Origin of conductive surface layer in annealed ZnO

Look, David C.; Claflin, B.; Smith, Helen

doi:10.1063/1.2903505

Cited by 57 publications

(41 citation statements)

References 14 publications

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“…The N d surf term describes the sharp N d eff increase within the outer few nanometers due to H and perhaps to impurity segregation from the bulk. 19 DLTS of both Zn-and O-face diodes reveal two dominant traps, including the well-known bulk traps E 3 ͑0.27 eV͒ and E 4 ͑0.49͒. 20,21 However, a surface-related trap, E s ͑Ͻ ϳ 95 nm deep, 0.49 eV͒ for the Pd-/Zn-face SBDs, made on 1 h ROP-treated ZnO, was not observed on the SBDs in this study.…”

Section: Figmentioning

confidence: 47%

Zn- and O-face polarity effects at ZnO surfaces and metal interfaces

Dong

Fang

Look

et al. 2008

Applied Physics Letters

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Depth-resolved cathodoluminescence spectroscopy, current-voltage, capacitance-voltage, and deep level transient spectroscopy of ZnO (0001) Zn- and (0001¯) O-polar surfaces and metal interfaces show systematically higher Zn-face near band edge emission and lower near-surface defect emission. Even with remote plasma decreases of the 2.5 eV near-surface defect emission, (0001)-Zn face emission quality still exceeds that of (0001¯)-O face. Ultrahigh vacuum-deposited Au and Pd diodes on as-received and O2/He plasma-cleaned surfaces display a strong polarity dependence that correlates with defect emissions, traps, and interface chemistry. A comprehensive model accounts for the polarity-dependent transport properties and their correlations with carrier concentration profiles.

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Section: Figmentioning

confidence: 47%

Zn- and O-face polarity effects at ZnO surfaces and metal interfaces

Dong

Fang

Look

et al. 2008

Applied Physics Letters

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“…Surface electron accumulation layers are known to have a pronounced effect on the measured conductivity and its temperature dependence [139,140,146,147], and so such a surface contribution cannot be ignored when considering conductivity in TCOs. Recent measurements also indicate that an accumulation of electrons can also be induced at the surfaces of other, more complex, oxides such as SrTiO 3 , indicating that this phenomenon may indeed be rather general across the oxide semiconductors [148,149].…”

Section: Surface Conductivitymentioning

confidence: 99%

Conductivity in transparent oxide semiconductors

King

Veal

2011

J. Phys.: Condens. Matter

225

180

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Abstract. Despite an extensive research effort for over 60 years, an understanding of the origins of conductivity in wide band-gap transparent conducting oxide (TCO) semiconductors remains elusive. While TCOs have already found widespread use in device applications requiring a transparent contact, there are currently enormous efforts to (i) increase the conductivity of existing materials, (ii) identify suitable alternatives, and (iii) attempt to gain semiconductor-engineering levels of control over their carrier density, essential for the incorporation of TCOs into a new generation of multifunctional transparent electronic devices. These efforts, however, are dependent on a microscopic identification of the defects and impurities leading to the high unintentional carrier densities present in these materials. Here, we review recent developments towards such an understanding. While oxygen vacancies are commonly assumed to be the source of the conductivity, there is increasing evidence that this is not a sufficient mechanism to explain the total measured carrier concentrations. In fact, many studies suggest that oxygen vacancies are deep, rather than shallow, donors, and their abundance in as-grown material is also debated. We discuss other potential contributions to the conductivity in TCOs, including other native defects, their complexes, and in particular hydrogen impurities. Convincing theoretical and experimental evidence is presented for the donor nature of hydrogen across a range of TCO materials, and while its stability and the presence of interstitial and substitutional species are still somewhat open questions, it is one of the leading contenders for unintentional conductivity in TCOs. We also review recent work indicating that the surfaces of TCOs can support very high carrier densities, opposite to the case of conventional semiconductors. In thin-film materials/devices and, in particular, nanostructures, the surface can have a large impact on the total conductivity in TCOs. We discuss models that attempt to explain both the bulk and surface conductivity based on features of the bulk band structure common across the TCOs, and compare these materials to other semiconductors. Finally, we briefly consider transparency in these materials, and its interplay with conductivity. Understanding this interplay, as well as the microscopic contenders for the conductivity of these materials, will prove essential to the future design and control of TCO semiconductors, and their implementation into novel multifunctional devices.

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“…This represents the lowest surface concentration for any ZnO sample that we have ever measured. Additionally, this low value for n sh,max indicates that no significant surface segregation of group III impurities, such as Al, Ga, or In, has occurred as has been observed 15 recently in thermally annealed samples. It is interesting to note that the two-layer model fit of the PPCrelaxed sample shows an increase in the concentrations of both the 50 and 400 meV donors as well as the acceptor concentration as compared to the as-received sample ͑see Table I͒.…”

Section: Resultsmentioning

confidence: 99%

“…[4][5][6][7] Changes in the electrical conductivity by several orders of magnitude have been attributed 3,[8][9][10][11] to surface accumulation or depletion layers produced by adsorbed gases such as H or O and can be enhanced 12,13 by UV light irradiation. More recent work [14][15][16][17] has demonstrated that the origin of the surface conductive layer in ZnO is more complicated than the simple adsorption model and has focused on the development of quantitative tools to analyze the surface donors and acceptors. In this work, we investigate changes in the surface conduction of hydrothermal ZnO which are produced by light exposure in vacuum and demonstrate that irradiation with UV light can effectively and permanently remove the conductive surface layer that is observed in almost all commercial ZnO samples.…”

Section: Introductionmentioning

confidence: 99%

UV light-induced changes to the surface conduction in hydrothermal ZnO

Claflin¹

2009

Journal of Vacuum Science &Amp; Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena

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High quality, bulk ZnO crystals grown by Tokyo Denpa using the hydrothermal process typically exhibit a room temperature carrier concentration in the 1013–1014 cm−3 range and a low mobility, conductive surface layer, observed at low temperature, with a sheet concentration on the order of 1012–1013 cm−2. In the sample discussed here, bulk conduction is controlled by two donor levels at 50 and 400 meV with concentrations of 1.2×1016 and 1.5×1016 cm−3, respectively. Temperature-dependent photo-Hall-effect measurements, using blue/UV light, in vacuum show an increase in the surface sheet carrier density to more than 1×1013 cm−2 at low to intermediate temperatures while the two bulk donors continue to dominate the high temperature behavior, up to 400 K. Long-lived persistent photoconductivity (PPC) is observed when the sample is returned to the dark. When the PPC is allowed to fully relax and the sample is exposed to air, there is surprisingly no longer any surface conduction at low temperature, while the two bulk donors remain unaffected. In this state, the 50 meV bulk donor level is observed to control the conduction over five orders of magnitude, down to a carrier concentration of 3.0×108 cm−3. This corresponds to an upper limit for the surface sheet carrier density of 1.6×107 cm−2. This is the lowest surface concentration we have ever observed in any ZnO sample and demonstrates that blue/UV light irradiation, in vacuum, at moderate temperatures is very effective at cleaning the surface. A subsequent 30 min anneal at 600 °C in forming gas (5% H2 in N2) increases the carrier concentration by almost two orders of magnitude. The forming gas anneal produces no changes in the concentrations of the 50 and 400 meV bulk donor levels and no new bulk donors are observed. However, the bulk acceptor concentration decreases from 2×1016 to 1.2×1016 cm−3, most likely as a result of passivation by hydrogen.

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Origin of conductive surface layer in annealed ZnO

Cited by 57 publications

References 14 publications

Zn- and O-face polarity effects at ZnO surfaces and metal interfaces

Zn- and O-face polarity effects at ZnO surfaces and metal interfaces

Conductivity in transparent oxide semiconductors

UV light-induced changes to the surface conduction in hydrothermal ZnO

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