JPS628579A - Photoconductive thin film and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain a photoconductive thin film characterized by broad spectral sensitivity and quick response speed, by the constituted, wherein the solid solution of CdS, CdSe and CdTe is a main component, 2-20mol% CdTe is included in the solid solution and Cu and Cl are also included. CONSTITUTION:The composition ratio of CdTe against CdS0.6Se0.4 is made to be (x). CdS0.6(1-x)Se0.4(1-x)Tex, wherein (x) is changed in the range of 0-0.3, is used as a main component. In this component, 0.2mol% CuCl2 is included, and a solid solution is formed. With the solid solution as an evaporation source, the thin solid solution film of CdS-CdSe-CdTe is formed on a glass substrate, whose dimensions are 50X280X1.2mm<3>, to a thickness of about 4,000Angstrom by vacuum evaporation. N2 is introduced in a quartz core tube at a rate of 0.3l/min and the sample is heat-treated in the tube at 550 deg.C for one hour. Al counter electrodes are formed by using a metal mask. Thus, the manufacturing method is simplified, and the photoconductive thin film, which is characterized by small change at a point of temperature change of Jp and broad spectral sensitivity can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a photoconductive thin film mainly composed of a Cd5-Cd5e-CdTe solid solution and a method for producing the same.

Conventional technology Conventionally, thin films have been formed by vacuum evaporation on glass substrates using CdS, CdSe, or their solid solutions with trace amounts of Cu, Ag, halogen, etc. added as an evaporation source.
By heating in a neutral atmosphere containing CdC11 or in air, photoconductivity was imparted and a photoconductor was manufactured.

For example, such a photoconductor is formed into a -dimensional linear array, and further electrodes are formed to form an optical sensor, and it can be used as a contact image sensor for reading documents such as facsimiles, in which the sensors are arranged in the same size as the document. The application of is progressing. Among such contact type image sensors that utilize thin films, those that use a solid solution of CdS and CdSe are already in practical use. Also, amorphous 5i
(a-8i) is also being developed.

On the other hand, devices using a single-crystal Si process have been developed, such as one in which COD chips are arranged in a straight line, and one in which bipolar transistors are formed on Si chips arranged in a straight line. Products using COD or bipolar transistors can achieve high performance because they can be processed at high speeds and microfabricated, but the process is complicated and the cost is high, and there is no way to create a complete lens that can reduce costs. Wireless type (e.g. "Study of direct reading contact type image sensor element configuration" Institute of Electronics and Communication Engineers technical research report IF
! 8l-35) is essentially impossible, and has the disadvantage of high overall cost including the cost of materials. In addition, for those using a-8i, a-8i and poly-5i (poly-8i) TPs can be formed in the same process.
Integration using T is also possible, but it has the disadvantage that the processing circuit becomes complicated because the signal is weak, and when using poly-8i, high temperature processing is required, so the substrate It also has disadvantages such as high cost. The solid solution of CdS and CdSe has the disadvantage of slow photoresponse speed, but since it is formed using a thin film process, it can be made without a lens, and the photocurrent is large, so the circuit configuration is simple. have.

Currently, increasing the speed of this optical sensor is a major technical issue, so for example, it is possible to increase the optical response speed by applying continuous bias light with a composition ratio (molar ratio) of CdS and CdSe of 2 = 8. has been attempted ('
"High-speed, high-sensitivity thin-film image sensor" 83rd Research Meeting of the Institute of Image Electronics Engineers, 84-05-8).

However, the problem to be solved by the present invention is that the solid solution of CdS and C-dse requires heat treatment at 500 to 600° C. in an atmosphere containing CdC11 and vapor in order to provide photosensitivity.

However, in order to process in an atmosphere of CdCa2 with a uniform concentration, one alumina boat is required for one substrate, which is a major drawback in production as only one substrate can be heat-treated at a time. There is.

In addition, the optical sensor based on Cd5-CdSe is CdSe
The higher the composition ratio, the faster the photoresponse speed will be.

The effect is particularly remarkable when using it while applying bias light, so the composition ratio of CdS and Cd55 is set to 2:8, and speeding up is achieved by using it while applying bias light. however.

The peak of the spectral sensitivity of this type of optical sensor moves to the long wavelength side when the CdSe composition ratio is increased, and to the short wavelength side when it is decreased.
It has a disadvantage that the sensitivity is low in the vicinity of 400 to 450 mm on the short wavelength side required for color sensors. In addition, the photocurrent Jp changes depending on the ambient temperature, and as the composition ratio of Cd5a increases, the photocurrent Jp changes significantly @Cd
In the case of S 6 , z S ea , s, Jp is about 176 in the range of -20°C to 60°C. For this reason, correction is required in terms of the circuit, which has the drawback of increasing costs. In order to eliminate these drawbacks, C
Although it is better to reduce the composition ratio of dSe, problems such as a decrease in sensitivity on the long wavelength side and a decrease in optical response speed occur.

The present invention solves the above-mentioned problems, and has a simpler manufacturing method than the conventional ones, small temperature changes in JP, wide spectral sensitivity, and response speed that is equal to or better than the conventional ones. The object of the present invention is to provide an excellent photoconductive thin film and a method for manufacturing the same.

Means for Solving the Problems In order to solve the above problems, the photoconductive thin film of the present invention is mainly composed of a solid solution of CdS, CdSe, and CdTe, and CdTe is contained in the solid solution in a composition ratio of 2 to 20 mol. Including %,
This has a structure containing Cu and (jL) as impurities.

Furthermore, the photoconductive thin film of the present invention is composed of CdS and Cd55.
and CdTe in the solid solution.

contains 2 to 20 mol% of CdTs, and CuC
It is obtained by heating and evaporating an evaporation source to which 9,2 has been added to form a thin film on a substrate, and then heat-treating this thin film in a neutral atmosphere or in air.

That is, a trace amount of impurity Cu formed by vapor deposition,
By heat-treating a Cd5-Cd5e-CdTe solid solution thin film containing Ca in a neutral atmosphere or air,
By forming a sensitizing center and a recombination center, the photocurrent JP is increased and the dark current J is decreased, thereby providing a function as a photosensor. It has practical sensitivity over a wide wavelength range,
Furthermore, a photoconductive thin film can be obtained in which the temperature change in photocurrent JP is small and the photoresponse speed is equivalent to or better than that of conventional ones.

Effect As mentioned above, the molar ratio (20 mol% or less) is such that temperature changes do not become a problem in the Cd5-Cd5e-CdTe system.
Cd5-Cd5a-CdTe with added CdTe
By heat-treating the solid solution thin film, it is possible to obtain a photoconductive thin film in which the sensitivity on the long wavelength side is increased without increasing the temperature change in JP, and in which Jp is sufficiently large and J is sufficiently small. .

EXAMPLE An example of the present invention will be described below based on the drawings. In the manufacturing method according to the present invention, Cd5o, , (, -2) 5e(1,4(t
-) OTe as the main component, and 0.2 mol% Cu
Create a solid solution including Caz, and use this solid solution as an evaporation source to form a Cd5-Cd5e-CdTe solid solution thin film with a film thickness of about 4000 by vacuum evaporation on a 50x280x1.2m+' glass substrate (Corning 7059). did. This sample was heated in a quartz furnace tube at O, 3n/f.
Heat treatment was performed at 550° C. for 1 hour while introducing f1 in of N2. For samples of 0 or more, a counter electrode (width 1 mm, length 2 m) was formed by AM using a metal mask.
where, photocurrent Jp, dark current Jd, rising response speed due to signal light τ1, falling response speed τd, rising response speed when applying bias light is b, falling response speed is db, spectral sensitivity, temperature dependence The sex was measured.

Note that τ is signal light (1001ux, IHz flashing light)
τ4 represents the time it takes for JP to reach 1/2 of its saturation value from 0 due to the rising edge of .

In addition, τ and b represent the time required for the increase in Jp due to the signal light to reach 1/2 of its saturation value from 0 when continuous bias light is applied, and τdb is the time required for the increase in Jp due to the signal light to reach 1/2 of its saturation value.
It represents the time until the increase in p decreases from the saturation value to 1/2 of the saturation value.

Furthermore, for comparison, a sample was prepared using a conventional method and similar measurements were performed. In the conventional manufacturing method, the molar ratio of CdS and CdSe is 2=8, that is, CdS,...Se
,,8 as the main component, and 0.2 mol% of CuC(L
The same < 50
280 x 1.2an” glass substrate (Corning 7
059) Cd5 with a film thickness of about 4000 by vacuum evaporation on
A -CdSe solid solution thin film was formed. Place this sample at the bottom.
d5-CdC11 powder was placed in an alumina boat, and heat treatment was performed at 550° C. for 1 hour.

A counter electrode (width 1
1II11. 2 mm in length) was formed by vapor deposition. Similar measurements were performed on the sample.

The spectral sensitivity and temperature dependence are as shown in FIGS. 1 and 2, and the curves (1) to (5) in FIG. 1 and the curves (1) to (6) in FIG. The curve (6) in FIG. 1 and the curve (7) in FIG. 2 show the conventional example.

In Figure 1, the spectral sensitivity region of the Cd5-Cd5e-CdTe optical sensor changes depending on the composition ratio of CdSe and CdTe, and this change is shown in Cd5o, Sel+4.
Cd5a, 5(t-x) 5ea4 with CdTe added to
A comparison of curves (1) and (6) shows the case where X=O~0.2 in the Cx-x)Tex system.

If CdTe is not included, by increasing Cdge.

As shown in curve (6), the peak only moves while keeping the same shape, and the sensitivity increases on the long wavelength side, and
The sensitivity on the short wavelength side decreases. However, curves (2) to (5)
As in, when CdTe is included, the sensitivity on the long wavelength side can be increased without reducing the sensitivity on the short wavelength side too much,
The sensitivity on the long wavelength side also increases with the amount of increase in Cd Te.

In addition, in Fig. 2, the temperature change of JP of the Cd5-Cd5e-CdTe optical sensor is different from that of CdSe and CdT.
It changes depending on the composition ratio of e. Here, curves (1) to (7
), if CdTe is not included, CdSe
It can be seen that the temperature change increases as Cd increases.
Depending on the increase in Te, as shown in curves (1) to (5), the temperature change up to 20 mol % of CdTe is not so large.

In addition, in this implementation method, at 20 so, for example, when x = 0.1, when DCIOV is applied, J p (
1001ux)=2.5X10-'(A), Jd:5
X 10-” (A), and the photoresponse speed was
In the same case < x = o, i, when the signal light intensity is 1001ux, t = 5.0m5ec, c d = 1.9m5ec, and when the signal light intensity is 4001ux, τP = 2.1m5ec,
t,1=1.1 m5ec. On the other hand, the response speed when applying bias light is frb = 0.48m when the signal light intensity is 1001ux and the bias light intensity is 401ux.
5ec, tdl, = 0.92m5ec.

On the other hand, in the conventional method, D
When CIOV is applied, J p (1001ux) =
2.8XIO-'(A), Jd=3×1O-1o(A
) was, also.

When the signal light intensity is 1001ux, the optical response speed is τ or =
5.0+wsec, t, 1 = 1, 2m5ec, and at 4001ux, τ1 = 1.5ssec,
td=0.4++sac. On the other hand, the response speed when applying negative light is that the signal light intensity is 1001ux,
When the bias light intensity is 401ux, τPb=0.75
m5ec, ? dl, = 0.74 m5ec.

Furthermore, it was confirmed that even when X in the present method is other than 0.1 within the range of 0.02 to 0.2, the optical response speed is equivalent to or higher than that of the conventional method.

In the above manufacturing method, Cd5-CdSe-Cd
The heat treatment of the solid solution thin film of Te may be carried out in a neutral atmosphere or in air, and the temperature at that time may be 480 to 600°C. This is because JP is small and This is because Jd is large, and at temperatures above 600° C., re-evaporation of the film occurs and JP becomes small again.

Furthermore, this is because it is formed using a thin film process.

It is clear that the present invention can also be used in a so-called complete contact type image sensor that does not require a lens.

Effects of the Invention According to the present invention, compared to conventional products, the manufacturing method is simpler, the change in JP temperature is small, the spectral sensitivity is wide, and the optical response speed is improved. It is possible to obtain an excellent photoconductive thin film with performance equivalent to or better than that of the conventional method, and its value is extremely high.

[Brief explanation of the drawing]

FIG. 1 is a characteristic diagram showing the spectral sensitivity of the photoconductive thin films of the present invention and the conventional example, and FIG. 2 is a characteristic diagram showing the temperature dependence of Jp of the photoconductive thin films of the present invention and the conventional example.

Claims (1)

[Scope of Claims] 1. A photoconductive thin film mainly composed of a solid solution of CdS, CdSe, and CdTe containing 2 to 20 mol% of CdTe, and containing Cu and Cl as impurities. 2. An evaporation source consisting mainly of a solid solution of CdS, CdSe, and CdTe containing 2 to 20 mol% of CdTe, to which CuCl_2 is added, is heated and evaporated to form a solid solution thin film on the substrate, and this thin film is neutralized. A method for producing a photoconductive thin film by heat treatment in an atmosphere or air. 3. The method for producing a photoconductive thin film according to claim 2, wherein the temperature of the heat treatment is 480 to 600°C.