WO2002033757A1 - Photodetecteur infrarouge lointain a heterojonction - Google Patents

Photodetecteur infrarouge lointain a heterojonction Download PDF

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Publication number
WO2002033757A1
WO2002033757A1 PCT/US2001/032403 US0132403W WO0233757A1 WO 2002033757 A1 WO2002033757 A1 WO 2002033757A1 US 0132403 W US0132403 W US 0132403W WO 0233757 A1 WO0233757 A1 WO 0233757A1
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group
photodetector
layer
emitter
group iii
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PCT/US2001/032403
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English (en)
Inventor
A. G. Unil Perera
Steven George Matsik
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Georgia State University Research Foundation, Inc.
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Priority to AU2002213326A priority Critical patent/AU2002213326A1/en
Priority to US10/492,372 priority patent/US7253432B2/en
Publication of WO2002033757A1 publication Critical patent/WO2002033757A1/fr

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/0304—Inorganic materials including, apart from doping materials or other impurities, only AIIIBV compounds
    • H01L31/03042—Inorganic materials including, apart from doping materials or other impurities, only AIIIBV compounds characterised by the doping material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/0304—Inorganic materials including, apart from doping materials or other impurities, only AIIIBV compounds
    • H01L31/03046—Inorganic materials including, apart from doping materials or other impurities, only AIIIBV compounds including ternary or quaternary compounds, e.g. GaAlAs, InGaAs, InGaAsP
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
    • H01L31/10—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors characterised by at least one potential-jump barrier or surface barrier, e.g. phototransistors
    • H01L31/101—Devices sensitive to infrared, visible or ultraviolet radiation

Definitions

  • the present invention generally relates to a photodetector. More particularly, the present invention relates to a far infrared photodetector utilizing a mechanism of detection based on free carrier absorption and internal photoemission over the bandgap offset of a heterojunction.
  • FIR detectors are of interest for various astronomy applications such as the Stratospheric Observatory For Infrared Astronomy (SOFIA) program and Explorer missions.
  • Stressed Ge[l] hereinafter “[n]” referring to the nth reference in the attached list of references at the end of the specification
  • blocked impurity band[2] detectors have been studied for almost 20 years as FIR detectors without being successful in making large arrays.
  • HIWIP homojunction interfacial workfunction internal photoemission infrared photodetectors
  • HIWIP detectors include successive highly doped emitter layers and undoped barrier layers. Detection takes place by free carrier absorption in the emitter layers followed by the internal photoemission of photoexcited carriers across the barrier and collection.
  • the cutoff wavelength is determined by the workfunction at the interface which is due to the bandgap narrowing caused by the doping in the emitter. By adjusting the device parameters, mainly the doping concentration in the emitter region, the cutoff wavelength may be tailored to the desired range.
  • HIWIP detectors have shown high responsivity and good detectivity in this range, but have their limitations. One is the high dark current associated with higher doping required for longer wavelength operation. Also the highest quantum efficiency reported was around 12.5 % for a 20 layer structure. [6].
  • the workfunction in HIWIPs is due to the bandgap narrowing effect in the highly doped emitter regions.
  • High density theory where only the dopant type (n or p) is considered but not the specific impurities, has been used to calculate the workfunction associated with doping concentration.[5] As the concentration is increased, the doping accuracy required to achieve a consistent workfunction for detection at wavelengths beyond 200 ⁇ m becomes more stringent.
  • the most common p-dopant, beryllium diffuses spontaneously at the concentrations required for response beyond 200 microns. The diffusion problems may be eliminated by using carbon as the dopant.
  • the present invention provides a photodetector.
  • the photodetctor has a plurality of N barriers, N being an integer greater than 1, each barrier being a layer of a material made from a first and a second group III elements and a first group V element and characterized by a bandgap.
  • the photodetector further has a plurality of N-l emitters, each emitter being a layer of material made from a third group III element and a second group V element and characterized by a bandgap different from that of the barriers and having at least one free carrier responsive to optical signals, wherein each emitter is located between two barriers so as to form a heterojunction at each interface between an emitter and a barrier.
  • each emitter is doped with a first group II, IV or VI element to cause free carriers in the emitter, wherein at least one construction parameter of each emitter causes at least one free carrier to occupy a range of substantially continuously distributed energies characterized by a three dimensional Fermi level and respond to optical signals having wavelength in the range of 3 to 100 ⁇ m with significant absorption.
  • each emitter is chosen from the group consisting of chemical identities of the third group III element, the first group II, IV or VI element and the second group V element, thickness of the emitter layer, doping concentration of the first group II, IV or VI element, the bandgap of the emitters, and the total number of the emitters.
  • the third group III element is Ga
  • the first group II element is Be
  • the second group V element is As
  • Other combinations of a group III element, a group II, JN or VI element and a group V element may be utilized to practice the present invention.
  • the thickness of each emitter layer is at least 140 A.
  • the thickness of each emitter layer is in the range of 140 to 200 A.
  • the doping concentration of Be is no more than 5 x 10 18 cm "3 .
  • the doping concentration of Be is in the range of 1 x 10 16 cm '3 to 5 x 10 18 cm '3 .
  • each emitter is characterized by a workfunction substantially in the range of 20 to 22 meV for each heterojunction, wherein the workfunction of each emitter is the difference between the three dimensional Fermi level of the emitter and the conduction band of a corresponding barrier. Additionally, the total number of the emitters is no smaller than 5. For example, the total number of the emitters can be chosen as an integer in the range of 5 to 50.
  • At least one construction parameter of each barrier causes a sharp interface between any pair of neighboring emitter and barrier and a dark current at least 100 times less than that of a comparable HIWIP at a given temperature.
  • the construction parameters of each barrier are chosen from the group consisting of chemical identities of the first and second group III elements and the first group V element, thickness of the barrier layer, relative concentration of the first and second group III elements, the bandgap of the barriers, and the total number of the barriers.
  • the first group III element is Al
  • the second group III element is Ga
  • the first group V element is As.
  • Other combinations of two group III elements and a group V element may be utilized to practice the present invention.
  • the concentration of the first group III element Al is characterized by a normalized amount x, x being in the range of 0 to 1
  • the concentration of the second group III element Ga is characterized by a normalized relative amount 1-x.
  • the thickness of each barrier layer is at least 600 A.
  • the thickness of each barrier layer can be chosen in the range of 600 to 1000 A.
  • the total number of the barriers is no smaller than 6.
  • the total number of the barriers is an integer in the range of 6 to 51.
  • the density of the dark current is somewhat inversely proportional to the total number of the barrier layers, N. In other words, the dark current is higher for a lower number of barriers but it is not a direct inverse proportionality.
  • the bandgap offset between the emitter and barrier materials is characterized by an activation energy in the range of 18 to 20 meV.
  • the bandgap offset is characterized by an activation energy substantially in the range of 18.5 to 19.5 meV.
  • the thickness of each barrier layer is greater than the thickness of each emitter layer.
  • the photodetector has a first contact layer in contact with one outmost barrier, wherein the first contact layer is made from a material substantially identical to the material from which emitters are made.
  • the first contact layer has a first surface and an opposite second surface, and an opening defined in the first surface to receive optical signals, the second surface being in contact with one outmost barrier.
  • the first contact layer has a high doping in order to get a good ohmic contact. This material will absorb IR so some of the first contact may be removed.
  • the photodetector also has a second contact layer opposite the first contact layer, wherein the second contact layer is made from a material substantially identical to the material from which emitters are made.
  • the photodetector further has a substrate in contact with the second contact layer, wherein the first contact layer is made from a material substantially identical to the material from which emitters are made. Moreover, conductive contacts located at the first and second contact layers, respectively, can be utilized for measuring the response of the photodetector to the optical signals.
  • the present invention provides a photodetector.
  • the photodetector has a plurality of N barriers, N being an integer greater than 1 , each barrier being a layer of a material made from a first and a second group III elements and a first group V element and characterized by a bandgap.
  • the photodetector also has a plurality of N-l emitters, each emitter being a layer of material made from a third group III element and a second group V element and characterized by a bandgap different from that of the barriers and having at least one free carrier responsive to optical signals, wherein each emitter is located between two barriers so as to form a heterojunction at each interface between an emitter and a barrier.
  • each emitter is doped with a first group II, IV or VI element to cause free carriers in the emitter.
  • At least one construction parameter of each barrier causes a sharp interface between any pair of neighboring emitter and barrier and a dark current at least 100 times less than that of a comparable HIWIP at a given temperature.
  • each barrier is chosen from the group consisting of chemical identities of the first and second group III elements and the first group V element, thickness of the barrier layer, relative concentration of the first and second group III elements, the bandgap of the barriers, and the total number of the barriers.
  • the first group III element is Al
  • the second group III element is Ga
  • the first group V element is As
  • the concentration of the first group III element Al is characterized by a normalized amount x, x being in the range of 0 to 1
  • the concentration of the second group III element Ga is characterized by a normalized relative amount 1-x.
  • each emitter causes at least one free carrier to occupy a range of substantially continuously distributed energies characterized by a three dimensional Fermi level and respond to optical signals having wavelength in the range of 3 to 100 ⁇ m with significant absorption.
  • the construction parameters of each emitter are chosen from the group consisting of chemical identities of the third group III element, the first group II, IV or VI element and the second group IV element, thickness of the emitter layer, doping concentration of the first group II, IV or VI element, the bandgap of the emitters, and the total number of the emitters.
  • the third group III element is Ga
  • the first group II element is Be
  • the second group V element is As.
  • the present invention provides a photodetector.
  • the photodetector has at least a first and a second barriers, each barrier being a layer of a first semiconductor material, and at least one emitter, the emitter being a layer of a second semiconductor material different from the first semiconductor material and having at least one free carrier responsive to optical signals.
  • the emitter is located between the first and second barriers so as to form a heterojunction at each interface between the emitter and the first and second barriers, and the at least one free carrier occupies a range of substantially continuously distributed energies characterized by a three dimensional Fermi level.
  • the first semiconductor material consists of a first and a second group III elements and a first group V element.
  • At least one construction parameter of each barrier causes a sharp interface at each interface between the emitter and the first and second barriers and a dark current at least 100 times less than that of a comparable HIWIP at a given temperature.
  • the construction parameters of each barrier are chosen from the group consisting of chemical identities of the first and second group III elements and the first group V element, thickness of the barrier layer, relative concentration of the first and second group III elements, and the bandgap of the barriers.
  • the first group III element is Al
  • the second group III element is Ga
  • the first group V element is As.
  • the concentration of the first group III element Al is characterized by a normalized amount x, x being in the range of 0 to 1
  • the concentration of the second group III element Ga is characterized by a normalized relative amount 1-x.
  • the thickness of each barrier layer is in the range of 600 to 1000 A.
  • the bandgap offset between the emitter and barrier materials is characterized by an activation energy substantially in the range of 18.9 to 19.1 meV.
  • the second semiconductor material consists of a third group III element and a second group V element.
  • the emitter is doped with a first group II, IV or VI element to cause free carriers in the emitter.
  • At least one construction parameter of the emitter causes significant absorption of optical signals having wavelength in the range of 3 to 100 ⁇ m.
  • the construction parameters of the emitter are chosen from the group consisting of chemical identities of the third group III element, the first group II, IV or VI element and the second group V element, thickness of the emitter layer, doping concentration of the first group II, IV or VI element, and the bandgap of the emitter.
  • the third group III element is Ga
  • the first group II element is Be
  • the second group V element is As.
  • each emitter layer is in the range of 140 to 200 A.
  • the doping concentration of Be is in the range of 1 x 10 16 cm “3 to 5 x 10 18 cm '3 .
  • the bandgap of the emitter is characterized by a workfunction substantially in the range of 20 to 22 meV for the heterojunction.
  • the thickness of each of the first and second barrier layers is greater than the thickness of the emitter layer.
  • the photodetector further has a first contact layer and a second contact layer, wherein the first contact layer and the second contact layer are made from a material substantially identical to the material from which the emitter is made, and the first and second barriers are located between the first contact layer and the second contact layer.
  • the first contact layer has a first surface and an opposite second surface, and an opening defined in the first surface to receive optical signals.
  • Conductive contacts are located at the first and second contact layers, respectively, for measuring the response of the photodetector to the optical signals.
  • the present invention provides a method of detecting far infrared optical signals.
  • the method includes the steps of forming a heterojunction at an interface between a layer of a first semiconductor material and a layer of a second semiconductor material different from the first semiconductor material, providing at least one free carrier responsive to the optical signals, wherein the at least one free carrier occupies a range of substantially continuously distributed energies characterized by a three dimensional Fermi level, and responding to optical signals having wavelength in the range of 3 to 100 ⁇ m with significant absorption characterized by a peak responsivity of approximately 60 A/W at a wavelength of approximately 6 ⁇ m.
  • the method further includes the step of doping the second semiconductor material with a first group II, IV or VI element to cause free carriers in the second semiconductor material.
  • the method also includes the step of measuring the absorption of the optical signals.
  • the present invention provides an apparatus for detecting far infrared optical signals.
  • the apparatus has means for forming a heterojunction at an interface between a layer of a first semiconductor material and a layer of a second semiconductor material different from the first semiconductor material, means for providing at least one free carrier responsive to the optical signals, wherein the at least one free carrier occupies a range of substantially continuously distributed energies characterized by a three dimensional Fermi level, and means for responding to optical signals having wavelength in the range of 3 to 100 ⁇ m with significant absorption characterized by a peak responsivity of approximately 60 A W at a wavelength of approximately 6 ⁇ m.
  • the apparatus further has means for doping the second semiconductor material with a first group II, IN or VI element to cause free carriers in the second semiconductor material. Additionally, the apparatus has means for measuring the absorption of the optical signals.
  • Fig. 1 is a cross-sectional view of a photodetector showing a mesa with a window etched in the top contact layer inside the metallic ring contact in one embodiment of the invention.
  • Fig. 2 illustrates a partial band diagram for the photodetector as shown in Fig.
  • Fig. 3 shows SIMS data showing the doping concentrations and the multilayers in a sample similar to the photodetector as shown in Fig. 1 in principle, the sample photodetector being identified as Sample #2283.
  • Fig. 4 illustrates dark current at various temperatures and data for related activation energy for the photodetector #2283 identified in Fig. 3.
  • Fig. 5 illustrates dark current measurements on 4 different mesas (two mesas of
  • Fig. 6 shows responsivity spectra for sample #2283 at biases from 15 to 500 mV obtained at 4.2 K and the responsivity at 32.5 ⁇ m for Sample #2283 and at 25 ⁇ m for Sample #2282.
  • Fig. 7 shows responsivity spectra for sample #2283 at biases 0.5, 2.0 and 3.5 kV/cm (0.1, 0.4 and 0.7 curves respectively) obtained at 4.2 K.
  • the arrow indicates the cutoff on the 3.5 kV/cm curve.
  • Fig. 8 illustrates the variation in cutoff frequency with Al fraction x.
  • a homojunction means a junction formed by two different electrical types of the same (band-gap) material.
  • a silicon p-n junction is a homojunction.
  • a heterojunction means a junction formed by two different electrical types of two chemically different materials, each having a band-gap different from that of the other.
  • An example of a heterojunction is a GaAs/Al(x)Ga(l-x)As junction, where x is a number satisfying 0 ⁇ x ⁇ 1.
  • the present invention provides a photodetector 100.
  • the photodetctor 100 has a plurality of barriers 106.
  • the total number of the plurality of barriers is N, where N is an integer greater than 1.
  • N is an integer greater than 1.
  • N 6
  • Each barrier 106 is a layer of a material made from a first and a second group III elements and a first group V element and characterized by a bandgap.
  • the first group III element is Al
  • the second group III element is Ga
  • the first group V element is As
  • the concentration of the first group III element Al is characterized by a normalized amount x, x being in the range of 0 to 1
  • the concentration of the second group III element Ga is characterized by a normalized relative amount 1-x.
  • the thickness of each bamer layer, identified as d b in Fig. 2 is at least 600 A.
  • the thickness of each barrier layer can be chosen in the range of 600 to 1000 A.
  • each sample being a photodetector in an embodiment of the present invention the thickness of each barrier layer is chosen as 800 A.
  • the total number of the barriers 106 is no smaller than 5.
  • the total number of the barriers 106 is an integer in the range of 6 to 51.
  • the total number N of the barriers 106 is chosen as 14 and 20, respectively.
  • the photodetector 100 further has a plurality of emitters 108.
  • the total number of emitters is N-l.
  • Each emitter 108 is a layer of material made from a third group III element and a second group V element and characterized by a bandgap different from that of the barriers and having at least one free carrier responsive to optical signals, wherein each emitter 108 is located between two barriers 106 so as to form a heterojunction at each interface 110 between an emitter 108 and a barrier 106.
  • each emitter 108 is doped with a first group II, Iv or VI element to cause free carriers in the emitter 108, wherein at least one construction parameter of each emitter 108 causes at least one free carrier to occupy a range of substantially continuously distributed energies characterized by a three dimensional Fermi level 112 as shown in Fig. 2 and respond to optical signals having wavelength in the range of 25 to 50 ⁇ m with significant absorption.
  • each emitter 108 is chosen from the group consisting of chemical identities of the third group III elements, the first group II, IV or VI element and the second group V element, thickness of the emitter layer 108, doping concentration of the first group II, IV or VI element, the bandgap of the emitters 108, and the total number of the emitters 108.
  • the third group III element is Ga
  • the first group II element is Be
  • the second group V element is As.
  • the thickness of each emitter layer is identified as d e in Fig. 2, is at least 140 A.
  • the thickness of each emitter layer 108 is in the range of 140 to 200 A.
  • the doping concentration of Be is no more than 5 x 10 18 cm '3 .
  • the doping concentration of Be is in the range of 1 x 10 16 cm '3 to 5 x 10 18 cm "3 .
  • the bandgap of each emitter 108 is characterized by a workfunction substantially in the range of 20 to 22 meV for each heterojunction.
  • At least one construction parameter of each barrier 106 causes a sharp interface 110 between any pair of neighboring emitter 108 and barrier 106 and a dark cu ⁇ ent at least 100 times less than that of a comparable HIWIP at a given temperature.
  • the construction parameters of each barrier 106 are chosen from the group consisting of chemical identities of the first and second group III elements and the first group V element, thickness of the barrier layer, relative concentration of the first and second group III elements, the bandgap of the barriers, and the total number of the barriers.
  • the first group III element is Al
  • the density of the dark cu ⁇ ent is inversely proportional to the total number of the barrier layers, N.
  • the bandgap of each barrier 106 is characterized by an activation energy in the range of 18 to 20 meV.
  • the bandgap of each barrier is characterized by an activation energy substantially in the range of 18.5 to 19.5 meV.
  • curve 418 is utilized to get an activation energy of 19 ⁇ 1 meV for Sample # 2283.
  • each barrier layer d b is greater than the thickness of each emitter layer d e .
  • the photodetector 100 has a first contact layer 102 in contact with one outmost barrier 106a, wherein the first contact layer 102 is made from a material substantially identical to the material from which emitters 108 are made.
  • the first contact layer 102 has a first surface 101 and an opposite second surface 103, and an opening 105 defined in the first surface 101 to receive optical signals 107, the second surface 103 being in contact with one outmost barrier 106a.
  • the opening 105 may be formed by partially etching the first surface 101 of the first contact layer 102.
  • the photodetector 100 also has a second contact layer 104 opposite the first contact layer 102, wherein the second contact layer 104 is made from a material substantially identical to the material from which emitters 108 are made.
  • the photodetector 100 further has a substrate 116 in contact with the second contact layer 104, wherein substrate 116 is made from a material substantially identical to the material from which emitters 108 made but being undoped.
  • conductive contacts 118, 120 located at the first and second contact layers 102, 104, respectively, can be utilized for measuring the response of the photodetector 100 to the optical signals 107.
  • the HEIWIP photodetector of the present invention uses thicker (larger than 140 A) emitter layers. In these thicker layers the carriers are not in states with specific energies but rather occupy a wide range of energies.
  • the abso ⁇ tion mechanism (free carrier abso ⁇ tion) in the HEIWIP photodetector of the present invention is different from the QWIP abso ⁇ tion and occurs for light with a wide wavelength range without decreasing at longer wavelengths.
  • the free carrier abso ⁇ tion strength increases as wavelength squared up to about 40 ⁇ m and then remains almost constant.
  • the HEIWIP photodetector of the present invention detectors at least has an advantage in terms of response strength and the wavelength range that can be covered in a single detector over a QWIP detector.
  • the material in the barrier layers between the emitter layers where abso ⁇ tion occurs and the level of doping in the emitter layers are two of the differences between the HEIWIP photodetector of the present invention and a prior art HIWIP detector.
  • the emitter and barrier layers are made of the same bandgap base material (homojunction) and the barrier is formed by the difference in allowed carrier energies in doped and undoped material. Because of the highly doped region next to the undoped region space charge develops at the interface, and along with movement of the doping material during the growth process reduces the interface sha ⁇ ness leading to increased dark cu ⁇ ent and detector noise.
  • the doping concentration must be very high ( ⁇ 10 19 cm “3 or higher), which increases dark cu ⁇ ent further, and any variations in the doping concentration will cause variations in the barrier and hence variations in the detector response.
  • quality of material goes down with increased doping giving rise to higher dark cu ⁇ ents due to defects.
  • the emitter and barrier layers are made of different bandgap base materials (heterojunction) with the barrier produced by differences in allowed carrier energies for the different materials.
  • the resulting structure has s a ⁇ interfaces between the layers of material reducing dark cu ⁇ ent (-200 times less than for a comparable HIWIP) and detector noise.
  • the doping concentration (10 16 -10 18 cm “3 ) of the HEIWIP photodetector of the present invention is at least an order of less than that of a prior art HIWIP detector for operating in the FIR range, which further reduces dark cu ⁇ ent.
  • the HEIWIP photodetector of the present invention provides the best properties of both types of prior art detectors: the improved response from the free carrier abso ⁇ tion mechanism of a prior art HIWIP detector with the lower dark cu ⁇ ent observed in a QWIP detector due to lower doping concentration and sha ⁇ er interfaces between layers.
  • the invention in one aspect provides a far infrared detector, or more specifically, a Heterojunction Internal Workfunction Internal Photoemission (HEIWIP) far infrared detector where the workfunction is primarily due to an AlGaAs layer next to a doped GaAs (emitter) layer.
  • the emitters 108 are doped to a sufficiently high level so that the carriers 122 form a 3- dimensional distribution in the emitters 108 and detection is by free carrier abso ⁇ tion.
  • the barriers 106 have a low Al fraction so that the workfunction (difference between the barrier conduction band 114 and the 3-D fermi level 112 in the emitters 108) will be small allowing operation at FIR wavelengths.
  • the cutoff wavelength can be tailored to any desired wavelength.
  • the doping in the emitters of the GaAs/AlGaAs structures in the cu ⁇ ent samples is kept low ( ⁇ 10 19 cm “3 ) to reduce the dark cu ⁇ ent to levels comparable or lower than those of QWIPs (quantum well infrared photodetectors), while the use of free carrier abso ⁇ tion in the emitter regions rather than the intersubband transition used in QWIPs will give the high responsivity observed in HIWIPs.
  • Increasing the doping in the emitter region should increase the abso ⁇ tion in the emitters but will also increase the dark cu ⁇ ent.
  • HEIWIPs can combine the best properties of the QWIP and HIWIP detectors leading to improved operation.
  • the device structure in one embodiment includes 158 A GaAs emitters and 800 A Al 002 Ga 098 As barriers.
  • the emitters were doped with Be to 3 x 10 18 cm “3 .
  • the top and bottom contacts were Be doped to 1 x 10 19 cm “3 with thicknesses 0.4 ⁇ m and 0.8 ⁇ m respectively.
  • Two samples were grown with 20 (sample #2283) and 14 (sample #2282) periods, respectively.
  • the devices were fabricated by etching 400 x 400 and 600 x 600 ⁇ m 2 mesas using standard wet etching techniques and then evaporating Ti/Pt/Au ohmic contracts onto the top and bottom layers.
  • a 260 x 260 ⁇ m or 460 x 460 ⁇ m window were opened respectively through the top contact to provide front illumination to the device.
  • Fig. 4 shows the dark cu ⁇ ents 410, 412, 414 and 416 for sample #2283 at various temperatures, respectively. Also shown is the 300 K background photocu ⁇ ent (as dashed line) coinciding with the 15 K dark cu ⁇ ent giving the BLIP temperature of 15 K for the detector, where BLIP (Background Limited Infrared Performance) is the operating condition where the photocu ⁇ ent produced by the background radiation exceeds the dark cu ⁇ ent. Under these conditions the detector performance is limited by the background rather than the detector dark cu ⁇ ent. Above 10 K the cu ⁇ ent is primarily thermionic and an A ⁇ henius plot 418 (inset in Fig.
  • the dark cu ⁇ ent at a given electric field is ⁇ 200 times less than for a HIWIP detector [7] with similar workfunction indicating the improvements in dark cu ⁇ ent provided by the present invention.
  • the improved dark cu ⁇ ent should lead to improved specific detectivity due to associated reduction in the detector noise.
  • Fig. 5 shows the dark cu ⁇ ent density 502, 504 for 4 mesas (of two different areas) from each sample at 4.2 K, respectively.
  • the dark cu ⁇ ent is almost identical for the small mesas (two higher density curves labeled a in Fig. 5) from each sample indicating good uniformity. Even considering the factor of 2.25 difference in area to the other two mesas (labeled b in Fig. 5) the cu ⁇ ent densities are still very similar.
  • the responsivity of the detectors was measured using a Perkin-Elmer System
  • Noise was measured using a low-noise preamplifier (SR 560) and a fast Fourier transform (FFT) spectrum analyzer (SR780) for sample 2283 at 4.2 K.
  • SR 560 low-noise preamplifier
  • FFT fast Fourier transform
  • SR780 fast Fourier transform spectrum analyzer
  • 1/f behavior was seen at low frequencies ( ⁇ 100 Hz) while at high frequencies the noise was relatively constant.
  • the noise was S j - 2 l0 "30 A 2 /Hz, which is much lower than the HIWIP values indicating an improvement in device quality.
  • the detectivity (denoted as D* ) was - 2 10 13 cm Hz 1/2 W which is a significant improvement over 5.9x 10 10 cm Hz 1 2 W obtained for a high performance HIWIP detector.
  • NEP for the detector was 1.4 ⁇ 10 "15 W/Hz 1/2 again about a factor of 100 better than for the HIWIP.
  • the photocu ⁇ ent efficiency for the detector was determined by dividing the photocu ⁇ ent by the number of incident photons, resulting in a peak efficiency of 23% for the HEIWIP detector, double the best HIWIP result reported to date of 12.5%[6] for a sample with the same number of layers and similar thicknesses.
  • Fig. 7 further shows responsivity spectra for sample #2283 at biases 0.5, 2.0 and 3.5 kV/cm (curves 710, 720 and 730 respectively) obtained at 4.2 K. The a ⁇ ow indicates the cutoff on the 3.5 kV/cm curve.
  • the increased D* compared to QWIPs can be understood by looking at the dark cu ⁇ ent and the abso ⁇ tion quantum efficiency.
  • the dark cu ⁇ ent can be predicted by a 3D carrier drift model[10] given by
  • ⁇ is the mobility
  • ⁇ is the electric field
  • v sat is the saturated drift velocity
  • m H is the carrier effective mass
  • ⁇ is the activation energy
  • ⁇ is the effective interface thickness.
  • the barrier lowering factor E ⁇ was chosen to co ⁇ espond to unifo ⁇ n field in the emitters and barriers.
  • the small difference in barrier height for different polarities can be due to possible variations in the Al fraction at the two ends.
  • the dark cu ⁇ ent is given by [11]
  • the experimentally observed dark cu ⁇ ent is ⁇ 2 orders of magnitude larger than for HEIWIPs, which could be due to the material quality associated with interchanging highly doped and intrinsic regions.
  • abso ⁇ tion quantum efficiency strongly favors the HEIWIP structure over the QWIP structure.
  • the free carrier abso ⁇ tion coefficient is ⁇ - N D ⁇ N where N D is the 3D doping density and N ⁇ 2-3 for wavelengths shorter than -30 ⁇ m and N - 0 for longer wavelengths.
  • N D is the 3D doping density
  • N ⁇ 2-3 for wavelengths shorter than -30 ⁇ m and N - 0 for longer wavelengths.
  • HEIWIPs are an exciting new approach to FIR detection.
  • Low Al fraction growth for the ba ⁇ iers and better determination of the feraii level in the emitter may lead to longer cutoff wavelength detectors with improved characteristics relative to the cu ⁇ ently available detectors.

Abstract

L"invention concerne un photodétecteur et un procédé de détection de signaux optiques dans l"infrarouge lointain. Dans une réalisation de l"invention, le photodétecteur (100) comprend N barrières (106), N étant un entier supérieur à 1, chaque barrière (106) étant constituée d"une couche d"un matériau fabriqué à partir d"un premier et d"un second éléments du groupe III et d"un premier élément du groupe V et caractérisé par une bande interdite. Le photodétecteur (100) comprend aussi N-1 émetteurs (108), chaque émetteur (108) étant constitué d"une couche de matériau fabriqué à partir d"un élément du groupe III et d"un second élément du groupe V et caractérisé par une bande interdite différente de celle des barrières (106) et possédant au moins un porteur libre (122) sensible à des signaux optiques, chaque émetteur (108) étant disposé entre deux barrières (106) de façon à former une hétérojonction (110) à chaque interface entre un émetteur (108) et une barrière (106). En outre, chaque émetteur (108) est dopé à l"aide d"un premier élément du groupe II, IV ou VI afin de favoriser la présence de porteurs libres (122) dans l"émetteur (108), au moins un paramètre de construction de chaque émetteur (108) amenant au moins un porteur libre (122) à occuper un domaine sensiblement continu d"énergies distribuées, caractérisé par un niveau de Fermi tridimensionnel (112) et à répondre à des signaux optiques (107) de longueur d"ondes comprise entre 3 et 10 νm, avec une absorption importante.
PCT/US2001/032403 2000-10-16 2001-10-16 Photodetecteur infrarouge lointain a heterojonction WO2002033757A1 (fr)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7838869B2 (en) 2005-10-21 2010-11-23 Georgia State University Research Foundation, Inc. Dual band photodetector

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5063419A (en) * 1988-11-15 1991-11-05 The United States Of America As Represented By The Secretary Of The Navy Heterostructure device useable as a far infrared photodetector
US5510627A (en) * 1994-06-29 1996-04-23 The United States Of America As Represented By The Secretary Of The Navy Infrared-to-visible converter

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5063419A (en) * 1988-11-15 1991-11-05 The United States Of America As Represented By The Secretary Of The Navy Heterostructure device useable as a far infrared photodetector
US5510627A (en) * 1994-06-29 1996-04-23 The United States Of America As Represented By The Secretary Of The Navy Infrared-to-visible converter

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7838869B2 (en) 2005-10-21 2010-11-23 Georgia State University Research Foundation, Inc. Dual band photodetector
US8093582B2 (en) 2005-10-21 2012-01-10 Georgia State University Research Foundation, Inc. Dual band photodetector

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