Pok-Kai Liao

Thermomigration of Te precipitates and improvement of (Cd,Zn)Te substrate characteristics for the fabrication of LWIR (Hg,Cd)Te photodiodes

Abstract(Cd,Zn)Te wafers containing Te precipitates have been annealed under well defined thermodynamic conditions at temperatures below and above the melting of Te. Results of the examination of the wafers with infrared microscopy before and after the anneals indicate a substantial reduction of the Te precipitates in wafers annealed at temperatures in excess of the melting point of Te compared with those annealed at temperatures below the melting point of Te. These results confirm the thermomigration of liquid Te precipitates to be the principally operative mechanism during annealing in the elimination of these precipitates in (Cd,Zn)Te wafers. The occurrence of Te precipitates in (Hg,Cd)Te epitaxial layers grown on (Cd,Zn)Te substrates containing Te precipitates is also explained on the basis of thermomigration of these precipitates during LPE growth from the substrates to the epilayers. Absence of occurrence of Te precipitates in (Hg,Cd)Te epilayers grown on annealed (Cd,Zn)Te substrates with negligible Te precipitates is also confirmed. Usefulness of annealing (Cd,Zn)Te substrates—to eliminate Te precipitates—prior to epilayer growth is confirmed via demonstration of improved long wavelength infrared (Hg,Cd)Te device array performance uniformity in epitaxial layers grown on (Cd,Zn)Te substrates with negligible Te precipitates after annealing.

Finite element thermal analysis on the crystal growth of HgCdTe by the travelling heater method

Abstract A finite element quasi-steady-state thermal model is described to simulate the growth of (Hg,Cd)Te by the traveling heater method. The ampoule region is incorporated in the thermal model and forms the basis for comparing the simulation results with experimental measurements. Sensitivity studies were carried out to explore the change of interface shape due to variations in thermophysical parameters, physical dimensions of the system, thermal boundary profile, and crystal growth conditions. It is found that the interface shape is always concave at the solvent/crystal/quartz-wall three-phase contact point regardless whether the macroscopic interface shape is concave or convex. This local concavity is due to the higher quartz-wall thermal conductivity with respect to that of the growing crystal. The modeling interface shapes are in good agreement with experimental observations and with the aid of the model simulation to search for the appropriate thermal and thermodynamic conditions, slightly macroscopic convex interface shapes can be achieved and (Hg,Cd)Te single crystals were grown.

Copper outdiffusion from CdZnTe substrates and its effect on the properties of metalorganic chemical vapor deposition-grown HgCdTe

We report that HgCdTe (MCT) epilayers grown by metalorganic chemical vapor deposition can be doped by copper outdiffusing from CdZnTe substrates. The copper content in the substrates was determined by the choice of the purity of the starting raw materials. Copper diffusion could be controlled by adjusting the tellurium precipitate density in the substrates. Growing on substrates with a high concentration of tellurium precipitates resulted in low doped MCT epilayers whereas a high copper concentration was found in MCT grown on substrates with a lower concentration of tellurium precipitates. A mechanism whereby tellurium precipitates getter copper during the post-growth cooldown of CdZnTe boules and trap copper in the substrates is proposed.

Advanced magneto-transport characterization of LPE-grown Hg 1−x Cd x Te by quantitative mobility spectrum analysis

We report the development, optimization, and testing of an advanced quantitative mobility spectrum analysis (QMSA) technique for determining free electron and hole densities and mobilities from field-dependent Hall and resistivity data. Application to temperature-dependent data for a series of 25 LPE-grown n-type and p-type Hg1−xCdxTe samples confirms that the fully automated procedure yields accurate and reliable results for all classes of samples, and also has greater sensitivity to minority carrier concentrations than previous mixed-conduction analysis methods. The QMSA is found to be a suitable standard tool for the routine electrical characterization of semiconductor materials and devices.

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

A study on preparation of Cd0.96Zn0.04Te(211)B substrates for growth of Hg1−xCdxTe 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 Hg1−xCdxTe layers. The results of this study indicate that, when the Cd0.96Zn0.04Te(211)B substrates are properly prepared, surface defects on the resulting MBE Hg1−xCdxTe films are reduced to minimum (size, ∼0.1 m and density ∼500/cm2) so that these MBE Hg1−xCdx Te films have surface quality as good as that of liquid phase epitaxial (LPE) Hg1−xCdxTe films currently in production in this laboratory.

Optical absorption coefficient of CdZnTe

The optical absorption coefficient of CdZnTe in the near and mid infrared spectral regions was measured at room temperature using FTIR transmission spectroscopy for several x-values in Cd1-xZnyTe grown by the horizontal Bridgman technique as well as for CdTe and ZnTe. The compositional dependence of the absorption coefficient near the band edge was used to determine the composition of bulk CdZnTe, an important material parameter in its application as a substrate for HgCdTe epitaxial growth. In the mid IR range, we find that the wavelength dependence of the absorption coefficient could be varied by adjusting the stoichiometry of the material, i.e., by annealing under various Cd overpressures. The shape of the mid IR Fourier transform infrared spectra is related to the type and the concentration of the free carriers as well as the resistivity of the material. For n-type material, the wavelength dependence of the absorption coefficient can be described by free carrier absorption.

HDVIP for low-background-flux and high-operating-temperature applications

An overview of the DRS HDVIP architecture for realization of large-area infrared focal plane arrays (IRFPAs) is given. Improvements needed to meet more stringent application requirements are discussed and modeled. Both theoretical and experimental data are presented.

Growth of (Hg,Cd)Te from Te-rich solutions

Mercury cadmium telluride has successfully been grown from tellurium rich solutions. Solution growth offers a number of advantages over other thin film and bulk growth techniques, e.g. purification, thin film and bulk growth, low dislocation density, and inexpensive growth equipment. The principal solution growth techniques that have received the most attention for (Hg,Cd)Te so far have been liquid phase epitaxy(LPE), the traveling heater method(THM) and the solid state recrystallization(SSR) process. These techniques have provided very high quality material for various device structures. The solid state recrystallization process, the traveling heater method and liquid phase epitaxy are reviewed and their similarities described.