Y. Nonogaki

Er-related luminescence in Er,O-codoped InGaAs/GaAs multiple-quantum-well structures grown by organometallic vapor phase epitaxy

We have grown Er,O-codoped InGaAs/GaAs multiple-quantum-well (MQW:Er,O) structures by low-pressure organometallic vapor phase epitaxy (OMVPE), and investigated their luminescence properties. The MQW structures are designed to emit light at 0.98 μm for direct excitation of Er ions. Degradation of the structures due to introduction of Er and oxygen is not observed in x-ray diffraction patterns. Er ions doped with oxygen exhibit a sharp and well-ordered photoluminescence spectrum predominantly from one kind of Er center (Er-2O center). Photoluminescence excitation measurements on MQW:Er,O and Er,O-codoped GaAs samples reveal that under below-GaAs-band-gap excitation (830–940 nm), the Er-related luminescence is observed only in the MQW:Er,O sample. It indicates that the luminescence originates from Er ions in InGaAs QWs by means of trap-mediated excitation process.

InAs dots grown on InP (001) by droplet hetero-epitaxy using OMVPE

Abstract We have successfully obtained InAs dots on InP (001) by droplet hetero-epitaxy and observed room-temperature photoluminescence (PL) spectrum with a peak at around 1.6 μm. Dependences of the surface morphology on the substrate temperature and on the trimethylindium (TMIn)-supply time were investigated. From atomic force microscopy (AFM), images of the samples prepared by varying the TMIn-supply time, a critical coverage for supplied In to form droplets was found to exist between 1.5 monolayers (MLs) and 3 MLs. This result was confirmed by low-temperature PL measurements. At the TMIn-supply of 3 MLs, which was above the critical coverage, small dots of height ≈5 nm were formed with the density of 9.8x10 9 cm −2 . A PL spectrum of the sample grown using the same sequence followed by the growth of a 10 nm InP cap-layer, exhibited a 1.6 μm emission, with a full width at half maximum (FWHM) of 150 meV at room temperature.

Formation of InAs islands on InP (001) by droplet hetero-epitaxy

Abstract We have successfully obtained InAs islands on InP (001) by a novel droplet hetero-epitaxy using low-pressure organometallic vapor phase epitaxy (LP-OMVPE). The island growth sequence was initiated by trimethylindium (TMIn) supply in H 2 atmosphere which was intended to form In droplets and followed by tertiarybutylarsine (TBAs) supply after the TMIn supply was stopped at a growth temperature. By this process, large islands of which height was above 100 nm with clear facets were observed under atomic force microscope (AFM). Using a modified sequence in which the TMIn supply was stopped and the substrate temperature was lowered before the TBAs supply, the appearance of the islands was drastically changed. In the specimen prepared by this sequence at the same substrate temperature, small islands of about 10 nm in height were observed together with the large islands. In order to form smaller islands and to suppress formation of the large islands, the substrate temperature dependence of the islands from 530 to 610°C were investigated. The smallest islands with the height of 5 nm were obtained at the substrate temperature of 565°C.

SR-stimulated etching and OMVPE growth for semiconductor nanostructure fabrication

Abstract Synchrotron radiation- (SR-)stimulated etching and selective area growth by organometallic vapor phase epitaxy were performed to form an ordered array of InP crystals on SiO 2 -patterned InP (001) substrate. The SR-stimulated etching was used to pattern the SiO 2 film, because photochemical reaction using SR was expected to provide smooth surfaces, vertical side walls and fine patterning. In the first place, we investigated the basic properties of the SR-stimulated etching by using a mm-size pattern of SiO 2 mask. The etched depth was observed to increase linearly with the irradiation dose. It was found that the etching depth was controlled very accurately. Next, we used μm-size patterns of SiO 2 masks for fabricating the ordered array of InP crystals. In a atomic force microscope image of the sample after etching, a steep side wall was observed. However, the etched surface was not smooth, contrary to our expectation. Moreover, some dust were observed on the surface. From this dust it was found that the SR-stimulated etching had a resolution of ≤100 nm at most.

Growth mode transition of InGaAs in OMVPE growth on GaP (001)

Abstract We investigated growth of InGaAs with different In compositions on GaP (001) substrates by low-pressure organometallic vapor phase epitaxy (LP-OMVPE). From atomic force microscope (AFM), plan-view transmission electron microscope (TEM), cross-sectional TEM and high-resolution TEM observations, it was found that the growth depended clearly on the ratio of TMIn flow rate to the total flow rate of group-III sources ( x TMIn ). In the low x TMIn region, InGaAs grew as a layer. In the high x TMIn region, on the other hand, island formation took place. At x TMIn =0.5, InGaAs islands were formed by the Stranski–Krastanow (S–K) growth mode.

Nanometer-scale InAs islands grown on GaP (001) by organometallic vapor phase epitaxy

We have successfully grown nanometer-scale InAs islands on GaP (001) by low-pressure organometallic vapor phase epitaxy (LP-OMVPE). Effects of substrate temperature and InAs deposition rate on the shape, size and areal density of InAs islands were investigated by ex-situ atomic force microscope (AFM) and high-energy electron diffraction (HEED). The AFM observations showed that the island size decreased with the substrate temperature while the areal density increased, indicating that migration play a role on island formation. The HEED patterns provided significant result that the island grown at high temperature (650°C) consisted of a few grains, while the island grown at low temperatures (550 and 500°C) was single crystalline.

Design and construction of UVSOR-BL4A2 beam line for nano-structure processing

Abstract We have designed and constructed a new beam line BL4A2 at UVSOR mainly for nano-structure fabrication based on synchrotron radiation stimulated surface photochemical reactions. In order to obtain high-photon flux, we use white ray beam focused with only one mirror. The beam line is connected with ultra-high vacuum scanning tunneling microscope for in-situ atomic scale observations, low energy electron diffraction and Auger electron spectroscope for surface crystal structure characterization, and photo-stimulated surface reaction chamber. In order to monitor the optical properties with atomic scale, a near field optical microscope is planned to be installed.

Nanostructure formation on Si (111) surface assisted by synchrotron radiation illumination: Characterization by scanning tunneling microscopy

Abstract The surface structures after the synchrotron radiation (SR) stimulated removal of native oxide on Si (111) exactly oriented and 4° misoriented surfaces were investigated by scanning tunneling microscopy. The exactly oriented surface showed large regions of atomically flat Si (111)-7×7 structure, and was characterized by the formation of single bilayer steps nicely registered to the underlying crystal structure, clearly different from the disordered step edge obtained by the usual high temperature thermal cleaning. The 4° misoriented sample showed nearly uniformly spaced step bunches and terraces terminated by 7×7 unit cells in both SR assisted and thermal cleanings.

Er-related luminescence from self-assembled InAs quantum dots doped with Er by organometallic vapor-phase epitaxy

We have grown self-assembled Er-doped InAs quantum dots (QDs) on GaAs (0 0 1) by organometallic vapor-phase epitaxy (OMVPE). InAs QDs are on average 7 nm in height and 20 nm in diameter, and the areal density is 6]1010 cm~2. They are independent of Er doping. Characteristic photoluminescence (PL) due to intra 4f-shell transitions of Er3‘ has been successfully observed for Er-doped InAs QDs. The luminescence appears only under the excitation with photon energy higher than the GaAs bandgap. It is discussed in relation to an energy-transfer model. ( 2000 Elsevier Science B.V. All rights reserved.

Effects of vicinal InP (001) surface on InAs dots grown by droplet hetero-epitaxy

Abstract We have systematically investigated the effects of InP substrate misorientation on the InAs dot formation by droplet hetero-epitaxy. The density and uniformity of the dots were investigated in the misorientation range between 0 and 4° from (001) toward [100]. Using 2°-off substrate, the density had the highest value of 2.2×1010 cm−2 and the uniformity was most improved, where average dot height and base diameter were 8.1 and 51 nm. Further, we found that the total volume of the dots increased with misorientation in spite of the other growth conditions remaining unchanged.