JPH0634407B2 - Photoelectric conversion element and manufacturing method thereof - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a photoelectric conversion element, and more particularly to a photoelectric conversion element used in an image input section of a facsimile or the like.

[Prior Art] Recently, in order to reduce the size of an image input unit of a facsimile or the like, a long reading element having the same size as a document has been actively developed. As the long reading element, a sandwich formed by using amorphous silicon (a-Si: H) as a photoconductor and sandwiching the amorphous silicon (a-Si: H) with a first electrode made of metal or the like and a transparent second electrode is used. A thin-film light receiving element having a structure is expected to have a wide range of uses because it has a fast photoresponse and excellent environment resistance.

Amorphous silicon has a larger area than single crystal silicon and can be deposited at a low temperature of about 300 ° C., so an element using this is easy to manufacture and has a light absorption coefficient of 1 in the visible region.
Since it is more than an order of magnitude larger, the film thickness is as thin as about 1 μm, which is drawing attention.

However, looking at the voltage-current characteristics of this element, there was the inconvenience that the dark current increased with the voltage and the bright / dark ratio could not be sufficiently taken. This is because the potential barrier is small at the interface between the transparent second electrode and amorphous silicon,
It is considered that this is because the injection of electrons from the transparent second electrode is likely to occur.

[Object of the Invention]

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a photoelectric conversion element that suppresses dark current and has a sufficiently large contrast ratio.

[Structure of Invention]

The photoelectric conversion element of the present invention has an amorphous silicon layer,
It is characterized in that the joint portion with the transparent electrode made of a metal oxide is a mutual diffusion layer of the both. That is,
The junction has a composition in which a metal oxide is contained in the amorphous silicon film, the band gap of the amorphous silicon is effectively increased, and the potential barrier between the metal oxide and the amorphous silicon is increased. Since the potential barrier blocks the injection of electrons from the electrode into the amorphous silicon layer, the dark current is 1/100 times that of conventional devices.
It is suppressed to 10 or less.

The diffusion layer of this joint portion is formed at a low temperature of about room temperature, and while suppressing the particle energy, after depositing a transparent electrode made of a metal oxide layer on the amorphous silicon layer, the diffusion layer in an oxygen atmosphere It is formed by applying a heat treatment (annealing) at a temperature of ℃ to 250 ℃. That is, while suppressing the particle energy so as not to give damage to the amorphous silicon layer, the second electrode is formed on the amorphous silicon layer, and then heat treatment is performed, whereby oxygen O is directly bonded to silicon, and Si -O bond is generated, and this Si-
An interdiffusion layer in which the constituent elements are truly diffused is formed without the O bond hindering the diffusion of oxygen O in the subsequent heat treatment. For example, when indium tin oxide is used as the second electrode, it is possible to form an interdiffusion layer having a state in which silicon is diffused in indium tin oxide and oxygen, tin, or indium is diffused in amorphous silicon, The presence of this interdiffusion layer effectively increases the bandgap of amorphous silicon, thereby increasing the potential barrier between the second electrode and the photoconductor layer.

Example of Invention

Hereinafter, the photoelectric conversion element of the embodiment of the present invention will be described with reference to the drawings.

The photoelectric conversion element of the embodiment of the present invention has a plan view in FIG. 1 and an AA cross section in FIG. 2, and an insulating glass substrate 1 and a plurality of films formed in a predetermined shape on the glass substrate. One chromium (Cr) electrode 2, an amorphous silicon layer 3 integrally formed on the chromium electrode 2, and a diffusion layer 4 formed on the amorphous silicon layer 3 so as to be integrated on the amorphous silicon layer 3. And an optically transparent indium tin oxide (ITO) electrode 5 formed.
Here, the chrome electrode 2 has a thickness of about 3000 .ANG., The amorphous silicon layer 3 has a thickness of about 1 .mu.m, and the indium tin oxide electrode has a thickness of 0.1 .mu.m.

Next, a method for manufacturing the photoelectric conversion element will be described.

First, a chromium thin film is deposited on the glass substrate 1 by an electron beam evaporation method to a thickness of about 3000 Å. At this time, the substrate temperature is
250 ℃ Then, the plurality of chromium electrodes 2 are formed by dividing into a desired shape by photolithography.

Furthermore, by the plasma chemical vapor deposition method (plasma CVD method),
An amorphous silicon layer as a photoconductor is deposited to a thickness of 1 μm. As the manufacturing conditions, monosilane (SiH ₄ ) gas is used, the substrate temperature is 200 to 300 ° C., the pressure is 0.2 to 0.5 Torr, and the power density is 20 to 70 mw / cm ² .

Then, an indium tin oxide film as a transparent electrode is deposited by about 0.1 μm by the DC sputtering method. The deposition conditions are as follows: Indium tin oxide (90 mol%)
Indium oxide In ₂ O ₃ +10 mol% tin oxide sno ₂ ) is used, oxygen partial pressure is 0.5 to 1.5 × 10 ⁻⁴ Torr, total pressure (argon Ar + oxygen O ₂ ) = 1 to 5 × 10 ⁻³ Torr substrate temperature 1
The temperature is set to 0 to 40 ° C., and the power density is initially 30 mw / cm ² to a film thickness of 100 Å, and then 200 mw / cm ² to a total film thickness of 0.1 μm.

Finally, the treatment is carried out in an oxygen + argon (Ar) atmosphere at 200 ° C. or in the air for about 30 minutes.

The current-voltage characteristics of the photoelectric conversion element thus formed are shown in FIG. In FIG. 3, the horizontal axis represents voltage (V), the vertical axis represents current (A), the curve A represents the photocurrent-voltage characteristic curve, and the curve B represents the dark current current-voltage characteristic curve.

In the measurement, the chromium electrode side was set to ground potential, and a negative bias was applied to the indium tin oxide film side. A green fluorescent lamp was used as a light source when measuring the photocurrent, and the illuminance was adjusted to be 100 lux (lx).

For comparison, the characteristic curve of the conventional photoelectric conversion element is shown by a dotted line. Dotted line C and dotted line D show the current-voltage characteristic curve of photocurrent and the current-voltage characteristic curve of dark current of the conventional photoelectric conversion element, respectively. This conventional photoelectric conversion element has no diffusion layer 4 as compared with the photoelectric conversion elements of the examples of the present invention shown by the curves A and B-that is, the final heat treatment is not performed in the manufacturing process. All are the same except for the differences, and the measurement conditions are also the same.

As is clear from the comparison between the curves A and B and the dotted lines C and D, the photoelectric conversion element of the present invention has a significantly reduced dark current as compared with the conventional photoelectric conversion element, and the light-dark ratio is improved by one digit or more. ing.

Furthermore, the film deposition conditions for the indium tin oxide electrode will be considered.

For comparison, the same process is performed up to the formation of the amorphous silicon layer, the power density is set to 200 mw / cm ² from the beginning during DC sputtering, the deposition rate is increased, and the indium tin oxide electrode is formed. The photocurrent and dark current characteristic curves of the photoelectric conversion element formed through the heat treatment step are shown by dashed-dotted lines E and F, respectively. The measurement conditions are the same as the measurement conditions of the examples of the present invention.

As is clear from the comparison between the curves A and B and the alternate long and short dash lines E and F, if the initial deposition rate of the indium tin oxide film by the DC sputtering method is high, the amount of dark current is reduced even after the diffusion process by heat treatment. It is hardly reduced but rather increased. Therefore, the contrast ratio is also poor.

FIG. 4 and FIG. 5 show the results of Auger analysis for the elemental composition in the vicinity of the amorphous silicon-indium tin oxide junction surface in the photoelectric conversion element of the present invention and the conventional example, respectively. Generally in Auger analysis,
Although only the element can be specified, in the case of Si, the energy shift due to oxidation is 13 eV (see "Practical Auger electron spectroscopy", published by Kyoritsu Shuppan Co., Ltd., edited by Shimizu and Yoshihara, page 106), and the presence or absence of oxidation can be identified. It is possible. Here, while etching is performed from the indium tin oxide film side to the amorphous silicon side by ion etching with xenon (Xe) gas, O, Sn, In, Si,
The height of the output peak at the energy position of Si—O was measured and plotted in FIGS. 4 and 5. In both FIG. 4 and FIG. 5, the vertical axis represents the content rate (%),
The horizontal axis shows the etching time (minutes).

As is clear from FIG. 4, in the photoelectric conversion element of the embodiment of the present invention, silicon Si is formed in the indium tin oxide film at the junction of indium tin oxide and amorphous silicon.
However, oxygen O and indium I are contained in amorphous silicon.
n and tin Sn are mutually diffused. Moreover, since the characteristic peak that appears at the time of the Si-O bond does not appear, there are few oxygen diffused in the amorphous silicon that is directly bonded to silicon, and rather, most of it is of indium tin oxide. It is considered that they are diffused in the amorphous silicon as fine crystals or amorphous.

FIG. 5 is a graph of the photoelectric conversion device whose measurement results are shown in FIG. As a result, a bond peak of silicon-oxygen (Si-O) appears at the joint between indium tin oxide and amorphous silicon, a reaction occurs at the joint, and it turns into silicon oxide SiO ₂ and almost no mutual diffusion occurs. You can see that it is not done.

The reason why the silicon oxide SiO ₂ is formed in this way is that the initial power density is high during sputtering,
It is considered that this is because the bonds of silicon-silicon (Si-Si) and silicon-hydrogen (Si-H) are broken during the sputtering and bond with oxygen O.

〔The invention's effect〕

As described above, according to the present invention, a photoelectric conversion device having a sandwich structure in which an amorphous silicon layer as a photoconductor is sandwiched between a first electrode and a translucent second electrode made of a metal oxide. In the conversion element, oxygen O is directly bonded to silicon at the bonding portion between the metal oxide and the amorphous silicon layer to generate a Si—O bond.
It is considered that the band gap of the amorphous silicon is widened by forming the interdiffusion layer in which the constituent elements are mutually diffused without the O bond hindering the diffusion of oxygen O in the subsequent heat treatment. Ie ITO
It is considered that the amorphous silicon in which the components are diffused has In and O contained in the amorphous silicon, and thus, unlike the amorphous silicon in the bulk, the electron affinity and the band gap are also changed. On the other hand, when a silicon oxide layer is formed at the interface between the amorphous silicon layer and the ITO layer as in the case of the conventional structure, positive charge is trapped in the silicon oxide at the interface, and the effective potential barrier height increases. Go down. Thus, according to the structure of the present invention, the amorphous silicon layer and the I
Compared with the case where a silicon oxide layer is formed at the interface with the TO layer, the potential barrier for electrons between indium tin oxide and amorphous silicon is increased, and injection of electrons from the indium tin oxide side into amorphous silicon is suppressed. As a result, the dark current can be reduced, and the light-dark ratio can be significantly improved.

[Brief description of drawings]

FIG. 1 is a plan view of a photoelectric conversion element according to an embodiment of the present invention, and FIG.
1 is a sectional view taken along the line AA in FIG. 1, FIG. 3 is a diagram showing current-voltage characteristic curves of the photoelectric conversion device of the present invention, and
FIG. 4 is a diagram showing the analysis results of the composition elements of the junction portion of the photoelectric conversion element of the embodiment of the present invention, and FIG. 5 is the analysis result of the conventional example for comparison with the results shown in FIG. FIG. 1 ... Substrate, 2 ... Chrome electrode, 3 ... Amorphous silicon layer, 4 ... Diffusion layer, 5 ... Indium tin oxide electrode A ... Current-voltage characteristic curve of photocurrent in the photoelectric conversion element of the embodiment of the present invention B ... Current-voltage characteristic curve of same dark current C, E ... Current-voltage characteristic curve of photocurrent in photoelectric conversion element of conventional example D, F ... Current-voltage characteristic curve of same dark current.

Claims (3)

[Claims] 1. A photoelectric conversion element comprising an amorphous silicon layer as a photoelectric body sandwiched between a first electrode and a translucent second electrode made of a metal oxide, wherein the second electrode and the amorphous silicon layer. At the junction with the layers,
A photoelectric conversion element comprising a mutual diffusion layer configured such that constituent elements Si and O, which are constituent elements, do not bond to each other and the constituent elements diffuse into each other to increase a potential barrier. 2. The photoelectric conversion element according to claim 1, wherein the metal oxide is any one of indium tin oxide, tin oxide, and cadmium tin oxide. 3. A first electrode forming step of forming a first electrode on a substrate, a photoconductor layer forming step of forming an amorphous silicon layer as a photoconductor layer, and a mutual reaction in the amorphous silicon layer. A second electrode forming step of forming a second electrode made of a metal oxide on the amorphous silicon layer while suppressing particle energy so that the energy does not exist, and at a joint portion between the amorphous silicon layer and the second electrode, A heating step of heating to 150 to 250 ° C. in an oxygen-containing atmosphere in order to form an interdiffusion layer in which constituent elements are mutually diffused without bonding Si and O, which are constituent elements, to each other. And a method for manufacturing a photoelectric conversion element.