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The role of silicon and boron carbide interface layers on thermal stability and electrical properties of tantalum, gold, and nickel contacts to 6H- and 4H-n-silicon carbide are presented in this report. Thin Ta/Si, Au/B 4 C, and Ni/B 4 C layers were deposited on SiC using electron-beam evaporation or sputter-deposition techniques. The structures were annealed either in ultra-high vacuum or in N–H ambient, at temperatures ranging from about 400 to 1150 °C. The samples were characterized using atomic force microscopy for surface topography, auger electron spectrometry for depth profiling, and glancing angle X-ray diffraction for microstructure and phase identification analyses. Transmission line model structures for current–voltage measurements and contact resistance evaluation were prepared using photolithography and lift-off techniques. Our results indicate that Ta in the Ta/SiC system decomposes SiC at about 800 °C, forming tantalum carbide with the accumulation of silicon at the TaC/SiC interface. In the Ta/Si/SiC system decomposition of SiC also occurs about the same temperature resulting in the structure TaC/Si/SiC. The Au/B 4 C/SiC system appears to be the most thermally stable with the lowest specific contact resistance of about 1×10 −6 Ω cm 2 for samples heat-treated above 1050 °C for 30 min.

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.

We present a systematic study of the sheet carrier density and valence-band offset in the GaN/AlxGa1-xN(0001) heterostructure as a function of x from ab initio density-functional methods. We find that the calculated sheet carrier density increases rapidly with x for xless than or equal to0.3 in good agreement with experiments, but beyond this concentration, it quickly saturates to a value of about 2x10(13) cm(-2). The band offset shows a small asymmetry between the Ga-face and N-face interfaces and changes more or less linearly with x, although a small bowing is found. The layer-projected densities of states indicate the formation of the two-dimensional electron gas at the Ga-face interface and confirm the absence of interface states in the gap. (C) 2004 American Institute of Physics.

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...

Nitrogen doped 4H–SiC and 6H–SiC epitaxial layers with net doping concentration of 1.5×1015 cm−3 were implanted with either Al, B, or Ar ions at 600 °C. The energy of the ions was 160 keV and at a dose of 2×1016 cm−2. After annealing at 1600 °C for 5–60 min, Schottky diodes were fabricated on the samples. The measured junction capacitance of ion implanted samples varies with both temperature and frequency. The thermal activation energy determined from the frequency break ωB ranges from 183 to 202 meV. The Ar implanted samples remain n-type and show no capacitive dispersion. Similarly, the junction capacitance of samples doped with Al does not exhibit dispersion. It is suggested that the implantation induced defects play a role in the observed dispersion. The data show that the series resistance is insignificant and does not account for the observed dispersion.

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.

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...

Temperature dependent Hall effect, optical admittance spectroscopy, and optical absorption measurements of semi-insulating bulk 4H–SiC are reported. Both intentionally vanadium doped material and commercial grade semi-insulating material were investigated. The carrier concentration versus inverse temperature results from Hall effect measurements up to 1000 K indicated the samples were dominated by one of two deep levels near midgap. In addition to the deep donor level of substitutional vanadium, Ec−1.6 eV, we observed another level at Ec−1.1 eV in some samples, indicating that levels other than the vanadium donor can pin the Fermi level in semi-insulating SiC. Optical admittance measurements on the semi-insulating material indicate the presence of levels at Ec−1.73 and 1.18 eV that were previously observed in conducting samples with this technique and we attribute these levels to the same defects producing the 1.1 and 1.6 eV levels seen by Hall effect. Secondary ion mass spectroscopy measurements of dopant and impurity concentrations are reported. Even though vanadium is present in all of these samples, along with other impurities we are at present unable to definitively identify the 1.1 eV level.

A magnetotransport study has been performed in II-VI ${\mathrm{ZnS}}_{0.06}{\mathrm{Se}}_{0.94}{/\mathrm{Z}\mathrm{n}}_{0.8}{\mathrm{Cd}}_{0.2}\mathrm{Se}$ quantum wells. We observed the quantum effects of weak localization and electron-electron interactions on the two-dimensional electron gas in the ${\mathrm{Zn}}_{0.8}{\mathrm{Cd}}_{0.2}\mathrm{Se}$ well. The effective mass of electrons in this highly correlated system, determined from Shubnikov--de Haas measurements, exhibits a strong magnetic field dependence: ${m}^{*}{/m}_{0}{=0.143+0.0027B}^{2}.$ The mass enhancement is attributed to the ``dressing'' of electrons by the many-particle effect due to electron-electron interactions.

We report on the growth and characterization of type-II infrared detectors with an InAs-GaSb superlattice active layer for the 8-12-/spl mu/m atmospheric window at 300 K. The material was grown by molecular beam epitaxy on semi-insulating GaAs substrates. Photoconductive detectors fabricated from the superlattices showed 80% cutoff at about 12 /spl mu/m at room temperature. The responsivity of the device is about 2 mA/W with a 1-V bias (E=5 V/cm) and the maximum measured detectivity of the device is 1.3/spl times/10/sup 8/ cm.Hz/sup 1/2//W at 11 /spl mu/m at room temperature. The detector shows very weak temperature sensitivity. Also, the extracted effective carrier lifetime, /spl tau/=26 ns, is an order of magnitude longer than the carrier lifetime in HgCdTe with similar bandgap and carrier concentration.

We have carried out saturation spectroscopy of cyclotron resonance in a semiconducting InAs/Al 0.5 Ga 0.5 Sb single quantum well using the UCSB free electron laser and have extracted an effective Landau level lifetime using an n -level rate equation model. The effective lifetime shows strong oscillations (>an order of magnitude) with frequency. Minima are shifted to higher frequencies than those given by the simple parabolic magnetophonon resonance condition due to large nonparabolicity in the InAs conduction band. We have also used this technique to investigate the origins of two lines: the X-line and cyclotron resonance in a “semimetallic” InAs/Al 0.1 Ga 0.9 Sb single quantum-well structure. Results show that the two lines are of different origin.

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.

By analyzing the temperature dependence of the capacitance and AC conductance of a zero-biased Schottky diode one obtains the activation energy of the primary shallow dopant in a semiconductor. This technique has become known as Thermal Admittance Spectroscopy. Thermal admittance spectroscopy has been used to characterize nitrogen donors in 4H-, 6H-, and 15R-SiC. p-type dopants (Al, B) have also been characterized in 6H-SiC. In addition, an activation energy attributable to hopping conduction has been obtained in n-type specimens of all three polytypes. The change in the admittance of a Schottky diode caused by illumination, measured as a function of wavelength, is called Optical Admittance Spectroscopy. Optical admittance spectroscopy has been used to study deep centers in silicon carbide where traditional electrical detection is made difficult by the requirement for very high measurement temperatures. Using this technique, the energies of the midgap donor-like levels attributed to vanadium atoms substitutionally occupying the inequivalent lattice sites have been determined. The bandgaps of 6H- and 4H-SiC polytypes have been measured, and the phonon spectra associated with the indirect transitions from the valence band to the conduction band have been determined.

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 electronic properties of heavily doped n-type 4H, 6H, and 15R SiC have been studied with temperature dependent Hall effect, resistivity measurements, and thermal admittance spectroscopy experiments. Hopping conduction was observed in the resistivity experiments for samples with electron concentrations of 1017 cm−3 or higher. Both band and hopping conduction were observed in all three polytypes in resistivity and Hall effect experiments. The hopping conduction activation energy e3 obtained from the resistivity measurements varied from 0.003 to 0.013 eV. The e3 value obtained from thermal admittance spectroscopy measurements were slightly lower. The nitrogen ionization levels were observed by thermal admittance spectroscopy only in those samples where hopping conduction was not detected by this experiment. Free carrier activation energy Ea for nitrogen was difficult to determine from temperature dependent Hall effect measurements because of the effects of hopping conduction. A new feature in the apparent carrier concentration vs inverse temperature data in the hopping regime was observed.

We present results of a detailed far-infrared magneto-optical study on a series of high-mobility InAs/${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ Sb (x=1.0, 0.8, 0.5, 0.4, 0.2, and 0.1) type-II single quantum wells. A wide range of phenomena arising from the unusual properties of two-dimensional (2D) electrons and holes and their Coulomb interaction in high magnetic fields has been revealed. Semiconducting samples (x\ensuremath{\geqslant}0.4), in which only 2D electrons exist in the InAs wells, exhibit cyclotron-resonance (CR) splittings due to large conduction-band nonparabolicity. Semimetallic samples (x=0.1 and 0.2), in which both 2D electrons (in InAs) and 2D holes (in ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ Sb) are present, show two additional lines (e and h-X lines) as well as electron and hole CR. The X-lines increase in intensity at the expense of CR with increasing electron-hole (e-h) pair density, decreasing temperature, or increasing magnetic field (at low field), suggesting that they are associated with e-h binding which is increased by the magnetic field. The electron CR shows strongly oscillatory linewidth, amplitude, and mass, part of which are interpreted in the light of the unusual 'antinonparabolic' band structure resulting from band overlap and coupling between conduction-band states in InAs and valence-band states in ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ Sb; part of these results are qualitatively consistent with the predictions of Altarelli and co-workers. The X lines are attributed to internal transitions of correlated electron e-h pairs (excitons) in high magnetic fields mediated by the excess electron density.

We have studied the lowest two subbands of two‐dimensional electron gas in δ‐doped AlInAs/GaInAs quantum wells by Shubnikov–de Haas, quantum Hall effect, and cyclotron resonance measurements. The effective masses determined by the field‐dependent cyclotron resonance measurements are 0.0576m0 and 0.0483m0 at electron densities of 17.3 and 3.6×1011 cm−2 for the first and second subbands, respectively. It was found that the electron in the first subband has heavier effective mass and shorter quantum lifetime than that in the second subband. Using the band gap of 810 meV and the band‐edge mass of 0.042m0 for Ga0.47In0.53As, we calculated the average distance of the two subbands from the conduction band edge to be 150 and 61 meV, respectively.

We have observed two oscillations due to the first two subbands of the two‐dimensional electron gas in δ‐doped AlAs0.56Sb0.44/Ga0.47In0.53As and Al0.48In0.52As/Ga0.47In0.53As quantum well by Shubnikov–de Haas measurements. From the electron densities of the two subbands we determined the energy difference between the first two subbands, ΔE01, to be 72 meV for the AlAs0.56Sb0.44/Ga0.47In0.53As, and 54 meV for the Al0.48In0.52As/Ga0.47In0.53As quantum well. By comparing with the data from an Al0.24Ga0.76As/Ga0.78In0.22As quantum well, we found that the larger band offset sample gives the higher ΔE01, but when the band offset is relatively small, the well width may determine the energy difference ΔE01.

The nitrogen levels in 4H‐SiC have been determined using thermal admittance spectroscopy. The values of Ec−0.053 eV for nitrogen at the hexagonal site and Ec−0.10 eV for nitrogen at the quasicubic site agree with those reported using other techniques. The deep levels in 4H‐SiC were studied using optical admittance spectroscopy. The optical admittance spectrum showed, besides the conductance peak corresponding to band to band transitions, four other conductance peaks. These peaks correspond to photoexcitation of carriers from the defect levels to the conduction band. It is inferred from a comparison with 6H‐SiC that the conductance peak b4 is due to excitation of electrons from the vanadium donor at Ec−1.73 eV. The photoconductance build up transients of the Ec−1.73 eV level are described fully by one exponential term. This suggests that only one center contributed to the observed conductance. The decay kinetics of persistent photoconductance due to the Ec−1.73 eV level follow the stretched exponential for...

The electronic properties of boron in bulk 6H-SiC have been studied by temperature dependent Hall effect, thermal admittance spectroscopy, and optical absorption. A single acceptor level located between 0.27 and 0.35 eV above the valence band is associated with boron on a silicon lattice site. The deep nature of this acceptor level prevents complete thermal activation of the level at room temperature and thus carrier concentration measurements at this temperature will not give the total boron concentration. A spread in the measured activation energy for boron is reported. Measurement of optical absorption is suggested as a nondestructive measure of boron concentration. No evidence for the D-center was observed in this material.

The vanadium donor level in high‐resistivity p‐type 6H‐SiC has been studied using optical admittance spectroscopy. Besides the conductance peak due to the band to band transitions, there are three conductance peaks in the spectra of most of the samples. These peaks correspond to photoexcitation of electrons from the valence band to the defect levels. The conductance peak due to the vanadium donor [V4+(3d1)] level at Ev+1.55 eV is identified. The build up of the photoconductance at this peak was studied and it was found that the conductance transients are completely described by a sum of two exponential expressions. The relevant parameters, α1, α2, Gmax(1) and Gmax(2), were determined as functions of temperature. The persistent photoconductance (PPC) due to this defect was also studied. The decay kinetics of the PPC follow the stretched exponential form. The potential barrier against recapture of carriers was determined to be 220 meV for the vanadium donor level.

We have examined the temperature dependence of the barrier height of Al, Ag, Au, and W metal‐semiconductor contacts on n‐type 6H‐SiC, and Al and Ag metal‐semiconductor contacts on p‐type 6H‐SiC. The barrier height was determined from the reverse biased capacitance‐voltage characteristics of the contacts at temperatures ranging from 300 to 670 K. The measurements were made at 1 MHz. These measurements were compared to the behavior predicted by standard models. All the diodes displayed a negative temperature dependence on n‐type SiC, and a positive temperature dependence on p‐type SiC. The temperature coefficient is related to the electronegativity of the metal by linear expression.

We have studied the electronic properties of \ensuremath{\delta}-doped ${\mathrm{Al}}_{0.48}$${\mathrm{In}}_{0.52}$As/${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As heterostructure samples by Shubnikov--de Haas and quantum-Hall-effect measurements. The well-defined two-subband occupied two-dimensional electron gas was detected. From the Fermi level at these two subbands, we calculate the energy difference between the lowest two subbands' minima to be 54 meV. After illuminating the sample at low temperature, we did observe a persistent photoconductivity. The electron densities of the first and second subbands for the sample with preannealed substrate were increased by the illumination from 17.3 to 18.2\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ and 3.6 to 4.1\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, respectively. The onset of the second subband occurs at ${\mathit{n}}_{0}$=10.3\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. We also found that the concentration of deep donor traps is about 1.4\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ and the deep donor traps are not affected by the preannealed substrate. We believe that the model to describe the persistent-photoconductivity effect in semiconductor heterostructure samples needs to be extended to those deep donor levels below Fermi energy and it does not have to be the DX center.

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.

Defects in n‐type 6H‐SiC have been studied using optical admittance spectroscopy. Six conductance peaks, which correspond to photoexcitation of electrons into the conduction band from defects and the valence band at different wavelengths, were clearly identified. Persistent photoconductance (PPC) due to a defect 1.07 eV below the conduction band was studied. The decay kinetics of the PPC follow the stretched exponential form. The relaxation time constant τ and the stretching factor β were systematically measured as functions of temperature. The thermal capture barrier of 61 meV was determined from these results. It was also found that the PPC can be quenched optically by illumination with sub band gap radiation. This is the first reported observation of optical quenching of PPC in n‐type 6H‐SiC. The lattice relaxation model is used to qualitatively explain these experimental results.

The growth of GaAs by chemical beam epitaxy using triethylgallium and trisdimethylaminoarsenic has been studied. Reflection high-energy electron diffraction (RHEED) measurements were used to investigate the growth behavior of GaAs over a wide temperature range of 300–550°C. Both group III- and group Vinduced RHEED intensity oscillations were observed, and actual V/III incorporation ratios on the substrate surface were established. Thick GaAs epitaxial layers (2–3 μm) were grown at different substrate temperatures and V/III ratios, and were characterized by the standard van der Pauw-Hall effect measurement and secondary ion mass spectroscopy analysis. The samples grown at substrate temperatures above 490°C showed n-type conduction, while those grown at substrate temperatures below 480°C showed p-type conduction. At a substrate temperature between 490 and 510°C and a V/III ratio of about 1.6, the unintentional doping concentration is n ∼2 × 1015 cm−3 with an electron mobility of 5700 cm2/V·s at 300K and 40000 cm2/V·s at 77K.

Deep level transient spectroscopy (DLTS) has been used to characterize deep impurity levels in n‐type 6H‐SiC single crystals. A defect level at Ec−0.71 eV with an electron capture cross section σ=5.63×10−20 cm2 was observed. Defect concentration profiles confirm that the defect is a bulk defect. Infrared absorption measurements in the spectral range of 7000–7700 cm−1 were made using these samples. The infrared absorption spectrum characteristic of vanadium in silicon carbide is composed of a group of three absorption lines in the spectral range of 7000–7700 cm−1. This infrared signature was seen in the specimens in which the DLTS spectrum revealed the presence of deep traps. This signature was absent in those specimens where no deep traps were indicated by DLTS. Correlating these facts, we have concluded that the observed peak at Ec−0.71 eV was due to vanadium atoms in silicon carbide.

Admittance spectroscopy has been used to study shallow levels in n‐type 6H‐SiC single crystals. A total of eight unintentionally doped n‐type samples obtained from three different sources were used in this study. Two of the samples were grown by the Lely method, while the others were grown by physical vapor transport. Two electron traps at EC−0.04 eV and EC−0.03 eV were detected in the more heavily n‐type (ND−NA=1018 cm−3) samples. These defects may be due to contaminants other than nitrogen. A defect level at EC−0.08 eV as detected in a sample with ND−NA=8.9×1017 cm−3. This level is associated with nitrogen at the hexagonal site (h). An electron trap at EC−0.11 eV was detected and is associated with nitrogen at the quasicubic sites (k1k2). This level was observed only in the lightly n‐type samples (ND−NA=4.7 ×1015–6.4×1017 cm−3).

Photocurrent transient effects in undoped semi‐insulating gallium arsenide have elicited conflicting explanations. The photocurrent quenching, enhanced photocurrent, and persistent photocurrent effects are reviewed and new results on these effects are reported. A comparison of the photocurrent transients with optical absorption quenching along with other experiments shows that photocurrent quenching is due to the metastable transformation of the deep donor EL2 and that the enhanced and persistent photocurrent effects are due to a seperate effect that is most likely a metastable transformation of another defect. Thermal recovery experiments show that the activated state responsible for the enhanced photocurrent recovers prior to the recovery of the metastable state of EL2. Furnace annealing experiments are presented showing that the enhanced and persistent photocurrent effects are significantly reduced after anneals at 500 °C, well below the annealing temperature of quenchable EL2. After reviewing existing...

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

We observed the carrier-concentration dependence of the magnetic-field-induced free electron and hole recombination in semimetallic Al x Ga 1-x Sb/InAs quantum wells. The electrons and holes recombined when the highest-hole Landau level crossed the Fermi level. The recombination continued with increasing field until all the holes vanished, leading to a semimetal-to-semiconductor transition. This transition shifted to a higher field when the electron concentration decreased

Low‐temperature (4.5 K) photoluminescence (PL) spectra of liquid phase electroepitaxially grown GaSb and GaInAsSb have been examined. The excitonic transitions observed in GaSb and GaInAsSb layers of compositions close to the GaSb corner of the phase diagram indicate an excellent quality of the grown layers. A systematic trend in the low‐temperature PL spectra is observed with the change in the alloy composition. The overall PL emission efficiency decreases and the number of excitonic transitions are fewer with the shift in the composition towards the lower band gap. Shift in the PL peak energy corresponding to the band to band transition with temperature was determined. The linear part of the shift above 100 K exhibits a slope of −0.3 meV/K.

We report on the liquid-phase electroepitaxial (LPEE) growth of GaSb and ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$${\mathrm{As}}_{\mathit{y}}$${\mathrm{Sb}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$ alloys on undoped (100) GaSb substrates. Alloys with room-temperature photoluminescence peak wavelengths as long as 2.32 \ensuremath{\mu}m have been grown. These layers were assessed by x-ray diffraction, energy-dispersive x-ray analysis, and low-temperature Fourier-transform photoluminescence (PL), with emphasis on the latter. The variation in the low-temperature photoluminescence bands as a function of the alloy composition has been investigated. The low-temperature (4.5 K) PL spectra of the alloys exhibited narrow peaks with full width half maxima in the range 3--7 meV, indicating an excellent quality of the LPEE-grown epilayers. The temperature and intensity dependences of the PL spectra were investigated to identify the nature of the recombination processes.

Application of the newly developed differential photoreflectance (DPR) technique to study layered GaAs/AlGaAs structures is presented and discussed. We show that in most instances DPR can provide selective photoreflectance (PR) response from layers buried within thin multilayer structures, where such responses may be superimposed on one another in ordinary PR technique. DPR measurement is achieved through alternative modulations from two laser pumps with different penetration depths, so that the modulation has a gradient with respect to the depth within the sample. The application of this technique is demonstrated for a buried heterojunction and buried multiple quantum wells. Comparison is made with ordinary PR. In one case the heterojunction of interest was buried under two highly doped 40‐nm‐thick GaAs and AlGaAs layers. We show that this heterojunction is barely distinguishable in an ordinary PR measurement. Nevertheless, at room temperature DPR shows distinct peaked signals which correspond to the pre...

Deep levels between 0.1 and 1.0 eV in semi‐insulating and high resistivity undoped horizontal Bridgman GaAs have been studied by temperature‐dependent Hall effect (TDH) and Fourier transform photoconductivity (FTPC). Activation energies at 0.77, 0.426, and 0.15 eV have been observed by TDH. Photoionization thresholds at 1.0, 0.8, 0.56, 0.44, and 0.25 are reported. The photoconductivity thresholds at 0.56 and 0.25 eV are reported for the first time. New features in the 0.44 eV threshold suggest that the defect responsible for this level has a small lattice relaxation and Frank–Condon shift. Possible associations of the FTPC and TDH energies with the deep‐level transient spectroscopy levels EL2, EL3, and EL6 are presented.

We report a demonstration of very long wavelength infrared detectors based on InAs/GaSb superlattices operating at T=80 K. Detector structures with excellent material quality were grown on an optimized GaSb buffer layer on GaAs semi-insulating substrates. Photoconductive devices with 50% cutoff wavelength of λc=17 μm showed a peak responsivity of about 100 mA/W at T=80 K. Devices with 50% cutoff wavelengths up to λc=22 μm were demonstrated at this temperature. Good uniformity was obtained over large areas even for the devices with very long cutoff wavelengths.

Mesure de l'effet Shubnikov-de Hass dans les heterostructures a mobilite elevee d'un gaz d'electrons bidimensionnel a deux sous-bandes peuplees. Determination de la concentration des porteurs de la premiere et de la seconde sous-bande par les oscillations a deux frequences. Chacune de ces concentrations croit apres l'etablissement de l'effet de la photoconductivite persistante. Determination des masses effectives de ces deux sous-bandes aux niveaux de Fermi

The hole-transport properties of Mg-doped AlGaN/GaN superlattices are carefully examined. Variable-temperature Hall-effect measurements indicate that the use of such superlattices enhances the average hole concentration at a temperature of 120 K by over five orders of magnitude compared to a bulk GaN film (the enhancement at room temperature is a factor of 9). An unusual modulation-doping scheme, which has been realized using molecular-beam epitaxy, has yielded high-hole-mobility superlattices and conclusively demonstrated the pivotal role of piezoelectric and spontaneous polarization in determining the band structure of the superlattices.

The effect of threading dislocations on electron transport in In0.24Ga0.76N/GaN multiple quantum wells has been studied by using transmission electron microscopy (TEM) and van der Pauw Hall effect measurements. From the cross-sectional TEM imaging, we observed the threading dislocations which “screw” through the multiple In0.24Ga0.76N/GaN quantum well. From the Hall effect measurement, we found that the Hall mobility decreases as the temperature decreases (μ∼T3/2) due to the threading dislocation scattering, and the Hall carrier concentration shows a transition from conduction-band transport to localized-state-hopping transport. The thermal activation energy of the residual donor level (probably Si) is about 20.2 meV.