Nae-Man Park

Band gap engineering of amorphous silicon quantum dots for light-emitting diodes

Amorphous silicon quantum dots (a-Si QDs), which show a quantum confinement effect were grown in a silicon nitride film by plasma-enhanced chemical vapor deposition. Red, green, blue, and white photoluminescence were observed from the a-Si QD structures by controlling the dot size. An orange light-emitting diode (LED) was fabricated using a-Si QDs with a mean size of 2.0 nm. The turn-on voltage was less than 5 V. An external quantum efficiency of 2×10−3% was also demonstrated. These results show that a LED using a-Si QDs embedded in the silicon nitride film is superior in terms of electrical and optical properties to other Si-based LEDs.

High efficiency visible electroluminescence from silicon nanocrystals embedded in silicon nitride using a transparent doping layer

We have fabricated light-emitting diodes with a transparent doping layer on silicon nanocrystals (nc-Si) embeded in silicon nitride matrix formed by plasma-enhanced chemical vapor deposition. Under forward biased condition, orange electroluminescence (EL) with its peak wavelength at about 600 nm was observed at room temperature. The peak position of the EL is very similar to that of the photoluminescence (PL) and the emitted EL intensity is proportional to the current density passing through the device. We suggest that the observed EL is originated from electron-hole pair recombination in nc-Si. By using indium tin oxide and n-type SiC layer combination as a transparent doping layer, we obtained high external quantum efficiency greater than 1.6%.

Epitaxial growth of ZnO nanowall networks on GaN/sapphire substrates

Heteroepitaxy of vertically well-aligned ZnO nanowall networks with a honeycomblike pattern on GaN∕c-Al2O3 substrates by the help of a Au catalyst was realized. The ZnO nanowall networks with wall thicknesses of 80–140nm and an average height of about 2μm were grown on a self-formed ZnO thin film during the growth on the GaN∕c-Al2O3 substrates. It was found that both single-crystalline ZnO nanowalls and catalytic Au have an epitaxial relation to the GaN thin film in synchrotron x-ray scattering experiments. Hydrogen-sensing properties of the ZnO nanowall networks have also been investigated.

ZnO film thickness effect on surface acoustic wave modes and acoustic streaming

Surface acoustic wave(SAW) devices were fabricated on ZnO thin films deposited on Si substrates. Effects of ZnOfilm thickness on the wave mode and resonant frequency of the SAWs have been investigated. Rayleigh and Sezawa waves were detected, and their resonant frequencies decrease with increase in film thickness. The Sezawa wave has much higher acoustic velocity and larger signal amplitude than those of Rayleigh mode wave.Acoustic streaming for mixing has been realized in piezoelectric thin filmSAWs. The Sezawa wave has a much better efficiency in streaming, and thus is very promising for application in microfluidics.

Lateral current transport path, a model for GaN-based light-emitting diodes: Applications to practical device designs

An advanced model to explain the current spreading phenomenon of a conventional GaN-based light-emitting diode is presented. For this work, an equivalent circuit, consisting of the two lateral resistance components of the p-transparent electrode and the n-type layer is proposed. Theoretical calculations clearly reveal that the current density crowds near the n or p pads according to the device parameters and has an exponential behavior as a function of the lateral length. Based on these results, appropriate device parameters including the critical transparent-electrode thickness were determined, leading to a perfectly uniform current distribution. It was even possible to demonstrate the ideal device geometry without the need for a transparent electrode such as an interdigitated structure.

Reactivation of Mg acceptor in Mg-doped GaN by nitrogen plasma treatment

Mg-doped GaN films, grown by metalorganic chemical vapor deposition, were treated with a nitrogen plasma after a conventional rapid thermal annealing (RTA). The conductivity of the p-type GaN film was greatly enhanced by nitrogen plasma treatment, and exhibited a higher sheet hole concentration as well as lower sheet resistance than the RTA samples. A photoluminescence (PL) band which peaked at 3.27 eV was new, and a band at 2.95 eV was markedly attenuated in the plasma treated samples. PL measurements suggest that self-compensation in a Mg-doped GaN caused by the nitrogen vacancies is effectively reduced by the nitrogen plasma treatment, leading to an enhanced p-type conductivity. In addition, the plasma-treated sample revealed a drastic reduction in specific contact resistance by three orders of magnitude, compared with the RTA samples.

Physics and Device Structures of Highly Efficient Silicon Quantum Dots Based Silicon Nitride Light-Emitting Diodes

An electrically driven light emitter from silicon is a long-standing problem in silicon photonics. Recently, significant progress has been made using silicon quantum dots (Si QDs) embedded in the silicon nitride thin films, transparent doping layers and electrodes, and surface-modified structures. This paper provides an overview of the progress in the device physics and fabrications of the Si QD light-emitting diodes (LEDs) including new device structures to improve the light extraction efficiency as well as highlights in the growth of the Si QDs and their quantum confinement effects (QCEs)

Band gap engineering of SiCN film grown by pulsed laser deposition

The band gap tuning of amorphous silicon carbon nitride thin (a-SiCN) film was demonstrated in the range of 2.3–3.0 eV by pulsed laser deposition using mixed targets. a-SiCN films were grown on silicon and quartz glass substrates at room temperature in a vacuum. Targets were fabricated by compacting a mixture of silicon carbide and silicon nitride powders. The stoichiometry of the film could be varied by the mixing ratio of the target. Ternary phase SiCN films were deposited at 30– 70 wt. % SiC in a target and their band gaps were controlled by appropriate adjustment of the carbon content. These findings indicate that the growth of a-SiCN films with a mixed target and their subsequent use as optoelectronic materials is a possibility.

Size-dependent charge storage in amorphous silicon quantum dots embedded in silicon nitride

Size-dependent charge storage was observed in metal–insulator–semiconductor structures containing amorphous Si quantum dots (a-Si QDs) grown by plasma-enhanced chemical vapor deposition. For a-Si QDs as large as 2 nm in diameter, one electron or one hole was stored in each a-Si QD. For small-sized a-Si QDs of 1.4 nm in diameter, however, the width of capacitance–voltage hysteresis was decreased, indicating that the charge density in the a-Si QDs was reduced. This can be attributed to the lowered tunneling barrier in the small-sized a-Si QDs resulting from a large quantum confinement effect. Long-term charge storage was observed in the fully charged a-Si QDs; this is attributed to a suppression of the discharge process by electrostatic repulsion among the charged dots.

Electron charging and discharging in amorphous silicon quantum dots embedded in silicon nitride

Electron charging and discharging were produced in metal-insulator-semiconductor structures containing amorphous silicon quantum dots (a-Si QDs) by increasing the applied voltage in a stepwise fashion without changing its sign. The metal-insulator-semiconductor structure was fabricated using an insulating silicon nitride film containing a-Si QDs by plasma-enhanced chemical vapor deposition. This charging behavior suggests that a-Si QDs in the silicon nitride are positively charged due to nitrogen dangling bonds. The surface state of the a-Si QDs is considered to play a dominant role in the charging properties such as electron storage and charge-loss rate in the a-Si QDs.