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We used atomic force microscopy to characterize the surface topography of p-GaN exposed to N2 and NH3 plasma under quasi-atmospheric pressure using a microwave-excited plasma source with a microstrip line structure. The exposure time was varied from 2 to 20 min at a substrate temperature of 700 °C. Under both N2 and NH3 plasma exposure for 2 min, the ridge-shaped features on the surface of as-grown p-GaN dulled immediately and the surface roughness decreased remarkably, whereas the atomic step structure of the surface was maintained. The step crossing and bunching of the surface disappeared with increasing exposure time to both types of plasma. However, increasing the NH3 plasma exposure time to 20 min led to the formation of pits and appearance of particles along the step edges, resulting in drastic roughening of the surface. Thus, GaN surfaces can be smoothed without the destruction of their step structures via moderate plasma exposure under quasi-atmospheric pressure, and these plasma sources could prospectively be used in metal-organic chemical vapor deposition systems for nitride semiconductor growth.

The dynamic characteristics of GaN-based vertical-cavity surface-emitting laser (VCSEL) with an AlInN/GaN semiconductor distributed Bragg reflector has been reported for the first time. The max dynamic lasing frequency of 1.1 GHz with a wavelength of 415 nm and a threshold current of 7 mA has been demonstrated. Moreover, at the pulse lasing operation, both the steep lasing waveforms and the tailing spontaneous emission waveforms were observed in driven with the square pulse waveforms. The dynamic lasing frequency driven by the sine waveform seems to be related to this steep lasing waveform at the square pulse operation. During these dynamic measurements, the lasing characteristics of the VCSEL were gradually degraded, and the lasing power is decreased by −7 dB at the same bias condition after several hours.

A ridge-shaped AlGaInP/GaInP quantum well (QW) structure with a ridge top width in the sub-wavelength region (500–600 nm) is successfully fabricated by selective area metal organic vapor phase epitaxial growth on patterned GaAs substrates, as the first step towards the realization of high-efficiency light-emitting diodes (LEDs) utilizing the evanescent wave coupling effect. It is confirmed by an X-ray diffraction study that AlGaInP layers grown on all the different facets of the ridge structure satisfy the lattice matching requirements for high-efficiency LED applications. A considerable enhancement of the light-extraction efficiency in the fabricated ridge-shaped QW structure is demonstrated by means of a photoluminescence analysis. The realization of the evanescent wave coupling effect is suggested by theoretical simulation. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We developed a high-resolution Michelson interferometer in order to stabilize the relative phase of double pulses by using a He-Ne two-frequency laser positioning system. A very stable phase-locked pulse pair whose phase difference was only 4° at 900 nm was realized. Using a high-resolution Michelson interferometer, we demonstrated coherent control of an exciton in a GaAs/AlGaAs crescent-shaped quantum wire. The dephasing time of the single quantum wire was 400 fs. © 2011 Wiley Periodicals, Inc. Electron Comm Jpn, 94(5): 25–32, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10238

A semiconductor light-emitting diode (LED) in which blocking of light by an opaque metal electrode, a serious problem hindering improvements in LED efficiency, could be completely eliminated is reported. The principle behind this is the spatial separation of the current-injection area from the light-emitting area by selectively forming the metal electrode on a high-bandgap-energy facet of a nonplanar substrate composed of multiple facets with varying bandgap energies. In this device, carriers injected from the opaque electrode are swept away from the electrode area towards the adjacent low-bandgap-energy area where they recombine by a potential energy gradient-driven carrier transfer process.

We have carried out an original study of longitudinal photoconductivity (PC) of undoped GaAs/AlGaAs V-grooved quantum wires (QWRs) by measuring the PC along the wire direction. The PC spectrum reveals several peak structures superimposed on a smooth background, the latter being related to the GaAs substrate. Some of these structures are connected with the QWRs. This is confirmed by the observed dependence of the PC spectrum on the exciting light polarization and by comparing this spectrum with photoluminescence and photoluminescence excitation spectra measured on a similar sample. A theoretical identification of the peak structures is given by calculating the optical transition energies of the studied QWRs in frames of the envelope function approximation. An original approach, based on the infrared preillumination of the sample, is proposed to reduce the contribution of the GaAs substrate to the PC, in this way favoring the QWR spectral structures.

We have investigated the ultrafast carrier dynamics in high-optical-quality V-grooved GaAs/AlGaAs quantum wires using pump-probe spectroscopy. It is found that, with resonant excitation, a blue shift due to exciton–exciton interaction occurs within 0.3 ps followed by a redshift due to exciton relaxation. Under nonresonant excitation, the redshift occurs due to the phonon emission during exciton formation. As a result, the exciton formation time and the exciton lifetime in the quantum wires are found to be 5 and 110 ps, respectively. This exciton lifetime corresponds to the free-exciton lifetime close to the theoretical intrinsic lifetime of one-dimensional excitons and proves that our quantum wires have little interface disorder and have high optical quality.

Femtosecond optical pulse propagation in a quantum well (QW) waveguide and a quantum wire (QWR) waveguide was investigated by cross-correlation frequency resolved optical gating (XFROG) spectroscopy. An optical pulse transmitted through the GaAs QW waveguide was found to stretch greatly from 140 fs to almost 1 ps due to nonlinear dispersion around the heavy hole exciton resonance at transverse electric polarization in a near resonant experiment. In contrast, only slight chirping of the pulse transmitted was observed either at transverse magnetic polarization or off resonance for both polarizations. In the GaAs QWR waveguide, the polarization anisotropy of a crescent shaped QWR could also be observed in terms of dispersion by XFROG spectroscopy in spite of the small absorption compared with that in the QW.

A series of nonlinear conductance phenomena is investigated in the GaAs V-grooved quantum wire (QWR) field effect transistors (FETs) prepared by the flow-rate modulated epitaxy (FME) technique. These are attributed to Coulomb blockade at a subthreshold gate bias, universal conductance steps near the threshold, real space transfer under a forward gate bias and a large source-drain bias condition. Weak carrier coupling between sidewall quantum wells and QWRis responsible for the

We have demonstrated ultrafast coherent control of excitons and exciton-polaritons in semiconductor quantum nanostructures. An InGaAs/AlGaAs multi-quantum-well Bragg structure and an AlGaAs/GaAs crescent-shaped quantum wire structure were used as samples for the control of the exciton-polariton and excitons in the nanostructure. In the multi-quantum-well Bragg sample, a two-stage decay due to the super-radiant exciton-polariton mode is clearly observed under the resonance conditions. Highly sensitive coherent control measurement enables us to observe the dephasing characteristics in the crescent-shaped quantum wire sample, and a very fast dephasing time T-2 of between 200 fs and 300 fs with powder dependence was observed.

Optical properties of V-shaped AlGaAs/GaAs single quantum wire with improved interface quality are investigated by means of photoluminescence (PL), PL excitation (PLE), and time-resolved PL measurements. Zero PLE absorption intensities are observed at low temperatures around ground state transitions. Excitation power density-dependent optical properties provide evidence that the zero PLE absorption intensities are due to an internal electric field created by the tiny strain-induced piezoelectric polarization along the wires, which causes complete spatial separation of the electron and hole wavefunctions along the wires.

A randomly disordered quantum wire superlattice (d-QWR-SL) structure was fabricated by means of metalorganic vapor phase epitaxy. Photoluminescence (PL) and the carrier lifetime properties were investigated systematically. The PL intensity was found to be independent of temperatures below 170 K. This thermally stable feature was due to dramatic suppression of the nonradiative recombination in the wire region. The carrier lifetime measurement confirmed the high thermal PL stability in the d-QWR-SL structure. This high thermal PL stability may be due to the state localization in the d-QWR-SL structures.

We have developed a new way of the constant growth technique to conserve a grating height of vertical-stacked V-groove AlGaAs/GaAs multilayer on submicron gratings up to 1.5 μm thickness by a low-pressure metalorganic chemical vapor deposition. The V-shaped GaAs buffer, grown on thermally deformed submicron gratings, has an important role in overcoming mass transport effects by recovering the deformed grating profile from sinusoidal to V-shaped. The low AlAs mole fraction is favorable to preserve the grating height up to a greater thickness. The constant growth technique is an important step to realize complex optoelectronic devices such as one-step grown distributed feedback lasers and two-dimensional photonic crystals.

We report the "rst successful growth of high-quality V-shaped AlGaAs/GaAs quantum wires (QWRs) using tertiarybutylarsine (TBAs) as the arsenic source by #ow rate modulated metalorganic vapor-phase epitaxy. We found that the fundamental structural qualities of QWRs grown using TBAs (for example, the growth selectivity, the sharpness of the V-groove bottom, and the uniformity of the QWR surface) are comparable to or better than those of QWRs grown using AsH 3 . A characteristic defect was observed mainly in the intersecting region of the (1 1 1)A side wall and the (0 0 1) #at facets at low growth temperatures, but it would not have an important in#uence on the size of the grown QWRs for a substrate with a su$ciently long (1 1 1)A side wall facet. In a preliminary optical investigation of a 4.5 nm thick QWR, we observed an energy separation as large as 58 meV between the ground and the "rst excited state and a ground state Stokes shift as small as 3.9 meV at 5 K. Both of these results are much better than the values of the sample grown using AsH 3 . ( 2000 Elsevier Science B.V. All rights reserved.

The effects of NH4OH : H2O2 : H2O (=1 : 3 : 50) etching of the initial V-grooved substrate and the use of an AlGaAs/GaAs short-period superlattice buffer layer on improvement of the interface uniformity of V-shaped AlGaAs/GaAs quantum wires (QWRs) were investigated by atomic force microscopy and micro-photoluminescence spectroscopy. We found that the surface roughness on the initial V-grooved substrates induced during V-groove preparation processes could be greatly reduced by the use of the NH4OH etching treatment. Further, by the combined use of the above two techniques, the QWR interface uniformity could be significantly improved, as revealed by both the elongation of the monolayer step islands formed on the (0 0 1) QWR facet and the suppression of step bunching on the (3 1 1)A QWR facet.

Crescent-shaped GaAs quantum wires were fabricated on a V-grooved GaAs(001) substrate using a flow rate modulation epitaxy technique in metalorganic chemical vapor phase deposition method. The microstructures of the quantum wires were investigated using electron microscopy. The optimum growth conditions for the quantum wire superlattice structures were discussed from the observed microstructures. The optical properties of the fabricated single quantum wires were also investigated using photoluminescence spectroscopy. The relationship between the microstructures of the quantum wires and the observed optical properties is discussed on the basis of computer simulations of the electronics structure.

A new self-limited growth induced by surface migration of Ga atoms during flow rate modulation epitaxy of GaAs on patterned substrate is demonstrated, which is considered to be very useful for the fabrication of many fine semiconductor structures, especially highly uniform quantum wires and quantum dots. The formation of stable surface reconstructions is proposed as a possible mechanism for this unique self-limiting phenomenon.

Microscopic photoluminescence and photoluminescence excitation spectroscopy are used in order to investigate exciton localization in quasi one dimensional systems such as crescent-shaped AlGaAs/GaAs quantum wires grown on a V-grooved substrate. We found that one wire is segmented into extended sections of hundreds of nanometres with slightly different confining potentials. Inside these sections one monolayer deep quantum boxes are present on the (001) top facet of the wire with lengths comparable to the exciton Bohr radius.

Barrier thickness dependences, namely coupling strength dependences, of optical properties in GaAs coupled quantum wires are investigated by the photoluminescence (PL), photoluminescence excitation (PLE) and radiative lifetime measurements. In the photoluminescence characteristics, the coupled states of symmetric and antisymmetric states are clearly observed with large barrier thickness dependence. Also, the temperature dependence of the radiative lifetime shows a barrier thickness dependence which might be due to the difference of the oscillator strength in the coupled quantum wires with different barrier thicknesses.

The optical properties of an AlGaAs (135 A)/GaAs (45 A) quantum wire superlattice (QWR-SL) grown on V-grooved substrate by flow rate modulation epitaxy are investigated using photoluminescence (PL) and PL excitation (PLE) spectroscopy. The QWR-SL showed step-like PLE spectra with the first step having almost no detectable PLE intensity which are completely different from the sawtooth-shaped PLE spectra of a 45 A thick single QWR sample. The unusual optical properties of the QWR-SL are probably due to the interaction of the one dimensional QWR electronic state with optical field since the QWR electronic states are electronically isolated by the thick AlGaAs barrier layer in this sample.

A gradual uncontrollable increase in the resistivity of the amorphous phase of phase-change alloys, such as Ge2Sb2Te5, known as drift, is a serious technological issue for application of phase-change memory. While it has been proposed that drift is related to structural relaxation, no direct structural results have been reported so far. Here, we report the results of Ge L3-edge x-ray absorption measurements that suggest that the drift in electrical conductivity is associated with the gradual conversion of tetrahedrally coordinated Ge sites into pyramidal sites, while the system still remains in the amorphous phase. Based on electronic configuration arguments, we propose that during this process, which is governed by the existence of lone-pair electrons, the concentration of free carriers in the system decreases resulting in an increase in resistance despite the structural relaxation towards the crystalline phase.

We report on a photoluminescence excitation study of a lateral array of GaAs quantum wires grown by flow rate modulation epitaxy on 4 μm pitch V-grooves. Several excitonic transitions exhibiting a strong polarization anisotropy and involving the ground and excited quantum wire subbands are clearly observed.

An AlGaAs(90 A)/GaAs(45 A) quantum wire superlattice (QWR-SL) with excellent size uniformity is grown on a 4.8 μm pitch V-grooved substrate by flow rate modulation epitaxy at a specific growth temperature (∼630 °C). In the low temperature (12 K) photoluminescence spectrum of the AlGaAs(90 A)/GaAs(45 A) QWR-SL, two excited subband emission peaks are observed at an excitation power density at least about four orders of magnitude lower than that needed for the observation of similar peaks from the single quantum wire (SQWR) reference sample. Another two side-peaks are also observed at the long wavelength side of the ground subband peak whose energy separations from the excited subband peaks are very close to the energies of GaAs optical phonons, implying the possibility of participation of optical phonons in the radiative transition processes of the QWR-SL. The ground subband emission peak of the QWR-SL sample shows a radiative lifetime (∼2.1 ns) almost four times longer than that of the SQWR sample. The above observations suggest that the carrier relaxation processes and the radiative transition probability of the QWR-SL seem to be considerably modified against the SQWR structure.

We present photoconductivity data on GaAs quantum wires grown on a V-grooved substrate by flow rate modulation epitaxy. They show that a moderate excitation power density, ∼1 W/cm2, allows the observation of the absorption structure of a single GaAs quantum wire embedded in a p-i-n diode. Furthermore, by increasing the number of active wires inside a diode, the photoconductivity signal is enhanced and additional details of the absorption structure are evidenced. And, finally, a rough quantitative agreement is obtained between the experimental absorption transitions and a simple calculation of the one-dimensional excitonic states using the envelope function approximation.

Si- and Zn-doping properties of GaP layers grown by organometallic vapor phase epitaxy (OMVPE) using tertiarybutylphosphine (TBP) are investigated. An electron concentration as high as 2.0 × 1019 cm−3 is easily achieved with a SiH4TMGa flow rate ratio of 2.2 × 10−3 and is higher than the value obtained by using gas source molecular beam epitaxy. The Si-doping efficiency seems to be considerably enhanced by the use of TBP compared with the case using PH3. The hole concentration of Zn-doped layers grown with TBP is comparable to that of PH3-OMVPE grown GaP layers, but the Hall mobilities are higher than those of PH3-OMVPE grown layers.

We report a novel low-power nitriding technique by utilizing a 2.45 GHz microwave-excited nitrogen radical flow system. Nitrogen plasma was produced at the nozzle with dimensions of 50 × 0.5 mm2 and blown onto the surface of a target substrate. A titanium substrate has been used as a target plate since it is easy to visualize a nitriding effect. The titanium substrate was treated under the conditions of 60 W microwave power, 20 Torr of nitrogen gas pressure, and a plate temperature of ∼800 °C. As a result, we have succeeded in nitriding of the titanium substrate in a quasi-atmospheric region of 20 Torr and of a very low power of 60 W with the hardness kept high, which is almost the same as the hardness processed by conventional nitriding methods.

Photoluminescence (PL) processes in (AlP) m /(GaP) m ( m =1–8 monolayers (ML)) short period superlattices grown by atmospheric pressure organometallic vapor phase epitaxy (OMVPE) using tertiarybutylphosphine (TBP) are investigated. Strong emission of bound excitons without phonon replicas is observed at low temperature. This suggests that the luminescence processes are of a direct transition. The transition probability is found to be very small since there is no band edge emission at high temperature. For the AlP/GaP superlattices with monolayer numbers greater than 4, the structural dependence of PL properties such as peak position and intensity qualitatively agree with the theoretically predicted results. However, direct to indirect transitions which are theoretically predicted for smaller monolayer numbers are not observed.

Short‐period superlattices of (AlP)m/(GaP)n (m, n=4–11.5) are grown by atmospheric pressure organometallic vapor phase epitaxy using tertiarybutylphosphine as a phosphorus source. Structural and optical properties of the grown superlattices are characterized by double‐crystal x‐ray diffraction and photoluminescence (PL) measurements, respectively. By comparing the measured and the calculated full width at half‐maximum of satellite peaks, we find that very high quality superlattices with atomically abrupt heterointerfaces are achieved. Strong emission of bound exciton is observed at low temperature. The exciton emission, however, disappears quickly as the temperature is increased, and only PL lines due to impurity recombination remain at high temperature (≳30 K). No evident phonon replica is observed in the PL spectra. From these results, a direct band‐gap structure seems to be realized as expected from the zone‐folding effect. However, the absence of band‐to‐band direct transition at high temperature sugg...

We report tiny strain-induced piezoelectric effects in an ultrahigh-quality AlGaAs/GaAs V-groove quantum wire structure. Zero photoluminescence excitation (PLE) absorption intensities are observed at low temperatures. Excitation power density-dependent optical properties provide evidence that the zero PLE absorption intensities are due to an internal electric field created by tiny strain-induced piezoelectric polarization along the wires, which causes complete spatial separation of the electron and hole wave functions along the wires. Absorption is enhanced by shining a He–Ne laser as a background in order to screen the electric field, which confirms the existence of piezoelectric field effects.

A novel light extraction technique based on coupling of evanescent waves in a ridge structure is reported. This technique can extract directly light outside the escape cone, which cannot be achieved with the conventional techniques.

AlxGa1−xP(0≤x≤1) epilayers are grown by OMVPE using TBP as a phosphorus source. GaP epilayers with a specular surface are obtained at growth temperatures higher than 740°C with a V/III ratio of 20. The conduction type of the grown layers is n-type, and the carrier concentrations of the GaP layers are as low as 7.4 × 1014cm−3. In the growth of AlxGa1−xP, the growth rate, the Al distribution coefficient, and the electrical properties of the AlxGa1−xP epilayers exhibit very different behavior compared with those of AlGaAs grown by OMVPE.

The disordered quantum-wire superlattice (d-QWRSL), in which individual layer thicknesses are randomly disordered, is grown. Luminescence characteristics are investigated with changing temperature and compared with those of a conventionally ordered quantum-wire superlattice (o-QWRSL). We observe the luminescence capability enhanced by disordering the atomic arrangement. The luminescence intensity of the d-QWRSL decays gently with increasing temperature and then becomes stronger above ∼100 K as compared with that of the o-QWRSL.

We report our comparative study on the luminescence recombination processes of V-grooved AlGaAs/GaAs quantum wires (QWRs) grown using tertiarybutylarsine (TBAs) and AsH3 as arsenic sources. Constant integrated photoluminescence (PL) intensity up to 120 K, as well as radiative efficiency of about 3000 times higher at room temperature, were observed for the TBAs sample compared with the AsH3 sample. Time-resolved PL measurements show a maximum decay time at temperature of as high as 240 K for the TBAs sample, which is about 100 K higher than that of similar samples grown using AsH3 as the arsenic source. These results suggest a dramatic reduction of nonradiative centers in QWR regions grown using TBAs as the arsenic source.

AlGaP layers and AlP/GaP superlattices were grown by atmospheric pressure OMVPE at low temperature on GaP substrates using tertiarybutylphosphine (TBP) as a phosphorus source for safety and using (NH 4 ) 2 S x solution for surface stabilization. High crystalline quality AlGaP epi-layers with specular surfaces were obtained at relatively low growth temperatures ( 4 ) 2 S x surface treatment. The AlP/GaP superlattices were grown at 725°C with a V/III ratio of 20 by using the (NH 4 ) 2 S x treatment. The X-ray diffraction profile showed satellite reflection peaks up to as high as fifth order for the (AlP) 17 /(GaP) 17 superlattice.

We report the first systematic investigation of the temperature-dependent carrier transferring processes of very short period V-grooved GaAs/AlGaAs quantum wire superlattice structures grown by flow rate-modulated metal-organic vapor phase epitaxy. The one monolayer (1 ML) fluctuation causes carrier confinement in sidewall (111) facet superlattice structures, and is shown to play an important role in the carrier transferring process. At low temperatures, the carrier transfer is blocked by the barriers of 1 ML fluctuation, while at high temperatures the carrier transfer from (111) superlattice to the wire region is shown to be very efficient after thermally overcoming the barriers. The temperature-dependent decay times of the different parts give direct evidence of the carrier transferring process, which demonstrates that the carriers can trap into the wire region within 50 ps at temperatures higher than 100 K.

International audience; We report on coherent control of the fundamental exciton state in a single quantum dot by means of two phase-locked picosecond laser pulses. By analyzing the experiments performed in the Rabi regime and comparing them to numerical simulations, we show that we can address the exciton coherence and get an insight into the involved dephasing processes. The total decoherence time T(2) (170 ps) is comparable to the effective exciton lifetime T(1) (200 ps), although not reaching the upper theoretical limit of 2T(1). We conclude that energy relaxation and pure dephasing processes due to virtual scattering with phonons both contribute on an equal footing to the loss of coherence in this kind of monolayer-step-induced GaAs quantum dots.

The use of a Pt/Al 2 O 3 catalyst during GaP growth using triethylphosphorus (TEP) OMVPE is presented. GaP layers are grown by atmospheric pressure OMVPE using TEP as a group-V source. Epilayers with mirror-like surface morphology are obtained with V/III ratio = 17 at 700°C. GaP growth was also conducted with a Pt/Al 2 O 3 catalyst introduced into the growth reactor. We show that carrier concentration in GaP is decreased by about an order of magnitude through the use of the Pt/Al 2 O 3 catalyst.

Photoluminescence is used to characterize the self-limiting effect observed during flow-rate modulation epitaxy (FME) of GaAs quantum wires (QWRs) on V-grooved substrate. Our study found that not only the central thickness but also the overall cross-sectional wire shape is maintained to an atomic level in the self-limited growth regions. This result suggests that the self-limited FME technique is a very promising method for an atomically controlled fabrication of high-quality QWRs.

We report on non-linear effects on the exciton ground state in a single quantum dot, driven by a strong resonant optical field. The resonant excitation is achieved by embedding the quantum dots in a waveguiding structure. Thus, the light–matter coupling is enhanced, allowing the observation of Rabi oscillations up to three periods. With increasing pulse area, a decrease of the exciton lifetime is observed leading to a very rapid damping of the oscillations amplitude. Resonant coherent control experiments show that the coherence time of the ground exciton state in a single quantum dot is fast, of the order of a few tens of picoseconds. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)