Anisotropy of the spatial distribution of In(Ga)As quantum dots in In(Ga)As-GaAs multilayer heterostructures studied by X-ray and synchrotron diffraction and transmission electron microscopy

High-resolution X-ray and synchrotron (crystal truncation rods) diffraction methods and transmission electron microscopy have been employed to study MBE-grown multilayer In(Ga)As-GaAs heterostructures with arrays of vertically coupled In(Ga)As quantum dots (QDs) in a GaAs matrix. Additional (vertical and lateral) spatial ordering of QDs in perfect crystalline structures, giving rise to undulations of the crystalline planes and quasi-periodic elastic strain, was shown to be essentially anisotropic with respect to crystallographic directions of the [110] type. The anisotropy of the QD formational system of can be accounted for by assuming that the spatial ordering of the QDs and the corrugation of the crystal planes are the initial stages of relaxation of the elastic strain introduced into the system by the QDs. The anisotropic relief of the crystal planes (corrugated growth surface) results from the formation of a system of spatially ordered quantum quasi-wires uniformly filled with QDs. In a multilayer heterostructure with high crystal perfection, the anisotropic relief of the crystal planes is inherited by overlying layers and its amplitude decreases gradually with increasing distance from the source of elastic strain—the superstructure containing In(Ga)As QDs in the given case.     

Lateral photoconductivity of multilayer Ge/Si structures with Ge quantum dots

Spectra of lateral photoconductivity of multilayer Ge/Si structures with Ge quantum dots, fabricated by molecular-beam epitaxy are studied. The photoresponse caused by optical transitions between hole levels of quantum dots and Si electronic states was observed in the energy range of 1.1–0.3 eV at T = 78 K. It was shown that the electronic states localized in the region of Si band bending near the Ge/Si interface mainly contribute to lateral photoconductivity. The use of the quantum box model for describing hole levels of quantum dots made it possible to understand the origin of peaks observed in the photoconductivity spectra. A detailed energy-level diagram of hole levels of quantum dots and optical transitions in Ge/Si structures with strained Ge quantum dots was constructed.     

Stress evolution aspects during InAs/InP (001) quantum wires self-assembling

We report here in situ measurements of stress evolution during molecular beam epitaxy growth of InAs/InP (001) quantum wires. The obtained results provide the necessary information to understand why quantum wires instead of quantum dots are formed in this heteroepitaxial system: a strong stress anisotropy along 〈110〉 directions is responsible for it. Moreover, during quantum wires growth As/P exchange reactions take place, which determine the total amount of InAs incorporated in the nanostructures and then their optical properties. In situ stress measurements have also been used to study the exchange process and they have allowed us to calculate the quantity of InAs that enters into the lattice because of it.     

Optical anisotropy of InGaAs quantum dots

Polarization studies of InGaAs/GaAs quantum dots (QDs) synthesized in the submonolayer deposition mode (SMLQDs) on a singular GaAs (100) surface are carried out using photoluminescence spectroscopy. The influence of the effective In content in InGaAs SMLQDs and the effect of a wide-gap AlGaAs matrix on the optical anisotropy of the QDs are investigated. The highest degree (>15%) of optical anisotropy between the [011] and [0 $ \bar 1 $ 1] directions in the emission corresponding to the ground state of InGaAs/GaAs SMLQDs is observed for an effective In content of ～40%. The use of a wide-gap AlGaAs matrix resulted in an increase in the optical anisotropy of InGaAs SMLQDs by a factor of 1.5. It is found that vertical stacking of In(Ga)As/AlGaAs SMLQDs in the vertical-coupling mode (with spacer-layer thicknesses of 5–10 nm) leads to a further increase in the degree of optical anisotropy, which becomes as high as 25% on average. According to the data of transmission electron microscopy, the optical anisotropy of the ground-state photo-luminescence is predominantly caused by the anisotropy of the lateral dimensions of QDs in the [011] and [0 $ \bar 1 $ 1] directions.     

Microstructural Aspects of Nucleation and Growth of (In,Ga)As-GaAs(001) Islands with Low Indium Content

Molecular beam epitaxy growth of multilayer In _x Ga_1-x As/GaAs(001) structures with low indium content (x = 0.20–0.35) was studied by X-ray diffraction and photoluminescence in order to understand the initial stage of strain-driven island formation. The structural properties of these superlattices were investigated using reciprocal space maps, which were obtained around the symmetric 004 and asymmetric 113 and 224 Bragg diffraction, and ω/2θ scans with a high-resolution diffractometer in the triple axis configuration. Using the information obtained from the reciprocal space maps, the 004 ω/2θ scans were simulated by dynamical diffraction theory and the in-plane strain in the dot lattice was determined. We determined the degree of vertical correlation for the dot position (“stacking”) and lateral composition modulation period (LCM) (lateral ordering of the dots). It is shown that initial stage formation of nanoislands is accompanied by LCM only for [110] direction in the plane with␣a period of about 50 to 60 nm, which is responsible for the formation of a quantum wire like structure. The role of In _x Ga_1-x As thickness and lateral composition modulation in the formation of quantum dots in strained In _x Ga_1-x As/GaAs structures is discussed.     

Resonant Raman scattering and atomic force microscopy of InGaAs/GaAs multilayer nanostructures with quantum dots

The transition from two-dimensional (2D) pseudomorphic growth to the three-dimensional (3D) (nanoisland) growth in In_xGa_1?xAs/GaAs multilayer structures grown by molecular-beam epitaxy was investigated by atomic force microscopy, photoluminescence, and Raman scattering. The nominal In content x in In_xGa_1?xAs was varied from 0.20 to 0.50. The thicknesses of the deposited In_xGa_1?xAs and GaAs layers were 14 and 70 monolayers, respectively. It is shown that, at these thicknesses, the 2D-3D transition occurs at x ≥ 0.27. It is ascertained that the formation of quantum dots (nanoislands) does not follow the classical Stranski-Krastanov mechanism but is significantly modified by the processes of vertical segregation of In atoms and interdiffusion of Ga atoms. As a result, the In_xGa_1?xAs layer can be modeled by a 2D layer with a low In content (x < 0.20), which undergoes a transition into a thin layer containing nanoislands enriched with In (x > 0.60). For multilayer In_xGa_1?xAs structures, lateral alignment of quantum dots into chains oriented along the \([\overline 1 10]\) direction can be implemented and the homogeneity of the sizes of quantum dots can be improved.     

Optical Gain of V—groove Zn1—xCdxSe／ZnSe Quantum Wires

The subband structures, distributions of electron and hole wave functions, state density, optical gain spectra, and transparency carrier density of the V-groove Zn 1-x Cd x Se/ZnSe quantum wires are investigated theoretically using four band effective-mass Hamiltonian, which takes into account the effects of the valence band anisotropy and the band mixing. The biaxial strain effect for quantum wires is included in the calculation. The compressive strain in the Zn 1-x Cd x Se wire region increases the energy separation between the uppermost subbands. The optical gain with xy -polarized light is enhanced, while optical gain with z -polarized light is strongly decreased. The xy -polarized optical gain spectrum has a peak at around 2.541 eV, with the transparency carrier density of 0.75×10 18 cm -3 . The calculated results also show that the strain tends to increase the quantum confinement and enhance the anisotropy of the optical transitions.     

Resonances related to an array of InAs quantum dots and controlled by an external electric field

Photoluminescence of multilayer structures with InAs quantum dots grown in the p-n junction in GaAs by molecular-beam epitaxy is studied. Formation of vertical columns of quantum dots is verified by the data of transmission electron microscopy. It is shown that a natural increase in the size of quantum dots from layer to layer brings about their vertical coalescence at the upper part of a column. An unbalance of electronic levels caused by the enlargement of quantum dots was compensated by an external electric field, so that the resonance of ground electronic states in the column was attained. The onset of resonances was checked by the methods of steady-state and time-resolved photoluminescence. It is shown that, in the case of a resonance, the photoluminescence intensity and the radiative lifetime of excitons increase (up to 0.6–2 ns), while the time of tunneling of charge carriers becomes shorter (shorter than 150 ps). Outside the resonances, tunneling of electrons is appreciably enhanced owing to the involvement of longitudinal optical phonons. If only these phonons are involved, the time of nonresonance tunneling between quantum dots becomes shorter than the time of relaxation of charge carriers from the barrier (100 and 140 ps, respectively).     

Tuning the energy spectrum of InAs/GaAs quantum dots by varying the thickness and composition of the thin double GaAs/InGaAs cladding layer

It is shown that the ground state transition energy in quantum dots in heterostructures grown by atmospheric-pressure MOCVD can be tuned in the range covering both transparence windows of the optical fiber at wavelengths of 1.3 and 1.55 μm by varying the thickness and composition of the thin GaAs/In_xGa_1−x As double cladding layer. These structures also exhibit a red shift of the ground state transition energy of the In_xGa_1−x As quantum well (QW) as a result of the formation of a hybrid QW In_xGa_1−x As/InAs (wetting layer) between the quantum dots (QDs). The Schottky diodes based on these structures are characterized by an increased reverse current, which is attributed to thermally activated tunneling of electrons from the metal contact to QD levels. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 4, 2004, pp. 448–454. Original Russian Text Copyright ? 2004 by Karpovich, Zvonkov, Levichev, Baidus, Tikhov, Filatov, Gorshkov, Ermakov.     

Self-organization of quantum dots in multilayer InAs/GaAs and InGaAs/GaAs structures in submonolayer epitaxy

The experimental results of RHEED and scanning tunneling microscopy investigations of multilayer structures of InGaAs/GaAs quantum dots, obtained by submonolayer epitaxy on singular and vicinal GaAS (100) substrates, are reported. The results presented show that spatial ordering of nano-objects exists in multilayer structures for InAs and heteroepitaxial InGaAs layers. Fiz. Tekh. Poluprovodn. 33, 733–737 (June 1999)     

Lateral photoconductivity of Ge/Si heterostructures with Ge quantum dots

The spectral dependences of the lateral photoconductivity of Ge/Si heterostructures with Ge quantum dots are studied. The photoresponse of the Ge/Si structures with Ge nanoclusters is detected in the range 1.0–1.1 eV at T = 290 K, whereas the photocurrent in the single-crystal Si substrate is found to be markedly suppressed. This result can be attributed to the effect of elastic strains induced in the structure on the optical absorption of Si. At temperatures below 120 K, the heterostructures exhibit photosensitivity in the spectral range 0.4–1.1 eV, in which the Si single crystal is transparent. The photocurrent in this range is most likely due to the transitions of holes from the ground states localized in the quantum dots to the extended states of the valence band.     

Influence of composition and anneal conditions on the optical properties of (In, Ga)As quantum dots in an (Al, Ga)As matrix

The optical properties of structures containing InGaAs quantum dots in GaAs and AlGaAs matrices grown by molecular-beam epitaxy are investigated. It is shown that increasing the In content in the quantum dots has the effect of raising the energy of carrier localization and increasing the energy distance between the ground state and the excited states of carriers in the quantum dots. An investigation of the influence of postgrowth annealing on the optical properties of the structures shows that the formation of vertically coupled quantum dots and the use of a wide-gap AlGaAs matrix enhances the thermal stability of the structures. Moreover, high-temperature (830 °C) thermal annealing can improve the quality of the AlGaAs layers in structures with vertically coupled InGaAs quantum dots in an AlGaAs matrix. The results demonstrate the feasibility of using postgrowth annealing to improve the characteristics of quantum dot lasers. Fiz. Tekh. Poluprovodn. 33, 91–96 (January 1999)     

Optical properties of germanium monolayers on silicon

Photoluminescence and Raman spectra of thin germanium layers grown on silicon at a low temperature (250°C) have been studied. In structures of this kind, in contrast to those grown at high temperatures, luminescence from quantum wells is observed at germanium layer thicknesses exceeding ～9 monolayers (ML). With the development of misfit dislocations, the luminescence lines of quantum wells are shifted to higher energies and transverse optical (TO) phonons involved in the luminescence are confined to a quasi-2D germanium layer. Introduction of an additional relaxed Si_0.95Ge_0.05 layer into the multilayer Ge/Si structure leads to a substantial rise in the intensity and narrowing of the luminescence line associated with quantum dots (to 24 meV), which points to their significant ordering.     

Photoconductivity of Si/Ge multilayer structures with Ge quantum dots pseudomorphic to the Si matrix

Longitudinal photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots grown pseudomorphically to the Si matrix are studied. Lines of optical transitions between hole levels of quantum dots and Si electronic states are observed. This allowed us to construct a detailed energy-level diagram of electron-hole levels of the structure. It is shown that hole levels of pseudomorphic Ge quantum dots are well described by the simplest “quantum box” model using actual sizes of Ge islands. The possibility of controlling the position of the long-wavelength photosensitivity edge by varying the growth parameters of Si/Ge structures with Ge quantum dots is determined.     

Band gap opening and charge modulation in AlGaAs lateral structures obtained by organized epitaxy

Quantum wires superlattices have been grown on GaAs vicinal surfaces. Their electrical resistance anisotropy, their low-field magnetoresistance (for current flow perpendicular to the wires) and, in the magnetic quantum limit, the van Hove singularities of the Landau bands, and the very sudden enhancement of the spin splitting above a critical field, all demonstrate unambiguously that a strong low-disordered periodic lateral potential modulation can be achieved as a result of significant atomic ordering by the steps.     

Anisotropic transport properties of quantum dot arrays fabricated by the edge-defined nanowires

The anisotropic transport properties of quantum dot arrays fabricated by the nanoimprinted multilayer quantum wires have been investigated. The vertical transport characteristics are estimated by the carrier capture time from the electron reservoir to the low dimensional structures, which turned out to be longer than the spontaneous emission lifetime. The lateral transport characteristics are modeled after the quantum conductance between quantum dot arrays which are readily available from the miniband structures. The resulting lateral transport revealed that lateral quantum conductance can be minimized when the Fermi level is kept at the center of the miniband gap when the gap is larger than 4 kT.     

Electron emission from multilayer ensembles of vertically coupled InAs quantum dots in an <Emphasis Type="Italic">n</Emphasis>-GaAs matrix

Electron emission from multilayer arrays of vertically coupled InAs quantum dots into the n-GaAs matrix in Schottky-barrier structures (electron concentration n ≈ 2 × 10¹⁶ cm^?3) is studied by admittance spectroscopy. It is established that, in the temperature region below ～70 K, electron emission in a rate range of 3 × 10⁴–3 × 10⁶ s^?1 proceeds via thermally activated tunneling through intermediate virtual states. As the number of layers in the quantum dot array increases from three to ten, a decrease in the electron emission rate is observed.     

Nanoscale selective area epitaxy for optoelectronic devices

Self-assembled quantum dots have been heavily researched in recent years because of the potential applications to quantum electronic and optoelectronic devices they present. The non-uniformity and random ordering resulting from the self-assembly processes, however, are detrimental to potential applications, prohibiting the type of engineering control necessary for complex systems. The work presented in this document has sought to overcome the limitations of self-assembly by combining selective area epitaxy via MOCVD with high-resolution electron beam lithography to achieve lateral control over semiconductor structures at the nanometer scale. Two different structures are presented. The first is patterned quantum dots which improve on the uniformity and order of similar self-assembled quantum dots. The second is an entirely novel structure, the nanopore active layer, which demonstrates the potential for this process to extend beyond the constraints of self-assembly. Experimental and theoretical results for both structures are presented.     

Optical properties of hybrid quantum-well–dots nanostructures grown by MOCVD

The deposition of In _x Ga_1–x As with an indium content of 0.3–0.5 and an average thickness of 3–27 single layers on a GaAs wafer by metalorganic chemical vapor deposition (MOCVD) at low temperatures results in the appearance of thickness and composition modulations in the layers being formed. Such structures can be considered to be intermediate nanostructures between ideal quantum wells and quantum dots. Depending on the average thickness and composition of the layers, the wavelength of the photoluminescence peak for the hybrid InGaAs quantum well–dots nanostructures varies from 950 to 1100 nm. The optimal average In _x Ga_1–x As thicknesses and compositions at which the emission wavelength is the longest with a high quantum efficiency retained are determined.     

Optical spectroscopy of self-organized nanoscale hetero-structures involving high-index surfaces

Similar effects are responsible for self-organization of periodically corrugated surface structures and ordered dot arrays on crystal surfaces. Strain relaxation on facet edges may result in the appearance of periodically corrugated surfaces for lattice-matched growth. Strain relaxation on facet edges and island interaction via the strained substrate act as driving forces for the formation of ordered arrays of uniform, strained lattice-mismatched islands on a crystal surface. A pseudoperiodic square lattice is manifested for the InAs-GaAs(100) system. Less ordered dots are formed on the GaAs(100) surface with a 4 monolayer GaSb deposition. New experimental methods are applied for the characterization of faceted nanoscale structures. For GaAs-AlAs multilayer structures grown on (311)A substrates, interface corrugation results in optical anisotropy of the same sign as expected from the low symmetry growth direction, making the main origin of the anisotropy unclear. Our quantitative optical reflectance and reflectance anisotropy studies show that the interface corrugation plays an important role for thin (less than 4 nm) GaAs layers. Mesa arrays from samples with InAs quantum dots grown on (100) surface are fabricated. The photoluminescence intensity is found to depend only weakly on the mesa size (1000 nm to 250 nm). The estimated electron-hole pair capture time into the InAs dot at room temperature is less than 1 ps. We also found a weak dependence of the threshold current density on the deep mesa stripe width (down to 3 μm) in the case of room temperature operated quantum dot injection lasers.