Effects of Surface Conduction on Hall-Effect Measurements in ZnO

Conduction processes in as-grown and annealed n-type ZnO crystals have been studied by means of Hall-effect measurements and X-ray photoelectron spectroscopy (XPS). For temperatures above approximately 50 K, the conductance is dominated by electrons in the bulk of the crystal, whereas below that temperature, surface conductance becomes more important. As the crystals are annealed in flowing N2 gas, the surface sheet carrier concentration increases, and concomitantly, the OH-bond peak in the XPS spectrum becomes a larger fraction of the total O peak. Conversely, either O2-plasma annealing or electron-beam irradiation produces a decrease in the surface conductance and the OH fraction. Thus, H-related donors are likely the cause of the surface conductance

Effects of Surface Conduction on Hall-Effect Measurements in ZnO

Conduction processes in as-grown and annealed n-type ZnO crystals have been studied by means of Hall-effect measurements and X-ray photoelectron spectroscopy (XPS). For temperatures above approximately 50 K, the conductance is dominated by electrons in the bulk of the crystal, whereas below that temperature, surface conductance becomes more important. As the crystals are annealed in flowing N2 gas, the surface sheet carrier concentration increases, and concomitantly, the OH-bond peak in the XPS spectrum becomes a larger fraction of the total O peak. Conversely, either O2-plasma annealing or electron-beam irradiation produces a decrease in the surface conductance and the OH fraction. Thus, H-related donors are likely the cause of the surface conductance

Nanoscale Depth-Resolved Cathodoluminescence Spectroscopy of ZnO Surfaces and Metal Interfaces

The electronic properties of ZnO surfaces and interfaces has until recently been relatively unexplored. We have used a complement of ultrahigh vacuum scanning electron microscope (SEM)-based, depth-resolved cathodoluminescence spectroscopy (DRCLS), temperature-dependent charge transport, trap spectroscopy, and surface science techniques to probe the electronic and chemical properties of clean surfaces and interfaces on a nanometer scale. DRCLS reveals remarkable nanoscale correlations of native point defect distributions with surface and sub-surface defects calibrated with capacitance trap spectroscopies, atomic force microscopy, and Kelvin probe force microscopy. The measurement of these near-surface states associated with native point defects in the ZnO bulk and those induced by interface chemical bonding is a powerful extension of cathodoluminescence spectroscopy that provides a guide to understanding and controlling ZnO electronic contacts

Nanoscale Depth-Resolved Cathodoluminescence Spectroscopy of ZnO Surfaces and Metal Interfaces

The electronic properties of ZnO surfaces and interfaces has until recently been relatively unexplored. We have used a complement of ultrahigh vacuum scanning electron microscope (SEM)-based, depth-resolved cathodoluminescence spectroscopy (DRCLS), temperature-dependent charge transport, trap spectroscopy, and surface science techniques to probe the electronic and chemical properties of clean surfaces and interfaces on a nanometer scale. DRCLS reveals remarkable nanoscale correlations of native point defect distributions with surface and sub-surface defects calibrated with capacitance trap spectroscopies, atomic force microscopy, and Kelvin probe force microscopy. The measurement of these near-surface states associated with native point defects in the ZnO bulk and those induced by interface chemical bonding is a powerful extension of cathodoluminescence spectroscopy that provides a guide to understanding and controlling ZnO electronic contacts

Remote Hydrogen Plasma Doping of Single Crystal ZnO

We demonstrate that remote plasma hydrogenation can increase electron concentrations in ZnO single crystals by more than an order of magnitude. We investigated the effects of this treatment on Hall concentration and mobility as well as on the bound exciton emission peak I4 for a variety of ZnO single crystals–bulk air annealed, Li doped, and epitaxially grown on sapphire. Hydrogen increases I4 intensity in conducting samples annealed at 500 and 600 °C and partially restores emission in the I4 range for Li-diffused ZnO. Hydrogenation increases carrier concentration significantly for the semi-insulating Li doped and epitaxial thin film samples. These results indicate a strong link between the incorporation of hydrogen, increased donor-bound exciton PL emission, and increased n-type conductivity

Surface and Near-Surface Passivation, Chemical Reaction, and Schottky Barrier formation at ZnO Surfaces and Interfaces

Using a combination of depth-resolved cathodoluminescence spectroscopy, electronic transport, and surface science techniques, we have demonstrated the primary role of native defects within ZnO single crystals as well as native defects created by metallization on metal - ZnO Schottky barrier heights and their ideality factors. Native defects and impurities resident within the ZnO depletion region as well as defects extending into the bulk from the intimate metal - ZnO interface contribute to barrier thinning of, carrier hopping across, and tunneling through these Schottky barriers. Chemical reactions at clean ZnO metal interfaces lead to metal-specific eutectic or oxide formation with pronounced transport effects. These results highlight the importance of bulk crystal quality, surface cleaning, metal interaction, and post-metallization annealing for controlling Schottky barriers. (C) 2008 Elsevier B. V. All rights reserved

Surface and Near-Surface Passivation, Chemical Reaction, and Schottky Barrier formation at ZnO Surfaces and Interfaces

Using a combination of depth-resolved cathodoluminescence spectroscopy, electronic transport, and surface science techniques, we have demonstrated the primary role of native defects within ZnO single crystals as well as native defects created by metallization on metal - ZnO Schottky barrier heights and their ideality factors. Native defects and impurities resident within the ZnO depletion region as well as defects extending into the bulk from the intimate metal - ZnO interface contribute to barrier thinning of, carrier hopping across, and tunneling through these Schottky barriers. Chemical reactions at clean ZnO metal interfaces lead to metal-specific eutectic or oxide formation with pronounced transport effects. These results highlight the importance of bulk crystal quality, surface cleaning, metal interaction, and post-metallization annealing for controlling Schottky barriers. (C) 2008 Elsevier B. V. All rights reserved