Experimental study of charge transport mechanisms in a hybrid metal/organic/inorganic device

H. Méndez,* I. Thurzo, and D. R. T. ZahnInstitut für Physik, Technische Universität Chemnitz, Reichenhainer Str. 70, D-09107 Chemnitz, Germany

�Received 19 September 2006; published 11 January 2007�

An hybrid metal/organic/inorganic semiconductor heterostructure was built under ultrahigh vacuum condi-tions �UHV� and characterized in situ. The aim was to investigate the influence of thin film layers of theorganic material Dimethyl-3,4,9,10-perylenetetracarboxylic diimide �DiMe-PTCDI� on the electrical responseof organic-modified Ag/GaAs Schottky diodes. The device was tested by combining photoemission spectros-copy �PES� with atomic force microscopy �AFM� and electrical measurements �current-voltage I-V,capacitance-voltage C-V, impedance and charge transient spectroscopies QTS�. The energy level alignmentthrough the heterostructure was deduced. This allows us to consider electrons acting as majority carriersinjected over a barrier by thermionic emission as a primary event in the charge transport. For thin organiclayers �below 8 nm thickness� most of the characterization techniques show an island-like growth leading toformation of voids. The coverage of the H+GaAs substrate as a function of the nominal thickness of DiMe-PTCDI was assessed via C-V measurements assuming a voltage independent capacitance of the organic layer.The frequency response of the device was evaluated through C-V and impedance measurements in the range1 kHz-1 MHz. The almost independent behavior of the capacitance in the measured frequency range confirmedthe assumption of a near geometrical capacitor, which was used for modeling the impedance of the device.QTS measurements performed on the heterostructure showed the presence of two relaxations induced bydeposition of the organic layer. The first one is attributed to a deep trap at the metal/organic interface, while thesecond one has very small activation energy ��20 meV� which is probably due to disorder at the organic film.With such information a fit of the I-V characteristics of DiMe-PTCDI organic modified diodes based on theanalytical expressions of a trapped charge limited current regime �TCLC� was intended. �Some figures areavailable in color only in the on-line version.�

DOI: 10.1103/PhysRevB.75.045321 PACS number�s�: 72.20.�i, 72.80.Le, 72.90.�y, 73.40.�c

I. INTRODUCTION

Nowadays, the field of organic electronics has reached thepoint where several applications are routinely available foruse, others are close to be commercialized, while others arein a design or testing phase for unique applications includingnanoscale technologies. This enormous progress in applica-tions of organic materials in optoelectronic devices is mainlydue to the fact that many aspects of organic device perfor-mance are already well understood, and have been thor-oughly documented in the scientific literature.1 But besidesthe increasing understanding of some physical properties oforganic materials, many others are not yet fully understood.For instance, the origin of the significant vacuum level offsetobserved at organic/inorganic interfaces, the charge injectionprocess at interfaces between metal �or inorganic semicon-ductor� electrodes and organic semiconductors, among manyother phenomena, remain as topics of research.

Almost without exception, the basic structure of organic-based devices consists of one or more active organic layerssandwiched between two metallic and/or semiconductingelectrodes, that either inject or remove charge carries andprovide electrical contact with other components. Withoutgoing into details, it seems to be clear that efficient chargetransport across the organic/inorganic interface is critical tothe device performance, and the commercial success of or-ganic electronics will strongly depend on making scientificprogress in understanding the injection process and the na-ture of the junction.1

The interface between the inorganic substrate, acting assupport, template, or contact, and the organic material plays

a key role in this context since it not only determines thestructure and morphology of the organic film and hence itsproperties. It also strongly influences the electric and opticalprocesses that occur at this interface, such as charge injectionor quenching of optical transitions. For instance, because ofthe different chemical bonding of different molecules at ametal or semiconductor surface, the band offset between thecontact and the organic layer can be “tailored” by choosing aproper organic interface coating or film material.2

However, the role of organic films is not properly passive.There is also evidence that organic molecules deposited onsolid surfaces can transfer molecular properties to the solid.Indeed, modifications of semiconductor or metal surfaces bymolecules with systematically varying properties can lead tocorresponding trends in the �electronic� properties of the re-sulting hybrid �molecule+solid� materials and devices madewith them.3 As examples, devices based on an “action at adistance” of carriers are cited. Such examples includemolecule-controlled diodes and sensors, where the electronsdo not need to go through the molecules. This point of viewconstitutes a new approach to molecule-based electronics:using the cooperative electron transport properties of metals�or inorganic semiconductors� and the controllable functionalversatility of molecules.

Besides metal/organic interfaces, organic/organic hetero-junctions have also been extensively investigated for theirrole in carrier transport in multilayer light emittingstructures.4 Organic/inorganic �OI� semiconductor junctions,however, have received only limited attention because theyare not central to current devices.5 They could, however, be-

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come relevant to future hybrid OI devices requiring materialswith complementary functionality which cannot be madewith structurally incompatible inorganic materials.6 A majordriving force in this area is the continuous quest to free mi-croelectronics from the limitations imposed on it by Siliconand III-V technologies.7 In order to pursue this goal, OI hy-brid devices are intended to be fabricated, using controlleddeposition of organic molecular thin films as an attempt forextending the properties of conventional semiconductors,and hence to take advantage of the individual properties ofboth materials. The main idea is to get control over the elec-tronic properties of conventional semiconductor devices �i.e.,the electron affinity �s and built-in potential Vb� by means ofthe properties of a molecular layer at the interface in suchdevices.

The present work deals with the modification of electronicproperties of Ag/GaAs�100� Schottky diodes by depositionof an organic interlayer, namely DiMe-PTCDI, under UHVconditions. In this way, the constituent parts and interfaces ofthe whole metal/organic/inorganic device are characterizedin situ by means of several electron spectroscopic techniquesand electrical measurements. For understanding the wholeperformance of the device, the mechanisms for generation,injection, and transport of free carriers through the hetero-structure were investigated. Since silver and n-GaAs with arelatively high doping concentration �ND�1�1016 cm−3�are the materials contacting the DiMe-PTCDI organic layer,the availability of generating free carriers is warranted.

The assumption of electron as majority carriers injectedinto the organic transport level over a barrier by thermionicemission as a primary event, is supported by the energy levelalignment deduced by PES and I-V measurements. Oncethere, before reaching the metal anode, carriers are stronglyscattered in the organic layer due to crystal imperfectionsand weak orbital overlapping leading to an almost-insulatingmaterial characterized by low mobility �org, conductivity�org and dielectric permitivity �org. Estimative values of thisparameters of the organic layer will be obtained after char-acterization of the metal/organic/inorganic hybrid devices byusing impedance spectroscopy, I-V and C-V measurements.Additionally, disorder, defects or dielectric relaxation effectsattributed to the organic layer can be monitored by chargetransient spectroscopy QTS.

II. EXPERIMENTAL

In general, an organic-modified GaAs�100� Schottky di-ode consists of a heterostructure as schematically shown inFig. 1�b�. By comparison with a traditional metal/GaAs�100�junction �i.e., the reference diode shown in Fig. 1�a�� there isan organic layer deposited under UHV conditions betweenthe metal and the GaAs�100� substrate. For building the as-sembly, a piece of GaAs�100� with a size of �8 mm�7 mm� is cut from the wafer �provided by MCP Wafertechnology Ltd.� and selected as a suitable inorganic semi-conductor substrate. The GaAs substrate is cleaned andchemically etched in order to remove the native insulatinglayer from the surface, with the goal of improving its surfaceelectronic properties. The procedure consists in consecutive

baths of acetone, ethanol and deionised water at 50 °C for5 min each, subsequent etching in HCl �38%� for 30 s anddrying in a N2 flow. After that, the etched GaAs substrate isplaced on a heated copper plate ��200 °C� containing asmall amount of melted indium �In�, in order to get an ohmicback contact with a small series resistance �5–9 �.

After transferring into a high vacuum chamber, the GaAssubstrate normally requires annealing for completing the pas-sivation procedure at 350 °C until a base pressure below 6�10−6 mbar is achieved. Then, hydrogen gas is introducedinto the chamber, getting a final pressure of �6.5±0.3��10−3 mbar. Afterwards, the remote hydrogen plasma is ac-tivated by applying 10 k VAC for 30 min. Details about ex-perimental conditions are provided elsewhere.8 When the hy-drogen plasma treatment is finished, the sample is transferredto an UHV chamber �P�5�10−11 mbar� and the organiclayer is deposited by organic molecular beam deposition�OMBD�. The thickness of the layer is monitored using aquartz microbalance previously calibrated by comparisonwith the results of AFM measurements. For DiMe-PTCDIthe sublimation temperature at a pressure of 5�10−10 mbaris �280 °C leading to a deposition rate of �0.5 nm/min.

Finally, the top contact consist of a set of silver dots de-posited by evaporation through a shadow mask onto the pre-viously deposited organic layer. Normally, silver sublimationstarts at 860 °C at a pressure of 5�10−9 mbar with a depo-sition rate of �1.7 nm/min. The mask was designed forthree different dot sizes �radius=0.5,0.4, and 0.3 mm, re-spectively� in order to check for surface leakage currents.Normally, one sample contains between 20 and 25 diodesand the electrical measurements are considered as represen-tative only when normalization with respect to the diode arearesults in a reproducibility higher than 70%.

III. RESULTS AND DISCUSSION

A. Photoemission spectroscopy (PES) and the occupied states

The carrier injection process depends strongly on the en-ergy level configuration through the whole hybrid metal/organic/inorganic device. The starting point is the location of

FIG. 1. �Color online� Assembly for in situ characterization byelectrical measurements under UHV conditions. �a� ReferenceAg/H+GaAs�100� diode, and �b� organic-modified H+GaAs�100�Schottky diode.

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electron occupied states measured at equilibrium conditionsby the PES technique. Under equilibrium, all the componentsof the device share a common energy level, which will betaken as a reference, and by definition corresponds to theFermi level. Thus, all the states below the Fermi level areoccupied and can be directly measured by the UPS tech-nique, but the information about unoccupied states, in ab-sence of inverse photoemission measurements �IPES�,should be obtained indirectly by complementary techniquessuch as I-V measurements, or optical absorption measure-ments. However, the interpretation of I-V and optical absorp-tion measurements is not straightforward for organic modi-fied diodes, and it will be mediated by direct comparison ofIPES measurements recorded on the DiMe-PTCDI/S-GaAs�100� system and additional considerations.

In order to get information about the energy level configu-ration of the Ag/DiMe-PTCDI/H+GaAs�100� heterostruc-ture, valence band spectra were measured in situ for a seriesof DiMe-PTCDI depositions onto a hydrogen plasma treatedGaAs�100� substrate. The UPS spectra were recorded imme-diately after each deposition of the organic layer withoutbreaking the vacuum in a stepwise manner, and the resultsare shown in Fig. 2.

The binding energy is calculated with respect to the Fermilevel EF. For the H+GaAs�100� substrate the valence bandmaximum �VBM� is located at �0.80±0.15� eV with respectto the Fermi level and therefore the offset between the con-duction band minimum �CBM� and EF should be around�0.62±0.15� eV. Regarding DiMe-PTCDI organic layers de-posited on H+GaAs�100�, the characteristic features of thehighest occupied molecular orbital �HOMO� become com-pletely defined only after reaching 4 nm to 8 nm thickness.UPS is a quite surface sensitive technique �with electronmean free path of �0.5 nm for helium I excitation source,h=21.22 eV�,9 able to detect subtle changes in the state ofthe H+GaAs�100� surface after depositing monolayers of or-ganic materials. Hence, UPS measurements suggest thatDiMe-PTCDI grows in an island-like mode, and the H+GaAssubstrate is completely covered only at some stage between4 nm and 8 nm of nominal thickness.

The weak features observed for the incomplete coverage�below 4 nm� and after a complete DiMe-PTCDI coverage

�8 nm nominal thickness� remain in the same energy posi-tion, with a binding energy of �2.10±0.15� eV. At this point,two statements should be highlighted. The first one concernsthe presence of characteristic occupied states calculated for asingle molecule. Indeed, theoretical calculations show thatthe HOMO consists of a single � orbital. The most pro-nounced feature is located around 4.5 eV binding energy andstems from the � and � bonds located at the imide, carboxyl,and methyl groups of the molecule.10,11 The very close agree-ment between the calculated energy positions for the HOMOand the measured ones suggests the absence of any stronginteraction between the organic molecule and the inorganicsubstrate. Thereby, no band bending in DiMe-PTCDI is ex-pected and possible changes in the HOMO energetic posi-tion, that the UPS technique is not able to detect �at leastwithin the resolution of the ARUPS 10 system whichamounts to 150 meV�, can be attributed only to the weak vander Waals bonding characteristic of organic crystals.

The second statement deals with the HOMO position it-self. According to the definition, it is the hole transportinglevel for organic semiconductors. Because all the consideredenergies are referred to the metal Fermi level, the injectionbarrier height for holes fits exactly with the HOMO position��2.1 eV�. For such a barrier, the most optimistic case of ametallic density of states �Nc�6.02�1023 cm−3� leads to anegligible hole density �p�2.7�1012 cm−3�. Thus, onlyelectron transport through the organic-modified heterostruc-ture will be considered for the electronic and transport prop-erties analysis.

A careful look at the spectra of DiMe-PTCDI layers de-posited on H+GaAs sketched in Fig. 2, reveals a gradual shiftof the secondary electron onset towards lower binding ener-gies with increasing thickness of the DiMe-PTCDI organiclayer until a nominal thickness of 8 nm is reached. For ad-ditional depositions of the organic layer the vacuum levelposition does not change anymore, supporting the idea of anincomplete coverage of the H+GaAs substrate below 8 nmnominal thickness of DiMe-PTCDI, as previously suggestedafter examination of the same set of spectra in the lowestbinding energy region. The final shift amounts to 0.2 eV, andis attributed to the formation of an interface dipole between

FIG. 2. �Color online� Valence band spectra of DiMe-PTCDI grown on hydrogen passivated GaAs�100� �H+GaAs�.

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the H+GaAs surface and the organic DiMe-PTCDI layer. Theionisation potential of H+GaAs ��4.9 eV� and DiMe-PTCDI��6.4 eV� are deduced from the spectra by considering thevacuum level position.

Following the study of the metal/organic/inorganic semi-conductor heterostructure, several depositions of silver onthe DiMe-PTCDI layer of 20 nm thickness were performed,and UPS measurements on the valence band region recorded�Fig. 3�. The HOMO position of DiMe-PTCDI does not shiftand gradually disappears until 2 nm of nominal thickness ofsilver. This confirms the expected absence of band bending atthe metal/organic interface and also suggests diffusion of themetal into the organic layer which is probably very roughand exhibits a high density of voids. This observation is alsosupported by looking at the secondary electron onset posi-tion, where there is no shift of the vacuum level below 2 nmof silver thickness.

For additional silver depositions the vacuum level shiftsgradually towards lower binding energies, reaching a finalvalue of 0.2 eV for a silver layer of 10 nm thickness. Thisgradual shift again points to a nonuniform formation of asilver layer below 10 nm of nominal thickness. The absenceof clearly resolved Ag 4d features �between 4 and 7.5 eVbinding energy� also indicates the presence of a very roughDiMe-PTCDI surface. It is known that the line shape of theAg 4d band strongly depends on bulk crystallographic direc-tions, surface morphology, and whether there is an adsorbateon the surface or not, and is considered as an indication ofthe crystallisation degree of the silver layer.12 Yet for the caseof a 10 nm silver layer deposited on organic thin films�10 nm thickness� of PTCDA and DiMe-PTCDI grown onsulphur passivated GaAs substrate, the Ag 4d states can beobserved.10,13 Thus, DiMe-PTCDI grown on H+GaAs exhib-its higher roughness that does not allow the formation ofrelatively large silver crystallites that could otherwise be de-tected as metallic 4d states.

After reaching 2 nm of silver thickness, two featuresemerge at about 0.5 eV and 1.7 eV in the HOMO-LUMOgap of the organic layer. Those bands are gap states that havealso been observed for interfaces such as PTCDA on Ag,14 orIn, Sn, Al, and Ti on PTCDA.15 These gap states are attrib-uted to a charge transfer between the metal atoms and themolecules resulting in polaron ��0.5 eV BE� and bipolaronlevels ��1.7 eV BE�.16 Normally, the bipolaron is occupiedwith two electrons and energetically positioned below theFermi level and the polaron is occupied with one electron

and should be positioned at the Fermi level. But, since or-ganic molecules represent small systems with localized elec-trons, the Coulomb repulsion between the first and secondelectrons shift the position of the occupied polaron state be-low the Fermi level,17 and therefore it can be detected byUPS.

In addition to the information obtained about the energylevel configuration already sketched in Fig. 3, there are twonew elements brought out from the analysis of the spectra ofsilver deposited on DiMe-PTCDI: the silver work functionvalue �4.50±0.15� eV and the presence of an additional in-terfacial dipole of 0.2 eV at the metal/organic interface. Themeasured work function for Ag deposited on DiMe-PTCDIis close to the value corresponding to a single Ag�110� crys-tal ��4.52 eV� which is above of the work function for poly-crystalline films of Ag ��4.25 eV�.18 However Ag films de-posited on DiMe-PTCDI showed a weak preferentialorientation along the �111� and �200� directions.16 This isprobably the reason for the measured work function of Ag onDiMe-PTCDI, since �Ag�111��4.75 eV, and the weak prefer-ential orientation increases the work function above the poly-crystalline value expected from the morphology of Ag grownon DiMe-PTCDI.

B. Energy level alignment and charge injection

Since the photoemission spectroscopy technique only pro-vides information about occupied states, the position of theconduction band minimum is deduced from the known valueof the energy bandgap of GaAs at room temperature�1.42 eV�. For completing the picture of the energy levelconfiguration at the organic/inorganic interface, the lowestunoccupied molecular orbital �LUMO� position of DiMe-PTCDI, which is thought to be the electron transport level inorganic semiconductors, must be deduced. Unfortunately, themethodology applied to the case of GaAs, an inorganic semi-conductor, does not hold for the case of organic semiconduc-tors. The difficulty arises from the big discrepancy betweenthe values of the optical and the transport bandgap, whichcan differ by as much as 1 eV.19 For the prototype perylene-derivative PTCDA for instance, the exciton binding energy isestimated to be around 0.6 eV.20

One indirect way to estimate the LUMO position is basedon previous observations made on inorganic semiconductorjunctions, where interface dipoles could be ascribed to thedifference in electron affinities between the two semiconduc-

FIG. 3. �Color online� Valence band spectrafor several depositions of silver on a 20 nm thinfilm of DiMe-PTCDI.

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tors in contact.21 By analogy, a strong correlation betweenthe interface dipole and the relative energy position ofPTCDA LUMO and CBM of GaAs, and thereby, the electronaffinity of the organic layer ��PTCDA� was found.22 The elec-tron affinity of the GaAs surface can be swept in a relativelybroad range ��1.5 eV�, depending on the surface treatment.The lowest GaAs electron affinity ��3.5 eV� was found dur-ing the development of the present work for hydrogenplasma treated GaAs �H+GaAs�, whereas the highest value��5.0 eV� corresponds to a selenium passivated GaAs sur-face �Se-GaAs�.23 The same correlation between the dipoleand the electron affinity of the materials at the DiMe-PTCDI/GaAs interface was intended and again a linear dependencewas found. The respective interface dipoles formed atPTCDA/GaAs and DiMe-PTCDI/GaAs interfaces as a func-tion of the GaAs electron affinity �s are indicated by blueand green dots in Fig. 4.

The interface dipole formed at both organic/GaAs�100�interfaces present an almost linear dependence on theGaAs�100� electron affinity �GaAs. Therefore the relationshipbetween the interface dipole and �GaAs is given by

= − a�GaAs + b , �1�

where it is explicitly described the negative slope. It can berearranged in the form

= − a��GaAs − �org� , �2�

which express the difference in electron affinities betweenthe organic and inorganic materials as the predominantmechanism for the interface dipole formation.

Using a linear fit the same slope value of a=0.67 wasfound for both organic materials. The interface dipole at thePTCDA/GaAs�100� interface is found to be zero at�GaAs�100�= �4.12±0.10� eV. This value also represents�PTCDA, assuming that the formation of the interface dipole isdriven by the difference or electron affinities between theorganic layer and the inorganic semiconductor. For DiMe-PTCDI an electron affinity value of �DiMe-PTCDI= �3.83±0.10� eV is obtained. All the considerations exposedabove for deducing the energy level alignment through thewhole metal / organic / inorganic heterostructure are depictedin Fig. 5.

According to the energy diagram DiMe-PTCDI has anelectron bandgap of �2.58±0.15� eV, in agreement with thevalue �2.64±0.15� eV deduced by combining PES and IPESmeasurements on DiMe-PTCDI layers deposited on sulphurpassivated GaAs.11 Such a value is very close to the onededuced for its homologous PTCDA ��2.55 eV�.16 By com-parison with the previously reported optical bandgap of bothmaterials �EDiMe-PTCDI

opt =2.14 eV, EPTCDAopt =2.22 eV� obtained

by absorption measurements.24 The exciton bindingenergies are EDiMe-PTCDI

exc = �0.44±0.15� eV and EPTCDAopt

= �0.33±0.15� eV, respectively.The organic transport level position �LUMO� of DiMe-

PTCDI is located below CBM of H+GaAs. It means that theorganic layer does not introduce any additional barrier forelectron transport compared to the reference Ag/H+GaAs di-ode. Results of I-V measurements performed on DiMe-PTCDI modified H+GaAs devices �Fig. 6� confirm this sup-position, since evaluation of the I-V measurements in therange 0.1−0.4 V using the thermionic emission theory re-

FIG. 4. �Color online� Interface dipole formed between theperylene derivatives PTCDA and DiMe-PTCDI, respectively, as afunction of the electron affinity of the GaAs substrate.

FIG. 5. �Color online� Energy level alignment at the Ag/DiMe-PTCDI/H+GaAs�100� heterostructure with an electron affinity ofDiMe-PTCDI��3.83 eV� as deduced from the experimental PESdata.

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sults in an effective barrier height �eff of �0.86±0.03�eV forall the diodes.25 The image force lowering effect due toGaAs ��GaAs�13.1�0� is estimated to be�120 meV andhence, the barrier height under equilibrium conditions isclose to 1 eV. By comparison with Fig. 5, this result suggestthat the contribution of the interface dipoles should be takeninto account for the determination of the barrier height fromPES measurements.

It was previously reported a study of PTCDA modifiedAg/H+GaAs diodes where a reduction of the effective barrierheight as a function of the PTCDA thickness was observed.26

The barrier lowering amounts to 180 meV for organic layerthickness up to 60 nm and was attributed to the image forceeffect due to the low value of the dielectric permitivity�DiMe-PTCDI�2�0.27 Such a reduction of the barrier was notobserved for the presented case of DiMe-PTCDI modified

diodes probably due to the morphology of the organic layer.A rough film induces voids through the organic layer thatallows direct contact between Ag and H+GaAs for very thinlayers, and thereby the influence of the organic layer in theimage force effect is strongly reduced.

C. Morphology of the organic thin films on H+GaAssubstrates

Figure 7 shows the morphology of DiMe-PTCDI layersdeposited on H+GaAs for thicknesses of 8 nm, 15 nm, and60 nm. From a macroscopic point of view, all three samplesare basically homogeneous as can be observed in the 4 �m�4 �m scan area of Figs. 7�a�, 7�c�, and 7�e�. Moreover, thezoom shown in the scanned 1 �m�1 �m area shown onFigs. 7�b�, 7�d�, and 7�f� allows us to observe the formationof large crystals ��200 nm for all thicknesses�. Followingthe thickness evolution of the samples, an island-like growthmode is apparent, since the grain size does not increase sig-nificantly as the film becomes thicker. The initial ribbonlikeshape for the crystals �at 8 nm thickness� is gradually substi-tuted by a spherical shape of slightly higher grain size �at60 nm thickness�, making the film more densely packed.

For the lowest thickness �8 nm� it is still possible to ob-serve uncovered zones attributed to the H+GaAs substrate.These characteristics coincide with the growth mode ofDiMe-PTCDI on sulphur passivated GaAs �S-GaAs�.24

Moreover, the incomplete coverage for thickness lower than8 nm, is in agreement with the experimental evidence sug-gested independently by photoemission, I-V and, as shall beshown next, by C-V measurements. The subsequent forma-tion of large crystals prevents the formation of voids along

FIG. 6. �Color online� I-V characteristics of Ag/DiMe-PTCDI/H+GaAs�100� for several thicknesses of the organic layer.

FIG. 7. �Color online� Morphology of three samples with different thicknesses of DiMe-PTCDI layers deposited on H+GaAs.

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the transport path for thickness higher than 15 nm and re-duces the influence of the H+GaAs substrate on the electricresponse of the organic modified device.

Capacitance-voltage �C-V� for the series of DiMe-PTCDImodified H+GaAs�100� Schottky diodes were measuredwhile varying the thickness of the organic layer �Fig. 8�.Taking the measured capacitance of Ag/H+GaAs�100�Schottky diodes as a reference, an increase of the capacitancefor DiMe-PTCDI organic layer thicknesses below 8 nm isvisible. For additional coverage the capacitance is reduced,becoming lower than the corresponding one to theAg/H+GaAs reference diode. In addition, a gradual decreasein the curvature of the C-V characteristics ��C /�V� is ob-served for thick layers ��8 nm� of DiMe-PTCDI modifieddiodes.

The decrease in the curvature ��C /�V� suggest that as theorganic film becomes thicker and more compact behaves likean insulator with a capacitance almost independent of thevoltage. The electron bandgap value for DiMe-PTCDI��2.6 eV� deduced from the combined analysis of PES,IPES and I-V measurements predicts an insulatorlike behav-ior of the organic layer. Hence, in the case of an ideal insu-lating organic layer �i.e., densely packed, without voids andtrap states� the capacitance of DiMe-PTCDI should be closeto the values of a geometrical capacitance Corg

Corg =�org

L�3�

with a dielectric permitivity �org and thickness L.Due to the capacitance of the organic layer behaving like

a geometrical one, a general failure in the classical theoreti-cal expressions derived for the behavior the capacitance of aSchottky barrier under reverse bias is expected. Indeed, aplot of C−2 as a function of the applied reverse bias does notresult in a linear behavior of the experimental data and pro-vide unrealistic values of the free carrier density in the deple-tion region and for the barrier height as well.

Based on the previous indications of an incomplete cov-erage of H+GaAs�100� by the organic DiMe-PTCDI layer atnominal thickness of the organic layer below 8 nm a simplemodel for the interpretation of C-V characteristics of the or-ganic modified diodes was created. The model assumes an

islandlike growth mode since the first monolayer of DiMe-PTCDI deposition on H+GaAs, and also that there is someinteraction between the metal/DiMe-PTCDI/H+GaAs�100�heterostructure able to change the effective built-in potentialthrough both interfaces. In other words, the built-in voltagein the H+GaAs�100� interfaced with DiMe-PTCDI is modi-fied with respect to the measured value of the referencebuilt-in potential of Ag/H+GaAs�100�, which amounts toVb0=1.14 eV.

Following the physical arguments exposed above, deposi-tion of the organic layer starts to cover the H+GaAs�100�surface while forming islands. As sketched in Fig. 9�a�, thetotal contribution of DiMe-PTCDI islands formed on theH+GaAs surface is represented by a uniform layer of theorganic material, with an area Ax and “effective thickness” x.The distinction between the effective thickness x and the“nominal thickness” L emerges from the assumption of anislandlike growth. For correlating the respective quantities Land x, special attention is paid to the fact that the quartzmicrobalance shift basically depends on the mass of the or-ganic layer �morg� deposited on the vibrating crystal. Thus,the desired relationship between the nominal thickness L andthe effective thickness x, is expressed by

morg = �orgAL = �orgAxx , �4�

where A refers to the diode area �i.e., the area of metallicsilver dots deposited on the DiMe-PTCDI�, Ax is the fractionof total diode area effectively modified by the organic mate-rial and �org is the specific mass of the organic material.

The situation is illustrated in the proposed equivalent cir-cuit shown in Fig. 9�b�. The left branch corresponds to theuncovered surface of the organic-modified diode. In otherwords, it is the contribution to the capacitance due to theAg/H+GaAs reference diode with a reduced effective area�A−Ax�. On the other hand, the right branch consists of aserial arrangement of the organic layer capacitance and theinorganic substrate capacitance. The effective area of thisserial arrangement is Ax �being Ax�A�.

FIG. 8. �Color online� C-V characteristics of DiMe-PTCDImodified diodes as a function of the organic layer thickness. Themeasurements were taken at a frequency of 1 MHz.

FIG. 9. �Color online� �a� Model for the interpretation of theC-V characteristics of DiMe-PTCDI organic modified Schottky di-odes. �b� Equivalent circuit proposed for the interpretation of C-Vmeasurements of DiMe-PTCDI modified H+GaAs diodes at a fixedac frequency.

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For the left branch, the expression for the capacitance isgiven by that corresponding to a Schottky barrier under re-verse bias with a reduced area25

CG = � e�GN

21/2 �A − Ax�

�Vb0 + V�1/2 , �5�

where N is the free carrier density, which at room tempera-ture is nearly the dopant density. The equivalent capacitanceof the right branch is a series combination of the organiclayer capacitance Corg and the capacitance of coveredH+GaAs surface Cx. At zero bias, the value of Cx is definedby changes in the free carrier density N, the covered area Axand the built-in potential Vbi. However, the absence of stronginteraction between the organic film and the substrate ex-cludes the possibility of significant changes in the free carrierdensity. Then, the capacitance of the covered H+GaAs sur-face is given by

Cx = � e�GN

21/2 Ax

�Vbi + V�1/2 . �6�

For the organic layer, the capacitance behaves nearly like ageometrical one

Corg =�orgAx

x, �7�

x being the effective thickness of the organic film. Theequivalent capacitance C of the circuit shown in Fig. 9�b�leads to the expression

C = CG +CorgCx

Corg + Cx. �8�

Substitution of �4�–�7� into �8� results in the expression �9�that reflects the change in capacitance after DiMe-PTCDImodification of the reference Schottky diode �C−CAg/H+GaAs= C�

C = � e�GN

21/2�Ax

A��Vbi + V�1/2 +

� e�GN

21/2

L

�org�Ax

A �

−1

− �Vb0 + V�−1/2 . �9�

The left hand term within square brackets belongs to thecapacitance of the organic modified diode while the righthand term is related to the capacitance of the reference diode.The unknown parameters are the covered area Ax, the built-inpotential of H+GaAs modified by the organic layer Vbi andthe electrical permitivity of the organic layer �org. Becausethere are too many unknown quantities, it is imperative toestimate them from at least three experimental curves. Acurve fitting of the set of C-V characteristics for six differentthicknesses of the DiMe-PTCDI layer should provide suffi-cient information.

In Fig. 10 the change in capacitance of a DiMe-PTCDImodified diode with respect to the capacitance of the un-modified Ag/H+GaAs diode is shown. The experimentaldata �represented by dots� are based on the measured ones,already shown in Fig. 8 and obtained by direct subtraction.Based on �9� with a constant value of �org for the whole set,a fit was performed for each thickness. The final results areplotted as solid lines in Fig. 10 and the desired parametersare reviewed in Table I. For the first two depositions of theorganic layer �2.0 and 4.0 nm of nominal thickness� there isa close agreement with the estimated ones �2.0 and 3.6 nm,respectively�. But the most important result is the indicationof incomplete coverage of the H+GaAs surface by the or-ganic layer for DiMe-PTCDI nominal thicknesses below8 nm, in agreement with the results of photoemission, a find-ing in support of the islandlike growth mode. Moreover, thethicknesses of compact layers obtained by fitting the C-Vplots are in all the cases lower than the measured ones, indi-cating a porous morphology �voids� of the organic layer. Therelatively big deviation of the two thicknesses for thick or-ganic films may be due to the model ignores the possibilityof potential drop across the organic thin film. Even changesin �org with increasing thickness and departures from theassumed voltage-independent capacitance of the organiclayer cannot be excluded.

The surface potential Vbi of GaAs in the presence of theorganic layer has an average value of �0.75±0.03� eV. Com-pared to the built-in potential for the reference diode�1.14±0.05� eV there is an effective reduction by

FIG. 10. �Color online� Change in the capacitance of the refer-ence diode induced by DiMe-PTCDI modification of the Schottkydiodes.

TABLE I. Physical parameters obtained after fitting the C-Vcharacteristics of DiMe-PTCDI modified diodes for several thick-ness of the organic layer.

Lexp /nm Coverage/% Vbi / eV �org/�0 Lfit /nm

2.0 53 0.75 2.5 2.0

4.0 85 0.72 2.5 3.6

8.0 100 0.78 2.5 13.0

15.0 100 0.77 2.5 20.0

30.0 100 0.75 2.5 25.1

60.0 100 0.75 2.5 31.4

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�0.39±0.05� eV in the surface potential for DiMe-PTCDImodified H+GaAs diodes. According to this result, probablythe reduction in the surface potential is induced by the pres-ence of the dipoles at both interfaces, which amounts to�0.40±0.15� eV.

D. Frequency response of Ag/DiMe-PTCDI/H+GaAs

In order to explore the frequency dependence of DiMe-PTCDI modified H+GaAs diodes, impedance spectroscopymeasurements in the range 1 kHz–1 MHz were performedin situ. The real and imaginary parts of the impedance aredisplayed in Fig. 11. No significant changes are observed forthe imaginary part, since the big resistance of the depletionlayer makes the ac current to flow through the equivalentcapacitor. Hence, the imaginary part of the impedance isgiven by the expression

Zim = 1/�Ceq. �10�

The equivalent capacitor Ceq for organic modified diodesconsist of an arrangement of the capacitance due to the or-ganic layer C� and the depletion region of H+GaAs CG. Thisconfiguration was already measured for a frequency of1 MHz �Fig. 8�. For a reverse bias of 0.1 V the capacitancevaries between 3.0�10−8 F cm−2 and 4.3�10−8 F cm−2.Thus, the expected variation in the imaginary component ofthe impedance for this frequency amounts to

�Zr =�Ceq

�Ceq2 =

1.3 � 10−8 F cm−2

106s−1�3 � 10−8 F cm−2�2 � 14 �11�

which is within the intrinsic error of the measurement tech-nique.

More significant are the observed changes in the realcomponent of the impedance. For modeling, an RC parallelcircuit composed by a resistor R� and a capacitor C� is as-sembled in a series configuration with the reference diode, assketched in Fig. 12. For fitting the reference diode it wasnecessary to use a combination of two RC circuits: one rep-resenting the depletion region of the semiconductor and the

FIG. 11. �Color online� �a� Real �Zr� and �b� imaginary �Zim�parts of the impedance for organic-modified diodes as a function ofthe organic layer thickness. The measured range was between1 KHz and 1 MHz.

FIG. 12. �Color online� Equivalent circuit for interpretation ofimpedance measurements on Ag/DiMe-PTCDI/H+GaAs diodes.

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second one taking into account the contribution of the backcontact. The depletion region fit resulted in a resistance RG=80 M and capacitance CG=1.3�10−10 F, while the bulkresistance of H+GaAs is estimated in 25 . On the otherhand, the back contact contributes with a resistance RX=120 and capacitance CX=6.3�10−9 F, attributed to theIndium/H+GaAs back contact with an effective radius of2.3 mm.

The influence of DiMe-PTCDI on organic-modified di-odes, as a function of the thickness can be observed on Fig.13. For thickness up to 8 nm, the same parameter valuesused for fitting H+GaAs and the back contact are kept, withinthe expected reasonable variations of the back contact fromone sample to another �RX=80–120 , CX=6.3–7.1�10−9 F�. An additional contribution comes into accountand it is illustrated in the DiMe-PTCDI organic modifieddiodes. At first glance this contribution to the impedancestems from the organic layer.

A careful examination of the fit parameters leading toR�=800 , C�=1.7–1.1�10−8 F values allows to concludethat it is a reasonable value for the capacitance only if theassumption of an almost perfect geometric capacitor inducedfrom the organic layer is considered as a gross estimate.Indeed, the geometric approach of the organic layer capaci-tance depends exclusively of the thickness, since the analysisof C-V measurements led to a dielectric permitivity value�2.5 �0 for DiMe-PTCDI. Thus, the capacitance for a 2 nmdeposited DiMe-PTCDI layer with a circular diode of half

millimetre radius, under the geometrical approach, results ina value of

C =�sA

L�

2.5 · 8.85 � 10−14 F cm−2��5 � 10−2 cm�2

2 � 10−7 cm

� 8.5 � 10−9 F �12�

which is near to what in principle is expected. However, thediscrepancy becomes larger with increasing thickness be-cause the capacitance should be reduced. Instead, an increaseof the capacitance up to C�=1.3�10−7 F was estimated for afilm of 60 nm of DiMe-PTCDI. At the same time a resistanceof 800 is excessive for a nonhomogeneous deposited or-ganic layer of 4 nm thickness. Both factors point to the pos-sibility of trap sites as the origin of this relaxation, an issueexplored by QTS in the next section. The contribution ofsuch electron traps then are represented for a frequency de-pendent resistor R��� with the form

R��� =R�

1 + ��/�0�2 �13�

with a resonance frequency f0�1.2–1.8�104 Hz. If thosetraps were also present in the reference diode they wouldresult in a value of R�=330 which is much smaller thanthe ones obtained after deposition of the organic layer, asshown in Table II. Therefore, they would have a negligiblecontribution to the impedance. Thus, this frequency depen-

FIG. 13. �Color online� Fitting of the real partof impedance measured on DiMe-PTCDI modi-fied diodes as a function of the organic layerthickness.

TABLE II. Fit parameters for impedance measurements of DiMe-PTCDI modified H+GaAs diodes.

Thickness/nm 0 4 8 15 30 60

R� / 330 800 800 1000 1500 1500

r / 25 28 28 115 120 340

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dent resistor is due entirely to the presence of the organiclayer.

In the same table the estimated values for the bulk resis-tance r are also presented, which contain contributions fromthe H+GaAs bulk, the DiMe-PTCDI layer and probably theback contact. Under the supposition of having a good backcontact with a resistance lower than 15 for the 60 nmthickness sample, and subtracting the contribution ofH+GaAs, a resistance of the organic layer close to 300 would be acceptable. In such case, the resistivity of uniformDiMe-PTCDI organic layers would be

� =RA

L=

300��4 � 10−2 cm�2

60 � 10−7 cm= 2.5 � 105 cm.

�14�

If all the carriers are supplied by H+GaAs with a density nearto the doping level n�1�1016 cm−3, the resulting mobilityis

� = �en��−1 = �1.6 � 10−19c1 � 1016 cm−32.5 � 105 cm�−1

= 2.5 � 10−3 cm2 V−1 s−1 �15�

which is a typical value for solid organic films.

E. In situ charge transient spectroscopy on Ag/DiMe-PTCDI/H+GaAs diodes

Several isothermal QTS spectra for DiMe-PTCDI modi-fied diodes with six different thickness of the organic layer�from 0 to 60 nm� were recorded at selected temperatures ina range of 200 K–300 K. Figure 14 shows the QTS spectrafor thin organic layers ��8 nm� taken at T=238 K and T=295 K, respectively. All the spectra were measured by ap-plying a positive pulse of +0.4 V without dc bias.

As a general trend, the intensity of the peaks decreaseswith increasing organic layer thickness. A fit with Gaussianfunctions of every spectrum registered at each temperaturewas carried out according to the procedure described in aprevious work.28 The A, B, and C processes with an activa-tion energy of 0.42 eV attributed to the electron level EC3 inGaAs are still detected,29 but their intensity decreases withincreasing DiMe-PTCDI thickness.

Reduction of the intensity of trap levels by deposition ofthe organic layer can be explained either by effective reduc-tion of the trap density, or simply by reduction in the amountof carriers emitted from H+GaAs due to scattering by theDiMe-PTCDI layer. Taking into account the incomplete cov-erage and the islandlike growth mode of the DiMe-PTCDIlayer, the former possibility agrees quite well with the sharp-ening of Ga 3d and As 3d core levels reported by PES on thesame organic layer deposited on sulphur passivated GaAssubstrates.22

The latter case points out to the possibility of an “slowingdown” of the characteristic emission time of the carriers dueto the presence of the organic layer. Thus, part of the emittedcharge carriers are scattered by the organic layer and its char-acteristic time response becomes slower and hence the peakD detected at lower time responses increases in intensity. Inaddition, a small broad peak of variance 0.6 located below10−2 s is present in several samples. This peak is attributed tocontributions from the back contact since they can be sepa-rated because it is not thermally activated and is present atfluctuating intensity from one sample to another.

At room temperature the characteristic H+GaAs featuresE and F, with an activation energy of 0.89 eV assigned toEL2 electron level, decrease with increasing thickness andfollow the same trend observed at low temperatures. How-ever, after 4 nm of DiMe-PTCDI coverage, a new feature Gwith a characteristic variance of 0.36 and activation energyof 0.72 eV appears. This is associated to the required energyfor electrons in the organic layer for reaching the metallicelectrode. The process includes the energy for charge carriers�0.2 eV� needed to overpass the opposite electric field at theorganic/metal interface. The feature becomes dominant foradditional coverage of the organic film as depicted on theright part of Fig. 15, while all the features originated fromH+GaAs are totally screened at room temperature after depo-sition of 15 nm thick film of DiMe-PTCDI. This led to con-clude that this relaxation is intrinsic to the organic layerprobably attributed to a trap level at the metal/organic inter-face.

For low temperatures the screening of H+GaAs feature isaccompanied by emergent broad peaks of variance 0.5 and asmall activation energy between 17 and 24 meV, which most

FIG. 14. �Color online� Fitted isothermal QTSspectra for DiMe-PTCDI thickness below 8 nmon H+GaAs substrates.

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probably are associated with disorder in the organic layer.Following the interpretation suggested by Bässler thesewould be the typical activation energies necessary for hop-ping transport processes in disordered organic solids.30

F. Analysis of the I-V characteristics of Ag/DiMe-PTCDI/H+GaAs diodes

For discussing the issue related to transport mechanismsin the hybrid heterostructure a fitting the I-V characteristicsof the device was performed �Fig. 16�. For the referencediode it was enough to consider a device were part of thevoltage is dropped through the depletion region and the re-maining one through the bulk series resistance. In order tohave an analytical expression for the voltage droppedthrough the depletion region of H+GaAs, the thermionicemission theory states that for voltages greater than V�3kT /e the current density J is given by

J = Js exp�eV/nkT� �16�

with Js being the saturation current, e the elementary charge,k the Boltzmann’s constant, and n the ideality factor. So, theAg/H+GaAs diode has an ideality factor of n=1.06±0.02and a series resistance of 5 . For the thinnest organic layer�4 nm� the same series resistance was obtained but a slight

increase in the ideality factor of n=1.09±0.02 was regis-tered. However, the model is too simple for fitting I-V curvescorresponding to thicker organic layers. A slightly morecomplex model includes an space charge limited current re-gime �SCLC� which obeys to the relationship

JSCLC =9

8�s�

V2

d3 . �17�

Keeping the series resistance of 5 for H+GaAs, the as-sumption of a thermionic emission injection process resultsin an increased ideality factor value of n=1.11±0.02. Addi-tionally, the estimated DiMe-PTCDI mobility was �=4�10−4 cm2 V−1 s−1, a value lower than the one deduced fromimpedance measurements. But the supposition of SCLC,proves to be not suitable for fitting I-V characteristics ofthicker organic layers. It constitutes a particular case of themost general situation of a trap charge limited currentregime.31 For this reason, expressions describing the TCLCregime

JTCLC = NC�e� �sl

eNt�l + 1��l�2l + 1

l + 1l+1 Vl+1

d2l+1 �18�

with l=Et /kT, were employed for organic layers thicker than15 nm. Table III resumes the fitting parameters. The results

FIG. 15. �Color online� Fitted isothermal QTSspectra for DiMe-PTCDI thicknesses �8 nm onH+GaAs substrates.

FIG. 16. �Color online� Best fitting results ofthe I-V characteristics for DiMe-PTCDI modifiedH+GaAs diodes.

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highlight the presence of a high density shallow trap, with anactivation energy varying between 15 and 24 meV. Thesevalues agree with those calculated from isothermal QTSmeasurements on the same layers, attributed to disorder onthe organic layer. Also, the mobility presents a closer agree-ment with the one extracted from the impedance analysis.

As it could be appreciated in Fig. 16 the fit using rela-tively simple expressions is not quite perfect. The main rea-son is that many conditions involved in deducing this singleequation are not fulfilled. Neglect of carrier diffusion, aninfinitely high carrier density at the contact and inadequateknowledge of the energy distribution and density of trapstates are counted as possible reasons. They had to be intro-duced at some extent arbitrarily, in order to have simple ana-lytical solutions for cases involving contributions from traplevels. Nevertheless the complexity of the device, this ap-proach used for the analysis of I-V curves points to the keyrole played by trap states in the transport mechanisms ofdevices based on organic layers. Moreover, by analogy withproperties of inorganic semiconductors, it allows a rough es-timation of important parameters for assessing the perfor-mance of operating devices.

IV. SUMMARY

The present work is an attempt to understand the influ-ence of organic layers for modifying the electronic properties

of traditional metal/semiconductor Schottky contacts. Withthis goal a metal/organic/inorganic semiconductor hetero-junction was investigated in situ through combined PES andelectrical measurements. Carrier injection is determined bythe energy the energy level alignment through the hetero-structure, which was assessed via PES measurements. In thissense, the dipole at the organic/inorganic interface seems tobe driven by the difference in electron affinities between thesemiconductor materials. Such observation provides a toolfor assessing the LUMO position in the organic semiconduc-tor, which qualitatively agrees with the I-V measurements. Inaddition, C-V and impedance spectroscopy allowed evalua-tion of the electrical behavior of the device in a narrow fre-quency range. A general increase in the value of the real partof the impedance after being modified by deposition of theDiMe-PTCDI organic layer was found. The increase is spe-cially remarkable at higher frequencies, opening the possibil-ity of increasing the operation range of GaAs-based rectify-ing diodes by deposition of organic layers. This processwould reduce leakage currents on GaAs at high frequencies.However this possibility depends strongly on how the or-ganic layers can influence other properties of GaAs, i.e., itsintrinsic defect density, mobility, etc. Finally, QTS measure-ments performed on the heterostructure showed evidence oftrap states and disorder in the organic layer. Those processeswith small activation energies proved to be determinant forfitting the I-V characteristics of DiMe-PTCDI organic modi-fied diodes using the expressions of a trapped charge limitedcurrent regime �TCLC�. Such a model was the best analyticalapproach found for fitting the I-V response. Further improv-ing probably will involve implementation of numerical cal-culations or additional considerations in the physics of thedevice.

ACKNOWLEDGMENT

The authors want to acknowledge to H. Kupfer in TU-Chemnitz for the AFM measurements.

*Author to whom correspondence should be addressed. Email ad-dress: h.mendez@physik.tu-chemnitz.de

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TABLE III. Parameters for fitting I-V characteristics of DiMe-PTCDI modified diodes under the supposition of a dominant trapcharge limited current regime �TCLC�.

Thickness/nm NC / cm−3 � / cm2 V−1 s−1 l Nt / cm−3

15 3�1016 7�10−4 0.75 1�1016

30 3�1016 7�10−4 0.65 1�1016

60 4�1016 7�10−4 0.90 1�1016

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