Epitaxial Deposition

Daniel Lentz

EE 518Penn State University

March 29, 2007Instructor: Dr. J. Ruzyllo

Outline

IntroductionMechanism of epitaxial growthMethods of epitaxial depositionProperties of epitaxial layersApplications of epitaxial layers

Epitaxial Growth Deposition of a layer on a

substrate which matches the crystalline order of the substrate

Homoepitaxy Growth of a layer of the

same material as the substrate

Si on Si Heteroepitaxy

Growth of a layer of a different material than the substrate

GaAs on Si

Ordered, crystalline growth; NOT epitaxial

Epitaxial growth:

Motivation

Epitaxial growth is useful for applications that place stringent demands on a deposited layer: High purity Low defect density Abrupt interfaces Controlled doping profiles High repeatability and uniformity Safe, efficient operation

Can create clean, fresh surface for device fabrication

General Epitaxial Deposition Requirements Surface preparation

Clean surface needed Defects of surface duplicated in epitaxial layer Hydrogen passivation of surface with water/HF

Surface mobility High temperature required heated substrate Epitaxial temperature exists, above which deposition is

ordered Species need to be able to move into correct

crystallographic location Relatively slow growth rates result

Ex. ~0.4 to 4 nm/min., SiGe on Si

General Scheme

Modified from http://www.acsu.buffalo.edu/~tjm/MOVPE-GaN-schematic.jpg

Thermodynamics Specific thermodynamics varies by process

Chemical potentials Driving force

High temperature process is mass transport controlled, not very sensitive to temperature changes

Steady state Close enough to equilibrium that chemical forces that drive growth

are minimized to avoid creation of defects and allow for correct ordering

Sufficient energy and time for adsorbed species to reach their lowest energy state, duplicating the crystal lattice structure

Thermodynamic calculations allow the determination of solid composition based on growth temperature and source composition

Kinetics

Growth rate controlled by kinetic considerations Mass transport of reactants to surface Reactions in liquid or gas Reactions at surface Physical processes on surface

Nature and motion of step growth Controlling factor in ordering

Specific reactions depend greatly on method employed

Kinetics Example Atoms can bond to flat surface,

steps, or kinks. On surface requires some critical

radius Easier at steps Easiest at kinks

As-rich GaAs surface As only forms two bonds to

underlying Ga Very high energy

Reconstructs by forming As dimers Lowers energy Causes kinks and steps on surface

Results in motion of steps on surface If start with flat surface, create step

once first group has bonded Growth continues in same way

http://www.bnl.gov/nsls2/sciOps/chemSci/growth.asp

Vapor Phase Epitaxy Specific form of chemical vapor deposition (CVD) Reactants introduced as gases Material to be deposited bound to ligands Ligands dissociate, allowing desired chemistry to

reach surface Some desorption, but most adsorbed atoms find

proper crystallographic position Example: Deposition of silicon

SiCl4 introduced with hydrogen Forms silicon and HCl gas Alternatively, SiHCl3, SiH2Cl2 SiH4 breaks via thermal decomposition

Precursors for VPE

Must be sufficiently volatile to allow acceptable growth rates

Heating to desired T must result in pyrolysis Less hazardous chemicals preferable

Arsine highly toxic; use t-butyl arsine instead VPE techniques distinguished by precursors

used

Varieties of VPE Chloride VPE

Chlorides of group III and V elements Hydride VPE

Chlorides of group III element Group III hydrides desirable, but too unstable

Hydrides of group V element Organometallic VPE

Organometallic group III compound Hydride or organometallic of group V element

Not quite that simple Combinations of ligands in order to optimize

deposition or improve compound stability Ex. trimethylaminealane gives less carbon

contamination than trimethylalluminum

http://upload.wikimedia.org/wikipedia/en/thumb/e/e5/Trimethylaluminum.png/100px-Trimethylaluminum.png,

http://pubs.acs.org/cgi-bin/abstract.cgi/jpchax/1995/99/i01/f-pdf/f_j100001a033.pdf?sessid=6006l3

Other Methods

Liquid Phase Epitaxy Reactants are dissolved in

a molten solvent at high temperature

Substrate dipped into solution while the temperature is held constant

Example: SiGe on Si Bismuth used as solvent Temperature held at

800°C High quality layer

Fast, inexpensive Not ideal for large area

layers or abrupt interfaces Thermodynamic driving

force relatively very low

Molecular Beam Epitaxy Very promising technique Elemental vapor phase

method Beams created by

evaporating solid source in UHV

Doping of Epitaxial Layers

Incorporate dopants during deposition Theoretically abrupt dopant distribution Add impurities to gas during deposition Arsine, phosphine, and diborane common

Low thermal budget results High T treatment results in diffusion of dopant into

substrate Reason abrupt distribution not perfect

Properties of Epitaxial Layer

Crystallographic structure of film reproduces that of substrate

Substrate defects reproduced in epi layer Electrical parameters of epi layer independent of

substrate Dopant concentration of substrate cannot be reduced Epitaxial layer with less dopant can be deposited

Epitaxial layer can be chemically purer than substrate

Abrupt interfaces with appropriate methods

Applications

Engineered wafers Clean, flat layer on top of

less ideal Si substrate On top of SOI structures Ex.: Silicon on sapphire Higher purity layer on lower

quality substrate (SiC) In CMOS structures

Layers of different doping Ex. p- layer on top of p+

substrate to avoid latch-up

More applications

Bipolar Transistor Needed to produce

buried layer

III-V Devices Interface quality key Heterojunction Bipolar

Transistor LED Laser

http://www.veeco.com/library/elements/images/hbt.jpg

http://www.search.com/reference/Bipolar_junction_transistor

Summary

Deposition continues crystal structure Creates clean, abrupt interfaces and high

quality surfaces High temperature, clean surface required Vapor phase epitaxy a major method of

deposition Epitaxial layers used in highest quality wafers Very important in III-V semiconductor

production

References P. O. Hansson, J. H. Werner, L. Tapfer, L. P. Tilly, and E. Bauser, Journal of Applied

Physics, 68 (5), 2158-2163 (1990). G. B. Stringfellow, Journal of Crystal Growth, 115, 1-11 (1991). S. M. Gates, Journal of Physical Chemistry, 96, 10439-10443 (1992). C. Chatillon and J. Emery, Journal of Crystal Growth, 129, 312-320 (1993). M. A. Herman, Thin Solid Films, 267, 1-14 (1995). D. L. Harame et al, IEEE Transactions on Electron Devices, 42 (3), 455-468 (1995). G. H. Gilmer, H. Huang, and C. Roland, Computational Materials Science, 12, 354-380

(1998). B. Ferrand, B. Chambaz, and M. Couchaud, Optical Materials, 11, 101-114 (1999). R. C. Cammarata, K. Sieradzki, and F. Spaepen, Journal of Applied Physics, 87 (3),

1227-1234 (2000). R. C. Jaeger, Introduction to Microelectronic Fabrication, 141-148 (2002). R. C. Cammarata and K. Sieradzki, Journal of Applied Mechanics, 69, 415-418 (2002). A. N. Larsen, Materials Science in Semiconductor Processing, 9, 454-459 (2006).