Hye-Won Seo

High optical absorption of indium sulfide nanorod arrays formed by glancing angle deposition.

Indium(III) sulfide has recently attracted much attention due to its potential in optical sensors as a photoconducting material and in photovoltaic applications as a wide band gap material. On the other hand, optical absorption properties are key parameters in developing photosensitive photodetectors and efficient solar cells. In this work, we show that indium sulfide nanorod arrays produced by the glancing angle deposition technique have superior absorption and low reflectance properties compared to conventional flat thin film counterparts. We observed an optical absorption value of approximately 96% for nanorods at wavelengths <500 nm in contrast to 79% for conventional thin films of indium sulfide. A superior photoconductivity (PC) response as high as about 40% (change in resistance upon illumination) was also observed in nanorod samples. This is mainly believed to be due to their high optical absorption, whereas only less than 1% PC change was detected in conventional thin films. We give a preliminary description of the enhanced light absorption properties of the nanorods by using the Shirley-George model, which predicts diffusion of light as a function of the roughness of the surface.

Epitaxial GaN nanorods free from strain and luminescent defects

Raman spectroscopy, cathodoluminescence imaging, and electron backscatter diffraction have been used to characterize the GaN nanorods as compared to their supporting matrix. The nanorods are strain free, distinguished from the mechanically and thermally stressed matrix that bears the brunt of all lattice mismatch and thermal strain, strain relaxation, and the related defect generation. This thus allows the loosely attached nanorods to grow to measurable perfection in electronic and crystal structures. The nanorods are crystallographically aligned with the matrix as well as the substrate.

InN nanotips as excellent field emitters

Unidirectional single crystalline InN nanoemitters were fabricated on the silicon (111) substrate via ion etching. These InN nanoemitters showed excellent field emission properties with the threshold field as low as 0.9V∕μm based on the criterion of 1μA∕cm2 field emission current density. This superior property is ascribed to the double enhancement of (1) the geometrical factor of the InN nanostructures and (2) the inherently high carrier concentration of the degenerate InN semiconductor with surface electron accumulation layer induced downward band bending effect that significantly reduced the effective electron tunneling barrier even under very low external field.

PiN InGaN nanorod solar cells with high short-circuit current

We report on the photovoltaic characteristics of molecular beam epitaxy-grown PiN InGaN nanorod solar cells. The glancing angle deposition process was adapted to grow continuous transparent metal layers on discontinuous nanorods. A short-circuit current density of 4.6 mA/cm2 and an open-circuit voltage of 0.22 V with a power conversion efficiency of 0.5% under 1 sun, air-mass 1.5, illumination were observed. The excellent light-generated current in the InGaN nanorod solar cells is considered to stem from the improved crystal quality owing to the strain-free nature as well as the enhanced light concentration effects in the nanorod configuration.

Photogated transistor of III-nitride nanorods

A III-nitride-based photogated transistor using photons to control the channel width of an otherwise gateless field effect transistor (FET) is investigated. This is accomplished by stacking sequential layers of p-GaN/InGaN/n-GaN on a Si substrate in an array of nanorods. The nitride p-i-n diode can be activated by light, whereupon the nanorod device shows phototransistor characteristics in forward bias but behaves like a photoconductor when in reverse bias. An optically pumped FET model, as justified by the low-dimensional nanogeometry, is used in analysis of the device. The resulting photogate efficiency and photocarrier mobilities are estimated to be ∼0.04 V/(W/cm2) and, ∼2000–3000 cm2/V s, respectively.

Using point-defect engineering to increase stability of highly doped ultrashallow junctions formed by molecular-beam-epitaxy growth

Stability of p+/n junctions remains a critical issue for device performance. We report that the technique of point-defect engineering (PDE) can substantially increase the stability of ultrashallow junctions formed by molecular-beam epitaxy. It is shown that an as-grown 15 nm, 2×1020/cm3 B-doped Si layer becomes unstable during 10 min thermal anneal above 650 °C. The thermal stability can be increased by performing a 5×1015/cm2 1 MeV Si ion implantation. The B profile with the MeV Si implant does not show significant diffusion during annealing up to 750 °C, and the final junction depth after an 800 °C/10 min anneal is about half that of an annealed unimplanted sample. Although with Mev implantation the as-implanted B profile becomes slightly deeper due to recoil implantation, and some of the B has been electrically deactivated by the MeV implantation, PDE is advantageous for postgrowth thermal processes above 700 °C. The mechanism causing the instability is discussed.

Evolution of nanoripples on silicon by gas cluster-ion irradiation

Si wafers of (100), (110) and (111) orientations were bombarded by gas cluster ion beam (GCIB) of 3000 Ar-atoms/cluster on average at a series of angles. Similar surface morphology ripples developed in different nanoscales. A simple scaling functional satisfactorily describe the roughness and wavelength of the ripple patterns as a function of dosage and angle of incidence. The ripples are formed orthogonal to the incident cluster-ions at large off-normal angles. An ellipsoidal pattern was created by two consecutive irradiations incident in mutually orthogonal directions with unequal exposure times between each irradiation, from 7:1 to 10:1, beyond which the original ripple imprints would be over-written. This work was inspired by use of the ripples to seed growth of controlled nanostructures without patterning by lithography or predeposition of catalysts.

Chain-oxygen ordering in twin-free YBa2Cu3O7-δ single crystals driven by 20-kev electron irradiation

We have examined the effects of 20 keV electron irradiation on [-Cu(1)-O(1)-]n chain oxygen arrangements in oxygen deficient but otherwise twin-free YBa2Cu3O6+x single crystals. Comparison of polarized Raman spectra of non-irradiated and irradiated areas provides evidence that electron bombardments instigate the collective hopping of oxygen atoms either from an interstitial at O(5) site to a vacant O(1) chain site or by reshuffling the chain segments to extend the average length of chains without changing the overall oxygen content. This oxygen ordering effect, while counter-intuitive, is analogous to that found in the photoexcitation induced ordering in which temporal charge imbalance from electron-hole pair creation by inelastic scattering of incident electrons causes a local lattice distortion which brings on the atomic rearrangements.

Competitive In and Ga incorporations for InxGa1-xN (0.29<x<0.36) nanorods grown at a moderate temperature

The kinetics of In and Ga incorporation into wurtzite InxGa1−xN nanorods, grown by plasma-assisted MBE under N-rich conditions at a moderate temperature, has been systematically investigated with Ga-flux set as a growth parameter at three distinct values while varying In-flux. The interplay of Ga and In fluxes in their contributions to the incorporation was found to disagree with the empirical Bottchers formula, of which the reliability is based on the assumption of preeminent Ga incorporation. The competition between Ga and In for incorporations involves, we believe, the displacement of In from the weaker In-N bonds by Ga to form the Ga-N bonds at high In and Ga fluxes.

p-GaN/InGaN/n-GaN pedestal nanorods: Effect of postgrowth annealing on the electrical performance

Pedestal p-GaN/InGaN/n-GaN nanorods have been fabricated on n-type Si (111) substrates by properly reducing the growth temperature of the p-GaN surface layer. Continuous p-GaN layers were formed on the top region by accelerated lateral growth, while keeping the underlying nanostructures and physical properties of InGaN and n-GaN intact, making it feasible for large-scale vertical integration. Growth of the p-GaN layer at 500 °C followed by annealing at 600–800 °C improved crystal structures and the overall electrical and luminescence properties of pedestal nanorods.