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Figure 5.34 shows the Raman spectra for each templates with excitation wavelength of 532 nm (solid lines) and 488 nm (short-dashed lines), in back scattering geometry. The allowed six Raman pho- non modes for sapphire with 532 nm (488 nm) excitation wavelength were observed at Eg378.0 cm

-1 (378.8 cm-1_), 1g A 416.0 cm-1 _{(417.0 cm}-1_), g E 429.0 cm-1 _{(430.0 cm}-1_), g E 446.6 cm-1 _{(448.6 cm}-1_), g E 576.7 cm-1 _{(576.1) cm}-1_, g

E 749.7 cm-1_{. Note that Raman spectrum with excitation wavelength of 488 nm for} sapphire is measured only up to 700 cm-1_{. Raman scattering intensities of all phonon modes except 749.9} cm-1 _{mode (since there is no measured data after 700 cm}-1 _{for 488 nm excitation wavelength) are same. In} the Raman spectrum of AlN/sapphire (Fig. 5.34 (b)), sapphire modes are stronger than AlN due to pene- tration depth into the sapphire for relatively thinner film (25 nm). A slight indication of A1LO peak is ob- served at 888 cm-1 _{though and there is no prominent peak of E}

2 high peak at 653 cm-1 observed.

Raman spectrum (excitation wavelength 532 nm) for Ga polar GaN film grown on AlN/sapphire is illustrated in the Fig. 5.34 (c). According to the Ref. [25], the peaks at 569.4 cm-1_{, and 734.8 cm}-1 _can be attributed to the E2 high, and A1LO phonon modes of GaN, respectively. As mentioned in the Ref. [25], with Lorentzian fit for the range 700-800 cm-1_{, E}

1LO phonon mode of GaN can be found at 742 cm- 1_{. Fig. 5.34 (d) is depicted the Raman spectra of N-polar GaN grown on sapphire for the excitation wave-} length of 532 nm and 488 nm. As well as, Raman spectra of n- and p-type doped GaN grown on i- GaN/AlN/sapphire are shown in the Fig. 5.34 (e), and (f) respectively. The Raman scattering from sap- phire are absence in the Raman spectra for each samples measured with 488 nm excitation wavelength. For this case, the Raman scattering measured using micro-Raman system and which allowed to focus the Raman features from the samples (in this case GaN) and sapphire scattering became much weaker. In fact, the intensity of A1LO phonon modes decreases for both n- and p-type samples. The absence of A1LO peak in N-polar GaN grown sapphire in both cases, 532 nm and 488 nm excitation wavelength and coinci- dence of A1LO with LPP modes and this was also observed from the FTIR reflectance spectrum.

Figure 5.34 Raman spectra of sapphire (a), AlN/sapphire (b), Ga-polar GaN/AlN/sapphire (c), N-polar- GaN/sapphire (d), n-type GaN/i-GaN/AlN/sapphire (e), p-type GaN/i-GaN/AlN/sapphire (f).

Solid line and short dashed lines are the Raman spectra for the excitation wavelength of 532 nm and 488 nm respectively. Dashed lines mark for the strongest sapphire-related Raman peaks.

Before arriving at conclusions about the InN samples, it is important to understand the quality of the templates or substrate since most of the dislocations can be originated from the substrate or template. Therefore, the Raman E2 high and A1LO phonon modes were analyzed in order to understand the struc- tural and electronic properties. The sifting of the Raman E2 high peak position from the ideal position is resulted by the stress or strain in semiconductors hence it can be used as a qualitative measure of those stress or strains. The stress is tensile as the E2 high peak exhibit the red shift (decrease of Raman fre- quency) and the stress is compressive as the E2 high peak has a blue shift (increase of Raman frequency).

Further, the full width half maximum (FWHM) of E2 high peak is expounded the short range ordering of the crystalline structure. The narrow E2 high FWHM is the better crystalline quality. The Raman E2 high and A1LO phonon modes FWHM maximum values and positions are listed in the Table 5.9.

Table 5.9 Raman E2 high and A1LO position and their FWHM values different templates. (532 nm of ex- citation wavelength).

Parentheses shows the values determined with 488 nm excitation wavelength. Substrate

E2 high (cm-1) A1 LO (cm-1)

FWHM Peak Position FWHM Peak Position

sapphire - - - - AlN/sapphire - - - 888.0 Ga polar GaN/AlN/sapphire 5.3 569.4 7.4 734.8 N polar GaN/AlN/sapphire 6.7 (4.4) 566.3 (565.8) - - n-GaN/AlN/sapphire 5.6 (2.7) 569.1 (568.1) 7.3 (5.6) 734.6 (733.3) p-GaN/AlN/sapphire 5.6 (3.7) 567.9 (569.5) 7.8 (6.3) 734.1 (733.9)

For the Ga polar GaN and n-GaN templates, Raman E2 high peak position has shown a blue shift of 1.4 cm-1 _{and 1.1 cm}-1_{, respectively, from the bulk value of 568 cm}-1 _{[24]. Raman E}

2 high peak position of the p-GaN is almost close to the bulk value. In contrast, N polar E2 high peak position exhibits a red shift of 2.3 cm-1 _{indicating it has tensile stress. A slight shift of E}

2 high modes with excitation wavelength is observed. Local crystalline order of the all GaN templates is almost same except N polar GaN template.

Figure 5.35 Raman spectra of InN films grown on sapphire (a), AlN/sapphire (b), Ga-polar GaN/AlN/sap- phire (c), N-polar-GaN/sapphire (d), n-type GaN/AlN/sapphire (e), p-type GaN/AlN/sapphire (f).

The Raman spectra of InN layers on each template with the excitation wavelength of 532 nm (solid lines) and 488 nm (short-dashed lines), in back scattering geometry, are depicted in Fig. 5.35. It is indicated that Raman scattering intensity and the broadening of the phonon modes are dependent on the excitation energy. The sapphire phonon modes (~378.7 cm-1_{) is absent in the Raman spectra with 488 nm} excitation wavelength. A1TO (445 cm-1) phonon mode is not allowed in the scattering geometry used in this study. However, for all above InN layers, the A1TO mode is detected. The E2 high and A1LO phonon modes of InN are observed ~487.5 cm-1 _{and ~590-593.2 cm}-1 _{with 532 excitation wavelength. Table 5.10} summarizes the Raman E2 high and A1LO phonon modes FWHM maximum values and positions.

The intensity enhancement of the A1LO phonon mode compare to the E2 high, is related to the high carrier concentration [26]. Raman spectra of InN layers performed with the excitation wavelength of 532 nm indicate that this wavelength excites the properties of the InN layer close to the templates. When compare the intensity of A1LO phonon mode with the intensity of E2 high mode (with excitation wave- length of 532 nm), InN grown on AlN/sapphire has the lowest A1LO phonon mode intensity. Thus, lowest carrier concentration. This confirms by the FTIR analysis results. See Table 5.7. The InN layer close to the AlN has the lowest carrier concentration comparison with the other samples.

The lowest Raman E2 high FWHM values of 6.8 cm-1 (with the 532 nm excitation source) is ob- served the InN grown on AlN/sapphire. Therefore, the InN layer grown on AlN template shows the better local crystalline order. The broadening of the E1TO phonon mode in IR reflectance spectra is related to the quality of the film [23]. From the FTIR analysis, lowest E1TO broadening value of 4.3 cm-1 was also obtained for this sample. It is further confirmed that InN on AlN has the better crystalline InN layer.

Table 5.10 Raman E2 high and A1LO position and their FWHM values for InN layers grown on different templates (532 nm of excitation wavelength).

Parentheses shows the values determined with 488 nm excitation wavelength. Sample

E2 high (cm-1) A1 LO (cm-1)

FWHM Peak Position FWHM Peak Position InN/sapphire 8.9 (7.6) 487.8 (489.1) 17.5 (19.6) 591.4 (587.8) InN/AlN/sapphire 6.8 487.7 17.1 591.9 InN/Ga polar GaN/AlN/sapphire 10.3 (7.9) 487.4 (490.5) 19.9 (21.7) 592.3 (590.7)

InN/N polar GaN/AlN/sapphire 8.3 (8.0) 487.6 (489.2) 17.4 (16.3) 592.7 (589.3) InN/n-GaN/AlN/sapphire 8.2 489.1 19.3 593.2 InN/p-GaN/AlN/sapphire 7.9 (7.5) 487.8 (491.4) 19.6 (19.6) 592.0 (591.1)