Electron cyclotron resonance plasma preparation of CdZnTe (211)B surfaces for HgCdTe molecular beam epitaxy

CdZnTe wafers were inserted into a multi-chamber processing facility without prior preparation, cleaned by exposure to an electron cyclotron resonance Ar/H₂ plasma, and used as substrates for molecular beam epitaxy of HgCdTe. Changes induced in the wafer near-surface region during the cleaning step were monitored using in situ spectroscopic ellipsometry. Ellipsometric data were subsequently modeled to provide the time evolution of the thickness of a native overlayer. Auger electron spectra were consistent with surfaces free of residual contamination and which had the stoichiometry of the underlying bulk. Surface roughness values of 0.4 nm were obtained ex situ using interferometric microscopy. Electron diffraction patterns of plasma prepared wafers heated to 185°C (the temperature required for HgCdTe molecular beam epitaxy) were streaked. Structural and electrical characteristics of epilayers grown on these substrates were found to be comparable to those deposited on wafers prepared using a conventional wet chemical process. This demonstrates an important step in an all-vacuum approach to HgCdTe detector fabrication.     

Heteroepitaxy of HgCdTe (211)B on Ge substrates by molecular beam epitaxy for infrared detectors

Epitaxial growth of (211)B CdTe/HgCdTe has been achieved on two inch germanium (Ge) by molecular beam epitaxy (MBE). Germanium was chosen as an alternative substrate to circumvent the weaknesses of CdZnTe wafers. The ease of surface preparation makes Ge an attractive candidate among many other alternative substrates. Best MBE CdTe growth results were obtained on (211) Ge surfaces which were exposed to arsenic and zinc fluxes prior to the MBE growth. This surface preparation enabled CdTe growth with B-face crystallographic polarity necessary for the HgCdTe growth. This process was reproducible, and produced a smooth and mirror-like surface morphology. The best value of the {422} x-ray double diffraction full width at half maximum measured from the HgCdTe layer was 68 arc-s. We present the 486 point maps of FWHM statistical values obtained from CdTe/Ge and HgCdTe/CdTe/Ge. High resolution microscopy electron transmission and secondary ion mass spectroscopy characterization results are also presented in this paper. High-performance middle wavelength infrared HgCdTe 32-element photodiode linear arrays, using the standard LETI/LIR planar n-on-p ion implanted technology, were fabricated on CdTe/Ge substrates. At 78K, photodiodes exhibited very high R₀A figure of merit higher than 10⁶ Ωcm⁻² for a cutoff wavelength of 4.8 μm. Excess low frequency noise was not observed below 150K.     

Dual-band infrared detectors made on high-quality HgCdTe epilayers grown by molecular beam epitaxy on CdZnTe or CdTe/Ge substrates

In this paper, we present all the successive steps for realizing dual-band infrared detectors operating in the mid-wavelength infrared (MWIR) band. High crystalline quality HgCdTe multilayer stacks have been grown by molecular beam epitaxy (MBE) on CdZnTe and CdTe/Ge substrates. Material characterization in the light of high-resolution x-ray diffraction (HRXRD) results and dislocation density measurements are exposed in detail. These characterizations show some striking differences between structures grown on the two kinds of substrates. Device processing and readout circuit for 128×128 focal-plane array (FPA) fabrication are described. The electro-optical characteristics of the devices show that devices grown on Ge match those grown on CdZnTe substrates in terms of responsivity, noise measurements, and operability.     

Molecular beam epitaxy growth of high-quality HgCdTe LWIR layers on polished and repolished CdZnTe substrates

We report here molecular beam epitaxy (MBE) mercury cadmium telluride (HgCdTe) layers grown on polished and repolished substrates that showed state-of-the-art optical, structural, and electrical characteristics. Many polishing machines currently available do not take into account the soft semiconductor materials, CdZnTe (CZT) being one. Therefore, a polishing jig was custom designed and engineered to take in account certain physical parameters (pressure, substrate rotational frequency, drip rate of solution onto the polishing pad, and polishing pad rotational velocity). The control over these parameters increased the quality, uniformity, and the reproducibility of each polish. EPIR also investigated several bromine containing solutions used for polishing CZT. The concentration of bromine, as well as the mechanical parameters, was varied in order to determine the optimal conditions for polishing CZT.     

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.     

分子束外延HgCdTe/CdZnTe(211)B表面缺陷研究

HgCdTe材料的表面缺陷是造成探测器性能下降的主要原因之一。采用聚焦离子束（Focused Ion Beam, FIB）、扫描电子显微镜（Scanning Electron Microscope, SEM）和能量色散X射线光谱仪（Energy Dispersive X-ray Spectrometer, EDX）研究了碲锌镉（CdZnTe)基HgCdTe外延层的表面缺陷。通过分析不同类型缺陷形成的原因,确定缺陷起源于HgCdTe材料生长过程。缺陷的形状与生长条件关系密切。凹坑及火山口状缺陷与Hg缺乏/稍高生长温度、分子束源坩埚中材料形状变化造成的不稳定束流有关。金刚石状缺陷和火山口状/金刚石状复合缺陷的产生与Hg/Te高束流比、低生长温度相关。在5 cm×5 cm大小的CdZnTe(211)B衬底表面上生长出了组分为0.216、厚度约为6.06~7 μm的高质量HgCdTe外延层。同时还建立了缺陷类型与HgCdTe薄膜生长工艺的关系。该研究对于制备高质量HgCdTe/CdZnTe外延层具有参考意义。     

Correlation of CdZnTe(211)B substrate surface morphology and HgCdTe(211)B epilayer defects

We present results on the surface morphology and recombination lifetimes of molecular-beam epitaxy (MBE)-grown HgCdTe (211)B epilayers and correlate them with the roughness of the CdZnTe substrate surfaces. The substrate surface quality was monitored by in-situ spectroscopic ellipsometry (SE) and reflection high-energy electron diffraction (RHEED). The SE roughness of the substrate was measured after oxide desorption in the growth chamber. The RHEED patterns collected show a strong correlation with the SE roughness. This proves that SE is a valuable CdZnTe prescreening tool. We also found a correlation between the substrate roughness and the epilayer morphologies. They are characterized by a high density of thin elongated defects, “needle defects,” which appear on most samples regardless of growth conditions. The HgCdTe epilayers grown on these substrates were characterized by temperature-dependent, photoconductive decay-lifetime data. Fits to the data indicate the presence of mid-gap recombination centers, which were not removed by 250°C/24-h annealing under a Hg-rich atmosphere. These centers are believed to originate from bulk defects rather than Hg vacancies. We show that Te annealing and CdTe growth on the CdZnTe substrates smooth the surface and lower substantially the density of needle defects. Additionally, a variety of interfacial layers were also introduced to reduce the defect density and improve the overall quality of the epilayer, even in the presence of less than perfect substrates. Both the perfection of the substrate surface and that of its crystalline structure are essential for the growth of high-quality material. Thus, CdZnTe surface polishing procedures and growth techniques are crucial issues.     

Transmission electron microscopy studies of defects in HgCdTe device structures grown by molecular beam epitaxy

Transmission electron microscopy (TEM) was used to evaluate the microstructure of molecular beam epitaxy (MBE) grown (211)B oriented HgCdTe films. TEM analysis of in-situ doped p-on-n and n-p-n device structures will be presented. Under fully optimized growth conditions the substrate-epilayer interface is free of threading dislocations and twins, and a high degree of structural integrity is retained throughout the entire device structure. However, under non-optimal growth conditions that employ high Hg/Te flux ratios, twins can be generated in the p-type layer of p-on-n device structure, resulting in roughness and facetting of the film surface. We propose a mechanism for twin formation that is associated with surface facetting. TEM evaluation of voids, threading dislocations and Te-precipitates in HgCdTe films are also discussed.     

High-resolution X-ray diffraction studies of molecular beam epitaxy-grown HgCdTe heterostructures and CdZnTe substrates

Lattice mismatch between substrates and epitaxial layers of different molefractions can create a variety of distortions and defects in Hg_(1−x)Cd_(x)Te epilayers, thus degrading the performance of infrared detectors fabricated from this material. X-ray diffraction is a sensitive nondestructive technique, which allows in-depth characterization of the crystal lattice prior to detector fabrication. We present results of triple-axis diffractometry performed on single- and double-layer HgCdTe films grown on (211)B CdZnTe substrates by molecular beam epitaxy (MBE). In this study, both the ω and 2θ diffraction angles have been recorded absolutely so that the diffraction peaks in the RSMs can be positioned directly in reciprocal space, without requiring reference to a substrate peak. The positions of both surface-symmetric and asymmetric diffraction peaks have been used to extract lattice spacings parallel and perpendicular to the (211) growth direction. The relaxed lattice parameter of each epilayer has been calculated assuming that the layers are elastically strained. The low symmetry of the (211) growth direction, coupled with the anisotropic elasticity of zinc-blende semiconductors, results in monoclinic distortion of the lattice, as observed in these samples. In double-layer samples, the mosaicity of both layers is greater than that observed in single epilayers. Layers subjected to a Hg-saturated anneal show greater lattice distortion than as-grown samples.     

Operation of a compact electron cyclotron resonance source for the growth of gallium nitride by molecular beam epitaxy (ECR-MBE)

The operation of an ASTeX compact electron cyclotron resonance plasma source and its effect on the growth of GaN thin films by electron cyclotron resonancemolecular beam epitaxy has been investigated. The role a flow limiting orifice plays in increasing plasma stability as well as reducing ion damage and impurities in resultant films has also been studied. Both optical emission spectroscopy as well as electrostatic (Langmuir) probe studies have been employed to elucidate the generation and transport of charged and neutral species. With the introduction of the flow orifice, a substantial decrease in ion induced damage as well as surface roughening in the films is observed. This can be accounted for in terms of a collisionally induced relaxation of the grad-B acceleration of charged species toward the substrate in plasma sources employing axial solenoidal fields.     

Ten-inch molecular beam epitaxy production system for HgCdTe growth

Growth of Hg_1−xCd_xTe by molecular beam epitaxy (MBE) has been under development since the early 1980s at Rockwell Scientific Company (RSC), formerly the Rockwell Science Center; and we have shown that high-performance and highly reproducible MBE HgCdTe double heterostructure planar p-on-n devices can be produced with high throughput for various single- and multiplecolor infrared applications. In this paper, we present data on Hg_1−xCd_xTe epitaxial layers grown in a ten-inch production MBE system. For growth of HgCdTe, standard effusion cells containing CdTe and Te were used, in addition to a Hg source. The system is equipped with reflection high energy electron diffraction (RHEED) and spectral ellipsometry in addition to other fully automated electrical and optical monitoring systems. The HgCdTe heterostructures grown in our large ten-inch Riber 49 MBE system have outstanding structural characteristics with etch-pit densities (EPDs) in the low 10⁴ cm⁻² range, Hall carrier concentration in low 10¹⁴ cm⁻³, and void density <1000 cm². The epilayers were grown on near lattice-matched (211)B Cd_0.96Zn_0.04Te substrates. High-performance mid wavelength infrared (MWIR) devices were fabricated with R₀A values of 7.2×10⁶ Ω-cm² at 110 K, and the quantum efficiency without an antireflection coating was 71.5% for cutoff wavelength of 5.21 μm at 37 K. For short wavelength infrared (SWIR) devices, an R₀A value of 9.4×10⁵ Ω-cm² at 200 K was obtained and quantum efficiency without an antireflection coating was 64% for cutoff wavelength of 2.61 μm at 37 K. These R₀A values are comparable to our trend line values in this temperature range.     

Electron microscopy of surface-crater defects on HgCdTe/CdZnTe(211)B epilayers grown by molecular-beam epitaxy

Surface-void defects observed in Hg_1−xCd_xTe (x ∼ 0.2–0.4) alloys grown by molecular-beam epitaxy (MBE) have been investigated using scanning and high-resolution transmission-electron microscopy (HRTEM) as well as atomic force microscopy (AFM). These surface craters, which have been attributed to Hg-deficient growth conditions, were found to originate primarily within the HgCdTe epilayer, rather than at the CdZnTe substrate, and they were associated with the local development of polycrystalline morphology. High-resolution observations established the occurrence of finely spaced HgCdTe/Te intergrowths with semicoherent and incoherent grain boundaries, as well as small HgCdTe inclusions embedded within the Te grains. This study is the first time that high-resolution electron microscopy has been used to investigate this type of defect.     

HgCdTe molecular beam epitaxy material for microcavity light emitters: Application to gas detection in the 2–6 µm range

Most pollution gases, CO, CO₂, NO_x, SO₂, CH₄ …, have fundamental optical absorption in the near infrared range. We report here on microcavity light sources emitting at room temperature between 2 and 6 μm integrated in a gas detection system. HgCdTe has been chosen for this application, among several semiconductor materials. Molecular beam epitaxy (MBE) is very well adapted to grow the suitable HgCdTe heterostructure. The quality of involved HgCdTe layers has to be optimized in order to have a good photoluminescence response at 300 K. For this study, we used the knowledge we acquired in the field of MBE HgCdTe growth for infrared focal plane arrays (IRFPAs). Especially, we took advantage of the substrate preparation before growing and the flux control. We show subsequently several characterization results concerning our material quality. The compact emitting system is formed by this microcavity structure coupled to a 0.8-μm external pumping source. The Fabry-Perot type microcavity is obtained by using two evaporated YF₃/ZnS dielectric multilayered Bragg mirrors. We developed several devices exhibiting emission wavelengths at 3.3 μm, 4.26 μm, and 4.7 μm for CH₄, CO₂, and CO gas measurements, respectively, and 3.7 μm for the reference beam. We measured less than 200 ppm CH₄ in a 1 bar mixed gas along a 10-cm-long cell.     

Lateral uniformity in HgCdTe layers grown by molecular beam epitaxy

The fabrication of high-quality focal plane arrays from HgCdTe layers grown by molecular beam epitaxy (MBE) requires a high degree of lateral uniformity in material properties such as the alloy composition, doping concentration, and defect density. While it is well known that MBE source flux nonuniformity can lead to radial compositional variation for rotating substrates, we have also found that composition can be affected significantly by lateral variations in substrate temperature during growth. In diagnostic experiments, we systematically varied the substrate temperature during MBE and quantified the dependence of HgCdTe alloy composition on substrate temperature. Based on these results, we developed a methodology to quickly and nondestructively characterize MBE-grown layers using postgrowth spatial mapping of the cutoff wavelength from the Fourier transform infrared (FTIR) transmission at 300 K, and we were able to obtain a quantitative relationship between the measured spatial variations in cutoff and the substrate temperature lateral distribution during growth. We refined this methodology by more directly inferring the substrate temperature distribution from secondary ion mass spectroscopy (SIMS) measurements of the As concentration across a wafer, using the fact that the As incorporation rate in MBE-grown p-type layers is highly sensitive to substrate temperature. Combining this multiple-point SIMS analysis with FTIR spatial mapping, we demonstrate how the relative contributions from flux nonuniformity and temperature variations on the lateral composition uniformity can be separated. This capability to accurately map the lateral variations in the substrate temperature has been valuable in optimizing the mounting and bonding of large substrates for MBE growth, and can also be valuable for other aspects of MBE process development.     

Molecular beam epitaxial growth of HgCdTe on CdZnTe(311)B

A series of n-type, indium-doped Hg_1−xCd_xTe (x∼0.225) layers were grown on Cd_0.96Zn_0.04Te(311)B substrates by molecular beam epitaxy (MBE). The Cd_0.96Zn_0.04Te(311)B substrates (2 cm × 3 cm) were prepared in this laboratory by the horizontal Bridgman method using double-zone-refined 6N source materials. The Hg_1−xCd_xTe(311)B epitaxial films were examined by optical microscopy, defect etching, and Hall measurements. Preliminary results indicate that the n-type Hg_1−xCd_xTe(311)B and Hg_1−xCd_xTe(211)B films (x ∼ 0.225) grown by MBE have comparable morphological, structural, and electrical quality, with the best 77 K Hall mobility being 112,000 cm²/V·sec at carrier concentration of 1.9×10⁺¹⁵ cm⁻³.     

CdZnTe(211)B衬底对MBE外延碲镉汞材料的影响分析

碲锌镉 CdZnTe（211）B 衬底广泛应用于碲镉汞（HgCdTe）分子束外延生长,其性能参数在很大程度上决定了碲镉汞分子束外延材料的质量。主要讨论了 CdZnTe（211）B 衬底几个关键性能参数对碲镉汞外延材料的影响,包括 Zn 组分及均匀性、缺陷（位错、孪晶及晶界和碲沉淀）以及表面状态（粗糙度和化学组成）,并且分析了对 CdZnTe（211）B 衬底进行筛分时各性能参数的评价方法和指标。     

HgCdTe molecular beam epitaxy technology: A focus on material properties

HgCdTe MBE technology is becoming a mature growth technology for flexible manufacturing of short-wave, medium-wave, long-wave, and very long-wave infrared focal plane arrays. The main reason that this technology is getting more mature for device applications is the progress made in controlling the dopants (both n-type and p-type in-situ) and the success in lowering the defect density to less than 2 x 10⁵/cm² in the base layer. In this paper, we will discuss the unique approach that we have developed for growing As-doped HgCdTe alloys with cadmium arsenide compound. Material properties including composition, crystallinity, dopant activation, minority carrier lifetime, and morphology are also discussed. In addition, we have fabricated several infrared focal plane arrays using device quality double layers and the device results are approaching that of the state-of-the-art liquid phase epitaxy technology.     

Suppression of strain-induced cross-hatch on molecular beam epitaxy (211)B HgCdTe

Because the performance of HgCdTe-based photodiodes can be significantly degraded by the presence of dislocations, we have systematically investigated and suppressed lattice-mismatch-induced cross-hatch formation and the associated generation of dislocations in (211)B HgCdTe/CdZnTe. A series of HgCdTe epilayers were deposited simultaneously on pairs of substrates with differing ZnTe mole fractions. Epilayers’ CdTe mole fraction and substrates’ ZnTe mole fractions were measured using optical-transmission spectra. Lattice mismatch and residual strain were estimated from room-temperature, x-ray diffraction, and double-crystal rocking-curve measurements (DCRC). It was found that cross-hatch patterns were suppressed in epilayers deposited on nearly lattice-matched substrates (|Δa/a_sub|<0.02%). Such epilayers exhibited excellent crystalline quality as revealed by defect-decoration etching (etch-pit density (EPD)<10⁵ cm⁻²) and x-ray diffraction (full-width at half-maximum (FWHM) ∼10 arcsec). In addition to determining the upper limits of lattice mismatch needed to eliminate cross-hatch, we investigated the use of reticulated substrates as a means to suppress cross-hatch. We found that growth on reticulated mesa structures (<100 μm) with edges parallel to [01-1] resulted in epilayers with substantially reduced cross-hatch-line densities despite large lattice mismatch (Δa/a_sub <0.04%). The use of reticulated substrates could suppress cross-hatch because of lateral-alloy variation in large substrates and complex multistack epilayers (e.g., multicolor detectors).     

Precise control of HgCdTe growth conditions for molecular beam epitaxy

A Hg_1-xCd_xTe growth method is presented for molecular beam epitaxy, which precisely controls growth conditions to routinely obtain device quality epilayers at a certain specific composition. This method corrects the fluctuation in composition x for run-to-run growth by feedback from the x value for the former growth to the fluxes from CdTe and Te cells. We achieved standard deviation Δx/ x of 3.3% for 13 samples grown consecutively. A substrate temperature drop was found during growth, which considerably degrades the crystal quality of epilayers. In this method, this drop is greatly diminished by covering the holder surface with a heavily doped Si wafer. Finally, etch pit density of 4 x 10⁴ cm-² and full width at half-maximum of 12 arc-s for the x-ray double-crystal rocking curve were obtained as the best values.     

Molecular beam epitaxy growth of high-quality arsenic-doped HgCdTe

We have initiated a joint effort to better elucidate the fundamental mechanisms underlying As-doping in molecular beam epitaxy (MBE)-grown HgCdTe. We have greatly increased the As incorporation rate by using an As cracker cell. With a cracker temperature of 700°C, As incorporation as high as 4×10²⁰ cm⁻³ has been achieved by using an As-reservoir temperature of only 175°C. This allows the growth of highly doped layers with high quality as measured by low dislocation density. Annealing experiments show higher As-activation efficiency with higher anneal temperatures for longer time and higher Hg overpressures. Data are presented for layers with a wide range of doping levels and for layer composition from 0.2 to 0.6.