Effects of In and Sb doping in LPE growth thermodynamics and in GaAs layer qualities

We have experimentally determined the distribution coefficients of In and Sb in liquid phase epitaxially (LPE) grown GaAs at several different temperatures and found them to be consistent with the values calculated from the pseudobinary phase diagram of the ternary system at a dilute In or Sb concentration. Both In and Sb are found to be effective in reducing the dislocation densities in the LPE grown GaAs epi-layers. Studies of the phase diagram and the surface morphology indicate that Sb is more effective than In. Based on the results from the surface morphology, x-ray broadening and etch pit density (EPD) data, the optimal Sb concentration was 2–3 × 10¹⁹ cm⁻³.     

Spectral characteristics of GaAs solar cells grown by LPE

This paper describes the spectral characteristics of GaAs solar cells grown by low-temperature liquid phase epitaxy (LPE). It demonstrates improvements in blue response and peak internal quantum efficiencies of 100 percent for an optimized cell structure with isovalent In doped base and ultrathin (<100Å) heavily doped cap p⁺-GaAs layer on the photosensitive surface. The conversion efficiency obtained from the optimized cells under one-sun AM 1.5 conditions is 23.4 percent. Our results indicate that the low-cost LPE-grown films are suitable for high-efficiency solar cells.     

Methods for reducing deep level emissions from ZnSe grown by organometallic vapor phase epitaxy

Methods for reducing deep level emission from ZnSe layers grown by photo-assisted organometallic vapor phase epitaxy were studied using photo-luminescence. A number of approaches to achieve reductions in deep level emissions were investigated. One of them was the use of a flow modulation technique. Reduction in the deep level emissions was observed when the layers were annealed under zinc-rich conditions during this growth process. The effect of cadmium as an isoelectronic dopant in ZnSe was also studied. It was observed that “doping” levels of cadmium resulted in considerable reduction in deep level emissions from ZnSe layers. Layers were grown under different II/VI ratios, and compared to cadmium doped layers of similar ratios. Reduction in deep level emissions were observed in all cadmium doped layers. Cadmium, therefore, seems to be the most promising isovalent dopant for reducing the deep level emissions in ZnSe.     

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.     

Modulation of electronic properties in liquid phase epitaxially grown p-n-p-n GaAs multilayers

Liquid phase epitaxy (LPE) is presented as an alternative method to molecular beam epitaxy (MBE) for growing p-n-p-n doped GaAs superlattices. LPE offers some advantages compared to MBE. Simple equipment, shorter growth times at comparable low growth temperatures, permits growing multilayers with a broad variety of single layer thickness 20 < d < 1000 nm at reasonably short growth times. Typical doping superlattice properties are tested in LPE multilayers, and demonstrated via some selected results: a) The simultaneous modulation of the conductivities in the n- and p-layer systems. It depends on the variation of the 2-dim. carrier concentrations and to a similar extent on the change of the mobilities with the effective channel thickness. b) The field effect transistor properties of the p-and n-doped systems are due to the special choice of doping concentrations and film thicknesses, c) The modulation of the effective band gap E _G ^ef is proved by cw and time resolved photoluminescence Good agreement is achieved between the expected shift E _G ^ef due to the LPE growth parameters and the observed shift of the peak energy of the luminescence spectra.     

New photoluminescence lines in Vanadium doped GaAs grown by MOVPE

We examined the electrical and optical properties of vanadium-doped GaAs grown by metalorganic vapour phase epitaxy using vanadium tetrachloride (VCl₄) as a novel dopant source. Samples with various vanadium incorporations were investigated. All samples were n type. The electron concentration dependence on the VCl₄ flow rate was established. At 15 K, by comparison with undoped layers grown in the same conditions, photoluminescence spectra of V-doped exhibited three new emission bands: at 1.41, 1 and 0.72 eV. The 1 and 0.72 eV band emissions were attributed to V²⁺ and V³⁺ intracenter emission, while the 1.41 eV band was suggested to be a donor-bound transition. The identity of the donor is tentatively attributed to a donor complex that associates vanadium to an arsenic vacancy. From Hall effect as function of temperature, the donor ionisation energy was estimated to be about 102±5 meV.     

Carbon doped GaAs grown in low pressure-metalorganic vapor phase epitaxy using carbon tetrabromide

Carbon tetrabromide was used as carbon source for heavily p-doped GaAs in low pressure metalorganic vapor phase epitaxy (MOVPE). The efficiency of carbon incorporation was investigated at temperatures between 550 and 670°C, at V/III ratios from 1 to 50 and carbon tetrabromide partial pressures from 0.01 to 0.03 Pa. Hole concentrations from 8 × 10¹⁷ to 5 × 10¹⁹ cm⁻³ in as-grown layers were obtained. After annealing in nitrogen atmosphere at 450°C, a maximum hole concentration of 9 × 10¹⁹ cm⁻³ and a mobility of 87 cm²/Vs was found. At growth temperatures below 600°C, traces of bromine were detected in the layers. Photoluminescence mapping revealed an excellent doping homogeneity. Thus, CBr₄ is found to be a suitable carbon dopant source in MOVPE.     

Electric current controlled liquid phase epitaxy of GaAs on N+ and semi-insulating substrates

Electric current controlled liquid phase epitaxy (LPE) of GaAs has been performed on both n⁺ and semi-insulating substrates. Growth is induced by current flow across the substrate-melt interface. The furnace temperature is held constant during growth so that direct electrical control of the growth process is achieved. The dependence of the growth rate on both the electric current density across the substrate-melt interface and the ambient furnace temperature was determined. Current densities from 5 to 20 A/cm² were employed and furnace temperatures ranging from 680 to 800°C were used. Sustained steady state growth rates as small as 0.022μm/min and as large as 1.4μm/min were obtained. For a given furnace temperature and current density, the measured growth rates on semi-insulating substrates range from 48% to 77% of the rates obtained on n⁺ n substrates. The surface morphology of the epitaxial layers is observed to depend on the electric current density employed during growth. Electric current controlled doping modulation was studied in epitaxial layers grown from unintentionally doped melts. The degree of doping modulation achieved is approximately proportional to the change in applied current density. Approximately a 40% increase in the net electron concentration is obtained by changing the current density from 10 to 30 A/cm² during growth. Preliminary experiments with tin doped epitaxial layers indicate that similar changes in the amount of tin incorporation can be achieved.     

Electrical and structural characterization of GaAs vertical-sidewall epilayers grown by atomic layer epitaxy

Electrical and structural measurements have been performed on novel test structures incorporatingp-type GaAs epilayers grown by organometallic vapor phase atomic layer epitaxy on the vertical sidewalls of semi-insulating GaAs rods formed by ion-beam-assisted etching. Preliminary results indicate that the vertical-sidewall epilayers have excellent crystal quality and sufficient electrical quality to support a sidewall-epitaxy device technology. Some examples of candidate electronic, electrooptic, and photonic devices for vertical-sidewall fabrication are FETs, resistors, waveguides, modulators, and quantum-wire and quantum-dot lasers.     

HgCdTe液相外延薄膜生长及缺陷表征

液相外延是制备HgCdTe薄膜材料的一项成熟技术,大尺寸的外延薄膜是研制HgCdTe红外焦平面列阵的基础。探讨了尺寸为30mm×40mm的HgCdTe液相外延薄膜生长技术,用红外透射光谱和X光貌相技术对材料进行了评价。并对HgCdTe薄膜表面的黑点缺陷、波纹起伏等特征形貌进行了讨论,指出黑点缺陷是杂质粒子在薄膜表面形成的包裹体,而表面波纹是由生长母液中的对流引起的。     

Intersubband absorption characteristics in OMVPE grown delta-doped GaAs/AlGAs multiple quantum well structures

A study was carried out using a series of multiple quantum well structures grown by organometallic vapor phase epitaxy (OMVPE) to evaluate the intersubband absorption characteristics, thus enabling a direct correlation of the delta doping parameters to device performance. Significant improvements in the absorption characteristics were obtained by adopting a localized delta-doping profile in the absorption quantum wells as opposed to homogeneously doped structures. The transition linewidths were observed to decrease from 40 meV in the uniformly doped sample to 20 meV in devices in corporating delta-doped quantum wells. In addition, the measured absorption strengths of the intersubband resonance grew in a linear fashion as the carrier sheet density in the well was increased to approximately 2 × 10¹² cm⁻².     

Structural properties of GaAsN grown on (001) GaAs by metalorganic molecular beam epitaxy

Detailed transmission electron microscopy (TEM) and transmission electron diffraction (TED) examination has been made of metalorganic molecular beam epitaxial GaAsN layers grown on (001) GaAs substrates. TEM results show that lateral composition modulation occurs in the GaAs_1−xN_x layer (x 6.75%). It is shown that increasing N composition and Se (dopant) concentration leads to poor crystallinity. It is also shown that the addition of Se increases N composition. Atomic force microscopy (AFM) results show that the surfaces of the samples experience a morphological change from faceting to islanding, as the N composition and Se concentration increase. Based on the TEM and AFM results, a simple model is given to explain the formation of the lateral composition modulation.     

Properties of Bi-doped PbTe layers grown by liquid phase epitaxy under controlled Te vapor pressure

Effects of Bi doping in PbTe liquid-phase epitaxial layers grown by the temperature difference method under controlled vapor pressure (TDM-CVP) are investigated. For Bi concentrations in the solution, x_Bi, lower than 0.2 at.%, an excess deep-donor level (activation energy E_d≈0.03–0.04 eV) appears, and Hall mobility is low. In contrast, for x_Bi>0.2 at.%, Hall mobility becomes very high, while carrier concentration is in the range of 10¹⁷ cm⁻³. Inductive coupled plasma (ICP) emission analysis shows that, for x_Bi=1 at.%, Bi concentration in the epitaxial layer is as high as N_Bi=2.3–2.7 × 10¹⁹ cm⁻³. These results indicate that Bi behaves not only as a donor but also as an acceptor, and the nearest neighbor or very near donor-acceptor (D-A) pairs are formed, so that strong self-compensation of Bi takes place. Carrier concentration for highly Bi-doped layers shows a minimum at a Te vapor pressure of 2.2 × 10⁻⁵ torr for growth temperature 470°C, which is coincident with that of the undoped PbTe.     

Investigation of high quality GaAs:In layers grown by liquid phase epitaxy

Indium-doped GaAs layers are investigated by low-field Hall effect, photoluminescence, and double crystal x-ray diffraction in order to study the influence of the In concentration on the electrical, optical, and crystallographic properties. The layers were grown by liquid phase epitaxy from solution with In concentrations in the range 0–10 at.%. It was found that epitaxial growth from the melt with 7 at.% In content produces the highest quality epitaxial layers.     

LPE HgCdTe on sapphire status and advancements

With the evolution of infrared arrays to over four million pixels, larger formats have demanded higher quality mercury cadmium telluride (MCT) wafers. Since single defects can easily degrade multiple diodes, high operability requires very homogeneous and nearly flawless epitaxial surfaces. Subsequent photolithography and hybridization also demand unprecedented levels of substrate flatness and low imperfections. To consistently and reliably produce large area arrays, Insaco Inc., The Boeing Company, and Rockwell International Corporation have developed major quality improvement procedures which address all three components of the infrared material wafer architecture. Centered on the producible alternative to cadmium telluride for epitaxy (PACE) process, technological advancements encompassed sapphire substrates, organometallic vapor phase epitaxy (OMVPE), cadmium telluride (CdTe) buffer layer growth, and liquid phase epitaxial (LPE) mercury cadmium telluride growth. Processed material from these runs mated to Conexant^™ fabricated multiplexers have successfully produced 1024 1024 and the first 2048 2048 IR short-wave (2.5 m at 80 K) hybrid focal plane arrays. Operabilities in these implanted n-on-p junction devices reach 99.98% with near 70% quantum efficiency in the astronomy ‘K’ band (2.2–2.4 microns).     

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.     

A new LPE growth method of semiconductor heterostructures with thickness profile variation of epitaxial layers

We have investigated and developed a method for the LPE growth of layers with approximately parabolic cross section. The channels were created during the growth process by modulating the liquid phase thickness with W or Mo wires parallel to the substrate. The main parameters of the channel can be controlled by changing the wire’s diameter and its distance from the substrate. This method can be incorporated directly into the growth process of a laser structure with an unstable resonator without the need of additional treatments as chemical etching, to produce the channel structure.     

Enhancement of Si-donor incorporation by Ga adatoms in Si delta-doped GaAs grown by molecular beam epitaxy

We report on the realization of a modified delta doping technique to obtain doping profiles in MBE grown GaAs, measured by capacitance-voltage (C-V) methods with full-widths at half-maximum (FWHM)s of 25 ± 5Å and peak concentrations of up to 1.1 × 10¹⁹ cm^?3. In this modified delta doping technique, both the Ga and Si shutters were opened for 15 sec during the delta doped layer growth while only the Si shutter is opened during conventional delta doping. Comparison of the two techniques under the same dopant flux and shutter-open-time interval shows that higher sheet-carrier concentrations with narrower FWHMs and higher peak concentrations are obtained with the modified delta doping than with the conventional delta doping method. This suggests that Si donor incorporation is enhanced by the Ga adatoms while broadening of the Si donor distribution is still negligible for this short time interval. The effects of the substrate temperature and the shutter-open time on the Si donor distribution have also been investigated.     

Effect of oxygen injection during vpe growth of GaAs Films

The effect of oxygen injection during VPE growth of silicon doped and unintentionally doped GaAs has been investigated for the Ga-AsCl₃-H₂ process. Depending on the oxygen concentration a reduction in the carrier concentration of up to four orders of magnitude is found for the case of Si doping. Without purposely adding Si, oxygen injection yields high resistivity layers. Hall mobility data suggest that the simultaneous injection of both silicon and oxygen is also responsible for the introduction of a deep center which is probably associated with a Si-0 complex. We compared our results on carrier concentration with values calculated from a thermody-namic model in which both the injected SiH₄ and oxygen are taken into account. The computed behavior shows the same trend as our experimental data. This finding also provides further support for the validity of the silicon contamination model as an explanation of the background doping of VPE GaAs.     

Characterization of liquid-phase epitaxially grown HgCdTe films by magnetoresistance measurements

In this paper, we demonstrate that measurements of the magnetoresistance can be used as a valuable alternative to conventional characterization tools to study transport properties of advanced semiconducting materials, structures, or devices. We have measured magnetoresistance on two different systems, namely, three liquid-phase epitaxially grown HgCdTe films and two GaAs-based high-electron-mobility-transistor (HEMT) structures. The results are analyzed by using a two-carrier model as a reference in the context of the reduced-conductivity-tensor scheme. The HEMT data are in quantitative agreement with the two-carrier model, but the HgCdTe data exhibit appreciable deviations from the model. The observed deviations strongly indicate a mobility spread and material complexity in the HgCdTe samples which are probably associated with inhomogeneities and the resulting anomalous electrical behavior.