Abstract
Zinc oxide is a novel material system for mid-infrared and THz optoelectronics. Especially its non-polar m-plane orientation is a promising candidate for the design of devices like quantum cascade lasers (QCLs) and detectors (QCDs). But for their realization novel fabrication schemes are needed. We present a new inductively coupled plasma reactive ion etching (ICP-RIE) process for etching of m-Zn(Mg)O heterostructures in a CH4-based chemistry. The process has been optimized for smooth vertical sidewalls together with high selectivity towards a SiN etch mask. This was achieved by combining the RIE etching with wet chemical etching in strongly diluted HCl. Similar to various types of semiconductor-based optoelectronic materials and devices (Sidor et al 2016 J. Electron. Mater. 45 4663–7; Ma et al 2016 Opt. Express 24 7823), including other wide-gap semiconductors like (In)GaN (Zhang et al 2015 Nanotechnology 26), we observe surface leakage currents in etched m-plane Zn(Mg)O structures. We show that they depend on the applied etching process and surface treatment techniques as well as the barrier composition in the Zn(Mg)O heterostructures. In addition, a treatment in hydrogen peroxide (H2O2) yields a significant surface leakage current suppression up to several orders of magnitude.
Highlights
The terahertz (THz) spectral region is a peculiar portion of the electromagnetic spectrum
THz quantum cascade lasers (QCLs) were demonstrated for the first time in 2002 by [14]
We developed a novel combined etching scheme including a first step of plasma-based dry etching in a reactive ion etching (RIE) machine, followed by a wet chemical etch and a final step of surface passivation to prevent surface leakage currents
Summary
The terahertz (THz) spectral region is a peculiar portion of the electromagnetic spectrum. We propose to use ZnO as suitable alternative material system for optoelectronic devices like (THz) QCLs and quantum cascade detectors (QCDs), owing to its high LO phonon energy mentioned previously. Significant effort has to be put in the characterization and analysis of its material parameters in the THz, like e.g. the effective mass and permittivity, as well as its electronic band parameters like the conduction band offset These efforts include mastering its high-quality epitaxial growth and the development of state-of-the-art device fabrication schemes. Like GaN, ZnO is a polar material (its main conformation is the wurtzite lattice [33, 41]), resulting in the formation of internal electric polarization fields [42] for its main crystallographic directions like the c-plane orientation (0001) This strongly increases the difficulty of designing and realizing complex ISB devices like QCLs or QCDs with their up to hundreds of precisely designed quantum wells. The whole etching procedure is optimized for smooth vertical sidewall profiles together with a high selectivity towards the etching mask and high, while still well-controllable, etch rates
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