Papers by Author: Cole W. Litton

Abstract: Improved structural quality and radiative efficiency were observed in GaN thin films grown by metalorganic chemical vapor deposition on in situ-formed SiN and TiN porous network templates. The room temperature carrier decay time of 1.86 ns measured for a TiN network sample is slightly longer than that for a 200 μm-thick high quality freestanding GaN (1.73 ns). The linewidth of the asymmetric X-Ray diffraction (XRD) (1012) peak decreases considerably with the use of SiN and TiN layers, 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: 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: Heteroepitaxial n-ZnO films have been grown on commercial p-type 6H-SiC substrates by plasma-assisted molecular-beam epitaxy, and n-ZnO/p-SiC heterojunction mesa structures have been fabricated and their photoresponse properties have been studied. Current-voltage characteristics of the structures had a very good rectifying diode-like behavior with a leakage current less than 2 x 10-4 A/cm2 at -10 V, a breakdown voltage greater than 20 V, a forward turn on voltage of ∼5 V, and a forward current of ∼2 A/cm2 at 8 V. Photosensitivity of the diodes, when illuminated from ZnO side, was studied at room temperature and photoresponsivity of as high as 0.045 A/W at -7.5 V reverse bias was observed for photon energies higher than 3.0 eV.

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: 4H SiC high power photoconductive semiconductor switching devices were fabricated. A highly doped n+-GaN subcontact epilayer was grown on SiC by organometallic vapor phase epitaxy in order to improve ohmic contact and avoid contact damage or degradation due to current filamentation, under high power operation. With an n+-GaN subcontact layer, the contact resistance was reduced and current crowding alleviated. Therefore the electrodes were not damaged or degraded at high power operation. Photocurrent up to 200 A and breakdown voltage up to 2900 V have been observed for the devices.