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Linear regime: results

The first observation of strong coupling in GaN was achieved in a modest Q factor (Q = 60) cavity (Antoine-Vincent 2003) at low temperature and then at room temperature (Semond 2005). Compared to GaAs, one peculiarity of nitrides is that the spin-orbit coupling is small and the three excitons A, B, and C related to the valence band maxima are close to each other and can couple to the cavity mode. At low temperature, this leads to a fine polariton structure where A and B polaritons can be identified (Sellers 2006a). At room temperature, A, B, and C excitons are not clearly resolved but still contribute to the coupling, with an oscillator strength depending on polarization (Butte 2006). It was pointed out that the oscillator strength is one order of magnitude larger in bulk GaN than in bulk GaAs. This explains why the strong coupling could be observed in modest Q-factor cavities in GaN, while the same observation in II-VIs or in arsenides required larger Q factors (Q = 350 in GaAs QWs (Weisbuch 1992), Q = 650 in ZnCdSe QWs (Kelkar 1995), Q = 415 in CdTe QWs (LeSiDang 1998)). In addition, the latter achievements in GaAs and II-VIs were based on QWs, i.e. less material than with a bulk active region. The strong coupling was also achieved with GaN QWs in a cavity with larger Q values (200-800) (Feltin 2006; Christmann 2006a). Again, it was pointed out that the oscillator strength in nitride QWs is ten times larger than in arsenide QWs, explaining that the strong coupling regime can be obtained in spite of broader exciton and photon modes. The impact of the inhomogeneous excitonic broadening on the strong coupling was analysed. The strong coupling condition is related to the exciton coherence time, and is thus related to the homogeneous broadening. However, when the inhomogeneous broadening increases, the number of excitons coupled to the mode decreases (or their effective oscillator strength decreases) and the strong coupling may disappear. The transition value for the inhomogeneous broadening was estimated to be 45 meV, i.e. larger than the Rabi splitting of 30 meV (Christmann 2006a). However, when GaN QWs are replaced by InGaN QWs, the much larger inhomogeneous broadening prevents any strong coupling observation, in contradiction to a claim for a strong coupling observation published by another group (Tawara 2004). The inhomogeneous broadening of the cavity mode due in plane fluctuations of the cavity thickness is also a key parameter. By reducing the area of the optical analysis the Q factor of the cavity is significantly increased from 200 to 2800 (Christmann 2006b). The nearly optimized cavity was finally obtained by increasing the number of QWs, leading to a Rabi splitting of 50 meV at 300 K (Christmann 2008).

Another specificity of nitride is the fact that they can be grown on various substrates, still keeping good optical and electrical properties. This has been demonstrated for LEDs or HEMTs, where high-performance devices have been fabricated on GaN, sapphire, SiC, and Si substrates. This is in contrast with arsenides, where homoepitaxy has been the only approach so far for obtaining device-quality material. The first observation of strong coupling was made in GaN grown on Si (Antoine-Vincent 2003), where the Si acts as a mirror. Latter, a bottom nitride Bragg mirror with larger reflectivity was added (Sellers 2006a). Cavities were also grown on sapphire. In this case, the bottom Bragg mirror was AlInN/GaN lattice-matched to GaN (Feltin 2006a), thus yielding both a high Q factor (a few thousands) and a good active region crystallographic quality (narrow exciton linewidth) (Feltin 2006b; Christopoulos 2007). Today, the highest reported Q factor in nitrides is 6400 (Butte 2009a). Cavities have also been grown on 6H-SiC (Tawara 2004) and on GaN substrates (Lu 2011). While the top mirror is almost always a dielectric Bragg mirror, various approaches have been used for the bottom mirror. In addition to epitaxial AlGaN/AlGaN or AlInN/AlGaN Bragg mirrors already mentioned, dielectric stacks are also used as bottom Bragg mirrors after substrate removal. This was done first for LEDs on Si in the weak coupling regime (Duboz 2003), and was extended to cavities in the strong coupling regime (Bejtka 2008). The technological process takes advantage of the easy removal of the Si substrate. Removing substrates others than Si is more difficult but has nevertheless been achieved with SiC by reactive ion etching, leading to a Q = 400 cavity after bottom-mirror deposition (Tawara 2004), and with sapphire by laser lift-off leading to a Q = 500-1000 after bottom-mirror deposition (Pattison 2007; Song 1999).

Interestingly enough, the technological development achieved in nitride cavities has allowed development of ZnO-based cavities which include an epitaxial

Table 10.1 Typical exciton energy (in bulk and in QWs) and observed Rabi splitting energies in various material systems.

Material

EXd (meV)

EjD (meV)

Rabi splitting (meV)

GaAs

4.8

8 - 14

6 - 20

CdTe

10

25

20 - 30

GaN

26

50

40 - 90

ZnO

60

> 80

100

nitride bottom mirror (Medard 2009; Guillet 2011a; Guillet 20116) and also development of perovskite microcavities (Lanty 2011). Table 10.1 shows that the larger exciton binding energy, associated to a larger oscillator strength, in GaN (also in ZnO) leads to larger Rabi splitting energies than in smaller band-gap materials. This chapter does not cover cavities in the weak coupling regime. We just recall that very early cavities were fabricated in the 1990s for LEDs and optically pumped VCSELs (Redwing 1996; Krestnikov 1999; Someya 1998). While the main differences lie in the active region (thickness, homogeneous broadening), the cavity fabrication is similar for weak and strong coupling regimes, and high Q cavities have been fabricated by hybrid approaches with a bottom epitaxial mirror (Kako 2002) or by a fully dielectric approach involving substrate removal (Song 2000a; Song 20006; Martin 2001).

 
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