Method for chemically passivating a surface of a product made of a III-V semiconductor material and the product obtained by such a method

This application is a 35 USC §371 U.S. national stage filing of International Patent Application No. PCT/FR2012/051310 filed on Jun. 12, 2012, and claims priority under the Paris Convention to French Patent Application No. FR 11 55157 filed on Jun. 14, 2011.

The present invention concerns methods for chemically passivating a surface of a product made of a III-V semiconductor material and the products obtained by such methods.

More specifically, the invention concerns a method for chemically passivating a surface of a product made of a III-V semiconductor material in which a) a P,N polymer film is formed on said surface by deposition in a solvent comprising liquid ammonia.

The article, “Fully Protective yet Functionalizable Monolayer on InP,” by Gonçalves et al., in Chemistry of Materials 22 (2010), 3114-3120, describes an example of such a method, in which a polyphosphazene film is deposited and has been shown to be very satisfactory. In effect, the passivation of the surface obtained by this method can be of high quality.

However, the film is formed with electrochemical assistance, which presents practical drawbacks that prevent the implementation of such a method on an industrial scale. Consequently, it is sought to simplify the production of such films.

To that end, a method of the type in question is characterized in that said film is formed by deposition, without electrochemical assistance, in said solvent, in the presence of an oxidizing chemical additive comprising phosphorous and generating electrical charge carriers in said surface.

Concerning deposition “without electrochemical assistance”, we can also speak of “open-circuit potential” deposition.

Surprisingly, it was found that the addition of such an additive comprising phosphorous in the solvent could be used to bring about the formation of a polyphosphazene-type film without electrochemical assistance. In this way, the complex operations needed for electrochemical methods, such as having to establish a connection for polarization, were eliminated. Additionally, we obtain an ultra-thin cover film.

Moreover, in preferred embodiments of the invention, we could have recourse to either of the following features:

for step a), we make use of dipping; the “V” material is phosphorous; the “III” material is indium; step a) simultaneously comprises dissolution of the surface semiconductor material, oxidation of the solvent, and formation of a monomer equivalent of polyphosphazene; step a) takes place between −40° C. and −100° C.; step a) takes place with an additive concentration comprised between 0.1 and 3 millimoles per liter (mM/L); we introduce, before step a), a step b) involving deoxidation of at least a portion of said surface; step b) is implemented for only a portion of the surface; the method also comprises, after step a), a step c) involving washing of a surface passivated in step a); said additive is PCl 5 .

According to another aspect of the invention, it also concerns a product comprising:

a III-V semiconductor material substrate presenting a surface, a P,N polymer film formed on said surface,

characterized in that said film presents, during x-ray photoelectron spectrometry (XPS), for N 1S , a spectrum having a first peak for a high binding energy and a second peak for a low binding energy, the first peak having an amplitude at least 20% greater than the second peak.

Other characteristics and advantages of the invention will appear from the following description of one of its embodiments, which is purely non-limiting, for which the attached drawings are provided.

In the drawings:

FIG. 1 is a schematic view of an installation for implementing a method according to an embodiment;

FIG. 2 a is a cutaway view of a product according to a first embodiment before application of the method;

FIG. 2 b is a corresponding view of 2 a following application of the method;

FIGS. 3 a and 3 b are corresponding views, respectively, of FIGS. 2 a and 2 b for a product according to a second embodiment;

FIG. 4 is a representative x-ray photoelectron spectrometry (XPS) curve for N 1S for a product obtained by the above method; and

FIG. 4 a is a curve similar to the curve shown in FIG. 4 for products obtained by methods involving electrochemical assistance.

In the different figures, the same reference numbers refer to identical or similar elements.

FIG. 1 is a schematic view representing an installation 1 for passivation of the surface of a product made from a III-V semiconductor material. An example of the product is shown in FIG. 2 a . Product 2 is formed of a III-V semiconductor material. It is made from one or more elements in column “III” of the periodic table of elements (notably indium) and one or more elements from column “V” of the periodic table of elements (notably antimony or phosphorous). For example, the material considered is indium phosphide, InP. Product 2 comprises a surface 2 a , which we seek to passivate.

As shown in FIG. 1 , installation 21 comprises passivation apparatus 3 . This comprises a recipient 4 , containing a liquid solvent. For example, the solvent in question consists of liquid ammonia, NH 3 . Apparatus 3 also comprises temperature regulating means 5 , such as a cryostat, suitable for maintaining the ammonia in the liquid state.

Product 2 is placed in the solvent in the presence of an additive. This additive is capable of oxidizing the III-V semiconductor material in said solvent. In particular, we employ an oxidizer:

that comprises phosphorous, that is compatible with the liquid ammonia solvent, that can generate electrical charge carriers (holes) in surface 2 a of product 2 .

A particular example of such an additive is phosphorous pentachloride (PCl 5 ).

For example, we use as a solvent liquid ammonia obtained by simple expansion in an open glass liquefaction cell kept at a temperature of −78° C. through the use of a refrigerant bath (for example, acetone filled with dry ice), confined in a Dewar flask. Alternatively, we could also work in a cryostat maintained between −40° C. and −100° C., preferably between −50° C. and −70° C., or in any other type of refrigerating container that can maintain a constant temperature in the above range. The liquid ammonia can be used in its neutral form, that is, totally pure, or acidic (for example, by introducing NH 4 Cl or Br at a concentration of 0.1 moles per liter of solvent).

For the III-V semiconductor material oxidizing additive, we use PCl 5 . The latter is introduced after weighing and put in solution using the standard method at a concentration of 1 to 3 millimoles per liter of solvent. If necessary, we can use lower concentrations of PCl5, for example, on the order of 0.1 to 1 millimoles per liter. The conditions are combined for deposition, without electrochemical assistance, of a polyphosphazene type P,N polymer film 6 on surface 2 a of product 2 (see FIG. 2 b ).

In general, polyphosphazene is a polymer of type (—RR′P═N—) n , where the R,R′ substituents are generally alkoxy, amino, R 2 N or halogens.

In the present case, we propose that the mechanism (in its electrochemical version) on which the reaction is based can be written, in the case of indium phosphide:

InP+9NH 3 +9 h + →(H 2 N—P═NH)+6NH 4 + +In 3+ +½N 2

This equation incorporates three phenomena of the mechanism leading to passivation:

dissolution of the material at the nanometric scale, oxidation of the solvent, and obtention of an equivalent monomer.

These together generate the continuous coating of surface 2 a , which creates the phenomenon of self-regulation through the limiting factor of coverage. In terms of roughness, this self-regulation maintains a final state that is very similar to the initial state, which excludes unwanted parasitic morphological disturbances in an optical configuration.

PCl 5 can be used to inject recoverable charge carriers in the surface to initiate a production mechanism for a polyphosphazene-type P,N polymer film similar to the mechanism described above in the electrochemical version. Additionally, this component is compatible with liquid ammonia.

The method is carried out entirely without electrochemical assistance (“electroless”), thereby enabling a surface film to be created without making use of the obligatory, complex contacting steps used for known electrochemical assistance methods.

The method described above is also compatible with the typical masking methods used for components of this type. As described, for example, in FIG. 3 a , product 2 carries mask 7 on its surface, for example, a mask of the Si 3 N 4 , SiO 2 , ZnS type, or a polymer compatible with liquid ammonia. In these cases, as shown in FIG. 3 b , passivation film 6 is formed uniquely on a portion of the product surface. In a variant, mask 7 could be formed of an oxide of the III-V material of product 2 . In effect, the oxides, in particular those of InP or GaP, are perfectly stable in liquid ammonia, even in the presence of PCl 5 .

For example, if we do not want such an oxide to interfere with the formation of passivation film 6 , installation 1 can also comprise deoxidation unit 8 upstream of passivation apparatus 3 itself. Thus, sample 2 first undergoes the deoxidation process before undergoing formation of the polyphosphazene-type film. For an example of a deoxidation method, we can, for example, dip the sample in deoxidizing solution 9 , typically 1M HCl in ultrapure water for several minutes, followed by rinsing 10 with ultrapure water, and drying 11 , for example, under an argon or nitrogen stream.

In this embodiment, the sample thus treated is immediately introduced into the liquid ammonia solution for passivation.

The method is, thus, especially compatible with the other industrial steps for the production of III-V semiconductor components.

After the step involving formation of a passivation film, we can also make use of an extraction step. For example, the cradle with the sample is rapidly withdrawn from treatment bath 4 , simply beneath exhaust hood 12 . We can then proceed to washing step 13 , for example, by plunging the sample into bath 14 of ultrapure ammonia. This step can continue or even be used for intermediate storage prior to final extraction. Final extraction occurs, for example, by a gradual increase of the temperature of the sample, maintained in its cradle, by means of temperature regulation device 15 . The processed sample is then stored in a conventional support for a semiconductor wafer.

It has elsewhere been observed that the P,N polymer film obtained without electrochemical assistance by the method described above is different from the polyphosphazene film obtained by methods involving electrochemical assistance found in the prior art. We can speak of a “polyphosphazene-type film” to the extent that it presents characteristics similar to polyphosphazene films without, however, being identical. In particular, the XPS photoelectronic spectrometry measurement associated with N 1S presents significant differences, as shown in FIG. 4 , in the method without electrochemical assistance described above and in FIG. 4 a for the prior electrochemical method. In FIG. 4 a , relative to the N 1Δ response of InP samples, curve A corresponds to a recently deoxidized InP surface, curve B is obtained after anodic treatment with electrochemical assistance in liquid ammonia and storage in air for at least one year, and curve B* is obtained by adding a step involving immersion for 16 hours in a deoxygenated aqueous solution of K 2 PtCl 4 (3 millimoles per liter) following the two steps of the method corresponding to curve B (this step induces a reaction of amine groups on the passivated surface with PtCl 4 2− and acts as a developer (see the publication referred to below)). As can be seen, in particular, in FIG. 4 , based on normalized intensity in arbitrary units, the spectrum presents a first peak with a binding energy equal to 399 electron volts and a second peak with a binding energy equal to 398 electron volts. The amplitude of the first peak (the highest binding energy) is clearly greater than that of the second peak (at least 20% greater), and this is easily reproducible. This spectrum characterizes the film obtained by the method described above and suggests a considerable surface order for one of the chemical environments of nitrogen in the film. By peak, we mean that the curve of intensity as a function of energy presents a maximum between an increasing part and a decreasing part. In comparison, we note that, for the electrolytic methods of FIG. 4 a , the spectrum presents two principal peaks with similar amplitudes.

Moreover, the film formed is very thin, which is demonstrated by the set of XPS data.

Therefore, we can consider an XPS spectrum measurement such as this to be a signature of the anelectrolytic implementation of the method of forming a polyphosphazene-type film.

The present description has been given principally with respect to indium phosphide (InP). It is presumed that the principles upon which the invention is based apply equally to other III-P materials, when all the precautions necessary for the use of another III-P material have been taken. In particular, we presume that the implementation of the above method with a gallium phosphide-type (GaP) material is possible.