TEM Characterization of HgCdTe/CdTe Grown on GaAs(211)B Substrates

A microstructural study of HgCdTe/CdTe/GaAs(211)B and CdTe/GaAs(211)B heterostructures grown using molecular beam epitaxy (MBE) was carried out using transmission electron microscopy and small-probe microanalysis. High-quality MBE-grown CdTe on GaAs(211)B substrates was demonstrated to be a viable composite substrate platform for HgCdTe growth. In addition, analysis of interfacial misfit dislocations and residual strain showed that the CdTe/GaAs interface was fully relaxed except in localized regions where GaAs surface polishing had caused small pits. In the case of HgCdTe/CdTe/GaAs(211)B, the use of thin HgTe buffer layers between HgCdTe and CdTe for improving the HgCdTe crystal quality was also investigated.     

分子束外延CdTe（211）B／GaAs（211）B的特性研究

ＣｄＴｅ／ＧａＡｓ是ＨｇＣｄＴｅ分子束外延的重要替代衬底材料。用Ｘ双晶衍射和光致发光测试研究了分子束外延生长的ＣｄＴｅ（２１１）Ｂ／ＧａＡｓ（２１１）Ｂ的晶体结构质量，表明外延膜晶体结构完整，具有很高的质量。用高分辨率的透射电镜研究其界面特性，观察到ＣｄＴｅ（２１１）Ｂ相对于ＧａＡｓ（２１１）Ｂ向着［１１１］方向倾斜一个小角度（约３°），界面的四面体键网发生扭曲，由于晶格失配，在界面存在很高的失配位错密度。用二次离子质谱分析仪分析了ＧａＡｓ衬底中的Ｇａ和Ａｓ向ＣｄＴｅ外扩散的情况。结果表明：如果要在ＧａＡｓ衬底上生长ＨｇＣｄＴｅ外延膜，必须先生长一层具有一定厚度的ＣｄＴｅ来阻止Ｇａ和Ａｓ向ＨｇＣｄＴｅ的外扩散和失配位错的延伸。     

在Si(211)衬底上分子束外延CdTe的晶格应变

文章利用高分辨率X射线衍射技术对分子束外延CdTe（211）B/Si（211）材料的CdTe外延薄膜进行了倒易点二维扫描,并通过获得的对称衍射面和非对称衍射面的倒易空间图,对CdTe外延层的剪切应变和正应变状况进行了分析.研究发现,对于CdTe/Si结构,随着CdTe厚度的增加,[1-1-1]、[01-1]两个方向的剪切角γ[1-1-1]和λ[01-1]都有变小的趋势,且γ[1-1-1]的大小约为γ[01-1]的两倍;对于CdTe/ZnTe/Si,ZnTe缓冲层的引入可以有效地降低CdTe层的剪切应变.CdTe层的正应变表现为张应变,主要来源于CdTe和Si的热膨胀系数存在差异,而在从生长温度280℃降至室温20℃的过程产生的热应变.     

Cyclic Annealing During Metalorganic Vapor-Phase Epitaxial Growth of (211)B CdTe on (211) Si Substrates

High-quality (211)B CdTe buffer layers on Si substrates are required to enable Hg_1–x Cd _x Te growth and device fabrication on lattice-mismatched Si substrates. Metalorganic vapor-phase epitaxy (MOVPE) of (211)B CdTe on Si substrates using Ge and ZnTe interlayers has been achieved. Cyclic annealing has been used during growth of thick CdTe layers in order to improve crystal quality. The best (211)B CdTe/Si films grown in this study display a low x-ray diffraction (XRD) rocking-curve full-width at half-maximum (FWHM) of 85 arcsec and etch pit density (EPD) of 2 × 10⁶ cm⁻². These values are the best reported for MOVPE-grown (211) CdTe/Si and are comparable to those for state-of-the-art molecular beam epitaxy (MBE)-grown CdTe/Si.     

Lattice Relaxation and Dislocation Reduction in MBE CdTe(211)B/Ge(211)

We have used x-ray diffraction to assess the thickness dependence of strain in molecular-beam epitaxial (MBE) CdTe(211)/Ge(211). For 25-nm-thick layers, we find tensile stress of 100 MPa and in-plane strain of ~1.5 × 10⁻³. This stress relaxes during growth and becomes zero beyond 1 μm. We use the Dunn and Koch formula to estimate the threading dislocation density from the full-width at half-maximum of the (224) rocking curve. We then estimate the annihilation radius of MBE-grown CdTe(211)B/Ge(211) to be ~10 nm. Our layers have etch pit densities between 5 × 10⁷ cm⁻² and 5 × 10⁶ cm⁻² for a thickness of 10 μm. The lowest densities were obtained by periodic annealing epitaxy. We discuss mechanisms for the saturation of the dislocation density.     

High-Quality (211)B CdTe on (211)Si Substrates Using Metalorganic Vapor-Phase Epitaxy

High-quality (211)B CdTe buffer layers are required during Hg_1−x Cd _x Te heteroepitaxy on Si substrates. In this study, direct metalorganic vapor-phase epitaxy (MOVPE) of (211)B CdTe on Si, as well as CdTe on Si using intermediate Ge and ZnTe layers, has been achieved. Tertiary butyl arsine was used as a precursor to enable As surfactant action during CdTe MOVPE on Si. The grown CdTe/Si films display a best x-ray diffraction rocking-curve full-width at half-maximum of 64 arc-s and a best Everson etch pit density of 3 × 10⁵ cm⁻². These values are the best reported for MOVPE-grown (211)B CdTe/Si and match state-of-the-art material grown using molecular-beam epitaxy.     

P-Type doping with arsenic in (211)B HgCdTe grown by MBE

Arsenic incorporation and doping in HgCdTe layers grown by molecular beam epitaxy (MBE) were examined in this paper. Arsenic incorporation into MBE-HgCdTe was carried out in two different ways: (1)ex-situ arsenic ion-implantation on indium-doped n-type HgCdTe layers, and (2) through a new approach called arsenic planar doping. We report onex-situ arsenic diffusion on indiumdoped MBE-HgCdTe layers at 450°C. In the investigated layers, arsenic redistribution occurs with a multi-component character. We obtained a diffusion coefficient of D_As = (1-3) × 10⁻¹³ cm²/s at 450°C. Results of differential Hall and fabricated p-n junctions suggest that during high temperature annealing, arsenic preferentially substitutes into Te sublattices and acts as acceptor impurities. In the second case, arsenic has been successfully incorporated during the MBE growth as an acceptor in the planar doping approach. Withoutex-situ annealing, as-grown layers show up to 50% activation of arsenic during the growth. These results are very promising forin-situ fabrication of infrared devices using HgCdTe material.     

Characterization of cross-hatch morphology of MBE (211) HgCdTe

We present the results of a detailed study of the nature and origin of cross-hatch patterns commonly observed on (211) HgCdTe epilayers deposited by molecular beam epitaxy. Cross-hatch patterns were examined using x-ray topography as well as Nomarski, interferometric, and atomic force microscopies. Cross-hatch patterns were generally comprised of three sets of lines, parallel to the [231],, [213], and [011] directions. The lines parallel to the [011] direction exhibited distinct properties compared to the two sets of lines parallel to [231] and [213]. Under growth conditions characterized by excessive Hg flux (low temperature), lines parallel to [011] were periodic and tended to dominate the cross-hatch pattern. In some cases, bands of dislocations, 10–100 m in width, formed parallel to [011]. Under optimized growth conditions, on very closely lattice-matched substrates, (dislocation densities <10⁵ cm⁻²) lines parallel to [011] vanished entirely, and lines parallel to [231] and [213] became sparse. The remaining lines were typically fragments terminated by either a single dislocation, a cluster of dislocations (micro-void), or the wafer's edge. The density of these line fragments tended to decrease as the dislocation density decreased. Under the best growth conditions on very closely lattice-matched substrates we have achieved dislocation densities of 5 10⁴ cm⁻², which is comparable to the dislocation density of the CdZnTe substrate.     

Impurity Gettering in (112)B HgCdTe/CdTe/Alternate Substrates

The crystalline structure and impurity profiles of HgCdTe/CdTe/alternate substrate (AS; Si and GaAs are possibilities) and CdTe/AS were analyzed by secondary-ion mass spectrometry, atomic force microscopy, etch pit density analysis, and scanning transmission electron microscopy. Impurities (Li, Na, and K) were shown to getter in as-grown CdTe/Si epilayers at in situ Te-stabilized thermal anneal (~500°C) interfaces. In HgCdTe/CdTe/Si epilayers, indium accumulation was observed at Te-stabilized thermal anneal interfaces. Impurity accumulation was measured at HgCdTe/CdTe and CdTe/ZnTe interfaces. Processing anneals were found to nearly eliminate the gettering effect at the in situ Te-stabilized thermal anneal interfaces. Impurities were found to redistribute to the front HgCdTe/CdTe/Si surface and p–n junction interfaces during annealing steps. We also investigated altering the in situ Te-stabilized thermal anneal process to enhance the gettering effect.     

In-situ ellipsometry studies of adsorption of Hg on CdTe(211)B/Si(211) and molecular beam epitaxy growth of HgCdTe(211)B

We study the adsorption of Hg on CdTe(211)B using an 88-wavelength spectroscopic ellipsometer mounted on a commercial, molecular beam epitaxy (MBE) chamber. A detailed analysis of the pseudo-dielectric function shows that Hg is present at the surface both in chemisorbed and physisorbed form. Effective medium models for a mixture of chemisorbed and physisorbed Hg on the microscopically rough CdTe surface could not fit our data. However, a proposed model in which a partial layer of physisorbed Hg sits on top of a partial layer of chemisorbed Hg fits the measured pseudo-dielectric function well and yields precise values for the thicknesses of the chemisorbed and the physisorbed Hg layers. These values change in the expected manner as a function of Hg flux, temperature, and Te coverage. An analysis of the uncertainty in the measured thicknesses is carried out in detail, and a study of the limitations of the ellipsometer used for this study is presented. The effects of these limitations on the precision and accuracy of in-situ data are enumerated.     

Impact of CdZnTe Substrates on MBE HgCdTe Deposition

The highest sensitivity, lowest dark current infrared focal plane arrays (IRFPAs) are produced using HgCdTe on CdZnTe substrates. As-received state-of-the-art CdZnTe 6 × 6 and 7 × 7.5 cm substrates were analyzed using Nomarski phase contrast microscopy, Auger electron spectroscopy, scanning electron microscopy/energy dispersive spectroscopy, and scanning profilometry. On all CdZnTe substrates tested, we observed as-received large area macro-defect contamination. Using a defect specification limit of 50 contiguous defective pixels, we identified non-compliant 1280 × 720 12-μm pitch focal plane arrays due to as-received substrate macro-defect contamination. Using the above specification, up to 20% IRFPA wafer yield loss is due to state-of-the-art as-received CdZnTe substrate macro-contamination.     

在Si(211)衬底上分子束外延CdTe的剪切应变

利用高分辨率X射线衍射技术对分子束外延CdTe（211）B/Si（211）材料的CdTe外延薄膜进行了倒易点二维扫描,并通过获得的对称衍射面和非对称衍射面的倒易空间图,对CdTe缓冲层的剪切应变状况进行了分析.研究发现,对于CdTe/Si结构,随着CdTe厚度的增加,[1-1-1]、[01-1]两个方向的剪切角γ[1-1-1]和γ[01-1]都有变小的趋势,且γ[1-1-1]的大小约为γ[01-1]的两倍;对于CdTe/ZnTe/Si结构,ZnTe缓冲层的引入可以有效地降低CdTe层的剪切应变.     

Recent Results on Growth of (211)B CdTe on (211)Si with Intermediate Ge and ZnTe Buffer Layers by Metalorganic Vapor-Phase Epitaxy

We report on the investigation of epitaxial cadmium telluride grown by metalorganic vapor-phase epitaxy (MOVPE) on (211)Si, with particular emphasis on studying the effect of changing the reactor parameters and thermal annealing conditions on the epilayer quality. The CdTe films were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and x-ray diffraction (XRD). The best CdTe films were observed when the Te/Cd precursor partial pressure ratio was close to 3.1. It was also observed that, though annealing improved the crystal quality, a slight increase in surface roughness was observed. Similar attempts were made to improve the growth conditions of ZnTe intermediate buffer layer, which showed similar trends with changes in precursor flows.     

MBE HgCdTe on Alternative Substrates for FPA Applications

Results of first-principles calculations and experiments focusing on molecular beam epitaxy (MBE) growth of HgCdTe on the alternative substrates of GaAs and Si are described. The As passivation on (2 × 1) reconstructed (211) Si and its effects on the surface polarity of ZnTe or CdTe were clarified by examining the bonding configurations of As. The quality of HgCdTe grown on Si was confirmed to be similar to that grown on GaAs. Typical surface defects in HgCdTe and CdTe were classified. Good results for uniformities of full width at half maximum (FWHM) values of x-ray rocking curves, surface defects, and x values of Hg_1−x Cd _x Te were obtained by refining the demanding parameters and possible tradeoffs. The sticking coefficient of As₄ for MBE HgCdTe was determined. The effects of Hg-assisted annealing for As activation were investigated experimentally and theoretically by examining the difference of the formation energy of As_Hg and As_Te. Results of focal-plane arrays (FPAs) fabricated with HgCdTe grown on Si and on GaAs are discussed.     

Metalorganic Vapor-Phase Epitaxial Growth of (211)B CdTe on (211) Si Using Ge Interfacial Layer

We have achieved metalorganic vapor-phase epitaxial growth of (211)B CdTe on Si without the requirement of a pregrowth high-temperature oxide desorption step. This was achieved by growing a thin Ge film on the starting (211) Si substrates. To get (211)B CdTe orientation, the Ge surface was exposed to As prior to the start of CdTe growth. A thin ZnTe interlayer between Ge and CdTe has been shown to improve the CdTe surface morphology.     

ZnCdTe衬底上HgCdTe分子束外延的位错密度

报道了用 MBE的方法 ,在 Zn Cd Te衬底上制备 Hg Cd Te薄膜的位错密度研究结果。研究发现Hg Cd Te材料的位错密度与 Zn Cd Te衬底的表面晶体损伤、Hg Cd Te生长条件以及材料组分密切相关。通过衬底制备以及生长条件的优化 ,在 Zn Cd Te衬底上生长的长波 Hg Cd Te材料 EPD平均值达到 4.2× 1 0 5cm- 2 ,标准差为 3 .5× 1 0 5cm- 2 ,接近 Zn Cd Te衬底的位错极限。可重复性良好 ,材料位错合格率为 73 .7%。可以满足高性能Hg Cd Te焦平面探测器对材料位错密度的要求     

Surface structure of plasma-etched (211)B HgCdTe

The surface of (211)B HgCdTe has been studied by reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). RHEED analysis of the as-grown Hg-rich molecular beam epitaxy (MBE) (211)B HgCdTe suggests the surface reconstructs by additional Hg incorporation. The plasma-etched (211)B HgCdTe surface is crystalline but stepped and facetted. RHEED analysis indicates asymmetric pyramids (base dimensions ≈0.5×1.1 nm) are formed to minimize surface Hg concentration. The AFM examination of plasma-etched (211)B HgCdTe reveals oriented mesoscopic features.     

Understanding the Evolution of CdTe Buffer Layer Surfaces on ZnTe/Si(211) and GaAs(211)B During Cyclic Annealing

We present the results of a detailed study of the changes that occur on CdTe buffer layer surfaces grown on ZnTe/Si(211) and GaAs(211)B during the routine thermal cyclic annealing (TCA) process. Observations indicate that CdTe buffer layer surfaces are Te saturated when the TCA is performed under Te overpressure. In the absence of Te flux during the TCA step, the CdTe surface loses CdTe congruently and the typical CdTe nanowires show the presence of nodules on their surfaces. The observed changes in reflection high-energy electron diffraction patterns during TCA are explained in terms of surface chemistry and topography observations. Overall, the Te overpressure is necessary to maintain a smoother and pristine surface to continue the molecular beam epitaxy (MBE) growth.