JPS6388561A - Electrophotographic sensitive body - Google Patents

Abstract

PURPOSE:To improve sensitivity of the titled body at a long wave length range by laminating an amorphous photoconductive layer comprising a silicon atom as a base material as a 1st. layer, and a amorphous photoconductive layer composed of the silicon atom and a germanium atom as a 2nd. layer on a conductive support. CONSTITUTION:The amorphous photoconductive layer 3 as the 1st. layer which contains 0.01-1atomic% nitrogen atom and 10<-4>-10<-2>atom% boron atom as the base material, and the amorphous photoconductive layer 4 as the 2nd layer which is composed of the silicon atom and the germanium atom, and contains 0.01-0.1atom% nitrogen atom and 5X10<-3>atom% boron atom, are formed on the conductive support. The conductive support is formed by a conductive substrate 1 and blocking layer 2. A surface protective layer 5 is finally provided on the layer 4. Said layer 5 is necessary to be translucent and high resistance and has preferably 500-5,000Angstrom film thickness. Thus, the titled body has sufficient sensitivity and electrification at a long wavelength range.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to improvement of an electrophotographic photoreceptor for a laser printer using amorphous silicon germanium (hereinafter referred to as "a-5iGexJ").

[Problems to be solved by conventional technology and invention] Conventionally,
The photoconductive layer of the electrophotographic photoreceptor is coated with fine powder such as CaS or ZNO dispersed in an organic substance, or A8 or T.
Amorphous selenium doped with e, organic semiconductors such as polyvinylcarbazole and trinitrofluorene have been used. However, resin dispersion materials such as Cd3% ZNO have problems with moisture resistance, amorphous selenium has problems with crystallization at high temperatures and toxicity of the material itself to the human body, and organic semiconductors have problems with moisture resistance. There is a problem with printing durability.

Amorphous silicon (hereinafter referred to as "a-siJ
) can be mentioned. A-31 is a pn by a group of 5pear professors at the University of Dundee in England.
It has been attracting attention since its successful control, and has advantages such as high printing durability as a photoreceptor due to its surface hardness comparable to that of diamond, the ability to control spectral sensitivity, and non-pollution. .

Electrophotographic laser printers have the advantages of high quality printing and image quality, as well as low noise.
It is expected that this product will be widely used in office automation equipment such as Japanese word processors. Semiconductor lasers have been widely used as light sources in laser printers due to the need to make devices smaller and lighter. At present, the stable oscillation wavelength of semiconductor lasers is in the long wavelength region of 7,600 m or more.

The optical bandgap of a-3i is usually /, 1 to /
, rθV, the maximum wavelength of spectral sensitivity during travel when used as a photoconductor is around &j Onm, and it can be used in ordinary copiers (ppc) and LED printers that use light emitting diode play as a light source. However, the oscillation wavelength of the semiconductor laser, which is the light source of the laser printer, is 760.
There is a problem in that it is difficult to achieve a practically sufficient sensitivity in the wavelength region longer than nm. Therefore, a-51cex is smaller than a-5i.

a-5IGeX is usually obtained as a glow discharge decomposition product of a mixture of SiH gas and GeH4 gas, but SiH,
By changing the gas flow rate ratio of
It is possible to control the ratio of 81 and Ge in the 7 films, and as a result, the optical band gap can be controlled.

Although the &-81GeK film has these advantages,
There is a drawback that the dark resistance decreases as the amount of Ge atoms increases, that is, as the band gap decreases. This is considered to be caused by an increase in thermally excited carriers as the band gap decreases. A like this
When an 81 G8z film is formed near the surface of a photoreceptor, charging characteristics deteriorate, such as a decrease in surface potential and an increase in dark decay. Although the deterioration of charging characteristics can be avoided by forming a layer near the substrate, the absorption of long wavelength light by the a-8i layer itself on the a-84Gex O cannot be ignored, and the sensitivity in the long wavelength region is sufficient. I can't even remember.

We have conducted intensive research to create an electrophotographic photoreceptor for laser printers that has sufficient sensitivity in the long wavelength region and good charging characteristics by designing the overall layer structure of the photoreceptor and optimizing the dopant in each layer. As a result, we have arrived at the present invention.

That is, the gist of the present invention is to form a first layer on a conductive support with a nitrogen atom content of 0.01 to /-2 at. %
, and the boron atom content is 10-4 to 10-2
at. %, an amorphous photoconductive layer with a silicon atom content as the second layer, and a nitrogen atom content of 0.0/-0
,/-2at. % and the boron atom content is 5×10 −2 at. % or less of silicon atoms and germanium atoms, and an electrophotographic photoreceptor characterized in that it forms an amorphous photoconductor consisting of silicon atoms and germanium atoms of 9% or less, and the inclusion of nitrogen atoms as a first layer on a conductive support. The amount is 0.0/
-/-2at. % and the boron atom content is io
~10 −2 at. %, the second layer has a nitrogen atom content of 0.01 to 0.1 at. % and a boron atom content of 1 x 10-3 at.% or less, an amorphous photoconductive layer consisting of silicon atoms and germanium atoms;
The content of nitrogen atoms as the third layer is 0.0 / − / −
2 at. % and the boron atom content is 5×10
~io-2at. The present invention relates to an electrophotographic photoreceptor characterized in that an amorphous photoconductive layer is formed having silicon atoms as a host material.

The present invention will be explained in detail below.

The evaluation method for the optical properties of a-8i and a-5icex films is usually described by E,^,Davie & N,F,Mo
tt(Philoe.

Mag, volume 2i, page 3, p. 1270). The optical bandgap is calculated from the following formula.

αhν=B (h sea kg)” ・・・・・・・・・
...(I) In equation (1), α is the absorption coefficient of the membrane, h is pla
nclc is a constant, ν is the optical frequency, and Eg is the optical band gap. Experimentally, the 1/co power of αhν obtained from the absorption coefficient measured at each frequency is greyed out with respect to hν, and the intercept between the straight line portion and the hν axis is defined as Kg (optical band gap).

This is shown in Figure/. Equation (1) OB reflects the slope of this straight line portion, and this value is hereinafter referred to as the B coefficient. If the absorption coefficient α is expressed in units of 4”jl ”” and the photon energy hν is expressed in units of ev, the B coefficient is 1-1·ev.
- expressed in units of 1.

The physical meaning of the B coefficient is considered to represent the band base state of an amorphous material.

This is shown in FIG. The band edge structure of an amorphous semiconductor is generally considered to have a shape as shown in FIG. 2, and this shape is reflected in light absorption, and the wider the base, the smaller the B coefficient.

FIG. 3 shows the layer structure of the photoreceptor according to the present invention.

1 is a conductive substrate, usually an aluminum drum. 3 is a photoconductive layer made entirely of silicon, and B and P are used to provide characteristics as a photoreceptor.

It is doped with trace amounts of N, O, O, etc. The content of nitrogen atoms in layer 3 is 0.01 to /-2 at. %, the boron atom content is 70 to 10 −2 at. %, which is undesirable because it causes a decrease in sex. In addition, if the content of boron atoms is less than the above range, the amount of Hiroshi 00 necessary for photoconductivity
To maintain a surface potential of more than volts! ~jo am
is set to

If necessary, amorphous silicon carbide (hereinafter [a-si
It's called cXJ. ), amorphous silicon nitride (hereinafter referred to as [a-siNxJ)], amorphous silicon oxide (
An insulating blocking layer 2t-/00-1000 angstroms thick, such as a layer 3 (hereinafter referred to as "a 5 to The amount is 0
．． 0/-0,/at.

The content of carbon atoms and boron atoms is less than or equal to I×IO”.

In the case of nitrogen atoms, if the content of nitrogen atoms and boron atoms in the layer is less than the above range, the dark resistance will decrease;
If the amount is too large, the photoconductivity decreases, which is not preferable. Further, in the case of boron atoms, if the amount exceeds the above range, the dark resistance and the density of the film will decrease, which is not preferable. In addition, 1hiro is usually formed into a film at a ratio of Go+(,/5iH4=0.03 to O1/!), and the ratio of germanium atoms to silicon atoms in the film is usually 0.2:0.lr. 5O0j
: It is preferable that it is within the range of 0).

If the ratio of germanium atoms in the film is smaller than this range, it is not preferable because if the band gap is too small, it will not be possible to obtain a material capable of producing long wavelengths. This layer μ plays the role of an absorption layer for long wavelength light, and its thickness is preferably set in the range of 0,i to 3°08 m. The surface image is on this 1 Hiro! 1
layer! is obtained at the end. The conditions required for this layer are translucency (optical bandgap is coeV or more)
and high resistance (specific resistance of 10120·α or more). A commonly used material that satisfies these conditions is a8iCzva-8INz.

a-0iOx etc. This surface protective layer improves the charging characteristics of the photoreceptor, increases the surface potential, lowers the dark decay rate, and stabilizes the photoreceptor during repeated charging.

is achieved. The m thickness of this surface protective layer is j0Q~5oo
It is preferable that the photoreceptor has a thickness of 0 angstroms. Figure ≠ shows the layer structure of the photoreceptor shown in Figure 3 with further improved charging characteristics. The improvements are that the surface protective layer/l and a-
20a-51 layer 10 is laminated between the siceX layer and the siceX layer.

For the same reason as in layer 3 and layer≠, the content of nitrogen atoms and boron atoms in Hio is 0.01 to /-2 at. Ch, the content of boron atoms! xlO""4 to 10" at 9% is preferred.

Also, this second a-S 1 layer 10 is the first a-S layer 10
It is not necessarily necessary that the film quality is the same as that of layer 1, but the optical band gap and doping amount (B, N
, Olo, p, etc.). (2g
The bandgap of the second a-8i layer 10 is
It is preferable to make it larger than the band gag in layer t.

) The thickness of the second a-3i layer is preferably 0.1 to 3 μm.

〔Example〕

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist thereof.

In addition, when the content of nitrogen and boron atoms in the photoreceptor was measured for each layer using secondary ion mass spectrometry (SIMS), it was found that the content in the film for the a-8i layer and the a-81Gez layer was higher than that of gas. The correspondence between flow rate ratio and l to l is shown.

(Example/Comparative Example 1) An aluminum substrate is placed on a substrate hole with a built-in heater in a vacuum container and heated to 320°C. Preliminarily evacuate the container using an oil diffusion pump to a 10 mcuff 5th power platform. Switch the exhaust system to an oil rotary pump, mass 70-
The controller controls SiH4 gas/O5C
IC! M%N, O gas 30 SCI:! Flow 700,800 M of argon gas into the above vessel. A throttle valve between the vessel and the oil rotary pump is adjusted so that the pressure within the vessel is 0.6 Torr. In this state, a voltage of about 300 volts is applied between the electrodes facing parallel to the aluminum substrate and separated by j1, and the substrate to cause glow discharge and deposit.

The glow discharge is temporarily stopped and the reaction gas is exhausted to reduce the pressure inside the container to below 0,000 torr. Mass flow controller VC Yori 5iH4fj Suwo3 IS OCM,
B, H, gas diluted to 160 PPm with hydrogen ≠j
Ammonia gas diluted to 7% with 500M He
SOC! M, respectively, when the total pressure inside the container is 0. Make it I tall. At this time, the flow rate ratio of ammonia to monosilane and N is 0° and 2%, and the flow rate ratio of diborane to monosilane is ≠θ? p”.

A layer is deposited on the a-8i light guide 1 at a power density of 0.0, O6 W/-.

After stopping the glow discharge again and exhausting the reaction gas, the Si
n, gas J OSCCM, G8 H4 gas diluted to 10% hydrogen/, t 5CIC! 1% in M%Hθ
Ammonia gas diluted to /. jsccM, respectively, and the pressure is o, r torr ((.G9H at this time
4 flow rate to S i H, the ratio to the flow rate is 0. Or, the flow rate ratio of ammonia to monosilane is 0, O1%
It is. a −5iG at a power density of 0,06W/−
Deposit the ex layer.

After stopping the glow discharge and exhausting the reaction gas, S i
The pressure was adjusted to 0.1 p) by flowing H, gas and N, O gas at a rate of jaccM each.

A film is deposited using a Hiromin method at a power density of 0, ozwy- to form a surface protective layer of a-slo:c/il with a thickness of about 100 angstroms.

In the layer structure of the photoreceptor described above, the film thickness of the a-siGex layer is 0.50 minutes when the film formation time of the a-siGex layer is A, /co, and 26% 50 minutes. j, /, 0% co, O, a. Opm, and in order to make the film thickness of the entire photoconductor equal to /μm thickness, the film forming time of the a-8% layer was changed to /31°13μm depending on the film thickness of a-3iGexffJ. ,lkoo,5Pt
It was a minute. Also, for comparison, a-81Gex
A photoreceptor without a layer was also produced (Comparative Example 1).

These photoreceptors were measured at a corona voltage of +4 KV using a surface potential measuring device (5P-4T Cor, manufactured by Kawaguchi Electric Co., Ltd.). The dark decay relationship was evaluated by the decrease in surface potential within 2.4% seconds of the initial stage of corona charging, and the residual potential was evaluated by the potential after irradiating white light colux for 10 seconds. In addition, using a monochromator, half-exposure sensitivity (E
jTO) was measured to evaluate the long wavelength sensitivity. Summarize the evaluation results in a table/table.

The sensitivity at 0/r 00 nm increases with respect to the film thickness of a-3iGex up to 2 μm, but it can be said that there is almost no increase in the effect at film thicknesses beyond that.

Furthermore, the photoreceptor of Comparative Example 1 had low long wavelength sensitivity and could not be used as a photoreceptor in practice.

(Example 2 and Comparative Example 2) Blocking layer and first a-si
After depositing the layer, ammonia gas diluted to 1% with H19 was added to 0,7 j SCC! Example other than M/
An a-5iCrex layer of about λμm is deposited under the same film forming conditions as . The second a-3 under the same conditions as the first a-8i layer.
The i-layer is subsequently deposited. A surface protective layer is deposited in the same manner as in Example/1 to produce a total of five layers of photoreceptor. At this time,
In order to investigate the influence of the position of the a5iGeX layer in the photoreceptor depth direction, a photoreceptor was created with the layer configuration shown below.Two surface layers/second a-Si layer (tIim)/a-8i
de, layer (2 μm, )/first a-Si layer ((1
0-t)μm)/charge injection blocking layer/aluminum substrate. The total film thickness of the photoreceptor is approximately 72 μm.

Table 2 shows the evaluation results of the long wavelength sensitivity of charging characteristics.

Table 2 (Example J) After performing the same heating and exhausting operations as in Example 1, approximately 300
A 5iOxllllI blocking layer of angstrom was used. After stopping the glow discharge and exhausting the reaction gas to reduce the pressure to below o, oi torr, SiH4 gas was diluted with iosccM and hydrogen to 26Qppm.
IH tatami for 2,3 secM, hydrogen gas for JJ130C
4800M ammonia gas diluted to 1% with M%He
, respectively, and the pressure is 0. f Adjust to tall. At this time, the flow ratios of ammonia and diborane to monosilane were 0.2 ppm and 20 ppm, respectively. Power density 0
,0 /T W /cfJ4 /CoQ 20 minutes film formation approx. 1
Jfk product the first a-8i layer of 0 μm. Stop the glow discharge and exhaust the gas again, S i H, gas i
rsacM, G e H4 gas diluted to 10% with hydrogen/, tE300M% B diluted to 260 ppm with hydrogen, H, gas o, is SCICM, 1 with He
% diluted ammonia gas at Q, 7.5 ccu, respectively, and adjust the pressure to I:), Hir). At this time, the flow ratios of ammonia and diborane to monosilane are each 0. O3% and/Oppm, GeH,
The flow rate ratio of SIH, to SIH, is θ,/. 0.0λW
/- for 10 minutes at a power density of about 0.34 m.
- Deposit the sicsx layer. Next, the first a-8i was used, except that the flow rate ratio of hydrogen to monosilane was i and o.
The second a-3 was deposited for 12 minutes under the same film forming conditions as IIl.
Leave an i-layer of approximately/μm. Further, the discharge is stopped and exhausted, and an a-8iO1 film is deposited as a surface protective layer in the same manner as in Example.

The total film thickness of the photoreceptor obtained in the above manner is / 0.1
It was tlm. In addition, for the %1〇a 31 layer and the second a-81 layer, the optical bandgap was measured by forming a single layer on a glass substrate, and the results were /, 7, respectively.
2 and i, r t ev were obtained. moreover,
When the composition of the a-8iGez layer was analyzed by the ICP method, the ratio of silicon atoms to germanium atoms was found to be Fitl: 32.

The charging characteristics of this photoreceptor are: surface potential = 263 volts, dark decay rate = / 1.1%/sec, residual potential 2μ,
6 volts, which is good, and 7 or 3 measured using an interference filter. The half-reduction exposure sensitivity at jnrn is 0. /
It showed sufficient value for I6d/erg and laser printer.

(Example 4L) GeH4 diluted to 2.3% with OM% hydrogen
Gas / J 5CIC! M, B2H diluted to 260 ppm with hydrogen.

Anthonia gas diluted to 7% with O, gsccM and He is Q, 7jSCC! M, hydrogen gas /, 230C
! M, respectively, and the pressure is O,! Adjust to tall.

At this time, the flow ratios of ammonia and diborane to mono7ran were each 0. Oj % and / Oppm and Ge
The flow filx ratio of H4 to SiH4 is o, or. After forming a film for 20 minutes at a power density of 0.021/cd, approximately 0.0.
Deposit a 31Gex layer of j ttm. Then the flow rate ratio of diborane to monosilane f/Opp
The second a-8t photoconductive layer and the surface protective layer are laminated by #ti in the same manner as in Example 3, except that m is used. The total film thickness of the obtained psychophotoreceptor was 10.0 μm.

The charging characteristics of this photoreceptor include: surface potential = 2.3 volts; dark decay rate = /J; 5'%/sec; residual potential =! ,
The Q bolt was in good condition. A diagram of the spectral sensitivity characteristics obtained using a monochromator! Shown below. a81Ge
200 compared to a conventional photoreceptor that does not contain z4.
Improvement in sensitivity is seen over a wide wavelength range of ~I 00 nm, which is useful not only for laser printers, but also for laser printers.
It can be said that it is superior to ppa and LED printers.

(Example j) Diameter rOtttx, length Jjryts, thickness! Simple 1 Surface finish o, is, material JISJOO,! Using an aluminum drum as a substrate, film formation is performed by generating glow discharge between the aluminum drum and a counter electrode arranged concentrically in a cylindrical shape (with a distance of !α). The film forming conditions in this case are shown below.

l) First a-81 layer 51H4: uooscaM Bs Hs / H Goose (Core Qppm) Niko ζ ASOC
! MNl(,/Hθ(1%):LPSOOMH,:
3t. +s c c M At this time, the flow ratios of ammonia and diborane to monosilane were 0.2% and 20 ppm, respectively.

Power density: 0. / W/i Film formation time: 733 minutes (approximately 20 μm) 2) a-3iG+5x layer SiH,: 111300M GeH4/Hl (5', J%): P/
,rscc! MBzH@ / Hl (270ppm
): J, / SOCMNH, / He (7%): 4
L, 7500M At this time, the flow rate ratio of ammonia and diborane to monosilane is 0.0t% and 10ppm, respectively.
, and the flow rate ratio of G 8 H4 to S i H4 is 0.7.

Power density: (7.
．． II! 300MNH,/He (7%): 41. j
sOc! MH,: /λj8ccM At this time, the flow ratios of ammonia and diborane to monosilane are Q, core and lOppm, respectively. S of Hl
The flow rate ratio for iH and Ic is /.

Power density: 0. /W/ad Film formation time: 1 minute (approx./ttm) Surface protective layer 5iH4: 20scaM N, O: 2oscabt Power density: Q, 02W/i Film formation time: 11 minutes (approx. 3000 angstroms)
) The total film thickness of the photoreceptor drum prepared by laminating each layer of Karari in sequence was 23 μm. I'm playing this drum for 7 seconds.
When the image was loaded into a printer using a semiconductor laser as a light source with an oscillation wavelength of , and reverse development was performed using positive charging, clear text and graphics were obtained. At this time, a black peta was taken out to examine the interference fringes, but there were no moiré fringes as described in JP-A-40-37/Hirohiko Publication.0 [Effects of the Invention] Electrophotographic photoreceptor of the present invention has sufficient sensitivity and charging characteristics in the long wavelength region, and is therefore suitable for electrophotographic photoreceptors for laser printers.

[Brief explanation of the drawing]

Figure 1 is a graph used when determining the optical band gap and the 8 coefficient, and is a graph in which the 772nd power of the product of the absorption coefficient α and the photon energy hν is plotted against the photon energy hν. FIG. 1 shows the band state of an amorphous semiconductor. FIG. 3 shows the layer structure of the photoreceptor, in which l is the substrate, c is the blocking 71, j is the a-8i photoconductive layer, and ≠ is asiGe.
x layer! denotes surface retention am, respectively. The figure ≠ shows the layer structure of a photoreceptor with improved charging characteristics, where 6 is a substrate and 7 is a blocking layer. r represents the first 0a81 photoconductive layer, ri represents the a-8iGez layer, 10 represents the second a-S L photoconductive layer, and l/ represents the surface protection 1-. figure! is a long-wavelength a-Si photoreceptor and a conventional a-S photoreceptor.
FIG. 3 is a diagram showing a comparison of spectral sensitivity characteristics of i photoreceptors. Phrase diagram 1 Figure 2 Figure 3 Figure 4; L length [nm1 Figure 5

Claims (15)

[Claims] (1) On the conductive support, the first layer has a nitrogen atom content of 0.01 to 1 at. % and the content of boron atoms is 10^-^4 to 10^-^2 at. %, and the second layer has a nitrogen atom content of 0.01 to 0.1 at. % and the boron atom content is 5×10^-^3 at. 1. An electrophotographic photoreceptor comprising an amorphous photoconductive layer comprising silicon atoms and germanium atoms in an amount of % or less. (2) The first layer is 5 to 50 μm, the second layer is 0.1 to 3 μm
The electrophotographic photoreceptor according to claim 1, having a film thickness of . (3) The second layer is GeH_4/SiH_4=0.03~
Claims characterized in that the film is formed at a ratio of 0.15, and the ratio of germanium atoms to silicon atoms in the film is within the range of 0.2:0.8 to 0.5:0.5. The electrophotographic photoreceptor according to any one of Items 1 and 2. (4) The electrophotographic photoreceptor according to any one of claims 1 to 3, further comprising a surface protective layer. (5) The electrophotographic photoreceptor according to claim 4, wherein the surface protective layer is made of insulating amorphous silicon oxide and has a thickness of 500 to 5000 angstroms. (6) The electrophotographic photoreceptor according to any one of claims 1 to 5, further comprising a charge injection blocking layer between the conductive support and the first layer. (7) The electrophotographic photoreceptor according to claim 6, wherein the charge injection blocking layer is made of amorphous silicon oxide and has a thickness of 100 to 1000 angstroms. (8) On the conductive support, the first layer has a nitrogen atom content of 0.01 to 1 at. % and the content of boron atoms is 10^-^4 to 10^-^2at. %, and the second layer has a nitrogen atom content of 0.01 to 0.1 at.%. % and the boron atom content is 5×10^-^3 at. % or less, an amorphous photoconductive layer consisting of silicon atoms and germanium atoms, the third layer having a nitrogen atom content of 0.01 to
1 at. %, and the boron atom content is 5 x 10^-
＾4〜10＾−＾2at. %. (9) The first layer is 5 to 50 μm, the second layer is 0.1 to 3 μm
9. The electrophotographic photoreceptor according to claim 8, wherein the third layer has a thickness of 0.1 to 3 μm. (10) The second layer is GeH_4/SiH_4=0.03
The film is formed at a ratio of ~0.15, and the ratio of germanium atoms to silicon atoms in the film is within the range of 0.2:0.8 to 0.5:0.5. The electrophotographic photoreceptor according to any one of Item 8 or Item 9. (11) The electrophotographic photoreceptor according to any one of claims 8 to 10, characterized in that the bandgap of the third layer is larger than the bandgap of the first layer. (12) The electrophotographic photoreceptor according to any one of claims 8 to 11, further comprising a surface protective layer. (13) The surface protective layer is made of insulating amorphous silicon oxide,
13. The electrophotographic photoreceptor according to claim 12, wherein the film thickness is 500 to 5000 angstroms. (14) Claims 8 to 1, characterized in that a charge injection blocking layer is provided between the conductive support and the first layer.
The electrophotographic photoreceptor according to any one of Item 3. (15) The electrophotographic photoreceptor according to claim 14, wherein the charge injection blocking layer is made of amorphous silicon oxide and has a thickness of 100 to 1000 angstroms.