RECEIVED DATE ( )We report Raman and time-resolved photoluminescence spectroscopic studies of multiwalled BN and B x C y N z nanotubes. The Raman spectroscopy shows that the as-grown B x C y N z nanotubes are radially phase separated into BN shells and carbon shells. The photoluminescence decay process is characterized by two time constants that are attributed to intra-and inter-BN sheet charge recombination, respectively. Comparison of the photoluminescence of BN nanotubes to that of hexagonal BN is consistent with the existence of a spatially indirect band gap in multi-walled BN nanotubes as predicted by theory.Hexagonal boron nitride (h-BN) is iso-structural with graphite and is the normal phase of BN that is stable at room temperature and ambient pressure. It is a wide band gap semiconductor with potential applications in optoelectronic devices. First-principles local-density calculations have shown that the lowest band gap (4.07 eV) is indirect, located near the Brillouin-zone edges. However, due to the quasitwo dimensional nature of the hexagonal structure, the lowest direct band gap is predicted to be close by

The temperature dependence of the photoluminescence (PL) transitions associated with various excitons and their phonon replicas in high-purity bulk ZnO has been studied at temperatures from 12 K to above room temperature (320 K). Several strong PL emission lines associated with LO phonon replicas of free and bound excitons are clearly observed. The room temperature PL spectrum is dominated by the phonon replicas of the free exciton transition with the maximum at the first LO phonon replica. The results explain the discrepancy between the transition energy of free exciton determined by reflection measurement and the peak position obtained by the PL measurement.

Chadi, Chang, and Walukiewicz Reply:We have recently proposed a structural model involving a large lattice relaxation for the DX center in GaAs which accounts for many of the physical properties of this defect. x The results of our calculations indicate that DX is stable in the proposed configuration only when negatively charged.In their Comment, Maude et al. 2 raise the question of whether this charge state is consistent with the observed pressure dependence of the mobility in heavily degenerate rt-type GaAs samples. 3 In these samples the Fermi level Ef at zero pressure lies 0.25-0.30 eV above the conduction-band minimum, close to the position of the DX resonance. As pressure is applied the energy of the Ti conduction state rises more rapidly than DX, forcing some electrons to become trapped on DX centers. This results in a decrease of the free-electron concentration but an increase in the mobility. Maude et al. 2 suggest that the observed trends are better explained by a neutral defect center. However, they are able to account for only half the observed percentage increases in the mobility ratios ^(P = 10-15 kbar)/juCP=0) for various samples. In the following we show that the experimental observations are consistent with a negatively charged DX center.The unique feature of the pressure and mobility measurements for the heavily doped GaAs samples 3 is the degeneracy of the energies of electrons in DX states with those at EF. In addition, at the doping levels and the temperature of 300 K at which pressure is applied, the electrons at £> and the DX centers are in full thermal equilibrium. The total density of electronic states at EF, therefore, which needs to be used in the calculation of the screening parameter is given by