Optical and Microstructural Characterization of the Effects of Rapid Thermal Annealing of CdTe Thin Films Grown on Si (100) Substrates

Journal of Electronic Materials, Jun 2005 by Neretina, S, Sochinskii, N V, Mascher, P

The effects of rapid thermal annealing (RTA) on CdTe/Si (100) heterostructures have been studied in order to improve the structural quality of CdTe epilayers. Samples of CdTe (111) polycrystalline thin films grown by vapor phase epitaxy (VPE) on Si (100) substrates have been investigated. The strained structures were rapidly thermally annealed at 400°C, 450°C, 500°C, 550°C, and 600°C for 10 sec. The microstructural properties of the CdTe films were characterized by carrying out scanning electron microscopy (SEM), x-ray diffraction (XRD), and atomic force microscopy (AFM). We have shown that the structural quality of the CdTe epilayers improves significantly with increasing annealing temperature. The optimum annealing temperature resulting in the highest film quality has been found to be 500°C. Additionally, we have shown that the surface nucleation characterized by the island size distribution can be correlated with the crystalline quality of the film.

Key words: Infrared (IR) detection, CdTe/Si heterostructures, rapid thermal annealing (RTA), structural characterization

INTRODUCTION

The epitaxial growth of CdTe continues to receive a great deal of attention in the areas of solar-energy conversion, gamma-ray detection, optoelectronics, and integrated optics. Because of chemical compatibility and the close lattice match with Hg^sub x^Cd^sub 1-x^Te, which is presently the most important material for infrared (IR) photodetectors, CdTe is also an ideal substrate for the epitaxial growth of high-quality Hg^sub x^Cd^sub 1-x^Te.1 In recent years, CdTe heteroepitaxial layers on Si substrates have been considered as promising alternative substrates for the subsequent growth of Hg^sub x^Cd^sub 1-x^Te over relatively expensive conventional bulk CdZnTe substrates. There are several reasons why there has been much interest in the use of silicon as a base substrate for the epitaxial growth of Hg^sub x^Cd^sub 1-x^Te/CdTe multilayer structures. The CdTe films have been grown on various substrate materials including GaAs,2 sapphire,3 InSb,4 and silicon, which are available in large areas of high quality. However, because of low cost and the commercial availability in the form of high-purity large diameter wafers, Si has been recognized as a promising substrate. Moreover, since signal processing is presently only available in silicon, Si-based technology allows the monolithic integration on a single chip of photonic and electronic components. If good-quality Hg^sub x^Cd^sub 1-x^Te/CdTe can be grown on Si, it could eventually become possible to integrate the Hg^sub x^Cd^sub 1-x^Te detectors directly to the silicon processor and produce large area monolithic infrared (IR) focal-plane arrays versus a currently used hybrid approach that transfers charge from the Hg^sub x^Cd^sub 1-x^Te detector to the silicon processor through indium bump bonds with all consequent limitations.

There are, however, several difficulties associated with the growth of CdTe on Si. The production of Hg^sub x^Cd^sub 1-x^Te detector arrays,5 for example, for thermal imaging application, requires producing large areas of high-quality CdTe films. High-quality CdTe epitaxial layers on Si substrates have been obtained by several growth methods such as molecular beam epitaxy,6 metal-organic chemical-vapor deposition,7,11 and hot wall epitaxy.8 The growth by vapor phase epitaxy (VPE) has also been considered because of the lower cost, high growth rate, and the capability of producing large area films. Since CdTe/Si structures have a large lattice mismatch (Δα/α [asymptotically =] 19% at 25°C) and thermal expansion coefficient difference (Δα/α = 46.9% at 25°C), the interface region contains dense arrays of dislocations resulting in deteriorating quality of the film. Many attempts have been made to investigate possible ways to improve the surface morphology and structural properties of strained CdTe films grown by different methods using postgrowth treatments. One of them is laser-assisted recrystallization by irradiation with an Ar ion laser beam.9 In this article, we present the experimental results on the remarkable improvement of the structural quality of the CdTe epilayers by postgrowth rapid thermal annealing.

EXPERIMENTAL DETAILS

The CdTe epilayers were grown by VPE on Si (100) substrates.10 The VPE growth was carried out at evaporation temperatures in the range of 700-800°C with no heating system for the substrates. Polycrystalline stoichiometric CdTe grown by the modified Bridgman method was used as a source material. The growth rate was in the range of 0.05-0.2 µm/min. With these conditions, the VPE growth provided planar 1-3-µm-thick CdTe epilayers formed uniformly over the entire substrate area.

The rapid thermal annealing system used in this work is a "heatpulse mini-pulse" built by A. G. Associates (Sunnyvale, CA). Mini-pulse is a rapid thermal processor that uses high intensity visible radiation to heat single wafers for short periods of time at precisely controlled temperatures. Tungsten halogen lamps and cold heating chamber walls allow fast wafer heating (heating rate of 50°C per second) and cooling rates. The rapid thermal annealing (RTA) process was performed in a nitrogen atmosphere. To protect CdTe/Si from any impurities within the mini-pulse system, samples were enclosed in a graphite boat. The RTA process was carried out in a temperature range between 400°C and 600°C at 50°C intervals for 10 sec.