Tailoring the Interface Quality between HfO₂ and GaAs via <i>in Situ</i> ZnO Passivation Using Atomic Layer Deposition

We investigated ZnO surface passivation of a GaAs (100) substrate using an atomic layer deposition (ALD) process to prepare an ultrathin ZnO layer prior to ALD–HfO₂ gate dielectric deposition. Significant suppression of both Ga–O bond formation near the interface and As segregation at the interface was achieved. In addition, this method effectively suppressed the trapping of carriers in oxide defects with energies near the valence band edge of GaAs. According to electrical analyses of the interface state response on p- and n-type GaAs substrates, the interface states in the bottom half of the GaAs band gap were largely removed. However, the interface trap response in the top half of the band gap increased somewhat for the ZnO-passivated surface

Electrical Properties of HfO₂ on Si_1–<i>x</i>Ge<i>_x</i> Substrates Pretreated Using a Y Precursor with and without Subsequent Oxidant Pulsing

We introduced Y–O bonds in the interfacial layer between HfO2 and Si1–xGex (x = 0, 0.15, and 0.3) using two different pretreatment methods to minimize the number of interfacial defects. The pretreatments involved the application of cyclic pulses of Y(CpBut)3 and N2, which proceeded with or without the injection of an oxidizing agent (H2O) at 250 °C, which was the temperature used for the subsequent in situ atomic layer deposition of HfO2. Both Y pretreatments were beneficial in reducing the leakage current and positive flatband voltage shift, which were induced by an increase in the Ge concentration of the substrate. In addition, the interface state density was significantly reduced by the pretreatments, and this effect was more pronounced when the oxidizing agent injection step was skipped. However, both pretreatments increased the capacitance-equivalent oxide thickness, thereby having an adverse effect, possibly owing to a change in the composition of the interfacial layer

Comparative Study of Atomic-Layer-Deposited Stacked (HfO₂/Al₂O₃) and Nanolaminated (HfAlO_<i>x</i>) Dielectrics on In_0.53Ga_0.47As

The high-k gate dielectric structures in stacked (HfO₂/Al₂O₃) and nanolaminated (HfAlO_<i>x</i>) forms with a similar apparent accumulation capacitance were atomic-layer-deposited on n-type In_0.53Ga_0.47As substrates, and their electrical properties were investigated in comparison with a single-layered HfO₂ film. Al-oxide interface passivation in both forms proved to be effective in preventing a significant In incorporation in the high-<i>k</i> film and reducing the interface state density. The measured valence band spectra in combination with the reflection electron energy loss spectra were used to extract the energy band parameters of various dielectric structures on In_0.53Ga_0.47As. A further decrease in the interface state density was achieved in the stacked structure than in the nanolaminated structure. However, in terms of the other electrical properties, the nanolaminated sample exhibited better characteristics than the stacked sample, with a smaller border trap density and lower leakage current under substrate injection conditions with and without voltage stressing

Improved Growth Behavior of Atomic-Layer-Deposited High‑<i>k</i> Dielectrics on Multilayer MoS₂ by Oxygen Plasma Pretreatment

We report on the effect of oxygen plasma treatment of two-dimensional multilayer MoS₂ crystals on the subsequent growth of Al₂O₃ and HfO₂ films, which were formed by atomic layer deposition (ALD) using trimethylaluminum and tetrakis-(ethylmethylamino)­hafnium metal precursors, respectively, with water oxidant. Due to the formation of an ultrathin Mo-oxide layer on the MoS₂ surface, the surface coverage of Al₂O₃ and HfO₂ films was significantly improved compared to those on pristine MoS₂, even at a high ALD temperature. These results indicate that the surface modification of MoS₂ by oxygen plasma treatment can have a major impact on the subsequent deposition of high-<i>k</i> thin films, with important implications on their integration in thin film transistors

Three-Dimensional Surface Treatment of MoS₂ Using BCl₃ Plasma-Derived Radicals

The realization of next-generation gate-all-around field-effect transistors (FETs) using two-dimensional transition metal dichalcogenide (TMDC) semiconductors necessitates the exploration of a three-dimensional (3D) and damage-free surface treatment method to achieve uniform atomic layer-deposition (ALD) of a high-k dielectric film on the inert surface of a TMDC channel. This study developed a BCl3 plasma-derived radical treatment for MoS2 to functionalize MoS2 surfaces for the subsequent ALD of an ultrathin Al2O3 film. Microstructural verification demonstrated a complete coverage of an approximately 2 nm-thick Al2O3 film on a planar MoS2 surface, and the applicability of the technique to 3D structures was confirmed using a suspended MoS2 channel floating from the substrate. Density functional theory calculations supported by optical emission spectroscopy and X-ray photoelectron spectroscopy measurements revealed that BCl radicals, predominantly generated by the BCl3 plasma, adsorbed on MoS2 and facilitated the uniform nucleation of ultrathin ALD–Al2O3 films. Raman and photoluminescence measurements of monolayer MoS2 and electrical measurements of a bottom-gated FET confirmed negligible damage caused by the BCl3 plasma-derived radical treatment. Finally, the successful operation of a top-gated FET with an ultrathin ALD–Al2O3 (∼5 nm) gate dielectric film was demonstrated, indicating the effectiveness of the pretreatment

Synthesis of Vertical MoO₂/MoS₂ Core–Shell Structures on an Amorphous Substrate via Chemical Vapor Deposition

Vertical MoO₂/MoS₂ core–shell structures were synthesized on an amorphous surface (SiO₂) by chemical vapor deposition at a high heating rate using a configuration in which the vapor phase was confined. The confined reaction configuration was achieved by partially covering the MoO₃-containing boat with a substrate, which allowed rapid buildup of the partially reduced MoO_3–<i>x</i> crystals in an early stage (below 680 °C). Rapid temperature ramping to 780 °C enabled spontaneous transition of the reaction environment from sulfur-poor to sulfur-rich, which induced a sequential phase transition from MoO_3–<i>x</i> to intermediate MoO₂ and finally to MoO₂/MoS₂ core–shell structures. The orthorhombic crystal structure of MoO_3–<i>x</i> contributed to the formation of vertical crystals on the amorphous substrate, whereas the nonvolatility of the subsequently formed MoO₂ enabled layer-by-layer sulfurization to form MoS₂ on the oxide surface with minimal resublimation loss of MoO₂. By adjustment of the sulfurization temperature and time, excellent control over the thickness of the MoS₂ shell was achieved through the proposed synthesis method

Ultrasensitive Room-Temperature Operable Gas Sensors Using p‑Type Na:ZnO Nanoflowers for Diabetes Detection

Ultrasensitive room-temperature operable gas sensors utilizing the photocatalytic activity of Na-doped p-type ZnO (Na:ZnO) nanoflowers (NFs) are demonstrated as a promising candidate for diabetes detection. The flowerlike Na:ZnO nanoparticles possessing ultrathin hierarchical nanosheets were synthesized by a facile solution route at a low processing temperature of 40 °C. It was found that the Na element acting as a p-type dopant was successfully incorporated in the ZnO lattice. On the basis of the synthesized p-type Na:ZnO NFs, room-temperature operable chemiresistive-type gas sensors were realized, activated by ultraviolet (UV) illumination. The Na:ZnO NF gas sensors exhibited high gas response (<i>S</i> of 3.35) and fast response time (∼18 s) and recovery time (∼63 s) to acetone gas (100 ppm, UV intensity of 5 mW cm^–2), and furthermore, subppm level (0.2 ppm) detection was achieved at room temperature, which enables the diagnosis of various diseases including diabetes from exhaled breath

Defect-Free Erbium Silicide Formation Using an Ultrathin Ni Interlayer

An ultrathin Ni interlayer (∼1 nm) was introduced between a TaN-capped Er film and a Si substrate to prevent the formation of surface defects during thermal Er silicidation. A nickel silicide interfacial layer formed at low temperatures and incurred uniform nucleation and the growth of a subsequently formed erbium silicide film, effectively inhibiting the generation of recessed-type surface defects and improving the surface roughness. As a side effect, the complete transformation of Er to erbium silicide was somewhat delayed, and the electrical contact property at low annealing temperatures was dominated by the nickel silicide phase with a high Schottky barrier height. After high-temperature annealing, the early-formed interfacial layer interacted with the growing erbium silicide, presumably forming an erbium silicide-rich Er–Si–Ni mixture. As a result, the electrical contact property reverted to that of the low-resistive erbium silicide/Si contact case, which warrants a promising source/drain contact application for future high-performance metal–oxide–semiconductor field-effect transistors