Metal Contact Materials

1. Elemental Metals Used for Semiconductor Contacts

Material	Symbol	Typical work function ${\varphi }_m$ (eV)	Resistivity at 20–25 °C ($\mu \Omega \cdot$cm)	Melting point (°C)	Common contact role	Common semiconductor/device use
Lithium	Li	$\approx 2.9$	9.3	181	Very-low-work-function contact/research	III–V, organic/novel devices
Magnesium	Mg	$\approx 3.7$	4.4	650	Low-work-function metal	n-type III–V, optoelectronics
Calcium	Ca	$\approx 2.9$	3.4	842	Very-low-work-function contact	Research/compound semiconductors
Beryllium	Be	$\approx 5.0$	4.0	1287	Diffusion/contact layer	III–V and specialized devices
Aluminum	Al	$\approx 4.28$	2.65	660	Low-${\varphi }_m$ ohmic metal; interconnect; electrode	Si, GaN, AlGaN/GaN, SiC, III–V
Scandium	Sc	$\approx 3.5$	55	1541	Low-work-function alloy/contact material	III-nitrides, research
Titanium	Ti	$\approx 4.33$	42	1668	Adhesion/contact metal; low-${\varphi }_m$ layer; reacts to form TiN/TiSi${}_x$/TiC	Si, SiC, GaN, AlGaN/GaN, III–V
Vanadium	V	$\approx 4.3$	20	1910	Ohmic-contact and refractory layer	GaN, SiC, III–V
Chromium	Cr	$\approx 4.5$	12.9	1907	Adhesion layer; contact metal	Si, GaAs, optoelectronics
Manganese	Mn	$\approx 4.1$	144	1246	Contact/alloy/research	Specialized devices
Iron	Fe	$\approx 4.5$	9.7	1538	Magnetic/contact material	Spintronics, research
Cobalt	Co	$\approx 5.0$	6.2	1495	Silicide-forming contact; barrier/contact metal	Si, SiC, CMOS
Nickel	Ni	$\approx 5.1$–$5.2$	6.9	1455	Ohmic/Schottky contact; silicide former	Si, SiC, GaN, Ge
Copper	Cu	$\approx 4.65$	1.68	1085	Low-resistivity interconnect/contact	Si CMOS, power devices
Zinc	Zn	$\approx 4.3$	5.9	420	p-type contact/alloying element	GaAs, GaN, ZnO, II–VI
Gallium	Ga	$\approx 4.2$	27	30	Liquid/low-temperature contact; alloying	GaAs, GaN, InGaN research
Indium	In	$\approx 4.1$	8.4	157	Low-temperature contact; solder; transparent-contact processing	InGaAs, InP, III–V, detector devices
Tin	Sn	$\approx 4.4$	11.5	232	Solder/contact/alloy	Packaging, Si/compound-semiconductor devices
Zirconium	Zr	$\approx 4.05$	42	1855	Refractory/contact/gate material	Si, high-$k$ CMOS, research
Niobium	Nb	$\approx 4.3$	15	2477	Refractory metal; superconducting contacts	Superconducting electronics, specialized devices
Molybdenum	Mo	$\approx 4.6$	5.3	2623	Refractory contact/gate/electrode	Si, GaN, SiC, power devices
Ruthenium	Ru	$\approx 4.7$	7.1	2334	Advanced CMOS contact/interconnect	Advanced CMOS, high-density metallization
Rhodium	Rh	$\approx 5.0$	4.7	1964	Noble/refractory contact	Specialized contacts
Palladium	Pd	$\approx 5.1$	10.8	1555	Schottky/contact; germanide/silicide former	Si, Ge, GaAs, III–V
Silver	Ag	$\approx 4.26$	1.59	962	Low-resistance electrode; optical devices	Solar cells, LEDs, specialized contacts
Hafnium	Hf	$\approx 3.9$	33	2233	Refractory/gate/contact material	Advanced CMOS, Si
Tantalum	Ta	$\approx 4.25$	13.5	3017	Refractory contact/barrier; silicide former	Si, SiC, GaN
Tungsten	W	$\approx 4.55$	5.6	3422	Plug/contact metal; refractory electrode	CMOS, power devices
Rhenium	Re	$\approx 5.0$	19	3186	Refractory electrode/contact	Specialized/high-temperature devices
Osmium	Os	$\approx 4.8$	8.1	3033	Specialized noble/refractory electrode	Research
Iridium	Ir	$\approx 5.3$	4.7	2446	Noble electrode; high-temperature contact	Ferroelectric/oxide electronics
Platinum	Pt	$\approx 5.6$	10.6	1768	Schottky metal; noble electrode; silicide former	Si, SiC, GaN, sensors
Gold	Au	$\approx 5.1$	2.44	1064	Noble top metal, bonding, contact/electrode	GaAs, GaN, LEDs, RF, packaging
Mercury	Hg	$\approx 4.5$	Liquid	-39	Specialized/research contact	Historical/specialized devices

Notes on elemental-metal work functions

The work function of a metal is not a single immutable number. Reported values depend on crystal orientation, surface cleanliness, adsorbates, deposition process, grain structure, temperature, and surface reconstruction. Representative values commonly used in semiconductor-contact calculations include Al $\approx 4.28$ eV, Ti $\approx 4.33$ eV, Ni $\approx 5.15$ eV, Au $\approx 5.10$ eV, Pd $\approx 5.12$ eV, Pt $\approx 5.65$ eV, Cu $\approx 4.65$ eV, Ag $\approx 4.26$ eV and W $\approx 4.55$ eV.

For a real device, do not determine whether a contact is ohmic solely from ${\varphi }_m$ versus $\chi$. Interface states, Fermi-level pinning, interfacial compounds, semiconductor doping, defects, reactions during annealing, and barrier thinning/tunneling can dominate.

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2. Metal Silicides

Silicides are among the most important contact materials in Si technology. A deposited metal reacts with Si during annealing to produce a low-resistance metal–silicon compound. The resulting process is commonly called salicidation when the silicide forms selectively on exposed Si/poly-Si regions.

Silicide	Approx. thin-film resistivity ($\mu \Omega \cdot$cm)	Typical formation/anneal range (°C)	Approx. barrier height to n-Si (eV)	Major use
TiSi	$\sim 60$	$\sim 500$	—	Historical/contact precursor
TiSi${}_2$ C49	60–70	500–700	—	Intermediate silicide phase
TiSi${}_2$ C54	13–16	700–900	$\sim 0.58$	Low-$R$ Si contacts; older CMOS
VSi${}_2$	45–55	$\sim 600$	—	Contact/research
CrSi${}_2$	$\gtrsim 250$ (strongly process dependent)	$\sim 450$	—	Specialized contacts
ZrSi${}_2$	35–40	—	$\sim 0.55$	Contact/research
HfSi${}_2$	$\sim 45$–60	—	—	Contact/gate-related research
NbSi${}_2$	$\sim 50$	—	—	Refractory contact/research
TaSi${}_2$	35–55	800–1000	$\sim 0.59$	Refractory silicide/contact
MoSi${}_2$	40–100	800–1000	$\sim 0.64$	Refractory electrode/contact
WSi${}_2$	30–70	$\sim 1000$	$\sim 0.67$	Refractory contact/gate/interconnect
MnSi	200–260	400–500	—	Specialized/research
FeSi	260–290	450–550	—	Specialized/research
FeSi${}_2$	High/semiconducting	$\sim 550$	—	Thermoelectric/optical rather than low-$R$ contact
Co${}_2$Si	$\sim 60$–70	300–500	—	Intermediate Co-silicide
CoSi	100–150	400–600	—	Intermediate Co-silicide
CoSi${}_2$	14–20	600–800	$\sim 0.65$	Low-resistance CMOS contacts
Ni${}_2$Si	20–30	200–350	—	Intermediate Ni-silicide
NiSi	12–20	350–600	—	Major modern Si contact material
NiSi${}_2$	40–50	600–800	$\sim 0.66$	High-temperature/end-phase silicide
Pd${}_2$Si	25–35	100–300	$\sim 0.74$	Schottky/contact to Si
PdSi	Process dependent	high temperature	—	Contact/research
Pt${}_2$Si	Process dependent	200–500	—	Intermediate Pt silicide
PtSi	28–35	250–400	$\sim 0.84$	Schottky diodes, IR detectors, contacts

Representative silicide values above are consistent with commonly cited semiconductor-contact data; values vary with film thickness, phase, stoichiometry, anneal, and microstructure.

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3. Metal Germanides

Germanium and SiGe devices use many of the same concepts as Si, but germanide formation is especially important because Ge is readily reacted with several contact metals.

Germanide / contact compound	Main metal	Typical application
NiGe	Ni	Low-resistance contacts to Ge/SiGe
Ni${}_2$Ge	Ni	Intermediate Ni–Ge reaction phase
CoGe	Co	Ge/SiGe contacts
Co${}_2$Ge	Co	Intermediate contact phase
PtGe	Pt	Ge Schottky/ohmic-contact research
Pt${}_2$Ge	Pt	Intermediate Pt–Ge phase
PdGe	Pd	Ge/III–V contact technology
Pd${}_2$Ge	Pd	Ge contact/intermediate phase
AuGe	Au–Ge alloy	Classic n-type GaAs ohmic contact
NiGeAu	Ni–Ge–Au	Ge/III–V ohmic-contact schemes
Au–Ge–Ni	Au–Ge–Ni alloy/stack	Classic GaAs ohmic metallization

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4. Important Metal Nitrides Used in Contacts and Metallization

Metal nitrides are frequently used as barriers, adhesion layers, diffusion barriers, contact layers, or reacted interface phases. Some are also gate electrodes.

Material	Approx. work function (eV)	Approx. resistivity ($\mu \Omega \cdot$cm)	Main role
TiN	$\sim 3.7$–5.0*	$\sim 20$–100	Contact, gate, barrier, diffusion barrier
TaN	$\sim 4.5$–5.0*	$\sim 100$–300	Diffusion barrier, resistor, gate/contact
WN / W${}_2$N	$\sim 4.5$–5.0*	$\sim 100$–300	Barrier/contact
MoN	process dependent	$\sim 100$	Contact/barrier
NbN	$\sim 4.5$–5.0*	process dependent	Superconducting electrode/contact
ZrN	$\sim 4.0$–4.6*	$\sim 10$–50	Contact/gate/barrier
HfN	$\sim 4.0$–4.5*	$\sim 50$–100	Gate/contact/barrier
AlN	insulating	—	Usually insulating/passivation, not a normal metal contact
GaN	semiconductor	—	Not a metal; can participate in conductive heterostructure contacts

*Nitride work functions are particularly process- and stoichiometry-dependent. Do not treat the listed range as a universal constant.

Important GaN example: Ti/Al contacts to n-GaN can react during annealing and produce TiN and an N-deficient interfacial region. The TiN formation and associated interface modification can substantially reduce the effective contact barrier.

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5. Metal Carbides Used in Contacts

Carbides are particularly important in contacts to SiC and other high-temperature semiconductors.

Material	Main role	Common semiconductor/use
TiC	Interfacial carbide; refractory contact	SiC, high-temperature contacts
TaC	Refractory carbide/contact	SiC, high-temperature devices
WC	Refractory carbide/contact	SiC, high-temperature devices
HfC	Ultra-high-temperature refractory material	Specialized/high-temperature devices
ZrC	Refractory contact/barrier	Specialized/high-temperature devices
NbC	Refractory contact	Specialized devices
Mo${}_2$C	Refractory contact/interface	SiC/research
Cr${}_3$C${}_2$	Refractory contact/coating	Specialized devices

For SiC, annealing refractory metals like Ti, Ta, and W can produce carbides plus silicides and ternary interfacial phases. The resulting interface determines the final electrical behavior.

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6. Contact Alloys and Multilayer Metallization

In practical fabrication, the “contact metal” is often a stack, not a single material. Different layers can perform different functions:

• semiconductor reaction / barrier reduction
• adhesion
• diffusion barrier
• low-resistance conduction
• oxidation protection
• wire bonding

Metallization	Typical semiconductor	Typical purpose
Al	Si, SiC, GaN	Simple low-${\varphi }_m$ ohmic/electrode metal
Ti/Al	n-GaN, AlGaN/GaN	Low-resistance n-type ohmic contact
Ti/Al/Ni/Au	n-GaN, AlGaN/GaN	Very common GaN HEMT ohmic stack
Ti/Al/Pt/Au	n-GaN, AlGaN/GaN	GaN ohmic contact with diffusion/barrier layer
Ti/Al/Ti/Au	n-GaN	Alternative GaN ohmic metallization
Ti/Al/Mo/Au	n-GaN	Alternative high-temperature GaN contact
V/Al/Pt/Au	n-GaN	Alternative GaN ohmic stack
Ni/Au	p-GaN	Common p-GaN contact stack
Pd/Au	p-GaN, p-type III–V	High-work-function p-type contact
Pt/Au	p-type III–V, GaN	High-work-function contact/electrode
Ni/Pt/Au	p-GaN/III–V	Contact + diffusion/barrier + top metal
Ni/Al	SiC	Ohmic contact, especially after annealing
Ni/Ti/Al	p-SiC	Low-temperature/modified SiC ohmic contact
Ni/Si/Al	p-SiC	Modified low-temperature SiC ohmic contact
Ti/Al	p-SiC	p-type SiC ohmic contact
Ti/Al/Ni	p-SiC	p-type SiC ohmic contact
Al/Ti/Ni	SiC	Ohmic metallization
Ti/Ni	SiC	Ohmic contact
W/Ni	SiC	High-temperature/refractory contact
Pt/Ti	SiC	Contact/alloy; can produce silicide/carbide phases
Ti/Au	III–V, GaN	Adhesion + top metal
Cr/Au	GaAs, LEDs, photonics	Adhesion + top metal
Ti/Pt/Au	III–V/GaN	Adhesion/contact + diffusion barrier + top metal
Ti/Pd/Au	III–V	Contact/barrier/top metal
Ti/W	Si/compound semiconductors	Adhesion/barrier/contact
Ti/TiN/W	CMOS	Contact/barrier/plug metallization
Ti/TiN/Cu	CMOS/interconnect	Barrier + Cu metallization
Ta/TaN/Cu	CMOS/interconnect	Diffusion barrier + Cu
W/Ti/TiN	CMOS	Contact/plug metallization
NiSi/Al	Si	Silicide + overlying metal
NiSi/TiN	Si CMOS	Silicide + barrier/cap
CoSi${}_2$/Al	Si	Silicide + overlying metal
PtSi/Au	Si/IR detectors	Silicide/contact + top metal

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7. Classic III–V Ohmic Contact Alloys

These are particularly important for GaAs, InP, GaSb, InAs and related devices.

Contact material / alloy	Semiconductor	Carrier type / role
AuGe/Ni/Au	GaAs	n-type ohmic
AuGe/Ni	GaAs	n-type ohmic
AuGe	GaAs	n-type ohmic
AuZn/Au	GaAs	p-type ohmic
AuBe/Au	GaAs	p-type ohmic
Pd/Zn/Pd/Au	GaAs	p-type ohmic
Pt/Ti/Pt/Au	III–V	Contact/electrode
Ni/Au	GaAs/GaN	p-type/contact/electrode
Ti/Pt/Au	GaAs/InP	Contact + diffusion barrier
Au/Sn	III–V packaging	Solder/bonding/contact
In/Au	III–V	Low-temperature bonding/contact
In/Sn	III–V	Solder/contact
Pd/Ge/Au	Ge/III–V	Ohmic contact
Ni/Ge/Au	Ge/III–V	Ohmic contact

Why AuGe is important

AuGe is a classic n-type GaAs ohmic-contact alloy. During annealing, Ge can diffuse into GaAs and produce a heavily doped near-surface region, while the reacted Au–Ge/GaAs interface produces the final contact structure.

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8. Transparent Conducting Contact Materials

For LEDs, photodetectors, solar cells and display/optoelectronic devices, the contact may need to conduct current while remaining optically transparent.

Material	Typical role	Common device
ITO — In${}_2$O${}_3$:Sn	Transparent electrode	LEDs, displays, photodetectors, solar cells
AZO — ZnO:Al	Transparent electrode	Solar cells, LEDs, optoelectronics
IZO — In${}_2$O${}_3$:Zn	Transparent electrode	Displays/optoelectronics
FTO — SnO${}_2$:F	Transparent electrode	Solar cells, optoelectronics
ATO — SnO${}_2$:Sb	Transparent electrode	Transparent electronics
ZnO:Ga	Transparent conducting oxide	UV/optoelectronics
ZnO:Al	Transparent conducting oxide	Solar/LED applications
CdO	Transparent conductor	Specialized optoelectronics
Graphene	Transparent conductor	Research/optoelectronics
Metal nanowires (Ag/Cu)	Transparent electrode	Flexible/transparent electronics

These are not all metals in the strict sense. They are included because they can function as electrical contact/electrode materials.

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9. Refractory and Advanced CMOS Contact Materials

As dimensions shrink, conventional Al and Cu metallization become increasingly constrained by diffusion, electromigration, line resistance and contact dimensions. Advanced technologies therefore use refractory metals and metal compounds.

Material	Main function
W	Tungsten contact plugs and local interconnects
Co	Advanced contact/interconnect material
Ru	Advanced contact/interconnect material
Mo	Advanced gate/contact/interconnect research and fabrication
TiN	Barrier, liner, gate, contact
TaN	Cu diffusion barrier
WN	Diffusion barrier/contact
TiSi${}_2$	Traditional salicide
CoSi${}_2$	Traditional/legacy salicide
NiSi	Modern low-temperature salicide
NiPtSi	NiSi stabilization / advanced CMOS salicide
PtSi	Schottky/IR/contact technology
WSi${}_2$	Refractory silicide
MoSi${}_2$	Refractory silicide
TaSi${}_2$	Refractory silicide

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10. Nickel-Based Contact Materials

Nickel deserves a separate section because it is one of the most important contact metals across several semiconductor families.

Si

$$\mathrm{Ni}+\mathrm{Si}\to \mathrm{N}{\mathrm{i}}_2\mathrm{Si}\to \mathrm{NiSi}\to \mathrm{NiS}{\mathrm{i}}_2$$

The useful low-resistance phase for modern Si contacts is commonly NiSi.

4H-SiC

Ni is one of the most widely investigated and used metals for n-type SiC ohmic contacts. High-temperature annealing can produce Ni–Si phases and carbon-rich/modified interfaces. Reported Ni–Si phases include:

• Ni${}_2$Si
• NiSi
• NiSi${}_2$

Representative properties reported for Ni-based SiC contact phases include:

Phase	Resistivity ($\mu \Omega \cdot$cm)	Work function (eV)
Ni	8–10	$\approx 4.86\pm 0.2$
Ni${}_2$Si	20–30	$\approx 4.46$
NiSi	10–20	$\approx 4.35$
NiSi${}_2$	35–50	—

The final SiC contact behavior depends on more than silicide formation; carbon precipitation, carbon vacancies, interface defects, heavy doping and tunneling can all contribute.

GaN

Ni is widely used in Ni/Au p-GaN contacts, while Ni-containing layers are also used as diffusion/barrier layers in n-GaN stacks such as Ti/Al/Ni/Au.

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11. Contact Material by Semiconductor

Semiconductor	Important contact materials
Si	Al, Ti, Ni, Co, Pt, W, Cu; TiSi${}_2$, CoSi${}_2$, NiSi, PtSi, WSi${}_2$
Ge	Al, Ti, Ni, Pt, Pd, Au; NiGe, CoGe, PtGe, PdGe
SiGe	Ti, Ni, Co, Pt; NiSiGe/NiGe-related reacted contacts
GaAs	AuGe/Ni/Au, AuGe, AuZn, AuBe, Pd, Pt, Ti, Ni, Au
InP	AuGe, AuZn, Pd, Pt, Ti, Ni, Au
InAs	Au, Ti, Ni, Pd, Pt, In, Al
InSb	Au, Ti, Ni, Pt, In
GaN	Ti, Al, Ni, Au, Pt, Pd, Mo, V; Ti/Al/Ni/Au; Ni/Au
AlGaN/GaN	Ti/Al/Ni/Au, Ti/Al/Pt/Au, Ti/Al/Ti/Au, V/Al/Pt/Au
p-GaN	Ni/Au, Pd/Au, Pt/Au
SiC	Ni, Ti, Al, Co, Ta, W, Pt, Pd; Ni-silicide/carbide-containing interfaces
ZnO	Al, Au, Ag, Pt, Ni, ITO, AZO
ZnS/ZnSe	Au, Pt, Ni, In, Al, transparent conductive oxides
CdTe	Au, Pt, Cu, Ni, ZnTe-based contact structures
Diamond	Ti, Mo, W, Ta, Pt, Au, Ni and carbide-forming/interfacial systems
2D materials	Au, Pt, Pd, Ti, Ni, Cr, graphene, graphene/metal stacks
Perovskites	Au, Ag, Al, Cu, ITO, FTO, organic/transparent conductive contacts

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12. Contact Material Selection — What Each Layer Actually Does

A multilayer contact should not be interpreted as though every layer directly contacts the semiconductor.

Semiconductor-contact layer

This is the layer that directly reacts with or touches the semiconductor.

Examples:

• Ti in Ti/Al/Ni/Au on n-GaN
• Ni in Ni/SiC
• Al in Al/p-SiC
• Ni in Ni/Si during salicidation
• Pt in PtSi/Si

Reaction layer

Annealing may convert the original metal into another compound.

Examples:

$$\mathrm{Ni}+\mathrm{Si}\to \mathrm{NiSi}$$ $$\mathrm{Ti}+\mathrm{Si}\to \mathrm{TiS}{\mathrm{i}}_{\mathrm{x}}$$ $$\mathrm{Ti}+\mathrm{N}\to \mathrm{TiN}$$ $$\mathrm{Ti}+\mathrm{SiC}\to \mathrm{TiC}+\mathrm{TiS}{\mathrm{i}}_{\mathrm{x}}+\text{other interfacial phases}$$

Therefore, the effective contact material after annealing may be completely different from the deposited metal.

Adhesion layer

Ti and Cr are commonly used because they adhere strongly to semiconductor surfaces and insulating layers.

Diffusion-barrier layer

Ni, Pt, TiN, TaN, WN and related materials can prevent undesirable interdiffusion between the contact and top metal.

Top metal

Au, Al, Cu and similar metals can provide a thick, low-resistance layer suitable for probing, bonding or interconnect.

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13. Important Formulae

Ideal Schottky barrier for an n-type semiconductor

$${\Phi }_{Bn}={\Phi }_m-{\chi }_s$$

where:

• ${\Phi }_{Bn}$ = ideal electron Schottky barrier height
• ${\Phi }_m$ = metal work function
• ${\chi }_s$ = semiconductor electron affinity

Ideal Schottky barrier for a p-type semiconductor

$${\Phi }_{Bp}=E_g-{\Phi }_{Bn}$$

or, under the ideal Schottky-Mott picture,

$${\Phi }_{Bp}={\chi }_s+E_g-{\Phi }_m$$

Work function of a semiconductor

For a semiconductor,

$${\varphi }_s=\chi +(E_C-E_F)$$

which is the same convention used in the semiconductor material reference table.

Contact resistivity

The specific contact resistance is commonly written as

$${\rho }_c={\left(\frac{\partial J}{\partial V}\right)}_{V=0}^{-1}$$

with units of $\Omega \cdot \mathrm{c}{\mathrm{m}}^2$.

For a thermionic-emission dominated contact,

$$J\propto A^{\ast }{T}^2\exp \left(-\frac{q{\Phi }_B}{k_{B}T}\right)$$

while heavily doped semiconductor contacts can enter thermionic-field-emission or field-emission regimes, where tunneling through a thin barrier becomes important.

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14. Important Questions

Ques: Why does a metal with a high work function not necessarily make a bad ohmic contact?

A high metal work function can produce a large ideal Schottky barrier on n-type material, but real contacts are controlled by interface states, Fermi-level pinning, reactions, defects, heavy doping and tunneling. Annealing can also create a new interfacial compound with a very different effective barrier. Therefore, a contact should not be classified using work function alone.

Ques: Why are Ti/Al contacts widely used on n-GaN?

Ti and Al have relatively low work functions compared with noble metals, and Ti/Al contacts undergo strong reactions during annealing. In common Ti/Al/Ni/Au structures, annealing can produce TiN and an N-deficient/interfacially modified region, while the heavily modified interface allows carrier tunneling. The Ni/Au layers additionally provide diffusion/oxidation protection and a practical top metal.

Ques: Why is Ni used so often on SiC?

Ni has a useful combination of thermal stability and strong interaction with SiC. On annealing it forms conductive Ni–Si phases and a modified SiC interface. Ni-based contacts are particularly important for n-type 4H-SiC.

Ques: What is salicidation?

Salicidation means self-aligned silicide formation. A metal is deposited over exposed Si or polysilicon and annealed so that it reacts with the exposed silicon. Unreacted metal can then be removed, leaving silicide only where silicon was exposed. Silicides such as TiSi${}_2$, CoSi${}_2$ and NiSi have been important in CMOS.

Ques: Why is NiSi preferred over NiSi${}_2$ in many modern Si contacts?

NiSi has a lower resistivity and can form at relatively low temperatures compared with many older silicides. NiSi is therefore well suited to scaled CMOS, although phase stability, agglomeration and thermal processing windows remain important.

Ques: Why are Au and Pt often used even though their work functions are high?

They are chemically stable, have useful electrical properties, and can serve as noble top electrodes or Schottky metals. In a multilayer contact, Au or Pt may not be the layer directly touching the semiconductor. For example, Ti/Al/Ni/Au on n-GaN uses Ti/Al at the semiconductor side and Au primarily as a top conductive/protective layer.

Ques: Why is Ti used in so many contact stacks?

Ti provides strong adhesion, is compatible with many deposition processes, has a relatively low work function, and readily reacts with Si, N and C. Those reactions can be useful for forming low-barrier or heavily modified interfaces.

Ques: What is the difference between a contact metal and a contact stack?

A contact metal is a single material used as an electrode. A contact stack contains several layers, each with a different purpose: semiconductor reaction, adhesion, barrier formation, diffusion control, oxidation protection and low-resistance top metallization.

Ques: Is the metal deposited in a contact always the material that determines the final Schottky barrier?

No. The actual interface after processing may contain silicides, nitrides, carbides, oxides, intermetallics, vacancies and reacted semiconductor material. The final interface is what determines the practical electrical barrier.

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15. Key References

1. S. M. Sze and K. K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley, 2007.
2. D. A. Neamen, Semiconductor Physics and Devices, 4th ed., McGraw-Hill, 2012.
3. O. Ambacher et al., “Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures,” Journal of Applied Physics 85, 3222 (1999).
4. C. Y. Hu et al., “Investigation on the different barrier effect of Ni and Pt in the Ti/Al/Pt/Au and Ti/Al/Ni/Au contacts to n-type GaN,” Journal of Crystal Growth 298, 804–807 (2007).
5. Z. Z. Chen et al., “Ohmic contact formation of Ti/Al/Ni/Au to n-GaN by two-step annealing method,” Materials Science and Engineering B 111, 36–39 (2004).
6. V. Kuchuk et al., “Ni-Based Ohmic Contacts to n-Type 4H-SiC: The Formation Mechanism and Thermal Stability,” Advances in Condensed Matter Physics (2016).
7. Stanford EE311, Silicides — semiconductor silicide/contact technology reference.
8. Standard semiconductor process-integration literature on TiSi${}_2$, CoSi${}_2$, NiSi, PtSi, Pd${}_2$Si, WSi${}_2$, MoSi${}_2$ and TaSi${}_2$.

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Reference values used in this file

The representative work-function values and common metal choices are consistent with standard semiconductor-contact references and tabulations. Representative silicide resistivity and barrier-height values are based on commonly reported thin-film contact data.

For example, representative published values include:

• Ni: $\sim 5.15$ eV work function
• Al: $\sim 4.28$ eV
• Ti: $\sim 4.33$ eV
• Au: $\sim 5.10$ eV
• Pd: $\sim 5.12$ eV
• Pt: $\sim 5.65$ eV
• Cu: $\sim 4.65$ eV
• Ag: $\sim 4.26$ eV
• W: $\sim 4.55$ eV

Representative silicide resistivities include:

• TiSi${}_2$: $\sim 13$–16 $\mu \Omega \cdot$cm
• CoSi${}_2$: $\sim 14$–20 $\mu \Omega \cdot$cm
• NiSi: $\sim 14$–20 $\mu \Omega \cdot$cm
• NiSi${}_2$: $\sim 40$–50 $\mu \Omega \cdot$cm
• PtSi: $\sim 28$–35 $\mu \Omega \cdot$cm
• Pd${}_2$Si: $\sim 25$–35 $\mu \Omega \cdot$cm
• WSi${}_2$: $\sim 30$–70 $\mu \Omega \cdot$cm
• MoSi${}_2$: $\sim 40$–100 $\mu \Omega \cdot$cm
• TaSi${}_2$: $\sim 35$–55 $\mu \Omega \cdot$cm

These values should be treated as representative engineering values, not universal constants. Contact properties are strongly dependent on surface preparation, crystal orientation, doping, annealing, phase, stoichiometry, thickness and interface chemistry.