FR2536911A1 - PHOTOSENSOR - Google Patents

Abstract

THE INVENTION RELATES TO PHOTODETECTOR TECHNOLOGY. A PHOTODETECTOR10 INCLUDES AN ELEMENT40 WHICH UNDERGOES A DETECTABLE INTERNAL PHYSICAL OR CHEMICAL CHANGE WHEN EXPOSED TO ELECTROMAGNETIC RADIATION, AS WELL AS MEANS30, 50 TO DETECT THIS CHANGE. THE PHOTODETECTOR ALSO INCLUDES A NEARLY PERIODIC SURFACE THAT INCREASES THE EFFICIENCY WITH WHICH THE INCIDENT ELECTROMAGNETIC RADIATION IS COUPLED TO THE INTERIOR OF THE PHOTODETECTOR. APPLICATION TO HIGH SENSITIVITY RAPID PHOTODETECTORS.

Description

The present invention relates to a photodetector comprising an element

  which experiences a detectable change when exposed to electromagnetic radiation, and which includes means for detecting this change when the element is exposed to electromagnetic radiation.

  A photodetector (as far as the invention is concerned) is a

  positive that undergoes a detectable internal change in its properties

  physical or chemical, for example, an internal change in its

  mechanical, electrical, electronic or optical

  exposed to electromagnetic radiation, and is a device that

  takes means, for example electrodes, to detect this change.

  For example, a photovoltaic device is a photode-

  the fact that such a device undergoes an internal change in its electrical properties, that is to say that a current is induced in the

  device when exposed to electromagnetic radiation.

  that, and the device typically comprises electrodes for sensing the current Two criteria that are often used to evaluate the usefulness of a photodetector for a particular purpose are its sensitivity and its speed of operation The first criterion relates to the value of the induced change under the effect of incident electromagnetic radiation, and the

  second is the delay: between a change in the intensity of

  electromagnetic incident and the corresponding detected variation

in the induced internal change.

  In many applications, it is desired to have a sensitivity

  high speed and high speed, which imposes contradictory constraints

  to a photodetector configuration In general, to obtain a high sensitivity, a relatively large amount of

  Electromagnetic radiation absorbing material is required for

  to induce a relatively large induced internal change, i.e. a large device structure is desirable On the contrary to generally achieve high speeds, one must employ a small device structure A small device decreases the internal capacity and / or the internal propagation distances of the change, and

therefore increases the speed.

I ::

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  We will have an example of the contradictory constraints that are

  photodetectors which must have both sensitivities and

  high capacities and high operating speeds, considering

  the contradictory physical constraints that are imposed on photo-

  metal-insulator-metal (MIM) type detectors to have both

  high operating speeds and high sensitivities

  MIM sensor typically includes a layer of material

  electrically insulating material, for example a metal oxide or a semi-

  insulation, interposed between two electrodes, for example two flat and smooth metal layers When using a metal oxide, it is called

  the photodetector a MOM device, such as Al-A 1 203-Ag.

  When using a semi-insulator with a transparent electrode (10% at

  less incident radiation is transmitted), photodetection is called

  MSM device, as for example-Al-Si amorphous AI A voltage difference (typically i to 2 volts) is applied between

  the electrodes during operation of the photodetector.

  Electromagnetic radiation falling on an electrode of a MOM device is reflected in a large proportion and is transmitted in the device, in a much lower proportion, by the excitation of electrons in one or both of the electrodes. These excited electrons then pass through the oxide layer, by tunnel effect,

  to the other electrode The time it takes for these electrons to work

  the oxide layer (with a characteristic thickness of 500 nm) is of the order of 10-14 seconds and these MOM devices are therefore able, in principle, to operate at very high operating speeds,

  characteristically limited by the capacity of the device Cepen-

  quantum yield (the number of electrons produced in

  positive MOM per incident radiation photon) of these devices is -4 extremely low, typically of the order of 10, due to

  only a relatively small fraction of incident radiation penetrates

  to be really in the device These MOM devices are therefore not

  interesting for many practical applications.

  When an electromagnetic radiation falls on the transparent electrode of an MSM device, it is believed that a fraction of the transmitted radiation is absorbed by the semi-insulating material, in which 3 3

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  electron-hole pairs are created Electrons and holes drift through the semi-insulating material, towards one or the other of the electrodes, under the influence of a voltage difference (typically

  1 to 2 volts) which is applied between the electrodes during operation.

  the device, to define a detectable current.

  MOM devices, MSM devices have operating speeds

  Because, for example, if the semi-insulating layer is only about 10 nm thick, charge carriers need only a time interval of the order of 10 seconds. for

  i O drift from one side of the semi-insulating layer to the other side,

  positive MSMs also have low quantum yields (typically about 10 -3), also because only one

  relatively small fraction of the incident radiation is coupled to the

  positive, that is to say that it is absorbed by the semi-insulating layer

  to produce electron-hole pairs.

  quantum measurements of MSM devices by increasing the thickness of the semi-insulating layer and thereby increasing the absorption of transmitted photons However, the increased thickness of the semi-insulating layer

  increases the propagation distance and thus reduces the speed of

tioning.

  Therefore, researchers working on the development

  photodetectors in general, and MIM photodetectors in particular.

  have long sought, without success so far, to

  to improve the sensitivities of photodetectors, these techniques -

  not requiring increases in photodetector sizes.

  According to the invention, these problems are solved in a photodetector characterized in that the radiation-sensitive element comprises a corrugated surface which is substantially periodic.

  The invention will be better understood on reading the description

  which will follow embodiments, and with reference to the accompanying drawings in which: Figures 1 to 3 are sections of photodetectors within the scope of the invention

  The invention relates to photodetectors, as well as to

  Electromagnetic radiation detection dice These methods include

-

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  the operation of detecting a physical or chemical change

  induced in a photodetector exposed to electromagnetic radiation.

  than. The photodetectors which fall within the scope of the invention have surfaces which are substantially periodic, ie surfaces whose heights vary substantially periodically (for example,

  relative to a reference surface) when moving along the surface

  faces of these photodetectors. These practically periodic surfaces are

  for example, external surfaces that receive radiation directly

  during the operation of the photodetectors.

  According to one variant, these surfaces are internal to a photodetector and are subjected to electromagnetic radiation which is transmitted through

  one or more layers of & material located above.

  In practice, the substantially periodic surfaces are formed by structuring, for example by etching, a surface which is initially relatively smooth, to form a bumpy surface. The resulting bumpy surface then constitutes the substantially periodic surface. According to one variant, a layer of material is deposited. on the bumpy surface, this material conforms to the shape of the bumpy surface, and the surface

  upper or lower material deposited is the practical surface

  Periodically, in both cases, an imaginary surface,

  which spatially coincides with the smooth, non-structured

  This reference surface is flat or curved, but for a structured curved surface to be a bumpy surface, the radius (or radii) of curvature of the reference surface is the reference surface referred to above. curve must be greater than the radius of curvature of the bumps, and preferably greater than

  ten times the radius of curvature of the bumps.

  In the context of the invention, a bumpy surface is "practically

  Periodically "if two criteria are satisfied The first criterion is that at least 75% of the" bumps "on a bumpy surface fulfill the condition that the spacing between the peaks (or bottoms of the hollows) of two" bumps " any adjacent-on the bumpy surface does not differ by more than about 0.2A from the mean spacing, A It should be noted that these "bumps" may have any

  shape, for example spherical, cylindrical or pyramidal.

  The second criterion that a substantially periodic surface must satisfy is the reflected light figure produced by such a surface. If a parallel light having a wavelength greater than the average spacing between the bumps is directed on a substantially periodic surface, , and a spot size greater than 10A, the reflected light will basically form a diffraction pattern. The angular position Q (relative to the incident light) of the center of

  each bright spot in the diffraction pattern that would be produced

  by a perfectly periodic surface (having no variation in the distance between bumps) is given by the relation (A) (sine) n in which A is the wavelength of the incident light and N is an integer ( 0.1, -2,) which indicates the diffraction order However, the angular position of a bright spot (in the diffraction pattern) produced by a substantially periodic surface may differ slightly * from the angular position of the corresponding bright spot which is produced by a perfectly periodic surface. This angular difference is, however, less than or equal to 0.2 G.

  It has been found that photodetectors having practically

  Periodically, as defined above, have sensitivities that are significantly higher (typically 10 times greater) than the sensitivities of similar types of photodetectors (having similar dimensions) that do not have such surfaces (provided that the thicknesses of comparable photodetectors

  are less than about 100 nm).

  A practically periodic surface of a photodetector (-entrant

  in the context of the invention) effectively couple the electromagnetic radiation

  magnetic incident towards the interior of the photodetector, in the form

  of an electromagnetic wave that propagates through the photodetec-

  adjacent to the substantially periodic surface of the photo-

  detector This surface coupling effect is absent, or occurs in an inefficient manner, when the surface of the photodetector is smooth or has only inequalities randomly distributed.

  the fact that incident electromagnetic radiation is more effective

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  coupled by the surface towards the inside of the photodetectors falling within the scope of the invention, the sensitivities of these photodetectors

  (compared to photodetectors that do not have practical surfaces

  Periodically) are increased without increasing the dimensions, for example thicknesses, of these photodetectors. All wavelengths of electromagnetic radiation falling on the substantially periodic surface of a photodetector

  (within the scope of the invention) are not coupled by the surface

  this towards the interior of the photodetector One can determine according to fundamental principles, for example the conservation of the momentum (see for example the work of H Raether, Excitation of Plasmon and Interband Transitions by Electrons, (Springer-Verlag , 1980), page 128), that the wavelength λ of the electromagnetic radiation which is coupled by the surface towards the interior of a photodetector is given by the relation: 27 sing 2 T 2 qr (neff)

__ _ _ + _ = _ _ A _ (1)

  0 here designates the angle made by the incident radiation with respect to

  a normal to the reference surface (defined above) of the photodetect

  (If the reference surface is curved, the orientation of the

  the, and therefore the angle C, will differ from point to point, as well as the wavelengths coupled in the surface) The parameter neff in equation (1) designates the effective refractive index of the photodetector If the photodetector consists of

  several different materials (for a single material, N eff is simple-

  The refractive index of this material is determined by, for example, exposing a test sample of the photodetector (having a known bump spacing, A) to known electromagnetic radiation (including many wavelengths). ) falling at a known angle,

  O The wavelength of the electromagnetic radiation is determined

  pleated by the surface towards the inside of the test sample in detec-

  both the wavelengths of the electromagnetic radiation that is

  transmitted by the test sample Knowing the wavelength

  plotted by the surface, A, the average distance between bumps, A, and the angle of incidence, 0, we can easily determine the value of neff from

  of equation (1) For most or most materials

  res composites, N ff is greater than 1 but less than about 5.

  It follows from equation (1) above that: (A) (neff) 1 (2) eff sinef f neff Both equations (1) and (2) clearly show that photodetectors

  within the scope of the invention are specific to both a.

  wavelength and angle, that is to say that the sensitivity of a

  todetector (characterized by a distance between bumps and an indi-

  This actual refraction N ff) will be obtained only for a particular wavelength, of electromagnetic radiation oriented at a particular angle of incidence. Moreover, it follows from equation (2) that for that a photodetector (within the scope of the invention)

  having an effective refractive index N eff detects a radiation 1 ee-

  tromagnetic wavelength A, having an angle of incidence e, the distance between bumps A must be chosen proportionally to A

  (the proportionality factor is (neff -s'in O)).

  As indicated above, the spacings between the bumps on the surfaces of the photodetectors within the scope of the invention may deviate from the average spacing, A, by up to about Therefore, the electromagnetic radiation coupled by the inward surface of a photodetector within the scope of the invention will not necessarily be limited to a length.

  waveform A (defined by equation (2)). On the contrary, there will be

  a band of wavelengths coupled by the surface towards the inside

  of the photodetector, the maximum width of this band (around a

  a), A, being given by: A Yx = - (neff sin O) a; A (3) eff at A denotes here the maximum deviation from the mean spacing, '/, and this maximum deviation is less than or equal to about 0.2 A. If a photodetector (within the scope of the invention) must detect two or more bands of overlapping wavelengths

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  not (or one or more wavelengths in each of the non-overlapping bands), or must detect a band of wavelengths that is wider than that which is detectable with a single set of bumps

  the photodetector is then manufactured, in accordance with

  according to the invention so as to have two or more sets of substantially periodic bumps. The average inter-bump spacing A for each set of bumps is chosen to be proportional to the wavelength, or the average wavelength, to detect In addition, the maximum deviation, AA, for each set of bumps must be large enough

  to ensure the coupling of the desired wavelength band A?).

  The amplitude of a bump on a practically periodic surface is defined as being half of the distance entered the planes that are

  respectively tangent to the top of the hump and to the bottom of an adjacent hollow.

  According to the invention, the average amplitude of the bumps on the portion of a substantially periodic surface that receives electromagnetic radiation ranges from about 0.1 A to about 10 μA. Average amplitudes less than about 0. , 1 A are undesirable because of an unfavorably low amount of electromagnetic radiation

  will be coupled by the surface towards the inside of the photodetector Amplifiers

  Higher average studies of about 10 A are not desirable because of

  manufacturing difficulties they entail.

  A convenient technique for making photodetectors having

  substantially periodic surfaces, in accordance with the invention,

  The material layer or layers of the photodetector are deposited on a substrate having a substantially periodic surface. At the time of deposition, the layer or layers of material conform (provided they are relatively

  thin) to the shape of the surface of the underlying substrate.

  A locally flat surface is made substantially periodic

  of a substrate using, for example, a holographic technique

  in this holographic technique, there is deposited on the surface of the substrate a masking layer-in which a pattern, for example a photographic resist material, can be defined. An interfering pattern is then formed in the photoresist material, in a

  first direction, for example by exposing the photoresist material

  light from two laser beams that meet each other.

  The first direction mentioned here is the direction of the line defined by the intersection of the plane containing the two laser beams and the surface of the substrate. If the light of each of the laser beams has a wavelength 1 'and each of the Laser beams make an angle O relative to a normal to the surface of the substrate, the gap between

  the resulting maximum intensity in the photoresist

  phique will be l / 2 sin O repeating this exposure procedure in a

  second direction, perpendicular to the first direction, and in

  then developing the photographic reserve material, a two-dimensional periodic figure is drawn in this material.

  subjacent substrate, using as an attack mask the material

  photographic reserve-in which one has formed a figure, one

  forms on the surface of the substrate a bidirectional network of bumps, which

  periodically (the average spacing between the bumps will be -

  ? 1/2 sin O) This same technique is naturally applicable to the surface

  this of the photodetector itself, to directly transform the surface of the photodetector into a substantially periodic surface

  Although the invention relates to detectors having surface

  these practically periodic, it also relates to a method of detecting electromagnetic radiation In accordance with the method of the invention, electromagnetic radiation is detected by exposing an electromagnetic radiation to a photodetector having a substantially periodic surface, and then measuring the resulting change product in the photodetector This procedure has the advantage of leading to the measurement of a relatively large signal, because of the sensitivity

  increased photodetector that results from the coupling effect by the sur-

  face that is produced by the substantially periodic surface.

  Although the invention applies to different types of photode-

  It also applies to fast photodetectors

  such as MIM photodetectors, and is of particular interest

  Particularly for the latter The MOM and MSM photodetectors within the scope of the invention will be described below as a teaching aid to allow a more complete understanding of the invention. The term "MOM photodetector" is used herein to denote a photodetector MIM whose photoelectric current is produced by electron transfer by

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tunnel effect.

  In general, each of the MOM and MSM photodetectors

  within the scope of the invention comprises an insulating layer between

  wedged between two electrodes, and a practically periodic surface

  that, in order to improve the efficiency with which the radiation

  electromagnetic is coupled by the surface towards the inside of the device.

  In the case of MOM photodetectors, the practically periodic surface

  that is typically the outer surface of an electrode that

  directly receives electromagnetic radiation during

  In the case of MSM photodetectors, the surface

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  substantially periodic is typically an internal surface of the device, i.e., the surface of the semi-insulating layer which is directly below an optically transparent electrode

  on which the electromagnetic radiation falls.

  During the operation of the MOM and MSM photodetectors falling within the scope of the invention, an electric field is applied across these devices to propel towards one or other of the

  electrodes the charge carriers created by electromagnetic radiation

  absorbed (to produce a detectable electric current under the effect of electromagnetic radiation) This field is created for example

  by applying an external voltage difference to the electro-

  According to one variant, in the case of MSM devices, the two elec-

  trodes are respectively formed from p-type and n-type semiconductor materials, to form p-i-n junctions which have internal electric fields, which eliminates the need for biasing by an external voltage. Yet another variant which is

  in the case of MSM photodetectors is to form a

  during the manufacture of the semi-insulating layer, this junction

  p-n also having an internal electric field.

  The practically periodic surfaces of the MOM photodetectors

  and MSM within the scope of the invention can be easily manufactured.

  However, and for convenience only, the practically periodic surfaces of the photodetectors contemplated below, and shown in FIGS. 1-3, are provided in various ways, such as for example by directly structuring these surfaces (relatively smooth at the origin); are described

  as having been made by depositing the layers of the constituent material

  by killing these photodetectors on a substantially periodic surface of a substrate Referring to FIG. 1, it can be seen that a MOM photodetector 10, within the scope of the invention, comprises a relatively thin insulating layer interposed between first and second second metal strips 30 and 50 (the metal strips 30 and 50 constitute the electrodes of the photodetector) The photodetector 10 is made, for example by depositing, for example by sputtering, the first metal strip 30 on the substantially periodic surface Of a substrate 20, at 1. 2 forming the insulating layer 40 on the surface of the metal strip 30, folds by depositing, for example by evaporation, the metal strip-50 on the

  Insulating layer 40 To reduce the area of the device, and therefore to reduce

  the capacity of the device, the metal strip 50 is deposited in a characteristic manner at an angle, for example at a right angle, with respect to

to the metal ribbon 30.

  The deposition (or formation) of the metal strips 30 and 50 and the insulating layer 40 on the substantially periodic surface 25 of the substrate 20 causes the top surface 60 of the metal strip 50 to assume a substantially periodic shape.

  the surface of the surface 60 effectively couples the electromagnetic radiation

  incidental geometrics (the wavelength and the angle of incidence of

  are coupled by equation (1) to the metal strip 50,

  in the form of energy that we consider to be what we call a "polari-

  surface plasmon tone "(i.e., an electromagnetic wave propagating in an electrically conductive material adjacent to the surface of the material) The surface plasmon polariton

  attenuated by a single-particle excitation mechanism, that is,

  say that electrons present in the metal ribbon 50 are excited to a higher energy state, and these electrons then go through the

  insulating layer 40, by tunnel effect, towards the metal strip 30.

  It has been found that the substantially periodic surface 60 of the metal strip 50 also functions to couple the electromagnetic radiation to the metal strip 50 (to enhance tunnel electron transfer) by a second mechanism, as important as the surface plasmon polariton It is believed that the second mechanism is a localized plasma resonance which is produced in each of the "bumps" on the surface 60 by incident electromagnetic radiation of appropriate frequency. This localized plasma resonance constitutes a collective oscillatory motion of electrons in each hump, in a direction transverse to the direction of propagation of the electromagnetic radiation, and this transverse movement is limited by the borders of each hump (if there were no such boundaries, there would be no no localized plasma resonances) A localized plasma resonance is produced in each bump when the frequency, f,

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  incident electromagnetic radiation is equal to V / d, denoting p by V the volume plasma frequency of the metal constituting the layer 50

  (see H Raether's book, above, page 50, for

  identifies the values-plasma-volume frequencies of many metals), and j is a geometrical factor that is defined by the shape of the bumps and

  by the average spacing, A, between the bumps.

  In practice, an iterative procedure is used to determine both a suitable metal for the metal strip 50 and an appropriate average value of the bump gap, A, which, taken together,

  effectively couple the specified electromagnetic radiation of long

  wavelength A and frequency f to local plasma resonances.

  According to this iterative procedure, we take the value A / 2 as the first approximation of the appropriate spacing between bumps, A, and we take j = 2 as the first corresponding approximation for the geometric factor. metal layer so that half the plasma frequency by volume for the metal, that is to say

  say V / j (= 2), equal to the frequency, f, of the electromagnetic radiation

  which must be coupled to the localized plasma resonances.

  However, since j = 2 is only an approximation of the value of j, the error inherent in this approximation is compensated for by choosing for the average spacing between bumps, -X, a new value which is appropriate. at the choice of the metal (and the shape of the bumps) One determines the distance between bumps, new and appropriate, by exposing for example to the radiation

  electromagnetic frequency f photodetect test samples

  10, having average spacings between bumps, which differ slightly

  the original approximation, that is, 2/2 (the maximum deviation from the original approximation, 5/2, should typically not be greater than about 50% The metal strip 50 of each of these test samples is made from the metal selected above. By detecting the frequency of the electromagnetic radiation that is transmitted by the test samples, the test sample is determined. which absorbs the frequency electromagnetic radiation, and

  so the proper spacing between bumps.

  It should be noted that plasma resonances localized in MOM photodetectors with smooth surfaces do not appear. Although localized plasma resonances do occur in

  film in the form of tlots consisting of electrically isolated particles

  triply (formed by depositing thin layers of a metal such as Ag,

  Au), the resonances are rapidly reduced and are unable to produce

  re a significant tunneling effect when the particles are brought into contact with each other (to make dandruff

electrically continuous).

  The localized plasma resonances that are produced in the

  bumps on the substantially periodic surface 60 are relatively insensitive

  at the angle at which incident radiation falls on the surface

  , and depend to a large extent only on the frequency of

  In addition, it has been discovered that the tunneling current that is produced by localized plasma resonances increases with increasing amplitude of bumps. In fact, the tunneling current that is produced by the resonances of localized plasma is much greater (by a factor of about 2) than the tunneling current that is produced by the surface plasmon polariton, once the amplitude of the bumps is greater than or equal to about 0.5 A Therefore, once

  the amplitude of the bumps is greater than or equal to approximately 0.5 J, the photo-

* MOM 10 detector is virtually insensitive to the angle of incidence of

electromagnetic radiation.

  The materials usable in the MOM photodetector 10 of the invention (see FIG. 1) vary depending on whether the photodetector 10 is to be used as a sensitive photodetector or insensitive to the angle of incidence of the electromagnetic radiation. However, in both

  In this case, the substrate material 20 of the MOM photodetector 10 must be preferably

  relatively electrically insulating (with a resistance

  at least 10 úL cm) in order to confine the electric currents

  within the MOM photodetector 10 A substrate material used

  readable for example consists of fused silica.

  Almost any metal is usable for the first time

  metal layer 30 of the MOM photodetector 10 (see FIG.

  in a configuration, and in order to more easily define the thickness of the insulating layer 40, the metal is preferably chosen

  so that it is easily oxidized, for example oxidized by plasma or

  The metals which are easily oxidized by plasma to form metal oxides include, for example, Al, Mg and Ni. The metal layer 30 must have a thickness

  at least 10 nm for this layer to be physical (and therefore electrically

  continuously) Although there is no theoretical upper limit

  on the thickness of the metal layer 30, practical considerations

  such as the need to minimize manufacturing difficulties,

  usually limit the metal layer 30 to a thickness not exceeding

  not a few hundred nanometers.

  The metal layer 50 may also consist of almost any metal, for example Au, Ag, Al The thickness of the

  metal layer 50 is between about 10 nm and about 30 nm.

  Thicknesses less than about 10 nm are undesirable for the reason

  its indicated above Thicknesses greater than about 30 nm are

  undesirable effects of the fact that the value of tunneling currents is

vorably weak.

  Layer 40 is a relatively thin layer of a material

  having a high dielectric strength (typically

-4

  than 10 V / cm) Among the materials which have a di-

  Electrical levels as high as those used for layer 40 include metal oxides such as A 1203, Mg O, and Ni Oi. The thickness of layer 40 is between about 1 nm and about 10 nm. A thickness of less than about 1 nm. is undesirable because "a layer as

  thin is likely to undergo electrical breakdown (even if the

  re is not physically discontinuous, it will become so under the effect of the application of a voltage difference) On the other hand, thicknesses greater than about 10 nm are undesirable because of tunneling currents unfavorably weak will cross the layer 40 Because of the ease with which one can define the thickness, one forms of

  preferably the layer 40 by oxidation of the metal layer 30, for example

  The insulating layer 40 is thus preferentially

  an oxide of the metal layer 30, hence the name photodetector

metal-oxide-metal -

  If the photodetector 10 is to be used as an angle of incidence detector, besides the fact that the surface 60 of the

  metai layer 50 must be practically periodic (for the purpose of

  the incident electromagnetic radiation to produce a surface plasmon polariton in the metal layer 50), the coefficient

  absorption of the metal of the metal layer 50 for the polariton of

  surface area should preferably be less than 1 / AJ Such a coefficient

  absorption will allow the surface plasmon polariton to

  to pager in the metal over a distance exceeding the average spacing between bumps Among the metals that have such an absorption coefficient are Ag, Au and Ai For other metals of this type, see for example

  an article by P B Johnson and R W Christy published in Physical Review, -

Vol B 6, 1962, page 4370.

  For the MOM photodetector to be sensitive to the angle of incidence

  dence, the amplitude of the bumps should be between approximately 0.1 A and

  about 0.3 J Amplitudes less than about 0.1 A are undesirable.

  because the resulting tunnel effect current is unfavorably low Amplitudes greater than about 0.3 L are

  fact that the influence of localized plasma resonances becomes

  high and that the detector becomes unfavorably insensitive

  at angle of incidence of electromagnetic radiation -

  If the MOM photodetector 10 is to be insensitive to the angle of incidence, besides the fact that the metal layer 50 and the mean spacing between the bumps are chosen so as to produce localized plasma resonances, the amplitude of the bumps on the surface 60 should be between about 0.5, A and about 10A. Amplitudes less than about 0.5A are undesirable because the influence of surface plasmon polaritons becomes unfavorably high and the detector becomes unfavorably sensitive to the angle of incidence of the radiation

  electromagnetic amplitudes greater than about 10 A are

  because surfaces with bumps of such magnitude

  are unfavorably difficult to manufacture.

  If, for example, the MOM photo detector 10 is to detect visible light with a wavelength of about 0.5 μm (and a frequency equal to about 6 × 10 Hz), and if the detector be

  insensitive to the angle, a first approximation that can be used for spreading

  The average amount between bumps, JA, is X / 2 = 0.25 μm, and a usable metal for la-layer 50 is, for example, Ag, whose volume plasma frequency is about 10 Hz (when this frequency is divided by 2, 14 -

  it gives a value close to 6 x 10 Hz) Using samples

  In addition, if the amplitude of the bumps was greater than The detector 10 is relatively insensitive to the angle of incidence. With reference to FIG. 2, there is shown an embodiment of the MSM photodetector 70 within the scope of the invention, which comprises

  a layer of semi-insulating material 100 (a material which has a resistivity

  at least 1 k Lcm) interposed between two electrodes, that is to say two ribbons of electrically conductive material 90 and 110 (having a resistivity of not more than 0.1 cm) MSM photodetector 70 is manufactured for example by successively depositing the layers 90, 100 and

  on the substantially periodic surface 85 of an electrical substrate

  insulation 80 (preferably having a resistivity greater than

  In order to reduce the capacity of the device, the tape 110 is deposited at an angle, for example a right angle, with respect to

ribbon 90.

  The main difference between the MSM 70 photodetector and the MOM 10 photodetector is that the electric currents

  produced in the detector 70 under the effect of electromagnetic radiation

  incident, are due generally to the absorption of electromagnetic radiation by the semi-insulating layer 100, and to the resulting formation of electron-hole pairs in this layer.

  formation of large numbers of electron-hole pairs, the conductive layer

  The electricity generator 110 is manufactured to be substantially transparent to incident electromagnetic radiation (the layer 110 transmits at least 10% of the electromagnetic radiation that falls on it). However, if the upper layer 110 is, for example, metal, the electric currents produced in the MSM photodetector

  may be due, at least in part, to excited electrons from

  110 of the metal layer 110, which pass through the semi-insulating layer 100 These excited electrons are formed, for example, by the attenuation

25369 1 1

  of a surface plasmon polariton in the metal layer 110, and this surface plasmon polariton is produced by the surface coupling of the electromagnetic radiation to the metal layer 110. These excited electrons may also result from localized plasma resonances in the bumps on the surface of the metal layer 110. The deposition of the layers 90, 100 and 110 on the surface substantially

  85 of the substrate 80 causes the surface 115 of the semiconductor layer

  insulating 100 takes a corresponding form, substantially periodic Equations (1) and (2) define the average spacing, AJ, between the bumps on the surface 115, as a function of the wavelength, A, of the electromagnetic radiation which must coupled to the semi-insulating layer 100 (in the form of an electromagnetic wave propagating adjacent the surface 115). As previously, the average amplitude of the

  bumps is from about 0.1 to about 10.

  The semi-insulating layer 100 is made from a material

  semi-insulating material of any such materials.

  all these semi-insulating materials are materials that have a resistivity of at least 1 k L cm, in order to eliminate, or at least

  to minimize currents of darkness (electrical currents

  in the photodetector 70 in the absence of electromagnetic radiation

  Among the semi-insulating materials that can be used is amorphous silicon (undoped) which has or has not been hydrogenated (with an atomic percentage of H up to 30%). The amorphous silicon is deposited (on the conducting layer Electricity 90) using, for example, conventional sputtering or chemical vapor phase deposition techniques. Monocrystalline silicon and polycrystalline silicon (undoped) are other useful semi-insulating materials Monocrystalline silicon is deposited. for example using techniques of

  chemical vapor deposition 8 or molecular beam epitaxy, tan

  Polycrystalline silicon is deposited using conventional sputtering or chemical vapor deposition techniques, for example. There are still other materials which can be used as appropriately doped III-V compounds (to produce resistivity). at least 1 k L cm), such as gallium arsenide, indium phosphide, indium gallium arsenide and indium gallium arsenide phosphide, and

  suitably doped II-VI compounds, such as sea telluride-

  According to the particular material, these materials are deposited by well known techniques of molecular beam epitaxy,

  chemical vapor deposition and in-liquid phase epitaxy.

  The thickness of the semi-insulating layer 100 is between about 10 nm and about 1 μm. Thicknesses less than about nm are undesirable because it is likely that layers of deposited material having such low thicknesses are physically discontinuous. Thicknesses greater than about 1 μm, although not prohibited, cause the semi-insulating layer 100 to be so thick (and thus absorb so much of the electromagnetic radiation) that

  the substantially periodic surface 115 provides only a small advantage.

  While the electrically conductive deposited layers, and 110, both perform the function of electrodes for detecting the photocurrent in the photodetector 70, the layer 110 must also be substantially transparent to the electromagnetic radiation (although the layer 90 is also useful if it is also transparent) If the layer 110 consists of a metal, for example Au, Ag or A 1, the thickness of this layer is between about 10 nm and about 30 nm. Thicknesses less than about 10 nm. are undesirable because such a deposited layer of metal, for example by sputtering, risks being physically (and therefore electrically) discontinuous. On the other hand, thicknesses greater than about 30 nm are undesirable because a layer of metal having such a large thickness will not be

  significantly transparent to electromagnetic radiation (it transmits

  will put less than about 10% of the incident electromagnetic radiation).

  Indium-tin oxide is another material that is used

  to make layer 110 and which is both conductive of

  electricity and practically transparent to electromagnetic radiation.

  indium-tin oxide is deposited on the semi-insulating layer 100, for example by cathodic sputtering. The thickness of the

  The indium-tin layer is from about 30 nm to about 2 μm.

  Thicknesses of less than about 30 nm give layers of indium tin oxide which have poorly conducting conductances. Thicknesses greater than about 2 μm, although not prohibited,

  require an unfavorably long deposition time.

  Other materials still usable for the manufacture of the layer 110 consist of appropriately doped semiconducting materials whose forbidden bands are larger than the forbidden band of the semi-insulating layer 100 Si, for example the semi-insulating layer 100 is a layer of gallium arsenide doped, for example, 18 -3 with chromium to a level of about 10 cm 3, the layer 110 is, for example, doped aluminum gallium arsenide, for example with 19 -3

  silicon to a level of about 10 cm 3 The semicon-

  layer 110 conductors are deposited on the semi-insulating layer using, for example, conventional low pressure chemical vapor deposition techniques. The thicknesses of these semiconductor materials are from about 30 nm to about 2 μm.

  thicknesses out of this range are undesirable for the indi-

  above (although thicknesses greater than about 2

not forbidden).

  Layer 90 does not need to be significantly transparent to electromagnetic radiation (although this is not

  forbidden), and layer 90 therefore consists of any of the

  considered above (in relation to layer 110) having the

  however, the upper limits of the thickness ranges that are imposed by the transmission requirement

do not apply.

  In a second embodiment of the MSM photodetector 70, which is generally similar to the first embodiment, described above, the semi-insulating material of the layer 100 is doped to produce a pn junction characterized by a field internal electrical circuit having a value equal to at least 10 V / cm. For example, such a pn junction is formed by depositing half the thickness of the OO layer and doping this half so as to produce a conductivity material.

  of type n, and then depositing the remaining half of the thickness of the.

  layer 100 and doping this remaining half to produce p-type conductivity material The existence of electric polarization

  internal link associated with the p-n junction removes the need for

  quer-a polarization by a voltage external to the electrically conductive layers 90 and 110 during operation of the photodetector 70. In a third embodiment of the photodetector MSM 70, which also eliminates the need for the application of a polarization by an external voltage, the electrode 110 (see FIG. 2) comprises a layer of semiconducting material, for example of p-type conductivity (in contact with the layer 100), whereas the electrode 90 comprises a layer of semiconducting material, for example, n-type de-conductivity (in contact with the layer 100) Thus, the two electrodes 90 and 110, and the semi-insulating layer 100, define a pin junction Although various semiconductor materials can be used for the electrodes

  and 110, the resulting p-i-n junction must have an electric field

  that extends over the thickness of the semi-insulating layer 100 and which has

  a value of at least 10 V / cm Lower electric field values

  at about 10 V / cm are undesirable because the time taken by the charge carriers to cross the layer thickness

  semi-insulating becomes unfavorably long.

  Of course, the photocurrent that is produced in the photode-

  70 is detected on the electrodes 90 and 110, and each electrode

  therefore includes a layer of metal that forms a low-resistance contact

  this (the resistance must be less than about 100 ohms) with the

  For example, if the electrode 110 comprises an indium phosphide layer doped with zinc (p-type conductivity), a zinc-gold alloy layer, for example, will form a low-resistance contact with the Indium phosphide If the electrode 90 comprises, for example, a layer of indium phosphide doped with silicon (n-type conductivity), a layer of gold-tin-nickel alloy, for example, will form a contact with low resistance with indium phosphide For the electrode 110 to be substantially transparent to electromagnetic radiation, the thickness of the metal layer that is used to form the low-resistance contact must be less than about 30 nm, while the thickness of the semiconductor layer must

be less than about 1 pm.

  Looking at Figure 3, we see a fourth way of realizing

  of the MSM 70 photodetector, which is generally similar to

-2536911

  first embodiment, and which comprises a layer of semi-

  insulator 100 (this material having a resistivity of at least 1k cnm) which is deposited directly on the substantially periodic surface 85 of an electrically insulating substrate 80 (the resistance of the substrate material is greater than that of the semiconductor material); This fourth embodiment of the photodetector also comprises two layers

  electrically conductive conductors 120 and 130, spaced apart and

  on the substantially periodic surface 115 of the semisolant layer, these layers 120 and 130 constituting the electrodes of the photodetector 70. In operation, when electromagnetic radiation falls on the substantially periodic surface 115, this surface couples the incident radiation to the semi-layer -solvent 100 in the form of an electromagnetic wave that propagates adjacent to the surface

  115 The passage of this wave leads to the formation of electron pairs

  hole near the surface 115, the electrons and the holes moving-to

  one or the other of the electrodes 120 or 130, under the influence of the

applied potential.

  The semi-insulating layer 100 is deposited on the substrate 80 in

  using, for example, conventional techniques for the chemical deposition

  steam, and it consists of any of the semi-

  insulation considered in relation to the semi-insulating layer of the first

  The photodetector embodiment 70 The thickness of the semiconductor layer

  100 is as large as desired, but this thickness must be at least 10 nm to ensure the physical continuity of the layer 100. In the fourth embodiment of the photodetector 70

  (see Figure 3), described above, the coupling of electromagnetic radiation

  magnetic-to the semi-insulating layer 100 is preferably confined in the space between the electrodes 120 and 130 Thus, the electrodes

  are preferably made from electrically conductive materials.

-3 ç -1

  tricity (having a conductivity of at least 10 (- cm)) which are practically opaque to electromagnetic radiation (the electrodes do not transmit more than about 10% of the incident radiation) If for example the electrodes 120 and 130 are made of a metal such as Au or Ag,

253691 1

  the thickness of the electrodes must be at least 20 nm so that they are practically opaque. Such metal electrodes have for example formed by cathodic sputtering on the surface 115 of the semi-insulating layer 100, using a layer in which a pattern has been defined, for example a layer of

  reserve in which a pattern has been defined, to prevent metallisation

  tion of the part of the surface 115 which must absorb the electromagnetic radiation. In yet another embodiment of the MSM photodetector

  70 (see FIG. 3), the electrodes 120 and 130 and the semiconductor layer

  insulating 100 form a junction p-i-n With the exception of the different positions of the electrodes, this junction-p-i-n is identical to that

  described above If the electrodes 120 and 130 are to be practically

  opaque to incident electromagnetic radiation, a thickness of at least 20 nm is given to the metal layers used to form

low resistance contacts.

Example 1

  A MOM Al-A 1203-Ag photodetector having a structure similar to that of the MOM photodetector shown in FIG. 1 was manufactured by first cleaning with conventional solvents a

  square plate of fused silica (2.5 cm square and-mm thick

  The fused silica plate was used as the substrate on which the MOM photodetector was made.

  The relative top surface was lithographically

  of the fused silica plate (to form a practically

  Periodically) by first evaporating a film of germanium about 80 nm thick on the plate. The germanium film was deposited immediately after the cleaning procedure to prevent the deposit of dust on the surface of the plate. The aim of the germanium film was to reduce light reflections on the surface of the

  fused silica plate during photolithographic treatment.

  It was then deposited on the germanium film by centrifugation.

  This photographic resist material is sold under the trade name AZ 1450 B by Shipley Company, 25369111 Newton, Massachusetts, USA. of a photographic resist with two intersecting laser light beams, each having a wavelength of 363.8 nm and an intensity of about 10 p W / cm, with an exposure time of about 30 seconds. oriented both

  beams of light so that they meet at an angle of

  in a plane perpendicular to the surface of the reservoir material.

  The fused silica plate was then rotated by 90 (in its own plane) and the photoresist material was again exposed to the two light beams for about a second. The water was then diluted with water ( five parts of water for

  part of a developer) a commercially available developer.

  this, sold under the trade name AZ 351 by the firm Shipley

  Company, and applied to the photographic reserve material.

  The germanium film was then subjected to reactive ionic etching in a C Br Y 3 plasma, using the photoresist in which a pattern was defined as an attack mask. we circulated

  in the reactive ionic attack chamber of C Br F 3 'with a flow rate of

  born at normal conditions of 10 cm 3 / min, while the pressure in the chamber was maintained at about 1.3 Pa and that the power density

  In these conditions, the film attack was about 0.05 W / cm 2

  Germanium particle over its entire thickness required about 10 minutes.

  The pattern traced in the photoresist material and in the germanium film was then transferred to the fused silica by reactive ion etching of the fused silica in a plasma of CHF 3 for about 25 minutes (using as a mask of attack

  the matter of photographic reserve and the film of germanium in the

  which one defined a motive) The conditions of attack were the

  same as those specified above, except that the den-

  The power density was approximately WO 5 W / cm. Photographic resist material and germanium films were then stripped with

  conventional solvents. It was known from micrographs

  Scanning tronic on previous samples that have undergone

  the same treatment, that the surface of the fused silica

25369 11

  was now a two-dimensional, almost periodic, bump system with a mean bump spacing of about 250 nm and an average bump amplitude (half the distance from the crest to

hollow) of about 125 nm.

  Conventional evaporation techniques were then used to evaporate an aluminum ribbon on the substantially periodic surface of the fused silica plate. This tape had a width of about 1 mm while its thickness was about 44 nm. then formed a layer of Al O on the surface of the aluminum ribbon, by plasma oxidation of the aluminum ribbon in an oxygen atmosphere, at a pressure of about 4 Pa and for about 10 minutes. of the layer of

  A 1203, deduced from measurements of capacity made later on the photo-

  MOM detector, was about 5 nm Then a silver ribbon was evaporated on the oxide layer, perpendicular to the aluminum layer,

  approximately 1 mm in width and approximately 18 nm in thickness. The photodetect

  MOM, defined by the intersection of the silver ribbon with the aluminum ribbon

  As a result, the surface of the silver electrode was practically periodic (it had conformed to the shape of the surface of the plate).

fused silica).

  Two additional MOM photodetectors have also been manufactured.

  Using the same general procedure as described above, one of these MOM photodetectors was fabricated on a substantially periodic surface (of a fused silica plate) whose bumps had an average amplitude of only 25 nm (FIG. obtained the 25 nm bump amplitude by reactive ionic etching of the plasma fused silica of CHF 3, for about 5 minutes only) The other MOM photodetector was fabricated as described above, but on a surface

smooth fused silica.

  It was then directed onto the photodetector MOM (on the silver electrode) whose practically periodic surface had bumps

  of an average amplitude equal to -25 nm, laser light (in polarization)

  tion p in the plane perpendicular to the reference surface of the photo-

  detector, that is to say the unstructured original surface of the fused silica plate), with a wavelength of 476.9 nm.

2536911 '

  turn the photodetector during its exposure to laser light,

  light sweeps a plane oriented perpendicular to the surface

  reference face of the photodetector During this procedure, we have

  the reflected light from the photodetector MOM with a wattmeter, and the intensity of the reflected light is recorded as a function of the angle of incidence of the light (the angle to a normal to the reference surface of the light). photodetector, this normal being in the

  light-swept plane). There was a sharp decrease in the intensity of

  reflected light (and thus an abrupt increase in the amount of light coupled to the interior of the photodetector) for an angle of incidence of 54, which is consistent with the formation of a surface plasmon polariton in the silver electrode of the photodetector,

  that is, in accordance with equation (1).

  The other two MOM photodetectors were also exposed to

  laser light, rotating them as described above.

  that the intensities of reflected light varied slowly

  function of the angle of incidence (there was no abrupt decrease

  at 540), from which it was concluded that there was little or no light

  plotted towards surface plasmon polaritons.

  The optical transmission properties of the MOM photodetector, whose substantially periodic surface had bumps of average amplitude equal to 125 nm (the average spacing between bumps on the surface of the silver was about 25 nm), were measured at using a

  commercial spectrometer (model No. 14) purchased from Cary Instru-

  Corporation, Monrovia, California This spectrometer included a tunable light source, from which light was

  directed under a normal incidence towards the photodetector (on the elec-

  silver detector), and a detector which measured the intensity of the light transmitted through the photodetector as a function of the wavelength of the incident light. The intensity of the transmitted light was measured for wavelengths ranging from from about 0.3 pm to about 0.9 pm The transmitted intensity was found to have a wide and important minimum

  around 0.5 μm, from which it has been deduced that a band of light wavelengths

  The 0.5 μm center was coupled to the photodetector by the formation of plasma resonances located in the silver electrode (the wavelength and the angle of incidence did not match the formation -d On the other hand, the intensities transmitted for the two other detectors did not have a

such minimum.

  The corresponding currents currents were then measured

  electron transfer by tunnel effect, which have been produced in each

  one of the three detectors under the effect of the incident laser light, whereas the silver electrode of each detector was polarized at + 1.5 V with respect to the aluminum electrode. wave equal to 476.9 nm, and intensity equal to about 0.2 W / cm fell on the silver electrode of each photodetector at an angle of 54 (as defined above) The light was cut at Hz and was given a p-polarization (as described above) The photocurrents produced in each photodetector were measured with a conventional lock-in amplifier. A photocurrent of

  0.12 A in the MOM photodetector which did not have a practical surface

  This corresponds to a quantum efficiency of about -5

  However, a photocurrent of 0.8 p A was measured and thus a

-4 -

  quantum deformation of 4 xl O in the MOM photodetector whose surface

  substantially periodic had a mean bump amplitude of 25 nm.

  The increased current and the increased quantum efficiency have been attributed to

  In addition, a photocurrent of 2 to A has been measured, and hence a quantum efficiency of about 10 'in the MOM photodetector, the surface of which is virtually the same. periodic had a mean bump amplitude of

  The value of the latter has been attributed to a large extent

  the formation of localized plasma resonance in the bumps on the silver electrode of the photodetector;

Example 2 -

  An MSM photodetector, similar to photodetector, has been manufactured.

  MSM shown in Figure 2, by cleaning and structuring the

  surface of a fused silica plate, as described in the example

  The resulting, substantially periodic, surface of the fused silica plate had bumps whose average amplitude was about

* 125 nm The average spacing between bumps was about 250 nm.

2536911 '

  Conventional evaporation techniques were then used to deposit aluminum tape on the substantially periodic surface of the fused silica plate. The width of the tape was about 1 mm, while the tape thickness was about Then, a layer of hydrogenated amorphous silicon, approximately 30 nm thick, was deposited on the central region of the aluminum strip. The silicon was deposited by sputtering at 200 ° C. from a target. silicon in an atmosphere containing 75% (by volume) Ar and 25% H 2 at a pressure of 0.13 Pa. The RF power density was about 2 W / cm 2, and the spraying was continued. cathodic for about 2 minutes Conventional masking techniques forming "shadow" zones were used to prevent the deposition of the amorphous silicon on the ends of the aluminum strip, so that an electrical contact

  can be established later with the ends of the aluminum ribbon

  The silver ribbon was then evaporated on the amorphous silicon.

  about 1 mm wide and about 15 nm thick, perpendicular to

aluminum tape.

  A second MSM photodetector has also been manufactured using

  the procedure described above, except that the surface

  of the fused silica plate was relatively smooth -

  Using the spectrometer described in Example 1, we have

  determined that the light of wavelength equal to about 700 nm

  bant (on the silver electrode) with normal incidence (relative to the reference surface of the photodetector), was effectively coupled

  by the inward surface of the MSM photodetector which had the surface

  this practically periodic (in accordance with equation (1)) With 4 pW of light, wavelength equal to 700 nm, falling (under normal incidence) on this photodetector MSM, with the light cut to 150 Hz,

  and with the silver electrode polarized at + 1.5 V compared to the electri-

  In the case of the aluminum detector, a photocurrent of 6 × 10 Å was measured in the MSM photodetector with the latch amplifier. This corresponds to a quantum yield of approximately 1.5%. For comparison, and under the same conditions, the photocurrent produced in the MSM photodetector which did not have a practically periodic surface was only

  4 x 109 A, which corresponds to a quantum amount of only

  4 x 10 A, which corresponds to a quantum yield of only 0.1%

25369 11

is lying -

Example 3

  An MSM photodetector, of the type shown in FIG.

  Figure 3, by cleaning and structuring the surface of a silicone hide

  This melted material, as described in Example 1 The resultant, substantially periodic surface of the fused silica plate had bumps the average amplitude of which was about 125 nm.

the bumps were about -250 nm.

  A layer of hydrogenated amorphous silicon, about 30 nm thick, was then deposited on the substantially periodic surface of the fused silica plate, using the procedure described in the example

  2 An aluminum ribbon was then deposited on the surface of the amorphous silicon.

  minium, with a gap of 25 pm at its center, using conventional evaporation and masking techniques The thickness of the aluminum tape was 50 nm, and the ribbon width was 0.5 mm Both halves aluminum ribbon constituted the two electrodes of the MSM photodetector. One half of the aluminum ribbon was then connected to a sampling oscilloscope, the output of which was connected to a Nicolet multichannel analyzer. The other half of the aluminum ribbon was connected to a DC 100 V polarization source. focused 10 s laser pulses on the space between the aluminum electrodes The wavelength of the laser pulses was 575 nm The number counted by the Nicolet analyzer, during a preselected number of signal, was proportional to the value of the current pulses produced in the photodetector MSM under the effect of the pulses of light was chosen at 1024 the number of

signal scans.

  Using 40 mW of laser power, with the light falling under normal incidence relative to the reference surface, the number corresponding to the peak output signal of the MSM photodetector was 9300. At an angle of incidence of 450, the light was efficiently coupled by the surface to the amorphous silicon (in accordance with equation (1)), and the number corresponding to the peak output signal was 70,000. As a comparison, the number corresponding to

25369 S 1

  peak output signal of a similar MSM photodetector, manufactured (as described above) on a smooth fused silica surface,

  under the same conditions, was only 3,000.

  It goes without saying that many modifications can be made to the device described and shown, without departing from the scope of the invention.

Claims (9)

claims-   1 photodetector, comprising an element (40; 100) which undergoes   a detectable change when exposed to electro-   magnetic means and comprising means (30; 130) for detecting this change.   when the element is exposed to electromagnetic radiation, characterized in that this element also comprises a corrugated surface which is substantially periodic 2 Photodetector according to claim 1, characterized in that the average spacing, J, between the projecting portions of the surface   waved is proportional to the wavelength of the electromagnetic radiation   which must be detected by the photodetector.   3 photodetector according to claim 2, characterized in that the amplitude of the projecting parts is between about 0.1 A and about 10 A. 4 photodetector according to claim 1, characterized in that   the element comprises a layer (40) of electrically insulative material   interposed between first (30) and second (50) metal layers, and in that the second metal layer comprises the corrugated surface which   is exposed to electromagnetic radiation - during operation- of the photodetector.   Photodetector according to claim 4, characterized in that   that the electrically insulating layer has a higher dielectric strength than 4 - than about 10 V / cm.   6 photodetector according to claim 5, characterized in that   that the electrically insulating layer is a metal oxide.   7 photodetector according to claim 5, characterized in that the thickness of the insulating layer is between about 1 nm and about 10 nm.   8 photodetector according to claim 7, characterized in that the thickness of the second metal layer is between about nm and about 30 nm.   9 photodetector according to claim 1, characterized in that   means to detect a change include first   re and second spaced electrodes (120, 130) in contact with the element 253691 1:   and the element is made of a semi-insulating material Photodetector according to claim 9, characterized in that each of the first and second electrodes comprises a layer of   electrically conductive material in contact with the semiconductor material   insulating. 11 photodetector according to claim 10, characterized in that the first electrode (130) covers a part of a surface of the semi-insulating material, the second electrode (120) covers a part   different from the same surface of the semi-insulating material, and this   face of the semi-insulating material comprises the corrugated surface which extends between the electrodes 12 Photodetector according to claim 9, characterized in that the first electrode comprises a layer of semiconducting material   whose bandgap is greater than the forbidden band of the substance semi-insulating.   13 photodetector according to claim 9, characterized in that   that the semi-insulating material comprises a p-n junction.   The photodetector according to claim 9, characterized in that the first electrode comprises a first semiconductor material layer, the second electrode comprises a second semiconductor material layer, and the conductivity of the first semiconductor material layer is opposed to that of the semiconductor material layer. second layer of semiconductor material.