Symposium Organizers
Osamu Ueda Kanazawa Institute of Technology
Mitsuo Fukuda Toyohashi University of Technology
Steve Pearton University of Florida
Edwin Piner Nitronex Corporation
Paolo Montangero Avago Technologies Italy S.R.L.
B1: Laser Reliability
Session Chairs
Monday PM, November 30, 2009
Liberty (Sheraton)
9:30 AM - **B1.1
Catastrophic Optical-damage in High-power, Broad-area Laser-diodes.
Aland Chin 1 , Rick Bertaska 2 , Martin Jaspan 3 , Allen Flusburg 3 , Stephen Swartz 3 , Maciej Knapczyk 3 , Israel Smilanski 3 , Jonah Jacob 3
1 , ALAND CHIN, LLC, Sharon, Massachusetts, United States, 2 , New England Analytical, LLC, Nashua, New Hampshire, United States, 3 , Science Research Laboratory, Inc., Somerville, Massachusetts, United States
Show AbstractFor modern, high-power laser-diodes, the remaining failure-mode is reported to be catastrophic optical-damage (COD). A brief description of the COD phenomena is as follows. A local region of the laser diode, generally at the front facet, is heated by absorption of the laser light so that the material melts. Since heat is generated by absorption of the laser light, the molten region is substantially confined to the active layer of the laser cavity. The surface of the molten region is optically reflective. Lasing is sustained in the optical cavity defined by the surface of the molten region and the back facet. The molten region propagates towards the back facet as the material exposed to the laser light continues to melt whereas the material on the opposite side, no longer heated by laser light, solidifies. Propagation of the molten region continues until there is insufficient gain in the optical cavity to maintain a liquid state. While catastrophic optical-damage (COD) of single-mode lasers involves only transverse modes, we recently discovered that COD of broad-area, multi-mode laser-diodes involves both transverse modes and ring-cavity modes. In 1972, internally-circulating modes in sawn-cavity, broad-area laser-diodes were proposed as an explanation for an observed bi-modal distribution in the efficiency of lasers that were fabricated in an essentially-identical fashion. The internally-circulating modes are the ring-cavity modes we presented in 2009.The presence of the ring-cavity modes accounts for many of the unusual features of COD, some of which have been reported but not explained. These features include:●Failure location at the facet not coincidental with a peak in the near-field intensity.●±16° branching and fan out of the COD track as it propagates●Random nature of failure by COD●Polycrystalline structure of a portion of the COD trackA detailed description of the phenomenon of COD in short (380μm cavity-length), 12μm aperture, proton-bombarded, double-heterostructure laser-diodes with uncoated facets was first presented in 1974. In these devices, COD generally initiates at the facets due to high optical-power density and propagate along transverse-mode filaments. To achieve reliable operation at high optical-power, broad-area laser-diodes have evolved to long (several-millimeter cavity-length), wide-aperture (50-200μm), dielectric-defined, broadened-waveguide separate-confinement, double-heterostructure, quantum-well laser-diodes with coated, passivated facets. COD in these devices involve both transverse modes and ring-cavity modes. As a result of these improvements in device structure, ring-cavity modes contribute to the COD process.This report provides a description of COD formation and propagation, with and without ring cavity modes, in broad-area laser-diodes.Approved for Public Release, Distribution Unlimited
10:00 AM - **B1.2
Failure Analysis Using Optical Evaluation Technique (OBIC) of LDs for Fiber Optical Communication.
Tatsuya Takeshita 1 , Hiromi Oohashi 1
1 Photonics Device Laboratory, NTT Corporation, Atsugi, Kanagawa Pref., Japan
Show AbstractThe introduction of high-speed services for fiber-optic access subscribers has led to a huge growth in data traffic. The rapid diversification of services means that next generation networks must be built quickly, economically and reliably. A high temperature laser allows us to eliminate the thermo-electric cooler conventionally needed in a transmitter module, which results in reductions in cost, power consumption and size. Moreover, a high-power laser provides a wide tolerance when coupling optical fibers. In addition, a high-power pump laser is needed to realize a wide-band and high-power erbium-doped fiber amplifier. This makes high-performance laser chips one of the keys to achieving highly reliable and cost-effective systems. In terms of laser reliability, we must clarify the degradation mechanism and postpone or suppress degradation if we are to achieve a reliable high-performance laser. We have analyzed degraded lasers using the optical beam induced current (OBIC) technique. When there are nonradiative recombination centers in the degraded region, the OBIC intensity decreases with increases in recombination density. This technique has the advantages of being non-destructive and highly sensitive. In addition, it provides high space resolution in degradation analyses. The OBIC is measured through the window of a 5.6-mm transistor outline (TO) can before and after aging. Then, by using the same LDs we can detect an OBIC change for several aging times. We can both detect the degraded region and layer, and estimate the degree of laser degradation by employing the relative OBIC intensity prior to aging. This OBIC technique is useful for analyzing the degree of laser degradation.Moreover, the incident wavelength can be changed by changing the optical source in the OBIC measurement setup, which in turn changes the absorption layer and the penetration distance. Some degraded laser layers are reveled by using these several wavelengths absorbed in different layers. In addition, degradation in the waveguide interior is detected by using an incident wavelength with long penetration. Thus, by monitoring the OBIC intensity at several wavelengths as well as before and after aging, we are able to discuss sudden and wear-out laser failures. In our presentation, we will introduce examples using the OBIC technique that contributed to the improvement of laser reliability.
10:30 AM - **B1.3
Performance of InP/InAlGaAs Light Emitting Transistors Using Zn and C as Base Dopant.
Russell Dupuis 1 , Yong Huang 1 , Jae-Hyun Ryou 1 , Forest Dixon 2 , Nick Holonayk 2 , Milton Feng 2
1 School of ECE, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Department of ECE, University of Illinois at Urbana-Champaign, Champaign-Urbana, Illinois, United States
Show AbstractLight emitting transistors (LETs) operating at around 1.55 µm were investigated using InP/InAlGaAs material system grown by metalorganic chemical vapor deposition (MOCVD). Device epitaxial structures were achieved by incorporating InGaAs quantum wells (QWs) in the base region of the N-InP/p-InAlGaAs/N-InAlAs heterojunction bipolar transistors (HBTs), and both current gain and long-wavelength light emission were demonstrated. It was found that control of the p-type doping profile in the base layer is one of the key factors that dictate the performance characteristics of long wavelength LETs.Epitaxial growth of the devices was carried out on (001) InP:Fe substrates between 600 °C and 650 °C at a reactor chamber pressure of 100 Torr. Both Zn and C were used as the p-type dopant in this study. A typical NpN LET structure consists of (from the bottom to the top) a Si-doped InP subcollector, an undoped In0.52Al0.48As collector, an undoped In0.53(AlxGa1-x)0.47As grading layer with xAl from 1 to 0.25, a Zn- or C-doped In0.53(Al0.25Ga0.75)0.47As base/active region with an undoped compressively-strained In0.58Ga0.42As QW embedded in the middle of the base layer, an InP:Si emitter and an emitter contact. The devices were fabricated using standard optical lithography and wet chemical etching to form an emitter and base mesa. AuGe/Ni/Au was used as the emitter and collector ohmic contacts and Au/Zn/Au as the base contact.Secondary ion mass spectrometry (SIMS) shows that in the LET with Zn-doped base Zn diffuses into both emitter and the adjacent graded collector, while in the C-doped LET, C stays in place and forms abrupt junctions. The turn-on voltages are 0.89 V and 0.78 V for the Zn-doped LET and the C-doped LET, respectively, indicating the presence of a potential spike at emitter-base junction due to Zn diffusion. In addition, Zn diffusion into the undoped QW region degrades the optical quality by creating non-radiative recombination centers, which is confirmed by the electroluminescence (EL) peak power for both devices. The C-doped SQW LET exhibits much higher light output owing to the intact active layer. C-doped LETs have a higher emitter injection efficiency and luminescence efficiency, but it is not granted without a problem. Whereas Zn will inevitably diffuse into active region and plague the QW, Zn can be contained only in the base region through proper engineering of growth conditions. It was found that Zn-doped LET has a much higher current gain. The diffusion-suppressed Zn-doped LET has a DC current gain of 45 in sharp contrast to the low current gain of about 0.25 in the C-doped LET. Short minority carrier lifetime in C-doped materials is considered to account for the low gain, which is possibly due to the low growth temperature and low V/III ratio used during C-doping.
11:30 AM - **B1.4
InGaN Laser Diode Degradation. Surface and Bulk Processes.
Piotr Perlin 1 2 , Lucja Marona 1 , Tadek Suski 1 , Przemek Wisniewski 1 2 , Mike Leszczynski 1 2 , Pawel Prystawko 1 , Michal Bockowski 1 2 , Robert Czernecki 1 2 , Irina Makarowa 2 , Bogdan Kowalski 3
1 , Institute of high Pressure Physics, Warsaw Poland, 2 , TopGaN Ltd, Warsaw Poland, 3 , Institute of Physics, Warsaw Poland
Show AbstractThe nature of degradation processes of nitride light emitting devices, particularly laser diodes is one of the most intriguing issues of the short-wavelength optoelectronics. In spite of the intensive research, no comprehensive mechanism of GaN based laser diodes degradation was so far presented. The present work focuses on the reliability study performed on InGaN laser diodes grown by MOVPE method on low and middle dislocation density GaN substrates (104-107cm-2). Our observation, in the agreement with the research performed by the others group, shows that that we can assign the degradation processes existing in the nitrides laser diodes into two categories: the surface reactions and bulk effects. What concerns the first category, the careful observation of the laser mirrors of degraded laser diodes, reveals the presence of carbon deposits. The formation of these deposits depends strongly on the atmosphere in which the aging process is being carried out. For instance the degradation is very fast if the diode operates in the atmosphere of dry nitrogen, and it is much slower if oxygen is added to the gas. All these mentioned above features resembles very closely so called PIF (Package Induced Failure) - mechanism of degradation characteristic for 980 nm, high-power laser diodes. This mechanism involves photochemical reactions of hydrocarbons decomposition on the surface of laser diode mirror. The second mode of degradation strongly depends on the density of dislocations. It primarily manifests by the increase of the threshold current of a device with relatively small variation of the slope efficiency. The degradation follows quite closely the square root of time-dependence, though it tends to deviated from this behavior at the late stage of the aging process. Interestingly no signs of degradation are visible in the cathodoluminescence images which makes tempting to think that the degradation effects do not consist in the development of the nonradiative recombination centers but rather enhance the scarier leakage. We will also briefly discuss the potential role of dislocation loop for the degradation of InGaN laser diodes.
12:00 PM - **B1.5
Structural Defects in GaN-based Materials and Their Relation to GaN-based Laser Diodes.
Shigetaka Tomiya 1 , Yuya Kanitani 1
1 Advanced Materials Laboratories, Sony Coporation, Atsugi, Kanagawa, Japan
Show AbstractReduction of structural defects in III-nitride based optical devices is critical important for high efficient and high reliable performance. In this presentation, three different types of structural defects observed in GaN-based laser diodes (LDs) are discussed. In the early stage of development of GaN-based laser diodes, a high density of threading dislocations (typically 108-1010/cm2) was inevitable, since samples were grown on sapphire substrates. In the conventional GaAs- and InP-based LDs, existence of a single dislocation in the stripe regions is harmful since dislocations can multiple easily by carrier recombination. On the other hand, an influence of dislocations for the GaN-based LDs is not predominant as much as that for the conventional LDs. First, we report on dislocation motion observed in GaN-based materials by electron beam irradiation and discuss relation between dislocation motion and device degradation. Next, we discuss on pyramidal defects observed in Mg doped p-type layers in GaN-based LDs. Since the Mg acceptor level is fairly deep, a high level of Mg doping is required to obtain a high concentration of free holes. However, as Mg concentration is increased, the pyramidal defects were nucleated. Here, we report on the detailed structural analysis of the pyramidal defects and discuss the formation mechanism of the defects. GaInN alloy layer is one of the most important elements for GaN-based optical devices. Technological problem of the growth of GaInN alloys with high indium concentrations is caused namely due to low miscibility of InN and GaN. During the course of the development of the high power pure blue laser diodes, we found the multiple defects nucleated at the GaInN MQWs. This type of the defects is very critical for the device lifetime. In this presentation, we report on the detailed structural analysis of this new type defects and will discuss the formation mechanism and the reduction methodology of these defects.
12:30 PM - B1.6
A Study of Degradation in High Power Multi-Mode InGaAs-AlGaAs Strained Quantum Well Lasers as Pump Lasers.
Yongkun Sin 1 , Nathan Presser 1 , Neil Ives 1 , Steven Moss 1
1 Electronics and Photonics Laboratory, The Aerospace Corporation, El Segundo, California, United States
Show AbstractHigh power broad-area InGaAs-AlGaAs strained quantum well (QW) lasers with emission wavelengths at 910-980nm are used as pump lasers to optically pump various fiber lasers and amplifiers. These applications mainly developed for industrial uses do not require stringent reliability from the pump lasers. Also, the fact that these pump lasers have not been deployed in high reliability communications systems including potential satellite systems necessitates careful study of reliability and degradation of these devices. Maximum output powers of both single-mode and broad-area lasers are limited by catastrophic optical mirror damage (COMD). However, unlike 980nm single mode lasers where a single failure mode is typically observed due to good facet passivation along with optimized structural designs, broad-area lasers have shown at least two different failure modes including COMD and bulk failure. There have been extensive reports on COMD, but very limited reports on bulk failure although catastrophic bulk failure has been identified as the dominant failure mode in broad-area InGaAs-AlGaAs strained QW lasers.We investigated reliability and degradation processes in commercial MOCVD-grown broad-area InGaAs-AlGaAs strained QW lasers at ~975nm by performing accelerated lifetests of these devices followed by failure mode analyses (FMA) with micro-analytical techniques including electron beam induced current (EBIC), time resolved electroluminescence (EL), and deep level transient spectroscopy (DLTS). Both passivated and unpassivated broad-area lasers were studied that yielded catastrophic failures at the front facet and also in the bulk. The lifetests performed typically under automatic current control mode generated failures at different stages of degradation. EBIC technique was employed to study dark line defects generated in degraded lasers stressed under different test conditions and was also used to estimate minority carrier diffusion lengths from laser diodes at different stages of degradation. Time resolved EL technique was employed to study initiation and progressions of dark spots and dark lines in real time as devices are aged. Lastly, DLTS technique was employed to study deep electron traps in degraded laser diodes. We will report our in-depth FMA results.
12:45 PM - B1.7
Reliability of Semiconductor Structures with Buried Quantum Dots.
Vladimir Chaldyshev 1 , Nikolay Bert 1 , Anna Kolesnikova 2 , Vladimir Nenedomsky 1 , Valerii Preobrazhenskii 3 , Mikhail Putyato 3 , Alexei Romanov 1 , Boris Semyagin 3
1 , Ioffe Institute, St.Petersburg Russian Federation, 2 , Institute of Problems in Mechanical Engineering, St.Petersburg Russian Federation, 3 , Institute of Semiconductor Physics, Novosibirsk Russian Federation
Show AbstractCrystal lattice mismatch in semiconductor structures leads to the generation and relaxation of elastic strains and corresponding mechanical stresses that in many cases determines the reliability of electronic and optoelectronic devices. For quantum dot (QD) structures it is usually assumed that the relaxation of mechanical stresses proceeds during their self-organization on the surface. However, burying of the QDs results in an increase in the stored elastic energy, which can affect the onset of relaxation in such semiconductor structures and impact reliability of the corresponding devices. We report on theoretical and experimental studies of general critical conditions for the onset of relaxation in stressed QDs embedded into a crystalline matrix. The fingerprint of the relaxation scenario is formation of specific satellite dislocation loops. The scenario is applicable to both the QDs nucleated and ripened in the bulk and the QDs formed on the surface and then buried by overgrowth. The results demonstrate that the critical relaxation radius for buried QDs is larger than critical thickness for stressed lattice mismatched films or surface islands.Transmission electron microscopy confirms the relaxation of stresses at InAs QDs buried in GaAs via formation of specific dislocation defects. It has been found that the dislocations formed in vicinity of selected InAs quantum dots do not exit to the free surface of the GaAs film. That means that the stress relaxation occurs in the bulk of the GaAs film near buried QDs, which were coherent to the matrix at the stage of their formation on the GaAs surface. The developed models can be used for the prediction of reliability of such devices as lasers and photodetectors based on the system of buried QDs.
B2: Degradation Mechanisms
Session Chairs
Russell Dupuis
T Takeshita
Monday PM, November 30, 2009
Liberty (Sheraton)
2:30 PM - **B2.1
Recombination-enhanced Dislocation Glides--The Current Status of Knowledge.
Koji Maeda 1
1 Applied Physics, The University of Tokyo, Tokyo Japan
Show AbstractAmong lattice imperfections greatly affecting the reliability of semiconductor devices, dislocations are the most common and important defects because they are usually harmful for the device performance. Especially in optoelectronic devices such as laser diodes, it is well known that the device degradation occurs as a consequence of multiplication of dislocations during device operation under minority carrier injection. More recently, device degradation in the similar mode was found to occur also in SiC p-i-n diodes for power-control applications [1,2]. Commonly, these phenomena are induced by the glide (and/or climb) motion of dislocations that is enhanced by carrier recombination at the dislocations. So far, it has been shown that the effect of recombination-enhanced dislocation glides (REDG) is observed in most semiconductors (elemental, III-V and II-VI compounds)[3]. The factors that determine whether the REDG effect actually brings about degradation or not are the presence of driving force, the presence of dislocation sources, and the magnitude of enhanced dislocation velocity. In most cases, the REDG effect is driven by a mechanical shear stress such as built in heterostructures by the lattice misfit. Therefore the lattice matching is a measure usually employed to suppress the degradation in the REDG mode. In SiC, however, the REDG of partial dislocations, which results in expansion of stacking faults, is exceptionally driven by a non-mechanical stress of unestablished origin [1]. Since nucleation of dislocations needs surmounting a large energy barrier or aggregation of point defects, the degradation in the REDG mode is difficult to occur in the absence of pre-existing dislocations. Therefore, the removal of as-grown dislocations is the most orthodox approach to the degradation-free devices. In GaN and related compounds, however, in spite of the high density of as-grown dislocations, the degradation in the REDG mode is not rapid, presumably due first to the absence of mechanical shear stress driving the dislocations and secondly to the exceptionally low rate of carrier recombination at dislocations. The enhanced dislocation glide velocity is characterized by the prefactor linearly dependent on the recombination rate and the activation energy reduced in magnitude which tends to increase with the bandgap energy of the crystal [3]. Although these features are explicable by the phonon kick mechanism, it is not yet exactly known what elementary process is enhanced. Measurements of excitation spectra in optical excitation may give us insight to this fundamental problem [2,4]. [1] S. Ha and M. Skowronski, Phys. Rev. Lett., 92 (2004) 175504. [2] A. Galeckas, J. Linnros and P. Prouz, Phys. Rev. Lett., 96 (2006) 025502. [3] K. Maeda and S. Takeuchi, Dislocation in Solids, 10 (1996) p.435. [4] Y. Ohno, T. Taishi and I. Yonenaga, phys. stat. sol., (2009) in press.
3:00 PM - **B2.2
Mechanism of Defect Reactions in Semiconductors.
Yuzo Shinozuka 1
1 Faculty of Systems Engineering, Wakayama University, Wakayama Japan
Show AbstractProposed mechanisms so far on defect reactions in semiconductors (defect creation, annihilation, multiplication, reconstruction, impurity diffusion, …) are reexamined with special attention to the instability of the atomic configuration for particular electronic states and the transient lattice vibration induced by successive carrier captures.1) Thermal activation process to overcome the potential barrier Un: The reaction rate is given by p0 exp(-Un/kBT), where p0 is the attempt frequency and Un depends on the electronic state n.2) Instability mechanism: An electronic transition at a defect promptly induces the lattice distortion along the reaction coordinate QR. A symmetry breaking distortion (Td to C3v) is inherent for the diamond and zincblende structures, and is favorable for hole localization. 3) Phonon kick mechanism: An electronic transition to or from the state n induces the lattice vibration along the lattice relaxation mode Q1. The vibration amplitude decreases because of the dephasing in relevant normal modes in Q1. If Q1 is not orthogonal to the defect reaction coordinate QR, the defect reaction rate is enhanced during the lattice relaxation time τ~ 2π/Δω, where Δω is the width of the frequency distribution of the normal modes. 4) Phonon kick mechanism (recombination): During the transient lattice vibration induce by an e (h) carrier capture the next h (e) capture process is enhanced. Thus the transient lattice vibration induced by successive e and h captures in turn enhances the next capture process. If N electrons and N holes are successively captured by a defect within a short period τ~2π/Δω, an electronic energy ~ N times Eg, the band gap energy, is transformed into the lattice vibration energy. The coherent vibration part Q1 is enhanced in step with each carrier capture. After a numerical simulation the probability of this positive feedback is found to critically depend on the carrier concentrations. The defect reaction rate is given by p0 exp(-Eiact/kBT) because only the first capture (i=e, h) is to be activated.With an animation we also show the meaning of the configuration coordinate diagram scheme in many electron representation, only which can properly relate the lattice distortion with the level position of a defect in the band gap in the multiphonon recombination process.
3:30 PM - B2.3
Bulk Catastrophic Optical-damage of 980nm, InxGa1-xAs/GaAs, High-power, Broad-area Laser-diodes Due to a Void in Au80Sn20 Solder.
Aland Chin 1 , Rick Bertaska 2 , Henry Eppich 3 , Martin Jaspan 3 , Jonah Jacob 3
1 , ALAND CHIN, LLC, Sharon, Massachusetts, United States, 2 , New England Analytical, LLC, Nashua, New Hampshire, United States, 3 , Science Research Laboratory, Inc., Somerville, Massachusetts, United States
Show Abstract Catastrophic optical-damage (COD) generally occurs at the front facet of laser diodes with unpassivated facets due to optical absorption that increase in time due to chemical reactions that are accelerated by temperature and light. When the facets are passivated, CODs often initiate at the front facet due to the combined intensity of a transverse mode and a ring-cavity mode. Additionally, in laser diodes with passivated facets, where COD occurrence at the facet is a random event, COD can sometimes initiate at a location within the optical cavity. In this study, we show a correlation between the location of a large solder-void and the location where COD initiates within the interior of the optical cavity of 980nm, InxGa1-xAs/GaAs, high-power, broad-area laser-diodes. The laser diodes are mounted epitaxial-side down onto Cu10W90 heat-sinks using Au80Sn20 eutectic solder. Using finite-element analysis (FEA), we propose a mechanism for COD formation within initially defect-free material due to a large void in the solder.Approved for Public Release, Distribution Unlimited
3:45 PM - B2.4
A Thermomechanical Approach to the Formation of Dark Defects in High Power Laser Diodes.
Alonso Martin 1 , Pilar Iniguez 2 , Juan Jimenez 1 , Myriam Oudart 3 , Julien Nagle 4
1 GdS Optron lab, Universidad de Valladolid, Valladolid Spain, 2 Física Teórica, Atómica y Óptica, Universidad de Valladolid, Valladolid Spain, 3 , Alcatel-Thales 3-5lab, Palaiseau France, 4 , Thales Research and Technology (TRT), Palaiseau France
Show AbstractGreat efforts are made to improve the optical power and the reliability of high power lasers, aiming to a growing number of applications (solid-state laser pumping, materials processing, optical communications, printing machines…). The research on the degradation mechanisms of these devices is a necessary way to improving their optical power and reliability. The increase in the optical power of the devices induces an important heating of the active parts, quantum well and waveguide layers, especially at the mirror facet where energy losses take place resulting in the degradation of the output power and being responsible of the end catastrophic optical damage (COD). COD may begin by nonradiative recombination at facet defects, which can transfer heat to the neighbor areas increasing the local temperature of the facet and producing band gap shrinkage with the subsequent enhancement of the laser light self-absorption. Besides, nonradiative recombination can transfer energy to the surrounding lattice allowing defect formation and motion by the mechanism of recombination enhanced defect reactions. The two processes feedback the local temperature increase during laser operation. Finally, a thermal runaway process leads to the catastrophic degradation of the device. A model providing a comprehensive description of the laser degradation is needed. Cathodoluminescence images of degraded devices unveil the presence of dark defects at the facet together with extended defects inside the cavity; these dark defects are formed by plastic deformation of the active layers of the laser structure. In this work, we put forward a microscopic scenario that accounts for extended defect creation and motion resulting in the rapid degradation of the laser. We will show that the idea of a high local temperature induced by REDR and feedback by laser light self absorption constitutes the most probable source of local stress leading to the formation of dislocations during laser operation. We model by a finite element analysis the distribution of thermal stresses induced by the local heating associated with defects at the facet mirror. A heat source located at the facet, in the active zone, is considered to simulate the defect acting as a local heat source fed by self-absorption of the laser light and nonradiative recombination processes. This heat source produces a temperature distribution in the device which is calculated solving the heat transfer equation of a solid. Local temperatures at the facet can be high, with prominent temperature gradients. These temperature gradients promote thermal stresses in the structure, predominantly in the vicinity of the active zone, which are calculated taking into account the principle of virtual work. According with the maximum shear stress theory or the maximum shear strain energy per unit volume, this procedure allows us to set up the conditions for the triggering of the plastic deformation in lasers under operation.
B3: Optical Devices and Reliability
Session Chairs
Ed Piner
Shigetaka Tomiya
Monday PM, November 30, 2009
Liberty (Sheraton)
4:30 PM - B3.1
Gain Saturation of 785-nm Laser Signal Amplified in Si-rich SiOx Strip-loaded Waveguides on Quartz and Si.
Chung-Lun Wu 1 , Cheng-Wei Lian 1 , Gong-Ru Lin 1
1 Graduate Institute of Photonics and Optoelectronics, National Taiwan University , Taipei Taiwan
Show AbstractSi light-emission devices have been successively demonstrated by using low-dimensional Si structure to overcome the indirect bandgap property of bulk Si with extremely low internal quantum efficiency. Low-dimension Si such as Si nanocrystal (Si-nc) can essentially improve its internal quantum efficiency of luminescence due to the quantum confinement of the self-trapped excitions. Since 2000, several groups have employed various-length pumping method to consecutively demonstrate the net modal gain of amplified spontaneous emission (ASE) from Si-ncs buried in the Si-rich SiOx planar waveguide up to 100 cm-1. Nonetheless, the characterization on the small-signal power gain of such waveguide structure under laser diode injection is mandatory towards the application of waveguide amplifiers in the future. In this work, we compare the laser-diode injection based small-signal power amplification and gain saturation properties of the SiO2/SiOx:Si-nc/SiO2 strip-loaded waveguide made on quartz and Si substrates.The Si-rich SiOx film with a thickness of 0.35 μm is deposited on quartz and Si substrates after growing 1µm-thick standard SiO2 buffer layer. After annealing the SiOx film in a quartz furnace to precipitate Si-nc, the SiO2 film of 1.5 μm is grown upon the SiOx film to form the waveguide. The sample with a 50μm-wide and 3cm-long photoresistive mask pattern is wet-etched by buffered oxide etchant (BOE) to form the SiO2/ Si-nc:SiOx/SiO2 strip-loaded waveguide. With a He-Cd laser based top pumping geometry and the variable strip length (VSL) method, the optical net modal gain and propagation loss of Si-nc is determined from the ASE ranged between 750-850 nm with 3dB spectral linewidth of 140 nm. The optical net modal gain of Si-nc at peak wavelength of 805 nm are 65 cm-1 and 85.7 cm-1, and the propagation loss is about 5 cm-1 and 21 cm-1 on quartz and Si substrates, respectively. Even with a higher net modal gain coefficient, the optical loss coefficient of the waveguide made on Si substrate is larger than that on quartz due to the more pronounced leakage of mode power to Si substrate. By injecting the directly modulated 785-nm laser diode signal with power of 1 nW into the 1.5cm-long waveguide, we further demonstrate the small-signal power amplification. Under the pumping power of 40 mW at 325 nm, the small-signal power gain is about 7 dB and 13 dB for waveguides made on quartz and Si substrates, respectively. We also observe the gain saturation effect and fit it with theoretical formula to find the peak gain of 27 and 36 dB, and the saturated power of 0.85 and 0.75 nW for the waveguide amplifiers made on quartz and Si substrates. Raising the saturation power relies strictly on increasing either the volume density of the buried Si-ncs or the SiOx film thickness.
4:45 PM - B3.2
Surface and Interface Effect of Durable and Efficient Polymer Solar Cells.
Dong Hwan Wang 1 , Jong Hyeok Park 2 , Sang Hyuk Im 3 , O Ok Park 1
1 Chemical & Biomolecular Engineering, KAIST, Daejeon, Daejeon, Korea (the Republic of), 2 Chemical Engineering, Sungkyunkwan University, Suwon, Suwon, Korea (the Republic of), 3 , KRICT, Daejeon, Daejeon, Korea (the Republic of)
Show Abstract Even though the fossil energy sources are much limited, the energy consumption is increasing rapidly every year. Therefore, new approaches for sustainable energy of solar energy systems are definitely urgent issues. Many of the different solar cells are under consideration by a lot of research groups all over the world such as Silicon, compound semiconductor and organic solar cells. Especially, polymer (plastic) based solar cell is a quite attractive one for its flexibility, low-cost possibility, light-weight, and semi-transparency. Here we focused on how to improve not only the efficiency and low cost processability (1) but also high-temperature durability will be considered. It can be demonstrated by introducing a protective layer of linear polymeric TiOx interlayer. It is attributed to an improved interfacial stability owing to relatively reduced morphology change at high temperature operation. The effectiveness of this unique feature makes it possible to fabricate more efficient organic solar cells by adopting a post annealing process. (2)-(3) Also we have tried to make bulk heterojunction photovoltaic cells with enhanced nanoscale morphology by adding an ionomer (partially sulfonated polystyrene (PSP)) into a regioregular P3HT/(6,6)-phenyl C61-butyric acid methyl ester (PCBM) blend.(4) And a patterned conducting IZO film with well-ordered periodic dot-structures with 50 nm or 200 nm deep features has been constructed by nanoimprinting technique as the anode of an organic solar cell. It was found that a highly ordered 2D-dot nano-patterned anode can enhance the device performance due to the large interfacial area between both of the electrodes and the active layer. This is possible because the nano-patterned structures can efficiently harvest electrons and holes from the active layer to each electrode in the IZO anode and Al cathode.(5) Finally, our group recently reported that bilayers active film with a concentration gradient has been successfully fabricated via solution process for the first time. The concentration variation has been confirmed by the Auger spectroscopy. The novel device showed an enhanced photocurrent density and power conversion efficiency compared to those of the BHJ PV prepared under the same fabrication condition. (6)1. SJ Yoon, JH Park, HK Lee and OO Park, Appl.Phys.Lett., 92 (2008) 1435042. DH Wang, SH Im, HK Lee, JH Park and OO Park, Langmuir, In revision (2009)3. TW Lee and OO Park, Appl.Phys.Lett., 77 (2000) 33344. KC Kim, JH Park and OO Park, Sol. Energy Mater. Sol. Cells, 92 (2008) 11885. DH Wang, DG Choi, KJ Lee, JH Jeong, JH Park and OO Park, Nanotechnology, In revision (2009)6. DH Wang, DG Choi, HK Lee, JH Park and OO Park, Appl.Phys.Lett., accepted (2009. 6)
5:00 PM - B3.3
A MHz Modulable Si-based LED Afforded by Engineering Light-emitting Defects in Si.
Norishige Tana-ami 1 , Jun Igarashi 1 , Yosuke Terada 1 , Yuhsuke Yasutake 1 , Susumu Fukatsu 1
1 , University of Tokyo, Tokyo Japan
Show AbstractDefects are often thought of as being “undesirable” whenever the reliability of devices is an issue. Intriguingly, however, there is a certain class of crystal defects that are of device grade in terms of their potentially useful optical and/or electrical properties. In this work, we demonstrate that a particular class of radiative defects created in Si helps build light-emitting-diodes (LEDs) with a MHz bandwidth. The significance of such a “defect-active dynamic LED” is two-fold. First, “defect engineering" opens up an avenue for boosting the light-emitting potential of Si, especially near-band-edge recombination, which has been a challenge over years in the field of photonics as well as materials science. Second, that the radiative defects in Si studied here are made to be responsive on the sub-µs time scale is remarkable, in view of slow dynamics of relaxation, >10µs, as in many of the defect-related systems. {311}-rod like defects were launched into Si in the course of programmed thermal relaxation of strain that had built up in the 1.5-µm device Si layer, lying on top of a Si-on-insulator substrate. Ambipolar LED geometry was defined by depositing Al Schottky pads. E-line electroluminescence (EL) due to {311}-defects survived above 150 K with decay times >>1µs increasing with temperature. A 2-MHz bandwidth of 100-% modulation amplitude was only achievable by biasing the LED at 4-8 V. The E-line intensity was at maximum at 20 K, at which temperature the modulation bandwidth also went through a maximum. This indicates that the radiative reco