Raman Analysis of Al x Ga 1-x N Films
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error in the linewidth and peak position measurements is ± 1.5 cm- 1 and 0.5 cm- 1 respectively. The AlxGal1 xN films were grown via the organometallic chemical vapor deposition (OMCVD) method at - 1100 oC on 6H-SiC(0001) substrates with a 100 nm AIN buffer layer. The thickness of the films is - 2 g.tm and the composition, x, which was determined via Rutherford backscattering (RBS), energy dispersive X-ray (EDX), and Auger spectroscopy, is: 0.06, 0.12, 0.22, 0.32, and 0.70. The Raman data points for x=0 and x=1 were obtained from GaN film and AIN crystallite
respectively. RESULTS AND DISCUSSION The Configurational Disorder in AlGaN Films Figure 1 shows the room-temperature Raman spectra of the E2 line from AlxGal-xN films of compositions 0.06, 0.12, 0.22, and 0.70, which exhibit linewidths I of 8, 13, 16, and 19 cm-1 respectively. As depicted in the figure, the spectral lineshape for films of x>O.06 exhibit asymmetric broadening and a peak shift toward higher frequency. Possible line broadening mechanisms applicable to alloys include thermal broadening, activation of a symmetry forbidden zone-center (q=O) mode which lies in the same frequency range as the investigated line, and broadening due to activation of a collection of modes of wavevectors q>0. The last two broadening mechanisms result from the elimination of the translational symmetry of the lattice due to alloying. To investigate the thermal contribution to the line broadening, Raman spectra were acquired at T=IOK, and no significant change was observed between the shapes and linewidths of the room and cold temperature spectra. Thus the linewidth in our sample is not strongly affected by temperature. The only effect of the low temperature is the shifting of the peak position by - 2 cm-1 toward higher frequency, which was also previously observed in GaN films [5] and crystals [6] and was attributed there to the thermal contraction of the bonding. The second possible mechanism is the activation of a q_0 symmetry forbidden mode which might be convoluted with the E2 line and cause the asymmetric broadening. However, the only mode in the frequency range of the E2 line that is forbidden in the back-scattering geometry is the AI(TO) mode of GaN at -560 cm-1 which lies at a lower frequency. A more plausible mechanism to account for the high frequency asymmetric linewidth in our Raman spectra is the spatial correlation model, also known as the confinement model. The model was developed to explain the asymmetry line in BN [7] and Si [8] and has been successfully applied to quantify the lineshape behavior of Gal-xAlxAs and Gal_xInxAs alloy systems [1]. The foundation of the model lies in attributing the relaxation of the q=0 Raman selection rules to the phonon confinement in a finite domain of size L. The size of L in an alloy system may be viewed as the average size of the ordered domains which are embedded in the configurational-disordered matrix. According to the model, as L gets smaller, the range of wavevectors Aq becomes larger: a wider range of frequencies are
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