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We report the investigation of ZnO thin films delta-doped with lithium and phosphorus introduced simultaneously. The films were deposited from high purity ceramic targets of ZnO and Li3PO4 on c-plane sapphire substrates by RF magnetron sputtering. An undoped ZnO film with a low background electron concentration was used as the buffer layer on the sapphire substrate. The doped films were prepared by carrying simultaneous sputtering from the ZnO and Li3PO4 ceramic targets. For uniform doped films, the simultaneous deposition from the ZnO and Li3PO4 was uninterrupted. For the delta-doped films on the other hand, deposition from the ZnO target was uninterrupted while that from the Li3PO4 was interrupted periodically using a shutter. Post-deposition annealing was carried using a rapid thermal processor in O2 at 900 oC for 3 min. Results obtained from photoluminescence spectroscopy measurements at 12 K revealed acceptor-related luminescence peaks at 3.35 eV, possibly due to the transition from exciton bound to a neutral acceptor. The x-ray diffraction 2θ-scans showed a single peak at about 34.4o. Hall effect measurements revealed p-type conductivities with an average Hall concentrations of 3.8 x 1013 cm-3 in uniform doped samples and 1.5 x 1016 cm-3 in delta doped samples. However, in some cases the Hall coefficients had both positive and negative values, making the determination of the carrier type inconclusive. The fluctuation in the carrier type could be due to the lateral inhomogeneity in the hole concentration caused by signal noise impacting the small Hall voltages in the measurements.
Thin films of ZnO co-doped with lithium and phosphorus were deposited on sapphire substrates by RF magnetron sputtering. The films were sequentially deposited from ultra pure ZnO and Li3PO4 solid targets. Post deposition annealing was carried using a rapid thermal processor in O2 and N2 at temperatures ranging from 500 °C to 1000 °C for 3 min. Analyses performed using low temperature photoluminescence spectroscopy measurements reveal luminescence peaks at 3.359, 3.306, 3.245 eV for the co-doped samples. The x-ray diffraction 2θ-scans for all the films showed a single peak at about 34.4° with full width at half maximum of about 0.17°. Hall Effect measurements revealed conductivities that change from p-type to n-type over time.
Zinc Oxide thin films were deposited on sapphire substrates by radio frequency (RF) magnetron sputtering from an ultra-high purity ZnO solid target. The ZnO films were deposited on sapphire substrates heated in oxygen and/or in vacuum prior to deposition. Additional parameters investigated included the substrate temperature varied from 25 °C to 600 °C, the deposition gas pressure varied from 5 mTorr to 40 mTorr and the gas flow rate varied from 5 to 30 standard cubic centimeter per minute (sccm). The resulting films were annealed using a rapid thermal processor in N2 gas at 900 °C for 5 min. Analyses carried out using photoluminescence spectroscopy (PL) and X-ray diffraction (XRD) measurements indicate that films deposited at 300 °C using Ar:O2 (1:1) had the best optical and microstructure qualities. Pre-heating the sapphire substrate in oxygen prior to deposition was found to create a smoother sapphire surface, and this produced a ZnO film with greatly improved qualities. This film had a luminescence peak at 3.362 eV with a full-width-half maximum (FWHM) value of 15.3 meV when measured at 11 K. The XRD 2θ-scans had peaks at 34.4° with the best FWHM value of only 0.10°. Production of high quality ZnO materials is a necessary step towards realizing highly conductive p-type doped ZnO materials which is currently a major goal in research efforts on ZnO.
We report the nanofabrication and characterization of triangular lattice array of photonic crystals (PCs) with diameter/periodicity as small as 100/180 nm on III-nitride Light Emitting Diodes (LEDs) using electron beam lithography and inductively-coupled-plasma dry etching. Under optical pumping, a maximum enhancement factor of 20 was obtained from the PCs for emission light intensity at the wavelength of 475 nm at room temperature. Under current injection, the total power at 20 mA of 300 × 300 μm2 unpackaged LED chips revealed an increase by 63% and 95% for 460 nm blue and 340 nm UV LEDs, respectively, as a result of the PC formation. Our results show that the fabrication of PCs enhances the power output significantly on the III-nitride LEDs, which currently have very low external quantum efficiency especially in the UV range.
Edge termination is an important aspect in the design of high power p-n junction devices. In this paper, we compare the breakdown characteristics of 4H-SiC p+-n diodes with oxide passivation and with edge termination using either low or high energy ion implantations. N- and p-type epilayers of 4H-SiC were grown by chemical vapor deposition on n+ 4H-SiC wafers. Circular mesa structures of different diameters were patterned and isolated by reactive ion etching. Four types of samples were fabricated. The first group was not implanted or passivated and was left for control. The second type consisted of oxide-passivated diode structures while the third and fourth types were ion implanted with 30 keV Ar+ and 2.2 MeV He+ ions, respectively. The time dependent breakdown characteristics were determined using a fast voltage ramp technique. The reverse bias breakdown voltages and leakage currents of these diodes were different for the different types of the edge termination. Diodes terminated using 2.2 MeV ion implantation yielded the best breakdown characteristics. A majority of the diodes exhibited abrupt breakdown.
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