Frank Szmulowicz

Band gap tuning of InAs/GaSb type-II superlattices for mid-infrared detection

The superlattice (SL) of a 40 period InAs∕GaSbSL structure were varied around the 20.5AInAs∕24AGaSb design in order to produce a device with an optimum mid-infrared photoresponse and a sharpest photoresponse cutoff. The samples for this study were grown by molecular beam epitaxy with precisely calibrated growth rates. Varying individual layer width around the nominal design, we were able to systematically change the photoresponse cutoff wavelength between 4.36 to 3.45um by decreasing the InAs width from 23.5 to 17.5A, and between 4.55 to 4.03μm by increasing the GaSb width from 18 to 27A. Therefore, the cutoff changes faster with decreasing InAs rather than increasing GaSb width. However, increasing GaSb width more effectively enhances the sharpness of photoresponse near band edge. The effect of design parameters on the photoresponse cutoff and other effects are explained by a nonperturbative, modified envelope function approximation (EFA) calculation that includes the interface coupling of heavy, light, ...

Recent Advances in InAs/GaSb Superlattices for Very Long Wavelength Infrared Detection

New infrared (IR) detector materials with high sensitivity, multi-spectral capability, improved uniformity and lower manufacturing costs are required for numerous long and very long wavelength infrared imaging applications. One materials system has shown great theoretical and, more recently, experimental promise for these applications: InAs/InxGa1-xSb type-II superlattices. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection beyond 15 microns. The infrared properties of various compositions and designs of these type-II superlattices have been studied. The infrared photoresponse spectra are combined with quantum mechanical modeling of predicted absorption spectra to provide insight into the underlying physics behind the quantum sensing in these materials. Results for superlattice photodiodes with cut-off wavelengths as long as 25 microns will be presented.

InAs/InGaSb superlattices for very long wavelength infrared detection

New infrared detector materials with high sensitivity, multi-spectral capability, improved uniformity and lower manufacturing costs are required for numerous long and very long wavelength infrared imaging applications. One materials system has shown great theoretical and, more recently, experimental promise for these applications: InAs/InxGa1-xSb type-II superlattices. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection beyond 10 microns. Far-infrared photoresponse of superlattices with cut-off wavelengths between 15 micrometers and 25 micrometers were studied. The measured photoresponse spectra for both photodiodes and photoconductors are compared to calculated absorption coefficient spectra. The electronic structure and the optical absorption of InAs/InxGa1-xSb superlattice infrared (IR) detector structures are calculated, for several values of x, using our implementation of the 8x8 envelope-function approximation (EFA) formalism. Good experimental-theoretical agreement is obtained regarding the long-wavelength threshold and absorption shape.

Short period superlattices : is thinner better?

For type-II superlattices with spatially indirect optical transitions across the band gap, short-period superlattices are often employed. The oscillator strength of intraband transitions, from holes states confined in one layer to electron states confined in a neighboring layer, are enhanced by increasing the wave function overlap of these states through reduced superlattice period. However, there are limits to accurately controlling an epitaxially grown semiconductor superlattice structure as the number of monolayers in each layer is decreased. For InAs/GaSb type superlattices, periods of 40Å or less are relevant to mid-infrared detection. Characterization and modeling results for a series of InAs/GaSb superlattices with periods ranging 50Å to 20Å will be presented. These results explore the break point between when thinner is better and when reducing the period no longer optimizes the superlattice optical performance.

Type-II InAs/InGaSb SL photodetectors

We report a set of high-quality InAs/InGaSb type-II photodetectors grown on GaSb substrates with cutoff wavelengths form 11 to 21 micrometers . The SL structural parameters were very repeatable between samples as evidenced by the consistency of the SL periods and the long wavelength photoresponse cut-off. The measured photoresponse spectra were in excellent agreement with the calculated absorption spectrum. Very low background carrier concentrations were achieved in this samples set. Based on the study, the optimum growth temperature for type-II photodetectors is between 390 to 410 C with a post growth annealing at 495 to 510 C. Thickness non-uniformity of type-II photodiodes was less than 1 percent across 2-inch wafers. We have also demonstrated photodetectors with good performance from 10 to 18 micrometers , directly grown on compliant InGaAs/GaAs substrates.

Type-II superlattice materials for mid-infrared detection

Type-II superlattices composed of alternating thin layers of InAs and GaSb, have been shown to be a highly flexible infrared materials system in which the energy band gap can be adjusted anywhere between 360 meV and 40 meV. These superlattices (SLs) are the III-V equivalent to the well established HgxCd1-xTe alloys used for infrared detection in the short, mid and long wavelength bands of the infrared spectrum. There are many possible designs for these superlattices that will produce the same narrow band gap by adjusting individual layer thicknesses and interface composition. Systematic growth and characterization studies were performed to determine optimum superlattice designs suitable for infrared detection in the 3 to 5 μm wavelength band. For these studies the individual layer thicknesses were less than 35Å. The effects of adding different thickness InSb-like interfaces were also studied. Through precision molecular beam epitaxy, design changes as small as 3Å to the SL layers could be studied. Significant changes were observed in the infrared photoresponse spectra of the various SL samples. The infrared properties of the various designs of these type-II superlattices were modeled using an 8-band Envelope Function Approximation. The infrared photoresponse spectra, combined with quantum mechanical modeling of predicted absorption spectra, were a key factor in the design optimization of the InAs/GaSb superlattices with band gaps in the range of 200 to 360 meV.

Whither P-type GaAs/AlGaAs QWIP?

P-type GaAs/AlGaAs quantum well infrared photodetectors (QWIP) represent a complementary technology to the well developed and already commercialized n-type GaAs/AlGaAs QWIP technology. Since n-QWIPs require grating couplings for normal incidence absorption, p-type GaAs/AlGaAs QWIPs have emerged as a viable alternative in some applications. In this paper, progress in optimizing the performance of p-type GaAs/AlGaAs QWIPs through modeling, growth, and characterization is described. Our approach begins with the theoretical design of p-QWIPs based on calculations of optical absorption. Next, samples are grown by MBE according to the theoretical designs and their characteristics measured. p-type QWIPs were optimized with respect to the well and barrier widths, alloy concentration, and dopant concentration; resonant cavity devices were also fabricated and the temperature dependent photoresponse was measured. Based on the progress to date, it is now possible to make some comparisons between the n- and p-type approaches. Further avenues for improvement of p-QWIP photoresponse are being explored by exploiting the rich physics of this coupled multi-band system.

GaAs/AlGaAs p-type multiple quantum wells for infrared detection at normal incidence: model and experiment

The development of devices for mid-, long-, and very long- wavelength IR detection has benefitted greatly from advances in band-gap engineering. Recently, there has been great progress in the development of n-type GaAs/AlGaAs quantum well infrared photoconductor (QWIP) detector arrays in all three technologically important wavelength windows. P-type GaAs.AlGaAs QWIPS represent a viable alternative to n-type GaAs/AlGaAs QWIPs, offering the advantage of normal incidence absorption without the need for grating couplers. The maturity of the MBE of GaAs/AlGaAs layered materials offers the possibility of mass producing low cost, high performance, large size, high uniformity, multicolor, high frequency bandwidth, two-dimensional imaging QWIP arrays. This paper describes progress in optimizing the performance of p- type GaAs/AlGaAs QWIPs through modeling, growth, and characterization. Using the 8x8 envelope-function approximation (EFA), a number of structures were designed and their optical absorption calculated for comparison with experiment. Samples were grown by MBE based on the theoretical designs and their photoresponse measured. P-type QWIPs were optimized with respect to the well and barrier widths, alloy concentration, and dopant concentration; resonant cavity devices were also fabricated and temperature dependent photoresponse was measured. The quantum efficiencies and the background-limited (BLIP) detectivities under BLIP conditions of our own p-QWIPs are comparable to those of n-QWIPs; however, the responsivities are smaller. For our mid-IR p-QWIPs, the 2D doping densities of 1- 2x1012 cm-2 maximized the BLIP temperature and dark current limited detectivity by operating at around 100K. At 80K, the detectivity of the optimum doped sample was (formula available in paper)at 10V bias. Barrier widths greater than 200 A were sufficient to impede the tunneling dark current; resonant cavities enhanced absorption five-fold.

Photoluminescence studies of beryllium doped GaAs/AlGaAs quantum wells

Abstract A photoluminescence (PL) investigation of beryllium doped GaAs/AlGaAs multiple quantum wells is reported. MBE grown samples with well widths 30–75 A and barrier thicknesses 100–500 A are included. The effect of beryllium doping in the well region of sheet carrier density 3×10 11 to 4×10 12 cm −2 on the position of the first conduction band-to-first heavy hole band (C1–HH1) free exciton line is investigated. The position of PL peak energies as a function of well width and doping is calculated using single particle energies from an envelope function approximation calculation and an estimate of many body effects, including a two-dimensional, screened exchange interaction. A very good agreement is found between the calculated and measured PL peak energies.

The role of InAs thickness on the material properties of InAs/GaSb superlattices

The epitaxial growth parameters optimized for mid-wavelength infrared (MWIR) InAs/GaSb superlattice (SL) growth are not directly applicable for long-wavelength infrared (LWIR) SL growth. We observed a two orders of magnitude drop in the spectral intensity of the measured photoresponse (PR) as the InAs layer thickness in the SL increases from 9 monolayers (MLs) to 16 MLs for a fixed GaSb layer thickness of 7 MLs. However, the theoretically calculated absorption strength decreases only by about a factor of two. So other factors affecting photoresponse, such as carrier mobility and lifetime, are likely responsible for the large drop in the PR of the LWIR SL in this sample set. In fact the measured Hall properties of MWIR and LWIR SLs are very different, with holes as the majority carriers in MWIR SLs and electrons as the majority carriers in LWIR SLs. Therefore we investigated the charge carrier density, carrier mobility, and carrier recombination dynamics in LWIR SL samples. Specifically we used temperature-dependent Hall effect and time-resolved pump-probe measurements to study the effect of adjusting several growth parameters on the background carrier concentrations and studied carrier lifetimes in LWIR SLs.