Reverse conductive nano array and manufacturing method of the same

A high-speed photodiode may include a photodiode structure having a substrate, a light-absorbing layer and a light-directing layer that is deposited on a top surface of the photodiode structure and patterned to form a textured surface used to change the angle of incident light to increase a light path of the incident light when entering the photodiode structure. In one embodiment, the light-directing layer may include a plurality of polygon such as triangular projections to refract the incident light to increase the light path thereof when entering the photodiode structure. In another embodiment, a plurality of nanoscaled sub-triangular projections can patterned on both sides of each triangular projection to more effectively increase the light paths. In a further embodiment, porous materials can be used to form the light-directing layer.

FIELD OF THE INVENTION

The present invention relates to opto-electronic devices, and more particularly to a high-speed photodiode having high absorption efficiencies without compromising the speed of the photodiode.

BACKGROUND OF THE INVENTION

Over the past few decades, photodiodes have been utilized in the areas including military, communication, information technology and energy. Photodiodes are operated by absorbing photons or charged particles to generate a flow of current in an external circuit, proportional to the incident power. In other words, photodiodes primarily have two functions: the absorption and conversion of light to an electrical signal, and the amplification of that electrical signal through multiplication.

The application of photodiodes in optical telecommunication can be seen inFIG. 1, where an electrical signal is converted into an optical signal at a transmitting end and then transmitted through an optical transmission line, such as an optical fiber. The converted optical signal is converted back to an electrical signal at a receiving end using a light-receiving element, such as a photodiode or photodetector.

Silicon photodiodes are semiconductor devices responsive to high energy particles and photons. A standard type is the PIN diode that basically includes an intrinsic semiconductor light-absorbing layer sandwiched between n-type and p-type semiconductor layers. As shown inFIG. 2, a PIN diode200may include a cathode210, an n-doped region220, an intrinsic light-absorbing layer230, a p-doped region240, and an anode250. When incident light270comes in, most photons are absorbed in the intrinsic layer230, and carriers generated therein can efficiently contribute to the photocurrent. The most common PIN diodes are based on silicon, and they are sensitive throughout the visible spectral region and in the near infrared up to the wavelength of 1 μm. InGaAs PIN diodes are available for longer wavelengths up to 1.7 μm. In addition, the PIN diode may have an anti-reflecting coating layer260on top of the p-doped region240.

An avalanche photodiode (APD) is another type of photodetector that exploits the photoelectric effect to convert light to electricity. Different from conventional PIN diodes, incoming photons trigger an internal charge avalanche in APDs, which may generate an internal current gain effect (around 100) due to this avalanche effect. As shown inFIG. 2a, a SACM (separate absorption, charge and multiplication) APD structure200′ may include at least an absorption layer210′, a charge layer220′ and a multiplication layer230′, where the charge layer220′ provides a sufficient electric filed drop between the absorption layer210′ and the multiplication layer230′ to secure effective avalanche multiplication.

The performance of the photodiodes is based on the achievable signal processing speed and noise, which are dependent on the absorption efficiencies.FIG. 3shows a simplified photodiode structure310with a thickness d, and the simplified structure310may include all layers in the PIN diode200shown inFIG. 2or the APD200′ inFIG. 2a. When incident light passes through the diode structure310, a light path length merely equals to the thickness of the structure d. The light path generally refers to the distance that an unabsorbed photon may travel within the device before it escapes out of the device. Without increasing the light path inside the diode structure310, the absorption efficiency is considered low. To enhance the absorption efficiency, some people proposed to increase the thickness of the light-absorbing layer, however, it limits the transit time of electrons and holes generated by the incident light and thus reduce the speed of the photodiode. Therefore, there remains a need for a new and improved photodiode having high absorption and multiplication efficiencies and high speed to overcome the problems stated above.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a high-speed photodiode device with high absorption efficiencies by increasing the light path of the incident light.

It is another object of the present invention to provide a high-speed photodiode device having a light-directing layer with a micro/nano-textured surface to change the angle of the incident light, and further increase the light path thereof.

It is a further object of the present invention to provide a high-speed photodiode device having a light-directing layer with a micro/nano-textured surface that includes a plurality of triangles, sub-triangles on the triangles, polygon-shaped projections, diamond-shaped projections, cone-shaped projections, or any combination of abovementioned shapes.

It is still a further object of the present invention to provide a PIN diode employing a light-directing layer with a micro/nano-textured surface to change the angle of the incident light, and further increase the light path thereof.

It is still a further object of the present invention to provide an avalanche photodiode (APD) employing a light-directing layer with a micro/nano-textured surface to change the angle of the incident light, and further increase the light path thereof.

In one aspect, a high-speed photodiode comprises a photodiode structure that includes a substrate, a light-absorbing layer, a charge layer and a multiplication layer; and a light-directing layer that is deposited on a top surface of the photodiode structure and patterned to form a textured surface used to change the angle of incident light to increase a light path of the incident light when entering the photodiode structure. In another aspect, the high-speed photodiode may be a PIN diode, which includes a substrate, an intrinsic light-absorbing layer and two contact layers.

In one embodiment, the light-directing layer may include a textured pattern such as a plurality of triangular projections to refract the incident light to increase the light path thereof when entering the photodiode structure, and the material of the triangular projections includes InP, GaAs, Si, Ge, InGaAs and InGaAsP. Furthermore, an increased light path can be obtained as d(sec(θ−θ2)−1), where d is thickness of the photodiode absorption layer, θ is an incident angle of the incident light, and θ2is a refractive angle of refracted light.

In another embodiment, a plurality of sub-triangular projections may be patterned on both sides of each triangular projection using nanolithography instruments including contact aligners, steppers or E-beam lithography. Furthermore, the incident light can be arranged to totally reflect in the triangular projection to significantly increase the light path when the refractive index of the sub-triangular projection is higher than the refractive index of corresponding triangular projection.

In a further embodiment, the light-directing layer is made by porous materials, and the incident light is deflected inside the porous material to increase the light path when entering the photodiode structure. It is noted that the incident light may be deflected more than one time in the porous material.

In another aspect, an optical communication system may include a transmitter, an optical fiber optically connecting to the transmitter, and an optical receiver optically coupled to the optical fiber. The optical receiver may include a high-speed photodiode having a photodiode structure including a substrate, a light-absorbing layer, a charge layer and a multiplication layer; and a light-directing layer that is deposited on a top surface of the photodiode structure and patterned to form a textured surface used to change the angle of incident light to increase a light path of the incident light when entering the photodiode structure.

DETAILED DESCRIPTION OF THE INVENTION

As stated above, speed is one of the key factors to determine the performance of photodiodes, especially when the photodiode is utilized in high-speed optical communication systems. The speed of the photodiode primarily depends on the absorption and multiplication efficiencies, which can be enhanced by changing the incident angle of the incident light to enable the light to travel a longer distance inside the photodiode, or using some “light-trapping” features to enable the light to bounce back and forth many times inside the photodiode.

In one aspect of the present invention, a high-speed photodiode400may include a diode structure410and a light-directing layer420that is disposed on top of the diode structure410, as shown onFIG. 4. In one embodiment, the high-speed photodiode400may be a PIN diode such as the PIN diode200. In another embodiment, the high-speed photodiode400may be an avalanche photodiode (APD). The substrate of the high-speed photodiode can be silicon, germanium, Si/Ge material system, Si/SixGe1−xmaterial system, InP/InGaAs material system or Inp/InGaAsP material system, according to the wavelength of the incident light.

In an exemplary embodiment, the light-directing layer420is deposited on top of the diode structure410and patterned to form a plurality of triangular projections with a facet angle θ. The size of the each triangle can range from a few micrometers to a few hundred nanometers or even smaller. The triangles can be formed by wet etching or other etching techniques in semiconductor fabrication process. As discussed above, the absorption efficiency of the photodiode can be enhanced by changing the incident angle of the incident light to increase the light path inside the photodiode. As can be see inFIG. 4afocusing on light reflection/refraction, when an incident light430is being incident on an angled surface of the light-directing layer420, the incident light430is refracted into the triangular projection of the light-directing layer420to generate a refracted light440entering the diode with a specific angle to travel a longer distance (than the thickness d) in the diode structure410. In other words, the light path of the incident light430increases to enhance absorption and multiplication efficiencies, and further enhance the speed of the photodiode400. It is noted that the projections of the light-directing layer420are not limited to triangles or sub-triangles on the triangles. The projections can be polygon-shaped, diamond-shaped, cone-shaped, or any combination of abovementioned shapes.

According to Snell's law, the ratio of sines of the angles of incidence and refraction is equivalent to the opposite ratio of the indices of refraction of the two media, which can be expressed as:
n1sin θ1=n2sin θ2
where θ1and θ2are the angles of incidence and refraction, which is measured relative to the normal plane N of the interface, while n1and n2are the refractive indices of the incident and refractive media, as shown inFIG. 4a. In one embodiment, the material of the light-directing layer420can be, but not limited to InP, GaAs, Si, Ge, InGaAs, InGaAsP. If the incident medium is air (refractive index about 1) and the refractive medium is silicon (refractive index about 3.97), the relationship between the incidence and refraction angles becomes sinθ1=3.97 sinθ2, and if the incidence angle θ1is known, the refractive angle θ2can be obtained accordingly.

Still referring toFIG. 4a, the incident light430can not only be refracted, but reflected to another triangular projection. A reflected light450can be considered the incident light of another triangular projection, and a second refracted light460can be generated, and enters the diode structure410with a longer light path. However, the magnitude of the refracted light460may be attenuated due to multiple reflection and refraction, and it may not be as effective (regarding electron/hole generation) as the first refracted light440.

As illustrated inFIG. 4b, the increased light path can be obtained using simple geometric calculations. A dash line430′ can be drawn from point where the incident light430interfaces with the angled surface, and an angle (θ−θ2) is formed between the dashed line430′ and the refracted light440. Therefore, the light path after the incident light430is being refracted is d sec(θ−θ2), and the increased light path is d(sec(θ−θ2)−1), and the light absorption efficiency of the diode is thus increased by sec(θ−θ2). For example, if the refractive index of the light-directing layer420is 3.1, the refractive angle is about θ/3, and the increased light path is

d⁡(sec⁡(23⁢θ)-1).
It is noted that the incident angle θ1is equal to the facet angle θ of the triangular projection.

In another embodiment, a high-speed photodiode500may include a diode structure510and a light-directing layer520that is disposed on top of the diode structure510, as shown onFIG. 5. Likewise, the high-speed photodiode500may be an APD200′ shown inFIG. 2aor a PIN diode200shown inFIG. 2. The substrate of the high-speed photodiode can be silicon, germanium, Si/Ge material system, Si/SixGe1−xmaterial system, InP/InGaAs material system or Inp/InGaAsP material system according to the wavelength of the incoming light.

While the light-directing layer520is similar to the light-directing layer420having a plurality of triangular projections, the light-directing layer520has a number of sub-triangular projections530on both sides of each triangular projection as shown inFIG. 5a. As discussed above, the triangular projections in the light-directing layer520can be formed by wet etching or other applicable semiconductor etching techniques, and the size thereof ranges from a few hundred micrometers to a few hundred nanometers. The sub-triangular projections530, which are even smaller than the triangular projections in the light-directing layer520, can be generated by nanolithography instruments, such as contact aligners, steppers or E-beam lithography.

Comparing with the light-directing layer420, the light-directing layer520with “nano-textured” sub-triangular projections530has much more angled surfaces to refract the incident light540to generate more refracted light (such as551to556) with different refractive angles to effectively increase the light paths as shown inFIG. 5b. Since the angle of refracted light can be controlled by the facet angle of the sub-triangular projections530as discussed above, the light path can be optimized either individually or collectively to effectively increase the electron/hole generation and further enhance the performance of the photodiode. In a further embodiment, if the incident light540passes from a high refractive index medium to a low refractive index medium, there is a possibility of total internal reflection. Namely, the incident light540can be trapped inside and make multiple travels to significantly increase the light path. For example, if the sub-triangular projection530is made by a material with a higher refractive index than the material used for the triangular projection, total internal reflection can be arranged to achieve when the incident light540passes from the sub-triangular projections530to the triangular projection.

A porous material is a material containing pores. The skeletal portion of the porous material is usually a solid, but structures like foams. In still a further embodiment shown inFIG. 6, a photodiode600has a diode structure610and a porous light-directly layer620patterned on top of the diode structure610. As can be seen inFIG. 6b, when the incident light630passes through the porous, the incident light630can be deflected to change the angle and further increase the light path (such as641and642) when entering the diode structure610. Moreover, the incident light630may be deflected more than once in the porous structure (such as643and644) before entering the diode structure610.