Masashi Akabori

Selective area MOVPE growth of two-dimensional photonic crystals having an air-hole array and its application to air-bridge-type structures

Abstract We describe a novel fabrication method of two-dimensional photonic crystals (2DPCs) by using selective area metal-organic vapor-phase epitaxial (SA-MOVPE) growth on (1 1 1) B masked substrates and its application to air-bridge-type structures with selective wet-chemical etching. This method is very promising as a way to form semiconductor 2DPCs because a PC membrane without process-induced damages can be obtained. To form uniform PC membranes having an air-hole array, we investigated the properties of SA-MOVPE regrowth on sub-micron-pitch masked substrates. We found that the optimum growth conditions strongly depend on the width of the GaAs opening as well as the pitch of the 2DPCs. An air-bridge structure having 250-nm-pitch 2DPC was also demonstrated by using a combination of SA-MOVPE regrowth and selective wet-chemical undercut etching.

Fabrication of semiconductor Kagome lattice structure by selective area metalorganic vapor phase epitaxy

Artificial two-dimensional semiconductor Kagome lattice structures formed by quantum wires can show ferromagnetism when the flatband is half filled, even though it does not have any magnetic elements. Experimental realization of such a Kagome lattice structure is reported. The structure, with different pattern periods, was formed with GaAs quantum wires by selective area metalorganic vapor phase epitaxy on GaAs (111)B substrates. To overcome the lateral overgrowth and to improve the shape of smaller period pattern, flow rate modulation epitaxy was employed and a GaAs Kagome lattice structure with 1 μm period was effectively grown.

Formation of AlxGa1-xAs periodic array of micro-hexagonal pillars and air holes by selective area MOVPE

Abstract A two-dimensional (2D) periodic array having air/semiconductor interfaces can be applied to photonic crystals (PCs), which are expected to control spontaneous emission and optical transports in the next-generation devices. In this paper, we report on the selective area metal-organic vapor phase epitaxial (SA-MOVPE) growth of a Al x Ga 1− x As 2D periodic array on a GaAs (1xa01xa01)B substrate for application to 2DPCs having GaAs/AlGaAs heterostructures. Al x Ga 1− x As ( x =0, 0.25 and 0.50) growth was carried out on triangular lattice array of hexagonal GaAs openings and hexagonal SiN x masks. A uniform Al 0.50 Ga 0.50 As hexagonal pillar array and a GaAs hexagonal air-hole array with a 1xa0μm-period were successfully obtained. The important growth parameter for uniform 2DPC structure formation by SA-MOVPE was clarified. Furthermore, we describe the successful demonstration of a 400xa0nm-period pillar array and an air-hole array, which corresponds to the optical communication wavelength λ =1.3–1.55xa0μm. The results indicate that SA-MOVPE method is very promising for the formation of uniform semiconductor 2DPCs without the occurrence of process-induced damages.

Dependence on In content of InxGa1-xAs quantum dots grown along GaAs multiatomic steps by MOVPE

Abstract We fabricated self-organized In x Ga 1− x As quantum dots on GaAs(0xa00xa01) vicinal surfaces having multiatomic steps by using metalorganic vapor phase epitaxy. The dots preferentially grew along the GaAs multiatomic step edges. We investigated the density and uniformity of the dots as a function of the substrate misorientation angle, the layer thickness, the In content, and the growth temperature. Observation of grown surfaces by atomic force microscopy showed that the dot formation depended strongly on the In content and layer thickness. In general, for lower In contents and/or thinner layers, no dot formation was observed because the interface stresses were insufficient for three-dimensional island growth. For higher In content and/or thicker layers, high-density dots formed on the GaAs multiatomic steps. However, these dots were not always uniform: they included small dots (∼40xa0nm) and large islands (>100xa0nm). Under optimum growth conditions, that is, 3.2-monolayer In 0.8 Ga 0.2 As growth, we obtained high-density (6.2×10 10 /cm 2 ) and highly uniform (10–25xa0nm) small quantum dots along the GaAs multiatomic step edges.

Formation and characterization of modulated two-dimensional electron gas on GaAs multiatomic steps grown by metalorganic vapor phase epitaxy

We used multiatomic steps spontaneously formed on vicinal surfaces during metalorganic vapor phase epitaxial (MOVPE) growth to introduce lateral potential modulation into two-dimensional electron gas (2DEG) and to realize a novel lateral surface superlattice (LSSL) with period of around 70 nm. Their electron transport properties were investigated at low temperatures and large anisotropies in the mobility and transconductance characteristics were found for the current parallel and perpendicular to the multiatomic steps. The potential modulation was further applied to form multiple quantum dots and tunnel junctions in combination with the squeezing of the width of the channel defined across the multiatomic steps. The Coulomb blockade type conductance oscillations were observed at low temperature transport measurement.

Large positive magnetoresistance in periodically modulated two-dimensional electron gas formed on self-organized GaAs multiatomic steps

Abstract We study magneto-transport properties of two-dimensional electron gas (2DEG) subjected to a periodic potential with a period of 60–70 nm, where the potential modulation is realized by periodically corrugated InGaAs channel layer formed on self-organized GaAs multiatomic steps. We find a large positive magnetoresistance (MR) when the current is perpendicular to the potential modulation, while such positive resistance is not observed for the current parallel to the potential modulation. It is also found that the amplitude of MR oscillation for current perpendicular to the potential modulation is extremely large over 3.5 T, and is modulated in 2–3.5 T. In addition, we also find the anisotropy in electron mobility. The results can be explained by the large potential modulation in the lateral surface superlattice (LSSL) and associating modulation of Landau subbands in LSSL.

Selective-area MOVPE fabrication of GaAs hexagonal air-hole arrays on GaAs(111)B substrates using flow-rate modulation mode

GaAs hexagonal air-hole arrays fabricated by selective-area metal-organic vapour phase epitaxy (SA-MOVPE) on patterned GaAs(111)B substrates are promising for applications to hexagonal air-hole-type two-dimensional photonic crystal (2D-PhC) slabs, because the grown structures exhibit smooth flat surfaces surrounded by crystal facets. In this paper, we describe SA-MOVPE carried out under various gas-flow sequences in order to reduce the growth temperature, and to obtain uniform air-hole arrays without lateral over-growth (LOG). We found that the growth rate in the pattern region and LOG were closely related to the effective As coverage and the desorption rate of the source materials. By optimizing SA-MOVPE, we obtained uniform hexagonal air-hole arrays with almost no LOG for arrays with 500?400?nm periodicity using alternate supply of the source materials (flow-rate modulation epitaxy mode). Finally, we successfully fabricated air-bridge-type hole arrays using selective etching of a sacrificial layer for vertical confinement of light in 2D-PhCs.

Effect of growth interruption time and growth temperature on the natural formation of InGaAs/AlGaAs quantum disk structures on GaAs (311)B substrates

We studied the effect of growth interruption time and growth temperature on the natural formation of InGaAs/AlGaAs quantum disk structures grown on high-index GaAs (311)B substrates by metalorganic vapor phase epitaxy at about 800u200a°C. The InGaAs/AlGaAs quantum disks were uniformly grown only with growth interruption time of 0 s. With increasing growth interruption time, the self-organized growth mode disappeared with the growth interruption time of 20 s. During growth interruption, strain for the natural formation of quantum disks was released by intermixing between In and Al. With decreasing growth temperature, the quantum disk did not show a hexagonal shape with clear facets. The quantum disks were formed at temperature range between 700 and 830u200a°C. The self-organization appeared through strain-driven mass transport. The strain-driven mass transport seems to have two characteristic lengths such as the amplitude and the periodicity determined by In composition and the InGaAs layer thickness from electron...

Theoretical and experimental investigation of an electron interference device using multiatomic steps on vicinal GaAs surfaces

Abstract We propose a new, lateral surface superlattice (LSSL) type of electron interference devices, where the period of LSSL is typically 60 nm, by utilizing multiatomic steps on a vicinal GaAs(0 0 1) surface. Conductivity of the device is theoretically studied by taking the effect of randomness in the LSSL into account. We also investigate its drain and transconductance characteristics experimentally at low temperatures, and found clear oscillations in g m − V G characteristics, which were ascribed to the electron interference effect.

The initial stage of InGaAs growth by MOVPE on multiatomic-stepped GaAs structures

We investigate the formation of self-organized InGaAs quantum wires (QWRs) on coherent multiatomic-stepped GaAs that had been grown on GaAs vicinal substrates by metal-organic vapor phase epitaxy. The key issues in the formation of InGaAs QWRs are the uniformity of the multiatomic steps and the large modulation in the lateral thickness of InGaAs on multiatomic-stepped GaAs structures. We use images obtained by atomic force microscopy and cross-sectional images of InGaAs layers obtained by transmission electron microscopy to investigate the surface morphology of InGaAs layers on multiatomic-stepped GaAs structures. As a result, we found that in the initial stages of InGaAs growth, Ga and In atoms preferentially attach themselves to the bottom edges of the multiatomic steps, and multiatomic-stepped structures with their own characteristic period and then reappear. These results suggest that the Schwoebel barrier, which causes the step bunching, may be decreased by the change of surface reconstruction after the introduction of small amount of In atoms to GaAs surfaces in the initial stage of growth. Furthermore, multiatomic-stepped structures formed during the growth of InGaAs layers have different periods from those on the underlying GaAs layer. The difference between the multiatomic-stepped structures on GaAs and InGaAs is thought to be caused by the difference of Schwoebel barriers between these two stepped surfaces.