Structural and morphological characteristics of planar (1120) a-plane gallium nitride grown by hydride vapor phase epitaxy Appl. Phys. Lett. 83, 1554 (2003); 10.1063/1.1604174 Modification of surface morphology and optoelectronic response in porous Si films by electrochemical methods

Experimental results given in Part I and II are interpreted with the help of a band scheme including five energy levels: β (1.91 eV above the VB, due to exciton—neutral acceptor complexes creation with 0.1 eV binding energy), A (0.55 eV above the VB, copper vacancies V Cu−), B (0.76 eV above A, [V Cu−—V O+] vacancy associations), C (0.97 eV above A, [VCu—VCu] vacancy associations) and D (0.38 eV below the CB, due to oxygen vacancies V+O). The strongest absorption bands, as well as the minima in the photoconductivity spectrum are explained by optical transitions from A to B and C levels. The defect density responsible for the level C is about 1017 cm−3. This scheme is consistent with results obtained by other authors on the electrical properties and the luminescence of Cu2O.

A large ^-direction dependence of the broad band close to the TO peak is observed and explained qualitatively and quantitatively. The TO peak is found to be repelled out of a two-phonon band by a strong third-order anharmonic interaction.In a crystal having the zinc-blende structure, the first-order Raman scattering allows detection of either the TO(g~0) phonon or the LO(#~0) phonon, according to the selected direction q of the transferred momentum. We have measured at 40 K the scattered intensities of a CuCl single crystal for various q directions (Fig. 1). Because all the spectra were recorded with the same spectral slit width (2 cm" 1 ), the heights (eventually above the continuum) of the sharp lines at 171 and 208 cm" 1 could be taken to fix a common intensity scale for the drawings of Fig. 1 by making use of the fact that the chosen geometries impose I B = 0.5(JA+IC)-The unusual features in CuCl are the broad and strong band of the TO spectrum 1 " 3 (curve A) in the 140-170 cm" 1 range, reaching a very sharp line at 171 cm" 1 with a maximum at 151 cm" 1 , and its replacement by a weak and featureless continuum in the LO spectrum (curve C). This observation of ^-dependent, first-orderlike spectra obviously differs from the recent results of Shand et al. 3 and excludes the assignment of the broad band to a pure second-order Raman scattering.We propose to explain these features by the existence of strong anharmonic interactions between one of the <7~0 optical phonons and a twophonon continuum limited on the high-frequency side by a P 3 type of singularity 4 due to a Bril-where i labels the TO or LO phonon with frequency w,, R* y is the usual first-order Raman tensor element, the anharmonic third-order interaction being given by the usual formula 5 :(F 3 0 2 [A(a>) + iT(a>)]^ ^ TJ vfiirqJu -qiJ 2 ) G (u,qih, -Qi^ QsJJs* -(fa^M^', 4^3> -Qs^) • ( 2 ) q 3 ,J 3f i 4

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