Raman analyses of the lifetimes of phonons in GaN and AlN crystallites of wurtzite structure are presented. In order to ensure the accuracy of the measurement of the phonon lifetimes, an experimental procedure to eliminate the broadening due to the finite slit width was performed. The lifetime analyses indicate that the phonon lifetimes in AlN as well as in GaN fall into two main time regimes: a relatively long time of the E 2 1 mode and much shorter times of the E 2 2 , E1͑TO͒, and A1͑TO͒ modes. The lifetimes of the E 2 1 , E 2 2 , E1͑TO͒, A1͑TO͒, and A1͑LO͒ modes of an high-quality AlN crystallite are 4.4, 0.83, 0.91, 0.76, and 0.45 ps, respectively. Moreover, the lifetime of the A1͑LO͒ mode found in this study is consistent with the current phonondecay model of that mode in wurtzite structure materials. The lifetimes of E 2 1 , E 2 2 , E1͑TO͒, and A1͑TO͒ of a GaN crystallite were found to be 10.1, 1.4, 0.95, and 0.46 ps, respectively. The A1͑LO͒ mode in the GaN was not observed and its absence is attributed to plasmon damping. The lifetime shortening due to impurities was also studied: the lifetimes of the Raman modes of an AlN crystallite, which contains about two orders of magnitude more Si and C impurities relative to the concentration of the high-quality crystallite were found to be 50% shorter. ͓S0163-1829͑99͒04419-7͔

We present photoluminescence (PL) studies of GaN and ZnO nanocrystallites and powders. Our studies show that in addition to the intrinsic photoluminescence characteristics, the photoluminescence properties of the porous media are also a strong function of conditions such as ensemble size and powder density, ultraviolet-laser excitation power, and vacuum state. PL redshifts up to 120 meV were observed for GaN and ZnO crystallites and were attributed to laser heating and heat trapping in the ensemble. The electron-phonon interaction model for GaN indicated ensemble temperature ∼550 K, which is consistent with the finding obtained via high-temperature PL and Raman experiments. The PL in the vacuum state exhibited a significant redshift, ∼80 meV relative to that in air, and the PL of a dense ZnO pellet was found to resemble that of the bulk more than does a loose powder. The PL analyses indicated an excitonic emission at room temperature for both GaN and ZnO crystallites with intensity saturation occurring for large ensembles at high laser power.

One of the key issues of phonon dynamics of nano- and micrometer-scale crystals is the identification of the observed Raman modes. Due to the tilted orientation of small crystallites, the usual Raman selection rules pertaining to the symmetry axes no longer hold, and mixed-symmetry modes need to be considered in order to explain the polar phonon properties of the crystallites. The Raman modes of ZnO crystallites of the wurtzite structure were investigated via micro-Raman scattering. The nonpolar E2 mode was the predominant mode in the spectra for out-of-resonant conditions. In resonance the crystallites exhibited a predominant mode at ∼580cm−1, intermediate to the frequencies of the A1(LO) and the E1(LO) modes of a reference ZnO single crystal at 568 and 586cm−1, respectively. Our analysis indicates that the observed frequency of the crystallite ensemble can be explained in terms of Loudon’s model of a quasimode behavior that is due to a preferential orientation of a crystallite ensemble. Additionally, model calculation of the quasi-LO frequency of totally random ensemble is presented.

Photoluminescence (PL) and Raman spectroscopy were employed to investigate the nature and sources of stress and the type and distribution of impurities and defects in thin diamond films grown on silicon substrates. The types of impurities and defects which were detected in the diamond films are the nitrogen, silicon, and the sp2-type bonding of the graphitic phase. Our Raman analyses indicate that the diamond films exhibit a net compressive stress. After compensating for the thermal interfacial stress and for the stress due to grain boundaries it was found that the residual internal stress is compressive in nature. From Raman line-shape analysis it was determined that the internal stress is due to the various impurities and defects present in the film. Moreover, the stress magnitude exhibits a strong correlation with the graphitic phase implying that the sp2 bonding produces a dominant compressive stress field. The PL analytical line-shape investigation of the nitrogen band at 2.154 eV indicates that the nitrogen centers are uniformly distributed in the film. The PL line shape exhibited a close fit to the Lorentzian–Gaussian convoluted line known as the Voit profile. The deconvolution of the line resulted in a dominant Gaussian component, corresponding to stress arising from line type defects, and a much smaller Lorentzian component corresponding to point defect stress. The Gaussian component was attributed to the graphitic phase implying that the sp2 bonding is not in the form of a point defect but rather takes the form of a line or extended defect. The line-shape investigation of the silicon band at 1.681 eV showed that the silicon centers are correlated with the silicon/diamond interfacial stress. Finally, the response of the nitrogen and silicon optical centers to the internal stress, which is manifested via the PL linewidth, was also studied. The silicon band exhibits the narrower linewidth which may indicate that the silicon center complies less to the internal stress than the nitrogen center or that the two optical centers are interacting with different types of stress sources.

This article presents a study of the quasi-longitudinal optical and quasi-transverse optical modes in wurtzite AlN which originate from the interaction of phonons belonging to the A1 and E1 symmetry groups. In order to analyze the allowed quasi as well as pure Raman modes, the modes were observed in a rotating crystallographic coordinate system, and the Raman tensors of the wurtzite crystal structure were calculated as a function of the crystallographic rotation. The frequencies of the quasimodes of wurtzite AlN were also analyzed in terms of the interaction of the polar phonons with the long range electrostatic field model. The experimental values of the Raman frequencies of the quasiphonons concur with these expected from the model, implying that the long range electrostatic field dominates the short range forces for polar phonons in AlN.

Raman and various photoluminescence (PL) techniques were employed to investigate the role of nitrogen doping on the optical spectra of chemical-vapor-deposited (CVD) diamond films and to determine the origin of the characteristic broadband luminescence which is observed from approximately 1.5 to 2.5 eV and centered at ∼2 eV. The PL transitions attributed to the zero-phonon lines (ZPL) of nitrogen centers are observed at 1.945 and 2.154 eV. A new possible nitrogen center at 1.967 eV is also observed as well as the band A luminescence centered at ∼2.46 eV. The experimental results preclude the possibility of the broadband PL being due to electron-lattice interaction of the nitrogen ZPL centers. We establish the presence of an in-gap state distribution in CVD diamond films attributed to the sp2 disordered phase and show that its optical transitions are the likely cause of the broadband luminescence. A model of the in-gap state distribution is presented which is similar to models previously developed for amorphous materials.

The photoluminescence ͑PL͒ in as-received and milled Si and SiO 2 powder is reported. The Si and SiO 2 powder is characterized by chemical analysis, Raman scattering, x-ray photoelectron spectra, infrared absorption, x-ray diffraction, and differential thermal analysis. The results indicate that the Si powder has amorphous Si oxide and suboxide surface layers. The milling of Si powder results in the formation of nanocrystalline/ amorphous Si components. An amorphous SiO 2 component is formed by milling crystalline SiO 2 . The PL spectra for as-received Si, milled Si, and SiO 2 powder exhibit similar peak shapes, peak maxima, and full width at half maximum values. For both the as-received and the milled Si powder, experimental results appear to exclude mechanisms for PL related to an amorphous Si component or Si-H or Si-OH bonds, or the quantum confinement effect. Similarly, for milled SiO 2 powder mechanisms for PL do not appear related to Si-H or Si-OH bonds. Instead the greatly increased intensity of PL for milled SiO 2 can be related to both the increased volume fraction of the amorphous SiO 2 component and the increased density of defects introduced in the amorphous SiO 2 upon milling. It is suggested that the PL for as-received Si, milling-induced nanocrystalline/ amorphous Si, and milled SiO 2 results from defects, such as the nonbridging oxygen hole center, in the amorphous Si suboxide and/or SiO 2 components existing in these powder samples. The PL measurement for milled SiO 2 is dependent on air pressure whereas that for as-received SiO 2 is not, suggesting that new emitting centers are formed by milling.

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