Extrinsic semiconductor optical filter

Abstract: An optical filter (18) includes a first layer (22) of semiconductor material having a bandgap and being doped with an impurity having an energy level in the bandgap such that the first layer absorbs optical energy below a first wavelength and transmits optical energy thereabove. The filter further includes a second multilayer coating (26) disposed on said first layer (22) which transmits optical energy at a second wavelength, above the first wavelength, and reflects optical energy at a third wavelength above the second wavelength. The first and second layers in combination provide a bandpass filter (18) with high transmission in-band, low transmission out-of-band and a sharp cutoff therebetween.

Description: The present invention relates to optical filters. More specifically the invention relates to semiconductor optical filters having a substrate and a multi-layer optical thin film deposited thereon. While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope of the invention as defined in the claims and additional fields in which the present invention would be of significant utility. Optical detectors are used in optical space sensors and other applications. In many cases, the detector is an infrared sensor combined with an optical filter. The optical filter defines the spectral band of radiation that will be sensed by the detector. One such optical filter is known as a long wavelength infrared (LWIR) multilayer dielectric (MLD) interference filter. LWIR MLD filters are constructed by coating a semiconductor or dielectric substrate with alternating layers of high refractive index and low refractive index optical thin film materials. By depositing high and low refractive index materials of the proper optical thickness, these interference filters can be made to reflect certain out-of-band wavelengths while transmitting in-band wavelengths. Thus by altering the thickness of each layer of the filter, a controlled amount of radiation in a narrow band of frequencies will be allowed to enter the detector. The typical design goals of conventional long wavelength infrared (LWIR) filters are to achieve: 1) broadband rejection, i.e., the filter should have a low out-of-band transmission; 2) high in-band transmission, allowing the selected transmission bands to enter the detector with minimum attenuation; and 3) a sharp cut-off slope, that is, the transition between transmitting and nontransmitting wavelength regions should be clearly defined. The need for broadband rejection requires an MLD filter of conventional construction with many layers. It is not uncommon for as many as 300 layers to be used in the construction of the filter. Each layer is one-quarter of the operating wavelength divided by the index of refraction. Thus with an operating wavelength of 20 microns and an index of refraction of say 2, each layer must be at least 2.5 microns thick. Three hundred such layers would be 750 microns thick. At this thickness, the interference filter may be thicker than the dielectric or semiconductor substrate. Thus, the filter must often be split between two or three substrates. This creates a packaging problem. Also the many layers of coating limits the speed (f-number) of the optical system. This is due to the fact that the spectral width of the transmitting region of these thick interference filters is shifted for incoming radiation at large angles of incidence (i.e. low f-number). Thus in low f-number systems the transition between the transmitting and nontransmitting regions would occur at the wrong wavelength. This limitation results in a larger optical system (i.e. high f-number) which is undesirable. These optical filters are designed to function in cryogenic environments and therefore must be durable as well as functional. In accordance with conventional design techniques, it is difficult to fabricate the filter for such environments due to stress and adhesion problems associated therewith. Hence, at present, only a few optical filter manufacturers have the capability of making these filters. Accordingly, the filters tend to be quite expensive. A prior art infrared photodetector is known from PATENT ABSTRACTS OF JAPAN Vol. 14, No. 444 (E-982) & JP-A-2177379. Further, a far infrared filter is known from APPLIED OPTICS Vol. 20, No. 1, 1981, USA; pages 31-39; J.S. Seeley et al: "Far infrared filters for the Galileo-Jupiter and other missions". Thus, there is a need in the art for a compact, durable, and cost effective filter which affords greater control of in-band and out-of-band transmission and is insensitive to the system f-number. With the foregoing in mind, the present invention provides a filter comprising: a first layer of semiconductive material having a bandgap and being doped with an impurity having an energy level in said bandgap such that said first layer absorbs optical energy below a first wavelength and transmits optical energy above said first wavelength, and a second layer of optical material disposed on said first layer, said second layer being arranged to transmit optical energy at a second wavelength above said first wavelength, and reflect optical energy at a third wavelength above said second wavelength, whereby said first and second layers in combination provide a passband for said filter with high transmission in-band, low transmission out-of-band and a sharp cutoff between the high and the low transmission regions. The present invention further provides an optical detector system comprising: a first layer of semiconductive material having a first bandgap and being doped with an impurity having an energy level in said first bandgap such that said first layer absorbs optical energy below a first wavelength and transmits optical energy above said first wavelength, and a detector, said detector comprising a second layer of material, in optical alignment with the first layer, having a second bandgap and being doped with an impurity having an energy level in said second bandgap such that said second layer absorbs optical energy above the first wavelength to thereby provide a bandpass filter/detector combination. According to another aspect of the present invention, there is provided a method of fabricating an optical filter including the steps of: a) depositing a layer of semiconductive material having a bandgap onto a substrate; b) doping said layer of semiconductive material with a dopant having an energy level in said bandgap such that said doped layer absorbs optical energy below a first wavelength and transmits optical energy above said first wavelength; and c) depositing an interference filter on said layer of semiconductive material, said interference filter being arranged to transmit optical energy at a second wavelength above said first wavelength, and reflect optical energy at a third wavelength above said second wavelength. The present invention allows for the achievement of out-of-band rejection two to three orders of magnitude greater than current state of the art long wave infrared (LWIR) optical filters. Thus, the teachings provided by the present invention allows for filter with substantially fewer coating layers for a given level of performance as most of the out-of-band rejection is achieved by absorption in the extrinsic semiconductor substrate. Specific embodiments of the present invention are now described, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is an operational block diagram of a typical optical sensing system. Fig. 2(a) is a cross-sectional representation of a conventional LWIR MLD optical filter. Fig. 2(b) is a graph representing the transmission characteristic of the conventional LWIR MLD filter. Fig. 3(a) is a cross-sectional representation of an illustrative LWIR MLD filter constructed in accordance with the teachings of the present invention. Fig. 3(b) is a graph representing the transmission characteristic of an illustrative LWIR MLD optical interference filter constructed in accordance with the teachings of the present invention. Fig. 4(a) is a graph representing the transmission characteristic of an illustrative LWIR MLD optical interference filter constructed of silicon doped with gallium and cooled to temperatures below 18°K in accordance with the teachings of the present invention. Fig. 4(b) is a graph of the illustrative LWIR MLD optical interference filter constructed of silicon doped with gallium and cooled to temperatures below 18°K then coated with an antireflection coating for radiation in the 20 to 28 micron range in accordance with the teachings of the present invention. Fig. 5(a) illustrates the transmission characteristics as a function of wavelength, λ in microns, of a filter constructed in accordance with the present teachings having a silicon substrate and various dopants shown. Fig. 5(b) illustrates the transmission characteristics as a function of wavelength, λ in microns, of a filter constructed in accordance with the present teachings having a germanium substrate and various dopants shown. Fig. 6 is an alternative embodiment of present invention in which the multiple layers of optical thin film material are replaced by doping of the detector in accordance with the present teachings to provide an optical arrangement with a precisely defined passband. Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention. Fig. 1 is an operational block diagram of a typical optical sensing system 10. The system 10 is comprised of front end optics 12, 14 which focus and direct incoming radiation 16 onto an optical filter 18. The filter 18 includes one or more substrates 22 each coated with multiple layers of optical thin film material 26 providing an interference filter therefor. The filter 18 defines a narrow band of wavelengths which enter an optical detector 20. The detector 20 includes sensors (not shown) adapted to absorb the wavelengths of interest and provide a corresponding output. Fig. 2(a) is a cross-sectional representation of a conventional LWIR MLD interference stack optical filter 18'. The filter 18' includes plural semiconductor or dielectric substrates 22' and 24'. As mentioned above, the conventional MLD filter 18' is constructed by coating the substrate 24' with alternating layers 26' of high refractive index and low refractive index optical thin film materials. By depositing high and low refractive index materials in an alternating sequence, the interference filter 18' may be constructed which will reflect certain wavelengths while transmitting others. Thus, by altering the thickness of each layer of the filter, a controlled amount of a narrow band of frequencies will be allowed to enter the detector. The typical design goals of conventional long wavelength infrared (LWIR) filters are to achieve: 1) broadband rejection, i.e., the filter should have a low out-of-band transmission; 2) high in-band transmission, allowing the selected transmission bands to enter the detector with minimum attenuation; and 3) a sharp cut-off slope, that is, the transition between transmitting and nontransmitting wavelength regions should be clearly defined. Fig. 2(b) is a graph representing the transmission characteristic of the conventional LWIR MLD filter 18'. The need for broadband rejection requires an MLD filter of conventional construction with many layers. It is not uncommon for as many as 300 layers to be used in the construction of such a filter. That is, in reference to Fig. 2(a), each layer 26' is one-quarter of the operating wavelength λ divided by the index of refraction n . With an operating wavelength λ of 20 microns and an index of refraction n of say 2, each layer must be at least 2.5 microns thick. Three hundred such layers would be 750 microns thick. At this thickness, the interference layers may be thicker than the dielectric or semiconductor substrate. Thus, the filter must often be split between two or three substrates. This makes the composite filter thicker and creates a severe packaging problem. Also the many layers of coating limits the speed (f-number) of the optical system due to the fact that the spectral width of the transmitting region of these thick interference filters will shift from the design value for incoming radiation at large angles of incidence (i.e. f-number). Further, LWIR MLD optical filters are designed to function in cryogenic environments and therefore must be durable as well as functional. In accordance with conventional techniques, it is difficult to fabricate the filter for such environments due to stress and adhesion problems associated therewith. That is, the multiple layers 26' of the filter 18' must be deposited at high temperature. This places each layer under considerable stress. The adhesion of the large number of layers under such high stress is problematic. Hence, at present, only a few optical filter manufacturers have the capability of making LWIR MLD interference optical filters. Accordingly, conventional LWIR MLD filters tend to be quite expensive. Thus, there is a need in the art for a compact, durable, and cost effective filter which affords greater control of in-band and out-of-band transmission and is insensitive to the optical system f-number. The need in the art is addressed by the optical filter 18 of the present invention which is illustrated in Fig. 3(a). In a most general sense, the filter 18 of the present invention includes a first layer of semiconductor material 22 which is doped with an impurity ion 23 (acceptor or donor) having an energy level in the bandgap of the semiconductor material such that the first layer 22 absorbs optical energy below a first (threshold) wavelength λ th and transmits optical energy thereabove. In the preferred embodiment, the first layer of material is a semiconductor substrate such as silicon (Si), germanium (Ge), cadmium telluride (CdTe), indium antimonide (InSb), etc. that is transparent in the long wavelength infrared region. A semiconductor with low lattice absorption should be used. When cooled, this doped or extrinsic semiconductor will absorb below a threshold wavelength λ th determined by the energy level of the impurity ion 23. The extrinsic semiconductor will also transmit at wavelengths greater than the threshold wavelength λ th. In the illustrative embodiment of Fig. 3(a), a second layer of optical thin film material 26 is disposed on the first layer 22. The second layer is a conventional MLD interference filter which transmits optical energy at a second wavelength, above the first wavelength λ th , and reflects optical energy at a third wavelength above the second wavelength. The first and second layers 22 and 26 in combination provide a filter 18 with high transmission in-band, low transmission out-of-band and a sharp cutoff therebetween. Fig. 3(b) is a graph representing the transmission characteristic of an illustrative LWIR MLD optical interference filter 18 constructed in accordance with the teachings of the present invention. The present invention allows for the achievement of out-of-band rejection two to three orders of magnitude greater than current state of the art LWIR optical filters with substantially fewer layers. Thus, the teachings provided by the present invention allow for a filter with substantially fewer coating layers for a given level of performance as most of the out-of-band rejection is achieved by absorption in the extrinsic semiconductor substrate. For example, silicon doped with gallium (SiGa), when cooled to temperatures below 18°K, absorbs at wavelengths less than 19 microns. Fig. 4(a) is a graph representing the transmission characteristic of an illustrative LWIR MLD optical interference filter 18 constructed of silicon doped with gallium and cooled to temperatures below 18°K in accordance with the teachings of the present invention. If the filter is then coated with an antireflection (AR) coating for radiation in the 20 to 28 micron range, an effective long wavelength pass (LWP) filter is provided with high out-of-band rejection as illustrated in Fig. 4(b). Transmission below λ th ≈ 19 µm is given by: T = e -αx For n ≈ 10 17 cm -3 , α will be ≈ 52 cm -1 and T = e -10.4 = 3 x 10 -5 for x = 2 mm. Thus, by using a SiGa (n≈10 17 cm -3 ) substrate 2 mm thick that is AR coated for the 20 to 28 µm band, an LWP filter can be constructed that offers improved performance and substantially lower cost relative to a filter constructed in accordance with conventional teachings. That is, to achieve the same performance with a conventional filter would require two substrates as illustrated in Fig. 2(a) with all four surfaces coated with a high number of layers of optical thin film material. The teachings of the present invention could be extended to other semiconductors and dopants. For example, Fig. 5(a) illustrates the transmission characteristics as a function of wavelength, λ in microns, of a filter, constructed in accordance with the present teachings, having a silicon substrate and various dopants shown. Fig. 5(b) illustrates the transmission characteristics as a function of wavelength, λ in microns, of a filter constructed in accordance with the present teachings having a germanium substrate and various dopants shown. Other possibilities for Si:Y and Ge:Y, where Y is the impurity type, are illustrated in Tables I and II, respectively, below. Substrate Dopant (Y) λ co (µm) Si As 28.00 Si Ga 19.07 Si Bi 17.97 Si Mg 11.30 Si S 11.30 Si Te 8.80 Si In 7.75 Substrate Dopant (Y) λ co (µm) Ge P 31.00 Ge Cr 24.80 Ge Sb 20.70 Ge Hg 16.10 Ge Te 12.40 Ge Ag 12.40 Ge Au 8.80 Returning to Fig. 3(a), the filter 18 is constructed by first performing a conventional design of an interference filter (typically on a computer) and then a) either depositing a layer of semiconductive material having a bandgap onto a substrate or providing a substrate of semiconductive material having a bandgap; b) doping the layer or substrate of semiconductive material with a dopant having an energy level in the bandgap thereof; and c) depositing an interference filter on the layer or substrate of semiconductive material in a conventional manner. The doping step further includes the following steps. First, the required attenuation αx is calculated based on a desired out-of-band transmission T for the filter in accordance with equation [1] above. Next, a type of dopant is selected based on the cutoff parameter of the transmission band desired for the filter. (The threshold wavelength, is determined by the energy level of the particular impurity ion used. The absorption of radiant energy is promoted by the migration of valence electrons from lower to higher energy levels. To insure a predictable absorption as well as a selectable band pass transmittance, while avoiding free carrier absorption, it is necessary to bring the doped substrate to its lowest energy level by placing it in a cryogenic environment.) The upper transmittance wavelength is determined by the band gap of the semiconductor substrate used. The lower transmittance wavelength is determined by the dopant's absorption properties. Given the desired thickness x of the layer or substrate of semiconductive material, the absorption coefficient α required for the filter is calculated by taking the log of both sides of equation [1]. The concentration of the chosen dopant is then determined from measured data in the literature based on the calculated absorption coefficient α. The layer of substrate of desired thickness x of semiconductive material is then doped with the dopant in accordance with the ascertained concentration. As previously stated, the doped substrate must be cooled below a critical temperature. The extrinsic semiconductor will then transmit at wavelengths greater than the threshold wavelength λ th and absorb wavelengths below the threshold wavelength λ th, thus forming a selectable cut off for a long wavelength band pass filter. Finally, the transmission characteristic of the interference filter is specified based on the transmission characteristic of the doped layer or substrate of semiconductive material. As is evident in the Tables above, the lower transmittance cut-off is determined by the absorption wavelength of the dopant. By changing the dopant, the lower cut-off of the passband can be adjusted. This is highly advantageous because the filter can be fabricated to accept greater fields of view without suffering a degradation in in-band transmittance. As most of the out-of-band rejection is achieved by absorption in the extrinsic semiconductor substrate, the present invention allows for the achievement of out-of-band rejection two to three orders of magnitude greater than current state of the art long wave infrared (LWIR) optical filters with substantially fewer coating layers for a given level of performance. Fig. 6 is an alternative embodiment of present invention in which the multiple layers of optical thin film material are replaced by doping of the detector in accordance with the present teachings. As illustrated in Fig. 6, the substrate of the detector 20'' may be doped with an impurity 25'' such that the filter 18'' provides a first layer of material having a bandgap and being doped with an impurity 23'' having an energy level in the bandgap such that the filter 18'' absorbs optical energy below a first wavelength and transmits optical energy thereabove. And the detector 20'' provides a second layer of material, in optical alignment with the first layer, having a bandgap and being doped with an impurity 25'' having an energy level in the bandgap thereof such that the detector absorbs optical energy above the first wavelength up to the threshold wavelength. The result of this filter/detector combination is an arrangement with a precisely defined passband without the need for MLD coating. Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications applications and embodiments within the scope of the invention as defined in the claims. For example, the invention is not limited to the combination of dopant and substrate used. Nor is the invention limited to manner in which the dopant is effected. It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.

Claims: A filter (18) comprising: a first layer of semiconductive material (22) having a bandgap and being doped with an impurity (23) having an energy level in said bandgap such that said first layer absorbs optical energy below a first wavelength and transmits optical energy above said first wavelength, and a second layer of optical material (26) disposed on said first layer, said second layer being arranged to transmit optical energy at a second wavelength above said first wavelength, and to reflect optical energy at a third wavelength above said second wavelength, whereby said first and second layers in combination provide a passband for said filter with high transmission in-band, low transmission out-of-band and a sharp cutoff between the high and the low transmission regions. The filter of claim 1 wherein said first layer (22) comprises a semiconductive substrate. The filter of claim 1 wherein said first layer (22) is disposed on a substrate. The filter of claim 1 wherein said second layer (26) comprises multiple sub-layers. The filter of claim 4 wherein said multiple sub-layers provide an interference filter. An optical detector system (10") comprising: a first layer of semiconductive material (22') having a first bandgap and being doped with an impurity (23') having an energy level in said first bandgap such that said first layer absorbs optical energy below a first wavelength and transmits optical energy above said first wavelength, and a detector (20"), said detector comprising a second layer of material, in optical alignment with the first layer, having a second bandgap and being doped with an impurity (25") having an energy level in said second bandgap such that said second layer absorbs optical energy above the first wavelength to thereby provide a bandpass filter/detector combination. A method of fabricating an optical filter (18) including the steps of : a) depositing a layer of semiconductive material (22) having a bandgap onto a substrate; b) doping said layer of semiconductive material with a dopant (23) having an energy level in said bandgap such that said doped layer absorbs optical energy below a first wavelength and transmits optical energy above said first wavelength; and c) depositing an interference filter (26) on said layer of semiconductive material, said interference filter being arranged to transmit optical energy at a second wavelength above said first wavelength, and reflect optical energy at a third wavelength above said second wavelength. The method of Claim 7 wherein said step b) includes the steps of: b1) calculating the attenuation according to a predetermined out-of-band transmission of the filter; b2) selecting said dopant based on the cutoff parameter of the predetermined out-of-band transmission of the filter; b3) calculating the absorption coefficient required for the filter according to the predetermined thickness of said layer of semiconductive material; b4) determining the concentration of the selected dopant based on the calculated absorption coefficient; and b5) doping said layer of semiconductive material having said predetermined thickness with said dopant in accordance with said concentration. The method of Claim 8 further including the step of specifying the transmission characteristic of said interference filter based on the transmission characteristic of said layer of semiconductive material.

Inventors: HOBROCK LOWELL M, HOBROCK, LOWELL M.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1993
Main classification: G02B5
IPC codes: G02B5, G02B1, H01L31