Strained-layer Superlattices Research Articles

Selective area heteroepitaxy of low dislocation density antiphase boundary free GaAs microridges on flat-bottom (001) Si for integrated silicon photonics

Integrating III–V gain elements in the silicon photonics platform via selective area heteroepitaxy (SAH) would enable large-scale and low-cost photonic integrated circuits. Here, we demonstrate antiphase boundary (APB)-free gallium arsenide (GaAs) microridges selectively grown on flat-bottom (001) silicon (Si) inside a recess. This approach eliminates the need for etching the patterned Si to form trapezoid or v-groove shapes, often leveraged for eliminating APBs. A low surface dislocation density of 8.5 × 106 cm−2 was achieved for 15-μm-wide GaAs microridges, quantified by electron channeling contrast imaging. The avoidance of APBs is primarily due to their self-annihilation, influenced by the sufficiently low temperature GaAs nucleation and subsequent higher temperature buffer overgrowth. Dislocation filtering approaches, namely, thermal cycle annealing and strained-layer superlattices, have been applied to effectively reduce the dislocation density. SAH of GaAs on trapezoidal-shaped Si pockets is also reported to illustrate the differing growth conditions for GaAs on (001) and (111) Si microplanes.

Perspective on advances in InAsSb type II superlattices grown on virtual substrates

Metamorphic InAs1−xSbx/InAs1−ySby strained layer superlattice (SLS) structures allow for great flexibility of engineering artificial band structures and, therefore, the design of new optical and electrical properties. By using tailored virtual substrates, the average lattice constant of the SLS can be chosen anywhere between 0.606 nm (InAs) and 0.648 nm (InSb), which allows for flexibility in the choice of compositions and thicknesses of the constituent layers. These parameters can then be tuned in a wide range, which is not possible when using binary substrates. Specifically, the layer thicknesses can be nearly arbitrarily small. Short period InAs1−xSbx/InAs1−ySby SLSs exhibit strong optical absorption and improved perpendicular carrier transport and can demonstrate Dirac-type carrier dispersion, a large g-factor, and deep band inversion. The prospects for the development of devices based on these structures are discussed.

Temperature dependence of diffusion length and mobility in mid-wavelength InAs/InAsSb superlattice infrared detectors

In the past decade, infrared detectors with InAs/InAsSb (Gallium-free) type-II strained layer superlattice absorbers became a technology of interest for many imaging applications. In this work, we study the dependence of minority carrier (hole) transport, absorption coefficient, and quantum efficiency (QE) of a 5.6 μm cutoff wavelength mid-wavelength infrared InAs/InAsSb detector on temperatures and applied bias. We found that the minority carrier lifetime is very long (τ ≈ 5.5 μs) and is temperature independent in the temperature range T = 50–150 K. The back-side illuminated QE without anti-reflection coating increases from ∼30% at T = 50 K to ∼60% at T = 180 K. The minority carrier (hole) diffusion length, Ldh, was found from QE and absorption coefficient. The hole diffusion length at T = 50 K is Ldh = 2.4 μm and increases monotonically to Ldh = 7.2 μm at T = 180 K. The hole mobility, calculated from diffusion length and minority carrier lifetime, is μh = 4.5 cm2/V s at T = 50 K and increases with temperature to reach μh = 7.2 cm2/V s at T = 150 K. In addition, we find that at lower temperatures where the diffusion length is shorter, the stronger QE dependence on applied bias is due to minority carrier collection from the depletion region, whose width increases with applied bias.

Thermal Camera-Based Fourier Transform Infrared Thermospectroscopic Imager.

In this technical note, we present an advanced thermospectroscopic imager based on a Fourier transform infrared (FT-IR) spectrometer and a thermal camera. This new instrument can image both thermal emission and multispectral absorbance fields in a few seconds at a resolution of 4 cm-1 or less. The setup is made of a commercial FT-IR spectrometer (ThermoFisher Nicolet iS50R) synchronized to an IR camera (indium antimonide and strained layer superlattice) as a detector to record the interferograms in each pixel of the images. A fast Fourier transform algorithm with apodization and Mertz phase correction is applied to the images, and the background is rationed to process the interferograms into the absorbance spectra in each pixel. The setup and image processing are validated using thin polystyrene films; during this processing, more than 1750 spectra per second are recorded. A spectral resolution equivalent to that of commercial FT-IR spectrometers is obtained for absorbance peaks valued less than two. The transient capability of the FT-IR thermospectroscopic imager is illustrated by measuring the heterogeneous thermal and absorbance fields during the phase change of paraffin over a few minutes. The complete mechanism of the thermochemical processes during a polymer solidification is revealed through the thermospectroscopic images, demonstrating the usefulness of such an instrument in studying fast transient thermal and chemical phenomena with an improved spectral resolution.

Passivation Study of InAs/GaSb Type-II Strained Layer Superlattice in Mid-wave Infrared Photodetector

This paper reports the results of a passivation study of InAs/GaSb type-II strained layer superlattice for a mid-wave infrared detector with p-on-n polarity. The cutoff wavelength of the fabricated photodiode was observed to be 5.5 µm at 77 K. At a negative bias of 0.1 V, the dark current density was measured to be 2.8 × 10−5, 8.2 × 10−5, and 3.1 × 10−2 A/cm2 for the diodes passivated with SiO2 and Si3N4 and the unpassivated diode, respectively. The photodiode passivated with SiO2 demonstrated a reduction in dark current density of approximately three times than that of the diode passivated with Si3N4. Therefore, the dynamic resistance-area product of the photodiode under the negative bias, passivated with SiO2, showed a larger value than that of the device passivated with Si3N4. In contrast, at 100–150 K, the resistance-area product at the zero-bias voltage of the photodiode passivated with Si3N4 showed a higher value than that of the device passivated with SiO2. It is believed that the relatively low thermal stress between GaSb and Si3N4 reduced the dark current density at zero bias and increased R0A in this temperature range.

InAs/InAsSb Type-II Strained-Layer Superlattice Infrared Photodetectors.

The InAs/InAsSb (Gallium-free) type-II strained-layer superlattice (T2SLS) has emerged in the last decade as a viable infrared detector material with a continuously adjustable band gap capable of accommodating detector cutoff wavelengths ranging from 4 to 15 µm and beyond. When coupled with the unipolar barrier infrared detector architecture, the InAs/InAsSb T2SLS mid-wavelength infrared (MWIR) focal plane array (FPA) has demonstrated a significantly higher operating temperature than InSb FPA, a major incumbent technology. In this brief review paper, we describe the emergence of the InAs/InAsSb T2SLS infrared photodetector technology, point out its advantages and disadvantages, and survey its recent development.

Investigation of AlGaN/GaN high electron mobility transistors on Silicon (111) substrates employing multi-stacked strained layer superlattice structures

Abstract AlGaN/GaN high electron mobility transistors (HEMTs) on Silicon (111) substrates are grown by using multi-stacked strained superlattices layer structures. Atomic force microscopy, X-ray diffraction and cross-sectional transmission electron microscopy are employed to investigate the structural properties of these AlGaN/GaN HEMT structures. The insertion of AlGaN/AlN strained superlattices layer modified with different numbers of GaN layers is revealed to reduce the pit density and threading dislocation density, resulting in the improvement of epilayer quality. The temperature dependent Hall effect measurements show that the two-dimensional electron gas characteristics are greatly improved by the employed modified superlattices layers in AlGaN/GaN HEMT structures throughout the whole measured temperature range. The superlattices layer engineering is shown to be important for achieving higher carrier mobility (>2340 cm2/V) and sheet carrier density (>8.22 × 1012 cm−2) at room temperature in AlGaN/GaN HEMTs.

Paving the way to dislocation reduction in Ge/Si(001) heteroepitaxy using C-based strained layer superlattices

Epitaxial Ge films were grown on Si(001) substrates by molecular beam epitaxy. During epitaxial growth, two carbon interlayers were deposited at varying substrate temperatures (140−620°C) and with varying C quantity (0−1.5monolayers). The influence of the second carbon interlayer on in-plane strain was investigated using high-resolution x-ray diffraction and transmission electron microscopy (TEM). All samples exhibited compressive strain, which was attributed to substitutional incorporation of carbon atoms. In-plane strain decreases with increasing substrate temperature during carbon deposition, indicating that enhanced surface mobility of carbon adatoms leads to formation of carbon clusters. This was confirmed by cross-sectional TEM investigations. Variation of C quantity at 180°C reveals maximum strain at an intermediate quantity of 0.8 monolayers. Omission of the second C interlayer results in much lower strain, indicating a mismatch between the two Ge layers separated by a C interlayer. This could be used to enforce dislocation filtering following the principle of strained layer superlattices. An upper estimate of 1×10−3 was found for the mismatch strain, resulting in a critical thickness for dislocation filtering of hc=153nm. A sample just exceeding hc exhibited a clear dislocation reduction effect as shown by TEM.

Strained CdZnTe/CdTe Superlattices As Threading Dislocation Filters in Lattice Mismatched MBE Growth of CdTe on GaSb

In this work, multiple sets of CdZnTe/CdTe strained-layer superlattices have been used as dislocation filtering layers for reducing the threading dislocations and improving the material quality of CdTe buffer layers grown by molecular beam epitaxy (MBE) on GaSb (211)B substrates. By incorporating a CdZnTe/CdTe superlattice filtering structure, a significant improvement in material quality has been achieved, with a low etch pit density of ∼ 1 × 105 cm−2 demonstrated for CdTe grown on GaSb, which is two orders of magnitude lower than previously reported values for CdTe grown directly on lattice mismatched substrates, and is comparable to values for state-of-the-art CdTe grown on lattice matched CdZnTe substrates. The filtering efficiency for each set of dislocation filtering layers has been determined to be approximately 70%. This approach provides a promising pathway towards achieving hetero-epitaxy of high quality HgCdTe on large-area lattice-mismatched alternative substrates with a low dislocation density for the fabrication of next generation infrared detectors with features of lower cost and larger array format size.

Electrical modulation of the LWIR absorption and refractive index in InAsSb-based strained layer superlattice heterostructures

InAsSb-based strained layer superlattices (SLS) have strong fundamental absorption, which can be easily modified in a controlled manner by injecting excess carriers. This makes them attractive for intensity modulation of infrared lasers as well as beam steering and spatial beam shaping with a nanosecond-scale time response. This paper reports the modulation of the fundamental absorption and the refractive index by carrier injection in a 3.45-nm-period InAsSb0.65/InAsSb0.35 SLS with a low temperature energy gap of 85 meV grown by molecular beam epitaxy on a GaSb substrate with a GaInSb metamorphic buffer. The SLS absorber was sandwiched by n- and p-type wider energy gap layers for electrical injection and confinement of excess carriers. The population of conduction band states was obtained by measuring the intensity modulation of a 10.6 μm CO2 laser for temperatures ranging from T = 77 to 200 K. An increase of the electron quasi-Fermi level with electrical injection up to 20 meV was observed. The experimental data imply a decrease in the Auger coefficient with temperature, from 3 × 1024 cm6/s at 77 K to 1 × 1024 cm6/s at T = 200 K attributed to recombination involving two electrons and a heavy hole. The refractive index changes obtained by electrical injection of excess carriers can reach 0.05 at T = 77 K and 0.035 at T = 200 K, which are at least three orders of magnitude greater than those obtained with electro-optical materials.

A superlattice-based resonant cavity-enhanced photodetector operating in the long-wavelength infrared

The design, fabrication, and characterization of a resonant cavity-enhanced photodetector (RCE PD) operating in the long-wavelength infrared regime are demonstrated. The incorporation of the low bandgap InAs/InAs0.70Sb0.30 type-II strained-layer superlattice into the absorber layer of the detector cavity, along with the high-reflectivity (Rm > 0.9) AlAs0.08Sb0.92/GaSb distributed Bragg reflector pairs, results in resonant enhancement at 7.7–7.8 μm, which is a spectral region relevant in applications in sensing of chemical warfare agents and in medical biomarker diagnostics. These resonant wavelength peaks also display a high quality factor in the range of 76–86 and a small temperature coefficient of 0.52 nm K−1. An nBn architecture, where an Al0.71Ga0.29As0.08Sb0.92 layer acts as a barrier for majority electrons while minimizing the valence band offset with the absorber, is also incorporated into the cavity in order to improve the electrical properties of the detector. Spectral response measurements yield a peak external quantum efficiency of 14.6% and a peak responsivity of 0.91 A W−1 at 77 K and −0.8 V; meanwhile, a dark current density of 2.0 × 10−4 A cm−2 at 77 K results in a specific detectivity of 3.7 × 1010 cm Hz1/2 W−1, coming close to the theoretical background-limited D* of an ideal broadband photovoltaic detector with the superlattice composition as that of the RCE PD.

High operating temperature pBn barrier mid-wavelength infrared photodetectors and focal plane array based on InAs/InAsSb strained layer superlattices.

Improving the operation temperature of the focal plane array (FPA) imagers is critical in meeting the demands to reduce the size, weight, and power (SWaP) for mid-infrared detection systems. In this work, we report the demonstration of a 15 µm-pitch 640×512 middle-format pBn FPA device with a 50% cutoff wavelength of 4.8 µm based on short period of InAs/InAsSb-based "Ga-free" type-II strained-layer superlattices, which achieves a high operating temperature (HOT) reaching 185 K. The pBn FPA exhibits a mean noise equivalent differential temperature (NETD) of 39.5 mK and an operability of 99.6% by using f/2.0 optics for a 300 K background at 150 K. The mean quantum efficiency is 57.6% without antireflection coating and dark current density is 5.39×10-5 A/cm2 at an operation bias of -400 mV, by which the mean specific detectivity(D*) is calculated as high as 4.43×1011 cm.Hz½/W.

Threading Dislocation Dynamics in ZnSSe Strained-Layer Superlattices

Strained-layer superlattices (SLSs) have been used extensively to modify the threading dislocation behavior in metamorphic semiconductor device structures, and have even been utilized as “dislocation filters.” However, up to the present time the application of SLSs in metamorphic structures has been impeded by the lack of detailed physical models for their behavior. In this work we apply a “weaving and jogging” model for dislocation interactions in ZnSSe SLSs to study their expected behavior and the dependence on the SLS design (placement, compositions, thicknesses, and growth conditions). Based on this study we make recommendations for the use of SLSs in the modification of threading dislocation behavior in ZnSSe/GaAs (001) device structures.

(Invited) Recent Advances in the Modeling of Strain Relaxation and Dislocation Dynamics in ZnSSe/GaAs (001) Heterostructures

In this talk we will describe state-of-the-art approaches to the modeling of strain relaxation and dislocation dynamics in ZnSSe/GaAs (001) heterostructures. Current modeling approaches are all based on the extension of the original Dodson and Tsao plastic flow model to include compositional grading and multilayers, dislocation interactions, and differential thermal expansion. Important breakthroughs in the last twenty-four months have greatly enhanced the utility of these modeling approaches in three respects: i) pinning interactions have been included in graded and multilayered structures for the first time, providing a better description of the limiting strain relaxation as well as the dislocation sidewall gettering; ii) a refined model for dislocation-dislocation interactions including jogging provides a more accurate physical description of compositionally-graded layers and step-graded layers; and iii) inclusion of back-and-forth weaving of dislocations provides a better description of dislocation dynamics in structures containing strain reversals, such as strained-layer superlattices or overshoot graded layers. We will describe these three key advances and illustrate applications of each to practical heterostructures.

Threading Dislocation Dynamics in ZnSSe Strained-Layer Superlattices

Strained-layer superlattices (SLSs) have been used extensively to modify the threading dislocation behavior in metamorphic semiconductor device structures, and are sometimes referred to as “dislocation filters.” However, up to the present time the application of SLSs in metamorphic structures has been impeded by the lack of detailed physical models for their behavior. In this work we apply a “weaving” and “jogging” model for dislocation interactions in ZnSSe SLSs to study their expected behavior and the dependence on the SLS design (placement, compositions, thicknesses, and growth conditions).

(Invited) Recent Advances in the Modeling of Strain Relaxation and Dislocation Dynamics in ZnSSe/GaAs (001) Heterostructures

In this paper we describe state-of-the-art approaches to the modeling of strain relaxation and dislocation dynamics in ZnSSe/GaAs (001) heterostructures. These are based on the extension of the 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 capability of the modeling approach in three respects: i) pinning interactions have been included in graded and multilayered structures for the first time, providing a better description of the limiting strain relaxation as well as the dislocation sidewall gettering; ii) a refined model for dislocation-dislocation interactions including jogging provides a more accurate physical description of compositionally-graded layers and step-graded layers; and iii) inclusion of back-and-forth weaving of dislocations provides a better description of dislocation dynamics in structures containing strain reversals, such as strained-layer superlattices and overshoot graded layers. Here we describe these three key advances and illustrate applications of each to practical heterostructures.

A Zagging and Weaving Model for Dislocation Interactions in Heterostructures Containing Strain Reversals

Strained-layer superlattices (SLSs) have been used to modify the threading dislocation behavior in metamorphic semiconductor device structures; in some cases they have even been used to block the propagation of threading dislocations and are referred to in these applications as “dislocation filters.” However, such applications of SLSs have been impeded by the lack of detailed physical models. Here we present a “zagging and weaving” model for dislocation interactions in multilayers and strained-layer superlattices, and we demonstrate the use of this model to the threading dislocation dynamics in InGaAs/GaAs (001) structures containing SLSs.

Recent Advances in the Modeling of Strain Relaxation and Dislocation Dynamics in InGaAs/GaAs (001) Heterostructures

In this paper we describe state-of-the-art approaches to the modeling of strain relaxation and dislocation dynamics in InGaAs/GaAs (001) heterostructures. Current approaches are all based on the extension of the original Dodson and Tsao plastic flow model to include compositional grading and multilayers, dislocation interactions, and differential thermal expansion. Important recent break-throughs have greatly enhanced the utility of these modeling approaches in four respects: i) pinning interactions are included in graded and multilayered structures, providing a better description of the limiting strain relaxation as well as the dislocation sidewall gettering; ii) a refined model for dislocation-dislocation interactions including zagging enables a more accurate physical description of compositionally-graded layers and step-graded layers; iii) inclusion of back-and-forth weaving of dislocations provides a better description of dislocation dynamics in structures containing strain reversals, such as strained-layer superlattices or overshoot graded layers; and iv) the compositional dependence of the model kinetic parameters has been elucidated for the InGaAs material system, allowing more accurate modeling of heterostructures with wide variations in composition. We will describe these four key advances and illustrate their applications to heterostructures of practical interest.

Defect engineering for high quality InP epitaxially grown on on-axis (001) Si

Heteroepitaxy of indium phosphide (InP) and its lattice-matched alloys on silicon (Si) show great promise for Si-based optoelectronic devices and photonic integrated circuits. Here, we report the monolithic growth of high crystalline quality InP on V-groove patterned (001) Si substrates by metalorganic chemical vapor deposition, demonstrating a low surface defect density of 4.5 × 107 cm−2, characterized by statistical electron channel contrast imaging. This advanced InP-on-Si virtual substrate is implemented by combining a compositionally graded indium gallium arsenide (InxGa1 − xAs) buffer and optimized In0.73Ga0.27As/InP strained-layer superlattices on gallium arsenide on a V-grooved Si template. These techniques gradually accommodate the lattice mismatch and effectively filter most of the generated dislocations. A comprehensive material characterization and the demonstration of room-temperature continuous-wave electrically pumped laser diodes on Si validate the suitability of using this InP-on-Si platform for monolithic integration of InP- and Si-based electronic and photonic devices.

InAs/InAsSb Strained-Layer Superlattice Mid-Wavelength Infrared Detector for High-Temperature Operation.

This paper reports an InAs/InAsSb strained-layer superlattice (SLS) mid-wavelength infrared detector and a focal plane array particularly suited for high-temperature operation. Utilizing the nBn architecture, the detector structure was grown by molecular beam epitaxy and consists of a 5.5 µm thick n-type SLS as the infrared-absorbing element. Through detailed characterization, it was found that the detector exhibits a cut-off wavelength of 5.5 um, a peak external quantum efficiency (without anti-reflection coating) of 56%, and a dark current of 3.4 × 10−4 A/cm2, which is a factor of 9 times Rule 07, at 160 K temperature. It was also found that the quantum efficiency increases with temperature and reaches ~56% at 140 K, which is probably due to the diffusion length being shorter than the absorber thickness at temperatures below 140 K. A 320 × 256 focal plane array was also fabricated and tested, revealing noise equivalent temperature difference of ~10 mK at 80 K with f/2.3 optics and 3 ms integration time. The overall performance indicates that these SLS detectors have the potential to reach the performance comparable to InSb detectors at temperatures higher than 80 K, enabling high-temperature operation.