Staffs

Students

A new type of multiple micro-cavity (MMC) laser was proposed and fabricated.

A GaInAsP/InP semiconductor collimating GRIN lens was proposed and fabricated for optical coupling improvement of SLA and LD to single mode fiber.

1. A new type of multiple micro-cavity (MMC) laser with etched mirrors was proposed, and it was found that it is possible to operate on sub-mA current.
2. Nearly vertical shape etched mirrors by using wet chemical etching was obtained
3. Threshold current density of 178A/cm² was obtained at room temperature under pulsed condition in MMC lasers.
4. CW operation was obtained at room temperature in MMC lasers.
5. A semiconductor collimating GRIN lens for vertical output beam divergence improvement of SLA and LD was fabricated, and integration with TTW-SLA was demonstrated.

Staffs

Students

GaInAsP/InP strained-quantum-film, -wire, and -box lasers have been studied both theoretically and experimentally. Temperature dependences of GaInAsP/InP compressively-strained single-quantum-well lasers with quantum-wire (Q-Wire) size active region were measured and compared with those of quantum-film (Q-Film) lasers. Better lasing properties of the Q-Wire laser over Q-Film lasers were confirmed at a temperture below 193K. A GaInAsP/InP compressively-strained quantum-box laser assisted with a thin quantum-film active layer was operated at 77K under CW condition.

1. Temperature dependences of GaInAsP/InP compressively-strained single-quantum-well lasers with quantum-wire (Q-Wire) size active region were measured and compared with those of quantum-film lasers. Lower threshold current as well as higher differential quantum effciency operation of Q-Wire laser than those of Q-Film laser at a temperture below 193K were obtained. Threthold current density of Q-Wire laser was 43A/cm² which was almost a half that of Q-Film laser being 85A/cm².
2. A GaInAsP/InP compressively-strained(CS,+1%) quantum-box laser assisted with a thin quantum-film active layer(CS,+1%,5nm) was operated at 77K under CW condition.The threshold current density was 603A/cm². The fabricated quantum-box size is 22nm diameter and 10nm thick with a period of 70nm.
3. By combining electron beam lithography and Cl₂-ECR dry etching with negative acceleration voltage, 10-40nm wide GaInAsP/InP multi-quantum-wire structures were fabricated. Moreover, low damage feature of this fabrication process was confirmed by PL observation. An introduction of a surface cleaning process with H₂ gas just after Cl₂-ECR dry etching was found to be effective for further reduction of damage.

Staffs

Visiting Researchers

Students

A novel ultra high-speed transistor utilizing high-velocity and wave properties of the ballistic electron was proposed and studied from the viewpoint of (1)ballistic transport in GaInAs/InP, in particular, the wave nature of the hot electron, (2)device concept using the wave-nature of the hot electron, and (3)nanostructure fabrication technology.

1. In order to evaluate the hot electron coherent length from the resonance characteristics of resonat tunneling diodes (RTDs), the dependence of the resonant level width on various broadening mechanisams is investigated. From the dependence of the resonant level width on the well width, it is shown that the individual contributions to resonant level broadening such as phase relaxation and well width variation can be analized separately. To investigate the coherent length at high temperature, the current voltage characteristics of a triple-barrier resonant tunneling diode (TBRTD) is analyzed. It is found that the phase coherent length can be estimated up to 100K using TBRTD.
2. Hot electron detection with a scanning probe microscope is discussed. To this end we have proposed a technique called Scanning Hot Electron Microscopy (SHEM), which allows to obtain both the spatial and energetic distribution of hot electrons in a device.
3. The scattering probability of a Fermi energy electron was theoretically analized for a semiconductor crystal where a finite number of doped impurity ions are placed periodically. Due to electron wave interface scattering for periodic placement is reduced in comparison with that for random placement. The scattering reduction depends on the boundary shape of the Burilluin zone of the superlattice of impurity ions, the number and concentration of imputity ions and the propagation direction.

Staffs

Students

Superlattices and ultrathin layers with the combination of metal and insulator were proposed as one of the candidates of the material for ultrahigh-speed electronic devices and optical devices because of the low resistivity of metals, low dielectric constant and wide band gap of insulators and high conduction band offset at metal/insulator heterointerface. A novel transistor using quantum interference in metal/insulator heterostructure has been proposed and it was shown that sub-pico second response can be expected at such devices.

1. Multiple negative differential resistance (NDR) due to quantum interference of hot electron waves was observed at 300K for the first time in a small area metal (CoSi2)/insulator(CaF2) quantum interference transistor structure fabricated by using electron-beam lithography. This was achieved by reducing voltage drop in collector series resistance.
2. The transfer efficiency a of hot electrons with monochromatic energy across a 1.9-nm-thick metal(CoSi₂) epilayer was estimated to be more than 0.96 from NDR characteristics of a metal(CoSi₂)/insulator(CaF₂) quantum interference transistor. Such a high value of a may be due to the very thin single-crystalline metallic layer. From this transfer efficiency, mean free path of hot electrons is estimated to be more than 50nm.
3. A small area metal(CoSi₂)/insulator(CaF₂) hot electron transistor using electron-beam lithography was fabricated. The emitter area was 0.9�~0.9mm². Although collector current increased without saturation due to leakage current through SiO₂ film under the outside electrode pads. Although the measured characteristics showed clear transistor action for the first time. The characteristics removed the leakage current exhibited the saturation, and current gain b>36 was obtained at 77K.

Staffs

Students

T.Matsunuma

T.Maruyama

New type of light emitting devices on Si substrate using silicon or cobalt siliside nanocrystals embedded in insulator (CaF₂) was investigated.

In a silicon quantum box embedded in insulator barriers, we can expect strong three dimensional quantum confinement effects, which can lead fascinating changes in the optical properties relative to those of the 3D bulk material.

1. Formation technique of nanocristal Si and CoSi₂ in single crystalline CaF₂ on Si(111) substrate was developed using co-deposition of Si, Co and CaF₂. Metal or semiconductor crystals less than 10nm in diameter were obtained using this technique, which was confirmed by Transmission Electron Microscopy cross sectional observation. Growth condition dependence of size and density were also clarified.
2. Visible light emission from nanocrystalline silicon embedded in CaF₂ was firstly observed in photoluminescence measurement even at room temperature. The blue or green light emissions could be observed for the first time by the naked eye using a He-Cd laser (=325nm) even at room temperature. The PL spectra observed at 20K had two peaks at about 450nm and 520nm. This range of wavelength corresponds to the energy between fundamental quantized levels of an electron and a hole in a Si quantum box of 1-2nm in diameter. As the growth temperature decreases from 300°C to room temperature, the PL intensity gets weaker probably due to degradation in crystalline quality. Effect of in-situ annealing of samples was also investigated.

Staffs

Students

Study of nanometer structure fabrication technique is important for the realization of quantum effect devices such as quantum-wire or -box devices and ballistic electron devices based on wave characteristics of electrons.

1. Nanostructure fabrication and alignment techniques were developed using e-beam direct writing and OMVPE. An alignment of 40 nmpitch buried GaInAs /InP double slit and 50 nmpitch electrodes was demonstrated.
2. Heterointerface of GaInAs/InP grown by OMVPE were observed by Atomic Force Microscope (AFM). Atomically flat regions was obtained on InP surface by optimized annealing prosess. To realize the atomically flat heterointerface, we investigated the relation between growth conditions and growth modes such as step-flow and/or two-dimensional nucleation for InP surface.
3. Anodization process of InP has been investigated with aiming at a novel fabrication technology of quality quantum-wire and quantum-box structures. As the result, a high density (space filling factor of 50%), uniformly shaped (triangle) and sized (40nm) vertical pillar structure was obtained on (111)A oriented InP substrate with a SiO₂ mask by adopting an EBX direct patterning followed by the anodization with HCl acid. Very small physical damage by anodization process was established from observing strong PL intensity from 40nm size-ordered triangle pillers.