The present invention relates to a method of manufacturing a semiconductor device in which a ZnO based oxide semiconductor layer is subjected to hetero epitaxial growth on a sapphire substrate, for example, in a light emitting device such as a light emitting diode (hereinafter referred to as an LED) using a ZnO based oxide semiconductor or a laser diode (hereinafter referred to as an LD), an SAW device such as an SAW (surface acoustic wave) filter or an SAW oscillating device, a pyroelectric device, a piezoelectric device, a gas sensor or the like. More specifically, the present invention relates to a method of manufacturing a semiconductor device having a high quality ZnO based oxide semiconductor layer which decreases dislocations or crystal defects being apt to generate in a grown film based on an atmosphere gas after growth, a stress caused by a difference in a coefficient of thermal expansion between a substrate and a ZnO based oxide semiconductor layer or the like.
A blue color based (a wavelength region from ultraviolet to yellow) light emitting diode (hereinafter referred to as an LED) to be used for a full color display, a signal light or the like and a blue laser diode (hereinafter referred to as an LD) for a very fine DVD light source for a next generation which continuously oscillates at a room temperature can be obtained by laminating GaN based compound semiconductor layers on a sapphire substrate and have recently attracted the attention. While the GaN based compound semiconductor is in major in the light emitting device having a short wavelength, it has also been investigated that a II-VI compound semiconductor such as ZnO is used. The ZnO has a band gap of 3.37 eV at a room temperature and it has also been expected that the ZnO based oxide can be applied to a transparent conductive film, a transparent TFT, a transparent conductive film, an SAW device, a piezoelectric device and the like in addition to the DVD light source.
The ZnO based oxide semiconductor is also a hexagonal crystal in the same manner as a GaN based compound semiconductor or sapphire and has a lattice constant close to that of GaN. Therefore, there has been proposed, as a substrate, sapphire which has been widely used industrially as a substrate for epitaxial growth of a GaN based compound semiconductor. However, the lattice constant (xe2x80x9caxe2x80x9d axial length) of the sapphire is 0.4758 nm, while the xe2x80x9caxe2x80x9d axial length of ZnO is 0.3252 nm. There is a problem in that a mismatch is great based on a difference in the lattice constant and a dislocation or a crystal defect is easily generated in an epitaxial grown layer. For this reason, it has been proposed a method of forming a buffer layer such as a ZnO layer on the sapphire substrate at a low temperature of approximately 350xc2x0 C. and then growing a ZnO based oxide semiconductor layer at a high temperature of approximately 600xc2x0 C.
As described above, the sapphire substrate is regarded as the optimum material which is currently proposed for such a substrate as to grow a ZnO based oxide semiconductor layer. However, there is a problem in that the dislocation or the crystal defect in the epitaxial grown layer cannot fully be decreased even if the effort such as interposing a buffer layer or the like is made when the ZnO based oxide semiconductor layer grows on the surface of the sapphire substrate, and a high quality ZnO based oxide semiconductor layer which has an excellent crystalline property cannot be thereby obtained.
In order to solve such a problem, the present inventors vigorously made investigations repetitively. As a result, it was found the following. More specifically, a countermeasure has conventionally been taken based on such a thought that a crystal defect is caused by a lattice mismatching based on a difference in a lattice constant between a substrate and a ZnO based oxide layer to be epitaxially grown, which should be solved. However, sapphire and ZnO have coefficients of thermal expansion of 7.3xc3x9710xe2x88x926 Kxe2x88x921 and 4.53xc3x9710xe2x88x926 Kxe2x88x921 respectively and a dislocation or a crystal defect is newly generated based on the difference in the coefficient of thermal expansion. In general, furthermore, after a semiconductor layer of this kind is completely grown, a substrate temperature is lowered while causing a gas of a material including a constituent element having a high vapor pressure to flow. When the ZnO based oxide semiconductor layer is left in the oxygen atmosphere, the dislocation or the crystal defect easily proceeds.
In consideration of such a situation, an object of the present invention is to provide a method of manufacturing a semiconductor device having a high quality ZnO based oxide semiconductor layer having an excellent crystalline property in which a dislocation or a crystal defect can be prevented from being generated over an epitaxial grown layer based on the atmosphere while a substrate temperature is lowered after the growth of the semiconductor layer and a difference in a coefficient of thermal expansion.
As described above, the present inventors vigorously made investigations repeatedly in order to improve the crystalline property of the ZnO based oxide semiconductor layer to be epitaxially grown on a sapphire substrate. As a result, it was found the following. More specifically, when the ZnO based oxide semiconductor layer is epitaxially grown at a high temperature of approximately 600xc2x0 C. and a heater for heating the substrate is turned off immediately after the growth, the temperature of the substrate is changed quickly and a stress is applied to both of the substrate and the ZnO based oxide semiconductor layer based on a difference in a coefficient of thermal expansion therebetween. Consequently, a dislocation or a crystal defect is newly generated in the epitaxially grown layer.
More specifically, even if the growth is carried out carefully such that the crystal defect is not generated during the epitaxial growth of the ZnO based oxide semiconductor layer, the dislocation or the crystal defect is newly generated when the substrate temperature is rapidly lowered after the growth is completed. Consequently, it was found that the characteristic of the device is greatly influenced by this new dislocation or crystal defect.
Furthermore, the following was found. After the growth of the ZnO based oxide semiconductor layer is completed, the supply of oxygen to be raw material of a ZnO based oxide is stopped and the lowering in the temperature is carried out at a low speed of 5 to 10xc2x0 C./minute or less, for example. Consequently, a ZnO based oxide semiconductor layer having an excellent crystalline property can be obtained. Moreover, the following was found. Conventionally, the gas of a constituent element having a high vapor pressure has been caused to flow when the substrate temperature is lowered. In the case of the ZnO based oxide, when the substrate temperature is lowered in the oxygen gas atmosphere, the surface is roughened by the oxygen, which is not preferable. By stopping the supply of the oxygen, a ZnO based oxide semiconductor layer having an excellent crystalline property can be obtained.
As a matter of course, a ZnO based oxide can be epitaxially grown also at a low temperature of approximately 400xc2x0 C. In the case in which the epitaxial growth is carried out at such a temperature, a new crystal defect is rarely generated because the difference from a room temperature is small even if a heater for heating a substrate is directly turned off to rapidly lower the temperature. But if the temperature of the epitaxial growth is low, a residual carrier concentration cannot be decreased. When the epitaxial growth is carried out at a high temperature of approximately 550 to 600xc2x0 C., the residual carried concentration can be decreased so that a semiconductor layer having a desirable carrier concentration can be obtained and the carrier concentration of a p-type layer can also be increased. The present inventors found these facts in another foregoing investigations.
So, it is preferable that the ZnO based oxide semiconductor layer should be epitaxially grown at a high temperature of approximately 500xc2x0 C. or more, and that a lowering the substrate temperature after the growth should be particularly controlled carefully.
A method of manufacturing a semiconductor device having a ZnO based oxide semiconductor layer according to the present invention comprises the steps of; growing hetero-epitaxially the ZnO based oxide semiconductor layer on a substrate having a coefficient of thermal expansion which is different from that of a ZnO based oxide semiconductor, at a high temperature of 500xc2x0 C. or more, and stopping the supply of an oxygen and cooling gradually until the temperature of the substrate is lowered to 350xc2x0 C. or less after the growth of the ZnO based oxide semiconductor layer is completed.
The ZnO based oxide semiconductor means an oxide including Zn and includes an oxide of IIA group and Zn, IIB group and Zn or IIA group, IIB group and Zn in addition to ZnO as a specific example.
By using this method, it is possible to prevent new dislocations or crystal defects from being induced during lowering the substrate temperature in the ZnO based oxide semiconductor layer. Consequently, a carrier concentration in an undoped state can be reduced and a mobility can be improved. As a result, the crystalline property of an active layer in a semiconductor light emitting device becomes excellent and a high light emission efficiency can be obtained. In addition, although the carrier concentration of a p-type layer or the like could not be increased fully, the crystalline property can be improved so that the mobility of a carrier can be improved and the carrier concentration can also be increased greatly.
It is preferable that a buffer layer comprising a ZnO based oxide semiconductor layer should be grown at a lower temperature than the high temperature at which the ZnO based oxide semiconductor layer is to be grown on the substrate before the semiconductor layer is grown at the high temperature of 500xc2x0 C. or more, and the irradiation (supply) of an oxygen should be then stopped when the substrate temperature is to be raised to grow the semiconductor layer at the high temperature.