Metallic conductivity and metal-semiconductor transition in Ga-doped ZnO

Bhosle, V.; Tiwari, Alok; Narayan, J.

doi:10.1063/1.2165281

Cited by 268 publications

(146 citation statements)

References 16 publications

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“…Recently, Gadoped ZnO achieved low sheet resistance and high transmittance, which is close to ITO and FTO. [13,14] Hence, both ZnO nanostructures and GZO films are promising materials for DSSCs. If ZnO nanostructures and GZO films can be sequentially grown on low-cost substrates at low growth temperature, it provides a low cost and high efficient photoelectrode for DSSC applications.…”

Section: Motivationmentioning

confidence: 99%

Dye-sensitized solar cells using ZnO nanotips and Ga-doped ZnO films

Chen

Pasquier

Saraf

et al. 2008

Semicond. Sci. Technol.

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Dye sensitized solar cell (DSSC) is a promising and low-cost photovoltaic device. In the DSSC, a monolayer of dye is anchored on the surface of a semiconductor oxide.Photoexcitation takes place inside the dye, and photogenerated charges are then separated at the dye/oxide interface. Nanostructured oxide films are particularly attractive for DSSCs as they provide large surface area for dye anchoring. The crystalline quality of oxide films has significant impact on the charge transport. It is important to reduce the charge traps in the oxide to speed up the charge transport. In the DSSC, the transparent conducting oxide serves as an optical window, which determines the amount of light entering the device, and as the electrode, which extracts photocurrent. In the cell design, the selection of semiconductor oxide and corresponding transparent electrode is critical to achieve efficient light harvesting, charge separation and extraction.ii In this dissertation, a novel photoelectrode, consisting of well-aligned ZnO nanotips and a Ga-doped ZnO (GZO) TCO film, is developed for DSSC applications. The n-type semiconductor ZnO nanotips provide large surface area for dye anchoring in conjunction with direct conduction pathways for charge transport, while the GZO film acts as the transparent electrode. ZnO nanotips and GZO films are grown by metalorganic chemical vapor deposition (MOCVD). GZO films (~400 nm) with sheet resistance of ~ 25 Ω/sq and transmittance over ~ 85% are achieved. ZnO nanotips have single crystalline quality and show free exciton emissions at room temperature. Photoelectrochemical cells are fabricated using liquid redox electrolyte and semi-solid gel electrolyte, respectively. UV light harvesting, which is directly generated by ZnO photoelectrode, is observed. The DSSC using liquid electrolyte exhibits a quantum efficiency at ~530nm of 65% and power conversion efficiency of 0.77%. By replacing the liquid electrolyte with gel electrolyte in the DSSC with the same structure, the open circuit voltage is increased from 610 mV to 726 mV and the overall power efficiency is increased to 0.89%. The aging testing shows that the DSSC using gel electrolyte has better stability than its liquid electrolyte counterpart.iii

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Section: Motivationmentioning

confidence: 99%

Dye-sensitized solar cells using ZnO nanotips and Ga-doped ZnO films

Chen

Pasquier

Saraf

et al. 2008

Semicond. Sci. Technol.

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“…This crossover from metal to insulator at a specific temperature has been observed in various oxide films [7,29,[38][39][40], presenting both a high carrier concentration leading to a degenerate semiconductor (metallic behavior) and a sufficient structural disorder leading to multiple scattering of the carriers [41]. The increase of the resistivity below the MIT critical temperature can be interpreted in the frame of the quantum corrections to the conductivity (QCC) to the semiclassical Boltzmann approach [41,42].…”

Section: Resultsmentioning

confidence: 99%

Transparent conductive Nd-doped ZnO thin films

Nistor¹,

Millon²,

Cachoncinlle³

et al. 2015

J. Phys. D: Appl. Phys.

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is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. Any correspondence concerning this service should be sent to the repository Administrator : archiveouverte@ensam.eu IntroductionDue to its specific electrical and optical properties (high electrical conductivity and high optical transparency in the visible domain), zinc oxide (ZnO) is a suitable material with applications in a large number of devices and components such as optical waveguides, piezoelectric transducers, transparent conductive electrodes, gas sensors, light emitting diodes, and others [1][2][3][4][5][6]. Moreover, because it is abundant and environmentally friendly, ZnO is an ideal wide bandgap material for applications in photovoltaics (PV). The ability to easily tune the conductivity, charge carrier concentration, and optical properties of ZnO films is highly desirable for these applications. Doping by well-suited elements is a way to modify or enhance physical properties or to induce new ones in oxide films [5][6][7][8][9]. In this frame, the best doping of ZnO thin films for both electrical conductivity and optical transparency comparable with those of the indium-tin oxide ( AbstractTransparent Nd-doped ZnO films with thickness in the range of 70 to 250 nm were grown by pulsed-laser deposition (PLD) on c-cut sapphire substrates at various oxygen pressures and substrate temperatures. A wide range of optical and electrical properties of the films were obtained and correlated to the composition and crystalline structure. The Nd-doped ZnO films are smooth, dense, and display the wurtzite phase. Different epitaxial relationships between films and substrate as a function of growth pressure and substrate temperature were evidenced by asymmetric x-ray diffraction measurements. By varying PLD growth conditions, the films can be tuned to have either metallic or semiconductor characteristics, with good optical transmittance in the visible range. Moreover, a low-temperature metal-insulator transition may be observed in Nd-doped ZnO films grown under low oxygen pressure. Resistivities as low as 6 × 10 −4 Ω cm and 90% optical transmittance in the visible range and different nearinfrared transmittance are obtained with approximately 1.0-1.5 at.% Nd doping and growth temperature of approximately 500 °C.

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“…For V TG = -20V, µ sat increases from 6.4 to 7.5 cm 2 /V·s with increasing the temperature from 25 to 70 o C. The increment of saturation mobility with increasing temperature in SG a-IGZO TFTs is explained as the thermal activation process with delocalization of free carriers from conduction band tail states. 14,15 In case of positive V TG = +20V, the mobility decreases from 19.1 to 15.4 cm 2 /V·s, with increasing the temperature from 25 to 70 o C. The negative temperature coefficients are commonly observed in metal or degenerated semiconductors when E F > E C . To find the activation energy (E a ) for various top gate biases, we plotted the saturation mobility vs. T −1 in Fig.…”

Section: -4mentioning

confidence: 99%

“…This can be explained by carrier scattering with phonons under the condition that Fermi level is positioned above the conduction band (E F > E C ). 14,15 Such temperature dependent mobility under various TG biases (V TG 's) are well understood as the transition of semiconducting to metallic conduction behavior in a-IGZO TFTs.…”

Section: Introductionmentioning

confidence: 99%

Semiconductor to metallic transition in bulk accumulated amorphous indium-gallium-zinc-oxide dual gate thin-film transistor

2015

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We investigated the effects of top gate voltage (VTG) and temperature (in the range of 25 to 70 oC) on dual-gate (DG) back-channel-etched (BCE) amorphous-indium-gallium-zinc-oxide (a-IGZO) thin film transistors (TFTs) characteristics. The increment of VTG from -20V to +20V, decreases the threshold voltage (VTH) from 19.6V to 3.8V and increases the electron density to 8.8 x 1018cm−3. Temperature dependent field-effect mobility in saturation regime, extracted from bottom gate sweep, show a critical dependency on VTG. At VTG of 20V, the mobility decreases from 19.1 to 15.4 cm2/V ⋅ s with increasing temperature, showing a metallic conduction. On the other hand, at VTG of - 20V, the mobility increases from 6.4 to 7.5cm2/V ⋅ s with increasing temperature. Since the top gate bias controls the position of Fermi level, the temperature dependent mobility shows metallic conduction when the Fermi level is above the conduction band edge, by applying high positive bias to the top gate.

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Metallic conductivity and metal-semiconductor transition in Ga-doped ZnO

Cited by 268 publications

References 16 publications

Dye-sensitized solar cells using ZnO nanotips and Ga-doped ZnO films

Dye-sensitized solar cells using ZnO nanotips and Ga-doped ZnO films

Transparent conductive Nd-doped ZnO thin films

Semiconductor to metallic transition in bulk accumulated amorphous indium-gallium-zinc-oxide dual gate thin-film transistor

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