Influence of excitation frequency on Raman modes of In1-xGaxN thin films

Low energy optical modes of MBE-grown In1-xGaxN thin films with different values of x are investigated using Raman spectroscopy. We also studied the influence of Raman excitation frequency using red and green lasers on scattering intensity of various Raman modes. For those In1-xGaxN alloys whose bandgap energy is close to the red laser, a huge enhancement in the intensities of A1(LO) mode and its 2A1(LO) replica is observed when excited with red laser as compared to the green laser excitation. We found that the energies of longitudinal optical modes (A1 (LO) and 2A1 (LO)) vary nonlinearly unlike the E2 mode with increasing Ga atomic fraction. A Raman mode ~ 540 cm-1 was observed in all In1-xGaxN films with low energy red laser excitation but was absent with green laser excitation. We attribute this mode to A1(TO) mode of the underneath GaN buffer layer.


Introduction
Currently various optical properties of In 1-x Ga x N alloys are extensively studied because of their potential applications in full-solar-spectrum photovoltaics [1,2,3], high-performance light-emitting and laser diodes [4] and, solid-state lighting [5]. In addition to the investigation of their bulk properties, the quantum well structures of these alloys are also currently investigated for a variety of applications including laser diodes and -photoluminescence spectroscopy with InGaN quantum disks [6]. High efficiency quantum well structure light-emitting-diodes in the blue-green region of the spectrum and laser diodes emitting violet light have already been made from group-III nitride semiconductors [7] and are in commercial use. In all these devices In 1-x Ga x N forms an active layer. The bandgap of these alloys can also be tuned from near-infrared (IR) to ultraviolet (UV) regimes and in addition these alloys can be fabricated to further increase their bandgap energy by quantum confinement effects [8].
Photoluminescence and other optical properties are strongly influenced by electron relaxation rates due to electron-electron and electron-phonon scattering [9,10]. So it is important to investigate their various phonon modes and how their spectral features can be influenced by the source excitation frequency. We studied the phonon modes of In 1-x Ga x N alloys using Raman spectroscopy and variations in their energies with increasing Ga atomic fraction in the InN compound matrices. We also investigated the effects of Raman excitation frequency on the scattering intensity of these modes for various values of x.
Thin buffer layers of AlN and GaN with thickness ~ 10 nm and 220 nm were grown to reduce the lattice mismatch between In 1-x Ga x N thin films and c-sapphire substrate. The thickness of the In 1-x Ga x N layer is ~ 0.6 m. The carrier concentrations and Hall mobilities of these thin films were determined at room temperature using Van der Pauw method and their values are listed in Table I. We noticed a small increase in carrier concentration and a substantial decrease in mobility with increasing Ga atomic fraction in the In 1-x Ga x N films. The well-defined interference fringe width from UV-Vis-NIR reflection spectrum was used to estimate the thickness of these thin films whose values are given in Table (1).

Results and Discussion
A strong electron-phonon Fröhlich interaction in wurtzite crystals can lead to a large enhancement in light scattering by polar LO phonons when an incident photon's energy is close to band gap energy of semiconductor [13,14]. In many cases it has also led to excitation of replica phonon modes. In wurtzite In 1-x Ga x N structure, there are six Raman active modes--E 2 (low), E 2 (high), A 1 (TO), A 1 (LO), E 1 (TO), and E 1 (LO) [15,16] which are observed in the first order Raman spectrum of In 1- x Ga x N films. Other optical modes --B 1 (low) and B 1 (high) --are silent in wurtzite nitrides but are occasionally observed in disordered films [17]. In this paper, we investigated the role of excitation energy on the Raman intensity of these modes by choosing two laser energies-the green laser (514.5 nm = 2.41 eV) having energy larger than the bandgap energy of all the In 1-x Ga x N films we have studied here and the red laser (785 nm = 1.58 eV) having energy between the band gap energy of x = 0.3 film (E g = 1.30 eV) and of x = 0.54 film ( E g = 1.85 eV). Out of the six Raman active modes, we observed E 2 (high) and the first-order A 1 (LO) Raman modes when the samples were excited by red laser (Fig. 1). Along with these modes, 2A 1 (LO) replica mode is also observed in all the samples. In between E 2 (High) and A 1 (LO) modes, one can also see a mode around 540 cm -1 in all the In 1-x Ga x N films. A similar mode has been observed [18] in post annealed InN thin films and has been marked as an unidentified feature. However, we associate this mode with A 1 (TO) mode of GaN originating from the GaN buffer layer used in all the films. This mode is not observed with green laser (Fig. 2(a)) because of the stronger absorption of the green laser by the top In 1-x Ga x N layer. As Ga atomic fraction in In 1-x Ga x N is increased, the energies of these modes (E 2 (High), A 1 (LO), and 2A 1 (LO)) increase as shown in Fig.3(a), as expected. However there is a dramatic decrease in the intensity of E 2 (high) mode relative to A 1 (LO) mode accompanied by increase in damping due to increase in the lattice disorder from Ga atoms substitution. At x = 0.54, E 2 (high) mode has almost disappeared into the noise. With increasing x value, the absorption of the laser energy decreases in the In 1-x Ga x N layer due to increase in the bandgap values and at x = 0.54, the bandgap value (E g = 1.86 eV) of In 1-x Ga x N layer becomes larger than the laser energy. This leads to a stronger transmission of the laser energy into the subsequent layers-GaN and Sapphire layers--and as a result intense excitations of many sapphire modes as marked by circles along with the A 1 (TO) of GaN marked as * (top panel, x = 0.54 in Fig. 1). We also observed an intense structure at 1070 cm -1 . Similar structure, however with less intensity, has been associated with the combination of E g (432 cm -1 ) GaN mode and A 1g (645 cm -1 ) sapphire mode [19]. Because of stronger excitation of the sapphire modes in this film, the observed mode could be a result of this combination.
In Fig. 2(a) we show the Raman spectra of In 1-x Ga x N films when excited off-resonance with green laser. A big change in the Raman intensities of these modes is observed when compared with the corresponding Raman spectral intensities obtained using red laser. The green laser energy is larger than the bandgap energies of the In 1-x Ga x N films, so there will be a negligible transmission of green laser energy into the GaN buffer layer underneath the In 1-x Ga x N films and sapphire substrate. As a result, no sapphire and GaN modes are observed in In 1-x Ga x N thin films with green laser, as shown in Fig 2(a). With increasing atomic fraction of Ga, the bandgap energy of In 1-x Ga x N layer increases and begins to tune with the laser energy. This increases the Fröhlich electron-phonon interaction leading to a stronger excitation of A 1 (LO) and 2A 1 (LO) phonons. At x = 0.54, the A 1 (LO) and 2A 1 (LO) phonons become quite intense compare to the E 2 modes. On the contrary, the Raman intensities of E 2 (low) and E 2 (high) modes decrease with increasing values of x and at x = 0.54, these modes cease to exist due to overdamping by the substitutional disorder of Ga atoms. Note that E 2 (low) mode could not be observed when excited with red laser. In Fig. 2(b), we also show the shift in the energy of E 2 (low) mode with increasing values of x.
The energies of the observed A 1 (LO) and 2A 1 (LO) modes increase nonlinearly ( Fig. 3(a)) while those of E 2 modes vary almost linearly with increasing Ga atoms concentration. We attribute this nonlinear behavior to the c-axial strain in our films due to their small thickness affecting lattice polarization along the c-axis more than in other directions [20]. Similar behavior of these modes is also observed for the red laser when Ga concentration is increased. Fig. 3(b) shows variation in the relative intensities of A 1 (LO) and E 2 (High) modes for red and green lasers. Clearly the ratio A 1 (LO) /E 2 (High) is quite large for red laser and varies quite strongly as compared to that of the green laser with increasing concentration of Ga.
At a particular value of x, there is no change in the frequency of these modes when energy of the laser is changed, however, the intensities of some of the modes change dramatically.

Summary
In summary, we have investigated the intensity of Raman scattering in In 1-x Ga x N thin films when their bandgap energy is close to the excitation frequency of the laser. We found that tuning of electronic bandgap transition of the films with the laser excitation frequency dramatically influences the scattering intensities of A 1 (LO), 2A 1 (LO) , E 2 (Low) and, E 2 (high) modes.