Search

The prospect of implementing quantum device architectures with technologically mature III–V semiconductors requires precisely controlled topologically protected edge states and bulk insulation. However, experimentally reaching this regime with III–V heterostructure epitaxy has been difficult due to charged bulk defects. Charged defects prevent bulk insulation and disturb the stability of edge states. Hence, we study carrier properties in a hybridized III–V InAs0.9Sb0.1/GaSb superlattice (SL) structure. We realize that an electron density (mobility) is limited to approximately 1012 cm−2 (104 cm2/V s). In order to understand these limits, the authors investigate the in-plane mobility of hybridized SLs as a function of current-carrying layer thickness, L, to determine scattering mechanisms that restrict carrier mobility. Although theory predicts the in-plane mobility is proportional to L6 dominated by interface roughness scattering (IRS) at low temperatures, we report that mobility follows the fourth power dependence, which is a weaker than expected from IRS theory. We attribute the discrepancy between experiment and the model to interface intermixing and wave function penetration into barrier regions. We use this understanding to develop a strategy for realizing high-performance topological materials. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The temperature dependent Hall effect measurements are conducted on a series of InAs/GaSb SLs with a fixed GaSb layer width and a variable InAs layer width d. The well width dependence of the measured mobilities closely matched the [mu][alpha][d.sup.6] dependence predicted by theory for the interface roughness scattering (IRS) dominated transport for samples.

The technique of transmission electron microscopy is used to study the growth and morphological properties of several short-period InAs/GaSb supperlattices (SLs), which exhibit large intervalance band separations, making room temperature operations possible. The analysis shows that the structural degradation of the SLs progresses and intensifies with the increase in the period, as SLs with extremely thin periods can also be obtained by using slower growth rates.

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 majoritycarrier 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. [ABSTRACT FROM AUTHOR]

The effect of subband gap illumination on transport properties of AIGaN/GaN heterostrcutures was investigated using the Shubnikov-de Haas and Hall effects.

A new study examines the electrical properties of Mg-doped GaN using variable-temperature Hall effect measurements.

Results demonstrate that the thin AlGaN and thick GaN layers are coheretly strained as determined by x-ray diffraction map analysis. Data on mobility spectra reveal single-carrier transport at low temperatures.

Nominally undoped n-type GaN layers grown by metalorganic chemical vapor deposition on silicon substrates were investigated using Thermal Admittance Spectroscopy and Optical Admittance Spectroscopy (OAS). A defect level was observed at Ec - 0.051 eV, and it is correlated with the nitrogen vacancy (Nv) which is a donor in GaN. Illuminating the samples with a monochromatic light with wavelengths ranging from 200 nm to 450 nm, the OAS spectrum was measured at different temperatures and with different excitation light intensities. A dominant peak was observed in the OAS spectrum at γ = 365 nm (3.40 eV); this is attributed to transitions from the valence band to the donor level. Our results show that the saturation level, Gm, of the photoconductance is a function of both light intensity and temperature. The photoconductance decay, after the illumination has been terminated, is non-exponential but it is fully described by the stretched exponential function. The value of β ranges from 0.78 to 0.86. The analysis suggests that the observed photoconductance decay is due to thermal emission of photo-excited carriers from the donor level. [ABSTRACT FROM AUTHOR]

The impact of post growth annealing on the electrical properties of a long wavelength infrared type-II superlattice (SL) was explored. Quarters of a single SL wafer were annealed at 440 °C, 480 °C, and 515 °C, respectively for 30 min. Changes in the electrical properties were followed using spectral photoconductivity, temperature dependent Hall effect, and time-resolved pump-probe measurements. The bandgap energy remained at ∼107 meV for each anneal, and the photoresponse spectra showed a 25% improvement. The carrier lifetime increased from 12 to ∼15 ns with annealing. The electron mobility was nearly constant for the 440 °C and 480 °C anneals, and increased from ∼4500 to 6300 cm2/Vs for the 515 °C anneal. [ABSTRACT FROM AUTHOR]

We investigate the optical and electrical properties of a series of InAs/GaSb superlattices (SLs) as a function of InAs layer width d, from 21 to 55 Å, with a fixed GaSb layer width of 24 Å, corresponding to SLs with the cutoff wavelengths between 4 and 19 μm. Since the higher electron mass in InAs/GaSb SLs than in mercury cadmium telluride should lead to lower photodiode tunneling currents, we also measured the cyclotron effective mass for a very long wavelength infrared design SLs. For d<40 Å, the SLs were p-type, with hole mobilities of approximately 8 000 cm2/V s. For a high mobility p-type sample no hole cyclotron resonance signal was detected. However, the SLs with d≥40 Å were n-type, with electron mobilities increasing from 865 to 6126 cm2/V s. Cyclotron resonance data on an n-type SL sample yielded an electron cyclotron mass of 0.068 m0, which is three times the InAs bulk value of 0.023 m0. The mass enhancement was only partially accounted for by conduction band nonparabolicity, based on our 8×8 envelope function calculation. [ABSTRACT FROM AUTHOR]

We report on a study of the in-plane carrier mobility in InAs/GaSb superlattices as a function of carrier density. Instead of using a number of differently doped samples, we use the persistent-photoconductivity effect to vary the carrier density over a wide range from n- to p-type in single samples and perform Hall effect measurements. Hence, our data are not obscured by sample to sample nonuniformities. We demonstrate that low-temperature in-plane mobilities are limited by screened interface roughness scattering (IRS), although present models of two-dimensional carrier screening of IRS lead to a limited agreement with our data. [ABSTRACT FROM AUTHOR]

Reducing residual background carriers in InAs/GaSb superlattices (SLs) is an essential task to increase the operating temperature of photoconductive devices. This paper discusses how low-temperature Hall measurements were used to tune several SL growth parameters for the minimum residual carriers in a typical midinfrared 21 Å InAs/24 Å GaSb SLs designed for the 4 μm cutoff wavelength. Among the three growth parameters studied, neither growth temperature nor in situ postannealing significantly affected the intrinsic carrier type and doping concentration. The lowest carrier density of 1.8×1011 cm-2 was achieved at 400 °C. All SLs grown at 400 °C maintained the lowest density around 1.6×1011 cm-2 with or without postannealing. However, in-plane carrier mobility showed a slight improvement with annealing, especially at temperatures above 450 °C. The growth parameter most sensitive to the carrier density was interface control. With a minor variation in interface shutter sequence, the carrier density dramatically increased from ∼2×1011 to 5×1012 cm-2, and the corresponding mobility dropped from 6600 to 26 cm2/V s, indicating dramatic degradation of interfacial quality. All SLs investigated in this study were residually p-type. [ABSTRACT FROM AUTHOR]

The purpose of this work is to explore materials for midinfrared detectors that can operate at room temperature. Shorter-period InAs/GaSb superlattices (SLs) have larger intervalance band separations, which are 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 grown and their morphological properties were carefully monitored by transmission electron microscopy. The effect of structural degradation caused by the period reduction on the optical properties was studied using low-temperature photoluminescence (PL). The samples with larger periods (50–32 Å) showed excellent structural qualities and produced narrow full width at half maximum (FWHM) of the PL peak (5 meV). 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. These structural degradations strongly influence optical properties causing significant broadening in the FWHM and higher residual carrier concentration in the shorter-period SLs. With slower growth rates, samples with periods as thin as 19 Å were grown without significant layer undulations. [ABSTRACT FROM AUTHOR]

We present results of transport measurements performed on AlGaN/GaN heterostructures grown on silicon substrates. Variable temperature Hall effect measurements revealed that the temperature dependence of the carrier density and mobility were characteristic of a two-dimensional electron gas (2DEG). Carrier densities greater than 1×1013 cm-2 and Hall mobilities in excess of 1500 cm2/V s 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. SdH measurements were repeated and again no oscillations were present following illumination. However, exposing the samples to radiation from an 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. SdH oscillations were observed in one AlGaN/GaN on silicon carbide sample following exposure to radiation from an UV LED. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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 properties were observed. The study suggests that surface effects can have profound effects on the properties of the two-dimensional electron gas in these heterostructures. [ABSTRACT FROM AUTHOR]

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 × 10[sup 16]/cm³ to 3.2 × 10[sup 18]/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. [ABSTRACT FROM AUTHOR]