Yudai Kamada

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...

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.

Analysis of Hump Characteristics in Thin-Film Transistors With ZnO Channels Deposited by Sputtering at Various Oxygen Partial Pressures

The electrical properties of thin-film transistors (TFTs) with ZnO channels which were deposited by radio-frequency magnetron sputtering at various oxygen partial pressures [p( O2)] are investigated. A negative shift of the turn-on voltage with a “hump” was observed, and donorlike traps were generated at intermediate energy levels from the conduction band when the ZnO channel was deposited at p(O2) below a critical pressure. Thermal desorption spectroscopy study revealed that the donorlike traps were generated when the ZnO film changed from O- to Zn-rich condition. The Zn-related native defects would be a possible origin of the donorlike traps generated at intermediate energy levels in the ZnO TFTs.

Linear-Source Ultrasonic Spray Chemical Vapor Deposition Method for Fabrication of ZnMgO Films and Ultraviolet Photodetectors

A linear-source ultrasonic spray chemical vapor deposition method has been developed and applied to fabricate ZnMgO ternary alloy thin films on glass substrates. A water solution of zinc acetate and magnesium acetate was ultrasonically atomized to form aerosol particles of water containing the sources, and then they were supplied onto the heated substrate by a nitrogen carrier gas through a nozzle with a linear aperture to form ZnMgO films. The source concentration ratios in the water solution successfully controlled the solid composition and hence raised the band gap of ZnMgO up to 3.75 eV, keeping the optical transmission higher than 90% for the visible-light region. An UV photodetector fabricated using the ZnMgO film showed the photoresponsivity to be as high as a few A/W, suggesting that this simple and cost-effective technique is promising for fabricating ZnMgO films for various applications.

Epitaxial ZnO Thin Films on a-Plane Sapphire Substrates Grown by Ultrasonic Spray-Assisted Mist Chemical Vapor Deposition

High-quality epitaxial ZnO thin films were grown by an ultrasonic spray-assisted mist chemical vapor deposition (CVD) on a-plane sapphire substrates with ZnO buffer layers. The ZnO thin films were grown with c-axis orientation without notable rotational domains. Surface morphologies and electrical properties were markedly improved as an effect of the ZnO buffer layers. The mobility in the ZnO epitaxial (main) layer was estimated to be 90 cm2/(Vs), which is reasonably high compared with those in ZnO layers grown by CVD processes. Photoluminescence at a low temperature (4.5 K) revealed a free A-exiton peak, and that at room temperature showed a strong band-edge peak with little deep-level luminescence.

Photo-Leakage Current of Zinc Oxide Thin-Film Transistors

The origin of photo-leakage current of zinc oxide thin-film transistors (ZnO TFTs) under light irradiation was investigated using a light shield technique. The irradiation position dependence revealed that the effect of light irradiation is much stronger near the source region in the channel than near the drain region. This can be explained by the enhanced carrier injection from the source electrode. The irradiation near the drain region, on the other hand, simply induced photocurrent, which is much smaller than the carrier injection on the source side. Therefore, completely transparent ZnO TFTs under visible light irradiation will be obtained, if the carrier injection from the source electrode is successfully suppressed.

Extraction of Trap Densities in ZnO Thin-Film Transistors and Dependence on Oxygen Partial Pressure During Sputtering of ZnO Films

Trap densities in the channel layers <i>Dt</i> of ZnO thin-film transistors have been extracted. First, the low-frequency (low-f) capacitance-voltage <i>C</i>-<i>V</i> characteristics are measured using the customized measurement system. Next, the surface potential is calculated from the low-f <i>C</i>-<i>V</i> characteristic, and the surface potential gradient is calculated by applying Gausss law. Finally, the spatial profile of the electric potential is calculated by applying Poisson equation and carrier density equations, and <i>Dt</i> is extracted. It is found that, generally, the deep states are flatly distributed in the energy gap apart from the conduction band, and the shallow states seem to be the tail states. Moreover, the dependence on the oxygen partial pressure during the sputtering of ZnO films [<i>p</i>(O₂)] has been analyzed. It is found that for <i>p</i>(O₂) = 0.50 ~ 0.75 Pa, <i>Dt</i> changes little, whereas for <i>p</i>(O₂) = 0.17 ~ 0.33 Pa, <i>Dt</i> increases. It is clarified that the abnormal shapes of the current-voltage and low-f <i>C</i>-<i>V</i> characteristics originate from the increase of <i>Dt</i>.

Effects of chemical stoichiometry of channel region on bias instability in ZnO thin-film transistors

We investigated effects of chemical stoichiometry of ZnO channel, controlled by oxygen partial pressure during deposition, on bias instability for ZnO thin-film transistors. Parallel threshold voltage shifts were mainly enhanced under gate bias stresses due to charge trapping when O-rich ZnO was used for channel layer. On the contrary, negative threshold voltage shifts were observed under both gate and drain bias stresses when Zn-rich ZnO was used for channel layer. This degradation was enhanced regardless of the bias polarity and the direction, attributing to electrically activated trap generations.

Positive Bias Instability of Bottom-Gate Zinc Oxide Thin-Film Transistors with a SiOx/SiNx-Stacked Gate Insulator

The positive bias instabilities of the zinc oxide thin-film transistors (ZnO TFTs) with a SiOx/SiNx-stacked gate insulator have been investigated. The film quality of a gate insulator of SiOx, which forms an interface with the ZnO channel, was varied by changing the gas mixture ratio of SiH4/N2O/N2 during plasma-enhanced chemical vapor deposition. The positive bias stress endurance of ZnO TFT strongly depended on the deposition condition of the SiOx gate insulator. From the relaxations of the transfer curve shift after imposition of positive bias stress, transfer curves could not be recovered completely without any thermal annealing. A charge trapping in a gate insulator rather than that in bulk ZnO and its interface with a gate insulator is a dominant instability mechanism of ZnO TFTs under positive bias stress.

Mechanism analysis of photoleakage current in ZnO thin-film transistors using device simulation

We analyzed the photoleakage current (Ileak) in ZnO thin-film transistors using device simulation. The dependences of Ileak on the location of light irradiation and drain voltage are reproduced by considering a Schottky barrier at the source contact using a two-dimensional device simulation. First, carrier generation is induced by light irradiation, the generated holes accumulate near the source contact, and some of these are captured in the donor traps. Next, the Schottky barrier becomes narrow, and electron injection increases via a tunneling effect. This discussion also suggests that the off-current is exceedingly low because the Schottky barrier prevents electron injection.