Surface and Near Surface Area Density of States for Magnetron-Sputtered ZnO and Al-ZnO: A MIES, UPS, and VBXPS Study Investigating Ultrahigh Vacuum Sputter Cleaning and UV Oxygen Plasma

Schmerl, Natalya M.; Gentle, Angus; Quinton, Jamie S.; Smith, G.B.; Andersson, Gunther G.

doi:10.1021/acs.jpcc.5b11631

Cited by 8 publications

(4 citation statements)

References 65 publications

(142 reference statements)

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“…III B 1, due to our slightly lower formation energies and usage of the experimental value of H (ZnO), our calculated E f (V O ) in O-poor conditions are significantly lower than those of other DFT studies, at about 0.05 eV for PBE0 and B97-2 and 0.19 eV for BB1k (see Table III). Such a low formation energy would indicate high nonstoichiometry in extreme reducing conditions, for which there is some experimental evidence [183][184][185], but measurements to investigate the intrinsic electrical conductivity properties of ZnO tend to be performed under less extreme conditions [36,88,186]. From such a low formation energy, we determine very high [V O ], of the order of 10 22 cm −3 for T > 400 K, using the PBE0 and B97-2 functionals.…”

Section: Charge Carrier and Defect Concentrationsmentioning

confidence: 98%

Deep vs shallow nature of oxygen vacancies and consequentn-type carrier concentrations in transparent conducting oxides

Buckeridge

Catlow

Farrow

et al. 2018

Phys. Rev. Materials

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The source of n-type conductivity in undoped transparent conducting oxides has been a topic of debate for several decades. The point defect of most interest in this respect is the oxygen vacancy, but there are many conflicting reports on the shallow versus deep nature of its related electronic states. Here, using a hybrid quantum mechanical/molecular mechanical embedded cluster approach, we have computed formation and ionization energies of oxygen vacancies in three representative transparent conducting oxides: In 2 O 3 , SnO 2 , and ZnO. We find that, in all three systems, oxygen vacancies form well-localized, compact donors. We demonstrate, however, that such compactness does not preclude the possibility of these states being shallow in nature, by considering the energetic balance between the vacancy binding electrons that are in localized orbitals or in effective-mass-like diffuse orbitals. Our results show that, thermodynamically, oxygen vacancies in bulk In 2 O 3 introduce states above the conduction band minimum that contribute significantly to the observed conductivity properties of undoped samples. For ZnO and SnO 2 , the states are deep, and our calculated ionization energies agree well with thermochemical and optical experiments. Our computed equilibrium defect and carrier concentrations, however, demonstrate that these deep states may nevertheless lead to significant intrinsic n-type conductivity under reducing conditions at elevated temperatures. Our study indicates the importance of oxygen vacancies in relation to intrinsic carrier concentrations not only in In 2 O 3 , but also in SnO 2 and ZnO.

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Section: Charge Carrier and Defect Concentrationsmentioning

confidence: 98%

Deep vs shallow nature of oxygen vacancies and consequentn-type carrier concentrations in transparent conducting oxides

Buckeridge

Catlow

Farrow

et al. 2018

Phys. Rev. Materials

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“…In order to be used as a transparent electrode, AZO films need to have a resistivity in the order of 10 –4 Ω·cm, while being transparent in the visible range. AZO films can be produced by various methods, such as magnetron sputtering, spray pyrolysis, or chemical vapor deposition . Recently, the possibility of depositing highly conductive AZO films close to room temperature (about 45 °C) has been shown using high power impulse magnetron sputtering (HiPIMS) .…”

Section: Introductionmentioning

confidence: 99%

Restoring the Properties of Transparent Al-Doped ZnO Thin Film Electrodes Exposed to Ambient Air

et al. 2017

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The properties of Al-doped ZnO (AZO) films are known to degrade with exposure to humidity. Different AZO films deposited using reactive direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS) have been aged in ambient laboratory conditions and annealed at temperatures between 160 and 180 °C in a N2 atmosphere. Their electrical and optical properties, which have been investigated both ex situ and in situ during the annealing, are improved. The results of the in situ measurements are interpreted in terms of a diffusion process, where hydroxyl groups are decomposed and water is diffusing out of the films. As hydroxyl groups are known to act as a trap for charge carriers in ZnO, their removal from the film can explain the improvement of the electrical properties by the annealing.

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“…The UPS spectra, which measure the DOS of the valence band, show two prominent peaks at 4.5 and 10.5 eV, which are from the O 2p orbitals (lone pair) and Zn 3d band, respectively. 35 In the UV−ozone treated sample, the peaks shift toward higher binding energies. Specifically, the two curves have the same shape, but the treated spectrum is shifted by 0.4 eV compared to the nontreated one.…”

Section: Resultsmentioning

confidence: 98%

“…The UPS spectra of the treated and nontreated ZnO films are plotted in Figure c. The UPS spectra, which measure the DOS of the valence band, show two prominent peaks at 4.5 and 10.5 eV, which are from the O 2p orbitals (lone pair) and Zn 3d band, respectively . In the UV–ozone treated sample, the peaks shift toward higher binding energies.…”

Section: Resultsmentioning

confidence: 99%

UV–Ozone Modified Sol–Gel Processed ZnO for Improved Diketopyrrolopyrrole-Based Hybrid Photodetectors

Pickett

Mohapatra

Ray

et al. 2019

ACS Appl. Electron. Mater.

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Organic–inorganic interfaces in photodiodes have recently gathered significant interest due to the realization of intrinsic p–n junctions and unique physical properties. Nanopatterned sol–gel ZnO films provide an alternate path for fullerene-free organic photodetectors. However, naturally occurring oxygen vacancies in ZnO often act as trap sites and can degrade device performance if left unchecked. Here, we focus on the role of UV–ozone treatment for filling oxygen vacancies in sol–gel processed ZnO for improving the hybrid interface with thienothiophene linked diketopyrrolopyrrole (DPP) films. The ZnO films are characterized by X-ray diffraction, ultraviolet photoelectron spectroscopy (UPS), cross-sectional electron microscope images, and electron energy loss spectroscopy (EELS). UV–ozone treatment shows no change in the crystal structure, but UPS indicates that the treated films are more resistive and have a higher oxygen concentration at the surface. The EELS spectra show gradual passivation of oxygen vacancies within the bulk of the ZnO films. Fullerene-free photodetectors fabricated from ZnO:DPP interfaces show dark currents reduced by half and photoresponsivities nearly doubled, on average, when the ZnO surface is UV–ozone treated compared to nontreated ZnO films, indicating this simple technique to be excellent for improving photodiode performance when ZnO is used as an electron transport layer.

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Surface and Near Surface Area Density of States for Magnetron-Sputtered ZnO and Al-ZnO: A MIES, UPS, and VBXPS Study Investigating Ultrahigh Vacuum Sputter Cleaning and UV Oxygen Plasma

Cited by 8 publications

References 65 publications

Deep vs shallow nature of oxygen vacancies and consequentn-type carrier concentrations in transparent conducting oxides

Deep vs shallow nature of oxygen vacancies and consequentn-type carrier concentrations in transparent conducting oxides

Restoring the Properties of Transparent Al-Doped ZnO Thin Film Electrodes Exposed to Ambient Air

UV–Ozone Modified Sol–Gel Processed ZnO for Improved Diketopyrrolopyrrole-Based Hybrid Photodetectors

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