Strained-layer Superlattices Research Articles

Photocapacitance study of GaSb: In, As for defect analysis in InAs/GaSb type-II strained layer superlattices

Steady-state photocapacitance measurements were used to characterize GaSb incorporated with In, As, and a control sample. Evidence of a trap level at 0.55eV was observed for all samples. The change in the capacitance for the sample with indium was about half the change for the other samples, indicating that the addition of indium modified the near-mid-gap trap levels. Another change in capacitance starting at 0.71eV was attributed to electrons from the valence band filling levels close to the conduction band.

Low operating bias InAs/GaSb strain layer superlattice LWIR detector

Minimization of operating bias and generation–recombination dark current in long wavelength infrared (LWIR) strained layer superlattice (SLS) detectors, consisting of a lightly doped p-type absorber layer and a wide band gap hole barrier, are investigated with respect to the band alignment between the wide band gap barrier and absorber layers. Dark current vs. bias, photoresponse, quantum efficiency, lifetime, and modeling are used to correlate device performance with the wide gap barrier composition. Decreases in dark current density and operating bias were observed as the conduction band of the wide gap barrier was lowered with respect to the absorber layer. The device achieved 95% of its maximum quantum efficiency at 0V bias, and 100% by 0.05V. This study demonstrates key device design parameters responsible for optimal performance of heterojunction based SLS LWIR detectors.

Chemical and physical passivation of type II strained-layer superlattice devices by means of thiolated self-assembled monolayers and polymer encapsulates

The efficacy of solution deposition of thiolated self-assembled monolayers (SAMs) has been explored for the purpose of passivating III–V type II superlattice (T2SL) photodetectors, more specifically a p-type heterojunction device. Sulfur passivation has previously been achieved on T2SL devices. However, degradation over time, temperature sensitivity and inconsistent reproducibility necessitate a physical encapsulate that can chemically bond to the chemical passivant. Thus, this research investigates two passivation methods, surface passivation with a thiol monolayer and passivation with a polymer encapsulant with a view toward future combination of these techniques. Analysis of the physical and chemical condition of the surface prior to deposition assisted in the development of ideal processes for optimized film quality. Successful deposition was facilitated by in situ oxide removal. Various commercially available functional (cysteamine) and non-functional (alkane) thiolated monolayers were investigated. Dark current was reduced by 3 orders of magnitude and achieved negligible surface leakage at low bias levels. The lowest dark current result, 7.69×10−6A/cm2 at 50mV, was achieved through passivation with cysteamine.

Post-etching mesa surface composition investigation of InAs/GaSb type-II strained layer superlattices using XPS characterization

XPS characterization was used to determine the surface chemistry of a mid-wave infrared T2SL treated by both an HCl-based and an H3PO4-based etching solution. This analysis, performed over both the etched and unetched portions of the sample, revealed that the HCl-based etch removed Ga and Sb oxides while the H3PO4-based etch removed In and As oxides. XPS imaging was also done on 200μm×200μm areas of the sample, and showed that HCl solution (Ga, and O) produced surfaces that were less stoichiometric than the H3PO4 solution (Ga2O3, Sb2O5, Sb in GaSb). Single-pixel, p-i-n test structures were fabricated using either etching solution, and an electrical comparison revealed over an order of magnitude improvement in dark current for the sample treated with the H3PO4 solution, compared to the HCl sample.

Radiometric characterization of an LWIR, type-II strained layer superlattice pBiBn photodetector

Type-II Strained Layer Superlattice (T2SLS) infrared photodetectors have been in development over the last decade. T2SLS offers a theoretically longer Auger recombination lifetime than traditional mercury cadmium telluride (MCT), which presumably translates to infrared detectors with lower dark-current and higher operating temperatures. However, these improvements did not materialize due to the presence of Shockley–Read–Hall (SRH) defects in T2SLSs, which limits the recombination lifetime well below the Auger-limit. With the recent introduction of the pBiBn, and other similar unipolar barrier detectors, T2SLS material has seen renewed interest since these designs ideally eliminate the SRH-generation and surface currents while retaining the other potential advantages of T2SLS: reduced manufacturing cost, better availability of a durable state-side manufacturing base, ability to tune the cutoff wavelength, and better uniformity. Here, an electrical and optical characterization of a long-wave, pBiBn detector with a T2SLS absorber is presented. Dark-current, spectral response and optical response were measured as functions of temperature and bias. Activation energy was then determined as a function of bias from the dark-current measurements. Quantum efficiency was also determined as a function of bias from the optical response measurements. Additionally, noise spectrum measurements were taken as a function of bias.

Dual color longwave InAs/GaSb type-II strained layer superlattice detectors

We report on the design, growth, fabrication and characterization of dual-band (long-/long-wave infrared) type-II InAs/GaSb strained layer superlattice (T2SL) detectors with pBp architecture. Under operating the bias of −200mV and +100mV, quantum efficiencies of 37% (∼11μm band) and 25% (∼9μm band) were realized, respectively. To reduce the dark current in a dual-band T2SL detector, the effect of a “restoration” chemical etch treatment and ZnTe passivation on device performance were investigated.

Use of 3-aminopropyltriethoxysilane deposited from aqueous solution for surface modification of III-V materials

Focal plane arrays of strained layer superlattices (SLSs) composed of InAs/GaSb are excellent candidates for infrared imaging, but one key factor limiting their utility is the lack of a surface passivation technique capable of protecting the mesa sidewall from degradation. Along these lines, we demonstrate the use of aqueous 3-aminopropyl triethoxysilane (APTES) deposited as a surface functionalizing agent for subsequent polymer passivation on InAs and GaSb surfaces following a HCl/citric acid procedure to remove the conductive oxide In2O3. Using atomic force microscopy, variable angle spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and modeling with density functional theory (DFT), we demonstrate that APTES films can successfully be deposited on III-V substrates by spin coating and directly compare these films to those deposited on silicon substrates. The HCl/citric acid surface preparation treatment is particularly effective at removing In2O3 without the surface segregation of In oxides observed from use of HCl alone. However, HCl/citric acid surface treatment method does result in heavy oxidation of both Ga and Sb, accompanied by segregation of Ga oxide to the surface. Deposited APTES layer thickness did not depend on the substrate choice, and thicknesses between 1 and 20nm were obtained for APTES solution concentrations ranging from 0.1 to 2.5 vol%. XPS results for the N1s band of APTES showed that the content of ionic nitrogen was high (∼50%) for the thinnest films (∼1nm), and decreased with increasing film thickness. These results indicate that APTES can indeed be used to form a silane surface layer to cover III-V materials substrates. Such APTES silane layers may prove useful in surface passivation of these materials alone, or as surface functionalizing agents for subsequent covalent binding with polymer overlayers like polyimide.

1.3 μm InAs/GaAs quantum‐dot laser monolithically grown on Si substrates operating over 100°C

A high-performance 1.3 μm InAs/GaAs quantum-dot laser directly grown on Si substrates has been achieved by using InAlAs/GaAs strained-layer superlattice serving as dislocation filter layers (DFLs). The Si-based laser achieves lasing operation up to 111°C with a threshold current density of 200 A/cm 2 and an output power exceeding 100 mW at room temperature.

Digital model for X-ray diffraction with application to composition and strain determination in strained InAs/GaSb superlattices

We propose a digital model for high quality superlattices by including fluctuations in the superlattice periods. The composition and strain profiles are assumed to be coherent and persist throughout the superlattice. Using this model, we have significantly improved the fit with experimental X-ray diffraction data recorded from the nominal InAs/GaSb superlattice. The lattice spacing of individual layers inside the superlattice and the extent of interfacial intermixing are refined by including both (002) and (004) and their satellite peaks in the fitting. For the InAs/GaSb strained layer superlattice, results show: (i) the GaSb-on-InAs interface is chemically sharper than the InAs-on-GaSb interface, (ii) the GaSb layers experience compressive strain with In incorporation, (iii) there are interfacial strain associated with InSb-like bonds in GaSb and GaAs-like bonds in InAs, (iv) Sb substitutes a significant amount of In inside InAs layer near the InAs-on-GaSb interface. For support, we show that the composition profiles determined by X-ray diffraction are in good agreement with those obtained from atom probe tomography measurement. Comparison with the kinetic growth model shows a good agreement in terms of the composition profiles of anions, while the kinetic model underestimates the intermixing of cations.

Minority Carrier Lifetime in Beryllium-Doped InAs/InAsSb Strained Layer Superlattices

Abstract : 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 NA = 6 1016 and 3 1017 cm (exp -3), electron lifetimes of tn = 45 ns and 8 ns were measured. The 6 1016 cm (exp -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( exp -3) level.

1.3-μm InAs/GaAs quantum-dot lasers monolithically grown on Si substrates using InAlAs/GaAs dislocation filter layers.

We compare InAlAs/GaAs and InGaAs/GaAs strained-layer superlattices (SLSs) as dislocation filter layers for 1.3-μm InAs/GaAs quantum-dot laser structures directly grown on Si substrates. InAlAs/GaAs SLSs are found to be more effective than InGaAs/GaAs SLSs in blocking the propagation of threading dislocations generated at the interface between the GaAs buffer layer and the Si substrate. Room-temperature lasing at ~1.27 μm with a threshold current density of 194 A/cm(2) and output power of ~77 mW has been demonstrated for broad-area lasers grown on Si substrates using InAlAs/GaAs dislocation filter layers.

Study on GaN-based light emitting diodes grown on 4-in. Si (111) substrate

GaN-based light emitting diodes (LEDs) grown on 4-in. Si (111) substrate by metal–organic chemical vapor deposition have been systematically characterized. The significantly smooth surface of the sample has been confirmed by an atomic force microscope. In X-ray diffraction measurement, two kinds of fringe peaks were observed, which are believed to originate from the strained-layer superlattice (SLS) and multiple quantum wells. Moreover, from cross-sectional scanning electron microscope images of the sample, it is found that the interfaces of SLS are smooth. The 100-pair AlN/GaN SLS can be employed to modulate the strain between the GaN layer and substrate, resulting in the improvement of GaN crystalline quality. The full width at half maximum of ω-scan of the GaN (0002) diffraction is around 630ʺ. In addition, the device properties have been investigated in detail, and the maximum light output power can reach 2.02mW with a high saturation injection current of 320mA.

InAs/GaSb Type-II Superlattice Detectors

InAs/(In,Ga)Sb type-II strained layer superlattices (T2SLs) have made significant progress since they were first proposed as an infrared (IR) sensing material more than three decades ago. Numerous theoretically predicted advantages that T2SL offers over present-day detection technologies, heterojunction engineering capabilities, and technological preferences make T2SL technology promising candidate for the realization of high performance IR imagers. Despite concentrated efforts of many research groups, the T2SLs have not revealed full potential yet. This paper attempts to provide a comprehensive review of the current status of T2SL detectors and discusses origins of T2SL device performance degradation, in particular, surface and bulk dark-current components. Various approaches of dark current reduction with their pros and cons are presented.

Characterization of InAlN/GaN high-electron-mobility transistors grown on Si substrate using graded layer and strain-layer superlattice

We report on In0.25Al0.75N/GaN heterostructures grown on Si using a graded buffer and a thick strained-layer superlattice. These heterostructures showed a smooth surface with a root mean square value of 0.2 nm, a high sheet carrier density of 1.7 × 1013 cm−2, and a Hall mobility of 1100 cm2 V−1 s−1. The In0.25Al0.75N/GaN high-electron-mobility transistors (HEMTs) showed a high drain current density of 755 mA/mm with a low specific on-resistance of 0.39 mΩ·cm2. Furthermore, In0.25Al0.75N/GaN HEMT with the gate–drain spacing of 20 µm showed a high power device figure-of-merit of 1.62 × 108 V2 Ω−1 cm−2.

Exploring cryogenic focused ion beam milling as a Group III–V device fabrication tool

In this paper, we compare the features observed on a Group III–V strained layer superlattice (SLS) materials system as a result of room temperature Ga+ focused ion beam (FIB) milling to the features observed as a result of cryogenic FIB (cryo-FIB) milling at –135°C under the same beam conditions (30kV:1nA). The features on the cryo-FIB milled material were observed both when the material was still cold and after it returned to room temperature. Although cryo-FIB milling yielded patterned features that were initially cleaner than comparable features defined by FIB milling at room temperature, we found that both room temperature FIB milling and cryo-FIB milling with subsequent sample warm-up resulted in the formation of Group III enriched features. These findings suggest that the structural and chemical properties of features fabricated by cryo-FIB milling are temperature-dependent, which is an important consideration when it comes to device fabrication. These dependencies will need to be better understood and controlled if cryo-FIB milling is to have future applications in this area.

Carrier lifetime studies in midwave infrared type-II InAs/GaSb strained layer superlattice

The authors report on an investigation of the dependence of the minority carrier lifetime in midwave infrared InAs/GaSb strained layer superlattices on a number of varied parameters: layer placement of two dopants (either Be or Te), and interface treatment between InAs and GaSb layers. In samples where the dopant and doping location was varied, it was found that the nonintentionally doped control sample exhibited the longest lifetimes (∼49 ns at 77 K under low injection), followed by the Be-doped and the Te-doped samples. Regardless of the type of doping, samples with dopants in only the InAs layer appeared to have longer lifetimes [low injection: 15 ns (Be), <3 ns (Te); high injection: 38 ns (Be), 16.2 ns (Te) at 77 K] compared to samples with dopants in the GaSb layer or all layers. However, because trap saturation behavior was observed in the transient photoluminescence (PL) decay, the intensity-dependent PL lifetime is a function of both the minority and majority carrier lifetimes, complicating the interpretation of the data. In samples where the treatment of the InAs/GaSb interface was varied, the sample that demonstrated the longest lifetime had a one-period growth sequence of InAs, an Sb soak, GaSb, and an InSb strain compensation layer. Of the three interface samples investigated, the sample (with a growth sequence of InAs, an Sb soak, GaSb, and a growth interrupt) that demonstrated the shortest lifetime also exhibited a fast initial decay for all injection levels, at only 110 and 150 K. This fast initial decay has been attributed to the appearance of another Shockley–Read–Hall trap level, contributing to a shorter carrier lifetime.

Infrared detectors: Advances, challenges and new technologies

Human knowledge of infrared (IR) radiation is about 200 years old. However it was in the late 20th century that we developed a wide range of smart technologies for detection and started to take advantage for our benefit. Today IR detector technology is in its 3rd generation and comes with challenging demands. Based on the propagation of IR radiation through free space it is divided into different transmission windows. The most interesting for thermal imaging are the mid-wave IR (MWIR) and the long-wave IR (LW IR). Infrared detectors for thermal imaging have a number of applications in industry, security, search & rescue, surveillance, medicine, research, meteorology, climatology and astronomy. Currently high-performance IR imaging technology is mainly based on epitaxially grown structures of the small-bandgap bulk alloy mercury-cadmium-telluride (MCT), indium antimonide (InSb) and GaAs based quantum-well infrared photodetectors (QWIPs), depending on the application and wavelength range. However, they operate at low temperatures requiring costly and bulky cryogenic systems. In addition there is always a need for better performance, which generates possibilities for developing new technologies. Some emerging technologies are quantum dot infrared photodetectors (QDIPs), type-II strained layer super-lattice, and QDIPs with type-II band alignment. In this report a brief review of the current and new technologies for high performance IR detectors, will be presented.

Growth and fabrication of a mid-wavelength infrared focal plane array based on type-II InAs/GaSbsuperlattices

We present the fabrication of a mid-wavelength infrared focal plane array (FPA) based on type-II InAs/GaSb strain layer superlattices (SLs). The detectors contain a 400-period 8 ML InAs/8 ML GaSb SL active layer, which is grown by solid source molecular beam epitaxy on GaSb (100) N type substrates. Lattice mismatch between the superlattices and GaSb substrate achieves 148.9 ppm. The full width at half maximum of the first order satellite peak from X-ray diffraction was 28 arcsec. Single element detectors and FPA with a 128 × 128 pixels were fabricated using citric acid based solution wet chemical etching. Chemical and physical passivation effectively reduces the surface leakage and this process was characterized by I—V measurement. The devices showed a 50% cut-off wavelength of 4.73 μm at 77 K. The photodiode exhibited an R0A of 103 Ω · cm2. The FPA was characterized with an integration time of 0.5 ms and F/2.0 optics at 77 K and the average blackbody detectivity of the detectors is 2.01 × 109 cm · Hz1/2/W.

Growth of Ge1−xSnx/Ge strained-layer superlattices on Si(1 0 0) by molecular beam epitaxy

Low-temperature molecular beam epitaxy was employed to grow Ge1−xSnx/Ge strained-layer superlattices (SLSs) on Si(100) substrates with a Ge buffer layer. The Ge1−xSnx and Ge layers in the SLSs were deposited at the same temperature as low as 180°C. Surface roughening during the growth was investigated by in situ reflection high-energy electron diffraction (RHEED). The periods, compositions, and quality of the SLSs were characterized by high resolution X-ray diffraction (HR-XRD), random and aligned Rutherford backscattering spectra (RBS), transmission electron microscopy (TEM), and scanning TEM. Besides, a Ge1−xSnx/Ge SLS sample was grown directly on Si(100) for comparison. It’s found that the quality of the Ge1−xSnx/Ge SLSs grown on a Ge buffer was significantly better than that grown directly on Si(100) substrate.

Design of Phosphorus-Containing MWIR Type-II Superlattices for Infrared Photon Detectors

Type-II strained layer superlattices (SLSs) offer a broad range of design degrees of freedom to help optimize their properties as absorber layers of infrared photon detectors. We theoretically examine a new class of mid-wavelength infrared (2-5 μm bandpass) Type-II structures with two-layer InGaSb/InPSb and four-layer InAs/GaSb/InAs/InPSb SLS periods. Phosphorous-containing SLSs are a promising approach to improving infrared photon detector performance due to providing a new set of material properties, including favorable valence band offsets. P-based SLSs of four-layer type InAs/GaSb/InAs/InPSb were found to be among the best 5-μm gap SLSs that we have modeled. Among the studied designs, the lowest dark current in an ideal structure is predicted for a four-layer 23.6 A InAs/20 A GaSb/23.6 Å InAs/60 Å InP 0.62Sb0.38 SLS. Its predicted ideal dark current is about 35 times lower than an n-type HgCdTe-based photodiode absorber and six times lower than a p-type HgCdTe one for the same bandgap, temperature, and dopant concentration. We also discuss a defect mitigation strategy that involves positioning the SLS gap in an energy range that avoids defect levels and show how this applies to the aforementioned P-containing SLS.