FR2612335A1 - HgCdTe photodiode with fast response - Google Patents

Abstract

The photodiode is produced in a p-type HgCdTe semiconductor layer 1. An n-type doped zone 2 is formed on one face of this layer. A metallisation 3 is in contact with a HgTe semimetal layer 5 arranged on the other face of the HgCdTe semiconductor layer 1, opposite the pn junction. Such a photodiode, responsive in the near infrared, has a low series resistance, thus endowing it with a fast response.

Description

The present invention relates to a photodiode, produced in a HgCdTe semiconductor layer of
type P, in which an area is formed on one side
N-type doped thus making a PN junction, and for
view of metallizations access to zones P and N.

Such photodiodes are sensitive in the infrared
near red, and the wavelength on which is
centered their maximum sensitivity depends on the compound
tion of the semiconductor crystal, defined composition
by the molar fraction x of the formula Hg1-xCdxTe, molar fraction which can vary from 0 to 1. These photodiodes are in particular used in telecommunications by optical fibers, at wavelengths of #, 1.30 and 1, 55 microns, for example.

 Photodiodes of the type defined above are already known, in which the HgCdTe layer is either massive or deposited by epitaxy on a substrate.

In both cases, these photodiodes have the disadvantage of having a relatively slow response, that is to say that they transmit poorly the high components of the frequency spectrum of the signals which modulate the light radiations which they detect. Currently, the cut-off frequencies of these photodiodes are from 500 to 800 MHz. It is obvious that this limits the rate of information that can be transmitted in the
fiber optic telecommunications systems. It is therefore essential to have photodiodes with a higher cut-off frequency. However, it is known that the cut-off frequency of a photodiode is all the higher as the RC product, the R series resistance and the capacitance C of its equivalent scheme, is weak. thus, one way to increase the cutoff frequency is to reduce the value of the series resistance R of the equivalent scheme.

 However, this resistance R is the total series resistance of the photodiode, and the two main components of this resistance are, on the one hand, the resistance of the P-type HgCdTe semiconductor layer, and on the other hand the contact resistance between this layer and the metallization which allows access to it.

However, in known photodiodes where the layer of
HgCdTe is massive, the thickness of this layer gives its resistance a relatively high value. Thus, and by way of example, a HgCdTe disc of type P, of resistivity of lon cm, of diameter 150 Fm, and of thickness 400 Wm has a resistance of 350 <. To reduce this resistance, it is possible, by mechanical polishing, to reduce the thickness of the layer, for example up to 50 rcm, which represents substantially the lower limit of what it is possible to do, taking into account the fragility of the material. We then obtain a resistance of 200n 1 which remains relatively high, all the more since it is necessary to add to this value the resistance of contact with metallization, which is difficult to reduce with the known technique which consists in interpose, between the P type semiconductor crystal and the metal of the metallization, an P + type overdoped layer. Indeed, as is known, the flgCdTe alloy is difficult to overdope, and there always remains a straightening effect between the P-type layer and the P + type layer, which increases the contact resistance.

In known photodiodes where the P-type HgCdTe layer is obtained by epitaxy, the thickness of this layer may be other small, for example of the order of 10, which reduces the contribution of the thickness of the layer to a few tens ohms. However, due to the fact that the P-type HgCdTe layer is deposited by epitaxy on a non-conductive substrate, the access metallization to zone P cannot be placed opposite the junction
PN and must be made by the layers of the layer of
P-type HgCdTe This results, on the one hand, in a deformation of the field lines detrimental to the proper functioning of the photodiode, and, on the other hand, a small contact surface between the metal of the access metallization zone P and the semiconductor. The contribution of the contact resistance to the total resistance is therefore significant and, despite the small contribution of the thickness of the P-type semiconductor layer, the total series resistance remains too high.

 The present invention aims to overcome the above drawbacks by providing a photodiode with low total series resistance, therefore with rapid response.

 To this end, it relates to a photodiode of the type defined above, characterized in that the metallization of access to the zone P is in contact with a layer of HgTe semimetal, disposed on the face of said semiconductor layer HgCdTe next to the PS junction.

 In the photodiode of the invention, the contribution, to the total series resistance, of the resistance of the contact between the metallization and the P-type semiconductor, is reduced, due to the use, instead of the conventional "metal on semiconductor ", from 1 semimetal contact to semiconductor", which uses the HgTe semimetal.

This material has intermediate electronic properties between that of metals and that of semiconductors.

As it is a semimetal, the contact is very good, that is to say ohmic, with the metal of the metallization. As, moreover, it is part of the HgCdTe family, it fits perfectly from a crystallographic point of view and since the HgTe semimetal layer is placed opposite the PN junction, there is no deformation of the lines of field, which remain perpendicular to the PN junction.

The contact resistance thus obtained between the access metallization to zone P and the zone P itself is thus only a few ohms.

Advantageously, said semiconductor layer
HgCdTe is deposited by epitaxy on the HgTe semimetal layer, itself deposited by epitaxy on a substrate.

 In such a photodiode, there is then, at the same time, a small contribution from the contact resistance and a small contribution from the resistance due to the thickness of the layer of P-type HgCdTe. total series resistance of the order of a few tens of ohms.

 Advantageously also, the metallization of access to zone P and the HgCdTe semiconductor layer are arranged on the same side of the HgTe semimetal layer, and the metallization of access to zone P passes through the HgCdTe semiconductor layer by a metallized hole. to come into contact with the HgTe semimetal layer.

In such a photodiode, the two metallizations of access to the zones P and N are arranged in the same plane, on the external face of the semiconductor layer
HgCdTe. This facilitates the manufacture and assembly of the connection wires and, moreover, allows the integration, on the same active substrate, for example in GaAs, or in Si, of photodiodes and electronic circuits to produce, on the same "chip "and without any transfer operation from one semiconductor block to another, both the detection function and that of milk or eletronical processing.

 The present invention will be better understood with the aid of the following description of several embodiments of the photodiode of the invention, and of their production methods, with reference to the appended drawings, in which - FIG. 1 represents a schematic view of a first type of photodiode according to the invention, - Figure 2 illustrates an example of use of the photodiode of Figure 1, and, - Figure 3 shows a schematic view of a second type of photodiode according to invention.

 FIG. 1 represents a photodiode, for detecting infrared radiation shown diagrammatically in the form of incident photons 8, and transforming this detected radiation into an electrical signal appearing between two metallizations 3 and 4.

In known manner, the photodiode comprises a junction
PN, result of the formation, on the upper face, in FIG. 1, of a layer 1 of HgCdTe semiconductor of type P, of a zone 2 of type P. Metallization 4, also arranged on the upper face of the layer 1 allows, through a passivation layer 9, access to zone 2 of type N. The metallization 3 is also arranged on the upper face of the layer 1 through which it passes, by a metallized hole 7 to come into contact with a layer 5 of HgTe semimetal placed on the lower face, in FIG. 1, of layer 1, and in particular facing the PN junction produced on the upper face of layer 1.

Thus, it can be said that the metallization 3 allows access to the zone P, that is to say to the layer 1 of HgCdTe of type P, by means of the semimetal layer.
HgTe, metallization 3 and layer 1 being arranged on the same side of layer 5.

 The photodiode of FIG. 1 is produced by epitaxy from a substrate 6, on which the layer 5 of HgTe is grown, with a thickness here equal to 2 Fm, then the layer 1 of HgCdTe of a thickness here equal to 10 m.

The substrate 6 is here mainly formed of a solid layer 62 of GaAs on which a thinner layer 61 of CdTe has been deposited. This type of substrate is designated by the expression CdTe-GaAs. Such a composition represents a particular case of the more general composition
HgCdTe-GaAs. Substrates can also be used
HgCdTe-A1203 or in HgCdTe-InSb, these two expressions designating as previously substrates formed of a massive layer of Al203 or InSb on which is deposited a layer of HgCdTe. It is also possible to use massive substrates in GaAs, CdTe, HgTe, CdZnTe, CdSeTe,
CdHgTe, or Si, for example.

 In FIG. 1, wires 10 and li connect the metallizations 3 and 4, respectively, to pins of the housing, not shown for the sake of simplicity, in which the photodiode is mounted.

 FIG. 2 shows another use of the photodiode of FIG. 1, which highlights the advantage of the arrangement of the two metallizations 3 and 4 on the upper face of the layer 1. In this case, on the same solid crystal 62 of GaAs are arranged on the one hand of circuits 12 and 3, of electronic processing, and, inside a well 14, a photodiode identical to that of FIG. 1; then we can give metallizations 3 and 4 forms such that they also allow access to circuits 12 and 13, to connect them directly to the photodiode. thus integrating, on the same chip 62, both the functions of photon detection 8 and electronic processing of the detected signal.

FIG. 3 represents another photodiode, this time obtained from a massive layer 1 of P-type HgCdTe semiconductor, on the underside of which a 5 'layer of epitaxy has been grown.
semimetal HgTe. A zone N 'of type N has been formed on the upper face, in the figure, so that the layer 5t is located opposite the PN junction thus obtained. Naturally, the thickness of the solid layer 1 ′ of HgCdTe semiconductor of FIG. 3, of the order of 100 μm is much greater than the thickness of the epitaxial layer 1 of HgCdTe semiconductor of FIG. 1, of the order of 10. The thickness of the layer 5 'of HgTe semimetal in FIG. 3 is of the same order of magnitude as the thickness of the layer 5 of HgTe semimetal in FIG. 1.

 In FIG. 3, a metallization 4 ′, disposed on the upper face, in FIG. 3, of the layer 1 ′, allows, through a passivation layer 9 ′, access to the zone 2t of type N. A wire 11t of connection is welded to metallization 42. On the lower face, in the figure, of layer 1, a metallization 3 'is in contact with the entire surface of layer 5' of HgTe semimetal. The metallization 3 'is bonded, using a layer of conductive adhesive 14, to a support 15 of metallized alumina.

 The photodiodes which have just been described are produced as will now be explained.

A first epitaxy method for producing the photodiode of FIGS. 1 and 2 is epitaxy by deposition of vapors of organometallic chemical compounds, or pyrolysis of organometallic vapors (MOCVD = etal
Organo Chemical Vapor Deposition "). This deposition is carried out using a carrier gas, here purified hydrogen, with, as sources of tellurium, cadmium and mercury, a flow of diethyltellide (C2H5) 2Te, a stream of dimethylcadmium (CH3) 2Cd, and a stream of mercury vapor, respectively.The elements combine on the heated substrate in a tube placed in an oven at 4000C by pyrolytic decomposition, or cracking, of organic molecules.

After temperature rise and stabilization, growth takes place at a speed of 4 tt per hour.

 In the case of the photodiode of FIG. 1, we start from a solid crystal of GaAs, of thickness around 500 Fm, which is first subjected to the flows of hydrogen, of (CH3) 2Cd and of ( C2115) 2Te, adjusted to deposit, in about a quarter of an hour, the "buffer" layer 61 of CdTe of thickness substantially equal to 1 Fm. The flow of (CH3) 2Cd is then stopped and replaced by the flow of mercury vapor, which is adjusted to deposit a layer of HgTe. The growth takes place for half an hour, to obtain the layer 5 of 2 Hm thick. The flow of (CH3) 2Cd is then restored by adjusting it, as well as the flow of (C2H5) 2Te and the flow of mercury vapor to deposit layer 1 of H61 Cd Te, the molar fraction x having the desired value for the centering of the photodiode response on the desired wavelength. The growth takes place for approximately two and a half hours, to obtain a layer 1 of approximately 10 Fm thick.

For a hydrogen flow rate of 1 # / min, the fluxes of (C2H532Te, (CH3) 2Cd and mercury vapor are of the order of 5 to 70 / min and their relative values determine the composition obtained A precise control of the temperature at @ 1 C makes it possible to obtain layers 61 of
CdTe, 5 of HgTe and 1 of HgCdTe homogeneous and uniform.

Rapid cooling by circulation of an inert gas, at the end of the deposition, ensures a P-type conductivity.

A second epitaxy method for producing the photodiode of FIGS. 1 and 2 is epitaxy in the liquid phase. In this method, the solid GaAs crystal is, after polishing and cleaning by chemical attack, placed in a graphite crucible, of known type, surmounted by a sliding lid, and provided with several bottomless wells each containing a bath, capable of thus be brought into contact with the crystal placed in the crucible. The crucible is placed in a tube open at both ends, for another subjected to a circulation of hydrogen under pressure, and the whole is placed in an oven heated to about 500 C. Four successive baths are provided here, the first of pure tellurium, the second of CdTe dissolved in tellurium, the third of HgTe dissolved in tellurium, and the fourth of Hgl XCdXTe dissolved in tellurium. The first bath allows a pickling, the second the growth of the "buffer" layer of CdTe, for one minute in order to obtain a layer of CdTe of 1? #M approximately, because the growth takes place at a speed of 1 #m / min substantially.The third and fourth baths allow the growth of the semi-metal layer 5
HgTe, for two minutes to obtain a thickness of approximately 2 m, then growth of the layer 1 of Hgl XCdxTe semiconductor, for ten minutes to obtain a thickness of approximately 10 fm.

 By cooling about 10C per minute during the growth of the layer of Hg, Cd Te, the desired P-type conductivity is obtained.

 Other epitaxy methods for producing the photodiode of FIGS. 1 and 2 are, for example, the method of epitaxy by molecular jet (EJM) described in the publication by J. FAURIE et al, published p. 1307 of the review "Applied Physic Letters" 45 (12) of December 15, 1984, or the EDRI vapor phase epitaxy method, described in French patent No. 2,460,545.

One method for producing the photodiode in FIG. 3 consists of starting from a massive layer of HgCdTe obtained for example by the T # IM (Traveling Heater) process.
Method) described in French Patent No. 2,502,190 or by the modified THM process described in French Patent Application No. 86 00 769. The solid layer is then thinned by mechanical polishing, to form the 1 ′ layer.

The 5t layer is deposited by epitaxy, by one of the methods above.

 It is then possible to form, for the photodiodes of FIGS. 1 and X, zones 2 and 2 ′, of type N, by proceeding in a known manner, for example by the planar technique of diffusion of mercury through passivation layers, as the sign teaches. French Patent No. 2,336,804, or by ion implantation of indium or aluminum, as taught by French Patent No. 2,488,048. A guard ring, of a type also known, can be produced.

 The hole 7 of the photodiode of FIGS. 1 and 2 is produced by selective chemical attack on the upper face, in the figure, protected by a line, except in the circular zone where the attack is to be made. . An acid and oxidizing solution based on potassium dichromate, the attack speed of which is calibrated, makes it possible to raise the HgCdTe without removing the HgTe if the duration is suitably chosen. Another method for making such a hole is ion machining, a technique using bombardment of the surface with low energy Argon ions. The parameters of this machining are chosen so as not to damage the rest of the surface. The metallizations 3 and 4 are then carried out simultaneously, and in a known manner, after metallization of the hole 7.

With a molar fraction, or composition, x close to 0.7, the photodiode produced using a crystal of Hg0.3 Cd Te will have a maximum sensitivity centered on
0.7 a wavelength of 1.3 tm. By choosing a different value, this maximum sensitivity can be moved, in a known manner, and, in particular, bring the value up to 12 Fm for x close to 0.2.

 It is also possible, and according to the teaching of French Patent No. 2,501,915, to choose the composition x so that the bandgap energies Eg and of the spin-orbit band are close, which provides a gain in current, by multiplication effect.

The main characteristics of the photodiodes produced with an epitiated layer of HgCdTe, operating at room temperature, with a circular sensitive surface of diameter 80 μm, adapted to the diameter of the optical fibers for telecommunications are the following, for x = 0.7
Reverse bias voltage: -10 Volts
Reverse current (V = -10 volts): <10 nA
Current response at A = 1.3 µm:> 0.7 A / W
Total capacity (V = -10 volts): 1 pF
Series resistance: 35 ohms
Reverse resistance: 1000 Megohms
Rise time: 200 picosec
Cutoff frequency (-3 dB): 1600 to 2000 Megahertz.

 As can be seen, the low value of the series resistance leads to a cutoff frequency twice as high as for the photodiodes of the prior art.

Claims (12)

Claims 1. Photodiode produced in a layer (1; 1?) Of HgCdTe semiconductor of type P, in which is formed, on one face, a zone (2; 2 ') doped of type N thus carrying out a PN junction, and provided with metallizations (3, 4; 31, 4t) of access to zones P and N, characterized in that the metallization (3; 3t) of access to zone P is in contact with a layer (5; 5t) of semimetal HgTe, disposed on the face of said layer (1; 1 ') of HgCdTe semiconductor opposite the PN junction. 2. The photodiode according to claim 1, in which said layer (î) of HgCdTe semiconductor is deposited by epitaxy on the layer (5) of HgTe semimetal, itself deposited by epitaxy on a substrate (6). 3. Photodiode according to claim 2, in which the metallization (3) of access to the zone P and the layer (1) of HgCdTe semiconductor are arranged on the same side of the layer (5) of HgTe semimetal, and the metallization ( 3) access to zone P crosses the layer (1) of HgCdTe semiconductor through a metallized hole (7) to come into contact with the layer (5) of HgTe semimetal. 4. Photodiode according to one of claims 2 or 3, in which the material of the substrate (6) is chosen from GaAs, CdTe, HoTe, CdZnTe, CdSeTe, CdHgTe, HgCdTeGaAs (61, 62), HgCdTe-A12O3, HgCdTe-InSb, and Si. 5. The photodiode according to claim 1, in which said layer (1Z) of HgCdTe semiconductor is massive and said layer (5 ') of HgTe semimetal is deposited by epitaxy on said layer (1') of EECdle semiconductor. 6. Photodiode according to one of claims 2 to 5, in which the epitaxial deposits are obtained by pyrolysis of organometallic vapors (llOCVD). 7. Photodiode according to one of claims 2 to 5, in which the epitaxial deposits are obtained in the liquid phase. 8. Photodiode according to one of claims 2 to 5, in which the epitaxial deposits are obtained by molecular jet (EJM). 9. Photodiode according to one of claims 2 to 5, in which the epitaxial deposits are obtained by epitaxy in the EDRI vapor phase.  10. Photodiode according to one of claims 1 to 9, in which the composition of the layer (1; 1l) of HgCdTe semiconductor being defined by the formula Hgl Cd Te, the molar fraction x is between substantially 0.2 and substantially 0.7. 11. Photodiode according to one of claims 1 to 4, in which the thickness of the semiconductor layer (1) HgCdTe is at most equal to 12 pm. 12. The photodiode as claimed in claim 3, in which the metallizations (3 4) for access to zones P and N, respectively, are also metallizations for access to electronic processing circuits (12, 13) integrated in the substrate, which are thus directly connected to the photodiode.