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The increased growth in the use of tip-based sensing, manipulations, and fabrication of devices in atomic force microscopy (AFM) necessitates the accurate prediction of the dynamic behavior of the AFM probe. The chip holder, to which the micro-sensing device is attached, and the rest of the AFM system can affect the overall dynamics of the probe. In order to consider these boundary effects, we propose a novel receptance coupling method to mathematically combine the dynamics of the AFM setup and probe, based on the equilibrium and compatibility conditions at the joint. Once the frequency response functions of displacement over force at the tool tip are obtained, the dynamic interaction forces between the tip and the sample in nanoscale can be determined by measuring the probe tip displacement. [ABSTRACT FROM AUTHOR]

We have performed a detailed contactless electroreflectance study of the interband excitonic transitions on both the Ga and N faces of a 200-μm-thick freestanding hydride-vapor-phase-epitaxy grown wurtzite GaN sample with low defect concentration in the temperature range between 20 and 300 K. The transition energies of the A, B, and C excitons and broadening parameters of the A and B excitons have been determined by least-square fits to the first derivative of a Lorentzian line shape. The energy positions and separations of the excitonic transitions in the sample reveal the existence of residual strain. At 20 K the broadening parameter of A exciton deduced for the Ga (5×10[SUP5] dislocation cm[SUP-2] and N (1×10[SUP7] dislocation cm[SUP-2]) faces are 3 and 7 meV, respectively, indicating a lower defect concentration on the former face. The parameters that describe the temperature dependence of the interband transition energies of the A, B, and C excitons as well as the broadening function of the A and B features are evaluated. The results from an analysis of the temperature dependence of the broadening function of excitons A and B indicate that GaN exhibits a very large exciton-phonon coupling. [ABSTRACT FROM AUTHOR]

Rotational Maker fringes, scaled with respect to χ11(2) of crystalline quartz, were used to determine the second order susceptibilities χ31(2) and χ33(2) for samples of thin AlxGa1-xN films, a thicker GaN film, and a free-standing GaN platelets. The pump wavelength was 1064 nm. The AlxGa1-xN samples, ranging in thickness from roughly 0.5 to 4.4 μm, were grown by metalorganic chemical vapor deposition (MOCVD) and hydride vapor-phase epitaxy (HVPE) on (0001) sapphire substrates. The Al mole fractions x were 0, 0.419, 0.507, 0.618, 0.660, and 0.666, for the MOCVD-grown samples, and x=0, 0.279, 0.363, and 0.593 for the HVPE-grown samples. An additional HVPE-grown GaN sample ∼70 μm thick was also examined. The free-standing bulk GaN platelets consisted of an HVPE grown film ∼226 μm thick removed from its growth substrate, and a crystal ∼160 μm thick grown by high-pressure techniques. For the AlxGa1-xN samples, the magnitudes of χ31(2) and χ33(2) decrease roughly linearly with increasing x and extrapolate to ∼0 for x=1. Furthermore, the constraint expected for a perfect wurtzite structure, namely χ33(2)=-2χ31(2), was seldom observed, and the samples with x=0.660 and x=0.666 showed χ31(2) and χ33(2) having the same sign. These results are consistent with the theoretical studies of nonlinear susceptibilities for AlN and GaN performed by Chen et al. [Appl. Phys. Lett. 66, 1129 (1995)]. The thicker bulk GaN samples displayed a complex superposition of high- and low-frequency Maker fringes due to the multiple-pass interference of the pump and second-harmonic generation beams, and the nonlinear coefficients were approximately consistent with those measured for the thin-film GaN sample. [ABSTRACT FROM AUTHOR]

Deep level electron and hole traps were studied by means of deep level transient spectroscopy with electrical and optical injection on a freestanding thick n-GaN sample with low dislocation density. It is shown that at both the upper and the lower surface of the sample there exists a thin, ~0.5 μm layer of damaged material with lowered concentration of electrons and enhanced density of deep centers. Deep in the bulk of the film the densities of the majority of the electron and hole traps are shown to be very low, but measurably higher on the lower face (N face), which was originally closer to the Al[SUB2]O[SUB3] substrate. The two faces are also shown to similarly differ in the density of deep hole traps whose concentration is deduced from low-temperature capacitance-voltage measurements in the dark and after illumination (such traps were previously associated with dislocation states). The concentration of persistent photocapacitance centers is shown to be very low on both sides and considerably lower than previously reported for other n-GaN samples with higher dislocation densities. Electron beam induced current measurements on both sides of the sample reveal the presence of dark spots whose density roughly correlates with the density of dislocations at the upper and lower faces. The reverse current at high voltages is shown to be considerably higher on the N face. The possible relationship of the observed phenomena to dislocations is discussed. [ABSTRACT FROM AUTHOR]

Photoluminescence of the dominant deep-level acceptor in high-purity freestanding GaN is studied over a wide range of excitation intensities. A yellow luminescence (YL) band at about 2.2 eV saturates with increasing excitation intensity, whereas a green luminescence (GL) band at about 2.5 eV increases as a square of the excitation intensity. The YL and GL bands are attributed to two charge states of the same defect, presumably a gallium vacancy-oxygen complex. [ABSTRACT FROM AUTHOR]

We have studied a broad photoluminescence band in high-mobility freestanding 200-μm-thick GaN template prepared by hydride vapor-phase epitaxy. Variable-excitation intensity and energy experiments showed two defect-related bands: a yellow luminescence (YL) band at about 2.15 eV and a green luminescence (GL) band at about 2.43 eV. In contrast to epitaxial GaN samples prepared by both vapor-phase and molecular-beam epitaxy, the YL in the sample studied is weak and can be easily saturated. However, the GL is dominant. We attribute the GL to isolated defects involving gallium vacancies and the YL to the same defect, but bound to dislocations, or possibly to structural surface defects. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Deep level defects responsible for the 2.4 eV photoluminescence (PL) band in a freestanding GaN template were studied by transient photoluminescence. A nonexponential decay of PL intensity observed at low temperature is attributed to a donor-acceptor pair recombination involving a shallow donor and a deep acceptor. At room temperature, a single-exponential PL decay with a lifetime of 30 μs was observed at the high-energy side of the band, whereas the second component with a lifetime of about 750 μs was detected at the low-energy side of the band. The PL decay and transformation of the PL spectrum at room temperature can be explained by transitions from the conduction band to two deep acceptors. Electron-capture cross section has been estimated as 4x10[sup -21] and 10[sup -19] cm[sup 2] for the yellow and green bands, respectively, contributing to the broad 2.4 eV band. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

High-quality free-standing GaN was obtained by hydride vapor-phase epitaxy (HVPE) growth and the subsequent removal of the sapphire substrates. In the photoluminescence (PL) spectra of the as-grown HVPE-GaN, we observed a strong phonon replica peak for temperatures higher than 80 K. Both the near-band-edge emission and the yellow emission in the cathodoluminescence spectra were inhomogeneous, and correlated with the crystalline structure. With the homoepitaxial regrowth by metal-organic chemical-vapor deposition (MOCVD) in the GaN substrates, these unusual optical properties were no longer observed and the PL peak became sharper than GaN grown by MOCVD on sapphire substrates, indicating that the free-standing GaN is suitable as substrates for the growth of device structures. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Microcathodolumunescence (MCL) spectra measurements, MCL and electron beam induced current (EBIC) imaging of the freestanding n-GaN samples grown by hydride vapor phase epitaxy were made. Dark-spot defects in plan-view EBIC and MCL images and dark line defects in MCL images taken on the cleaved surface of the samples could be associated with dislocations. MCL spectra measurements in the vicinity of dislocations and in the matrix do not reveal specific luminescence bands that could be attributed to dislocations but rather suggest that dislocation regions have higher density of deep nonradiative traps. [ABSTRACT FROM AUTHOR]

Improved structural quality and radiative efficiency were observed in GaN thin films grown by metalorganic chemical vapor deposition on TiN porous network templates formed by in situ thermal annealing of Ti in ammonia. The room-temperature decay times obtained from biexponential fits to time-resolved photoluminescence data are longer than ever reported for GaN. The carrier lifetime of 1.86 ns measured for a TiN network sample is slightly longer than that for a 200 µm thick high-quality freestanding GaN. The linewidth of the asymmetric x-ray diffraction (XRD) (10...) peak decreases considerably with the use of TiN layer and with increasing in situ annealing time, indicating the reduction in threading dislocation density. However, no direct correlation is yet found between the decay times and the XRD linewidths, suggesting that point defect and impurity related nonradiative centers are the main parameters affecting the lifetime. [ABSTRACT FROM AUTHOR]

We report measurements of the Schottky barrier heights of Ni/Au contacts on Ga-polarity and N-polarity n-GaN under hydrostatic pressure and applied in-plane uniaxial stress. Under hydrostatic pressure the two different polarities of GaN yield significantly different rates of Schottky barrier height increase with increasing pressure. Uniaxial stress parallel to the surface affects the Schottky barrier height only minimally. The observed changes in barrier height under stress are attributed to a combination of band structure and piezoelectric effects. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Pt Schottky rectifier arrays were fabricated on 200-μm-thick, freestanding GaN layers. Even with the reduced dislocation density in these layers (∼10[sup 5] cm[sup -2]) relative to conventional GaN on sapphire (>10[sup 8] cm[sup -2]), rectifiers fabricated on the freestanding GaN show a strong dependence of reverse breakdown on contact diameter. We show that by interconnecting the output of many (∼130) smaller (500 μm×500 μm) rectifiers, we can achieve high total forward output current (161 A at 7.12 V), low forward turn-on voltage of ∼3 V, and maintain the reverse breakdown voltage. The on/off ratio of the rectifier array was ∼8×10[sup 7] at 5 V/-100 V. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We studied time-resolved photoluminescence PL over a temporal range 10[SUP-6]- 10[SUP3]s in high-purity freestanding GaN templates. Red, yellow, green, blue, and shallow donor-acceptor emission bands can be resolved in the PL spectrum. Observation of luminescence long after the excitation is switched off is a striking feature of our study. The persistent PL observed for all above bands, except for the green band, is primarily attributed to the donor-acceptor-pair-type recombination. An unusually slow, nonexponential decay of radiative transitions from the conduction band to the shallow acceptor was also observed, pointing to some additional mechanism for the persistent PL. Possible role of the surface states in this effect is discussed. [ABSTRACT FROM AUTHOR]

Positron annihilation measurements reveal show that negative Ga vacancies are the dominant acceptors in n-type gallium nitride grown by hydride vapor phase epitaxy. The concentration of Ga vacancies decreases, from more than 10[sup 19] to below 10[sup 16] cm[sup -3], as the distance from the interface region increases from 1 to 300 µm. These concentrations are the same as the total acceptor densities determined in Hall experiments. The depth profile of O is similar to that of VGa, suggesting that the Ga vacancies are complexed with the oxygen impurities. [ABSTRACT FROM AUTHOR]

White-colored materials synthesized by a thermal annealing of milled GaN powders at 930 °C in a nitrogen atmosphere were identified to be monoclinic β-Ga[sub 2]O[sub 3] nanowires by x-ray diffraction and scanning electron microscopy. High-resolution transmission electron microscopy revealed that these nanowires are single nanocrystals, and energy dispersive x-ray indicated that these nanomaterials are free of any metals. In addition, bundles of these crystalline nanowires in the rectangular-pole shape are a few centimeters in length.© 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) studies were performed on a 1.5-μm-thick GaN layer grown by molecular-beam epitaxy on a freestanding GaN template that in turn was grown by hydride vapor-phase epitaxy. PL spectra from both the epilayer and the substrate contain a plethora of sharp peaks related to excitonic transitions. We identified the main peaks in the PL spectrum. Taking advantage of the observation of donor bound exciton peaks and their associated two-electron satellites, we have determined the binding energies of two distinct shallow donors (28.8 and 32.6 meV), which are attributed to Si and O, respectively. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A free-standing GaN template grown by hydride vapor phase epitaxy has been characterized by transmission electron microscopy (TEM). The TEM investigation was augmented by x-ray diffraction, defect delineation etching process followed by imaging with atomic force microscopy and variable temperature photoluminescence. The density of dislocations near the N face was determined to be, in order, 3±1x10[sup 7], 4±1x10[sup 7], and about 1x10[sup 7] cm[sup -2] by cross-sectional TEM, plan-view TEM, and a defect revealing etch, respectively. The same methods on the Ga face revealed the defect concentration to be, in order, less than 1x10[sup 7] cm[sup -2] by plan-view TEM, less than 5x10[sup 6] cm[sup -2] by cross-sectional TEM, and 5x10[sup 5] cm[sup -2] by defect revealing hot H[sub 3]PO[sub 4] acid, respectively. The full width at half maximum of the symmetric (0002) x-ray diffraction peak was 69 and 160 arc sec for the Ga and N faces, respectively. That for the asymmetric (101_4) peak was 103 and 140 arc sec for Ga and N faces, respectively. The donor bound exciton linewidth was about 1 meV each at 10 K, and a green band centered at about 2.44 eV was observed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Schottky barrier diodes, on both Ga and N faces of a ∼300-μm-thick free-standing GaN layer, grown by hydride vapor phase epitaxy (HVPE) on Al[sub 2]O[sub 3] followed by laser separation, were studied by capacitance-voltage and deep level transient spectroscopy (DLTS) measurements. From a 1/C[sup 2] vs V analysis, the barrier heights of Ni/Au Schottky contacts were determined to be different for the two polar faces: 1.27 eV for the Ga face, and 0.75 eV for the N face. In addition to the four common DLTS traps observed previously in other epitaxial GaN including HVPE-grown GaN a new trap B[sup ′] with activation energy E[sub T]=0.53 eV was found in the Ga-face sample. Also, trap E[sub 1] (E[sub T]=0.18 eV), believed to be related to the N vacancy, was found in the N-face sample, and trap C (E[sub T]=0.35 eV) was in the Ga-face sample. Trap C may have arisen from reactive-ion-etching damage. [ABSTRACT FROM AUTHOR]

High peak electron mobilities were observed in freestanding c-plane GaN layers. Two well-defined electrical layers, a low mobility degenerate interface layer, and a high mobility nondegenerate bulk layer, were present in these samples. The carrier concentrations and mobilities for the layers were extracted using two methods: (1) magnetic field dependent Hall effect analysis; and (2) a simple two layer Hall model with the assumption that one of the layers is degenerate. The electron Hall mobility of the bulk layer is found to peak at nearly 8000 cm2/V s at low temperature using the magnetic field dependent Hall effect analysis. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a correction to an article published in the journal 'Applied Physics Letters,' volume 82.

Edge-terminated Schottky rectifiers fabricated on quasibulk GaN substrates showed a strong dependence of reverse breakdown voltage V[sub B] on contact dimension and on rectifier geometry (lateral versus vertical). For small diameter (75 μm) Schottky contacts, V[sub B] measured in the vertical geometry was ∼700 V, with an on-state resistance (R[sub ON]) of 3 mΩ cm2, producing a figure-of-merit V[sub B][sup 2]/R[sub ON] of 162.8 MW cm-2. Measured in the lateral geometry, these same rectifiers had V[sub B] of ∼250 V, R[sub ON] of 1.7 mΩ cm2 and figure-of-merit 36.5 MW cm-2. The forward turn-on voltage (V[sub F]) was ∼1.8 V (defined at a current density of 100 A cm-2), producing V[sub B]/V[sub F] ratios of 139–389. In very large diameter (∼5 mm) rectifiers, V[sub B] dropped to ∼6 V, but forward currents up to 500 mA were obtained in dc measurements. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A free-standing 300-μm-thick GaN template grown by hydride vapor phase epitaxy has been characterized for its structural and optical properties using x-ray diffraction, defect delineation etch followed by imaging with atomic force microscopy, and variable temperature photoluminescence. The Ga face and the N face of the c-plane GaN exhibited a wide variation in terms of the defect density. The defect concentrations on Ga and N faces were about 5x10[sup 5] cm[sup -2] for the former and about 1x10[sup 7] cm[sup -2] for the latter. The full width at half maximum of the symmetric (0002) x-ray diffraction peak was 69 and 160 arc sec for the Ga and N faces, respectively. That for the asymmetric (10-14) peak was 103 and 140 arc sec for Ga and N faces, respectively. The donor bound exciton linewidth as measured on the Ga and N faces (after a chemical etching to remove the damage) is about 1 meV each at 10 K. Instead of the commonly observed yellow band, this sample displayed a green band, which is centered at about 2.44 eV. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Γ[sub 5] and Γ[sub 6] free excitons have been identified in GaN from emission measurements. Another emission peak is also observed which we believe to be the longitudinal free exciton. These measurements along with electrical measurements, which show the sample to have very high peak mobility, attest to the high quality of the sample. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]