Self-organized quantum wires and dots: New opportunities for device applications

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

Heinrichsdorff F., Mao M.-., Kirstaedter N., Krost A., Bimberg D., Kosogov A.O., Werner P.

We report on quantum dot (QD) lasers made of stacked InAs dots grown by metalorganic chemical vapor deposition. Successful growth of defect-free binary InAs/GaAs QDs with high lateral density (dl⩾4×1010 cm−2) was achieved in a narrow growth parameter window. The room-temperature photoluminescence (PL) intensity is enhanced up to a factor of 3 and the PL peak width is reduced by more than 30% when a thin layer of In0.3Ga0.7As is deposited onto the InAs QDs. A QD laser with a single sheet of such InAs/InGaAs/GaAs QDs exhibits threshold current densities as low as 12.7 and 181 A/cm2 at 100 and 300 K, respectively. Lasers with threefold stacked QDs show ground-state lasing and allow for cw operation at room temperature.

Ledentsov N.N., Bimberg D., Shernyakov Y.M., Kochnev V., Maximov M.V., Sakharov A.V., Krestnikov I.L., Egorov A.Y., Zhukov A.E., Tsatsul’nikov A.F., Volovik B.V., Ustinov V.M., Kop’ev P.S., Alferov Z.I., Kosogov A.O., et. al.

Transmission electron microscopy studies of low indium composition InxGa1−xAs insertions (x<0.4) in a GaAs matrix deposited by molecular beam epitaxy or by metal-organic chemical vapor deposition reveal nanoscale quasiperiodic compositional and morphological modulations. The luminescence peak wavelength is found to be a strong function of deposition parameters and can be tuned from 1.1 to 1.3 μm for the same x and average thickness of the deposit. Strained (In, Ga, Al)As lasers with such a laterally modulated InxGa1−xAs active region demonstrate significant depolarization of the electroluminescence. For short cavity length, lasing occurs at energies corresponding to transitions between InxGa1−xAs conduction band and light holelike valence band states. Devices lase at low threshold current densities and demonstrate continuous wave operation up to 3 W at room temperature.

Nakamura S., Senoh M., Nagahama S., Iwasa N., Yamada T., Matsushita T., Sugimoto Y., Kiyoku H.

The emission spectra of InGaN multi-quantum-well (MQW) laser diodes (LDs) under continuous-wave (CW) operation at room temperature (RT) showed periodic subband emissions with an energy separation of 2 meV. The peak wavelength of the laser emission was measured as a function of the operating current. The peak wavelength showed mode hopping toward higher energy with increasing operating current. Each single-mode laser emission was located at a peak of each periodic subband emission. These periodic subband emissions probably result from the transitions between the subband energy levels of the InGaN quantum dots formed from In-rich regions in the InGaN well layers. High-power InGaN MQW LDs with an output power of 40 mW were performed under RT CW operation.

Schur R., Sogawa F., Nishioka M., Satomi Ishida S.I., Yasuhiko Arakawa Y.A.

We report the growth, characterization and lasing of vertical microcavity lasers with an active layer of self-organized InGaAs/GaAs quantum dots. The quantum dots are formed by spinodal phase separation in low-In-content (x=0.03) In x Ga1- x As epilayers deposited with decreasing growth temperature in a hot-wall metalorganic chemical vapor deposition reactor. Optical transitions involving ground and excited states of the quantum dots were investigated using photoluminescence at moderately high excitation densities. Lasing oscillation was observed at 77 K by optical pumping. The coupling parameter β of the spontaneous emission into the lasing mode was estimated to be ∼8×10-3.

Maksimov M.V., Gordeev N.Y., Zaitsev S.V., Kop’ev P.S., Kochnev I.V., Ledentsov N.N., Lunev A.V., Ruvimov S.S., Sakharov A.V., Tsatsul’nikov A.F., Shernyakov Y.M., Alferov Z.I., Bimberg D.

Gaseous phase epitaxy from metal organic compounds is used to obtain a low-temperature injection laser with an active region based on In0.5Ga0.5As/GaAs quantum dots. Optimizing the growth conditions and geometric parameters of the structure has made it possible to increase the range of ultrahigh thermal stability in the threshold current (the characteristic temperature is T 0=385 K) up to 50 °C.

Kosogov A.O., Werner P., Gösele U., Ledentsov N.N., Bimberg D., Ustinov V.M., Egorov A.Y., Zhukov A.E., Kop’ev P.S., Bert N.A., Alferov Z.I.

Annealing at higher temperature (700 °C) of structures with two-dimensional and three-dimensional arrays in InAs–GaAs quantum dots (QDs) results in an increase in the size and in a corresponding decrease in the indium composition of the QDs. The change in the In composition is monitored by the contrast pattern in the plan-view transmission electron microscopy (TEM) images viewed under the strong beam imaging conditions. Increase in the size of the QDs is manifested by the plan-view TEM images taken under [001] zone axis illumination as well as by the cross-section TEM images. We show that the dots maintain their geometrical shape upon annealing. Luminescence spectra demonstrate a shift of the QD luminescence peak toward higher energies with an increase in the annealing time (10–60 min) in agreement with the decrease in indium composition revealed in TEM studies. The corresponding decrease in the QD localization energy results in an effective evaporation of carriers from QDs at room temperature, and the intensity of the QD luminescence decreases, and the intensity of the wetting layer and the GaAs matrix luminescence increase with the increase in the annealing time.

Ledentsov N.N., Shchukin V.A., Grundmann M., Kirstaedter N., Böhrer J., Schmidt O., Bimberg D., Ustinov V.M., Egorov A.Y., Zhukov A.E., Kop’ev P.S., Zaitsev S.V., Gordeev N.Y., Alferov Z.I., Borovkov A.I., et. al.

Alternate short-period GaAs-InAs deposition following InAs pyramid formation on a GaAs (100) surface leads to the creation of vertically split pyramids. This splitting is driven by the energetics of the Stranski-Krastanow growth mode. The strain energy is reduced due to the successive transfer of InAs from the buried part of the pyramid to the uncovered part. The resulting arrangement represents a laterally ordered array of nanoscale structures inserted in a GaAs matrix, where each structure is composed of several vertically merging InAs parts. Results of optical studies demonstrate the expected electronic coupling in vertical direction. Coupling is found to decrease the radiative lifetime and to result in low-energy shifts of the corresponding peaks in luminescence and absorption spectra. Vertically coupled quantum dots exhibit injection lasing at very low current densities. \textcopyright{} 1996 The American Physical Society.

Ledentsov N.N., Böhrer J., Bimberg D., Kochnev I.V., Maximov M.V., Kop’ev P.S., Alferov Z.I., Kosogov A.O., Ruvimov S.S., Werner P., Gösele U.

We demonstrate direct growth of electronically coupled zero-dimensional structures forming a super-quantum dot using metal-organic chemical vapor deposition. After the first sheet with InGaAs pyramids is formed on GaAs surface, alternate short-period GaAs-InGaAs deposition leads to spontaneous formation of layered structures driven by the energetics of Stranski–Krastanow growth. As a result columnlike InGaAs structures each having a characteristic lateral size of ∼23 nm at the top and composed of many closely packed InGaAs parts are formed. The full width at half maximum of superdot luminescence of 28 meV at 8 K indicates good average uniformity of the superdot ensemble. Absorption is found to be resonant with luminescence.

Richter W., Zettler J.-.

This paper summarizes recent results in optical real time analysis of epitaxial growth. Emphasis is placed on reflectance anisotropy spectroscopy (RAS) and spectroscopic ellipsometry (SE) which by their accuracy and sensitivity seem to be the most promising tools for analysis in the three main epitaxial methods: metal-organic vapour phase epitaxy (MOVPE), molecular beam epitaxy (MBE) and metal-organic-MBE (MOMBE). Both optical techniques exploit the polarisation changes imposed by the surface upon reflection. Thereby RAS, sensing the anisotropic part of the reflectance, turns out to be extremely surface sensitive in materials which are otherwise (in the bulk) optically isotropic. This is the case for all semiconductors with diamond or zincblende structure. After giving a basic understanding of RAS, chemical trends in the presently available data of different III–V-semiconductor surfaces are discussed. Using GaAs(001) as a well studied example, differences in growth dynamics and growth mechanisms in the three epitaxial growth techniques are presented. From these basic studies useful applications are derived for growth monitoring including substrate conditioning before growth (group V stabilisation, temperature), control of thickness and stoichiometry during growth as well as monitoring the switching procedures during the growth of heterostructures.

Heinrichsdorff F., Krost A., Grundmann M., Bimberg D., Kosogov A., Werner P.

In0.5Ga0.5As/GaAs (001) quantum dots (QDs) were grown by metalorganic chemical vapor deposition (MOCVD) on exactly (001) oriented substrates using the Stranski–Krastanow growth mode. The dot density and their relative geometrical arrangement are found to be strongly dependent on the substrate temperature. The dots have identical square shaped bases oriented along 〈100〉. For high densities a preferential relative alignment of the dots along the 〈110〉 directions is found. These dots tend to be arranged in a chainlike pattern with decreasing dot size towards the ends of the chains. From these observations the dot formation process for In0.5Ga0.5As quantum dots is suggested to be driven by energetics whereas the relative orientation is governed by kinetic effects.

Aggarwal R.L., Maki P.A., Molnar R.J., Liau Z.‐., Melngailis I.

Molecular-beam epitaxy was used to grow a 100 nm Al0.1Ga0.9N/100 nm GaN/500 nm Al0.1Ga0.9N double heterostructure on a 10-μm-thick GaN buffer layer grown with hydride vapor phase epitaxy on (0001) sapphire. Lasing from the 100 nm GaN active layer has been obtained at ∼359 nm at liquid-nitrogen temperature (77 K) and at ∼365 nm at room temperature (295 K), using transverse optical pumping at 337.1 nm with a 600 ps transversely excited atmospheric pressure pulsed nitrogen laser. Threshold pump fluences were measured to be 0.3 and 0.5 mJ/cm2 at 77 and 295 K, respectively, for a laser with 65 μm cavity length. In a laser of 23 μm cavity length, longitudinal cavity modes were observed with 0.56 nm spacing, corresponding to a group refractive index of 5.0 at the lasing wavelength.

Krost A., Heinrichsdorff F., Bimberg D., Darhuber A., Bauer G.

The structural properties of highly strained buried InxGa1−xAs layers on GaAs substrates are investigated by high-resolution x-ray diffraction. Such layers of a few monolayers in thickness serve for the formation of self-organized quantum dots by the Stranski–Krastanow growth mode. Exceeding a critical layer thickness the growth mode changes from two-dimensional Frank–van der Merwe to the three-dimensional Stranski–Krastanow mode resulting in the formation of coherently strained InxGa1−xAs islands. X-ray spectra of such structures below the growth mode transition can be perfectly simulated using dynamical theory allowing for determination of layer thickness with submonolayer sensitivity and composition within 5%. Dot formation manifests itself in a decrease of the effective In content of the wetting layer.

Shchukin V.A., Ledentsov N.N., Kop'ev P.S., Bimberg D.

The energetics of an array of three-dimensional coherent strained islands on a lattice-mismatched substrate is studied. The contribution of the edges of islands to the elastic relaxation energy always has a minimum as a function of the size of an island $L$, and the total energy $E(L)$ may have a minimum at an optimum size ${L}_{\mathrm{opt}}$. Among different arrays of islands on the (001) surface of a cubic crystal, the total energy is minimum for the 2D periodic square lattice with primitive lattice vectors along ``soft'' directions [100] and [010]. This is a stable array of islands which do not undergo ripening.

Shchukin V.A., Borovkov A.I., Ledentsov N.N., Kop’ev P.S.

Syrbu N., Dorogan A., Dorogan V., Zalamai V.

The polarization dependences of reflection and wavelength modulated reflection spectra of quantum wells In0.68Al0.1Ga0.13As/In0.42Al0.22Ga0.24As were investigated. Spectral dependences of refractive indices, extinction coefficients, real and imaginary parts of dielectric constants of quantum wells structures for different polarizations were calculated by the Kramers–Kronig analysis. A phenomenon of birefringence and an interference of polarized light waves in quantum wells were researched. The isotropic wavelength λ0 = 1.246 μm was found out. Interference spectra changes the density of fringes and refractive indices (Δni, ΔnKK = nР,Р–nS,S) intersect zero axis at energy of isotropic wavelength (0.955 eV).

Syrbu N.N., Dorogan V., Dorogan A., Vieru T., Ursaki V.V., Zalamai V.V.

The transparency, reflection and luminescence spectra of In0.3Ga0.7As structures with 8 nm thickness and quantum wells limited by the barrier layer GaAs of a 9 nm (upper layer) and 100 nm (bottom layer) thickness had been studied in the region of photon energy 0.5–1.6 eV. Lines associated with the transitions hh,lh1-e1(1s,2s,3s), hh2,lh2-e2(1s,2s,3s), hh1,lh1-e2(1s) and hh3,lh3-e3(1s) had been revealed in reflection spectra. The shapes of the reflection and transparency lines had been calculated using a single oscillator model of dispersion relations and the Kramers–Kronig integrals. The binding energy of hh,lh1-e1 excitons, the effective mass mhh∗ and mlh∗ and the damping factor for the optical transitions to QW and QD had been determined. The lifetime of charge carriers on quantum dots varies in the range of 0.04–0.1 ps, while the radiative lifetime of excitons in quantum wells in the considered structure is around 2 ps.

Syrbu N., Dorogan A., Dragutan N., Vieru T., Ursaki V.

Emission maxima related to the recombination of excitons (e1–hh1, e1–lh1, e2–hh2, and e2–lh2) were observed in photoluminescence spectra of GaAs/In 0.3 Ga 0.7 As/GaAs quantum wells . The emission bands due to e1–hh1 and e1–lh1 transitions were found to have a doublet character explained by the exchange interaction of excitons in quantum wells. Emission bands due to radiative E b –hh1, E b –lh1 transitions in the buffer GaAs layer are observed in the region of 1.5 eV. ► Luminescence due to recombination of excitons in GaAs/In 0.3 Ga 0.7 As quantum wells studied. ► Emission maxima related to excitons formed with heavy and light holes found. ► Emission bands due to e1–hh1 and e1–lh1 transitions found to have a doublet character. ► Doublet emission is explained by the exchange interaction of excitons in quantum wells. ► Emission bands due to radiative transitions in the buffer GaAs layer observed around 1.5 eV.

Melnik R.V., Zotsenko K.N.

Nanostructures, created by confinement of the motion of an electron from all three dimensions and known as quantum dots (QDs), provide materials scientists with a wide range of potential applications. These structures are produced today with advances of QD growth technology, and computational tools are fundamental in providing a better understanding of such structures. In this paper QD nanostructures are analysed with due account for coupling effects between electronic states in the dot and the wetting layer regions. The analysis, performed on the basis of the finite element methodology and Arnoldi iterations, demonstrates that the effect of coupling may be essential. The numerical procedure applied here is more efficient compared to the QR algorithm typically used in the context of modelling low-dimensional nanostructures. We report results of computational experiments for cylindrical and truncated conical QDs and compare them with the earlier results obtained for fully conical QD nanostructures.

Ni Y., He L.H.

Continuum phase-field model is adopted to study the phase separation of a binary epilayer grown on the surface of a solid with subsurfacial dislocation array. It is found that there exists interesting competition between the contributions to pattern formation from non-uniform surface stress and the buried dislocations. Numerical simulation demonstrates that various well-ordered composition patterns can be achieved, and their sizes, shapes and spatial orders are controllable in broad ranges.

Ledentsov N.N.

Injection lasers for the spectral range beyond 1.2 μm play an important role in many areas, from material processing to telecom applications. The wavelength range near 1.28–1.32 μm is particularly important for datacom applications at distances up to 2–10 km, with an advantage of ultrafast data links based on time division multiplexing (TDM) and, more recently, coarse wavelength division multiplexing (CWDM) systems. High-power single transverse mode long-wavelength laser diode pumps are required for the 14xx–15xxnm wavelength range, where Raman fiber amplifies for long-haul telecom applications are used for dense wavelength division multiplexing (DWDM) systems. Cost-effective wavelength-tunable vertical cavity surface emitting lasers (VC-SELs) are needed for the 1.5–1.65 μm range used in DWDM. However, this classification became to some extent obsolete after the appearance of Lucent Technologies™ ‘AllWave™’ fiber which does not contain an OH+ absorption hump between the 1.3 μm and 1.5 μm fiber transparency windows. In the near future all applications between 1.2 and 1.7 μm may be merged in unified CWDM/DWDM systems (transmitters, Raman amplifiers). Long-wavelength lasers are also needed for eye-safe lidar for terrestrial and free-space applications.

Tsatsul'nikov A.F., Krestnikov I.L., Lundin W.V., Sakharov A.V., Kartashova A.P., Usikov A.S., Alferov Z.I., Ledentsov N.N., Strittmatter A., Hoffmann A., Bimberg D., Soshnikov I.P., Litvinov D., Rosenauer A., Gerthsen D., et. al.

Coherent ultrathin GaAsN insertions are formed in a GaN matrix by predeposition of an ultrathin GaAs layer on a GaN surface, followed by annealing in an NH3 atmosphere and overgrowth with GaN. During the overgrowth, most of the As atoms are substituted by N, with a dense array of coherent GaAsN nanodomains with lateral sizes of about 3-4 nm formed in the GaN matrix. We report a green luminescence due to GaAsN insertions, surviving at high observation temperatures and excitation densities.

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

We have studied quantum dots (QDs) fabricated by activated spinodal decomposition (ASD) of an InGa(Al)As alloy deposited on top of self-organized InAs nanoscale stressors on GaAs substrate. Such a growth sequence results in a strong red shift of the PL emission down to 1.3 μm at 300 K. This red shift is caused by the formation of In-rich areas in the vicinity of the InAs islands, which increase the effective dot size. Beyond a certain critical InAs composition or nominal thickness of the InGa(Al)As layer the PL line shifts back towards higher energies. Adding Al to the alloy increases the red shift for a given In concentration. Room temperature lasing near 1.3 μm with threshold current densities of about 85 A / cm 2 was achieved for lasers based on three-fold stacked ASD-formed QDs, with a maximum cw output power of 2.7 W.

Bimberg D., Grundmann M., Heinrichsdorff F., Ledentsov N.N., Ustinov V.M., Zhukov A.E., Kovsh A.R., Maximov M.V., Shernyakov Y.M., Volovik B.V., Tsatsul’nikov A.F., Kop’ev P.S., Alferov Z.I.

Semiconductor heterostructures with self-organized quantum dots (QDs) have experimentally exhibited properties expected for zero-dimensional systems. When used as active layer in the injection lasers, these advantages help to strongly increase material gain and differential gain, to improve temperature stability of the threshold current, and to provide improved dynamic properties. Molecular beam epitaxy (MBE) represents a developed technology well suited for fabrication of self-organized QDs. Optimization of deposition parameters can ensure that the self-organized islands are small (∼10 nm), have a similar size and shape and form dense arrays. Saturation material gain is as high as 150000 cm −1 compared with QW values of about 3000 cm −1 . Maximum differential gain reported for QD lasers approaches 10 −12 cm 2 and exceeds the QW laser values by about three orders of magnitude. Direct observation of relaxation oscillations reveals present cut-off frequencies close to 10 GHz. High internal (>96%) and differential (70%) efficiencies at 300 K are realized. Using the novel concept of electronically-coupled QDs and oxide-defined 10 μm apertures, CW lasing with J th =180 A/cm 2 , is realized in surface-emitting QD lasers (300 K). Wall-plug efficiencies are up to 16%. Total currents as low as 68 μA are measured for 1μm apertures. GaAs-based lasers for the 1.3 μm range with low J th (65 A/cm 2 ) at room temperature (RT) are realized using InAs/InGaAs/GaAs QDs obtained by activated spinodal decomposition. In stripes the lasing occurs via the QD ground state ( J th =90 A/cm 2 ) for cavity lengths L >1 mm (uncoated). Differential efficiency is 55% and internal losses are 1.5 cm −1 . A characteristic temperature near RT is 160 K. 3W CW operation at RT is achieved. The recent progress in lasers based on self-organized MBE QDs already made it possible to fabricate devices with dramatically improved characteristics as compared to recent QW devices for the most important commercial applications.

Ledentsov N.N., Grundmann M., Heinrichsdorff F., Bimberg D., Ustinov V.M., Zhukov A.E., Maximov M.V., Alferov Z.I., Lott J.A.

Quantum-dot (QD) heterostructures are nanoscale coherent insertions of narrow-gap material in a single-crystalline matrix. These tiny structures provide unique opportunities to modify and extend all basic principles of heterostructure lasers and advance their applications. Despite early predictions, fabrication of QD heterostructure (QDHS) lasers appeared to be a much more challenging task, as compared to quantum well (QW) devices. The breakthrough occurred when techniques for self-organized growth of QD's allowed the fabrication of dense arrays of coherent islands, uniform in shape and size, and, simultaneously, free from undesirable defects. Recently, the figure of merit of QDHS lasers surpasses some of the key characteristics of QW devices in some of the most important applications.

Sun Y., Miyasato T., Wigmore J.K.

A variety of nanoscale SiC structures, in particular the grain, whisker, and flake, displaying, respectively, zero, one, and two dimensions, has been grown by hydrogen plasma sputtering of a SiC target in the presence of a small amount of oxygen. Growth of the different nanoscale structures takes place by various mechanisms. The SiC whiskers are initiated by SiO2 seed crystals in the SiC film matrix at temperatures around 700 °C. On the other hand, the grains and flakes are separated by insertions of amorphous and graphitic carbon in the films at higher temperatures, around 950 °C. Both these processes result from the reaction of oxygen with the growing SiC film.

Ledentsov N.N.

Despite its early age, laser based on arrays of self-organized quantum dots modified all the basic commandments of the heterostructure laser. Excitonic gain mechanism and discrete energy spectrum in a quantum dot provide principally new ways to control optical properties of the media. Extension of the spectral range using the same substrate will probably soon lead to the appearance of quantum dot lasers on the market.

Volovik B.V., Tsatsul’nikov A.F., Bedarev D.A., Egorov A.Y., Zhukov A.E., Kovsh A.R., Ledentsov N.N., Maksimov M.V., Maleev N.A., Musikhin Y.G., Suvorova A.A., Ustinov V.M., Kop’ev P.S., Alferov Z.I., Bimberg D., et. al.

When an array of strained InAs nanoislands formed on a GaAs surface is overgrown by a thin (1–10 nm) layer of an indium-containing solid solution, stimulated decomposition of the solid solution is observed. This process causes the formation of zones of elevated indium concentration in the vicinity of the nanoislands. The volume of newly formed InAs quantum dots increases as a result of this phenomenon, producing a substantial long-wavelength shift of the photoluminescence line. This effect is enhanced by lowering the substrate temperature, and it depends weakly on the average width of the band gap of the solid solution. The indicated approach has been used successfully in achieving room-temperature emission at a wavelength of 1.3 µm.