Implant Confined 1850nm VCSELs Matthew M. Dummer *, Klein Johnson, Mary Hibbs-Brenner, William K. Hogan Vixar, 2950 Xenium Ln. N. Plymouth MN 55441 ABSTRACT Vixar has recently developed VCSELs at 1850nm, a wavelength of interest for neural stimulation applications. This paper discusses the design and fabrication of these new long-wavelength lasers, and reports on the most recent performance results. The VCSELs are based on InP-compatible materials and incorporate highly strained InGaAs quantum wells to achieve 1850nm emission. Current confinement in the VCSEL is achieved by ion implantation, resulting in a planar fabrication process with a single epitaxial growth step. Continuous wave lasing is demonstrated for aperture sizes varying from 8 to 50µm with threshold currents of 1-17mA. The devices demonstrate peak power of 7mW at room temperature and CW operation up to 85 C. Keywords: VCSEL, vertical cavity, long wavelength, ion implantation, neural stimulation 1. INTRODUCTION Long wavelength vertical cavity surface emitting lasers (VCSELs) are becoming increasingly desirable for many applications. For example, low-loss fiber optic communication (1550nm) [1] and trace gas detection (1800-2500nm) [2] are two areas where the unique characteristics of VCSELs (high speed, narrow linewidth, temperature stability, etc.) are especially advantageous. Another application which has recently gained attention is infrared neural stimulation (INS). INS is a new field of research that uses low power infrared light to excite neural tissue [3]. INS has several advantages over traditional electrical stimulation methods including improved spatial specificity, a larger difference between the stimulation and damage thresholds, and the lack of a stimulation artifact following neural response [3-4]. Wavelengths of 1840-1880nm have been the primary focus for INS because the steep increase in the absorption coefficient of tissue in this range allows the optical penetration depth to be specifically tailored for many neurologic applications [5]. Thus far the majority of INS research has been conducted in laboratory environments using bench-top lasers coupled to optical fibers to deliver stimulation signals. However, the development of an implantable compact optical source is a critical step toward making INS a clinically useful technology. VCSELs have been proposed as an attractive solution due to their small size, simple packaging geometry, and potential for high efficiency at low power [6]. In this work we present the design and characterization of a new VCSEL device with a target wavelength of 1850 nm. We believe this device shows significant potential for future INS applications. 2. LONG WAVELENGTH VCSEL CHALLENGES The 1850nm VCSELs in this work are based on indium phosphide, a material system that is necessary to achieve long wavelength emission but has historically proven challenging for VCSEL fabrication [7-8]. One of the fundamental hurdles is the very thick epitaxy that must be grown, since the thicknesses of the layers scale proportionately with wavelength. Also, because of the relatively low index contrast available in these materials, achieving high reflectivity distributed Bragg reflectors (DBRs) requires more periods further adding to the total thickness. In most cases, the mirrors must be comprised at least partially of quaternary materials, which leads to inherently poor thermal conductivity. * mdummer@vixarinc.com, (763) 746-8045, www.vixarinc.com
It should also be noted that there are no common materials lattice-matched to InP that can be used to achieve emission at 1850nm. Therefore compressively strained InGaAs layers must be incorporated in the active region to produce sufficiently low-bandgap quantum wells. Achieving good electron confinement in the active region is difficult due to the low conduction band offset. Non-radiative recombination is also problematic, since Auger processes become dominant as the quantum well bandgap decreases. Therefore low gain and poor temperature performance are critical issues. Another complication for long wavelength VCSELs is that unlike AlGaAs devices, there is no straightforward process to form oxide apertures. Instead, other methods for current confinement must be implemented. To date the most common approaches have been selective lateral etching for undercut apertures [9], or regrowing over lithographically defined buried tunnel junctions [10]. Both of these methods complicate fabrication in comparison to standard oxidation processes. To simplify the fabrication in this work, we have developed instead an ion implantation method for current confinement. Our approach to an implant confined longwavelength VCSEL is described below. Figure 1. Cross sectional drawing of an implant-confined long wavelength VCSEL 3. DEVICE DESIGN The structure of the 1850nm VCSEL is shown in Fig 1. The device uses a hybrid mirror design to achieve very high reflectivity while minimizing the total semiconductor thickness. The bottom DBR is all-epitaxial, while the top mirror consists of a partial semiconductor DBR followed by a deposited dielectric stack. The combination of the two reflective mediums produces a total reflectivity greater than 99.9% for the upper mirror. With this mirror scheme, a bottom emitting geometry is preferred since the lower DBR is more transmissive. The resonant cavity between the two mirrors contains a strained-ingaas MQW as the active region. An intracavity tunnel junction is also included above the active region, allowing both DBRs to be doped n-type for lower loss and higher conductivity. Current confinement in the device is achieved by a two step ion implantation process. The first implant creates a narrow gain-guide aperture placed at the tunnel junction interface. A second larger diameter implant step renders the upper DBR layers non-conductive to provide isolation between separate devices on the wafer. Using this approach, current can be applied through intra-mirror contacts on the top surface allowing current to spread laterally into the desired active region. The ion implantation method is advantageous since it results in a planar fabrication process and requires only a single epitaxial growth step. 4. ION IMPLANTATION FOR CURRENT CONFINEMENT As shown above, the VCSEL relies on ion implantation to control the flow of current through the device. This is primarily accomplished by suppressing carrier transport across the tunnel junction with a proton implant. The TJ consists of a delta-doped n+/p+ interface where the overlap of the conduction and valence bands in adjacent
materials allows carriers to easily tunnel between the bands. Introducing a high concentration of protons to the TJ region pins the Fermi level mid bandgap to effectively create a barrier that impedes tunneling. Test structures with varying diameter of gain guide implant have been measured to characterize the effectiveness of this approach. Figure 2a compares I-V curves measured for circular apertures from 0 to 50µm diameter. For the cases of 5µm- 50µm the diodes exhibit a normal turn-on voltage of 0.7 V, and the series resistance scales inversely proportional to active area. However, when the aperture is completely implanted (diameter=0µm), the diode turn-on characteristic is no longer apparent and current flow is significantly reduced. These results verify that ion implantation can successfully be used to produce a current aperture in the VCSEL. As shown in Fig. 2a, a finite amount of current is still present after the TJ has been implanted. However, for normal operating biases less than 2V, the proportion of leakage current outside the desired aperture is negligible. (a) (b) Figure 2. (a) Comparison of IV characteristics for VCSEL test structures with varying implant aperture diameter. (b) Measured series resistance for dot diode test structures compared to VCSELs with ring contacts. Measurements of lateral current spreading have also been conducted to determine the impact of the buried implant on the upper semiconductor layers conductivity. Fig. 2b compares the resistance of the dot diode structures used in the previous measurement with VCSELs of various diameters on the same wafer. Current is applied in the VCSELs using a ring contact at the edge of the aperture. Therefore the VCSEL measurement includes both the lateral and vertical resistance in the structure, whereas the dot diode resistance is only in the vertical direction. As shown, the resistance of both contact geometries is comparable for all aperture sizes, with the VCSEL (ring contact) resistance typically being 2-3Ω higher. This difference can be attributed to the lateral current spreading resistance in the top DBR. Again the difference here is very small and does not significantly impact the VCSEL performance.
Figure 3. Continuous wave room temperature LIV characteristics for VCSELs of varying aperture diameter 5. VCSEL RESULTS Figure 3 shows the light versus current and voltage (LIV) results for VCSELs measured continuous wave at room temperature. Lasing is clearly observed for aperture sizes varying between 8 and 50µm. Typical threshold current densities are 1-2kA/cm 2 and the lowest lasing threshold current occurs at 1.1 ma for the smallest device. Larger devices exhibit increased threshold but also achieve higher output power. The maximum CW power measured is 7.2 mw for a 50µm diameter device. To our knowledge this is the highest CW power reported for an 1850nm VCSEL. The power is limited by self-heating which reduces the VCSEL efficiency as the current density increases, resulting eventually in thermal rollover. Voltage characteristics are similar to those of the test structures measured in Fig 2., showing a typical turn on of 0.7 V and series resistance that varies between 10-50Ω depending on the device size. The useful range of operation typically occurs with biases of 1.0V to 1.5V. Figure 4. (a) Threshold current, (b) maximum CW output power, (c) peak differential quantum efficiency and (b) peak wall-plug efficiency for VCSELs with varying aperture diameter.
Figure 4 compares basic laser parameters for VCSELs of various sizes. As shown in figure 4a, threshold current varies from 1mA to as high as 17mA for the largest aperture sizes. The corresponding increase in output power with size is shown in Fig. 4b. Differential quantum efficiency has also been measured, (Fig. 4c) and falls within the range of 15-25%. This translates to a slope efficiency of 0.10-0.17 mw/ma. There is a clear size dependence to the differential efficiency, with the smaller aperture devices exhibiting lower slope after reaching threshold. This is most likely caused by self-heating, since the thermal impedance increases for smaller active area devices. The total CW power conversion (wall-plug) efficiency has been calculated in Fig 4d. The trade-off between high resistance for small devices and high threshold in larger devices results in an optimal device diameter in terms of efficiency. In this case maximum wall-plug efficiency is 9.5% CW measured in a 20µm aperture VCSEL. (a) (b) Fig. 5 (a) LI measurement for a 12µm VCSEL at various operating temperatures. (b) Comparison of maximum output power vs. temperature for different VCSEL sizes. The performance of the VCSELs over temperature has also been characterized. Fig 5a shows the LI characteristic for a 12µm aperture VCSEL at various temperatures. Like most VCSELs, the device exhibits an increase in threshold, and reduced slope efficiency as the temperature is increased. The CW lasing threshold can be observed for temperatures as high as 85 C. Figure 5b compares the temperature performance for other device sizes. For all devices maximum output power is highly dependent on the external temperature. Also, the maximum lasing temperature decreases as the diameter increases. For the 50µm VCSEL continuous wave operation can only be achieved up to 60 C. Figure 6. Comparison of VCSEL spectrum for 15µm and 35µm aperture.
Output spectra have been measured to verify lasing at 1850nm. Figure 6 compares the output spectrum of a 15µm and 35µm design biased at 2.5I th. The smaller device has a single spectral mode with a linewidth less than 0.1nm, limited by the resolution of the spectrum analyzer. The larger device is clearly multimode with a spectral width less than 2nm. Both devices exhibit a peak wavelength in the target 1850nm range. 6. CONCLUSION We have proposed and demonstrated a new long wavelength VCSEL operating at 1850nm. The device utilizes a hybrid mirror design and ion-implanted current confinement to achieve a simple fabrication process with a single epitaxial growth. Electrical characterization confirms that the implantation method is effective for confining current in the VCSEL, and that low vertical and lateral resistance can be achieved. Measurements at room temperature demonstrate continuous wave lasing for a wide range of VCSEL sizes, with threshold currents as low as 1mA. The maximum output power at 22C was 7.2mW, and CW operation has been observed as high as 85 C. The peak wall plug efficiency was 9.5%. These devices show promise as an optical source for infrared neurostimulation, and continued improvements on output power and efficiency will help to widen the scope of future INS applications. ACKNOWLEDGMENTS Vixar would like to acknowledge Lockheed Martin Aculight for collaboration and financial support during this project. REFERENCES [1] M. Ortsiefer, R. Shau, F. Mederer, R. Michalzik, J. Rosskopf, G. Bohm, F. Kohler, C. Lauer, M. Maute, and M.C. Amann, High-speed modulation up to 10 Gbit/s with 1.55 µm wavelength InGaAlAs VCSELs, Electronics Letters, vol.38 (20) pp. 1180-1181, 2002 [2] G. Boehm, M. Ortsiefer, R. Shau, J. Rosskopf, C. Lauer, M. Maute, F. Kohler, F. Mederer, R. Meyer, M.C. Amann, InP-based VCSEL technology covering the wavelength range from 1.3 to 2.0 µm, Journal of crystal growth, vol. 251, pp. 748-753, 2003 [3] J. Wells, C. Kao, K. Mariappan, J. Albea, E. Jansen, P. Konrad, and A. Mahadevan-Jansen, Optical stimulation of neural tissue in vivo," Opt. Lett. 30 (5), pp. 504-507, 2005. [4] A. Izzo, J. Walsh, E. Jansen, M. Bendett, J. Webb, H. Ralph, and C. Richter, Optical parameter variability in laser nerve stimulation: a study of pulse duration, repetition rate, and wavelength," IEEE Transactions on Biomedical Engineering, 54(6), pp. 1108{1114, 2007. [5] J.Walsh and J. Cummings, Effect of the dynamic optical properties of water on midinfrared laser ablation," Lasers in surgery and medicine 15(3), pp. 295-305, 1994. [6] M. Hibbs-Brenner, K. Johnson, and M. Bendett, VCSEL technology for medical diagnostics and therapeutics," in Proc. SPIE, 7180, pp. 71800-71810, 2009. [7] J.S. Harris, GaInNAs long-wavelength lasers: progress and challenges Semiconductor science and technology, 17, p. 880, 2002 [8] C.J Chang-Hasnain, Progress and prospects of long-wavelength VCSELs IEEE. Communications Magazine, 41(2), pp. S30-S34, 2003 [9] D. Feezell, D.A. Buell, L.A. Coldren, InP-based 1.3-1.6µm VCSELs with selectively etched tunnel-junction apertures on a wavelength flexible platform, IEEE Photonics Technology Letters, 17(10), pp. 2017-2019, 2005 [10] M.C. Amann, W. Hofmann, "InP-Based Long-Wavelength VCSELs and VCSEL Arrays," IEEE Journal of Selected Topics in Quantum Electronics, 15(3), pp. 861-868, 2009