Agostini, Giovanni

Agostino, Angelo

Cagliero, Stefano

Codato, S.

Gianolio, Diego

Lamberti, Carlo

Martinez Criado, G.

Mino, Lorenzo

Piovano, Andrea

Truccato, Marco

English

Institutional Research Information System University of Turin

Multi quantum well electroabsorption-modulated laser characterised by hard X-raymicroprobeA multi quantum well electroabsorption-modulated laser is a semiconductor deviceemployed in long-distance, high-frequency optical-fibre communication. The most advanceddevices involve monolithic integration of two functions, electroabsorption modulator anddistributed feedback laser, at the chip level. Devices are manufactured by state-of-the-artmethods of substrate masking and vapour deposition, but the results of this process arejudged empirically. Here scientists have used an ESRF hard X-ray microprobe tocharacterise the semiconductor material constituents of such a device with unprecedenteddetail.Characterisation of a laser/modulator device using the ESRF hard X-ray microprobe.Multi quantum well (MQW) electroabsorption modulated lasers (EML) are widely used in opticalcommunication systems.  MQW structures are based on quaternary III-V semiconductor alloys depositedon suitable substrates. High performance optoelectronic devices require monolithic integration of differentfunctions at chip level. Excellent results in the development of monolithic integration have been reachedwith the selective area growth (SAG) technique [1]. SAG exploits the perturbation of the growth fluxesinduced by a dielectric mask (usually SiO2): when the metallorganic precursors collide with the dielectricmask, they are deflected and they can desorb or migrate through the unmasked semiconductor where thegrowth can start. In this way the reactive species coming from the gas phase are enriched by those deflectedby the mask and a different material is obtained with respect to the unmasked region. Since the reactivespecies have different diffusion lengths [2], the result is a variation in composition and thickness ofsemiconductors grown near (SAG region) and far (field region) from the mask (Figure 1a).Multi quantum well electroabsorption-modulated laser characterised by ha... http://www.esrf.fr/news/spotlight/spotlight107/spotlight107/1 of 4 18/05/2010 09:42Figure 1. a) Optical micrograph of the SiO2 stripe-patterned InP substrate allowing SAG growth. The blackrectangle, magnified in part b), shows the region sampled in the XRF maps. b) Spatial maps of thefluorescence counts of the principal element’s lines (beam energy 17 keV). The last map shows the ratiobetween Ga(Kα) and As(Kβ) counts highlighting the change in the average (well plus barrier)AlxGayIn1-x-yAs quaternary composition in the SAG region. The fluorescence is scaled in arbitrary units.One of the most promising applications of SAG is EML [3] obtained by monolithic integration of anelectroabsorption modulator (EAM) with a distributed feedback laser. A voltage modulation at 10 GHzapplied to the EAM switches it between an opaque and a transparent state by means of the Stark effect [4]and ensures the modulation of the laser emission, allowing long-distance communications (up to 80 km) athigh frequency (10 Gb/s). The SAG EML device investigated in the present work is an AlxwGaywIn1-xw-ywAs/AlxbGaybIn1-xb-ybAs (compressive-strained well/tensile-strained barrier) MQW structure grown onInP by metallorganic vapour phase epitaxy. The SAG mask used for the growth (Figure 1a) featured 20 µmwide SiO2 stripes with a 30 µm opening width between them.Figure 1b shows the results of X-ray fluorescence (XRF) mapping at beamline ID22: the maps reveal thatGa Kα and As Kβ counts are higher in the SAG region owing to material enrichment caused by the SiO2stripes. The effectiveness of the SAG technique in modulating the chemical composition of the quaternaryalloy is proven by the map reporting the ratio between Ga and As counts in which a gradient in the averagewell/barrier chemical composition is clearly visible. Since the Ga/As ratio is lower in the SAG region than inthe field, we can assert that the average Ga content of the MQW structure progressively increases bymoving from the SAG to the field in the middle of a couple of stripes (along the region labelled “Y-line” inFigure 1a). This is due to the SAG effect that exploits the shorter effective diffusion length (D/k) of In withrespect to that of Ga and Al [2].The structural parameters of the sample were investigated by µ-X-ray diffraction (µ-XRD): 35 differentspatial points were sampled along the Y-line (Figure 2a). With such data, it is possible to obtain the width(wb, ww, Figure 2c) and the mismatch (mb, mw, Figure 2d) of the barrier and of the well by fitting theobserved pattern [5,6] using a model based on the dynamic theory of X-ray diffraction (Figure 2b). It isMulti quantum well electroabsorption-modulated laser characterised by ha... http://www.esrf.fr/news/spotlight/spotlight107/spotlight107/2 of 4 18/05/2010 09:42possible to see that wb and ww undergo a modulated increase moving from field to SAG regions: this is thedirect measure of the material enrichment in the SAG region. Moreover, both mb and mw values increasealmost monotonically moving from the field to the SAG region, reflecting the expected modulation of theAlxGayIn1-x-yAs composition of barrier and well layers, that makes possible two MQW heterostructureswith two different functions monolithically integrated in the same epitaxial growth.Figure 2. a) Selection of some of the 35 full X-ray diffraction patterns collected along the Y-line starting 30µm before the end of the stripes in the SAG region. b) Example of comparison between experimental andsimulated XRD patterns in the field region. c) Barrier and well widths and period as a function of theposition along the Y-line, obtained by simulation of the 35 experimental XRD patterns (the image at thebottom shows the location of the masked region). d) As part c) for the well, barrier and overall mismatches.This key information, coupled with the µm-determination of the energy gap (Figure 3) allows us tounderstand the structural gradient of the MQW structure along the Y-line from the field to the SAG region.The µm-space resolved photoluminescence study, reported in Figure 3, also highlights that thedistributed feedback laser (SAG region) emits at the desired wavelength for third window communications(l = 1.55 µm) and that the EAM (field region) has a higher energy gap. The latter point implies that themodulator is transparent to the radiation emitted by the laser, when the negative potential is not applied.Multi quantum well electroabsorption-modulated laser characterised by ha... http://www.esrf.fr/news/spotlight/spotlight107/spotlight107/3 of 4 18/05/2010 09:42Figure 3. a) Selection of seven (out of 100) photoluminescence spectra collected along a 1500 µm-longY-line covering the complete mask from field to field. b) Top: maximum of the emission for each spectrumvs. position along the Y-line. Bottom: optical micrograph correlating the photoluminescence maximum(top) with the position on the sample. The seven coloured points highlight the positions where the fullspectra reported in a) were collected.This unprecedented characterisation allowed us to determine the structure of the grown heterostructurewith a monolayer-resolution (~ 5 Å) along the growth axis and with µm-resolution in the growth plane. Thisknowledge yields the appropriate feedback needed to improve the growth process, previously based onlyon a trial/error approach. A new generation of devices may arise by applying this method to other SAGgrowths with different stripes and opening sizes [7]. References[1] A.M. Gibbon, J.P. Stagg, C G. Cureton, et al., Semicond. Sci. Technol. 8, 998 (1993).[2] J. Decobert, N. Dupuis, P.Y. Lagree, et al., J. Cryst. Growth 298, 28 (2007).[3] R.A. Salvatore, R.T. Sahara, M.A. Bock, and I. Libenzon, IEEE J. Quantum Electron. 38, 464 (2002).[4] D.A.B. Miller, D.S. Chemla, and T.C. Damen, Phys. Rev. Lett. 53, 2173 (1984).[5] C. Lamberti, Surf. Sci. Rep. 53, 1 (2004).[6] C. Ferrari and C. Bocchi, in Characterization of Semiconductor Heterostructures and Nanostructures,edited by C. Lamberti (Elsevier, Amsterdam, 2008), p. 93.[7] L. Mino, A. Agostino, S. Codato and C. Lamberti, J. Anal. At. Spectrom. 25, (2010), in press, DOI:10.1039/c000435a. Principal publication and authorsL. Mino (a), D. Gianolio (a), G. Agostini (a), A. Piovano (a), M. Truccato (b), A. Agostino (c), S. Cagliero (c),G. Martinez-Criado (d), S. Codato (e) and C. Lamberti (a), Structural Characterization of Multi-QuantumWells in Electroabsorption-Modulated Lasers by using Synchrotron Radiation Micrometer-Beams, Adv.Mater. 22, 2050–2054 (2010).(a) Department of Inorganic, Materials and Physical Chemistry, University of Turin (Italy)(b) Department of Experimental Physics, University of Turin (Italy)(c) Department of General and Organic Chemistry, University of Turin (Italy)(d) ESRF(e) Avago Technologies Italy S.r.l., Turin (Italy) European Synchrotron Radiation FacilityMulti quantum well electroabsorption-modulated laser characterised by ha... http://www.esrf.fr/news/spotlight/spotlight107/spotlight107/4 of 4 18/05/2010 09:42

Multi quantum well electroabsorption-modulated laser characterised by hard X-ray microprobe

https://iris.unito.it/bitstream/2318/77183/1/10SAG_Spotlight107.pdf

Multi quantum well electroabsorption-modulated laser characterised by hard X-ray microprobe

Abstract

Similar works

Full text

Available Versions

Institutional Research Information System University of Turin