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Molecular Beam Epitaxy (MBE). By Mohammad Junaebur Rashid, PhD. Post Doctoral Researcher. Solar Energy Research Institute ( SERI ) , University of Kebangsaan Malaysia ( UKM ) . Outline. Introduction. What is epitaxy?. Different epitaxy techniques. Molecular Beam Epitaxy (MBE).

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slide1

Molecular Beam Epitaxy (MBE)

By

Mohammad Junaebur Rashid, PhD

Post Doctoral Researcher

Solar Energy Research Institute (SERI),

University of Kebangsaan Malaysia (UKM).

slide2

Outline

  • Introduction
  • What is epitaxy?
  • Different epitaxy techniques
  • Molecular Beam Epitaxy (MBE)
  • System
  • Working principles and conditions
  • Growth monitoring
  • Growth Mechanism
  • Conclusion
slide3

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

Overlayeris called an epitaxial film or epitaxial layer.

  • Registry between the film and the substrate

Epitaxial atom

Substrate atom

slide4

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

asubis the lattice parameter of the substrate

slide5

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
slide6

MBE

  • System
  • 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.

Schematics of MBE

slide7

MBE

  • System

LN2Cryopanel

Rotation system

Turbo

RHEED Gun

Substrate Holder

Effusion Sources

http://www.uni-giessen.de/cms/

slide8

MBE

  • System

Sample Transfer

Mass Spectrometer

In-situ Measurement System

LayTec

slide9

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)

slide10

MBE

  • Control mechanisms
  • Independent heating of material sources in the effusion cell.
  • Water cooling system
  • Both solid and gas source can be used
  • Drawback of gas (thin layer formation on the chamber’s wall)
  • Memory effect of the sources and dopants
  • The beams can be shuttered in a fraction of a second

High resolution TEM of the lattice image shows the sharp interface between AlN and Si(111)

  • Control composition and doping of the growing structure at monolayer level
  • Viacomputer / manual controlled shutters.

Nearly atomically abrupt transition from one material to another.

  • Growth rates are typically on the order of a few A°/s.
slide11

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) providesan angular distribution of atoms or molecules in a beam.
  • The gaseous elements cancrack / 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.
slide12

MBE

  • Working principles and conditions

Mean free path for N2molecules at 300 K

  • The mean free path (l) of the particles > geometrical size of the chamber (10-5 Torr is sufficient)
  • Outgassingfrom 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.
slide13

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)

slide14

Growth Monitoring

  • In-situ growth monitoring
  • Growth rate can be obtained using beam flux monitor
  • Should use before and after the deposition
  • Apyrometer 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.]

slide15

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)

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

Strain

(Deduced from Stoney’s equation)

[Valid for hf/ hs << 1 and for small value of stress (linear approximation). For large bending non-linear theory will be applicable.]

slide16

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
  • 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

slide17

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

http://www.physik.uni-kl.de/hillebrands/research/methods/molecular-beam-epitaxy/

slide18

Growth Mechanism

  • Modes of epitaxial growth regarding kinetics
slide19

Growth Mechanism

  • Modes of epitaxial growth regarding thermodynamics
  • i.e., competition between surface / interface energies.
slide20

Growth Mechanism

  • Modes of epitaxial growth regarding thermodynamics
  • i.e., competition between surface / interface energies.
slide21

Growth Mechanism

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

(Layer & island growth mode)

slide22

Growth Mechanism

  • Thin film growth process
  • Surface diffusion and island density

2

4

1

3

5

7

8

6

  • The larger the diffusion coefficient, D, the lower the island density, N.
slide23

Summary

  • Presented the MBE system
  • 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
slide25

Growth Mechanism

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