Electron scattering mechanisms in GZO films grown on a-sapphire substrates by plasma-enhanced molecular beam epitaxy

Authors

H. Y. Liu, Virginia Commonwealth University
V. Avrutin, Virginia Commonwealth UniversityFollow
N. Izyumskaya, Virginia Commonwealth UniversityFollow
Ü. Özgür, Virginia Commonwealth UniversityFollow
A. B. Yankovich, University of Wisconsin-Madison
A. V. Kvit, University of Wisconsin-Madison
P. M. Voyles, University of Wisconsin-Madison
H. Morkoç, Virginia Commonwealth UniversityFollow

Document Type

Article

Original Publication Date

2012

Journal/Book/Conference Title

Journal of Applied Physics

Volume

111

Issue

10

DOI of Original Publication

10.1063/1.4720456

Comments

Originally published at http://dx.doi.org/10.1063/1.4720456

Date of Submission

October 2015

Abstract

We report on the mechanisms governing electron transport using a comprehensive set of ZnO layers heavily doped with Ga (GZO) grown by plasma-enhanced molecular-beam epitaxy on a-plane sapphire substrates with varying oxygen-to-metal ratios and Ga fluxes. The analyses were conducted by temperature dependent Hall measurements which were supported by microstructural investigations as well. Highly degenerate GZO layers with n > 5 × 10²⁰ cm⁻³ grown under metal-rich conditions (reactive oxygen-to-metal ratio c-direction. For highly conductive GZO layers, ionized-impurity scattering with almost no compensation is the dominant mechanism limiting the mobility in the temperature range from 15 to 330 K and the grain-boundary scattering governed by quantum-mechanical tunnelling is negligible. However, due to the polar nature of ZnO having high crystalline quality, polar optical phonon scattering cannot be neglected for temperatures above 150 K, because it further reduces mobility although its effect is still substantially weaker than the ionized impurity scattering even at room temperature (RT). Analysis of transport measurements and sample microstructures by x-ray diffraction and transmission electron microscopy led to a correlation between the grain sizes in these layers and mobility even for samples with a carrier concentration in the upper 10²⁰ cm⁻³ range. In contrast, electron transport in GZO layers grown under oxygen-rich conditions (reactive oxygen-to-metal ratio >1), which have inclined grain boundaries and relatively smaller grain sizes of 10–20 nm by x-ray diffraction, is mainly limited by compensation caused by acceptor-type point-defect complexes, presumably (Ga_Zn-V_Zn), and scattering on grain boundaries. The GZO layers with n <10²⁰ cm⁻³grown under metal-rich conditions with reduced Ga fluxes show a clear signature of grain-boundary scattering governed by the thermionic effect in the temperature-dependent mobility but with much higher RT mobility values compared to the samples grown under oxygen-rich conditions [34 vs. 7.5 cm²/V·s]. Properties of GZO layers grown under different conditions clearly indicate that to achieve highly conductive GZO, metal-rich conditions instead of oxygen-rich conditions have to be used.

Rights

Liu, H. Y., Avrutin, V., & Izyumskaya, N., et al. Electron scattering mechanisms in GZO films grown on a-sapphire substrates by plasma-enhanced molecular beam epitaxy. Journal of Applied Physics, 111, 103713 (2012). Copyright © 2012 American Institute of Physics.

Is Part Of

VCU Electrical and Computer Engineering Publications