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Ultraviolet photoluminescence, transmission electron microscopy and KOH etching were used to characterize extended defects in 4H-SiC epilayers grown at high growth rates (18 μm/h). Layers exhibited high densities of in-grown stacking faults and dislocation half-loops. The stacking faults were 8H Shockley-type faults. The Burgers vector of the dislocation half-loops was in the (0001) basal plane. Both defects nucleate within the epilayer at early stages of growth. Defect nucleation is directly correlated with high initial growth rate and is not related to any defects/heterogeneities in the substrate or epilayer. Epilayer growth by nucleation of two-dimensional islands is proposed as a possible mechanism for the formation of both defects, through nucleation of faulted Si-C bilayers. [ABSTRACT FROM AUTHOR]

This manuscript presents the first observation of the space-charge-limited current (SCLC) conduction mechanism in individual heterostructure nanowires (NWs). This effect is exploited to extract size-dependent carrier densities and to demonstrate surface-dominated behavior for these technologically relevant nanostructures. Mobile carrier densities were shown to increase from 2.5 × 1016 to 5.6 × 1017 cm-3, as NW width decreased from 200 to 50 nm. This size-dependent behavior is a consequence of the increasing influence of near-surface confined carriers as widths decrease. Traps impact the SCLC response and were characterized as an exponential band edge tail with an average characteristic energy of 75 meV. In addition to the specific materials properties extracted, these results further demonstrate the tendency for low-dimensional materials (1D NWs) to exhibit SCLC at much lower injection fluxes compared to their higher dimensional (2D heterostructure field-effect transistors) counterparts. [ABSTRACT FROM AUTHOR]

Atomic layer deposition was employed to deposit relatively thick (∼30 nm) aluminum oxide (Al2O3) using trimethylaluminum and triply-distilled H2O precursors onto epitaxial graphene grown on the Si-face of silicon carbide. Ex situ surface conditioning by a simple wet chemistry treatment was used to render the otherwise chemically inert graphene surface more amenable to dielectric deposition. The obtained films show excellent morphology and uniformity over large (∼64 mm2) areas (i.e., the entire sample area), as determined by atomic force microscopy and scanning electron microscopy. X-ray photoelectron spectroscopy revealed a nearly stoichiometric film with reduced impurity content. Moreover, from capacitance-voltage measurements a dielectric constant of ∼7.6 was extracted and a positive Dirac voltage shift of ∼1.0 V was observed. The graphene mobility, as determined by van der Pauw Hall measurements, was not affected by the sequence of surface pretreatment and dielectric deposition. [ABSTRACT FROM AUTHOR]

The dominant recombination processes controlling the carrier lifetime in n-type 4H–SiC epitaxial layers grown with low concentrations of the Z1/2 defect (the dominant bulk lifetime killer), where Z1/2 no longer determines the lifetime, have been investigated by studying the variation in the carrier lifetime with temperature. The temperature dependent lifetimes were obtained primarily by low-injection photoluminescence decay for several low-Z1/2 epilayers over a wide temperature range. The results were fitted to simulations of the temperature dependent recombination rate, where bulk, surface and interface recombination was considered. No significant contribution from other bulk defects was observed, and upper limits to the bulk recombination rate were found to be consistent with the low Z1/2 concentrations measured in these materials. There was also no significant contribution from carrier capture at the epilayer/substrate interface, which is consistent with behavior expected at low injection for low-doped epilayers grown on n+ substrates. Corresponding high-injection measurements exhibited very different behavior, consistent with the surface/interface under flat-band conditions. Consequently, it is concluded that for low-Z1/2 materials, control of the carrier lifetime has not been transferred from Z1/2 to another bulk defect, but is instead dominated by surface and interface recombination. Simulations suggest that further enhancement of the total lifetime under the high injection conditions of a device structure would require very thick epilayers, effectively passivated surface and interface recombination and a further reduction in the remaining Z1/2 concentrations. The temperature dependence of the low-injection carrier lifetime was also found to provide a method to estimate the surface band bending and the surface defect density. [ABSTRACT FROM AUTHOR]

The impact of surface depletion on the electronic properties of semiconductor nanowires (NWs) is explored both theoretically and experimentally. The impact of dopant concentration, surface barrier height, and NW radius on surface depletion and extracted material properties are determined by solving Poisson’s equation for the cylindrical system. The theoretical results reveal a size-dependent systematic error in carrier concentration extraction, which is verified through experiment. Interrogation of GaN NWs with radii from 15 to 70 nm exposed an error that reaches over an order of magnitude for the samples studied. These data compared favorably to an analytical treatment assuming physically reasonable material properties. While this manuscript focuses on GaN, the systematic error discussed will be present for any semiconducting NW, which exhibits surface band bending and therefore influences the behavior and characterization of a wide range of semiconducting nanoelements. [ABSTRACT FROM AUTHOR]

The impetus for dislocation motion in thin films is generally understood in terms of Peach-Koehler forces. For the case of III-nitride films grown on step-free 4H-SiC mesas, however, it is the gradient of the strain energy from the mesa edge that is capable of driving misfit dislocations. Using the strain profile as a function of the distance from the mesa edge and the line tension of the c-plane threading arms, we have calculated the excess stress driving the half loop from the mesa edge into the mesa interior. We have also compared the half-loop excess stress with the excess stress driving the tilt of threading edge dislocations, which has been proposed as one of the principal strain relief mechanisms in III-nitride films. The excess stress driving c-plane half loops ranges from a few 1000 MPa at the mesa edge to few 100 MPa towards the mesa interior, while the excess stress driving the tilt of threading edge dislocations is in excess of 20 000 MPa. The greater excess stress driving dislocation tilt, however, does not dominate strain relief for III-nitride films on step-free SiC mesas due to the difficulty in nucleating threading dislocations in the absence of interfacial steps. [ABSTRACT FROM AUTHOR]

Bulk growth of 4H-SiC is challenging due to the required high growth temperatures and gradients used in sublimation physical vapor transport that are difficult to control, particularly over large diameter boules. We used x-ray diffraction mapping to show concave crystal plane curvature in substrates from five commercial suppliers with two suppliers producing wafers with <=2° curvature. The extent of curvature varied little for substrates from any particular supplier. Maps of peak position and full width at half maximum from symmetric and asymmetric reflections were used to identify defective regions in the crystal. Closer examination of the rocking curves in the defective regions found a low density of low angle grain boundaries only in substrates from one supplier. [ABSTRACT FROM AUTHOR]

Presents a study that characterized electron cyclotron resonance plasmas by vacuum ultraviolet spectroscopy. Method of the study; Results and discussion; Conclusion.

Provides information on a study that reported the growth of gallium nitride films using the electron cyclotron resonance microwave plasma-assisted molecular beam epitaxy method. Methodologies used; Results and discussion; Conclusions.

Recent breakthroughs in bulk crystal growth of β-Ga2O3 by the edge-defined film-fed technique has led to the commercialization of large-area β-Ga2O3 substrates. Standard epitaxy approaches are being utilized to develop various thin-film β-Ga2O3 based devices including lateral transistors. This article will discuss the challenges for metal organic chemical vapor deposition (MOCVD) of β-Ga2O3 and the design criteria for use of this material system in power electronic device structures. [ABSTRACT FROM AUTHOR]

Details a study which investigated the influence of plasma etching on the electrical properties of n-type gallium nitride thin films. Experimental procedures; Influence of the presence of hydrogen in the etching plasma on free-electron density; Description of the annealing behavior of the thin films.

This study compares the physical, chemical and electrical properties of Al2O3 thin films deposited on gallium polar c- and nonpolar m -plane GaN substrates by atomic layer deposition (ALD). Correlations were sought between the film's structure, composition, and electrical properties. The thickness of the Al2O3 films was 19.2 nm as determined from a Si witness sample by spectroscopic ellipsometry. The gate dielectric was slightly aluminum-rich (Al:O=1:1.3) as measured from X-ray photoelectron spectroscopy (XPS) depth profile, and the oxide-semiconductor interface carbon concentration was lower on c -plane GaN. The oxide's surface morphology was similar on both substrates, but was smoothest on c -plane GaN as determined by atomic force microscopy (AFM). Circular capacitors (50-300 μm diameter) with Ni/Au (20/100 nm) metal contacts on top of the oxide were created by standard photolithography and e-beam evaporation methods to form metal-oxide-semiconductor capacitors (MOSCAPs). The alumina deposited on c -plane GaN showed less hysteresis (0.15 V) than on m -plane GaN (0.24 V) in capacitance-voltage (CV) characteristics, consistent with its better quality of this dielectric as evidenced by negligible carbon contamination and smooth oxide surface. These results demonstrate the promising potential of ALD Al2O3 on c -plane GaN, but further optimization of ALD is required to realize the best properties of Al2O3 on m -plane GaN. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The benefits of dry oxidation of n -GaN for the fabrication of metal-oxide-semiconductor structures are reported. GaN thin films grown on sapphire by MOCVD were thermally oxidized for 30, 45 and 60 minutes in a pure oxygen atmosphere at 850 °C to produce thin, smooth GaOx layers. The GaN sample oxidized for 30 minutes had the best properties. Its surface roughness (0.595 nm) as measured by atomic force microscopy (AFM) was the lowest. Capacitance-voltage measurements showed it had the best saturation in accumulation region and the sharpest transition from accumulation to depletion regions. Under gate voltage sweep, capacitance-voltage hysteresis was completely absent. The interface trap density was minimum (Dit = 2.75×1010 cm-2eV-1) for sample oxidized for 30 mins. These results demonstrate a high quality GaOx layer is beneficial for GaN MOSFETs. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Thin AlN layers were grown at 200-650 °C by plasma assisted atomic layer epitaxy (PA-ALE) simultaneously on Si(111), sapphire ([formula]), and GaN/sapphire substrates. The AlN growth on Si(111) is self-limited for trimethyaluminum (TMA) pulse of length > 0.04 s, using a 10 s purge. However, the AlN nucleation on GaN/sapphire is non-uniform and has a bimodal island size distribution for TMA pulse of ≤0.03 s. The growth rate (GR) remains almost constant for Tg between 300 and 400 °C indicating ALE mode at those temperatures. The GR is increased by 20% at Tg = 500 °C. Spectroscopic ellipsometry (SE) measurement shows that the ALE AlN layers grown at Tg ≤ 400 °C have no clear band edge related features, however, the theoretically estimated band gap of 6.2 eV was measured for AlN grown at Tg ≥ 500 °C. X-ray diffraction measurements on 37 nm thick AlN films grown at optimized growth conditions (Tg = 500 °C, 10 s purge, 0.06 s TMA pulse) reveal that the ALE AlN on GaN/sapphire is (0002) oriented with rocking curve full width at the half maximum (FWHM) of 670 arc sec. Epitaxial growth of crystalline AlN layers by PA-ALE at low temperatures broadens application of the material in the technologies that require large area conformal growth at low temperatures with thickness control at the atomic scale. [ABSTRACT FROM AUTHOR]

Epitaxial graphene grown on SiC (0001) showed significant changes in electrical resistance upon exposure to polar protic and polar aprotic vapors in the ambient atmosphere. The dipole moment of these chemicals was found to have a strong impact on the magnitude of the sensor response, which increases with increasing dipole moment. Using the combination of low-frequency noise and Hall measurements, we demonstrate that the chemical sensing mechanism in epitaxial graphene is based on fluctuations of the charge carrier density induced by vapor molecules adsorbed on the surface of the graphene. [ABSTRACT FROM AUTHOR]

Vertical diodes of epitaxial graphene on n- 4H-SiC were investigated. The graphene Raman spectra exhibited a higher intensity in the G-line than the 2D-line, indicative of a few-layer graphene film. Rectifying properties improved at low temperatures as the reverse leakage decreased over six orders of magnitude without freeze-out in either material. Carrier concentration of ∼1016 cm-3 in the SiC remained stable down to 15 K, while accumulation charge decreased and depletion width increased in forward bias. The low barrier height of 0.08 eV and absence of recombination-induced emission indicated majority carrier field emission as the dominant conduction mechanism. [ABSTRACT FROM AUTHOR]

There has been significant research in the study of in-plane charge-carrier transport in graphene in order to understand and exploit its unique electrical properties; however, the vertical graphene–semiconductor system also presents opportunities for unique devices. In this letter, we investigate the epitaxial graphene/p-type 4H-SiC system to better understand this vertical heterojunction. The I –V behavior does not demonstrate thermionic emission properties that are indicative of a Schottky barrier but rather demonstrates characteristics of a semiconductor heterojunction. This is confirmed by the fitting of the temperature-dependent I–V curves to classical heterojunction equations and the observation of band-edge electroluminescence in SiC. [ABSTRACT FROM PUBLISHER]

Electrical characteristics of Cr/Au and Ti/Au metal contacts on epitaxial graphene on 4H-SiC showed significant variations in resistance parameters at 300 K. These parameters decreased substantially as the temperature increased to 673 K. The work function, binding energy, and diffusion energy of the deposited metals were used to explain these observed variations. The quantitative analysis of our data demonstrates that non-reactive metals with higher work functions result in lower contact resistance, which can be further decreased by 70% using appropriate annealing. These results provide important information when considering epitaxial graphene for high temperature applications. [ABSTRACT FROM AUTHOR]

Ambient-environment Kelvin probe microscopy of many (10 μm)2 areas of single-layer graphene on SiC(0001) shows area-to-area rms surface potential variation of 12 meV. Electronic transport data are consistent with the minimum conductivity regime. Together the data indicate a highly uniform carrier concentration with a small magnitude (<1012 cm-2). We conclude that the previously reported large spread in carrier densities from Hall measurements on similar samples is an artifact of electron-hole puddling in the minimum conductivity regime. [ABSTRACT FROM AUTHOR]

In this letter, we present both the \1/f noise and phase noise performance of top-gated epitaxial graphene field-effect transistors (FETs) in nonlinear circuit applications for the first time. In the case of frequency doublers, the fundamental signal is suppressed by 25 dB below the second harmonic signal. With a phase noise of -110 dBc/Hz measured at a 10-kHz offset, a carrier-to-noise degradation (\Delta CNR) of 6 dB was measured for the frequency doubler. This implies noiseless frequency multiplication without additional \1/f noise upconversion during the nonlinear process. The frequency multiplication was demonstrated above the gigahertz range. The \1/f noise of top-gated epitaxial graphene FETs is comparable or lower than that of exfoliated graphene FETs. [ABSTRACT FROM PUBLISHER]

Epitaxial graphene layers were grown on the C-face of 4H–SiC and 6H–SiC using an argon-mediated growth process. Variations in growth temperature and pressure were found to dramatically affect the morphological properties of the layers. The presence of argon during growth slowed the rate of graphene formation on the C-face and led to the observation of islanding. The similarity in the morphology of the islands and continuous films indicated that island nucleation and coalescence is the growth mechanism for C-face graphene. [ABSTRACT FROM AUTHOR]

Epitaxial graphene field effect transistors were fabricated, characterized, and studied. Both the capacitance and transport measurements were performed on the same devices using an electrochemical gate. The quantum capacitance of the epitaxial graphene was extracted, which was similar to that of exfoliated graphene near the Dirac point, but it exhibits a large sublinear behavior at high carrier densities. The recently developed self-consistent theory for charged impurities in graphene is found to provide a reasonable description of the transport data, but a more complete theory is needed to explain both the transport and quantum capacitance data for the epitaxial graphene devices. [ABSTRACT FROM AUTHOR]

Epitaxial graphene (EG) films were grown in vacuo by silicon sublimation from the (0001) and [formula] faces of 4H-SiC and 6H-SiC. Hall effect mobilities and sheet carrier densities of the films were measured at 300 and 77 K and the data depended on the growth face. About 40% of the samples exhibited holes as the dominant carrier, independent of face. Generally, mobilities increased with decreasing carrier density, independent of carrier type and substrate polytype. The contributions of scattering mechanisms to the conductivities of the films are discussed. The results suggest that for near-intrinsic carrier densities at 300 K epitaxial graphene mobilities will be ∼150 000 cm2 V-1 s-1 on the (0001) face and ∼5800 cm2 V-1 s-1 on the (0001) face. [ABSTRACT FROM AUTHOR]

The paths of basal plane dislocations (BPDs) through 4H-SiC epitaxial layers grown on wafers with an 8° offcut were tracked using ultraviolet photoluminescence imaging. The reduction of BPDs by conversion to threading edge dislocations was investigated at ex situ and in situ growth interrupts. For ex situ interrupts, BPDs are imaged after each of several growths. The wafer remains in the reactor for in situ interrupts and BPDs are imaged after the growth is finished. For in situ interrupts, a combination of temperature, propane flow, and duration has been determined, which achieve a BPD reduction of 98%. [ABSTRACT FROM AUTHOR]

Threading dislocations in metal-organic chemical-vapor grown GaN films were imaged nondestructively by the electron channeling contrast imaging (ECCI) technique. Comparisons between ECCI and cross-sectional transmission electron microscopy indicated that pure edge dislocations can be imaged in GaN by ECCI. Total threading dislocation densities were measured by ECCI for various GaN films on engineered 4H-SiC surfaces and ranged from 107 to 109 cm-2. A comparison between the ultraviolet electroluminescent output measured at 380 nm and the total dislocation density as measured by ECCI revealed an inverse logarithmic dependence. [ABSTRACT FROM AUTHOR]

Threading dislocations in thin (<200 nm) AlN nucleation layers (NLs) grown by metal-organic chemical vapor deposition on top of 4H-SiC on-axis mesas with atomic-scale steps were analyzed by transmission electron microscopy. The AlN NL controlled threading dislocations in an overlying ∼2 μm GaN layer through two identified mechanisms: threading half-loop formation and dislocation bending at V-shaped pits. Threading dislocations in the AlN film could be traced directly to bilayer 4H-SiC steps at the substrate/film interface. These observations reveal several approaches to extended defect reduction in GaN films grown on 4H-SiC. [ABSTRACT FROM AUTHOR]

Direct imaging of atomic step morphologies and individual threading dislocations in on-axis epitaxial 4H-SiC surfaces is presented. Topographically sensitive electron images of the crystalline surfaces were obtained through forescattered electron detection inside a conventional scanning electron microscope. This technique, termed electron channeling contrast imaging (ECCI), has been utilized to reveal the configuration of highly stepped, homoepitaxial 4H-SiC films grown on 4H-SiC mesa structures. Individual threading dislocations have been consistently imaged at the core of spiral atomic step morphologies located on the 4H-SiC surfaces. The ability of ECCI to image atomic steps was verified by atomic force microscopy. [ABSTRACT FROM AUTHOR]

An approach to reduce vertical threading dislocations in the active regions of III-nitride devices is described. The approach involves confined homo- or heteroepitaxy of GaN materials using sputtered oxide masks to delineate growth regions and conventional metal-organic chemical vapor deposition. The resulting confined epitaxial material is terminated with equilibrium crystal facets, which form hexagonal mesas, and contains a reduced dislocation density and reduced strain compared to the underlying template layer for homoepitaxial growth. Characterization of pn junction diodes grown with this approach reveals significantly reduced leakage currents in as-grown, unpassivated structures (as low as 1×10-7 A cm-2). [ABSTRACT FROM AUTHOR]

Distributed Bragg reflectors (DBRs) composed of an AlN/AlGaN superlattice were grown of Si (111) substrates. The first high-reflectance III-nitride DBR on Si was achieved by growing the DBR directly on the Si substrate to enhance the overall reflectance due to the high index of refraction contrast at the Si/AlN interface. For a 9× DBR, the measured peak reflectance of 96.8% actually exceeded the theoretical value of 96.1%. The AlN/AlGaN superlattice served the added purpose of compensating for the large tensile strain developed during the growth of a crack-free 500 nm GaN/7× DBR/Si structure. This achievement opens the possibility to manufacture high-quality III-nitride optoelectronic devices without optical absorption in the opaque Si substrate. [ABSTRACT FROM AUTHOR]

We report that very low threading dislocation densities (8×107/cm2) were achieved in uniform GaN layers grown by metalorganic chemical vapor deposition on (0001) 4H-SiC mesa surfaces 50 μm×50 μm in area that were completely free of steps. Transmission electron microscopy (TEM) indicated that all observable GaN film threading dislocations were of edge type. TEM analysis of the defect structure of the nucleation layer (aluminum nitride, AlN) revealed a lack of c-component dislocations, and the clean annihilation of lateral, a-type dislocations within the first 200 nm of growth, with no lateral dislocations developing threading arms. These results indicate that the elimination of steps on the initial (0001) 4H-SiC growth surface may play an important role in the removal of mixed and c-type dislocations in subsequently grown AlN and GaN heteroepitaxial layers. [ABSTRACT FROM AUTHOR]