L. Grazulis

Interfaces as design tools for short-period InAs/GaSb type-II superlattices for mid-infrared detectors

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. Although the oscillator strengths are predicted to be larger for thinner period superlattices, the absorption coefficients are comparable because of the compensating effect of larger band widths. However, larger intervalence band separations for thinner-period samples should lead to longer minority electron Auger lifetimes and higher operating temperatures in p-type SLs. In addition, the hole masses for thinner-period samples are on the order the free-electron mass rather than being effectively infinite for the wider period samples. Therefore, holes should also contribute to photoresponse. 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.

Band gap formation in graphene by in-situ doping

We report the formation of band gaps in as-grown stacks of epitaxial graphene with opposite doping. Control of in-situ doping during carbon source molecular beam epitaxy growth on SiC was achieved by using different carbon sources. Doping heterostructures were grown by stacking n-type material from a C60 source on p-type material from a graphite filament source. Activation energies for the resistivity and carrier concentration indicated band gaps up to 200 meV. A photoconductivity threshold was observed in the range of the electrical activation energies. Band gap formation is attributed to electric fields induced by spatially separated ionized dopants of opposite charge.

Control of residual background carriers in undoped mid-infrared InAs/GaSb superlattices

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.

Exploring optimum growth window for high quality InAs/GaInSb superlattice materials

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.

Homoepitaxy of 6H-SiC by solid-source molecular beam epitaxy using C60 and Si effusion cells

Abstract High quality homoepitaxial 6H-SiC films have been grown by solid-source molecular beam epitaxy (MBE) using C 60 and Si effusion cells. Scanning electron micrographs show terraced surfaces indicative of step-flow growth. Cross-sectional transmission electron microscopy results demonstrate extremely good epitaxial growth with no hint of dislocations, double-positioning boundaries, or 3C inclusions. We believe this is the first report of homoepitaxy of 6H-SiC using C 60 and the first instance of silicon carbide (SiC) epitaxy using a Si effusion cell in the evaporation rather than the sublimation mode. This combination of solid-source MBE and determination of appropriate growth conditions have led to superior homoepitaxial growth of 6H-SiC.

Type-II superlattice materials for mid-infrared detection

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.

Stability studies of lead sulfide colloidal quantum dot films on glass and GaAs substrates

The stability of colloidal PbS quantum dot (QD) films deposited on various substrates including glass and GaAs was studied. Over a period of months, the QD film sample was re-tested after being left unprotected in air under ambient conditions. Despite exposure to 532 nm laser excitation and cooling to cryogenic temperatures, the initial photoluminescence (PL) remained stable between tests. We also retested a set of samples that had remained under ambient conditions for over 2 years. To track potential changes to the QDs over time, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), optical microscopy, UV-Vis-NIR spectrophotometry and atomic force microscopy (AFM) were employed. Evidence points towards oxidation enforced shrinking of the active QD volume causing a blue shift of the absorption and photoluminescence. The presented studies are important for reliability expectations of light emitters based on PbS QDs.

The role of InAs thickness on the material properties of InAs/GaSb superlattices

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.

Study of strain balance in long wavelength infrared InAs/GaSb superlattice materials

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.

Electrical and structural characterization of a single GaSb∕InAs∕GaSb quantum well grown on GaAs using interface misfit dislocations

Interface misfit formation has been used for the growth of high mobility GaSb∕InAs single quantum wells (SQW) formed on GaAs substrates. The SQW structure was topped with 800A GaSb, followed by 100A GaSb:Si (5×108cm−3), 10nm GaSb, 10nm InAs, and finally 250nm GaSb on a GaAs substrate. The structural quality was examined using high resolution x-ray diffraction and transmission electron microscopy. Reciprocal space mapping indicated that the GaSb was completely relaxed. A high resolution x-ray rocking curve showed good agreement between the proposed structure and the simulation, assuming that all layers were relaxed to the GaSb lattice, and clearly showed interference fringing from individual layers. Atomic force microscopy showed the film appeared textured, and that the final growth occurred by step flow growth. The observed peak-to-peak roughness was 7nm over a 100×100μm2 square area. Plane view transmission electron microscopy analysis showed a nearly regular array of Lomer dislocations responsible for t...