Zhaoqiang Fang

Deep level characteristics in n-GaN with inductively coupled plasma damage

The effects of energetic ion-induced damage on deep traps in n-GaN have been investigated using deep level transient spectroscopy. The energetic ions were produced in an inductively coupled plasma reactive ion etching (ICP-RIE) system. The electrons captured at the trap levels E1 (0.25 eV) and E2 (0.62 eV), in a control sample, were found to depend logarithmically on the duration of the filling pulse, indicating a relationship to dislocations. The dramatic increase in the concentration of deep level E1 traps, as a function of etching-bias voltage, is thought to indicate the introduction of a VN-related complex. On the other hand, the concentration of deep level E2 traps shows an initial increase at an etching-bias of −50 V, followed by a decrease at higher etching-bias voltages. This trend was also observed in the room-temperature yellow luminescence spectra and x-ray photoelectron spectroscopy, which suggests that the deep level E2 is associated with point defects in the form of VGa-impurity complexes.

An Experimental Set-Up for In Situ Hall Measurements Under High-Energy Electron Irradiation for Wide-Bandgap Materials

A dc Hall-effect apparatus, based on conventional van der Pauw specimen geometry, has been developed for in situ measurements under van de Graaff electron irradiation (0.7–2.2 MeV). An associated cryostat permits ambient temperatures of 95–300 K. A key element is a flat permanent magnet of field strength 3.55 kG. Initial test measurements have been performed on wide-bandgap semiconductor materials, including an HVPE-grown AlGaN/GaN heterostructure field-effect transistor structure.

Deep centers in conductive and semi-insulating GaN

Unintentionally doped conductive and carbon doped semi-insulating GaN films have been characterized by deep level transient spectroscopy, thermally stimulated current spectroscopy, and photoluminescence (PL). Based on correlations with dislocation density, point defects created by electron irradiation, and deep PL bands, the major traps in GaN can be tentatively associated with N vacancies, Ga vacancies, and N interstitials.

Defect Chemistry Study of Nitrogen Doped ZnO Thin Films. Final report

Our team has investigated the defect chemistry of ZnO:N and developed a thermal evaporation (vapor-phase) method to synthesis p-type ZnO:N. Enhanced p-type conductivity of nitrogen doped ZnO via nano/micro structured rods and Zn-rich Co-doping process were studied. Also, an extended X-Ray absorption fine structure study of p-type nitrogen doped ZnO was conducted. Also reported are Hall-effect, photoluminescence, and DLTS studies.

Electrical Measurements in GaN: Point Defects and Dislocations

Defects can be conveniently categorized into three types: point, line, and areal. In GaN, the important point defects are vacancies and interstitials; the line defects are threading dislocations; and the areal defects are stacking faults. The authors have used electron irradiation to produce point defects, and temperature-dependent Hall-effect (TDH) and deep level transient spectroscopy (DLTS) measurements to study them. The TDH investigation has identified two point defects, an 0.06-eV donor and a deep acceptor, thought to be the N vacancy and interstitial, respectively. The DLTS study has found two point-defect electron traps, at 0.06 eV and 0.09 eV, respectively; the 0.06-eV trap actually has two components, with different capture kinetics. With respect to line defects, the DLTS spectrum is as-grown GaN includes an 0.45-eV electron trap, which has the characteristics of a dislocation, and the TDH measurements show that threading-edge dislocations are acceptor-like in n-type GaN. Finally, in samples grown by the hydride vapor phase technique, TDH measurements indicate a strongly n-type region at the GaN/Al{sub 2}O{sub 3} interface, which may be associated with stacking faults. All of the defects discussed above can have an influence on the dc and/or ac conductivity of GaN.