Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate
Reexamination Certificate
2002-12-13
2004-11-09
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
C118S725000, C118S728000, C156S345420, C438S723000
Reexamination Certificate
active
06815316
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating a compound semiconductor device employed for an LED (light emitting diode) or the like and an apparatus for fabricating a compound semiconductor device, and more particularly, it relates to a method of fabricating a semiconductor device for a ZnSe-based LED employed for the backlight of a liquid display unit or the like and an apparatus for fabricating a compound semiconductor device.
2. Description of the Prior Art
In order to drive a compound semiconductor device such as an LED, it is necessary to form an electrode on a compound semiconductor. For example, an electrode of ohmic contact must be formed on the back surface of a ZnSe substrate, in order to produce a ZnSe-based LED chip. However, such an electrode of ohmic contact cannot be readily formed on the ZnSe substrate for the following reasons:
(a) The upper bound of the carrier concentration of the ZnSe substrate is limited to the latter half of the 10
17
mark. No ZnSe substrate having a carrier concentration exceeding the latter half of the 10
17
mark has heretofore been fabricated. In particular, the carrier concentration of a p-type ZnSe substrate cannot even reach the 10
17
mark.
(b) Oxides are readily formed on the surface of the substrate.
(c) The treatment temperature for forming the electrode must be not more than 250° C., in order to protect an active layer or a cladding layer of an emission part.
In general, In which is a low melting point metal is known as an electrode metal implementing ohmic contact. An electrode of ohmic contact can be formed also on the aforementioned ZnSe substrate by fusing In.
In a ZnSe-based LED prepared by fusing In, however, various inconveniences result from the low melting point of In in solder reflow or transfer molding. When the ZnSe-based LED is heated to 200° C. to 250° C., for example, ball-up results from the low melting point of In of about 157° C., and hence a flat interface cannot be obtained. Therefore, uniform ohmic contact cannot be attained despite implementation of partial ohmic contact. When a flat electrode having ohmic contact is not formed, an unnecessarily high voltage must be applied to the overall LED, which in turn requires a large number of batteries and cannot be readily applied to the backlight for a liquid crystal display screen of a portable telephone or the like. Thus, strongly awaited is development of an electrode, not prepared from In, capable of attaining thermally and mechanically stable ohmic contact with a compound semiconductor.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of fabricating a compound semiconductor device having an electrode attaining stable ohmic contact with a compound semiconductor without employing a low melting point metal such as In and an apparatus for fabricating a compound semiconductor device.
The method of fabricating a compound semiconductor device according to the present invention comprises a substrate cleaning step including a first cleaning step of heating a compound semiconductor substrate containing a first conductivity type impurity in a temperature range of not more than 250° C. for etching its surface with hydrogen chloride and a second cleaning step of performing a radical hydrotreatment on the compound semiconductor substrate etched with hydrogen chloride after the first cleaning step.
When etching the surface of the compound semiconductor substrate with hydrogen chloride, an oxide film resulting from atmospheric exposure can be removed. In this hydrogen chloride etching, the temperature of the compound semiconductor substrate must be not more than 250° C., in order to prevent damage of an active layer and a cladding layer already formed on the opposite surface of the compound semiconductor substrate. When performing the hydrogen chloride treatment at a temperature of not more than 250° C., the surface of the compound semiconductor substrate adsorbs Cl although the oxide film or a carbide can be removed from this surface. Therefore, the radical hydrotreatment is performed for removing the adsorbed Cl. Cl can be removed by the radical hydrotreatment. Radical hydrogen has high reaction activity, and hence a sufficiently high radical hydrogenation reaction rate can be ensured also when setting the temperature of the compound semiconductor substrate to not more than 250° C. A clean surface of a compound semiconductor can be obtained also at a temperature of not more than 250° C. due to the hydrogen chloride treatment and the radical hydrotreatment, so that an epitaxial compound semiconductor film can be formed without forming interfacial levels. The compound semiconductor substrate includes not only the bare substrate in the initial stage of the treatments but also the compound semiconductor substrate formed with a thin film such as an optical active layer in the process of the treatments. The present invention is mainly directed to a case of forming an ohmic electrode layer on the back surface of a compound semiconductor substrate already formed with an optical active layer or the like on its surface without damaging the optical active layer or the like.
The method of fabricating a compound semiconductor device according to the present invention can further comprise a compound semiconductor film forming step of epitaxially growing a compound semiconductor film containing the first conductivity type impurity in a higher concentration than the compound semiconductor substrate on the compound semiconductor substrate after the cleaning step and a conductive electrode film forming step of forming a conductive electrode film on the compound semiconductor film.
When directly forming a conductive electrode layer of a material other than In on the compound semiconductor substrate, ohmic contact cannot be attained if the compound semiconductor substrate has a low carrier concentration. Therefore, the compound semiconductor film containing a conductive impurity in a higher concentration than the compound semiconductor substrate is epitaxially grown for forming the conductive electrode film on the epitaxial film having a high carrier concentration and ensuring ohmic contact. If merely performing cleaning by general etching after exposing the surface of the compound semiconductor substrate to the atmosphere, a desired compound semiconductor film cannot be obtained due to a large quantity of impurities remaining on the surface. Therefore, when performing film formation at a low temperature of not more than 250° C., for example, an epitaxial film having a high carrier concentration cannot be obtained with a small number of interfacial levels on the surface of the compound semiconductor substrate.
If the interface between the compound semiconductor film and the conductive electrode film is not clean, large interfacial resistance is formed to cause such large potential difference on the interface that ohmic contact is not implemented, or voltage applied to the overall LED cannot be reduced even if ohmic contact is implemented. According to the aforementioned structure of the present invention, a highly clean surface can be obtained on the back side of the compound semiconductor substrate without damaging an optical active layer or the like already formed on its surface, for readily forming an electrode of ohmic contact thereon.
An active layer and a cladding layer serving as emission parts are formed on another surface of an n-type ZnSe substrate. The active layer and the cladding layer stably emit blue light, while the ZnSe substrate receiving this blue light emits yellow light. Therefore, highly stable white light can be obtained through the low-priced compound semiconductor device. According to the present invention, the electrode of ohmic contact is provided on the back surface of the aforementioned n-type ZnSe substrate for reducing a contact potential, thereby enabling reduction of the number of batteries necessary for the backlight of a liquid crystal display unit for a p
Berry Renee R.
Fasse W. F.
Fasse W. G.
Sumitomo Electric Industries Ltd.
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