High performance AlGaAs-based laser diodes: Fabrication, characterization and applications

We clarified the mechanism of forward-biased electrostatic-discharge-induced degradation of InP-based laser diodes. This degradation was caused by melting of the active layer as a result of light absorption. We observed a reduction in tolerance on aging in uncoated laser diodes. This reduction was suppressed by facet coating.

Gallium nitride (GaN)‐based semiconductor laser diodes (LDs) have garnered significant attention due to their promising applications. However, high‐power LDs face serious degradation issues that limit their practical use. This study investigates the degradation factors of 437 nm and 6.3 W LDs by comparing light–current–voltage (L–I–V) characteristics, transmission electron microscopy (TEM), cathodoluminescence (CL), and secondary ion mass spectroscopy (SIMS) before and after 1000‐h aging. The diffusion of mirror coating from the resonant cavity surface is identified as a key factor contributing to high‐power LD degradation, which has not been reported in milliwatt‐level LDs. Meanwhile, the mechanisms behind the LD degradation are profiled and summarized together with the diffusion and other factors. On basis of the mechanism exploration, an anti‐aging technology for high‐power GaN‐based LDs is developed by using aluminum nitride for passivation layer and sapphire materials for mirror film. This anti‐aging technology has been verified, and a nearly ten‐time degradation suppression is achieved from 1000 h. This study elucidates the degradation mechanisms of high‐power GaN LDs and provides an effective technology to extend their lifespan, thereby prompting the practical applications of high‐power LDs.

A unified theoretical model for the degradation of II-VI blue-green laser diodes and light-emitting diodes (LEDs) is presented. The model is based on the recombination-enhanced defect generation. Based on this model and the corresponding rate equations for the laser diodes and LEDs, we show a general picture to explain the aging behaviors of both the optical output power of the LEDs under a constant current aging condition and the increase of the operation current of laser diodes under constant optical power operation. We found a universal curve for the optical output decay of the II-VI LEDs as a function of the aging time. The power decay is associated with the increase of defect density caused by electron-hole recombination enhancement. Our model also explains very well the increase of the operation current of laser diodes under the constant optical output aging condition.

Gradient index micro lenses for laser diode applicationsR. Goering, T. Possner, B. MesserschmidtFraunhofer Institution for Applied Optics and Precision EngineeringSchillerstrasse 1 , 07745 Jena, GermanyABSTRACTCollimating and diverging cylindrical micro lenses with N.A. up to 0.60 fabricated by thermal silver ion exchangein a special designed glass type are reported. Cylindrical microlens arrays are fabricated by field assistet ion-exchange. Applications to fiber coupling as well as to diode bar collimation are discussed.Keywords: ion-exchange, GRIN-components, laser diode beam shaping, laser bar collimation, fiber coupling1. INTRODUCTIONMicrooptical GRIN-lenses with high N.A. are very useful for the transformation of the strongly divergent andasymmetric beam emitted from semiconductor laser diodes and laser diode bars to a well-collimated beam orto a focused spot. Preferred applications are the coupling between laser diodes and optical fibres, the pumpingof solid state lasers as well as optical scanning. High N.A., a nearly diffraction limited pertormance and lensdimensions of a few hundred microns are required. In a previous paper we presented the formation of one-dimensional cylindrical microlenses by a two step silver ion-exchange process1. Here we would like to discussthe optimization of these components and a number of new elements fabricated by similar ion-exchangeprocesses and applications which could not be realized by standard SELFOC-lenses.Figures 1 and 2 show flat GRIN-components and ion-exchange techniques for their realization. The formationof diverging lenses is possible in a single ion-exchange step. For the generation of cylindrical collimating lenseswe use a two step process, proposed by Ohmi2. Cylindrical microlens-arrays are produced by a field-assistedexchange using a masked substrate.Fig. 1: Thermal ion-exchanged GRIN components

Catastrophic optical damage of high power InGaAs/AlGaAs laser diodes

The application of indium tin oxide (ITO) as the p‐cladding layer of II‐VI compound semiconductor laser diodes (LDs) on InP substrates was investigated. The waveguide analysis of the LD structures revealed that the optical confinement effect around the active layer was obviously improved by changing the p‐cladding layer from the conventional MgSe/BeZnTe superlattice to ITO. For example, the estimated optical confinement factors were 0.15 and 0.27 for the conventional and ITO LD structure, respectively, when the emission wavelength was 580 nm. In addition, we investigated optimum LD structures, considering the optical and carrier confinements at the active layer. In experiments, light emitting devices with an ITO layer were fabricated on InP substrates via molecular beam epitaxy and radio‐frequency (RF) magnetron sputtering. Yellow emissions at 582 nm were observed by current injections at room temperature. These results indicate that ITO is a promising p‐cladding layer material for II‐VI LDs on InP substrates. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

630-nm band AlGaInP laser diodes are very attractive light sources for high-density optical disk systems. These visible laser diodes are required for high performances such as self-pulsating operation, low threshold current, and so on. Recently, we developed self-pulsating 630-nm band AlGaInP laser diodes with a tensile strained multi-quantum well (S-MQW) active layer by optimizing structural parameters. However, the threshold current was not sufficiently low because of the thick active layer. On the other hand, we have investigated a strain-compensated multi-quantum well (SC-MQW) structure and achieved a single longitudinal mode visible laser diode with a low threshold current. In this study, we report the self-pulsating 630-nm band AlGaInP laser diodes with the lowest threshold current by introducing the SC-MQW structure into the active layer for the first time.

The deep-level emission at 1.0 eV is analyzed to be associated with the slow degradation of GaAlAs DH laser diodes. The intensity of this deep-level emission increases at the same rate as the increase in the threshold current during the slow degradation. The carrier lifetime does not change correspondingly to the increase in the threshold current. This deep level is considered to act as the photon-absorption center to cause the increase in the threshold current.

High power diode lasers have been widely used in many fields. To meet the requirements of high power and high reliability, passively cooled single bar CS-packaged diode lasers must be robust to withstand thermal fatigue and operate long lifetime. In this work, a novel complete indium-free double-side cooling technology has been applied to package passively cooled high power diode lasers. Thermal behavior of hard solder CS-package diode lasers with different packaging structures was simulated and analyzed. Based on these results, the device structure and packaging process of double-side cooled CS-packaged diode lasers were optimized. A series of CW 200W 940nm high power diode lasers were developed and fabricated using hard solder bonding technology. The performance of the CW 200W 940nm high power diode lasers, such as output power, spectrum, thermal resistance, near field, far field, smile, lifetime, etc., is characterized and analyzed.

An empirical method for lifetime projection of 1.55 /spl mu/m InGaAs/InP MQW-DFB laser diodes (LD) is presented. On the basis of experimental results of accelerated aging test for 1500 hours, relationship between LD degradation, operating voltage, and ambient temperature has been determined. The presented method makes it possible to predict the lifetime of LDs by determining the thermal voltage ratio.

This PhD thesis describes the experimental study of wurtzite III-nitride semiconductors grown on non-polar crystal orientations, namely (1120) a- and (1010) m-planes. Hindered by poor material quality, they were not as extensively investigated as the polar basal (0001) c-plane. Nevertheless, the renewed interest in this topic has been gaining momentum since 2000, thanks to the improvement in material quality. The motivation is to overcome the quantum-confined Stark effect (QCSE) in heterostructures grown on c-plane, which is induced by the intrinsic spontaneous and piezoelectric polarization field. QCSE weakens the performance of c-plane optoelectronic devices such as light emitting diodes (LEDs) and laser diodes and it can be avoided by growing these devices on non-polar planes. In addition, non-polar LEDs are also featured by the spontaneously polarized light emission and thus have a large potential for liquid crystal display backlighting application. This thesis includes two aspects: (1) Since previous study in our laboratory was lacking before this work, the first target was to establish the heteroepitaxial growth procedures for a- and m-plane GaN layers on r- and m-plane sapphire substrates, respectively, by hydride vapor phase epitaxy (HVPE). Then, smooth planar templates have been obtained. Particularly, m-plane GaN has been demonstrated on m-plane sapphire. Further analysis revealed that actually three orientations (two semi-polar and one non-polar) can be grown on m-plane sapphire through the modification of sapphire surface chemistry and/or growth procedure. Nevertheless, the material quality of these non-polar templates is inferior to that of c-plane as they are characterized by the presence of high-density stacking faults and threading dislocations. To reduce the defect density, epitaxial lateral overgrowth (ELO) technique has been applied. As a result, material quality close to that of planar c-plane was achieved in GaN layers overgrown on the mask areas. Overall, the experimental results revealed peculiar characteristics of non-polar orientation, induced by the intrinsic anisotropy of wurtzite structure parallel and perpendicular to the basal plane. It also lays the foundation for further improvement in material quality. (2) Based on these aforementioned results, the next stage was to investigate the heterostructures grown on those HVPE ELO templates. GaN/AlGaN single quantum wells (QWs) and AlN/GaN distributed Bragg reflectors (DBR) have been grown by NH3-source molecular beam epitaxy (MBE). The experiments confirm that QWs are free from QCSE and their optical properties are comparable to those grown on planar c-plane templates. Furthermore, nitride-based microcavities (MCs) have been demonstrated on a-plane ELO templates. Their optical properties are encouraging and open the way for more sophisticated structure aimed at polariton physics. In conclusion, non-polar a- and m-plane GaN planar and ELO templates have been obtained through epitaxy on r- and m-plane sapphire substrates, respectively, by HVPE. Nanostructures such as QWs, DBRs, and MCs have been grown on non-polar HVPE GaN ELO templates by MBE.

Over the last two decades III-nitride optoelectronic devices have experienced an impressive evolution in terms of performance. However, their potential is far from being fully exploited. Although they offer bandgaps from the deep UV to the infrared spectral range, lasing is currently restricted to the 336?536 nm wavelength region. Several challenges still need to be overcome. For example, the thermal stability of the InGaN active region becomes an issue in long-wavelength (? > 500 nm) laser diodes: high indium content (In > 25%) quantum wells (QWs) have been shown to markedly degrade when kept above 900 °C. For this reason, the growth temperature of the subsequent p-type waveguide and cladding layers must be sufficiently low to avoid QW degradation, which is accompanied by a reduction in the internal quantum efficiency. Thanks to the high vacuum environment, molecular beam epitaxy (MBE) allows growing these layers at a much lower temperature compared to metalorganic vapor phase epitaxy (MOVPE), and could be used to reduce the thermal budget on the active region, further extending the wavelength range. Thus the aim of this thesis is the realization of long wavelength laser diodes by combining high efficiency active regions grown by MOVPE with p-doped layers grown by ammonia-MBE. Hence, the p-type doping by ammonia-MBE is first scrupulously studied and optimized on 500 nm thick (Al)GaN layers. In order to comprehensively address the physical processes governing the doping efficiency, the role of the growth temperature, the V-III ratio, and the growth rate is investigated. In addition, the Mg cell configuration resulting in the best doping profile control and reproducibility is studied as well. Electrochemical capacitance-voltage profiling is extensively used to study the compensation mechanisms. The optimization leads to the achievement of layers exhibiting low dopant compensation (below 5%) up to high doping levels and resistivity values as low as 0.4 Ohm­ cm. These layers are then implemented on top of active regions grown by MOVPE and lasing is obtained for the first time in hybrid structures exhibiting state-of-the-art electrical characteristics. The threshold voltage is as low as 4.3 V in an index guided structure with 800x2 ?m² ridge dimension designed for violet emission (? = 400 nm). These excellent current-voltage characteristics are achieved owing to the doping profile control in the cladding and contact layers which is confirmed by the low contact resistance value of 5E-4­ cm². Lasing is also demonstrated beyond 500 nm, keeping state-of-the-art electrical characteristics, despite the reduced doping in the cladding layer. These devices also exhibit values for internal optical losses as low as 6 cm?1. The growth of tunnel junctions is also attempted. Low specific resistance values are demonstrated in ammonia-MBE grown GaN tunnel homojunctions. The absence of hydrogen passivation and the reduced memory effects of MBE are considered to be crucial for the realization of these devices together with the stability of the doping at reduced growth temperatures. n-p-n structures featuring a tunnel junction in series with a p-n junction are then used to spread the current across light-emitting diodes. A device architecture featuring a buried tunnel junction to be used for current injection inmicro-LEDs or vertical cavity surface emitting lasers is also demonstrated and excellent current confinement properties are obtained.

We have studied the aging-induced degradation effect and the related mechanism of blue and ultraviolet (UV) laser diodes (LDs). First of all, the F parameter value, leakage current, and yellow luminescence intensity of LDs all increase after 24 h of the aging process, indicating that one of the reasons for the degradation of UV LDs may be the increase of the non-radiative recombination center in the material. Second, irreversible damage may be found on the front cavity surface of the UV and blue LDs. Due to the large UV photon energy, water molecules in the environment atmosphere are ionized to form OH− ions, which combine with dust in air to form SiO2 sediments and then attach to the front cavity surface. In addition, a large photon energy may cause damage to the anti-reflection film on the front cavity surface and lead to a too-high local temperature near the cavity surface, resulting in molten Ga droplets. Both sediment and the precipitation of molten GaN on the cavity surface will directly affect the function of the front cavity surface and the output power of the LD. In order to improve the reliability of the GaN-based UV LDs, it is necessary to reduce the density of material defects, select more stable coating materials on cavity facets, and improve the sealing property of the device package.

Diffraction-limited high-power narrow-spectrum red diode lasers are attractive for many applications, such as photodynamic therapy, laser display, and as a pump source to generate UV light by second harmonic generation (SHG). High-power, diffraction-limited diode lasers can be realized by the technology of lasers with a tapered gain-region (Kintzer et al., 1993; Donnelly et al., 1998; Wenzel et al., 2003; Paschke et al., 2005; Sumpf et al., 2009; Fiebig et al., 2010). The tapered laser devices can be used in applications where narrowspectrum is not needed such as photodynamic therapy, but for other applications such as a pump source for UV light generation, the spectral quality of these devices has to be improved. In order to improve the spectral quality of a tapered laser, different techniques are applied, such as a monolithically integrated master oscillator power amplifier by forming Bragg gratings in the semiconductor material (O’Brien et al., 1993, 1997a), injection locking to an external single-mode laser (Goldberg et al., 1993; Mehuys et al., 1993b; O’Brien et al., 1997b; Wilson et al., 1998; Ferrari et al., 1999; Spieβberger et al., 2011), and different external-cavity feedback techniques (Jones et al., 1995; Cornwell & Thomas, 1997; Morgott et al., 1998; Goyal et al., 1998; Pedersen & Hansen, 2005; Chi et al., 2005; Lucas-Leclin et al., 2008; Tien et al., 2008; Sakai et al., 2009). Up to 1 W output power at 668 nm from a Fabry-Perot tapered diode laser was obtained with a beam quality factor of 1.7, and the spectral width was smaller than 0.2 nm (Sumpf et al., 2007). Around 670 nm, tunable narrow-linewidth diffraction-limited output was also achieved from an injection-locking tapered diode laser system seeded with a single-mode external-cavity diode laser (Haring et al., 2007); the output power was up to 970 mW. A 670 nm micro-external-cavity tapered diode laser system was demonstrated with a reflecting volume Bragg grating as a feedback element; in continuous wave (CW) mode, more than 0.5 W output power was obtained, and in pulse mode, 5 W peak power was obtained with a beam quality factor of 10 and a spectral width below 150 pm (Tien et al., 2008). Up to 1.2 W output power at 675 nm from a tapered laser

In this paper thermal characteristic and front facet of the 808 nm GaAs-Based laser diodes (LDs) before and after degradation are presented and discussed. The relation between thermal characteristic and the change of facet is analyzed. Aging tests are carried out under the conditions of the constant current stress (3 A), and the junction temperature of LD is fixed at 70°C. The total thermal resistance increases from 2.9 to 6.5 *C/W before and after degradation. Furthermore, the contribution of each component to the total thermal resistance has been obtained from the differential structure function and cumulative structure function. The thermal resistance of chip becomes slightly larger in magnitude after degradation than that before degradation, while significant increase in the thermal resistance of solder layer (indium attaching material) and package body is observed after degradation. In addition, some spots and lines of melted materials are observed using laser microscope after degradation, which lead to temperature increase of front facet and the optical power decrease. We can see that the total thermal resistance increase with the appearance of these spots and lines defect in facet of LDs. Consequently, from the results above we can conclude that the performance of LDs could be improved through optimizing of the front facet and package body of LDs.