Zinc blende ~ZB! GaN quantum dots have been grown by plasma-assisted molecular-beam epitaxy
on AlN buffer layers using 3C-SiC~001! substrates. The two- to three-dimensional growth mode
transition is studied by following the evolution of the reflection high-energy electron diffraction
pattern. ZB GaN island layers are further examined by atomic force microscopy and transmission
electron microscopy, extracting a mean island height of 1.6 nm and a mean diameter of 13 nm at a
density of 1.331011 cm22. Embedded ZB GaN quantum dots show strong ultraviolet
photoluminescence without any thermal quenching up to room temperature.SFERERegion Rhône-AlpesConsejo Nacional de Ciencia y Tecnologí

Adelmann, Christoph

Chabuel, F.

Daudin, B.

Feuillet, G.

Mariette, H.

Martínez-Guerrero, Esteban

Mula, Guido

Pelekanos, N.T.

Simon, Julia

Applied Physics Letters

Repositorio Institucional del ITESO

Instituto Tecnológico y de Estudios Superiores de Occidente 2000-08-07 Self-assembled zinc blende GaN quantumdots grown Martínez-Guerrero, Esteban; Adelmann, Christoph; Chabuel, F.; Simon,Julia; Pelekanos, N.T.; Mula, Guido; Daudin, B.; Feuillet, G.; Mariette, H. E. Martinez-Guerrero, C. Adelmann, F. Chabuel, J. Simon, N. T. Pelekanos, Guido Mula,a) B.Daudin, G. Feuillet, and H. Mariette, Self-assembled zinc blende GaN quantum dots grown, APPLIEDPHYSICS LETTERS Vol. 77, N. 6, pp 809 - 811, (2000)  Enlace directo al documento: http://hdl.handle.net/11117/652 Este documento obtenido del Repositorio Institucional del Instituto Tecnológico y de Estudios Superiores deOccidente se pone a disposición general bajo los términos y condiciones de la siguiente licencia:http://quijote.biblio.iteso.mx/licencias/CC-BY-NC-ND-2.5-MX.pdf (El documento empieza en la siguiente página)Repositorio Institucional del ITESO rei.iteso.mxDepartamento de Electrónica, Sistemas e Informática DESI - Artículos y ponencias con arbitrajeSelf-assembled zinc blende GaN quantum dots grownby molecular-beam epitaxyE. Martinez-Guerrero, C. Adelmann, F. Chabuel, J. Simon, N. T. Pelekanos,Guido Mula,a) B. Daudin, G. Feuillet, and H. Marietteb)CEA-CNRS Joint Group ‘‘Nanophysique et Semiconducteurs,’’ De´partement de Recherche Fondamentalesur la Matie`re Condense´e, SPMM, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France~Received 20 April 2000; accepted for publication 12 June 2000!Zinc blende ~ZB! GaN quantum dots have been grown by plasma-assisted molecular-beam epitaxyon AlN buffer layers using 3C-SiC~001! substrates. The two- to three-dimensional growth modetransition is studied by following the evolution of the reflection high-energy electron diffractionpattern. ZB GaN island layers are further examined by atomic force microscopy and transmissionelectron microscopy, extracting a mean island height of 1.6 nm and a mean diameter of 13 nm at adensity of 1.331011 cm22. Embedded ZB GaN quantum dots show strong ultravioletphotoluminescence without any thermal quenching up to room temperature. © 2000 AmericanInstitute of Physics. @S0003-6951~00!01032-9#The current interest in self-assembled semiconductorquantum dots ~QDs! is partly motivated by the potential toachieve lasers with low threshold currents and improvedtemperature characteristics.1 Moreover, the nanometric sizeand the coherent growth of self-assembled QDs are particularadvantages in situations where the surrounding crystal matrixdisposes of a large number of structural defects. This is thecase of group III nitrides which generally exhibit highthreading dislocation densities of the order of108 – 1010 cm22.To date, self-assembled group III nitride QDs have beengrown exclusively in the wurtzite ~WZ! structure; for theGaN/AlN system, such QDs have been achieved both bymetalorganic chemical vapor deposition ~CVD!2 as well asmolecular-beam epitaxy ~MBE! using Stranski–Krastanowgrowth mode.3,4Despite the above-mentioned achievements for the WZphase of nitrides, its zinc blende ~ZB! counterparts presentadditional interest due to the absence of internal electricfields. It has been established that the optical properties ofWZ GaN/AlGaN quantum wells5–7 and GaN/AlN QDs8,4 arestrongly influenced by the presence of a huge internal elec-tric field. This internal field, which can reach several MV/cm, stems from both spontaneous polarization and piezoelec-tric effects since WZ nitride samples are usually grown alongthe polar @0001# axis. For optoelectronic applications, theexistence of the field can be detrimental since it brings aboutlow exciton oscillator strengths and, consequently, long ra-diative lifetimes, ensuing from spatial separation of electronsand holes.In contrast, ZB nitride structures are usually grownalong the nonpolar @001# axis on GaAs~001!9–11 or3C-SiC~001!9,12 substrates, so that no internal field is presentin heterostructures. Then, the above-mentioned problemscan, in principle, be eluded. It is the aim of the present letterto demonstrate that self-assembled GaN QDs in the ZBphase can be grown by MBE.The samples were grown in a commercial MECA 2000MBE growth chamber equipped with Ga and Al effusioncells. Active N was generated by a radio-frequency plasmacell from EPI company. The substrate consisted of a 3-mm-thick 3C-SiC~001! layer grown by CVD on Si~001!.Before GaN deposition, ZB AlN buffer layers have beengrown at substrate temperatures of TS5720 °C and growthrates of 300 nm/h. The reflection high-energy electron dif-fraction ~RHEED! pattern was found to be streaky after thedeposition of about 5 nm, showing no WZ traces duringnucleation. A RHEED image of such a smooth AlN bufferlayer is shown in Fig. 1~a!.The growth mode experienced by ZB GaN on AlN atTS5720 °C was studied by RHEED. Figures 1~a!–1~c! showthe evolution of the RHEED pattern during GaN deposition:a!Permanent address: INFM and Dipartimento di Fisica, Universita` di Ca-gliari, Cittadella Universitaria, Strada Provinciale Monserrato-Setsu km0.7, 09042 Monserrato, Cagliari, Italy.b!Also at: Laboratorie de Spectrome´trie Physique, Universite´ Joseph FourierGrenoble I, France.FIG. 1. Evolution of the RHEED pattern during GaN deposition on AlN. ~a!Zinc blende AlN buffer. ~b! GaN wetting layer. ~c! GaN islands. Variationof the in-plane lattice parameter a during the growth of ~d! a single layer ofGaN islands and ~e! a ten-period multilayer sample ~the numbers label thelayer order!. Where shown, the shaded areas correspond to GaN deposition.APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 6 7 AUGUST 20008090003-6951/2000/77(6)/809/3/$17.00 © 2000 American Institute of Physicsstarting from a smooth AlN layer @Fig. 1~a!#, the RHEEDimage remains streaky during the initial stage of GaN growth@Fig. 1~b!#, but changes abruptly after 2–3 monolayers ~ML!to a superposition of streaks and Bragg spots @Fig. 1~c!#,which is ascribed to GaN islanding. Note the appearance ofweak $111% facets.These GaN islands were subsequently overgrown withAlN, recovering a smooth surface after typically 15 nm, asindicated by a streaky RHEED pattern, similar to that de-picted in Fig. 1~a!. By repeating this cycle, multilayersamples were achieved. Raman and x-ray diffraction mea-surements ascertain the ZB phase purity of these samples.12To further investigate the RHEED observations, real-time measurements of the in-plane lattice parameter a wereperformed by recording the relative variation of the RHEEDstreak separation Da/a0 with respect to the initial AlN latticeparameter a0 . For fully relaxed GaN on AlN, Da/a0 equals2.7% using the values of aAlN5a054.38 Å ~Ref. 13! andaGaN54.50 Å ~Ref. 14!. Figure 1~d! shows the variation ofDa/a0 during the growth of a GaN island layer, subse-quently capped by AlN. In keeping with the RHEED pattern,no variation of the in-plane lattice parameter is observed forthe first 2–3 ML. Thereafter, correlated to the appearance ofBragg spots, Da/a0 increases rapidly to a level of about1.7%, attributed to relaxation by free surface formation onthe islands.Finally, Fig. 1~e! shows the variation of Da/a0 duringthe growth of a ten-period multilayer sample. During AlNovergrowth of the GaN islands, Da/a0 decreases progres-sively, reaching the initial fully relaxed level after about 10nm. The reversibility of the relaxation further corroboratesthe interpretation of elastic relaxation by GaN islanding, asexpected for Stranski–Krastanow growth. As a whole, oneobserves a good reproducibility of the growth cycle. Notehowever, that the maximum relaxation slightly decreaseswith higher layer numbers. The explanation for this behaviorremains unclear at the moment.The GaN island formation evidenced by RHEED hasbeen further examined by two techniques: first, GaN islandssurface layers were analyzed by atomic force microscopy~AFM! and, second, multilayer samples were investigated bytransmission electron microscopy ~TEM!.Figure 2 shows a topographic AFM image of a singleGaN island layer grown on a 10-nm-thick AlN buffer layer.The extracted average island height above the wetting layeris 1.660.5 nm. The average dot diameter is 13 nm, at adensity of 1.331011 cm22. This leads to an aspect ratio ofabout 1/8, in contrast to a value of 1/5 in the case of WZGaN quantum dots.3 This is much smaller than the aspectratio value of 1/& expected for pyramids formed by $111%facets, as observed by RHEED, therefore the GaN islands aretruncated.Following Ref. 15, the relaxation of an island can beapproximately written as a function of the ratio of its heighth and diameter d only asexx~h !5exx~0 !S 12 2phd D , ~1!where exx(h) and exx(0) are the deformations on the top andat the bottom of the nanostructure, respectively. Applyingthis formula, we obtain a residual deformation in our ZBFIG. 2. ~a! Topographic AFM image of a single GaN QD surface layer. ~b!Distribution of the GaN island height above the wetting layer as extractedfrom AFM measurements.FIG. 3. ~a! Dark-field cross-sectional TEM image of four layers of GaNislands. ~b! High-resolution TEM image of a single GaN island surroundedby $111% stacking faults.810 Appl. Phys. Lett., Vol. 77, No. 6, 7 August 2000 Martinez-Guerrero et al.GaN islands corresponding to a partial relaxation equivalentto Da/a052.1%, in good agreement with the RHEED data@see Fig. 1~e!#. Note that the same approach applied to ourWZ GaN islands leads to complete relaxation of the in-planelattice parameter, as also observed experimentally.16A (1¯10) cross-sectional TEM dark-field image in Fig.3~a! clearly reveals that the GaN islands are formed on top ofa two-dimensional wetting layer. Although it is difficult todeduce the wetting layer thickness from these images, the2–3 ML estimated from RHEED data are in qualitativeagreement. Furthermore, the size of the GaN QDs agreesfairly well with that measured by AFM. One can also noticethe presence of numerous stacking faults along the $111%planes. The high-resolution TEM image in Fig. 3~b! showsthat the islands grow preferentially within each domainbounded by these stacking faults. The large density of stack-ing faults may be at the origin of the broad size distributionof the islands since they can lead to local relaxation at thegrowth front and, thus, to an inhomogeneous strain distribu-tion on the surface.In Fig. 4 we compare the photoluminescence ~PL! spec-tra of a stack of ten QD layers embedded in AlN at T510 K and T5300 K, illuminated by 10 W/cm2 of afrequency-tripled Ti-sapphire laser with lX5266 nm (EX54.67 eV). For this photon energy, the GaN QDs are ex-cited by absorption directly in the QD excited states. Theenergy shift of the maximum of the PL line between 10 and300 K corresponds exactly to the variation of the bulk ZBGaN band gap17 for this temperature range. The PL peak istypically centered at 3.8 eV, blueshifted by more than 500meV with respect to the bulk ZB GaN band gap ~Eg53.26 eV at 10 K!. This confinement energy is much stron-ger than that of small WZ GaN QDs,8 confirming the ab-sence of strong electric fields in these ZB nanostructures.The PL spectral shape and the linewidth remain practi-cally unchanged with temperature, which is consistent withintrinsic QD PL emission coming from an ensemble of un-coupled QDs. Moreover, the linewidth of the broad lumines-cence spectra is in agreement with the large dispersion of thedot size revealed by both AFM and TEM measurements.The inset in Fig. 4 shows the spectrally integrated PLintensity as a function of the sample temperature. As ex-pected for QDs, no thermal quenching of the PL intensity isobserved up to room temperature. Surprisingly, the intensityrather increases by a factor of 2 between 200 and 300 K.This could be a consequence of an increase in the absorptioncoefficient of QDs at the excitation wavelength, resultingfrom the decrease of the GaN band gap energy at highertemperatures.In conclusion we have demonstrated that growth of ZBGaN on AlN at TS5720 °C follows a Stranski–Krastanowmode. As a consequence, we have grown ZB GaN QDs witha mean diameter of 13 nm and a mean height of 1.6 nmabove the wetting layer. The QDs show strong ultraviolet PLemission that is blueshifted with respect to the ZB bulk GaNband gap energy. Furthermore, the emission energy is higherthan that of WZ GaN QDs of the same height, even thoughthe GaN band gap is smaller for the ZB than for the WZstructure. This supports the absence of a large internal elec-tric field in ZB nanostructures. The PL intensity is found topersist up to room temperature, as expected for zero-dimensional confinement.The authors are very grateful to P. Aboughe´-Nze´ and Y.Monteil ~Universite´ Claude Bernard, Lyon! for providing theSiC substrates. E. Martinez-Guerrero acknowledgesCONACYT-Mexico and SFERE-France for financial sup-port.1 Y. Arakawa and H. Sakaki, Appl. Phys. Lett. 40, 939 ~1982!.2 S. Tanaka, S. Iwai, and Y. Aoyagi, Appl. Phys. Lett. 69, 4096 ~1996!.3 B. Daudin, F. Widmann, G. Feuillet, Y. Samson, M. Arlery, and J. L.Rouvie`re, Phys. Rev. B 56, R7069 ~1997!; F. Widmann, B. Daudin, G.Feuillet, Y. Samson, J. L. Rouvie`re, and N. T. Pelekanos, J. Appl. Phys.83, 7618 ~1998!.4 B. Damilano, N. Grandjean, F. Semond, J. Massies, and M. Leroux, Appl.Phys. Lett. 75, 962 ~1999!.5 J. S. Im, H. Kollmer, J. Off, A. Sohmer, F. Scholz, and A. Hangleiter,Phys. Rev. B 57, R9435 ~1998!.6 M. Leroux, N. Grandjean, J. Massies, B. Gil, P. Lefebvre, and P. Bigen-wald, Phys. Rev. B 60, 1496 ~1999!.7 J. Simon, R. Langer, A. Barski, and N. T. Pelekanos, Phys. Rev. B 61,7211 ~2000!.8 F. Widmann, J. Simon, B. Daudin, G. Feuillet, J. L. Rouvie`re, N. T.Pelekanos, and G. Fishman, Phys. Rev. B 58, R15 989 ~1998!.9 H. Okumura, K. Ohta, G. Feuillet, K. Balakrishnan, S. Chichibu, H.Hamaguchi, P. Hacke, and S. Yoshida, J. Cryst. Growth 178, 113 ~1997!.10 O. Brandt, H. Yang, A. Trampert, W. Wassermeier, and K. H. Ploog,Appl. Phys. Lett. 71, 473 ~1997!.11 D. J. As, T. Simonsmeier, B. Schoettker, T. Frey, D. Schikora, W. Krieg-seis, W. Burkhardt, and B. K. Meyer, Appl. Phys. Lett. 73, 1835 ~1998!.12 B. Daudin, G. Feuillet, J. Hu¨bner, Y. Samson, F. Widmann, A. Philippe,C. Bru-Chevallier, G. Guillot, E. Bustarret, G. Bentoumi, and A. Deneu-ville, J. Appl. Phys. 84, 2295 ~1998!.13 I. Petrov, E. Mojab, R. C. Powell, and J. E. Greene, Appl. Phys. Lett. 60,2491 ~1992!.14 S. Strite, J. Ruan, Z. Li, A. Salvador, H. Chen, D. J. Smith, W. J. Choyke,and H. Morkoc¸, J. Vac. Sci. Technol. B 9, 1924 ~1991!.15 R. Kern and P. Mu¨ller, Surf. Sci. 392, 103 ~1997!.16 C. Adelmann, G. Mula, G. Feuillet, and B. Daudin ~unpublished!.17 J. Petalas, S. Logothetidis, S. Boultadakis, M. Alouani, and J. M. Wills,Phys. Rev. B 52, 8082 ~1995!.FIG. 4. Photoluminescence spectra of a ten-period multilayer sample at 10and 300 K. The inset shows the spectrally integrated PL intensity as afunction of sample temperature.811Appl. Phys. Lett., Vol. 77, No. 6, 7 August 2000 Martinez-Guerrero et al.

Self-assembled zinc blende GaN quantum dots grown

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Self-assembled zinc blende GaN quantum dots grown

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