Method for forming Schottky diode

A method for forming a diode provided with electrodes and a semiconductive layer. Such method comprises applying ion beam irradiation to a substrate having a protruding portion at a desired position for monocrystalline diamond formation. In this manner the substrate is and subjected to surface modification thereby effecting a process for diamond crystal growth on the substrate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a Schottky diode utilizing diamond as a 
semiconductive material. 
2. Related Background Art 
Diamond is known as a unique semiconductor. It is suitable for use in 
electronic devices operating at high speed, high power or under harsh 
climatic conditions. For example, diamond exhibits electron and positive 
hole mobility of 2,000 and 2,100 respectively, which are sufficiently high 
in comparison with those of Si (1,500; 450), GaAs (8,500; 400) or 3C-SiC 
(1,000; 70). Also, the thermal conductivity and the energy band gap of 
diamond are significantly larger than those of other semiconductive 
materials. Tab. 1 shows the comparison of these properties, extracted from 
N. Fujimori, "Handbook of Synthetic Diamond" and Setaka et al;., Science 
Forum, Tokyo, 1989. 
TABLE 1 
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Si GaAs 3C-SiC Diamond 
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Energy gap[eV] 
1.1 1.4 2.2 5.5 
Thermal conductivity 
1.5 0.5 4.9 20.0 
[W/cmK] 
Electron mobility 
1500 8500 1000 2000 
Positive hole mobility 
450 400 70 2100 
[cm.sup.2 /V.sec] 
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However, these excellent properties can only be exhibited in a 
monocrystalline layer, and can hardly be reflected in electronic devices 
in case of an amorphous or polycrystalline structure involving many 
imperfections. Up to the present, diamond in monocrystalline structure 
could only be obtained as a natural product, by synthesis under high 
temperature and high pressure, or by homoepitaxial growth of thin 
monocrystalline layer on such natural or synthesized diamond. Consequently 
diamond is unfavorable in terms of productivity, economy and limitation in 
space as a substrate in comparison with other semiconductive materials. 
On the other hand, deposition of diamond on Si or SiO.sub.2 does not 
provide a monocrystalline layer but a polycrystalline layer in which 
minute monocrystalline domains are mutually separated by domain boundaries 
as reported by B. V. Derjaguin et al., J. Crystal Growth 2 (1986) 380; S. 
Matsumoto et al., Jpn. J. Appl. Phys. 21 (1982) L183; and H. Kawarada et 
al., Jpn. J. Appl. Phys. 26 (1987) L1031. 
A device formed on such polycrystalline layer cannot exhibit sufficient 
device characteristics due to a potential barrier against carrier 
movement, formed by the domain boundary. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a method for forming a 
Schottky diode with improved device characteristics in a manner which 
artificially avoids the undesirable influence of grain boundaries on a 
substrate with reduced limitation on productivity, economy and area. 
The above-mentioned object can be attained, according to the present 
invention, by a method for forming a diode with an electrode and a 
semiconductive layer, characterized by applying ion irradiation to a 
substrate having a projecting portion in a desired position for 
monocrystalline diamond growth and subjected to surface modification, and 
effecting diamond crystal growth.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The object of the present invention can be attained by a method having the 
features described in the appended claims. More specifically, the present 
invention allows to obtain a Schottky diode with reduced fluctuation in 
performance and improved device characteristics particularly with 
excellent inverse voltage resistance. This is accomplished by growing a 
diamond layer on a silicon substrate in such a manner that the active area 
of the device is excluded from grain boundaries. Thereby reducing 
limitation on productivity, economy and area. 
In general, the crystalline structure of a deposited layer is determined by 
and follows that of a substrate if an epitaxial relationship stands 
between both crystalline structures. For example, a monocrystalline 
silicon layer can be grown on a monocrystalline silicon substrate. Also a 
monocrystalline GaAs layer can be grown on a monocrystalline GaAs 
substrate. Such relationship generally stands when the lattice constant of 
the deposited layer is the same as that of the substrate, or when the 
chemical properties of the deposited layer are close to those of the 
substrate. The incoming atoms of the deposited layer diffuse on the 
surface of the substrate, then preferentially remain on kinks or steps on 
the substrate surface, and grow into a layer, forming a terrace. On the 
other hand, when GaAs is deposited on a monocrystalline Si substrate, a 
monocrystalline GaAs layer can be formed under suitable depositing 
conditions, even though the lattice constants of both layers are 
significantly different. In this case, however, because of elevated 
interfacial energy resulting from the difference in chemical properties, 
the growth does not take place in a layer but follows a process of 
three-dimensional nucleation effecting growth and amalgamation of such 
grown nuclei into a continuous layer. In said amalgamation, no grain 
boundaries are formed because the directions of crystallization of the 
nuclei are three dimensionally defined by that of the monocrystalline 
substrate. Consequently the electronic device formed on thus obtained 
monocrystalline semiconductor layer can function reflecting the excellent 
properties of said semiconductor layer. 
On the other hand, if the epitaxial relationship does not stand between two 
layers, as in the case of diamond and silicon, three-dimensional nuclei 
with disorderly directions of crystallization are formed in arbitrary 
positions on the substrate as shown in FIG. 1A, and grow individually. As 
a result, mutually neighboring nuclei collide with each other to form a 
boundary of uncontrolled position at the interface, so that a 
polycrystalline structure is eventually obtained as a whole, as shown in 
FIG. 1B. Although a monocrystalline structure is present within a domain 
surrounded by grain boundaries, the area of such monocrystalline domain is 
inevitably random because of the random position of the grain boundary 
resulting from the random nucleation positions. The electronic device 
formed on such random polycrystalline layer is unable to reflect the 
excellent properties of the material of said layer. In fact the 
characteristics of such device are predominantly determined by the height 
and quantity of potential barries of the grain boundaries, and are 
therefore fluctuating and insufficient. 
However the present inventors have found that in a system without epitaxial 
relationship, the positions of nucleation can be controlled by a structure 
shown in FIGS. 2A and 2B. It is therefore rendered possible to grow a 
monocrystalline area of a size enough for device formation, in an 
arbitrary position. It is also possible to prevent the formation of 
boundaries, by terminating the growth of monocrystalline islands prior to 
the collisions thereof. Furthermore, even after the collision of the 
growing monocrystalline islands, the area for forming the device can be 
made free from the grain boundaries because such boundaries can be formed 
in desired positions. It is therefore rendered possible to form the 
devices without the grain boundaries or with predetermined quantity and 
position of the grain boundaries, and to obtain device characteristics 
without fluctuation and reflecting the properties of the material, as in 
the case of device formed on a monocrystalline layer. 
Schottky diodes prepared in this manner are naturally much more uniform, in 
performance than those formed on a random polycrystalline layer. 
Now reference is made to FIGS. 3A to 3D for explaining an embodiment of the 
present invention. At first, surface irregularities are formed on the 
surface of a silicon substrate 32. Then said substrate 32 is immersed in 
liquid containing fine diamond particles 31 and is subjected to vibration, 
for example by ultrasonic wave to modify the surface of said silicon 
substrate 32. In this step the substrate surface is given a stress energy, 
minute coarseness or deposition of small diamond crystals, for increasing 
the nucleation density of gaseous grown diamond. Diamond nucleation 
scarcely takes place on the silicon substrate 32 without the 
above-mentioned step of surface modification. If diamond is deposited in 
the state shown in FIG. 3A, a polycrystalline diamond layer is formed over 
the entire surface of the substrate 32 without any selectivity. For the 
purpose of forming a single diamond nucleus selectively in a desired 
position, the present inventors have found a method of diagonal ion beam 
irradiation with an incident angle .theta., as shown in FIG. 3B. The 
modified surface layer, formed by diamond particles as explained above, is 
removed by the ion beam irradiation, but said surface layer, essential for 
the nucleation, remains in the farthest position, with respect to the 
direction of ion beam irradiation, among four corners of the protruding 
portion formed on the substrate surface. Thus the single nucleus of 
diamond is formed only in this position by a diamond forming process as 
shown in FIG. 3C. The modification of silicon surface by diamond particles 
takes place most strongly at the corners of the protruding portion, so 
that the sites for diamond nucleation are formed more densely in said 
corners. The diagonal ion beam irradiation has least effect of removal of 
said nucleation sites in the farthest corner in the direction of 
irradiation, so that the sites can be selectively left in said corner. The 
diamond nucleus grows by the continuation of the above-mentioned diamond 
forming process, and eventually contacts the neighboring diamond crystals. 
FIG. 4 shows the structure of Schottky diodes formed on diamond crystals 
having sufficiently large monocrystalline areas selectively grown in 
predetermined positions as explained above. The Schottky diode was 
prepared by forming the above-explained diamond layer 42 on a P.sup.+ Si 
substrate 43, then forming an ohmic electrode 44 on the bottom face of 
said substrate and forming an aluminum electrode 41 on the top face of 
said diamond layer. 
For the purpose of comparison, a Schottky diode was prepared in a similar 
manner on a polycrystalline diamond layer with random nucleation. FIG. 5 
shows the measured voltage-current characteristics, in which a solid line 
indicates the performance of the device formed on the diamond crystal with 
controlled nucleation position, while a broken line indicates that of the 
device formed on the diamond layer with random nucleation position. 
Though the performance in the forward direction is almost the same for both 
devices, the breakdown voltage in the reverse direction in the device 
formed on the selectively grown crystal is about twice as large as that of 
the device formed on the randomly grown crystals. Stated differently, the 
inverse breakdown voltage of the Schottky diode increases drastically when 
it is formed on the diamond crystal which lacks the grain boundary 
functioning as a path for leakage current. 
In order to remove the modified surface layer of the substrate while 
leaving enough area of said layer for the formation of single diamond 
nuclei, the incident angle of the ion beam irradiation is preferably 
within a range from 5.degree. to 90.degree., more preferably from 
10.degree. to 80.degree., and most preferably from 20.degree. to 
70.degree.. 
Examples of irradiating ions include argon, silicon, germanium, phosphor 
and boron. 
The acceleration energy of the irradiating ions is preferably within a 
range from 1 to 10 keV, more preferably from 2 to 8 keV, and most 
preferably from 3 to 7 keV. 
Examples of the substrate include a monocrystalline silicon substrate 
(orientation of crystal (100), (110), (111) etc.), a polycrystalline 
silicon substrate, and a glass substrate principally composed of 
SiO.sub.2. 
Also the depth of surface irregularities formed on said substrate is 
preferably within a range from 0.1 to 1 .mu.m, more preferably from 0.2 to 
0.8 .mu.m, and most preferably from 0.12 to 0.5 .mu.m, in view of 
formation of a modified surface layer suitable for obtaining single 
diamond nuclei in desired positions on the substrate. 
Though the protruding portions of the substrate have been assumed to be 
rectangular in the foregoing description, said protruding portions are not 
limited to such shape. 
The atmospheric pressure at the aforementioned ion beam irradiation onto 
the substrate is preferably within a range from 10.sup.-6 to 10.sup.-2 
Torr. Below the lower limit mentioned above, the ion current becomes 
insufficient and the selectivity for diamond deposition becomes deficient. 
On the other hand, above the upper limit, the ion current becomes 
excessively large and the selectivity again becomes deficient. The former 
deficiency results from excessively high diamond nucleation density, while 
the latter results from excessively low diamond nucleation density. 
Also the area of each protruding portion formed on the substrate is 
desirably so small as to allow formation of a single crystal per each 
protruding portion, and is preferably not exceeding 16 .mu.m.sup.2, and 
more preferably not exceeding 4 .mu.m.sup.2. 
EXAMPLE 
On the surface of a P-type silicon monocrystalline (100) substrate, there 
were formed, by reactive ion etching, rectangular protruding portions of a 
size of 2.times.2 m.sup.2 and a height of 0.2 .mu.m, mutually spaced by 
10, 20, 30 or 40 .mu.m. The specific resistivity of the substrate was 
10.sup.-2 .OMEGA.cm. The substrate was then immersed in aqueous dispersion 
of diamond particles of Ca. 30 .mu.m in diameter, and was subjected to 
ultrasonic vibration for 3 minutes (cf. FIG. 3A). After drying, the 
substrate was subjected to irradiation with argon ion beam with an 
incident angle .theta.=30.degree. (cf. FIG. 3B). The atmospheric pressure 
during said irradiation was ca. 10.sup.-4 Torr. The ion beam current and 
acceleration voltage were respectively 50 .mu.A and 5 kV, and the 
irradiation time was 12 minutes. 
Subsequently diamond was grown on said substrate by a CVD process utilizing 
microwave plasma (cf. FIG. 3C) under following conditions: 
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substrate temperature 150.degree. C. 
pressure 35 Torr 
microwave power 310 W 
gas CO (5%)/H.sub.2 
growth time 15 hours 
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The above-mentioned growth provided single crystals of diamond, each about 
10 .mu.m in diameter, uniformly arrange with a pitch of 10, 20, 30 or 40 
.mu.m. The thickness of said crystals was about 5 .mu.m. For the purpose 
of comparison, diamond deposition conducted under the same conditions on a 
substrate which has been subjected to surface modification with diamond 
particles without formation of protruding portions and without ion beam 
irradiation provided a polycrystalline diamond layer with widely varying 
grain sizes from 1 to 5 .mu.m, as shown in FIG. 1B. 
Though the raw material was not doped with impurities, boron present at a 
high concentration in the substrate was diffused into the diamond in the 
course of growth thereof. 
Schottky diode was prepared in the following manner. Aluminum electrodes 
41, 44, of a thickness of 0.5 .mu.m, were formed by electron beam 
evaporation respectively on the bottom face of the Si substrate 43 and on 
the top face of the diamond layer 42. The aluminum functions as an ohmic 
electrode 44 for the silicon of low resistance, and as a Schottky 
electrode 41 for the diamond layer. As shown in FIG. 5, the inverse 
breakdown voltage of the diode formed on monocrystalline diamond was about 
twice as large as that of the diode formed on polycrystalline diamond. 
As explained in the foregoing, the Schottky diode formed on a sufficiently 
large selectively deposited monocrystalline diamond crystal having a 
sufficiently large monocrystalline grown in a predetermined position 
according to the present invention is superior, particularly in the 
inverse breakdown voltage, to the Schottky diode formed on a 
polycrystalline diamond layer with random nucleations.