Sz. Fujita

Carrier concentration dependence of band gap shift in n-type ZnO:Al films

Al-doped ZnO (AZO) thin films have been prepared by mist chemical vapor deposition and magnetron sputtering. The band gap shift as a function of carrier concentration in n-type zinc oxide (ZnO) was systematically studied considering the available theoretical models. The shift in energy gap, evaluated from optical absorption spectra, did not depend on sample preparations; it was mainly related to the carrier concentrations and so intrinsic to AZO. The optical gap increased with the electron concentration approximately as ne2∕3 for ne≤4.2×1019 cm−3, which could be fully interpreted by a modified Burstein–Moss (BM) shift with the nonparabolicity of the conduction band. A sudden decrease in energy gap occurred at 5.4−8.4×1019 cm−3, consistent with the Mott criterion for a semiconductor-metal transition. Above the critical values, the band gap increased again at a different rate, which was presumably due to the competing BM band-filling and band gap renormalization effects, the former inducing a band gap widen...

Self-assembled ZnO quantum dots with tunable optical properties

Self-assembled ZnO quantum dots (QDs) were achieved by a vapor phase transport process. ZnO nanodots were naturally formed on solid substrates in the Volmer-Weber growth mode. Size control of nanodots could be readily realized by varying the growth time. The as-prepared ZnO QDs are of high quality and very stable after formation. The blueshift of band gap energies derived from quantum confinement effects was confirmed by optical absorption spectra. Photoluminescence spectra revealed the tunable behavior of ultraviolet luminescence due to exciton localization. The realization of size-tuned color from ZnO QDs makes them more promising for practical applications.

Carrier concentration induced band-gap shift in Al-doped Zn1-xMgxO thin films

Transparent conducting Al-doped Zn1−xMgxO thin films were grown on glass substrates by chemical vapor deposition. The resistivity could be lowered to 10−3Ωcm with optical transmittance above 85% in visible regions. The influence of carrier concentration on band-gap shift in Zn1−xMgxO alloys was systematically studied. The shift of energy gap could be fully explained by the Fermi-level band filling and band-gap renormalization effects. As the Mg content increased, the electron effective masses in Zn1−xMgxO (x=0–0.21) alloys increased from 0.30m0 to 0.49m0. The Al-doping efficiency was reduced with the increase in alloy composition.

ZnO quantum dots synthesized by a vapor phase transport process

A vapor phase transport growth process has been developed to synthesize ZnO quantum dots (QDs) on Si substrates. The characteristics were investigated for as-prepared ZnO QDs without any additional treatment. The formation of ZnO QDs with 6 nm in height and 15 nm in diameter is confirmed by scanning electron microscope and atomic force microscopy. Room-temperature photoluminescence reveals that the as-prepared ZnO QDs exhibit a predominant ultraviolet emission at 3.32 eV while the low energy defect-related blue-green emission is significantly quenched. The band gap of ZnO QDs is determined to be 3.41 eV, which evidently indicates the quantum confinement effects.

ZnO growth on Si substrates by metalorganic vapor phase epitaxy

Diethylzinc(DEZn) and nitrous oxide (N 2O) as a source gas combination in the metalorganic vapor phase epitaxy (MOVPE) of zinc oxide (ZnO) has produced high-quality layers on sapphire, but no growth was confirmed on Si. This problem was overcome by using an underlying layer of ZnO grown directly onto the Si using nitrogen oxide (NO2 )a s a more reactive oxidation source. The main ZnO layer grown in this way on the ZnO/Si pretreated at 8001C possessed a c-axis orientation and exhibited bound exciton (BX) emission as narrow as 3 meV at full-width at half-maximum together with a free exciton (EX) at 9 K. These results demonstrated the high potential of MOVPE technology for the growth of ZnO on Si using appropriate surface treatments for optical and electrical applications. r 2002 Elsevier

Growth of Polycrystalline GaN on Silicon (001) Substrates by RF Plasma Chemical Vapor Deposition with ZnO Buffer Layer

Polycrystalline GaN was grown at 650 °C on silicon (001) substrates by radio frequency (rf) plasma enhanced chemical vapor deposition (PECVD) with ZnO buffer layer. X-ray diffraction (XRD) measurements revealed that the ZnO buffer layer was oriented along the (0002) planes while GaN thin films were along (0002) and (101-1) planes. The ZnO buffer layer was found to be effective for the enhanced photoluminescence (PL) from the polycrystalline GaN. The low temperature (24 K) PL spectrum was dominated by the band-edge luminescence peaking at about 3.35 eV without appreciable deep level emissions. Temperature-dependent PL measurements indicated that the band-edge PL spectrum red-shifted with increasing temperature, where the nonradiative recombination is considered to be more dominant. Slope between the PL photon energy and the measurement temperature higher than 200 K up to room temperature well agreed with that of polycrystalline GaN by molecular beam epitaxy (MBE).

Homoepitaxial Growth of ZnO by Metalorganic Vapor Phase Epitaxy

Homoepitaxial technique of metalorganic vapor phase epitaxy (MOVPE) was used for the growth of high quality epitaxial ZnO layers. Two conditions, proper thermal treatment of substrate prior to the growth for obtaining flat surface and high flow rate ratios of source materials, nitrous oxide (N 2 O) and diethylzinc (DEZn), were found to be important. Surface roughness below 1 nm as well as strong free exciton emission at 15 K of an MOVPE-ZnO layer on a bulk ZnO substrate have demonstrated the high potential of MOVPE for ZnO.

Thermal nitridation of silicon in nitrogen plasma

Direct thermal nitridation of silicon in a nitrogen plasma has been performed above 1000 °C. From Auger electron spectroscopy analysis, the formed films contain some oxygen and are identified as oxynitride films. Refractive indices of these films varied from 1.5 to 1.9 with nitridation time. The film thickness is about 40 A after nitridation of 10 h at 1145 °C and the film growth is saturated at this value. Capacitance‐voltage characteristics of Al gate metal‐nitride‐semiconductor capacitors show a stable behavior. The fixed charge density Qss is estimated to be on the order of 1012 cm−2.

ZnO growth toward optical devices by MOVPE using N2O

Wide bandgap semiconductor zinc oxide (ZnO) layers were grown by metalorganic vapor phase epitaxy (MOVPE) using nitrous oxide (N2O). Strong ultraviolet (UV) photoluminescence emissions with 1000 times less deep ones at room temperature were observed from ZnO layers grown on sapphire. Alow temperature (500 C)-grown buffer layer of ZnO was effective to enhance the initial nucleation process and to achieve high quality ZnO layers on it at higher growth temperatures (600–700 C). ZnO layers grown on III–V semiconductor substrates showed dominant UV luminescence in spite of low temperature growth. These results imply the abilities of high quality ZnO growth by MOVPE.

Formation and deterioration mechanisms of low-resistance TaTi ohmic contacts for p-GaN

We demonstrated in our previous paper [Appl. Phys. Lett. 74, 275 (1999)] that TaTi contacts annealed at temperature of 800 °C provided a specific contact resistance (ρc) of around 3×10−5 Ω cm2 for p-GaN epilayers with hole concentration of 7.0×1017 cm−3. The reduction of the contact resistances was believed to be due to reactivation of Mg atoms doped in the GaN epilayer by removing H. In the present article, diffusion behavior of hydrogen atoms in the p-GaN was extensively studied by secondary-ion mass spectroscopy and correlated with the electrical properties of the TaTi contacts. It was found that formation of the low-resistance TaTi ohmic contacts was well explained by diffusion behavior of hydrogen in the p-GaN. Although the deterioration rates of the TaTi contacts during low temperature storage after ohmic contact formation were coincident with the diffusion rates of hydrogen atoms in the GaN, the deterioration mechanism could not be explained simply by hydrogen release from the TaTi contacts into th...