Molecular Beam Epitaxy MBE Mustafa Yorulmaz S Çigdem

Molecular Beam Epitaxy (MBE) Mustafa Yorulmaz* S. Çigdem Yorulmaz** Izzet Yildiz* *Koç University, Material Science and Engineering, Rumelifeneri Yolu, Sariyer-34450, Istanbul **Koç University, Department of Physics, Rumelifeneri Yolu, Sariyer-34450, Istanbul

Outline n Introduction q q n Working Principle of MBE q q q n n n What is Epitaxy? Epitaxy Techniques Working Conditions Setup & Operation Results and Control Mechanisms Benefits and Drawbacks of MBE Applications Conclusion December 17, 2008 MASE 570 Micro and Nanofabrication, Fall 2008 2

What is Epitaxy? n Epitaxy: Deposition and growth of monocrystalline structures/layers. n Epitaxial growth results in monocrystalline layers differing from deposition which gives rise to polycrystalline and bulk structures. n MBE growth mechanism * Epitaxy types: q q Homoepitaxy: Substrate & material are of same kind. (Si-Si) Heteroepitaxy: Substrate & material are of different kinds. (Ga-As) MBE growth mechanism ** * Veeco Instruments © 2008, http: //www. veeco. com ** Dan Connelly, © 2007, http: //oz. irtc. org December 17, 2008 MASE 570 Micro and Nanofabrication, Fall 2008 3

Epitaxy Techniques n Vapor-Phase Epitaxy (VPE) q q q n Modified method of chemical vapor deposition (CVD). Undesired polycrystalline layers Growth rate: ~2 µm/min. Liquid-Phase Epitaxy (LPE) q q q Crystal layers are from the melt existent on the substrate. Hard to make thin films Growth rate: 0. 1 -1 µm/min. n Molecular Beam Epitaxy (MBE) q q Relies on the sublimation of ultrapure elements, then condensation of them on wafer In a vacuum chamber (pressure: ~10 -11 Torr). “Beam”: molecules do not collide to either chamber walls or existent gas atoms. Growth rate: 1µm/hr. December 17, 2008 MASE 570 Micro and Nanofabrication, Fall 2008 4

MBE: Working Principle Ø Epitaxial growth: Due to the interaction of molecular or atomic beams on a surface of a heated crystalline substrate. Ø The solid source materials sublimate Ø They provide an angular distribution of atoms or molecules in a beam. Ø The substrate is heated to the necessary temperature. Ø The gaseous elements then condense on the wafer where they may react with each other. A typical MBE system* Ø Atoms on a clean surface are free to move until finding correct position in the crystal lattice to bond. Ø Growth occurs at the step edges formed: More binding forces at an edge. Molecular Beam Epitaxy** * “Basics of molecular beam epitaxy (MBE)” by Fernando Rinaldi ** http: //iramis. cea. fr/en/Phocea/Vie_des_labos/Ast/ast_sstechnique. php? id_ast=494 December 17, 2008 MASE 570 Micro and Nanofabrication, Fall 2008 5

Scheme of typical MBE system Monitor residual gases, source beams In situ growth control Deposition Example for Ga. As: • As (As 4 or As 2 through a cracker • Ga • Al • In • Be (p-doping) • Si (n-doping) Sample rotation

MBE: Working Conditions § The mean free path (l) of the particles > geometrical size of the chamber (10 -5 Torr is sufficient) Mean free path for Nitrogen molecules at 300 K * §Ultra-high vacuum (UHV= 10 -11 Torr) to obtain sufficiently clear epilayer. §Gas evalution from materials has to be as low as possible. Pyrolytic boron nitride (PBN) is chosen for crucibles (Chemically stable up to 1400°C) §Molybdenum and tantalum are widely used for shutters. §Ultrapure materials are used as source. * “Basics of molecular beam epitaxy (MBE)” by Fernando Rinaldi December 17, 2008 MASE 570 Micro and Nanofabrication, Fall 2008 7

MBE: Operation http: //www. virlab. virginia. edu/VL/MBE. htm/state/0 December 17, 2008 MASE 570 Micro and Nanofabrication, Fall 2008 8

MBE: Results and Control Mechanisms Ø Control of composition and doping of the growing structure at monolayer via computer controlled shutters Ø Growth rates are typically on the order of a few A°/s and the beams can be shuttered in a fraction of a second allowing nearly atomically abrupt transition from one material to another. ) Ø Independent heating of material sources Ø RHEED (Reflection High Energy Electron Diffraction) for monitoring the growth of the crystal layers. RHEED oscillations * Ø Mass spectrometer for monitoring the residual gases and checking source beams for leaking Ø A cryogenic screening around the substrate as a pump for residual gases. * “Basics of molecular beam epitaxy (MBE)” by Fernando Rinaldi December 17, 2008 MASE 570 Micro and Nanofabrication, Fall 2008 9

In situ monitoring by RHEED

In situ monitoring by RHEED (continued …) Physical nature of RHHED oscillations

Benefits and Drawbacks of MBE Advantages n Clean surfaces, free of an oxide layer n In-situ deposition of metal seeds, Disadvantages n Expensive (10 $ per MBE chamber) n Asaro-Tiller-Grinfeld (ATG) instability semiconductor materials, and dopants an elastic instability when there is a mismatch between the lattice sizes of the growing film and the supporting crystal - film breaks into isolated islands n Low growth rate (1μm/h) n Very complicated system n Precisely controllable thermal evaporation n Epitaxial growth under ultra-high vacuum 6 conditions n Seperate evaporation of each component n Substrate temperature is not high n Ultrasharp profiles December 17, 2008 MASE 570 Micro and Nanofabrication, Fall 2008 12

Applications n n Novel structures as quantum devices Silicon/Insulator/Metal Sandwiches Superlattices Microelectronic Devices TEM image of MBE Growth of Ultra-Thin In. Ga. As/Al. As. Sb Quantum Wells* * http: //www. photonics. ethz. ch/research/core_competences/technology/epitaxial_growth/mbe December 17, 2008 MASE 570 Micro and Nanofabrication, Fall 2008 13

Brolis Semiconductors offers molecular beam epitaxy foundry service of custom structures using in-house multi-wafer MBE facility. Possible multi-wafer growth capacity: 14 x 2 -inch, 7 x 3 -inch, and 4 x 4 -inch. Available materials sources are Aluminium (Al), galium (Ga), indium (In), Arsenic (As) and Antimony (Sb). For doping purposes berylium (Be), silicon (Si) and galium telluride (Ga. Te) are available. In-house wafer qualificatioin includes: – HRXRD (rocking curve, reciprocal space mapping) – Photoluminescense 10 K – 300 K – Room-temperature photoluminescence mapping up to 4 -inch wafer diameter – Optical microscopy – Van der Pauw measurements for Hall mobility and carrier concentration – FTIR reflectivity measurements in the spectral range 800 nm – 15 000 nm Wafer growth, qualification and packaging is carried out in the class ISO 6 (class 1000) cleanroom environment. We have provided molecular beam epitaxy foundry service for a variety of different material combinations for a variety of applications: Ga. As/Al. Ga. As/In. Ga. As, Al. In. As/In. Ga. As/In. P, Al. Ga. As. Sb/Ga. In. As. Sb/Ga. Sb, In. As/Ga. Sb/Al. Sb, etc Our experimental room-temperature electron mobility is up to 9000 cm 2/(Vs) for Ga. In. As/Al. Ga. As/Ga. As and 26 000 cm 2/(Vs) for In. As/Al. Sb/Ga. As structures. In order to get a quotation, please send us you structure information, which would include: – Layer thickness and material compositions – Layer doping information – Desirable substrate and the size of the substrate – Allowed tolerances – Quantity of wafers

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Conclusions n n n n Typically in ultra-high vacuum Deposition rates are very low (1 monolayer/second) Very well controlled (Shuttering: 0. 1 s) Grow films with good crystal structure Often use multiple sources to grow alloy films Deposition rate is so low that substrate temperature doesn’t need to be as high Expensive Sophisticated system December 17, 2008 MASE 570 Micro and Nanofabrication, Fall 2008 17