Bulk unrelaxed InAsSb alloys with Sb compositions up to 65% were grown on compositionally graded GaInSb and AlInSb buffers on GaSb substrates by molecular beam epitaxy. The minimum energy gap for these materials at T = 77 K was estimated to be 90 meV. Benchmark material parameters were measured for barrier photodetector heterostructures with 1- μ m-thick InAs 0.6 Sb 0.4 absorbers. A minority hole lifetime of 185 ns and a diffusion length of 9 μ m at T = 77 K were determined from the transient response of barrier heterostructures. The data imply a hole mobility of 10 3 cm 2 /Vs, which was confirmed with frequency response measurements. A 100- μ m square mesa contact nBn heterostructure demonstrated a −3 dB frequency response bandwidth of 50 MHz.

Growth of unrelaxed and unstrained AlzIn1−zAsySb1−y with a lattice constant=6.23Å was demonstrated. InAs1-xSbx with this lattice constant produces a bandgap corresponding to absorption in the long-wavelength infrared range. The structures were grown on GaSb substrates, using a lattice constant shifting buffer layer. Good photoluminescence intensity was shown, ranging from 2.0 to 4.5μm, demonstrating the potential for development of multi-color infrared detectors that can cover both the mid- and long-wavelength infrared bands.

A new method for determination of minority carrier lifetimes, mobilities and diffusion lengths is demonstrated. From the transient response of long wavelength infrared barrier detectors with a cut-off of 10 micron, the mobility of the minority holes and their lifetime in bulk InAs0.6Sb0.4 at T = 77 K was determined to be 800 cm2 V−1s−1 and 165 ns, respectively.

Bulk unrelaxed InAsSb alloys with Sb compositions up to 65% and layer thicknesses up to 3 μm were grown by molecular beam epitaxy. The photoluminescence (PL) peak energy as low as 0.10 eV was demonstrated at T = 77 K. The electroluminescence and quantum efficiency data demonstrated with unoptimized barrier heterostructures from T = 80 to 150 K suggested large absorption and carrier lifetimes sufficient for the development of long wave infrared detectors and emitters with high quantum efficiency. The minority hole transport was found to be adequate for development of the detectors and emitters with large active layer thickness.

Minority carrier lifetimes in undoped and Beryllium-doped Type-2 Ga-free, InAs/InAsSb strained layer superlattices (SLS) with energy gaps as low as 0.165 eV were determined from photoluminescence kinetics. The minority carrier lifetime of 450 ns at 77 K in the undoped SLS confirms a high material quality. In similarly-grown structures that were p-doped to N A = 6 × 1016 and 3 × 1017 cm−3, electron lifetimes of τ n = 45 ns and 8 ns were measured. The 6 × 1016 cm−3 doping level is a factor of 6 greater than the typical background doping level in long-wave infrared (LWIR) Ga-containing InAs/GaSb SLS with similar bandgap and electron lifetime. This suggests that LWIR photodetectors with InAs/InAsSb SLS absorbers can be designed with smaller minority carrier concentrations and diffusion dark current densities. A relatively slow decrease of the lifetime with doping suggests a minor role of Auger recombination in the studied Ga-free SLS at T = 77 K with p-doping up to mid-1017 cm−3 level.

The energy gaps were studied in two types of structures: unrelaxed bulk InAs1−x Sb x layers with x = 0.2 to 0.46 grown on metamorphic buffers and type II InAs1−x Sb x /InAs strained-layer superlattices (SLS) with x = 0.225 to 0.296 in the temperature range from T = 13 K to 300 K. All structures were grown on GaSb substrates. The longest wavelength of photoluminescence (PL) at low temperatures was observed from bulk InAs0.56Sb0.44 with a peak at 10.3 μm and full-width at half-maximum (FWHM) of 11 meV. The PL data for the bulk InAs1−x Sb x materials of various compositions imply an energy gap bowing parameter of 0.87 eV. A low-temperature PL peak at 9.1 μm with FWHM of 13 meV was observed for InAs0.704Sb0.296/InAs SLS. The PL spectrum of InAs0.775Sb0.225/InAs SLS under pulsed excitation revealed a second peak associated with recombination of electrons in the three-dimensional (3D) continuum with holes in the InAs0.775Sb0.225. This experiment determined the conduction-band offset in the InAs0.775Sb0.225/InAs SLS. The energies of the conduction and valence bands in unstrained InAs1−x Sb x and their bowing with respect to the Sb composition are discussed.

InAs0.6Sb0.4/Al0.75In0.25Sb-based barrier photodetectors were grown metamorphically on compositionally graded Ga1−xInxSb buffer layers and GaSb substrates by molecular beam epitaxy. At the wavelength of 8 μm and T = 150 K, devices with 1-μm thick absorbers demonstrated an external quantum efficiency of 18% under a bias voltage of 0.45 V.

Metamorphic heterostructures containing bulk InAs1−xSbx layers and AlInAsSb barriers were grown on GaSb substrates. The lattice mismatch (up to 2.1%) between the GaSb substrates and the InAsSb layers was accommodated by the growth of GaInSb linearly graded buffers. The 1 μm thick InAsSb0.44 layer with an absorption edge above 9 μm exhibited an in-plane residual strain of about 0.08%. InAs1−xSbx structures with x = 0.2 and x = 0.44 operated as light emitting diodes at 80 K demonstrated output powers of 90 μW and 8 μW at 5 μm and 8 μm, respectively.

Bulk unrelaxed InAsSb alloys with Sb compositions up to 44 % and layer thicknesses up to 3 mu m were grown by molecular beam epitaxy. The alloys showed photoluminescence (PL) energies as low as 0.12 eV at T = 13 K. The electroluminescence and quantum efficiency data demonstrated with unoptimized barrier heterostructures at T=80 and 150 K suggested large absorption and carrier lifetimes sufficient for the development of long wave infrared detectors and emitters with high quantum efficiency. The minority hole transport was found to be adequate for development of the detectors and emitters with large active layer thickness.

The band gap energy of the alloy InAsSb has been studied as a function of composition with special emphasis on minimization of strain-induced artifacts. The films were grown by molecular beam epitaxy on GaSb substrates with compositionally graded buffer layers that were designed to produce strain-free films. The compositions were precisely determined by high-resolution x-ray diffraction. Evidence for weak, long-range, group-V ordering was detected in materials exhibiting residual strain and relaxation. In contrast, unstrained films having the nondistorted cubic form showed no evidence of group-V ordering. The photoluminescence (PL) peak positions therefore corresponds to the inherent band gap of unstrained, unrelaxed, InAsSb. PL peaks were recorded for compositions up to 46% Sb, reaching a peak wavelength of 10.3 mu m, observed under low excitation at T = 13 K. The alloy band gap energies determined from PL maxima are described with a bowing parameter of 0.87 eV, which is significantly larger than measured for InAsSb in earlier work. The sufficiently large bowing parameter and the ability to grow the alloys without ordering allows direct band gap InAsSb to be a candidate material for low-temperature long-wavelength infrared detector applications. DOI: 10.1103/PhysRevB.86.245205

The strong bandgap bowing in the InAsxSb1−x alloy system allows it to potentially be used for infrared photodetection in the middle and long wavelength range. The authors have used compositionally graded metamorphic buffer layers to accommodate the misfit strain between InAsxSb1−x alloys and GaSb and InSb substrates in order to reach the long wave infrared range. In this work, we present the characterization of metamorphically grown InAsxSb1−x films that demonstrate strong photoluminescence in the spectral range from 5 to 9 μm.

Thick InAsBi layers were grown for photoluminescence (PL) characterization. The As to In overpressure ratio was carefully characterized and adjusted to achieve Bi-droplet-free surfaces. A closed loop feedback system was used to maintain the As overpressure during a 5-h deposition sequence. Despite a high degree of control of the growth parameters, evidence for local phase separation was observed in the PL spectra.

The optical properties of bulk unrelaxed InAsSb layers having a low temperature photoluminescence (PL) peak up to 10 mu m are presented. The materials were grown on GaSb substrates by molecular beam epitaxy. The lattice mismatch between the epilayers and GaSb substrates was accommodated with linearly graded GaAlInSb buffers. An 11-meV width of PL at full-width half-maximum was measured for InAsSb with Sb compositions of 20 and 44%. The best fit for the dependence of the energy gap on Sb composition was obtained with a 0.9-eV bowing parameter. Temperature dependences of the energy gap for InAsSb alloys with 20 % and 44% Sb were determined from PL spectra in the temperature range from 12 to 300 K. A T=77 K minority carrier lifetime up to 350 ns in undoped InAsSb layers with 20% Sb was determined from PL kinetics.

Minority carrier lifetime in long-wave infrared (LWIR) type II InAs/GaSb superlattices was studied using the optical modulation response (OMR) technique in wide ranges of excitation and temperature. The measured carrier lifetime was found to increase superexponentially with decreasing excitation power density below the level of 1 mW/cm2 to 2 mW/cm2. The phenomenon was qualitatively explained by the presence of shallow trapping centers.

Diode lasers operating at 3 mu m in continuous wave mode at room temperature were fabricated using metamorphic molecular beam epitaxy. The laser heterostructures have a lattice constant 1.3-1.6% bigger than that of the GaSb substrates. The mismatch between the epi-structure and the substrate lattice constants was accommodated by a network of misfit dislocations confined within linearly compositionally graded buffer layers. Two types of the buffers were tested-GaInSb and AlGaInSb. The laser heterostructures with Al-containing buffer layers demonstrated better surface morphology and produced devices with lower threshold and higher efficiency. At the same time the use of Al-containing buffers caused an excessive voltage drop across the laser heterostructure. Thus, a maximum continuous wave output power of 200 mW was obtained from lasers grown on GaInSb buffers, while only 170 mW was obtained from those grown on AlGaInSb buffers.

Infrared (IR) detector materials based on III-V semiconductors are an affordable alternative to HgCdTe, which is the current material of choice for most high performance IR focal plane array systems fielded in the Army. Based on the assumption that III-V compounds do not have an inherently small bandgap enabling operation out to 10 mu m, the long wave infrared (LWIR) range, there has been substantial research investment in superlattice (SL) approaches. For this reason, quantum structures such as quantum well IR photodetectors (QWIPs) and typeII strained-layer superlattices (SLS) are grown to take advantage of confinement effects and to induce an effective bandgap in the desired range. The drawbacks of these approaches include inherently low quantum efficiencies and operating temperature for QWIPs, and very short minority carrier lifetimes for SLS detectors. In this presentation, we will discuss approaches for developing III-V, direct bandgap, dilute N and Bi alloys grown by molecular beam epitaxy (MBE) for LWIR applications.

GaInSb and AlGaInSb compositionally graded buffer layers grown on GaSb by MBE were used to develop unrelaxed InAs1-XSbXepilayers with lattice constants up to 2.1 % larger than that of GaSb. The InAsSb buffer layer was used to grow InAs0.12Sb0.88 layer on InSb. The structural and optical characterization of 1-μm thick InAs1-xSbx layers was performed together with measurements of the carrier lifetime.

We have examined the growth of strained layer superlattice (SLS) structures for the purpose of characterizing and improving the minority carrier lifetime. Structures with different SL periods but with same absorption wavelength were first studied. Despite a doubling of the number of interfaces per thickness unit, no significant change was seen in the carrier lifetime. This observation points away from the interfaces as the location of lifetime limiting defect centers. To gain further insights into the spatial location of the defect centers, a series of binary InAs and GaSb layers grown with different substrate temperatures, were studied. We found that higher growth temperatures were beneficial for both binaries, although the improvement for GaSb was less than that of InAs. The substrate temperature was also varied in SLS structures and characterized with high-resolution x-ray diffraction. By using the peak width from the SLS zero-order diffraction as a figure of merit, we found a shallow growth window of ∼±20° around an optimum temperature of 440°C. Outside this temperature window the material quality deteriorated very rapidly. Unfortunately, the substrate temperatures that would provide an improvement in the binary lifetimes fall mainly above the SLS growth window, thus limiting this parameter as a means of improving lifetimes in the SLS. A model that qualitatively relates bulk and SLS lifetimes through native defects is proposed and strategies for improving the lifetimes are discussed.

Minority carrier lifetime, photoluminescence (PL), and interband absorption in midinfrared range of spectra were measured in InAs/GaSb strained-layer superlattices (SLS) grown by molecular beam epitaxy (MBE) on GaSb substrates. The carrier lifetime was determined by time-resolved PL (TRPL) and from analysis of PL response to sine-wavemodulated excitation. Studies of the PL kinetics in the frequency domain allowed for direct lifetime measurements in SLS structures with an excess carrier concentration level of 3.5×1015 cm-3. The minority carrier lifetime at T = 77 K was obtained from the dependence of the carrier lifetime on excitation power. SLS structures with similar absorption wavelengths but with different InAs and GaSb layer thicknesses and with different amounts of strain were investigated and compared with mercury cadmium telluride (MCT) samples. No apparent trend was seen in structures with different number of interfaces per unit length. All SLS lifetime values measured so far are more than an order of magnitude lower than those of MCT.