Materials Science Forum Vols. 527-529

Abstract: Strain relaxation in the GaN/AlN/6H-SiC epitaxial system grown by vicinal surface epitaxy (VSE) is investigated and compared with that in on-axis epitaxy. High resolution x-ray diffraction (HRXRD) measurements show that GaN films grown by VSE have improved crystalline quality. High resolution transmission electron microscope (HRTEM) studies reveal that there are two types of misfit dislocations (MDs) at AlN/6H-SiC interfaces: 60˚ complete dislocations along <1120 > directions with Burgers vector 1/3<1120 > and 60˚ Shockley partials along <10 10 > directions with Burgers vector 1/3<10 10 >. The latter are usually geometrical partial misfit dislocations (GPMDs) that are dominant in VSE to accommodate the lattice mismatch and stacking sequence mismatch simultaneously. In VSE, it is the high-density GPMDs formed at the vicinal surface steps that facilitate rapid strain relaxation at the initial stage of deposition and hence lead to superior crystalline quality of the subsequently grown GaN films.

Abstract: The phonon anisotropy property of the GaN wurtzite crystal is studied using angular dependent Raman spectroscopy both theoretically and experimentally. The polarized Raman scattering spectra were recorded from cross-sections of c-axis oriented GaN films as a function of the angle between the incident laser polarization direction and the film normal direction in three different configurations. The Raman intensity of A1(TO) showed a sinusoidal dependence on the rotating angle, as also did the E1(TO) mode, while the E2 mode has a quite different behavior. The theoretical fit takes into account the susceptibility contribution and the phase differential of different vibrating elements.

Abstract: Using a combination of synchrotron white beam x-ray topography (SWBXT) and high resolution x-ray diffraction (HRXRD), the structural quality of AlN crystals grown by various sublimation-based techniques have been non-destructively analyzed. Spontaneously nucleated AlN crystals are characterized by very low defect densities but their size is small. Self-seeding results in nucleation of multiple grains of different orientations, a few of which are of good quality while most are highly strained. Using readily available commercial 4H and 6H-SiC substrates, several growth runs have been carried out using different growth conditions to obtain thick AlN layers, either attached to the seed or free-standing. While attached layers are typically cracked and highly strained, crack-free free-standing layers can be obtained by delamination or SiC decomposition. X-ray characterization reveals these crystals have good purity but moderately high defect densities.

Abstract: Both n-type and p-type GaN MOS capacitors with plasma-enhanced CVD-SiO2 as the gate oxide were characterized using both capacitance and conductance techniques. From a n type MOS capacitor, an interface state density of 3.8×1010/cm2-eV was estimated at 0.19eV near the conduction band and decreases deeper into the bandgap while from a p type MOS capacitor, an interface state density of 1.4×1011/cm2-eV 0.61eV above the valence band was estimated and decreases deeper into the bandgap. Unlike the symmetric interface state density distribution in Si, an asymmetric interface state density distribution with lower density near the conduction band and higher density near the valence band has been determined.

Abstract: It is generally accepted that the Schottky barrier height (SBH) is affected by the initial band bending at the bare nGaN surface as well as by an additional contribution following metal deposition. In this work the effect of processing used for device fabrication on the surface band bending of bare c-plane nGaN was studied by surface potential electric force microscopy (SP-EFM). An increase of the initial upward band bending from 1.0 ± 0.1eV for the as-grown GaN to 1.9 ± 0.1eV after RTA treatment in N2 ambient was observed. No significant dependence of band bending on N2 or Ar as ambient gas during the RTA treatment was observed. The increase of the initial upward band bending was also confirmed by photoluminescence (PL) measurements. We suggest that the RTA treatment causes a high density of surface states, possibly as a result of high temperature reaction of ambient gas and remnant contamination.

Abstract: Carrier transport properties of AlGaN/GaN heterostructures have been analyzed with the quantitative mobility spectrum analysis (QMSA) technique. The nominally-undoped Al0.15Ga0.85N/GaN sample was grown by metal-organic vapor phase epitaxy. Variable-magneticfield Hall measurements were carried out in the temperature range of 4-160 K and magnetic field range of 0-6.6 T. QMSA was applied to the experimental variable-field data to extract the concentrations and mobilities associated with the high-mobility 2DEG and the relatively lowmobility bulk electrons for the temperature range investigated. For temperatures below 100 K the calculated mobility and carrier density values were close to the experimental results. No bulk conduction was observed in this temperature range. At 160 K, QMSA results show that parallel conduction in 3 mm thick GaN layer started to affect the average electron mobility.

Abstract: The investigated AlGaN epitaxial layers were grown by hydride vapor phase epitaxy (HVPE) on a commercial P+ SiC substrate or on an N+ SiC Lely substrate with a p+ SiC layer previously grown by sublimation epitaxy. To investigate the electrical characteristics of the n-p heterojunction, mesa structures of 100, 200 and 1500 microns in diameter were fabricated by reactive ion etching. Investigation of electrical characteristics shows good quality of grown n- AlGaN/p-SiC heterojunctions. This shows applicability of this technological combination for producing n-AlGaN/p-SiC bipolar or FET transistors.

Abstract: GaN PiN diodes with a 4 μm Si-doped n--GaN drift layer (n~7×1016 cm-3) were grown on free-standing GaN using metalorganic chemical vapor deposition. Atomic force microscopy showed smooth surfaces with a step structure indicating good 2D growth. The dislocation density and impurity incorporation in the drift layer were remarkably reduced compared to a similar diode structure grown on sapphire. The full width at half maximum of the (0002) rocking curve was 79 arcsec, much smaller than 230 arcsec for the heteroepitaxial structure. The diodes on GaN demonstrated rectification up to –265 V, corresponding to a critical electric field ~2.7×106 V/cm. The maximum value of the figure of merit is ~2.4 MW cm-2, which represents a 2.2× improvement over the diodes on sapphire.

Abstract: Characterization of n+-GaN/p−-SiC and n+-GaN/p+-SiC heterojunctions as well as fabrication of GaN/SiC heterojunction bipolar transistors (HBTs) using these heterojunctions is presented. The electroluminescence spectrum from n+-GaN/p+-SiC heterojunction diodes under forward bias clearly indicates electron injection from n+-GaN into p+-SiC. HBTs consisting of n+-GaN emitter /p+-SiC base/n−-SiC collector/n+-SiC substrate have been fabricated. Although clear common-base properties were obtained, the current gain was very low (10-4). SiC homojunction bipolar junction transistors (BJT) using the same base-collector junction exhibited a current gain value of 0.5, suggesting the low current gain of GaN/SiC HBTs originates from low emitter efficiency.

Abstract: We report on simple techniques for extracting the electrical properties of 1-dimensional semiconductor nanowires using standard ultraviolet (UV) photo-lithography instead of e-beam lithography (EBL), which is a commonly used technique for the fabrication of nanoscale electrical devices. For electrical transport measurement the gallium nitride nanowires (GaN NWs) were prepared by a horizontal hot-wall chemical vapor deposition (CVD) with metallic Ga and NH3 gas for Ga and N sources, and GaN nanowire field effect transistor (FET) structures on a 8×8 mm2 silicon wafer were fabricated by ordinary 2-mask photo-lithography processes. The estimated carrier mobility from the gate-modulation characteristics is on the order of 60 ∼ 70 cm2/V⋅s. We found that our approach is a powerful and simple technique to extract the electrical properties of semiconductor nanowires. The material characteristics of GaN nanowires are also discussed.