Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
2000-05-05
2002-01-01
Christianson, Keith (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
Reexamination Certificate
active
06335218
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a group III nitride semiconductor device (also simply referred to as a device, hereinafter) and, more particularly, to a method of producing the device.
2. Description of the Related Art
In the field of light emitting devices such as light emitting diodes, semiconductor laser diodes or the like, a semiconductor light emitting device having a crystal layer obtained by adding a group II element such as magnesium (Mg), zinc (Zn) or the like into a single crystalline group III nitride semiconductor (Al
x
Ga
1−x
)
1−y
In
y
N (0≦x≦1, 0≦y≦1) attracts much attention as a device which can emit a blue light.
Epitaxial growth of nitride semiconductor is generally performed by a metalorganic chemical vapor deposition (MOCVD) method. An as-grown layer of a group III nitride semiconductor crystal (Al
x
Ga
1−x
)
1−y
In
y
N (0≦x≦1, 0≦y≦1) element such as Mg, Zn has been added by using MOCVD, however, usually exhibits extremely high resistivity. Attempts to achieve a blue light emitter using this material have been hampered by this inability of p-type conduction.
In recent years, it has been reported that low-resistivity p-type could be obtained by performing a special treatment to the high-resistivity (Al
x
Ga
1−x
)
1−y
In
y
N (0≦x≦1, 0≦y≦1) doped with group II element such as Mg. H. Amano et al. found out that low-resistivity p-type conduction was realized by performing a low-energy electron-beam irradiation to the crystal (H. Amanoetal. :Jpn. J. Appl. Phys. Vol.28, 1989, pp.L2112-2114). S. Nakamura et al. has found out that a low-resistivity p-type crystal can be also realized by performing a heat treatment to the crystal at a temperature in a range from approximately 700 to 800° C. in nitrogen ambient under atmospheric pressure or under high pressure (S. Nakamura et al.: Jpn. J. Appl. Phys. Vol.31, 1992, pp. L139-142).
The mechanism of the treatment for establishing those p-type conduction is interpreted in such a manner that, the hydrogen atoms which passivate the group II acceptor impurities such as Mg or the like by combining with them in the grown film are dissociated by the above treatments.
According to the above method of the low-energy electron-beam irradiation, an extremely high room temperature hole concentration on the order of E18/cc is obtained in a resultant p-type crystal. However, a treated depth is limited within a penetration depth of the electron beam and, for example, the treated depth is equal to about 0.3 &mgr;m with an accelerating voltage of 6 to 30 kV (S. Nakamura et al.: Jpn. J. Appl. Phys. Vol.31. 1992, pp. L139-142). Since the low-energy electron-beam irradiation treatment is performed by scanning the wafer surface with an electron beam in a vacuum vessel, the equipment is apt to be bulky and moreover a long treatment time is needed, so that this method is unfavorable for the mass production of the laser devices.
On the other hand, the above method of the heat treatment is free from the strict limitation of the treated depth as in the case of the low-energy electron-beam irradiation process. It is considered that the heat treatment is suitable for mass production since a number of wafers can be treated in a batch by a heating furnace.
In the case of producing a semiconductor laser device by using the heat treatment as mentioned above, a contact resistance at an electrode of the device remains as a problem, since the room-temperature hole concentration adjacent to the electrode is equal to approximately 3E17/cc. If a treatment temperature is raised in an attempt to increase the hole concentration of the film as a whole, electric characteristics of the device are contrarily deteriorated. It is thought that this is because nitrogen vacancies are generated near the film surface due to nitrogen dissociation, the nitrogen vacancies act as donors and compensate the acceptors, and the hole concentration in a region near the surface contrarily decreases. A contact characteristic of the electrode is, consequently, deteriorated.
OBJECT AND SUMMARY OF THE INVENTION
Several countermeasures against the nitrogen dissociation problem as mentioned above are conceivable.
For example, as a first countermeasure, there is a method of extremely raising the pressure of nitrogen ambient for the heat treatment. It has been found that if the heat treatment is performed under a high nitrogen pressure of approximately 90 atm, no surface degradation occurs even at a temperature of approximately 1000° C. (S. Nakamura et al.: Jpn. J. A.ppl. Phys. Vol.31, 1992, pp. L1258-1266).
In the case of using the above method, however, since an extremely high pressure and temperature used in the process requires a special chamber for the heat treatment, resulting in an obstacle to the realization of mass production.
There is a second countermeasure against the nitrogen dissociation problem in which a protecting film or cap layer made of another material is formed on a crystal layer of the group III nitride semiconductor doped with the group II impurity element and, thereafter, the heat treatment is performed. The material of the protecting film is required to have hydrogen permeability so as not to obstruct the elimination of hydrogen while preventing the dissociation of nitrogen from the crystal. Silicon dioxide (SiO
2
), silicon nitride (Si
3
N
4
), aluminum nitride (AlN) and the like can be mentioned as candidate materials which can endure the temperature condition to be used for the heat treatment as mentioned above.
The protecting film SiO
2
is stable at a comparative high temperature and can be easily removed by wet etching using hydrofluoric acid (HF) or the like. Since GaN is hardly eroded by HF, the protecting film SiO
2
formed on a GaN layer has such an advantage that the HF etching is automatically stopped at a time when the proper amount of film is etched. There are, however, the following drawbacks.
An SiO
2
film is formed by a sputtering method or the like, o (oxygen) deficiency fends to occur during the film formation to become SiO
x
(where subscripted x denotes an atom ratio), so that sputtering has to be performed while adding an oxygen gas. In this instance, oxygen penetrates into the nitride semiconductor film. Since oxygen acts as a donor in the group III nitride semiconductor to compensates the acceptor in the crystal of the group III nitride semiconductor doped with group II impurity, the hole concentration in a region near the surface of the group III nitride semiconductor is contrarily decreased. As a result, an expected effect cannot be derived.
A cap layer of Si
3
N
4
also has sufficient high-temperature stability, however, due to its extremely high chemical stability, it is difficult to be removed by chemical etching, An RIE (reactive ion etching) therefore, has to be used for removing the Si
3
N
4
film. In removal of the cap layer, etch selectivity mentioned above becomes a serious requirement. The ideal selectivity is that obtained for the etching of SiO
2
layer on GaN layer by HF.
The thickness of each constituent layer of a device is of great importance, especially for laser diode. The uppermost layer, namely, a contact layer (Mg doped GaN layer in this case) for an electrode of the device is formed to have a relatively small thickness of 0.1 &mgr;m. Since high selectivity cannot in RIE, it is extremely difficult to remove the Si
3
N
4
cap layer completely while leasing the contact layer with a predetermined thickness.
To realize a practical laser device, it is necessary to form some refractive index waveguide structure in the device. The waveguide structure most generally used is what is called a ridge structure. Although a ridge structure can be formed by removing some portions of the semiconductor crystal layer (while) leaving a thin ridge portion, a precise control is required on the removal. Since the ridge forming step is performed after the cap layer removing step, the error caused in the cap layer removing st
Kimura Yoshinori
Miyachi Mamoru
Ota Hiroyuki
Christianson Keith
Morgan & Lewis & Bockius, LLP
Pioneer Corporation
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