Volovik_24ICPS.doc submitted to World Scientific : 26.04.00 : 13:16 1/5 LASING IN STRUCTURES WITH InAs SUBMONOLAYER INSERTIONS IN AN AlGaAs MATRIX WITHOUT EXTERNAL OPTICAL CONFINEMENT B.V. VOLOVIK, A.YU. EGOROV, P.S. KOP'EV, A.R. KOVSH, I.E. KOZIN, I.L. KRESTNIKOV, M.V. MAXIMOV, A.V. SAKHAROV, A.F. TSATSUL'NIKOV, V.M. USTINOV, A.E. ZHUKOV, ZH.I. ALFEROV A.F.Ioffe Physico-Technical Institute of the Russian Academy of Sciences, Politekhnicheskaya 26, St.-Petersburg 194021, Russia N.N. LEDENTSOV*, M. STRASSBURG, A. HOFFMANN, D. BIMBERG Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germany I.P. SOSHNIKOV*, P. WERNER Max-Planck-Institut für Mikrostrukturphysik, 06120 Halle, Germany We study structural and optical properties of sets of InAs submonolayer (SML) insertions in AlGaAs matrix. Stacked InAs SML deposition causes formation of uniform arrays of vertically stacked monolayer-high nanoscale InAs islands having a lateral size of about 10 nm. InAs SML insertions result in a photoluminescence peak, which dominates the spectra up to high excitation densities and observation temperatures. Magnetooptical data reveal significantly reduced exciton radii due to exciton localization at SML islands and formation of quantum dot (QD) like states. Resonant waveguiding and lasing under photoexcitation are demonstrated from QDs in structures without external cladding of the active region by layers having a lower average refractive index. 1 Introduction Submonolayer (SML) depositions of narrow gap material on singular [1] and vicinal [2,3] surfaces were proposed for fabrication of quantum dots. Scanning transmission microscopy studies of InAs growth on GaAs (100) [3] revealed formation of arrays of uniform in size two-dimensional islands having a width of 4 nm and elongated in the [0-11] direction. InAs-GaAs SML insertions were proven to exhibit a remarkably strong exciton oscillator strength even for ultrathin coverage [4]. However, very little is known about InAs SMLs in an AlGaAs matrix and about the possibility to fabricate vertically-correlated structures in this system. More recently, II-VI submonolayer structures were proposed for fabrication of quantum dots (QD), QD lasers and devices [5], based on the effect of resonant enhancement of the refractive index at the low energy side of exciton absorption peak as it follows from the Kramers-Kronig equations.
Transcript
Volovik_24ICPS.doc submitted to World Scientific : 26.04.00 : 13:16 1/5
LASING IN STRUCTURES WITH InAs SUBMONOLAYER INSERTIONS IN AN AlGaAs MATRIX WITHOUT EXTERNAL OPTICAL
V.M. USTINOV, A.E. ZHUKOV, ZH.I. ALFEROV A.F.Ioffe Physico-Technical Institute of the Russian Academy of Sciences,
Politekhnicheskaya 26, St.-Petersburg 194021, Russia
N.N. LEDENTSOV*, M. STRASSBURG, A. HOFFMANN, D. BIMBERG Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstrasse 36, D-10623
Berlin, Germany
I.P. SOSHNIKOV*, P. WERNER Max-Planck-Institut für Mikrostrukturphysik, 06120 Halle, Germany
We study structural and optical properties of sets of InAs submonolayer (SML) insertions in AlGaAs matrix. Stacked InAs SML deposition causes formation of uniform arrays of vertically stacked monolayer-high nanoscale InAs islands having a lateral size of about 10 nm. InAs SML insertions result in a photoluminescence peak, which dominates the spectra up to high excitation densities and observation temperatures. Magnetooptical data reveal significantly reduced exciton radii due to exciton localization at SML islands and formation of quantum dot (QD) like states. Resonant waveguiding and lasing under photoexcitation are demonstrated from QDs in structures without external cladding of the active region by layers having a lower average refractive index.
1 Introduction
Submonolayer (SML) depositions of narrow gap material on singular [1] and vicinal [2,3] surfaces were proposed for fabrication of quantum dots. Scanning transmission microscopy studies of InAs growth on GaAs (100) [3] revealed formation of arrays of uniform in size two-dimensional islands having a width of 4 nm and elongated in the [0-11] direction. InAs-GaAs SML insertions were proven to exhibit a remarkably strong exciton oscillator strength even for ultrathin coverage [4]. However, very little is known about InAs SMLs in an AlGaAs matrix and about the possibility to fabricate vertically-correlated structures in this system.
More recently, II-VI submonolayer structures were proposed for fabrication of quantum dots (QD), QD lasers and devices [5], based on the effect of resonant enhancement of the refractive index at the low energy side of exciton absorption peak as it follows from the Kramers-Kronig equations.
Volovik_24ICPS.doc submitted to World Scientific : 26.04.00 : 13:16 2/5
In this work we study structural and optical properties of SML InAs insertions in an AlGaAs matrix. We demonstrate formation of nanoscale quantum dots (QDs) in this system and lasing under photoexcitation in structures without external optical confinement. Investigated structures are grown by molecular-beam epitaxy on GaAs(100) substrates. An Al0.3Ga0.7As layer with a thickness of 0.7 µm is deposited first on a GaAs buffer layer, and followed by an active region consisting of 20 periods of 0.5 ML InAs insertions placed in the central parts of 1nm-thick GaAs quantum wells (QW) and separated by 5 nm width Al0.3Ga0.7As barriers. A 0.1 µm-thick Al0.3Ga0.7As layer is grown on top.
2 Results and discussion
In the Fig. 1 we show plan-view transmission electron microscopy (TEM) (a) and cross-section high resolution electron microscopy (XHREM) images of the investigated structure. The average lateral size of the InAs islands seen in plan-view TEM is 12 nm. The average density of these dots is 1.1 × 1011 cm-2. The observed lateral size is larger than one revealed for the InAs-GaAs system in [3]. Indeed, for InAs SMLs in a GaAs matrix we saw only few dots of such a size in plan-view TEM images, pointing to the fact that in the InAs-GaAs case most of the islands are smaller and the growth occurs without vertical correlation [6]. XHREM image demonstrates stripy contrast related to GaAs QW insertions. No three-dimensional islands are revealed in the XHREM images.
Figure 1. Bright-field plane view TEM (a) and XHREM (b) images for the structure with InAs SML insertions in AlGaAs matrix.
InAs insertions result in appearance of a new PL line, which dominates in the spectra up to high excitation densities and observation temperatures [7]. To prove exciton localization at SML islands, we have performed magnetooptical studies of the structures (see Fig. 2a). The estimated according to [8] extent of the wavefunction is 5 nm in lateral and 4 nm in vertical direction. Thus, the exciton volume is about 20 times smaller than the volume of the GaAs bulk exciton [9].
Volovik_24ICPS.doc submitted to World Scientific : 26.04.00 : 13:16 3/5
In Fig. 2b we show gain spectra of the structure. We note that the gain peak
remains narrow and the absorption peak does not disappear up to very high excitation densities. As strong resonant absorption results in a strong enhancement
of the refractive index on the low energy side of absorption peak, the observed gain behaviour is in a full agreement with the concept of resonant waveguiding [5].
Volovik_24ICPS.doc submitted to World Scientific : 26.04.00 : 13:16 4/5
Figure 2. a) PL spectra of the investigated structure. In inset PL peak position vs. magnetic field is shown. b) Gain spectra at different excitation densities
In Fig. 3 the integral PL intensity detected from the edge of the Fabri-Perot cavity with a length of 1 mm, as a function of excitation density is shown. The lasing occurs at the low energy side of the exciton feature revealed in the optical reflectance spectrum.
Figure 3. a) Integral PL intensity vs excitation density. b) PL spectra detected from the edge of Fabri-Perot cavity.
Volovik_24ICPS.doc submitted to World Scientific : 26.04.00 : 13:16 5/5
At temperatures below 50K the threshold excitation density is almost temperature-insensitive in agreement with the QD nature of the excitons trapped at InAs islands. At higher temperatures the threshold density increases exponentially with characteristic temperature of T0 = 30 K, which we relate to insufficient localization energies of the trapped excitons and nonradiative recombination in the low-temperature grown AlGaAs matrix regions.
To conclude, we demonstrate resonant waveguiding and lasing in SML InAs/AlGaAs QD structures. We expect similar structures to be attractive for improvement of optical confinement in lasers operating in the visible spectral range, for creation of resonant waveguides and, also, vertical cavity lasers with self-adjusted gain spectra and cavity modes.
This work was supported by the Russian Foundation of Basic Research, INTAS and Volkswagen Foundation.
References
1. * On leave from A.F. Ioffe Physico-Technical Institute 1 P.D.Wang, N.N.Ledentsov, C.M.Sotomayor Torres, P.S.Kop’ev, and
V.M.Ustinov, Appl.Phys.Lett 64, 1526 (1994), ibid. 66, 112 (1995). 2 O. Brandt, G.C. La Rocca, A. Heberle, A. Ruiz and K. Ploog, Phys. Rev. B 45,
3803 (1992). 3 V. Bressler-Hill, A. Lorke, S. Varma, P.M. Petroff, K. Pond, and
6 M. Strassburg, V. Kutzer, U.W. Pohl, A. Hoffmann, I. Broser, N.N. Ledentsov, D. Bimberg, A. Rosenauer, U. Fisher, D. Gerthsen, I.L. Krestnikov, M.V. Maximov, P.S. Kop’ev, Zh.I. Alferov, Appl. Phys. Lett. 72, 942 (1998)
8 I.E. Itskevich, M. Henini, H.A. Carmona, L. Eaves, P.C. Main, D.K. Maude, J.C. Portal, Appl. Phys. Lett. 70, 505 (1997)
9 A.J. Shields, M. Pepper, P.C.M. Christianen, J.C. Maan, M.Y. Simmons, D.A. Ritchie, Proc. of 12th Int. Conf. “High Magnetic Fields in the physics of semiconductors, Würzburg, 1996, p. 737
