Surface structure of (111)A HgCdTe

The surface of (111)A HgCdTe has been studied by reflection high-energy electron diffraction and atomic force microscopy (AFM). The as-grown liquid-phase epitaxy (LPE) surface has bilayer (3.7 ± 0.2 Å) step/terrace structures, macro-steps, and cross-hatch patterns. Macro-steps occur about the $[11\bar 2]$ and are from 10–40 Å in height. AFM and x-ray measurements indicate the as-grown epilayer is ≈0.2° off-cut (random polar angle) from the (111). 〈110〉 cross-hatch lines consistent with bilayer (step height=3.9 ± 0.2 Å) {111} slip dislocation are observed. The native oxide/carbon layer for the as-grown LPE (111)A HgCdTe is ≈8 Å. The experimental results suggest that the as-grown LPE surface approximates an equilibrium vicinal crystal structure. The 0.1% Br:ethylene glycol wet chemically etched surfaces retained the macro-step structure, but numerous small protrusions (10–100 Å height, ≈300 Å diameter) developed. The plasma-etched (111)A HgCdTe surface is crystalline, but exhibits surface disorder and is roughened.     

Surface Morphology and Defect Formation Mechanisms for HgCdTe (211)B Grown by Molecular Beam Epitaxy

The surface morphology and crystallinity of HgCdTe films grown by molecular beam epitaxy (MBE) on both CdZnTe and CdTe/Si (211)B substrates were characterized using atomic force microscopy (AFM), as well as scanning (SEM) and transmission (TEM) electron microscopy. Crosshatch patterns and sandy-beach-like morphologies were commonly found on MBE (211) HgCdTe epilayers grown on both CdZnTe and CdTe/Si substrates. The patterns were oriented along the , , and directions, which were associated with the intersection between the (211) growth plane and each of the eight equivalent HgCdTe slip planes. This was caused by strain-driven operation of slip in these systems with relative large Schmid factor, and was accompanied by dislocation formation as well as surface strain relief. Surface crater defects were associated with relatively high growth temperature and/or low Hg flux, whereas microtwins were associated with relatively low growth temperature and/or high Hg flux. AFM and electron microscopy were used to reveal the formation mechanisms of these defects. HgCdTe/HgCdTe superlattices with layer composition differences of less than 2% were grown by MBE on CdZnTe substrates in order to clarify the formation mechanisms of void defects. The micrographs directly revealed the spiral nature of growth, hence demonstrating that the formation of void defects could be associated with the Burton, Cabrera, and Frank (BCF) growth mode. Void defects, including microvoids and craters, were caused by screw defect clusters, which could be triggered by Te precipitates, impurities, dust, other contamination or flakes. Needle defects originated from screw defect clusters linearly aligned along the directions with opposite Burgers vector directions. They were visible in HgCdTe epilayers grown on interfacial superlattices. Hillocks were generated owing to twin growth of void or needle defects on (111) planes due to low growth temperature and the corresponding insufficient Hg movement on the growth surface. Therefore, in addition to nucleation and growth of HgCdTe in the normal two-dimensional layer growth mode, the BCF growth mode played an important role and should be taken into account during investigation of HgCdTe MBE growth mechanisms.     

Anisotropic Surface Roughness in Molecular-Beam Epitaxy CdTe (211)B/Ge(211)

We characterize the surface of molecular-beam epitaxy (MBE)-grown CdTe(211)B/Ge(211) by atomic-force microscopy (AFM), optical interference microscopy, and generalized ellipsometry (GE). We find that, for substrate temperatures above 300°C, the surface is rough and hazy; the AFM root-mean-square roughness is of the order of 150 Å. It appears from GE that the optical response is anisotropic, the principal axes of anisotropy being along the $ [\overline{1} 11] We characterize the surface of molecular-beam epitaxy (MBE)-grown CdTe(211)B/Ge(211) by atomic-force microscopy (AFM), optical interference microscopy, and generalized ellipsometry (GE). We find that, for substrate temperatures above 300°C, the surface is rough and hazy; the AFM root-mean-square roughness is of the order of 150 ?. It appears from GE that the optical response is anisotropic, the principal axes of anisotropy being along the and directions. For a substrate temperature of approximately 300°C, the surface is smooth and mirror-like and the AFM roughness is as low as 45 ?. The sample is still anisotropic, even though the magnitude of the cross-polarized reflection coefficients are very small in this case. It appears that the anisotropy originates from the surface roughness, not the bulk.     

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.     

Lattice mismatch induced morphological features and strain in HgCdTe epilayers on CdZnTe substrates

HgCdTe epilayers on CdZnTe substrates can exhibit a cross-hatch pattern of periodically varying strain and surface undulations as revealed by x-ray topography, and in some cases by Nomarski optical microscopy. On { 111 } oriented material, the pattern appears as three sets of parallel lines in the 〈 110 〉 slip directions (60° apart). To investigate this phenomenon and its impact on photovoltaic device performance, we have characterized several liquid phase epitaxy (LPE)-grown HgCdTe epilayer samples by means of Lang x-ray reflection topography, synchrotron white beam x-ray topography (SWBXT), etch pit density, and other techniques. The cross hatching generally shows a correlation with the ZnTe mole fraction of the substrate. In particular, the pattern is likely to appear when the natural lattice parameter of the layer at room temperature is slightly larger or smaller than that of the substrate in the same region. We also find the corresponding pattern in { 211 } oriented layers grown by MBE. Although substantial compositional interdiffusion occurs at the layer/substrate interface during LPE growth at around 500°C, this is not a necessary condition for the cross-hatch pattern, as demonstrated by the occurrence of the pattern in MBE material grown at less than 200°C. In terms of device performance, the pattern is manifested as lines of diodes in an array having greater leakage than their neighbors. In addition to these results, we have investigated other anomalies, by means of SWBXT applied to large-area diodes that have been electrically tested. A novel technique called absorption edge contour mapping, using synchrotron white beam x-rays with a molybdenum filter, was applied to reveal the longer range lattice strain.     

The structure of the Si (211) surface

Silicon (211) has been proposed as an alternative substrate for CdTe/HgCdTe molecular beam epitaxial growth. Silicon has a clear advantage over other substrates because of its low cost, high strength, and thermal-expansion coefficient, which matches that of the silicon readout integrated circuit. The (211) orientation has been shown to yield high-quality CdTe and HgCdTe/CdTe layers over other orientations. The reconstruction and faceting of the Si (211) surface is poorly understood despite the importance of the (211) orientation. The results of low-energy electron diffraction (LEED) studies have been contradictory, and their conclusions are inconsistent with recent scanning tunneling microscopy (STM) studies. LEED and STM images were used to determine the most probable Si (211) surface facet structure as a function of annealing temperature. Samples annealed at a high temperature (i.e., >1260°C) allowed the formation of ordered LEED spot patterns as opposed to the typically reported $[\bar 111]$ streaks. The pattern in the $[0\bar 11]$ direction gave a consistent 2× (7.68 Å) reconstruction.     

Passivation effect on optical and electrical properties of molecular beam epitaxy-grown HgCdTe/CdTe/Si layers

The effects of passivation with two different passivants, ZnS and CdTe, and two different passivation techniques, physical vapor deposition (PVD) and molecular beam epitaxy (MBE), were quantified in terms of the minority carrier lifetime and extracted surface recombination velocity on both MBE-grown medium-wavelength ir (MWIR) and long-wavelength ir HgCdTe samples. A gradual increment of the minority carrier lifetime was reported as the passivation technique was changed from PVD ZnS to PVD CdTe, and finally to MBE CdTe, especially at low temperatures. A corresponding reduction in the extracted surface recombination velocity in the same order was also reported for the first time. Initial data on the 1/f noise values of as-grown MWIR samples showed a reduction of two orders of noise power after 1200-Å ZnS deposition.     

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.     

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.     

Characterization of PbSnSe/CdTe/Si (211) Epilayers Grown by Molecular Beam Epitaxy

Narrow-bandgap PbSnSe has received much attention as a promising alternative material for mid- and long-wavelength high performance of infrared detection at relatively high operating temperatures owing to the weak composition dependence of its bandgap, which can intrinsically result in better uniformity. Additionally, it possesses a high dielectric constant that is anticipated to be much more tolerant to defects. In addition, its growth by molecular beam epitaxy (MBE) can be easily accomplished in comparison with HgCdTe and many III–V quantum well and superlattice materials. However, overcoming the high lattice and thermal mismatches between PbSnSe and CdTe/Si substrates and improving the crystalline quality of PbSnSe grown on CdTe/Si substrates are challenges that require further study. Additionally, interdiffusion between CdTe and PbSnSe can take place and lead to nonuniform distributions of elements in PbSnSe. Epitaxial crystal PbSnSe alloy films were grown by MBE and were investigated by scanning and high-resolution transmission electron microscopy (STEM/HRTEM). Etch pit density (EPD) measurements were done to determine the density of threading defects in the films. EPD measurements on PbSnSe surfaces gave values in the mid-10⁶ cm⁻² range. The dislocations exposed as etch pits were found to accumulate and form small-angle grain boundaries lined up along the () direction, which is the intersection line between (100) and (211) growth planes.     

Selective epitaxy of cadmium telluride on silicon by MBE

CdTe B was grown on As-terminated Si(111) by molecular beam epitaxy (MBE). Nucleation and interface properties were studied by photoelectron spectroscopy, scanning tunneling microscopy, electron diffraction, and energy-dispersive spectroscopy of x-rays. Selective growth on Si(111) was investigated either by using SiO₂ as a mask, or by growing on a patterned CdTe seed layer. The highest temperature where CdTe nucleates on As-terminated Si(111) surfaces is typically in the range of 220–250°C. On a SiO₂ mask, CdTe nucleates at the same temperatures, leading to polycrystalline growth. However, homoepitaxy of CdTe is possible around 300°C. Hence, CdTe can be grown selectively on a patterned CdTe seed layer on Si(111). This is confirmed by scanning electron microscopy and scanning Auger microscopy.     

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.     

Optical Properties and Microstructure of Silver-Copper Nanoparticles Synthesized by Pulsed Laser Deposition

Utilizing a pulsed laser deposition (PLD) method, silver-copper (Ag-Cu) nanoparticles have been synthesized by changing the surface area ratio of the target (S _R = S _Cu/(S _Ag + S _Cu)) from 0 to 30%. The peak absorption attributed to surface plasmon resonance (SPR) increased when increasing S _R up to 15%, above which it decreased. The peak shifts seem to be induced by the changes in the conductivity and morphology of the Ag-Cu nanoparticles. Additionally, the interplanar spacings of the Ag-Cu nanoparticles prepared at S _R = 15% corresponded to the Ag {111}, {200}, {220}, and Cu {111} planes. However, since the interplanar spacings attributed to the Cu {200} and {220} planes were not detected, the Ag-Cu nanoparticles were believed to possess a lattice constant (a) close not to the Cu phase (a = 3.615 Å) but to the Ag phase (a = 4.086 Å). Moreover, confirming the presence of Cu atoms in the nanoparticles using energy dispersive X-ray (EDX) spectra, Ag-Cu nanoparticles may be a solid solution in which Cu atoms partially replace Ag atoms in the fcc structure.     

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.     

Strain Reduction in Selectively Grown CdTe by MBE on Nanopatterned Silicon on Insulator (SOI) Substrates

Silicon-based substrates for the epitaxy of HgCdTe are an attractive low-cost choice for monolithic integration of infrared detectors with mature Si technology and high yield. However, progress in heteroepitaxy of CdTe/Si (for subsequent growth of HgCdTe) is limited by the high lattice and thermal mismatch, which creates strain at the heterointerface that results in a high density of dislocations. Previously we have reported on theoretical modeling of strain partitioning between CdTe and Si on nanopatterned silicon on insulator (SOI) substrates. In this paper, we present an experimental study of CdTe epitaxy on nanopatterned (SOI). SOI (100) substrates were patterned with interferometric lithography and reactive ion etching to form a two-dimensional array of silicon pillars with ～250 nm diameter and 1 μm pitch. MBE was used to grow CdTe selectively on the silicon nanopillars. Selective growth of CdTe was confirmed by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Coalescence of CdTe on the silicon nanoislands has been observed from the SEM characterization. Selective growth was achieved with a two-step growth process involving desorption of the nucleation layer followed by regrowth of CdTe at a rate of 0.2 Å s^?1. Strain measurements by Raman spectroscopy show a comparable Raman shift (2.7 ± 2 cm^?1 from the bulk value of 170 cm^?1) in CdTe grown on nanopatterned SOI and planar silicon (Raman shift of 4.4 ± 2 cm^?1), indicating similar strain on the nanopatterned substrates.     

CdTe/GaAs(111)B薄膜的MBE生长研究

采用分子束外延方法,在GaAs(111)B衬底上,生长CdTe薄膜,以求研制出用于液相外延生长碲镉汞(HgCdTe)薄膜的CdTe/GaAs(111)B复合衬底.通过理论分析和实验探索,优化了生长温度和Te/Cd束流比等重要生长参数,获得了质量较好的CdTe薄膜,再通过循环热处理,使CdTe/GaAs(111)B复合衬底的质量得到进一步的提高,X-射线回摆曲线半峰宽(FWHM)有明显的降低.为LPE-HgCdTe薄膜的生长打下了较好基础.     

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).     

Recent Progress in MBE Growth of CdTe and HgCdTe on (211)B GaAs Substrates

Alternate substrates for molecular beam epitaxy growth of HgCdTe including Si, Ge, and GaAs have been under development for more than a decade. MBE growth of HgCdTe on GaAs substrates was pioneered by Teledyne Imaging Sensors (TIS) in the 1980s. However, recent improvements in the layer crystal quality including improvements in both the CdTe buffer layer and the HgCdTe layer growth have resulted in GaAs emerging as a strong candidate for replacement of bulk CdZnTe substrates for certain infrared imaging applications. In this paper the current state of the art in CdTe and HgCdTe MBE growth on (211)B GaAs and (211) Si at TIS is reviewed. Recent improvements in the CdTe buffer layer quality (double crystal rocking curve full-width at half-maximum?≈?30?arcsec) with HgCdTe dislocation densities of ≤10⁶?cm^?2 are discussed and comparisons are made with historical HgCdTe on bulk CdZnTe and alternate substrate data at TIS. Material properties including the HgCdTe majority carrier mobility and dislocation density are presented as a function of the CdTe buffer layer quality.     

GaSb: A New Alternative Substrate for Epitaxial Growth of HgCdTe

In this work, GaSb is proposed as a new alternative substrate for the growth of HgCdTe via molecular beam epitaxy (MBE). Due to the smaller mismatch in both lattice constant and coefficient of thermal expansion between GaSb and HgCdTe, GaSb presents a better alternative substrate for the epitaxial growth of HgCdTe, in comparison to alternative substrates such as Si, Ge, and GaAs. In our recent efforts, a CdTe buffer layer technology has been developed on GaSb substrates via MBE. By optimizing the growth conditions (mainly growth temperature and VI/II flux ratio), CdTe buffer layers have been grown on GaSb substrates with material quality comparable to, and slightly better than, CdTe buffer layers grown on GaAs substrates, which is one of the state-of-the-art alternative substrates used in growing HgCdTe for the fabrication of mid-wave infrared detectors. The results presented in this paper indicate the great potential of GaSb to become the next generation alternative substrate for HgCdTe infrared detectors, demonstrating MBE-grown CdTe buffer layers with rocking curve (double crystal x-ray diffraction) full width at half maximum of ～60 arcsec and etch pit density of ～10⁶ cm^?2.     

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.