Photocathode and electron tube having enhanced absorption edge characteristics

The present invention relates to a photocathode having a structure for improving the quantum efficiency and sharpening the absorption edge characteristic on the long wavelength side within the wavelength range of incident light to improve the photosensitivity, and an electron tube having the same. The photocathode according to the present invention comprises at least a p-type GaAlN layer for absorbing incident light to excite photoelectrons, a p-type GaN layer which covers the second major surface of the p-type GaAlN layer, the second major surface opposing a first major surface that faces a substrate, and a surface layer provided to sandwich the p-type GaN layer with the p-type GaAlN layer and mainly containing an alkali metal or an alkali metal oxide.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a photocathode applicable to an 
image-pickup tube or photometry device and a phototube having the same 
and, more particularly, a photocathode having a plurality of layers each 
mainly containing a compound semiconductor material and functioning so as 
to emit photoelectrons which are excited by incident light. 
2. Related Background Art 
Conventionally, a compound semiconductor layer of CsTe is applied to a 
photocathode for the UV range. The spectral sensitivity characteristic of 
this photocathode has a high radiation sensitivity to incident light 
having a wavelength of about 180 to 320 nm. However, to improve the 
quantum efficiency or absorption edge characteristic in the long 
wavelength side within the wavelength range of incident light, there is a 
decisive difficulty caused by a defect called a color center. 
As a photocathode for the UV range for which various operation 
characteristics are expected to be improved as compared with the 
photocathode having the CsTe layer, a photocathode having a compound 
semiconductor layer mainly containing Ga.sub.1-x Al.sub.x N (0&lt;x&lt;1) is 
available. The spectral sensitivity characteristic of this photocathode is 
adjusted by changing the absorption edge characteristic in the long 
wavelength side within the incident wavelength range of about 200 to 350 
nm in correspondence with the composition of Ga.sub.1-x Al.sub.x N, i.e., 
an alloy of AlN and GaN. 
Prior arts associated with such a photocathode having a layer mainly 
containing a compound semiconductor material of Ga.sub.1-x Al.sub.x N are 
disclosed in, e.g., 
"U.S. Pat. No. 3,387,161 (1968)", 
"U.S. Pat. No. 3,986,065 (1976)", and 
Japanese Patent Laid-Open No. 61-267374. 
SUMMARY OF THE INVENTION 
The present inventors have found the following problems upon examining the 
above prior arts. 
In the conventional photocathode, to improve the quantum efficiency, a Cs 
monomolecular layer is deposited on the Ga.sub.1-x Al.sub.x N layer. With 
this structure, a negative electron affinity acts on photoelectrons 
excited in the conduction band of the Ga.sub.1-x Al.sub.x N layer, so that 
the photoelectrons are easily emitted into the vacuum (outside the 
photocathode) through the Cs monomolecular layer. 
However, the compound semiconductor material of Ga.sub.1-x Al.sub.x N is 
easily oxidized because it contains Al. For this reason, when the 
phototube is to be manufactured, in the process of temporarily extracting 
the Ga.sub.1-x Al.sub.x N layer from a vacuum system in which crystal 
growth is performed, transferring the Ga.sub.1-x Al.sub.x N layer to 
another vacuum system, and sealing the Ga.sub.1-x Al.sub.x N layer as a 
phototube, the surface of the Ga.sub.1-x Al.sub.x N layer is easily 
oxidized upon contacting air. 
To obtain a high quantum efficiency, a monomolecular layer of Cs oxide is 
formed on the Ga.sub.1-x Al.sub.x N in place of the Cs monomolecular layer 
in some cases. With this structure, the negative electron affinity which 
acts on photoelectrons excited in the conduction band of the Ga.sub.1-x 
Al.sub.x N layer further increases to the negative side. In the process of 
forming this monomolecular layer of the Cs oxide, the surface of the 
Ga.sub.1-x Al.sub.x N layer is oxidized at a high probability upon 
contacting introduced oxygen. 
When the monomolecular layer of Cs or Cs oxide is formed on this oxidized 
surface of the Ga.sub.1-x Al.sub.x N layer, the work function of the 
surface of the Ga.sub.1-x Al.sub.x N does not sufficiently decrease. No 
sufficient negative electron affinity can be obtained from the 
monomolecular layer of Cs or Cs oxide, resulting in a degradation in 
quantum efficiency or broadening of the absorption edge characteristic in 
the long wavelength side of incident light. 
An object of the present invention is to provide a photocathode having a 
structure for preventing oxidation of the surface of a Ga.sub.1-x Al.sub.x 
N layer to improve the quantum efficiency and also sharpens the absorption 
edge characteristic in the long wavelength side within the wavelength 
range of incident light to improve the photosensitivity, and an electron 
tube having the same. 
A photocathode according to the present invention has a function of 
emitting photoelectrons excited by incident light. In a reflection type 
photocathode, a substrate for supporting the Ga.sub.1-x Al.sub.x N layer 
is provided, and the photocathode is set at a predetermined position in 
the vacuum container of a phototube. In a transmission type photocathode, 
the Ga.sub.1-x Al.sub.x N layer is provided on the inner wall of the 
vacuum container of a phototube, and part of the vacuum container is used 
as a substrate for supporting the Ga.sub.1-x Al.sub.x N layer. With these 
arrangements, the photocathode is applied to the phototube. 
More specifically, a photocathode according to the present invention 
comprises at least a light absorption layer for absorbing the incident 
light to excite the photoelectrons, the light absorption layer being a 
p-type compound semiconductor layer mainly containing Ga.sub.1-x Al.sub.x 
N (0&lt;x&lt;1) and having a first major surface and a second major surface 
opposing the first major surface; a photoelectric emission layer being a 
p-type compound semiconductor layer mainly containing GaN, for drifting 
the excited photoelectrons, the photoelectric emission layer being 
covering and in direct contact with the first major surface of the light 
absorption layer; and a surface layer for emitting the excited 
photoelectrons outside the photocathode, the surface layer comprising at 
least one of an alkali metal and an alkali metal oxide and being provided 
at a position opposing the light absorption layer through the 
photoelectric emission layer. 
Particularly, the photoelectric emission layer has a bandgap energy lower 
than that of the light absorption layer. The surface layer has a vacuum 
level lower than that of the conduction band of the photoelectric emission 
layer. With this structure, the energy level of the conduction band in the 
energy diagram of this photocathode is lowered from the light absorption 
layer toward the surface layer through the photoelectric emission layer. 
The photocathode according to the present invention may have a substrate 
which is set at a predetermined position of the vacuum container of the 
phototube or constitutes part of the vacuum container. In both cases, the 
substrate is arranged at a position being opposite to the photoelectric 
emission layer through the light absorption layer. In addition, the 
substrate is preferably a plate member mainly containing sapphire. 
The photocathode according to the present invention may have a contact 
layer provided between the substrate and the light absorption layer, the 
contact layer being a p-type compound semiconductor layer mainly 
containing GaN. The photocathode may have an electron shielding layer 
provided between the substrate and the light absorption layer, the 
electron shielding layer mainly containing a semiconductor material having 
a bandgap energy higher than that of the light absorption layer. The 
semiconductor material is preferably p-type AlN. 
In the photocathode according to the present invention, the photoelectric 
emission layer mainly containing p-type GaN and the surface layer mainly 
containing an alkali metal or alkali metal oxide are sequentially 
laminated on the light absorption layer mainly containing p-type 
Ga.sub.1-x Al.sub.x N (0&lt;x&lt;1). The major component of the light absorption 
layer, i.e., p-type Ga.sub.1-x Al.sub.x N has a bandgap energy of about 
3.5 to 6.0 eV. The major component of the photoelectric emission layer, 
i.e., p-type GaN has a bandgap energy lower than that of the light 
absorption layer. In addition, the major component of the surface layer, 
i.e., the alkali metal or alkali metal oxide has a vacuum level lower than 
that of the conduction band of the photoelectric emission layer. For this 
reason, the energy level of the conduction band in the energy diagram of 
this photocathode is lowered from the light absorption layer toward the 
surface layer through the photoelectric emission layer. 
With this structure, when photons incident in the photocathode of the 
present invention have a predetermined energy, the photons are absorbed by 
the light absorption layer. At this time, electrons existing in the 
valence band of the light absorption layer are excited to the conduction 
band and become free electrons. For this reason, the photons are diffused 
or drifted along the conduction band which is lowered in level from the 
light absorption layer toward the photoelectric emission layer. The 
photons which are diffused or drifted from the light absorption layer to 
the photoelectric emission layer are emitted into the vacuum (outside the 
photocathode through the surface layer) by the negative electron affinity 
of the surface layer. 
Since the photoelectric emission layer is a p-type compound semiconductor 
layer mainly containing p-type GaN, Al is not contained in the 
composition, unlike the light absorption layer. Therefore, the 
photoelectric emission layer is not easily oxidized, unlike the light 
absorption layer. The surface of the light absorption layer is covered 
with the photoelectric emission layer and therefore is not oxidized. In 
addition, the surface layer mainly containing the alkali metal or alkali 
metal oxide is provided on the photoelectric emission layer. Since the 
work function of the surface of the photoelectric emission layer is 
sufficiently decreased, a negative electron affinity by the surface layer 
can be sufficiently obtained. Therefore, in this photocathode, the quantum 
efficiency can be improved, and the absorption edge characteristic in the 
long wavelength side within the wavelength of incident light can be 
sharpened. 
The major component of the photoelectric emission layer, i.e., p-type GaN 
can lattice-match the major component of the light absorption layer, i.e., 
p-type Ga.sub.1-x Al.sub.x N. For this reason, the photoelectric emission 
layer having a satisfactory crystallinity is epitaxially grown on the 
light absorption layer. Since almost no crystal defects are occurred in 
the photoelectric emission layer, the photocathode can obtain satisfactory 
photoelectron diffusion properties. 
When an electron shielding layer is provided between the substrate and the 
light absorption layer, an energy barrier is formed between the light 
absorption layer and the electron shielding layer in the conduction band 
in the energy diagram of the photocathode because the electron shielding 
layer is a p-type layer and has a bandgap energy higher than that of the 
light absorption layer. For this reason, photoelectrons excited in the 
light absorption layer are diffused or drifted along the conduction band 
E.sub.C which is lowered in level from the light absorption layer toward 
the surface layer through the photoelectric emission layer without being 
diffused or drifted to the electron shielding layer side. Therefore, the 
quantum efficiency of the photocathode is further improved. 
In particular, the major component of the electron shielding layer, i.e., 
AlN can lattice-match the major component of the light absorption layer, 
i.e., p-type Ga.sub.1-x Al.sub.x N. For this reason, the light absorption 
layer and photoelectric emission layer having a satisfactory crystallinity 
are epitaxially grown on the electron shielding layer. Since almost no 
crystal defects are occurred in the light absorption layer and the 
photoelectric emission layer, the photocathode can obtain satisfactory 
photoelectron diffusion properties. 
In the phototube to which the photocathode (reflection type photocathode 
having a substrate) according to the present invention is applied, the 
photocathode and the anode are accommodated in the vacuum container to 
face each other. When a predetermined voltage is applied between the 
photocathode and the anode, an electric field is generated from the anode 
toward the photocathode. When photons having an energy higher than the 
bandgap energy of the light absorption layer are incident in the 
photocathode through the vacuum container, some photons are absorbed by 
the photoelectric emission layer, although most photons are transmitted 
through the photoelectric emission layer and absorbed by the light 
absorption layer. To reduce the number of photons absorbed by the 
photoelectric emission layer, the thickness of the photoelectric emission 
layer must be adjusted. According to the above-described function of the 
photocathode of the present invention, photoelectrons emitted into the 
vacuum through the surface layer travel while being accelerated by the 
electric field generated between the anode and the photocathode, are 
accepted by the anode, and detected. 
In the phototube to which the photocathode (transmission type photocathode 
having a substrate constituting part of the vacuum container) according to 
the present invention is applied, the substrate of the photocathode is 
arranged as the window portion of the vacuum container, and the anode is 
accommodated in the vacuum container to face the photocathode. When a 
predetermined voltage is applied between the photocathode and the anode, 
an electric field is generated from the anode toward the photocathode. 
When photons having an energy higher than the bandgap of the light 
absorption layer are incident on the photocathode through the substrate 
(part of the vacuum container) of the photocathode, the photons are 
absorbed by the light absorption layer. According to the above-described 
function of the photocathode, photoelectrons emitted into the vacuum 
through the surface layer travel while being accelerated by the electric 
field generated between the anode and the photocathode, are accepted by 
the anode, and detected. 
The present invention will be more fully understood from the detailed 
description given hereinbelow and the accompanying drawings, which are 
given by way of illustration only and are not to be considered as limiting 
the present invention. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will be apparent to those skilled in the 
art from this detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The arrangements and functions of embodiments of a photocathode according 
to the present invention and a phototube having the photocathode will be 
described below in detail with reference to FIGS. 1 to 12. The same 
reference numerals denote the same elements throughout the drawings, and a 
detailed description thereof will be omitted. The dimensional ratio in the 
drawings does not necessarily match that in the description. 
First Embodiment 
As shown in FIGS. 1 and 2, the phototube of the first embodiment has a 
so-called reflection type photocathode. Particularly, FIG. 2 is a 
sectional view of the phototube of the first embodiment, which is taken 
along a line I--I in FIG. 1. 
As shown in FIGS. 1 and 2, in a phototube 10 of the first embodiment, a 
photocathode 30 and an anode 40 are accommodated in a vacuum container 20 
to face each other by a predetermined distance. The vacuum container 20 is 
a hollow cylindrical glass container whose interior is held in a high 
vacuum state at a pressure of about 10.sup.-8 Torr. The photocathode 30 is 
supported by a metal lead pin 51 through a metal support table 50. The 
lead pin 51 extends from the bottom portion of the photocathode 30 through 
the bottom portion of the vacuum container 20 and is electrically 
connected to the cathode output terminal of an external power supply (see 
FIG. 2). A predetermined voltage is applied to the photocathode 30 through 
the lead pin 51 so that the potential of the photocathode 30 is set to be 
lower than that of the anode 40. The anode 40 is a metal electrode having 
a rectangular shape and is supported by a metal lead pin 52. The lead pin 
52 extends from the bottom portion of the anode 40 through the bottom 
portion of the vacuum container 20 and is electrically connected to the 
anode output terminal of the external power supply (see FIG. 2). A 
predetermined voltage is applied to the anode 40 through the lead pin 52 
so that the potential of the anode 40 is set to be higher than that of the 
photocathode 30. 
In the photocathode 30, a substrate 32 is set on a predetermined surface 
area of a metal support plate 31. The support plate 31 mainly contains Mo 
and is shaped into a rectangular plate. The substrate 32 mainly contains 
of sapphire and is shaped into a rectangular plate. A contact layer 33, a 
light absorption layer 34, and a photoelectric emission layer 35 are 
sequentially laminated on the substrate 32 as various semiconductor 
layers. 
The contact layer 33 is a p-type compound semiconductor layer mainly 
containing GaN and epitaxially grown so as to cover the entire surface 
area of the substrate 32. The contact layer 33 has a thickness of about 50 
nm and is doped with a p-type dopant, i.e., Mg or Zn at a concentration of 
about 5.times.10.sup.18 cm.sup.-3. 
The light absorption layer 34 is a p-type compound semiconductor layer 
mainly containing Ga.sub.0.6 Al.sub.0.4 N and epitaxially grown so as to 
cover a predetermined surface area of the contact layer 33. The light 
absorption layer 34 has a thickness of about 200 nm and is doped with a 
p-type dopant, i.e., Mg or Zn at a concentration of about 
1.times.10.sup.17 to 1.times.10.sup.18 cm.sup.-3. 
The photoelectric emission layer 35 is a p-type compound semiconductor 
layer mainly containing GaN and epitaxially grown so as to cover the 
entire surface area of the light absorption layer 34. The photoelectric 
emission layer 35 has a thickness of about 10 nm and is doped with a 
p-type dopant, i.e., Mg or Zn at a concentration of about 
5.times.10.sup.18 cm.sup.-3. With this structure, the first major surface 
of the light absorption layer 34, which faces the sapphire substrate 32, 
contacts the contact layer 33 provided between the substrate 32 and the 
light absorption layer 34, and the second major surface of the light 
absorption layer 34, which faces the first major surface, contacts the 
photoelectric emission layer 35. 
A surface layer 36 containing an alkali metal or an alkali metal oxide is 
deposited on the photoelectric emission layer 35. The surface layer 36 is 
a monomolecular layer containing Cs oxide and provided so as to cover the 
entire surface area of the photoelectric emission layer 35. 
The photocathode 30 is arranged such that the surface of the support plate 
31 is set parallel to a tube axis AX1 of the vacuum container 20 and along 
the side wall of the vacuum container 20. The plate-like support table 50 
extending to be perpendicular to the tube axis AX1 is set at the bottom 
portion of the support plate 31 while contacting the side portions of the 
substrate 32 and contact layer 33 mainly containing Mo. The rod-like lead 
pin 51 extending along the tube axis AX1 and comprising a Kovar metal is 
attached to the central bottom portion of the support table 50. 
The anode 40 is a metal ring having an opening at its center and arranged 
at a position facing the surface layer 36 of the photocathode 30. The 
rod-like lead pin 52 extending along the tube axis AX1 and comprising a 
Kovar metal is attached to the bottom portion of the anode 40. 
As shown in FIG. 3 showing the energy diagram of the reflection type 
photocathode 30 of the present invention having the above structure, the 
energy level of a conduction band E.sub.C is lowered from the light 
absorption layer 34 toward the surface layer 36 through the photoelectric 
emission layer 35. The major component of the light absorption layer 34, 
i.e., p-type Ga.sub.0.6 Al.sub.0.4 N has a bandgap energy of about 4.27 eV 
as the energy difference between the conduction band E.sub.C and a valence 
band E.sub.V. The absorption edge on the long wavelength side within the 
wavelength range of incident light is at a wavelength of about 290 nm. On 
the other hand, the major component of the photoelectric emission layer 
35, i.e., p-type GaN has a bandgap energy lower than that of the light 
absorption layer 34. The major component of the surface layer 36, i.e., 
the Cs oxide has a work function smaller than the energy difference 
between the conduction band E.sub.C and a Fermi level E.sub.F of the 
photoelectric emission layer 35 and has a vacuum level lower than that of 
the conduction band E.sub.C of the photoelectric emission layer 35. 
The function of the first embodiment will be described below. 
When a predetermined voltage is applied between the photocathode 30 and the 
anode 40 from the external power supply (see FIG. 2) through the lead pins 
51 and 52, an electric field is generated from the anode 40 toward the 
photocathode 30. After this preparation, photons transmitted through the 
vacuum container 20 are incident on the photocathode 30 through the 
surface layer 36. When the photons have an energy higher than the bandgap 
energy of the light absorption layer 34, some photons are absorbed by the 
photoelectric emission layer 35, although most photons are transmitted 
through the photoelectric emission layer 35 and absorbed by the light 
absorption layer 34. To reduce the number of photons absorbed by the 
photoelectric emission layer 35, the thickness of the photoelectric 
emission layer 35 is adjusted to about 10 nm. 
In the light absorption layer 34, electrons e.sup.- existing in the 
valence band E.sub.V are excited to the conduction band E.sub.C and become 
free electrons. The generated photoelectrons e.sup.- are diffused or 
drifted along the conduction band E.sub.C which is lowered in level from 
the light absorption layer 34 toward the surface layer 36 through the 
photoelectric emission layer 35 and are emitted into the vacuum (in the 
vacuum container 20 outside the photocathode 30) by the negative electron 
affinity of the surface layer 36. The emitted photoelectrons e.sup.- 
travel while being accelerated by the electric field generated between the 
anode 40 and the photocathode 30, are accepted by the anode 40, and 
detected by an external ammeter. 
Since the photoelectric emission layer 35 is a p-type compound 
semiconductor layer mainly containing GaN, Al is not contained in the 
composition, unlike the light absorption layer 34. Therefore, the 
photoelectric emission layer 35 is not easily oxidized, unlike the light 
absorption layer 34. The surface of the light absorption layer 34 is 
covered with and in direct contact with the photoelectric emission layer 
35, and therefore it is not easily oxidized. The surface layer 36 mainly 
containing the Cs oxide is provided on the photoelectric emission layer 
35. Since the work function of the surface of the photoelectric emission 
layer 35 is sufficiently decreased by the surface layer 36, a negative 
electron affinity by the surface layer 36 can be obtained. Therefore, in 
the reflection type photocathode 30, the quantum efficiency is improved, 
and the absorption edge characteristic on the long wavelength side within 
the wavelength range of incident light is sharpened. 
The major component of the photoelectric emission layer 35, i.e., p-type 
GaN can lattice-match the major component of the light absorption layer 
34, i.e., p-type Ga.sub.0.6 Al.sub.0.4 N. For this reason, the 
photoelectric emission layer 35 having a satisfactory crystallinity is 
epitaxially grown on the light absorption layer 34. Since almost no 
crystal defects are occurred in the photoelectric emission layer 35, the 
photocathode 30 can obtain satisfactory photoelectron diffusion 
properties. 
The photoelectric emission layer 35 has a bandgap energy lower than that of 
the light absorption layer 34 so the conduction band E.sub.C of the 
photoelectric emission layer 35 is lower than that of the light absorption 
layer 34. Since photoelectrons excited in the light absorption layer 34 
are efficiently diffused or drifted along the electric field directing the 
surface layer 36, photoelectron emission can be achieved at a high quantum 
efficiency. 
A method of manufacturing the photocathode according to the first 
embodiment will be described below. 
In this manufacturing method, conventional MOCVD (Metal Organic Chemical 
Vapor Deposition) is used. First, the substrate 32 is set in a reaction 
vessel. After the reaction vessel is evacuated, hydrogen gas is introduced 
as a carrier gas. Next, while holding the interior of the reaction vessel 
at a predetermined pressure, the substrate 32 is heated to a predetermined 
temperature, and reaction gases are introduced into the reaction vessel. 
In this process, by controlling the flow rate of each source gas to be 
mixed as a reaction gas to a predetermined rate, various semiconductor 
layers are epitaxially grown on the substrate 32. 
First, as source gases, Ga(CH.sub.3).sub.3, NH.sub.3, and Mg(C.sub.5 
H.sub.5).sub.2 or Zn(CH.sub.3).sub.2 are introduced into the reaction 
vessel to form the contact layer 33 on the substrate 32. The flow rates of 
the source gases are as follows: about 10 to 20 cm.sup.3 /min for 
Ga(CH.sub.3).sub.3, about 11 to 21 cm.sup.3 /min for NH.sub.3, and about 
0.8 to 2.6 cm.sup.3 /min for Mg(C.sub.5 H.sub.5).sub.2 or 
Zn(CH.sub.3).sub.2. The surface temperature of the substrate 32 is about 
940 to 1,100.degree. C. The internal pressure of the reaction vessel is 
about 760 Torr. The growth time is about 1.5 to 2 min. 
Next, as source gases, Ga(CH.sub.3).sub.3, Al(CH.sub.3).sub.3, NH.sub.3, 
and Mg(C.sub.5 H.sub.5).sub.2 or Zn(CH.sub.3).sub.2 are introduced into 
the reaction vessel to form the light absorption layer 34 on the contact 
layer 33. The flow rates of the source gases are as follows: about 10 to 
20 cm.sup.3 /min for Ga(CH.sub.3).sub.3, about 11 to 21 cm.sup.3 /min for 
NH.sub.3, about 10 to 20 cm.sup.3 /min for Al(CH.sub.3).sub.3, and about 
0.4 cm.sup.3 /min to 1.5 cm.sup.3 /min for Mg(C.sub.5 H.sub.5) or 
Zn(CH.sub.3).sub.2. The surface temperature of the substrate 32 is about 
940 to 1,100.degree. C. The internal pressure of the reaction vessel is 
about 760 Torr. The growth time is about 6 to 8 min. 
Subsequently, as source gases, Ga(CH.sub.3).sub.3, NH.sub.3, and Mg(C.sub.5 
H.sub.5).sub.2 or Zn(CH.sub.3).sub.2 are introduced into the reaction 
vessel to form the photoelectric emission layer 35 on the light absorption 
layer 34. The flow rates of the source gases are as follows: about 10 to 
20 cm.sup.3 /min for Ga(CH.sub.3).sub.3, about 11 to 21 cm.sup.3 /min for 
NH.sub.3, and about 0.8 to 2.6 cm.sup.3 /min for Mg(C.sub.5 H.sub.5).sub.2 
or Zn(CH.sub.3).sub.2. The surface temperature of the substrate 32 is 
about 940 to 1,100.degree. C. The internal pressure of the reaction vessel 
is about 760 Torr. The growth time is about 20 to 25 sec. 
The substrate 32 on which the various semiconductor layers are laminated is 
temporarily removed from the reaction vessel and subjected to patterning 
by conventional photolithography. In this patterning, an etching mask 
layer having a predetermined pattern is patterned on the photoelectric 
emission layer 35. The photoelectric emission layer 35 and the light 
absorption layer 34 are formed into a rectangular pattern by conventional 
wet etching. Thereafter, the etching mask layer on the photoelectric 
emission layer 35 is removed. 
The support plate 31 is bonded to the lower surface of the substrate 32 
which has undergone the above process. At the same time, the support table 
50 is bonded to the side portions of the substrate 32 and the contact 
layer 33. Thereafter, the substrate 32 is set in the vacuum container 20. 
The Cs oxide is deposited, by conventional vacuum deposition, on the 
surface of the photoelectric emission layer 35 which is provided on the 
substrate 32 set at a predetermined position in the vacuum container 20, 
to form the surface layer 36. The vacuum container 20 is sealed in a high 
vacuum state, thereby obtaining the phototube 10 having the reflection 
type photocathode 30. 
A comparison experiment for the phototube (the first embodiment shown in 
FIGS. 1 and 2) having the reflection type photocathode of the present 
invention and the conventional phototube will be described below. 
For the phototube of the first embodiment, the light absorption layer and 
photoelectric emission layer of the photocathode were formed of p-type 
compound semiconductor layers mainly containing Ga.sub.0.6 Al.sub.0.4 N 
and GaN, respectively, as described above. The conventional phototube has 
almost the same structure as that of the first embodiment except the 
photocathode. More specifically, the light absorption layer of the 
photocathode is formed of a compound semiconductor layer mainly containing 
CsTe. Short-wavelength light is irradiated on these phototubes, and the 
quantum efficiencies are measured. 
FIG. 4 is a graph showing the spectral sensitivity characteristics of the 
phototube of the first embodiment and the conventional phototube. 
According to this graph, for light having a wavelength of about 200 to 350 
nm, the quantum efficiency of the phototube of the first embodiment is 
higher than that of the conventional phototube. In addition, the 
absorption edge of the phototube of the first embodiment on the long 
wavelength side within the wavelength range of incident light is sharper 
than that of the conventional phototube. 
Second Embodiment 
The phototube of the second embodiment has a so-called transmission type 
photocathode, as shown in FIGS. 5 and 6. Particularly, FIG. 6 is a 
sectional view of the phototube of the second embodiment (FIG. 5), which 
is taken along a line II--II in FIG. 5. 
As shown in FIGS. 5 and 6, in a phototube 10 of the second embodiment, a 
photocathode 30 and an anode 40 serving as the window portions of a vacuum 
container 20 are accommodated in the vacuum container 20 to face each 
other by a predetermined distance. The vacuum container 20 is constituted 
by a hollow cylindrical glass container whose one end is open and a 
substrate 32 of the photocathode 30. The vacuum container 20 is 
hermetically sealed by the glass container and the substrate 32 while 
holding its interior in a high vacuum state at a pressure of about 
10.sup.-8 Torr. With this structure, part of the vacuum container 20 
functions as the substrate 32 of the photocathode 30. 
The photocathode 30 is supported by the side wall portion of the vacuum 
container 20 to hermetically seal the vacuum container 20. The 
photocathode 30 is connected to a metal lead pin 51 through a metal wiring 
layer 53. The lead pin 51 extends from the end portion of the wiring layer 
53 through the bottom portion of the vacuum container 20 and is 
electrically connected to the cathode output terminal of an external power 
supply (see FIG. 6). A predetermined voltage is applied to the 
photocathode 30 through the lead pin 51 so that the potential of the 
photocathode 30 is set to be lower than that of the anode 40. 
The anode 40 is a metal electrode having a circular plate-like shape and is 
supported by a metal lead pin 52. The lead pin 52 extends from the end 
portion of the anode 40 through the bottom portion of the vacuum container 
20 and is electrically connected to the anode output terminal of the 
external power supply (see FIG. 6). A predetermined voltage is applied to 
the anode 40 through the lead pin 52 so that the potential of the anode 40 
is set to be higher than that of the photocathode 30. 
In the photocathode 30, the substrate 32 is joined to the side wall portion 
of the vacuum container 20 and functions as the window portion of the 
vacuum container 20. An electron shielding layer 37, a light absorption 
layer 34, and a photoelectric emission layer 35 are sequentially laminated 
on the substrate 32 as various semiconductor layers. 
The electron shielding layer 37 is a p-type compound semiconductor layer 
mainly containing AlN and epitaxially grown so as to cover the entire 
surface area of the substrate 32. The electron shielding layer 37 has a 
thickness of about 75 nm and is doped with a p-type dopant, i.e., Mg or Zn 
at a concentration of about 1.times.10.sup.18 to 5.times.10.sup.18 
cm.sup.-3. 
The light absorption layer 34 is a p-type compound semiconductor layer 
mainly containing Ga.sub.0.6 Al.sub.0.4 N and epitaxially grown so as to 
cover a predetermined surface area of the electron shielding layer 37. The 
light absorption layer 34 has a thickness of about 200 nm and is doped 
with a p-type dopant, i.e., Mg or Zn at a concentration of about 
1.times.10.sup.17 to 1.times.10.sup.18 cm.sup.-3. 
The photoelectric emission layer 35 is a p-type compound semiconductor 
layer mainly containing GaN and epitaxially grown so as to cover the 
entire surface area of the light absorption layer 34. The photoelectric 
emission layer 35 has a thickness of about 10 nm and is doped with a 
p-type dopant, i.e., Mg or Zn at a concentration of about 
5.times.10.sup.18 cm.sup.-3. With this structure, the first major surface 
of the light absorption layer 34, which faces the sapphire substrate 32, 
contacts the electron shielding layer 37 provided between the substrate 32 
and the light absorption layer 34, and the second major surface of the 
light absorption layer 34, which faces the first major surface, contacts 
the photoelectric emission layer 35. 
A surface layer 36 comprising an alkali metal oxide is deposited on the 
photoelectric emission layer 35. The surface layer 36 is a monomolecular 
layer including Cs oxide and provided so as to cover the entire surface 
area of the photoelectric emission layer 35. 
The photocathode 30 is set to be joined to the side wall portion of the 
vacuum container 20 such that the surface of the substrate 32 is arranged 
to be perpendicular to an tube axis AX2 of the vacuum container 20. The Al 
wiring layer 53 coating the side wall of the vacuum container 20 along the 
tube axis AX2 is arranged aside the photocathode 30. The Al wiring layer 
53 electrically contacts the surface of the electron shielding layer 37 
and the side portion of the light absorption layer 34. The rod-like lead 
pin 51 extending along the tube axis AX2 and comprising a Kovar metal is 
attached to the end portion of the Al wiring layer 53. 
The anode 40 is arranged at a position facing the surface layer 36 of the 
photocathode 30. The rod-like lead pin 52 extending along the tube axis 
AX2 and comprising a Kovar metal is attached to the end portion of the 
anode 40. 
As shown in FIG. 7 showing the energy diagram of the photocathode 30 of the 
present invention having the above structure, the energy level of a 
conduction band E.sub.C is lowered from the electron shielding layer 37 
toward the surface layer 36 through the light absorption layer 34 and the 
photoelectric emission layer 35. The major component of the light 
absorption layer 34, i.e., p-type Ga.sub.0.6 Al.sub.0.4 N has a bandgap 
energy of about 4.27 eV as the energy difference between the conduction 
band E.sub.C and a valence band E.sub.V. The absorption edge on the long 
wavelength side within the wavelength range of incident light is at a 
wavelength of about 290 nm. 
On the other hand, the major component of the photoelectric emission layer 
35, i.e., p-type GaN has a bandgap energy lower than that of the light 
absorption layer 34. The major component of the electron shielding layer 
37, i.e., p-type AlN has a bandgap energy higher than that of the light 
absorption layer 34. The major component of the surface layer 36, i.e., 
the Cs oxide has a work function smaller than the energy difference 
between the conduction band E.sub.C and a Fermi level E.sub.F of the 
photoelectric emission layer 35 and has a vacuum level lower than that of 
the conduction band E.sub.C of the photoelectric emission layer 35. 
The function of the second embodiment will be described below. 
When a predetermined voltage is applied between the photocathode 30 and the 
anode 40 from the external power supply (FIG. 6) through the lead pins 51 
and 52, an electric field is generated from the anode 40 toward the 
photocathode 30. After this preparation, photons transmitted through the 
substrate 32 (part of the vacuum container 20) of the photocathode 30 are 
incident in the photocathode 30. When the photons have an energy lower 
than that of the electron shielding layer 37 and higher than the bandgap 
energy of the light absorption layer 34, photons are transmitted through 
the electron shielding layer 37 and absorbed by the light absorption layer 
34. 
In the light absorption layer 34, electrons e.sup.- existing in the 
valence band E.sub.V are excited to the conduction band E.sub.C and become 
free electrons. Since an energy barrier is present between the light 
absorption layer 34 and the electron shielding layer 37, the generated 
photoelectrons e.sup.- are diffused or drifted along the conduction band 
E.sub.C which is lowered in level from the light absorption layer 34 
toward the surface layer 36 through the photoelectric emission layer 35 
without being diffused or drifted into the electron shielding layer 37, 
and emitted into the vacuum (in the vacuum container 20 outside the 
photocathode) by the negative electron affinity of the surface layer 36. 
The emitted photoelectrons e.sup.- travel while being accelerated by the 
electric field generated between the anode 40 and the photocathode 30, are 
accepted by the anode 40, and detected by an external ammeter. 
Since the photoelectric emission layer 35 is a p-type compound 
semiconductor layer mainly containing GaN, Al is not contained in the 
composition, unlike the light absorption layer 34. Therefore, the 
photoelectric emission layer 35 is not easily oxidized, unlike the light 
absorption layer 34. The surface of the light absorption layer 34 is 
covered with and in direct contact with the photoelectric emission layer 
35, and therefore it is not easily oxidized. In addition, the surface 
layer 36 mainly containing the Cs oxide is provided on the photoelectric 
emission layer 35. Since the work function of the surface of the 
photoelectric emission layer 35 is sufficiently decreased by the surface 
layer 36, a negative electron affinity by the surface layer 36 can be 
obtained. Therefore, in the photocathode 30, the quantum efficiency is 
improved, and the absorption edge characteristic on the long wavelength 
side within the wavelength range of incident light is sharpened. 
The major component of the electron shielding layer 37, i.e., p-type AlN 
and the major component of the photoelectric emission layer 35, i.e., 
p-type GaN can lattice-match the major component of the light absorption 
layer 34, i.e., p-type Ga.sub.0.6 Al.sub.0.4 N. For this reason, the light 
absorption layer 34 and photoelectric emission layer 35 having a 
satisfactory crystallinity are epitaxially grown on the electron shielding 
layer 37. Since almost no crystal defects are occurred in the light 
absorption layer 34 and the photoelectric emission layer 35, the 
photocathode 30 can obtain satisfactory photoelectron diffusion 
properties. 
The electron shielding layer 37 has a bandgap energy higher than that of 
the light absorption layer 34 so the conduction band E.sub.C of the 
electron shielding layer 37 is higher in level than that of the light 
absorption layer 34. Since photoelectrons excited in the light absorption 
layer 34 are diffused or drifted along the electric field directing the 
surface layer 36, the photoelectrons are not diffused or drifted into the 
electron shielding layer 37. On the other hand, the photoelectric emission 
layer 35 has a bandgap energy lower than that of the light absorption 
layer 34 so the conduction band E.sub.C of the photoelectric emission 
layer 35 is lower in level than that of the light absorption layer 34. 
Since photoelectrons excited in the light absorption layer 34 are 
efficiently diffused or drifted along the electric field generated from 
the light absorption layer toward the surface layer 36. The photoelectrons 
excited in the light absorption layer 34 are diffused or drifted into the 
photoelectric emission layer 35 without being diffused or drifted into the 
electron shielding layer 37. Therefore, the photocathode 30 can obtain a 
higher quantum efficiency (efficient photoelectron emission). 
A method of manufacturing the photocathode according to the second 
embodiment will be described below. 
In this manufacturing method as well, conventional MOCVD is used. First, 
the substrate 32 is set in a reaction vessel. After the reaction vessel is 
evacuated, hydrogen gas is introduced as a carrier gas. Next, while 
holding the interior of the reaction vessel at a predetermined pressure, 
the substrate 32 is heated to a predetermined temperature, and reaction 
gases are introduced into the reaction vessel. In this process, by 
controlling the flow rate of each source gas to be mixed as a reaction gas 
to a predetermined rate, various semiconductor layers are epitaxially 
grown on the substrate 32. 
First, as source gases, Al(CH.sub.3).sub.3, NH.sub.3, and Mg(C.sub.5 
H.sub.5).sub.2 or Zn(CH.sub.3).sub.2 are introduced into the reaction 
vessel to form the electron shielding layer 37 on the substrate 32. The 
flow rates of the source gases are as follows: about 10 to 20 cm.sup.3 
/min for Al(CH.sub.3).sub.3, about 11 to 21 cm.sup.3 /min for NH.sub.3, 
and about 0.8 to 2.6 cm.sup.3 /min for Mg(C.sub.5 H.sub.5).sub.2 or 
Zn(CH.sub.3).sub.2. The surface temperature of the substrate 32 is about 
940 to 1,100.degree. C. The internal pressure of the reaction vessel is 
about 760 Torr. The growth time is about 6 to 8 min. 
Next, as source gases, Ga(CH.sub.3).sub.3, Al(CH.sub.3).sub.3, NH.sub.3, 
and Mg(C.sub.5 H.sub.5).sub.2 or Zn(CH.sub.3).sub.2 are introduced into 
the reaction vessel to form the light absorption layer 34 on the electron 
shielding layer 37. The flow rates of the source gases are as follows: 
about 10 to 20 cm.sup.3 /min for Ga(CH.sub.3).sub.3, about 11 to 21 
cm.sup.3 /min for NH.sub.3, about 10 to 20 cm.sup.3 /min for 
Al(CH.sub.3).sub.3, and about 0.4 cm.sup.3 /min to 1.5 cm.sup.3 /min for 
Mg(C.sub.5 H.sub.5) or Zn(CH.sub.3).sub.2. The surface temperature of the 
substrate 32 is about 940 to 1,100.degree. C. The internal pressure of the 
reaction vessel is about 760 Torr. The growth time is about 6 to 8 min. 
Subsequently, as source gases, Ga(CH.sub.3).sub.3, NH.sub.3, and Mg(C.sub.5 
H.sub.5).sub.2 or Zn(CH.sub.3).sub.2 are introduced into the reaction 
vessel to form the photoelectric emission layer 35 on the light absorption 
layer 34. The flow rates of the source gases are as follows: about 10 to 
20 cm.sup.3 /min for Ga(CH.sub.3).sub.3, about 11 to 21 cm.sup.3 /min for 
NH.sub.3, and about 0.8 to 2.6 cm.sup.3 /min for Mg(C.sub.5 H.sub.5).sub.2 
or Zn(CH.sub.3).sub.2. The surface temperature of the substrate 32 is 
about 940 to 1,100.degree. C. The internal pressure of the reaction vessel 
is about 760 Torr. The growth time is about 20 to 25 sec. 
The substrate 32 on which the various semiconductor layers are laminated is 
temporarily removed from the reaction vessel and subjected to patterning 
by conventional photolithography. In this patterning, an etching mask 
layer having a predetermined pattern is formed on the photoelectric 
emission layer 35. The photoelectric emission layer 35 and the light 
absorption layer 34 are shaped into a circular pattern by conventional wet 
etching. Thereafter, the etching mask layer on the photoelectric emission 
layer 35 is removed. 
The side portion of the substrate 32 which has undergone the above process 
is fused to the side wall portion of the vacuum container 20 to constitute 
part of the vacuum container 20. In addition, the wiring layer 53 is 
electrically connected to the surface of the electron shielding layer 37 
and the side portion of the light absorption layer 34. Thereafter, the Cs 
oxide is deposited, by conventional vacuum deposition, on the surface of 
the photoelectric emission layer 35 to form the surface layer 36. The 
vacuum container 20 is sealed in a high vacuum state, thereby obtaining 
the phototube 10 having the transmission type photocathode 30. 
A comparison experiment for the phototube (the second embodiment shown in 
FIGS. 5 and 6) having the transmission type photocathode of the present 
invention and the conventional phototube will be described below. 
For the phototube of the second embodiment, the electron shielding layer, 
the light absorption layer, and the photoelectric emission layer of the 
photocathode were laminated of p-type compound semiconductor layers mainly 
containing AlN, Ga.sub.0.6 Al.sub.0.4 N and GaN, respectively, as 
described above. The conventional phototube had almost the same structure 
as that of the second embodiment except the photocathode. More 
specifically, the light absorption layer of the photocathode was formed of 
a compound semiconductor layer mainly containing CsTe. Short-wavelength 
light was irradiated on these phototubes, and the quantum efficiencies 
were measured. 
FIG. 8 is a graph showing the spectral sensitivity characteristics of the 
phototube of the second embodiment and the conventional phototube. 
According to this graph, for light having a wavelength of about 200 to 350 
nm, the quantum efficiency of the phototube of the second embodiment is 
higher than that of the conventional phototube. The sapphire substrate 
used for the phototube of the second embodiment has a minimum 
transmittance with respect to light having a wavelength of about 200 nm. 
Since the phototube has a transmission type photocathode having the above 
structure, the spectral sensitivity characteristic on the short wavelength 
side within the wavelength range of incident light is largely limited by 
the optical characteristics of the substrate. As is apparent from FIG. 8, 
the absorption edge characteristic of the phototube on the long wavelength 
side within the wavelength range of incident light is improved as compared 
with the conventional phototube. 
Third Embodiment 
In the third embodiment (FIGS. 9 and 10), the phototube has a so-called 
reflection type photocathode having almost the same structure as that of 
the first embodiment (FIGS. 1 and 2). Particularly, FIG. 10 is a sectional 
view of the phototube of the third embodiment (FIG. 9), which is taken 
along a line III--III in FIG. 9. 
As shown in FIGS. 9 and 10, a phototube 10 of the third embodiment is 
different from the phototube 10 of the first embodiment in that an 
electron shielding layer 37 as in the photocathode 30 shown in FIGS. 5 and 
6 is provided between a substrate 32 and a light absorption layer 34 of a 
photocathode 30. More specifically, the electron shielding layer 37 is a 
p-type compound semiconductor layer mainly containing AlN and epitaxially 
grown so as to cover the entire surface area of the substrate 32. The 
electron shielding layer 37 has a thickness of about 75 nm and is doped 
with a p-type dopant, i.e., Mg or Zn at a concentration of 
1.times.10.sup.18 to 5.times.10.sup.18 cm.sup.-3. 
In the energy diagram of the photocathode 30, like the photocathode 30 
shown in FIGS. 5 and 6, the energy level of a conduction band E.sub.C is 
lowered from the electron shielding layer 37 toward a surface layer 36 
through the light absorption layer 34 and the photoelectric emission layer 
35 (FIG. 7). 
According to this structure, the photocathode 30 can be obtained by almost 
the same manufacturing method as for the photocathode shown in FIGS. 5 and 
6. However, the substrate 32, the electron shielding layer 37, the light 
absorption layer 34, and the photoelectric emission layer 35 are shaped 
into rectangular shapes. In addition, a support plate 31 is bonded to the 
bottom portion of the substrate 32, and a support table 50 is bonded to 
the side portions of the substrate 32 and the electron shielding layer 37. 
The photocathode 30 is set at a predetermined position in the vacuum 
container 20. After this setting, the surface layer 36 is formed on the 
surface of the photoelectric emission layer 35. The vacuum container 20 is 
sealed in a high vacuum state, thereby obtaining the phototube 10 having 
the reflection type photocathode 30. 
The function of the third embodiment will be described below. 
When a predetermined voltage is applied between the photocathode 30 and the 
anode 40 from an external power supply (FIG. 10) through lead pins 51 and 
52, an electric field is generated from the anode 40 toward the 
photocathode 30. After this preparation, photons transmitted through the 
vacuum container 20 are incident on the photocathode 30 through the 
surface layer 36. When the photons have an energy higher than the bandgap 
energy of the photoelectric emission layer 35, some photons are absorbed 
by the photoelectric emission layer 35, although most photons are 
transmitted through the photoelectric emission layer 35 and absorbed by 
the light absorption layer 34. To reduce the number of photons absorbed by 
the photoelectric emission layer 35, the thickness of the photoelectric 
emission layer 35 is adjusted to about 10 nm. 
In the light absorption layer 34, electrons e.sup.- existing in a valence 
band E.sub.V are excited to the conduction band E.sub.C and become free 
electrons. Since an energy barrier is present between the light absorption 
layer 34 and the electron shielding layer 37, the generated photoelectrons 
e.sup.- are diffused or drifted along the conduction band E.sub.C which 
is lowered from the light absorption layer 34 toward the surface layer 36 
through the photoelectric emission layer 35 without being diffused or 
drifted into the electron shielding layer 37, and emitted into the vacuum 
(in the vacuum container 20 outside the photocathode 30) by the negative 
electron affinity of the surface layer 36. The emitted photoelectrons 
e.sup.- fly while being accelerated by the electric field generated from 
the anode 40 toward the photocathode 30, are accepted by the anode 40, and 
detected by an external ammeter. 
Therefore, the phototube 10 exhibits almost the same operation 
characteristics as those of the above-described phototube (FIGS. 5 and 6). 
Fourth Embodiment 
In the fourth embodiment (FIGS. 11 and 12), the phototube has a so-called 
transmission type photocathode having almost the same structure as that of 
the second embodiment (FIGS. 5 and 6). Particularly, FIG. 12 is a 
sectional view of the phototube of the fourth embodiment (FIG. 11), which 
is taken along a line IV--IV in FIG. 11. 
As shown in FIGS. 11 and 12, a phototube 10 of the fourth embodiment is 
different from that of the second embodiment in that a contact layer 33 as 
in the photocathode 30 shown in FIGS. 1 and 2 is provided between a 
substrate 32 and a light absorption layer 34 of a semiconductor 
photocathode 30. More specifically, the contact layer 33 is a p-type 
compound semiconductor layer mainly containing GaN and epitaxially grown 
so as to cover the entire surface area of the substrate 32. The contact 
layer 33 is thinner (thickness: about 10 nm) than that of the photocathode 
shown in FIGS. 1 and 2 and is doped with a p-type dopant, i.e., Mg or Zn 
at a concentration of 5.times.10.sup.18 cm.sup.-3. 
In the energy diagram of the photocathode 30, like the photocathode 30 
shown in FIGS. 1 and 2, the energy level of a conduction band E.sub.C is 
lowered from the light absorption layer 34 toward a surface layer 36 
through a photoelectric emission layer 35 (FIG. 3). 
According to this structure, the transmission type photocathode 30 can be 
obtained by almost the same manufacturing method as for the photocathode 
shown in FIGS. 1 and 2. However, the substrate 32, the contact 33, the 
light absorption layer 34, and the photoelectric emission layer 35 are 
laminated into circular shapes. In addition, the side portion of the 
substrate 32 is fused to the side wall portion of a vacuum container, and 
a wiring layer 53 is electrically connected to the surface of the contact 
layer 33 and the side portion of the light absorption layer 34. 
Furthermore, the surface layer 36 is formed on the surface of the 
photoelectric emission layer 35. The vacuum container 20 is sealed in a 
high vacuum state, thereby obtaining the phototube 10 having the 
transmission type photocathode 30. 
The operation of the fourth embodiment will be described below. 
When a predetermined voltage is applied between the photocathode 30 and an 
anode 40 from an external power supply (FIG. 12) through lead pins 51 and 
52, an electric field is generated from the anode 40 toward the 
photocathode 30. After this preparation, photons transmitted through the 
substrate 32 (part of the vacuum container 20) of the photocathode 30 are 
incident in the photocathode 30. When the photons have an energy higher 
than the bandgap energy of the light absorption layer 34, some photons are 
absorbed by the contact layer 33, although most photons are transmitted 
through the contact layer 33 and absorbed by the light absorption layer 
34. To reduce the number of photons absorbed by the contact layer 33, the 
thickness of the contact layer 33 is adjusted to about 10 nm. 
In the light absorption layer 34, electrons e.sup.- existing in a valence 
band E.sub.V are excited to the conduction band E.sub.C and become free 
electrons. The generated photoelectrons e.sup.- are diffused or drifted 
along the conduction band E.sub.C which is lowered in level from the light 
absorption layer 34 toward the surface layer 36 through the photoelectric 
emission layer 35 and emitted into the vacuum (in the vacuum container 20 
outside the photocathode 30) by the negative electron affinity of the 
surface layer 36. The emitted photoelectrons e.sup.- travel while being 
accelerated by the electric field generated from the anode 40 toward the 
photocathode 30, are accepted by the anode 40, and detected by an external 
ammeter. 
Therefore, the phototube 10 exhibits almost the same operation 
characteristics as those of the above-described phototube shown in FIGS. 1 
and 2. 
The present invention is not limited to the above embodiments, and various 
changes and modifications can be made. 
In each of the embodiments, as the composition of the p-type Ga.sub.1-x 
Al.sub.x N (0&lt;x&lt;1) constituting the light absorption layer of the 
photocathode, p-type Ga.sub.0.6 Al.sub.0.4 N is used. However, when the 
flow rates of the source gases, i.e., Ga(CH.sub.3).sub.3, 
Al(CH.sub.3).sub.3, NH.sub.3, and Mg(C.sub.5 H.sub.5).sub.2 or 
Zn(CH.sub.3).sub.2 are adjusted in epitaxial growth of the light 
absorption layer, various compositions of the p-type Ga.sub.1-x Al.sub.x N 
(0&lt;x&lt;1) can be set. The spectral sensitivity characteristic of the 
photocathode is adjusted by changing the absorption edge characteristic in 
the long wavelength side within the wavelength range of incident light 
within the wavelength range of about 200 to 350 nm in correspondence with 
the composition of Ga.sub.1-x Al.sub.x N, i.e., an alloy of AlN and GaN. 
As has been described above in detail, in the photocathode according to the 
present invention, the photoelectric emission layer mainly containing 
p-type GaN and the surface layer mainly containing an alkali metal or an 
alkali metal oxide are sequentially laminated on the light absorption 
layer mainly containing p-type Ga.sub.1-x Al.sub.x N (0&lt;x&lt;1). The p-type 
GaN as the major component of the photoelectric emission layer has a 
bandgap energy lower than that of the light absorption layer. In addition, 
the alkali metal or alkali metal oxide as the major component of the 
surface layer has a vacuum level lower than that of the conduction band of 
the photoelectric emission layer. For this reason, in the energy diagram 
of this photocathode, the energy level of the conduction band is lowered 
from the light absorption layer toward the surface layer through the 
photoelectric emission layer. 
When photons incident in the photocathode has a predetermined energy, the 
photons are absorbed by the light absorption layer. At this time, 
electrons existing in the valence band of the light absorption layer are 
excited to the conduction band and become free electrons. For this reason, 
the photons are diffused or drifted along the conduction band which is 
lowered in level from the light absorption layer toward the photoelectric 
emission layer. The photons which are diffused or drifted from the light 
absorption layer to the photoelectric emission layer are emitted into the 
vacuum (outside the photocathode) by the negative electron affinity of the 
surface layer. 
Since the photoelectric emission layer is a p-type compound semiconductor 
layer mainly containing GaN, Al is not contained in the composition, 
unlike the light absorption layer. Therefore, the photoelectric emission 
layer is not easily oxidized, unlike the light absorption layer. The 
surface of the light absorption layer is covered with the photoelectric 
emission layer and therefore is not easily oxidized. In addition, the 
surface layer mainly containing the alkali metal or alkali metal oxide is 
provided on the surface of the photoelectric emission layer. Since the 
work function of the surface of the photoelectric emission layer is 
sufficiently decreased, a negative electron affinity by the surface layer 
can be obtained. 
According to the present invention, since the quantum efficiency is 
improved, and the absorption edge characteristic on the long wavelength 
side within the wavelength range of incident light is sharpened, a 
semiconductor photocathode which achieves a high sensitivity as a 
so-called solar blind can be provided. 
In the phototube to which the reflection type photocathode according to the 
present invention is applied, the photocathode and the anode are 
accommodated in the vacuum container to face each other. In the phototube 
to which the transmission type photocathode according to the present 
invention is applied, the substrate of the photocathode is arranged as the 
window portion of the vacuum container, and the anode is accommodated in 
the vacuum container to face the photocathode. When a predetermined 
voltage is applied between the photocathode and the anode, an electric 
field is generated from the anode toward the photocathode. When photons 
having a predetermined energy are incident in the photocathode, the 
photons are absorbed by the light absorption layer. Photoelectrons emitted 
from the surface layer into the vacuum (outside the photocathode) by the 
above-described function of the photocathode travel while being 
accelerated by the electric field between the anode and the photocathode, 
are accepted by the anode, and detected. 
According to the present invention, since the quantum efficiency is 
improved, and the absorption edge characteristic on the long wavelength 
side within the wavelength range of incident light is sharpened, a 
semiconductor photocathode which achieves a high sensitivity as a 
so-called solar blind can be provided. 
From the invention thus described, it will be obvious that the invention 
may be varied in many ways. Such variations are not to be regarded as a 
departure from the spirit and scope of the invention, and all such 
modifications as would be obvious to one skilled in the art are intended 
for inclusion within the scope of the following claims. 
The basic Japanese Application No. 6-231317 (231317/1994) filed on Sep. 27, 
1994 is hereby incorporated by reference.