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Silver Schottky contacts were fabricated on (112̄0) n-ZnO epilayers, which were grown on R-plane sapphire substrates by metalorganic chemical-vapor deposition. The flatband barrier height was determined to be 0.89 and 0.92 eV by current–voltage and capacitance–voltage measurements, respectively. The ideality factor was found to be 1.33.

Abstract-ZnO is a wide bandgap semiconductor with a direct bandgap of 3.32eV at room temperature. It is a candidate material for ultraviolet LED and laser. ZnO has an exciton binding energy of 60 meV, much higher than that of GaN. It is found to be significantly more radiation hard than Si, GaAs, and GaN, which is critical against wearing out during field emission. Furthermore, ZnO can also be made as transparent and highly conductive, or piezoelectric. ZnO nanotips can be grown at relatively low temperatures, giving ZnO a unique advantage over the other nanostructures of wide bandgap semiconductors, such as GaN and SiC. In the present work, we report the selective growth of ZnO nanotips on various substrates using metalorganic chemical vapor deposition. ZnO nanotips grown on various substrates are single crystalline, n-type conductive and show good optical properties. The average size of the base of the nanotips is 40 nm. The room temperature photoluminescence peak is very intense and sharp with a full-width-half-maximum of 120 meV. These nanotips have potential applications in field emission devices, near-field microscopy, and UV photonics.

INTRODUCTIONZinc oxide (ZnO) is a wide energy bandgap semiconductor that has potential applications for photodetectors, 1,2 light-emitting diodes, lasers, 3,4 and modulators 5 operating in the ultraviolet range. The direct bandgap of ZnO can be tuned from 3.3 eV to 4.0 eV by alloying ZnO with MgO to form the ternary compound, magnesium zinc oxide (Mg x Zn 1Ϫx O). The ZnO/Mg x Zn 1Ϫx O heterostructure is promising for a wide range of electronic and photonic devices. 6,7 Metal/Mg x Zn 1Ϫx O contacts, including ohmic and Schottky types, play important roles in device performance. In the case of ohmic-contact technology, nonalloyed ohmic contact with low specific contact resistance is preferred, particularly for shallow junction and low voltage devices, as it provides smooth metal-semiconductor interfaces resulting from the limited interface reaction. In this paper, we report the results of Al nonalloyed ohmic contacts on epitaxial Mg x Zn 1Ϫx O (0 Յ ϫ Յ 0.34) thin films. EXPERIMENTALZnO and Mg x Zn 1Ϫx O films (200-500 nm) were grown on R-plane sapphire (R-Al 2 O 3 ) substrates using metal organic chemical vapor deposition (MOCVD). The R-plane sapphire was chosen as a substrate instead of the commonly used C-plane sapphire as it offers two advantages: (1) the c-axis of ZnO lies in the surface plane, with the lattice mismatch parallel to the c-axis of ZnO being lower than the mismatch perpendicular to the c-axis (1.53% versus 18.3%), resulting in overall less mismatch; and (2) in-plane anisotropy in the ZnO/R-Al 2 O 3 system can be used to make novel optical, electrical, and piezoelectric devices. 8 The composition of the Mg x Zn 1Ϫx O films were evaluated using the Rutherford backscattering spectroscopy (RBS) technique. The surface morphology was inspected using fieldemission scanning electron microscopy (FESEM). Xray diffraction (XRD) was used to analyze the crystallinity of the material. After the growth of the Mg x Zn 1Ϫx O films, metal layers (Al/Au: 100 nm/100 nm) were deposited using an electron-beam evaporator to form the nonalloyed ohmic contacts. Aluminum was chosen as the ohmic contact metal because of its low barrier height on ZnO. 9 Gold was used as a cover layer to prevent the surface oxidation after the contact formation, as well as for bonding. The transmission line method (TLM) was used to measure the specific contact resistances. The linear TLM metallization pattern consists of seven square contact pads (area 80 ϫ 80 m 2 ), separated by different spacing, Al nonalloyed ohmic contacts were fabricated and characterized on Mg x Zn 1-x O (0 Յ ϫ Յ 0.34) epilayers, which were grown on R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). Specific contact resistances were evaluated by the transmission line method (TLM). A specific contact resistance of 2.5 ϫ 10 Ϫ5 ⍀cm 2 was obtained for Al contact to ZnO with an electron concentration of 1.6 ϫ 10 17 cm Ϫ3 . The current flow mechanism was studied by investigating the dependence of specific contact resistances on electron concentra...

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