Patent Publication Number: US-9412856-B2

Title: Semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-052374, filed Mar. 14, 2014, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The disclosure relates to a semiconductor device. 
     BACKGROUND 
     A semiconductor element such as a switching element or a diode is used in a circuit such as a switching power source or an inverter. These semiconductor elements are required to have high breakdown voltage and low ON resistance. In general, there exists a tradeoff based on a material of the semiconductor element between the breakdown voltage and the ON resistance. 
     The lowering of the ON resistance of a semiconductor element is believed to be approaching the limit of silicon, which is a primary material of semiconductor device elements. Thus, to further lower the ON resistance, it is necessary to use elements other than silicon. With the use of a nitride semiconductor such as GaN or AlGaN or a wide band gap semiconductor such as a silicon carbide (SiC) as a material of a switching element, the tradeoff relationship can be improved, thus enabling a lowering of the ON resistance of a semiconductor device. 
     A High Electron Mobility Transistor (HEMT) which uses the AlGaN/GaN heterojunction structure is a nitride semiconductor with a low ON resistance, for example. The HEMT achieves the low ON resistance through the high mobility of electrons at a heterojunction interface channel and a high electron concentration generated by polarization. 
     Since electrons are generated by polarization in the HEMT a high concentration of electrons exist below a gate electrode. Accordingly, a HEMT is usually a normally-ON element (that is, the device channel is conductive unless a voltage is applied to the gate electrode) and the threshold voltage of the HEMT is generally negative. However, in view of a safe operation, there has been a demand for a normally-OFF element where a threshold voltage is positive. Further, it is desirable to increase a gate-on threshold voltage for increasing an ON current in the normally-OFF element. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view illustrating a semiconductor device according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present disclosure provides a semiconductor device having an increased gate-on threshold voltage. 
     A semiconductor device includes a first and second nitride semiconductor layer. The second nitride semiconductor layer has a band gap larger the first nitride semiconductor layer. Source and drain electrodes are spaced from each other on the second nitride semiconductor layer. A third nitride semiconductor layer is on the second nitride semiconductor layer between the source and drain electrodes. A gate electrode is formed on the third nitride semiconductor layer between the source and drain electrodes. The third nitride semiconductor layer comprises a plurality of stacked layers which includes at least two high-dopant concentration layers and at least one a low-dopant concentration layer. The low-dopant concentration has a lower p-type dopant concentration than the high-dopant concentration layer. The low-dopant concentration also has a band gap larger than the high-dopant concentration layer. The lowermost layer (the layer nearest the second nitride semiconductor layer) and the uppermost layer (the layer nearest the gate electrode) in the third nitride semiconductor layer stack are high-dopant concentration layers. 
     In general, according to one embodiment, a semiconductor device includes: a first nitride semiconductor layer; a second nitride semiconductor layer that is formed on the first nitride semiconductor layer and has a band gap larger than a band gap of the first nitride semiconductor layer; a source electrode that is formed on the second nitride semiconductor layer; a drain electrode that is formed on the second nitride semiconductor layer; a third nitride semiconductor layer that is formed on the second nitride semiconductor layer between the source electrode and the drain electrode, and is formed by alternately laminating a high-dopant concentration layer which contains a p-type dopant and a low-dopant concentration layer which has lower p-type dopant concentration than the high-dopant concentration layer and a band gap larger than a band gap of the high-dopant concentration layer, the lowermost layer and the uppermost layer being the high-dopant concentration layer; and a gate electrode that is formed on the third nitride semiconductor layer. 
     In the disclosure, identical or substantially similar elements are given the same reference symbols, and repeated explanation of these parts may be omitted when appropriate. 
     In this disclosure, “nitride semiconductor” includes a GaN-based semiconductor, for example. “GaN-based semiconductor” is a general term for a semiconductor material which includes at least one of gallium nitride (GaN), aluminum nitride (AlN), and indium nitride (InN), or mixtures of these materials. 
     In the disclosure, “undoped” means that a dopant is not introduced intentionally to the relevant material or layer. 
     The semiconductor device according to the embodiment includes: a first nitride semiconductor layer; a second nitride semiconductor layer which is formed on the first nitride semiconductor layer and has a band gap larger than that of the first nitride semiconductor layer; a source electrode which is formed on the second nitride semiconductor layer; a drain electrode which is formed on the second nitride semiconductor layer; a third nitride semiconductor layer which is formed on the second nitride semiconductor layer between the source electrode and the drain electrode, and is formed by alternately laminating a high-dopant concentration layer which contains a p-type dopant and a low-dopant concentration layer which has lower p-type dopant concentration than the high-dopant concentration layer and a band gap larger than that of the high-dopant concentration layer, the lowermost layer and the uppermost layer being the high-dopant concentration layer; and a gate electrode which is formed on the third nitride semiconductor layer. 
       FIG. 1  is a schematic cross-sectional view illustrating a semiconductor device according to an example embodiment. The semiconductor device according to the example embodiment is an HEMT including a GaN-based semiconductor. 
     As illustrated in  FIG. 1 , the semiconductor device (HEMT)  100  includes: a substrate  10 ; a buffer layer  12 ; a channel layer (first nitride semiconductor layer)  14 ; a barrier layer (second nitride semiconductor layer)  16 ; a source electrode  18 ; a drain electrode  20 ; a cap layer (third nitride semiconductor layer)  22 ; and a gate electrode  26 . 
     The substrate  10  is made of silicon (Si), for example. Sapphire (Al 2 O 3 ) or silicon carbide (SiC) can be also used in place of silicon, for example. 
     The buffer layer  12  is formed on the substrate  10 . The buffer layer  12  has a function of alleviating a lattice mismatch between the substrate  10  and the channel layer  14 . The buffer layer  12  has the multilayer structure formed of aluminum gallium nitride (Al w Ga 1-w N (0&lt;W&lt;1)), for example. 
     The channel layer  14  is formed on the buffer layer  12 . The channel layer  14  is made of undoped Al z Ga 1-z N (0≦Z&lt;1), for example. To be more specific, the channel layer  14  is made of undoped GaN, for example. A film thickness of the channel layer  14  is between 0.5 μm and 3 μm, inclusive, for example. 
     The barrier layer  16  is formed on the channel layer  14 . A band gap of the barrier layer  16  is larger than a band gap of the channel layer  14 . The barrier layer  16  is made of undoped Al u Ga 1-u N (0&lt;U≦1, where U&gt;Z), for example. To be more specific, the barrier layer  16  is made of undoped Al 0.20 Ga 0.80 N, for example. A film thickness of the barrier layer  16  is between 20 nm and 50 nm, inclusive, for example. 
     A heterojunction interface is formed between the channel layer  14  and the barrier layer  16 . When the HEMT  100  is turned on, a two-dimensional electron gas (2DEG) is formed at the hetero-junction interface, and the gas becomes a carrier. 
     The source electrode  18  and the drain electrode  20  are formed on the barrier layer  16 . The source electrode  18  and the drain electrode  20  are respectively formed of a metal electrode, for example. The metal electrode is an electrode which is mainly made of aluminum (Al), for example. It is desirable that an ohmic contact be formed between the source electrode  18  and the barrier layer  16  as well as between the drain electrode  20  and the barrier layer  16 . A distance between the source electrode  18  and the drain electrode  20  is approximately 18 μm, for example. 
     The cap layer  22  is formed on the barrier layer  16  between the source electrode  18  and the drain electrode  20 . The cap layer  22  has a function of increasing a potential of the channel layer  14  thus increasing a threshold value of the HEMT  100 . 
     The cap layer  22  has the structure where a high-dopant concentration layer  22   a  and a low-dopant concentration layer  22   b  are alternately laminated. The lowermost layer and the uppermost layer of the cap layer  22  are formed of the high-dopant concentration layer  22   a.    
     Although the case where three high-dopant concentration layers  22   a  and two low-dopant concentration layers  22   b  are laminated to each other is illustrated in  FIG. 1  as an example, the number of layers of the high-dopant concentration layer  22   a  is not limited to three layers, and the number of layers of the low-dopant concentration layers  22   b  is not limited to two layers. Furthermore, the composition, a p-type dopant concentration, and a film thickness of the high-dopant concentration layer  22   a  may differ for each layer—that is, each layer  22   a  need not be the same as each other layer  22   a , but may vary in composition, dopant concentration, and/or thickness. In the same manner, the composition, a p-type dopant concentration and a film thickness of the low-dopant concentration layer  22   b  may differ for each layer—that is, each layer  22   b  need not be the same as each other layer  22   b , but may vary in composition, dopant concentration, and/or thickness. 
     The high-dopant concentration layer  22   a  contains a p-type dopant. The p-type dopant is Mg (magnesium), for example. From a viewpoint of increasing a threshold value of the HEMT  100 , it is desirable that the p-type dopant concentration in the high-dopant concentration layer  22   a  is 1×10 18  atoms/cm 3  or more. 
     Further, from a viewpoint of bringing a junction between the cap layer  22  and the gate electrode  26  into an ohmic contact, it is desirable that a p-type dopant concentration in the cap layer  22  become the largest in the high-dopant concentration layer  22   a  which is the uppermost layer in contact with the gate electrode  26 . It is desirable that the p-type dopant concentration in the high-dopant concentration layer  22   a  which is the uppermost layer be 1×10 19  atoms/cm 3  or more. 
     The high-dopant concentration layer  22   a  is made of p-type Al x Ga 1-x N (0≦X&lt;1), for example. To be more specific, the high-dopant concentration layer  22   a  is made of p-type GaN, for example. A film thickness of the high-dopant concentration layer  22   a  between 1 nm and 10 nm, inclusive, for example. The high-dopant concentration layer  22   a  is made of a single crystal. 
     A dopant concentration can be analyzed by SIMS (Secondary Ion Mass Spectrometry), for example. 
     The low-dopant concentration layer  22   b  has a lower p-type dopant concentration than the high-dopant concentration layer  22   a . Further, the low-dopant concentration layer  22   b  has a larger band gap than the high-dopant concentration layer  22   a.    
     The low-dopant concentration layer  22   b  is made of undoped Al y Ga 1-y N (0&lt;Y≦1, where Y&gt;X), for example. To be more specific, the low-dopant concentration layer  22   b  is made of undoped Al 0.40 Ga 0.60 N, for example. A film thickness of the low-dopant concentration layer  22   b  between 1 nm and 10 nm, inclusive, for example. The low-dopant concentration layer  22   b  is made of a single crystal. 
     The gate electrode  26  is formed on the cap layer  22 . The gate electrode  26  is a metal electrode, for example. The metal electrode is an electrode having a laminated structure of platinum (Pt) and gold (Au), for example. It is desirable that an ohmic contact be formed between the gate electrode  26  and the high-dopant concentration layer  22   a  which forms the uppermost layer. 
     Next, one example of a method of manufacturing the semiconductor device according to this embodiment is explained. 
     Firstly, a substrate  10 , for example, a Si substrate is prepared. Next, the buffer layer  12  is formed on the Si substrate by an epitaxial growth method, for example. 
     Next, an undoped GaN layer (which becomes the channel layer  14 ) and an undoped Al 0.20 Ga 0.80 N layer (which becomes the barrier layer  16 ) are formed on the buffer layer  12  by an epitaxial growth method. 
     Next, a p-type GaN layer (which forms the high-dopant concentration layer  22   a ) and an undoped Al 0.40 Ga 0.60 N layer (which forms the low-dopant concentration layer  22   b ) are formed by an epitaxial growth method in an alternating layer stack, so as to form the cap layer  22 . 
     The cap layer  22  is selectively grown on a surface of the barrier layer  16  by forming a patterned insulation film on a surface of the barrier layer  16 , for example, before forming the cap layer  22 . 
     Next, the source electrode  18  and the drain electrode  20  are formed on the surface of the barrier layer  16  by forming a metal film on a surface of the barrier layer  16  and by patterning to the metal film. The gate electrode  26  is formed on a surface of the cap layer  22  by forming a metal film on a surface of the cap layer  22  and patterning the metal film. 
     The semiconductor device  100  illustrated in  FIG. 1  is manufactured by the above-mentioned manufacturing method. 
     Next, the manner of operation and advantageous effects of the semiconductor device  100  according to this example embodiment are explained. 
     In the HEMT  100 , the cap layer  22 , which contains the p-type dopant, is present directly below the gate electrode  26  and hence, a potential of the channel layer  14  is increased. Accordingly, the generation of a two-dimensional electron gas is suppressed so that a threshold value of the HEMT  100  is increased as compared to a case where the cap layer  22  is not present. When an energy level at a lower end of a conductive band on a heterojunction interface falls on a higher energy side than a Fermi level, the channel layer  14  is depleted even when a gate voltage is 0 V so that the HEMT  100  is of a normally-off operation type. 
     To increase an ON current of the HEMT  100 , it is desirable to increase a gate-on (threshold) voltage applied to the gate electrode  26  at the time of turning on the HEMT  100 . However, when the gate-on voltage is excessively increased at the time of turning on the HEMT  100 , a gate leak current, which flows between the source electrode  18  and the gate electrode  26 , is increased, for example, so that there exists a possibility that power consumption will be increased or an operation failure will occur. 
     When a band gap of the cap layer  22  is increased, a forward rising voltage of a pn junction between the barrier layer  16  and the cap layer  22  is increased and hence, a gate-on voltage can be increased as a result. 
     On the other hand, in the GaN-based semiconductor, the larger the band gap, the lower a p-type dopant activation rate becomes and hence, it is difficult to form a p-type semiconductor. For example, to compare GaN with AlGaN or to compare GaN with AlN, GaN which has a smaller band gap exhibits a higher p-type dopant activation rate. Accordingly, it is difficult for a semiconductor having a large band gap to increase a potential of the channel layer  14  by increasing a p-type dopant concentration. 
     In this example embodiment, the high-dopant concentration layer  22   a  having a small band gap and the low-dopant concentration layer  22   b  having a large band gap are laminated to each other. Due to such a configuration, the high-dopant concentration layer  22   a  having a high p-type dopant activation rate contributes to the increase of a p-type dopant concentration in the cap layer  22  as a whole. On the other hand, the low-dopant concentration layer  22   b  having a large band gap contributes to the increase of a band gap of the cap layer  22  as a whole. 
     Accordingly, it is possible to achieve the nitride semiconductor layer where the cap layer  22  effectively has a large band gap and a high p-type dopant concentration when considered as a whole. Accordingly, it is possible to achieve the HEMT  100  which performs a normally-off operation and has a high gate-on voltage. 
     A film thickness of the low-dopant concentration layer  22   b  is desirably from 1 nm or more to 10 nm or less, and is more desirably from 2 nm or more to 6 nm or less. 
     When the film thickness of the low-dopant concentration layer  22   b  becomes lower than the above-mentioned range, it is difficult to increase a band gap of the cap layer  22  as a whole. On the other hand, when the film thickness of the low-dopant concentration layer  22   b  exceeds the above-mentioned range, there exists a possibility that a p-type dopant concentration in the cap layer  22  becomes insufficient as a whole. 
     A film thickness of the high-dopant concentration layer  22   a  is desirably from 1 nm or more to 10 nm or less, and is more desirably from 2 nm or more to 6 nm or less. 
     When the film thickness of the high-dopant concentration layer  22   b  becomes lower than the above-mentioned range, it is difficult to increase a p-type dopant concentration of the cap layer  22  as a whole. On the other hand, when the film thickness of the high-dopant concentration layer  22   b  exceeds the above-mentioned range, there exists a possibility that a band gap of the cap layer  22  will become extremely small as a whole. 
     From a viewpoint of allowing the cap layer  22  to have a characteristic which is acquired by averaging the property of the high-dopant concentration layer  22   a  and the property of the low-dopant concentration layer  22   b , it is desirable that a film thickness of the low-dopant concentration layer  22   b  and a film thickness of the high-dopant concentration layer  22   a  each be set to 10 nm or less. Further, from a viewpoint of allowing the cap layer  22  to have a characteristic which is acquired by averaging the property of the high-dopant concentration layer  22   a  and the property of the low-dopant concentration layer  22   b , it is desirable that the cap layer  22  have a superlattice structure. 
     It is desirable that an average aluminum composition of the cap layer (third nitride semiconductor layer)  22  be lower than an average aluminum composition of the barrier layer (second nitride semiconductor layer)  16 . When a band gap of the cap layer  22  becomes larger than a band gap of the barrier layer  16 , there is a possibility that an amount of piezo-polarization generated directly below the gate electrode  26  will be increased so that a threshold voltage level of the HEMT  100  is lowered. 
     The average aluminum composition of the cap layer  22  is obtained by dividing a sum of the products of aluminum compositions and film thicknesses of the respective high-dopant concentration layers  22   a  and the products of aluminum compositions and film thickness of the respective low-dopant concentration layers  22   b  by a total, combined film thickness of the cap layer  22 . 
     For example, assuming that a composition of the ith (1≦i) high-dopant concentration layer  22   a  is expressed as Al Xi Ga 1-Xi N, where Xi is a respective composition value for the ith high-dopant concentration layer  22   a , a film thickness of the ith high-dopant concentration layer  22   a  is expressed as Tai, a composition of the ith (1≦i) low-dopant concentration layer  22   b  is expressed as Al Yi Ga 1-Yi N, where Yi is a respective composition value for the ith low-dopant concentration layer  22   b , and a film thickness of the ith low-dopant concentration layer  22   b  is expressed as Tbi, when the number of layers of the high-dopant concentration layers  22   a  is n (1≦i≦n), the average aluminum composition of the cap layer  22  is expressed by the following formula. 
     
       
         
           
             
               
                 
                   
                     
                       
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     Accordingly, assuming that a composition of the barrier layer  16  is expressed by Al u Ga 1-u N, for example, it is desirable that the following formula be satisfied. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     An average aluminum composition of the cap layer  22  can be obtained by analyzing layer composition in a film thickness direction by SIMS. When the cap layer  22  has a superlattice structure, for example, the average aluminum composition of the cap layer  22  can be obtained also by a photoluminescence method. 
     As has been explained, according to this embodiment, there is provided the HEMT  100  is a normally-off transistor and has a high gate-on (threshold) voltage. 
     Although the explanation has been made by taking the example case where GaN or AlGaN is used as a material for forming the nitride semiconductor layer, it is also possible to use InGaN, InAlN or InAlGaN, which contains indium (In), for forming the nitride semiconductor layer, for example. Furthermore, AlN can be also used as a material for forming the nitride semiconductor layer. 
     Although undoped AlGaN is explained as a material for forming the barrier layer as an example, n-type AlGaN can also be used as a material for forming the barrier layer. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.