Tunnel QW-QDs InGaAs-InAs High Gain Medium for All-Epitaxial VCSELs
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Tunnel QW-QDs InGaAs-InAs High Gain Medium for All-Epitaxial VCSELs Vadim Tokranov, Michael Yakimov, Jobert van Eisden, and Serge Oktyabrsky College of Nanoscale Science and Engineering, University at Albany - SUNY, 251 Fuller rd, Albany, NY, 12203
ABSTRACT Structures of tunnel-coupled pairs consisting of InGaAs (In composition was varied from 29 to 36%) quantum wells (QW) grown on top of shape-engineered self-assembled InAs quantum dots (QW-on-QDs) were employed to increase the maximum saturated gain of QDbased laser active medium. Room temperature optical properties of tunnel-coupled well-on-dots structures at low excitation were found to be sensitive to energy separation between ground states (GS) energies of QDs and QW. The spectra also show that QD-related photoluminescence (PL) tends to peak at discrete energy separations from the QW peak, in this case multiples of ~ 35meV (LO phonon energy). The optimized GS energy separation between QW and QDs was found to be close to the energy of the LO phonon. This structure demonstrated narrowing of the QD PL line down to 21.6 meV at T=77K, indicating efficient resonant tunneling of carriers from QW into QD ensemble states. All-epitaxial vertical cavity surface emitting lasers (VCSELs) with triple-pair tunnel QW-on-QDs as active medium demonstrated continuous wave mode lasing. Tunnel QDs-QW VCSELs exhibited 1.8 mA (Jth ~ 800 A/cm2) minimum threshold current at QD GS emission wavelength, 1135 nm, with 0.7mW optical power and 12% light-current efficiency. INTRODUCTION During the last decade quantum dots (QD) have evolved from a predicted [1] superior gain medium through a variety of advances in laser diode characteristics including ultralow threshold current density [2] and thermal stability [3]. Nevertheless, QD medium has very few demonstrations comparable to quantum well (QW) medium in high gain applications such as allepitaxial vertical cavity surface emitting lasers (VCSEL) [4, 5]. A relatively broad QD size distribution as well as the limitations in carrier capture and thermalization are still limiting the maximum saturated gain of QD-based lasers. Recently, QD-coupled-to-QW structures with tunnel injection of carriers into QDs were proposed and resulted in demonstration of superior performance parameters of laser diodes [6, 7]. Carrier capture in QWs is more efficient than in QDs, so QW may provide a path for fast carriers transfer into the dots - for example by resonant phonon assisted tunneling with emission of longitudinal optical (LO) phonon [7, 8]. Reported tunneling lasers generally employed a QDs grown on QW configuration with a few nm thin tunnel barriers. These “dots-on-well” structures contained one carrier-collecting QW coupled to QDs. The stress-driven formation process of QDs is very sensitive [9] to barrier thickness and strained coupled QW parameters. Compared to this “dots-grown-on-well” approach, we may expect more controllable QD growth leading to more effective tunneling using a “well-grownon-dots” design. To the best of our knowledg
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