In this paper, the reported experimental data in [Sci. Rep., 2012, 2, 533] related to electrical transport properties in bulk ZnO, ZnMgO/ZnO, and ZnMgO/ZnO/ZnMgO single and double heterostructures were analyzed quantitatively and the most important scattering parameters for controlling electron concentration and electron mobility were obtained. Treatment of intrinsic mechanisms included polar-optical phonon scattering, piezoelectric scattering and acoustic deformation potential scattering. For extrinsic mechanisms, ionized impurity, dislocation scattering, and strain-induced fields were included. For bulk ZnO, the reported experimental data were corrected for removing the effects of a degenerate layer at the ZnO/sapphire interface via a two layer Hall effect model. Also, donor density, acceptor density and donor activation energy were determined via the charge balance equation. This sample exhibited hopping conduction below 50K and dislocation scattering closely controlled electron mobility closely. The obtained results indicated that the enhancement of electron mobility in double sample, compared with the single one, can be attributed to the reduction of dislocation density, two dimensional impurity density in the potential well due to background impurities, and/or interface charge and strain-induced fields, which can be related to better electron confinement in the channel and enhancement in the sheet carrier concentration of 2DEG in this sample.
Plastic relaxation of the misfit stress in core-shell semi-conducting nanowires can lead to structural defects, detrimental to applications. Core-shell Zn{0.7}Mg{0.3}O/ZnO quantum well heterostructures were deposited on ZnO nanowires. Strain along the a and c axes of the wurtzite structure is relaxed through the glide of dislocation half-loops from the free surfaces, within pyramidal and prismatic planes. Some half-loops are closed up in the barriers to accommodate the misfit at two consecutive interfaces of the quantum well stack. Dislocations are also observed within the nanowire core: contr…
Nanowire-based light-emitting devices require multi-quantum well heterostructures with high room temperature optical efficiencies. We demonstrate that such efficiencies can be attained through the use of ZnO/Zn(1-x)MgxO core shell quantum well heterostructures grown by metal organic vapour phase epitaxy. Varying the barrier Mg concentration from x=0.15 to x=0.3 leads to the formation of misfit induced dislocations in the multi quantum wells. Correlatively, temperature dependant photoluminescence reveals that the radial well luminescence intensity decreases much less rapidly with increasing tem…
Two-dimensional electron gases (2DEGs) at the ZnO/ZnMgO interface are promising for applications in spintronics and quantum computing due to the combination of low spin-orbit coupling and high electron mobility. Growing high mobility 2DEGs requires high quality substrates with low impurity densities. In this work we demonstrate a ZnO substrate sample treatment combining high temperature rapid thermal annealing and chemical etching to improve the surface quality for the subsequent growth of 2DEGs. This process enables the growth of a 2DEG with low-temperature mobility of $4.8\times10^4$~cm$^2$V…
In this paper, we report a novel photovoltaic phenomenon in a low cost manganite/ZnO p-n heterojunction grown on ITO glass substrate by pulsed laser depositon (PLD) under relative low growth temperature. The heterostructure ITO/La0.62Ca0.29K0.09MnO3(LCKMO)/ZnO/Al exhibits reproducible rectifying characteristics and consists of four parts: aluminum top electrode, ITO bottom electrode, manganite and ZnO semiconductor films. Moreover, the light current can generate under continuous laser (λ=325nm) irradiation. In this article, we investigate the influence of manganite and ZnO film thickness on th…
We report on the growth of epitaxial ZnO thin films and ZnO based heterostructures on sapphire substrates by laser molecular beam epitaxy (MBE). We first discuss some recent developments in laser-MBE such as flexible ultra-violet laser beam optics, infrared laser heating systems or the use of atomic oxygen and nitrogen sources, and describe the technical realization of our advanced laser-MBE system. Then we describe the optimization of the deposition parameters for ZnO films such as laser fluence and substrate temperature and the use of buffer layers. The detailed structural characterization b…
Recently the developments in the field of II-VI-oxides have been spectacular. Various epitaxial methods has been used to grow epitaxial ZnO layers. Not only epilayers but also sufficiently good-quality multiple quantum wells (MQWs) have also been grown by laser molecular-beam epitaxy (laser-MBE). We discuss mainly the experimental aspect of the optical properties of excitons in ZnO-based MQW heterostructures. Systematic temperature-dependent studies of optical absorption and photoluminescence in these MQWs were used to evaluate the well-width dependence and the composition dependence of the ma…
The band alignment in ZnO/GaN and related heterostructures are crucial for the uses in solar harvesting technology. Here, we report our density functional calculations of the band alignment and optical properties of ZnO/GaN and ZnO/(Ga1-xZnx)(N1-xOx)/GaN heterostructures using a Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. We found that the conventional GGA functionals underestimate not only the band gap but also the band offset of these heterostructures. Using the hybrid functional calculations, we show that the (Ga1-xZnx)(N1-xOx) solid solution has a direct band gap of about 2.608 eV, in…
Electronic, optical and transport properties of the graphene/ZnO heterostructure have been explored using first-principles density functional theory. The results show that Zn12O12 can open a band gap of 14.5 meV in graphene, increase its optical absorption by 1.67 times covering the visible spectrum which extends to the infra-red (IR) range, and exhibits a slight non-linear I-V characteristic depending on the applied bias. These findings envisage that a graphene/Zn12O12 heterostructure can be appropriate for energy harvesting, photodetection, and photochemical devices.
ZnO/ZnS heterostructures as hole reservoir to boost Ni foam energy storage performance
University of Catania（卡塔尼亚大学）；IMM-CNR（意大利国家研究委员会材料研究所）；Catalan Institute of Nanoscience and Nanotechnology (ICN2)（加泰罗尼亚纳米科学和技术研究所）；CSIC（西班牙国家研究委员会）；BIST（巴塞罗那科学技术研究院）；ICREA（加泰罗尼亚卓越研究员）
Recent technological advancement in ZnO heterostructures has expanded the possibility of device functionalities to various kinds of applications. In order to extract novel device functionalities in the heterostructures, one needs to fabricate high quality films and interfaces with minimal impurities, defects, and disorder. With employing molecular-beam epitaxy (MBE) and single crystal ZnO substrates, the density of residual impurities and defects can be drastically reduced and the optical and electrical properties have been dramatically improved for the ZnO films and heterostructures with MgxZ…
Van de Waals heterostructures (VDWH) is an emerging strategy to engineer the electronic properties of two-dimensional (2D) material systems. Motivated by the recent discovery of MoSi$_2$N$_4$ - a synthetic septuple-layered 2D semiconductor with exceptional mechanical and electronic properties, we investigate the synergy of \ce{MoSi2N4} with wide band gap (WBG) 2D monolayers of GaN and ZnO using first-principle calculations. We find that MoSi$_2$N$_4$/GaN is a direct band gap Type-I VDWH while MoSi$_2$N$_4$/ZnO is an indirect band gap Type-II VDWH. Intriguingly, by applying an electric field or…
Oxide based ferromagnet/semiconductor heterostructures offer substantial advantages for spin electronics. We have grown (111) oriented Fe3O4 thin films and Fe3O4/ZnO heterostructures on ZnO(0001) and Al2O3(0001) substrates by pulsed laser deposition. High quality crystalline films with mosaic spread as small as 0.03 degree, sharp interfaces, and rms surface roughness of 0.3 nm were achieved. Magnetization measurements show clear ferromagnetic behavior of the magnetite layers with a saturation magnetization of 3.2 muB/f.u. at 300 K. Our results demonstrate that the Fe3O4/ZnO system is an intrig…
We report measurements of the spin susceptibility and the electron effective mass for two-dimensional electrons confined at the interfaces of MgxZn1-xO/ZnO single heterostructures (x = 0.05, 0.08, and 0.11), grown by molecular-beam epitaxy on (0001) ZnO substrates. By tuning the built-in polarization through control of the barrier composition, the electron density was systematically varied in the range of 5.6 x 10^11 to 1.6 x 10^12 cm^-2, corresponding to a range of 3.1 < rs < 5.2, where rs is the average electron spacing measured in units of the effective Bohr radius. We used the coincidence …
The magneto-transport properties of single proton-implanted ZnO and of Li(7\%)-doped ZnO microwires have been studied. The as-grown microwires were highly insulating and not magnetic. After proton implantation the Li(7\%) doped ZnO microwires showed a non monotonous behavior of the negative magneto-resistance (MR) at temperature above 150 K. This is in contrast to the monotonous NMR observed below 50 K for proton-implanted ZnO. The observed difference in the transport properties of the wires is related to the amount of stable Zn vacancies created at the near surface region by the proton implan…
Heterostructures of Co-doped ZnO and Permalloy were investigated for their static and dynamic magnetic interaction. The highly Co-doped ZnO is paramagentic at room temperature and becomes an uncompensated antiferromagnet at low temperatures, showing a narrowly opened hysteresis and a vertical exchange bias shift even in the absence of any ferromagnetic layer. At low temperatures in combination with Permalloy an exchange bias is found causing a horizontal as well as vertical shift of the hysteresis of the heterostructure together with an increase in coercive field. Furthermore, an increase in t…
We realize a two-dimensional electron system (2DES) in ZnO by simply depositing pure aluminum on its surface in ultra-high vacuum, and characterize its electronic structure using angle-resolved photoemission spectroscopy. The aluminum oxidizes into alumina by creating oxygen vacancies that dope the bulk conduction band of ZnO and confine the electrons near its surface. The electron density of the 2DES is up to two orders of magnitude higher than those obtained in ZnO heterostructures. The 2DES shows two $s$-type subbands, that we compare to the $d$-like 2DESs in titanates, with clear signature…
Heterostructures of wurtzite based devices have attracted great research interests since the tremendous success of GaN in light emitting diodes (LED) industry. Among the possible heterostructure material candidates, high quality GaN thin films on inexpensive and lattice matched ZnO substrate are both commercially and technologically desirable. However, the energy of ZnO polar surfaces is much lower than that of GaN polar surfaces. Therefore, the intrinsic wetting condition forbids such heterostructures. As a result, poor crystal quality and 3D growth mode were obtained. To dramatically change …
Enhanced interfacial charge transfer by Z-scheme in defect-mediated ZnO-CdS nano-composite with rGO as a solid-state electron mediator for efficient photocatalytic applications
We prospose a novel heterostructure system consisting of compounds with chemical formula A$_2$Mo$_3$O$_8$ (A, B : Zn,Mg,Cd) that can host a two dimensional electron/hole gas (2DEG/2DHG). We study spontaneous polarization and piezoelectric properties of these compounds using first principles methods and Berry phase approach. We show that these kind of heterostructures are very stable due to extreamly low interfacial strain. The formation of a 2DEG/2DHG has been investigated in case of Zn$_2$Mo$_3$O$_8$/Mg$_2$Mo$_3$O$_8$ and polarization discontinuity has been found to be driving mechanism. The …
Physics-Informed Long-Range Coulomb Correction for Machine-learning Hamiltonians
Key Laboratory of Computational Physical Sciences (Ministry of Education)（计算物理科学国家重点实验室（教育部））；Institute of Computational Physical Sciences（计算物理科学研究院）；State Key Laboratory of Surface Physics（表面物理国家重点实验室）；Department of Physics, Fudan University, Shanghai, 200433, China.（复旦大学物理系，上海，200433，中国）
The ferrimagnetic spinel oxide Zn(x)Fe(3-x)O(4) combines high Curie temperature and spin polarization with tunable electrical and magnetic properties, making it a promising functional material for spintronic devices. We have grown epitaxial thin films with 0<=x<=0.9 on MgO(001) substrates with excellent structural properties both in pure Ar atmosphere and an Ar/O2 mixture by laser molecular beam epitaxy. We find that the electrical conductivity and the saturation magnetization can be tuned over a wide range during growth. Our extensive characterization of the films provides a clear picture of …
