Abstract:
A photosensing device with a photovoltage sensing mechanism, a graphene layer and a semiconductor layer. The graphene layer is sandwiched between the semiconductor layer and a substrate. The photovoltage sensing mechanism senses the photovoltage created by light impinging on the graphene-semiconductor heterojunction. The strength of the photovoltage is used to indicate the level of illumination of the impinging light.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation in part (CIP) of and claims priority to currently pending U.S. patent application Ser. No. 14/291,007, filed May 30, 2014. 
     
    
     FIELD 
       [0002]    Embodiments of the present disclosure relate to semiconductor devices, and particularly to a photosensing semiconductor device. 
       BACKGROUND 
       [0003]    Most photosensing devices utilize photodiodes to convert light energy into electronic signals. Conventional photodiodes are p-n junctions or PIN structures that produce a photocurrent when light of certain intensity strikes the photodiodes. The light energy in the form of photons of sufficient energy excites the electrons in the photodiodes to produce electron-hole pairs. The electron moves towards the conduction band from the valence band thereby producing a photocurrent. 
         [0004]    Because most photosensing devices use this photocurrent to represent the intensity of light impinging on the photodiodes, the photosensing devices are vulnerable to high light intensity which may saturate the output signal of photosensing devices, and low light intensity which may induce too little photocurrent and reset circuitry is often needed to reset the photodiode. Therefore there is room for improvement in the art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0005]    Implementations of the present technology will now be described, by way of example only, with reference to the attached figures. 
           [0006]      FIG. 1A  is a perspective view of an array of photosensing devices having photodiodes in accordance with a first embodiment of the present disclosure. 
           [0007]      FIG. 1B  is a cross-sectional view taken along line  1 B- 1 B in  FIG. 1A . 
           [0008]      FIG. 2A  is a perspective view of an array of photosensing devices in accordance with a second embodiment. 
           [0009]      FIG. 2B  is a cross-sectional view taken along line  2 B- 2 B in  FIG. 2A . 
           [0010]      FIG. 3A  shows biasing of a graphene-semiconductor heterojunction. 
           [0011]      FIG. 3B  shows a graph of photovoltage responsivity vs. incident power of a graphene-semiconductor heterojunction. 
           [0012]      FIG. 4  is a graph showing photovoltage vs. illuminance of a graphene-semiconductor heterojunction. 
           [0013]      FIG. 5  is a graph showing photovoltage vs. illuminance of a graphene-semiconductor heterojunction. 
           [0014]      FIG. 6A  is a graph of diagrammatic view showing the sensing of the photovoltage of the photodiodes in  FIGS. 1A and 1B . 
           [0015]      FIG. 6B  is a graph of diagrammatic view showing the sensing of the photovoltage of the photodiodes in  FIGS. 1A and 1B . 
           [0016]      FIG. 7A  is a graph of diagrammatic view showing the sensing of the photovoltage of the photodiodes in  FIGS. 1A and 1B . 
           [0017]      FIG. 7B  is a graph of diagrammatic view showing the sensing of the photovoltage of the photodiodes in  FIGS. 1A and 1B . 
           [0018]      FIG. 8  is a graph showing the process of manufacturing of graphene-sensing heterojunction of a graphene-semiconductor heterojunction. 
           [0019]      FIG. 9  shows a block diagram of a module using the photosensing device of  FIG. 1A  or  2 A. 
           [0020]      FIG. 10  shows a block diagram of a system using the photosensing device of  FIG. 1A  or  2 A. 
           [0021]      FIG. 11  is a cross-sectional view of a photosensing device in accordance with another embodiment. 
           [0022]      FIG. 12  is a cross-sectional view of a photosensing device in accordance with another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The present disclosure, including the accompanying drawings, is illustrated by way of examples and not by way of limitation. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean “at least one.” 
         [0024]      FIGS. 1A and 1B  shows an embodiment of a photosensing device  1 . The photosensing device  1  comprises multiple active pixel regions  30 . The multiple active pixel regions  30  are shown as a matrix of rows and columns, however, in one embodiment, the matrix of rows and columns can be one row or one column. In another embodiment, as shown in  FIG. 9 , the photosensing device  1  are connected to at least one decoder, including row decoder circuits  101 , and column decoder circuits  102 , and multiplexer circuits  103  as a module  100  to extract the information from each active pixel region  30 . In a further embodiment, as shown in  FIG. 10 , the module  100  is part of a system  200  where the extracted information from each active pixel region  30  is processed and/or displayed on a display screen  201  and/or stored in a storage unit  202  of the system, the system  200  may further comprises a controller  203  and/or an input module  204 . 
         [0025]    Each active pixel region  30  includes photodiodes  25  and a transistor. In this embodiment, the transistor may be a MOS transistor, such as a CMOS sensing circuit  35 . Each photodiode  25  comprises a graphene layer  10  and a semiconductor layer  15 . In this embodiment, the semiconductor layer  15  is a silicon-based layer, which may be but not limited to high opacity polycrystalline silicon or amorphous silicon. The CMOS sensing circuit  35  is an illustration of a CMOS sensing circuit, other variation of CMOS sensing circuitry may also be adopted. The CMOS sensing circuit  35  includes metal layers (e.g., M 1 , M 2 , etc.) separated by inter-metal dielectrics (e.g., IMD 1 , IMD 2 , etc.) and inter-connected by vias  28 . The CMOS sensing circuit also includes a silicon substrate, a P-well and an N-well on top of the Si substrate and transistors circuitry disposed on the P and N-well. 
         [0026]    In  FIGS. 1A and 1B , the graphene-semiconductor photodiode  25  is on top of the CMOS sensing circuit  35 . In  FIGS. 2A and 2B , the graphene-semiconductor photodiode  31  is adjacent to the CMOS sensing circuit  35 . 
         [0027]    The graphene layer  10  and the semiconductor layer  15  forms a graphene-semiconductor heterojunction. The semiconductor layer can be an n-type or a p-type semiconductor. In this embodiment, the semiconductor layer is of n-type conductivity. For graphene-semiconductor junction, the excitation of electrons by light energy occurs in the semiconductor, for example an n-type silicon, and the graphene is the carrier collector. The semiconductor layer  15  is a silicon-based layer, which may be but is not limited to high opacity polycrystalline silicon or amorphous silicon. As shown in  FIG. 1B , the graphene-semiconductor photodiode  25  is implemented on the CMOS sensing circuit  35 , the thickness of semiconductor layer  15  may be varied to allow only a certain range of wavelength band (e.g. visible light) to be absorbed. In conjunction with the low optical absorption (˜2.3%) of graphene over a wide range of wavelength, infrared (IR) light may not be absorbed by the heterojunction, allowing only the light of certain wavelengths (e.g. visible light) to pass through, and the need for IR-cut filters, which are necessary in conventional CMOS image sensor modules, is eliminated while ensuring high amount of visible light is absorbed for photoexcitation. 
         [0028]    CMOS image sensors, such as active pixel imaging sensors (APS), demand high pixel density (image resolution) in order to suit a wide variety of applications and consumers&#39; needs. These CMOS image sensors can be applied to portable electronic devices such as cameras and cell phones. The size of the sensor and the pixel density (i.e. image resolution) are interrelated and may directly affect the total photo-sensing area and the corresponding sensor performances including signal-to-noise ratio and operational dynamic range. For example, a CMOS image sensor with a higher pixel density (the sensor size being constant) may lead to smaller pixel size with reduced photo-sensing area and requires higher total amount of transistors in a chip, which effectively reduce the total photo-sensing area and thereby reduce photo responsivity and corresponding dynamic range. 
         [0029]    By implementing graphene-semiconductor heterojunction on top of the CMOS IC chip, where high photo-responsivity at low light levels, low optical absorption, intrinsic signal suppression mechanism, high operational dynamic range, elimination of fill factor limits, reduced photodiode area, and straightforward implementation of the graphene-semiconductor heterojunction on semiconductor substrates may be realized, the aforementioned detrimental effects can be eliminated while maintaining the performance of CMOS image sensor. 
         [0030]    Further, having the graphene-semiconductor heterojunction on the CMOS IC chip eliminates the limit of fill factor because the photodiode is not located in the same plane with the CMOS circuits. As shown in  FIG. 1A , the photodiode locates above the circuits, on the CMOS IC chip, and do not have to share area with each other. Furthermore, with the photo-voltage sensing mechanism, the area of photodiode needs not to be large, thus breaking the conventional resolution-sensor size tradeoff for CMOS image sensors. Also, as explained below, sensing the photovoltage, the circuit for each pixel becomes simplified since many circuit blocks such as reset circuit are not necessary. 
         [0031]    As shown in  FIG. 3A , when a reverse bias (V r   bias ) is applied to the graphene-semiconductor junction, the Fermi level of graphene (E f (Gr)) moves higher with respect to the Fermi level of the n-type semiconductor (E f (Si)). This feature allows for a greater number of accessible states for the photoexcited holes from the valence band of the semiconductor. Under low lighting condition where less photoexcited carriers are available due to limited amount of incident photons, these carriers may be collected more efficiently. Because of the low density of state property near the Fermi level of graphene (E f (Gr)), the electric potential of graphene is highly sensitive to the amount of charges. Thus sensing the photovoltage of the graphene-semiconductor photodiode instead of the photocurrent, the photo-sensitivity of the photodiode becomes much greater than conventional photodiodes. 
         [0032]    By observing the open-circuit voltage, it is found that graphene-semiconductor photodiode is highly sensitive to incident light power. As shown in  FIG. 3B , the photovoltage responsivity of the graphene-semiconductor heterojunction increases with decreasing incident light power. This inversely proportional correlation provides an intrinsic signal suppression mechanism, that is, the photovoltage (V) increases logarithmically with increasing illuminance (lux) (see  FIGS. 4 and 5 ). Thus, the graphene-semiconductor heterojunction photodiode absorbs more photons (e.g. higher illuminance under direct sunlight) than conventional photodiodes. This achieves a higher operational dynamic range as image sensor without employing conventional signal suppression techniques, which either require more transistors in each pixel to mimic the logarithmic relation or need to implement complex control circuits to separately deal with the signals at low and high illumination levels. 
         [0033]      FIGS. 6A and 6B  show diagrammatic view of one way of sensing a photovoltage of a graphene-semiconductor photodiode. As illustrated in  FIGS. 6A and 6B , the first or the second terminal of the graphene-semiconductor diode can be connected to a reference voltage source. In the embodiments of  FIGS. 6A and 6B , the reference voltage source is ground and a constant voltage source respectively. In  FIG. 6A , the graphene-semiconductor photodiode  250 , wherein the semiconductor is of n-type conductivity, is connected to a transistor, such as a MOSFET M 1  in a source follower configuration. The graphene terminal is an anode, and is connected to the gate  70  of the MOSFET M 1  and the semiconductor terminal is a cathode, and is connected to ground. The source  71  of the MOSFET is grounded through resistor  72  and the drain  73  is connected to voltage V dd    74 . The output V out  is taken at the source  71  of the MOSFET M 1 . In  FIG. 6B , the graphene-semiconductor photodiode  251 , wherein the semiconductor is of p-type conductivity, is connected to a transistor, such as a MOSFET M 1  in a source follower configuration. The graphene terminal is a cathode, and is connected to the gate  70  of the MOSFET M 1  and the semiconductor terminal is an anode, and is connected to a constant voltage source Vconst  75 . The source  71  of the MOSFET is grounded through resistor  72  and the drain  73  is connected to voltage V dd    74 . The output V out  is taken at the source  71  of the MOSFET M 1 . 
         [0034]      FIGS. 7A and 7B  show diagrammatic view of another way of sensing a photovoltage of a graphene-semiconductor photodiode. As illustrated in  FIGS. 7A and 7B , the first or the second terminal of the graphene-semiconductor diode can be connected to a reference voltage source. In the embodiments of  FIGS. 7A and 7B , the reference voltage source is ground and a constant voltage source respectively. In  FIG. 7A , the graphene-semiconductor photodiode  250  is connected to an operational amplifier (op-amp)  80  in a voltage buffer configuration. The input transistors of the op-amp may be transistors, such as MOSFETs. The graphene terminal of the photodiode  250  is an anode, and is connected to the non-inverting input terminal  81  of the op-amp, and the semiconductor terminal is a cathode, and is connected to ground. The inverting input  82  of the op-amp is connected to the output  83  of the op-amp. In  FIG. 7B , the graphene-semiconductor photodiode  251  is connected to an operational amplifier (op-amp)  80  in a voltage buffer configuration. The input transistors of the op-amp may be transistors, such as MOSFETs. The graphene terminal of the photodiode  251  is a cathode, and is connected to the non-inverting input terminal  81  of the op-amp, and the semiconductor terminal is an anode, and is connected to constant voltage source Vconst  84 . The inverting input  82  of the op-amp is connected to the output  83  of the op-amp. 
         [0035]    Because negligible or no current flows to the gate  70  of the MOSFET M 1  or into the non-inverting input terminal  81  of the op-amp, the photovoltage at the graphene terminal of the graphene-semiconductor photodiode  250  or the photovoltage at the graphene terminal of the graphene-semiconductor photodiode  251  is sensed by MOSFET M 1  or the op-amp  80 . Because the photovoltage, not the photocurrent is being sensed, reset transistor may not be needed because the photosensing device is capable of accepting higher intensity of light before saturation occurs. 
         [0036]    The graphene may be disposed on a substrate by techniques including but not limited to: chemical vapor deposition (CVD) and graphene transfer. In CVD, the chemical vapors of material elements interact and then deposit on the surface of wafer. In the case of CMOS image sensor, since the metal layers already on the CMOS IC chip cannot endure high temperature, the graphene may be deposited by using low-temperature processes. Therefore, the CVD processes for growing graphene in this embodiment may be the ones with low growth temperature but assisted by ionized gasses, such as plasma-enhanced CVD (PECVD) or electron-cyclotron resonance CVD (ECRCVD). In graphene transfer, as shown in  FIG. 8 , the graphene  41  is first grown on a copper foil  40  by CVD. Then the foil is coated with polymethyl methacrylate (PMMA)  42 . The graphene along with the PMMA layer  42  is separated from the Cu foil  40  by H 2  bubbles  50  using the so-called H 2  bubbling process in a NaOH solution  45  or by directly etching away the copper foil  40  in a FeCl 3  solution. The graphene-PMMA  55  is then placed onto the substrate. The graphene adheres to the substrate due to Van der Waals force. The PMMA can then be washed away by normal chemical etching. 
         [0037]    The graphene-semiconductor heterojunction is created by disposing semiconductor material on top of the previously grown graphene by sputtering, bonding another substrate (on which the semiconductor layer already exists, to the surface of graphene), or chemical vapor deposition (in the case of CMOS image sensor implementation, a CMOS post process compatible CVD, such as PECVD or ECRCVD, may be used). 
         [0038]    In another embodiment, the graphene-semiconductor heterojunction may be implemented on various semiconductor substrates (such as Si, GaAs, or other semiconductors) as discrete photodetectors for applications such as ambient light sensor, range finder, or proximity sensor. 
         [0039]    In another embodiment of the present disclosure, the graphene-semiconductor heterojunction may be implemented on large substrates (such as glass or plastic) as image sensors. The thickness of the glass may vary to provide different application needs, such as a thin glass with certain flexibility. The plastic may be PEN (polyethylene naphthalate), PES (polyethersulfone), PET (polyester), PI (polyimide) and so forth. In the case of adopting plastic as substrates, low temperature manufacturing processes are preferred, such as transfer, coating, sputtering, low-temperature CVD and so forth). These image sensors may be applied to larger camera for 3C (such as on large screen), large camera for surveillance, vehicles, defense (such as on windows or mirrors), large camera for medical imaging. 
         [0040]    In a further embodiment, the graphene-semiconductor heterojunction may be implemented as X-ray image sensor, wherein the graphene is disposed by CVD or graphene transfer process on crystalline silicon substrate or amorphous silicon on glass substrate after the pixel circuit has been processed. The graphene may also be disposed on flexible plastic substrate, with low temperature manufacturing processes, such as transfer, coating, sputtering, low-temperature CVD and so forth. Reflective material such as aluminum may be disposed on top of the graphene layer, wherein a plurality of scintillators such as CsI:Tl may be enclosed within the reflective material. The graphene is covered and protected by scintillators. The reflective material allows X-ray to pass through and reflects the visible light emitted from scintillator. With the aforementioned higher sensitivity of graphene-semiconductor photodetectors, the dose of X-ray may be reduced, thus lowering the radiation exposure to patients. 
         [0041]    In a further embodiment, the graphene-semiconductor heterojunction may be integrated with a metal oxide semiconductor field effect transistor (MOSFET)  110  as a photosensing device by interposing a graphene layer  112  between the gate insulation layer  111  and the gate layer  113  of MOSFET  110  device as shown in  FIG. 11 . The gate stack  114  of such MOSFET  110  device is consisted of gate insulation layer  111 /graphene layer  112 /gate layer  113  and formed on the silicon substrate from bottom to top. The gate layer  113  of MOSFET devices may be made of silicon-based material but not limited to high opacity polycrystalline silicon or amorphous silicon. The spectral response of the photosensing device can be adjusted by controlling the thickness of the gate layer  113 . The gate insulation layer  111  of MOSFET  110  devices may be made of electrical insulation materials but not limited to silicon dioxide or high-k dielectric insulation layer. The MOSFET  110  devices may further include a source  115 /drain  116  region formed on the substrate by ion implantation process with a high doping concentration typically larger than 10 20 /cm 3  for signal pick-up and amplification of photosensing device. The graphene layer  112  directly contacts with the gate layer  113  of MOSFET  110  devices for forming a semiconductor heterojunction and reducing the parasitic interconnect resistance and capacitance. The graphene layer  112 /gate layer  113  heterojunction structure has the advantages of less parasitic resistance and capacitance and simple fabrication process for improving the performance of photosensing devices of the invention, wherein the graphene layer  112  is disposed by CVD or graphene transfer process on crystalline silicon substrate or amorphous silicon on glass substrate after the signal readout circuit has been processed. The graphene layer  112 /gate layer  113  heterojunction structure may also be disposed on flexible plastic substrate, with low temperature manufacturing processes such as transfer, coating, sputtering, low-temperature CVD and so forth. In some cases, the photosensing devices with graphene layer  112 /gate layer  113  heterojunction are arranged in one- or two-dimensional arrays for various applications. 
         [0042]    In a further embodiment, the graphene-semiconductor heterojunction is formed by directly disposing a graphene layer on the silicon substrate as shown in  FIG. 12 . In this case, the silicon substrate is a p-type silicon material and the graphene layer  122  is an opposite type material to the p-type and vice versa. The graphene  122  terminal of the heterojunction is electrically connected  124  to a gate layer  123  of MOSFET  120  device for photo-signal pick-up and amplification. The MOSFET  120  device has the structure consisted of silicon substrate, gate insulation layer  121 , gate layer  123 , and source  126 /drain  125  region as conventional. The gate layer  123  of MOSFET  120  device may be made of silicon-based material but not limited to high opacity polycrystalline silicon or amorphous silicon. The gate insulation layer  121  of MOSFET  120  device may be made of electrical insulation materials but not limited to silicon dioxide or high-k dielectric insulation layer. The source  126 /drain  125  region of the MOSFET  120  is formed on the substrate by ion implantation process with a high doping concentration typically larger than 10 20 /cm 3  for signal pick-up and amplification of photosensing device. The graphene layer  122  directly contacting with the silicon substrate has the advantages of excellent near-IR sensing capability and simple fabrication process for improving the performance of photosensing devices of the invention, wherein the graphene layer  122  is disposed by CVD or graphene transfer process on crystalline silicon substrate or amorphous silicon on glass substrate after the signal readout circuit has been processed. The graphene layer  122  may also be disposed on flexible semiconductor substrate, with low temperature manufacturing processes such as transfer, coating, sputtering, low-temperature CVD and so forth. In some cases, the photosensing devices with graphene-semiconductor heterojunction are arranged in one- or two-dimensional arrays for various applications. 
         [0043]    The described embodiments are merely possible examples of implementations, set forth for a clear understanding of the principles of the present disclosure. Many variations and modifications may be made without departing substantially from the spirit and principles of the present disclosure. All such modifications and variations are intended to be comprised herein within the scope of this disclosure and the described inventive embodiments, and the present disclosure is protected by the following claims.