Patent Publication Number: US-2011057198-A1

Title: TECHNIQUE FOR DEVELOPMENT OF HIGH CURRENT DENSITY HETEROJUNCTION FIELD EFFECT TRANSISTORS BASED ON (10-10)-PLANE GaN BY DELTA-DOPING

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. Section 119(e) of co-pending and commonly assigned U.S. Provisional Application Ser. No. 61/238,056, filed on Aug. 28, 2009, by Tetsuya Fujiwara, Stacia Keller, and Umesh K. Mishra, entitled “TECHNIQUE FOR DEVELOPMENT OF HIGH CURRENT DENSITY HETEROJUNCTION FIELD EFFECT TRANSISTORS BASED ON (10-10)-PLANE GaN BY DELTA-DOPING,” attorney&#39;s docket number 30794.312-US-P1 (2009-612-1), which application is incorporated by reference herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     This invention was made with Government support under Grant No. N00014-05-1-0419 awarded by the Office of Naval Research, MINE and MURI. The Government has certain rights in this invention. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to nonpolar gallium nitride (GaN) based devices, and in particular delta-doped (δ-doped) (10-10)-plane GaN transistors. 
     2. Description of the Related Art 
     There exist expectations that (10-10)-plane GaN transistors should realize high threshold voltages, which are required for power switching devices. However, low current density (˜30 milliamps (mA)/millimeter(mm)) has been a problem for (10-10)-plane GaN transistors. More current, i.e. more power, is required for high power switching devices. 
     Thus, there is a need for increasing the current density on (10-10)-plane GaN transistors. The present invention satisfies that need. 
     SUMMARY OF THE INVENTION 
     The present invention discloses an improved (10-10)-plane GaN transistor having a current density ten times higher than a conventional (10-10)-plane GaN transistor, which is obtained by delta-doping (δ-doping). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
         FIG. 1  is a cross-sectional schematic of a δ-doped (10-10)-plane GaN Heterojunction Field Effect Transistor (HFET) structure. 
         FIG. 2  plots the omega-2theta X-ray diffraction profile of (10-10)-plane AlGaN/GaN heterostructures (Intensity, in arbitrary units (a.u.) vs. 2theta in degrees (°)). 
         FIG. 3  is an atomic force microscope (AFM) image of the surface morphology of a (10-10)-plane AlGaN/GaN heterostructure. 
         FIG. 4  plots drain-source current (I ds ), in mA/mm, as a function of drain source voltage V ds (I ds V ds  characteristics) of δ-doped (10-10)-plane GaN HFETs. 
         FIG. 5  plots I ds -V ds  characteristics of conventional (uniform-doped) (10-10)-plane GaN HFETs. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     Fabrication 
       FIG. 1  shows the schematic structure of the δ-doped (10-10)-plane GaN HFET. The (10-10)-plane AlGaN/GaN heterostructure was grown by metal organic chemical vapor deposition on (10-10)-plane GaN substrates. The growth was initiated with the deposition of a 1 micrometer (μm) thick unintentionally doped (u.i.d.) GaN layer. Then, a 1.5-μm-thick Fe doped GaN layer was grown by using bis-cyclopentadienyl-iron. A 0.8-μm-thick u.i.d. GaN layer was grown as the channel layer. A 2.5 nanometer (nm) thick spacer Al 0.32 Ga 0.68 N layer was deposited. A δ-doped layer was formed by flowing SiH 4  and NH 3 . A 22.5 nm-thick Al 0.32 Ga 0.68 N cap layer was deposited. 
     An omega-2theta X-ray diffraction profile of the epitaxial film is shown in  FIG. 2 . 
     A surface morphology image of the epitaxial film, taken by AFM, is shown in  FIG. 3 . 
     Ti(20 nm thick)/Al(120 nm thick)/Ni(30 nm thick)/Au(50 nm thick) stacks were deposited by e-beam evaporation as ohmic contact metals, and subsequently subjected to a rapid thermal annealing at 870° C. for 30 seconds in an N 2  atmosphere. A Cl 2  based dry etch was carried out for mesa isolation. 
     A Ni (30 thick)/Au(250 thick)/Ni(50 nm thick) stack was deposited by e-beam evaporation as the Schottky gate metal. 
     A 160 nm thick Si x N y  passivation film was deposited by plasma-enhanced thermal chemical vapor deposition. The Si x N y  was etched with CF 4  dry etching. 
     Ti(20 nm thick)/Au (250 nm thick) pad metals were deposited by e-beam evaporation. 
     Characterization 
       FIG. 4  shows the Ids-Vds characteristics of δ-doped (10-10)-plane GaN HFETs. 380 mA/mm of maximum drain current was obtained.  FIG. 5  shows the Ids-Vds characteristics of conventional (uniform-doped) (10-10)-plane GaN HFETs. Therefore, at least 10 times higher current density was obtained by δ-doping. Further optimization of the present invention&#39;s device can increase the current density even further. 
     Possible Modifications 
     One possible for developing high current density (10-10)-plane GaN transistors is that (11-20)-plane GaN can be used instead of (10-10)-plane GaN described above, because (11-20)-plane GaN also has no polarization. Therefore, (11-20)-plane GaN transistors can also have high current density by delta doping. 
     Moreover, although the present invention is described as comprising GaN, other (Al,Ga,In)N materials may be used as well. The term “(Al,Ga,In)N” as used herein is intended to be broadly construed to include respective nitrides of the single species, Al, Ga, and In, as well as binary, ternary and quaternary compositions of such Group III metal species. These compounds are also referred to as Group III nitrides, or III-nitrides, or just nitrides, or by (Al,Ga,In)N, or by Al (1-x-y) In y Ga x N where 0≦x≦1 and 0≦y≦1. 
     Advantages and Improvements 
     The present invention has great advantages compared to the other ways for developing high current density (10-10)-plane GaN transistors. Usually, a uniform Si doped technique has been used to increase current density. However, parallel conduction occurred by increasing the Si doping concentration. In the present invention, the maximum carrier density without the parallel conduction was significantly improved by δ-doping, because the doping layer can be set at a close distance from the heterointerface that induces the two-dimensional-electron gas. For example, in delta-doping, all dopants are set within several nm of the interface, while in uniform doping, some dopants may exist more than 10 nm from the interface. 
     Appendix 
     Further information on the present invention can be found in the Appendix of the parent provisional application identified above and incorporated by reference herein, wherein the Appendix comprises a publication by Tetsuya Fujiwara, Stacia Keller, Masataka Higashiwaki, James S. Speck, Steven P. DenBaars, and Umesh K. Mishra, entitled “Si Delta-Doped m-Plane AlGaN/GaN Heterojunction Field-Effect Transistors,” found in Applied Physics Express, Vol. 2, No. 061003 (Jun. 12, 2009), and is incorporated by reference herein. 
     Conclusion 
     This concludes the description of the preferred embodiment of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.