High-performance wafer-bonded bottom-emitting 850-nm VCSEL's onundoped GaP and sapphire substrates

We have demonstrated high performance wafer-bonded bottom-emitting 850-nm vertical-cavity surface-emitting lasers (VCSEL's) on transparent substrates. The free-carrier absorption of the substrate was avoided by using undoped GaP or sapphire substrates. The maximum external quantum efficiency approaches 48% while the threshold current remains as low as 550 μA for the 6×6 μm² VCSEL's bonded on GaP substrates. VCSEL's with 8.6×8.6 μm² aperture bonded on sapphire substrates also exhibit threshold currents of 800 μA and external quantum efficiencies of 33.2%. The difference in efficiency between these two devices results from the change of the refractive index of the exit medium     

Uniform wafer-bonded oxide-confined bottom-emitting 850-nm VCSELarrays on sapphire substrates

Uniform bottom-emitting 850-nm vertical-cavity surface-emitting laser (VCSEL) arrays on sapphire substrates have been demonstrated using wafer bonding technology to transfer the epitaxially-grown VCSEL structures from GaAs substrates onto sapphire substrates. The uniformity of the VCSEL arrays were improved by placing thin oxide aperture at the standing wave node to reduce scattering loss for small aperture devices. The averaged threshold current of a 5×5 VCSEL array is as low as 346 μA, while the averaged external quantum efficiency approaches 57%. The maximum wall-plug efficiency is 25% and the single-mode output power is more than 2 mW under continuous-wave current excitation at room temperature. We have also demonstrated a large (10×20) VCSEL array with variations of threshold current and external quantum efficiency less than 4% and 2%, respectively     

Small-signal characteristics of bottom-emitting intracavitycontacted VCSEL's

We characterize the small-signal frequency response of intracavity contacted ~970 nm VCSEL's designed for flip-chip bonding to electronic circuits and then modified with reduced capacitance and a novel lateral current injection method to obtain a high-speed response. We measure 3-dB bandwidths in excess of 10 GHz and an extracted K-factor limited f-max of 95 GHz for fabricated single-mode ~970-nm lasers     

Efficient single-mode oxide-confined GaAs VCSEL's emitting in the850-nm wavelength regime

Single- and multimode vertical-cavity surface-emitting lasers (VCSELs) with three unstrained GaAs quantum wells (QWs) and emission wavelengths around 850 nm have been fabricated using molecular beam epitaxy (MBE) for crystal growth. Wet chemical etching and subsequent selective oxidation are applied for current confinement. The influence of oxide layer position on lateral index guiding is studied in detail in order to increase maximum single-mode output power. A device of 3-μm active diameter and reduced index guiding shows maximum single-mode output power of 2.25 mW with a side-mode suppression ratio (SMSR) of more than 30 dB for high-efficiency oxidized VCSELs     

Development of bottom-emitting 1300-nm vertical-cavity surface-emitting lasers

We present experimental results on the development of bottom-emitting GaInNAs vertical-cavity surface-emitting lasers (VCSELs) operating at wavelengths near 1300 nm. This development effort is based on the modification of oxide-apertured top-emitting structures to allow emission through the GaAs substrate. Similar device performance was seen in both the top- and bottom-emitting structures. Single-mode output powers (adjusted for substrate absorption) of /spl sim/0.75 mW, with threshold currents of 1.3 mA, were achieved with /spl sim/3.5-/spl mu/m aperture diameters. Larger multimode devices exhibited a maximum adjusted output power of 2.2 mW. To the best of our knowledge, these are the first bottom-emitting flip-chip compatible 1300-nm VCSELs fabricated with GaInNAs-GaAs active regions.     

850-nm diode lasers based on AlGaAsP/GaAs heterostructures

Laser heterostructures with waveguides and emitter layers made of AlGaAs and AlGaAsP alloys are grown by hydride metal-organic vapor-phase epitaxy on a GaAs substrate and studied. It was shown that the heterostructure containing AlGaAsP layers has a large curvature radius in comparison with the structure consisting of AlGaAs layers. Ridge waveguide lasers with an aperture of 100 ??m are fabricated based on the AlGaAsP/GaAs laser heterostructure and studied. The internal optical loss of the lasers is 0.75 cm^?1; the characteristic parameter T ₀ = 140 K in the temperature range of 20?C70°C. The maximum optical output power per mirror reached 4.1 W; cw lasing was retained at a heat sink temperature of 120°C.     

Ultralow-threshold sapphire substrate-bonded top-emitting 850-nm VCSEL array

Oxide-confined top-emitting vertical-cavity surface-emitting-laser (VCSEL) 8 /spl times/ 8 arrays were designed and fabricated with ultralow thresholds. The arrays were flip-chip bonded onto sapphire substrates and mounted in pin-grid-array packages as optical transmitter arrays. By using the offset-contact bonding process, we were able to obtain very high yield for hybridized devices without damaging the VCSEL mesas. Room-temperature lasing thresholds below 70 /spl mu/A were found from some of these packaged VCSELs with measured oxide apertures 2.6 /spl mu/m in diameter. The emission spectrum at an injection current of 70 /spl mu/A showed a full-width at half-maximum linewidth of less than 2.5 A. Polarization properties were also confirmed from the output of the device. The superior performance was attributed to the optimized size and placement of the confinement aperture and the precise alignment of the gain profile of the active region to the mode of the resonant cavity.     

Properties of GaP light-emitting diodes grown on spinel substrates

Red and green GaP electroluminescent diodes have been successfully fabricated from GaP grown heteroepitaxially on spinel substrates by a vapor phase/liquid phase two-stage process. Current-voltage and light emission characteristics of the diodes are compared with those grown on bulk substrates. Quantum efficiencies up to 0·1 per cent in the red and 0·01 per cent in the green have been obtained.     

Diffusion and solid solubility of Zn in GaP at 850°C

Zinc was diffused at 850°C from ternary sources containing 1–20 atom percent Zn and 0.5–1.0 atom percent P in Ga into n-type GaP grown by liquid phase epitaxy (LPE) and the liquid encapsulated Czochralski (LEC) process. Data on the surface concentration, and thereby the solid solubility of Zn as a function of the source composition, were obtained from⁶⁵Zn radiotracer analysis and electrical measurements. At 850°C these values range from 8×l0¹⁷ cm⁻³ for a 1 atom percent Zn to 5×10¹⁸ cm⁻³ for a 20 atom percent Zn concentration in the Ga/P/Zn ternary solution. These data are self-consistent, although they are somewhat lower than the previously reported values . Qualitatively, the results agree with recent thermodynamic calculations. The diffusion kinetics were found to be non-ideal as indicated by departures from a linear-square-root-of-time model and by reproducible changes in the junction depth following pre-diffusion heat treatments in different ambients.     

Growth of direct bandgap GalnP quantum dots on GaP substrates

GaInP has a direct bandgap for In concentrations higher than approximately 30%, and the band-lineup between GaInP and GaP is type-II for In concentrations less than 60%. Therefore, in order to use GaInP as the active light-emitting layer in an optoelectronic device grown on GaP, the strain induced by the lattice mismatch between GaInP and GaP has to be somehow managed such that formation of crystal defects is suppressed. One method is to grow the layer thinner than the critical thickness. Another method that recently received much attention is to grow strain-induced Stranski-Krastanov islands (sometimes referred to as self-assembled quantum dots). Small droplets of highly latticmismatched materials have been embedded into single crystals without generating defects such as threading dislocations and stacking faults using this method. We have grown a series of GaInP/GaP layers by metalorganic chemical vapor deposition and have studied the light emission from them. Ordered GaInP islands were found to be responsible for the light emission. We present the light emission characteristics of these ordered GaInP/GaP islands, and their dependence on various growth parameters.     

Widely tunable 850-nm metal-filled asymmetric cladding distributed Bragg reflector lasers

Broadly tunable distributed Bragg reflector (DBR) lasers utilizing metal-filled surface-etched diffraction gratings were fabricated on a GaAs-unstrained quantum well with AlGaAs core and cladding layers. Devices with 200- and 400-/spl mu/m gratings sections were fabricated. Single-mode devices are fabricated over a broad spectral range, exhibiting over 55 mW of single-facet output power. The optical properties of this structure are analyzed in terms of effective index step between the peak and valley of the grating and scattering loss in the DBR. Discrete devices were fabricated with lasing wavelengths between 846.6 and 862.9 nm at intervals of 1.44nm, representing a range of 16.3 nm. Wavelength tuning by current injection into the DBR section is explored, and a broad tuning range of over 18 nm is measured while single-mode performance is maintained. The spectral linewidth of these devices is measured. The temperature dependence of light versus current, threshold current values, and spectral characteristics are also examined.     

High-speed modulation of 850-nm intracavity contacted shallowimplant-apertured vertical-cavity surface-emitting lasers

GaAs-AlGaAs quantum-well (850 nn) vertical-cavity surface-emitting lasers, with lateral current injection and shallow implanted apertures, show small signal modulation bandwidths of at least 11 GHz and large signal data rates of at least 10 Gb/s. The devices achieved a maximum output power of 2.1 mW, with a threshold current and voltage of 1 mA and 1.71 V, respectively. The shallow implantation step provides photolithographically precise aperture formation (using O⁺ ions), for efficient lateral current injection into the quantum-well active region of the laser, from intracavity contacts. The device aperture was 7 μm in diameter, and the opening in the annular top contact was 13 μm in diameter. The optical spectrum showed several transverse modes     

Degradation behavior of 850-nm vertical-cavity surface-emittinglasers with an air-post index-guide structure

The degradation mechanism of a vertical-cavity surface-emitting laser (VCSEL) with an air-post structure is analyzed for stable optical parallel interconnection communication. It is clarified that the degradation is caused by the behavior of latent defects in the GaAs active layer. Decreasing the defects in the active region as well as decreasing the threshold current is important for obtaining a long-lifetime VCSEL     

Ultraviolet semiconductor laser structures with pseudomorphic ZnCdSSe quaternary alloys on GaP substrates

For short-wavelength laser diodes operating in the ultraviolet region, a ZnCdSSe/ ZnSSe or ZnCdSSe/ZnCdS quantum well (QW) structure on a GaP substrate is proposed. The physical parameters of Zn_1−xCd_xS_ySe_1−y alloys in the range of 0 ≤ x ≤ 0.3 and 0.7 ≤ y ≤ 1 are estimated to design the device structure. It is shown that this system can be grown coherently on GaP substrates and is expected to possess the so-called type I QW structures that allow the effective confinement of both electrons and holes. ZnCdSSe/ZnSSe double heterostructures grown by gas-source molecular beam epitaxy exhibited relatively strong photoluminescence from the quaternary active layer at 4.2K, which suggested carrier confinement in the active layer.     

Performance, uniformity, and yield of 850-nm VCSELs deposited by MOVPE

Vertical-cavity surface-emitting lasers (VCSELs) emitting near 850 nm and fabricated with the metal-organic vapor phase epitaxy (MOVPE) epitaxial growth technique and a planar proton implant process have been demonstrated with excellent performance, uniformity, and yield across a 3-in wafer. Four thousand lasers were tested on a three-inch-diameter wafer, with a yield of 99.8%. This translates into a yield of 94% for fully functional 34/spl times/1 arrays. The average threshold current, threshold voltage, and dynamic resistance at 10 mA operating current were 3.07 mA, 1.59 V, and 34 ohms, respectively. Uniformity of better than /spl plusmn/9% in threshold current, /spl plusmn/1% in threshold voltage, and /spl plusmn/1.5% in maximum optical output power across a 34-element array was demonstrated.     

Performance and Reliability of 12.5-Gb/s Oxide-Free 850-nm Mesa VCSELs

Oxide-free mesa vertical-cavity surface-emitting lasers (VCSELs) emitting at 850 nm have been designed for short reach datacom applications at data rates up to 12.5 Gb/s. The top distributed Bragg reflector is etched away creating a mesa that provides both current and photon confinement. The devices exhibit low threshold current and a donut-shaped far-field profile that is suited for transmission on both legacy and laser-optimized multimode fibers. Open eye diagrams with high margin are observed in on-wafer testing of 8-10 mum VCSELs at 10.3125 Gb/s over 5degC-95degC. Accelerated aging tests indicate a long device lifetime, with the time for a cumulative failure of 1% estimated to be 15 million h at 40degC for 12-mum VCSELs.     

High-Power and High-Speed Zn-Diffusion Single Fundamental-Mode Vertical-Cavity Surface-Emitting Lasers at 850-nm Wavelength

We demonstrate a high-performance Zn-diffusion single-mode 850-nm vertical-cavity surface-emitting laser, which has a low threshold current (0.5 mA), high differential efficiency (80%), high modulation current efficiency (8.2 GHz/mA), and can sustain the single fundamental-mode output with a maximum output power of 7.3 mW under the full range of bias currents. With this device we can achieve 10 Gb/s eye-opening at a low bias current (1.8 mA) and a peak-to-peak driving-voltage of 0.5 V, which corresponds to a very high data-rate/power-dissipation ratio of 6.5 Gps/mW.     

0.5-W single transverse-mode operation of an 850-nm diode-pumped surface-emitting semiconductor laser

We report the power scaling of a diode-pumped GaAs-based 850-nm vertical external-cavity surface-emitting laser, by use of an intracavity silicon carbide (SiC) heatspreader optically contacted to the semiconductor surface. To our knowledge, this is the first demonstration of bonding of SiC to a III-V semiconductor structure using the technique of liquid capillarity. High output power of >0.5 W in a circularly symmetric, TEM/sub 00/ output beam has been achieved with a spectral shift of only 0.6 nm/W of pump power. No thermal rollover was evident up to the highest pump power available, implying significant further output-power scaling potential using this approach.     

An optoelectronic oscillator using an 850-nm VCSEL for generating low jitter optical pulses

We demonstrate an optoelectronic oscillator using a gain-switched vertical-cavity surface-emitting laser in a fiber-feedback configuration. We simultaneously generate a 2-GHz optical pulse stream at 850 nm with 750-fs timing jitter (over 100 Hz-10 MHz range) along with an electrical signal that is locked to the repetition rate of the optical pulses. The timing jitter performance is confirmed by measuring higher harmonic phase noise.