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.     

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.     

分子束外延CdTe（211）B／Si复合衬底材料

报道了用MBE的方法，在3英寸Si衬底上制备ZnTe／CdTe（211）B复合衬底材料的初步研究结果，该研究结果将能够直接应用于大面积Si基HgCdTe IRFPA材料的生长．经过Si（211）衬底低温表面处理、ZnTe低温成核、高温退火、高温ZnTe、CdTe层的生长研究，用MBE方法成功地获得了3英寸Si基ZnTe／CdTe（211）B复合衬底材料．CdTe厚度大于10μm，XRD FWHM平均值为120arc sec，最好达到100arc sec，无（133）孪晶和其他多晶晶向．     

Dislocation profiles in HgCdTe(100) on GaAs(100) grown by metalorganic chemical vapor deposition

We studied dislocation etch pit density (EPD) profiles in HgCdTe(lOO) layers grown on GaAs(lOO) by metalorganic chemical vapor deposition. Dislocation profiles in HgCdTe(lll)B and HgCdTe(lOO) layers differ as follows: Misfit dislocations in HgCdTe(lll)B layers are concentrated near the HgCdTe/CdTe interfaces because of slip planes parallel to the interfaces. Away from the HgCdTe/CdTe interface, the HgCdTe(111)B dislocation density remains almost constant. In HgCdTe(lOO) layers, however, the dislocations propagate monotonically to the surface and the dislocation density decreases gradually as dislocations are incorporated with increasing HgCdTe(lOO) layer thicknesses. The dislocation reduction was small in HgCdTe(lOO) layers more than 10 μm from the HgCdTe/CdTe interface. The CdTe(lOO) buffer thickness and dislocation density were similarly related. Since dislocations glide to accommodate the lattice distortion and this movement increases the probability of dislocation incorporation, incorporation proceeds in limited regions from each interface where the lattice distortion and strain are sufficient. We obtained the minimum EPD in HgCdTe(100) of 1 to 3 x 10⁶ cm^-2 by growing both the epitaxial layers more than 8 μm thick.     

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.     

束源杂质引发的MBE HgCdTe表面缺陷

研究了利用GaAs作为衬底的HgCdTe MBE薄膜的表面缺陷，发现其中一类缺陷与Hg源中杂质有关。采用SEM对这类缺陷进行正面和横截面的观察，并采用EDX对其正面和横截面进行成分分析。并设计了两个实验：其一，在CdTe／GaAs衬底上，低温下用Hg源照射20min，再在其上继续高温生长CdTe；其二，在CdTe／GaAs衬底上，一直用Hg源照射下高温生长CdTe。两个实验后CdTe表面都出现与HgCdTe表面相比在形状和分布上类似的表面缺陷，采用光学显微镜和SEM对CdTe表面缺陷进行了观察，通过CdTe表面缺陷和HgCdTe表面缺陷的比较，我们证实了这类表面缺陷的成核起源于Hg源中杂质。     

Orientation dependence of HgCdTe epitaxial layers grown by MOCVD on Si substrates

Orientation dependence of HgCdTe epilayers grown by MOCVD on Si substrates was studied. Substrate orientation is considered to be one of the most sensitive factors to enable hetero-epitaxial growth on silicon substrates, especially in the case of a low temperature growth process. The present work was carried out with characterized features of a low temperature process for HgCdTe growth on Si and using a thin CdTe buffer layer. The (100), (100) misoriented toward [110], (311), (211), (111), and (331) oriented Si substrates were used in the present work. The best results were obtained on (211)Si substrates with an x-ray full width at half maximum of 153 arc sec for a 5 (im thickness HgCdTe layer and 69 arc sec for a 10 um thickness layer. It was found that the effective lattice mismatch of CdTe/Si heterosystem was reduced to 0.6% (for the 611 lattice spacing of CdTe and 333 spacing of Si) in the case of (133)CdTe/(211)Si.     

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.     

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.     

Heteroepitaxy of HgCdTe(112) infrared detector structures on Si(112) substrates by molecular-beam epitaxy

High-quality, single-crystal epitaxial films of CdTe(112)B and HgCdTe(112)B have been grown directly on Si(112) substrates without the need for GaAs interfacial layers. The CdTe and HgCdTe films have been characterized with optical microscopy, x-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. HgCdTe/Si infrared detectors have also been fabricated and tested. The CdTe(112)B films are highly specular, twin-free, and have x-ray rocking curves as narrow as 72 arc-sec and near-surface etch pit density (EPD) of 2 × 10⁶ cm⁻² for 8 μm thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 76 arc-sec and EPD of 3-22 × 10⁶ cm⁻². These MBE-grown epitaxial structures have been used to fabricate the first high-performance HgCdTe IR detectors grown directly on Si without use of an intermediate GaAs buffer layer. HgCdTe/Si infrared detectors have been fabricated with 40% quantum efficiency and R₀A = 1.64 × 10⁴ Ωm₂ (0 FOV) for devices with 7.8 μm cutoff wavelength at 78Kto demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.     

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.     

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.     

Modeling of in situ monitored laser reflectance during MOCVD growth of HgCdTe

An effective way to in situ monitor the metalorganic chemical vapor deposition (MOCVD) of HgCdTe/CdTe/ZnTe on GaAs or GaAs/Si substrates is presented. Specular He-Ne laser reflectance was used to in situ monitor the growth rates, layer thickness, and morphology for each layer in the grown multilayer structure. In situ monitoring has enabled precise measurements of ZnTe nucleation and CdTe buffer layer thicknesses. Monitoring the constancy of reflectance during the thicker CdTe buffer growth where absorption in the CdTe reduces reflectance to just the surface component has led to optimum buffer growth ensuring good quality of subsequently grown HgCdTe. During the interdiffused multilayer process (IMP) HgCdTe growth, because multiple interfaces are present within the absorption length, a periodic reflectance signal is maintained throughout this growth cycle. A theoretical model was developed to extract IMP layer thicknesses from in situ recorded experimental data. For structures that required the growth of a larger band gap HgCdTe cap layer on top of a smaller band gap active layer, in situ monitored reflectance data allowed determination of alloy composition in the cap layer as well. Continuous monitoring of IMP parameters established the stability of growth conditions, translating into depth uniformity of the grown material, and allowed diagnosis of growth rate instabilities in terms of changes in the HgTe and CdTe parts of the IMP cycle. A unique advantage of in situ laser monitoring is the opportunity to perform “interactive” crystal growth, a development that is a key to real time MOCVD HgCdTe feedback growth control.     

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.     

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.     

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.     

分子束外延HgCdTe薄膜的CdTe缓冲层特性研究

CdTe是GaAs衬底上分子束外延(MBE)HgCdTe薄膜时的缓冲层,引入缓冲层的目的是减小失配位错,CdTe缓冲层的生长直接影响到后续HgCdTe薄膜的制备质量,然而目前现有文献鲜有报道CdTe缓冲层的最佳厚度.采用X射线双晶衍射、位错腐蚀坑密度(EPD)、FT-IR和椭圆偏振光谱的方法,从CdTe缓冲层厚度对位错密度的影响入手,分析并确定了理想的CdTe缓冲层厚度.     

In-situ ellipsometric measurements of the MBE growth of CdTe/HgTe and CdTe/ZnTe superlattices

Phase modulated ellipsometric data recorded during molecular beam epitaxial growth of CdTe/HgTe and CdTe/ZnTe superlattices on (100) and (211)B oriented Cd_0.96Zn_0.04Te and GaAs substrates are presented. The measurements provide a continuous monitor of the growth process, thickness, growth rate, compositional data, and evidence of interdiffusion in CdTe/HgTe superlattices at elevated temperatures. The thickness measurements are independent of growth kinetics and surface orientation and agree well with those obtained from x-ray diffraction and reflection high energy electron diffraction. Ellipsometry shows that the incorporation of Hg in CdTe is significantly higher on (100) oriented surfaces than on (211)B oriented surfaces. Fine structure in the data from CdTe/ZnTe superlattices may be associated with a surface reconstruction during deposition of each CdTe layer. The experimental results for CdTe/HgTe superlattices compare well with results of thin film multi-layer calculations. The general applicability of ellipsometry as an in-situ analytical technique for epitaxial growth of a range of semiconductor superlattices is discussed.     

Electrical and structural properties of epitaxial CdTe/HgCdTe interfaces

In this study, CdTe epilayers were grown by metalorganic chemical vapor deposition on epitaxial HgCdTe with the purpose of developing suitable passivation for HgCdTe photodiodes. Two types of CdTe layers were investigated. One was grown directly,in situ, immediately following the growth of HgCdTe. The second type of CdTe was grown indirectly, on top of previously grown epitaxial HgCdTe samples. In this case, the surface of the HgCdTe was exposed to ambient atmosphere, and a surface cleaning procedure was applied. The material and structural properties of the CdTe/HgCdTe interfaces were investigated using secondary ion mass spectroscopy, Auger electron spectroscopy, Rutherford back scattering, and x-ray double crystal diffractometry techniques. Electrical properties of the CdTe/HgCdTe heterostructure were determined by capacitance-voltage (C-V) characterization of Schottky barrier devices and metal insulator semiconductor devices. Also, a preliminary current-voltage characterization of n⁺ p photodiodes was performed. A theoretical model suitable for analysis of graded heterojunction devices was used for interpretation of C-V measurements.     

Analysis of Mesa Dislocation Gettering in HgCdTe/CdTe/Si(211) by Scanning Transmission Electron Microscopy

Due to its strong infrared absorption and variable band-gap, HgCdTe is the ideal detector material for high-performance infrared focal-plane arrays (IRFPAs). Next-generation IRFPAs will utilize dual-color high-definition formats on large-area substrates such as Si or GaAs. However, heteroepitaxial growth on these substrates is plagued by high densities of lattice-mismatch-induced threading dislocations (TDs) that ultimately reduce IRFPA operability. Previously we demonstrated a postgrowth technique with the potential to eliminate or move TDs such that they have less impact on detector operability. In this technique, highly reticulated mesa structures are produced in as-grown HgCdTe epilayers, and then subjected to thermal cycle annealing. To fully exploit this technique, better understanding of the inherent mechanism is required. In this work, we employ scanning transmission electron microscopy (STEM) analysis of HgCdTe/CdTe/Si(211) samples prepared by focused ion beam milling. A key factor is the use of defect-decorated samples, which allows for a correlation of etch pits observed on the surface with underlying dislocation segments viewed in cross-section STEM images. We perform an analysis of these dislocations in terms of the general distribution, density, and mobility at various locations within the mesa structures. Based on our observations, we suggest factors that contribute to the underlying mechanism for dislocation gettering.