JPH0241744B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a photoreceptor that exhibits excellent photosensitivity characteristics, particularly in the near-infrared wavelength region.

Prior Art In recent years, amorphous silicon (hereinafter abbreviated as a-Si), amorphous germanium (hereinafter abbreviated as a-Ge) or amorphous silicon produced by glow discharge decomposition method or sputtering method has been developed. Germanium (hereinafter abbreviated as a-Si:Ge)
The application of this to electrophotographic photoreceptors has been attracting attention. This is because these a-Si, a-Ge, a-Si:Ge
This is because it is far superior to conventional selenium and CdS photoreceptors in terms of environmental stainability, heat resistance, abrasion resistance, etc.

Especially in the case of a-Si:Ge, since the band gap of Ge is smaller than that of a-Si,
Adding an appropriate amount of Ge can be expected to extend the photosensitivity characteristics toward longer wavelengths, and its application to semiconductor laser beam printers, whose development has been remarkable in recent years, can be considered.

For example, a photoreceptor with a so-called single-layer structure in which an a-Si:Ge photoconductive layer is formed on a substrate with a thickness of several tens of microns has been proposed. Since the mobility of the generated charge carriers is small, many of the charge carriers are trapped in the photoconductive layer, resulting in an increase in residual potential and a decrease in photosensitivity.
Therefore, the Ge content is limited, making it difficult to obtain a photoreceptor with high sensitivity in the long wavelength region.

Purpose of the invention The present invention has been made in view of the above facts.
The purpose is to provide a photoreceptor that is highly sensitive in the near-infrared region and is capable of obtaining good images with excellent charge retention.

Summary of the Invention The summary of the present invention is to provide a conductive substrate with a thickness of 5 to 5
10 to 40 atomic% hydrogen at 100 microns,
An amorphous silicon semiconductor layer containing up to 20,000 ppm of boron and oxygen is formed on top of the amorphous silicon semiconductor layer with a thickness of 0.1 to 2 microns and a molar ratio of silicon to germanium of 1:1 to 19:1.
The photoreceptor comprises an amorphous silicon-germanium photoconductive layer containing 40 atomic percent hydrogen, up to 20,000 ppm boron, and at least 10 ^-3 atomic percent oxygen.

FIG. 1 shows the structure of a photoreceptor according to the present invention, 1
is a conductive substrate with a-Si semiconductor layer 2 and a-
It is formed by sequentially laminating Si:Ge photoconductive layers 3.

The a-Si semiconductor layer 2 formed on the substrate 1 is formed to a thickness of about 5 to 100 microns, preferably about 10 to 60 microns, by glow discharge decomposition or sputtering. The thickness of this a-Si semiconductor layer 2 depends on the thickness of the a-Si:Ge photoconductive layer 3, which will be described later.
When the thickness is 1 micron or less, especially 0.5 micron or less, it functions as a photoconductive layer that guarantees photosensitivity in the visible light region to some extent. Along with that a-
The Si semiconductor layer 2 functions as a main charge retention layer. When the a-Si semiconductor layer also functions as a photoconductive layer, in addition to having the above-mentioned thickness, it also contains about 10 to 40 atomic% of hydrogen and about 5×10 ^-2 to 10 ^-5 atomic% of hydrogen. Oxygen and about 10 to
Contains 20,000 ppm of Group A impurities of the periodic table (preferably boron). In other words, the a-Si semiconductor layer 2 has a dark resistance with hydrogen alone, as detailed in Japanese Patent Application Laid-Open No. 156834/1983 by the same inventor as the present application.
10 Less than ¹⁰ Ω・cm and necessary as a charge retention layer
A dark resistance of 10 ¹³ Ωcm cannot be achieved. However, if it contains oxygen and impurities in the above range in addition to hydrogen, approximately
A dark resistance of 10 ¹³ Ω・cm or more is guaranteed and it can function as a charge retention layer. The reason for keeping oxygen below 0.05 atomic% is to ensure good photosensitivity.
The reason why it is set to 10 ^-5 atomic % or more is to ensure a dark resistance on the order of 10 ¹³ Ω·cm when used in combination with Group A impurities of 10 ppm or more. In addition, impurities should be maximized.
The reason why it is set at 20,000 ppm is because the dark resistance decreases rapidly if it is added more than that. On the other hand, the photosensitivity decreases as the oxygen content increases, but as mentioned above, the oxygen content is at most 0.05 atomic%, a very small amount, so high sensitivity is maintained, especially at
At a wavelength of 700 nm, Se and PVK-TNF (molar ratio 1:
It has several times higher sensitivity than 1) and is also excellent as a photoconductive layer.

The reason why the dark resistance of a-Si is significantly improved by the addition of oxygen is thought to be due to the effective elimination of dangling bonds, although much remains unclear. That is, a-Si uses SiH ₄ , Si ₂ H ₆ , etc. as its starting material, hydrogen is used as a carrier gas in the glow discharge decomposition method, and even B ₂ H when it contains boron. Since ₆ is used, it generally contains hydrogen on the order of 10 to 40 atomic percent. However, hydrogen alone can only solve the dangling bonds insufficiently, and the dark resistance does not improve much. However, the inclusion of oxygen eliminates most of the dangling bonds and improves the dark resistance to 10 ¹³ Ω·cm or more. Further, since a-Si has a wide band gap and high charge carrier mobility as its inherent property, it functions effectively as a charge transport layer. In addition, if the desired dark resistance can be achieved without significantly impairing the photosensitivity characteristics, a-Si
The content of the photoconductive layer 2 is not limited to those mentioned above, but is arbitrary.

If the a-Si semiconductor layer 2 is not to have a function as a photoconductive layer but only as a charge retention guarantee layer, a larger amount of oxygen may be added. Further, nitrogen or carbon may be added instead of oxygen.

The a-Si:Ge photoconductive layer 3 is also formed on the a-Si semiconductor layer 2 to a thickness of about 0.1 to 2 microns by a glow discharge decomposition method or a sputtering method.
It contains at least about 10 to 40 atomic percent hydrogen and a suitable amount of boron. Contains hydrogen as a starting material
This is because SiH ₄ , GeH _{4 ,} etc. are used, and as will be described later, it is convenient to use hydrogen as a carrier gas for SiH ₄ and GeH ₄ in the glow discharge decomposition method. However, in the case of the a-Si:Ge photoconductive layer 3 containing only hydrogen, the dark resistance is low and surface charges flow in the lateral direction, causing image disturbance. In other words, a-Si
Semiconductor 2 guarantees a certain degree of charge retention due to its high dark resistance, but the surface layer a-Si:
If the resistance of the Ge photoconductive layer 3 is too low, it is impossible to prevent charges from escaping in the lateral direction. For this reason, a-Si:
The Ge photoconductive layer contains an appropriate amount of boron. The inclusion of boron works to improve the dark resistance to some extent and is effective in eliminating the above-mentioned disadvantages.

More preferably, 10 ^-3 to 5 x 10 ^-2 atomic % of oxygen is contained in addition to hydrogen and boron. Oxygen significantly improves the dark resistance of the a-Si:Ge photoconductive layer, reliably prevents charge flow in the lateral direction, and improves the charging potential. Setting the oxygen content to a maximum of about 5 x 10 ^-2 atomie% is a-
This is because the original photosensitivity of Si:Ge is lost, and
The reason for setting the value to 10 ^-3 atomic % is that oxygen less than 10 -3 atomic % does not significantly improve the dark resistance. Note that the inclusion of oxygen also increases the efficiency of boron addition, making it possible to contain up to about 20,000 ppm of boron. That is 5×10 ^-2
The content of oxygen in an amount of 10 ^-3 atomic % eliminates most of the dangling bonds in the a-Si:Ge photoconductive layer without significantly lowering the photosensitivity, improving the dark resistance, and improving the localization level of the mobility gap. This greatly improves the efficiency of adding boron.

The a-Si:Ge photoconductive layer 3 guarantees excellent photosensitivity in the near-infrared region, particularly in the long wavelength region of 700 to 900 nm, since the bandgap of Ge is smaller than that of a-Si. In other words, Ge improves the photosensitivity in the long wavelength region, where a-Si alone has low sensitivity,
This will enable application to semiconductor laser beam printers that use a wavelength of around 800 nm as an exposure source. In order to improve the sensitivity on the long wavelength side, Ge can be contained in a molar ratio of a-Si to a maximum of 1:1 and a minimum of 19:1. That is, when a-SixGe _1-x is used as a photoconductive layer, x is 0.5 to 0.95. The reason why the molar ratio is set to at least 19:1 is because if the Ge content is less than that, no improvement in sensitivity in the long wavelength region can be expected, whereas if it is more than 1:1, the sensitivity will decrease. This is because the bandgap of Ge is considerably narrower than that of a-Si, so when a large amount of Ge is added, charge carriers generated in the a-Si:Ge photoconductive layer 3 are transferred to the interface with the a-Si semiconductor layer 2. This is thought to be due to being trapped. In addition, a-Si:Ge
If only the photoconductive layer guarantees sensitivity from visible light to near-infrared light, the more Ge it contains, the lower the overall sensitivity will be.
The molar ratio is limited to 1:1.

As mentioned above, the a-Si:Ge photoconductive layer 3 is preferably about 0.1 to 2 microns thick; The thickness should be approximately 0.1 to 0.5 microns. This point will _be explained _in detail with reference to FIG.
25 atomic%, oxygen approximately 0.01 atomic%, boron approximately
It shows the light transmittance (%/μ) per 1 micron of film thickness at wavelengths from 400 to 1000 nm of 40 ppm (containing 40 ppm), and as is clear from this, the same layer completely transmits light at short wavelengths up to about 600 nm per 1 micron of film thickness. absorb light. This means that when the film thickness of the a-Si:Ge photoconductive layer 3 is at least 1 micron or more, 600 nm
This means that the following shortwave light does not reach the a-Si semiconductor layer 2, and the guaranteed sensitivity for wavelengths of 600 nm or less is a-Si:
This will depend on the Ge photoconductive layer. The light transmittance is
Rise at 600nm, approx. 40% at 700nm, approx. at 800nm
60%, and about 70% at 900nm. Therefore a-Si:
The Ge photoconductive layer 3 has high light absorption at short wavelengths and low light absorption at long wavelengths. However, since it still absorbs long wavelengths, sensitivity can be guaranteed over both short and long wavelengths. The above is the light absorption rate per 1 micron, so if the film thickness is made thinner than 1 micron, the a-Si:Ge photoconductive layer will transmit at least a small amount of light, even shortwave light of 600 nm or less, in the visible light region of the a-Si semiconductor layer 2. It is meaningful to have the role of guaranteeing the sensitivity of This effect becomes particularly noticeable at a thickness of 0.5 micrometers or less, and in this case, it is desirable to provide the a-Si semiconductor layer with the photoconductive layer function as described above.

The reason why the thickness of the a-Si:Ge photoconductive layer 3 is about 0.1 micron or more is to ensure photosensitivity especially in the long wavelength region, and if it is less than that, sufficient light absorption cannot be achieved.
The reason why the thickness is about 2 microns or less is because Ge has a narrow band gap and low mobility of the charge carrier. In other words, charge carriers generated in the same layer must be transported to the substrate 1 side via the a-Si semiconductor layer 2, but if the film thickness is 2 microns or more, it will be trapped at the interface with the a-Si semiconductor layer 2, reducing sensitivity. This is because it causes a decrease.

Next, an inductively coupled glow discharge decomposition method for manufacturing the photoreceptor according to the present invention will be explained.

In FIG. 3, the first, second, third, and fourth tanks 4, 5, 6, and 7 contain SiH ₄ , B ₂ H ₆ , GeH ₄ ,
_O2 gas is sealed. _SiH4 , _{B2H6 , GeH4}
The carrier gas for all gases is hydrogen, but
There is no problem even if it is Ar or He. These gases are supplied to the corresponding first, second, third and fourth regulating valves 8, 9,
It is released by opening mass flow controllers 10, 11, and its flow rate is controlled by mass flow controllers 12, 13, 1.
4, 15, and the gas in the first and second tanks 4, 5 is regulated to the first main pipe 16 and the third tank 6.
The GeH ₄ gas from
O ₂ gas from tank 7 is sent to third main pipe 18 . In addition, 19, 20, 21, 22 are flow meters, 2
3, 24, 25 are check valves. The gas flowing through the first, second, and third main pipes 16, 17, and 18 is sent into the reaction tube 26, but a resonant vibration coil 27 is wound around this reaction tube, and its own high-frequency power is Preferably, the frequency is about 0.1 to 3 kilowatts, and the frequency is 1 to 50 MHz.
is appropriate. Inside the reaction tube 26, there is a-
aluminum on which the Si:Ge photoconductive layer 2 is formed;
A substrate 28, such as stainless steel, NESA glass, etc., is placed on a turntable 30 which is rotatable by a motor 29, and the substrate 29 itself is heated to about 100-400° C., preferably about 400° C., by suitable heating means.
It is heated uniformly to a temperature of 150 to 300°C. The interior of the reaction tube 26 is connected to a rotary pump 31 and a diffusion pump 32 because a high degree of vacuum (discharge voltage: 0.5 to 2 Torr) is required during layer formation.

In the glow discharge decomposition apparatus described above, first, in order to form the a-Si semiconductor layer 2 containing hydrogen on the substrate, the first regulating valve 8 is opened and the first tank 4 is opened.
When containing SiH ₄ gas, open the second regulating valve 9 and supply B ₂ H ₆ gas from the second tank 5.
Furthermore, when oxygen is contained, the fourth regulating valve 11 is also opened to release O ₂ gas. The amount of each gas released is regulated by the mass flow controllers 12, 13, and 15, and SiH ₄ gas or a gas mixed with B ₂ H ₆ gas is passed through the first main pipe 16 to form SiH ₄
Oxygen gas in a certain molar ratio to the third main pipe 1
8 into the reaction tube 26. The inside of the reaction tube 26 is in a vacuum state of about 0.5 to 2.0 Torr, the substrate temperature is 100 to 400°C, and the resonant vibration coil 2
Coupled with the fact that the high frequency power of 7 is set at 0.1 to 3 kilowatts and the frequency is set at 1 to 50 MHz, glow discharge occurs, gas decomposes, and a-Si semiconductor containing hydrogen, boron, and oxygen is deposited on the substrate. 2 is about
Forms as quickly as 0.5 to 5 microns/60 minutes.

Once the a-Si semiconductor layer with a thickness of 5 to 100 microns is formed, the glow discharge is temporarily interrupted. After that, SiH ₄ , B ₂ H ₆ , GeH ₄ ,
Under the same conditions as above, a-Si was released by releasing _O2 gas.
An a-Si:Ge photoconductive layer 3 containing at least hydrogen and boron is formed on the semiconductor layer 2 to a thickness of 0.1 to 2 microns.

The photoreceptor of the present invention can also be produced using a capacitively coupled glow discharge decomposition apparatus shown in FIG. Note that the same parts as in FIG. 3 are given the same reference numerals and the explanation thereof will be replaced. In Figure 4, 33,
34 are fifth and sixth tanks sealed with SiH ₄ , GeH ₄ , and hydrogen as a gas carrier gas; 35;
36 are fifth and sixth regulating valves, 37 and 38 are mass flow controllers, and 39 and 40 are flowmeters.
Inside the reaction chamber 41 are first and second electrode plates 43 and 44 connected to a high frequency power source 42 in proximity to the substrate 28.
are arranged in parallel, and are connected to fourth and fifth main pipes 45 and 46, respectively. Note that the first and second electrode plates are electrically connected by a conductive wire 47.

The first electrode plate 43 has rectangular parallelepiped-shaped first and second electrode plates.
It has a structure in which two conductors 48 and 49 are overlapped, and the front wall facing the substrate 28 has a large number of gas release holes, the intermediate wall of the overlapping part has a small number of gas release holes, and the back wall has a fourth gas release hole. A gas introduction hole is formed to be connected to the main pipe 45. First, the gas from the fourth main pipe 45 is temporarily stored in the first conductor 48, and then gradually released from the hole in the intermediate wall and then released from the discharge hole of the second conductor 49. It is now released evenly. At the same time as the gas is released, the high frequency power source 42 generates a
Power of 1.5 kilowatts (frequency 1 to 50 MHz) is applied to the first and second electrode plates 43 and 44 to cause glow discharge and form a photoconductive layer on the substrate 28. At this time, the substrate 28 is kept electrically grounded or a DC bias voltage is applied to itself. This device has the advantages of uniform discharge of the electrode plate, uniform layer formation distribution, excellent gas decomposition efficiency and fast film formation rate, easy gas introduction, and simple construction.

Experimental examples will be explained below.

Experimental Example 1 The glow discharge decomposition apparatus shown in Fig. 3 is used, except that a Pyrex tube with a diameter of 100 mm and a height of 600 mm is used as the reaction tube 26, and a Pyrex tube with a diameter of 130 mm and a height of 130 mm is used as the reaction tube 26.
A photoreceptor according to the present invention was constructed by winding a 90 mm, 10 turn resonant vibration coil.

An aluminum drum with a diameter of 80 mm is placed on a turntable as a substrate 28, and the temperature is raised to about 200°C.
The inside of the reaction tube 26 is evacuated to 10 ^-6 Torr using the rotary pump 31 and the diffusion pump 32, and then only the rotary pump is used. Next, from the first tank 4, SiH ₄ gas with hydrogen as a carrier gas (for hydrogen
SiH ₄ 10%) was released into the reaction tube 26 under a flow rate of 70 sccm, 18 sccm of B ₂ H ₆ gas (80 ppm in hydrogen) was released from the second tank 5, and 0.8 sccm of O ₂ gas was released from the fourth tank 7. Introduced coil 27 to 160watts (frequency
1 micron/60 by applying high frequency power of 4MHs)
Approximately 25 atomic% hydrogen in a thickness of 20 microns in minutes
An a-Si semiconductor layer 2 containing 0.01 atomic % of oxygen and 40 ppm of boron in addition to the above was formed. Note that the discharge voltage at this time was 1 Torr.

Next, 70 sccm of SiH ₄ gas was added from the first tank 4.
18 sccm of B ₂ H ₆ gas was released from the second tank 5, and 14 sccm of GeH ₄ gas (10% in hydrogen) was released from the third tank 6, and under the same conditions as above, a layer of 0.1 in thickness was released on the a-Si semiconductor layer 2. Approximately 25 atomic% hydrogen and
An a-Si _0.75 Ge _0.25 photoconductive layer 3 containing 40 ppm of boron was formed.

A photoconductor with the same structure under similar conditions, except that sample A was a photoconductor with an a-Si _0.75 Ge _0.25 photoconductive layer 8 further containing approximately 0.01 atomic percent oxygen, and sample B was a photoconductor that was the same as sample A but with an a-Si A photoreceptor was prepared in which the thickness of the _0.75 Ge _0.25 photoconductive layer 3 was 2 microns.

Each of these photoreceptors was charged to 400V, and the light irradiation was varied in the wavelength range of 500 to 850nm in 50nm increments using a monochromator, and the relationship with the light energy required to reduce the surface potential by half was measured. The spectral sensitivity was investigated.

The results are shown in Figure 5, with curves A and B
correspond to samples A and B, respectively. Note that curve C is the spectral sensitivity of a photoreceptor having only an a-Si semiconductor layer on a substrate. As is clear from the figure, the photoreceptor of the present invention has significantly improved photosensitivity, particularly in the long wavelength region of 700 nm or more. Sample A, in which the a-Si _0.75 Ge _0.25 photoconductive layer contains hydrogen, boron, and oxygen, has considerably high sensitivity as shown by curve A. On the other hand, sample B in which the thickness of the a-Si _0.75 Ge _0.25 photoconductive layer was increased to 2 microns has higher long wavelength sensitivity as shown by curve B, making it possible to apply it to high-speed semiconductor laser beam printers. .

On the other hand, although the photosensitivity in the visible light region is lower than that of curve C, it is still low at 600 nm.
It has a sufficiently high sensitivity of 0.1 cm ² /erg or more. In the case of sample A, the sensitivity is higher than that of sample B, partly because the a-Si semiconductor layer functions as a photoconductive layer.
On the other hand, Sample B, which has a large a-Si:Ge photoconductive layer, has the lowest thickness, and from this fact, the limit is about 2 microns.

Next, use the same photoreceptor as sample A, but a-Si _0.75 Ge _0.25
When a photoreceptor was prepared in which the photoconductive layer contained 0.05 atomic percent of oxygen and its spectral sensitivity was measured, it was found that the sensitivity was lower than curve A for both visible light and long wavelengths. However, it was confirmed that the sensitivity was higher than that of curve C. However, if more oxygen is contained, it is expected that there will be no significant difference from curve C, and in this sense, it is desirable that the oxygen content be at most about 0.05 atomic %.

Furthermore, photoreceptors identical to Sample A were prepared, except that the a-Si _0.75 Ge _0.25 photoconductive layer contained 200, 2000, and 20000 ppm of boron in addition to hydrogen and oxygen, respectively, and the spectral sensitivities of each were measured. Curve A in the wavelength range
A slightly lower sensitivity was measured as the boron content increased. However, both curves C
It was much more sensitive than that.

Finally, the same photoreceptor as sample A, but a-Si:Ge
The molar ratio of Si and Ge in the photoconductive layer was 19:1 and 10:1, respectively.
When photoreceptors with ratios of 1, 2:1, and 1:1 were made and the spectral sensitivities were measured, a trace amount of 19:1 was found.
Even with Ge, the sensitivity on the long wavelength side improves, and as the amount of Ge increases, the sensitivity becomes higher. In fact, when the amount of Ge is 2:1, the sensitivity is about 1.4 to 1.9 times higher than curve A. However, when the molar ratio of Si to Ge is 1:1, the sensitivity is lower than when it is 2:1. The reason for this is unknown, but the bandgap of Ge is much narrower than that of a-Si, so a large amount of
This is believed to be because when Ge is added, carriers generated in the a-Si:Ge photoconductive layer are trapped at the interface with the a-Si semiconductor layer and cannot move. In this sense, the maximum molar ratio of Si and Ge is 1:1.

In the image forming experiment, a photoreceptor of curve A was set in a laser beam printer. Then, it is positively charged with a corona charger, and the oscillation wavelength is 780 nm, 2 mW.
The semiconductor laser beam was directly modulated and scanned by a rotating polygon mirror, negative exposure was performed on the photoreceptor, and positive polarity toner was used to perform reversal magnetic brush phenomenon, transfer, cleaning, and charge removal. A4 photoreceptor moving speed is 130mm/sec.
When I printed 40 sheets of size paper per minute,
Prints that reproduced extremely clear characters at 10 dots/mm were obtained. The print quality remained clear even after printing 100,000 copies.

[Brief explanation of the drawing]

FIG. 1 is a laminated configuration diagram of a photoconductor according to the present invention, FIG. 2 is a diagram showing the light transmittance of an amorphous silicon-germanium photoconductive layer, and FIGS. 3 and 4 are diagrams showing a photoconductor according to the present invention. FIG. 5 is a diagram showing the spectral sensitivity of the photoreceptor according to the present invention. 1... Conductive substrate, 2... a-Si semiconductor layer, 3
... a-Si:Ge photoconductive layer, 4 ... first tank (SiH ₄ gas), 5 ... second tank (B ₂ H ₆ gas),
6... Third tank (GeH ₄ gas), 7... Fourth tank (O ₂ gas), 27... Resonant vibration coil.

Claims (1)

[Claims] 1. An amorphous silicon semiconductor layer having a thickness of 5 to 100 microns and containing 10 to 40 atomic% hydrogen, boron and oxygen up to 20,000 ppm is formed on a conductive substrate, and an amorphous silicon semiconductor layer having a thickness of 0.1 to 2 microns is formed on the conductive substrate. It guarantees photosensitivity in the near-infrared region, and the molar ratio of silicon and germanium is 1:1 to 19:1.
A photoreceptor comprising an amorphous silicon-germanium photoconductive layer containing from 40 atomic percent hydrogen, up to 20,000 ppm boron, and at least 10 ^-3 atomic percent oxygen.