JP2011077396A - Terahertz-wave radiating element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly efficient and high power terahertz-wave radiation element. <P>SOLUTION: The terahertz-wave radiation element includes: a multilayer structure in which at least two heterojunction structures are laminated, in which a substrate, a layer composed of a nitride semiconductor on the substrate, and a layer consisting of a nitride semiconductor having a bigger band gap than the nitride structure are laminated; a Schottky electrode to which the multilayer structure is Schottky-connected; and an ohmic electrode to which the multilayer structure is ohmic-connected, wherein the Schottky electrode has a radiation field conversion function for converting the electron plasma oscillation of a terahertz-frequency band generated in the multilayer structure to an electromagnetic wave of the terahertz-frequency band. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a terahertz wave radiating element, and more particularly to a terahertz wave radiating element including a semiconductor element that can radiate electromagnetic waves in a terahertz frequency band.

  Terahertz band electromagnetic waves have the transparency of radio waves and the broadband, coherent, and light-collecting properties of light, and are expected to be applied in various fields such as security, biotechnology, environment, and communications. Has been.

  As a device for emitting terahertz electromagnetic waves, for example, a photoconductive element, a terahertz parametric generator, a quantum cascade laser, a carbon dioxide laser, a backward wave tube, and the like are used. However, these apparatuses require a large-scale and expensive femtosecond laser or picosecond laser, or need to operate by cooling to a very low temperature. Therefore, there is a problem that it is difficult to reduce the size or cost of these devices.

In order to overcome this problem, that is, to realize a small, inexpensive, and cooling-free terahertz radiation apparatus, research and development of devices using electron plasma existing in semiconductors has been vigorously advanced. Here, as an electron plasma in a semiconductor, a field effect transistor (FET) structure or a two-dimensional electron gas (2-dimensional electron gas) in a high electron mobility transistor (HEMT) is used. Is used (for example, Non-Patent Document 1). The speed of the 2DEG plasma wave, which is the collective vibration of electrons, is on the order of 10 ⁸ cm / s, which is one digit higher than the speed of a single electron, and is a general electron using a single electron. Operation in a higher frequency terahertz frequency band is possible compared to devices. The operating principle of this device will be described below.

  FIG. 6 is a cross-sectional view showing the structure of the terahertz wave radiating element shown in FIG. The terahertz wave radiating element 900 includes a source electrode 908, a gate electrode 909, a drain electrode 910, a semiconductor layer 911, a two-dimensional electron gas region 912, and a gate insulating film 913.

  In the terahertz wave radiating element 900, when a bias voltage is applied to the source electrode 908 and the drain electrode 910, a two-dimensional electron gas flow is generated. The two-dimensional electron gas flow becomes unstable due to reflection at the source electrode 908 end and the drain electrode 910 end. This instability induces electron plasma oscillation in the terahertz frequency band, and causes radiation of terahertz electromagnetic waves.

  Furthermore, in recent years, the above-described technology has been developed, and in 2006, radiation of terahertz electromagnetic waves was confirmed at room temperature (for example, Non-Patent Document 2). This non-patent document 2 discloses a technique that can obtain a terahertz electromagnetic wave output of about 0.1 μW from an AlGaN / GaN material HEMT.

  Thus, research and development of semiconductor devices capable of emitting terahertz waves at room temperature using electron plasma have progressed.

Physical Review Letters, Vol. 71, no. 15, pp 2465-2468 (1993) Applied Physics Letters, 88, 141906 (2006).

However, in a conventional semiconductor device that can emit a terahertz wave, that is, a terahertz wave radiating element, the intensity of the radiable terahertz electromagnetic wave is still as weak as 1 μW or less, and the output is insufficient for various applications. Further, the conventional terahertz wave radiating element has a very low power conversion efficiency of 1 × 10 ^−7, and an input power of about 1000 mW is required to obtain an output of 0.1 μW.

  Therefore, in order to put the terahertz wave radiating element, which is a semiconductor element capable of terahertz radiation, into practical use, improvement in output and efficiency is indispensable.

  Therefore, the present invention has been made in view of such problems, and an object thereof is to provide a high-power and high-efficiency terahertz wave radiating element.

  To achieve the above object, a terahertz wave radiating element according to the present invention includes a substrate, a layer made of a nitride semiconductor on the substrate, and a layer made of a nitride semiconductor having a band gap larger than that of the nitride semiconductor. A stacked structure in which at least two heterojunction structures are stacked, a Schottky electrode that is Schottky connected to the stacked structure, and an ohmic electrode that is ohmically connected to the stacked structure. The Schottky electrode has a radiation field conversion function for converting the electron plasma vibration in the terahertz frequency band generated in the laminated structure into an electromagnetic wave in the terahertz frequency band.

  With this configuration, the flow rate of the two-dimensional electron gas that is the generation source of the terahertz wave can be increased by the number of heterojunction structures, so that the terahertz wave output can be improved.

  Thereby, a high-output and high-efficiency terahertz wave radiating element can be realized.

  Here, preferably, the Schottky electrode is formed in a metal grating shape.

  With this configuration, plasma vibration (plasmon) of a two-dimensional electron gas generated in the terahertz band can be efficiently converted into a radiated electromagnetic field that can be emitted to the outside of the device.

Preferably, the multilayer structure includes a first GaN layer formed on the substrate, and a first GaN layer formed on the first GaN layer and having a band gap larger than that of the first GaN layer. Al a first heterojunction structure comprising _x Ga _1-x N layer, the second GaN layer formed on the first Al _x Ga _1-x N layer, and said second GaN layer a second heterojunction structure is formed consisting of a second Al _y Ga _1-y N layer having a large band gap than the second GaN layer, formed on the second Al _y Ga _1-y N layer And a third heterojunction structure comprising a third Al _z Ga _{1 -z} N layer formed on the third GaN layer and having a band gap larger than that of the third GaN layer Are laminated.

According to this configuration, even if the AlGaN layer is not doped with n-type impurities, a two-dimensional electron gas is induced by polarization at the AlGaN / GaN heterojunction interface. Then, two-dimensional electron gas is present at the interface between the second GaN layer and the second Al _y Ga _1-y N layer and at the interface between the third GaN layer and the third Al _z Ga _1-z N layer. Since it is induced, the flow rate of the two-dimensional electron gas that becomes the source of the terahertz wave can be doubled.

  Moreover, since the second GaN layer and the third GaN layer have a double hetero structure in which both sides are sandwiched between AlGaN layers, the induced two-dimensional electron gas is efficiently confined in the GaN layer. For this reason, it is possible to generate a high-density two-dimensional electron gas flow and further improve the output of the terahertz wave.

Preferably, the second GaN layer, the third GaN layer, and the second Al _y Ga _1-y N layer each have a thickness of 10 nm or less.

  Thereby, the second GaN layer and the third GaN layer become quantum wells, and a higher density two-dimensional electron gas flow can be obtained.

Preferably, the Al composition x, y and the first Al _x Ga _{1 -x} N layer, the second Al _y Ga _{1 -y} N layer, and the third Al _z Ga _{1 -z} N layer, There is a relationship of x> y and y <z between z.

With this configuration, since the second Al _y Ga _1-y N layer, which is a barrier layer between quantum wells, has a low Al composition, the mutual flow of the two-dimensional electron gas flow in the second GaN layer and the third GaN layer is reduced. The action becomes stronger. This interaction enhances the instability of the plasma, so that terahertz waves can be generated with high efficiency.

  Preferably, the ohmic electrode is formed on a side surface of the multilayer structure.

  With this configuration, since the ohmic electrode is directly formed on the heterojunction structure, the resistance between the electrode and the two-dimensional electron gas can be reduced. Therefore, a highly efficient terahertz wave radiation device with little loss can be realized.

  Preferably, the substrate is a high-resistance Si substrate.

  According to this configuration, since the high-resistance Si substrate has a small absorption coefficient with respect to the terahertz wave, the generated terahertz wave can be emitted from the device without being attenuated. Further, since the absorption coefficient is small, it is possible to emit light from the substrate side.

  According to the present invention, a high-power and high-efficiency terahertz wave radiating element can be realized.

  Specifically, the terahertz wave radiating element of the present invention has features that are not found in conventional terahertz wave radiating elements, such as small size, low cost, and no cooling. Therefore, applications using terahertz electromagnetic waves such as security, biotechnology, environment, and communication are used. It can be expected to greatly contribute to the progress of the field. In addition, since the terahertz wave radiating element of the present invention can be manufactured at low cost using an existing semiconductor process, the practical application and popularization of applications can be greatly promoted. Thus, the practical value of the present invention is very great.

It is sectional drawing which shows the structure of the terahertz wave radiation element in embodiment of this invention. It is a figure which shows the band lineup of the terahertz wave radiation element in embodiment of this invention. It is a figure for demonstrating the manufacturing method of the terahertz wave radiation element in embodiment of this invention. It is sectional drawing which shows the structure of the terahertz wave radiation element of the modification in embodiment of this invention. It is a figure which shows the band lineup of the terahertz wave radiation element of the modification in embodiment of this invention. It is a figure which shows the structure of the conventional terahertz wave radiation element.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a cross-sectional view showing the structure of the terahertz wave radiating element of the present embodiment. In the terahertz wave radiating element 100 shown in FIG. 1, a laminated structure 12 is laminated on a substrate 11, and a source electrode 108, a gate electrode 109, and a drain electrode 110 are formed on the laminated structure 12.

  The substrate 11 is, for example, a Si substrate, preferably high resistance Si.

The laminated structure 12 includes a plurality of heterojunction structures in which a layer made of a nitride semiconductor and a layer made of a nitride semiconductor having a band gap larger than that of the nitride semiconductor are laminated. Are stacked. Specifically, in the laminated structure 12, the first GaN layer 102, the first Al _x Ga _1-x N layer 103, the second GaN layer 104, and the second Al _y Ga _1-y N The layer 105, the third GaN layer 106, and the third Al _z Ga _{1 -z} N layer 107 are sequentially stacked.

  The first GaN layer 102 is formed on the substrate 11.

The first Al _x Ga _1-x N layer 103 is a nitride semiconductor layer having a band gap larger than that of the first GaN layer 102, and is formed on the first GaN layer 102. The first Al _x Ga _1-x N layer 103 forms a heterojunction structure with the first GaN layer 102.

The second GaN layer 104 is formed on the first Al _x Ga _1-x N layer 103. The film thickness of the second GaN layer 104 is, for example, 10 nm or less. The second Al _y Ga _1-y N layer 105 is a nitride semiconductor layer having a larger band gap than the second GaN layer 104, and is formed on the second GaN layer 104. The second Al _y Ga _1-y N layer 105 forms a heterojunction structure with the second GaN layer 104. The second GaN layer 104 has a double hetero structure in which the upper and lower sides are sandwiched between AlGaN layers. Note that the Al composition of the second Al _y Ga _1-y N layer 105 has a relationship of x> y.

The third GaN layer 106 is formed on the second Al _y Ga _1-y N layer 105. The film thickness of the third GaN layer 106 is, for example, 10 nm or less.

The third Al _z Ga _{1 -z} N layer 107 is a nitride semiconductor layer having a larger band gap than the third GaN layer 106, and is formed on the third GaN layer 106. The third Al _z Ga _{1 -z} N layer 107 forms a heterojunction structure with the third GaN layer 106. The third GaN layer 106 has a double hetero structure in which the upper and lower sides are sandwiched between AlGaN layers. Note that the Al composition of the third Al _z Ga _{1 -z} N layer 107 has a relationship of y <z.

The gate electrode 109 is formed on the laminated structure 12 and has a radiation field conversion function. Specifically, the gate electrode 109 is formed on the third Al _z Ga _{1 -z} N layer 107. The gate electrode 109 is a Schottky electrode with respect to the AlGaN layer, and is made of Ni, Pt, Au, and Ti. Further, the gate electrode 109 is formed in a grating shape having a periodic opening as shown in FIG. 1 as a radiation field conversion function for converting the plasma vibration of the two-dimensional electron gas into a radiation electromagnetic field. That is, the radiation field conversion function is formed by the gate electrode 109 being made of a metal grating having a periodic opening.

The source electrode 108 and the drain electrode 110 are formed in the multilayer structure 12. Specifically, each of the source electrode 108 and the drain electrode 110 extends from the region without the gate electrode 109 on the third Al _z Ga _{1 -z} N layer 107 to the side surface of the stacked structure 12 (the end of the stacked structure 12). Part). The source electrode 108 and the drain electrode 110 are electrodes that are ohmic with respect to the GaN layers (the first GaN layer 102, the second GaN layer 104, and the third GaN layer 106) of the stacked structure 12, respectively. And is composed of Ti and Al.

  As described above, the terahertz wave radiating element 100 is configured.

  FIG. 2 is a diagram showing a band lineup of the terahertz wave radiating element of the present embodiment.

  As shown in FIG. 2, a two-dimensional electron gas is formed in the second GaN layer 104 and the third GaN layer 106 by the polarization effect at the heterointerface between the AlGaN layer and the GaN layer. That is, by configuring the terahertz wave radiating element 100 as described above, two-dimensional electron gas is induced by polarization at the AlGaN / GaN heterojunction interface without doping the AlGaN layer with n-type impurities.

  Further, as described above, the second GaN layer 104 and the third GaN layer 106 have a double hetero structure in which both sides (upper and lower sides of the layer) are sandwiched between AlGaN layers. Therefore, the second GaN layer 104 and the third GaN layer 106 can efficiently (strongly) confine the two-dimensional electron gas in the GaN layer.

Thus, in the terahertz wave radiating element 100, two or more channel FETs, that is, multi-channel FETs using the two-dimensional electron gas in the second GaN layer 104 and the third GaN layer 106 as channels are formed. Has been. In other words, at least two interfaces are provided between the second GaN layer 104 and the second Al _y Ga _1-y N layer 105 and the third GaN layer 106 and the third Al _z Ga _1-z N layer 107. Since the two-dimensional electron gas is induced, the flow rate of the two-dimensional electron gas that is the source of the terahertz wave can be increased at least twice.

  Next, the operation principle of the terahertz wave radiating element 100 configured as described above will be described.

  When a voltage is applied between the source electrode 108 and the drain electrode 110, a current using the two-dimensional electron gas in the second GaN layer 104 and the third GaN layer 106 as a carrier flows. The two-dimensional electron gas flow induces an unstable state (plasma instability) due to reflection at both ends of the region below the gate electrode 109.

  When the gate of the FET formed in the terahertz wave radiating element 100 has a short gate length, plasma oscillation (plasmon) at the terahertz frequency is induced due to this plasma instability. Here, the induced plasmon is a longitudinal wave (non-radiating field) propagating through the channel. However, the induced plasmon is radiated to the outside as a terahertz electromagnetic wave (radiation field) by being coupled with the metal grating of the gate electrode 109.

  In this way, by passing a current between the source electrode 108 and the drain electrode 110, that is, a current using the two-dimensional electron gas formed in the second GaN layer 104 and the third GaN layer 106 as a carrier is passed. As a result, a terahertz electromagnetic wave is generated below the gate electrode 109.

  In order to generate the terahertz electromagnetic wave more efficiently, the FET constituting the terahertz wave radiating element 100 may be operated in the saturation region with the source electrode 108 and the gate electrode 109 short-circuited.

  Further, the frequency of the terahertz electromagnetic wave can be set to a desired value by adjusting the voltage of the gate electrode 109.

  Next, a method for manufacturing the terahertz wave radiating element 100 having the structure configured as described above will be described.

  FIG. 3 is a diagram for explaining a method of manufacturing the terahertz wave radiating element in the embodiment of the present invention.

First, the first GaN layer 102, the first Al _x Ga _1-x N layer 103, the second GaN layer 104, the second Al _y Ga _1-y N layer 105, and the like are formed on the substrate 11 by epitaxial growth. A third GaN layer 106 and a third Al _z Ga _{1 -z} N layer 107 are sequentially stacked. That is, the laminated structure 12 is laminated on the substrate 11 by epitaxial growth (FIG. 3A).

  Next, the peripheral portion of the epitaxial layer of the laminated structure 12 is removed by etching to a predetermined depth (FIG. 3B).

Next, a metal material is deposited on the third Al _z Ga _{1 -z} N layer 107 and around the epitaxial layer exposed by etching, and patterned to form the source electrode 108, the gate electrode 109, and the drain electrode 110. (FIG. 3C).

  The terahertz wave radiating element 100 is manufactured as described above.

  As described above, since the terahertz wave radiating element 100 according to the present invention has at least two channels in the second GaN layer 104 and the third GaN layer 106, compared with a conventional element having one channel, It is possible to increase the flow rate of the two-dimensional electron gas that is a generation source of the terahertz wave at least twice. This makes it possible to realize a high-power terahertz wave radiating element 100.

  In the terahertz wave radiating element 100 according to the present invention, since the gate electrode 109 is formed in a grating shape having a periodic opening, plasmons are efficiently converted into a radiation field. Thereby, a highly efficient terahertz wave radiating element 100 can be realized.

  Further, in the terahertz wave radiating element 100 according to the present invention, the second GaN layer 104 and the third GaN layer 106 have a double hetero structure in which both sides (upper and lower sides of the layer) are sandwiched between AlGaN layers. The induced two-dimensional electron gas is strongly confined in the GaN layer. Therefore, a high-density two-dimensional electron gas flow is formed, and a high-power terahertz wave radiating element 100 can be obtained.

In particular, if the thickness of the second GaN layer 104, the third GaN layer 106, and the second Al _y Ga _1-y N layer 105 is 10 nm or less, the second GaN layer 104 and the third GaN layer 104 Since the GaN layer 106 becomes a quantum well, a higher-density two-dimensional electron gas flow can be obtained.

In this case, the Al composition x of the first Al _x Ga _{1 -x} N layer 103, the second Al _y Ga _{1 -y} N layer 105, and the third Al _z Ga _{1 -z} N layer 107, About y and z, it sets so that it may have the relationship of x> y and y <z. That is, when the Al composition of the second Al _y Ga _1-y N layer 105, which is a barrier layer between quantum wells, is reduced, the second GaN layer 104 and the third GaN layer 106 exceed the barrier layer. It becomes easier for the electrons inside to go back and forth. That is, the interaction of the two-dimensional electron gas flow is strengthened. This leads to enhanced plasma instability, and as a result, it is possible to generate terahertz waves with high efficiency.

  Further, in the terahertz wave radiating element 100 according to the present invention, as shown in FIG. 1, ohmic electrodes are directly formed on the two channels on the side surface of the device, that is, the side surface of the terahertz wave radiating element 100. Bonding with the two-dimensional electron gas can be reduced. Therefore, a highly efficient terahertz wave radiating element 100 with little loss of input power can be realized.

  As described above, according to the present invention, a high-power and high-efficiency terahertz wave radiating element can be realized.

  Note that the material of the substrate is preferably a material having a small absorption coefficient in the terahertz band. For example, a high resistance Si substrate can be used as the substrate having a small absorption coefficient. Thereby, the generated terahertz wave can be emitted from the device without being attenuated. When a substrate having a small absorption coefficient is used, it is possible to emit terahertz electromagnetic waves from the substrate side.

  In the present invention, the case where the gate electrode 109 is formed in a grating shape has been described. However, the present invention is not limited thereto, and the gate electrode 109 may be formed as a single gate electrode, for example. Further, for example, metals that are not electrically connected to each other may be formed side by side in a grating shape next to a single gate electrode.

In the terahertz wave radiating element 100 according to the present invention, the interface between the second GaN layer 104 and the second Al _y Ga _1-y N layer 105, and the third GaN layer 106 and the third Al _z Ga. It has been described that two-dimensional electron gas is induced at the interface with the _1-z N layer 107. Furthermore, in the terahertz wave radiating element 100, a two-dimensional electron gas is induced by polarization at the interface between the first GaN layer 102 and the first Al _x Ga _1-x N layer 103 to form a channel. The channel induced at the interface between the first GaN layer 102 and the first Al _x Ga _1-x N layer 103 also increases the flow rate of the two-dimensional electron gas that is the source of the terahertz wave, and outputs the terahertz wave. It contributes to improving. Accordingly, the channel induced at the interface between the first GaN layer 102 and the first Al _x Ga _1-x N layer 103 is used to improve the output of the terahertz wave, as in the modification described below. It may be structured.

(Modification)
FIG. 4 is a sectional view showing the structure of a terahertz wave radiating element according to a modification of the embodiment of the present invention. FIG. 5 is a diagram showing a band lineup of a terahertz wave radiating element according to a modification of the embodiment of the present invention. Elements similar to those in FIGS. 1 and 2 are denoted by the same reference numerals, and detailed description thereof is omitted.

The terahertz wave radiating element 200 shown in FIG. 4 does not include the third GaN layer 106 and the third Al _z Ga _{1 -z} N layer 107 as compared with the terahertz wave radiating element 100 shown in FIG. Different.

As shown in FIG. 5, the terahertz wave radiating element 200 includes not only the interface between the second GaN layer 104 and the second Al _y Ga _1-y N layer 105 but also the first GaN layer 102 and the first Al _A channel is also formed at the interface with the _x Ga _1-x N layer 103 by inducing a two-dimensional electron gas by polarization. Thereby, in the terahertz wave radiating element 200, the flow rate of the two-dimensional electron gas that is a generation source of the terahertz wave can be increased twice, and the output of the terahertz wave can be improved.

  As described above, according to the present invention, a high-power and high-efficiency terahertz wave radiating element can be realized.

  As described above, the terahertz wave radiating element of the present invention has been described based on the embodiment. However, the present invention is not limited to this embodiment. Unless it deviates from the meaning of this invention, the form which carried out the various deformation | transformation which those skilled in the art can consider to this Embodiment, and the form constructed | assembled combining the component which those skilled in the art can think are also included in the scope of the present invention.

  The present invention can be used for a terahertz wave radiating element, and in particular, can be used for a terahertz wave radiating element used in an imaging apparatus, a nondestructive inspection apparatus, a large-capacity communication apparatus, and the like.

11 substrate 12, 22 stacked structure 100,200,900 terahertz wave radiating element 102 first GaN layer 103 first Al _x Ga _1-x N layer 104 second GaN layer 105 a second Al _y Ga _{1- y} N layer 106 Third GaN layer 107 Third Al _z Ga ₁ -z N layer 108, 908 Source electrode 109, 909 Gate electrode 110, 910 Drain electrode 911 Semiconductor layer 912 Two-dimensional electron gas region 913 Gate insulating film

Claims (7)

A substrate,
A stacked structure in which at least two heterojunction structures in which a layer made of a nitride semiconductor and a layer made of a nitride semiconductor having a larger band gap than the nitride semiconductor are stacked are stacked on the substrate; ,
A Schottky electrode that is Schottky connected to the multilayer structure;
An ohmic electrode that is ohmically connected to the laminated structure;
The Schottky electrode is a terahertz wave radiating element having a radiation field converting function for converting an electron plasma vibration in a terahertz frequency band generated in the laminated structure into an electromagnetic wave in a terahertz frequency band. The terahertz wave radiating element according to claim 1, wherein the Schottky electrode is formed in a metal grating shape. The laminated structure is
A first GaN layer formed on the substrate and a first Al _x Ga _1-x N layer formed on the first GaN layer and having a band gap larger than that of the first GaN layer. 1 heterojunction structure;
A second GaN layer formed on the first Al _x Ga _1-x N layer, and a second Al having a band gap larger than that of the second GaN layer formed on the second GaN layer. _a second heterojunction structure comprising _y Ga _1-y N layers;
A third GaN layer formed on the second Al _y Ga _1-y N layer, and a third Al formed on the third GaN layer and having a larger band gap than the third GaN layer. _z Ga _1-z N a third heterojunction structure formed by stacking consisting layers according to claim 1 or 2 THz wave radiating device according to. The terahertz wave radiating element according to claim 3, wherein film thicknesses of the second GaN layer, the third GaN layer, and the second Al _y Ga _1-y N layer are each 10 nm or less. Between the Al compositions x, y and z in the first Al _x Ga _{1 -x} N layer, the second Al _y Ga _{1 -y} N layer and the third Al _z Ga _{1 -z} N layer The terahertz wave radiating element according to claim 3, wherein x> y and y <z. The terahertz wave radiating element according to any one of claims 1 to 5, wherein the ohmic electrode is formed on a side surface of the multilayer structure. The terahertz wave radiating element according to any one of claims 1 to 6, wherein the substrate is made of a high-resistance Si substrate.

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