528 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 3, MARCH 2005
High-Speed Modulation of
InGaAs : Sb–GaAs–GaAsP Quantum-Well
Vertical-Cavity Surface-Emitting Lasers
With 1.27-
m Emission Wavelength
H. C. Kuo, Member, IEEE, Y. H. Chang, H. H. Yao, Y. A. Chang, F.-I. Lai, M. Y. Tsai, and S. C. Wang, Member, IEEE
Abstract—1.27- m InGaAs : Sb–GaAs–GaAsP vertical-cavity
surface-emitting lasers (VCSELs) were grown by metal–organic chemical vapor deposition and exhibited excellent performance and temperature stability. The threshold current changes from 1.8 to 1.1 mA and the slope efficiency falls less than 35% as the temperature raised from room temperature to 70 C. 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 measured to be 10.7 GHz with modulation current efficiency factor (MCEF) 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.
Index Terms—Characterization, InGaAsSb, laser diodes, metal–organic chemical vapor deposition (MOCVD), optical fiber devices, semiconducting.
I. INTRODUCTION
L
ONG-WAVELENGTH vertical-cavity surface-emit-ting lasers (VCSELs) are key devices in optical fiber metropolitan-area networks. Recently, Koyama et al. [1] and Tansu et al. [2] employed of 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 very promising perfor-mance and continuous-wave operation at up to 120 C as well as 10-Gb/s operation [3]. 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 poorly 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 [4]. The authors have observed that adding Sb into samples with high In content sharply increases the PL intensity, and have found that the alloy thus formed not only behaves as a surfactant but contributes significantly to red-shift of the optical emission [4]. Additionally, adding a surfactant suchManuscript received June 4, 2004; revised October 13, 2004. This work was supported by the National Science Council of Republic of China (R.O.C.) in Taiwan under Contract NSC 92-2215-E-009-015 and Contract NSC 92-2112-M-009-026.
The authors are with the Institute of Electro-optical Engineering, National Chiao-Tung University, Hsin-Tsu 30050, Taiwan, R.O.C. (e-mail: [email protected]).
Digital Object Identifier 10.1109/LPT.2004.840042
as Sb [5] and Te [6] significantly increases the critical layer thickness of InGaAs on GaAs. This work presents high-per-formance InGaAs : Sb–GaAs–GaAsP QWs VCSELs grown by metal–organic chemical vapor deposition (MOCVD). The light output and voltage versus current ( – – ) performance and high-speed performance proved these VCSELs a good candidate of long wavelength VCSEL.
II. EXPERIMENT
All structures were grown on semi-insulating GaAs (100) substrates by low-pressure MOCVD. The group-V pre-cursors were the hydride sources AsH and PH . The trimethyl alkyls of gallium (Ga), aluminum (Al), indium (In), and antimony (Sb) were the group-III precursors. The epitaxial structure was as follows (from bottom to top)—n -GaAs buffer, 40.5-pair n -Al Ga As/n -GaAs (Si-doped) DBR, undoped active region, p-Al Ga As oxidation layer, 25-pair p -Al Ga As/p -GaAs DBR (carbon-doped) and p -GaAs (carbon-doped) contact layer. The graded-index separate confinement heterostructure ac-tive region consisted mainly of a double QW’s acac-tive 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 and to ) confinement layers (growth temperature C with AsH –TMSb flow ratio 50). The thickness of the cavity active region was . Carbon was used as the p-type dopant in the DBR to increase the carrier concentration cm . The interfaces of both the p-type and n-type Al Ga As–GaAs DBR layers are linearly graded to reduce the series resistance. The optical characteristics of QWs were optimized through PL measurement and structural analysis. The details of growth optimization will be published elsewhere [7].
Fig. 1 compares the PL spectra of In Ga As QW with various 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 P strain-compensating layer. The PL peak emission wave-lengths 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. The full-width at half-maximum (FWHM) of the PL emission peak from In Ga As : Sb is larger and the PL intensity slightly
KUO et al.: HIGH-SPEED MODULATION OF InGaAs : Sb–GaAs–GaAsP QW VCSELs WITH 1.27- m EMISSION WAVELENGTH 529
Fig. 1. Comparison of the PL spectra of InGaAs with different In composition and In Ga As with Sb incorporation.
Fig. 2. Schematic cross section of high-speed VCSEL structure. The oxide-confined aperture is 5m and the surface relief size is 3.5 m.
lower. The red-shift of InGaAs : Sb can be attributed to the Sb in the alloy constituent and red-shifts the optical emission. As the Indium composition increases to 0.42, the FWHM of the PL emission increases significantly and the PL intensity drops dramatically.
Fig. 2 schematically depicts the VCSEL structure. A pro-cessing sequence involved six photomasks to fabricate oxide-confined polyimide-bridged VCSELs with coplanar waveguide probe pads. This process was designed to minimize capacitance while keeping a reasonably low resistance [8]. Device fabrica-tion began with the formafabrica-tion of cylindrical mesas with a diam-eter of 30 m by etching the surrounding semiconductor into the bottom n-type mirror to a depth of 5 m, using an induc-tively coupled plasma reactive ion etching 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/min for the Al Ga As layer, so the oxide extended 12.5 m from the mesa sidewall. Ti–Au was evaporated to form the p-type contact ring, and AuGeNiAu was evaporated 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 using rapid thermal annealing. After the contact formation, the photosensitive polyimide was spun on the sample to form in-sulation. Ti–Au was deposited to a thickness of from 200 to 3000 to form the metal interconnects and coplanar wave-guide probe pads. Heat treatment following metal deposition was applied to strengthen the metal-to-polyimide adhesion. Fi-nally, surface relief etching with a diameter of 3.5 m was per-formed to a depth of for single-mode operation [9].
Fig. 3. Temperature-dependentL–I–V curves. Inset is the emission spectra of InGaAs : Sb–GaAs–GaAsP VCSELs at different driving currents.
Fig. 4. (a) Small signal modulation measurement on VCSELs; the maximal bandwidth measured to be 10.7 GHz with MCEF is5.25 GHz=(mA) . (b) 3-dB bandwidth(f ) is plotted as a function of the bias current above threshold.
Fig. 3 plots curves of temperature-dependent – – . The de-vices exhibit single transverse mode characteristics at a lasing wavelength of 1.27 m, with a sidemode suppression ratio of 30 dB, as in the inset in Fig. 3, over the whole operating range. Notably, the maximal single-mode output power exceeds 1.2 mW at room temperature (0.8 mW at 70 C). Output power rollover occurs as the current increases above 10 mA at 25 C (9.5 mA at 70 C). The threshold current changes between 1.8 and 1.1 mA with temperatures from 25 C to 70 C and the slope efficiency drops less than 35% from 0.17 to 0.11 mW/mA
530 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 3, MARCH 2005
Fig. 5. (a) Room temperature (b) 70 C eye diagram of our VCSEL data up to 10-Gb/s and 6-dB extinction ratio. The time scale is 15 ps/div and the vertical scale is 90 mV/div for (a) and 60 mV/div for (b).
due to the large gain-cavity offset. The resistance of the VCSEL is 120 and the capacitance is 0.1 pF. Accordingly, para-sitic effects limit the devices to a frequency response of around 13 GHz.
Fig. 4(a) indicates the modulation frequency increases with the bias current, until it flattens at a bias of about 7 mA. At a bias current of only 5 mA, the maximum 3-dB modulation frequency response is measured as 8.36 GHz, which is suitable for 10-Gb/s operation. In Fig. 4(b), the 3-dB bandwidth is plotted as a function of the bias current. The maximal bandwidth is measured to be 10.7 GHz with a modulation current efficiency factor (MCEF) of 5.25 GHz mA and the differential gain at room temperature was estimated to cm .
To measure the high-speed VCSEL under large signal mod-ulation, microwave and light wave probes were used in con-junction with a 10-Gb/s pattern generator (MP1763 Anritsu) with a pseudorandom bit sequence of and a 12.5-GHz photoreceiver. Eye diagrams were obtained for back-to-back (BTB) transmission on VCSEL. Fig. 5(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 of 41 ps, and jitter (p-p) 20 ps. The VCSELs also exhibit superior performance at high temperature. The reason-ably open eye diagram in Fig. 5(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. Fig. 6 shows average re-ceived power dependence of the BER at 10-Gb/s modulation under BTB transmission and temperature of 25 C and 70 C. In this experiment, multimode fiber was aligned to realize the highest coupling efficiency. An error rate of was achieved without any indication of a BER floor. This result further veri-fies the superior performance of the presented VCSELs.
III. CONCLUSION
High-performance InGaAs : Sb–GaAs–GaAsP QWs VC-SELs with an emission wavelength of 1.27 m were success-fully demonstrated. The VCSELs 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 comparable to that of GaInAsN
Fig. 6. (a) 25 C and (b) 70 C average received power dependence of the BER at 10-Gb/s modulation under BTB transmission.
VCSELs, but with better thermal stability. Longer wavelength is possible with incorporating more Sb or In content into the QW by reducing of growth temperature. Thus, the InGaAs : Sb VCSEL should be viable low-cost light source of optical fiber data link systems.
ACKNOWLEDGMENT
The authors would like to thank Prof. N. Tansu of Lehigh University, Dr. C. Kuo of LuxNet Corportaion, and Dr. C. Sung and Dr. J. Y. Chi of the Industrial Technology Research Institute for their valuable discussions and technical support.
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