Piezoelectric Level Splitting in GaInN/GaN Quantum Wells
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G 3.66 Mat. Res. Soc. Proc. Vol. 537 © 1999 Materials Research Society
EXPERIMENTAL Pseudomorphic Gaj.-In.N/GaN heterostructures were grown along the c-axis by metal organic vapor phase epitaxy (MOVPE) on sapphire using low temperature deposited AIN buffer layers [6]. Samples consist of 5 sequences of nominally undoped L,=3 nm Ga1 _InXN wells embedded in 6 nm GaN barriers. The maximum composition is estimated to be x = 0.2. At this barrier width coupling of states between the wells is strongly suppressed. The set is grown either directly on 2 4m GaN or embedded in a GaN pn-junction with the p-side and a transparent contact on top. For reflection measurements we used a Xe white light source and a 325 nm 40mW HeCd laser for modulation (PR). The same laser was used for PL. In ER a sinusoidal voltage of 0.2 Vpp at 2 kHz and a variable offset was applied to modulate reflection in the pn-diode sample. All experiments were performed at room temperature.
PHOTOREFLECTION AND LUMINESCENCE IN GaInN/GaN MQW PL and PR on nine samples with different well composition are presented in Fig. I in the sequence of the PL peak energy. A N2 NI C N1o N3 F series of narrow oscillations (MV/cm) arbitrarily labeled Co ... C 4 appears in
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C
0.66 G 0.60 F 0.55 E 0.35 D
PR
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0.23 ,
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0.24 B
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0,24 SA
2.2
2.6
3.0
3.4
Energy (eV) Figure 1 Photoreflection and photoluminescence oj strained 3 nm GaInN/6nm GaN MQWs for 9 different compositions. Associated features are connected by dashed lines and grouped in features Co ... C 4 c nd No... N 3. In all samples a PR contribution appearsclose to the PL maximum. Interpreted electric field v6 lues are indicatedon the right.
the range of 3.2 - 3.7 eV range.
Even sharper modulation appears near 3.42 eV (for the present purpose labeled No). A series of weaker but broad PR contributions N, - N3 appears at lower energy the lowest of which has its maximum very close to the PL peak energy of the respective sample. This finding is in contrast to results by Chichibu et al. [11] where PR signal can be identified only at higher energies. Those levels would possibly correspond to levels N1 or N 2 in our samples. A different signal amplitude may have contributed in From the clear this discrepancy. signature of N3, both in PL and PR, we propose that this PL originates in a well defined electronic level rather than a logarithmic edge of a randomly distributed property such as proposed in earlier work.
The sharp oscillation in No is attributed to excitons in the GaN / / barriers or the GaN epilayer underneath A G // the MQW. The splitting in the order of "* E // 30 meV into the different excitons is >0.10 D below the current interest of our SX cx /y7 interpretation. Oscillations in the / / A N / / vicinity of No, namely Co ... C 4 strongly // // ,AI/ /_/' resemble FKOs above a critical point in the joint density of states (DOS) in the presence of a large electric field F. ,0.05 0/,.. w"7 ///.7 K.. One minimum is close to No and a distortion of both signals must be expected
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