Integrating organic molecules with solid-state electronicdevices is a topic of great current interest.1–3 By combiningthe electronic properties of semiconductors with the chemi-cal, biological, and molecular electronic versatility of or-ganic materials, a variety of novel devices can be realized.This approach may enable significant advances in chemicaland biological sensing, electronic devices, energy conver-sion, and a variety of other applications. Although well-developed theories exist to describe the physics of electronicdevices and molecular materials independently, the elec-tronic properties of hybrid devices are poorly understood. Todescribe such structures in a meaningful, physically realisticmanner, theoretical and analytical techniques must beadapted to address their unique properties.
One class of hybrid device that has been extensivelystudied is the molecularly modified Schottky diode.4–8 Suchdevices consist of a molecular layer bound to a moderatelydoped semiconductor with a metal contact. There is an offsetbetween the metal work function and the semiconductorelectron affinity, yielding a substantial Schottky barrier and adepletion region in the semiconductor at zero bias; theseelectrostatic properties result in devices exhibiting classicSchottky diodelike behavior.9–11 In such devices, the molecu-lar layer modulates electronic properties due both to molecu-
lar electronic effects and to it’s electrostatic influence on thesubstrate.12,13 It has been demonstrated that dipolar molecu-lar layers deposited on semiconductor surfaces can modulatethe surface potential in a controlled manner.4 The influenceof dipolar molecular layers on metal/molecule/ semiconductor MMS device electrostatics is considerablymore complicated. Metal-molecule interactions are oftencritically important in determining device behavior.14 It hasbeen found that both the magnitude and direction of Schottky
barrier height modification can be influenced by suchinteractions.14,15 In addition to changing the device electro-statics, the molecular layers can modify junction propertiessuch as semiconductor surface state densities and distribu-tions, which can in turn influence the measured device char-acteristics. In most studies, the devices are analyzed usingstandard experimental methods for relatively ideal Schottky-barrier diodes;16,17 however additional physical insight canbe gained from more detailed theoretical consideration of the
junction properties.In this study, we present complementary capacitance ex-
periments and calculations to explore various physical pro-cesses that can occur in MMS devices. Gold/molecule/ n-type
silicon MMS diodes with a series of substituted aryl molecu-lar layers were fabricated and capacitance-voltage CV mea-surements were performed. The devices were analyzed usingstandard methods for Schottky diodes. Calculations were de-veloped to determine device electrostatics and resulting CV
characteristics including molecular capacitive effects, mo-lecular charge density, and Si surface states. Experimentally,it is found that the addition of a dipolar layer induces modestchanges in the apparent Schottky barrier height. By compar-ing the experimental results and calculations, it is possible todetermine the physical effects responsible for the observedmodulation in device behavior. Silicon surface states, mo-lecular capacitive effects, molecular charge density, and
metal-molecule interactions are explored to determine theirinfluence on experimentally determined Schottky barrierheights.
II. EXPERIMENTAL
MMS diodes, as shown schematically in Fig. 1, werefabricated. 111-orientation n-type P-doped, N d =31015 cm−3 Si wafers were cleaned, hydrogen terminated,and functionalized with the substituted aryl molecular spe-cies shown in Fig. 1. Details of surface modification andaElectronic mail: [email protected].
experimental trends are qualitatively similar. From a linearfit, the theoretical prediction for 2 Å top spacing has a slopeof 0.3, whereas the 3 Å top spacing calculation has a slope of 0.1. The experimental results are more consistent with thecalculations for devices with fairly intimate contact betweenthe metal and the molecular headgroup, indicating that sig-nificant screening of molecular charge takes place. Surfacepotential modulation of Si with the same molecular modifiers
has been studied previously using photovoltage measure-ments. The as-deposited molecular layers were found to in-duce surface potential changes of 25 mV /D when thesample was in an electrolyte environment.36 Collectively,these observations suggest that molecular dipole-inducedsurface potential modulation is present; however there is par-tial screening of this effect due to the presence of the goldcontact.
As shown in Fig. 10, the presence of high surface-statedensities in such devices can affect both the Schottky barrierheight and the validity of the barrier height inferred fromcapacitance measurements. In MMS devices, the molecularlayers generally cannot occupy every site on the semicon-
ductor surface for steric reasons; however the surfaces stillshow remarkable chemical stability.37 It is possible that ad-ditional electronic states exist at the surface due to the re-maining hydrogen-terminated surface sites, oxidation of thesilicon surface, and defects in the molecular monolayer. Priorspectroscopic studies of these molecular layers indicate thatsome Si backbond oxidation occurs and the extent of thisoxidation is dependent on the molecular substituent.18,20 Thegood agreement between the nominal doping density and theexperimentally determined doping density as well as the lowideality factors from the IV measurements suggest that theelectronic surface state densities in these devices are quitelow. It appears that the molecular layers have a stabilizingeffect on the surface even though they do not passivate everydangling bond. Low surface state densities ranging from 3109 to 31011 cm−2 have been reported for alkyl-terminated Si 111 surfaces.38,39 Several other mechanismsfor charge storage within the junction can occur, includingmolecular40 and metal41 effects. These effects, if interpretedincorrectly, can lead to errors in the inferred barrier heightfrom capacitance measurements. The determination of physi-cally meaningful Schottky barrier heights from capacitancemeasurements requires that the devices have low interfacestate densities, as is the case in this study, or that appropriateanalysis is used to correct for this effect.
The findings and methods presented here can be gener-alized to a variety of organic molecule-functionalized semi-conductor surfaces. In particular, sensing of chemical andbiological materials using charge-based solid-state ap-proaches is a topic of great current interest. Although a va-riety of devices have been experimentally demonstrated, theobserved changes in semiconductor surface potential havegenerally been small. The importance of top contact screen-ing effects, either due to a metal or ions in solution, canexplain the relatively modest responses realized to date.42
The methodology presented here for calculating expectedchanges in semiconductor surface potential can be applied tothis class of problems through the use of appropriate molecu-
lar charge densities and top metal contacts or counterions.The results suggest that improved electrostatic control oversemiconductor surfaces in MMS devices can be achieved byreducing the interaction between the molecular charge andthe metal, perhaps by adding a very thin insulating layer orinsulating molecular functional group adjacent to the metal.
V. CONCLUSION
A series of gold-molecule-silicon diodes with dipolarmolecular layers have been fabricated and characterized us-ing CV measurements. Complementary calculations wereperformed to determine the effects of molecular charge den-sity, Si surface states, and metal-molecule interaction effectson the overall device capacitive properties. By correlatingexperimental results with calculations, it was determined thatSi surface state densities in these structures are sufficientlylow that they do not affect the capacitance characterization.Experimentally observed trends in Schottky barrier heightwere consistent with calculated trends in Si surface potential,indicating that the charge distribution within the molecule is
modulating the device characteristics. The modest magnitudeof this effect is consistent with calculations in which the goldtop contact is relatively close to the top atom of the molecu-lar layer, indicating that screening of molecular charge due tothe top metal contact is an important effect in such devices.
ACKNOWLEDGMENTS
The authors would like to thank Avik Ghosh and SmithaVasudevan for helpful discussion. This work is supported byNSF Grant No. ECE0506802, NASA-URETI Grant No.NCC3-1363, DoD MURI program, and the Office of NavalResearch. A.S. was supported by a NSF Graduate Research
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