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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)

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.

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.

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.

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.

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.

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.

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...

In this article, we make a systematic comparison of the photoluminescence (PL) properties of AlGaAs/GaAs V-grooved quantum wire (QWR) structures with their counterpart quantum well (QWL) structures grown in the same system. The QWR structures exhibit constant photoluminescence (PL) intensity up to a temperature Tc of 90, 140, and 170 K for a 2, 5, and 9 nm QWR, respectively. The 9 nm QWR sample showed the highest relative thermal stability with respect to its counterpart 9 nm QWL sample, its PL intensity being stronger than that of the 9 nm QWL up to 270 K. This temperature is very close to room temperature, at which most opto-electronic devices operate. The time-resolved PL measurements provide further confirmation of the PL results. These data show that the PL thermal stability of the V-grooved QWR structures, despite the increased interface to volume ratio, is now comparable to that of the counterpart QWL structures, which is a basic condition for the superior properties of QWR structures to be brought...

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.

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.

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.

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 have fabricated coupled quantum wires, consisting of two GaAs quantum wires as small as 5 nm thick and 30 nm in width (15 nm in effective width) separated by a 1–3 nm thick AlGaAs barrier, using flow rate modulation epitaxy (FME) on a V‐grooved substrate and observed its optical characteristics. A clear energy splitting between the fundamental coupled state and the higher‐order coupled state of the coupled quantum wires was observed from the room‐temperature photoluminescence. Furthermore, the splitting energy increases as the thickness of barrier decreases from 3 to 1 nm, which is predicted theoretically for coupled quantum systems.

The temperature dependence of photoluminescence (PL) properties of AlGaAs/GaAs quantum wire (QWR) grown on V‐grooved substrates by flow rate modulation epitaxy is investigated. PL from a 7.1 nm thick QWR is easily observed even at room temperature. The full width at half‐maximum (FWHM) of the QWR emission peak increases linearly with increasing temperature at low temperatures and becomes almost independent of temperature at high temperatures, while that of a quantum well layer (QWL) sample increases with increasing temperature up to room temperature. The FWHM of QWR is found to be considerably narrower than that of the QWL sample at high temperatures, which is expected theoretically from the sharp one‐dimensional density of states of QWR but has not been clearly observed experimentally.

Flow rate modulation epitaxy (FME) is applied to the low‐temperature growth of AlGaAs/GaAs quantum wires (QWRs) on nonplanar substrates. The growth selectivity is found to be enhanced greatly by the use of FME, as compared with the conventional metalorganic chemical vapor deposition due to the enhanced migration of Ga species. An AlGaAs/GaAs QWR with a central thickness of about 9 nm and a lateral width of about 28 nm is grown at 600 °C on a V‐grooved substrate. Good photoluminescence properties are observed from the grown QWR, with the peak energy being in good agreement with the calculated energy level of a parabolic shape lateral confinement potential.

The growth interruption time in the organometallic vapor phase epitaxial growth of AlP/GaP layers using tertiarybutylphosphine is optimized for the realization of an AlP/GaP monolayer superlattice. In the growth interruption, there exists an optimum time (2 s) beyond which the full width at half‐maximum (FWHM) of the x‐ray diffraction satellite peaks of an (AlP)5/GaP)5 superlattice becomes broader. The monolayer (AlP)1/(GaP)1 superlattice is realized for the first time in a AlP/GaP system. An average heterointerface roughness of as small as 0.12 A is estimated from the FHWM of the (001) diffraction peak of the (AlP)1/(GaP)1 superlattice.

Electroluminescent diodes with active layers of Al0.5Ga0.5P bulk alloy (b‐AL), (AlP)5/(GaP)5 ordered superlattice (o‐SL), and (AlP)m/(GaP)n (m,n=3,6,9) disordered superlattice (d‐SL) are fabricated by organometallic vapor phase epitaxy using tertiarybutylphosphine. Stronger injection electroluminescence is observed from the d‐SL diode as compared with the b‐AL and the o‐SL diodes. The total light output of the d‐SL diode is about 4–5 times stronger than that of the b‐AL and the o‐SL diodes. The result indicates the remarkable luminescence capability of the d‐SL.

The influence of surface roughness on the initial V-grooved substrate on the interface uniformity of V-shaped AIGaAs/GaAs quantum wires (QWRs) is investigated by direct atomic force microscopy observation of V-groove surface. We found that roughness on the initial V-grooved substrate induced during V-groove preparation processes can severely affect the interface uniformity of QWRs grown on it. We also found that roughness on the initial substrate can be greatly reduced by a simple etching treatment using a NH4OH:H2O2:H2O=1:3:50 solution, and this resulted in significant improvement of QWR interface quality.

The subbands structure of crescent shaped GaAs coupled quantum wires is clearly observed by photoluminescence excitation (PLE) measurements. In the PLE spectra of single quantum wires (wire thickness=4.5 nm), sharp exciton peaks of the first two heavy hole-like transitions are observed with large energy difference of 47 meV. However, two adjacent peaks with small energy splitting of 24 meV are observed in the coupled quantum wires (wire thickness=5 nm, barrier thickness=3 nm). From the measurements of the barrier thickness dependence, these peaks agree well with the symmetric and antisymmetric states of the coupled quantum wires calculated by the finite element method.

A new self-limited growth based on the control of surface migration of Ga atoms during now rate modulation epitaxial growth of GaAs on patterned substrates is demonstrated. By the use of this new growth technique, atomically uniform GaAs quantum wires and quantum dots can be realized easily, despite the existence of pattern size fluctuations in the initial substrate induced by pattern preparation processes.

The photoconductivity (PC) of undoped GaAs/AlGaAs quantum wires (QWRs) along the wire direction is studied for the first time. The PC spectrum reveals a strong substrate-related background as well as several small structures, some of which could be connected with the QWRs. This suggestion is confirmed by the observed polarization dependence of the PC and by photoluminescence (PL) and photoluminescence excitation (PLE) measurements on a similar sample. Prolonged pre-illumination of the sample with infrared light (hν=1.18 eV) considerably reduces the background in the PC spectrum, and makes the QWR structures better resolved.

International audience; In this paper we present a review on major advances achieved over the past ten years in the field of fabrication of semiconductor quantum wires (QWRs) using epitaxial growth techniques and investigation of their optical properties. We begin the review with a brief summary on typical epitaxial QWRs developed so far. We next describe the state-of-the-art structural qualities of epitaxial QWRs in terms of (i) size uniformity between wires, (ii) heterointerface uniformity, (iii) crystal purity, and (iv) strength of lateral quantum confinement. Several prominent breakthroughs have been accomplished concerning the improvements of wire qualities, including (i) realization of V-shaped GaAs/AlGaAs QWRs in the ``real one-dimensional'' (1D) regime in which exciton states can extend coherently over distances exceeding 1 mu m, (ii) reduction of residual impurity concentrations in V-shaped GaAs/AlGaAs QWRs to a level comparable to that in an equivalent quantum well (QWL), which resulted in the semiconductor QWR with room-temperature photoluminescence efficiency exceeding that of a QWL, and (iii) reduction of the multimonolayer (ML) interface fluctuations on the second-grown arm QWL surface, in old-generation T-shaped GaAs/AlGaAs QWRs, to the single-ML level. The second part of this article is devoted to the discussion of optical properties of epitaxial QWRs, such as exciton dynamics, fine structure of exciton levels, and nonlinear effects, studied by means of high-spatial resolution spectroscopy, i.e., microphotoluminescence experiments. We will concentrate our discussions on V-shaped GaAs/AlGaAs QWRs and put an emphasis on demonstrating how the interface quality influences wire's optical properties. The properties of QWRs in the ``zero-dimensional quantum box regime'' and QWRs in the real 1D regime will be presented in separate sections. We will show that the realization of QWRs in the real 1D regime makes possible the investigation of intrinsic 1D effects by focusing on a single perfect 1D wire region using microscopic techniques. This has led to important results, for instance, (i) the demonstration of the square-root dependence of 1D exciton radiative recombination lifetimes down to a temperature as low as 10 K (limited by the experimental setup) and (ii) the clear demonstration of the existence of Mott transition in a 1D exciton system which is a fundamental problem under long debate. (c) 2006 American Institute of Physics.

Femtosecond optical pulse propagation in a quantum wire (QWR) waveguide was investigated by two-color sum-frequency cross-correlation frequency-resolved optical gating (XFROG) spectroscopy. The polarization anisotropy of the crescent-shaped GaAs QWR was observed in terms of absorption and refractive index dispersion by XFROG spectroscopy.

We report the spectral characteristics of AlGaAs–GaAs multiple-quantum-wire (QWR) lasers fabricated by flow rate modulation epitaxy on V-grooved substrates. Room-temperature lasing from the fundamental state (1e-1hh) is generally observed in devices longer than 350 µm; however, secondary peaks at the adjacent higher subbands are immediately detected following current injection. The evolution of the stimulated emission spectra is investigated here. Typical threshold current and the wavelength tuning rate of current are observed to be 6.2 mA and 0.012 nm/mA, respectively, for 800-µm-long uncoated cavities.

A new self-limited growth technique based on control of surface migration in flow rate modulation epitaxial growth on nonplanar substrates is proposed. The proposed self-limited growth is experimentally confirmed by growing AlGaAs/GaAs quantum wires on V-grooved substrates. Using this new self-limited growth technique, atomically uniform semiconductor quantum nanostructures are expected to be fabricated easily.