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Surface studies on α–sapphire for potential use in GaN epitaxial growth
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
2009 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This Licentiate thesis summarizes the work carried out by the author the years 2004 to 2008 at the University of Iceland and the Royal Institute of Technology (KTH) in Sweden. The aim of the project was to investigate the structure of sapphire (alpha-Al2O3) surfaces, both for pure scientific reasons and also for potential use as substrate for GaN-growth by molecular beam epitaxy.

More generally the thesis describes some surface science methods used for investigating the substrates; the general physical back ground, the experi- mental implementation and what information they can give. The described techniques are used for surface analysis on sapphire substrates which have been treated variously in order to optimize them for use as templates for epi- taxial growth of GaN or related III-V compounds.

The thesis is based on three published papers.

The first paper focuses on the formation a thin AlN layer on sapphire, which may act as a buffer layer for potential epitaxial growth of GaN or any related III-V materials. Two types of sapphire substrates (reconstructed and non- reconstructed) were exposed to ammonia resulting in the formation of AlN on the surface. The efficiency of the AlN formation (nitridation efficiency) for the two surfaces was then compared as a function of substrate temperature through photoelectron spectroscopy and low electron energy diffraction. The reconstructed surface showed a much higher nitridation efficiency than the non-reconstructed surface.

In the second paper, the affect of different annealing processes on the sapphire morphology, and thus its capability to act as a template for GaN growth, was studied. Atomic force microscopy, X-ray diffraction analysis together with ellipsometry measurements showed that annealing in H2 ambient and subse- quent annealing at 1300 °C in O2 for 11 hours resulted in high quality and atomically flat sapphire surface suitable for III-V epitaxial growth.

The third paper describes the effect of argon sputtering on cleaning GaN surfaces and the possibility of using indium as surfactant for establishing a clean and stoichiometric GaN surface, after such sputtering. Soft sputtering, followed by deposition of 2 ML of indium and subsequent annealing at around 500 °C resulted in a well ordered and clean GaN surface while hard sputtering introduced defects and incorporated both metallic gallium and indium in the surface.

Place, publisher, year, edition, pages
KTH , 2009. , xi, 49 p.
Series
Trita-ICT/MAP, 2009:3
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-10669ISBN: 978-91-7415-286-9 (print)OAI: oai:DiVA.org:kth-10669DiVA: diva2:223677
Presentation
2009-06-05, Sal/Hall C1, KTH-Electrum, Isafjordsgatan 26, Kista, 10:15 (English)
Opponent
Supervisors
Available from: 2009-06-22 Created: 2009-06-13 Last updated: 2010-11-03Bibliographically approved
List of papers
1. Influence of initial surface reconstruction on nitridation of Al2O3 (0001) using low pressure ammonia
Open this publication in new window or tab >>Influence of initial surface reconstruction on nitridation of Al2O3 (0001) using low pressure ammonia
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2007 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 101, no 1, 013519- p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this study is to investigate the effect of initial surface reconstruction on the nitridation process of Al2O3 (0001). This was done by exposing differently reconstructed sapphire substrates at different temperatures to low pressure ammonia (NH3). Structural and chemical analysis were carried out using low-electron energy diffraction and x-ray photoelectron spectroscopy. The experiments revealed that using low pressure ammonia (P-NH3 < 1 X 10(-5) Torr), no nitridation takes place on (1x1) unreconstructed surfaces. However, when the unreconstructed surface starts to change to a (root 31 x root 31) R +/- 9 degrees reconstructed surface, with increasing substrate temperature, the nitridation becomes successful. When using the initially reconstructed surface, the nitridation is successful even from the lowest temperature used. These results suggest that the initial surface reconstruction has a major effect on the nitridation process. This kinetic behavior has not been reported before, with most nitridation studies mainly focusing on the effect of surface temperature on the resulting surface morphology, rather than the actual kinetics of the process itself.

Keyword
molecular-beam epitaxy, sapphire nitridation, alpha-alumina, gan growth, xps, plasma, films, temperature, alpha-al2o3, substrate
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-16331 (URN)10.1063/1.2384807 (DOI)000243585200039 ()2-s2.0-33846332036 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2010-12-10Bibliographically approved
2. Characterisation of high-temperature annealing effects on alpha-Al2O3(0001) substrates
Open this publication in new window or tab >>Characterisation of high-temperature annealing effects on alpha-Al2O3(0001) substrates
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2008 (English)In:  PROCEEDINGS OF THE 17TH INTERNATIONAL VACUUM CONGRESS/13TH INTERNATIONAL CONFERENCE ON SURFACE SCIENCE/INTERNATIONAL CONFERENCE ON NANOSCIENCE AND TECHNOLOGY    / [ed] Johansson LSO; Andersen JN; Gothelid M; Helmersson U; Montelius L; Rubel M; Setina J; Wernersson LE, 2008, Vol. 100Conference paper, Published paper (Refereed)
Abstract [en]

High temperature annealing in air has been applied as an effective ex-situ surface treatment for the alpha-Al2O3(0001) substrates used in molecular beam epitaxy (MBE) growth of III-nitrides. The method is based on the criterion that atomically smooth surface of terrace-and-step like structure, which is considered to be crucial in obtaining a high quality epilayer, could be produced upon high temperature annealing. The annealed surface was mostly studied by atomic force microscopy (AFM) imaging. In this work, the effects of high temperature annealing on the surface morphology, crystalline quality, optical quality and surface reconstruction behaviour of alpha-Al2O3(0001) substrates were fully studied using AFM, triple-axis high resolution X-ray diffraction (THRXRD), spectroscopic ellipsometry (SE) and insitu reflection high-energy electron diffraction (RHEED). A new strategy, H-2 thermal cleaning at 1100 degrees C followed by O-2 annealing at 1300 degrees C was proposed as an efficient surface treatment for alpha-Al2O3 (0001) substrates for MBE growth.

Series
Journal of Physics Conference Series, ISSN 1742-6588 ; 100
Keyword
AFM, Atomically smooth surface, Characterisation, Crystalline quality, Efficient surface, Ex situ, High quality, High resolution X ray diffraction, High-temperature annealing, III-nitrides, In-situ, MBE growth, New strategy, Optical qualities, Thermal cleaning
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-25035 (URN)10.1088/1742-6596/100/4/042020 (DOI)000275655200068 ()
Conference
17th International Vacuum Congress/13th International Conference on Surface Science/Internatinal Conference on Nanoscience and Technology. Stockholm, SWEDEN, JUL 02-06, 2007
Note
QC 20101006Available from: 2010-10-06 Created: 2010-10-06 Last updated: 2010-10-06Bibliographically approved
3. Investigation on the role of indium in the removal of metallic gallium from soft and hard sputtered GaN (0001) surfaces
Open this publication in new window or tab >>Investigation on the role of indium in the removal of metallic gallium from soft and hard sputtered GaN (0001) surfaces
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2009 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 517, no 21, 6023-6026 p.Article in journal (Refereed) Published
Abstract [en]

Cleaning of GaN by argon sputtering and subsequent annealing introduces metallic gallium on the GaN surface. Once formed, this metallic gallium can be difficult to remove. it has a strong influence on the Fermi level position in the band gap and poses a problem for subsequent epitaxial growth on the surface. We present a method of removing metallic gallium from moderately damaged GaN surfaces by deposition of indium, and formation of an In-Ga alloy that can be desorbed by annealing at similar to 550 degrees C. After the In-Ga alloy has desorbed, photoemission spectra show that the Ga3d bulk component becomes narrower indicating a smoother and more homogeneous surface. This is also reflected in a sharper low energy electron diffraction pattern. On heavily damaged GaN surfaces, caused by hard sputtering, larger amount of metallic gallium forms after annealing at 600 degrees C. This gallium readily alloys with deposited indium, but the alloy does not desorb until a temperature of 840 degrees C is reached and even then, traces of both indium and metallic gallium could be found on the surface.

Keyword
Gallium nitride, Surfactant, Cleaning, Indium, Argon sputtering, Surface defects, Photoemission spectroscopy, stoichiometric gan(0001)-1 x-1, core-level photoemission, molecular-beam epitaxy, electron accumulation, ion-bombardment, n-type, deposition, nitride
Identifiers
urn:nbn:se:kth:diva-18636 (URN)10.1016/j.tsf.2009.05.023 (DOI)000268438200013 ()2-s2.0-67449127658 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2010-12-10Bibliographically approved

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