A review of impurity behavior in bulk and epitaxial Hg1−xCdxTe

Abstract

Extrinsic doping using elements which produce stability of electrical properties will become increasingly important in future infrared device structures based on Hg1−xCdxTe (MCT). This paper reviews the incorporation and activation of dopants in the most widely used bulk and epitaxial growth techniques. Stoichiometry at the growth temperature is demonstrated to be the critical factor which affects dopant activation. A number of factors, including stoichiometry, can affect the as-grown electrical properties of MCT and the importance of determining the type of conduction in the as-grown state, if successful extrinsic doping is to be accomplished, is stressed. The minimum criterion for confirmation of dopant activity is established as agreement between electrical and chemical data on the same low temperature Hg-annealed sample. At low concentrations of dopants, an additional requirement is to confirm the absence of other potential impurity dopants at equivalent levels. Most elements are active dopants in accordance with their relative position in the periodic table but several important exceptions exist, notably group V elements in Te-rich material. Slow-diffusing dopants are preferred and techniques are described which produce stable doped/undoped heterostructures, using As as the acceptor element in metalorganic vapour phase epitaxy growth. Data on dopant segregation behavior, in growth from liquids, acceptor ionization energies and minority carrier lifetimes are presented and their importance is discussed. Ionization energies can be used to differentiate doped from undoped material, providing the degree of compensation is known. Doping using extrinsic acceptors has been shown to improve minority carrier lifetimes in material grown by certain techniques but is unsuccessful in other types of material. Some recent data on annealing undoped Te-rich liquid phase epitaxial material will be presented which suggests that higher minority carrier lifetimes can be achieved purely by defect control.

Ion milling-assisted study of defect structure of HgCdTe films grown by liquid phase epitaxy

“…Interestingly, in samples Lg1 and Lg3, this concentration was of the order of~10 14 cm -3 , and continued to decrease with further ageing. This speaks of extreme purity of these samples, which is below the typical level of residual donor doping in LPE HgCdTe [9]. The obtained values can be compared only to these in intentionally doped bulk HgCdTe crystals [10], where low residual concentration was achieved by the introduction of Se.…”

Nonequilibrium Operation of Arsenic Diffused Long-Wavelength Infrared HgCdTe Photodiodes

“…Such a low level of p-type doping has never been demonstrated in a controlled way by MBE for HgCdTe. [6][7][8] The goal of obtaining a reduced p-type carrier concentration will be to reduce the Auger generation-recombination carrier dynamics and to provide the enhanced carrier diffusion required for high quantum efficiency and low dark currents, resulting in enhanced D*. In this work, arsenic was used as a source of extrinsic p-type carriers with the proper arsenic doping level obtained by a novel diffusion technique based on diffusion by thermal annealing to obtain P + /p/N planar devices.…”

“…This fact allows for stable and well-controlled p-n junctions with sharp transitions. 8 The necessary control of $10 15 cm -3 p-type doping for the P + /p/N + structure has never been achieved in more than 20 years of research on HgCdTe development by MBE. Present technology limits p-type doping with As values down to $10 16 cm -3 and even this amount of doping is usually not obtained reproducibly.…”

Background donor concentration in HgCdTe

“…In p−type sam− ples ##13, 15 and 17 this concentration was of the order of 10 14 cm -3 , and continued to decrease with further ageing. This spoke of extreme purity of these samples, which was below the typical level of BDC in LPE−grown MCT [5]. The obtained values could be compared only to these in bulk MCT where low residual concentration was achieved by the introduction of Se, as described in the previous section.…”

“…As is known, most typical donor dopants in MCT are In Hg , Ga Hg , Al Hg , I Te , As Hg , P Hg [5]. Among other donors listed in the litera− ture are (F, Cl, Br) Te , also O, Ge, Sn and Si.…”