ICMCTF2015 Session C5: Thin Films for Active Devices

Piezoelectric AlN films for electroacoustic devices are typically deposited by magnetron sputtering. Sputtering is compatible with standard microelectronic fabrication processes and requires lower deposition temperatures than other techniques. In order to enhance the texture of AlN, metal seed layers, such as molybdenum, are usually used. Low temperature growth of AlN films for devices where the seed layer cannot be used is challenging.

Here we report on the growth of thin textured (002) AlN layers directly on Si substrates without any metal seed layer. The films were deposited by reactive High Power Impulse Magnetron sputtering (HiPIMS) from an aluminium target in argon/nitrogen atmosphere. Because in HiPIMS very high degree of ionization of the sputtered material is achieved, this technique provides highly ionized flux to the substrate and thus promotes surface diffusion. Moreover, nitrogen dissociation which occurs in the high density HiPIMS plasma increases reactivity of the nitrogen. For comparison, pulsed DC sputtering was also performed under identical conditions.

We show that for 200 nm thick AlN films grown on (100) Si, the HiPIMS process produces well textured (002) films already at room temperature while the pulsed DC films are very poor. At 400°C, which is the optimal temperature for pulsed DC deposition, the HiPIMS films are superior with the FWHM value of 5.1 and 14.2° for the HiPIMS and pulsed DC, respectively. No appreciable stresses were observed in the films. The HiPIMS deposition process is more robust than standard DC sputtering and provides sufficient energy input even for configurations with relatively large target-to-substrate distance. It is therefore suitable also for co-sputtering of ternary nitrides based on AlN.

Vanadium dioxide (VO₂) has been extensively studied due to its ability to reversibly transform from a semiconducting monoclinic phase (low-temperature) to a metallic tetragonal phase (high-temperature) when it is thermally, electrically, or optically triggered. Upon transitioning to the metallic phase, the electrical resistivity decreases by as much as five orders of magnitude and the optical transmittance in the near-IR decreases significantly. These VO₂ films can be incorporated into conventional metamaterial devices such as split-ring resonators (SRRs) at terahertz frequencies in order to provide a simple design concept and demonstrate tunable resonances and frequencies. We have synthesized high quality VO₂ epitaxial thin films by pulsed laser deposition and their semiconductor-to-metal transitions were characterized as a function of film growth conditions. Films grown at optimized conditions exhibited a significant resistivity drop (>10⁴ Ω-cm) and large optical transmittance change (> 60 %) in the near-infrared region across the transition. We have fabricated various active metamaterials at terahertz frequencies using the phase transition in VO₂ thin films. We will present details on the electrical and optical properties of VO₂ films and discuss examples of active metamaterials based on VO₂ thin films.

This work was funded by the Office of Naval Research (ONR) through the Naval Research Laboratory Basic Research Program.

We have proposed and successfully developed unique ALE-like InN deposition processes to fabricate novel coherent structure 1 ML-thick InN/GaN matrix-based QWs by using a custom-made MBE equipped with a spectroscopic ellipsometry (SE) system. The attached SE system is quite useful and effective as an in-situ monitoring, deep understanding, and precise controlling tool for the growth processes. The novel InN QWs were fabricated under self-ordering and self-limiting processes achieved at remarkably higher growth temperatures (600-700 C) than the higher side critical temperature (~500 C) for continuous InN film growth under +c-polarity growth regime by MBE. The fundamental structure of the novel QWs consisted of 1 ML-thick InN well coherently embedded in GaN matrix, and the QW could be controlled in fractional ML-thick and also 2 ML-thick InN wells in the GaN matrix depending on the growth conditions. Then, these InN QWs are generically expressed here as “1 ML” InN QWs.

On the basis of above mentioned development on coherent structure “1 ML” InN/GaN matrix QWs, we have also been investigating to extend them to (InN)_m/(GaN)_n short-period superlattices (SPSs), where we simply call them as SMART structure and/or SMART process. Here SMART stands for “Superstructure Magic Alloys fabricated at Raised Temperature”. We expect that the InN/GaN SPS-based SMART structures can play a role as high quality artificial or digital InGaN ternary alloys. Of course, the GaN barrier layer thickness must be also ultimately reduced in this case down to a few monolayers or several monolayers level so that localized wave functions at the InN wells can be overlapped with each other. Furthermore, it should be noted that the SMART process has been successfully developed first in MBE but it has been extended to MOVPE now. It was found that the structural quality of the SMART structure devices is getting worse in the case of MOVPE than that of MBE, but MOVPE-grown devices often show better device performances than those for MBE due to higher growth temperatures.

In this paper, we first summarize what is the novel “1 ML”-InN/GaN matrix QW system and then also discuss how and where those “1 ML”-InN QWs are fabricated on or inside the GaN matrix by MBE. Next, the basic idea for their potential application to InN/GaN SPS-based artificial or digital InGaN ternary alloys, i.e. SMART structure, will be reported. Furthermore, our idea and recent efforts for applying the “1 ML” InN QW/GaN matrix QW system and the SMART structure to develop blue-green light emitters and high efficiency III-N tandem solar cells, respectively, are also discussed, and their initial stage results will be reported.