Large area depositon of Cd1-xZnxTe on GaAs and Si substrates by metalorganic chemical vapor deposition

Results of large-area (up to 1000 cm²/run) Cd_1-xZn_xTe heteroepitaxy on both GaAs and GaAs/Si substrates by metalorganic chemical vapor deposition (MOCVD) are presented. Cd_1-xZn_xTe (x = 0-0.1) films exhibited specular surface morphology, 1% thickness uniformity (standard deviation), and compositional uniformity (Δx) of ±0.002 over 100 mm diam substrates. For selected substrate orientations and deposition conditions, the only planar defects exhibited by (lll)B Cd_1-xZn_xTe/GaAs/Si films were lamella twins parallel to the CdTe/GaAs interface; these do not propagate through either the Cd_1-xZn_xTe layer or subsequently deposited liquid phase epitaxy (LPE) HgCdTe layer(s). Background Ga and As-impurity levels for Cd_1-xZn_xTe on GaAs/Si substrates were below the secondary ion mass spectroscopy detection limit. Preliminary results of HgCdTe liquid phase epitaxy using a Te-rich melt on Si-based substrates resulted in x-ray rocking curve linewidths as narrow as 72 arc-sec and etch-pit densities in the range 1 to 3 x 10⁶ cm².     

The influence of As and Ga prelayers on the metal-organic chemical vapor deposition of GaAs/Ge

GaAs epilayers were grown on Ge by metal-organic chemical vapor deposition (MOCVD) with As or Ga prelayers. The grown epilayers were examined for surface morphology, antiphase domain (APD) presence, and optical quality using optical interference contrast microscopy, molten potassium hydroxide (KOH) etching, and photoluminescence (PL) spectroscopy. The As prelayer results in smooth, shiny, and APD-free epilayers with good optical quality. In contrast, the Ga prelayer results in a rough surface with APDs and higher carbon incorporation.     

Electron microscope studies of InxGav1−xAs/GaAs/Si grown by metalorganic chemical vapor deposition

The microstructure of In_xGa_1−xAs/GaAs (5 nm/5 nm, x < 0 to 1.0), as grown by a metalorganic chemical vapor deposition two-step growth technique on Si(100) at 450‡C, and subsequently annealed at 750‡C, is investigated using plan-view and cross-sectional transmission electron microscopy. The variations in resultant island morphology and strain as a function of the In content were examined through the comparison of the misfit dislocation arrays and moirés observed. The results are discussed in relation to the ways in which the island relaxation process changes for high In content.     

Hall effect analysis of high purity p-type GaAs grown by metalorganic chemical vapor deposition

Variable temperature Hall effect and low temperature photoluminescence measurements have been performed on high purityp- andn-type GaAs grown at atmospheric pressure by metalorganic chemical vapor deposition. These high purity epitaxial GaAs layers were grown as a function of the arsine (AsH₃) to trimethylgallium (TMG) ratio (V/III ratio). The accurate quantitative assessment of the electronic properties of thep-type layers was emphasized. Analysis of the material focussed on the variation of the concentration of the shallow impurities for different V/III ratios. Surface and interface depletion effects are included to accurately estimate the concentration of impurities. The model of the merging of the excited states of the acceptor with the valence band is used to include the dependence the activation energy of the impurity on the acceptor concentration as well as on acceptor species identity. The characteristicp- ton-type conversion with increasing V/III ratio was observed in these samples and the reason for thep- ton-type conversion is that the background acceptor concentration of carbon decreases and the germanium donor concentration increases as the V/III ratio is increased.     

Zinc telluride films by photoenhanced metalorganic chemical vapor deposition

Polycrystalline films of zinc telluride (ZnTe) have been deposited on glass and conducting semiconductor coated glass substrates at 270°-350° C by photoenhanced metalorganic chemical vapor deposition (PECVD) using the reaction of dimethylzinc (DMZn) or diethylzinc (DEZn) and diisopropyltellurium (DIPTe) in hydrogen under atmospheric pressure. The deposited films are always ofp-type conductivity. Their properties are affected by the DMZn/DIPTe or DEZn/DIPTe molar ratio in the reaction mixture. The optimum DMZn/DIPTe ratio has been found to be approximately 0.9 on the basis of the open-circuit voltage of ZnTe/CdS heterojunctions and photoconductivity measurements. Without intentional doping, the deposited films are of high resistivity (>10⁷ ohm-cm) at room temperature, and the resistivity of these films has been controlled by using arsine as a dopant. The structural, optical, and electrical properties of ZnTe films have been characterized. Supported by the Solar Energy Institute under Subcontract XL-8-18091-1.     

MOCVD grown CdZn Te/GaAs/Si substrates for large-area HgCdTe IRFPAs

Large-area HgCdTe 480×640 thermal-expansion-matched hybrid focal plane arrays were achieved by substituting metalorganic chemical vapor deposition (MOCVD)-grown CdZnTe/GaAs/Si alternative substrate in place of bulk CdZnTe substrates for the growth of HgCdTe p-on-n double-layer heterojunctions by controllably-doped mercury-melt liquid phase epitaxy (LPE). (100) CdZnTe was grown by MOCVD on GaAs/Si using a vertical-flow high-speed rotating disk reactor which incorporates up to three two-inch diameter substrates. Layers having specular surface morphology, good crystalline structure, and surface macro defect densities <50 cm⁻² are routinely achieved and both the composition uniformity and run-to-run reproducibility were very good. As the composition of the CdZnTe layers increases, the x-ray full width at half maximum (FWHM) increases; this is a characteristic of CdZnTe grown by VPE techniques and is apparently associated with phase separation. Despite a broader x-ray FWHM for the fernary CdZnTe, the FWHM of HgCdTe grown by LPE on these substrates decreases, particularly for [ZnTe] compositions near the lattice matching condition to HgCdTe. An additional benefit of the ternary CdZnTe is an improved surface morphology of the HgCdTe layers. Using these silicon-based substrates, we have demonstrated 78K high-performance LWIR HgCdTe 480×640 arrays and find that their performance is comparable to similar arrays fabricated on bulk CdZnTe substrates for temperatures exceeding approximately 78K. The performance at lower temperatures is apparently limited by the dislocation density which is typically in the low-mid 10⁶ cm⁻² range for these heteroepitaxial materials.     

GaAs/Ge heterojunction grown by metal-organic chemical vapor deposition and its application to high efficiency photovoltaic devices

We have studied the heteroepitaxial growth of GaAs on Ge substrates by metal-organic chemical vapor deposition (MOCVD). Different growth conditions and substrate orientations were employed to examine the properties of GaAs grown upon Ge substrates, and in particular the GaAs/Ge interface. The interface properties were found to strongly depend on growth conditions. By small changes in the growth temperature, the GaAs/ Ge interface was altered from active to passive. Only a narrow temperature window (600 to 630° C) for the initial GaAs layer growth gave the passive-Ge junction together with good surface morphology. Accordingly, a high efficiency (19%, AMO) GaAs solar cell was grown by atmospheric pressure MOCVD on a Ge substrate without any junction in the Ge.     

Structural characterization of thick, high-quality epitaxial Ge on Si substrates grown by low-energy plasma-enhanced chemical vapor deposition

In this paper, we report on the growth of epitaxial Ge on a Si substrate by means of low-energy plasma-enhanced chemical vapor deposition (LEPECVD). A Si_1?xGe_x graded buffer layer is used between the silicon substrate and the epitaxial Ge layer to reduce the threading dislocation density resulting from the lattice mismatch between Si and Ge. An advantage of the LEPECVD technique is the high growth rate achievable (on the order of 40 Å/sec), allowing thick SiGe graded buffer layers to be grown faster than by other epitaxial techniques and thereby increasing throughput in order to make such structures more manufacturable. We have achieved relaxed Ge on a silicon substrate with a threading dislocation density of 1 × 10⁵ cm^?2, which is 4?10x lower than previously reported results.     

Mass spectroscopy study of GaN metalorganic chemical vapor deposition

Metalorganic chemical vapor phase deposition of GaN on (100) GaAs has been studied using mass spectroscopy. With increasing substrate temperature, the amount of decomposed trimethylgallium (TMGa) was observed to increase exponentially with a characteristic energy of 1.5 eV. The presence of NH₃ was found to suppress the production of CH₃ in the gas phase. This implies that CH₃ of TMGa reacts with the hydrogen atom of NH₃, forming CH₄ as a main gas product. Studies of nitrogen evaporation from the growth surface when TMGa flow was off lead to the conclusion that increased growth rate could result in decreased background electron concentration due to nitrogen vacancy. The presence of NH₃ significantly promotes the decomposition of TMGa. Desorption of excess Ga atoms from the growth surface at low NH₃ flow rates takes place as suggested by the increased ratio of peak intensity of Ga (m/e = 69) to that of DMGa ((CH₃)₂Ga, m/e- 99) with decreasing NH₃ flow rate.     

High-Quality GaN heteroepitaxial films grown by metalorganic chemical vapor deposition

In this paper, we describe the growth and characterization of high-quality GaN heteroepitaxial films grown on basal-plane sapphire substrates using metalorganic chemical vapor deposition. The quality of these films is analyzed by a variety of methods, including high-resolution x-ray diffraction, optical transmission spectroscopy, transmission electron microscopy (TEM), room temperature photoluminescence, and room-temperature Hall measurements. The x-ray diffraction full width at half maximum value of ΔΘ ～37 arc s is the narrowest reported to date for any III-V nitride film on any substrate. The x-ray rocking curves for ～0.48 μm thick GaN/Al₂O₃ heteroepitaxial layers exhibit Pendellösung fringes, indicating that even relatively thin films can be of high quality. High-resolution TEM lattice images further attest to the excellent structural quality, showing the films to be completely free of stacking faults. Furthermore, no evidence of columnar growth is observed.     

UV photon assisted control of interface charge between CdTe substrates and metalorganic chemical vapor deposition CdTe epilayers

The technology to control the interface charge density between CdTe substrates and CdTe epilayers grown by metalorganic chemical vapor deposition is studied. The interface charge is determined by the modified built-in potential derived from capacitance-voltage characteristics of Schottky contacts formed on the epilayers. Novel ultraviolet (UV) photon assisted and photo thermal surface pretreatments that control the interface between p-type CdTe substrates and CdTe epilayers are reported. The substrates are exposed to UV radiation provided by a high pressure Hg lamp operating at 600 W with a wide emission spectrum between 190 and 300 nm. The UV photon assisted surface pretreatment with hydrogen is compared with additional surface pretreatments: thermal pretreatment with hydrogen (without UV photons) and UV photo thermal pretreatment with hydrogen. The UV photon assisted and the UV photo thermal surface pretreatments with hydrogen reduce the interface charge density to a practically negligible value. In addition, the p-type doping level of the substrate is reduced considerably in a layer of few microns adjacent to the interface.     

Growth of high mobility InSb by metalorganic chemical vapor deposition

Thin films of InSb have been grown on insulating GaAs substrates using the metalorganic chemical vapor deposition technique with trimethyl indium and trimethyl antimony as reactants. We find that the mobilities obtained are usually low unless indium is predeposited onto the substrate. This indium predeposition technique greatly improves the yield of InSb films with mobilities of ~50000 crn²V⁻¹S⁻¹ at room temperature and a typical thickness of 2 microns. With this predeposition technique, the electron mobilities of these films become relatively independent of the vapor stroichiometry during growth and of the growth temperature. The electron mobilities are also very uniform across a wafer. These properties are obtained even when the film growth rate exceeds 2 μm/h.     

Selective growth of CdTe on Si and GaAs substrates using metalorganic vapor phase epitaxy

Epitaxial lateral overgrowth (ELO) is a new technique to grow low-defect-density thin films on lattice-mismatched substrates. For the ELO growth of CdTe and HgCdTe on Si substrate to be successful, the first requirement is that the growth should be selective. To that end, we have used several mask materials, and several growth conditions in order to obtain selective growth. A number of growth-experiments have been carried out, with temperatures in the range from 380°C to 550°C, and pressures in the range from 380 torr to less than 20 torr. Perfectly selective growth of CdTe has been achieved on Si and GaAs substrates using Si₃N₄ as the mask layer. Successful lateral epitaxial growth of CdTe was also achieved.     

Initial stages of GaN/GaAs (100) growth by metalorganic chemical vapor deposition

We have investigated the growth of GaN buffers by metalorganic chemical vapor deposition (MOCVD) on GaAs (100) substrates. Atomic force microscope (AFM) and reflection high-energy electron diffraction (RHEED) were employed to study the dependence of the nucleation on the growth temperature, growth rate, annealing effect, and growth time. A two-step growth sequence must be used to optimize and control the nucleation and the subsequent growth independently. The size and distribution of islands and the thickness of buffer layers have a crucial role on the quality of GaN layers. Based on the experimental results, a model was given to interpret the formation of hexagonal-phase GaN in the cubic-phase GaN layers. Using an optimum buffer layer, the strong near-band emission of cubic GaN with full-width at half maximum (FWHM) value as small as 5.6 nm was observed at room temperature. The background carrier concentration was estimated to be in the range of 10¹³ ∼ 10¹⁴ cm⁻³.     

Growth,characterization, and modeling of ternary InGaAs-GaAs quantum wells by selective-area metalorganic chemical vapor deposition

A computational diffusion model is used to predict thickness and composition profiles of ternary In_xGa_1-xAs quantum wells grown by selective-area, atmospheric pressure metalorganic chemical vapor deposition (MOCVD), and its accuracy is investigated. The model utilizes diffusion equations and boundary conditions derived from basic MOCVD theory, with reaction parameters derived from experimental results, to predict the concentration of each column III constituent throughout the concentration boundary layer. Solutions to these equations are found using the two-dimensional, finite element method. The growth thickness profiles of GaAs, InP, and In_xGa_1-xAs deposited by selective-area MOCVD are observed by conventional profilometry, and compositions are measured indirectly by laser emission wavelengths. The data presented show that the model accurately predicts growth thickness and composition profiles of ternary III-V materials grown by selective-area MOCVD.     

Characterization of n-type Ge layers on Si (100) substrates grown by rapid thermal chemical vapor deposition

Phosphorus-doped n-type Ge layers were grown on p-type Si (100) wafers (8 in. in diameter, resistivity 5–15 Ω cm) using rapid thermal chemical vapor deposition (RTCVD). The surface morphology was very smooth, with a root mean square (RMS) surface roughness of 0.29 nm. The in-plane lattice constant calculated from high-resolution X-ray diffraction (HR-XRD) data was 0.5664 nm, corresponding to in-plane tensile strain of ～0.47%. The Raman Ge peak for each location indicates tensile strain from the Ge wafer. We estimated the in-plane strain as tensile strain of ～0.45%, in excellent agreement with the XRD analysis. Initial photocurrent spectrum experiments on the sample confirm valence band splitting of the direct gap induced by tensile strain. The temperature dependence of the direct bandgap energy E_Γ1 of Ge can be described by the empirical Varshni expression E_Γ1(T)=0.864–5.49×10^–4T ²/(T+296).     

InGaAs quantum dots formed in tetrahedral-shaped recesses on GaAs (111)B grown by metalorganic chemical vapor deposition

Novel semiconductor quantum dots (QDs), grown in tetrahedral-shaped recesses (TSRs) formed on a (111)B GaAs substrate, are described from both material science and device application points of view. After explaining the fabrication procedure for TSRs, growth of InGaAs QDs and their optical properties are explained. It is revealed that an InGaAs QD of indium-rich chemical composition is formed spontaneously at the bottom of each TSR. The mechanism of the QD formation is discussed in detail. It is proved from magneto-photoluminescence that the QDs actually have optical properties peculiar to zero-dimensional confinement. Several experimental results indicating excellent growth controllability of the QDs are presented. Finally, recent challenges to apply the QDs to electronic memory devices are reported. Two kinds of devices, where the position of individual QD is artificially controlled, are proposed for the first time and the preliminary experimental results are explained.     

Growth of (GaAs)1_x(Ge2)x by metalorganic chemical vapor deposition

We report deposition of (GaAs)_{1_x}(Ge₂)_x on GaAs substrates over the entire alloy range. Growth was performed by metalorganic chemical vapor deposition at temperatures of 675 to 750°C, at 50 and 760 Torr, using trimethylgallium, arsine, and germane at rates of 2–10 μ/h. Extrinsic doping was achieved using silane and dimethylzinc in hydrogen. Characterization methods include double-crystal x-ray rocking curve analysis, Auger electron spectroscopy, 5K photoluminescence, optical transmission spectra, Hall-effect, and Polaron profiling. Results achieved include an x-ray rocking curve full-width at half maximum as narrow as 12 arc-s, Auger compositions spanning the alloy range from x = 0.03 to x = 0.94, specular surface morphologies, and 5K photoluminescence to wavelengths as long as 1620 nm. Undoped films are n type, with n ≈ 1 × 10¹⁷ cm⁻³. Extrinsic doping with silane and dimethylzinc have resulted in films which are n type (10¹⁷ to 10¹⁸ cnr⁻³) or p type (5 × 10¹⁸ to 1 × 10²⁰ cm⁻³). Mobilities are generally ≈ 50 cm²/V-s and 500 cm²/V-s, for p and n films, respectively.     

Metalorganic chemical vapor deposition growth and characterization of InGaP/GaAs superlattices

Lattice-matched In_0.49Ga_0.51P/GaAs superlattices were grown on (001) GaAs substrates using metalorganic chemical vapor deposition. The interface properties were characterized by photoluminescence, transmission electron microscopy, and x-ray diffraction. By varying the growth temperature, the precursor flow rates, and the growth interruption at the interfaces, we found that, while arsenic and phosphorus carry over have some effect on the formation of a low-bandgap InGaAsP quaternary layer at the interfaces, the In surface segregation seems to play an important role in the formation of the interface quaternary layer. Evidence of this indium segregation comes from x-ray and photoluminescence studies of samples grown at different temperatures. These studies show that the formation of an interfacial layer is more prominent when the growth temperature is higher. Growing a thin (∼1 monolayer thick) GaP intentional interfacial layer on top of the InGaP before the growth of the GaAs layer at the P→As transition effectively suppresses the formation of the low-bandgap unintentional interface layer. On the other hand, the growth of a thin GaAsP (or GaP) layer before the growth of the InGaP layer, at the As→P transition increases the formation of a low-bandgap interfacial layer. This nonequivalent effect of a GaP layer at the two interfaces on the PL properties is discussed.     

MOCVD growth of AlGaAs/GaAs structures on nonplanar {111} substrates: Evidence for lateral gas phase diffusion

We investigate the MOCVD growth characteristics of AlGaAs on nonplanar {111}A and {111}B substrates. Growth over features etched into the {111} substrates is found to be highly anisotropic and asymmetric. The ratio of growth rates on adjacent facets is strongly dependent on the depth of the etched feature during growth, and is strikingly different between AlGaAs and GaAs layers. These observations suggest a large difference in the surface chemistry of Al and Ga species under these growth conditions and indicate that the column III element determines the relative growth rates of different facets during nonplanar growth. The results also provide strong evidence that lateral gas phase diffusion of reactants can be perhaps more significant than surface migration as a mechanism determining the incorporation sites of column III elements. Growth characteristics on nonplanar {111} substrates are markedly different than those observed for nonplanar growth on {100} substrates, creating a new set of design tools for the single step growth of guided wave devices such as lasers, modulators and waveguides.