A review is given of recent results in developing improved fabrication processes for ZnO devices with the possible application to UV light emitters, spin functional devices, gas sensors, transparent electronics, and surface acoustic wave devices. There is also interest in integrating ZnO with other wide band-gap semiconductors, such as the AlInGaN system. In this article, we summarize recent progress in controlling n- and p-type doping, materials processing methods, such as ion implantation for doping or isolation, Ohmic and Schottky contact formation, plasma etching, the role of hydrogen in the background n-type conductivity of many ZnO films, and finally, the recent achievement of room-temperature ferromagnetism in transition-metal (Mn or Co)-doped ZnO. This may lead to another class of spintronic devices, in which the spin of the carriers is exploited rather than the charge as in more conventional structures.

Recent advances in processing of ZnO

Over the past two decades, several advances have been made in micromachined sensors and actuators. As the field of microelectromechanical systems (MEMS) has advanced, a clear need for the integration of materials other than silicon and its compounds into micromachined transducers has emerged. Piezoelectric materials are high energy density materials that scale very favorably upon miniaturization and that has led to an ever-growing interest in piezoelectric films for MEMS applications. At this time, piezoelectric aluminum-nitride-based film bulk acoustic resonators (FBAR) have already been successfully commercialized. Future innovations and improvements in inertial sensors for navigation, high-frequency crystal oscillators and filters for wireless applications, microactuators for RF applications, chip-scale chemical analysis systems and countless other applications hinge upon the successful miniaturization of components and integration of piezoelectrics and metals into these systems. In this article, a comprehensive review of micromachined piezoelectric transducer technology will be presented. Piezoelectric materials in bulk and thin film forms will be reviewed and fabrication techniques for the integration of these materials for microsensor applications will be presented. Recent advances in various piezoelectric microsensors will be presented through specific examples. This review will conclude with a critical assessment of the future trends and promise of this technology.

https://iopscience.iop.org/article/10.1088/0957-0233/20/9/092001/pdf

Piezoelectric MEMS sensors: state-of-the-art and perspectives

ZnO is attracting considerable attention for its possible application to light-emitting sources due to its advantages over GaN. We review the recent progress in the growth of ZnO epitaxial films, doping control, device fabrication processes including etching and ohmic contact formation, and finally the prospects for fabrication and characteristics of ZnO light-emitting diodes.

https://iopscience.iop.org/article/10.1088/0022-3727/40/22/R01/pdf

ZnO thin films and light-emitting diodes

CuCrO2 is the most promising Cu-based delafossite for p-type optoelectronic devices. Despite this, little is known about the p-type conduction mechanism of this material, with both CuI/CuII and CrIII/CrIV hole mechanisms being proposed. In this article we examine the electronic structure, thermodynamic stability and the p-type defect chemistry of this ternary compound using density functional theory with three different approaches to the exchange and correlation; the generalized-gradient-approximation of Perdew, Burke and Ernzerhof (PBE), PBE with an additional correction for on-site Coulombic interactions (PBE + U) and the nonlocal, screened-exchange hybrid functional HSE06. The fundamental band gap of CuCrO2 is demonstrated to be indirect in nature. Under all growth conditions, the dominant intrinsic p-type defect will be the Cu vacancy, with hole formation centered solely on the Cu sublattice. Mg doping is found to be significantly lower in energy than intrinsic defect formation, explaining the large increases in conductivity seen experimentally. Cu-rich/Cr-poor growth conditions are found to be optimal for both intrinsic and extrinsic (Mg doping) defect formation, and should be adopted to maximize performance.

Understanding the p-type defect chemistry of CuCrO2

An O–H stretching line at 3326.3cm−1 was previously assigned to a shallow donor that is introduced into ZnO by H. This infrared line has been found to appear in as-grown ZnO samples when they are annealed near 400 °C without an external source of H, showing that there is an H-containing defect in commercially available ZnO that is not seen by infrared spectroscopy that can be converted into a shallow donor. The interstitial H2 molecule in ZnO is suggested as a candidate for the “hidden” H species.

``Hidden'' Hydrogen in As-Grown ZnO

The etch rate of bulk ZnO in Cl2/Ar high density plasmas was found to be thermally activated with an activation energy of ∼0.31 eV at 200 °C).

Temperature-dependent Cl2/Ar plasma etching of bulk single-crystal ZnO

Wide bandgap semiconductors have many properties that make them attractive for high power, high temperature device applications. In this paper we review wet etching of three important materials, namely ZnO, GaN and SiC. While ZnO is readily etched in many acid solutions including HNO3/HCl and HF/HNO3, and in the nonacid acetyleacetone, the group III nitrides and SiC are very difficult to wet etch and generally dry etching is used. Various etchants for GaN and SiC have been investigated, including aqueous mineral acid and base solutions, and molten salts. Wet etches have a variety of applications to wide bandgap semiconductor technology, including defect decoration, polarity and polytype (for SiC) identification by producing characteristic pits or hillocks, and device fabrication on smooth surfaces. Electrochemical etching is successful at room temperature in some situations for GaN and SiC. In addition, photo-assisted wet etching produces similar rates independent of crystal polarity.

https://iopscience.iop.org/article/10.1149/1.2731219/pdf

Wet Chemical Etching of Wide Bandgap Semiconductors-GaN, ZnO and SiC

Temperature-dependent cw- and time-resolved photoluminescence (PL), as well as optically detected magnetic resonance (ODMR) measurements are employed to evaluate effects of deuterium (2H) doping on optical properties of ZnCdO∕ZnO quantum well structures grown by molecular beam epitaxy. It is shown that incorporation of H2 from a remote plasma causes a substantial improvement in radiative efficiency of the investigated structures. Based on transient PL measurements, the observed improvements are attributed to efficient passivation by hydrogen of competing nonradiative recombination centers via defects. This conclusion is confirmed from the ODMR studies.

Effects of hydrogen on the optical properties of ZnCdO∕ZnO quantum wells grown by molecular beam epitaxy

Ohmic contact formation on p-type Mg-doped CuCrO2 layers grown by pulsed-laser deposition was investigated. While the current-voltage characteristics from Ti∕Au contacts showed back-to-back Schottky behavior, a specific contact resistance of ∼1×10−4Ωcm2 was achieved by using Ni instead of Ti. The contact resistivity was fairly independent of measurement temperature, suggesting that tunneling is the dominant transport mechanism. The contact resistance remained practically constant upon annealing in the 100–400°C range. Above 500°C, the morphology became rough and the contact showed rectifying behavior. This degradation resulted from both the out-diffusion of oxygen and the in-diffusion of Ni and Au in CuCrO2.

W.T. Lim

Papers

Temperature-dependent Cl2/Ar plasma etching of bulk single-crystal ZnO

Wet Chemical Etching of Wide Bandgap Semiconductors-GaN, ZnO and SiC

Effects of hydrogen on the optical properties of ZnCdO∕ZnO quantum wells grown by molecular beam epitaxy

Ni∕Au Ohmic contacts to p-type Mg-doped CuCrO2 epitaxial layers

Dry etching of zinc-oxide and indium-zinc-oxide in IBr and BI3 plasma chemistries