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High power laser systems require the use of optical isolators to prevent coupling of reflected light into the pump laser. Terbium Gallium Garnet (TGG) and Potassium Terbium Fluoride (KTF) are materials used as optical isolators and while they have been grown for many years, advances in crystal growth and processing make a new set of measurements of the Verdet coefficients of these materials desirable. We present new measurements of the Verdet coefficients of TGG and KTF from 0.405 μ to 1.55 μ and derive expressions for the spectral behavior of the Verdet coefficients.

International audience; The oxide/semiconductor structure is key to controlling current in electronic devices and HfO$_2$ is a common gate material in conventional electronic devices due to its favorable dielectric properties. Graphene devices also require insulating gates. We demonstrate a unique direct growth approach to obtain the bottom gate structure (graphene/HfO$_2$/n-SiC). The present approach involves transfer of graphene grown by chemical vapor deposition (CVD) on Cu to oxidized Si wafers, a complex process prone to low yield and reduced performance. Furthermore, HfO$_2$ is preferred to SiO$_2$ because of its superior properties. The proposed concept consists of the direct deposition of graphene by solid carbon molecular beam epitaxy on Hf metal coated n-type SiC, followed by oxygen intercalation to form HfO$_2$. The oxygen intercalation will then convert the underlying Hf into HfO$_2$ due to the strong affinity of Hf with oxygen. We identify the graphene/HfO 2 formation by Raman, X-ray photoelectron spectroscopy (XPS), Low energy electron diffraction (LEED), Low energy electron microscopy (LEEM) and electrical properties including Hall mobility and leakage current measurement.

Through magneto-transport measurements and analysis of the observed Shubnikov de Haas oscillations in (010) (AlxGa1-x)2O3/Ga2O3 heterostructures, spin-splitting of the Landau levels in the (010) Ga2O3 two-dimensional electron gas (2DEG) has been studied. Analysis indicates that the spin-splitting results from the Zeeman effect. By fitting the both the first and second harmonic of the oscillations as a function of magnetic field, we determine the magnitude of the Zeeman splitting to be 0.4$\hbar\omega_c$, with a corresponding effective g-factor of 2.7, for magnetic field perpendicular to the 2DEG., Comment: 7 pages, 5 figures

Titanium nitride (TiN) is a recent material of interest for plasmonics and electrodes due to its high-temperature stability, robustness, and CMOS-compatibility. Previously we found that optical plasmonic quality correlated with the microstructural quality of TiN, and both properties depended on deposition temperature: TiN grown at 550 °C resulted in favorable optical properties for plasmonic materials. The present work investigates strain and stoichiometry of TiN films as a result of varying deposition temperature. Six (111)-oriented TiN films were grown on c-sapphire with reactive magnetron sputtering; substrate temperatures ranged from 280 °C to 760 °C. The TiN films were analyzed with x-ray photoelectron spectroscopy, x-ray diffraction, and high-resolution transmission electron microscopy to examine composition, strain, lattice constants, unit cell angles, dislocation defects, mosaicity, and low-angle tilt boundaries. We found that lattice constants only remained a constant 4.242 A at temperatures above 490 °C. Above this substrate temperature the deviation from a 90˚ unit cell angle remained at a minimum, indicating less strain. When combined with the previously-found plasmonic material properties of the films, this trend suggests that more relaxed films grow above 490 °C and yield lower relative optical loss, and favorable plasmonic material quality factors. Rocking curves suggested fewest dislocation defects and least mosaicity in the optically favorable 550 °C film. The results of this work indicate that microstructure, most significantly strain and defects, correlates with optical properties in TiN. This work emphasizes the importance of specific deposition conditions—in this case, substrate temperature—to minimize strain and defect density for achieving optimal plasmonic properties for TiN thin films.

We investigate the minority carrier lifetime of Be-doped InAs/InAsSb type-II superlattices as a function of doping density and temperature using time-resolved photoluminescence (TRPL) to determine if switching the superlattice type from the typical n-type residual carrier concentration to p-type may improve device performance by improving the lifetime–mobility product. The introduction of the Be dopant to the superlattice reduces the carrier lifetime, first by a factor of ∼3 for doping densities near or below the n-type residual carrier concentration, then by an order of magnitude for samples doped well above the residual carrier concentration. Further, the higher-doped p-type samples demonstrate two distinct TRPL decay regimes and two peaks in the PL spectra, suggesting the formation of an additional acceptor-related recombination pathway leading to the observed shorter carrier lifetime.

Authors discuss how anion incorporation was controlled during the epitaxial growth process to develop InAs/GaInSb superlattice (SL) materials for very long wavelength infrared applications. A SL structure of 47.0 A InAs/21.5 A Ga0.75In0.25Sb was selected to create a very narrow band gap. Although a molecular beam epitaxy growth developed can produce a strain balanced ternary SL structure with a precisely controlled band gap around 50 meV, the material quality of grown SL layers is particularly sensitive to growth defects formed during an anion incorporation process. Since Group III antisites are the dominant structural defects responsible for the low radiative efficiencies, the authors focus on stabilizing III/V incorporation during SL layer growth by manipulating the growth surface condition for a specific anion cracking condition. The optimized ternary SL materials produced an overall strong photoresponse signal with a relatively sharp band edges and a high mobility of ~10,000 cm2/V s that is important for developing infrared materials.

Guanine and poly(methyl methacrylate) (PMMA) layers 60 nm thick were fabricated by physical vapor deposition and spin-coating, respectively, for potential application as bio-based field-effect transistors. A few layers of graphene on a variety of non-flexible and flexible supports were used as substrates for the guanine and PMMA layers. The interactions between these layers and graphene were studied and their effects on the electrical properties of the graphene were monitored over an extended period of time. Guanine had no noticeable effects on the transport properties of graphene. The presence of PMMA reduced the charge-carrier mobility of graphene by up to 42%. This combination has potential for use as a device in biosensors, environmental science, computer-processing, and graphene-based electronics.

In order to limit cooling requirements, it is important to operate superlattice devices such as infrared detectors at the highest possible temperatures consistent with maintaining satisfactory figures of merit regarding signal and noise. One of the characteristics governing the device performance is vertical carrier mobility, although only horizontal mobilities are routinely measured. Recently, we calculated low-temperature vertical and horizontal mobilities, as limited by interface roughness scattering, for type-II InAs/GaSb superlattices as a function of SL dimensions and the degree of roughness. We found that the horizontal mobility was a double-valued function of the roughness correlation length, Λ. Here, we show that the indeterminacy of Λ can be overcome by comparing the temperature dependence of the calculated and measured mobilities; hence, we extend the calculation to higher temperatures. While the scattering mechanism itself is temperature independent, the band structure and the carrier distribution are temperature-dependent. As a function of temperature, we find that as a function of the correlation length, mobilities can increase, decrease, or remain constant. This behavior is explained on the basis of the physics of the problem.

There are continuing efforts to develop type-II superlattice (SL) materials for very long wavelength infrared (VLWIR) detector applications. However, the SLs have high residual electron background doping densities that depend on SL growth conditions, which lead to shorter minority carrier lifetime and lower performance parameters than theoretically predicted. In this study, the authors compare the technical advantages of using InAs/GaInSb over InAs/GaSb SL with respect to reducing the electron doping levels. Our temperature-dependent electrical transport measurements show that the InAs/GaInSb SL design has a lower electron density than the InAs/GaSb SL with the same bandgap and have electron densities (mobilities) on the order of the mid 1011 cm−2 (25 000 cm2/V s). Since small period InAs/GaInSb SLs also produce greater Auger recombination suppression for a given VLWIR gap than the large period InAs/GaSb SL, the InAs/GaInSb SL appears to be a better candidate for long lifetime IR materials for future very long wavelength infrared devices.

Titanium nitride (TiN) is a mechanically robust, high-temperature stable, metallic material receiving considerable attention for resilient plasmonics. In this work, the authors fabricated six heteroepitaxial TiN films on sapphire using controllably unbalanced reactive magnetron sputtering. They examined the effect of substrate growth temperature on the plasmonic and crystalline quality of the film. Optical properties of all films were obtained from spectroscopic ellipsometry; plasmonic quality factors were determined from the real and imaginary parts of the dielectric function. The authors determined crystallinity using x-ray diffraction and surface morphology using atomic force microscopy. X-ray diffraction showed (111) TiN peaks with Pendellosung fringes indicating consistent heteroepitaxy. Atomic force microscopy showed smooth surfaces with root mean square surface roughness ranging from 0.2 to 2.6 nm. Based on this characterization, the authors determined that the substrate deposition temperature of 550 °C yielded (111)-oriented heteroepitaxial TiN with minimal surface roughness. The authors found that 550 °C also gave highest plasmonic quality factors for all wavelengths, approaching the values of today's best plasmonic materials (such as Au and Ag). Further, the Q-factors at wavelength 1550 nm inversely correlated with calculated lattice constants. Their results indicate that the plasmonic response of TiN is directly linked with structural quality of the film.Titanium nitride (TiN) is a mechanically robust, high-temperature stable, metallic material receiving considerable attention for resilient plasmonics. In this work, the authors fabricated six heteroepitaxial TiN films on sapphire using controllably unbalanced reactive magnetron sputtering. They examined the effect of substrate growth temperature on the plasmonic and crystalline quality of the film. Optical properties of all films were obtained from spectroscopic ellipsometry; plasmonic quality factors were determined from the real and imaginary parts of the dielectric function. The authors determined crystallinity using x-ray diffraction and surface morphology using atomic force microscopy. X-ray diffraction showed (111) TiN peaks with Pendellosung fringes indicating consistent heteroepitaxy. Atomic force microscopy showed smooth surfaces with root mean square surface roughness ranging from 0.2 to 2.6 nm. Based on this characterization, the authors determined that the substrate deposition temperature of 5...

The mid-infrared 21 A InAs/24 A GaSb superlattices (SLs) designed for the 4 μm cutoff wavelength were grown by molecular beam epitaxy at growth temperatures between 370 and 430 °C in order to reduce residual background carriers. The lowest density of 1.8 × 10 11 cm -2 was obtained from the SLs grown at 400 °C. With increasing growth temperature, in-plane hole mobility decreased from 8740 to 1400 cm 2 / Vs due to increased interfacial roughness, while the photoluminescence (PL) intensity increased due to a decrease in the number of nonstoichiometric nonradiative defects. Further reduction of carrier density to 1 × 10 11 cm -2 was achieved by increasing barrier width. As GaSb layer width increases from 24 to 48 A, the cutoff wavelength decreased from 4.1 to 3.4 μm, which is still in the mid-infrared detection window. More importantly, a dramatic improvement on the PL intensity and the full width at half maximum was achieved from the SL samples with the wider GaSb widths. All mid-infrared SL samples investigated in our studies were residually p-type.

Electrical properties, Fe concentration, and deep centers in semi-insulating Fe-doped GaN substrates grown by hydride vapor phase epitaxy (HVPE) were characterized by temperature-dependent Hall-effect measurements, secondary ion mass spectroscopy, and thermally stimulated current (TSC) spectroscopy. Five adjacent samples from a low-[Fe] wafer displayed very high resistivity, dominated by a center at 0.94 eV. At least six traps were observed in the samples by TSC, with trap B (0.56-0.60 eV) being dominant. A metastable trap A1 at ∼0.82 eV appeared after white-light illumination at 300 K. A sample from a high-[Fe] wafer displayed a lower resistivity, dominated by a center at 0.58 eV. The largest TSC peak in this sample was trap A1, although trap B also appeared. These TSC traps are compared with deep-level-transient-spectroscopy traps reported in conductive epitaxial and bulk HVPE-GaN. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The main goal of this research was to maintain the bulk charge carrier mobility of graphene, after deposition of the gate dielectric layer used for making transistor devices. The approach was introducing a thin film of deoxyribonucleic acid (DNA) nucleobase purine guanine, deposited by physical vapor deposition (PVD), onto layers of graphene that were transferred onto various flexible substrates. Several test platforms were fabricated with guanine as a standalone gate dielectric, as the control, and guanine as a passivation layer between the graphene and PMMA. It was found that the bulk charge carrier mobility of graphene was best maintained and most stable using guanine as a passivation layer between the graphene and PMMA. Other transport properties, such as charge carrier concentration, conductivity type and electrical resistivity were investigated as well. This is an important first step to realizing high performance graphene-based transistors that have potential use in bio and environmental sensors, computer-processing and electronics.

Heterostructures consisting of two-dimensional materials have shown new physical phenomena, novel electronic and optical properties and new device concepts not observed in bulk material systems or purely three dimensional heterostructures. These new effects originated mostly from the van der Waals interaction between the different layers. Here we report that a new optical and electronic device platform can be provided by heterostructures of 2D graphene with a metal oxide (TiO2). Our novel direct synthesis of graphene/TiO2 heterostructure is achieved by C60 deposition on transition Ti metal surface using a molecular beam epitaxy approach and O2 intercalation method, which is compatible with wafer scale growth of heterostructures. As-grown heterostructures exhibit inherent photosensitivity in the visible light spectrum with high photo responsivity. The photo sensitivity is 25 times higher than that of reported graphene photo detectors. The improved responsivity is attributed to optical transitions between O 2p orbitals in the valence band of TiO2 and C 2p orbitals in the conduction band of graphene enabled by Coulomb interactions at the interface. In addition, this heterostructure provides a platform for realization of bottom gated graphene field effect devices with graphene and TiO2 playing the roles of channel and gate dielectric layers, respectively.

Significant attention has recently been given to photoluminescence (PL) spectra and lifetime measurements on InAs/InAsSb superlattices, as high quality optical material with long carrier lifetimes are required for infrared detectors. The standard sample structure for PL measurements includes energy barriers to block photo-generated carriers from being lost through non-radiative recombination at interfaces between the superlattice and the surface or between the superlattice and the buffer/substrate. However, defect, surface, and/or interface states in AlSb, a commonly used barrier material, are known to contribute carriers to InAs quantum wells. Due to the similarity of the AlSb interface with the InAs/InAsSb superlattice, the effects of the barriers on the electrical and optical properties of the superlattice were investigated. Structures with AlSb barriers at the top and bottom of the superlattice, with no AlSb barriers, and with an AlSb barrier only at the top of the superlattice structure were studied. Hall Effect measurements revealed little change in the sheet carrier concentration at 10 K due to the barriers, but significant increases in low temperature mobility and a two-dimensional-like mobility temperature dependence were observed when barriers were present. Further high magnetic field measurements are necessary, however, to understand the transport properties of these samples due to the likelihood that multiple carriers are present. The photoluminescence (PL) spectra were almost identical regardless of the barriers, except for a 15% increase in intensity with the AlSb barrier between the buffer layer and the superlattice. The surface AlSb barrier had little effect on the intensity. The barriers are therefore recommended for PL measurements to increase the signal intensity; however, they complicate the analysis of single-field Hall Effect measurements.

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 A InAs/21.5 A Ga 0.75 In 0.25 Sb, 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.

densities greater than 1 x 10 13 cm -2 and Hall mobilities in excess of 1500 cm 2 /Vs were measured at room temperature. Variable field Hall measurements at low temperatures, and in magnetic fields up to 6 T, indicated that conduction is dominated by a single carrier type in these samples. Shubnikov de-Haas (SdH) measurements were also performed, but no oscillations were observed in fields up to 8 T and at temperatures as low as 1.2 K. Illuminating some of the samples with a blue (λ = 470 nm) light emitting diode (LED) induced a persistent increase in the carrier density. Shubnikov de-Haas measurements were repeated and again no oscillations were present following illumination. However, exposing the samples to radiation from an ultraviolet (UV) (λ = 395 nm) LED induced well defined SdH oscillations in fields as low as 4 T. The observation of SdH oscillations confirmed the presence of a 2DEG in these structures. It is hypothesized that small angle scattering suppressed the oscillations before exposure to UV light. This conclusion is supported by the observed increase in the quantum scattering time, τq, with the carrier density and the calculated quantum to transport scattering times ratio, τq/τc. For instance, in one of the samples the τq increased by 32% while the τc changed by only 3% as the carrier density increased; an indication of an increase in the screening of small angle scattering. The absence of SdH oscillations in fields up to 8 T and at temperatures as low as 1.2 K is not unique to AlGaN/GaN on silicon. This behavior was observed in AlGaN/GaN on sapphire and on silicon carbide. Shubnikov-de Haas oscillations were observed in one AlGaN/GaN on silicon carbide sample following exposure to radiation from a UV LED.

A study of aging effects on the electron transport properties of AlGaN/GaN heterostructures grown on sapphire and silicon carbide substrates using temperature dependent Hall effect measurements is presented with the focus on the variations in the mobility, carrier concentration, and resistivity of these structures over time. The transport parameters for several of these structures were measured at various times after the initial measurement, including times exceeding one year. These studies show that these parameters are not stable in most of the samples. The most common effects are an increase in carrier concentration and a decrease in mobility. Changes on the order of several tens of percent were common. Since surface oxidation and other variations in surface states were suspected as possible sources of the observed changes, one sample was treated with HCl and KOH and measured again after each treatment. However, we were unable to recover the initial results although further changes in the electrical pr...

We report on the electrical characteristics of InSb and n-type doping of InSb layers grown on GaAs substrates using a SnTe captive source by molecular beam epitaxy (MBE). The undoped epilayers are n-type in the temperature range of 10 to 300 K investigated. Doped layer with carrier concentrations ranging from 2×1016/cm3 to 3.2×1018/cm3 with corresponding x-ray full width at half maxima varying from 170–200 arcsec have been achieved. High carrier mobility of 94 098 cm2/V s on lightly doped samples has been achieved. These results suggest SnTe source as being one of the donor dopants of choice for MBE grown InSb epilayers. Temperature and magnetic field dependent Hall and resistivity measurements with various multicarrier conduction analysis techniques indicate three conduction channels for undoped InSb and two conduction channels for doped InSb. They have been used successfully to explain the temperature and thickness dependence of the electrical properties of MBE grown undoped and doped InSb epilayers.

Magneto-transport measurements have been made on strain-free In0.25Ga0.75Sb/InAs superlattices grown on GaSb substrates. The superior structural quality of these superlattices compared to older materials in the literature resulted in a much higher resolution of the Shubnikov–de Haas oscillations. Three separate conduction channels were observed. Measurements with the magnetic field in the plane of the sample indicated that the channel with the longer period was due to three dimensional-like conduction within the superlattice while the other two channels showed two dimensional behavior most likely associated with conduction at the surface or interfaces between the superlattice and bulk-like layers. Comparison with multi-carrier analysis indicated that the channel with intermediate period was due to hole conduction while the others were due to electron conduction. Oscillations were observed at temperatures up to 20 K. The concentration of the superlattice channel at low temperature was 5.93 × 1015 cm−3 whil...

In this study, we investigated the effect of substrates on the electrical properties of transferred graphene. A wide range of substrates such as silicon carbide (SiC), glass, kapton, photo-print paper, polydimethylsiloxane (PDMS) and Willow glass were selected based on their surface properties, flexibility and lattice match. Four monolayers of graphene were transferred onto each of these substrates. A comparative study of the electrical characteristics of the transferred graphene film only and graphene/guanine film on the different substrates was undertaken.

Twisted graphene is of particular interest due to several intriguing characteristics, such as its the Fermi velocity, van Hove singularities and electronic localization. Theoretical studies recently suggested the possible bandgap opening and tuning. Here, we report a novel approach to producing epitaxial twisted graphene on SiC (0001) and the observation of its intrinsic bandgap behaviour. The direct deposition of C60 on pre-grown graphene layers results in few-layer twisted graphene confirmed by angular resolved photoemission spectroscopy and Raman analysis. The strong enhanced G band in Raman and sp(3) bonding characteristic in X-ray photoemission spectroscopy suggests the existence of interlayer interaction between adjacent graphene layers. The interlayer spacing between graphene layers measured by transmission electron microscopy is 0.352 ± 0.012 nm. Thermal activation behaviour and nonlinear current-voltage characteristics conclude that an intrinsic bandgap is opened in twisted graphene. Low sheet resistance (~ 160 Ω □(-1) at 10 K) and high mobility (~2,000 cm(2) V(-1) s(-1) at 10 K) are observed.

We have used the Shubnikov–de Haas and the Hall effects to investigate the effect of subband gap illumination on the transport properties of a very high mobility, μ=54 000 cm2/V s at T=1.2 K, Al0.09GaN0.91/GaN heterostructure. We have found that this illumination resulted in a photocurrent that was persistent at low temperatures. This photocurrent, which led to an increase in the carrier density, was used to study the dependence of the mobility and the quantum scattering time on the carrier density. Unlike in previous studies of persistent photoconductivity in GaN and AlGaN/GaN by other researchers, we have found that the mobility did not always increase with the carrier density as a result of illumination. For small duration of illumination, the mobility increased with the carrier density. However additional illumination resulted in a decrease of the mobility despite the increase in the carrier density. We believe that screening is responsible for the initial increase in the mobility, while ionization of...

The electrical properties of p-type Mg-doped GaN are investigated through variable-temperature Hall effect measurements. Samples with a range of Mg-doping concentrations were prepared by metalorganic chemical vapor phase deposition. A number of phenomena are observed as the dopant density is increased to the high values typically used in device applications: the effective acceptor energy depth decreases from 190 to 112 meV, impurity conduction at low temperature becomes more prominent, the compensation ratio increases, and the valence band mobility drops sharply. The measured doping efficiency drops in samples with Mg concentration above 2×1020 cm−3.

An AlxGa1−xN/GaN two-dimensional electron gas structure with x=0.13 deposited by molecular beam epitaxy on a GaN layer grown by organometallic vapor phase epitaxy on a sapphire substrate was characterized. X-ray diffraction maps of asymmetric reciprocal lattice points confirmed that the thin AlGaN layer was coherently strained to the thick GaN layer. Methods for computing the aluminum mole fraction in the AlGaN layer by x-ray diffraction are discussed. Hall effect measurements gave a sheet electron concentration of 5.1×1012 cm−2 and a mobility of 1.9×104 cm2/V s at 10 K. Mobility spectrum analysis showed single-carrier transport and negligible parallel conduction at low temperatures. The sheet carrier concentrations determined from Shubnikov–de Haas magnetoresistance oscillations were in good agreement with the Hall data. The electron effective mass was determined to be 0.215±0.006 m0 based on the temperature dependence of the amplitude of Shubnikov–de Haas oscillations. The quantum lifetime was about one...

The results of an electrical transport study of an AlGaN/GaN heterostructure grown on a bulk AlN substrate are presented. Temperature dependent Hall effect and magnetoresistance measurements were made on an MOCVD grown heterostructure. The temperature dependences of the carrier concentration and mobility are characteristic of a two-dimensional electron gas (2DEG). The carrier concentration was 1.19x1013 cm–2 at room temperature and 1.02x1013 cm–2 at 77 K while the mobility was 1100 cm2/Vs at room temperature and 5280 cm2/Vs at 77 K. The magnetoresistance measurements revealed the presence of Shubnikov – de Haas (SdH) oscillations. Illumination of the sample resulted in a persistent photocurrent, and an enhanced SdH oscillation amplitude with an increased quantum scattering time. The ratio of the classical to quantum scattering times, τc/τq, was 4.3, which is similar to values for AlGaN/GaN structures on other substrates. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Hall and Shubnikov-de Haas effects measurements were performed to identify the impact of inserting a 1nm AlN interlayer between AlGaN and GaN. The transport properties of this AlGaN/AlN/GaN structure were compared to the conventional structure (AlGaN/GaN). Our measurements indicate the AlN interlayer resulted in improved transport in the temperature range studied (300 K-1.2 K). However, the impact of the AlN layer was found to be much more significant at low temperatures. The improvement is believed to be the result of reduced alloy scattering arising from a stronger confinement of the 2DEG. At 300 K the carrier density and mobility for the AlGaN/AlN/GaN structure were roughly 1.00 x 1013 cm–2 and 1937 cm2/Vs. Whereas those of the conventional structure were 8.57 x 1012 cm–2 and 1523 cm2/Vs. Shubnikov-de Haas oscillations were observed in both samples at low T. This is an indication that a high quality 2DEG is present in each structure. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have experimentally determined the effective mass (m*) of GaN, the classical (τc), and quantum (τq) scattering times for a two-dimensional electron gas residing at the interface of an AlGaN/GaN heterostructure, using the Shubnikovde Haas effect. The ratio of the two scattering times, τc/τq, suggests that, at low temperatures, the scattering mechanism limiting the mobility is due to remote ionized impurities located in AlGaN. This study should provide sample growers with information useful for improving the quality of the nitride heterostuctures.

We have investigated the Kosterlitz-Thouless (KT) transition in a series of proximity-coupled Josephson junction arrays of varying widths. Our results indicate that the KT transition in any experimentally realizable sample is almost always obscured by the presence of thermally generated, finite-size-induced free vortices. While the existence of these finite-size-induced free vortices has been known for some time, our work suggests that they are much more prevalent and thus have a far greater effect on the transition than had been previously thought. As a consequence of this, the vortex-unbinding transition temperature T{sub KT} may not occur when the experimentally measured current-voltage exponent a(T)=3, but in fact may occur at significantly higher temperatures. We present a detailed picture of these finite-size effects applied specifically to arrays, but which may have implications for other two-dimensional systems. {copyright} {ital 1998} {ital The American Physical Society}

We have studied the effect of well thickness on the two-dimensional electron-hole system in semimetallic ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Sb/InAs quantum wells by Shubnikov--de Haas (SdH) measurements. The number of hole carriers in the ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Sb barriers was changed by a negative persistent photoconductivity effect. From the amplitude of SdH oscillation for the sample with the thinnest well, we found that the interface roughness scattering dominated when the number of hole carriers is small. After the number of holes is increased by the negative persistent photoconductivity effect, the electron-hole scattering becomes more important, and results in a reduction of the electron quantum lifetime. The competition between electron-hole scattering and interface roughness scattering depends on the amount of holes in the barriers.

The material properties of p-type InAs/InAsSb superlattices are of interest for infrared photodiodes, but InAs/InAsSb superlattices are residually n-type and p-type superlattices have not been investigated thus far. This study examines the material properties of a mid-wavelength infrared InAs/InAsSb superlattice design doped with Be concentrations from 0.5-7x10 16 cm -3 . High-resolution x-ray diffraction revealed slight structural variation throughout the ~500 nm thick superlattice layer, but the RMS surface roughness was reasonable. Hall Effect measurements, taken at 10 K to remove any conduction effects from the undoped GaSb substrate, revealed the superlattice converting from ntype to p-type at Be:3x10 16 cm -3 . The maximum hole mobility achieved at the two highest Be doping levels was ~24,000 cm 2 /Vs, which is high for mid-wavelength infrared superlattices. The doped superlattices all had photoluminescence (PL) peaks 12 - 34 meV lower in energy than the undoped sample, and the PL peak FWHMs increased as the average superlattice mismatch increased, as expected. Comparing the photoresponse to the PL allowed the Be acceptor binding energy in the superlattice (13 meV) to be determined, which agreed with the reported Be acceptor binding energy in InAs.

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 A InAs/21.5 A In 0.25 Ga 0.75 Sb 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 (T g ) 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 T g . For the SLs grown from 390 to 470 °C, a PR signal gradually increases as T g 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 cm 2 /Vs. The mobility of the SL grown at 440 °C was approximately 10,000 V/cm 2 with a sheet carrier concentration of 5 × 10 11 cm -2 , but the mobility precipitously dropped to 21 cm 2 /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.

The electrical properties of In0.25Ga0.75Sb/InAs superlattices designed for use as very long wavelength infrared detectors were studied with magnetic field dependent transport measurements and multicarrier analysis. Two electron channels and one hole channel were identified. The low concentration, high mobility electron channel was identified with conduction within the superlattice. Residual electron concentrations in the superlattice are found to be in the high 1010 cm-2 range with mobilities on the order of 40 000 cm2/V s. The other electron and hole channels have significantly lower mobilities with concentrations in the 1011 cm-2 range and can have a significant impact on the resistivity, carrier concentration, and mobility as measured at a single magnetic field value in conventional transport measurements.

A long-lived persistent photoconductivity has been observed at low temperatures in thin, unintentionally doped Ga 0.51 In 0.49 P/GaAs quantum wells. The two-dimensional electron gas was studied by low field Hall and Shubnikov-de Haas measurements. Both the low field mobility and quantum scattering time increased with increasing carrier concentration after illumination with red light. The increase in carrier concentration is attributed to photoexcitation of carriers from a deep level located at 0.9 eV below the conduction band of InGaP. Parallel conduction due to extended illumination has also been observed in the anomalous behavior of the quantum Hall plateaus.

We report on positive and negative persistent photo-effects observed in some red light emitting diode (LED)-illuminated type-II InAs/GaSb superlattices (SLs) grown on a p-type GaSb. By analyzing the time dependence of the transverse resistance during and after the illumination, we show that the rise and decay curves are logarithmic, a behavior which points to hierarchically constrained carrier dynamics. Accordingly, negative persistent effects are explained by diffusion and trapping of photo-excited carriers in the p-type buffer layer and their subsequent tunneling back to the SL. On the other hand, positive persistent effects are explained by a low density of majority-carrier trapping centers in the buffer layer. Hence, persistent photo-effects upon red-LED irradiation provide a diagnostic of the quality of the superlattice-buffer interface.

Electronic transport coefficients of very thin undoped GaInP/GaAs quantum wells have been measured at low temperatures by the low field Hall effect and by the Shubnikov-de Haas effect. Unlike the case in undoped AlGaAs/GaAs, strong Shubnikov-de Haas oscillations were observed in this structure after increasing the carrier concentration via the persistent photoconductivity effect. Low temperature mobilities close to 70 000 cm 2 V -1 s -1 at a carrier concentration of 6.5 × 10 11 cm -2 were observed in a 25 A quantum well. The results are compared with the theory of interace roughness scattering and we find that the mobility of our samples goes as L 1.3 where L is the well width. We conclude that the barrier height of the confining potential plays a very important role in determining the effect of interface roughness scattering on the mobility of very thin quantum wells.

Temperature-dependent minority carrier lifetimes of InAs/InAs1-xSbx type-II superlattices are presented. The longest lifetime at 11 K is 504 ± 40 ns and at 77 K is 412 ± 25 ns. Samples with long periods and small wave function overlaps have both non-radiative and radiative recombination mechanisms apparent, with comparable contributions from both near 77 K, and radiative recombination dominating at low temperatures. Samples with short periods and large wave function overlaps have radiative recombination dominating from 10 K until ~200 K. The improved lifetimes observed will enable long minority carrier lifetime superlattices to be designed for high quantum efficiency, low dark current infrared detectors.

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 cm 2 /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.

Al0.35−δInδGa0.65N/GaN heterostructures have been studied by using transmission electron microscopy, photoluminescence, and Shubnikov–de Haas (SdH) measurements. In the sample of δ

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.

Direct deposition of graphene from carbon sources on foreign substrates without the use of metal catalysts is shown to be an effective process with several advantages over other growth techniques. Carbon source molecular beam epitaxy (CMBE) in particular provides an additional control parameter in carbon flux and enables growth on substrates other than SiC, including oxidized Si and sapphire. CMBE using thermally evaporated C60 and a heated graphite filament on SiC is reported here. The graphene films were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy and Hall effect. Graphene films on Si-face SiC grown using the C60 source have Bernal-like stacking and n-type conduction while those grown using the graphite filament have turbostratic stacking and p-type conduction. The sheet concentration for both n- and p-type doping is linearly dependent on film thickness.

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.

We report on the results of transport studies of MBE-grown InAs/GaSb superlattices. We demonstrate that the in-plane mobility is limited by interface roughness scattering by showing that, as a function of InAs layer width L, the in-plane mobility behaves as μ ∝ L5.3, which closely follows the classic sixth power dependence expected from theory for interface-roughness-limited mobility. Fits to the mobility data indicate that, for one monolayer surface roughness, the roughness correlation length is about 35 Å. Next, we show that the in-plane carrier mobility in InAs/GaSb superlattices is inversely proportional to carrier density in n- and p-type samples, the result of screened interface roughness scattering.

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 A InAs/24 A 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 (1.8x10 11 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 ~2x10 11 to 5x10 12 cm -2 , and the corresponding mobility dropped from 6600 to 26 cm 2 /Vs, indicating a degradation of interface quality. However, the carrier density was further reduced to 1x10 11 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 A, the photoluminescence peak position shifts from 4.1 to 3.4 μm, for a fixed InAs width of 21 A, indicating a photodiode based on these wider designs would fall short of fully covering the 3 to 5 μm mid-IR spectral region.