Effects of annealing on the electrical properties of CdxHg1−xTe

Abstract

CdxHg1−xTe with 0.17<x<0.31 has been annealed using closed-tube and open-tube methods. In both methods the mercury vapor pressure during an anneal was controlled by a mercury reservoir held at a temperature either the same as the CdxHg1−xTe (isothermal anneal), or lower than the CdxHg1−xTe (two-temperature anneal). Isothermal anneals carried out in closed-tube and open-tube systems convert material which is initially p type by native defects to n type. Two-temperature, closed-tube anneals can be used to convert n type to p type, the acceptor concentration being controlled by the CdxHg1−xTe temperature and the mercury vapor pressure. Two-temperature, open-tube anneals also result in conversion from n to p, however, the mercury vapor pressure (over the range studied) does not influence the final acceptor concentration but does affect the time required to reach equilibrium. The results are discussed in terms of the pressure-temperature diagram and the defect/impurity balance in CdxHg1−xTe .

Bulk Growth of Cadmium Mercury Telluride (CMT)

“…Low n-type levels ( 1 × 10 15 cm −3 ) were achieved in the as-grown state for x = 0.2-0.3, an obvious advantage in terms of reduced handling for Bridgman growth as no annealing stage was required-unlike other bulk methods. Bridgman material was used (Jones et al [29]) to establish the pressure-temperature diagram, by a combination of isothermal and twotemperature annealing. On the Hg-rich side at temperatures below 320 • C the n-type carrier concentration is controlled by residual impurities.…”

“…Material with x > 0.3 is n-type as-grown, at a level above that found for Bridgman crystals but, at x < 0.3, the material is p-type. Such material converts to low (< 1 × 10 15 cm −3 ) n-type on the standard anneal treatment (Jones et al, [29]) indicating that metal vacancies are the cause of the p-type behaviour. Groups I and III elements are acceptors and donors, respectively, on the metal sites as they are in Bridgman, with the exception of Au.…”

Isothermal vapour-phase epitaxy of mercury-cadmium telluride (Hg,Cd)Te

“…It is known [105,[115][116][117][118] that Hgt-~Cd~Te is a defect material with electrically active native defects. If the crystal is grown in Te-saturated conditions, the most important defects are doubly charged Hg vacancies.…”

“…The process of low-temperature annealing can be performed isothermally (isothermal annealing), or with the source and the substrate at different temperatures (two-temperature annealing) [115]. In this case, the p ~ n conversion is performed more slowly if the mercury-zone temperature is lower than the substrate temperature.…”

“…The acceptor concentration, N,, depends on the sample temperature, Tt and on the mercury reservoir temperature, T2, if a closed tube is used for the annealing. For a given T2, N, increases [115].…”