We are in the midst of the second quantum revolution. Research institutes and companies worldwide are working toward harnessing the power of quantum physics for technological applications. Gapless surface states on topological insulators are protected from elastic scattering on nonmagnetic impurities, which makes them promising candidates for low-power electronic applications. Conventional III–V infrared (IR) materials have the flexibility to engineer topologically protected surface states that can be resistant to ambient environments. In particular, largely hybridized band structures provide thermodynamically stable edge currents at the higher operating temperatures, which are important for IR sensing applications. Hence, we focused on optimizing two critical components for establishing ambient topological insulator; one for enlarging the hybridization gap, Δ, and the other for reducing bulk conduction in InAsSb/InGaSb structures. We performed a modelling study, and achieved an approximately 79 meV from InAs/InGaSb superlattices (SLs) lattice matched to AlSb, which is one of the largest reported value by far. Based on this modeling study, we selected a baseline SL design of InAsSb/GaSb on GaSb with Δ of ~62 meV to address key material issues such as finite bulk carrier conduction in undoped region of SLs. Systematic growth/processing optimization was performed in order to reduce the bulk charge carriers. The origin of constrained carrier dynamics in largely hybridized SL system and their effects on the designed topological structure were discussed.
To test whether conventional infrared materials can be used to control the electronic wavefunction to form a topological state, a 6.2 Å metamorphic (InAs/InGaSb/InAs) quantum well (QW) absorber with ~60 meV of hybridization gap (Δ) was investigated. We developed a thick metamorphic InGaSb buffer layer on GaAs wafer to create a 6.2 Å lattice constant for the QW growths. The lattice constant of virtual substrates (VSs) was very close to the target value of 6.2 Å, however the resulting crystalline quality of the VSs was inefficient for topological insulator. The cross-sectional transmission electron microscopy image revealed that the dislocation density in the InGaSb buffer layer was high closer to the GaAs substrate and gradually reduced upon continued growth. However some mismatch-related defects were propagated into the absorber region, consequently degraded the transport quality of absorber. The QW absorber grown on VS had a low mobility. The mobility was dramatically improved by selecting pseudomorphic QW or superlattice absorber with a small Δ that was grown on a lattice-matched GaSb substrate. Hence, in order for the proposed 6.2 Å materials to be viable for sensing applications, a critical effort will be the development of better optimized metamorphic buffers for the design or of highlyhybridized psedomorphic designs that can be grown on lattice-matched substrates.
Quantum photonics opens doors for applications in sensing, data transfer, and quantum computing. Application areas in many of these technologies require in some manner tunable single photon sources. Hyperbolic metamaterials, composed of metallic building blocks embedded in dielectric media control emission lifetime by modifying the photon density of states. However, no previous efforts have explored the transient modification of metamaterials to modulate emission. Antimony-based semiconductor hyperbolic metamaterials (SHMMs) offer a route to modulation of these resonances at the mid-infrared (IR) wavelength range, which would modulate emission. In this work, we demonstrate the ability to create an ultrafast hyperbolic momentum state in metallic InAsSb/dielectric GaSb stacks and explore the possibility of transient modification of metamaterials by controlling the optical properties of photon emission. If successful, this study will establish a new platform for deterministic single photon emission that can be integrable into opto-electronic platforms and dramatically advance optical quantum technologies. Properly engineered quantum well structures are grown by molecular beam epitaxy with Si-doping in order to convert the InAsSb layers from dielectric to metallic at IR frequencies. The carrier excitation scheme of the engineered hyperbolic stacks is investigated in a variety of excitation levels using pump–probe measurements. The photo-excited carriers in the structure with a metal fraction of ∼0.5 show a polarization dependent reflectivity change, which indicates a transient hyperbolic metamaterial state in the heterostructure induced by the pump laser.
Hyperbolic metamaterials are valuable potential single-photon emitters because of their large density of states at phase boundaries. We grow metamaterial stacks using alternating layers of undoped GaSb (dielectric) and Si-doped InAsSb (metallic). This combination can act as a dielectric in the sample plane, but a metal perpendicular to the plane, forming a hyperbolic metamaterial (HMM) state depending on the density of free electrons. We demonstrate this behavior by injecting free electrons using an ultrafast 1300-nm pump laser, while probing the differential reflectivity and transmittivity with a linearly polarized probe in the range of 4-5 µm. The difference in results for s- and p-polarized probes demonstrates the anisotropic nature of the hyperbolic state, suggesting that single photons at mid infrared (MIR) frequencies may be efficiently emitted in a highly directional manner. The HMM state is also dependent on the metal fraction, which we control via the relative thicknesses of the layers. Additionally, spectroscopic ellipsometry reveals that the metal fraction is consistently lower than the nominal value, a phenomenon we attribute to doped carriers being squeezed to the center of the InAsSb layers. Our analysis of the pump-related shift of the metal/dielectric/HMM phase diagram shows that our sample structure is a highly tailorable avenue to MIR spontaneous photon emission.
We engineer the nonlinear response of multilayered hyperbolic metamaterials systems by employing the optical response of Arsenide semiconductor materials. The photoexcited carriers lead to spectral, temporal, and polarization dependent optical response that is described in this talk. The response is suggestive of generating a transient hyperbolic materials response in this system. Exploration of this response and the use of these materials as a basis for nonlinear metasurfaces will be discussed.
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.
Significantly improved carrier lifetimes in very long-wave infrared (VLWIR) InAs/GaInSb superlattice (SL) absorbers are demonstrated using time-resolved microwave reflectance (TMR) measurements. A nominal 47.0 Å InAs/21.5 ÅGa0.75In0.25Sb SL structure that produces an ∼25 μm response at 10 K has a minority carrier lifetime of 140±20 ns at 18 K, which is an order-of-magnitude improvement compared with previously reported lifetime values for other VLWIR detector absorbers. This improvement is attributed to the strain-engineered ternary SL design, which offers a variety of epitaxial advantages and ultimately leads to the improvements in the minority carrier lifetime by mitigating defect-mediated Shockley–Read–Hall (SRH) recombination centers. By analyzing the temperature dependence of TMR decay data, the recombination mechanisms and trap states that currently limit the performance of this SL absorber were identified. The results show a general decrease in the long-decay lifetime component, which is dominated by SRH recombination at temperatures below ∼30 K and by Auger recombination at temperatures above ∼45 K. Since the strain-balanced ternary SL design offers a reasonably good absorption coefficient and many epitaxial advantages during growth, this VLWIR SL material system should be considered as a competitive candidate for VLWIR photodetector technology.
Significantly improved carrier lifetimes in very long wavelength infrared (VLWIR) InAs/GaInSb superlattice (SL) absorbers are demonstrated by using time-resolved microwave reflectance (TMR) measurements. A nominal 47.0 Å InAs/21.5 Å Ga0.75In0.25Sb SL structure that produces an approximately 25 μm response at 10 K has a minority carrier lifetime of 140 ± 20 ns at 18 K, which is an order-of-magnitude improvement compare to previously reported lifetime values for other VLWIR detector absorbers. This improvement is attributed to the strain-engineered ternary SL design, which offers a variety of epitaxial advantages and ultimately leads to the improvements in the minority carrier lifetime by mitigating defect-mediated Shockley-Read-Hall (SRH) recombination centers. By analyzing the temperature dependence of TMR decay data, the recombination mechanisms and trap states that currently limit the performance of this SL absorber are identified. The results show a general decrease in the long-decay lifetime component, which is dominated by SRH recombination at temperatures below ~30 K, and by Auger recombination at temperatures above ~45 K. This result implies that minimal improvement can be made in the minority carrier lifetime at temperatures greater than 45 K without further suppressing Auger recombination through proper band engineering, which suggests that the improvement to be gained by mitigation of the SRH defects would not be substantial at these temperatures. At temperatures lower than 30 K, some improvement can be attained by mitigated of the SRH recombination centers. Since the strain-balanced ternary SL design offers a reasonably good absorption coefficient and many epitaxial advantages during growth, this VLWIR SL material system should be considered a competitive candidate for VLWIR photodetector technology.
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.
KEYWORDS: Stereolithography, Antimony, Superlattices, Interfaces, Atomic force microscopy, Temperature metrology, Long wavelength infrared, Indium arsenide, Surface roughness, Transmission electron microscopy
The objective of this work is to establish molecular beam epitaxy (MBE) growth processes that can produce high
quality InAs/GaInSb superlattice (SL) materials specifically tailored for very long wavelength infrared (VLWIR)
detection. To accomplish this goal, several series of MBE growth optimization studies, using a SL structure of 47.0 Å
InAs/21.5 Å Ga0.75In0.25Sb, were performed to refine the MBE growth process and optimize growth parameters.
Experimental results demonstrated that our “slow” MBE growth process can consistently produce an energy gap near 50
meV. This is an important factor in narrow band gap SLs. However, there are other growth factors that also impact the
electrical and optical properties of the SL materials. The SL layers are particularly sensitive to the anion incorporation
condition formed during the surface reconstruction process. Since antisite defects are potentially responsible for the
inherent residual carrier concentrations and short carrier lifetimes, the optimization of anion incorporation conditions, by
manipulating anion fluxes, anion species, and deposition temperature, was systematically studied. Optimization results
are reported in the context of comparative studies on the influence of the growth temperature on the crystal structural
quality and surface roughness performed under a designed set of deposition conditions. The optimized SL samples
produced an overall strong photoresponse signal with a relatively sharp band edge that is essential for developing
VLWIR detectors. A quantitative analysis of the lattice strain, performed at the atomic scale by aberration corrected
transmission electron microscopy, provided valuable information about the strain distribution at the GaInSb-on-InAs
interface and in the InAs layers, which was important for optimizing the anion conditions.
We show here that n-type InAs/InGaSb superlattices can be electrically isolated from lightly doped n-type GaSb
substrates at much higher temperatures than from the more common undoped p-type GaSb substrates without the use of
a large band gap insulating buffer layer. Temperature dependent Hall effect measurements show superlattice conduction
up to near room temperature, which is significantly higher than the 20 K observed for p-type substrates. Multi-carrier
analysis of magnetic field dependent transport data demonstrate the absence of a substrate related conduction channel.
We argue that the isolation is due to the depletion layer at the p-n junction between the p-type buffer layer and the n-type
substrate.
We report ternary growth studies to develop a largely strained InAs/InGaSb superlattice (SL) material for very long wavelength infrared (VLWIR) detection. We select a SL structure of 47.0 Å InAs/21.5 Å In0.25Ga0.75Sb that theoretically designed for the greatest possible detectivity, and tune growth conditions for the best possible material quality. Since material quality of grown SLs is largely influenced by extrinsic defects such as nonradiative recombination centers and residual background dopings in the grown layers, we investigate the effect of growth temperature (Tg) on the spectral responses and charge carrier transports using photoconductivity and temperature-dependent Hall effect measurements. Results indicate that molecular beam epitaxy (MBE) growth process we developed produces a consistent gap near 50 meV within a range of few meV, but SL spectral sensing determined by photoresponse (PR) intensity is very sensitive to the minor changes in Tg. For the SLs grown from 390 to 470 °C, a PR signal gradually increases as Tg increases from 400 to 440 °C by reaching a maximum at 440 °C. Outside this growth window, the SL quality deteriorates very rapidly. All SLs grown for this study were n-type, but the mobility varied in a variety of range between 11,300 and 21 cm2/Vs. The mobility of the SL grown at 440 °C was approximately 10,000 V/cm2 with a sheet carrier concentration of 5 × 1011 cm-2, but the mobility precipitously dropped to 21 cm2/Vs at higher temperatures. Using the knowledge we learned from this growth set, other growth parameters for the MBE ternary SL growth should be further adjusted in order to achieve high performance of InAs/InGaSb materials suitable for VLWIR detection.
Annealing effect on the quality of long wavelength infrared (LWIR) InAs/GaSb superlattices (SLs) has been
investigated using atomic force microscopy (AFM), photoconductivity, temperature dependent Hall, and time-resolved
differential transmission measurements using an electronically delayed pump-probe technique. Quarters of a single SL
wafer were annealed at 440, 480, and 515 °C, respectively for 30 minutes under a Sb-over pressure. Morphological
qualities of the SL surface observed by AFM did not show any indication of improvement with annealing. However, the
spectral intensity measured by photoconductivity showed an approximately 25 % improvement, while the band gap
energy remained at ~107 meV for each anneal, The electron mobility was nearly unaffected by the 440 and 480 °C
anneals, however showed the improvement with the 515°C anneal, where the mobility increased from ~4500 to 6300
cm2/Vs. The minority carrier lifetime measured at 77 K also showed the improvement with annealing, increasing from
12.0 to 15.4 nanoseconds. In addition to the longer lifetimes, the annealed samples had a larger radiative decay
component than that of unannealed sample. Both the longer measured lifetime and the larger radiative decay component
are consistent with the modest improvement in the quality of the annealed SL sample. Overall the qualities of LWIR SL
materials can be benefit from a post growth annealing technique we applied.
The strain distribution across interfaces in InAs/GaSb superlattices is investigated by scanning transmission electron
microscopy (STEM), using an aberration corrected probe. Atomic resolution images of the superlattices (grown on
(100)-GaSb substrates) were acquired using the high-angle annular dark field (HAADF) imaging technique. For
quantitative strain analysis, the peak-pair algorithm was used to determine the local atomic displacements across
interfaces and within individual layers in the structure. The measured displacements were then used to calculate the
strain map with respect to a reference lattice in the GaSb-substrate region. To precisely identify the local regions in the
strain map Fourier transformation of the HAADF-STEM image was performed to obtain the chemically-sensitive (200)-
Fourier component of the image. A comparison of these images with strain profiles determined from the strain maps
revealed that the GaSb-on-InAs interface is GaAs-like, with a tensile strain of - 0.018 ± 0.003, whereas the overall strain
at the InAs-on-GaSb interface was negligible. In addition, the strain within the GaSb layers was found to be
compressive, with a magnitude of 0.008 ± 0.003, indicating In incorporation in these layers.
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.
The epitaxial growth parameters optimized for mid wavelength infrared (MWIR) InAs/GaSb superlattice (SL)
growth are not necessarily the best parameters for very long wavelength infrared (VLWIR) SL growth. While the cutoff
wavelength of the SL structure can be easily extended from a MWIR to a VLWIR spectral range by increasing InAs
layer thickness with a fixed GaSb layer thickness, the structural and optical properties of SLs are changing as well, and
these changes are not necessarily beneficial to the material quality of VLWIR SLs. For instance, tensile strain in the SL
rapidly increases as InAs layer thickness increases. This impacts the interface growth processes used to strain balance
the average lattice constant of the SL to match the GaSb substrate, the interface engineering in a VLWIR SL is very
different than that in a MWIR SL. Using a baseline SL design of 16 monolayers (MLs) InAs/7 MLs GaSb, a systematic
study of controlling the Sb/As background pressure and shutter sequence during interface formation was performed in
order to minimize tensile strain in the VLWIR SLs. The effect of various shutter sequences on the SL morphological
quality and their impact on optical spectral response is reported. By inserting 0.5 MLs of InSb-like interfaces, using a
migration-enhance-epitaxy technique, in the baseline SL design, while maintaining a total SL period of 23 MLs, we
achieved a high structural quality, strain balanced LWIR SL with a photoresponse onset at 15 μm.
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.
The performance and operating temperature of infrared (IR) detectors is largely limited by thermal generation and noise processes in the active region of the device. Particularly, excess background charge carriers enhance Auger recombination and dark currents, and depress the detector figures of merit. Therefore, reducing background carriers in the undoped region of pin diodes is an important issue for developing high-operating temperature IR detectors. In this
paper, we discuss how, through low-temperature Hall measurements, we optimized several growth and design parameters to lower residual carrier densities in various mid-IR InAs/GaSb superlattice (SL) designs. Among the growth/processing parameters investigated in the 21 Å InAs/24 Å GaSb SLs, neither growth temperature nor in-situ
post-growth annealing significantly affected the overall carrier type and density. All of the mid-IR SL samples
investigated were residually p-type. The lowest carrier density (1.8x1011 cm-2) was achieved in SLs grown at 400 °C and
the density was not reduced any further by a post-growth anneal. The growth parameter that most affected the carrier
density was interface composition control. With a minor variation in interface shutter sequence, the carrier density
dramatically increased from ~2x1011 to 5x1012 cm-2, and the corresponding mobility dropped from 6600 to 26 cm2/Vs,
indicating a degradation of interface quality. However, the carrier density was further reduced to 1x1011 cm-2 by
increasing the GaSb layer width. More importantly, a dramatic improvement on the photoluminescence intensity was
achieved with wider GaSb SLs. The disadvantage is that as GaSb layer width increases from 24 to 48 Å, the photoluminescence peak position shifts from 4.1 to 3.4 μm, for a fixed InAs width of 21 Å, indicating a photodiode based on these wider designs would fall short of fully covering the 3 to 5 μm mid-IR spectral region.
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.
The purpose of this work is to explore mid-infrared (IR) photodetector materials that can operate at room temperature. Shorter-period InAs/GaSb superlattices (SLs) have larger intervalance band seperations, which is beneficial for reducing Auger recombination and tunneling current, thus making room temperature operation possible. To test these possibilities, several short-period SLs ranging from 50 to 11 Å were designed for 4 μm detection threshold and molecular beam epitaxy was used to grow specially designed structures. Since morphological degradation is generally expected in shorter-period SLs, their structural qualities were monitored by transmission electron microscopy. The effect of layer properties on the optical and electrical properties was studied using low temperature photoconductivity measurements and magnetic field dependent Hall measurements. The samples with larger-periods (50 to 31 Å) showed excellent structural qualities, leading to sharper photoresponse band edge (5 meV) and lower residual background carrier concentrations (8x1010 cm-2). As the period approached 24 Å, slight layer thickness undulations within the SLs were observed and these undulations intensified as the period further reduced to 17 Å. Evidently, these structural degradations strongly influence their optical properties causing significant broadening in photoresponse band edge (9 meV). In the thinner samples with the period below 17 Å, no optical signal was detected. With slower growth rates, samples with periods as thin as 19 Å were grown without significant layer thickness variations.
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.
We investigated a novel possibility to attain all-optical logical gates. The host of the device was a thin-film semiconductor (CdS, GaAs, InP) on glass produced by various methods (pulsed-laser deposition and metal organic chemical vapor deposition). In the thin-film two visible laser beams, the primary and secondary ray, were crossed in the same spot. In this way, the secondary beam caused a transmission decrease in the primary beam. Laser crossing is an extremely undemanding concept based on electronic absorption alterations. Apparently, every semiconductor can be used for laser crossed all-optical logics and, in contrary to other semiconductor based concepts, laser crossing does not demand specific materials, material qualities or nonlinear features. The unmatched overall simplicity and possible THz operations recommend laser crossing for the realization of all-optical digital devices.
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.
One of the most straightforward methods possible is presented and investigated to form thin film GaAs. The film was deposited on unheated glass in vacuum (10-6 Torr) by the ablation from a GaAs wafer with the emission of a pulsed Nd:YAG laser (532 nm, 6 ns, 10 Hz). The photoluminescence, photocurrent, transmission and micro-Raman measurements of the films demonstrate that films with promising optoelectronic properties have been formed. Most importantly, from the viewpoint of light emitting and optoelectronic device production, the films show photoluminescence of comparable intensity with the bulk material without emissions owing to impurities, although the films show a rather flat absorption edge which indicates tail states. The observed photocurrent was in the μA/W range driven by rather moderate electric fields on the order of 100 V/cm. Concerning the material quality, the films have an extremely smooth surface as demonstrated with scanning electron microscopy. Grown GaAs films on glass substrates were amorphous evidenced by X-ray diffraction measurements, however, micro-Raman measurements showed crystalline phonon modes, suggesting that localized crystalline structure might co-exist in amorphous GaAs films.
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.
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