Patent Publication Number: US-2005121747-A1

Title: Automatically passivated n-p junction and a method for making it

Description:
FIELD OF THE INVENTION  
      This invention relates to passivated n-p junctions in semiconductor materials, arrays formed of the same and a method or methods for producing passivated n-p junctions and arrays of the same. The invention has particular, but not exclusive, utility in the construction of infrared (IR) photodiodes and detectors that function as two-dimensional staring arrays fabricated using mercury cadmium telluride (HgCdTe).  
      Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.  
     BACKGROUND ART  
      Mercury cadmium telluride (MCT) n-on-p junctions are typically formed on p-type MCT using ion milling or ion implantation so as to form an n-type region.  
      Typically, a masking layer is formed on the surface of a p-type body of MCT. The masking layer has windows formed therein which expose the surface of the p-type body where the n-p junctions are to be formed. Ion implantation or ion milling is then performed to create the n-type regions, thus forming an n-on-p junction. However, the processes of ion implantation and ion milling both result in damage occurring to the MCT crystal lattice. In order to counter such damage, the MCT is annealed after forming the n-p junctions at high temperature. Annealing serves to repair the crystal lattice and/or activate the n-type regions. However, either or both ion implantation/milling and the annealing process tend to degrade the interface between the masking layer and the MCT, which results in degradation of the n-p junction surface and thus poor performance of the n-p junction. In order to overcome the above problem, the masking layer is commonly removed and the surface repassivated. However, this exposes the n-on-p junction at the surface to subsequent processing steps, which tend to also degrade the quality of the n-on-p junction. Although passivants have been used as part of a masking layer, the processes of ion milling and ion implantation and/or annealing affect the nature of the passivant such that after exposure to the ion implantation or ion milling and/or annealing process, the MCT/passivant interface is degraded. Thus, a new passivation layer needs to be formed.  
      Degradation of the n-on-p junction surface not only leads to a reduction in performance of the particular n-on-p junction, but also leads to variations in the performance of individual junctions within an array of junctions. Diode arrays formed using existing techniques can vary in performance criteria by as much as an order of magnitude across a wafer.  
     DISCLOSURE OF THE INVENTION  
      In contrast with the prior art methods, the present invention provides a simpler process for producing automatically passivated n-p junctions, without utilising high temperature annealing, and in which the n-p junction at the surface is never exposed.  
      The present invention also provides for the construction of planar semiconductor arrays having multi-spectral characteristics that may function as IR photodiode detectors.  
      In accordance with one aspect of this invention, there is provided a method for forming an automatically passivated n-p junction, comprising the steps of: providing a p-type body containing Group II and Group VI elements, one of which is mercury; forming a passivation layer having at least one window provided therein on a surface of the p-type body; subjecting said p-type body to a reactive ion etching process using the passivation layer as a mask to form the n-p junction; and forming ohmic contacts to the n-type and p-type regions.  
      The lateral extension of the type conversion from p to n resulting from the reactive ion etching process, results in the n-on-p junction at the surface being under the passivation layer, resulting in an automatically passivated junction which is never exposed during subsequent processing. This, along with the lack of post-processing annealing, results in n-on-p junctions of superior quality and uniformity compared with existing techniques. The lack of post-processing annealing is due to the fact that n-on-p junctions formed by reactive ion etching do not require an activation anneal and do not suffer from the same degree of lattice damage caused by ion implantation/milling.  
      Preferably, the p-type body comprises mercury cadmium telluride.  
      Preferably, the step of forming a passivation layer with windows provided therein comprises the steps of forming a passivation layer on the p-type body and etching windows therein.  
      The passivation layer can be formed from any suitable material. Examples of suitable materials include ZnS, wider bandgap MeT and bi-layer passivants such as ZnS/Si 3 N 4  or ZnS/SiO 2 . Advantageously, the process doesn&#39;t degrade the performance of the passivant.  
      Preferably, the passivation layer is relatively thick to prevent type conversion of areas covered by the passivation layer. In this regard, a passivation layer thickness of 0.3 μm of ZnS has achieved good results. It is envisaged however that the passivation layer may be formed thinner than 0.3 μm and still provide good results, or that it may consist of a layer of two materials in which the upper layer is subsequently removed.  
      In accordance with a second aspect of this invention, there is provided an n-p junction formed in accordance with the method of the first aspect of this invention.  
      In accordance with a third aspect of this invention, there is provided a method for forming an array of n-p junctions on a semiconductor body having a plurality of p-type material layers containing Group II and Group VI elements, one of which is mercury, comprising the steps of: 
          etching the body to expose a portion of each layer;     forming a passivation layer over the body;     forming a window in the passivation layer in each portion;     subjecting the body to a reactive ion etching process using the passivation layer as a mask to form an n-p junction in each layer;     forming an ohmic contact to each of the n-type regions; and     forming an ohmic contact to a common p-type layer.        

      Preferably, the step of etching the body exposes a plurality of portions within each layer at spaced locations across the body, so as to form a plurality of multi-wavelength detectors.  
      Preferably, the method includes the step of etching a channel between adjacent detectors.  
      More preferably, the channel passes through all of the p-type layers except the common p-type layer.  
      In accordance with a fourth aspect of this invention, there is provided a method for forming an array of n-p junctions on a semiconductor body having a plurality of p-type material layers containing Group II and Group VI elements, one of which is mercury, comprising the steps of: 
          etching the body to expose a portion of each layer;     forming a passivation layer over the body;     forming a window in the passivation layer;     subjecting the body to a reactive ion etching process to form n-p junctions that extend substantially to the substrate;     forming an ohmic contact to each of the common n-type regions; and     forming ohmic contacts to each layer on said portions.        

      Preferably, the step of etching the body exposes a plurality of portions of each layer at spaced locations across the body to form a plurality of detectors.  
      Preferably, adjacent detectors are separated by an n-p junction.  
      In accordance with a fifth aspect of the present invention, there is provided a semiconductor material comprising an n-p junction, the material including: 
          a substrate;     a layer of p-type material surmounting said substrate;     a region of converted n-type material formed on a localised portion of the surface of said p-type material, defining an n-p junction between the p-type and n-type material;     a passivation layer surmounting the surface of the p-type material and the n-p junction, including windows respectively exposing part of the surface of the converted n-type material and a portion of the surface of the p-type material distant from the n-type material for disposing ohmic contacts on the respectively exposed surfaces, without exposing the n-p junction.        

      Preferably, the semiconductor material includes said ohmic contacts so as to form an electronically connectable component.  
      Preferably, said passivation layer is formed on the surface of the p-type material prior to conversion of the n-type material.  
      Preferably, said p-type material is MCT.  
      Preferably, said conversion is performed by a plasma induced process.  
      Preferably, said plasma induced process is a reactive ion etching process that creates a laterally displaced n-on-p junction beneath the surface of the passivation layer, without degrading the passivation layer.  
      Preferably, said passivation layer is formed of zinc sulphide (ZnS).  
      Preferably, said passivation layer has a thickness of 0.3 μm.  
      In a particular embodiment of this aspect of the invention, a layer of n-type material may be interposed between said substrate and said layer of p-type material so that said p-type layer surmounts said n-type layer, and said n-type layer surmounts said substrate so as to form a junction isolated n-on-p diode.  
      In this embodiment, preferably the region of n-type material extends through the p-type layer to the n-type layer.  
      Preferably, said region is annular, being arranged to circumscribe a portion of said layer of p-type material, isolating the circumscribed portion from the remainder of said layer of p-type material.  
      Preferably, a plurality of discrete regions of n-type material are provided in the layer of p-type material to form an array of n-p junctions therein, whereby a said window is disposed to expose a portion of said circumscribed portion of p-type material for disposing an ohmic contact thereon.  
      In another embodiment of this aspect of the invention, a multi-wavelength detector may be formed by interposing between the layer of p-type material and the passivation layer: 
          an isolating layer of p-type material;     a second layer of p-type material; and     another isolating layer of p-type material; 
 
 whereby the isolating layer surmounts the first layer of p-type material, the second layer surmounts the isolating layer, the other isolating layer surmounts the third layer, and the passivation layer surmounts the other isolating layer. 
       

      Other layers such as a third layer and a further isolating layer may be interposed between the other isolating layer and the passivation layer in an accumulative manner to allow for the formation of additional wavelength detectors.  
      According to this embodiment, preferably the layers of p-type material are each formed of a thickness corresponding to a predetermined cut-off wavelength.  
      In order to function as a detector, the semiconductor material is arranged so that incident light impinges the substrate side thereof. In this arrangement, it is preferred that the thickness of each layer is such that the cut-off wavelength of the first layer is less than the second layer, and the second layer is less than any third layer and so on.  
      Preferably, the isolating layers are formed of semiconductor material having a wider band gap than the semiconductor material used for forming the p-type layers.  
      Preferably, the p-type layers and their surmounting isolating layers are arranged in pairs that are recessed such that a portion of each corresponding p-type layer and isolating layer pair constitutes the final layer pair of that portion of the semiconductor material and is surmounted by said passivation layer.  
      In one type of multi-wavelength detector, a said converted n-type region may be formed in each final layer pair and extends through the isolating layer to the layer of p-type material thereof; and said windows are provided in the passivation layer of each final layer pair to expose part of each converted n-type region for disposing said ohmic contacts thereon.  
      Preferably, a channel is formed extending through the outer layers of p-type material and the isolating material so that the passivation layer directly surmounts the first isolating layer at prescribed locations on the semiconductor material to divide the same into predetermined detector elements or pixels constituting the array. In this manner, the channel reduces cross talk between adjacent detector elements.  
      In another type of multi-wavelength detector, a said converted n-type region may be formed in each final layer pair comprising: (i) the first p-type layer and the corresponding surmounting isolating layer thereof, and (ii) the last p-type layer and the corresponding surmounting isolating layer thereof; whereby the converted n-type region extends through to the substrate, and said windows are provided in the passivation layer of: (i) said first and last final layer pairs to expose part of each converted n-type region, and (ii) each final layer pair distant from said converted n-type region to expose part of the isolating layer thereof; for disposing said ohmic contacts on the exposed parts of the semiconductor material.  
      In this manner, the converted n-type regions divide up the semiconductor material into predetermined detector elements or pixels constituting the array.  
      Preferably, in either type of multi-wavelength detector, each detector element comprises a single final layer pair from each of the layers constituting the semiconductor material, whereby each final layer pair is adjacent to another layer pair within said detector element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be described with reference to four embodiments thereof and the accompanying drawings, in which:  
       FIG. 1  shows a passivation layer and a photoresist layer provided on a p-type body, in accordance with the first embodiment of the invention;  
       FIG. 2  shows the formation of a window in the passivation layer of  FIG. 1 ;  
       FIG. 3  shows the p-type body of  FIG. 2  after being subjected to a reactive ion etching process;  
       FIG. 4  shows the p-type body of  FIG. 3  after forming ohmic contacts;  
       FIGS. 4A  to  4 C show successive steps in forming a junction-isolated n-on-p diode according to the second embodiment of the invention; and  
      FIGS.  5  to  13  show successive steps in forming a multi-wavelength detector according to the third embodiment of the invention; and  
      FIGS.  14  to  19  show the steps in forming a multi-wavelength detector according to a fourth embodiment of the invention. 
    
    
     BEST MODE(S) FOR CARRYING OUT THE INVENTION  
      The first embodiment is directed towards a method of forming an n-on-p junction.  
       FIG. 1  shows a p-type body  10  of MCT provided on a substrate  12 . Firstly, a passivation layer  14  of ZnS is formed on the p-type body  10  by thermal deposition. Other methods such as electron beam deposition may be used as appropriate.  
      Next, photoresist  16  is applied to the passivation layer  14 . The photoresist  16  is selectively removed from the passivation layer  14  wherever a window is desired to be formed within the passivation layer.  
      Windows  18  are then formed in the passivation layer  14  by etching through the removed photoresist  16 , whereby regions of the passivation layer  14  not covered by the photoresist  16  are removed during the etching. The windows  18  extend to the surface of the p-type body  10 . The photoresist  16  is then removed from the remaining passivation layer  14 , as shown in  FIG. 2 .  
      Next, the exposed surface of the p-type body  10  is subjected to a reactive ion etching (RIE) process. This may be the same process that was used to etch windows  18  in the passivation layer  14 . In the embodiment, a parallel plate reactor is used. The following conditions are used during the reactive ion etching process: hydrogen flow rate of 27 sccm; methane flow rate of 5 sccm; total pressure of 415 mTorr; DC bias of 200V; cathode temperature of 18° C. During the RIE process, the p-type body  10  and the substrate  12  are mounted on the RIE cathode.  
      The p-type body is then subjected to reactive ion etching for a period of 2 minutes. This results in the exposed surface of the p-type body  10  being etched to a depth of 0.4 μm, with type conversion from p-type to n-type occurring to a depth of approximately 3 μm in a region  20  adjacent to the exposed surface. Importantly, the lateral extension of the n-type region  20  results in the n-p junction  22  at the surface being beneath the passivation layer  14 , resulting in an automatically passivated n-p junction.  
      Advantageously, it has been discovered that whilst reactive ion etching process parameters including time and total pressure can be used to control the depth of the n-p junction these do not significantly increase the width of the junction. As a result, by using this method of forming an n-on-p junction, it is possible to control the depth of the junction. If so desired, the n-on-p junction could extend to the substrate in order to reduce cross talk between adjacent n-on-p junctions.  
      Next, electrical contact to the n-type region  20  is made by depositing Cr/Au metal at  24 . Electrical contact to the remaining p-type body can be formed in the usual manner.  
      It is to be noted that the process described in the present embodiment does not degrade the passivant/MCT interface, and does not require annealing to be performed after formation of the n-type region, which is believed to improve the quality and uniformity of the n-on-p junctions. In addition, the automatic passivation of the n-on-p junctions due to the n-on-p junction surface never being exposed, also assists in the quality and uniformity of the formed junctions.  
      The second embodiment is directed towards a method of forming junction-isolated n-on-p diodes. The method is illustrated in  FIGS. 4A  to  4 C. In this embodiment n-type regions are used to isolate p-type regions and form junction-isolated n-on-p diodes.  
       FIG. 4A  shows a body  50  on which the junction-isolated n-p diodes are to be formed. The body comprises a substrate  52 , on which the following layers are grown, in order: 
          a first layer of n-type material  54 ; and     a second layer of p-type material  56 .        
      The layers  54  and  56  are formed from mercury cadmium telluride. A passivation layer  58  is then applied to the body  50 . The passivation layer is formed of ZnS and in this particular embodiment is deposited to a thickness of 0.38 μm. Photoresist (not shown) is applied to the body  50  and photolithographically patterned. Windows  60  are then etched in the passivation layer  58 . The result is shown in  FIG. 4A .  
      Next, the body  50  is subjected to reactive ion etching in a similar manner to that described in the first embodiment. The reactive ion etching forms n-type regions  62  beneath each window  60 . The process parameters of the reactive ion etching are controlled so as to ensure the n-type regions  62  extend to the n-type layer  54 . As a result the portion of the p-type layer  56  between the n-type regions  62  is isolated from the remaining p-type layer  56  and forms an n-on-p junction therewith. The result is shown in  FIG. 4B . It should be apparent that although  FIG. 4B  shows a cross-section of the body, the principle can be readily applied to form n-on-p junctions isolated in two dimensions by forming an n-type region  62  that encloses a portion of the p-type layer  56 .  
      Photoresist (not shown) is then applied to the body  50  and photolithographically patterned. A window  64  is then etched in the passivation layer  58  above the isolated portion of the p-type layer  56 . Next, metal contacts  66  are attached to the n-type regions  62  and to the isolated portion of the p-type region  56 . The result is shown in  FIG. 4C .  
      The third embodiment is directed towards a method of forming a multi-wavelength detector. The steps in the method are illustrated in FIGS.  5  to  16 . The. embodiment will describe a method for forming a detector responsive to three wavelengths, λ 1 , λ 2  and λ 3 .  
       FIG. 5  shows a body  100  on which the detectors are to be formed. The body comprises a substrate  102 , on which the following layers are grown, in order: 
          a first layer of p-type material  104 ;     an isolating layer of p-type material  106 ;     a second layer of p-type material  108 ;     another isolating layer of p-type material  110 ;     a third layer of p-type material  112 ; and     a final isolating layer of p-type material  114 .        
      The layers  104 ,  108  and  112  of p-type material are formed from mercury cadmium telluride and are designed so as to have cut-off wavelengths of λ 1 , λ 2  and λ 3 , respectively.  
      In this embodiment, the detector will be used with light incident upon the substrate  102 , and accordingly the cut-off wavelengths are chosen such that λ 1 &lt;λ 2 &lt;λ 3 .  
      The isolating layers  106 ,  110  and  114  are also formed from MCT, however they are such that their band gaps are wider than those of the MCT layers  104 ,  108  and  112 .  
      First, a mask is applied using a layer of photoresist. The mask is then photolithographically patterned. Next, the body  100  is etched in order to remove the isolating layer  114  and the third layer  112  in specific locations as determined by the patterning. The remaining photoresist is then removed. The result is shown in  FIG. 6 .  
      Next, a mask of photoresist is again applied to the body  100  and photolithographically patterned. The body  100  is then etched so as to remove the isolating layer  110  and the second layer  108  where it is not required. The photoresist is then removed. The result is shown in  FIG. 7 .  
      Next, a further mask of photoresist is applied and photolithographically patterned. The body  100  is then etched so as to form a channel  116  extending through the second and third layers  108  and  112  and the isolating layers  110  and  114 . The channel  116  reduces cross talk between adjacent detectors. The result is shown in  FIG. 8 , wherein the dashed lines indicate adjacent detectors. Note that each detector has a portion  118  of the third layer  112  or  114  exposed, a portion  120  of the second layer  108  or  110  exposed and a portion  122  of the first layer  104  or  106  exposed.  
      Next, a passivation layer  124  is applied to the body  100 . The passivation layer is formed of ZnS and in this particular embodiment is deposited to a thickness of 0.3 μm. The result is shown in  FIG. 9 .  
      Next, photoresist is applied to the body  100  and photolithographically patterned. Windows  126  are then etched into the passivation layer  124 . A window  126  is then formed on each of the portions  118 ,  120  and  122  on each pixel. The result is shown in  FIG. 10 .  
      Next, the body  100  is subjected to reactive ion etching in a similar manner to that described in the first embodiment. The reactive ion etching forms n-type regions  130  beneath each window  126 . The n-type regions  130  formed on each of the portions  118 ,  120  and  122  serve to form n-p junctions with layers  112 ,  108  and  104 , respectively in each detector. The result is shown in  FIG. 11 .  
      Next, a metal ohmic contact  132  is attached to each of the n-type regions  130 . The result is shown in  FIG. 12 .  
      The final step is to attach a metal ohmic contact to the p-type region  104 , which is a common p-type region in the embodiment. The metal contact is denoted  134  as shown in  FIG. 13 . By utilising the method for forming an n-on-p junction of the invention, the n-on-p junctions formed between the n-type regions  130  and the layers  104 ,  108  and  112  are automatically passivated. This provides a more consistent n-on-p junction which results in a more uniform detector. This is particularly important where an array of detectors are produced in a single body.  
      The fourth embodiment is also directed towards a method for producing a multi-wavelength detector capable of detecting three wavelengths. The fourth embodiment makes use of a body  200  of the same form as the third embodiment and like parts in the fourth embodiment use the same reference numerals as those in the third embodiment with  100  added thereto. Thus, the body  102  in the third embodiment is designated  202  in the fourth embodiment. The method of the fourth embodiment is illustrated in FIGS.  14  to  18 .  
      The first steps of the embodiment are to etch the layers  212  and  208  in a similar manner to that described in the third embodiment. After the layers  212  and  208  have been etched, the body  200  will be as shown in  FIG. 14 .  
      Next, a passivation layer  224  is formed on the body  200 . The passivation layer  224  is also made from ZnS and deposited to a depth of 0.3 μm. The result is shown in  FIG. 15 .  
      Next, photoresist is applied on top of the passivation layer  224  and photolithographically patterned. The passivation layer  224  is then etched to reveal windows  226 .  
      In contrast to the third embodiment, where a window  116  was formed in each of the portions  118 ,  120  and  122 , in this embodiment the windows are formed at the boundaries between adjacent detectors. The body is then subjected to reactive ion etching for sufficient time for the n-type regions  230  to extend to the substrate  202 . The result is shown in  FIG. 16 .  
      Next, metal contacts  232  are attached to each of the n-type regions  230 . This result is shown in  FIG. 17 . The n-type regions  230  form horizontal n-on-p junctions with each of the layers  212 ,  208  and  204  in each detector.  
      The final step is to form a metal contact  234  to the p-type region in each portion  218 ,  220  and  222  of each detector. The result is shown in  FIG. 18 .  
       FIG. 19  shows an example of arranging a plurality of detectors of the fourth embodiment as an array on a single body. The metal contacts  232  and  234  shown in  FIG. 19  provide convenient access to the regions  230  and layers  212 ,  208  and  204 . Each metal contact  232  and  234  is shown using two forms of hatching in  FIG. 19 , although it should be appreciated that the contact will be formed as a contiguous element. The cross-hatching represents the portion of the metal contacts  232  and  234  that extend to the region  230  or layer  212 ,  208  or  204  as appropriate. The single hatching represents the remainder of the metal contact  232  or  234 , which provides a larger working area. Further, the remainder of the metal contacts  234  to layers  208  and  204  allow the metal contact to extend onto the same surface as the metal contacts to the region  230  and layer  212 .  
      A similar arrangement may be adopted in order to produce a plurality of detectors of the third embodiment as an array on a single body.  
      It should be appreciated that this invention is not limited to the particular embodiments described above.