Infrared absorption behavior in CdZnTe substrates

Infrared (IR) optical transmission measurements of polished CdZnTe wafers can provide useful information about excess impurities, stoichiometry, and inhomogeneities (precipitates and inclusions). We have investigated the IR transmission behavior of Cd_0.96Zn_0.04Te between 8 m and 20 m at room temperature. The measurements were made before and after thermal treatments involving control of the Cd and Zn overpressures, which served to minimize the Cd (cation) vacancy population. Our results support the polar optical phonon scattering theory of Jensen, according to which the absorption in donor dominated CdZnTe varies as^m with m=3. For material dominated by acceptors, we show that the theoretical absorption by inter-valence band transitions can be approximated by a similar power law with exponent m=1, and that Cd-vacancy dominated wafers are in reasonable agreement with this. We find some wafers in which the asgrown condition exhibits partial compensation of impurity donors by Cd vacancy acceptors, and demonstrate removal of the compensation by annealing to fill the vacancies. In a separate group of wafers, we find that an observed increase in absorption occurring during growth of a HgCdTe layer by liquid phase epitaxy can be explained in terms of an increase in Cd vacancies caused by diffusion of Cd to Te precipitates. This effect can be reversed by annealing in Cd−Zn vapor, which fills vacancies and eliminates some precipitates. Impurity concentrations were measured by glow discharge mass spectrometry (GDMS).     

Cd1—yZnyTe晶片Te夹杂的观察

本文观察了Cd0.96Zn0.04Te晶片的8层红外透射分层域聚焦显微像,分析了各个层域的Te夹杂情况及Te夹杂密度,指出了找到CdZnTe较小夹杂密度层域面并用之来做外延HgCdTe衬底表面的方法。     

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.     

X-ray Diffraction Imaging of Improved Bulk-Grown CdZnTe(211) and Its Comparison with Epitaxially Grown CdTe Buffer Layers on Si and Ge Substrates

Large-area high-quality Hg_1–x Cd _x Te sensing layers for infrared imaging in the 8 μm to 12 μm spectral region are typically grown on bulk Cd_1–x Zn _x Te substrates. Alternatively, epitaxial CdTe grown on Si or Ge has been used as a buffer layer for high-quality epitaxial HgCdTe growth. In this paper, x-ray topographs and rocking-curve full-width at half-maximum (FWHM) data will be presented for recent high-quality bulk CdZnTe grown by the vertical gradient freeze (VGF) method, previous bulk CdZnTe grown by the vertical Bridgman technique, epitaxial CdTe buffer layers on Si and Ge, and a HgCdTe layer epitaxially grown on bulk VGF CdZnTe.     

Cd1-yZnyTe的Zn组分面分布及均匀性控制

随着HgCdTe红外探测器尺寸规模的扩大,晶格匹配的Cd1-yZnyTe衬底组份均匀性越来越受到重视。由于在CdZnTe晶体中Zn的分凝系数大于1,晶片中Zn组分分布不均匀,从而使HgCdTe外延薄膜不同区域因晶格失配而产生位错的情况不同, 精确测量CdZnTe衬底的Zn组分及其均匀性是十分必要的。高分辨率X射线衍射是精确测量晶格常数和Zn组分的有效方法之一,但不适合常规面分布测定,而采用红外透射光谱法则能够在短时间内及实验容许的误差范围内快速获得CdZnTe中Zn组份的分布。另外,对锭条上不同方位的晶片进行径向和横向面分布红外透射光谱测量,可为组份均匀性控制提供依据。     

HgCdTe／CdZnTe晶格失配与X光衍射貌相的关系研究

文章采用富Te水平推舟液相外延工艺在CdZnTe衬底上生长了HgCdTe外延薄膜。研究了外延薄膜／衬底晶格失配度、X光衍射貌相、红外焦平面器件探测率三者之间的关系。对于HgCdTe外延层的X光衍射貌相我们将其大致分为五类，分别是Crosshatch貌相、混合貌相、均匀背景貌相、Mosaic貌相以及由衬底质量问题引起的沟壑状貌相，采用Crosshatch貌相和混合貌相材料所制备的红外焦平面器件，平均来说其探测率(D*)较高。X射线双轴衍射的实验结果表明，当外延层与衬底的晶格失配度为～0.03％时，外延层会呈现明显的Crosshatch貌相；而当失配度减小时，会逐渐呈现出混合貌相、均匀背景貌相、直至失配度为负值时呈现Mosaic貌相。因此，对于特定截止波长的HgCdTe焦平面器件，可以通过控制HgCdTe／CdZnTe之间的失配，生长出符合我们要求的貌相的碲镉汞外延材料，从而来提高焦平面器件的性能。     

Detailed study of above bandgap optical absorption in HgCdTe

HgCdTe remains the material of choice for high-performance infrared (IR) detectors due to its tunable direct bandgap energy corresponding to the IR spectral region, and the advancement of HgCdTe materials growth and processing technologies. Accurate knowledge of the HgCdTe optical absorption coefficient is important for IR detector design, layer screening, and device analysis. The spectral response for IR detectors is dependent on optical absorption above the bandgap energy, where much of the study of absorption coefficient in HgCdTe has focused on the bandtail region. In this work, the optical absorption coefficient was studied by theoretical bandstructure calculations and experimental measurements on HgCdTe layers using techniques of IR spectroscopic ellipsometry and IR transmission. The theoretical and experimental results suggest that the absorption coefficient between 600 cm⁻¹ and 5,000 cm⁻¹ is related to energy relative to bandgap with a fractional exponent between 0.6 and 1, rather than the previously used expressions relating to a parabolic or hyperbolic bandstructure. The fitting parameters for Hg_1-xCd_xTe with x=0.22–0.60 are presented to develop a model for the optical absorption coefficient spectra. The calculated detector spectral response using the new and previously reported absorption coefficient models suggests that next generation IR detectors employing multilayer structures with graded compositional profiles will likely benefit from this new model.     

Role of Dislocation Scattering on the Electron Mobility of n-Type Long Wave Length Infrared HgCdTe on Silicon

It has been reported that the basic electrical properties of n-type long wave length infrared (LWIR) HgCdTe grown on silicon, including the majority carrier mobility (μ _e) and minority carrier lifetime (τ), are qualitatively comparable to those reported for LWIR HgCdTe grown on bulk CdZnTe by molecular beam epitaxy (MBE). Detailed measurements of the majority carrier mobility have revealed important differences between the values measured for HgCdTe grown on bulk CdZnTe and those measured for HgCdTe grown on buffered silicon substrates. The mobility of LWIR HgCdTe grown on buffered silicon by MBE is reported over a large temperature range and is analyzed in terms of standard electron scattering mechanisms. The role of dislocation scattering is addressed for high dislocation density HgCdTe grown on lattice-mismatched silicon. Differences between the low temperature mobility data of HgCdTe grown on bulk CdZnTe and HgCdTe grown on silicon are partially explained in terms of the dislocation scattering contribution to the total mobility.     

Advancements in THM-Grown CdZnTe for Use as Substrates for HgCdTe

The focus of this work is to evaluate the suitability and substrate potential of Cd_0.9Zn_0.1Te and Cd_0.96Zn_0.04Te crystals grown by the traveling heater method (THM). THM-grown Cd_0.9Zn_0.1Te crystals used for gamma spectroscopy have shown very good spectral performance owing partly to the very low concentration of Te inclusions and precipitates. Inspection in the infrared (IR) of annealed THM-grown CdZnTe wafers reveals no inclusions >3 μm, and Fourier-transform infrared measurements show IR transmission values in excess of 60%. Wafer etch pit density values are typically less than 4 × 10^?4 pits/cm², and double-crystal x-ray rocking-curve measurements show full-width at half-maximum values approaching 40 arcsec. 〈211〉 wafers have been produced with off orientation within 0.3°. (111)-Oriented, seeded THM growth runs have the ability to provide 10 60 mm × 60 mm × 2 mm wafers from a 75-mm-diameter boule or 20 90 mm × 90 mm × 2 mm wafers from a 100-mm-diameter boule.     

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。     

The annealing of Cd1−xZnxTe in CdZn vapors

As-grown CdZnTe usually contains defects, such as twins, subgrain boundaries, dislocations, and Te precipitates. It is always important to anneal CdZnTe slices in Cd vapor to eliminate these defects, especially Te precipitates. The exchange of Zn atoms between the slices and the vapor plays an important role during the annealing process. In this paper, the effects of Zn partial pressure on the properties of the annealed slices are studied carefully by measuring the concentration profiles, the infrared (IR) transmission spectra, and the x-ray rocking curves. It was found that a surface layer with different compositions and possibly different structure from the bulk crystal formed during the annealing of CdZnTe samples in the saturated Zn vapor. The accumulation of excess Te in the surface layer helps to increase the IR permeability of the bulk crystal greatly. To improve the crystallization quality, a lower Zn-pressure annealing should be used following the high Zn-pressure annealing. The diffusion of Zn in the bulk crystal has also been analyzed at the temperatures of 700°C and 500°C. Calculations determined that D_Zn (700°C)=4.02 × 10⁻¹² cm²s⁻¹ and D_Zn (500°C)=1.22 × 10⁻¹³ cm²s⁻¹.     

磁光—光致发光分析CdZnTe单晶带边浅杂质能级

通过对Bridgeman方法生长的CdZnTe单晶样品进行光致发光(Photoluminescence,PL)光谱测量,发现CdZnTe样品表面Te沉淀物的存在明显影响能量低于1.5 eV的深能级发光过程.进一步对CdZnTe晶锭的不同位置取样进行低温变磁场光致发光光谱测试,获得高分辨光谱信息.拟合分析结果表明:(1)在不含Te沉淀物的CdZnTe样品内部存在应力分布,并因此导致轻、重空穴带分裂;(2)1.57 eV发光特征源于浅施主杂质与价带间的复合过程.     

Optical-absorption model for molecular-beam epitaxy HgCdTe and application to infrared detector photoresponse

Accurate knowledge of the optical-absorption coefficient in HgCdTe is important for infrared (IR) detector design, production process (layer screening), and interpretation of detector performance. Measurements of the optical-absorption coefficient of HgCdTe layers with uniform composition are presented with the goal of developing a revised model in the interest of IR detector technology. Existing methods of determining HgCdTe alloy composition from IR transmission measurements are compared, where one self-consistent method is suggested and shown to agree with experimental detector data. An exponential Urbach and hyperbolic model are presented to represent band tail and above-bandgap absorption regions, respectively. Parameters associated with these models are extracted for Hg_1−xCd_xTe compositions of x=0.22–0.60 and temperatures of T=40–300 K using samples of varying thickness to obtain accurate data for varying spectral regions of the absorption coefficient. An initial analytical expression for the absorption coefficient is presented and compared to experimental detector-response data. Detector-response simulations indicate that accurate optical-absorption models are needed, where detector structures with thin layers and arbitrary composition profiles in current and future IR detectors will be the most demanding.     

HgCdTe组分异质结的生长与表征

采用富碲水平推舟液相外延生长方式,在(111)晶向的碲锌镉衬底上生长了双层组分异质HgCdTe外延薄膜,并对生长后的薄膜质量进行了评价.使用染色法和红外透射光谱数值拟合的方法,对两层薄膜的厚度进行了表征,并建立了一个双层薄膜纵向组分分布模型;对材料的电学参数进行测量的结果显示,双层异质液相外延样品中长波层的载流子迁移率较之单层液相外延样品略高,原因可能是中波覆盖层对长波外延层起到了钝化作用.     

液相外延HgCdTe薄膜组分均匀性对红外透射光谱的影响

用多层模型和膜系传递矩阵计算了HgCdTe/CdZnTe外延薄膜的红外透射光谱，结果表明组分扩散区主要影响透射光谱的干涉条纹和透射率小于10％的区域，而组分梯度区则影响吸收边斜率．横向组分波动也将影响透射光谱的吸收边斜率，当组分均方差小于0.005时，横向组分波动对透射光谱影响可以忽略．用新的组分分布模型计算了HgCdTe/CdZnTe液相外延薄膜的理论透射光谱，并运用非线性二乘法使理论曲线能够很好地与实验结果吻合，从而获得了更加可信的HgCdTe外延薄膜的纵向组分分布和厚度参数．     

Substrate quality impact on the carrier concentration of undoped annealed HgCdTe LPE layers

The impact of Te precipitates and impurities, in CdZnTe or CdTe substrates, on grown liquid phase epitaxy (LPE) HgCdTe layer hole concentrations was studied. The carrier concentrations in capped annealed LPE HgCdTe layers grown on CdZnTe substrates with large densities of Te precipitates are frequently significantly higher than those expected for HgCdTe annealed under Hg-deficient conditions. The carrier concentration in the LPE layer, due to the diffusion of copper ions from contaminated CdTe substrates into the layer, is strongly affected by the polarity of the (111)-oriented substrates. Layers grown on the (111)A face showed very high concentrations of Cu, whereas in those grown on the (111)B face normal carrier concentrations were achieved. These phenomena are discussed on the basis of defects formed either in the epilayer or in the layer-substrate interface.     

碲镉汞材料提纯研究

杂质是影响碲镉汞器件性能的重要因素之一.对于碲锌镉衬底晶体和窄禁带碲镉汞材料来说,杂质的影响更加显著.主要论述了碲镉汞材料中常见的杂质类型以及杂质在材料中的作用,并分析了影响器件性能的主要杂质.采用辉光放电质谱法(Glow Discharge Mass Spectrometry,GDMS)测试了材料中的杂质含量,同时通...     

CdZnTe substrate producibility and its impact on IRFPA yield

The need for cost effective production of HgCdTe infrared detectors and focal plane assemblies has led to increased attention to the availability of high quality large-area CdZnTe substrates. Reasonable yield of large-area substrates (≥4 cm×6 cm format) is necessary for fabrication of focal plane assemblies (FPAs) now in production, and for future infrared (IR) detectors which are growing in size and complexity. Raytheon’s infrared materials producibility (IRMP) program has addressed this issue, after identifying critical drivers of FPA yield coming from substrates, and targeted certain improvements in substrate process steps for highest impact on large-area substrate yield. Three specific areas of improvements in the substrate process were addressed: (1) compounding of a large 6 kg charge of CdTe; (2) vertical Bridgman growth of 92 mm diameter CdZnTe boules in both quartz and pyrolytic boron nitride (PBN) crucibles; and (3) optimized Cd overpressure control during growth and cool-down of the boule. It was shown that the Cd overpressure and the cooling schedule had the strongest effects on defect populations. The resulting improvements include a 33% increase in wafer yield per unit starting weight, an estimated 50% reduction in substrate cost per cm², better morphology of epitaxial HgCdTe layers, and improved yield of satisfactory IR detectors. The criteria for selecting substrates have also improved as a result of this work. In addition, photovoltaic detectors were fabricated on wafers from a variety of sources, and tested. Results compare favorably with those on baseline (earlier process) substrates.     

CdZnTe substrate impurities and their effects on liquid phase epitaxy HgCdTe

Impurity levels were tracked through the stages of substrate and liquid phase epitaxy (LPE) layer processing to identify sources of elements which degrade infrared photodetector performance. Chemical analysis by glow discharge mass spectrometry and Zeeman corrected graphite furnace atomic absorption effectively showed the levels of impurities introduced into CdZnTe substrate material from the raw materials and the crystal growth processes. A new purification process(in situ distillation zone refining) for raw materials was developed, resulting in improved CdZnTe substrate purity. Substrate copper contamination was found to degrade the LPE layer and device electrical properties, in the case of lightly doped HgCdTe. Anomalous HgCdTe carrier type conversion was correlated to certain CdZnTe and CdTe substrate ingots.     

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.