Epitaxial Deposition 5j543v

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Ou Introduction 2 Mechanism of epitaxial growth 2 Methods of epitaxial deposition 2 Properties of epitaxial layers 2 Applications of epitaxial layers 2

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àeposition of a layer on a substrate which matches the crystalline order of the substrate Homoepitaxy 2

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Growth of a layer of the same material as the substrate Si on Si

Heteroepitaxy 2

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Growth of a layer of a different material than the substrate GaAs on Si

Ordered, crystalline growth; NOT epitaxial

Epitaxial growth:

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Epitaxial growth is useful for applications that place stringent demands on a deposited layer: 2 2 2 2 2 2

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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

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Surface preparation 2 2 2

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Clean surface needed àefects of surface duplicated in epitaxial layer Hydrogen ivation of surface with water/HF

Surface mobility 2 2 2 2

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



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

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Specific thermodynamics varies by process 2 2

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Chemical potentials àriving 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

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Growth rate controlled by kinetic considerations 2 2 2 2

Mass transport of reactants to surface Reactions in liquid or gas Reactions at surface Physical processes on surface 2 2

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Nature and motion of step growth Controlling factor in ordering

Specific reactions depend greatly on method employed

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Atoms can bond to flat surface, steps, or kinks. 2 2 2

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As-rich GaAs surface 2 2

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Lowers energy Causes kinks and steps on surface

Results in motion of steps on surface 2 2

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

As only forms two bonds to underlying Ga Very high energy

Reconstructs by forming As dimers 2

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On surface requires some critical radius Easier at steps Easiest at kinks

If start with flat surface, create step once first group has bonded Growth continues in same way

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Specific form of chemical vapor deposition (CVà 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: àeposition of silicon 2 2 2 2

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

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Must be sufficiently volatile to allow acceptable growth rates Heating to desired T must result in pyrolysis Less hazardous chemicals preferable 2

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Arsine highly toxic; use t-butyl arsine instead

VPE techniques distinguished by precursors used

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Chloride VPE 2

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Chlorides of group III and V elements

Hydride VPE 2

Chlorides of group III element 2

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Hydrides of group V element

Organometallic VPE 2 2

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Group III hydrides desirable, but too unstable

Organometallic group III compound Hydride or organometallic of group V element

Not quite that simple 2 2

Combinations of ligands in order to optimize deposition or improve compound stability Ex. trimethylaminealane gives less carbon contamination than trimethylalluminum

http://.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

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Liquid Phase Epitaxy 2

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Reactants are dissolved in a molten solvent at high temperature Substrate dipped into solution while the temperature is held constant Example: SiGe on Si 2 2

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Bismuth used as solvent Temperature held at 800°C

High quality layer

2 2

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Fast, inexpensive Not ideal for large area layers or abrupt interfaces Thermodynamic driving force relatively very low

Molecular Beam Epitaxy 2 2

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Very promising technique Elemental vapor phase method Beams created by evaporating solid source in UHV

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Incorporate dopants during deposition 2 2 2

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

Low thermal budget results 2

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High T treatment results in diffusion of dopant into substrate Reason abrupt distribution not perfect

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Crystallographic structure of film reproduces that of substrate Substrate defects reproduced in epi layer Electrical parameters of epi layer independent of substrate 2 2

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àopant 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

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Engineered wafers 2

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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 2 2

Layers of different doping Ex. p- layer on top of p+ substrate to avoid latch-up

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Bipolar Transistor 2

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

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III-V àevices 2 2

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

Needed to produce buried layer

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Interface quality key Tranzistor bipolar cu jonctiune diferita (Heterojunction ?? LEà Laser

u 2 2 2 2 2 2

àepunerea continua structura cristalina Creaza interfete curate,abrupte(rapide ?? si suprafete de calitate inalta Temperaturi inalte, se cer suprafete curate Faza de vapori epitaxy ± o metoda majora de depunere Straturile epitaxiale folosite in (wafers ?? de calitate inalta (highest quality wafers Foarte important in productia semiconductoarelor III-V



2 2 2 2 2 2 2 2 2 2 2 2

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