Electric device with grains and an insulating layer

An electric device comprises a crystalline film deposited on a substrate and an electrode formed on the film. The crystalline film consists of a number of colomnar crystal grown at right angles from the surface of the substrate. There are many grain boundaries passing through the crystalline film from the substrate surface to the electrode. The direct contact between the electrode and the grain boundaries is prevented by means of an insulating coating applied only to the portion of surfaces of the columnar crystals where the boundaries appear.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to electric devices and a method of 
manufacturing the devices and, more particularly to electric devices and a 
manufacturing method utilizing crystalline films such as light emitting 
devices. 
2. Description of the Prior Art 
For emission of reddish light rays, GaAs semiconductors have been utilized 
to manufacture light emitting devices for more than a decade. The emission 
of blue or green light, as well as white light, however, has long been 
hardly realized by means of solid state devices. 
The inventor has before proposed to make a light emitting device from 
diamond which can emit light at short wave lengths, for example, as 
described in Japanese Patent Application No. sho 56-1469360 filed on Sept. 
17, 1981. Diamond is promising, as a light emitting substance for mass 
production, because of its high thermal resistance, chemically stabilities 
and low price, in view of a great demand for light emitting devices in the 
market. It is, however, very difficult to manufacture diamond light 
emitting devices at a high yield required for commercialization because 
there are formed a large proportion of products whose efficiencies are 
undesirably too low to satisfy the requirement of the application thereof. 
Furthermore, the performance of prior art diamond devices tends to quickly 
age by actual operation. For example, prior art diamond light emitting 
devices were heated up to 50.degree. C. only by application of 30 V for 10 
min and the performances were then significantly degraded. 
BRIEF SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an electric device with 
a crystalline film having a high performance just expected from nature of 
the crystalline film. 
It is another object of the present invention to provide a method of 
manufacturing electric devices at a high yield in mass production. 
Additional objects, advantages and novel features of the present invention 
will be set forth in the description which follows, and in part will 
become apparent to those skilled in the art upon examination of the 
following or may be learned by practice of the present invention. The 
object and advantages of the invention may be realized and attained by 
means of the instrumentalities and combinations particularly pointed out 
in the appended claims. 
The present invention has been made based upon the discovery of the origin 
of the low yield of diamond light emitting device manufacture. The light 
emitting action of diamond light emitting devices takes place when a 
relatively large current is passed through diamond crystals by applying a 
voltage between a pair of electrodes sandwiching the diamond crystals. The 
electric energy carried by the current, however, is consumed largely only 
to produce heat rather than to emit light rays. The inventor succeeded in 
the discovery of the origin of the low efficiencies and the heat 
generation. As a result, the following fact has been found. 
When deposited, diamond tends to form a polycrystalline film 2 composed of 
columnar crystals 2 grown at right angles on a substrate as illustrated in 
FIG. 1. There are formed many grain boundaries 4 extending through the 
diamond film 2. It was found by Raman spectroscopic analysis that these 
grain boundaries consist of segregation of carbon graphite which has a 
resistivity substantially lower than that of diamond crystals, e.g. by a 
factor of 10.sup.2 .about.10.sup.4. Furthermore, these boundaries tend to 
gather metallic ions occuring inadvertently in the film 5. The metallic 
ions also function to elevate the conductivity of the boundaries. Because 
of this, a large proportion of current flows across the film along the 
boundaries rather than through the diamond crystals. It is for this reason 
that prior art diamond electric devices can not exhibit sufficient 
performances which are inherently expected from the characteristics of 
diamond itself. For example, the current passing through the boundaries 
has no contribution to light emitting but only function to generate heat. 
Another origin of heat generation is existence of pinholes 5 passing 
through the film 2 which are undesirably but often formed during 
deposition. When an upper electrode 3 is deposited, short current passages 
are formed undesirably. 
To achieve the foregoing and other object, and in accordance with the 
present invention, as embodied and broadly described herein, it is avoided 
to form graphite paths which bypass diamond crystals and conceal the 
inherent electric characteristics of diamond. The present invention 
utilizes the fact that diamond films are deposited with grain boundaries 
exposed on the perimeters of the crystals which are recessed from the 
center top surfaces of the columnar crystals. These recesses are filled 
with and insulated by an insulating material such as a photoresist or a 
molten glass, e.g. molten SiO.sub.2 doped with boron or phosphorus. The 
insulated material prevents an upper electrode formed on the diamond film 
from being in contact with the graphite material in the boundaries. 
By this structure, the electrode makes electrical contact only with diamond 
crystals and therefore current passed through the insides of the crystals. 
The current induces recombination of electron-hole pairs between mid-gap 
states (radiation centers), between the mid-gap states and a valence band, 
between a conduction band and the mid-gap states and between the 
conduction band and the valence band. The spectrum of light emitted from a 
diamond film is determined by differential energy levels between the 
mid-gap states, the bottom of the conduction band and the apex of the 
valence band. Depending upon the differential levels, it is possible to 
emit blue or green light or radiation having continuous spectrum of 
wavelengths over a relatively wide range such as white light. 
The above discussion can be applied for general natures of crystalline 
films consisting of columnar crsytals, and therefore the present invention 
has general applicability to other crystalline material than diamond.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 2(A) to 2(C), a method of forming a light emitting 
device in accordance with a first embodiment of the invention will be 
explained. A diamond coating 2 is deposited on a single crystalline 
silicon semiconductor substrate 1 to a thickness of 0.5 to 3.0 
micrometers, e.g. 1.3 micrometers, by a known microwave-assisted plasma 
CVD method in a magnetic field. The CVD methods of such a type include ECR 
CVD or MCR CVD as described in Japanese Patent Application No. sho 
61-292859 filed on Dec. 9th, 1986 and U.S. patent application Ser. No. 
07/178,362 of the applicant. The substrate 1 has been heavily doped with 
boron ions in order to be a p-type impurity semiconductor substrate having 
a low resistivity. The surface of the substrate to be coated is finely 
scratched by putting it in a liquid in which diamond fine particles are 
dispersed and applying ultrasonic waves thereto for 10 min to 1 hour. The 
scratchs form focuses on the surface of the substrate for growing diamond 
crystals therefrom. 
The microwave CVD process of the diamond film is carried out in a plasma 
CVD chamber provided with a microwave oscillator capable of emitting 
microwaves at 2.45 GHz at a maximum output power of 10 KW and a pair of 
Helmholtz coils. The coils are energized during the deposition to induce a 
magnetic field having a maximum strength of 2K Gauss and a resonating 
strength of 875 Gauss at the surface of the substrate to be coated. The 
deposition of the carbon coating will be carried out in accordance with 
the following procedure. 
The scratched substrate is disposed in the chamber in order to experience 
the magnetic field of 755 Gauss. After evacuating the inside of the 
chamber to 10.sup.-3 to 10.sup.-6 Torr, a reactive gas is introduced to a 
pressure of 0.01 to 50 Torr, e.g. 0.1 Torr. The reactive gas comprises an 
alcohol such as methyl alcohol (CH.sub.3 OH) or ethyl alcohol (C.sub.2 
H.sub.5 OH) diluted with hydrogen at 1 to 15 vol %, e.g. 5 vol %. Then, 
microwaves are applied at 2.45 GHz in the direction parallel to the 
direction of the magnetic field to cause ionized particles of the reactive 
gas in the form of plasma to resonate therewith in the magnetic field. As 
a result, a number of columnar crystals 21' of diamond grow on the 
substrate. During the deposition of diamond crystals, carbon graphite is 
also deposited. However, the graphite, which is relatively chemically 
unstable as compared with diamond, reacts with oxygen and hydrogen ions 
which also occur in the plasma of the alcohol and is removed from the 
deposited film in the form of gases, i.e. carbon dioxide or methane. The 
graphite then occurs only in the grain boundaries between the diamond 
crystals. 
The deposited carbon film eventually includes a number of grain boundaries 
4 and some pinholes 5. It should be noted that grain boundaries extend 
upward from the substrate and terminate in recesses located between 
adjacent crystals 2' at the film surface. In accordance with the present 
invention, these recesses and pinholes are filled with an insulating 
material. Namely, a positive photoresist 6 is coated on the diamond film 2 
by a spin coating method. The photoresist is preferably chosen from 
transparent photoresists such as OFPR 800-60CP having a viscosity of rank 
C which is diluted four times by a thinner to have an appropriate 
viscosity. The photoresist is then baked at 80.degree. C. for 20 min and 
exposed to ultraviolet light illumination from the upper side at 2 
mW/cm.sup.2 for about 6 seconds. The duration of this exposure to 
illumination is precisely adjusted in order to develop the photoresist 
only to a level designated by numeral 11 from the upper surface thereof 
(FIG. 2(B)). Namely, the photoresist is only developped to a plane which 
is perpendicular and slightly lower than the top surface of the diamond 
crystals, leaving the portion below the plane undeveloped filling the 
pinholes 5' and the recesses 4'. The developed portion of the photoresist 
is removed by a solution (NMD-3). The undeveloped portion is left as it is 
or by thermally cured (hardened) if necessary. 
By this procedure, the pinholes and the grain boundaries are hidden from 
the upper side surface of the diamond film while the top surface of the 
diamond crystals are left exposed. The upper surface of the film 2 is then 
coated with an upper electrode 7 made of indium tin oxide (ITO) is direct 
electrical contact with the diamond crystals. The electrode 7, however, 
shall not make direct contact with the pinholes 5 or the grain boundaries 
4 by virtue of the photoresist. On the electrode 7 is formed a lead 
electrode 12 which is made from a multilayered film consisting of alminum, 
silver and/or molibdenum films. The lead electrode 12 may be formed by an 
alminum film alone or a dual film consisting of an upper cromium or 
molibdenum film and a lower alminum film coated thereon. The upper cromium 
or molibdenum film is high heat-resistant and has a high stability while 
the lower alminum film is suitable for wire bonding. As a result, a light 
emitting device with a light emitting area of 1 to 10 mm square e.g. 3 mm 
square is formed. 
When a voltage of 10 to 200 V (e.g. 60 V) was applied across the diamond 
film 5 of a diamond light emitting device formed in accordance with the 
above procedure by means of the upper electrode 7 and the substrate 1 
functioning as the counterpart lower electrode, diamond emitted green 
visual light at 12 cd/m.sup.2 by virtue of current passing therethrough. 
The voltage may be applied as a DC voltage or as a pulse train at 100 Hz 
of a duty ratio of 50%. The illumination was not appreciably reduced even 
when application of 60 V was continued for a month. 
Referring now to FIG. 3, a second embodiment of the present invention will 
be described. The upper surface of a single crystalline silicon 
semiconductor substrate 20 is insulated by coating of a silicon nitride 
film of 0.5 to 1 micrometer thickness. On the insulated surface, a diamond 
film is deposited to a thickness of 0.5 to 3 micrometers, e.g. 1.2 
micrometers in the same manner as the film 2 of the first embodiment. The 
upper surface of the diamond film 2 is given fine scratchs in the same 
manner as the substrate 1 of the first embodiment is given. The scratched 
surface is schematically shown with broken line 22. The scratchs form a 
number of recombination centers (light emitting centers) on the surface of 
the film 2. The filling procedure or recesses and pinholes with a 
photoresist 8 is same as that of the first embodiment. A pair of 
electrodes 7 and 17 are formed on the upper surface of the substrate in 
order to pass current parallel to the surface. When a 40 V was applied 
between the electrodes of a light emitting device formed by this 
procedure, 6 cd/cm.sup.2 light emission was observed. 
Referring now to FIGS. 4(A) to 4(D), a third embodiment of the present 
invention will be explained. Similar numerals are used for designating 
corresponding parts as those used in FIGS. 2(A) to 2(C) and redundant 
explanations therefore will not be repeated. After a diamond film 2 is 
deposited on a semiconductor substrate 2 in the same manner as in the 
first embodiment, a silicon film 23 is deposited to a thickness of 300 to 
3000 angstroms over the upper surface of the diamond film 2 by the 
decomposition of disilane or monosilane in the same chamber as the diamond 
film. The silicon film 23 is then heated at 500.degree. to 1000.degree. C. 
in a hydrogen atmosphere to form a silicon carbide coating 24 on the upper 
surface of the diamond film through alloying reaction between the diamond 
films 2 and the silicon film 23 as illustrated in FIG. 4(B). 
The upper surface of the diamond film 2 provided with the silicon carbide 
film 24 is coated next with an organic glass such as (CH.sub.3).sub.3 OH 
and (C.sub.2 H.sub.5).sub.4 O.sub.4 Si by a spin coating method as shown 
in FIG. 4(C). The organic component of the glass coating is driven off by 
heating it at 200.degree. to 500.degree. C., e.g. 400.degree. C. in order 
to solidify the coating. In this way, the upper surface of the diamond 
film 2 is covered with an inorganic glass film 25. The portion of the 
glass film no lower than the top surfaces of the diamond crystals is 
removed by etching for the same purpose as in the first embodiment. An 
upper electrode 7 and a lead electrode 12 are formed on the exposed upper 
surface of the diamond film 2 in the same manner as the first embodiment. 
In this embodiment, undesirable reaction between the inorganic glass and 
the diamond is prevented by means of the intervening silicon carbide 
coating 24 and a long-term reliability can be obtained. Of course, a 
similar structure can be formed with an insulating substrate and a pair of 
upper electrodes as described in FIG. 3. 
Similar electric devices can be formed by use of crystalline boron nitride 
films instead of diamond film in accordance with any aboce embodiments in 
the same manner. The method and structure are substantially same as 
described above except particularly provided in this paragraph. In this 
case, suitable substrates to be coated with crystalline boron nitride 
films are made from, for example, beryllium nitride, diamond, a silicon 
semiconductor, or any other pane whose surface is insulated by an 
insulating material such as amorphous boron nitride in advance. The 
suitable reactive gases are, for example; boron source gases such as 
diborane (B.sub.2 H.sub.6), boron fluoride (BF.sub.3), organic boron 
compounds, methylboron (B(CH.sub.3).sub.3) and nitrogen source gas such as 
ammonia (NH.sub.3), nitrogen (N.sub.2), nitrogen fluoride (NF.sub.3). The 
boron source gases and the nitrogen source gases are used in combination. 
Some gaseous compounds such as borofluoric ammonium (NH.sub.4 BF.sub.4) 
have both functions of the boron and nitrogen sources and can be used 
alone. The pressure of reactive gases during deposition is 0.01 to 10 
Torr, typically 0.1 to 1 Torr, e.g. 0.26 Torr. The input power of 
microwaves is 2 to 10 KW. The thickness of crsytalline boron nitride film 
consisting of columnar crystals is 0.5 to 5 micrometers, e.g. 1.3 
micrometers on the average. The substrate temperature 400.degree. to 
1200.degree. C., e.g. 1000.degree. C. Other conditions are same as used in 
the above embodiments. Of course, crystalline diamond films and 
crystalline boron nitride films can be used in combination to form 
multilayered films. 
The foregoing description of preferred embodiments has been presented for 
purposes of illustration and description. It is not intended to be 
exhaustive or to limit the invention to the precise form described, and 
obviously many modifications and variations are possible in light of the 
above teaching. The embodiment was chosen in order to explain most clearly 
the principles of the invention and its practical application thereby to 
enable others in the art to utilize most effectively the invention in 
various embodiments and with various modifications as are suited to the 
particular use contemplated. Although the photoresist is positive in the 
first and second embodiments, a negative photoresist can be used when the 
electric device is constructed on a transparent substrate. In this case, 
the negative photoresist is cured by illumination effected from the 
substrate side. 
The present invention is broadly applicable to any electric device which 
utilizes a polycrystalline film since the principle is based only upon the 
general feature of deposited crystalline film including grain boundaries. 
These electric devices can be formed on a single substrate, i.e. an 
integrated circuit device which may consist of diamond light emitting 
devices, diamond diodes, diamond transistors, diamond resistances, diamond 
capacitors and the like. Of course, it is possible to sever a single 
substrate, after a number of diamond devices are formed on the substrate, 
into individual separate devices.