De De. De M Q fix = const PR R/R Intensity (arb. inits) Energy (ev) a) b)

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1 PIEZOELECTRIC EFFECTS IN GaInN/GaN HETEROSTRUCTURES AND QUANTUM WELLS C. WETZEL, T. TAKEUCHI, S. YAMAGUCHI, H. KATO, H. AMANO, and I. AKASAKI High Tech Research Center, Meijo University, Shiogamaguchi, Tempaku-ku, Nagoya Japan The electronic bandstructure of strained GaInN/GaN multiple quantum wells is studied as an example for piezoelectric wurtzite nitrides. In photoreection, photoluminescence and electroreection a minimum of four discrete levels are identied. In the limit of large strain levels form a Stark-like ladder the step size of which is given by the piezoelectric dipole across the strained well layer. Electric elds up to 0.9 MV/cm are identied. Electroreectance under an external bias eld identies the direction of the piezo eld to point from growth surface to substrate for biaxially strained GaInN. Piezoelectric elds are found to strongly aect the electronic bandstructure. 1 Introduction The wurtzite wide bandgap system of GaInN/GaN exhibits superb optoelectronic properties 1. Despite considerable device progress the nature of the light emission process remains highly controversial 2;3;4. In order to elucidate the bandstructure in GaInN/GaN multiple quantum wells (MQWs) we performed photoreection (PR), electroreection (ER) and photoluminescence (PL) spectroscopy. 2 Experimental GaInN/GaN heterostructures were grown in metal organic vapor phase epitaxy on sapphire 5. MQWs consist of 5 sequences of nominally L z = 30 A GaInN wells embedded in 60 A GaN. The set is grown either on 2 m GaN or embedded in a GaN pn-junction with the p-side and a transparent contact on top. Pseudomorphic strain was identied in x-ray mapping of both lattice constants. Reection measurements were performed using a white light source and a 325 nm HeCd laser for modulation. PL was performed using the same laser. In electroreection (ER) a sinusoidal voltage of 0.2 V pp at 2 khz and a variable oset was applied. All experiments were performed at room temperature. 1

2 PR R/R Intensity (arb. inits) De De De M 1 M 0 M 3 M 2 + Q fix = const a) b) Figure 1: a) PR and PL spectra of GaInN/GaN MQW structures. Corresponding features are indicated by dashed lines identifying four major interband transitions M 0 M 3 and narrow FKOs E 0 E 4. Derived electric eld values F are indicated. b) Schematic of the bandstructure at every strained GaN/GaInN/GaN heterointerface. Spatially direct transitions are identied. Dipole D is induced by the xed piezoelectric -layers. 3 Photoreection and Photoluminescence PR and PL spectra of the set of samples is shown in Fig. 1a) in the sequence of decreasing PL energy, i.e. increasing InN-fraction x. In PR a narrow excitonic double feature at 3.43 ev marks the GaN bandgap energy labeled here M 0. This signal is superimposed on a strong oscillation that marks an abrupt onset labeled M 1. At even lower energy a transition occurs that splits into M 2 and M 3 for decreasing PL energy. The PL maximum follows the lowest level M 3. The oscillations above level M 1 resemble Franz-Keldysh oscillations (FKOs) previously identied in strained GaInN/GaN single heterostructures 2. Oscillations are perturbed in part by the excitonic features at the GaN bandgap but their strong amplitude allows to directly determine the piezoelectric eld in the structures (see labels in Fig. 1a). An equidistant spacing of levels M 0, M 1,M 2, and M 3 at high elds is readily seen. 4 Electroreection To identify the role of the electric eld ER under variable bias voltage was performed (Fig. 2). Three levels M 0, M 2 0 and M 3 are clearly observed in the spectra (Fig. 2 a) while more features are seen in the grayscale presentation (Fig. 2 c). For increasing reverse bias from -2 V to +10 V level M 0 shifts to the red while M 2 0 initially shows a blueshift and levels o around -8 V. A stronger blueshift is seen for M 3. M 3 and M 2 0 merge around U bias =,6 V clearly 2

3 indicating that splitting is controlled by the electric eld and that a xed piezoelectric eld exists in the well pointing from the growth surface to the substrate. This agrees with supports corresponds to the ndings by Takeuchi et al. interpreting the quantum conned Stark eect in PL 5. The connection between M 2 and M 2 0 is subject of ongoing work. 5 Discussion The FKOs identify that M 1 is the edge of a three dimensional density of states gap. The eld F increases with strain or InN fraction. In direct correlation all the splittings i;i+1 = E Mi,E Mi+1 grow with F : Delta i;i+1 = FeL z;eff,(i = 1 3) with L z;eff 1:2L z. The splitting can be as large as 300 mev. Under biaxial stress piezoelectric -charges Q fix of opposite polarity are induced at the GaInN-GaN heterointerfaces. Carriers traversing the capacitor-like dipole layer of the well gain or loose the energy W t = FeL z, which is identical to 01. M 1 therefore corresponds to transitions with nal states on opposite sides of the well. Transitions ending on the same side correspond to level M 0, the barrier material bandgap energy. For emission correlated pairs of electrons and holes entering the well at energy level M 1 screen and depolarize the xed piezo charges. The energy in the capacitor W n = n 2 e 2 L z =(2 0 r A) reduces by W n, W n,1 =(2n, 1) 2 e 2 L z =(2 0 r A), which again corresponds to the energy for traversing the dipole (approximation of xed eld) W t = ne 2 L z =( 0 r A) W n, W n,1. Upon annihilation the same amount is returned to the capacitor by reducing the PL energy. An identical pair of levels is expected in the GaInN well. The Stokes shifted emission path coincides with level M 3 and a respective level M 2 0 is expected at E M2 0 = E M3 + Delta 20 3 and experimentally coincides with M 2. Consequently for high piezoelectric elds a Stark-like ladder is formed in the GaInN well. The level spacing is given by the piezoelectric dipole D of the strained GaInN well layer with thickness L z. A Coulombic Stokes shift gives rise to discrete levels at lower energies in the recombination path. However, clear PR signal is also seen in M 3. At present no distinction is seen between M 2, the Stokes shifted barrier gap level, and M 2 0, the GaInN well level. Equivalent transitions may have been observed by Chichibu et al. 4. In contrast to their work we nd a PR level M 3 that corresponds to the PL energy. We conclude, that the apparent discrepancy between emission and absorption is a discrete splitting rather than a localization process coupled to any uctuation mechanism. 3

4 M 3 M 2 M 0 ER p GaN MQW n GaN sapphire M 3 M 2 M a) b) c) Figure 2: Electro reection of a GaInN/GaN MQW pn-diode for variable bias voltage. Spectra of xed U bias a) and a colorcoded equi-signal plot c) indicate the merging of M 3 and M 2 for reverse bias eld corresponding to F bias =0:3 MV/cm as derived from the eld shift of the M 0 barrier transition. The structure is indicated in b). 6 Conclusion The dominance of piezo electric eld eects in the electronic band structure of strained wurtzite nitride heterostructures has been demonstrated in the example of GaN/GaInN/GaN MQW structures. A Stark-like ladder system is found and well described by the piezoelectric dipole generated by xed charges at the well interfaces. Electric eld strengths of 0.9 MV/cm are directly identied. The system contains abundant new physics for future work. This work was supported by JSPS Research for the Future Program in the Area of Atomic Scale Surface and Interface Dynamics under the project of "Dynamic Process and Control of Buer Layer at the Interface in a Highly Mismatched System". References 1. I. Akasaki and H. Amano, Jpn. J. Appl. Phys. 36, 5393 (1997). 2. C. Wetzel, H. Amano, I. Akasaki, T. Suski, J.W. Ager, E.R. Weber, E.E. Haller, and B.K. Meyer, Proc. Mater. Res. Soc. 482, 489 (1998). 3. C. Wetzel, T. Takeuchi, H. Amano, and I. Akasaki, Proc. Mater. Res. Soc. (Wide-Bandgap Semiconductors for High Power, High Frequency and High Temperature, Symposium F Spring Meeting 1998) in print. 4. S. Chichibu, T. Azuhata, T. Sota, and S. Nakamura, Appl. Phys. Lett. 69, 4188 (1996). 4

5 5. T. Takeuchi, S. Sota, M. Katsuragawa, M. Komori, H. Takeuchi, H. Amano, and I. Akasaki, Jpn. J. Appl. Phys. 36, L 382 (1997). 5