T 0221/98 28-02-2002

Reasons for the Decision

1. The appeal is admissible.

2. Main request

2.1. The main request (see in particular claim 1) concerns a light-emitting device of gallium nitride compound semiconductor material having a first layer of gallium nitride compound semiconductor material doped with a p-type impurity, a second layer of n-type gallium nitride compound semiconductor material, a first layer electrode for the first layer, and a second layer electrode for the second layer, these electrodes comprising respective layers with specific metals and/or alloys in contact with the layers of gallium nitride.

The independent claims of the application as filed, ie, claims 1, 4 and 6, concern light-emitting devices of gallium nitride compound semiconductor having an n-layer of n-type gallium nitride compound semiconductor and an i-layer of i-type gallium nitride compound semiconductor doped with a p-type impurity.

This is also the case for the whole content of the application as filed (see the text and drawings) and, thus, claim 1 of the main request is distinguished therefrom inter alia by the removal of the feature "an i-layer of i-type".

2.2. The appellant has argued that according to T 331/87, OJ EPO 1991, 22, the replacement or removal of a feature from a claim is allowable if the three conditions mentioned therein are satisfied (cf. point IV of the Summary of Facts and Submissions, here above).

The following is to be noted in this respect:

Indeed, it has not been disputed that the feature that the first layer of the gallium nitride compound semiconductor doped with a p-type impurity is an i-layer of i-type is not explained in the application as filed as being essential, so that the condition (1) of T 331/87 is fulfilled.

From the whole content of the application as filed (see in particular page 1, penultimate paragraph to page 3, second paragraph, second line), it is derivable that the present invention addresses the problem or disadvantage of known light emitting diodes arising because of the electrode of aluminum being formed in direct contact with a layer made i-type by doping with a p-type impurity, this resulting in non-uniform emission of light from coarse dots rather than a uniform plane.

The appellant has argued that the definition of the present invention cannot be interpreted to comprise the case that first layer of gallium nitride semiconductor compound is n-conductive because, in particular, if the first layer were to be n-conductive by either no doping impurity at all or only a small amount of p-type impurities, the device would not function as a light-emitting device, so that the device of inter alia claim 1 does not comprise a n-conductive layer.

However, these arguments cannot convince for the following reasons:

Since it is admitted that the first layer of a device as defined in claim 1 of the present main request can be n-conductive if the mentioned p-type doping comprises only a small amount of p-type impurities and that such a device would not function as a light-emitting device, then the feature of the application as filed that the first layer is i-type can be considered as indispensable for the function of the invention in the light of the technical problem it serves to solve in the sense that its removal leads inter alia to a device wherein the first layer is of n-type conductivity and which admittedly does not work.

It follows therefrom that the removal of the feature of the application as filed that the first layer is i-type also requires real modification of other features to compensate for the change in that sense that it is necessary to specify that the first layer is not of n-type gallium nitride compound semiconductor material.

Thus, since the conditions (2) and (3) in the above-mentioned decision T 331/87 are not fulfilled, it appears with respect to claim 1 of the present main request that the removal of the feature "an i-layer of i-type" from the main claims of the application as filed contravenes Article 123(2) EPC.

2.3. The appellant filed in preparation for the oral proceedings several technical publications showing inter alia that, before the filing date of the present application, it was generally known that doping with p-impurities was necessary to obtain both i-type and p-type conductivity, whereby p-type conductivity could also be obtained without an additional irradiation with electrons. In particular, it is derivable from the technical documents cited that specific process steps such as annealing can be necessary to obtain p-type.

However, there is no indication in the application as filed, which consistently discloses a first layer being i-type, that any such specific process step is carried out, so that the relevant information in this respect was not derivable from the application as filed.

The appellant has also pointed out that document D4 (see the abstract) shows a device of the same type with a p-type first layer (5) doped with a p-type impurity (Zn), in contact with an Al layer (2), and characteristics of the device, in particular 30 mcd in light emission brightness being the same as the prior art characteristics mentioned in the application as filed (see page 11, second complete paragraph, second sentence).

However, document D4 is not mentioned in the application as filed. Moreover, the conventional device mentioned in the application as filed (see the passage cited above) in this respect is specified as having an i-type layer. There is no indication either that p-n devices were disclosed in the application as filed.

Moreover, it is to be noted that light-emitting devices of gallium nitride including an i-type layer are the subject-matter of eg document D3 (see in particular column 1, lines 19 to 42), which specifies that the device comprises no p-n junction but is made by joining the i-layer and n-layer, and of documents D1 and D2.

Thus, for the reader of the application as filed, there was no reason to consider that the p-doped i-layer could be a layer of a conductivity type different from i-type.

2.4. Therefore, the main request does not satisfy the requirement of Article 123(2) EPC that a European patent application may not be amended in such a way that it extends beyond the content of the application as filed.

3. Auxiliary request I

3.1. Claim 1 of the auxiliary request I specifies that the layer of gallium nitride compound semiconductor material doped with a p impurity is a i-type layer; the device can comprise, in contact with the i-layer, an alloy including Ti, and, in contact with the other layer, Al or an alloy including Al.

A light-emitting device of gallium nitride compound semiconductor material is known from document D2 (see the abstract) having an i-layer (3) of gallium nitride compound semiconductor material, a second layer (2) of n-type gallium nitride compound semiconductor material, an electrode (44) for the i-layer (3), and a second layer electrode (55) for the second layer (2); the electrode (44) for the i-layer comprises a layer (4') of Ti in contact with the i-layer.

It is to be noted that, from document D2, it is not derivable how the i-layer is obtained. However, it is generally known to people skilled in the art, for instance from document D4 (see the abstract) or document D3 (see column 2, lines 13 to 23; column 7, lines 19 to 37, column 8, lines 7 to 12 and Figure 1; see also column 9, lines 25 to 32 and Figure 9), that gallium nitride compound semiconductor material of i-type can be obtained by doping the material with a p-type impurity, Zn.

It is also to be noted that it is not directly and unambiguously derivable from the abstract of document D2, which is the material for the electrode layer in contact with the n-type region. However, for the same type of device, an Al layer in contact with a n-type layer of a gallium nitride i-n junction device is known from any of the documents D1 (see the abstract) or D3 (see column 7, lines 26 to 28; Figure 1).

Thus, this information can be used in an obvious way to complete the device of document D2.

However, it is not derivable from document D2 that an alloy of titanium can be used in place of Ti as the material of the electrode layer in contact with the i-layer of the device.

Starting from document D2, the main problem to be solved is to find an alternative material for the material of the electrode layer in contact with the i-layer of the device.

In document D2, it is stated that electrode (44) on the i-type gallium nitride layer (3) has an underlayer which is made of metal (Ti) which repels solder but well adheres to crystal surface. In this respect, it is to be noted that, in the application as filed, no information is made available showing, for the electrode layer in contact with the i-layer, any advantage of titanium alloys as compared to titanium, and these materials are thus derivable as being equivalent.

The appellant has not provided any specific argument in this respect.

Thus, since titanium and alloys containing e.g. a large amount of titanium can be understood as having similar properties, there was at least an incentive to try such materials for the electrode in contact with the i-type layer in the device known from D2.

Therefore, having regard to the state of the art, the subject-matter of claim 1 of the auxiliary request I is obvious to a person skilled in the art and it thus lacks an inventive step in the sense of Article 56 EPC.

4. Auxiliary request II

In claim 1 of the auxiliary request II, the i-layer electrode comprises a layer formed directly on said i-layer and being made of Ni, Ag, or an alloy containing any of them.

Thus, the claim can concern a device wherein the i-layer electrode comprises a layer formed directly on said i-layer and being made of an alloy containing Ni, wherein moreover the n-layer and the i-layer have their respective electrodes which are either on the same surface of the device, or on opposite surfaces.

In the application as filed,

- there are embodiments wherein a Ni layer is formed in contact with the i-layer of the device, said Ni layer functioning as an electrode therefor (see claim 1), whereby Ag or Ti or an alloy thereof may replace the Ni underlayer (see page 9, last paragraph to page 10, second paragraph and Figures 3f and 4g; page 11, second paragraph, three first lines).

- in specific further embodiments, the electrode for the i-layer of the device is made of Ni, Ag or Ti, or an alloy containing any of them, i.e. also an alloy including Ni (see claims 4 and 6).

In these specific further embodiments, as well as in all the embodiments shown in the drawings, the n-layer and the i-layer have their respective electrodes on the same surface.

However, there is no basis in the application as filed for a light-emitting device of gallium nitride compound semiconductor material with a n-layer and an i-layer wherein an i-layer electrode comprises a layer formed directly on said i-layer and which is made of an alloy containing Ni, and wherein the n-layer and the i-layer have their respective electrodes which are not on the same surface.

Therefore, the auxiliary request II does not satisfy the requirement of Article 123(2) EPC.

5. Auxiliary request III

5.1. Admissibility of the amendments

Claim 1 of the auxiliary request III specifies that the n-layer (3, 4) and the i-layer (5) of the light-emitting device have their respective electrodes formed on the same surface and is otherwise similar to claim 1 of the auxiliary request II.

A light-emitting device according of claim 1 is disclosed in particular in independent claim 4 of the application as filed.

Since moreover the further claims of the auxiliary request III are only dependent claims for devices comprising particular features which have a basis in the original disclosure, the request satisfies the requirement of Article 123(2) EPC.

5.2. Inventive step

A light-emitting device is known from document D4 (see the abstract of the PAJ and the abstract provided with applicant's letter dated 30 May 1997) having a p-type layer (5) of gallium nitride AlxGa1-xN, x = 0, doped with Zn, and an n-layer (4) of a gallium nitride compound thereunder, an electrode (6) for said Zn-doped p-layer (5), and an n-layer electrode (7) for said n-layer (4), whereby said p-layer electrode (6) is in contact with the p-layer (5) and is made of Al. Other metallic compositions, e.g. Ni, Ag and alloys thereof, are mentioned for the electrode (6) on the p-layer (5).

As set forth here above (cf. item 2.3 of the reasons), Zn is a p-type impurity in gallium nitride.

In the opinion of the Board, document D4 represents the closest prior art in that sense that it also concerns a gallium nitride light-emitting device and that the same metallic materials are cited for the electrode of the layer doped with p-type impurities.

As regards the disclosure in document D4, the appellant argued as follows:

All the examples of document D4 consistently use aluminium for the metal of the electrode in contact with the layer doped with p-type impurities. Although this document mentions several other metals, e.g. Ni or Ag or alloys thereof as materials of this electrode, a specific combination of an electrode of Ni, Ag or an alloy thereof in contact with a p-doped i-type GaN layer is not disclosed in document D4.

The Board agrees with the above submissions, since in document D4, the Zn-doped layer (5) is a p-type layer, and not an i-type layer and, although it can be derived from document D4 that Ni, Ag or alloys thereof can be substituted for Al for a p-type layer, there is however no such indication for an i-type layer.

In the patent application in suit, the light-emitting patterns of light-emitting devices with Al, Ni and Ag as the electrode material for the i-type layer are shown in Figures 5A, 5B and 5C respectively. It follows from a comparison of these patterns that with Ni and Ag as the electrode material, the light is emitted uniformly from a plane rather than from dots as in the device with Al electrode (see Figure 5A).

The appellant submitted that although document D4 mentions Ni and Ag as substitute for Al as an electrode material, there is no hint in the document that Ni or Ag as an electrode material in contact with an i-type layer would provide uniform light-emission from a plane. The Board accepts this submission and finds that the choice of Ni, Ag or alloys thereof as in the claimed subject-matter was a purposive selection with a view to improving light-emission characteristics of the device, and therefore would not have been obvious from the teaching of document D4.

The abstract of document D2 only discloses Ti for the electrode layer in contact with the i-type GaN layer. The abstract of document D5 is not relevant since the conductivity type of the gallium nitride layer (105) in contact with the conductive layer (106) is not specified, and also this GaN layer (105) is adjacent to a light-emitting layer (104) of a different material, i.e., silicon carbide layer. Moreover, the material of the conductive layer (106) is not indicated either, so that the device is different from the device of claim 1.

For the foregoing reasons, the Board comes to the conclusion that the subject-matter of claim 1 of the auxiliary request III is not obvious to a person skilled in the art and thus involves an inventive step in the sense of Article 56 EPC.

Therefore, claim 1 is patentable in the sense of Article 52(1) EPC. Since the remaining claims are dependent device claims concerning specific embodiments, they are also patentable for the same reasons.

The description, however, needs to be adapted to the claims.