JP2007042559A - Transmission type photoelectric surface and photodetector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photoelectric surface improved in linearity characteristics while suppressing the pollution of the emission face of photoelectrons by introducing a conductive layer which maintains sufficient ultraviolet region transmittance, and a photoelectric cathode. <P>SOLUTION: The photoelectric surface 1 is provided with a glass substrate 11 of which on one face ultraviolet rays enter, an adhesion layer 12 formed on the other face opposed to one face of the glass substrate 11, a group III nitride semiconductor layer 14 which is located on the side of the other face of the glass substrate 11 and generates photoelectrons according to the incidence of the ultraviolet rays, and an ultraviolet region transparent conductive layer 13 which is located between the adhesion layer 12 and the group III nitride semiconductor layer 14. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a transmission type photocathode including a group III nitride semiconductor layer and a photodetector using the same.

  For example, a group III-V compound semiconductor is used for the photocathode such as a photomultiplier tube. Among them, a group III nitride semiconductor has excellent characteristics such as high quantum efficiency and high reliability, It is also a good material in terms of environmental impact.

As a transmission type photocathode using a group III nitride semiconductor, a UV glass substrate, a laminate composed of a SiO ₂ layer, a group III nitride semiconductor layer and an alkali metal-containing layer formed on the UV glass substrate, 2. Description of the Related Art A transmission type photocathode that is formed on a peripheral portion of a body and includes an electrode portion for applying a potential to a stacked body is known (see Patent Document 1).
Japanese Patent No. 3623068

In general, the p-type is selected for the characteristics of the group III nitride semiconductor on the transmission type photocathode. However, the activation energy Ea of Mg, which is a p-type dopant, is very large with Ea = 150 meV in GaN and Ea = 300 meV in Al _0.3 GaN, and the carrier generation rate is low. Although it is conceivable to increase the dopant concentration in order to increase the carrier concentration, there is a limit to increasing the concentration because the crystal quality of the active layer is lowered.

Furthermore, the photocathode using a group III nitride semiconductor not only has a large Ea as described above, but also has a high sheet resistance because it is a thin film. In the transmission type photocathode, the required film thickness of the nitride semiconductor layer is about 100 nm. In this case, the sheet resistance is several GΩ / □ (sq.) For GaN, and several 100 GΩ / □ (for Al _0.3 GaN). sq.). Therefore, the high-speed response of the photocathode is deteriorated and the linearity characteristic is greatly reduced. For example, when used in a photodetector such as a photoelectric tube or a photomultiplier tube, the linearity characteristic of the photodetector is also greatly reduced. In addition, when used in the proximity image intensifier, the gate characteristics are greatly deteriorated.

  In view of this, the present inventor has studied the introduction of a conductive layer in a transmissive photocathode including a group III nitride semiconductor layer. Specifically, studies were made on forming a metal film or the like on the surface of the active layer on the vacuum side of the transmission type photocathode in an extremely thin or lattice or mesh form.

  When a conductive layer is introduced over the vacuum side surface of the active layer, that is, the entire surface of the photoelectron emission surface of the photoelectron emission layer, the conductive layer becomes a layer that tunnels photoelectrons, so that the characteristics of tunneling photoelectrons can be ensured. It is necessary to make the thickness of the conductive layer extremely thin. However, even if the film thickness is about several nanometers, the photoelectron transmission rate in the conductive layer is very poor at 10% or less, and in addition, contamination of the surface of the photoelectron emission layer is inevitable in the process of forming the conductive layer on the photoelectron emission surface. The characteristics as the photocathode are degraded.

  On the other hand, when the conductive layer is formed in a lattice shape or a mesh shape, photoelectrons are emitted in a region where the lattice electrode portion is not formed, but the photoelectrons cannot pass through the lattice electrode portion, and the characteristics are greatly deteriorated. For this reason, the uniformity of the entire light receiving surface cannot be ensured. For example, when used in an image tube or the like, there arises a problem that a lattice pattern appears in an output image image.

  The present invention has been made on the basis of the knowledge obtained by the above-described studies. Even when the conductive layer is introduced, the sufficient sensitivity in the ultraviolet region is maintained without contaminating the photoelectron emission surface of the photoelectron emission layer. It is an object of the present invention to provide a transmission type photocathode using a group nitride semiconductor layer and a photodetector using the same.

  The transmission type photocathode of the present invention includes a glass substrate on which ultraviolet light is incident on one surface, an adhesive layer formed on the other surface facing the one surface of the glass substrate, and the other surface of the glass substrate. And a group III nitride semiconductor layer that generates photoelectrons in response to incidence of ultraviolet light, and an ultraviolet transparent conductive layer positioned between the adhesive layer and the group III nitride semiconductor layer. .

  According to the transmissive photocathode of the present invention, a thin film of the ultraviolet transparent conductive layer is formed on the light incident side of the photoelectron emission layer, so that sufficient ultraviolet sensitivity and linearity characteristics can be obtained and photoelectron emission is achieved. The photoemission surface of the layer is not contaminated in the manufacturing process.

The adhesive layer may include any one of a SiO ₂ layer, a SiN layer, and a Si ₃ N ₄ layer. By using a SiO ₂ layer, a SiN layer, or a Si ₃ N ₄ layer as a layer for adhering the glass substrate to the ultraviolet transparent electrode layer, the selection of the material for forming the ultraviolet transparent electrode layer depends on the material UV It is only necessary to consider the region transmittance characteristics. In the manufacturing process, the crystal growth of the group III nitride semiconductor is not restricted at all, and a high-quality group III nitride semiconductor can be formed.

  The ultraviolet transparent conductive layer may be a metal layer or a metal oxide layer, and the ultraviolet transparent conductive layer is formed between the metal layer, the metal layer, and the group III nitride semiconductor layer. It is good also as providing an AlN layer. By configuring the ultraviolet transparent conductive layer in such a configuration, a very thin layer having a high ultraviolet transmittance can be formed. In particular, when the ultraviolet transparent conductive layer is a thin metal layer, it is possible to obtain a very thin layer of several nm to several tens of nm with good ultraviolet transmittance.

  The photodetector of the present invention includes the transmissive photocathode and an anode for taking out electrons emitted from the transmissive photocathode as an output signal. With this configuration, a photodetector having high ultraviolet sensitivity and improved linearity characteristics can be obtained.

  According to the present invention, the conductive layer is formed on the ultraviolet incident side of the group III nitride semiconductor layer that generates photoelectrons on the transmission type photocathode, so that sufficient sensitivity in the ultraviolet region is obtained without contaminating the photoelectron emission surface. And a transmissive photocathode having high linearity characteristics can be obtained.

  Hereinafter, a transmission type photocathode and a photodetector according to an embodiment of the present invention will be described. The same reference numerals are used for the same elements or elements having the same function, and redundant description is omitted.

  FIG. 1 is a sectional view of a transmissive photocathode according to an embodiment of the present invention, and FIG. 3 shows a band model of the transmissive photocathode shown in FIG.

  The transmission type photocathode 1 shown in FIG. 1 (hereinafter simply referred to as “photocathode” when not necessary) includes a glass substrate 11 on which ultraviolet rays are incident on one surface 11 a and one surface of the glass substrate 11. An adhesive layer 12 formed on the other surface 11b opposite to 11a and a group III nitride semiconductor layer 14 (photoelectron) that is located on the other surface 11b side of the glass substrate 11 and generates photoelectrons in response to the incidence of ultraviolet rays. Emission layer), and an ultraviolet transparent conductive layer 13 located between the adhesive layer 12 and the group III nitride semiconductor layer 14. Further, the surface 14b on the side where the ultraviolet transparent conductive layer 13 is not formed and from which the photoelectrons generated in the group III nitride semiconductor layer 14 are emitted is an alkali layer or an alkali oxide layer. A certain layer 15 is formed. Here, in the photocathode 1, when ultraviolet light is incident from one surface 11a of the glass substrate 11, photoelectrons are generated, and a photoelectron emission surface (in this embodiment, the layer 15 is opposite to the surface 11a on which the light is incident). Photoelectrons are emitted from the surface 15b). Here, the layer 15 is not necessarily formed.

The glass substrate 11 is made of, for example, UV glass that is transparent in the ultraviolet region. The UV glass exemplified here is a glass in which the threshold wavelength limit wavelength on the short wavelength side of a general borosilicate glass (for example, Kovar glass) is extended to 185 nm among borosilicate glasses. 9781 and 8337, 8337B of Schott. The adhesive layer 12 is a layer for adhering the glass substrate 11 to the ultraviolet transparent conductive layer 13 described later, and as a material, SiO ₂ , SiN, Si ₃ N ₄ or the like can be used.

  The ultraviolet transparent conductive layer 13 is a layer formed as a metal thin film or a metal oxide thin film.

  As a metal thin film used as the ultraviolet transparent conductive layer 13, a metal thin film made of a refractory metal such as Cr, Ti, W, Mo, Zr, Ta, or Nb, or a silicide thin film or a nitride thin film containing these metal components Can be used. As a method of forming such a metal thin film, there is an IBS method (ion beam sputtering method), and the surface of the group III nitride semiconductor layer 14 (for example, GaN layer) on which the metal thin film is deposited is sufficiently flat. Therefore, a continuous film can be formed even with an extremely thin film of several nm to several tens of nm. Regarding the ultraviolet transmittance, the transmittance in the ultraviolet region of 400 nm or less is 45% or more with a film thickness of 14 nm. Although the metal thin film has a problem that the adhesive strength with the GaN surface used for the group III nitride semiconductor layer 14 is low, it is sufficient to form a Cr film on the surface of the GaN layer in advance. Strength is obtained.

On the other hand, the metal oxide thin film formed as the ultraviolet transparent conductive layer 13 includes materials having an oxide-based wide band gap such as Ga ₂ O ₃ , ITO, and ZnO. Ga ₂ O ₃ has Eg = 4.9 eV, and the short wavelength threshold of the transmission wavelength is 250 nm. Furthermore, Ga ₂ O ₃ can be grown as a single crystal, and the ultraviolet transmittance is as high as 55% when the film thickness is 100 nm, and the conductivity can be an n-type film of 1 S / cm or less. ITO has an Eg of 3.9 eV and a short wavelength threshold of transmission wavelength of 320 nm, ZnO has an Eg of 3.4 eV and a short wavelength threshold of transmission wavelength of 365 nm, and is suitable for an InGaN-based transmission type photocathode. Can be used as

  As shown in FIG. 2, the transmission type photocathode 10 includes a metal film 13A as an ultraviolet transparent electrode layer, and an AlN layer 13B formed between the metal film 13A and the group III nitride semiconductor layer 14. You may make it prepare. AlN has a band gap Eg = 6.2 eV and is suitable for a transparent electrode layer, and can form a high-quality interface with few defects from the group III nitride semiconductor layer. Further, a growth technique for n-type AlN having a very small sheet resistance of several tens of Ω / □ (sq.) By Si doping has been developed. The n-type AlN layer 13B is formed epitaxially on the group III nitride semiconductor layer 14 after the group III nitride semiconductor layer 14 is grown, for example, by MOCVD. The thickness may be 10 nm to several tens of nm. Thereafter, a metal thin film (metal film 13A) is further formed on the AlN layer 13B.

  The group III nitride semiconductor layer 14 is a layer formed as a nitride-based semiconductor layer including, for example, p-type GaN doped with Mg. The layer 15 may be either an alkali layer or an alkali oxide layer, and is formed as a layer of Cs—O, Cs—I, Ce—Te, Sb—Cs, or the like.

  In the photocathode 1 configured as described above, as shown in the band model of FIG. 3, the ultraviolet transparent conductive layer 13 does not become a layer through which photoelectrons pass. Therefore, the conductive layer can be introduced while maintaining the characteristics as the photocathode.

  The transmission type photocathodes 1 and 10 shown in FIGS. 1 and 2 have been described so far, but when incorporated in a photodetector, the electrode portion for applying a potential to the group III nitride semiconductor layer 14 is a photoelectron emission. In order to prevent this, the peripheral portion of the surface 14b from which the photoelectrons of the group III nitride semiconductor layer 14 are emitted may be covered. Specifically, as shown in FIG. 4, for example, a Cr metal layer can be formed as the electrode portion 21 from the side surface 11 s to a part of the surface 14 b of the glass substrate 11 by a vacuum deposition method. FIG. 4 shows a case where conduction with the electrode portion 21 is achieved by removing a part of the group III nitride semiconductor layer 14 and exposing the ultraviolet transparent conductive layer 13 at several places.

  FIG. 5 shows a schematic cross-sectional view of the phototube 100 using the transmission type photocathode 2 shown in FIG. The vacuum hermetic container of the photoelectric tube 100 shown in FIG. 5 has a light receiving face plate in which the photocathode 2 is formed on the inner side of the glass substrate 11 in a cathode atmosphere on the cathode lead plate 131 provided at the opening end of the bulb 140. It is formed by fixing via a low melting point metal such as. Further, a disc-shaped anode 110 is provided in the airtight container so as to face the photocathode 2. The photocathode 2 and the cathode lead plate 131 are electrically connected via a chromium layer 120 as an electrode part and a low melting point metal such as In, and the anode 110 is electrically connected to a lead wire 132. When a photon is incident on the photocathode 2 in a state where a constant voltage higher than that of the photocathode 2 is applied to the anode 110 via the lead wire 132, photoelectrons corresponding to the amount of incident light are emitted from the photocathode 2. In this way, incident light is detected as an electrical signal. The photocathode according to the present invention includes, in addition to the phototube described above, a photomultiplier tube provided with a dynode (not shown) between the photocathode 2 and the anode 110 shown in FIG. 5, an image intensifier, and the like. It can be used for various photodetectors using a photocathode.

Subsequently, UV glass is used for the glass substrate 11, an ultraviolet transparent conductive film of Ga ₂ O ₃ is used for the ultraviolet transparent conductive layer 13, and p-type GaN is used for the group III nitride semiconductor layer 14 as an example. A method for manufacturing a transmissive photocathode according to an embodiment will be described.

First, in order to grow an active layer to be a group III nitride semiconductor layer 14, a Si (111) substrate is prepared as a crystal growth substrate, and a multilayer film of AlN or AlN and GaN is grown on the main surface thereof. A buffer layer is formed. Thereafter, GaN constituting the active layer is epitaxially grown on the main surface of the buffer layer. On the GaN surface of the crystal-grown wafer, an ultraviolet transparent conductive film of Ga ₂ O ₃ is formed as the ultraviolet transparent conductive layer 13. The Ga ₂ O ₃ film can be formed by a pulse laser deposition method or the like in a vacuum using a sintered β-GaO ₃ target. When forming the film, the wafer is heated to, for example, 880 ° C., and a KrF excimer laser is used as the laser, whereby a Ga ₂ O ₃ ultraviolet transparent conductive film can be formed. In addition, the resistance value can be further reduced by doping Sn ions during the formation of the ultraviolet transparent conductive film and simultaneously performing deposition under a low O ₂ partial pressure of 10 ⁻⁵ Pa or less. The film thickness of Ga ₂ O ₃ may be about 100 nm. In that case, the light transmittance for incident light of 280 nm is about 75%.

A SiO ₂ layer is formed as an adhesive layer 12 with the glass substrate 11 on the surface of the formed Ga ₂ O ₃ ultraviolet transparent conductive film. The SiO ₂ layer may be formed with a thickness of about 200 nm using, for example, a thermal CVD method. After forming the SiO ₂ layer, the UV glass prepared as the glass substrate 11 together with the wafer is heated to near the softening point temperature of the UV glass in vacuum or in an inert gas. After the temperature has risen, the wafer and UV glass are bonded together and then weighted and bonded. (Glass bonding method)

After bonding the wafer and the UV glass, the exposed portion of the UV glass is coated with picein or the like to protect the exposed portion of the UV glass from etching with hydrofluoric acid. Then, the Si (111) substrate is removed by etching with a hydrofluoric acid etching solution. Further, the buffer layer made of AlN or a multilayer film of AlN and GaN or the like is removed by etching with a KOH-based or H ₃ PO _4- based etching solution. After removing the buffer layer, Cr metal is formed at a desired position by vapor deposition to form the electrode part 21. After the electrode portion 21 is formed and configured into a device shape, the photoelectron emission surface 14b of the group III nitride semiconductor layer 14 is activated with an alkali or an alkali oxide in vacuum in order to improve the photoelectron emission. .

  The effects of the transmission type photocathode and the photodetector of the present invention described above will be described.

  In the photocathode of the present invention, since the thin film of the conductive layer is formed on the light incident side of the active layer serving as the photoelectron emission layer, the photoelectron emission surface of the photoelectron emission layer is not easily contaminated in the manufacturing process. In particular, when a film containing a metal component is used as the conductive layer, it is possible to form a very thin layer of several nm to several tens of nm having a high ultraviolet transmittance of 45 to more than 50%. Ultraviolet sensitivity and linearity characteristics can be obtained.

In addition, since the photocathode of the present invention uses a SiO ₂ layer, a SiN layer or a Si ₃ N ₄ layer as an adhesive layer, it is possible to form a conductive layer after growing a group III nitride semiconductor layer. It is. Therefore, the formation of the conductive layer does not affect the crystal growth of the group III nitride semiconductor layer, and a high-quality crystal can be obtained. Another advantage is that the photocathode can be formed over a large area simply by introducing a simple vapor deposition process.

It is sectional drawing of the transmission type photocathode which concerns on embodiment of this invention. It is sectional drawing of the transmission type photocathode which concerns on another embodiment of this invention. It is a band model of the transmissive photocathode shown in FIG. It is a figure which shows the principal part of the aspect which formed the electrode part in the transmissive photocathode shown by FIG. It is a schematic sectional drawing of the photoelectric tube which concerns on embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2, 10 ... Transmission type photocathode, 11 ... Glass substrate, 12 ... Adhesion layer, 13 ... Conductive layer, 14 ... Group III nitride semiconductor layer (photoelectron emission layer), 15 ... Alkali layer or alkali oxide layer, DESCRIPTION OF SYMBOLS 21 ... Electrode part, 100 ... Phototube, 110 ... Anode, 120 ... Electrode part, 131 ... Cathode lead plate, 132 ... Lead wire, 140 ... Valve | bulb.

Claims (5)

A glass substrate on which ultraviolet rays are incident on one surface;
An adhesive layer formed on the other surface opposite to one surface of the glass substrate;
A group III nitride semiconductor layer located on the other surface side of the glass substrate and generating photoelectrons in response to incidence of ultraviolet rays;
An ultraviolet transparent conductive layer located between the adhesive layer and the group III nitride semiconductor layer;
A transmissive photocathode. The transmission type photocathode according to claim 1, wherein the adhesive layer includes any one of a SiO ₂ layer, a SiN layer, and a Si ₃ N ₄ layer.   The transmission type photocathode according to claim 1 or 2, wherein the ultraviolet transparent conductive layer is a metal layer or a metal oxide layer.   The said ultraviolet transparent conductive layer is provided with the metal layer and the AlN layer formed between the said metal layer and the said group III nitride semiconductor layer, The Claim 1 or Claim 2 characterized by the above-mentioned. Transmissive photocathode. 5. A photodetector comprising a transmissive photocathode according to any one of claims 1 to 4 and an anode for taking out electrons emitted from the transmissive photocathode as an output signal inside a vacuum vessel. .

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