Departm en t of Micro- an d Nan oscien ces Aa lto- Lauri Riuttanen Riuttanen Lauri DD 213 Diffusion injected light / 2015 emitting diode Diffusion injected light emitting diode diode emitting light injected Diffusion Lauri Riuttanen 9HSTFMG*agfibg+ 9HSTFMG*agfibg+ ISB N 978 -952-60-658 1-6 (pr in ted) B USINESS + ISB N 978 -952-60-658 2-3 (pdf) ECONOMY ISSN-L 1799-493 4 ISSN 1799-493 4 (pr in ted) ART + ISSN 1799-4942 (pdf) DESIGN + ARCHITECTURE Aalto Un iversity Aalto Un iversity School of Electrical En gin eerin g SCIENCE + Departm en t of Micro- an d Nan oscien ces TECHNOLOGY www.aalto.fi CROSSOVER DOCTORAL DOCTORAL DISSERTATIONS DISSERTATIONS 2015 Aalto University publication series DOCTORAL DISSERTATIONS 213/2015 Diffusion injected light emitting diode Lauri Riuttanen A doctoral dissertation completed for the degree of Doctor of Science (Technology) to be defended, with the permission of the Aalto University School of Electrical Engineering, at a public examination held at the large seminar room of the Micronova-building on 18 December 2015 at 12. Aalto University School of Electrical Engineering Department of Micro- and Nanosciences Supervising professor Professor Markku Sopanen, Aalto University, Finland Thesis advisor Doctor Sami Suihkonen, Aalto University, Finland Preliminary examiners Doctor Rachel Oliver, University of Cambridge, United Kingdom Professor Magnus Borgström, Lund University, Sweden Opponent Professor Charles Thomas Foxon, University of Nottingham, United Kingdom Aalto University publication series DOCTORAL DISSERTATIONS 213/2015 © Lauri Riuttanen ISBN 978-952-60-6581-6 (printed) ISBN 978-952-60-6582-3 (pdf) ISSN-L 1799-4934 ISSN 1799-4934 (printed) ISSN 1799-4942 (pdf) http://urn.fi/URN:ISBN:978-952-60-6582-3 Unigrafia Oy Helsinki 2015 Finland Abstract Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Lauri Riuttanen Name of the doctoral dissertation Diffusion injected light emitting diode Publisher School of Electrical Engineering Unit Department of Micro- and Nanosciences Series Aalto University publication series DOCTORAL DISSERTATIONS 213/2015 Field of research Optoelectronics Manuscript submitted 10 August 2015 Date of the defence 18 December 2015 Permission to publish granted (date) 17 November 2015 Language English Monograph Article dissertation (summary + original articles) Abstract Lighting plays a major role in consumption of electrical energy in the world. Thus, increasing the efficiency of light sources is one key element in reducing the green house gas emissions. Light emitting diodes (LEDs) are gaining a foothold in general lighting. Despite their rapid development in light output and their superior efficiency compared to other light sources, LEDs still need improvements in order to become the ultimate lighting technology. A typical LED is a double heterojunction (DHJ) structure, in which the active region fabricated from a lower band gap material is sandwiched between higher band gap p- and n-doped regions. By biasing such a structure electrons and holes are transferred by current into the active region, where they recombine releasing energy as photons. The carrier injection in a conventional LED structure is typically efficient. However, in more exotic novel structures based on nanowires or near surface nanostructures, fabricating a DHJ becomes difficult. This thesis presents the experimental studies on a novel current injection method for light emitting applications. The method is based on bipolar diffusion of charge carriers. Unlike in the conventional method, the active region does not have to placed between the p- and n-layers of the pn-junction. The diffusion injection method is experimentally demonstrated by two types of prototype structures. The first prototype was fabricated using a multi quantum well (MQW) stack buried under the pn-junction. The second prototype was fabricated using a near surface quantum well (QW) placed on top of the pn-junction. The first prototype showed that the diffusion current components can be used to excite an active region outside of the pn-junction. The second prototype showed a large improvement in injection efficiency as well as the suitability of the method for exciting surface structures. The applications of diffusion injection can be found in blue galliun nitride based LEDs studied in this thesis as well as in green solid-state light sources, light sources integrated into silicon technology and devices based on nanostructures and plasmonics. Keywords gallium nitride, III-nitrides, LED, diffusion ISBN (printed) 978-952-60-6581-6 ISBN (pdf) 978-952-60-6582-3 ISSN-L 1799-4934 ISSN (printed) 1799-4934 ISSN (pdf) 1799-4942 Location of publisher Helsinki Location of printing Espoo Year 2015 Pages 121 urn http://urn.fi/URN:ISBN:978-952-60-6582-3 Tiivistelmä Aalto-yliopisto, PL 11000, 00076 Aalto www.aalto.fi Tekijä Lauri Riuttanen Väitöskirjan nimi Diffusiolla viritetty loistediodi Julkaisija Sähkötekniikan korkeakoulu Yksikkö Mikro- ja nanotekniikan laitos Sarja Aalto University publication series DOCTORAL DISSERTATIONS 213/2015 Tutkimusala Optoelektroniikka Käsikirjoituksen pvm 10.08.2015 Väitöspäivä 18.12.2015 Julkaisuluvan myöntämispäivä 17.11.2015 Kieli Englanti Monografia Yhdistelmäväitöskirja (yhteenveto-osa + erillisartikkelit) Tiivistelmä Valaistus kuluttaa merkittävän osan sähköenergiasta maailmanlaajuisesti. Valaistuksen hyötysuhteen parantaminen on siis yksi merkittävimmistä menetelmistä vähentää kasvihuonepäästöjä. Vaikka loistediodien (LEDs, engl. light emitting diodes) nopea kehitys ja niiden ylivoimainen hyötysuhde kilpaileviin teknologioihin on antanut LED:eille jalansijaa myös yleisvalaistuksessa, LED-teknologia tarvitsee vielä tutkimusta, jotta ne voisivat täydellisesti korvata vaihtoehtoiset teknologiat. Perinteinen LED perustuu kaksoisheteroliitokseen (DHJ, engl. double heterojunction), jossa aktiivinen alue on valmistettu p- ja n-tyyppiseksi seostettujen alueiden väliin. Kytkemällä jännite rakenteen yli elektronit ja aukot liikkuvat sähkökentän mukaisesti ajautumisvirtana seostetutuista alueista kohti aktiivista aluetta, jossa ne rekombinoituvat vapauttaen energiaa fotoneina. Varaustenkuljettajien syöttö on perinteisessä LED-rakenteessa tyypillisesti erittäin hyvä. Haasteita ilmenee uusissa, eksoottisissa LED-rakenteissa, jotka perustuvat lähellä pintaa oleviin nanorakenteisiin. Näissä tapauksissa DHJ:n valmistaminen on hankalaa ja varaustenkuljettajien syöttö on usein heikkoa. Tämä väitöskirja esittelee kokeellisia tutkimuksia uudesta diffuusioon perustuvasta LED- rakenteiden viritysmenetelmästä. Menetelmässä aktiivisen alueen ei tarvitse olla seostettujen alueiden välissä. Kokeelliset todisteet menetelmän toimivuudesta näytetään kahdella eri tyyppisellä prototyyppirakenteella. Ensimmäinen prototyyppi on pn-liitoksen alle haudattu monikvanttikaivorakenne. Toisessa prototyypissä yksittäinen kvanttikaivo on pn-liitoksen päällä. Ensimmäinen koerakenne osoitti, että pn-liitoksen ulkopuolelle valmistetun aktiivisen alueen viritys onnistuu. Toisella rakenteella havaittiin merkittävä hyötysuhteen parannus. Se myös osoitti, että pinnassa sijaitseva rakenne voidaan virittää sen alla olevalla pn-liitoksella. Mahdollisia sovelluksia menetelmälle löytyy jo mainittujen sovellusten lisäksi vihreiden puolijohdevalolähteiden valmituksessa sekä pii-teknologiaan integroitavissa valolähteissä. Avainsanat galliunnitridi, III-nitridit, LED, diffuusio ISBN (painettu) 978-952-60-6581-6 ISBN (pdf) 978-952-60-6582-3 ISSN-L 1799-4934 ISSN (painettu) 1799-4934 ISSN (pdf) 1799-4942 Julkaisupaikka Helsinki Painopaikka Espoo Vuosi 2015 Sivumäärä 121 urn http://urn.fi/URN:ISBN:978-952-60-6582-3 Preface The work presented in this thesis was carrier out in Department of Micro- and Nanosciences in School of Electrical Engineering of Aalto University. Firstly, I want to thank professor Markku Sopanen for giving me this op- portunity to work, study and perform my research in his group under his guidance. Secondly, I want to express my gratitude for Dr. Sami Suihko- nen for instructing me in my research as well as for his insight and ex- pertise in the field. I would also like to thank my colleagues, who have become more than co-workers but also good friends. In addition to their work-related discussions, the good company and off-topic discussions have made my research more than just work but also a pastime. Especially, I would like to thank Dr. Henri Nykänen for his expertise in processing and cleanroom work, Dr. Pyry Kivisaari and Dr. Jani Oksanen for their simulation work and helping me to understand the physics of LEDs, and Olli Svensk for his know-how in MOVPE growth. My gratitudes also go for the University of Jyväskylä, whose National Doctoral Programme in Nanoscience (NGS-Nano) funded a significant part of my research. My friends and family have been the most supportive during my time in stud- ies and my life in general. My final gratitudes goes to my lovely wife Sini for her love and support. He, who controls ammonia, controls MOVPE. Ammonia must flow. Espoo, November 30, 2015, Lauri Riuttanen i Preface ii Contents Preface i Contents iii List of Publications v Author’s Contribution vii List of abbreviations ix List of symbols xi 1. Introduction 1 2. Background 5 2.1 History of LEDs .......................... 5 2.2 History of III-nitrides ....................... 6 3. Electrical properties of semiconductors 9 3.1 Band theory ............................ 9 3.2 Current transport in semiconductors .............. 10 3.3 Polarisation fields and piezoelectricity