Strained Quantum Well Lasers Research Articles

Catastrophic Optical Bulk Damage – A New Failure Mode in High-Power InGaAs-AlGaAs Strained Quantum Well Lasers

ABSTRACTHigh-power single-mode (SM) and multi-mode (MM) InGaAs-AlGaAs strained quantum well (QW) lasers with emission wavelengths of 910 − 980 nm are extensively used in various fiber lasers and amplifiers for both telecom and defense applications. In particular, underseas and satellite communication systems require stringent reliability from these lasers. Since these lasers predominantly fail by catastrophic and sudden degradation due to COD, it is crucial especially for space satellite applications to investigate reliability, failure modes, and degradation mechanisms of these lasers. Catastrophic optical mirror damage (COMD) was known to be the only failure mode until our group reported a new failure mode in MM and SM InGaAs-AlGaAs strained QW lasers in 2009 and 2016, respectively. Our group reported that bulk failure due to catastrophic optical bulk damage (COBD) has become the dominant failure mode of both SM and MM lasers. Since there have been limited reports on COBD compared to COMD, the intent of this paper is to introduce our studies on COBD that have spanned the last decade. We investigated reliability, failure modes, and degradation processes in SM and MM InGaAs-AlGaAs strained QW lasers by performing short-term step-stress tests and long-term accelerated life-tests as well as failure mode analyses using various nondestructive and destructive micro-analytical techniques including electron beam induced current (EBIC), time-resolved electroluminescence (EL), deep level transient spectroscopy (DLTS), focused ion beam (FIB), and high-resolution TEM. EBIC and EL techniques were employed to study dark line defects generated in degraded lasers stressed under different test conditions. Time-resolved EL techniques were employed to study initiation and progressions of dark spots and dark lines in real time as lasers were aged. DLTS techniques were employed to study electron traps in both pristine and degraded lasers. Lastly, FIB and high-resolution TEM were employed to prepare cross sectional and plan view TEM specimens to study DLD areas in post-aged lasers. We also report our current understanding on degradation mechanisms responsible for COBD in both SM and MM lasers.

Catastrophic Optical Bulk Damage in High-Power InGaAs-AlGaAs Strained Quantum Well Lasers

High-power single-mode (SM) and multi-mode (MM) InGaAs-AlGaAs strained quantum well lasers are critical components for terrestrial and space satellite communications systems. Thus, it is crucial to understand reliability, failure modes, and degradation mechanisms of these lasers for the high-reliability applications. However, a little has been reported on failure modes and degradation mechanisms of state-of-the-art SM and MM InGaAs-AlGaAs strained quantum well (QW) lasers. This study addresses the aforementioned issues by performing long-term life tests, failure mode analysis (FMA), and physics of failure (root causes) investigation. We performed long-term accelerated life tests on SM and MM InGaAs-AlGaAs strained QW lasers. Our life tests have accumulated over 25 000 test hours for SM lasers and over 35 000 test hours for MM lasers. FMA was performed on degraded lasers using electron-beam-induced current, electroluminescence, focused ion beam, and high-resolution TEM. All the SM failures showed catastrophic and sudden degradation and all of these failures were bulk failures. Our group previously reported that bulk failure or catastrophic optical bulk damage (COBD) is the dominant failure mode of MM InGaAs-AlGaAs strained QW lasers. To the best of our knowledge, we are the first group demonstrating that the dominant failure mode of both SM and MM InGaAs-AlGaAs strained QW lasers is bulk failure. Since degradation mechanisms responsible for COBD are still not well understood, we also employed other techniques for the physics of failure investigation including deep level transient spectroscopy, time-resolved electroluminescence, and time-resolved photoluminescence to investigate COBD processes in SM and MM lasers.

Reliability and Degradation Mechanisms in High Power Broad-Area InGaAs-AlGaAs Strained Quantum Well Lasers

ABSTRACTReliability and degradation processes in broad-area InGaAs-AlGaAs strained quantum well (QW) lasers are under intensive investigation because these lasers are the key components for fiber lasers and amplifiers that have found both industrial and military applications in recent years. Unlike single-mode lasers that were developed for high reliability telecom applications, broad-area lasers were mainly targeted for applications that require less stringent reliability of the lasers until recently. Especially, the lack of field reliability data is a concern for satellite communication systems where high reliability is required of lasers for long-term duration. For our present study, we addressed this concern by performing long-term life-tests of broad-area InGaAs-AlGaAs strained QW lasers and also by studying mechanisms that are responsible for catastrophic degradation of the lasers.

Spontaneous Radiative Efficiency and Gain Characteristics of Strained-Layer InGaAs–GaAs Quantum-Well Lasers

The optical gain spectra, unamplified spontaneous emission spectra, and spontaneous radiative efficiency are extracted from the measurement of amplified spontaneous emission (ASE) on a single pass, segmented contact 0.98-mum-emitting aluminum-free InGaAs-InGaAsP-GaAs quantum-well (QW) laser diode. These measurements provide a baseline for which to compare higher strain InGaAs QW lasers emitting near 1.2 mum. The peak gain-current relationship is extracted from gain spectra and the peak gain parameter go is found to agree within 25% of the value extracted using conventional cavity length analysis for 0.98-mum-emitting devices. The spontaneous radiative current is extracted using the fundamental connection between gain and unamplified spontaneous emission, which in turn gives an estimate of the amount of nonradiative recombination in this material system. The spontaneous radiative efficiency, the ratio of spontaneous radiative current to total current, at room temperature of 0.98-mum-emitting InGaAs QW laser material is found to be in the range of 40%-54%, which is 2.5-3.5 times larger than that of highly strained InGaAs QW laser emitting near lambda = 1.2 mum. Whereas the gain parameter, g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = dg/d(ln j), was measured to be 1130 and 1585 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for the 0.98-mum- and 1.2-mum-emitting materials, respectively. From the calculated below threshold current injection efficiency of 75%-85%, we deduce that the internal radiative efficiency of the QW material is ~ 20% higher than the ratio of internal radiative current to external injected current extracted directly from ASE measurements.

Failure analysis of InGaAs/GaAs strained-Layer quantum-well lasers using a digital OBIC monitor

The mechanisms that cause the sudden failure of InGaAs/GaAs strained-layer quantum-well lasers (LD) are analyzed by monitoring an optical beam-induced current (OBIC). Statistically, the LDs that suddenly fail have different n-OBIC/sub ridge/ intensity profiles (p-n junction OBIC under the ridge normalized by that under the electrode) from LDs with conventional catastrophic optical damage. It appears that dislocations are generated in the vicinity of an antireflection (AR) facet and extend to the AR facet, and meltdown finally occurs there. The suppression of dislocation generation around the AR facet is important in regards to preventing sudden failure.

Tunneling Injection Quantum-Dot Lasers

ABSTRACTWe investigate a tunneling injection InP quantum-dot (QD) laser theoretically and experimentally. The device consists of a single compressively strained InP QD layer coupled closely to two tensile strained InGaP quantum wells (QWs). While most tensile strained QW lasers in this wavelength (red) range lase in the transverse-magnetic (TM) polarization, our QD laser lases in the transverse-electric (TE) polarization from the first excited state of the compressively strained QDs, which is coupled to the ground state of the tensile-strained InGaP QWs. When we measure TE and TM modal gain spectra, a typical QW gain evolution behavior is observed at low injection currents, which can be theoretically explained by the quasi-equilibrium of carrier distribution. When the injection current is increased near threshold, a TE gain narrowing and a simultaneous TM gain pinning are observed in the measured modal gain spectra, which cannot be explained via the quasi-equilibrium model. We propose a polarization-dependent photon-mediated carrier re-distribution in the QD-coupled-QW structure to explain this TE and TM gain evolution behavior. When the injection current is just below threshold, the strong carrier depletion via stimulated emission due to coupling between the InP QD and InGaP QW states plays an important role in carrier re-distribution, which depends on the optical transition energy and polarization. This polarization-dependent photon-mediated carrier re-distribution explains the TE gain narrowing and TM gain pinning behavior. To quantitatively demonstrate the photon-mediated carrier re-distribution near the threshold current, a set of coupled rate equations are solved taking into account the polarization-dependent stimulated emission processes. The calculated polarization power ratio based on the coupled rate equations explains the experimental observations.

Band structure calculations for dilute nitride quantum wells under compressive or tensilestrain

We have calculated the band structure of InGaAsN/GaAs(N)/GaAscompressively strained quantum wells (QW) emitting at1.3 µm using the band anticrossing model and an eight-band Hamiltonian. The calculated interband optical transition energies have been compared tothe experimental ones deduced from photocurrent, photoluminescence and excitation ofphotoluminescence spectroscopy experiments and measured laser characteristics extractedfrom the recent literature. Because of the high compressive strain in the QW,strain-compensated structures may be required in order to grow stable multiple QWs; in viewof this we have studied the band structure of InGaAsN/GaAsP/GaAs QWs emitting at1.3 µm. Dilutenitride structures also offer the possibility of growing tensile strained QW lasers on InP substrate emittingin the 1.55 µm emission wavelength range. In order to evaluate the potentialities of such structures wehave determined the band characteristics of InGaAsN/InGaAsP/InP heterostructures witha TM polarized fundamental transition.

High-temperature performance of InGaAs-InGaAlAs-InP 1.79-/spl mu/m diode lasers grown in solid-source molecular-beam epitaxy

1.79-μm InGaAs-InGaAlAs strained-layer quantum-well diode lasers have been fabricated. A characteristic temperature of 72 K has been achieved. At a temperature as high as 100/spl deg/C, a continuous-wave output power of more than 6.5 mW per facet has been demonstrated with lasers using as-cleaved facets as mirrors.

The influence of quantum-well composition on the performance of quantum dot lasers using InAs-InGaAs dots-in-a-well (DWELL) structures

The optical performance of quantum dot lasers with different dots-in-a-well (DWELL) structures is studied as a function of the well number and the indium composition in the InGaAs quantum well (QW) surrounding the dots. While keeping the InAs quantum dot density nearly constant, the internal quantum efficiency /spl eta//sub i/, modal gain, and characteristic temperature of 1-DWELL and 3-DWELL lasers with QW indium compositions from 10 to 20% are analyzed. Comparisons between the DWELL lasers and a conventional In/sub 0.15/Ga/sub 0.85/As strained QW laser are also made. A threshold current density as low as 16 A/cm/sup 2/ is achieved in a 1-DWELL laser, whereas the QW device has a threshold 7.5 times larger. It is found that /spl eta//sub i/ and the modal gain of the DWELL structure are significantly influenced by the quantum-well depth and the number of DWELL layers. The characteristic temperature T/sub 0/ and the maximum modal gain of the ground-state of the DWELL structure are found to improve with increasing indium in the QW It is inferred from the results that the QW around the dots is necessary to improve the DWELL laser's /spl eta//sub i/ for the dot densities studied.

High-performance 1.3-μm InAsP strained-layer quantum-well ACIS (Al-oxide confined inner stripe) lasers

We have demonstrated high-performance 1.3-/spl mu/m InAsP strained-layer quantum-well lasers with Al-oxide confined inner stripe (ACIS) structure. The oxidized layers are consisted of strain-compensated AlAs-InP-AlInAs (6 nm/2 ML/3 nm) superlattice (SAS) layers and grown on InP substrate using gas-source molecular beam epitaxy (GS-MBE). For the structural optimization of the ACIS laser, the dependence of quantum efficiency, threshold current and lateral mode characteristics on the initial ridge width and the current aperture width have been investigated. A very low-threshold current of 2.0 mA, a high differential quantum efficiency of 67% and stable single lateral mode operation of over 40 mW were obtained for the optimized ACIS lasers. The excellent characteristics obtained in the ACIS lasers are attributed to both the current and the optical confinements by the Al-oxide layer. In addition, we confirmed high uniformity of the threshold current and the oxidized width. This device is very promising candidate for low-cost access networks and optical interconnects.

Carrier lifetime and recombination in long-wavelength quantum-well lasers

We present a novel analysis for correcting the measured differential carrier lifetime to account for carrier population in both the barrier and separate confinement heterostructure (SCH) regions of quantum-well (QW) lasers. This analysis uses information obtained from the measured spontaneous emission spectra to correct the measured lifetime and obtain the intrinsic well lifetime. Once the intrinsic well lifetime is obtained, the intrinsic well recombination coefficients can also be obtained. We show that the carrier population in the barrier/SCH layers can significantly affect the measured carrier lifetime and the extracted recombination coefficients. We also show that this analysis yields transparency carrier density and differential gain numbers which are very different from those obtained with the traditional analysis and much closer to what is predicted for highly strained QW lasers. These differences indicate the importance of accounting for barrier/SCH carriers on the measurement of basic QW laser material properties.

Determination of active-region leakage currents in ridge-waveguide strained-layer quantum-well lasers by varying the ridge width

This paper is concerned with the investigation of lateral current spreading in InGaAsP strained-layer, ridge-waveguide multiple-quantum-well (MQW) semiconductor lasers consisting of nine quantum wells (QWs) and having ridge widths varying from 1.5 to 5 /spl mu/m. By using a simple analytical model, it is possible to demonstrate that as much as 60% of the injected current escapes out of the active region of a 1.5-/spl mu/m laser at threshold. For the first time, such an analysis is extended into the above-threshold regime to examine the effects of lateral current spreading on the external differential efficiency.

Non-Markovian gain of strained-layer wurtzite GaN quantum-well lasers with many-body effects

A theoretical model for the optical gain of strained-layer wurtzite GaN quantum-well (QW) lasers is developed taking into account valence-band mixing, many-body effects and non-Markovian relaxation. The valence-band structure is calculated from a 6/spl times/6 multiband effective mass Hamiltonian for the wurtzite structure taking into account built-in strain due to lattice mismatch. The theoretical foundation for the optical processes is based on the time-convolutionless reduced-density operator formalism given in previous papers for an arbitrary driven system coupled to a stochastic reservoir. Many-body effects are taken into account within the time-dependent Hartree-Fock approximation and the optical gain with Coulomb (or excitonic) enhancement is derived by integrating the equation of motion for the interband polarization. It is predicted that the Coulomb enhancement of gain is pronounced with increasing magnitude of compressive strain in the QW.

Theory of non-Markovian gain in strained-layer quantum-well lasers with many-body effects

A non-Markovian model for the optical gain of strained-layer quantum-well lasers is developed taking into account the valence-band mixing, strain effects, many-body effects, and the non-Markovian relaxation using the time-convolutionless reduced-density operator formalism given in previous papers for an arbitrary driven system coupled to a stochastic reservoir. Many-body effects are taken into account within the time-dependent Hartree-Fock approximation and the valence-band structure is calculated from the 6/spl times/6 Luttinger-Kohn Hamiltonian. The optical gain with Coulomb (or excitonic) enhancement is derived by integrating the equation of motion for the interband polarization. It is shown that the vertex function for the interband polarization can be obtained exactly without relying on the Pade approximation. As a numerical example, an In/sub x/Ga/sub 1-x/As-InP quantum well (QW) is chosen for its wide application in optical communication systems. It is predicted that the Coulomb enhancement of gain is pronounced in the cases of compressive and unstrained QWs while it is negligible in the case of tensile strained QW.

Dependence of optical gain on crystal orientation in wurtzite–GaN strained quantum-well lasers

Optical gains in wurtzite–GaN strained quantum-well (QW) lasers are estimated theoretically for various crystallographic directions. The calculation of the valence subbands is based on the k⋅p theory, where deformation potentials are determined by a semiempirical tight-binding method. It is found that the gains in GaN strained QW lasers with non-(0001) orientations, particularly around the (101̄5) orientation, are markedly high and anisotropic, unlike those in (0001)-oriented lasers.

Diode laser spectroscopy of gaseous HCl

Concentrations of gaseous HCl less than 1 ppm were measured with a strained-layer quantum-well distributed-feedback laser emitting the wavelength 1.74 μm at room temperature. Second-harmonic wavelength-modulation spectroscopy was used to reach a detection limit of 0.1 ppm m length with a signal-averaging time of 4 s. The measurements were carried out on gaseous HCl mixed with nitrogen (N2) at 25oC when the pressure was 1 bar.

A1.3 μm strained quantum well laser on a graded InGaAs buffer with a GaAs substrate

We grew a 1.3 μm strained-layer quantum well (SL-QW) laser with InGaP cladding layers on a lattice-relaxation buffer layer by metalorganic vapor phase epitaxy. For the lattice-relaxation buffer, we used a compositionally graded InGaAs/GaAs structure. The significantly reduced surface roughness of the InGaP cladding layers achieved by supplying a large amount of H2Se enables CW operation of our 1.3 μm SL-QW laser. We achieved a low threshold current of less than 10 mA and a high characteristic temperature of 100K around room temperature.

Study on the dominant mechanisms for the temperature sensitivity of threshold current in 1.3-μm InP-based strained-layer quantum-well lasers

We study the basic physical mechanisms determining the temperature dependence of the threshold current (I/sub th/) of InP-based strained-layer (SL) quantum-well (QW) lasers emitting at a wavelength of 1.3 /spl mu/m. We show that I/sub th/ exhibits a different temperature dependence above and below a critical temperature T/sub c/. It is indicated that T/sub c/ is the maximum temperature below which the threshold gain exhibits a linear relationship with temperature. We demonstrate that below T/sub c/ the Auger recombination current dominates the temperature dependence of I/sub th/. On the other hand, above T/sub c/ a significant increase in both the internal loss and radiative recombination current in the separate-confinement-heterostructure region, which is mainly due to electrostatic band-profile deformation, is found to play a major role in determining the temperature sensitivity of I/sub th/. On the basis of the comparison between the theoretical analysis and the experimental results, we conclude that the temperature dependence of the threshold current in 1.3-/spl mu/m InP-based SL-QW lasers is dominated by different mechanisms above and below T/sub c/.

Explanation for the temperature insensitivity of the Auger recombination rates in 1.55 μm InP-based strained-layer quantum-well lasers

We study the temperature sensitivity of the Auger recombination rates in 1.55 μm InP-based strained-layer (SL) quantum-well (QW) lasers on the basis of the band structures obtained by the self-consistent numerical solution of the Poisson equation, the scalar effective-mass equation for the conduction band, and the multiband effective mass equation for the valence band. The results of the theoretical analysis are then compared with the recent experimental results to clarify the basic physical mechanism which determines the Auger recombination rates in SL-QW lasers. It is shown that the recent temperature sensitivity measurements of Auger recombination coefficients can be consistently explained in terms of the direct band-to-band Auger process in the quasi-two-dimensional system. We demonstrate that the Auger recombination process in 1.55 μm InP-based SL-QW lasers is mainly dominated by the direct band-to-band Auger process regardless of QW structures in the temperature range of 273–398 K.

Very high characteristic temperature and constant differential quantum efficiency 1.3-μm GaInAsP-InP strained-layer quantum-well lasers by use of temperature dependent reflectivity (TDR) mirror

A very high characteristic temperature T/sub 0/ of 150 K (25-70/spl deg/C) or 450 K (25-50/spl deg/C) and an almost constant differential quantum efficiency operation in the temperature range of 25-70/spl deg/C were achieved in 1.3-/spl mu/m GaInAsP-InP strained-layer quantum-well (SL-QW) lasers by use of a novel temperature dependent reflectivity (TDR) mirror composed of multiple quarter-lambda thickness /spl alpha/-Si-SiOx dielectric films with quarter-lambda shift in the vicinity of center portion, The mechanism of high T/sub 0/ and constant differential quantum efficiency were explained using the structural parameters, transparent current density and gain coefficient of a SL-QW laser that are derived experimentally. The effect of TDR mirror was confirmed by measuring the temperature dependence of net gain of a SL-QW laser with TDR mirror. It was found that less temperature dependent net gain due to the decrease of mirror loss with temperature played an important role for improving the temperature characteristics of threshold current. Almost constant differential quantum efficiency over a wide temperature range is attributed to the increase of the facet reflectivity with temperature.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>