Ji-Myon Lee

Low-resistance and nonalloyed ohmic contacts to plasma treated ZnO

Low-resistance and nonalloyed ohmic contacts to epitaxially grown n-ZnO were formed by exposing n-ZnO to an inductively coupled hydrogen and an argon plasma. Using Ti/Au, the specific contact resistivity of the ohmic contact was drastically decreased from 7.3×10−3 to 4.3×10−5 Ω cm2 by hydrogen plasma treatment. The photoluminescence spectrum of the hydrogen plasma treated ZnO showed a large enhancement in band-edge emission and a strong suppression in deep-level emission. These results suggest that the low contact resistivity can be attributed to an increase in carrier concentration on the ZnO surface. The specific contact resistivity of the Ar-plasma treated sample was also decreased to 5.0×10−4 Ω cm2, presumably due to the formation of shallow donor on the ZnO surface by ion bombardment.

Modeling of a GaN-based light-emitting diode for uniform current spreading

The characteristics of the GaN/InGaN multiquantum-well light-emitting diode (LED) have been examined from the view point of uniform current spreading. By means of simple modeling, it was found that the current density and the length of the lateral current path through the transparent layer represent dominant parameters in determining uniform current spreading. In this regard, we studied the effect of current density on the reliability characteristics of the LED. We were able to significantly improve the electrical, optical, and reliability characteristics of the LED in terms of reducing the length of the lateral current path through the transparent layer.

Improvement in light-output efficiency of InGaN/GaN multiple-quantum well light-emitting diodes by current blocking layer

The fabrication and characterization of an InGaN/GaN multiple-quantum well (MQW) light-emitting diode (LED) with a SiO2 current blocking layer inserted beneath the p-pad electrode is described. The light-output power and external quantum efficiency for the InGaN/GaN MQW LED chip with a current blocking layer were significantly increased compared to those for the conventional InGaN/GaN MQW LED chip. The increase in the light-output power can be attributed to the injection of additional current into the light-emitting quantum well layer of the LED by the SiO2 current blocking layer and a reduction in parasitic optical absorption in the p-pad electrode.

Dry Etching of ZnO Using an Inductively Coupled Plasma

The dry etching characteristics of ZnO using an inductively couple plasma (ICP) have been investigated, for the first time, as functions of plasma chemistry, radio frequency (rf) table power, and ICP power. The CH 4 /H 2 etchant gases resulted in the highest etch rate of ZnO, suggesting that the etching of Zn in ZnO largely involves a process in which a volatile metallorganic zinc compound, such as Zn(CH 3 ) y is formed. The etch rate was increased with increasing rf table power, and the highest etch rate of 2000 A/min was achieved at an rf table power of 200 W (dc bias: -80 V). As the ICP power was increased, the etch rate also increased, which suggests that the plasma density is also an important factor in this process. Furthermore, it was observed that hydrogen-containing plasma etching enhances the band-edge photoluminescence of the ZnO film.

Dry etch damage in n-type GaN and its recovery by treatment with an N2 plasma

We report on etch-induced damage in n-type GaN caused by an inductively coupled plasma, and damage recovery by means of treatment with an N2 plasma. As the plasma dc bias was increased by increasing the rf table power during etching, the optical and electrical properties of the etched GaN films deteriorated as the result of etch-induced damage. However, an N2 plasma treatment for the etched samples effectively removed the etch-induced defects and damage on the surface, leading to improved surface morphology, photoluminescence, and ohmic contact in n-type GaN.

Temperature dependence of photoluminescence of InGaN films containing In-rich quantum dots

The temperature dependence of the photoluminescence (PL) of InGaN films, grown by metalorganic chemical vapor deposition, has been investigated. A strained InGaN thin film which contains composition-fluctuated regions shows the so-called S-shaped temperature dependence of the dominant PL peak energy. However, an InGaN thick film which contains quantum dot-like In-rich regions shows a sigmoidal temperature dependence of the dominant PL peak energy, as the result of a transfer of carriers from the band-edge related luminescent centers to quantum dot-like In-rich regions. It is also found that the activation energy for the thermal quenching of PL intensity in the InGaN thick film which contains quantum dot-like In-rich regions is larger than that in the strained InGaN thin film which contains composition-fluctuated regions.

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.

Cl2 ‐ Based Dry Etching of GaN and InGaN Using Inductively Coupled Plasma The Effects of Gas Additives

The effects of added H 2 , Ar, and CH4 gases on the etch characteristics of GaN and InGaN were studied using an inductively coupled Cl 2 -based plasma. Each added gas had a unique effect on the etch rate, anisotropy, surface roughness, and sidewall morphology. The most anisotropic etch profile was obtained using Cl 2 but the etched surface showed the roughest morphology and was covered with etch residues, the origins of which were the micromasking of the sputtered dielectric. When H 2 gas was added to the Cl 2 plasma the etch residues were removed and the surface roughness was decreased, even though the etch rate was slightly decreased. The etch rate of GaN by Cl 2 /H 2 /Ar plasmas was saturated above an Ar flow rate of 16 sccm and the surface roughness of the etched GaN was lower, compared with Cl 2 /H 2 plasmas at low source power. Finally, it was found that the In compound was etched as a result of reaction with CH 4 .

InGaN/GaN multiple quantum well light-emitting diodes with highly transparent Pt thin film contact on p-GaN

The fabrication and characterization of an InGaN/GaN multiple quantum well light-emitting diode (LED) with a highly transparent Pt thin film as a current spreading layer are described. The room temperature electroluminescence exhibits a strong emission at 453 nm. Pt-contacted LEDs show good electrical properties and high light-output efficiency compared to Ni/Au-contacted ones. The light transmittance and the specific contact resistance of a Pt thin film with a thickness of 8 nm on p-GaN was determined to be 85% at 450 nm and 9.12×10−3 Ω cm2, demonstrating that a Pt thin film can be used as an effective current spreading layer with high light transparency.

Nitridation of sapphire substrate and its effect on the growth of GaN layer at low temperature

A remote plasma enhanced-ultrahigh vacuum chemical vapor deposition system equipped with a radio frequency (RF) — inductively coupled plasma which produces the reactive nitrogen species was employed to grow GaN layers at low temperature. The X-ray photoelectron spectroscopy analysis of nitrogen composition on the nitridated substrate surface indicated that the nitridation process on the substrate surface is largely a⁄ected by RF power at low temperature. However, the atomic force microscope images indicated that the protrusion density on the nitridated sapphire surface is critically dependent on the nitridation temperature. It was possible to nitridate the sapphire surface without the production of protrusions by controlling the RF power and nitridation temperature even at low temperatures. The crystallinity of the GaN grown at 450iC was found to be much improved when the sapphire substrate was nitridated at low temperature prior to the GaN layer growth. Moreover, lateral growth of the GaN layer was enhanced not only by an increase in growth temperature but also by the nitridation of sapphire substrate prior to the GaN layer growth. ( 1999 Elsevier Science B.V. All rights reserved.