Photocathode comprising a plurality of openings on an electron emission layer

A semiconductor photocathode 1 includes: a transparent substrate 11; a first electrode 13, formed on the transparent substrate 11 and enabling passage of light that has been transmitted through the transparent substrate 11; a window layer 14, formed on the first electrode 13 and formed of a semiconductor material with a thickness of no less than 10 nm and no more than 200 nm; a light absorbing layer 15, formed on the window layer 14, formed of a semiconductor material that is lattice matched to the window layer 14, is narrower in energy band gap than the window layer 14, and in which photoelectrons are excited in response to the incidence of light; an electron emission layer 16, formed on the light absorbing layer 15, formed of a semiconductor material that is lattice matched to the light absorbing layer 15, and emitting the photoelectrons excited in the light absorbing layer 15 to the exterior from a surface; and a second electrode 18, formed on the electron emission layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photocathode.

2. Related Background Art

Photocathodes are used in photodetectors and other measuring devices, and for example, a transmission type photocathode, described in (Patent Document 1: Japanese Published Unexamined Patent Application No. H9-199075) is used. This transmission type photocathode is sensitive to near infrared light and has a window layer, formed of InAlGaAs on a light incidence side of a light absorbing layer, formed of an InGaAs-based material.

With photocathodes, forming of a transparent conductive film on a light incidence side of a light absorbing layer to lower a surface resistance that obstructs analysis of high-speed phenomena, etc., using a photodetector is known (see Patent Document 2: Japanese Published Examined Patent Application No. H4-30706). The provision of a mesh electrode or an island electrode at a light incidence side to apply a bias voltage in a photocathode used in a photodetector is also known (Patent Document 3: Japanese Patent No. 2902708).

Meanwhile, as an application example of a photodetector, fluorescence lifetime analysis, in which a sample is excited by light and a variation in time of an intensity of a fluorescence emitted by the sample is measured, can be cited. A photodetector used for fluorescence lifetime analysis has an electron tube, such as a photomultiplier tube, an image intensifier tube, or a streak tube, in which a photocathode is incorporated. Generally in fluorescence lifetime analysis using a photodetector, a pulsed light of a short wavelength (such as visible laser light) is used as the light for sample excitation and a fluorescence of a longer wavelength than the pulsed light (for example, infrared fluorescence) is measured.

SUMMARY OF THE INVENTION

However, because the photocathode described in Patent Document 1 is narrow in the wavelength band of incident light that can excite photoelectrons, though the photocathode has adequate sensitivity to wavelengths of infrared fluorescence, it does not have sensitivity to wavelengths of visible laser light as well as wavelengths of an ultraviolet band. The inventions described in Patent Documents 2 and 3 do not provide an effect of widening the wavelength band in terms of the sensitivity of a photocathode. Thus, conventionally, separate photocathodes have to be used according to the wavelength of the detected light, and separate photodetectors are prepared for the excitation light and for the fluorescence.

An object of the present invention is thus to provide a semiconductor photocathode having a flat sensitivity for light of a wide wavelength band.

The present inventor examined various laminated structures as well as shapes and materials of light absorbing layers and other layers, mainly in terms of semiconductor photocathodes that are made to operate by application of a bias voltage. As a result, the present inventor noted that with conventional semiconductor photocathodes, before reaching a light absorbing layer, in which photoelectrons are excited in response to incidence of light, light of a wavelength band of sensitivity (especially visible to ultraviolet light) is blocked by a layer (such as a window layer) disposed at a light incidence side of the light absorbing layer, and thus came to conceive the present invention.

A semiconductor photocathode according to the present invention includes: a transparent substrate; a first electrode, formed on the transparent substrate and enabling passage of light that has been transmitted through the transparent substrate; a light absorbing layer, formed on the first electrode and in which photoelectrons are excited in response to an incidence of light; a window layer, interposed between the first electrode and the light absorbing layer and being formed of a semiconductor material that is wider in energy band gap than the light absorbing layer, is lattice matched to the light absorbing layer, and has a thickness of no less than 10 nm and no more than 200 nm; an electron emission layer, formed on the light absorbing layer, formed of a semiconductor material that is lattice matched to the light absorbing layer, and emitting the photoelectrons excited in the light absorbing layer to the exterior from a surface; and a second electrode, formed on the electron emission layer.

With the semiconductor photocathode according to the present invention, though the window layer that is lattice matched to the semiconductor material of the light absorbing layer is formed on the light incidence side, the thickness of the window layer is extremely thin. Thus, in a state in which a bias voltage is applied, light of a wide wavelength band, from an ultraviolet band to a near infrared band, that is transmitted through the transparent substrate passes through the first electrode and, without hardly being blocked thereafter by the window layer, is made incident on the light absorbing layer, and photoelectrons are thereby excited. The excited photoelectrons are emitted to the exterior via the electron emission layer. A semiconductor photocathode with sensitivity to light of a wide wavelength band is thus provided.

Also, with the above-described semiconductor photocathode, the first electrode may be a metal material layer with a thickness of no less than 5 nm and no more than 100 nm. With this arrangement, even if the first electrode is formed of a metal material, light of a wide wavelength band can be transmitted with the first electrode having a thickness that is controllable in terms of manufacture.

Also, the first electrode may be a metal material layer with a thickness of no less than 10 nm and no more than 50 nm. With this arrangement, when the first electrode is formed of a metal material, light of an even wider wavelength band can be transmitted toward the light absorbing layer while applying the bias voltage uniformly on the semiconductor photocathode.

Also, the first electrode may be formed of a metal material having openings. With this arrangement, even if the first electrode is arranged as a metal material layer, light can be passed through toward the light absorbing layer via the openings.

Also, the first electrode may be formed of at least one type of transparent conductive material selected from the group consisting of ITO, ZnO, In2O3, and SnO2. By using a transparent conductive material that transmits light in the first electrode, the light transmitted through the transparent substrate can be passed through toward the light absorbing layer while providing the function of an electrode.

With the above-described semiconductor photocathode, the thickness of the window layer may be no less than 20 nm and no more than 100 nm. By making the thickness of the window layer be in this range, the bias voltage can be applied satisfactorily even with a thickness that enables the forming of a uniform layer and light of a wide wavelength band can be transmitted satisfactorily.

The above-described semiconductor photocathode may furthermore include: a contact layer, interposed between the electron emission layer and the second electrode and formed of a semiconductor material that is lattice matched to the electron emission layer. Because by providing a contact layer, a contact resistance between the electron emission layer and the second electrode can be lowered, the bias voltage can be applied effectively.

The above-described semiconductor photocathode may furthermore include: an insulating film, interposed between the transparent substrate and the first electrode. By providing such an insulating film, an effect of improving the adherence of the transparent substrate and semiconductor materials is provided.

The above-described semiconductor photocathode may furthermore include: an antireflection film, interposed between the transparent substrate and the first electrode. By providing the antireflection film, reflectance at desired wavelengths can be reduced in regard to light that is made incident on the light absorbing layer and the photoelectron emission efficiency can be improved.

Thus, with the present invention, light of a wide wavelength band from an ultraviolet band to a near infrared band can be made incident on the light absorbing layer in the state in which the bias voltage is applied. A semiconductor photocathode with a flat sensitivity for a wide wavelength band can thus be provided.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Semiconductor photocathodes according to embodiments of the present invention shall now be described with reference to the attached drawings. Elements that are the same shall be provided with the same symbol, and overlapping description shall be omitted.

First Embodiment

FIG. 1is a plan view of a transmission-type semiconductor photocathode1according to a first embodiment, andFIG. 2is a sectional view taken on line II-II ofFIG. 1.

The semiconductor photocathode1includes a transparent substrate11, an intermediate film (insulating film or antireflection film)12, a first electrode13, a window layer14, a light absorbing layer15, an electron emission layer16, a contact layer17, and a second electrode18. The window layer14, the light absorbing layer15, the electron emission layer16, and the contact layer17are arranged as a semiconductor multilayer film that serves the role of photoelectric conversion.

The transparent substrate11is formed of a material that is not restricted in the short wavelength sensitivity end and transmits an incident light hv of a wide wavelength band spanning from an ultraviolet band to a near infrared band. As the material of the transparent substrate11, for example, glass or quartz is used. The transparent substrate11is a portion that maintains the mechanical strength of the semiconductor photocathode1and may be a portion of a vacuum container in a case of incorporation in an electron tube.

By providing the intermediate layer12as an insulating film interposed between the transparent substrate11and the first electrode13, adherence of the transparent substrate11and semiconductor materials can be improved. Also, by providing intermediate layer12as an antireflection film interposed between the transparent substrate11and the first electrode13, reflectance at desired wavelengths can be reduced in regard to light made incident on the light absorbing layer15and the photoelectron emission efficiency can be improved.

The first electrode13is arranged as a metal material layer of extremely thin thickness that is formed on the transparent substrate11and is arranged as an incidence side electrode that enables passage of the light transmitted through the transparent substrate11. The first electrode13is formed of a material, such as W (tungsten), Mo (molybdenum), Ni (nickel), Ti (titanium), and Cr (chromium), and the thickness thereof is preferably no less than 5 nm and no more than 200 nm and more preferably no less than 10 nm and no more than 50 nm. For example, the first electrode13can be formed of tungsten of 10 nm thickness.

By arranging the first electrode13in this manner, light arriving at the first electrode13can be made to pass through toward the light absorbing layer15across a wide wavelength band with the first electrode13having a thickness that is controllable in terms of manufacture. Also, light of a wide wavelength band from an ultraviolet band to a near infrared band can be transmitted favorably while applying a bias voltage uniformly on the semiconductor photocathode. Especially in a case where the thickness is made no less than 10 nm and no more than 50 nm, because both a more uniform film quality and a low surface resistance can be realized at the same time, an effect of enabling a uniform bias field to be formed while maintaining a high transmittance is provided.

The window layer14is arranged as a layer of a semiconductor material of extremely thin thickness that is formed on the first electrode13. This window layer14is formed of a p-type semiconductor material (for example, InP) that is lattice matched to a semiconductor material of the light absorbing layer15to be described later, and functions not only as a window layer that transmits the incident light hv but also as a p contact layer having a function for application of a bias voltage. Furthermore and as shall be described later, the window layer14is wider in energy band gap than the light absorbing layer15and thus also has a function of preventing the photoelectrons, generated at the light absorbing layer, from diffusing to the transparent substrate side. Here, that a certain crystal is lattice matched to the semiconductor material of the window layer means that when the window layer is formed of InP, the difference between the lattice constant of the crystal and the lattice constant of InP is within ±0.5% of the lattice constant of InP.

The thickness of the window layer14is preferably no less than 10 nm and no more than 200 nm and more preferably no less than 20 nm and no more than 100 nm. For example, the window layer14may be formed of p-type InP of 50 nm thickness. By thus arranging the window layer14, the bias voltage can be applied favorably with the window layer14having a thickness by which a uniform layer can be formed, and light of a wide wavelength band from an ultraviolet band to a near infrared band can be transmitted favorably. Especially in a case where the thickness of the window layer14is made no less than 20 nm and no more than 100 nm, the effects of transmitting the incident light hv with good efficiency and blocking the diffusion of the photoelectrons, excited at the light absorbing layer15, to the first electrode and thereby transferring the photoelectrons to the electron emission layer16side efficiently are provided. A carrier concentration of the window layer14is preferably no less than 1×1017cm−3and no more than 1×1019cm−3. In this case, the effect that a uniform bias voltage can be applied to the light absorbing layer15is provided.

A semiconductor material, besides p-type InP, that is lattice matched to the light absorbing layer15and has an energy gap greater than that of the light absorbing layer15may be used as the material of the window layer14.

The light absorbing layer15excites photoelectrons in response to the incident light hv and is formed on the window layer14. The light absorbing layer15is formed of a semiconductor material (for example, p-type InGaAs of high resistance) that is narrower in energy band gap than the window layer14and is latticed matched to the window layer14. The light absorbing layer15may be made no less than 20 nm and no more than 5000 nm in thickness and no less than 1×1015cm−3and no more than 1×1017cm−3in carrier concentration. As the material of the light absorbing layer15, a compound semiconductor, selected from among p-type InGaAsP, p-type InAlGaAs, etc., may be used besides p-type InGaAs.

The electron emission layer16is wider in energy band gap than the light absorbing layer15, emits the photoelectrons excited at the light absorbing layer15to the exterior from a surface, and is formed on the light absorbing layer15. This electron emission layer16is formed of a semiconductor material (such as p-type InP) that is lattice matched to the light absorbing layer15. In the electron emission layer16, openings16T of approximately 1000 nm width are formed in the form of stripes to enable electrons to be emitted to the exterior. In the case of the semiconductor photocathode1shown inFIGS. 1 and 2, the openings16T are formed in the form of stripes and openings of the same shape are formed in the contact layer17and the second electrode18as well. Though with the example shown inFIG. 1, the openings16T are formed in the form of stripes, these may instead be formed in the form of a mesh and the shape is not restricted in particular as long as openings of uniform shape are provided.

The electron emission layer16may be made no less than 50 nm and no more than 2000 nm in thickness and no less than 5×1015cm−3and no more than 1×1017cm−3in carrier concentration. The openings16T may be made no less than 100 nm and no more than 100000 nm in line width, and the openings16T may be made no less than 100 nm and no more than 100000 nm in pitch. Besides p-type InP, a semiconductor material that is lattice matched to the light absorbing layer15and has an energy gap greater than that of the light absorbing layer15may be used as the material of the electron emission layer16.

The contact layer17is interposed between the electron emission layer16and the second electrode18and is formed of a semiconductor material that is lattice matched to the electron emission layer16. The contact layer17is an additional layer for making the bias voltage be applied effectively by lowering a contact resistance between the electron emission layer16and the second electrode18and is formed, for example, of n-type InP. In a case where p-type semiconductor materials are used in the light absorbing layer15and the electron emission layer16and an n-type semiconductor material is used as the contact layer17, the contact layer17is an n contact layer17. The contact layer17may be made no less than 50 nm and no more than 10000 nm in thickness and no less than 1×1017cm−3and no more than 1×1019cm−3in carrier concentration. Besides n-type InP, a semiconductor material that is lattice matched to the light absorbing layer15and has an energy gap greater than that of the light absorbing layer15may be used as the material of the contact layer17.

The second electrode18is formed on the electron emission layer16and is formed, for example, of Ti. By providing the second electrode18, the bias voltage can be applied to the light absorbing layer15and the electron emission layer16. In the present embodiment, the second electrode18is formed on the contact layer17and is arranged as a photoelectron emitting side electrode. The second electrode18may be made no less than 5 nm and no more than 1000 nm in thickness. Al, Pt, Ag, Au, Cr, or an alloy of these, etc., may be used besides Ti as the material of the second electrode18.

(Operation of the Semiconductor Photocathode)

An operation of the semiconductor photocathode1shall now be described. To apply a bias voltage of a reverse direction from the exterior, a high potential terminal side of a bias voltage source50is connected to the second electrode18and a low potential terminal side of the bias voltage source50is connected to the first electrode13as shown inFIG. 13.

With the semiconductor photocathode1thus connected, when the incident light hv is made incident from the transparent substrate11side in the state in which the bias voltage is applied, though a portion of the light is reflected or absorbed by the first electrode13and the window layer14, the rest of the light reaches the light absorbing layer15. Electrons resulting from photoelectric conversion at the light absorbing layer15are then emitted to the exterior from the surface of the electron emission layer16.

(Method for Manufacturing the Semiconductor Photocathode)

A method for manufacturing the semiconductor photocathode according to the embodiment shall now be described.FIGS. 3A,3B,3C,4A,4B, and4C are sectional views of a manufacturing process of the semiconductor photocathode1.

First, an InP substrate42is prepared. Then, by MOCVD (metal organic chemical vapor deposition), crystal growths of an etching stop layer41, formed of InGaAs, the contact layer17(for example, n-type InP), the electron emission layer16(for example, p-type InP), the light absorbing layer15(for example, p-type InGaAs), and the window layer14(for example, p-type InP) are carried out successively on the InP substrate42. Subsequently, the first electrode13(for example, tungsten) is vacuum deposited onto the window layer14(FIG. 3A).

Then, after depositing the intermediate film12(for example, a silicon dioxide film) by plasma-enhanced CVD (plasma-enhanced chemical vapor deposition), the wafer is adhered onto the transparent substrate11(for example, glass) by thermocompression bonding (FIG. 3B).

By etching the wafer, made integral with the transparent substrate11, by immersing the wafer in heated hydrochloric acid, the entirety of the InP substrate42is removed. This etching step is stopped automatically by the etching stop layer41(FIG. 3C).

Thereafter, by etching the etching stop layer41by a sulfuric-acid-based etchant, a substrate, having the contact layer17as a top surface and the transparent substrate11as the rear surface, is prepared (FIG. 4A).

The second electrode18is then vacuum deposited, and by photolithography and RIE dry etching (reactive ion etching), a stripe pattern is formed on the electron emission layer16, the contact layer17, and the second electrode18. Electron emitting portions for emitting electrons to the exterior of the semiconductor photocathode1are thereby formed in the electron emission layer16(FIG. 4B).

Lastly, by photolithography and chemical etching using hydrochloric acid and sulfuric acid type etchants, the first electrode13is exposed, and the semiconductor photocathode1, shown inFIG. 2, is prepared (FIG. 4C).

(Characteristics of the Semiconductor Photocathode)

FIG. 5is a diagram of characteristics data of the semiconductor photocathode according to the first embodiment. As shown inFIG. 5, with the semiconductor photocathode according to the present embodiment, a flat trend of low fluctuation width is obtained for the sensitivity over a wide wavelength band from 350 nm in the ultraviolet band to 1650 nm. Especially in a wavelength band from 450 nm to 1600 nm, a flat trend of low fluctuation width is obtained at higher sensitivity.

Effects of the semiconductor photocathode according to the embodiment with the above arrangement shall now be described. With the semiconductor photocathode according to the embodiment, though the window layer14that is lattice matched to the semiconductor material of the light absorbing layer15is formed on the light absorbing layer15in order to form the light absorbing layer15, the thickness of the window layer14is made extremely thin. Thus, for a wide wavelength band from an ultraviolet band to a near infrared band, light that is transmitted through the transparent substrate in the state in which the bias voltage is applied passes through the first electrode and is made incident on the light absorbing layer to excite photoelectrons without being blocked by the window layer. A semiconductor photocathode having a flat sensitivity for light of a wide wavelength band is thus provided.

In other words, with the semiconductor photocathode1, in the state in which the bias voltage is applied, not only light of the near infrared band exceeding 780 nm but light of the visible band and light of the ultraviolet band from 350 nm to 450 nm can be made to reach the light absorbing layer15. Because a single semiconductor photocathode can thus be provided with sensitivity for a wide wavelength band from an ultraviolet band to a near infrared band, separate photocathodes do not have to be used according to the wavelength of the detected light in incorporation into a photomultiplier tube, image intensifier tube, streak tube, or other electron tube. Thus, not only can improvements be made in regard to the lowering of precision due to preparing separate photodetectors for excitation light and for fluorescence, but the structure of a measuring device can also be simplified to enable size reduction and cost reduction.

Specifically, for time-resolved fluorescence measurement, because simultaneous measurement of excitation light pulses (generally of a shorter wavelength than a fluorescence wavelength) and fluorescence is enabled, not only can the measurement precision be improved but size reduction and cost reduction of a device can be realized as well. Also, by combining with a compact, maintenance-free cooler, a photodetector that can accommodate a wide wavelength band can be manufactured.

Second Embodiment

A transmission-type semiconductor photocathode according to a second embodiment of the present invention shall now be described.

FIG. 6is a sectional view of the transmission-type semiconductor photocathode2according to the second embodiment. Because the plan view of the semiconductor photocathode2is the same asFIG. 1, description shall be omitted by providing elements corresponding to those ofFIG. 1with the same symbols.

A difference of the present embodiment with respect to the first embodiment is a first electrode23that is disposed at the light incidence side, and other elements are the same as those of the first embodiment. With the present embodiment, the first electrode23differs from that of the first embodiment in being arranged as a metal material layer having openings23B. Specifically, as shown in the plan view ofFIG. 7, by providing the plurality of openings23B in the first electrode23, the first electrode23is patterned in the form of stripes.

The metal material that forms the first electrode23is not restricted in particular, and as with the first electrode13according to the first embodiment, the first electrode23can be formed from a material among W (tungsten), Mo (molybdenum), Ni (nickel), Ti (titanium), Cr (chromium), etc. The thickness of the first electrode23is also not restricted in particular, and in a case where tungsten is used as the metal material, the thickness may be 100 nm.

The semiconductor photocathode2that is thus arranged can be made to operate by applying the bias voltage using the bias voltage source50in the same manner as in the first embodiment. With the present embodiment, because the plurality of openings23B are provided in the form of stripes, though light that is made incident on the transparent substrate11is blocked by substantially 100% by line portions23A and edges23C, the light passes through without being blocked at openings23B. The light transmitted through the transparent substrate can thus be passed through toward the light absorbing layer15.

In the present embodiment, though the number of openings23B is not restricted in particular, to make the light transmitted through the transparent substrate pass through efficiently, an opening percentage β, expressed by the following equation, with w1being a line width of each line portion23A and w2being a pitch width at which openings23B are to be provided, is preferably made as large as possible.
β={1−(w1/w2)}×100  (Equation)

For example, the line width w1of the line portion23A may be set to 5000 nm and the pitch w2of the openings23B may be set to 100000 nm. In this case, the opening percentage β is 95%.

Preferably, with the openings23B, the line width w1is made no less than 500 nm and no more 50000 nm, and the pitch w2is made no less than 500 nm and no more than 500000 nm. By setting the line width w1and the pitch w2in the above ranges, the bias voltage can be applied effectively to the semiconductor photocathode, and the semiconductor photocathode can be formed with good reproducibility by photolithography. ThoughFIG. 7shows a case where the plurality of openings23B are aligned in the form of stripes, a plurality of openings may instead be aligned in a different mode, such as in the form of a mesh, concentric circles, etc.

The method for manufacturing the semiconductor photocathode2according to the present embodiment is substantially the same as the method for manufacturing the semiconductor photocathode1according to the first embodiment. However, a difference with respect to the manufacturing method for the first embodiment is that after the step of vacuum depositing the first electrode13on the window layer14, shown inFIG. 3A, a step of forming the plurality of openings23B by a photolithography process and RIE dry etching is added.

Third Embodiment

A transmission-type semiconductor photocathode according to a third embodiment shall now be described. Because the semiconductor photocathode according to the present embodiment is the same as the first embodiment in plan view and sectional view, corresponding elements shall be provided with the corresponding symbols and description thereof shall be omitted.

A difference between the present embodiment and the first embodiment is a first electrode33that is disposed at the light incidence side of the semiconductor photocathode3(seeFIG. 2), and other elements are the same as those of the first embodiment. Specifically, the present embodiment differs from the first embodiment in that the first electrode33is formed of a transparent conductive material. The transparent conductive material making up the first electrode33may be at least one type of material selected from the group consisting of ITO, ZnO, In2O3, and SnO2. ITO, ZnO, In2O3, and SnO2are all transparent oxide semiconductors. The thickness of the first electrode33is preferably no less than 100 nm and no more than 5000 nm and more preferably no less than 200 nm and no more than 1000 nm.

The semiconductor photocathode3thus arranged can be made to operate by applying the bias voltage using the bias voltage source50in the same manner as the first embodiment. In the present embodiment, because the first electrode33is formed of a transparent conductive material, it has a property of transmitting light while having the function of an electrode. Light transmitted through the transparent substrate can thus be passed through toward the light absorbing layer15.

The method for manufacturing the semiconductor photocathode3according to the present embodiment is substantially the same as the method for manufacturing the semiconductor photocathode1according to the first embodiment. However, a difference with respect to the manufacturing method for the first embodiment is that in the step of vacuum depositing the first electrode13onto the window layer14shown inFIG. 3A, the first electrode33, formed of the transparent conductive material, is formed in place of the first electrode13, formed of the metal material.