JOURNAL OF APPLIED PHYSICS
VOLUME 88, NUMBER 5
1 SEPTEMBER 2000
Effect of heterobarrier leakage on the performance of high-power 1.5 m InGaAsP multiple-quantum-well lasers L. Shterengas Department of Electrical Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794
R. Menna Sarnoff Corporation, 201 Washington Road, Princeton, New Jersey 08543
W. Trussell Night Vision and Electronic Sensors Directorate, 10215 Burbeck Road, Fort Belvoir, Virginia 22060-5806
D. Donetsky and G. Belenkya) Department of Electrical Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794
J. Connolly and D. Garbuzov Sarnoff Corporation, 201 Washington Road, Princeton, New Jersey 08543
共Received 21 April 2000; accepted for publication 20 June 2000兲 Broad stripe 1.5 m InGaAsP/InP multiple-quantum-well graded-index separate-confinement heterostructure lasers with different waveguide widths and doping profiles were designed, fabricated, and characterized. A record value of more than 16 W of pulsed optical power was obtained from lasers with a broadened waveguide design. Studies of the characteristics of lasers with different p-doping profiles as well as modeling data show that the heterobarrier electron leakage is responsible for the effect of the device optical power saturation with current. © 2000 American Institute of Physics. 关S0021-8979共00兲01319-0兴
I. INTRODUCTION
showed that the heterobarrier leakage is responsible for power saturation in 1.5 m InGaAsP/InP lasers. Lasers with a higher Zn concentration at the SCH/p-cladding interface exhibited increased output power. We attribute this improvement to the effect of the heterobarrier leakage suppression as a result of higher SCH/p-cladding interface doping. Our simulation results support this conclusion.
Eye-safe semiconductor lasers are in demand for development of laser range finder and countermeasure systems. These applications require a high level of pulsed optical power. The optical power of semiconductor lasers is limited by the catastrophic optical damage 共COD兲 of mirrors1,2 and light–current (L – I) characteristic rollover.3–7 For highpower 1.5 m InP-based lasers, the COD threshold is high1 and the L – I rollover limits the maximum optical power level. Different mechanisms can be considered to explain the effect of power saturation in these devices: device overheating, carrier leakage, gain saturation, and spatial hole burning or filamentation. In this work, we carried out a comprehensive study of the nature of power saturation in 1.5 m high-power InGaAsP/InP multiple-quantum-well 共MQW兲 two-step graded separate-confinement heterostructure 共SCH兲 lasers. We measured the light–current dependence at different temperatures and duty cycles, voltage–current characteristics, near- and far-field emission patterns, laser spontaneous emission, optical gain spectra, and optical losses. We studied devices with different waveguide width and doping profiles. Broadened waveguide 共BW兲 devices (W ⫽710 nm) with 200 m stripe width and 1 mm cavity length gave more than 16 W of pulsed optical power at a current of about 60 A. Study of the lasers with a narrow waveguide 共NW兲 of 260 nm and with two different doping profiles
II. DEVICE STRUCTURE AND EXPERIMENT
Broad-area 共100 and 200 m stripe width兲 1.5 m InGaAsP/InP MQW BW and NW lasers were studied. Two different p-cladding doping profiles were used for NW lasers with W⫽260 nm. All layers except the quantum wells were grown lattice matched to the InP substrate. Details of the structure 共Fig. 1兲 can be found elsewhere.3 Front and back mirrors were coated to low 共3%–5%兲 and high reflectivity 共95%兲, respectively. Figure 2共a兲 shows the secondary ion mass spectroscopy 共SIMS兲 profiles of the two types of NW lasers studied. The As signal indicates the location of the active region. The structure with a SCH/p-cladding Zn concentration of 2⫻1017 cm⫺3 will be referred to as low doped, the one with 7⫻1017 cm⫺3 as doped. BW and low-doped NW devices have similar p-cladding doping profiles. Lasers with BW, 200 m stripe width and 1 mm cavity length exhibit superior performance having high slope efficiency of 0.54 W/A in the linear part of the L – I and more than 16 W of pulsed optical power at 60 A 关Fig. 2共b兲兴. The high slope efficiency of the BW structures is a result of the low value of the internal optical loss.3 We utilized a spatial
a兲
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FIG. 1. Schematic energy-band diagram of the laser structure.
filtering technique8 to register modal gain of the on-axis mode in multimode broad stripe lasers. Modal gain spectra for BW 200 m stripe width and 500 m cavity length lasers were measured at different injection currents. Figure 3 shows that for photon energies less than 0.81 eV the material gain is FIG. 3. Current dependence of the modal gain spectrum of a BW laser with 500 m cavity length and 200 m stripe width at 20 °C.
equal to zero. The absolute value of the modal gain is equal to a total optical loss of 40 cm⫺1. The mirror loss for a coated device with 500 m cavity length is 36 cm⫺1. Thus, the internal optical loss is about 4 cm⫺1. NW structures had an internal optical loss of about 24 cm⫺1 and lower slope efficiency of 0.44 W/A in the linear part of the L – I for both low-doped and doped 500 m cavity length devices. At the high level of injection the L – I rollover was observed 关Fig. 2共b兲兴. The power saturation effect was less pronounced for doped NW lasers. An optical output power of 14 W for the doped NW lasers with 100 m stripe width and 500 m cavity length was registered at 60 A compared to 9 W for the low-doped devices 关Fig. 2共b兲兴. In order to demonstrate the effect of high current density on device external efficiency, we chose the NW devices with short cavities 共500 m兲 and 100 m stripe width, where the L – I rollover is more pronounced. The L – I were measured using short pulses 共50 ns兲
FIG. 2. 共a兲 SIMS doping profiles for NW low-doped and doped structures. 共b兲 L – I characteristics of BW lasers with 1 mm cavity length and 200 m stripe width 共circles兲, low-doped 共down triangles兲, and doped 共up triangles兲 NW lasers with 500 m cavity length and 100 m stripe width.
FIG. 4. I – V characteristics of BW lasers with 1 mm cavity length and 200 m stripe width 共circles兲, low-doped 共down triangles兲, and doped 共up triangles兲 NW lasers with 500 m cavity length and 100 m stripe width.
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FIG. 5. L – I characteristics of low-doped 共a兲 and doped 共b兲 NW devices registered at different current pulse widths. FIG. 7. Dependences of the far-field emission pattern width on output optical power.
and repetition rates of 10 kHz to minimize the effect of heating, a significant factor at high injection levels 共60 A corresponds to a current density of about 120 kA/cm2兲. Measured current–voltage (I – V) characteristics 共Fig. 4兲 allow one to estimate the dissipated power P diss⫽IV⫺ P, where P is optical power, about 100 W for high currents in the cw regime. Such thermal power leads to device overheating and the dependence of the L – I on pulse width, especially for short cavity 共500 m兲 devices, and strong thermal L – I rollover 共Fig. 5兲. In order to determine the role of optical effects on power saturation, detailed measurements of the near- and far-field emission patterns were carried out. No filamentation was observed in the near-field emission patterns 共Fig. 6, inset兲. The transverse far-field emission pattern shows single-mode operation and no current dependence. The difference in the rates of the lateral far-field emission pattern broadening with current reflects the difference in the output optical power of low-doped and doped devices. The broadening of the farfield emission pattern with power shown in Fig. 7 can be explained by the effect of spatial hole burning. This broad-
We used the PADRE modeling procedure9 to simulate the performance of NW devices with two doping profiles. Fragments of band diagrams for NW lasers with two different doping profiles are presented in Fig. 9, showing the effect of heterobarrier suppression by injection.10 Figure 9 indicates
FIG. 6. Current dependence of the lateral far-field emission pattern of lowdoped 共a兲 and doped 共b兲 NW devices at 25 °C. Insets show the corresponding near-field emission patterns at 50 A.
FIG. 8. Spontaneous emission intensity from the SCH layer registered at different pumping currents for low-doped 共a兲 and doped 共b兲 NW devices at 25 °C.
ening cannot be responsible for the difference in the rates of the L – I saturation of the low-doped and doped NW devices. Studies of the spontaneous emission intensity from the laser waveguide show that an increase of the injection current leads to a higher carrier concentration in the SCH layer. Figure 8 indicates that the spontaneous emission intensity from the 1.13 eV InGaAsP layer 共see Fig. 1兲 is five times higher in the doped NW sample compared the low-doped one. A redshift in the emission peak, which was observed when using 300-ns-wide current pulses, clearly indicates device overheating. This effect disappears for a 50 ns pulse. III. MODELING AND DISCUSSION
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L – I and the simulation results 共Fig. 10兲. The modeling explains qualitatively all experimental observations. The quantitive discrepancy can be understood taking into account two factors: device overheating even for short pulse operation at high current densities and the increase of the internal loss with current above threshold. Such loss increase is caused by carrier accumulation in the SCH region and corresponding enhancement of intervalence-band absorption.12 We suggest that the increase of the doping level of SCH/p-cladding interface in the case of BW will maximize the output optical power. The doping level should change abruptly on the interface to eliminate unwanted waveguide doping leading to optical loss enhancement and device efficiency degradation. This is the subject of further studies. FIG. 9. Simulated conduction-band profile 共solid lines兲 for 1 A 共threshold兲 and 60 A currents for low-doped 共a兲 and doped 共b兲 NW devices at 25 °C. Dashed lines: electron quasi-Fermi levels at 60 A.
the barrier for thermionic emission of electrons from the SCH layer into the p-cladding layer is 30% higher for lasers with the higher-doped interface. The higher-energy barrier limits the electron leakage into the p-ladding layer, increasing the carrier accumulation in the SCH layer.11 Calculations show that under high injection the electron gas in the SCH layer is degenerate, implying that considerable radiative recombination within the SCH layer can be expected. The positions of the quasi-Fermi levels with respect to the conduction-band edges under high injection are consistent with higher spontaneous emission intensity from the SCH layer of the doped structure. We calculated the internal efficiency normalized to its threshold value for NW devices from both the experimental
IV. CONCLUSION
We carried out a comprehensive characterization of high-power broad-area 1.5 m InGaAsP/InP MQW lasers with two different waveguide widths and different p-doping profiles. 16 W of pulsed optical power was registered for broadened waveguide devices. Narrow waveguide lasers with a doped p-cladding/SCH interface achieve output powers of 14 W at 60 A of pulsed current compared to 9 W for low-doped lasers. The experimental data and modeling suggest that doping of the SCH/p-cladding interface suppresses the power saturation effect by reducing electron heterobarrier leakage. We concluded that heterobarrier carrier leakage is responsible for limitation of the device output power in 1.5 m high-power InGaAsP/InP lasers. ACKNOWLEDGMENTS
The authors acknowledge M. Hybertsen and R. Martinelli for useful discussions, and ARO Grant No. DAAD 1999910122 for support. O. Ueda, Microelectron. Reliab. 39, 1839 共1999兲. K. H. Park, J. K. Lee, D. H. Jang, H. S. Cho, C. S. Park, K. E. Pyun, J. Y. Jeong, S. Nahm, and J. Jeong, Appl. Phys. Lett. 73, 2567 共1998兲. 3 D. Garbuzov, L. Xu, S. R. Forest, R. Menna, R. Martinelli, and J. C. Connolly, Electron. Lett. 32, 1717 共1996兲. 4 D. Z. Garbuzov, R. U. Martinelli, H. Lee, P. K. York, R. J. Menna, J. C. Connolly, and S. Y. Narayan, Appl. Phys. Lett. 69, 2006 共1996兲. 5 R. U. Martinelli, R. J. Menna, G. H. Olsen, and J. S. Vermaak, IEEE Photonics Technol. Lett. 6, 1415 共1994兲. 6 T. Makino, J. D. Evans, and G. Mak, Appl. Phys. Lett. 71, 2871 共1997兲. 7 I. K. Han, S. H. Cho, J. S. Heim, D. H. Woo, S. H. Kim, J. H. Song, F. G. Johnson, and M. Dagenais, IEEE Photonics Technol. Lett. 12, 251 共2000兲. 8 D. V. Donetsky, G. L. Belenky, D. Z. Garbuzov, H. Lee, R. U. Martinelli, G. Taylor, S. Luryi, and J. C. Connolly, Electron. Lett. 35, 298 共1999兲. 9 R. F. Kazarinov and H. R. Pinto, IEEE J. Quantum Electron. 30, 49 共1994兲. 10 G. L. Belenky, C. L. Reynolds, Jr., R. F. Kazarinov, S. Swamatanian, S. Luryi, and J. Lopata, IEEE J. Quantum Electron. 32, 1450 共1996兲. 11 G. L. Belenky, C. L. Reynolds, D. V. Donetsky, G. E. Shtengel, M. S. Hybertsen, M. A. Alam, G. A. Baraff, R. K. Smith, R. F. Kazarinov, J. Winn, and L. E. Smith, IEEE J. Quantum Electron. 35, 1515 共1999兲. 12 J. Taylor and V. Tolstikhin, J. Appl. Phys. 87, 1054 共2000兲. 1 2
FIG. 10. Simulated 共solid lines兲 and experimental 共dots兲 values of normalized internal efficiency for low-doped and doped NW devices at 25 °C.
