Long-Wavelength InGaAs/GaAs Quantum Dot Lasers

Glas F., Guille C., Hénoc P., Houzay F.

Lott J.A., Ledentsov N.N., Ustinov V.M., Maleev N.A., Zhukov A.E., Kovsh A.R., Maximov M.V., Volovik B.V., Alferov Z.I., Bimberg D.

Pulsed lasing at 1.3 /spl mu/m via the exciton ground state is demonstrated for vertical cavity surface emitting lasers containing three uncoupled sheets of InAs quantum dot active layers. The structures are grown directly on GaAs substrates and when fabricated include selectively oxidised AlO current apertures, intracavity metal contacts, and AlO/GaAs distributed Bragg reflectors. Experimental devices operate pulsed at room temperature with threshold currents below 2 mA and differential slope efficiencies above 40%.

Steinle G., Riechert H., Egorov A.Y.

The authors report on electrically pumped MBE-grown VCSELs on GaAs substrate with an InGaAsN active region, emitting above 1.28 µm with record characteristics. The CW output power at room temperature exceeds 500 µW with an initial slope efficiency of 0.17 W/A.

Choquette K.D., Klem J.F., Fischer A.J., Blum O., Allerman A.A., Fritz I.J., Kurtz S.R., Breiland W.G., Sieg R., Geib K.M., Scott J.W., Naone R.L.

Selectively oxidized vertical cavity lasers emitting at 1294 nm using InGaAsN quantum wells are reported for the first time which operate continuous wave at and above room temperature. The lasers employ two n-type Al{sub 0.94}Ga{sub 0.06}As/GaAs distributed Bragg reflectors each with a selectively oxidized current aperture adjacent to the optical cavity, and the top output mirror contains a tunnel junction to inject holes into the active region. Continuous wave single mode lasing is observed up to 55 C. These lasers exhibit the longest wavelength reported to date for vertical cavity surface emitting lasers grown on GaAs substrates.

Lott J.A., Ledentsov N.N., Ustinov V.M., Egorov A.Y., Zhukov A.E., Kop'ev P.S., Alferov Z.I., Bimberg D.

Ground state lasing is reported for vertical cavity lasers containing three-period InGaAs/GaAs vertically coupled quantum dot active regions. The structures include selectively oxidised AlO current apertures and AlO/GaAs reflectors. Experimental devices emitting near 1.0 µm operate continuous wave at 20°C with threshold currents

Ribbat C., Sellin R., Grundmann M., Bimberg D., Sobolev N.A., Carmo M.C.

The influence of high energy proton irradiation on the device properties of InGaAs/GaAs quantum dot and quantum well lasers has been investigated. In the regime of spontaneous emission, quantum dot lasers show a much enhanced radiation hardness compared to quantum well lasers, manifested in a smaller increase of threshold current density. However, in the lasing regime the device characteristics are similarly influenced. Internal differential quantum efficiencies are reduced, internal optical losses remain constant.

Shernyakov Y.M., Bedarev D.A., Kondrat'eva E.Y., Kop'ev P.S., Kovsh A.R., Maleev N.A., Maximov M.V., Mikhrin S.S., Tsatsul'nikov A.F., Ustinov V.M., Volovik B.V., Zhukov A.E., Alferov Z.I., Ledentsov N.N., Bimberg D.

Low threshold current density (J/sub th/=65 A/cm/sup 2/) operation near 1.3 /spl mu/m at room temperature (RT) is realised for lasers using InAs-InGaAs-GaAs quantum dots (QDs). The lasing occurs via the QD ground state for cavity length L>1 mm. The differential efficiency is 40% and internal losses are 1.5 cm. The characteristic temperature near RT is 160 K.

Ledentsov N.N., Shchukin V.A., Bimberg D., Ustinov V.M., Cherkashin N.A., Musikhin Y.G., Volovik B.V., Cirlin G.E., Alferov Z.I.

We report on reversible and irreversible phenomena in size-limited InAs island growth (SLIG) on GaAs(001) surface. We found that, with increasing the substrate temperature, the island density of the SLIG islands decreases, the lateral size of the islands increases and the islands strongly flatten. The average volume is either decreased or weakly affected. The total amount of InAs accumulated in quantum dots (QDs) strongly decreases in favour of the gas of In adatoms on the surface. Both unidirectional and reversible tuning of the substrate temperature after formation of the islands causes reversible changes in the island shape and density. We show the possibility of dramatically increasing the volume and the density of QDs approaching the strategically important 1.3 µm wavelength range via adatom condensation with cooling of the substrate after the formation of QDs. We also demonstrate that the substrate temperature cycling procedure may remarkably reduce the defect density in QD structures.

Ledentsov N.N., Litvinov D., Rosenauer A., Gerthsen D., Soshnikov I.P., Shchukin V.A., Ustinov V.M., Egorov A.Y., Zukov A.E., Volodin V.A., Efremov M.D., Preobrazhenskii V.V., Semyagin B.P., Bimberg D., Alferov Z.I.

GaAs-AlAs corrugated superlattices (CSL) are formed on spontaneously nanofaceted (311)A surfaces. Using high-resolution transmission electron microscopy (HRTEM) along the $$[\bar 233]$$ zone axis with an appropriate image evaluation technique to enhance the contract between GaAs and AlAs we found two distinct lateral periodicities along the $$[0\bar 11]$$ directions for two different CSL layer thickness regimes. For multilayer deposition with GaAs layer thickness exceeding 1 nm the lateral periodicity of 3.2 nm is clearly revealed. The contrast originates from the thickness modulation of both AlAs and GaAs layers with a period of 3.2 nm in the $$[0\bar 11]$$ direction. The corrugation height is about 1 nm and it is symmetric for both upper and lower GaAs-AlAs interfaces. Thicker sections of the thickness-modulated AlAs and GaAs layers of the CSL are shifted by a half period with respect to each other. In the regime when the GaAs deposited average thickness is below 1 nm, which is necessary for complete coverage of the AlAs surface, a lateral periodicity of ≈1.5–2 nm is additionally revealed. We attribute this effect to the formation of local GaAs clusters dispersed on a corrugated (311)A AlAs surface resulting in a local phase reversal of the AlAs surface in their vicinity upon subsequent overgrowth. This reversal can be explained by the same effect as the phase shift of the surface corrugation upon heteroepitaxy on (311)A. In our model AlAs does not wet the GaAs cluster surface, unless different more energetically favorable scenario is possible. This causes accumulation of AlAs in the vicinity of the GaAs cluster and, as a result, the local phase reversal of the AlAs surface. The AlAs corrugated surface domains with different phases coexist on the surface resulting in an additional periodicity revealed in the HREM contrast modulation. Additionally HRTEM studies indicate that the AlAs-GaAs interface inclination angles in both regimes are 40° and 140° with respect to the flat (311) surface in an argreement with the {331} facet geometry model proposed by R. Nötzel, N.N. Ledentsov, L. Däweritz, M. Hohenstein, and K. Ploog.

Solomon G.S., Pelton M., Yamamoto Y.

The spontaneous emission from an isolated semiconductor quantum dot state has been coupled with high efficiency to a single, polarization-degenerate cavity mode. The InAs quantum dot is epitaxially formed and embedded in a planar epitaxial microcavity, which is processed into a post of submicron diameter. The single quantum dot spontaneous emission lifetime is reduced from the noncavity value of 1.3 ns to 280 ps, resulting in a single-mode spontaneous emission coupling efficiency of 78%.

Krestnikov I.L., Cherkashin N.A., Sizov D.S., Bedarev D.A., Kochnev I.V., Lantratov V.M., Ledentsov N.N.

A new method for obtaining InGaAs nanodomains on the surface of GaAs or (Al,Ga)As is suggested. At the first stage, an InGaAs layer with a thickness above the critical value for dislocation formation is deposited onto the substrate surface by metalorganic CVD. Then the InGaAs film is coated with a thin AlAs layer and annealed at an elevated temperature. The “repulsion” of AlAs from plastically relaxed regions near dislocations and the high temperature stability of AlAs result in that evaporation is restricted to the regions containing defects. The self-organization effects favor the formation of an ordered array of coherent nanodomains that can be used for obtaining buried low-dimensional nanostructures and/or nanoheteroepitaxial inclusions.

Volovik B.V., Kovsh A.R., Passenberg W., Kuenzel H., Grote N., Cherkashin N.A., Musikhin Y.G., Ledentsov N.N., Bimberg D., Ustinov V.M.

Structural and optical properties of thin InGaAsN insertions in GaAs, grown by molecular beam epitaxy using an RF nitrogen plasma source, have been investigated. Nitrogen incorporation into InGaAs results in a remarkable broadening of the luminescence spectrum as compared with that of InGaAs layer with the same indium content. Correspondingly, a pronounced corrugation of the upper interface and the formation of well defined nanodomains are revealed in cross-sectional and plan-view transmission electron microscope (TEM) images, respectively. Raising the indium concentration in InGaAsN (N

Krestnikov I.L., Ledentsov N.N., Hoffmann A., Bimberg D.

Ultrathin insertions of a narrow band-gap material in wide band-gap matrices represent a challenging medium in view of aspects of growth phenomena, unique optical properties, and non-trivial approaches for structural characterization. In a very general case ultrathin submonolayer insertions may form arrays of islands due to the principally discrete nature of the growth front. If the islands are large enough, these islands may act as locally formed quantum well (QW) insertions. If, however, the islands' size is comparable to the Bohr radius and the band-gap difference between the insert and the matrix material is large enough, quantum dots (QD) are formed. Realization of the first or the second regime depends on the surface properties of the substrate and the deposit, particularly, on the tensors of the intrinsic surface stress of both materials and on the lattice mismatch. In this work we consider in detail the case of ultrathin CdSe insertions in wide gap ZnMgSSe matrices: that the nominal thickness is chosen below the critical thickness for three-dimensional (3D) island formation. We give an overview of the experimental results available for these structures obtained by submonolayer or about-one monolayer CdSe depositions. A comparison with similar phenomena observed in conventional III-V and III-N systems is given and possible growth scenarios are discussed. We also discuss practical device applications of the structures based on ultrathin insertions for non-traditional devices. Examples of resonant waveguiding and lasing in edge geometry, of surface emitting lasers with low finesse cavities, and of broad-miniband high-frequency Esaki-Tsu anti-dot superlattices are given.

Maximov M.V., Tsatsul’nikov A.F., Volovik B.V., Sizov D.S., Shernyakov Y.M., Kaiander I.N., Zhukov A.E., Kovsh A.R., Mikhrin S.S., Ustinov V.M., Alferov Z.I., Heitz R., Shchukin V.A., Ledentsov N.N., Bimberg D., et. al.

Strain-driven decomposition of an alloy layer is investigated as a means to control the structural and electronic properties of self-organized quantum dots. Coherent InAs/GaAs islands overgrown with an InGa(Al)As alloy layer serve as a model system. Cross-section and plan-view transmission electron microscopy as well as photoluminescence (PL) studies consistently indicate an increase in height and width of the island with increasing indium content and/or thickness of the alloy layer. The increasing island size is attributed to the phase separation of the alloy layer driven by the surface strain introduced by the initial InAs islands. The decomposition is enhanced by the addition of aluminum to the alloy layer. The ground-state transition energy in such quantum dots is significantly (up to 200 meV) redshifted compared to the original InAs/GaAs quantum dots, allowing to reach the 1.3 \ensuremath{\mu}m spectral region maintaining the high PL efficiency and the low defect density typical for Stranski-Krastanow growth. The possibility of degradation less stacking of such quantum dot layers enables injection lasing on the ground-state transition with a differential efficiency of 57% and a continuous-wave output power of 2.7 W.

Tsatsul’nikov A.F., Kovsh A.R., Zhukov A.E., Shernyakov Y.M., Musikhin Y.G., Ustinov V.M., Bert N.A., Kop’ev P.S., Alferov Z.I., Mintairov A.M., Merz J.L., Ledentsov N.N., Bimberg D.

Quantum dots (QDs) formed on GaAs(100) substrates by InAs deposition followed by (Al,Ga)As or (In,Ga,Al)As overgrowth demonstrate a photoluminescence (PL) peak that is redshifted (up to 1.3 μm) compared to PL emission of GaAs-covered QDs. The result is attributed to redistribution of InAs molecules in the system in favor of the QDs, stimulated by Al atoms in the cap layer. The deposition of a 1 nm thick AlAs cover layer on top of the InAs–GaAs QDs results in replacement of InAs molecules of the wetting layer by AlAs molecules, leading to a significant increase in the heights of the InAs QDs, as follows from transmission electron microscopy. This effect is directly confirmed by transmission electron microscopy indicating a transition to a Volmer–Weber-like QD arrangement. We demonstrate an injection laser based on this kind of QDs.

Springholz G., Schwarzl T., Heiss W.

In conclusion, lead salt vertical-cavity surface-emitting lasers offer attractive properties as coherent infrared laser sources. In particular, they feature single mode operation, emit circularly shaped parallel beams with extremely small beam divergence, and exhibit very sharp emission lines widths below 12 µeV. Based on the use of high finesse infrared microcavity structures, pulsed mode operation has been achieved well above room temperature as well as CW-operation up to 120 K, which is expected to be significantly increased in the near future. In comparison with quantum cascade lasers, the lead salt VCSELs show a substantially larger wavelength tunability, which is of crucial importance for spectroscopy applications. In addition, lead salt VCSELs can be grown on readily available substrate materials. This not only drastically reduces costs and facilitates the laser fabrication, but also offers improved heat dissipation due to the higher substrate thermal conductivity. Up to now, lead salt VCSELs have operated only under optical excitation but with some technological efforts electrically pumped lasers should also become feasible. Alternatively, one can envision hybrid structures in which low cost and readily available NIR pump lasers are integrated in one package with the lead salt VCSELs to obtain easy to use and cost-efficient mid-infrared laser sources. This would certainly open many promising applications in a variety of different fields.