Woo-Shik Jung

Controllable Nondegenerate p-Type Doping of Tungsten Diselenide by Octadecyltrichlorosilane

Despite heightened interest in 2D transition-metal dichalcogenide (TMD) doping methods for future layered semiconductor devices, most doping research is currently limited to molybdenum disulfide (MoS2), which is generally used for n-channel 2D transistors. In addition, previously reported TMD doping techniques result in only high-level doping concentrations (degenerate) in which TMD materials behave as near-metallic layers. Here, we demonstrate a controllable nondegenerate p-type doping (p-doping) technique on tungsten diselenide (WSe2) for p-channel 2D transistors by adjusting the concentration of octadecyltrichlorosilane (OTS). This p-doping phenomenon originates from the methyl (-CH3) functional groups in OTS, which exhibit a positive pole and consequently reduce the electron carrier density in WSe2. The controlled p-doping levels are between 2.1 × 10(11) and 5.2 × 10(11) cm(-2) in the nondegenerate regime, where the performance parameters of WSe2-based electronic and optoelectronic devices can be properly designed or optimized (threshold voltage↑, on-/off-currents↑, field-effect mobility↑, photoresponsivity↓, and detectivity↓ as the doping level increases). The p-doping effect provided by OTS is sustained in ambient air for a long time showing small changes in the device performance (18-34% loss of ΔVTH initially achieved by OTS doping for 60 h). Furthermore, performance degradation is almost completely recovered by additional thermal annealing at 120 °C. Through Raman spectroscopy and electrical/optical measurements, we have also confirmed that the OTS doping phenomenon is independent of the thickness of the WSe2 films. We expect that our controllable p-doping method will make it possible to successfully integrate future layered semiconductor devices.

N-Channel Germanium MOSFET Fabricated Below 360

Below 360°C, we demonstrate germanium (Ge) n+/p junction diode and n-channel Ge metal-oxide-semiconductor field-effect transistor (MOSFET) with a low temperature Al/Al₂O₃/GeO₂ gate stack for monolithic 3-D integration using a metal-induced dopant activation (MIDA) technique. In particular, the cobalt (Co) MIDA phenomenon is investigated on Ge damaged by an implantation process. Shallow (~100 nm) source/drain junctions with very low resistivity (5.2 × 10^-4 Ω-cm) are then achieved at very low temperature by the Co MIDA technique. Consequently, high diode and transistor current on/off ratios (~10⁴ and ~10³, respectively) are obtained in this n-channel Ge MOSFET.

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Despite growing interest in doping two-dimensional (2D) transition metal dichalcogenides (TMDs) for future layered semiconductor devices, controllability is currently limited to only heavy doping (degenerate regime). This causes 2D materials to act as metallic layers, and an ion implantation technique with precise doping controllability is not available for these materials (e.g., MoS2, MoSe2, WS2, WSe2, graphene). Since adjustment of the electrical and optical properties of 2D materials is possible within a light (nondegenerate) doping regime, a wide-range doping capability including nondegenerate and degenerate regimes is a critical aspect of the design and fabrication of 2D TMD-based electronic and optoelectronic devices. Here, we demonstrate a wide-range controllable n-doping method on a 2D TMD material (exfoliated trilayer and bulk MoS2) with the assistance of a phosphorus silicate glass (PSG) insulating layer, which has the broadest doping range among the results reported to date (between 3.6 × 10(10) and 8.3 × 10(12) cm(-2)) and is also applicable to other 2D semiconductors. This is achieved through (1) a three-step process consisting of, first, dopant out-diffusion between 700 and 900 °C, second, thermal activation at 500 °C, and, third, optical activation above 5 μW steps and (2) weight percentage adjustment of P atoms in PSG (2 and 5 wt %). We anticipate our widely controllable n-doping method to be a starting point for the successful integration of future layered semiconductor devices.

by Cobalt-Induced Dopant Activation for Monolithic Three-Dimensional-ICs

Vacancy defects in germanium (Ge) adversely impact the electrical performance of Ge based metal-oxide-semiconductor field-effect transistor (MOSFET) in several ways. They behave as an acceptor site, thereby deactivating n-type dopants in the source/drain region. They can also increase substrate leakage currents and impact carrier lifetime in the channel region. In this paper, we characterize and verify the electrical behavior of vacancy defects in Ge using spreading resistance profiling (SRP). Effect of thermal annealing on the vacancy concentration is studied. Finally, passivation of these defects using fluorine (F) ion-implant is shown to demonstrate the feasibility of performance enhancement in Ge-MOSFETs.

Wide-range controllable n-doping of molybdenum disulfide (MoS2) through thermal and optical activation.

A WSe2 -based vertical graphene-transition metal dichalcogenide heterojunction barristor shows an unprecedented on-current increase with decreasing temperature and an extremely high on/off-current ratio of 5 × 10(7) at 180 K (3 × 10(4) at room temperature). These features originate from a trap-assisted tunneling process involving WSe2 defect states aligned near the graphene Dirac point.

Fluorine passivation of vacancy defects in bulk germanium for Ge metal-oxide-semiconductor field-effect transistor application

In this work, we investigate a low temperature boron (B) and phosphorus (P) activation in amorphous (α)-Ge using metal-induced crystallization technique. Eight candidates of metals (Pd, Cu, Ni, Au, Co, Al, Pt, and Ti) are used to crystallize the α-Ge at a low temperature. Resistivity measurement, transmission electron microscopy, and x-ray diffraction analyses reveal behaviors of the metal-induced dopant activation process using the metals reacting with α-Ge. It is revealed that Co achieves the highest B and P activation ratio in Ge below 360 °C with a slow diffusion rate. This method can be utilized to activate gate, source, and drain of transistors on upper layers in three-dimensional integrated circuits, where low temperature processing is critical.

Extremely Large Gate Modulation in Vertical Graphene/WSe2 Heterojunction Barristor Based on a Novel Transport Mechanism

High aspect-ratio three-dimensional (3D) a-Si:H solar cells have been fabricated to enhance a light absorption path while maintaining a short carrier collection length. Substantial efficiency enhancement in 3D solar cells was achieved due to the boost in JSC with no degradation of FF which is comparable to FF obtained from 2D solar cells.

Metal-induced dopant (boron and phosphorus) activation process in amorphous germanium for monolithic three-dimensional integration

In this letter, we investigate the electrical behavior of vacancy <i>V</i>_Ge defects in Ge at various thermal annealing conditions through electrochemical capacitance-voltage analysis. Then, the effects of the annealing process on Ge n⁺/p junction diodes were also studied with <i>J</i>-<i>V</i>, transmission electron microscopy, and secondary ion mass spectroscopy measurements in the aspects of point-defect healing and dopant diffusion/loss phenomena. The <i>V</i>_Ge defects tend to heal by recombining with Ge interstitial atoms as the annealing process temperature increases. However, the diffusion/loss problems of P atoms in Ge become severe at above 500^°C. Therefore, an optimal postfabrication annealing process at 600^°C is proposed in terms of point-defect healing and dopant diffusion/loss reduction.

9.4% Efficient Amorphous Silicon Solar Cell on High Aspect-Ratio Glass Microcones

In this work, the effect of Ge point defect healing process between 550 °C and 650 °C is investigated, in the aspect of leakage (off) current and junction depth of Ge n+/p junction diodes using ECV, TEM, J-V, and SIMS analyses. After 600 °C anneal, off-current density (2 × 10−4 A/cm2) is dramatically reduced due to the defect healing phenomenon that decreases the number of point defects, subsequently providing a higher on/off-current ratio of 5 × 103. In spite of the high healing temperature, junction diodes seem not to suffer from the deep diffusion of phosphorus (P) in Ge because those diffuse mostly through VGe. In addition, it is also confirmed that Ti is an appropriate material in terms of diffusion barrier and diffusivity for Ge n+/p junction contact metal.

Effects of Thermal Annealing on In Situ Phosphorus-Doped Germanium

In this letter, we investigate the impact of the thermal recovery (annealing) process on the electrical characteristics and the stability of IGZO TFTs in terms of: 1) vertical/lateral diffusion of Ti atoms into the IGZO channel region from the source/drain electrode and 2) recovery or local rearrangement of ions in IGZO. Although low thermal recovery temperatures <;300 °C are required to avoid the Ti diffusion that degrades IGZO TFT electrical characteristics, the TFT devices fabricated below 300 °C becomes very unstable in gate/drain electrical-stresses as a tradeoff.