LO phonon replica and exciton many

Mo-P.052
Mo-P.052
LO phonon replica and exciton many-body effects of GaN Nanocolumns
Yuta Inose‡, Kazuya Kinjo‡, Kazuhiro Ema‡§,
Jun Yoshida‡, Kouji Yamano‡, and Katsumi Kishino‡§
‡
Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo
102-8554, Japan
§
Sophia Nanotechnology Research Center, Sophia University
Gallium nitride (GaN) nanocolumn [1] is one of the most attractive systems for studying
physical properties in which many interesting phenomena can be observed. In our previous
work, we observed the column diameter dependence of exciton and biexciton energies in selforganized GaN nanocolumns [2]. However, the observation was limited in the statistically
averaged phenomenon over the distribution of the column diameters. In this study, we report a
clear diameter dependence of the optical properties in regularly-arrayed GaN nanocolumns [3].
We measured photo-excited emission of regularly-arrayed GaN nanocolumns at 5 K using
femtosecond pulses as excitation source. The column diameters are uniform in their size in the
region of 50~200 nm, the period is 200~300 nm, and the height is about 500 nm. We have
observed that the PL intensities of the defect emission decrease drastically with decreasing
column diameter, namely, the crystals with a smaller diameter have a higher quality. In
addition, we have found that the LO phonon replica component decreases with decreasing
column diameter arising from the nanocrystal effect [4], as shown in Fig. 1. We also
measured emission of GaN nanocolumns with 200 nm in diameter at high carrier densities as
shown in Fig. 2. We observed all kinds of exciton many-body effects in addition to exciton
emission (X), that is, biexciton (M) emission, exciton-exciton scattering (P), and electron-hole
plasma (EHP). We estimated that exciton binding energy and exciton Mott transition density
are 22 meV and 1019 cm-3, respectively. We also found that peak energy of biexciton emission
show red-shift due to a rise in effective exciton temperature. Since we also measured timeresolved PL at such high densities, we can also discuss the dynamics of the transition among
each stage of the many-body phenomena.
Fig. 1. Column diameter dependence of
PL spectra, normalized to the peak
intensity of exciton emission.
Fig. 2. Excitation power dependence of PL
spectra at high carrier densities,
normalized to the peak intensity.
References
[1] M. Yoshizawa et al., Jpn. J. Appl. Phys. 36, L459 (1997).
[2] K.Kouyama et al., Phys. Stat. Sol. (c) 6, 141 (2009).
[3] K. Kishino et al., J. Cryst. Growth 311, 2063 (2009).
[4] C. H. Chia et al., J. Appl. Phys. 109, 063526 (2011).