Photodetector

Abstract

There is provided a photodetector, comprising a semiconductor heterostructure having in sequence: a first collection layer having substantially uniform doping of a first doping type; a radiation-absorbing layer having substantially uniform doping of the first doping type and having a band gap less than or equal to that of the first collection layer; and a barrier layer having a band gap greater than that of the radiation-absorbing layer, the top of the valence band of the barrier layer being substantially equal in energy to that of the radiation-absorbing layer where the first doping type is n-type or the bottom of the conduction band of the barrier layer being substantially equal in energy to that of the radiation-absorbing layer where the first doping type is p-type; wherein a first portion of the barrier layer is of the first doping type and a second portion of the barrier layer is of a second doping type, the first portion of the barrier layer being adjacent to the radiation-absorbing layer, forming a heterojunction within the barrier layer which gives rise to a depletion region within each portion of the barrier layer.

Claims

1. A photodetector, comprising a semiconductor heterostructure having in sequence: a first collection layer having substantially uniform doping of a first doping type; a radiation-absorbing layer having substantially uniform doping of the first doping type and having a band gap less than or equal to that of the first collection layer; and a barrier layer having a band gap greater than that of the radiation-absorbing layer, the top of the valence band of the barrier layer being substantially equal in energy to that of the radiation-absorbing layer where the first doping type is n-type or the bottom of the conduction band of the barrier layer being substantially equal in energy to that of the radiation-absorbing layer where the first doping type is p-type; wherein a first portion of the barrier layer is of the first doping type and a second portion of the barrier layer is of a second doping type, the first portion of the barrier layer being adjacent to the radiation-absorbing layer, forming a heterojunction within the barrier layer which gives rise to a depletion region within each portion of the barrier layer, wherein the band gap of the barrier layer is the same in the first and second portions of the barrier layer.

2. A photodetector according to claim 1, wherein the thickness of the first portion of the barrier layer is substantially equal to the width of the depletion region in the first portion of the barrier layer.

3. A photodetector according to claim 1, further comprising: a second collection layer adjacent to the second portion of the barrier layer, the second collection layer having substantially uniform doping of the second doping type.

4. A photodetector according to claim 3, wherein the band gap of the second collection layer is less than that of the barrier layer.

5. A photodetector according to claim 3, wherein either: where the first doping type is n-type, the top of the valence band of the second collection layer is substantially equal in energy to that of the barrier layer and the radiation-absorbing layer; or where the first doping type is p-type, the bottom of the conduction band of the second collection layer is substantially equal in energy to that of the barrier layer and the radiation-absorbing layer.

6. A photodetector according to claim 3, wherein either: where the first doping type is n-type, the top of the valence band of the second collection layer is higher in energy than that of the barrier layer and the radiation-absorbing layer; or where the first doping type is p-type, the bottom of the conduction band of the second collection layer is lower in energy than that of the barrier layer and the radiation-absorbing layer.

7. A photodetector according to claim 6, wherein either: where the first doping type is n-type, the energy difference between the top of the valence band of the second collection layer and the bottom of the conduction band of the radiation-absorbing layer is approximately equal in magnitude to the potential difference across the barrier layer; or where the first doping type is p-type, the energy difference between the bottom of the conduction band of the second collection layer and the top of the valence band of the radiation-absorbing layer is approximately equal in magnitude to the potential difference across the barrier layer.

8. A photodetector according to claim 1, wherein the thickness of the second portion of the barrier layer is substantially equal to or greater than the width of the depletion region in the second portion of the barrier layer, such that the thickness of the whole barrier layer is substantially equal to or greater than the width of the depletion region of the heterojunction within the barrier layer.

9. A photodetector according to claim 8, wherein the second portion of the barrier layer acts as a second collection layer, the thickness of the second portion of the barrier layer being greater than the width of the depletion region in the second portion of the barrier layer.

10. A photodetector according to claim 8, further comprising a second collection layer adjacent to the second portion of the barrier layer, the second collection layer having substantially uniform doping of the second doping type, and wherein the thickness of the second portion of the barrier layer is substantially equal to the width of the depletion region in the second portion of the barrier layer such that the thickness of the whole barrier layer is substantially equal to the width of the depletion region of the heterojunction within the barrier layer.

11. A photodetector according to claim 1, wherein the radiation-absorbing layer has a lower doping level than the first collection layer.

12. A photodetector according to claim 1, wherein the doping level of the first portion of the barrier layer is lower than the doping level of the second portion of the barrier layer.

13. A photodetector according to claim 1, wherein the first collection layer and the radiation-absorbing layer are formed of the same material.

14. A photodetector according to claim 1 wherein the first doping type is n-type doping and the second doping type is p-type doping.

15. A photodetector according to claim 1 wherein the first doping type is p-type doping and the second doping type is n-type doping.

16. A photodetector according to claim 1, wherein the barrier region has a thickness of at least 20 nm, preferably 50 nm, more preferably 75 nm and most preferably 100 nm.

17. A photodetector according to claim 1, wherein the barrier region has a maximum thickness of 500 nm, preferably 250 nm, more preferably 150 nm and most preferably 100 nm.

18. The photodetector of claim 1, wherein the barrier region has a sufficient thickness to prevent the tunnelling of carriers across the barrier region.

19. A photodetector according to claim 1, wherein the radiation-absorbing region has a thickness approximately equal to or greater than the absorption length of the radiation to be detected.

20. A photodetector according to claim 1, wherein the radiation-absorbing region has a thickness of at least 5% greater than the absorption length of the radiation to be detected, preferably at least 15%, more preferably at least 25% and most preferably at least 30%.

21. A photodetector according to claim 1, wherein the radiation-absorbing region has a maximum thickness of 150% greater than the absorption length of the radiation being detected, preferably 100%, more preferably 75% and most preferably 50%.

22. A photodetector according to claim 1, wherein the barrier region has a band gap of between 800 meV and 3000 meV.

23. A gas sensor comprising a photodetector according to claim 1.

Description

(1) Examples of photodetectors in accordance with the present invention will now be described and contrasted with conventional photodetectors with reference to the accompanying drawings, in which:

(2) FIG. 1 shows an example of a conventional n-p photodetector;

(3) FIG. 2 is an energy level diagram illustrating the operation of the photodetector of FIG. 1;

(4) FIG. 3 is an energy level diagram illustrating the operation of an example of a conventional nBn photodetector without external bias;

(5) FIG. 4 is an energy level diagram illustrating the operation of the photodetector of FIG. 3 with an applied external bias;

(6) FIG. 5 is an energy level diagram illustrating the operation of an example of a conventional nBp photodetector;

(7) FIG. 6 is an energy level diagram illustrating the operation of a photodetector in accordance with a first embodiment of the present invention;

(8) FIG. 7 is an energy level diagram illustrating the operation of a photodetector in accordance with a second embodiment of the present invention; and

(9) FIG. 8 is an energy level diagram illustrating the operation of a photodetector in accordance with a third embodiment of the present invention.

(10) Preferred embodiments of the present invention are depicted in FIGS. 6, 7 and 8 and will be described below. In all cases the described examples are based on a nBp type structure, i.e. where the layer in which the radiation is absorbed is of the negative doping type (n-type), and the minority carriers diffuse across the barrier to a positively-doped region (p-type). However, all the principles can be transposed to structures of the opposite polarity, i.e. pBn type devices, in which the radiation-absorbing layer is p-type and the minority carriers diffuse across the barrier to an n-type region. To arrive at such structures, the doping type indicated for each layer in the description below should be inverted, i.e. changed from n-type to p-type and vice versa. It will be appreciated that in each of the embodiments described below, it should be assumed that the Fermi levels are equalised throughout the structure, unless explicitly stated otherwise (for example, if an external bias is applied to the device).

(11) As shown in FIG. 6, in a first embodiment the semiconductor heterostructure forming the photodetector 30 comprises three layers: a first collection layer 31, which here is n-type (n-contact) and preferably highly doped; a radiation-absorbing layer 32 (absorber) which is of the same doping type as the first collection layer (here, n-type); and a barrier layer 33. The absorber 32 should have a smaller band gap (i.e. energy difference between the top of its valence band E.sup.A.sub.V and the bottom of its conduction band E.sup.A.sub.C) than that of the first collection layer 31, and also preferably has a lower degree of doping than the first collection layer 31. The band gap of the barrier layer 33 (i.e. E.sup.B.sub.CE.sup.B.sub.V) must be larger than that of the absorber 32, preferably much greater. The top of the valence band of the barrier layer 33 (E.sup.B.sub.V) is substantially equal to that of the absorber layer 32 (E.sup.A.sub.V) and of the first collection layer 31.

(12) The barrier layer 33 comprises a first portion 33a which is of the same doping type as the absorber layer 32 (hence n-type in this example) and is adjacent to the absorber layer 32. A second portion 33b of the barrier layer 33 is of the other doping type (hence p-type in this example). Preferably, the first portion 33a is weakly doped whilst the second portion 33b is highly doped. The oppositely doped portions 33a, 33b give rise to a depletion region existing within the barrier layer 33, resulting in a potential difference V and hence an electric field across the barrier layer 33. This accelerates the minority carriers (here, holes) across the barrier 33 from the absorber 32 such that the photocurrent detected in the external circuit C is more accurately representative of the received radiation. No external bias is required to enable the photocurrent or to establish this electric field. The wide band gap of the barrier layer 33 prevents significant thermal generation of electron-hole pairs in the barrier layer itself due to the large activation energy that would be required for such generation.

(13) The concentration of donors N.sub.D in the first portion 33a of the barrier is related to the thickness d.sub.N of that portion and to the concentration of acceptors N.sub.A in the second barrier portion 33b. In this case, the thickness is chosen so that substantially all the donors in the first portion 33a are ionized and hence the thickness d.sub.N is substantially equal to the depletion region thickness in the first portion 33a of the barrier 33. In other examples, the thickness d.sub.N of this region could be arranged to be greater than (or, less preferably, lesser than) the depletion region thickness. Hence, N.sub.D and d.sub.N are related as follows:

(14) $\begin{matrix} d_{N} \sqrt{\frac{2 .Math.}{e} .Math. \frac{N_{A}}{N_{D} (N_{A} + N_{D})} .Math. V} & (1) \end{matrix}$
Where is the dielectric constant, e is the electron charge, and V is the contact potential difference of the p-n junction formed in the barrier 33. V can be written as:

(15) $\begin{matrix} V \frac{E}{e} + \frac{kT}{e} \ln (\frac{N_{D}^{a} N_{A}^{c}}{N_{C}^{a} N_{V}^{c}}), N_{C, V}^{a, c} = 2 {(\frac{2 m_{c, v} kT}{{(2)}^{2}})}^{3 / 2} & (2) \end{matrix}$

(16) Here, E=E.sub.C.sup.absorberE.sub.v.sup.barrier is the difference between the energies of the bottom of the absorber conduction band and top of the barrier valence band, k is the Boltzmann constant, T is the temperature, m.sub.c,v are the effective masses of electrons in the absorber and holes in the second collection layer, N.sub.D.sup.a is the concentration of donors in the absorber layer, N.sub.A.sup.c is the concentration of the acceptors in the second collection layers, h is Planck's constant. The expressions (1) and (2) are obtained on the assumption that the charge carriers are non-degenerate.

(17) By arranging the thickness d.sub.N of the first portion 33a to be substantially equal to the depletion region width in the same region, the electric field established by the p-n junction is substantially constrained to the barrier layer and in particular does not extend significantly into the absorption layer 22. This allows the thickness of the absorption layer 22 to be increased without leading to an increase in the degree of generation-recombination noise.

(18) Whilst not essential, it is preferred that the thickness d.sub.p of the second portion 33b region of the barrier should be greater than or substantially equal to the width of the corresponding depletion region in that portion. Hence, preferably:

(19) $\begin{matrix} d_{P} \sqrt{\frac{2 .Math.}{e} .Math. \frac{N_{D}}{N_{A} (N_{A} + N_{D})} .Math. V} & (3) \end{matrix}$

(20) In the present embodiment, the outermost part of the second portion 33b of the barrier layer (represented by reference numeral 33b in FIG. 6) acts as a second collection layer of the device in the sense that the minority carriers diffuse towards it. In order to improve the effectiveness of this part of the barrier for this purpose, it is preferred that the thickness d.sub.P of the second portion 33b of the barrier is greater than (not equal to) the width of the depletion region in this portion. This results in the outermost part 33b of the portion 33b being non-depleted and outside of the electric field, optimising this part of the device to function as a collector.

(21) It will be appreciated that a pBn photodetector in line with the above principles can be formed using a p-type first collection layer, a p-type absorber and a barrier region in which the first portion is p-type and the second is n-type. In this case since the minority carriers are now electrons it is the conduction band energies of the barrier and absorber (E.sup.B.sub.C and E.sup.A.sub.C) which should substantially match one another, the obstruction presented by the barrier occurring in the valence band so as to prevent the flow of majority carriers (holes).

(22) A second embodiment of the invention is shown in FIG. 7. Here, the detector structure 40 comprises four layers: a first collection layer 41, which here is n-type (n-contact) and preferably highly doped; a radiation-absorbing layer 42 (absorber) which is of the same doping type as the first collection layer (here, n-type); a barrier layer 43; and a second collection layer 44 which is of the opposite doping type as that of the first collection layer 41. The first collection layer 41, radiation-absorbing layer 42 and barrier layer 43 each have substantially the same properties in terms of their doping levels and band gaps as the corresponding layers in the FIG. 6 embodiment, although different considerations apply to the thickness of the barrier 43 as described further below. In particular, the barrier 43 comprises a first n-type portion 43a and a second p-type portion 43b. The second collection layer 44 has a smaller band gap than that of the barrier layer 43 and is preferably of a high doping level (e.g. equal to that of the second portion 44b of the barrier). Preferably, the top of the valence band of the second collection layer 44 (E.sup.C.sub.V) is substantially equal to that of the barrier 43, the absorber 42 and the n-contact 41. The purpose of the additional collection layer 44 is to protect the device surface from oxidation as may especially occur where an aluminium alloy is used to form the barrier layer 43. In a particular embodiment the first collection layer is made of the same material as absorber.

(23) The same considerations as regards the thickness d.sub.N of the first portion 43a of the barrier as described in the first embodiment apply equally here. However, in the present embodiment, since an additional second collection layer 44 is provided, the barrier layer 43 no longer needs to act as a collector. As such it is preferred that the thickness d.sub.P of the second portion 43b of the barrier layer be substantially equal to (not greater than or lesser than) the width of the depletion region in the same portion 43b. This again can be determined in accordance with equation (3) given above. As such, the electric field established by the p-n junction in the barrier layer does not extend substantially into the second collection layer 44, which remains non-depleted.

(24) Again, a pBn-type photodetector in line with the above principles can be formed using a p-type first collection layer, a p-type absorber, a barrier region in which the first portion is p-type and the second is n-type, and an n-type second collection layer. In this case since the minority carriers are now electrons it is the conduction band energies of the second collection layer, barrier and absorber (E.sup.C.sub.C, E.sup.B.sub.C and E.sup.A.sub.C) which should substantially match one another, the obstruction presented by the barrier occurring in the valence band so as to prevent the flow of majority carriers (holes).

(25) A third embodiment of the invention is shown in FIG. 8. Again, the photodetector 50 comprises four layers, namely: a first collection layer 51, which here is n-type (n-contact) and preferably highly doped; a radiation-absorbing layer 52 (absorber) which is of the same doping type as the first collection layer (here, n-type); a barrier layer 53; and a second collection layer 54 which is of the opposite doping type as that of the first collection layer 51. The first collection layer 51 and radiation-absorbing layer 52 each have substantially the same properties in terms of their doping levels and band gaps as the corresponding layers in the embodiments of FIGS. 6 and 7, although different considerations apply to the barrier 53 and second collection layer 54 as described further below.

(26) There are two main differences between the photoconductor of the present embodiment and that of FIG. 7. First is the material used for the second collection layer 54 (p-contact). The top of the valence band of this material (E.sup.CV) is higher than the top of the valence bands of the absorber 52 (E.sup.A.sub.V) and the barrier 53 (E.sup.B.sub.V). Hence there is an energy difference E.sub.1 existing in the valence band between the barrier layer 53 and the collection layer 54. Secondly, the barrier doping is implemented differently. Again, in this embodiment the barrier comprises a first portion 53a which is n-type and a second portion 53b which is p-type. However in this case both of the portions 53a, 53b have a low doping level so a substantially symmetrical p-n junction is formed (although this is not essential). The barrier width d (=d.sub.N+d.sub.P) and the dopant concentrations N.sub.A (acceptors in the p-region) and N.sub.D (donors in the n-region) are chosen so that the width of the barrier d is substantially equal to the width of the depletion region formed by the p-n junction:

(27) Hence,

(28) $\begin{matrix} d \sqrt{\frac{2 .Math.}{e} .Math. \frac{N_{D} + N_{A}}{N_{D} N_{A}} .Math. V} & (4) \end{matrix}$
where V is defined by expression (2). This requirement means that all the donors and acceptors in the barrier are ionized.

(29) Further, it is preferable that the material of the second contact layer (p-contact) 54 is chosen so that the bottom of the absorber's conduction band (E.sup.A.sub.C) and the top of the p-contact's valence band (E.sup.C.sub.V) are related as follows:
E.sub.v.sup.CE.sub.c.sup.AV(5)

(30) In other words, the energy difference E.sub.2 between the bottom of the absorber's conduction band (E.sup.A.sub.C) and the top of the p-contact's valence band (E.sup.C.sub.V) is approximately equal to V, e.g. to within +/50%, more preferably +/25%, still preferably +/10%. This has the result that only a small amount of charge transfer is required for the Fermi levels to equalise across the device and hence excessive charge transfer between the absorber 52 and the contact 54 is suppressed. This prevents the onset of a built-in electric field in the absorber 52 which could otherwise promote generation-recombination noise.

(31) Again, a pBn-type photodetector in line with the above principles can be formed using a p-type first collection layer, a p-type absorber, a barrier region in which the first portion is p-type and the second is n-type, and an n-type second collection layer. In this case since the minority carriers are now electrons it is the conduction band energies of the barrier and absorber (E.sup.B.sub.C and E.sup.A.sub.C) which should substantially match one another, the obstruction presented by the barrier occurring in the valence band so as to prevent the flow of majority carriers (holes). The energy difference E.sub.1 will now exist in the conduction band between the barrier layer 53 and the collection layer 54, and the energy difference E.sub.2 will be between the top of the absorber's valence band (E.sup.A.sub.v) and the bottom of the p-contact's conduction band (E.sup.C.sub.C).

(32) In all embodiments, it is preferred that the barrier region has a thickness of at least 20 nm, preferably 50 nm, more preferably 75 nm and most preferably 100 nm. Also preferably, the barrier region may have a maximum thickness of 500 nm, preferably 250 nm, more preferably 150 nm and most preferably 100 nm. Advantageously, the barrier region should have a sufficient thickness to prevent the tunneling of carriers across the barrier region. The band gap of the barrier region should be sufficiently high to present a significant energy barrier to majority carriers and also a high activation energy for the formation of electron-hole pairs, so as to prevent generation of thermalized carriers within the barrier itself. Preferably, the barrier region has a band gap of between 800 meV and 3000 meV.

(33) The radiation-absorbing region preferably has a thickness approximately equal to or greater than the absorption length of the radiation to be detected. In particularly preferred embodiments, the radiation-absorbing region has a thickness of at least 5% greater than the absorption length of the radiation to be detected, preferably at least 15%, more preferably at least 25% and most preferably at least 30%. Also preferably, the radiation-absorbing region may have a maximum thickness of 150% greater than the absorption length of the radiation being detected, preferably 100%, more preferably 75% and most preferably 50%. The target radiation may be, for example, infrared radiation, e.g. in the waveband 3-5 microns.

(34) In accordance with third embodiment, exemplary materials, dopant concentrations and thicknesses of the first collection layer, the radiation-absorbing layer, and the first and second portions of the barrier layer and the second collection layer are given in the table below. This specific configuration of photodetector may have a peak responsivity to light of wavelength around 3.3 m. It will be appreciated however that the exemplary materials, dopant concentrations and thicknesses of the first collection layer, the radiation-absorbing layer, and the first and second portions of the barrier layer and the second collection layer (if applicable) may also be used with the first and second embodiments of the present invention.

(35) TABLE-US-00001 Doping, Thickness, Layer Material cm.sup.3 m First collection InAs n-type, 200 layer 5 10.sup.17 Radiation- InAs n-type, 5 absorbing layer 1-2 10.sup.16 First portion of AlAs.sub.0.16Sb.sub.0.84 n-type, 0.15 barrier layer 4 10.sup.16 Second portion of AlAs.sub.0.16Sb.sub.0.84 p-type, 0.15 barrier layer 4 10.sup.16 Second collection In.sub.0.2Ga.sub.0.8As.sub.0.26Sb.sub.0.74 p-type, 0.4 layer 1 10.sup.19

Photodetector

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Inventors

Cpc classification

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H10F39/191

ELECTRICITY

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H10F39/103

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H10F39/193

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H10F39/184

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H10F30/222

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H10F39/021

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H01L27/14

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H01L27/146

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H01L27/144

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Abstract

Claims

Description