Abstract:
The present invention includes methods directed to improved processes for producing a monolayer of sulfur on the surface of a semiconductor. As a surface layer, it functions to passivate the surface; if annealed, it provides a doping element.

Description:
TECHNICAL FIELD 
     The present invention is directed towards an improved process for creating a sulfur monolayer on the surface of a semiconductor material. 
     BACKGROUND OF THE INVENTION 
     As semiconductor technology moves toward ever faster and smaller devices, new challenges arise. In particular, when contemplating devices that require shallow (&lt;9 nm) defect-free extensions, conventional techniques fall short. Cluster implantation, plasma immersion ion implantation and solid phase epitaxial regrowth are known for sub-micron device fabrication, but their use to make nanoscale devices is problematic. Monolayer doping is attractive, but the known means for providing a monolayer are not. Solution passivation with ammonium sulfide on binary and ternary semiconductors is known, but the procedures described in the literature would require the use and disposal of industrial quantities of 20% aqueous (NH 4 ) 2 S x . Ammonium polysulfide and its precursor, ammonium sulfide, are toxic; both storage and disposal raise safety issues. Twenty percent aqueous ammonium sulfide and polysulfide must be stored under the most stringent conditions and, after use, must be disposed of by collection and off-site decontamination. Contact with acidifying substances in wastewater generates hydrogen sulfide, which is itself volatile, offensive and toxic. There is a general need for a process for producing a sulfur monolayer on a semiconductor, which process is practical on an industrial scale. 
     Although the use of solutions of ammonium polysulfide as low as 0.2% has been reported for passivating the surface of InAs nanowires to fabricate an ohmic contact [Suyatin et al. Nanotechnology 18, 105307 (2007)], in general, systematic studies of ammonium sulfide passivation of InGaAs [O&#39;Connor et al. J. Appl. Phys. 109, 024101 (2011)] have suggested that the optimal concentration for that purpose was 10%. Similarly, U.S. Pat. No. 6,924,218 describes a 2:9 mixture of 20% ammonium sulfide and 30% aqueous ammonia for passivating the surface of an InAlAs Schlottky layer. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an improved process for producing a monolayer of sulfur on the surface of a semiconductor. As a surface layer, it functions to passivate the surface; if annealed, it provides a doping element. 
     In one aspect, the invention relates to a method for passivating a IV or III-V semiconductor material. The method comprises exposing the surface of the semiconductor material to an aqueous solution of (NH 4 ) 2 S at a concentration between 0.001% by weight and 0.1% by weight. 
     In a second aspect, the invention relates to a method for making an n-region in a semiconductor comprising:
         (a) providing a IV or III-V semiconductor material substrate;   (b) exposing the material to an aqueous solution of (NH 4 ) 2 S at a concentration between 0.001% by weight and 0.1% by weight to provide a semiconductor material having a surface layer of sulfur;   (c) capping the surface layer of sulfur; and   (d) annealing the semiconductor material having a capped surface layer of sulfur, whereby the sulfur diffuses into the semiconductor material.       

    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The use of ammonium sulfide and polysulfide for passivating III-V surfaces is known. However, such use in the past has been largely limited to the use of aqueous solutions containing about 10-20% ammonium sulfide or polysulfide. As noted above, 20% aqueous ammonium sulfide and polysulfide must be stored under the most stringent conditions and, after use, must be disposed of by collection and off-site decontamination. This adds greatly to the hazard and expense of using ammonium sulfide solutions for passivation. It has now been surprisingly found that passivation of semiconductor substrates suitable for nanometer scale devices can be carried out with an aqueous solution of (NH 4 ) 2 S at a concentration between 0.001% by weight and 0.1% by weight. This concentration is orders of magnitude smaller than previously thought possible, and it makes the storage and disposal of reagent much simpler. At that degree of dilution, standard sanitary sewers can be safely used for disposal. 
     The use of super-dilute solutions of ammonium sulfide allows the creation of a monolayer of sulfur, which in turn makes possible the fabrication of junctions of less than 10 nm dimension. It also avoids damage to the substrate, which occurs when ion implantation is employed to create sub-10 nm features. Moreover, the technique is readily adapted to non-planar structures, and the equipment and processing costs are lower than for ion implantation processes. The process of the invention is thus valuable in making sub-10 nm devices. 
     The semiconductors that are the substrates for the process of the invention are known in the art as IV and III-V semiconductors. Examples of common IV semiconductors are silicon, germanium and silicon-germanium. Examples of common III-V semiconductors are GaAs, InP, InAs, GaP, InGaAs, InAlAs, InAlGa and GaInP. Examples of other, less common binary and ternary III-V materials include: AlSb, GaSb, GaP, InSb, AlGaAs, GaAsP, InGaN. For some uses quaternary III-V materials, such as AlGaInP and InGaAsSb, can be employed. The binary semiconductors GaAs, InP, InAs, GaP and ternary and quaternary alloys of GaAs, InP, InAs and GaP are preferred. The InAs/GaAs ternary alloy can be characterized as In x Ga 1-x As where x is the proportion of InAs and  1-x  is the proportion of GaAs. The most convenient substrate for In x Ga 1-x As is InP. Since In x Ga 1-x As with 53% InAs has the same lattice constant as InP, the combination leads to very high quality thin films, and In x Ga 1-x As with 53% InAs is often called “standard InGaAs” without bothering to note the values of “ x ” or “ 1-x ”. Unless otherwise noted, the examples below employed the standard 53% InGaAs. 
     The ammonium sulfide solution employed in the processes described herein is an aqueous solution of (NH 4 ) 2 S at a concentration between 0.001% by weight and 0.1% by weight. In some embodiments, the aqueous solution contains 0.02±0.019% by weight of ammonium sulfide. 
     The ammonium sulfide solution may be optionally fortified by the addition of from 0.0001% by weight to 0.01% by weight of elemental sulfur. In some embodiments, the aqueous solution includes an additional 0.001±0.0009% by weight of sulfur. “An additional” amount of sulfur means an amount beyond the stoichiometric amount present as (NH 4 ) 2 S. Presumably the additional sulfur is incorporated into ammonium polysulfide, so that the passivation solution in this case would comprise ammonium sulfide with a minor amount of ammonium polysulfide. As can be seen below, the presence of the ammonium polysulfide (or excess sulfur) is unnecessary for the passivation. 
     The sulfur monolayer created by the process described herein can be whatever thickness is desired. In most cases the desired thickness is from 5 Å to 50 Å. In some embodiments, it will be 10 Å to 20 Å thick. The sulfur monolayer can be left in place as a passivation layer, or the structure can be annealed to disperse the sulfur into the substrate and create an n-region of defined morphology. If the sulfur-coated substrate is to be annealed, it is advantageous to cap the sulfur to prevent its loss by vaporization during annealing. This ensures that substantially all of the sulfur that has been deposited diffuses into the substrate rather than only a portion thereof diffusing into the substrate. 
     The capping material may be any material typically used as a chemical barrier. Oxides or nitrides that can be conformally coated function in this capacity. The most common example is silicon nitride. 
     Annealing is commonly carried out in an inert atmosphere such as argon, at temperatures from 300° C. to 1100° C. In certain embodiments the temperatures are from 400° C. to 800° C. The expression “from 300° C. to 1100° C.” means that the process is carried out either by maintaining any temperature between 300° C. and 1100° C. or by varying the temperature within that range. 
     EXAMPLES 
     The substrates used in the examples were standard InGaAs. The InGaAs surfaces were first degreased by dipping in acetone and rinsing with isopropyl alcohol. Surface oxide was then removed by a 30 second to 3-minute dip in aqueous HF (10:1 or 1000:1) at room temperature followed by a dip rinse in H 2 O. Passivation was carried out at room temperature by dipping the substrate in ammonium sulfide solutions at the concentrations shown below for 5 minutes followed by a dip rinse in H 2 O. In various experiments not shown, the dip time ranged between 1 minute and 15 minutes. Longer times, e.g. 30 minutes or 60 minutes could be employed, but for most purposes, there is no reason to do so. The sulfur was capped by physical vapor deposition (sputtering) of a 200 Å film of silicon nitride using a single crystal silicon target doped with phosphorus (99.999% purity) and a flow rate of argon 35 SCCM and nitrogen 36 SCCM at 300 W power at ambient temperature. The capped substrates were annealed under argon at 400° C. to 800° C. for 30 seconds. 
     The following examples are not construed as limiting and are offered by way of illustration. While the substrates used in all examples were standard InGaAs substrates, the specific substrate used in a given example may have come from a manufacturing run different from that of other examples. 
     Example 1 
     20% ammonium sulfide plus 13.3 g/L of sulfur 
     Example 2 
     20% ammonium sulfide 
     Example 3 
     1000:1 dilution=0.3 mL of solution from example 1 diluted in 300 mL distilled water=0.2 g/L of (NH 4 ) 2 S with 1.3 mg/L of additional sulfur 
     Example 4 
     10,000:1 dilution=0.3 mL of solution from example 1 diluted in 3 L distilled water=20 mg/L of (NH 4 ) 2 S plus 0.13 mg/L of sulfur 
     Example 5 
     1000:1 dilution=0.3 mL of solution from example 1 diluted in 300 mL distilled water=0.2 g/L of (NH 4 ) 2 S with 1.3 mg/L of additional sulfur 
     Example 6 
     1000:1 dilution=0.3 mL of solution from example 2 diluted in 300 mL distilled water=0.2 g/L of (NH 4 ) 2 S without additional sulfur 
     Example 7 
     10,000:1 dilution=0.3 mL of solution from example 1 diluted in 3 L distilled water=20 mg/L of (NH 4 ) 2 S plus 0.13 mg/L of sulfur 
     Example 8 
     10,000:1 dilution=0.3 mL of solution from example 2 diluted in 3 L distilled water=20 mg/L of (NH 4 ) 2 S without additional sulfur 
     A comparison of examples is shown in Table 1: 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Total 
                 R sh   
                 n 
                   
               
               
                   
                 Conc. of 
                 dopant 
                 (ohm/sq) 
                 (1 × 10 12   
                 Mob 
               
               
                 Example # 
                 (NH 4 ) 2 S 
                 (×10 13 ) 
                 Hall 
                 cm −2 ) 
                 (cm 2 V −1 s −1 ) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                     20% 
                 3.49 
                 104 
                 23.4 
                 2570 
               
               
                 2 
                     20% 
                 2.58 
                 123.7 
                 21.4 
                 2360 
               
               
                 3 
                  0.1% 
                 6.2 
                 64.6 
                 36 
                 2690 
               
               
                 4 
                 0.01% 
                 4.35 
                 107.3 
                 24.9 
                 2340 
               
               
                 5 
                  0.1% 
                 2.75 
                 115.2 
                 20.0 
                 2710 
               
               
                 6 
                  0.1% 
                 1.90 
                 174.7 
                 14.0 
                 2550 
               
               
                 7 
                 0.01% 
                 1.47 
                 187 
                 13.6 
                 2450 
               
               
                 8 
                 0.01% 
                 2.14 
                 178.6 
                 14.0 
                 2490 
               
               
                   
               
             
          
         
       
     
     When the substrate is analyzed by secondary atom mass spectrometry (SIMS), it is found that the sulfur concentration (in atoms/cm 3 ) is comparable for all four samples both at the surface and as a function of depth, all of them being&gt;10 19  at the surface and dropping below 10 17  by 100 nm depth. 
     While several aspects of the present invention have been described and depicted herein, alternative aspects may be affected by those skilled in the art to accomplish the same objectives. It should be understood that the present disclosure is to be considered as an exemplification of the principles of this invention and is not intended to limit the invention to the embodiments illustrated.