Photovoltaic converters based on GaAs and AlGaAs epitaxial layers on GaAs substrates with developed surface area

MOCVD and LPE technologies of deposition of GaAs and AlGaAs layers onto (100) GaAs substrates with a developed surface area are developed. Porous GaAs layers and surface microprofiles of dendrite and quasi-grating types were fabricated on these substrates. The quality of layers was determined in comparative studies of the surface morphology and X-ray diffraction. Further, photovoltaic converters based on these layers have been devised. The best parameters among the samples under study were attained in photovoltaic converters based on the layers with the dendrite-type microprofile of substrate, which had the most developed area of the working surface and the dislocation density of 10⁴ cm^?2. In particular, at the wavelength of 0.65 μm, the external quantum efficiency of these photovoltaic converters was 150% higher than in the reference samples produced on a smooth surface.     

Low-threshold InGaAsP/InGaP lasers at 810 nm grown on GaAs substrate by LPE

InGaAsP/InGaP double-heterostructure lasers emitting at 810 nm have been fabricated on GaAs substrates using liquid-phase epitaxy (LPE). A threshold current as low as 2.0 kA/cm2 with an external differential quantum efficiency of 54% is obtained. Thus it has been shown that high-quality InGaAsP/InGaP lasers can be obtained by LPE growth.     

Metal-organic vapor-phase epitaxy growth and characterization of thick (100) CdTe layers on (100) GaAs and (100) GaAs/Si substrates

The growth characteristics and crystalline quality of thick (100) CdTe-epitaxial layers grown on (100) GaAs and (100) GaAs/Si substrates in a metal-organic vapor-phase epitaxy (MOVPE) system for possible applications in x-ray imaging detectors were investigated. High-crystalline-quality epitaxial layers of thickness greater than 100 μm could be readily obtained on both types of substrates. The full width at half maximum (FWHM) values of the x-ray double-crystal rocking curve (DCRC) decreased rapidly with increasing layer thickness, and remained around 50–70 arcsec for layers thicker than 30 μm on both types of substrates. Photoluminescence (PL) measurement showed high-intensity excitonic emission with very small defect-related peaks from both types of epilayers. Stress analysis carried out by performing PL as a function of layer thickness showed the layers were strained and a small amount of residual stress, compressive in CdTe/GaAs and tensile in CdTe/GaAs/Si, remained even in the thick layers. Furthermore, the resistivity of the layers on the GaAs substrate was found to be lower than that of layers on GaAs/Si possibly because of the difference of the activation of incorporated impurity from the substrates because of the different kinds of stress existing on them. A heterojunction diode was then fabricated by growing a CdTe epilayer on an n⁺-GaAs substrate, which exhibited a good rectification property with a low value of reverse-bias leakage current even at high applied biases.     

Scanning force microscopy studies of GaAs films grown on offcut Ge substrates

The surface morphology of GaAs films grown on offcut Ge substrates is studied using a scanning force microscope (SFM). We investigated the effects of the Ge buffer layer, growth temperature, film thickness, and prelayer on the GaAs surface morphology. The starting Ge substrates are offcut 6° toward the [110] direction to minimize single steps on the substrates before molecular beam epitaxial film growth. We find that comparing with GaAs samples grown without Ge buffer layers or with unannealed Ge buffer layers, samples with annealed Ge buffer layers are much smoother and contain no antiphase boundaries (APBs) on the surface. For thick (≥1 μm) GaAs films with an annealed Ge buffer layer, the surfaces display crosshatch lines and elongated mounds (along , which are associated with the substrate offcut direction. As the film thickness increases, the crosshatch lines become shorter, denser and rougher, and the mounds grow bigger (an indication of GaAs homoepitaxial growth). We conclude that annealed Ge buffer layers are crucial for growing high quality GaAs films with few APBs generated during the growth. In addition, under optimal conditions, different prelayers make little difference for thick GaAs films with annealed Ge buffer layers.     

HgCdTe液相外延材料组分分布的红外透射光谱评价技术

对红外透射光谱法测定HgCdTe液相外延材料纵向组分分布技术进行了深入的研究.红外透射光谱的理论计算采用了王庆学提出的组分分布模型,并考虑了光穿越组分梯度区时产生的干涉效应.通过测量同一样品在不同外延层厚度下的一组红外透射光谱,该方法的有效性得到了实验验证.进一步对组分模型中参数(即外延总厚度、组分互扩散区厚度、材料表面组分和HgCdTe层组分梯度)的拟合方法进行了讨论,并确定了各拟合参数的拟合精度.结果显示,该方法可作为测定HgcdTe液相外延材料组分特性的一种有效的测试评价技术.     

Te-rich liquid phase epitaxial growth of HgCdTe on Si-based substrates

The growth of high quality (111)B oriented HgCdTe layers on CdZnTe/GaAs/Si and CdTe/Si substrates by Te-rich slider liquid phase epitaxy (LPE) is reported. Although the (111) orientation is susceptible to twinning, a reproducible process yielding twin-free layers with excellent surface morphology has been developed. The electrical properties and dislocation density in films grown on these substrates are comparable to those measured in HgCdTe layers grown on bulk CdTe substrates using the same LPE process. This is surprising in view of the large lattice mismatch that exists in these systems. We will report details of both the substrate and HgCdTe growth processes that are important to obtaining these results.     

Molecular beam epitaxy growth of InAs-AlSb-GaSb interband tunneling diodes

This paper presents our studies of the growth of InAs/GaAs and GaSb/GaAs heterojunctions by molecular beam epitaxy and their applications in fabricating the InAs-AlSb-GaSb interband tunneling devices. The Hall effect and x-ray diffraction were used to determine the optimum growth conditions for the layers. In addition, the qualities of the InAs and GaSb epilayers, grown under their optimum conditions, were compared. The full width at half maximum (FWHM) from the x-ray diffraction for a GaSb epilayer is about 50 arcsec narrower than an InAs epilayer of the same thickness. The narrower FWHM and excellent surface morphology of the GaSb layer have led us to grow the polytype heterostructure on a p⁺-GaAs substrate using a p⁺-GaSb as the buffer layer. The polytype tunneling structures grown on GaAs substrates under these conditions show good negative differential resistance properties. Five different interband tunneling structures are compared and discussed in terms of their peak-current densities and peak-to-valley current ratios.     

Detailed structural analysis of GaAs grown on patterned Si

The growth of GaAs on patterned Si substrates is essential for the integration of GaAs and Si devices. Moreover this growth may have to be done in wells to planarize the surfaces of the Si and GaAs devices for their interconnection. In this study, GaAs is grown by MBE on such patterned Si substrates, with window width down to 3 μm, and a complete structural characterization of the material is made by electron microscopy. In every case, SEM observations show a very good definition of the pattern by the contrast of the flat surface of the monocrystalline GaAs compared to the very rough surface of the polycrystalline GaAs. Cross-sectional observations by STEM or TEM on non-etched samples reveal that the quality of the monocrystalline GaAs is at least as good in terms of defect density as that of the standard GaAs on Si. On samples etched with a plasma to produce wells, grains often form against the Si sidewall. Chemical etching with lateral etching avoids contact of the growing GaAs with the Si sidewall and subsequent grain formation. The crystalline quality obtained on etched samples is not as good as on non etched samples. A way of preparing the wells to improve this crystalline quality is proposed.     

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.     

掺In半绝缘GaAs衬底上外延GaAs的晶格失配研究

本文报道了在掺In半绝缘GaAs衬底上的液相和汽相外延生长,并用x射线双晶衍射和光学显微等方法研究外延层和衬底之间的晶格失配.结果表明,当衬底中In组分x<0.004时,外延层失配应力主要由弹性形变调节,不出现失配位错,并可得到很好的表面形貌;当x≥0.006时,外延层产生失配位错,失配应力主要由失配位错调节,液相外延层表面出现沿[110]和[110]方向的十字网络.当外延层产生范性形变时衬底中的临界In组分x_c在0.004和0.006之间.     

New model of LPE growth: Growth rate calculation

Expressions for the LPE growth rate are developed on the basis of the new steady-state growth model assuming nucleation in the solution. The solution thickness can be calculated at which the LPE growth rate reaches a maximum for any solution cooling rate. The initial and the volume nucleation parameters, formation times, critical supersaturations and supercoolings of the solution have been determined from experimental GaP and GaAs LPE data. Calculated growth rates fit very closely with experimental rates of GaP and GaAs LPE growth.     

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.     

Si/CdTe复合衬底HgCdTe液相外延材料的生长与性能分析

通过改进推舟液相外延技术,成功地在(211)晶向Si/CdTe复合衬底上进行了HgCdTe液相外延生长,获得了表面光亮的HgCdTe外延薄膜.测试结果表明,(211)Si/CdTe复合衬底液相外延HgCdTe材料组分及厚度的均匀性与常规(111)CdZnTe衬底HgCdTe外延材料相当;位错腐蚀坑平均密度为(5～8)×105 cm-2,比相同衬底上分子束外延材料的平均位错密度要低一个数量级;晶体的双晶半峰宽达到70″左右.研究结果表明,在发展需要低位错密度的大面积长波HgCdTe外延材料制备技术方面,Si/CdTe复合衬底HgCdTe液相外延技术可发挥重要的作用.     

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.     

高质量HgCdTe薄膜的液相外延生长

用固态HgTe作为Hg补偿源，采用开管、水平滑块推舟的富Te液相外延(LPE)方法在碲锌镉(Cd1-yZnyTe，y=0．04)衬底上外延生长大面积Hg1-xCdxTe(x=0．2)薄膜材料，通过适当的生长条件得到组分均匀、结构完整、表面形貌良好的长波碲镉汞薄膜。测试结果表明本方法生长的HgCdTe薄膜能满足目前研制红外焦平面器件的要求。     

Substrate quality impact on the carrier concentration of undoped annealed HgCdTe LPE layers

The impact of Te precipitates and impurities, in CdZnTe or CdTe substrates, on grown liquid phase epitaxy (LPE) HgCdTe layer hole concentrations was studied. The carrier concentrations in capped annealed LPE HgCdTe layers grown on CdZnTe substrates with large densities of Te precipitates are frequently significantly higher than those expected for HgCdTe annealed under Hg-deficient conditions. The carrier concentration in the LPE layer, due to the diffusion of copper ions from contaminated CdTe substrates into the layer, is strongly affected by the polarity of the (111)-oriented substrates. Layers grown on the (111)A face showed very high concentrations of Cu, whereas in those grown on the (111)B face normal carrier concentrations were achieved. These phenomena are discussed on the basis of defects formed either in the epilayer or in the layer-substrate interface.     

Ga_(1-x)In_xAs_ySb_(1-y)/GaSb LPE生长及其性质研究

报导了在n型(100)GaSb衬底上,温度为520—530℃时,用液相外延的方法实现了组分在0≤x≤0.19,0≤y≤0.14范围内的Ga_(1-x)In_xAs_ySb_(1-y)四元合金半导体的生长。X射线双晶衍射,电子探针及光学显微镜的观察和分析测试表明:所得外延层的表面形貌和界面特性优良,组分分布和层厚均匀,晶格匹配及单晶性能良好。对外延层表面的氧化情况使用Auger能谱仪进行测试分析。另外,对生长中存在的一些问题进行了讨论。     

As掺杂碲镉汞富碲液相外延材料特性的研究

对富碲液相外延As掺杂碲镉汞(HgCdTe)材料的研究发现,其电学性能存在着不稳定性,材料霍尔参数的实验数据与均匀材料的理论计算结果也不能很好的吻合.通过采用剥层变温霍尔测量和二次离子质谱(SIMS)测试对材料纵向均匀性进行检测的结果显示,外延材料中的As在高温富汞激活退火过程中具有向材料表面扩散的效应,导致在表面形成了高于主体层浓度1～2个量级的高浓度表面层,并导致了AsTe受主的浓度在HgCdTe薄膜中呈非均匀分布.考虑这一效应并采用双层模型的霍尔参数计算方法后,As掺杂HgCdTe液相外延材料的电学行为得到了较好的解释,并较为准确地获得了退火后材料表面层与主体层的受主浓度及受主能级等电学参数.     

Structural and electrical contact properties of LPE grown GaAs doped with indium

We have studied the effects of In doping on the structural and electrical properties of a liquid phase epitaxially (LPE) grown GaAs. The results of surface morphology studies show that macroscopically, a terrace-free area in certain regions can be seen on the surface of a GaAs layer doped with In of 2.4 × 10¹⁹ cm³. The full widths at half-maximum (FWHM) of x-ray double crystal rocking curves show that a GaAs epi-layer of good crystalline quality can be obtained by doping In to a concentration up to 4.3 × 10¹⁹ cm^-3, beyond which a sharp increase in the FWHM is observed. Etch pit density (EPD) study also shows that the dislocation density is reduced by doping the epi-layer with In. At an optimum In concentration, 2.4 × 10¹⁹ cm^-3, the EPD was reduced by a factor of 20 when measured at the surface of a 9μm thick epi-layer. The I-V characteristics of Au-GaAs Schottky diodes show, for the layer with an optimal In concentration, an ideality factor close to 1.04 over more than seven decades of current. For the same layer, the reverse I-V characteristics are close to an ideal Schottky diode and could be fitted by a theoretical curve, combining the thermionic field emission and thermionic emission. For doping levels higher than 6 × 10¹⁹ cm^-3, the epitaxial layer quality deteriorated. We report the results obtained from the Nomarski optical microscope, double crystal x-ray rocking curves, etch pit density, forward and reverse I-V characteristics, and the theoretical current transport models.     

(Al)GaInP multiquantum well LEDs on GaAs and Ge

MOVPE growth of AlGaInP layers on GaAs and Ge have been demonstrated. The surface morphology of the epilayers was smooth under optimized growth conditions. The epilayers showed good PL intensity on both GaAs and Ge substrates. It has been observed that at room temperature the PL intensity drops in the first few seconds after excitation and attains a steady state. The Zn-doped AlGaInP did not show any signs of H-passivation. The MQW LEDs on both the substrates produced electroluminescence which increased with applied current. Results indicate the feasibility of AlGaInP LEDs on Ge.