Patent Publication Number: US-2021193820-A1

Title: Semiconductor structure and forming method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/CN2018/104556 filed on Sep. 7, 2018, the disclosure of which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to semiconductor technology, and in particular, to a semiconductor structure with a metal doped layer and a method for forming the same. 
     BACKGROUND 
     A group III-V based semiconductor material (e.g., GaN, AlGaN or the like) has many advantages in terms of its electrical, physical and chemical properties, such as wide band gaps, high electron mobility, high breakdown voltage, and excellent chemical stability. Therefore, this material is particularly well suited to high-power, high-frequency, and high-temperature applications. Semiconductor devices for these types of applications exhibit high electron mobility and can withstand high voltage, while operating at a high frequency. For example, these devices may include high electron mobility transistors (HEMTs), heterojunction field-effect transistors (HFETs), or modulation doped field effect transistors (MODFETs). 
     SUMMARY 
     In some embodiments of the disclosure, a method of forming a semiconductor structure is provided. The method of forming a semiconductor structure includes: providing a substrate; forming a discontinuous metal atomic layer on the substrate; and forming an epitaxial layer on the discontinuous metal atomic layer, wherein metal atoms of the discontinuous metal atomic layer are driven into the epitaxial layer during the growth of the epitaxial layer, so that at least a part of the epitaxial layer is doped with metal atoms. Owing to the discontinuous metal atomic layer, a phenomenon of current leakage corresponding to diffusion of atoms from the substrate is eliminated, and the epitaxial layer has smoother surface morphology, better crystalline quality and higher resistivity because of doping with metal atoms. 
     In some embodiments of the disclosure, the method further includes forming a nucleation layer between the substrate and the discontinuous metal atomic layer. 
     In some embodiments of the disclosure, the nucleation layer includes GaN, AlGaN, AlInGaN, or a combination thereof. 
     In some embodiments of the disclosure, the epitaxial layer includes GaN, AlGaN, AlInGaN, or a combination thereof. 
     In some embodiments of the disclosure, the metal of discontinuous metal atomic layer comprises Fe, Mn, Sb, Bi, Cd, Zn, Mg, Na, or a combination thereof. 
     In some embodiments of the disclosure, the method further includes forming a heterojunction on the epitaxial layer; and forming a gate structure, a source contact, and a drain contact on the heterojunction. 
     In some embodiments of the disclosure, a semiconductor structure is provided, the semiconductor structure includes: a substrate, and an epitaxial layer disposed on the substrate, wherein the epitaxial layer comprises a metal-doped layer doped with metal atoms disposed on the substrate, the doping concentration of the metal atoms is decreased form a bottom surface to the top surface of the metal-doped layer and the doping concentration of the metal atoms at the bottom surface of the metal-doped layer is larger than 1×10 17  atoms/cm 3 . 
     In some embodiments of the disclosure, the semiconductor structure further includes a nucleation layer between the substrate and the metal-doped layer. 
     In some embodiments of the disclosure, the nucleation layer includes GaN, AlGaN, AlInGaN, or a combination thereof. 
     In some embodiments of the disclosure, the epitaxial layer includes AlN, AlGaN, AlInN, AlInGaN, or a combination thereof. 
     In some embodiments of the disclosure, the metal atom includes Fe, Mn, Sb, Bi, Cd, Zn, Mg, Na, or a combination thereof. 
     In some embodiments of the disclosure, the semiconductor structure further includes a heterojunction on the epitaxial layer; and a gate structure, a source contact, and a drain contact on the heterojunction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present disclosure can be further understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1A  to  FIG. 1E  illustrate some semiconductor structures corresponding to various stages of a method  200  in accordance with some embodiments. 
         FIG. 2  shows the method  200  of forming a semiconductor structure according to some embodiments of this disclosure. 
         FIG. 3  illustrates a relationship between the doping concentration of the metal atoms and the total thickness of a traditional metal-doped layer. 
         FIG. 4  illustrates a relationship between the doping concentration of the metal atoms and the total thickness of the metal-doped layer as the embodiment shown in  FIG. 1D . 
         FIG. 5A  is a 5×5 μm 2  AFM scan of an epitaxial layer without metal doping. 
         FIG. 5B  is a 5×5 μm 2  AFM scan of an epitaxial layer with metal doping. 
         FIG. 6A  is a graph showing X-ray diffraction (XRD) omega rocking curve of the (002) face of a GaN epitaxial layer without metal doping in one embodiment. 
         FIG. 6B  is a graph showing XRD omega rocking curve of the ( 002 ) face of a GaN epitaxial layer with metal doping in one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following description is of the best-contemplated mode of carrying out the disclosure. This description is made for the purpose of illustrating the general principles of the disclosure and should not be taken in a limiting sense. These are, of course, merely examples and are not intended to be limited. In addition, the disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Referring to  FIG. 1A  to  FIG. 1E  and  FIG. 2 ,  FIG. 2  shows a method  200  of forming a semiconductor structure according to embodiments of this disclosure,  FIG. 1A  to  FIG. 1E  illustrate some semiconductor structures corresponding to various stages of the method  200  in accordance with some embodiments. 
     Additional operation steps can be provided before, during, and/or after the steps described in  FIG. 2 . Moreover, in different embodiments, some of the steps that are described in  FIG. 2  can be replaced or eliminated. In some embodiments, the semiconductor device  200  is implemented as a transistor (e.g., HEMT, HFET, or MOSFET). 
     As shown in  FIG. 2  and  FIG. 1A , the method  200  includes a step  202 : forming a nucleation layer  102  on a substrate  100 . In some embodiments, the substrate  100  is formed of a material that is suitable for growing a semiconductor structure including Group III nitride material (e.g., GaN, AlGaN, AlInGaN, AN). For example, the substrate  100  may be formed of Si, sapphire (Al2O3), SiC, or another suitable material. 
     In some embodiments, the nucleation layer  102  is optional and may include AN, GaN, AlGaN, AlInGaN, or a combination thereof and have a thickness in a range of about 0.1 nm to 500 nm. In some embodiments, the nucleation layer  102  may be grown onto the sapphire, silicon carbide, or silicon substrate by metal-organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), metal organic VPE (MOVPE), plasma-enhanced CVD (PECVD), or the like. 
     Referring to  FIG. 2  and  FIG. 1B , the method  200  includes a step  204 : forming a discontinuous metal atomic layer  104  on an upper surface  109  of the nucleation layer  102 . In some embodiments, the discontinuous metal atomic layer  104  may be employed to provide doped impurities (i.e., metal atoms) for achieving a semiconductor material layer with high resistivity. In some embodiments, the discontinuous metal atomic layer  104  may be formed of a material containing at least one metal that is selected from the group of Fe, Mn, Sb, Bi, Cd, Zn, Mg, and Na. In some embodiments, the discontinuous metal atomic layer  104  is formed by an atomic layer deposition (ALD) process, MOCVD process or another suitable deposition process. In these cases, the discontinuous metal atomic layer  104  may be a single atomic layer and expose parts of the nucleation layer  102 . 
     Referring to  FIG. 1C  and  FIG. 2 , the method  200  includes a step  206 : forming an epitaxial layer (e.g., a semiconductor material)  110  on the discontinuous metal atomic layer  104  and parts of the nucleation layer  102  uncovered by the discontinuous metal atomic layer  104 . In some embodiments, the epitaxial layer  110  includes Group III nitride material (e.g., GaN, AlGaN, AlInGaN, AN). In some embodiments, the epitaxial layer  110  may be a buffer layer. In some embodiments, the epitaxial layer  110  is formed by MOCVD, HYPE, LPE, MBE, MOVPE, PECVD, or another suitable deposition. 
     In some embodiments, metal atoms (e.g., Fe, Mn, Sb, Bi, Cd, Zn, Mg, Na, or a combination thereof) of the discontinuous metal atomic layer  104  may be driven into (indicated by the arrows shown in  FIG. 1C ) the epitaxial layer  110  during the growth of the epitaxial layer  110  due to an auto-doping effect, so that at least a part of the epitaxial layer  110  is doped with metal atoms. As shown in  FIG. 1D , since some metal atoms of the discontinuous metal atomic layer  104  are driven into a part of the epitaxial layer  110  (as shown in  FIG. 1C ) due to the unintentional auto-doping effect, a metal-doped layer  110   a  is doped with the metal atoms therein and an overlying epitaxial layer  110   b  is not doped with the metal atoms. In some embodiments, the epitaxial layer  110  may only include the metal-doped layer  110 a, that is, the overlying epitaxial layer  110   b  is eliminated. Owing to the metal dopant, the metal-doped layer  110   a  has smoother surface morphology, better crystalline quality and higher resistivity. 
     Referring to  FIG. 5A  and  FIG. 5B ,  FIG. 5A  is a 5×5 μm 2  atomic force microscope (AFM) scan of an epitaxial layer without metal doping in one embodiment, and the epitaxial layer in this embodiment is GaN.  FIG. 5B  is a 5×5 μm 2  AFM scan of an epitaxial layer with metal doping in one embodiment, and the epitaxial layer in this embodiment is GaN. In  FIG. 5A , the root mean square (Rms) roughness of the interface of the epitaxial layer without metal doping is approximately 0.59 nm. In  FIG. 5B , the root mean square (Rms) roughness of the interface of the epitaxial layer with metal doping is approximately 0.19 nm. Obviously, the epitaxial layer with metal doping has smoother surface morphology. 
     Referring to  FIG. 6A  and  FIG. 6B ,  FIG. 6A  is a graph showing X-ray diffraction (XRD) omega rocking curve of the (002) face of an GaN epitaxial layer without metal-doped in one embodiment, and the full width at half maximum (FWHM) of the XRD omega rocking curve is 694 arcsec.  FIG. 6B  is a graph showing XRD omega rocking curve of the (002) face of a GaN epitaxial layer with metal doping in one embodiment, and the FWHM of the XRD omega rocking curve is 139 arcsec. Obviously, the GaN epitaxial layer with metal doping has better crystalline quality. 
     Referring to  FIG. 1E  and  FIG. 2 , in some embodiments, the method  200  may also include a step  208 : forming a heterojunction  112 . The heterojunction  112  may include a GaN/AlGaN heterojunction. As shown in  FIG. 1E , the heterojunction  112  may include a channel layer  112 b and a barrier layer  112 a. In some embodiments, the heterojunction  112  is formed by MOCVD, HYPE, LPE, MBE, MOVPE, PECVD, or another suitable deposition. 
     Referring to  FIG. 1E  and  FIG. 2 , in some embodiments, the method  200  may also include a step  210 : forming a gate structure  120 , a source contact  122 , and a drain contact  124  on the heterojunction  112  to form a semiconductor device  200  such as a transistor (e.g., HEMT, HFET, or MOSFET). In some embodiments, the gate structure  120  includes a gate dielectric layer  116  and a gate contact  118  on the gate dielectric layer  116 . Moreover, the source contact  122  and the drain contact  124  are arranged on both sides of the gate structure  120 . The gate contact  118 , the source contact  122 , and the drain contact  124  may be formed of a conductive material, such as metal (e.g., Al, TiN, Au, Ni, Ti, tantalum (Ta), tungsten (W), or a combination thereof) or another suitable electrode material. In some embodiments, the gate dielectric layer  116  may be formed of SiN, SiCN, SiO 2 , SiAlN, Al 2 O 3 , AlON, SiON, HfO 2 , HfAlO, or a combination thereof. After formation of the gate structure  120 , the source contact  122 , and the drain contact  124 , a fabrication of the semiconductor device (e.g., a transistor)  1  is completed. 
     Referring to  FIG. 2 , in some embodiments, the step  202  may be eliminated. In some embodiments, the sequence of the step  202  and the step  204  may be adjusted, that is, the discontinuous metal atomic layer  104  may be formed on the substrate  100 , and then the nucleation layer  102  may be formed on the discontinuous metal layer  104 , and then the epitaxial layer  110  may be formed on the nucleation layer  102 . 
     Referring to  FIG. 3 ,  FIG. 3  illustrates a relationship between the doping concentration of the metal atoms and the total thickness of a traditional metal-doped layer. The metal of the traditional metal-doped layer is achieved by directing a flow of metal atoms source to a growth chamber during growing a semiconductor layer. Because of memory effect, the metal may not be doped into the semiconductor layer promptly, so that the doping concentration of the traditional metal-doped layer is increased gradually from the bottom surface (near the nucleation layer) as shown in  FIG. 3 . Because some atoms in the substrate may diffuse into above layers forming on the substrate, for example, Si atoms in Si, SiC or SiO2 substrate and O atoms in SiO2, MgO or Al 2 O 3  may diffuse into the semiconductor layer above the substrate, a phenomenon of current leakage is aroused in the semiconductor layer. 
     Referring to  FIG. 4 ,  FIG. 4  illustrates an embodiment of a relationship between the doping concentration of the metal atoms and the thickness of the metal-doped layer  110   a  shown in  FIG. 1D . The metal-doped layer  110   a  according to above-mentioned method in this disclosure has the doping concentration decreased form the bottom surface (near the nucleation layer) to the top surface. The metal-doped layer  110   a  has a maximum doping concentration (e.g., 1×10 19  atoms/cm 3 ) of metal atoms at a bottom surface near the substrate, and the metal can increase the resistance, so a phenomenon of current leakage corresponding to diffusion of atoms from the substrate is eliminated. 
     In some embodiments, the doping concentration of metal atoms at the bottom surface of the metal-doped layer  110   a  is larger than 1×10 17  atoms/cm 3 , such as 2×10 17  atoms/cm 3 , 1×10 18  atoms/cm 3 . 
     While the disclosure has been described by way of example and in terms of the preferred embodiments, it should be understood that the disclosure is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.