Selected Papers: Basic Research Related to Novel

Computational Physics Technologies forDeveloping Novel Functional Nanostructures
Hiroyuki Kageshima†
Abstract
This paper presents an overview of recent progress in computational physics technologies. In developing novel functional nanostructures, it is necessary to take into consideration the fact that all materials consist of atoms. Computational physics technologies enable us to study the motion of the fundamental components of atoms, their nuclei and electrons, based on quantum dynamics.

1. Introduction
New functional nanoscale devices, or nanodevices,
are now being studied intensively because nanostructures are expected to exhibit novel physics. Their
extremely small size, however, makes nanodevice
formation and observation difficult for experimental
researchers. Moreover, it is not clear what physics
really appears in experiments or how it appears. Nanodevice research and development are, therefore, currently expected to advance via theoretical methods.
Nanodevices cannot be described by traditional treatments based on Newtonian dynamics and classicalelectromagnetism with continuum models. It is necessary to take into consideration the fact that all materials consist of separate particles, namely atoms.
Computational physics technologies enable us tostudy the motions of the fundamental components ofatoms, their nuclei and electrons, on the basis ofquantum dynamics.
NTT Basic Research Laboratories has a long history of developing such computational physics technologies. Among these technologies, first-principles
calculations are the most powerful tools for studying
the physics of nanostructures [1]-[6]. They enable us
to calculate the electronic states that govern the physical properties of materials, such as the bonding ofatoms and the energy gaps of the valence and conduction bands, by inputting the position and element† NTT Basic Research Laboratories
Atsugi-shi, 243-0198 Japan
Email: kages@nttbrl.jp

12

number of each atom constituting the material. In thecalculations, the movements of electrons are calculated using the Schrödinger equation, which is the quantum dynamics counterpart of the Newtonian equationin classical mechanics. We can also change any parameters at will, which we generally cannot do inexperiments, so we can thus create virtual experiments in computers and obtain ideal results.
In the early days, first-principles calculationsstrained the limits of available computational
resources because they calculate the complex coupled motions of the huge number of electrons in amaterial, so they could be applied to only very limited materials and physical properties. NTT Basic
Research Laboratories started developing technologies based on first-principles calculations in the1980s and has been developing and improving themfor about 20 years. During this period, the first-principles calculation methods were merged with supercomputer technologies, and the possible targets wereextended from simple crystals to defects, surfaces,interfaces, nanostructures and so on. Similarly, theintroduction of new technologies extended the methods to include materials with a second row and transition metal elements, such as diamond, oxides,nitrides, iron, manganese, and palladium. They werealso extended to evaluate optimized atomic structuresand reaction energies as well as to calculate the density of states and band structures.
2. Approaches for nanodevices
We are studying how to develop novel functional
NTT Technical Review

Functional Nanostructures

Nano-device
Gate metal
Insulating film
Semiconductor
with dopantsContinuum model

Design ofdevice fabrication
processes

Design offunctional devicematerials

Atomic model

Design ofdevice
characteristics

and integrate novel materials into the nanodevices,
the characteristics of the fabricated nanodevices may
still not be clear because novel physics is expected toappear in nanostructures. Thus, designing and clarifying the device characteristics is also an importantstep. Moreover, to design all these things theoretically, we must develop new methods based on computational physics technologies because new physics canbe much more efficiently studied if the new physicalproperties can be calculated.
3. Design of fabrication processes

Development of computational physics technologies

Firsth2 2
n r′ion(r ) + ∫dr ′ – +µ XC (r ) |φ i〉principles ε i|φ i〉 = – – ∇ + V loc
2m|r –r ′|
calculation

Fig. 1. Approaches to the design of novel functional
nanodevices.

nanodevices by four approaches: designing device
fabrication processes, designing functional devicematerials, designing device characteristics, anddeveloping computational physics technologies tosupport the three design approaches (Fig. 1). As atypical example of nanodevices, let us consider one
that uses a metal-insulator-semiconductor field-effecttransistor (MISFET). This device is formed ofstacked layers: the semiconductor, insulating film,and gate metal layers. The semiconductor layer might
include dopants or impurities. Since the thickness of
the insulating film layer is on the atomic scale, theroughness of the upper and lower interfaces and theuniformity of the insulating film layer are the key factors. This means that it is very important to control
the fabrication of such structures. The design of thefabrication processes is thus important for nanodevices.
When we develop novel functional nanodevices,
we expect to alter the semiconductor layer by meansof a layer with novel functional materials, such as
magnetic materials, strongly correlated materials,organic materials, or superconducting materials. As
shown by the dilute magnetic semiconductor GaMnAs, for example, dopants in the semiconductor
layer are expected to show new functionality. It may
be possible to integrate such new functional materials
into nanodevices. For this, the design of such materials is another important issue.
Even if we fabricate nanostructures appropriately
Vol. 4 No. 11 Nov. 2006

The thermal oxidation process of silicon is very
important in the fabrication of silicon-based devices.
This process forms a silicon oxide insulating film
layer on the semiconductor layer of the MISFET
device structure. Despite its importance, the atomicscale details of the process have not yet been clarified. The surface of the silicon substrate is covered
with the silicon oxide film, so an oxygen moleculemust first diffuse through the oxide film before it canreact with the silicon. Since this process is thus aninterfacial reaction process and the volume of newly
formed oxide is much larger than that of the reacted
silicon substrate, a large expansion in volume should
occur at the interface. Furthermore, it is known thatthe interfacial reaction rate changes with the strain at
the interface. Thus, it is important to clarify how theinterfacial reaction process occurs and how it isaffected by interfacial strain.
Using computational physics technologies, weevaluated the path and the energy landscape for theinterfacial reaction of the oxygen molecule with thesilicon substrate (Fig. 2) [7],[8]. The results show thatthe process consists of two stages. Initially, the oxygen molecule is situated in a cage formed by the bond
network of the silicon oxide film. In the first stage, it
rotates as it approaches the substrate and is adsorbedon the substrate at the interface. Then, in the secondstage, the oxygen molecule breaks up and each oxygen atom separately penetrates into the silicon substrate to form new silicon oxide. The first stage of thisreaction emits a reaction heat of about 3 eV per oxygen molecule or 70 kcal/mol and the second stageemits about 7 eV per oxygen molecule or 160kcal/mol.
With the help of macroscopic simulation based ona continuum approximation, these results can beclosely compared with experimental results, giving
us useful information for designing the process [9].

13Selected Papers

O2 molecule
700–1100°C CO2 gas
Si-oxide

Si-oxide
Si substrateStress/strain
Volume expansion
Stress/strain
Si substrate

Energy (eV)

0.0
–2.0
–4.0
–6.0
–8.0

–10.005 10 15 20 25 30 35 40
Reaction coordinate

Fig. 2. Thermal silicon oxidation process clarified by computational physics technologies.
90°e–e––Q

Q
Electrode + –2+ Charged

HSS

Q+ – Electrode
2– Charged90°

HS

ISD

+1e–12%±0e–1ewith transformation+1%

37°±0eH

HS

S

without transformation+2e–1eS90°

90°

ISD

15°

H+ Charged

Single-molecule single-electron (SE)device with 4,4”-terphenyldithiol

–1eH

23°

+1eH

H

S

S–2e90°11°+2e–12%

Vg

HSH

S

Charge injection induces structural transformation.

Fig. 3. Single-electron device characteristics of an organic molecule clarified
by computational physics technologies.

4. Other approaches
Single organic molecules are very attractive materials for novel nanodevices of the future. Those molecules could be especially useful as the single-electronisland of single-electron devices because the size of14

the island, the key to the device characteristics, is definitely limited by the size of the molecule itself. To
clarify the possibility of using a molecule as a single
electron island, we evaluated the device characteristics (Fig. 3) [10]. The results on a single 4,4”-terphenyldithiol molecule show that the atomic strucNTT Technical Review

Selected Papers

Classical theory

Quantum theory

2+Q
∆µ–QQ
F = – – Q∆ µ2C

F = FKS [{ψi }] – Q [{ψi }] ∆µMinimize

Minimize

Q = C∆ µ

New Schrödinger equation

Fig. 4. Principle of newly developed EFED method.

CathodeTi

AnodeSrO

Increase in charge
density ∆ρZ(Capacitance)–1 C–1 [HR/e2]

AnodeTotal
2

Electrostatic part
1
Anode part
Cathodepart000.1
0.2
0.3
Applied electric bias ∆µ [HR]z coordinate

Fig. 5. Device characteristics of a nanocapacitor with repeated cathode-anode pairs made of atomically thin SrTiO2electrodes clarified by the EFED method. HR is Hartree (1 HR = 27.2 V) and e is the elementary charge.

ture of the molecule changes if the charge on the molecule is modified by the gate voltage. The two thiolgroups at the ends and the phenyl group at the middle
rotate independently when the charge changes. As aresult, the separation of the source-drain current peakis greatly modified by these rotations in the atomicstructure.
Among the device characteristics, capacitance isvery important because the FET is essentially acapacitor. However, the capacitance characteristics
cannot be calculated by current first-principles calculations. To overcome this difficulty, we have developed a completely new method, which we call theenforced Fermi-energy difference (EFED) method(Fig. 4) [11]. This method takes into account theexternal work (Q∆ µ) by the electric bias (∆ µ) to fix
the polarized charge in the system. By adding thiswork to the quantum free energy of the system, weobtained a new Schrödinger equation for the system
under electric bias.
Using the EFED method, we have been studying
what characteristics appear in nanocapacitance (Fig.
Vol. 4 No. 11 Nov. 2006

5) [12]. When an electric bias (∆ µ) is applied, ourmethod surely induces positive charges on the cathodes and negative charges on the anodes as shown inthe bottom-left figure (∆ µ is the increase in charge
density). The results show that the capacitance is notconstant as the classical theory would predict.
Instead, it is governed by the electronic states of theelectrodes as well as by their shapes and separation.
The right figure shows that the inverse of the capacitance is the sum of electrostatic, cathodic, and anodic
parts. The cathodic and anodic parts are governed by
the density of states of the corresponding electrode.
The electrostatic part is governed by classical electrostatics, but a detailed analysis shows that thecharges on the electrodes expand to the vacuum andthe estimated effective electrode separation changes
with the applied electric bias while the real separation
is fixed. This means that we can expect quantum
effects in nanocapacitors. Since the EFED method isvery simple and works well with any first-principles
method, it should clarify the physics in many nanodevices in the future.
15

Selected Papers

References[1]T. Ohno, “Stability of the (InAs)1/(GaAs)1 monolayer superlattice,”
Phys. Rev. B, Vol. 38, No. 18, pp. 13191-13196, 1988.
[2] T. Ohno, K. Shiraishi, and T. Ogawa, “Intrinsic origin of visible-lightemission from silicon quantum wires — electronic-structure and geometrically restricted exciton,” Phys. Rev. Lett., Vol. 69, No. 16, pp.
2400-2403, 1992.
[3] K. Takeda, K. Shiraishi, M. Fujiki, M. Kondo, and K. Morigaki, “Photocreated metastable states in polysilanes,” Phys. Rev. B, Vol. 50, No.
8, pp. 5171-5179, 1994.
[4] H. Kageshima and K. Shiraishi, “Momentum-matrix-element calculation using pseudopotentials,” Phys. Rev. B, Vol. 56, No. 23, pp.
14985-14992, 1997.
[5] A. Taguchi, and H. Kageshima, “First-principles investigation of theoxygen negative-U center in GaAs,” Phys. Rev. B, Vol. 57, No. 12,pp. R6779-R6782, 1998.
[6] H. Kageshima, and K. Shiraishi, “First-principles study of oxide
growth on Si(100) surfaces and at SiO2/Si(100) interfaces,” Phys.
Rev. Lett., Vol. 81, No. 26, pp. 5936-5939, 1998.
[7] T. Akiyama, H. Kageshima, and T. Ito, “First-principles analyses of
O2 molecules around ultrathin SiO2/Si(100) interface,” Jpn. J. Appl.
Phys., Vol. 43, No. 11B, pp. 7903-7908, 2004.
[8] T. Akiyama, and H. Kageshima, “Reaction mechanisms of oxygen at
SiO2/Si(100) interface,” Surf. Sci., Vol. 576, No. 1-3, pp. L65-L70,
2005.
[9] M. Uematsu, H. Kageshima, K. Shiraishi, M. Nagase, S. Horiguchi,
and Y. Takahashi, “Two-dimensional simulation of pattern-dependent oxidation of silicon nanostructures on silicon-on-insulator substrates,” Solid-State Electronics, Vol. 48, No. 6, pp. 1073-1078, 2004.
[10] K. Uchida, H. Kageshima, and H. Inokawa, “Charge Injection Effects
in a Single 4,4”-Terphenyldithiol Molecule,” Jpn. J. Appl. Phys., Vol.
44, No. 12, pp. 8759-8763, 2005.
[11] K. Uchida, H. Kageshima, and H. Inokawa, “First-principles study offield-effect doping in nano-scale systems by the enforced Fermi-energy difference method,” e-J. Surf. Sci. Nanotech., Vol. 3, pp. 453-456,2005.
[12] K. Uchida, H. Kageshima, and H. Inokawa, “Quantum effects in thecapacitance between a pair of thin and slightly separated SrTiO3
slabs: A first-principles study,” Phys. Rev. B, Vol. 74, No. 3, pp.
035408-1-6, 2006.

16

Hiroyuki Kageshima
Senior Research Scientist, Nano-Devices
Research Group, Physical Science Laboratory,
NTT Basic Research Laboratories.
He received the B.S. degree in physics fromWaseda University, Tokyo, in 1986, and theM.S. and D.S. degrees in physics from the University of Tokyo, Tokyo, in 1988 and 1991,respectively. Since joining NTT in 1991, he has
been studying the physics of semiconductor
nanostructures and computational physics technologies. He received the Best Poster Paper
Award at the Fall Meeting of the Materials
Research Society, Boston, USA, in 1997 and theJJAP Paper Award of the Japan Society ofApplied Physics in 2000. He is a member of thePhysical Society of Japan, the Japan Society ofApplied Physics, and the Surface Science Society of Japan.

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