By Mohammad Junaebur Rashid, PhD

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ByMohammad Junaebur Rashid, PhD

Solar Energy Research Institute (SERI), University of Kebangsaan Malaysia (UKM).

Post Doctoral Researcher

Molecular Beam Epitaxy (MBE)

Introduction What is epitaxy?

Different epitaxy techniques

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Molecular Beam Epitaxy (MBE)

Working principles and conditions

Outline

Growth Mechanism

Conclusion

System

Growth monitoring

What is epitaxy?

The term “epitaxy” comes from the

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Introduction

Greek roots epi (ἐπί) mean «above», and

taxi (τάξις) mean «in ordered manner».

Epitaxy Growth of a single crystal film on top of a crystalline substrate

Registry between the film and the substrate

Overlayer is called an epitaxial film or epitaxial layer.

Substrate atom

Epitaxial atom

Types of epitaxy

Homoepitaxy: substrate and material are of same kind (Si-Si).

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Introduction

Heteroepitaxy: substrate and material are of different kinds (Si-Ge, AlN/Si).

In-plane lattice mismatch: %

sub

subfilm

aaa

Allows for optoelectronic structures and band gap engineered devices.

Leads to unmatched lattice parameters.

Causes strained or relaxed growth lead to interfacial defects.

Altered the electronic, optic, thermal and mechanical properties of the films.

Lattice mismatch:

where, afilm is the lattice parameter of the film and asub is the lattice parameter of the substrate

Effect on the film

Different epitaxial techniques

Chemical vapor deposition (CVD)

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Introduction

The semiconductor is dissolved in the melt of another material (example: InP)

Undesired polycrystalline layers

Liquid-phase epitaxy (LPE)

Growth rate: ~2 µm/min.

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

System

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MBE

Schematics of MBE

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.

Knudsen effusion cells: used as sources evaporators.

System

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MBE

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

TurboRotation system

Effusion Sources

RHEED Gun

Substrate Holder

LN2 Cryopanel

System

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MBE

Sample Transfer

Mass Spectrometer

LayTec

In-situ Measurement System

System

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MBE

RHEED Monitor

RHEED controller

Crystal Oscillator(Beam flux monitor)

Sample Load Lock

RIBER 21 MBE systems: 8 sources (Al, Ga, C, NH3, Si, SiH4)

LayTec

Turbo

(In-situ Measurement System)

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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)

Working principles and conditions

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MBE

Epitaxial growth occurs because

the substrate is heated to the necessary temperature

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.

The solid source (sublimation) provides an angular distribution of atoms or molecules in a beam.

interaction of molecular or atomic beams on a surface of a heated crystalline substrate.

Working principles and conditions

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MBE

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

Mean free path for N2 molecules at 300 K

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.

RHEED

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Growth Monitoring

RHEED (Reflection High Energy Electron Diffraction) for monitoring the growth of the layers.

Growth rate can be obtained from RHEED oscillation.

Probe only few monolayers

Information about the crystallinity.

Measure the lattice parameter.

Information about the state of the layers (2D, 3D etc.)

GaN QDs(chevron like

shape)

In-situ growth monitoring

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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). 

[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.]

Temperature range 450°C ... 1400°C

Growth Monitoring

In-situ growth monitoring

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Wafer-selective curvature measurements

Light beams send nearly perpendicular to the surface in the center region of the wafer while rotating the wafer (8 – 10 rpm).

LAYTEC curvature measurement system based on two parallel laser beams (635 nm)

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 diameterM = biaxial modulus and h = thickness.Subscripts f and s referring to the film and substrate.

1𝑅− 1

𝑅0=𝑆−𝑆0

2𝐷𝐿Curvature, R

Strain(Deduced from Stoney’s equation)

Bow, b 𝑏= 1𝑅 (1− cos (𝑊𝑅

2 ))

𝜀=𝑅𝑀 𝑠h𝑠

2

6𝑀 𝑓 h𝑓[Valid for hf / hs << 1 and for small value of stress (linear approximation). For large bending non-linear theory will be applicable.]

Growth Monitoring

In-situ growth monitoring

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950 nm, 633 nm and 405 nm

Reflectance at different wavelengths (using LAYTEC)

Growth rate, layer thickness and roughness

Measuring growth rate

Choose the reflectance wavelength

Growth rate per hour:

λ2𝑛

3600𝑡 𝑓 −𝑡 𝑖

Ref

lect

ance

Time / s

Example:tf = 2300 sec, ti = 2000 sec, n = 3.25 @ 950 nm (for Ga0.5In0.5P)Growth rate: 1.75 µm/hr (app.)

http://www.semiconductor-today.com/news_items/2011/NOV/LAYTEC_141111.html

Growth Monitoring

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Growth Mechanism

In a typical MBE-deposition process the material that needs to be deposited is heated in UHV and forms a molecular beam.

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

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

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Modes of epitaxial growth regarding kinetics

Growth Mechanism

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Modes of epitaxial growth regarding thermodynamics

i.e., competition between surface / interface energies.

Growth Mechanism

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Growth Mechanism

Modes of epitaxial growth regarding thermodynamics

i.e., competition between surface / interface energies.

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Modes of epitaxial growth regarding thermodynamics

i.e., competition between surface / interface energies.

Growth Mechanism

(Layer & island growth mode)

Thin film growth process

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Surface diffusion and island density

Growth Mechanism

1 2 3 4

5 6 7 8

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

Presented the MBE system

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Behavior of adatoms in the surface diffusion process

Summary

Control mechanisms

Growth monitoring (by RHEED, growth rate, curvature, etc.)

Working principles and conditions

Growth mechanisms

Learned different growth modes

Thank you very much for your attention

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Thin film growth process

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Surface diffusion and island density

Growth Mechanism

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