Laser reflectance monitoring of the nucleation and growth of CdTe on basal plane sapphire substrates for focal plane arrays

The nucleation of CdTe onto basal plane sapphire and the subsequent growth of a CdTe buffer layer has been studied using in-situ laser reflectance (probe wavelength 633 nm, HeNe laser). The production of midwave infrared focal plane arrays requires the growth of typically 10 micrometer of CdTe (111)B buffer layer in order to grow out problems due to stacking faults, dislocation clusters and twinning. A-face and B-face growth of CdTe is seen to produce different reflectance 'signatures' within the first 6000 angstroms of growth, so enabling the early identification of problems with the growth process. Laser reflectance was also successfully demonstrated to predict the thickness of the buffer layer. Oscillations in the laser reflectance are attenuated due to absorption by the film at the probe wavelength used after approximately 6000 angstrom. However by the on-line calculation of the growth rate at every half wavelength oscillation, it is possible to extrapolate a film thickness for the total growth time. This extrapolated value is seen to be in good agreement with the thickness calculated ex-situ by beta-back scattering. The dependence on the buffer layer growth on the nucleation conditions was also investigated. The determination of whether the buffer layer grows A-face or B-face is seen to be more influenced by the II:VI ratio than the temperature during nucleation. For a nucleation temperature of 400 degrees Celsius, with a II:VI ratio of 6:1 the growth of the buffer layer is seen to be 100% A-face. As the II:VI ratio is increased the degree of A-face growth is seen to decline and the material becomes dominated by B-face growth. At a II:VI ratio of 60:1 the material is entirely B-face and predominantly untwinned. The difference in the two growth modes is manifested in the laser reflectance. Greater scattering of the laser light occurs during B-face growth due to the increased roughening compared to A-face growth. Consequently the reflectance signal in the B-face signature is seen to fall away more rapidly than is the case with A-face growth.