C. M. Park

Photoluminescence studies of GaN nanorods on Si (111) substrates grown by molecular-beam epitaxy

We have investigated the optical properties of dislocation-free vertical GaN nanorods grown on (111) Si substrates by radio-frequency plasma-assisted molecular-beam epitaxy. The hexagonal shape nanorods with lateral diameters from 80 to 190 nm are obtained. They are fully relaxed and have a very good crystal quality characterized by extremely strong and narrow photoluminescence excitonic lines near 3.47 eV. Three distinct features are observed in photoluminescence. First, free exciton transition is observed at 3.477 eV for GaN nanorods of decreased diameter. Second, the photoluminescence spectra show an abnormal behavior with increasing temperature. The third feature is the size effect in that the PL peak energies are blueshifted with decreasing diameter of the GaN nanorod. The activation energy of the free exciton for the GaN nanorods with different diameters was evaluated.

Raman characterization of electronic properties of self-assembled GaN nanorods grown by plasma-assisted molecular-beam epitaxy

We have investigated the Raman scattering of the aligned gallium nitride (GaN) nanorods grown by plasma-assisted molecular-beam epitaxy. It was determined by Raman spectroscopy that the GaN nanorods are relatively strain free. The free carrier concentration, as well as electron mobility of the GaN nanorods, was obtained by the line shape analysis of the coupled A1longitudinal-optical(LO) phonon-plasmon mode. The electron concentration and mobility of electron obtained from line shape analysis are 3.3×1017cm−3 and 140cm2Vs, respectively. The local temperature of the nanorod sample was estimated based on the ratio of Stokes to anti-Stokes Raman peak intensity. Since the position of the LO phonon peak was found to be dependent on both the temperature and the LO phonon-plasmon coupling, it is crucial to consider the temperature effect in determining the frequency of the uncoupled LO phonon mode for the line shape analysis. The frequency of the A1(LO) mode of an undoped bulk GaN was used as a reference to dete...

Time-resolved and time-integrated photoluminescence studies of coupled asymmetric GaN quantum discs embedded in AlGaN barriers

We have investigated exciton dynamics in asymmetric GaN quantum discs embedded in AlGaN barriers with an Al content of 50% using time-integrated and time-resolved micro-photoluminescence measurements. Emission from the quantum discs emerges at lower energy than that from the GaN nanocolumns, which suggests that GaN quantum discs are strongly affected by the built-in electric field. The lifetimes of localized excitons in quantum discs were obtained. Nonlinear emission from quantum discs under high excitation power was attributed to tunneling of carriers to larger discs from smaller discs.

Electron trap level in a GaN nanorod p-n junction grown by molecular-beam epitaxy

We have studied the electrical properties of a GaN nanorod p-n junction diode by deep level transient spectroscopy measurements. The p-n junction nanorods were patterned on a SiO2 substrate by using e-beam lithography. In order to confirm the formation of p-n junction, cathodoluminescence and current-voltage measurements, as a function of temperature, were made. The current-voltage curve exhibits strong temperature dependence, suggesting that thermionic emission over a barrier dominates. This barrier most likely corresponds to emission from a deep level in the band. The deep level appears to be an electron trap at Ec-0.40eV below the conduction band with a capture cross section of 2.22×10cm2 near the depletion region of the p-n junction.

Self-assembled AlxGa1−xN nanorods grown on Si(001) substrates by using plasma-assisted molecular beam epitaxy

Hexagonal Al(x)Ga(1-x)N nanorods were grown by plasma-assisted molecular beam epitaxy (PAMBE) on Si(001) substrates. The Al mole fraction was determined from x-ray diffraction (XRD) measurement and its value was varied from 0 to 15. It is found that, under group III-rich conditions, the growth rate of the Al(x)Ga(1-x)N nanorods decreases and the diameter increases due to the possibility of incorporation of aluminium and gallium. In order to study structural and optical properties, x-ray diffraction and cathodoluminescence (CL) measurements were carried out. The Al content (x) is calculated from these measurements and their values are compared.

Deep level defects in Si-doped AlxGa1−xN films grown by molecular-beam epitaxy

The deep trap levels of AlxGa1−xN films with x in the range from 0 to 0.15 grown on c-plane sapphire substrates using rf-plasma-assisted molecular-beam epitaxy have been investigated by deep level transient spectroscopy measurements. Two distinct defect levels (denoted as Ei and Di) were observed. The origins of the Ei and the Di are associated with point defects such as the N vacancies and extended defects, such as the threading dislocations, respectively. According to Al content (x), the activation energy and capture cross section for the Di defect ranged from 0.19to0.41eV and 1.1–6.6×10−15cm2, respectively. The trap energy levels of Di defects in AlxGa1−xN were calculated and the values were nonlinear with Al content. The bowing parameter of AlxGa1−xN films was determined to be b=1.22.

The recombination mechanism of Mg-doped GaN nanorods grown by plasma-assisted molecular-beam epitaxy

Magnesium-doped GaN nanorods were grown on Si(111) substrates by plasma-assisted molecular-beam epitaxy. Time-integrated and time-resolved photoluminescence measurements were carried out to study the optical transitions. Two emission lines corresponding to blue emission at about 3.26 and 3.18 eV, with their corresponding phonon replicas, were observed. These peaks are attributed to conduction band to shallow acceptor transitions and to defects associated with column/substrate interface–shallow Mg acceptor complexes, respectively.

Effects of different potential barriers on the structural and optical properties of GaN∕AlxGa1−xN∕GaN coupled multiquantum wells

Coupled multiquantum well structures, GaN∕AlxGa1−xN∕GaN bounded by AlxGa1−xN barriers with varying Al content x, are characterized by using high-resolution x-ray diffraction and photoluminescence measurements. It is clearly demonstrated that the structural and optical properties strongly depend on the Al content x. Photoluminescence peaks continue to be redshifted with respect to the bulk GaN band gap with increasing x up to 0.5, but, in turn, become blueshifted beyond this value. This behavior is understood in terms of the different screening effects of the piezoelectric field induced by a strain with increasing Al content. It is found that the strain starts to be relatively relaxed around x=0.5, leading to the reduction in the screening of the piezoelectric field.

Two-photon excitation spectroscopy of coupled asymmetric GaN/AlGaN quantum discs

By using two-photon excitation spectroscopy we take advantage of the increased spatial resolution to perform time-integrated and time-resolved photoluminescence measurements on several discrete stacks of GaN quantum discs. The stack structure consisted of coupled asymmetric GaN quantum discs with embedded AlGaN barriers, which were grown at the tip of a GaN nanocolumn. We observed that with increasing optical excitation power the carrier lifetime decreased due to free-carrier screening, with an enhancement of the screening effect in the larger quantum disc due to carrier tunnelling from the smaller quantum disc.

Free carrier screening in coupled asymmetric GaN quantum discs

We present an investigation of free-carrier screening in coupled asymmetric GaN quantum discs with embedded AlGaN barriers using time-integrated and time-resolved micro-photoluminescence measurements, supported by three-dimensional multi-band k.p computational modeling. We observe that with increasing optical excitation the carrier lifetime decreases and emission energy blue-shifts. This originates from the screening of built-in piezo- and pyroelectric fields in the quantum discs by photo-generated free-carriers. Due to non-resonant tunneling of carriers from the smaller disc to the larger disc, free carrier screening is enhanced in the larger disc. The non-resonant tunneling was found to have a significant role in samples with a thin barrier, as the screening decreased with barrier thickness (i.e. decreased tunneling). Computational modeling was in good agreement with the experimental results.