WO1989006052A1

WO1989006052A1 - Reticulated junction photodiode having enhanced responsivity to short wavelength radiation

Info

Publication number: WO1989006052A1
Application number: PCT/US1988/003492
Authority: WO
Inventors: Michael G. Farrier
Original assignee: Santa Barbara Research Center
Priority date: 1987-12-14
Filing date: 1988-10-13
Publication date: 1989-06-29

Abstract

A reticulated diode junction (26) has heavily doped regions (42) separated by undoped regions (44). Short wavelength (SWL) radiation which enters the diode structure through the undoped regions is absorbed within these regions and generates photocarriers. The recombination of these photocarriers within the undoped regions does not occur at a rate comparable to the recombination within the doped regions and as a result, a lateral charge carrier mobility is achieved whereby the SWL generated photocarriers are collected by laterally disposed portions of the reticulated junction, thereby permitting the SWL generated photocarriers to contribute to the overall photodiode current. The reticulated structure is achieved by an implantation or diffusion blocking means, such as a mask (62).

Description

RETICULATED JUNCTION PHOTODIODE HAVING ENHANCED RESPONSIVITY TO SHORT WAVELENGTH RADIATION

FIELD OF THE INVENTION

This invention relates generally to photodiodes and, in particular, relates to a photodiode having a reticulated pn junction for enhanced short wavelength photoresponse.

BACKGROUND OF THE INVENTION

Photodiodes, such as photodiodes constructed of silicon (Si) , mercury-cadmiun-telluride (HgCdTe) and indium antimonide (InSb) absorb incident radiation and generate charge carriers therefrom. " Depending on the doping and biasing potential of a diode junction, either holes or electrons are collected by the junction to generate a diode current. This current is related to the intensity of the incident radiation so that, by measuring the output current of the diode, the intensity of the incident radiation may be determined.

One problem that arises in the use of such photodiodes is that incident radiation is absorbed within the diode structure nonuniformly. That is, short wavelength (SWL) radiation has a lesser depth of penetration into the diode structure than does long wavelength_^ (LWL) radiation. Inasmuch as the diode junction is typically formed adjacent to an upper surface of the diode structure, the upper surface also being the surface through which the radiation enters the diode, photocarriers generated by the absorption of SWL radiation typically are generated within the junction doping layer itself. Due to the relatively heavy doping of the junction top layer these photocarriers have a relatively short lifetime and are quickly recombined. The recombination of these photocarriers within the junction top layer does not therefore contribute to the diode current. Thus, it can be appreciated that SWL radiation may not significantly contribute to the overall diode current.

It has been known in the art to improve the SWL responsivity of a silicon photodiode by fabricating a junction having a doubly doped profile. Such a doubly doped junction profile provides an electrical field which is generated at the surface of the diode as a result of the relatively steep doping gradient. Photocarriers which are generated at or near the surface are repelled from the surface by this field, the photocarriers thus being also repelled from the heavily doped region. In general, this region of heavily doped silicon is made extremely thin (approximately 500 angstroms) in order to minimize recombination losses within this region. Such a structure does enhance SWL responsivity but is structurally complex and requires significant control of ion implantation and other silicon processing parameters during the construction of the diode. For example, additional high temperature processing steps may be required. Thus, the overall complexity and cost of the photodiode is significantly increased. A further problem which results from this type of steeply graded junction is that ,, due to the ion implantation of the surface of the diode surface, defects which arise from the implantation process result in an overall increase in surface states. That is, such photodiodes have a relatively large diode dark current and, hence, a reduction in diode signal to noise ratio.

SUMMARY OF THE INVENTION

The aforementioned and other problems are overcome by a photodiode which, in accordance with the method and apparatus of the invention, has a reticulated diode junction having heavily doped regions separated by undoped regions. SWL radiation which enters the diode structure through the undoped regions is absorbed within these regions and generates photocarriers. The

^~ recombination of these photocarriers within the undoped regions does not occur at a rate comparable to the recombination within the doped regions and, as a result, a lateral charge carrier mobility is achieved whereby the SWL generated photocarriers are collected by laterally disposed portions of the reticulated junction, thereby permitting the SWL generated photocarriers to contribute to the overall photodiode current. The reticulated structure is achieved by a relatively simple implantation or diffusion blocking means, such as mask. The overall junction depth is not of primary importance and may be made shallow in order to improve the responsivity of the doped regions as well. Furthermore, by not heavily doping certain areas of the photodiode surface, surface state generation due to the doping process, ^• such as implantation or diffusion, is minimized in these, undoped regions therefore resulting in a diode having an increased signal to noise ratio. The^' use of the invention provides for design flexibility by allowing a choice of substrate doping and bias conditions related to the size of the undoped area and, therefore, also the amount of responsivity improvement.

In accordance with an embodiment of the invention there is provided a semiconductor photodiode for absorbing incident radiation and generating charge carriers therefrom, comprising a substrate comprised of semiconductor material doped with a first type of dopant to a first concentration of dopant atoms and a photodiode junction within a top surface region of the substrate, the junction having a depth and being doped with a second ^'type of dopant to a given concentration of dopant atoms, the second concentration being greater than the first concentration, and wherein the junction is a reticulated junction having a total junction surface area defined by a first area doped to the second concentration surrounding one or more second areas doped with the first type of dopant to the first concentration.

In accordance with a method of the invention there is provided a method of increasing the responsivity of a semiconductor photodiode to incident radiation having wavelengths of approximately 0.5 microns or less comprising the steps of providing a substrate comprised of semiconductor material doped with a first type of dopant to a first concentration and forming a reticulated diode junction within a top surface region of the substrate to a given depth by doping selected areas of the top surface with a second type of dopant to a second concentration which exceeds the first concentration, the junction being formed such that portions of the substrate extend upwardly to the substrate surface at a plurality of positions within the junction such that photocarriers generated by radiation having wavelengths of approximately 0.5 microns or less to a depth approximately equal to the junction depth within the upwardly extending substrate portions are not immediately reσombined.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be made ^'more apparent herein in the detailed description of the invention ^" and also in reference to the drawings wherein:

Fig. 1 shows a cross-sectional view of a conventional photodiode wherein the undesirable recombination of substantially all SWL generated photocarriers occurs within the doped junction region;

Fig. 2 shows a cross sectional view of a photodiode constructed in accordance with a method and apparatus of the invention in which SWL generated photocarriers within an undoped region adjacent to the surface of the photodiode are not recombined and have lateral mobility; Fig. 3 is a top- view of the photodiode of Fig. 2 showing a representative reticulated junction structure;

Fig. 4 is a top view of an exemplary photodiode constructed in accordance with the method and apparatus of the invention showing the dimensions of the various diode structures;

Fig. 5 is a graph showing the carrier concentration per cubic centimeter as a function of depth into the junction of the photodiode of Fig. 4; and

Fig. 6 is a graph showing the difference in responsivity as a function of wavelength of the exemplary photodiode of Fig. 4 and of a photodiode constructed in accordance with the prior art.

DETAILED DESCRIPTION OF THE INVENTION

Although the method and apparatus of the invention will be described in the context of a specific embodiment wherein a substrate of n-type (100) silicon has a p-type impurity, such as boron, implanted therein to form a pn junction, it should be understood that the use of the invention may be advantageously employed in any front side illuminated photodiode structure wherein the absorption depth of the incident photons in a particular semiconductor material is shallow, such that a significant fraction of photogenerated carriers are reco bined in the heavily doped p+(n+) region. For example, infrared detecting devices constructed of semiconductor material such HgCdTe, which has a high radiation absorption coefficient (c< ₀ ^l~~- 3000) , where the majority of photocarriers are generated within the top 3 microns of the detector surface, also benefit from the use of the invention. Photodiodes constructed of InSb also benefit from the teaching of the invention. Thus, the teachings of the invention should be understood to apply to photodiodes constructed of a number of different types of semiconductor materials and are not meant to be limited to those constructed only of silicon.

Referring first to Fig. 1 there is shown a conventional silicon photodiode having a substrate 12, a depletion region 14 and a pn junction 16. As can be seen, the substrate 12 may be doped at a lower portion with an n type impurity to form an n-type layer 13 and in an upper portion with a p-type impurity of higher impurity

- —concentration for forming p-type layer 17. Coupled to the layer 17 is a terminal 18 for applying a negative, or reverse, bias voltage (~V_β) , the application of which induces the formation of the depletion region 14. Radiation having a wavelength associated with short wavelength (SWL) radiation enters the photodiode through the top surface of layer 17 wherein the SWL radiation is absorbed and generates photocarriers, the photocarriers being holes and electrons. Due to the relatively high doping concentration of the layer^' 17 as compared to the layer 13 these photocarriers have a relatively short lifetime before they are recombined. This recombination of photocarriers within the layer 17 is disadvantageous in that these charge carriers do not contribute to the diode current. It can also be seen that radiation having longer wavelengths, that is long wavelength (LWL) radiation, penetrates through the layer 17 and is absorbed within th.e depletion region 14. Due to the lesser concentration of majority carriers within the depletion region 14 the generated minority photocarriers have a relatively longer lifetime than those photocarriers within the layer 17. The holes are attracted to the junction 16 due to the operation of -V_ to contribute to the diode current.

As has been previously stated, the recombination of SWL generated photocarriers within the p-layer 17 region is disadvantageous in that these photocarriers do not contribute to the diode current. Also, due to the laterally continuous nature of the junction across the surface of the diode the steps used to form the junction, such as ion implantation or diffusion, result in the generation of increased surface states which adversely affect the signal to noise ratio of the diode.

Referring now to Fig. 2 there is shown a portion of a photodiode 20 constructed in accordance with the method and apparatus of the invention. As can be seen, photodiode 20 is comprised of an n-type substrate 22, a depletion region 24 and a reticulated p-type layer 42 which forms a reticulated pn junction having elements 26 and 28.

Referring briefly to Fig. 3 it can be seen that the view shown in Fig. 2 is a portion of a photodiode having the p-layer 42 which is not laterally continuous over the entire surface of the diode but which, in accordance with the invention, has a plurality of openings 44 made therein, thus accounting for the reticulated nature of the junction.., As can be seen, this reticulated structure results in the depletion region 24 extending to the surface of the diode, this extension of the depletion region 24 to the surface occurring within the openings 44. A single terminal 30 may be conductively coupled to the p-layer 42 as shown in Fig. 3 to provide a bias voltage -V_ to the entire junction region. SWL radiation impinging on the top 0 surface of the diode 20 within the opening 44 can be seen to penetrate into the depletion region 24 wherein the SWL radiation is absorbed generating photocarriers. Inasmuch as these photocarriers are generated in a lightly doped surface region of the diode, as opposed 5 to the heavily doped p-layer 42, the photocarriers have a relatively longer lifetime. Thus, a lateral mobility of the holes is possible within the region 44, enabling

^' " the holes- to be collected by one of the adjacent junctions, such as the junction 28. LWL radiation 0 penetrates to a greater depth within the diode before being absorbed and generating photocarriers, these photocarriers contributing to the diode current as in the device shown in Fig. 1.

5 Although the diode shown in Fig. 2 has a substrate doped with an n-type donor material and a junction layer doped with a p-type acceptor material it should be realized that the doping of the junction and the substrate may be reversed, that is, the substrate may 0 be doped with a p-type material and the junction layer doped with an n-type material. Thus, the use of the invention may be advantageously employed in diodes having either type of doping. Of course, if the upper portion is doped with an ή-type material the polarity of the bias voltage would also be reversed, that is, a positive bias voltage would be applied to the junction.

The top surface of the diode of Figs. 2 and 3 may also be provided with a layer of insulating material and/or with an anti-reflective coating, the coatings not being shown in the Figures.

Due to the reticulated structure of the diode junction layer 42, the resulting photodiode demonstrates improved short wavelength visible responsivity over a conventional photodiode, such as the photodiode shown in Fig. 1. In general, in the conventional photodiode SWL radiation (being that radiation from the near UV to approximately 0.5 microns) is absorbed and generates photocarriers in a relatively shallow region near the silicon interface with a surface insulator (not shown) and well within the finite depth (d) of the heavily doped p-layer 17. Any photocarriers generated in this heavily doped layer 17 are almost instantaneously recombined and do not survive to be swept into the depletion region 14 as a photocurrent. The invention overcomes this problem by blocking the p layer dopant from selected regions of the surface of the photodiode, thereby allowing photocarriers generated within a shallow surface region of the diode, and within the non p-type region, to be collected and contribute to the diode current.

In general, the relative size of the blocked surface area of the photodiode is related to the lateral depletion width which is a function of substrate dopant concentration and the reverse bias magnitude of the diode. It has been found that the non-p-type doped region 44 is preferably less than approximately twice the lateral depletion width (assuming that the lightly doped region 44 is surrounded by a junction region) . Thus, by optimizing the ratio of the areas of the doped to the non-p-type doped area the net shallow photocarrier lifetime and resultant SWL responsivity is significantly improved.

SWL responsivity enhancement is generally limited in the SWL absorption region by surface recombination effects which are related to surface traps present at any silicon insulator interface. In general, it has been found that surface recombination is predominant within the first 200 angstroms to 300 angstroms of silicon beneath- a silicon insulator interface. Photocarriers generated by ultraviolet radiation (having Wavelengths below 0.4 microns) largely reside within the surface capture cross-section of the surface traps. A photodiode constructed in accordance with the method and apparatus of the invention beneficially reduces the surface state trap density due to the removal of implant or diffusion induced stress and contamination within the regions 44. Responsivity enhancement is, in general, limited for radiation longer than 0.5 microns wavelength because the absorption depth for this longer wavelength radiation is significantly deeper than the junction depth (d) and thus the quantity of photocarriers recombined in the heavily doped junction regions is a small fraction of the total photo-generated carrier density. τ_ EXAMPLE

An example of a photodetecting device constructed in accordance with the invention will now be given. A 5 substrate of 500 ohm-cm to 600 ohm-cm n-type (100) float zone refined 4 inch diameter silicon was provided. Such float zone refined silicon typically has n-type impurity present within a concentration

1 _? _L6 range of 10 to 10 atoms per cubic centimeter. Upon _^0 top surface of the substrate were implanted a plurality of p-type junctions. The junctions were formed by the implantation of boron to achieve a doping concentration profile as shown in Fig. 5. The junction depth was typically 0.7 microns and the peak ₁₅ concentration of boron atoms was approximately 2 X 10 18 atoms per cubic centimeter. The boron was implanted such that a plurality of photodiod arrays were formed, consisting of four 16 diode arrays having 0.004 inch,by

0.004 inch photodiode elements and one 32 element array

2_Q having 0.002 inch by 0.00185 inch photodiode elements. One array of 16 photodiodes was processed in a conventional manner by not blocking the boron implantation from the active region of each of the photodiodes. This array of conventionally processed

25 photodiodes served as a responsivity standard against which the responsivity enhancement of the photodiodes constructed in accordance with the invention were measured. In the other arrays of photodiodes portions of the active area of each of the photodiodes was

3_Q blocked from doping in such a way that six rectangular regions of undoped semiconductor material were formed within each of the photodiodes. In Fig. 4 there can be seen a plurality of masks which are utilized in the construction of. the photodiode of the invention and which thereby also define the shape and dimensions of the various structures described above. As can be seen, a light shield aperture mask 52 defines a region wherein radiation is incident upon the photodiode 50. Aperture mask 52 is surrounded by a contact and oxide thinning mask 54 which defines a region within which a shallow boron implant is accomplished. Aperture mask 52 is also contained within an isoplanar mask 56 which defines the peripheral boundary of the PN junction. A p+ diffusion mask 58 defines a region within which a boron diffusion is applied in order to create a relatively deep PN junction beneath an overlying metal contact which is itself defined by the metal interconnect mask 60. In accordance with the invention the lateral surface continuity of the p-type boron implanted layer is regularly interrupted by a plurality of boron implant blocking masks 62. Implant blocking masks 62 are disposed, in this embodiment of the invention, as a plurality of substantially parallel rectangular masks beneath which substantially n-type silicon is preserved during the implantation of boron within the mask 54. Thus, the underlying pn junction is "reticulated" due to the presence of the implant blocking masks 62 during the implantation of the device.

Of course, more c-^~ less than six such blocking masks 62 may be employed and the masks may have other than a rectangular shape as shown. Finally, a p+ contact mask 64 serves to "etch" a hole in an oxide coating (not shown) . for conductively coupling the overlying metalization defined by mask 60 to the p+ diffusion defined by mask 58.

The dimensions of the exemplary photodiode of Fig. 4 are given below in the following table:

TABLE

DESIGNATION DIMENSIONS (MICRONS)

Thus, the area of the undoped region is, for the illustrated embodiment, given by the difference in areas between the total area of the masks 62 and the photoactive area defined by aperture mask 52. Of course, depending on the relative areas of masks 62 and 52 other doped/undoped ratios are within the scope of the invention.

A source of negative bias voltage was coupled to each of the junctions of the array and the arrays of photodiodes were illuminated by a source of radiation having a variable wavelength output. The graph of Fig. 6 illustrates the responsivity of the photodiode arrays plotted against wavelength in microns. The solid line A corresponds to the average responsivity of the photodiode arrays constructed in accordance with the method and apparatus of the invention. The dotted line B shows the responsivity of the reference photodiodes constructed in a conventional manner.

As can be seen, the responsivity of the photodiodes constructed in accordance with the invention have an improvement of responsivity of from approximately 55 percent at 0.42 microns to an average of approximately 11 percent at 0.5 microns. The slope of the responsivity curve suggests that an improvement of approximately 140 percent would result at a wavelength of 0.4 microns.

Inasmuch as the responsivity enhancement at 0.42 microns is higher than the percent of the region of the photodiode blocked- from doping (or 33 percent of the diode photodetecting area) the additional improvement in responsivity is due to, at least in part, to a reduction in surface trap density in the undoped region 52 which results in additionally increased carrier lifetime within these regions beyond that which would normally occur from the absence of the p-type dopant.

As can be realized, a number of modifications may become evident to those skilled in the art. For example and has been previously stated, a reticulated junction photodiode constructed in accordance with the method and apparatus of the invention may be employed with semiconductor materials other than silicon, such as with photodiodes comprised of HgCdTe and InSb. In an HgCdTe photodiode it-may be desirable to implant boron, which functions as a donor (n+) in p-type HgCdTe, at similar concentrations and to a similar depth as described above. Of course, p+ on n HgCdTe photodiodes are also within the scope of the invention however, an additional annealing process may be required to repair implant related lattice damage. A p+ implanted photodiode constructed of InSb may be constructed by implanting, for example. Be to a similar concentration and to a similar depth as described above.

Also, the total percentage of the area of undoped photoreceptive region of each photodiode may be varied, in conjunction with other photodiode parameters such as the bias voltage and the doping of the substrate, to achieve other reticulated shapes and patterns. Thus,

^" the^" invention is not to be considered to be limited to the embodiments described herein, the invention is instead to be limited only as defined by the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A semiconductor photodiode for absorbing incident radiation and generating charge carriers therefrom, including charge carriers generated by incident SWL radiation, comprising:

a substrate comprised of semiconductor material doped with a first type of dopant to a first concentration of dopant atoms; and

a photodiode junction region within a top surface region of said substrate, - - .- said junction region having a given depth and being doped with a second type of dopant of opposite electrical conductivity to said first type, said second type of dopant having a second concentration of dopant atoms, said second concentration being greater than said first concentration, wherein

said junction region has a shape for defining a reticulated junction having a total junction region area defined by a first area doped to said second concentration surrounding one or more second areas doped with said first type of dopant to said first concentration. and wherein charge carriers generated by

SWL radiation within..said second areas and within said given depth have an increased lifetime over charge carriers generated within said first area.

2. A photodiode as defined in Claim 1 wherein a ratio of a total second surface area to said junction region area is approximately one third.

3. A photodiode as defined in Claim 2 wherein said first area is a substantially square surface area and wherein said second areas each have a substantially rectangular surface area, each of said second areas having edges along a length thereof substantially parallel one to another and substantially parallel to at least one edge of said first area.

4. A photodiode as defined in Claim 3 wherein each of said second areas has a length of approximately 80 microns and a width of approximately seven microns and wherein said edge of said first area has a length of approximately 100 microns.

5. A photodiode as defined in Claim 4 wherein said second type of dopant is implanted into said top surface to a depth of approximately 0.70 microns for defining a junction depth of said junction.

6. A photodiode as defined in Claim 5 wherein said first concentration of dopant atoms is such that substantially all of the charge carriers generated within one of said second areas within the junction depth are not reσόmbined but are collected by an adjacent one of said first areas.-

7. A photodiode as defined in Claim 6 wherein said substrate is comprised of n-type silicon and wherein said second type of dopant is comprised of boron and wherein said implanted boron has a maximum ccoonncceennttrraattiiion of approximately 2 X 10 18 atoms per cubic centimeter.

8. A photodiode as defined in ^'Claim 6 wherein said substrate is comprised of HgCdTe and wherein said second type of dopant is comprised of boron and wherein said implanted boron has a maximum ccoonncceennttrraattiion of approximately 2 X 10 18 atoms per cubic centimeter.

9. A photodiode as defined in Claim 6 wherein said substrate is comprised of InSb and wherein said second type of dopant is comprised of beryllium and wherein said implanted beryllium has a maximum concentration of approximately 2 X 10 18 atoms per cubic centimeter.

10. A photodiode as defined in Claim 1 further comprising means for coupling a reverse bias potential to said junction, said reverse bias potential having a magnitude sufficient to induce a depletion region within said photodiode, said depletion region extending through each of said second areas and also to a depth within said substrate.

11. A photodiode as defined in Claim 1 wherein said semiconductor material is silicon and wherein said first type of dopant has an n-type of conductivity and wherein said second..type of dopant has a p-type of conductivity.

12. A photodiode as defined in Claim 1 wherein said semiconductor material is silicon and wherein ^" said first type of dopant has a p-type of conductivity and wherein said second type of dopant has an n-type of conductivity.

13. A method .of enhancing the responsivity of a semiconductor photodiode to incident radiation having wavelengths of approximately 0.5 microns or less comprising the steps of:

providing a substrate comprised of semiconductor material doped with a first type of dopant to a first concentration; and

forming a reticulated diode junction at a given depth within a top surface region of the substrate by doping selected areas of the top surface with a second type of dopant to a second concentration which exceeds the first concentration, the junction being formed such that portions of the substrate extend upwardly through the junction at a plurality of positions within the junction such that photocarriers generated by radiation having wavelengths of approximately 0.5 microns or less are not immediately recombined within the upwardly extending substrate portions.

14. A method as defined in Claim 13 wherein the step of forming is accomplished by the steps of:

masking predefined areas of the top surface; and

ion implanting the second type of dopant in a pattern which overlies and encompasses the masked areas therein, the mask substantially preventing the implantation of ions therethrough.

15. A method as defined in Claim 14 wherein the step . of ion implanting is- accomplished by implanting boron atoms to a maximum depth of approximately 0.70 microns into the top surface.

16. A method as defined in Claim 15 wherein each of the predefined areas has a substantially identical rectangular shape and wherein the pattern is a substantially square pattern.

17. A method as defined in Claim 13 wherein the step of forming is accomplished by the steps of:

masking predefined areas of the surface of the depletion region; and diffusing the second type of dopant in a pattern which overlies and encompasses the masked areas therein, the mask substantially preventing the diffusion of ions therethrough.

18. A method as defined in Claim 17 wherein each of the predefined areas has a substantially identical rectangular shape and wherein the pattern is a substantially square pattern.