Materials and reliability handbook for semiconductor optical and electron devices


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Materials and reliability handbook for semiconductor optical and electron devices

  1. 1. Chapter 8 InGaN Laser Diode Degradation Piotr Perlin and Łucja Marona Abstract We discuss the current knowledge of degradation processes in InGaN laser diodes. It is quite surprising that after quite a few years of intensive studies, there is still no clear picture of physical mechanisms lying behind these aging processes. First of all, in contrast to GaAs counterparts, the nitride laser degradation seems to be independent from extended defects movement and multiplication, is uniform across the device surface, and is rather unrelated with optical phenomena. Involvement of point defects in this process is very tempting but not yet sufficiently experimentally supported hypothesis. Bipolar GaN devices will very likely teach us new things about the physics and technology of wideband gap devices – a truly new thing in the optoelectronics. 8.1 Introduction Since the first appearance of nitride optoelectronic devices, the problem of their reliability and understanding the mechanism of degradation processes turned out to be crucial. The early studies that focused on light-emitting diodes (LEDs) revealed that the degradation of these devices is dominated by the packaging issues (epoxy darkening and carbonization) [1, 2]. Packaging improvement enabled manufacturing blue-green LEDs operating at current densities close to 20 A/cm2 and of the lifetime of 50–100 kh, the value satisfying for the most of the manufacturers. P. Perlin (*) Institute of High Pressure Physics, Semiconductors Laboratory, Polish Academy of Sciences, Sokołowska 29/37, 01-142 Warsaw, Poland TopGaN Ltd., Warsaw, Poland e-mail: Ł. Marona Institute of High Pressure Physics, Semiconductors Laboratory, Polish Academy of Sciences, Sokołowska 29/37, 01-142 Warsaw, Poland O. Ueda and S.J. Pearton (eds.), Materials and Reliability Handbook for Semiconductor Optical and Electron Devices, DOI 10.1007/978-1-4614-4337-7_8, # Springer Science+Business Media New York 2013 247
  2. 2. In contrast to LEDs, nitride laser diodes (LDs) reliability issues were found to be much more complex. The early laser diodes were grown on sapphire substrates which are not lattice matched to GaN. Therefore, these devices were characterized by a very large density of defects of the order of 109 to 1010 dislocation per square centimeter. It was not too surprising that the lifetime of the first InGaN laser diodes was very short. For example, the first CW operated laser demonstrated by Nakamura lived only 1 s [3]. An obvious approach was to increase the lifetime of laser diodes by reducing the dislocation density. Epitaxial lateral overgrowth (ELOG) technology allowed to reduce the dislocation density approximately by two orders of magnitude [4]. In parallel, availability of freestanding GaN substrates (dislocation densities <104 cm2 ) was gradually improving. Using of both, ELOG technique and GaN substrates, allows to increase lifetime of nitride laser diodes to desirable 10,000 h [4]. Though the described progress in the longevity of nitride laser diodes was impressive, the mass production of these devices started with a significant delay (6–7 years), which may mean that the production yield and reliability problems were not completely solved at that time. At present, though a limited number of companies apparently possess a long-living nitride laser diodes know-how, our understanding of the degradation processes in this important mate- rial system is still very limited. 8.2 Structure of InGaN Laser Diode Nitride-based laser diode structures are grown by two epitaxial method: mostly by metal-organic vapor phase epitaxy (MOVPE) [4] but also by molecular beam epitaxy (MBE) [5] on bulk GaN substrates. There are only few GaN substrates suppliers, and they use different fabrication technologies. Sumitomo Electrics uses “DEEP” method [6], Ammono Ltd. uses ammonothermal growth method [7], TopGaN Ltd. is specializing in high-pressure gallium nitride growth [8], and Furukawa, Hitachi Cables, Mitsubishi, Lumilog, and Kyma use various modifications of hydride vapor phase epitaxy (HVPE) to manufacture GaN substrates. Gallium nitride substrates have many quality disadvantages like crystal bowing, dislocations, and too small available sizes. Substrate problems influence epitaxial growth often leading to cracking, defects propagation, and poor uniformity of growth. Ability to grow good quality nitride layers is a crucial issue for obtaining satisfying parameters of the device. The conventional epistructure of InGaN laser diode is a separate confinement heterostructure (SCH) consisting of 1–3 InGaN quantum wells [4]; see Fig. 8.1. Usually on top of the active layers, a p-type AlGaN layer, acting as a electron blocker, is placed. In minority cases, EBL can be moved up and put between the upper waveguiding layer and the upper cladding layer. Active region is covered by waveguides, usually formed by GaN layers of the thickness of approximately 100 nm. Waveguiding layers may be Si- or Mg-doped, but they may be also left 248 P. Perlin and Ł. Marona
  3. 3. undoped. Whole structure is sandwiched by n- and p-doped AlGaN claddings to avoid optical mode leakage. 8.3 Packaging of InGaN Laser Diodes Nitride laser diodes are typically mounted into 5.6 mm TO cans (TO18 package) (Fig. 8.2) or quite rarely into 9 mm cans. In most cases, the laser diodes are assembled in a p-up configuration. Depending on the manufacturer, laser chips are mounted on the top of a highly thermally conducting submount (heat spreader) or directly on the stem of the package. Very high thermal conductivity of the GaN allows for heat-spreader elimination. The important aspect of nitride laser diodes is hermeticity of the package. Since the high photon energy of the light emitted from a laser enhances the chemical reactions on the mirrors, it is important to keep moisture and hydrocarbons out of the operating devices. This issue will be discussed in more detail in the next chapters. 8.4 Symptoms of the Degradation Degradation of the semiconductor laser diodes can be divided into three groups depending on the device region where a damage occurs as shown in the Table 8.1 below. In the next sections, problems with facet, active layer, and contact degradation will be discussed in more details. Fig. 8.1 Scheme of nitride laser diode epitaxial structure 8 InGaN Laser Diode Degradation 249
  4. 4. 8.4.1 Facet Degradation Quite frequently, the first symptom of laser diodes degradation occurs at their facets. Mirror degradation is a phenomenon well known in case of arsenide devices [9]. We can roughly divide it into two classes: first one is a catastrophic optical mirror damage (COMD), a process which manifests through the sudden increase of the mirror’s temperature leading to melting of the facets. Increase of the tempera- ture is caused by an optical absorption in the near-facet area of a device and a positive feedback loop between the increase of device temperature and the increase of the optical absorption. The threshold of COMD was determined to be around 40–57 MW/cm2 [10, 11] for InGaN laser diodes. These values are an order of magnitude higher than for GaAs counterparts, thus showing the potential of nitride lasers for high optical power emission. However, a mechanism of COMD is far from being well understood in the nitride system. The second effect related with mirror degradation is the formation of carbon deposits on the surface of the output mirror. It was discovered that under the operation in the oxygen-free atmosphere, a complicated process of photo-assisted hydrocarbons decomposition occurs leading to the formation of carbon deposit and fast laser diode Fig. 8.2 Packaged nitride laser diode: (a) nitride laser diode installed on top of diamond heat spreader in TO 18 package, (b) laser chip soldered directly to the stem of the package (A courtesy of TopGaN Ltd.) Table 8.1 Degradation mechanisms in nitride laser diodes Region of a device Phenomena Factor Laser facet Catastrophic mirror damage (COD), oxidation, different chemical reactions Humidity, light, atmospheres, surface states, temperature Active layer Nonradiative recombination centers Current flow, temperature Contacts Metal diffusion Current flow, temperature 250 P. Perlin and Ł. Marona
  5. 5. degradation. Similar processes were observed in GaAs-based lasers and were called package-induced failure (PIF). This term was introduced by Julia Sharps to name the phenomenon of very fast degradation of 980 nm, high power laser diode operated in hermetic packages under neutral gas atmosphere [12]. The fingerprint of this degra- dation mode was the formation of carbon deposit on the mirrors of the laser during its operation and high, positive, susceptibility to the presence of oxygen in the laser package [13]. It was shown that the reaction is related to the hydrocarbon photoin- duced decomposition and the source of the hydrocarbons may be the package atmosphere or the package material [13]. Quite the same observations have been made recently by groups studying the facet degradation of nitride laser diode. Schoedl et al. [14] found that operating laser diode in dry nitrogen atmosphere leads to the formation of a thick deposit. This deposit can be altered or even remove by adding oxygen to the atmosphere. The authors also claim that the presence of a water vapor speeds up the mirror oxidation. Observation of the carbon deposit formation was also reported by Kim et al. [15]. They limited this process by an additional cleaning and careful sealing of the package. Our experiments performed on laser diodes in argon or nitrogen atmospheres with or without an addition of oxygen show the following: 1. The fastest degradation occurs in dry nitrogen atmosphere. 2. The addition of the oxygen slows down the degradation but does not eliminate it completely until the proper cleaning (oxygen plasma asher and UV ozone cleaning) of the structure is performed. 3. The facet degradation involves the formation of carbon deposit not only in the region of the laser stripe but also around the chip at the level of p-n junction. 4. The progress of degradation is irregular; the oscillation of the threshold current may occur, related to the deposit growth and delamination. 5. The source of hydrocarbons is most likely the electroplated gold on the laser chips but also very likely gold layers on the commercial package material. The carbon deposit formed on the facet of our laser diode is presented in Fig. 8.3. Judging from our experience, the carbon deposit formation may limit the lifetime of devices from few tens up to hundreds of hours. The chemistry of this process is still not well understood, and the resemblance to classical 980 nm PIF mechanism has to be investigated. 8.4.2 Active Layer Degradation Elimination of facet degradation reveals much more fundamental process occurring in the whole volume of the active area of a nitride laser diode. As a first step, let us analyze symptoms observed during the aging of a device. Degradation in InGaN- based laser diodes occurs the most frequently through the increase of the threshold current like it is shown in Fig. 8.4 [16, 17]. What can be easily seen is that the strong increase of the threshold current is not accompanied by the proportional change in the slope efficiency. 8 InGaN Laser Diode Degradation 251
  6. 6. Very slow evolution of slope efficiency of the degraded devices seems to be quite reproducible feature of nitride LDs [16, 17]. This behavior agrees with the simple model of the increase of the nonradiative recombination within the active layer [16]. According to this model, constant slope efficiency implies constant Fig. 8.3 Carbon deposit protruding from the facet of an InGaN laser diode (Courtesy of TopGaN Ltd.) 0 40 80 120 160 200 240 0.0 2.5 5.0 outputpower(mW) current (mA) t=0 h t=600 h t=900 h Fig. 8.4 The evolution of light-current curves with the degradation time 252 P. Perlin and Ł. Marona
  7. 7. injection efficiency and no increase of optical losses (in nitride devices cavity losses are dominated by the optical absorption on magnesium states). The slope efficiency (SE) can be expressed by: SE ¼ j am ai þ am ; (8.1) where j is injection efficiency, am mirror losses, and ai internal losses. The threshold current was reported [16–18] to increase with the square root of time like it is shown in Fig. 8.5. The square root dependence was quite frequently associated with diffusion according to the Einstein-Smoluchowski formula: Dx ¼ ffiffiffiffiffi Dt p ; (8.2) where Dx is change of the stoichiometry along one direction, D is diffusion coefficient, and t is time. However, the question arises: what actually diffuses? Though the interdiffusion of magnesium was a tempting candidate for the degrada- tion mechanism, there is only one very indirect evidence provided by secondary ion mass spectroscopy (SIMS)-smeared magnesium profile measured at large area light-emitting diode [19]. The lack of SIMS profiles measured at the laser diodes tested under standard conditions is related to the difficulty in performing 200 400 1.0 1.1 1.2 1.3 square root - fit experiment Relativechangeoftheoperatingcurrent Aging time (hours) Fig. 8.5 The increase of operation current as a function of time in comparison with a square root function 8 InGaN Laser Diode Degradation 253
  8. 8. SIMS measurements over very tiny area of the laser stripe. However, we recently managed to accurately measure SIMS profiles of many dopants (Mg, H, Si) [20] over a stressed area of a laser diode (Fig. 8.6). In contrast to previously reported data, our new results show no magnesium nor hydrogen profile changes with the degradation time. Thus, we can safely say that these two elements are most likely not involved directly in the relevant diffusion process. This of course does not exclude local rearrangements of atoms around defects centers, complexes, etc. Additionally, there is still open question concerning possibility of native defects diffusion. Though the square root dependence of degradation dominates in some cases, also linear behavior of the current versus time is observed like the one shown in Fig. 8.7. At the present moment, it is not clear why in some cases linear degradation mode dominates and if it means that the mechanism of the degradation is different. Degradation of laser diodes is often characterized by a value of so-called activation energy (EA). This value can be estimated by using Arrhenius formula: D ¼ C exp EA kT ; (8.3) where D is degradation rate, C is a constant, EA activation energy, T temperature, and k Boltzmann constant. Degradation rate can be defined in various ways, as, for example, change of the threshold current in the unit of time or a number of accumulated failures as a function of time. Arrhenius equation shows that degrada- tion is thermally activated process. The values of the activation energy for nitrides are reported to be between 0.3 [11, 16] and 0.5 eV [21]. Finally, the additional argument that the degradation is related to the active area of the device (volume) and is nor related with complicated laser diode topology was provided by Meneghini et al. [17]. They showed that if instead of laser diode you fabricate a completely planar light-emitting diode (LED) of the identical epistructure to the laser, then it degrades in the identical manner scaled only by 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1016 1017 1018 1019 1020 initial after aging concentration(cm−3 ) Distance from the surface (µm) MgA Fig. 8.6 Magnesium SIMS profiles measured over the aged area and virgin area of the heavily stressed laser diode (10,000 h of operation) 254 P. Perlin and Ł. Marona
  9. 9. the value of the operating current density (much lower for LEDs). The authors establish also a linear relation between the aging rate and the current density for all tested devices. Optical Gain in Stressed Laser Diodes Optical gain is a decisive parameter which defines the laser threshold. Optical gain is a function of the carrier density in the active area of the laser diode and thus depends on the recombination kinetics. One of the first studies of the optical gain in the stressed laser diode was performed by Goddard [22] from Xerox Palo Alto Group. They demonstrate that the laser diode degradation (an increase of the threshold current) is accompanied by a gradual decrease of the optical gain. We performed systematic study of the gain spectra taken during laser diode aging. On Fig. 8.8, we can see the results of our study. The gain spectra were recorded using a Hakki-Paoli method. We see that after degradation a magnitude of the gain peak is decreased when measured under constant current conditions. However, if current is increased again, we can see that gain curve returns to its original strength and form. This result can be interpreted in such a way that under same operating current, due to an increase nonradiative recombination or carrier escape from the quantum wells, the number 0 50 100 150 200 250 300 60 70 80 90 100 ILD[uA] t[hours] LD5800 d41 Fig. 8.7 An example of a close to linear degradation behavior of nitride laser diode. Vertical axis shows a device operating current measured at constant optical power of 10 mW 8 InGaN Laser Diode Degradation 255
  10. 10. of carriers in the InGaN quantum wells is lower which lowers the gain value correspondingly. The stability of the gain spectral shape confirms the material stability excluding, for example, the quantum well interdiffusion. Nonradiative Lifetime Measurements As we mentioned before, an expected reason for laser diode degradation is genera- tion of nonradiative recombination centers within the active layer of the stressed device. This is why it is so important to determine the nonradiative recombination time (or rate) during the aging process. Direct measurements of the recombination rates on completely processed and packaged devices are hard to perform. There- fore, an important approach to determine these parameters is to use an analysis of subthreshold light-current characteristics of laser diodes. The starting point for such an analysis is always the so-called A-B-C rate equation [23]: jI qV ¼ AN þ BN2 þ CN3 ; (8.4) where j is an injection efficiency, I is a current, q is a charge on electron and V is an active area volume, A is nonradiative recombination rate coefficient, B is 400 410 420 430 440 450 460 470 −50 −40 −30 −20 −10 0 10 gaincm−1 Wavelenght (nm) before aging after aging, 50 mA after aging, 80mA Fig. 8.8 Gain spectra in the aged device. Black curve shows an initial gain peak, a red curve presents the spectrum measured at the same current, and a green curve shows the gain for the device with increased current to compensate the gain loss 256 P. Perlin and Ł. Marona
  11. 11. bimolecular radiative rate coefficient, and C is typically related with Auger recom- bination or similar mechanism. Optical power is related with the recombination coefficient through the follow- ing equation: P ¼ kBN2 : (8.5) Using the above two formulas, relation between optical power and current can be derived: I ffiffiffi P p ¼ a þ b ffiffiffi P p ; (8.6) where parameters a and b are defined by: a ¼ qV j A ffiffiffiffiffiffi kB p ; (8.7) b ¼ qV j 1 k : (8.8) The application of the Eq. 8.6 for the determination of the nonradiative lifetime in nitrides laser diode was demonstrated by Ryu et al. [24]. On Fig. 8.9, we show the measured a parameter which is basically proportional to nonradiative recombination rate A (we assume here that bimolecular radiative recombination B is constant). As it is visible, a parameter steadily grows suggesting a steady increase of nonradiative recombination rate. The increase of the nonradiative recombination in the active layer of the device can be intuitively associated with the increasing density of deep levels 0 20 40 60 80 100 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 parametera(arb.units) Time (h) Fig. 8.9 Depending of nonradiative recombination on the aging time of the device 8 InGaN Laser Diode Degradation 257
  12. 12. (within Schottky-Read model). One of the few methods suitable for determining the concentration of these nonradiative recombination centers is deep level transient spectroscopy (DLTS). In this method, we measure the junction-related capacitance transient in relatively broad temperature range. Meneghini et al. [25] performed DLTS measurements during the degradation of InGaN laser diodes. They detected a new signal of the amplitude increasing with the degradation time. This signal can be attributed to the electron trap situated 0.35–0.45 eV below the conduction band minimum. Meneghini et al. pointed out that such a level was previously attributed to VGa-(ON)3 complex [26]; however, this important problem needs further studies. Role of the Extended Defects From the very beginning of nitride laser diode fabrication, the dislocations were considered as a key factor influencing the lifetime of these devices. As we men- tioned before, the first CW operated violet laser of Nakamura et al. lased only for one second [3] and contained most likely around 109 dislocations per centimeter squared. Systematic study performed by Takeya et al. [18] demonstrated very steep dependence of the laser diodes’ lifetime on the number of threading dislocations existing in the laser diode structure. The lifetime of the device varies from hundreds to ten thousand hours when defect density decreases from 107 down to 106 cmÀ2 . Though the strong dependence of the laser diode lifetime on the defect density is a fact, the physical interpretation of this relation is far from being clear. In the classical III-V systems, the dislocation glide and multiply, leading to the formation of so-called dark lines [9] is a main cause of degradation. Movement and multipli- cation lead to the effective increase of the number of nonradiative recombination centers. In the nitride laser diodes, the dislocation does not multiply. Gallium nitride, with its large cohesion energy and strong, short interatomic bonds, should not be too much susceptible to these phenomena. Indeed, most of the existing papers claim that the dislocations in the stressed and unstressed regions of the aged laser diodes look the same, like it is demonstrated by Tomiya et al. [27] by using electron transmission microscopy (TEM) characterization. It means that no new dislocations are created, though the authors of that paper reports on the observed recombination-enhanced dislocation glide (REDG) mechanism. The dislocation activity was observed in situ in TEM microscope by irradiating the laser structures with electrons. These dislocations were, most likely, the dislocation loops lying in (0001) basal plane. However, the authors of the previously discussed paper claim that this mechanism cannot lead to multiplication of threading dislocations being extended along the c axis of the crystal. The only reports on the formation of the localized dislocation network come from UK Sharp group [28, 29]. This group fabricated their devices by using a gas source MBE method. After a few-hour-long operation, the laser diodes were deprocessed and examined by TEM. The electron microscopy revealed the network of dislocations confined in the active region of the device. The authors claim that the mechanism of dislocation network formation may be similar to REDG 258 P. Perlin and Ł. Marona
  13. 13. mentioned before [27]. However, considering the uniqueness of this observation, the discussion about the true importance of this mechanism for nitride laser diode degradation has to be postponed until more facts are gathered. Another piece of the puzzle which does not fit too well is that the laser diode containing 106 dislocations in square centimeter has only around 10 dislocations within its stripe. If the diffusion is fast along the dislocation, the nonradiative recombination centers should be concentrated close to the dislocations forming dark areas. This effect is up to our knowledge never observed. 8.4.3 Contact Degradation: Operation Voltage and the Series Resistance Another typical feature of the degradation is an increase of the diode operation voltage under the prolonged operation. Figure 8.10 shows the I-V curves measured for the virgin laser diode and after 900 h of operation at 5 mW optical output power. The increase of 0.5 V of the operation voltage at the current of 150 mA was observed. Gradual degradation of the device series resistance and the operating voltage is most likely related to the deterioration of the metal contact to the p-type layer. There is however a lack of broader experimental base to evaluate this phenomenon. Data coming from the transistor reliability studies show no metal diffusion to semiconductor from the hardly stresses metal contacts. 0 50 100 150 200 250 0 1 2 3 4 5 6 7 voltage(V) current (mA) t=900 h t=0 h Fig. 8.10 Comparison between current–voltage characteristics of virgin and stressed laser diode 8 InGaN Laser Diode Degradation 259
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