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High performance AlGaAs-based laser diodes: Fabrication, characterization and applications
Microelectronies Journal 29 (1998) 97-104 © 1998 Published by Elsevier Science Limited Printed in Great Britain. All rights reserved 0026-2692/98/$19.00
ELSEVIER
i~i~il ~ii PH:SOO26-2692(97)OOO27-X
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High performance AIGaAs-based laser diodes: fabrication, characterization and applications V. lakovlev*, A. Sarbu, A. Mereutza, G. Suruceanu, A. Caliman, O. Catughin, A. Lupu and S. Vieru Optoelectronics Centre of Technical University of Moldova, 168 Stefan cel Mare, Chisinau, 277012, Moldova
Data are presented on high performance A1GaAs/GaAs and InGaAs/A1GaAs quantum well laser diodes fabricated by low temperature liquid phase mesa melt-etching and regrowth. The base laser structures were grown either by molecular beam epitaxy or metal-organic vapour phase epitaxy MOVPE. Native oxides were used as a mask in the melt-etcl~ting and regrowth processes. Cylindrical microlenses were used for obtaining equal divergence angles in both planes, perpendicular and parallel to the active layer plane at more than 1 W of continuous wave operation o p t i c a l power. Characterization of mirror catastrophical degradation of single mode laser diodes using traditional electrooptical parameters as well as the parameters of internal secondharmonic generation and measurements of excess mirror temperature were performed. The thermal resistance, active layer mirror temperature and spectral width have been determined as well. © 1998 Published by Elsevier Science Ltd. *Author to whom correspondence should be addressed. Tel: (373)-2-248133. E-mail: [email protected].
1. Introduction o w temperature (<900K) mesa meltetching and regrowth proved to be very efficient for high performance buried heterostructure laser diode fabrication [1-3]. L o w threshold (Ith<5 mA) laser diodes were fabricated using molecular beam epitaxy (MBE)-grown strained InGaAs/A1GaAs (2--980 nm) and metalorganic chemical vapour deposition ( M O C V D ) - g r o w n unstrained AIGaAs/GaAs (2=810nm) epitaxial structures [4]. Degradation o f these laser diodes is caused by physicochemical processes that take place on the mirrors during operation and are enhanced by high optical power densities and increased temperatures [5]. Here we present data on electrooptical and thermal properties and characterization o f degradation processes o f these lasers. The degradation process was accelerated by
L
97
V. lacovlev et al./High performance AIGaAs-based laser diodes
single current pulses. Electrooptical and internal second-harmonic (SH) generation measurements were performed at different degradation steps. The mirror temperature values were measured by microphotoluminescense [6] and photothermodeflection methods [7]. Excess mirror temperature values of 4 and 10K were determined for 980nm InGaAs/AlGaAs and 810nm AlGaAs/GaAs, respectively, for laser diodes operating at 60 mA current and 15 m W per facet. In this paper we also present the results on optical and thermal characteristics studies of 960 nm strained quantum well continuous wave (CW) AlGaAs/InGaAs buried heterostructure lasers with 0.1 mm active layer widths and 0.95 m m cavity lengths, emitting 1 W and more optical power. Far-field patterns of these laser diodes were modified using cylindrical microlenses. Laser diodes operated continuously at 1W optical power for 1000 h with no essential degradation. 2. Fabrication
Single spatial mode laser diodes with threshold current as low as 3-5 mA at 3/lm active layer width and 0.4-1.3 mm cavity length were fabricated using selective mesa melt-etching and liquid phase epitaxial (LPE) regrowth [1-3]. In the present work the quality of buried heterostructures was improved considerably by using AlGa_As native oxides as a mask in melt-etching and regrowth LPE processes [4]. As reported earlier [8], melt-etched mesa shapes depend on the mask adhesiveness to the epitaxial structure in undersaturated Ga-Al-As solutions at temperatures around 900 K. Besides the fact that adhesion of traditional masking layers such as SiO2 and Al203 is much worse than that of A1GaAs native oxide, the adhesion of these oxide layers is not
98
uniform and depends very much on the state of the multilayer epitaxial structure surface leading to non-reproducible results. The growth mechanism of native oxides provides high material quality and uniform interface with no impurities on it. Native oxide layers were grown by anodic oxidation of AlxGal_xAs (x=0.5-0.6) in ammonium citrate electrolyte. This oxide is stabilized to resist harsh conditions of melt-etching by postgrowth thermal treatment. It was shown that the masking properties of AlGaAs-based native oxide improve dramatically by increasing the MAs composition of MGaAs. In order to form such a mask an additional A1GaAs layer with a thickness of 200-300nm is grown on the surface of the multilayer structure. To form a necessary mask pattern this layer is anodized through a photoresist mask. When using A1GaAs-based native oxide mask no undercutting under the mask was observed in the low temperature LPE mesa melt-etching. The anisotropy and selectivity of melt-etching ensure that the active layer area is about two times smaller than the contact area. As a result the contact resistivity can be minimized. Only masks of very good quality permit the formation of these reverse mesa structures. Mirrors of some laser diodes were first coated with epitaxial ZnSe with a thickness of 120nm [9], then with highly reflective (HR) and antireflective (AR) Si-SiO2-based layers. Reflectivity of laser diode mirrors was 0.05 for front and 0.9 for rear mirrors. Laser chips were mounted on copper heat sinks p-side down. In order to reduce the beam divergence in the plane perpendicular to the active layer plane a silica cylindrical microlens with a diameter of 0.2mm and 5 m m length was fixed in the nearest vicinity (0.04-0.1 mm) of the laser diode mirror in such a way that the laser diode axis and the microlens axis intersected at a right angle. Epoxy was used to fix the microlens [10].
Microelectronics Journal, Vol. 29, No. 3
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i
i
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tmA Fig. 2. Light-current characteristics of InGaAs/A1GaAs (A=980nm) strained quantum well active region uncoated laser diodes for different cavity lengths: (a) 0.6mm; (b) 0.9 mm; (c) 1.3 mm.
3. Characterization Light-current characteristics o f InGaAs/A1GaAs and AIGaAs/GaAs B H uncoated laser diodes (LDs) were measured in a current range that was close to but did :not exceed the limit o f catastrophic optical degradation (COD) (Fig. 1, unstrained quantum well active region, 2=810nm; Fig. 2, strained quantum well active region, 2=980 nm). The devices operate at high marror power densities (up to 1 0 m W per micron o f active layer width for 810 n m LD and up to 20 m W per rrficron o f active layer width for 980 n m LD). A typical near-field pattern o f these LDs is presented in Fig. 3, curve a. Nearfield and far-field patterns were stable up to the degradation limit. Shown in Fig. 4 are the typical active layer temperature versus driving current for InGaAs and A1GaAs laser,;, determined using the method described by Paoli [11]. We attribute the better thermal behaviour o f 980 n m InGaAs/ A1GaAs to the fac,: that both electrical and thermal resistances o f these lasers are lower than respective values fbr 8 1 0 n m A1GaAs/GaAs
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Fig. 3. Typical near-field pattern of laser diodes. (a, b) Fundamental harmonic (FH) radiation (a) before and (b) after COD. (c) Second-harmonic radiation after COD. lasers. Laser mirror temperatures for 9 8 0 n m InGaAs/A1GaAs lasers were determined by the microphotoluminescence method [6]. The results are presented in Fig. 5. It is important to note that laser diodes which have practically the same light-current (P-/) and active layer temperature Tal versus current (Tal-/) characteristics have different mirror temperature versus current characteristics. W e presume that mirror temperatures o f uncoated laser diodes are depen-
99
V. lacovlev et al./High performance AIGaAs-based laser diodes
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Fig. 5. Mirror temperatures versus driving current for three InGaAs/A1GaAs(A=980nm) laser diodes which have practically the same light-current (P-/) and active layer temperature Tal versus current (Tal-/) characteristics.Note different mirror temperatures for different laser diodes at the same current value and optical power.
dent on the mirror surface state densities, which are different for different laser diodes. This parameter does not influence considerably either P I or T~1-1characteristics. Indirect bandgap zone structure o f cladding layers in 8 1 0 n m AlGaAs/GaAs lasers makes it very difficult to perform microphotoluminescence measurements. Mirror temperatures for such laser diodes were measured by a photothermodeflection technique [7]. The principle o f this method is very simple. The deflection angle o f a
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Fig. 6. Plots of active layer temperature (solid line) and mirror temperature (dashed line, determined from photothermal deflection angle data) versus driving current for MGaAs/GaAs (A=810nm) unstrained quantum well active region uncoated laser diodes with 0.5 mm cavitylength. probe laser beam passing in the nearest vicinity o f a laser mirror is proportional to the mirror temperature. The higher mirror temperature results in a higher deflection angle. The problem with this method is the correct calibration. All our previous measurements o f mirror and active layer temperatures have shown that at low current values near the threshold these temperatures practically do not differ. This fact was used for the calibration o f the phototherrnodeflection method. Shown in Fig. 6 are the plots o f active layer temperature and photothermal deflection angle data versus driving current. As we can see in Figs 5 and 6, the difference between mirror temperature Tm versus driving current (Tm-/) characteristics o f InGaAs/ AlGaAs and AlGaAs/GaAs lasers is that AlGaAs/ GaAs lasers have a superlinear (Tm-/) characteristic. This superlinear dependence o f mirror temperature versus driving current explains the m u c h lower C O D limit o f AlGaAs/GaAs lasers comparative to InGaAs/A1GaAs strained quantum well laser diodes. The weak SH radiation at the frequency 2co that can be observed in the output beam spectrum along with the fundamental frequency co originates in a thin near-surface layer, adjacent to the mirror facet o f a laser. As we have reported
Microelectronics Journal, Vol. 29, No. 3
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120
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Current puls amplitude, mA
Fig. 7. The impact of s:ingle electrical current pulses on LD efficiency (solid line) and SH power (dotted line, arbitrary units), for an unstrained A1GaAs active layer LD emitting at 810nm (LD1). The fundamental harmonic output power of LD1 was maintained at a constant value of 10 mW per facet.
earlier [12, 13], the :intensity of SH radiation is indicative of the laser diode working regime: the bigger SH intensity corresponds to increased optical power density on the mirror and greater risk of COD. The :moment of transition from t h e gradual degradation to C O D in C W laser diodes is very difficult to predict. We use single current pulse technique as a procedure of LD degradation study [14]. Short current pulses (z
The next 140mA pulse caused a factor of three decrease of efficiency, NF pattern alteration (Fig. 3, curve b) and two times SH power increase. Instead of the central lobe in the nearfield distribution at the beginning of study (Fig. 3, curve a), a minimum appeared on this distribution after degradation (curve b). Note that the SH NF pattern (Fig. 3, curve c) reflects more evidently the non-uniform field distribution in a laser waveguide than the FH NF pattern. The next single current pulse with an amplitude of 150mA caused COD. As a result of the degradation black spots appeared on both laser mirrors. For LD2 a C W FH power of 15 m W per facet was maintained during the study. Three single current pulses from 0.1 to 0.25 A did not change the value of SH signal or laser diode efficiency (Fig. 8), or the nearfield pattern. A 0.3A current pulse caused a small decrease of efficiency and increasing SH power. After the 0.5A pulse LD did not generate at all. Both investigated LD chips after failure were cleaved into three parts: one from the central part of the initial chip and two from the side parts with cavity length of about 0.1 mm The central part of the chips were operable again, while the side ones did not generate at all. Considerable increasing of the intensity of SH signal before the catastrophic optical degradation of the laser mirrors is caused by the appearance of the non-uniformity in the near-field distribution of FH emission. As one can s e e in Fig. 3 the impact of single electrical pulses on an operating laser diode results in near-field alterations. A dark spot is formed on the laser diode mirror. The area of the luminous body decreases and, at a constant emitting power, the optical power density on the mirror increases. Knowing that the SH intensity is proportional to the square of FH intensity [12, 13], the reason why SH intensity increases in the process of laser diode gradual degradation becomes clear. The light-current characteristic of laser diodes with 0.1ram active layer widths and 0.95mm
101
V. lacovlev et al./High performance AIGaAs-based laser diodes
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Fig. 8. The impact of single electrical LD efficiency (solid line) and SH power trary units) for a strained InGaAs active at 980nm (LD2). The fundamental
current pulses on (dashed line, arbilayer LD emitting harmonic output
power of LD2 was maintained at a constant value of 15 mW per facet.
cavity length is linear from a threshold current of 1 8 0 m A up to 1 . 7 W optical power at 2A. Series resistance of these lasers was found to be 0.30.5~2. Thermal resistance typically did not exceed the value of 5 K / W , and minimal measured value was 1.6 K/W. In order to obtain non-absorbing mirrors, both laser diode facets were coated with epitaxial ZnSe [9]. In the case of wide stripe lasers far-field pattern asymmetry is quite large and results in many difficulties w h e n these lasers are applied for pumping solid state lasers. In order to overcome this problem we used silica microlenses that were properly aligned and fixed in the vicinity of laser diode mirrors. Far-field pattern distributions in the planes perpendicular (a) and parallel (b) to the active layer plane of a laser diode with 0.1 m m active layer width are shown in Fig. 9. After fixing the microlens the far-field distribution in the parallel plane practically does not change, while the perpendicular distribution changes dramatically (c) and is nearly the same as (b). Optical power measurements have shown that 80% of all optical power is concentrated in a cone with an angle o f 6 °.
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Angle, deg. Fig. 9. Far-field pattern in the planes (a) perpendicular and (b, c) parallel to the active layer plane (b) before and (c) after fixing the microlens.
A cylindrical microlens, placed in the nearest vicinity of a laser diode mirror, has a strong effect on the emission spectra of laser diodes. Without a microlens the full width at half amplitude (FWHA) of laser diode emission spectra at 0 . 2 W emitting power was 1.4nm, while F W H A of emission spectra after a properly placed cylindrical microlens is three times narrower, 0.4nm. We attribute this spectral narrowing effect to lateral mode suppression in a coupled cavity, formed by a concave surface o f a microlens and a laser diode mirror [15]. It is interesting to note that the spectral width has its minimal value at the same distance o f the microlens from the laser mirror at which symmetrical far-field distribution of the laser beam is obtained. Excess mirror temperatures relative to the bulk temperature of the laser diode have been determined either from the shift of microphotoluminescence spectra of the mirror facets [6] or from measurements of deflection angles. T h e excess mirror temperature value o f 10 K, determined from microphotoluminescence distribution at 1 W emitted power, was used as a reference point for calibrating the photothermodeflection data [10]. Because the laser diodes studied in this paper did not show
Microelectronics Journal, Vol. 29, No. 3
any substantial degradation during 1000h o f continuous operation, we may conclude that excess mirror temperatures o f the order o f 10 K do not cause catastrophic optical degradation.
4. Applications A1OaAs laser diodes with emission wavelength around 810 n m and emitting single spatial mode have been successfully used for pumping N d doped optical fibre lasers. Laser diode mirrors were first coated with epitaxial ZnSe with a thickness o f 120 nm then with H R and AP,. SiSiO2-based layers. This procedure allow us to increase the emitting power per facet up to 100 mW. This optical power was sufficient to pump a N d - d o p e d single mode fibre laser with a length o f 3 m. Art InGaAs/MGaAs laser with an emission wavelength o f 950 n m and optical power o f 2 0 - 5 0 r o W at current lower than 100mA was used to fabricate portable laser therapy apparatus that has been used successfully in urology, dermatology, stomatology, etc.
5. Summary The quality o f A1GaAs-based buried heterostructure lasers has been improved considerably by using an A1GaAs native oxide mask for in situ mesa melt-etching and LPE regrowth. In this case there is no undercutting and the shape o f the mesas is dictated only by anisotropy o f melt-etching. Uncoated buried heterostructure laser diodes with an active layer width o f 3/lm and less emit in a single spatial mode. The m a x i m u m optical power densities are 10 m W per micron o f active layer width for AIGaAs/GaAs lasers and 20 m W per micron o f active layer width for InGaAs/ A1GaAs lasers. The higher optical power o f InGaAs/A1GaAs lasers is attributed to lower electrical and thermal resistances and to linear mirror temperature versus current characteristics. Internal second-harmonic generation studies have confirmed that: A1GaAs-based laser diodes
degrade because o f the mirror damage at high optical power densities and increased temperature. A1GaAs/InGaAs strain quantum well buried heterostructure laser diodes emitting at 960 n m with an active layer width 0.1 mm, small electrical (0.3 f~) and thermal (1.6-5K/W) resistivities, high efficiency (0.7 W/A), symmetrical farfield pattern (8 ° F W H A in both directions) and narrow spectral width (0.4nm FWHA) have been obtained. Buried heterostructure A1GaAs-based laser diodes have been used successfully in pumping optical fibre lasers and in medicine.
References [1] Alferov, Zh.I., Andreyev, V.M., Syrbu, A.V., Mereutza, A.Z. and Yakovlev, V.P. Extremely low threshold A1GaAs buried heterostructure quantum well laser diodes grown by LPE, Appl. Phys. Lett., 57 (1990) 2873-2875. [2] Chand, N., Chu, S.N.G., Dutta, N.K., Lopata, J., Geva, M., Syrbu, A.V., Mereutza, A.Z. and Yakovlev, V.P. Growth and fabricationof high performance 980-nm strained InGaAs quantum well lasers for erbium doped fibre amplifiers, IEEE J. Quantum Electron., 302 (1994) 424-440. [3] Syrbu, A.V., Mereutza, A.Z., Yakovlev, V.P., Suruceanu, G.I. and Lupu, A.T. Buried heterostructure A1GaAs photonic devices fabricatedby low temperature mesa melt etching and regrowth, J. Electron. Mater., 5 (1994) 710-714. [4] Iakovlev, V. High performance A1GaAs-based laser diodes: fabrication, characterisationand applications, Proc. 20th Int. Conf. on Microelectronics (MIEL'95), Vol. 1, Nis, Serbia, 12-14 September 1995, pp. 435-439. [5] Epperlein, P.W. Microtemperature measurement on semiconductorlaser mirrors by reflectancemodulation a newly developedtechnique for laser characterisation, Jpn.J. Appl. Phys. Part 1, 3212 (1993) 5514-5522. [6] Katsavetz, N.I., Garbuzov, D.Z., Grishina, T.A., Kudrik, I.E. and Pitkianen, P.V. Study of correlation between optical characteristics and mirror facet temperature of the active region in high power SCH SQW InGaAs/GaAs and InGaAs/GaAs laser diodes, in P.C. Chen, L.A. Johnson and H. Temkin (eds), Proc. SPIE 2148 Laser diode technology and Applications
V/, 1994, pp. 152-158.
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[7] Bertolotti, M., Liakhou, G.L., Li Voti, R., Wang, R.P., Sibilia, C., Syrbu, A. and lakovlev, V.P. An experimental and theoretical analysis of the temperature profile in semiconductor laser diodes using the photothermodeflection technique, Meas. Sci. Technol., 6 (1995) 1278-1290. [8] Mereutza, A.Z., Syrbu, A.V. and Yakovlev, V.P. Buried heterostructure formation processes for high performance devices, Proc. 15th Annual Semiconductor Conf., Sinaia, Romania, October 1992. [9] Syrbu, A.V., Yakovlev, V.P., Suruceanu, G.I., Mereutza, A.Z., Mawst, L.J., Bhattacharya, A., Nesnidal, M., Lopez, J. and Botez, D. ZnSe-facetcoated InGaAs/InGaAsP/InGaP diode lasers of high CW power and 'wallplug' efficiency, Electron. Lett. 32 (1996) 352-354. [10] Syrbu, A., Mereutza, A., Suruceanu, G., Iakovlev, V., Vieru, S., Catughin, O., Predescu, M., Popescu, I. and Ispasoiu, R. AIGaAs/InGaAs buffed heteros~ucture laser diodes for pumping solid state lasers, Proc. 1995 Int. Semiconductor Conf., Sinaia, pp. 453-456. [11] Paoli, T.L. A new technique for measuring the thermal impedance of junction lasers, IEEEJ. Quantum Electron., QE-11 (1975) 498-503.
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[12] Kravetsky, I.V., Kulyuk, L.L., Lupu, A.T., Shutov, D.A., Suruceanu, G.I., Syrbu, A.V. and Yakovlev, V.P. Internal second harmonic generation in CW A1GaAs SQW lasers: the facet degradation monitoring, AppI. Surf. Sci., 69 (1993) 424-428. [13] Yakovlev, V.P., Lupu, A.T., Syrbu, A.V., Suruceanu, G.I., Kravetsky, I.V., Kulyuk, L.L. and Chand, N. Characterisation of strained quantum well InGaAs/ A1GaAs buried heterostructure lasers using internal second-harmonic generation, in P.C. Chen, L.A. Johnson and H. Temkin (eds), Proc. SPIE 2148, Laser Diode Technology and Applications VI, 1994, pp. 440448. [14] Yakovlev, V.P., Lupu, A.T., Syrbu, A.V., Mereutza, A.Z., Kravetsky, I.V. and Kulyuk, L.L. Characteffsation of the degradation processes in the buffed heterostructure quantum well laser diodes using internal second harmonic emission, Presented at MRS Fall Meeting, Boston, 28 November-2 December 1994. [15] Petermann, K. External optical feedback phenomena in semiconductor lasers, IEEE J. Selected Topics Quantum Electron., 12 (1995) 480-489.
ELSEVIER
i~i~il ~ii PH:SOO26-2692(97)OOO27-X
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High performance AIGaAs-based laser diodes: fabrication, characterization and applications V. lakovlev*, A. Sarbu, A. Mereutza, G. Suruceanu, A. Caliman, O. Catughin, A. Lupu and S. Vieru Optoelectronics Centre of Technical University of Moldova, 168 Stefan cel Mare, Chisinau, 277012, Moldova
Data are presented on high performance A1GaAs/GaAs and InGaAs/A1GaAs quantum well laser diodes fabricated by low temperature liquid phase mesa melt-etching and regrowth. The base laser structures were grown either by molecular beam epitaxy or metal-organic vapour phase epitaxy MOVPE. Native oxides were used as a mask in the melt-etcl~ting and regrowth processes. Cylindrical microlenses were used for obtaining equal divergence angles in both planes, perpendicular and parallel to the active layer plane at more than 1 W of continuous wave operation o p t i c a l power. Characterization of mirror catastrophical degradation of single mode laser diodes using traditional electrooptical parameters as well as the parameters of internal secondharmonic generation and measurements of excess mirror temperature were performed. The thermal resistance, active layer mirror temperature and spectral width have been determined as well. © 1998 Published by Elsevier Science Ltd. *Author to whom correspondence should be addressed. Tel: (373)-2-248133. E-mail: [email protected].
1. Introduction o w temperature (<900K) mesa meltetching and regrowth proved to be very efficient for high performance buried heterostructure laser diode fabrication [1-3]. L o w threshold (Ith<5 mA) laser diodes were fabricated using molecular beam epitaxy (MBE)-grown strained InGaAs/A1GaAs (2--980 nm) and metalorganic chemical vapour deposition ( M O C V D ) - g r o w n unstrained AIGaAs/GaAs (2=810nm) epitaxial structures [4]. Degradation o f these laser diodes is caused by physicochemical processes that take place on the mirrors during operation and are enhanced by high optical power densities and increased temperatures [5]. Here we present data on electrooptical and thermal properties and characterization o f degradation processes o f these lasers. The degradation process was accelerated by
L
97
V. lacovlev et al./High performance AIGaAs-based laser diodes
single current pulses. Electrooptical and internal second-harmonic (SH) generation measurements were performed at different degradation steps. The mirror temperature values were measured by microphotoluminescense [6] and photothermodeflection methods [7]. Excess mirror temperature values of 4 and 10K were determined for 980nm InGaAs/AlGaAs and 810nm AlGaAs/GaAs, respectively, for laser diodes operating at 60 mA current and 15 m W per facet. In this paper we also present the results on optical and thermal characteristics studies of 960 nm strained quantum well continuous wave (CW) AlGaAs/InGaAs buried heterostructure lasers with 0.1 mm active layer widths and 0.95 m m cavity lengths, emitting 1 W and more optical power. Far-field patterns of these laser diodes were modified using cylindrical microlenses. Laser diodes operated continuously at 1W optical power for 1000 h with no essential degradation. 2. Fabrication
Single spatial mode laser diodes with threshold current as low as 3-5 mA at 3/lm active layer width and 0.4-1.3 mm cavity length were fabricated using selective mesa melt-etching and liquid phase epitaxial (LPE) regrowth [1-3]. In the present work the quality of buried heterostructures was improved considerably by using AlGa_As native oxides as a mask in melt-etching and regrowth LPE processes [4]. As reported earlier [8], melt-etched mesa shapes depend on the mask adhesiveness to the epitaxial structure in undersaturated Ga-Al-As solutions at temperatures around 900 K. Besides the fact that adhesion of traditional masking layers such as SiO2 and Al203 is much worse than that of A1GaAs native oxide, the adhesion of these oxide layers is not
98
uniform and depends very much on the state of the multilayer epitaxial structure surface leading to non-reproducible results. The growth mechanism of native oxides provides high material quality and uniform interface with no impurities on it. Native oxide layers were grown by anodic oxidation of AlxGal_xAs (x=0.5-0.6) in ammonium citrate electrolyte. This oxide is stabilized to resist harsh conditions of melt-etching by postgrowth thermal treatment. It was shown that the masking properties of AlGaAs-based native oxide improve dramatically by increasing the MAs composition of MGaAs. In order to form such a mask an additional A1GaAs layer with a thickness of 200-300nm is grown on the surface of the multilayer structure. To form a necessary mask pattern this layer is anodized through a photoresist mask. When using A1GaAs-based native oxide mask no undercutting under the mask was observed in the low temperature LPE mesa melt-etching. The anisotropy and selectivity of melt-etching ensure that the active layer area is about two times smaller than the contact area. As a result the contact resistivity can be minimized. Only masks of very good quality permit the formation of these reverse mesa structures. Mirrors of some laser diodes were first coated with epitaxial ZnSe with a thickness of 120nm [9], then with highly reflective (HR) and antireflective (AR) Si-SiO2-based layers. Reflectivity of laser diode mirrors was 0.05 for front and 0.9 for rear mirrors. Laser chips were mounted on copper heat sinks p-side down. In order to reduce the beam divergence in the plane perpendicular to the active layer plane a silica cylindrical microlens with a diameter of 0.2mm and 5 m m length was fixed in the nearest vicinity (0.04-0.1 mm) of the laser diode mirror in such a way that the laser diode axis and the microlens axis intersected at a right angle. Epoxy was used to fix the microlens [10].
Microelectronics Journal, Vol. 29, No. 3
b
120
a
80 E
40
10
/// ~f-( '
0
I
_ ,
0
i
l
50
J
,
J
I
0
100
I, mA Fig. 1. Lfight-current characteristics of A1GaAs/GaAs (A=810nm) unstrained ,quantum well active region uncoated laser diodes for different cavitylengths: (a) 0.8 ram; (b) 0.5 ram; (c) 0.35 mm.
i
i
100
150
0 50
200
tmA Fig. 2. Light-current characteristics of InGaAs/A1GaAs (A=980nm) strained quantum well active region uncoated laser diodes for different cavity lengths: (a) 0.6mm; (b) 0.9 mm; (c) 1.3 mm.
3. Characterization Light-current characteristics o f InGaAs/A1GaAs and AIGaAs/GaAs B H uncoated laser diodes (LDs) were measured in a current range that was close to but did :not exceed the limit o f catastrophic optical degradation (COD) (Fig. 1, unstrained quantum well active region, 2=810nm; Fig. 2, strained quantum well active region, 2=980 nm). The devices operate at high marror power densities (up to 1 0 m W per micron o f active layer width for 810 n m LD and up to 20 m W per rrficron o f active layer width for 980 n m LD). A typical near-field pattern o f these LDs is presented in Fig. 3, curve a. Nearfield and far-field patterns were stable up to the degradation limit. Shown in Fig. 4 are the typical active layer temperature versus driving current for InGaAs and A1GaAs laser,;, determined using the method described by Paoli [11]. We attribute the better thermal behaviour o f 980 n m InGaAs/ A1GaAs to the fac,: that both electrical and thermal resistances o f these lasers are lower than respective values fbr 8 1 0 n m A1GaAs/GaAs
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2
um
Fig. 3. Typical near-field pattern of laser diodes. (a, b) Fundamental harmonic (FH) radiation (a) before and (b) after COD. (c) Second-harmonic radiation after COD. lasers. Laser mirror temperatures for 9 8 0 n m InGaAs/A1GaAs lasers were determined by the microphotoluminescence method [6]. The results are presented in Fig. 5. It is important to note that laser diodes which have practically the same light-current (P-/) and active layer temperature Tal versus current (Tal-/) characteristics have different mirror temperature versus current characteristics. W e presume that mirror temperatures o f uncoated laser diodes are depen-
99
V. lacovlev et al./High performance AIGaAs-based laser diodes
35
28 27
o a0!
26 0
L~
20 ~
I- 23 22
0
20
40
60
~._
0
50
100
180
I, m A
Fig. 4. Typical active layer temperature versus driving current for InGaAs (solid line b) and A1GaAs (dashed line a) lasers.
a
70 [
60 [
....
50
....... "
20
0
50
100
I, mA
Fig. 5. Mirror temperatures versus driving current for three InGaAs/A1GaAs(A=980nm) laser diodes which have practically the same light-current (P-/) and active layer temperature Tal versus current (Tal-/) characteristics.Note different mirror temperatures for different laser diodes at the same current value and optical power.
dent on the mirror surface state densities, which are different for different laser diodes. This parameter does not influence considerably either P I or T~1-1characteristics. Indirect bandgap zone structure o f cladding layers in 8 1 0 n m AlGaAs/GaAs lasers makes it very difficult to perform microphotoluminescence measurements. Mirror temperatures for such laser diodes were measured by a photothermodeflection technique [7]. The principle o f this method is very simple. The deflection angle o f a
100
~"
I, m A
21
0
i
25
25 24
20
J
Fig. 6. Plots of active layer temperature (solid line) and mirror temperature (dashed line, determined from photothermal deflection angle data) versus driving current for MGaAs/GaAs (A=810nm) unstrained quantum well active region uncoated laser diodes with 0.5 mm cavitylength. probe laser beam passing in the nearest vicinity o f a laser mirror is proportional to the mirror temperature. The higher mirror temperature results in a higher deflection angle. The problem with this method is the correct calibration. All our previous measurements o f mirror and active layer temperatures have shown that at low current values near the threshold these temperatures practically do not differ. This fact was used for the calibration o f the phototherrnodeflection method. Shown in Fig. 6 are the plots o f active layer temperature and photothermal deflection angle data versus driving current. As we can see in Figs 5 and 6, the difference between mirror temperature Tm versus driving current (Tm-/) characteristics o f InGaAs/ AlGaAs and AlGaAs/GaAs lasers is that AlGaAs/ GaAs lasers have a superlinear (Tm-/) characteristic. This superlinear dependence o f mirror temperature versus driving current explains the m u c h lower C O D limit o f AlGaAs/GaAs lasers comparative to InGaAs/A1GaAs strained quantum well laser diodes. The weak SH radiation at the frequency 2co that can be observed in the output beam spectrum along with the fundamental frequency co originates in a thin near-surface layer, adjacent to the mirror facet o f a laser. As we have reported
Microelectronics Journal, Vol. 29, No. 3
0,6 0,5 ! 0,4 ,_ 0,3
~
0,2[ ~O,1 0
100
120
140
Current puls amplitude, mA
Fig. 7. The impact of s:ingle electrical current pulses on LD efficiency (solid line) and SH power (dotted line, arbitrary units), for an unstrained A1GaAs active layer LD emitting at 810nm (LD1). The fundamental harmonic output power of LD1 was maintained at a constant value of 10 mW per facet.
earlier [12, 13], the :intensity of SH radiation is indicative of the laser diode working regime: the bigger SH intensity corresponds to increased optical power density on the mirror and greater risk of COD. The :moment of transition from t h e gradual degradation to C O D in C W laser diodes is very difficult to predict. We use single current pulse technique as a procedure of LD degradation study [14]. Short current pulses (z
The next 140mA pulse caused a factor of three decrease of efficiency, NF pattern alteration (Fig. 3, curve b) and two times SH power increase. Instead of the central lobe in the nearfield distribution at the beginning of study (Fig. 3, curve a), a minimum appeared on this distribution after degradation (curve b). Note that the SH NF pattern (Fig. 3, curve c) reflects more evidently the non-uniform field distribution in a laser waveguide than the FH NF pattern. The next single current pulse with an amplitude of 150mA caused COD. As a result of the degradation black spots appeared on both laser mirrors. For LD2 a C W FH power of 15 m W per facet was maintained during the study. Three single current pulses from 0.1 to 0.25 A did not change the value of SH signal or laser diode efficiency (Fig. 8), or the nearfield pattern. A 0.3A current pulse caused a small decrease of efficiency and increasing SH power. After the 0.5A pulse LD did not generate at all. Both investigated LD chips after failure were cleaved into three parts: one from the central part of the initial chip and two from the side parts with cavity length of about 0.1 mm The central part of the chips were operable again, while the side ones did not generate at all. Considerable increasing of the intensity of SH signal before the catastrophic optical degradation of the laser mirrors is caused by the appearance of the non-uniformity in the near-field distribution of FH emission. As one can s e e in Fig. 3 the impact of single electrical pulses on an operating laser diode results in near-field alterations. A dark spot is formed on the laser diode mirror. The area of the luminous body decreases and, at a constant emitting power, the optical power density on the mirror increases. Knowing that the SH intensity is proportional to the square of FH intensity [12, 13], the reason why SH intensity increases in the process of laser diode gradual degradation becomes clear. The light-current characteristic of laser diodes with 0.1ram active layer widths and 0.95mm
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V. lacovlev et al./High performance AIGaAs-based laser diodes
0,8
1
0,6
7 /' ;iI
.~ 0,4
,
k
IJ ~ 0,2
•i
•
II
/ ,1' b
0,5
0
0,2
0,4
0,6
Current pulse amplitude, A -40
Fig. 8. The impact of single electrical LD efficiency (solid line) and SH power trary units) for a strained InGaAs active at 980nm (LD2). The fundamental
current pulses on (dashed line, arbilayer LD emitting harmonic output
power of LD2 was maintained at a constant value of 15 mW per facet.
cavity length is linear from a threshold current of 1 8 0 m A up to 1 . 7 W optical power at 2A. Series resistance of these lasers was found to be 0.30.5~2. Thermal resistance typically did not exceed the value of 5 K / W , and minimal measured value was 1.6 K/W. In order to obtain non-absorbing mirrors, both laser diode facets were coated with epitaxial ZnSe [9]. In the case of wide stripe lasers far-field pattern asymmetry is quite large and results in many difficulties w h e n these lasers are applied for pumping solid state lasers. In order to overcome this problem we used silica microlenses that were properly aligned and fixed in the vicinity of laser diode mirrors. Far-field pattern distributions in the planes perpendicular (a) and parallel (b) to the active layer plane of a laser diode with 0.1 m m active layer width are shown in Fig. 9. After fixing the microlens the far-field distribution in the parallel plane practically does not change, while the perpendicular distribution changes dramatically (c) and is nearly the same as (b). Optical power measurements have shown that 80% of all optical power is concentrated in a cone with an angle o f 6 °.
102
-20
0
20
40
Angle, deg. Fig. 9. Far-field pattern in the planes (a) perpendicular and (b, c) parallel to the active layer plane (b) before and (c) after fixing the microlens.
A cylindrical microlens, placed in the nearest vicinity of a laser diode mirror, has a strong effect on the emission spectra of laser diodes. Without a microlens the full width at half amplitude (FWHA) of laser diode emission spectra at 0 . 2 W emitting power was 1.4nm, while F W H A of emission spectra after a properly placed cylindrical microlens is three times narrower, 0.4nm. We attribute this spectral narrowing effect to lateral mode suppression in a coupled cavity, formed by a concave surface o f a microlens and a laser diode mirror [15]. It is interesting to note that the spectral width has its minimal value at the same distance o f the microlens from the laser mirror at which symmetrical far-field distribution of the laser beam is obtained. Excess mirror temperatures relative to the bulk temperature of the laser diode have been determined either from the shift of microphotoluminescence spectra of the mirror facets [6] or from measurements of deflection angles. T h e excess mirror temperature value o f 10 K, determined from microphotoluminescence distribution at 1 W emitted power, was used as a reference point for calibrating the photothermodeflection data [10]. Because the laser diodes studied in this paper did not show
Microelectronics Journal, Vol. 29, No. 3
any substantial degradation during 1000h o f continuous operation, we may conclude that excess mirror temperatures o f the order o f 10 K do not cause catastrophic optical degradation.
4. Applications A1OaAs laser diodes with emission wavelength around 810 n m and emitting single spatial mode have been successfully used for pumping N d doped optical fibre lasers. Laser diode mirrors were first coated with epitaxial ZnSe with a thickness o f 120 nm then with H R and AP,. SiSiO2-based layers. This procedure allow us to increase the emitting power per facet up to 100 mW. This optical power was sufficient to pump a N d - d o p e d single mode fibre laser with a length o f 3 m. Art InGaAs/MGaAs laser with an emission wavelength o f 950 n m and optical power o f 2 0 - 5 0 r o W at current lower than 100mA was used to fabricate portable laser therapy apparatus that has been used successfully in urology, dermatology, stomatology, etc.
5. Summary The quality o f A1GaAs-based buried heterostructure lasers has been improved considerably by using an A1GaAs native oxide mask for in situ mesa melt-etching and LPE regrowth. In this case there is no undercutting and the shape o f the mesas is dictated only by anisotropy o f melt-etching. Uncoated buried heterostructure laser diodes with an active layer width o f 3/lm and less emit in a single spatial mode. The m a x i m u m optical power densities are 10 m W per micron o f active layer width for AIGaAs/GaAs lasers and 20 m W per micron o f active layer width for InGaAs/ A1GaAs lasers. The higher optical power o f InGaAs/A1GaAs lasers is attributed to lower electrical and thermal resistances and to linear mirror temperature versus current characteristics. Internal second-harmonic generation studies have confirmed that: A1GaAs-based laser diodes
degrade because o f the mirror damage at high optical power densities and increased temperature. A1GaAs/InGaAs strain quantum well buried heterostructure laser diodes emitting at 960 n m with an active layer width 0.1 mm, small electrical (0.3 f~) and thermal (1.6-5K/W) resistivities, high efficiency (0.7 W/A), symmetrical farfield pattern (8 ° F W H A in both directions) and narrow spectral width (0.4nm FWHA) have been obtained. Buried heterostructure A1GaAs-based laser diodes have been used successfully in pumping optical fibre lasers and in medicine.
References [1] Alferov, Zh.I., Andreyev, V.M., Syrbu, A.V., Mereutza, A.Z. and Yakovlev, V.P. Extremely low threshold A1GaAs buried heterostructure quantum well laser diodes grown by LPE, Appl. Phys. Lett., 57 (1990) 2873-2875. [2] Chand, N., Chu, S.N.G., Dutta, N.K., Lopata, J., Geva, M., Syrbu, A.V., Mereutza, A.Z. and Yakovlev, V.P. Growth and fabricationof high performance 980-nm strained InGaAs quantum well lasers for erbium doped fibre amplifiers, IEEE J. Quantum Electron., 302 (1994) 424-440. [3] Syrbu, A.V., Mereutza, A.Z., Yakovlev, V.P., Suruceanu, G.I. and Lupu, A.T. Buried heterostructure A1GaAs photonic devices fabricatedby low temperature mesa melt etching and regrowth, J. Electron. Mater., 5 (1994) 710-714. [4] Iakovlev, V. High performance A1GaAs-based laser diodes: fabrication, characterisationand applications, Proc. 20th Int. Conf. on Microelectronics (MIEL'95), Vol. 1, Nis, Serbia, 12-14 September 1995, pp. 435-439. [5] Epperlein, P.W. Microtemperature measurement on semiconductorlaser mirrors by reflectancemodulation a newly developedtechnique for laser characterisation, Jpn.J. Appl. Phys. Part 1, 3212 (1993) 5514-5522. [6] Katsavetz, N.I., Garbuzov, D.Z., Grishina, T.A., Kudrik, I.E. and Pitkianen, P.V. Study of correlation between optical characteristics and mirror facet temperature of the active region in high power SCH SQW InGaAs/GaAs and InGaAs/GaAs laser diodes, in P.C. Chen, L.A. Johnson and H. Temkin (eds), Proc. SPIE 2148 Laser diode technology and Applications
V/, 1994, pp. 152-158.
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[7] Bertolotti, M., Liakhou, G.L., Li Voti, R., Wang, R.P., Sibilia, C., Syrbu, A. and lakovlev, V.P. An experimental and theoretical analysis of the temperature profile in semiconductor laser diodes using the photothermodeflection technique, Meas. Sci. Technol., 6 (1995) 1278-1290. [8] Mereutza, A.Z., Syrbu, A.V. and Yakovlev, V.P. Buried heterostructure formation processes for high performance devices, Proc. 15th Annual Semiconductor Conf., Sinaia, Romania, October 1992. [9] Syrbu, A.V., Yakovlev, V.P., Suruceanu, G.I., Mereutza, A.Z., Mawst, L.J., Bhattacharya, A., Nesnidal, M., Lopez, J. and Botez, D. ZnSe-facetcoated InGaAs/InGaAsP/InGaP diode lasers of high CW power and 'wallplug' efficiency, Electron. Lett. 32 (1996) 352-354. [10] Syrbu, A., Mereutza, A., Suruceanu, G., Iakovlev, V., Vieru, S., Catughin, O., Predescu, M., Popescu, I. and Ispasoiu, R. AIGaAs/InGaAs buffed heteros~ucture laser diodes for pumping solid state lasers, Proc. 1995 Int. Semiconductor Conf., Sinaia, pp. 453-456. [11] Paoli, T.L. A new technique for measuring the thermal impedance of junction lasers, IEEEJ. Quantum Electron., QE-11 (1975) 498-503.
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[12] Kravetsky, I.V., Kulyuk, L.L., Lupu, A.T., Shutov, D.A., Suruceanu, G.I., Syrbu, A.V. and Yakovlev, V.P. Internal second harmonic generation in CW A1GaAs SQW lasers: the facet degradation monitoring, AppI. Surf. Sci., 69 (1993) 424-428. [13] Yakovlev, V.P., Lupu, A.T., Syrbu, A.V., Suruceanu, G.I., Kravetsky, I.V., Kulyuk, L.L. and Chand, N. Characterisation of strained quantum well InGaAs/ A1GaAs buried heterostructure lasers using internal second-harmonic generation, in P.C. Chen, L.A. Johnson and H. Temkin (eds), Proc. SPIE 2148, Laser Diode Technology and Applications VI, 1994, pp. 440448. [14] Yakovlev, V.P., Lupu, A.T., Syrbu, A.V., Mereutza, A.Z., Kravetsky, I.V. and Kulyuk, L.L. Characteffsation of the degradation processes in the buffed heterostructure quantum well laser diodes using internal second harmonic emission, Presented at MRS Fall Meeting, Boston, 28 November-2 December 1994. [15] Petermann, K. External optical feedback phenomena in semiconductor lasers, IEEE J. Selected Topics Quantum Electron., 12 (1995) 480-489.