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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...

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.

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.

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 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 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.

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...

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.

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 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.

Short-period superlattices consisting of alternating layers of GaN:Mg and AlGaN:Mg were grown by low-pressure organometallic vapor phase epitaxy. The electrical properties of these superlattices were measured as a function of temperature and compared to conventional AlGaN:Mg layers. It is shown that the optical absorption edge can be shifted to shorter wavelengths while lowering the acceptor ionization energy by using short-period superlattice structures instead of bulk-like AlGaN:Mg.

The persistent-photocurrent effect in bulk, semi-insulating GaAs has been studied as a function of temperature. The persistent photocurrent is activated by illumination with 1.13-eV light and is preceded by an enhanced photocurrent during long-term illumination after the initial photocurrent has decayed in a quenching process similar to that of the deep donor EL2. Both the magnitude of the persistent photocurrent, and its decay rate are strongly temperature dependent and show significant reduction above 40 K. These results indicate that while the metastable transformation of EL2 is required for the activation of the persistent photocurrent, another unidentified metastable process after this is also required.

Photoreflectance is used to measure AlxGa1−xAs composition, and to determine carrier concentrations in Si‐doped AlGaAs epilayers capped with GaAs. Undoped caps are generally depleted, and do not show a usual GaAs photoreflectance. However, photoreflectance from the cap/(doped AlGaAs) interface produces a broad signal which distorts the entire spectrum, making it hard to locate the GaAs and AlGaAs band edges precisely. A similar broad signal from modulation‐doped heterostructures is apparently associated with samples that show the presence of two‐dimensional electron gas.

Room‐temperature photoreflectance measurements were used to determine the radial and axial distribution of low levels of indium in 3‐in.‐diameter semi‐insulating bulk GaAs materials grown by the liquid‐encapsulated Czochralski method. These results were compared with 4.2‐K photoluminescence data and found to be accurate and more convenient for this application. Room‐temperature photoreflectance allows an accurate determination of the indium content in the range of mole fraction 0.1%–2.0% with standard deviation of 0.03%. Two types of radial inhomogeneity were found in commercially available GaAs wafers. This is discussed in terms of indium segregation and shape of solid and liquid interface during the crystal growth.

Thermal admittance spectroscopy has been used to determine the ground-state energies of the boron impurity in 6H-SiC. The background doping, NA−ND, of the samples used in this study ranged from 3×1016 to 1×1018 cm−3. From electron spin resonance studies, it is known that boron substitutes for silicon in the silicon carbide lattice occupying three inequivalent sites. Using admittance spectroscopy the ground state energies of Ev+0.27 eV, Ev+0.31 eV, and Ev+0.38 eV were determined for the shallow boron acceptor in 6H-SiC. The free carrier concentration does not appear to be the only determining factor for which the boron acceptor level is observed.

We have measured the Shubnikov–de Haas (SdH) effect in δ-doped AlAs0.56Sb0.44/In0.53Ga0.47As heterostructures and observed a population of the second subband. Using the persistent photoconductivity effect we increased the electron density from 26.73 to 28.20×1011 cm−2 in the first subband and 6.61 to 7.20×1011 cm−2 in the second. The onset of the second subband population occurs when the first subband is filled to a density of 11.56×1011 cm−2. From the nonparabolic band approximation we calculated the effective masses in both subbands before illumination. The effective mass for the second subband was evaluated using the temperature dependence of the SdH amplitude. Its value agrees well with the values obtained from the k⋅p approximation and infrared cyclotron resonance measurements.

Impurity conduction (or hopping conduction) has been observed in the more heavily n‐type 4H–SiC samples by both temperature dependent resistivity measurements and thermal admittance spectroscopy. The measured activation energies e3 for hopping were 4–5 meV and 2.3–3.0 meV, respectively. No evidence of hopping conduction was seen by either method in the sample where ND–NA

Optical admittance spectroscopy has been used to study defects in 4H–SiC; the vanadium donor level at EC‐1.73 eV has been identified. The optical admittance spectrum of 4H–SiC is similar to that of 6H–SiC, where the vanadium donor level is at EC‐1.59 eV. The band gaps of 6H–SiC and 4H–SiC were measured. The values of 3.10±0.03 eV for 6H–SiC and 3.41±0.03 eV for 4H–SiC are in reasonable agreement with reported values. Using the vanadium donor level in both 4H–SiC and 6H–SiC as a common reference, the band offsets for 6H–SiC/4H–SiC heterojunction are estimated to be ΔEC=0.14 eV and ΔEV=0.17 eV.

The slow buildup of photoconductance due to a peak centered at 1.13 eV below the conduction band in n‐type 6H–SiC samples has been studied using optical admittance spectroscopy. The background doping of the samples ranges from 3.2×1015 cm−3 to 2×1018 cm−3. The results indicate that the carriers responsible for the observed conductance are excited from defect levels that are energetically close. The photoconductance transients are completely described by a sum of two exponential expressions. The relevant parameters, α1,α2,Gmax(1), and Gmax(2), are determined as functions of temperatures. The background doping ND‐NA has no effect on the conductance buildup process, this indicates that the nitrogen donors play no role either in the formation of the defect or in the buildup process of the photoconductance.

We have observed the population of the second two‐dimensional electron subband in δ‐doped Al0.48In0.52As/Ga0.47In0.53As heterostructures by Shubnikov–de Haas measurements. After illuminating the samples at low temperature, the electron density increases from 17.3 to 18.2×1011 cm−2 for the first subband and from 3.6 to 4.1×1011 cm−2 for the second subband. The population of the second subband begins when the first subband is filled at a density of 10.3×1011 cm−2. The effective mass of the second subband is equal to (0.045±0.003)m0, indicating significant band nonparabolicity in the Ga0.47In0.53As well.

Persistent photoconductivity has been observed at low temperatures in thin, unintentionally doped GaInP/GaAs/GaInP quantum wells. The two‐dimensional electron gas was studied by low field Hall and Shubnikov–de Haas effects. After illumination with red light, the electron concentration increased from low 1011 cm−2 to more than 7×1011 cm−2 resulting in an enhancement of both the carrier mobility and the quantum lifetime. The persistent photocarriers cannot be produced by DX‐like defects since the shallow dopant concentration in the GaInP layers is too low to produce the observed concentration. We suggest that the persistent carriers are produced by photoionization of deep intrinsic donors in the GaInP barrier layer. We also report observation of a parallel conduction path in GaInP induced by extended illumination.

Electronic transport properties of very thin undoped GaInP/GaAs quantum wells have been measured by temperature dependent low field Hall effect and by Shubnikov–de Haas effect. Strong Shubnikov–de Haas oscillations were observed after increasing the electron concentration via the persistent photocurrent effect. Low temperature mobilities of up to 70 000 cm2/V s at carrier concentrations of 6.5×1011 cm−2 were observed in a 20 A quantum well. The results are compared with the theory of interface roughness scattering which indicates extremely smooth interfaces; however, discrepancies between experiment and theory are observed.

We have observed a negative persistent photoconductivity effect in In0.25Ga0.75Sb/InAs quantum wells with Shubnikov–de Haas measurements. The saturated reduction of the electron density in the InAs well was about 10%. The electron effective mass was found to be (0.048±0.004) m0 for an electron density of 18.0×1011 cm−2. The electron quantum lifetime decreased as the electron density was reduced by the negative persistent photoconductivity effect due to electron‐hole interaction.

We report on the growth and characterization of type-II IR detectors with a InAs/GaSb superlattice active layer in the 15-19 micrometers wavelength range. The material was grown by molecular beam epitaxy on semi-insulating GaAs substrates. The material was processed into photoconductive detectors using standard photolithography, dry etching, and metalization. The 50 percent cut-off wavelength of the detectors is about 15.5 micrometers with a responsivity of 90mA/W at 80K. The 90 percent-10 percent cut-off energy width of the responsivity is only 17meV which is an indication of the uniformity of the superlattices. These are the best reported values for type-II superlattices grown on GaAs substrates.

Thermal admittance spectroscopy was used to characterize the shallow dopants in chemical vapor deposition ( CVD) grown thin films and in sublimation sandwich method ( SSM) grown 4H-SiC layers. The values of the activation energy levels of EC − 0.054 eV for Nitrogen at the hexagonal site and of EC − 0.10 eV for Nitrogen at the cubic site were indices of comparison. The net carrier concentrations ( ND − NV ) of the films were determined by capacitance-voltage measurements. The net carrier concentrations for the SSM films ranged from 2 × 1017 to 7 × 1017 cm−3. The two Nitrogen levels were observed in the CVD films. Hopping conduction with an activation energy of EC −0.0058 eV was observed in one SSM sample having ND − NV = 7 × 1017 cm−3.

Impurity conduction (or hopping conduction) has been observed in the more heavily n-type 4H-SiC samples by both temperature dependent resistivity measurements and thermal admittance spectroscopy. The measured activation energies ɛ 3 for hopping were 4–5 meV and 2.3–3.0 meV respectively. No evidence of hopping conduction was seen by either method in the sample where ND-NA < 1018 cm-3. The thermal admittance spectrum of the lightly n-type sample showed the two nitrogen levels at 53 and 100 meV.

Nitrogen is the common n-type dopant of the various polytypes of silicon carbide. The nitrogen levels in 4H-SiC (at EC-53 meV and EC-100 meV) and in 6H-SiC (at EC-89 meV, EC- 100 meV, and EC-125 meV) have been studied in detail by temperature dependent Hall effect measurements, electron spin resonance (ESR), and thermal admittance spectroscopy. Until now, such detailed studies of the nitrogen levels in 15R-SiC have not been carried out.Lely-grown 15R samples were used in these studies. The net carrier concentrations (ND-NA), determined by room temperature CV measurements, ranged from 1 × 1018 to 3 × 1018 cm−3. The nitrogen levels in 15R-SiC were studied using thermal admittance spectroscopy. Optical admittance spectroscopy (OAdS) was used to study the deeper defects in this polytype. It was found that optical transitions to the conduction band were inhibited in the heavily doped material.

We have fabricated AlxGa1−xN/GaN heterostructures with high two-dimensional electron gas (2DEG) mobilities and high sheet carrier densities by metalorganic vapor phase epitaxy (MOVPE). The 2DEG sheet density and mobility exhibit a compositional dependence on the Al fraction of the electron donor layer. The highest mobility (5750 cm2/Vs at 16K) was measured in a sample with x=0.15 that had a sheet carrier density of 8.5×1012 cm−2. The undoped AlxGa1−xN layers have low background carrier concentrations and can be intentionally doped n-type using SiH4. The effect of intentional n-type doping of the AlxGa1−xN donor layer on the electrical properties of the 2DEG was studied in structures that included an undoped AlxGa1−xN spacer layer of varying thickness. Higher 2DEG mobilities were obtained when a 100Å thick undoped layer was included in the structure due to spatial separation of the 2DEG from ionized impurities in the doped AlxGa1−xN. These initial results demonstrate that the electrical properties of AlxGa1−xN/GaN heterostructures can be controlled by intentional doping and appropriate layer design.

In this paper, we report chemical beam epitaxy (CBE) of GaAs, and for the first time, Arion laser-assisted CBE using triethylgallium (TEGa) and tris-dimethylaminoarsenic (TDMAAs), a safer alternative to arsine. Samples grown at substrate temperatures above 490° C show n-type conduction, while those grown at lower temperatures show p-type conduction. An unintentional doping concentration of n∼lx1016 cm-3 with an electron mobility of 5200 cm2/V.s at 300 K and 16000 cm2/V.s at 77 K have been achieved. These are the best results reported for GaAs grown with TDMAAs. Laser-assisted CBE of GaAs is studied in the substrate temperature range of 240-550°C. There are two different substrate-temperature regions for laser-enhanced growth, 265 to 340°C and 340 to 440°C, which are believed to be caused by different TEGa decomposition mechanisms. The laser-assisted growth with TDMAAs, compared to AS4 or AsH3, shows a wider range of growth enhancement at low substrate temperatures.

Zinc germanium phosphide, an important frequency-conversion material for producing mid-infrared lasers, is plaqued by a detect-related absorption band extending from the fundamental edge (0.62 microns) to ∼3 microns. The level of absorption varies with melt composition, and can be reduced by post-growth annealing treatments. In these experiments, further reduction of the near-band-edge absorption was achieved by irradiating with 1-1.5 MeV electrons at cumulative fluence levels up to 2.75xlOI8cm2. Ge-rich, ZnP2-rich, and both as-grown and annealed stoichiometric crystals were studied. The near-edge absorption of the higher-loss, nonstoichiometric samples decreased monotonically with each subsequent irradiation, whereas the absorption in the lower-loss, stoichiometric samples was minimized after cumulative electron fluences of 2 x 10I8cnv2 and 1 x 10I8cm2 for as-grown and annealed material respectively. The minimum absorption coefficient achieved at 1/im was ∼4.4cnr’ in both stoichiometric samples, representing a factor two decrease for the as-grown crystal. Further exposure after reaching saturation served only to increase the losses at longer wavelengths.

The performance and operating temperature of infrared (IR) detectors are largely limited by thermal generation and noise processes in the active region of the device. Particularly, excess background charge carriers enhance dark currents and depress the detector figures of merit. Therefore, reducing the overall defects and background carriers in the undoped region of p-i-n diodes is an important issue for developing high-operating temperature IR detectors. In this article, the authors discuss how several postgrowth annealing conditions and interface shutter sequences are optimized to reduce the density of nonradiative defect trap centers and background carriers and studied their relevance to the photoluminescence (PL) emission qualities of typical mid-IR InAs∕GaSb superlattices (SLs). Among the several in situ postgrowth annealing temperatures investigated, the SLs annealed at 450°C had the highest carrier density and the lowest PL intensity, while the SLs annealed at 475°C had the lowest carrier density a...

We report the use of rooni-Lemperature photorefleetance measuremenLs to deLenitine radial arid axial nonuniformity of low levels of indium in 3-inch diameter semi-insulaLing GaAs bulk materials grown by the liquid-encapsulaLed Cochralski method. These resulLs were compared with umc. Types of inhomogeneities are discussed in Lerms of indium segregation and the shape of the solid and liquid interface during crystal growth.

A study of deep levels in high purity semi-insulating 4H-SiC has been made using temperature dependent Hall effect (TDH), thermal and optical admittance spectroscopies, and secondary ion mass spectrometry (SIMS). Thermal activation energies from TDH varied from a low of 0.55eV to a high of 1.65eV. All samples studied showed n-type conduction with the Fermi level in the upper half of the band gap. Fits of the TDH data to different charge balance equations and comparison of the fitting results with SIMS measurements indicated that the deep levels are acceptorlike even though they are in the upper half of the band gap. Carrier concentration measurements indicated that the deep levels are present in concentrations in the low 1015cm−3 range, while SIMS results demonstrate nitrogen and boron concentrations in the low to mid-1015-cm−3 range. The results suggest that compensation in this material is a complex process involving multiple deep levels.

Temperature dependent Hall effect (TDH), low temperature photoluminescence (LTPL), secondary ion mass spectrometry (SIMS), optical admittance spectroscopy (OAS), and thermally stimulated current (TSC) measurements have been made on 6H-SiC grown by the physical vapor transport technique without intentional doping. n- and p-type as well semi-insulating samples were studied to explore the compensation mechanism in semi-insulating high purity SiC. Nitrogen and boron were found from TDH and SIMS measurements to be the dominant impurities that must be compensated to produce semi-insulating properties. The electrical activation energy of the semi-insulating sample determined from the dependence of the resistivity was 1.0eV. LTPL lines near 1.00 and 1.34eV, identified with the defects designated as UD-1 and UD-3, were observed in all three samples but the intensity of the UD-1 line was almost a factor of 10 more in the n-type sample than in the the p-type sample with that in the semi-insulating sample being inter...

Deep levels and measured band discontinuities in Si/SiGe heterostructures grown by gas‐source molecular‐beam epitaxy have been identified. Hole traps in the p‐type, undoped, single layers of Si1−xGex (0.05≤x≤0.26) have activation energies ranging from 0.029 to 0.45 eV and capture cross sections (σ∞) ranging from 10−9 to 10−20 cm2. Possible origins of these centers are discussed. It is found from admittance spectroscopy measurements that the band discontinuity is almost entirely in the valence band for 0

Temperature‐dependent Hall‐effect and photoluminescence measurements have been performed on a series of antimony‐doped bulk GaAs samples that were otherwise undoped. A new donor level located 0.48 eV below the conduction‐band edge has been detected by both experiments in all antimony‐doped samples studied. This level reduces the resistivity of antimony‐doped material below the semi‐insulating limit. Comparison with known intrinsic levels in undoped material have been made and it is shown that the 0.48‐eV donor is distinct from any of these. It is concluded that the defect responsible for the 0.48‐eV donor involves an impurity antisite SbGa either isolated or in a complex with intrinsic defects.