Phonons and Free Carriers in a Strained Hexagonal GaN-AlN Superlattice Measured by Infrared Ellipsometry and Raman Spect
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age biaxial stress, σxx, was given using elastic constants (Cij), and phonon deformation potential constants (aλ, bλ) for α-GaN given in Refs. [3] and [4], respectively. Spectroscopic ellipsometry (SE) is known to be an excellent technique for measurement of thin-film optical properties. Successful application of infrared (IR) SE for measurement of phonon and free-carrier effects in group-III nitride heterostructures was reported previously [5,6]. SE is an indirect technique, and calculated model spectra need to be fit to experimental data. If appropriately chosen, model parameters, which parameterize the dielectric functions of the sample constituents, can provide physically meaningful quantities, such as transverse-optical (TO), longitudinal-optical (LO) frequencies and broadenings, static or high-frequency dielectric constants, and layer thickness. Because free-carrier absorption affects the IR dielectric response, concentration and mobility parameters can be extracted from the IRSE lineshape analysis if the carrier effective mass is known [7]. When combined with electrical Hall measurements, the IRSE data can further provide information about the effective mass parameter, as recently demonstrated for n- and p-type α-GaN [6]. The focus of this work is to investigate stress and free-carrier effects in a strained αGaN-AlN-SL by measurement of the long-wavelength dielectric response using IRSE for the first time, in combination with µ-Raman investigations. EXPERIMENTAL One SL with 16 periods of alternating wurtzite GaN (8 nm) and AlN (3 nm) layers was grown by MOVPE on (0001) sapphire. A 1-µm-thick GaN layer was deposited on a ~15 nm AlN buffer layer prior to the SL (for more details see Ref. [8]). The sample was measured at room temperature by IRSE and polarized µ-Raman scattering. Ellipsometric parameters were acquired at multiple angles of incidence (57°, 72°), and for wavenumbers from 333 cm-1 to 1200 cm-1. A rotating-polarizer, rotating-compensator, Fouriertransform-based variable-angle-of-incidence spectroscopic ellipsometer was used. A detailed description of the IRSE approach is given in Refs. [5,7] and references therein. The µ-Raman spectra were recorded with a XY-Dilor spectrometer at five different configurations in backscattering geometry for wavenumbers from 200 cm-1 to 1200 cm-1. The excitation wavelength was 488 nm (Ar+ laser). The incident laser light power was 150 mW. The diameter of the laser focus at the sample surface was typically 1 µm. The sample orientations during Raman and IRSE measurement are shown in Figs. 1b, and 2b, respectively. DISCUSSION Fig. 1a shows the µ-Raman spectra in five different backscattering configurations. The spectra reveal the A1(TO), E1(TO) (labeled by “1”, and “2”, respectively), and E2, A1(LO), and E1(LO) phonon modes of the 1-µm-thick GaN layer which supports the SL.
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{AlN3nm/GaN8nm}16x/GaN
1 2 c a Raman intensity (a.u.)
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