G. Badano

Mercury cadmium telluride/tellurium intergrowths in HgCdTe epilayers grown by molecular-beam epitaxy

Surface crater defects in HgCdTe epilayers grown by molecular-beam epitaxy have been investigated using cross-sectional scanning and transmission electron microscopy, as well as atomic force microscopy. These defects originated primarily within the HgCdTe films, and were shown to be associated with the local development of polycrystalline morphology. High-resolution observations established the occurrence of finely spaced HgCdTe/Te intergrowths with either semicoherent or incoherent grain boundaries, as well as small HgCdTe inclusions embedded within Te grains.

Formation mechanism of crater defects on HgCdTe/CdZnTe (211) B epilayers grown by molecular beam epitaxy

Crater defects on the surfaces of HgCdTe epilayers grown by molecular beam epitaxy have been investigated. A semiempirical model coupled with observations by transmission electron microscopy was used to analyze the defect formation mechanism. We find that Te2 dissociation plays an important role. The defect density can be controlled by adjusting growth conditions such as the substrate growth temperature, Hg flux, growth rate and composition. Tight control over the pretreatment procedures before molecular beam epitaxy growth is also necessary.

HgCdTe/CdTe/Si infrared photodetectors grown by MBE for near-room temperature operation

Conventional HgCdTe infrared detectors need significant cooling in order to reduce noise and leakage currents resulting from thermal generation and recombination processes. Although the need for cooling has long been thought to be fundamental and inevitable, it has been recently suggested that Auger recombination and generation rates can be reduced by using the phenomena of exclusion and extraction to produce nonequilibrium carrier distributions. The devices with Auger suppressed operation requires precise control over the composition, and donor and acceptor doping. The successful development of the molecular beam epitaxy (MBE) growth technique for multi-layer HgCdTe makes it possible to grow these device structures. Theoretical calculations suggest that the p n+ layer sequence is preferable for near-room temperature operation due to longer minority carrier lifetime in lightly doped p-HgCdTe absorber layers. However, because the low doping required for absorption and nonequilibrium operation is easier to achieve in n-type materials, and because Shockley-Read centers should be minimized in order to obtain the benefits of Auger suppression, we have focused on p+ n structures. Planar photodiodes were formed on CdTe/Si (211) composite substrates by As implantation followed by a three step annealing sequence. Three inch diameter Si substrates were employed since they are of high quality, low cost, and available in large areas. Due to this development, large area focal plane arrays (FPAs) operated at room temperature are possible in the near future. The structures were characterized by FTIR, x-ray diffraction, temperature dependent Hall measurements, minority carrier lifetimes by photoconductive decay, and in-situ ellipsometry. To study the relative influence of bulk and surface effects, devices with active areas from 1.6 10−5 cm2 to 10−3 cm2 were fabricated. The smaller area devices show better performance in terms of reverse bias characteristics indicating that the bulk quality could be further improved. At 80 K, the zero bias leakage current for a 40 m 40 m diode with 3.2 m cutoff wavelength is 1 pA, the R0A product is 1.1 104-cm2 and the breakdown voltage is in excess of 500 mV. The device shows a responsivity of 1.3 107 V/W and a 80 K detectivity of 1.9 1011 cm-Hz1/2/W. At 200 K, the zero bias leakage current is 5 nA and the R0A product 2.03-cm2, while the breakdown voltage decreases to 40 mV.

Electrical activation and electrical properties of arsenic-doped Hg 1- x Cd x Te epilayers grown by MBE

The annealing and electrical properties of extrinsic in situ doped mercury cadmium telluride epilayers grown by molecular beam epitaxy (MBE) on B CdTe/Si and CdZnTe substrates are studied. The doping is performed with an elemental arsenic source. HgCdTe epilayers of CdTe mole fraction in the range of mid-wavelength IR are grown at substrate temperatures of 175-185 degrees C. The temperature dependent Hall effect characteristics of the grown samples are measured by the van der Pauw technique. A magnetic field of up to 0.8 T is used in these measurements. The analysis of the Hall coefficient in the temperature range of 40-300 K with a fitting based on a three-band non-parabolic Kane model, a fully ionized compensating donor concentration, and tow independent discrete acceptor levels is reported. Both as-grown and annealed samples are used in this study. All of the as-grown samples showed-type characteristics whereas annealed samples showed p-type characteristics. Activation annealing at different temperatures was performed. Conversion to p-type at lower than conventional annealing temperatures was achieved. Theoretical models are utilized to understand the dependence of the activated arsenic concentration on the annealing temperature.

Temperature-dependent adsorption of Hg on CdTe(211)B studied by spectroscopic ellipsometry

The adsorption of Hg on CdTe(211)B is studied by reflective high-energy electron diffraction and by spectroscopic ellipsometry in the range of 1.8–4.1 eV. We use a Hg molecular beam to create a high equilibrium coverage of Hg on CdTe. We find that at least two types of Hg are present at the surface: A physisorbed form, which displays a Drude-type dielectric function, and a chemisorbed form, which gives rise to a dielectric function similar to that of bulk Hg1−xCdxTe, but with sharper structure characteristic of lower dimensionalities. The dependence of the relative amounts of these two forms of Hg on the temperature and the impinging flux is obtained from the data analysis.

Ellipsometric study of the nucleation of (2 1 1) HgCdTe on CdZnTe(2 1 1)B

We have grown numerous samples under Hg-deficient conditions, in order to study the formation of void defects. The surface morphology of all samples grown was characterized using optical and SEM microscopy. At the same time, the substrates were studied prior to nucleation using in situ spectroscopic ellipsometry (SE). We find a correlation between the ellipsometric signal prior to nucleation and the final morphology of the layers grown. We studied the Hg layer present at the surface before nucleation by RHEED, XPS and SE. A better model is proposed, which reflects the physical characteristics of the substrate surface.

MBE growth of HgCdTe HOT detector heterostructures

Specially designed mercury cadmium telluride (Hg1-xCdxTe) p-ν-n+ heterostructures were grown by molecular beam epitaxy (MBE) on CdTe/Si and CdZnTe (211)B-oriented substrates for infrared photo-detector operation at near room temperature. Growth of this structure requires precise control over the crystal quality, compositional profiles, and donor and acceptor doping levels. The doping levels and density of Shockley-Read-Hall centers in the absorber layer must be low enough to realize the benefits of Auger suppression under non-equilibrium device operation. In order to avoid possible contamination from chemical compounds used in traditional substrate mounting methods, non-contact (In-free) substrate mounting was used to grow the structures. High-energy electron diffraction (RHEED) was implemented to develop a substrate thermocouple temperature ramping curve that maintains a constant epilayer temperature. The structures were characterized by FTIR, x-ray diffraction, and temperature dependent Hall measurements. High operating temperature (HOT) detectors were fabricated on these materials and showed good room-temperature response.

Improved model of HgCdTe's pseudo dielectric function for in-situ ellipsometry data analysis during MBE growth

A Woollam M88 spectroscopic ellipsometer was used to characterize the molecular beam epitaxy growth nucleation of Hg1-xCdxTe layers on CdZn0.035Te substrates and the substrate temperature prior to the growth. We developed a new approach to ellipsometry data analysis to better determine the substrate temperature. It is based on the accurate determination of the critical point energies and linewidths, which display strong temperature dependence in the CdZnTe system. The new model was able to resolve temperature differences of the order of +/-2.5oC. We also show that ellipsometry can be used to characterize the nucleation of Hg1-xCdxTe on CdZnTe substrates. More work is in progress to assess the run-to-run reproducibility of our temperature measurement, and to further investigate Hg1-xCdxTe nucleation.