Polarity control and residual strain in ZnO epilayers grown by molecular beam epitaxy on (0001) GaN/sapphire

Ullah, Md. Barkat; Avrutin, Vitaliy; Li, Si Qian; Das, Saikat; Monavarian, Morteza; Toporkov, Mykyta; Özgür, Ü.; Ruterana, P.; Morkoç̌, H.

doi:10.1002/pssr.201600189

Cited by 22 publications

(18 citation statements)

References 27 publications

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“…Single-crystalline ZnO thin films were grown on GaN(0001)/ c-sapphire templates following the methods previously reported. 16 Briefly, Ga-polar (0001) GaN was deposited onto c-plane sapphire by metal-organic chemical vapor deposition. After cleaning in aqua regia and aqueous HCl in order to remove potential metal contamination, the GaN templates were loaded into a plasma enhanced molecular beam epitaxy (PE-MBE) system wherein the GaN surface was further thermally cleaned followed by exposure to a Zn beam.…”

Section: Growth Of Zno Thin Filmsmentioning

confidence: 99%

Influence of ZnO thin film crystallinity on in vitro biocompatibility

Lewinski

Avrutin

Izadi

et al. 2018

Toxicology Research

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This study evaluated the cytocompatibility of single- and poly-crystalline ZnO thin films using extract and direct contact methods. Exposure to poly-crystalline ZnO extract resulted in reduced cell viability, on average 82%/70% as measured by MTS/LDH assays, respectively. Direct exposure to both single- and poly-crystalline ZnO thin films resulted in reduced cell viability, which was attributed to anoikis due to inhibition of cell adhesion to the substrate by zinc. Intracellular zinc imaging suggests that single crystalline ZnO thin films do not result in a significant change in intracellular zinc concentrations. Overall, the results suggest that single-crystalline ZnO thin films have better short-term (24 h) cytocompatibility and support their potential to serve as a biocompatible sensor material.

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Section: Growth Of Zno Thin Filmsmentioning

confidence: 99%

Influence of ZnO thin film crystallinity on in vitro biocompatibility

Lewinski

Avrutin

Izadi

et al. 2018

Toxicology Research

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“…Note that the broadening of the reflection from Be 0.02 Mg 0.26 ZnO is due to its thinness. The tensile biaxial strain in the ZnO layer is an indication of the Zn-polar heterostructure, as investigated in our previous study 13 . Be and Mg contents in the BeMgZnO quaternary were calculated from the Bragg angle of its XRD (0002) reflection and emission photon energy in LT-photoluminescence (LT-PL) spectrum measured at 13 K (not shown).…”

Section: Representative Resultsmentioning

confidence: 67%

“…For the growth of ZnO-based heterostructures, however, more reports on the formation of 2DEG are realized by MBE at the present time prior to the commercialization of the potential applications 10 11 12 . Recently, we have developed MBE growth of high quality ZnO heterostructures with an accurate control of surface polarity on Ga-polar GaN templates 13 . It was found that with Zn pre-exposure treatment, ZnO layers so grown exhibited Zn-polarity when nucleated with low VI/II ratios (<1.5), while those nucleated with VI/II ratios above 1.5 exhibited O-polarity.…”

Section: Introductionmentioning

confidence: 99%

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

Ding

Avrutin

Izioumskaia

et al. 2018

JoVE

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Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.

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“…Experimental Procedures : Zn‐polar Be 0.02 Mg 0.26 ZnO/ZnO heterostructures were grown by plasma‐enhanced molecular beam epitaxy (PE‐MBE) on GaN/sapphire templates that were prepared by metal‐organic chemical vapor deposition (MOCVD). Zn polarity was maintained by using a low VI/II ratio during the nucleation of ZnO at 300 °C on GaN . The thickness of the Be 0.02 Mg 0.26 ZnO barrier and the ZnO channel layer was 30 and 300 nm, respectively.…”

Section: Electron Mobilities Sheet Carrier Concentrations Schottky mentioning

confidence: 99%

Characterization of Ag Schottky Barriers on Be_0.02Mg_0.26ZnO/ZnO Heterostructures

Ullah

Ding

Nakagawara

et al. 2017

Physica Rapid Research Ltrs

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The BeMgZnO/ZnO heterostructures are capable of producing sufficiently high sheet electron densities to allow field effect transistor operation near or at the LO phonon plasmon resonance frequency for minimal LO phonon lifetimes and high electron velocity. Schottky barriers are imperative for the implementation of the aforementioned devices. Therefore, we have undertaken fabrication and characterization of Ag Schottky barriers on Zn-polar Be 0.02 Mg 0.26 ZnO/ZnO heterostructures, exhibiting the said two-dimensional electron gas (2DEG), grown by molecular beam epitaxy. Ag Schottky barriers are characterized by current-voltage (I-V) measurements in the temperature range from 80 to 400 K. At room temperature, the highest barrier height of 1.07 eV and an ideality factor of 1.22 are achieved with a rectification ratio of about eight orders of magnitude. Richardson constants of 38 AE 22 and 29 AE 20 A cm À2 K À2 are found using modified Richardson plots from Schottky diodes fabricated on two different structures. These values are consistent with the theoretical estimate of 36 A cm À2 K À2 for Be 0.02 Mg 0.26 ZnO. The temperature variation of barrier heights and ideality factors, an aberration from pure thermionic behavior, has been explained with plausible spatial inhomogeneity of barrier height with three Gaussian distributions in the temperature ranges of 80-200, 220-280, and 300-400 K.ZnO as a wide bandgap semiconductor [1] (%3.3 eV at room temperature) with high electron saturation velocity [2] (theoretical 3.5 Â 10 7 cm s À1 ), has garnered popularity owing to its potential for applications in thin film transparent transistors, transparent electrodes for solar cells, light emitters, chemical and biological sensors etc. [1,[3][4][5] Alloying with other II-VI oxides with higher bandgap (MgO and BeO) paves the way for heterojunction field effect transistors with polarization induced two-dimensional electron gas (2DEG). [6,7] Performance of such devices at high electric field, where electron transport is highly controlled by longitudinal optical (LO) phonons, [8] suffers due to accumulation of non-equilibrium hot LO phonons owing to strong electron-LO phonon coupling. Devices operating at or near plasmon-LO phonon resonance exhibit high electron drift velocity [9][10][11] allowed by ensuing ultrafast decay of hot LO phonons, [12,13] which leads to faster electron energy relaxation [14] and optimum device operation conditions (higher frequencies, lower degradation). [8] Plasmon-LO phonon resonance occurs when the energy of the LO phonon becomes similar to the plasmon energy, which occurs in ZnO at a bulk carrier concentration of %7 Â 10 18 cm À3 (ZnO LO phonon energy 72 meV). [14] Clearly, this is far beyond the MESFET operating window (typically 1 Â 10 17 À5 Â 10 17 cm À3 ). The necessity of high electron density makes the use of ZnO heterostructures with 2DEG imperative.Sheet carrier densities up to 8 Â 10 12 cm À2 (corresponding bulk concentration of 2 Â 10 19 cm À3 ) have been reported with MgZnO/ZnO heterostructu...

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Polarity control and residual strain in ZnO epilayers grown by molecular beam epitaxy on (0001) GaN/sapphire

Cited by 22 publications

References 27 publications

Influence of ZnO thin film crystallinity on in vitro biocompatibility

Influence of ZnO thin film crystallinity on in vitro biocompatibility

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

Characterization of Ag Schottky Barriers on Be_0.02Mg_0.26ZnO/ZnO Heterostructures

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Polarity control and residual strain in ZnO epilayers grown by molecular beam epitaxy on (0001) GaN/sapphire

Cited by 22 publications

References 27 publications

Influence of ZnO thin film crystallinity on in vitro biocompatibility

Influence of ZnO thin film crystallinity on in vitro biocompatibility

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

Characterization of Ag Schottky Barriers on Be0.02Mg0.26ZnO/ZnO Heterostructures

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Characterization of Ag Schottky Barriers on Be_0.02Mg_0.26ZnO/ZnO Heterostructures