Laser stimulated selective area growth of quantum dots

Wankerl A., Emerson D.T., Shealy J.R.

We report on selective area growth of AlGaAs from trimethylgallium, trimethylaluminum, and arsine by UV laser stimulated organometallic vapor phase epitaxy. Using a wavelength tunable pulsed laser system, we investigated the stimulation mechanism and wavelength dependence of the growth enhancement in the range from 235 to 255 nm. Adlayer photolysis constitutes the dominant stimulation mechanism at these wavelengths, since the observation of laser-induced, submicron periodic surface ripples restricts the growth enhancement to a surface process and laser enhanced pyrolysis can be excluded by temperature simulations. We observe an abrupt wavelength edge at 252 nm, below which the stimulation rate is constant and above which the growth enhancement ceases. The growth contrast under our experimental conditions is 3.2:1 and the selectively grown AlGaAs is of optical quality.

Gotoh H., Ando H.

We investigate the excitonic optical properties in thin quantum boxes in the intermediate regime between the two-dimensional (2D) and zero-dimensional (0D) with a theoretical analysis that rigorously treats excitonic confinement effects. It is found that the exciton binding energy is substantially enhanced and that the oscillator strength concentrates to the lowest excitonic transition, even in a thin box whose lateral width is considerably (about five times) larger than the Bohr radius. Novel optical properties experimentally observed in semiconductor quantum disks, which are the intense photoluminescence spectrum and ultranarrow photoluminescence excitation spectrum, are explained well by the theoretical results. We also calculate exciton absorption in a thin box in which an electric field is applied in the lateral direction. The present theory can simulate how the electroabsorption evolves from the quantum confined Stark effect in the 0D to the quantum confined Franz–Keldysh effect in the 2D with an increase in the lateral size of the box. In the intermediate regime between 2D and 0D, a strong excitonic electric-field effect, distinct from the well-known electroabsorption effects at 0D and 2D, is found. These theoretical results demonstrate that even though the lateral confinement is weak, it considerably enhances the electron–hole Coulomb interaction and alters excitonic optical features markedly in the thin quantum box.

Frank Heinrichsdorff F.H., Alois Krost A.K., Nils Kirstaedter N.K., Ming-Hua Mao M.M., Marius Grundmann M.G., Dieter Bimberg D.B., Alexander Olegovich Kosogov A.O., Peter Werner P.W.

InAs quantum dots (QDs) have been grown by metalorganic chemical vapor deposition on exactly (001) oriented GaAs using the Stranski-Krastanow growth mode. The samples exhibit a high average dot density of 4 ×1010 cm-2 with no defects over macroscopic areas. The QDs show bright room temperature luminescence at around 1.1 eV. Vertical dot stacks consisting of up to 5 QD sheets with various GaAs separation layer thicknesses have been produced. Transmission eletron microscope images show pronounced QD ordering in the growth direction. For thin separation layers the dot luminescence is red shifted by ∼ 70 meV for the stacked dots as compared to single dot sheets. A low threshold (100 A/cm2 at 77 K) separate confinement heterojunction laser with a five-fold dot stack as an active medium operating at up to room temperature is demonstrated.

Miller M.S., Landin L., Jeppesen S., Petersson A., Maximov I., Kowalski B., Samuelson L.

We demonstrate the manipulation of InAs island size through growth on patterned GaAs substrates. Using chemical beam epitaxy, we deposited islands on patterns consisting of concentric circular trenches. One sample was left uncapped for atomic force microscopy, and another was capped with GaAs for micro-photoluminescence. Luminescence images taken at particular energies show that at different orientations along the circular arcs, the islands have different luminescence energies and thus different sizes. In correlating the island distributions found by atomic force microscopy images with the luminescence, we conclude that the InAs islands that form in high-density one-dimensional chains are smaller than those that are not in chains.

Cirlin G.E., Petrov V.N., Golubok A.O., Tipissev S.Y., Dubrovskii V.G., Guryanov G.M., Ledentsov N.N., Bimberg D.

Using scanning tunneling microscopy we have studied the influence of initial stage (up to 1.5 monolayers) growth kinetics on the surface morphology of 3 ML InAs GaAs (100) grown on vicinal surfaces (misoriented by 3° and 7° towards [011] direction) by different modifications of molecular beam epitaxy. The results presented clearly show the influence of growth kinetics on the arrangement of InAs GaAs quantum dot arrays on vicinal surfaces.

Geiger M., Bauknecht A., Adler F., Schweizer H., Scholz F.

Self-organized InAs quantum dot structures were prepared by low pressure metalorganic vapor phase epitaxy (MOVPE) using the Stranski-Krastanow growth mode. We present photoluminescence (PL) and atomic force microscopy (AFM) studies of the transition from a 2D InAs wetting layer to 3D quantum sized islands. The very narrow window where this transition takes place could be studied in detail by taking advantage of the slight growth rate gradient of our MOVPE system. After the growth of a critical layer thickness, first quantum dots occur. From the PL spectra obtained at different positions on the wafer, we conclude that a part of the wetting layer is consumed by the dot formation process. This is further confirmed by AFM measurements. Also a qualitative correlation between dot density and PL intensity is described. Moreover, we observed that the dot formation can be suppressed by instantaneous overgrowth. Besides the overgrowth temperature the growth interruption between the InAs film and the GaAs cap layer was found to be of great importance for the dot formation.

Grundmann M., Ledentsov N.N., Stier O., Böhrer J., Bimberg D., Ustinov V.M., Kop'ev P.S., Alferov Z.I.

Electronic transitions in nm-scale pyramid-shaped InAs/GaAs quantum dots feature only ground-state electrons. Allowed optical transitions involving excited hole states in addition to the ground-state transition are revealed in absorption and photoluminescence spectra. The experimental data agree with detailed theoretical calculations of the electronic structure, including strain, piezoelectric, and excitonic effects, and lead to unambiguous assignment of the transitions. We find as upper bound for the relative standard deviation of the size fluctuation $\ensuremath{\xi}l~0.04$. The hole sublevel separation is consistent with a pyramid shape fluctuation between {101} and {203} side facets.

Grundmann M., Christen J., Ledentsov N.N., Böhrer J., Bimberg D., Ruvimov, ‡ S.S., Werner P., Richter U., Gösele U., Heydenreich J., Ustinov V.M., Egorov A.Y., Zhukov A.E., Kop'ev P.S., et. al.

We report ultranarrow $(<0.15\mathrm{meV})$ cathodoluminescence lines originating from single InAs quantum dots in a GaAs matrix for temperatures up to 50 K, directly proving their $\ensuremath{\delta}$-function-like density of electronic states. The quantum dots have been prepared by molecular beam epitaxy utilizing a strain-induced self-organizing mechanism. A narrow dot size distribution of width $12\ifmmode\pm\else\textpm\fi{}1\mathrm{nm}$ is imaged by plan-view transmission electron microscopy. Cathodoluminescence images directly visualize individual dot positions and recombination from a single dot. A dense dot array $(\ensuremath{\sim}{10}^{11}\mathrm{dots}/{\mathrm{cm}}^{2})$ gives rise to a distinct absorption peak which almost coincides with the luminescence maximum.

Ruvimov S., Werner P., Scheerschmidt K., Gösele U., Heydenreich J., Richter U., Ledentsov N.N., Grundmann M., Bimberg D., Ustinov V.M., Egorov A.Y., Kop’ev P.S., Alferov Z.I.

Morphology evolution of molecular-beam-epitaxy-grown InAs and ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As layers as a function of deposition thickness, range from 1 to 10 ML, is studied by transmission-electron microscopy to characterize the formation and the self-organization of pseudomorphic quantum dots. For deposition (at 450--480 \ifmmode^\circ\else\textdegree\fi{}C) of 3--7 ML of InAs and 5--10 ML of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As, respectively, well-developed and crystallographically perfect dots with typical base length of 12 nm and small size dispersion form, which exhibit short-range ordering on a primitive two-dimensional square lattice along 〈100〉. The luminescence from all samples with coherent dots exhibits high quantum efficiency. For 4-ML InAs dots, coincidence of luminescence and absorption is demonstrated. Arrangement of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As dots in chains along [110] is the result of ordering during deposition at even lower temperatures (\ensuremath{\sim}320 \ifmmode^\circ\else\textdegree\fi{}C).

Maayan E., Kreinin O., Veinger D., Thon A., Bahir G., Salzman J.

A GaAs laser diode has been used as the light source for the selective epitaxial growth of GaAs by photoenhanced metalorganic chemical vapor deposition (PE-MOCVD). The use of GaAs/InGaP layered structures as substrates for these experiments permitted to accurately control the availability of photoexcited carriers reaching the GaAs growth surface. The experimental results obtained rule out pure pyrolysis and photolysis as possible mechanisms responsible for the PE growth rate, and indicate that photoexcited carriers are directly involved in the PE process. The experimental data are consistent with a kinetic model that invokes surface recombination as the main physical mechanism for the PE growth rate effect.

Ustinov V.M., Egorov A.Y., Zhukov A.E., Ledentsov N.N., Maksimov M.V., Tsatsul'nikov A.F., Bert N.A., Kosogov A.O., Kop'ev P.S., Bimberg D., Alferov Z.I.

Vertically coupled InAs quantum dots have been synthesized by MBE through successive deposition of InAs dot sheets and thin GaAs spacers. The energy of ground state transition in PL spectra has been found to depend on a number of dot sheets and the spacer width. Injection laser based on vertically coupled quantum dots demonstrated lasing via the ground state of quantum dots in the entire 80K-300K temperature range. Lower threshold current density and wider range of the thermal stability of threshold current density as compared to the single sheet quantum dot laser have been observed.

Miller M., Jeppesen S., Georgsson K., Kowalski B., Malm J., Pistol M., Samuelson L.

We report vertically-aligned InAs islands separated by GaAs barriers thin enough for electronic coupling. Thinner barriers reduced the InAs critical-thickness for island formation. Transmission electron microscopy revealed well-aligned islands with all detected islands in complete stacks. Atomic force microscopy showed the top islands of uncapped stacks are fully formed. The photoluminescence peak was sharper and shifted to lower energy compared to a single-layer growth. We attribute this shift to island-to-island electronic coupling and to the smaller compressive strain in the center of the composite structure.

Brandt O., Tapfer L., Ploog K., Bierwolf R., Hohenstein M.

We study the segregation of In during the overgrowth of an InAs monolayer (ML) with GaAs by molecular beam epitaxy. The presence of segregating In adatoms (In floating layer) at the growth surface is observed in situ by reflection high-energy electron diffraction. We demonstrate (i) that the segregation process causes a spatial spread-out of 0.4 ML of In into the first 4–5 ML of the GaAs overlayer and (ii) that this spread-out can be inhibited by the thermal desorption of the In floating layer in the initial stage of overgrowth (flash-off). The flash-off approach creates in fact a single InAs ML in the GaAs matrix.  

Fareell T., Armstrong J.V., Joyce T.B., Bullough T.J., Kightley P., Goodhew P.J.

The effect of 12 ns, 308 nm (XeCl) excimer laser pulses on the CBE growth rate of GaAs, at temperatures below the maximum non-laser assisted growth rate, G max , has been studied as a function of laser fluence and repetition frequency. There is a threshold fluence for growth rate enhancement, above which the growth rate is dependent on repetition frequency, being restored to G max at 20 Hz. The growth rate in the laser spot is measured by dynamic optical reflectivity (DOR).

Donnelly V.M., McCaulley J.A.

We report selected area growth of GaAs by XeF excimer laser induced pyrolysis of triethylgallium (TEGa) adsorbed on GaAs(100). TEGa dissociatively chemisorbs at 400 °C to form a stable layer which decomposes further under laser irradiation to liberate hydrocarbon products. The Ga left behind on the surface reacts with As2 and As4 (formed by pyrolysis of trimethylarsine or triethylarsine in a side tube) to grow GaAs in irradiated areas. Patterned films with feature sizes of ∼70 μm (limited by the projection system) were grown by this method. Interference between the incident beam and light scattered along the surface causes a substructure of parallel lines, with a spacing about equal to the laser wavelength (0.35 μm), to form on the features. This indicates that the ultimate spatial resolution is comparable to that predicted by thermal diffusion calculations (∼0.3 μm).

Silveira J.V., Savu R., Canesqui M.A., Filho J.M., Swart J.W., Filho A.G., Moshkalev S.A.

In this work we used a new approach for electrical contact improvement between multi-wall carbon nanotubes and metallic electrodes by localized laser heating. The nanotubes were suspended, using the dielectrophoresis technique, over a gap of 1μm width and 5μm depth connecting the ends of the patterned electrodes. Subsequently, the as deposited nanotubes were directly heated, in ambient atmosphere, by a laser having a wavelength of 473nm. The Raman signal of the nanotubes, through its G band displacement, was used to determine the process temperature and this parameter was controlled by calibrating the incident power density. The changes in the nanotubes morphology were evaluated using scanning electron microscopy, Raman spectroscopy and electrical measurements. After calibration, this method was employed for improving the electrical contact between suspended multi-wall carbon nanotubes and different electrodes (W, Ti and Au). The reduction in the electrical resistance was between 80 - 99%, 80 - 95% and 10 - 90% for W, Ti and Au electrodes, respectively, resulting in contact resistivity as low as ~1 kΩμm 2 .

Koo Hahn C., Motohisa J., Fukui T.

Position-controlled InAs single/double self-assembled quantum dots (SAQDs) were formed by selective area (SA) metalorganic vapor-phase epitaxy (MOVPE). Narrow GaAs (001)-areas, in the range of 70-200 nm in width/length, were constructed by SA-MOVPE on partially SiN x masked substrates. The number of InAs SAQDs formed on the width-controlled GaAs (001)-facet strongly depends on the width of the top facet. It is also found out that the two-dimensional (2D)-to-3D growth mode transition of InAs in selective MOVPE growth is determined not only by the growth conditions but also by the pattern itself. Different vapor-phase diffusions of source materials depending on the pattern together with dissimilar interfacet surface migration of adatoms are thought to be the origin.

Hahn C., Motohisa J., Fukui T.

Position- and number-controlled InAs self-organized quantum dots (SOQDs) were formed by selective-area-metal-organic chemical vapor phase epitaxy on the partially SiNx patterned GaAs (001) substrate. The mask layer was patterned along the [110] direction, and somewhat wider rectangular openings were also attached to the line. As a result of GaAs SA growth, a pyramidal shaped structure was formed on the rectangular region of the pattern. The top area of the pyramidal structure is a very narrow hexagonal-shaped (001) facet which is surrounded by two-{111}B and four-{124} facets. The SOQD was preferentially formed on the top (001) facet because the growth rate on the (001) facet is far much higher than on the surrounding sidewalls. It is found that the number of SOQDs formed is strongly dependent on the width of the top (001)-facet so that control of single, double, and multiple SOQD(s) is possible.

Paki P., Leonelli R., Isnard L., Masut R.A.

We have investigated the effect of the misorientation (001) InP substrates on the optical properties of submonolayers of InAs in InP grown by metalorganic chemical vapor deposition. InAs submonolayers were systematically studied using low temperature photoluminescence (PL), photoluminescence excitation spectroscopy and temperature-dependent, excitation density PL. For submonolayer samples with oriented substrates, the observed PL linewidths and energies are satisfactorily explained within a two-dimensional (2D) quantum well picture. The formation of InAs isolated quantum dots which is found in the submonolayer samples with misoriented substrates towards (110) orientations, however, results in 0D exciton localization.

Pinnington T., Levy Y., MacKenzie J.A., Tiedje T.

We present real-time measurements of surface structure evolution during quantum dot growth in InAs/GaAs grown by molecular-beam epitaxy. The measurements were made using an ultraviolet light-scattering technique in which the 254 nm line of a mercury lamp is used as the light source. This technique provides sensitivity to roughness on lateral lengthscales as low as 154 nm for our setup. Using this technique, we can detect the onset of quantum dot formation in this system, as indicated by reflection high-energy electron-diffraction measurements. The continuous increase in the scattering signal after the dots have formed, is explained in terms of diffusion-limited growth and ripening of the large islands that coexist with the quantum dots.

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.