Sun-Ghil Lee

First-principles study of the self-interstitial diffusion mechanism in silicon

We study the stability and migration mechanism of self-interstitials in Si through first-principles self-consistent pseudopotential calculations. The neutral Si interstitial is lowest in energy at a [110]-split site, with energy barriers of 0.15-0.18 eV for migrating into hexagonal and tetrahedral interstitial sites, while the migration barrier from a hexagonal site to a tetrahedral site is lower, 0.12 eV. These migration barriers are further reduced through successive changes in the charge state at different sites, which allow for the athermal diffusion of interstitials at very low temperatures. The [110]-split geometry is also the most stable structure for negatively charged states, while positively charged self-interstitials have the lowest energy at tetrahedral sites. Apart from the migration barrier, the formation energy of the [110]-split interstitial is estimated to be about 4.19 eV; thus, the resulting activation enthalpy of about 4.25 eV is in good agreement with high-temperature experimental data.

Atomic model for blue luminescences in Mg-doped GaN

We investigate the origin of the broad luminescence observed around 2.7-2.9 eV in heavily Mg-doped GaN through first-principles pseudopotential calculations. We find that a defect complex composed of an Mg interstitial and an N vacancy gives rise to optical transition levels around 2.8 eV above the valence band maximum, suggesting that the blue luminescences are caused by deep-donor-to-valence-band transitions. The formation of the - complex is enhanced by hydrogenation and is more preferable in p-type samples grown under Ga-rich conditions.

Optical properties of ZnSSe/ZnMgSSe quantum wells

We study the electronic structure and excitonic properties of single quantum wells through an exact treatment of the Pikus - Bir Hamiltonian. In the ZnSe well, we find three distinguished exciton peaks in the absorption spectra, which are associated with the n = 1 and 2 heavy holes (HH1 and HH2) and n = 1 light hole (LH1), in good agreement with experiments. As the S content of the well increases, the HH1 exciton peak increases more rapidly than the LH1 peak, and a severe overlap between these two peak appears at x = 0.1. The binding energy of the HH1 exciton also increases with the S content; however, its maximum value decreases because of the decreasing confinement effect in the well.

Dimensional crossover and temperature dependence of the heat capacity in two-direction double-barrier resonant-tunnelling structures

We calculate the heat capacity of electrons as a function of the electron density and temperature in two-direction double-barrier resonant-tunnelling structures. The strength of the barrier potential increases in one direction, so that the system becomes 2D; the heat capacity as a function of the electron density goes to a step-like shape, which is similar to the one in the DOS. From to to 1D, the heat capacity reflects the peaks in the DOS with increasing electron density, and becomes a sawtooth-like shape in 1D. However, an asymmetric peak in the DOS makes two peaks in the heat capacity because the available DOS in thermal excitations becomes smaller as the chemical potential approaches the peak in the DOS. The heat capacity shows a linear dependence on the temperature in 3D and 2D, but not in 1D even at low temperatures. It exhibits a square root T dependence when the chemical potential is located near a pole in the DOS.