We report, for the first time, the performance of a GaAs/GaAlAs distributed Bragg reflector (DBR) laser using a focused ion beam implanted grating (FIB‐DBR). Stripes of Si+ + with a period of 2300 Å and a dose ∼1014 cm−2 are directly implanted into the passive large optical cavity layer to provide the distributed feedback. Surface‐emitting light from the second‐order grating is observed. Threshold current of 110 mA and single DBR mode operation from 20 to 40 °C are obtained. The wavelength tuning rate with temperature is 0.8 Å/°C. The coupling coefficient is estimated to be 15 cm−1. The results show that FIB technology is practical for distributed feedback and DBR lasers and optoelectronic integrated circuits

Boenke, M. M.

Clark, W. M., Jr.

Stevens, E. H.

Utlaut, Mark

Wang, S.

Wu, M. C.

English

University of Portland

University of PortlandPilot ScholarsPhysics Faculty Publications and Presentations Physics7-25-1988GaAs/GaAlAs distributed Bragg reflector laserwith a focused ion beam, low dose dopantimplanted gratingM. C. WuM. M. BoenkeS. WangW. M. Clark Jr.E. H. StevensSee next page for additional authorsFollow this and additional works at: http://pilotscholars.up.edu/phy_facpubsPart of the Physics CommonsThis Journal Article is brought to you for free and open access by the Physics at Pilot Scholars. It has been accepted for inclusion in Physics FacultyPublications and Presentations by an authorized administrator of Pilot Scholars. For more information, please contact library@up.edu.Citation: Pilot Scholars Version (Modified MLA Style)Wu, M. C.; Boenke, M. M.; Wang, S.; Clark, W. M. Jr.; Stevens, E. H.; and Utlaut, Mark, "GaAs/GaAlAs distributed Bragg reflectorlaser with a focused ion beam, low dose dopant implanted grating" (1988). Physics Faculty Publications and Presentations. 1.http://pilotscholars.up.edu/phy_facpubs/1AuthorsM. C. Wu, M. M. Boenke, S. Wang, W. M. Clark Jr., E. H. Stevens, and Mark UtlautThis journal article is available at Pilot Scholars: http://pilotscholars.up.edu/phy_facpubs/1GaAs/GaAIAs distributed Bragg reflector laser with a focused ion beam, low dose dopant implanted grating M. C. Wu, M. M. Boenke, and S. Wang Department of Electrical Engineering and Computer Sciences, Electronies Research Laboratory, University a/California, Berkeley, California 94720 W. M. C!ark, Jr., E. H. Stevens, and M. W. Utiaut Hughes Research Laboratories, Malibu, California 90265 (Received 18 March 1988; accepted for publication 31May1988) We report, for the first time, the performance of a GaAs/GaAlAs distributed Bragg reflector (DBR) laser using a focused ion beam implanted grating (FIB-DBR). Stripes of Si++ with a period of 2300 A and a dose - l 0 14 cm - 2 are directly implanted into the passive large optical cavity layer to provide the distributed feedback. Surface-emitting light from the second-order grating is observed. Threshold current of 110 mA and single DBR mode operation from 20 to 40 •care obtained. The wavelength tuning rate with temperature is 0.8 A.IC. The coupling coefficient is estimated to be 15 cm - 1• The results show that FIB technology is practical for distributed feedback and DBR lasers and optoelectronic integrated circuits. Distributed feedback (DFB) lasers1 and distributed Bragg reflector (DBR) lasers2- 5 are the most important sin-gle-wavelength light source for optical communications and optoelectronic integrated circuits. Conventional gratings providing the distributed feedback are usually fabricated with holographic lithography and etching. 1•5 With this tech-nique it is difficult to fabricate lower order gratings, which have the benefits oflarger coupling efficiency and lower radi-ation loss. A large coupling constant, K, is important for sta-ble single-mode operation under high-frequency modula-tion6 and for wider wavelength tuning range in DBR lasers. 7 Electron beam lithography8 has the capability of writing first-order gratings; however, this requires the use of very thin layers ofphotoresist ( ::::::200 A) for which proper step coverage is often a problem. Use of focused ion beam (FIB) implantation offers sig-nificant advantages to the fabrication of these gratings be-cause the technique is freed from the resolution and align-ment difficulties associated with resist-mask-dependent processing< With FIB it is possible to vary the dose, stripe width and spacing, and ion and implant energy with great flexibility to make the grating structure. These advantages have been demonstrated previously in semiconductor elec-tronic device fabrication including metal-semiconductor field-effect transistors (MESFET's) with a striped channel structure9 and laterally profiled active area implants to re-duce the Kirk effect in bipolar transistors. w In semiconduc-tor lasers FIB micromachining has been utilized to provide facet etching and coupled-cavity lasers. 11 The FIB implant and prolonged high-temperature annealing was used to pro-duce disordering in superlattices. 12 These uses of the FIB require either implant doses much higher than 1015 cm -2 , which necessitate very long FIB processing times, or long annealing time, which precludes the fabrication of fine fea-tures, and do not emphasize the flexible advantages of the tool. Our theoretical calculations show that grating coupling strengths on the order of 10-- l 00 cm - 1 that are suitable for DFB and DBR lasers would be possible using dopant im-plants more than ten times less. n Operation of FIB-implanted grating is based on the free-carrier-induced index change. Free carriers in semiconduc-tors will reduce the refractive index n through carrier-in-duced shift of absorption edge (on/aN- 10-20 cm3 ) 14 and free-carrier-plasma effect (()n/oN-10·- 21 cm3 ) <Such index change (tin -0.05) is somewhat smaller than that produced by a conventional composition-varied grating (.in~ 0.2). Therefore, the laser cavity must be designed to maximize the effect. In this letter, we report the performance of a DBR laser with a second-order FIB-implanted grating. The schematic of the laser structure is shown in Fig. l. A coupled active-passive large optical cavity (LOC) structure is used. The layers above the LOC are removed in the passive section, and the grating is formed by direct FIB implantation into the LOC layer to maximize the ovedap of the field and the grating. The fabrication process is described in the fol-lowing. First, five !ayers are grown successively by liquid phase epitaxy: 1.5-µm-thick n-Ga0 .7 Al0 _3 As cladding layer, 0. 5-µm-thick n-Gaa.ss Al0 15 As LOC layer, 0.1-µm-thick un-doped GaAs active layer, 1.3-µm-thick p-Gao 7 Al0 .3 As up-per cladding layer, and finally 0.3-µm-thick p,. -GaAs cap layer. After growth, the active part is masked by Si3N4 and the upper cladding layer in the passive section is selectively removed by KI-I2 (l 13 g Kl, 65 g 12 , and 100 cc H 20). The exposed active layer is then etched away by 1: 1 :20 ratio of NH40H:H20 2 :H20. A ridge waveguide of width 10 ,um is formed in the active area along the [011 J direction, and a 16-pm-wide shallow ridge waveguide is also formed in the pas-sive LOC section to improve lateral guiding and matching with the active guide. The gratings were produced via FIB implants of Si. Sili-con was chosen because ( l ) the free-carrier-induced index change is larger for n-type dopants 14 and (2) the low diffusi-vity of Si ensures minimal spreading of the grating pattern during annealing. The Si stripes were produced by scanning the FIB from a three-lens, mass-separating, variable-energy ion focusing column. 15 Dose was varied from 4 to 8 X 1014 cm - 2 by changing the number of repetitive scans by the beam over the stripe. In calculating the number of scans 265 App!. Phys. Lett. 53 (4), 25 July HISB 0003-6951 /88/300265-03$01.00 © 1968 American Institute of Physics 265 PASSIVE FIB-IMPLANTED GRATING ACTIVE RIOGE-LOC I FIG. l. Schematic drawing of the active-passive LOC DBR laser with FIB-implani-ed grating. L- n+ GaAs SUBSTRATE required for a given stripe dose, the stripe width was as-sumed to be that of the beam, that is, a full width at half maximum intensity (FWHM) of approximately 700 A for a Gaussian current density profile. System accuracy for beam placement is estimated to be within 450 A. The gratings were implanted at 100 keV and the second-order period was ap-proximately 2300 A. The wafer is then capped with thermal chemical vapor deposited (CVD) Si3 N4 and rapid thermal annealed at 950 °C for 10 s. The encapsulation of the Si3N 4 is very important to reduce the diffusivity of Si atoms in GaAs, 15 and negligible diffusion is obtained in this case. After metallization and lapping, the laser is cleaved and mounted p-side up for measurement There is no coating on either facet. First, the laser is inspected from the top surface to observe the surface-emitted light from the second-order grating. Figure 2 shows the near-field distribution along a laser with a long grating section (LG = 320 µm) and a short active section (LA = 160 µm). The cross-sectional schemat-ic of the laser is shown in Fig. 2(a). Figure 2(b) shows the picture of the near field in which the white bar in the middle corresponds to the surface-emitted light deflected by the sec-ond-order grating. The circular spot at the left end of the white bar is due to imperfect matching of the waveguides at the transition region. From the line-scanned profile of the near field [Fig. 2(c)], the value of K is estimated 17 to be 15 cm- 1• The DBR mode is observed when the lasing wavelength is close to the Bragg wavelength /L 8 ( ;:::;; 8530 A). Figure 3 shows the spectrum at various temperatures for a laser with LA = 290 µm, LG= 335 µm, and biased at 220 mA. The measurement is done under pulsed operation with a pulse width of l µsand repetition rate of 1 kHz. At 16.8 °C, the DBR mode at 8528 A is far away from the gain peak at 8485 A and the lasing is dominated by Fabry-Perot ( FP) modes. The FP mode spacing of 3.1 A indicates the lasing is between the cleaved front facet and the etched facet at the transition. As temperature increases, the gain peak shifts towards A,0 and the DBR mode gradually dominates and suppresses the 266 Appl. Phys. Lett, Vol. 53, No. 4, 25 July i9S8 FP modes. The gain peak coincides with A a at about 35 °C, at which strongest light intensity is observed. At higher tem-perature, side mode suppression becomes worse because of the increase in threshold current. The variation of threshold current and lasing wave-length versus current is summarized in Fig. 4. The laser lases in single wavelength from 20 to 40 °C. The wavelength tun-ing rate of 0.8 A/C indicates that i.t is DBR mode. The threshold current variation with temperature follows a straight line with a characteristic temperature T0 = 110 K except for a dip around 35 °C. The dip corresponds to the coincidence of the gain peak and A. B. At room temperature, the threshold current is 110 mA. Theoretically, the implanted grating can be approxi-~L_•t--L~L~ C$i&seosoese I I (al (b) (cl Fi G. 2. (a) Schematic cross section of the DBR laser, ( b) near-field picture of the surface-emitting light from the second-order FIB-implanted grating, and ( c) the line-scanned profile of the near field. Wu eta!. 266 (#C-161 8450 8500 8550 WAVELENGTH (A) FIG. 3. Spectrum of the DBR laser at various temperatures. At low tem-perature ( 16.8 "C), FP modes (8480-8490 Al of the active cavity domi-nate. As temperature increases, the gain peak shift toward Bragg wave-length ( ::::::8530 Al and DBR mode emerge and suppress the FP modes. mated by an array of two-dimensional Gaussian functions of vertical straggle !:..RP, horizontal straggle ll.R 0 period A, and centered at R,, below the surface. The activated elec-trons at the peak ( ~ 3 X 1018 cm-- 3 for our dose and anneal-ing condition) decrease the refractive index by 6.n = 3 X 1 o- 2, The corresponding change in effective re-fractive index 6.netr = 0.004 for RP= 0.10 µm and ARP = 0.045 µm. The second-harmonic component of this index perturbation is t.n~~) = (..jfiiAn 0tril.R,I A) ·-(8n2<l.R;/A') F AR 350 Ao 2 I A f2)'/1 e . or .,. , = , K = 1T 1.:>n~lf I An = 18 cm - i, in good agreement with the experiment data. More complete theoretical calculation will be published else-where. 13 Though K is not very large at present, it can be improved by a factor of 5 by reducing the thickness of the LOC layer, which effectively increases the overlap of light and grating. Reduction of the FIB linewidth would permit implanting a first-order grating, with which another factor of two increase in K is expected. fa conclusion, we have demonstrated, for the first time, a GaAs/GaA!As active-passive DBR laser with a FIB-im-planted second-order grating. The laser has a threshold cur-rent of 110 mA and operates in a single DBR mode from 20 to 40 °C with a 0.8 A../°C tuning rate. The coupling constant 267 Appl. Phys. Lett., Vol. 53, No. 4, 25 July 1988 150 ;( 1 1,. /' 140 ! 1SC 1-J(~l( 120 z w )(---i::::=> 110 a: .,,...-- a: 100 :;) (,) 8560 o;( ::c 8540 I-Cl z 8520 w ...I GDBR mode w ~ 6500 "'FP mode ;: ,, A " A 8480 15 20 25 30 35 40 45 5!) FIG.4. Variation of threshold current l,h and lasing wavelength with tem-perature, The l,h followed a straight line except for a dip at around 35 °C, corresponding to the coincidence of the gain peak and the Bragg wave-length. Single wavelength operation is observed from 20 to 40 °C. is estimated to be 15 cm - 1• An improvement in K by an order of magnitude is achievable by reducing the thickness of the passive waveguide and with a first-order FIB-implanted grating. This technology is compatible with molecular beam epitaxy (MBE) and the FIB implant can be done in situ inside the MBE chamber. The experimental results show that the FIB is a promising technology for future DFB and DBR lasers and optoelectronic integrated circuits. This work was supported by Eastman Kodak, BeH Communica-tions Research, and National Aeronautics and Space Ad-ministration grant No. NAG-1-580. 'Y, Nakano and K. Tada, Appl. Phys. Lett. 49, 1145 ( 1986). 2F. K. Reinhart, R. A. Logan, and C. V. Shank, Appl. Phys. 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Miyau-chi, and H. Nakashima, Appl. Phys. Lett. 51, 109 ( 1987). nM. M. Boenke, M. C. Wu, S. Wang, W. M. Clark, Jr., E. H. Stevens, and M. W. Ut!aut (unpublished). 140. D. Sell, H. C. Casey, Jr., and K. W. Wecht, J. Appl. Phys. 45, 2650 (1974). 15V. Wang. J. W. Ward, and R. L. Seliger, J. Vac. Sci. Technol. 19, 119 (1981 ). 16M. E. Greiner and J. F. Gibbons, J. Appl. Phys. 57, 518! (1985). 17S. Wang, in Semiconductors and Semimetals, edited by W. T. Tsang (Aca-demic, New York, 1985), Vol 22, part E, p. 6!. Wu eta!. 267 

GaAs/GaAlAs distributed Bragg reflector laser with a focused ion beam, low dose dopant implanted grating

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GaAs/GaAlAs distributed Bragg reflector laser with a focused ion beam, low dose dopant implanted grating

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