Method of creating CIGS photodiode for image sensor applications

Embodiments disclosed herein include photodiodes and methods of forming such photodiodes. In an embodiment, a method of creating a photodiode, comprises disposing an absorber layer over a first contact, wherein the absorber layer comprises a first conductivity type, and disposing a semiconductor layer over the absorber, wherein the semiconductor layer has a second conductivity type that is opposite from the first conductivity type. In an embodiment, the method further comprises disposing a hole blocking layer over the semiconductor layer, wherein the hole blocking layer is formed with a reactive sputtering process with a processing gas that comprises oxygen, and disposing a second contact over the hole blocking layer.

BACKGROUND

Embodiments relate to the field of semiconductor manufacturing and, in particular, to systems and methods for improved photodiode efficiency.

2) Description of Related Art

CuIn1-xGaxSe2(CIGS) thin films have been used as one layer in a diode for photodiode applications. The other layer of the diode may include a Ga2O3layer. The use of Ga2O3has provided improvements in performance (over the use of CdS layers) in part because of the wider bandgap of Ga2O3(i.e., 4.9 eV) and high transmittance of visible light. Ga2O3is also more environmentally friendly than CdS due to the presence of cadmium.

However, CIGS based photodiodes with Ga2O3have not been optimized for characteristics such as quantum efficiency and reduction of leakage current (also referred to as dark current). Particularly, CIGS photodiodes are particularly susceptible to high dark current levels. Accordingly, the signal to noise (e.g., signal to dark current) ratio of CIGS photodiodes is not currently adequate for most uses.

SUMMARY

Embodiments disclosed herein include photodiodes and methods of forming such photodiodes. In an embodiment, a method of creating a photodiode, comprises disposing an absorber layer over a first contact, wherein the absorber layer comprises a first conductivity type, and disposing a semiconductor layer over the absorber, wherein the semiconductor layer has a second conductivity type that is opposite from the first conductivity type. In an embodiment, the method further comprises disposing a hole blocking layer over the semiconductor layer, wherein the hole blocking layer is formed with a reactive sputtering process with a processing gas that comprises oxygen, and disposing a second contact over the hole blocking layer.

Embodiments may also include a photodiode that comprises an absorber layer, wherein the absorber layer has a first conductivity type, and a semiconductor layer over the absorber layer, wherein the semiconductor layer has a second conductivity type. In an embodiment, the photodiode further comprises a hole blocking layer over the semiconductor layer, wherein the hole blocking layer comprises an excess concentration of oxygen.

Embodiments may also include a photodiode that comprises a first contact, a P-type absorber over the first contact, wherein the absorber comprises a CuIn1-xGaxSe2(CIGS) film, and an N-type semiconductor layer over the absorber, wherein the semiconductor layer comprises Ga2O3and Sn. In an embodiment, the photodiode further comprises a hole blocker over the semiconductor layer, wherein the hole blocker comprises Ga2O3with an excess atomic percentage of oxygen, and a second contact over the hole blocker.

DETAILED DESCRIPTION

Systems and methods described herein include processes for modulating the resistance of various layers in a photodiode by increasing and/or decreasing the oxygen content of various layers. In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments. It will be apparent to one skilled in the art that embodiments may be practiced without these specific details. In other instances, well-known aspects are not described in detail in order to not unnecessarily obscure embodiments. Furthermore, it is to be understood that the various embodiments shown in the accompanying drawings are illustrative representations and are not necessarily drawn to scale.

As noted above, photodiodes with diodes comprising a CIGS film and a Ga2O3film have demonstrated improvements over photodiodes where the Ga2O3is replaced with CdS. However, such photodiodes still suffer from non-optimal quantum efficiencies and high dark currents. Accordingly, embodiments disclosed herein include photodiode constructions that provide improved quantum efficiencies and/or reduced dark currents. Particularly, embodiments disclosed herein include processes that allow for modulation of the resistance and/or dopant concentration of various layers in the photodiode. Generally, increases in the resistance will decrease the dark current, and increasing the dopant concentration will increase quantum efficiency by driving the depletion region deeper into the CIGS film.

Referring now toFIG. 1, a cross-sectional illustration of a photodiode100is shown, in accordance with an embodiment. In an embodiment, the photodiode100may comprise a first contact126. The first contact126may be one or more suitable conductive materials. For example, the first contact126may comprise molybdenum (Mo) or the like. In an embodiment, the first contact126may have a thickness that is between approximately 100 nm and approximately 1 μm. The first contact layer126may be disposed over a substrate (not shown). For example, the substrate may comprise a glass substrate, a semiconductor substrate, or any other suitable material on which a photodiode100may be fabricated.

In an embodiment, an absorber layer125is disposed over the first contact126. The absorber layer125may be a semiconductor material with a first conductivity type. For example, the first conductivity type may be P-type. In a particular embodiment, the absorber layer125is a CuIn1-xGaxSe2(CIGS) film. The variable X may be chosen to provide desired electrical properties and may be between 0 and 1. The absorber layer125may have any suitable thickness. For example, the absorber layer125may have a thickness that is between approximately 0.5 μm and 5 μm.

In an embodiment, a semiconductor layer124is disposed over the absorber layer125. The semiconductor layer124may have a second conductivity type that is opposite from the first conductivity type. For example, where the absorber layer125is P-type, the semiconductor layer124is N-type. Accordingly, a P—N diode is formed between the semiconductor layer124and the absorber layer125.

In an embodiment, the semiconductor layer124comprises Ga2O3. In some embodiments, the semiconductor layer124may further comprise a dopant to increase conductivity of the semiconductor layer. For example, a Ga2O3semiconductor layer124may be doped with tin (Sn). In an embodiment, the atomic percentage of Sn may be between approximately 5% and approximately 10%. Increasing the conductivity of the semiconductor layer124drives the depletion region deeper into the absorber layer and improves quantum efficiency. As will be described in greater detail below, the resistance of the semiconductor layer124may also be decreased by integrating excess oxygen into the semiconductor layer124. For example, excess oxygen may be supplied by a reactive sputtering process that utilizes a processing gas comprising oxygen. In an embodiment, the semiconductor layer124may have a thickness between approximately 10 nm and approximately 100 nm.

In an embodiment, the photodiode100may further comprise a hole blocker123disposed over the semiconductor layer124. In an embodiment, the hole blocker123comprises a material with a high potential barrier in order to suppress the injection of holes from the second contact122to the P—N diode. The blocking of hole injection reduces the dark current.

In a particular embodiment, the hole blocker123may also comprise Ga2O3. However, whereas the doping concentration of the semiconductor layer124is increased to improve quantum efficiency, the hole blocker123is primarily used to mitigate dark current. Accordingly, in addition to blocking the injection of holes, the resistance of the hole blocker123is also increased relative to the resistance of the semiconductor layer124. For example, the resistance of the hole blocker123may be increased by increasing the atomic percentage of oxygen in the Ga2O3. The oxygen concentration may be increased by using a reactive sputtering process with a processing gas that comprises oxygen. The use of an oxygen partial pressure during the sputtering reduces the number of oxygen vacancies and, therefore, increases the resistance. In an embodiment, the atomic percentage of oxygen may be increased by up to approximately 5% above the concentration of oxygen typical of the hole blocker material (e.g., Ga2O3) In some embodiments, the oxygen partial pressure may be increased or decreased during the sputtering to provide a graded (e.g., non-uniform) oxygen concentration through the thickness of the hole blocker123. In an embodiment, the hole blocker123may have a thickness between 25 nm and 200 nm.

In an embodiment, the photodiode100may further comprise a second contact122disposed over the hole blocker123. The second contact122may be any suitable conductive material that is substantially optically transparent to a desired bandwidth of electromagnetic radiation. For example, the second contact122may comprise indium tin oxide (ITO) or the like. In an embodiment, the second contact122may have a thickness that is between approximately 50 nm and 500 nm.

In an embodiment, the photodiode100may further comprise an antireflective coating (ARC)121over the second contact122. The ARC121may improve efficiency of the device by reducing reflected electromagnetic radiation. In an embodiment, the ARC121may comprise SiO2or the like. The thickness of the ARC121may be any thickness suitable for a reducing or substantially eliminating reflections. For example, the thickness of the ARC121may be between approximately 10 nm and approximately 50 nm.

In the description ofFIG. 1above, the photodiode100is particularly described as a CIGS photodiode with a Ga2O3semiconductor layer and hole blocker. However, it is to be appreciated that embodiments disclosed herein are applicable to many different photodiode constructions. Particularly, embodiments disclosed herein include modifications to the semiconductor layer124and the hole blocker123that improve quantum efficiency and reduce dark current. Generally, quantum efficiency is improved by increasing the doping concentration of the semiconductor layer124to drive the depletion region deeper into the absorber layer125, and the dark current is reduced by increasing the resistance of the hole blocker123by adding excess oxygen. Such modifications may be similarly applicable to other material systems.

Referring now toFIG. 2, an equivalent circuit diagram200of a photodiode is shown, in accordance with an embodiment. The equivalent circuit diagram200is useful in illustrating the benefits of an increased resistance of various layers with respect to the reduction in dark current.

In an embodiment, the equivalent circuit comprises a first resistor221that represents the contact resistance and a second resistor222that represents the resistance of the hole blocker123. The third resistor223is the shunt resistance (also referred to as the “on resistance”). The equivalent circuit200further comprises a diode230and a current source231that are in parallel with the third resistor223.

The total resistance RTotalof the equivalent circuit diagram200is represented by Equation 1. As shown, the total resistance RTotalis equal to the sum of the contact resistance RContact, the hole blocker resistance RHoleBlocker, and the shunt resistance RShunt. The dark current is dictated in part by the total resistance RTotal. That is, increasing the total resistance RTotalwill result in a decrease in the dark current. In a particular embodiment, the total resistance RTotalis increased by increasing the hole blocker resistance RHole Blocker. As noted above, the hole blocker resistance RHole Blockeris increased by increasing the atomic percentage of oxygen in the hole blocker123.
RTotal=RContact+RHole Blocker+RshuntEquation 1

However, it is to be appreciated that increasing the total resistance RTotalmay also negatively affect the signal intensity. Accordingly, there is a tradeoff between the signal intensity and dark current. Embodiments disclosed herein, therefore, include a process for identifying an optimum balance between the two parameters.

An exemplary graph of the resistance versus intensity with the signal341and the dark current342is shown inFIG. 3. The plots shown inFIG. 3utilize a fictional set of data points and may or may not reflect the exact relationship between the two parameters. However, the graph inFIG. 3does illustrate that slopes and shapes of the two parameters may not be directly correlated with each other. That is, the ratio of signal intensity to dark current intensity may not be uniform for all resistance values. Accordingly, an optimum resistance value may be selected that provides the highest signal to dark current ratio. Once the desired resistance value is determined, the oxygen content of the hole blocker123needed to achieve that resistance can be determined in order to provide an optimized photodiode100.

Embodiments disclosed herein also comprise methods of fabricating such optimized photodiodes with processing operations that are easily tailored to provide the desired resistances and dopant concentrations. An example of one such processing method is described with respect to the process flow diagram inFIG. 4and the corresponding cross-sectional illustrations of each processing operation inFIGS. 5A-5D.

Referring now toFIG. 4, a process flow diagram of a process450is shown, in accordance with an embodiment. In an embodiment, the process450may begin with operation451which comprises disposing an absorber layer525with a first conductivity type over a first contact. In an embodiment, the first conductivity type may be a P-type semiconductor. In a particular embodiment, the absorber layer is a CIGS film. The absorber layer may be deposited with any suitable deposition process. As shown in the partially formed photodiode500inFIG. 5A, the absorber layer525may be disposed over a first contact526. The first contact526may be disposed over a substrate519(e.g., glass, silicon, etc.).

In an embodiment, process450may continue with operation452which comprises disposing a semiconductor layer524with a second conductivity type over the absorber layer525, as shown inFIG. 5B. In an embodiment where the first conductivity type is a P-type semiconductor, the second conductivity type is an N-type semiconductor. For example, the semiconductor layer524may comprise Ga2O3. In some embodiments, the Ga2O3may be doped with Sn. Doping with Sn may allow for an increase in the N-type doping (e.g., to provide N+or N++doping) of the semiconductor layer524. For example, an atomic percentage of Sn in the semiconductor layer524may be between approximately 5% and approximately 10%. Increasing the doping drives the depletion region of the diode (formed by the absorber layer525and the semiconductor layer524) further into the absorber layer525. Driving the depletion region further into the absorber layer525increases the quantum efficiency of the photodiode500.

In some embodiments, the semiconductor layer524may be disposed over the absorber layer525with any suitable process. For example, a PVD process, such as sputtering may be used to deposit the semiconductor layer524. In some embodiments, excess oxygen may also be incorporated into the semiconductor layer524in order to selectively increase the resistance of the semiconductor layer524. Additional oxygen may be incorporated into the semiconductor layer524with processes such as reactive sputtering with the use of an oxygen containing process gas.

In an embodiment, process450may continue with operation453which comprises disposing a hole blocking layer523over the semiconductor layer524with a reactive sputtering process with a processing gas comprising oxygen, as shown inFIG. 5C. In some embodiments, the hole blocking layer523may comprise Ga2O3with an excess of oxygen. That is, the atomic percentage of oxygen in the hole blocking layer523may be up to approximately 5% higher than in a typical Ga2O3layer. Accordingly, the resistance of the hole blocking layer523may be increased in order to reduce the dark current in the system. Those skilled in the art will recognize that the use of reactive sputtering to provide the excess oxygen is particularly useful as the oxygen concentration of the hole blocking layer523may be easily controlled by changing the partial pressure of oxygen in the reactive sputtering chamber. That is, different sputtering targets are not needed in order to modulate the oxygen content of the hole blocking layer523as is necessary in processes, such as pulsed laser deposition (PLD). Furthermore, reactive sputtering also allows for the oxygen concentration to be graded within the thickness of the hole blocking layer523by increasing or decreasing the oxygen partial pressure during the deposition of the hole blocking layer523.

In an embodiment, process450may continue with operation454which comprises disposing a second contact522over the hole blocking layer523, as shown inFIG. 5D. In an embodiment, the second contact522may be any conductive layer that is substantially transparent to a desired bandwidth of electromagnetic radiation. For example, the second contact522may comprise ITO or the like. In some embodiments, an ARC521may also be disposed over the second contact522.

Referring now toFIG. 6, a plan view illustration of a system660that comprises a plurality of photodiodes600is shown, in accordance with an embodiment. In an embodiment, the system660may comprise a substrate661. In an embodiment, the substrate661may comprise a glass substrate, a silicon substrate, or any other suitable substrate. An array of photodiodes600may be arranged over the substrate661. The photodiodes600may be substantially similar to the photodiodes described above. For example, each of the photodiodes600may comprise a CIGS absorber layer, a Ga2O3:Sn semiconductor layer, and a Ga2O3hole blocker layer with excess oxygen. In an embodiment, the system660may be used for any application that utilizes photodiodes, such as imaging, sensing, solar cells, or the like.

Referring now toFIG. 7, a block diagram of an exemplary computer system760of a processing tool is illustrated in accordance with an embodiment. In an embodiment, computer system760is coupled to and controls processing in the processing tool. Computer system760may be connected (e.g., networked) to other machines in a Local Area Network (LAN), an intranet, an extranet, or the Internet. Computer system760may operate in the capacity of a server or a client machine in a client-server network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. Computer system760may be a personal computer (PC), a tablet PC, a set-top box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a server, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single machine is illustrated for computer system760, the term “machine” shall also be taken to include any collection of machines (e.g., computers) that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies described herein.

System processor702represents one or more general-purpose processing devices such as a microsystem processor, central processing unit, or the like. More particularly, the system processor may be a complex instruction set computing (CISC) microsystem processor, reduced instruction set computing (RISC) microsystem processor, very long instruction word (VLIW) microsystem processor, a system processor implementing other instruction sets, or system processors implementing a combination of instruction sets. System processor702may also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal system processor (DSP), network system processor, or the like. System processor702is configured to execute the processing logic726for performing the operations described herein.

The computer system760may further include a system network interface device708for communicating with other devices or machines. The computer system760may also include a video display unit710(e.g., a liquid crystal display (LCD), a light emitting diode display (LED), or a cathode ray tube (CRT)), an alphanumeric input device712(e.g., a keyboard), a cursor control device714(e.g., a mouse), and a signal generation device716(e.g., a speaker).

The secondary memory718may include a machine-accessible storage medium731(or more specifically a computer-readable storage medium) on which is stored one or more sets of instructions (e.g., software722) embodying any one or more of the methodologies or functions described herein. The software722may also reside, completely or at least partially, within the main memory704and/or within the system processor702during execution thereof by the computer system760, the main memory704and the system processor702also constituting machine-readable storage media. The software722may further be transmitted or received over a network720via the system network interface device708. In an embodiment, the network interface device708may operate using RF coupling, optical coupling, acoustic coupling, or inductive coupling.

In the foregoing specification, specific exemplary embodiments have been described. It will be evident that various modifications may be made thereto without departing from the scope of the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.