Richard Sewell

Generation-recombination effects on dark currents in CdTe-passivated midwave infrared HgCdTe photodiodes

The effect of an abrupt CdTe∕HgCdTe passivation heterointerface on generation recombination and dark currents in n-on-p midwave infrared photodiodes with 5.2-μm cut-off wavelength has been investigated. Experimentally, it was observed that the zero-bias-dynamic resistance, R0, at low temperatures scales with the perimeter of the n-on-p junction, rather than with the junction area, suggesting that surface effects are dominant. The diode current–voltage characteristics at low temperatures indicate significant contributions from tunneling effects, which is the dominant leakage current mechanism for reverse bias greater than approximately 30mV. These two observations suggest that the region where the junction terminates at the CdTe∕HgCdTe abrupt interface is responsible for the above effects. A two-dimensional model has been developed to investigate the dark current mechanisms in the vicinity of the junction termination at CdTe∕HgCdTe interface, which also takes into account the effect of dislocations on gene...

Mercury cadmium telluride resonant-cavity enhanced photoconductive infrared detectors

Resonant-cavity-enhanced Hg1−xCdxTe photoconductive detectors for midwave infrared wavelengths are investigated for use in multi- and hyper-spectral sensor systems. Resonant-cavity-enhanced performance is modeled, and compared with measured performance of fabricated devices. The responsivity of fabricated devices shows resonant cavity enhancement, with performance limited by surface recombination.

Nanoindentation of HgCdTe prepared by molecular beam epitaxy

Nanoindentation has been used to investigate the elastoplastic behavior of Hg0.7Cd0.3Te prepared by molecular beam epitaxy. It was found that Hg0.7Cd0.3Te had a modulus of elasticity of ∼50GPa and hardness of ∼0.66GPa. The HgCdTe response to nanoindentation was found to be purely elastic for low loads and developed into ∼10% elastic and ∼90% plastic response for higher-load indentation exhibiting significant amounts of creep. The onset of plasticity has been observed to be marked by discontinuities or “pop-in” events in the indenter load-penetration curves at sheer stresses of ∼1.8GPa, and has been correlated with the homogeneous nucleation and propagation of dislocations.

Incorporation and activation of arsenic in MBE-grown HgCdTe

Research into p-type doping of HgCdTe with arsenic has concentrated on the use of a conventional effusion cell and optimization of growth conditions to achieve an increase in incorporation efficiency. This study investigates the use of a cracker cell, which is now the preferred method of doping HgCdTe due to its higher arsenic incorporation efficiency under optimum growth conditions. A detailed investigation of a number of arsenic doped HgCdTe layers grown on CdZnTe substrates by molecular beam epitaxy using a cracker cell as a source of arsenic is presented. Growth parameters influencing the amount of arsenic incorporated, such as the cracker-cell bulk temperature and substrate temperature, were investigated. Arsenic depth profiles were obtained via detailed secondary ion mass spectrometry where all major constituents in the epilayers were analysed. Magneto-transport Hall measurements were performed on as-grown material and those that underwent high-temperature anneals typical for arsenic activation. Using the quantitative mobility spectrum analysis technique, contributions to total conductivity arising from various carriers present in the samples have been separated. As-grown samples were found to exhibit n-type behaviour consistent with arsenic incorporating on cation sublattice, while samples that underwent high-temperature annealing show partial activation of arsenic with electron compensation.

Electrical type conversion of p-type HgCdTe induced by nanoimprinting

A nanoimprinting method was used to generate square imprints and arrays of imprints ranging in lateral dimension from 1 μm to 50 μm in p-type HgCdTe. Laser Beam Induced Current (LBIC) characterization shows electrical type conversion around each imprint and imprint array. The LBIC signal intensity surface maps of imprinted regions and their dependence with measurement temperature correspond well with surface maps of n-on-p HgCdTe photodiodes formed by conventional techniques.

Characterisation of arsenic doped HgCdTe grown by Molecular Beam Epitaxy

Extrinsic p-type doping of Mercury Cadmium Tel-luride (HgCdTe) epilayers grown by Molecular Beam Epitaxy (MBE) was carried out with an arsenic (As) cracker cell. As-grown samples were characterised via Fourier Transform Infrared Transmission Spectrometry (FTIR), Secondary Ion Mass Spectrometry (SIMS) as well as variable field magneto-transport measurements coupled with the quantitative mobility spectrum analysis (QMSA) to study the Hall effect characteristics. Arsenic activation annealing of the samples were performed and magneto-transport measurements repeated. Results indicate that as-grown samples show n-type behaviour indicating that arsenic incorporate as donors in the material with annealed samples showing p-type characteristics with heavy compensation.

Accurate determination of composition profiles in abrupt MBE-grown HgCdTe heterostructures

Heterostructured Hg(1-x)Cd(x)Te photodetectors are important for the next generation of high performance Infra-Red (IR) sensing applications. The measurement of the composition and thickness of each layer in double layer HgCdTe heterostructures is examined in this paper, in particular, the use of infrared transmission and Secondary Ion Mass Spectroscopy techniques. Several authors have published models of the optical absorption coefficient and refractive index in HgCdTe, and these models have been assessed on their suitability for use in modelling the infrared transmission characteristics of multilayer HgCdTe films. No data is available for the refractive index of HgCdTe for photon energies around the bandgap energy, so a modified Sellmeier equation has been used to approximate the refractive index in this range. A versatile mathematical model of the infrared transmission of multilayer HgCdTe films is presented, based on the characteristic matrix of each layer. The model is then fit to experimental data, where the composition and thickness of each of the HgCdTe layers are fitting parameters. While some film parameters may be determined with high accuracy from infrared transmission, Secondary Ion Mass Spectroscopy (SIMS) is useful as a complimentary technique which enables the measurement of the composition of the wider bandgap HgCdTe layer in double layer HgCdTe films, as well as measurement of the interface abruptness and layer uniformity. A method of calibrating SIMS data is presented, which gives results consistent with those obtained from infrared transmission fitting. Room temperature infrared transmission spectra and SIMS depth profiles of HgCdTe layers grown by Molecular Beam Epitaxy at the University of Western Australia are presented, and are compared with theoretical composition vs. depth profiles which have been determined using elements of each measurement technique.

Photovoltaic detectors fabricated by direct imprinting of mercury cadmium telluride

This is the first report of photovoltaic detectors fabricated by direct imprinting of a semiconductor. Evidence is reported that is consistent with the indented region of p-type HgCdTe type converted to n-type HgCdTe.

Strain and orientation effects in mercury cadmium telluride grown by molecular beam epitaxy

Quantitative mobility spectrum analysis of variable magnetic field Hall measurements is a proven technique for determining the mobility, type and concentration of multiple charge carriers in semiconductor materials. When applied to mercury cadmium telluride, the technique often generates spurious results, the origin of which is under investigation in this work. An underlying assumption of Hall measurements in the Van-der-Pauw configuration is that conduction in the sample is laterally isotropic. To test this assumption, several six contact Hall bars have been fabricated on a sample of HgCdTe grown by molecular beam epitaxy. The Hall bars are aligned with significant crystallographic directions on the (211) oriented surface. X-ray reciprocal space maps of the same sample have been measured to quantify the strain state and orientation of the epilayers. Nomarski contrast microscopy of the sample surface reveals surface undulations on the sample, aligned with the intersection of {111} slip planes and the (211) surface. Results of mobility spectrum analysis show no significant differences with respect to the orientation of Hall bars in the heavily doped sample under investigation.

Nanoimprint induced electrical type conversion in HgCdTe

We report supporting evidence that is consistent with p- to n-type conversion of HgCdTe that is induced by surface indentation, as the progress towards the fabrication of photovoltaic infrared detectors by direct imprinting.