Date post: | 14-Jan-2017 |
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Molecular Beam Epitaxy
Enrique Flores FontauIEE3030-Microelectronics
Tallinn University of Technology
Introduction What is Epitaxy? Epitaxy types Growth modes
Molecular Beam Epitaxy Working principle MBE Growth process MBE features In situ monitoring
Materials SS-MBE
Benefits and Problems Applications Conclusions
Contents
IntroductionWhat is Epitaxy?
Epitaxy is the process of growing a thin crystalline layer on a crystalline substrate.
Epitaxial layer is always thinner than the substrate
Epitaxial grow techniques:
What is epitaxy?
Vapor-Phase Epitaxy Liquid Phase-Epitaxy Molecular Beam Epitaxy
VPE is a modification of chemical vapor deposition
LPE is a method to grow semiconductor crystal layers from the melt on solid
substrates.
MBE is based on an UHV(Ultra High Vacuum) technique.
Chemical reactions involved Chemical reactions involved No chemical reactions involved.
Homoepitaxy
The deposition substrate is the same material as we are depositing from the beam. (e.g Si on Si)
HeteroepitaxySubstrate and material are of different composition in order to fabricate integrated crystalline layers of different materials. (e.g GaAs on Si)
Epitaxy types
There are three main growth modes that can occur depending upon the substrate temperature, the deposition rate and available surface energy
Growth modes
Molecular Beam Epitaxy
What is Molecular Beam Epitaxy?
• Pumping Systems
• Growth Chamber, epitaxial growth happens.
• Load lock facilitates the introduction and removal of samples
• Auxiliary chamber host analytical and process equipment
MBE system
Gas sources are heated in separate k-cells or electron beam evaporators to achieve molecular or atom beams.
No interaction with each other until they reach the Surface.
During the deposition, the interactions of the atoms produce the epitaxial growth.
MBE Growth process
Controlling , via shutters and the temperature of the source, will control the rate of impinging materials.
The temperature of the substrate will control the rate of diffusion and desorption.
Background gases help to avoid monolayr contamination.
MBE Growth process
Deposition rate (): 1-5 s
Growth temperature (): 550
Thickness control (Å): 5
Interface width (Å): 5
Shuttering control: 0.1 s
MBE features
Reflection High Energy Electron Diffraction (RHEED) Observe removal of contaminants from the substrate surface
Calibrate growth rates
Estimate the substrate temperature
Determine the stoichiometry
Analyze surface morphology – RHEED pattern
Study growth kinetics – RHEED intensity oscillations
In Situ Monitoring
Materials
What kind of materials are used?
Materials used on MBE
Different materials are used depending the type of MBE, but we will focus on Solid Source MBE type.
Molecular Beams Substrate target
Group III – V molecular beams III-V Semiconductors
SS- MBEGroup II – VI molecular beams II-VI Semiconductors
Others IV-VI Semiconductors, Heusler alloys, silicides, metals ...
Typically, the substrate target is a semiconductor material with useful electronic properties.
The molecular beam quite often is composed of evaporated elemental substances such as gallium and arsenic
Materials used on MBE
III-V semiconductors offer high electron mobility and a direct high band gap.
II-VI semiconductors exhibit direct large band gaps , but have some problems with conductivity.
IV – VI Semiconductors also offer a narrow band gap.
Benefits and Problems
Is it worth to use ?
Clean surfaces. Monitoring in situ. Independent vaporization of each
material. Multiple sources are used to grow
alloy films and heterostructures.Deposition is controlled at
submonolayer level. Extremely flexible technique since
growth parameters are varied independently.
Benefits/Problems of MBEVery low deposition rates: 1um –
100nm per hour are used. High equipment cost and long set
up time. Extreme Flexibility (uncontrolled
flexibility = chaos!).
Many Boring Evenings!Mostly Broken Equipment!Mega-Buck Evaporation!
Applications
Are there any electronic applications?
Applications
The driving force today is the fabrication of advanced electronic and optoelectronic devices.
Transistors (HEMT,HBT):
Microwave devices (IMPATT)
Optoelectronic devices (MQW) laser
Conclusions
Key points of the topic
Very well controlled and clean result.
High equipment cost and long setup time
In situ monitoring
High Speed electronic and optoelectronic applications
III-V semiconductors as GaAs are the most common used in Electronic and OptoElectronics devices.
Conclusions
Any Questions?
Ah, I get it now!