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.     

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.     

Use of electron cyclotron resonance plasmas to prepare CdZnTe (211)B substrates for HgCdTe molecular beam epitaxy

Without any additional preparation, Cd_1−yZn_yTe (211)B (y∼3.5%) wafers were cleaned by exposure to an electron cyclotron resonance (ECR) Ar/H₂ plasma and used as substrates for HgCdTe molecular beam epitaxy. Auger electron spectra were taken from as-received wafers, conventionally prepared wafers (bromine: methanol etching, followed by heating to 330–340°C), and wafers prepared under a variety of ECR process conditions. Surfaces of as-received wafers contained ∼1.5 monolayers of contaminants (oxygen, carbon, and chlorine). Conventionally prepared wafers had ∼1/4 monolayer of carbon contamination, as well as excess tellurium and/or excess zinc depending on the heating process used. Auger spectra from plasma-treated CdZnTe wafers showed surfaces free from contamination, with the expected stoichiometry. Stoichiometry and surface cleanliness were insensitive to the duration of plasma exposure (2–20 s) and to changes in radio frequency input power (20–100 W). Reflection high energy electron diffraction patterns were streaked indicating microscopically smooth and ordered surfaces. The smoothness of plasma-etched CdZnTe wafers was further confirmed ex situ using interferometric microscopy. Surface roughness values of ∼0.4 nm were measured. Characteristics of HgCdTe epilayers deposited on wafers prepared with plasma and conventional etching were found to be comparable. For these epilayers, etch pit densities on the order of 10⁵ cm⁻² have been achieved. ECR Ar/H₂ plasma cleaning is now utilized at Night Vision and Electronic Sensors Directorate as the baseline CdZnTe surface preparation technique.     

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.     

Minority carrier lifetime in indium-doped HgCdTe(211)B epitaxial layers grown by molecular beam epitaxy

We have studied the minority-carrier lifetime on intentionally indium-doped (211)B molecular beam epitaxially grown Hg_1-xCd_xTe epilayers down to 80K with x ≈ 23.0% ± 2.0%. Measured lifetimes were explained by an Auger-limited band-to-band recombination process in this material even in the extrinsic temperature region. Layers show excellent electron mobilities as high as ≈2 x 10⁵ cm²v^-1s^-1 at low temperatures. When the layers are compensated with Hg vacancies, results show that the Schockley-Read recombination process becomes important in addition to the band-to-band processes. From the values of τ_n0 and τ_p0 of one sample, the obtained defect level is acceptor-like and is somewhat related to the Hg vacancies.     

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.     

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.     

Laser-beam-induced current mapping of spatial nonuniformities in molecular beam epitaxy As-grown HgCdTe

The formation of dislocations and corresponding built-in electric fields in molecular beam epitaxy (MBE)-grown HgCdTe can have a major impact on the performance and yield of photodetectors fabricated from this material. This paper investigates the presence of such built-in electric fields arising from dislocation segregation in MBE as-grown HgCdTe, and their subsequent removal via a low-temperature Hg-saturated anneal. The electrical properties and surface morphology of an HgCdTe layer grown on a thin CdTe buffer layer are compared with those of an HgCdTe layer grown directly on the CdZnTe substrate. Laser-beam-induced current (LBIC) imaging is a nondestructive technique capable of mapping built-in electric fields present in a semiconductor material, which, in the present case, has been used to reveal dislocation distributions present in as-grown, unintentionally doped, MBE-grown Hg_0.71Cd_0.29Te. Two-dimensional scanning LBIC measurements at 160 K allow spatial mapping of electric fields across the HgCdTe wafer. Subsequent isothermal annealing of the wafer in an Hg atmosphere has been found to decrease the magnitude of the built-in electric fields to below the LBIC detection limit. However, of particular note, is that before and after annealing, crosshatch patterns can be seen using Nomarski microscopy, with the crosshatching being predominantly in the [01 ] direction and, to a lesser extent, in the [ 31] and [ 13] directions. Defect-decoration etching of the annealed wafer reveals dislocation banding parallel to the [01 ] direction, which closely resembles the contrast observed in the LBIC image of the wafer before annealing. These Nomarski and LBIC images are compared with those of a second wafer, which incorporates a 40-nm CdTe buffer layer. The second wafer does not show significant Nomarski or LBIC contrast, indicating a flat, electrically uniform as-grown layer. Variable magnetic-field Hall measurements at 77 K and quantitative mobility-spectrum analysis (QMSA) indicate predominately p-type conduction with a doping density of 2×10¹⁵ cm⁻³ in the as-grown layer. After Hg annealing at 240°C, no LBIC signals are observed at 160 K, and Hall measurements at 77 K indicate the presence of two n-type carriers, with a combined doping density of 2×10¹⁵ cm⁻³. Double-crystal x-ray diffraction measurements show no evidence of twinned crystal volumes in the layers before or after annealing, or any change in the full-width at half-maximum (FWHM) (41 arcsec) of the (422) reflection. The similarity between the dislocation density distribution, as revealed by defect decoration, and the LBIC image suggests that Hg out-diffusion during growth is expedited in regions of high dislocation concentration, thus creating a nonuniform Hg vacancy-acceptor concentration. The as-grown acceptor concentration, in turn, modulates the hole concentration, creating p⁺/p⁻ junctions and built-in electric fields in the material. Low-temperature annealing in a saturated-Hg atmosphere does not remove the crosshatch patterns or dislocation banding, but it fills the Hg vacancies, revealing the uniformly distributed n-type background, thus reducing the magnitude of any built-in electric fields. The LBIC mapping of MBE as-grown HgCdTe samples is, thus, capable of revealing defect distributions that would otherwise require a destructive technique, such as defect-decoration etching, to determine.     

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。     

采用(211)碲锌镉衬底制备碲镉汞液相外延薄膜

文中对(211)晶向的CdZnTe衬底进行液相外延生长HgCdTe。获得的碲镉汞液相外延材料的组分为0. 30 ～0. 33,薄膜厚度为10 ～15μm,表面缺陷密度为500cm- 2 ,材料的FWHM达到24弧秒,位错腐蚀坑密度约为2 ×105 cm- 2 ,该材料的表面形貌与采用(111)晶向衬底的HgCdTe外延材料有较大区别。     

Shear deformation and strain relaxation in HgCdTe on (211) CdZnTe

Shear strain is present in Hg_0.68Cd_0.32Te epitaxial layers grown by molecular beam epitaxy on (211)-oriented Cd_1−yZn_yTe substrates. Differences in the substrate zinc composition led to lattice mismatch between the epitaxial layer and the substrate. The shear strain induced by the mismatch was measured using reciprocal space maps in the symmetric (422) and asymmetric (511) and (333) reflections. In addition, strain relaxation through the formation of misfit dislocations was confirmed using double crystal x-ray topography. Both the shear strain and the misfit dislocation density increased with increasing mismatch between the epitaxial layer and the substrate. Lattice-matched layers were free of misfit dislocations and exhibited triple axis diffraction rocking curve widths of approximately 6 arcsec. The combination of a thick epitaxial layer, a low index substrate, and the potential for lattice mismatch indicates that both shear strains and misfit dislocations must be considered in the structural analysis of HgCdTe/CdZnTe heterostructures.     

Surface Structure of Molecular Beam Epitaxy (211)B HgCdTe

The as-grown molecular beam epitaxy (MBE) (211)B HgCdTe surface has variable surface topography, which is primarily dependent on substrate temperature and substrate/epilayer mismatch. Nano-ripple formation and cross-hatch patterning are the predominant structural features observed. Nano-ripples preferentially form parallel to the \( [\bar {1}11] \) and are from 0 Å to 100 Å in height with a wavelength between 0.1 μm and 0.8 μm. Cross-hatch patterns result from slip dislocations in the three {111} planes intersecting the (211) growth surface. The cross-hatch step height is 4 ± 1 Å (limited data set). This indicates that only a bi-layer slip (Hg/Cd + Te) in the {111} slip plane occurs. For the deposition of MBE (211)B HgCdTe/CdTe/Si, the reorientation of multiple nano-ripples coalesced into “packets” forms cross-hatch patterns. The as-grown MBE (211)B CdTe/Si surface is highly variable but displays nano-ripples and no cross-hatch pattern. Three types of defects were observed by atomic force microscopy (AFM): needle, void/hillock, and voids.     

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.     

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。     

Analysis of low doping limitation in molecular beam epitaxially grown HgCdTe(211)B epitaxial layers

We report the results of the transport properties and the recombination mechanisms of indium-doped HgCdTe(211)B (x ≈ 23.0% ± 2.0%) layers grown by molecular beam epitaxy. We have investigated the origin(s) of the background doping limitation in these layers. Molecular beam epitaxially grown layers exhibit excellent Hall characteristics down to indium levels of 2 x 10¹⁵ cm⁻³. Electron mobilities ranging from (2-3) x 10⁵ cm²/v-s at 23K were obtained. Measured lifetime data fits very well with the intrinsic band-to-band recombinations. However, below 2 x 10¹⁵ cm⁻³ doping levels, mobility vs temperature curves starts to reflect nonuniformity in carrier distribution. Also, when we reduced the Hg vacancy concentration down to 10¹² cm⁻³ range, by annealing at 150°C, Hall characteristics shows an increase in the nonuniformity in the epilayers. It was found that after annealed at 150°C, the obtained SR defect level has a different origin than the previously obtain Hg-vacancy related defect level.     

Doping of molecular beam epitaxy HgCdTe using an arsenic cracker source (As2)

This study investigated the incorporation of arsenic dimers (As₂), delivered from a “cracker” effusion cell. The HgCdTe epilayers were deposited under standard growth conditions. During deposition, arsenic was incorporated using both a standard arsenic effusion cell and a cracker cell. It was found that arsenic concentration rose dramatically as a function of cracker-zone temperature, particularly at temperatures above 600°C. This behavior was consistent with the temperature dependence of the effusion cell’s cracking efficiency, as determined by residual gas analysis. The temporal stability of the arsenic source was excellent. Arsenic concentrations of 2.8×10²⁰ cm⁻³ were achieved at a cracker temperature of 800°C. The arsenic beam-equivalent pressure, estimated from an uncorrected, nude ion-gauge reading, was ∼8×10⁻⁷ mbar.     

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.     

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.     

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.     

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.