Subhashis Das

SiC is an indirect bandgap semiconductor with material properties ideal for power electronics but not so much as an optical emitter. Meanwhile, gallium arsenide (GaAs) is a material known for high-performance optical devices due to its direct bandgap and carrier lifetime. Integrating GaAs with silicon carbide (SiC) can result in the best of both materials. However, integrating the two presents a significant challenge due to the large lattice mismatch between the two materials. In this paper, we investigate the growth of high-quality GaAs directly on 4H-SiC and on AlAs/4H-SiC substrates. The thin films were characterized using key techniques for structural and optical analyses, such as x-ray diffraction, atomic force microscopy, and photoluminescence (PL) spectroscopy. The 3D-island nature of growth of GaAs directly on SiC results in weak in-plane correlation with the substrate but high photoluminescence. This was demonstrated with an observed PL intensity comparable to the PL observed from a GaAs substrate with a similar buffer layer. Introduction of a thin AlAs nucleation layer results in improved wetting of the substrate, better in-plane correlation with substrate, and overall improved crystalline quality and is now under further study.

Germanium films were grown on c-plane sapphire with a 10 nm AlAs buffer layer using molecular beam epitaxy. The effects of Ge film thickness on the surface morphology and crystal structure were investigated using ex situ characterization techniques. The nucleation of Ge proceeds by forming (111) oriented three-dimensional islands with two rotational twin domains about the growth axis. The boundaries between the twin grains are the origin of the 0.2% strain and tilt grains. The transition to a single-grain orientation reduces the strain and results in a better-quality Ge buffer. Understanding the role of thickness on material quality during the Ge(111)/Al₂O₃(0001) epitaxy is vital for achieving device quality when using group IV material on the sapphire platform.

Germanium films were grown on c-plane sapphire with a 10 nm AlAs buffer layer using molecular beam epitaxy. The effects of Ge film thickness on the surface morphology and crystal structure were investigated by ex-situ characterization techniques. The nucleation of Ge proceeds by forming (111) oriented three-dimensional islands with two rotational twin domains about the growth axis. The boundaries between the twin grains are the origin of 0.2% strain and tilt grains. The transition to a single-grain orientation reduces the strain and results in a better-quality Ge buffer. Understanding the role of thickness on material quality during the Ge(111)/Al2O3(0001) epitaxy is vital for achieving device quality when using group IV material on the sapphire platform.

There are indelible challenges related to transistor action and realization of emerging two-dimensional van der Waals (vdW) multilayer (2D $_{\text{ml}}\text{)}$ field-effect transistors (FETs), to the post silicon technology era. For scalability, a cost-effective large area ultrafine thin films interface and band alignment of multilayer channel material with compatible gate dielectric are essential. Here, 2D $_{\text{ml}}$ hafnium disulfide (HfS $_{\text{2}}\text{)}$ and ZrO $_{\text{2}}$ are employed as channel material and gate dielectric, respectively, and anticipated that vdW interaction of said structures entails the high-quality interface with trivial dangling bonds and defects caused by lattice mismatch. The investigated Al/ZrO $_{\text{2}}$ /HfS $_{\text{2}}$ /Al $_{\mu\text{-IDE}}$ FETs exhibit the subthreshold swing (SS) $\sim$ 65 mV/dec, $\textit{I}_{\biosc{on}}$ / $I_{\biosc{off}}$ ratio of $\sim$ 104, transconductance of $\sim$ 3.99 $\mu$ S, effective mobility of $\sim$ 74 cm $^{\text{2}}$ /Vs at $\textit{V}_{\text{gs}}$ of 2 V, and leakage current density of $\sim$ 33.8 nA/cm $^{\text{2}}$ at $\textit{V}_{\text{gs}}$ of $-$ 1 V. Thus, the steep SS, sturdy current saturation, low-voltage operation ( $\sim$ 3 V), and leakage current establish the potential candidature of HfS $_{\text{2}}$ and ZrO $_{\text{2}}$ -based 2-D FETs for both conventional and ubiquitous electronics.

A new and astonishing resistive gas sensor based on All-IDE/HfS2, synthesized by chemical route, has been developed for precise detection of methanol vapor at room temperature. A high sensitivity of 1.29 has been found at room temperature with the exposure of 500 ppm of methanol vapor at an operating bias of 1 V. Sensitivity has been obtained from transient response analysis. Most importantly, very fast response/recovery characteristics with good baseline restoration have been witnessed. The response time and recovery time have been recorded in the range of ∼12.12 s to ∼21.14 s and ∼23.72 s to ∼39 s. Cross-sensitivity with other interfering species has also been studied. A comprehensive justification, including Langmuir adsorption-desorption isotherm for appreciable sensing response, has also been delineated.

A new and interesting field-effect transistor (FET) structure based on multilayer HfS₂ as a channel material, integrated with Al μ-interdigitated electrodes (μ-IDEs) and HfO₂ as a gate dielectric is reported for the first time. The electrical performance of Al/HfO₂/HfS₂/Al)μ -IDE-based FETs exhibited the threshold voltage of ~ -2.72 V, generous subthreshold swing (SS) of ~ 70 mV/dec, substantial ION/IOFF ratio of ~ 10⁴, transconductance of ~ 1.5 μS, a significantly high electron mobility of ~ 56.7 cm²/Vs at Vds = 1 V, and low gate leakage current of ~ 47 nA/cm² at Vgs = -1 V. Therefore, moderate threshold voltage, generous SS, robust current saturation, ultralow off-state leakage current, and low operating voltage of the fabricated Al/HfO₂/HfS₂/Alμ -IDE, FETs show its potential for next-generation (NG) low-power FET applications.