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Fabrication of Indium Arsenide Quantum Dot Structure for Semiconductor Optical Amplifiers

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Fabrication of Indium Arsenide Quantum Dot Structure for Semiconductor Optical Amplifiers Henri Jussila Fabrication of Indium Arsenide Quantum Dot Structure for Semiconductor Optical Amplifiers Faculty of Electronics, Communications and Automation Thesis submitted for examination for the degree of Master of Science in Technology. Espoo 30.3.2010 Thesis supervisor: Prof. Markku Sopanen Thesis instructors: Doc. Teppo Hakkarainen Dr. Abuduwayiti Aierken Aalto University School of Science A’’ and Technology aalto university abstract of the school of science and technology master's thesis Author: Henri Jussila Title: Fabrication of Indium Arsenide Quantum Dot Structure for Semiconductor Optical Amplifiers Date: 30.3.2010 Language: English Number of pages:7+41 Faculty of Electronics, Communications and Automation Department of Micro- and Nanotechnology Professorship: Optoelectronics Code: S-104 Supervisor: Prof. Markku Sopanen Instructors: Doc. Teppo Hakkarainen, Dr. Abuduwayiti Aierken In this thesis, the properties of self-assembled InAs/InGaAsP/InP quantum dots (QD) have been investigated. The focus of this work was to fabricate such a structure that can be used in a telecommunication wavelength semiconductor op- tical amplifier (SOA). The examined InAs QD samples were fabricated by met- alorganic vapor-phase epitaxy technique. Photoluminescence (PL) and atomic force microscopy (AFM) were used in the characterization of the optical and the structural properties of the samples, respectively. InAs island size and the areal density was first optimized for SOAs by growing self-assembled InAs islands di- rectly on InP. These results were used to finalize the structure containing also InGaAsP waveguide layer. The AFM results of the final structure show that InAs islands were grown quite homogenously on InGaAsP with an average height of 10 nm and an average base diameter of 30 nm. The low temperature PL peak of InAs/InGaAsP/InP quantum dots was tuned to 1.56 µm wavelength. Keywords: Indium phosphide, indium arsenide, indium gallium arsenide phos- phide, quantum dot, semiconductor optical amplifier, metalorganic vapor phase epitaxy. aalto-yliopisto diplomityon¨ teknillinen korkeakoulu tiivistelma¨ Tekij¨a:Henri Jussila Ty¨onnimi: Indiumarsenidikvanttipisterakenteen valmistaminen optiseen puolijohdevahvistimeen P¨aiv¨am¨a¨ar¨a:30.3.2010 Kieli: Englanti Sivum¨a¨ar¨a:7+41 Elektroniikan, tietoliikenteen ja automaation tiedekunta Mikro- ja nanotekniikan laitos Professuuri: Optoelektroniikka Koodi: S-104 Valvoja: Prof. Markku Sopanen Ohjaajat: Dos. Teppo Hakkarainen, Tkt. Abuduwayiti Aierken T¨am¨a diplomity¨o tutkii itseorganisoidusti valmistettuja InAs kvanttipis- teit¨a InGaAsP:n ja InP p¨a¨all¨a. Ty¨on tavoitteena on ollut sellaisen kvanttipisterakenteen valmistaminen, joka soveltuu optiseen puolijohdevahvis- timeen (SOA) tietoliikenneaallonpituuksilla. Ty¨ot¨avarten valmistettiin InAs- kvanttipisten¨aytteit¨ametallo-organisella kaasufaasiepitaksialla. Ty¨onn¨aytteet karakterisoitiin fotoluminesenssi- ja atomivoimamikroskopiamittausmenetelmill¨a. InAs-saarekkeiden koko ja tiheys optimoitiin ensiksi sopivaksi SOA-rakennetta varten valmistamalla itseorganisoituvia InAs-saarekkeita suoraan InP p¨a¨alle. N¨ait¨atuloksia k¨aytettiin optimoidun InGaAsP-kerroksen sis¨alt¨av¨anrakenteen valmistuksessa. Atomivoimamikroskopiamittaus paljasti, ett¨aoptimoidun raken- teen InAs-saarekkeet olivat keskim¨a¨arin 10 nm korkeita ja 30 nm leveit¨a.Kvant- tipisteiden luminesenssipiikin maksimi havaittiin 1.56 µm aallonpituudella 10 K l¨amp¨otilassa. Avainsanat: Indiumarsenidi, indiumfosfidi, indiumgalliumarsenidifosfidi, kvant- tipiste, optinen puolijohdevahvistin, metallo-organinen kaasu- faasiepitksia. iv Preface This thesis has been carried out in Optoelectronics group at the Department of Micro and Nanosciences at Aalto University. I would like to thank Professor Markku Sopanen for providing me with the oppurtunity to work in the Optoelectronics group while similarly carring out this thesis. I also would like to thank him for everything I have learned during numerous interesting discussions with him related to this thesis. Furthermore, I would like to thank my thesis instructor Docent Teppo Hakkarainen for all the help and guidance he has given me during this work. In addition to this, I would like to thank my other thesis instructor Dr. Abuduwayiti Aierken for teaching me the secrects behind the MOVPE apparatus, and also always providing help when I have needed it. I want to thank M.Sc P¨aiviMattila and Dr. Nagarajan Subramaniyam for all the support and help they have given me during this thesis work. I own great compliments to them. I also want to thank the whole personnel of the department of the Micro and Nanosciences for the inspiring and nice athmosphere that have encouraged me to come to work excited day after day. Finally, I want to thank my parents and my brother for all the support they have given me. Without you this thesis would never have been possible. Otaniemi, 30.3.2010 Henri Jussila v Contents Abstract ii Abstract (in Finnish) iii Preface iv Contents v List of acronyms vii 1 Introduction 1 2 Theoretical 2 2.1 Solid state physics . .2 2.1.1 Crystal structure . .2 2.1.2 Energy band structure . .3 2.1.3 III-V compound semiconductors . .5 2.2 Quantum structures . .6 2.2.1 Semiconductor quantum wells and quantum dots . .7 2.2.2 Real quantum dot systems . .8 2.3 Semiconductor optical amplifier . .9 2.3.1 Indium arsenide quantum dot semiconductor optical amplifier 12 3 Metalorganic vapor phase epitaxy 13 3.1 Source materials and pyrolysis . 13 3.2 Surface processes and epitaxial growth . 14 3.2.1 Surface processes and heteroepitaxy . 14 3.2.2 Stranski-Krastanow growth mode . 17 3.3 Mass-transport controlled growth . 17 3.4 Setup . 18 3.5 Growth of InP and lattice-matched InGaAsP on InP substrates and As/P exchange reaction . 20 4 Characterization methods 22 4.1 Atomic force microscopy . 22 vi 4.2 Photoluminescence measurement . 23 4.3 X-ray diffraction . 25 5 Results 26 5.1 Fabrication of InAs QD samples . 26 5.2 Self-assembled growth of InAs islands on InP . 27 5.3 Optical properties of InAs/InP QDs . 30 5.4 Tuning the QD peak position to 1,55 µm with InGaAsP . 33 6 Conclusions 38 References 39 vii List of acronyms AFM atomic force microscopy AlGaN aluminium gallium nitride DOS density of state EDFA erbium doped fiber amplifier FCC face centered cubic FM Frank-van der Merve growth mode FWHM full-width-at-half-maximum GaAs gallium arsenide GaN gallium nitride InAs indium arsenide InGaAsP indium gallium arsenide phosphide InP indium phosphide LT-PL low-temperature photoluminescence MFC mass flow controller ML monolayer MOVPE metalorganic vapor phase epitaxy PL photoluminescense QD quantum dot QW quantum well RT-PL room-temperature photoluminescence SK Stranski-Krastanow growth mode SOA semiconductor optical amplifier TBAs tertiarybutylarsine TBP tertiarybutylphosphine TEM transmission electron microscopy TMGa trimethylgallium TMIn trimethylindium VW Volmer-Weber growth mode WL wetting layer XRD X-ray diffraction 1 1 Introduction In recent years, continuous improvements in optical data transfer systems have en- abled the modern information society. At present, vast amount of information coded in optical signals propagates all around the world inside optical fibers. Optical sig- nals in long-haul networks have a wavelength of 1.55 µm which corresponds to the absorption minimum of light in silica fibers. However, the intensity of these signals still considerably decays without amplification. Therefore, efficient optical ampli- fiers are required. Previous research has provided many suitable amplifier solutions for operation at telecommunication wavelengths [1]. For instance, erbium doped fiber amplifiers (EDFAs) are able to amplify optical signals inside the fibers [2]. Semiconductor optical amplifiers (SOA) are used in applications that amplify the optical signals outside the optical fibers [3]. Previously, telecommunication wavelength region SOAs have been fabricated from indium gallium arsenide (phosphide)/indium phosphide (InGaAs(P)/InP)-based bulk and quantum well (QW) materials [4, 5]. In recent years, indium arsenide (InAs) quantum dot (QD) and quantum dash structures grown on InP substrates have been examined and fabricated as a new solution for telecommunication wavelength region components [6, 7]. In order to realize the full potential of these components, a significant amount of effort has been addressed to characterizing the properties of InAs/InP QDs [8-14]. It has been shown that InAs QD layer can provide higher light emission efficiency than conventional InGaAsP/InP QW layer [11]. Therefore, InAs QDs on InP substrates could potentially provide a material that enables telecom- munication wavelength SOAs to operate more efficiently. Although these materials are promising, many problems still remain to be solved for developing high-quality materials for commercial applications. In this Master0s thesis, InAs QDs have been grown by metalorganic vapor-phase epitaxy (MOVPE) technique on InP substrates, and their properties have been in- vestigated. Photoluminescence (PL), atomic force microscopy (AFM), and X-ray diffraction (XRD) were used in the characterization of the optical and the struc- tural properties of the samples, respectively. InAs island size and the areal den- sity was first optimized for SOAs by growing self-assembled InAs islands directly on InP. These results were used to finalize the structure containing also InGaAsP waveguide layer. AFM results show that InAs islands, which eventually form the QDs, were grown quite homogenously on InGaAsP
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