Strained-layer Superlattices Research Articles

Hole diffusion length and mobility of a long wavelength infrared InAs/InAsSb type-II superlattice nBn design

We demonstrated high-performance 8.9 μm cutoff wavelength nBn InAs/InAsSb type-II strained-layer superlattice (T2SL). These detectors exhibit a long minority carrier (hole) lifetime of 1.2 μs at 80 K, high quantum efficiency of 40% for back-side illuminated devices without antireflection coating, and low dark current density of 4.6 × 10−6 A/cm2 at 80 K. We measured absorption, minority carrier (hole) lifetime, quantum efficiency, and spectral response as a function of the temperature and applied bias. We investigated the temperature dependence of the hole diffusion length and mobility and found that their values increase with temperature from 1.3 μm and 6.5 cm2/Vs at 30 K to 6.5 μm and 36 cm2/Vs at T = 90 K. We compared the measured diffusion length and mobility of holes in long-wavelength infrared (LWIR) T2SL with these parameters of a high-performance mid-wavelength infrared (MWIR) T2SL. Unexpectedly, hole mobility in LWIR T2SL was found to be higher than in MWIR that is contrary to the theoretical predictions.

Prediction of Shockley–Read–Hall lifetimes in strained layer superlattices for mid-wave and long-wave infrared photodetectors

We have calculated carrier nonradiative recombination lifetimes limited by Shockley–Read–Hall (SRH) centers in strained layer superlattices (SLSs) for mid-wave and long-wave infrared applications. The capture rate of an electron (hole) in the SLS's conduction (valence) band by the defect level is dominated by a multi-phonon process, which is orders-of-magnitude more efficient than the radiative process. Long-range polar coupling between electrons and optical phonons can account for the observed SRH lifetimes in a variety of SLSs reported in the literature. The capture rate depends on temperature rather weakly, consistent with experimental observations. The efficient capture is caused by the comparable electronic difference and lattice relaxation energy, Ect∼Sℏω, with S and ℏω being the Huang–Rhys factor and optical photon energy in the SLSs. A weaker polar coupling would give rise to a smaller capture cross section, which, for InAs/InAsSb SLSs, can be achieved by increasing Sb in the alloy region.

Hybrid Signal Processing for Automatic Detection and Removal of Material Absorption and Noise From Microsphere-Lens-Enhanced Midwave Infrared Type II Strained Layer Superlattice Photodetectors

Hybrid Signal Processing for Automatic Detection and Removal of Material Absorption and Noise From Microsphere-Lens-Enhanced Midwave Infrared Type II Strained Layer Superlattice Photodetectors

Evaluating Phonon Characteristics by Varying the Layer and Interfacial Thickness in Novel Carbon-Based Strained-Layer Superlattices

Systematic results of lattice dynamical calculations are reported as a function of m and n for the novel (SiC)m/(GeC)n superlattices (SLs) by exploiting a modified linear-chain model and a realistic rigid-ion model (RIM). A bond polarizability method is employed to simulate the Raman intensity profiles (RIPs) for both the ideal and graded (SiC)10-Δ/(Si0.5Ge0.5C)Δ/(GeC)10-Δ/(Si0.5Ge0.5C)Δ SLs. We have adopted a virtual-crystal approximation for describing the interfacial layer thickness, Δ (≡0, 1, 2, and 3 monolayers (MLs)) by selecting equal proportions of SiC and GeC layers. Systematic variation of Δ has initiated considerable upward (downward) shifts of GeC-(SiC)-like Raman peaks in the optical phonon frequency regions. Our simulated results of RIPs in SiC/GeC SLs are agreed reasonably well with the recent analyses of Raman scattering data on graded short-period GaN/AlN SLs. Maximum changes in the calculated optical phonons (up to ±~47 cm−1) with Δ = 3, are proven effective for causing accidental degeneracies and instigating localization of atomic displacements at the transition regions of the SLs. Strong Δ-dependent enhancement of Raman intensity features in SiC/GeC are considered valuable for validating the interfacial constituents in other technologically important heterostructures. By incorporating RIM, we have also studied the phonon dispersions [ωjSLq→] of (SiC)m/(GeC)n SLs along the growth [001] as well as in-plane [100], [110] directions [i.e., perpendicular to the growth]. In the acoustic mode regions, our results of ωjSLq→ have confirmed the formation of mini-gaps at the zone center and zone edges while providing strong evidences of the anti-crossing and phonon confinements. Besides examining the angular dependence of zone-center optical modes, the results of phonon folding, confinement, and anisotropic behavior in (SiC)m/(GeC)n are compared and contrasted very well with the recent first-principles calculations of (GaN)m/(AlN)n strained layer SLs.

The growth of low-threading-dislocation-density GaAs buffer layers on Si substrates

Monolithic integration of III–V optoelectronic devices on Si platform is gaining momentum, since it enables advantages of low cost, less complexity and high yield for mass production. With the aim of achieving advances in monolithic integration, the challenges associated with lattice mismatch between III–V layers and Si substrates must be overcome, as a low density of threading dislocations (TDs) is a prerequisite for the robustness of the integrated devices. In this paper, we have investigated and compare different types of dislocation filter layers (DFLs) from InGaAs asymmetric step-graded buffer layer (ASG), InGaAs/GaAs strained-layer superlattices, and quaternary alloy InAlGaAs ASG, on the functionality of reducing TD density (TDD) for GaAs buffer layers on Si. Compared to other DFLs, the sample with InAlGaAs ASG buffer layer shows the lowest average TDD value and roughness, while the decrease of TDD in the sample with InAlGaAs ASG buffer layer can be understood in terms of the hardening agent role of aluminium in the InAlGaAs ASG. By further optimising the InAlGaAs ASG through thermal cyclic annealing, we successfully demonstrate a low surface TDD of 6.3 ± 0.1 × 106 cm−2 for a 2 μm GaAs/InAlGaAs ASG buffer layer grown on Si. These results could provide a thin buffer design for monolithic integration of various III–V devices on Si substrates.

GaAs/Si Tandem Solar Cells with an Optically Transparent InAlAs/GaAs Strained Layer Superlattices Dislocation Filter Layer

Epitaxial growth of III–V materials on Si is a promising approach for large-scale, relatively low-cost, and high-efficiency Si-based multi-junction solar cells. Several micron-thick III–V compositionally graded buffers are typically grown to reduce the high threading dislocation density that arises due to the lattice mismatch between III–V and Si. Here, we show that optically transparent n-In0.1Al0.9As/n-GaAs strained layer superlattice dislocation filter layers can be used to reduce the threading dislocation density in the GaAs buffer on Si while maintaining the GaAs buffer thickness below 2 μm. Electron channeling contrast imaging measurements on the 2 μm n-GaAs/Si template revealed a threading dislocation density of 6 × 107 cm−2 owing to the effective n-In0.1Al0.9As/n-GaAs superlattice filter layers. Our GaAs/Si tandem cell showed an open-circuit voltage of 1.28 V, Si bottom cell limited short-circuit current of 7.2 mA/cm2, and an efficiency of 7.5%. This result paves the way toward monolithically integrated triple-junction solar cells on Si substrates.

Complementary Barrier Infrared Detector Architecture for Long-Wavelength Infrared InAs/InAsSb Type-II Superlattice

We describe the challenges for long- and very long-wavelength InAs/InAsSb type-II strained-layer superlattice infrared detectors, and provide an overview of progress in device architecture development for addressing them. Specifically, we have explored the complementary barrier infrared detector (CBIRD) that contains p-type InAs/InAsSb T2SLS absorbers for enhancing quantum efficiency, while also suppressing surface shunt current. We describe selected device results, and also provide references to additional results and more in-depth discussions.

Reduced Dislocation of GaAs Layer Grown on Ge-Buffered Si (001) Substrate Using Dislocation Filter Layers for an O-Band InAs/GaAs Quantum Dot Narrow-Ridge Laser.

The development of the low dislocation density of the Si-based GaAs buffer is considered the key technical route for realizing InAs/GaAs quantum dot lasers for photonic integrated circuits. To prepare the high-quality GaAs layer on the Si substrate, we employed an engineered Ge-buffer on Si, used thermal cycle annealing, and introduced filtering layers, e.g., strained-layer superlattices, to control/reduce the threading dislocation density in the active part of the laser. In this way, a low defect density of 2.9 × 107 cm−2 could be achieved in the GaAs layer with a surface roughness of 1.01 nm. Transmission electron microscopy has been applied to study the effect of cycling, annealing, and filtering layers for blocking or bending threading-dislocation into the InAs QDs active region of the laser. In addition, the dependence of optical properties of InAs QDs on the growth temperature was also investigated. The results show that a density of 3.4 × 1010 InAs quantum dots could be grown at 450 °C, and the photoluminescence exhibits emission wavelengths of 1274 nm with a fullwidth at half-maximum (FWHM) equal to 32 nm at room temperature. The laser structure demonstrates a peak at 1.27 μm with an FWHM equal to 2.6 nm under a continuous-wave operation with a threshold current density of ∼158 A/cm2 for a 4-μm narrow-ridge width InAs QD device. This work, therefore, paves the path for a monolithic solution for photonic integrated circuits when III−V light sources (which is required for Si photonics) are grown on a Ge-platform (engineered Ge-buffer on Si) for the integration of the CMOS part with other photonic devices on the same chip in near future.

Demonstration of room-temperature continuous-wave operation of InGaAs/AlGaAs quantum well lasers directly grown on on-axis silicon (001)

Room-temperature continuous-wave operation of InGaAs/AlGaAs quantum well lasers directly grown on on-axis silicon (001) has been demonstrated. A 420 nm thick GaAs epilayer completely free of antiphase domains was initially grown on the silicon substrate in a metal-organic chemical vapor deposition system and the other epilayers, including four sets of five-period strained-layer superlattices and the laser-structural layers, were successively grown in a molecular beam epitaxy system. The lasers were prepared as broad-stripe Fabry–Pérot ones with a stripe width of 21.5 μm and a cavity length of 1 mm. Typically, the threshold current and the corresponding threshold current density are 186.4 mA and 867 A/cm2, respectively. The lasing wavelength is around 980 nm, and the slope efficiency is 0.097 W/A with a single-facet output power of 22.5 mW at an injection current of 400 mA. This advancement makes the silicon-based monolithic optoelectronic integration relevant to quantum well lasers more promising with an enhanced feasibility.

Measuring Non-Destructively the Total Indium Content and Its Lateral Distribution in Very Thin Single Layers or Quantum Dots Deposited onto Gallium Arsenide Substrates Using Energy-Dispersive X-ray Spectroscopy in a Scanning Electron Microscope.

The epitaxial deposition of a precise number, or even fractions, of monolayers of indium (In)-rich semiconductors onto gallium arsenide (GaAs) substrates enables the creation of quantum dots based on InAs, InGaAs and indium phosphide (InP) for infrared light-emitting and laser diodes and the formation of indium antimonide (InSb)/GaAs strained layer superlattices. Here, a facile method based on energy-dispersive X-ray spectroscopy (EDXS) in a scanning electron microscope (SEM) is presented that allows the indium content of a single semiconductor layer deposited on a gallium arsenide substrate to be measured with relatively high accuracy (±0.7 monolayers). As the procedure works in top-down geometry, where any part of a wafer can be inspected, measuring the In content of the surface layer in one location without destroying it can also be used to map the lateral indium distribution during quantum dot formation and is a method suitable as an in-situ quality control tool for epitaxy.

Comparative Study on Planar Type-II Strained-Layer Superlattice Infrared Photodetectors Fabricated by Ion-Implantation

Recent progress in Type-II strained layer superlattice (SLS) material systems has offered viable alternatives towards achieving large format, small-pitch, and low-cost focal plane arrays for different military and commercial applications. For focal plane array fabrication, in order to address difficulties associated with mesa-isolation etching or the complex surface treatment/ passivation process, planar structures have been considered. In this work, a comparative study on the recent progress on the planar SLS photodetector using ion-implantation for device isolation is presented. The devices presented here are nBn and pBn heterostructure InAs/InAsSb SLS photodetectors, where Zn and Si were chosen as the ion implants, respectively. The electrical and optical performance of the planar devices were compared to each other and with associated mesa-etched fabricated devices, to give a deeper view of the device performance.

Prediction of Shockley-Read-Hall Centers in Strained Layer Superlattices for Mid-Wave Infrared Photodetectors

Prediction of Shockley-Read-Hall Centers in Strained Layer Superlattices for Mid-Wave Infrared Photodetectors

Compact thermal imager: a flight demonstration of infrared technology for Earth observations.

During 2019, an infrared camera, the compact thermal imager (CTI), recorded 15 million images of the Earth from the International Space Station. CTI is based on strained-layer superlattice (SLS) detector technology. The camera covered the spectral range from 3 to 11 µm in two spectral channels, 3.3-5.4 and 7.8-10.7 µm. Individual image frames were 26×21km2 projected on the ground, with 82 m pixel resolution. A frame time of 2.54 s created continuous image swaths with a 13% along-track image overlap. Upper limits determined on the ground and in flight for the electronic offset, read noise, and dark current demonstrated the stability of the SLS detector and camera over many months. Temperature calibration was established using a combination of preflight and in-flight measurements. A narrowband approximation of temperature as a function of photon counts produced an analytic relationship covering a temperature range of 0°-400°C. Examples of CTI images illustrate temperature retrievals over sea ice, urban and agricultural areas, desert, and wildfires.

Short-period InAsSb-based strained layer superlattices for high quantum efficiency long-wave infrared detectors

Infrared detector barrier heterostructures with strained layer superlattice (SLS) absorbers with different periods were compared. The first was a reference using a conventional barrier heterostructure with a low temperature energy gap corresponding to a wavelength of 10 μm in a 2-μm-thick undoped absorber using a 10.9 nm period with InAs/InAsSb0.36 compositions grown directly on a GaSb substrate. The second structure, in contrast, used a significantly shorter 4.3 nm period absorber with InAsSb0.3/InAsSb0.55 compositions, similar energy gap, and absorber thickness, which were grown on a 6.2 Å lattice constant GaIn0.3Sb virtual substrate on GaSb. It was found that in the short period SLS, the vertical hole mobility and minority carrier lifetime in the temperature range of 80–150 K were a factor on 2–3 greater than in the reference structure. The improvement of the vertical hole mobility was attributed to the effect of hole delocalization. The latter results in an increase in the optical absorption coefficient and the quantum efficiency.

Low Dark Current and High Speed InGaAs Photodiode on CMOS-Compatible Silicon by Heteroepitaxy

Top-illuminated proof-of-concept indium gallium arsenide (InGaAs) photodiode (PD) array and high speed InGaAs PDs were realized on (001) silicon (Si) substrate by direct heteroepitaxy using metal–organic chemical vapor deposition. The PDs containing InGaAs active layer lattice-matched to InP were grown on Si substrates employing InP-on-Si template with growth technique including GaAs on V-grooved Si, thermal cycle annealing and strained layer superlattice defect filters. Dry etched mesa structure with polyimide passivation was implemented. 8 × 8 InGaAs PD arrays with mesa diameters of 20, 30 and 40 µm were realized on (001) Si, and their performances were benchmarked with identical devices on a native InP substrate. A low dark current density of 0.45 mA/cm² and a responsivity of 0.72 A/W at 1550 nm were measured for 40-µm-diameter device on Si. Directly modulation has also been performed showing a 3-dB bandwidth up to 11.2 GHz and open eye diagram up to 25 Gbps. The imaging performance of InGaAs PD arrays on Si were also demonstrated for the first time by capturing the output beam profile of a single mode fiber at 1550 nm.

Approaches to low-cost infrared sensing.

The Air Force Research Laboratory's Sensors Directorate has multiple missions, including the development of next generation infrared sensors. These sensors reflect advancements in both academic and research communities, as well as requirements flow-down from operators. There has been a multitude of developments over the past decade in each community. However, there has also been consilience that low-cost infrared sensing will be necessary for the Air Force. This paradigm stands in contrast to the current generation of high performance infrared sensors, i.e., cryogenically cooled, hybridized HgCdTe, InSb, and III/V strained layer superlattices. The Sensors Directorate currently has a multi-pronged approach to low-cost infrared sensing to meet this paradigm shift, including research in silicides, SiGeSn, and lead salts. Each of these approaches highlights our integration of materials, devices, and characterization.

InAsSb-based detectors on GaSb for near-room-temperature operation in the mid-wave infrared

III-Sb barrier detectors suitable for the mid-wave infrared were grown on GaSb by molecular beam epitaxy. Using both bulk-InAsSb and an InAsSb–InAs strained layer superlattice, operation close to room temperature was demonstrated with cutoff wavelengths of 4.82 and 5.79 μm, respectively, with zero-bias operation possible for the bulk-InAsSb detector. X-ray diffraction, temperature dependent dark current, and spectral quantum efficiency were measured, and an analysis based on calculated specific detectivity was carried out. 1/f noise effects are considered. Results indicate that these optimized devices may be suitable as alternatives to InSb, or even HgCdTe, for many applications, especially where available power is limited.

(Invited) Recent Advances in the Modeling of ZnSySe1-y / GaAs (001) Heterostructures with Application to Dislocation Sidewall Gettering

In this talk we will describe state-of-the-art approaches to the modeling of strain relaxation and dislocations in ZnSySe1-y/GaAs (001) heterostructures, with applications to dislocation sidewall gettering (DSG) in devices. Current modeling approaches are based on the extension of the original Dodson and Tsao plastic flow model [B. W. Dodson and J. Y. Tsao, Appl. Phys. Lett., 51, 1325 (1987); Appl. Phys. Lett., 52, 852 (1988)] to include compositional grading and multilayers, dislocation interactions, and differential thermal expansion. Important recent breakthroughs have greatly enhanced the utility of these modeling approaches in three respects: i) pinning interactions have been included in graded and multilayered structures, providing a better description of the rate of strain relaxation as well as the limiting strain; ii) a refined model describing the interaction length for dislocation-dislocation interactions was formulated to include jogging in compositionally-graded and step-graded layers; and iii) inclusion of back-and-forth weaving of dislocations provides a more accurate description of heterostructures containing strain reversals, such as strained-layer superlattices or overshoot graded layers. We will describe these three key advances and use reasonable estimates of the kinetic parameters for ZnSySe1-y to explain and illustrate practical features of dislocation sidewall gettering in II-VI heterostructures.

(Invited) Recent Advances in the Modeling of ZnSySe1-y / GaAs (001) Heterostructures with Application to Dislocation Sidewall Gettering

In this paper we describe state-of-the-art approaches to the modeling of strain relaxation and dislocations in ZnSSe/GaAs (001) heterostructures, with applications to dislocation sidewall gettering (DSG) in devices. Current modeling approaches are based on the extension of the original Dodson and Tsao plastic flow model [B. W. Dodson and J. Y. Tsao, Appl. Phys. Lett., 51, 1325 (1987); Appl. Phys. Lett., 52, 852 (1988)] to include compositional grading and multilayers, dislocation interactions, and differential thermal expansion. Important recent breakthroughs have greatly enhanced the utility of these modeling approaches in three respects: i) pinning interactions have been included in graded and multilayered structures, providing a better description of the rate of strain relaxation as well as the limiting value; ii) a refined model describing the interaction length for dislocation-dislocation interactions was formulated to include jogging in compositionally-graded and step-graded layers; and iii) inclusion of back-and-forth weaving of dislocations provides a more accurate description of heterostructures containing strain reversals, such as strained-layer superlattices or overshoot graded layers. We will describe these three key advances and use reasonable estimates of the kinetic parameters for ZnSSe to explain and illustrate practical features of dislocation sidewall gettering in II-VI heterostructures.

E‐band InAs quantum dot laser on InGaAs metamorphic buffer layer with filter layer

InAs quantum dots (QDs) are receiving attention as next-generation E-band light source that offers high-temperature operation and temperature insensitive operation. However, high-density crystal defects occur at the interface between the InGaAs buffer layer and GaAs, resulting in reduced device performance and shortened lifetime. Here, E-band QD lasers are demonstrated on InGaAs buffer layer, which suppressed the spread of dislocation by introducing a high-temperature annealing and a strained layer superlattice filter. In the device, the peak wavelength at room temperature is measured to be 1427 nm and the threshold current density was 440 A/cm2. This result indicates that E-band QD laser structures on low threading dislocation density are promising for the realisation of high-performance E-band lasers.