Abstract:
A photodetector is formed from a body of semiconductor material substantially surrounded by dielectric surfaces. A passivation process is applied to at least one surface to reduce the rate of carrier generation and recombination on that surface. Photocurrent is read out from at least one electrical contact, which is formed on a doped region whose surface lies entirely on a passivated surface. Unwanted leakage current from un-passivated surfaces is reduced through one of the following methods. (a) The un-passivated surface is separated from the photo-collecting contact by at least two junctions (b) The un-passivated surface is doped to a very high level, at least equal to the conduction band or valence band density of states of the semiconductor (c) An accumulation or inversion layer is formed on the un-passivated surface by the application of an electric field. Electrical contacts are made to all doped regions, and bias is applied so that a reverse bias is maintained across all junctions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/210,223 filed by Rafferty et al. on Aug. 23, 2005 and entitled: “Low-Noise Semiconductor Photodetectors.” The foregoing application Ser. No. 11/210,223 is incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The United States Government has certain rights to this invention pursuant to NSF Award DMI-0450487. 
    
    
     FIELD OF INVENTION 
     This invention relates to semiconductor photodetectors for visible and infrared light, and in particular, to low-noise semiconductor photodetectors and methods for making them. 
     BACKGROUND OF THE INVENTION 
     Semiconductor photodiodes are widely used for the detection of light, both visible and infrared. They exploit the internal photoelectric effect, where electron-hole pairs are generated in the semiconductor by photon absorption and contribute to electrical conduction inside the device, leading to a corresponding current at the contacts of the detector. Such detectors are fabricated singly, or in linear arrays for spectroscopy, or in two-dimensional (2-D) arrays for imaging. 
     To create highly sensitive detectors, low noise is desired. Low noise requires that all sources of leakage current in the photodiode should be suppressed to the greatest degree possible. Leakage currents in a semiconductor photodiode arise by a variety of mechanisms, including leakage at surface traps, leakage through bulk traps or defects, quantum-mechanical tunneling between the valence and conduction bands in the semiconductor, spontaneous electron-hole generation through thermal energy, impact ionization, and junction diffusion current. 
     Tunneling leakage can be reduced by employing moderate doping levels and low voltages. Bulk leakage can be reduced by using high-purity materials and by using growth techniques which avoid the formation of crystal defects such as stacking faults, twins, and dislocations. Spontaneous electron-hole generation and impact ionization are negligible in detectors made of an indirect bandgap material such as silicon or germanium. When all these leakage mechanisms have been reduced, surface leakage and diffusion current remain as the dominant leakage mechanisms. 
     Surface leakage is caused by traps at the interface between the semiconductor and any dielectric surfaces which contact it. The traps typically originate due to dangling bonds which result when the semiconductor lattice is terminated. Two types of surface leakage can be distinguished: leakage arising where a depletion region intersects a surface, and leakage where the semiconductor adjoining the interface is doped and charge-neutral. In both cases, leakage will arise whenever an electron-hole pair is generated at a trap on the surface, and the electron and hole make their way to different junctions, causing current to flow in an external circuit. Leakage at a depleted surface is proportional to the intrinsic carrier concentration and therefore depends on temperature as exp (−Eg/2kT) where Eg is the semiconductor bandgap. Leakage at a doped interface varies as exp (−Eg/kT) and is typically much lower. A semiconductor photodetector using the photoelectric effect, such as a P-N photodiode, cannot avoid having a depletion layer intersecting the semiconductor surface. The larger the depletion layer, the more surface leakage. The un-depleted surfaces will also give rise to leakage current even if some means is found to suppress leakage at the depleted surface. 
     Diffusion current is an intrinsic aspect of a diode and cannot be eliminated, though it can be reduced. It arises whenever voltage bias is applied to the diode. The applied voltage disturbs the minority carrier concentrations at the edge of the diode junction from their equilibrium values. The minority carrier concentrations at the contacts are always equal to their equilibrium values. Consequently there is a gradient of minority carriers between the junction and the contacts, giving rise to a steady diffusion current of minority carriers. Under reverse bias, the condition where a photodiode is normally operated, minority carriers flow from the contacts to the junction, where they are continuously swept away by the field to become majority carriers on the other side of the junction. 
     All these sources of leakage current compete with the photocurrent generated by incoming light, and therefore compete with the signal and reduce the signal-to-noise ratio. 
     Photodiodes formed in silicon exploit the highly optimized silicon/silicon dioxide surface. These surfaces, which have extremely low surface recombination velocities, are referred to as passivated surfaces. Such photodiodes are widely used in CCDs and CMOS imagers. However it is desirable to form photodetectors in other materials besides silicon, in order to form images using light of wavelengths to which silicon is not sensitive, e.g., infrared light. 
     Germanium is one material which can be used to form infrared-sensitive photodiodes. Germanium photodiodes have been reported to have undesirably high dark current for many applications. Reported leakage current densities for germanium diodes grown on silicon are of order 1 mA/cm 2 . See references designated [1][2] in the attached Appendix. This is approximately equal to the photocurrent that would be generated by bright sunlight, and represents a high level of leakage. Germanium photodiodes formed in bulk germanium have reported leakage 10-100 times lower [3][4], but this is still not sufficient for imaging indoors or in twilight conditions. To form low leakage detectors, improved devices and processes are needed. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a low noise photodetector comprises a body of semiconductor material substantially surrounded by dielectric material. A portion of the body surface is passivated by a high quality dielectric and a portion is unpassivated. The semiconductor body includes a p-n junction for operation as a photodetector to minimize leakage, the p-n junction (including its depletion region) intersects the semiconductor surface within the passivated portion of the surface, and leakage from the unpassivated surface is minimized by one or more of the following: 1) the body includes opposite polarity p-n junctions (n-p and p-n) in the electrical path between the surface and the photocurrent collector, 2) the body includes a highly doped region in contact with the dielectric, 3) a doped semiconductor outside a thin dielectric provides a charge accumulation region adjacent the interface. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The nature, advantages and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in connection with the accompanying drawings: 
         FIG. 1   a  is a cross sectional view of a first embodiment of a low noise photodiode according to the invention. 
         FIG. 1   b  shows a plan view of the  FIG. 1   b  photodiode. 
         FIGS. 2   a ,  2   b  and  2   c  show a process sequence to create the photodiode of  FIGS. 1   a  and  1   b.    
         FIGS. 3   a  and  3   b  show a cross-section and plan view respectively of a second embodiment of a low noise photodiode. 
         FIGS. 4   a ,  4   b  and  4   c  show an exemplary process sequence to create the photodetector of  FIG. 3 . 
         FIGS. 5   a  through  5   e  show an alternative process sequence to create the detector of  FIG. 3 . 
         FIGS. 6   a  and  6   b  show a cross-section and plan view of a third low noise photodiode. 
         FIGS. 7   a  through  7   g  show a process sequence to create the detector of  FIG. 6 . 
         FIG. 8 , which is prior art, shows a conventional contacting scheme for a photodiode useful in understanding an advantageous additional feature of the invention. 
       And  FIGS. 9A and 9B  illustrate how different embodiments of the invention can be combined in a low noise photodetector. 
     
    
    
     It is to be understood that these drawings are for illustration of the concepts of the invention and are not to scale. 
     DETAILED DESCRIPTION 
       FIGS. 1   a  (cross-section) and  1   b  (plan-view) illustrate a low noise semiconductor photodetector. The semiconductor body  10  is substantially surrounded by dielectric material or materials. A first portion of the semiconductor surface is passivated, and a second portion is unpassivated. Here a high-quality dielectric  12  with a low surface recombination velocity is formed on the top surface of the semiconductor body  10  to passivate the top surface. The body is peripherally surrounded by low quality dielectric  20  that does not passivate the peripheral surface. The semiconductor body  10  is doped p-type. An n-type area  14  is formed in the body, peripherally enclosed by the p-type layer and forming a junction  24  between the n-type and p-type layer. A second p-type layer  16  is formed in the n-type layer  14 , peripherally enclosed by the n-type layer and forming a junction  22  between the inner p-type and n-type layers ( 16 , 14 ). Ohmic metal contacts  30 ,  32 , and  34  are formed to all the doped semiconductor layers ( 16 ,  14 , and  10 ). Photocurrent is detected on contact  30 , which is biased relative to contact  32  so that a reverse bias exists on the junction  22 . Bias is applied to contact  34  so that either zero or reverse bias exists across the junction  24 . 
     The p-n junctions  22 ,  24  including their respective depletion regions intersect the surface of the body  10  in respective intersection regions  22 A,  24 A. Leakage is minimized by keeping these intersection regions within the passivated portion of the semiconductor surface. 
     Moreover, in this embodiment, any carrier generated at the interior unpassivated dielectric surfaces  40  must cross two junctions of opposite polarity (p to n and n to p) in order to reach region  16  and the contact  30 . If the carrier is a hole, it will preferentially stay in the p-type layer  10  and be collected at contact  34 . If it is an electron, it will enter the n-type layer  14  and will then preferentially stay there, to be collected at contact  32 . Thus both types of carriers generated at the unpassivated surface will be prevented from reaching the photo-collecting contact  30 . 
     A further optimization of the structure to improve the quantum efficiency is to grade the middle doped layer (n-type in this example) so that the doping is lower near the center of the well and higher near the edge. This creates a barrier for photocarriers generated in the n-type region so that the photogenerated holes from the n-region  14  will preferentially be collected at the center “p” contact  30  rather than at the perimeter “p” contact  34 . Such a graded doping profile is likely to arise naturally if the doping is created by ion implantation, but the effect can be enhanced by judicious choice of implant energy and dose. 
     Although the device has been described as p-n-p, it should be appreciated that a corresponding n-p-n implementation is equally practical by appropriate choice of doping. 
     A process sequence to create the  FIG. 1  device is illustrated by  FIGS. 2   a ,  2   b , and  2   c . A semiconductor body  10  is formed within a cavity inside a dielectric layer  20 , for instance by the technique disclosed in U.S. patent application Ser. No. 10/453,037 filed by J. Bude et al. on Jun. 3, 2003 and entitled “Semiconductor Devices with Reduced Active Region Defects and Unique Contacting Schemes” which is incorporated herein by reference. The &#39;037 application claims the benefit of U.S. Provisional Application Ser. No. 60/434,359, filed by J. Bude et al. on Dec. 18, 2002, and this provisional application is also incorporated herein by reference. A passivation process is then applied to the top surface of the semiconductor  10  to create a high-quality dielectric layer  12 . Such methods are known to those skilled in the state of the art. For silicon, passivation can be achieved by growing a high quality silicon oxide dielectric on the surface. For germanium, a highly passivated surface can be created using germanium oxynitride, as demonstrated in reference [5]. 
     Referring to  FIG. 2   b , photoresist  50  is then applied to the wafer and an opening is formed above the semiconductor body. N-type ions, such as, phosphorus or arsenic, are implanted to form the n-type layer  14 . If a strongly graded profile is desired, as described above, a series of chained implants can be used, combining high dose, high energy and lower dose, lower energy implants. The first level of photoresist is then removed. 
     As shown in  FIG. 2   c , a second level of photoresist  52  is then deposited and patterned to form a hole above the interior of the n-type layer  14 . P-type ions, such as boron, are implanted to form the p-type layer  16 . The second level of photoresist is removed. Ohmic contacts are then made to the semiconductor surface using methods familiar to those skilled in the art. 
       FIG. 3   a  (cross section) and  FIG. 3   b  (plan view) show a second photodetector in accordance with the invention. The semiconductor body  210  is doped p-type, and a portion of the surface is passivated with a high-quality dielectric  212 . An n-type layer  242  is created inside the body. Around the surface of body  210 , a p-type layer  244  of heavy doping is created. The doping should be at least as high as the density of states in the material in order to suppress carrier generation around the dielectric/semiconductor interface  240 . A contact  250  is made to the n-type layer and photocurrent is read out from this contact. A p-type contact  252  is formed to the p-type layer, and bias is applied so that the junction between the n-type and p-type regions is reverse-biased. 
     A process sequence to create the  FIG. 3  device is illustrated by  FIGS. 4   a ,  4   b , and  4   c . A crystalline semiconductor body  210  doped p-type is surrounded by dielectric material or materials  220 . A passivation process is then applied to the exposed top surface of the semiconductor  210  to create a high-quality dielectric layer  212  (the passivation may take place after implantation to save a step). Such methods are known to those skilled in the state of the art. For the semiconductor germanium, a highly passivated surface can be created using germanium oxynitride, as demonstrated for example in reference [5]. A layer of photoresist  260  is deposited and patterned to protect most of the semiconductor region. Ion implantation is then carried out to create a high level of p-type doping  244  around the sides, and the photoresist is removed. 
     As shown in  FIG. 4   b , a second ion implantation is carried out with the energy adjusted so that the peak of the implant is close to the bottom of the doped layer. By this combination of implants, a continuous high level of doping is created around the semiconductor/dielectric interface  240 . A second level of photoresist  262  is deposited ( FIG. 4   c ) and patterned to create a hole inside the lightly doped body. An n-type layer  242  is formed by ion implantation to create the cathode of the diode. Metal contacts are formed to the n-type and p-type layers in the usual way. 
     An alternative process sequence for this device is illustrated in  FIGS. 5   a - e . Before the semiconductor is grown to fill the cavity in the dielectric layer  220 , a second dielectric layer  270  doped with a high level of boron is deposited to coat the inside of the dielectric cavity ( FIG. 5   a ). The semiconductor body  210  is formed as before ( FIG. 5   b .) A high quality dielectric layer  212  is formed on the top surface of the semiconductor ( FIG. 5   c ). During this or subsequent heat treatments, boron diffuses out of the surface of the dielectric  270  and enters the semiconductor  210 , thus forming heavily boron doped region  244 . The device is then masked with photoresist  262  and patterned with a hole above the semiconductor, and n-type dopants are implanted through the hole to form the n-type layer  242  ( FIG. 5   d ). Contacts are created to the n-region and p-region in the usual manner. 
     If the surface doping layer  244  is not wide enough to allow a contact to be easily formed, a supplementary mask  272  and ion implantation ( FIG. 5   e ) may be employed to create additional p-type doping  274  on the top surface, to which a contact may be made. 
     Although the device has been described as p-n, it should be appreciated that a corresponding n-p implementation is equally practical by appropriate choice of doping. 
     A third embodiment of a low noise photodetector is illustrated in  FIGS. 6   a ,  6   b . A crystalline semiconductor body  310  doped p-type is peripherally surrounded by low quality dielectric material or materials  320 . A high-quality dielectric  312  with a low rate of generation and recombination passivates the top surface of the semiconductor body. Surrounding the outside of dielectric  320  is a polysilicon semiconductor  316  doped heavily with the same polarity as the body of the semiconductor body  310 . By appropriate choice of the thickness of the dielectric layer  320 , an accumulation layer of “holes” will be formed on the interface  340  between the crystalline semiconductor body  310  and the dielectric layer  320 . An n-type area  342  is formed in the body, peripherally enclosed by the p-type layer and forming a junction  324  between the n-type and p-type layer. The p-type body  310  and the n-type area  342  should have a doping level high enough not to be depleted of mobile carriers. A highly doped p-type area  344  is formed so that it touches the accumulation layer at the interface  340 . Ohmic metal contacts  350   352  are formed to the doped semiconductor layers  342  and  310 , respectively. Photocurrent is detected on the center contact  350 , which is biased relative to contact  352  so that a reverse bias exists on the junction  324 . 
     A process sequence for creating the detector of  FIG. 6  is shown in  FIGS. 7   a - g . A prepared dielectric cavity  300  is coated with a polysilicon semiconductor layer  316 . The layer may be doped as it is grown by in-situ doping, or it may be doped subsequently by exposure to appropriate gases or to ion implantation. A dielectric layer  320  is then deposited on the polysilicon semiconductor layer  316  ( FIG. 7   b ). The thickness of the dielectric layer  320  is chosen to allow the field of the polysilicon doped layer to attract holes to the surface of the crystalline semiconductor which will be subsequently formed and to ensure that dielectric pinhole failures do not compromise the epitaxial growth of the semiconductor body  310 . The thickness is preferably in the range 5-50 nm. The crystalline semiconductor body  310  is then formed, advantageously by the technique as disclosed in the aforementioned Ser. No. 10/453,037. A high-quality dielectric layer  312  is formed on the crystalline semiconductor body  310  as previously described ( FIG. 7   d ). A resist mask  362  is deposited and patterned to form a hole above the body of the semiconductor  310  ( FIG. 7   e ). N-type ions are implanted to form an n-type layer  342 . The resist is removed. A resist mask  364  is deposited and patterned to form a hole adjoining the perimeter of the semiconductor  310 . ( FIG. 7   f ). P-type ions are implanted to form a p-type layer  344 . The resist is removed. In  FIG. 7   e , the surface dielectric and the polysilicon are removed from the field. Contacts are formed to the n-type and p-type doped areas in the usual way. 
     Although the  FIG. 6  embodiment has been described using an accumulation layer of holes, an inversion layer could also be created by doping the polysilicon layer  316  the opposite to the body  310 . The doped area  344  should then also be n-type, and a separate contact to the p-type body  310  should be made on the top surface. 
     It will also be appreciated that the scope of the invention also includes a corresponding device similar to  FIG. 6  with all the polarities reversed. 
     A further feature of the photodiodes described herein is can be seen by comparing  FIG. 3   b  or  FIG. 6   b  with the conventional photodiode of  FIG. 8 . In  FIGS. 3   b  and  6   b  the annular doped region is contacted at only one point. This is an explicit design decision. In the conventional contacting scheme shown in  FIG. 8  there are many contacts covering much of the area where light might otherwise enter the device. Conventional wisdom teaches that the annular region should be fully contacted by metal in order to minimize contact resistance. As many vias as possible should be sunk from the metal layer to the semiconductor layer, consistent with the via formation design rules. In contrast, applicants here have recognized that each contact contributes to the diffusion current of the diode, and that the number of contacts should in fact be minimized. At the low current levels at which most photodetectors operate, the contact resistance is unimportant. It is much more important to limit the dark current, which competes with the signal. Simulations show that reducing the number of contacts in a square detector from 28 to 4, with one at each corner, should reduce diffusion current by about a factor of 10. Reducing the number of contacts further, to just one, should lead to a further reduction in diffusion current. Therefore low-noise photodetectors of the type described herein advantageously have no more than 30% and preferably no more than about 25% of the available heavily doped area actually covered by contacts. 
     It is also possible to combine two or more of the approaches described in connection with  FIGS. 1 ,  3  and  6  in a single device. For instance, one unpassivated face may be neutralized by the approach of  FIG. 1  and the other unpassivated faces neutralized by the approach of  FIG. 3 , and so on. 
       FIG. 9   a  (cross-section) and  FIG. 9   b  (plan-view) exemplify such a combination. A semiconductor body  410  doped p-type has a passivating dielectric  412  on the top surface and unpassivating dielectrics  420  on the sides and bottom. A double junction is formed in the vertical direction, for instance by ion implantation, so that an n-type layer  444  separates the upper and low p halves of the device. Heavy n-type doping  446  is used to reduce the minority carrier density along the sidewalls  440 . It also creates a conducting path  446  from the n-contact  452  to the buried n layer  444 . Photocurrent is read on contact  450 . The bottom p section of the well can be contacted from below. The device combines the double junction technique to neutralize the bottom and the heavy doping technique to neutralize the sidewalls. 
     It can now be seen that one aspect of the invention is a low noise photodetector comprising a body of semiconductor material. The body has a surface substantially surrounded by dielectric material and comprising a first portion that is passivated and a second portion that is unpassivated. The body also comprises a first region doped to a first type of conductivity (p or n) and a second region doped to the second type of conductivity (n or p), the two regions forming a first p-n junction. 
     The first p-n junction intersects the surface of the body in an intersection region that is within the passivated portion of the body surface, and the device is adapted to minimize leakage current from the unpassivated second portion of the body surface by one or more of the following: 
     1) the body includes a third doped region to form a second p-n junction in the path between the unpassivated surface (or a part thereof) and the first region, the second p-n junction having a polarity opposite the first junction, 
     2) the region of the semiconductor body that is adjacent the unpassivated surface portion is highly doped to suppress carrier generation at the unpassivated surface, and 
     3) a highly doped semiconductor is disposed around and in contact with the dielectric adjacent the unpassivated portion of the semiconductor surface to form an accumulation layer or an inversion layer on the unpassivated surface. 
     While the above description contains many specific examples, these should not be construed as limitations on the scope of the invention, but rather as examples of several preferred embodiments. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents. 
     APPENDIX OF REFERENCES 
     
         
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