High-gain excitonic lasing from a single InAs monolayer in bulk GaAs
A. R. Goñi, M. Stroh, C. Thomsen, F. Heinrichsdorff, V. Türck, A. Krost, D. Bimberg
1998
Applied Physics Letters
We report the observation of highly efficient laser emission from a single InAs layer with an effective thickness of 1.5 monolayers ͑ML͒ embedded in bulklike GaAs. Lasing action is obtained at the wavelength of the InAs thin-layer luminescence ͑870 nm͒ by cw optical pumping with a threshold power density of 0.9(3) kW/cm 2 at 10 K. Gain measurements yield a very high material gain of 1.0(5)ϫ10 4 cm Ϫ1 for the InAs layer when pumped with ϳ10 kW/cm 2 at low temperatures. The 0 dimensional
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... of the emission as determined from cathodoluminescence and the absence of band-gap renormalization with increasing pump level speak for an excitonic mechanism of population inversion. © 1998 American Institute of Physics. ͓S0003-6951͑98͒00812-2͔ Nowadays, much effort is being devoted to the fabrication of optoelectronic devices based on low-dimensional semiconductor nanostructures, 1-5 because of the recently demonstrated improvement in device performance, for example by the ␦-function density of states in zero dimensions. 6,7 This advantage of low-dimensional systems is often partly impaired by large fluctuations in size and shape, which lead to strongly inhomogeneously broadened emission lines. A structure consisting of a very thin InAs layer in GaAs, whose thickness is in the monolayer ͑ML͒ range, presents several peculiarities which make such structures very promising candidates for device applications. Monolayerthick InAs/GaAs structures exhibit an isomorphic epitaxial growth despite the high built-in strain (ϳ7%) which affects the energies of confined electron and hole states. 8, 9 Furthermore, monolayer-width fluctuations play a fundamental role in localizing free carriers in the plane of the monolayer 10 with the consequent enhancement of excitonic effects. One would also expect that the optical properties of the ML samples will resemble that of quantum dots systems. 11 In fact, this structure exhibits a very strong but narrow ͓full width half maximum ͑FWHM͒Ϸ8 meV͔ photoluminescence ͑PL͒ line. 8-10 Although lasing action in thin InAs/GaAs ML structures has been previously reported, 12,13 no investigation of the lasing performances in relation to an excitonic kind of mechanism for population inversion or in connection to zerodimensional ͑0D͒ characteristics of the emission was undertaken. In this letter we report the observation of stimulated emission from excitons in a single InAs monolayer in bulklike GaAs. Laser emission occurs at constant energy, independent of the pump power, at about 100 meV below the GaAs band gap involving radiative recombination processes from ground-state excitons bound to the InAs layer. 10 Cathodoluminescence measurements show that these excitons are localized in the InAs plane by layer-width fluctuations. Very striking is the large material gain of 1.0(5) ϫ10 4 cm Ϫ1 achieved at 12 kW/cm 2 excitation power ͑10 K͒, which is higher for InGaAs/InGaAsP multiple quantum wells ͑QWs͒, 14 but comparable to the values measured recently in InAs/GaAs quantum dot lasers. 15 The very high gain values obtained for the InAs/GaAs ML sample, the insensitivity of its emission energy upon pump power levels and the localized nature of the exciton states all together give strong evidence for an excitonic mechanism of stimulated emission similar to that observed in quantum dots. The sample was grown by metalorganic chemical vapor deposition ͑MOCVD͒ on a semi-insulating ͑001͒ GaAs substrate as described elsewhere. 16 The laser structure consists of a single InAs layer ͑effective thickness about 1.5 ML͒ sandwiched between 300 nm thick GaAs layers. A waveguide is formed by two cladding layers of undoped Al 0.5 Ga 0.5 As. Short cavities from 250 to 400 m in length with mirrorlike surfaces perpendicular to the plane of the InAs ML were obtained by cleaving the sample after it was mechanically thinned from the backside to a total thickness of 150 m. Standard PL was excited with the 514 nm line of an Ar ϩ -ion laser. Gain measurements were performed in 90°scattering geometry at low temperatures between 2 and 120 K. Light emission was excited by a continuous wave ͑cw͒ Ti:sapphire laser tuned to 750 nm, which is absorbed only by the GaAs. The pump laser was incident on the ͑001͒ growth surface and was focused to a 40 m wide stripe of variable length. Light emitted from one of the cleavage mirrors was analyzed. Figure 1 shows representative PL spectra of the InAs/ GaAs ML sample measured in backscattering from the ͑001͒ growth surface at different excitation powers and at 2 K. The optical emission is dominated by a single intense and narrow line (FWHMϷ10 meV), which is redshifted by 93 meV from the GaAs gap energy. The much weaker structures at around 1.48-1.50 eV correspond to transitions involving carbon impurities in bulk GaAs. The energy level scheme is also shown in Fig. 1 . Tight-binding calculations indicate that only a single electron state and two almost-degenerate hole levels are bound to the InAs layer. Their wave functions are the largest at the InAs, but penetrate up to 10 ML's in GaAs. 9 The main PL peak was demonstrated to arise from a radiative recombination of excitons bound to the thin InAs layer. 8, 9, 17 We noted that the energy position and the bandwidth of the main PL peak remain unchanged even though APPLIED PHYSICS LETTERS VOLUME 72, NUMBER 12
doi:10.1063/1.120586
fatcat:ptay7k35gvflfpjabdh6qkqxsi