Knigge, A.

Wenzel, H.

Weyers, M.

Zeimer, U.

Zorn, M.

English

Università del Salento: ESE - Salento University Publishing

Comparison of AlGaAs and AlInP cladding layers for red edge−emitting lasersM. Zorn 1) ,  H. Wenzel 1) ,  A. Knigge 1) ,  U. Zeimer 1) ,  M. Weyers 1)1) Ferdinand−Braun−Insitut fuer Hoechstfrequenztechnik (FBH), Albert−Einstein−Str. 11, D−12489 Berlin, GermanyIntroductionRed edge−emitting lasers are of potential interest e.g. for visible applications, data storage and consumerelectronics (e.g. laser printers and scanners). In the wavelength range between 630 nm and 690 nm the laserparameters like threshold current density and efficiency are critical due to the unfavourable material properties inthe AlInGaP material system which is required for this wavelength range. Red edge−emitting lasers normallyconsist of InGaP quantum wells with AlGaInP waveguide layers embedded in AlInP cladding layers [1]. However,growth and processing of AlInP layers is difficult. Especially high p−doping cannot be achieved in AlInP [2].In our work we investigated the possibilities of increasing the p−doping of AlInP by changing the growthparameters, especially the growth temperature, in metal−organic vapour phase epitaxy (MOVPE). Furthermore, toovercome the problems caused by the AlInP cladding layers, these were replaced by AlGaAs layers. This givesadvantages in the laser processing and the possibility of higher p−doping levels [3]. Furthermore, the dopant zinc(Zn) which easily diffuses through the epitaxial structure can then be replaced by carbon (C) in the claddinglayer. Optical in−situ monitoring by reflectance anisotropy spectroscopy (RAS) was used during optimisationof the growth process for example at the InGaP/AlInP interface.ExperimentalAll epitaxial layer structures presented in this work were grown in an Aixtron 200/4 low−pressure MOVPEsystem. The sources used include trimethylgallium (TMGa), trimethylaluminum (TMAl), trimethylindium (TMIn),arsine (AsH3) and phosphine (PH3). Silicon (Si) from Si2H6 was used for n−doping. Zinc (Zn) from dimethylzinc(DMZn) and carbon (C) from tetrachlorbromide (CBr4) were utilized for p−doping. The growth pressurewas 150 hPa and the main growth temperature was 770°C. Epiready n+ GaAs (100) substrates misoriented 6°towards [111]A were used.The MOVPE system is equipped with low−strain UV transparent viewports for normal incidence optical accessand with a corresponding hole in the liner−tube. In−situ measurements were performed using a LayTecEpiRAS−200 spectrometer that allows combined reflectance anisotropy spectroscopy (RAS) [4,5] and reflectance(R) measurements under standard device growth conditions [6].Growth OptimizationFig. 1 shows a complete RAS fingerprint taken during growth of a red laser structure with AlInP cladding layers.This fingerprint (also called colorplot) was obtained by taking RAS spectra in the range between 1.5 eV (826nm) and 5 eV (248 nm) continously. Afterwards the spectra are color−coded and plotted versus growth time[6]. The different growth stages (deoxidation etc.) as well as the growth of the different layers with theircorrespondig composition and doping levels can clearly be distinguished. Failures in the growth process can beidentified easily and the reproducibility can be checked by simply comparing the fingerprints of different growthruns. Fig. 2 shows two transients taken at 3.8 eV (326 nm) in time−resolved mode during growth of two differentlaser structures. Here also the growth of the layers with their correspondig doping level can clearly bedistinguished. The RAS signal behaves as reported earlier [7,8]: for n−type doping the RAS signal decreases whilep−type doping leads to an increase in the RAS signal at this photon energy (wavelength). Furthermore, it can beseen that during growth of the first laser structure (dashed line) the RAS signal shows a strong increase up to 36RAS units during heating of the n:InGaP from 600°C up to 770°C (grey circle). This is a sign for surfaceroughening due to phosphorus desorption.10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.V.09M. Zorn et al Fig. 1: Typical RAS fingerprint (colorplot) taken during growth of a red laser structure. The different growthstages and layers with their correspondig doping types and levels can clearly be distinguished.The second laser grown with an optimised procedure shows only a small increase in the RAS signal up to 5 RASunits at the n:InGaP/n:AlGaInP interface. This shows that in this case nearly no surface degradation has occured atthis interface. Ex−situ microscope measurements support these results: The first sample showed a bad morphologywhile the second sample is mirror−like.When growing laser structures using AlGaAs cladding layers, phosphorus desorption is not a problem because noInGaP layer is needed between the GaAs and the AlInP to reduce the band offsets. In the AlGaAs system the sameeffect can be achieved more easily by introducing a graded index layer between the GaAs and the AlGaAs layer.The doping profiles of the different laser structures where investigated by SIMS. Fig. 3 shows doping profiles oflaser structures with Zn:AlInP cladding layers and for comparison with Zn:AlGaAs cladding layers. It can be seenthat the p−doping level in the AlGaAs is much higher than in the AlInP grown at 770°C. The achivable p−dopinglevel in AlInP can be increased by lowering the growth temperature. At a growth temperature of 660°C the Znconcentration is comparable to the one in AlGaAs. However, the growth of AlInP at lower growth temperaturesleads to a reduced crystal quality. Furthermore, the higher Zn incorporation at 660°C in AlInP obviously leads to ahigher Zn diffusion in the waveguide layer. When lowering the growth temperature, broad area devices show adecrease of the threshold current density and an increase of the differential efficiency. In the lasers with AlGaAscladding layers a strong improvement can be achieved by replacing Zn by C from CBr4 in the p:AlGaAs layer.Due to the reduced Zn diffusion the threshold current density decreases and the differential effiency inceases.10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.V.09M. Zorn et al Fig. 2: RAS transients taken at 3.8 eV (326 nm) during growth of two different red laser structures (lower part).The grown layers are indicated. The upper part shows the respective growth temperature.Fig. 3: SIMS profiles showing the Zn incoorporation in the laser structures with: a) Zn:AlInP cladding layersgrown at 660C, b) Zn:AlInP cladding layers grown at 770C, c) Zn:AlGaAs cladding layers grown at 770C.10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.V.09M. Zorn et al As result of the optimization process we focused on two different layer structures for the growth of the laserstructures: One with AlInP cladding layers using Zn and a growth temperature of 660°C on the p−side and oneconsisting of AlGaAs using C for p−type doping grown at 770°C.Laser DataFor the growth of laser structures we used compressively strained double quantum wells for an emissionwavelength of 653 nm. In pulsed operation our lasers with AlInP cladding layers show a low threshold currentdensity of 530 A/cm2 with an external differential efficiency of 69% for 100 µm x 1000 µm stripes which seems tobe comparable to results reported in the literature [9,10]. For lasers containing AlGaAs cladding layers weobtained a threshold current density of 590 A/cm2 and an external differetial efficiency of 77 % under the sameconditions. Due to the different optical properties the lasers with AlGaAs cladding layers have a reduced verticalfar field angle of 30° in comparison to 36° for the lasers with AlInP cladding layers. Therefore, it should bepointed out that although the vertical far field angle is reduced (resulting in a smaller quantum well confinementfactor) the threshold current density increases only slightly. Furthermore, the external differential efficiency iseven higher. Coated low mesa stripe lasers fabricated from layer structures with AlGaAs cladding layers usingcarbon doping on the p−side achieved an output power of about 0.5 W at 650 nm and 20°C in cw operation.In summary, replacing the Zn:AlInP cladding layers bei C:AlGaAs seems to be a promising way to grow high−power red laser structures. However, further investigations (e.g. life time tests) have to be carried out to prove thisassumption.References[1] D.P. Bour, R.S. Geels, D.W. Treat, T.L. Paoli, F. Ponce, R.L. Thornton, B.S. Krusor, R.D. Bringans, D.F.Welch, IEEE J. Quant. Electron. 30 (1994) 593[2] Y. Nishikawa, Y. Tsuburai, C. Nozaki, Y. Ohba, Y. Kokubun, H. Kinoshita, Appl. Phys. Lett. 53 (1988) 2182[3] P. Unger, G.−L. Bona, R. Germann, P. Roentgen, D.J. Webb, IEEE J. Quant. Electron. 29 (1993) 1880[4] D.E. Aspnes, Mater. Sci. Eng. B 30 (1995) 109[5] J.−T. Zettler, K. Haberland, M. Zorn, M. Pristovsek, W. Richter, P. Kurpas, M. Weyers, J. Cryst. Growth 195(1998) 151[6] K. Haberland, P. Kurpas, M. Pristovsek, J.−T. Zettler, M. Weyers, W. Richter, Appl. Phys. A 68 (1999) 309[7] K. Haberland, A. Bhattacharya, M. Zorn, M. Weyers, J.−T. Zettler, W. Richter, J. Electron. Mater. 29 (2000)468[8] M. Zorn, K. Haberland, A. Knigge, A. Bhattacharya, M. Weyers, J.−T. Zettler, W. Richter, J. Cryst. Growth235 (2002) 25[9] S. Orsila, M. Toivonen, P. Savolainen, V. Vilokkinen, P. Melanen, M. Pessa, SPIE 3628 (1999) 203[10] P. Raisch, R. Winterhoff, W. Wagner, M. Kessler, H. Schweizer, T. Riedl, R. Wirth, A. Hangleitner, F.Scholz, Appl. Phys. Lett. 74 (1999) 215810th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.V.09M. Zorn et al 

Comparison of AlGaAs and AlInP cladding layers for red edge-emitting lasers

ESE - Salento University Publishing

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Comparison of AlGaAs and AlInP cladding layers for red edge-emitting lasers

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