Patent Publication Number: US-9425341-B2

Title: P-I-N photodiode with dopant diffusion barrier layer

Description:
PRIORITY CLAIM 
     This application claims the benefit of priority of Singapore Patent Application No. 201207481-1, filed 8 Oct. 2012, the benefit of priority of which is claimed hereby, and which is incorporated by reference herein in its entirety. 
     FIELD OF INVENTION 
     The invention relates generally to a p-i-n (pin) photodiode. 
     BACKGROUND 
     Effort has been devoted in Silicon (Si) compatible photonics during the last two decades, especially in the field of active devices. Active building blocks have been fabricated on Si optical modulators and Germanium (Ge) photodetectors, with some showing integration with standard complementary metal-oxide-semiconductor (CMOS) circuits or other optical components. 
     Ge has been investigated for active photonics devices, particularly for light detection at optical communication wavelengths (of around 1.3 to 1.55 μm). High performance Ge photodetectors have been realized due to its favorable absorption coefficient at these wavelengths. 
     A Ge photodetector needs low dark current, high responsivity and high bandwidth for optimum performance. Doping profiles must be optimized such that the electron-hole generation region remains intrinsic during operation. Phosphorus diffusion in Ge is extremely fast, resulting in non-abrupt junctions and higher dopant concentrations in intrinsic regions. This effectively increases device capacitance at a fixed operating bias and reduces photodetector bandwidth (RC-limit). 
     A low annealing temperature at 500° C. would be able to restrict dopant diffusion. However, this is at the expense of higher dopant activation and limits device integration compatibility with backend CMOS processes which uses temperatures of up to 600-700° C. 
     There is thus a need to address the above drawbacks for existing photodetectors. 
     SUMMARY 
     According to one aspect of the invention, there is provided a pin photodetector comprising a dopant diffusion barrier layer disposed within an active light absorbing region of the pin photodetector. 
     According to another aspect of the invention, there is provided A method of fabricating a pin photodetector, the method comprising: providing a first semiconductor layer having a region doped to a first polarity; providing a second semiconductor layer having a region doped to a second polarity, the second polarity being opposite to the first polarity; providing a dopant diffusion barrier layer between the first semiconductor layer and the second semiconductor layer; and doping the first semiconductor layer to a higher doped portion within the region doped to a first polarity, wherein the dopant diffusion barrier layer reduces doping of the first polarity to form an intrinsic layer between the dopant diffusion barrier layer and the second semiconductor layer, so that the dopant diffusion barrier layer is disposed between the first semiconductor layer and the intrinsic layer, and the intrinsic layer is disposed between the dopant diffusion barrier layer and the second semiconductor layer, wherein the intrinsic layer has a region doped to the first polarity to a concentration less than that of the higher doped portion of the first semiconductor layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention, in which: 
         FIG. 1  shows a cross-section structure of a pin photodetector according to a first embodiment of the invention. 
         FIG. 2  shows a cross-section structure of a pin photodetector according to a second embodiment of the invention. 
         FIG. 3A  shows a top view of a pin photodetector according to a preferred embodiment of the invention.  FIG. 3B  shows a cross-section structure of the pin photodetector taken along line A-A′ of  FIG. 3A . 
         FIGS. 4A and 4B  show a method to fabricate a pin photodetector, according to various embodiments of the invention. 
         FIGS. 5A to 5D  each show a stage within an exemplary process to fabricate a vertical waveguided Ge-based photodetector. 
         FIGS. 6A, 6B and 6C  show transmission electron microscope (TEM) images of the cross-section of a germanium (Ge) epitaxial stack after phosphorus (P) implantation and annealing. 
         FIG. 7A  shows secondary ion mass spectrometer (SIMS) data for the Ge stack, having a Si interlayer (IL) and  FIG. 7B  shows SIMS data for a Ge stack without a Si IL. 
         FIG. 8  plots 3 dB bandwidth (RC-limited) results for Ge epitaxial stacks with different doping concentrations and varying operating bias. 
     
    
    
     DEFINITIONS 
     The following provides sample, but not exhaustive, definitions for expressions used throughout various embodiments disclosed herein. 
     The term “p-i-n photodetector” may refer to a diode with an intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor. The p-i-n photodetector (interchangeably referred to as a “pin photodetector”) is typically operated under reverse bias such that charge carriers are emitted within the intrinsic region of the semiconductor electronic device as a consequence of absorption of energy from light of a certain wavelength. 
     The term “dopant diffusion barrier layer” may mean a semiconductor region that seeks to confine dopant, introduced during fabrication of the pin photodetector, to within a targeted region. The dopant diffusion barrier layer seeks to prevent spreading, through diffusion, of the dopant to surrounding regions where the presence of dopant is not desired as this would impair the desired properties of the pin photodetector. In various embodiments, the dopant diffusion barrier layer may be able to effectively restrict the dopant to the targeted region, so that the dopant diffusion barrier layer prevents dopant from diffusing across and provides anneal conditions whereby the dopants will not penetrate this layer. However, given that the depth of dopant diffusion penetration also depends on additional parameters, such as the thickness of the dopant diffusion barrier layer and thermal budget, the dopant diffusion barrier layer may serve to reduce the concentration of dopant that diffuses across. 
     The term “layer” is not necessarily a flat planar structure and may take the shape of an earlier layer upon which the layer is formed. 
     The term “active light absorbing region” may mean a depletion region present in the pin photodetector, which generates external carriers when exposed to light, the depletion region being enhanced when the pin photodetector is placed under reverse bias to encompass the dopant diffusion barrier layer under a sufficiently large bias. 
     The term “semiconductor” may mean materials that include silicon (Si), germanium (Ge), gallium arsenide (GaAs), indium gallium arsenide (InGaAs) and indium phosphide (InP). 
     The terms “first polarity” and “second polarity” may be understood in the context of forming the p-n junction of the pin photodetector. Accordingly, the structure, with which a respective one of either of these terms is associated, forms part of the p-n junction of the pin photodetector. 
     The term “higher doped portion” may mean a portion, within an already doped region, having a higher concentration of the same dopant. When a region is doped to a first polarity, there may be a higher doped portion having a higher concentration of dopant of the first polarity. Similarly, when a region is doped to a second polarity, there may be a higher doped portion having a higher concentration of dopant of the second polarity. 
     The term “intrinsic layer” may mean a semiconductor region that may be undoped or doped to a lesser extent compared to the other layers that comprise the pin photodetector. 
     DETAILED DESCRIPTION 
     In the following description, various embodiments are described with reference to the drawings, where like reference characters generally refer to the same parts throughout the different views. 
     It has been found that when fabricating a pin photodetector, dopant (such as phosphorus) diffuses extremely quickly in semiconductor material, like germanium, which results in non-abrupt junctions and higher dopant concentration in intrinsic region. This effectively increases device capacitance at a fixed operating bias and reduces photodetector bandwidth (as derived from an RC-limit of such pin photodetectors). 
     A low annealing temperature at 500° C. has been used to restrict dopant diffusion. However, this is at the expense of higher dopant activation and limits device integration compatibility with backend CMOS processes which uses temperatures up to 600-700° C. 
     Various embodiments of the present invention address the above shortcomings.  FIG. 1  shows a cross-section structure of a pin photodetector  100  according to a first embodiment of the invention. The pin photodetector  100  includes a dopant diffusion barrier layer  102  disposed within an active light absorbing region  104  of the pin photodetector  100 . 
     As part of fabricating the pin photodetector  100 , the pin photodetector  100  is exposed to doping (represented by reference numeral  160 ). The in-situ placement of the dopant diffusion barrier layer  102  causes the dopant diffusion barrier layer  102  to completely prevent or at least limit the dopant that can diffuse past (represented by reference numeral  180 ). Accordingly, the dopant is confined within a targeted region  162  and the spreading of the dopant to a surrounding region  182  is limited. The dopant diffusion barrier layer  102  thus allows optimisation of doping profiles such that the electron-hole generation region remains intrinsic during operation. 
     Further embodiments of the invention incorporate the dopant diffusion barrier layer  102  of  FIG. 1  as described below. 
       FIG. 2  shows a cross-section structure of a pin photodetector  200  according to a second embodiment of the invention. 
     The pin photodetector  200  comprises a first semiconductor layer  206  having a region doped to a first polarity and a second semiconductor layer  208  having a region  212  doped to a second polarity. In the second embodiment of the invention, the entire first semiconductor layer  206  may be doped to the first polarity or the doping is confined to a specific region of the first semiconductor layer  206 .  FIG. 2  also shows that only a specific region (that which is labeled  212 ) of the second semiconductor layer  208  is doped to the second polarity. However, the entire second semiconductor layer  208  may be doped to the second polarity. The second polarity is opposite to the first polarity. The first polarity may be from doping the first semiconductor layer  206  with an n-type dopant and the second polarity may be from doping the second semiconductor layer  208  with a p-type dopant. 
     A dopant diffusion barrier layer  202  is disposed between the first semiconductor layer  206  and the second semiconductor layer  208  for reducing doping of the first polarity into a region  218  between the dopant diffusion barrier layer  202  and the second semiconductor layer  208 . This dopant diffusion barrier layer  202  may also extend the spreading of dopant to a region  220  that overlaps into the second semiconductor layer  208 . Similar to the pin photodetector  100 , the dopant diffusion barrier layer  202  is disposed within an active light absorbing region  204  of the pin photodetector  200 . 
     The region doped to the first polarity of the first semiconductor layer  206  may have a higher doped portion  210 . Similarly, the region  212  doped to the second polarity of the second semiconductor layer  208  may have a higher doped portion  216 . The higher doped portion  216  of the second polarity and the higher doped portion  210  of the first polarity form a pn junction of the pin photodetector  200 . 
     The region  218  between the dopant diffusion barrier layer  202  and the second semiconductor layer  208  may be an intrinsic layer  214 . The intrinsic layer  214  is located so that the dopant diffusion barrier layer  202  is disposed between the first semiconductor layer  206  and the intrinsic layer  214 , and the intrinsic layer  214  is disposed between the dopant diffusion barrier layer  202  and the second semiconductor layer  208 . The intrinsic layer  214  has a region doped to the first polarity to a concentration less than that of the higher doped portion  210  of the first semiconductor layer  206  due to the presence of the dopant diffusion barrier layer  202 , for the reasons mentioned above with respect to the dopant diffusion barrier layer  102  of  FIG. 1 . 
     As shown in  FIG. 2 , the first semiconductor layer  206 , the dopant diffusion barrier layer  202  and the intrinsic layer  214  are provided as a stack  222  on the second semiconductor layer  208 . 
     The dopant diffusion barrier layer  202  may comprise material that is different from the first semiconductor layer  206 , whereby the first semiconductor layer  206  and the dopant diffusion barrier layer  202  may comprise materials from any one or more of Group IV semiconductors, an alloy of Group IV semiconductors (such as SiGe, SiC, GeC or GeSn) and Group III-V semiconductor compounds. For instance, suitable semiconductor pairs for the first semiconductor layer  206  and the dopant diffusion barrier layer  202  include germanium and silicon; germanium and a silicon-germanium alloy (such as Si 1-x Ge x ); or a silicon-germanium alloy and silicon. 
     The intrinsic layer  214  may comprise material that is the same as that of the first semiconductor layer  206 . 
     The first semiconductor layer  206  and the dopant diffusion barrier layer  202  may respectively comprise a Group IV semiconductor and a Group III-V semiconductor compound pairing, such as germanium and silicon, germanium and a silicon-germanium alloy or a silicon-germanium alloy and silicon. The second semiconductor layer may comprise silicon, germanium or a silicon-germanium alloy. 
       FIG. 3A  shows a top view of a pin photodetector  300  according to a preferred embodiment of the invention.  FIG. 3B  shows a cross-section structure of the pin photodetector  300  taken along line A-A′ of  FIG. 3A . 
     The pin photodetector  300  comprises a first semiconductor layer  306 , a second semiconductor layer  308 , a dopant diffusion barrier layer  302 , an intrinsic layer  314  and metal layers  324 . 
     The first semiconductor layer  306  has a region doped to a first polarity and the second semiconductor layer  308  has a region  312  doped to a second polarity. The second polarity is opposite to the first polarity. The first polarity may be from doping the first semiconductor layer  306  with an n-type dopant and the second polarity may be from doping the second semiconductor layer  308  with a p-type dopant. The region doped to the first polarity of the first semiconductor layer  306  may have a higher doped portion  310 . Similarly, the region  312  doped to the second polarity of the second semiconductor layer  308  may have a higher doped portion  316 . The higher doped portion  316  of the second polarity and the higher doped portion  310  of the first polarity form a pn junction of the pin photodetector  300 . Each of the metal layers  324  is in contact with each of the higher doped portion  316  of the second polarity and the higher doped portion  310  of the first polarity. 
     The dopant diffusion barrier layer  302  is disposed within an active light absorbing region of the pin photodetector  300 . The dopant diffusion barrier layer  302  is disposed between the first semiconductor layer  306  and the intrinsic layer  314 . The intrinsic layer  314  is disposed between the dopant diffusion barrier layer  302  and the second semiconductor layer  308 . The intrinsic layer  314  has a region doped to the first polarity to a concentration less than that of the higher doped portion  310  of the first semiconductor layer  306  due to the presence of the dopant diffusion barrier layer  302 , for the reasons mentioned above with respect to the dopant diffusion barrier layer  102  of  FIG. 1 . 
     As shown in  FIG. 3B , the first semiconductor layer  306 , the dopant diffusion barrier layer  302  and the intrinsic layer  314  are provided as a stack  322  on the second semiconductor layer  308 , such that the respective bases of the first semiconductor layer  306 , the dopant diffusion barrier layer  302  and the intrinsic layer  314  extend to be in contact with a surface of the second semiconductor layer  308 . 
     In the preferred embodiment, germanium is used for the first semiconductor layer  306  and the intrinsic layer  314 . Silicon is used for the dopant diffusion barrier layer  302 . Silicon is also used for the second semiconductor layer  308 . The top surface of the Ge first semiconductor layer  306  is n++ doped with phosphorus dopant to form the higher doped portion  310  of the first polarity. The top surface of the Si second semiconductor layer  308  is doped with boron to form the p+ region  312  doped to the second polarity and the p++ higher doped portion  316  of the second polarity. 
     As shown in  FIG. 3B , metal electrodes (i.e. the metal layers  324 ) are placed at the sides and top of the Ge first semiconductor layer  306  to form a vertical pin diode. The Ge epitaxial stack  322  has a Si interlayer (IL) sandwiched in between the Ge layers used to realise the first semiconductor layer  306  and the intrinsic layer  314 . A vertical electric field is applied to the photodetector  300  for photocurrent generation during operation. From  FIG. 3A , an optical input  352  may be evanescently-coupled via a crystalline Si waveguide (WG)  350  into the Ge detector  300 . 
     The Ge pin photodetector  300  may employ in-situ grown Si interlayer (i.e. the dopant diffusion barrier layer  302 ) for controlled dopant diffusion. The Si interlayer prevents phosphorus dopant diffusion downwards into Ge during activation anneal and other post Ge epitaxy annealing processes. This enables abrupt dopant junction formation in Ge and retains integrity of the intrinsic region  314 . 
       FIG. 4A  shows a method  400  to fabricate a pin photodetector, according to various embodiments of the invention. The method comprises the following steps: 
     In step  402 , a first semiconductor layer having a region doped to a first polarity is provided. In step  404 , a second semiconductor layer having a region doped to a second polarity is provided, with the second polarity being opposite to the first polarity. In step  406 , a dopant diffusion barrier layer is provided between the first semiconductor layer and the second semiconductor layer. 
     In step  408 , the first semiconductor layer is doped to form a higher doped portion within the region doped to a first polarity. The dopant diffusion barrier layer reduces doping of the first polarity to an intrinsic layer between the dopant diffusion barrier layer and the second semiconductor layer. This causes the dopant diffusion barrier layer to be disposed between the first semiconductor layer and the intrinsic layer, and the intrinsic layer to be disposed between the dopant diffusion barrier layer and the second semiconductor layer. Due to the blocking effect of the dopant diffusion barrier layer, the intrinsic layer has a region doped to the first polarity to a concentration less than that of the higher doped portion of the first semiconductor layer. 
     Steps  402 ,  404 ,  406  and  408  are to be understood as not necessarily occurring in sequence. In one embodiment of the invention, the first semiconductor layer, the dopant diffusion barrier layer and the intrinsic layer may be formed from a multilayer structure that sandwiches the dopant diffusion barrier layer between two layers. Doping the stack causes an exposed layer of the two layers to form the first semiconductor layer, while the dopant diffusion barrier layer inhibits the dopant from diffusing deeper into the stack, thereby creating the intrinsic layer within the other of the two layers. The intrinsic layer formed in step  408  may comprise material that is the same as that of the first semiconductor layer. 
     In a preferred embodiment, the dopant diffusion barrier layer is provided as an in-situ grown Si interlayer for controlled dopant diffusion. Consider the above multilayer structure with such an interlayer acting as an interface between the two layers. When fabricating a pin photodetector comprising such a multilayered structure, a first of the two layers is exposed to dopant, while the interlayer serves to reduce, or even prevent, the dopant from diffusing into a second of the two layers. The dopant diffusion barrier layer allows a higher anneal temperature to be used (as compared to the dopant diffusion barrier layer being absent) for better implant activation and defect repair without adversely affecting the intrinsic region. In known pin photodetectors, an ex-situ additional semiconductor layer (e.g. doped poly-Si) is used to form the higher doped regions mentioned above. The dopant diffusion barrier layer eliminates the need to use this additional layer. 
     To complete the fabrication of the pin photodetector referred to in  FIG. 4A , the following further steps may be carried out, as shown in  FIG. 4B . These further steps  410 ,  412  and  414  are to be understood as not necessarily occurring in sequence. 
     In step  410 , the second semiconductor layer (of step  404  of  FIG. 4A ) is doped to form a higher doped portion within the region doped to the second polarity. In step  412 , the first semiconductor layer, the dopant diffusion barrier layer and the intrinsic layer (mentioned in one or more of the steps  402  to  408  of  FIG. 4A ) may be provided as a stack on the second semiconductor layer. In step  414 , a metal layer is formed to be in contact with each of the higher doped portion of the second polarity and the higher doped portion of the first polarity (respectively from step  410  of  FIG. 4B  and step  408  of  FIG. 4A ). 
       FIGS. 5A to 5D  each show a stage within an exemplary process, which implements the method  400  described above in respect of  FIGS. 4A and 4B , to fabricate a vertical waveguided Ge-based photodetector. The process comprises the following stages: i) Si waveguide/passives formation; ii) bottom P+ substrate and P++ contact implant (using, for example, boron) and activation anneal; iii) Ge selective epitaxy growth with an in-situ Si interlayer growth (Si interlayer thickness&lt;=5 nm); iv) top Ge N++ implant (using, for example, phosphorus) and activation anneal; and v) metallization and deep trench etch. Further details of the method  400  are as follows. 
       FIG. 5A : Si WG etch and p implant into Si 
     Firstly, a Si WG (not shown in  FIG. 5A , but see  FIG. 3A ) is patterned and p ion implantation (using, for example, boron) is performed into a SOI (silicon on insulator) substrate  550  that comprises a top Si slab  552  and a buried oxide layer  508 . With reference to  FIGS. 4A and 4B , this results in the formation of the second semiconductor layer having a region doped to the second polarity of step  404  and, within this region, a greater doped portion to the second polarity of step  410 . The Si slab  552  and the WG height may follow the initial thickness of an SOI wafer, which is around 220 nm. The Si WG width may be around 500 nm with a nanotaper width of around 200 nm at the ends to allow efficient fibre-to-WG coupling and vice versa. 
       FIG. 5B : Selective Ge epitaxy with Si interlayer 
     A dielectric layer  540  (using, for example, SiO 2 ) may first be grown as a mask for selective epitaxy (growth of the layers  514 ,  502  and  506 ) shown in  FIG. 5B . The regions with the dielectric layer  540  will prevent epitaxy growth on the Si slab  552 . Next, a Ge stack  522  is formed by a selective epitaxy process using ultra-high vacuum chemical vapour deposition (UHV-CVD). Other CVD processes can be used, e.g. reduced pressure CVD (RPCVD). Approximately 500 nm of Ge may be grown to form a bulk region  514 . This was followed by in-situ deposition of a 5 nm thick silicon interlayer (Si IL)  502  (resulting in the formation of the dopant diffusion barrier layer of step  406  of  FIG. 4A ) and finally an approximately 50 nm thick top Ge epitaxial layer  506  (resulting in the formation of the first semiconductor layer having a region doped to the first polarity of step  402  of  FIG. 4A ) for junction formation. 
     The epitaxy process to grow the bulk region  514  may be performed using a low temperature Ge seed at around 400° C. and a higher temperature cyclic Ge growth at around 550° C. The Si interlayer  502  may also be grown at around 550° C., while the top Ge epitaxial layer  506  may be grown at around 400° C. 
       FIG. 5C : n-implant and anneal 
     After the Ge epitaxial stack  522  formation, the top Ge epitaxial layer  506  is doped by n ion implantation (using, for example, phosphorus) using a photoresist mask (not shown), which results in the formation of the first semiconductor layer having the higher doped region of the first polarity of step  408  of  FIG. 4A . The Si interlayer  502  serves to counter diffusion of the n dopant into the bulk region  514 , thereby forming the intrinsic layer (of step  408  of  FIG. 4A ) within the bulk region  514 . This implant activation anneal may be generally done at temperatures up to 700° C., but temperatures higher than 700° C. with a shorten anneal time can possibly be used. 
       FIG. 5D : Metallization 
     The implant activation anneal of  FIG. 5C  is followed by interlayer dielectric (ILD)  542  deposition. Contact holes are formed within the ILD  542  that expose portions of the Si slab  552  and portions of the top Ge epitaxial layer  506  for Al metallization  524  to form the metal layer of step  414  of  FIG. 4B . An optional 5 nm cap (not shown) may then be provided for passivation. 
     For material analysis, Ge epitaxial stacks with Si interlayer thicknesses of 2.5 and 5 nm were grown and implanted with phosphorus. Secondary ion mass spectrometer (SIMS) analysis was subsequently carried out to analyze phosphorus (P) diffusion into bulk Ge after different anneals for Ge epitaxial stacks with and without Si interlayer. 
     Transmission electron microscope (TEM) images of the cross-section of a Ge epitaxial stack after P implantation and annealing at around 600° C. for 5 min are shown in  FIGS. 6A, 6B and 6C , where the Ge epitaxial stack has a Si interlayer of around 5 nm thickness (see  FIG. 6C ). The thickness of the top Ge layer can be controlled to fix junction depth. 
       FIG. 6B  shows an enlarged view of portion  602  of  FIG. 6A , while  FIG. 6C  in turn shows an enlarged view of portion  604  of  FIG. 6B . The images show that the Si interlayer remains intact after annealing and little intermixing between Si and Ge has occurred. The Si interlayer is seen to be continuous, which facilitates uniformly preventing dopant diffusion downwards into the Ge stack. 
       FIG. 7A  shows SIMS data for the Ge stack, having the Si interlayer as described above with respect to  FIGS. 6A to 6C . It can be observed that the Si interlayer is effective in preventing P diffusion into bulk Ge (denoted using the reference numeral  702 ). 
       FIG. 7B  shows SIMS data for a Ge stack without Si interlayer. High P diffusion has occurred, which increases the P concentration in Ge (denoted using the reference numeral  704 ) to about 1e18 to 1e19 cm −3 . This increases dark current and reduces photodetector bandwidth due to increased device capacitance at a particular operating voltage. Thus, in comparison, using a Ge stack with a Si interlayer, as shown in  FIGS. 6A to 6C , provides for a photodetector with better bandwidth. 
     Based on the SIMS results, 3 dB RC-limited bandwidths achievable for different Ge stacks of around 500 nm thickness, with and without Si interlayer, are calculated.  FIG. 8  plots these 3 dB bandwidth (RC-limited) results for these Ge epitaxial stacks with different doping concentrations and varying operating bias, where the Ge photodetector is taken to be 8×25 um. After 600° C. anneal, the intrinsic doping concentration is 1e16 cm −3  and 1e18 cm −3  for Ge epitaxial stack with and without Si interlayer, respectively. The RC-limited 3 dB bandwidth (f 3dB,RC ) is calculated using: 
                     f       3   ⁢   d   ⁢           ⁢   B     ,   RC       ≈     1     2   ⁢   π   ⁢           ⁢   RC               (   1   )               
whereby R is the series resistance and C is the device capacitance. The series resistance is estimated to be 100Ω. It is clearly seen that with a lower intrinsic bulk concentration of 1e16 cm −3 , a higher bandwidth is achieved for a photodetector using a Ge stack with a Si interlayer, compared to a photodetector using a Ge stack without a Si interlayer. At Ge thickness of 500 nm for vertical pin Ge PDs, the device bandwidth is not limited by the transit time, since the distance between vertical electrodes is small. Therefore, the calculated RC-limited bandwidth is a good representation of the device bandwidth.
 
     In addition, for a photodetector with thin intrinsic regions (&lt;500 nm), it is preferable to have sharp and abrupt junction profiles. Any variation in intrinsic region thickness due to dopant diffusion would have a significant effect on the device capacitance. It is observed that the junction depth for Si interlayer split is around 30 to 60 nm shallower than the control Ge epitaxial stack. The junction depth is taken when P concentration hits 1e16 cm −3 , since the intrinsic doping concentration of Ge is around 1e16 cm −3 . There is uncertainty on the exact location of the junction because of the knock-on effect of frontside SIMS, and a more precise location of the junction can be known using backside SIMS profiling. Nevertheless, an estimation of the bandwidth can be made. The 3 dB RC-limited bandwidth is calculated to be higher for Ge photodetector with Si interlayer with the same physical epitaxial thickness as one without the Si interlayer. With a targeted i-Ge thickness of 300 nm, the Si interlayer split is estimated to have 10 to 25% higher bandwidth. When the i-Ge thickness drops to 200 nm, this bandwidth gain increases to 20 to 40% over the control Ge photodetector. 
     From the above, a preferred embodiment of the invention provides a Ge epitaxial stack for photodetectors. This epitaxial stack comprises a thin Si interlayer that acts as a diffusion barrier to prevent dopant diffusion into bulk Ge and that can form abrupt junctions. The Ge epitaxy stack has a sandwiched thin in-situ silicon epitaxial layer. Through this Ge stack, shallow and abrupt junctions can be achieved at higher annealing temperatures like 625° C. Enhancement in device bandwidth is predicted from precise control of junction profile in the Ge photodetector using this epitaxial stack. A simple fabrication process flow is used to fabricate such a Ge photodetector, with only an insertion of a thin silicon interlayer during Ge epitaxy process. A design and fabrication process scheme can be implemented to realise vertical pin structure photodetector with applications in both surface-illuminated and waveguided detectors. 
     It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the embodiments without departing from a spirit or scope of the invention as broadly described. The embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.