R.J. Westerhout

Dark current modelling of midwave infrared HgCdTe gated photodiodes

HgCdTe midwave-infrared gated photodiodes were studied both experimentally and theoretically to determine the dark current mechanisms present under different surface conditions. By modifying HgCdTe photodiode equations to account for surface band-bending we are able obtain a first-order approximation to the behaviour of these gated photodiodes. A good fit to the data has been obtained for band-to-band tunnelling and surface generation-recombination current, but not for trap-assisted-tunnelling (TAT) current. This is attributed to the exclusion of interface states in the model, the complicated TAT behaviour, as well as two-dimensional effects.

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.

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.

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.

HgCdTe MWIR PECVD SiN passivated photodiodes

To determine a suitable passivation and insulation layer for HgCdTe photodiodes, the properties of various low temperature plasma-enhanced chemical vapour deposition (PECVD) SiN films were investigated. Metal-insulator-metal (MIM) capacitors were fabricated on silicon to determine a suitable PECVD SiN film for insulation of HgCdTe diodes. It was found that PECVD process temperature of 125degC, at high power, and low process pressure gave the best film characteristics. These films were stable in atmosphere, as shown by FTIR measurements, which also shows oxidation of low power, high process pressure films. HgCdTe photodiodes were fabricated with PECVD SiN films with 200 W RF power, 300 mTorr pressure, and a SiN4:NH3:N2 5:45:100 gas ratio. Gated diode measurements indicated that R0A values of 3.0times106 Omega cm2 are achievable with fine-tuning of the SiN charge.

Sidewall effects of MBE grown CdTe for MWIR HgCdTe photoconductors

The semiconductor-passivating layer interfaces, as well as their dielectric properties, play important and very often dominant roles in determining HgCdTe device performance. In this work, photoconductors were fabricated on p-type mid-wave infrared Hg1-xCdxTe material using molecular beam epitaxy (MBE) grown CdTe film as the passivation layer. In order to emphasize the importance of the CdTe passivating layer, HgCdTe photoconductors with and without the sidewall CdTe passivation were fabricated. The photoconductor with its sidewalls passivated shows significantly higher responsivity than the one without sidewall passivation. Conductance-voltage characteristics of capacitor using CdTe between HgCdTe and metal contacts were made in studying insulating property of the passivation film. Scanning electron microscopy (SEM) micrographs and X-ray diffraction (XRD) spectra were included in order to investigate the structural properties of the CdTe passivation film.

Laser beam induced current for qualitative evaluation of HgCdTe van der Pauw sample uniformity

HgCdTe is the preferred semiconductor for fabrication of high-performance infrared (IR) detectors. This material also typically contains multiple carrier species for charge transport, which makes characterisation of the mobility and concentration of each species particularly difficult. Accurate carrier transport characterisation can be achieved by quantitative mobility spectrum analysis of variable-magnetic-field Hall and resistivity data, but this imposes certain conditions concerning sample uniformity and homogeneity. Laser beam induced current (LBIC) is a non-destructive characterisation technique that can be used to qualitatively investigate the electronic properties of HgCdTe samples, specifically including electrically-active defects and p-n junctions. Non-uniformity in the LBIC profiles can also be related to ill-conditioned Hall and resistivity data.

1/f noise in HgCdTe infrared gated photodiodes

In this work gated midwave infrared (MWIR) Hg1-xCdxTe photodiodes are used to investigate the physical origin of 1/f noise generation. Gated photodiodes were fabricated on liquid phase epitaxy p-type HgCdTe MWIR material with a vacancy doped concentration of 1.6 x 1016cm-3 and x = 0.31. CdTe was thermally deposited and used as both a passivant for the HgCdTe and a mask for the plasma-based type conversion, and ZnS was used as an insulator. Fabricated devices show a R0A of 1-5x104&OHgr;cm2 at 77K with zero gate bias. Application of 2V to the gate improves the R0A by more than two orders of magnitude to 6.0 x 106&OHgr;cm2, which corresponds to the p-type surface being at transition between depletion and weak inversion. Trap-assisted tunelling (TAT) current was observed at negative gate biases and reverse junction biases. For gate biases greater than 3V a field-induced junction breakdown was observed. Gated photodiodes show diffusion limited behaviour at zero bias above 200K, and TAT, band-to-band tunnelling, and generation-recombination (GR) limited behaviour below, for gate biases from -8V to 8V. Field-induced junction breakdown current was also observed to be temperature independent. Noise current, In = &agr;I&bgr;f-0.5 trend was observed above 200pA reverse bias dark current, with &agr; = 3.5 x 10-5 and &bgr; = 0.82, which corresponds to the TAT dominated region. Below 200pA, junction GR current starts to dominate and this previously mentioned trend for In is no longer observed. Junction GR current was not seen to be correlated with 1/f noise in these photodiodes.