JPS63204263A - Electrophotographic sensitive body - Google Patents

Abstract

PURPOSE:To enhance electrostatic chargeability, to lower residual potential, and to enhance photosensitivity in a wide wavelength region by forming the photoconductive layer of an electrophotographic sensitive body composed of a charge retaining layer and a charge generating layer, each having a layer region composed of specified laminated semiconductor films, and forming superlattice structure. CONSTITUTION:The electrophotographic sensitive body is formed by laminating on a conductive substrate 1 a barrier layer 2, the photoconductive layer 3 composed of the charge retaining layer 5 and the charge generating layer 6, and on this layer a surface layer 4. The layer 6 is formed by alternately laminating the thin films of microcrystalline Si different in crystallization degree, and said thin films of at least one type of the 2 types change in crystallization degree in each film in the layer thickness direction, thus permitting the obtained photosensitive body to be enhanced in transfer performance, resistivity, and chargeability characteristics by using the laminated thin films different in optical band gap from each other having superlattice structure for the photoconductive layer.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrophotographic photoreceptor having excellent charging characteristics, dark decay characteristics, photosensitivity characteristics, environmental resistance, etc.

[Prior art] Amorphous silicon containing hydrogen (H) (hereinafter referred to as a)
-81:H) has recently attracted attention as a photoelectric conversion material, and has been applied to photoreceptors in electrophotographic processes as well as solar cells, thin film transistors, image sensors, and the like.

Conventionally, inorganic materials such as CdS, ZnO, Se, or 5e-Te, or f-N-vinylcarbazole (PVC) have been used as materials constituting the photoconductive layer of electrophotographic photoreceptors.
z) or trinitronefluorenone (TNF'). However, a-81
:H is a non-polluting substance compared to these inorganic or organic materials, so it does not require recovery treatment, has high spectral sensitivity in the visible light region, and has high surface hardness and wear resistance. It has advantages such as excellent impact resistance. For this reason, a-8i:H is attracting attention as a photoreceptor material for electrophotographic processes.

This a-81:H is being studied as a photoreceptor material based on the Carlson method, but in this case, high resistance and photosensitivity are required as photoreceptor characteristics. However, it is difficult to satisfy both of these characteristics with a single photoreceptor, so a barrier layer is provided between the photoconductive layer and the conductive support, and a surface charge retention layer is provided on the photoconductive layer. These requirements can be met by creating a laminated structure.

[Problems to be Solved by the Invention] By the way, a-8i:H is usually formed by a glow discharge decomposition method using a silane gas, but at this time,
a-8t: Hydrogen is incorporated into the H film, and the difference in the amount of hydrogen causes significant changes in the staining and optical properties. That is, a-
When the amount of hydrogen that enters the 8t:H film increases, the optical bandgap becomes narrower and the resistance of a-8i:H increases, but the photosensitivity to long wavelength light decreases accordingly. Therefore, it is difficult to use it, for example, in a laser beam printer equipped with a semiconductor laser. Furthermore, when the hydrogen content in the a-8t:H film increases, depending on the film formation conditions, those having bonding structures such as (SiH2)n and 5IH2 may occupy most of the area in the film.

Then? Since the number of id increases and the number of dangling bonds of silicon increases, the photoconductive properties deteriorate and the photoconductor becomes unusable as an electrophotographic photoreceptor. As the amount of hydrogen incorporated into the inverse K, a-8t:H decreases, its optical bandgap becomes smaller and its resistance decreases, but its photosensitivity to longer wavelength light increases. However, when the hydrogen content is low, there is less hydrogen available for silicon dangling to bond with and reduce the bond. For this reason, the mobility of the generated carriers is reduced, the life span is shortened, and the photoconductive properties are deteriorated, making it difficult to use as an electrophotographic photoreceptor.

In this way, the photoconductive layer of the electrophotographic photoreceptor can be formed into a single a-8
If it is composed of only the 1:H layer, there is a problem that the characteristics change greatly depending on the manufacturing conditions of the a-81:H layer, and desired characteristics cannot be obtained.

The present invention has been made in view of such circumstances, and
To provide an electrophotographic photoreceptor that has excellent charging ability, low residual potential, high photosensitivity over a wide wavelength range up to the near infrared region, good adhesion to a substrate, and excellent environmental resistance. The purpose is to

[Structure of the Invention] (Means for Solving the Problems) As a result of various studies, the present inventors have constructed a photoconductive layer of an electrophotographic photoreceptor consisting of a charge retention layer and a charge generation layer. ,
The present inventors have discovered that the above object can be achieved by stacking a plurality of predetermined semiconductor films, that is, forming a region with a superlattice structure, and have completed the present invention.

That is, the electrophotographic photoreceptor of the present invention is an electrophotographic photoreceptor comprising an electrically conductive support and a photoconductive layer, the photoconductive layer comprising a charge generation layer and a charge retention layer, The holding layer is formed by alternately laminating amorphous silicon thin films and amorphous silicon thin films containing at least one element among carbon, oxygen, and nitrogen, and the charge generation layer has different crystallinity. It is constructed by alternately stacking microcrystalline silicon thin films, and is characterized in that the crystallinity of at least one of the microcrystalline silicon thin films varies from film to film in the layer thickness direction. - Microcrystalline silicon (μc-8i) used in the present invention is
It is thought to be formed of a mixed layer of highly crystallized cricon with a grain size of about several tens of Angstroms and amorphous silicon, and has the following physical characteristics. First, X-ray diffraction measurements show a crystal diffraction pattern with 2θ in the vicinity of 28 to 28.5°, and it can be clearly distinguished from amorphous a-8t in which only a halo appears. Second, the dark resistance of μc-8t is 10'°
It can be adjusted to Ω・α or more, and the dark resistance is 1050・
It is also clearly distinguished from the polycrystalline silicon of α.

The optical bandgap (Eg') of the μc-8i used in the present invention is preferably 1.55 eV, for example. However, it can be set arbitrarily within a certain range. It is preferable to add the required amount of hydrogen to each to obtain the desired Ego and use it as μc-8i:H. This compensates for silicon dangling bonds,
Dark resistance and bright resistance are balanced, and photoconductive properties are improved.

Note that an actual μc-8t film often takes on a weak P type or N type depending on specific factors such as manufacturing conditions (it is particularly susceptible to becoming an N type). Therefore, in order to obtain the I-type required for forming a superlattice structure, it is desirable to lightly dope impurities having opposite conductivity types to cancel out the P-type or N-type.

The crystallinity of the microcrystalline silicon thin film constituting the charge generation layer is preferably 60 to 90, more preferably 60 to 80.
It is best to change it within a range of %.

(Function) In the electrophotographic photoreceptor of the present invention, since the photoconductive layer is provided with the superlattice structure, carriers generated in this region have a long life and a high mobility. Although the theory has not yet been fully established, there is no doubt that this is a quantum effect due to the periodic well potential characteristic of the superlattice structure, and this is particularly referred to as the superlattice effect.

In this way, the mobility of carriers in the photoconductive layer is large.
Furthermore, the sensitivity of the electrophotographic photoreceptor is significantly improved due to the longer life of the carrier.

(Example) FIG. 1 is a diagram showing a cross-sectional structure of an electrophotographic photoreceptor according to an example of the present invention. In the figure, conductive support 1
A barrier layer 2 is formed thereon, and a photoconductive layer 3 consisting of a charge retention layer 5 and a charge generation layer 6 is formed thereon.

Further, a surface layer 4 is formed on the charge generation layer 6.

Note that both the charge retention layer 5 and the charge generation layer 6 have a superlattice structure.

Hereinafter, the structure of the electrophotographic photoreceptor shown in FIG. 1 will be explained in detail.

The conductive support 1 usually consists of a drum made of aluminum.

The barrier layer 2 may be formed using μc-8t or a-8i:H, or may be formed using a-BN:H (amorphous boron to which nitrogen and hydrogen are added). Furthermore, an insulating film may be used. For example, a high-resistance insulating barrier layer can be formed by incorporating one or more elements selected from carbon C1 nitrogen N and oxygen 0 into μc-8t:H and a-81:H.

The thickness of the barrier layer 2 is preferably 100X to 10 μm.

The barrier layer 2 suppresses the flow of charge between the conductive support 1 and the charge generation layer 5! 7 It is formed to enhance the charge retention function of the surface of the photoreceptor and to increase the charging ability of the photoreceptor. Therefore, when a Carlson type photoreceptor is constructed using a semiconductor layer as a barrier layer, the barrier layer 2 is made to be P type or N type in order not to reduce the ability to retain charges charged on the surface. That is, when the surface of the photoreceptor is positively charged, the barrier layer 2 is made of P type to prevent electrons that neutralize the surface charge from being injected into the charge generation layer.

Conversely, when the surface is negatively charged, the barrier layer 2 is made of N type,
This prevents holes that neutralize surface charges from being injected into the charge generation layer. Since carriers injected from the barrier layer 2 become noise with respect to carriers generated in the charge generation layer 6 upon incidence of light, preventing carrier injection as described above improves sensitivity. In addition, μc-8t:H
or a-8t: In order to make H the P type, the first
It is preferable to dope with elements belonging to the group, such as boron B, aluminum At, gallium Ga, indium In, and thallium Tt. In addition, in order to make μc-8t:H and a-81:H N-type, elements belonging to Group V of the periodic table, such as nitrogen, phosphorus P, arsenic As, antimony Sb, and bismuth Bi, are doped. It is preferable to do so.

The charge generation layer 6 generates carriers when light is incident thereon, and carriers of one polarity neutralize the charges on the surface of the photoreceptor, and the other carriers travel within the charge retention layer 5 and become conductive. The sexual support 1 is reached.

Both the charge retention layer 5 and the charge generation layer 6 are constructed by alternately laminating two types of thin layers.

These thin layers have different optical bandgaps and each have a thickness in the range of 30-200X. In this way, by stacking thin layers with mutually different optical bandgaps, the optical bandgap can be made larger based on the layer with the smaller optical bandgap, regardless of the size of the optical bandgap itself. A superlattice structure with periodic potential barriers is formed in which the layers act as barriers. In this superlattice structure, the barrier thin layer is extremely thin, so
Due to carrier tunneling in the thin layer, carriers pass through the barrier and travel into the superlattice structure. Further, in such a superlattice structure, a large number of carriers are generated upon incidence of light. Therefore, it has high photosensitivity. In addition,
By changing the bandgap and layer thickness of the thin layer of the superlattice structure, the apparent bandgap of the layer having the Peter junction superlattice structure can be freely adjusted.

a-81 constituting the charge retention layer 5 and charge generation layer 6:
The hydrogen content in H and μe-8i:H is 0.
01 to 30 atoms are preferred, and 1 to 25 atoms are even more preferred. Such a hydrogen content compensates for the dangling bonds of silicon, brings the dark resistance and bright resistance into balance, and improves the photoconductive properties.

a-81: To simulate the H layer by the glow discharge decomposition method, silane gases such as SiH4 and 512H5 are introduced into the reaction chamber as raw materials, and a high frequency jiglow discharge is applied to the thin layer. H can be added to.

If necessary, hydrogen or helium gas can be used as a carrier gas for silanes. On the other hand, SiF4
Silicon halides such as gas and s i ct4 gas can be used as source gases. Further, a-81:H containing H can be similarly formed by reacting with a mixed gas of silane gas and silicon halide gas. Note that these thin layers can be formed not by the glow discharge decomposition method but also by a physical method such as sputtering.

Similarly to a-81:H, the μc-8i layer can also be formed using a high-frequency glow discharge decomposition method using silane gas as a raw material. In this case, the temperature of the support is a-8t:
When forming an H, the height is set high and the high frequency power is also
In the case of 8i:H, if shi is also set high, μe-8t:H
becomes easier to form. Furthermore, by increasing the support temperature and high frequency power, the flow rate of raw material gas such as silane gas can be increased, and as a result, the film formation rate can be increased.

In addition, by using gas obtained by diluting high-order silane gas such as 5IH4 and 512H6 with hydrogen,
μc-8i:H can be formed with higher efficiency.

In order to make Ac-8t:H and a-8t:H P-type, elements belonging to Group 1 of the periodic table, such as boron B,
Aluminum At, gallium Ga, indium In.

It is preferable to dope with thallium Tt, etc.
To make μc-8t:H and a-81:H n-type, they are doped with elements belonging to Group Ⅰ of the periodic table, such as nitrogen N, phosphorus P1 arsenic As, antimony Sb1, and bismuth B1. It is preferable. This doping with p-type impurities or n-type impurities prevents charges from moving from the support side to the photoconductive layer. On the other hand, μc-8i
:H and a-8i:H contain at least one element selected from carbon C1 nitrogen N and oxygen 0, thereby providing high resistance and increasing surface charge retention ability.

The content of these elements is 5 to 40 atoms, preferably 10
~30 atoms.

A surface layer 4 is provided on the charge generation layer 6.

Since the charge generation layers 60a-8i:H and the like have a relatively large refractive index of 3 to 3.4, light reflection easily occurs on the surface. When such light reflection occurs, the proportion of the amount of light absorbed by the photoconductive layer or the charge generation layer decreases, resulting in increased light loss. For this reason, it is preferable to provide the surface layer 4 to prevent reflection. Further, by providing the surface layer 4, the charge generation layer 6 is protected from damage. Furthermore, by forming a surface layer, the charging ability is improved, and the charge can be easily deposited on the surface. The material forming the surface layer is a-
These include inorganic compounds such as 8iN:H, a-810:H, and a-8iC:H, and organic materials such as divinyl chloride and polyamide.

When the surface of the electrophotographic photoreceptor constructed in this manner is charged with a positive voltage of about 500 V by corona discharge, a potential barrier is formed. When light (hv) is incident on this photoreceptor, carriers of electrons and holes are generated in the superlattice structure of the charge generation layer 6. Electrons in the conduction band are accelerated toward the surface layer 4 side by the electric field in the photoreceptor, and holes are accelerated toward the conductive support 1 side. In this case, the number of carriers generated at the boundary between thin layers with different optical bandgaps is extremely larger than the number of carriers generated in the bulk. Therefore, this superlattice structure has high photosensitivity. In addition, in the potential well layer, due to quantum effects, compared to the case of a single layer without a superlattice structure,
Carrier life is 5 to 10 times longer. Furthermore, in a superlattice structure, a periodic barrier layer is formed due to bandgap discontinuity, but carriers easily pass through the bias layer due to the tunnel effect, so the effective carrier mobility is The mobility of the carrier is equivalent to that in the bulk (1), and the carrier has excellent running properties.

Similarly, in the case of the charge retention layer 5, due to quantum effects, the lifetime of carriers in the potential well layer is 5 to 10 times longer than in the case of a single layer without a superlattice structure.

In addition, in a superlattice structure, a periodic barrier layer is formed due to bandgap discontinuity, but carriers can easily pass through the bias layer due to the tunnel effect.
The effective mobility of the carrier is equivalent to the mobility in bulk, and the carrier has excellent mobility. As mentioned above,
A superlattice structure in which thin layers with different optical band gaps are laminated can provide high photoconductivity and provide clearer images than conventional photoreceptors.

Referring to FIG. 2, an apparatus and a manufacturing method for manufacturing the electrophotographic photoreceptor of the above embodiment by a glow discharge method will be described below. In the figure, gas cylinders 21, 22, 23.
24, for example, SiH4°B2H6, H2, CH
A second grade source gas is accommodated.

These gases are controlled by a valve 26 for adjusting the flow rate.
and is supplied to a mixer 28 via a pipe 27. A pressure gauge 25 is installed in each gompe,
By adjusting the valve 26 while monitoring the pressure gauge 25, the flow rate and mixing ratio of each source gas to be supplied to the mixer 28 can be adjusted.

The gases mixed in the mixer 28 are supplied to a reaction vessel 29. A rotating shaft 30 is attached to the bottom 31 of the reaction vessel 29 so as to be rotatable in a vertical direction. The rotating shaft 3
At the upper end of 0, a disk-shaped support 32 has its surface aligned with the rotation axis 30.
It is fixed vertically. Inside the reaction vessel 29, a cylindrical electrode 33 is installed on the bottom 31 with its axial center aligned with the axial center of the rotating shaft 30. A drum base 34 of a photoreceptor is placed on a support base 32 with its axial center aligned with the axial center of the rotating shaft 30.
A heater 35 for heating the drum base is disposed inside the drum. A high frequency power source 36 is connected between the electrode 33 and the drum base 34.
A high frequency current is supplied between them. The rotating shaft 30 is rotationally driven by the motor 3BK. Reaction container 29
The internal pressure is monitored by a pressure gauge 37, and the reaction vessel 29 is connected via a dart valve 38 to a suitable evacuation means such as a vacuum pump.

When manufacturing a photoreceptor using the above-mentioned manufacturing apparatus, after installing the drum base 34 in the reaction container 29, the gate pulp 39 is opened and the inside of the reaction container 29 is heated to about 0.1 Torr.
Evacuate to below pressure. Next, cylinder 21.22,2
s, 24, the required reaction gases are mixed at a predetermined mixing ratio and introduced into the reaction vessel 29.

In this case, the flow rate of the gas introduced into the reaction vessel 29 is set so that the pressure within the reaction vessel 29 is 0.1 to 1.0 Torr. Next, the motor 38 is operated to rotate the drum base 34, and the heater 35 rotates the drum base 34.
is heated to a constant temperature, and a high frequency current is supplied between the stain electrode 33 and the drum base 34 by the high frequency power supply 36 to form a glow discharge between them. As a result, a-8l:H is deposited on the drum base 34. Note that N20°NH3, No2. N2. CH4, C2
By using H4,0,2 gas or the like, carbon, oxygen, and nitrogen can be contained in a-8t:H.

As described above, the electrophotographic photoreceptor according to the present invention can be manufactured using a closed system manufacturing apparatus, and therefore is safe for the human body.

Next, the results of testing the electrophotographic properties of an electrophotographic photoreceptor according to the present invention formed into a film will be described.

Test Example 1 If necessary, to prevent interference, an aluminum drum base with a diameter of 80 m+ and a width of 350 fins, which has been subjected to acid treatment, alkali treatment, and sandblasting, is installed in the reaction vessel. The vacuum was evacuated to 10-5 Torr. While heating the drum base to 250°C and rotating it at 10 rpm, 500 SCCM of SjH4 gas, B2
H6 gas was introduced into the reaction vessel at a flow rate ratio of 10 to 5IH4 gas, and the pressure inside the reaction vessel was reduced to ITo.
rrK regulated. Then, a high frequency power of 13.56 MHz was applied to generate plasma to form a p-type a-81C:H barrier layer on the drum base.

Next, SiH4 gas was introduced into the reaction vessel at a flow rate of 5008 CCM, C) (4 gas was introduced into the reaction vessel at a flow rate of 308 CCM,
Applying 00W high frequency power, 120X a-8iC:
A thin H layer was formed. Next, CH4 gas was set to 0, and B2
H6 gas was introduced into the reaction vessel at a flow rate ratio of 10-6 to SiH4 gas, and a high frequency power of 400 W was similarly applied to form a 1201 l-type a-8t:H thin layer. Repeat this operation to create 500 layers of A-8i.
1 consisting of a C:H thin layer and 500 a-8i:H thin layers
A charge retention layer having a superlattice structure of 2 μm was formed.

Next, SiH4 gas was introduced at a flow rate of 50 SCCM and H2 gas was introduced at a flow rate of 500 SCCM, and a high frequency power of 1.2 kW was applied with the pressure inside the reaction vessel set to ITorr to form a 100X μc-81:H thin layer. The crystallinity of this thin layer was 85 degrees. Next, the high frequency power is
Under the same conditions except that it was set to 00W, 100X μc-8t:H
A single layer of Thin 21 was formed. The crystallinity of this thin layer was 60%.

These operations were repeated to form a charge generation layer having a superlattice structure of 3 μm. However, in the case of the former μC-8i:H thin layer, the high frequency power was gradually lowered as each thin layer was formed, and the degree of crystallinity was varied from 85 to 65. □ Finally, a surface layer consisting of a 0.5 μm thin a-8iC:H layer was formed.

When the surface of the photoreceptor thus formed is positively charged at about 500 V and exposed to white light, this light is absorbed by the charge generation layer and carriers of electron-hole pairs are generated. In this test example, a large number of carriers were generated, the carriers had a long life, and high running performance was obtained. As a result, clear and high quality images were obtained. In addition, when the photoreceptor manufactured in this test example was repeatedly charged, the reproducibility and stability of the transferred image were extremely good. It has been proven that it has excellent durability.

The photoreceptor manufactured in this manner has high sensitivity even to long wavelength light of 780 to 79 Qnm, which is the oscillation wavelength of a semiconductor radar. When this photoreceptor was installed in a semiconductor radar printer and multiple images were formed using the Carlson process, clear, high-resolution images could be obtained even when the exposure amount on the photoreceptor surface was 25 erg. Ta.

Test Example 2 Electrophotographic exposure was carried out in the same manner as in Test Example 1, except that an a-8iN:H thin layer was formed in place of the a-8iC:H thin layer, which is one of the constituent layers of the charge retention layer. manufactured a body.

Note that the a-8iN:)L thin layer is made of 50% SiH gas.
It was obtained by introducing 0 SCCM, N, , f into the reaction vessel at a flow rate of 80 SCCM and applying 500 W of high frequency power.

When an image was formed using this photoreceptor in the same manner as in Test Example 1, a clear and high quality image was obtained.

Test Example 3 Test Example 1 except that the crystallinity of the μC-8i:H thin layer, which is one of the constituent layers of the charge generation layer, was changed as shown in FIGS. 3(a) to (f). An electrophotographic photoreceptor was manufactured in the same manner as described above.

When images were formed using these photoreceptors in the same manner as in Test Example 1, clear and high quality images were obtained.

Note that the types of thin layers constituting the charge retention layer and the charge generation layer are not limited to two types as in the above test example, but three or more types of thin layers may be laminated. It is sufficient to form boundaries between different thin layers.

〔Effect of the invention〕

According to this invention, since the photoconductive layer uses a superlattice structure composed of laminated thin layers with mutually different optical band gaps, carrier mobility is high, and the charging property is high and has high resistance. An excellent electrophotographic photoreceptor can be obtained. In particular, the present invention has the advantage that by appropriately combining the materials forming the thin layer, it is possible to obtain a photoreceptor having optimal photoconductive properties for light in any wavelength band.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an electrophotographic photoreceptor according to an embodiment of the present invention, FIG. 2 is a diagram showing an apparatus for manufacturing an electrophotographic photoreceptor according to an embodiment of the invention, and FIG. 3 is a thin layer of crystals. FIG. 3 is a diagram showing changes in the degree of DESCRIPTION OF SYMBOLS 1... Conductive support, 2... Barrier layer, 3... Photoconductive layer, 4... Surface layer, 5... Charge retention layer, 6...
Charge generation layer. Applicant's agent Patent attorney Takehiko Suzue Figure 1 Layer list (a) Layer list (C) 47! (b) (d) 4 A (e) Book 4 (f) Figure 3

Claims (9)

[Claims] (1) In an electrophotographic photoreceptor having a conductive support and a photoconductive layer, the photoconductive layer is composed of a charge generation layer and a charge retention layer, and the charge retention layer is composed of an amorphous silicon thin film and a carbon , and an amorphous silicon thin film containing at least one element selected from oxygen and nitrogen, and the charge generation layer is formed by alternately stacking microcrystalline silicon thin films having different degrees of crystallinity. An electrophotographic photosensitive member characterized in that the crystallinity of at least one of the microcrystalline silicon thin films varies from film to film in the layer thickness direction. (2) The electrophotographic photoreceptor according to claim 1, wherein the thin film has a thickness of 30 to 200 Å. (3) The photoconductive layer contains at least one element selected from elements belonging to Group III and Group V of the periodic table. Photographic photoreceptor. (4) The electrophotographic photoreceptor according to any one of claims 1 to 3, wherein the charge generation layer contains at least one element selected from carbon, oxygen, and nitrogen. . (5) The photoconductive layer contains at least one element selected from carbon, oxygen, and nitrogen, and the concentration thereof changes in the layer thickness direction.
The electrophotographic photoreceptor according to any one of item 4. (6) A patent claim characterized in that a barrier layer made of an amorphous material or a semiconductor material in which at least a portion thereof is crystallized is formed between the conductive support and the photoconductive layer. The electrophotographic photoreceptor according to item 1. (7) The electrophotographic photoreceptor according to claim 6, wherein the barrier layer contains at least one element selected from elements belonging to Group III and Group V of the periodic table. . (8) The electrophotographic photoreceptor according to claim 6 or 7, wherein the barrier layer contains at least one element selected from carbon, oxygen, and nitrogen. (9) The electrophotographic photoreceptor according to claim 1, wherein a surface layer is formed on the photoconductive layer.