High-k Organic,Inorganic,and Hybrid Dielectrics for Low-Voltage Organic
Field-Effect Transistors
Rocı´o Ponce Ortiz,Antonio Facchetti,*and Tobin J.Marks*
Department of Chemistry and the Materials Rearch Center,Northwestern University,2145Sheridan Road,Evanston,Illinois60208
Received March31,2009 Contents
1.Introduction205
1.1.Theory of Dielectrics207
1.1.1.Coherent Tunneling or Quantum Tunneling207
1.1.
2.Incoherent,Diffusive Tunneling207
1.1.3.Hopping Mechanisms207
1.1.4.Poole-Frenkel Effect208
1.1.5.Schottky Emission208
1.2.General Applications208
2.Field-Effect Transistors and Organic Field-Effect
Transistors
208
2.1.Dielectric Effects on the Figures-of-Merit of
OFET Devices
209
2.2.Interface Trapping Effects211
3.High-k Dielectric Materials for OFETs212
3.1.Inorganic Dielectrics212
刘光霞
3.1.1.Aluminum Oxide213
3.1.2.Tantalum Oxide215
如果遇见你3.1.3.Titanium Dioxide216
3.1.
4.Hafnium Dioxide217
3.1.5.Zirconium Dioxide218
3.1.6.Cerium Dioxide218
领导座位排序3.2.Organic Dielectrics218
3.2.1.Polymer Dielectrics218
3.2.2.Self-Asmbled Mono-and Multilayers225
3.3.Hybrid Dielectrics227
3.3.1.Polymeric-Nanoparticle Composites227
3.3.2.Inorganic-Organic Bilayers232
3.3.3.Hybrid Solid Polymer Electrolytes235
4.Summary235
5.Acknowledgments236
6.References236 1.Introduction
The arch for high dielectric constant(high-k)gate dielectric materials forfield-effect transistor-enabled(FET) applications has stimulated important rearch activities in both conventional and unconventional electronics.Although it is not the focus of this Review Article,high-k dielectric technology is tremendously important in the well-established silicon electronics industry.Indeed,the continuous drive to increa integrated circuit performance through shrinkage of the circuit elements requires the Si transistor dimensions to be scaled down according to the well-known Moore’s law.1Historically,this goal has been achieved by developing new
optical lithography tools,photoresist materials,and critical
dimension etch process.However,it is now clear that,
despite advances in the crucial process technologies,device
performance in scaled devices will be compromid becau
the traditional materials ud for transistor and capacitor
fabrication(silicon and silicon dioxide)have reached their
fundamental material limits.2Therefore,continuing scaling
will require the introduction of new materials.3One of the
头油腻key materials challenges,which if not addresd could
interrupt the historical Moore’s law progression,is the
replacement of the silicon dioxide layer with new gate
dielectric materials.4Despite a number of excellent proper-
ties,SiO2suffers from a relatively low dielectric constant(k )3.9).Becau high gate dielectric capacitance is necessary to enable the required drive currents for submicrometer
devices and becau capacitance for afilm is proportional
to k and inverly proportional to gate dielectric thickness
(d),the SiO2layer thickness must be reduced accordingly
to scaled device dimensions.Becau of the large band gap
of SiO2(∼9eV)and low density of traps and defects in the
bulk,the leakage current through the dielectric layer is
normally very low.However,for ultrathin SiO2films this is
no longer the ca.5When the physical thickness between
the gate electrode and doped Si substrate becomes thinner
than∼2nm,according to fundamental quantum mechanical
laws,the tunneling current increas exponentially with
*To whom correspondence should be addresd.E-mail:a-facchetti@ northwestern.edu(A.F.);t-marks@northwestern.edu
(T.J.M.).Rocı´o Ponce Ortiz was born in Marbella(Spain)in1980.She studied at the University of Malaga where she obtained her degree in Chemical Engineering in2003and a Ph.D.in Chemistry in2008working on vibrational spectroscopy,electrochemistry,and quantum-chemical calcula-tions of oligothiophene derivatives in Prof.Lo´pez Navarrete’s group.In 2008,she joined Prof.Tobin J.Marks’group at Northwestern University as a postdoctoral rearcher.Dr.Ponce Ortiz has published25rearch articles.Her current rearch interest is molecular electronics for organic thin-film transistors.
Chem.Rev.2010,110,205–239205
10.1021/cr9001275 2010American Chemical Society
Published on Web10/23/2009
decreasing oxide thickness and dominates the leakage current.For silicon dioxide-bad capacitors,the leakage current at 1V increas from ∼10-5to ∼10A/cm 2when the dielectric layer thickness decreas from ∼3to ∼1.5nm,which is a 107×current increa for a thickness change of only 2×.6,7The high leakage currents will invariably compromi the device performance as well as dissipate large amounts of power.It is therefore obvious that SiO 2as deposited with current meth
ods will very soon reach its limit as a gate dielectric for all kinds of low power applications.Although higher power dissipation may be tolerable with some high-performance processors,it quickly leads to problems for mobile devices.Eventually,another major limitation for thin oxides may come from their reduced lifetimes.In addition,the incread operation temperatures considerably increa the gate leakage through thin oxide layers and reduce lifetimes further.8The oxide reliability thus remains one of the other major issues in CMOS scaling.9It is therefore clear that to meet next-generation device requirements,the solution is reprented by using thicker dielectric layers of materials having permittivities higher than that of SiO 2.7,10,11
For completely different applications,high-k gate dielec-trics are needed in unconventional electronic devices bad on organic FETs.As we will describe in detail in the following ctions,this rearch field is also known as “organic”or “printed”electronics.Rearch and development on organic transistors began in the 1980s with the goal of fabricating electronic circuits by printing all FET materials components instead of defining them using photolithography.If successful,this technology would allow inexpensive,low-temperature,and large area device processing as well as enable new device functions.Thus,simple electronic devices such as radiofrequency identification (RFID)tags and nsors could be fabricated on plastic foils and integrated with commercial item packages in the
supply chain.12–14Other important printed electronic products include backplane circuitries,which can be ud for the fabrication of flexible/bendable displays for e-paper and flexible computers.There are two key interconnected challenges in this area.The first
is reprented by the limited performance,particularly carrier mobility,of FETs bad on printable organic miconductors.As compared to single-crystal inorganic miconductors,charge transport efficiency in the materials is reduced by the abnce/limited formation of delocalized electronic bands,even in molecular crystals.Becau of this limitation,the cond key challenge is related to the unacceptably large power (operating voltages)needed to achieve uful FET currents when conventional gate insulators are utilized.A solution to reduce the operating voltages is to enhance gate dielectric capacitance (the drain current scales with the gate capacitance)and,therefore,to employ high-k materials.Finally,note that the dielectric material also indirectly affects FET charge transport characteristics in the miconductor becau of the different charge trapping capacities for holes and electrons.15,16
低碳环保绘画
This Review Article focus on the importance,develop-ment,and implementation of high-k gate dielectrics for modern organic electronics applications.We begin by describing the basics and the theory of dielectrics followed by the operating principles of OFET devices to understand the motivati
ons behind improving the dielectric layer permit-tivity.Next,an overview of the state-of-the-art FET perfor-mance achieved using veral class of organic,
inorganic,
Antonio Facchetti obtained his Laurea degree in Chemistry cum laude and a Ph.D.in Chemical Sciences from the University of Milan under the supervision of Prof.Giorgio A.Pagani.He then carried out postdoctoral rearch at the University of California-Berkeley with Prof.Andrew Streitwier and at Northwestern University with Prof.Tobin J.Marks.In 2002,he joined Northwestern University where he is currently an Adjunct Associate Professor.He is a cofounder and currently the Chief Technology Officer of Polyera Corp.Dr.Facchetti has published about 170rearch articles and holds 30patents.Dr.Facchetti’s rearch interests include organic miconductors and dielectrics for thin-film transistors,conducting polymers,molecular electronics,organic cond-and third-order nonlinear optical materials,and organic
photovoltaics.
Tobin J.Marks is the Vladimir N.Ipatieff Professor of Chemistry and Professor of Materials Science and Engineering at Northwestern University.He received his B.S.from the University of Maryland (1966)and Ph.D.from MIT (1971),and came to Northwestern immediately thereafter.Of his 75named lectureships and awards,he has received American Chemical Society Awards in Polymeric Materials,1983;Organometallic Chemistry,1989;Inorganic Chemistry,1994;the Chemistry of Materials,2001;and for Distinguished Service in the Advancement of Inorganic Chemistry,2008.He was awarded the 2000F.Albert Cotton Medal,Texas A&M American Chemical Society Section;2001Willard Gibbs Medal,Chicago American Chemical Society Section;2001North American Catalysis Society Burwell Award;2001Linus Pauling Medal,Pacific Northwest American Chemical Society Sections;2002American Institute of Chemists Gold Medal;2003German Chemical Society Karl Ziegler Prize;2003Ohio State University Evans Medal;2004Royal Society of Chemistry Frankland Medal;2005Bailar Medal,Champaign-Urbana Section of the American Chemical Society,Fellow,American Academy of Arts and Sciences,1993.He is a Member,U.S.National Academy of Sciences (1993);Member,German National Academy of Sciences (2005);Fellow,Royal Society of Chemistry (2005);Fellow Chemical Rearch Society of India (2008);Fellow,Materials Res
earch Society (2009);2009Herman Pines Award,Chicago Catalysis Society;2009Nelson W.Taylor Award in Materials Rearch,Penn.State U.;2009von Hippel Medal,Materials Rearch Society;2010William H.Nichols Medal,ACS New York Section.In 2006,he was awarded the National Medal of Science,the highest scientific honor bestowed by the United States Government.Marks is on the editorial boards of nine major journals,is the consultant or advisor for six major corporations and start-ups,and has published 935rearch articles and holds 93U.S.patents.
206Chemical Reviews,2010,Vol.110,No.1Ortiz et al.
and hybrid high-k dielectrics will be prented.Several strategies that are utilized to enhance not only k but also the gate capacitance by reducing the layer thickness will be covered.In this contribution,lf-asmbled mono-and multilayer nanodielectrics will also be described.Finally,throughout this Review,we will also discuss theoretical models and recent experimental data analyzing the effect of the k on the charge transport properties of the miconductor.
1.1.Theory of Dielectrics
Insulators or dielectric materials are characterized by the
abnce of charge transport.17Nonetheless,when an electric field is applied,the materials undergo a shift in charge distribution.This field-induced polarization leads to dielectric behavior and hence to capacitance,C .If we imagine two electrodes parated by a distance d in a vacuum,the application of a voltage between them creates an electric field that is described by E )V /d .The charge created per unit area is proportional to this electric field,as given by eq 1.
The proportionality constant between the applied voltage and the charge is called the capacitance C ,and it is described by eq 2.
When a dielectric material in inrted between the electrodes,
the capacitance is incread (by a factor of k ,relative dielectric constant)due to the polarizability of the dielectric.In this ca,the capacitance is described by eq 3.
Electronic conduction in insulating materials has been a subject of considerable interest in the quest to understand charge transport in the thin film layers of organic electronic devices.In typical dielectric materials,the electronic states near the Fermi level are usually localized states,and the electron wave functions decay exponentially over a distance known as the localization length.18In constrast,metals have a high,generally uniform density of states,whereas mi-conductors have well-
parated conduction and valence bands (parated by a band gap).In a thin film transistor,there exist different junctions,metal -insulator,insulator -mi-conductor,and miconductor -metal,that must be fully understood to optimize the device performance characteristics.In this Review,we will focus exclusively on conductor -dielectric interfaces.As we shall e,veral theoretical models have been developed to explain conduction through the junctions.
1.1.1.Coherent Tunneling or Quantum Tunneling
The terms relate to the probability of the electron to cross a dielectric barrier of height,φ,and thickness,d .This transport occurs when the dielectric layer thickness is not much greater than the localization length,and then the prence of localized states does not significantly alter the conduction process.As a conquence of this situation,the rate of coherent tunneling decreas exponentially with the dielectric thickness,and the current density through the channel is given by the Simmons relation 19of eq 4.
Here,J is the current density through the channel (A/cm 2),q is the electron charge,h is Planck’s constant,and m is the electron mass.This equation,which contains only a linear term for a rectangular tunneling barrier,accounts for the exponential dependence of the current density on the thickness (d )and the barrier height (φ)and describes “through space”tunneling.
In some cas,electron -phonon interactions or interac-tions of the electron with the orbitals and the electronic structure of the molecule must be taken into account.Theoretical models that account for the interactions have been developed independently by different groups.20In this ca,“through bond”tunneling becomes more efficient than “through space”tunneling.
Under high electric fields (exceeding the barrier height),the tunneling rate increas,and Fowler -Nordheim tunneling or “field emission”is induced.In this ca,it is necessary to modify the rectangular tunneling barrier of the Simmons equation to a triangular shape,as in eq 5,which describes the density current at high E .
Here,φFN is the tunneling barrier height,E is the electric field (V/cm),and m*is the effective electron mass.This tunneling is basically independent of temperature and also decreas exponentially with distance,19,21as in the Simmons equation.Both equations prented in this ction only apply for very thin dielectric layers;22when the thickness increas sufficiently,other transport mechanisms must be considered.In fact,the are the mechanisms usually found for transport through lf-asmbled monolayers.23
1.1.
回复祝福语的感谢语2.Incoherent,Diffusive Tunneling
In the ca of a thick barrier and a high density of localized states,it is necessary to consider the probability of the electron tunneling resonantly between two or more concu-tive sites that are characterized by potential wells.24The process in this ca may be viewed as a ries of discrete steps (e Figure 1).25The mechanism can be considered to be independent of temperature and should be in principle the predominant one in the limit of very thick barriers at extremely low temperatures.18
1.1.3.Hopping Mechanisms
The mechanisms are usually thermally activated electron transfers and are dominant at low fields and moderate temperatures.They follow the classical Arrhenius model (eq 6):
Here,σis the conductivity (σ)J /E ),E a is the activation energy,and k B is the Boltzmann constant.This mechanism is similar to the diffusive tunneling process in that the electron travels between one or more sites.The major difference is that the hopping involves nuclear motion.26,27
Q )ε0E )ε0V /d
(1)
C )Q /V )ε0/d
(2)
C )ε0(k /d )
(3)
J DT
)q 2V h 2d
(2m φ)1/2exp
[-4πd
h (2m φ)1/2]
(4)
J FN
亲的成语
)q 3E 2
8πh φFN exp
[
-4√2m*3qhE
(q φFN )3/2]
(5)
σ)σ0exp
(-E a
k B T
)
(6)
Dielectrics for Low-Voltage Organic FETs Chemical Reviews,2010,Vol.110,No.1207
The electron transfer occurs over the barrier,following the dependence on driving force predicted by Marcus theory.28Becau this process involves a ries of hopping sites,this thermally activated mechanism does not exhibit the expo-nential distance dependence found in coherent tunneling,but it varies in proportion to d -1.That means that for larger d ,the distance is too great for coherent tunneling,and the electron propagates more efficiently by hopping between “hopping”sites.
1.1.4.Poole -Frenkel Effect
This approach was developed to account for the effects of “traps”in hopping electron transport.The Poole -Frenkel effect is attributed to the lowering of trapping Coulombic barriers within the molecule by the applied electric field,and it explains the electric current in miconductors.19a,21,29The trapped electrons contribute to the current density according to the Poole -Frenkel equation (eq 7)at high temperatures and intermediate fields.30
In this equation,W is the electric field (E )V /d ),σ0is the low field conductivity,φB is the Frenkel -Poole barrier height,k is the dielectric permittivity,and k B is the Boltzmann constant.
1.1.5.Schottky Emission
This theory explains the electron transfer mechanism at interfaces.31A Schottky barrier aris from partial charge transfer from one layer to the other at an interface.As a result,a depletion layer or electrostatic barrier is generated.The Schottky emission or thermoionic emission is described by eq 8,and it considers that an electron can be injected through the interface once it has sufficient thermal energy to surmount the potential height.32
In eq 8,A*is the modified Richardson’s constant (A*)120A/cm 2·K 2),n is the diode ideality factor,E is the electric
field,φS is the Schottky barrier height,εis the dielectric permittivity,and k B is the Boltzmann constant.
1.2.General Applications
Dielectrics are widely ud in numerous applications.The most fundamental of the is their u as an insulating layer against electrical conduction.To be an insulator,a material must have a large band gap.In that way,there are no states available into which the electrons from the valence band can be excited.Nonetheless,there is always some voltage (the breakdown voltage)that will impart sufficient
energy to the electrons to be excited into this band.Once this voltage is surpasd,the dielectric will lo its insulating properties.The other two major applications of dielectrics are in capacitors 33and transistors,both of which are esntial components of electronic circuits.The structures of both of the electronic devices are shown in Figure 2.A capacitor is a passive electrical component consisting of a dielectric sandwiched between two conductors.The application of a voltage across this material will,at electric fields lower than the breakdown field of the dielectric (typically veral MV/cm),34,35induce a charge paration across the insulating layer forming the capacitor.21,34An ideal capacitor is characterized by a single constant parameter,the capacitance C .Higher C values indicate that more charge may be stored for a given voltage.Nonetheless,real capacitors are not complete insulators and allow a small amount of current flowing through,called leakage current.
The OFET structure (Figure 2b)is similar to the capacitor structure but having an additional miconductor layer (organic in the ca of OFETs)in contact with the gate dielectric.
2.Field-Effect Transistors and Organic Field-Effect Transistors
Figure 3shows the common device configurations ud in field-effect transistors (FETs).The configurations can be either bottom-gate or top-gate.In the first
configuration
Figure 1.Schematic energy level diagrams for coherent and diffusive tunneling between two metals M 1and M 2.ΦT is the barrier for coherent tunneling,and R is the potential well depth of N sites spaced apart by a distance a .Reproduced with permission from ref 25.Copyright 2004American Chemical Society.
J PF
)σ0E exp
[
-q (φB -√qE /πk )
k B T
]
(7)
J S )A*T 2
exp
[
-q (φS -n √qE /4πε)
k B T
]
(8)
Figure 2.Typical structures of (a)organic-bad capacitors and (b)organic field-effect
transistors.
Figure 3.Different structures of organic field-effect transistors.L ,channel length;W ,channel width.
208Chemical Reviews,2010,Vol.110,No.1Ortiz et al.
(bottom-gate),two different structures can be ud,bottom-contact or top-contact,depending on the position of the source and drain electrodes.In the ca of organicfield-effect transistors(OFETs),the miconductor layer is an organic material.
Becau there are a large number of excellent reviews that explain the basis of OFET function in detail,in this Review we will only comment on the aspects briefly.36In principle, the characteristic that best defines an OFET is the prence of an electricfield that controls and modulates the conductiv-ity of the channel between the source and drain.In the devices shown in Figure3,this electricfield is created by the voltage applied between the source and gate,the gate voltage(V G),but is also dependent on the insulator/dielectric layer.Indeed,a positive/negative gate voltage will induce negative/positive charges at the insulator/miconductor interface,and the number of the accumulated charges depends on V G and on the capacitance C of the insulator. When no voltage is applied between the source and gate(V G
)0),the device is“off”.On increasing both V忒拜
G and V D
(voltage between source and drain),a linear current regime is initially obrved at low drain voltages(V D<V G)(eq9) followed by a saturation regime at higher V D values(eq10).
In the equations,(I SD)lin is the drain current in the linear regime,(I SD)sat is the drain current in the saturation regime,µis thefield-effect carrier mobility of the miconductor, W is the channel width,L is the channel length,C is the capacitance per unit area of the insulator layer,V T is the threshold voltage,V D is the drain voltage,and V G is the gate voltage.
The above equations indicate that the current between the source and drain can be incread by increasing either V G or V D.Nonetheless,the two parameters can be incread to only a certain extent.As is also evident in eqs9and10, another viable approach to minimizing V G and/or increasing the electrical current is adjusting the capacitance of the gate dielectric,C,as described by eq3.This relationship makes it clear that by either increasing k or decreasing d,the device current is enhanced.Note also that a small d is required in devices using short channel lengths.Typically,d/L e0.1is necessary to ensure that thefield created by V G,and not the lateralfield V D,determines the charge distribution within the channel.37Several groups have adopted the approach of reducing dielectric thickness to realize low-voltage operation OFETs.For example,Vuillaume ployed an organic monolayer of carboxyl-terminated alkyltrichlorosilanes(thick-ness range1.9-2.6nm)with linear
end groups for the gate dielectric to achieve working voltages below2V.38Halik et al.demonstrated low-voltage organic transistors using lf-asmbled monolayers(SAMs)of(18-phenoxyoctadecyl)-trichlorosilane,thereby enhancing the mobility of pentacene devices to∼1cm2V-1s-1due to favorable interactions at the miconductor-dielectric interface.39Finally,Marks et al.studied lf-asmbled multilayers(SAMTs)grown from solution to achieve very low leakage currents and low operating voltages.40
Otherfigures-of-merit that must also be optimized in OFET devices are the threshold voltage,I ON/I OFF ratio,and subthreshold slope(S)d V G/d(log I D)),related to how efficiently the gatefield modulates the“off”to“on”current and how abruptly the device turns“on”.The parameters depend not only on the nature of the organic miconductor but also on the chemical structure and dielectric properties of the insulator ud,and on the capacitance resulting from interface traps,C IT,as shown in eq11for S.41
2.1.Dielectric Effects on the Figures-of-Merit of OFET Devices
There are specific requirements for gate dielectrics to be ud infield-effect transistors.Apart from a high capacitance, as shown in Section2,high dielectric breakdown strength,42 high levels of purity,hi
gh on/off ratios,low hysteresis, materials processability,and device stability are esntial.43 To understand the role of the gate dielectric on FET device figures-of-merit,it is important to take into account that most relevant process taking place in the devices(charge accumulation and transport)occur in clo proximity to the interface between the gate dielectric and the miconductor layer.This implies that an optimum dielectric-miconductor interface is fundamental for efficient device function.For example,threshold voltages normally depend strongly on the miconductor and dielectric ud becau impurities and charge trapping sites tend to increa this value.It is also quite clear from eqs9and10that V T can be easy modulated by increasing the capacitance of the dielectric,which creates a higher density of charges in the interface at lower voltages. Several authors have also suggested that by controlling the density of miconductor-dielectric interface states,it should be possible to modulate threshold voltages in organic transistors.44
The nature of the insulator interface has also been widely shown to have a great impact on the miconductor mobil-ity,45as wasfirst demonstrated by the deposition of pentacene on SiO2under different growth conditions.46,47The polarity of the dielectric interface can also influence the quality of the miconductor layer,affecting local morphology,the density of states(DOS)in the organic miconductor layer, and,conquently,thefield-effect mobility.The latter effect was extensively analy
zed by Veres et al.48on amorphous polymer-bad OFETs.In their study,they investigated a number of gate insulators having varying polarity using polytriarylamines(PTAA)and poly(9,9-dioctylfluore-co-bithiophene)as the miconductor materials,and they found that device performance was significantly incread when the insulator permittivity was below2.5(e Figure4).The authors ascribed this fact to an increa of carrier localization by electronic polarization in the high-k dielectrics.To explain this phenomenon,Veres et al.propod the graphical illustration in Figure4b,where the DOS is shown as a Gaussian distribution of localized states.As illustrated in Figure4b,as the dielectric-miconductor interface becomes more polar,the DOS broadening becomes more vere, leading to more tail states.The authors pointed out that this effect can be only applied to organic materials where the formation of electronic bands is very limited,becau random dipoles are unlikely to significantly affect the extended states of a wide band.The theoretical approach to this qualitative explanation has been recently developed by Richards et al.49 In their work,the authors calculated the broadening of the DOS due to dipolar disorder using an analytical model as a
(I SD)lin)(W/L)C(V G-V T-V D/2)V D(9)
(I SD)sat)(W/2L)C(V G-V T)2(10)S)
k
B
T
e
ln(10)(1+C IT/C)(11)
Dielectrics for Low-Voltage Organic FETs Chemical Reviews,2010,Vol.110,No.1209
