Abstract:
A semiconductor device includes: a semiconductor layer disposed above a substrate; an insulating film formed by oxidizing a portion of the semiconductor layer; and an electrode disposed on the insulating film, wherein the insulating film includes gallium oxide, or gallium oxide and indium oxide.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional Application of Ser. No. 13/315,682 filed on Dec. 9, 2011, which claims the benefit of priority from Japanese Patent Application No. 2011-35117 filed on Feb. 21, 2011, the entire contents of both of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field 
     The embodiments discussed herein relate to a semiconductor device and a method for manufacturing the semiconductor device. 
     2. Description of Related Art 
     Materials including GaN, AlN, InN, and a mixed crystal of these materials, which are included in nitride semiconductors, have a wide band-gap and are used in high-output electronic devices, short-wavelength light-emitting devices, and the like. Field-effect transistors (FETs), for example, high electron mobility transistors (HEMTs) are used in high-output-high-efficiency-amplifiers or high-power switching devices, and the like. In an HEMT including an electron supply layer having AlGaN and an electron transit layer having GaN, strain due to a difference in a lattice constant between AlGaN and GaN may occur in AlGaN and piezoelectric polarization may occur. Since a high-concentration two-dimensional electron gas is generated by the piezoelectric polarization, a high-output device may be provided. 
     For example, related are is disclosed in Japanese Laid-open Patent Publication Nos. 2002-359256, 2007-19309, and 2009-76845. 
     SUMMARY 
     According to one aspect of the embodiments, a semiconductor device includes: a semiconductor layer disposed above a substrate; an insulating film formed by oxidizing a portion of the semiconductor layer; and an electrode disposed on the insulating film, wherein the insulating film includes gallium oxide, or gallium oxide and indium oxide. 
     Additional advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become more apparent to those skilled in the art upon examination of the following or upon learning by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary semiconductor device; 
         FIGS. 2A to 2I  illustrate an exemplary method for producing a semiconductor device; 
         FIG. 3  illustrates an exemplary a characteristic of supercritical water; 
         FIG. 4  illustrates an exemplary a semiconductor device; 
         FIG. 5  illustrates an exemplary a discrete-packaged semiconductor device; 
         FIG. 6  illustrates an exemplary power supply device; and 
         FIG. 7  illustrates an exemplary a high-frequency amplifier. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In an HEMT which is used in a high-output-high-efficiency-amplifier, a high-power switching device, or the like, normally-off may be applied thereto, and the HEMT may have a high breakdown voltage. In order to achieve a normally-off operation, a portion of a semiconductor layer located under a gate electrode is removed to form a gate recess. In a gate recess structure, the threshold voltage becomes positive without increasing the resistance between electrodes. In order to obtain a high drain breakdown voltage and a high gate breakdown voltage, a metal insulator semiconductor (MIS) structure including a gate insulating film is used in an FET or HEMT having a lateral structure. The gate recess structure and the MIS structure may be applied to an HEMT including a GaN-based semiconductor material. 
     In a semiconductor device having the gate recess structure and the MIS structure, a portion of a semiconductor layer formed by epitaxial growth or the like is removed by, for example, etching, and an insulating film may be formed on the etched portion. Consequently, the semiconductor device may be damaged by the etching. 
     In embodiments, substantially the same components and the like are assigned the same reference numerals, and the description of those components may be omitted or reduced. 
       FIG. 1  illustrates an exemplary semiconductor device. The semiconductor device illustrated in  FIG. 1  may include a transistor called HEMT. The semiconductor device includes a semiconductor layer including an electron transit layer  21 , a spacer layer  22 , an electron supply layer  23 , and a cap layer  24 , the semiconductor layer being provided on a buffer layer  20  formed on a substrate  10  corresponding to a semiconductor or the like. The semiconductor layer is formed by epitaxial growth such as metal-organic vapor phase epitaxy (MOVPE). A source electrode  42  and a drain electrode  43  that are coupled to the electron supply layer  23  are provided. An insulating film  30  corresponding to a gate insulating film is provided on a region of the electron transit layer  21  where a gate electrode  41  is formed. The gate electrode  41  is provided on the insulating film  30 . The source electrode  42  and the drain electrode  43  may be coupled to the electron transit layer  21 . A protective film including an insulator may be provided so as to cover the cap layer  24 . 
     The substrate  10  may be any one of a Si substrate, a SiC substrate, a sapphire (Al 2 O 3 ) substrate, and the like. For example, when a Si substrate is used as the substrate  10 , the buffer layer  20  may be provided. When a material other than silicon is used as the substrate  10 , the buffer layer  20  may not be provided. The electron transit layer  21  may be composed of i-GaN. The spacer layer  22  may be i-AlGaN. The electron supply layer  23  may be composed of n-AlGaN. The cap layer  24  may be an n-GaN. A two-dimensional electron gas (2DEG)  21   a  is generated on the electron supply layer  23  side of the electron transit layer  21 . 
     The gate electrode  41 , the source electrode  42 , and the drain electrode  43  may include a metal material. The insulating film  30  corresponding to the gate insulating film is formed by oxidizing the spacer layer  22 , the electron supply layer  23 , and the cap layer  24 . For example, by oxidizing i-AlGaN corresponding to the spacer layer  22 , n-AlGaN corresponding to the electron supply layer  23 , and n-GaN corresponding to the cap layer  24 , Ga 2 O 3  and Al 2 O 3  are formed. The Ga 2 O 3  and Al 2 O 3  may correspond to the insulating film  30 . Alternatively, the electron supply layer  23  and the cap layer  24  may be oxidized. Alternatively, only the cap layer  24  may be oxidized. 
     The semiconductor layer may include GaN and AlGaN. Alternatively, the semiconductor layer may include a nitride semiconductor such as InAlN or InGaAlN. The insulating film  30  corresponding to the gate insulating film may be Ga 2 O 3 , In 2 O 3 , Al 2 O 3 , and the like formed by oxidizing InAlN and InGaAlN. 
       FIGS. 2A to 2I  illustrate an exemplary a method for producing a semiconductor device. 
     As illustrated in  FIG. 2A , a buffer layer  20  is formed on a substrate  10 . A semiconductor layer including, for example, an electron transit layer  21 , a spacer layer  22 , an electron supply layer  23 , and a cap layer  24  is formed on the buffer layer  20  by epitaxial growth such as MOVPE. A silicon nitride (SiN) film  61  for forming a mask is formed on the cap layer  24 . The substrate  10  may be a substrate composed of Si, SiC, sapphire (Al 2 O 3 ), or the like. The buffer layer  20  for epitaxially growing the electron transit layer  21  and other layers is formed on the substrate  10 . The buffer layer  20  may be, for example, an undoped i-AlN layer having a thickness of 0.1 μm. The electron transit layer  21  may be an undoped i-GaN layer having a thickness of 3 μm. The spacer layer  22  may be an undoped i-AlGaN layer having a thickness of 5 nm. The electron supply layer  23  may be an n-Al 0.25 Ga 0.75 N layer having a thickness of 30 nm. The electron supply layer  23  is doped with Si serving as an impurity element at a concentration of 5×10 18  cm −3 . The cap layer  24  may be an n-GaN layer having a thickness of 10 nm. The cap layer  24  is doped with Si serving as an impurity element at a concentration of 5×10 18  cm −3 . The silicon nitride (SiN) film  61  having a thickness of 200 nm is deposited on the cap layer  24  by plasma enhanced chemical vapor deposition (PECVD). 
     As illustrated in  FIG. 2B , a resist pattern  62  is formed on the silicon nitride film  61 . For example, a photoresist is applied onto the silicon nitride film  61 . The photoresist is exposed by an exposure apparatus and then developed to form the resist pattern  62 . The resist pattern  62  may have an opening in a region where an insulating film corresponding to a gate insulating film is to be formed. 
     As illustrated in  FIG. 2C , the silicon nitride film  61  in a region where the resist pattern  62  is not formed is removed by dry etching such as reactive ion etching (RIE). The resist pattern  62  may also be removed. A mask  64  including the silicon nitride film  61  having an opening  63  is formed. The silicon nitride film  61  is used as the material of the mask  64 . Alternatively, other materials that are not easily oxidized may be used. 
     As illustrated in  FIG. 2D , the spacer layer  22 , the electron supply layer  23 , and the cap layer  24  that are exposed in the opening  63  of the mask  64  are oxidized to form an insulating film  30  corresponding to a gate insulating film. For example, the insulating film  30  may be formed by oxidizing the spacer layer  22 , the electron supply layer  23 , and the cap layer  24  using supercritical water. For example, the silicon nitride film  61  having the opening  63  is placed at a predetermined position in a chamber filled with pure water. The inside of the chamber is set to a high-temperature, high-pressure state, and the pure water in the chamber becomes supercritical water. Since supercritical water has a strong oxidizing power, materials that are oxidized at high temperatures are oxidized at lower temperatures by using supercritical water. For example, GaN and AlGaN are thermally oxidized at a temperature of 1,000° C. or higher. However, the crystal structures of the epitaxially grown semiconductor layers may be broken by heating at such a high temperature. In oxidation with supercritical water, supercritical water at a temperature of 700° C. or lower, for example, about 380° C. is used, and thus a semiconductor layer in a predetermined region that is in contact with supercritical water is oxidized. A nitride semiconductor is oxidized without causing disorder of the crystal structure of the semiconductor layer. The insulating film  30  corresponding to the gate insulating film is formed by oxidizing a part of a semiconductor layer including a nitride semiconductor without degrading characteristics of the semiconductor device. 
       FIG. 3  illustrates an exemplary characteristic of supercritical water. The term “supercritical water” refers to water in a state at a temperature of 374° C. or higher and a pressure of  218  atmospheres or more, and refers to, for example, water in a state where gas and liquid are not distinguished from each other. Supercritical water has properties different from those of normal water. The properties of supercritical water may accelerate a chemical reaction. By using supercritical water, GaN and AlGaN, which are oxidized at high temperatures, are oxidized at relatively low temperatures. For example, by oxidizing the spacer layer  22  corresponding to i-AlGaN, the electron supply layer  23  corresponding to n-AlGaN, or the cap layer  24  corresponding to n-GaN, the insulating film  30  corresponding to Ga 2 O 3  and Al 2 O 3  is formed. The spacer layer  22 , the electron supply layer  23 , and the cap layer  24  are oxidized by using supercritical water at a temperature of 700° C. or lower, for example, 550° C. or lower to form the insulating film  30 . Subcritical water, which is in a state at a temperature and a pressure slightly lower than those of the critical point, also has a strong oxidizing power. The oxidizing power of water in a state close to subcritical water is also relatively strong. Accordingly, GaN and AlGaN may be oxidized at low temperatures by using water that is in a liquid state at a temperature higher than 100° C., as in the case of supercritical water. For example, a semiconductor layer including a nitride semiconductor may be oxidized at a temperature equal to or lower than a temperature at which the source electrode  42  and the drain electrode  43  are brought into ohmic contact with each other. 
     A recess is formed in the oxidized region formed by oxidizing the spacer layer  22 , the electron supply layer  23 , and the cap layer  24 , and in addition, an insulating film is formed in the recess. A process of forming the recess and a process of forming the insulating film are performed contemporaneously, and thus the manufacturing process may be simplified. 
     Similarly, in a semiconductor device including a nitride semiconductor other than GaN and AlGaN, an oxidized film corresponding to an insulting film may be formed. For example, also in a semiconductor device in which a nitride semiconductor such as InAlN or InGaAlN is used as a semiconductor layer, an insulating film including In 2 O 3 , Ga 2 O 3 , and Al 2 O 3  may be formed by oxidation using supercritical water or the like. 
     As illustrated in  FIG. 2E , the silicon nitride film  61  is removed. For example, the silicon nitride film  61  is removed by wet etching using hot phosphoric acid. 
     As illustrated in  FIG. 2F , a resist pattern  65  is formed. For example, a photoresist is applied onto the surface of the cap layer  24 . The photoresist is exposed by an exposure apparatus and then developed to form the resist pattern  65 . The resist pattern  65  has openings in regions where a source electrode  42  and a drain electrode  43  are to be formed. Before the formation of the resist pattern  65 , an element isolation region (not illustrated) may be formed. A resist pattern (not illustrated) having an opening in the element isolation region is formed. Dry etching using a chlorine-based gas or ion implantation is conducted in the opening to form the element isolation region. The resist pattern (not illustrated) used for forming the element isolation region is removed. 
     As illustrated in  FIG. 2G , dry etching such as RIE is conducted using a chlorine-based gas to remove the cap layer  24  in the openings of the resist pattern  65 . The resist pattern  65  is also removed by an organic solvent or the like. Thus, the cap layer  24  is removed in the regions where the source electrode  42  and the drain electrode  43  are to be formed. 
     As illustrated in  FIG. 2H , the source electrode  42  and the drain electrode  43  are formed. For example, a resist pattern (not illustrated) having openings in the regions where the source electrode  42  and the drain electrode  43  are to be formed is formed. A photoresist is applied onto the surface on which the cap layer  24  is formed. The photoresist is exposed by an exposure apparatus and then developed to form the resist pattern. A metal film, for example, a Ta film having a thickness of about 20 nm or an Al film having a thickness of about 200 nm is formed over the entire surface by vacuum deposition or the like. The metal film deposited on the resist pattern is then removed by lift-off using an organic solvent. The metal film in regions where the resist pattern is not formed is used as the source electrode  42  and drain electrode  43  which are disposed on the electron supply layer  23  and includes Ta/Al. The metal film, which includes Ta and is in contact with the cap layer  24 , is heat-treated in a nitrogen atmosphere at a temperature in the range of 400° C. to 700° C., for example, at 550° C., thereby establishing ohmic contact between the source electrode  42  and the drain electrode  43 . In the case where the ohmic contact is established without heat treatment, the heat treatment may not be conducted. 
     As illustrated in  FIG. 2I , a gate electrode  41  is formed. For example, a resist pattern (not illustrated) having an opening in a region where the gate electrode  41  is to be formed is formed. A photoresist is applied onto the surface on which the cap layer  24  is formed. The photoresist is exposed by an exposure apparatus and then developed to form the resist pattern. A metal film, for example, a Ni film having a thickness of about 40 nm or a Au film having a thickness of about 400 nm is formed over the entire surface by vacuum deposition. The metal film deposited on the resist pattern is then removed by lift-off using an organic solvent. The metal film in a region where the resist pattern is not formed is used as the gate electrode  41  which is disposed on the insulating film  30  and includes Ni/Au. Heat treatment or the like may then be conducted as required. 
       FIG. 4  illustrates an exemplary semiconductor device. As illustrated in  FIG. 4 , a protective film  50  may be formed in regions where the cap layer  24  is exposed. For example, the protective film  50  may include an insulator. For example, the protective film  50  may be formed by depositing an aluminum oxide film or a silicon nitride film by plasma atomic layer deposition (ALD) or the like. 
       FIG. 5  illustrates an exemplary discrete-packaged semiconductor device. The semiconductor device illustrated in  FIG. 1  or  4  may be discrete-packaged.  FIG. 5  schematically illustrates the inside of the discrete-packaged semiconductor device. Therefore, the arrangement of electrodes etc. illustrated in  FIG. 5  may be different from the structure illustrated in  FIG. 1  or  4 . 
     For example, the semiconductor device illustrated in  FIG. 1  or  4  is cut by dicing or the like to form a semiconductor chip  410  of HEMT including a GaN-based semiconductor material. The semiconductor chip  410  is fixed on a lead frame  420  with a die attaching agent  430  such as solder. 
     A gate electrode  441  is coupled to a gate lead  421  with a bonding wire  431 . A source electrode  442  is coupled to a source lead  422  with a bonding wire  432 . A drain electrode  443  is coupled to a drain lead  423  with a bonding wire  433 . The bonding wires  431 ,  432 , and  433  may include a metal material such as Al. The gate electrode  441  may be a gate electrode pad, and is coupled to, for example, the gate electrode  41 . The source electrode  442  may be a source electrode pad, and is coupled to, for example, the source electrode  42 . The drain electrode  443  may be a drain electrode pad, and is coupled to, for example, the drain electrode  43 . 
     A resin seal is performed by a transfer molding method using a molding resin  440 , thus producing a semiconductor device in which the HEMT including the GaN-based semiconductor material is discrete-packaged. 
       FIG. 6  illustrates an exemplary power supply device. A power supply device  460  illustrated in  FIG. 6  includes a high-voltage primary side circuit  461 , a low-voltage secondary side circuit  462 , and a transformer  463  provided between the primary side circuit  461  and the secondary side circuit  462 . The primary side circuit  461  includes an AC power supply  464 , for example, a bridge rectifier circuit  465 , and a plurality of switching elements, for example, four switching elements  466  and one switching element  467 , etc. The secondary side circuit  462  includes a plurality of switching elements, for example, three switching elements  468 . The semiconductor device illustrated in  FIG. 1  or  4  may be used as the switching elements  466  and  467  of the primary side circuit  461 . The switching elements  466  and  467  of the primary side circuit  461  may each be a normally-off semiconductor device. The switching elements  468  of the secondary side circuit  462  may each be a metal-insulator-semiconductor field-effect transistor (MISFET) containing silicon. 
       FIG. 7  illustrates an exemplary high-frequency amplifier. A high-frequency amplifier  470  illustrated in  FIG. 7  may be applied to a power amplifier for a base station of mobile phones. The high-frequency amplifier  470  includes a digital pre-distortion circuit  471 , mixers  472 , a power amplifier  473 , and a directional coupler  474 . The digital pre-distortion circuit  471  compensates for non-linear distortion in an input signal. One of the mixers  472  mixes the input signal in which the non-linear distortion is compensated for with an alternating current signal. The power amplifier  473  amplifies the input signal mixed with the alternating current signal. The power amplifier  473  illustrated in  FIG. 7  may include the semiconductor device illustrated in  FIG. 1  or  4 . The directional coupler  474  performs, for example, monitoring of an input signal and an output signal. For example, based on switching of a switch, the other mixer  472  may mix an output signal with an alternating current signal and transmit the mixed signal to the digital pre-distortion circuit  471 . Since the semiconductor device illustrated in  FIG. 1  or  4  is used in the power amplifier  473 , a power supply device and an amplifier may be provided at a low cost. 
     Example embodiments of the present invention have now been described in accordance with the above advantages. It will be appreciated that these examples are merely illustrative of the invention. Many variations and modifications will be apparent to those skilled in the art.