Schottky barrier infrared sensor

A solid-state infrared sensor using a Schottky barrier diode. The sensor has a first layer of a semiconductor of a first conductivity type and a second layer of a metal or a metal silicide and the first and second layer are joined to each other to form the Schottky barrier diode. Further, the sensor includes a third layer disposed in the depletion layer formed in the first layer out of contact with the Schottky junction interface. The third layer contains an impurity which is introduced for positioning an effective barrier formed in the depletion layer under an image force, closely to the junction interface. Intensity of an infrared radiation is detected using a multiple reflection effect of hot carriers. According to the infrared sensor, since the position of the effective barrier generated by an influence of a image force is close to the Schottky junction, the attenuation of the energy of the hot carriers is suppressed and the injection yield of the hot carriers passing through the Schottky barrier is increased.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a Schottky barrier infrared sensor or a 
Schottky barrier infrared detector. 
2. Description of the Prior Art 
Rapid advances in the research and development of Schottky barrier infrared 
detectors have been experienced in the art since the proposal and 
practical application of such a Schottky barrier infrared detector by W. 
Kosonocky et al. of RCA laboratories in U.S.A. (Hammam Elabd and Walter F. 
Kosonocky, RCA Review, Vol. 43, pages 569-589, December 1982). Recent 
years have seen the development of several Schottky barrier infrared image 
sensors having a practical level of pixel numbers. The Schottky barrier 
infrared sensor proposed by W. Kosonocky et al. comprises a Schottky 
barrier diode of platinum silicide (PtSi) and p-type silicon doped with 
boron at a concentration ranging from 10.sup.14 to 10.sup.15 cm.sup.2, and 
employs the photoelectric conversion of the Schottky barrier diode for its 
operation principles. It is known that the photoelectric conversion of the 
Schottky barrier infrared sensor can be increased by reducing the 
thickness of the PtSi layer to a level of several nanometers (nm), based 
on the multiple reflection effect of hot holes that are generated by the 
photoelectric conversion. 
Japanese Patent laid-open patent No. Sho-63-237583 (JP, A, 63-237583) of S. 
Toyama discloses an infrared sensor composed of a degenerated 
semiconductor and a nondegenerated semiconductor which are joined to each 
other for increasing the life of hot carriers to increase the efficiency 
and allowing an optimum design of cut-off wavelengths. The degenerated 
semiconductor layer is used to absorb the infrared radiation to generate 
hot carriers, as with the metal film of a Schottky barrier infrared 
sensor. The nondegenerated semiconductor is composed of a potential 
barrier region substantially corresponding to a region where a depletion 
layer is formed and a carrier injection region into which carriers are 
injected from the degenerated semiconductor, with the potential barrier 
region and the degenerated semiconductor layer being directly contacted to 
each other. 
Schottky barrier infrared sensors are grouped into front-irradiated 
Schottky barrier infrared sensors and back-irradiated Schottky barrier 
infrared sensors that are classified according to the surface to which the 
infrared radiation is applied. FIG. 1 of the accompanying drawings shows a 
conventional back-irradiated Schottky barrier infrared sensor. As shown in 
FIG. 1, a p-type silicon (Si) substrate 2 has opposite surfaces ground to 
a mirror finish, one of the surfaces serving as a surface to which an 
infrared radiation 13 is applied and which is coated with an 
anti-reflection film 14 over its entire area. On the other surface of the 
Si substrate 2, there is deposited a PtSi film 1 in the form of a circular 
island having a thickness t. The Si substrate 2 and the PtSi film 1 
jointly make up a Schottky barrier diode. The infrared radiation applied 
to the Schottky barrier diode from the side of the Si substrate 2 is 
absorbed by the PtSi film 1, which generates hot carriers for thereby 
detecting the applied infrared radiation. 
An n.sup.- guard ring 16 is formed in the p-type Si substrate 2 in 
surrounding relation to the PtSi film 1 for lessening an electric field 
concentration to suppress a leak current. The n.sup.- guard ring 16 
extends from the surface of the p-type Si substrate 2 to a certain depth 
therein. The surface of the Si substrate 2 around the PtSi film 1 is 
covered with a thermal oxidization film 17 of SiO.sub.2 which is also 
effective to reduce any leak current. An insulating film 18 is deposited 
on the PtSi film 1, thereby capping the PtSi film 1. The insulating film 
18 is made of silicon oxide (SiO.sub.2) formed by sputtering or the like. 
An ohmic contact 19 is formed in the p-type Si substrate 2, and a contact 
20 is formed in electric contact with the PtSi film 1. A DC power supply 
21 is connected between the ohmic contact 19 and the contact 20 for 
applying a DC voltage across the infrared sensor to maintain a positive 
potential at the PtSi film 1 for thereby keeping the infrared sensor 
reverse-biased. Since infrared radiations to be detected by the infrared 
sensor are generally weak in intensity, attempts have been made to 
increase the detection sensitivity as by utilizing the multiple reflection 
effect of hot carriers in the PtSi film 1. 
FIG. 2 of the accompanying drawings is a potential diagram illustrative of 
operation principles of the conventional infrared sensor described above. 
Since the infrared sensor comprises a Schottky barrier diode, a depletion 
layer is created in the outermost surface and interior region of the 
outermost surface of the p-type Si substrate 2 which contacts the PtSi 
film 1. If not subjected to an image force, the Schottky barrier diode has 
a Schottky barrier height .PHI.b.sub.0 which is reduced quadratically in a 
transverse direction across the p-type Si substrate 2 according to the 
Poisson's equation. A potential in the absence of the image force is 
indicated by the broken-line curve 4 in FIG. 2. Actually, however, since a 
short-range force as the image force acts in the vicinity of a metal or a 
metal silicide, the Schottky barrier diode has a potential distribution 
indicated by the solid-line curve 5 in FIG. 2. The bottom of the potential 
distribution represents the position of an effective barrier which is 
located at a depth d from the Schottky junction interface, the effective 
barrier having a height .PHI.b.sub.effect. If the Schottky barrier diode 
has a junction interface between the p-type Si substrate 2 with a boron 
concentration of 5.times.10.sup.14 cm.sup.-2 and the PtSi film 1 and a 
reverse bias of 0 V is applied, then the depth d of the effective barrier 
from the junction interface is of about 10 nm, the Schottky barrier height 
.PHI.b.sub.0 is of 0.27 eV, and the effective barrier height 
.PHI.b.sub.effect is of about 0.22 eV. 
When an infrared radiation is applied to the Schottky barrier diode, the 
applied infrared radiation is absorbed mainly in the PtSi film 1, which 
generates hot holes. While there is a possibility for the hot holes to 
move in every direction, those of the hot holes moving toward the Si 
substrate 2 which satisfy given conditions at the position of the 
effective barrier further move into the Si substrate 2, and are detected 
as a signal charge when entering deeply in the Si substrate 2 along the 
forward electric field of the depletion layer. Stated otherwise, because 
hot holes are injected as excessive carriers into the p-type Si substrate, 
the applied infrared radiation can be detected by detecting the amount of 
the excessive carriers. 
When hot holes 7 having an energy E.sub.0 are generated in the PtSi film 1 
in response to the application of an infrared radiation to the Schottky 
barrier diode, as shown in FIG. 2, some of the hot holes 7 reach the 
position of the effective barrier in the Si substrate 2. At this time, 
they lose part of their energy, and become hot holes 8 having an energy 
E.sub.1. The momentum of hot holes has the same probability in every 
direction at all times because of the scattering effect at a shorter 
distance than the energy attenuation length. If the component of the 
momentum of a hot hole which is perpendicular to the barrier is greater 
than .sqroot.2m*.PHI.b.sub.effect at the position of the effective 
barrier where m* is the effective mass of the hot hole, then the hot hole 
moves over the effective barrier and enters deeply into the Si substrate 
2. Otherwise, the hot hole is reflected by the effective barrier. After 
being scattered and reflected, the reflected hot hole returns as an energy 
particle 9 having an energy E.sub.3 back to the position of the effective 
barrier. Some of such energy particles move over the effective barrier as 
with the hot hole having the energy E.sub.1. 
The number of hot holes which move over the effective barrier increases 
through the above process. This process is referred to as the multiple 
reflection effect of hot holes, and is repeated until the energy of hot 
holes decreases to a level lower than an effective Schottky barrier energy 
Eb.sub.effect. To promote the multiple reflection effect, W. Kosonocky et 
al. have proposed to reduce the thickness t of the PtSi film to several nm 
to suppress the energy attenuation per reflection, and indicated the 
effects of the proposal. W. Kosonocky et al. have also indicated that if 
the quantum efficiency of the infrared sensor is represented by Y*, the 
Planck's constant by h, the frequency of the infrared radiation by .nu., 
and a parameter derived from the potential distribution by Cl*, then the 
following equation is satisfied: 
##EQU1## 
W. Kosonocky et al. have ignored the attenuation of the energy of hot holes 
in the interval from the Schottky junction interface to the position of 
the effective barrier. While W. Kosonocky et al. have derived the above 
equation (1) based on the assumption of the equation: 
EQU h.nu..apprxeq..PHI.b.sub.effect ( 2) 
from an appropriate equation: 
##EQU2## 
However, the precondition represented by the equation (2) is not correct, 
and hence the equation (1) is in error. 
Nevertheless, the equation (1) is in general use because its error has gone 
unnoticed. For Schottky barrier infrared sensors proposed after the 
proposal by W. Kosonocky et al., their characteristics have been studied 
without any concern at all about the attenuation of the energy of hot 
holes in the interval from the Schottky junction interface to the position 
of the effective barrier. For example, Japanese Patent laid-open No. 
Hei-4-111467 (JP, A, 4-111467) discloses nothing about the distance d from 
the junction interface to the effective barrier position while a reference 
is made to the control of the effective barrier with the image force. 
As described above, no considerations have heretofore been given to the 
attenuation of the energy of hot holes in the interval from the Schottky 
junction interface to the position of the effective barrier. In PtSi/Si 
Schottky barrier diodes, however, the distance d is large enough not to be 
neglected because it is of several nm, which also represent an optimum 
thickness for the PtSi film. Although the PtSi film should be sufficiently 
thick as it serves as a layer for absorbing applied infrared radiation, 
its thickness is selected to be of several nanometers to promote the 
multiple reflection effect. Consequently, in order to utilize the multiple 
reflection effect, it is necessary to take into account the distance d 
which is about the same as the thickness of the PtSi film. In the 
conventional Schottky barrier infrared sensors, the distance from the 
Schottky junction interface to the position of the effective barrier is 
large under the influence of the image force, and hence the energy which 
is lost by reflected hot carriers before they are injected into the Si 
substrate is so large that the sensitivity is not increased as much as 
expected according to the multiple reflection effect. It is important to 
reduce the distance d down to the effective barrier position for 
increasing the sensitivity of infrared detection because a reduction in 
the distance d can result in an increase in the sensitivity according to 
the multiple reflection effect. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a Schottky barrier 
infrared sensor which has a reduced distance from a Schottky junction 
interface to the position of an effective barrier for achieving an 
increase in the sensitivity of infrared detection according to the 
multiple reflection effect. 
According to the present invention, the above object can be achieved by a 
Schottky barrier infrared sensor comprising a Schottky barrier diode 
composed of a first layer comprising a semiconductor of a first 
conductivity type and a second layer comprising a metal or a metal 
silicide, the first layer and the second layer being joined to each other, 
for detecting an incident infrared radiation based on the amount of 
excessive carriers generated in the first layer in response to passage of 
hot carriers through a depletion layer formed in the first layer in a 
position corresponding to a junction interface between the first layer and 
the second layer, said hot carriers being generated in the second layer by 
the incident infrared radiation, and a third layer disposed in the 
depletion layer formed in the first layer out of contact with the junction 
interface, the third layer containing an impurity which is introduced for 
positioning an effective barrier formed in the depletion layer under an 
image force, closely to the junction interface. 
The above and other objects, features, and advantages of the present 
invention will become apparent from the following description when taken 
in conjunction with the accompanying drawings which illustrate preferred 
embodiments of the present invention by way of example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 shows a Schottky barrier infrared sensor according to a first 
embodiment of the present invention. Those parts of the Schottky barrier 
infrared sensor shown in FIG. 3 which are identical to those of the 
conventional Schottky barrier infrared sensor shown in FIG. 1 are denoted 
by identical reference numerals. 
The Schottky barrier infrared sensor shown in FIG. 3 differs from the 
conventional Schottky barrier infrared sensor shown in FIG. 1 in that a 
very thin n-type impurity-introduced layer 10 is formed in a depletion 
layer region of a Schottky diode in a p-type Si substrate 2. The n-type 
impurity-introduced layer 10 is disposed immediately below the Schottky 
junction interface of the p-type Si substrate 2 out of direct contact with 
the Schottky junction interface. Specifically, the n-type 
impurity-introduced layer 10 lies at a depth ranging from 1 nm to 5 nm 
from the Schottky junction interface. The n-type impurity-introduced layer 
10 may be formed by molecular beam epitaxy (MBE) growth or delta (.delta.) 
doping techniques such as ion implantation. 
FIG. 4 shows a potential distribution across the Schottky barrier infrared 
sensor shown in FIG. 3. In FIG. 4, the height of a Schottky barrier in the 
absence of the image force is represented by .PHI.b.sub.0, and a potential 
curve in the absence of the image force is represented by the broken line 
4a. The height of an actual Schottky barrier in the presence of the image 
force is represented by .PHI.b.sub.effect ', and a potential curve in the 
presence of the image force is represented by the solid line 5a. The 
distance d' from the Schottky junction interface to the position of an 
effective barrier is smaller than the distance d in the conventional 
Schottky barrier infrared sensor shown in FIG. 3. The potential curve is 
relatively flat in the vicinity of the position of the effective barrier. 
When hot holes 7 having an energy E.sub.0 are generated in the PtSi film 1 
in response to the incidence of an infrared radiation to the Schottky 
barrier diode, as shown in FIG. 4, some of the hot holes 7 reach the 
position of the effective barrier in the Si substrate 2, as hot holes 8a 
having an energy E.sub.1 '. Some of these hot holes 8a move over the 
effective barrier into the Si substrate 2. After being scattered and 
reflected, some of the hot holes which cannot move over the effective 
barrier reach the position of the effective barrier again as hot holes 9a 
having an energy E.sub.3 '. Some of the hot holes 9a move over the 
effective barrier into the Si substrate 2, thus increasing a signal 
charge. 
If the energy attenuation constant of hot holes in the p-type Si substrate 
2 is fixed irrespective of the impurity concentration in the substrate, 
then since the relationships: E.sub.1 '&gt;E.sub.1 'E.sub.3 '&gt;E.sub.3 are 
satisfied by the reduction of the distance from the Schottky junction 
interface to the effective barrier position from d to d', the probability 
that hot holes move over the effective barrier is increased. Provided that 
the governing factor for determining the attenuation constant in the Si 
substrate 2 is phonon scattering, the assumption that the energy 
attenuation constant is fixed irrespective of the impurity concentration 
is appropriate. In the present embodiment, therefore, since the 
attenuation of the energy of hot holes is suppressed by reducing the 
distance from the Schottky junction interface to the effective barrier 
position, the number of hot holes moving over the effective barrier is 
increased, thereby increasing the sensitivity of infrared detection. 
The difference between the impurity introduction according to the present 
embodiment and the interface impurity introduction for controlling the 
Schottky barrier height will be described below. According to the present 
invention, the impurity is introduced not for controlling the Schottky 
barrier height, but for controlling the position of the effective barrier. 
The Schottky barrier height is sensitive to the condition of the interface 
between a semiconductor (which is made of Si in the embodiment) and a 
metal (or a metal silicide). In the present embodiment, the impurity is 
introduced out of contact with the Schottky junction interface in order to 
achieve the same Schottky barrier height as if no impurity-introduced 
layer were formed in the depletion layer. According to the interface 
impurity introduction for controlling the Schottky barrier height, 
however, the impurity-introduced layer and the metal are directly joined 
to each other at the Schottky junction interface. In the infrared sensor 
disclosed in Japanese patent laid-open No. 63-237583, the degenerated 
semiconductor absorbs the incident infrared radiation, and the degenerated 
semiconductor and the potential barrier region, which corresponds to the 
impurity-introduced layer in the present embodiment, are directly joined 
to each other, unlike the infrared sensor in this embodiment. 
FIG. 5 shows a Schottky barrier infrared sensor according to a second 
embodiment of the present invention. Those parts of the Schottky barrier 
infrared sensor shown in FIG. 5 which are identical to those of the 
Schottky barrier infrared sensor shown in FIG. 3 are denoted by identical 
reference numerals. 
The Schottky barrier infrared sensor shown in FIG. 5 differs from the 
Schottky barrier infrared sensor shown in FIG. 3 in that a p.sup.+ 
impurity-introduced layer 11 is employed in place of the n-type 
impurity-introduced layer. The p.sup.+ impurity-introduced layer 11 is 
formed in a depletion layer region of a Schottky diode in a p-type Si 
substrate 2. The p.sup.+ impurity-introduced layer 11 is disposed 
immediately below the Schottky junction interface of the p-type Si 
substrate 2 out of direct contact with the Schottky junction interface. 
Specifically, the p.sup.+ impurity-introduced layer 11 lies at a depth 
ranging from 1 nm to 5 nm from the Schottky junction interface. The 
p.sup.+ impurity-introduced layer 11 may be formed by molecular beam 
epitaxy (MBE) growth or delta doping techniques such as ion implantation. 
FIG. 6 illustrates a potential distribution across the Schottky barrier 
infrared sensor shown in FIG. 5. In FIG. 6, the height of a Schottky 
barrier in the absence of the image force is represented by .PHI.b.sub.0, 
and a potential curve in the absence of the image force is represented by 
the broken line 4b. The height of an actual Schottky barrier in the 
presence of the image force is represented by .PHI.b.sub.effect ", and a 
potential curve in the presence of the image force is represented by the 
solid line 5b. The distance d" from the Schottky junction interface to the 
position of an effective barrier is smaller than the distance d in the 
conventional Schottky barrier infrared sensor shown in FIG. 3. The 
potential curve is steeper than the potential curve in FIG. 2 in the 
vicinity of the position of the effective barrier. 
When hot holes 7 having an energy E.sub.0 are generated in the PtSi film 1 
in response to the incidence of an infrared radiation to the Schottky 
barrier diode, as shown in FIG. 6, some of the hot holes 7 reach the 
position of the effective barrier in the Si substrate 2, as hot holes 8b 
having an energy E.sub.2 ". Some of these hot holes 8b move over the 
effective barrier into the Si substrate 2. After being scattered and 
reflected, some of the hot holes which cannot move over the effective 
barrier reach the position of the effective barrier again as hot holes 9b 
having an energy E.sub.3 ". Some of the hot holes 9b move over the 
effective barrier into the Si substrate 2, thus increasing a signal 
charge. If the energy attenuation constant of hot holes in the p-type Si 
substrate 2 is fixed irrespective of the impurity concentration in the 
substrate, then since the relationships: E.sub.2 "&gt;E.sub.2 ", E.sub.3 
"&gt;E.sub.3 are satisfied by the reduction of the distance from the Schottky 
junction interface to the effective barrier position from d to d", the 
probability that hot holes move over the effective barrier is increased, 
resulting in an increase in the sensitivity of infrared detection. 
The principles of the present invention are also applicable to general 
infrared sensors composed of a Schottky barrier diode which comprises a 
semiconductor and a metal or a metal silicide. For example, an n-type Si 
substrate may be used as a semiconductor substrate, and hot electrons may 
be employed as hot carriers. Semiconductors other than silicon may also be 
used. A metal or a metal silicide for forming a Schottky junction is 
selected depending on the semiconductor used and the desired wavelength to 
be detected. 
Although certain preferred embodiments of the present invention have been 
shown and described in detail, it should be understood that various 
changes and modifications may be made therein without departing from the 
scope of the appended claims.