Patent Publication Number: US-9431570-B2

Title: Process for producing layered member and layered member

Description:
This is a continuation application of copending application Ser. No. 12/761,898, having a filing date of Apr. 16, 2010, which is a divisional application of application Ser. No. 10/592,325, now abandoned, having a §371 date of Jun. 21, 2007, which is a national stage filing based on PCT International Application No. PCT/JP2005/003879, filed on Mar. 7, 2005. The application Ser. No. 12/761,898, which is now U.S. Pat. No. 8,888,914 and application Ser. No. 10/592,325, which is now abandoned, are incorporated by reference herein in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a layered member and a manufacturing method for a layered member comprising a layer formed by a nitride type semiconductor material. 
     BACKGROUND ART 
     An example of a layered member comprising a layer formed by a nitride type semiconductor material is a photoelectric surface comprising a GaN layer as an active layer (for instance, see cited patent 1). 
     Cited patent 1: Japanese Patent Application Laid-open No. H10-241554. 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     With a conventional photoelectric surface, the quantum efficiency when an excited photoelectron is emitted by light entering a nitride semiconductor crystalline layer as a light absorbing layer has been increasing, but even higher quantum efficiency and lower cost are being demanded for photoelectric surfaces. 
     An object of the present invention is to provide a layered member and a manufacturing method for a layered member which can further increase quantum efficiency and achieved lower costs. 
     Means for Solving the Problems 
     In order to achieve the aforementioned object, the present inventors have performed evaluations from many aspects. For material costs and productivity, sapphire substrates have a high material cost, and an extremely long time is required when mechanically processing so the price becomes even higher. In contrast, silicon substrates with high quality are supplied at low cost in large sheets. Furthermore, using a glass bonding method, the productivity of the photoelectric surface process is excellent compared to sapphire substrates. Recently, the market has been demanding lower prices as well as demanding higher performance. From this point of view, there is demand to satisfy both of these requirements. Therefore the present inventors first focused attention to the polarization of nitride type semiconductor materials. Nitride type semiconductor materials have material specific polarization properties which include spontaneous polarization along the c axis of the crystal and piezo polarization. To illustrate, if these polarization properties are used in a photoelectric surface such as a photoelectron multiplier tube or the like, the positive charge will increase above the surface level because of polarization, and therefore strong band bending will occur at the surface. Therefore the quantum efficiency of the active layer is increased by utilizing the surface emission of the photoelectrons. Furthermore, by widening the depleted layer, a built-in field and active layer will be formed and the diffusion length will be extended. 
     However, in order to utilize this polarization, the topmost layer of the photoelectric surface must be a −c surface (surface in the negative c polar direction, N surface direction) and a very smooth surface must be achieved. However, with the MOCVD growth method used for normal sapphire substrates (surface orientation of main surface is (0001)c), the +c plane (plane in the positive c polar direction, plane in the Group III element surface direction) will be the growth direction. With this crystal growth method, the growth of the −c surface is difficult to control and a highly smooth surface can not be obtained. 
     As a result of further investigations into this point by the present inventors, the following finding was made. Namely, when a wafer is obtained using this crystal growth method, the surface on the opposite side of the +c polar direction (hereinafter, +c surface) is the −c polar direction surface (hereinafter, −c surface). Furthermore, it was discovered that the plane orientation of the crystal growth substrate used to grow the nitride type semiconductor material also has an effect of achieving a smooth surface. The present invention was achieved based on these findings. 
     The manufacturing method of the layered member of the present invention comprises the steps of: preparing a substrate for crystal growth which is a crystalline substance with the main surface in the (111) plane orientation; forming a buffer layer along the main surface of the substrate for crystal growth; forming a nitride semiconductor crystal layer on the buffer layer by crystal growth in the Group III element surface (positive c polar) direction using a nitride type semiconductor material; forming an adhesive layer on the nitride semiconductor crystal layer; adhesively fixing the substrates onto the adhesive layer; and removing the substrate for crystal growth to obtain the buffer layer with a negative c polar surface. 
     With the layered member manufacturing method of the present invention, the plane orientation is (111) for the main surface of the substrate for crystal growth which forms a nitride semiconductor crystal layer by crystal growth through a buffer layer, so the surface of the substrate side for growing crystals of the nitride type semiconductor material can be the −c layer. Furthermore, the substrate for growing crystals is removed after the nitride semiconductor crystal layer and the substrate are adhesively fixed together by an adhesive layer so the −c surface of the buffer layer can be the topmost surface layer. 
     Furthermore, the layered member manufacturing method of the present invention preferably further comprises, after the step of removing the substrate for crystal growth, a step of removing the buffer layer to obtain a nitride semiconductor crystal layer which has a negative c polar surface. The buffer layer is removed, so the −c layer of the nitride semiconductor crystal layer can be the topmost surface layer. 
     Furthermore, the layered member manufacturing method of the present invention preferably further comprises, after the step of removing the substrate for crystal growth, a step of causing crystal growth of the semiconductor material on the negative c polar surface of the buffer layer. Crystal growth will occur on the negative c polar surface so favorable crystal growth is possible. 
     Furthermore, the layered member manufacturing method of the present invention preferably further comprises, after the step of removing the buffer layer, a step of causing crystal growth of the semiconductor material on the negative c polar surface of the nitride semiconductor crystal layer. Crystal growth will occur on the negative c polar surface so favorable crystal growth is possible. 
     Furthermore, the layered member manufacturing method of the present invention preferably further comprises, prior to the step of removing the substrate for crystal growth, a step of forming a protective layer which covers at least the periphery of the substrate. The periphery of the substrate will be covered by a protective layer, and therefore, when removing the buffer layer and the substrate for forming crystals by etching, for instance, erosion of the substrate can be reduced. 
     The layered member of the present invention is comprising a nitride semiconductor crystal layer which is a crystalline layer formed by a nitride type semiconductor material and in which the direction from the first surface thereof to the second surface thereof is the N surface (negative c polar) direction of the crystal; an adhesive layer formed along the first surface of the nitride semiconductor crystal layer; and a substrate which is adhesively fixed to the adhesive layer such that the adhesive layer is located between the substrates and the nitride semiconductor crystal layer. 
     With the layered member of the present invention, the direction from the first surface to the second surface of the nitride semiconductor crystal layer is the negative c polar direction, so the second surface will be the −c surface. 
     Furthermore, with the layered member of the present invention, the first surface is an incidence plane where the light enters, the second surface is an emission plane which emits the photoelectron, and the substrate is a glass substrate formed to transmit light, and the layered member is preferably used as a photoelectric surface member which emits photoelectrons which have been excited by incident light. The second surface is the emission plane, so the emission plane of the photoelectric surface member can be the −c surface. 
     With the present invention, a layered member can be produced where the topmost layer is the −c surface. Therefore, a layered member and a manufacturing method for a layered member which can have even higher quantum efficiency can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram for describing the manufacturing method of a photoelectric surface member which is an embodiment of the present invention; 
         FIG. 2  is a diagram for describing the manufacturing method of a photoelectric surface member which is an embodiment of the present invention; 
         FIG. 3  is a diagram for describing the materials used in manufacturing the photoelectric surface member which is an embodiment of the present invention; 
         FIG. 4  is a diagram for describing the effect of the photoelectric surface member which is an embodiment of the present invention; and 
         FIG. 5  shows the energy distribution properties for p type +c and −c GaN. 
     
    
    
     DESCRIPTION OF THE SYMBOLS 
       1   a —photoelectric surface member,  10 —nitride semiconductor crystal layer,  12 —adhesive layer,  14 —glass substrate,  16 —cathode electrode,  18 −Cs—O layer. 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The findings of the present invention can easily be understood by considering the following detailed description while referring to the attached drawings which are shown only as examples. Continuing, the best mode for carrying out the present invention will be described while referring to the attached drawings. Where possible, the same code has been attached to the same parts and duplicate descriptions have been omitted. Furthermore, the scale of the dimensions in the drawings do not necessarily match that of the descriptions. 
     The manufacturing method of the photoelectric surface material which is an embodiment of the present invention will be described while referring to  FIG. 1A-E  and  FIG. 2A-E .  FIG. 1  A-E and  FIG. 2  A-E are cross-section diagrams for describing the manufacturing steps of the photoelectric surface member. 
     First, a silicon (111) substrate was prepared as the substrate  50  for crystal growth (see  FIG. 1A ). The substrate  50  for crystal growth which is a silicon (111) substrate is a crystalline material and the surface orientation of the main surface  501  is (111). Al x Ga 1−x N (0&lt;X≦1) is grown to approximately several tens of nanometers, and buffer layer  52  is formed on the main surface  501  of the silicon (111) substrate  50  (see  FIG. 1B ). 
     A nitride semiconductor crystal layer  10  with a thickness of approximately several hundred nanometers is formed on the main surface  521  of the buffer layer  52  by epitaxial growth using a Group III-V nitride semiconductor gas material comprising Ga and N (see  FIG. 1C ). The nitride semiconductor crystal layer  10  is doped with magnesium to a level between approximately E19 and E20. As has already been described, the surface orientation of the main surface  501  of the substrate  50  for crystal growth is (111), so the first surface  101  of the nitride semiconductor crystal layer  10  is the +c surface, and the second surface  102  is the −c surface. 
     A layer of silicon dioxide was overlaid with a thickness of between 100 and 200 nm on to the first surface  101  of the nitride semiconductor crystal layer  10  using the CVD method to form the adhesive layer  12  (see  FIG. 1D ). Next, the glass substrate  14  was prepared. The glass substrate  14  preferably has a thermal expansion coefficient similar to the thermal expansion coefficient of the substrate  50  for crystal growth, and preferably contains prescribed alkali ion elements. Corning&#39;s 9741 and Schott&#39;s 8337B are examples of these types of glass substrates  14 . 
     After cleaning the glass substrate  14 , the glass substrate  14  and a multilayered sheet with the configuration shown in  FIG. 1D  (substrate  50  for crystal growth, buffer layer  52 , nitride semiconductor crystal layer  10 , and adhesive layer  12  successively overlaid) were rapidly heated to the glass softening point while the main surface  121  of the adhesive layer  12  was brought into contact with the glass substrate  14 . At this time, a prescribed loading was applied, and the multilayered sheet and the glass substrate  14  were thermocompression bonded through the adhesive layer  12  (see  FIG. 1E ). 
     In the condition shown in  FIG. 1E , at least the glass substrate  14  was covered by an adhesive Teflon sheet  54  (see  FIG. 2A ). Next, etching was performed at room temperature using (1 HF+1 HNO 3 +1 CH 3 COOH) as the etchant. The substrate  50  for crystal growth was etched by this etching process, and the etching was stopped by the buffer layer  52  (see  FIG. 2B ). Therefore, the buffer layer  52  acted as the stopping layer. 
     Next, etching was performed using (1 KOH+10 H 2 O+0.01 H 2 O 2 ) as the etchant to remove the buffer layer  52  (see  FIG. 2C ). Normally, the etching speed of +c surface AlN and GaN is extremely slow, but with this embodiment, the −c surface side is etched so etching can be performed using the aforementioned etchants. Note, the timing to complete the etching of the buffer layer  52  is determined by the elapsed time, the confirmation results of the flatness of the second surface  102  of the nitride semiconductor crystal layer  10 , and the transmissivity or the like of the nitride semiconductor crystal layer  10 . 
     When etching of the buffer layer  52  was complete, the adhesive Teflon sheet  54  was removed. Next, a cathode electrode  16  was formed by the vapor deposition from the glass substrate  14  to the second surface  102  of the nitride semiconductor crystal layer  10  (see  FIG. 2D ). Cr, Al, and Ni or the like may be used as the material of the cathode electrode. 
     Finally, after cleaning the second surface  102  of the nitride semiconductor crystal layer  10 , a Cs—O layer  18  was formed on the second layer  102  to obtain a photoelectric surface member  1   a  ( FIG. 2E ). Note, any one or combination of Cs—I, Cs—Te, or Sb—Cs or the like may be used as a layer containing alkali metal in place of the Cs—O layer  18 . 
     In the aforementioned process, the buffer layer  52  surface obtained by removing the substrate  50  for crystal growth was a flat −c polar surface. Using this −c polar surface as a substrate for crystal growth (re-growth substrate), various devices with excellent characteristics which use semiconductor materials can be manufactured by growing one or more layers of high quality semiconductor crystals such as Al x Ga 1−x N (0≦X≦1) on the buffer layer  52 . 
     Furthermore, the surface of the nitride semiconductor crystal layer  10  obtained after removing the buffer layer  52  has a flat −c polar surface. The aforementioned manufacturing process was described as a process for manufacturing a photoelectric surface, but if this nitride semiconductor crystal layer  10  is used as a substrate for crystal growth (regrowth substrate), various devices with excellent characteristics which use semiconductor materials can be manufactured by growing one or more layers of high quality semiconductor crystals such as Al x Ga 1−x N (0≦X≦1) or InN or the like. 
     Note, the materials used for the layers and substrates are not restricted to those described above.  FIG. 3  shows an example of a material which enables nitride semiconductor crystal layer  10  +c surface growth and flattening of the nitride semiconductor crystal layer  10  second surface  102 . In the example shown in  FIG. 3 , if the material of the substrate  50  for crystal growth is silicon and the surface orientation is (111), AlN or AlN/GaN superlattice is preferably used as the buffer layer  52  material, and GaN, AlGaN, or InGaN is preferably used as the material of the nitride semiconductor crystal layer  10 . Furthermore, if the material of the substrate  50  for crystal growth is GaAs and the surface orientation is (111)A, InGaAsN is preferably used as the material of the buffer layer  52 , and GaN, AlGaN, or InGaN is preferably used as the material for the nitride semiconductor crystal layer  10 . Furthermore, if the material of the substrate  50  for crystal growth is GaP and the surface orientation is (111)A, InGaPN is preferably used as the material for the buffer layer  52  and GaN, AlGaN, or InGaN is preferably used as the material for the nitride semiconductor crystal layer  10 . Furthermore, in order to increase the quantum efficiency of the photoelectron surface for any of these cases, a step of forming an electron stopping layer  10  with a larger bandgap after the step of forming the crystal layer is preferable. This is because a potential barrier is formed on the opposite side to the vacuum surface of the nitride semiconductor crystal layer, and, of the photoelectrons generated, the photoelectrons moving towards the opposite direction to the vacuum escape surface direction are repelled to the opposite direction, and therefore photoelectrons move towards the vacuum escape surface direction. The material in this case is preferably AlN, AlGAN, or BGaN. 
     The effect of this embodiment will be described. With the manufacturing method of this embodiment, the surface orientation will be (111) for the main surface  501  of the substrate  50  for crystal growth for forming a nitride semiconductor crystal layer  10  using crystal growth through a buffer layer  52 , and therefore the surface of the substrate  50  for crystal growth side of the nitride semiconductor crystal layer  10  can be a −c surface. Furthermore, after adhesively fixing the nitride semiconductor crystal layer  10  and the glass substrate  14  through the adhesive layer  12 , the substrate  50  for crystal growth and the buffer layer  52  are removed, so the −c surface of the nitride semiconductor crystal layer  10  can be the second surface  102  which is the topmost layer. 
     The effect of making the topmost layer of the nitride semiconductor crystal layer  10  or in other words the second surface  102  (surface which emits photoelectrons) to be the −c surface will be described while referring to  FIG. 4  and  FIG. 5 .  FIG. 4  is a bandgap diagram for a photoelectric surface, and the broken line shows the case where the topmost layer is the +c surface, and the solid line shows the case where the topmost layer is the −c surface. Generally, the surface energy band of a p type semiconductor curve downward. To this is added the effect of spontaneous polarization and piezo polarization. This polarization effect is reversed depending on whether the surface is the +c surface or the −c surface, and the latter case acts effectively. In other words, if the N surface (−c surface) is the electron emission plane, the polarity direction will change from bulk towards the emission plane with both polarizations (spontaneous polarization and piezo polarization) (fixed electric charge of the polarity but the emission plane side is positive). In order to block this, the density of the acceptors which undergo electron disassociation near the surface is increased, the depletion layer is widened, and the downward facing curve effect will increase as shown by the solid line in  FIG. 4 . As a result, the vacuum level will be lowered by that amount so the photoelectrons can more easily escape and the quantum efficiency of the photoelectric surface will be increased. Furthermore, because of the widening of the depletion layer, a built-in field will be formed in the nitride semiconductor crystal layer, and the diffusion length will increase (d 1  and d 2  in  FIG. 4 ). Therefore, electrons in the deep location can reach the surface and escape. As shown in  FIG. 5 , the energy distribution properties determined from the relationship of the electron current to the applied voltage show that the high-energy component of the −cGaN is higher than that of +cGaN, and the acceleration effect due to polarization is also shown. As a result, electrons which have energy above the vacuum level (V.L.) of the surface are increased, and the number of photoelectrons which can escape will be higher.