Liquid phase epitaxial growth of CdxHg1-xTe for infrared detection

The current status of liquid phase epitaxy, LPE, of the infrared (IR) detecting material cadmium mercury telluride, CdxHg1-xTe (CMT) is described. The basic process is centred on a tellurium-rich melt and the use of high quality CdZnTe substrates, both 20 x 30 and 30 x 40 mm in size. Growth takes place in a high purity graphite boat in flowing Pd-diffused hydrogen. A separate HgTe source provides the necessary Hg overpressure control. Growth occurs by ramp cooling from similar to 500 degrees C to produce layers with thicknesses between 25 and 35 mm. An in-situ anneal at similar to 300 degrees C following growth is used to set the acceptor concentration to the level required for photodiode fabrication. Assessment of the layers includes Fourier transform infrared (FTIR) spectrometry to determine cut-on wavelength (related to x) and layer thickness. FTIR spectrometry is also used to map the lateral variations in wavelength and thickness. There is a grade in x through the thickness of the layer and this must be determined and allowed for during device processing. Defect densities should also be kept to a minimum to reduce the numbers of poor or dead diodes on large area focal plane arrays. A further measure of the quality of the LPE layers is the background donor level. This is determined by Hall effect measurements after a low temperature Hg anneal. Levels of < 1 x 10(15) cm(-3) are normally required for photodiode arrays, while lower levels (< 4 x 10(14) cm(-3)) are necessary to make the highest performance photoconductive devices. Various chemical analysis techniques have been developed to assess the purity of the material grown and an example of an impurity survey by mass spectrometry will be presented.