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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]

We have performed spatially resolved polar magneto-optical Kerr effect microscopy measurements on as-grown and annealed Ga0.95Mn0.05As thin films with tensile strain. We find that the films exhibit very strong perpendicular magnetic anisotropy which is increased upon annealing. During magnetic reversal, the domain walls propagate along the direction of surface ripples for the as-grown sample at low temperatures and along the [110] direction for the annealed sample. This indicates that the magnetic domain pattern during reversal is determined by a combination of magnetocrystalline anisotropy and a distribution of pinning sites along the surface ripples that can be altered by annealing. These mechanisms could lead to an effective method of controlling domain wall propagation. [ABSTRACT FROM AUTHOR]

We present Mn L3,2 x-ray absorption measurements from two dilute magnetic semiconductor systems: p-type (Ga,Mn)As, which is ferromagnetic with TC=140 K; and n-type (Ga,Mn)N, in which the magnetic impurities are predominantly paramagnetic. After removing a Mn-rich oxide surface layer by chemical etching, the Mn L3,2 spectra from (Ga,Mn)As appear less localized than in previous reports, which is ascribed to screening due to p–d hybridization. Our results suggest that previous studies may have been influenced by the quality of the surface. In contrast, in the (Ga,Mn)N film the Mn ground state is closer to pure d5. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Scanning Hall probe microscopy has been used to study the magnetic domain structure of GaMnAs thin films after various low temperature anneals to increase the Curie temperature (TC). Samples with in-plane magnetization, which received short low temperature anneals in vacuum directly after growth, exhibit very small (∼2–5 μm) rather random domains. In stark contrast similar samples, which additionally received very long low temperature anneals in air, display large (10–100 μm) domains, which still contain clearly resolvable magnetic disorder. Preliminary scans of air-annealed samples with out-of-plane magnetization also reveal a very irregular, rather fine (1–3 μm) domain structure. In all samples micron-sized regions at domain walls frequently remain ferromagnetic well above the average TC, indicating either the presence of ferromagnetic precipitates (e.g., MnAs) or material there with higher than average Mn concentration. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on measurements of the hole density, ferromagnetic transition temperature TC, and magnetization in a series of as-grown and annealed (Ga,Mn)As samples. Estimating the fraction of incorporated Mn occupying interstitial and substitutional sites allows a direct comparison of the predictions of mean field theory with experiment, and a determination of the magnetic moment per substitutional Mn. The saturation of TC at high Mn concentration is consistent with the mean field prediction. The estimated magnetic moment per Mn is close to the expected 5μB for all samples studied if an antiferromagnetic coupling between interstitial and substitutional Mn is assumed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have imaged the micromagnetic structure of 50 nm Ga[sub 0.91]Mn[sub 0.09]As films after different low-temperature anneals. Samples annealed in vacuum for 10 min display a very random domain structure with small (∼3–5 μm) domains. In contrast a sample which was further annealed in air for 50 h exhibited the highest Curie temperature and very large (∼100 μm) domains. Even within large domains we resolve magnetic disorder which has not been removed by the annealing procedure. Micron-sized regions near domain walls remain ferromagnetic well above T[sub C] in all the films, possibly indicating the presence of regions with above average Mn density or very small MnAs precipitates, which act as pinning centers and strongly influence the coercive fields of the films. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The resistivity, temperature, and magnetic field dependence of the anomalous Hall effect in a series of metallic Ga[SUB1-x]Mn[SUBx]As thin films with x=0.015 to 0.08 is presented. A quadratic dependence of the anomalous Hall resistance on the resistivity is observed, with a magnitude which is in agreement with Berry phase theories of the anomalous Hall effect in dilute magnetic semiconductors. [ABSTRACT FROM AUTHOR]

Presents a study which investigated the resonant polaron effects on donor metastable states in n-gallium arsenide using far infrared Fourier transform magneto-spectroscopy. Information on the interaction of electrons with optical phonons; Computational approach; Results.

Examines the mobility of two-dimensional electron gas structures. Influence of aluminum gallium arsenide on the mobility; Trend of carrier density and mobility with undoped spacer thickness observed following the cleanup phase; Production of a sample with mobility in excess of 3×10[sup6] cm²V[sup-1]s[sup-1].

Recent developments with group III nitrides suggest AlxGa1-xN based LEDs can be new alternative commercially viable deep ultra-violet light sources. Due to a significant difference in the lattice parameters of GaN and AlN, AlxGa1-xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE) using a novel RF plasma source. Thick wurtz-ite AlxGa1-xN films were grown by PA-MBE on 2-inch GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1-xN samples. Growth rates of AlxGa1-xN up to 3 µm/h have been demonstrated. Our novel high efficiency RF plasma source allowed us to achieve free-standing bulk AlxGa1-xN layers in a single day's growth, which makes our MBE bulk growth technique commercially viable. [ABSTRACT FROM AUTHOR]

In this letter, we study the optical properties of GaN1-xSbx thin films. Films with an Sb fraction up to 42% were synthesized by alternating GaN-GaSb layers at a constant temperature of 325 °C. The measured optical absorption data of the films are interpreted using a modified band anticrossing model that is applicable to highly mismatched alloys such as GaN1-xSbx in the entire composition range. The presented model allows us to more accurately determine the band gap as well as the band edges over the entire composition range thereby providing means for determining the composition for, e.g., efficient spontaneous photoelectrochemical cell applications. [ABSTRACT FROM AUTHOR]

We have studied time-integrated and time-resolved photoluminescence (PL) spectra and the transport properties of cubic GaN layers doped with different amounts of either magnesium (p-type) or silicon (n-type). In Mg doped material a broad blue peak was observed whose origin in cubic material has not been well established. Our temperature dependent and time resolved PL results as well as the observation of strong self-compensation in Mg-doped cubic GaN have allowed us to present a model for the origin of this emission in cubic material. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We investigate the dependence of the magnetic and structural properties of (Ga,Mn)As/(Al,Ga,Mn)As bilayer films on the stoichiometry of the interface region. For films incorporating a thin As-deficient layer at the interface, the out-diffusion of interstitial Mn from the bottom layer is strongly suppressed, resulting in a large difference in TC and magnetic anisotropy between the two layers. X-ray reflectivity measurements show that the suppression of interstitial diffusion is correlated with an increased interface roughness. When the As-deficient interface layer is thicker than 2.5 nm, the in-plane uniaxial magnetic easy axis rotates from the [1-10] to the [110] crystalline axis. [ABSTRACT FROM AUTHOR]

Transmission Electron Microscopy of Mg doped GaN1-xAsx samples, grown by MBE at low temperatures, show substantial structural changes for samples that are semi-insulating and those with high or low conductivity. The conductive samples show p-type conductivity as evidence from the positive thermopower values. All the Mg doped samples show phase segregation: cubic GaAs and GaN grains (a mixture of cubic and some hexagonal) phases within an amorphous matrix. The best conductive samples show cubic GaAs grains with high density of stacking faults embedded into an amorphous matrix. The samples that are less conductive have lower ratio of the amorphous to the crystalline phase of the samples and much lower density of stacking faults. Higher Mg concentration is expected in the amorphous parts of the samples The semi-insulating samples that have either low Mg concentration or low As show grains of GaAs and GaN attached to each other with no evidence of the amorphous phase between them. There are no SFs in these grains. It is possible that the presence of the GaN between the GaAs grains lead to semi-insulating material properties since p-type doping of GaN is more difficult. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We have studied the growth of GaN layers by plasma-assisted electroepitaxy (PAEE). We have used an RF-plasma source to produce high concentrations of N species in the Ga melt and a high DC electric current to transfer the N species from the Ga surface to the growth interface. We have achieved continuous GaN layers by PAEE at growth temperatures as low as ∼650oC. We have also investigated PAEE growth of GaN layers on GaN nanocolumn templates. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We have discovered materials with a wide tunability of the band gap that opens the possibility to achieve a desirable material for multi-junction solar cells. Two different alloy systems have been studied. GaN1-xAsx alloys over the entire composition range were grown by plasma-assisted MBE at low temperature on crystalline and amorphous (glass) substrates. For growth on sapphire the alloys are amorphous in the composition range of 0.171-xAsx alloys ranges from 3.4-0.7 eV, covering a large part of the solar spectrum, therefore this material is potentially useful for solar energy conversion. The substitution of N by Bi in GaN forming GaN1-x Bix is extremely difficult since Bi and N have even larger differences in size and electronegativity. TEM studies of GaN1-xBix with x up to 0.2 show that the alloy is not entirely amorphous. Small crystallites embedded in an amorphous matrix are observed and Bi segregation was confirmed by Electron Energy Loss studies. A strong reduction in band gap is observed in samples with increasing Bi, suggesting the formation of a random alloy but low energy absorption tail may be related to the presence of the crystallites observed by TEM (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]