Evidence of anomalous behavior in low-n-type mercury cadmium telluride induced by extended defects

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Abstract
Suitable improvements of our liquid phase epitaxy (LPE) growth process enable us to obtain excellent quality p-type as-grown mercury cadmium telluride (MCT) epilayers, well adapted to high-performance photovoltaic (PV) detector technology. Although our technology only needs undoped p-type MCT as starting material, we also convert a lot of these epilayers to n type to check their electrical properties in view of two fundamental problems: anomalies of electrical behavior and residual doping level. After n-type annealing, most of our LPE layers have a classical behavior versus temperature even under a high magnetic field value of 10 000 G: at 77 K, values of n77d =2×1014 cm−3 and μ77 =105 cm2 V−1 s−1 and at 27 K n27d =2×1014 cm−3 and μ27 =2 to 3×105 cm2 V−1 s−1 are routinely obtained, for a cutoff wavelength of 6.3 μm (as given by IR transmission at room temperature); this corresponds to a cutoff wavelength of 10.6 μm at 77 K, as measured on our PV diodes. Transmission electron microscopy (TEM) observations show that these layers are defect-free and this is confirmed by x-ray topography which shows only the misfit dislocation pattern. However, some samples have an anomalous behavior featured by a ‘‘kink’’ on the mobility curve between 100 and 77 K, the nd value being in the low-1014 range. TEM observations show in this case the presence of dislocations, the density of which is directly correlated to a more or less significant kink on the mobility curve; also, the aspect of the x-ray topography changes from a sharp misfit structure (low density of loops) to a mixed structure (higher density of loops). If this density of loops is still increased, the x-ray topography becomes hazy and the Hall-effect measurement indicates the epilayer to be p type even after an n-type annealing. These loops are assumed to generate doping microinhomogeneities responsible for anomalous Hall-effect behavior. So, extended defects have to be avoided to achieve high-quality MCT material; a way to reach this goal is to grow buffered structures, the electrical properties of which are presented together with TEM observations.

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