Patent Publication Number: US-11393857-B2

Title: Image sensor and image sensing-enabled display apparatus including the same, and method of making the image sensor

Description:
FIELD 
     The disclosure relates to an image sensor, and more particularly to an image sensor including an intrinsic semiconductor layer having a crystallinity gradient, an image sensing-enabled display apparatus including the image sensor, and a method of making the image sensor. 
     BACKGROUND 
     Mobile apparatuses, such as smart phone, smart tablet computer, and laptop computer, have become a daily necessity for most people nowadays. The mobile apparatuses generally have a display member in common. With the development of display technology toward a flexible display member, the flexible display member has been used in a foldable mobile apparatus or a rollable mobile apparatus so as to provide new appearance and additional function different from those of conventional rigid mobile apparatuses. For example, a foldable smart phone can be unfolded to become a smart tablet computer, and can serve as a laptop computer when it is used with an external keyboard. With the inclusion of the flexible display member, multiple mobile apparatuses having different functions can be integrated in a single apparatus which possesses thin, light and portable properties. For example, a flexible display overcenter assembly and a flexible display flexure assembly are disclosed in U.S. Pat. Nos. 8,804,324 B2 and 9,176,535 B2, respectively. 
     A conventional display member, such as a liquid crystal display (LCD) member or an active matrix organic light emitting display (AMOLED) member, includes a plurality of thin-film transistors (TFTs) and a plurality of pixels that are arranged in an array. Each of the pixels is driven by a respective one of the thin-film transistors for displaying an image. Each of the TFTs is a field-effect transistor that includes a semiconductor layer made from a semiconducting material such as amorphous silicon, crystalline silicon, indium gallium zinc oxide (IGZO), a nanocarbon material-mixed organic material, and combinations thereof. The TFTs may be formed on a flexible substrate, such as a heat resistant polymeric substrate including polyimide. 
     Since photodetection diodes may be manufactured using the above semiconducting material and since production equipment of the photodetection diodes is incorporable into production equipment of the TFTs array, an image sensor including a plurality of photodetection diodes and a plurality of TFTs arranged in an array is manufactured using the production equipment of the TFTs array and is utilized indifferent fields, such as an X-ray flat panel detector disclosed in Chinese Invention Patent Publication No. CN 103829959 B, and an X-ray image sensing element and a sensing module disclosed in Chinese Invention Patent Publication No. CN 102903721 B. Compared with image sensors made from conventional crystalline materials, the semiconducting material for making the photodetection diodes of the conventional image sensor has a band gap for absorbing substantial visible light, and the conventional image sensor has a relatively low signal-to-noise ratio (SNR) due to less interference from environmental visible light. Hence, the application of the image sensor focuses primarily on the technical aspects of X-ray sensing, such as the abovementioned X-ray flat panel detector and X-ray image sensing element and sensing module. For alleviating the interference from the environmental visible light, the abovementioned X-ray flat panel detector and the X-ray image sensing element and sensing module include at least one fluorescent layer or flickering layer for converting an incident X-ray light, which has a relatively short wavelength and a relatively high collimation, to the visible light. The visible light subsequently emits on the photodetection diodes. 
     When the image sensor is incorporated with the flexible substrate and then is integrated with the display member to be disposed in a flexible display device, the integration of the image sensor with the display member can be contemplated so as to realize the flexible display device with a photodetection function, i.e., a flexible image-sensing display device. However, since the conventional display member is limited by its thickness and a pixel aperture ratio thereof, an image generated from the light detected by the photodiodes is distorted because of optical diffraction. Besides, since incident light has to pass through multiple layers of the conventional display member before being detected by the photodetection diodes, and since there are optical display signals and touch sensing signals being transmitted in the flexible image sensing display device, it is difficult to retrieve useful optical signals from the environment inside the flexible image sensing display device with a low signal-to-noise ratio. The difficulty level in retrieving the useful optical signals is approximately equal to that of single-photon imaging. An original image generated by the electrical signals has to be resolved by reconstruction through an algorithm on the basis of an optical theory. In order to deal with the difficulty of optical-signal retrieval, it is proposed to further dispose an optical reinforcing member in the conventional flexible image-sensing display device or to dispose the image sensor on one side of the display member, such as a display module disclosed in Chinese Invention Patent Publication No. 101359369 B, so as to reconstruct the image through light that is not vertically incident on the side of the display member. However, inclusion of the optical reinforcing member disadvantageously increases the thickness of the flexible image-sensing display device, and side-arrangement of the photodiodes on the display member tends to obstruct full-screen viewing. When the flexible image-sensing display device is applied to the foldable mobile apparatus, it is difficult to integrate multiple functions in the foldable mobile apparatus including the flexible image-sensing display device. 
     Referring to  FIG. 1 , a conventional image sensor includes a photodetection diode that includes an amorphous silicon n-type layer  93 , an amorphous silicon intrinsic layer  92  formed on the amorphous silicon n-type layer  93 , and an amorphous silicon p-type layer formed on the amorphous silicon intrinsic layer  92 . The image sensor has a relatively low photoelectric conversion efficiency and the amorphous silicon intrinsic layer  92  thereof has a relatively serious staebler-Wronski effect occurred at the initial stage of light exposure. Thus, the conventional image sensor does not satisfy the high light sensitivity required by a conventional thin film apparatus, such as a flexible display apparatus, and cannot be easily applied to a flexible display apparatus with photodetection function. Besides, the additional difficulty in integration of the flexible image sensor with the flexible display apparatus comes from maintenance, reliability and the lifetime of the flexible display apparatus, which depends on water resistance and oxygen barrier properties of the flexible display apparatus. For example, an flexible AMOLED is required to have a water vapor transmission rate of at least less than 10 −6  g/m 2  per day, and an oxygen transmission rate of at least less than 10 −6  cm 3 /m 2 ·bar per day. Hence, it is also a focal point to develop the flexible image-sensing display apparatus with satisfactory water resistance and oxygen resistance. 
     Therefore, there is plenty of room for improving the image sensor included in the image-sensing display apparatus and for improving a method of making the image sensor so as to expand the wavelength range received by the image sensor and to increase the photoelectric conversion efficiency of the image sensor. 
     SUMMARY 
     Therefore, an object of the disclosure is to provide an image sensor that can alleviate at least one of the drawbacks of the prior art. 
     According one aspect of the disclosure, an image sensor includes a plurality of pixel sensing portions that are arranged in m columns and n rows. Each of m and n is a positive integer not less than 1. Each of the pixel sensing portions includes: a thin film transistor, and a photodetection diode that is electrically connected to the thin film transistor and that includes an n-type semiconductor layer, an intrinsic semiconductor layer formed on the n-type semiconductor layer, and a p-type semiconductor layer formed on the intrinsic semiconductor layer. 
     The intrinsic semiconductor layer of the photodetection diode of each of the photodetection pixel portions has a crystallinity gradient that varies from an amorphous silicon structure to a microcrystalline silicon structure along a first direction (L 1 ) extending from the p-type semiconductor layer toward the n-type semiconductor layer. 
     According another aspect of the disclosure, an image sensing-enabled display apparatus includes a display unit defining an image-sensing region, and at least one the above image sensor disposed below the image-sensing region. 
     According another aspect of the disclosure, a method of making an image sensor includes: forming a removable adhesive layer on a template; forming a substrate on the removable adhesive layer; forming a thin-film transistor on the substrate; forming a photodetection diode on the substrate by forming an n-type semiconductor layer on the substrate, forming an intrinsic semiconductor layer on the n-type semiconductor layer, and forming a p-type semiconductor layer on the intrinsic semiconductor layer that has a crystallinity gradient that varies from an amorphous silicon structure to a microcrystalline silicon structure alone a first direction extending from the p-type semiconductor layer toward the n-type semiconductor layer; and removing the removable adhesive layer and the template from the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the disclosure will become apparent in the following detailed description of the embodiments with reference to the accompanying drawings, of which: 
         FIG. 1  is a schematic view illustrating a conventional photodetection diode of a conventional image sensor; 
         FIG. 2  is a schematic cross-sectional view illustrating one of pixel sensing portions of an embodiment of an image sensor according to the disclosure; 
         FIG. 3  is a schematic view illustrating a photodetection diode of the embodiment of the image sensor; 
         FIG. 4  illustrates two configurations (a) and (b) of incorporation of an optical film into the photodetection diode of  FIG. 3 ; 
         FIG. 5  is a schematic view illustrating three configurations (a) to (c) of incorporation of two or more optical films into the photodetection diode of  FIG. 3 ; 
         FIG. 6  is a schematic view illustrating another configuration of the photodetection diode of the embodiment of the image sensor; 
         FIG. 7  is a schematic view illustrating two configurations (a) and (b) of incorporation of an optical film into the photodetection diode of  FIG. 6 ; 
         FIG. 8  is a schematic view illustrating three configurations (a) to (c) of incorporation of two or more optical films into the photodetection diode of  FIG. 6 ; 
         FIG. 9  is a schematic view illustrating two forms (a) and (b) of an embodiment of an image sensing-enabled display apparatus according to the disclosure; 
         FIG. 10  is a fragmentary partly cross-sectional view illustrating incorporation of a waterproof and oxygen barrier structure into the image sensor of  FIG. 2 ; 
         FIG. 11  is a flow chart of an embodiment of a method of making an image sensor according to the disclosure; and 
         FIG. 12  is a schematic view illustrating consecutive steps of the method of making an image sensor of FIG.  11 . 
     
    
    
     DETAILED DESCRIPTION 
     Before the disclosure is described in greater detail, it should be noted that where considered appropriate, reference numerals or terminal portions of reference numerals have been repeated among the figures to indicate corresponding or analogous elements, which may optionally have similar characteristics. 
     Referring to  FIGS. 2 and 3 , an embodiment of an image sensor  32  according to the disclosure is illustrated. The embodiment of the image sensor includes a substrate  42 , a plurality of pixel sensing portions  320  (only one is shown in  FIG. 2 ) that are formed on the substrate  42  and that are arranged in m columns and n rows. Each of m and n is a positive integer not less than 1. The substrate  42  may be a glass sheet or a flexible polymeric sheet. Each of the pixel sensing portions  320  includes a thin film transistor  11 , and a photodetection region  322  that includes a photodetection diode  13  that is electrically connected to the thin film transistor  11  and that includes an n-type semiconductor layer  16 , an intrinsic semiconductor layer  15  formed on the n-type semiconductor layer  16 , and a p-type semiconductor layer  14  formed on the intrinsic semiconductor layer  15 . The intrinsic semiconductor layer  15  has a crystallinity gradient that varies from an amorphous silicon structure to a microcrystalline silicon structure along a first direction (L 1 ) extending from the p-type semiconductor layer  14  toward the n-type semiconductor layer  16 . 
     The thin film transistor  11  of each of the pixel sensing portions  320  is used for transmitting an electrical signal from the photodetection diode  13 . In the embodiment, the substrate  42  is a polymeric substrate that is disposed to the thin film transistor  11  and the photodetection diode  13  of each of the pixel sensing portions  320 . The polymeric substrate  42  includes polyimide. 
     The thin film transistor  11  includes a gate electrode metal layer  102 , a first electrically isolating layer  103 , an intrinsic amorphous silicon channel layer  104 , a n + -doping amorphous silicon source/drain contact layer  105 , a source/drain electrode layer  106 , and a second electrically isolating layer  107  sequentially formed on the polymeric substrate  42  in that order. The thin film transistor  11 , the gate electrode metal layer  102 , the first and second electrically isolating layers  103 ,  107 , the intrinsic amorphous silicon channel layer  104 , the n + -doping amorphous silicon source/drain contact layer  105 , and the source/drain electrode layer  106  are not of the essence of the present disclosure and therefore will not be further elaborated for the sake of brevity. 
     The photodetection region  322  of each of the pixel sensing portions  320  further includes a transparent metal-oxide electrode layer  108  formed on the photodetection diode  13  and a portion of layers of the photodetection region  322  are formed together with the layers of the thin film transistor  11 , such as the gate electrode layer  102 , the first electrically isolating layer  103 , the source/drain electrode layer  106 , the second isolating layer  107 . In other words, when the gate electrode layer  102 , the first isolating layer  103 , the source/drain electrode layer  106  and the second isolating layer  107  of each of the pixel sensing portions  320  are formed, these layers extend from the thin film transistor  11  to the photodetection region  322 . 
     Crystalline silicon normally has a tetrahedron structure where each of silicon atoms is tetrahedrally and covalently bonded to four neighboring silicon atoms. The tetrahedral structure of the crystalline silicon (c-Si) is able to continue over a large range, thus forming a well-ordered crystal lattice. However, for amorphous silicon, the long range of the tetrahedral structure as with the c-Si is not present and silicon atoms form a continuous random network. In other words, not all of the silicon atoms of the amorphous silicon are arranged as the tetrahedral structure. Hence, a portion of the silicon atoms of the amorphous silicon have dangling bonds due to such random structural arrangement, and cannot be covalently bonded to the neighboring silicon atoms. However, electrical conductivity of the amorphous silicon is reduced by the dangling bonds. In practical use, the dangling bonds may be passivated by hydrogen, and thus density of dangling bonds of the hydrogenated amorphous silicon can be reduced drastically so as to satisfy the requirement for a semiconducting material. However, since the number of the dangling bonds in the hydrogenated amorphous silicon is still much greater than that of the crystalline silicon, application of this hydrogenated amorphous silicon to a p-doping semiconductor structure or an n-doping semiconductor structure will result in more defects than application of crystalline silicon to the p-doping or n-doping semiconductor structures. Furthermore, when the hydrogenated amorphous silicon is applied to an intrinsic semiconductor structure of a photodetection diode, Staebler-Wronski effect occurs in the intrinsic semiconductor structure during the initial stage of light exposure. 
     In this embodiment, the intrinsic semiconductor layer  15  of the photodetection diode  13  of each of the pixel sensing portions  320  has a crystallinity gradient that varies from an amorphous silicon structure to a microcrystalline silicon structure along the first direction (L 1 ). Hence, the photodetection wavelength range of the photodetection diode  13  of the embodiment can expand from the visible light region to the infrared light region or near the infrared light region, and thus the photoelectric conversion efficiency can be improved. More specifically, the intrinsic semiconductor layer  15  of the photodetection diode  13  of each of the pixel sensing portions  320  defines a plurality of intrinsic semiconductor sub-layers  151  extending along a second direction (L 2 ) perpendicular to the first direction (L 1 ). In this embodiment, four of the intrinsic semiconductor sub-layers  151  are shown. Each of the intrinsic semiconductor sub-layers  151  has a homogenous crystallinity. When the first direction (L 1 ) extends in a Z-axis of a coordinate system, each of the intrinsic semiconductor sub-layers  151  that extends along the second direction (L 2 ) extends over and is formed uniformly in the x-y plane of the coordinate system. For example, one of the intrinsic semiconductor sub-layers  151  which intersects the Z axis at a point z 1  may uniformly has a crystallinity of 8%, and another one of the intrinsic semiconductor sub-layers  151  below the one of the intrinsic semiconductor sub-layers  151  relative to the n-type semiconductor layer  16  may uniformly has a crystallinity of 16% or 20%. In other words, the crystallinity of each of the intrinsic semiconductor sub-layers  151  is uniformly distributed in the x-y plane, and the intrinsic semiconductor layer  15  which defines the intrinsic semiconductor sub-layers  151  has a crystallinity gradient gradually increased in the first direction (L 1 ) from 0% (corresponding to the amorphous silicon structure) to larger than 40% (corresponding to the crystalline silicon structure). Hence, the photoelectric conversion efficiency (e.g., photoelectric conversion quantum efficiency) of the photodetection diode  13  can be improved, and the image sensor  32  including the photodetection diode  13  can be used in a high photosensitivity field, e.g., an image sensing-enabled display apparatus having a relatively large sensing area. 
     Specifically, when there are more than four of the intrinsic semiconductor sub-layers  151  present in the intrinsic semiconductor layer  15 , the crystallinities of the intrinsic semiconductor sub-layers  151  may vary by a fixed amount, such as 8%, along the first direction (L 1 ), i.e., the crystallinities of the intrinsic semiconductor sub-layers  151  superimposed on one another in the first direction (L 1 ) may respectively be 0, 8%, 16%, 24%, 32%, 40%, etc. In another form, the crystallinities of the intrinsic semiconductor sub-layers  151  may vary by a variable amount along the first direction (L 1 ). For example, the crystallinities of the intrinsic semiconductor sub-layers  151  superimposed in the first direction (L 1 ) may respectively be 0, 6%, 15%, 22%, 30%, 40%, etc. Hence, the crystallinity gradient of the intrinsic semiconductor layer  15  may be determined based on the actual use within the predetermined threshold range, i.e., 0% of the amorphous silicon structure to 40% of the crystalline silicon structure. 
     In this embodiment, the intrinsic semiconductor layer  15  of the photodetection diode  13  of each of the pixel sensing portions  320  may be made from silanes and hydrogen gas using the CVD techniques. Hydrogen gas ratio, deposition temperature, and deposition power are three process parameters for forming the intrinsic semiconductor layer  15 . The hydrogen gas ratio, deposition temperature, and deposition power for forming the microcrystalline silicon structure are higher than that for forming the amorphous silicon structure. By means of gradually increasing the hydrogen gas ratio, the deposition temperature, and the deposition power during the CVD process along with the increase in the crystallinity, the resulting intrinsic semiconductor layer  15  has a band gap decreasing from 1.7 eV along the first direction (L 1 ) and an average band gap less than 1.7 eV so as to have the required crystallinity gradient. 
     Referring to  FIGS. 3 and 6 , the p-type semiconductor layer  14  of the photodetection diode  13  of each of the pixel sensing portions  320  may have a single-layered structure or a multi-layered structure, and may be made from silanes, hydrogen gas and trimethyl boron (B(CH 3 ) 3 ) using the CVD techniques or may be made from silianes, hydrogen gas and borane using the CVD techniques. More specifically, as shown in  FIG. 6 , the multi-layered structure of the p-type semiconductor layer  14  includes a lower p-type semiconductor layered portion  142  formed on the intrinsic semiconductor layer  15  and an upper p-type semiconductor layered portion  141  formed on the lower p-type semiconductor layered portion  142 . The upper p-type semiconductor layered portion  141  has a band gap greater than 1.7 eV and a p-type dopant in an amount not less than two times of that of the lower p-type semiconductor layered portion  142 , so that the p-type semiconductor layer  14  has a band gap greater than 1.7 eV. In the embodiment, the p-type dopant is boron and is provided from trimethyl boron (B(CH 3 ) 3 ) or borane. By virtue of the structure of the p-type semiconductor layer  14 , when the incident light passes from a top surface of the p-type semiconductor layer  14  to the n-type semiconductor layer  16  through the intrinsic semiconductor layer  15 , the incident light absorbed by the p-type semiconductor layer  14  can be reduced, and thus the light passing through the p-type semiconductor layer  14  and absorbed by the intrinsic semiconductor layer  15  can be increased. Hence, the photoelectric conversion efficiency of the image sensor  32  of the disclosure can be enhanced so as to satisfy the actual application requirements. 
     Specifically, in the multi-layered structure of the p-type semiconductor layer  14 , the upper p-type semiconductor layered portion  141  of the p-type semiconductor layer  14  has a non-crystalline silicon structure and is heavily doped with the p-type dopant of borane, which leads to the increase in dangling bonds. If the p-type semiconductor layer  14  only includes the heavily doped upper p-type semiconductor layered portion  141 , uniformity of built-in voltage of the photodetection diode  13  tends to be further damaged or destroyed. Inclusion of the lower p-type semiconductor layered portion  142  will alleviate the damage or destruction of the uniformity of built-in voltage of the photodetection diode  13 . The lower p-type semiconductor layered portion  142  is normally doped and has the p-dopant in the amount not greater than half of that of the upper p-type semiconductor layered portion  141 . In addition, the lower p-type semiconductor layered portion  142  has a thickness lower than that of the single-layered structure of the p-type semiconductor layered portion  14 . The decrease in the thickness of the lower p-type semiconductor layered portion  142  will result in the decrease in the light absorbed by the p-type semiconductor layer  14 , and thus the light entering and absorbed by the intrinsic semiconductor layer  15  is increased. Furthermore, since the lower p-type semiconductor layered portion  142  has a concentration of the p-type dopant of borane within a normal level, the uniformity of the built-in voltage of the photodetection diode  13  can be effectively prevented from being destroyed. 
     More specifically, the lower p-type semiconductor layered portion  142  of the p-type semiconductor layer  14  of the photodetection diode  13  of each of the pixel sensing portions  320  is formed using the CVD techniques under a first hydrogen gas ratio, a first deposition temperature, and a first deposition power. The upper p-type semiconductor layered portion  141  is formed using the CVD techniques under a second hydrogen gas ratio, a second deposition temperature, and a second deposition power. The first hydrogen gas ratio, the first deposition temperature and the first deposition power are respectively greater than the second hydrogen gas ratio, the second deposition temperature and the second deposition power, such that the lower p-type semiconductor layered portion  142  has a microcrystalline silicon structure having the crystallinity greater than 40%, and the band gap of the microcrystalline silicon structure is greater than 1.7 eV. 
     In one form, the multilayered structure of the p-type semiconductor layer  14  may include more than two layered portions. For example, the p-type semiconductor layer  14  may include the lower and upper p-type semiconductor layered portions  142 ,  141 , and an intermediate p-type semiconductor layered portion (not shown) disposed between the lower and upper p-type semiconductor layered portions  142 ,  141 . The upper p-type semiconductor layered portion  141  has the amorphous silicon structure and is heavily doped with the p-type dopant of borane, and each of the lower p-type semiconductor layered portion  142  and the intermediate p-type semiconductor layered portion has the microcrystalline structure, a concentration of the p-type dopant of borane within the normal level and a reduced thickness as mentioned above. Similarly, decrease in the thicknesses of the lower and intermediate p-type semiconductor layered portions  142  will result in the decrease in the light absorbed by the lower and intermediate p-type semiconductor layered portions  142 , and thus the light entering into and absorbed by the intrinsic semiconductor layer  15  is increased. Furthermore, since each of the lower and intermediate p-type semiconductor layered portions  142  has the concentration of the p-type dopant of borane within the normal level, damage in uniformity of the built-in voltage of the photodetection diode  13  can be prevented. It is noted that when the p-type semiconductor layer  14  has more than two of the layered portions that are stacked one above another, the topmost layered portion, which is the most distant from the intrinsic semiconductor layer  15 , is heavily doped with the p-type dopant of borane, and the other ones of the layered portions have the concentration of the p-type dopant of borane within the normal level. 
     Referring back to  FIGS. 3 and 6 , the n-type semiconductor layer  16  may have a single-layered structure or a multi-layered structure. The n-type semiconductor layer  16  may be made from silanes, hydrogen gas and phosphine using the CVD techniques. As shown in  FIG. 6 , the multi-layered structure of the n-type semiconductor layer  16  of the photodetection diode  13  of each of the pixel sensing portions  320  includes an upper n-type semiconductor layered portion  161  formed on the intrinsic semiconductor layer  15  and a lower n-type semiconductor layered portion  162  formed on the upper n-type semiconductor layered portion  161 . The lower n-type semiconductor layered portion  162  has a band gap greater than 1.7 eV and an n-type dopant in an amount not less than two times of that of the upper n-type semiconductor layered portion  161 . The n-type dopant may be phosphorus, which is provided from a phosphorus-containing gas which includes phosphine, thereby forming the upper n-type semiconductor layered portion  161  having a microcrystalline silicon structure with a crystallinity greater than 40%. 
     Specifically, in the multi-layered structure of the n-type semiconductor layer  16 , the lower n-type semiconductor layered portion  162  of the n-type semiconductor layer  16  has the non-crystalline silicon structure and is heavily doped with phosphorus, and thus the dangling bonds in the lower n-type semiconductor layered portion  162  are increased. If the n-type semiconductor layer  16  only includes the lower n-type semiconductor layered portion  162 , uniformity of the built-in voltage of the photodetection diode  13  tends to be further damaged or destroyed. Inclusion of the upper n-type layered portion  161  will alleviate the damage or destruction of the uniformity of the built-in voltage of the photodetection diode  13 . The upper n-type semiconductor layered portion  161  is normally doped and has the n-dopant in the amount not greater than half of that of the lower n-type semiconductor layered portion  162 . In addition, the upper n-type semiconductor layered portion  161  has a thickness lower than that of the single-layered structure of the n-type semiconductor layered portion  16 . The decrease in the thickness of the upper n-type semiconductor layered portion  161  will result in the decrease in the light absorbed by the n-type semiconductor layer  16 , and thus the light that is reflected on the n-type semiconductor layer  16  and absorbed by the intrinsic semiconductor layer  15  is increased. Hence, the photoelectric conversion efficiency of the photodetection diode  13  can be improved. Besides, the dangling bonds in the n-type semiconductor layer  16  are reduced, and thus the uniformity of the built-in voltage of the photodetection diode  13  is improved. 
     Specifically, the upper n-type semiconductor layered portion  161  is formed using the CVD techniques under a third hydrogen gas ratio, a third deposition temperature, and a third deposition power. The lower n-type semiconductor layered portion  162  is formed using the CVD techniques under a fourth hydrogen gas ratio, a fourth deposition temperature, and a fourth deposition power. The third hydrogen gas ratio, the third deposition temperature and the third deposition power are respectively greater than the fourth hydrogen gas ratio, the fourth deposition temperature, and the fourth deposition power. Therefore, the upper n-type semiconductor layered portion  161  has the microcrystalline silicon structure greater than 40% and the band gap of the microcrystalline silicon structure is greater than 1.7 eV. 
     In one form, the multilayered structure of the n-type semiconductor layer  16  may include more than two layered portions. For example, the n-type semiconductor layer  16  may include the lower and upper n-type semiconductor layered portions  162 ,  161 , and an intermediate n-type semiconductor layered portion (not shown) disposed between the lower and upper n-type semiconductor layered portions  162 ,  161 . The lower n-type semiconductor layered portion  162  has the amorphous silicon structure and is heavily doped with the n-type dopant of phosphorus, and each of the upper n-type semiconductor layered portion  161  and intermediate n-type semiconductor layered portion has the microcrystalline structure, a concentration of the n-type dopant of phosphorus within a normal level, and a reduced thickness as mentioned above. Similarly, the decrease in the thicknesses of the upper n-type semiconductor layered portion  161  and the intermediate n-type semiconductor layered portion will result in the decrease in the light absorbed by the upper n-type semiconductor layered portion  161  and the intermediate n-type semiconductor layered portion, and thus the light that is reflected on the n-type semiconductor layer  16  and then absorbed by the intrinsic semiconductor layer  15  is increased. In addition, since each of the upper and intermediate n-type semiconductor layered portions  161  has the concentration of the n-type dopant of phosphorus within a normal level, damage in the uniformity of the built-in voltage of the photodetection diode  13  can be prevented. It is noted that when the n-type semiconductor layer  16  includes more than two of the layered portions that are stacked one above another, the bottommost layered portion, which is the most distant from the intrinsic semiconductor layer  15 , is heavily doped with the n-type dopant of phosphorus, the other ones of the layered portions disposed on the bottommost layer have the concentration of the n-type dopant of phosphorus within the normal level. 
     Referring to the configuration (a) of  FIG. 4  and the configuration (a) of  FIG. 7 , the photodetection diode  13  of each of the pixel sensing portions  320  may further include a first optical film  21  that is immediately disposed on the p-type semiconductor layer  14 . The first optical film  21  is used for reducing a reflection rate of light from a top surface  140  of the p-type semiconductor layer  14  or a refraction angle of light in the p-type semiconductor layer  14 . Since the refraction angle of light in the p-type semiconductor layer  14  is reduced, the light passing through the p-type semiconductor layer  14  is close to a normal line of the p-type semiconductor layer  14  that is perpendicular to the p-type semiconductor layer  14 . Hence, a light flux passing through the p-type semiconductor layer  14  and absorbed by the intrinsic semiconductor layer  15  is increased, and the photoelectric conversion efficiency of the photodetection diode  13  is thus improved. Specifically, in the configuration (a) of  FIG. 7 , the first optical film  21  may be formed on a top surface of the upper p-type semiconductor layered portion  141  of the p-type semiconductor layer  14 . 
     In one form, the first optical film  21  of the photodetection diode  13  of each of the pixel sensing portions  320  has a structure selected from a photonic crystal structure with a refractive index varied periodically, a microlens array structure with a refractive index varied periodically, an incident light-scattered crystal structure with a refractive index varied non-periodically, and an incident light-diffused crystal structure with a refractive index varied non-periodically. The first optical film  21  has a refractive index smaller than that of the p-type semiconductor layer  14 . When the light is refracted by an interface of the intrinsic semiconductor layer  15  and the p-type semiconductor layer  14 , the angle of refraction is smaller than an angle of incidence, so that the light entering into the p-type semiconductor layer  14  is close to the normal line of the p-type semiconductor layer  14  that is perpendicular to the p-type semiconductor layer  14 . 
     Referring to the configuration (b) of  FIG. 4  and the configuration (b) of  FIG. 7 , the photodetection diode  13  of each of the pixel sensing portions  320  may further include a second optical film  22  that is immediately disposed on the n-type semiconductor layer  16 . The second optical film  22  is used for reflection of light from the n-type semiconductor layer  16  to the intrinsic semiconductor layer  15  when the light passes through the n-type semiconductor layer  16 , so that the light reflected by the second optical film  22  is absorbed again by the intrinsic semiconductor layer  15 . More specifically, when the light passing through the p-type semiconductor layer  14 , the intrinsic semiconductor layer  15  and the n-type semiconductor layer  16  is reflected by the second optical film  22 , a portion of the reflected light is absorbed by the intrinsic semiconductor layer  15  again, and the other portion of the reflected light is reflected again by the second optical film  22  and then enters into the intrinsic semiconductor layer  15 . Therefore, the light can be reflected by the second optical film  22  multiple times for increasing the absorption of the intrinsic semiconductor layer  15 . Specifically, in the configuration (b) of  FIG. 7 , the second optical film  22  is immediately disposed on a bottom surface of the lower n-type semiconductor layered portion  162  opposite to the intrinsic semiconductor layer  15 . 
     The second optical film  22  has a structure selected from a photonic crystal structure with a refractive index varied periodically, an incident light-scattered crystal structure with a refractive index varied non-periodically, and an incident light-diffused crystal structure with a refractive index varied non-periodically. Hence, since the light passing through the n-type semiconductor layer can be reflected by the second optical film, the reflected light can be absorbed again by the intrinsic semiconductor layer, and the wavelength range of the light that is able to be absorbed by the intrinsic semiconductor layer  15  is expanded. Thus, the photoelectric current produced in the intrinsic semiconductor layer is increased. 
     In one form, each of the pixel sensing portions  320  may include two or more of the photodetection diodes that are electrically connected in series and superimposed on one another in the first direction (L 1 ). Taking one of the pixel sensing portions  320  including two of the photodetection diodes  13  superimposed on one another in the first direction (L 1 ) as an example, the n-type semiconductor layer  16  of one of the photodetection diodes  13  is proximate to the p-type semiconductor layer  14  of the other of the photodetection diodes  13 . In the exemplified pixel sensing portion  320 , the second optical film  22  may be disposed on the bottom surface of the n-type semiconductor layer  16  of the one of the photodetection diodes  13  so that the light passing through the n-type semiconductor layer  16  of the one of the photodetection diodes  13  can be reflected by the second optical film  22  and absorbed by the intrinsic semiconductor layer  15  of the one of the photodetection diodes  13  again. Similarly, the second optical film  22  may be disposed on the bottom surface of the other one of the photodetection diodes  13  so that the light passing through the n-type semiconductor layer  16  of the one of the photodetection diodes  13  can be reflected by the second optical film  22  and absorbed by the intrinsic semiconductor layer  16  of the other one of the photodetection diodes  13  again. Therefore, the photoelectric conversion efficiency of the pixel sensing portions  320  is improved. 
     Referring to  FIGS. 5 and 8 , the two or more of the photodetection diodes  13  included in one of the pixel sensing portions  320  may have the first optical film and/or the second optical film  22 . The abovementioned pixel sensing portion  320  including the two photodetection diodes  13  superimposed on one another in the first direction (L 1 ) is taken as an example, where the p-type semiconductor layers  14  of the photodetection diodes  13  have the single-layered structure or the multi-layered structure and the n-type semiconductor layers  16  of the photodetection diodes have the single-layered structure or the multi-layered structure. Referring to the configurations (a) of  FIGS. 5 and 8 , the one of the photodetection diodes  13  has the first optical film  21  disposed on the top surface of the p-type semiconductor layer  14  and the second optical film  22  disposed on the bottom surface of the n-type semiconductor layer  16 . Referring to the configurations (b) of  FIGS. 5 and 8 , two of the second optical films  22  are disposed on the bottom surfaces of the n-type semiconductor layers  16  of the two photodetection diodes  13 , respectively. Referring to the configurations (c) of  FIGS. 5 and 8 , the one of the photodetection diodes  13  has the first optical film  21  disposed on the top surface of the p-type semiconductor layer  14  and each of the photodetection diodes  13  has the second optical film  22  disposed on the bottom surface of the n-type semiconductor layer  16 . 
     The first optical film  21  and the second optical film are separately made from an oxygen-containing compound or a nitrogen-containing compound using the CVD techniques or sputtering techniques. The oxygen-containing compound is selected from a group consisting of silicon oxide having a formula of SiO x  with x being not less than 1, niobium pentaoxide (Nb 2 O 5 ), zinc oxide (ZnO), indium tin oxide (ITO), and titanium dioxide (TiO 2 ). The nitrogen-containing compound has a formula of SiN y  with y being not less than 1. 
     In another embodiment of the image sensor  32  of the disclosure, each of the pixel sensing portions  320  includes two of the photodetection diodes  13  that are electrically connected in series and vertically superimposed on one another. Each of the photodetection diodes  13  includes the p-type semiconductor layer  14 , an intrinsic semiconductor layer and the n-type semiconductor layer  16 . The intrinsic semiconductor layer of the one of the photodetection diodes  13  that bears a light incident surface has an amorphous silicon structure so as to receive the incident light having the wavelength range within the visible light region, and the intrinsic semiconductor layer of the other one of the photodetection diodes  13  that is disposed below the one of the photodetection diodes  13  has an microcrystalline silicon structure or a silicon-germanium structure so as to receive the light having the wavelength ranging from the visible light region to the infrared light region or near infrared light region. Therefore, application field of the image sensor  32 , which may serve as a TFT image sensing array film, can be expanded. 
     The band gap is an important physical parameter of the semiconducting material and is determined by the band structure of the semiconducting material, which is relevant to the crystal structure and binding properties of atoms, etc. The silicon-germanium structure is made from silane, germane and hydrogen gas using the CVD techniques and has a band gap less than 1.7 eV. The silicon-germanium structure may include a non-crystalline silicon-germanium structure or a microcrystalline silicon-germanium structure. At room temperature (300K), the band gap of the germanium is 0.66 eV, and the band gap of the intrinsic semiconductor layer of the other one of the above photodetection diodes  13  is decreased when germanium is doped in saline. When the band gap of the intrinsic semiconductor layer of the other one of the above photodetection diodes is less than 1.7 eV, the intrinsic semiconductor layer of the other one of the above photodetection diodes can absorb the light having the wavelength ranging from the visible light region to the infrared light region (or near infrared light region). In this embodiment, by controlling the concentration of germane (GeH 4 ), the photodetection diode  13  of each of the pixel sensing portions  320  having one of the non-crystalline silicon-germanium structure and the microcrystalline silicon-germanium structure can absorb the light having an expanded wavelength ranging from 600 nm to 1000 nm. 
     Referring to  FIG. 9 , an embodiment of an image sensing-enabled display apparatus  3  according to the disclosure is illustrated. The embodiment of the image sensing-enabled display apparatus  3  includes a display unit  31  and at least one of the image sensor  32  of the disclosure. The display unit  31  includes a display member  311 , a glass cover  312  disposed on the display member  311  for protecting the display member  311 , and a driving integrated circuit  313  made of a flexible printed circuit (FPC). In one form, the display unit  31  may further include a touch sensor (not shown) disposed on a bottom surface of the glass cover  312  that faces the display member  311  such that the image sensing-enabled display apparatus  3  can have touch-sensing function. The display unit  31  defines an image-sensing region  314 . The at least one of the image sensor  32  is disposed below the image-sensing region  314  and serves as a photodetector. As mentioned above, the image sensor  32  may include a plurality of the pixel sensing portions  320  arranged in an array. In actual use, the image sensor  32  may disposed below the display member  311  and perform image sensing within the range of the display member  311 . 
     The image sensing-enabled display apparatus  3  may be an electronic apparatus equipped with a touch display screen, such as a portable apparatus (e.g., a cell phone, a tablet, a personal digital assistant (PDA), etc.), a personal computer, and an industrial computer. The image sensing-enabled display apparatus  3  may detect a fingerprint, a face, an eyeball or a posture of a user, etc. Taking the eyeball detection as an example, the image sensing-enabled display apparatus  3  may be incorporated with an optical imaging device (not shown) that is disposed between the display unit  31  and an eye of the user. When an eye of the user is imaged on the optical imaging device, the projection formed within an eye gaze tracking area (not shown) is defined in the display unit  31  and then is captured by a transmitting and sensing unit (not shown) disposed below the eye gaze tracking area. By virtue of the cooperation of the image sensor  32  and the display unit  31 , the image sensing-enabled display apparatus  3  can serve as virtual reality (VR) equipment. 
     The display member  311  of the display unit  31  includes a thin film transistor for driving the display member  311  and transmitting electrical signals. The display member  311  is selected from one of an active matrix organic light emitting diode (AMOLED) image-sensing display device, a liquid crystal display (LCD) display device, a quantum dot image-sensing display device, and an electronic ink (E-ink) image-sensing display device. When the display member  311  is the LCD device, the image sensing-enabled display apparatus  3  further includes a backlight unit  33  that is disposed below the at least one image sensor  32  such that the at least one image sensor  32  is disposed between the display unit  31  and the backlight unit  33 . The backlight unit  33  is used for emitting light. The backlight unit  33  may be a light emitting diode (LED) backlight module, or other electronic device that can emit light. Alternatively, when the display member  311  is the OLED device that is self-luminous, the image sensing-enabled display apparatus  3  may be free of the backlight unit  33 . 
     In one form, the display unit  31  may define at least two of the image sensing regions  314 . The image sensing-enabled display apparatus  3  includes at least two of the image sensors  32 , each of which corresponds in position to a respective one of the image sensing regions  314 . In the embodiment, the image sensing-enabled display apparatus  3  may further include a drive and control circuit unit  34  that is an FPC-connected integrated circuit for image reading. The drive and control circuit unit  34  is configured to turn on and turn off the image sensors  32  upon receiving turn-on and turn-off signals inputted by a user, respectively. 
     When the number of the image-sensing regions  314  of the display unit  31  and the number of the image sensors are respectively exemplified to be two, the image-sensing regions  314  may be respectively located at the top and the bottom of the display unit  31 , or may be respectively located at left and right sides of the display unit  31 . More specifically, each of the image sensors  32  is disposed beneath a respective one of the image-sensing regions  314 . The image sensors  32  are turned on and turned off by the turn-on and turn-off signals inputted by the user. In one form, the image-sensing regions  314  cooperatively cover the entire area of the display member  311 , so that all of the light passing through the image-sensing regions  314  of the display unit  31  can be absorbed by the image sensors  32 . In one form, the image sensors  32  may cover two-thirds or three-fourths of the area of the display member  311 . The image sensors  32  may be controlled in such a manner that one of the image sensors  32  is turned on and the other one of the image sensors  32  is turned off. 
     In one form, the number of the image-sensing regions  314  may be varied based on actual use, and the turn-on or turn-off states of the image sensors  32  may be separately controlled by the user. 
     In the embodiment, since the polymeric substrate  42  including polyimide is disposed to the pixel sensing portions  320 , the image sensor  32  of the disclosure can be disposed below the display member  311 , which is flexible, and thus the resulting image sensing-enabled display apparatus  3  can meet the market requirements of being thin and flexible. 
     Referring to  FIG. 10 , in one form, each of the image sensors  32  includes a waterproof and oxygen barrier structure  35  that is disposed between the pixel sensing portions  320  and the polymeric substrate  42 . More specifically, the waterproof and oxygen barrier structure  35  is immediately formed on the polymeric substrate  42 . The waterproof and oxygen barrier structure  35  may include a plurality of inorganic layers (not shown) and a plurality of organic layers (not shown) alternately formed on one another. Each of the inorganic layers includes aluminum oxide (Al 2 O 3 ), silicon oxide (SiO x ) with x being greater than 1, or silicon nitride (SiO y ) with y being greater than 1. Each of the organic layers includes an acrylic-based polymeric material or poly-p-xylylene-based polymeric material. In the waterproof and oxygen barrier structure  35 , each of the inorganic layers is water resistant, and a flow of oxygen in the waterproof and oxygen barrier structure  35  is impeded by the organic layers. In addition, when the image sensing-enabled display apparatus  3  is flexible, the organic layers can serve as buffer layers. Therefore, the weak-light imaging can be realized in the image sensing-enabled display apparatus  3  including the image sensors  32  that have the waterproof and oxygen barrier structure  35  and the polymeric substrate  42  and that can be integrated with the display member  311 , which is flexible. 
     Referring to  FIGS. 11 and 12 , an embodiment of a method of making the embodiment of the image sensor  32  which has relatively high photoelectric conversion efficiency is illustrated. The method includes steps S 701  to S 705 . 
     In Step S 701 , a removable adhesive layer  43  is formed on a template  41 . In the embodiment, the template  41  is a glass plate, and the removable adhesive layer  43  is made from an adhesive solution adapted for being coated on the template  41 . In one form, the template  41  may be made from other material useful for film formation thereon. 
     In Step S 702 , the substrate  42  is formed on the removable adhesive layer  43  such that the removable adhesive layer  43  is disposed between the template  41  and the substrate  42 . 
     In Step S 703 , the thin film transistor  11  is formed on the substrate  42 . 
     In step S 704 , the photodetection diode  13  is formed on the substrate  42  by forming the n-type semiconductor layer  16  on the substrate  42 , forming the intrinsic semiconductor layer  15  on the n-type semiconductor layer  16 , and forming the p-type semiconductor layer  14  on the intrinsic semiconductor layer  15 . 
     As mentioned in the above, the p-type semiconductor layer  14  may include the multilayered structure, and formation of the photodetection diode  13  on the substrate  42  is carried out by forming the n-type semiconductor layer  16  on the substrate  42 , forming the intrinsic semiconductor layer  15  on the n-type semiconductor layer  16 , forming the lower p-type semiconductor layered portion  142  on the intrinsic semiconductor layer  15 , and forming the upper p-type semiconductor layered portion  141  on the lower p-type semiconductor layered portion  142 . 
     In step S 705 , the removable adhesive layer  43  and the template  41  are removed from the substrate  42  so as to form the image sensor  32 . 
     In one form, when the image sensor  32  including the pixel sensing portions  320  is used for manufacturing the image sensing-enabled display apparatus  3 , the image sensor  32  with the removable adhesive layer  43  and the template  41  may be cut for conforming a contour of the display member  311  prior to step S 705 . In one form, the image sensing-enabled display apparatus  3  may include the cut image sensors  32  that are put together to be conformed with the contour of the display member  311 . 
     As mentioned in the above, the substrate  42  may be the polymeric substrate including polyimide. In this embodiment, the forming of the polymeric substrate  42  on the removable adhesive layer  43  including coating a polyimide solution on the removable adhesive layer  43 , and baking the polyimide solution to cure the polyimide solution. 
     In one form, the method may further include forming the waterproof and oxygen barrier structure  35  on the substrate  42  by coating techniques prior to the forming of the photodetection diode  13 . The forming of the waterproof and oxygen barrier structure  35  includes: alternately forming on the substrate  42  a plurality of the inorganic layers and a plurality of the organic layers on the substrate  42 . 
     In addition, the method may further include forming the first and second optical films  21 ,  22  as mentioned previously. 
     To sum up, the merits of the image sensor  320  according to the disclosure are mentioned as below: (1) by the controlling of the band gap of the p-type semiconductor layer  14  of the photodetection diode  13  of each of the pixel sensing portions  320  to be greater than 1.7 eV, the wavelength range of the incident light that is able to pass through the p-type semiconductor layer  14  is expanded; (2) by the design of the crystallinity gradient that varies from the amorphous silicon structure to the microcrystalline silicon structure along the first direction (L 1 ), the wavelength range of the light that is able to be absorbed by the intrinsic semiconductor layer  15  is expanded so that the photoelectric conversion efficiency is enhanced and the Staebler-Wronski effect is alleviated; (3) by the design and the process of forming the n-type semiconductor layer  16  of the photodetection diode  13  of each of the pixel sensing portions  320 , the n-type semiconductor layer  16  has the amorphous silicon structure or the microcrystalline silicon structure, and thus the number of the dangling bonds in the n-type semiconductor layer  16  is reduced. Hence, the p-type and n-type semiconductor layers  14 ,  16  cooperatively maintain the uniformity of the built-in voltage of the photodetection diode  13 ; (5) by the design of the photodetection diode  13 , the photodetection wavelength range of the photodetection diode  13  can be expanded to be from the visible light region to the infrared light region or near the infrared light region, and thus the photoelectric conversion efficiency is improved. In addition, the merits of the image sensing-enabled display apparatus  3  including the image sensor  32  are mentioned as below: (1) in actual use, since the image sensor  32  may be disposed below the display unit  31  or integrated with the active array thin film transistor layer of the display member  311 , the image sensing-enabled display apparatus  3  can perform image sensing function and display function; (2) by the design of the polymeric substrate  42  including polyimide, and the inclusion of the waterproof and oxygen barrier structure  35  on the polymeric substrate  42 , the image sensing-enabled display apparatus  3  is flexible and the low light imaging can be realized. 
     In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiment. It will be apparent, however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. It should also be appreciated that reference throughout this specification to “one embodiment,” “an embodiment,” an embodiment with an indication of an ordinal number and so forth means that a particular feature, structure, or characteristic may be included in the practice of the disclosure. It should be further appreciated that in the description, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects, and that one or more features or specific details from one embodiment may be practiced together with one or more features or specific details from another embodiment, where appropriate, in the practice of the disclosure. 
     While the disclosure has been described in connection with what is considered the exemplary embodiment, it is understood that this disclosure is not limited to the disclosed embodiment but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.