Fabricated by Impurity-Free Vacancy. Diffusion with a Novel Masking Technique. P. Cusumano, J. H. Marsh, Senior Member, IEEE, M. J. Rose, and J. S. Roberts.
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High-Quality Extended Cavity Ridge Lasers Fabricated by Impurity-Free Vacancy Diffusion with a Novel Masking Technique P. Cusumano, J. H. Marsh, Senior Member, IEEE, M. J. Rose, and J. S. Roberts
Abstract— By using phosphorous doped (5% wt P) silica as masking material and standard silica capping to promote quantum well interdiffusion, GaAs–AlGaAs ridge lasers with integrated transparent waveguides were fabricated. With a selective differential blue-shift of 30 nm in the absorption edge, devices with 400-m/2.73-mm-long active/passive sections exhibited threshold currents of 8 mA in CW operation, only 1 mA higher than that for normal lasers of the same active length and from the same chip. This 14% increase in threshold current was accompanied by a slope efficiency decrease of 40%. Losses of 3.2 cm01 were measured in the passive waveguides at the lasing wavelength using the Fabry–Perot resonance method. This value is among the lowest reported so far using an impurity-free disordering technique. Index Terms— Diffusion processes, integrated optoelectronics, optical losses, quantum-well lasers, semiconductor waveguides.
I. INTRODUCTION
T
HE INTEGRATION of photonic components on III–V semiconductors containing a multiple quantum-well (QW) active layer inside the waveguide core can be achieved after the epitaxial growth using spatially selective QW intermixing techniques [1]. For the fabrication of devices with integrated transparent waveguiding structures, intermixing techniques that do not rely on the introduction of impurities or structural defects are advantageous because optical losses due to free carriers and scattering can be kept to a minimum. In the GaAs–AlGaAs system, the impurity-free vacancy diffusion (IFVD) process, using SiO capping and rapid thermal annealing (RTA), is based on the fast out-diffusion of Ga atoms into the cap creating Group III vacancies [2]. The diffusion of these vacancies toward the multiple QW layer promotes atomic exchange of Ga and Al between barriers and wells which, in turn, leads to a blue shift of the optical Manuscript received October 10, 1996; revised November 7, 1996. This work was supported in part by EPSRC and MoD under Grant GR/K 45968. The work of P. Cusumano was supported by the European Community within the frame of the Human Capital and Mobility program. P. Cusumano is with the Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K. on leave from the Dipartimento di Ingegneria Elettrica, Universit`a degli Studi di Palermo, Viale delle Scienze, I-90128 Palermo, Italy. J. H. Marsh is with the Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K. M. J. Rose is with the Department of Applied Physics and Electronic Mechanical Engineering, University of Dundee, Dundee DD1 4HN, U.K. J. S. Roberts is with the Department of Electronics and Electrical Engineering, University of Sheffield, Sheffield S1 3JD, U.K. Publisher Item Identifier S 1041-1135(97)01928-9.
absorption edge by several tens of nm. Simple photonic circuits fabricated by IFVD include laser-modulator [3], [4], laser-waveguide-detector [5], [6] and modulator-waveguide [7]. Recently, the use of phosphorous doped silica (SiO :P) as dielectric cap for promoting QW intermixing in both the GaAs–AlGaAs and InGaAsP–InP systems has been demonstrated by Rao et al. [8]. In their case, however, the multiple QW GaAs–AlGaAs structure was nominally undoped and the P content of the film was 1% wt P [9]. In an attempt to reproduce similar experiments, but using a QW separateconfinement double heterostructure pin laser material, we found that SiO :P with 5% wt P, deposited by plasma enhanced chemical vapor deposition (PECVD), acts as a very effective mask in suppressing Ga out-diffusion during the IFVD process for both pin and nip QW structures. More detailed results of our study will be published elsewhere [10]. A possible explanation of the masking properties of the SiO :P with high P content is that the film is more dense and void-free and has a reduced built-in stress as compared with SiO [11] making it impermeable to Ga atoms. In this letter, we report on the characteristics of CW operated extended cavity single mode ridge lasers fabricated by IFVD using SiO :P capping layer as a mask. The influence of the losses in the passive section, measured using the Fabry–Perot resonance method, on threshold current and slope efficiency is addressed. Our results indicate that TE losses in the transparent integrated waveguides at the lasing wavelength are as low as 3.2 cm for a differential shift of 30 nm obtained by IFVD. II. DEVICE FABRICATION A double QW separate-confinement heterostructure was grown by MOCVD on an n-type GaAs wafer and consisted of the following layers: a 1.5- m-thick n-doped Al Ga As lower cladding layer, an undoped 0.23 m thick Al Ga As core containing in its center two 10-nm GaAs quantum wells separated by a 10-nm Al Ga As barrier, a 0.7- m-thick pdoped Al Ga As upper cladding layer and finally a 0.1- m highly p-doped GaAs contact layer. The fabrication process involved first selective IFVD and then waveguide formation and ohmic contact deposition. A 200-nm-thick SiO :P film containing 5% wt P was deposited by PECVD and removed only from the passive sections by photolithography and wet etching in buffered HF solution.
1041–1135/97$10.00 1997 IEEE
CUSUMANO et al.: HIGH-QUALITY EXTENDED CAVITY RIDGE LASERS
Fig. 1. Photoluminescence wavelength shift versus temperature of rapid thermal annealing for 60 s in samples containing both intermixed and masked regions.
The samples were then completely capped with a 200-nm-thick SiO film deposited by PECVD and rapid thermal annealed for 60 s in the range of temperature 900 C–950 C. After RTA, all the samples showed excellent surface morphology under optical microscope inspection, particularly at the edge interface between SiO :P and SiO . Fig. 1 shows the wavelength shift, with respect to the as-grown material, in the photoluminescence (PL) spectrum excited with an argon-ion laser 514 nm) and measured at 77 K for test samples containing both intermixed and masked regions. From the graph the masking property of the SiO :P is quite evident, limiting the wavelength shift to less than 7 nm in the range of temperature studied, whereas shifts up to 50 nm are measured for the SiO capped region. Broadening in the PL width of the interdiffused areas was observed as compared to that of the masked region but, even for the sample that underwent the highest degree of blue shift, the PL width less than doubles (9 nm against 5 nm for the masked region and as-grown material). Devices were fabricated from samples 7 9 mm in size with a differential shift of 30 nm after RTA at 940 C for 60 s using a simple self-aligned technique. Single mode ridge waveguides (3 m wide, 0.7 m deep) were formed by dry etching in SiCl using photoresist as a dry etching mask. After e-gun evaporation of 250 nm of SiO at room temperature, lift-off in acetone was used to remove the silica from the top of ridges. The top GaAs layer was selectively (in respect to Al Ga As) dry etched in SiCl to prevent current from spreading to the integrated transparent waveguides. Finally, bars of solitary and extended cavity lasers were cleaved and soldered on Cu mounts for testing in CW operation at 20 C. III. RESULTS
AND
DISCUSSION
Transmission spectra of both disordered and undisordered ridge waveguides cleaved from the same sample were measured using a tunable Ti:sapphire laser. As shown in Fig. 2, a blue-shift of about 27 nm, in good agreement with the results from 77-K PL measurement, is present in the absorption edge of the disordered waveguides. Light/current curves and related spectra for a solitary 400m-long laser and for an integrated device with 400 m/2.73mm-long active/passive sections, reflecting the typical behav-
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Fig. 2. Transmission spectra of ridge waveguides fabricated on intermixed and masked regions of the same sample after RTA at 940 C for 60 s.
Fig. 3. Light–current (L–I ) curves and spectra of normal and extended cavity ridge lasers.
ior of the fabricated devices, are shown in Fig. 3. The extended cavity lasers showed equal power from both active and passive ends with emission wavelengths between 853 and 855 nm. As shown in Fig. 3, due to the losses introduced by the integrated waveguide, the threshold current increases by 14%, from 7 mA for the solitary laser to 8 mA for the extended cavity laser, and the slope efficiency decreases by 40%, from 0.32 W/A per facet to 0.19 W/A per facet. We directly measured the losses in the passive waveguides cleaved from the same sample, as a function of the wavelength and for TE polarization, using the Fabry–Perot resonance method [12] and the results are shown in Fig. 4. The losses exhibit a broad minimum of about 3 cm around the lasing wavelength of 855 nm, increasing for both longer and shorter wavelengths due to, respectively, free carrier absorption in the doped cladding layers and resonant absorption in the partially intermixed QW. Assuming a logarithmic dependence of the gain versus current density [13], from the balance between gain and losses at threshold the following formulas are obtained:
(1)
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measurements were done, we could not see any degradation of laser performance during the measurements. IV. CONCLUSION
Fig. 4. TE polarization losses versus wavelength in the integrated ridge waveguides.
(2)
where and are the losses and length, respectively, of the active and passive section, is the optical overlap factor per well, the gain constant and the number of wells. In deriving (1) and (2), coupling losses between the active and passive waveguide have been assumed to be zero because no mode mismatch due to refractive index change is introduced by the IFVD process. From the measured losses and using (1) we obtain a value for the product of 81.7 cm . Furthermore from (2), assuming a modal reflectivity of 0.32, we calculate the losses in the active section to be 3.4 cm , in good agreement with the value of 3 cm obtained from broad-area lasers fabricated in the as-grown material. The mirror losses are 28.5 cm and the losses introduced by the integrated waveguide amount to 22 cm due to the quite high ratio of 6.87. This explains the 40% decrease in slope efficiency. Due to the high quality material and the logarithmic gain current density relation in quantum wells, leading to higher differential optical gain as compared with bulk material, the treshold gain in the extended cavity lasers is attained with only a 14% increase in treshold current. For increasing losses in the pumped active section, the contribution of the losses in the integrated waveguide to the denominator of (2) would be less significant and the decrease in slope efficiency would be less dramatic. The losses achieved in the passive waveguides at the lasing wavelength are about the same as those in the active sections indicating little band edge absorption in the partially intermixed QW, which basically exhibit a transparency behavior but with no current injection. Transparency could be also achieved by current injection in a nonintermixed QW but this, for long integrated devices, requires a second contact, electrically insulated from the active region, and the subsequent increase in the total injected current not always is compatible with CW operation. Losses in the integrated waveguides are limited mainly by p-type free carrier absorption with a possible contribution from acceptorlike Ga vacancies [14] that have diffused into active region. A simple way to reduce the losses could be by tailoring the doping profiles together with the structural parameters of the laser structure [15]. Although no systematic lifetime
We reported the characteristics of CW operated extended cavity ridge lasers fabricated by IFVD using SiO :P capping layer as a mask. The influence of the losses in the passive section, measured using the Fabry–Perot resonance method, on threshold current and slope efficiency have been addressed. Our results indicate that losses at the lasing wavelength in the transparent integrated waveguides are as low as 3.2 cm for a differential shift of 30 nm obtained by IFVD. To the best of our knowledge, this is among the lowest values reported so far using an impurity-free disordering technique. The selective IFVD process using SiO :P with 5% wt P is highly reproducible and is being used for the fabrication of a number of photonic integrated devices such as mode-locked lasers with good performance uniformity. REFERENCES [1] J. H. Marsh, “Quantum well intermixing,” Semiconduct. Sci. Technol., vol. 8, no. 6, pp. 1136–1155, 1993. [2] L. J. Guido, N. Holonyak, Jr., K. C. Hsieh, R. W. Kaliski, W. E. Plano, R. D. Burnham, R. L. Thornthon, J. E. Epler, and T. L. Paoli, “Effect of dielectric encapsulation and As overpressure on Al-Ga interdiffusion in Alx Ga10x As quantum-well heterostructures,” J. Appl. Phys., vol. 55, no. 6, pp. 540–542, 1989. [3] S. O’Brien, J. R. Shealy, and G. W. Wicks, “Monolithic integration of an (Al)GaAs laser and an intracavity electroabsorption modulator using selective partial interdiffusion,” Appl. Phys. Lett., vol. 58, no. 13, pp. 1363–1365, 1991. [4] A. Ramdane, P. Krauz, E. V. K. Rao, A. Amoudi, A. Ougazzaden, D. Robein, A. Gloukhian, and M. Carre’, “Monolithic integration of InGaAsP-InP strained-layer distributed feedback laser and external modulator by selective quantum-well interdiffusion,” IEEE Photon. Technol. Lett., vol. 7, pp. 1016–1018, Sept. 1995. [5] J. Werner, E. Kapon, N. G. Stoffel, E. Colas, S. A. Schwarz, C. L. Schwartz, and N. Andreakis, “Integrated external cavity GaAs/AlGaAs lasers using selective quantum well disordering,” Appl. Phys. Lett., vol. 55, no. 6, pp. 540–542, 1989. [6] D. Hofstetter, H. P. Zappe, J. E. Epler, and P. Riel, “Monolithically integrated DBR laser, detector, and transparent waveguide fabricated in a single growth step,” IEEE Photon. Technol. Lett., vol. 7, pp. 1022–1024, Sept. 1995. [7] P. Cusumano, T. F. Krauss, and J. H. Marsh, “High extinction ratio GaAs/AlGaAs electroabsorption modulators integrated with passive waveguides using impurity-free vacancy diffusion,” Electron. Lett., vol. 31, no. 4, pp. 315–317, 1995. [8] E. V. K. Rao, A. Hamoudi, Ph. Krauz, M. Juhel, and H. Thibierge, “New encapsulant source for III–V quantum well disordering,” Appl. Phys. Lett., vol. 66, no. 4, pp. 472–474, 1995. [9] E. V. K. Rao, private communication. [10] P. Cusumano, B. S. Ooi, A. Saher Helmy, S. G. Ayling, A. C. Bryce, J. H. Marsh, B. Voegele, and M. J. Rose, “Suppression of quantum well intermixing in doped GaAs/AlGaAs structures using phosphorus-doped SiO2 encapsulant layer,” Appl. Phys. Lett., submitted for publication. [11] S. K. Ghandi, VLSI Fabrication Principles, 2nd ed. New York: Wiley, 1994, pp. 530–532, and references therein. [12] R. G. Walker, “Simple and accurate loss measurement technique for semiconductor optical waveguides,” Electron. Lett., vol. 21, no. 13, pp. 581–583, 1985. [13] P. W. A. McIlroy, A. Kurobe, and Y. Uematsu, “Analysis and application of theoretical gain curves to the design of multiquantum well lasers,” IEEE J. Quantum Electron., vol. QE-21, pp. 1958–1963, 1985. [14] S. O’Brien, J. R. Shealy, F. A. Chambers, and G. Devane, “Tunable (Al)GaAs lasers using impurity-free partial interdiffusion,” J. Appl. Phys., vol. 71, no. 2, pp. 1067–1069, 1992. [15] R. G. Waters, D. S. Hill, and S. L. Yellen, “Efficiency enhancement in quantum well lasers via tailored doping profiles,” Appl. Phys. Lett., vol. 52, no. 24, pp. 2017–2018, 1988.
