Scaling Properties of Lithographic VCSELs Abdullah Demir*a,b, Guowei Zhaoa, Sabine Freisema,b Xiaohang Liua, Dennis G. Deppea,b a CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, FL 32816; b sdPhotonics LLC ABSTRACT Data are presented demonstrating lithographic vertical-cavity surface-emitting lasers (VCSELs) and their scaling properties. Lithographic VCSELs have simultaneous mode- and current-confinement defined only by lithography and epitaxial crystal growth. The lithographic process of these devices allows getting uniform device size throughout a wafer and easy scaling to manufacture very small lasers. The semiconductor’s high thermal conductivity enables the small lithographic VCSEL to have lower thermal resistance than an oxide-aperture VCSEL, while the lithographic fabrication produces high VCSEL uniformity even at small size. Very dense packing is also possible. Devices of 3 μm to 20 μm diameters are fabricated and scaling properties are characterized. 3 μm lithographic VCSELs produce output power of 4.1 mW, with threshold current of 260 μA and slope efficiency of 0.76 W/A at emission wavelength of ~980 nm. These VCSELs also have single-mode single-polarization lasing without the use of a surface grating, and have >25 dB sidemode-suppression-ratio up to 1 mW of output power. Lifetime tests demonstrate that 3 μm VCSEL operates for hundreds of hours at high injection current level of 85 kA/cm2 with 3.7 mW output power without degradation. Scaling properties and low thermal resistance of the lithographic VCSELs can extend the VCSEL technology to manufacturable and reliable small size lasers and densely packed arrays with long device lifetime. Keywords: Semiconductor laser, VCSEL, lithographic, oxide-free, single-mode, single-polarization, thermal resistance
1. INTRODUCTION In this paper, we demonstrate lithographically-defined VCSELs in which the transverse mode and optical cavity are defined using only lithography and epitaxial crystal growth. We show that eliminating the oxide aperture reduces the thermal resistance. A low thermal resistance can increase the output power saturation before thermal rollover and when it is combined with better mode matching to gain for smaller devices, high output power densities are possible from small devices. Lithographic processing gives the ability to reach small sizes that are difficult to achieve reproducibly for oxide-aperture VCSELs. The demonstration of lithographic laser diodes with good scaling properties could therefore be an important step toward producing ultra-small size laser diodes with good size control, manufacturability and high reliability. Also important in this new technology is the ability to lithographically control polarization in the same fabrication step used to form the new VCSEL aperture. VCSELs of various sizes are fabricated and characterized to study the size effect on thermal resistance and output power density. Oxide-confined edge-emitting lasers (EELs)1,2,3 and VCSELs4,5 have been heavily studied using conversion of buried AlAs layer to native oxide of AlxOy by selective oxidation6,7. The improvements of VCSEL performance have been significant due to the introduction of mode- and current-confinement through oxide-aperture. The low threshold and low power consumption have produced high speed modulation and high efficiency VCSELs5,8. Oxide-confined quantum-dot edge emitting laser was shown to reach low threshold current densities3. However, the oxide is dramatically disparate in its material properties with respect to the surrounding semiconductor, and it is this disparity that creates roadblocks for further developments. Oxide VCSELs has historically suffered problems but remains in use due to the oxide’s ability to provide lateral mode and current confinement needed for efficient operation of the VCSELs. The problems are introduced by the oxide-aperture incorporation such as dislocation formation after oxidation, strain and low thermal conductivity of the oxide and size variation of oxide-aperture. The associated issues of oxide cause early device failure *
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Vertical-Cavity Surface-Emitting Lasers XV, edited by James K. Guenter, Chun Lei, Proc. of SPIE Vol. 7952, 79520O · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.875579
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and it also limits the manufacturability and yield of VCSELs for small aperture VCSELs. This paper presents the results for a novel method of lithographic laser technology that eliminates the need for oxide, and enables improved designs in mode and current confinement.
2. DEVICE FABRICATION The lithographic VCSELs are grown using solid source molecular beam epitaxy and they are defined lithographically9. Figure 1 shows the optical photograph of the VCSEL wafer (on the left) and a single device with a mesa at the center of the p-metal contact pad (on the right). The device array of various sizes used in the fabrication ranges from 3 μm to 20 μm in diameter. The VCSEL consists of 21 n-type AlAs/GaAs quarter-wave bottom mirror pairs with Al0.1Ga0.9As onewavelength cavity spacer and three InGaAs/GaAs quantum wells placed at the center of the cavity and completed with p-type Al0.7Ga0.3As/GaAs mirror stack of 20 pairs. Both mode and current confinement are achieved using a phaseshifting epitaxial layer within the VCSEL cavity. P-metal
12 μm – 20 μm
10 μm – 3 μm
Figure 1. The optical photograph of the VCSEL wafer after its completed growth (on the left) and higher magnification of a 10 µm VCSEL following p-metallization (on the right).
3. EXPERIMENTAL RESULTS The VCSELs are operated under continuous-wave operation in an epi-up configuration without wafer thinning, bonding, or other heat sinking methods. They are merely probed on an electrical probe station, with power dissipated through the semiconductor substrate and into the metal probe stage. Devices of a range of sizes throughout the fabricated wafer are tested and they show good uniformity in terms of light output-current and current-voltage characteristics. Figure 2 shows the light output versus current characteristics and the lasing spectrum at 1 mA for 3 μm diameter VCSEL. The device has low threshold current of 260 μA. The slope efficiency of the device is 0.76 W/A corresponding to 59 % differential quantum efficiency, which is comparable to the best results achieved by well-developed oxideconfined VCSELs5. The maximum output power is reached at injection current level of 9 mA by producing 4.1 mW of laser output. The low threshold and linear trend of output power with injected current indicate stable operation of the laser. The VCSEL characteristics are quite uniform. The good uniformity of the VCSELs is due to the lithographic control process, while the high drive levels and output power shown in figure 2 are due to the elimination of strain causing oxide and improvements in the VCSELs’ thermal resistance. Figure 3 shows the measured side-mode-suppression-ratio (SMSR) for a 3 μm VCSEL at different output power levels. The optical beam remains polarization locked in its fundamental mode for all power levels and SMSR remains higher than 25 dB for power levels of up to above 1 mW. The SMSR goes lower for higher power levels since output of orthogonal polarization has an increasing fractional power with increasing current. Various devices of the small size VCSELs have been tested and they all show similar SMSR characteristics concluding high uniformity of the VCSELs due to high controllability of the lithographic process.
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Figure 2. Output power versus current characteristics of 3 μm diameter VCSEL device with threshold current of 260 μA producing upto 4.1 mW laser output.
Figure 3. Measured side-mode suppression ratio (SMSR) of the 3 µm VCSEL showing single-mode singlepolarization emission. The SMSR remains greater than 25 dB for power levels slightly above 1 mW.
A study of the impact of the change from the oxide-aperture to the oxide-free lithographic confinement on thermal resistance has been performed. These results are shown in figure 4. The lithographic VCSELs are tested without any wafer thinning, bonding or heat sinking. This testing shown in figure 4 demonstrates that thermal impedance is already at record level for VCSELs of these sizes, when compared against the best values reported in the literature for oxide confinement. In the oxide-aperture VCSEL, the heat is essentially blocked by the oxide so that the influence of the upper semiconductor mirror is greatly diminished. The reduction in thermal resistance over oxide is due to better heat spreading that can occur with the oxide-free III-V structure through the upper epitaxial mirror of the lithographic VCSEL. Lower thermal resistance leads to less heat accumulation and fluctuations around the active region and provide more stable operation with longer device lifetime for these devices. This improved thermal impedance is expected to be a significant advantage for the single mode VCSELs used as sensors and for high speed VCSELs of active optical cables.
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Heat spreading is the major means by which the VCSEL cools under the very high operating power densities. Figure 5 demonstrates the power density versus current density curves showing that the low thermal resistance enables increased power density and current density as the VCSEL size is reduced. This is due to more effective 3-dimensional heat flow for the smaller sizes, combined with mode and current confinement that enables efficient operation for small VCSEL size. It shows that the output power density is the highest for the smallest device size of 3 μm diameter. The output power density for 3 μm device is two times more than what is achievable by 8 μm diameter VCSEL. The increase of the output density with smaller sizes also indicates that the overlap between the gain and optical mode is better for smaller sizes among the device sizes fabricated in this study. Since modulation speed tracks stimulated emission rate, which in turn tracks power density, these high power density VCSELs are expected to be capable of high modulation speed. High output power density of individual devices is also very promising to get high efficiency high power VCSEL arrays and closely packed arrays are possible when combined with better heat spreading characteristic of these devices.
Oxide VCSEL
Lithographic VCSEL
Figure 4. Thermal resistance versus device diameter for the lithographic VCSEL and comparison to the best results from the literature for oxide VCSELs.
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Figure 5. Power density vs. current density curves showing that the low thermal resistance enables increased power density and current density as the VCSEL size is reduced.
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The reliability performance of VCSELs is also very critical for long operation lifetime of these devices. There have been reliability studies of oxide-VCSELs10,11 concerning the reliability of these devices under various conditions and environments. It was found that high humidity and high temperature operating conditions cause device failure modes due to built-in stress of the oxide. Size dependence of the VCSEL reliability has been studied in detail as well12 because of its importance to get low power operation and high speed modulation of VCSELs. The reliability of oxide-aperture VCSELs has shown to be very dependent on device size with larger devices being better. Even though there are various definitions of reliability and different ways to measure it, more reliable device means a device of longer lifetime. The lithographic VCSELs’ oxide-free structure is expected to outperform oxide-VCSELs in terms of the reliability due to its strain-free, low thermal resistance nature without point defects. Since the reliability gets worse for smaller devices, we tested the smallest lithographic VCSEL of 3 μm diameter fabricated in this study. Lithographic 3 μm VCSEL is driven to put out 3.7 mW of output power at injection current level of higher than 20 times the threshold (6.0 mA), which corresponds to very high injection current density of 85 kA/cm2. Figure 6 shows the reliability result of the device showing that its output power is constant for the tested operation time of around 200 hours. 3 μm lithographic VCSEL does not show any sign of degradation after 200 hours operation. This is already quite different than for oxide VCSELs, for which the smallest oxide VCSELs are quite sensitive to operating current density due to localized heating and thermally induced strain caused by the thermal mismatch of the oxide with the surrounding semiconductor material. Thus our initial testing indicates that lithographic VCSELs will produce devices that are more robust over thermal excursions and more robust under high operating current density than the oxide VCSELs. This may be expected due to the material problems of oxide VCSELs.
Figure 6. Output power versus time for 3 µm diameter VCSEL device showing the constant output power level even after 200 hours of operation on a probe stage.
4. SUMMARY We have demonstrated the scaling properties of lithographic GaAs-based VCSEL that can extend the VCSEL technology to high reliability and reproducibility. This is an important step toward producing lasers of ultra-small sizes with grating-free polarization-locked single-mode VCSELs. The device characteristic of the lithographic laser showed high output power of 4.1 mW from 3 μm device and the lowest thermal resistance for a VCSEL, which are critical for VCSEL applications. We show that eliminating the oxide aperture reduces the thermal resistance with increased power density found for VCSEL size of 3 μm used in this study. In addition to scaling and uniformity advantage of lithographic VCSELs compared to oxide-VCSELs, oxide-free lithographic VCSELs exhibited more reliable performance. The initial reliability tests showed that 3 μm lithographic VCSELs do not show any power degradation in long operation times in the lab while oxide-VCSELs show decrease in output power even in a shorter time. In summary, grating-free single-
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mode single-polarization lasing and more reliable performance of these lasers show that lithographic VCSELs can be the VCSELs of next generation to be used in sensing, high power arrays and possibly high speed optical communication.
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