INDEX-GUIDED MULTIQUANTUM- WELL AlGaAs LASERS

Transcription

1 Philips J. Res. 45, , 1990 R 1233 INDEX-GUIDED MULTIQUANTUM- WELL AlGaAs LASERS by H.P.M.M. AMBROSIUS, H.J.M. BOERRIGTER-LAMMERS, S.H. HAGEN, M. BOERMANS, M. CHANG and G.A. ACKET Philips Research Laboratories, P. O. Box 80000,5600 JA Eindhoven, The Netherlands Abstract Various types of index-guided multiquantum-well laser structures are described. The results are compared and differences are explained. The choice between the structures depends on the type of application. Keywords: diode lasers, quantum wells 1. Introduetion The advantages of quantum well lasers over normal lasers are well known 1.2). In short, they are lower threshold current, narrower linewidth and high power handling capability of the laser mirrors. In the AlGaAs system in particular, lasers with quantum well active regions enclosed in a separate optical waveguide may reach threshold current densities as low as a few hundred amperes per square centimetre at room temperature. In principle, either gain-guided or index-guided lasers can be made with such structures. However, it turns out that only index-guided lasers fully exploit the potential advantages of the use of quantum wells. There are two reasons for this. First, in gain-guided lasers the lateral mode is determined by antiguiding 3) due to the negative contribution ofthe injected carriers to the real part ofthe refractive index. This antiguiding leads to diffraction of optical power on both sides of the stripe into the absorbing side regions, thus causing an internal opticalloss and hence a higher threshold current. Second, gain-guided lasers with quantum well active regions usually exhibit a complicated dynamic behaviour in which thermal effects 3.4) are very important. This is due to local heating effects, resulting in an increase in refractive index and therefore a change in the lateral Philips Journalof Research Vol.45 Nos 3/

2 H.P.M.M. Ambrosius et al. optical guiding in the laser structure. Low threshold currents and a good dynamic response can be obtained using index guiding 5); hence this approach was chosen. In this paper we describe the properties of three types of index-guided multiquantum-well lasers. The details of the manufacturing processes used have been described 'elsewhere 6.7) and will not be repeated here. We limit ourselves to a short review of the structures. 2. Description of the structures used The structures will be described in chronological order, which is also the order of decreasing threshold current The self-aligned structure A schematic diagram of the structure is presented in fig. 1. The growth and processing steps have been described in ref. 6. The characteristic feature of this structure is the presence of an n-gaas current blocking layer which is entirely surrounded by p-algaas. Furthermore, outside the groove, the active layer is rather close to the current blocking layer. The current blocking layer absorbs radiation emitted from the active region. The presence of such an absorbing current blocking layer deforms the wave travelling in the active layer in the transverse (i.e. perpendicular to the active layer) direction, pushing it into the underlying cladding material. Since the cladding material has a lower refractive index this deformation leads to a slight lowering of the refractive index of the wave on both sides of the groove and these refractive index steps confine the lasing mode laterally. Experimental results are presented in Sec The ridge waoequide laser This structure is shown in fig. 2. The current flows only through the contact on top of the ridge. Furthermore, as well as the ridge the material has been partially removed by etching up to close to the active layer so that here also the mode in the active layer is deformed and pushed into the lower cladding,. again resulting in a somewhat lower effective index of the lasing mode below the etched regions. A marked difference in the index guiding compared with the SAS laser described earlier is that outside the ridge no absorbing layers are present. This means that the refractive index step at the ridge edges is real whereas it is complex in the case of the SAS (absorption being equivalent to an imaginary contribution to the complex refractive index). Experimental 166 Philips Journalof Research Vol.4S Nos 3/4 1990

3 Index-guided multiquantum-well AlGaAs lasers p-aigaas (0_2-0_3I-'m) n-gaas ~... ';;;;;;... f!- active..._."'... ". region..',-m''''""",:,,.'. i\.-4i-'m-/ n-gaas substrate epitaxy step 1 technology step 1 current blocking layer metal epitaxy step 2 technology step 2 Fig. 1. The self-aligned structure (SAS) laser: schematic representation. active --j. region p-gaas I P. A.IG.aAs. ~ n-aigaas '.. metal current blocking layer n-gaas substrate epitaxy step technology step Fig.2. The ridge waveguide laser (RWL): schematic representation. Philips Journalof Research vet, 45 Nos 3/

4 H.P.M.M. Ambrosius et al. results will be presented in Sec. 5. It will be seen that the structural differences between SAS and RWL indeed lead to differences in the electro-optic behaviour of the laser diodes The buried heterostructure laser In the two previous structures, the index guiding resulted from a mode deformation at both sides of the stripe. However, it is also possible to obtain a lateral index step by creating a lateral composition difference, for instance by surrounding the active layer at both sides of the stripe by AIGaAs of a higher aluminium content, which also implies a lower refractive index. In practice this is often realized by making so-called buried structures in which the outer parts of the active layer are removed by etching after which a second burying epitaxy step of cladding material leads to the buried heterostructure. However, such etching of the active layer may result in a reliability risk since after etching the edges of the active layer are exposed to the environment. Therefore, we have used an alternative burying technique. Its schematics are shown in fig. 3. It is realized in one growth step by making use of the anisotropy of MOVPE growth on a profiled GaAs substrate 7). The results are presented in Sec. 6. It will be shown there that this type of laser is capable of reaching even lower threshold currents than the ridge guide laser. This is due to the tighter charge carrier and optical confinement. The low power dissipation makes high-temperature operation possible. 3. Experimental results on the self-aligned structure lasers In fig. 4 we present the continuous wave (CW) light output L vs. current i characteristic of an SAS laser with a quantum-well active region. Threshold currents of the order of 30 ma are usually obtained near room temperature. The L-i curves remain kink free up to about 40 mw. In fig. 5 the lateral far-field distributions are shown. It is seen that the far-field distribution remains stable up to high power. More details about the electro-optic behaviour of this type of laser and its use for optical recording purposes are found in the paper by Harm in this issue. 4. Experimental results on the ridge waveguide laser A typical L-i characteristic of the ridge guide laser is shown in fig. 6. The corresponding lateral far-field distribution is shown in fig. 7. Although the lasers can be kink free up to very high power levels (70 mw), usually kinks 168 Philips Journni of Research Vol.45 Nos 3/4 1990

5 Index-guided multiquantum-well AlGaAs lasers (100) mesa n-gaas substrate technology step 1 epitaxy step Zn-diftusion technology step 2 Fig. 3. All-metal-organic (MO) vapour phase epitaxy (YPE) buried heterostructure (BHS) laser: schematic representation. in the L-i characteristic occur at power levels below that of the SAS laser. However, threshold currents of the RWL are definitely lower than those of the SAS and the differential efficiency above threshold is higher by about a factor of 1.5. Both facts indicate that the opticallosses in the RWL are lower than those in the SAS. 5. Experimental results on the buried heterostructure laser A typical L-i characteristic of a BHS laser is shown in fig. 8. The corresponding lateral far-field distributions are presented in fig. 9. It is noted that the far field is considerably wider as compared with the previous structures because of the tighter lateral optical confinement. In fig. 10 we also show the temperature dependence of the L-i characteristics of these types of laser. It is seen that CW operation at 100 C is obtained without any thermal saturation. Philips Journalof Research Vol.45 Nos 3/

6 H.P.M.M. Ambrosius et al. 40 L- CW30 C.. t 30 L (mw) J 1 b'm O+-~~~---r--'---~------~ o i (ma)_ Fig. 4. Typical light output L vs. current i characteristic of an SAS laser together with the variation in output wavelength with i at 30 oe. The facets are uncoated. Because of the very low threshold currents, which are even lower than those of the RWL and considerably lower than those of the SAS, the low thermal dissipation is clearly helpful in reaching CW operation at high temperatures. 6. Discussion The results presented above indicate that all three concepts for creating lateral optical index guiding of the lasing mode in index-guided quantum well lasers work and generate a lowest order lasing mode up to a reasonable output power. The particular choice which has to be made depends on the laser properties desired. The results on the RWL and also on the BHS indicate a very high differential efficiency above threshold. If this efficiency is expressed as a total quantum efficiency of the two mirrors combined, efficiency values of up to 0.80 are often obtained. From these values, knowing that the equivalent mirror loss is around 40 cm - 1, the internal mode loss by absorption and/or scattering can be 170 Philips Journalor Research Vol.4S Nos 3/4 1990

7 Index-guided multiquantum-well AlGaAs lasers Far Field CW 30 C 35mW Ir 20mW 10mW 1mW -40 o 40 Fig. 5. Typical lateral far-field distribution of relative intensity Ir vs. 0 of SAS lasers at various values of the output power L at 30 oe. -40 e (deg) o L (mw) r E5nm O~~~--~~"*"~'~ o i(ma) Fig.6. Typical light output L vs. current i characteristic of an RWL at 30 oe. The structure contains four 50 A GaAs wells. Philips Journalor Research Vol.45 Nos 3/

8 H.P.M.M. Ambrosius el al. 75mW 50mW 25mW ij (deg) 1mW Fill. 7. Typicallateral far-field distribution ofthe same RWL at various values ofthe output power. Light-current characteristic L(mW) CW 30'C 5 O~-L~----~--~----~----L---~~ o i(ma) --- Fig. 8. Typical light output L vs. current i characteristic of the all-mo VPE BHS laser. estimated. It turns out to be very low, of the order of 10 cm - 1. This is a substantially lower loss than in conventional double-heterojunction lasers and is probably due to the fact that these quantum well lasers are provided with an internal waveguide which is undoped, so that free carrier absorption within this waveguide is very small. This contrasts with the situation in a normal double-heterostructure laser where the active layer is surrounded by doped cladding layers so that the flux of the optical mode penetrates into doped 172 Philips Journalof Research Vol.45 Nos 3/4 1990

9 Index-guided multiquantum-well AlGaAs lasers o (deg) Fig. 9. Typicallateral far-field distributions of a BHS laser. L(mW) 4 t goog so-c ':-i-i-i G i (ma) Fig. ID. Temperature variation of L-; characteristic from 30 C to 100 C. cladding material, which does create a sizeable free carrier absorption. It is very interesting to see that these extremely high differential efficiencies lead to a very high overall electrical-to-optical power conversion efficiency, especially at high powers. For power lasers, the value of the threshold current is of less interest. Here, high power output and beam stability at these power levels, especially the so-called 'kink' power, which is the power up to which the L-i characteristic Philips Journalof Research Vol.45 Nos 3/

10 H.P.M.M. Ambrosius et al. is exactly linear, are of interest. Generally, in the index-guided lasers, the lasing mode below the kink is the lowest order lateral mode, whereas above the kink the next higher order lateral mode is also present (see ref. 8). From the results mentioned it is clear that this mode stability is highest for the SAS laser structure, which is also the reason that this structure has been optimized for high power applications (see also the paper by Harm in this issue). The fact that the beam stability is better in the SAS laser than in the other two index-guiding structures can be understood by considering the modal gain of the next higher order lateral mode (first-order mode) of the structure. This mode has the maxima of its power distribution rather close to the stripe edges. However, at the edges ofthe stripe, in the SAS laser, the current blocking layer is present. Usually this current blocking layer consists of GaAs which is absorbing at the lasing wavelength. Hence, the higher order mode experiences a higher internal absorption loss than the fundamental lateral mode, which has the maximum of the power distribution at the centre of the stripe. This means that the structure internally suppresses the higher order lateral modes and favours the fundamental mode. There is a price to be paid for this improved mode stability. Because the exponential tails of the fundamental mode extend slightly beyond the stripe edges, the fundamental mode also suffers from some absorption owing to the presence of the current blocking layer. This means that the internal absorption coefficient of the mode in the SAS structure is higher than that of the ridge guide structures and BHS. This internal absorption is found experimentally to be of the order of 50 cm -1, a higher value which accounts for the difference in threshold current and in the differential efficiency 9). Ideally, one would like to have high power lasers with the very high differential efficiencies of the ridge laser and still having a good discrimination between fundamental and higher order modes. This requires the incorporation of additional mode filtering elements into the ridge structure. Work towards such improved power lasers is in progress at our laboratories. REFERENCES ') L.C. Liu and A. Yariv, J. Lumin., 30, 551 (1985). 2) P. Blood, Proc. Soc. Photo-Opt. Instrum. Eng., 861, 34 (1988). 3) J.W.M. Biesterbos, R.P. Brouwer, A. Valster, J.A. de Poorterand G.A. Acket, IEEE J. Quantum Electron., 19,961 (1983). 4) F.C. Prince, T.J.S. Mattos, N.B. Patel, D. Kasemset and C.S. Hong, IEEE J. Quantum Electron., 21, 634 (1985). 5) G.A. Acket, Introduetion to this Special Issue. 6) H.F.J. van 't Blik and H.J. M. Boerrigter-Lammers, J. Cryst. Growth, 92, 165 (1988). 7) H.P.M.M. Ambrosius, H.J.M. Boerrigter-Lammers, S.H. Hagen, R.P. Tijburg and H.F.J. van 't Blik, Conference on Advanced Processing and Characterisation Technology, Tokyo, October Philips Journalof Research Vol. 45 Nos 3;4 1990

11 Index-guided multiquantum-well AlGaAs lasers H) B. Garrett and 1. Whiteaway, lee Proc., 134 (Pt J), 11 (1987). 9) H.F.J. van 't Blik, H.J.M. Boerrigter-Lammers, A. Valsterand G.A. Acket, Int. Conf. on Optical Science and Engineering, Hamburg, 1988, Paper , SPIE Publ Authors H.P.M.M. Ambrosius: Drs. degree (chemistry), Catholic University of Nijmegen, The Netherlands, 1977; Ph.D., Catholic University of Nijmegen, The Netherlands, 1981; Philips Research Laboratories, Eindhoven, ; Laboratoires d'electronique Philips, Limeil- Brevannes, ; Philips Research Laboratories, Eindhoven, He has been working in the lield of epitaxial growth orinp/lngaasp by LPE ( ) and of GaAs/AIGaAs by MOVPE (1988- ). Jo le in Boerrigter-Lammers: Ing. degree (chemical engineering), HTS Eindhoven, The Netherlands, 1986; Philips Research Laboratories, Eindhoven, Her studies are related to the epitaxial growth of GaAs and AIGaAs using metal organic vapour phase epitaxy and the application to quantum well semiconductor laser devices. S.H. Hagen: Drs. degree (physical chemistry), University of Groningen, The Netherlands, 1957; Ph.D., University of Amsterdam, 1966; staff member, Van der Waals Laboratory, University of Amsterdam, ; Philips Research Laboratories, Eindhoven, His thesis work was on electron spin resonance of copper ions in ammonium chloride. At Philips Research Laboratories he was engaged in research on the physical properties of semiconductor materials such as silicon carbide, amorphous semiconductors, semiconductor heterojunctions and in the physics of devices based on such materials. Currently he is doing research on the physical properties of GaAs/AIGaAs and InGaP/AlInGaP laser devices. M.l. B. Boerma ns: Ing. degree (physical engineering), HTS Eindhoven, The Netherlands, 1985; Philips Research Laboratories, Eindhoven, His work is concerned with the characterization of GaAs-AIGaAs as well as InGaP-AIGalnP (visible) semiconductor laser diodes. C.V.J.M. Chang: Industrieel Ingenieur (chemistry-option biochemistry), Industriele Hogeschool, Ghent, 1986; Philips Research Laboratories, Eindhoven, Her thesis work was on the isolation and characterization of a Lactobacillus bacterium from corn soakwater. At Philips she has been working on the technology of laser structures generally and primarily on wet-chemical etching. At present her main work concerns reactive ion etching. G.A. Ack ct : Drs. degree (physics), University of Utrecht, 1960; Ph.D. degree, University of Utrecht, 1965; Philips Research Laboratories, Eindhoven, He is department head of the Short Wavelength Optical Devices group. From 1981 to 1988 he was part-time professor at the Technical University of Delft. His interests are opto-electronics, lasers, optical waveguides, optical communication and optical recording. Philips Journalof Research Vol.45 Nos 3/