Strained-layer Superlattices Research Articles

Effect of growth technological conditions on the heterointerface thickness in the InAs/GaSb strained-layer superlattices grown by MOCVD

In this article, we investigated the effect of technological growth parameters by metal-organic chemical vapor deposition (MOCVD) method on the thickness of the transition layers, which affect the value of tensile strain in the structure, in the InAs/GaSb superlattice. We consider that the thickness of transition layers depends on the roughness of the growth surface and the technological conditions of growing single layers in the superlattice. At the first stage, we have determined the optimal annealing parameters for the minimization of the substrate roughness as T = 650 °C and t = 8 min, with a minimum value of 1.1 nm. At the second stage, studies were carried out on the effect of the sequence of elements and the delay time of reagent supply during the growth of InAs and GaSb layers on the quality of heterointerface. The stress analysis of the obtained structures was performed by reflectance anisotropy spectroscopy. It was shown that a superlattice becomes less tense with the increase in the number of pairs of alternating layers.

Transition levels of intrinsic defects in type-II InAs/InAs0.5Sb0.5 strained-layer superlattices

We report a first-principles study of the formation energies and transition energy levels of intrinsic point defects, including In and As vacancies, antisites, and interstitials, in the InAs and InAs0.5Sb0.5 regions of the type-II InAs/InAs0.5Sb0.5 strained-layer superlattices (SLSs). Both strain and the quantum confinement effects are thoroughly studied. The transition levels of the defects calculated from the strained bulk InAs and InAsSb are aligned to the band edge states of the SLS. The calculations reveal that both the strain and the change of the SLS band edges have significant effects on the transition levels and change in turn the role of these defects in the recombination of carriers through the Shockley-Read-Hall mechanism.

Threading Dislocation Behavior in InGaAs/GaAs (001) Superlattice Buffer Layers

We conducted a modeling study of the threading dislocation behavior in chirped and unchirped InGaAs/GaAs (001) strained-layer superlattices (SLSs) using a Dodson & Tsao / Kujofsa & Ayers (DTKA) type plastic flow model. Four types of SLSs were investigated: type I was chirped using compositional modulation, type II was chirped using layer thickness modulation, type III was unchirped with alternating layers of InGaAs and GaAs, and type IV was unchirped with alternating layers of InGaAs having two different compositions. Generally the surface and average values of the dislocation density decreased with increasing total thickness. The dependence on top indium composition was more complex, due to dislocation compensation and multiplication effects, but for type II and IV superlattices, the average and surface threading dislocation densities increased in nearly monotonic fashion with top indium composition. Based on these results, the compositionally-modulated chirped (type I) and InGaAs/GaAs unchirped (type III) superlattices appear to be best suited as buffer layers for metamorphic devices, while the chirped superlattices with layer thickness modulation (type II) and InGaAs/InGaAs unchirped (type IV) superlattices appear to be poorly suited for use as buffer layers for devices containing high indium content.

A Modeling Study of Dislocation Behavior in InGaAs/GaAs (001) and InAlGaAs/GaAs (001) Heterostructures Utilizing Strained-Layer Superlattices

There have been a number of studies of dislocation filtering by strained-layer superlattices incorporated into semiconductor heterostructures. Despite the extensive nature of this body of work, a general understanding of the phenomenon has proved elusive, preventing wide application of SLS dislocation filters in practical devices. However, recent advances in the understanding of dislocation dynamics and the consequent development of detailed plastic flow models allow the prediction of threading dislocation density profiles in general semiconductor heterostructures. In this work, we have applied such a plastic flow model to InGaAs/GaAs (001) heterostructures containing an InGaAs-based SLS, and we have studied the dependence of the surface and average threading dislocation densities on the placement and design of the SLS.

Strained-layer-superlattice-based compact thermal imager for the International Space Station.

The compact thermal imager (CTI) is a dual-band, strained-layer-superlattice (SLS) detector-based instrument that was installed on the exterior of the International Space Station (ISS) in conjunction with the third Robotic Refueling Mission 3 (RRM3) in 2018. The CTI serves as a pathfinder for future thermal infrared capability on Landsat. The CTI incorporates an SLS hybrid, a dual-band 3-5 and 8-10 μm, electrically switchable, 320×256 array with 30 μm2 pixels, bonded to an Indigo ISC0903 Readout Integrated Circuit (ROIC). The telescope was built around an integrated detector cryocooler assembly developed under a NASA Small Business Innovative Research award with QmagiQ, LLC. The cooler is a Ricor K508 and the front-end optics is a custom-designed, doublet lens telescope with a 150 mm focal length. The ground resolution is 80 meters/pixel from the ISS altitude of 400 km. A filter creates two spectral channels from the dual bands, 3.3-5.4 and 7.8-10.2 μm. The detector hybrid control electronics is a custom-developed system based on the Teledyne Imaging Systems SIDECAR Application-Specific Integrated Circuit. This module provides the electronic interface from the RRM3 SpaceCube on-board processor to the detector/ROIC assembly. The primary goal of this mission was to perform a technology demonstration of the SLS technology and the commercial cooler technology elevating the Technology Readiness Level (TRL) to TRL 9 on a bare-bones budget and relatively fast development cycle. Some science objectives include locating fires, approximating land surface temperatures, and monitoring evapotranspiration, sea ice, and glacier dynamics. In this paper, we will present the design of the focal plane, optics, electronics, and mechanical structure of the CTI. We will also describe the operation and qualification tests that were performed to bring the CTI to the NASA TRL 6 in preparation for the launch on a SpaceX Dragon from the Kennedy Space Center.

A Modeling Study of Dislocation Behavior in InGaAs/GaAs (001) and InAlGaAs/GaAs (001) Heterostructures Utilizing Strained-Layer Superlattices

There have been a number of studies of dislocation filtering by strained-layer superlattices (SLSs) incorporated into semiconductor heterostructures. Despite the extensive nature of this body of work, a general understanding of the phenomenon has proved elusive, preventing wide application of SLS dislocation filters in practical devices. However, recent advances in the understanding of dislocation dynamics and the consequent development of detailed plastic flow models allow the prediction of threading dislocation density profiles in general semiconductor heterostructures. In this work, we have extended the usefulness of the plastic flow models by accounting for the weaving and zagging of dislocations at mismatched interfaces when calculating the effective interaction length for annihilation and coalescence reactions between dislocations. The refined plastic flow model has been applied to InGaAs/GaAs (001) and InGaAlAs / GaAs (001) heterostructures containing InGaAs and AlGaAs SLSs, and we have studied the dependence of the threading dislocation density profiles on the placement and design of the SLS. The refined model shows qualitative agreement with experimental results for SLS dislocation filters.

Development of InAs/InAsSb Type II Strained-Layer Superlattice Unipolar Barrier Infrared Detectors

We recently reported mid-wavelength infrared (MWIR) InAs/InAsSb type II strained-layer superlattice (T2SLS) unipolar barrier detectors and focal-plane arrays with significantly higher operating temperature than InSb. Herein, we document the development leading to the MWIR InAs/InAsSb T2SLS detectors at the NASA Jet Propulsion Laboratory. We also briefly compare the InAs/InAsSb T2SLS with some other approaches.

Optical and Electrical Study of InAs/GaSb Type II Strained Layer Superlattice for Mid-wave Infrared Detector

This paper reports the results of modeling of optical and electrical characteristics of InAs/GaSb type II strained layer superlattice (SLS) for the mid-wave infrared detection with n-on-p polarity. The band gap calculation of the SLS was conducted as a function of the InAs and GaSb thickness, using a modified Kronig-Penny model. The cut off wavelength of the fabricated diode was ~ 6 μm (~ 0.2 eV) at 120 K. The product of zero-bias resistance and area (R0A) as a function of an applied bias was investigated in detail to analyze the dark current mechanisms. The thermal diffusion and generation-recombination current were dominant factors under low positive bias. However, as the reverse bias voltage increased, the trap assisted component became one of the dominant dark current factors.

InAs/InAsSb Type-II Superlattice Mid-Wavelength Infrared Focal Plane Array With Significantly Higher Operating Temperature Than InSb

We report focal plane array (FPA) results on a mid-wavelength InAs/InAsSb type-II strained layer superlattice (T2SLS) unipolar barrier infrared detector with a cutoff wavelength of 5.4 μm. For 300 K background in the 3–5-μm band, f/2 aperture, an FPA operating at 150 K exhibits a mean noise equivalent differential temperature (NEDT) of 18.5 mK, and an NEDT operability of 99.7%. The NEΔT distribution has a width of 8 mK, with no noticeable distribution tail, indicating excellent uniformity. The mean noise-equivalent irradiance is 9.1 × 1011 photons/sec-cm2. The mean quantum efficiency is 49.1% without antireflection coating, and the mean specific detectivity (D*) is 2.53 × 1011 cm-Hz½/W. Benefitting from an absorber material with a much longer Shockley–Read–Hall minority carrier lifetime, and a device architecture that suppresses generation-recombination and surface-leakage dark current, the InAs/InAsSb T2SLS barrier infrared detector FPA has demonstrated a significantly higher operating temperature than the mid-wavelength infrared market-leading InSb.

Interfacial Mixing Analysis for Strained Layer Superlattices by Atom Probe Tomography

Quantum wells and barriers with precise thicknesses and abrupt composition changes at their interfaces are critical for obtaining the desired emission wavelength from quantum cascade laser devices. High-resolution X-ray diffraction and transmission electron microscopy are commonly used to calibrate and characterize the layers’ thicknesses and compositions. A complementary technique, atom probe tomography, was employed here to obtain a direct measurement of the 3-dimensional spatially-resolved compositional profile in two InxGa1−xAs/InyAl1−yAs III-V strained-layer superlattice structures, both grown at 605 °C. Fitting the measured composition profiles to solutions to Fick’s Second Law yielded an average interdiffusion coefficient of 3.5 × 10−23 m2 s−1 at 605 °C. The extent of interdiffusion into each layer determined for these specific superlattices was 0.55 nm on average. The results suggest that quaternary active layers will form, rather than the intended ternary compounds, in structures with thicknesses and growth protocols that are typically designed for quantum cascade laser devices.

1.3-μm InAs/GaAs quantum dots grown on Si substrates**Project supported by the National Key Research and Development Program of China (Grant No. 2018YFA0306101), the Scientific Instrument Developing Project of Chinese Academy of Sciences (Grant No. YJKYYQ20170032), and the National Natural Science Foundation of China (Grant Nos. 61790581, 61435012, and 61505196).

We compare the effect of InGaAs/GaAs strained-layer superlattice (SLS) with that of GaAs thick buffer layer (TBL) serving as a dislocation filter layer. The InGaAs/GaAs SLS is found to be more effective than GaAs TBL in blocking the propagation of threading dislocations, which are generated at the interface between the GaAs buffer layer and the Si substrate. Through testing and analysis, we conclude that the weaker photoluminescence for quantum dots (QDs) on Si substrate is caused by the quality of capping In0.15Ga0.85As and upper GaAs. We also find that the periodic misfits at the interface are related to the initial stress release of GaAs islands, which guarantees that the upper layers are stress-free.

Effect of substrate orientation on Sb-based MWIR photodetector characteristics

Effects of GaSb substrate orientation and surface polarity on performance of MWIR photodetectors (PD) were evaluated by comparing devices fabricated on (1 0 0), (2 1 1)A and B, and (3 1 1)A and B oriented substrates. Two types of PDs were evaluated: bulk InAsSb barrier PD with wavelength ∼4 µm, and type-II strained layer superlattice (T2SL) PD with wavelength 5.5 µm. Epitaxial structures were grown by solid source molecular beam epitaxy (MBE) on substrates with various orientations side-by-side in the same growth run. Material performance was evaluated by AFM, Nomarski microscopy, x-ray diffraction, 77 K photoluminescence (PL), and PD current-voltage and spectral testing. All wafers demonstrated reasonable surface morphology, with some variability in roughness from wafer to wafer. Bulk nBn devices fabricated on the high-index substrates show a blue shift up to 0.15 µm for both 77 K PL and for spectral cutoff wavelength compared to the same structure on the (1 0 0) substrate. Growth on high-index substrates also showed moderate reduction of quantum efficiency (QE) and variations in dark current (Jd). The (3 1 1)A and (2 1 1)A oriented structures exhibited the most significant Jd reduction, by a factor of ∼3 and ∼6, respectively. Substrate orientation induces more variation in the T2SL PD parameters, especially in Jd and QE. Here, the (2 1 1)B orientation demonstrates a red shift of the PL and cutoff wavelength by about 1 µm. These results suggest that high-index substrates can be explored to fine-tune and manipulate unique PD characteristic properties for specific applications.

Two-colour In0.5Ga0.5As quantum dot infrared photodetectors on silicon

An InGaAs quantum dot (QD) photodetector is directly grown on a silicon substrate. GaAs-on-Si virtual substrates with a defect density in the order of 106 cm−2 are fabricated by using strained-layer superlattice as dislocation filters. As a result of the high quality virtual substrate, fabrication of QD layer with good structural properties has been achieved, as evidenced by transmission electron microscopy and x-ray diffraction measurements. The InGaAs QD infrared photodetector is then fabricated on the GaAs-on-Si wafer substrate. Dual-band photoresponse is observed at 80 K with two response peaks around 6 and 15 μm.

Bonding of cysteamine on InAs surfaces

Passivation of InAs surfaces in InAs/GaSb type-II strained layer superlattices is important in many applications, and cysteamine has been effectively used for this purpose. However, there is a lack of fundamental understanding of the bonding between cysteamine and InAs. Here, first principles modeling, X-ray photoelectron spectroscopy (XPS), and high temperature XPS were used to characterize the adsorption of cysteamine on InAs surfaces. Our surface modeling results on InAs(1 0 0) showed that the computed adsorption energies were highest for InS and AsS bonds (both approaching 43 kcal/mol), and these values were influenced by the local reconstruction of the InAs(1 0 0) surface and, in some cases, the presence of neighboring cysteamine molecules. XPS results of as-received, control and cysteamine treated InAs(1 0 0) surfaces were consistent with InS and AsS bonding of cysteamine and indicated an indium-rich surface. High temperature XPS results up to 250 °C showed that sulfur persisted at the interface at higher temperatures while carbon and nitrogen were removed, suggesting that the InS and AsS bonds are more stable than the CS bond. Finally, near edge X-ray absorption fine structure (NEXAFS) measurements confirmed that the primary amine group of cysteamine was available for covalent bonding with potential potting materials.

Electroluminescence and photoluminescence of type-II InAs/InAsSb strained-layer superlattices in the mid-infrared

There is continuing interest in the development of superlattices for use in photonic devices operating in the technologically important mid-infrared spectral range. In this work type-II strained-layer superlattices of InAs/InAs1-xSbx (x = 0.04 and x = 0.06) were grown on InAs (1 0 0) substrates by MBE. Structural analysis of the samples revealed good crystalline quality but a non-uniform distribution of Sb within the QWs which originated from segregation effects during growth. Bright photoluminescence emission was obtained at low temperature (4 K) which persisted up to 300 K. Two prototype samples were grown containing the corresponding superlattices in the active region and fabricated into LEDs. Mid-infrared electroluminescence was obtained from both these LEDs over the temperature range 7–300 K and both devices exhibit emission coincident with the main CO2 absorption band near 4.2 μm at room temperature. These LEDs produced output powers of 8.2 µW and 3.3 µW under 100 mA injection current at room temperature and are of interest for CO2 detection and further development for mid-infrared gas sensing applications.

Ultra-short period Ga-free superlattice growth on GaSb

This work describes a thorough investigation of the structural properties of intended binary InAs/InSb strained layer superlattices (SLS) on GaSb substrates for infrared detection. The designed periods were as short as possible, with the InSb layers approaching one to two molecular monolayers. None of the examined growth conditions produced complete InSb layers. All samples showed a significant loss of Sb. The Sb that was incorporated was found to exhibit a spread in the growth direction, mainly due to step formation, and secondly due to forward diffusion. All structures, therefore became an InAs1-xSbx/InAs1-ySby SLS. The intended InAs layer had a Sb composition of x ∼ 0.003–0.010 and the intended InSb layer had composition y, ranging from ∼0.24–0.43. All terrace steps appeared to be in the same direction, resulting in a weak tilt of the SLS relative to the substrate normal. We discuss the implications for growth of high-Sb-concentration InAsSb/InAs structures.

Many-body perturbation theory study of type-II InAs/GaSb superlattices within the GW approximation

Recent studies suggest that the many-body perturbation theory in the partially self-consistent GW (GW0) approximation significantly improves the prediction of band gaps in various semiconductors. In this work, we employed GW formalism to study the electronic structure of type-II InAs/GaSb strained-layer superlattices (T2SLs). T2SLs considered in this study, denoted by (monolayers of InAs, monolayers of GaSb) are (), (), (), (), and (). The InSb-type interfacial layer was introduced in the structures to resemble the actual growth condition in our laboratories. The electronic band gaps are indirect in all the structures. The band gaps at the center of the Brillouin zone show good agreement with experimental data. This study is the first step to investigate the electronic, optical, and defect characteristics of T2SLs within a parameter-free ab initio method.

Development of a Long-Wave Infrared Band-Edge (LWIR BE) thermometry instrument.

Accurate measurement of substrate temperature is one of the most critical process control parameters for molecular beam epitaxy (MBE) growth. Band-edge thermometry instruments have proven to be a valuable tool for process control during MBE growth of semiconductor films, providing as high as ±1 °C temperature resolution. The increasing use of InAs, GaSb, and AlSb iii-v materials necessitates a method for accurately measuring the temperature of their closely lattice-matched GaSb substrates. Current-technology instruments typically rely on InGaAs detector materials which have a maximum wavelength λ detection of ∼1.7 μm, but GaSb substrates have a band gap energy corresponding to λ > 2 μm. A band-edge thermometry instrument capable of λ > 2 μm has been developed using an InAs/InGaSb strained-layer superlattice detector sensitive to 2-9.5 μm long-wave IR wavelengths.

Origin and suppression of critical deep pit in high-electron-mobility transistor structure using GaN on Si technology with strained-layer superlattice

For the utilization of high-electron-mobility transistor (HEMT) devices fabricated on GaN on Si structure as high-power devices, deep pits, which are known as inverted pyramid-shaped defects formed in the surface of a HEMT structure, are critical because they act as leakage pathways and degrade the breakdown property. We investigated in detail the origin of pits in the HEMT structure with a strained-layer superlattice (SLS). The pits were found to emerge from rotation domains, in which the crystal structure rotates around the c-axis, in the AlN buffer layer. The analysis by ultra low voltage scanning electron microscopy (ULV-SEM) revealed that the domains were formed on dense Al regions in Al preseeding layer. By optimizing Al pre flow conditions, we eliminated dense Al regions in Al preseeding layer, consequently, no rotation domains in the AlN buffer layer. An extremely low pit density of the HEMT structure with fine breakdown property was successfully achieved.

Enhancement of breakdown voltage for fully-vertical GaN-on-Si p-n diode by using strained layer superlattice as drift layer

We have demonstrated a vertical GaN-on-Si p-n diode with breakdown voltage (BV) as high as 839 V by using a low Si-doped strained layer superlattice (SLS). The p-n vertical diode fabricated by using the n−-SLS layer as a part of the drift layer showed a remarkable enhancement in BV, when compared with the conventional n−-GaN drift layer of similar thickness. The vertical GaN-on-Si p-n diodes with 2.3 μm-thick n−-GaN drift layer and 3.0 μm-thick n−-SLS layer exhibited a differential on-resistance of 4.0 Ω · cm2 and a BV of 839 V.