structured layers of GaAs. Singlecrystalline
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GaAs treated by the proposed method is characterized by expansion of the spectral interval of photosensitiv ity up to 2 μm in the longwavelength range. REFERENCES 1. H. Xu, B. W. Zeiger, and K. S. Suslick, Chem. Soc. Rev. 42, 2555 (2013). 2. D. L. Miller, Progr. Biophys. Mol. Biol. 93 (1), 314 (2007). 3. F. Chemat and M. K. Khan, Ultrason. Sonochem. 18 (4), 813 (2011). 4. Critical Cleaning Process Applications Management Safety and Environmental Concerns, Ed. by B. Kanegs berg and E. Kanegsberg (CRC Press, 2011). 5. J. H. Bang and K. Suslick, Adv. Mater. 22, 1039 (2010). 6. A. Ye. Baranchikov, V. K. Ivanov, and Yu. D. Tretyakov, Russ. Chem. Rev. 76, 133 (2007). 7. R. K. Savkina, Recent Patents on Electrical and Elec tronic Engineering 6 (3), 157 (2013). 8. Y. Arata and Y.Ch. Zhang, Appl. Phys. Lett. 80 (13), 2416 (2002).
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Translated by P. Pozdeev
