Ramanathan Sh. (Ed.) Thin Film Metal-Oxides: Fundamentals and Applications in Electronics and Energy

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170 Y. Hikita and H.Y. Hwang

Fig. 5.1 The perovskite crystal structure. The B-site ion in the body center is coordinated

octahedrally by oxygen ions with the A-sites occupying the cube corners

or rare-earth ion as depicted in Fig. 5.1. By varying the B-site elements, drastic

differences in the physical properties can be obtained. Examples include ferro-

electricity, high temperature superconductivity (in perovskite-derived layered com-

pounds), colossal magnetoresistance, metal–insulator transitions, and various mag-

netic ground states. From a structural point of view, most of these materials fall

within a small range of lattice constants due to the small variation in the ionic radii

of the transition metal ions [4]. This is a signiﬁcant advantage when growing epitax-

ial heterostructures compared with compound semiconductors in which the lattice

constants can easily vary by tens of percents. Furthermore, the strongly correlated

nature of the electrons and their strong coupling with the lattice and spins in these

systems make them sensitive to external perturbations, hence phase transitions can

be triggered readily by chemical substitution, light illumination, temperature, and

magnetic or electric ﬁelds. This high sensitivity gives these materials new degrees

of freedom to engineer sensors and devices based on these concepts.

The fabrication techniques developed for perovskite oxides in the past few

decades have enabled growth of oxide interfaces with atomic scale control. Among

many thin ﬁlm fabrication techniques, pulsed laser deposition (PLD) has been

widely used to grow artiﬁcial oxide structures, including much of the work de-

scribed here (Fig. 5.2). PLD utilizes a focused excimer laser as the energy source

to ablate a ceramic target onto a heated substrate. The high energy density of the

laser enables evaporation of materials with high melting points while generally

keeping stoichiometry [5]. The growth process can be monitored in-situ by using

reﬂection high-energy electron diffraction (RHEED), which is often differentially

pumped to operate in high oxygen partial pressure. In addition to the basic thin ﬁlm

growth processes, substrate preparation techniques to prepare well-deﬁned substrate

5 Complex Oxide Schottky Junctions 171

Fig. 5.2 Schematic diagram of a pulsed laser deposition system equipped with RHEED for in-situ

observation of the deposition process

surfaces have played a major role in advancing the studies of interfaces in these ma-

terials, such as the wet-etching method to selectively terminate substrate surfaces

[6,7] or optimizing annealing conditions for step-terrace structures [8].

In parallel with the advances in the growth techniques, recent progress in the

physical understanding of perovskite interfaces have raised the ﬁeld into a new

era. Two such examples can be found in structures with metallic interfaces in-

duced between insulating perovskites. First is the microscopic characterization of

the LaTiO

=SrTiO

superlattice which demonstrated that the charge penetration in

a delta-doping heterostructure is conﬁned to a signiﬁcantly smaller region compared

with those of conventional semiconductors [9]. The second is the discovery of inter-

face dependent electrical conductivity at the LaAlO

=SrTiO

.100/ interface [10].

Both of these results suggest a novel concept of charge reconstruction at interfaces

owing to the variable valence degree of freedom in the perovskites. Triggered by

these ﬁndings, a variety of experimental and theoretical studies have been made to

explore charge, spin, and orbital reconstructions at oxide interfaces [11–24]. These

studies have expanded from the original interface between oxide insulators to inter-

faces involving ferroelectric, magnetic or superconducting materials.

Meanwhile, these new concepts developed from the capability to tailor artiﬁcial

structures with atomic scale control have given an ideal platform to investigate het-

erostructures which can be directly related to device applications using perovskites.

Metal–semiconductor interfaces are among the most fundamental interfaces in

electronic devices and require understanding of various important physical concepts

such as band bending in semiconductors, band alignments, Schottky barrier heights

172 Y. Hikita and H.Y. Hwang

(SBHs), and the formation of interface states. It is not clear, however, whether the

existing concepts of Schottky junctions in conventional semiconductors can be ap-

plied directly to perovskites. For example, the multiple valence accessibility and the

strong interaction between electrons, spins, and the lattice are not accounted for in

the existing Schottky junction concepts. Therefore, expanding the existing knowl-

edge of interface semiconductor physics to incorporate the complexity found in the

perovskites and exploring phenomena unique to these oxides are vital steps toward

establishing a platform for designing electronic devices.

In this context, one of the initial motivations to study complex oxide Schottky

junctions in detail was as part of structures such as superconducting FETs or su-

perconducting base transistors. After the discovery of superconducting cuprates

and their sensitivity to the hole concentration, efforts to modulate their properties

by ﬁeld effect were carried out in metal–insulator–superconductor (MIS) structures

(Fig. 5.3). Here, the superconductor and the insulator were typically YBa

6 C•

and SrTiO

, respectively [25–27]. The leakage properties of the SrTiO

gate-

insulator were investigated by capacitance–voltage .C V/ and current–voltage

.I V/ techniques, which revealed the formation of dead layers and reduced

relative permittivity at interfaces. However, the main focus was on the ﬁeld effect

on the cuprates and less attention was paid to the current processes across the MIS

structure.

Another approach in using high temperature superconductivity in electronic

devices was in high speed transistors using the superconductor as the base

in a metal–insulator–superconductor–semiconductor structure originally pro-

posed more than half a century ago [28]. The main experimental focus was

on In=Ba

1x

BiO

=NbWSrTiO

[29]orIn=Ba

1x

BiO

=NbWSrTiO

[30],

in which a native oxide layer was intrinsically formed at the In=Ba

1x

BiO

.Ba

1x

BiO

/ interface. The Schottky junctions of the Ba

1x

BiO

=NbW

SrTiO

interface were investigated extensively [31, 32] and here detailed analyses

of the dielectric properties of NbWSrTiO

under strong electric ﬁelds were studied.

Fig. 5.3 (a) Schematic diagram of an MIS structure for ﬁeld effect measurements. (b) Temperature

dependence of the drain-source resistance .R

/ for three applied gate voltages: C10 V(solid

squares), 0 V (open squares), and 10 V(solid circles). The behavior of R

in the vicinity of T

is shown in the insets for the same sample (lower right) and a different sample (upper left)(From

[26], by permission)

5 Complex Oxide Schottky Junctions 173

One of the important achievements of this time was the derivation of a functional

formula for treating the electric ﬁeld dependent relative permittivity which will be

given in detail later in this chapter. Similarly, an alternative structure for current

injection type transistors was studied in the so-called dielectric-base transistors in

YBa

6C•

=NdGaO

=NbWSrTiO

heterostructures [33], which also led to the

study of YBa

6C•

=NbWSrTiO

interface electrical characteristics [34].

Attempts to incorporate ferroelectrics at interfaces have been studied using

.Pb; Zr/TiO

and PbTiO

. Ideally, ferroelectrics are insulators, but the intrinsically

forming defects readily make these materials semiconducting. Bistable conduction

characteristics have been achieved at Au=PbTiO

Schottky junctions [35], and band

alignment control between .Pb; Zr/TiO

and NbWSrTiO

[36] was applied in design-

ing high efﬁciency photodiodes [37].

The remainder of this chapter is organized as follows. In Sect. 5.2,abrief

review of the basic characteristics of Schottky junctions is given. In Sect. 5.3,re-

cent advances in the understanding of perovskite Schottky junction formation are

presented. New applications and novel functionalities of complex oxide Schottky

junctions will be discussed in Sect. 5.4, and a summary and outlook presented in

Sect. 5.5.

5.2 Schottky Junctions

Detailed reviews regarding semiconductor Schottky junctions can be found in

[38–41].

5.2.1 Basic Concepts

When a metal and an n-type semiconductor are brought into contact, the difference

in the metal work function and the electron afﬁnity of the semiconductor drives elec-

tron diffusion from the semiconductor to the metal surface region so as to sustain

a ﬂat Fermi level throughout the structure. This requires the presence of a negative

charge on the surface of the metal balanced by a positive charge in the semicon-

ductor. The charge on the surface of the metal consists simply of extra conduction

electrons contained within a screening length, and the positive charge is provided by

the uncompensated donor ions in a region depleted of electrons. The much smaller

donor concentration in the semiconductor with respect to that of the metal results in

a depletion width much larger in the semiconductor side, associated with the band

bending shown in Fig. 5.4. The SBH is given as

D 

 ; (5.1)

where 

is the metal work function and  is the electron afﬁnity of the semi-

conductor. The quantitative expressions for band bending and depletion width are

174 Y. Hikita and H.Y. Hwang

Fig. 5.4 Schematic band

diagram of an n-type

semiconductor Schottky

junction. E

VA C

is the vacuum

level, 

is the work function

of the metal,  is the electron

afﬁnity of the semiconductor,

W is the depletion width, ˚

the Schottky barrier height,

the Fermi level, E

is the semiconductor

conduction (valence) band

minimum (maximum), and

is the built-in potential

n-type semiconductorMetal

VAC

Donor charge density r

Electric field E(x)

Electrostatic potential f(x)

n-type

semiconductor

Metal

−

Fig. 5.5 (a) Donor charge density ,(b) electric ﬁeld E.x/,and(c) electrostatic potential .x/in

the semiconductor as a function of the distance x from the interface in the depletion approximation

given by solving Poisson’s equation under the depletion approximation where the

uncompensated donor charge density diminishes abruptly at the depletion edge.

The depletion width W , the electric ﬁeld E.x/, and the electrostatic potential

.x/ in the depletion region are given as (Fig. 5.5):

5 Complex Oxide Schottky Junctions 175

W D

(5.2)

E.x/ D

.x  W/ (5.3)

.x/ D



Wx 



 V

: (5.4)

Here, "

and N

are the relative permittivity and the dopant concentration of the

semiconductor, V

is the built-in potential deﬁned as the energy difference between

the conduction band minimum of the semiconductor at the interface and at the

depletion width, q is the elementary charge, and "

the vacuum permittivity. The

formation of the Schottky barrier described above is known as the Schottky–Mott

limit, which assumes (1) no surface dipole contribution to 

, (2) no localized

states on the surface of the semiconductor, and (3) that there is perfect contact

between the semiconductor and the metal. Deviations from the Schottky–Mott limit

are frequently observed in experiments [42], which have been studied intensively

by incorporating interface effects such as semiconductor surface states [43], metal-

induced gap states [44], defect related states [45], or disorder induced gap states

[46]. Recently, a more comprehensive theory of SBH formation known as bond

polarization theory has been proposed [47], which effectively take the previously

mentioned factors into account by considering the effect of charge redistribution

at the interface by deﬁning an interface speciﬁc region (ISR) in the proximity

of the interface. Therefore the interface structure can be depicted as metal–ISR–

semiconductor, and the ISR contains all the effects arising from the interfaces,

namely additional dipoles. It is now generally clear that the SBH is determined not

only by the bulk properties but also is strongly affected by the interface local charge

distribution.

5.2.2 Current Transport Processes

The most important feature of a Schottky junction determining its device perfor-

mance is the current transport characteristics. In addition to acting as a rectiﬁer in

circuits, it displays various transport characteristics as a function of temperature and

doping concentration in the semiconductor (Fig. 5.6).

The classical current transport mechanism across the interface is the thermionic

emission process which is expressed as

J D J



exp





 1



; (5.5)

where the saturation current density for thermionic emission J

is given by

D A



exp





q˚



: (5.6)

176 Y. Hikita and H.Y. Hwang

Fig. 5.6 Transport processes

in a forward-biased Schottky

junction. (a) Thermionic

emission over the barrier,

(b) thermionic-ﬁeld emission,

and (c) ﬁeld emission

n-type semiconductorMetal

Here, V is the applied voltage, k is the Boltzmann constant, T is the temperature,



D4 m



q=h

is the Richardson constant, m



is the effective mass of the elec-

tron in the semiconductor, and h is Plank’s constant. In real junctions, the transport

process is usually more complicated, and to take account of the deviation from ideal

thermionic behavior, a phenomenological parameter called the ideality factor n is in-

troduced in (5.5). The general expression for classical interface transport is therefore

J D J



exp



nkT



 1



: (5.7)

Either when the semiconductor doping concentration is high (hence the depletion

region is small), or the thermal energy is insufﬁcient to overcome the barrier, ﬁeld

emission (electron tunneling) is dominant. A formulation of the ﬁeld emission pro-

cess derived based on the WKB approximation results in the simpliﬁed form

J D J



exp





 1



; (5.8)

where

4



: (5.9)

The saturation current density for the ﬁeld emission process J

is given as [48]:

2A



exp .q˚



q˚

qV



oi

sin

kT



q˚

qV



oi

: (5.10)

Here,  is the energy difference between the conduction band minimum and the

Fermi level in the bulk semiconductor. The physical signiﬁcance of E

is that it

is the equivalent of the built-in potential of the Schottky barrier such that the trans-

mission probability for an electron whose energy coincides with the bottom of the

5 Complex Oxide Schottky Junctions 177

conduction band at W is equal to e

1

. Therefore the ratio kT=E

is a measure of the

relative dominance of the thermionic and ﬁeld emission processes. We expect ﬁeld

emission if kT E

, and thermionic emission if kT E

. In the region where

kT  E

, the carriers are thermally excited to an energy less than the SBH and

are ﬁeld emitted through the barrier (thermionic-ﬁeld emission), and the current–

voltage relation is similar to that for ﬁeld emission:

J D J

TFE



exp





 1



; (5.11)

where

D E

coth





(5.12)

and the saturation current density for thermionic-ﬁeld emission is given as:

TFE



E

.q˚

 qV C /

k cosh .E

=kT/

exp







q˚

C 



: (5.13)

Equation (5.12) demonstrates that by plotting E

.DnkT/ against the thermal en-

ergy, kT, the curve indicates the dominant transport process for a given temperature

range. The case for an Au/n-GaAs junction is shown in Fig. 5.7 [48]. Since J

TFE

is a weakly varying function of V , by plotting ln



TFE

cosh .E

=kT/=T



against

1=E

, the SBH can be obtained from the linear slope.

Fig. 5.7 Experimental values of E

as a function of temperature for an Au/n-GaAs junction. The

solid line denotes a ﬁt using (5.12)(From[48], by permission)

178 Y. Hikita and H.Y. Hwang

5.2.3 Barrier Height Characterization Techniques

The most critical parameter governing the operation of a Schottky junction is the

SBH. Here the widely used techniques for its measurements are summarized.

 IV Characteristics

From the IV characteristics in the forward-bias voltage, the ln J–V plot is a

straight line. The linearly extrapolated value on the ln J axis gives the saturation

current density J

, from which the SBH can be evaluated with the knowledge

of the Richardson constant. This is the simplest method although it has three

drawbacks, (1) the transport mechanism must be assumed, (2) the Richardson

constant must be known, and (3) ambiguity arising from the effective area of the

current path.

 C V Characteristics

The barrier height can also be determined by measuring the capacitance arising

from the space charges in the depletion region. The capacitance is related to the

reverse-bias voltage by

 V/; (5.14)

which states that the intercept on the voltage axis in a 1=C

V plot gives the

built-in potential V

. From the built-in potential, the barrier height can be calcu-

lated using the relation, q˚

D qV

C :  can be calculated from the doping

concentration of the semiconductor, which is readily obtained from the slope of

the 1=C

V plot. The C V measurement tends to capture the spatial average

of the built-in potential but suffers from drawbacks such as (1) the assumption

of the depletion approximation in deriving (5.14), and (2) inclusion of any unin-

tended extrinsic charge modulating at the frequency of the measurement such as

deep level traps and interface states [49].

 Photoemission Spectroscopy (PES)

The shift in the core-levels in the semiconductor under thin metal ﬁlms by pho-

toemission can also be a probe for measuring the SBH. PES offers a unique

ability to study the development of band bending as a function of the cover-

age of metal overlayers deposited on the semiconductor surface. Monochromatic

X-rays excite core electrons to states high enough in energy to traverse the metal

and escape into vacuum [50]. The drawbacks associated with the interpretation of

SBHs from PES are that the intense light radiation generates electron-hole pairs

that locally charge the isolated metal clusters and induce signiﬁcant changes in

the observed band bending [51]. Furthermore, this surface sensitive technique

may include the effects of surface potentials.

 Internal Photoemission (IPE)

Internal photoemission (IPE) uses tunable monochromatic light to photo-excite

the electrons in the metal. If the excited electrons have kinetic energy sufﬁcient to

surmount the SBH, they will diffuse into the semiconductor and will be detected

5 Complex Oxide Schottky Junctions 179

Fig. 5.8 (a) IPE spectra for an Au/n-Si junction, (b) SBH change (points) and the energy gap

change (solid line) as a function of temperature (From [53], by permission)

as a net photocurrent because of the strong internal electric ﬁeld in the depletion

region. In usual semiconductors, the electric ﬁeld at the interface, E.x D 0/,isof

the order of 10

–10

Vcm

1

, thus large collection efﬁciency of the photoemitted

carriers can be achieved without application of an external voltage. The SBH is

determined using the Fowler relation which relates the incident photon energy

h with the normalized photoexcited electron density, the photoyield Y [52],

Y / .h  ˚

: (5.15)

From (5.15),

Y vs. h shows a linear dependence and the extrapolated value

on the abscissa is the SBH. An example of the SBHs obtained from IPE spectra

is shown in Fig. 5.8 for the case of an Au/n-Si junction taken at different temper-

atures, which clearly demonstrates that the temperature dependence of the SBH

can be explained by the temperature dependence of the energy gap of Si [53]. De-

spite its practical effectiveness, microscopic interactions occurring in the metal

[54] or reﬂection from the metal surface and the interface due to the ﬁnite thick-

ness of the metal layer [55] are not included in (5.15).

5.3 Physics of Complex Oxide Schottky Junctions

5.3.1 Dielectric Properties of SrTiO

The semiconductor used most often in perovskite Schottky junctions is NbWSrTiO

The parent compound SrTiO

is a cubic perovskite band insulator with an energy

gap of 3.2 eV [56]. Conductivity is induced by introducing heterovalent ions such as

Nb or La to the host, or by creating oxygen vacancies [57–59], inducing electron car-