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A systematic investigation on the effects of growth temperature, Ga flux, and Sb flux on the incorporation of Sb, film structure, and optical properties of the GaN1-xSbx highly mismatched alloys (HMAs) was carried out. We found that the direct bandgap ranging from 3.4 eV to below 1.0 eV for the alloys grown at low temperature. At the growth temperature of 80 °C, GaN1-xSbx with x > 6% losses crystallinity and becomes primarily amorphous with small crystallites of 2-5 nm. Despite the range of microstructures found for GaN1-xSbx alloys with different composition, a well-developed absorption edge shifts from 3.4 eV (GaN) to close to 2 eV for samples with a small amount, less than 10% of Sb. Luminescence from dilute GaN1-xSbx alloys grown at high temperature and the bandgap energy for alloys with higher Sb content are consistent with a localized substitutional Sb level ESb at ~1.1 eV above the valence band of GaN. The decrease in the bandgap of GaN1-xSbx HMAs is consistent with the formation of a Sb-derived band due to the anticrossing interaction of the Sb states with the valence band of GaN. [ABSTRACT FROM AUTHOR]

Typically only dilute (up to ∼10%) highly mismatched alloys can be grown due to the large differences in atomic size and electronegativity of the host and the alloying elements. We have overcome the miscibility gap of the GaN1-xAsx system using low temperature molecular beam epitaxy. In the intermediate composition range (0.10 < x < 0.75), the resulting alloys are amorphous. To gain a better understanding of the amorphous structure, the local environment of the As and Ga atoms was investigated using extended x-ray absorption fine structure (EXAFS). The EXAFS analysis shows a high concentration of dangling bonds compared to the crystalline binary endpoint compounds of the alloy system. The disorder parameter was larger for amorphous films compared to crystalline references, but comparable with other amorphous semiconductors. By examining the Ga local environment, the dangling bond density and disorder associated with As-related and N-related bonds could be decoupled. The N-related bonds had a lower dangling bond density and lower disorder. [ABSTRACT FROM AUTHOR]

Recent work has shown that GaN1-xAsx can be grown across the entire composition range by low temperature molecular beam epitaxy with intermediate compositions being amorphous, but control of the electrical properties through doping is critical for functionalizing this material. Here we report the bipolar doping of GaN1-xAsx with high As content to conductivities above 4 S/cm at room temperature using Mg or Te. The carrier type was confirmed by thermopower measurements. Doping requires an increase in Ga flux during growth resulting in a mixed phase material of polycrystalline GaAs:N embedded in amorphous GaN1-xAsx. [ABSTRACT FROM AUTHOR]

The steady state and time-resolved photoluminescence (PL) spectra of cubic AlxGa1-xN have been measured for 0 < x < 1. The intensity of the room temperature PL increases by an order of magnitude when the AlN content increases from x = 0 to x = 0.95. Additionally, the PL decay slows down with the decrease of temperature and increase of x. These results show that strong localization of carriers on alloy composition fluctuations plays a large role in determining the intensity and temporal evolution of the PL. The activation energy for the localized carriers increases with the increase of x and reaches the value of 55 meV at x = 0.95. [ABSTRACT FROM AUTHOR]

Alloying is a commonly accepted method to tailor properties of semiconductor materials for specific applications. Only a limited number of semiconductor alloys can be easily synthesized in the full composition range. Such alloys are, in general, formed of component elements that are well matched in terms of ionicity, atom size, and electronegativity. In contrast there is a broad class of potential semiconductor alloys formed of component materials with distinctly different properties. In most instances these mismatched alloys are immiscible under standard growth conditions. Here we report on the properties of GaN1-xAsx, a highly mismatched, immiscible alloy system that was successfully synthesized in the whole composition range using a nonequilibrium low temperature molecular beam epitaxy technique. The alloys are amorphous in the composition range of 0.170.2, and to the upward movement of the valence band for alloys with x<0.2. The unique features of the band structure offer an opportunity of using GaN1-xAsx alloys for various types of solar power conversion devices. [ABSTRACT FROM AUTHOR]

The degradation of 1.3 μm GaInNAs lasers was investigated using accelerated aging tests. This was followed by comprehensive characterization, including standard light-current-voltage (L-I-V) characterization, capacitance measurements, photoluminescence microscopy (PLM), on-axis amplified spontaneous emission (ASE) spectra measurements, and photocurrent (PC) and electroluminescence (EL) spectroscopies. The slope efficiency of the device dropped by 50% with a 300% increase in the threshold current after the accelerated aging test. The ideality factors of the aged devices are higher than those of the unaged devices. PLM images showed no evidence of catastrophic optical mirror damage. The measured capacitances of the aged devices are all similar to those of the unaged devices, indicating that there was no significant dopant diffusion in the junction region. Fourier transforms of the ASE spectra showed that no intracavity defects were present in the aged lasers, suggesting that intracavity defects are not responsible for the rapid degradation of the aged devices. Although the PC measurements showed defects at 0.88–0.95 eV and at ∼0.76 eV, these defect signatures did not increase with aging. On the other hand, EL measurements revealed that radiative deep level defects were generated during the aging tests, which may be related to the degradation of the devices. Based on the above measurement results, we identify the generation of radiative deep level defects as the main causes of degradation of these devices. [ABSTRACT FROM AUTHOR]

An x-ray method is described for the structure characterization of semiordered GaN nanorods. In contrast to other works based on synchrotron radiation, the method uses a standard x-ray laboratory equipment so that it is suitable for a rapid characterization of the nanorods in a technological laboratory. The method uses a grazing-incidence diffraction setup and it makes it possible to determine the mean size of the rods and their angular twist with respect to the crystalline substrate. The applicability of the method is demonstrated on a series of GaN nanorod samples and the parameters of the nanorods are compared with the results of scanning electron microscopy. [ABSTRACT FROM AUTHOR]

We present a far-infrared transmission study on group-III nitride thin films. Cubic GaN and AlN layers and c-oriented wurtzite GaN, AlN, and AlxGa1-xN (x<0.3) layers were grown by molecular beam epitaxy on GaAs and Si(111) substrates, respectively. The Berreman effect allows us to observe simultaneously the transverse optic and the longitudinal optic phonons of both the cubic and the hexagonal films as transmission minima in the infrared spectra acquired with obliquely incident radiation. We discuss our results in terms of the relevant electromagnetic theory of infrared transmission in cubic and wurtzite thin films. We compare the infrared results with visible Raman-scattering measurements. In the case of films with low scattering volumes and/or low Raman efficiencies and also when the Raman signal of the substrate material obscures the weaker peaks from the nitride films, we find that the Berreman technique is particularly useful to complement Raman spectroscopy. [ABSTRACT FROM AUTHOR]

We have studied the magnetic and electronic properties of (Ga,Mn)As ferromagnetic semiconductor layers containing low concentrations of room temperature magnetic MnAs precipitates, using x-ray magnetic circular dichroism in x-ray absorption spectroscopy. Measurements below and above the Curie temperature of the (Ga,Mn)As show localized multiplet structures and broad metallic-like line shapes, respectively. By applying magnetic fields below the coercivity of the (Ga,Mn)As layer, the magnetization reversal behavior of the ferromagnetic semiconductor and the MnAs precipitates can be resolved and are found to be independent of each other. [ABSTRACT FROM AUTHOR]

We have used scanning Hall probe microscopy to image domain structures and magnetization reversal in optimally annealed Ga0.91Mn0.09As films grown on (311)B GaAs substrates. Unmagnetized films exhibit a disordered mazelike domain structure consistent with a composite state of regions with magnetization along [0,1,0] and [0,0,1] out-of-plane easy axes. The characteristic stripe width of ∼3 μm exhibits almost no temperature dependence in the range of 5–90 K, consistent with recent theoretical predictions, while the peak domain fields drop almost linearly with increasing temperature. With an applied field perpendicular to the zero-field-cooled film magnetization proceeds by the motion of rather ordered stripe-shaped domains which form preferentially along one of the [0,1,0]/[0,0,1] easy axes. Surprisingly, stripelike domains are not clearly observed during reversal from the magnetized state, which appears to involve the propagation of magnetic “bubbles.” Weak image contrast in the magnetized state points to the existence of residual magnetic disorder in the films on an ∼2–3 μm length scale. Abrupt breaks within single images indicate the occurrence of large Barkhausen events when domain walls suddenly jump over ∼1 μm distances. This implies the existence of strong pinning sites on this length scale and this, as well as the residual magnetic disorder, may be related to microscopic Mn-rich regions formed during sample growth. [ABSTRACT FROM AUTHOR]

We have investigated the effect of the internal electric fields on radiative and nonradiative processes in a series of AlN/GaN superlattices. The results of measurements involving direct detection of phonons emitted as a result of nonradiative recombination and carrier energy relaxation are compared with time resolved photoluminescence studies. Using these complementary techniques we have observed directly the effect of free carrier screening on the radiative and nonradiative recombination processes in the superlattice samples. We find that at high excitation power, photoinjected carriers screen the strong internal fields, resulting in an enhanced radiative recombination. As the carriers recombine, the descreening effect results in an increase in recombination via nonradiative processes, which we observe directly as a delayed phonon signal in the time of flight measurements. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The energy relaxation rate for hot electrons in n-type GaN epilayers has been measured over the temperature range 1.5-300 K. Several samples grown by molecular-beam epitaxy and having different electron concentrations have been studied. At low electron temperatures (T[sub e]<20 K), the energy relaxation is via acoustic phonon emission. The magnitude and temperature dependence of the energy relaxation are found to be in good agreement with theoretical calculations using appropriate values of the deformation potential and piezoelectric coupling constants and ignoring screening. For T[sub e]>=70 K, the dominant mechanism of energy loss is optic phonon emission. For the several samples studied, consistent values of the optic phonon energy and electron-optic phonon relaxation time, 90±4 meV and 5-10 fs, respectively, are measured. The energy agrees well with values obtained by other methods and the relaxation time is consistent with theoretical calculations of the Fro¨hlich interaction and indicate that hot phonon effects are absent up to 10[sup -8] W/electron dissipation. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the local microstructure of hexagonal AlN through x-ray absorption. Features of the two polytypes of AlN; Experimental details; Reduction of distances in the first, second and third shells in the presence of oxygen; Comparison of Al-O and Al-N distances; Determination of maximum electron charge densities.

Examines the band gaps of both hexagonal and cubic thin films deposited by a modified molecular-beam-epitaxy process on gallium arsenide and gallium phosphide substrates. Significance of the method of supplying nitrogen to the growing film; Application of x-ray diffraction and arsenic beam; Factors that may cause errors in band-gap measurements; Structural properties of the film.

Deals with a study that presented a model for the spectral response of gallium arsenide and Al[subx]Ga[sub1-x]As p-n and p-i-n solar cells based on a standard solution of the minority-carrier equations. Application of the quantum well solar cell; Capability of the model; Strengths and weaknesses of the model; Disadvantages from the use of intercell connection.

Examines the identification and field-dependent ionization energies of a number of the metastable states. Mechanisms which lead to photoconductivity; Influence of the presence of ionized impurities on transition probabilities and energy shifts of shallow donor states; Structure of the transition to the metastable state with and without band-gap radiation.

Focuses on a study which discussed the effects of practical properties of gallium arsenide-aluminum gallium arsenide heterostructures on the accuracy of a quantum-Hall resistance standard. Methods of conduction that were addressed; Details of the measurement concerning the influences of sample size and mobility; Aspects of mobility that were investigated; Recommendations on sample geometry and mobility.

Presents information on a study which investigated the temperature dependence of the photoluminescence decay kinetics of a series of gallium arsenide (GaAs)/aluminum arsenide (AlAs) quantum-well structures where GaAs thickness was kept constants and AlAs was varied. Methodology of the study; Results and discussion; Conclusions.

Deals with a study that investigated the effects of undoped spacer layers on the electrical properties of single barrier heterostructures by measuring incremental slope resistance. Aspects that increase with spacer thickness; Capability of arsenic/gallium arsenide double barrier resonant tunneling devices; Components of the devices.

We present an experimental investigation of the magnetic, electrical, and structural properties of Ga0.94Mn0.06As1-yPy layers grown by molecular beam epitaxy on GaAs substrates for y≤0.3. X-ray diffraction measurements reveal that the layers are under tensile strain, which gives rise to a magnetic easy axis perpendicular to the plane of the layers. The strength of the magnetic anisotropy and the coercive field increases as the phosphorous concentration is increased. The resistivity of all samples shows metallic behavior with the resistivity increasing as y increases. These materials will be useful for studies of micromagnetic phenomena requiring metallic ferromagnetic material with perpendicular magnetic anisotropy. [ABSTRACT FROM AUTHOR]