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Topic Name: Confinement of Electrons to Diamond Isotopes : Super lattice structure realized using only carbon
Category: Chemical
Research persons: Shinichi Shikata ,Hideyuki Watanabe
Location: Tsukuba, Japan
Details
Among all available materials, diamond has the optimal characteristics with
respect to hardness, thermal conductivity, light transmission wavelength range,
and chemical stability. Furthermore, as a semiconducting material, diamond shows
excellent characteristics in terms of breakdown electric field and carrier
mobility, suggesting prospective application to electrochemical and
semiconductor devices, in addition to mechanical and optical applications. As a
semiconductor, diamond appears to have a promising future in high-performance
materials for power devices and quantum computers. Studies are still underway
because the aspects of diamond as a semiconducting material have not yet been
fully explored.
In this study, diamonds were synthesized using a microwave plasma-assisted
chemical vapour deposition (CVD) method, from methane (CH4) gases
containing only 12C or 13C carbon isotopes. A super
lattice structured diamond was prepared by alternately depositing 25 diamond
layers, each containing only 12C or 13C, with thickness of
30 nm. The layered diamond was irradiated with an electron beam and the
electron-hole recombination was measured. It was found that recombination
occurred only in the 12C-containing diamond layers, indicating that
electrons and holes were confined to 12C layers. So far, the
confinement of electrons and holes has been reported only with a combination of
different materials (GaAs/AlGaAs, InGaAs/InP, etc.); this study reports the
first success with a single material. The result makes structural design of a
single material using the semiconductor band-gap engineering possible, and it
offers an effective means for developing ultrafast devices and quantum
functional devices.
Social Background for Research:
Diamond is an insulating material normally, but it is also a semiconducting
material, in which the resistivity is controlled to 16 orders of magnitude by
adding impurities. One of excellent properties of diamond is that the thermal
conductivity is 6 times as large as those of the widely adopted heat sink
materials including copper. The prospective electronic applications using
diamond include power devices that are needed for the power control of electric
appliances, lying at the center of energy-saving technologies. The saving of
electrical power energy through upgraded power devices is one of the prioritized
energy-related innovative technologies in the 'Cool Earth – Innovative Energy
Technology Program' of the Ministry of Economy, Trade and Industry for drastic
reduction of CO2. Furthermore, diamond is now a prospective material
for quantum bits in future quantum computers. Today, this feasibility has become
even more promising: room temperature operation is made feasible; the lifetime
of quantum bits is improving; and quantum entanglement, needed for enabling
quantum calculation, is substantiated.
History of Research:
Aiming at developing new applied technologies, the Diamond Research Center of
AIST conducts studies on diamond integrating semiconducting properties of
diamond with its hardness, thermal conductivity, elastic modulus, optical
transmittance, chemical stability, and electrochemical properties, those are the
most outstanding characteristics among all materials. In the past, the center
developed a technology for fabricating a large-sized diamond single crystal .
Including the present study, the center has also studied various devices and
their basic material aspects, aiming at the development of diamond power devices
.
Details of Research:
In this study, diamond was synthesized by microwave plasma-assisted CVD.
Source gases were methane (CH4) and hydrogen (H2); when
methane was made with only the 12C isotope, the deposited diamond
layer contained only 12C, and when methane was made with only the
13C isotope, only 13C is contained in the deposited layer.
A super lattice structure was fabricated by alternately stacking 25 diamond
layers, each with thickness of 30 nm. Figure 1 shows the schematic illustration
of a super lattice structure and the results of chemical composition analysis in
the vertical direction using secondary ion mass spectrometry (SIMS) for the
13C/12C diamond layer stack. The results very clearly show that
12C and 13C diamond layers are stacked. This is the first
study reporting the fabrication of a nanometer-sized super lattice structure
with diamond.
Irradiating an electron beam on the stacked thin-film structure to generate
electrons and holes, their recombination processes was measured. It was found
that recombination occurred only in the diamond layers with 12C,
suggesting that electrons and holes were confined there. As Fig. 2 shows, in the
super lattice structure, only light due to recombination of electrons and holes
in the layers with 12C. For comparison, measurements were made on the
samples with only a single nanometer-sized layer, and recombination was detected
both in the 12C and the 13C layers. The energy difference
(difference in band gaps) of 20 mV between the two diamond layers, as shown in
Fig. 3, indicates that electrons and holes move away from the 13C
diamond layers into the 12C layers and electrons and holes are
confined.
So far, the confinement of electrons and holes has been realized only with a
super lattice structure using heterogeneous junctions of different materials (GaAs/AlGaAs,
InGaAs/InP, etc.), which differ in energy, while the lattice constants are
similar. High electron mobility transistors and semiconductor lasers are the
semiconductor devices designed and fabricated by using this important
phenomenon. The new type of confinement using homojunctions(same material),
which was previously considered impossible and has been achieved in this study,
is attributable to the relatively large energy difference between the isotopes
of diamond. The homojunction is made of the same crystal and offers the merit
that the electrons and holes do not recombine at the junction interface, and so
various device structures will be fabricated easily.
The confinement of electrons and holes using isotopic homojunctions is an
important finding. Using only diamond, it is now possible to control the energy
state of electrons and holes and their distribution, which intrinsically depend
on the material, by using the semiconductor band-gap engineering. Thus, we now
have an effective means for realizing ultrafast devices and quantum functional
devices. This finding may revolutionize the field of diamond applications.
Future Schedule :
For practical device applications of diamond, even higher material quality is
needed, and our challenges include the reduction of defects, epitaxial film
growth on large-sized wafers, and control of electrons and holes. Furthermore,
regarding the confinement of electrons and holes using isotopes, the lifetime of
electrons and holes inside the isotopes, recombination at the homojunction
interface, and mobility of electrons and holes must be evaluated in detail, and
the data will be examined whether they can effectively be used to design quantum
functional devices. The technology shall be developed horizontally, such as for
creating 13C quantum bits. We shall establish basic technologies
through future studies on new electronic applications for diamond.
About the Researchers:
1. Shinichi Shikata
Deputy Director of Diamond Research Center and concurrently Leader
2. Hideyuki Watanabe (Research Scientist)
Others of the Device R&D Team, The Diamond Research Center (Director: Naoji
Fujimori) of the National Institute of Advanced Industrial Science and
Technology (AIST)
President: Tamotsu Nomakuchi
| Tags: |
Super lattice structure - Diamond Isotopes - |
| Research Documents: |
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