Classifications

• • 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
  • H01L31/02—Details
  • H01L31/0232—Optical elements or arrangements associated with the device
  • H01L31/02327—Optical elements or arrangements associated with the device the optical elements being integrated or being directly associated to the device, e.g. back reflectors
• • 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
  • H01L31/02—Details
  • H01L31/0232—Optical elements or arrangements associated with the device

Definitions

• the invention relates to a photodetection device and in particular an infrared photodetector.
• Such a device comprises an absorbent semiconductor layer having a determined thickness.
• Infrared quantum photodetectors are already known. These must be cooled well below room temperature, to minimize or even eliminate in the semiconductor, the carrier generation process, or dark current, which competes with the free carrier photo-generation. , or useful signal.
• the decrease of the detector temperature is therefore the current means in the state of the art for maximizing the signal-to-noise ratio.
• the thickness of the semiconductor layer is several micrometers, which ensures maximum absorption of infrared light.
• a decrease in the semiconductor volume present in the photodetector also reduces the dark current. Decreasing the thickness of the semiconductor layer has other advantages: reducing the cost of materials and increasing the speed of detection.
• This structure can in particular take the form of a structured metallic mirror which is placed on the rear face of a semi-circular layer. absorbent conductor, so as to obtain a strengthening of the absorption in this semiconductor layer.
• Such a mirror may especially be used in quantum well detectors, in the spectral range of the medium and far infrared.
• This article describes a detector comprising a stack of layers forming quantum wells. In normal incidence to the detector, the latter is not absorbent.
• the mirror makes it possible to reflect the incident wave with an angle so that it is absorbed by at least one quantum well.
• Such a mirror has also been proposed for improving the efficiency of thin-film solar cells of amorphous Si in the visible and near-infrared regions.
• a structured metal mirror can diffractably couple the incident radiation to the semiconductor absorber layer.
• the metal mirror can be structured in the form of a two-dimensional network, in particular an array of pads in various configurations, for example square studs in square grating or round studs in square or hexagonal gratings.
• the height of the pads or holes of the mirror used is of the order of a quarter of the wavelength of use, to optimize the absorption of quantum wells.
• the structures With a material encasing the structure of the network whose index is 2.5, the structures will have a depth of about 400 nm for a wavelength of 4 pm. This leads to mirrors with a thickness of about 500 nm, the thickness being counted between the bottom of the mirror and the top of the structures.
• This large thickness can pose a number of technological difficulties of realization.
• the quantum efficiency of these photodetectors depends considerably on the angle of light incident on the photodetector.
• the absorption spectrum of the semiconductor layer present in the photodetector reveals a resonance peak of the absorption which is a function of the angle of the incident light. It is found that with known mirrors, the position and the shape of the absorption peak vary significantly with the angle of incidence of the wave on the detector.
• the article by Andersson et al., J. Appl. Phys. 171 (1992) 3600 tends to show a very wide angular response with insensitivity up to at least 15 °.
• hyperspectral detection that is to say the detection of several wavelengths within the same detection range (for example 4 different "colors" in the same range 8 -12 Mm) without the colors mixing when the radiation arrives at an angle of incidence.
• the object of the invention is to overcome the disadvantages of photodetectors of the state of the art by providing a photodetector comprising a semiconductor thin film and a structured metal mirror and which has a reduced sensitivity to the incidence angle of the infrared light radiation used, while maintaining the quantum efficiency of absorption of this light radiation, measured for example as the ratio between the light power absorbed in the semiconductor and the incident light power.
• the invention relates to a photodetector for a light radiation of a given wavelength ( ⁇ ) comprising a stack of layers with:
• a spacing layer made of a material transparent to said wavelength
• a structured metal mirror the distance (g) between the apex of said mirror and said spacer layer being less than ⁇ and said mirror having an array of holes defining an array of metal reliefs, with a pitch P of between 0.5 ⁇ / nSC and 1, 5 ⁇ / nSC where nSC is the real part of the refractive index of the semiconductor material, a width L of relief between 9P / 10 and P / 2 and a depth h of hole between ⁇ / 100 and ⁇ / 15.
• Such a photodetector has a lower angular sensitivity and the spectral width of the resonance is increased.
• this photodetector makes it possible to envisage the easier realization of matrices of pixels that are sensitive to different wavelengths (multispectral detection). This results in particular:
• the pitch P is preferably between 3P / 4 and 9P / 10.
• the network of said mirror has a beam of hollow lines.
• this network has two crossed beams of parallel hollow lines. They can also be parallel or not to the edges of the photodetector.
• the semiconductor material is selected from Si, Ge, SiGe, InAs, InSb, GaSb, PbS, PbSe, PbTe or Hg Cd x Te -x (x ⁇ 0.9), ternary alloys such as InGaAs , AlInAs, AlInSb, InAsSb or InGaSb, quaternary alloys such as InGaAsP or InGaAsSb and quinary alloys such as GalnAsSb or GalnAsSbP, or a type II superlattice, for example InAs / InSb on GaSb.
• ternary alloys such as InGaAs , AlInAs, AlInSb, InAsSb or InGaSb
• quaternary alloys such as InGaAsP or InGaAsSb
• quinary alloys such as GalnAsSb or GalnAsSbP
• the material of the spacer layer is selected from air, ZnS, CdTe, S1O2 or III-V materials.
• the spacer layer has a thickness of at least 10 nm, in order to fulfill a diffusion barrier function.
• the distance g is less than ⁇ / 50.
• the photodetector may comprise on said layer of semiconductor material, a substrate transparent to said wavelength.
• This substrate may for example be the growth substrate of the absorbing semiconductor layer, for example a CdZnTe substrate in the case where it is desired to grow a layer of CdHgTe.
• the substrate may also contribute to the mechanical cohesion of the final photodetector.
• this transparent substrate is made of a material whose index is greater than that of air.
• FIG. 1 is a sectional view of an exemplary photodetector according to the invention
• FIG. 2 is a curve showing, in an example of a one-dimensional photodetector according to the invention, the quantum efficiency of the photodetector as a function of the wavelength for an incident light perpendicular to the photodetector,
• FIG. 3 shows a network of curves similar to those of FIG. 2 corresponding to different angles of incidence
• FIG. 4 shows four curves representing the quantum efficiency of a photodetector according to the invention, as a function of the angle of incidence, for different values of the parameter g.
• FIG. 5 illustrates another embodiment of the photodetector according to the invention.
• FIG. 6 presents a network of curves showing, as a function of the wavelength and for several angles of incidence of the incident light, the quantum efficiency of another example of a one-dimensional photodetector comprising a transparent substrate,
• FIG. 7 shows a network of curves similar to those of FIG. 6 and for a photodetector without a transparent substrate
• FIGS. 8a and 8b are each a curve showing, as a function of wavelength, the quantum efficiency of two photodetectors according to the invention comprising a mirror structured in two dimensions and for an incident light perpendicular to the photodetector, and
• FIG. 9 comprises five curves representing the quantum efficiency of a photodetector according to the angle of incidence, for different values of the width L of the reliefs and of the depth h of the hole.
• FIG. 1 illustrates an example of a photodetector 1 according to the invention.
• This photodetector comprises a stack of layers, illustrated in section in FIG. 1, with a substrate 10 which is transparent at the wavelength ⁇ of the light radiation used.
• the arrows F illustrate the propagation of the incident light radiation at the front face 100 of the photodetector.
• the light radiation propagates inside the substrate 10, undergoing a refraction phenomenon at the passage of the air-substrate interface, inducing a modification of its angle of incidence according to the Snell-Descartes law. Since the substrate has an index greater than that of air, the angle of incidence in the substrate will be lower than the angle of incidence in the air. This substrate will act as a growth substrate of the semiconductor, or else an element ensuring the mechanical cohesion of the photodetector.
• This transparent substrate can be made in CdZnTe.
• a layer 11 of a semiconductor material In contact with the transparent substrate 10, there is provided a layer 11 of a semiconductor material.
• This transparent substrate may be omitted in other embodiments of the photodetector according to the invention.
• the fact that the semiconductor layer is bare decreases the life of the photodetector device, or else induces a strong recombination of photogenerated carriers on the surface of this layer and therefore a reduction in the collection efficiency of the photodetector. photogenerated charges.
• an electrical passivation layer may then be provided on the layer 11.
• This layer should be thin enough not to affect the optical characteristics of the device (typically it will be chosen small in front of the wavelength). It may, for example, be a layer of CdTe coated with a layer of ZnS, both having a thickness of about 100 nm.
• the semiconductor material and its thickness are chosen so that it is partially absorbent.
• this layer of semiconductor material is chosen such that the semiconductor layer 11 is not totally absorbent. It can advantageously respond to the following relation (1):
• kSC is the imaginary part of the refractive index of the semiconductor material.
• This material may be chosen from the following materials: Si, Ge, SiGe, InAs, InSb, GaSb, PbS, PbSe, PbTe or Cd x Hg -x Te (with x ⁇ 0.9), ternary alloys such as InGaAs, AlInAs , AlInSb, InAsSb or InGaSb, quaternary alloys such as InGaAsP or InGaAsSb and quinary alloys such as GalnAsSb or GalnAsSbP, or a type II superlattice, for example InAs / InSb on GaSb.
• the stack further comprises a metal mirror 13 and, between the semiconductor layer 11 and the mirror 13, a spacer layer 12.
• the mirror comprises, on its surface 130, in contact with the spacer layer 12, a series of reliefs 131.
• the top of the mirror 13 corresponds to the top of the relief 131 The highest.
• the distance g between the apex of the mirror 13 and the semiconductor layer 11 is chosen to be smaller than the wavelength ⁇ of the light radiation used.
• the structuring of the mirror 13 induces a guided mode parallel to the semiconductor layer 11 with an evanescent electric field which is maximum at the apex of the mirror and which decreases towards the substrate 10.
• the absorbing layer 11 is placed sufficiently close to the mirror to be in this region of reinforcement of the electromagnetic field. This corresponds to a distance g from the surface of the mirror smaller than ⁇ .
• This spacer layer 12 is made of a material transparent to the wavelength of the light radiation used.
• This material may be air or else: ZnS, CdTe, III-V materials according to the group to which the absorbing semiconductor belongs.
• the distance g is less than ⁇ / ne, where ⁇ is the wavelength of the light radiation used and where is the optical index of the material of the spacer layer 12.
• this distance g can be substantially zero.
• this structuring can be done in one or two dimension (s).
• the reliefs 131 extend, on the surface 130, along substantially parallel lines.
• the mirror array is defined by a bundle of hollow or raised lines.
• the resonance of the absorption will be generated only in TM polarization, that is to say for a magnetic field of the incident light radiation parallel to the lines of the network.
• Structuring can also be provided in two dimensions, so as to make the photodetector independent of the polarization.
• the structuring then consists of a network of reliefs in the form of pads, for example a square network of round or square studs, or a hexagonal network of round studs.
• this network then has two crossed beams of parallel lines, hollow.
• these lines may or may not be parallel to the edges of the photodetector.
• the materials that can be used to make the mirror 13 are noble metals such as gold, aluminum and copper as well as all metals to the extent that their conductivity is not more than twenty times lower than that of gold.
• the structures may include a thin layer of metal for securing the previous metal on the spacer layer 12, for example Ti titanium.
• another coupling in play is the coupling of the incident light with the pair of coupled and guided modes parallel to the plane of the semiconductor layer, obtained through the network formed on the mirror.
• the strength of the latter coupling will be determined by the shape of the network (period or not P, fill factor L / P and depth h of the structures). It is preferable for the invention that this coupling network is strong, in order to obtain a high quantum detection efficiency.
• the period of the network sets the torque (wavelength, angle of incidence) of the resonance peak.
• nSC is the real part of the refractive index of the absorbing semiconductor material.
• the pitch P will be chosen included in the range 0.5A / nSC and
• the width L of the reliefs will be chosen between 9P / 10 and P / 2.
• the depth of the structuring of the mirror that is to say the height h of the reliefs 131 or the depth of the holes, will be between ⁇ / 15 and ⁇ / 100 and it will be chosen typically substantially equal to ⁇ / 50. In general, this height h will be adjusted according to the thickness e of the layer 11 of the semiconductor and the optical index of the spacer layer 12.
• MCT telluride cadmium mercury
• the wavelength ⁇ of the incident light radiation is between 3 and 5 ⁇ .
• the imaginary part of the refractive index of the semiconductor material is of the order of 0.2 for a wavelength of 4 ⁇ m.
• the relation (1) leads to a thickness of the semiconductor layer of less than about 2 ⁇ .
• the thickness of the semiconductor layer will be chosen equal to 400 nm.
• the layer of spacing material is made of ZnS.
• the metal mirror is made of gold and the distance g between the metal mirror and the spacer layer is equal to 50 nm. It is therefore between ⁇ / 100 and ⁇ / 60, for the range of wavelength retained.
• the thickness between the surface 130, corresponding to the bottom of the structures, and the bottom of the mirror, corresponding to the surface 132 opposite the surface 130 is at least equal to the thickness of the skin at the wavelength considered.
• the latter will be of the order of 25 nm for a gold mirror at a wavelength of 4 ⁇ m.
• the pitch of the grating is chosen equal to 1450 nm and the width L of the metallic lines is chosen equal to 800 nm.
• P is well located in the range 0.5A / nSC and 1, ⁇ / nSC, for the range of wavelength retained.
• L is chosen substantially equal to 0.55 P.
• the depth of the structuring of the mirror that is to say the height h, is 125 nm. It is therefore between ⁇ / 40 and ⁇ / 24 for the range of wavelength considered.
• FIG. 2 illustrates, for this photodetector example, the quantum efficiency of the photodetector (that is to say the ratio between the power absorbed and the incident power or the ratio between the number of electron-hole pairs actually generated by absorption in the semiconductor layer and the number of incident photons) as a function of the wavelength (expressed in nm), for an incident light perpendicular to the polarization photodetector TM and for an operating temperature of 77 K.
• the quantum efficiency of the photodetector that is to say the ratio between the power absorbed and the incident power or the ratio between the number of electron-hole pairs actually generated by absorption in the semiconductor layer and the number of incident photons
• This absorption layer shows a resonance peak, a function of the angle of incidence and the resonance wavelength, which is here about 4.44 ⁇ .
• This curve shows that the optical absorption in the semiconductor layer is about 70% of the incident power for the resonant wavelength.
• the value of the pitch P makes it possible to adjust the resonance wavelength.
• a variation of P between 1.015 ⁇ and 1.6 ⁇ makes it possible to vary the resonance wavelength between 3.6 and 4.8 ⁇ m.
• the resonance wavelength is adjusted thanks to the geometrical parameters of the structure, such as the nature and the thicknesses of the materials put in contact with the semiconductor layer, the metal constituting the structured mirror, the thickness of the semiconductor layer or the value of g.
• variations of these parameters are of the second order on the adjustment of the resonance wavelength, with respect to the choice of P.
• FIG. 9 illustrates the influence of the lateral width L of the reliefs and the depth of hole h on the angular sensitivity of the photodetector.
• the different curves R 1 to R 5 give the quantum efficiency of the photodetector as a function of the angle of incidence on the semiconductor layer.
• the curve f3 ⁇ 4 corresponds to the photodetector referenced for FIGS. 2 and 3.
• the ratio L / P is equal to 0.55 and h is equal to 125 nm. These two values are in the selected ranges.
• the other curves correspond to a photodetector having the same characteristics, except that concerning the width L and possibly the height h.
• the curve Ri corresponds to a photodetector for which L is equal to 1000 nm and h is equal to 125 nm.
• the L / P ratio is 0.69. It is well located in the chosen range, as h.
• the curve R3 corresponds to a photodetector for which L is equal to 600 nm and h is equal to 170 nm.
• the ratio L / P is 0.41 and is not in the range chosen, unlike h.
• the curve R 4 corresponds to a photodetector for which L is equal to 350 nm and h is equal to 600 nm.
• the L / P ratio is 0.24. This report, like h, is not in the ranges selected.
• the curve R 5 corresponds to a photodetector for which L is equal to 700 nm and h is equal to 700 nm.
• the L / P ratio is 0.48. This report and h are also not in the ranges selected.
• FIG. 3 illustrates a network of curves similar to that of FIG. 2 and corresponding to angles of incidence varying between 0 and 4 °. These curves were obtained with a photodetector identical to the example described above and corresponding to FIG.
• the curve Ci in solid line corresponds to the curve shown in Figure 2.
• the light radiation is perpendicular to the surface of the photodetector.
• Curve C2 also corresponds to a curve showing the quantum efficiency of the photodetector as a function of the wavelength, but for a light incident on the layer 11 forming an angle of 1 ° with respect to the normal.
• the curve C3 corresponds to a variation of the angle of incidence of 2 °
• the curve C 4 to a variation of 3 °
• the curve C 5 to a variation of 4 °.
• This beam of curves shows that the quantum efficiency of the photodetector does not vary as a function of the angle of incidence, if this variation is ⁇ 1, 5 ° in the transparent substrate 10, which corresponds to a variation of ⁇ 4 ° in the air.
• the response of the detector is divided substantially by two for a variation of the angle of incidence greater than ⁇ 4 ° in the substrate 10, ie ⁇ 10 ° in the air.
• the angular sensitivities of a photodetector comprising a transparent CdZnTe substrate 10 (FIG. 6) and a photodetector without a transparent substrate (FIG. 7) will now be compared.
• the photodetectors are one-dimensional and comprise a MCT semiconductor layer whose thickness is 400 nm.
• the metal mirror is made of gold, the height of the reliefs is 125 nm for the photodetector comprising a CdZnTe substrate and 225 nm for the one which does not have one.
• the thickness between the bottom of the structures and the bottom of the mirror will for example be 100 nm. Here it is greater than the skin thickness of the metal at the wavelength considered, which is 25 nm for gold.
• the distance g between the metal mirror and the spacer layer is 50 nm.
• the pitch P of the grating is chosen equal to 1450 nm and the width L of the reliefs or lines is chosen equal to 1000 nm.
• the solid line curve C'i illustrates the quantum efficiency of the CdZnTe substrate photodetector as a function of the wavelength, for a light radiation perpendicular to the surface of the photodetector and for an operating temperature of 77K.
• Curve C 2 is a similar curve for an angle of incidence of 1 °.
• the curves C'3 and C ' 4 respectively correspond to a variation of the angle of incidence of 2 ° and 3 ° relative to the perpendicular to the surface of the photodetector.
• the solid line curve K 1 illustrates the quantum efficiency of the substrateless photodetector, as a function of wavelength, for a light radiation perpendicular to the surface of the photodetector and for the same operating temperature.
• the curves K 2 to K 5 are similar curves respectively corresponding to a variation of the angle of incidence of 5 °, 10 °, 15 ° and 20 °, relative to the perpendicular to the surface of the photodetector.
• Ki to K5 shows that the photodetector without substrate is less sensitive to the angle of incidence of the light radiation that the photodetector has a CdZnTe substrate.
• the response of the photodetector varies substantially to the same extent for a variation of the angle of incidence between 0 ° and 15 ° for the photodetector without substrate and between 0 ° and 2 ° for the photodetector with substrate (which corresponds to a variation between 0 ° and 5 ° in the air present above the substrate).
• the photodetector with transparent substrate has the advantage of being more mechanically robust.
• FIG. 4 shows several curves making it possible to compare the quantum efficiency of the photodetector, as a function of the angle of incidence on the semiconductor layer, for different values of the parameter g.
• the photodetector considered has the same characteristics as that taken with reference to FIGS. 2 and 3, with regard to the values of e, P and L / P.
• h retained here is between 125 and 150 nm.
• the parameter g is modified here in each of the photodetectors considered.
• the curve Gi corresponds to a photodetector for which g is equal to 10 nm.
• g is equal to 50 nm
• g is equal to 160 nm
• g is equal to 300 nm.
• the curve G2 corresponds to that obtained with the photodetector described above and which are associated with Figures 2 and 3. It corresponds to the curve F3 ⁇ 4 of Figure 9.
• the detector response will remain insensitive to a variation of the angle of incidence of only ⁇ about 0.3 °.
• the performance of the photodetector i.e., its angular sensitivity, can be considered as being determined by the width of the absorption-angle curve at the half-height of the curve.
• this performance can be considered good when the width of the curve corresponds to an angular amplitude of about 5 °.
• FIG. 8a is a curve showing the quantum efficiency of the photodetector as a function of the wavelength for a photodetector according to the invention having a two-dimensional structured gold mirror.
• the material is ZnS and the CdZnTe substrate, and the mirror has square shaped pads with a width L of 1200 nm and a pitch P of 1450 nm.
• Figure 8b is the corresponding curve for a photodetector identical to that used for Figure 8a, except for the width L of the pads which is here 800 nm.
• FIGS. 8a and 8b show that the photodetector having a width L substantially also at 4P / 5 (FIG. 8a) makes it possible to obtain a single resonance peak, unlike the photodetector having a width L substantially equal to P / 2 ( Figure 8b).

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