Direct measurement of density-of-states effective mass and scattering parameter in transparent conducting oxides using second-order transport phenomena

Young, David L.; Coutts, T. J.; Kaydanov, V.; Gilmore, A.; Mulligan, William P.

doi:10.1116/1.1290372

Cited by 108 publications

(60 citation statements)

References 10 publications

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“…9 For TiO 2 , the deviation from linearity for f l Ͼ 1 J/m 2 suggests that the effective mass in this material is density dependent: these deviations occur for N av Ͼ 1 ϫ 10 25 m −3 ͑ p Ͼ 70 THz͒, 19 similar to zinc oxide. 22 For Si, band calculations 3 indicate that the electron effective mass is constant over the density range investigated here, partly due to the sixfold degenerate band structure in Si.…”

Section: Using 17mentioning

confidence: 99%

Reduction of carrier mobility in semiconductors caused by charge-charge interactions

et al. 2007

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We investigate the effect of charge-charge interactions on carrier mobility in titanium dioxide ͑TiO 2 ͒ and silicon ͑Si͒ using terahertz spectroscopy. Charge scattering times and plasma frequencies are directly determined as a function of charge density. In Si, a linear increase in scattering rate for densities exceeding 10 21 m −3 is attributed to electron-hole scattering. In contrast, in TiO 2 , charge-charge interactions are suppressed due to dielectric screening, highlighting the vastly different dielectric properties for these two materials. DOI: 10.1103/PhysRevB.75.233202 PACS number͑s͒: 72.40.ϩw, 72.10.Di, 72.20.Dp Charge-charge interactions determine many of the properties of highly doped or photoexcited semiconductors. Carrier-carrier collisions, for instance, can strongly affect the conductivity, and the decrease of electron and hole drift mobilities with increasing charge carrier densities is an important factor in determining the current-voltage characteristics of, for example, semiconductor diodes at increased current densities 1 and solar cells under strong illumination conditions. 2 Charge mobility depends on two quantities: its effective mass and scattering time ͑i.e., the average time in-between momentum-randomizing events, due to, e.g., electronphonon scattering͒. Theory has indicated that both of these quantities can be strongly dependent on the free charge density in semiconductors. [3][4][5] Experimental investigations of carrier-carrier interactions are, however, intrinsically more complex, as most experimental techniques ͑such as optical reflectivity measurements 6,7 ͒ cannot easily distinguish between changes in scattering time and effective mass. Terahertz time-domain spectroscopy ͑THz-TDS͒, with its ability to determine both the real and imaginary components of the complex conductivity, 8 allows us to differentiate between density-dependent changes in effective mass from changes in scattering time.Here, we compare the high-density photoconductivity of single crystal silicon ͑Si͒ and rutile-type titanium dioxide ͑TiO 2 ͒, two technologically relevant materials with low and high dielectric functions, respectively. By photoexciting with intense, femtosecond ultraviolet laser pulses and probing with THz pulses, we can generate and measure high-density plasmas in semiconductors on ultrafast time scales, accessing plasma densities spanning 5 orders of magnitude from 10 21 to 10 26 m −3 . In Si, we find that the scattering rate increases linearly for densities Ͼ10 21 m −3 , an effect characteristic of direct charge-charge scattering. For TiO 2 , the variation of the scattering rate with density is much weaker and is caused by band filling effects. The difference between these two materials is shown to originate from their vastly different dielectric screening properties.The TiO 2 ͑1 mm thick 001 rutile crystal grown commercially by the Verneuil method by Crystal-GmbH͒ and silicon ͑0.5 mm thick undoped Si wafer, grown by the Czochralski method with estimated impurity density Ͻ10 13 cm −3 ,...

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Section: Using 17mentioning

confidence: 99%

Reduction of carrier mobility in semiconductors caused by charge-charge interactions

et al. 2007

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“…Ionized impurity scattering is usually considered as the limiting factor for heavily doped ZnO. 5,6,18 But, as reviewed by Ellmer,5 several other mechanisms could explain this low mobility value, such as formation of impurity clusters, higher charge states of ionized donors ͑due to self-doping by oxygen vacancies͒, or extrinsic dopants on interstitial sites. Further investigations, such as temperature dependence of mobility measurements, are needed to gain insight into this phenomenon.…”

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confidence: 99%

Transition between grain boundary and intragrain scattering transport mechanisms in boron-doped zinc oxide thin films

Steinhauser

Faÿ

Oliveira

et al. 2007

Applied Physics Letters

236

127

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A comprehensive model for the electronic transport in polycrystalline ZnO:B thin films grown by low pressure chemical vapor deposition is presented. The optical mobilities and carrier concentration calculated from reflectance spectra using the Drude model were compared with the data obtained by Hall measurements. By analyzing the results for samples with large variation of grain size and doping level, the respective influences on the transport of potential barriers at grain boundaries and intragrain scattering could be separated unambiguously. A continuous transition from grain boundary scattering to intragrain scattering is observed for doping level increasing from 3 ϫ 10 19 to 2 ϫ 10 20 cm −3Transparent conductive oxides ͑TCOs͒ are an essential part of thin-film silicon solar cells. To contact the cell and act as a transparent window, TCOs have to exhibit a high conductivity and a high optical transmittance. In addition, they have to scatter the light at the TCO-cell interface in order to increase the effective absorption of light within the active layers.1,2 Boron-doped zinc oxide ͑ZnO:B͒ layers deposited by low pressure chemical vapour deposition ͑LPCVD͒ have been intensively developed in our institute.3 This material is especially attractive for thin-film solar cell technology, because of its low cost, and of the wide availability of its constituent raw materials. Furthermore, the LPCVD technique is well suited for large-scale device fabrication. 4 These films are constituted of large grains with a pronounced 9-18 preferential crystallographic direction. The extremities of the grains appear at the growing surface as large pyramids, which yield an as-grown rough surface texture that efficiently diffuses the light that enters into the solar cell. 1,3The optical and electrical properties of TCO films have been extensively investigated and recently reviewed. Ellmer 5 and Minami 6 discussed the limit of the resistivity of such films by analyzing reported data for ZnO films deposited by different techniques. They found that the electron mobility in undoped films is mainly limited by grain boundary scattering, whereas for doped layers intragrain scattering mechanism is predominant. But the transition between these two mechanisms within a given ZnO film series could not be evidenced yet. In the present work, the scattering mechanism limiting the electron mobility is determined by the comparison, of the value of the optical mobility ͑as evaluated by using the classical Drude model͒ with the value of the Hall mobility. When both values are markedly different, the mechanism limiting the electron mobility is attributed to grain boundary scattering. When both values are similar, the limiting mechanism is attributed to intragrain scattering. This approach is commonly applied to other TCOs. [7][8][9][10][11] The wide range of carrier density easily achievable in boron-doped LPCVD ZnO by varying the gas flow ratio allows us to observe within one single film system the transition from one transport mechanism to the other. T...

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“…In narrow-gap, direct-gap semiconductors, such as InSb, (Hg,Cd)Te, PbSe, (Pb,Sn)Te, non-parabolicity significantly affects electronic transport properties at Fermi levels of only E F ∼ 0.1eV, or even lower. For wide-gap semiconductors, such as TCOs, non-parabolicity has not been much investigated but for heavily doped ZnO, CdO, and zinc stannate it has been established that there is a progressive increase of the density-of-states effective mass with carrier concentration [46][47][48].…”

Section: Influence Of Non-parabolicitymentioning

confidence: 99%

“…The four-coefficient instrument created at NREL has already been described and discussed in detail in [45,47]. Here we present only a brief review of its principal features and operation.…”

Section: Measurement Techniques and Experimental Proceduresmentioning

confidence: 99%

“…Dr. Mulligan created the first system that provided measurements of the four coefficients in typical TCO thin films and performed preliminary experiments on cadmium stannate [44]. Dr. Young has developed new, more perfect and sophisticated measurement equipment and performed systematic comparative studies of a variety of TCO thin polycrystalline film materials [45][46][47][48].…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Process Development and Basic Studies of Electrochemically Deposited CdTe-Based Solar Cells; Annual Technical Report, Phase II, 16 May 1999-13 May 2000

Kaydanov¹,

Ohno

2001

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Direct measurement of density-of-states effective mass and scattering parameter in transparent conducting oxides using second-order transport phenomena

Cited by 108 publications

References 10 publications

Reduction of carrier mobility in semiconductors caused by charge-charge interactions

Reduction of carrier mobility in semiconductors caused by charge-charge interactions

Transition between grain boundary and intragrain scattering transport mechanisms in boron-doped zinc oxide thin films

Process Development and Basic Studies of Electrochemically Deposited CdTe-Based Solar Cells; Annual Technical Report, Phase II, 16 May 1999-13 May 2000

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