GROWTH OF WIDE BANDGAP SEMICONDUCTORS USING PULSED ELECTRON BEAM DEPOSITION (PED) PROCESS

Wide bandgap materials hold a great potential in solid state lighting, power electronics, and radio frequency (RF) device applications. The cumulative market for all these applications is forecasted to be approximately $75 billion within a decade. The most promising wide bandgap materials are gallium nitride (GaN), aluminum nitride (AlN), their pseudobinary ternary alloys (Al1-xGaxN), and zinc oxide (ZnO). This dissertation describes the development of growth processes for binary GaN, AlN, and ZnO films on various cost-effective substrates (sapphire and silicon) using the pulsed electron beam deposition (PED) technique. PED is a physical vapor deposition process that deposits materials through generation of high energy plasma from a target material using 10 to 15 keV electron beam pulse bombardment on the target surface at a 1-10 Hz pulse rate with a peak current density of ~106 A/cm2 during the 70 to 100 ns pulse duration. ZnO was grown on sapphire at 300oC, 500oC, and 700, and on Si(100) at 300oC and 500oC. The grown samples were studied using structural (scanning electron microscope, x-ray diffraction (XRD), Rutherford back-scattering, electron back-scattered diffraction), optical (photoluminescence (PL)), and electrical (Hall effect measurement) characterization methods. The ZnO/Si(100) samples, which had thicknesses between 68 nm and 130 nm, showed crystalline morphology along with near-band edge optical emission at 370 nm (3.36 eV) with very weak luminescence from defect bands. Measured electron densities and mobilities were in the range of 1020 cm-3 and 44.74 cm2/Vs, respectively. By contrast, ZnO layers grown on sapphire substrates exhibited PL emission in the 500-700 nm (1.78-2.49 eV) indicating high density of defects with energy levels in the ZnO band gap and a much lower electron mobility of ~10 cm2/Vs. GaN was grown on sapphire, Si(111), and Ge/Si(111) at 600oC. Measurement of the GaN/Si(111) sample showed only a polar (0002) x-ray diffraction peak, near band edge PL at 368.5 nm (3.37 eV), and an electron mobility of 300 cm2/Vs at room temperature. Passivation of this material using hydrogen implantation improved the PL emission intensity by more than 100%. GaN was also grown on sapphire using a ZnO buffer layer, with a growth rate of 0.31 Å/pulse. PL characterization showed presence of near band edge emission (at 371.5 nm at 4K) with a strong defect band due to nitrogen vacancies. EBSD scan of the GaN surface demonstrated c-plane (0002) orientation. AlN nanowires (NW) were grown on sapphire and Si(111) substrates at low temperatures such as 500oC. SEM imaging reported that the nanowires were 100 µm long with an average diameter of 2.5 µm. Also EBSD scan verified the NW structures as hexagonal wurtzite structures. This dissertation concludes with suggestions for future improvements for the PED equipment and the growth processes. Additional tasks for future application of this tool are (i) add a butterfly valve for better control of the chamber pressure, (ii) add a shutter to protect the growth substrate during first stages of plasma formation. In addition, initial results involving the use of compliant buffer layer materials such as IV-VI semiconductors grown by molecular beam epitaxy (MBE) to reduce strain effects in GaN growth are presented in the concluding section