SEMICONDUCTOR PHOTOELECTRIC SURFACE

Abstract: In a photocathode 10 using a photocathode of the present invention as a photocathode 30, within a hermetic envelope 20 maintained under vacuum, the photocathode 30 and an anode 40 are disposed so as to oppose each other while voltages are applied thereto by way of lead pins 51, 52, respectively. In the photocathode 30, a sapphire substrate 32 on which a matching layer 33 made of a-AlN, an active layer 34 made of p-type GaN, and a surface layer 35 made of CsO are stacked is secured to a metal support plate 31. The dopant concentration of the active layer 34 increases from 1 × 10 16 cm -3 in the surface to 5 × 10 17 cm -3 at a depth of 100 nm, while being 1 × 10 18 cm -3 only in the deepest portion of 10 nm. As a consequence, the crystallinity of the active layer 34 improves, and the diffusion length increases, whereby the quantum efficiency and the sharp-cuffing property improve.

Description: Technical Field The present invention relates to a semiconductor photocathode, a group III-V semiconductor photocathode in particular, which emits a photoelectron under vacuum in response to a photon incident thereon. Background Art It is preferable that a semiconductor photocathode used in a photomultiplier or the like have a high efficiency of photoelectric emission. As such a semiconductor photocathode, one disclosed in U.S. Patent No. 3,387,161 has been known. This semiconductor photocathode has an active layer in which a surface of a p-type semiconductor having a dopant concentration of at least 1 × 10 18 cm -3 but not greater than 1 × 10 19 cm -3 is activated with an alkali metal. As a consequence of such a configuration, downward energy band bending is formed in the surface of the photocathode on the vacuum emission side, so as to lower the vacuum-level barrier in the surface, thereby making it easier for photoelectrons to escape therefrom, and also making it easier for photoelectrons generated within the active layer distanced from the surface on the vacuum emission side to reach the emission-side surface. It is due to the fact that the diffusion length can be enhanced without lowering the probability of electron emission. Disclosure of the Invention If a high dopant concentration is employed, however, then an absorption near a band end will tend to occur due to a defect or the like generated in a crystal, thereby deteriorating the sharp-cutting property in optical absorption characteristics. Also, from the viewpoint of solar blindness, it will be preferable if the dopant concentration is low. It is due to the fact that the decrease in crystallinity can be suppressed more as the dopant concentration is lower. Nevertheless, though a low dopant concentration can increase the diffusion length, the probability of electron emission will decrease, whereby the quantum efficiency will decrease. Therefore, it has conventionally been difficult to further lower the dopant concentration. In view of the problems mentioned above, it is an object of the present invention to provide a semiconductor photocathode yielding a high quantum efficiency with a low dopant concentration. The semiconductor photocathode in accordance with the present invention is a semiconductor photocathode for emitting a photoelectron under vacuum in response to an incident photon, comprising an active layer made of a p-type doped group III-V compound semiconductor whose surface on the photoelectron emission side is activated with an alkali metal or alkali metal oxide, characterized in that a matching layer is provided on a surface of said active layer and that said active layer has an energy band gap which is at least twice as much as a work function of the alkali metal or alkali metal oxide, and a surface dopant concentration of not greater than 1 × 10 17 cm -3 on the photoelectron emission side. As a consequence, the crystallinity is prevented from decreasing, whereby the dispersion length increases. Also, since the crystallinity is favorable, the probability of electrons reaching the emission-side surface is high, and the probability of electron emission can be prevented from deteriorating, whereby the quantum efficiency can be kept high. Also, electrons are easily emitted from the surface. An electron supply layer may be provided on a side of the active layer different from the photoelectron emission side. Also, the dopant concentration of the active layer may be 1 × 10 17 cm -3 or less near the photoelectron emission surface and 1 x 10 18 to 1 × 10 19 cm -3 on the inside thereof. Alternatively, the dopant concentration of the active layer may gradually increase from near the photoelectron emission surface toward the inside, so as to become 1 × 10 18 to 1 × 10 19 cm -3 in the deepest portion on the inside. According to these configurations, the diffusion length becomes longer, whereas the electric field inside the diffusion layer is configured so as to move electrons toward the emission surface, whereby the probability of electrons reaching the emission surface would improve. It is further preferred that the thickness of a region where the dopant concentration is 1 × 10 18 to 1 × 10 19 cm -3 in the deepest portion on the inside of the active layer be a few nm or less. In this case, of the photoelectrons generated in the high-concentration doped layer, the amount of photoelectrons which disappear by migrating to the side opposite to the emission-side surface is suppressed. Therefore, it can be applied to a transmission type photocathode structure. Alternatively, a Schottky electrode formed on the active layer surface may be provided, so that an external bias is applied to the active layer. As a consequence, the photoelectrons generated within the active layer are efficiently guided to the emission-side surface by the external bias. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings. They are given by way of illustration only, and thus should not be considered limitative of the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it is clear that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, and various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. Brief Description of the Drawings Fig. 1 is a schematic view of a photoelectric tube utilizing the photocathode of the present invention; Fig. 2 is a chart showing the dopant concentration distribution in the active layer of the photocathode of Fig. 1; Fig. 3 is a chart comparing wavelength characteristics of the photocathode of the present invention and a conventional product; Fig. 4 is a graph showing a relationship between the dopant concentration and the quantum efficiency; Fig. 5 is a chart showing an example of the dopant concentration distribution in the active layer of the photocathode of the present invention; Fig. 6 is a chart showing another example of the dopant concentration distribution in the active layer of the photocathode of the present invention; and Fig. 7 is a view showing still another example of the dopant concentration distribution in the active layer of the photocathode of the present invention. Best Mode for Carrying Out the Invention In the following, embodiments of the present invention will be explained with reference to the drawings. Fig. 1 is a schematic view of a transmission type photoelectric tube utilizing the semiconductor photocathode in accordance with the present invention. This photoelectric tube 10 is constructed by accommodating a photocathode 30 utilizing the semiconductor photocathode in accordance with the present invention and an anode 40 into a hermetic envelope (vacuum envelope) 20 maintained under vacuum. This vacuum envelope 20 is a hollow columnar envelope made of glass, and its inside is held at a pressure of about 10 -8 Torr or lower. The photocathode 30 is supported by a metal lead pin 51 by way of a metal support plate 31 having a hole at the center thereof and a metal support table 50. On the other hand, the anode 40 is a metal electrode shaped like a rectangular frame, and is supported by a metal lead pin 52. The lead pins 51, 52 each penetrate through the bottom part of the vacuum envelope 20 and are connected to an external power source, such that a higher voltage is applied to the anode 40 than that applied to the photocathode 30. A substrate 32 made of sapphire is secured to the rectangular frame-shaped metal support plate 31, and a matching layer 33, an active layer 34, and a surface layer 35 are successively stacked thereon, so as to form the photocathode 30. For example, the matching layer 33 is made of amorphous AlN epitaxially grown on the substrate 32. This matching layer 33 has a layer thickness of about 10 nm and lattice-matches the active layer 34, thus allowing the crystal of the active layer 34 to grow favorably. Also, it is provided in order to prevent photoelectrons generated in the active layer 34 from traveling backward. The active layer 34 is made of p-type GaN epitaxially grown on the matching layer 33. This active layer 34 has a thickness of 100 nm or greater and is doped with Mg or Zn as a p-type dopant. Its concentration distribution is as shown in Fig. 2. It has a first layer, located near the surface, having a thickness of 100 nm and a second layer, formed at least inside its light-entrance surface, having a thickness of 1 nm. In the first layer, the dopant concentration is 1 × 10 16 cm -3 near its surface and increases toward the second layer, so as to reach 5 × 10 17 cm -3 at the boundary with respect to the second layer. The dopant concentration of the second layer is 1 × 10 18 cm -3 , which is higher than that of the first layer. For growing the matching layer 33 and active layer 34, various crystal growth methods such as MOCVD, MBE, HWE, and the like can be used. On the surface of the active layer 34, the surface layer 35 made of an alkali metal or its oxide, e.g., Cs or CsO, is formed by vapor deposition. This surface layer 35 is formed as a monoatomic layer. The energy band gap for vacuum emission in the surface layer 35 is 1.4 eV when Cs is utilized as the alkali metal, and 0.9 eV when CsO is utilized, thus being one half or less of the energy band gap, 3.4 eV, of GaN in the active layer. The operation of this photoelectric tube will now be explained. When light is incident on the photocathode 30 from the substrate 32 side, the incident light passes through the hole of the metal support plate 31 and then is transmitted through the substrate 32 and the matching layer 33, so as to reach the active layer 34. Photons are mainly absorbed by the first layer in the active layer 34, whereby photoelectrons are generated. The band gap energy within the active layer 34 would have a form substantially corresponding to the dopant concentration. As a result, the photoelectrons generated in the first layer move within the first layer as if they slid down a slope, thereby reaching the surface layer 35. Since there is a large band gap with respect to the surface layer 35, whereas the surface layer 35 is very thin, the photoelectrons are easily emitted under vacuum. The emitted photoelectrons reach the anode 40 due to the electric field between the photocathode 30 and the anode 40, and are detected as a current. For comparing performances of a conventional photocathode and those of the photocathode of the present invention in accordance with Fig. 1, the inventor compared their wavelength characteristics. Fig. 3 shows the results in comparison with each other. Here, a product in which the active layer was formed as one layer having a dopant concentration of 1 × 10 18 cm -3 was used as the conventional product for comparison. The broken line and solid line indicate the respective wavelength characteristics of quantum efficiencies in the photocathode of the conventional product and the present invention. It has been confirmed that, as compared with the conventional product, the product of the present invention exhibits a higher quantum efficiency at a wavelength of 350 nm or shorter, and a lower quantum efficiency at a wavelength of 400 nm or longer, thereby improving the sharp-cutting property and improving characteristics in a short wavelength region. It is assumed to be because of the fact that, along with an increase in diffusion length, the probability of photoelectrons reaching the surface improves due to the improvement in crystallinity, thereby improving the efficiency in emission of photoelectrons from the surface as well. Fig. 4 is a graph comparing quantum efficiencies at a wavelength of 254 nm, for example, concerning various test products with different dopant concentrations in their active layers. Though quantum efficiencies considerably vary among the test products, it has been confirmed that, as a whole, the products with a lower dopant concentration (1 × 10 17 cm -3 or less) in accordance with the present invention have a quantum efficiency equal to or greater than that of the conventional products having a higher dopant concentration (1 × 10 18 to 1 × 10 19 cm -3 ). Other embodiments of the present invention will now be explained. The concentration distribution of the active layer 34 may not only be as shown in Fig. 2, but also can be constituted, as shown in Fig. 5, by first and second layers, with their respective concentrations changing from each other like a step. As a consequence of such a configuration, photoelectrons generated by photons entering from the side opposite to the emission surface can effectively be guided to the emission side. The photocathode of the present invention is also applicable to a reflection type photocathode which emits photoelectrons from the same side on which photons are incident. In this case, the matching layer 33 may be made of amorphous AIN or GaN epitaxially grown on the substrate 32, for example. Figs. 6 and 7 show the active layer concentration distributions in the reflection type photocathodes corresponding to the transmission type photocathodes of Figs. 2 and 5, respectively. In either case, photoelectrons generated in the layer with a higher dopant concentration can efficiently be guided to the emission-side surface. Their dopant concentration control can easily be set by controlling the supply of dopant material. Though a high concentration region is preferably disposed at a portion distanced from the emission surface, it is not essential and may not be provided as well. Alternatively, an external bias voltage may be applied to the active layer, so as to form a gradient in the energy band gap level therewithin, thereby forcibly guiding photoelectrons to the emission-side surface as well. In this case, the dopant concentration therewithin may be either made uniform or provided with a predetermined distribution as mentioned above. Though the foregoing explanation relates to examples in which GaN is used as the active layer; Ga, In, Al, B, and the like can be used as a group III material, whereas N, P, As, and the like can be used as a group V material. Also, Cs, CsO, and the like can be used as the alkali metal of the surface layer. In accordance with the present invention, as explained in the foregoing, the active layer with a low dopant concentration stabilizes the crystallinity and increases the diffusion length, whereby a photocathode having a high quantum efficiency and an improved sharp-cutting property can be obtained. Further, as a semiconductor having a wide energy band is used in the active layer, photoelectrons are reliably emitted from the surface. Also, as the dopant concentration distribution of the active layer is adjusted, the photoelectrons generated inside the active layer can reliably be guided to the emission-side surface. From the foregoing explanations of the invention, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. Industrial Applicability The photocathode in accordance with the present invention is not only usable in photoelectric tubes but also applicable to photocathodes carrying out various kinds of photoelectric conversion.

Claims: A semiconductor photocathode (30) for emitting a photoelectron under vacuum in response to an incident photon, comprising: an active layer (34) made of a p-type doped group III-V compound semiconductor whose surface on the photoelectron emission side is activated with an alkali metal or alkali metal oxide, characterized in that a matching layer (33) is provided on a surface of said active layer (34) and that said active layer (34) has an energy band gap which is at least twice as much as a work function of said alkali metal or alkali metal oxide, wherein at least the surface of said active layer (34) on the photoelectron emission side has a dopant concentration of not greater than 1 x 10 17 cm -3 . A semiconductor photocathode according to claim 1, further comprising high dopant concentration layer on a side of said active layer (34) different from the photoelectron emission side. A semiconductor photocathode according to claim 1, wherein the dopant concentration of said active layer is not greater than 1 × 10 17 cm -3 near the photoelectron emission surface and 1 × 10 18 to 1 × 10 19 cm -3 on the inside thereof. A semiconductor photocathode according to claim 3, wherein a region with the dopant concentration of 1 × 10 18 to 1 × 10 19 cm -3 on the inside of said active layer has a thickness of a few nm or less. A semiconductor photocathode according to claim 1, wherein the dopant concentration of said active layer gradually increases from near the photoelectron emission surface toward the inside and is 1 × 10 18 to 1 × 10 19 cm -3 in the deepest portion on the inside. A semiconductor photocathode according to claim 5, wherein a region with the dopant concentration of 1 × 10 18 to 1 × 10 19 cm -3 on the inside of said active layer has a thickness of a few nm or less. A semiconductor photocathode according to claim 1, further comprising a Schottky electrode formed on said active layer surface, wherein an external bias is applied to said active layer.

Inventors: NIHASHI TOKUAKI, NIHASHI, TOKUAKI
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1998
Main classification: H01J1
IPC codes: H01J40, H01J1