KR101704664B1 - Terahertz modulator - Google Patents

Abstract

A terahertz modulator according to the present invention includes a substrate; A first metal provided on the substrate; A second metal provided on the substrate; At least three contacts in which the first metal and the second metal are in indirect contact; And a closed circuit comprising the first metal, the second metal, and the at least three contacts, wherein at least one resonator for controlling the transmission of the terahertz wave can be included. According to the present invention, the modulation index can be increased to 80%, the modulation bandwidth can be increased to ~ GHz, and the operating voltage can be lowered. In addition, various advantages can be obtained according to the nature of the invention, and the description thereof will be made more specifically in the detailed description.

Description

Terahertz modulator {Terahertz modulator}

The present invention relates to a terahertz, and more particularly to a technique for modulating a wave of a terahertz frequency.

Although the terahertz wave (hereinafter sometimes referred to as a terahertz wave or terahertz wave) has advantages such as being harmless to the human body, it is difficult to control the signal and difficult to obtain the intensity that can be used commercially. .

On the other hand, the development of the component technology for the oscillation of the terahertz wave is very active, but the technology for the modulation and control of the terahertz wave is not yet fully realized. This is because the terahertz wave is hardly affected by the electric field and the magnetic field as compared with the electromagnetic wave below the microwave frequency, and it is difficult to control the terahertz band signal.

As a conventional technique for terahertz modulation, Non-Patent Document 1 discloses a technique of modulating using a split ring resonator (SRR) arrangement and a Schottky diode. This technique electrically controls the metamaterial of the SRR structure. Using the operation that the resistance of the split gap of the SRR changes depending on the depletion region according to the voltage applied to the Schottky diode, the frequency of the metamaterial It is to change the characteristics.

The operation of the non-patent document 1 will be described in more detail. GaAs semiconductor substrate, n-GaAs is grown thereon, and a Schottky contact having an SRR shape is formed thereon with a single metal. Therefore, since the entire SRR is connected in a single metal structure, the entire SRR functions as a schottky contact, and the entire lower surface of the metal forms a depletion region. Further, the ohmic contact is disposed at the edge of the substrate, so that the Schottky contact serves as the anode and the ohmic contact serves as the cathode.

When a reverse voltage is applied to the Schottky diode, a depletion is generated in the semiconductor region where the center gap of the SRR is located. The size of this region depends on the applied voltage. The larger the reverse voltage becomes, the larger the depletion region becomes, and as a result, the entire split gap becomes the depletion region. Then, the resistance of the above split gap changes depending on the presence or absence of the depletion region. As a result, the resonance frequency of the SRR changes, and the transmission characteristic of the terahertz passing through the SRR changes, thereby causing modulation.

As a result, the resonance frequency of the SRR changes according to the change of the resistance due to the change of the depletion region, and the transmittance of the terahertz is changed at a specific frequency according to the varying resonance frequency, which is applied to the terahertz modulator.

According to the present technique, the modulation index is moderate at 50%, the modulation bandwidth is as low as ~ MHz, and the operating voltage is as high as 10V.

Non-Patent Document 2 discloses a technique of controlling the transmittance of terahertz by electrically controlling a GaAs HEMT device and using the property that a THz wave is reflected or absorbed according to a change in conductivity of a channel located under the gate have. According to the present technique, the modulation index is very low as 0.3%, and the modulation bandwidth is very low as ~ kHz.

Non-Patent Document 3 discloses a technique of phase-modulating terahertz by electrically controlling the carrier concentration of the parabolic quantum well using a parabolic quantum well (PQW: Parabolic Quantum Well) and a Schottky diode. According to this technique, the bandwidth is very low as ~ kHz.

Non-Patent Document 4 discloses a technique for optically controlling 1-D photon crystals and modulating terahertz using a photo-excited carrier generated by irradiating a GaAs surface with a high-power laser. The technology is a light modulation technology with a bandwidth of ~ GHz as a very good level and a modulation index as low as 50%.

As can be seen from the above description, among the terahertz modulation techniques introduced so far, the non-patent reference 1 is the best. However, it can be seen that the modulation index is still low, the modulation bandwidth is very low, and the operating voltage is as high as 10V.

Non-Patent Document 1: Hou-Tong Chen, Willie J. Padilla, Joshua M. O. Zide, Arthur C. Gossard, Antoinette J. Taylor and Richard D. Averitt, "Active terahertz metamaterial devices", Nature 444, 597, 2006. Non-Patent Document 2: Kleine-Ostmann, T., Dawson, P., Pierz, K., Hein, G. & Koch, "Room-temperature operation of an electrically driven driven terahertz modulator", Appl. Phys. Lett 84, 2004. Non-Patent Document 3: R. Kersting, G. Strasser, and K. Unterrainer, "Terahertz phase modulator", Electronics Letters 36, 1156, 2000. Non-Patent Document 4: Petr Kuzel and Filip Kadlec, "Tunable structures and modulators for THz light", Comptes Rendus Physique '9, 197, 2008.

The inventor of the present invention, Patent Document 1 has been intensively observed over a long period of time to solve the problem of a low modulation index, a problem of a very low modulation bandwidth, a problem of a high operating voltage, and a variety of problems A terahertz modulator has been developed to obtain the effect.

A terahertz modulator according to the present invention includes: a substrate; A first metal provided on the substrate; A second metal provided on the substrate; At least three contacts in which the first metal and the second metal are in indirect contact; And a closed circuit comprising the first metal, the second metal, and the at least three contacts, wherein at least one resonator is provided for controlling the transmission of the terahertz wave. According to the present invention, the efficiency of the resonator can be improved by varying the material providing the contact point.

At least two resonators may be provided in an array on the substrate.

At least one of the contacts may be provided as a fixed capacitance capacitor, the first metal and the second metal overlap, a dielectric layer may be interposed at the interface, and the fixed capacitance capacitor Two can be provided on both sides of the resonator, respectively.

At least one of the contacts may be provided as a variable capacitance capacitor, the variable capacitance capacitor may include: a semiconductor layer provided on the substrate; And a protective film on the semiconductor layer, and the metal may be contacted through two open portions of the protective film. The metal in contact with the semiconductor layer may contact the semiconductor layer at a nanoscale and form a Schottky contact. Preferably, the semiconductor layer may be provided as a hemat. The variable capacitance capacitor may be provided at a central portion of the resonator, and a depletion region may be provided only at a lower portion of the variable capacitance capacitor.

Of the at least three contacts, the at least one contact is a variable capacitance capacitor, the at least two contacts are fixed capacitance capacitors, and the fixed capacitance capacitor in the middle of the variable capacitance capacitors may be provided on both sides.

According to another aspect of the present invention, there is provided a terahertz modulator comprising: a substrate; A first metal provided on the substrate in a predetermined shape; A second metal provided on the substrate in a predetermined shape; And a capacitor provided between the metal and at least two locations where the first metal and the second metal are in contact with each other.

Here, the capacitor includes at least one variable capacitance capacitor and at least one fixed capacitance capacitor to form a resonator, and the resonator may be provided to form an array on the substrate. Two fixed capacitance capacitors are provided, one variable capacitance capacitor is provided, the fixed capacitance capacitor is short-circuited when viewed at a high frequency of terahertz, can be opened at a low frequency, the resonator is provided with a square, The longer direction can be provided.

According to another aspect of the present invention, there is provided a terahertz modulator comprising: a substrate; A first metal provided on the substrate in a predetermined shape; A second metal provided on the substrate in a predetermined shape; And an array of resonators capable of controlling the transmission of the terahertz wave, wherein the resonator includes a capacitor provided at each of at least two positions where the first metal and the second metal are in contact with each other, A resonator can be provided. Wherein the capacitor comprises: a variable capacitance capacitor provided at a center portion of the split ring resonator; And fixed capacitance capacitors provided on both sides of the split ring resonator, respectively.

According to the present invention, the modulation index can be increased to 80%, the modulation bandwidth can be increased to ~ GHz, and the operating voltage can be lowered. In addition, various advantages can be obtained according to the nature of the invention, and the description thereof will be made more specifically in the detailed description.

1 is a view for explaining the operation of a terahertz modulator according to an embodiment;
2 is a diagram illustrating a terahertz modulator according to an embodiment;
3 is an enlarged view of one of the resonators.
4 and 5 are cross-sectional views taken along the line I-I 'of FIG. 3 and the line II-II' of FIG. 3, respectively.
Fig. 6 is an equivalent circuit diagram of a resonator at a low frequency, and Fig. 7 is an equivalent circuit diagram of a resonator at a high frequency.
8 and 9 are views for explaining the difference in transmission characteristics depending on the shape of the resonator.
10 is a result obtained by measuring the capacitance characteristics while varying the gate length of the Schottky contact in the semiconductor layer.
Figure 11 is an embodiment of a ham based on InP as an example.
12 shows an embodiment of a hamster based on GaAs.
FIG. 13 shows an embodiment of a GaN-based hamter.

Hereinafter, specific embodiments of the present invention will be described in detail with reference to the drawings. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. It will be understood that they are also included within the scope of the present invention.

Although the accompanying drawings in the description of the embodiments can be exaggeratedly shown differently from the actual product, they should be understood in the scope of description of the invention. Also, in the following description, in the case where there is no special designation, the prior art means the technique introduced in the non-patent document 1.

1 is a view for explaining the operation of a terahertz modulator according to an embodiment.

Referring to FIG. 1, a terahertz wave enters a front of a terahertz modulator 1 to which a control signal is applied. When the terahertz wave passes through the terahertz modulator 1, A modulated modulation terahertz wave is emitted. The modulated signal may be used in various ways such as image processing and compression sensing.

2 is a diagram illustrating a terahertz modulator according to an embodiment.

2, the THz modulator according to the embodiment is provided with an array of resonators 2 on a substrate, in which the first metal 11 and the second metal 12 are different from each other in the resonator 2 It is provided as a structure that makes indirect contact with the intervention of matter. Therefore, the Schottky contact is provided only where the contact of the metal 11 (12) is located, and the depletion region 3 by the Schottky contact is formed only where the contact of the metal 11 (12) / RTI > Therefore, the depletion region is narrower than in the prior art, and accordingly, the time constant of the resonator is low, so that the modulation bandwidth can reach ~ GHz. In addition, the effect that the resonator can operate sufficiently even with a low applied voltage can be expected.

A bias voltage is applied to the metals 11 and 12 by a bias power source 13 so that a shape of the metal 11 and 12 forming a closed circuit and a shape of an indirect contact between the metal 11 and the metal 12 So that the resonator 2 can be provided.

Hereinafter, the resonator will be described in more detail.

3 is an enlarged view of one of the resonators.

Referring to FIG. 3, the first metal 11 and the second metal 12 are indirectly contacted with each other at three places in the resonator 2. The contacts of the first metal 11 and the second metal 12 may have a shape of two squares. Each contact will be described. First, a first fixed capacitance capacitor 5 and a second fixed capacitance capacitor 6, to which a capacitance component is fixed, are provided at the contacts of both side edges, and a variable capacitance capacitor 4 whose capacitance component is variable is provided in the middle portion . The form of this resonator can be called a split ring resonator (SRR). It can be seen that the depletion region 3 is provided only on the lower side of the variable capacitance capacitor 4.

4 and 5 are cross-sectional views taken along the line I-I 'of FIG. 3 and the line II-II' of FIG. 3, respectively.

3 to 5, the variable capacitance capacitor 4 includes a semiconductor layer 7 provided on a substrate 9, a protective film 10 provided in a structure covering the semiconductor layer 7, The first and second metal layers 11 and 12 are in contact with the semiconductor layer 7 at both sides where the protective film is removed. The fixed capacitors 5 and 6 are provided with a structure in which the first and second metal layers 11 and 12 overlap each other with the dielectric layer 21 interposed therebetween. The dielectric layer 21 and the protective film 10 may be made of SiN _x .

The semiconductor layer 7 can advantageously exemplify a ham transistor (HEMT: High Electron Mobility Transistor). The hamster generates a two-dimensional region (two-dimensional electron gas: 2DEG) in which the mobility of electrons is extremely high at the interface of the heterojunction semiconductor. At this time, the carrier is hardly susceptible to ion scattering, Use what had. Therefore, since a high reaction rate can be obtained, it can be suitably applied to a terahertz modulator.

The contact will be described in more detail.

First, in the variable capacitance capacitor 4, the first and second metal layers 11 and 12 form Schottky junctions with the semiconductor layer 7 in the substantially central portion of the resonator 2. The Schottky junction may be a nanoscale gate contact for switching the capacitance component in the THz region. If the nanoscale Schottky contact is provided in the semiconductor plane as a gate, it can have varactor characteristics. That is, the variable capacitance capacitor 4 can be called a varactor. More specifically, the collector is provided as a metal (11) -semiconductor layer (7) -metal (12) and may be referred to as an MSM (Metal Semiconductor Metal) varactor. The capacitance of the variable capacitance capacitor 4 can be varied according to the bias voltage applied from the bias power supply 13.

The variable capacitance of the variable capacitance capacitor 4 as described above will be described. First, two nanoscale Schottky contacts provided with the first and second metals 11 and 12 and the semiconductor layer 7 are provided with a Schottky layer provided on the surface and a Schottky layer provided on the surface of the Schottky layer (E. G., 2DEG (8)) capable of electron transfer provided therein. In this case, the capacitance C1 by the Schottky layer and the capacitance C2 by the electron-transportable layer can be regarded as capacitances connected in series from the metal 11 and 12. The capacitance C1 by the Schottky layer is much larger than the capacitance C2 by the electron mobility layer. Therefore, since the total amount of capacitances at the time of serial merging of the two capacitors C1 and C2 is a reciprocal of the sum of the reciprocals of the respective capacitances, the capacitance of the MSM varactor is determined by the capacitance C2 by the electronically movable layer Dominated. In this case, the capacitance C2 of the electronically drivable layer is symmetrically different from that of electrons moving in both directions within the electron-transportable layer in accordance with the bias voltage. As a result, the capacitance C2 due to the electron-transportable layer changes only within a certain bias voltage range, and in other cases, the same capacitance is generated, because the bias C2 caused by the electron- The voltage is given as approximately ± 3V in the embodiment.

The fixed capacitance capacitors 5 and 6 will now be described. The first metal 11 and the second metal 12 are provided so as to overlap each other on both sides of the resonator 2. The capacitive component can be obtained by overlapping each other. The overlay structure is a structure of the metal 11, the dielectric layer 21, and the metal 12, and constitutes a metal-insulator-metal (MIM) capacitor. At this time, the capacitance of the fixed capacitance capacitors 5 and 6 depends on the area A where the metal overlaps, the dielectric constant? _R of the dielectric layer 21, and the thickness t of the dielectric layer 21, It is decided together.

The capacitance of the fixed capacitance capacitors 5 and 6 given by Equation 1 can be designed to be very large so as to have a very low impedance in a terahertz frequency region (for example, 1 Terahertz). The capacitance of the fixed capacitance capacitor can be, for example, 318 fF (femtofarad) or more.

As described above, when the capacitance of the fixed capacitance capacitors 5 and 6 is increased, there is no fixed capacitance capacitors 5 and 6 provided on both sides of the resonator 2 in terms of the terahertz signal It seems to be short-circuited. However, the DC signal can be biased by the bias power supply 13 as it is opened by the fixed capacitance capacitors 5 and 6.

 Fig. 6 is an equivalent circuit diagram of a resonator at a low frequency, and Fig. 7 is an equivalent circuit diagram of a resonator at a high frequency (in the embodiment, a terahertz frequency).

6 and 7, the capacitance of the variable capacitance capacitor 4 changes according to the bias voltage. The fixed capacitance capacitors 5 and 6 appear to be opened at a low frequency and short-circuited at a high frequency. Therefore, in the high frequency region of the terahertz, the resonator 2 forms a resonant circuit. When the terahertz wave is incident on the resonator, when the terahertz wave is at a frequency that is matched with the resonant frequency of the resonator 2, it is possible to prevent the resonator 2 from transmitting. In this situation, the modulation effect can be derived. The resonant frequency of the resonator may be given by Equation (2).

Where L is the inductance of the shape of the metal (11) 12 as a closed circuit, and C _var is the capacitance of the variable capacitance capacitor (4). Therefore, it is possible to change the resonance frequency by changing the shape of the metal (11) and the metal (12), which are generally shown as rectangles, in various ways to change the inductance of the resonator, and the resonance frequency of the capacitors It can be determined whether or not modulation is performed by turning the state on or off (i.e., bistable).

Figs. 8 and 9 are views for explaining the difference in transmission characteristics depending on the shape of the resonator.

Referring to FIGS. 8 and 9, it can be seen that the modulation index can be adjusted by varying the aspect ratio of the resonator provided as a quadrangle. In detail, the second resonance frequency of the resonator 2 changes according to the dipole resonance of the metal 11 and 12, and the metal 11 and 12 of the resonator 2 9), the secondary resonance frequency can be shifted to a higher position, and the spectrum can be provided in a smoother form.

Observing the shaded circle shows that the transmissivity is about 20% in the case of 1 fF in FIG. 9, but almost 100% in the case of 3 fF. However, in the case of Fig. 8, the transmittance is about 20% in the case of 1fF, but 90% to 95% in the case of 3fF. Therefore, by shifting the secondary resonance frequency to a higher position, it is possible to improve the modulation efficiency and obtain a smoothed spectrum response.

10 shows the result of measuring the capacitance characteristics while varying the gate length of the Schottky contact in the semiconductor layer. As already explained, the schottky contacts provided at nanoscale, with the distance between the gates being 4 m.

Referring to FIG. 10, a change in capacitance occurs when the bias voltage is approximately. + -. 3V, which means that the change in capacitance C2 by the electron transportable layer begins to occur at. + -. 3V. The capacitance C2 due to the electron mobility layer changes sharply during a certain period G and the capacitance C2 due to the electron mobility layer hardly changes within +/- 2V. Therefore, it can be seen that the capacitance of the variable capacitance capacitor 4 can be stably operated at two points (for example, 1 fF and 3 fF in the embodiment).

As the gate length L of the Schottky contact of the nanoscale becomes larger, the capacitance difference becomes larger as the variable capacitance capacitor 4 is turned on / off. It is possible to obtain an optimum terahertz modulator suitable for the use state by appropriately combining them.

As an application example of the semiconductor layer 7, it has been described that a ham capable of achieving a high reaction rate can be used. Fig. 11 shows an embodiment of a ham based on InP, Fig. 12 shows an embodiment of a ham based on GaAs, and Fig. 13 shows an embodiment of a ham based on GaN. In the case of InP or GaAs based hammers, it is possible to realize a small time constant based on high electron mobility. In the case of GaN based hammers, the band gap characteristics can strongly resist the collapse.

The THz modulator according to the present invention improves the problem of low modulation index, the problem of very low modulation bandwidth, and the problem of high operating voltage. Thus, by providing a highly efficient terahertz modulator, the application of terahertz to actual use can be further shortened.

1: terahertz modulator
3: depletion region
4: variable capacitance capacitor
5, 6: Fixed capacity capacitors

Claims (20)

Board;
A first metal provided on the substrate;
A second metal provided on the substrate;
At least three contacts in which the first metal and the second metal are in indirect contact; And
A closed circuit comprising the first metal, the second metal, and the at least three contacts, wherein the closed circuit comprises at least one resonator for controlling the transmission of the terahertz wave,
At least one of the contacts is a variable capacitance capacitor,
And a depletion region is provided only in a lower portion of the variable capacitance capacitor. The method according to claim 1,
Wherein at least two resonators are provided in an array on the substrate. The method according to claim 1,
Wherein at least one of the contacts is a fixed capacitance capacitor. The method of claim 3,
The fixed-
Wherein the first metal and the second metal are overlapped and a dielectric layer is interposed at the interface. The method of claim 3,
Wherein the fixed capacitance capacitors are provided on both sides of the resonator, respectively. delete The method according to claim 1,
In the variable capacitance capacitor,
A semiconductor layer provided on the substrate; And
A protective film on the semiconductor layer,
And the metal is contacted through two open portions of the protective film. 8. The method of claim 7,
Wherein the metal contacting the semiconductor layer is in contact with the semiconductor layer at a nanoscale. 8. The method of claim 7,
Wherein the metal in contact with the semiconductor layer is a Schottky contact. 8. The method of claim 7,
Lt; RTI ID = 0.0 > THz < / RTI > The method according to claim 1,
Wherein the variable capacitance capacitor is provided at a central portion of the resonator. delete The method according to claim 1,
Wherein at least one contact of the at least three contacts is a variable capacitance capacitor and the at least two contacts are fixed capacitance capacitors and the fixed capacitance capacitor is provided on both sides of the middle variable capacitance capacitor. Board;
A first metal provided on the substrate in a predetermined shape;
A second metal provided on the substrate in a predetermined shape; And
And a capacitor provided between the metal and at least two locations where the first metal and the second metal are in contact with each other,
Wherein the capacitor includes at least one variable capacitance capacitor and at least one fixed capacitance capacitor to form a resonator,
And a depletion region is provided only in a lower portion of the variable capacitance capacitor. 15. The method of claim 14,
Wherein the resonator comprises an array on the substrate. 16. The method of claim 15,
Wherein two fixed capacitance capacitors are provided, and one of said variable capacitance capacitors is provided. 16. The method of claim 15,
Wherein the fixed capacitance capacitor is short-circuited when viewed at a high frequency of terahertz and open at a low frequency. 16. The method of claim 15,
Wherein the resonator is provided in a rectangular shape, and the longitudinal direction is longer. Board;
A first metal provided on the substrate in a predetermined shape;
A second metal provided on the substrate in a predetermined shape; And
An array of resonators capable of controlling the transmission of the terahertz wave is included,
Wherein the resonator includes a capacitor provided at each of at least two positions where the first metal and the second metal are in contact with each other,
The resonator forms a split ring resonator,
The capacitor
A variable capacitance capacitor provided at a center portion of the split ring resonator and a fixed capacitance capacitor provided at both sides of the split ring resonator,
And a depletion region is provided only in a lower portion of the variable capacitance capacitor. delete

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