a. Band-Offset At Solid Interfaces

     As the demand for more and better functionalities in electronic and photonic devices continues to rise, the search for better materials and better structures also intensifies and expands in its scope. A general trend for the development of future electronic devices has been to integrate an astronomical number of minuscule units together, each consisting of a large variety of materials. When materials with widely different atomic structures and electronic structures are tightly integrated into a small dimension, the control of the electrical integrity of the conglomerate becomes a formidable issue and is of paramount importance to the overall performance of the device. Carrier transport across solid interfaces is governed by a parameter generally referred to as the “band-offset” of that interface. For specific material interfaces, this parameter is also known as the “Schottky barrier height”, the “molecular level alignment”, etc.1, 2, 3 Understanding the formation mechanism and being able to control the magnitude of this band-offset are prerequisite to advancing future device structures to higher functionality and smaller physical dimensions. Unfortunately, the formation mechanism of band-offset at even the traditional inorganic semiconductor interfaces has yet to reach a level of understanding that garners wide acceptance. And with that state of the matter, our abilities to vary or “tune” the barrier height at specific interfaces range from modest to none. Great strides need to be quickly made in the science and technology of the “band-offset” to ensure that the rapid pace of the technology boom that we are presently enjoying can continue.

     The basic diagram to analyze the energy level alignment is shown in Fig. 1 for a metal-organic interface. The same analysis and diagram are also valid for band-offsets at essentially all other kinds of solid interfaces. One expects the bulk band structure of either the metal or the organic material to be found in the interior of either material (at the left- and right-hand sides of Fig. 1). There exists an “interface specific region (ISR)” with a width of a few atomic/molecular spacings (1-2nm), whose electronic structure is unlike that of either of the bulk materials. The electronic structure of this region is highly system-specific. However, simply from general principles, it is clear that the electrostatic potential has to be continuous across the ISR and the electronic structure of the ISR is such that it minimizes the interface energy. The offset of the highest occupied molecular orbital (HOMO) for this interface, F^o_homo, which is the energy difference between the HOMO state on the organic side and the Fermi level on the metal side, can be read off of Fig. 1 as

F^o_homo = m_M - m_OR - eD_ISR         (1)

In Eq. (1), m_M and m_OR are quantities having to do with only the bulk metal and organic materials, respectively. Therefore, the contribution from the interface, as far as the band-offset is concerned, is only in the form of a difference in the average crystal potential across the ISR, eD_ISR . Understanding and being able to predict this potential difference, which is intimately related to the “interface dipole” more commonly discussed in the literature, are at the heart and soul of understanding the whole interface energy level alignment problem.

     Until recently, the most popular models of the interface dipole have been based on interface gap states and the concept of charge neutrality level (CNL).4 There have been well-known problems with these models, both from a self-contradiction between the bias- and the metal- dependencies of the barrier height5 and from a disagreement with the experimentally observed dependence of the barrier height on the interface atomic structure.6 Recently, it was shown that the magnitude of the interface dipole at metal-semiconductor interfaces could be well explained, and that the aforementioned inconsistencies regarding interface dipole could all be removed, if the interface dipole was assumed to arise solely from polarized bonds at the interface.7, 8 Since its publication, this “bond polarization” view of interface dipole formation has received warm support,9, 10 although its validity must still be regarded as untested. An interesting prediction of this theory is the “unpinning” of the Fermi level when the interface bonds are weakened. This immediately suggests what may be a long-awaited solution to the problem of tuning the band offset, namely, by forming metastable, non-interacting, interface structures. If demonstrated, the ability to tune the band-offset would considerably relax the constraints on the choice of materials for modern integrated devices and enhance the functionality of existing technologies and materials. This document proposes the development of a versatile instrument that enables the fabrication and the in-situ characterization of a variety of interfaces under conditions of reduced reaction and well-controlled cleanliness. The proposed instrument will be well-suited to test the fundamental concept of band-offset formation, explore the tunability of band offset in interfaces formed with the new technique, and develop strategies and methods for advanced ohmic contact technologies in current and future electronic and photonic devices.

b. Energy Level Alignment at Organic Interfaces

     Electronic and optoelectronic devices and structures based on organic thin films and self-assembled monolayer (SAM) molecules have attracted much recent interest. The performance of these devices often depends on the quality of the ohmic contact.11, 12 Because of the delicacy of the organic materials, the usual contact method of physical deposition of metal usually leads to severe damage to the organic layer, as schematically shown in Fig. 2. Generally speaking, “soft” contact technologies, i.e. those that are less disruptive to the integrity of the organic material, are preferred for organic devices. For SAM molecular layers in particular, there is zero tolerance for damage, because an entire SAM layer often consists of a single layer of molecules. For that reason, the electronic transport across SAM layers has largely been studied using special techniques such as “controlled break junctions”,13, 14 conducting-probe atomic force microscope (CP-AFM),15-19 and mercury droplet contacts.20-22 Without an energetic deposition step, these specialized contact methods do not encroach on the organic molecules, although they are obviously not practical for real applications. Recently, a few novel soft-contact methods have been reported. In the lift-off float-on (LOFO) method, soft gold or aluminum pads are picked-up off the meniscus of a liquid and onto a surface covered with a SAM layer.23, 24 The dipole of the molecular layer was found to survive the formation of the interface, and was utilized to demonstrate a modification of the Au-GaAs Schottky barrier height.25 In another approach, a pair of “crossed wires” is first exposed to molecules and then brought to precise distances from each other using controlled contact forces. The cross wires are then used as contact electrodes to study the electronic transport across molecules between the two wires.26, 27 In yet another technique, known as nanoscale transfer printing (nTP), Au layers are first deposited on the patterned surface of an elastomeric stamp and then the Au on the raised areas of the pattern is transferred to the SAM layer by contact printing.28, 29 The nTP method is very attractive for practical applications because of its ability to make nanoscale patterned contact without lithography. Related to contact printing, the lift-off of metal layer on organic devices has also been demonstrated by cold-welding with a hard substrate.30

     After some initial discussions, views on the electrical conduction through short-length molecules or alkyl- and Xe-chains molecular layers appear to be converging.31 It is increasingly clear that the conduction for materials with large HOMO-LUMO gaps is dominated by tunneling and is largely “through-bond” rather than “through-space”.32 What remains less settled is the mechanism(s) for stochastic switching observed at metal-molecule-metal junctions.33, 34 For metal-organic interfaces formed on thick (>100nm) organic layers, experimental investigations have shown that the dipole at metal-organic interfaces can be quite significant,35-37 whereas the dipole at organic-organic interfaces is usually negligible.36, 38 Very recent results seem to suggest that the interface dipole can also be quite significant even for organic-organic interfaces, if dopant molecules are added to one of the organic materials.39-41 These general trends are in good agreement with the bond-polarization based picture of interface dipole formation,7, 8 described above. Much of existing work on metal-molecular junctions has been performed with noble metals as the electrode material. The advantages of using noble metals are that the strong sulfur-metal bonds lead to very robust self-assembly of molecular layers on the metal, and that the stability of the noble metal allows the fabrication to be carried out under ambient conditions. The use of noble metals, however, limits the charge-transfer at the metal-organic to be in the direction of from the metal to the molecule. To obtained a less biased view of the molecular level alignment picture at metal-organic interfaces and, in particular, to focus on the relationship between interface dipole and interface bonding, there is a need to study junctions formed with a wide variety of metals, preferably without sacrificing the cleanliness and the ease of interface fabrication. However, one notices that none of the existing techniques allow the formation of junction between molecules and clean metals that are electropositive, in a fashion that imparts minimal damage to the organic layer. This capability will be realized in the instrument that is being proposed.

c. Schottky Barrier Height and Ohmic Contact

     The formation of Schottky barrier height at metal-semiconductor interfaces and the related problem of heterojunction band-offset formation have been subjects of intensive research and debate for several decades.1, 2, 42 The lack of a strong dependence of the barrier height on the metal work function, a phenomenon known as “Fermi level pinning”,43, 44 has prompted the attribution of the barrier height/band-offset formation to bulk effects in early theoretical models. However, experimental and theoretical work on carefully controlled epitaxial interfaces45-48 and recent discovery of the varied behavior associated with inhomogeneous barrier height49, 50 have demonstrated the important role played by interface bonding on barrier height formation. The recent theoretical work7, 8 on interface bonds seems to have reconciled the perceived conflict between the structure dependence of the barrier height45, 46 and the apparent pinning of the barrier height.43, 44

     There are many applications in which the ability to tune the barrier height/band-offset is strongly desirable. For example, the contact resistance to a semiconductor can be dramatically improved with a reduction in its Schottky barrier height. The ohmic contact issue is particularly poignant for wide band gap semiconductors with doping difficulties, such as the p-type GaN.51-53 Another interface where the ability to tune the Schottky barrier height will be greatly appreciated is between high permittivity (high-K) gate dielectrics and metal gates, which is an important element of next-generation ULSI devices. Beside good electrical conductivity, metal gates help to keep the crucial effective oxide thickness (EOT) small by avoiding reaction with the high-k dielectric and thereby obviating the need for a (lower-k) buffer layer.54 One philosophy for metal gate is to choose a metal with a work function which matches roughly the mid-gap point of the semiconductor.55 However, to be able to maintain the threshold gate voltage for the field effect transistor at a convenient voltage, especially at scaled-back power supply voltages, it is desirable to have separate Fermi level positions for the gates on n-type and p-type channels. For this purpose, one needs to control the Schottky barrier height between the metal gate and the high-K dielectric. Previously, there have been different approaches to modify the SBH. The most successful scheme has been to insert a very thin layer of material between the metal and the semiconductor. For example, layers of insulators,56, 57 semiconductors,58-60 molecular dipoles,23, 61, 62 and chemical passivation,63, 64 formed on the semiconductor surface, have been shown to modify the barrier height of Schottky contact. The manner by which the SBH is affected by the interlayer is rather unpredictable and system-specific. The creation of two new interfaces, one between the metal and the interlayer and one between the interlayer and the semiconductor, and the likelihood of charge transfer across the interlayer make the experimental “interlayer” situation rather difficult to analyze. It hasn’t been possible to make meaningful comparison with theoretical models nor to deduce information from these studies, which is of general relevance.

     Recent development in the theoretical picture of interface dipole formation7, 8 suggests an obvious method to “tune” the barrier height and, at the same time, perform a evaluation of the this view of barrier height formation. Since the interface dipole is expected to be small at weakly bonded metal-semiconductor interfaces, the fabrication of such interfaces could lead to a large swing of Schottky barrier height with different metals. In the literature, there are already indications that such an approach may be fruitful: barrier height of contacts formed in air with mercury probe was found to follow the Schottky-Mott prediction.65 Additionally, the “barrier height” controlling the electron transfer rate at a semiconductor-electrolyte interface tends to follow the Schottky-Mott rule,66-68 which is in good agreement with the bond-polarization picture, because the molecules in the liquid are not bonded to the semiconductor electrode. One should also mention that there was an early attempt to eliminate metal-semiconductor reaction during deposition, by using a condensed solid xenon film to cushion the incoming metal flux,69, 70 although that work resulted in metal island formation. So, the question at hand seems to be “How to weaken the interface bonds?”. A straightforward method to reduce the strength of bonding, or to control the extent of bonding, at metal semiconductor interfaces is to join together pre-fabricated metal and semiconductor layers. The vast majority of metal-semiconductor interfaces are formed by the deposition of metal, atom by atom, onto the surface of a semiconductor. It has long been pointed out that metal adatoms, with high chemical reactivity, large surface mobility, and significant kinetic energy with the thermal evaporation or sputtering processes, can react readily with the semiconductor surface. By bringing pre-existing metal and semiconductor surfaces together, one takes advantage of the saturated bonds on both surfaces to dramatically reduce the chemical activity between the two materials, which should lead to a barrier height closer to the Schottky-Mott limit and opens the possibility to tune the Schottky barrier height.