CONFINAMIENTO NANOSC´OPICO EN ESTRUCTURAS ... - It works!

166 Publicaciones4518 J. Appl. Phys., Vol. 94, No. 7, 1 October 2003 Movilla, Climente, and Planellestial barrier including QD size reduction when d0 nm) isnot the only source of the experimental PL blueshift it isworth pointing out that using a more optimistic, somewhatunrealistic, set of parameters such as unstrained InAs mass,homegeneous QD composition and reduced InAs/GaAs bandoffset does not significantly improve the agreement with theexperiment. Since the incorporation of very small amountsof Al in InAs QDs has been identified as a dominant mechanismfor Al-related PL blueshift in some QD systems, 13,39 wesuggest that some Al diffusion plays also a role in this case.This mechanism would allow a simple explanation of theenhanced PL blueshift as coming from an enlargement of theQD energy band gap. The presence of Al in the QD wouldcome from the Al atoms existing in the WL when the dot isgrown on an AlAs surface. 14 The question is how could Al beincorporated in the dot when the AlAs layer is 1 or 2 nmbelow the WL, especially considering the low mobility ofthis cation. Since InAs islands are known to grow by takingmaterial not only from the WL, but also from the substrate ifthe synthesis is carried out at temperatures over 420 °C, 34and QDs in Ref. 17 are grown at 440° the substrate beingpreviously grown at 580 °C, what would produce a moderateAl diffusion toward the WL, they may presumably drainsmall Al quantities from the ‘‘contaminated’’ WL and/or substrate.This interpretation is in agreement with other experimentalstudies dealing with thin AlAs layers. Rebohle et al. 20designed a sample similar to structure C with d1 nm, butin which the substrate layers were deposited at a temperatureof 485 °C in order to reduce Al, Ga, and In intermixing. Theresulting PL blueshift was of only 33 meV, smaller than thatof Ref. 17 for the same case but still larger than the theoreticalvalue coming only from confinement potential effects.In addition, related structures where the substrate wasdeposited at 610 °C yield much larger transition energies. 21Our calculations indicate that the PL blueshift observedin Ref. 17, when the QD is directly grown on a thin AlAslayer, is consistent with an average QD composition ofroughly In 0.93 Al 0.07 As. Since the presence of Al in InAs selfassembledQDs produces kinetic changes in the dotformation, 39 reflection high-energy electron diffraction studiesmay be helpful for a definitivy assessment of the role ofAl in the discussed blueshift.IV. CONCLUDING REMARKSWe studied the influence of a thin AlAs layer, locatedinside a GaAs matrix, on the electron energy levels of InAsQDs deposited nearby. Our results show that, in good agreementwith experiment, thin AlAs capping layers produce amoderate PL blueshift. In this case, the ground-state energyis mainly affected by the vertical confinement while the firstexcitedstate is additionally sensitive to the lateral one, aresult that may be useful to tune the energy spacing betweenthe two lowest-lying QD states. When the AlAs layer isgrown in the substrate, the AlAs high potential barrier alonecannot account for the large experimental PL blueshift. Ourresults suggest that, in this case, small amounts of Al diffuseinto the QDs. For QDs grown directly on the AlAs layer, weestimate an average In 0.93 Al 0.07 As QD composition to accountfor the experimental data of Kim et al. 17ACKNOWLEDGMENTSContinuous support from Generalitat Valenciana, UJI-Bancaixa, and MEC-FPU grants are acknowledged.1 Y. Masumoto and T. Takagahara, Semiconductor Quantum Dots Springer,Berlin, 2002.2 Y. Arakawa and H. Sakaki, Appl. Phys. Lett. 40, 939 1982.3 V. Rhyzii, I. Khmyrova, V. Mitin, M. Stroscio, and M. Willander, Appl.Phys. Lett. 78, 3523 2001.4 J. Phillips, J. Appl. Phys. 91, 4590 2002.5 H. C. Liu, J.-Y. Duboz, R. Dudek, Z. R. Wasilewski, S. Fafard, and P.Finnie, Physica E 17, 631 2003.6 M. Grundmann, N. N. Ledentsov, O. Stier, D. Bimberg, V. M. Ustinov, P.S. Kop’ev, and Z. I. Alferov, Appl. Phys. Lett. 68, 9791996.7 M.-Y. Kong, X.-L. Wang, D. Pan, Y.-P. Zeng, J. Wang, and W. Ge, J. Appl.Phys. 86, 1456 1999.8 C. Lobo, R. Leon, S. Fafard, and P. G. Piva, Appl. Phys. Lett. 72, 28501998.9 H. C. Liu, M. Gao, J. McCaffrey, Z. R. Wasilewski, and S. Fafard, Appl.Phys. Lett. 78, 792001.10 J. Phillips, K. Kamath, X. Zhou, N. Chervela, and P. Bhattacharya, Appl.Phys. Lett. 71, 2079 1997.11 R. Leon, S. Fafard, D. Leonard, J. L. Merz, and P. M. Petroff, Appl. Phys.Lett. 67, 521 1995.12 Y. S. Kim, U. H. Lee, D. Lee, S. J. Rhee, Y. A. Leem, H. S. Ko, D. H.Kim, and J. C. Woo, J. Appl. Phys. 87, 241 2000.13 A. Polimeni, A. Patanè, M. Henini, L. Eaves, and P. C. Main, Phys. Rev.B 59, 5064 1999.14 P. Ballet, J. B. Smaters, H. Yang, C. L. Workman, and J. Salamo, J. Appl.Phys. 90, 481 2001.15 Z. Ma, K. Pierz, U. F. Keyser, and R. J. Haug, Physica E 17, 1172003.16 U. H. Lee, D. Lee, H. G. Lee, S. K. Noh, J. Y. Leem, and H. J. Lee, Appl.Phys. Lett. 74, 1597 1999.17 J. S. Kim, P. W. Yu, J.-Y. Leem, M. Jeon, S. K. Noh, J. I. Lee, G. H. Kim,S.-K. Kang, J. S. Kim, and S. G. Kim, J. Appl. Phys. 91, 50552002.18 F. Ferdos, S. Wang, Y. Wei, M. Sadeghi, Q. Zhao, and A. Larsson, J. Cryst.Growth 251, 1452003.19 M. Arzberger, U. Käsberg, G. Bohm, and G. Abstreiter, Appl. Phys. Lett.75, 3968 2002.20 L. Rebohle, F. F. Schrey, S. Hofer, G. Strasser, and K. Unterrainer, Appl.Phys. Lett. 81, 2079 2002.21 K. W. Berryman, S. A. Lyon, and M. Segev, Appl. Phys. Lett. 70, 18611997.22 A. D. Stiff, S. Krishna, P. Bhattacharya, and S. W. Kennerly, IEEE J.Quantum Electron. 37, 1412 2001.23 M. Califano and P. Harrison, Phys. Rev. B 61, 10959 2000.24 Y. Li and O. Voskoboynikov, Solid State Commun. 120, 792001.25 N. Nishiguchi and K. Yoh, Jpn. J. Appl. Phys., Part 1 36, 3928 1997.26 Z. Barticevic, M. Pacheco, and A. Latge, Phys. Rev. B 62, 69632000.27 M. Grundmann, O. Stier, and D. Bimberg, Phys. Rev. B 52, 11969 1995.28 C. Pryor, Phys. Rev. B 57, 7190 1998.29 G. Bastard, Wave Mechanics Applied to Semiconductor HeterostructuresLes Editions de Physique, Les Ulis, 1990.30 W. E. Arnoldi, Quart. J. Applied Mathematics 9, 171951; Y. Saad,Numerical Methods for large Scale Eigenvalue Problems Halsted, NewYork, 1992; R. B. Morgan, Math. Comp. 65, 1213 1996.31 R. B. Lehoucq, D. C. Sorensen, P. A. Vu, and C. Yang, ARPACK: Fortransubroutines for solving large scale eigenvalue problems, Release 2.1; R.B. Lehoucq, D. C. Sorensen, and C. Yang, ARPACK User’s Guide: Solutionof Large-Scale Eigenvalue Problems with Implicit Restarted ArnoldiMethods SIAM, Philadelphia, 1998.32 I. Vurgaftman, J. R. Meyer, and L. R. Ram-Mohan, J. Appl. Phys. 89,5815 2001.33 S. S. Li, J. B. Xia, Z. L. Yuan, Z. Y. Xu, W. Ge, X. R. Wang, Y. Wang, J.Wang, and L. L. Chang, Phys. Rev. B 54, 11575 1996.34 P. B. Joyce, T. J. Krzyzewski, G. R. Bell, B. A. Joyce, and T. S. Jones,Phys. Rev. B 58, R15981 1998.35 This composition profile pushes the electron density toward the top of the