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Study of Ohmic Contact Formation on Algan/Gan Heterostructures

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Study of Ohmic Contact Formation on Algan/Gan Heterostructures DEGREE PROJECT IN INFORMATION AND COMMUNICATION TECHNOLOGY, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2019 Study of ohmic contact formation on AlGaN/GaN heterostructures KAI-HSIN WEN KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE DF KTH Royal Institute of Technology Study of ohmic contact formation on Al- GaN/GaN heterostructures Master’s thesis in Nanotechnology Kai-hsin Wen Information and Communication Technology KTH ROYAL INSTITUTE OF TECHNOLOGY Stockholm, Sweden 2019 Master’s thesis 2019 Study of ohmic contact formation on AlGaN/GaN heterostructures Kai-hsin Wen Information and Communication Technology KTH Royal Institute of Technology Stockholm, Sweden 2019 Study of ohmic contact formation on AlGaN/GaN heterostructures Kai-hsin Wen © Kai-hsin Wen, 2019. Supervisors: Niklas Rorsman, Chalmers University of Technology Ding-yuan Chen, Chalmers University of Technology Examiner: Mattias Hammar, KTH Royal Institute of Technology Master’s Thesis 2019 Information and Communication Technology KTH Royal Institute of Technology SE-100 44 Stockholm Telephone +46 8 790 60 00 Cover: The contour plot of the obtained Rc from the laser focus/dose matrix. Typeset in LATEX, template by David Frisk Printed by KTH Stockholm, Sweden 2019 iv Study of ohmic contacts formation on AlGaN/GaN heterostructures Kai-hsin Wen Information and Communication Technology KTH Royal Institute of Technology SE-100 44 Stockholm Abstract It is challenging to achieve low-resistive ohmic contacts to III-nitride semiconductors due to their wide bandgap. A common way to reduce the contact resistance is to recess the ohmic area prior to metallization. In the minimization of the contact resistance, parameters like the recess depth, anneal temperature and design of the metal stack are commonly optimized. In this work, three other approaches have been evaluated. All experiments were performed on AlGaN/GaN heterostructures. The fabricated ohmic contacts were recess etched, metallized with a Ta/Al/Ta stack, and annealed at 550-575◦C. Firstly, it is shown that the laser writer intensity, transmittance and focus offset during optical lithography affect the contact resistance. The reason is believed to be the variation in the resist profile, which has an impact on the metal coverage. At the optimum intensity/transmittance/focus condition, which generates a relatively medium undercut, a contact resistance of 0.23 Ωmm was obtained. In the second approach, the metal layer of annealed contacts was removed by wet etching, followed by the re-deposition of a metal stack and annealing. The purpose was to increase the amount of N vacancies in the AlGaN, which are responsible for the contact formation. A minimum contact resistance of 0.41 Ωmm was achieved with this method, compared to 0.28 Ωmm with the regular method (without re- metallization). In the last approach, the bottom Ta layer was sputtered, whereas evaporation was used in all other cases. The minimum contact resistance was found to be 0.6 Ωmm, which was higher than for the evaporated contacts. The reason was assumed that the thickness of sputtered Ta should be thinner than the evaporated Ta due to its higher density. Moreover, the obtained lower sheet resistance is assumed to caused by the atomic scale damage due to the high energy ions during sputtering. Keywords: ohmic contacts, wide bandgap, Ta-based, recess etch, N-vacancies v Sammanfattning En utmaning med III-nitrid-halvledare är att uppnå låg-resistivitetskontakter, på grund av deras breda bandgap. Ett konventionellt tillvägagångsätt för att reducera kontaktresistansen är att fördjupa ohmska ytan före metallisering. I strävandet av att minska den ohmska resistansen sker vanligtvis en optimering av följande parame- trar, recessddjup, anlöpningstemperatur och metallagersdesign. I detta arbete så har samtliga tre parametrar evaluerats. Alla experiment utfördes på AlGaN/GaN- heterostrukturer. De tillverkade ohmska kontakterna var recesssetsade, metalliser- ade med ett Ta/Al/Ta lager och anlöpt vid 550-575◦C. Den primära undersökningen, visar att laserritar-intensitet, -transmission och - fokusförskjutning under optisk litografi inverkar på kontaktresistansen. Anledningen antas vara variation i resistprofilen, vilket påverkar metallbeläggningen. Vid opti- mal intensitet/transmission/fokus-förhållanden, (som genererar en underskärning), blev den resulterande kontaktresistansen 0.23 Ωmm uppmätt. I en sekundär undersökning, avlägsnas ohmska kontaktens metallager genom våtet- sning, följt av en återdeponering av ett nytt metallager, samt anlöpning. Syftet var att öka mängden N-vakanser i AlGaN-lagret, som formar ohmska kontakten. Min- sta kontaktresistansen uppmätt var 0.41Wmm, att jämföras med 0.28 Ωmm, som uppnåddes genom den konventionella metoden (utan återmetallisering). Den sista undersökningen jämförde sputtrade med evaporerade bottenlager av Ta, (evaporation användes som standardmetod i de tidigare undersökningarna). Med sputtrning blev den minsta kontakresistansen 0.6 Ωmm, (högre än de evaporerade kontakterna). En hypotetisk förklarning kan vara att det sputtrade Ta-lagret är tunnare än det evaporerade Ta-lagret, på grund av en dess högre densitet. Därutöver, den uppmätta lägre skiktresistansen antas bero på den skada i atomskala som sker vid de höga energi-kollisioner som joner skapar vid sputtrning. vi Acknowledgements This thesis work is pursued at the Department of Microtechnology and Nanoscience - MC2, Microwave Electronics Laboratory. It is a great experience for me to work here and during this period I have gained so much knowledge. I would like to sincerely thank my supervisor Niklas Rorsman for giving me the chance to come to Chalmers and work with him. Despite his tight schedule, his door is always open for discussion whenever I have any question. Furthermore I would like to express my deepest gratitude to my daily supervisor Ding-yuan Chen for his patience and guidance. Thank you for teaching me the processing techniques in the cleanroom and all the measurements used in this thesis. I would also like to thank Hans Hjelmgren for helping me build a simulation model and teaching me to use TCAD simulation. Also, I would like to appreciate all the people in Microwave Electronics Laboratory for creating such a fantastic working environments. Finally, I am deeply grateful to my family and all my friends for always being supportive encouraging me to push on. Kai-hsin Wen, Gothenburg, June 2019 viii x Contents List of Figures xiii List of Tables xv 1 Introduction1 1.1 Thesis objectives and summarized results................2 1.2 Thesis outline...............................3 2 Ohmic contact technology5 2.1 Metal-semiconductor contact.......................5 2.1.1 Current transport mechanisms..................6 2.2 Ohmic contact mechanism........................8 2.3 Ohmic contact types...........................9 2.3.1 Planar contacts..........................9 2.3.2 Recessed contacts......................... 10 2.3.3 n+-GaN regrowth contacts.................... 12 2.4 Physical modelling............................ 13 3 Fabrication Process 17 3.1 Standard Cleaning............................ 17 3.2 LPCVD.................................. 18 3.3 Mesa.................................... 18 3.3.1 Photolithography......................... 19 3.3.2 Plasma Ashing.......................... 20 3.3.3 Plasma Etching.......................... 21 3.4 Ohmic Contact.............................. 21 3.4.1 Metal Deposition......................... 21 3.4.2 Re-metalization.......................... 22 3.4.3 Sputtered Ta........................... 23 4 Characterization 25 4.1 Scanning electron microscopy...................... 25 4.2 Transmission line method........................ 26 4.2.1 TLM structure.......................... 26 4.2.2 Epi-layer sheet resistance Rsh .................. 27 4.2.3 Contact resistance Rc and specific contact resistivity ρc .... 27 xi Contents 5 Results 31 5.1 Laser writer focus/intensity test..................... 31 5.2 Ohmic contact re-metallization..................... 33 5.3 Sputtered Ta............................... 36 6 Conclusion and future work 39 Bibliography 41 xii List of Figures 1.1 Schematic of GaN HEMT structure....................1 2.1 Band diagram of metal-semiconductor in equilibrium [18].......5 2.2 Schematic of three different carrier transport mechanism for different Nd.[24]..................................7 2.3 E00 plotted as a function of doping concentration for GaN at T= 300K.8 2.4 Illustration of three different recess depth cases. (a)the barrier is still present (b)the barrier is still present but it is too thin to retain 2DEG under it (c)the barrier is completely removed in etching process.... 11 2.5 The simulation model structure(a)model from software (b)schematic of the model................................ 13 2.6 Schematic of the simulation model structure and the obtained results of various doping depth, concentration and the temperature...... 15 3.1 Schematic of the ohmic structure in this work.............. 17 3.2 The schematic of the position of photoresist and the laser beam.... 19 3.3 The etchant versus etching target material (X: The target material can not be etched by the etchant, : The target material can be etched by the etchant, –: Not found from the literature)........ 23 4.1 (a)Schematic TLM strcture (b)Microscope image of the TLM structure 26 4.2 (a)A schematics shows the different components of Rtot. (b) Total resistance Rtot plotted as a function of isolation distance
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