Kikuchi Akihiko

Intersubband transition (ISBT) in GaN/AlN multiple quantum structure is an attractive candidate for the mechanism of next generation optical communication devices due to optical communication wavelength (1.55μm) carrier transition with ultrafast carrier relaxation time (140fs). On the other hand, the GaN nanocolumn has high crystalline due to dislocation free nature. In this study, we fabricated to photodetector structure and detection characteristics were evaluated, GaN nanocolumn were grown on Si(111) substrate inserting GaN/AlN multiple quantum disk (MQD) by RF-plasma assisted molecular beam epitaxy (RF-MBE). The absorption peak at 1.59μm by P-polarization light was observed. On the nanocolumn wafer Ti/Al thick electrode was deposited. The photodetection of GaN/AlN-MQD was occurred by irradiating the 1.47μm wavelength laser light at room temperature for the first time. The backside illumination has five times larger detectioncurrent than the surface illumination. Moreover, to confirm whether the principle of operation was ISBT, the dependence of polarization and wavelength etc. were evaluated. Those measurements were suggested that the origin of photodetection be ISBT because behavior to follow to the ISBT phenomenon of photocurrent had been seen.

GaN nanocolumn which is a columnar single crystalline GaN nano-crystal having small diameter around 100nm. In this study, GaN nanocolumns were grown by RF-plasma assisted molecular beam epitaxy on c-plane sapphire substrates and temperature dependency of integrated photoluminescence (PL) intensity was investigated. The PL intensity at 300K was as high as about 40% of the intensity at 15K. This result indicates that GaN nanocolumns have high emission efficiency that is amount of nonradiative recombination components due to surface recombination, threading dislocations and point defects are relatively small. The electroluminescence (EL) peak wavelength shift of InGaN/GaN multiple quantum disk nanocolumn LEDs grown on n-type Si (111) substrates was evaluated as a function of injection current. The EL peak wavelength observed for whole electrode area showed large blue shift with increasing the injection current. But remarkable shift was not observed for the microsopic EL measured for an area with about 3μm-diameter. These results suggested that the large wavelength shift of the nanocolumn LED was caused by intensity competition between nanocolumns with different emission wavelengths. It is expected that growth of uniform nanocolumns brought about improved performance of InGaN nanocolumn LEDs.

Recently, the fundamental bandgap of InN was reported to be 0.65-0.8eV due to the improvement of crystal quality of InN filmes. Therfore InN will be a new material for the optical communication devices. In this study, InN/In_<0.75>Ga_<0.25>N multiple quantum wells (MQWs) were fabricated by rf plasma assisted molecular beam epitaxy (RF-MBE). From the high-resolution transmission electron microscopy (HR-TEM) measurement, smooth and sharp hetero-interface with one-two monolayer interface thickness fractuation was observed. Clear satellite peaks up to the fifth order were observed in high resolution ω-2θ scan XRD spectra. Room temperature photoluminescence emission at from 1.58 to 1.97 corresponds to the InN well width change was observed for the first time. The PL peak wavelengths were longer than calculated transition wavelengths using a square potential model. It can be explained that large piezoelectric field exist in the InN well layers.

The optical property of GaN nanocolumns grown on (0001) Al_2O_3 substrates was characterized by room temperature photolummescence (PL) measurement For the case of low excitation condition, the PL peak intensity of the GaN nanocolumns was about 20〜400 times stronger than that of an undoped GaN continuous film grown by metalorganic chemical vapor deposition with a dislocation density of 3〜5x10^9cm^<-2> and thickness of 37 μm For the higher excitation condition, stimulated emission was observed with a low threshold power density of 198 kW/cm^2 We also grown GaN nanocolumns with InGaN/GaN multiple quantum well. Bright PL emission at wavelength from 436 to 614 nm was observed InGaN/GaN multiple quantum well nanocolumn LEDs were grown on n-type (111) Si The LEDs showed clear rectifying behavior Green to red-orange electro-luminescence was obtained at room temperature

ISBT in GaN/AlN MQW has been expected to be a ultra fast all-optical switch Now, a scheme of inducing ISBT, whose wavelength of 155 μm, by IBT is investigated. This scheme gives us the applications for #1 ON-OFF controlling of ISBT devices by IBT resonant light, #2 hi-speed ultraviolet detector The latter device is expected to show hi-speed detection is same as IBT relaxation time For realization of those devices, we investigated the IBT excitation dependences of intersubband absorption As the result, a relatively high ISBT extinction ratio of 38 % at IBT excitation power of 3 mW was obtained, and also ISBT extinction ratio of over 99 % at 40 mW by a calculation These results show the possibilities of realization for those devices

Recently, the fundamental bandgap of InN was reported to be 0.65-0.8 eV, contrary to the previous knowledge of 1.9 eV, due to the improvement of crystal quality of InN filmes. Therfore InN will be a new material for the optical communication devices. In this study, In-rich InGaN films and double-heterostructures were grown by rf-plasma assisted molecular beam epitaxy (RE-MBE). The photoluminescence emission from well layer was observed at 1.45 μm. Furthermore, for modeling the InN-system optical waveguides, the refractive indices of InN, GaN and AlN were measured in the wavelengths range from 1.8 to 2.5 μm. The refractive indices for InN at the IR region were evaluated to be 2.7〜2.55.

Intersubband transition (ISBT) in GaN/AlN super lattice system is very attractive phenomenon due to ultrafast absorption relaxation (〜100fsec) and possibility of application in optical communication wavelength region. In this study, GaN/AlN superlattices were grown by RF-plasma assisted molecular beam epitaxy on (0001) sapphire substrate. By use of SiO_2 rib-waveguide structure, light polarization dependency of ISBT was observed. The ISBT absorption coefficient was systematically evaluated for wide carrier concentration range from 1x10^<18> to 3x10^<20>cm^<-3>. The free carrier absorption coefficient was also estimated by waveguide measurement.

Recently, the fundamental bandgap of InN was reported to be 0.8-0.7eV, contrary to the previous knowledge of 1.9eV, due to the improvement of crystal quality of InN films. Therefore InN will be a new attractive material for the optical communication. In this study, high quality InN films with the low residual carrier density were grown by rf-plasma assisted molecular beam epitaxy (RF-MBE) by changing the V/III supply ratio and growth temperature. The room temperature electron density of InN was reduced to be 2-3×10^<18>cm^<-3> and the electron mobility was 1190cm^2/Vs. Furthermore, the growth conditions of InN nano-columns were investigated and the growth of InN/GaN/InN hetero-nano-columns were also demonstrated.

Resonant cavity-enhanced (RCE) photodetector (PD) is achieved a high responsivity and excellent wavelength selectivity for a thin absorption layer. In this investigation, we have fabricated AlGaN-based RCE ultraviolet (UV) PD by rf-plasma-assisted molecular beam epitaxy (RF-MBE). We have successfully improved the responsivity of RCE-UV-PD by grown on sapphire substrate without metalorganic chemical vapor deposition (MOCVD)-GaN template. Dark current was 66pA at 5V bias voltage. We observed clear resonant detection peaks by resonant cavity effect. Peak responsivities were 1.5A/W at 357nm and 1.2A/W at 364nm with 9V bias voltage, for a thin absorption layer of 40nm.

Intersubband transition (ISBT) at the optical communication wavelength range from 1.14 to 1.61 μm was systematically investigated in (GaN)m/(AlN)n superlattices (SLs). The SL samples were grown directly on (0001) sapphire substrates by molecular beam epitaxy using rf-plasma nitrogen as a source (rf-MBE). The SLs consisted of 90 periods of GaN-well (m=2〜10 mono-layer (ML) in thickness) and AlN barrier (n〜11 ML). For a 4-ML GaN well, the ISBT absorption wavelength reached down to 1.14 μm, close to a theoretically predicted limitation, and it was shifted monotonically up to 1.61 μm with increasing the well thickness to 9.5 ML. The linewidth of the absorption spectra as narrow as 61 meV was observed at 1.54 μm.

The polarity control technology for Ga-polar GaN with molecular beam epitaxy using rf-plasma nitrogen source (RF-MBE) was established. Inserting high-temperature grown AlN multiple interlayers (HT-AlN-MIL) into MBE-grown GaN at 750℃ suppressed the propagation of dislocations into upper GaN, and the electrical and optical properties were improved. The HT-AlN-MIL was deposited on GaN templates grown by metal organic chemical vapor deposition (MOCVD), followed by GaN grown by MBE. The dislocations with screw character in MBE-grown GaN were reduced by about two orders of magnitude compared to those of the bottom GaN template, which produced the step-like surface morphology for MBE-GaN. AlN/GaN double barrier resonant tunneling diodes (RTD) were fabricated by RF-MBE on MOCVD-GaN templates, using this dislocation reduction technique. Clear negative differential resistance with a peak-to-valley ratio over 30 has been obtained.

The resonant cavity-enhanced photodetector (RCE-PD) is expected to have wavelength selectivity, high quantum efficiency and high speed characteristics. The photosensitivity characteristics of AlGaN based RCE-PD was theoretically investigated. The AlGaN distributed Bragg reflectors for at blue and UV region were grown by molecular beam epitaxy using RF-plasma excited nitrogen. Relatively high reflectivity of 95% and 92% was achieved at the wavelength of 444nm and 377nm, respectively.

GaN was grown by migration enhanced epitaxy(MEE)using molecular beam epitaxy(MBE)with rf-radical nitrogen source. We observed pit free smooth surface morphologies of MEE GaN by scanning electron microscope(SEM). 3.5μm thick GaN : Si layer was grown on MEE GaN layer by standard MBE with a high growth rate of 1.2μm/hr. The carrier density and mobility at room temperature were 1.2×10^<17>cm^<-3> and 372cm^2/Vs, respectively. Sharp photoluminescence(PL)spectrum without broad emission from 550nm to 580nm was observed at 15K. We also achieved relatively high growth rate of 2.6μm/hr by standard RF-MBE. The carrier density of Si doped n typr GaN was controlled from 4.9×10^15cm^-3 to 5.3×10^<20>cm^<-3>.

GaN nanostructures (GaN nano-column) were self-organized on (0001 Al_2O_3 by rf-radical source molecular beam epitaxy (RF-MBE). An averaged diameter of GaN nano-column was minimized to 41nm by the control of V/III supply ratio. Self-organization process of GaN nano-column was applied for the fabrication of GaN/Al_<0.18>Ga_<0.82>N multi quantum disk (MQD). The GaN nano-columns with an average diameter of 46nm was fabricated, in which the 10pair of GaN (6nm)/Al_<0.18>Ga_<0.82>N (9nm) multi-layer structure was built-in to form quantum disks. The blue-shift in a photoluminescence (PL) peak wavelength at room temperature was observed for the MQD sample, probably due to the quantum-size effect.

III-Nitrides were grown on sapphire substrate by molecular beam epitaxy (MBE) using RF-radical nitrogen sources. First, we demonstrated the growth of III-Nitrides-nano-structures which have uniform column figure (nano-columns) keeping c-axis perpendicular to a substrate surface. Its dependency on nitrogen flow rate and RF-input power were investigated respectively. The minimum column diameter of 53nm was formed. Next, we controlled AI contents in AIGaN quasi ternary alloy consisted of GaN/AIN short period binary super lattice using the shutter control method. And then, we fabricated GaN/AIGaN/AIGaN-SCH structures using this method.

The purpose of this study is the realization of optical modulator of II-VI MgZnCdSe compounds lattice-matching to InP substrate. In this study, quantum confined stark effect of ZnCdSe/MgZnCdSe multiple quantum well(MQW) grown on InP substrate by molecular beam epitaxy(MBE) was investigated. The reflectance measurement for the MQW structure at room temperature showed the maximum variation of the reflectance at a wavelength near the lowest excitonic transition gap was -6.3% which corresponded to the refractive index venation of -3.4% under 1.87x10^5V/cm electric field apply. This value was larger than that of III-V materials. It is expected that the high performance optical modulator is realized due to the large excitonic effect on II-VI compounds. Fabry-Parot optical modulator which consisted of the spacer layer of the MQW between two multiple layer reflectors were also designed, and the extinction ratio and the insertion loss of the modulator were estimated. Furthermore MgZnCdSe multiple layer reflectors were grown an maximum reflectance of 98.0% was obtained at 596nm

Mg_x(Zn_yCd_<1-y>)Se(x=0〜0.63) compounds and ZnCdSe/MgZnCdSe doublehetero-structures lattice-matching to InP substrates were grown by molecular beam epitaxy (MBE). The bandgap energy of these layers from 2.17 to 3.12 eV at 15K were evaluated by photoluminescence (PL) measurement. Refractive indices of MgZnCdSe compounds were calculated from reflectance measurements. ZnCdSe/MgZnCdSe quantum-well structures were fabricated. Sharp PL spectrum without deep level emission at 15K and strong PL emission from ZnCdSe quantum-well layer at room temperature were observed. And then, ZnCdSe/MgZnCdSe multi-quantum-well structures were fabricated. Electro-luminescence emissions under pulsed operation at 77K were observed for the first time in MgZnCdSe systems.

Self-organization of strained quantum-wires (QWRs) in (GaP)_m/(InP)_m short period binary superlattice (SPBS) was systematically investigated by changing monolayer number m and substrate misorientation. It was largely depended on substrate misorientation angle (SMA) toward [011] direction. Self-organization on QWRs occurred at m over 1.2 ML for SMA of 0° and even at 1ML for SMA of 5°. While for SMA of 15°, self-organization of QWRs was suppressed by small step width. GaInP/AlInP compressively strained multi-quantum-wire lasers with (GaP)_<1.5>/(InP)_<1.5> SPBS active region was fabricated and low threshold current density with 257 A/cm^2 was obtained under the room temperature pulsed condition.

Self-organization of GaInP strained quantum-wires in the growth of (Gap)_2/(InP)_2 short period binary superlattice on (100)GaAs substrates is a very effective way to realize strained quantum-wire lasers. The conditions for self-organization of strained quantum-wires, however, were not studied sufficiently. Thus, the condition under which quantum-wires were self-organized in is period (GaP)_m/(InP)_m short period binary superlattices grown by gas source MBE was investigated with changing monolayer number systematically, occurring self-organization of quantum-wires for the monolayer number m over 1.2. Room temperature low threshold current densities (J_<th>) Operation of self-organized GaInP/AlInP strained quantum-wire lasers with J_<th> of 294A/cm^2 (under continuous wave operation) and 278A/cm^2 (under pulsed condition) were described.