Abstract—The 1.27- m InGaAs:Sb-GaAs-GaAsP vertical cavity surface emitting lasers (VCSELs) were grown by met-alorganic chemical vapor deposition and exhibited excellent performance and temperature stability. The threshold current varies from 1.8 to 1.1 mA and the slope efficiency falls less than 35% from 0.17 to 0.11 mW/mA as the temperature is raised from room temperature to 75 C. The VCSELs continuously operate up to 105 C with a slope efficiency of 0.023 mW/mA. With a bias current of only 5 mA, the 3-dB modulation frequency response was measured to be 8.36 GHz, which is appropriate for 10-Gb/s operation. The maximal bandwidth is estimated to be 10.7 GHz with modulation current efficiency factor of 5.25 GHz/(mA)1 2. These VCSELs also demonstrate high-speed modulation up to 10 Gb/s from 25 C to 70 C. We also accumulated life test data up to 1000 h at 70 C/10 mA.
Index Terms—Characterization, InGaAsSb, laser diodes, met-alorganic chemical vapor deposition (MOCVD), optical fiber de-vices, semiconducting.
I. INTRODUCTION
L
ONG-WAVELENGTH vertical cavity surface-emit-ting lasers (VCSELs) are key devices in optical fiber metropolitan-area networks (MANs) [1]. The most promising results regarding low-cost long-wavelength lasers or VCSELs, have been obtained using GaInAsN quantum wells (QWs) grown on GaAs substrates [1]–[7]. The large conduction band offset improves the temperature performance over that of conventional InP-based materials. The GaAs system provides high-performance AlGaAs/GaAs DBR mirrors and permits the use of the well-established oxide-confined GaAs-based VCSEL manufacturing infrastructure. However, GaInAsN is a very challenging material system from a growth perspective: 1) incorporating N into the InGaAs QW is difficult and increases the nonradiative (monomolecular and Auger) recombination, reducing the material gain and increasing the transparency carrier density [8]; 2) during post-growth annealing, the ni-trogen diffuses out from the quantum well and blue-shifts theManuscript received July 15, 2004; revised October 26, 2004. This work was supported in part by the National Science Council of the Republic of China in Taiwan under Contract NSC 90-2215-E-009-102, in part by the Aca-demic Excellence Program of the ROC Ministry of Education under Contract 88-FA06-AB, and in part by .the Institute of Nuclear Energy under Contract 922001InER015.
The authors are with the Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: [email protected]).
Digital Object Identifier 10.1109/JSTQE.2004.841696
optical emission. Recently, Koyama et al. [9] and Tansu et al. [10] employed highly strained InGaAs QW active lasers to extend the emission wavelength to 1.2 m. Highly strained InGaAs VCSELs with a photoluminescence (PL) peak at 1.205 m and a laser emission wavelength of 1.26–1.27 m have demonstrated a promising performance and continuous-wave (CW) operation at up to 120 C as well as 10 Gb/s operation [11]. However, the emission wavelength of 1.26 m barely meets optical communication standards, such as IEEE 820.3ae 10 Gb/s Ethernet. Furthermore, this laser performs relatively poor at room temperature due to the large negative gain-cavity offset. Antimony (Sb) present during GaInAsN growth has been believed to act as a surfactant and improve PL perfor-mance [12]. The authors have observed that adding Sb into samples with high In content sharply increases the intensity and the alloy thus formed exhibits significant red-shift in optical emission [12]. Additionally, adding a surfactant such as Sb [13] or Te [14] significantly increases the critical layer thickness of InGaAs on GaAs. This work presents high-performance In-GaAs:Sb-GaAs-GaAsP QWs VCSELs grown by metal organic chemical vapor deposition. These VCSELs exhibit excellent performance and temperature stability. The threshold current changes from 1.8 to 1.1 mA and the slope efficiency falls less than 35% from 0.17 to 0.11 mW/mA as the temperature is raised from room temperature to 75 C. The VCSELs contin-uously operate up to 105 C with a slope efficiency of 0.023 mW/mA. With a bias current of only 5 mA, the 3-dB modu-lation frequency response is measured to be 8.36 GHz, which is suitable for 10 Gb/s operation. The maximal bandwidth is
10.7 GHz with MCEF of 5.25 GHz mA . II. STRUCTURE ANDPHYSICALMODEL
All structures were grown on semi-insulating GaAs (100) substrates by low pressure metalorganic chemical vapor de-position. The group-V precursors were the hydride sources AsH and PH . The trimethyl alkyls of gallium (Ga), alu-minum (Al), indium (In), and Antimony (Sb) were the group-III precursors. The epitaxial structure was as fol-lows (from bottom to top)— GaAs buffer, 40.5-pair Al Ga As GaAs (Si-doped) distributed Bragg reflector (DBR), undoped active region, Al Ga As oxidation layer, 25-pair Al Ga As/p GaAs DBR (carbon-doped) and GaAs (carbon-doped) contact layer. The graded-index separate confinement heterostructure
Fig. 1. Numerical calculation of the critical thickness for In Ga As0GaAs as a function of indium composition x in the range 0.25 < 2 < 0.45. Full symbols represent the QWs where there defects were not observed. Open symbols indicate that QWs show faceting or dislocations. Square symbols represent InGaAs grown on GaAs, while circle symbols represent InGaAs grown on GaAs with1.5% Sb incorporation.
(GRINSCH) active region consisted mainly of a double QWs active region In Ga As Sb GaAs GaAs P (60/100/100 ), with PL emission at 1.214 m, embedded between two linear-graded Al Ga As ( to 0.6 and to 0) confinement layers (growth temperature C with AsH TMSb flow ration 50). The cavity was formed in one-wavelength thickness. Carbon was used as the -type dopant in the DBR to increase the carrier concentration 2 3 10 cm . The interfaces of both - and -type Al Ga As/GaAs DBR layers are linearly graded to reduce the series resistance. The growth rate is m/hr for In-GaAs:Sb QW region and 2 m/hr for Al Ga As/GaAs DBR region. The optical properties of QWs were optimized through PL measurement and structural analysis as described as followed. The FP-dip wavelength is at 1270 nm, which was determined by reflection measurement.
To optimize growth parameters, we need to investigate crit-ical layer thickness (CLT) of InGaAs grown on GaAs with the effect of Sb surfactant. In lattice-mismatched systems, the strain of the layers is accommodated elastically in layers thinner than a misfit-dependent critical thickness as described by Matthews and Blakeslee [15]. Fig. 1 shows the numerical calculation of the critical thickness for In Ga As GaAs as a function of indium composition in the range 0.25 . Substi-tuting the critical thickness as a function of Indium composi-tion for the width of the quantum well, the energy states are calculated for the first level in the QW. The maximal achievable wavelength is less than 1100 nm. However, recent publication demonstrated that the critical thickness of InGaAs on GaAs is larger than Matthews and Blakeslee’s predication and the max-imal wavelength can be achieved is 1200 nm [16], [17]. People and Bean have proposed an alternative description for the crit-ical thickness by defining a balance between the strain energy in an epitaxial layer and the formation energy of an isolated screw dislocation [18]. Additionally, the critical thickness of In-GaAs on In-GaAs can be enhanced by additional strain compen-sating layer or surfactant [13]. Secondary Ion mass spectroscopy
Fig. 2. Comparison of the photoluminescence spectra of InGaAs with different In composition and In Ga As with Sb incorporation.
(SIMS) measurement was performed to investigate whether Sb being incorporated into the lattice or just as a surfactant while the TEM measurements were used to determine the formation of faceting or dislocations. This critical thickness is plotted in Fig. 1. Full symbols represent the QWs where the defects were not observed. Open symbols indicate that QWs show faceting or dislocations. Square symbols represent InGaAs grown on GaAs, while circle symbols represent InGaAs grown on GaAs with 1.5% Sb incorporation (measured by SIMS). It is clear that the Sb do incorporate into the crystal and the CLT of InGaAs on GaAs increases with adding of Sb. No dislocations were ob-served for 75 In Ga As Sb/GaAs, 60 In Ga As Sb/GaAs and 50 In Ga As Sb/GaAs QWs. These re-sults are consistent with the observations of [13].
Fig. 2 compares the PL spectra of In Ga As QW with var-ious In contents ( and ), and that of In Ga As QW with incorporated Sb. All samples have a well thickness of 60 with a GaAs spacer and a GaAs strain-com-pensating layer. The PL peak emission wavelengths of the grown In Ga As/GaAs/GaAsP and In Ga As Sb/GaAs/GaAsP QWs are 1.194 and 1.214 m, respectively. A red-shift of 20 nm is observed. For In Ga As Sb, the full-width at half-maximum (FWHM) of the PL emission peak from In Ga As Sb is larger and the PL intensity is slightly lower. The red-shift of InGaAs:Sb can be attributed to the Sb in the alloy constituent. The incorporation of more in-dium or the lowering of the conduction band also explains the red-shift of the optical emission. The FWHM of the PL emission increases significantly and the PL intensity drops dramatically when the indium composition increases to 0.42. These changes can be explained by the dislocations and the formation of nonra-diative defects of the 60 In Ga As/GaAs/GaAsP QW where 60 exceeds the critical layer thickness. From the re-flectivity spectrum of an as-grown VCSEL structure, the cavity mode is centered at 1.266 m and the stop band is over 90 nm which is due to the large index contrast of AlGaAs/GaAs DBRs. The cavity mode and the gain mode offset is determined to be 52 nm.
Fig. 3 schematically depicts the VCSEL structure. A pro-cessing sequence involved six photomasks to fabricate
oxide-confined polyimide-planarized VCSELs with coplanar wave-guide probe pads. This process was designed to minimize ca-pacitance while keeping a reasonably low resistance [19]. De-vice fabrication began with the formation of cylindrical mesas with a diameter of 30 m by etching the surrounding semicon-ductor into the bottom -type mirror to a depth of 5 m, using an ICP-RIE system. The sample was wet-oxidized in a 420 C steam environment for 20 min to form the current aperture and provide lateral index guiding to the lasing mode. The oxidation rate was 0.6 m for the Al Ga As layer, so the oxide extended 12.5 m from the mesa sidewall. Ti/Au was evapo-rated to form the p-type contact ring, and AuGeNiAu was evap-orated onto the etched n-buffer layer to form the n-type contact, which is connected to the semi-insulating substrate. Contacts were alloyed for 30 s at 420 C with rapid thermal annealing (RTA). After the contact formation, the photosensitive poly-imide was spun on the sample to form insulation and planariza-tion. Ti-Au was deposited to a thickness of from 200 to 3000 to form the metal interconnects and coplanar waveguide probe pads. Heat treatment following metal deposition was applied to strengthen the metal-to-polyimide adhesion. Finally, surface re-lief etching with a diameter of 3.5 m was performed to a depth of near half wavelength for single-mode operation [20].
III. RESULTS ANDDISCUSSIONS
The DC characteristics of completed VCSELs were measured with a probe station, an Agilent 4145 A semiconductor param-eter analyzer and an InGaAs photodiode. Fig. 4 plots curves of temperature-dependent light output and voltage versus current (LIV). The maximal output power exceeds 1.2 mW at room tem-perature (0.4 mW at 95 C) and output power rollover occurs as the current increases above 10 mA at 25 C (8.7 mA at 95 C). Furthermore, a large drop in the maximal power was observed in the temperature range between 75 C 95 C. The large drop in maximal power might be resulted from the higher free carrier absorption and higher Auger recombination rate in high temperature. Improvement in output power might be achieved by 1) optimizing the doping profile in the cavity region and 2) further pushing the emission wavelength longer to reduce the cavity-gain detuning.
The threshold current and slope efficiency were plotted as a function of heat sink temperature, as shown in Fig. 5. The large cavity-gain offset resulted in decreasing threshold current den-sities as the temperature was increased. When the gain max-imum and the emission wavelength were aligned, a minmax-imum threshold current of 1.1 mA was achieved at 70 C–75 C.
Fig. 4. Temperature-dependent light output and voltage versus current (LIV) curves.
Fig. 5. Threshold current and slope efficiency as a function of heat sink temperature.
The slope efficiency was 0.17 mW/mA at room temperature and fall less than 35% as the temperature was raised to 75 C. The VCSEL’s continuously operated up to 105 C with a slope efficiency of 0.023 mW/mA. The resistance of the VCSEL is 120 and the capacitance is 0.1 pF. Accordingly, parasitic effects limit the devices to a frequency response of 13 GHz.
Lateral mode characteristic is an important feature for long wavelength VCSELs since it strongly affects the transmission properties, especially the long haul optical communication. The spectra of the InGaAs:Sb-GaAs-GaAsP VCSELs were measured using an Advantest Q8381 A optical spectrum analyzer with a resolution of 0.1 nm. The devices exhibit single transverse mode characteristics at a lasing wavelength of 1.27 m, with a side mode suppression ratio (SMSR) of 30 dB over the whole operating range, as depicted in Fig. 6.
The small signal response of VCSELs as a function of bias current was measured using a calibrated vector network ana-lyzer (Agilent 8720 ES) with wafer probing and a 50 m mul-timode optical fiber connected to a New Focus 25 GHz pho-todetector. Fig. 7 indicates the modulation bandwidth increases with the bias current, until it flattens at a bias of about 7 mA. At
Fig. 6. Emission spectra of InGaAs:Sb-GaAs-GaAsP VCSELs at different driving currents.
Fig. 7. Small signal modulation measurement on InGaAs:Sb-GaAs-GaAsP VCSELs.
a bias current of only 5 mA, the maximum 3-dB modulation fre-quency is measured as 8.36 GHz, which is suitable for 10 Gb/s operation. In Fig. 8, the 3-dB bandwidth dB is plotted as a function of the bias current. At low-bias currents, the band-width increase in proportion to the square root of the current above threshold, as expected from the rate equation analysis. The saturation of bandwidth was clear observed as bias current increases above 7 mA which can be attributed to heating effect. The maximal bandwidth is estimated to 10.7 GHz with modula-tion current efficiency factor (MCEF) of 5.25 GHz mA . To measure the high-speed VCSEL under large signal modulation, microwave and light wave probes were used in conjunction with a 10 Gb/s pattern generator (MP1763 Anritsu) with a pseudorandom bit sequence of 2 and a 12.5-GHz photoreceiver with an OC-192 low pass filter. Eye diagrams were obtained for back-to-back (BTB) transmission on VC-SELs. Fig. 9(a) demonstrates that the room temperature eye diagram of the presented VCSEL biased at 6 mA, with data up to 10 Gb/s and an extinction ratio of 6 dB. The clear open eye pattern indicates good performance of these InGaAs:Sb VCSELs with the rise time of 30 ps, the fall time
Fig. 8. The 3-dB bandwidth(f dB) is plotted as a function of the bias current.
Fig. 9. (a) Room temperature (b) 70 C eye diagram of our VCSEL data up to 10 Gb/s and 6 dB extinction ratio. The scale is 15 ps/div.
of 41 ps, and jitter (p-p) 20 ps. The VCSELs also exhibit superior performance at high temperature. The reasonably open eye-diagram in Fig. 9(b) demonstrates the high-speed performance of the VCSEL (biased at 7 mA) at 10 Gb/s with an extinction ratio of 6 dB at 70 C. This result further verifies the superior performance of the presented VCSELs.
To guarantee the device reliability is always a tough work but a natural task for the components supplier in the data commu-nication markets. We have accumulated life test data up to 1000 h at 70 C/10 mA. As shown in Fig. 10, the threshold current is plotted versus time scale for the InGaAs:Sb VCSEL chips on TO-46 header under the high temperature operation lifetime
Fig. 10. HTOL (70 C/10 mA) performance of the InGaAs:Sb-GaAs-GaAsP VCSEL.
(HTOL) test at 70 C/10 mA. The threshold current was drop slightly at first 72 h which might be attributed to initial material stability. All samples show pretty stable performance except one sample show slight degradation after 1000 h.
IV. CONCLUSION
High-performance InGaAs:Sb-GaAs-GaAsP QWs VC-SELs with an emission wavelength of 1.27 m were suc-cessfully demonstrated. The VCSEL’s exhibit a very low threshold current, good temperature performance, and a high modulation bandwidth of 10.7 GHz with MCEF of 5.25 GHz mA . The VCSELs also demonstrate high-speed modulation up to 10 Gb/s from 25 C to 70 C. The results reveal the performance of the InGaAs:Sb VCSELs com-parable to that of GaInAsN VCSELs, but with better thermal stability. They are, therefore, very promising candidates for 1.3 m commercial applications.
ACKNOWLEDGMENT
The authors would like to thank Dr. C. Kuo of LuxNet Cor-poration, Dr. C. H. Lin of Agilent Technologies, Dr. C. P. Sung and Dr. J. Chi of ITRI, and Prof. N. Tansu of Lehigh University for useful discussions and technical support.
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Hao-Chung Kuo (S’94–M’99), received the B.S. degree in physics from
Na-tional Taiwan University, Taipei, Taiwan, R.O.C., in 1990, the M.S. degree in electrical and computer engineering from Rutgers University, New Brunswick, NJ, in 1995, and the Ph.D. in electrical and computer engineering from the Uni-versity of Illinois, Urbana–Champaign, in 1999.
He has an extensive professional career both in research and industrial re-search institutions. He has been a Rere-search Consultant with Lucent Technolo-gies, Bell Laboratories, Holmdel, NJ, (1995–97); an R&D Engineer with the Fiber-Optics Division, Agilent Technologies, Palo Alto, CA, (1999–2001); and an R&D Manager with LuxNet Corporation, Fremont, CA, (2001–2002). Since September 2002, he has been a Faculty Member with the Institute of Electro-Op-tical Engineering, National Chiao Tung University, Hsinchu, Taiwan, R.O.C. He has authored or co-authored over 60 publications. His current research interests include the epitaxy, design, fabrication, and measurement of high speed InP and GaAs based vertical cavity surface emitting lasers, as well as GaN-based Lighting emitting devices and nanostructures.
Ya-Hsien Chang was born in Taipei, Taiwan, R.O.C., on Jan 12, 1976. He
received the B.S. and M.S. degrees in electrical engineering from National Tsing-Hua University, Hsinchu, Taiwan, R.O.C. He is currently working to-ward the Ph.D. degree at the Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu, Taiwan, R.O.C.
His research is focused on characterizing III-nitride material and modeling the high speed VCSEL’s.
Yi-An Chang was born in Taipei, Taiwan, R.O.C., on March 13, 1978. He
re-ceived the B.S. and M.S. degrees in physics from the National Changhua Uni-versity of Education (NCUE), Changhua, Taiwan, R.O.C., in 2001 and 2003, re-spectively. He is currently working toward the Ph.D. degree in electrooptical en-gineering at the Institute of Electro-Optical Enen-gineering, National Chiao-Tung University, Hsinchu, Taiwan, R.O.C.
He joined the Laboratory of Lasers and Optical Semiconductors, NCUE, in 2000, where he was engaged in research on passiveQ-switching with solid-state saturable absorbers and III-nitride semiconductor materials for light-emitting diodes and semiconductor lasers. His research interests include InGaN and In-GaAsN semiconductor lasers.
Fang-I Lai was born in Taipei, Taiwan, on October 4, 1975. She received the
B.S. degree in physics from Tung-Hai University, Taichung, Taiwan, R.O.C., and the M.S. degree from the Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu, Taiwan, R.O.C., where she is currently working toward the Ph.D. degree.
Her research interests include the study of process techniques of VVCSELs.
Jung-Tang Chu was born in Taiwan, on July 12, 1978. He received the B.S.
degree in physics from National Chung-Hsing University, Taichung, Taiwan, R.O.C., and the M.S. degree from the Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu, Taiwan, R.O.C., where he is cur-rently working toward the Ph.D. degree at the Semiconductor Laser Technology Laboratory.
His research is focused on fabrication of III-V semiconductor light emitting devices.
Min-Ying Tsai received the B.S. degree in physics from Cheng Kung
Univer-sity, Tainan, Taiwan, R.O.C. She is currently working toward the M.S. degree at the Semiconductor Laser Technology Laboratory, Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan.
Her research is focused on characterizing III-nitride and InGaAsN materials.
Shing-Chung Wang (M’79) received the B.S. degree from National Taiwan
University, Taipei, Taiwan, R.O.C., in 1957, the M.S. degree from National To-hoku University, Sendai, Japan, in 1965, and the Ph.D. from Stanford University, Stanford, CA, in 1971, all in electrical engineering.
He has an extensive professional career both in academic and industrial re-search institutions. He has been a Faculty Member with National Chiao Tung University, Hsinchu, Taiwan, R.O.C. (1965–67); a Research Associate at Stan-ford University (1971–74); a Senior Research Scientist at Xerox Corporation, Palo Alto, CA, (1974–85); and a Consulting Scientist at Lockheed-Martin Palo Alto Research Laboratories (1985–95), Palo Alto, CA. In 1995, he rejoined Na-tional Chiao Tung University as a Faculty Member of the Institute of Electro-Optical Engineering. He has authored or co-authored over 160 publications. His current research interests include semiconductor lasers, vertical cavity surface emitting lasers, blue and UV lasers, quantum confined optoelectronic structures, optoelectronic materials, diode-pumped lasers, and semiconductor laser appli-cations.
