AVALANCHING SEMICONDUCTOR DEVICE HAVING AN EPITAXIALLY GROWN LAYER

Abstract: An avalanche diode device that has a first layer of semiconductor material (12) and a second layer of semiconductor material (18) positioned thereon so as to define an interface (14). The first layer of semiconductor material (12) is a substrate and the second layer (18) comprises an avalanche layer wherein a depletion region is generated at the interface. The second layer (18) is preferably deposited on the first layer (12) through chemical vapor deposition epitaxial growth techniques. When photons enter the avalanche layer, a minority charge carrier is produced and, when the device is reversed biased, an avalanche of charge carriers is produced in the avalanche layer. Since the avalanche layer is deposited using epitaxial growth techniques, the uniformity of the layer produces higher gain uniformity. Further, the avalanche layer can be configured using the epitaxial growth techniques to have a gain region of higher dopant concentration located adjacent the interface with a spectral absorption layer with a lower dopant concentration interposed between the gain region and the surface that receive the photons. Hence, a higher quantum efficiency can be produced in an avalanche photodiode having this configuration while still maintaining a high degree of gain uniformity and low noise characteristics.

Description: AVALANCHING SEMICONDUCTOR DEVICE HAVING AN EPITAXIALLY GROWN LAYER Background of the Invention Field of the Invention The present invention relates to semiconductor avalanche devices and, in particular, concerns an avalanche photodiode device that has two layers of semiconductor material that form a p-n junction wherein an avalanche of charge carriers is produced when the diode is reverse biased in response to generation of one or more charge carriers in one of the layers wherein one of the layers is grown using epitaxial growth techniques. Description of the Related Art Avalanche semiconductor devices are devices that are comprised of two layers of semiconductor material that are positioned adjacent one another so that an electric field and a depletion region is formed at the interface between the two layers. One such device is an avalanche photodiode (APD) which is typically comprised of a p-n junction that is reversed biased by an applied voltage so that when a photon impinges on a first layer of the avalanche photodiode and is absorbed, a charge carrier is produced in the first layer and the electric field moves the charge carrier towards the interface. In an avalanche photodiode, the first layer is configured so that an avalanche of charge carriers are generated in response to the charge carrier produced as a result of the absorption of the photon travelling towards the interface. These types of devices are particularly useful in applications where an electrical signal is desired in response to the sensing of light. One desired characteristic of an avalanche photodiode is gain uniformity over the surface of the device. In other words, the number of charge carriers produced in one region of the APD in response to a single photon impinging on that region should be substantially the same as the number of charge carriers produced in a different region of the APD in response to a photon impinging on the surface of the different region of the APD. Gain uniformity is particularly desired for large area avalanche photodiode (LAAPD) applications. Presently, LAAPD devices are available that have surface areas in excess of 20 mm 2 . It is desirable that over the entire surface area of the APD, that the electrical signal produced by the device is substantially uniform regardless of where the photon impinges on the device. It is understood that gain uniformity is a function of the doping profile of the layer of the APD that receives the photons. Specifically, doping inconsistencies within this layer can result in the electric field between the layers varying from region to region within the APD. These variations in the electric field can further result in fewer or greater charge carriers being produced by different regions of the device in response to a photon impinging upon the region of the layer containing the doping inconsistencies. Typically, in the prior art, the p-n junctions which form the APD devices are fabricated by diffusing a p- doped region into an n-doped substrate using well-known diffusion techniques. One difficulty with the well-known diffusion techniques used in producing APD's is that there is often disuniformity of the doping profile. Consequently, prior art APD's suffer from non-uniformity of gain across the surface area of the APD. This non-uniformity of gain is exacerbated in LAAPD devices. A further difficulty that results from forming p-n junctions for APD devices using diffusion techniques is that the interface between the p-region and the n-region is generally diffuse. Specifically, the exact position of the interface over the entire surface area of the APD varies across the surface area of the interface. In some regions the p-dopant will have diffused farther into the n-substrate than in other regions. Consequently, in some regions of the APD, the avalanching of charge carriers will occur at a region that is farther from the p-n interface than in other regions. This results in a reduction of gain uniformity and also produces an increase in the noise of the circuit. Overall, the diffuse interface between the p and the n region produces a less sensitive APD and this problem is exacerbated in large area avalanche photodiode applications (LAAPD). Another desired characteristic of APD devices is high quantum efficiency. Quantum efficiency is defined as the ratio of electron hole pairs that is created in a region of an APD to the number of photons that are incident on the region. It will, of course, be appreciated that increased quantum efficiency results in more sensitive APD devices. It is understood that the quantum efficiency of a semiconductor is dependent upon several factors, one being the characteristic absorption depth of the photons impinging upon the device. In particular, photons impinging upon semiconductor material will be absorbed so as to generate an electron hole pair within a certain distance. A measure of this distance is the characteristic absorption depth. For certain wavelengths of photons, the characteristic absorption depth is much greater than the typical thickness of the p-region which receives the photons in an APD device. For example, many APD devices have a p-layer that is on the order of 30 microns wherein the characteristic absorption depth of a photon having a wavelength of 1.06 microns is on the order of 770 microns. Hence, since the thickness of the p-layer is so small when compared to the characteristic absorption depth, the percentage of photons that are absorbed to form electron hole pairs that will ultimately avalanche is small. It is possible to increase the thickness of the absorbing region to improve the quantum efficiency, however, using existing diffusion techniques, the doping of the layer at any given depth is typically cross-sectionally non- uniform across the layer. Increasing the thickness of the region containing this doping results in more non-uniformity and less uniform gain across the device due to increased probability of avalanching occurring at different distances from the interface. Consequently, it is difficult to produce a sensitive APD that has a high quantum efficiency and still maintains a high uniformity of gain across the surface area of the device. This difficulty is particularly exacerbated for APD's that are designed to be used with wavelengths that have a long characteristic absorption depth such as 1.06 microns wavelength light. Hence, there is a need for an APD device and a process for manufacturing the same that has improved uniformity of doping characteristic at any cross-section to the junction formed by the interface between the p and the n regions of the semiconductor material comprising the APD, as well as a precise uniform junction that does not vary in its depth. There is also a need for an APD device that has improved quantum efficiency and is therefore more sensitive that still maintains a high degree of gain uniformity and low noise characteristics. Summary of the Invention The aforementioned needs are satisfied by the avalanche photodiode device of the present invention which comprises a first semiconductor layer having a first doping characteristic and a first crystal structure and defining a first surface and a second semiconductor layer deposited on the first semiconductor layer so that the second semiconductor layer forms an interface with the first layer at the first surface and extends outward therefrom. The second semiconductor layer has a second doping characteristic and the first crystal structure and defines an outer surface. Further, when the second layer is positioned on the first layer, a depletion region is formed at the interface between the layers so that when the junction between the layers is reversed biased, an avalanche of charge carriers is produced in response to a photon penetrating into the device. In the preferred embodiment, the first semiconductor layer is comprised of a substrate of n-type silicon and the second semiconductor layer is comprised of a layer of p-type silicon that is epitaxially grown using chemical vapor deposition techniques on the n-type substrate. This results in a p-n junction that has a very sharp interface between the p and the n region. This results in a reduction in the non-uniformity of gain across the surface of the interface that occurs as a result of the avalanching of charge carriers occurring at different distances from the interface at different positions on the device. Further, using chemical vapor deposition techniques, an improvement in the uniformity of doping characteristics of the p-type silicon region can be achieved which can result in improved gain uniformity for the device. Consequently, the devices of the preferred embodiment have an avalanching layer that is formed using epitaxial growth processes. This allows for tailoring the characteristics of the avalanching layer so as to optimize the performance characteristics of the device. In the preferred embodiment, the avalanching layer can be optimized so that the avalanching layer achieves a high current gain in response to a photon impinging on the avalanching region while still maintaining a high degree of gain uniformity. Additionally, the use of epitaxial growth processes to form the avalanching layer allows for modifications of the avalanching layer to obtain desired performance characteristics. In particular, another aspect of the present invention is that the second layer of the avalanche photodiode has a doping profile wherein a region of the second layer that is located a first distance from the interface between the first and second layers has a first doping concentration and a region of the second layer that is located a second distance from the interface between the first and second layers has a second doping concentration. The first doping concentration is greater than the second doping concentration and the first distance is less than the second distance. In the preferred embodiment, the epitaxially grown p-layer is the avalanching layer and it has a region of increased doping concentration that is located adjacent to the interface between the p and the n layers. It will be appreciated that using chemical vapor deposition epitaxial growth techniques, buried layers of increased doping concentration can be positioned within a region of an avalanching layer that receives photons. Consequently, one application of this aspect of the invention is that a uniform region having a high doping concentration that is selected to provide a particular gain characteristic for the device can be grown in the region and then a thicker spectral absorption region having a lower doping concentration can then be grown using the epitaxially growth techniques. This can be used to form an avalanche photodiode that has an avalanche layer having a thick spectral absorption region and a more highly doped buried gain region positioned adjacent the interface. The thicker spectral absorption region increases the quantum efficiency of the APD and therefore makes the device more sensitive. However, the buried gain region maintains a desired current gain for the device without sacrificing gain uniformity. Specifically, in the prior art device, thicker absorption regions were typically only possible by increasing the thickness of the layer so that the entire layer had a uniform high doping concentration. This increases the possibility of avalanching occurring at different distances from the interface. Using the epitaxial growth process, however, a thinner buried gain region can be grown with a thicker lightly doped spectral absorption layer positioned between the light receiving surface and the buried gain region. Another aspect of the present invention is that a process is provided wherein the doping profile of one of the layers of the avalanche device can be tailored to achieve particular performance characteristics. Specifically, using chemical vapor deposition epitaxial layer growth, different regions of the layer can be grown to have particular doping concentrations that will result in particular performance characteristics. In one embodiment of this aspect of the invention, the process comprises growing an epitaxial layer that has a doping profile such that there is a region that has a dopant concentration of 1 x 10' 6 dopant atoms per cubic centimeter that is on the order of 7 microns thick and is positioned immediately adjacent the interface. The layer also includes a second region having a dopant concentration of approximately 5 x 10" dopant atoms per cubic centimeter wherein the second region is positioned on the surface of the first region that is located distal from the interface and is approximately 100 microns thick. It will be appreciated that in this aspect of the present invention, the process allows for the region of increased dopant concentration to be positioned at varying distances from the interface to achieve different performance characteristic of the resulting device. From the foregoing, the present invention provides an avalanche device that has improved performance characteristics. In particular, the device has improved uniformity of gain and lower noise characteristics. Further, the process of fabricating the device of the present invention allows for great flexibility in configuring the layer of the device to achieve particular performance characteristics. These and other objects and advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings. Brief Description of the Drawings Figure 1A is a schematic illustration of a first embodiment of a bevelled edge avalanche photodiode having a first layer and a second layer extending outward therefrom wherein the second layer is formed using epitaxial growth techniques; Figure 1B is a graph illustrating the doping profile of the device of Figure 1A; Figure 2A is a schematic illustration of a second embodiment of a bevelled edge avalanche photodiode wherein the avalanche photodiode includes a first layer of semiconductor material and a second layer of semiconductor material and wherein the second layer is formed using epitaxial growth techniques so that it has a region of higher doping concentration immediately adjacent an interface between the first and second layers; Figure 2B is a graph which illustrates the doping profile of the device of Figure 2A; Figure 3A is a schematic illustration of a third embodiment of a bevelled edge avalanche photodiode device having a first layer and a second layer positioned thereon to form an interface wherein the second layer is formed using epitaxial growth techniques so that it has a region of increased doping concentration that is positioned within the second layer at a position that is spaced from the interface; and Figure 3B is a chart illustrating the doping profile of the device of Figure 3A. Detailed Description of the Preferred Embodiment Reference will now be made to the drawings wherein like numerals refer to like parts throughout. Figure 1A illustrates a first embodiment of an avalanche photodiode 10 that is prepared using a unique fabrication technique. Specifically, Figure 1A shows a first semiconductor layer 12 which is comprised in this embodiment of an n-type single crystal silicon substrate having resistivity of typically 60 to 90 ohm cm. The substrate 12 defines a first surface 14. A second semiconductor layer 16, which in this embodiment comprises an avalanching layer, is formed on the first surface 14 of the substrate 12 so that the first surface 14 defines an interface between the substrate 12 and the avalanche layer 16. The avalanche layer 16 is the layer where the majority of an avalanche of charge carriers will be produced in response to one or more photons impinging upon the device 10. It will be appreciated, however, that a small amount of charge carriers will be produced by avalanche in other regions of the device 10 including the substrate 12. In the preferred embodiment, the avalanche layer 16 is comprised of a p-type single crystal surface layer that is epitaxially grown on the n-type single crystal substrate 12 in a manner that will be described in greater detail hereinbelow. The avalanche layer 16 has an outer surface 19. In the preferred embodiment, a contact layer 20 is diffused onto an outer surface 19 of the avalanche layer 16 using well-known diffusion techniques. An ohmic contact 26 comprised of a thin n+ + doped silicon layer can also be formed on a bottom surface 24 of the substrate 12 through diffusion techniques. Figure 1A also illustrates an electrode structure 21 formed on the upper surface 19 of the device 10. In the preferred embodiment, the electrode structure 21 is preferably comprised of silicon and provides a contact for a voltage that is applied to the avalanche photodiode device 10. It will be appreciated that the electrode structure 21 may also comprise a reactive metal such as molybdenum, tungsten, titanium or other contact metals such as gold, aluminum and platinum. It will also be appreciated that the electrode structure 21 is generally shaped as a ring to expose a large area of the outer surface 15 of the avalanche layer 16 to light as possible. It is understood that positioning the p-type avalanche layer 16 on the n-type substrate 12 will result in a depletion region 25 (represented by dashed lines in Figure 1A) being formed in the substrate 12 and the avalanche layer 16, when a biasing voltage is applied between the electrode structure 21 and the contact 26. In the preferred embodiment, the doping process of the p-type layer 14 is performed so that the depletion layer extends almost to the upper surface 15 of the avalanche region 16. Figure 1A also illustrates that the avalanche photodiode 10 has bevelled side surfaces 28a and 28b that are bevelled so that when a biasing voltage is applied between the electrode structure 21 and the contact 26, the strength of the electric field at the edge of the device 10 does not exceed the strength of the electric field within the device 10 in a manner that is known in the art. In the preferred embodiment, the avalanche photodiode device 10 is circular in configuration and the bevelled edges 28a and 28b extend around the entire outer circumference of the device 10. Figure IB shows the dopant profile of the avalanche photodiode 10 of Figure 1A in greater detail. In particular, the avalanche layer 16 is comprised of a layer of p-type material that has a dopant concentration of approximately 2 x 10' 5 dopant atoms per cm 3 and has a thickness of approximately 12 microns. Figure 1B also illustrates that the n-type substrate 12 in this preferred embodiment has a dopant concentration of approximately 1 x 10" dopant atoms per cm 3 and is greater than approximately 300 microns thick. Figure 1B also illustrates that the contact region 20 which has a dopant density of approximately 1 x 10 1β dopant atoms per cm 3 and is approximately 0.2 microns thick. Similarly, the n-contact 24 has a dopant concentration of approximately 1 x 10 20 dopant atoms per cm 3 and is approximately 0.2 microns thick. It will be appreciated that positioning the avalanche layer 16 on the n-substrate 12 with the dopant concentration shown in Figure 1B results in an electric field being formed within the depletion region 25 of the device 10, when a biasing voltage is applied between the electrode structure 21 and contact 26. The electric field (represented by dashed lines in Figure IB) has a peak value of approximately 2 x 10 s volts/cm. It will be further appreciated that Figures 1A and 1B, as well as Figures 2A, 2B, 3A and 3B described hereinbelow are not drawn to scale and are simply illustrative of the basic configuration of the devices. As illustrated in Figure 1B, the dopant profile of the avalanche layer 16 is such that there is a very sharp or abrupt interface between the p-doped avalanche layer 16 and the n-doped substrate 12. In particular, using the epitaxial growth process that is described below, the interface between the p and the n layers is very abrupt in that the layer of p-doped silicon essentially begins at the outer surface 14 of the n-doped substrate 12 so that the p- doped region 14 has the same crystalline structure as the n-doped region 12. In operation, a reverse biased voltage is applied between the electrode structure 21 and the contact 26 that is between approximately 1000-3000 volts. Consequently, when a photon enters the avalanche layer 16 through the contact layer 20, an electron hole pair is preferably formed within the undepleted region and then diffuses to the depletion region 25 and a minority charge carrier, i.e., electrons in this embodiment, is then swept towards the interface 14 by the electric field created by the applied reverse bias voltage. This results in an avalanche of charge carriers being swept towards the interface between the avalanche layer 16 and the n-substrate 12 and the device 10 preferably exhibits a current gain on the order of 200. Preferably, the avalanche of charge carriers results in a measurable electrical signal in response to the photon impinging upon the avalanche layer 16. It will be appreciated that since the avalanche layer 16 has a uniform dopant profile as a result of the growth of the avalanche layer 16 using CVD epitaxial growth techniques, that there is a very high degree of spatial uniformity of the dopant atoms within the avalanche layer 16. This results in a high degree of uniformity of the current gain resulting from the avalanching process. Further, the epitaxial growth process also results in an absence of crystalline processing defects. For example, using the process described herein an APD having a gain of 300 and an area of more than 40 mm 2 had a gain uniformity of 3% as compared to 5% for comparable prior art devices. This reduction in the crystalline defect rate and the improvement in doping uniformity results in an improvement in the gain uniformity characteristics of the device. Additionally, because there is a sharp interface between the substrate 12 and the avalanche layer 16, the uniformity of the gain across the interface is also very high and the resultant noise is very low as the avalanching effect is likely to occur at substantially the same distance from the interface 14 throughout the entire surface area of the avalanche photodiode 10. The p-region 16 is preferably grown using a chemical vapor deposition epitaxial growth process. Initially positioned within a prior art chemical vapor deposition epitaxial chamber is a graphite susceptor that provides mechanical support for the wafer and to transfer thermal energy to the wafers. The wafers are processed sequentially, wherein they are placed one at a time on the susceptor in a cassette to cassette process. Prior to epitaxial deposition, the susceptor must be passivated. In the epitaxial chamber the passivation process comprises an in situ gas phase etching of the susceptor surface by using a mixture of HCI (etchant gas) and H 2 (carrier gas) containing H 2 + HCI at about 1 90°C for 5-7 minutes. The etch is typically followed by a CVD process wherein a silicon source gas is flowed through the chamber in combination with a hydrogen carrier gas. A typical silicon source gas such as trichlorosilane (SiHCLj) thermally reacts at the surface of the susceptor causing deposition of silicon. The resulting deposition passivates the susceptor surface and prevents potential contamination of the silicon wafers during the subsequent epitaxial growth process. Following the surface passification of the susceptor, an n-type silicon wafer is placed on the susceptor in the CVD chamber in a manner that is known in the art and the wafer is preheated in the H 2 carrier gas at approximately 1150°C for three minutes to remove any remaining nascent oxide from the surface of the silicon wafers. Subsequently, the p-type avalanche layer 16 is then deposited using chemical vapor deposition by flowing a typical gas combination of a silicon source gas such as trichlorosilane (SiHCI 3 at 0.18 standard liters per minute), a p-type dopant gas such as diborane (B 2 H Θ ) at 0.18 standard liters per minute and a carrier gas such as hydrogen (H j ) at 48 standard liters per minute through the CVD epitaxial chamber at temperatures of between 1100 and 1200 C at approximately atmospheric pressure. This results in CVD deposition of the p-type layer 16 that has a growth rate of approximately 4.26 microns per minute. This growth rate is continued until the desired thickness of the p- layer is achieved which, in the embodiment shown in Figures 1A and 1B is 12 microns. Further, the p-type dopant gas in the CVD chamber is diluted to provide the doping concentration that is desired for the avalanche layer 16 which, in the embodiment shown in Figure 1, is 2 x 10 *5 dopant atoms per cm 3 . Two key parameters inherent in the available state of the art epitaxial reactor insure uniform growth of the epitaxial layer with the desired doping characteristics over very large areas. The utilization of a rectangular shaped shower head gas injection system provides planar flow of the reactant gases to the wafer surface allowing growth of highly uniform thickness epitaxial layers. The use of advanced computer controlled mass flow controllers further allow precise metering of reactant gas flow resulting in extraordinary control and reproducibility of epitaxial layer doping concentration. The above-described epitaxial growth procedure results in a very uniform avalanche layers 16 being grown to a desired thickness on the n-substrate 12. The CVD epitaxial growth technique further allows for precise control of the dopant rate and the dopant profile within the layer 16. Moreover, it permits growing layers that have a thickness of between less than 1 to greater than 100 microns over an area at least as large as 20 mm 2 while maintaining a low defect density (on the order of 10 or less over a wafer layer having a 100 mm diameter). Further, using the epitaxial growth method described hereinabove reduces the need for a commonly used post doping and annealing step that is used with diffusion and implantation based techniques for forming APD's to reduce non- uniformity and crystalline defects. Figure 2A illustrates another embodiment of an avalanche photodiode 30 having an epitaxially grown avalanche layer. The avalanche photodiode 30 includes a substrate 40 which, in the preferred embodiment, is comprised of an n-doped layer that has a first surface 42. A second layer of a semiconductor material 44 comprising an avalanche layer is then deposited on the surface 42 of the substrate 40 so as to define an interface at the surface 42. In this embodiment, the avalanche layer 44 is comprised of a p-doped layer of silicon wherein the p- doped layer of silicon includes a gain region 48 of heavily p-doped silicon and a more lightly doped spectral absorption region 46 of /7-doped silicon. The avalanche layer 44 is epitaxially grown on the surface 42 using an epitaxial growth process similar to the process described hereinabove, except that the growth process initially results in the more heavily doped gain region 48 being grown first immediately adjacent the interface 42. Subsequently, the more lightly doped rr region 46 is then grown on top of the more heavily doped p-region 48. Further, in the embodiment shown in Figure 2A, a more heavily ρ+ doped depletion stop layer 50 is then grown on an outer surface 45 of the π region 46 using CVD epitaxial growth techniques. A p+ + doped contact surface 52 is then grown on an outer surface 51 of the p+ region 50 using known diffusion techniques. An electrode structure 54 similar to the electrode structure 21 described hereinabove is then positioned on an upper photon receiving surface 53 of the device 30. Further, an electrical contact 56 is grown on a bottom surface 55 of the semiconductor substrate 40. In the preferred embodiment, the depletion stop region which comprises a more highly doped region of semiconductor material, is positioned on the outer surface of the avalanche region so that the depletion region does not extend to the electrode structure. Figure 2B illustrates the doping profile of the avalanche photodiode 30 in greater detail. In particular, the substrate 40 is comprised of n-doped silicon having a dopant concentration of approximately 6 x 10" dopant atoms per cm 3 and a resistivity of 60-90 ohm cm. The buried p-doped region 48 has a dopant concentration of approximately 2 x 10 15 dopant atoms per cm 3 and is approximately 10 microns thick. The π region 46 has a dopant concentration of 5 x 10 12 dopant atoms per cm 3 and is approximately 100 microns thick. The depletion stop layer 50 has a dopant concentration of approximately 10 18 dopant atoms per cm 3 and is approximately 5 microns thick. The photon receiving surface 52 and ohmic contact 56 both have a dopant concentration of approximately 10' a dopant atoms per cm 3 and each is approximately .2 μm thick. It will be appreciated that using CVD epitaxial growth process of the preferred embodiment, that an avalanche photodiode can be produced that has a region which has a higher doping concentration located adjacent the interface 42 between the n-substrate 40 and the p-region 46. In other words, the doping profile of the epitaxial grown p layer 43 can be varied to achieve different performance characteristics of the device. In particular, the avalanche photodiode 30 shown in Figure 2A is configured to maximize the response to photons having a wavelength on the order of 1.06 microns. It is understood that the quantum efficiency of the device is increased when the thickness of the avalanche layer of the avalanche photodiode is increased. However, in the prior art the difficulty with increasing the avalanche layer is that the avalanche layer generally has to be of a single dopant concentration that is selected to provide the desired gain characteristics. An increase of the thickness of an avalanche layer that is more highly doped to provide a desired gain characteristic can result in a device that has less gain uniformity and more noise characteristics due to avalanche occurring at different distances from the interface between the avalanche layer and the substrate as discussed above. In contrast, in the avalanche photodiode 30 the thickness of the avalanche layer 44 includes the spectral absorption region 46 formed of more lightly doped /7-type silicon. Hence, the thickness of the avalanche layer 44 is increased while reducing the effects of avalanche occurring at different distances from the interface by decreasing the dopant concentration in this region. However, the desired gain characteristic is achieved by epitaxially growing the more highly p-doped buried gain region 48 which is buried beneath the absorption layer. In the preferred embodiment, the photons impinge on the photosensitive surface 52 and travel inward into the spectral absorption region 46 wherein a minority charged carrier is preferably produced. This minority charged carrier then is swept towards the gain region 48 by the electric field that results as a result of the reverse bias and an avalanche of charged carriers is preferably produced in the relatively thin gain region 48 immediately adjacent the interface 42. Consequently, the avalanche photodiode 30 is less sensitive to avalanching occurring at different distances from the interface 42. Specifically, since the spectral absorption layer has a low doping concentration when compared to the gain region, if photon absorption occurs at different locations within the spectral absorption region 48, there is less variation in the resulting current gain as the most significant portion of the avalanching is most likely to occur in the relatively thin gain region. Further, under the reverse bias conditions the depletion region preferably reaches through the π region and provides stable avalanche gain through the avalanche region. Because the spectral absorption layer 46 has increased the thickness of the avalanche layer 44, the quantum efficiency of the device has also been increased. In this embodiment, the quantum efficiency is approximately 12% at 1.06 microns for carriers that undergo full amplification. This compares to a quantum efficiency for an avalanche photodiode having comparable gain characteristics formed using diffusion based techniques of less than 5%. Hence, using CVD epitaxial growth techniques to form buried regions of increased doping concentration can result in a device that has a greater quantum efficiency and is therefore more sensitive without any decrease in the gain or noise characteristics. Figures 3A and 3B further illustrate that the doping profile of an avalanche layer of a device can be varied to achieve different performance characteristics for the device. In particular, Figure 3A illustrates another embodiment of an avalanche photodiode 60 that is comprised of a substrate 70 which is preferably an n-doped substrate. The substrate includes a first surface 72 wherein a second layer 74 of semiconductor material is deposited thereon so that an interface is defined at the surface 72. The second layer 74 in this embodiment is comprised of p-doped silicon that is grown using CVD epitaxial growth techniques so that it has a varied dopant profile and defines an avalanche layer for the device. In particular, the avalanche layer 74 includes a spacer layer 76 of lightly /7-doped material that is positioned immediately adjacent the interface 72. A more heavily doped gain region 78 is then grown on an outer surface 77 of the π-doped spacer region 76 using the CVD epitaxial growth techniques described above. A spectral absorption region 80 is then grown on an outer surface 79 of the gain region 78 using epitaxial growth techniques. The spectral absorption region 80 is more lightly doped than the gain region 78 but increases the thickness of the avalanche layer 74 so as to improve the quantum efficiency of the device 60 in the manner described above. Subsequently, a depletion layer stop region 82 is formed on an outer surface 81 of the spectral absorption region 80 for the same purpose that was described above. A contact layer 84 of a very heavily p-doped silicon is then diffused into the depletion stop region 82 in the manner described above and an electrode structure 86 is then positioned on an upper surface 83 of the device. Further, an ohmic contact 88 is formed on a bottom surface 89 of the substrate 70 so that the electrode structure 86 and the ohmic contact 88 permit the reverse biasing voltage to be applied to the APD device. As shown in Figure 3B, the primary difference between the avalanche photodiode 60 and the avalanche photodiode 30 described above in reference to Figures 2A and 2B is that there is a 7 micron thick spacer layer 76 positioned between the gain region 78 and the interface 72. This spacer region has the effect of maintaining a substantially constant electric field (shown in dashed lines) at or near interface. The operation of the device 60 is similar to the operation of the device 30 described in reference to Figures 2A and 2B and the device 60 simply illustrates that the doping profile of the second layer can be varied to achieve specific performance characteristics of the device. From the foregoing, it will be appreciated that by growing the avalanche layer of silicon material using CVD epitaxial growth processes results in devices that have greater gain uniformity over the area of the device. This is particularly important for large area avalanche photodiode device application such as those devices having an area of greater than 20 mm 2 . Further, the APD devices formed using the epitaxial process described hereinabove also results in devices that have significantly lower noise characteristics. In particular, it is possible to produce avalanche photodiodes using the epitaxial growth process described hereinabove that has a range of noise equivalent power (NEP) of between 1.8 x 10" and 6.3 x 10 ,s watts per Hz ' for an area of greater than 20 mm 2 in response to receiving light having a wavelength of between 300 nm and 850 nm. Further, the spatial uniformity of the dopant atoms which directly impacts the gain uniformity and the energy resolution of an avalanche photodiode, is also improved in an avalanche photodiode that has a p-region formed using CVD epitaxial growth. For example, in tests performed by the Applicant, a 10 mm large area avalanche photodiode formed using the above described techniques had a 9.5% FWHM in response to a 55 Fe 5.9 keV X-ray irradiation. The specification for standard 10 mm LAAPD formed through diffusion techniques is that the full width at half maximum of the κ Fe distribution should be less than 18%. Hence, experimental data obtained by the applicant indicates that the spatial uniformity of a p layer grown using epitaxial growth techniques of the preferred embodiment result in a p-layer that has much greater spatial uniformity and consequently greater gain uniformity and energy resolution. Consequently, the avalanche devices of the preferred embodiment have greater gain uniformity, less noise and greater performance characteristics than the avalanche devices of the prior art. Further, growing the p-region using CVD epitaxial growth techniques allows for greater control over the doping profile of the region which can then be used to obtain devices that have specific performance characteristics. While the embodiments of the invention have been described in terms of avalanche photodiode devices, it will be appreciated by a person skilled in the art that any avalanching device which experiences current gain as a result of generation of a charge carrier in an avalanche layer can be made using the techniques described above without departing from the spirit of the present invention. Consequently, although the preferred embodiment of the present invention has shown, described, and pointed out the fundamental novel features of the invention as applied to these embodiments, it will be understood that various omissions, substitutions, and changes in the form of the detail of the device as illustrated may be made by those skilled in the art without departing from the spirit of the present invention. Consequently, the scope of the invention should not be limited to the foregoing discussion, but should be defined by the claims which follow.

Claims: WHAT IS CLAIMED IS: 1. An avalanche photodiode device (APD) comprising a body of silicon semiconductor material having an upper surface, a lower surface and a bevelled side surface wherein said body of silicon semiconductor material is configured such that a biasing voltage applied between said upper surface and said lower surface produces an electric field having an edge electric field intensity along said bevelled side surface which does not exceed an interior electric field intensity within said body of silicon semiconductor material, said body of silicon semiconductor material further comprising: an n-doped silicon semiconductor substrate having a first doping characteristic, a first crystal structure and a first surface; and an epitaxial p-doped silicon semiconductor layer having a second doping characteristic and said first crystal structure, wherein said epitaxial p-doped silicon semiconductor layer is grown on said first surface of said n-doped silicon semiconductor substrate by means of chemical vapor deposition (CVD) such that said epitaxial p-doped silicon semiconductor layer forms a p-n junction with said n-doped silicon semiconductor substrate at said first surface of said n-doped silicon semiconductor substrate and a depletion region is formed around said p-n junction which extends outward therefrom, such that when said p-n junction is reverse biased, an avalanche of charge carriers is produced in said APD in response to a photon penetrating into said APD. 2. The avalanche photodiode device of Claim 1, wherein said epitaxial p-doped silicon semiconductor layer is deposited on said n-doped silicon semiconductor substrate so that a doping profile is created in said epitaxial p-doped silicon semiconductor layer, said doping profile comprising: a first uniform doping concentration in a first substantially uniformly thick region of said epitaxial p-doped silicon semiconductor layer which is located a first distance from said p-n junction; and a second doping concentration in a second region of said epitaxial p-doped silicon semiconductor layer which is located a second distance from said p-n junction, wherein said first uniform doping concentration is greater than said second doping concentration and said first distance is less than said second distance. 3. The avalanche photodiode device of Claim 2, wherein said first region of said epitaxial p-doped silicon semiconductor layer is formed using epitaxial growth so that said first region has a spatial uniformity such that said first uniform doping concentration at any point in a plane parallel to said p-n junction is substantially the same. 4. The avalanche photodiode device of Claim 3, wherein said first region of said epitaxial p-doped silicon semiconductor layer has said spatial uniformity over an area that is greater than 20 mm 2 . 5. The avalanche photodiode device of Claim 3, wherein a first quantity of charge carriers produced in said epitaxial p-doped silicon semiconductor layer at a first point in a plane parallel to said p-n junction in response to a photon entering said upper surface at a corresponding first point in said upper surface is substantially the same as a second quantity of charge carriers produced at a second point in said plane in response to a photon entering said upper surface at a corresponding second point in said upper surface. 6. The avalanche photodiode device of Claim 5, wherein said first doping characteristic of said n-doped silicon semiconductor substrate and said second doping characteristic and said doping profile of said epitaxial p-doped silicon semiconductor layer are selected so that said avalanche photodiode device has a range of noise equivalent power (NEP) of between 1.8 x 10 " to 6.3 x 10 ' 5 watts per Hz 1 for an area of greater than 20 mm 2 in response to receiving light having a wavelength of between 300 nm to 850 nm. 7. The avalanche photodiode device of Claim 5, wherein said doping profile of said epitaxial p-doped silicon semiconductor layer is selected so that said first region comprises a gain region that is selected to provide a first pre selected gain characteristic for said photodiode and wherein an absorption spectral response region is formed on said gain region that has a thickness and doping concentration selected to maximize absorption and electrical response to a photon of a first pre selected wavelength. 8. The avalanche photodiode device of Claim 7, wherein said doping profile of said epitaxial p-doped silicon semiconductor layer is selected so that said gain region is approximately 10 μm thick and wherein said absorption spectral response region comprises a p-type semiconductor material that is approximately 100 μm thick and is positioned adjacent said gain region. 9. The avalanche photodiode device of Claim 8, wherein said gain region has a doping concentration of greater than 2 X 10"/cm 3 and wherein said absorption spectral response region has a doping concentration of less than 5 X 10 "/cm 3 and wherein said avalanche photodiode device produces an electrical signal in response to light having a wavelength of 1.06 microns impinging on said depletion region. 10. The avalanche photodiode device of Claim 2, wherein said doping profile in said epitaxial p-doped silicon semiconductor layer is selected so that said depletion region extends from said p-n junction to substantially adjacent to said upper surface. 11. An avalanche photodiode device (APD) comprising: a body of silicon semiconductor material comprising n-type material and p-type material, said body of silicon semiconductor material having first and second body portions, said first body portion comprising a layer of said n-type material having a surface area of at least about 20 mm 2 and said second body portion comprising an epitaxial layer of said p-type material disposed on said first body portion of said n-type material so as to form a single p-n junction, said epitaxial layer having a surface opposite said p-n junction for receiving photons, said epitaxial layer having a thickness of at least 10 microns, said p-type material having a doping profile that varies from said photon receiving surface to said p-n junction, said profile exhibiting a doping concentration which increases at a location (i) spaced from said photon receiving surface and (ii) between said photon receiving surface and said p-n junction, said p-n junction being reverse biased at a voltage which produces an electric field within said body of silicon semiconductor material, said electric field causing charge carriers to avalanche in response to receipt of said photons, wherein a side surface of said body of silicon semiconductor material is geometrically configured so that said electric field has a magnitude internal to said body of silicon semiconductor material that is greater than that on said side surface of said body of silicon semiconductor material. 12. The avalanche photodiode device of Claim 11 , wherein said doping concentration abruptly increases at said location. 13. A method of producing a bevelled-edge avalanche photodiode device (APD) comprising the steps of: providing an n-type semiconductor substrate having a first doping characteristic and a first crystal structure; growing a p-type epitaxial layer of semiconductor material on a first surface of said n-type semiconductor substrate using chemical vapor deposition (CVD) so that said p-type epitaxial layer has a second doping characteristic and said first crystal structure, wherein a depletion region is formed at an interface between said n-type semiconductor substrate and said p-type epitaxial layer so that a junction between said n-type semiconductor substrate and said p-type epitaxial layer is reverse biased so that an avalanche of charge carriers is produced in response to a photon impinging on an outer surface of said bevelled-edge avalanche photodiode device, wherein said step of growing an epitaxial layer further comprises the steps of: growing a first region of said p-type epitaxial layer to a first thickness from said interface such that said first region of said p-type epitaxial layer has a first doping density; and growing a second region of said p-type epitaxial layer from said first thickness to an upper surface of said p-type epitaxial layer wherein said second region of said p-type epitaxial layer has a second doping density and a second thickness, wherein said first doping density is greater than said second doping density and is selected to provide a first desired gain of said avalanche photodiode device and said second thickness is selected so as to increase said avalanche of charge carriers in response to said photon having a first wavelength entering said p-type epitaxial layer; and providing contacts for applying a voltage to said p-type epitaxial layer relative to said n-type semiconductor substrate to further reverse bias said junction. 14. A bevelled-edge avalanche photodiode device comprising: a substrate of an n-type semiconductor material which defines a first surface; and an avalanche layer of a p-type semiconductor material positioned on said first surface so as to define an interface, wherein said avalanche layer is epitaxially grown on said first surface and wherein a depletion region is formed around said interface such that a reverse biased p-n junction formed at said interface between said substrate and said avalanche layer produces an avalanche of charge carriers in response to generation of a minority charge carrier within said avalanche layer. 15. The device of Claim 14, wherein: said avalanche layer is configured to have an outer surface that receives photons; and said avalanche of charge carriers is produced in response to a photon being absorbed in said avalanche layer and producing said minority charge carrier. 16. The device of Claim 15, wherein said avalanche layer comprises: a gain region positioned substantially adjacent said interface, wherein said gain region has a first doping concentration and a first thickness; and a spectral absorption region interposed between said gain region and said outer surface, wherein said spectral absorption region has a second doping concentration which is less than said first doping concentration. 17. The device of Claim 16, wherein said first doping concentration and said second doping concentration are selected so that substantially all of said avalanche of charge carriers occurs in said gain region regardless of where said photons are absorbed in said spectral absorption region. 18. The device of Claim 17, further comprising a spacer region interposed between said gain region and said interface. 19. The device of Claim 18, wherein said gain region is approximately 7 microns thick and has a doping concentration of approximately 1 X 10 1S dopant atoms per cm 3 and wherein said spectral absorption region is approximately 100 microns thick and has a doping concentration of approximately 5 X 10 12 dopant atoms per cm 3 . 20. The device of Claim 18, wherein said spacer region is approximately 7 microns thick and has a doping concentration of approximately 5 X 10' 2 dopant atoms per cm 3 . 21. An avalanche photodiode device comprising: a substrate of n-type semiconductor material which defines a first surface; and an avalanche layer of p-type semiconductor material epitaxially grown on said first surface so as to define an interface comprising a p-n junction, wherein a depletion region is formed around said interface when said junction is reverse biased to produce an avalanche of charge carriers in response to generation of a minority charge carrier within said avalanche layer, said avalanche layer having a light receiving surface and comprising a first region of a first doping concentration and a second region of a second doping concentration, said first doping concentration being greater than said second doping concentration, and said first region being interposed between said second region and said interface, the periphery of said photodiode device being geometrically configured so that the electric field strength at the periphery of the device is not substantially greater than the electric field strength interior to the periphery. 22. A method of fabricating an avalanche photodiode device comprising the steps of: providing a substrate of n-type semiconductor material which defines a first surface; epitaxially growing an avalanche layer of p-type semiconductor material on the first surface to form an interface between the substrate and the avalanche layer, said step of epitaxially growing comprising forming the avalanche layer to produce a p-n junction at said interface which, when reversed biased, causes an avalanche of charge carriers in response to generation of a minority charge carrier within said avalanche layer, said step of forming the avalanche layer comprising forming first and second regions in said avalanche layer by epitaxially growing said regions such that the first region is interposed between said interface and said second region, and said step of forming said regions comprising the step of doping the first region with a doping concentration greater than that of the second region; and geometrically configuring the periphery of said photodiode device so that the electric field strength at the periphery of the device is not substantially greater than the electric field strength interior to the periphery.

Inventors: BOISVERT JOSEPH CHARLES, JOSTAD LEON LESLIE, MONTROY JOHN THOMAS, BOISVERT, JOSEPH, CHARLES, JOSTAD, LEON, LESLIE, MONTROY, JOHN, THOMAS
Applicants: ADVANCED PHOTONIX INC, ADVANCED PHOTONIX, INC.
Application year: 1997
Main classification: H01L31
IPC codes: H01L31