GaInPjAIGaInP VISIBLE-LIGHT EMITTING LASER DIODES GROWN BY METAL ORGANIC VAPOUR PHASE EPIT AXY

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1 Philips J. Res. 45, , 1990 R1240 GaInPjAIGaInP VISIBLE-LIGHT EMITTING LASER DIODES GROWN BY METAL ORGANIC VAPOUR PHASE EPIT AXY by A. VALSTER, J. v.d. HEIJDEN, M. BOERMANS and M. FINKE Philips Research Laboratories, P.O. Box 80000, 5600 JA Eindhoven, The Netherlands Abstract The realization of laser diodes emitting in the nm wavelength range is described. For emission near 670 nm, lngaalp double heterostructures with an lngap active layer are used; for realization of diodes at shorter wavelengths lngap quantum wells are of great importance. The growth of these materials by metal organic vapour phase epitaxy is also described. Keywords: metal organic vapour phase epitaxy, visible lasers. 1. Introduetion Visible-light emitting lasers have attractive potential applications in future optical-information processing systems such as high-density optical recording and high-speed laser printers. Another attractive application of visible lasers is the replacement of the widely used helium-neon gas lasers in for instance bar code readers and laser pointers. Advantages of such a replacement are the simple battery supply instead of the high-voltage one, the reduction in size and weight and the expected longer lifetime of the semiconductor lasers. The quaternary I1I-V alloy semiconductor Al(x)Ga(l_x_y)In(y)P has the largest band gap among the III-V semiconductor compounds that can be matched to a GaAs substrate (y = 0.50). The alloy spans a direct band gap range from about 1.85 ev (at x = 0) to about 2.25 ev near the F -X cross-over at x = 0.35 (ref. 1). The growth of the AlGalnP alloy using liquid phase epitaxy (LPE) is not possible because of the large aluminium segregation coefficient from the liquid to the solid phase in Al-In-P systems"), A real breakthrough in this field was made by the use of the metal organic vapour phase epitaxy Philips Journalof Research Vol.45 Nos 3/

2 A. Vals/er et al. (MOVPE) growth technique. High-quality material has been grown successfully by low-pressure MOVPE and results will be demonstrated. By using a simple one-step growth followed by a simple wet-chemical etching step, we realized a novel GalnPjAIGalnP stripe-geometry laser structure for the first time. The result will be described. For emission at 675 nm, the active layer consists ofga(1_y)ln(y)p alloy with y ~ It is shown that the structure can be made highly reliable provided the mirrors are suitably coated. There is also considerable interest in obtaining laser emission at wavelengths shorter than 675 nm. One of the reasons is that the sensitivity of the eye increases strongly towards shorter wavelengths. Various ways to shorten the wavelength have been studied. Substitution of gallium by aluminium results in higher band gap AIGalnP active layers. Continuous wave (CW) operation at room temperature at 645 nm with x = 0.08 has been achieved in this way. Another way of reducing the wavelength is the use of quantum well active layers. In this paper a separate confinement multiple quantum well structure is described for emission in the 630 nm wavelength region; in this way CW operation near 630 nm at room temperature, and even considerably above this temperature, was achieved for the first time. 2. MOVPE growth of the AIGalnP double heterostructure laser materials Epitaxial growth was carried out under low-pressure conditions by MOVPE in a horizontal reactor with a rectangular cross-section. A vent-run switching manifold with minimized dead volume at the reactor inlet allows fast switching of the growth species. Trimethylindium (TMIn), trimethylgallium (TMGa) and trimethylaluminium (TMAI) were used as the metal alkyl group III sources and pure phosphine (PH 3 ) was used as the hydride group V source. Hydrogen selenide (H 2 Se, 1000 ppm in H 2 ) and dimethylzinc (DMZn) were employed as the n-type and p-type doping sources respectively. For the carrier gas we used palladium-diffused H 2 with a total gas flow of 5.5 standard I min -1. The epitaxial layers were grown on (001 )-oriented silicon-doped 2 in GaAs substrates held on an SiC-coated graphite susceptor heated by IR lamps. A schematic representation of the reactor cell containing the susceptor is shown in fig. 1. The capacity is two 2 in GaAs substrates. The growth temperature is 700 C, the V/Hl ratio in the gas phase is 300 and the reactor pressure is 50 mbar. These reactor conditions result in a growth rate of l.4,um min -1. Prior to the deposition ofthe AIGalnP layers a thin GaAs layer was usually grown on top ofthe substrate. Special attention was paid to the lattice matching of the epitaxial layers to the substrate. The lattice mismatch between the 268 I'hilips Journalof Research Vol.45 Nos 3/4 1990

3 GalnPjAIGalnP visible-light emitting laser diodes / outertube ~ ~---li-ne-r-tu-be DMZn TMln TM AI TMGa H 2 IRlamp heater inlet reactor exhaust 'Fig. I. Reactor cell containing susceptor and two 2 in GaAs substrates. epitaxial layers and the substrate was determined by double-crystal X-ray diffraction 3). In order to prevent the formation of misfit dislocations alllayers were lattice matched to the substrate within ±O.l %. This mismatch is very critical to the indium content y of the material. Photoluminescence mapping across the wafer shows a variation in the photoluminescence peak wavelength ofless than 10nm for ternary as well as quaternary layers, which demonstrates the excellent homogeneity of the material across a 2 in wafer. The effective minority-carrier lifetime 4) of the ternary GalnP epitaxial layer is measured for the first time by the decay of the bandedge luminescence by optical excitation with 581 nm radiation from a mode-locked dye laser. Figure 2 shows a decay curve of an AIGalnP double heterostructure as indicated in the inset and the slope yields the non-radiative carrier lifetime of 210 ns, demonstrating the high optical quality of the MOVPE-grown material. When aluminium was introduced into the GalnP layer the lifetime decreased steeply probably because some oxygen and moisture were present in the system after loading the substrate (see also Sec. 4). 3. Laser structure for emission near 675 nm The laser structure used for emission near 675 nm is shown schematically in fig. 3. A normal AlGalnPjGalnP double heterostructure, where the GalnP is the active layer material and the n-type and p-type AIGalnP layers are the cladding layers, is grown on top of the substrate in the way described. The refractive index difference and the band gap difference between the active Philips Journalof Research Vol.45 Nos 3/

4 A. Valster et al. rp [arb.units] 104~----r , 581nm excitation /~' ===I=}===== GainP {Alo.sGao.4>o.slno.sP active "========== layer GaAs substrate 10 o t [ns] Fig. 2. Decay curve (photon flux cp as function of time I) of the band edge luminescence of an AlGalnP double heterostructure, as indicated in the inset. GalnP layer and the quaternary cladding layers is responsible for the optical and carrier confinement. On top of the p-type AIGalnP cladding layer a thin GalnP inserted layer, also p-type, has been grown as can be seen in the figure, followed by a heavily doped p-type GaAs contacting layer (fig. 3a). The lateral stripe-geometry structure is defined as follows. By use of standard photolithography and wet-chemical etching, the p-gaas top layer is selectively removed everywhere except for a narrow, approximately 7-JJ.m wide striped region with an etchant which stops at the GalnP layer. On top of the structure a contact metallization is used which makes an ohmic contact to the p-type GaAs, but makes a current-blocking contact to the exposed part of the intermediate GalnP layer. Hence, the current is effectively laterally confined. After thinning the substrate the n-side ohmic contact was formed (see fig. 3b). It is clear that this structure is essentially gain guided and will have a somewhat astigmatic beam, but for many applications in the visible wavelength region this is not a problem at all. An advantage of this structure is that it requires only one growth step and one simple etching step. The electro-optical properties of these lasers are presented in figs 4 and 5. Figure 4 shows the light output L versus current i characteristics at various temperatures. Although the structure is gain-guided, the threshold current at 270 Philips Journalof Research Vol.45 Nos 3/4 1990

5 GalnP/AIGalnP visible-light emitting laser diodes p" GaAs top layer -{' pgalnp inserted layer {:===================P' p-aigalnp cladding layer GainP active layer -J=:===================P' n-aigalnp cladding layer n-gaas substrate a) ridge waveguide b) GalnP/AIGalnP ridge waveguide double heterostructure laserdiode Fig. 3. Schematic representation of 670 nm laser structure. room temperature is seen to be quite low. This is attributed to the relatively low conductivity of the p-type cladding layer, which makes current spreading between the contact layer and the active layer less important than in gain-guided AIGaAs lasers. The increase in threshold current at higher temperatures is related to leakage of injected electrons towards the p-type cladding layer (see ref. 5), and hence will not critically depend on the device Philips Journalof Research Vol. 4S Nos 3/

6 A. Vals/er et al. S T = BQ 90'C L[mW] l- t 4 t- t- 3 t- t- 2 t- I-..,,/~ ol_~~~~~~~i~~_i_j o ~i[ma] Fig. 4. Typical L-i (light output L as function of current ij characteristics as a function of temperature for 670 nm lasers. structure. The far-field distributions perpendicular and parallel to the junction plane are presented in fig. 5. Typically, the far-field widths are around 33 and 7, respectively. This means that the aspect ratio is somewhat larger than that of AIGaAs lasers. The astigmatic distance is of the order of 30 J1m, which is comparable with the depth of focus limited by the relatively small numerical aperture of the beam parallel to the junction plane. Lasers with coated mirrors show hardly any degradation during life tests of more than 8000 h at 50 C and 3 m W (see fig. 6). 4. Possibility of laser emission below 675 nm Because ofthe steep increase in the response curve ofthe human eye towards shorter wavelengths, there is a strong interest in obtaining laser emission at 272 Philip. Journalof Research Vol.45 Nos 3/4 1990

7 GalnP/AIGalnP visible-light emitting laser diodes Poz2mW t Fig. 5. Far-field distributions (relative intensity I, as a function of the angle from the central axis a) parallel and perpendicular to the junction plane for 670 nm lasers. t ~ MOVP 7 LD 3027 À. = 670 nm coated mirrors T", 50 e 0 P=3mW 40 O~--~--~--~--~--~--~--~--~---L--~ o _..t[hr] Fig. 6. Aging characteristics (the threshold current i'h as a function of time t) of the AlGalnP low ridge laser. these wavelengths, notably near 633 nm which is the well-known wavelength of the widely used helium-neon laser. The most direct way to reduce the wavelength is to increase the band gap by the addition of aluminium to the active layer, possibly combined with increasing the aluminium content in the cladding layers, in complete analogy to what is generally done in the AlGaAs system.lfthis is done, one usually observes an increase in the threshold current at room temperature and an even sharper increase in the temperature Philips Journalof Research Vol.45 Nos 3/

8 A. Valster et al. Ndopedx o Fig. 7. Quantum-well 30A -- Pdoped laser structure for emission at 626 nm wavelength (x is the aluminium content). dependence of the threshold current. Part of the increase at room temperature is related to the formation ofnon-radiative recombination centres in the active layer, as is for instance evidenced by a decrease of photoluminescence efficiency when aluminium is added to the materialof the active layer. The increase in the temperature dependence of threshold current is related to an increased leakage of electrons towards the p-type cladding layer since at higher aluminium concentrations the band gap in the cladding layer becomes indirect and hence an additional increase in the aluminium content gives only a marginal increase in the band gap. The decrease of the radiative recombination efficiency in the active layer can be prevented by using quantum wells rather than the addition of aluminium to shorten the wavelength of the emission. An example of a laser structure in which this has been applied is shown in fig. 7. The quantum weil laser structure presented is of the separate confinement type, the inner part of which also contains the quantum wells and acts as an optical waveguide layer embedded between the outer cladding layers. The L-Î characteristics of lasers made from such a structure are shown in fig. 8. The use of the quantum wells has shifted the wavelength of the optical transitions down to to 626 nm. At room temperature, the threshold is about 120mA, which is rather high but not excessive. More seriously, however, the temperature dependence of the threshold has increased even more so that at room temperature only pulsed operation is possible. More detailed analysis, however, reveals that this is due to charge carrier leakage from the quantum wells towards the barriers. It is well known that this leakage is much less pronounced for somewhat wider wells. Therefore, we have increased the well width from about 30 A up to about 50 A and have optimized the growth. As a result of these measures, the threshold current at room temperature came down considerably, but the temperature dependence of the threshold current decreased even more. As a result we obtained the first CW operating lasers at the "magic" wavelength 274. Philips Journalof Research Vol.45 Nos 3/4 1990

9 GalnP/AIGalnP oisible-liqht emittinq laser diodes L[mW] t 5 MOV218 LD31061#25 4 pulse 3 T=20 C 300C 40 C ~i[ma] Fig. 8. Typical L-i characteristics for 626 nm lasers at several temperatures. 125r ~ ti MOV386 LD s um ridge CW T z 25 C P=2mW O~~~~_~~A~~A~~A~~~ ~~ ~ ~I..[A] Fig. 9. Emission spectrum (intensity I as function of wavelength J.) for 633 nm laser diodes. Philips Journalof Research Vol.45 Nos 3/

10 A. ValsIer el al. L[mW] t 5r ~ 4 3 MOV386 LD 3363 CW À.25'C = 633 nm L=300 um T = 'C 'i[ma] Fig. 10. Typical L-; characteristics as a function of temperature for 633 nm laser diodes. of 633 nm in the world and it appeared possible to obtain CW operation even up to substantially higher temperatures. By now, depending on the laser chip length, the maximum operating temperature is around C. The first successful life testing experiments have already been carried out at 40 C. The first results indicate an operating lifetime of at least 1000h at this temperature, which is quite a promising result considering the fact that the structure has not yet been optimized. A typical spectrum of our 633 nm lasers is presented in fig. 9; a typical set of L-i characteristics as a function of temperature is presented in fig. 10. The far-field distributions parallel and perpendicular to the junction plane are similar to those of the 670 nm lasers. More details are given in ref. 6. Finally, it should be mentioned that the emission wavelength can be reduced stili further by cooling. There are three reasons for this. Firstly, when lowering the temperature, the threshold current decreases. Secondly, the thermal conductivity of semiconductor materials increases towards lower temperatures. And thirdly, the band gap increases towards lower temperatures leading to a shortening of the emission wavelength. By cooling lasers which lased in the pulsed mode at room temperature at 610nm, an emission wavelength of585 nm was achieved at 77 K, which is in the yellow region. This was achieved under CW operation with a threshold current of only IOmA. 276 Philips Journalor Research Vol. 45 Nos 3/4 1990

11 GalnPjAIGalnP visible-light emitting laser diodes 5. Conclusions It has been found possible to fabricate very reliable low-threshold 670 nm laser diodes from InGaAlP material grown by MOVPE. Furthermore, significant advances have been made with the shortening of the emission wavelength. The resulting laser diodes will have a significant impact in important product areas such as barcode readers, optical pointers, optical printers and optical recording. REFERENCES 1) H.C. Casey and M.B. Panish, Heterostructure Lasers, Part B, Academic Press, New York, 1978, p. 7. 2) M. Kazumura, I. Ohta and I. Teramoto, Jpn. J. App!. Phys., 22, (1983). 3) W.J. Bartels, J. Vac. Sci. Techno!., BI, 388 (1983). 4) G.W. 't Hooft, C. v. Opdorp and A.T. Vink, Acta Electron., 25, (1983). 5) S.H. Hagen, A. Valster, M.J.B. Boermans, J. M.M. van der Heijden and G. Acket, Proc. 12th IEEE Int. Semiconductor Laser Conf., Davos, September 9-13,1990, paper C.4. 6) A. Valster, C.T.H.F. Liedenbaum, J.M.M. v.d. Heijden, M.N. Finke, A.L.G. Severens and M.J. B. Boermans, Proc. 12th IEEE Int. Semiconductor Laser Conf., Davos, September 9-13, 1990, paper C.1. ~; Authors A. Valster. Ir. degree (Chemistry), Technical University Eindhoven, The Netherlands, 1979; Philips Research Laboratories, Eindhoven, At Philips he started fabrication of AIGaAs/GaAs and GaInAsP/InP semiconductor lasers for optical communication systems. From September 1985 until September 1986 he spent a year at the Central Research Laboratory ofhitachi in Kokubunji, Japan. Currently he is investigating the processing ofvisible light-emitting semiconductor lasers. M.J.B. Boermans. see p J. M. M. van der Heijden. Ing. degree (Chemistry), H.T.S. Eindhoven, The Netherlands, 1983; Philips Research Laboratories, Eindhoven, His thesis work was on the crystallization of inorganic salts in a laboratory scale reactor. At Philips his current responsibilities are in the technology of IR and visible semiconductor laser diodes. M. N. Fi n ke. Ing. degree (Chemical engineer), H.T.S. Eindhoven, The Netherlands, 1985; Philips Research Laboratories, Eindhoven, He has been concerned with the LPE growth of special structures of GaAs-AIGaAs laser diodes. In 1989 he switched to the MOVPE growth of AIGaInP visible laser diodes. Philips Journalof Research Vol.45 Nos 3/