InAs-GaSb Superlattices Research Articles

Comment on “A study of vertical and in-plane electron mobility due to interface roughness scattering at low temperature in InAs-GaSb superlattices” [J. Appl. Phys. 114, 053712 (2013)

The purpose of this comment is to point out that the paper by Safa, Asgari, and Faraone [J. Appl. Phys. 114, 053712 (2013)] (SAF) on electronic transport in superlattices contains a number of errors in physics and execution. By dealing with a finite number of periods and forcing the wave function to be zero at the upper and lower boundaries of the superlattice stack, SAF have turned the system into a quantum well for which the momentum along the growth axis is not a good quantum number, so that the bands in the growth direction are flat and the corresponding carrier velocities and vertical mobilities are zero. A number of other errors allow the authors to get nonzero results and to reach conclusions that qualitatively mirror those of Szmulowicz, Haugan, Elhamri, and Brown [Phys. Rev. B 84, 155307 (2011)].

Demonstration of a 1024 $\times$ 1024 Pixel InAs–GaSb Superlattice Focal Plane Array

We describe the demonstration of a 1024 × 1024 pixel long-wavelength infrared focal plane array based on an InAs-GaSb superlattice absorber surrounded by an electron-blocking and a hole-blocking unipolar barrier. An 11.5-μm cutoff focal plane without antireflection coating based on this complementary barrier infrared detector design has yielded noise equivalent differential temperature of 53 mK at operating temperature of 80 K, with 300 K background and f/2 cold-stop.

High-Power MWIR Cascaded InAs–GaSb Superlattice LEDs

Midwave IR LEDs operating at 3.8 mum with output powers approaching 25 mW at 77 K are reported. Devices based on the InAs-GaSb superlattice material system grown by solid source molecular beam epitaxy are demonstrated in a cascaded active region configuration as high-power IR emitters. Optical and electronic characteristics of 16-stage devices with variable mesa size were examined to assess the performance dependence on device size and injection current. The results are suggestive that output power saturation was due to thermal management limitations and carrier leakage out of the active region. Reported output power measurements were taken without the use of an immersion lens or other collection optics, thus representing the upper hemisphere output. Devices were also demonstrated to generate an upper hemisphere power exceeding 1.4 mW at 220 K under quasi-DC excitation conditions.

Quantum Efficiency Analysis of InAs–GaSb Type-II Superlattice Photodiodes

We compare the experimentally measured and theoretically calculated quantum efficiency (QE), where an analytical drift-diffusion photocurrent model is used, of n+ -on-p InAs-GaSb superlattice (SL) photodiodes. With inputs of the transport parameters obtained by the electron-beam-induced current technique and absorption coefficient spectra calculated by the eight-band kldrp method for the p-SL, n+ -SL, and depletion region, taking into account the band filling effect, we show that the drift-diffusion photocurrent model is a good approximation for the InAs-GaSb type-II SL photodiodes, which implies that the SL depletion region in InAs-GaSb SL photodiodes is as effective as that in bulk semiconductor photodiodes in terms of collecting the photo-excited electron-hole pairs. Using this theoretical model, we also find that the high doping density in n-type SL degrades the QE by reducing the absorption coefficient. As a result, the n-type doping density is suggested to be below 1times1017cm-3 in order to optimize the QE for the studied InAs-GaSb SL photodiodes.

Background Limited Performance of Long Wavelength Infrared Focal Plane Arrays Fabricated From M-Structure InAs–GaSb Superlattices

The recent introduction of a M-structure design improved both the dark current and R0 A performances of type-II InAs-GaSb photodiodes. A focal plane array fabricated with this design was characterized at 81 K. The dark current of individual pixels was measured between 1.1 and 1.6 nA, 7 times lower than previous superlattice FPAs. This led to a higher dynamic range and longer integration times. The quantum efficiency of detectors without antireflective coating was 74%. The noise equivalent temperature difference reached 23 mK, limited only by the performance of the testing system and the read out integrated circuit. Background limited performances were demonstrated at 81 K for a 300 K background.

Active Region Cascading for Improved Performance in InAs–GaSb Superlattice LEDs

Cascading of active regions in InAs-GaSb superlattice light-emitting diodes (LEDs) grown by molecular beam epitaxy is demonstrated as an effective means of increasing optical emission. Devices were fabricated into 120 x 120 mum2 mesas to demonstrate suitability for high resolution projection systems. Devices with 1, 4, 8, and 16 stages were designed for midwave infrared emission at 3.8 mum operating at 77 K, and quasi-continuous-wave output powers in excess of 900 muW from a 16-stage LED have been demonstrated. External quantum efficiency is shown to improve substantially with cascading, approaching 10% for a 16-stage device.

High-Performance Focal Plane Array Based on InAs–GaSb Superlattices With a 10-$\mu{\hbox {m}}$ Cutoff Wavelength

We report on the demonstration of a focal plane array based on Type-II InAs-GaSb superlattices grown on n-type GaSb substrate with a 50% cutoff wavelength at 10 mum. The surface leakage occurring after flip-chip bonding and underfill in the Type-II devices was suppressed using a double heterostructure design. The R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> A of diodes passivated with SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> was 23 Omegamiddotcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> after underfill. A focal plane array hybridized to an Indigo readout integrated circuit demonstrated a noise equivalent temperature difference of 33 mK at 81 K, with an integration time of 0.23 ms.

Electroluminescence of InAs–GaSb Heterodiodes

The electroluminescence of a Type II InAs-GaSb superlattice heterodiode has been studied as a function of injection current and temperature in the spectral range between 3 and 13 /spl mu/m. The heterodiode comprises a Be-doped midwavelength infrared (MWIR) superlattice with an effective bandgap around 270 meV and an undoped long wavelength infrared (LWIR) superlattice with an effective bandgap of 115 meV. At high injection currents and elevated temperatures the band to band transitions of both superlattices can be observed. By increasing the temperature the intensity of the MWIR emission component shows a well defined thermally activated increase. The activation energy of the Beryllium doping was evaluated to be 28 meV.

Metal organic chemical vapor deposition of metaphorphic InAs–GaSb superlattices on (0 0 1) GaAs substrates for mid-IR photodetector applications

InAs/GaSb superlattices (SLs) were grown on (0 0 1) GaAs substrates by metal organic chemical vapor deposition. A thin low temperature (LT) GaSb nucleation layer followed by a 250 nm thick high temperature (HT) GaSb buffer layer were introduced before the growth of the SLs and their growth conditions were optimized. The effects of growth temperature and GaSb growth rate on the morphology and structural properties of the SL were systematically studied. Atomic force microscopy, photoluminescence, transmission electron microscopy and X-ray diffraction were used to characterize the grown structures. Generally, nanopipes, about 100 nm in diameter, were found in the grown structures and their formation depends strongly on the growth temperature and the GaSb growth rate in growing the SL. By optimizing these parameters, high-quality InAs/GaSb SLs without nanopipes and with sharp interfaces have been realized.

Negative and positive luminescence in midwavelength infrared InAs-GaSb superlattice photodiodes

The quantum efficiency of negative and positive luminescence in binary type-II InAs-GaSb superlattice photodiodes has been investigated in the midinfrared spectral range around the 5-/spl mu/m wavelength. The negative luminescence efficiency is nearly independent on temperature in the entire range from 220 to 325 K. For infrared diodes with a 2-/spl mu/m absorbing layer, processed without anti-reflection coating, a negative luminescence efficiency of 45% is found, indicating very efficient minority carrier extraction. The temperature dependent measurements of the quantum efficiency of the positive luminescence enables for the determination of the capture cross section of the Shockley-Read-Hall centers involved in the competing nonradiative recombination.

Bond Orbital Model with Microscopic Interface Effects

The interface perturbations at the atomic scale are neglected in the bond orbital model (BOM) due to the unit-cell-scale orbitals. This makes the breakdown of the higher symmetry Hamiltonian of BOM used for calculating the microscopic interface phenomena. To improve this problem, we introduce the intracell effects into the BOM by expanding the BOM basis in terms of the tetrahedral (anti)bonding orbitals and using the potential operator instead of the scalar potential for the extraction of the microscopic information. The modified BOM method provides the direct insight into the microscopic symmetry of the crystal chemical bonds in the vicinity of the heterostructure interfaces. It indicates more physical meanings and easier manipulations than the other methods in the mathematical calculations to discuss the symmetry breaking phenomena. The InAs–GaSb superlattice as an example can support the proposed model well.

Long-wavelength type-II photodiodes operating at room temperature

The operation of uncooled InAs-GaSb superlattice photodiodes with a cutoff wavelength of /spl lambda//sub c/=8 /spl mu/m and a peak detectivity of 1.2/spl times/10/sup 8/ cmHz/sup 1/2//W at zero bias is demonstrated. The detectivity is similar to the best uncooled HgCdTe detectors and microbolometers. However, the R/sub 0/A product is more than two orders of magnitude higher than HgCdTe and the device is more than four orders of magnitude faster than microbolometers. These features combined with their low 1/f noise and high uniformity make these type-II photodiodes an excellent choice for uncooled high-speed IR imaging arrays.

Uncooled InAs-GaSb type-II infrared detectors grown on GaAs substrates for the 8-12-μm atmospheric window

We report on the growth and characterization of type-II infrared detectors with an InAs-GaSb superlattice active layer for the 8-12-/spl mu/m atmospheric window at 300 K. The material was grown by molecular beam epitaxy on semi-insulating GaAs substrates. Photoconductive detectors fabricated from the superlattices showed 80% cutoff at about 12 /spl mu/m at room temperature. The responsivity of the device is about 2 mA/W with a 1-V bias (E=5 V/cm) and the maximum measured detectivity of the device is 1.3/spl times/10/sup 8/ cm.Hz/sup 1/2//W at 11 /spl mu/m at room temperature. The detector shows very weak temperature sensitivity. Also, the extracted effective carrier lifetime, /spl tau/=26 ns, is an order of magnitude longer than the carrier lifetime in HgCdTe with similar bandgap and carrier concentration.

Electronic Properties of Semimetallic Heterojunctions

The properties of InAs–GaSb heterojunctions containing a semimetallic electron–hole double layer at the heterointerface have been considered. By self-consistent calculations of interface accumulation and inversion layers, band diagrams, capacitance–voltage and current–voltage characteristics were analyzed for all possible combinations of the semiconductor doping types, namely, for n–p, n–n, p–p and p–n-heterojunctions. The n-InAs–p-GaSb junction containing accumulation layers in both semiconductors, has the most unusual properties including the capacitance growth with the increase of contact potential and metallic conductivity caused by activationless generation–recombination processes at the interface. A phenomenological theory of these processes is developed, which can also describe the conductivity of long-period InAs–GaSb superlattices. One more remarkable effect is the strong frequency dispersion of capacitance in p-InAs–n-GaSb junctions caused by the re-charging of a single or double inversion layer at the interface.

Growth of [formula omitted] strained layer superlattices by MOVPE III. Use of UV absorption to monitor alkyl stability in the reactor

InAsGaSb strained layer superlattices have been grown by atmospheric pressure MOVPE and the growth conditions optimised by observing, in real time, the in-situ UV absorption of the alkyls in the growth chamber. The Raman scattering of folded longitudinal acoustic phonons in the superlattices has been used as a probe of the periodicity of the superlattice. Atomic force microscopy has also been used to give information about the final surface morphology and RMS roughness of the superlattices. By combining all three techniques, optimum conditions have been found for the growth of short period InAsGaSb superlattices. These have been used to sandwich a long period superlattice designed for transport measurements. The use of the short period superlattices eliminated additional conducting layers at each end of the semimetallic superlattice and produced structures where the hole and electron densities are equal. Such structures exhibit a dramatic new quantum transport effect where the Hall resistance goes to zero at high pressures and low temperatures.

Polaron in a superlattice with δ-like potentials

The problem of the polaron spectrum is studied in a superlattice having narrow quantum wells and relatively wide potential barriers. A δ-like superlattice potential is chosen to solve the problem. This model is adequate, if the penetration depth of the electron wave function into the barrier region is much greater than the width of the quantum well. A weak-coupling polaron at low temperature is studied. Only volume phonons are considered. Expressions are obtained for the polaron mass and the shift of the polaron energy under these assumptions. To test the model, numerical calculations were performed for an InAs-GaSb superlattice, whose quantum wells are quite deep (the energy offset of the conduction bands in InAs and GaSb equals 830 meV), narrow (the width of a quantum well corresponds to the width of an InAs monolayer 6 A), and the barrier width corresponding to the thickness of the GaSb layers equals 150 A. The assumption that the penetration depth of the wave function is much greater than the barrier width holds well.

Interface and layer thickness dependence of the effective mass in [formula omitted] superlattices studied by high field cyclotron resonance

The effective mass has been studied in a series of semiconducting and semimetallic InAs GaSb superlattices as a function of superlattice period and band gap. The mass is found to be determined almost entirely by the superlattice period, and is relatively insensitive to both the ratio of the InAs to GaSb thickness, and the structure of the interface layer, which was grown as both a monolayer of InSb and GaAs. Using pulsed magnetic fields of up to 180 T it was possible to observe spin-split cyclotron resonance at room temperature for all samples studied. On cooling the spin-splitting was found to be temperature dependent. This is attributed to the importance of electron-electron interactions which couple the two spin transitions as observed recently in high purity GaAs GaAlAs heterojunctions at low temperatures.

Recombination lifetime in InAs–Ga1−xInxSb superlattices

We report an experimental investigation of Shockley–Read and Auger lifetimes in InAs–Ga1−xInxSb and InAs–GaSb superlattices, based on measurements of the photoconductive response to excitation by a frequency-doubled CO2 laser (4.63 μm) at intensities up to 100 kW/cm2. Results at 77 K for low excitation levels yield Shockley–Read lifetimes between 0.13 and 6 ns. The scaling of the lifetime with carrier concentration also provides the first determinations of Auger coefficients in narrow-gap type II superlattices: γ3≊8×10−25 cm6/s at 300 K and γ3≊1.3×10−27 cm6/s at 77 K. The observed Auger lifetime at 77 K is two orders of magnitude longer than that in Hg1−xCdxTe with the same energy gap, which has highly favorable implications for InAs–Ga1−xInxSb superlattices in both IR detector and nonlinear optical applications.

TYPE-II SUPERLATTICES AND VARIABLE OVERLAP SUPERLATTICES IN TOTAL INTERNAL REFLECTION SWITCHES FOR THE LONGWAVE INFRARED

We derive explicit criteria for the properties required of a semiconductor nonlinear medium suitable for use in all-optical switching devices employing total internal reflection. Transmission as a function of laser intensity and film thickness has been calculated using a realistic model for penetration of the evanescent beam under TIR conditions. Requirements based on these results include a large nonlinear refractive index, large index change at saturation and small absorption coefficient. We show that unlike previously-studied semimetals and narrow-gap semiconductors, Type-II superlattices such as InAs-GaSb and variable-overlap superlattices (variants of Type-II which include a spacer between the layer containing the conduction-band minimum and that containing the valence band maximum) such as InAs-AlSb-GaSb hold prospects for satisfying all of these requirements simultaneously. As the free carrier lifetime will have a crucial influence on device performance, we have initiated a systematic experimental study of electron-hole recombination in InAs-based superlattices. From degenerate and nondegenerate four-wave mixing experiments, we have also determined nonlinear optical coefficients as a function of difference frequency and intensity. An InAs-GaSb superlattice has been found to display a refractive index change of ≈ 0.1, as well as device figures of merit which slightly surpasses any previously reported for weakly-saturating nonlinearities at CO 2 wavelengths. It is anticipated that future experiments on Type-II superlattices with longer lifetimes may yield nearly two orders of magnitude additional improvement in the nonlinear refractive index.

Electronic structure of the InAs-GaSb superlattice studied by the renormalization method.

The electronic band structure of the InAs-GaSb superlattice is studied within the localized-orbital framework by the renormalization method. The tight-binding Hamiltonian includes spin-orbit coupling and an accurate description of the composing crystals near relevant band edges. The tight-binding renormalization-group method is described in detail, and is shown to be conceptually simple and operatively efficient. The superlattice band structure is calculated for several unit-cell widths, and the semiconductor-semimetal transition is analyzed. A negative indirect band gap appears for long-period superlattices; its origin and the effects of band anisotropy and spin-orbit splitting are discussed.