Vaidya Nathan

Uncooled operation of type-II InAs∕GaSb superlattice photodiodes in the midwavelength infrared range

We report high performance uncooled midwavelength infrared photodiodes based on interface-engineered InAs∕GaSb superlattice. Two distinct superlattices were designed with a cutoff wavelength around 5μm for room temperature and 77 K. The device quantum efficiency reached more than 25% with responsivity around 1A∕W. Detectivity was measured around 109cmHz1∕2∕W at room temperature and 1.5×1013cmHz1∕2∕W at 77 K under zero bias. The devices were without antireflective coating. The device quantum efficiency stays at nearly the same level within this temperature range. Additionally, Wannier–Stark oscillations in the Zener tunneling current were observed up to room temperature.

Near bulk-limited R0A of long-wavelength infrared type-II InAs∕GaSb superlattice photodiodes with polyimide surface passivation

Effective surface passivation of type-II InAs∕GaSb superlattice photodiodes with cutoff wavelengths in the long-wavelength infrared is presented. A stable passivation layer, the electrical properties of which do not change as a function of the ambient environment nor time, has been prepared by a solvent-based surface preparation, vacuum desorption, and the application of an insulating polyimide layer. Passivated photodiodes, with dimensions ranging from 400×400to25×25μm2, with a cutoff wavelength of ∼11μm, exhibited near bulk-limited R0A values of ∼12Ωcm2, surface resistivities in excess of 104Ωcm, and very uniform current-voltage behavior at 77K.

Type-II M structure photodiodes: an alternative material design for mid-wave to long wavelength infrared regimes

In this work, an AlSb-containing Type II InAs/GaSb superlattice, the so-called M-structure, is presented as a candidate for mid and long wavelength infrared detection devices. The effect of inserting an AlSb barrier in the GaSb layer is discussed and predicts many promising properties relevant to practical use. A good agreement between the theoretical calculation based on Empirical Tight Binding Method framework and experimental results is observed, showing the feasibility of the structure and its properties. A band gap engineering method without material stress constraint is proposed.

High-performance type-II InAs/GaSb superlattice photodiodes with cutoff wavelength around 7 μm

We report the most recent result in the area of type-II InAs/GaSb superlattice photodiodes that have a cutoff wavelength around 7 μm at 77 K. Superlattice with a period of 40 A lattice matched to GaSb was realized using GaxIn1−x type interface engineering technique. Compared with significantly longer period superlattices, we have reduced the dark current density under reverse bias dramatically. For a 3 μm thick structure, using sulfide-based passivation, the dark current density reached 2.6×10−5A∕cm2 at −3 V reverse bias at 77 K. At this temperature the photodiodes have R0A of 9300Ωcm2 and a thermally limited zero bias detectivity of 1×1012cmHz1∕2∕W. The 90%–10% cutoff energy width was only 16.5 meV. The devices did not show significant dark current change at 77 K after three months storage in the atmosphere.

Recent Advances in LWIR Type-II InAs/GaSb Superlattice Photodetectors and Focal Plane Arrays at the Center for Quantum Devices

In recent years, Type-II InAs/GaSb superlattice photodetectors have experienced significant improvements in material quality, structural designs, and imaging applications. They now appear to be a possible alternative to the state-of-the-art HgCdTe (MCT) technology in the long (LWIR) and very long wavelength infrared regimes. At the Center for Quantum Devices, we have successfully realized very high quantum efficiency, very high dynamic differential resistance R0A-product LWIR Type-II InAs/GaSb superlattice photodiodes with efficient surface passivation techniques. The demonstration of high-quality LWIR focal plane arrays that were 100% fabricated in-house reaffirms the pioneer position of this university-based laboratory.

Arsenic activation in molecular beam epitaxy grown, in situ doped HgCdTe(211)

Photovoltaic p-n junctions are the most significant active components of both current infrared photodetectors and advanced ones being developed. It is of the utmost importance to control both p- and n-type extrinsic doping. This letter addresses the issue of activating arsenic as a p-type dopant of Hg1−xCdxTe at temperatures sufficiently low that the integrity of p-n junctions and the intrinsic advantages of molecular beam epitaxy as a growth technique will not be compromised. The p-type activation of arsenic in (211)B Hg1−xCdxTe is reported after a two-stage anneal at temperatures below 300 °C for Cd compositions suitable for the sensing of long wavelength infrared radiation.

Narrow gap HgCdTe absorption behavior near the band edge including nonparabolicity and the Urbach tail

An analytical model describing the absorption behavior of HgCdTe is developed that simultaneously considers the contributions from nonparabolic conduction and light hole bands as calculated by a 14×14 matrix k∙p method as well as the Urbach tail. This model is capable of smoothly fitting experimental absorption coefficient curves over energies ranging from the Urbach tail region to the intrinsic absorption region up to 300meV above the band gap. Comparisons to the experimental results give good agreement.

Development of gold-doped Hg0.79Cd0.21Te for very-long-wavelength infrared detectors

Gold-doped Hg1−xCdxTe samples of x=0.2067 (in the very-long-wavelength infrared spectral band, with cutoff wavelengths ∼13.2 μm at 77 K) were prepared by tellurium-melt liquid-phase epitaxy. The samples were doped with indium to ∼2×1014 cm−3 and gold to ∼7×1015 cm−3, and were characterized by secondary ion mass spectroscopy, Hall measurements, and minority carrier lifetime measurements. State-of-the-art minority carrier lifetime of ∼0.82 μs was obtained.

Optical absorption in Hg1−xCdxTe

The theory of optical absorption due to interband transitions in direct-gap semiconductors is revisited. An analytical expression for the linear absorption coefficient in narrow-gap semiconductors is obtained by including the nonparabolic band structure due to Keldysh [Sov. Phys.–JETP 6, 763 (1958)] and Burstein–Moss shift. Numerical results are obtained for Hg1−xCdxTe for several values of x and temperature, and compared with recent experimental data. The agreement is found to be good.

Improvement of the accuracy of the in-situ ellipsometric measurements of temperature and alloy composition for MBE grown HgCdTe LWIR/MWIR structures

Spectroscopic ellipsometry (SE) has proven to be a very reliable technique for the in-situ monitoring of the substrate temperature and alloy composition during the HgCdTe epitaxy. In this work, the influence of the variations in the angle of incidence and the spectral wavelength shift on the measured accuracy of the growth temperature and alloy composition are studied, and a method for precisely determining these variations independent of the modeling of the SE data has been developed. It is shown that the stability of the fittings of the optical models for in-situ applications increases and that the couplings between model parameter decreases upon either eliminating the angle of incidence as an independent model parameter or correcting for the shifts of the wavelength offset. The variations in the angle of incidence and wavelength shift, which arise in the M88 ellipsometer from reflected beam deflections, were precisely calibrated in two dimensions as a function of alignment parameters, using a thick thermally grown SiO2/Si sample and were parameterized for our experimental geometry. A new extension of theWVASE software was developed to correct the raw SE data in real time for wavelength shift and the angle of incidence drift. A comparison of the corrected and uncorrected results of in-situ temperature measurements for HgCdTe and CdZnTe(211) B/Si(211) clearly demonstrates that the proposed method significantly enhances the accuracy of temperature and composition readings over a broad range of values in these parameters.