Litton C.W., . Reynolds D.C., Collins T.C. Zinc Oxide Materials for Electronic and Optoelectronic Device Applications

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4.5 The Inﬂuence of ZnO Polarity on Schottky Barriers

Several studies indicate that, as with air-exposed ZnO contacts, metals on Zn-polar

surfaces yield signiﬁcantly higher F

than on O-polar surfaces. This is evident from

Table 4.1, and Figures 4.4 and 4.5 for several metals. It can be attributed to higher sub-

surface defect concentrations for O-polar crystals.

[9]

DRCLS, C

2

–V, and deep level

transient spectroscopy (DLTS) studies show that Zn-face diodes have higher barrier

heights, lower sub-surface deep level defects, and lower sub-surface free carrier con-

centrations than O-face diodes of the same ZnO crystal. Figure 4.11(a) illustrates the ratio

DEF

NBE

of 2.5 eV deep level defect to near-band-edge emission for Zn and O faces of the

same ZnO crystal, both before and after chemical and ROP cleaning and as a function of

depth below the ZnO surface. Defect emissions are always higher for O-face surfaces.

[9]

Figure 4.11(b) shows the effect of thes e differences on Au and Pd Schottky barriers on the

Figure 4.11 (a) Ratio I

DEF

NBE

of 2.5 eV deep level defect to near-band-edge emission for Zn

and O faces of the same ZnO crystal, both before and after chemical and ROP cleaning. Zn-

terminated surfaces have lower deep level defect densities.

[9]

(b) C

2

V plots for Au and Pd

diodes on Zn-polar and O-polar surfaces of ROP-cleaned ZnO.

[9]

Inset shows higher carrier

densities for the O-polar surfaces. (c) DLTS trap emissions vs depth for 32 nm Au on ZnO(000



showing an increase in trap E8 with decreasing depth. Reprinted from Z.-Q. Fang, et al., Surface

traps in vapor-phase-grown bulk ZnO studied by deep level transient spectroscopy, J. Appl.

Phys.104, 063707. Copyright (2008) with permission from American Institute of Physics

102 ZnO Surface Properties and Schottky Contacts

same ROP-cleaned surface s. Zn-polar surfaces yield higher Schottky barriers than O-polar

surfaces for both Au and Pd UHV-deposited contacts formed immediately after ROP

cleaning.

The inset in Fig. 4.11(b) illustrates the change in carrier concentration vs depth obtained

from C

2

–V measurements for all four diodes. The O-face ZnO exhibits substantially

higher carrier densities vs the Zn-terminated surface, apparently from additional acceptors

near the Zn-polar surface.

[48]

Furthermore, strong increases in carrier density are apparent

for both surfaces in the 70–100 nm depth range. ( C–V measurements at shallower depths

may be subject to forward bias effects.)

Associated with these defect and doping effects, DLTS reveals a strong increase in trap

states approaching the surface. Figure 4.11(c) illustrates DLTS spectra taken from one of

the Au- ZnO(000



I) diodes in (b), showing an increase in trap feature E8 located 0.9 eV

below the conduction band.

[48]

This feature increases by over an order of magnitude

relative to the bulk E3 trap signal as depth decreases from 180 to 60 nm. Thus both

doping and trap densities are changing dramatically with distance to the free surface. It is

also signiﬁcant that the trap depth of 0.9 eV corresponds closely to an energy level 2.5 eV

above the valence band, consistent with the 2.5 eV defect emissions in Figures 4.3 and 4.7.

Overall, Figure 4.11 emphasizes the strong coupling between polarity, native point defects

and doping at or near the ZnO surface.

4.6 The Inﬂuence of Chemistry

Chemistry also plays a role at metal–ZnO interfaces. There is a qualitative difference

between reactive vs nonreactive metal–ZnO interfaces. Metals such as Al, Ta, Ti, and Ir

that react with ZnO to form oxide s produc e defects associated with oxygen vacancies,

whereas metals such as Au, Pd, and Pt that form eutectics with Zn produce defects

associated with zinc vacancies. Depending on the density of defects and the extent of

reaction in the sub-surface region, reactive metals can form either ohmic or blocking

contacts.

[14,15]

Finally, native point defects created by chemical reaction and diffusion at the

metal–ZnO interface produce large changes in barrier height. Figure 4.12 illustrates the

creation of new native point defects at the Au– and Al–ZnO interfaces.

[14]

The ability to

excite CL emission from interfaces is particularly useful for studying localized electronic

states at metal –semiconductor interfaces. In turn, these states can be related to the

Schottky barriers that form with charge transfer across the interfaces. Figure 4.12

illustrates CL spectra for ZnO(000



I) surfaces with and without a metal overlayer. The

Figure 4.12(a) insert illustrates a metal overlayer on clean, ordered ZnO along with the

incident electron beams and exciting photons. For the 30 nm thick metal overlayers shown,

the probe depth of 2 keV into the bare ZnO is equivalent to the depth excited by 5 keV

through the metal.

In the case of Au on ZnO(000



I) both the metallized and bare surface ZnO show almost

identical spectra with defect emission almost three orders of magnitude below the NBE

emission. This Au/ZnO diode exhibits rectifying I–V characteristics. In the case of Al on

ZnO(000



I), Figure 4.12(b), the metallized surface reveals an increase in the deep level

The Inﬂuence of Chemistry 103

3.53.02.52.01.5

100

1000

10000

100000

CL Intensity

Energy (eV)

3.53.02.52.01.5

100

1000

10000

100000

Energy (eV)

100

1000

10000

100000

650ºC

350ºC

550ºC

450ºC

-related

Luminescence Intensity

Energy (eV)

-related

ZnOBand Gap

3.36 eV

No anneal

3.5

3.0

2.5

2.0

1.5

ZnO

(c)(b)(a)

Figure 4.12 CL spectra of bare ZnO(000



1) vs with and without a Au (a), Al (b) or annealed Au (c) diode overlayer on the semiconductor.

Reprinted from L.J. Brillson, et al., Dominant effect of near-interface native point defects on ZnO Schottky barriers, Appl. Phys. Lett. 90, 102116.

104 ZnO Surface Properties and Schottky Contacts

emission centered at 2.5 eV by nearly an order of magnitude. This increase signiﬁes the

creation of new defect states within a depth of only a few nanometers from the metal

interface. Signiﬁcantly, these new states occur at the same energy of native point defects

that are intrinsic to the ZnO crystal. This Al/ZnO diode displays ohmic I–V characteristics.

In the case of the same Au/ZnO(000



I) diode at different annealing temperatures,

Figure 4.12(c), little or no change takes place in the spectra for temperatures up to 550



With a 650



C anneal, however, a new emission appears centered at 2.0 eV. This peak

corresponds to emission frequently seen in bulk ZnO and has been attributed to Zn

vacancies. Recent experiments on implanted ZnO in fact show a strong correlation as a

function of depth between the density of Zn vacancies or vacancy clusters obtained from

positron annihilation spectroscopy (PAS) with the intensity of the 2 eV ZnO deep level

emission measured by DRCLS.

[49]

Likewise, the 2.5 eV feature is commonly associated

with oxygen vacancies.

[14]

Consistent with the effect of interface metal–ZnO chemis try on Schottky barriers, the

defect emissions and the Schottky barrier characteristics show corresponding changes as a

function of annealing temperature. The CL features shown in Figure 4.12 match the

changes in I–V characteristics as a function of temperature. Figure 4.13 illustrates I–V

characteristics for Al, Au, and Ta vs 1-h anneal at the temperatures indicated. For Al,

Figure 4.13(a) shows that the diode retains its ohmic character with increasing anneal

temperature. This is expected since Al continues to react at higher temperatures with O in

ZnO to create more O vacancies. Figure 4.13(b) shows the dramatic change from

rectifying to ohmic as the annealing temperature nears the Zn–Au eutectic temperature

and Zn vacancy-related defects are created.

[15]

Figure 4.13(c) displays another chemically

driven change in diode properties . Here Ta reacts strongly with ZnO to produce Ta oxide

and forms a blocking contact in addition to new O vacancy-related defects. The result is

that the reverse current characteristic decreases by nine orders of magnitude.

[15]

Interest-

ingly, even the zero bias point of the I–V curve is displaced due to a built-in dipole at the

Ta–ZnO interface. For ZnO crystals with higher initial defect densities, this blocking

feature also appears at intermediate temperatures. However, reverse current leakage

increases with higher temperature annealing as the additional defects provide hopping

conduction pathways through the barrier layer.

Figures 4.12 and 4.13 lead to several conclusions: (1) metals on semiconductors can

induce the formation of localized interface states; (2) these new states appear at energies

corresponding to point defects native to the bulk semiconductor; and (3) the nature of the

chemical interaction between metal and ZnO determ ines what states form and at what

temperature. The appearance of new states in Figure 4.12(c) occurs at a temperature just

above the Au–Zn eutectic temperature (625



C), consistent with the extraction of Zn from

the ZnO. Conversely, the formation of new states at Al–ZnO interfaces is consistent with

the formation of a metal oxide that extracts oxygen from the ZnO. Indeed, the behavior

exhibited by these Au and Al diodes on ZnO are characteristic of many other metals. Thus

relatively unreactive metals such as Au, Pd, and Pt that form eutectics with Zn display

rectifying I–V characteristics and increases in 2.1 eV defect emission, particularly at

elevated temperatures. Conversely, more reactive metals such as Al, Ta, and Ir react

with the crystal’s oxygen to form oxides and display ohmic behavior. This can explain the

vs DH

plots for metals on ZnO that show characteristic “S-shape” plateaus in barrier

height for reactive vs unreactive metals.

[4]

The Inﬂuence of Chemistry 105

210-1-2

1E-5

1E-4

1E-3

0.01

Amperes

Vo l t a g e

(a) (b) (c)

1 Hr RTO

Plasma

350

º C Anneal

450º C Anneal

550º C Anneal

3210-1-2

-3

20-2

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

I (Amperes)

1 Hr RT O

Plasma

350 Anneal O

Plasma

450 Anneal O

Plasma

550 Anneal O

Plasma

3210-1-2-3

-2 0 2

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1 Hr RTO

Plasma

350

C Anneal

450

C Anneal

550

C Anneal

1 Hr RTO

Plasma

350

C Anneal

450

C Anneal

550

C Anneal

Voltage

Figure 4.13 I–V characteristics for (a) Al, (b) Au, and (c) Ta diodes on clean ZnO(000



106 ZnO Surface Properties and Schottky Contacts

Recently, Allen and Durbin recognized the role of metal–ZnO chemical reaction in

forming oxygen vacancies and gap states that “pin” the Fermi level, pointing to the

increase in F

with free energy of formation of its metal oxide

[50]

[see Figure 4.14(a)].

Robertson et al. ﬁnd a similar trend for an even wider set of metal oxides

[51]

(see

Figure 4.14(b)]. Allen and Durbin noted that their Ag oxide contacts were unlikely to form

such oxides with ZnO and thus enabled less E

pinning. The role of native point defects in

stabilization was ﬁrst noted in studies of metal–InP interfaces, where outdiffusion of

anions or cations promoted by the interface chemistry leads to two distinct Schottky

barrier levels, depending on the anion/cation nonstoichiometry.

[52,53]

Metals that react

Figure 4.14 (a) Free energy of metal oxide formation per O atom vs metal work function for

common metal contacts to ZnO. Reprinted from M.W. Allen and S.M. Durbin, Inﬂuence of

oxygen vacancies on Schottky contacts to ZnO, Appl. Phys. Lett. 92, 122110. Copyright (2008)

with permission from American Institute of Physics (b) Experimental values of MO

per O atom

vs work function for a wider range of metals. Reprinted from J. Robertson, O. Sharia, and A. A.

Demkov, Fermi level pinning by defects in HfO2-metal gate stacks, Appl. Phys. Lett. 91,

132912. Copyright (2007) with permission from American Institute of Physics (c) Transition in

Schottky barrier with interface chemical reactivity for ZnO and other compound semiconduc-

tors. Reprinted from L.J. Brillson, Transition in Schottky Barrier Formation with Chemical

Reactivity, Phys. Rev. Lett. 40, 260. Copyright (1978) with permission from American Physical

Society

The Inﬂuence of Chemistry 107

strongly with semiconductor anions form low or zero barriers while relatively unreactive

metals form high barriers. Figure 4.14(c) illustrates the transi tion in F

for ZnO with

interface heat of reaction,

¼ð1=xÞ½H

ðCAÞH

ðM

AÞ ð4:6Þ

for the reaction M þ(1/x)CA )(1/x)[M

A þC] for metal M, anion A, and cation C,

where M

A is the most stable metal–anion reaction product.

[54]

In this case, C ¼Zn and

A ¼O. Metals that form oxides form low barriers while relatively unreactive metals that

only form eutectics with Zn form barriers in the 0.7 eV range. This transition in Schottky

barrier between two limiting values is common to most if not all compound semicon-

ductors, regardless of ionicity. Together with the observed formation of native point

defects at the metal–semiconductor interface with electronic states in the band gap,

Figure 4.14(c) demonstrates that the same chemically induced defect principle applies to

ZnO as well. Overall, these interface-speciﬁc states indicate that native point defects can

play an important role in Schottky barrier formation.

Along with the increase in defect emission measured with DRCLS at the intimate

metal–ZnO contact, the corresponding I–V characteristics show increased leakage or

current blockage, depending on whether or not the metal forms an interfacial oxide. These

ZnO results demonstrate that native point defects resident in the bulk crystal as well as

similar defects produced by chemical interaction at the ZnO interface have a strong effect

on Schottky barrier formation. This ﬁnding has broad signiﬁcance for Schottky barrier

studies given Figure 4.7 and the orders-of-magnitude difference in defect densities and

density variations with depth between ZnO crystals grown by different growth methods –

hydrothermal, melt, vapor phase transport, and molecular beam epitaxy, the same growth

method but different sources, and even crystals grown by the same method from the same

source! Low defect ROP-cleaned ZnO displays higher Schottky barriers and lower

idealities than the corresponding metal diode on ROP-cleaned high defect material.

[14]

Similarly, low defect ZnO shows much lower increases in defect creation and depth

redistribution than high defect ZnO. Thus considerable evidence exists to suggest that

Schottky barrier features depend heavily on native point defect densities resident in the

crystal prior to diode formation. Therefore, a knowledge of these defect concentrations and

their depth distribution is a prerequisite for Schottky barrier studies.

4.7 Charge Transport and Extended Metal–ZnO Schottky Barriers

The effects described thus far can be represented schematically in terms of a competition

of several interface charge transport mechanisms as well as in terms of chemically active

interface material structures. Figure 4.15(a) represents these charge transport mechanisms

schematically. Here thermionic emission of carriers over the Schottky barrier is augmented

by tunneling through the barrier and hopping transport through states within the band gap.

Impurities or defects that increase carrier density inside the surface space charge region

will decrease the barrier width, thereby increasing the contribution of tunneling to the

overall current. Similarly, increasing defect concentrations near the semiconductor surface

enables charge movement through the semiconductor depletion region, especially at defect

108 ZnO Surface Properties and Schottky Contacts

densities at which the gap state wave functions begin to overlap. Indeed, if enough defects

are introduced, e.g., by near-surface segregation or surface mechanical damage, the

otherwise Schottky barriers become ohmic.

Beyond the multiple channel charge transport pictured in Figure 4.15(a), Figure 4.15(b)

illustrates how chem ical interactions at the microscopic metal–ZnO interface alter the

classical Schottky barrier picture.

[55,56]

Metal–ZnO reactions can introduce new dielectric

layers that block transport or change the effective work function at the intimate junction.

The chemical interactions evident in Figure 4.12 demonstrate that such interactions are

common in ZnO and other oxides. Figure 4.15(b) also shows changes in the band bending

region of the sem iconductor due to semiconductor anion and/or cation outdiffusion as well

as metal or other impurity indiffusion that change the effective carrier concentration at

different depths. Such chemical changes thereby contribute to the tunneling and/or

hopping transport pictured in Figure 4.15(a ).

These charge transport mechanisms are in fact more general than ZnO alone,

extending to compound semiconductors overall. They highlight the importance of initial

semiconductor crystal quality, surface preparation, contact format ion, subsequent thermal

or other processing, and the resultant interface chemical reaction and/or and diffusion

leading to the formation of electronically active states within the semiconductor

band gap.

Based on these factors, controllable ZnO Schottky barriers require at minimum:

(1) high quality single crystals whose defect conce ntrations are several orders of

magnitude below unintentionally doped crystal concentrations; (2) surface preparation

that removes chemical contamination such as hydroxides and hydrocarbons without

creating surface asperities beyond monolayer roughness levels; (3) contact formation

by metal deposition that does not introduce addi tional near-interface contamination,

reaction, or other semiconductor disruption; and (4) any subsequent thermal processing

kept below temperatures at which that particular metal–semiconductor reaction can occur.

Figure 4.15 (a) Competing charge transport mechanisms in ZnO including thermionic

emission, tunneling, and hopping transport through defect levels in the band gap. (b)

Schematic band diagram of extended metal–semiconductor interface including reacted and

interdiffused regions with associated changes in band bending and interface dielectric phases

Charge Transport and Extended Metal–ZnO Schottky Barriers 109

For high barriers, Table 4.1 shows that high work function metals such as Au, Pd, and Pt

are good candidates although ambient gas diffusion through Au and Pd introduces

interface dipoles that can alter the barriers. Ag provides relatively high barriers once

oxidized and if the ﬁnal reaction product is thermally stabilized. Ta reacts to form Ta oxide

interlayers that block transport effectively. However, excessive annealing coupled with

high background defect densities can degrade the blocking layer. In this regard, Ir is found

to yield relatively high barriers due to its high work function, weak reactivity with ZnO,

and stability in air. A new strategy to avoid interface chemical interaction is to use high

work function and chemically inert conducting polymers to form high barriers.

[36]

For

ohmic contacts, low work function metals such as Al, Ti and In are good candidates in low

defect ZnO as long as they do not react to form blocking oxides. Reactive metals such as

Al and Ti will also form interface defects that promote hopping transport.

4.8 Conclusion

Schottky barriers to ZnO show a wide range of phenomena and electronic behavior. These

can all be directly tied to the chemical and structural changes that researchers are now

beginning to probe systematically on an atomic scale. Work on ZnO contact has only

recently been carried out on high quality material, nearly free from complicating factors

such as impurities, morphological and native point defects. These advances will help

accelerate our understanding and control of ZnO electrical contacts.

Acknowledgements

The author gratefully acknowledges support for this work by National Science Foundation

Grant No. DMR-0513968, DMR-0803276 (Verne Hess). He acknowledges the technical

contributions of Drs H. L. Mosbacker, Y. Strzhemechny, Y. Dong and M. Hetzer as well as

PhD student D. Doutt.

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