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This paper addresses the broadening mechanism of the near band gap photoluminescence in GaN films doped n-type with silicon. The films were produced by plasma assisted MBE and their carrier concentration was varied systematically from 1015 to 1020 cm−3. The photoluminescence was excited with a 10 mW He-Cd laser at 77K. At low carrier conentration ( < 1017 cm−3 ) the photoluminescence peak has FWHM of about 18 meV, while at high carrier concentration (>1018 cm−3 ) the FWHM increase monotonically with the carrier concentration up to about 120 meV. The broadening of the line at high carrier concentration is attributed to impurity band broadening and tailing of the density of states. The data were quantitatively analyzed, as a function of carrier concentration and compensation ratio, using the impurity band broadening model of Morgan [1] and the agreement between model and experimental data supports the model's validity and suggest a potential method of determining the compensation in degenerate semiconductors.

AlxGa1−xN-based UV emitters in the form of a graded-index separate confinement heterostructure were grown on 6H-SiC. The devices consist of 100 nm AlxGa1−xN compositionally graded from x = 0.3 to 0 and x = 0 to 0.3 in two sides. Owing to polarization doping from naturally occurring or intentionally incorporated dopants, the two sides are of the doped p- and n-types, giving rise to a p–n junction. The doping level was calculated to be 1018 cm−3 for the p- and n-sides. It was found experimentally that this doping level requires the intentional incorporation of Mg and Si, and those devices show better rectification and stronger electroluminescence emission.

In this paper we are addressing some of the fundamental materials issues for the development of vertical GaN-based power devices. Major components of such device are the n+ GaN freestanding substrate on which a thick (~50 µm), low defect density and low carrier concentration (

Creation of short-period InN/GaN superlattices is one of the possible ways of conducting band gap engineering in the green-blue range of the spectrum. The present paper reports results of photoluminescence experiments, including pressure effects, on a superlattice sample consisting of unit cells with one monolayer of InN and 40 monolayers of GaN. The results are compared with calculations performed for different types of superlattices: InN/GaN, InGaN/GaN, and InN/InGaN/GaN with single monolayers of InN and/or InGaN. The superlattices are simulated by band structure calculations based on the local density approximation (LDA) with a semi-empirical correction for the “LDA gap error”. A similarity is observed between the results of calculations for an InGaN/GaN superlattice (with one monolayer of InGaN) and the experimental results. This indicates that the fabricated InN quantum wells may contain some Ga atoms due to interdiffusion.

The authors report on the growth, structure, and emission properties of AlGaN double heterostructures having a graded-index-separate-confinement-heterostructure design. These devices were grown on the Si-face of 6H-SiC substrates by plasma-assisted molecular-beam epitaxy. The active region of the device consists of 75-nm thick Al0.72Ga0.28N film, confined by two 50-nm thick compositionally graded AlxGa1−xN films (x = 1–0.8 and x = 0.8–1) and two AlN cladding layers. X-ray diffraction and transmission electron microscopy provide evidence that the compositionally graded AlGaN layer may also be serving as a strain transition buffer, by blocking threading defects in the vicinity of the AlN/AlGaN heterointerface. Polarization dependent photoluminescence studies indicate that the emission from these structures at 257 nm is transverse magnetic polarized. Simulation studies indicate that the vertical confinement of the optical mode in these structures is 32.5% and simulations of the band structure indicate the formation of a p-n junction resulting from polarization-induced doping. Electron-beam pumping of these structures provides evidence of the onset of stimulated emission at room temperature.

Measurements of photoluminescence and its dependence on hydrostatic pressure are performed on a set of InN/nGaN superlattices with one InN monolayer and with different numbers of GaN monolayers. The emission energies, EPL, measured at ambient pressure, are close to the value of the band gap, Eg, in bulk GaN, in agreement with other experimental findings. The pressure dependence of the emission energies, dEPL/dp, however, resembles that of the InN energy gap. Further, the magnitudes of both EPL and dEPL/dp are significantly higher than those obtained from ab-initio calculations for 1InN/nGaN superlattices. Some causes of these discrepancies are suggested

We investigate the deep-UV optical emission and gain properties of AlxGa1−xN/AlyGa1−yN multiple quantum wells structures. These structures were grown by plasma-assisted molecular-beam epitaxy on 6H-SiC substrates, under a growth mode which promotes various degrees of band-structure potential fluctuations in the form of cluster-like features within the wells. The degree of inhomogeneities in these samples was determined by cathodoluminescence mapping. We measured the TE-polarized amplified spontaneous emission in the sample with cluster-like features and quantified the optical absorption/gain coefficients and gain spectra by the variable stripe length technique under ultrafast optical pumping. A maximum net modal gain of about 120 cm−1 is measured at 4.9 eV. On the other hand, we found that samples with homogeneous quantum wells lead to absorption. Numerical simulations are performed to support our experimental findings.

Positron beam analysis has been performed on autodoped n-type, semi-insulating and Mg doped p-type epitaxially grown layers of GaN on sapphire. Doppler Broadening measurements clearly indicate the presence of vacancies in the intrinsically autodoped n-type GaN by an increase in the annihilation Doppler lineshape S-parameter of 1.04 relative to the value for the high resistivity sample. This value is typical for vacancy-type defects in compound semiconductors such as GaAs. Results of experiments with higher sensitivity to core-electrons are also presented. These two detector coincidence measurements yield information on the chemical environment surrounding the vacancies. The results are consistent with the presence of Ga vacancies in the autodoped n-type sample.

The electronic structure of thin film wurtzite GaN has been studied using a combination of angle resolved photoemission, soft x-ray absorption and soft x-ray emission spectroscopies. We have measured the bulk valence and conduction band partial density of states by recording Ga L- and N K- x-ray emission and absorption spectra. We compare the x-ray spectra to a recent ab initio calculation and find good overall agreement. The x-ray emission spectra reveal that the top of the valence band is dominated by N 2p states, while the x-ray absorption spectra show the bottom of the conduction band as a mixture of Ga 4s and N 2p states, again in good agreement with theory. However, due to strong dipole selection rules we can also identify weak hybridization between Ga 4s- and N 2p-states in the valence band. Furthermore, a component to the N K-emission appears at approximately 19.5 eV below the valence band maximum and can be identified as due to hybridization between N 2p and Ga 3d states. We report preliminary results of a study of the full dispersion of the bulk valence band states along high symmetry directions of the bulk Brillouin zone as measured using angle resolved photoemission. Finally, we tentatively identify a non-dispersive state at the top of the valence band in parts of the Brillouin zone as a surface state.

We report the growth of AlxGa1−xN alloys on (0001) sapphire by the method of Electron Cyclotron Resonance-assisted Molecular Beam Epitaxy (ECR-MBE). The films were doped n-type with silicon at carrier concentration levels from 1016 to 1019 cm−3. SEM studies reveal smooth surface morphology consistent with the observed 3×4 surface reconstruction in the RHEED pattern. Independent determination of the Al-concentration and the lattice constant of the alloys shows that Vegard’s rule is obeyed in the pseudo-binary GaN-AlN system. The bandgap of the alloys, determined by transmission and photoluminescence measurements, was found to depend linearly on Al-concentration.

AlxGal−xN films (x≤0.60) were grown on c-plane sapphire and (0001) 6H-SiC substrates using ECR plasma assisted Molecular Beam Epitaxy. Evidence of long range ordering in the investigated AlxGal−xN films is presented. Without intentional dopants the films are semi-insulating with resistivities ranging from 103 to 105 Ω.cm. The films were doped n-type with Si and p-type with Mg. The carrier concentration in the Si doped films, as determined by Hall effect measurements, was between 1016 to 1019 cm−3. At constant Si cell temperature, the carrier concentration was found to be reduced with AlN mole fraction, consistent with the observation that the donor ionization energy increases with Al content. Correspondingly, the electron mobility decreases with Al concentration, a result attributed to alloy scattering. The Mg doped films were found to exhibit p-type conductivity by thermoelectric power measurements with resistivities varying from 3 to 30 Ω-cm.

GaN and All-xGaxN films were grown by the method of ECR-MBE. Absorption constants as a function of wavelength were determined from transmission measurements. Photoconducting detectors were fabricated from these films and characterized in terms of their spectral response and photoconductive gain. Mobility-lifetime products were determined from the measurement of photoconductive gain. The resistivity and mobility-lifetime products of the films were varied from 10-109 ohm-cm and 10-3 -10-8 cm2/V respectively by changing the microwave power in the ECR discharge from 20-60 watts. The change in the mobility-lifetime product is attributed to change in the lifetimes of the photogenerated carriers. This assumption is supported by direct measurement of detector response times. Finally, we report for the first time, the detection of alpha particles using GaN detectors.

X-ray absorption measurements at the N and O-K-edges are used to study the local microstructure in cubic and hexagonal GaN films grown by ECR-MBE and HVPE. A distortion in the local microstructure is identified in the 1st nearest neighbor (nn) shell, consisting of Ga atoms, in both the cubic and hexagonal samples. Two N-Ga distances are identified, R1 and R2, where R1 is the expected distance of 1.95Å while R2=R1+0.25Å. The same distortion is detected in the next nn shell containing Ga atoms, where the two distances are 3.7Å and 4.1Å. All the reported distance variations are larger than the error-bar. The nitrogen 2nd nn neighbor is found at the expected distance of 3.12Å while N deficiency is not detected. Finally, the O-Ga distance is found equal to 1.60Å and therefore it can be proposed that the oxygen atom occupies interstitial positions.

We report the growth of InGaN/AIGaN MQWs on c-plane sapphire by electron cyclotron resonance assisted molecular beam epitaxy (ECR-MBE). Two types of structures were investigated; one employing a GaN and the other a A1GaN barrier layer. The first structure consists of five periods of 80 Å thick In0.09Ga0.91N wells separated by 90 Å thick GaN barriers. The second structure consists of|seven periods of 120 Å thick In0.35Ga0.65N wells and Al0.1Ga0.9N barriers. The substrate temperature was kept constant during the growth of both the wells and the barriers, thus avoiding the need for any temperature cycling during the growth, which may lead to interfacial contamination. The films were characterized by cross sectional transmission electron microscopy (TEM), room temperature photoluminescence (PL) and sub-micron resolution luminescence microscopy. TEM images show sharp and abrupt interfaces, thus confirming the high interfacial quality of the MQW structures. Both structures exhibit strong RT luminescence emission peaking at 387 nm (FWHM = 16nm) for the In0.09Ga0.91N/GaN structure and at 463 nm (FWHM = 28nm) for the In0.35Ga0.65N/A10.1Ga0 9N structure. The high resolution luminescence microscopy studies reveal that the radiative recombination for the InGaN quantum wells is 60–70 times more efficient than for the underlying GaN film.

We report for the first time, fabrication of photoconducting UV detectors made from GaN films grown by molecular beam epitaxy. Semi-instilating GaN films were grown by the method of electron cyclotron resonance microwave plasma-assisted molecular beam epitaxy (ECR-MBE). Photoconductive devices with interdigitated electrodes were fabricated and their photoconducting properties were investigated. In this paper we report on the performance of the detectors in terms of UV responsivity, gain-quantum efficiency product, spectral response and response time. We have measured responsivity of 125A/W and gain-quantum efficiency product of 600 at 254nm and 25V. The response time was measured to be on the order of 20ns for our detectors, corresponding to a bandwidth of 25Mhz. The spectral response showed a sharp long-wavelength cutoff at 365nm, and remained constant in the 200nm to 365nm range. The response of the detectors to low-energy x-rays was measured and found to be linear for x-rays with energies ranging from 60kVp to 90kVp.

We report on methods for the growth of GaN by MBE directly on 6H-SiC substrates. The films were doped p-type by the incorporation of Mg and the samples were characterized by studying their structure and morphology by RHEED, XRD and SEM and their recombination properties by photoluminescence measurements. The undoped films were found to be atomically smooth with 2×2 surface reconstruction and have an x-ray rocking curve with a FWHM of 3.5 arcmin. The photoluminescence spectra indicate that recombination is dominated by transition across the gap. The p-type doped films have a rocking curve with FWHM of 6.5 arcmin. and the majority of recombination occurs through D-A transitions at 3.26 eV.

The epitaxial growth of wurtzite and zincblende GaN on (0001) sapphire and (001) Si by the Electron Cyclotron Resonance-assited Molecular Beam Epitaxy (ECR-MBE) method is discussed. We show that films can be grown in the layer-by-layer mode when growth occurs in Ga-rich regime. Surface roughening mechanisms are addressed. The similarity of photoluminescence data of Mg-doped wurtzite GaN films with those of undoped zincblende GaN films suggests that Mg doping facilitates the formation of stacking faults in the wurtzite structure which are nucleation sites for zincblende domains.

Silicon multiple quantum wells confined by ZnS barriers have been grown by MOCVD. Calculations indicate that well widths must be less than 15Å for visible light emission, basically independent of the band offsets. No near-infrared photoluminescence (except bulk Si band-edge emission) was observed from samples with 70Å or greater Si layers, using stimulation at 325 nm with a He-Cd laser; hole trapping in ZnS may play a role. However, we found evidence of yellow light emission from samples which possibly contain Si quantum dots embedded in ZnS.

We report the growth of InGaN alloys over practically the entire composition range at the growth temperature of GaN (700–800 °C) by MBE. We found that when the grown films are thick (> 0.3 μm), incorporation of more than about 30% indium results in phase separation of InN, which is consistent with spinodal decomposition. On the other hand we discovered that such phase separation is absent in thin InGaN films ( < 600Å) grown as GaN/InGaN/GaN heterostructures. In such configurations we were able to incorporate up to 81% In, which is the highest yet reported.

UV photon echoes from a ∼120 nm GaN thin film exhibit a biexponential decay attributed to the coherence loss of slowly and rapidly decaying excitons corresponding to excitons with different degrees of localization or trapping in this material. Both exciton populations are strongly inhomogeneously broadened. For one exciton type, T2 is ∼300 fs at 10 K and ≤25 fs at 300 K; for the other, T2 is pulse-width limited (≤25 fs) at all temperatures. At low temperatures, coherence decay predominantly results from impurity and defect site interactions. At higher temperatures, strong exciton-LO phonon coupling dominates dephasing.

Far-infrared photoconductive detectors based on intersubband transitions in III-nitride semiconductor quantum wells are demonstrated. The device active material is based on a double-step quantum-well design, where two different (Al)GaN compositions are used both in the wells and in the barriers. With this approach, one can create a virtually flat multiple-quantum-well potential energy profile, where the deleterious effects of the intrinsic spontaneous and piezoelectric fields of nitride heterostructures are almost completely eliminated. Photocurrent spectra centered at a wavelength of 23 μm (13 THz frequency) are resolved up to 50 K, with responsivity of approximately 7 mA/W.

Extended x-ray absorption fine structure has been used to study the local structure of ordered AlxGa1−xN films with nominal concentrations of x = 0.20 and x = 0.45. Strong cation ordering is found in alternating (0001) planes for both samples, with the 20% sample exhibiting nearly ideal local order compared to the 45% sample. Significant distortions in the nearest-neighbor Ga-N distances compared to the virtual crystal approximation are observed, although these distortions are found to be smaller than for disordered films. This result is counter to current theoretical predictions.

The polarization properties of the amplified spontaneous emission (ASE) in the deep-UV wavelengths from AlGaN/AlN structures with 70% Al content have been investigated. Samples are prepared through a novel liquid phase epitaxy mode leading to strong compositional fluctuations. Large net modal gain is quantified to 230 nm after femtosecond UV optical pumping by the variable-stripe length technique. A strong transverse electric (TE)-polarization mode of the amplified emission has been demonstrated, together with an unpolarized emission giving rise to absorption only. These measurements provide an important observation in order to understand the origin of the optical gain in AlGaN alloys with high Al content.

Deep-UV optical gain has been demonstrated in Al0.7Ga0.3N/AlN multiple quantum wells under femtosecond optical pumping. Samples were grown by molecular beam epitaxy under a growth mode that introduces band structure potential fluctuations and high-density nanocluster-like features within the AlGaN wells. A maximum net modal gain value of 118 ± 9 cm−1 has been measured and the transparency threshold of 5 ± 1 µJ/cm2 was experimentally determined, corresponding to 1.4 × 1017 cm−3 excited carriers. These findings pave the way for the demonstration of solid-state lasers with sub-250 nm emission at room temperature.

Electrostatic potential profiles across wurtzite AlN/GaN quantum dot (QD) superlattices grown by molecular beam epitaxy have been measured using off-axis electron holography. The profiles for individual GaN QDs show large phase shifts which can be understood in terms of spontaneous polarization and piezoelectric fields and the accumulation of positive and negative charge at the GaN/AlN interfaces. An electric field with magnitude of ∼7.8 ± 2 MV/cm was measured across the center of a GaN QD, in reasonable agreement with reported simulations.

Ultraviolet electroabsorption modulators based on bulk GaN films and on GaN/AlGaN multiple quantum wells were developed and characterized. In both types of devices, the absorption edge at room temperature is dominated by excitonic effects and can be strongly modified through the application of an external electric field. In the bulk devices, the applied voltage causes a broadening and quenching of the excitonic absorption, leading to enhanced transmission. In the quantum-well devices, the external field partially cancels the built-in polarization-induced electric fields in the well layers, thereby increasing the absorption. Unlike optical modulators based on smaller-bandgap zinc blende semiconductors, the bulk devices here are shown to provide similar performance levels as the quantum well devices, which is mainly a consequence of the uniquely large exciton binding energies of nitride semiconductors.

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We report the development of AlGaN based deep ultraviolet light emitting diodes (UV-LEDs) by molecular beam epitaxy. By growing the AlGaN well layer under Ga-rich conditions to produce strong potential fluctuations, internal quantum efficiency of a quantum well structure emitting at 300 nm was found to be 32%. By combining such Ga-rich growth condition in the active region with polarization field enhanced carrier injection layers, deep UV-LEDs emitting at 273 nm were obtained with output power of 0.35 mW and 1.3 mW at 20 mA continuous wave and 100 mA pulsed drive current, respectively. The maximum external quantum efficiency was 0.4%.

We report electron-spin-resonance measurements on zinc-blende GaN. The observed resonance has an isotropic g value of 1.9533\ifmmode\pm\else\textpm\fi{}0.0008 independent of temperature, a Lorentzian line shape, and a linewidth (18 G at 10 K) which depends on temperature. The spin-lattice relaxation time at 10 K was estimated to be ${\mathit{T}}_{1\mathit{e}}$=(6\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ sec. Using a five-band model a g value consistent with the experimental results was obtained and a conduction-electron effective mass ${\mathit{m}}^{\mathrm{*}}$/${\mathit{m}}_{0}$=0.15\ifmmode\pm\else\textpm\fi{}0.01 was calculated. The observed signal, together with conductivity data, was attributed to nonlocalized electrons in a band of autodoping centers and in the conduction band.

Defects in diamond films, produced by the filament-assisted chemical-vapor deposition of methane and hydrogen as a function of total gas pressure and substrate temperature, were investigated by electron-paramagnetic-resonance measurements. We found an isotropic g value (2.0028±0.0002) independent of growth conditions. The peak-to-peak linewidth increases with pressure from 3.5 to 5 G and the spin density decreases with increasing pressure and temperature and varies between 10 19 and 10 17 spins/cm 3

Reactive ion etching of GaN grown by electron-cyclotron-resonance, microwave plasma-assisted molecular beam epitaxy on (0001) sapphire substrates was investigated. A variety of reactive and inert gases such as CC12F2, SF6, CF4, H2/CH4 mixtures, CF3Br, CF3Br/Argon mixtures and Ar were investigated. From these studies we conclude that of the halogen radicals investigated, Cl and Br etch GaN more effectively than F. The etching rate was found to increase with decreasing pressure at a constant cathode voltage, a result attributed to larger mean free path of the reactive species.

A comparative study of the effects of hydrogen in n-type (unintentionally and Si-doped) as well as p-type (Mg-doped) MBE-grown GaN is presented. Hydrogenation above 500°C reduces the hole concentration at room temperature in the p-type material by one order of magnitude. Three different microscopic effects of hydrogen are suggested: Passivation of deep defects and of Mg-acceptors due to formation of hydrogen-related complexes and the introduction of a hydrogenrelated donor state 100 meV below the conduction band edge.

Vertical electronic transport in periodic GaN/AlGaN multiple-quantum-well structures grown on free-standing GaN substrates is investigated. Highly nonlinear current-voltage characteristics are measured, displaying a clear transition from a high-resistance state near zero applied bias to a low-resistance state as the voltage is increased. The measurement results, including their temperature dependence and the variations in turn-on voltage with subband structure and bias polarity are in full agreement with a picture of sequential tunneling through the ground-state subbands of adjacent coupled quantum wells. Scattering-assisted tunneling due to interface roughness or structural defects appears to be the dominant transport mechanism. The potential role of photon-assisted tunneling is also investigated.

This paper reports the growth by molecular beam epitaxy of AlN and GaN thin films on R-plane sapphire substrates. Contrary to previous findings that GaN grows with its (112¯0) A-plane parallel to the (11¯02) R-plane of sapphire, our results indicate that the crystallographic orientation of the III-nitride films is strongly dependent on the kinetic conditions of growth for the GaN or AlN buffer layers. Thus, group III-rich conditions for growth of either GaN or AlN buffers result in nitride films having (112¯0) planes parallel to the sapphire surface, and basal-plane stacking faults parallel to the growth direction. The growth of these buffers under N-rich conditions instead leads to nitride films with (112¯6) planes parallel to the sapphire surface, with inclined c-plane stacking faults that often terminate threading dislocations. Moreover, electron microscope observations indicate that slight miscut (∼0.5°) of the R-plane sapphire substrate almost completely suppresses the formation of twinning defects i...

Langmuir probe measurements and optical emission spectroscopy were employed to characterize electron cyclotron resonance plasmas with two different magnetic field configurations. The plasma tern-perature was directly measured by a thermocouple immersed inside the plasmas. To deposit diamond atlow pressure( lOOmTorr) it is required for the feed sources to contain a large amount of carbon as wellas OH radicals. However, the C, H, and 0 ratios still fall into the "diamond domain" of Bachmann's dia-gram. The oxygen addition to the CHiJH2 plasma leads to the improvement of diamond films prepared at 800mTorr, which is due to the OH radicals formed in the plasma. The addition of oxygen does not increase the plasma temperature. Our results of ECR diamond deposition under various pressures supportthat neutral-neutral reactions dominate in diamond growth process. 1. INTRODUCTION It is technologically important and scientifically interesting to synthesize diamond thin films by elec-tron cyclotron resonance microwave plasma-assisted CVD. The diffused plasma at low pressure of theECR process makes diamond growth possible on large areas and at relatively low temperatures. ECR-PACVD is also a promising technique for the growth of c-BN and thus can be used to grow c-BN/dia-mond heterostructures.Hiraki's group1 at Osaka university first studied ECR-PACVD using various sources including CH/H2, CO2IH2

Optically smooth diamond films were deposited on Al and Si substrates by the ECR-PACVD method. The films on Al were grown on relatively low temperatures and pressures from CH3OH/H2O and were found to have highly defective nanocrystalline structure. The films on c-Si were grown at higher temperatures and pressures from CH4/H2/O2 and were found to consist of highly oriented (100) faceted crystals. The optical constants of the diamond films on c-Si are similar to those of natural diamond.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Material growth is an inherently non-equilibrium process. However, thermodynamic considerations often provide important insight into material growth, the structure of grown materials, and process control parameters. In essence, thermodynamic considerations are important when activated processes are either slow or fast on the time scale of the growth. Activated kinetic processes are important when their time scale is the same as that of growth. Realistic ab-initio calculations of material structure and dynamics can provide a microscopic understanding of both thermodynamics and the kinetics of material growth. The primary focus of this article is a recently proposed defect-assisted multiple-regrowth stabilization of cubic phases. in this theory the incorporation of vacancies at the growth face changes the relative binding energy of cubic versus hexagonal phases so that diamond and cubic boron nitride can nucleate and grow. This theory predicts that diamond nucleation and growth is enhanced under electron rich or positive ion conditions. Experimental results on growth of both diamond and cubic boron nitride that motivate and support theoretical predictions are described. Cubic boron-nitride grows under off-stoichiometric conditions. The nucleation rate of diamond is increased by many orders of magnitude when a flux of electrons impinges upon the surface. Raman line broadening and ESR measurements indicate the presence of significant concentrations of point defects. Predictions and experimental evidence for both n and p type doping will be discussed. Ab-initio calculations of key kinetic processes and thermodynamic quantities for diamond and boron nitride growth are described.

High electron mobility autodoped GaN films were produced by the ECR assisted MBE method. The net electron concentration was varied systematically from 2×1019 to 2×1017cm−3 by controlling the active nitrogen overpressure. Correspondingly, the electron mobility increased from 20 to 210 cm2V−1sec−1. The line through the experimental data also predicts the electron mobilities of GaN films produced by the CVD methods.

Diamond films were produced at a relatively low pressures (2 (95%) and traces of oxygen at substrate temperatures from ambient (no intentional heating) to 1050°C. Faceted surface morphologies were observed even at the lowest temperature of growth. The microstructure is dominated by octahedral crystals below 600°C, by cubic crystals at 800–900°C, and by multiply twined (111) crystals at temperatures higher than 950°C. The weak temperature dependence of the growth rate is consistent with hydrogen abstraction from the growing surface being the rate controlling step.

GaN films were grown on c-plane (0001), a-plane (1120) and r-plane (1102) sapphire substrates by the ECR-assisted MBE method. The films were grown using a two-step growth process, in which a GaN buffer is grown first at relatively low temperatures and the rest of the film is grown at higher temperatures. RHEED studies indicate that this growth method promotes lateral growth and leads to films with smooth surface morphology. The epitaxial relationship to the substrate, the crystalline quality and the surface morphology were investigated by RHEED, X-ray diffraction and SEM studies.

Defects in BN thin films, produced by reactive sputtering, were investigated by Electron Paramagnetic Resonance (EPR) measurements. The EPR signals of films produced with up to 10% N2in the argon discharge are consistent with films having a cubic structure, and becoming more ordered with nitrogen incorporation in the films. The concentration of spins is in the order of 1019spins/g and they are attributed to nitrogen vacancies with an electron trapped in. Carbon incorporation changes the EPR signal and increases the concentration of spins significantly. This result is consistent with the notion that carbon doping stabilizes the electron in a nitrogen vacancy.

Chemically synthesized single-crystal silver nanoparticles are used to demonstrate plasmon enhanced visible light emission from nitride semiconductor quantum wells. For ease of assembly and testing, the nanoparticles are embedded onto the surface of flexible resin films, which are then simply adhered on top of the light emitting samples. Large enhancements in photoluminescence efficiency are correspondingly measured at emission wavelengths near the nanoparticle plasmonic resonance. At the same time, when samples emitting at a sufficiently far detuned wavelength are used, the measured efficiency is not affected by the nanoparticles, which confirms the plasmonic origin of the observed enhancement.

Wide-bandgap semiconductors such as GaN∕AlGaN and ZnO∕MgZnO quantum wells are promising for improving the spectral reach and high-temperature performance of terahertz quantum cascade lasers, due to their characteristically large optical phonon energies. Here, a particle-based Monte Carlo model is developed and used to quantify the potential of terahertz sources based on these materials relative to existing devices based on GaAs∕AlGaAs quantum wells. Specifically, three otherwise identical quantum cascade structures based on GaN∕AlGaN, ZnO∕MgZnO, and GaAs∕AlGaAs quantum wells are designed, and their steady-state carrier distributions are then computed as a function of temperature. The simulation results show that the larger the optical phonon energies (as in going from the AlGaAs to the MgZnO to the AlGaN materials system), the weaker the temperature dependence of the population inversion. In particular, as the temperature is increased from 10to300K, the population inversions are found to decrease by facto...

We report the development of Al0.7Ga0.3N/AlN quantum wells with high internal quantum efficiency. All samples had identical well and barrier thickness but the III/V flux ratio was varied during growth by increasing the Ga flux. The luminescence spectra show single peaks which vary from 220 nm (III/V∼1) to 250 nm (III/V⪢1) with internal quantum efficiency varying from 5% to 50%, respectively. To account for these results, a growth model was proposed in which at III/V∼1 the growth proceeds via vapor phase epitaxy, while at III/V⪢1 the growth proceeds via liquid phase epitaxy.

Optically pumped pulsed emission of short-wave infrared radiation based on intersubband transitions in GaN/AlN quantum wells is demonstrated. Nanosecond-scale pump pulses are used to resonantly excite electrons from the ground states to the second-excited subbands, followed by radiative relaxation into the first-excited subbands. The measured room-temperature output spectra are peaked near 2 μm with integrated powers of a few hundred nanowatts. The intersubband origin of the measured luminescence is confirmed via an extensive study of its polarization properties and pump wavelength dependence, as well as simulations of the quantum well subband structure.

We report piezoresistance studies in microcrystalline silicon films produced by reactive sputtering from a silicon target in an atmosphere of hydrogen and argon. The microcrystalline silicon films are two phase materials consisting of 50-100Å diameter silicon crystallites embedded in an amorphous Si-Hx matrix. The conductivity of the films was found to decrease significantly when the films were put under compression. Conductivity decreases of up to 100% were observed; this large conductivity changes with strain indicate that microcrystalline silicon is ideally suited for highly sensitive strain gauge applications. The results can be qualitatively accounted for in a model which assumes quantum confinement of carriers in 50Å diameter silicon crystallites separated by tunnelable amorphous Si-Hx barriers.

The refractive-index nonlinearities of intersubband transitions in GaN/AlN quantum-well waveguides are investigated. A large spectral broadening of TM-polarized near-infrared pulses is observed after propagation through these devices due to intersubband self-phase modulation. From the measured data, a nonlinear refractive index n2 of 1.8×10−12 cm2/W is estimated at the operating wavelength of 1550 nm. A detailed theoretical model of the intersubband refractive index as a function of wavelength and optical intensity is then presented. This model assumes an inhomogeneously broadened transition line and is based on experimentally determined material and device parameters. The results of this study are finally used to discuss the prospects of nitride quantum wells for all-optical switching via cross-phase modulation.