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!