Growth kinetics of GaSb by metalorganic vapor phase epitaxy

The growth rates of GaSb by metalorganic vapor phase epitaxy were studied as functions of growth temperatures and partial pressures of precursors. A Langmuir-Hinshelwood model was used to explain the GaSb growth rate in the chemical reaction controlled regime. The relationship between growth kinetics and epilayer qualities was discussed and properties of GaSb were obtained.     

Deep level transient spectroscopy assessment of silicon contamination in AlGaAs layers grown by metalorganic vapor phase epitaxy

A systematic silicon contamination has been detected by deep level transient spectroscopy in undoped and n-type doped (Te, Se, Sn) AlGaAs layers, grown in two different metalorganic vapor phase epitaxy reactors. DX center generation by substitutional donors, with very specific capture and emission thermal barriers (fingerprints), is the key to unambiguously identifying their presence, with detection limits well below the standard secondary ion mass spectroscopy capability. We comment on the potential sources of Si contamination (most common in this epitaxial technique), and on the relevance of such contamination to interpreting correctly experimental data related to the microscopic structure of DX centers.     

Optical properties and recombination mechanisms in GaN and GaN:Mg grown by metalorganic vapor phase epitaxy

Photoluminescence (PL) and reflection spectra of undoped and Mg-doped GaN single layers grown on sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) were investigated in a wide range of temperatures, excitation intensities, and doping levels. The undoped layers show n-type conductivity (μ=400 cm²/Vs, n=3×10¹⁷ cm⁻³). After annealing at T=600–700°C, the Mg-doped layers showed p-type conductivity determined by the potential-profiling technique. A small value of the full width at half maximum (FWHM=2.8 meV) of the excitonic emission and a high ratio between excitonic and deep level emission (≈5300) are evidences of the high layer quality. Two donor centers with activation energies of 35 and 22 meV were observed in undoped layers. A fine structure of the PL band with two narrow lines in the spectral range of the donor-acceptor pair (DAP) recombination was found in undoped layers. An anomaly was established in the temperature behavior of two groups of PL lines in the acceptor-bound exciton and in donor-acceptor pair regions in Mg doped layers. The lower energy line quenched with increasing temperature appreciably faster than the high energy ones. Our data does not agree with the DAP recombination model. It suggests that new approaches are required to explain the recombination mechanisms in undoped and Mg-doped GaN epitaxial layers.     

Electrical and optical properties of lodine doped CdZnTe layers grown by metalorganic vapor phase epitaxy

Electrical and photoluminescence properties of iodine doped CdZnTe (CZT) layers grown by metalorganic vapor phase epitaxy have been studied. Doped layers showed an n-type conductivity from the Zn composition x=0 (CdTe) to 0.07. Above x=0.07, resistivities of doped layers increased steeply up to 10⁶ Ω-cm. Resistivities of doped CZT layers were higher than those of undoped layers above x=0.6. Photoluminescence intensity of doped layers increased compared to undoped layers. Doped CdTe and ZnTe layers showed neutral donor bound exciton emission lines at the exciton related region. Also, these layers showed an increase in emission intensity at the donor acceptor pair recombination bands. Sharp emission lines were observed in doped CZT layers at around 1.49 eV. These emission lines were considered to be originated from GaAs substrates which were optically excited by the intense emission from doped CZT layers.     

Morphology and defect structures of GaSb islands on GaAs grown by metalorganic vapor phase epitaxy

The initial nucleation of GaSb on (001) GaAs substrates by metalorganic vapor phase epitaxy has been investigated using transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). TEM results showed that the GaSb islands experience a morphological transition as the growth temperature increases. For growth at 520°C, the islands are longer along the [110] direction; at 540°C, they are nearly square, and at 560°C, they are longer along the direction. Possible mechanisms are proposed to describe such a transition. TEM and HREM examination showed that lattice misfit relaxation mechanisms depend on the growth temperature. For the sample grown at 520°C, the lattice mismatch strain was accommodated mainly by 90° dislocations; for the sample grown at 540°C, the misfit strain was relieved mostly by 90° dislocations with some of 60° dislocations, and for the sample grown at 560°C, the strain was accommodated mainly by 60° dislocations which caused a local tilt of the GaSb islands with respect to the GaAs substrate. The density of threading dislocations was also found to be dependent on the growth temperature. Mechanisms are proposed to explain these phenomena.     

ZnMgSSe/ZnSSe/ZnSe-heterostructures grown by metalorganic vapor phase epitaxy

ZnMgSSe heterostructures have been grown in a low-pressure metalorganic vapor phase epitaxy system with the precursors dimethylzinc triethylamine, ditertiarybutylselenide, tertiarybutylthiol, and biscyclopentadienylmagnesium at 330°C and a total pressure of 400 hPa. The optimization of the single layers was carried out by means of low temperature photoluminescence. Only the near band edge emission was observable with negligible deep levels. The heterostructures consisting of a triple ZnSe quantum well showed intense luminescence which hints at an effective carrier confinement. Scanning transmission electron microscopy investigations of the heterostructures still showed structural detects since the layers were not lattice matched to the GaAs substrate yet.     

HgCdTe growth on (552) oriented CdZnTe by metalorganic vapor phase epitaxy

We report the growth of HgCdTe on (552)B CdZnTe by metalorganic vapor phase epitaxy (MOVPE). The (552) plane is obtained by 180 rotation of the (211) plane about the [111] twist axis. Both are 19.47 degrees from (111), but in opposite directions. HgCdTe grown on the (552)B-oriented CdZnTe has a growth rate similar to the (211)B, but the surface morphology is very different. The (552)B films exhibit no void defects, but do exhibit ∼40 μm size hillocks at densities of 10–50 cm⁻². The hillocks, however, are significantly flatter and shorter than those observed on (100) metalorganic vapor phase epitaxy (MOVPE) HgCdTe films. For a 12–14 μm thick film the height of the highest point on the hillock is less than 0.75 μm. No twinning was observed by back-reflection Laue x-ray diffraction for (552)B HgCdTe films and the x-ray double crystal rocking curve widths are comparable to those obtained on (211)B films grown side-by-side and with similar alloy composition. Etch pit density (EPD) measurements show EPD values in the range of (0.6–5)×10⁵ cm⁻², again very similar to those currently observed in (211)B MOVPE HgCdTe. The transport properties and ease of dopant incorporation and activation are all comparable to those obtained in (211)B HgCdTe. Mid-wave infrared (MWIR) photodiode detector arrays were fabricated on (552)B HgCdTe films grown in the P-n-N device configuration (upper case denotes layers with wider bandgaps). Radiometric characterization at T=120–160 K show that the detectors have classical spectral response with a cutoff wavelength of 5.22 μm at 120 K, quantum efficiency ∼78%, and diffusion current is the dominant dark current mechanism near zero bias voltage. Overall, the results suggest that (552)B may be the preferred orientation for MOVPE growth of HgCdTe on CdZnTe to achieve improved operability in focal plane arrays.     

Direct growth of high-quality thick CdTe epilayers on Si (211) substrates by metalorganic vapor phase epitaxy for nuclear radiation detection and imaging

Direct growth of high-quality, thick CdTe (211) epilayers, with thickness up to 100 μm, on Si (211) substrates in a vertical metalorganic vapor phase epitaxy system is reported. In order to obtain homo-orientation growth on Si substrates, pretreatment of the substrates was carried out in a separate chamber by annealing them together with pieces of GaAs at 800–900°C in a hydrogen environment. Grown epilayers had very good substrate adhesion. The full-width at half-maximum (FWHM) value of the x-ray double-crystal rocking curve from the CdTe (422) reflection decreased rapidly with increasing layer thickness and remained between 140–200 arcsec for layers >18 μm. Photoluminescence measurement at 4.2 K showed high-intensity, bound excitonic emission and very small defect-related deep emissions, indicating the high crystalline quality of the grown layers. Furthermore, a CdTe/n⁺-Si heterojunction diode was fabricated that exhibited clear rectifying behavior.     

Structural properties of ZnSy Se1−yZnSe/GaAs (001) heterostructures grown by photoassisted metalorganic vapor phase epitaxy

ZnS_ySe_1−yZnSe/GaAs (001) heterostructures have been grown by photoassisted metalorganic vapor phase epitaxy, using the sources dimethylzinc, dimethylselenium, diethylsulfur, and irradiation by a Hg arc lamp. The solid phase composition vs gas phase composition characteristics have been determined for ZnSy_ySe_1−y grown with different mole fractions of dimethylselenium and different temperatures. Although the growth is not mass-transport controlled with respect to the column VI precursors, the solid phase composition vs gas phase composition characteristics are sufficiently gradual so that good compositional control and lattice matching to GaAs substrates can be readily achieved by photoassisted growth in the temperature range 360°C ≤ T ≤ 400°C. ZnSe/GaAs (001) single heterostructures were grown by a two-step process with ZnSe thicknesses in the range from 54 nm to 776 nm. Based on 004 x-ray rocking curve full width at half maximums (FWHMs), we have determined that the critical layer thickness is h_c ≤200 nm. Using the classical method involving strain, lattice relaxation is undetectable in layers thinner than 270 nm for the growth conditions used here. Therefore, the rocking curve FWHM is a more sensitive indicator of lattice relaxation than the residual strain. For ZnS_ySe_1−y layers grown on ZnSe buffers at 400°C, the measured dislocation density-thickness product Dh increases monotonically with the room temperature mismatch. Lower values of the Dh product are obtained for epitaxy on 135 nm buffers compared to the case of 270 nm buffers. This difference is due to the fact that the 135 nm ZnSe buffers are pseudomorphic as deposited. For ZnS_ySe_1−y layers grown on 135 nm ZnSe buffers at 360°C, the minimum dislocation density corresponds approximately to room-temperature lattice matching (y ∼ 5.9%), rather than growth temperature lattice matching (y ∼ 7.6%). Epitaxial layers with lower dislocation densities demonstrated superior optical quality, as judged by the near-band edge/deep level emission peak intensity ratio and the near band edge absolute peak intensity from 300K photoluminescence measurements.     

Self-organization phenomenon of strained InGaAs on InP (311) substrates grown by metalorganic vapor phase epitaxy

We have demonstrated that a self-organization phenomenon occurs in strained InGaAs system on InP (311) substrates grown by metalorganic vapor phase epitaxy. This suggests that a similar formation process of nanocrystals exists not only on the GaAs (311)B substrate but also on the InP (311)B substrate. However, the ordering and the size homogeneity of the self-organized nanocrystals are slightly worse than those of the InGaAs/AlGaAs system on the GaAs (311)B substrate. The tensilely strained condition of a InGaAs/InP system with growth interruption in a PH₃ atmosphere reveals a surface morphology with nanocrystals even on the InP (100) substrate. It was found that strain energy and high growth temperature are important factors for self-organization on III-V compound semiconductors. Preliminary results indicate that the self-organized nanostructures in strained InGaAs/InP systems on InP substrates exhibit room temperature photoluminescent emissions at a wavelength of around 1.3 p.m.     

Integrated in situ monitoring of a metalorganic vapor phase epitaxy reactor for II–VI epitaxy

The focus of this paper is the integrated use of system and wafer monitors to obtain a complete picture of the growth process and to identify the major causes of variance. In situ monitoring of a growing layer of Hg_1-xCd_xTe using laser reflectometry has been compared with the outputs of various system monitors such as pyrometer, organometallic concentration, and flow. Differences between the expected and measured concentrations were corrected by initially adjusting the organometallic flows, and the remaining variance in CdTe interdiffused multilayer process layer thickness was then corrected by increasing the CdTe cycle time. The remainder of the layer gave a homogenized alloy composition of 0.256 for a target value of 0.258. This example shows how different in situ monitors have been integrated to give a comprehensive picture of the growth process, which were then related to known kinetic behavior. As a result of this monitoring, it was possible to identify critical parameters for control.     

Lateral epitaxial overgrowth of GaN films on SiO2 areas via metalorganic vapor phase epitaxy

Lateral epitaxial growth and coalescence of GaN regions over SiO₂ masks previously deposited on GaN/AlN/6H-SiC(0001) substrates and containing 3 μm wide rectangular windows spaced 7 μm apart have been achieved. The extent and microstructural characteristics of these regions of lateral overgrowth were a complex function of stripe orientation, growth temperature, and triethylgallium (TEG) flow rate. The most successful growths were obtained from stripes oriented along 〈1 00〉 at 1100°C and a TEG flow rate of 26 μmol/min. A density of ∼10⁹ cm⁻² threading dislocations, originating from the underlying GaN/AlN interface, were contained in the GaN grown in the window regions. The overgrowth regions, by contrast, contained a very low density of dislocations. The surfaces of the coalesced layers had a terrace structure and an average root mean square roughness of 0.26 nm.     

Interdiffusion and relaxation in metalorganic vapor phase epitaxy grown InGaAs/GaAs strained layer quantum wells

Low pressure metalorganic vapor phase epitaxy grown strained InGaAs/GaAs quantum well structures have been characterized by photoluminescence and x-ray diffraction. It is shown that beyond the pseudomorphic limit, these structures show considerable gallium/indium interdiffusion at the interfaces and partial strain relaxation in the quantum well layers.     

Reaction kinetics control in preparation of CdTe and HgCdTe by hot wall metalorganic vapor phase epitaxy

The growth rates of CdTe and HgCdTe by hot wall metalorganic vapor phase epitaxy were studied as functions of growth temperatures and partial pressures of precursors. It is suggested that the growth of CdTe and HgCdTe was controlled by reaction kinetics. The relationship between growth processes and epilayer properties was discussed and high quality epilayers were obtained.     

Recent results on metalorganic vapor phase epitaxially grown HgCdTe heterostructure devices

The ability to grow complex multilayer structures in Hg_1-xCd_xTe by epitaxial techniques has made it possible to produce a range of new devices such as infrared LEDs, lasers, and two-color infrared detector arrays. The devices described here, however, are designed to operate at temperatures above 145K and include both infrared sources and detectors. Three layer ppn structures, where the underlined symbols mean wider gap, have close to Auger limited R_oAs at temperatures above 145K. Under reverse bias, the devices exhibit Auger suppression leading to useful detectivities at room temperature. The diodes exhibit forward biased electroluminescence at room temperature although the efficiency of this emission is found to fall rapidly as the peak wavelength is increased toward 9 μm due to increased Auger recombination rates. By reverse biasing them, however, the devices show negative luminescence as a result of reducing the electron and hole densities below their thermal equilibrium value. The diode emitters have a higher quantum efficiency when used in this mode due to Auger suppression of the dark current.     

Analysis of reactor geometry and diluent gas flow effects on the metalorganic vapor phase epitaxy of AIN and GaN thin films on α(6H)-SiC substrates

The influence of diluent gas on the metalorganic vapor phase epitaxy of AlN and GaN thin films has been investigated. A computational fluid dynamics model using the finite element method was employed to improve film uniformity and to analyze transport phenomena. The properties of AlN and GaN thin films grown on α(6H)-SiC(0001) substrates in H₂ and N₂ diluent gas environments were evaluated. Thin films of AlN grown in H₂ and N₂ had root mean square (rms) roughness values of 1.5 and 1.8 nm, respectively. The surface and defect microstructures of the GaN thin films, observed by scanning and transmission electron microscopy, respectively, were very similar for both diluents. Low temperature (12K) photoluminescence measurements of GaN films grown in N₂ had peak intensities and full widths at half maximum equal to or better than those films grown in H₂. A room temperature Hall mobility of 275 cm²/V·s was measured on 1 μm thick, Si-doped, n-type (1×10¹⁷ cm⁻³) GaN films grown in N₂. Acceptor-type behavior of Mg-doped GaN films deposited in N₂ was repeatably obtained without post-growth annealing, in contrast to similar films grown in H₂. The GaN growth rates were ∼30% higher when H₂ was used as the diluent. The measured differences in the growth rates of AlN and GaN films in H₂ and N₂ was attributed to the different transport properties of these mixtures, and agreed well with the computer model predictions. Nitrogen is shown to be a feasible alternative diluent to hydrogen for the growth of AlN and GaN thin films.     

Growth characteristics of CdZnTe layers grown by metalorganic vapor phase epitaxy using dimethylzinc, dimethylcadmium, diethyltelluride, and dimethyltelluride as precursors

Growth characteristics of (100)-oriented CdZnTe layers grown by atmospheric-pressure metalorganic vapor phase epitaxy have been studied using dimethylzinc (DMZn), dimethylcadmium (DMCd), diethyltelluride (DETe), and dimethyltelluride (DMTe) as precursors. Variations of Zn composition and layer growth rate were examined by changing the DMZn supply ratio, defined as DMZn/(DMCd+DMZn), where the precursors are expressed in appropriate units of flow rate, from 0 (no DMZn) to 1.0 (no DMCd), while keeping the total group II supply rate constant. The growth rate of CdZnTe layers was found to decrease monotonically with increase of the DMZn supply ratio. On the other hand, the Zn composition x of grown layers increased gradually up to x=0.04 with increase of the DMZn supply ratio from 0 to 0.8, beyond which the Zn composition increased abruptly to ZnTe. The abrupt transition of Zn composition was suppressed by increasing the VI/II ratio. The growth mechanism of CdZnTe layers was studied based on the observed growth characteristics of CdTe and ZnTe. A higher desorption rate from the growth surface for Zn species than for Cd species, and a higher rate of CdTe formation than ZnTe formation are believed to cause the observed growth characteristics. CdZnTe layers with high crystal quality were grown in a wide range of Zn compositions. The full-width at half-maximum values for x-ray double-crystal rocking-curve measurements were lower than 320 arc-sec for x<0.3 and x>0.75.     

Thermal annealing effects on p-type conductivity of nitrogendoped ZnSe grown by metalorganic vapor phase epitaxy

Post-growth thermal annealing (e.g., 500°C, 30 min), is proposed as one of the promising techniques to realize and to improve the quality of p-type ZnSe layers grown by metalorganic vapor phase epitaxy (MOVPE). The layers were grown at low temperature (350°C) by photo-assisted MOVPE with doping nitrogen from tertiarybutylamine (t-BuNH₂). The flow rate of t-BuNH₂ was limited to be relatively low, in order to avoid heavy doping, with which as-grown layers exhibited electrically high-resistivity; but the thermal annealing converted the layers to p-type. As the as-grown layers exhibited the stronger donor-to-acceptor pair recombination lines or the weaker donor-bound excitonic emission (I_x) lines in photoluminescence, the annealed layers resulted in higher net acceptor concentration, which was 1 x 10¹⁷ cm⁻⁴ at the optimum conditions at present.     

The role of surface adsorbates in the metalorganic vapor phase epitaxial growth of (Hg,Cd)Te onto (100) GaAs Substrates

It has been established that a compound present as an impurity in the propan-2-ol used in the preparation of GaAs (100) substrates for the metalorganic vapor phase epitaxy growth of (Hg,Cd)Te has a marked effect on the crystalline perfection and surface morphology of the resulting layers. In particular, the presence of this species, which contains Na, ensures that (i) the epitaxial overgrowth is of (100) orientation without the need for ZnTe nucleation layers, and (ii) the density of pyramidal hillocks on the surface can be reproducibly < 10 cm⁻².     

Low temperature growth of (100) HgCdTe layers with DtBTe in metalorganic vapor phase epitaxy

Growth characteristics of (100) HgCdTe (MCT) layers by MOVPE at low temperature of 275°C were studied using ditertiarybutyltelluride as a tellurium precursor. Growths were conducted in a vertical narrow-spacing growth cell at atmospheric pressure. Cd composition of MCT layers were controlled from 0 to 0.98 using dimethylcadmium (DMCd) flow. The growth rate was constant for increase of DMCd flow. During the growth, Cd was incorporated preferentially into the MCT layers. Enhancement of Cd incorporation in the presence of Hg was also observed. Crystal quality and electrical properties were also evaluated, which showed that high quality MCT layers can be grown at 275°C. Strain in CdTe layers grown at 425 and 275°C was also evaluated. Lattice parameter of layers grown at 425°C approached bulk value at thickness of 5 μm, while layers grown at 275°C relaxed at 1 μm. The rapid strain relaxation of layers grown at 275°C was considered due to the layer growth on the strain relaxed buffer layer. The effect of the thermal stress on the relaxation of CdTe lattice strain was also discussed.