Bum Ho Choi

Electrical and Optical Properties of Index-Matched Transparent Conducting Oxide Layers for Liquid Crystal on Si Projection Displays

In this study, the characteristics of several kinds of index-matched transparent conducting oxide layers (TCO) were investigated for applications to liquid crystals on silicon (LCoS) projection displays. The purpose of exploring the characteristics of these materials is to find alternatives to currently used index-matched indium-doped tin oxide (ITO) layers, since index-matched ITO suffers from high production costs and materials shortages. Based on zinc oxide (ZnO) TCO layers, metal dopants such as Al, In, or Ga were added to improve the optical and electrical properties; 15 nm thick ZnO, aluminum-doped ZnO (AZO), indium–gallium-doped ZnO (IGZO), indium-doped ZnO (IZO), and gallium-doped ZnO (GZO) layers were sputtered on glass substrates using radio frequency (RF) and direct current (DC) magnetron sputtering. The measured transparency in the visible radiation range was above 94% for all prepared index-matched TCO layers. Among them, the transparency of AZO layers was the highest, reaching 97.5%. The sheet resistance of the TCO layers was around 100 Ω cm-2, with 82.6 Ω cm-2 being the lowest measured value obtained from a 15 nm thick AZO layer. Furthermore, the sheet resistance uniformity measured by samples with an area of 200×200 mm2 was below 5%. Atomic force microscopy measurement results show that the root-mean-square surface roughness values were lower than 0.01 nm in ZnO and AZO, and 0.128 and 0.261 in IGZO and GZO, respectively. The contact angle, which is another key factor in index-matched TCO-coated substrates, was around 25°, which meets the requirements for LCoS projection display panels. Among the tested TCO layers, AZO exhibited superior characteristics in terms of optical and electrical properties. Therefore, AZO represents an alternative to currently used index-matched ITO layers in LCoS projection displays.

Stress-Driven Formation of InGaAs Quantum Dots on GaAs with Sub-Micron Platinum Pattern

We study the formation of self-assembled InGaAs quantum dots on GaAs substrate with a sub-micron platinum stripe pattern. Islands or quantum dots preferentially nucleate at the boundary of metal patterns. In addition, island density reduced region near the boundary of the metal pattern is found. Those results are attributed to the stress between metal stripe and GaAs surface, which produces a laterally stressed region around the metal stripe. Adatoms on this region preferentially migrate toward the edge of metal stripes with maximum stress. This result may show a possible way for the interconnection between randomly distributed self-assembled quantum dots and metal stripes.

Suppression of Cross Contamination in Multi-Layer Thin Film Prepared by Using Rotating Hexagonal Sputtering Cathode.

In this study, single- and multi-layered thin films were prepared on a glass substrate using a newly developed rotating hexagonal sputtering cathode in a single chamber. The rotatinghexagonal sputtering cathode can install up to six different sputtering targets or six single targets in a cathode. Using the rotating hexagonal cathode, we prepared a single-layered AZO film and a multi-layer film to evaluate the performance of hexagonal gun. Cross-contamination, which is often observed in multi-layer thin film preparation, was suppressed to nearly zero by controlling process parameters and revising hardware. Energy-saving effects of five-layered glass were also verified by measuring the temperature.

Continuous Synthesis of Carbon Nanotubes Using a Plasma-Enhanced Chemical Vapor Deposition System at Atmospheric Pressure

Continuous synthesis of carbon nanotubes (CNTs) without replenishment of catalyst was investigated using a plasma-enhanced chemical vapor deposition system at atmospheric pressure for the first time. CNTs were successfully grown in a continuous manner on an iron-catalyzed substrate up to a height of 750 µm by two cycle of continuous growth. In the first growth step, the height of the CNT forest was 427 µm under the optimum synthesis conditions. The high-density Fe nanoparticles remained on the surface after the first growth step, although the density was slightly reduced. The second growth step was performed after harvesting the grown CNTs mechanically. The height of the CNTs obtained in the second growth step was 320 µm, which is 100 µm shorter than that of the CNTs obtained in the first growth step. By the same procedure, the third growth step was carried out. Unlike the results of the first two growth steps, the CNT forest was sparsely formed with a height of less than 30 µm, suggesting that the catalytic material was used up during the second growth step. The results show that a 20-nm-thick catalytic material was consumed for the growth of CNTs up to a height of 750 µm. This study opens up the possibility of continuous synthesis of CNTs and mass production at low cost for industrial applications.

Self-Assembled Quantum Dot Single Electron Devices

Single electron tunneling and its application to future VLSI systems has been an important subject extensively studied for the last decade [l]. Many types of materials and ideas have been applied to fabricate and implement single electron devices operating at high temperatures. The self-assembled quantum dot (SAQD) system is one of the attractive candidates for single electron devices since high quality Coulomb islands can be obtained in one-step growth processes. Furthermore, the characteristic energy scale of the devices would enhance because the quantum energy is expected to be added to the classical charging energy. on InGaAs SAQDs. lever-arms with nm spacings. staircases at 77 K and higher temperatures. Figure 1 (a) and (b) show an AFM photos of typical SAQD single electron devices fabricated by the lever-arm technique. The InGaAs SAQDs we have used were grown by an MOCVD technique and the typical diameter of the dots is approximately 20 nm [2]. The aluminum lever-arms with spacings from 200 to 40 nm were fabricated by a standard ebeam exposure and a lift-off process. Figure 2 (a) and (b) show the 77 K current-voltage (I V) and its differential conductance - voltage characteristics.(dUdV - V) of lever - arm device with the gap of 40 nm. Several staircases are clearly identified in both the I-V and the dVdV-V. dI/dV-V of the device with the gap of 150 nm. Clear staircases are also seen. These staircases are originated from the single electron tunneling through SAQDs located in the shortest current path between two lever - arms. In conclusion, self-assembled guantum dot single electron devices are made by the lever-arm technique with the minimum gap spacing of 40 nm and clear staircases are observed in the I-V characteristics. The result of more complicated devices with multiple lever-arms will also be presented at the conference.

Electrical Properties of Electron-Beam Exposed Silicon Dioxides and Their Application to Nano-devices

Electrical properties of the electron-beam induced carbon contamination layers have been reported. Contacts to the contamination layers are achieved by a simple deposition of aluminum and the current-voltage characteristics are successfully measured. A double junction structure, with the size smaller than 10 nm, has been fabricated by a one-step electron beam irradiation and it exhibits Coulomb staircases at room temperature.