1 On the evolution of InAs thin films grown by molecular beam epitaxy on the GaAs(0 0 1) surface vorgelegt von Diplom-Physiker Jan Grabowski aus Güstrow Von der Fakultät II Mathematik und Naturwissenschaften der Technischen Universität Berlin zur Erlangung des akademischen Grades Doktor der Naturwissenschaften Dr. rer. nat. genehmigte Dissertation Promotionsausschuss: Vorsitzender: Prof. Dr. rer. nat. Michael Lehmann Berichter: Prof. Dr. rer. nat. Mario Dähne Berichter: Dr. rer. nat. Lutz Geelhaar Tag der wissenschaftlichen Aussprache: 14. Dezember 2010 Berlin 2010 D 83
3 Der Beginn aller Wissenschaften ist das Erstaunen, dass die Dinge sind, wie sie sind. Aristoteles
4 Zusammenfassung Halbleiternanostrukturen sind zurzeit von großem Interesse für weitreichende Anwendungen in der Elektronik und Optoelektronik. Viele dieser Bauteile, insbesondere für die optische Datenübertragung im Bereich großer Wellenlängen, die von grundlegender Bedeutung in der modernen Kommunikationstechnik ist, basieren auf InAs/GaAs-Quantenpunktstrukturen (QD). Auch wenn die Eigenschaften der InAs/GaAs-QDs bereits intensiv untersucht wurden, so ist doch immer noch nur sehr wenig über die Benetzungsschicht (WL) bekannt. Im Rahmen dieser Arbeit wurde der Verlauf der Entstehung dieses InAs-WL im Detail untersucht. Dazu wurden mittels Molekularstrahlepitaxie (MBE) dünne InAs-Schichten im Bereich einer Monolage (ML) auf die GaAs(0 0 1) Oberfläche aufgedampft und anschließend mit reflektiver hochenergetischer Elektronenbeugung (RHEED) und insbesondere mit der Rastertunnelmikroskopie (STM) untersucht. Die dünnen InAs Schichten wurden in den beiden typischen Wachstumsbereichen gewachsen, auf der GaAs-c(4 4) und der GaAs-β2(2 4) rekonstruierten Oberfläche, mit variabler Schichtdicke von Submonolagen mit 0,09 ML InAs bis zu 1,65 ML InAs, bei der die kritische Schichtdicke für das QD-Wachstum überschritten wird. Dabei wurden drei grundsätzliche Wachstumsphasen entdeckt. Bei niedrigen InAs-Bedeckungen adsorbiert das Indium bevorzugt an energetisch günstigen Positionen auf der Oberfläche in Ansammlungen von durchschnittlich acht Indiumatomen. In den STM-Aufnahmen erscheinen diese Ansammlungen als deutliche helle Signaturen. Es werden ein Strukturentwicklungsmodell vorgestellt und die elektronischen Eigenschaften sowie die Gitterverspannung diskutiert. Bei einer InAs-Bedeckung von 0,67 ML transformiert die ursprüngliche Oberfläche in eine (4 3) rekonstruierte In 2/3 Ga 1/3 As-ML und deren detaillierte Struktur und Verspannungseigenschaften werden aufgezeigt. Weiter aufgedampftes InAs bildet dann eine zweite Lage auf der InGaAs-ML, gekennzeichnet durch eine typische zick-zack Anordnung von (2 4) rekonstruierten Einheitszellen, die eine abwechselnde α2/α2-m Konfiguration besitzen. Im Gegensatz zu den vorherigen Oberflächenrekonstruktionen, bei denen die strukturelle Gitterverspannung effizient abgebaut werden kann, staut sich in dieser zweiten (2 4) rekonstruierten Schicht mit dem weiteren Einbau von Indiumatomen eine kompressive Gitterverspannung an. Wenn diese zweite Schicht vervollständigt ist, beinhaltet der resultierende doppelschichtige WL eine Gesamtmenge von 1,42 ML InAs. An diesem Punkt führt die angestaute Gitterverspannung zum Stranski - Krastanow (SK) Wachstumsübergang vom zweidimensionalen zum dreidimensionalen Wachstum, und weiteres aufgewachsenes InAs sammelt sich in typischen dreidimensionalen Inseln, den QDs. Darüber hinaus führt die Gitterverspannung im WL zur Verlagerung von Material aus dem WL in die QDs. Dieser Reifungsprozess, letztendlich auch auf Kosten von Teilen des WL, kann eingeschränkt werden, wenn das Substrat direkt nach dem Wachstum sehr schnell abgekühlt wird (quenching).
5 Abstract Semiconductor nanostructures are currently of high interest for a wide variety of electronic and optoelectronic applications. A large number of devices, in particular for the optical data transmission in the long-wavelength range, essential in modern communication, are based on InAs/GaAs quantum dot (QD) structures. Though the properties of the InAs/GaAs QDs have been extensively studied, only little is known about the formation and structure of the wetting layer (WL) yet. In the present work, the pathway of the InAs WL evolution is studied in detail. For this purpose, InAs thin films in the range of one monolayer (ML) are deposited on the GaAs(0 0 1) surface by molecular beam epitaxy (MBE) and studied by reflection high energy electron diffraction (RHEED) and in particular by scanning tunneling microscopy (STM). The InAs thin films are grown in both typical growth regimes, on the GaAs-c(4 4) and the GaAs-β2(2 4) reconstructed surface, in a variety of thicknesses starting from submonolayers with 0.09 ML of InAs up to 1.65 ML of InAs exceeding the critical thickness for QD growth. In principle, three growth stages are found. At low InAs coverages, the indium adsorbs in agglomerations of typically eight In atoms at energetically preferable surface sites. In the STM images, the signatures of these In agglomerations appear with a clear bright contrast. A structural model for the initial formation of these signatures is presented, and its electronic and strain related properties are discussed. At an InAs coverage of about 0.67 ML the initial surface transforms into a (4 3) reconstructed In 2/3 Ga 1/3 As ML and the detailed structure and strain properties of this surface are unraveled. On top of the InGaAs ML further deposited InAs forms a second layer, characterized by a typical zig-zag alignment of (2 4) reconstructed unit cells, with an alternating α2/α2-m configuration. In contrast to the previous surface reconstructions, where structural strain is sufficiently reduced, this second (2 4) reconstructed InAs layer accumulates unfavorable amounts of compressive strain from the InAs incorporation. With a fully evolved second layer the complete two-layer WL contains a total amount of 1.42 ML of InAs. At this point, the accumulated amount of strain induces the Stranski - Krastanow (SK) growth transition from two-dimensional to three-dimensional growth, and further deposited InAs accumulates in typical three-dimensional islands, the QDs. Moreover, the unfavorable strain in the WL leads to a relocation of InAs material from the WL into the QDs. This QD ripening, eventually at the account of parts of the WL, can be reduced by rapid quenching of the substrate immediately after growth.
7 List of publications Parts of this work have been published in: Evolution of the InAs wetting layer on GaAs(001)-c(4 4) on the atomic scale, J. Grabowski, C. Prohl, B. Höpfner, M. Dähne, and H. Eisele, Appl. Phys. Lett. 95, (2009) Atomic structure of the (4 3) reconstructed InGaAs monolayer on GaAs(001), H. Eisele, B. Höpfner, C. Prohl, J. Grabowski, and M. Dähne, Surf. Sci. 604, 283 (2010) Atomic structure and strain of the InAs wetting layer growing on GaAs(001)-c(4 4), C. Prohl, B. Höpfner, J. Grabowski, M. Dähne, and H. Eisele, J. Vac. Sci. Technol. B 28, C5E13 (2010) Further publications: Low Budget UHV STM Built by Physics Students for Use in a Laboratory Exercises Course, R. Timm, H. Eisele, A. Lenz, S. K. Becker, J. Grabowski, T.-Y. Kim, L. Müller-Kirsch, K. Pötschke, U. W. Pohl, D. Bimberg, and M. Dähne, AIP Conf. Proc. 696, 216 (2003) Structure and intermixing of GaSb/GaAs quantum dots, R. Timm, H. Eisele, A. Lenz, S.K. Becker, J. Grabowski, T.-Y. Kim, L. Müller-Kirsch, K. Pötschke, U.W. Pohl, D. Bimberg, and M. Dähne, Appl. Phys. Lett. 85, 5890 (2004) Formation and atomic structure of GaSb nanostructures in GaAs studied by cross-sectional scanning tunnelling microscopy, R. Timm, J. Grabowski, H. Eisele, A. Lenz, S.K. Becker, L. Müller-Kirsch, K. Pötschke, U.W. Pohl, D. Bimberg, and M. Dähne, Physica E 26, 231 (2005) A cross-sectional scanning tunneling microscopy study of GaSb/GaAs nanostructures, R. Timm, A. Lenz, J. Grabowski, H. Eisele, and M. Dähne, Springer Proceedings in Physics 107, 479 (2005) Formation and Atomic Structure of GaSb Quantum Dots in GaAs Studied by Cross-Sectional Scanning Tunneling Microscopy, R. Timm, A. Lenz, J. Grabowski, H. Eisele, K. Pötschke, U.W. Pohl, D. Bimberg, and M. Dähne, in: Proceedings of EW-MOVPE XI, ed. by E. Kapon and A. Rudra (Lausanne 2005), p. 39
9 Acknowledgments In the very beginning, I would like to express my gratitude to the many people, who contributed to these studies and to the completion of this work. First of all, my sincere thanks belong to Prof. Dr. Mario Dähne, who gave me this great opportunity to explore challenging fields of science, to study and research freely in his group, and to complete my work in writing this thesis. I would like to further thank Dr. Lutz Geelhaar for his interest in my work and his willingness to referee this thesis, and Prof. Dr. Michael Lehmann for his willingness to chair the final discussion. Sincere thanks go to Prof. Dr. Karl Jacobi, who kindly provided the experimental setup for this study, and certainly to Dr. Yevgeniy Temko, who introduced me to the mysteries of MBE and this setup in particular, and who teached me essential know-how of conducting growth experiments using this setup. Spasibo bolьxoe. Special thanks belong to Peter Geng, who gave me a helping hand dealing with many technical issues especially during the early days of my studies. Furthermore, my sincere thanks go to Kathrin Schatke for her supportive assistance in dealing with all the different kinds of harmful and hazardous substances and for many pieces of advice. I am very thankful to my former students and co-workers Sylvia Hagedorn, Britta Höpfner, and Christopher Prohl for their great contributions to this work. Thank you for your assistance during experiments and also repair, for fruitful discussions and a nice working environment, and for your effort in establishing our small MBE-STM-Team here. I would also like to acknowledge Dr. Holger Eisele for his scientific contributions. Many thanks belong to the former and present members of the Dähne working group, who created a nice and friendly atmosphere and whom I enjoyed sharing time with. My special thanks go to Dr. Rainer Timm and Dr. Andrea Lenz, who introduced me to the STM technique, provided continuous support during my first STM steps and shared their knowledge with me. I would like to thank Martin Franz, Dr. Kai Hodeck, Angela Berner and Gerd Pruskil for many nice discussions and a great time. I would like to express my special thanks to Prof. Dr. Hans-Eckhardt Gumlich for his motivation and kindness and exceptionally for initiating great discussions about the ethics and philosophy of science. My deepest thanks belong to Dr. Lena Ivanova and Dr. Martina Wanke for so many great years of working together. Thank you for your support and for becoming such great friends. Schlußendlich möchte ich mich auch bei meinen Freunden und meiner Familie bedanken. Ich danke Anja Brostowski und Tai-Yang Kim, die mich durchs Studium begleitet haben, und dabei halfen, so manche Durststrecke zu überstehen. Ich danke allen meinen Freunden, die schon viel Rücksicht auf mich und meine Arbeit nehmen mußten und trotzdem immer noch geduldig mit mir sind und hinter mir stehen. Ein ganz besonderer Dank aber gilt meiner Familie für das Interesse an meiner Arbeit und die Unterstützung über die vielen Jahre meines Studiums und meiner wissenschaftlichen Arbeit. Vielen lieben Dank!
11 11 Contents 1. Introduction 15 Part I. Theoretical and experimental background 2. Semiconductor surfaces properties and growth Gallium arsenide a III/V semiconductor The electron counting rule The GaAs(0 0 1) surface The GaAs(0 0 1)-c(4 4) surface reconstruction The GaAs(0 0 1)-β2(2 4) surface reconstruction Semiconductor nanostructures Epitaxial growth Stranski - Krastanow growth of InAs/GaAs quantum dots Molecular beam epitaxy (MBE) Ultra high vacuum (UHV) Evaporation Theory of evaporation Knudsen cells Material distribution Material flux Beam equivalent pressure Surface growth mechanisms Reflection high energy electron diffraction (RHEED) Formation of diffraction patterns Growth control Scanning tunneling microscopy (STM) The STM principle Surface imaging Modes of operation Contrast mechanisms
12 12 Contents 5. Experimental setup The UHV chamber system setup The MBE setup The STM setup The setup for preparation and further analysis Sample storage and handling Preparation of the experimental setup Preparation of the STM tips Preparation of the sample substrate Calibration of the sample temperature Part II. Results and discussion 6. Homoepitaxial growth on GaAs(0 0 1) Introduction Experimental details Sample preparation Determination of the growth rate The GaAs(0 0 1)-c(4 4) reconstructed surface Sample growth parameters STM results Summary The GaAs(0 0 1)-β2(2 4) reconstructed surface Sample growth parameters STM results Summary InAs thin film growth on GaAs(0 0 1)-c(4 4) Introduction Experimental details Sample preparation Determination of the growth rate InAs thin films at submonolayer coverages Sample growth parameters STM results Discussion: Formation and structure of the InAs signatures Summary InAs thin films at coverages close to one monolayer Sample growth parameters STM results
13 Contents Discussion: Formation and structure of the In 2/3 Ga 1/3 As monolayer Discussion: Formation and structure of the InAs islands on top of the In 2/3 Ga 1/3 As monolayer Summary InAs thin films during quantum dot growth Sample growth parameters STM results Discussion: The InGaAs/InAs wetting layer during quantum dot formation Summary Strain effects during the evolution of the InAs wetting layer Strain at the GaAs(0 0 1)-c(4 4) surface Strain related to the InAs signatures on the GaAs-c(4 4) surface Strain at the In 2/3 Ga 1/3 As-(4 3) reconstructed monolayer Strain at the InAs-(2 4) reconstructed second layer Strain during quantum dot formation Summary InAs thin film growth on GaAs(0 0 1)-β2(2 4) Introduction Experimental details Sample preparation Estimation of the growth rate InAs thin films on GaAs(0 0 1)-β2(2 4) Sample growth parameters STM results Discussion Summary Conclusion 129 Appendix 133 A. Data of the beam equivalent pressure (BEP) B. Determination of the deposited InAs coverage C. Sample heating data List of Abbreviations 145 Bibliography 147
14 14 Contents
15 15 1. Introduction In the 20 th century, the ongoing industrialization with its growing need for advanced technology has become a strong motor for scientific progress in many fields. Yet, among all those fields, the fast evolving information and communication technology probably has branded this century most . Its electronic devices have become so essential for everyday life, a world without seems hardly imaginable. In retrospective, the introduction of the semiconductor-based transistor in 1948  may be considered the starting point of this technological development and of the semiconductor industry. Semiconductors soon became the key material for electronic and optoelectronic applications, for their compounds are versatilely tunable in electronic and structural properties to meet many applicational needs . Since then, there has been an ongoing progress to further improve the efficiency of such devices, while decreasing their size concurrently. Computers that once filled whole rooms now fit the palm of one s hand. Undoubtedly, this process of miniaturization eventually facilitated the broad acceptance of electronics in everyday life and fundamentally changed the way we live. However, the ongoing ambition to further minimize the size of electronics has also created novel challenges to modern physics in recent decades . On the nanoscale, quantum effects that were negligible before, now become dominating the electronic properties of such devices . Lateral confinement of charge carriers causes the quantization of energy states, leading to a complete discretization when charge carriers are localized in all three dimensions [6,7]. In such a quantum dot (QD) the occupation of only certain energy levels is allowed, comparable to the electronic structure of a single atom . Low-dimensional semiconductor structures (LDS) were predicted to be promising for novel applications, as low-threshold laser diodes [9 12], high-speed transistors , single photon detectors/emitters  or quantum memories [15, 16]. An important step towards the systematic usage of quantum effects by manipulating matter on the nanoscale is marked by the discovery of self-organization mechanisms in epitaxial growth of lattice mismatched compound semiconductor material systems in 1985 . Basic material for most optoelectronic nanostructure applications are III/V compound semiconductors. Their energy band gap can be tuned by adapting the chemical composition of the alloy [18, 19] which allows, e.g., the adjustment of the light emission wavelength . Likewise the lattice constant of the crystal structure is tunable to some ex-
16 16 1. Introduction tent [21, 22]. Although many material compositions have been investigated for their structural and electronic properties, only some are yet used commercially. Among those, the indium arsenide/gallium arsenide (InAs/GaAs) material system is currently one of the technologically most important. Low-dimensional InAs/GaAs nanostructures, e.g. QDs, are currently of high interest for the use in ultra-fast electronics , storage devices  and in particular in optoelectronic applications, especially for optical data transmission in the long-wavelength range [25 29]. Recently, also InAs/GaAs QD precursors, so-called submonolayer quantum dots have come into focus for the use in high power disk lasers , infrared photo detectors , as well as very fast light emitters for optical chip-to-chip connections [32 34]. Thus, the InAs/GaAs QD system has been studied extensively in the recent years. It is well established that the quantum dots form in the Stranski - Krastanow growth mode [8, 35, 36] and their size and shape have been investigated in detail [37 43]. However, detailed knowledge about the evolution of an InAs monolayer (ML) before the transition from twodimensional growth to three-dimensional growth (2D 3D transition) is still missing . On the other hand, information on the atomic structure and the strain induced formation principles of the intermixed InGaAs wetting layer (WL) is fundamental for the detailed understanding of QD growth and even more for the formation of submonolayer QDs. Moreover, the atomic ordering in the WL creates a strain profile at the surface, affecting the adsorption of further deposited material and influencing its crystal structure and its optoelectronic properties . In this work, the evolution of the InAs/GaAs wetting layer from the bare GaAs(0 0 1) surface to the critical thickness of QD formation is investigated on the atomic scale. Increasing amounts of InAs material were deposited on the GaAs(0 0 1) surface by molecular beam epitaxy (MBE) and studied in-situ by reflection high electron diffraction (RHEED) and in particular by scanning tunneling microscopy (STM). In the first part of this thesis, a brief introduction on the material system and growth is given in Chapter 2. Chapters 3 and 4 focus on the experimental techniques MBE/RHEED and STM, respectively. Chapter 5 then provides an overview on the experimental setup and the sample preparation. The second part of this thesis is dedicated to the presentation and interpretation of the experimental results. In Chapter 6 the principles of homoepitaxy are introduced and homoepitaxy on both the GaAs(0 0 1)-c(4 4) and the GaAs(0 0 1)-β2(2 4) surface is used to derive basic information on the growth parameters, such as the appropriate V/III ratio and the growth rate or on the calibration of the substrate temperature. In addition, STM images of these well-known surface reconstructions are used to calibrate the STM instrument. Chapter 7 presents the experimental findings on the InAs thin film evolution during InAs growth on the GaAs(0 0 1)-c(4 4) surface from very low submonolayer coverages to the critical thickness for QD growth. The discussion hereby mainly focuses on the formation processes and the structural properties of the different growth stages of the InAs thin film
17 17 and on the effects of structural strain. Chapter 8 presents the experimental findings on the InAs thin film evolution during growth on the GaAs(0 0 1)-β2(2 4) surface. It is shown that the principal growth mechanisms during the InAs thin film evolution are similar in both growth regimes. Finally, Chapter 9 summarizes the fundamental conclusions that could be drawn from the presented findings and gives an outlook on prospective experiments to unravel further aspects of InAs/GaAs growth.
18 18 1. Introduction
19 19 Part I. Theoretical and experimental background
21 21 2. Semiconductor surfaces properties and growth 2.1. Gallium arsenide a III/V semiconductor Gallium arsenide (GaAs) is a binary semiconductor, and similarly to most III-V materials it crystallizes in the zincblende crystal structure. As the chemical bond between arsenic and gallium atoms is of both ionic and covalent nature, the covalence leads to the formation of sp 3 -hybridized electron orbitals of the bulk atoms. This sp 3 -hybridization is responsible for the tetrahedral alignment of the bulk atoms, characteristic for the zincblende crystal structure. Each atom is thereby equidistantly surrounded by four atoms of the other kind as illustrated in Fig. 2.1 [45, 46]. The energies of the hybridized orbitals from As and Ga atoms are located in the valence band (VB) and the conduction band (CB) of the semiconductor, respectively . If the symmetry of the crystal structure is reduced, e.g. at the surface where the crystal is no longer infinite, the hybridized orbitals cannot form bonds and will remain unsaturated (dangling bonds). This surface structure is commonly referred to as bulk-truncated. These Figure 2.1: The GaAs zincblende bulk crystal structure. The lattice parameter a = nm  is defined as the side length of the cubic unit cell. Gallium atoms are illustrated in blue and arsenic atoms in light gray.
22 22 2. Semiconductor surfaces properties and growth dangling bonds are partly filled with electrons and thus cause additional surface states, which increase the surface energy of the semiconductor . On the GaAs surface this bulk-truncated surface energy can be lowered by an electron transfer from the Ga dangling bonds in the CB to the As dangling bonds in the VB. With a depleted CB and a completely filled VB the semiconductivity at the surface is restored. Naturally, such an electron transfer is only trivial on stoichiometric non-polar surfaces, where the number of group-iii and group-v atoms is equal, e.g. the GaAs(1 1 0) oriented surface [50 52]. Surface Ga atoms would swap from sp 3 -hybridization to sp 2 -hybridization providing the excess electrons to fill the dangling bonds of the As atoms, allowing them to remain in sp 3 -hybridization. The now sp 2 -hybridized Ga atoms finally cause a change in the crystal surface geometry leading to a relaxation of Ga and As atoms, the so-called buckling, as illustrated in Fig. 2.2 [53 55]. Most low-index crystal surfaces, including the GaAs(0 0 1)-surface that was used for the investigations in this work, do not fulfill the stoichiometry criteria for such a simple relaxation and therefore undergo rearrangement processes to form stable reconstructed surfaces that will regain the semiconductivity at the surface as well as minimize the number of dangling bonds. The reconstructed surface thereby usually is the one with least free energy in the kinematically available range of possible reconstructions (see, e.g., ). Yet, as this kinematically available range is highly affected by the ambient conditions such as temperature and vapor pressure, different reconstructions for the same bulk-truncated surface are possible . High-index bulk-truncated surfaces usually transform into reconstructions of small areas of low-index surfaces (facets) or into the terraced reconstruction of a low-index surface with similar orientation (vicinal surfaces). On the other hand, stable high-index GaAs surfaces, such as GaAs(1 1 4) [58, 59] and GaAs(2 5 11) [60, 61], have also been found. Figure 2.2: Side view of the relaxed GaAs(1 1 0) surface. The hybridization transition of the uppermost Ga atoms from sp 3 to sp 2 leads to a retraction of these Ga atoms under the surface plane and thus an outstretching of neighboring surface As atoms. White arrows indicate this so-called buckling. The corresponding filled As dangling bonds are illustrated by gray ovals, empty Ga dangling bonds by white ovals. Smaller circles depict atoms located lower than the figure plane.
23 2.2. The electron counting rule The electron counting rule The complete annihilation of surface charges caused by dangling bonds on the bulktruncated surface is an appropriate criterion for the possible existence of stable surfaces and reconstructions, resulting in the principle of the electron counting rule (ECR). The ECR requires the number of available surface state electrons (i.e. from states that are not of bulk nature) to exactly fill all dangling bond states in the VB, leaving those in the CB completely depleted, in order to restore the surface semiconductivity [47, 55, 62, 63]. For III-V semiconductors in zincblende structure, such as GaAs, each atom has covalent bonds to four tetrahedrally aligned atoms of the other kind. Each bond is filled with two electrons of which gallium, as the trivalent part, contributes 3 /4 electrons and arsenic, as the pentavalent part, 5 /4 electrons. As an example, on the GaAs(1 1 0) surface, a filled As dangling bond requires two electrons and the Ga dangling bond has to be empty. The surface stoichiometry delivers 3 /4 electrons from Ga and 5 /4 electrons from As, so that the electron transfer during surface relaxation [49, 55] will fulfill the ECR (Eq. 2.1): electrons available : 3/4 e (Ga db ) + 5 /4 e (As db ) = 2 e (2.1) electrons required : 0 e (Ga db ) + 2 e (As db ) = 2 e Although the vast majority of stable surface configurations fulfill the ECR, it is only an indicating condition, as stable configurations that do not fulfill the ECR have been found as well . The determining factor is the minimization of surface free energy, which does not necessarily require a semiconducting surface The GaAs(0 0 1) surface Figure 2.3: (a) Top view and (b) side view of the GaAs(0 0 1) bulk-truncated surface. The surface unit cell is marked by a yellow square. The As dangling bonds are illustrated by gray ovals. Atoms below the figure plane are depicted by smaller circles. For reasons of clarity the lower Ga atom plane is not shown in the top view image.
24 24 2. Semiconductor surfaces properties and growth The bulk-truncated GaAs(0 0 1) surface, as shown in Fig. 2.3, is either group-iii or group- V terminated. To lower the surface energy, rearrangement processes of the surface atoms lead to surface reconstructions that minimize the number of dangling bonds. Thereby the GaAs(0 0 1) surface shows a large variety of reconstructions, depending on the ambient circumstances, reaching from highly Ga-rich to extremely As-rich stages . A schematic overview of the different reconstruction domains observed by RHEED is given in Fig In principle two growth regimes for epitaxial growth of InAs on the GaAs(0 0 1) surface exist, separated by the phase transition of the GaAs(0 0 1) surface reconstruction from the As-rich GaAs-c(4 4) reconstructed surface to the As-rich GaAs(0 0 1)-β2(2 4) reconstructed surface between C . However, this transition is not completely abrupt and thus hysteresis effects led to the assumption of an interstitial stage of (2 1) periodicity in the RHEED observations in Ref. . Recently, the co-existence of both the c(4 4) and the β2(2 4) reconstruction was observed during the phase transition. These STM studies found no evidence for a stable interstitial reconstruction . The c(4 4) and the β2(2 4) surface reconstructions are both of high relevance for applications and thus the basic substrate reconstructions for the studies in this work. Therefore both will be discussed here in more detail. Figure 2.4: The surface phase diagram of reconstruction phases observed by RHEED on GaAs(0 0 1) in relation to the substrate temperature T S and the As 4 /Ga flux ratio, published in Ref. . The narrow region of (2 1) structure between the c(4 4) and the (2 4) structure is ascribed to hysteresis effects of the transition.
Epitaxy Epitaxial Growth Epitaxy means the growth of a single crystal film on top of a crystalline substrate. For most thin film applications (hard and soft coatings, optical coatings, protective coatings)
Control of r. f. nitrogen plasma source in solid source MBE Courtesy of Yoon Soon Fatt. Used with permission. Mass Flow Controller Ultra High Vacuum Leak Valve RF nitrogen N 2 Gas Ultra Pure Four channel
MOCVD growth system General MOCVD growth features Advantages Faster growth than MBE, can be a few microns per hour; multi-wafer capability easily achievable Higher temperature growth; growth process is
Molecular Beam Epitaxy Klaus Ploog Paul Drude Institut Tutorial Session #1 Epitaxial Growth 27 th International Conference on the Physics of Semiconductors Flagstaff, AZ, 2004 Molecular Beam Epitaxy (MBE)
1 Layer Deposition: Thermal Oxidation and CVD Rupesh Gupta IIT Delhi Supervisor: Dr. Chacko Jacob 2 OUTLINE Thermal Oxidation and Model o Factors Affecting Kinetics o Future Trends: Oxidation o CVD and
Electronic Materials Devices and Fabrication Layering: Thermal Oxidation Dr. S. Parasuraman Department of Metallurgical and Materials Engineering Indian Institute of Technology, Madras Module - 01 Lecture
1.5 Light absorption by solids Bloch-Brilloin model L e + + + + + allowed energy bands band gaps p x In a unidimensional approximation, electrons in a solid experience a periodic potential due to the positively
Name: Class: Date: ID: A Unit 12 Practice Test Multiple Choice Identify the choice that best completes the statement or answers the question. 1) A solid has a very high melting point, great hardness, and
Chemical Vapor Deposition Physical Vapor Deposition (PVD) So far we have seen deposition techniques that physically transport material from a condensed phase source to a substrate. The material to be deposited
Physics 441/2: Transmission Electron Microscope Introduction In this experiment we will explore the use of transmission electron microscopy (TEM) to take us into the world of ultrasmall structures. This
Lecture 3: Electron statistics in a solid Contents Density of states. DOS in a 3D uniform solid.................... 3.2 DOS for a 2D solid........................ 4.3 DOS for a D solid........................
s 1 Measurement and Problem Solving 2-3 Weeks Measurements are subject to errors which need to be accounted for. 1. How do you accurately measure, using significant figures, in the metric system? Understand
Introduction to Thin Film Introduction to Thin Film Verfahrenstechnik der Oberflächenmodifikationen Prof. Dr. Xin Jiang Lecture Institut für Werkstofftechnik der Uni-Siegen Sommersemester 2007 Introduction
4.1 SOLAR CELL OPERATION Y. Baghzouz ECE Department UNLV SOLAR CELL STRUCTURE Light shining on the solar cell produces both a current and a voltage to generate electric power. This process requires a material
Doped Semiconductors Dr. Katarzyna Skorupska 1 Doped semiconductors Increasing the conductivity of semiconductors by incorporation of foreign atoms requires increase of the concentration of mobile charge
Condensed Matter Physics Prof. G. Rangarajan Department of Physics Indian Institute of Technology, Madras Lecture - 36 Semiconductors We will start a discussion of semiconductors one of the most important
QUANTUM DOTS Technology White Papers nr. 13 Paul Holister Cristina Román Vas Tim Harper QUANTUM DOTS Technology White Papers nr. 13 Release Date: Published by Científica Científica, Ltd. www.cientifica.com
Low energy ion scattering study of 4 on Cu(1) Chapter 8. Low energy ion scattering study of 4 on Cu(1) 8.1. Introduction For a better understanding of the reconstructed 4 surfaces one would like to know
How MOCVD Works Deposition Technology for Beginners Contents MOCVD for Beginners...3 MOCVD A Definition...4 Planetary Reactor Technology...5 Close Coupled Showerhead Technology...6 AIXTRON MOCVD Production
Covalent Crystals - covalent bonding by shared electrons in common orbitals (as in molecules) - covalent bonds lead to the strongest bound crystals, e.g. diamond in the tetrahedral structure determined
Chemical Sputtering von Kohlenstoff durch Wasserstoff W. Jacob Centre for Interdisciplinary Plasma Science Max-Planck-Institut für Plasmaphysik, 85748 Garching Content: Definitions: Chemical erosion, physical
Peculiarities of Titanium Compound Films Formation on Heat Sensitive Substrates by Electric Arc Vaporization A. Vovsi and Y. Lipin, J/S Co Sidrabe, Latvia Keywords: darc vaporization; Polymer substrates;
Graphene a material for the future by Olav Thorsen What is graphene? What is graphene? Simply put, it is a thin layer of pure carbon What is graphene? Simply put, it is a thin layer of pure carbon It has
U. Scotti di Uccio Surface characterization of oxygen deficient SrTiO 3 films and single crystals Coherentia-CNR-INFM Napoli, Italy Prof. R. Vaglio R. Di Capua, G. De Luca, M. Radovic, N. Lampis, P. Perna,
Satisloh Italy S.r.l. Coating Technology: Evaporation Vs Sputtering Gianni Monaco, PhD R&D project manager, Satisloh Italy 04.04.2016 V1 The aim of this document is to provide basic technical information
Chemical Synthesis Spontaneous organization of molecules into stable, structurally well-defined aggregates at the nanometer length scale. Overview The 1-100 nm nanoscale length is in between traditional
Chapter 6 CO oxidation on RuO 2 (101) 6.1 Introduction In this chapter, we will pay attention to the CO adsorption on bulk RuO 2 (101) and its activity in the CO oxidation. The objective is to probe experimentally,
Technical articles X-ray thin-film measurement techniques II. Out-of-plane diffraction measurements Toru Mitsunaga* 1. Introduction A thin-film sample is two-dimensionally formed on the surface of a substrate,
Heterogeneous Catalysis and Catalytic Processes Prof. K. K. Pant Department of Chemical Engineering Indian Institute of Technology, Delhi Module No. #03 Lecture - 08 So, last time we were talking about
Sputtering Vacuum Evaporation Recap Use high temperatures at high vacuum to evaporate (eject) atoms or molecules off a material surface. Use ballistic flow to transport them to a substrate and deposit.
Pulsed laser deposition of organic materials PhD theses Gabriella Kecskeméti Department of Optics and Quantum Electronics University of Szeged Supervisor: Dr. Béla Hopp senior research fellow Department
Lecture 2: Semiconductors: Introduction Contents 1 Introduction 1 2 Band formation in semiconductors 2 3 Classification of semiconductors 5 4 Electron effective mass 10 1 Introduction Metals have electrical
Lecture 6 Scanning Tunneling Microscopy (STM) General components of STM; Tunneling current; Feedback system; Tip --- the probe. Brief Overview of STM Inventors of STM The Nobel Prize in Physics 1986 Nobel
contamination analysis for compound semiconductors ANALYTICAL SERVICES B u r i e d d e f e c t s, E v a n s A n a l y t i c a l g r o u p h e l p s y o u C O N T R O L C O N T A M I N A T I O N Contamination
EXPERIMENTAL METHODS IN COLLOIDS AND SURFACES PARTICLE SURFACE AREA FROM GAS ADSORPTION TYPES OF ADSORPTION Physical adsorption: rapid, depends on adsorbate bulk concentration, multiple molecular layers
F. G. Tseng Lec6, Fall/2001, p1 Lecture 6 PVD (Physical vapor deposition): Evaporation and Sputtering Vacuum evaporation 1. Fundamental of Evaporation: The material to be evaporated is heated in an evacuated
Physics 610/Chemistry 678 Semiconductor Processing and Characterization Quiz I July 18, 2014 Part I: Short-answer questions on basic principles. (5 points each) 1. Briefly describe the CZ method and the
Tunnel Effect: - particle with kinetic energy E strikes a barrier with height U 0 > E and width L - classically the particle cannot overcome the barrier - quantum mechanically the particle can penetrated
Properties of optical thin films produced by reactive low voltage ion plating (RLVIP) Antje Hallbauer Thin Film Technology Institute of Ion Physics & Applied Physics University of Innsbruck Investigations
Outline Exponential Attenuation Chapter 3 F.A. Attix, Introduction to Radiological Physics and Radiation Dosimetry Simple exponential attenuation and plural modes of absorption arrow-beam vs. broad-beam
Module 6 : PHYSICS OF SEMICONDUCTOR DEVICES Lecture 34 : Intrinsic Semiconductors Objectives In this course you will learn the following Intrinsic and extrinsic semiconductors. Fermi level in a semiconductor.
Nanoelectronics 09 Atsufumi Hirohata Department of Electronics 12:00 Wednesday, 4/February/2015 (P/L 006) Quick Review over the Last Lecture ( Field effect transistor (FET) ): ( Drain ) current increases
Electronic Supplementary Material (ESI) for Nanoscale. This journal is The Royal Society of Chemistry 2014 Electronic supporting information Misfit dislocation free InAs/GaSb core-shell nanowires grown
Chapter 7-1 Atomic Layer Deposition (ALD) Definition of ALD Brief history of ALD ALD process and equipments ALD applications 1 Definition of ALD ALD is a method of applying thin films to various substrates
Basic types of solid materials. Overview The theory of bands provides a basis for understanding the classification and physical properties of solid materials such as electrical conductivity, optical behavior
AN OVERVIEW Semiconductors form the active regions of \intelligent" devices. The following gures provide an overview of why semiconductors are important, what the various demands in semiconductor technologies
MTE 585 Oxidation of Materials Part 1 Ref. Ch. 11 in Superalloys II Ch. 8 in Khanna Ch. 14 in Tien & Caulfield Introduction To illustrate the case of high temperature oxidation, we will use Ni-base superalloys.
Hydrogen Adsorption on Nanoporous Biocarbon Mikael Wood, Jacob Burress, Cintia Lapilli, Peter Pfeifer, Parag Shah, Galen Suppes University of Missouri-Columbia Phillip Parilla, Anne Dillon National Renewable
Lecture 19: Solar cells Contents 1 Introduction 1 2 Solar spectrum 2 3 Solar cell working principle 3 4 Solar cell I-V characteristics 7 5 Solar cell materials and efficiency 10 1 Introduction Solar cells
OpenStax-CNX module: m38278 1 BET Surface Area Analysis of Nanoparticles Nina Hwang Andrew R. Barron This work is produced by OpenStax-CNX and licensed under the Creative Commons Attribution License 3.0
State of the art in reactive magnetron sputtering T. Nyberg, O. Kappertz, T. Kubart and S. Berg Solid State Electronics, The Ångström Laboratory, Uppsala University, Box 534, S-751 21 Uppsala, Sweden D.
A Remote Plasma Sputter Process for High Rate Web Coating of Low Temperature Plastic Film with High Quality Thin Film Metals and Insulators Dr Peter Hockley and Professor Mike Thwaites, Plasma Quest Limited
Modern Physics (PHY 3305) Lecture Notes Modern Physics (PHY 3305) Lecture Notes Solid-State Physics: The Theory of Semiconductors (Ch. 10.6-10.8) SteveSekula, 30 March 2010 (created 29 March 2010) Review
Understanding the p-n Junction by Dr. Alistair Sproul Senior Lecturer in Photovoltaics The Key Centre for Photovoltaic Engineering, UNSW The p-n junction is the fundamental building block of the electronic
Forensic Science Standards and Standard 1: Understands and applies principles of scientific inquiry Power : Identifies questions and concepts that guide science investigations Uses technology and mathematics
Shuji Nakamura Gerhard Fasol The Blue Laser Diode GaN Based Light Emitters and Lasers With 246 Figures and 49 Tables Springer Contents 1. Introduction 1 1.1 LEDs and LDs 1 1.2 Group-III Nitride Compound
Report on the Thermal Diffusivity, Specific Heat, and Thermal Conductivity of Aluminum Oxide and Pyroceram 9606 This report presents the results of phenol diffusivity, specific heat and calculated thermal
LS Scienza dei Materiali - a.a. 2005/06 Fisica delle Nanotecnologie part 8 Version 4, Dec 2005 Francesco Fuso, tel 0502214305, 0502214293 - email@example.com http://www.df.unipi.it/~fuso/dida Tecnologie
Advanced Materials Science - Lab Intermediate Physics University of Ulm Solid State Physics Department Electrical Conductivity Translated by Michael-Stefan Rill January 20, 2003 CONTENTS 1 Contents 1 Introduction
Matter, Materials, Crystal Structure and Bonding Chris J. Pickard Why should a theorist care? Where the atoms are determines what they do Where the atoms can be determines what we can do Overview of Structure
1. Photon Beam Damage and Charging at Solid Surfaces John H. Thomas III 1. Introduction............................. 2. Electrostatic Charging of Samples in Photoemission Experiments............................
Detailed simulation of mass spectra for quadrupole mass spectrometer systems J. R. Gibson, a) S. Taylor, and J. H. Leck Department of Electrical Engineering and Electronics, The University of Liverpool,
Modern Construction Materials Prof. Ravindra Gettu Department of Civil Engineering Indian Institute of Technology, Madras Module - 2 Lecture - 2 Part 2 of 2 Review of Atomic Bonding II We will continue
CHEMISTRY STANDARDS BASED RUBRIC ATOMIC STRUCTURE AND BONDING Essential Standard: STUDENTS WILL UNDERSTAND THAT THE PROPERTIES OF MATTER AND THEIR INTERACTIONS ARE A CONSEQUENCE OF THE STRUCTURE OF MATTER,
Hello and Welcome to this presentation on LED Basics. In this presentation we will look at a few topics in semiconductor lighting such as light generation from a semiconductor material, LED chip technology,
Applied Surface Science 186 2002) 453±457 Exploring the deposition of oxides on silicon for photovoltaic cells by pulsed laser deposition Lianne M. Doeswijk a,*, Hugo H.C. de Moor b, Horst Rogalla a, Dave
USING MERCURY PROBES TO CHARACTERIZE USJ LAYERS James T.C. Chen and Wei Liu Four Dimensions, Inc. Hayward, CA 94545 It is well established that uniformity and dosage of drain-source ion implantation on
Lecture: 33 Solidification of Weld Metal This chapter presents common solidification mechanisms observed in weld metal and different modes of solidification. Influence of welding speed and heat input on
Improved predictive modeling of white LEDs with accurate luminescence simulation and practical inputs TracePro Opto-Mechanical Design Software s Fluorescence Property Utility TracePro s Fluorescence Property
Lecture 13 and 14 Thin Film Deposition and Epitaxy (Chemical Vapor Deposition, Metal Organic CVD and Molecular Beam Epitaxy) Reading: Chapters 13 and 14 Chemical Vapor Deposition Chemical gas sources are
Assessment Chapter Test A Chapter: States of Matter In the space provided, write the letter of the term or phrase that best completes each statement or best answers each question. 1. The kinetic-molecular
Adsorption and Catalysis Dr. King Lun Yeung Department of Chemical Engineering Hong Kong University of Science and Technology CENG 511 Lecture 3 Adsorption versus Absorption H H H H H H H H H Adsorption
Sebastian Volz (Ed.) Microscale and Nanoscale Heat Transfer With 144 Figures and 7 Tables In Collaboration with Remi Carminati, Patrice Chantrenne, Stefan Dilhaire, Severine Gomez, Nathalie Trannoy, and
1 P a g e Physics Notes Class 12 Chapter 14 Semiconductor Electronics, Materials, Devices and Sample Circuits It is the branch of science which deals with the electron flow through a vacuum, gas or semiconductor.
Semiconductor lasers and LEDs read Agrawal pp. 78-116 Objectives, understand the following: Stimulated emission, spontaneous emission, and absorption in semiconductors Design of an LED and laser diode:
International Journal of Arts and Sciences 3(1): 18-26 (2009) CD-ROM. ISSN: 1944-6934 InternationalJournal.org Usage of Carbon Nanotubes in Scanning Probe Microscopes as Probe Bedri Onur Kucukyildirim,