A Slightly Oxidizing Etchant for Polishing of CdTe and CdZnTe Surfaces

New iodine-evolving etching solutions were developed for surface treatment of CdTe and CdZnTe crystals. The nature of the chemical dissolution of CdTe samples with (100), (110), and (111) orientations, and also of (211) Cd_0.96Zn_0.04Te samples, in KIO₃-KI-lactic acid solution was studied. It was established that this dissolution is diffusion controlled. Surface roughness measurements revealed highly polishing properties of the etchant. A study of the chemical composition and structure of the (211)B Cd_0.96Zn_0.04Te surfaces etched under different conditions was carried out. The thickness of the Te oxide layer was evaluated in the course of heating in vacuum. KIO₃-KI-lactic acid treatment appears to be preferable in terms of elimination of Te oxides from the surface as compared with standard Br₂-methanol solution. The etching compositions were shown to be useful for chemical polishing of CdTe-based surfaces.     

Chemical Interaction of CdTe and CdZnTe with Aqueous Solutions of H2O2-HI-Tartaric Acid

An iodine-based etching system, H₂O₂-HI-tartaric acid, was tested on CdTe samples with (110), (100), (111)A, and (111)B orientations. The etching rate of CdTe was shown to depend on the sample orientation, ratio of etchant composition components, temperature, and rotation speed of the disc. A study of the chemical composition and structure of the (211)B Cd_1−x Zn _x Te surfaces etched under different conditions was carried out. X-ray photoelectron spectroscopy (XPS) measurements showed that a nearly stoichiometric surface was achieved after heating of the etched surface in vacuum. Reflection high-energy electron diffraction (RHEED) measurements revealed a very good single-crystalline surface layer in samples etched with HI-based solutions as compared with bromine-methanol treatment.     

Chemical Polishing of CdTe and CdZnTe in Iodine–Methanol Etching Solutions

Iodine-containing polishing etchants were tested on CdTe and Cd_1−x Zn _x Te surfaces with different orientations. The nature of the chemical dissolution of CdTe with (111)A, (111)B, (110), and (100) orientations in the I₂–methanol solution was studied. It was established that this dissolution is diffusion controlled. Study of the chemical composition and structure of the (211)B Cd_1−x Zn _x Te surfaces etched under different conditions was carried out. X-ray photoelectron spectroscopy (XPS) measurements showed that a stoichiometric surface was achieved after short heating of the etched surface in vacuum. Reflection high-energy electron diffraction (RHEED) measurements reveal a high-quality single-crystalline surface layer in samples etched with iodine–methanol solutions as compared with bromine–methanol treatment.     

Chemical Etching of CdTe in Aqueous Solutions of H<Subscript>2</Subscript>O<Subscript>2</Subscript>-HI-Citric Acid

An iodine-based etching system, H₂O₂-HI-citric acid, has been developed and tested on CdTe samples with orientations (111)A, (111)B, (110), and (100). The etching velocity of CdTe was shown to depend on sample orientation and other etching conditions. The surface roughness was comparable with that of the surfaces after Br-methanol treatment. A comparative study of the chemical composition of the (211)B CdZnTe surfaces etched under different conditions was performed. X-ray photoelectron spectroscopy (XPS) measurements showed that all treated surfaces of CdZnTe samples are enriched with Te. The HI-based treatment seems to be more acceptable than the Br-methanol treatment in terms of elimination of Te oxides from the surface, however.     

Chemical–Mechanical Polishing of CdTe and ZnxCd1?xTe Single Crystals by H2O2(HNO3)–HBr–Organic Solvent Etchant Compositions

Chemical–mechanical polishing of CdTe and Zn _x Cd_1−x Te single-crystal surfaces by bromine-evolving compositions based on aqueous solutions of H₂O₂(HNO₃)–HBr–solvent has been investigated. The dependences of the chemical–mechanical polishing rate on the dilution of the base polishing etchant for various organic components have been determined. The surface condition after such polishing has been investigated using profilometry. The polishing etchant compositions for CdTe and Zn _x Cd_1−x Te single-crystal surfaces and the chemical polishing conditions have been optimized.     

Fast Detection of Precipitates and Oxides on CdZnTe Surfaces by Spectroscopic Ellipsometry

We study the surface chemistry of Cd_0.96Zn_0.04Te(211)B by X-ray photoelectron spectroscopy and ellipsometry. We obtain first the dielectric functions of amorphous Te and Cd on Cd_0.96Zn_0.04Te(211)B by in␣situ ellipsometry after growing thin films of each material by molecular beam epitaxy. We then study their oxidation in air and show that Cd oxidizes primarily as a hydroxide whereas Te is present as TeO₂. In neither case is the oxidation of the films complete, as a substantial amount of either metallic Te or Cd remains even after several days of oxidation. We subsequently exploit an electroless chemical etchant based on Ce⁴⁺ to produce Te-rich Cd_0.96Zn_0.04Te(211)B surfaces which oxidize very little in air. Another etchant containing KCN reduces the amount of α-Te substantially. Finally, we construct a tentative optical model for the Cd_0.96Zn_0.04Te(211)B surface that yields the abundance of amorphous Te on the surface.     

Transient behavior of Hg1−xCdxTe film growth on (111)B CdTe substrates by chemical vapor transport

As part of a systematic investigation of the effects of substrate surfaces on epitaxial growth, the transient behavior of Hg_1−xCd_xTe film growth on (111)B CdTe by chemical vapor transport (CVT) has been studied as a function of growth time under vertical stabilizing (hot end on top) and vertical destabilizing (hot end at bottom) ampoule orientations. The experim ental results show the morphological transition of the Hg_1−xCd_xTe deposition on (111)B CdTe at 545°C from three-dimensional islands to layers within about 0.5 and 0.75 h for the growth under vertical stabilizing and destabilizing conditions, respectively. The combined effects of small convective flow disturbances on the growth morphology and defect formation are measurable. The overall trends of the time dependent growth rates and compositions of the Hg_1−xCd_xTe epitaxial layers under stabilizing and destabilizing conditions are similar. The system atically higher growth rates of the Hg_1−xCd_xTe films by about 10% under vertical destabilizing conditions could be influenced by a small convective contribution to the mass transport. The combined results show that improved Hg_1−xCd_xTe epitaxial layers of low twin density on (111)B CdTe substrates can be obtained by CVT under vertical stabilizing conditions.     

Large area depositon of Cd1-xZnxTe on GaAs and Si substrates by metalorganic chemical vapor deposition

Results of large-area (up to 1000 cm²/run) Cd_1-xZn_xTe heteroepitaxy on both GaAs and GaAs/Si substrates by metalorganic chemical vapor deposition (MOCVD) are presented. Cd_1-xZn_xTe (x = 0-0.1) films exhibited specular surface morphology, 1% thickness uniformity (standard deviation), and compositional uniformity (Δx) of ±0.002 over 100 mm diam substrates. For selected substrate orientations and deposition conditions, the only planar defects exhibited by (lll)B Cd_1-xZn_xTe/GaAs/Si films were lamella twins parallel to the CdTe/GaAs interface; these do not propagate through either the Cd_1-xZn_xTe layer or subsequently deposited liquid phase epitaxy (LPE) HgCdTe layer(s). Background Ga and As-impurity levels for Cd_1-xZn_xTe on GaAs/Si substrates were below the secondary ion mass spectroscopy detection limit. Preliminary results of HgCdTe liquid phase epitaxy using a Te-rich melt on Si-based substrates resulted in x-ray rocking curve linewidths as narrow as 72 arc-sec and etch-pit densities in the range 1 to 3 x 10⁶ cm².     

Characterization of Plasma Etching Process Damage in HgCdTe

Microstructural and electrical damage to n-type long-wavelength infrared Hg_1?x Cd _x Te (MCT) following CH₄-H₂-based inductively coupled plasma etching has been investigated. While the damage from such etching processes to MCT has previously been characterized for planar full-wafer etching, in this communication we present the results of an investigation of the damage incurred to etched sidewalls, whose faces constitute the majority of the etched surface in novel architectures. Auger electron spectroscopy was used to monitor the evolution of X _Cd beneath etched surfaces. So far, no X _Cd evolution has been detected underneath etched surfaces within a ΔX _Cd = 0.02 resolution. Conductivity and minority-carrier lifetime have been studied on patterned photoconductors, from which it is possible to extract a surface recombination velocity (SRV). These studies have shown surface conductivity variations and SRV shifts of several orders of magnitude, depending on the etching process used.     

Effect of surface orientation of CdxHg1−x Te crystals on the properties of surface anodic oxides

Electron spectroscopy for chemical analysis was used to study the effect of surface orientation of Cd_xHg_1?x Te crystals (x=0.19) on the composition of anodic oxide. It is found that anodic oxide compositions on the (111) and (100) crystal surfaces are identical and change gradually from the substrate to the outer surface, whereas the oxide on the (110) surface contains an interlayer of unoxidized Te. This result correlates with some properties known for Cd_xHg_1?x Te oxides.     

The surface morphology,structural properties and chemical composition of Cd1−xZnxTe polycrystalline thick films deposited by close spaced vacuum sublimation

Thick polycrystalline Cd_1−xZn_xTe films with x ranged from 0.37 to 0.80 were obtained by the close spaced vacuum sublimation method. In order to investigate properties of the films structural, PIXE and Raman studies were carried out. Determination of chemical composition of the films by EDS, PIXE and XRD has shown good correlation of results. Raman spectroscopy reveals the relation between zinc concentration and vibrational properties of the films. Studies of the spatial distribution of the chemical elements on the film surface by micro-PIXE and micro-Raman spectroscopy have shown that films are uniform and free of secondary phases such as CdTe, ZnTe and Te inclusions.     

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.     

Positron-defect profiling in Cd<Subscript>1−x</Subscript>Zn<Subscript>x</Subscript>Te wafers after saw cutting

Near-surface damage induced by saw cutting of ingots of Cd_1−xZn_xTe was investigated by positron-defect depth profiling. The damage extends to several micrometers depth and depends on the cutting apparatus. The samples were polished and etched repeatedly, followed each time by positron-depth profiling. New subsurface damage created during the polishing process is observed. No new damage is observed after etching. Positron-depth profiling is suggested as a diagnostic tool to monitor the quality of sample surfaces.     

Characteristics of island growth of Hg1−xCdx Te on (100) CdTe substrates by chemical vapor transport

The characteristics of island growth of Hg_1−xCd_xTe (MCT) by chemical vapor transport on (100) CdTe substrates have been studied, for the first time, by optical microscopy, by infrared transmission spectroscopy, and by chemical etching. Nonuniformities of the growth thickness, of the growth rate, and of the composition of islands are observed. Based on the experimental results, the origin and the inhibition of island growth of MCT on (100) CdTe substrates are discussed.     

Defect reduction in Hg1−xCdxTe grown by molecular beam epitaxy on Cd0.96Zn0.04Te(211)B

A study on preparation of Cd_0.96Zn_0.04Te(211)B substrates for growth of Hg_1−xCd_xTe epitaxial layers by molecular beam epitaxy (MBE) was investigated. The objective was to investigate the impact of starting substrate surface quality on surface defects such as voids and hillocks commonly observed on MBE Hg_1−xCd_xTe layers. The results of this study indicate that, when the Cd_0.96Zn_0.04Te(211)B substrates are properly prepared, surface defects on the resulting MBE Hg_1−xCd_xTe films are reduced to minimum (size, ∼0.1 m and density ∼500/cm²) so that these MBE Hg_1−xCd_x Te films have surface quality as good as that of liquid phase epitaxial (LPE) Hg_1−xCd_xTe films currently in production in this laboratory.     

Determining and Controlling the Magnesium Composition in CdTe/CdMgTe Heterostructures

The relationships between Mg composition, band gap, and lattice characteristics are investigated for Cd_1?x Mg _x Te barrier layers using a combination of cathodoluminescence, energy dispersive x-ray spectroscopy, variable angle spectral ellipsometry, and atom probe tomography. The use of a simplified, yet accurate, variable angle spectral ellipsometry analysis is shown to be appropriate for fast determination of composition in thin Cd_1?x Mg _x Te layers. The validity of using high-resolution x-ray diffraction for CdTe/Cd_1?x Mg _x Te double heterostructures is discussed. The stability of CdTe/Cd_1?x Mg _x Te heterostructures are investigated with respect to thermal processing.     

The temperature-composition phase diagram and the miscibility gap of Hg1-xCdxTe solid solutions by dynamic mass-loss measurements

The dynamic mass-loss technique has been employed to measure Hg partial pressures over Te-saturated Hg_1-xCd_xTe solid solutions with x = 0.40, 0.54, and 0.70 in the 10^-1 to 10^-4 atm range. The relative chemical potentials of HgTe in Hg_1-xCd_xTe solid solutions have been calculated using the measured Hg partial pressures at temperatures below 413°C, and fitted into an analytical expression. A Gibbs-Duhem integration yielded the relative chemical potentials of CdTe. By combining the relative chemical potentials of the binary components HgTe and CdTe, an expression for the Gibbs free energy of mixing was derived. The binodal (miscibility gap) and spinodal curves of the Hg_1-xCd_xTe solid solutions have been established with the critical temperature and composition of 221°C and Hg_0.40Cd_0.60Te.     

Fine tuning of the emission of ultra-thin quantum wells of CdSe and CdTe by modification of the growth temperature

Ultra-thin quantum wells (UTQWs) of CdTe and CdSe should present a emission energy as a function of thickness, however, we have observed that depending on substrate temperature the peak energy is modified: the higher the growth temperature the larger the blue shift. Considering (i) a chemical interaction that produces the substitution of Cd atoms by Zn atoms at the QW-barrier interface, and, (ii) the large Bohr radius of the excitons in II-VI semiconductors, we argue that a few percent substitution of Cd atoms by Zn atoms is perceived as a change in average composition and not as a thickness fluctuation. Since the Cd substitution is a thermally activated process, a larger blue shift is expected at the higher temperatures. Therefore, the UTQWs can be described as made of Zn_1−xCd_xSe or Zn_1−xCd_xTe alloys with high Cd content, x∼1 at the lower substrate temperatures (T_s). Then, the proper selection of T_s can be advantageously employed for fine tuning of the excitonic emission in the energy region between that of UTQWs of the pure binary compound with thickness difference of 1 ML, making possible to cover continuously the visible spectral region with CdTe and CdSe UTQWs.     

In-situ control of temperature and alloy composition of Cd1−xZnxTe grown by molecular beam epitaxy

In this work in-situ spectroscopic ellipsometry (SE) has been applied for the simultaneous determination of the growth temperature and alloy composition for the epitaxial Cd_1−xZn_xTe(211)/Si(211) structure. The optical dielectric functions of CdTe and Cd_0.96Zn_0.04Te (CZT) epilayers were studied as a function of temperature both ex-situ and in-situ in the range from 1.6 eV to 4.5 eV. We employed parametric models for the simulation of the optical properties of CZT at and between the critical points (CP) E₀, E₀ + Δ₀, E₁, E₁ + Δ₁, E₂(Σ) and E₂(Σ). Critical point energies and line widths for Cd_1−xZn_xTe were obtained through the fitting process, which included both zero order and higher order derivatives of the SE pseudo dielectric function. The dependence of the different critical points on Zn concentration x is discussed. It has been demonstrated that the energy of the weak E₀ + Δ₀ transition can be used to measure composition, while the E₁ energy can be used as a real-time temperature measure. The model parameters were optimized through the simultaneous analysis of multiple data sets, and the temperature dependent model was developed for in-situ application. Our analysis is estimated to produce uncertainties of only ±0.5°C in measuring the temperature and ±0.5% in measuring the composition of only the zero order dielectric function is being fitted. The effects of a surface overlayer, of reflected beam deflections, and of other experimental problems on the overall accuracy, are discussed as well as ways to improve the in-situ SE data quality.     

Thermodynamic stability of bulk and epitaxial CdHgTe,ZnHgTe, and MnHgTe alloys

The thermodynamic stability of Cd_1?xHg_xTe, Mn_xHg_1?xTe, and Zn_xHg_1?xTe alloys is studied. Calculations performed in the context of the δ lattice-parameter model indicate that CdHgTe and ZnHgTe alloys are stable over the entire range of compositions at typical growth temperatures. At the same time, a miscibility gap is found in Mn_xHg_1?xTe at 0.33 < x < 1 at T = 950 K, which is consistent with the known experimental data. It is shown that the biaxial strains observed in Mn_xHg_1?xTe/CdTe and Mn_xHg_1?xTe/Cd_0.96Zn_0.04Te thin epitaxial films lead to a narrowing of the miscibility gap and to insignificant lowering of critical temperatures.