Patent Application
20230197866
The invention relates generally to photodetectors and in particular to an electron-photon barrier layer utilized in single band and dual band photodetectors.
A photodetector is a device that detects or responds to incident light by using the electrical effect of absorbed individual photons. Photodetectors may be designed to detect incident light within a particular spectral frequency range (e.g., visible, infrared, etc.).
A dual band photodetector is one that is capable of operating in a first mode to detect light in a first spectral range and a second mode to detect light in a second spectral range. Typically, a first absorber layer (i.e., Band 1 region) is reverse biased to detect photons absorbed in a first spectral range and a second absorber layer (i.e., Band 2 region) is reverse biased to detect photons absorbed in a second spectral range, wherein a minority-majority carrier pair is created in response to absorption of a photon (light) within the given spectral range. The barrier layer blocks majority carriers from propagating between the respective absorber layers (e.g., the barrier layer blocks electrons in nBn detectors and holes in pBp detectors). The reverse biasing of the corresponding absorber layer allows the minority carrier to be collected which is proportional to the number of photons absorbed. In the first mode, minority carriers are collected from the first absorber layer, and in the second mode minority carriers are collected from the second absorber layer. A blocking layer comprised of high band gap layer is typically positioned between the first absorber layer and the second absorber layer to block majority carrier transport between the respective absorption layers. The blocking layer ensures that minority carriers collected during a first mode originated in the first absorber layer and minority carriers collected during a second mode originated in the second absorber layer, thereby preventing (or at least reducing) crosstalk between the respective absorber layers.
Typical blocking layers only block majority carriers (e.g., electrons in nBn detectors or holes in pBp detectors). In dual band photodetectors, when imaging in the higher spectral range (e.g., detecting light absorbed by second absorber layer), light may be generated in the other region (i.e., first absorber layer) via radiative recombination. In some cases, the wavelength of the light generated via radiative recombination in the first absorber layer may propagate and be absorbed in the second absorber layer, resulting in undesirable crosstalk between the respective bands or layers. It would therefore be desirable to prevent or reduce this type of optical crosstalk between respective bands.
According to one aspect, a dual band photodetector includes a first band absorber layer configured to absorb incident light in a first wavelength spectral band and a second band absorber layer configured to absorb incident light in a second wavelength spectral band. The dual band photodetector further includes an electron-photon blocking (EPB) layer located between the respective layers and includes at least one high band gap layer and at least one intervening layer. The difference in refractive index between the at least one high band gap layer and the at least one intervening layer form a distributed brag reflector (DBR) designed to reflect wavelengths corresponding with radiative recombination photons emitted from at least the first absorber layer to reduce optical crosstalk between the first band absorber layer and the second band absorber layer.
According to another aspect, a photodetector includes a first band absorber layer configured to absorb incident light in a first wavelength spectral band and an electron-photon blocking (EPB) layer located adjacent to the first band absorber layer. The EPB layer includes at least one high band gap layer having a first refractive index and at least one intervening layer having a second refractive index different than the first refractive index, wherein a difference in refractive index between the at least one high band gap layer and the at least one intervening layer form a distributed brag reflector (DBR) designed to reflect a wavelength or subset of wavelengths within the first wavelength spectral band.
According to another aspect, a dual band imaging device includes an imaging lens, a readout integrated circuit (ROIC), and a dual band photodetector. The dual band photodetector includes a plurality of dual band pixels configured to receive incident light from the imaging lens and to generate an electrical response, each pixel comprising a first band absorber layer, a second band absorber layer, and an electron-photon blocking (EPB) layer located between the first band absorber layer and the second band absorber layer. The EPB layer includes at least one high band gap layer and at least one intervening layer, wherein the high band gap layer is configured to block majority carrier transport between the first band absorber layer and the second band absorber layer and a difference in refractive index between the at least one high band gap layer and the at least one intervening layer form a distributed brag reflector (DBR) designed to reflect wavelengths corresponding with radiative recombination photons emitted from at least the first absorber layer to reduce optical crosstalk between the first band absorber layer and the second band absorber layer.
According to some aspects, the present disclosure is directed to photodetectors, and in particular to photodetectors utilizing an electron-photon blocking (EPB) layer. In some embodiments, the EPB layer is comprised of one or more alternating stacks of high band gap layers defined by a first refractive index separated by an intervening layer having a second refractive index different than the first refractive index. In some embodiments, the first refractive index of the high band gap layer is less than the second refractive index associated with the intervening layer. The contrast in refractive indexes between the respective layers of the EPB create a distributed brag reflector (DBR). In a dual band detector having a first band region (configured to detect light in a first spectral band range) and a second band region (configured to detect light in a second spectral band range), the EPB layer is located between the first band region and the second band region. The high band gap material included in the EPB layer blocks majority carriers (e.g., electrons, holes depending on the detector type) from propagating between the respective bands, while the DBR acts to reflect light generated as a result of radiative recombination. The EPB layer therefore acts as both an electron blocking layer (i.e., majority carrier) as a well a photon blocking layer that reduces optical crosstalk between the respective detectors. The forward biasing of the respective band by the ROIC causes photogenerated minority carriers from being collected while the other respective band is reverse biased.
According to another aspect, the EPB layer may be utilized in a single band photodetector. The EPB layer may be utilized to reflect unabsorbed light to increase absorption efficiency by the absorption layer (in this case, a single absorption layer). In some embodiments, the increased efficiency may allow for the thickness of the absorption layer to be decreased.
Each of the plurality of dual band pixels 116 includes first band absorber layer 118, an electron-proton blocking (EPB) layer 120, a second band absorber layer 122, a contact layer 124 and metal contact pad 126. In some embodiments, the first absorber layer 118 is configured to absorb light within a first spectral band and the second absorber layer 122 is configured to absorb light within a second spectral band. In some embodiments, the first spectral band is lower in wavelength than the second spectral band (i.e., the first band absorber layer 118 absorbs higher energy light than the second band absorber layer 122). In this way, incident light in the first spectral band propagates through the anti-reflective coating layer 108 and the substrate 110 and is absorbed by the first band absorber layer 118. Incident light in the second spectral band propagates through the anti-reflective coating layer 108, the substrate 110, the first band absorber layer 118 and the EPB layer 120 and is absorbed by the second band absorber layer 122. Because the incident light in the second spectral band is lower energy (i.e., higher wavelength), the incident light in the second spectral band is not absorbed by the first band absorber layer 118, but instead passes through the first band absorber layer 118 and the EPB layer 120 and is absorbed by the second band absorber layer 122. The energy gained through the absorption of light—whether in the first band absorber layer 118 or second band absorber layer 122—generates an electron-hole pair with an electron moving from the valence band to the conduction band. Minority carriers (either electrons or holes) are collected based on the mode of operation of the device. The dual band photodetector collects these minority carriers (e.g., electrons or holes) from either the first band absorber layer 118 or the second band absorber layer 122, allowing the photodetector to image in either the first spectral band in a first mode of operation or the second spectral band during a second mode of operation. Operation within the first mode or second mode is a result of the polarity of the bias direction. In the first mode, the polarity of the bias voltage set by the ROIC 106 results in the collection of minority carriers created in the first band absorber layer 118. In the second mode, the polarity of the bias voltage is reversed by the ROIC 106 and the minority carriers are collected from the second band absorber layer 122. As discussed in more detail with respect to
In some embodiments, reflectivity efficiency is related, at least in part, on the number of layers within the EPB layer 120 (described in more detail in
wherein λ is the wavelength of the light to be reflected and n is the refractive index of the material the light is propagating within. m is an integer number that can be any odd number starting from 1 such as 1, 3, 5, 7, 9, etc.
In some embodiments, the thickness of the high bandgap layer 132 (which has a first refractive index) will be different than the thickness of the intervening layer 134 (which has a second refractive index). For example, the high bandgap layer 132—having a lower refractive index n1—may be thicker than the corresponding intervening layer 134 characterized by a high refractive index n2. However, because layer thickness may vary with multiples m of the wavelength, in some embodiments the high bandgap layer 132 may be thinner than the intervening layer 134 despite the lower refractive index. In addition, in some embodiments a plurality of layers, wherein increasing the number of layers increases the reflectivity efficiency of the EPB layer 120. In some embodiments, the DBR created by the one or more layers is selected based on the bandgap of the first band absorber layer 118 (or second band absorber layer 122), wherein the bandgap determines the wavelength of light generated via radiative recombination (referred to herein as the cutoff wavelength). Knowledge of the bandgap allows the DBR to be tuned to the wavelength of light generated via radiative recombination, thereby reflecting photons created via this process that might otherwise result in optical cross-talk between the respective first and second band absorber layers. In some embodiments, the intervening layer 134 is comprised of one or more of InP, GaAs, InGaAs, InAlAs, GaSb, InGaSb, AlGaSb, InAsSb, AlGaAsSb, and/or InGaAsSb, InGaAsP, InAlAsP.
In the energy band diagram within the device 116 shown in
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
