Site-controlled growth of InP/GaInP quantum dots on GaAs substrates

A technology platform for the epitaxial growth of site-controlled InP quantum dots (QDs) on GaAs substrates is presented. Nanoholes are patterned in a GaInP layer on a GaAs substrate by electron beam lithography and dry chemical etching, serving as QD nucleation centers. The effects of a thermal treatment on the structured surfaces for deoxidation are investigated in detail. By regrowth on these surfaces, accurate QD positioning is obtained for square array arrangements with lattice periods of 1.25?μm along with a high suppression of interstitial island formation. The optical properties of these red-emitting QDs (λ?～?670?nm) are investigated by means of ensemble- and micro-photoluminescence spectroscopy at cryogenic temperatures.     

Optical anisotropy of InAs quantum dots

The optical anisotropy of InAs quantum dots (QDs) synthesized in the regime of either continuous or submonolayer deposition on a singular GaAs(100) surface have been studied using polarized photoluminescence measurements. It is established that an isolated array of QDs formed in a continuous deposition regime possesses a weak (<1–2%) optical anisotropy, whereas the vertical matching (coupling) of such QDs via less than 15-nm-thick spacer layers leads to an 8% linear polarization of PL along the [0$ bar 1 $ bar 1 1] crystallographic direction. QDs formed in the regime of submonolayer deposition exhibit a strong (17–20%) anisotropy of emission from the ground and excited states of QDs in the same crystallographic direction.     

Fabrication of InAs quantum dots on silicon

Reflection high-energy electron diffraction and scanning tunneling microscopy have been used to demonstrate for the first time that InAs quantum dots may be fabricated directly on Si(100) by molecular beam epitaxy. It is shown that heteroepitaxial growth in an InAs/Si system takes place by the Stranski-Krastanow mechanism and the surface morphology depends strongly on the substrate temperature. Pis’ma Zh. Tekh. Fiz. 24, 10–15 (April 26, 1998)     

Stacked layers of InAs self-assembled quantum dots

Carrier injection and subsequent radiative recombination in two vertically stacked (but electronically only weakly coupled) layers of InAs/GaAs self-assembled quantum dots (SADs) embedded in the intrinsic region of a double hetero p-i-n structure was investigated by electroluminescence (EL) spectroscopy in the temperature range from 20 to 300 K. In such structures the filling of the SADs by charge carriers strongly depends not only on the applied voltage, but also on the relative position of the SAD layers within the i-region and on the temperature. The experimental data provide evidence of the dominant role of hole dynamics in the recombination processes in the stacks of SADs. The difference of the electronic structure of the SADs in the top and bottom layers is reflected by independent contributions of the two quantum dot layers to the electroluminescence from the SADs. The possibility to tune the emission spectra by varying the thickness of the GaAs layer between neighbouring SAD layers and by using the indium flush technique is demonstrated.     

Self-aligned charge read-out for InAs nanowire quantum dots

A highly sensitive charge detector is realized for a quantum dot in an InAs nanowire. We have developed a self-aligned etching process to fabricate in a single step a quantum point contact in a two-dimensional electron gas and a quantum dot in an InAs nanowire. The quantum dot is strongly coupled to the underlying point contact that is used as a charge detector. The addition of one electron to the quantum dot leads to a change of the conductance of the charge detector by typically 20%. The charge sensitivity of the detector is used to measure Coulomb diamonds as well as charging events outside the dot. Charge stability diagrams measured by transport through the quantum dot and charge detection merge perfectly.     

Optically-induced charge storage in self-assembled InAs quantum dots

We present results on spectrally resolved photo-resistance studies of optically-induced charge storage effects in self-organized InAs quantum dots (QDs). The stored charge can be detected and erased electrically. The investigated structure designed for electron or hole storage in the QDs consists of a modulation doped two-dimensional channel which was grown on top of a layer of InAs QDs, separated by an asymmetric tunnel barrier. Our results show that optical QD charging with spectral resolution provides information on the charging dynamics and on the quantity and spectral dependence of stored charges in the QDs. This is a novel technique by which QD excitation spectra can be studied. Spectrally resolved storage effect measurements on electrons as well as on holes allowed to investigate thermal redistribution of carriers in the quantum dot layer. It was found that only at low temperatures carriers can be stored selectively over long time scales in the InAs QDs. The charge storage effect is observable for several hours at temperatures up to 170 K, for several seconds up to 250 K due to an increase in thermal emission of stored charges.     

Morphology and composition of InAs/GaAs quantum dots

Surface compositional maps of self-organized InAs/GaAs quantum dots were obtained with laterally resolved photoemission spectroscopy. We found a surface In concentration of about 0.85 at the center of the islands which decreases to 0.75 on the wetting layer. Comparison with concentration values found in the core of similar dots suggests a strong In segregation on the topmost surface layers of the dots and on the surrounding wetting layer. Furthermore, the morphological properties of the dots such as size and density have been measured with plan-view transmission electron microscopy and low energy electron microscopy.     

Level structure of InAs quantum dots in two-dimensional assemblies

The electronic level structure of colloidal InAs quantum dots (QDs) in two-dimensional arrays, forming a QD-solid system, was probed using scanning tunneling spectroscopy. The band gap is found to reduce compared to that of the corresponding isolated QDs. Typically, the electron (conduction-band) ground state red shifts more than the hole (valence-band) ground state. This is assigned to the much smaller effective mass of the electrons, resulting in stronger electron delocalization and larger coupling between electron states of neighboring QDs compared to the holes. This is corroborated by comparing these results with those for InAs and CdSe nanorod assemblies, manifesting the effects of the electron effective mass and arrangement of nearest neighbors on the band gap reduction. In addition, in InAs QD arrays, the levels are broadened, and in some cases their discrete level structure was nearly washed out completely and the tunneling spectra exhibited a signature of two-dimensional density of states.     

Scanned probe imaging of quantum dots inside InAs nanowires

We show how a scanning probe microscope (SPM) can be used to image electron flow through InAs nanowires, elucidating the physics of nanowire devices on a local scale. A charged SPM tip is used as a movable gate. Images of nanowire conductance versus tip position spatially map the conductance of InAs nanowires at liquid-He temperatures. Plots of conductance versus backgate voltage without the tip present show complex patterns of Coulomb-blockade peaks. Images of nanowire conductance identify their source as multiple quantum dots formed by disorder along the nanowire--each dot is surrounded by a series of concentric rings corresponding to Coulomb blockade peaks. An SPM image locates the dots and provides information about their size. In this way, SPM images can be used to understand the features that control transport through nanowires. The nanowires were grown from metal catalyst particles and have diameters approximately 80 nm and lengths 2-3 microm.     

Nucleation sequence of InAs quantum dots on cross-hatch patterns

InAs quantum dots (QDs) are grown via molecular beam epitaxy on cross-hatch pattern (CHP) templates that result from lattice-mismatched epitaxy of In(x)Ga(1-x)As on (100)-GaAs substrates. Growth of InAs on low-(x = 0.10) and medium-(x = 0.13) mismatch CHPs with InAs thickness grading from sub- to beyond critical thickness show different stages of QD nucleation that is dictated mainly by surface steps. Tangential surface stress fields arising from the buried network of (110) misfit dislocations (MDs) at the InGaAs/GaAs interface are simulated in two dimensions and found to have a direct correlation to QD height at various locations, implying sequential QD nucleation at the surface intersection of the glide plane of dislocation T-section, cross-hatch intersection, threading dislocation, [1-10] MD line, and [110] MD line, followed by nucleation on the flat areas. Deviations from this nominal sequence is possible due to material anisotropy and are accounted for in the stress calculation by dislocation-specific scaling factors.     

Self-assembling InAs and InP quantum dots for optoelectronic devices

Stranski–Krastanov growth in molecular beam epitaxy allows the preparation of self assembling InAs and InP quantum dots on GaAs and Ga_0.52In_0.48P buffer layers, respectively. InAs dots in GaAs prepared by slow growth rates and low temperature overgrowth provide intense photoluminescence at the technologically important wavelength of 1.3 μm at room temperature. Strain induced vertical alignment, size modification and material interdiffusion for stacked dot layers are studied. A blue shift of the ground state transition energy is observed for the slowly deposited stacked InAs dots. This is ascribed to enhanced strain driven intermixing in vertically aligned islands. For very small densely stacked InP and InAs dots the reduced confinement shift causes a red shift of the ground state emission. The InP quantum dots show intense and narrow photoluminescence at room temperature in the visible red spectral range. First InP/Ga_0.52In_0.48P quantum dot injection lasers are prepared using threefold stacked InP dots. We observe lasing at room temperature in the wavelength range between 690–705 nm depending on the size of the stacked InP dots.     

Deep levels induced by InAs/GaAs quantum dots

Self-organised InAs/GaAs quantum dots (QDs) were formed by molecular beam epitaxy using the Stranski–Krastanov growth mode. Deep-level transient spectroscopy as well as secondary ion mass spectrometry have been used to characterise structures containing the QDs. DLTS depth profiling procedures indicate that deep level-related defects are localised in GaAs in the vicinity of the QD plane. For the first time, we report the presence of a deep level-related trap with an extremely high thermal activation energy of E_c − 1.03 eV. An electron trap at E_c − 0.78 eV can be identified as the well-known level related to the EL2 family. We conclude that a third trap revealed at E_c −0.57 eV is the familiar PL killer related to the intrinsic point defect-oxygen complex. The latter is confirmed by results of the SIMS study, which indicates that the amount of oxygen accumulated at the InAs/GaAs heterointerface is increased. This paper demonstrates that the EL2 and oxygen-related deep-level centers occur by the presence of InAs/GaAs QDs. We present the hypothesis that deep states could be a factor limiting the efficiency of QD-based devices.     

内嵌InAs量子点的场效应晶体管特性研究

研究了内嵌InAs量子点的异质结场效应晶体管在室温和低温下的电学特性,获得了量子点影响下器件的输出特性曲线。在室温下,通过分别测试在近红外光照和量子点充电条件下器件的Ⅰ-Ⅴ特性,证明了量子点通过类似纳米悬浮栅的作用,对邻近沟道的二维电子气施加影响。在低温下观察到器件漏电流出现负微分电导现象。这一现象可由2DEG和量子点之间的共振隧穿来解释。这些结果提供了一种新的操作传统场效应晶体管的方法,并有望制成新型量子点存储器。     

Statistics of electron emission from InAs/GaAs quantum dots

A theoretical treatment for thermal and tunneling emission of electrons from InAs/GaAs quantum dots is performed to achieve “effective emission rates” corresponding to experimentally obtained quantities. From these results, Arrhenius graphs are calculated using parameter values for quantum dots with 20/10 nm base/height dimension. Emission from the electron s shell as direct transitions, as two-step transitions from the s to the p shell, as thermal transitions from s to p followed by tunneling and as direct tunneling from the s and the p shell to the GaAs conduction band is taken into account. Due to the varying emission possibilities, Arrhenius graphs appear with complicated shapes depending on quantities originating from structural and electronic properties of the quantum dots.     

Transformation of self-assembled InAs/InP quantum dots into quantum rings without capping

Transformation of self-assembled InAs quantum dots (QDs) on InP(100) into quantum rings (QRs) is studied. In contrast to the typical approach to III--V semiconductor QR growth, the QDs are not capped to form rings. Atomic force micrographs reveal a drastic change from InAs QDs into rings after a growth interruption in tertiarybutylphosphine ambient. Strain energy relief in the InAs QD is discussed and a mechanism for dot-to-ring transformation by As/P exchange reactions is proposed.     

Defects in nanostructures with ripened InAs/GaAs quantum dots

InAs/GaAs quantum dot (QD) structures were grown by molecular beam epitaxy (MBE) with InAs coverages θ continuously graded from 1.5 ML to 2.9 ML. A critical coverage of 2.23 ML is found, above which the islands undergo ripening, which causes a fraction of quantum dots to increase in size and to eventually relax through the formation of pure, edge-type misfit dislocations which propagate towards the surface in the form of V-shaped defects. Concomitant with ripening, extended-defect related traps with activation energies of 0.52 and 0.84 eV were observed, and regarded as the cause of the significant worsening of the optical and electrical properties in high coverage structures. Their relationship with the observed dislocations is discussed.     

InSb/InAs quantum dots grown by liquid phase epitaxy

The first original results on the growth of quantum dots (QDs) in the InSb/InAs system by liquid phase epitaxy (LPE) are reported. The density and dimensions of QDs were studied by methods of scanning probe microscopy and atomic force microscopy. The surface density, shapes, and dimensions of LPE-grown nanoislands depend on the growth conditions (temperature, cooling rate, and solution melt-substrate contact time). In the interval of temperatures T = 420–445°C, homogeneous arrays of InSb quantum dots on InAs(100) substrates were obtained with an average height of H = 3.4 ± 1nm, a radius of R = 27.2 ± 7.5 nm, and a density of up to 1.9 × 10¹⁰ cm^?2.     

Few electron double quantum dots in InAs/InP nanowire heterostructures

We report on fabrication of double quantum dots in catalytically grown InAs/InP nanowire heterostructures. In the few-electron regime, starting with both dots empty, our low-temperature transport measurements reveal a clear shell structure for sequential charging of the larger of the two dots with up to 12 electrons. The resonant current through the double dot is found to depend on the orbital coupling between states of different radial symmetry. The charging energies are well described by a capacitance model if next-neighbor capacitances are taken into account.     

Comparative study of low temperature growth of InAs and InMnAs quantum dots

The evolution of InAs and In(0.85)Mn(0.15)As quantum dots grown at 270?°C is studied as a function of coverage. We show that, in contrast to what occurs at high temperature, the two-dimensional to three-dimensional transition is not abrupt but rather slow. This is due to the finding that part of the deposited material also contributes to the wetting layer growth after quantum dot formation. This aspect is particularly accentuated in In(0.85)Mn(0.15)As deposition. The Voronoi area analysis reveals a significant spatial correlation between islands.