Reaction chemistry and resulting surface structure of HgCdTe etched in CH4/H2 and H2 ECR plasmas

We report on several new aspects of etching of Hg_1−xCd_xTe (x = 0.22), HgTe, and CdTe in CH₄/H₂/Ar plasmas generated by an electron cyclotron resonance plasma source. Using a residual gas analyzer, we have identified elemental Hg, TeH₂, Te(CH₃)₂, and Cd(CH₃)₂ as the primary reaction products escaping from a HgCdTe surface during the plasma exposure. We have also demonstrated that a bias is not needed to etch HgCdTe at moderate temperatures (30-40°C), as previously suggested by other researchers. We have also developed a technique that avoids the formation of hydrocarbon polymer films on a HgCdTe sample during etching. Moreover, we have examined by x-ray photoelectron spectroscopy analysis and ellipsometry the surface condition of HgCdTe resulting from etching with this technique at zero bias. After exposure to the CH₄/H₂Ar plasma (or to a H₂/Ar plasma only), the HgCdTe samples exhibited a depletion of the HgTe component in the near surface region (increase of the x-value). The depletion covered a range from virtually x = 1 after H₂/Ar (10:2 in sccm) etching to values 0.4 < x < 0.5 after CH₄/H₂Ar (7:7:2 in seem) etching. Exposures to the plasmas were found to result in surface roughening of HgCdTe, however, plasmas rich in H₂ were observed to cause significantly rougher surfaces than plasmas with small H₂/CH₄ ratios. This difference in the resulting surface condition is attributed solely to chemical effects since the respective ion energies are considered to be below the damage threshold for HgCdTe in both cases. We also investigated the etching of HgTe and CdTe single crystals. The etch rate of HgTe was found to be over one order of magnitude higher than that of CdTe under similar conditions. This large difference in etch rates is assumed to be responsible for the observed preferential etching of the HgTe component indicated by the HgTe depletion of the HgCdTe surface region.     

位错对布里奇曼HgCdTe体晶结构完整性的影响

本文根据Ｘ射线形貌相、扫描电镜和金相显微镜的实验结果和有关位错、相图的理论，分析了位错和亚晶界、亚晶界和Ｘ射线形貌相的关系以及住错对布里奇曼技术生长的ＨｇＣｄＴｅ体晶结构完整性的影响。因位错降低能量的要求，ＨｇＣｄＴｅ体晶中的密度位错要形成亚晶界和三维位错网络等亚结构。在Ｘ射线形貌相中，亚结构的取向衬度是比位错直接象更强的成象机构。正常凝固长晶过程中ＨｇＣｄＴｅ材料的组分要变化，并使其固液界面的温度梯度也发生相应变化，由此引入的热应力在结晶过程中会在ＨｇＣｄＴｅ晶体中产生高密度位错。位错形成的亚晶界和三维位错网络等亚结构严重破坏了ＢｒｉｄｇｍａｎＨｇＣｄＴｅ体晶结构完整性，这使其Ｘ射线形貌相往往不好。     

Use of ellipsometry to characterize the surface of HgCdTe

Ellipsometry is a sensitive, rapid, and nondestructive optical technique for characterizing materials, especially surfaces and films. By measuring the change in the state of polarization of a light beam reflecting from the sample, one may infer certain characteristics of the sample. We present a review of the applications of ellipsometry to HgCdTe and related materials. The fundamentals of the technique are discussed briefly and the optical parameters at the wavelength 6328A for several materials of interest to infrared technology are listed. The emphasis of this paper is on the interpretation of the ellipsometric data, expressed in terms of the usual parameters Ψ and Δ obtained at a single wavelength. Methods and limitations of the analysis of single films, both nonabsorbing and absorbing, are discussed. Examples of an acceptance window for process monitoring are presented. The ellipsometric signatures of amorphous Te films and microroughness are described, along with a graphical method for interpreting the readings from very thin films. Spectroscopic applications and in situ monitoring of molecular beam epitaxial growth processes are briefly reviewed.     

Characterization of Hg1−xCdxTe heterostructures by thermoelectric measurements

P-on-n mercury cadmium telluride (MCT) heterostructures grown by MOCVD with As and In as n- and p-type dopants, respectively, are examined by measuring the Seebeck and Hall coefficients between 20 and 320K. The results are analyzed regarding doping and composition of the layers by least squares fitting the experimental profiles with the calculated temperature dependencies. The electron and hole densities of the layers are calculated taking into account Fermi-Dirac statistics, a nonparabolic conduction band, a parabolic valence band, a discrete acceptor level, and fully ionized donors. For the Seebeck coefficient, the relation we previously showed to be valid for p-type MCT¹ is used. This relation relies on the thermoelectric effect in a temperature gradient resulting from the diffusion of nondegenerate carriers scattered by LO-phonons. It also fits the observed thermoelectric properties of n-type MCT in a wide temperature range. The doping and structural parameters determined from the thermoelectric measurements agreed very well with As and In profiles obtained from secondary ion mass spectroscopy measurements and the data obtained from analyses of infrared transmission measurements.     

Hall effect characterization of LPE HgCdTe P/n heterojunctions

The field and temperature dependence of the Hall coefficient has been used to simultaneously extract information about the p and n layers in very long wave length infrared P/n HgCdTe heterojunctions. The field dependence allows the effects of high mobility electrons to be separated from those of low mobility holes. The higher the magnetic field, the higher the sensitivity to the parameters of the P layer. For a maximum magnetic field of 8000 gauss, the hole sheet concentration must be at least five times the electron sheet concentration to obtain accurate results for the P layer. This criterion is satisfied for typical liquid phase epitaxy (LPE) heterostructures. The analysis determines the hole sheet resistance (concentration times mobility), rather than the hole concentration or mobility separately. Independent knowledge of the P layer thickness and the relationship between hole concentration and resistivity are needed to convert the Hall measurement results to hole concentrations. Analysis of the field-dependent Hall data is complicated by the finding that at least three electrons of different mobilities are needed to fit the field dependence of the Hall coefficient in n-type LPE HgCdTe layers. These results are consistent with previous conclusions that electrons with different mobilities are needed to model bulk n-HgCdTe, and with a range of mobilities in the graded composition interface between the LPE layer and CdTe substrate. Consistent results are obtained for the concentrations and mobilities of the three types of electrons in the n-HgCdTe layer with and without the P layer present. N and P type carrier concentrations are also consistent with dopant concentrations measured by secondary ion mass spectroscopy.     

HgCdTe光导芯片室温电阻率不稳定问题探讨

集中讨论 MCT 光导芯片电阻率随时间变化的问题。研究结果表明:MCT芯片中 Hg 原子的逐渐逸出导致了材料组分即 x 值随时间变化,这是芯片室温电阻率变化的主要原因;另外,在某些条件下(如加温),芯片表面可形成高浓度的 n 型载流子薄层,也将引起芯片的室温电阻率的变化。     

Growth of high quality CdTe on Si substrates by molecular beam epitaxy

We have systematically studied the growth of CdTe (lll)B on Si(001)with different atomic step structures, defined uniquely by miscut tilt angle and direction. X-ray double crystal rocking curve (DCRC) analysis has been used to evaluate the crystalline quality and twin content of the films. High-resolution electron microscopy has been used to examine the CdTe(lll)B/Si(001) interface and to follow the microstructural evolution as a function of distance from the interface. Our results show that the formation of double domains and twins is very sensitive to the tilt parameters. When growth conditions are optimized, twins are not observed at distances greater than about 2.5 microns from the substrate surface. The best quality films exhibit a DCRC FWHM of 60 arc sec, for a film thickness of 17 μm, the lowest value ever reported for heteroepitaxial growth of CdTe on Si or GaAs. In efforts to improve the nucleation process, precursors such as Te and As have been used, and we have shown that they improve the stability of the heterointerface.     

Influence of Hg pressure on diffusion coefficient of As in HgCdTe

Arsenic diffusion coefficients were measured in HgCdTe at 350°C within the single phase field. The diffusion coefficients displayed a strong dependence on Hg pressure, increasing by more than 1×10³ with decreasing Hg pressure. These measurements were performed by growing As doped HgCdTe films by Hg-rich liquid phase epitaxy on undoped or In-doped base layers, where the growth temperature ranged between 330 and 350°C. Use of these low growth temperatures under Hg-rich conditions permitted attainment of virtual step profiles in As, with negligible diffusion into the base layers. These provided ideal starting points for subsequent diffusion anneals. Diffusion of arsenic under selected low Hg pressures was then employed to tune the positioning of the p/n junction for double layer heterojunction films, by locating it ahead of the heterointerface. Formation of valence band barriers to the photogenerated minority carriers across the junction could thus be avoided. When on the other hand, diffusion experiments were performed under Hg saturated conditions, the heterointerface moved at a faster rate than the p/n junction, leading to the formation of valence band barriers.     

Observation of indium-vacancy and indium-hydrogen interactions in Hg1−xCdxTe

We have used a nuclear hyperfine technique, perturbed γγ angular correlation (PAC), to study the interactions between¹¹¹In and native defects and impurities in Hg_1−xCd_xTe. The PAC technique uses the quadrupole interaction of¹¹¹In with local electric field gradients to characterize the local environment of this donor dopant. We observed that when In was diffused into a bulk or thin film sample of Hg_1−xCd_xTe (x=0.21 and x=0.3) at 350°C and the sample was slow cooled, the In occupied sites with near-cubic symmetry, presumably the substitutional metal site. However, when the sample was quenched, a fraction of the In was incorporated into defects characterized by quadrupole interaction strengthsv _Q1 andv _Q2 and asymmetries of ν₁=ν₂=0.08. These defects are attributed to the trapping of a metal vacancy at a next-nearest neighbor site to the In atom. The introduction of hydrogen by boiling the samples in distilled water for >4h eliminated the previously observed PAC signals and created defects characterized byv _Q3=35 MHz, ν₃ <0.1 andv _Q4=MHz, ν₄ <0.1. These defects are attributed to the decoration of the In-V_Hg complex by a hydrogen atom. Hall effect measurements showed that hydrogenation increased the hole concentration in p-type quenched samples and even converted n-type indium-doped samples to p-type. A possible model for hydrogen incorporation which includes self-compensation by vacancy creation is suggested.     

引晶技术在HgCdTe体单晶生长中的应用

本文论述引入籽晶技术定向生长ＨｇＣｄＴｅ体单晶，通过石英管相对于加热器的缓慢移动来使熔区移过均匀的合成锭达到晶体生长的目的。其结构完整性及组分均匀性得到改善，并且电参数优良。已制出多元光导及光伏器件。