We report the catalyst-free growth of ZnO nanotips by metalorganic chemical vapor deposition (MOCVD) on various substrates, including c-sapphire, (100) Si, titanium, glass and SiO2. Structural, optical, and electrical properties of ZnO nanotips are investigated. ZnO nanotips are found to be single crystalline and oriented along the c-axis normal to the growth plane. The nanotips exhibit dominant free excitonic transition and enhanced luminescence efficiency with negligible deep-level emission. Controllable in situ Ga doping during MOCVD growth reduces the resistivity of ZnO nanotips. Selective growth of ZnO nanotips has been achieved on patterned Ti/r-Al2O3, SiO2/r-Al2O3, and silicon-on-sapphire (SOS) substrates. It provides the potential to integrate ZnO nanotips and ZnO epitaxial films on a single patterned substrate for various device applications.
The hot carrier relaxation dynamics are studied in both intrinsic and n-type ZnO films grown on R-plane sapphire by metalorganic chemical vapor deposition. An ultrafast UV pump/UV probe experiment was used to study the relaxation process. Absorption saturation and band-gap renormalization are observed. A novel femtosecond pump-probe technique is also used in which the electrons present in n-type ZnO are excited by an infrared pump and the electron dynamics are monitored by a tunable near UV probe. Complex transients, showing bleaching and induced absorption, are observed. The results from those two samples are discussed.
An optical circuit was designed and built to facilitate the testing of a free-space optical interconnect. Details of the optical design are presented. The interconnect was based on a hybrid CMOS/GaAs chip which had a 16 X 16 array of detectors on a 250 micrometers pitch interlaced with a 16 X 16 array of modulators. The optomechanics enabled two such chips, bonded to printed circuit boards, to be mounted, positioned and aligned relative to each other. The operating wavelength of the devices was 827 nm. The optical system worked successfully and alignment could be achieved across the array with ease and high precision.
AlGaN based interdigital metal-semiconductor-metal (MSM) photodetectors with 14 percent Al have been successfully grown and fabricate don sapphire substrates. The devices exhibit large gains up to 106 at high bias voltages, but with very high dark currents, > 1 mA and very long detector responses, > 60 seconds. A negative temperature coefficient for the breakdown voltage was observed indicating that tunneling is occurring. However, at high bias voltages, avalanche breakdown also appears to be present since a constant breakdown field of 105 V/cm was obtained independent of MSM geometry. Avalanche breakdown is nucleated at the non-uniform field distribution at the edge of the MSM finger.
We report a novel approach to normal incidence multiple quantum well light modulators. The quantum-confined Stark effect is utilized to tune the polarization rotation and phase retardation created by a thermally induced in-plane anisotropic strain. An exceedingly high contrast ratio of 4800:1 is demonstrated for a normally-on device at room temperature.
The Vapor Transport Epitaxy (VTE) thin film deposition technique for the deposition of III - V and II - VI compound semiconductors and material results are reviewed. The motivation for development of the VTE technique is the elimination of several problems common to molecular beam epitaxy/chemical beam epitaxy and metalorganic chemical vapor deposition systems. In VTE, vapors from sources feed through throttling valves into a common manifold which is located directly below the inverted wafer. A high degree of film uniformity is achieved by controlling the flux distribution from the common manifold. The technique operates in the 10-4 - 10-6 Torr range using elemental, metalorganic or gaseous precursors. The system is configurated for 2 inch diameter wafers but the geometry may easily be scaled for larger diameters. Using elemental sources, we have demonstrated oval defect free growth of GaAs on GaAs (100) and (111) 2 degree(s) off substrates, through several microns of thickness at growth rates up to ten microns per hour. GaAs films which were grown without the manifold exhibit classic oval defects. The deposition rate of ZnSe films as a function of elemental flux, VI/II ratio, and growth temperature are described. The ZnSe films exhibited smooth surface morphologies on GaAs (100) 2 degree(s) off substrates. X- ray analysis shows that III - V and II - VI films exhibited crystallinities comparable to films produced by molecular beam epitaxy and metalorganic chemical vapor deposition techniques.
The Fermi level position in low temperature (LT) GaAs is studied by photoreflectance (PR). The experiments show that the Fermi level in both the as-grown and the annealed LT-GaAs is firmly pinned, however, the pinning position occurs at different energies: 0.47 eV below the conduction band edge for the as-grown samples and 0.65 eV below the conduction band edge for the annealed samples. The pinning in the as-grown LT-GaAs is the result of a high degree of charge compensation of deep levels, while the pinning in the annealed LT-GaAs is due to the depletion of carriers by the Schottky barrier at the metallic As precipitates. From the measured Fermi level and ionization ratio of As antisites, the (0/+) donor level of the As antisite is found to be at Ec - 0.57 eV.