Charles Musca

Determination of mechanical properties of PECVD silicon nitride thin films for tunable MEMS Fabry–Pérot optical filters

This paper reports an investigation on techniques for determining elastic modulus and intrinsic stress gradient in plasma-enhanced chemical vapor deposition (PECVD) silicon nitride thin films. The elastic property of the silicon nitride thin films was determined using the nanoindentation method on silicon nitride/silicon bilayer systems. A simple empirical formula was developed to deconvolute the film elastic modulus. The intrinsic stress gradient in the films was determined by using micrometric cantilever beams, cross-membrane structures and mechanical simulation. The deflections of the silicon nitride thin film cantilever beams and cross-membranes caused by in-thickness stress gradients were measured using optical interference microscopy. Finite-element beam models were built to compute the deflection induced by the stress gradient. Matching the deflection computed under a given gradient with that measured experimentally on fabricated samples allows the stress gradient of the PECVD silicon nitride thin films introduced from the fabrication process to be evaluated.

Current status and issues in the surface passivation technology of mercury cadmium telluride infrared detectors

Surface passivation has been recognized as a crucial step in the fabrication of mercury cadmium telluride photoconductive as well as photovoltaic detectors. The subject has attracted considerable attention in the past and several reviews existed by 1991. The subject matter, however, received added impetus with the development of techniques like MOCVD and MBE and recently there has been considerable work on MCT passivation using in situ grown II-VI semiconductors. In this report, we have tried to give the present status and identify the issues particularly with reference to the recent work on the subject.

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...

Interpretation of current flow in photodiode structures using laser beam-induced current for characterization and diagnostics

This paper presents an interpretation of the physical mechanisms involved in the generation of laser beam-induced current (LBIC) in semiconductor p-n junction diodes. LBIC is a nondestructive semiconductor characterization technique that has been used in a qualitative manner for a number of years and is especially useful for examining individual photodiodes within large two-dimensional arrays of devices. The main thrust of this work is the analysis of LBIC in terms of nonzero steady-state circulatory current flow within the device and, hence, the interpretation of LBIC line profiles to diagnose the patterns of current flow within the structure. This provides an important basis for future studies seeking to relate LBIC to indicators of p-n junction performance and integrity such as dark current components and reverse bias saturation current. In particular, this paper examines the ideal cases of a single isolated p-n junction diode structure, and also considers an array of such devices in close proximity to each other. Modifications to the idealized theory that are required to account for localized junction leakage and surface recombination are presented, and the effect of Schottky contacts is discussed. Numerical simulations based on the HgCdTe family of semiconductors are presented to support the theory.

Stress in low-temperature plasma enhanced chemical vapour deposited silicon nitride thin films

Two experimental techniques have been investigated to examine residual stress in low-temperature plasma enhanced chemical vapour deposited (PECVD) SiNx thin films: one that measures the stress-induced substrate curvature, and the other that takes advantage of the stress-induced deformation of freestanding diagnostic microstructures. A general linear dependence of residual stress on SiNx deposition temperature is observed, with the magnitude of stress changing linearly from ~300 MPa tensile stress to ~600 MPa compressive stress as the deposition temperature is decreased from 300 to 100 °C. However, the results deviate from the linear dependence by a different degree for both measurement techniques at low deposition temperatures. The stress values obtained via the substrate curvature method deviate from the linear dependence for deposition temperatures below 200 °C, whereas the values obtained via the diagnostic microstructures method deviate from the linear dependence for deposition temperatures below 100 °C. Stress uniformity over the deposition area is also investigated.

p-to-n type-conversion mechanisms for HgCdTe exposed to H2/CH4 plasmas

The role of hydrogen incorporation in H2/CH4 reactive ion etching (RIE) induced type-conversion of p-type HgCdTe is investigated. A model is proposed in which hydrogen is incorporated into the HgCdTe crystal lattice in at least three different forms. It is proposed that the junction formation mechanism is a mixture of RIE-induced damage and Hg interstitial formation to which hydrogen forms strong bonds, and hydrogen-induced neutralization of acceptors. Confirmation of the model is presented based on experimental secondary ion mass spectroscopy of RIE-induced junctions, transport measurements reported previously, and initial diode bake stability testing.

Characterization of electrically active defects in photovoltaic detector arrays using laser beam-induced current

One of the main limitations in the operability of modern infrared focal plane arrays of p-n junction diodes formed on molecular-beam epitaxy (MBE)-grown HgCdTe is the effect of localized defects. Such defects, including voids, triangles and microvoids, are a feature of the MBE growth regime and can compromise the performance of devices fabricated within the vicinity of electrically active defects. While such defects can often be identified visually, not all defects are electrically active such that they provide a current leakage path shunting the p-n junction of the individual photodiode. In this paper, the use of laser beam-induced current is proposed as a nondestructive characterization technique, and quantitative aspects of its use in the study of electrically active defects in photodiode arrays are examined.

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.

Characterisation of reactive-ion-etching-induced type-conversion in p-type HgCdTe using scanning laser microscopy

Abstract In this work we characterise the n-type converted region occurring in both vacancy and extrinsically doped p-type Hg 1− x Cd x Te ( x ≈ 0.3) after standard reactive-ion-etch (RIE) process. The laser beam induced current (LBIC) technique is used to characterise parameters such as lateral and vertical conversion depth. Furthermore, by fitting a theoretically determined LBIC signature to the measured LBIC over the temperature range 80–300 K, it is possible to estimate the donor level density of the n-type converted region.