106 Zhukov, Ustinov, Egorov, Kovsh, Tsatsul’nikov, Maximov, Ledentsov, Zaitsev, Gordeev, Kopchatov, Shernyakov, Kop’ev, Bimberg, and Alferov 106 Regular Issue Paper Journal of Electronic Materials, Vol. 27, No. 3, 1998 INTRODUCTION Remarkable improvement of the threshold cur- rent density (J th ) and its thermal stability have been predicted for a quantum dot (QD) laser. 1 Self-ordered QDs formed in a wider band-gap matrix are promis- ing candidates for laser applications. Significant progress in QD lasing in (In,Ga)As/(Al,Ga)As and (In,Ga)P/InP systems has been recently reported on. 2–7 Using the concept of vertically coupled quantum dots (VECQDs) of InGaAs in a GaAs matrix, we have achieved J th values as low as 100 A/cm 2 at room temperature for lasing via the QD ground state. 8 It was found that carrier evaporation out of QDs leads to increase in threshold current density (J th ) at elevated temperatures. The carrier localization energy in QDs can be increased by replacing a GaAs matrix material with an AlGaAs one. In the present work, we study threshold, spectral, and power characteristics of low- threshold lasers based on InGaAs QDs in an AlGaAs matrix. EXPERIMENTAL Laser structures were grown on (100) Si-doped GaAs substrates by solid-source molecular beam epi- taxy in a Riber 32P machine using atomic and tet- ramer species of group III and group V elements, respectively. The active region consisted of three- or ten-period In 0.5 Ga 0.5 As/Al 0.15 Ga 0.85 As VECQD array (N = 3 or 10). The substrate temperature was 485°C for the deposition of the QD region and 10 nm thick AlGaAs covering layer and 700°C for the rest of the structure. VECQDs self-organize during successive deposition of several sheets of In 0.5 Ga 0.5 As quantum dots with effective thickness of 1.2 nm separated by 5 nm wide AlGaAs spacers. Formation of QDs was controlled by monitoring reflection high energy elec- tron diffraction (RHEED) patterns during the deposi- tion. The active region was inserted into the middle of an undoped 0.4 μm thick graded index Al x Ga 1–x As waveguiding region (x = 0.45 ÷ 0.15) confined by 1.5 μm thick Al 0.45 Ga 0.55 As cladding layers. A 0.6 μm thick GaAs:Be contact layer was grown on top of the laser structure. The laser diodes were fabricated in both a (Received July 1, 1997; accepted October 31, 1997) Injection Lasers Based on InGaAs Quantum Dots in an AlGaAs Matrix A.E. ZHUKOV, 1 V.M. USTINOV, 1 A.YU. EGOROV, 1 A.R. KOVSH, 1 A.F. TSATSUL’NIKOV, 1 M.V. MAXIMOV, 1 N.N. LEDENTSOV, 1 S.V. ZAITSEV, 1 N.YU. GORDEEV, 1 V.I. KOPCHATOV, 1 Y.M. SHERNYAKOV, 1 P.S. KOP’EV, 1 D. BIMBERG, 2 and ZH.I. ALFEROV 1 1—A.F.Ioffe Physico-Technical Institute, Russian Academy of Sciences, 194021, Politekhnicheskaya 26, St. Petersburg, Russia. 2—Institut für Festkrperphysik, Technische Universität Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany Arrays of vertically coupled InGaAs quantum dots (QDs) in an AlGaAs matrix have been used in injection lasers. Increase in the band gap of a matrix material by replacement of a GaAs matrix with an AlGaAs one led to dramatic increase in quantum dot localization energy. By using this approach, we reduced the thermal population of the matrix and wetting layer states and thus decreased room temperature threshold current density to 63 A/cm 2 , increased differential efficiency up to 65%, and achieved room temperature continuous wave operation with output power of 1 W. Negative characteristic temperature has been observed in temperature dependence of threshold current density of these lasers in some temperature range. A qualitative explanation assuming a transition from non-equilibrium to Fermi population of QD states is proposed. Key words: Continuous wave (CW) operation, InGaAs, injection laser, quantum dot, threshold current