Hiroto Sekiguchi

Effect of Mg codoping on Eu3+ luminescence in GaN grown by ammonia molecular beam epitaxy

The effect of Mg codoping on the Eu3+ luminescence in GaN was investigated by photoluminescence measurements. Two notable emission peaks associated with the 5D0→7F2 transition in the Eu3+ ions were governed by Mg codopants, which corresponded to the change of the dominant peak wavelength from 622.3 to 620.3 nm with an increase in Mg concentration. An optimal amount of Mg also led to enhancement of approximately 20 times of the Eu3+ luminescence. These results indicated that the Mg codopants selectively activated the optical site of 620.3 nm emission due to the elimination of nonradiative deexcitation paths from the 5D0 state.

Red-Light-Emitting Diodes with Site-Selective Eu-Doped GaN Active Layer

Mg codoping into Eu-doped GaN (GaN:Eu) changed the dominant optical site and increased the photoluminescence (PL) intensity at room temperature (RT). From the ratio of PL integrated intensity at 25 K to that at 300 K, PL efficiency of the GaN:Eu,Mg layer was evaluated to be as high as 77%. On the basis of this experiment, GaN:Eu-based LEDs grown by NH3 MBE were fabricated. Clear rectification characteristics with a turn-on voltage of 3.2 V were observed and a pure red emission was observed by the naked eye at RT. For the electroluminescence (EL) spectra, two predominant peaks of higher-efficiency optical sites A and C were selectively enhanced and the EL intensity was improved. This result suggests that GaN:Eu was very effective for realizing red-light-emitting devices using the nitride semiconductor.

Effect of Growth Mode on Eu-Incorporation and Luminescence of Eu-Doped GaN Epitaxial Film Grown by Plasma-Assisted Molecular Beam Epitaxy

The growth mode of europium (Eu)-doped GaN epitaxial films grown on a GaN template by rf plasma-assisted molecular beam epitaxy (PAMBE) was investigated with different III/V ratios under a constant Eu beam equivalent pressure ratio [PEu/(PEu+PGa)]. The reflection high-energy electron diffraction (RHEED) patterns and atomic force microscopy (AFM) images revealed the transition of the growth mode from three-dimensional (3D) to step-flow/two-dimensional (2D) by increasing the III/V ratio. When the films were grown in the 3D growth mode, Eu concentrations estimated by Rutherford backscattering spectrometry/channeling (RBS/channeling) were almost constant, although the III/V ratios varied. However, when the growth mode was transferred from 3D to step-flow/2D, precipitates on the surface abruptly increased while the Eu concentration abruptly decreased, indicating the abrupt degradation of Eu-incorporation in the film. Luminescence sites of Eu3+ were sensitive to the III/V ratio, and Eu atoms have different luminescence sites in both growth modes. Furthermore, luminescence efficiency abruptly increased when the growth mode was transferred from 3D to step-flow/2D.

Demonstration of a large-area AlGaN/GaN Schottky barrier photodetector on Si with high detection limit

A high-quality AlGaN/GaN Schottky barrier photodetector was demonstrated on Si. The device exhibited a very low dark current of 0.94 fA at −2.5 V and 2.51 pA at −10 V. There were two spectral response peaks located at 280 and 340 nm, respectively, and the relative intensity of these peaks changed with applied bias. The responsivity exhibited optical power density dependence measured at 340 nm with different bias. The zero bias differential

Structural and optical properties of Eu-doped GaN nanocolumns on (111) Si substrates grown by RF-plasma-assisted molecular beam epitaxy

Eu-doped GaN (GaN:Eu) shows a sharp line emission and the thermal stability of the emission wavelength. To improve their optical properties, GaN:Eu nanocolumns were grown on GaN nanocolumn platforms with high crystalline quality by RF-plasma-assisted molecular beam epitaxy. The GaN:Eu growth temperature strongly affected the nancolumn configuration. Although a high growth temperature of more than 700 °C enhanced lateral growth, a low growth temperature of 600 °C led to independent GaN:Eu nanocolumns. Although it was revealed that an increase in Eu concentration brought about polycrystalline growth, the optically active site concentration increased with increasing Eu concentration without concentration quenching, which suggests that the nanocolumn crystal is a valuable material for developing the novel optical devices utilizing GaN:Eu.

Optical sites in Eu- and Mg-codoped GaN grown by NH3-source molecular beam epitaxy

Mg codoping can improve the luminescence properties of Eu-doped GaN. However, the enhanced optical sites differ depending on the fabrication method. In this study, the optical sites in Eu- and Mg-codoped GaN [GaN:(Eu, Mg)] grown by NH3-source molecular beam epitaxy (MBE) were evaluated. The optical properties of an Eu–Mg-related site grown by NH3-MBE were highly stable against thermal annealing. Although the luminescence at sites A (622.3 and 633.8 nm) and B (621.9 and 622.8 nm) was dominant under indirect excitation of Eu ions through GaN, four different optical site groups in addition to sites A and B were observed under resonant excitation. These optical sites are inconsistent with the Eu–Mg-related sites reportedly observed in GaN:(Eu, Mg) fabricated by organometallic vapor phase epitaxy, indicating that the optical site constitution strongly depends on the growth method. Furthermore, site A, with a high cross section, contributed to as much as 22% of the total photoluminescence (PL) integrated intens...

Organometallic chemical vapor deposition of silicon nitride films enhanced by atomic nitrogen generated from surface-wave plasma

Organometallic chemical vapor deposition of silicon nitride films enhanced by atomic nitrogen generated from surface-wave plasma is investigated. Feasibility of precursors of triethylsilane (TES) and bis(dimethylamino)dimethylsilane (BDMADMS) is discussed based on a calculation of bond energies by computer simulation. Refractive indices of 1.81 and 1.71 are obtained for deposited films with TES and BDMADMS, respectively. X-ray photoelectron spectroscopy (XPS) analysis of the deposited film revealed that TES-based film coincides with the stoichiometric thermal silicon nitride.

Fabrication of Si/SiO2/GaN structure by surface-activated bonding for monolithic integration of optoelectronic devices

A Si/SiO2/GaN-light-emitting-diode (LED) wafer is proposed as a new structure for the monolithic integration of both Si circuits and GaN-based optical devices. Surface-activated bonding was performed to transfer a Si layer from a silicon-on-insulator substrate to a SiO2/GaN-LED substrate. Transmission electron microscopy observation revealed that a defect-free Si layer was formed on the SiO2/GaN-LED substrate without interfacial voids. The crystalline quality of the Si layer, which is characterized by an X-ray rocking curve, was markedly improved by flattening the SiO2/GaN-LED substrate before bonding. Finally, a micro-LED array was successfully fabricated on the Si/SiO2/GaN-LED wafer without the delamination of the Si layer.

Monolithic integration of Si-MOSFET and GaN-LED using Si/SiO2/GaN-LED wafer

In this report, we present a monolithic integration method for a Si-MOSFET and a GaN-LED onto a Si/SiO2/GaN-LED wafer as an elemental technology for monolithic optoelectronic integrated circuits. To enable a Si-MOSFET device process, we investigated the thermal tolerance of a thin top-Si and GaN-LED layer on a Si/SiO2/GaN-LED wafer. The high thermal tolerance of the Si/SiO2/GaN-LED structure allowed for the monolithic integration of a Si n-MOSFET and a GaN-µLED without degrading the performance of either device. A GaN-µLED driver circuit was fabricated using a Si n-MOSFET and a µLED of 30 × 30 µm2, with the modulation bandwidth of the circuit estimated to be over 10 MHz.

Chemical vapor deposition of silicon nitride film enhanced by surface-wave plasma for surface passivation of AlGaN/GaN device

Surface passivation of AlGaN/GaN device by silicon nitride (SiN) deposition using surface-wave plasma enhanced chemical vapor deposition (SPECVD) technique which is proposed by authors group has been studied. SiN deposition condition by SPECVD using bis(dimethylamino)dimethylsilane (BDMADMS) precursor was investigated. Effect of passivation in terms of current-voltage characteristics of gateless AlGaN/GaN device is discussed by comparing the characteristics without passivation layer and with passivation layer by SPECVD and the conventional plasma chemical vapor deposition (PCVD). Increase of the current and decrease of the current hysteresis in AlGaN/GaN device were observed in SiN passivated samples by SPECVD technique. These results revealed that passivation by SPECVD technique is promising for AlGaN/GaN-based high output current transistors and their stable operation.