[Paper Review] Molecular Electronics by Chemical Modification of Semiconductor Surfaces

Ayelet Vilan, David Cahen|arXiv (Cornell University)|Dec 11, 2016

Molecular Junctions and Nanostructures211 references109 citations

TL;DR

A review of how chemical modification of molecular monolayers at metal/semiconductor interfaces can tune energy level alignment, passivate states, induce dipoles, and drive charge rearrangements to impact transport in hybrid devices.

ABSTRACT

Inserting molecular monolayers within metal / semiconductor interfaces provides one of the most powerful expressions of how minute chemical modifications can affect electronic devices. This topic also has direct importance for technology as it can help improve the efficiency of a variety of electronic devices such as solar cells, LEDs, sensors and possible future bioelectronic devices, which are based mostly on non-classical semiconducting materials (section 1). The review covers the main aspects of using chemistry to - control alignment of energy levels at interfaces (section 2): - passivate interface states (section 3), - insert molecular dipoles at interfaces (section 4), - induce charge rearrangement at and around interfaces (section 5). After setting the stage, we consider the unique current-voltage characteristics that result from transport across metal / molecular monolayer / semiconductor interfaces. Here we focus on the interplay between the monolayer as tunneling barrier on the one hand, and the electrostatic barrier within the semiconductor, due to its space-charge region (section 6), on the other hand, as well as how different monolayer chemistries control each of the these barriers. Section 7 provides practical tools to experimentally identify these two barriers, and distinguish between them, after which section 8 concludes the story with a summary and a view to the future. While this review is concerned with hybrid semiconductor / molecular effects (see Refs. 1,2 for earlier reviews on this topic), issues related to formation of monolayers and contacts, as well as charge transport that is solely dominated by molecules, have been reviewed elsewhere[3-6], including by us recently[7].

Motivation & Objective

• Explain how chemistry controls energy level alignment at metal/semiconductor interfaces.
• Discuss passivation of interface states to improve device performance.
• Describe how molecular dipoles inserted at interfaces influence electrostatics and transport.
• Analyze charge rearrangements at and around interfaces due to molecular layers.
• Summarize practical experimental approaches to identify and distinguish interfacial barriers.

Proposed method

• Review the roles of molecular monolayers as tunneling barriers and their interaction with semiconductor space-charge regions.
• Discuss how monolayer chemistry modulates interfacial energy alignment and dipole moments.
• Outline experimental techniques to separate electrostatic and tunneling barriers in M-MOS (metal–molecule–semiconductor) structures.
• Synthesize prior work on nanostructured interfaces and hybrid semiconductor/molecular effects.

Experimental results

Research questions

• RQ1 How do molecular monolayers modify energy level alignment at metal/semiconductor interfaces?
• RQ2 What mechanisms govern passivation of interface states by molecular modification?
• RQ3 How do molecular dipoles and charge rearrangements affect transport across hybrid interfaces?
• RQ4 How can one experimentally distinguish tunneling barriers from semiconductor space-charge barriers in these systems?

Key findings

• Molecular monolayers can shift interfacial energy levels through dipole moments.
• Chemical modification can passivate interface states to improve device behavior.
• Monolayers act as tunneling barriers whose effect competes with the semiconductor space-charge region.
• Monolayer chemistry controls electrostatic barriers and charge distribution at and around interfaces.
• Practical tools exist to experimentally identify and distinguish between the two main barriers.
• The review emphasizes the interplay between molecular and semiconductor effects in transport across metal/molecule/semiconductor interfaces.