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Spatial distribution of photoluminescence (PL) spectra has been assessed in violet light emitting InGaN single-quantum-well (SQW) structures by employing a scanning near-field optical microscope (SNOM) at room temperature. Clear correlation has been found between PL intensity and PL peak wavelength under illumination collection (I-C) mode, where strong PL was observed in the area emitting at lower energy bands. However, dark spots having diamter of about a few hundreds nm appeared also in lower energy region. Moreover, multi-mode SNOM-PL with a spatial resolution of 200 nm revealed that the difference of PL-intensity image between I-C mode and illumination (I) mode is not so large comparing with a blue light emitting InGaN-SQW. These findings suggest that threading dislocations act as nonradiative recombination centers in the violet sample unlike blue ones, and that the diffusion length to such centers is larger than that to localized centers leading to radiative recombination. Furthermore, nonradiative recombination centers originating from point defects have been spatially identified in the violet sample grown on epitaxially laterally overgrown (ELO) GaN template. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We fabricated GaN-based ultraviolet laser structures having air/semiconductor distributed Bragg reflectors (DBR) as a cavity mirror by focused ion beam (FIB) etching. The designed structure consisted of 5 pairs of 3λ/4 air gaps and 7λ/4 semiconductor layers. A scanning electron microscope (SEM) measurement showed that the fabricated DBR had vertical sidewalls and that the widths of air gaps and semiconductor layers were 275–305 nm and 260–290 nm, respectively. The numerical calculation taken a diffraction loss in the air gap into account demonstrated that the reflectivity of the fabricated DBR mirror should be higher than 95%. The photo-pumped lasing measurement revealed that the threshold excitation power density of the laser structure with the DBR mirror at one end of the laser cavity was reduced by 27% compared with the one without DBR mirrors. This reduction in the lasing threshold corresponds to the improvement of the reflectivity from 17% for facet mirrors to 52–74% for the DBR mirrors, suggesting that the fabricated DBR mirror has high potential for application to integrated photonic components. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) See erratum physica status solidi (c) Vol. 2, No. 7, 2932 (2005)

Starting with next generation high density optical disc, commercial use of GaN based violet laser diodes (LD) has been adopted in many new fields, such as bio-medical, reprographic, exposure and etc. Recently, the lasing wavelength has broadened to cover from ultraviolet (UV) to blue-green region, which enabled other new applications. In this paper, current status of GaN based LDs in UV to Blue-green region are reported and next target is discussed.

We review the recent progress of GaN-based light emitting diodes (LEDs) and laser diodes (LDs) and discuss the availability for ultraviolet (UV) emission optoelectronic devices by using GaN related compound semiconductors. We fabricated UV LEDs and LDs with an emission wavelength of 365 nm which are useful for various industrial uses because of same wavelength as i-line of high-pressure mercury vapor lamps. Regarding UV LEDs, practical 365 nm UV LED with high efficiency was realized by optimizing device structure. For fabricating the ultraviolet LDs, we used the AlInGaN active layer instead of InGaN one. It was investigated that the relationship between the threshold current density and the lasing wavelength in the UV region. We demonstrated the shortest LDs with a lasing wavelength 365 nm under the continuous-wave operation.

Since 1993, InGaN light-emitting diodes (LEDs) have been improved and commercialized, but these devices have not fulfilled their original promise as solid-state replacements for light bulbs as their light-emission efficiencies have been limited. Here we describe a method to enhance this efficiency through the energy transfer between quantum wells (QWs) and surface plasmons (SPs). SPs can increase the density of states and the spontaneous emission rate in the semiconductor, and lead to the enhancement of light emission by SP-QW coupling. Large enhancements of the internal quantum efficiencies (eta(int)) were measured when silver or aluminium layers were deposited 10 nm above an InGaN light-emitting layer, whereas no such enhancements were obtained from gold-coated samples. Our results indicate that the use of SPs would lead to a new class of very bright LEDs, and highly efficient solid-state light sources.

We studied the differentiation of thyroid nodules using fine-needle aspiration biopsy (FNA) and Tl-201 scintigraphy quantitative analysis. One-hundred and thirty-one thyroid nodules were examined: 83 follicular lesions (58 benign and 25 malignant lesions) and 48 non-follicular lesions (8 benign and 40 malignant lesions). During Tl-201 scintigraphy examinations, an early and a delayed image were acquired 10 and 120 min after an intravenous injection, respectively. The T/N ratio (counts of nodular lesion/counts of contralateral normal thyroid tissue) of each image was calculated quantitatively. We assessed the ability of the Tl-201 scintigraphy and of the FNA analysis to differentiate benign and malignant lesions and determined the cut-off levels for the assays. For the follicular lesions, the area under the ROC (Receiver Operating Characteristic) curve (Az) for the Tl-201 scintigraphy data was greater than that for the FNA data. For the non-follicular lesions, the Az for the FNA data was greater than that for the Tl-201 scintigraphy data. We set cut-off levels at 1.370 for follicular lesions, and 1.070 for non-follicular lesions. The sensitivity and specificity were 76% and 82.7% for follicular lesions, and 90% and 87.5% for non-follicular lesions, respectively. The overall accuracy of the analysis was 84.0%.

We have investigated the thermal activation process of Mg dopant in Mg-doped GaN by secondary ion mass spectrometry (SIMS) and Hall measurement with the van der Pauw configuration. We have found a systematic relationship between the amount of the hydrogen dissociated from the Mg-doped GaN layer by thermal annealing and the electronic conductive properties of the layer, i.e., Hall mobility, Hall carrier density and resistivity of the layer. The hydrogen in the Mg-doped GaN layer has been classified into at least two different modes. The first mode involves relatively larger activation energy of the dissociation process from the Mg-doped GaN layer, and the dissociation energy from the Mg-doped GaN to the atmosphere is 0.8–1.5 eV. The second mode has the dissociation energy of 0.2–0.5 eV. The former mode may be associate with the hydrogen passivation of the Mg dopant in GaN and the latter one may be closely related to the passivation of the electrical compensation mechanism for the positive hole carriers in the Mg-doped GaN.

Optical and structural properties of an InGaN double-quantum-well (DQW) laser diode (LD) wafer that lased at 450 nm were investigated to discuss an enormous impact of a polarization-induced electric field on the recombination dynamics in InGaN quantum structures. The quantum-well (QW) structure was shown to have the well thickness as thin as approximately 1 nm and InN molar fraction x of approximately 14%. The gross effective electric field in the QW (FQW) was estimated to be 490 kV/cm from the Franz–Keldysh oscillation (FKO) period in the electroreflectance (ER) spectrum, implying that an internal piezoelectric field (Fpiz) of approximately 1.4 MV/cm was cancelled by the pn junction built-in field (Fbi) and Coulomb screening due to carriers in the DQW. The magnitude of FQW can be further weakened by applying reverse bias (VR) on the junction; the decrease in the photoluminescence (PL) lifetime at low temperature measured under VR was explained to be due to a recovery of electron-hole wavefunction overlap for small VR (|VR

We review the recent progress of nitride-based light-emitting diodes (LEDs) and discuss the relation between dislocation density and quantum efficiency. We also discuss how to improve the external quantum efficiency of nitride-based LEDs. Secondly, the group-III nitride laser diodes (LDs), which emit from near-ultraviolet to pure-blue, are reviewed. Reducing threading dislocations can increase the lifetime of nitride LDs. Using the epitaxial lateral overgrowth technique, a dislocations density of the order of 105 cm−2 has been obtained. The relation between the lifetime of nitride LDs and the density of dislocations is discussed. Finally, near-ultraviolet LDs and pure-blue LDs are described. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigated the hot carrier dynamics in InGaN multi-quantum wells using photoluminescence, time-resolved photoluminescence (TRPL) and white-light pump and probe methods at room temperature. The pump and probe spectra or ΔOD spectra have not reflected the screening of electric field by photoexcited carriers but the distribution of hot carriers. It reflects the band filling of the tail state in lower energy side and Maxwell-Boltzmann-like tail of hot carriers on the higher energy side. ΔOD spectra in 1.5 ps rose in the observed spectral region, which indicates that thermalization among carriers is very fast. It is because of carrier-LO phonon and carrier-carrier interaction. After 2 ps ΔOD spectra are red shifted as increasing the time delay. At 40 ps time delay, the Maxwell-Boltzmann-like tail shows still the high temperature of carriers. This slow cooling is likely due to the hot phonon effects where the nonequilibrium phonon population may lead to a reabsorption of phonons by photoexcited carriers.

Bound and unbound biexcitons in a free-standing bulk GaN are investigated by time-integrated and spectrally-resolved four-wave mixing measurements, where the formation of hetero-biexcitons that consist of A and B excitons (XX AB ) as well as A biexcitons and their unbound biexciton are clearly observed. Through the FWM spectra and delay-time dependence, it is found that the interaction between A and B excitons gives rise to the phase shifts of the quantum beating and the energy shifts of the spectra, which is considered as the effect of the unbound state of XX AB (i.e. XX * AB ). This interaction is found to play an important role in the FWM signals for all polarizations.

Optical properties induced by two major effects, potential fluctuation and piezoelectric fields, have been assessed to interpret the emission mechanism in low-dimensional nitride semiconductors because the former leads to the exciton/carrier localization, and the latter to the quantum confined Stark effect (QCSE). Degenerated white-light pump-and-probe spectroscopy has been employed to assess which factor plays an important role in the series of In x Ga 1-x N multiple quantum well (MQW) structures whose well widths are 3 nm, 5 nm and 10 nm. Moreover, photoluminescence (PL) mapping with scanning near-field optical microscopy (SNOM) has revealed the dense distribution of island-like structures, the size of which ranges from 20 nm to 70 nm in a 3 nm-thick In x Ga 1-x N single quantum well (SQW) structure emitting at blue spectral region.

We have remarkably improved the emission efficiency of 365 nm ultraviolet (UV) light emitting diode (LED). Rugged pattern is fabricated on the surface of LED chips in order to enhance the extraction efficiency. As a result, the extraction efficiency of this LED approximately increased by 50%. When this UV LED was operated at a forward-bias pulsed current of 500 mA at room temperature (RT), the peak wavelength, the output power (Po), the operating voltage (Vf) and the external quantum efficiency (ηex) were 365 nm, 240 mW, 4.5 V and 14.1%, respectively. On the other hand, at a forward-bias direct current of 500 mA at RT, Po, Vf and ηex were 210 mW, 4.3 V and 12.4%, respectively. The ηex at 365 nm increased to double that of the reported value before. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A low resistive p-type GaN layer highly doped with Mg (1020 cm-3) was studied by conductance frequency spectroscopy. Three peaks were found in the G/f–f curve of the reversed-biased Mg doped p–n junction at temperatures ranging from 300 K to 77 K. The activation energies associated with two peaks are 63 meV and 90 meV, respectively around room temperatures, and decrease with decreasing temperatures. The activation energy of one more peak appearing at low temperatures is 16 meV, the value of which is very shallow compared to the reported value. Analysis using a multi-deep level model is given, and the experimental results are well explained by this analysis. These results suggest that the Mg acceptor in GaN has not a simple hydrogen-like structure, but a complex multi-level structure.

We fabricated a high-color-rendering, red-enhanced white-light-emitting diode (LED) using a new red phosphor and a short-wavelength YAG phosphor. When the new white-LED was operated at a forward-bias current of 20 mA at room temperature (RT), color temperature (Tcp), the general color rendering index (Ra) and luminous efficiency were 4670 K, 87.7 and 25.5 lm/W, respectively. Most of the color-rendering indexes (CRIs) of the new white-LED were larger than those of current white-LEDs, in which only YAG is used. In particular, the CRI-No.9 value, which shows the color reproduction in the red region, is improved from -2.5 to 62.6.

Photoluminescence and reflectance spectra of Si-, Ge- and Sn-doped GaN epilayers grown on sapphire substrates at 10 K have been investigated, by which ionization energies on the Ga site were determined as 29, 30 and 33 meV, respectively. To investigate the coherent properties of excitons, the dephasing times of excitons in undoped and Si-doped GaN on sapphire substrates and freestanding GaN were measured using the degenerate four-wave mixing technique. The resulting homogeneous broadenings in undoped and Si-doped GaN on sapphire substrates were about twice as large as that in freestanding GaN, which indicates that defect-induced scattering is stronger in the former than in the latter.

It is well known that GaN-based semiconductors are the most excellent material for short wavelength light-emitting devices. In this paper, we review the development of GaN-based laser diodes (LDs). 400 nm InGaN-LDs are mainly described. Additionally, ultraviolet (UV) and purely-blue LDs are demonstrated, and the emission-wavelength dependence of the threshold current density is studied.

We have measured photothermal signals of a bulk GaN crystal by the photothermal divergence (PTD) method in the temperature range from 110 K to 370 K. The thermal conductivity and the scattering time of phonons contributing to the thermal conductivity have been evaluated from the photothermal signals. A phonon-defect interaction has been expected to play a crucial role in determining the thermal properties of the sample employed here in the above temperature range.

First, we review the recent progress on nitride-based light-emitting diodes (LEDs) and discuss the relation between dislocation density and luminous efficiency. We also discuss a method to improve the external quantum efficiency of nitride-based LEDs. Secondly, group-III nitride laser diodes (LDs) which emit from near-ultraviolet to pure-blue are reviewed. Reducing the threading dislocation density can increase the lifetime of nitride LDs. Using an epitaxial lateral overgrowth technique, a dislocation density of the order of 10 5 cm -2 has been obtained. The relation between the lifetime of nitride LDs and the dislocation density are discussed. Finally, near-ultraviolet LDs and pure-blue LDs are described.

The crystal orientation of GaN films deposited on fused quartz substrates by rf planar magnetron sputtering using a mesh to guard against plasma exposure was investigated by X-ray diffraction. Sample films were deposited at constant N 2 gas pressure and input power at a range of substrate temperatures between 84°C and 600°C. It was found that GaN films with good crystal orientation can be produced by rf sputtering with the c -axis of GaN crystals oriented normal to the substrate surface in almost all films produced. The crystal 0 0·2 orientation was good at substrate temperatures exceeding 300°C, however, films deposited at 600°C peeled from the substrate. The fine mesh installed to protect the film from exposure to plasma was found to be very useful for depositing films with a good crystal orientation of 0 0·2, and the best crystal orientation was realized for a film deposited by sputtering with a target to mesh separation of 30 mm.

Electroluminescence (EL) mapping has successfully been performed for In x Ga 1-x N single quantum well (SQW)-based light emitting diodes (LEDs) by employing scanning near-field optical microscopy (SNOM) at room temperature. The relative EL intensity fluctuates in the range from 1 to 4 with the spatial scale less than a few hundred nanometers. Clear correlation has been found between EL peak wavelengths and EL intensities where the regions of long wavelength correspond to those of strong EL intensity, suggesting that the injected carriers redistribute toward local potential minima within In x Ga 1-x N SQWs.

InGaN multiple quantum well laser diode (LD) wafer that lased at 400 nm was shown to have the InN mole fraction, x , of only 6% in the wells. Nanometer-probe compositional analysis showed that the fluctuation of x was as small as 1% or less, which is the resolution limit. However, the wells exhibited a Stokes-like shift (SS) of 49 meV and an effective localization depth E 0 was estimated by time-resolved photoluminescence (TRPL) measurement to be 35 meV at 300 K. Since the effective electric field due to polarization in the wells is estimated to be as small as 286 kV/cm, SS is considered to originate from an effective bandgap inhomogeneity. Because the well thickness fluctuation was insufficient to produce SS or E 0 , the exciton localization is considered to be an intrinsic phenomenon in InGaN material. Indeed, bulk cubic In 0.1 Ga 0.9 N, which does not suffer any polarization field or thickness fluctuation effect, exhibited an SS of 140 meV at 77 K and similar TRPL results. The origin of the localization is considered to be due to the large bandgap bowing and In clustering in InGaN material. Such shallow and low density localized states are leveled by injecting high density carriers under the lasing conditions for the 400 nm LDs.

Violet laser diodes (LDs) with higher power and longer lifetime are required for some applications such as read/write laser light sources of digital versatile disks (DVDs). We achieved that the reliability of the LDs for DVDs was improved by using low dislocation density free standing GaN substrates. Moreover, we fabricated blue and ultraviolet (UV) LDs in order to spread the emission wavelength. The details of LD characteristics in the blue and UV region were investigated. In the blue LDs, there is a strong relationship between the threshold current density and the emission wavelength of LDs. For near UV LDs, we fabricated three types of LDs whose active layers are (I) ternary InGaN, (II) quaternary AlInGaN, (III) binary GaN single quantum well structures (SQW). We demonstrated the binary GaN SQW LDs under continuous-wave operation for the first time. From our experiments, we successfully expanded the range of GaN-based LDs emission wavelength, from UV (365 nm) to pure blue (465 nm).

Ultraviolet (UV) laser diodes (LDs) were grown on epitaxially laterally overgrown GaN substrates by a metalorganic chemical vapor deposition method. We fabricated three types of UV LDs whose active layers were (I) ternary InGaN, (II) quaternary AlInGaN and (III) binary GaN single quantum well (SQW) structures. We investigated the LD characteristics in detail in the near UV region. LDs whose active layers were quaternary Al0.03In0.02Ga0.95N were demonstrated under the pulsed-current-biased conditions at room temperature. The emission wavelength of these LDs was the shortest wavelength (366.4 nm) in our experiments. We also demonstrated binary GaN SQW LDs for the first time. The threshold current density, voltage and emission wavelength of these LDs under 25°C continuous-wave (cw) operation were 3.5 kA/cm2, 4.6 V and 369.0 nm, respectively. The estimated lifetime was approximately 2000 h under 25°C cw operation at an output power of 2 mW.

Carrier dynamics in two types of InGaN-based semiconductors were investigated using nondegenerated pump and probe spectroscopy. The samples consist of an In 0.1 Ga 0.9 N active layer, 30 nm thick (sample a) or an In 0.1 Ga 0.9 N/GaN multiple quantum well with ten periods, 3 nm/10 nm thick (sample b). Pump and probe spectroscopy revealed that photo-generated carriers pumped at 370 nm (3.350 eV) rapidly reach the bottom of levels in the active layers within a time scale of several ps for both samples. For the sample a, a positive peak at 3.21 eV was sandwiched between two negative peaks at 3.29 and 3.17 eV in the spectra of photo-induced change of absorption coefficient (Aa) because Franz-Keldysh and Stark effects on excitonic absorption were almost eliminated due to the screening of internal electric field. However, for the sample b, only negative signals were observed in Δα, indicating that the band filling of localized states plays a more important role. This is because the effect of electric field on the optical transition property is different depending on well widths.

Halide vapor phase epitaxy of thick GaN films was demonstrated on ScAlMgO4 (SCAM) substrates, and their self-separation was achieved. The 320-µm-thick GaN film was self-separated from the SCAM substrate during the cooling process after the growth. This separation phenomenon occurred because of both the c-plane cleavability of SCAM and the difference in the thermal-expansion coefficients between GaN and SCAM. The dark-spot densities for the GaN films on the SCAM substrates were approximately 30% lower than those on sapphire substrates. These results indicate that SCAM substrates are promising for fabricating a high-quality freestanding GaN wafer at a low cost.

Carrier dynamics in GaN and InGaN/GaN SQW structures were observed by using the transient grating (TG) method with sub-picosecond pulsed laser at room temperature. The diffusion coefficients (D) of photo-created carriers were estimated by the decay rate time of TG signals and the photoluminescence (PL) lifetime. It was found that D depends on the emission wavelength (In composition). The relationship between the emission efficiencies and carrier diffusion was considered in terms of the spatial inhomogeneity of In composition.

Spatial distribution of photoluminescence (PL) spectra has been assessed in an InGaN single quantum well (SQW) structure by means of fluorescence microscopy and scanning near-field optical microscopy (SNOM) under illumination-collection mode. The PL intensity of fluorescence image is uniform at 77 K, but the dark spot areas were extended with increasing temperature. The near-field PL images revealed the variation of both peak energy and intensity in PL spectra according to the probing location with the scale less than a few hundreds nm.

We review the progress in the field of InGaN-based light-emitting diodes (LEDs) and discuss the issue of threading dislocations and the luminous efficiency. The first candela-class blue LEDs have been developed. An InGaN layer was used to produce these LEDs instead of a GaN active layer. The quantum-well structure InGaN active layer dramatically improved the external quantum efficiency. There are a number of threading dislocations in nitride-based LEDs. InGaN LEDs, however, have quite high external quantum efficiency. With regard to this, it is thought that the fluctuation of the indium mole fraction is strongly related to the high external quantum efficiency. Considering the density of threading dislocations in the nitride-based LEDs, we discuss what can improve the external quantum efficiency of nitride-based LEDs.

Recombination dynamics of spontaneous and stimulated emissions have been assessed in InGaN-based light emitting diodes (LEDs) and laser diodes (LDs), by employing time-resolved photoluminescence and pump and probe spectroscopy. As for an In0.02Ga0.98N ultraviolet LED, excitons are weakly localized by 15 meV at low temperature, but they become almost free at room temperature (RT). It was found that addition of a small amount of In results in the reduction of nonradiative recombination centres originating from point defects. The internal electric field does exist in InGaN active layers, and induces a large modification of excitonic transitions. However, it alone does not explain the feature of spontaneous emission observed in an In0.3Ga0.7N blue LED such as an anomalous temperature dependence of peak energy, almost temperature independence of radiative lifetimes and mobility-edge type behaviour, indicating an important role of exciton localization. The lasing mechanism was investigated for In0.1Ga0.9N near ultraviolet (390 nm), In0.2Ga0.8N violet-blue (420 nm) and In0.3Ga0.7N blue (440 nm) LDs. The optical gain was contributed from the nearly delocalized states (the lowest quantized levels (LQLs) within quantum wells) in the violet LD, while it was from highly localized levels with respect to the LQL by 250 meV for the violet-blue LD, and by 500 meV for the blue LD. It was found that the photo-generated carriers rapidly (less than 1 ps) transferred to the LQL, and then relaxed to the localized tail within the timescale of a few ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of the LQL under higher photo-excitation.

Defects in GaN grown using metalorganic chemical vapor deposition were studied through the use of monoenergetic positron beams. For Mg-doped GaN, no large change in the diffusion length of positrons was observed before and after activation of Mg. This was attributed to the scattering of positrons by potentials caused by electric dipoles of Mg–hydrogen pairs. For Si-doped GaN, the line-shape parameter S increased as carrier density increased, suggesting an introduction of Ga vacancy due to the Fermi level effect. Based on these results, we discuss the effects of the growth polar direction of GaN on optical properties in this article. Although the optical properties of a GaN film grown toward the Ga face direction exhibited excitonic features, a film grown toward the N face (−c) direction exhibited broadened photoluminescence and transmittance spectra, and a Stokes shift of about 20 meV was observed. This difference was attributed to extended band-tail states introduced by high concentrations of donors and ...

Time-resolved optical characterization is an indispensable tool to study the recombination mechanisms of excitons and/or carriers based on radiative, non-radiative, localization and many-body processes. In this paper, we review the instrumentation of various spectroscopic techniques for the assessment of In x Ga 1 -x N-based semiconductors such as time-resolved photoluminescence (TRPL), time-resolved electroluminescence (TREL), transient grating (TG) method to probe photothermal processes, microscopic TRPL using optical microscope, submicroscopic TRPL using scanning near field optical microscopy (SNOM) and pump-and-probe spectroscopy for the measurement of transient absorption/gain spectra. The obtained results are cited in the references.

To understand the etching phenomenon or preferential etching in the GaN crystal that consisted of two elemental atoms, dry etchings were carried out using rare gases with a simple structure. n-type GaN crystals were RF-plasma-etched with Ar, Kr, and Xe gases at various gas pressures and etching times. The etched surfaces were analysed with the XPS method. The surface composition (N/Ga ratio) of the samples etched at Ar pressure of 10 mTorr and 200 mTorr almost agreed with the calculation values of the plasma particle model (PIS). N/Ga ratio of Ar etched sample at the pressure of 50 – 100 mTorr was approximately one and did not agree with the PIS calculation. This discrepancy seems to be related to the pit-like defect formation at the pressure range. The values of the N/Ga ratio of the samples etched with Kr and Xe gases were approximately one and did not agree with the theoretical PIS calculation, which predicts N/Ga > 1. This discrepancy may have occurred by N2 molecule formation with excess N atoms, and which may have been desorbed from the surface. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The surface morphology of n-GaN etched by the He plasma is independent of gas pressure and etch time: it is observed to be as smooth as that of the as-grown. This result is significantly different from that by the Ar plasma etch which causes surface defects at a high gas pressure (50∼100 mTorr) and long etch time (> 60 min) due to synergy effect of plasma ions and UV lights emitted. This difference would result from a difference between UV light spectra emitted: for the He plasma there is no UV light less than 380 nm in wavelength, whereas for the Ar plasma there is the UV light emission. The simulation shows that the N/Ga ratio at the surface etched by the He plasma at each gas pressure (Ga-rich) is much lower than that by the Ar plasma. The results simulated for the He plasma are in disagreement with those measured by XPS, which would probably indicate additional removal of Ga from the Ga-rich surface by volatilisation/evaporation due to thermal effect. For the Ar plasma, the experimental N/Ga ratio at a low gas pressure (10 mTorr) is reproduced by physical effect, whereas at the high gas pressure it is not reproduced, which would be closely related to occurrence of the surface defects. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties of the InGaN single-quantum-well amber light-emitting-diodes were investigated to verify the importance of localized quantum well (QW) excitons in their spontaneous emission mechanisms. The internal piezoelectric field (FPZ) across the QW of the order of 1.4 MV/cm was confirmed to point from the surface to the substrate. Transmittance and photovoltaic spectra exhibited a broad band tail state, and the emission lifetime increased with decreasing detection photon energy. The electroluminescence spectra did not show remarkable energy shift between 10 and 300 K. The spectra exhibited an exponential tail and the higher energy portion increased more rapidly than that of the lower energy one, reflecting a thermal distribution of the localized carriers within the tail states. Since the well thickness is only 2.5 nm and is atomically flat, the device is considered to exhibit reasonably efficient emission with the external quantum efficiency of 5% at 20 mA in spite of the presence of FPZ and larg...

Optical spectra of GaN, In-doped GaN, and InGaN single quantum well (SQW) structures were compared to explore the role of In for the emission mechanisms in the SQW light emitting diodes (LEDs). The internal electric field, F, due to spontaneous and piezoelectric polarization in strained quantum wells (QWs) naturally induces the quantum confined Stark effect (QCSE) that reduces the electron–hole (e–h) wavefunction overlap due to rapid ionization of excitons causing a serious decrease of the emission efficiency especially for the GaN SQW, although a clear QW excitonic absorption peak was observed. The low energy tail of the emission spectrum of InGaN amber SQW LEDs extends below the bulk InN bandgap, indicating that F is not completely screened by injected carriers. However, doping and alloying of In in GaN improve the emission efficiency despite the fact that the effective bandgap inhomogeneity is increased with increasing InN mole fraction. In is considered to support effective localization of QW excitons, and the Coulomb interaction between the e–h pair is maintained provided that the QW thickness is smaller than the free exciton Bohr radius.

Optical gain of InGaN quantum well laser diodes and its internal field dependences are investigated. Gain spectra are experimentally measured by the Hakki-Paoli method. Inhomogeneous broadening of the gain spectra as well as of the electroluminescence spectra increases with In content of the active layers due to potential fluctuation. From measured carrier decay times and current density, we estimated the carrier density for which modal gain overcomes internal loss. We present gain calculation including quantum confined Stark effect, Coulomb screening and valence band structure simultaneously. Calculated and measured carrier densities are in good agreement. In InGaN quantum well laser diodes working at 470 nm, the quantum confined Stark effect reduces the optical transition probability down to 30% of its value without internal field. Wavelength (nm) 520 500 480 460 440 420 400 380 2.4 2.6 2.8 3.0 3.2 3.4 Photon energy (eV) Typical electroluminescence spectra of InGaN quantum well laser diodes with different peak wavelengths. Injected current is 10 mA.

We fabricated three types of high luminous efficiency white light emitting diodes (LEDs). The first is the white LED, which had a high luminous efficiency (ηL) of 161 lm/W with the high luminous flux (v) of 9.89 lm at a forward-bias current of 20 mA. The blue LED had a high output power (e) of 42.2 mW and a high external quantum efficiency (ηex) of 75.5%. The second is the high luminous efficiency white LED with a low forward-bias voltage (Vf) of 2.80 V, which was almost equal to the theoretical limit. ηL and wall-plug efficiency (WPE) were 169 lm/W and 50.8%, respectively, at 20 mA. They were approximately twice higher than those of a tri-phosphor fluorescent lamp (90 lm/W and 25%). The third is the high power white LED fabricated from the high power blue LED with high e of 651 mW at 350 mA. v, ηL, and WPE of the high power white LED were 145 lm, 134 lm/W, and 39.6% at 350 mA, respectively.

Ultraviolet (UV) InGaN/AlGaN double-heterostructure (DH) light-emitting diodes (LEDs) with an external quantum efficiency of 7.5%, an output power of 5 mW and an emission wavelength of 371 nm were developed. High-power UV LEDs are obtained using an InGaN active layer with a thickness of 400 A instead of a GaN active layer. The localized energy states caused by In composition fluctuation in the InGaN active layer are related to the high efficiency of the InGaN-based LEDs.

The use of wildflowers has become popular for creating aesthetic public landscaping. As weeding costs are rather expensive, automation of weeding would help reduce maintenance costs. In this study, as a step towards automation of weeding, identification of plants for wildflower gardens was performed by image processing. Shape feature analysis was applied to the binary images to identify the plant as one of four species (aster, pansy, treasure flower, and California Poppy). As features, roundness, first invariant central moment, area, and perimeter were studied for each plant type. Mahalanobis’ generalized distance was used in identification of the plants. The results showed that the best shape features for identifying these plants were roundness and first invariant central moment.

A low on-resistance and high-breakdown-voltage enhancement-mode (E-mode) AlGaN/AlN/GaN high electron mobility transistor (HEMT) was fabricated without the recessed-gate process. A thin AlGaN barrier layer (4.5 nm) was used for the normally-off operation. In order to decrease the on-state resistance, two different techniques are used. One is the side-ohmic contact, which has a low contact resistance, due to the direct contact with the two-dimensional electron gas (2DEG). The other is SiO2 passivation, which induces the sheet carriers and decreases the sheet resistance. As a result, an on-state resistance of 1.9 mΩcm2 and an off-state breakdown voltage of 610 V were achieved for the E-mode HEMT within a threshold voltage of approximately -0.1 V.

We fabricated the high luminous efficiency white light emitting diode (LED) and the high power white LED by using the patterned sapphire substrates and an indium–tin oxide (ITO) contact as a p-type electrode. The high luminous efficiency white LED was the yellow YAG-phosphors-coated small-size (240 ×420 µm2) high efficiency blue LED with the quantum efficiency of 63.3% at a forward-bias current of 20 mA. The luminous flux (Φ), the forward-bias voltage (Vf), the correlated color temperature (Tcp), the luminous efficiency (ηL), and the wall-plug efficiency (WPE) of the high luminous efficiency white LED are 8.6 lm, 3.11 V, 5450 K, 138 lm/W, and 41.7%, respectively. The luminous efficiency is 1.5 times greater than that of a tri-phosphor fluorescent lamp (90 lm/W). The high power white LED was fabricated from the larger-size (1 ×1 mm2) blue LED with the output power of 458 mW at 350 mA. Φ, Vf, Tcp, ηL, and WPE of the high power white LED are 106 lm, 3.29 V, 5200 K, 91.7 lm/W, and 27.7%, respectively, at 350 mA. The WPE is greater than that of a fluorescent lamp (25%) in the visible region. Moreover, the luminous flux of the high power white LED reaches to 402 lm at 2 A, which is equivalent to the total flux of a 30 W incandescent lamp.

We have studied the optical gain and its suppression mechanism of In x Ga 1− x N multiple quantum well structures with different indium compositions. The variable stripe length method and the up-conversion method were employed in this research. It was found that the optical gain spectra became broad and the differential gain decreased with increasing the indium composition. These results indicated that inhomogeneous broadening affected not only spontaneous emission but also stimulated emission. The rise time of photoluminescence under a strong excitation condition was changed from 0.95 ps to 12.4 ps depending on the indium composition. Assuming the rise time reflects the relaxation lifetime from the absorption edge energy to population inversion levels, our calculation suggested that long rise times led to gain suppression in In x Ga 1− x N laser diodes with larger indium compositions.