Piezoelectric polarization in the radiative centers of GaInN/GaN quantum wells and devices. C. Wetzel, 1 T. Detchprohm, 1 T. Takeuchi, 1;2 H. Amano, 1

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1 Piezoelectric polarization in the radiative centers of GaInN/GaN quantum wells and devices. C. Wetzel, T. Detchprohm, T. Takeuchi, ;2 H. Amano, ;2 and I. Akasaki ;2 High Tech Research Center, Meijo University, -5 Shiogamaguchi, Tempaku-ku, Nagoya , Japan 2 Department of Electrical and Electronic Engineering, Meijo University, -5 Shiogamaguchi, Tempaku-ku, Nagoya , Japan Abstract We identify, quantify, and correlate the polarization dipole across the well of device-typical piezoelectric GaInN/GaN heterostructures with the luminescence properties of the well. This quantity reects in the asymmetry of the barrier height on either side of the well. By a detailed comparison of photore- ection, electroreection, low and high excitation density photoluminescence we nd that a very similar splitting occurs in the emission characteristics of the well. We therefore conclude that the electronic band structure within the wells is also to a very large extent controlled by the quantity of the polarization dipole in such polarization heterostructures. PACS: Cr, Bw, 7.7.Ej, Ly Keywords: GaInN/GaN, quantum well, polarization, bandstructure, luminescence, reectance, level splitting Typeset using REVTEX Electronic mail: Wetzel@meijo-u.ac.jp J. Electronic Materials 2 in print

2 I. INTRODUCTION Signicant advances in characterization, processing, and growth have led to the development of group-iii nitrides as a versatile high performance electronic material system. {3 However, at present the interplay of materials physics and system performance is insuciently elucidated. The wide electronic band gap and strong ionic bonding contributions may explain part of the observations, but according to our ndings, 4{8 it is the strong polarization that dominates the optoelectronic properties of the system. Large polarization eects had been observed in piezoresistivity experiments 9 and in theory. Alternate interpretations emphasize the relevance of spatial inhomogeneities, very large alloy uctuations and quantum dot formation as important processes to enhance the luminous eciency. {4 We here present comparative results of photoreection (PR), electroreection (ER), and photoluminescence (PL) in high quality GaInN/GaN quantum wells (QWs) and AlGaInN diode structures to clarify the underlying concepts. II. EXPERIMENTAL A set of two Ga,x In x N/GaN multiple QW structures of ve sequences of well width L z = 3 A and barrier width L b = 6 A was grown by metal organic vapor phase epitaxy on c-plane sapphire, low temperature deposited AlN buer layers, and 2 m GaN epi-layers. The InN-fraction x =:2 and x =:8 was determined by a dynamical rocking analysis of the x-ray diraction. A fourth Ga,x In x N/GaN (x = :2) multiple QW sample (L z = 2 A, L b = 6 A) was Si doped to N D = 2 8 cm,3 in both wells and barriers. Further a device structure consisted of ve Ga,x In x N/GaN (x = :5) L z = 3 A, L b = 6 A QWs embedded in a pn-junction. The n-side underneath the undoped QWs was 3 m GaN doped to N D = 2 8 cm,3. The p-layer was formed by a p-al :2 Ga :8 N layer (6 A) and a p-gan layer (2 A), both doped to N A = 5 9 cm,3. A transparent contact was formed on the top-most p-layer. PR was performed with a Xe white light source and periodic 325 nm modulation at low power densities of 25 mw/cm 2 from a HeCd laser. Low density PL under identical conditions was performed by blocking the white light. High excitation density PL was performed using a pulsed 337 nm N 2 laser at mj energy. For ER a sinusoidal voltage at V amplitude was applied as modulation and a variable bias voltage U b superimposed. All experiments were performed at room temperature. III. RESULTS AND DISCUSSION ER as a function of variable bias voltage in the range of +5 V to - V is shown in Fig.. A sequence of oscillations mark the band gap energy in the GaN barriers and/or the p-contact region (N ) and the lowest level in the QW (N 3 ). As a function of increasing reverse bias voltage the rst shifts to lower energies reecting the increased potential drop along the GaN layer by the Franz-Keldysh eect in the presence of the electric eld. The lowest level in the QW initially shows a strong blue shift and then levels o at around -8V. Additional features in the intermediate energy range indicate a rapid shift towards lower energies along the same bias variation. A detailed discussion of the latter has been given elsewhere. 5 The important 2

3 feature here is the observation of the two phases of N 3, the blue shift and the saturation at nite bias voltage. For increased forward bias electroluminescence appears. The close level correspondence in both modes indicates, that luminescence and ER signal originate in the same level of the QW. A continuous shift of the PL in a similar structure has previously been described by the quantum conned Stark eect in the presence of a large electric eld within the well. 6 The identical eect is now observed here in an absorption type experiment. This correspondence of emission and absorption experiments supports the interpretation, that luminescence is governed by recombination between discrete energy levels rather than tails of any distribution dominated by inhomogeneities. 5;8 The observation of a saturation point of the blue shift clearly marks the at band condition of the well (F w = ). At this eld a large electric eld F b is active in the barriers as evidenced by the red shift of the GaN band gap signal. The actual eld in the well F w is the combination of the polarization component of the QW itself, the built-in potential of the pn-junction, and the externally applied bias. The at band condition in the barriers (F b = ) is expected close to the forward bias voltage U b (F b = ) = E g (GaN)=e =3:4 V. This is consistent with the trend of the GaN band gap signal. This result clearly supports that a nite polarization acts within the well and it produces a eld F w within the wells that is oset from the externally applied bias eld. In the next step we shall quantify the polarization acting within the individual wells. PR in the composition set of samples is presented in Fig. 2. In both samples a strong oscillation is identied in the vicinity of the barrier band gap energy near N and N. These can be identied as Franz-Keldysh-oscillations (FKO) in the joint density of states (DOS) in the presence of a large electric eld F, where charge carriers are free to move along the eld. 7;7 For a eld perpendicular to a set of QW layers this is possible either in the barriers or in the QW region for resonant states above the respective barrier levels. From the interpretation of the FKO period we derive electric eld values in the wells F w = :55 MV/cm (x = :2) and F w = :82 MV/cm (x = :8). we sthe oscillations mark the minimum of the respective DOS in N at an energy below the band gap energy of the barriers. The apparent localization corresponds to the potential step across the well and is induced by the polarization acting across the well. At lower energy we observe several contributions in PR. Clear maxima are identied in the spectra and are labeled N 2 and N 3, respectively. PL performed under identical low UV excitation densities is shown in the same graphs. Due to the identical geometry employed, signal intensities can be compared directly. For both spectra, the PL intensity is one to two orders of magnitude below the PR signal. A possible cross-talk of both signals can therefore be excluded. This was further supported by double modulation experiments. We observe a very close correspondence of the peak positions with the PR maximum in N 3. This shows, that this luminescence band marks a well dened discrete level in the joint DOS. Spectra of high excitation density PL are also shown. Under pulsed high density optical excitation stimulated emission occurs near N 2. The respective shift from N 3 to N 2 has previously been associated with the quantum conned Stark eect. 4 Similar shifts of the luminescence peak and merging with the level of stimulated emission have also been associated with the gradual lling of localized states before the level of highest gain is reached above a certain mobility edge. 8 Our comparison with PR, however, suggests, that a discrete level at this energy (N 2 ) also exists at very low excitation density. Consequently strong focus of interest must be centered around the origin of the level associated with N 2. 3

4 In the next step we compare the respective level splittings N { N, N 2 { N 3, the energy separation of stimulated emission and low excitation density PL, and the quantity F w el z with respect to the InN fraction x (Fig. 3). The polarization dipole is given by PL z = r (F w,f b )el z and for the case L b >> L w can be approximated by r F w el z. We observe a remarkable correspondence of all the values and vanishing discrepancy for the sample with the higher composition, strain, and piezoelectric polarization of x = :8. Apparently the polarization dipole across the QW is not only responsible for asymmetric barrier heights on either side of the QW but furthermore controls the electronic band structure within the depth of the QW. A similar splitting is observed in the PL of the doped sample under pulsed high density excitation density as a function of excitation power (Fig. 4). A pair of two levels with linear response to the excitation power accompany a center line with superlinear dependence indicating stimulated emission for the highest excitation energy. Within the tenfold power variation no change of the respective line positions can be observed. The photo generation of electron-hole pairs apparently is not sucient to appreciably vary the polarization conditions in the wells. This condition is found to be valid up to the highest excitation power applied, where stimulated emission occurs. This observation is in good agreement with recent selfconsistent bandstructure calculations. 9 This is also veried by comparing the polarization charge density 6 of P=e =5 2 cm,2 with the sheet carrier pair density t = N t L z =3 2 cm,2 required to achieve transparency at the expected volume level of N t 9 cm,3. Consequently bipolar injection at the lasing threshold should be insucient to compensate the polarization eect. These experimental values of electric elds and associated polarization charges are signicantly below those that would have been expected from the results of rst principles calculations. Our here determined values are in excellent agreement with values derived from a large set of some 2 samples with variable composition as presented in Refs. 8, 6. Possible sources of discrepancies on the experimental side are the superposition of elds induced by screening charges. Detailed investigations even in highly doped samples, however, show that such an eect can merely amount to some - 2 % of the experimental values. 2 It can not explain a discrepancy F theory =F experiment 7 which is of the order of the static dielectric constant r = :4. The facts that the joint density of state mass is the only material dependent parameter in our interpretation and that it enters in a sublinear power of =2 result in rather small systematic errors in the determination of the actual elds. We see, that the polarization dipole induces a level splitting at the barrier band gap energy introducing a transition at the energy E g (GaN), F w el z below the barrier band gap energy. According to these results a similar process apparently holds for the splitting in the lowest levels of the GaInN QW. Due to the very similar splitting energies, especially in the limit of large x, we draw parallels between N and N 2, and N and N 3, respectively. We therefore propose that similarly to the level splitting at the GaN band gap energy, also the splitting in the well is mainly caused by the polarization dipole induced at the heterointerfaces of the layered system. 4

5 IV. CONCLUSION In conclusion we have identied a close correlation between the polarization dipole induced at the polarization heterointerfaces across the pseudomorphic GaN/GaInN/GaN layer structure and the interband transition scheme at the barrier band gap energy as well as in the QW. We conclude that the quantity of the polarization dipole does not only aect the electronic level scheme at the GaN barrier band gap but also controls the various discrete levels identied in PR, low excitation PL and in stimulated emission under high excitation density. This can be expressed as a piezoelectric bandstructure control within such an AlGaInN polarization heterostructure. ACKNOWLEDGEMENT This work was partly supported by the 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 the Buer Layer at the Interface in a Highly-Mismatched System and the Ministry of Education, Science, Sports and Culture of Japan (contract No. 483). 5

6 REFERENCES I. Akasaki and H. Amano, Jpn. J. Appl. Phys. 36, 5393 (997). 2 I. Akasaki, in Nitride Semiconductors, Eds. F.A. Ponce, S.P. DenBaars, B.K. Meyer, S. Nakamura, and T. Strite, Mat. Res. Soc. Symp. Proc. 482, 3 (998). 3 I. Akasaki and C. Wetzel, Proc. of the IEEE. 85(), 75 (997). 4 T. Takeuchi, S. Sota, M. Katsuragawa, M. Komori, H. Takeuchi, H. Amano, and I. Akasaki, Jpn. J. Appl. Phys. 36, L 382 (997). 5 C. Wetzel, H. Amano, I. Akasaki, T. Suski, J.W. Ager, E.R. Weber, E.E. Haller, and B.K. Meyer, Mat. Res. Soc. Symp. Proc. 482, 489 (998). 6 C. Wetzel, S. Nitta, T. Takeuchi, S. Yamaguchi, H. Amano, and I. Akasaki, MRS Internet J. Nitride Semicond. Res. 3, 3 (998). 7 C. Wetzel, T. Takeuchi, H. Amano, and I. Akasaki, Jpn. J. Appl. Phys. 38, L 63 (999). 8 C. Wetzel, T. Takeuchi, H. Amano, and I. Akasaki, J. Appl. Phys. 85, 3786 (999). 9 A.D. Bykhovski, V.V. Kaminski, S. Shur, Q.C. Chen, and M.A. Khan, Appl. Phys. Lett. 68, 88 (996). F. Bernardini, V. Fiorentini, and D. Vanderbilt, Phys. Rev. B 56, R24 (997). Y. Narukawa, Y. Kawakami, M. Funato, S. Fujita, S. Fujita, and S. Nakamura, Appl. Phys. Lett. 7, 98 (997). 2 S. Chichibu, T. Azuhata, T. Sota, and S. Nakamura, Appl. Phys. Lett. 7, 2822 (997). 3 N.A. El-Masry, E.L. Piner, S.X. Liu, and S.M. Bedair, Appl. Phys. Lett. 72, 4(998). 4 T. Wang, D. Nakagawa, M. Lachab, T. Sugahara, and S. Sakai, Appl. Phys. Lett. 74, 328 (999). 5 C. Wetzel, T. Takeuchi, H. Amano, and I. Akasaki, unpublished 6 T. Takeuchi, C. Wetzel, S. Yamaguchi, H. Sakai, H. Amano, I. Akasaki, Y. Kaneko, S. Nakagawa, Y. Yamaoka, and N. Yamada, Appl. Phys. Lett. 73, 69 (998). 7 C. Wetzel, T. Takeuchi, S. Yamaguchi, H. Katoh, H. Amano, and I. Akasaki, in Blue Laser and Light Emitting Diodes II. eds. K. Onabe, K. Hiramatsu, K. Itaya, Y. Nakano, Tokyo, Japan: Ohmsha, 998. p A. Satake, Y. Masumoto, T. Miyajima, T. Asatsuma, F. Nakamura, and M. Ikeda, Phys. Rev. B, 57, R 24 (998). 9 F. della Sala, A. di Carlo, P. Lugli, P.F. Bernardini, V. Fiorentini, R. Scholz, and J.-M. Jancu, Appl. Phys. Lett. 74, 22 (999). 2 C. Wetzel, H. Amano, and I. Akasaki, Jpn. J. Appl. Phys. (999) in press. 6

7 Bias Voltage (V) FIGURES Electroreflectance Signal ( -3 ) 8 4 N 3 N Photon Energy (ev) FIG.. Electroreectance signal in an AlGaInN pn-diode as a function of bias voltage. N 3 marks the lowest state in the GaInN QW and shows the quantum conned Stark eect in ER and electroluminescence. For large forward bias luminescence dominates the signal. In this case narrow line features appear due to the inappropriate normalization to the DC reectance. N corresponds to the GaN bandedge in the top p-layer and shows the Franz-Keldysh eect. - Luminescence Intensity (arb. units) x=.8 N PL low x=.2 high N 3 low PL high N Photon Energy (ev) FIG. 2. Comparison of photoreection, low, and high excitation density photoluminescence of GaInN/GaN QWs with dierentwell composition. A splitting of the interband transition involving the barriers into N and N allows to quantify the polarization dipole across the wells. In the depth of the well a very close correspondence occurs between the dierent luminescence levels and the maxima in the PR. The simultaneous occurrence of transitions N 2 and N 3 in PR indicate, that stimulated emission and low excitation density PL arise from two dierent states. N N 2 N N N PR PR P6899A.o 2 PR Signal ( -4 ) -2-7

8 Splitting Energy (ev).3.2. PR N 2 -- N 3 PR N -- N PL high -- low FeLz..5.2 x in Ga -x In x N FIG. 3. Splitting energies of N { N, N 2 { N 3, PL under high and low excitation density, and the polarization dipole respectively versus the InN-fraction x. Values are very similar within both samples and nearly coincide for higher x and F in the x = :8 sample. The splitting energies increase with x and associated piezoelectric eld. P6899A.o Photoluminescence Intensity (rel. units) Photon Energy (ev) FIG. 4. Photoluminescence under variable high density excitation in a doped GaInN/GaN QW structure. Stimulated emission occurs between the two further luminescence lines. The line positions and respective splittings do not vary appreciably despite a variation of the electron-hole pair concentration by a factor of ten. p6499b.c 3.2 8