Young-Zo Yoo

S doping in ZnO film by supplying ZnS species with pulsed-laser-deposition method

S-doped ZnO (ZnO:S) film was fabricated by supplying ZnS species from laser ablation of a ZnS target during ZnO growth. Variations of lattice constants and band gaps with respect to S content did not follow Vegard’s law. The ZnO:S film showed semiconducting behavior with lower activation energy and resistivity than those of ZnO owing to higher carrier concentration. Despite the absence of magnetic elements, the large magnetoresistance amount of 26% was observed at 3 K from ZnO:S film.

Reduced defect densities in the ZnO epilayer grown on Si substrates by laser-assisted molecular-beam epitaxy using a ZnS epitaxial buffer layer

Nonradiative photoluminescence (PL) lifetime (τnr) and point defect density in the (0001) ZnO epilayer grown on (111) Si substrates by laser-assisted molecular-beam epitaxy (L-MBE) using a (0001) ZnS epitaxial buffer layer were compared with those in the ZnO films on (111) and (001) Si substrates prepared by direct transformation of ZnS epilayers on Si by thermal oxidation [Yoo et al., Appl. Phys. Lett. 78, 616 (2001)]. Both the ZnO films exhibited excitonic reflectance anomalies and corresponding PL peaks at low temperature, and the density or size of vacancy-type point defects (Zn vacancies), which were measured by the monoenergetic positron annihilation measurement, in the L-MBE epilayer was lower than that in the films prepared by the oxidation transformation. The ZnO epilayer grown on a (0001) ZnS epitaxial buffer on (111) Si exhibited longer τnr of 105ps at room temperature.

V defects of ZnO thin films grown on Si as an ultraviolet optical path

V defects were observed in the ZnO films epitaxially grown on the ZnS-buffered Si. Although the defects were located on the surface, strong near-bandedge emission confined to the {1011} facets of V defects was observed at room temperature. The near-bandedge emission spreads out over the whole film centering at V defects at 30 K. The detailed optical characterization shows that activation of excitonic absorption is responsible for this unique optical behavior.

45° rotational epitaxy of SrTiO3 thin films on sulfide-buffered Si

Sulfide was employed as a buffer layer for the growth of SrTiO3 (STO) thin films on Si. In order to utilize a relationship of a 45° rotational lattice match between Si and STO, ZnS, with almost the same lattice constant as Si, was used as the buffer. The buffer layer showed a partially disordered region at the ZnS/Si interface, owing to steady interdiffusion between ZnS and Si. STO film on ZnS buffered Si showed the rotational epitaxy with respect to Si and sharp STO/ZnS interface. Propagation of stacking faults from the ZnS/Si interface was observed, but those plane defects were terminated at the rotational STO/ZnS interface, resulting in high-quality STO films. The dielectric constant of the STO/ZnS film was 34.

Nonpolar a-plane GaN film on Si(100) produced using a specially designed lattice-matched buffer: A fresh approach to eliminate the polarization effect

Using pulsed laser deposition, nonpolar a-plane GaN thin films were grown on Si(100) substrates coated with a nonpolar MnS(100) plane buffer layer. The film showed an epitaxial relationship of GaN(112¯0)∥MnS(100)∥Si(100) with an in-plane alignment of GaN[1¯100]∥MnS[010]∥Si[010]. The high-resolution cross-sectional transmission electron microscopy image of the GaN∕MnS interface showed an abrupt atomic interface. A strong band-edge emission from the GaN film was observed at 364.3nm (=3.4eV) in cathodoluminescence measurements at 30K. This result in controlling the growth plane provides GaN films free of polarization effects in the direction of film growth, which favor the integration of optoelectronic devices combined with silicon.

Epitaxial Growth of the Wurtzite(1120) AlN Thin Films on Si(100) with MnS Buffer Layer.

A growth of the wurtzite (110) AlN thin films on Si(100) with MnS buffer layer was demonstrated for the first time by KrF pulsed excimer laser deposition. The presence of an epitaxial MnS layer on Si(100) substrate was found to affect the orientation of the AlN growth. AlN films was grown epitaxially on the MnS buffer at a substrate temperature higher than 700°C. The AlN/MnS/Si heterostructure is investigated by X-ray diffraction and in situ reflection high-energy electron diffraction. An AlN(110)∥MnS(100)∥Si(100) with in-plane alignment of AlN[0001]∥MnS[010]∥Si[010] and AlN[1100]∥MnS[001]∥Si[001] was observed. This result indicates a possibility of non-polar epitaxial AlN films on Si(100) with an expectation of a new substrate for GaN based optoelectronic devices on silicon.

Heteroepitaxy of hexagonal ZnS thin films directly on Si (111)

Epitaxial Wurtzite ZnS (W-ZnS) films were grown directly on Si (111) at temperatures as high as 800°C by pulsed laser deposition. A detailed deconvolution method was used to discriminate the ZnS reflection from superimposed peaks overlapped by both ZnS and Si peaks in X-ray measurement. From the peak deconvolution, the rocking full width at half maximum and lattice constant of the ZnS film were determined to be 0.28° and 6.26 A, respectively. A tilt between W-ZnS and Si was observed and the tilt angle decreased with increasing growth temperature. The W-ZnS (0001) film on Si (111) showed the epitaxial relationship of Si [220]//ZnS[1015]. An X-ray diffraction scan along the L direction in reciprocal space and a high-resolution transmission electron microscopy image revealed that the W-ZnS film partially had stacking fault domains in the hexagonal phase. Free excitonic emission from the W-ZnS film was observed at 4 K.

Effects of Single-Crystalline GaN Target on GaN Thin Films in Pulsed Laser Deposition Process

The effects of a GaN single-crystal target (SC-GaN) of laser-ablated GaN thin films on sapphire were investigated. GaN thin films were grown on a single-crystalline c-plane sapphire substrate using the SC-GaN. Unlike the usual growth behavior in laser ablation of a sintered ceramics GaN target (CR-GaN), the rocking curves of the obtained GaN films were superimposed patterns composed of sharp and broad components. The interface between the GaN film and sapphire substrate was atomically flat and sharp. To investigate the effects of the target on the plume of a pulsed laser deposition process in detail, the plume was analyzed by Langmuir probe methods and optical emission spectroscopy (OES). The Langmuir probe results showed that the electron temperature for the plume of an SC-GaN was nearly one third of that of a CR-GaN, although the ion density in the plume of the SC-GaN was found to be greater than the ion density in that of the CR-GaN. OES data showed a significant fraction of the oxygen species in the plume of the CR-GaN. These are thought to be the primary cause of damage and/or impurity to growing films in the plume of the CR-GaN.

Sulfide and Oxide Heterostructures For the SrTiO3 Thin Film Growth on Si and Their Structural and Interfacial Stabilities

Epitaxial SrTiO3 (STO) thin films with high electrical properties were grown on Si using ZnS single- and SrS/MnS hetero-buffer layers. STO films on both ZnS-buffered and SrS/MnS-buffered Si showed two growth orientations, (100) and (110). The temperature dependence of the growth orientation for STO films was different for the ZnS single-buffer layer in comparison with the SrS/MnS heterobuffer layers. (100) growth of STO films on SrS/MnS-buffered Si became dominant at high temperatures about 700 ?C, while (100) growth of STO films on ZnS-buffered Si became dominant at a relatively low growth temperature of 550 ?C. STO(100) films on ZnS-buffered and SrS/MnS-buffered Si showed lattice and domain matches for epitaxial relationships with [001]ZnS?[011]STO and SrS[001]?[011]STO, respectively via 45? in-plane rotation of STO films relative to both ZnS and SrS layers. The ZnS buffer layer contained many stacking faults because of the mismatch between ZnS and Si, however, those defects were terminated at the ZnS/STO interface. In contrast, the MnS buffer was very stable against stacking defect formation. Transmission electron microscopy measurements revealed the presence of a disordered region at the ZnS/Si and MnS/Si interfaces. Auger electron spectroscopy and transmission electron microscopy results showed that a good MnS/Si interface at the initial growth stage degraded to a SiS2-x-rich phase during MnS deposition and again into a SiO2-x-rich phase during STO deposition at the high growth temperature of 700 ?C. It was also observed that STO on SrS/MnS-buffered Si showed a markedly high dielectric constant compared with that of STO on ZnS-buffered Si.