Young Yi Kim's research while affiliated with Sungkyunkwan University and other places

While tin oxides such as SnO and SnO2 are widely used in various applications, surprisingly, only a limited number of reports have been presented on the microstructural characteristics of tin oxide thin films grown under various growth conditions. In this paper, the effects of the substrate temperature and content of foreign Zn ion on the microstructural characteristics of tin oxide thin films grown by radio-frequency magnetron sputtering were investigated. The increase in substrate temperature induced change in the stoichiometry of the thin films from SnO1+x to SnO2−x . Additionally, the phase contrast in the transmission electron microscopy image revealed that SnO1+x and SnO2−x phases were alternating in thin films and the width of each phase became narrower at high substrate temperature. The ternary zinc tin oxide thin films were deposited using the co-sputtering method. As the ZnO target power increased, the crystallinity of the thin films became poly-crystalline, and then showed improved crystallinity again with two types of phases.

The development of p-channel tin oxide thin-film-transistors spurred the research into microstructural analysis of tin oxide phases and control of conduction type, as it is widely known that tin oxide thin films exhibit both n- and p-type conduction depending on growth conditions. This study reports the relationship between the microstructural properties and the ambiguity of the electrical conduction type observed in nonstoichiometric tin oxides. Nonstoichiometric tin oxide thin films have been produced by RF magnetron sputtering with a dependence on the growth gas atmosphere. The crystal phase of the tin oxide deposited under low ambient oxygen content was mainly SnO1+x with relatively stable p-type conduction. On the other hand, for deposition under high ambient oxygen content, phase separation with structural modulation in the tin oxide film occurred in SnO-like and SnO2-like regions. These phases with different conduction types caused electrically unstable dual conduction types in the tin oxide films, despite their low electrical resistivity.

Vertically aligned Ga-doped ZnO (ZnO:Ga) nanorods were grown on sapphire substrates without buffer layers under conditions designed to ensure rapid growth rates with a magnetron sputtering system. Unlike the undoped ZnO layer with microsize protrusions, the 1 and 2wt% Ga doping induced the formation of uniform nanorod arrays with epitaxial growth behavior. In the initial growth stage, the ZnO:Ga layers exhibited island growth with pyramidal shapes due to enhanced misfit strain. This promoted preferential growth along the c-axis, resulting in the formation of nanorod arrays ,without buffer layers. However, the increased stress stored in the ZnO:Ga layers generated high density of stacking faults, which led to an intense donor–acceptor pair emission at 3.31eV. In addition, the sample doped with a high Ga amount (2wt%) had rotated crystal phases, resulting in rough sidewalls with nano-branch or sawtooth shapes.

This study examined the effect of the synthesis temperatures on the characteristics of vertically aligned Ga-doped ZnO (GZO) nanorods grown on a ZnO template by thermal evaporation using Zn and Ga sources. The increase in synthesis temperature at less than 700 degrees C induced stress relaxation relative to the ZnO template due to the suppression of defect generation by the formation of nanorods, while a further increase resulted in an increase in compressive strain due to dominant Ga doping. The increase in Ga concentration in the GZO nanorods with increasing synthesis temperature was also confirmed by X-ray photoelectron spectroscopy and photoluminescence. The best conductivity was observed in the GZO nanorods grown at 800 degrees C. On the other hand, the GZO nanorods synthesized at 900 degrees C showed less conductivity and weak near-band-edge emission properties due to the generation of defects from the excess Ga.

We grew heterojunction light emitting diode (LED) structures with various n-type semiconducting layers by magnetron sputtering on p-type GaN at high temperature. Because the undoped ZnO used as an active layer was grown under oxygen rich atmosphere, all LED devices showed the EL characteristics corresponding to orange-red wavelength due to high density of oxygen interstitial, which was coincident with the deep level photoluminescence emission of undoped ZnO. The use of the Ga doped layers as a top layer provided the sufficient electron carriers to active region and resulted in the intense EL emission. The LED sample with small quantity of Mg incorporated in MgZnO as an n-type top layer showed more intense emission than the LED with ZnO, in spite of the deteriorated electrical and structural properties of the MgZnO film. This might be due to the improvement of output extraction efficiency induced by rough surface.

Phosphorus (P)-doped ZnO thin films with amphoteric doping behavior were grown on c-sapphire substrates by radio frequency magnetron sputtering with various argon/oxygen gas ratios. Control of the electrical types in the P-doped ZnO films was achieved by varying the gas ratio without post-annealing. The P-doped ZnO films grown at a argon/oxygen ratio of 3/1 showed p-type conductivity with a hole concentration and hole mobility of 1.5×1017cm−3 and 2.5cm2/Vs, respectively. X-ray diffraction showed that the ZnO (0002) peak shifted to lower angle due to the positioning of P3− ions with a larger ionic radius in the O2− sites. This indicates that a p-type mechanism was due to the substitutional PO. The low-temperature photoluminescence of the p-type ZnO films showed p-type related neutral acceptor-bound exciton emission. The p-ZnO/n-Si heterojunction light emitting diode showed typical rectification behavior, which confirmed the p-type characteristics of the ZnO films in the as-deposited status, despite the deep-level related electroluminescence emission.

n-ZnO:Ga/i-ZnO/p-Si heterojunction light-emitting diodes were fabricated on patterned Si substrates with increased interface area where hole carriers were transported to the i-ZnO layer. The patterned Si substrates were prepared by electrochemical etching, and the n-type ZnO:Ga films were deposited by high-temperature sputtering. In the patterned LED, the lower breakdown and greater leakage current under a reverse bias was attributed to the formation of a high density of grain boundaries and random tilting of the c-axis. Compared to an LED without patterning, the patterned substrates resulted in approximately 75% improvement in the output power of visible emission, which was attributed to a 1.33-fold increase in the heterojunction area and the increase in grain boundary density due to grain tilting.

The dependence of the MgO sputtering power on the structural and optical properties of epitaxially grown MgZnO thin films on GaN/sapphire substrates by radio-frequency magnetron sputtering was investigated. The photoluminescence investigation showed blue shift of 170 meV in MgZnO film grown at the MgO power of 300 W, compared with the ZnO films grown at the MgO power of 0 W, which was attributed to the enhancement of the Mg incorporation at higher power. In addition, increase in Mg mole fraction with increase in sputtering power of MgO was observed from the PL results, and a maximum of 6.6 at.% Mg was obtained at the MgO power of 300 W. The high-resolution X-ray diffraction and transmission electron microscopy (TEM) investigations revealed that the threading dislocation density in the MgZnO thin films increased with increase in sputtering power. Furthermore, microstructural analysis performed by TEM revealed formation of a thin cubic-like phase in the interface between GaN template and MgZnO thin film, together with increased thickness of the interfacial layer with sputtering power.

MgZnO/ZnO heterostructures with different Mg compositions were grown on c-sapphire substrates by radio frequency magnetron co-sputtering using two targets, and their microstructural and optical properties were investigated. The analysis showed a blue shift in the ultraviolet emission and a decrease in lattice spacing with increasing power to the MgO target (i.e. increasing Mg composition in the film) up to 200 W at a ZnO target power of 300 W. Further increases in the MgO target power showed an opposite trend, which was attributed to the phase separation into Mg-rich and Zn-rich phases. The film phase separated naturally along the vertical direction and consisted of bottom and top regions, which corresponded to hexagonal and cubic structures, respectively.

The microstructural characteristics and crystallographic evolutions of Ga-doped ZnO (GZO) films grown at high temperatures were examined by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The GZO films with various film thicknesses were grown on (0 0 0 1) Al2O3 substrates at 750 °C by RF magnetron sputtering using a 2 wt% Ga-doped ZnO single target. The (0 0 0 2) ZnO peaks in the XRD patterns shifted to a higher angle with increasing film thickness and an additional (1 0 1¯ 1) ZnO peak was observed in the final stage of film growth. HRTEM showed the epitaxial growth of GZO films in the initial growth stage and the formation of surface protrusions in the intermediate stage due to elastic relaxation. The surface protrusions consisted of {1 0 1¯ 1}, {1 0 1¯ 3}, and {0 0 0 2} planes. After the surface protrusions had formed, a GZO film with many c-axis tilted grains formed due to plastic relaxation, where the tilted grain boundaries had an angle of 62° to the substrate. The formation of the protrusions and c-axis tilted grains was closely related to the strain status of the film induced by Ga incorporation, high-temperature growth and a high film thickness.