Amir Afshar

Growth mechanism of atomic layer deposition of zinc oxide: A density functional theory approach

Atomic layer deposition of zinc oxide (ZnO) using diethylzinc (DEZ) and water is studied using density functional theory. The reaction pathways between the precursors and ZnO surface sites are discussed. Both reactions proceed by the formation of intermediate complexes on the surface. The Gibbs free energy of the formation of these complexes is positive at temperatures above ∼120 °C and ∼200 °C for DEZ and water half-reactions, respectively. Spectroscopic ellipsometry results show that the growth per cycle changes at approximately the same temperatures.

Electrical Comparison of

A low-temperature atomic layer deposition technique for high-κ dielectric films on GaN templates was investigated for MOS applications. This improved growth method produced capacitance densities and a field effect mobility approaching 375 cm2/Vs for ZrO2 and 250 cm2/Vs for HfO2 films on GaN. Furthermore, the low density of dielectric-semiconductor interface traps confirmed a reliable cohesion between the high-κ and GaN. The improved gate dielectric deposition technique has the capabilities to improve the overall quality of GaN-based MOSFETs.

{\rm HfO}_{2}

This paper presents a capacitance model and mobility extraction method through the use of tapered transmission line theory for accumulation-mode MOSCAP test structures. The analytical model accounts for the discrepancies commonly found when measuring the capacitance of nontraditional MOSCAP architectures. Through fabrication of a planar MOSCAP, this model accurately reproduced consistent capacitance density measurements for several device dimensions and high-κ dielectric thicknesses. In this paper, the theoretical basis of the model extracts the effective electron mobility of the accumulation channel in the semiconductor without fabricating a transistor.

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Wide-bandgap, metal-oxide thin-film transistors have been limited to low-power, n-type electronic applications because of the unipolar nature of these devices. Variations from the n-type field-effect transistor architecture have not been widely investigated as a result of the lack of available p-type wide-bandgap inorganic semiconductors. Here, we present a wide-bandgap metal-oxide n-type semiconductor that is able to sustain a strong p-type inversion layer using a high-dielectric-constant barrier dielectric when sourced with a heterogeneous p-type material. A demonstration of the utility of the inversion layer was also investigated and utilized as the controlling element in a unique tunnelling junction transistor. The resulting electrical performance of this prototype device exhibited among the highest reported current, power and transconductance densities. Further utilization of the p-type inversion layer is critical to unlocking the previously unexplored capability of metal-oxide thin-film transistors, such applications with next-generation display switches, sensors, radio frequency circuits and power converters.

{\rm ZrO}_{2}

ZrO2 has been deposited on GaN by Atomic Layer Deposition. Multiple Metal-Oxide-Semiconductor Capacitors with 4.4 nm, 5.4 nm, and 8.5 nm of ZrO2 oxide were fabricated with Cr electrodes. Capacitance measurements produce capacitance densities as high as 3.8 μF/cm2. Current densities of 0.88 A/cm2 at 1 V for the 4.4 nm oxides and hysteresis values of less than 6 mV were observed for the 5.8 nm oxide, indicating an interfacial Dit not greater than 6.4 × 1010 cm2. Temperature dependent current measurements revealed no signature Poole-Frankel component. Comprehensive assessment of these measurements indicates a low defect density oxide formed on GaN with a low number of interface states.

Gate Dielectrics on GaN

We have fabricated ZnO source-gated thin film transistors (SGTFTs) with a buried TiW source Schottky barrier and a top gate contact. The ZnO active channel and thin high-κ HfO2 dielectric utilized are both grown by atomic layer deposition at temperatures less than 130 °C, and their material and electronic properties are characterized. These SGTFTs demonstrate enhancement-mode operation with a threshold voltage of 0.91 V, electron mobility of 3.9 cm2 V−1 s−1, and low subthreshold swing of 192 mV/decade. The devices also exhibit a unique combination of high breakdown voltages (>20 V) with low output conductances.

Capacitance Modeling and Characterization of Planar MOSCAP Devices for Wideband-Gap Semiconductors With High-

Organic solar cells (OSCs) are a complex assembly of disparate materials, each with a precise function within the device. Typically, the electrodes are flat, and the device is fabricated through a layering approach of the interfacial layers and photoactive materials. This work explores the integration of high surface area transparent electrodes to investigate the possible role(s) a three-dimensional electrode could take within an OSC, with a BHJ composed of a donor-acceptor combination with a high degree of electron and hole mobility mismatch. Nanotree indium tin oxide (ITO) electrodes were prepared via glancing angle deposition, structures that were previously demonstrated to be single-crystalline. A thin layer of zinc oxide was deposited on the ITO nanotrees via atomic layer deposition, followed by a self-assembled monolayer of C60-based molecules that was bound to the zinc oxide surface through a carboxylic acid group. Infiltration of these functionalized ITO nanotrees with the photoactive layer, the bulk heterojunction comprising PC71BM and a high hole mobility low band gap polymer (PDPPTT-T-TT), led to families of devices that were analyzed for the effect of nanotree height. When the height was varied from 0 to 50, 75, 100, and 120 nm, statistically significant differences in device performance were noted with the maximum device efficiencies observed with a nanotree height of 75 nm. From analysis of these results, it was found that the intrinsic mobility mismatch between the donor and acceptor phases could be compensated for when the electron collection length was reduced relative to the hole collection length, resulting in more balanced charge extraction and reduced recombination, leading to improved efficiencies. However, as the ITO nanotrees increased in height and branching, the decrease in electron collection length was offset by an increase in hole collection length and potential deleterious electric field redistribution effects, resulting in decreased efficiency.

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To enable scalable MOSFET technology in III-V semiconductor platforms, high quality semiconductor-oxide interfaces are essential. In this paper, a novel low-temperature plasma-enhanced atomic layer deposition (PEALD) technique was applied to deposit nanoscale high-k dielectrics on several III-V substrates, including InP, GaAs, InAs, and GaN. Approximately 7 nm of ZrO2 was grown and patterned to form MOSCAP structures, which were subsequently analyzed through electrical characterization to evaluate dielectric and interface quality. The oxide films fabricated were found to have interface trap densities ranging from 1010 - 1013 eV-1cm-2, and showed high capacitance densities (~ 2.5 μF/cm2). GaN and InP MOSCAPs with ZrO2 dielectric layers were found to have gate currents in line with direct tunneling phenomena and MOS mobilities approaching that of doped bulk semiconductors. Scaled InP MOSFET devices using these experimental values were also simulated using an optimized device structure.

Dielectrics

Contact effects at the source/drain (S/D) electrodes in thin film transistors (TFTs) based on zinc oxide (ZnO) utilizing the top gate, bottom contact geometry are investigated using electrical and material studies. Low temperature atomic layer deposition (ALD) is employed to directly grow the ZnO active layer on Au, Ru, and TiW bottom electrodes. TFT performance varies significantly depending on the S/D metallization scheme. Au contacts are seen to uniquely promote the growth of a highly n-doped ZnO channel that degrades the TFTs output impedance and controllability. On the other hand, TiW contacts suffer from poor carrier injection because of Schottky barrier formation. The best overall performance is demonstrated by Ru/ZnO TFTs based on their excellent switching capabilities alongside a moderately high mobility. From X-ray photoelectron spectroscopy measurements, the superior performance of Ru is the consequence of an oxidation-free interface that inhibits the adsorption of hydroxide surface dopants. Thus, the surface chemistry of the metal contact plays a large role on the ZnO growth and metal/ZnO interface energetics.

Sustained hole inversion layer in a wide-bandgap metal-oxide semiconductor with enhanced tunnel current

Metamaterials mediate long-range dipole-dipole interactions between quantum emitters. Dipole-dipole interactions (Vdd) between closely spaced atoms and molecules are related to real photon and virtual photon exchange between them and decrease in the near field connected with the characteristic Coulombic dipole field law. The control and modification of this marked scaling with distance have become a long-standing theme in quantum engineering since dipole-dipole interactions govern Van der Waals forces, collective Lamb shifts, atom blockade effects, and Förster resonance energy transfer. We show that metamaterials can fundamentally modify these interactions despite large physical separation between interacting quantum emitters. We demonstrate a two orders of magnitude increase in the near-field resonant dipole-dipole interactions at intermediate field distances (10 times the near field) and observe the distance scaling law consistent with a super-Coulombic interaction theory curtailed only by absorption and finite size effects of the metamaterial constituents. We develop a first-principles numerical approach of many-body dipole-dipole interactions in metamaterials to confirm our theoretical predictions and experimental observations. In marked distinction to existing approaches of engineering radiative interactions, our work paves the way for controlling long-range dipole-dipole interactions using hyperbolic metamaterials and natural hyperbolic two-dimensional materials.