David R. Rhiger

Infrared absorption behavior in CdZnTe substrates

Infrared (IR) optical transmission measurements of polished CdZnTe wafers can provide useful information about excess impurities, stoichiometry, and inhomogeneities (precipitates and inclusions). We have investigated the IR transmission behavior of Cd0.96Zn0.04Te between 8 m and 20 m at room temperature. The measurements were made before and after thermal treatments involving control of the Cd and Zn overpressures, which served to minimize the Cd (cation) vacancy population. Our results support the polar optical phonon scattering theory of Jensen, according to which the absorption in donor dominated CdZnTe varies asm with m=3. For material dominated by acceptors, we show that the theoretical absorption by inter-valence band transitions can be approximated by a similar power law with exponent m=1, and that Cd-vacancy dominated wafers are in reasonable agreement with this. We find some wafers in which the asgrown condition exhibits partial compensation of impurity donors by Cd vacancy acceptors, and demonstrate removal of the compensation by annealing to fill the vacancies. In a separate group of wafers, we find that an observed increase in absorption occurring during growth of a HgCdTe layer by liquid phase epitaxy can be explained in terms of an increase in Cd vacancies caused by diffusion of Cd to Te precipitates. This effect can be reversed by annealing in Cd−Zn vapor, which fills vacancies and eliminates some precipitates. Impurity concentrations were measured by glow discharge mass spectrometry (GDMS).

Solid‐state quaternary phase equilibrium diagram for the Hg–Cd–Te–O system

For the Hg–Cd–Te–O system an approximate three‐dimensional diagram of quaternary phase equilibrium at room temperature has been constructed for the first time, to aid in the study of Hg1−xCdxTe surface oxides. Thermodynamic calculations based on standard Gibbs energies of formation were performed to determine which phases are mutually stable. Stability relationships were checked by reaction experiments in mixtures of bulk reference compounds, using Raman spectroscopy for analysis. Elemental Hg is shown to coexist in equilibrium with all of the oxide and telluride phases. Thus, it is likely to be present whenever oxidation is incomplete. CdTeO3 is shown to be stable with respect to the other oxides and tellurides. Several instabilities are indicated, in which an oxide can react with the Hg1–xCdxTe to release elemental Hg or Te, or HgTe.

Use of ellipsometry to characterize the surface of HgCdTe

Ellipsometry is a sensitive, rapid, and nondestructive optical technique for characterizing materials, especially surfaces and films. By measuring the change in the state of polarization of a light beam reflecting from the sample, one may infer certain characteristics of the sample. We present a review of the applications of ellipsometry to HgCdTe and related materials. The fundamentals of the technique are discussed briefly and the optical parameters at the wavelength 6328A for several materials of interest to infrared technology are listed. The emphasis of this paper is on the interpretation of the ellipsometric data, expressed in terms of the usual parameters Ψ and Δ obtained at a single wavelength. Methods and limitations of the analysis of single films, both nonabsorbing and absorbing, are discussed. Examples of an acceptance window for process monitoring are presented. The ellipsometric signatures of amorphous Te films and microroughness are described, along with a graphical method for interpreting the readings from very thin films. Spectroscopic applications and in situ monitoring of molecular beam epitaxial growth processes are briefly reviewed.

Progress with type-II superlattice IR detector arrays

We report progress in the development of long wavelength infrared (LWIR) focal plane arrays (FPAs) built on type-II strained layer InAs/GaSb superlattice materials. Work at Raytheon Vision Systems and Jet Propulsion Laboratory has led to successful devices with cutoff wavelengths in the 10 to 12 μm range. Pixels have been formed by wet etching and surface passivation by plasma-deposited silicon dioxide. We present test results on arrays hybridized with indium bump bonding to silicon readout integrated circuits, as well as analyses of current-voltage characteristics of individual diodes. In particular, we find that, at temperatures below about 70 K the leakage current is dominated by generation-recombination effects near zero bias and by trap-assisted tunneling in reverse bias. Although other authors have demonstrated imaging for SWIR and MWIR type-II superlattice devices, to our knowledge no one has done so prior to 2006 in the LWIR range. We have obtained both still and video imaging with 256×256 arrays with 30-μm pixels operating at 78 K, having high operability and a cutoff wavelength of 10.5 μm.

Infrared imaging arrays based on superlattice photodiodes

We report on the status of focal plane arrays (FPAs) based on GaSb/InAs type-II superlattice diodes grown by molecular beam epitaxy (MBE) and designed for infrared absorption in the 2-5μm and 8-10μm bands. Recent LWIR devices have produced differential resistance-area product greater than 100 Ohmcm2 at 80K with a long wavelength cutoff of approximately 10μm. The measured quantum efficiency of these front-side illuminated devices is close to 25% in the 8-9 μm range. MWIR devices have produced detectivities as high as 8x1013 Jones with a differential resistance-area product greater than 3x107 Ohmcm2 at 80K with a long wavelength cutoff of approximately 3.7μm. The measured quantum efficiency of these front-side illuminated MWIR devices is close to 40% in the 2-3μm range at low temperature and increases to over 60% near room temperature. Initial results on SiO2 and epitaxial-regrowth based passivation techniques are also presented, as well as images from the first lot of 1kx1k MWIR arrays and our latest 256x256 LWIR arrays.

Development of large single crystal (3-inch ingot) CdZnTe for large-volume nuclear radiation detectors

Further progress has been made in the development of the Modified Vertical Bridgman method for the growth of CdZnTe crystals for fabrication of x-ray and gamma-ray detectors to operate at room temperature. Specifically, the diameter of the grown ingots has been increased from 2 to 3 inches. High quality, large volume (up to 6 in3) twin-free single crystals have been produced. Detectors fabricated with this material show sharp energy resolution and good uniformity.

Atomic scale analysis of the effect of the SiO2 passivation treatment on InAs/GaSb superlattice mesa sidewall

We have analyzed by electron microscopy techniques the effect of the deposition of a SiO2 passivation layer on an InAs/GaSb type-II superlattice (SL) mesa with applications as a photodetector. Our images reveal good conformal coverage by the SiO2 upon an undulating edge of the SL mesa. However, we have observed scarce As clusters at the interface between the SL mesa and the passivation layer and some degree of oxidation of the mesa sidewall. The strong reduction in surface leakage currents demonstrates that the observed imperfections do not have a substantial detrimental effect on the passivation capabilities of the SiO2 layer.

Demonstration of Mid and Long-Wavelength Infrared Antimonide-based Focal Plane Arrays

We have demonstrated the use of bulk antimonide based materials and type-II antimonide based superlattices in the development of large area mid wavelength infrared (MWIR) focal plane arrays (FPAs) as well as smaller format long wavelength infrared (LWIR) arrays. Barrier infrared photodetectors (BIRDs) and superlattice-based infrared photodetectors are expected to outperform traditional III-V MWIR and LWIR imaging technologies and are expected to offer significant advantages over II-VI material based FPAs. We have used molecular beam epitaxy (MBE) technology to grow InAs/GaSb superlattice pin photodiode and bulk InAsSb structures on GaSb substrates. The coupled quantum well superlattice device offers additional control in wavelength tuning via quantum well sizes and ternary composition. Furthermore, we have fabricated mid-wavelength 1024x1024 pixels superlattice imaging FPAs, 640x512 MWIR arrays based on the BIRD concept, and 256x256 LWIR arrays based on pin superlattice structures. These initial FPA have produced excellent infrared imagery.

HgCdTe/CdZnTe p-i-n high-energy photon detectors

Classical solid-state detection of x-ray and gamma-ray radiation consists of a high voltage applied between two metallic contacts sandwiching a high resistivity, high dielectric strength material; high voltage and high resistivity are required to enable complete charge collection while minimizing the resolution-degrading leakage current (dark current). We report here the conception and successful fabrication and test of a new device construct which changes this paradigm. P-type and n-type layers are fabricated by mercury cadmium telluride (HC.T) liquid phase epitaxy (LPE) on opposite sides of a high-quality wafer of CdZnTe (CZT) in order to construct a p-i-n diode structure. Wafers up to 9 cm2 area have been grown. This diode structure provides an extremely high effective resistivity and barrier to the flow of dark current in the device. Several wafer lots have repeatably yielded p-i-n detectors which exhibit typical diode current-voltage (I-V) curves with very low dark currents at very high bias voltages. Spectra obtained from these detectors produce exceptionally sharp photopeaks which exhibit very little low-energy tailing.

Mid-wavelength InAsSb detectors based on nBn design

The development of InAsSb detectors based on the nBn design for the mid-wave infrared (MWIR) spectral region is discussed. Comparisons of optical and electrical properties of InAsSb photodetectors with two different barrier material, namely, AlAs 0.15Sb0.75 (structure A) and AlAs0.10Sb0.9 (structure B) are reported. The dark current density in the AlAs0.15Sb0.85 is lower possibly due to the larger valence band offset. Clear room temperature spectral responses is observed and a specific detectivity (D*) of 1.4x1012 and 1.01x1012 cmHz1/2/W at 0.2 V, and a responsivity of 0.87 and 1.66 A/W under 0.2 V biasing at 77 K and 3.5 μm, assuming unity gain, was obtained for structures A and B, respectively.