GUARD RING STRUCTURE

Abstract: An avalanche photodiode having a beryllium guard ring. The beryllium is implanted at a dosage of at least 5 x 10<14> per cm<2> and preferably at 3 x 10<15> per cm<2> and subsequently annealed to provide a guard ring profile with a p+ core and a superlinearly graded tail. The doping profile of the guard ring immediately adjacent the core is superlogarithmic.

Description: GUARD RING STRUCTURE This invention relates to beryllium guard ring structures and fabrication thereof, and in particular to guard ring structures for avalanche photodiodes. In photodiodes (APD's), avalanche photocurrent is multiplied by impact ionisation of primary photogenerated carriers accelerated by a high field. In III - V APD devices for use at wavelengths in the regions of 1.3 and 1.5 microns a material such as indium gallium arsenide or suitable quaternary compounds may be used for the region where photogeneration occurs. If the multiplication also takes place in this material, which has a comparatively small band gap, high dark currents resulting from tunnelling occur at the high fields required for multiplication, giving rise to excessive noise. To avoid this problem the carrier pairs are photogenerated in indium gallium arsenide and then the holes are swept into a wider band gap indium phosphide layer containing the pn junction where avalanche multiplication takes place. This type of device is known as the separate absorption and multiplication (SAM) structure. A further modification is the SAGM which has a -graded layer of intermediate composition (usually a quaternary) between the indium phosphide and indium gallium arsenide layers. The thickness and doping level of the indium phosphide multiplication layer has to be carefully controlled in order to achieve the correct field for avalanche multiplication near the pn junction, with a sufficiently low field at the interface with the indium gallium arsenide layer to- avoid tunnelling. Both planar and mesa APD structures are known, and in general planar structures have proved superior, and are therefore preferred. t In planar APD devices the pn junction, which is generally formed by p+ implantation or diffusion into a nominally n type layer r will have a curved edge and is therefore subject to edge beakdown unless the boundary shape is modified to minimise the effects. Thus it is usual for a p type guard ring, also diffused or implanted, to be fabricated around the outer edges of the p type junction region. The structure and doping concentrations within the guard ring influence the extent to which edge breakdown is prevented. Accordingly the present invention provides an avalanche photpdiode comprising a semiconductor substrate, an n type light absorption layer, an n type indium phosphide multiplication layer, a p type region formed on the multiplication layer and providing an abrupt pn junction with the multiplication layer and a grade p type guard ring surrounding said p type region, characterised in that the guard ring comprises ion implanted beryllium at an 14 2 implantation dosage of at least 5 x 10 per cm . The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic cross section through a SAGM APD with a guard ring, and Figure 2 is a graph showing the dopant profile of the guard ring. Referring to Figure 1, a preferred planar SAGM, APD structure is shown which comprises an n+ indium phosphide substrate 1, doped for example with sulphur to a level of 8 x 10 18 per cc. Over the substrate there is a 0.5 micron no type indium phosphide buffer layer 2, doped for 17 example to a level of 10 per cc, a 3 micron layer of nominally undoped indium gallium arsenide 3 which in fact 15 has a residual n type level of 10 per cc and a 0.1 micron layer 4 of indium gallium arsenide phosphide with a 1.3 micron equivalent band gap. The layer 4 serves to grade the band gap between the layer 3 and the next layer 5 while maintaining lattice match. Layer 5 16 comprises sulphur doped 10 per cc indium phosphide and, overlying layer 5, is a final layer 6 of nominally 15 undoped, actually 10 per cc n type, mdium phosphide. The indium phosphide layer 6 is of the order of 1 to 2 microns thick, and into its top surface a diffusion of zinc dopes a portion 7 p+ to a level of the order of 18 10 per cc. The p+ diffusion preferably extends to a depth of about half of the layer 6 but may extend up to the interface with layer 5. Around the p+ region 7 (and in fact fabricated prior to it) a guard ring 8 of beryllium is provided by the technique described below. The device layers 1 to 6, including the area into which zinc will subsequently be diffused are masked to define an implantation target zone and the target zone is implanted with berrylium ions with an energy in the range of 100 to 14 2 200 KeV to a dosage of at least 5 x 10 per cm and 15 2 preferably in the range of 2 to 4 x 10 per cm , most 15 2 . . preferably 3 x 10 per cm . After implantation the device is annealed; preferably annealing is continued for about three hours at a temperature in the range of 625 to / SUBSTSTUTESHEET 750°C, most*preferably for 3 hours at 700°C. To prevent loss of phosphorus from the device surface the annealing stage is conducted under flowing phosphine and hydrogen. Further protection may be provided by coating exposed surfaces with silicon nitride. The technique of annealing in an open tube with a flow of phosphine and hydrogen is found more convenient than sealing the device in an ampoule with red phosphorus. The layers are then remasked with the zinc diffusion region exposed and a zinc diffusion stage follows. Any suitable zinc diffusion technique jπay be utilised, for example a sealed ampoule technique with typical conditions of 20 minutes heating at 525°C with a ZnAs source. Figure 2 shows dopant concentration (plotted on a logarithmic scale) versus depth for a beryllium guard ring fabricated as described. It will be realised that the profile after implantation and before annealing will follow a substantially Gaussian distribution centred around a mean range that increases in depth with the implantation energy. Apart from relatively small difference in mean range and spread the resulting profiles after annealing are of substantially similar shape irrespective of implantation energy, the maximum mean range being less than 1 micron at 400 KeV and about 0.5 microns at 200 KeV with correspondingly wider spreads are also being .obtained at higher implantation energy. The profile shown in Figure 2 may conveniently be divided into three sections. The first section 'A 1 denotes a p+ core region extending from the surface of comparatively high doping level.. This region 'A 1 extends deeper than the main junction depth and to a depth generally of the order of 1 to 2 microns. After the p+ region 'A' the beryllium concentration level begins to fall off at a superlogarithmic rate as shown in zone *B', which ma$ typically extend for up to one micron and is followed by *._* UBSTJTUTESHEET a lower rate superlinear fall off 'C for about 1 to 1.5 microns, after which the profile falls to an abrupt tail. The superlinear guard ring region should have a doping level gradient which decreases continuously from the surface such that the average slope over the thickness of the 16 —"3 1 multiplication layer is less than 5 x 10 cm " μm~ , lfi —"_ —1 more preferably less than 3 x 10 cm " μm~ and most 16 — * •? —1 preferably less than 1 x 10 cm " μm " . The superlogarithmic guard ring region should have a gradient of the log of the doping level which decreases continuously from the surface such that the average slope of the log doping over the thickness of zone B is less than 3 decades μm-1 , and greater than 0.5 decades μm~ , most preferably about 1.5 decades μm~. The profile obtained is surprising in view of reported work on beryllium doping of n-type indium phosphide (Devlin et al, Inst. Phys. Conf. Ser. No.45: Chapter 6 pp 510-518) in which for lower dosages of implanted beryllium smoothly reducing dopant profiles were obtained. It was also further thought that dosages as high as 3 x 10 15 per cm 2 would result in excessive damage to the crystal structure (Slater et. al. Nuclear Instrumentation Methods B 7/8 , p 429, 1985). The guard ring structure provided by the high dosage beryllium implant and subsequent annealing approximates closely to the requirements of an ideal guard ring which are generally: a laterally and vertically graded junction with sufficiently low doping gradient to reduce field enhancement due to edge curvature, and a guard ring junction depth greater than and laterally beyond the main p+n junction in order to suppress the field enhancement at the curved edge of the main junction, and more specifically: provide a higher p+ core region (the 'A' region) near to the surface to prevent back depletion to the main p+n junction also to suppress curved edge field enhancement; permits sufficient back depletion into the p type guard ring to prevent significant enhancement of the electric field in the ternary absorption layer (which would result in tunnelling current); avoids abrupt changes in doping concentration within the depletion region to prevent local enhancement of electric field, and avoids significant p-type doping of the ternary absorption layer to prevent tunnell-ing current, because the dopant concentration rapidly falls to a low value in the absorption layer. It should be particularly noted that outside the p+ core region 'A' the dopant distribution reduces smoothly and continuously without discontinuity, the low doping gradient in the superlinear region 'c' providing a graded pn junction in the multiplication layer without the very wide depletion widths associated with a linearly graded junction. It is found that the shape of the doping gradient is determined by implant dose and does not depend upon anneal time or temperature. An implant dosage of 2 x 10 14 per cm 2 does not result, after annealing, in the desired profile. The doping profile is obtained early in the annealing process (as early as 15 minutes) but at that stage there is still residual implantation damage which reduces with increasing anneal time. For practical purposes an annealing time in the range of 30 minutes to 4 hours is preferred, most preferably about 3 hours. Advantages given by the high dosage beryllium implanted guard ring over existing guard ring structures are that the doping profile renders the position of the guard ring much less critical, the insensitivity to annealing conditions enables high reproducibility and the superlinear region of the doping profile allows a more compact guard ring structure than linearly graded structures.

Claims: CLAIMS 1. An avalanche photodiode comprising a semiconductor substrate, an n type light absorption layer, an n type indium phosphide multiplication layer, a p type region formed on the multiplication layer and providing an abrupt pn junction with the multiplication layer and a graded p type guard ring surrounding said p type region, characterised in that the guard ring comprises ion implanted beryllium at an implantation dosage of at 14 2 least 5 x 10 per cm . 2. An avalanche photodiode according to claim 1 wherein the guard ring is subsequently annealed to provide a p core region extending to at least the depth of the abrupt pn junction and a graded region with a superlinear doping profile extending through the multiplication layer beyond the abrupt junction. 3. An avalanche photodiode according to claim 2 in which the doping profile of the graded region comprises a region adjacent the core having a superlogarithmic doping profile. 4. An avalanche photodiode according to any preceding claim in which the implantation dosage is in the range of 2 to 4 x 10 5. A method of fabricating an avalanche photodiode guard ring comprising ion implanting beryllium at a dosage of at least 5 x 10 14 per cm2 to form a guard ring in an n type indium phosphide layer and annealing the implantation in the presence of a phosphorus overpressure. 6. A method according to claim 5 in which the implantation is annealed in the presence of a flow of phosphine and hydrogen. 7. A method according to claim 5 or claim 6 in which the anneal temperature is in the range of 625°C to 750°C and the anneal time is at least 15 minutes. 8. A method according to any of claims 5 to 7 in which the implantation dosage is in the range of 2 to 4 x 10 15 per cm2. 9. An avalanche photodiode substantially as hereinbefore described with reference to the accompanying drawings. 10.An avalanche photodiode comprising a semiconductor substrate, an n type light absorption layer, an n type indium phosphide multiplication layer, a p type region formed on the multiplication layer and providing an abrupt pn junction with the multiplication layer and a graded p type guard ring surrounding said p type region, characterised in that the guard ring comprises ion implanted beryllium at an implantation dosage of at 14 2 least 5 x 10 per cm and has a doping profile substantially as shown in figure 2. 11. A method of forming a guard ring substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: RIMINGTON JULIE JOAN, ROBERTSON MICHAEL JAMES, RODGERS PAUL MICHAEL, RIMINGTON, JULIE, JOAN, ROBERTSON, MICHAEL, JAMES, RODGERS, PAUL, MICHAEL
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: H01L31
IPC codes: H01L21, H01L31