Challenges and opportunities of ZnO-related single crystalline heterostructures

Abstract

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 Mg x Zn 1-x O. Here, we overview such recent technological advancement from various aspects of application. Towards optoelectronic devices such as a light emitter and a photodetector in an ultraviolet region, the development of p-type ZnO and the fabrication of excellent Schottky contact, respectively, have been subjected to intensive studies for years. For the former, the fine tuning of the growth conditions to make Mg x Zn 1-x O as intrinsic as possible has opened the possibilities of making p-type Mg x Zn 1-x O through NH 3 doping method. For the latter, conducting and transparent polymer films spin-coated on Mg x Zn 1-x O was shown to give almost ideal Schottky junctions. The wavelength-selective detection can be realized with varying the Mg content. From the viewpoint of electronic devices, two-dimensional electrons confined at the Mg x Zn 1-x O/ZnO interfaces are promising candidate for quantum devices becauseof high electron mobility and strong electron-electron correlation effect. These wonderful features and tremendous opportunities in ZnO-based heterostructures make this system unique and promising in oxide electronics and will lead to new quantum functionalities in optoelectronic devices and electronic applications with lower energy consumption and high performance.

The S concentration dependence of lattice parameters and optical band gap of a-plane ZnOS grown epitaxially on r-plane sapphire

“…Band gap engineering of ZnO is a key issue for its use in optoelectronic devices [3]. Substitution of the cation Zn 2+ or anion O 2À by an isoelectronic element can modify the band gap of ZnO.…”

Switchable conductivity at the ferroelectric interface: Nonpolar oxides

“…36 The combination of two polar materials (Mg x Zn 1-x O/ZnO) led to a 2DEG at the interface due to charge being driven there by the polarization of the materials; however, these materials are not switchable. 37 There is evidence that ferroelectrics may allow for the creation of surface charge. In the bulk of a ferroelectric, the material cannot be metallic, due to screening of the polarization due to conduction electrons.…”

Chemical Vapour Deposition of Gas Sensitive Metal Oxides

“…Zinc oxide (ZnO) is a II-VI semiconductor with a wide direct band gap (3.37 eV), large exciton binding energy (60 meV), spontaneous polarization and piezoelectric constants which make it an attractive material for electronic, optoelectronic, energy generator and photocatalytic applications [34]. This material has been widely used for gas detection since the early 1960s, and it is still among the most reported metal oxide materials used for gas sensing.…”

“…Most of the interesting functionalities of ZnO originate from its wurtzite crystal structure, which can be described as a number of alternating planes composed of tetrahedrally coordinated O 2´a nd Zn 2+ ions, stacked along the c-axis. This crystal does not possess inversion symmetry, having a large spontaneous polarization along the [0001] crystalline direction [34,35].…”