S. Chevtchenko

Surface band bending of a-plane GaN studied by scanning Kelvin probe microscopy

We report the value of surface band bending for undoped, a-plane GaN layers grown on r-plane sapphire by metalorganic vapor phase epitaxy. The surface potential was measured directly by ambient scanning Kelvin probe microscopy. The upward surface band bending of GaN films grown in the [112¯0] direction was found to be 1.1±0.1V. Because polarization effects are not present on a-plane GaN, we attribute such band bending to the presence of charged surface states. We have modeled the surface band bending assuming a localized level of surface states in the band gap on the surface. It should be noted that the band bending observed for a-plane layers is comparable to that obtained on polar c-plane layers, and both a-plane and c-plane GaN films with similar surface treatments demonstrate comparable band bending behavior, indicating that charged surface states dominate band banding in both cases.

Comparative study of the (0001) and (0001) surfaces of ZnO

The authors compare the surface and optical properties of the Zn-polar (0001) and O-polar (0001¯) surfaces of bulk ZnO samples. For optical characterization, steady-state photoluminescence using a He–Cd laser was measured at 15 and 300K. At room temperature, the (0001¯) surface demonstrates nearly double the near-band-edge emission intensity seen for the (0001) surface. Using scanning Kelvin probe microscopy, the authors have measured surface contact potentials of 0.39±0.05 and 0.50±0.05V for the (0001) and (0001¯) surfaces, respectively. The resulting small difference in band bending for these two surfaces indicates that charge transfer between the surfaces is not a dominant stabilizing mechanism. Conductive atomic force microscopy studies show enhanced reverse-bias conduction in localized regions on the (0001¯) vs (0001) surface. The differences in surface conduction and band bending between the two polar surfaces can be attributed to their chemical interactions with hydrogen and water in the ambient.

GaN resistive hydrogen gas sensors

GaN epilayers grown by organometallic vapor phase epitaxy have been used to fabricate resistive gas sensors with a pair of planar ohmic contacts. Detectible sensitivity to H2 gas for a wide range of gas mixtures in an Ar ambient has been realized; the lowest concentration tested is ∼0.1% H2 (in Ar), well below the lower combustion limit in air. No saturation of the signal is observed up to 100% H2 flow. Real-time response to H2 shows a clear and sharp response with no memory effects during the ramping cycles of H2 concentration. The change in current at a fixed voltage to hydrogen was found to change with sensor geometry. This appears to be consistent with a surface-adsorption-induced change of conductivity; a detailed picture of the gas sensing mechanism requires further systematic studies.

Study of SiNx and SiO2 passivation of GaN surfaces

The optical properties of GaN films have been found to be sensitive to SiNx and SiO2 surface passivation. The main effect of such passivation on photoluminescence (PL) data is an increase of the PL intensity for near-band-edge emission. This effect is attributed to the removal of oxygen from the surface of GaN and the subsequent formation of a protective layer during passivation. The increase in PL intensity is more pronounced for samples passivated with SiO2, which demonstrate initially lower PL intensity and a lower equilibrium concentration of free electrons. A nearly constant band bending of approximately 1.0 eV at the surface has been observed for as-grown and passivated samples by scanning Kelvin probe microscopy (SKPM). This constant value is explained by pinning of the Fermi level at the surface. In addition, we have demonstrated that passivation of the GaN surface between the contacts of a Schottky diode leads to a reduction of the leakage current observed at reverse bias. It was found that the s...

AlGaN/GaN MOS transistors using crystalline ZrO2 as gate dielectric

Epitaxial growth of ZrO2 has been achieved on MOCVD-grown GaN(0001) templates by oxides molecular beam epitaxy using reactive H2O2 for oxygen and organometallic source for Zr. Utilizing a low temperature buffer layer followed by high temperature insitu annealing and high-temperature growth, monoclinic (100)-oriented ZrO2 thin films were obtained. The full width at half maximum of ZrO2 (100) rocking curve was 0.4 arc degree for 30-nm-thick films and the rms roughness for a 5&mgr;m by 5 &mgr;m AFM scan was 4 Å. The employment of epitaxial ZrO2 layer in the AlGaN/GaN heterojunction field effect trasnsistor as a gate dielectric has resulted in the increase of the saturation-current density and pinch-off voltage as well as in near symmetrical gate-drain I-V behavior.

Defect reduction in GaN epilayers grown by metal-organic chemical vapor deposition with in situ SINx nanonetwork

Line and point defect reductions in thin GaN epilayers with single and double in situ SiNx nanonetworks on sapphire substrates grown by metal-organic chemical vapor deposition were studied by deep-level transient spectroscopy (DLTS), augmented by x-ray diffraction (XRD), and low temperature photoluminescence (PL). All samples measured by DLTS in the temperature range from 80to400K exhibited trap A (peak at ∼325K) with an activation energy of 0.55–0.58eV, and trap B (peak at ∼155K) with an activation energy of 0.21–0.28eV. The concentrations of both traps were much lower for layers with SiNx nanonetwork compared to the reference sample. The lowest concentration was achieved for the sample with 6min deposition SiNx nanonetwork, which was also lower than that for a sample prepared by conventional epitaxial lateral overgrowth. In concert with the DLTS results, PL and XRD linewidths were reduced for the samples with SiNx network indicating improved material quality. Consistent trend among optical, structural, ...

Structural and electrical properties of Pb(Zr,Ti)O3 films grown by molecular beam epitaxy

Single-crystal, single-phase Pb(ZrxTi1−x)O3 films (x=0–0.4) were grown on (001) SrTiO3 and SrTiO3:Nb substrates by molecular beam epitaxy. Layer-by-layer growth of the Pb(Zr,Ti)O3 films was achieved by using PbTiO3 buffer layers between the SrTiO3 substrates and the Pb(Zr,Ti)O3 films. The layers with low Zr content showed high crystallinity with full width at half maximum of ω-rocking curves as low as 4arcmin, whereas increase in Zr concentration led to pronounced angular broadening. The PbZr0.07Ti0.93O3 films exhibited remanent polarization as high as 83μC∕cm2, but local areas suffered from nonuniform leakage current.

Reactive ion etch damage on GaN and its recovery

Surface properties of GaN subjected to reactive ion etching and their impact on device performance have been investigated by transport, optical, and surface potential measurements. Different etching conditions were studied to minimize plasma-induced damage. Higher etch rates could be obtained at high powers and low pressures, with the accompanying roughening of the surface. Surface potential for the as-grown samples was found to be in the range of 0.5–0.7V using scanning Kevin probe microscopy. However, after etching at a power level of 300W, the surface potential decreased to 0.1–0.2V. An almost linear reduction was observed with increasing power. Additionally, the intensity of the near band edge photoluminescence decreased and the free carrier density increased after etching. These results suggest that the changes in the surface potential may originate from the formation of possible nitrogen vacancies and other surface oriented defects. To recover the etched surface, N2 plasma, rapid thermal annealing, ...

I-V characteristics of Au∕Ni Schottky diodes on GaN with SiNx nanonetwork

Room temperature and temperature dependent current-voltage characteristics of Ni∕Au Schottky diodes fabricated on undoped GaN prepared with and without in situ SiNx nanonetwork by metal organic chemical vapor deposition have been studied. The features of the Schottky diodes depend strongly on the SiNx deposition conditions, namely, its thickness. Reduction in the point and line defect densities caused the Schottky barrier height to increase to 1.13eV for 5min SiNx deposition time as compared to 0.78eV without SiNx nanonetwork. Similarly, the breakdown voltage also improved from 76V for the reference to 250V when SiNx nanonetwork was used. With optimized SiNx nanonetwork, full width at half maximum values of (0002) and (101¯2) x-ray rocking curves improved to 217 and 211arcsec, respectively, for a 5.5μm thick layer, as compared to 252 and 405arcsec for a reference sample of the same thickness, which are comparable to literature values. The photoluminescence linewidth also reduced to 2.5meV at 15K with free...

Comparative study of deep levels in GaN grown on different templates

Deep levels in thin GaN epilayers grown by metal-organic chemical vapor deposition on different templates were studied by photocapacitance spectroscopy and deep-level transient spectroscopy (DLTS) using Schottky barrier diodes. We observed the reduction of electrically and optically active traps in GaN grown with in situ SiNx nanonetwork and SiO2 striped mask or conventional epitaxial lateral overgrowth technique (ELO) as compared to a typical control layer on a sapphire substrate. All samples measured by DLTS in the temperature range from 80 K to 400 K exhibited traps with activation energies 0.55-0.58 eV and 0.21-0.28 eV. The lowest concentration of both traps was achieved for the sample with 6 min deposition of SiNx nanonetwork, which was lower than that for the sample prepared by conventional ELO, and much lower than that in the control. The steady-state photocapacitance spectra of all samples taken at 80 K over the spectral range 0.75-3.50 eV demonstrated a similar trend for all the layers. The photocapacitance spectra exhibited defect levels with optical threshold energies of 1.2-1.3, 1.6, 2.2 and 3.1 eV. The determined concentrations of traps were compared and the results were consistent with DLTS measurements. The layer with SiNx nanonetwork has the lowest concentrations of optically active traps with the standard GaN control layer being the worst in terms of trap concentrations. The consistent trend among the photocapacitance spectroscopy and DLTS results suggests that SiNx network can effectively reduce deep levels in GaN, which otherwise can deteriorate both optical and electrical performance of GaN-based devices.