In-Hoon Choi

Evolution of stress relaxation and yellow luminescence in GaN/sapphire by Si incorporation

We report a systematic study accomplished with a series of undoped and Si-doped GaN epilayers grown on sapphire (0001) with the carrier concentration of 4.0×1017−1.6×1019 cm−3 in order to investigate the evolution of stress relaxation and yellow luminescence by Si incorporation. As the Si doping becomes higher, the bound exciton peaks are gradually shifted to lower energy due to relaxation of the thermal residual stress with the linear coefficient of ΔE/Δσ∥=42 meV/GPa. The present results show that both the full width at half maximum of double-crystal x-ray diffractometry and the photoluminescence intensity ratio of the yellow luminescence to edge emission gradually increase as the Si incorporation becomes heavier. We suggest that the Si doping in GaN epilayers induces overall defects and gives rise to stress relaxation during the cool-down process, and the yellow luminescence may be originated from a complex of VGa and the Si-induced defect.

Room-temperature ferromagnetism of Cu-implanted GaN

1MeV Cu2+ ion was implanted into GaN with a dose of 1×1017cm−2 at room temperature. After implantation, the samples were subsequently performed by rapid thermal annealing at 700, 800, and 900°C for 5min. Both nonmagnetic Cu ion implanted samples annealed at 700 and 800°C exhibit the ferromagnetism at room temperature, and the saturation magnetization of these samples is estimated to be 0.057μB and 0.27μB per Cu atom from M-H curve, respectively. However, the sample annealed at 900°C does not show ferromagnetism due to clustering of Cu during the annealing process.

Fabrication of vanadium oxide thin film with high-temperature coefficient of resistance using V2O5/V/V2O5 multi-layers for uncooled microbolometers

Vanadium oxide thin film is a promising material for uncooled microbolometers due to its high temperature coefficient of resistance (TCR) at room temperature. It is, however, very difficult to deposit vanadium oxide thin films having a high temperature coefficient of resistance and low resistance because of the process limits in microbolometer fabrication. We present a novel fabrication method for vanadium oxide thin films having good electrical properties. Through the formation of a sandwich structure of V2O5 (100 A)/V (∼80 A)/V2O5 (500 A) by a conventional sputter method and post-annealing at 300 °C in oxygen, a mixed phase of VOx is formed. The results show that the mixed phase formed by this process has a high TCR of more than −2%/°C and low resistivity of <0.1 Ω cm at room temperature.

Stress relaxation in Si-doped GaN studied by Raman spectroscopy

We report the Si-doping-induced relaxation of residual stress in GaN epitaxial layers grown on (0001) sapphire substrate by the metalorganic vapor phase epitaxy technique. Micro-Raman spectroscopy is used to assess stress situation in the films with systematically modulated doping concentration from 4.0×1017 up to 1.6×1019 cm−3. As the Si-doping concentration increases, a monotonic decrease of the E2 phonon frequency is observed, which signifies gradual relaxation of the stress in the film. The layers are fully relaxed when electron concentration exceeds 1.6×1019 cm−3. The linear coefficient of shift in Raman frequency (ω) induced by the in-plane biaxial compressive stress (σ∥) is estimated to be Δω/Δσ∥=7.7 cm−1/GPa. We suggest that Si doping increases density of misfit dislocation, judging from linewidth of x-ray rocking curve.

Enhanced characteristics of an uncooled microbolometer using vanadium–tungsten oxide as a thermometric material

To produce a highly sensitive uncooled microbolometer, the development of a high-performance thermometric material is essential. In this work, amorphous vanadium–tungsten oxide was developed as a thermometric material at a low temperature of 300°C, and the microbolometer, coupled with the material, was designed and fabricated using surface micromachining technology. The vanadium–tungsten oxide showed good properties for application to the microbolometer, such as a high-temperature coefficient of resistance of over −4.0%∕K and good compatibility with the surface micromachining and integrated circuit fabrication process due to its low fabrication temperature. As a result, the uncooled microbolometer could be fabricated with high detectivity over 1.0×109cmHz1∕2∕W at a bias current of 7.5μA and a chopper frequency of 10–20Hz.

Effect of Be doping on the properties of GaMnAs ferromagnetic semiconductors

We have studied two series of molecular beam epitaxy grown Ga1−xMnxAs epilayers with several different Be doping levels. Two Mn concentrations x were chosen for this study: 0.03 and 0.05, and these values were maintained constant in each series. These samples were characterized by using SQUID and magnetotransport measurements. A systematic increase of the Curie temperature TC was observed in SQUID measurements on the series of Ga1−xMnxAs with x=0.03. The resistivity measured at zero magnetic field shows a local maximum near the Curie temperature, reflecting the effects of critical scattering near TC. The observed increase of TC in Ga1−xMnxAs for this low range of x can be explained by the increase of the free carrier concentrations in the system arising from Be doping. However, in the series of Ga1−xMnxAs with the higher concentration of Mn (x=0.05), the measurements reveal that the TC systematically decreases with increasing Be doping level. We discuss this effect in terms of a fundamental limitation of ...

The effect of ZrO2 buffer layer on electrical properties in Pt/SrBi2Ta2O9/ZrO2/Si ferroelectric gate oxide structure

Abstract We have investigated metal/ferroelectric/insulator/semiconductor (MFIS) structure with strontium bismuth tantalate (SBT) as ferroelectric thin film and ZrO 2 as the insulating buffer layer. Sr 0.8 Bi 2.4 Ta 2 O 9 thin films were prepared by metal organic deposition (MOD) method and ZrO 2 films were deposited by r.f.-sputtering. Coercive field that decisively affects the memory window was increased greatly by inserting the ZrO 2 insulator between SBT and SiO 2 and, thus, the memory window also increases with an electric field to the SBT. Memory windows of MFIS structure were in the range of 0.3–2.6 V when the gate voltage varied from 3 to 10 V. Memory windows of MFIS structure were found to be dependent on the thickness of the buffer layer. We observed the maximum memory window in MFIS with a 28-nm thickness of ZrO 2 layer. Auger electron spectroscopy (AES) depth profile and high resolution transmission electron microscopy (HRTEM) of SBT/ZrO 2 (28 nm)/Si structure showed that the ZrO 2 thin films as a buffer layer helped to prevent the formation of interfacial layer and interdiffusion between SBT and Si.

Mobility enhancement and yellow luminescence in Si-doped GaN grown by metalorganic chemical vapor deposition technique

Abstract We have studied the Si-doping characteristics in GaN on (0 0 · 1) sapphire substrates. The films were grown by horizontal metalorganic chemical vapor deposition technique at 100 Torr. Prior to Si-doping, achievement of optimum growth conditions was preceded in order to insure doping controllability. For undoped film, background carrier concentration was as low as 4 × 10 17 cm −3 . 10 K-photoluminescence (PL) in undoped film was dominated by the luminescence from two sharp free-excitons and one bound-exciton. n-Type doping with SiH 4 was carried out and electron concentration of up to 2 × 10 19 cm −3 was achieved. As the electron concentration increased, the activation efficiency of Si donor also increased. In spite of the increased carrier concentration, Hall mobility in the doped films was increased by a factor of up to two (220 cm 2 /V s) compared to the undoped films. The mobility behavior is attributed to a transport mechanism involving defect band conduction and does not correlate with the line width of the X-ray rocking curve. In addition, as the doping concentration increased, yellow luminescence occurring at around 2.2 eV gradually dominated PL spectra at 10 K and room temperature. Our PL study suggests that gallium vacancy is responsible for the yellow luminescence.

Effects of Bi-Pt alloy on electrical characteristics of Pt/SrBi2Ta2O9/CeO2/Si ferroelectric gate structure

Interface morphology and electrical properties of Pt/SrBi2Ta2O9(SBT)/CeO2/Si ferroelectric gate structure are characterized by considering the interactions among Bi, O, and Pt atoms during annealing process. It is found that the interfacial roughness of the Pt/SBT might be reduced during the annealing at 800 °C because the bottom side of the Pt electrode reacts with Bi atoms outdiffused from the SBT and the Bi–Pt alloys are molten at 765 °C, and the metallic Bi atoms are consumed by forming Bi oxide. Additionally, the capacitance and memory window of the ferroelectric gate structure annealed at 800 °C decrease to 69% and 80% of those values of the as-deposited gate structure, respectively, due to the additional capacitance and the voltage drop at the low dielectric Bi-oxide capacitor. In contrast, the leakage current characteristics are improved by two orders of magnitude after annealing at 800 °C for 30 min.

Improvement of electrical properties of ferroelectric gate oxide structure by using Al2O3 thin films as buffer insulator

Abstract SrBi2Nb2O9 (SBN) and Al2O3 thin films were prepared by r.f.-sputtering as a ferroelectric material and as a buffer insulator for metal/ferroelectric/insulator/semiconductor (MFIS) structure, respectively. The electrical properties of those films on Si substrate were studied. Coercive field that decisively affects the memory window was greatly increased by inserting an Al2O3 insulator between SBN and Si, and thus the memory window also increased with the increasing electric field to the SBN. The Al2O3 intermediate layer between the perovskite SBN film and Si substrate prevent SBN from the serious inter-diffusion into Si substrate. Memory windows of MFIS structure were in the range of 0.7–3.4 V when the gate voltage varied from 3 to 9 V. Memory windows of MFIS structure were found to be dependent on the thickness and stoichiometry of the buffer layer. We obtained the maximum memory window in the MFIS structure with an optimized insulator thickness of 11.4 nm.