WO2017007265A1

WO2017007265A1 - Terahertz modulator

Info

Publication number: WO2017007265A1
Application number: PCT/KR2016/007386
Authority: WO
Inventors: 장재형; 김동주
Original assignee: 광주과학기술원
Priority date: 2015-07-07
Filing date: 2016-07-07
Publication date: 2017-01-12
Also published as: KR101704664B1; KR20170006129A

Abstract

A terahertz modulator according to the present invention can comprise: a substrate; a first metal provided on the substrate; a second metal provided on the substrate; at least three contact points with which the first metal and the second metal indirectly come in contact; and at least one resonator for adjusting the transmission of terahertz waves, as a closed circuit including the first metal, the second metal, and the at least three contact points. According to the present invention, a modulation index can be increased by 80%, a modulation bandwidth can be increased in GHz, and an operating voltage can be lowered.

Description

Terahertz modulator

FIELD OF THE INVENTION The present invention relates to terahertz and, more particularly, to techniques for modulating waves at terahertz frequencies.

Waves in the terahertz band (hereinafter also referred to as terahertz or terahertz para) have advantages such as harmless to the human body, but are still unexplored due to difficulties in controlling signals and difficult to obtain commercially available strengths. It remains as it is.

On the other hand, while the development of component technology for oscillation of the terahertz wave, etc. is very active, the technology for modulation and control of the terahertz wave has not been made sufficiently yet. This is because terahertz waves are hardly affected by electric and magnetic fields as compared to radio waves below the microwave frequency, making it difficult to control signals in the terahertz band.

As a prior art for terahertz modulation, Non-Patent Document 1 (Hou-Tong Chen, Willie J. Padilla, Joshua MO Zide, Arthur C. Gossard, Antoinette J. Taylor & Richard D. Averitt, "Active terahertz metamaterial devices", Nature 444, 597, 2006. has introduced a technique for modulating using a split ring resonator (SRR) array and a Schottky diode. The present technology electrically controls the metamaterial of the SRR structure, and uses the operation in which the resistance of the split gap of the SRR varies depending on the depletion region according to the voltage applied to the Schottky diode. It is to change the characteristics.

The action of Non-Patent Document 1 will be described in more detail. Using a GaAs semiconductor substrate, n-GaAs was grown thereon, and a Schottky contact having an SRR shape thereon was formed of a single metal. Therefore, since the entire SRR is connected by one metal structure, the entire SRR serves as a Schottky contact, so that the entire lower surface of the metal forms a depletion region. In addition, 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 region is generated in the semiconductor region where the center gap of SRR is located. The size of this area depends on the applied voltage. As the reverse voltage increases, the depletion region becomes large, so that 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. Accordingly, the resonance frequency of the SRR changes and the transmission characteristics of the terahertz passing through the SRR change, causing modulation.

As a result, the resonant frequency of the SRR varies according to the change of the resistance due to the change in the depletion region, and the transmittance of the terahertz at a specific frequency varies according to the changing resonant frequency.

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

Non-Patent Document 2 (Kleine-Ostmann, T., Dawson, P., Pierz, K., Hein, G. & Koch, “Room-temperature operation of an electrically driven terahertz modulator”, Appl. Phys. Lett 84, 2004 In (.), A technique of controlling the transmittance of terahertz by electrically controlling the GaAs HEMT device and reflecting or absorbing the terahertz wave according to the change in conductivity of the channel located under the gate has been introduced. According to the present technology, the modulation index is very low as 0.3%, and the modulation bandwidth is very low as ~ kHz.

Non-Patent Document 3 (R. Kersting, G. Strasser, and K. Unterrainer, "Terahertz phase modulator", Electronics Letters 36, 1156, 2000.) includes parabolic quantum wells (PQW) and Schottky diodes. And a technique of phase modulating terahertz by electrically controlling the carrier concentration of the parabolic quantum well. According to the technique, the bandwidth is very low as ~ kHz.

Non-Patent Document 4 (Petr Kuzel and Filip Kadlec, "Tunable structures and modulators for THz light", Comptes Rendus Physique '9, 197, 2008.) has shown that a high-power laser is a GaAs surface for optically controlling 1-D photonic crystals. We introduce a technique for modulating terahertz using photo-excited carriers generated by irradiation with. This technology is optical modulation technology, bandwidth is very good as ~ GHz and modulation index is low as 50%.

As can be seen from the description above, it can be seen that the above-mentioned Non-Patent Document 1 is the best among the terahertz modulation techniques introduced to date. However, it can be seen that the modulation index is still low, the modulation bandwidth is very low, and the operating voltage is high as 10V.

The present invention improves the problem of low modulation index, very low modulation bandwidth, and problem of high operating voltage in the prior art introduced in Non-Patent Document 1, and in addition, it is possible to obtain various effects that are manifested as the essence of the present invention. Developed a terahertz modulator.

The 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 indirectly contacting the first metal and the second metal; And a closed circuit including the first metal, the second metal, and the at least three contacts, wherein at least one resonator is configured to control the transmission of terahertz waves. According to the present invention it is possible to improve the efficiency of the resonator by changing the material providing the contact.

At least two or more 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 fixed capacitance capacitor may overlap the first metal and the second metal, a dielectric layer may be interposed at an interface thereof, and the fixed capacitance capacitor may be Two may be provided on each side of the resonator, respectively.

At least one of the contacts may be provided as a variable capacitance capacitor, and the variable capacitance capacitor may include a semiconductor layer provided on the substrate; And a passivation layer on the semiconductor layer, and the metal may be in contact with each other through two places in which the passivation layer is opened. The metal that is in contact with the semiconductor layer may be in nanoscale contact with the semiconductor layer and make a Schottky contact. Preferably, the semiconductor layer may be provided as hept. In addition, the variable capacitance capacitor may be provided at the center portion of the resonator, and a depletion region may be provided only at a lower portion of the variable capacitance capacitor.

Among the at least three contacts, at least one contact may be a variable capacitance capacitor, at least two contacts may be a fixed capacitance capacitor, and the variable capacity capacitor may be provided at both sides of the variable capacity capacitor.

According to another aspect of the present invention, a terahertz modulator includes: 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 metals in at least two places where the first metal and the second metal are in contact with each other.

Here, the capacitor may include at least one variable capacitor and at least one fixed 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 shorted at high frequency in terahertz, open at low frequency, the resonator is provided in a square, The longitudinal direction may provide longer.

Another aspect of the terahertz modulator according to the present invention includes 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 regulating the transmission of terahertz waves, wherein the resonators include capacitors provided at at least two locations where the first metal and the second metal contact each other, and the resonator has a split ring. A resonator can be provided. The capacitor may include a variable capacitance capacitor provided at a central portion of the split ring resonator; And fixed-capacitors respectively provided at both sides of the split ring resonator.

According to the present invention, the modulation index can be increased to 80% level, 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 a description thereof will be made apparent in the detailed description.

1 illustrates the operation of a terahertz modulator according to an embodiment.

2 illustrates a terahertz modulator according to an embodiment.

3 is an enlarged view of any one resonator.

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

6 is an equivalent circuit diagram of a resonator at low frequencies, and FIG. 7 is an equivalent circuit diagram of a resonator at high frequencies.

8 and 9 are diagrams illustrating a difference in transmission characteristics according to the shape of a resonator.

10 is a result of measuring capacitance characteristics while varying the length of a gate of a Schottky contact on a semiconductor layer.

11 is an embodiment of a hamt based on InP by way of example.

12 is an embodiment of a hamt based on GaAs.

Figure 13 is an embodiment of a hamt based on GaN.

Hereinafter, with reference to the drawings will be described in detail a specific embodiment of the present invention. However, the spirit of the present invention is not limited to the embodiments set forth below, and those skilled in the art who understand the spirit of the present invention may add, change, delete, add, etc. to other embodiments within the scope of the same idea. It may be easily proposed by the present invention, but this will also be included within the scope of the present invention.

The accompanying drawings in the description of the embodiments may be exaggeratedly shown differently from the actual product, but it should be understood as a scope for explanation of the inventive idea. In addition, in the following description, when there is no special reference, a prior art shall mean the technique introduced by the said nonpatent literature 1.

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

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

2 illustrates a terahertz modulator according to an embodiment.

Referring to FIG. 2, in the terahertz modulator according to the embodiment, an array of resonators 2 is provided on a substrate, and each of the resonators 2 is different from the first metal 11 and the second metal 12. It is provided in a structure that makes indirect contact with the substance. Thus, the Schottky contact is provided only where the contacts of the

metals

11 and 12 are located, and the depletion region 3 by the Schottky contact is only where the contacts of the

metals

11 and 12 are located. Is provided. Therefore, the depletion region is narrower than the prior art, and thus the time constant of the resonator is low, and thus the modulation bandwidth can reach ~ GHz. In addition, the effect that the resonator can operate sufficiently even at a low applied voltage can be expected.

A bias voltage by a bias power source 13 is applied to the

metals

11 and 12, and the indirect contact between the shapes of the

metals

11 and 12 forming a closed circuit and the

metals

11 and 12 is applied. It may be provided that the resonator (2).

Hereinafter, the resonator will be described in more detail.

3 is an enlarged view of any one resonator.

Referring to FIG. 3, the resonator 2 indirectly contacts the first metal 11 and the second metal 12 at three locations. The first metal 11, the second metal 12, and a contact thereof may form two quadrangular shapes. Each contact is demonstrated. Firstly, the first fixed capacitance capacitor 5 and the second fixed capacitance capacitor 6 on which the capacitive components are fixed are provided at the contacts at both edges, and the variable capacitance capacitor 4 on which the capacitive components are variable is provided at the center portion. . This resonator may be referred to as a split ring resonator SRR. It can be seen that the depletion region 3 is provided only below the variable capacitance capacitor 4.

3 to 5, the variable capacitance capacitor 4 includes a semiconductor layer 7 provided on the substrate 9, a protective film 10 provided in a structure covering the semiconductor layer 7, and the First and

second metals

11 and 12 contacting the semiconductor layer 7 may be included at both side portions spaced apart from each other to remove the protective film. The fixed

capacitance capacitors

5 and 6 are provided in a structure in which the first and

second metals

11 and 12 are overlapped with each other with the dielectric layer 21 interposed therebetween. The dielectric layer 21 and the passivation layer 10 may preferably use SiN _x .

The semiconductor layer 7 may preferably exemplify a High Electron Mobility Transistor (HEMT). The Hamp allows the formation of a two-dimensional region (two-dimensional electron gas: 2DEG) at the interface of the heterojunction semiconductor is very high, the carrier is difficult to receive the ion scattering, the carrier has a higher mobility than the carrier in the general semiconductor Use what you have. Therefore, the present invention can be preferably applied to a terahertz modulator because there is an effect of obtaining a high reaction rate.

The contact point will be described in more detail.

First, in the variable capacitance capacitor 4, the first and

second metals

11 and 12 form a Schottky junction with the semiconductor layer 7 at approximately the center of the resonator 2. The Schottky junction can be made with nanoscale gate contacts for the switching of capacitive components in the terahertz region. When the nanoscale schottky contact is provided to the semiconductor plane as a gate, it may have a varactor characteristic. That is, the variable capacitance capacitor 4 may be referred to as a varactor. More specifically, the varactor may be referred to as a metal semiconductor metal (MSM) varactor, which is provided in the configuration of the metal 11-semiconductor layer 7 and the metal 12. The variable capacitance capacitor 4 may have a variable capacitance according to a bias voltage applied from the bias power source 13.

As described above, the variable capacity of the variable capacitor 4 will be described. First, two nanoscale schottky contacts provided by contacting the first and

second metals

11 and 12 and the semiconductor layer 7 include a schottky layer provided on a surface and a schottky layer. An electron moveable layer (eg, 2DEG 8) provided therein is included. In this case, when viewed from the

metals

11 and 12, the capacitance C1 by the Schottky layer and the capacitance C2 by the electron movable layer may be viewed as capacitances connected in series. The capacitance C1 by the Schottky layer is much larger than the capacitance C2 by the electron-movable layer. Therefore, since the total amount of capacitance in series merging of two capacitances C1 and C2 is an inverse of the sum of the inverses of each capacitance, the capacitance of the MSM varactor is determined by the capacitance C2 by the electron-movable layer. Is dominated. In this case, the capacitance C2 of the electron moveable layer is symmetrically different from each other as electrons move in both directions in accordance with the bias voltage. As a result, the capacitance C2 due to the electron-movable layer varies only within a range of a constant bias voltage, and in other cases, the capacitance C2 is different, and the bias C2 caused by the electron-movable layer starts to vary. The voltage is given in the example of approximately ± 3V.

The fixed

capacitance capacitors

5 and 6 will be described. The first metal 11 and the second metal 12 are provided in a structure overlapping each other at both sides of the resonator 2. By overlapping each other, it may have a capacitive component. The overlap structure is a structure of the metal 11, the dielectric layer 21, and the metal 12, and constitutes a metal-insulator-metal (MIM) capacitor. In this case, the capacitance of the fixed

capacitance capacitors

5 and 6 is represented by Equation 1 by the area A where the metal overlaps, the dielectric constant ε _r of the dielectric layer 21, and the thickness t of the dielectric layer 21. Is determined as follows.

The capacitance of the fixed

capacitance capacitors

5 and 6 given by Equation 1 may be designed to be very large so as to have a very low impedance in the terahertz frequency region (for example, 1 terahertz). The capacitance of the fixed capacitance capacitor can be, for example, provided at 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 the terahertz signal. It seems to be shorted. However, the DC signal is opened by the fixed

capacitance capacitors

5 and 6, and the bias voltage can be applied by the bias power supply 13.

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

6 and 7, the capacitance of the variable capacitance capacitor 4 changes according to a bias voltage. The fixed

capacitance capacitors

5 and 6 appear to be open at low frequencies and shorted at high frequencies. Therefore, the resonator 2 forms a resonant circuit in the terahertz high frequency region. When the terahertz wave enters the resonator, when the terahertz wave is a frequency matched with the resonant frequency of the resonator 2, the terahertz wave can lead to an operation that cannot penetrate the resonator 2. In this situation, modulation can be induced. The resonant frequency of the resonator may be given by Equation 2.

L is an inductance formed by the shapes of the

metals

11 and 12 as a closed circuit, and C _var is the capacitance of the variable capacitance capacitor 4. Therefore, the resonance frequency can be changed by changing the inductance of the resonator by changing the shapes of the

metals

11 and 12, which are generally illustrated as quadrangles, in various ways, and the capacitance of the capacitor of the variable capacitance capacitor 4 can be changed. By turning the state on or off (ie, bistable), one can determine whether to modulate.

8 and 9 are views for explaining the difference in transmission characteristics according to the shape of the resonator as an example.

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

metals

11 and 12, and the

metals

11 and 12 of the resonator 2 are changed. By changing the length in the longitudinal direction to be shorter than in the transverse direction (ie, FIG. 9), the secondary resonant frequency can be shifted to a higher position, and the spectrum can be provided in a more smooth form.

Observation of the circle of hatched in Fig. 9 shows a transmittance of 20% in the case of 1fF, but almost 100% in the case of 3fF. However, in the case of FIG. 8, it can be observed that the transmittance of 20% in the case of 1fF, but 90% to 95% in the case of 3fF. Therefore, by shifting the secondary resonant frequency to a higher place, the modulation efficiency can be improved and a smooth spectrum response can be obtained.

10 is a result of measuring capacitance characteristics while varying the length of the gate of the Schottky contact in the semiconductor layer. As already explained, the Schottky contacts were provided on a nanoscale, with the distance between the gates being 4 μm.

Referring to FIG. 10, a change in capacitance occurs when the bias voltage is approximately ± 3V, which starts to change in capacitance C2 by the electron-movable layer at ± 3V. It is shown that the capacitance C2 due to the electron-movable layer changes rapidly during a predetermined period G, and that the capacitance C2 due to the electron-movable layer hardly changes within ± 2V. Therefore, it can be seen that the capacitance by the variable capacitance capacitor 4 can be stably operated at two points (for example, 1fF and 3fF in the embodiment).

As the gate length L of the schottky contact of the nanoscale becomes larger, the difference in capacitance as the variable capacitor 4 is turned on and off becomes larger. By combining them properly, an optimal terahertz modulator suitable for the state of use will be obtained.

As an example of application of the semiconductor layer 7, it has been described that a hamt capable of obtaining a high reaction rate can be used. FIG. 11 illustrates an embodiment of a ham based on InP, FIG. 12 illustrates an embodiment of a ham based on GaAs, and FIG. 13 illustrates an embodiment of a ham based on GaN. Hamps based on InP or GaAs can implement small time constants based on high electron mobility, and Hamps based on GaN can be strongly resistant to collapse due to bandgap characteristics.

The terahertz modulator according to the present invention improves the problem of low modulation index, very low modulation bandwidth, and high operating voltage. Thus, by providing high-efficiency terahertz modulators, the application to terahertz's actual use can be further advanced.

Claims

Board;

A first metal provided on the substrate;

A second metal provided on the substrate;

At least three contacts indirectly contacting the first metal and the second metal; And

And a closed circuit comprising the first metal, the second metal, and the at least three contacts, the terahertz modulator comprising at least one resonator for controlling transmission of terahertz waves.
The method of claim 1,

And at least two resonators are provided in an array on the substrate.
The method of claim 1,

At least one of the contacts is a terahertz modulator.
The method of claim 3, wherein

The fixed capacitance capacitor,

And a terahertz modulator, wherein the first metal and the second metal overlap, and a dielectric layer intervenes at an interface thereof.
The method of claim 3, wherein

And two fixed capacitance capacitors each provided at both sides of the resonator.
The method of claim 1,

At least one of the contacts is a terahertz modulator.
The method of claim 6,

The variable capacitance capacitor,

A semiconductor layer provided on the substrate; And

A protective film on the semiconductor layer is included,

And a terahertz modulator in which the metal is in contact with each other through two open portions of the protective film.
The method of claim 7, wherein

And the metal in contact with the semiconductor layer is in contact with the semiconductor layer on a nanoscale.
The method of claim 7, wherein

And the metal in contact with the semiconductor layer makes a Schottky contact.
The method of claim 7, wherein

The terahertz modulator of the semiconductor layer is a hemp.
The method of claim 6,

And the variable capacitance capacitor is provided at the center of the resonator.
The method of claim 6,

And a terahertz modulator provided with a depletion region only below the variable capacitance capacitor.
The method of claim 1,

Wherein said at least one contact is a variable capacitance capacitor, at least two contacts are fixed capacitance capacitors, and said variable capacitance capacitor is provided at both sides of said terahertz modulator.
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 metals in at least two places where the first metal and the second metal are in contact with each other.
The method of claim 14,

The capacitor includes at least one variable capacitance capacitor and at least one fixed capacitance capacitor to form a resonator.

And the resonator is an array on the substrate.
The method of claim 15,

And two fixed-capacitance capacitors and one variable-capacitance capacitor.
The method of claim 15,

Wherein said fixed capacitance capacitor is shorted at high frequencies in terahertz and open at low frequencies.
The method of claim 15,

Wherein said resonator is provided in a quadrangle and has a longer longitudinal direction.
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 is included that can control the transmission of terahertz waves,

The resonator includes a capacitor provided at each of at least two places where the first metal and the second metal contact each other,

And the resonator is a terahertz modulator forming a split ring resonator.
The method of claim 19,

The capacitor,

A variable capacitance capacitor provided at the center of the split ring resonator; And

And a terahertz modulator including fixed capacitance capacitors respectively provided at both sides of the split ring resonator.