Properties Of Epilayers Research Articles

Studies of the Crystal Structure and the Optical Properties of ZnTe Epilayers

Studies of the Crystal Structure and the Optical Properties of ZnTe Epilayers

Observations of the Crystal Structure and the Optical Properties of ZnS Epilayers by Using Spectroscopic Ellipsometry

Observations of the Crystal Structure and the Optical Properties of ZnS Epilayers by Using Spectroscopic Ellipsometry

Effects of substrate temperature upon optical properties of ZnTe epilayers grown on (100) GaAs substrates by MOVPE

AbstractThe photoluminescence (PL) properties of the ZnTe epilayers grown on (100) GaAs substrates with various substrate temperatures have been investigated. It has been demonstrated that the PL spectra depend strongly on the substrate temperature. A too low (390 °C) or too high (440 °C) substrate temperature deteriorates the epilayer quality due to the formation of defects or/and to the inclusion of impurities. A moderate substrate temperature (around 420 °C) is suitable to obtain a high‐quality epilayer with the narrow spectral width and strong emission intensity. This is attributed to the moderate substrate temperature improving the crystallinity of the ZnTe epilayer by enhancing recrystallization of the adsorbed precursors.

Effect of reactor pressure on the electrical and structural properties of InN epilayers grown by high-pressure chemical vapor deposition

The influence of super-atmospheric reactor pressures (2.5–18.5 bar) on the electrical and structural properties of InN epilayers deposited on GaN/sapphire (0001) templates by high-pressure chemical vapor deposition has been studied. The epilayers were analyzed by Raman, x-ray diffraction (XRD), and Fourier transform infrared reflectance spectrometry to determine the structural properties as well as the phonon frequencies, dielectric function, plasma frequency, layer thickness and damping parameters of the epilayers. For the studied process parameter space, best material properties were achieved at a reactor pressure of 12.5 bar and a group-V/III ratio of 2500 with a free carrier concentration of 1.5 × 1018 cm−3, a mobility of the bulk InN layer of 270 cm2 V−1 s−1, and a Raman (E2 high) FWHM value of 10.3 cm−1. This study shows that the crystalline layer properties—probed by XRD 2θ–ω scans—improve with increasing reactor pressure.

Characterization of epitaxial ZnTe layers grown on GaAs substrates by transmission electron microscopy and photoluminescence

The authors have investigated the effects of a low-temperature ZnTe buffer layer on structural and optical properties of ZnTe epilayers grown on (100) GaAs substrates by transmission electron microscopy and photoluminescence measurements. It has been found that the low-temperature ZnTe buffer layer can reduce the defects such as disordered region in the ZnTe buffer layer and suppress the dislocations at the ZnTe surface region, resulting in the formation of a high quality ZnTe epilayer grown on the low-temperature buffer layer.

Residual strains and optical properties of ZnO thin epilayers grown on r-sapphire planes

ZnO epilayers with a thickness of ∼360 nm were directly grown on r-sapphire planes at different temperatures (550, 500, 450 °C) with metal-organic chemical vapor deposition. Residual strains along the vertical direction and optical properties of the epilayers were investigated by using a variety of techniques including x-ray diffraction, low-temperature photoluminescence (PL), cathodoluminescence and Raman scattering. Compressive strains were revealed to exist in all the epilayers. Their strengths were found to reduce with increasing growth temperature. In addition, the optical properties of the epilayers were studied and the relationship between the strain and PL peak position was unveiled.

Elaboration of Monocrystalline Si Thin Film on 3C-SiC(100)/Si Epilayers by Low Pressure Chemical Vapor Deposition

The growth of continuous silicon monocrystalline thin films on 3C-SiC epilayers deposited on silicon substrates is presented in this study. Such heterostructures can be beneficial for the fabrication of Micro Electro Mechanical Systems or electronic applications. The elaboration of these heterostructures was carried out using Low Pressure Chemical Vapor Deposition. X-ray Diffraction, Fourier Transformed Infra-Red spectroscopy and Scanning Electron Microscopy have been used to investigate the structural properties of Si epilayers and their dependence on growth conditions. Monocrystalline Si (110) films are obtained on 3CSiC(100)/Si (100) substrates, only when using growth temperatures close to 850°C. The strong influence of the underlying 3C-SiC film on the final structural properties of Si epilayer is evidenced.

AlN插入层对aAlGaN的外延生长的影响

The effect of AlN interlayer on strain states and its effect on optical properties of a-AlGaN epilayers grown by using metal organic chemical vapor deposition (MOCVD) method are investigated. The strain is characterized by the frequency shift based on Raman spectroscopy measurement. The results show that residual strain in a-AlGaN grown on the AlN interlayer is relaxed due to AlN interlayer act as a stable and compliant substrate induced weakening of mechanical strength. Accordingly, the near band edge emission (NBE) peak shows red shift in room temperature photoluminescence measurement. In addition, the introduction of AlN interlayer lead to the red shift of NBE photo-luminescence peaks, which can be contribute to the strain determined by Raman spectra.

Study of InN surface by high resolution electron energy loss spectroscopy (HREELS )

Understanding the surface structure and termination of semiconductor thin films during epitaxial growth is critical since it affects the structural, interfacial and electronic properties of epilayers and device structures. The structural properties and surface bonding configuration of InN layers grown by high pressure chemical vapor deposition (HPCVD) have been characterized using high resolution electron energy loss spectroscopy (HREELS). The HREELS analysis of these InN layers shows that the surface is predominantly terminated with NH2 species. This is in contrast to previous work on HPCVD-grown InN layers, which showed only NH species on the nitrogen terminated surface. The presence of NH2 related modes of vibration is the indication of the orientation of the surface other than c-plane.Key words: HREELS; Indium nitride; NH<sub>2</sub>The Himalayan Physics Vol.2, No.2, May, 2011Page: 35-37Uploaded Date: 1 August, 2011

Growth of GaN epilayers on c-, m-, a-, and (20.1)-plane GaN bulk substrates obtained by ammonothermal method

GaN epilayers were grown by metalorganic chemical vapor deposition (MOCVD) on c-, m-, a-, and (20.1)-plane GaN substrates obtained by the ammonothermal method. The influence of (i) the surface preparation of substrates, (ii) MOCVD growth parameters, and (iii) the crystallographic orientation of substrates on the structural and optical properties of GaN epilayers was investigated and carefully analyzed. It was observed that the polishing of substrates and their misorientation have strong impact on the quality of GaN epilayers grown on these substrates. The MOCVD growth process was optimized for epilayers grown on m-plane GaN substrates. The best structural and optical properties were achieved for epilayers deposited at 1075 °C and the total reactor pressure of 50 mbar. These conditions were applied to grow GaN epilayers on substrates with other ( c-, a-, and (20.1)-plane) crystallographic orientations in the same MOCVD process. Particularly good optical properties were obtained for GaN epilayers deposited on polar and non-polar ( m- and a-plane) substrates, whereas slightly worse optical properties were observed for epilayers deposited on the semi-polar substrate. It therefore means that MOCVD growth conditions optimized for a given crystallographic direction ( m-plane direction in this case) work rather well also for other crystallographic directions.

Determination of the transport properties in 4H-SiC wafers by Raman scattering measurement**Project supported by the National Natural Science Foundation of China (Grant No. 60876003) and the Knowledge Innovation Project of Chinese Academy of Sciences (Grant Nos. Y072011000 and ISCAS2008T04) and the Science and Technology Projects of the State Grid Corporation of China (ZL71-09-001).

The free carrier density and mobility in n-type 4H-SiC substrates and epilayers were determined by accurately analysing the frequency shift and the full-shape of the longitudinal optic phonon—plasmon coupled (LOPC) modes, and compared with those determined by Hall-effect measurement and that provided by the vendors. The transport properties of thick and thin 4H-SiC epilayers grown in both vertical and horizontal reactors were also studied. The free carrier density ranges between 2×1018 cm−3 and 8×1018 cm−3 with a carrier mobility of 30–55 cm2/(V·s) for n-type 4H-SiC substrates and 1× 1016−3×1016 cm−3 with mobility of 290–490 cm2/(V·s) for both thick and thin 4H-SiC epilayers grown in a horizontal reactor, while thick 4H-SiC epilayers grown in vertical reactor have a slightly higher carrier concentration of around 8.1×1016 cm−3 with mobility of 380 cm2/(V·s). It was shown that Raman spectroscopy is a potential technique for determining the transport properties of 4H-SiC wafers with the advantage of being able to probe very small volumes and also being non-destructive. This is especially useful for future mass production of 4H-SiC epi-wafers.

Influence of thermally diffused aluminum atoms from sapphire substrate on the properties of ZnO epilayers grown by metal-organic chemical vapor deposition

In this study, the authors investigate the evolution of the structural and electrical properties of ZnO epilayers grown by the metal-organic chemical vapor deposition method on c-sapphire substrates. The inserting of a low-temperature ZnO buffer layer not only significantly improves the structural quality of the high-temperature (HT)-grown ZnO epilayer on a sapphire substrate but also results in high background electron concentration in it from the Hall-effect measurement. After subtracting the conductive contribution from a thin degenerated layer mostly formed between the buffer layer and the substrate based on the two-layer model, the deduced electron-carrier concentration is still in the order of 1018 cm−3, which is much larger than the 1016 cm−3 obtained from capacitance-voltage measurement near the top surface. This indicates that a much thicker layer with high carrier concentration should be formed in the HT-grown ZnO epilayer, which is significantly different from that observed in GaN epitaxy, where only a thin degenerated interfacial layer is suggested to form in the GaN buffer layer. Al atoms’ distribution acquired from secondary-ion mass spectrometry shows a strong dependence on the temperature of the ZnO growth process, indicating that a thermally enhanced diffusion mechanism should be responsible for the observation of the enhanced Al atom concentration in the HT-grown ZnO epilayer. As substituted Al atoms on the Zn site act as donors in ZnO, the one-to-one correspondence between Al content and the carrier concentration, as well as the analysis of temperature-dependent Hall-effect measurement, indicates that diffusion-induced gradient-distributed AlZn shallow donors should be the main origin of the high background-carrier concentration in the HT-grown ZnO epilayers.

Structural and Optical Properties of Nonpolar $(11 \bar{2}0)$ ZnO Films Grown by Plasma-Assisted Molecular-Beam Epitaxy

Structural and optical properties of ZnO epilayers grown on r -plane sapphire substrates by plasma-assisted molecular-beam epitaxy have been investigated using various techniques. A high-purity wurtzite structure and a smooth surface with an a-plane orientation were obtained. The resonant Raman scattering spectra show a typical multiphonon vibration characteristic from wurtzite ZnO. A good near band gap luminescence property is revealed in photoluminescence spectra.

N incorporation and optical properties of GaAsN epilayers on (3 1 1)A/B GaAs substrates

We compared the N incorporation and optical emission in GaAsN epilayers grown on (3 1 1)A/B and (1 0 0) GaAs substrates using a chemical beam expitaxy system. Over the growth-temperature range 420 –460 °C, N composition was enhanced 2–3 times for the epitaxial growth following [3 1 1]B orientation, but reduced in the [3 1 1]A direction. Both (3 1 1) A and B substrates are effective to weaken the photoluminescence emission from the deep levels as compared with the (1 0 0) plane. The deep-level emission can be further suppressed for all substrates by increasing the growth temperature and/or performing postgrowth annealing. However, in contrast to the continuous increase in total emission intensities of (3 1 1)B sample, a decreasing tendency was recorded for (3 1 1)A with the rise in growth temperature. The optimum growth temperature and annealing conditions for better crystal quality were found to depend on the growth orientation and surface polarity. These results present a potential approach to improving the N incorporation efficiency in Ga(In)AsN materials through adopting high-index substrates such as (3 1 1)B.

Optical and magnetic properties of GaN epilayers implanted with ytterbium

We have studied the optical and magnetic properties of ytterbium implanted GaN epilayer grown on (0001) sapphire by metalorganic chemical vapor by deposition (MOCVD). Samples were implanted at room temperature with Yb ions at dose 4?10 15 cm −2 and energy of 150 keV. The implanted samples were annealed at 1000 ?C in N 2 at atmospheric pressure to recover implantation damages. The photoluminescence (PL), PL excitation (PLE), and PL kinetics have been studied with continuous and pulse photo-excitations in 360-1100 nm spectral range at different temperatures. The characteristic Yb 3+ ion emission spectra were observed in the spectral range between 970-1050 nm. Theoretical fittings of the experimental PL temperature and PL kinetics data suggest that Yb 3+ ions are involved in at least two major luminescence centers. The PLE spectra indicate that excitation of the Yb 3+ ion occurs via electron-hole pair generation and complex processes. Magnetization versus magnetic field curves shows an enhancement of magnetic order for Yb-implanted samples in 5 K to 300 K temperature range. The Yb-implanted GaN sample showing weak ferromagnetic behavior was compared with the ferromagnetic in situ doped GaYbN material.

Effect of Heat Treatment on Structural and Optoelectronic Properties of GaN Epilayers

GaN epilayers were grown on sapphire by metal-organic chemical vapor deposition (MOCVD), and the samples were annealed with rapid thermal processor (RTP) at 650, 750, 850 and 950oC, respectively. The effect of heat treatment on structural and optoelectronic properties of GaN epilayers was investigated. X-ray diffraction (XRD) analysis shows that the full width at half maximum (FWHM) of the rocking curves becomes smaller as the annealing temperature increases. Photoluminescence (PL) spectra at room temperature demonstrate that the yellow band decreases with the increase of annealing temperature. Hall-effect measurements reveal that carrier concentration of the GaN epilayers raise with the increase of annealing temperature. The results suggest that the structural and optoelectronic properties of GaN epilayers could be significantly improved by heat treatment.

Effect of growth temperature of AlN interlayers on the properties of GaN epilayers grown on c‐plane sapphire by metal organic chemical vapor deposition

AbstractThe effect of growth temperature of AlN interlayers on the properties of GaN epilayers grown on c‐plane sapphire by metal organic chemical vapor deposition has been investigated by high resolution X‐ray diffraction (HRXRD) and Raman spectroscopy. It is concluded that the crystalline quality of GaN epilayers is improved significantly by using the high temperature AlN (HT‐AlN) interlayer in GaN buffers. The density of threading dislocation is reduced especially for edge type dislocations. Higher compressive stress exists in GaN epilayers with HT‐AlN interlayer than with low temperature AlN (LT‐AlN) interlayer, which is related to the reduction of strain relaxation caused by the formation of misfit dislocation. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interfacial structure of semipolar AlN grown on m -plane sapphire by MBE

The interfacial properties of epilayers grown on m-plane sapphire by plasma-assisted molecular-beam epitaxy (PAMBE) were investigated in the context of achieving high-quality semipolar GaN/AlN heterostructures with reduced internal polarization. Epilayers grown under metal-rich conditions with and without nitridation pre-treatment of the sapphire surface were compared using transmission electron microscopy (TEM). In addition to the dominant semipolar orientation, the films exhibited an interfacial zone comprising AlN nanocrystals with the m-plane orientation. The size of the nonpolar nanocrystals increased significantly with sapphire nitridation. A good epitaxial relationship with the substrate was attained by both the semipolar as well as the nonpolar AlN. However, the interfacial structure was perturbed by interfacial roughness and pit formation. The surface nanofacets were correlated to the increased introduction of nanocrystals with m-plane orientation.

Comparative study of different properties of GaN films grown on (0001) sapphire using high and low temperature AlN interlayers

Comparative study of high and low temperature AlN interlayers and their roles in the properties of GaN epilayers prepared by means of metal organic chemical vapour deposition on (0001) plane sapphire substrates is carried out by high resolution x-ray diffraction, photoluminescence and Raman spectroscopy. It is found that the crystalline quality of GaN epilayers is improved significantly by using the high temperature AlN interlayers, which prevent the threading dislocations from extending, especially for the edge type dislocation. The analysis results based on photoluminescence and Raman measurements demonstrate that there exists more compressive stress in GaN epilayers with high temperature AlN interlayers. The band edge emission energy increases from 3.423 eV to 3.438 eV and the frequency of the Raman shift of E2(TO) moves from 571.3 cm−1 to 572.9 cm−1 when the temperature of AlN interlayers increases from 700 °C to 1050 °C. It is believed that the temperature of AlN interlayers effectively determines the size, the density and the coalescence rate of the islands, and the high temperature AlN interlayers provide large size and low density islands for GaN epilayer growth and the threading dislocations are bent and interactive easily. Due to the threading dislocation reduction in GaN epilayers with high temperature AlN interlayers, the approaches of strain relaxation reduce drastically, and thus the compressive stress in GaN epilayers with high temperature AlN interlayers is high compared with that in GaN epilayers with low temperature AlN interlayers.

Probing the relationship between structural and optical properties of Si-doped AlN

Much efforts have been devoted to achieve conductivity control in the ultrahigh band gap (∼6.1 eV) AlN by Si doping. The effects of Si-doping on the structural and optical properties of AlN epilayers have been investigated. X-ray diffraction studies revealed that accumulation of tensile stress in Si-doped AlN is a reason for the formation of additional edge dislocations. Photoluminescence (PL) studies revealed that the linewidths of both band-edge and impurity related transitions are directly correlated with the density of screw dislocations, Nscrew, which increases with the Si doping concentration (NSi). Furthermore, it was formulated that the band-edge (impurity) PL emission linewidth increases linearly with increasing Nscrew at a rate of ∼3.3±0.7 meV/108 cm−2 (26.5±4 meV/108 cm−2), thereby establishing PL measurement as a simple and effective method to estimate screw dislocation density in AlN epilayers.