JP5988193B2 - Planar light modulator - Google Patents

Description

  The present invention is a planar optical modulator that has an i-GaAs layer and allows light to enter in a direction perpendicular to the main surface of the i-GaAs layer, and changes the absorption coefficient of the i-GaAs layer by applying a reverse voltage. The present invention relates to a planar light modulator that changes the intensity of light.

  In recent years, in the optical communication field and the optical information processing field, high speed and large capacity have been promoted, and realization of a high-speed response, low voltage drive, and high extinction ratio spatial semiconductor optical modulator is required. In the case of aiming at a two-dimensional arrangement of switch elements or integration, a planar light modulator that allows light to enter from a direction perpendicular to the main surface is desired because of the ease of two-dimensional arrangement.

  As such a planar light modulator, the one described in Patent Document 1 is known. The planar optical modulator described in Patent Document 1 includes an n-GaAs substrate, an i-GaAs layer formed on the n-GaAs substrate, a Schottky electrode formed on the i-GaAs layer, and an n− And an ohmic electrode formed on the surface opposite to the i-GaAs layer forming side of the GaAs substrate. The Schottky electrode has a ring shape with a window at the center, and the ohmic electrode also has a window at the center. In this planar light modulator, light is incident vertically from the window portion, and a reverse voltage is applied between the ohmic electrode and the Schottky electrode to change the absorption coefficient of the i-GaAs layer, thereby increasing the light intensity. It is something to change. The change in absorption coefficient is due to the Franz-Keldish effect.

  In this planar optical modulator, in order to reduce the drive voltage and achieve a high extinction ratio, the thickness of the depletion layer when applying a reverse voltage is made as large as possible, and the electric field strength in the i-GaAs layer is made uniform. is necessary. Therefore, it is necessary to reduce the electron concentration of the i-GaAs layer as much as possible. In order to realize a high-speed response, it is necessary to increase the mobility of the i-GaAs layer as much as possible. Patent Document 1 discloses a method of forming an i-GaAs layer having such a low electron concentration and high mobility.

JP2011-249650

  However, the planar optical modulator of Patent Document 1 using the Franz-Keldish effect cannot sufficiently reduce the drive voltage, and further reduction of the drive voltage has been demanded.

  The present invention is a planar optical modulator based on a novel operating principle, and an object thereof is to realize a planar optical modulator driven at a low voltage.

According to the first aspect of the present invention, the mobility at a temperature of 77K is 2.0 × 10 ^{5 to} 3.1 × 10 ⁵ cm ² / V · s, and the electron concentration at a temperature of 77K is 1 × 10 ¹² to 1 × 10. ^An i-GaAs layer having a thickness of ¹³ / cm ³ and a thickness of 20 to 30 μm, a transparent layer located on the i-GaAs layer, forming a depletion layer in the i-GaAs layer, and being in Schottky contact with the i-GaAs layer An electrode and a back electrode positioned opposite to the transparent electrode side of the i-GaAs layer, the transparent electrode is circular in plan view, and its diameter is 2 times the thickness of the i-GaAs layer. The absorption coefficient spectrum of the i-GaAs layer has a light absorption peak due to excitons at room temperature, and decreases sharply on the longer wavelength side than the light absorption peak. By increasing the reverse voltage applied between the backside electrodes, Shifted to the long side, the intensity of light to be transmitted in a direction perpendicular to the main surface of the i-GaAs layer, a positive potential with respect to the transparent electrode and the reverse voltage (transparent electrode side back electrode side to between the back electrode The light that is modulated and transmitted by the change in the absorption coefficient of the i-GaAs layer due to the application of voltage) is near the wavelength longer than the light absorption peak and has a wavelength of 870 to 900 nm, and the extinction ratio is 5 V It is a planar light modulator characterized by being 10 dB or more at -12 V.

In the planar light modulator of the present invention, light is incident in a direction perpendicular to the main surface of the i-GaAs layer, and a reverse voltage is applied between the electrode structure and the back electrode (the back electrode side is set to a positive potential with respect to the electrode structure side). Is applied to change the absorption coefficient of the i-GaAs layer and modulate the intensity of the light transmitted through the i-GaAs layer. Further, the present invention is a planar optical modulator having a high extinction ratio capable of setting the extinction ratio to 10 dB or more at a driving voltage of 5 to 12V.

  The feature of the present invention is not an operation principle using the conventional Franz-Keldish effect but an operation principle using exciton absorption.

  The operation principle of the planar light modulator of the present invention will be described below. By setting the mobility and electron concentration of the i-GaAs layer within a predetermined range, a light absorption peak due to excitons occurs in the absorption coefficient spectrum of the i-GaAs layer. On the longer wavelength side than this light absorption peak, the absorption coefficient decreases rapidly. Further, when the reverse voltage applied between the electrode structure and the ohmic electrode is increased, the light absorption peak shifts to the long wavelength side. Therefore, when light in the vicinity of the long wavelength side of the light absorption peak due to excitons is incident, the absorption coefficient of the i-GaAs layer varies greatly with changes in the applied reverse voltage. As a result, the planar light modulator of the present invention can realize an operation with a high extinction ratio by low voltage driving.

In order to improve the effect of exciton absorption, the i-GaAs layer has a mobility at a temperature of 77K of 1.0 × 10 ^{5 to} 3.1 × 10 ⁵ cm ² / V · s, and an electron concentration at a temperature of 77K. Is preferably 1 × 10 ¹² to 1 × 10 ¹⁴ / cm ³ . More desirably, the mobility of the i-GaAs layer at a temperature of 77K is 1.5 × 10 ^{5 to} 3.1 × 10 ⁵ cm ² / V · s, and the electron concentration at a temperature of 77K is 1 × 10 ^{12 to} 3 ×. The range is 10 ¹³ / cm ³ . More preferably, the mobility of the i-GaAs layer at a temperature of 77K is 2.0 × 10 ^{5 to} 3.1 × 10 ⁵ cm ² / V · s, and the electron concentration at a temperature of 77K is 1 × 10 ¹² to 1 ×. 10 ¹³ / cm ³ .

  The thickness of the i-GaAs layer is desirably 5 to 30 μm. If it is thinner than 5 μm, the effect of exciton absorption cannot be sufficiently exhibited, which is not desirable. On the other hand, if it is thicker than 30 μm, light absorption by the i-GaAs layer increases and the drive voltage also rises, which is not desirable. A more desirable thickness is 10 to 30 μm, and even more desirably 20 to 30 μm. Further, the thickness of the i-GaAs layer and the electron concentration so that the thickness of the depletion layer extending from the interface between the electrode structure and the i-GaAs layer toward the i-GaAs layer is larger than the thickness of the i-GaAs layer. Need to be set. This is because the electric field strength in the i-GaAs layer becomes uniform, and both low voltage driving and a high extinction ratio can be achieved to achieve stable operation.

The electrode structure is a structure in which an electrode in direct Schottky contact is formed on an i-GaAs layer, or an MIS type structure in which an insulating film is formed on an i-GaAs layer and an electrode is formed on the insulating film. Also good.

  In the case of using a transparent material (transparent means transparent to the wavelength of light modulated by the planar light modulator of the present invention, the same applies hereinafter) for these electrode structures. It is desirable to be provided in a region on which light is incident (light transmission region). This is because the electric field strength in the i-GaAs layer under the electrode structure can be made uniform, and the driving voltage of the planar light modulator can be reduced. As a material for the transparent electrode that comes into Schottky contact, a transparent conductive oxide such as ITO or a transparent organic conductive material such as polyaniline can be used.

  When an opaque material (such as Au) is used as the electrode structure, it is desirable to open a window in the light transmission region to suppress light absorption by the electrode structure. As the pattern in which the window is opened, a ring-shaped pattern (a pattern having a circular window at the center) in a plan view can be adopted. In the case of a ring shape, the diameter of the window is preferably 2 to 5 times the thickness of the i-GaAs layer. This is because the electric field strength in the i-GaAs layer under the electrode structure can be made uniform, and the driving voltage of the planar light modulator can be reduced.

  The i-GaAs layer may be formed on a substrate such as an n-GaAs substrate, or may not be formed on the substrate. The case where it is not formed on the substrate means that the substrate is removed by etching or polishing after growing an i-GaAs layer on the substrate, for example. Further, an AlGaAs layer may be provided between the substrate and the i-GaAs layer. By providing an AlGaAs layer having a larger band gap energy than the i-GaAs layer, leakage current can be reduced, driving voltage can be reduced, and stable operation can be realized. More preferably, the Al composition ratio of the AlGaAs layer to the group III metal is 20 to 30 mol%, and the thickness of the AlGaAs layer is 0.1 to 0.5 μm. The effect of reducing leakage current is further increased, and more stable operation can be realized. The AlGaAs layer can also be used as an etching stopper layer when the substrate is removed by etching.

  The wavelength of light modulated by the planar light modulator of the present invention is preferably on the longer wavelength side than the exciton absorption peak of the i-GaAs layer, and on the exciton absorption peak side as much as possible. This is because a high extinction ratio can be obtained. Specifically, the thickness is desirably 870 to 920 nm.

In the present invention, the electrode structure is a Schottky electrode in Schottky contact with the i-GaAs layer, and the pattern in plan view is a ring shape having a circular window, and the diameter of the window is that of the i-GaAs layer. may I 2-5 Baidea of thickness.

In the present invention, the Schottky electrode may be Au or a multilayer film including an Au film .

In the present invention, the electrode structure may be an MIS type structure constituted by an insulating film located on the i-GaAs layer and an electrode located on the insulating film .

In the present invention, a planar light modulator characterized by having an AlGaAs layer between the i-GaAs layer and the ohmic electrode may be used.

In the present invention, the AlGaAs layer may have an Al composition ratio of 20 to 30 mol% and a thickness of 0.1 to 0.5 μm with respect to the group III metal .

In the present invention, the back electrode may be an ohmic electrode that is in ohmic contact with the i-GaAs layer .

  When the present inventors prototyped and evaluated a planar optical modulator using the method of forming an i-GaAs layer disclosed in Patent Document 1, a light absorption peak due to exciton absorption was observed in the spectrum of the absorption coefficient. It was found that the absorption coefficient abruptly decreased on the longer wavelength side than the light absorption peak. It was also found that the light absorption peak due to the exciton shifts to the longer wavelength side when the reverse voltage applied to the planar light modulator is increased.

  The present invention is a planar light modulator manufactured based on this discovery, and uses exciton absorption as an operating principle. According to this planar light modulator, a high extinction ratio can be achieved at a very low driving voltage due to a sharp decrease in absorption coefficient on the longer wavelength side than the exciton absorption peak and a shift of the exciton absorption peak due to application of a reverse voltage Can be obtained. Further, since the i-GaAs layer of the present invention has a very high mobility, it has a very high speed response of the order of 10 to 100 ps.

FIG. 3 is a diagram illustrating a configuration of a planar light modulator according to the first embodiment. FIG. 3 is a diagram illustrating an operation principle of the planar light modulator according to the first embodiment. The graph which showed the applied voltage dependence of the transmission spectrum. A graph showing the extinction ratio. FIG. 5 is a diagram illustrating a configuration of a planar light modulator according to a second embodiment. The graph which showed the relationship between the transmittance | permeability and a reverse voltage. FIG. 6 is a diagram illustrating a configuration of a planar light modulator according to a third embodiment. FIG. 6 is a diagram illustrating a configuration of a planar light modulator according to a fourth embodiment.

  Hereinafter, specific examples of the present invention will be described with reference to the drawings. However, the present invention is not limited to the examples.

  FIG. 1 is a diagram illustrating a configuration of a planar light modulator according to the first embodiment, and FIG. 1A is a cross-sectional view. The planar light modulator according to the first embodiment includes an n-GaAs substrate 10, an i-GaAs layer 11 positioned on the n-GaAs substrate 10, a transparent electrode 12 positioned on the i-GaAs layer 11, and an n- It is comprised by the electrode 13 (back surface electrode of this invention) located in the GaAs substrate 10 back surface (surface on the opposite side to the i-GaAs layer 11 formation side).

The n-GaAs substrate 10 has a main surface of (100), a thickness of 25 μm, and an electron concentration of 3 × 10 ¹⁸ / cm ³ . The n-GaAs substrate 10 is thinned to a thickness of 25 μm by forming the i-GaAs layer 11 on the 350 μm thick n-GaAs substrate 10 and then polishing the back surface of the n-GaAs substrate 10.

The i-GaAs layer 11 has a thickness of 10 μm, a mobility of 1 × 10 ^{5 to} 3.1 × 10 ⁵ cm ² / V · s at a temperature of 77 K, and an electron concentration of 1 × 10 ¹² to 1 × 10 at a temperature of 77 K. ¹⁴ / cm ³ .

In order to further exhibit the effect of exciton absorption described later, the mobility of the i-GaAs layer 11 at a temperature of 77K is set to 1.5 × 10 ^{5 to} 3.1 × 10 ⁵ cm ² / V · s at a temperature of 77K. The electron concentration is desirably 1 × 10 ^{12 to} 3 × 10 ¹³ / cm ³ . More desirably, the mobility at a temperature of 77K is 2 × 10 ^{5 to} 3.1 × 10 ⁵ cm ² / V · s, and the electron concentration at a temperature of 77K is 1 × 10 ¹² to 1 × 10 ¹³ / cm ^3. is there.

  In the first embodiment, the thickness of the i-GaAs layer 11 is 10 μm, but the thickness is not limited to this and may be in the range of 5 to 30 μm. If it is thinner than 5 μm, the effect of exciton absorption described later cannot be sufficiently exhibited, which is not desirable. On the other hand, if it is thicker than 30 μm, the absorption of light by the i-GaAs layer 11 increases and the drive voltage also rises, which is not desirable. A more desirable thickness is 10 to 30 μm, and even more desirably 20 to 30 μm.

  The i-GaAs layer 11 was produced by the following method.

First, raw material GaAs and metal Ga were placed on a sample stage. As the raw material GaAs, polycrystalline GaAs having an electron concentration of 1 × 10 ¹⁵ to 8 × 10 ¹⁵ / cm ³ was used. Ga was 6 g, and GaAs was 35 mg. Next, a sample stage on which GaAs and Ga were arranged was inserted into the reaction tube, the inside of the reaction tube was depressurized to 10 ⁻⁸ torr, and heated to 800 ° C. Next, hydrogen gas was flowed into the reaction tube at 59 sccm. In this state, the inside of the reaction tube is maintained at 800 ° C., and kept for 24 to 25 hours in a state in which hydrogen gas is flown to produce a mixed melt of Ga and GaAs, and impurities contained in the mixed melt are caused by hydrogen gas. It was made to discharge. The mobility and electron concentration of the i-GaAs layer 11 to be formed can be controlled within a target range by this holding time.

Next, the reaction tube was lowered to room temperature, the sample stage was taken out of the reaction tube, and the n-GaAs substrate 10 was placed on the sample stage. At this time, the surface of the n-GaAs substrate 10 was covered with another GaAs substrate so that the surface of the n-GaAs substrate 10 was not exposed to the environment before coming into contact with the mixed melt of GaAs. Next, the sample stage was returned to the reaction tube. Next, the inside of the reaction tube was depressurized to 10 ⁻⁸ torr and heated to 800 ° C. Next, hydrogen gas was flowed into the reaction tube at 59 sccm.

  Next, cooling was started at an ambient temperature of 2 ° C./min. When the ambient temperature reached 798 ° C., which is the growth temperature of GaAs, the n-GaAs substrate 10 and the mixed melt of GaAs were brought into contact with each other. Then, GaAs was grown in a liquid phase on the n-GaAs substrate 10 while continuing cooling at a rate of 2 ° C./min. This crystal growth was terminated when the ambient temperature reached 753 ° C. Thus, the i-GaAs layer 11 was formed on the n-GaAs substrate 10.

  The transparent electrode 12 is made of polyaniline having a thickness of 0.12 μm. Further, as shown in FIG. 1B, a circular pattern is formed in the central portion of the element in plan view. The circular diameter is 2 to 5 times the thickness of the i-GaAs layer 11. FIG. 1B is a diagram of the planar light modulator according to the first embodiment viewed from above (transparent electrode 12 side). The transparent electrode 12 is in Schottky contact with the i-GaAs layer 11. By this Schottky contact, a depletion layer extends in the thickness direction from the surface of the i-GaAs layer 11 with which the transparent electrode 12 contacts to the n-GaAs substrate 10 side. The thickness and electron concentration of the i-GaAs layer 11 are set so that the depletion layer is thicker than the i-GaAs layer 11. As a result, the electric field strength in the i-GaAs layer 11 becomes uniform, and both low voltage driving and a high extinction ratio can be achieved, and a stable operation can be realized. The transparent electrode 12 may be made of a transparent conductive organic material other than polyaniline, such as PEDET: PSS (polyethylenedioxythiophene: polystyrenesulfonic acid), ITO (indium tin oxide), ZnO (zinc oxide). A transparent conductive oxide such as may be used.

  The electrode 13 is Ni / Au / Ge (where the symbol “/” means stacking, and A / B means that A is deposited and then B is deposited. A GaAs substrate 10 having a structure in which Ni, Au, and Ge are stacked in this order from the back side. Further, as shown in FIG. 1C, a pattern is formed in which a slit-like window 13a is opened at the center of the element in plan view. FIG. 1C is a diagram of the planar optical modulator of the first embodiment viewed from below (electrode 13 side). The electrode 13 is in ohmic contact with the i-GaAs layer 11. The transparent electrode 12 and the electrode 13 are connected to a DC power source, and a reverse voltage (a voltage that makes the electrode 13 side a positive potential with respect to the transparent electrode 12 side) is applied.

  The electrode 13 may be made of a Schottky contact material (for example, the same material as that of the transparent electrode 12). However, in order to reduce the driving voltage, a material that makes ohmic contact as in the first embodiment is desirable.

  Next, the operation of the planar light modulator according to the first embodiment will be described.

  Light enters the planar light modulator in a direction perpendicular to the element main surface from the transparent electrode 12 side. Then, light passes through the transparent electrode 12, the i-GaAs layer 11, and the n-GaAs substrate 10, and light is output from a window 13 a opened in the electrode 13.

  Then, by applying a reverse voltage (a voltage at which the electrode 13 side is a positive potential with respect to the transparent electrode 12) between the transparent electrode 12 and the electrode 13, and changing the absorption coefficient of the i-GaAs layer 11 according to the voltage value. The light intensity output from the planar light modulator is changed.

  Conventionally, the operation principle of a planar optical modulator having such a structure is based on the Franz-Keldish effect, which utilizes the phenomenon that the absorption edge shifts to the long wavelength side when a high voltage is applied. It was.

On the other hand, in the planar light modulator of Example 1, the mobility of the i-GaAs layer 11 at a temperature of 77 K is 1 × 10 ^{5 to} 3.1 × 10 ⁵ cm ² / V · s, and the electron concentration at a temperature of 77 K is also the same. By setting it to 1 * 10 < ¹² > -1 * 10 < ¹⁴ > / cm < ³ >, a planar light modulator is implement | achieved by the operation principle different from the past.

  FIG. 2A is a diagram for explaining the operating principle of the planar optical modulator of the first embodiment, and FIG. 2B is an operating principle of the conventional planar optical modulator based on the Franz-Keldish effect. It is a figure explaining. These two figures are image diagrams in which the horizontal axis indicates the wavelength and the vertical axis indicates the absorption coefficient.

  In the planar optical modulator of Example 1, the mobility and electron concentration of the i-GaAs layer 11 are in the above-described ranges, so that the absorption coefficient spectrum of the i-GaAs layer 11 has a light absorption peak due to excitons at room temperature. However, it can be seen (see FIG. 2A). Such a light absorption peak due to excitons has not occurred in conventional GaAs unless the temperature is extremely low. The absorption coefficient spectrum of the i-GaAs layer 11 rapidly decreases on the longer wavelength side than the light absorption peak due to excitons. Further, when the magnitude of the reverse voltage applied between the transparent electrode 12 and the electrode 13 is increased, the light absorption peak due to excitons shifts to the longer wavelength side. Therefore, if the wavelength of the light incident on the planar optical modulator of the first embodiment is close to the long wavelength side of the light absorption peak due to excitons, the absorption coefficient changes greatly due to the change in the magnitude of the reverse voltage. From the above operation principle, the driving voltage of the planar optical modulator of the first embodiment can be made very low.

  On the other hand, in the conventional planar light modulator based on the Franz-Keldish effect, as shown in FIG. 2B, the absorption voltage is increased by shifting the applied reverse voltage to the longer wavelength side by increasing the applied reverse voltage. The coefficient is changed. As can be seen by comparing FIGS. 2 (a) and 2 (b), the change in the absorption coefficient when the reverse voltage value is changed from V1 to V3 (V1 <V3) is as follows when exciton absorption is used. In comparison, the change is very small when the Franz-Keldish effect is used. Therefore, the planar light modulator based on the Franz Keldish effect has a higher drive voltage than the planar light modulator of the first embodiment that operates by exciton absorption.

  As described above, according to the planar light modulator of the first embodiment, the absorption coefficient can be greatly changed as compared with the conventional planar light modulator, and the driving voltage can be reduced.

  If the operation principle of the planar light modulator of the first embodiment is described more precisely, the conventional Franz Keldish effect is not lost in the planar light modulator of the first embodiment. In addition to this, the effect of exciton absorption is considered to be synergistic.

  Further, in the planar light modulator of the first embodiment, the transparent electrode 12 that is in Schottky contact is formed on the light transmission region of the i-GaAs layer 11, so that the i-GaAs layer 11 is directly under the transparent electrode 12. The depletion layer extends toward the light transmission region, and the electric field strength can be made uniform even by applying a small reverse voltage in the light transmission region. Therefore, when the applied voltage is increased, the applied voltage value until the light absorption peak due to excitons is actually shifted can be reduced, and as a result, the driving voltage of the planar light modulator is reduced. be able to.

  FIG. 3 shows a transmission spectrum of the planar light modulator of the first embodiment. The transmission spectrum was measured in five cases where the reverse voltage applied between the transparent electrode 12 and the electrode 13 was 0V, 5V, 8V, 10V, and 11V. As shown in FIG. 3, it can be seen that a light absorption peak due to excitons is observed in the vicinity of 870 nm at any applied voltage. Further, it can be seen that the transmitted light intensity rapidly increases on the longer wavelength side than the light absorption peak due to excitons. It can also be seen that when the applied voltage exceeds 5 V, the light absorption peak due to excitons shifts to the longer wavelength side as the applied voltage increases. Thus, FIG. 3 supports that the principle of operation of the planar optical modulator of Example 1 is that the light absorption peak due to excitons shifts to the longer wavelength side as the applied voltage increases.

  FIG. 4 is a graph showing the wavelength dependence of the extinction ratio. The extinction ratio is the ratio of the output light intensity when the applied voltage is 5V, 8V, 10V, and 11V with respect to the output light intensity when the applied voltage between the transparent electrode 12 and the electrode 13 is 0V. It can be seen that at 871 to 872 nm, extinction ratios of about 4 dB at an applied voltage of 8 V, about 7.5 dB at 10 V, and about 10 dB at 11 V are obtained. Note that a voltage of 5 V is required until the light absorption peak shifts, that is, until the planar optical modulator operates. This offset voltage is reduced by lowering the electron concentration of the i-GaAs layer 11. It is possible. In principle, an extinction ratio of 10 dB or more can be obtained at an applied voltage of 5 V by minimizing the offset voltage. Therefore, it is presumed that an extinction ratio of 10 dB or more can be obtained at an applied voltage of 5 to 12V.

  FIG. 5 is a diagram illustrating the configuration of the planar optical modulator according to the second embodiment. The difference from the planar light modulator of the first embodiment is that an electrode 22 is provided instead of the transparent electrode 12. Other structures are the same as those in the first embodiment. The electrode 22 is made of Au and is in Schottky contact with the i-GaAs layer 11. Further, the electrode 22 is a ring-shaped pattern in which a circular window 22a is opened at the center in a plan view. The diameter of the window 22 a is 2 to 5 times the thickness of the i-GaAs layer 11. If the diameter of the window 22a is smaller than twice, it takes time to reduce the diameter of the light incident on the planar light modulator, and it is difficult to match the position of the light incident position with the position of the window 22a. On the other hand, if it is more than 5 times, the reverse voltage required to obtain a uniform electric field strength up to the center of the window increases, and the driving voltage of the planar optical modulator increases, which is not desirable. Light enters the i-GaAs layer 11 perpendicularly from the window 22a. In addition, the electrode 22 and the electrode 13 are connected to a DC power source, and a reverse voltage (a voltage that makes the electrode 13 side a positive potential with respect to the electrode 22) is applied between the electrode 22 and the electrode 13. The planar light modulator according to the second embodiment has the same operation principle as that of the planar light modulator according to the first embodiment, and the driving voltage is reduced as compared with the conventional planar light modulator.

  As described above, the planar light modulator of the second embodiment employs the ring-shaped pattern electrode 22 having the circular window 22a instead of using the transparent material. Compared to the first embodiment, an opaque material can be used for the electrode 22, which is excellent in that the material selection range is wide. Further, since the diameter of the window 22a of the electrode 22 is 2 to 5 times the thickness of the i-GaAs layer 11, the applied voltage value required for the light transmission region of the i-GaAs layer 11 to have uniform electric field strength is set. When the applied voltage is increased, the applied voltage value until the light absorption peak due to excitons is actually shifted can be reduced. As a result, the driving voltage of the planar light modulator can be reduced.

  A more desirable diameter of the window 22a is 3 to 5 times the thickness of the i-GaAs layer 11, and further desirably 3 to 4 times.

  FIG. 6 shows the planar light modulator of the second embodiment, wherein the diameter of the window 22a is 300 μm, which is 10 times the thickness of the i-GaAs layer 11, the outer diameter of the electrode 22 is 325 μm, and the wavelength of incident light is It is the graph which showed the applied voltage dependence of the transmittance | permeability of the flat type optical modulator of a comparative example set to 897 nm. As shown in FIG. 6, the transmittance is almost 100% at 0V, 15% at 10V, 10% at 15V, 1% at 30V, 8.2dB at 10V, 10dB at 15V, 20dB at 30V. It was found that the extinction ratio was obtained. If the diameter of the window 22a is made smaller than 10 times the thickness of the i-GaAs layer 11, it is presumed that the drive voltage can be further reduced, and the diameter of the window 22a of the electrode 22 is made smaller than the thickness of the i-GaAs layer 11. If it is 2 to 5 times, it is considered that an extinction ratio of 10 dB or more can be obtained at a driving voltage of 5 to 15V.

  FIG. 7 is a diagram illustrating the configuration of the planar optical modulator according to the third embodiment. The planar light modulator according to the third embodiment has a structure in which an AlGaAs layer 30 is provided between the n-GaAs substrate 10 and the i-GaAs layer 11 in the planar light modulator according to the first embodiment. Other structures are the same as those of the planar light modulator of the first embodiment. The AlGaAs layer 30 has an Al composition ratio with respect to the group III metal of 20 to 30% and a thickness of 0.1 to 0.5 μm. By providing the AlGaAs layer 30 in this manner, the leakage current when a reverse voltage is applied is reduced, so that the driving voltage of the planar optical modulator can be further reduced, and more stable operation can be achieved. it can.

FIG. 8 is a diagram illustrating the configuration of the planar optical modulator according to the fourth embodiment. The planar light modulator of the fourth embodiment employs an electrode structure described below instead of the transparent electrode 12 of the planar light modulator of the first embodiment. The electrode structure is a MIS type structure composed of a light-transmitting insulating film 40 located in contact with the i-GaAs layer 11 and an electrode 41 located in contact with the insulating film 40. The insulating film 40 is formed on the entire surface of the i-GaAs layer 11, and the electrode 41 is formed in a ring-like pattern in which a circular window is opened in a region where light is incident, like the electrode 22 of the second embodiment. ing. In the case where an opaque material is used as the insulating film 40, the insulating film 40 may be a ring-shaped pattern like the electrode 41. Further, when a transparent material is used for the insulating film 40 and a transparent material is used for the electrode 41, the electrode 41 may be formed in a circular pattern in a region where light is incident like the electrode 12 of the first embodiment. For the insulating film 40, materials such as SiO ₂ , Al ₂ O ₃ , SiN, Si ₃ N ₄ , and TiO ₂ can be used. A material such as Au or In can be used for the electrode 41.

  As in the fourth embodiment, a depletion layer is formed in the i-GaAs layer 11 immediately below the electrode 41 even when the MIS type structure is used instead of the Schottky contact electrodes in the first to third embodiments. Therefore, it is possible to realize a planar optical modulator having the principle of exciton absorption as in the first to third embodiments.

  In the first, second, and fourth embodiments, an AlGaAs layer may be inserted between the n-GaAs substrate 10 and the i-GaAs layer 11 as in the third embodiment. As in the third embodiment, the leakage current can be reduced, the driving voltage of the planar optical modulator can be reduced, and a stable operation can be realized.

  In the first to fourth embodiments, the i-GaAs layer 11 is positioned on the n-GaAs substrate 10, but the n-GaAs substrate 10 may be removed by etching or polishing. Since light absorption by the n-GaAs substrate 10 can be reduced, insertion loss can be reduced. Further, if an AlGaAs layer is formed between the n-GaAs substrate 10 and the i-GaAs layer 11, the AlGaAs layer can be used as an etching stopper layer when the n-GaAs substrate 10 is etched.

  In Examples 1 to 4, light is incident from above the i-GaAs layer 12 (electrodes 12, 22, and 41 side), but light may be incident from the opposite side (electrode 13 side).

  The wavelength of the light modulated by the planar light modulator of the present invention is longer than the light absorption peak due to excitons (approximately 870 nm at room temperature and applied voltage of 0 V), and preferably as close to the exciton absorption peak as possible. Is desirable. This is because a high extinction ratio can be obtained. Specifically, the thickness is desirably 870 to 920 nm, and more desirably 870 to 900 nm.

  The planar light modulator of the present invention can be used for an optical switch, a projector, etc. in the field of optical communication.

10: n-GaAs substrate 11: i-GaAs layer 12: transparent electrode 13, 22, 41: electrode 30: AlGaAs layer 40: insulating film

Claims (7)

The mobility at a temperature of 77K is 2.0 × 10 ^{5 to} 3.1 × 10 ⁵ cm ² / V · s, the electron concentration at a temperature of 77K is 1 × 10 ¹² to 1 × 10 ¹³ / cm ³ , and the thickness is 20 An i-GaAs layer of ˜30 μm ;
A transparent electrode located on the i-GaAs layer, forming a depletion layer in the i-GaAs layer, and in Schottky contact with the i-GaAs layer ;
A back electrode located on the opposite side of the i-GaAs layer from the transparent electrode ;
Have
The transparent electrode is circular in plan view, and its diameter is 2 to 5 times the thickness of the i-GaAs layer,
The absorption coefficient spectrum of the i-GaAs layer has a light absorption peak due to excitons at room temperature, and decreases sharply on the longer wavelength side than the light absorption peak.
The light absorption peak is shifted to the long wavelength side by increasing the reverse voltage applied between the transparent electrode and the back electrode,
Modulating the intensity of light transmitted in a direction perpendicular to the main surface of the i-GaAs layer by changing the absorption coefficient of the i-GaAs layer by applying a reverse voltage between the transparent electrode and the back electrode;
The light to be transmitted is near the longer wavelength side than the light absorption peak and has a wavelength of 870 to 900 nm,
The extinction ratio is 10 dB or more at a driving voltage of 5 to 12 V.
Planar optical modulator, characterized in that. The transparent electrode is planar optical modulator according to claim 1, characterized in that the ITO. The planar light modulator according to claim 1, wherein the transparent electrode is a conductive organic material. The planar light modulator according to claim 3, wherein the transparent electrode is polyaniline. Planar type optical modulator according to, any one of claims 1 to 4, characterized in that it has an AlGaAs layer between the back electrode and the i-GaAs layer. 6. The planar light modulator according to claim 5 , wherein the AlGaAs layer has an Al composition ratio of 20 to 30 mol% and a thickness of 0.1 to 0.5 [mu] m with respect to a group III metal. The back electrode is an ohmic electrode which is in ohmic contact with the i-GaAs layer, planar optical modulator according to any one of claims 1 to 6, characterized in that.