Excess Carrier Lifetimes in (HgCd)Te Grown by MOCVD Interdiffused Multilayer Process
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EXCESS CARRIER LIFETIMES IN (HgCd)Te GROWN BY MOCVD INTERDIFTUSED MULTILAYER PROCESS P. Mitra, T. R. Schimert, Y. L. Tyan, A. J. Brouns and F. C. Case Loral Vought Systems Corp., P. 0. Box 650003, Dallas, TX 75265 ABSTRACT Transient lifetime measurements on p- and n-type Hgl-xCdxTe epitaxial films, grown by the MOCVD-interdiffused multilayer process (IMP) on CdTe and CdZnTe, are reported. Lifetimes have been measured on undoped n-type, vacancy and arsenic doped p-type (HgCd)Te for x-values of 0.20-0.28 over the 25K-300K temperature range. Lifetime characterization has been carried out primarily by non-contact transient millimeter wave reflectance as well as by standard photoconductive decay. It is shown that Auger limited lifetimes are achievable in undoped n-type material. N-type lifetime results are analyzed within the framework of a multilevel Shockley-Read model which provide insight into the nature and density of defect states in the material. In p-type (HgCd)Te, for the same carrier concentration, longer lifetimes are obtained by As-doping than in vacancy doped material. INTRODUCTION The excess carrier lifetime of Hgl-xCdxTe (MCT) is a key material parameter that governs the performance of photoconductive and photovoltaic infrared detectors. Lifetimes are strongly dependent on carrier concentrations and the presence of material defects and its effects in these devices are manifested in detectivity, frequency response and dark current. Lifetime characterization and the recombination mechanisms in MCT has been reviewed recently [1]. In recent years MOCVD grown MCT has improved to a point where high quality photodiodes are beginning to be realized [2-4]. It is therefore necessary to scrutinize carrier lifetime and its relationship to defects [5] in such material in detail. In this paper transient lifetime results are reported on MOCVD grown n-type and p-type MCT on CdTe and lattice matched (CdZn)Te substrates with alloy composition in the range x-0.20-0.28. The p-type epilayers are either vacancy or arsenic doped. The n-type layers are intrinsic and obtained by a Hg-anneal from the as-grown undoped p-type material. The lifetime measurements were made primarily by the non-contact transient millimeter wave reflectance (TMR) technique (6,7] and in some cases by standard photoconductive decay (PCD) [1]. The n-type lifetime data is analyzed in terms of Auger, radiative and ShockleyRead (S-R) recombination processes. S-R recombination effects are calculated within the framework of a recently developed 3-level model [8]. The three levels include a divalent Hg-vacancy acceptor [5] and an independent deep level [9] which has been recently linked to the presence of Hg interstitials [10]. These defect states, which lie within the forbidden energy-gap, act as recombination centers affecting excess carrier lifetimes. Finally, lifetime results on As-doped p-type MCT are compared with vacancy doped material with the same hole concentrations. MATERIALS GROWTH AND MEASUREMENTS MOCVD growth of MCT was performed using the interdif
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