Growth, properties and applications of HgCdTe

Abstract

This paper provides primarily a review of the methods used to grow HgCdTe with a summary of some of its basic properties and applications. Methods of crystal growth fall generally into three classes: growth from the melt, from solution and from the vapor phase. All three methods have been and are being used to grow HgCdTe. The high vapor pressure of HgCdTe at the melting point, combined with a large segregation coefficient, have effectively limited the use of Czochralski or zone melting techniques, but two melt growth techniques have survived: (1) a variation of Bridgman growth called quench-anneal wherein a dendritic crystal is formed by quenching the melt and is homogenized by solid state recrystallization below the melting point, (2) a variation of freezing from a large volume called slush-growth wherein a melt is held in a temperature gradient for several weeks while a crystal grows. Growth from solution has taken the form of liquid phase epitaxy (LPE) on CdTe with the LPE systems including growth from Hg-rich, HgTe-rich and Te-rich solutions and using tipping, vertical dipping, vertical sliding and horizontal sliding. Vapor phase growth is very promising but is not yet in production. Techniques include growth by isothermal close spaced epitaxy in which HgTe is transported isothermally by chemical potential onto CdTe, molecular beam epitaxy (MBE) in which elements are evaporated in a high vacuum, and metal organic chemical vapor deposition (MOCVD) in which some of the metal atoms are carried to the substrate bound to organic radicals before being freed by pyrolysis. In all these methods, control of Hg pressure is a major concern. The fundamental properties discussed briefly are those of prime interest to detector manufacturers: energy gap (Eg), intrinsic carrier concentration (ni), and electrical activity of dopants. A reasonable fit to the Eg data from ȣ 20 papers is given by Eg = -0.302+1.93x+5.35×10-4T(1ȡ2x)-0.810x2+0.832x3. This gap, combined with k·p calculations, gives the intrinsic carrier concentration as a function of composition and temperature: ni = (5.585-3.820x+1.753×10-3T-1.364×10-3xT)×1014E3/4gT3/2 exp(-Eg/2kT). HgCdTe is typically used in the manufacture of infrared (IR) detectors, with both commercial applications such as medical thermography and building heat loss analysis and military applications such as surveillance of activities on the surface of the earth and terminal guidance of missiles. Detectors operating in the atmospheric windows are able to see both in the dark and through clouds. HgCdTe is the material of choice for the 3ȁ5 μm and the 8ȁ12 μm wavelength IR detectors. HgCdTe is not strong or easy to work with, but the technology is maturing and sophisticated devices are now being built.