InP-In<inf>1-x</inf>Ga<inf>x</inf>As<inf>y</inf>P<inf>1-y</inf>embedded mesa stripe lasers

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IEEE JOURNAL O F QUANTUM ELECTRONICS, VOL. QE-16, NO. 10, OCTOBER 1980

-In 1 -xGaxAsyP1-y Embedded Mesa Stripe Lasers FRANCISCO C. PRINCE, NAVIN B. PATEL, AND DOUGLAS J. BULL

(Invited Paper)

Abstract-A laser structure was fabricated using two-step liquid-phase epitaxy, employing the melt-backtechnique.Thefabricationand properties of thisstructurearedescribed in detail. Good linearity of the power output up to power levelsof 20 mW was obtained. The threshold current density at 300 K is 9-12 kA/cm2. This high value is mainly due to Zn-diffusion from the third to the buffer layer during the second growth step of the fabrication process. The external differential quantumefficiency is 30-35 percent under pulsed operation at25'C.The puked thresholdcurrenthasanexponentialbehavior with temperature where To = 60'C. The far-field beam divergences in the directions parallelandperpendicular to the junctionplaneare 12-15'and35-40',respectively.Transversemodestabilizationwas improved with this laser structure.

I. INTRODUCTION INCE the first operation [ 1] - [ 2 ] and the reliability demonstration [3] , 141 of the In, -xGaxAsyP,-y DH laser, intensive work has been done to improve laser characteristics and performance. Using the low loss and low dispersion wavelength region of the optical fibers and a 1.3 pm InGaAsP DH laser as a source, the feasibility of large-capacity [5] and long distance [6] optical transmission systems has been demonstrated.In such systems, good linearity of the source and its operation in a stable fundamental transverse mode are indispensable. In the GaAlAslaser system, where the technology iswell developed, several structures have been fabricated and good characteristics have been demonstratedfor applications in optical transmission systems. In all laser structures, the basic factor to obtain linearity and stable transverse mode is the introduction of a built-in wave-guiding mechanism in the active layer. The laser structure developed and characterized in this work isverysimilar to the channeled substrate planar (CSP)laser [7], where the guiding mechanism introduced in the active layer is due to the spatial modulation of the losses in regions near the active layer of the device. A schematic diagram of the structure of our laser is shown in Fig. l(a). As in the GaAlAs lasers, nonlinearities are significantly reduced by using this built-in passive waveguiding mechanism. In this paper we present the fabrication and optical characterization of this new device.

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11. LASERDESIGN InGaAsP is a new laser material. The first laser operation wasachieved in 1975 [ l ] , [ 2 ] and little is known about the refractive index variation with material parameters, gain coefficients, and other parameters that are necessary foran exact calculation of the laser properties. Thus, the optimization of the laser parameters can only be done in an estimated manner. Our first design was as similar as possibleto a GaAlAs laser [16] in which the guidingwasachieved byabuilt-in mechanism rather than a spatial gain profile. A recent report on a self-aligned structure [17]justifies why our laser works in a satisfactory way [15]. Based on calculations made by Yano et al. [ 171, we can estimate the refractive index step An created along the junction plane for our case. This step is created by radiation losses of the mode outside the stripe region and its value is very insensitive to the absorption coefficient value of the top absorbing quaternary layer. Using their data, in Fig. 2 we plot An as a function of the parameter Manuscriptreceived May 21, 1980. This workwassupported by t (the thckness of the binary InP top layer which is left near Telebras. The authors are with the Instituto de Fisica, Universidade Estadual de the shoulder after the melt-back) for an activer layer thickness 0.2 pm. Since t for our lasers falls in the range of 0.3-0.5 pm, Campinas, 13100 Campinas /SP, Brazil. 0018-9197/80/1000-1034$00.75 0 1980 IEEE

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TABLE I DETAILS OF THE LAYER THICKNESS AND DOPING. THERESIDUAL DOPIKG OF THE SYSTEM IS 2-5 X 10l6

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Fig. 3. Surface morphology (a) before and (b) after the second growth cycle, showing the wave pattern observed.

The solid composition and thelattice parameter of the InGaAsP active layer was measured in a similarly grown 2 pm thick layer using electron microprobe analysis [ 131 and a divergentX-ray source [ 141, respectively. The solid compositionfoundinthis layer was x = 0.23, y = 0.56, andthe mismatch was 0.07 percent. 2.5 3xda(m) InP I After this first growth cycle, a 3000 ii SiO, film was deposited over the wafer by electron-beam evaporation. Then, 1"0.77Ga0.23As0d6p044 I = by standard photolithographic procedure, stripes 7 pm wide the value of the refractive index difference for our devices is witha separation of 250 pm were defined. After removing estimated to beinthe (1-4) X range, which is sufficient photoresist, the surface was thoroughly cleaned and put back in the growth reactor and the second cycle was initiated. The to stabilize the transverse mode. melt-back took place at 640°C for 10 s using a melt with 10°C 111. DEVICEFABRICATION of undersaturation. The growth of the top quaternary layer was done during 60 s withthe same melt used in the first To fabricate the embedded mesa stripe laser, a melt-back growth cycle. step was carried out in the second liquid-phase epitaxial It was found that near the edges of the stripes the thickness growth. This method is quite useful in the growth of special of the layer removed by the melt-back is more than between structures in Inl ,Ga,As,P, -.y quaternary alloy [8], [9] where the thermal damage of the substrate surface is a severe the lines. This effect is due to the small curvature in the melt caused by the presence of the SiO, fiim. This thickness difproblem. The sequence of the process to grow the embedded mesa ference causes a faster growth near the shoulder and a conse(EMS) laser was the following: after an overnight bake of the quent indium inclusion, on the top,near the stripe edges. This four melts of pure In, the boat was loaded with InP, InAs, does not affect the laser properties since it is sufficiently far GaAs, and Sn. Liquid composition was chosen for the (1 1l)B from the region of the high optical field. In Fig. 1 we show substrate based on the data of Nakajima et al. [lo], togive an an SEM picture together with a schematic drawing of the cross emission wavelength near 1.3 pm at room temperature. After section of the EMS laser. loading the boat, a bake at 650°C during 4 h is necessary for Another perturbation caused by the presence of the SiO, homogenization of the melts. The substrate is then put into mask during the growth was waves running in the stripe directhe LPE system, which issimilar to the apparatus described tion. These waves can not be due to some misorientation of for growth of GaAs [ 111. The substrate is put in the growth thecrystal since in the first growth cycle no terraces were reactor last to minimize the surface degradation. This way, present on the layers. Also, the wave pattern was reproducible a melt-back is not necessary in the first growth cycle. Together in different growth runs. with the substrate, in this last step, we also add zinc in the In Fig. 3 we can compare the morphology before and after third melt to prevent melt contaminationand possible mis- the second growth cycle. placed junction [ 121. Finishing the second growth cycle, the surface was etched After putting the substrate in the growth chamber and let- with H,O :Hz02:HF = 8 :3 : 2 to remove the SiO, film and ting the temperature stabilize, a cooling was initiated with a the indium droplets. A new SiO, film was then deposited on rate of 0.7"C/min. The growth of the first layer was initiated the wafer and 10 pm stripe windows were opened over the at 645°C and the active layer was grown at 640°C during a mesas. This step is necessary to protect the top quaternary time of 10 s, which gives a 0.2 pm thicklayer. Details of layer from being converted to p-type material when we carry layer thicknesses and doping are shown in Table I. out a shallow Zn diffusion to improve the ohmic contact.

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IEEE JOURNAL OF QUANTUM ELECTRONICS,VOL.QE-16,NO. 10, OCTOBER 1980

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Finally, the wafer was metallized with AuZn on the p-side and AuSn on the n-side. Bars, 300 pm wide, were cleaved to form the mirrors, and individual devices were diced with the help of a diamond scriber. Theuse of a (100) substrate to fabricate the lasers would facilitate the device fabrication, but we found that the shoulder [15], which is left on the side of the mesa, was not in a sharp trapezoidal form, and that the shallow melt-back always leaves melt over the wafer due to nonuniform surface dissolution.

IV. DEVICEPROPERTIES A. Light-versus-CurrentCharacteristics A typical set of light-versus-current characteristics of embedded mesa stripe lasers is shown in Fig. 4. The curves were measured under pulsed operation at lo3 pulses/s and a pulse width of 1 ps. The top transparent layer thickness t near the shoulder is between 0.3-0.5 pm, so that the estimated refractive index difference is between 1-4 X The threshold current density is in the 9-12 kA/cm2 range. 5 102 tIO I5 20 25 35 40 45 SO 55 80 (a) Thejunction position was determined byelectron beam TEMPERATURE TP"') induced current meassrements (EBIC). It was found that the Fig. 6. (a) Plot of pulsed threshold current and (b) external differential junction position was within the buffer layer, 0.3 pm away quantum efficiency fora device, showingatransitiontemperature from the lower interface of the active layer. This of course is of 45°C. due to zinc diffusion from the topzinc-doped layer during the second growth cycle. This affects the threshold current mainly B . Temperature Dependenceof Threshold in two ways: first , there is an increase in the free carrier ab- Current andEfficiency sorption losses, and second, the holes are not confined in the The light-versus-current characteristics of the embedded 0.2 ,um quaternary active layer whch causes carrier losses by mesa stripe lasers for temperatures between 20 and 50°C are hole injection and recombination in the InP regions near the shown in Fig. 5. In Fig. 6(a) and (b) we plotthe pulsed junction. The injection mechanism is similar to that present in threshold current and external differential quantum efficiency remote junction heterostructure lasers [18], where, however, (from both mirrors) versus heat-sink temperature, respectively. the hole injection is minimized by an additional barrier at the The threshold current has an exponential behavior with temperature where To = 60°C. This value is comparable to that junction. Together with these carrier losses, there is also the additional reported by Hsieh [19] for the zinc diffused stripe lasers. The optical loss caused by the very nature of the device where the value of To persists upto temperatures of 3O-5O0C, with slight variation for different devices. For higher temperatures guided mode has radiation losses in the top quaternary layer. a decrease can be seen. Theobserved differential quantum efficiency fromboth Together with the slight decrease of the value of T o ,a strong mirrors is typically 30 percent at 25"C, and is strongly dependecrease inthe external differential quantum efficiency is dent of theheat-sink temperature as we will discuss.

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Fig. 7. Near field along the junction plane. m e near-field spot size at half powex is 3 Nm and remains constantat different injection levels.

Fig. 9. Lasing spectraof the laser at different injection levels.

respectively, which are values slightly lower than those measuredinconventional DH InGaAsPlasersdesigned for the 1.3 pmwavelength [21].

D. Lasing Spectra For use in optical communications, single-mode operation is a dispensable characteristic of a 1.3 pm light source, since at this wavelength the dispersion becomes minimum. Due to the built-in index guiding mechanismalong the junction plane, single-mode operation was almost obtained in the EMS lasers. Pulsed lasing spectra for several injection levels are shown in Fig. 9. The spectral modebehavior issimilar to thestrip-20 -10 0 10 20 buried heterostructure laser [22]. 0 e Near threshold the laser operates in multiple longitudinal Fig. 8. (a) Far field of the EMS laser parallel and (b) perpendicular to modes. As the injection current is increased almost all power the junction plane. The beam divergences are14 and 35" in directions is concentrated in a single longitudinal mode. If we continue parallel and perpendicularto the junction plane, respectively. to increase the current, a jumpof the laser power to the next seen. We believe that there was an increase of carrier leakage higher wavelength longitudinal mode is observed due to heatacross the confining current barriers, consequently, for these ing of the active layer. This effect is commonly observed in temperaturesthe stripe defined bythe p-n junction is no the structure when an index guiding mechanismis introduced longer effective. Similar behavior for thresholdcurrentand along the junction plane [l6], [22] , [23]. efficiency was found previously in lasers of the transverse V. CONCLUSION junction stripe [20]. A laser structure, the embedded mesa stripe laser, emitting C. Optical Mode Patterns in the 1.3 pm region, was fabricated by a twb-stepLPE growth In the design of optical transmission systems the knowledge method. The effective refractive index step that was built-in along the of the laser mode properties are very important. Efficiency of laser-fiber coupling is directly related to beam divergence of junction plane was estimated based on calculations made by Yano et al. [17]. The crystal growthand laser fabrication the optical source. The embedded mesa stripe laser operates in the fundamental procedure were described in detail. mode in both directions, perpendicularand parallel tothe The threshold current and external quantum efficiency were junction plane. The mode patterns remain unchanged up to measured at different heat-sink temperatures. The value of To power levels of more than 10 mW. for this laser was found to be 60°C. The differential external The near field in the direction parallel to the junctionplane quantum efficiency is very sensitive to device temperature. is shown in Fig. 7 for five different injection levels. The nearThe device operates in the fundamental transverse mode up field full width at half maximum is 3 pm, and remains con- to powerlevelsof more than 10 mW. Far-field beam diverstant with varying injection. The far-field parallel and perpen- gences of 12-15' and 35-40" weremeasured in directions dicular to the junction piane is shown in Fig. 8(a) and (b), parallel and perpendicular to the junction plane, respectively. respectively. The beam divergences are 12-15' and 35-40" in Almost single-longitudinal mode operation was obtained for the direction parallel and perpendicular to the junction plane, currents above threshold.

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The device properties under high injection levels were also investigated. A saturation in output power was observed for power levels above 25 mW. We believe that this was caused due to high sensitivity of the external efficiency to the device temperature. ACKNOWLEDGMENT We wish to thank J . L. Gonqalves, A. Celso Ramos, A. A. G. Von Zuben, and P. C. Silva for help during the laser fabrication, and Dr. W. Ruhle and Dr. J. Harris, Jr. for useful discussions. REFERENCES [l] A. P. Bogatov, L. M. Dolginov, L. V. Druzhinina, P. G. Eliseev, B.N. Sverdlov, and E. G.Shevchenko,“Heterojunction lasers made of GaxInl-xAsyP1-y and AlxGal-xSbyAsl-y solid solutions,” Sov.J. Quantum Electron.,vol. 4,pp. 1281, Apr. 1975. [2] J. J. Hsieh, “Room temperature operation of GaInAsP/InP double-heterostructure diode laser emitting at 1.1 pm,” Appl. Phys. Lett.,vol. 28,pp. 283-285, Mar. 1976. [ 31 C. C. Shen, J. J. Hsieh, and T. A. Lind, “1500 h continuous CW operation of double-heterostructure GaInAsP/InP lasers,” Appl. Phys. Lett.,vol. 30, pp. 353-354, June 1976. [4] T. Yamamoto, K. Sakai, and S. Akiba, “10 000 h continuous CW operation of Inl-xGaxAsyPl-y/InP DH lasers at room temperapp. ture,” ZEEE J. Quantum Electron., vol. QE-15, . . 684-687, Aug. 1979. 151 J. Yamada. M. Saruwatari. K. Asatani, H. Tsuchiva. A. Kawana, K. Sugiyama, and T. Kimura, IEEE J. Quantum-Electron., vol: QE-14, pp. 791-799, NOV.1978. [6] T. Ito, K. Nakagawa, S. Shimada, K. Ishihara, Y. Ohmorp, and K. Sugiyama, in h o c . Znt. Conf. Topical Meeting Opt. Fiber Commun., 1979. [7] K. Aiki, M. Nakamura, T. Kuroda, J. Umeda, R. Ito, N. Chinone, injection and M. Maeda, “Transverse mode stabilized A1,Gal,As lasers with channeled-substrate planar structure,” IEEE J. Quantum Electron., vol. QE-14, pp, 89-94, Feb. 1978. [8] H. Kano, K. Oe, S. Ando, and K. Sugiyama, “Buried stripe GaInAsPlInP DH laser preparedby using melt-back method,” Japan. J. Appl. Pkys.,vol. 17,pp. 1887-1888, Oct. 1978. [9] H. Kano and K. Sugiyama, “Operation characteristics of buriedstripe GaInAsP/InP DH lasers madeby melt-back method,” J . Appl. Phys., vol. 50, pp. 7934-7938, Dec. 1979. [lo] K. Nakajima, T. Kusunoki, K. Akita, and T. Kotami, “Phase diagram of In-Ga-As-P quaternary system and LPE growth conditionsforlattice matching on InP substrates,” J. Electrockem. Soc.,vol. 125,pp. 123-127, Jan. 1978. [ l l ] L. R. Dawson, “Near-equilibrium LPE growth of GaAsdouble-heterostructures,” J. OystalGrowth, vol. Gal-,A1,As 27, pp. 86-96,1974. [ 121 F. R. Mash and J. J. Coleman, “Zinc contamination and misplaced p-n junctions in InP-GaInPAs DH lasers,” Electron. Lett., vol. 14, pp. 558-559, Aug. 1978. [ 131 J. W. Colby, in Proc. 6th Nat. Conf. Electron Probe Analysis, vol. 17,1971. [14] S. L. Chang, N. B. Patel, Y. Nannichi, and F. C. Prince, “Determination of lattice mismatch in Gal-,AlxAs on GaAs substrate by using a divergent x-ray source,” J. Appl. Phys., vol. 50, pp. 2975-2976, Apr. 1979. [IS] F. C . Prince, N. B. Patel, and D. J. Bull, “(Inca) (AsP)/InP embedded mesa stripe lasers,” Appl. Phys. Lett., vol. 35,pp. 577-580, Oct. 1979.

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[16] K. Aiki, M. Nakamura, T. Kuroda,and J. Umeda, “Channeledsubstrate planar structure (A1Ga)As injection lasers,” Appl. Pkys. Lett.,vol. 30, pp. 649-651, June 1977. [ 171 M. Yano, H. Nishi, and M. Takusagawa, “Oscillation characteristics in InGaAsP/InP DH lasers with self-aligned structure,” IEEE J. Quantum Electron.,vol. QE-15, pp. 1388-1395, Dec. 1979. [ 181 T. Kobayashi and Y. Furukawa, “Recombination enhanced annealing effect in AlGaAs/GaAs remote junction lasers,” ZEEE J. Quantum Electron., vol. QE-15, pp. 674-684, Aug. 1979. [ 191 J. J. Hsieh, “Zn-diffused, stripe geometry, double-heterostructure GaInAsP/InP diode lasers,” IEEE J. Quantum Electron., vol. QE15, pp. 694-697, Aug. 1979. [ 201 H. Namizaki, “Transverse-junction-stripe lasers with a GaAs p-n homojunction,” IEEE J. Quantum Electron., vol. QE-11, pp. 427-431, July 1975. [21] Y . Itaya, Y. Suematsu, S. Katayama, K. Kishino, and S. Arai, “Low threshold current density (100) GaInAsP/InP DH lasers,” Japan. J. Appl. Pkys.,vol. 18,pp. 1795-1805, Sept. 1979. [22] W. T. Tsang and R. A. Logan, “GaAs-AlxGa1,As strip buried heterostructure lasers,” IEEE J. Quantum Electron., vol. QE-15, pp. 451-469, June 1979. 1231D. Botez, “Single-mode cw operation of “double-dovetail’’ constricted DH(A1Ga)As diode lasers,” Appl. Pkys. Lett., vol. 33, pp. 872-874, NOV. 1978.