Seong-Ju Park

Deep centers in a free-standing GaN layer

Schottky barrier diodes, on both Ga and N faces of a ∼300-μm-thick free-standing GaN layer, grown by hydride vapor phase epitaxy (HVPE) on Al2O3 followed by laser separation, were studied by capacitance–voltage and deep level transient spectroscopy (DLTS) measurements. From a 1/C2 vs V analysis, the barrier heights of Ni/Au Schottky contacts were determined to be different for the two polar faces: 1.27 eV for the Ga face, and 0.75 eV for the N face. In addition to the four common DLTS traps observed previously in other epitaxial GaN including HVPE-grown GaN a new trap B′ with activation energy ET=0.53 eV was found in the Ga-face sample. Also, trap E1 (ET=0.18 eV), believed to be related to the N vacancy, was found in the N-face sample, and trap C (ET=0.35 eV) was in the Ga-face sample. Trap C may have arisen from reactive-ion-etching damage.

Identification of the Γ5 and Γ6 free excitons in GaN

The Γ5 and Γ6 free excitons have been identified in GaN from emission measurements. Another emission peak is also observed which we believe to be the longitudinal free exciton. These measurements along with electrical measurements, which show the sample to have very high peak mobility, attest to the high quality of the sample.

Deep electron and hole traps in freestanding n-GaN grown by hydride vapor phase epitaxy

Deep level electron and hole traps were studied by means of deep level transient spectroscopy with electrical and optical injection on a freestanding thick n-GaN sample with low dislocation density. It is shown that at both the upper and the lower surface of the sample there exists a thin, ∼0.5 μm layer of damaged material with lowered concentration of electrons and enhanced density of deep centers. Deep in the bulk of the film the densities of the majority of the electron and hole traps are shown to be very low, but measurably higher on the lower face (N face), which was originally closer to the Al2O3 substrate. The two faces are also shown to similarly differ in the density of deep hole traps whose concentration is deduced from low-temperature capacitance–voltage measurements in the dark and after illumination (such traps were previously associated with dislocation states). The concentration of persistent photocapacitance centers is shown to be very low on both sides and considerably lower than previous...

Electrical and Optical Properties of GaN/Al2O3 Interfaces

Hall-effect, photoluminescence (PL), cathodoluminescence (CL), and Raman scattering measurements have been used to characterize the Ga (top) and N (bottom) faces of free-standing GaN layers grown by hydride vapour phase epitaxy on Al2O3. The material near the bottom has higher carrier concentration, lower mobility, larger PL linewidths, brighter CL emission, and stronger Raman plasmon–phonon lines than the material near the top. All results are consistent with the diffusion of O from the Al2O3 substrate, sometimes covering a distance of several tens of micrometres. The O donor is compensated by Ga vacancy acceptors, known to exist from positron annihilation experiments. However, Raman and CL profiling show that the poor interface region ends rather abruptly, giving excellent material near the top (Ga) face.

Microcathodoluminescence and Electron Beam Induced Current Observation of Dislocations in Freestanding Thick n-GaN Sample Grown by Hydride Vapor Phase Epitaxy

Microcathodolumunescence (MCL) spectra measurements, MCL and electron beam induced current (EBIC) imaging of the freestanding n-GaN samples grown by hydride vapor phase epitaxy were made. Dark-spot defects in plan-view EBIC and MCL images and dark line defects in MCL images taken on the cleaved surface of the samples could be associated with dislocations. MCL spectra measurements in the vicinity of dislocations and in the matrix do not reveal specific luminescence bands that could be attributed to dislocations but rather suggest that dislocation regions have higher density of deep nonradiative traps.

White light-emitting diodes with phase-separated InGaN active layers

We fabricated the white light-emitting diodes without phosphor materials using phase-separated InGaN active layers. The white luminescence was attributed to the broad distributions of indium composition and size of quantum dot- like In-rich regions in the phase-separated InGaN Ternary alloys.

Synthesis of Graphene Films by Chemical Vapor Deposition for Transparent Conducting Electrodes of GaN Light‐Emitting Diodes

This work demonstrales large‐scale simultaneous fabrication of patterned graphene‐based GaN light‐emitting diodes (LEDs). Graphene sheets were synthesized using a chemical vapor deposition (CVD) technique on nickel films and showed typical CVD‐synthesized film properties, possessing a sheet resistance of ∼605u2009Ω/⃞ with a transparency of more than 85% in the 400–800 nm wavelength range, and was applied as transparent condueting electrodes of GaN‐based blue LHDs, The light output performance of GaN LEDs with graphene electrodes was comparable to that of conventional ITO‐electrode LEDs over the range of input current up to 150 mA.

High electron mobility in free-standing GaN substrates

High peak electron mobilities were observed in free-standing c -plane GaN substrates. Two layers, a low mobility degenerate layer and a high mobility bulk layer, were present in these samples. The carrier concentrations and mobilities for the layers were extracted using two methods: 1) magnetic field dependent Hall effect analysis and 2) a simple two carrier model with the assumption that one of the layers is degenerate. In addition, measurements were performed after etching away the degenerate layer. The mobility of the bulk layer is found to peak at nearly 8000 cm 2 /Vs at 60K using the magnetic field dependent Hall effect data. Record room temperature mobility for bulk GaN of 1190 cm 2 /V s was measured.