Molecular Beam Epitaxy (MBE) By Mohammad Junaebur Rashid, PhD Post Doctoral Researcher Solar Energy Research Institute (SERI), University of Kebangsaan Malaysia (UKM).

Outline Introduction Molecular Beam Epitaxy (MBE) Growth Mechanism What is epitaxy? Different epitaxy techniques Molecular Beam Epitaxy (MBE) System Working principles and conditions Growth monitoring Growth Mechanism Conclusion

Introduction What is epitaxy? The term “epitaxy” comes from the Greek roots epi (ἐπί) mean «above», and taxi (τάξις) mean «in ordered manner». Growth of a single crystal film on top of a crystalline substrate Epitaxy Overlayer is called an epitaxial film or epitaxial layer. Registry between the film and the substrate Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate, where there is registry between the overlayer and the substrate. The overlayer is called an epitaxial film or epitaxial layer. For most technological applications, it is desired that the deposited material form a crystalline overlayer that has one well-defined orientation with respect to the substrate crystal structure (single-domain epitaxy). Epitaxial atom Substrate atom

Introduction Types of epitaxy Homoepitaxy: substrate and material are of same kind (Si-Si). Heteroepitaxy: substrate and material are of different kinds (Si-Ge, AlN/Si). Leads to unmatched lattice parameters. Causes strained or relaxed growth lead to interfacial defects. Effect on the film Altered the electronic, optic, thermal and mechanical properties of the films. Allows for optoelectronic structures and band gap engineered devices. Lattice mismatch: In-plane lattice mismatch: where, afilm is the lattice parameter of the film and asub is the lattice parameter of the substrate

Introduction Different epitaxial techniques Chemical vapor deposition (CVD) Undesired polycrystalline layers Growth rate: ~2 µm/min. Liquid-phase epitaxy (LPE) The semiconductor is dissolved in the melt of another material (example: InP) Hard to make thin films Growth rate: 0.1-1 µm/min Molecular beam epitaxy (MBE) Relies on the sublimation of ultra-pure elements, then molecular beam arrive at 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 (even lower). Others: MOCVD, HVPE, MOMBE

MBE System Knudsen effusion cells: used as sources evaporators. Schematics of MBE Knudsen effusion cells: used as sources evaporators. Typical Knudsen cell contains a crucible made of pyrolytic boron nitride, quartz, tungsten or graphite heating filaments (often made of metal tantalum), water cooling system, heat shields and opening shutter.

MBE System LN2 Cryopanel Rotation system Turbo RHEED Gun Substrate Holder Effusion Sources http://www.uni-giessen.de/cms/

In-situ Measurement System MBE System Sample Transfer Mass Spectrometer In-situ Measurement System LayTec

(In-situ Measurement System) MBE System Crystal Oscillator (Beam flux monitor) Turbo Sample Load Lock RHEED Monitor LayTec (In-situ Measurement System) RHEED controller RIBER 21 MBE systems: 8 sources (Al, Ga, C, NH3, Si, SiH4)

MBE Control mechanisms Independent heating of material sources in the effusion cell. The beams can be shuttered in a fraction of a second Both solid and gas source can be used Via computer / manual controlled shutters. Water cooling system Drawback of gas (thin layer formation on the chamber’s wall) Memory effect of the sources and dopants Growth rates are typically on the order of a few A°/s. Nearly atomically abrupt transition from one material to another. Control composition and doping of the growing structure at monolayer level High resolution TEM of the lattice image shows the sharp interface between AlN and Si(111)

MBE Working principles and conditions Epitaxial growth occurs because the substrate is heated to the necessary temperature interaction of molecular or atomic beams on a surface of a heated crystalline substrate. The solid source (sublimation) provides an angular distribution of atoms or molecules in a beam. The gaseous elements can crack / condense on the wafer where they may react with each other. Atoms on a clean surface are free to move until finding correct position in the crystal lattice to bond.

MBE Working principles and conditions Mean free path for N2 molecules at 300 K The mean free path (l) of the particles > geometrical size of the chamber (10-5 Torr is sufficient) Outgassing 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.

Growth Monitoring RHEED RHEED (Reflection High Energy Electron Diffraction) for monitoring the growth of the layers. Probe only few monolayers Information about the state of the layers (2D, 3D etc.) Information about the crystallinity. Measure the lattice parameter. Growth rate can be obtained from RHEED oscillation. GaN QDs (chevron like shape)

Growth Monitoring In-situ growth monitoring Growth rate can be obtained using beam flux monitor Should use before and after the deposition A pyrometer is a type of thermometer used to measure high temperatures.  Emissivity Corrected Pyrometer (ECP).  Temperature range 450°C ... 1400°C [Substrate temperature is one of the key parameters during epitaxial growth. → Influences the growth rate, the composition of ternary and quaternary compounds and the doping level. → Impact on the quality of the grown layer and its roughness, thereby influencing the performance of devices based on such epitaxial layers.]

Growth Monitoring In-situ growth monitoring Wafer-selective curvature measurements LAYTEC curvature measurement system based on two parallel laser beams (635 nm) Light beams send nearly perpendicular to the surface in the center region of the wafer while rotating the wafer (8 – 10 rpm). Curvature range: from -7000 km-1 (convex) to +800 km-1 (concave) The radius of curvature is obtained measuring displacements of two laser beam with two split diode and two differential amplifiers. 1 𝑅 − 1 𝑅 0 = 𝑆− 𝑆 0 2𝐷𝐿 Here, D = displacements of two laser beam, R = radius of curvature, L = distance between the layer and detectors, S = displacements of detected signals, W = wafer diameter M = biaxial modulus and h = thickness. Subscripts f and s referring to the film and substrate. Curvature, R Bow, b 𝑏= 1 𝑅 (1− cos 𝑊𝑅 2 ) Strain (Deduced from Stoney’s equation) 𝜀= 𝑅 𝑀 𝑠 ℎ 𝑠 2 6 𝑀 𝑓 ℎ 𝑓 [Valid for hf / hs << 1 and for small value of stress (linear approximation). For large bending non-linear theory will be applicable.]

MBE In-situ growth monitoring Wafer-selective curvature measurements LAYTEC curvature measurement system based on two parallel laser beams (635 nm) Measurements are performed with light beams nearly perpendicular to the surface in the center region of the wafer while rotating the wafer (8–10 rpm). Curvature range: from -7000 km-1 (convex) to +800 km-1 (concave) The radius of curvature is obtained measuring displacements of two laser beam (one reference and one signal) with two split diode and two differential amplifiers.

Growth Monitoring In-situ growth monitoring Reflectance at different wavelengths (using LAYTEC) 950 nm, 633 nm and 405 nm Growth rate, layer thickness and roughness Measuring growth rate Choose the reflectance wavelength Growth rate per hour: Reflectance λ 2𝑛 3600 𝑡 𝑓 − 𝑡 𝑖 Example: tf = 2300 sec, ti = 2000 sec, n = 3.25 @ 950 nm (for Ga0.5In0.5P) Growth rate: 1.75 µm/hr (app.) Time / s http://www.semiconductor-today.com/news_items/2011/NOV/LAYTEC_141111.html

Growth Mechanism In a typical MBE-deposition process the material that needs to be deposited is heated in UHV and forms a molecular beam. The atoms of the beam are then adsorbed (adhesion of atoms) by the sample surface (adatoms). During the deposition, the adatoms interact with the atoms of the surface. This interaction depends on the type of adatoms, the substrate, and the temperature of the substrate. Behavior of adatoms in the surface diffusion process It is responsible for the nucleation and the subsequent growth in the form of thin layers on the substrate. To achieve good-quality film growth the growth rate must be small (typical growth rate: 0.05 ... 1 Å/s) and therefore the vacuum pressure in the ultrahigh vacuum regime is typically only a few 10−11 mbar. http://www.physik.uni-kl.de/hillebrands/research/methods/molecular-beam-epitaxy/

Growth Mechanism Modes of epitaxial growth regarding kinetics

Growth Mechanism Modes of epitaxial growth regarding thermodynamics i.e., competition between surface / interface energies.

Growth Mechanism Modes of epitaxial growth regarding thermodynamics i.e., competition between surface / interface energies.

Growth Mechanism Modes of epitaxial growth regarding thermodynamics i.e., competition between surface / interface energies. (Layer & island growth mode)

Growth Mechanism Thin film growth process Surface diffusion and island density 1 2 3 4 5 6 7 8 The larger the diffusion coefficient, D, the lower the island density, N.

Summary Presented the MBE system Growth mechanisms Control mechanisms Working principles and conditions Growth monitoring (by RHEED, growth rate, curvature, etc.) Growth mechanisms Behavior of adatoms in the surface diffusion process Learned different growth modes

Thank you very much for your attention

Growth Mechanism Thin film growth process Surface diffusion and island density The larger the diffusion coefficient, D, the lower the island density, N.