ApplPhysLett_91_142113-nakano
Schottky contact on a ZnO…0001…single crystal with conducting polymer M.Nakano,A.Tsukazaki,R.Y.Gunji,K.Ueno,A.Ohtomo,and T.Fukumura a?
Institute for Materials Research,Tohoku University,Sendai980-8577,Japan
M.Kawasaki
Institute for Materials Research,Tohoku University,Sendai980-8577,Japan and CREST,Japan Science
and Technology Agency,Tokyo102-0075,Japan
?Received14July2007;accepted4September2007;published online4October2007?
High quality Schottky junctions were fabricated on a ZnO?0001?bulk single crystal by spin coating
a commercial conducting polymer,poly?3,4-ethylenedioxythiophene?:poly?styrenesulfonate?
?PEDOT:PSS?,as the metal electrodes.The junctions exhibited excellent rectifying behavior with a
typical ideality factor of1.2.Such parameters as Schottky barrier height??b?and built-in potential
?V bi?show negligible variation among junctions.The electron af?nity of ZnO derived from?b and
qV bi values show a slight deviation??0.2eV?,suggesting the existence of spontaneously formed
interfacial dipole layer between ZnO?0001?polar surface and anionic PSS molecules.?2007
American Institute of Physics.?DOI:10.1063/1.2789697?
A wide-gap semiconductor zinc oxide?ZnO?,for which p-n junction light emitting diode and quantum Hall effect have been already demonstrated,1–3is one of the promising compounds for realizing transparent oxide electronics.4–7For developing various electronic devices using ZnO,high qual-ity electric contacts on ZnO,both Ohmic and Schottky con-tacts,have to be formed with desired properties.Metal/ semiconductor Schottky junction is very useful for evaluating the electronic states of a semiconductor.8From the current and capacitance measurements,such electronic properties of the semiconductor as the ionized dopant con-centration,depletion layer width,and band line-up param-eters can be evaluated.Also,capacitance measurement can be used to probe carrier distribution and energy band discon-tinuity in semiconductor heterostructures.9Moreover,high-quality Schottky junction can be used for such applications as metal-semiconductor?eld-effect transistors and photodetectors.10–12
In order to form Schottky contact on an n-type ZnO, various noble metals with large work function such as Au, Ag,Pt,and Pd have been employed.13–15However,the junc-tion properties were often found to be sensitive to the prepa-ration methods of the metal deposition and/or the surface treatment condition,resulting in irreproducible properties if these processes were not well optimized.On the other hand, p-type conducting polymer,poly?pyrrole?,was demonstrated to serve as a high quality Schottky contact on n-type inor-ganic compound semiconductors such as InP.16,17Solution based soft fabrication process may avoid the formation of interfacial trap states possibly due to small physical and/or chemical stresses at the interface,giving rise to better junc-tion properties.
Here,we report on the realization of high quality Schottky junctions by simply spin coating a commercially available p-type conducting polymer on a ZnO?0001?single crystal.
ZnO single crystal substrate?Tokyo Denpa?was chemi-cally etched with a diluted HCl solution,followed by ultra-sonication in organic solvents and UV-ozone exposure for eliminating surface contaminants.The Zn-polar surface was then spin coated with poly?3,4-ethylenedioxythiophene?:
poly?styrenesulfonate??PEDOT:PSS?aqueous solution ?H.C.Starck,Baytron PH500?and dried in a vacuum oven at200°C for30min in a glove box with Ar atmosphere.
PEDOT:PSS coated glass substrates were also prepared through the same procedure for electrical measurements of the PEDOT:PSS thin?lms.The work function and the resis-tivity of PEDOT:PSS?lms were measured in air by photo-electron yield spectroscopy?Riken Keiki,AC-2?and a four point contact probe,respectively.
Figure1?a?shows the chemical structures of PEDOT ?left?and PSS?right?.Polythiophene backbone of PEDOT forms the pathway for hole transport??-conjugated system, dotted line?,and PSS acts as an acceptor.Figure1?b?shows photoelectron yield spectrum of a PEDOT:PSS?lm on glass substrate.From the linear?t to?Photoelectron yield?1/2
a?Electronic mail:
fukumura@imr.tohoku.ac.jp
FIG.1.?a?Chemical structures of PEDOT?left?and PSS?right?.?b?Pho-
toelectron yield spectrum of a PEDOT:PSS?lm coated on glass substrate.
Broken straight lines denote the?tting lines to evaluate the work function
??m?.The inset shows the?lm thickness and the electrical resistivity of
PEDOT:PSS?lms formed on glass substrates as a function of the rotation
speed during spin coating.
APPLIED PHYSICS LETTERS91,142113?2007?
0003-6951/2007/91?14?/142113/3/$23.00?2007American Institute of Physics
91,142113-1
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??h ???m ??h ?is photon energy ?,the work function ??m ?of PEDOT:PSS was evaluated to be 5.0eV,which is similar to the value reported in literatures.18–20The ?lm thickness of PEDOT:PSS was controlled by the rotation speed of spin-coating ranging from 40to 80nm with no signi?cant change in the resistivity as low as ?1m ?cm,as shown in the inset of Fig.1?b ?.In the present study,the rotation speed was set at 4000rpm and the thickness of PEDOT:PSS ?lm was about 50nm.It is worth noting that the optical transmittance of a 50nm thick PEDOT:PSS ?lm showed excellent internal transmittance of ?90%for a wide region of wavelength from 250to 1000nm ?not shown ?,which is suitable for op-tical application such as Schottky photodetectors.
Figure 2?a ?shows the schematic structure of the PEDOT:PSS/ZnO Schottky junction.Au contact electrode was deposited on the PEDOT:PSS/ZnO by thermal evapora-tion.Circular mesa structures with a diameter of 350?m and ring-shaped Ohmic electrodes ?Ti/Au ?were formed on the ZnO substrate by Ar ion milling and electron-beam evapora-tion through photolithography technique,where the junction area was 9.6?10?4cm 2.The junction properties were exam-ined by current density–voltage ?J -V ?and capacitance-voltage ?C -V ?measurements at room temperature in air un-der dark condition,using semiconductor parameter analyzer
?Agilent Technologies,4155C ?and LCR meter ?Agilent Technologies,4284A ?,respectively.The forward bias ?posi-tive V ?corresponds to the current ?ow from PEDOT:PSS to ZnO.Figure 2?b ?shows a typical J -V characteristics in linear ?right ?and log ?left ?scales.The junction shows a good rec-tifying behavior:the recti?cation ratio ?J ?2V ??/?J ??2V ??is as high as ?107.From the thermionic emission model,8a Schottky junction under forward bias has the J -V relation of J =J 0exp
?qV
nk B T
?
?V ?3k B T /q ?,?1?
where J 0is the saturation current density,q is the elementary charge,V is the applied voltage,n is the ideality factor,k B is Boltzmann’s constant,and T is the absolute temperature.J 0is expressed as
J 0=A *T 2exp ???b
k B T
?
,?2?where A *is the effective Richardson constant with A *
=36A cm ?2K ?2for ZnO ?m e *=0.3m 0?,
21
and ?b is the Schottky barrier height.The slope and the J intercept from the linear ?t to the semilog plot for V =0.4?0.5V yield in n =1.2and ?b =0.9eV,respectively.The n value close to unity indicates the high quality of the junction.The resultant recti?cation ratio and n are comparable to the best value using Pt and Ag electrode in recent reports,12,22,23in spite of the simple spin-coating process in this study.The inset of Fig.2?b ?shows the histogram of ?b variation among different 37junctions on a ZnO substrate,indicating the considerably small deviation in their properties.
Figure 2?c ?shows the 1/C 2-V characteristics of the same device measured at 500kHz.The 1/C 2-V characteristics show good linearity in the measured voltage range,re?ecting the small leakage current ?observed tangent ?was as low as 0.01?.Also,the linearity suggests the uniform distribution of the ionized donors in the ZnO substrate and the voltage in-dependent relative permittivity ??s ?of ZnO.The Schottky junction under reverse bias has a C -V relation of
1C 2=2?V bi ?V ?q ?s ?0N D
,?3?
where C is the capacitance in unit area,V bi is the built-in potential,?0is the vacuum permittivity,and N D is the ionized donor concentration in the depletion layer.The depletion layer width ?W d =?s ?0/C ?is expressed as
W d =
?
2?s ?0
qN D
?V bi ?V ?.?4?
From Eq.?3?,the V intercept and the slope from the linear ?t in Fig.2?c ?for V =?3–0V yield in V bi =0.6V and N D =1.4?1017cm ?3,assuming ?s =8.21The value of N D is consistent with the value of the electron concentration in ZnO substrate obtained from Hall measurement with van der Pauw method,implying that the depletion layer is formed not in the PEDOT:PSS layer but in the ZnO layer.From Eq.?4?,the depletion layer width spans from W d =61nm at 0V to 150nm at ?3.0V.The inset of Fig.2?c ?shows the frequency dependence of the junction capacitance at V =0V.The almost frequency independent behavior may suggest the small effect of interfacial trap states in the junc-tion.All the different 37junctions show negligible variation in V bi as well as ?b ?not shown ?
.
FIG.2.?a ?The schematic cross section of the PEDOT:PSS/ZnO Schottky junction.?b ?The current density–voltage ?J -V ?characteristics for the PEDOT:PSS/ZnO Schottky junction.The ideality factor ?n ?and the Schottky barrier height ??b ?are also shown.The histogram of ?b among 37junctions on a ZnO substrate is shown in the inset.?c ?The capacitance-voltage ?C -V ?characteristics for the PEDOT:PSS/ZnO Schottky junction measured at 500kHz.The built-in potential ?V bi ?,ionized donor concentra-tion ?N D ?,and the depletion layer width ?W d ?at V =0V are also indicated.The frequency dependence of the junction capacitance at V =0V is shown in the inset.
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In a metal/n -type semiconductor Schottky junction,the electron af?nity ??s ?of the semiconductor can be determined
from the J -V measurement using a simple relation of ?s JV
=?m ??b .In the present case,?m =5.0eV and ?b =0.9eV
lead to ?s
JV
=4.1eV.While ?s can also be deduced from C -V characteristics using a relation of ?s
CV
=?m ??qV bi +??,where ?is the energy difference between the bottom of the conduc-tion band and the Fermi energy ?E C ?E F ?,expressed as ?=k B T ln ?N C /N D ?,where N C stands for the effective density of states at the conduction band bottom in ZnO.N C is de-duced to be 4?1018cm ?3at room temperature from N C =2?2?m *k B T /h 2?3/2,and then ?yields in 0.1eV.From
?m =5.0eV and qV bi =0.6eV,?s CV
is evaluated to be 4.3eV.
The obtained values of ?s JV and ?s CV
are within the range of
reported values of 4.1–4.4eV.24–26Note that ?s
JV
is smaller than ?s CV
representing opposite tendency to the conventional Schottky junctions.
Figure 3shows the resultant energy band diagram of the PEDOT:PSS/ZnO Schottky junction.The obtained junction parameters are also shown.The discrepancy of ?s obtained from J -V or C -V measurements could be attributed to a di-pole layer existing at PEDOT:PSS/ZnO ?0001?interface,that is caused by static Coulomb interaction between the ZnO ?0001?polar surface and the anionic PSS molecules in PE-DOT:PSS.The importance of such a large spontaneous po-larization along the c axis 27–29was already pointed out in our quantum Hall effect experiments 3and other experiments.23It is well-known that such interfacial dipole causes the vacuum level shift ???for organic/metal and organic/organic interfaces.30This sort of dipole will shift the Schottky barrier
height,giving rise to the difference between ?s
JV and ?s CV
,as shown in Fig.3.Range of ?for all the junction is 0.1–0.2eV depending on the variation in ?b and V bi ,where ?increases linearly with the decreasing n .It should be men-tioned that the magnitude of ??0.2eV is in the same order with the vacuum level shift observed at the interface between PEDOT:PSS and organic molecules.18
In summary,high quality Schottky junctions were formed on a ZnO ?0001?single crystal by employing a simple technique of spin-coating PEDOT:PSS as the metal electrode.The high recti?cation ratio and small n value near unity were reproducibly realized.The discrepancy in ?s val-ues obtained from J -V and C -V measurements suggest the existence of the interfacial dipole layer.
The authors thank the Evaluation Division of Fundamen-tal Technology Center,Research Institute of Electrical Com-munication,Tohoku University for photoelectron yield spec-trum measurement.One of the authors ?M.N.?is supported by Research Fellowships of the Japan Society for the Promo-tion of Science for Young Scientists.Another one of the authors ?M.K.?thanks for the support from JFE 21st Century Foundation.
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FIG. 3.The schematic energy band diagram of the PEDOT:PSS/ZnO Schottky junction.The evaluated junction parameters are also shown.E VAC ,E F ,and E C are the vacuum level,the Fermi energy,and the bottom of the conduction band,respectively.?is the energy difference between E C and E F .?s JV and ?s CV are the electron af?nity from J -V and C -V measurements,respectively.?is the vacuum level shift.The positive and negative symbols represent the electric dipole layer spontaneously formed at the interface.
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