Shigeru Kohmoto

Site-controlled self-organization of individual InAs quantum dots by scanning tunneling probe-assisted nanolithography

A nanometer-scale site-control technique for individual InAs quantum dots (QDs) has been developed by using scanning tunneling microscope (STM) -assisted nanolithography and self-organizing molecular-beam epitaxy. We find that nanometer-scale deposits can be created on a GaAs surface by applying voltage and current pulses between the surface and a tungsten probe of the STM, and that they act as “nanomasks” on which GaAs does not grow directly. Accordingly, subsequent thin GaAs growth produces GaAs nanoholes above the deposits. By supplying 1.1 ML InAs on this surface, QDs are self-organized at the hole sites, while hardly any undesirable Stranski–Krastanov QDs are formed in the flat surface region. Using this technique with nanometer precision, a QD pair with 45 nm pitch is fabricated.

Site-controlled InAs single quantum-dot structures on GaAs surfaces patterned by in situ electron-beam lithography

We studied a site-control technique for InAs quantum dots (QDs) on GaAs substrates using a combination of in situ electron-beam (EB) lithography and self-organized molecular-beam epitaxy. In small, shallow holes formed on prepatterned mesa structures by EB writing and Cl2 gas etching, QDs were selectively formed, without any formation on the flat region between the patterned holes. The density of the QDs in each hole was dependent on the hole depth, indicating that atomic steps on the GaAs surfaces act as migration barriers to In adatoms. In an array of holes including 5–6 monolayer steps, a single QD was arranged in each hole.

SITE CONTROL OF SELF-ORGANIZED INAS DOTS ON GAAS SUBSTRATES BY IN SITU ELECTRON-BEAM LITHOGRAPHY AND MOLECULAR-BEAM EPITAXY

We studied a site-control method for self-organized InAs quantum dots on GaAs substrates by a combination of in situ electron-beam (EB) lithography and molecular-beam epitaxy (MBE) using an ultrahigh-vacuum multichamber system. Small and shallow holes were patterned on a MBE-grown GaAs (001) surface by in situ EB writing and Cl2-gas etching. When more than a 1.4 monolayer of InAs was applied to the patterned surface, In(Ga)As dots were preferentially self-organized in the holes while dot formation around the holes was sufficiently suppressed. When we further increased the amount of InAs, the dots enlarged remarkably, presumably due to a stress-relaxation effect.

Spontaneous lateral alignment of In0.25Ga0.75As self-assembled quantum dots on (311)B GaAs grown by gas source molecular beam epitaxy

Spontaneous lateral alignment was observed in InGaAs quantum dots formed by self-assembly on (311)B GaAs by gas source molecular beam epitaxy. The alignment occurred in a direction inclined about 60° from the [011] direction on a (3-11) [(311)B] surface. A typical base diameter of the dots was about 120±10 nm. The heights varied from 3 to 13 nm as the nominal thickness of the InGaAs layer increased from 4 to 8 nm. The formation mechanism for the alignment is studied based on the growth thickness dependence of the dot structures. A photoluminescence linewidth of 24 meV was obtained from 9 nm high dots at 77 K, indicating the formation of a uniform dot structure.

Site-controlled self-organization of InAs quantum dots

In situ site-control techniques for self-organized InAs quantum dots (QDs) have been developed using an electron beam (EB) and a scanning tunneling microscope (STM) probe combined with molecular beam epitaxy. In the in situ EB-assisted process, InAs dots are preferentially formed in shallow, sub-μm-size GaAs holes with the InAs supply. We find that the specific slope of a hole acts as a favorable site for dot formation. In the in situ STM probe-assisted process, the size and pitch of the holes are considerably reduced into nanoscale. InAs QDs are then self-organized only at the hole sites due to strain-induced selective nucleation. Using this process, two- and three-dimensional QD arrays are fabricated with nanometer precision.

Chlorine-Based Dry Etching of III/V Compound Semiconductors for Optoelectronic Application.

Chlorine-based dry etching of III/V compound semiconductors for optoelectronic applications has been reviewed. The advantages of the ultrahigh-vacuum (UHV)-based electron cyclotron resonance (ECR)-plasma reactive ion beam etching (RIBE) over conventional RF-plasma reactive ion etching (RIE) were emphasized as the capability to use carbon-free, chlorine (Cl2) gas plasmas, controllability of ion energies and compatibility with other UHV-based chambers such as a molecular beam epitaxy (MBE) chamber. The RIBE technique was shown to exhibit excellent laser diode performances, such as extremely low threshold-current, high polarization-controllability and a lifetime of more than 3000 h for structures with more than 1-µm-wide etched-mesa width. The degree of etching-induced damage was evaluated in terms of the nonradiative surface recombination velocity Sr and the possibilities of practical applications of the dry-etched devices were discussed using the Sr values.

InAs-Dot/GaAs Structures Site-Controlled by in situ Electron-Beam Lithography and Self-Organizing Molecular Beam Epitaxy Growth

A novel site-control technique for InAs dot fabrication on GaAs has been demonstrated by a combination of in situ electron-beam (EB) lithography and self-organizing molecular beam epitaxy (MBE) using an ultrahigh-vacuum multichamber system. On an MBE-grown GaAs (001) surface, shallow holes of submicron size were patterned by in situ EB writing and Cl2 gas etching. By supplying more than 1.4 monolayer of InAs onto the patterned surface, In(Ga)As dots were preferentially self-organized in the holes, while dot formation around the holes was sufficiently suppressed, due to the selectivity of In atom incorporation in the (111)B-like slope in the hole. This indicates the usefulness of such a technique in fabricating arbitrarily arranged quantum-dot structures.

Flexible heat-flow sensing sheets based on the longitudinal spin Seebeck effect using one-dimensional spin-current conducting films

Heat-flow sensing is expected to be an important technological component of smart thermal management in the future. Conventionally, the thermoelectric (TE) conversion technique, which is based on the Seebeck effect, has been used to measure a heat flow by converting the flow into electric voltage. However, for ubiquitous heat-flow visualization, thin and flexible sensors with extremely low thermal resistance are highly desired. Recently, another type of TE effect, the longitudinal spin Seebeck effect (LSSE), has aroused great interest because the LSSE potentially offers favourable features for TE applications such as simple thin-film device structures. Here we demonstrate an LSSE-based flexible TE sheet that is especially suitable for a heat-flow sensing application. This TE sheet contained a Ni0.2Zn0.3Fe2.5O4 film which was formed on a flexible plastic sheet using a spray-coating method known as “ferrite plating”. The experimental results suggest that the ferrite-plated film, which has a columnar crystal structure aligned perpendicular to the film plane, functions as a unique one-dimensional spin-current conductor suitable for bendable LSSE-based sensors. This newly developed thin TE sheet may be attached to differently shaped heat sources without obstructing an innate heat flux, paving the way to versatile heat-flow measurements and management.

Effects of lateral quantum dot pitch on the formation of vertically aligned InAs site-controlled quantum dots

We discuss the effects of lateral quantum dot pitch on the formation of vertically aligned InAs site-controlled quantum dots (SCQDs) separated by thin spacers on GaAs substrates. Bilayers of InAs SCQD square arrays of various quantum dot-array pitches (100 nm, 75 nm, 50 nm, and 40 nm) are fabricated based on nanometer-scale site-control techniques and self-organizing epitaxy. In situ scanning tunneling microscope observations reveal that the vertical pairing probability between the two layers of SCQDs decreases with decreasing pitch, with a dramatic reduction when the pitch falls below 75 nm. Moreover, room-temperature μ-photoluminescence measurements show that the linewidth of the bilayers of vertically aligned SCQDs increases with declining pitch. Again, we found a remarkable increase in photoluminescence linewidth when the pitch falls below 75 nm. The observed results are attributed to lateral interactions between elastic strain fields induced by the first layer of buried InAs SCQDs.

Smooth and vertical InP reactive ion beam etching with Cl2 ECR plasma

Smooth and vertical InP reactive ion beam etching has been achieved with electron cyclotron resonance Cl2 plasma at high ion energy (≥900 eV), high temperature (230°C) and relatively low Cl2 pressure (~10-4 Torr). Smooth etching of an InP system by Cl2 plasma has often been reported as difficult compared to that of the GaAs system due to low volatility of reactive products such as InClx. In the present work, precise control of incident ion energy and Cl2 pressure contributed to the improvement of both the vertical profile and bottom smooth surface under high substrate temperature (~200°C). Vertical profiles were easily achieved even at high temperatures by varying the Cl2 pressure. While etching conditions suitable for vertical wall-formation were maintained, surface morphology was drastically improved by increasing ion energy above 900 eV and the bottom roughness became less than 100 nm at 1450 eV.