Masato Ohmori

Formation of Ultra-low Density (

We have studied a self-assembled growth technique to form ultra-low density InAs quantum dots on GaAs by molecular beam epitaxy. After growing a GaAs layer under a particular condition, we have deposited an InAs layer of far less than the critical thickness and performed an annealing process. By optimizing these process steps, the density of dots is successfully controlled over a wide range from 104 to 108 cm-2, at which the average interdot distance gets as long as 100 µm. Photoluminescence spectra of low dot density samples have shown discrete single-dot features even under a macroscopic optical excitation. These dots are found to be formed preferentially on GaAs mounds especially when the dot density is around 2.5×105 cm-2.

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Photoluminescence is observed from CdSe nanocrystal quantum dots filling in the core of a 1 m long photonic bandgap fiber. Photoluminescence at the peak wavelength of 609 nm is observed with 532 nm, 10 mW pumping

cm-2) Self-Organized InAs Quantum Dots on GaAs by a Modified Molecular Beam Epitaxy Method

Photoluminescence is observed from PbSe nano-crystal quantum dots at 1550 nm wavelength filling in the core of a 1 m long photonic bandgap fiber. Photoluminescence at the peak wavelength of 1554 nm is observed with 1535 nm, 10 mW pumping.

Observation of photoluminescence of semiconductor nanocrystal quantum dots in the core of photonic bandgap fiber

Photoluminescence (PL) characteristics have been systematically studied in a set of novel InAs quantum dot (QD) systems, where the average interdot distance is set in the range between 2 and 20 μm by controlling the self-assembled growth on GaAs. By investigating the ratio of the PL intensity of QDs to that of the wetting layer (WL), the diffusion of excitons in the WL and their subsequent trapping by QDs are examined. It is found that the diffusion length LD of excitons is about 0.45 μm at 11 K, but increases with temperature, reaching 4 μm at 60 K.

Photoluminescence of semiconductor nanocrystal quantum dots at 1550 nm wavelength in the core of photonic bandgap fiber

AlGaInAs nanowires or rods of 20–40 nm diameter were formed by depositing an AlGaAs/GaAs/InAs short-period superlattice onto self-organized InAs quantum dots on GaAs. The In content is found to be substantially higher in the rods than in the superlattice matrix, implying that rods serve as favorable paths for electrons. Transport properties measured at 4.2 K on a sample where 79-nm-long rods are buried between n+-GaAs electrodes show that rods are indeed far more conductive than their matrix barrier. Photoluminescence study has indicated that photogenerated carriers recombine mostly in the seed dot portion of rods.

Diffusion process of excitons in the wetting layer and their trapping by quantum dots in sparsely spaced InAs quantum dot systems

We observed photoluminescence (PL) spectra in 735 and 535 nm region from CdTe and CdSe colloidal quantum dots (QDs) filling in the air hole of the photonic bandgap fiber (PBF). We show the PL of these QDs is efficiently transmitted in the PBF.

Formation of InAs/AlGaAs/GaAs Nanowire Structures by Self-Organized Rod Growth on InAs Quantum Dots and Their Transport Properties

We fabricate GaAs-based triangular barrier photodiodes (TBPs), in which type-II GaSb quantum dots (QDs) are embedded in the vertex part of their triangular barriers. Their current–voltage characteristics and photo-responses are studied at low temperatures to show that GaSb QDs enhance the number and lifetime of photo-generated holes that are trapped by QDs in the barrier, resulting in the increase in the electron current around positively charged QDs. An extremely high responsivity of 109 A/W is achieved.

Photoluminescence spectroscopy of semiconductor colloidal quantum dots in the photonic bandgap fiber

To make power devices with lower losses and higher power capabilities, superjunctions (SJs), based on GaN, GaAs, and related materials, are attractive. It is because they have higher breakdown fields than Si and allow the use of heterostructures, where highly conductive layers of 2-D electrons and holes can be formed. As a step toward such SJ devices, we have fabricated AlGaAs/GaAs four-terminal devices in which a pair of electron/hole channels is embedded. Transport properties of electrons and holes are studied by operating the devices as FETs in which the conductance of one channel is controlled by using the other as a gate. Current-voltage and capacitance-voltage characteristics between the two channels are evaluated by operating the devices as p-n diodes. Measured characteristics are examined by comparing them with device simulations with a focus on the carrier depletion process in each channel.

GaAs-based triangular barrier photodiodes with embedded type-II GaSb quantum dots

We have fabricated a GaAs-based triangular-barrier photodiode, in which self-assembled InGaAs quantum rods (Q-rods) are embedded in its barrier region. Transport study at 100 K has shown that electrons start to flow mainly through Q-rods when a bias is set above a threshold. Upon illumination, photo-generated holes are found to accumulate in the middle portion of Q-rods and efficiently lower the local barrier height, yielding the responsivity as high as 105 A/W at the incident light of 1 fW.

Electrical Characteristics of AlGaAs/GaAs Heterostructures With a Pair of 2-D Electron and Hole Channels

AlGaAs/GaAs/AlGaAs double-heterostructures having two-dimensional electron and hole channels at the respective interfaces are studied by measuring their photocurrent and photoluminescence characteristics. Under the weak photoexcitation, it is found that photo-generated electrons and holes are driven by a built-in electric field between the two channels and flow out mostly as a photocurrent to the respective electrodes, making the photoluminescence negligibly small. When the excitation reaches a certain level, some of photo-generated electrons and holes accumulate in each channel and weaken the built-in field, leading to an exponential increase in photoluminescence or the radiative recombination of electrons and holes. When the excitation gets strong, photo-generated carriers are lost mostly in the form of photoluminescence, resulting in the saturation of photocurrent. A theoretical model to explain these findings is presented. A possibility of using this type of study to clarify operating mechanisms of su...