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This report reveals that diffusion of hydrogen induces gradual degradation in InGaN-based laser diodes (LDs). The increase in nonradiative recombination centers (NRCs) in the LDs has been attributed to diffusion-related phenomena. Factors other than NRCs, such as the threshold carrier density Nth, can increase threshold current Ith. Those factors, however, were not fully investigated. Moreover, the diffusant responsible for the degradation of the LDs has not been univocally identified yet. To separately evaluate the roles of NRCs and Nth in increasing Ith, this report analyzes the stress-induced variation of nonradiative recombination lifetime τnr and lasing wavelength λl. It is revealed that the density of NRCs increases at the first stage of gradual degradation, followed by a rise in Nth. In addition, this report proposes a novel model for the time-variation of 1/τnr to investigate the diffusion-related degradation. By using this model, we extrapolate the value of the diffusion coefficient of diffusants involved in the degradation in InGaN-based LDs. The proposed analysis methods and obtained results are useful for understanding the physics of LD degradation.

High-power InGaN-based laser diodes (LDs) are expected as light-sources of various high-power applications. The catastrophic-optical-damage (COD) is a major failure mechanism to limit the output power, where optical absorption at a light-emitting facet causes an irreversible damage to the LDs. In this paper, a window structure in InGaN-based LDs to suppress the COD is demonstrated for the first time. The structure is formed by the metal organic chemical vapor deposition growth over a recess placed besides of the ridge-waveguide. The In composition in the quantum well which is corresponding to the bandgap energy is reduced by controlling the local tilt angle at the sidewall of the recess. Thus, the formed window structure eliminates undesired optical absorption at the cleaved facet so that extremely high light output power over 2 W in narrow-stripe ridge-waveguide InGaN-based blue-violet LDs is achieved without any CODs.

This report describes a crystallization method we developed for amorphous (a)-Si film by using 405-nm laser diodes (LDs). The proposed method has been used to fabricate bottom gate (BG) microcrystalline (µc)-Si TFTs for the first time. A µc-Si film with high crystallinity was produced and high-performance BG µc-Si TFTs with a field effect mobility of 3.6cm2/Vs and a current on/off ratio exceeding 108 were successfully demonstrated. To determine the advantages of a 405-nm wavelength, a heat flow simulation was performed with full consideration of light interference effects. Among commercially available solid-state lasers and LDs with wavelengths having relatively high optical absorption coefficients for a-Si, three (405, 445, and 532nm) were used in the simulation for comparison. Results demonstrated that wavelength is a crucial factor for the uniformity, efficiency, and process margin in a-Si crystallization for BG µc-Si TFTs. The 405-nm wavelength had the best simulation results. In addition, the maximum temperature profile on the gate electrode through the simulation well explained the actual crystallinity distributions of the µc-Si films.

With this paper we analyze the correlation between the degradation of InGaN-based laser diodes (LDs) and the increase in the non-radiative recombination rate in the active region. Several 405 nm MOCVD LDs have been submitted to CW stress, for 2000 h (stress current in the range 40-100 mA, case temperature = 75 °C). During stress, we extensively evaluated the optical characteristics of the LDs: a technique for the evaluation of the non-radiative recombination lifetime (τ nr ) in the active material was developed and used for the analysis of the stress effects. We demonstrate the following: (1) degradation determines the increase in LDs threshold current (I th ) and the decrease in the τ nr ; (2) degradation of I th and τ nr have similar kinetics; and (3) the degradation rate of the LDs is almost linearly related to the stress current level. The degradation process is therefore ascribed to the decrease of internal quantum efficiency caused by the increase of the non-radiative recombination rate in the active region.

The purpose of this work is an in depth reliability analysis of Blu-Ray InGaN laser diodes (LD) submitted to CW stress at different currents and temperatures. During reliability tests, LD devices exhibit a gradual threshold current increase, while slope efficiency is almost not affected by the ageing treatment. Furthermore we demonstrate that: (i) the degradation rate shows a linear correlation with stress current level; (ii) the I th increase is correlated to the decrease in non-radiative lifetime ( τ nr ); (iii) stress temperature acts as an accelerating factor for LD degradation; (iv) pure thermal storage does not significantly degrade LDs characteristics.

This paper reports on the degradation of InGaN-based laser diodes for Blu-ray technology. The devices have been submitted to stress under: 1) constant current, different temperatures; 2) high temperature, no bias; and 3) constant temperature, several current levels. The tests carried out within this paper demonstrate that stress determines the increase in the threshold current, according to the square root of stress time. The degradation rate has been found to be strongly determined by the stress current level, while the optical field had only a limited role in determining the degradation kinetics. The impact of temperature on device degradation is also limited, as confirmed by the activation energy value of 250 meV extrapolated by measurements carried out at different temperatures. On the basis of the evidence collected within this paper, we attribute the degradation of the lasers to an electrothermally activated process that induces an increase in the nonradiative recombination rate with subsequent decrease in the optical efficiency of the active layer.

This paper reports on high-power and high-temperature operation of an AlGaInP-based high-power red laser diode with magnesium (Mg)-doped quaternary-alloy cladding layer. The use of Mg dopant with small diffusion coefficient enables abrupt doping profiles as well as high carrier concentrations when compared to conventional zinc (Zn) dopant. It was also found that the metal-organic vapor phase epitaxial (MOVPE) growth of Mg-doped quaternary AlGaInP alloy is not affected by so-called reactor memory effects, while unintentional incorporation of Mg is observed in GaAs after the growth of Mg-doped GaAs layers. The higher carrier concentration in the p-type cladding layer enhanced carrier confinement in the active layer so that device performance at high temperature is improved. The abrupt doping profile suppressing dopant diffusion into the active layer eliminates the nonradiative recombination in the active layer resulting in higher external quantum efficiency. The characteristic temperature of the Mg-doped red laser with a lasing wavelength of 659 nm is as high as 167 K while the Zn-doped laser exhibits a temperature of 127 K. High kink-free output power of 150 mW is achieved at 75/spl deg/C.

780 nm-band AlGaAs and 650 nm-band AlGaInP laser diodes are monolithically integrated on a GaAs substrate as the light source in DVD/CD optical pickups. In order to reduce internal losses for improved characteristics and to ensure better flexibility of device design, real-refractive index-guided self-aligned (RISA) structure is employed for both 780 nm and 650 nm-band LDs. As a result, low noise characteristics due to self-sustained pulsation in the both wavelength bands have been achieved. A dual-wavelength laser with a high power ( > 100 mW) 780 nm-band LD has also been developed for recording of CD-R/RW disks.

Low operating current and high-temperature operation has been demonstrated in 650-nm AlGaInP high-power laser diodes with real refractive index guided self-aligned (RISA) structure. The RISA structure features an Al/sub 0.5/In/sub 0.5/P current blocking layer, which leads to small internal loss in the waveguide and substantially reduced operating carrier density. The resultant operating current for 50-mW continuous-wave at 70/spl deg/C is as low as 98 mA, which is almost a half of the lowest value ever reported.

Real refractive index guided self-aligned (RISA) AlGaAs laser diodes have various advantages over conventional complex refractive index guided lasers, which include low operating current, stablefundamental transverse-mode operation up to high power levels, good controllability of current and optica distributions, and so on. In this review, the concept of RISA lasers and their applications to optical data storage systems are presented. Taking advantage of low operating current characteristics of the RISA lasers, a number of optical elements which compose a conventional optical pickup are successfully integrated into a very small plastic package, which results in substantial reduction of pickup size in various optical disk systems.

We have developed 100-mW high-power angled-stripe superluminescent diodes with a new angled-stripe real refractive-index guided self-aligned structure. The structure has a GaAlAs optical confinement layer on a planar active layer and an inclined current injection stripe by 5/spl deg/ with respect to the facet normal. The structure gives small internal loss (/spl sim/10 cm/sup -1/) and facet power reflectivity less than the order of 10/sup -6/. As a result, the output power as high as 105 mW at a low operating current of 270 mA is obtained with less than 3% spectral modulation and 10.5 nm full width at half maximum (FWHM) spectral width.

Self-sustained pulsation in narrow stripe geometry AlGaAs laser diodes is investigated numerically and experimentally. By taking account of the time-dependent two-dimensional optical field, carrier spreading in the cladding layers, and multilongitudinal modes, we succeeded in clarifying the mechanism for the pulsation. Fluctuation of the optical field plays an important role in forming saturable absorption. The numerical analysis clearly shows that our newly proposed real refractive index-guided structure is very suitable for a pulsation laser in which the optical field distribution and current spreading can be independently controlled. Furthermore, me show that the new structure gives much lower threshold current than that of a conventional loss-guided structure. These predicted advantages of the real refractive index-guided structure are verified experimentally with good agreement. >

We have developed a high-power self-sustained pulsation laser by using a multiple-quantum-well real refractive index guided self-aligned (RISA) structure that allows formation of large saturable absorbers outside a stripe region. As a result, a peak power as high as 800 mW was obtained in the laser with the stripe width of 1.0 /spl mu/m. >

A 405 nm LDs crystallization method of a-Si has been applied to the processing of bottom gate (BG) type microcystalline (μc-) Si TFT for the first time. We have successfully demonstrated superior I-V characteristics of BG μc- Si TFTs. In order to verify the validity of our process, we performed a heat flow simulation and compared commercially available lasers having wavelengths of 405, 445 and 532 nm. The simulation explained well the experimental results and showed that the wavelength is a crucial factor on uniformity, energy efficiency, and process margin and the 405 nm gave the best results among the three wavelengths.

High power over 2W free from catastrophic-optical-damage is achieved in InGaN-based lasers with epitaxially-formed window structure. Epitaxial growth over a recess reduces the In composition which eliminates the undesired optical absorption only at the facet.

With this paper we describe an extensive analysis of the reliability of InGaN-based laser diodes, emitting at 405 nm. These devices have excellent characteristics for application in the next-generation optical data storage systems. The analysis aims at describing the degradation process, as well as at investigating the role of current in determining the degradation rate. The results obtained within this paper suggest that the degradation of the laser diodes is correlated to the increase in the non-radiative recombination rate, with subsequent worsening of the optical properties of the devices. Furthermore, our findings support the hypothesis that current is the main driving force for degradation, while temperature and optical power play only a limited role in determining the degradation kinetics.

This paper reports an analysis of the degradation of laser diodes for Blu-Ray technology. The study has been carried out by means of optical and electrical characterization techniques. We demonstrate that: (i) stress at constant current level determines the increase of the threshold current of the devices, and the decrease of the sub-threshold emission signal; (ii) degradation rate has an almost linear dependence on the stress current level, thus suggesting that current is a significant driving force for devices degradation. Furthermore, thanks to an accurate study of the optical parameters of the devices, we demonstrate that the degradation of the samples is due to the decrease of the non-radiative recombination lifetime of the carriers in the active layer, with subsequent decrease of the optical efficiency of the devices. Degradation can be attributed to the generation/propagation of defects in the active layer of the devices, that can be also responsible for the modification of the electrical parameters of the laser diodes.

It is generally speculated that the increase of threshold current Ith in GaN-based blue laser diodes (LDs) during aging is caused by the increase of nonradiative recombination centers in the InGaN MQW active layers. However, no reports directly supported this speculation. In this report, by analyzing current-light output characteristics below threshold, we have proved that the nonradiative recombination lifetime decreases during degradation. The observation directly verifies that the density of defects increases during aging. The increase of Ith is mainly caused by the decrease of carrier lifetime. These results will be useful to comprehend the underlying physics on degradation in GaN-based LDs.

This paper describes an analysis of the reliability of GaN-based laser diodes, submitted to constant current, constant optical power and high temperature stress. We demonstrate that constant current and constant optical power stress induce the increase of the threshold current of the devices, that varies according to the square-root of time. The threshold current increase is found to be strongly correlated to the decrease of the sub-threshold emission of the devices, thus suggesting that stress determines the increase of the non-radiative recombination paths in the active layer. Degradation rate is found to depend on stress temperature and on current level, while it does not significantly depend on the optical field in the cavity. The evidences presented within this work support previous literature results, that attribute devices degradation to the diffusion of impurity species towards the active layer, with subsequent increase of the non-radiative recombination rate. The identified degradation process is shown to be electro-thermally activated.

This paper presents an analysis of the role of current in the long-term degradation of InGaN-based laser diodes (LDs). The analysis has been carried out by means of a wide set of stress tests under different driving conditions. During stress, the optical and electrical characteristics of the samples have been continuously monitored, with the aim of determining the physical mechanisms responsible for degradation and the dependence of the degradation rate on the driving conditions. The results reported in this paper demonstrate that: (i) constant current stress determines the increase of the threshold current of the devices, that is well related to the decrease of the sub-threshold emission; (ii) stress induces the increase of the reverse current of the devices, but does not affect their forward-bias characteristics; (iii) degradation rate has an almost linear dependence on stress current level; (iv) degradation takes place also below lasing threshold and for very low stress current levels, i.e. with very limited optical field levels. The results described in this paper indicate that the degradation kinetics are mostly determined by the stress current level used during operation: optical field and temperature are thought to play only a minor role in determining the degradation rate. Furthermore, the strong correlation between the threshold current increase and the sub-threshold emission decrease suggests that degradation can be ascribed to the increase of the non-radiative recombination rate in the active region of the devices. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This letter describes an analysis of the degradation of InGaN-based laser diodes. The influence of current, temperature, and optical power level on the degradation kinetics has been analyzed by means of a wide set of stress tests carried out under different operating conditions. We demonstrate the following: 1.) the degradation rate is strongly related to the operating current level; 2.) high-temperature stress does not determine significant degradation of lasers characteristics; and 3.) the intensity of the optical field does not significantly influence the degradation rate. Degradation process is found to be electrothermally activated and is ascribed to the increase of the nonradiative recombination rate in the active layer, with subsequent decrease of the efficiency of the devices.

We have successfully attained low‐noise and high‐power operation in GaAlAs lasers by applying high reflectivity facet coating approach to nonabsorbing mirror lasers. High reflectivity coating is used to reduce feedback induced noise coming from hopping between the diode cavity modes and between the external cavity modes. A nonabsorbing mirror structure is used to obtain high‐power operation by suppressing mirror degradation and catastrophic optical damage effectively even with the use of high reflectivity coating. Nonabsorbing mirror lasers with the front facet reflectivity 50% showed the relative intensity noise level less than −130 dB/Hz at 3 mW under 0%–10% optical feedback and showed the cw output power as high as 50 mW in fundamental spatial mode operation. Under 30‐mW cw operation at 50 °C, on the other hand, obvious degradation has not been observed over 4000 h in the nonabsorbing mirror lasers with 50% front facet reflectivity. Thus the present laser is useful as a low‐noise source at a low‐power level and also as a high‐power source with high reliability.

In this paper, we demonstrate Mg-doped AlGalnP LDs with high characteristic temperature. We found that abrupt Mg profiles can be obtained in quaternary alloys like AlGalnP, although significant memory effects are observed in GaAs. Based on this effect, we fabricated AlGalnP LDs with a highly Mg-doped cladding layer. As a result, reliable 650-nm-band LDs with characteristic temperature over 133 K are achieved.

High temperature self-sustained pulsation has been realized with a real refractive index guided self-aligned (RISA) structure which has a compressively strained-MQW for the active region. Self-sustained pulsation is maintained up to 75/spl deg/C at an average power of 7 mW CW and the maximum operating current is as low as 60 mA. The maximum operating temperature for self-sustained pulsation is the highest ever reported.

We demonstrate for the first time low-noise (RIN 150 mW) self-sustained pulsation GaAlAs MQW laser diodes with small-astigmatism (

We have developed for the first time 100 mW high-power angled-stripe superluminescent diodes with a new real refractive index guided self-aligned structure. The structure has a GaAlAs optical confinement layer on a planar active layer and an inclined current injection stripe by 5/spl deg/ with respect to the facets. That gives small internal loss (/spl sim/10 cm/sup -1/) and facet power reflectivity less than the order of 10/sup -6/. As a result, the output power as high as 105 mW at a low operating current of 270 mA is obtained with less than 3% spectral modulation and 10.5 nm full width at half maximum spectral width.

High-power (>100mW) 820 nm-band distributed Bragg reflector (DBR) laser diodes (LDs) with stable fundamental transverse mode operation and continuous wavelength tuning characteristics have been developed. To obtain high-power LDs with a stable fundamental transverse mode in 820 nm wavelength range, an AlGaAs narrow stripe (2.0 micrometers ) real refractive-index-guided self-aligned (RISA) structure is utilized. In the RISA structure, the index step between inside and outside the stripe region ((Delta) n) can be precisely controlled in the order of 10 -3 ). To maintain a stable fundamental transverse mode up to an output power over 100 mW, (Delta) n is designed to be 4x10 -3 . Higher-order transverse modes are effectively suppressed by a narrow stripe geometry. Further, to achieve continuous wavelength tuning capability, the three-section LD structure, which consists of the active (700micrometers ), phase control (300micrometers ), and DBR(500micrometers ) sections, is incorporated. Our DBR LDs show a maximum output power over 200mW with a stable fundamental transverse mode, and wavelength tuning characteristics ((Delta) (lambda) ~2nm) under 100 mW CW operation.

Low operating current and high temperature operation has been achieved in 650 nm AlGaInP high power double quantum well laser diodes with a real refractive index guided self-aligned (RISA) structure. The attained operating current for 50 mW CW at 70/spl deg/C is as low as 98 mA, which is almost a half of the lowest value ever reported.

We have integrated the surface photonic crystal (PhC) on GaN-based blue light-emitting diodes (LEDs) for the first time in order to enhance the extraction efficiency of the LEDs. With the finite-difference time-domain method, we have calculated 3.6-fold enhancement in light output. The theoretical calculations have revealed that the optimum pitch of the PhC is much longer than the emission wavelength when the distance between the PhC and the active layer of LEDs is short. This design enables PhC formation on chemically stable GaN surfaces. In addition, an indium tin oxide (ITO)-based transparent electrode is formed directly on the surface of PhC to realize light emission from the whole area of the LED. The fabricated PhCs have increased the light output of blue LEDs by 1.5 times compared with the LEDs without PhC. We have demonstrated that PhC will realize highly efficient solid-state lighting with GaN-based LEDs.

Monolithically integrated 780-nm-band and 650-nm-band self-sustained pulsating (SSP) lasers, which are desirable for simplified optical pickups in digital versatile disk (DVD) systems, have been developed for the first time. The real refractive index guided self-aligned (RISA) waveguide structure is adapted to reduce absorption loss in the current blocking layers. In order to obtain stable SSP, a saturable absorber formed in the active layer outside the current stripe, and a saturable absorbing layer above the active layer are utilized for the 780-nm-band and 650-nm-band laser diodes (LDs), respectively. Relative intensity noise less than -130 dB/Hz is maintained at temperatures of up to 80°C at an output power of 7 mW for the 650 nm band and 10 mW for the 780 nm band, which suggests that stable SSP operations have been realized.

We demonstrate for the first time a real refractive index guided GaAlAs laser structure that has both very low-noise characteristics at a low output power (reading mode) and a stable fundamental transverse mode up to a high power level (writing mode) sufficient for optical read-write systems. The low-noise characteristics are realized by self-sustained pulsation which is induced by large saturable absorbers outside a stripe, and the high power is achieved by using a thin active layer which confines the optical flux very loosely. In order to optimize the device parameters, numerical simulation of the structure is proposed with consideration of interaction between optical field and carrier diffusion under gain nonlinearity and carrier diffusion taken into account. The calculated results are verified experimentally. The lasers showed relative intensity noise level less than -135 dB/Hz under 0-20% optical feedback and output power higher than 60 mW.