Final Report for AOARD Grant #114013 (FA2386-11-1-4013) “Investigation and characterization of defects in epitaxial films for ultraviolet light emitting devices using FUV time-resolved photoluminescence, time-resolved cathodoluminescence, and spatio-time-resolved cathodoluminescence excited using femtosecond laser pulses” 5/22/2013 Principal Investigator:

Impacts of structural and point defects on the carrier (exciton) recombination dynamics in wide bandgap semiconductors such as AlN, high AlN mole fraction AlxGa1-xN alloys, and GaN were studied in this project. For this purpose, we quantified the radiative lifetimes (R) and nonradiative lifetimes (NR) for the near-band-edge (NBE) emission by measuring the luminescence lifetimes () and equivalent values of internal quantum efficiencies (int) as a function of temperature by means of deep ultraviolet (DUV) time-resolved photoluminescence (TRPL) and time-resolved cathodoluminescence (TRCL) measurements using a frequency-quadrupled (4) femtosecond Al2O3:Ti laser and pulsed electron beams generated using an in-house pulsed photoelectron(PE-) gun driven by the a frequency-tripled (3) femtosecond laser, respectively. By comparing these lifetime data with the results of structural characterizations and positron annihilation spectroscopy (PAS) measurement, we correlated the lifetimes and cation vacancy concentrations. We also developed a spatio-time-resolved cathodoluminescence (STRCL) measurement system first equipped with a rear-excitation configuration pulsed PE-gun and later replaced by a front-excitation configuration one for probing local carrier dynamics in wide bandgap semiconductors. This technique enabled to measure spatiallyand time-resolved luminescence signals even at DUV wavelengths. For demonstrating the improved PE-gun performances, spatially-resolved cathodoluminescence (SRCL) and local TRCL measurements were carried out on GaN, AlGaN, AlN, and hexagonal BN powders using the new STRCL system. Introduction: Aluminium nitride (AlN) and high AlN mole fraction AlxGa1-xN alloys have attracted considerable interest for applications in UV-C (200280 nm) DUV LEDs, because the bandgap energy (Eg) of AlN is 6.01 eV at 300 K [1,2]. Taniyasu et al. have demonstrated [3] the shortest wavelength electroluminescence peak at 210 nm from an AlN p-i-n homojunction LED. However, its external quantum efficiency (ext) was as low as 10 [3], and the highest ext for the state-of-the-art AlGaN DUV LEDs is limited to be approximately 10% for the emission Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.