High electron mobility of epitaxial ZnO thin films on c-plane sapphire grown by multistep pulsed-laser deposition

Abstract

A multistep pulsed-laser deposition (PLD) process is presented for epitaxial, nominally undoped ZnO thin films of total thickness of 1 to 2 μm on c-plane sapphire substrates. We obtain reproducibly high electron mobilities from 115 up to 155 cm2/V s at 300 K in a narrow carrier concentration range from 2 to 5×1016 cm−3. The key issue of the multistep PLD process is the insertion of 30-nm-thin ZnO relaxation layers deposited at reduced substrate temperature. The high-mobility samples show atomically flat surface structure with grain size of about 0.5–1 μm, whereas the surfaces of low-mobility films consist of clearly resolved hexagonally faceted columnar grains of only 200-nm size, as shown by atomic force microscopy. Structurally optimized PLD ZnO thin films show narrow high-resolution x-ray diffraction peak widths of the ZnO(0002) ω- and 2Θ-scans as low as 151 and 43 arcsec, respectively, and narrow photoluminescence linewidths of donor-bound excitons of 1.7 meV at 2 K.

Weak localization and mobility in ZnO nanostructures

“…We report a Hall mobility of 103±5 cm 2 V s for 0 V gate bias at 275 K. By increasing gate bias to +30 V at 275 K, the mobility increases to 131 cm 2 V s . These values are comparable to those reported in the literature for ZnO thin films grown on sapphire by PLD 49 and MBE, 50,51 but are an order of magnitude larger than those reported on ZnO nanowires, 21,34 with the exception of nanowires with surface passivation treatments that have field-effect mobilities above 1000 cm 2 V s . 22 As temperature decreases, electron mobility first increases due to reduced electron-phonon scattering, reaches a peak, and then decreases to the lowest temperature we measured, 1.9 K, where we report a mobility of 31±10 cm 2 V s at 0 V gate bias and 92±2 cm 2 V s at +30 V gate bias.…”

Contrasting transparent conductive properties of ZnO films on amorphous and crystalline substrates in view of thickness dependence

“…For this purpose, sapphire c-plane substrates offer an opportunity to investigate the effect of reducing the thickness of the transitional region. The epitaxial characteristics of ZnO films on sapphire substrate have been well documented, [8][9][10][11][12][13][14][15] because growing high quality crystals is the main concern with regard to creation of light emission devices comprised of p/n junctions. However, only a few papers have reported on transparent conductive ZnO films on epitaxial substrates.…”

Facile construction of nanofibrous ZnO photoelectrode for dye-sensitized solar cell applications

“…2 In addition, ZnO is widely available with high electron mobility. 3 Up to date, the highest efficiency for ZnO-based DSSCs is reported to be 6.58%, 4 which is lower than that for devices based on TiO 2 photoelectrodes. This is due to the poor chemical stability of ZnO in acidic dye solution and the formation of Zn 2+ /dye complexes that could block the injection of electrons from the dye molecules to the semiconducting electrodes.…”