Article pubs.acs.org/JPCC
Multilevel Operation of Resonant Tunneling with Binary Molecules in a Metal−Insulator−Semiconductor Configuration Hoon-Seok Seo, Ryoma Hayakawa, Toyohiro Chikyow, and Yutaka Wakayama* International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan S Supporting Information *
ABSTRACT: We attained multilevel manipulation of resonant tunneling in a metal−insulator−semiconductor (MIS) structure, in which heterogeneous molecules of copper hexadecafluorophthalocyanine (F16CuPc) and copper phthalocyanine (CuPc) were embedded in an insulating layer to form a double-tunneling junction. Resonant tunneling was observed in samples with either molecule, revealing that the molecules worked as intermediate electrodes for the tunneling current. The carrier transport was ascribed to resonant tunneling through the energy levels of individual molecules. That is, the threshold voltage of resonant tunneling can be controlled according to the energy levels of the molecules. We achieved multilevel resonant tunneling operation with binary molecules of F16CuPc and CuPc, in which the carriers were injected into the respective molecules at corresponding threshold voltages. Our findings point to a new multilevel operation of resonant tunneling through organic molecules in a practical MIS device structure.
1. INTRODUCTION Organic molecules have been used to develop nanoscale devices.1,2 The scanning tunneling microscope (STM) is widely used to investigate electrical transport through single molecules.3−12 For instance, Pan et al. used STM to investigate the electronic states and transport properties of single molecules of pyridine-σ-C60, C59N, cobalt phthalocyanine, and melamine.3 Using ultrahigh-vacuum STM, Guisinger et al. demonstrated a negative differential resistance at room temperature through individual styrene molecules on silicon surfaces.4 Kowalzik et al. showed single-electron tunneling (SET) effects at room temperature in a double tunnel junction of an STM-tip/single molecule (tailored arylthio-coronene)/ substrate structure.5 These reports show the high potential of organic molecules as media for controlling tunneling currents at the single-electron level. This advantage stems from the discrete energy levels of organic molecules, which are similar to the quantum effects of semiconductor dots. If such tiny amounts of electrical current can be successfully manipulated, electronic devices with ultra-low power consumption, such as singleelectron memory devices and transistors, will be possible. However, these approaches are still far from practical applications, because STM-based techniques are effective only for examining properties of single molecules and are unrealistic for large-scale integration. Therefore, it is necessary to regulate the tunneling current through molecules in practical device configurations. For applications of SET, semiconductor quantum dots have been widely used for memory devices13,14 and transistors.15,16 For example, the Coulomb blockade effect is the operating © 2014 American Chemical Society
principle of single-electron memory devices, which makes it possible to inject carriers into dots at the single-electron level. In addition, such devices would allow multilevel memory operation and ultra-low power consumption. However, singleelectron memory devices have not been realized so far. A major problem is the difficulty in controlling the size of quantum dots at the nanometer scale; the non-uniform size of quantum dots hinders stable device operation at room temperature. Another obstacle is the low controllability of the density of dots in the range of 1011−1012 cm−2.13,17,18 A high density of dots is needed to improve memory capacity. Thus, the precise control of quantum-dot size at the nanometer scale and an increase in density are essential prerequisites for creating single-electron devices. The main purpose of this study was to solve these problems through the use of organic molecules as quantum dots, particularly for controlling tunneling current. In this study, we incorporated molecules in a metal−insulator−semiconductor (MIS) structure with the aim of creating a new way to integrate molecular functions into a practical device structure.19,20 Organic molecules have many advantages over inorganic quantum dots: First, the molecules have a uniform size at the nanometer scale. Second, their nanometer-scale size permits higher densities of dots. Third, their energy levels are tunable through the attachment of functional groups, such as electron-withdrawing (or -donating) groups. These features Received: November 19, 2013 Revised: February 28, 2014 Published: February 28, 2014 6467
dx.doi.org/10.1021/jp411386s | J. Phys. Chem. C 2014, 118, 6467−6472
The Journal of Physical Chemistry C
Article
Figure 1. Schematic illustration of the metal−insulator−semiconductor structure with F16CuPc and CuPc molecules.
Figure 2. (a) I−V characteristics. (b) Differential conductance curve of a sample with F16CuPc. (c,d) Energy diagrams of the Au/Al2O3/F16CuPc/ SiO2/n-Si structure at (c) positive and (d) negative bias voltages. Here, the energetic position of the Fermi level in the Si substrate was estimated to be 0.1 eV above the midgap from capacitance−voltage measurements at 60 K. The values of the HOMO and LUMO levels were estimated from the peak positions in the dI/dV curve.
make organic molecules superior to inorganic molecules for making quantum dots. We employed hexadecafluorophthalocyanine (F16CuPc) and copper phthalocyanine (CuPc) to achieve multilevel SET operation in an MIS structure. The energy levels of these molecules can be varied markedly just by fluoridation, without changing the basic molecular frame. Photoemission spectroscopy and scanning tunneling spectroscopy measurements clarified that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
levels of F16CuPc molecules are entirely about 1 eV lower than those of CuPc molecules because of the strong electron affinity of the fluorine atoms.21,22 In fact, this distinction allows F16CuPc and CuPc molecules to work as n- and p-type semiconductors, respectively. First, we examined tunneling through F16CuPc and CuPc molecules in MIS structures. The origin of SET is discussed in terms of resonant tunneling through the energy levels of the individual molecules. Next, we fabricated a binary-molecule-based double tunneling junction, in which both F16CuPc and CuPc molecules were incorporated 6468
dx.doi.org/10.1021/jp411386s | J. Phys. Chem. C 2014, 118, 6467−6472
The Journal of Physical Chemistry C
Article
Figure 3. (a) I−V characteristics. (b) Differential conductance curve of a sample with CuPc. (c, d) Energy diagrams of the Au/Al2O3/CuPc/SiO2/nSi structure at (c) positive and (d) negative bias voltages. Here, the energetic position of the Fermi level in the Si substrate was estimated to be 0.1 eV above the midgap from capacitance−voltage measurements at 60 K.
Information, Figure S1). Finally, circular Au thin films with an area of 3.1 mm2 were deposited on top of the Al2O3 layer through a metal shadow mask by electron-beam deposition. In these structures, the Au film and n-Si substrate function as top and bottom electrodes, respectively; the Al2O3 and SiO2 thin films act as tunneling barrier layers; and the F16CuPc and CuPc molecules are embedded in the insulating layers as intermediate electrodes to form double tunneling junctions. The current− voltage (I−V) characteristics were measured with a semiconductor device analyzer (Agilent B1500A) in a vacuum (10−5 Pa) at 60 K.
together in an insulating layer. We observed multiple staircases reflecting the energy levels of the respective molecules. These findings indicate the potential for the multilevel operation of resonant tunneling with multiple molecules, which is not obtainable with inorganic quantum dots.
2. EXPERIMENTAL SECTION Figure 1 illustrates a multilayer structure of Au/Al2O3/organic molecules/SiO2/n-Si(100) and the molecular structures of F16CuPc and CuPc. To form the multilayer structure, highly doped n-type silicon substrates (
