Kolla Lakshmi Ganapathi

Integration of high-k dielectrics with two-dimensional (2D) layered semiconductors is one of the bottleneck in realization of the devices in a viable technology. In this work we report on the optimization and integration of electron beam (e-beam) evaporated, ultrathin (5 nm) HfO2 thin films on a few layer (5-7 nm) MoS2 substrate, without the need for any functionalization and buffer layers. The effect of HfO2 film thickness on optical, compositional and electrical properties of the material has been evaluated using ellipsometry, Rutherford backscattering, x-ray photoelectron spectroscopy and electrical measurements. The estimated dielectric constant and leakage current density values have been correlated with the refractive index and density of the films. It has been observed that the dielectric constant decreased drastically in ultrathin (5 nm) HfO2 film in case of Al as a gate electrode due to the formation of interfacial layer. The effect of gate electrode material and its processing on dielectric properties of ultra-thin films has been studied in detail and it is found that e-beam evaporated Ni seems to be a promising gate electrode. Equivalent oxide thickness of 1.2 nm and leakage current density (J) of 1.1 × 10-4 A cm-2 have been achieved for 5 nm films with Ni as gate electrode. Top gated MoS2 transistors with, 5 nm HfO2 gate dielectric show very good performance with I ON to I OFF ratio of 106. We thus demonstrate e-beam evaporation as a promising technique to directly integrate high-k dielectrics (HfO2) with 2D materials.

We report on the direct current (dc) current-voltage (I-V) characteristics of few-layer muscovite mica (MuM) flakes exfoliated and transferred onto SiO2/Si substrate, under different substrate dc bias voltages. Contrary to usual observations in conventional two-dimensional systems, we observe an increase in the in-plane electrical conductivity with a reducing thickness of MuM flakes. At a given voltage, the electrical conductivity of approximately five-layered MuM flake (T3) is 3 orders of magnitude larger than that in approximately ten-layered MuM flake (T2). The I-V characteristics are used to analyze the mechanism of conduction. The model-based analysis reveals the hopping-conduction mechanism to be dominant as compared to the Poole-Frenkel effect. The thickness-dependent work function is measured using Kelvin probe force microscopy for a MuM flake on Si substrate. Assuming that the measured work function is correlated with the Fermi level, we report an upward movement of the Fermi level, toward the conduction band with the reducing thickness of MuM flakes, indicating an increase in the conduction-band carrier density. The observed increase in conductivity in T3 when compared to T2 may be attributed to surface doping due to the increased contribution from K+ ions and lattice relaxation. Our results show that there is a possibility of using few-layer mica as a wide-band-gap semiconductor and that it can open up different avenues for two-dimensional electronic devices.

The lack of techniques for counter doping in two dimensional (2D) semiconductors has hindered the development of p/n junctions, which are the basic building blocks of electronic devices. In this work, low-energy argon ions are used to create sulfur vacancies and are subsequently “filled” with oxygen to create p-doped MoS2−xOx. The incorporation of oxygen into the MoS2 lattice and hence band-structure modification reveal the nature of the p-type doping. These changes are validated by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Raman spectroscopy, and photoluminescence measurements combined with density functional theory calculations. Electrical measurements reveal a complete flip in carrier polarity from n-type to p-type, which is further examined using temperature-dependent transport measurements. The enhancement of p-field-effect transistor characteristics is facilitated by employing top-gated transistors and area-selective vacancy engineering only in the contact regions. Finally, on the same flake, an in-plane MoS2 (n)/MoS2−xOx (p) type-I (straddling) heterojunction with rectifying behavior and excellent broadband photoresponse is demonstrated and explained using band diagrams. The spatial/metallurgical abruptness (<100 nm) of the heterojunctions is ascertained using Raman mapping. This process of vacancy engineering, which enables air-stable, area-selective, controlled, CMOS-compatible doping of 2D semiconductors is envisioned to open new vistas in the development of high-performance electronic and optoelectronic devices.

We report on the band offset and interfacial electronic properties of e-beam evaporated HfO2 gate dielectrics on III-nitride device stacks on Si. A conduction band offset of 1.9 eV is extracted for HfO2/GaN along with a very low density of fixed bulk and interfacial charges for optimally annealed samples. Normally-ON HfO2/AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors exhibit negligible shifts in threshold voltage, transconductances of 120 mS/mm for 3- mu m gate length devices, and three-terminal off-state gate leakage currents of 55 nA/mm at a V D of 100 V. Dynamic capacitance dispersion measurements in the temperature range of 25 °C-200 °C show two peaks at the AlGaN/GaN interface corresponding to slow and fast interface traps with a peak D it of 5.5× 10-13 and 1.5× 10-13 eV-1cm-2 respectively as a function of Fermi level position above E-C. The HfO2/AlGaN interface exhibits a reasonably constant peak D it of 2 × 10-13 - 4.4× 10-13 eV-1cm-2 at trap levels of 0.42-0.72 eV below E C. Hysteretic pulsed ID - VG measurements revealed a negative shift in threshold voltages indicative of unoccupied donor-like trap states at the HfO2/AlGaN interface and comparable Dit to that inferred from conductance measurements.

We report on multiple excitonic resonances in bilayer tungsten diselenide (BL-WSe2) stacked at different angles and demonstrate the use of the stacking angle to control the occurrence of these excitations. BL-WSe2 with different stacking angles were fabricated by stacking chemical vapour deposited monolayers and analysed using photoluminescence measurements in the temperature range 300-100 K. At reduced temperatures, several excitonic features were observed and the occurrences of these exitonic resonances were found to be stacking angle dependent. Our results indicate that by controlling the stacking angle, it is possible to excite or quench higher order excitations to tune the excitonic flux in optoelectronic devices. We attribute the presence/absence of multiple higher order excitons to the strength of interlayer coupling and doping effect from SiO2/Si substrate. Understanding interlayer excitations will help in engineering excitonic devices and give an insight into the physics of many-body dynamics.

In this work, the authors report the application and influence of slot plane antenna plasma oxidation (SPAO) on the quality of Ge/high-k based metal-oxide-semiconductor capacitors. The effect of SPAO exposure on the Ge/high-k interface during atomic layer deposition of the dielectric along with the reliability characteristics has been studied. A significant improvement in the electrical properties has been observed when the high-k stacks are exposed to SPAO treatment. The devices treated with SPAO after Al2O3/ZrO2 deposition (CASE-1) show slightly better equivalent oxide thickness, low leakage current density, and marginally better breakdown characteristics compared to the devices treated with SPAO in-between Al2O3/ZrO2 deposition (CASE-2). This can be attributed to the densification of the gate stack as the plasma exposed to the total stack and the formation of the thick interfacial layer as evident from the X-ray photoelectron spectroscopy (XPS) measurements. A stable and thin interfacial layer formation was observed from XPS data in the samples treated with SPAO in-between high-k stack deposition compared to the samples treated with SPAO after high-k stack deposition. This leads to the low interface state density, low hysteresis, comparable dielectric breakdown, and reliable characteristics in CASE-2 compared to CASE-1. On the other hand, XPS data revealed that the interface is deteriorated in the samples treated with SPAO before high-k stack deposition (CASE-3) and leads to poor electrical properties.

Despite the concerted effort of several research groups, a detailed experimental account of defect dynamics in high-quality single- and few-layer transition-metal dichalcogenides remains elusive. In this paper we report an experimental study of the temperature dependence of conductance and conductance fluctuations on few-layer MoS2 exfoliated on hexagonal boron nitride and covered by a capping layer of high-κ dielectric HfO2. The presence of the high-κ dielectric made the device extremely stable against environmental degradation as well as resistant to changes in device characteristics upon repeated thermal cycling, enabling us to obtain reproducible data on the same device over a timescale of more than 1 year. Our device architecture helped bring down the conductance fluctuations of the MoS2 channel by orders of magnitude compared to previous reports. The extremely low noise levels in our devices made it possible to detect the generation-recombination noise arising from charge fluctuation between the sulfur-vacancy levels in the band gap and energy levels at the conductance band edge. Our work establishes conduction fluctuation spectroscopy as a viable route to quantitatively probe in-gap defect levels in low-dimensional semiconductors.

We demonstrate high performance broadband UV-to-NIR detection, which is a critical issue associated with ZnO-based photodetectors. The as-synthesized ZnO-ZnCr2O4 nanowalls were first time utilized for broadband, i.e., 250-850 nm photo-detection (both in front and back illumination configurations). The dark current was found to be as low as 0.12 nA. The device has shown peak sensitivity for the UV region ( λex= 350 nm) with the photo-sensitivity of ~1.28× 105 , photo-responsivity of 5.49 AW-1, photo-detectivity of 1.91 × 1013 cmHz1/2W-1, linear dynamic range of 82 dB, and external quantum efficiency of 1900%. In addition, the white light emission (CIE coordinates of 0.32 and 0.34) was also observed in the ZnO-ZnCr2O4 nanowalls. This letter will open new directions in oxide semiconductors-based optoelectronic devices.

Diamond due to its outstanding optical, electrical, mechanical and thermal properties finds an important place in electronic, opto-electronic and quantum technologies. Recent progresses showing superconductivity in diamond by boron doping has opened up many avenues including its applications in SQUID devices especially with polycrystalline diamond films. Granular boron doped diamond films find applications in quantum inductance devices where high surface inductance is required. Particularly important are the defect centers in diamond like nitrogen-vacancy (N-V), silicon vacancy (SiV) and other color centers which are ideal candidates for next generation quantum hardware systems. For efficient device applications, an indispensable need remains for a substitutional donor in diamond lattice that yields a lower thermal activation energy at room temperature. In this review, a comprehensive summary of research and the technological challenges has been reported including some of the results on nitrogen doping in polycrystalline diamond to understand the transport phenomenon emphasizing on its possible future applications.

High-performance solar-blind photo-detection using highly transparent CuO nanostructures (with a bandgap of 4.15 eV) has been demonstrated. The device shows the dark current as low as 0.2 nA (-10 V applied bias) and no signature of breakdown even at a bias up to ±175 V. The device has shown record photo-sensitivity of 610, photo-responsivity of 14.02 A/W and photo-detectivity of 3.59× 1013 cmHz 1/2W-1 in the UV-C region. The ratio of photo-responsivities at 210 nm and 500 nm i.e. R210/R500 was found to be 5.05× 104. Additionally, the device has shown external quantum efficiency of 5900 % at 210 nm excitation. This letter will establish CuO as one of the most promising ultra-wide bandgap semiconductors for cost-effective solar blind photo-detection.