Optical properties of non-polar and semipolar InGaN QWs
The reduced impact of the QCSE in semipolar InGaN QWs can be seen in the smaller shift of the emission wavelength with increasing well-width (Craven et al., 2004a) and increasing injection-current (Sharma et al., 2005). In addition, as the wavefunction overlap in non-and semipolar QWs increases, the recombination lifetime decreases (Netzel et al., 2010a). This could of course be caused by a short non-radiative life time—even at 10 K—which we could exclude by comparison to c-plane samples (Netzel et al., 20116). In addition, the exciton binding energy in non- and semipolar QWs is enhanced (Netzel et al., 20106). This allows the observation of excitonic recombination rather than bimolecular recombination at room temperature for (10l0) and (1122) InGaN quantum wells. PL measurements also revealed a delayed S-shape of the quantum-well luminescence emission energy (Netzel et al., 2011a; Wernicke et al., 2012). This is a sign for an increased localization energy (Li et al., 2005). InGaN QWs on (0001) c-plane typically exhibit an S-shape turning point of ~150K, whereas semi- and non-polar InGaN QWs can exhibit turning points at temperatures exceeding 300K (Netzel et al., 20116). This is accompanied by an increased full-width-half-maximum (FWHM) of the emission spectra. In order to differentiate between long-range fluctuations and microscopic localization, micro-PL measurements were performed. These scans show very small fluctuations of the emission energy in 5 x 5 pm2 areas as shown in Fig. 8.12. The emission energy of the (1122) Ino.24Gao.76N varies by only 12meV and the minimum intensity is 78 % of the local maximum, which is small in comparison to c-plane QWs. Single spectra (spatial resolution is 400 nm) reveal a local FWHM that increases
Fig. 8.12. 5 x 5 pm2 micro-PL image of an (1122) InGaN QW. Courtesy of Lukas Schade (University of Freiburg).
Fig. 8.13. Local FWHM from micro-PL of a variety of non- and semipolar QWs. A reduced transition energy leads a strong increase of the local FWHM. Courtesy of Lukas Schade (University of Freiburg).
strongly with the emission wavelength (Fig. 8.13). This is another indication to highly localized states in non- and semipolar QWs.
The reduced quantum confined Stark effect is not the only difference for the recombination in semipolar and c-plane QWs. The anisotropic and shear strain that causes the zero crossing of the piezoelectric field (see Section 8.1) also affects the band structure (Schade et al., 20116), causing polarized emission and a shift of the transition energy (Wernicke et al., 2012). The reduction of surface symmetry lifts the isotropic symmetry of the hole wavefunction within the basal plane. The three typical valance subbands of (0001) InGaN (light-hole LH, heavy-hole HH, and split-off hole SH band) mix into new subbands. The symmetry of the wavefunction defines the selection rules and thus the polarization of the emitted light. In c-plane GaN the light emitted from the LH and HH band is polarized perpendicular to the c-axis isotropically in the basal plane, and the SH is polarized parallel to the c-axis. For non-polar InGaN the topmost valence band (called A1) is formed from a mix of LH and HH, and is polarized mostly perpendicular to the c-direction. For semipolar QWs, the valence bands are formed again from a mix of LH and HH bands, and with increasing shear strain (maximum at 45°) a portion of the SH band. This leads to a reduced degree of polarization of the single subbands. The calculations also predict an orthogonal polarization of the second-highest (B1) subband (Fig. 8.14), at least for small k-values (Schade et al., 2011a). The model also describes the polarization switching (change of the subband polarization from perpendicular to c towards parallel to c) observed for (1122) InGaN quantum wells by increasing the indium content. The switching point is present for all indium content, and dominant c'
Fig. 8.14. Optical polarization degree of the top-most subbandlevel A1 and the second top-most subbandlevel B1 of an Ino.15Gao.85N QW strained to GaN in dependence of the tilt angle to the c-plane. A polarization degree of 1 represents total polarization with E perpendicular to the c axis. A polarization degree of -1 corresponds to a polarization of the subband parallel to c' (the projection of the c-axis). Measurement points represent the polarization at 5K of violet-emitting InGaN QWs (~ 15% indium content). At this temperature the B1 subband is not populated, and the A1 polarization can be measured directly (Schwarz and Scheibenzuber, 2011; Schade et al., 20126). Reprinted with permission from Schade et al. (20116), © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
polarization was experimentally verified for (10l2) InGaN QWs. The switching point increases to higher angles with increasing indium content to a maximum angle of 69° (calculated for InN on GaN). The (2021) and (10l0) cannot be affected by polarization switching. Since subband polarization and distance depend strongly on strain, strain relaxation influences the polarization (Koslow et al., 2012). The polarization degree of an InGaN QW at room temperature is not only given by the subband polarization, but is further reduced by occupation of the orthogonally polarized B1 subband. Therefore, a high degree of polarization can be achieved only for inclination angles >65° for a wide range of indium content, due to high subband polarization and subband spacing (Schade et al., 2011a; Schade et al., 20116). Also, inclination angles between 15° and 55° might allow for a high degree of polarization for sufficiently high indium content if the subband distance is large enough. Only the (10l1) exhibits a rather low degree of polarization for all indium content. Experimentally observed polarization values agree mostly with the calculated ones (Schade et al., 2011a). The trend is even observed in localization centers with different indium content causing the inhomogeneous broadening (Schade et al., 2012a). Surprisingly, the polarization observed in (2021) QWs is much lower than expected (Schade et al., 2011a).
Although the subband polarization fits the calculated values very well, the measured subband spacings are much larger, leading to a higher degree of polarization at room temperature than expected from the calculation (Schade et al., 20116).
This effect of split subbands can be used to produce LEDs with strongly polarized emission, and also increases the optical gain in lasers (see Section 8.6). The effect can also be used to extract the quasi-Fermi level and thereby the carrier density in the quantum well (Schade et al., 20126).