There has been great progress in recent years in advancing the state-of-the-art of Ga-free InAs/InAsSb
superlattice (SL) materials for infrared detector applications, spurred by the observation of long minority carrier
lifetimes in this material system. However, compositional and dimensional changes through antimony (Sb) segregation
alter the detector properties from those originally designed. For this reason, in this work, the authors explore epitaxial
conditions that can mitigate this segregation in order to produce high-quality SL materials for optimum detector
performance. A nominal SL structure of 7.7 nm InAs/3.5 nm InAs0.7 Sb0.3 tailored for an approximately six-micron
response at 5 K was used to optimize the epitaxial parameters. Since the growth of mixed AsSb alloys is complicated by
the potential reaction of As with Sb surfaces, the authors vary the substrate temperature (Ts) in order to control the As
surface reaction on a Sb surface. Experimental results indicate that the SL sample grown at the lowest investigated Ts
produces the highest Sb-mole fraction x of ~0.3 in InAs1-x Sbx layers, which then decreases by 21 % as the Ts increases
from 395 to 440 °C. This reduction causes an approximately 30 meV blueshift in the position of the excitonic
photoluminescence (PL) peak. This finding differs from the results obtained from the Ga-containing InAs/GaSb SL
equivalents, where the PL peak position remains constant at about 220 meV, regardless of Ts. The Ga-free SLs generally
generate a broader PL linewidth than the corresponding Ga-containing SLs due to the higher spatial Sb distribution at the
hetero-interfaces engendered by Sb segregation. In order for this newly proposed Ga-free SL materials to be viable for
detector applications, the material problem associated with Sb segregation needs to be adequately controlled and further
mitigated.
Gallium (Ga)-free InAs/InAsSb superlattices (SLs) are being actively explored for infrared detector applications due to the long minority carrier lifetimes observed in this material system. However, compositional and dimensional changes through antimony (Sb) segregation during InAsSb growth can significantly alter the detector properties from the original design. At the same time, precise compositional control of this mixed-anion alloy system is the most challenging aspect of Ga-free SL growth. In this study, the authors establish epitaxial conditions that can minimize Sb surface segregation during growth in order to achieve high-quality InAs/InAsSb SL materials. A nominal SL structure of 77 Å InAs/35 Å InAs0.7Sb0.3 that is tailored for an approximately six-micron response at 150 K was used to optimize the epitaxial parameters. Since the growth of mixed-anion alloys is complicated by the potential reaction of As2 with Sb surfaces, the authors varied the deposition temperature (Tg) under a variety of Asx flux conditions in order to control the As2 surface reaction on a Sb surface. Experimental results reveal that, with the increase of Tg from 395 to 440 °C, Sb-mole fraction x in InAs1-xSbx layers is reduced by 21 %, under high As flux condition and only by 14 %, under low As flux condition. Hence, the Sb incorporation efficiency is extremely sensitive to minor variations in epitaxial conditions. Since a change in the designed compositions and effective layer widths related to Sb segregation disrupts the strain balance and can significantly impact the long-wavelength threshold and carrier lifetime, further epitaxial studies are needed in order to advance the state-of-the-art of this material system.
The epitaxially grown system comprising semimetallic ErAs (erbium arsenide) nanoparticles (MNP) and InAs (indium arsenide) single quantum dots (QDs) hold great promise for plasmonic applications. The distinguishing feature of the ErAs-InAs hybrid is that the hybrid separation can be varied and accurately controlled in the molecular-beam epitaxial growth. In order to assess the potential of this system for plasmonic applications, this paper aims to estimate and optimize the expected magnitude of the plasmonic enhancement. We use the Sun–Khurghin theory to estimate the expected absorption and photoluminescence (PL) enhancement. Using a carefully selected set of materials’ parameters as input, we predict about 500-fold plasmonic PL enhancement for this system under resonant conditions.
The epitaxial growth parameters optimized for mid-wavelength infrared (MWIR) InAs/GaSb superlattice (SL)
growth are not directly applicable for long-wavelength infrared (LWIR) SL growth. We observed a two orders of
magnitude drop in the spectral intensity of the measured photoresponse (PR) as the InAs layer thickness in the SL
increases from 9 monolayers (MLs) to 16 MLs for a fixed GaSb layer thickness of 7 MLs. However, the theoretically
calculated absorption strength decreases only by about a factor of two. So other factors affecting photoresponse, such as
carrier mobility and lifetime, are likely responsible for the large drop in the PR of the LWIR SL in this sample set. In
fact the measured Hall properties of MWIR and LWIR SLs are very different, with holes as the majority carriers in
MWIR SLs and electrons as the majority carriers in LWIR SLs. Therefore we investigated the charge carrier density,
carrier mobility, and carrier recombination dynamics in LWIR SL samples. Specifically we used temperature-dependent
Hall effect and time-resolved pump-probe measurements to study the effect of adjusting several growth parameters on
the background carrier concentrations and studied carrier lifetimes in LWIR SLs.
Operation of InAs/GaSb superlattice-based devices requires efficient transport of carriers perpendicular to
superlattice layers by drift and/or diffusion. While transverse mobility measurements are performed routinely, vertical
transport measurements are difficult and nonstandard, so that very little is known about their value and dependence on
material quality, which is important in device modeling. In such a situation, model calculations can help fill the void. In
this work, both the horizontal and vertical electron transport in InAs/GaSb superlattices qua superlattices, not quantum
wells, as in Gold's model or its extensions, are modeled. The respective Boltzmann equations in the relaxation time
approximation are solved, using the interface roughness scattering as the dominant mobility-limiting mechanism. In
absence of screening, a universal relation that the vertical relaxation rates are always smaller than horizontal relaxation
rates is derived; hence vertical mobilities are generally smaller than horizontal mobilities. We calculate vertical and
horizontal mobilities as a function of such superlattice parameters as layer widths and the correlation length of interface
roughness. The calculated ratios of the vertical to horizontal mobilities can be used to estimate vertical mobilities from
measurements of horizontal mobilities.
Type-II superlattice (SL) materials research in the Materials & Manufacturing Directorate of the Air Force Research
Laboratory began in 1988. This materials system holds great promise as the III-V equivalent to HgCdTe alloys for
infrared detection. Great progress has been made on the epitaxial growth of InAs/Ga1-xInxSb superlattices in the past
twenty years by a number of research groups. However, not all of the materials issues have been solved. To continue
to resolve these limiting materials issues, basic superlattice materials, without photodiode fabrication, are used to
characterize the impact of growth processes and SL design on the structural, electrical and optical properties. An
integrated approach of theoretical modeling, in-house molecular beam epitaxy, and a host of materials measurement
techniques is employed to study the optimization of the superlattices for infrared detection. In the past few years the
majority of the samples grown in-house have been designed for the middle wavelength infrared (MWIR) band.
However, there are challenges in applying MWIR SL growth optimization to longer wavelength SLs. Recent
progress on understanding the complex interplay between InAs/GaSb superlattice composition and fundamental
electrical and optical properties will be covered.
The past decade has witnessed rapid progress in the development of techniques for correcting lens aberrations in high-resolution
transmission electron microscopy (HRTEM), resulting in significant enhancement in the directly interpretable
spatial resolution in HRTEM images. Furthermore, in combination with advanced image processing and analysis, it is
now possible to employ HRTEM as a quantitative technique for structural and chemical analysis at the atomic scale. In
this paper we have applied these developments to investigate interfaces in InAs/GaSb superlattices, the main objectives
being the mapping of changes in chemical composition and strain at each interface. For examining changes in
composition we use the focal series reconstruction technique, which retrieves the quantum-mechanical electron wave
function at the exit surface of the sample. The phase images of the electron wave function are then analyzed by linear
multivariate statistical analysis to independently quantify change in the In/Ga and As/Sb contents across each interface.
The strain profiles across interfaces are determined from HRTEM images, obtained from a TEM equipped with a
spherical aberration corrector, employing the "peak-pair analysis" (PPA) algorithm. Finally, the high-angle annular
dark-field imaging technique (HAADF), using a monochromated and probe corrected TEM, is also employed to examine
interfaces.
This paper derives a new fundamental equation for the frequency spectra ω(q) of one-dimensional photonic
crystals as a function of Brillouin wave vector q in the form of a novel factored expression,
tan2qd / 2 = tan(kNaN -αN) × tan(kNaN - βN), where N the number of layers per period is, d is the unit cell width, and ki = niω/c is the local wave vector in the ith layer of width 2ai and refractive index ni. Angles(αN,βN) depend on the parameters of all N layers but are
independent of aN . For two layers, (α2, β2) correspond to the even/odd parity solutions at the center and the edge of the
Brillouin zone. The derived spectral expression provide separate eigenvalue conditions for consecutive band edges at the
center and the edge of the Brillouin zone for any N and is useful in finding the Bloch phase that is necessary in finite
crystal calculations. The formalism is convenient for tailoring band gaps and for calculating impurity modes in dielectric
stacks.
The performance of infrared focal plane arrays and quantum cascade lasers manufactured from InAs/GaSb type-
II superlattices (SLs) depends on the mobility of carriers along the growth axis. In turn, the longitudinal mobility
depends on the quality of SL interfaces. In-plane transport is a sensitive measure of interface quality and the degree of
interface roughness scattering (IRS). In this paper, we demonstrate the IRS-limited transport regime in InAs/GaSb SL
samples grown for this study. We find that the in-plane mobility
μ as a function of InAs layer width L behaves as
μ ∝ L5 , which closely follows the classic sixth power dependence expected from theory. Fits to the mobility data
indicate that, for one monolayer surface roughness, the roughness correlation length is about 35 Å.
This paper discusses the potential attributes of (110)-grown InAs/GaSb superlattices for infrared detection
applications. In comparison to (001)-grown structures, (110) SLs will be thinner, have higher mobilities, diffusion
lengths, quantum efficiencies, and gains. Unless growth issues arise, they should also have higher minority carrier
lifetimes, higher responsivities, lower noise, and higher detectivities. The first 8x8 envelope-function approximation
calculation for a (110)-oriented structure shows the bands to be slightly anisotropic and the oscillator strengths to be
polarization dependent. Layer widths for specific absorption thresholds were calculated.
A new formalism for calculating the photonic band structure of multi-layer photonic band gap (PBG) materials
is presented. The formalism expresses all boundary conditions in terms of tangents rather than exponential functions.
The formalism is compact, algorithmically simple, and physically appealing, and provides a new conceptual framework
for describing the photonic band structure of layered materials. Its simplicity makes it possible to represent
eigenfrequency conditions using geometric constructs, find a factored form of the secular equation, and derive analytic
eigenfrequency conditions and analytic wave functions for multi-layer structures. Computationally, the new formalism
makes it possible to find explicitly
the complete band structure of multi-layer PBG materials with integer ratios of
optical path lengths (i.e., any combination of quarter-wave, half-wave, etc., stacks) through a single
diagonalization of a
low order secular equation, the alternative being an implicit
root search via the transfer matrix formalism. The
formalism is demonstrated on multi-layered structures arranged in the Fibonacci sequence and a half-wave-quarterwave-
eighth-wave PBG.
For type-II superlattices with spatially indirect optical transitions across the band gap, short-period superlattices are often employed. The oscillator strength of intraband transitions, from holes states confined in one layer to electron states confined in a neighboring layer, are enhanced by increasing the wave function overlap of these states through reduced superlattice period. However, there are limits to accurately controlling an epitaxially grown semiconductor superlattice structure as the number of monolayers in each layer is decreased. For InAs/GaSb type superlattices, periods of 40Å or less are relevant to mid-infrared detection. Characterization and modeling results for a series of InAs/GaSb superlattices with periods ranging 50Å to 20Å will be presented. These results explore the break point between when thinner is better and when reducing the period no longer optimizes the superlattice optical performance.
The effect of interface anisotropy on the electronic structure of InAs/GaSb type-II superlattices is exploited in the design of thin-layer superlattices for mid-IR detection threshold. The design is based on a theoretical envelope function model that incorporates the change of anion and cation species across InAs/GaSb interfaces, in particular, across the preferred InSb interface. The model predicts that a given threshold can be reached for a range of superlattice periods with InAs and GaSb layers as thin as a few monolayers. The model also predicts that the absorbance for the 4 micron superlattices is inversely proportional to their period so that smaller period superlattices can be grown proportionately thinner. An added advantage of thin-period superlattices is a smaller hole mass and a lower Auger recombination rate, which should lead to higher detector operating temperatures. A number of superlattices with periods ranging from 50.6 to 21.2 Å for the 4 μm detection threshold were grown by molecular beam epitaxy based on the model design. Low
temperature photoluminescence and photoresponse spectra confirmed that the superlattice band gaps remained constant at 330 meV although the period changed by the factor of 2.5. Overall, the present study points to the importance of interfaces as a tool in the design and growth of thin superlattices for mid-IR detectors for room temperature operation.
Type-II superlattices composed of alternating thin layers of InAs and GaSb, have been shown to be a highly flexible infrared materials system in which the energy band gap can be adjusted anywhere between 360 meV and 40 meV. These superlattices (SLs) are the III-V equivalent to the well established HgxCd1-xTe alloys used for infrared detection in the short, mid and long wavelength bands of the infrared spectrum. There are many possible designs for these superlattices that will produce the same narrow band gap by adjusting individual layer thicknesses and interface composition. Systematic growth and characterization studies were performed to determine optimum superlattice designs suitable for infrared detection in the 3 to 5 μm wavelength band. For these studies the individual layer thicknesses were less than 35Å. The effects of adding different thickness InSb-like interfaces were also studied. Through precision molecular beam epitaxy, design changes as small as 3Å to the SL layers could be studied. Significant changes were observed in the infrared photoresponse spectra of the various SL samples. The infrared properties of the various designs of these type-II superlattices were modeled using an 8-band Envelope Function Approximation. The infrared photoresponse spectra, combined with quantum mechanical modeling of predicted absorption spectra, were a key factor in the design optimization of the InAs/GaSb superlattices with band gaps in the range of 200 to 360 meV.
In the very long wavelength infrared (VLWIR) band, λ>14 microns, the detector materials are currently limited to extrinsic semiconductors. These extrinsic materials can be either heavily doped bulk semiconductor, like silicon or germanium, or a doped quantum well heterostructure. An alternative choice that provides the opportunity for higher temperature operation for VLWIR sensing is an intrinsic material based on a type-II InAs/Ga(In)Sb superlattice. There are many possible designs for these superlattices which will produce the same narrow band gap by adjusting individual layer thicknesses, indium content or substrate orientation. The infrared properties of various compositions and designs of these type-II superlattices have been studied. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection beyond 15 microns. An overview of the status of this material system will be presented. In addition, the latest experimental results for superlattice photodiodes with cut-off wavelengths as long as 30 microns will be covered.
New infrared (IR) detector materials with high sensitivity, multi-spectral capability, improved uniformity and lower manufacturing costs are required for numerous long and very long wavelength infrared imaging applications. One materials system has shown great theoretical and, more recently, experimental promise for these applications: InAs/InxGa1-xSb type-II superlattices. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection beyond 15 microns. The infrared properties of various compositions and designs of these type-II superlattices have been studied. The infrared photoresponse spectra are combined with quantum mechanical modeling of predicted absorption spectra to provide insight into the underlying physics behind the quantum sensing in these materials. Results for superlattice photodiodes with cut-off wavelengths as long as 25 microns will be presented.
P-type GaAs/AlGaAs quantum well infrared photodetectors (QWIP) represent a complementary technology to the well developed and already commercialized n-type GaAs/AlGaAs QWIP technology. Since n-QWIPs require grating couplings for normal incidence absorption, p-type GaAs/AlGaAs QWIPs have emerged as a viable alternative in some applications. In this paper, progress in optimizing the performance of p-type GaAs/AlGaAs QWIPs through modeling, growth, and characterization is described. Our approach begins with the theoretical design of p-QWIPs based on calculations of optical absorption. Next, samples are grown by MBE according to the theoretical designs and their characteristics measured. p-type QWIPs were optimized with respect to the well and barrier widths, alloy concentration, and dopant concentration; resonant cavity devices were also fabricated and the temperature dependent photoresponse was measured. Based on the progress to date, it is now possible to make some comparisons between the n- and p-type approaches. Further avenues for improvement of p-QWIP photoresponse are being explored by exploiting the rich physics of this coupled multi-band system.
New infrared detector materials with high sensitivity, multi-spectral capability, improved uniformity and lower manufacturing costs are required for numerous long and very long wavelength infrared imaging applications. One materials system has shown great theoretical and, more recently, experimental promise for these applications: InAs/InxGa1-xSb type-II superlattices. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection beyond 10 microns. Far-infrared photoresponse of superlattices with cut-off wavelengths between 15 micrometers and 25 micrometers were studied. The measured photoresponse spectra for both photodiodes and photoconductors are compared to calculated absorption coefficient spectra. The electronic structure and the optical absorption of InAs/InxGa1-xSb superlattice infrared (IR) detector structures are calculated, for several values of x, using our implementation of the 8x8 envelope-function approximation (EFA) formalism. Good experimental-theoretical agreement is obtained regarding the long-wavelength threshold and absorption shape.
Hui Liu, Chunying Song, Aidong Shen, Richard Dudek, Ming Gao, Emmanuel Dupont, Zbigniew Wasilewski, Margaret Buchanan, P. Wilson, Brad Robinson, David Thompson, Gail Brown, Frank Szmulowicz, James Ehret
Quantum wells, especially those made of GaAs and InP related compounds, have enabled several unique infrared devices. Two prime examples are quantum well infrared photodetectors (QWIP) and quantum cascade lasers. This paper discusses a few examples of QWIP related devices: (1) QWIPs are well suited for high speed and high frequency applications--work on achieving high absorption efficiency and high operating temperature has been carried out. (2) A variation of conventional QWIP structures can lead to simultaneous visible and infrared detection, and demonstrations using both GaAs and InP based structures have been made. (3) P- type structures may achieve competitive performance and lend to easy fabricating of large focal plane arrays, and good performance has been achieved in resonant-cavity enhanced p- QWIPs.
The development of devices for mid-, long-, and very long- wavelength IR detection has benefitted greatly from advances in band-gap engineering. Recently, there has been great progress in the development of n-type GaAs/AlGaAs quantum well infrared photoconductor (QWIP) detector arrays in all three technologically important wavelength windows. P-type GaAs.AlGaAs QWIPS represent a viable alternative to n-type GaAs/AlGaAs QWIPs, offering the advantage of normal incidence absorption without the need for grating couplers. The maturity of the MBE of GaAs/AlGaAs layered materials offers the possibility of mass producing low cost, high performance, large size, high uniformity, multicolor, high frequency bandwidth, two-dimensional imaging QWIP arrays. This paper describes progress in optimizing the performance of p- type GaAs/AlGaAs QWIPs through modeling, growth, and characterization. Using the 8x8 envelope-function approximation (EFA), a number of structures were designed and their optical absorption calculated for comparison with experiment. Samples were grown by MBE based on the theoretical designs and their photoresponse measured. P-type QWIPs were optimized with respect to the well and barrier widths, alloy concentration, and dopant concentration; resonant cavity devices were also fabricated and temperature dependent photoresponse was measured. The quantum efficiencies and the background-limited (BLIP) detectivities under BLIP conditions of our own p-QWIPs are comparable to those of n-QWIPs; however, the responsivities are smaller. For our mid-IR p-QWIPs, the 2D doping densities of 1- 2x1012 cm-2 maximized the BLIP temperature and dark current limited detectivity by operating at around 100K. At 80K, the detectivity of the optimum doped sample was (formula available in paper)at 10V bias. Barrier widths greater than 200 A were sufficient to impede the tunneling dark current; resonant cavities enhanced absorption five-fold.
We report a set of high-quality InAs/InGaSb type-II photodetectors grown on GaSb substrates with cutoff wavelengths form 11 to 21 micrometers . The SL structural parameters were very repeatable between samples as evidenced by the consistency of the SL periods and the long wavelength photoresponse cut-off. The measured photoresponse spectra were in excellent agreement with the calculated absorption spectrum. Very low background carrier concentrations were achieved in this samples set. Based on the study, the optimum growth temperature for type-II photodetectors is between 390 to 410 C with a post growth annealing at 495 to 510 C. Thickness non-uniformity of type-II photodiodes was less than 1 percent across 2-inch wafers. We have also demonstrated photodetectors with good performance from 10 to 18 micrometers , directly grown on compliant InGaAs/GaAs substrates.
We report on the growth and characterization of InAs/InGaSb type-II superlattices (SLs) designed with a photoresponse cut-off wavelength of 10 micrometers . The structural parameters, layer thicknesses and compositions, were chosen to optimize the IR absorption for a superlattice with an energy band gap of 120 meV. The energy band structure and optimized absorption coefficient were determined with an 8 X 8 envelope function approximation model. The superlattices were grown by molecular beam epitaxy and comprised of 100 periods of 43.6-angstrom InAs and 17.2-angstrom In.23Ga.77Sb strain balanced to the GaSb substrates. In order to reduce the background carrier concentrations in this material, SLs grown with different substrate temperatures were compared before and after annealing. The measured photoresponse cut-off energies of 116 +/- 6 meV is in good agreement with the designed value for the SLs. The intensity of the measured mid-IR photoresponse was found to improve by an order of magnitude for the SLs grown at the lower substrate temperature and then annealed at 520 degrees C for 10 minutes. However, the x-ray diffraction spectra were very similar before and after annealing. The temperature dependent Hall measurements at low temperatures were dominated by holes with quasi 2D behavior.
We present results on p-type quantum well IR photodetectors (QWIPs) based on GaAs substrates, and discuss issues related to the optimization of their performance. Due to the fact that a p-QWIP allows normal incidence absorption, the simplicity in device fabrication makes it interesting for implementing a pixel-less imaging device based on the integration of QWIP and light emitting diode.
The absorption of infrared radiation at normal incidence in p-type GaAs/AlGaAs quantum wells, unlike in n-type, is fundamentally allowed. We have measured and theoretically modeled the bound-to-continuum absorption in these p-type materials. The infrared absorption coefficient was calculated are based on the electronic structure, wave functions and optical matrix elements obtained from an 8 X 8 envelope-function approximation (EFA) calculation. The 8 X 8 EFA Hamiltonian incorporates the coupling between the heavy, light, spin-orbit, and conduction bands. In calculating the continuum states for bound-to- continuum intersubband absorption, we do not enclose the well in an artificial box with infinite walls. A comparison of the theoretical absorption and measured photoresponse results verified the accuracy of our model and provided a basis for optimizing the design of p-type quantum wells for infrared detection.
Exciton and biexciton binding energies, and wave functions are calculated with a three parameter variational model in an effective mass approximation for a rectangular GaAs quantum well wire surrounded by an AlGaAs cladding. Moreover, the Al interdiffusion into the wire and the finite band offsets between the wire and the cladding have been included. The Coulomb interaction terms are treated exactly in their full 3D form throughout the calculation, especially in the case of the biexciton, a more physically realistic procedure then used in previous calculations which employed an effective 1D potential. Our treatment is unique in the use of a 2D Fourier expansion in the Coulomb potential terms. For the range of dimensions studied, the inclusion of the Al interdiffusion had a pronounced affect on the binding energies when compared to those obtained from the infinite barrier model. Using the results of the exciton and biexciton calculation, we calculate the third-order nonlinear optical susceptibility as a function of pump-probe frequencies in a small range about the exciton absorption resonance. We have found, depending upon wire dimensions and the amount of pump detuning, values of the susceptibilities to be on the order of 10-1 esu and a large off-resonance optical gain due to biexciton formation.
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