Description
Self-assembly of molecular nanodielectrics on Ge and III-V materials for next generation transistors.
Silicon does not allow the development of very high mobility and high frequency transistors. The use of germanium or III-V semiconductors as a conduction channel is the most studied alternative at present because of their excellent electrical properties. The main obstacle to the development of III-V MOS transistors, whose properties rival or exceed those of Si CMOS, is that the insulating layer of germanium and III-V materials is neither of good quality nor thermodynamically stable, in contrast to the exceptional insulating and passivation properties of SiO2 on Si. Thus, the SAGe III-V project proposes to design, fabricate and evaluate new ultra-insulating (high K) thin films, fabricated from self-assembled monolayers of organic compounds on Ge and III-V materials in order to give them these viable properties allowing their use in the next generations of devices.
Objectifs
The objective for the Nanostructuration team of ISEN Yncréa Méditerranée was to prepare high permittivity dielectric layers on Ge based on chemically grafted self-assembled molecular monolayers.
Project partners
Main teacher-researchers involved
Project Director
Lionel PATRONE
Teacher-researcher
Virginie GADENNE
Teacher-researcher
And also: G. DELAFOSSE
More details
The aim here is to highlight the link between organization and electrical properties (ANR SAGe III-V project). The molecules, synthesized at CEA/Saclay (B. Jousselme), have a thiol grafting function presenting a pi-conjugated system () in the middle of two alkyl chains (), forming a system --. Two pi-conjugated systems were chosen: one N-type (acceptor) based on naphthalene, the other P-type (donor) based on terthiophene. The SAMs of these molecules should thus exhibit a negative differential resistance (RDN) effect obtained by resonant tunneling through a molecular level of the potential well represented by the pi-conjugate system isolated from silicon and the upper metal contact by the double tunnel junction constituted by the two alkyl chains. These molecules were chosen on the basis of their differing molecular levels, enabling the position of the RDN peak to be modulated.
For SAMs on gold, the molecular levels of these molecules are positioned asymmetrically in relation to the Fermi level of the electrodes: for 3TSH, which is a P-type donor, the HOMO is closer to the Fermi level, while for NaPhSH, which is an N-type acceptor, the LUMO is closer. This, added to the asymmetry of the electrical contacts (contact is stronger with the gold substrate on which the molecule is anchored), which translates into an asymmetry of the potential profile across the molecule, should result in current rectification at different polarities for the two molecules.
Results obtained
The images and current-voltage measurements performed by STM on these SAMs on Au(111)/mica (Fig. 1) have allowed us to better understand the organization within these monolayers and to locally study their electrical behavior. While a short-range organization could be observed on the 3TSH SAMs following an √3x√3R30° arrangement typical of thiols on Au(111), no organization could be detected on the NaPhSH SAMs. This may be related to its larger steric hindrance. The poorer organization of NaPhSH SAMs was confirmed by infrared spectroscopy and water contact angle measurements.
The organization and electronic properties of the SAMs were determined through analyses performed by STM (Fig. 1) and which were completed by conductive AFM at IEMN, and within our team at IM2NP in Marseille by UPS and IPES spectroscopies which allowed to measure HOMO and LUMO positions respectively (Fig. 2) for the first time in the team, thus paving the way for a methodology to successfully study the structure-electronic property relationships of SAMs through the complementarity between STM and UPS/IPES.
Figure 1: STM image and schematic of the SAM on gold of the donor (left) and acceptor (right) molecules. The SAM of the donor molecule is organized, as shown by the lattice appearing in the STM image, resulting in a single HOMO level involved in the electronic transport measured under the microscope tip. In contrast, the SAM disorder of the acceptor molecule leads to a dispersion of the three LUMO electron levels involved in electron transport. Below the STM images, we can see the rectification characteristic of the donor or acceptor character of the molecule on the current-voltage characteristics, as well as the greater dispersion of the rectification voltage for the unorganized SAM (acceptor).
Figure 2: UPS and IPES spectra to determine the position of the highest-energy occupied "HOMO" and lowest-energy unoccupied "LUMO" molecular levels, respectively, of the two SAMs of the donor and acceptor molecules studied. The IPES spectrum reveals two LUMO levels for the SAM without acceptor organization.
These different analyses have allowed us to highlight the relationship between structure and electrical properties. In particular, a single HOMO level is involved in the transport through the organized layer of the donor molecule, while for the disorganized layer of the acceptor molecule several LUMO levels are involved.
Publications over the period
X. Lefèvre, F. Moggia, O. Segut, Y.-P. Lin, Y. Ksari, G. Delafosse, K. Smaali, D. Guérin,V. Derycke, D. Vuillaume, S. Lenfant,* L. Patrone,* B. Jousselme*
Influence of molecular organization on the electrical characteristics of π-conjugated self-assembled monolayers.
J. Phys. Chem. C 119(10) pp. 5703-5713 (2015) / DOI: 10.1021/jp512991d
