Modeling the Influence of Nitrogen Concentration on Optical Gain Performance in Gallium Nitride Arsenide/Gallium Arsenide Quantum Wells Vertical-cavity Surface-emitting Lasers

Remarkable progress made in vertical cavity surface emitting lasers (VCSELs) emitting at 850 and 980 nm has led them to find an increasing number of applications in high speed data communications as well as in potential space satellite systems. However, little has been reported on reliability and failure modes of InGaAs VCSELs emitting at ~980 nm although it is crucial to understand failure modes and underlying degradation mechanisms in developing these VCSELs that exceed lifetime requirements for space missions. The active layer of commercial VCSELs that we studied consisted of two or three InGaAs quantum wells. The laser structures were fabricated into deep mesas followed by a steam oxidation process to form oxide-apertures for current and optical confinements. Our multi- mode VCSELs showed a laser threshold of ~ 0.5 mA at RT. Failures were generated via accelerated life-testing of VCSELs. For the present study, we report on failure mode analysis of degraded oxide-VCSELs using various techniques. We employed nondestructive techniques including electroluminescence (EL), optical beam induced current (OBIC), and electron beam induced current (EBIC) techniques as well as destructive techniques including focused ion beam (FIB) and high-resolution TEM techniques to study VCSELs that showed different degradation behaviors. Especially, we employed FIB systems to locally remove a portion of top-DBR mirrors of degraded VCSELs, which made it possible for our subsequent EBIC and OBIC techniques to locate damaged areas that were generated as a result of degradation processes and also for our HR-TEM technique to prepare TEM cross sections from damaged areas. Our nondestructive and destructive physical analysis results are reported including defect and structural analysis results from pre-aged VCSELs as well as from degraded VCSELs life-tested under different test conditions.

The electro-optical efficiency of vertical-cavity surface-emitting lasers (VCSELs) strongly depends on the efficient carrier injection into the quantum wells (QWs) in the laser active region. Carrier injection degrades with increasing temperature, which limits VCSEL performance in high-power applications where self-heating imposes high-operating temperatures. In a numerical model, we investigate the transport of charge carriers in an 808-nm AlGaAs multi-quantum-well structure with special attention to the temperature dependence of car- rier injection into the QWs. Experimental reference data were extracted from oxide-confined, top-emitting VCSELs. The transport simulations follow a drift-diffusion-model complemented by an energy-resolved car- rier-capture model. The QW gain was calculated in the screened Hartree-Fock approximation. With the combi- nation of the gain and transport model, we explain experimental reference data for the injection efficiency and threshold current. The degradation of the injection efficiency with increasing temperature is not only due to increased thermionic escape of carriers from the QWs, but also to state filling in the QWs initiated from higher threshold carrier densities. With a full opto-electro-thermal VCSEL model, we demonstrate how changes in VCSEL properties affecting the threshold carrier density, like mirror design or optical confinement, have con- sequences on the thermal behavior of the injection and the VCSEL performance. © 2015 Society of Photo-Optical

A lattice temperature model is derived for oxide-confined vertical cavity surface emitting lasers (VCSELs) based on carrier transport and the conservation of energy. Peltier heat is caused by the bandedge and quasi-Fermi level discontinuities at a heterojunction. However, considering the device size, Peltier heat needs to be distributed and is not just generated at the interface, otherwise, an anomolous spike in temperature will occur. We have developed a novel treatment to model the Peltier heat at a heterojunction by use of a Monte Carlo simulation. Peltier heat is found to be a major heat contributor, and it results in a rapid and high temperature rise in the separate confinement heterostructure (SCH) region of the laser diode. We have also shown that the carrier thermal conductivities for materials with high mobilities must be included at high carrier densities because they contribute to additional spreading of the thermal energy. Subsequently, this lattice temperature model is coupled self-consistently to electronic and optical solvers to form a complete simulator for VCSELs. Self-heating causes a fast temperature rise when the VCSEL is operated under continuous wave conditions, causing resonant wavelength changes and an eventual thermal rollover. The resonant wavelength shift has been shown to be caused mainly by the heating of the distributed Bragg reflectors even though the peak temperature occurs within the SCH region. Possible physical factors causing the thermal rollover have also been examined with our complete simulator. The Auger recombination process is found to be one of the main factors causing the thermal rollover in 980 nm oxide-confined VCSELs while the photon lifetime is a factor in determining the position of the thermal rollover. We have also achieved a very good match between our simulated results and experimental data.

In vertical‐cavity surface‐emitting lasers (VCSELs), the cavity length defines the resonance wavelength, which is directly related to the laser detuning, that is, the difference between resonance wavelength and gain peak. A low detuning maximizes the modal gain leading to a reduction of the threshold. Therefore, controlling the cavity length of VCSELs is of great importance. Here optically pumped ultraviolet‐C (wavelength 280 nm) VCSELs with precise cavity length control are demonstrated. The VCSEL structure is formed by an AlN cavity with 5 Al0.40Ga0.60/Al0.70Ga0.30N quantum wells and a top HfO2 spacer layer with dielectric SiO2/HfO2 distributed Bragg reflectors on both sides of the cavity. To access the N‐face side of the cavity, a new methodology referred to as photo‐assisted electrochemical etching is employed for substrate removal. Across a 0.9 mm 1.2 mm area, the lasing wavelength varies a maximum of 1.17 nm between different UVC VCSELs, exhibiting threshold pump power densities from 0.7 MW/cm2 to 3.7 MW/cm2 and detuning values between 0 to 2 nm. The results show that VCSELs with a cavity length variation lower than 1 can be obtained with this technology.

A highly sensitive and easy-to-fabricate hydrogen sensor based on a plasmonic ‘gold nanowire array on a palladium layer deposited on a metallic substrate' is proposed. Plasmonic waveguide modes are excited in the gaps between the nanowires in this ‘gold nanowire array on a palladium spacer layer deposited on a metallic substrate' system. As incident light is coupled into the plasmonic modes, a dip in the reflectance spectra is observed at the resonant wavelength, i.e., the wavelength at which the incident light is coupled into plasmonic modes. On exposure to hydrogen, the palladium spacer layer transforms to palladium hydride (PdHx), where x, the atomic ratio of H:Pd, increases as the hydrogen concentration increases. This transformation changes the optical properties of the Pd layer, and hence the position of the resonance wavelengths (λres), i.e., the position of the reflection dips in the reflectance spectra of the Au-Pd-Au system, for various concentrations of hydrogen. The difference between the positions of the resonant wavelengths of PdHx and Pd, (λres(PdHx)−λres(Pd)), is used as a measure of the sensitivity of the proposed hydrogen sensor. Analysis of this shift in the plasmon resonance wavelength is done numerically, using Rigorous Coupled Wave Analysis (RCWA) for various values of d, the side length of the nanowires; t, the thickness of the Pd spacer; g, the gap between the adjacent nanowires and θ, the angle of the incident radiation. It is found that, in the presence of hydrogen, the maximum shift in the resonance wavelength for the proposed sensor is ~41 nm as compared to the case when hydrogen is absent. This shift in the resonance wavelength is higher than many currently employed plasmonic Pd-based hydrogen sensors. Thus, the proposed ‘gold nanowire array on a palladium spacer layer deposited on a metallic substrate' is an easy-to-fabricate, selective and sensitive hydrogen sensor.

The analogue modulation characteristics, including second order harmonic and third order intermodulation distortion, relative intensity noise (RIN) and spurious free dynamic range (SFDR), of single mode, GaAs-based 1.28 μm vertical cavity surface emitting laser (VCSELs) with highly strained InGaAs quantum wells have been investigated. The VCSELs utilise an oxide aperture for current and optical confinement and an inverted surface relief (SR) for suppression of higher order transverse modes. The inverted SR structure also has the advantage of suppressing oxide modes that, otherwise, appear in VCSELs with a large detuning of the cavity resonance with respect to the gain peak, which is needed to extend the emission wavelength. RIN levels comparable with those of single mode VCSELs emitting at 850 nm are demonstrated, with values from 140 to 150 dB/Hz in the 2 5 GHz range. SFDR values of 100 and 95 dBHz2/3 are obtained at 2 and 5 GHz, respectively. These values are in the range of those required in radio-over-fibre links.

Long wavelength (LW) vertical cavity surface emitting lasers (VCSELs) are important low cost and low power consumption sources for optical interconnects in data centers, sensing and spectroscopy [1,2]. We discuss here the recent progress in the design, fabrication and industrialization of such devices made by using wafer fusion, which allows integration of GaAs-based distributed Bragg reflectors (DBRs) with InP-based active structures and an independent optimization of the mirror and active cavity properties before the fusion. Both the GaAs-based mirrors and InP-based optical gain structures are grown by metalorganic vapor phase epitaxy (MOVPE). The VCSEL structure comprises an InP-based active region with 5 to 6 InAlGaAs compressively strained quantum wells (QWs) as the gain medium and tunnel junction (TJ) aperture for current and optical confinement, double-fused to two GaAs/AlGaAs-based Bragg mirrors (Fig.1a). By epitaxial growth, device design and processing optimization 1.3- and 1.5μm waveband VCSELs emitting single mode power of 6–8-mW at room temperature and up to 3-mW at 80°C were demonstrated (Fig.1b) [3,4]. Moreover, industrially manufactured 10-Gb/s full CWDM wavelength-set VCSEL devices for coarse wavelength division multiplexing systems with high yield and Telcordia-reliability have been developed [5–6].

As a key part of vertical cavity surface emitting laser (VCSEL), active region will seriously affect the threshold and efficiency of the device. To obtain the appropriate laser wavelength and material gain, the design of In<sub>0.18</sub>Ga<sub>0.82</sub>As strain compensated quantum well is optimized. The relationship between the lasing wavelength of multiple quantum wells (MQWs) and the thickness is calculated. Considering the influence between the active region temperature and the lasing wavelength, the thickness of the quantum well is chosen as 6 nm, and the quantum barrier thickness is chosen as 8 nm, corresponding to the lasing wavelength of 929 nm. The material gain characteristics of the MQWs at different temperatures are simulated by Rsoft. The material gain exceeds 3300/cm at 300 K, and the temperature drift coefficient of the peak wavelength is 0.3 nm/K. In this work, Al<sub>0.09</sub>Ga<sub>0.91</sub>As and A<sub>l0.89</sub>Ga<sub>0.11</sub>As are chosen as the high- and the low-refractive index material of distributed Bragg reflector (DBR), and 20 nm graded layer is inserted between two types of materials. The influence of the graded layer thickness of DBR on the valence band barrier and reflection spectrum are calculated and analyzed. The increase of graded layer thickness can lead the band barrier peak and the reflection spectrum bandwidth to decrease. The reflection spectrum and phase spectrum of P-DBR and N-DBR are calculated by the transmission matrix mode (TMM): the reflectance of DBR is over 99% and the phase shift is zero at 940 nm. The optical field distribution of the whole VCSEL structure is simulated, in which the standing wave peak overlaps with the active region, and the maximum gain can be obtained. Using the finite element method (FEM), the effect of oxidation confined layer on the injection current is simulated. The current in the active region is effectively limited to the position corresponding to the oxidation confined hole, and its current density is stronger and more uniform. The optical field distributions in different modes of photonic crystal-vertical cavity surface emitting laser (PC-VCSEL) are simulated, and different modes have different resonant wavelengths. The values of quality factor Q in different modes of VCSEL and PC-VCSEL are calculated, Q of the fundamental mode is higher than that of higher transverse mode. It is demonstrated that the photonic crystal air hole structure can realize the output of basic transverse mode by increasing the loss of high order transverse mode. The VCSEL and PC-VCSEL with oxidation hole size of 22 μm are successfully fabricated, in which the photonic crystal period is 5 μm, the air pore diameter is 2.5 μm, and the etching depth is 2 μm. Under continuous current test, the maximum slope efficiency of VCSEL is 0.66 mW/mA, the output power is 9.3 mW at 22 mA, and the lasing wavelength is 948.64 nm at 20 mA injection current. Multiple wavelengths and large spectrum width are observed in the spectrum of VSCEL, which is an obvious multi-transverse mode. The maximum fundamental transverse mode output of PC-VCSEL reaches 2.55 mW, the side mode suppression ratio (SMSR) is more than 25 dB, and the spectrum width is less than 0.2 nm, indicating that the photonic crystal air hole has a strong control effect on the transverse mode, and the laser wavelength is 946.4 nm at 17 mA.

The physical and optical properties of compressively strained InGaAsP/InGaP quantum wells for 850-nm vertical-cavity surface-emitting lasers are numerically studied. The simulation results show that the maximum optical gain, transparency carrier densities, transparency radiative current densities, and differential gain of InGaAsP quantum wells can be efficiently improved by employing a compressive strain of approximately 1.24% in the InGaAsP quantum wells. The simulation results suggest that the 850-nm InGaAsP/InGaP vertical-cavity surface-emitting lasers have the best laser performance when the number of quantum wells is one, which is mainly attributed to the non-uniform hole distribution in multiple quantum wells due to high valence band offset.

Long-wavelength vertical-cavity surface-emitting lasers (VCSELs) are desirable as low-cost sources for optical metropolitan-area and access networks. In the development of 1.3-µm VCSELs, most attention today is given to monolithic GaAs-based solutions, although no established active material exists in this wavelength region. This thesis investigates the possibility of reaching the 1.3-µm telecom wavelength window using GaInNAs quantum wells (QWs) or 1.2-µm InGaAs QWs in conjunction with negative gain-cavity detuning in VCSELs. The work includes metal-organic vapor-phase epitaxy and characterization of InGaAs and GaInNAs QWs, realization of 1.3-µm InGaAs VCSELs as well as elements of optimization and analysis of such lasers. The evaluation of GaInNAs and InGaAs QWs has been performed using a number of characterization methods such as photoluminescence (PL), high-resolution x-ray diffraction, secondary-ion mass spectroscopy, and atomic-force microscopy as well as fabrication and evaluation of broad-area lasers (BALs). Both performance and growth reproducibility of GaInNAs QWs are considered and could be improved by using high V/III ratios. Nontrivial relations between PL and laser performance are pointed out and the technologically important but problematic combination of AlGaAs and GaInNAs in the same epitaxial structure is studied. Parallel to the work on GaInNAs, the possibility of extending the wavelength of InGaAs QWs towards 1.3 µm has been investigated. Generally better luminescence efficiency and laser performance are obtained for InGaAs than for GaInNAs QWs, but the gain-peak wavelength for InGaAs QWs is presently limited to about 1.24 µm due to strain-induced degradation. In this work the InGaAs QW growth is optimized for long wavelength and high luminescence. It is demonstrated that multiple QW structures can be grown with strain similar to that of single QWs, which is interesting for VCSEL applications. Record BALs with two to five InGaAs/GaAs QWs have low threshold current densities,  70 A/cm2 per QW at 1.24 µm. The main advantage of InGaAs QWs compared to GaInNAs QWs is that they represent a better-known material system with less complex and more stable growth. However, InGaAs QWs > 1.2 µm are on the verge of strain relaxation, and the possible consequences for laser production and reliability have to be considered. Using 1.2-µm InGaAs QWs, high-performance 1.3-µm VCSELs were achieved by negative gain-cavity detuning. Dynamic performance and surface reliefs to improve the single-mode operation have been investigated. The VCSELs have excellent high-temperature performance due to a smaller spectral distance between the gain-peak and the laser mode at elevated temperature. More specifically, a 1.27-µm single-mode device showed maximum output powers of 1.1 and 0.5 mW at 20 and 140oC, which is state-of-the-art for GaAs-based long-wavelength VCSELs. In all, two methods for 1.3-µm GaAs-based VCSELs, GaInNAs and InGaAs QWs, have been investigated. GaInNAs is a difficult material but is still promising and several companies have predicted a near-future market introduction. However, the growth of GaInNAs is both complex and sensitive to growth fluctuations. On the other hand, gain-cavity detuned InGaAs-QW VCSELs show state-of-the-art performance at 1260-1290 nm with straightforward growth and processing. The devices exhibit good static and dynamic performance, and preliminary reliability tests indicate that there is no intrinsic problem. Both approaches are promising for application in real-world optical networks and deserve further attention.

Distributed Bragg reflectors (DBRs) are typically used as the highly reflecting mirrors of vertical-cavity surface-emitting lasers (VCSELs). In order to provide optical field confinement, oxide apertures are often incorporated in the process of the selective wet oxidation of high aluminum-content DBR layers. This technology has some potential drawbacks such as difficulty in controlling the uniformity of the oxide aperture diameters across a large-diameter (≥ 6 inch) production wafers, high DBR series resistance especially for small diameters below about 5 μm despite elaborate grading and doping schemes, free carrier absorption at longer emission wavelengths in the p-doped DBRs, reduced reliability for oxide apertures placed close to the quantum wells, and low thermal conductivity for transporting heat away from the active region. A prospective alternative mirror is a high refractive index contrast grating (HCG) monolithically integrated with the VCSEL cavity. Two HCG mirrors potentially offer a very compact and simplified VCSEL design although the problems of resistance, heat dissipation, and reliability are not completely solved. We present an analysis of a double HCG 980 nm GaAs-based ultra-thin VCSEL. We analyze the optical confinement of such a structure with a total optical thickness is ~1.0λ including the optical cavity and the two opposing and parallel HCG mirrors.

A comprehensive numerical model for investigating the thermal, electrical and optical characteristics of vertical cavity surface emitting lasers with a diffused quantum wells structure is presented. In general, this model performs a self-consistent calculation of quasi 3D distribution of temperature, voltage and optical field. The quasi 2D diffusion and the recombination of carrier concentration inside the quantum well active layer are also introduced into the model. In particular, this model includes the calculation of the quasi 3D ion-implantation profile. In addition, the influence of impurity induced compositional disordering on the optical gain and refractive index of the quantum wells active layer is also taken into consideration. Using this model, the steady state characteristics of diffused quantum wells vertical cavity surface emitting lasers are studied theoretically. It is shown that significant improvement of stable single-mode operation can be obtained using diffused quantum wells structure.

Study on effect of quantum well number on performance characteristics of GaN-based vertical cavity surface emitting laser

We present a combination of spectroscopy and device measurements on GaAsSb/GaAs vertical-cavity surface-emitting laser (VCSEL) structures to determine the temperature at which the wavelength of the VCSEL cavity mode (CM) aligns with that of the quantum well (QW) ground-state transition (GST), and therefore the gain peak. We find that, despite the achievement of room temperature (RT) continuous wave lasing in VCSEL devices, the QW transition and the CM are actually slightly misaligned at this temperature; room temperature electroluminescence measurements from a cleaved edge of the VCSEL wafer indicate that the 300 K QW GST energy is at 0.975 ± 0.005 eV, while the CM measured in the VCSEL surface reflectivity spectra is at 0.9805 ± 0.0002 eV. When the wafer sample is cooled, the CM and QW GST can be brought into alignment at 270 ± 10 K, as confirmed by temperature-dependent electro-modulated reflectance (ER) and edge-electroluminescence spectroscopic studies. This alignment temperature is further confirmed by comparing the temperature dependence of the emission energy of a fabricated VCSEL device with that of an edge-emitting laser structure with a nominally identical active region. The study suggests that for further device improvement, the room temperature CM and QW GST energies should be more closely matched and both designed to a smaller energy of about 0.95 eV, somewhat closer to the 1.31 μm target. The study amply demonstrates the usefulness of non-destructive ER characterisation techniques in VCSEL manufacturing with GaAsSb-based QWs.

GaInNAs is a new active material for optical fiber communications. It makes the long wavelength laser diode on GaAs substrates possible with hetero-epitaxial growth techniques. Due to a larger energy offset in the conduction band, it is expected that the temperature characteristics of GaInNAs lasers are better than those of conventional InP-based lasers. Fabrication of GaInNAs vertical-cavity surface-emitting laser is also easier because of well-established GaAs/AlGaAs distributed Bragg reflectors and Al/sub x/O/sub y/ confinement layers. Until now, continuous-wave operation of edge emitting laser at 1.3 /spl mu/m and pulsed operation of vertical-cavity surface-emitting laser at 1.18 /spl mu/m were reported. In the report, we present the optical gain and related properties of the GaInNAs quantum well calculate by the k.p method with the envelope function approximation. We focus on the effects of each material composition in the quantum well structures on emission wavelength, transparency carrier density, differential gain, and carrier leakage.