U.S. patent application number 10/566722 was filed with the patent office on 2006-08-24 for single crystal of highly purified hexagonal boron nitride capable of far ultraviolet high-luminance light emission, process for producing the same, far ultraviolet high-luminance light emitting device including the single crystal, and utilizing the device, solid laser and solid light emitting unit.
This patent application is currently assigned to NATIONAL INSTITUTE FOR MATERIALS SCIENCE. Invention is credited to Hisao Kanda, Masayuki Katagiri, Satoshi Koizumi, Nesladek Milos, Takashi Taniguchi, Kenji Watanabe, Takatoshi Yamada.
Application Number | 20060185577 10/566722 |
Document ID | / |
Family ID | 34623569 |
Filed Date | 2006-08-24 |
United States Patent
Application |
20060185577 |
Kind Code |
A1 |
Watanabe; Kenji ; et
al. |
August 24, 2006 |
Single crystal of highly purified hexagonal boron nitride capable
of far ultraviolet high-luminance light emission, process for
producing the same, far ultraviolet high-luminance light emitting
device including the single crystal, and utilizing the device,
solid laser and solid light emitting unit
Abstract
A highly pure hexagonal boron nitride single crystal not
influenced by impurities and capable of high-luminance short wave
ultraviolet light emission reflecting inherent characteristics is
provided; a high-luminance ultraviolet light emitting element is
provided by using the above single crystal; and utilizing the above
element, a simple compact low-cost long-lived far ultraviolet
solid-state laser and far ultraviolet solid-state light emitting
apparatus are provided. A highly pure hexagonal boron nitride
single crystal having a single light emission peak in the far
ultraviolet region of up to a wavelength of 235 nm is produced by
melting said boron nitride crystal as raw material in the presence
of a highly pure solvent under high-temperature and high-pressure,
followed by crystallization. A light emitting element or a light
emitting layer comprised of the obtained crystal is excited with
electron beams, and the thus generated far ultraviolet light
resonated or without resonation is taken out.
Inventors: |
Watanabe; Kenji; (Ibaraki,
JP) ; Taniguchi; Takashi; (Ibaraki, JP) ;
Koizumi; Satoshi; (Ibaraki, JP) ; Kanda; Hisao;
(Ibaraki, JP) ; Katagiri; Masayuki; (Ibaraki,
JP) ; Yamada; Takatoshi; (Ibaraki, JP) ;
Milos; Nesladek; (Ibaraki, JP) |
Correspondence
Address: |
KANESAKA BERNER AND PARTNERS LLP
SUITE 300, 1700 DIAGONAL RD
ALEXANDRIA
VA
22314-2848
US
|
Assignee: |
NATIONAL INSTITUTE FOR MATERIALS
SCIENCE
IBARAKI
JP
|
Family ID: |
34623569 |
Appl. No.: |
10/566722 |
Filed: |
November 17, 2004 |
PCT Filed: |
November 17, 2004 |
PCT NO: |
PCT/JP04/17434 |
371 Date: |
February 2, 2006 |
Current U.S.
Class: |
117/2 |
Current CPC
Class: |
B01J 3/065 20130101;
C09K 11/63 20130101; C30B 29/38 20130101; B01J 2203/066 20130101;
B01J 2203/069 20130101; B01J 3/062 20130101; B01J 2203/0645
20130101; H01S 5/32341 20130101 |
Class at
Publication: |
117/002 |
International
Class: |
H01L 21/322 20060101
H01L021/322 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 18, 2003 |
JP |
2003-388467 |
Feb 12, 2004 |
JP |
2004-035501 |
Sep 8, 2004 |
JP |
2004-260480 |
Claims
1. Highly pure hexagonal boron nitride single crystals with far
ultraviolet light emission characteristics emitting far ultraviolet
light having the maximum light emission peak in the far ultraviolet
region at a wavelength of 235 nm or shorter.
2. The highly pure hexagonal boron nitride single crystals with the
far ultraviolet light emission characteristics in claim 1, wherein
said far ultraviolet light is far ultraviolet light having the
maximum light emission peak at a wavelength of 210 nm to 220 nm,
remarkably at 215 nm.
3. A method for producing highly pure hexagonal boron nitride
single crystals with far ultraviolet light emission
characteristics, wherein the highly pure hexagonal boron nitride
single crystals with far ultraviolet light emission characteristics
emitting far ultraviolet light having the maximum light emission
peak in the far ultraviolet region at a wavelength of 235 nm or
shorter are produced through the procedures of mixing boron nitride
crystals with a highly pure solvent, melting the same by heating
under high-temperature and high-pressure, and recrystallizing the
same.
4. The method for producing highly pure hexagonal boron nitride
single crystals with the far ultraviolet light emission
characteristics in claim 3, wherein said far ultraviolet light has
the maximum light emission peak at a wavelength of 210 nm to 220
nm, remarkably at 215 nm.
5. The method for producing highly pure hexagonal boron nitride
single crystals with the far ultraviolet light emission
characteristics in claim 3, wherein said solvent is selected from
nitride or boronitride of alkali metal or alkali earth metal.
6. A solid-state far ultraviolet light emitting element consisting
of a highly pure hexagonal boron nitride single crystal, excited by
electron beam irradiation to emit far ultraviolet light having the
maximum light emission peak in the far ultraviolet region at a
wavelength of 235 nm or shorter.
7. The solid-state far ultraviolet light emitting element in claim
6, wherein said far ultraviolet light is single-peaked
high-luminance light with the peak at a wavelength of 210 nm to 220
nm, remarkably at 215 nm.
8. A solid-state far ultraviolet laser characterized by the
generation of laser light with a far ultraviolet region wavelength,
using a highly pure hexagonal boron nitride crystal with far
ultraviolet light emission characteristics as a direct-type
semiconductor solid-state light emitting element and combining
therewith an electron beam irradiation apparatus as an exciting
source.
9. The solid-state far ultraviolet laser in claim 8, wherein said
light in the far ultraviolet region generated thereby is the
single-peaked high-luminance laser light with a peak at a
wavelength of 210 nm to 220 nm, remarkably at 215 nm.
10. A solid-state far ultraviolet light emitting apparatus, wherein
a light emitting layer consisting of a highly pure hexagonal boron
nitride single crystal capable of emitting far ultraviolet light
with a single emission peak in far ultraviolet region at a
wavelength of 235 nm or shorter and an exciting means for exciting
said light emitting layer are combined and encapsulated together
into a vacuum container, and the light emitting layer is excited to
emit far ultraviolet light by operation of the exciting means.
11. The solid-state far ultraviolet light emitting apparatus in
claim 10, wherein said far ultraviolet light has a single peak at a
wavelength of 210 nm to 220 nm, remarkably at 215 nm.
12. The solid-state far ultraviolet light emitting apparatus in
claim 10, wherein said exciting means for exciting the light
emitting layer is an electron beam emitting means.
13. The solid-state far ultraviolet light emitting apparatus in
claim 12, wherein said exciting means by electron beam emitting
means consists of an anode electrode attached to the back face of a
light emitting layer consisting of a hexagonal boron nitride
crystal, an electron beam emitting substrate attached to the light
emitting layer through an insulating spacer, a cathode electrode
attached to the back surface of the electron beam emitting
substrate, and a means to apply voltage between both electrodes;
and the electron beam is emitted from said electron beam emitting
substrate toward the light emitting layer by application of voltage
between both electrodes.
14. The solid-state far ultraviolet light emitting apparatus in
claim 13, wherein said electron beam emitting substrate attached
through said insulating spacer is a diamond substrate.
15. The solid-state far ultraviolet light emitting apparatus in
claim 14, wherein the structure of said diamond substrate wherein a
large number of pyramid-shaped protrusions for emitting the
electron beam are arranged in a lattice-like manner on the surface
facing the light emitting layer.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to (i) a highly pure hexagonal
boron nitride single crystal capable of emitting high-luminance far
ultraviolet light with a single emission peak at a wavelength of
235 nm or shorter, particularly at 210 nm to 220 nm and remarkably
at 215 nm, a producing method thereof, and a far ultraviolet light
emitting element consisting of said single crystal. The present
invention also relates to (ii) a solid-state laser using a
solid-state light emitting element consisting of said highly pure
hexagonal boron nitride single crystal. Further, the present
invention relates to (iii) a solid-state far ultraviolet light
emitting apparatus which includes said highly pure hexagonal boron
nitride crystal as the light emitting layer with an exciting means
incorporated thereinto. More particularly, the present invention
relates to a solid-state far ultraviolet light emitting apparatus,
wherein said exciting means of the light emitting layer is an
electron beam. Also further particularly, the present invention
relates to a solid-state far ultraviolet light emitting apparatus,
wherein said exciting means of the light emitting layer comprises a
diamond substrate emitting an electron beam.
BACKGROUND OF THE INVENTION
[0002] Development of high-luminance ultraviolet light emitting
elements is recently progressing toward practical use. Light
emitting elements with the emission wavelength of the order of 300
nm have been proposed using various materials such as gallium
nitride and solid solution thereof. For the changeover to the
shorter wavelength of the emitting wavelengths of these solid-state
light emitting elements, there is a large demand in fields such as
high densification of recording media and others. To date, as the
candidates for far ultraviolet light emitting element on a
wavelength of the order of 200 nm, diamond, cubic boron nitride
crystal (hereinafter, denoted by cBN) and aluminum nitride have
been proposed and studies for application thereof are in
progress.
[0003] In searching for materials for a high-luminance light
emitting element in the far ultraviolet region, important
characteristics include: having broad band gaps and chemical
stability, and preferably to be direct transition type
semiconductors, and the like. Except for above described materials,
the solid-state light emitting materials with far ultraviolet light
emission characteristics of the order of 200 nm include hexagonal
boron nitride crystal (hereinafter, denoted by hBN) having about
5.8 eV band gap and being a direct transition type semiconductor.
But there have been factors to prevent its realization. hBN has
been used for a long time as a chemically stable insulator
material, is synthesized by gas phase reaction of boron oxide and
ammonia, and is now utilized in many forms (such as powder,
sintered body, and film form).
[0004] However, hBN obtained by the above described gas phase
reaction has contained impurities to make it difficult to obtain
hBN having the far ultraviolet light emission characteristics
corresponding to its specific band gap. In order to use this
material as the high-luminance light emitting element in far
ultraviolet region, it is necessary first to establish methods to
synthesize highly pure single crystals, but there has been no
report until now that the highly pure hBN single crystal with
expected light emission characteristics has been successfully
obtained by a hBN synthesizing method, aiming at its potential
ability as a solid-state far ultraviolet light emitting element
with the emission wavelength of the order of 200 nm.
[0005] As for the synthesizing method, hBN has been known to be
synthesized by the thermal decomposition reaction or by the gas
phase reaction between boron compounds such as boron oxide and
ammonia, but it has been difficult to obtain highly pure single
crystals by these reactions. Especially, they have never been
considered established as the manufacturing methods of single
crystal materials to use for semiconductors or the like.
[0006] On the other hand, cubic boron nitride crystal, a
high-pressure phase of hBN, has been known to be synthesized by
using hBN or the like as the raw material and boronitride of alkali
metal or alkali earth metal as a solvent and by recrystallizing
said raw material in said solvent under high-temperature and
high-pressure of 55,000 atmospheric pressure and 1,600.degree. C.
Obtained cBN single crystal has high hardness next to the diamond,
and is widely used as a super hard material, and this procedure for
synthesizing cBN has already been established industrially.
[0007] Because cBN synthesized in this way also has a broad band
gap (Eg: 6.3 eV), it has been studied for a long time as a
solid-state short wavelength light emitting element. However, every
cBN single crystal hitherto reported is colored in amber, orange or
the like, and the light emitting behavior corresponding to cBN
specific band gap has not yet been able to be observed in this
situation. As a possible cause thereof, large effect of impurities
contained in the cBN crystal may be nominated. Therefore, in order
to use the cBN single crystal as a material having specific light
emission characteristics corresponding to the band gap of said
crystal, establishment of synthesis reaction to achieve higher
purification of cBN single crystal has become an important subject
to study, as well as full understandings of light emission
characteristics specific to cBN.
[0008] Under such background, it has been reported that synthesis
of hBN single crystal was tried under the condition of cBN
synthesis daringly changing the temperature and pressure conditions
to those at which hBN is produced stably (non-patent literature 1).
However, from the crystal-growth solvent used in the synthesis
experiment in this report, only colored cBN crystals were obtained,
and about the hBN crystal that was formed concurrently as a
by-product, there was no description on the light emission behavior
thereof at all or no suggestion on short wavelength light emission
thereof.
[0009] In such a situation, the inventors of the present invention
intensively studied the synthetic conditions for obtaining highly
pure cBN single crystals. Consequently, they found factors
necessary to obtain highly pure cBN single crystals, and thus
succeeded in synthesizing highly pure cBN single crystals having
optical characteristics specific to the cBN crystal, and reported
in an academic literature (non-patent literature 2). This synthetic
procedure was, in short, after establishing a clean and dry
nitrogen-atmosphere, crystals were grown using highly pure solvent
(such as barium boronitride) purified with utmost care. By this
procedure, highly pure cBN single crystals were successfully
obtained (non-patent literature 2).
[0010] Non-patent literature 1; H. Akamaru, A. Onodera, T. Endo, O.
Mishima, J. Phys. Chem. Solids, 63, 887 (2002).
[0011] Non-patent literature 2; T. Taniguchi, S. Yamaoka, J. Cryst.
Growth, 222, 549 (2001).
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0012] Above is the present situation of the hBN material or cBN,
the high-pressure phase thereof, expected to have light emission
characteristics in far ultraviolet range. Especially, hBN, a wide
band gap semiconductor, is of direct transition type, and so is
expected as a high-luminance and short wavelength solid-state light
emitting element, but the present situation is as described above.
In order to live up to the above expectation, it is urgent to
derive the original characteristics of the substance, that is, to
establish methods for synthesis of highly pure single crystals not
being affected by contaminations, and it is expected to be
realized.
[0013] Moreover, as light emitting apparatuses in the ultraviolet
region, laser apparatuses using various kinds of gases or
semiconductor light emitting apparatuses are known so far. But
these apparatuses need cooling units and are large scale
apparatuses, and are complicated and expensive with pn-junction,
pin-junction and the like, so that ultraviolet light emitting
apparatuses being simple, compact, low cost, and highly efficient
are desired.
[0014] The present invention intends to meet these requirements.
That is, the problem to be solved by the present invention is to
synthesize highly pure hBN single crystals which has been
impossible to be produced by the conventional hBN synthetic
procedure, and using these, to provide an element capable of far
ultraviolet high-luminance light emission reflecting the
characteristics specific to hBN. Moreover, making use of said
hexagonal boron nitride crystal having specific light emission
characteristics, the invention intends to provide simple, compact,
low cost, and highly efficient solid-state far ultraviolet lasers
and solid-state far ultraviolet region high-luminance light
emitting apparatuses, instead of conventional large-scale
apparatuses using gases or complicated and expensive semiconductor
apparatuses. That is to say, the invention intends to provide
solid-state light emitting apparatuses utilizing the far
ultraviolet solid-state light emitting elements adopting highly
pure hexagonal boron nitride crystals having far ultraviolet light
emission characteristics as active mediums.
Means for Solving the Problems
[0015] For the above purpose, the inventors of the present
invention studied on the synthesis experiments reported in the
above non-patent reference 2, for obtaining highly pure cBN single
crystals starting from the raw hBN material using a clean and dry
nitrogen atmosphere and purified solvents. They have tried
experiments to survey in detail and to control the critical
conditions to synthesize highly pure cBN single crystals, and have
found that highly pure hBN single crystals can be obtained by
appropriately adjusting conditions of the temperature and the
pressure.
[0016] They then surveyed in detail the optical characteristics of
said highly pure hBN single crystal obtained from the above
findings, and found and clarified the following optical
characteristics. That is, obtained crystals were colorless,
transparent and highly pure crystals with high electric resistance.
When the crystal was excited by electron beam irradiation with
cathode luminescence, markedly high-luminance light emission at a
wavelength of 215 nm was observed at room temperature. Also, at 83
K, light emission was observed at the wavelength of 210 nm to 235
nm. According to a light absorption experiment, an absorption
spectrum showing light absorption at 208 nm and 213 nm was
obtained. When this was compared with ultraviolet light emission
from a highly pure diamond single crystal measured under the same
condition, it was found that light emission intensity at the
wavelength of 215 nm of the hBN single crystal at room temperature
showed a value about 1000 times or more stronger than the
diamond.
[0017] That is, as the result of intensive studies for obtaining
highly pure hBN single crystals on the basis of prior arts
described in the above references (non-patent literatures 1 and 2)
as the preceding techniques, the present invention succeeded in the
synthesis of highly pure hBN single crystals having a single light
emission peak in the far ultraviolet region near a wavelength of
215 nm responding to electron beam irradiation only, by setting the
hBN single crystal growing conditions to the reported synthetic
conditions for obtaining highly pure cBN single crystals in the
above non-patent literature 2.
[0018] Also, utilizing the above highly pure hexagonal boron
nitride crystal as a light emitting element or a light emitting
layer, and configuring and incorporating thereinto an exciting
means by an electron beam irradiation, the inventors of the present
invention have succeeded in easily designing and providing a
simple, small-sized, highly efficient, solid-state far ultraviolet
light emitting apparatus, unlike a conventional large-scale
solid-state laser apparatus using gas necessary to be equipped with
a water cooler, or conventional light emitting apparatus using a
costly semiconductor solid-state light emitting device produced by
repeating multiple layers of complicated pn-junctions and
pin-junctions.
[0019] The present invention has been prosecuted based on a series
of the above described findings and successes, and its embodiments
are described in the following (1) to (15). Among them, a group of
the inventions concerning highly pure hexagonal boron nitride
single crystals, synthetic methods thereof, and light emitting
elements consisting of said single crystals, given in (1) to (7),
are denoted by the first group inventions. Also, the inventions
concerning solid-state lasers capable of emitting far ultraviolet
laser lights given in (8) and (9), wherein light emitting elements
comprising said single crystals are combined with electron beam
emitting means, are denoted by the second group inventions.
Further, the inventions concerning the solid-state light emitting
apparatus generating far ultraviolet light given in (10) to (15),
wherein the light emitting layer consisting of said single crystal
and the exciting means are integrally incorporated into a vacuum
chamber, are denoted by the third group inventions.
The First Group Inventions
[0020] (1) Highly pure hexagonal boron nitride single crystals with
far ultraviolet light emission characteristics emitting far
ultraviolet light having the maximum light emission peak in the far
ultraviolet region at a wavelength of 235 nm or shorter.
[0021] (2) The highly pure hexagonal boron nitride single crystals
with the far ultraviolet light emission characteristics described
in (1), wherein said far ultraviolet light is far ultraviolet light
having the maximum light emission peak at a wavelength of 210 nm to
220 nm, remarkably at 215 nm.
[0022] (3) A method for producing highly pure hexagonal boron
nitride single crystals with far ultraviolet light emission
characteristics, characterized in that the highly pure hexagonal
boron nitride single crystals with far ultraviolet light emission
characteristics emitting far ultraviolet light having the maximum
light emission peak in the far ultraviolet region at a wavelength
of 235 nm or shorter are produced through the procedures of mixing
the boron nitride crystals with a highly pure solvent, melting it
by heating under high-temperature and high-pressure, and
recrystallizing it.
[0023] (4) The method for producing highly pure hexagonal boron
nitride single crystals with the far ultraviolet light emission
characteristics described in (3), wherein said far ultraviolet
light has the maximum light emission peak at a wavelength of 210 nm
to 220 nm, remarkably at 215 nm.
[0024] (5) The method for producing highly pure hexagonal boron
nitride single crystals with the far ultraviolet light emission
characteristics described in (3) or (4), wherein said solvent is
selected from nitride or boronitride of alkali metal or alkali
earth metal.
[0025] (6) A solid-state far ultraviolet light emitting element
consisting of a highly pure hexagonal boron nitride single crystal,
excited by electron beam irradiation to emit far ultraviolet light
having the maximum light emission peak in the far ultraviolet
region at a wavelength of 235 nm or shorter.
[0026] (7) The solid-state far ultraviolet light emitting element
described in (6), wherein said far ultraviolet light is a
single-peaked high-luminance light with the peak at a wavelength of
210 nm to 220 nm, remarkably at 215 nm.
The Second Group Inventions
[0027] (8) A solid-state far ultraviolet laser characterized by the
generation of laser light with a far ultraviolet region wavelength,
using a highly pure hexagonal boron nitride crystal with far
ultraviolet light emission characteristics as a direct-type
semiconductor solid-state light emitting element and combining
therewith an electron beam irradiation apparatus as an exciting
source.
[0028] (9) The solid-state far ultraviolet laser described in (8),
wherein said light in the far ultraviolet region generated thereby
is the single-peaked high-luminance laser light with a peak at a
wavelength of 210 nm to 220 nm, remarkably at 215 nm.
The Third Group Inventions
[0029] (10) A solid-state far ultraviolet light emitting apparatus,
characterized in that a light emitting layer consisting of a highly
pure hexagonal boron nitride single crystal capable of emitting far
ultraviolet light with a single emission peak in far ultraviolet
region at a wavelength of 235 nm or shorter and an exciting means
for exciting said light emitting layer are combined and
encapsulated together into a vacuum container, and the light
emitting layer is excited to emit far ultraviolet light by
operation of the exciting means.
[0030] (11) The solid-state far ultraviolet light emitting
apparatus described in (10), wherein said far ultraviolet light has
a single peak at a wavelength of 210 nm to 220 nm, remarkably at
215 nm.
[0031] (12) The solid-state far ultraviolet light emitting
apparatus described in (10), wherein said exciting means for
exciting the light emitting layer is an electron beam emitting
means.
[0032] (13) The solid-state far ultraviolet light emitting
apparatus described in (12), characterized in that said exciting
means by the electron beam emitting means consists of an anode
electrode attached to the back surface of the light emitting layer
consisting of a hexagonal boron nitride crystal, an electron beam
emitting substrate attached to the light emitting layer through an
insulating spacer, a cathode electrode attached to the back surface
of the electron beam emitting substrate, and a means to apply
voltage between both electrodes; and an electron beam is emitted
from said electron beam emitting substrate toward the light
emitting layer by application of voltage between both
electrodes.
[0033] (14) The solid-state far ultraviolet light emitting
apparatus described in (13), wherein said electron beam emitting
substrate attached through said insulating spacer is a diamond
substrate.
[0034] (15) The solid-state far ultraviolet light emitting
apparatus described in (14), characterized by the structure of said
diamond substrate wherein a large number of pyramid-shaped
protrusions for emitting the electron beam are arranged in a
lattice-like manner on the surface facing the light emitting
layer.
Effects of the Invention
[0035] In the present invention, the first group inventions make it
possible to create hexagonal boron nitride single crystals having
specific light emission characteristics showing a strong and
high-luminance light emission at a wavelength of 235 nm or shorter,
particularly at 210 nm to 220 nm, remarkably at 215 nm in
wavelength, not obtained by the prior art. Hereby, designing of
solid-state high-luminance ultraviolet light emitting elements have
become possible and various requirements for such as developments
of more and more highly densified recording mediums and stronger
sterilization by higher power output are able to be satisfied.
[0036] Also, in the present invention, the second group inventions
make it possible to provide a compact solid-state light emitting
element and a small solid-state laser having an oscillation
wavelength of around 200 nm which has been difficult to be provided
hitherto, by using a simple means to excite the element consisting
of a highly pure hexagonal boron nitride single crystal with an
electron beam.
[0037] Moreover, in the present invention, the third group
inventions make it possible to provide a solid-state high-luminance
light emitting apparatus compact, low cost, highly efficient, long
lived, and having a single peak at the wavelength of 210 nm to 220
nm, especially at 215 nm at room temperature, by using a highly
pure boron nitride crystal as the emitting layer and by
incorporating integrally this emitting layer and an exciting means,
especially an electron beam exciting means utilizing a substrate
having an electron beam emitting part consisting of a diamond, into
a vacuum chamber.
[0038] As described above, the present invention has succeeded in
providing the compact solid-state light emitting element and the
compact solid-state light emitting apparatus having the oscillation
wavelength of 210 to 220 nm, especially at 215 nm, which has been
difficult hitherto to be realized, and is expected to contribute
largely to the development of various industrial fields. The
compact, high power output, low cost and long lived solid-state far
ultraviolet light emitting element and the solid-state laser, or
the solid-state light emitting apparatus are desired in many
fields, and the range of their utilization has broad divergence
such as the field of semiconductors (for making the
photolithography highly minute), the field of the information (next
generation high capacity optical discs), the field of the medical
care and living body (ophthalmological treatment, DNA cleavage and
the like), and environmental field (sterilization and the like),
and the benefit obtained therefrom may be immeasurable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 is a schematic condition diagram showing the region
for synthesis of recrystallized hBN.
[0040] FIG. 2 shows an example of light emission spectra excited by
electron beams at room temperature.
[0041] FIG. 3 shows an absorption spectrum and a light emission
spectrum excited by the electron beam at low temperature.
[0042] FIG. 4 shows a laser-oscillation spectrum excited by the
electron beam.
[0043] FIG. 5 shows excitation current dependence of the
laser-oscillation spectrum excited by the electron beam.
[0044] FIG. 6 shows excitation current dependence of both the light
emission intensity and the longitudinal mode width (the fringe
width) excited by the electron beam.
[0045] FIG. 7 illustrates an embodiment wherein a plane different
from the light emission output plane is excited.
[0046] FIG. 8 shows a light emission spectrum excited by the
electron beam at low temperature (83 K).
[0047] FIG. 9 illustrates a schematic diagram of a solid-state
laser wherein the laser light is generated and taken out from a
parallel plate sample excited by an electron beam utilizing the
accelerated electron beam of an electron microscope.
[0048] FIG. 10-1 illustrates a preliminary step of a silicon
substrate for producing a diamond electron emitting device,
vapor-deposited with a SiO.sub.2 layer.
[0049] FIG. 10-2 illustrates a process wherein a photoresist
pattern is formed.
[0050] FIG. 10-3 illustrates processes of SiO.sub.2 etching and
SiO.sub.2 mask pattern formation.
[0051] FIG. 10-4 illustrates a step of forming concave
pyramid-shaped pits on the Si substrate and the sectional view of
the Si substrate after completion of the step.
[0052] FIG. 10-5 illustrates a process to produce a diamond device
by the CVD method using the etched Si substrate as the
template.
[0053] FIG. 10-6 illustrates a sectional view of the diamond device
having protruded structures formed after removal of the Si
substrate.
[0054] FIG. 10-7 illustrates an element which is made by mounting
the obtained diamond device on a platinum electrode substrate
through a Ti/Au electrode.
[0055] FIG. 11 illustrates the structure of a solid-state far
ultraviolet light emitting apparatus of the present invention.
[0056] FIG. 12 shows light emission characteristics of the
ultraviolet light emitting element.
EXPLANATION OF SYMBOLS
[0057] 1: a solid-state laser
[0058] 2: an electron gun using a LaB.sub.6 filament
[0059] 3: an accelerated electron beam flow
[0060] 4, 6: electron beam focusing lenses
[0061] 5: a diaphragm
[0062] 7: an electron beam objective lens
[0063] 8: an ellipsoidal mirror
[0064] 9: a parallel plate of the hexagonal boron nitride
crystal
[0065] 10: ultraviolet laser light
[0066] 11: a spectrograph
[0067] 12: a Si substrate
[0068] 13: a SiO.sub.2 layer
[0069] 14: photoresist patterning
[0070] 15: SiO.sub.2 layer etching
[0071] 16: etching of Si
[0072] 17: a diamond layer and pyramid-shaped diamond
[0073] 18: a Ti/Au electrode
[0074] 19: a platinum electrode
[0075] 20: an extraction electrode of Au
[0076] 21: a glass plate
[0077] 22: an electron beam
[0078] 23: far ultraviolet light
[0079] 24: an anode electrode of Ti/Au
[0080] 25: a substrate of a hexagonal boron nitride crystal
The Best Embodiments for Carrying Out the Invention
[0081] Hereinafter, the best embodiments for carrying out the
present invention are explained in a sequential order from the
first group inventions to the third group inventions.
[0082] The first group invention of the present invention relates
to highly pure hBN single crystals capable of emitting ultraviolet
light in far ultraviolet region, synthesis processes thereof, and
the light emitting element consisting of said single crystal.
[0083] The highly pure hBN single crystal capable of emitting
ultraviolet light in the far ultraviolet region is produced by
processes to treat the raw material of hBN at high-temperature and
under high-pressure in the presence of highly pure solvent of
alkali metal or alkali earth metal boronitride, followed by
recrystallization.
[0084] By recrystallization, the hBN single crystal free from
impurity having high-luminance ultraviolet light emission at the
wavelength of 235 nm or shorter, especially at 210 nm to 220 nm,
remarkably at 215 nm, can be obtained. The temperature and pressure
conditions therefor need high-temperature and high-pressure. As a
tentative guidepost, 20,000 atmospheric pressure and 1,500.degree.
C. or higher are preferable.
[0085] These conditions are the temperature and pressure under
which the raw material, boron nitride, is recrystallized into hBN
under the coexistence of the solvent. Boronitride of alkali metal
or alkali earth metal used as the solvent must exist stably without
oxidization or decomposition during the process. Especially, it is
effective to forward the reaction under high-pressure. This
suppresses decomposition of the solvent and enables crystal growth
for a long time necessary for the synthesis of large and highly
pure crystals, and thus is preferable.
[0086] However, attention should be paid to the fact that under too
high-pressure, the raw material hBN may make phase transition to a
high-pressure phase, cBN. That is, in order to obtain intended
highly pure hBN single crystals, the temperature and pressure
conditions in the region free from cBN single crystal generation
are needed. FIG. 1 shows the temperature and pressure conditions to
recrystallize hBN. According to this figure, recrystallization of
hBN is possible even under the thermodynamically stable conditions
of cBN, but transfer to cBN proceeds more easily with the increase
in pressure, and so higher reaction temperature, the condition for
stable hBN, is necessary in order to forward hBN
recrystallization.
[0087] That is, as the upper limit pressure for hBN
recrystallization, around 6 GPa may be appropriate. Under higher
pressure than this, the synthesis conditions must be set at the
thermodynamically stable conditions of hBN and the temperature on
this occasion is near 3,000.degree. C. This condition is not
appropriate for obtaining crystals large enough in size. Therefore,
considering the economical efficiency in industrial manufacturing,
the upper limit for the synthesis condition of said single crystal
may be about 60,000 atmospheric pressure. As for the lower limit,
even under 1 atmospheric pressure or lower, synthesis of
high-luminance light emitting highly pure hBN crystals through
recrystallization may be possible, if decomposition and oxidization
of the solvent can be repressed. In the present invention, the
high-luminance light emitting highly pure hBN crystals were
synthesized in hBN recrystallization region shown by netting in
FIG. 1.
[0088] On the other hand, boronitride of alkali metal or alkali
earth metal and the like easily react with water and oxygen. hBN
recrystallized from the reaction system containing these oxides and
the like as impurities was affected by the impurities such as
oxygen and the like, and the hBN single crystal capable of showing
light emission phenomenon in the short wavelength region at or
below 300 nm, could not be obtained. In contrast to this, the
present invention can provide highly pure hBN single crystals
showing light emission in shorter wavelength region such as at a
wavelength of 235 nm or shorter, especially showing high-luminance
ultraviolet light emission at a wavelength of 210 nm to 220 nm,
remarkably at 215 nm, by using commercially available so-called low
pressure phase boron nitride as the raw material and by dissolution
thereof into the highly pure solvent followed by recrystallization,
which has not been possible to obtain by the conventional
techniques or prior arts.
[0089] Next, the first group inventions are explained specifically
based on examples and figures. However, these examples, etc. are
disclosed for a help of easy understanding of the invention, and
the present invention is never limited by these examples and the
like.
EXAMPLE 1
[0090] Hexagonal boron nitride crystal sintered body (about 0.5
.mu.m grain size) on which deoxidation processing by heat treatment
in vacuum at 1,500.degree. C. and in nitrogen gas stream at
2,000.degree. C. had been applied, was loaded into a molybdenum
capsule in a high-pressure cell together with a barium boronitride
solvent. The preparation of the solvent and loading the sample into
the capsule were all performed under dry nitrogen atmosphere. The
high-pressure reaction cell was treated at the pressure and
temperature conditions of 25,000 atmospheric pressure and
1,700.degree. C. for 20 hours by a belt type high-pressure
apparatus. The increasing rate of temperature was around 50.degree.
C./min. After cooling with the rate of about 500.degree. C./min,
the cell was decompressed and the sample was recovered together
with the molybdenum capsule in the high-pressure cell.
[0091] The molybdenum capsule was removed by mechanical or chemical
treatment (mixed solution of hydrochloric acid and nitric acid),
and the sample was recovered. Colorless and transparent hexagonal
prism form crystals (around 1 to 3 mm) were obtained, and on the
crystals, identification of the phase by optical microscopic
observation, SEM observation and X-ray diffraction, and assessment
of optical characteristics (transmittance, cathode luminescence)
were prosecuted. It was confirmed that the crystal was a single
phase of hBN by X-ray diffraction patterns of the crystal
grains.
[0092] By the cathode luminescence observation, single-peaked
high-luminance ultraviolet light emission was observed near a
wavelength of 215 nm at room temperature as shown in FIG. 2, and,
an ultraviolet light emission bands (as shown by up arrows in the
figure) were observed at 210 nm to 235 nm at the temperature of 83
K, as shown in FIG. 3.
[0093] In a light absorption observation, high transmittance was
shown at a wavelength of 2,500 nm to near 200 nm, and light
absorption structures (as shown by down arrows in the figure) were
observed at the wavelengths of 208 nm and 213 nm at the temperature
8 K as shown in FIG. 3.
EXAMPLE 2
[0094] A hexagonal boron nitride crystal sintered body (about 0.5
.mu.m grain size), on which deoxidation processing had been applied
by heat treatments in vacua at 1,500.degree. C. and in nitrogen gas
stream at 2,000.degree. C., was loaded into the molybdenum capsule
together with the solvent of mixed barium boronitride and lithium
boronitride 1:1 by weight ratio. High-pressure treatment was
applied in the same manner as in Example 1 and the sample was
recovered.
[0095] The recovered sample had a same morphology as in Example 1,
and ascertained to be hBN crystal. By cathode luminescence
measurement, a broad light emission was observed near 300 nm,
together with a high-luminance light emission at a wavelength of
215 nm.
EXAMPLE 3
[0096] A hexagonal boron nitride crystal sintered body (about 0.5
.mu.m grain size) on which deoxidation processing by heat treatment
in vacuum at 1,500.degree. C. and in nitrogen gas stream at
2,000.degree. C. had been applied, was loaded into a molybdenum
capsule together with the solvent of mixed barium boronitride and
lithium boronitride 1:1 by weight ratio. The preparation of this
solvent and loading of the sample into capsule were all performed
under dry nitrogen atmosphere. The molybdenum reaction cell was
processed in nitrogen gas stream at the pressure and temperature
conditions of 1 atmospheric pressure and 1,500.degree. C. for two
hours. The rate of temperature increase was about 10.degree.
C./min. The molybdenum capsule was recovered after cooling with the
rate of about 20.degree. C./min.
[0097] Then, the molybdenum capsule was removed by mechanical or
chemical treatment (mixed solution of hydrochloric acid and nitric
acid), and the sample inside was recovered. The solvent portion
partly showed an aspect of decomposition, but in part,
recrystallization was seen at the interface with the hBN raw
material. Solvent component was removed by acid treatment. After
washing, on the obtained hBN crystal, identification of the phase
by optical microscopic observation, SEM observation and X-ray
diffraction was prosecuted, and assessment thereof through the
optical characteristics tests (transmittance, cathode luminescence)
was done.
[0098] As a result, a broad light emission near 300 nm was observed
by cathode luminescence measurements together with a high-luminance
light emission at a wavelength of 215 nm.
[0099] In other cases than the above examples 1 to 3, measurements
on many samples produced under a little different synthetic
conditions made it clear that the maximum luminescence peaks were
concentrated in particular at a wavelength of 210 nm to 220 nm,
remarkably at 215 nm. Although these maximum luminescence peak
widths are narrow, they distribute with a considerable widths.
Causes thereof are not altogether clear, but ununiformity of the
crystallinity due to defects or minor components such as impurities
may concern.
COMPARATIVE EXAMPLE 1
[0100] Deoxidation processing by thermal treatment in vacuum at
1,500.degree. C. and in nitrogen gas stream at 2,000.degree. C. had
been applied to the commercially available hBN sintered body and
hBN powder, and then light emitting behaviors thereof were measured
by the cathode luminescence. As a result, no single-peaked strong
light emission near 215 nm was observed.
COMPARATIVE EXAMPLE 2
[0101] In the case that the solvent used in the processes described
in Example 1 contained oxygen impurities due to oxidation in part,
recrystallized hBN single crystals could be synthesized by re-using
this solvent in hBN synthesis experiments, mixing the raw material
with the solvent and treating them with high-temperature and
high-pressure. However, by the cathode luminescence measurement, a
broad and strong light emission was observed near a wavelength of
300 nm, rather than 215 nm. It is considered that by the effects of
impurities such as oxygen, the high-luminance short-wavelength
light emission characteristics were inhibited.
[0102] The above comparative example 2 instructs that
recrystallization using highly pure solvents is important to
produce the highly pure hBN single crystals and to make them
express good high-luminance light emission characteristics. These
examples and comparative examples show that, in the synthetic
conditions, atmosphere and high grade purification of the solvents
used are important in producing highly pure high-luminance light
emitting hBN single crystals in the present invention.
[0103] Based on the above findings, using low pressure phase boron
nitride as the raw material and highly pure solvents such as barium
boronitride, recrystallization of boron nitride was conducted. Then
hexagonal boron nitride single crystals showing a behavior of
single-peaked high-luminance light emission at a wavelength of 215
nm was obtained.
[0104] Next, the second group inventions of the present invention
are explained based on examples and figures. However, these
specific explanations are disclosed as a help for easy
understanding of the invention, and the invention is never limited
by these examples. The used materials or numerical conditions such
as impurity concentrations or film widths described in the
following explanations are some of the examples only, and the
invention is never limited by these examples.
EXAMPLE 4
[0105] First, both faces of a parallel plate were formed by
delamination along the cleavage plane utilizing cleavability of the
c plane of the highly pure hexagonal boron nitride crystal obtained
in Example 1, and a Fabry-Perot etalon consisting of the parallel
plate of several tens of .mu.m in thickness was formed.
[0106] FIG. 9 shows a far ultraviolet solid-state laser element
constructed using this parallel plate and an accelerated electron
beam of an electron microscope. In the figure, the element utilizes
an electron microscope constructed of machine components: from the
electron gun 2 using an LaB.sub.6 filament to the electron beam
objective lens 7. Electron beam flow 3 emitted from the LaB.sub.6
filament of the electron gun was accelerated and incident on the c
plane of said parallel plate sample with the energy of 20 KeV and
860 mA/cm.sup.2. Emitted light from the sample was then collected
by an ellipsoidal mirror 8 to be analyzed by a spectrograph 11.
[0107] As a result, it was clarified that far ultraviolet laser
light around wavelength region at 215 nm was emitted from the
sample excited by the electron beam. FIG. 4 is the
laser-oscillation spectrum at that time, from the c plane of the
parallel plate sample about 10 .mu.m in thickness. As shown in FIG.
4, there appeared sharp spectrum structures like fine comb-teeth in
the light emission centering on near 215 nm. These spectrum
structures having shapes of comb-teeth indicate that longitudinal
modes of the Fabry-Perot etalon formed by front and back sides of
the parallel plate are optically amplified by the induced emission
of the hexagonal boron nitride crystal excited by electron beams,
and it became clear that laser-oscillation operation took
place.
EXAMPLE 5
[0108] As in Example 4, making use of cleavability of the boron
nitride single crystal obtained in Example 1, a parallel plate
sample about 6 .mu.m in thickness was prepared, oscillated and
measured in the same way as in Example 4. FIG. 5 and FIG. 6 show
results of the measurements. According to these figures, due to the
incompleteness of the cleavability, a laser threshold of the
electron beam density was elevated and the threshold values of the
laser-oscillation operation and light emission operation were
observed.
[0109] As shown in the lower figure of FIG. 6, when electron beam
density (excitation current) is gradually increased, light emission
output suddenly starts to increase more rapidly at a certain
electron beam density. This electron beam density (excitation
current value) can be defined as the threshold value. In FIG. 5,
from the spectrum with the largest light emitting intensity to the
one with the fifth intensity correspond to the measured points
above the threshold at which light emission output suddenly starts
to increase rapidly in the lower figure of FIG. 6.
[0110] In a resonance mode of Fabry-Perot etalon, that is, a
wavelength position of the longitudinal mode shown by in FIG. 5,
these spectra show the width-narrowing of fringe-like spectra in
the excitation current value range greater than or equal to the
threshold value as is shown in FIG. 6 above, and shows that at each
wavelength position of the longitudinal mode the laser-oscillation
operates above the threshold value. In this way, with the
laser-oscillation threshold value as a borderline, the element is
shown to be usable as a laser element at or above the threshold
value, and as a solid-state ultraviolet light emitting element
other than a laser element below the threshold value.
[0111] Laser oscillation operation in the above mentioned example
refers to the laser-oscillation operation of the sample, the boron
nitride, produced under the specific synthetic condition obtained
in Example 1, but this type of laser-oscillation operation is not
limited to the one obtained in Example 1. Other than Example 1,
similar results were observed on the boron nitride grown under the
synthetic conditions of Example 2 or 3.
[0112] In the above described Examples 4 and 5, the parallel plate
Fabry-Perot etalon was used. However, there is a method, wherein,
instead of the parallel plate, the hBN crystal is processed into
the shape of rectangular waveguide as is shown in FIG. 7. This
structure allows light to reflect at both end faces of the
waveguide to resonate. The side face, not containing the face to
take out the laser light or the emitted light, is excited. Adopting
this method, due to the fact that the face excited by electron is
different from the faces providing laser resonator mirrors, damages
such as pollution and element face brake-down of the laser end face
and the excitation end face can be repressed, and also
amplification region can be set over the whole waveguide. Also, by
optimizing the shape of the light waveguide, single mode
oscillations in both transverse mode and longitudinal mode are
possible.
[0113] Moreover, although a LaB.sub.6 filament was used as the
source of accelerated electron beam in above described Examples 4
and 5, it is possible to drastically decrease the element size by
utilizing, for example, small cathodes such as carbon nano-tube
emitter or a diamond emitter.
[0114] In above mentioned Examples 4 and 5, laser-oscillation and
light emission phenomenon of the light emission band with the peak
at a wavelength of 215 nm were described. The light emission bands
in the wavelength of 210 nm to 235 nm obtained by cooling the above
mentioned sample also show laser-oscillation operation, which can
be understood by the remarkable increase in light emission
intensity at each energy positions of the longitudinal mode as is
shown by the spectrum in FIG. 8. Thus, these bands are possible to
be utilized as lasers.
[0115] In Example 4, as the acceleration energy condition of the
electron beam, acceleration voltage of 20 keV and electron density
of 860 mA/cm.sup.2 were adopted, but the laser-oscillation is not
restricted to this condition, but should be determined by the
optical loss at both end faces of the laser resonator and the
optical loss in the waveguide. With the sample showing the spectrum
in FIG. 4, similar oscillation operation is confirmed, for example,
at the electron density of 0.2 mA/cm.sup.2.
[0116] In Examples 4 and 5, the cleavage planes without
modification were utilized as reflection planes of the Fabry-Perot
etalon. But it is possible to obtain positively a high reflectivity
by adopting an embodiment to deposit suitable metals (Al,
MgF.sub.2) and the like on the cleavage planes to increase the Q
value of the resonator and decrease the threshold value. This
procedure may be expected as an effective means.
[0117] Furthermore, in Examples 4 and 5 described above, an example
was disclosed wherein the single crystal obtained in the embodiment
of the first group invention was used to design a solid-state
laser. This suggests that the boron nitride single crystal itself
can be made into a structure appropriate to resonate the light.
However, it is evident that the present invention has a function as
far ultraviolet generation solid-state light emitting elements, not
restricted to the laser element. Therefore, the present invention
involves an embodiment as solid-state light emitting elements other
than the laser elements. In this case, it is hardly necessary to
say that the boron nitride crystal does not need to be constructed
into a special structure like the resonanator structure of a laser
element. The single crystal has only to be cut into a suitable size
and shape, whereto an electron beam emitting apparatus is combined,
and is used.
[0118] Next, the third group inventions of the present invention
are explained based on examples and figures. However, also these
examples disclosed here are disclosed for a help of easy
understanding of the invention, and the invention is never limited
by them.
[0119] The third embodiments of the present invention provide
specific utilization methods for the invention of the highly pure
hexagonal boron nitride single crystals with far ultraviolet light
emission characteristics obtained in the first embodiments of the
present invention, and propose specifically a solid-state light
emitting apparatus of electron beam excitation type emitting far
ultraviolet light having a single light emission peak at 215
nm.
[0120] FIGS. 10-1 to 10-7 are process drawings illustrating each of
producing steps of the electron emitting device based on a diamond
substrate that causes the light emitting element or the light
emitting layer consisting of said single crystal of the present
invention to emit light. FIG. 11 illustrates structure of a
solid-state far ultraviolet light emitting apparatus of the present
invention produced by this process and FIG. 12 shows far
ultraviolet light emission characteristics of this apparatus.
EXAMPLE 6
[0121] Production processes of the light emitting layer consisting
of the highly pure hexagonal boron nitride crystal are disclosed
here.
[0122] The highly pure hexagonal boron nitride single crystals were
produced according to the same processes as in Example 1.
[0123] The resultant crystals were analyzed and assessed by various
analytical means such as identification of the phase with optical
microscopic observation, SEM observation and X-ray diffraction, and
optical characteristics tests (transmittance, cathode
luminescence). As a result, the crystal was ascertained to be of
the single hBN phase. By the cathode luminescence observation,
single-peaked high-luminance ultraviolet light emission was
observed near a wavelength of 215 nm at room temperature as shown
in FIG. 2, and, an ultraviolet light emission spectrum (as shown by
in the figure) was observed at 210 nm to 235 nm at the temperature
of 83 K, as shown in FIG. 3.
[0124] In a light absorption measurement, high transmittance was
shown from the wavelength around 2,500 nm to 200 nm, and light
absorption structures (shown by in the figure) were observed at the
wavelengths of 208 nm and 213 nm at the temperature 8 K as shown in
FIG. 3.
[0125] As the obtained single crystal had strong cleavability along
the c plane, slice-shaped thin films of about several millimeters
square in area were cut out, making use of this cleavability.
Thickness from the extent of several tens of .mu.m to several .mu.m
may be enough and preferable. Ti/Au (about 15 nm in thickness) was
applied on the back face thereof to form an anode, and was used as
the light emitting layer in the solid-state far ultraviolet light
emitting apparatus shown in the next Examples 7 and 8.
EXAMPLE 7
[0126] Production processes of an electron emitting device made of
diamond for exciting the light emitting layer obtained in Example 6
is disclosed. These processes consist of steps illustrated in from
FIG. 10-1 to FIG. 10-7.
[0127] As shown in FIG. 10-1, a silicon (100) substrate 12 is
provided, and SiO.sub.2 layer 13 of about 200 nm in thickness is
formed on the substrate. Next, after photoresist was applied
uniformly, square pits with one side 70 .mu.m in length were formed
at intervals of 7 .mu.m (FIG. 10-2) using a photoresist pattern 14,
and then naked SiO.sub.2 part was etched 15 by hydrogen fluoride
aqueous solution to form a mask pattern on the SiO.sub.2 layer 13
(FIG. 10-3). Next, concave pyramid-shaped pits consisting of four
(111) planes are formed on the Si (100) substrate 12 by 15%
(CH.sub.3).sub.4NOH solution heated to 90.degree. C. (FIG.
10-4).
[0128] After this photoresist and SiO.sub.2 on the substrate are
removed using hydrogen fluoride aqueous solution or the like, a
boron-added diamond plane is formed by using hot filament CVD
method or the like and mixing diborane gas (B.sub.2H.sub.6) to make
the boron atom/carbon atom concentration ratio of about 100 ppm
(FIG. 10-5). Here, as the diamond plane must support itself, a
thickness of about tens of .mu.m is needed. Next, the Si substrate
12 forming the mold is dissolved away by a liquid mixture of
HF:HNO.sub.3=1:1 to form a diamond substrate 17 with pyramid-shaped
structures (FIG. 10-6). Designating the face with diamond minute
protrusion structures as the front face, a Ti/Au contact 18 for an
electrode is formed on the back face, and then the diamond plate is
placed on an electrically conductive substrate such as platinum
substrate 19 (FIG. 10-7).
EXAMPLE 8
[0129] Construction processes of a far ultraviolet light emitting
apparatus (FIG. 11).
[0130] A glass plate 21 (about 100 .mu.m in thickness) for
insulation was provided on the electron emitting element produced
by the procedure like in Example 7, a circular hole with a diameter
of about 500 .mu.m was formed, and gold (Au) 20 was vapor-deposited
on the surface around the hole edge with thickness of about 50
.mu.m as shown in the figure. On this gold-deposited plane 20, the
thin film of the hexagonal boron nitride crystal produced in
Example 6 was placed so that Ti/Au-deposited face thereof contacts
with the gold-deposited plane, and thus an electron emission device
having the face 17 with the pyramid-shaped minute diamond
protrusions as a cathode and the Ti/Au face on the hexagonal boron
nitride film as an anode 24 is formed. In this case, the
gold-deposited plane on the glass plate works as an extraction
electrode for the anode. The ultraviolet-emission window of this
ultraviolet light emitting element is encapsulated in a glass tube
having an window of quartz or the like, electrodes are pulled out,
and the glass tube is made to be vacuum (for example, high vacuum
at 1.times.10.sup.-5 Torr or lower).
EXAMPLE 9
[0131] Operation procedure of the far ultraviolet light emitting
apparatus constructed as described above is shown.
[0132] By grounding the electrode on the platinum substrate of the
far ultraviolet light emitting apparatus and applying a voltage of
about 1 kV or higher on the anode extraction electrode 24,
electrons are emitted from the emitting source of the diamond
pyramid-shaped minute protrusions 17 and excite the hexagonal boron
nitride crystal 25. The excited hexagonal boron nitride crystal 25
exhibited light emission with the peak at 215 nm at room
temperature. The emitted ultraviolet light is taken out from the
back surface of the hexagonal boron nitride crystal, and is
obtained through the ultraviolet-emission window. FIG. 12 shows a
light-emission spectrum (with a peak at about 215 nm and also
light-emission bands at 300 nm) of this light emitting
apparatus.
EXAMPLE 10
[0133] Procedures of laser-oscillation operation of the far
ultraviolet light emitting apparatus are shown.
[0134] Experimental data already known in the art (non patent
literature 3) have shown that the hexagonal boron nitride crystal
plate can perform laser-oscillation by an acceleration voltage of
about 20 kV when the excitation current density is set to about 0.2
mA/cm.sup.2. At the acceleration voltage of 1 kV in this case,
number of pairs of an electron and a positive hole equivalent to
the above condition can be achieved at about 4 mA/cm.sup.2. With
the current of about 10 .mu.A, the far ultraviolet light emitting
apparatus is considered to perform laser operation.
[0135] Also, by depositing an appropriate metal (Al, MgF.sub.2) or
the like on the upper face of the cleavage plane, effects to obtain
high reflectivity to increase the Q-value of the resonator, and to
decrease the threshold value, are expected. Further, by using a
uniform Al film instead of the Ti/Au film on the lower surface of
the hexagonal boron nitride crystal, similar increase in the
Q-value and decrease in the threshold value are expected.
[0136] As shown in the above examples, the present invention has
succeeded in obtaining a compact and highly efficient ultraviolet
light emitting element or an apparatus completely different from
conventional far ultraviolet light emitting apparatuses. These
examples show just only some embodiments thereof, and the present
invention is not limited to the above examples. For example, the
far ultraviolet light emitting apparatus in the above described
examples uses the boron nitride produced under the specific
synthetic condition obtained in Example 1, and the far ultraviolet
light-emission by electron beam excitation of this boron nitride is
referred to here. However, the light emission like this is not
limited to the one obtained in Example 1. Besides Example 1,
similar results were observed on the boron nitride grown under the
synthetic conditions of Example 2 and 3.
[0137] In the above examples, the diamond emitter is used as an
electron beam source, but, for example, a carbon nano-tube emitter
or the like may also be utilized.
[0138] Moreover, as for the pyramid-shaped minute protrusions, by
further increasing in number, arranging in a lattice-like manner,
and controlling individual protrusion independently, a patterned
electron beam emission and far ultraviolet light emission can be
obtained and utilized for display apparatus and the like, for
example.
[0139] Non-patent literature 3: Nature Materials, vol. 3, 404-409
(2004)
INDUSTRIAL APPLICABILITY
[0140] The present invention provides a hexagonal boron nitride
single crystal showing a strong high-luminance light emitting
behavior at the wavelength of 235 nm or shorter, especially at 210
to 215 nm, having been never obtained by the prior arts. Due to
this, a solid-state high-luminance ultraviolet light emitting
element has become possible to be easily designed. In addition, it
is of great significance that the invention has provided a basic
material capable of responding to recent increasing needs for
developing higher density recording media, and the present
invention is expected to contribute largely to the development of
industry. Also, needs for sterilization treatment by ultraviolet
light have recently been taken up seriously as one of the important
environmental measures.
* * * * *