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.