JP3271589B2 - Ultra high frequency oscillator and ultra high frequency oscillator using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-high frequency oscillator using a diamond film and an apparatus using the same.

[0002]

2. Description of the Related Art By irradiating an ultrashort optical pulse to an ultrahigh-frequency oscillator made of a semiconductor material or a second-order nonlinear optical material, a TH corresponding to the Fourier component of the envelope component of the optical pulse is obtained.
z Electromagnetic waves are emitted. THz electromagnetic waves can be applied to spectroscopic analysis, gas analysis, imaging, and the like.

The generation of a THz electromagnetic wave from an ultrahigh frequency oscillation element has been explained by the fact that the dipole of the electrode changes due to the time change of the photoexcitation current. That is, to generate a THz wave, it is necessary to quickly generate a photo-excitation current and quickly extinguish it. Therefore, in order to generate carriers, it is more advantageous to irradiate with excitation light having a shorter pulse width. This is because carrier generation continues for the irradiation time. Further, in order to increase the intensity of electromagnetic wave radiation, it is necessary to increase the intensity of the electric field applied to the electrodes, and the material is required to have high withstand voltage.

Conventionally, this type of THz element is described in, for example, "199
October 1995, Applied Physics Letter, No. 7
Volume 1, Issue 16 (APPLIED PHYSICS LETTER, VOL.71, N
O.16, OCTOBER, 1997) ”, a superconducting material, YBCO, was formed on MgO, and this YBCO was shaped into a dipole antenna, and then irradiated with a titanium sapphire laser. In this method, THz waves are radiated by utilizing fast reduction and recovery of superconducting current.

[0005] Also, March 1997, Japanese Journal of Applied Physics, 36th edition.
Vol.3B (JAPANESE JOURNAL OF APPLIED PHYSICS
VOL.36, NO.3B, MARCH, 1997), in order to shorten the traveling distance of the carrier, use a tunnel microscope or an atomic force microscope and set the interval between the parts where the carrier is generated to several tens nm. Has been taken. In this method, Ti serving as an electrode is formed on a GaAs substrate, and the Ti is oxidized by an atomic force microscope to form a portion where photocarriers are generated. Since the width of Ti oxide made with an atomic force microscope is as narrow as several tens of nanometers, the traveling distance of photocarriers can be shortened.
As a result, THz waves are emitted.

Also, "September 1997, Applied Physics, No. 66
As shown in Vol. 9, No. 9, a sample is placed in an electric field to emit THz waves, thereby successfully emitting THz with high brightness.

In recent years, an element using a diamond having the highest withstand voltage has been developed. (IEEE JOURNAL OF
QUANTUM ELECTRONICS, VOL.QE-22, NO.1, JANUARY, 198
As shown in “6),” a super-high-frequency oscillation element was fabricated on single crystal diamond, and a pulse waveform with a full width at half maximum of about 400 ps was obtained using an Nd: YAG laser capable of generating a ps pulse as excitation light.

[0008]

However, the above-mentioned prior art has the following problems.

[0009] An ultra-high frequency oscillation element using a superconducting material,
A problem is that a special device is required in addition to the element for generating THz. High strength THz in this type of element
This is because, in order to generate a wave, it is necessary to cool the sample to use a superconducting substance or to install a magnet for applying a magnetic field.

In the case of an ultra-high frequency oscillation element using a semiconductor material such as GaAs, a problem arises in that a special fine processing technique is required to manufacture the element. It is necessary to shorten the distance between the electrodes in order to shorten the travel distance of the optical carrier, which requires a processing method using an atomic force microscope or a tunnel microscope instead of the conventional lithographic technology Becomes The processing by the atomic force microscope or the tunnel microscope is complicated in operation, and at the same time, it is difficult to perform a wide range of processing. In this respect, there is room for improvement.

In the case of an ultra-high frequency oscillator using single crystal diamond, the carrier lifetime is long and it is difficult to obtain an element with high efficiency, and in particular, it is extremely difficult to obtain a high frequency THz wave. This is because there are not enough recombination centers in single crystal diamond. Also,
Since single crystal diamond has a wide band gap, the laser used needs to have energy higher than the energy of this band gap. A laser with a narrow pulse width, such as a femtosecond laser, has a long wavelength and cannot be used because it is smaller than the band gap energy of single crystal diamond.

Also, the above-mentioned conventional ultrahigh frequency oscillation element has insufficient withstand voltage characteristics, and when a high voltage is applied between the electrodes, the element may not function properly due to an increase in leakage current or dielectric breakdown. there were. Therefore, it is difficult to generate sufficiently high-intensity THz waves, and there is room for improvement in this respect.

The present invention has been made in view of the above problems, and has as its object to provide an ultrahigh frequency oscillation element and an ultrahigh frequency oscillation device capable of generating a high-intensity and high-frequency THz wave. .

[0014]

According to the present invention, there is provided a substrate, a diamond film provided on the substrate, and at least one pair of electrodes provided separately on the diamond film. An ultra-high frequency oscillation element that irradiates excitation light to a light irradiation location in the diamond film to generate an electromagnetic wave, wherein the light irradiation location in the diamond film is made of polycrystalline diamond. Provided.

Further, according to the present invention, there is provided an ultra-high-frequency oscillator having a light source for emitting excitation light, an ultra-high frequency oscillation element irradiated with the excitation light, and a means for applying a voltage to an electrode provided on the ultra-high frequency oscillation element. In the high-frequency oscillator, there is provided an ultra-high-frequency oscillator, wherein the ultra-high-frequency oscillator is the above-described ultra-high-frequency oscillator of the present invention.

The ultra-high frequency oscillation element is a device in which a pair of electrodes are provided on a diamond film at a distance, and an electromagnetic wave is generated by irradiating excitation light such as a pulse laser while applying a voltage between the electrodes. The light irradiation location in the diamond film is, for example, a gap portion sandwiched between the pair of electrodes, but the entire surface of the diamond film may be the light irradiation location. In the present invention, the irradiated portion of the pulse laser is made of polycrystalline diamond. Since a diamond crystal has a higher dielectric strength than other semiconductor materials, a high voltage can be applied between the electrodes, and a high-luminance high-frequency electromagnetic wave can be emitted when excitation light is irradiated.

Further, in the ultrahigh frequency oscillation element of the present invention, since the density of crystal grains is set to a certain value or more, the number of grain boundaries is increased as compared with the conventional element. This increases the recombination centers,
Generated carriers can be quickly eliminated.

Further, in the ultrahigh frequency oscillation element of the present invention, the density of the crystal grains of the polycrystalline diamond constituting the element is set to a certain value or more, and a level due to the influence of the grain boundary is intentionally formed in the forbidden band. . By utilizing this level, it is possible to use excitation light having a relatively long wavelength. For example, a femtosecond laser whose wavelength was too long to be used in the prior art can be used as excitation light for the ultrahigh frequency oscillation device of the present invention.

Hereinafter, the configuration and operation of the ultrahigh frequency oscillation device of the present invention will be described in more detail.

In order to operate the ultrahigh frequency oscillator at a high speed, it is necessary to quickly extinguish the optical carriers generated by the excitation light by the recombination center. For this reason, it is necessary to introduce a large number of recombination centers and minimize the distance between the electrodes. In addition, in order to perform a high output operation, it is necessary to apply a voltage as high as possible between the electrodes, and therefore, a high insulating property is required for a constituent material of the element. If this insulation property is low, when a high voltage is applied, the leakage current increases and the S /
N deteriorates or dielectric breakdown occurs, and the element does not operate. In other words, in order to realize high speed and high output, it is important that the material has high withstand voltage and that there are many recombination centers of optical carriers.

On the other hand, in a conventional ultrahigh-frequency oscillation element using diamond, a laser having a wavelength shorter than 225 nm has been used as excitation light because its forbidden band width is 5.5 eV. However, lasers near this wavelength have a pulse half-width of the order of ps (picoseconds). Therefore, when this type of laser is used as the excitation light, the lifetime of the generated optical carrier is ps (picoseconds) or more. Titanium sapphire lasers and other lasers with femtosecond pulse half-widths have also been developed.For example, titanium sapphire lasers have a long wavelength of 760 nm, and even when using the second harmonic, the valence band of diamond is reduced. Existing electrons cannot be directly excited to the conduction band. For this reason, the femto laser cannot be used as it is for an ultra-high frequency oscillation element composed of single crystal diamond composed only of carbon.

In the present invention, polycrystalline diamond is used as the material of the ultrahigh frequency oscillation element. The grain boundary of the polycrystalline diamond forms a level shallower than the valence band because not only defects due to discontinuity of the crystal structure but also defects due to impurities and stress during growth are present. Therefore, if the density of crystal grains is increased, the number of portions having a level shallower than the valence band increases. This shallow level results in a recombination center. Thus, even if a femtosecond laser having a long wavelength is used, carriers can be generated by excitation from a level, not from a valence band. That is, a femtosecond laser, which cannot be used in a conventional ultra-high frequency oscillation device of diamond, can be applied, and high-speed electric pulses can be generated.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the crystal grain density of polycrystalline diamond is preferably 10 ⁶ / cm ² or more. By doing so, the elimination speed of the generated carriers can be increased, and the frequency of the generated THz electromagnetic wave can be increased. Although there is no particular upper limit for the crystal grain density, a sufficiently high frequency THz electromagnetic wave can be obtained, for example, at about 10 ⁹ / cm ² .

In the present invention, the region between the pair of electrodes and at least a part of the pair of electrodes may be covered with an overcoat film made of polycrystalline diamond. Thereby, the withstand voltage characteristics can be further improved. In this case, the polycrystalline diamond constituting the overcoat film preferably has a crystal particle density of 10 ⁶ / cm ² or more. As a result, the rate of disappearance of carriers generated in the overcoat film can be increased, and the frequency of the generated THz electromagnetic wave can be increased. Although there is no particular upper limit for the crystal grain density, a sufficiently high frequency THz electromagnetic wave can be obtained, for example, at about 10 ⁹ / cm ² .

In the present invention, the density of the crystal grains of the polycrystalline diamond is set to a certain value or more, and a level is intentionally formed in the forbidden band due to the influence of the grain boundary. By utilizing this level, the energy of the excitation light can be made smaller than the band gap energy of the polycrystalline diamond film, and the range of laser selection can be greatly expanded. For example, a femtosecond laser (laser having a pulse half-width of less than 1.0 ps), which had a wavelength too long to be used in a conventional ultra-high-frequency oscillator using diamond, was changed to a pump light of the ultra-high-frequency oscillator of the present invention. Can be used as

[0026]

The present invention will be described in more detail with reference to the following examples.

(First Embodiment) FIG. 1 is a schematic view of an ultra-high frequency oscillator according to a first embodiment of the present invention. The substrate 1 serves as a base when the polycrystalline diamond film 2 is grown.
This substrate may be any one that can form a diamond film by the CVD method, such as silicon, molybdenum, platinum, copper, and Si.
C, a ceramic substrate, or the like can be used. The polycrystalline diamond film 2 is a portion where photocarriers are generated by the excitation light 5 and is produced by a CVD method.

A voltage is applied to the electrode 3, and when carriers generated in the polycrystalline diamond film 2 by the excitation light 5 flow through this electrode, high-frequency electromagnetic waves are emitted into space. The overcoat diamond layer 4 is made of polycrystalline diamond and has a role of preventing a discharge from occurring between the electrodes when a high voltage is applied to the electrodes 3, and is usually formed on the electrodes by a CVD method. The excitation light 5 is applied to the polycrystalline diamond film 2 and the overcoat diamond layer 4 to generate photo carriers in the polycrystalline diamond film 2 and the overcoat diamond layer 4. The THz electromagnetic wave 11 is generated by irradiation of the excitation light 5.

The ultrahigh frequency oscillation device shown in FIG. 1 was manufactured in the following procedure. First, a polycrystalline diamond film 2 is formed on a silicon substrate 1 by a CVD method using hydrogen gas and methane gas.
Grew. At this time, by controlling the growth temperature, the concentration ratio of hydrogen gas and methane gas, and the film forming pressure, the particle diameter of the polycrystalline diamond constituting the polycrystalline diamond film 2 can be adjusted. That is, by optimizing these conditions, the density of the crystal grains constituting the polycrystalline diamond film 2 can be controlled. Utilizing this phenomenon, a diamond film 4 having a thickness of 10 μm was formed on a silicon substrate so that the crystal grain density became 10 ⁶ / cm ² . The CVD conditions are, for example, a substrate temperature of 600 to 800 ° C., a film forming pressure of 1 to 200 torr, and a gas flow ratio of (CH4) / (H2 + CH4) = 0.005 to 0.03.

Next, a metal film constituting the electrode 3 was formed on the polycrystalline diamond film 2 by a sputtering method. One or two or more pairs of electrodes 3 are formed. In this embodiment, platinum is used for the electrode 3. However, other metals can be used as long as they can make ohmic contact with the polycrystalline diamond film 2. Further, the surface of the polycrystalline diamond film 2 may be polished before forming the metal. After the formation of platinum, a pattern was formed by lithography to produce a desired electrode shape. The length of the electrode gap sandwiched between the pair of electrodes was 6 μm. If the gap between the electrodes is too large, there is a disadvantage that the capacity of the power supply needs to be increased when obtaining a high applied electric field strength. Therefore, the gap between the electrodes is preferably set to 1 mm or less.

After the electrodes were formed, an overcoat diamond layer 4 for preventing dielectric breakdown was formed by a CVD method in a portion where the gap between the electrodes and the electrode 3 were opposed to each other.
(B)). In this embodiment, when forming the overcoat diamond layer 4, the gap between the electrodes and the electrode 3 near the gap are polished in advance with diamond powder. Overcoat diamond layer 4 is a polycrystalline diamond film
Crystals grown from 2 and microcrystals with newly generated nuclei are mixed.

The crystal grain density of the ultrahigh frequency oscillation device manufactured as described above was measured. The crystal particle density was measured by taking an SEM photograph of the surface and measuring the crystal particles in the photograph. The crystal grain density (number density) of the ultrahigh frequency oscillation device of this example was about 10 ⁶ / cm ² .

FIG. 2 shows a high-frequency electromagnetic wave generation system using the ultrahigh-frequency oscillation element of this embodiment. The laser 6 emits pump light 5 for generating carriers in the ultrahigh frequency oscillation device 10.
Occurs. The excitation light 5 is irradiated into the ultra-high frequency oscillation element 10 to generate carriers in the ultra-high frequency oscillation element 10. The delay 7 is composed of the laser 6 and the
Device (not shown) for measuring electric pulses generated from
A device that synchronizes with The ultrahigh frequency oscillation element 10 is the same as that shown in FIG. 1, and an optical carrier is generated by the excitation light 5. The mirror 8 changes the traveling direction of the laser. The power supply 9 applies a voltage between the electrodes, and gives the carrier transfer energy.

Next, the operation of the diamond ultra-high frequency oscillator of this embodiment will be described in detail with reference to FIG. In order to operate the ultrahigh frequency oscillator, it is necessary to first generate optical carriers. Then, the excitation light 5 is applied to the polycrystalline diamond film 2 and the overcoated diamond layer 4.
Irradiation. In the present embodiment, a KrF laser having a pulse half width of 2 ps was used as the excitation light. When the diamond film 4 is irradiated with the excitation light 5, electrons and holes are generated as photocarriers. This optical carrier moves in the polycrystalline diamond film 2 or the overcoat diamond layer 4. The more the amount of generated carriers and the number of carriers reaching the electrode 3 emitting high frequency electromagnetic waves, the greater the intensity of the generated high frequency electromagnetic waves 10. Becomes stronger. In addition, since high-frequency electromagnetic waves are generated due to a change in the photoexcitation current, it is preferable that the generated optical carriers disappear immediately.

FIG. 3 shows the relationship between the intensity of the electric field applied to the electrodes and the relative intensity of the generated high-frequency electromagnetic waves. from this result,
A high electric field of 1 MV / cm or more can be applied to the diamond ultra-high frequency oscillation element shown in this embodiment, and a THz electromagnetic wave of sufficient intensity is generated by applying such a high electric field. You can see that. In addition, it can be seen that particularly high intensity THz electromagnetic waves can be obtained by increasing the electric field intensity applied to the electrodes.

(Second Embodiment) Next, another embodiment of the present invention will be described in detail with reference to the drawings. FIG. 4 is a cross-sectional view of an ultra-high frequency oscillator according to a second embodiment of the present invention. This ultra-high frequency oscillator is manufactured by the same manufacturing method as in the first embodiment. The structure differs from the ultrahigh frequency oscillation device of the first embodiment in that the crystal grain density is 10% over the entire polycrystalline diamond film 4 in the first embodiment.
Whereas it has a ⁶ / cm ² or more, only the gap portion sandwiched between the electrodes of the polycrystalline diamond film 4 is in the crystalline particle density 10 ⁶ / cm ² or more in the second embodiment Is a point. This gap portion becomes a laser irradiation location.

Next, the operation of the ultrahigh frequency oscillation device shown in FIG. 4 will be described in detail with reference to the drawings.

In order to operate the ultrahigh frequency oscillator, it is necessary to first generate optical carriers. Then the excitation light 5
Is applied to the polycrystalline diamond film 2a. In the present embodiment, a KrF laser having a pulse half width of 2 ps was used as the excitation light. When the polycrystalline diamond film 2a is irradiated with the excitation light 5, electrons and holes are generated as photocarriers. The generated electrons and holes are directed in the direction of the biased electrode (electrons are +
Side, holes start to move to-side). At this time, electrons and holes move through the polycrystalline diamond, and pass through the crystal grain boundaries of the diamond before reaching the electrode 3. Since these grain boundaries act as recombination centers,
The optical carrier will disappear when passing through it. In other words, if the crystal grain density of diamond is increased,
Since the distance until the generated photocarrier reaches the crystal grain boundary is shortened, the photocarrier disappears quickly after generation, resulting in high-speed operation. FIG. 5 is a diagram for explaining that the higher the crystal grain density, the more the carrier disappearance points. The grain boundary of the diamond crystal 21 is the recombination center 2
Acts as zero. Since the crystal grain boundaries increase as the crystal grain density increases, the higher the crystal grain density, the greater the number of places where the carrier disappears.

Table 1 shows the intensity (relative value) of the radiated electromagnetic wave generated when the diamond ultra-high frequency oscillators having different crystal densities are irradiated and the half width of the electric pulse. As a result, it was found that when the crystal grain density was 10 ⁶ / cm ² or more, the pulse half width was 20 ps or less. In addition, the electric pulse intensity is shown as a relative value with 1 at the time of 10 ⁶ / cm ^2. At this time, the intensity of the peak is 100 times or more of the background,
The S / N ratio for the measurement was sufficient.

[0040]

[Table 1]

As described above, when the crystal grain density in the diamond film is set to 10 ⁶ / cm ² or more, the generated THz
The frequency of the electromagnetic wave can be increased.

[0042] Although this embodiment has only the electrode gap portion sandwiched between in the polycrystalline diamond film and the crystal particle density 10 ⁶ / cm ² or more, the crystal grain density of 10 throughout the polycrystalline diamond film ⁶ / Even with cm ² or more, the same results as in Table 1 were obtained.

(Third Embodiment) Next, a third embodiment of the present invention will be described.
Examples will be described in detail with reference to the drawings. Figure 6
It is a figure which shows the super-high frequency oscillation element of the 3rd Example of light.
Substrate 1a is the part that becomes the base when growing diamond
It is. The polycrystalline diamond film 2a is excited by the excitation light 5a.
This is where photocarriers are generated.
It is. Excitation light 5a Polycrystalline diamond film 2a and overcoat
Carriers are generated in the diamond layer 4b. Electrode 3a
Is the part to which the electric field is applied, and the generated electromagnetic wave
Release. This electrode 3a has 16 pairs of multi-array
A, electrode wire width 40μm, electrode gap 1mm,
The area of the part surrounded by the electrode is 7cm^TwoAnd This circle
H-array structure: electrode processing, applied voltage, THz radiation intensity, etc.
In consideration of the above, the electrode line width is 1 μm or more consisting of one or more pairs,
The gap between the electrodes is 1mm or less, and the area surrounded by the electrodes is 0.1cm
^TwoIt is desirable that this is the case. Overcoat diamo
The conductive layer 4b serves to prevent discharge between the electrodes and excites
It is a portion that generates carriers by light. THz wave 1
Reference numeral 1 denotes an electromagnetic wave generated by irradiation with the excitation light 5a. Fig. 7
Is a high-frequency electromagnetic wave generation system according to a third embodiment of the present invention.
Show. Titanium sapphire laser 23 is femtosecond pal
Can be generated and the key
The excitation light 5a for generating carriers is generated. 2nd harmonic
The generator 22 is based on the fundamental wave of the titanium sapphire laser 23
This device generates two harmonics. Second harmonic that is the excitation light
The wave is applied to the ultra-high frequency oscillator 10a,
A carrier is generated in the child 10a. Delay 7 is titanium
Generated from sapphire laser 23 and super-high frequency oscillator
A device synchronized with a device (not shown) that measures electric pulses
It is a place. Ultra high frequency oscillator 10a emits Thz wave
The one shown in FIG. 6 was used. Mirror 8 is laser advance
It is a device that changes the direction. Power supply 9 applies voltage between electrodes
And has a role to propagate the carrier as a signal.

Next, the operation of the super-high frequency oscillator of FIG. 6 will be described in detail with reference to the drawings.

In order to operate the ultrahigh frequency oscillator, it is necessary to first generate optical carriers. Then the excitation light 5
Irradiate a on the diamond film. In this embodiment, the second harmonic of a titanium sapphire laser having a pulse half width of 100 fs was used as the excitation light. The second harmonic is generated by the second harmonic generator 22 shown in FIG. When the excitation light 5 is applied to the polycrystalline diamond film 2a and the overcoat diamond layer 4b, electrons and holes are generated as photocarriers. The wavelength of the second harmonic of the titanium sapphire laser is about 380 nm,
There is not enough energy to excite electrons from the valence band of diamond to the conductor. Therefore, almost no photocarriers are generated from the inside of the bulk, and photocarriers are generated at crystal grain boundaries that create levels in the band gap. Therefore, as the crystal density of the film increases, the existence of grain boundaries increases, and the generation of photocarriers increases. Thereby, the intensity of the electric pulse is increased, and the S / N ratio can be improved.

The generated electrons and holes begin to move toward the biased electrode (electrons are on the + side and holes are on the-side). At this time, electrons and holes move through the polycrystalline diamond, and pass through the diamond crystal grain boundaries before reaching the electrode 3a. Since these crystal grain boundaries act as recombination centers, the photocarriers disappear when passing through them. In other words, if the crystal density of diamond is increased, the distance before the generated photocarriers reach the crystal grain boundary is shortened, so that the photocarriers disappear quickly after generation, and as a result, they can operate at high speed and emit faster electromagnetic waves. Become. Since the radiation intensity of the THz electromagnetic wave is proportional to the square of the irradiation area of the excitation light, high luminance THz can be obtained by using a multi-array electrode structure.

In this embodiment, the grain boundaries are increased by increasing the crystal grain density. As described above, these grain boundaries are the locations where carriers are generated when a femtosecond laser is used, and at the same time, photocarriers are generated. Is the recombination center of That is, in this embodiment, by increasing the crystal particle density, the number of places where carriers are generated by the femtosecond laser and the number of recombination centers are increased, and the ultrahigh frequency oscillation element is designed to have high speed and high output.

Table 2 shows a titanium safa having a pulse half width of 100 fs.
Ultra-high frequency diamonds with different crystal densities
The intensity of the radiated electromagnetic wave generated when the oscillator
Shows the full width at half maximum of the air pulse. As a result, the crystal grain density was 1
0⁶Months / cm^TwoAbove this, the pulse half width becomes 1ps or less
It has been found. The electric pulse intensity is 10⁶Months / cm ^Two
The time is shown as a relative value with 1 as the value.
The degree is more than 100 times the background, and the S / N ratio for performing the measurement
Was enough.

[0049]

[Table 2]

[0050]

As described above, according to the present invention, the light-irradiated portion in the diamond film is made of polycrystalline diamond. For this reason, the grain boundary of the polycrystalline diamond acts as a carrier recombination center, and the carrier life can be shortened, so that an ultra-high frequency oscillation element that emits a high-intensity and high-frequency THz electromagnetic wave can be obtained. In particular, if the crystal grain density of polycrystalline diamond is 10 ⁶ / cm ² or more,
It is possible to obtain an ultrahigh-frequency oscillation element that further increases the annihilation rate of generated carriers and emits THz electromagnetic waves with higher intensity and higher frequency.

Further, since the grain boundary of polycrystalline diamond forms an energy level shallower than the valence band, laser light having a relatively long wavelength can be used as excitation light. This allows the use of femto lasers,
It is possible to obtain an ultra-high frequency oscillation element that emits a THz electromagnetic wave having a higher frequency than before.

Further, by covering an area between the pair of electrodes and at least a part of the pair of electrodes with an overcoat film made of polycrystalline diamond, the withstand voltage characteristic of the ultrahigh frequency oscillation element is improved. This makes it possible to apply a high electric field of, for example, 1 MV / cm or more between the electrodes. By applying such a high electric field, a higher intensity THz electromagnetic wave can be generated.

[0053]

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an ultrahigh frequency oscillation device according to the present invention.

FIG. 2 is a diagram showing a schematic structure of an ultrahigh frequency oscillation device of the present invention.

FIG. 3 is a diagram showing a relationship between an electric field intensity and an emitted electromagnetic wave intensity when the ultrahigh frequency oscillation device of the present invention is used.

FIG. 4 is a cross-sectional view of the ultrahigh frequency oscillation device of the present invention.

FIG. 5 is a diagram for explaining that as the crystal particle density is higher, the number of locations where the carrier disappears is larger.

FIG. 6 is a cross-sectional view of the ultrahigh frequency oscillation device of the present invention.

FIG. 7 is a diagram showing a schematic structure of an ultrahigh frequency oscillation device according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 1a Substrate 2, 2a Diamond film 3 Electrode 4, 4a, 4b Overcoat diamond film 5 Excitation light 6 Laser 7 Delay 8 Mirror 9 Power supply 10, 10a, 10b Ultrahigh frequency oscillation element 11 THz wave 20 Recombination center 21 Diamond crystal 22 Second harmonic generator 23 Titanium sapphire laser

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-6-350120 (JP, A) Tani et al, "Esm. "Light-speed photoconductive switch using diamond thin film", Chemistry and Industry, Japan, The Chemical Society of Japan, Vol. 1042-1044. GONON et al, "Photoconductivity as associated with depth levels in polycrystalline diamond films", PHILOSOPIC AL MAGAZINE VETTERS. 72, No. 4, pp. 257 −261. (58) Field surveyed (Int.Cl. ⁷ , DB name) H01S 1 / 02,3 / 00,5 / 00 H01L 31/00-31/119 G01T 1/00-3/08 G01J 1 / 00-1/60

Claims (7)

(57) [Claims] 1. An ultra-high frequency oscillation element utilizing a photoexcitation current by irradiation of an ultrashort light pulse, comprising: a base, a diamond film provided on the base, and a separation film provided on the diamond film. A light irradiation site in the diamond film is made of polycrystalline diamond having a crystal grain density of 10 ⁶ / cm ² or more, and a high electric field of at least 1 MV / cm is applied between the electrodes. An ultrahigh frequency oscillation element, wherein the energy of the excitation light of the ultrashort light pulse is smaller than the bandgap energy of the polycrystalline diamond film, and the energy of the excitation light of the ultrashort light pulse is generated by irradiating the light irradiation location with the applied voltage. 2. The device according to claim 1, wherein a region between the pair of electrodes and at least a part of the pair of electrodes are covered with an overcoat film made of polycrystalline diamond. Ultra high frequency oscillation element. 3. The ultra-high frequency oscillation device according to claim 2, wherein the polycrystalline diamond forming the overcoat film has a crystal grain density of 10 ⁶ / cm ² or more. 4. The ultrahigh frequency oscillation device according to claim 1, wherein the excitation light of the ultrashort light pulse is a laser beam having a pulse half width of less than 1.0 picosecond. 5. An ultra-high frequency oscillation device comprising: a light source for emitting excitation light; an ultra-high frequency oscillation element irradiated with the excitation light; and means for applying a voltage to an electrode provided on the ultra-high frequency oscillation element. The ultrahigh frequency oscillation device according to claim 1, wherein the ultrahigh frequency oscillation device is the ultrahigh frequency oscillation device according to claim 1. 6. The ultrahigh frequency oscillation device according to claim 5, wherein the energy of the excitation light emitted from the light source is smaller than the band gap energy of the polycrystalline diamond film. 7. The light source according to claim 5, wherein the laser has a pulse half width of less than 1.0 picosecond.
Or the ultrahigh frequency oscillator according to 6.