Patent Publication Number: US-2020287060-A1

Title: Schottky barrier diode

Description:
TECHNICAL FIELD 
     The present invention relates to a Schottky barrier diode and, more particularly, to a Schottky barrier diode using gallium oxide. 
     BACKGROUND ART 
     A Schottky barrier diode is a rectifying element utilizing a Schottky barrier generated due to bonding between metal and a semiconductor and is lower in forward voltage and higher in switching speed than a normal diode having a PN junction. Thus, the Schottky barrier diode is sometimes utilized as a switching element for a power device. 
     When the Schottky barrier diode is utilized as a switching element for a power device, it is necessary to ensure a sufficient backward withstand voltage, so that, silicon carbide (SiC), gallium nitride (GaN), or gallium oxide (Ga 2 O 3 ) having a larger band gap is sometimes used in place of silicon (Si). Among them, gallium oxide has a very large band gap (4.8 eV to 4.9 eV) and a large breakdown field (7 MV/cm to 8 MV/cm), so that a Schottky barrier diode using gallium oxide is very promising as the switching element for a power device. An example of the Schottky barrier diode using gallium oxide is described in Patent Document 1 and Non-Patent Document 1. 
     In the Schottky barrier diode described in Non-Patent Document 1, a plurality of trenches are formed so as to overlap an anode electrode in a plan view, and the inner wall of each of the trenches is covered with an insulating film. With this structure, when a backward voltage is applied, a mesa region positioned between adjacent trenches becomes a depletion layer, so that a channel region of a drift layer is pinched off. Thus, a leak current upon application of the backward voltage can be significantly reduced. 
     CITATION LIST 
     Patent Document 
     
         
         [Patent Document 1] JP 2017-045969 A 
       
    
     Non-Patent Document 
     
         
         [Non-Patent Document 1] Ga 2 O 3  Schottky Barrier Diode with Trench MOS Structure (The 64th Spring Meeting of the Japan Society of Applied Physics, 2017 [15p-315-13]) 
       
    
     SUMMARY OF INVENTION 
     Problem to be Solved by the Invention 
     However, in the Schottky barrier diodes described in Patent Document 1 and Non-Patent Document 1, an electric field concentrates on the end portion of the anode electrode, so that when a high voltage is applied, dielectric breakdown occurs in this portion. For example, in the Schottky barrier diodes described in Non-Patent Document 1, an electric field concentrates on an edge part of the trench positioned at the end portion. 
     It is therefore an object of the present embodiment to provide a Schottky barrier diode using gallium oxide, which is less likely to cause dielectric breakdown due to concentration of an electric field. 
     Means for Solving the Problem 
     A Schottky barrier diode according to the present invention includes: a semiconductor substrate made of gallium oxide; a drift layer made of gallium oxide and provided on the semiconductor substrate; an anode electrode brought into Schottky contact with the drift layer; and a cathode electrode brought into ohmic contact with the semiconductor substrate. The drift layer has an outer peripheral trench formed at a position surrounding the anode electrode in a plan view. 
     According to the present invention, an electric field is dispersed by the presence of the outer peripheral trench formed in the drift layer. This alleviates concentration of the electric field on the corner of the anode electrode, making it unlikely to cause dielectric breakdown. 
     The Schottky barrier diode according to the present invention may further include an insulator embedded in the outer peripheral trench. This enhances an electric field dispersion effect. 
     In the present invention, the drift layer may further have a plurality of center trenches formed at a position overlapping the anode electrode in a plan view. In this case, the inner wall of each of the plurality of center trenches may be covered with an insulating film. With this configuration, a mesa region positioned between the adjacent center trenches becomes a depletion layer upon application of a backward voltage, so that a channel region of the drift layer is pinched off. Thus, a leak current upon application of the backward voltage can be significantly reduced. 
     In this case, the width of the outer peripheral trench may be larger than the width of the center trench, the depth of the outer peripheral trench may be larger than the depth of the center trench, and the mesa width between the outer peripheral trench and the center trench positioned closest to the outer peripheral trench may be smaller than the mesa width between the plurality of center trenches. With this configuration, concentration of an electric field is further alleviated, making it further unlikely to cause dielectric breakdown. 
     Advantageous Effects of the Invention 
     As described above, according to the present invention, there can be provided a Schottky barrier diode using gallium oxide, which is less likely to cause dielectric breakdown due to concentration of an electric field. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a top view illustrating the configuration of a Schottky barrier diode  100  according to a first embodiment of the present invention. 
         FIG. 2  is a cross-sectional view taken along line A-A in  FIG. 1 . 
         FIG. 3  is a cross-sectional view illustrating the configuration of a Schottky barrier diode  200  according to a second embodiment of the present invention. 
         FIG. 4  is a cross-sectional view illustrating the configuration of a Schottky barrier diode  300  according to a third embodiment of the present invention. 
         FIG. 5  is a view illustrating the simulation result of comparative example 1. 
         FIG. 6  is a view illustrating the simulation result of example 1. 
         FIG. 7  is a view illustrating the simulation result of comparative example 2. 
         FIG. 8  is a view illustrating the simulation result of example 2. 
         FIG. 9  is a graph illustrating the relationship between the depth of the outer peripheral trench and the electric field strength. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Preferred embodiments of the present invention will be explained below in detail with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a top view illustrating the configuration of a Schottky barrier diode  100  according to the first embodiment of the present invention.  FIG. 2  is a cross-sectional view taken along line A-A in  FIG. 1 . 
     As illustrated in  FIGS. 1 and 2 , the Schottky barrier diode  100  according to the present embodiment has a semiconductor substrate  20  and a drift layer  30 , both of which are made of gallium oxide (β-Ga 2 O 3 ). The semiconductor substrate  20  and drift layer  30  are each introduced with silicon (Si) or tin (Sn) as an n-type dopant. The concentration of the dopant is higher in the semiconductor substrate  20  than in the drift layer  30 , whereby the semiconductor substrate  20  and the drift layer  30  function as an n +  layer and an n-layer, respectively. 
     The semiconductor substrate  20  is obtained by cutting a bulk crystal formed using a melt-growing method, and the thickness (height in the Z-direction) thereof is about 250 μm. Although there is no particular restriction on the planar size of the semiconductor substrate  20 , the planar size is generally selected in accordance with the amount of current flowing in the element and, when the maximum amount of forward current is about 20 A, the widths in the X- and Y-directions may be set to about 2.4 mm. 
     The semiconductor substrate  20  has an upper surface  21  positioned on the upper surface side in a mounted state and a back surface  22  positioned on the lower surface side in a mounted state. The drift layer  30  is formed on the entire upper surface  21 . The drift layer  30  is a thin film obtained by epitaxially growing gallium oxide on the upper surface  21  of the semiconductor substrate  20  using a reactive sputtering method, a PLD method, an MBE method, an MOCVD method, or an HVPE method. Although there is no particular restriction on the film thickness of the drift layer  30 , the film thickness is generally selected in accordance with the backward withstand voltage of the element and, in order to ensure a withstand voltage of about 600 V, the film thickness may be set to, e.g., about 7 m. 
     An anode electrode  40  brought into Schottky contact with the drift layer  30  is formed on an upper surface  31  of the drift layer  30 . The anode electrode  40  is formed of metal such as platinum (Pt), palladium (Pd), gold (Au), nickel (Ni), or the like. The anode electrode  40  may have a multilayer structure of different metal films, such as Pt/Au, Pt/Al, Pd/Au, Pd/Al, Pt/Ti/Au, or Pd/Ti/Au. On the other hand, a cathode electrode  50  brought into ohmic contact with the semiconductor substrate  20  is formed on the back surface  22  of the semiconductor substrate  20 . The cathode electrode  50  is formed of metal such as titanium (Ti). The cathode electrode  50  may have a multilayer structure of different metal films, such as Ti/Au or Ti/Al. 
     The drift layer  30  has formed therein an outer peripheral trench  10  at a position not overlapping the anode electrode  40  in a plan view (as viewed in the Z-direction) so as to surround the anode electrode  40 . The outer peripheral trench  10  can be formed by etching the drift layer  30  from the upper surface  31  side. 
     The outer peripheral trench  10  is formed for alleviating an electric field concentrating on the end portion of the anode electrode  40 . In the present embodiment, the inside of the outer peripheral trench  10  is filled with an insulator  11 . In the present invention, the inside of the outer peripheral trench  10  may not necessarily be filled with the insulator  11  and may be left hollow, or may be filled partially or wholly with a conductor. However, when the inside of the outer peripheral trench  10  is filled with a conductor, the conductor needs to be electrically isolated from the anode electrode  40 . When the inside of the outer peripheral trench  10  is filled with the insulator  11 , an electric field dispersion effect can be enhanced as compared to when the inside of the outer peripheral trench  10  is left hollow. 
     As described above, in the Schottky barrier diode  100  according to the present embodiment, the outer peripheral trench  10  is formed in the drift layer  30 , so that an electric field concentrating on the end portion of the anode electrode  40  is alleviated by the outer peripheral trench  10 . This can prevent dielectric breakdown due to concentration of an electric field. 
     Second Embodiment 
       FIG. 3  is a cross-sectional view illustrating the configuration of a Schottky barrier diode  200  according to the second embodiment of the present invention. 
     As illustrated in  FIG. 3 , in the Schottky barrier diode  200  according to the second embodiment, a plurality of center trenches  60  are formed in the drift layer  30 . The center trenches  60  are all formed at a position overlapping the anode electrode  40  in a plan view. The inner wall of each of the center trenches  60  is covered with an insulating film  61  made of HfO 2  or the like. The inside of each center trench  60  is filled with the same material the anode electrode  40 . When the center trenches  60  are formed as in the present embodiment, the anode electrode  40  may be made of a material having a low working function, such as molybdenum (Mo) or copper (Cu). Further, in the present embodiment, the dopant concentration of the drift layer  30  is increased up to about 5×10 16  cm −3 . Other configurations are basically the same as those of the Schottky barrier diode  100  according to the first embodiment, so the same reference numerals are given to the same elements, and overlapping description will be omitted. 
     A part of the drift layer  30  positioned between the adjacent center trenches  60  constitutes a mesa region M 1 . The mesa region M 1  becomes a depletion layer when a backward voltage is applied between the anode electrode  40  and the cathode electrode  50 , so that a channel region of the drift layer  30  is pinched off. Thus, a leak current upon application of the backward voltage can be significantly reduced. 
     In the Schottky barrier diode having such a structure, an electric field concentrates on the bottom portion of a center trench  60   a  positioned at the end portion, making it likely to cause dielectric breakdown at this portion. However, in the Schottky barrier diode  200  according to the present embodiment, the outer peripheral trench  10  is formed at the outer periphery of the center trenches  60 , an electric field concentrating on the center trench  60   a  at the end portion is alleviated. 
     As illustrated in  FIG. 3 , a part of the drift layer  30  positioned between the center trench  60   a  at the end portion and the outer peripheral trench  10  constitutes a mesa region M 2 . Although there is no particular restriction on the relationship between a mesa width W 1  of the mesa region M 1  and a mesa width W 2  of the mesa region M 2 ,
         W 1 ≥W 2  is preferably satisfied, and   W 1 &gt;W 2  is more preferably satisfied.
 
This is because that the mesa width W 1  of the mesa region M 1  needs to be ensured to some extent in order to reduce on-resistance and that the smaller the mesa width W 2  of the mesa region M 2  is, the higher the electric field dispersion effect becomes. However, the lower limit of the mesa width W 2  of the mesa region M 2  is restricted by processing accuracy.
       

     Although there is also no particular restriction on the relationship between a width W 3  of the center trench  60  and a width W 4  of the outer peripheral trench  10 ,
         W 3 ≤W 4  is preferably satisfied, and   W 3 &lt;W 4  is more preferably satisfied.
 
This is because that the width W 3  of the center trench  60  needs to be reduced to some extent in order to reduce on-resistance and that the larger the width W 4  of the outer peripheral trench  10  is, the higher the electric field dispersion effect becomes.
       

     As described above, the Schottky barrier diode  200  according to the present embodiment has an effect that can reduce a leak current upon application of a backward voltage, in addition to the effect obtained by the Schottky barrier diode  100  according to the first embodiment. Further, in the present embodiment, the outer peripheral trench  10  and the center trench  60  have the same depth, and thus, they can be formed in the same process. 
     Further, although the inner wall of the center trench  60  is covered with the insulating film  61 , and the inside thereof is filled with the same material as the anode electrode  40  in the present embodiment, the inside of the center trench  60  may be filled with a semiconductor material of an opposite conductivity type (p-type, in the present embodiment) without the use of the insulating film  61 . 
     Third Embodiment 
       FIG. 4  is a cross-sectional view illustrating the configuration of a Schottky barrier diode  300  according to the third embodiment of the present invention. 
     As illustrated in  FIG. 4 , in the Schottky barrier diode  300  according to the third embodiment, a depth D 2  of the outer peripheral trench  10  is larger than a depth D 1  of the center trench  60 . Other configurations are basically the same as those of the Schottky barrier diode  200  according to the second embodiment, so the same reference numerals are given to the same elements, and overlapping description will be omitted. 
     When the depth D 2  of the outer peripheral trench  10  is small, the electric field dispersion effect cannot sufficiently be obtained; however, by making the depth D 2  of the outer peripheral trench  10  larger than the depth D 1  of the center trench  60 , the electric field dispersion effect can be enhanced. 
     It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention. 
     Example 1 
     A simulation model of example 1 having the same configuration as the Schottky barrier diode  100  illustrated in  FIGS. 1 and 2  was assumed, and electric field strength was simulated with a backward voltage applied between the anode electrode  40  and the cathode electrode  50 . The dopant concentration of the semiconductor substrate  20  was set to 1×10 18  cm −3 , and the dopant concentration of the drift layer  30  was to 1×10 16  cm −3 . The thickness of the drift layer  30  was set to 7 μm. For comparison, a simulation model of comparative example 1 having a structure obtained by removing the outer peripheral trench  10  and insulator  11  from the simulation model of example 1 was assumed, and electric field strength was simulated with a backward voltage applied between the anode electrode  40  and the cathode electrode  50 . 
       FIG. 5  is a view illustrating the simulation result of comparative example 1. In the simulation model of comparative example 1, an electric field concentrated on the corner of the anode electrode  40 , and the maximum value thereof was 8.3 MV/cm. 
       FIG. 6  is a view illustrating the simulation result of example 1. Also in the simulation model of example 1, an electric field concentrated on the corner of the anode electrode  40 ; however, the electric field was dispersed by the outer peripheral trench  10 , with the result that the maximum value thereof was reduced to 6.8 MV/cm. 
     Example 2 
     A simulation model of example 2 having the same configuration as the Schottky barrier diode  200  illustrated in  FIG. 3  was assumed, and electric field strength was simulated with a backward voltage applied between the anode electrode  40  and the cathode electrode  50 . The depth D 1  and the width W 3  of the center trench  60  were set to 3 μm and 1 μm, respectively, the mesa width W 1  of the mesa region M 1  was set to 2 μm, and the insulating film  61  formed on the inner wall of the center trench  60  was an HfO 2  film having a thickness of 50 nm. On the other hand, the depth D 2  and the width W 4  of the outer peripheral trench  10  were set to 3 m and 5 μm, respectively, and the mesa width W 2  of the mesa region M 2  was set to 2 μm. The dopant concentration of the drift layer  30  was set to 5×10 16  cm −3 . Other conditions are the same as those of the simulation model of example 1. 
     For comparison, a simulation model of comparative example 2 having a structure obtained by removing the outer peripheral trench  10  and insulator  11  from the simulation model of example 2 was assumed, and electric field strength was simulated with a backward voltage applied between the anode electrode  40  and the cathode electrode  50 . 
       FIG. 7  is a view illustrating the simulation result of comparative example 2. In the simulation model of comparative example 2, an electric field concentrated on the bottom portion of the center trench  60   a  positioned at the end portion, and the maximum value thereof was 12.1 MV/cm. 
       FIG. 8  is a view illustrating the simulation result of example 2. Also in the simulation model of example 2, an electric field concentrated on the bottom portion of the center trench  60   a  positioned at the end portion; however, the electric field was dispersed by the outer peripheral trench  10 , with the result that the maximum value thereof was reduced to 11.6 MV/cm. 
     On the other hand, the electric field concentrated to some extent also at the bottom portion of the center trench  60  located at a position other than the end portion; however, no difference was observed between example 2 and comparative example 2, and the maximum values thereof were both 9.4 MV/cm. 
     Example 3 
     A simulation model of example 3 having the same configuration as the Schottky barrier diode  300  illustrated in  FIG. 4  was assumed, and electric field strength was simulated with a forward voltage applied between the anode electrode and the cathode electrode. In the simulation model of example 3, the depth D 2  of the outer peripheral trench  10  was set to 5 μm. Other conditions are the same as those of the simulation model of example 2. 
       FIG. 9  is a graph illustrating the relationship between the depth of the outer peripheral trench and the electric field strength. In  FIG. 9 , E 1  denotes the maximum electric field applied to the semiconductor substrate  20  near the center trench  60  located at a position other than the end portion, E 2  denotes the maximum electric field applied to the semiconductor substrate  20  near the center trench  60   a  located at the end portion, E 3  denotes the maximum electric field applied to the insulating film  61  covering the inner wall of the center trench  60  located at a position other than the end portion, and E 4  denotes the maximum electric field applied to the insulating film  61  covering the inner wall of the center trench  60   a  located at the end portion. 
     As illustrated in  FIG. 9 , the electric field applied to the semiconductor substrate  20  and insulating film  61  near the center trench  60   a  positioned at the end portion was more alleviated as the depth of the outer peripheral trench  10  was increased. On the other hand, the electric field applied to the semiconductor substrate  20  and insulating film  61  near the center trench  60  positioned at a position other than the end portion was constant regardless of the depth of the outer peripheral trench  10 . 
     REFERENCE SIGNS LIST 
     
         
           10  peripheral trench 
           11  insulator 
           20  semiconductor substrate 
           21  upper surface of semiconductor substrate 
           22  back surface of semiconductor substrate 
           30  drift layer 
           31  upper surface of the drift layer 
           40  anode electrode 
           50  cathode electrode 
           60  center trench 
           60   a  center trench at end portion 
           61  insulating film 
           100 ,  200 ,  300  Schottky barrier diode 
         M 1 , M 2  mesa region