Patent Application
20050186435
1. Field of the Invention
The present invention relates to a light emitting device and a method for manufacturing the same.
2. Related Background Art
In recent years, β-FeSi2 has received great attention. β-FeSi2 is abundant as a resource, and a harmless and chemically stable semiconductor. β-FeSi2 is a direct transition-type semiconductor whose forbidden bandgap is approximately 0.85 eV. It is possible to epitaxially grow β-FeSi2 on a Si substrate. Thus, β-FeSi2 is expected to be a material having less environmental burden for next-generation light emitting/light receiving elements.
However, the characteristics of β-FeSi2 have not been known well yet. No report to the effect that light emission was observed from a continuous β-FeSi2 film has ever been made. There exists a report to the effect that a photoluminescence (PL) emission from FeSi2 microcrystals embedded in a Si (100) substrate by an ion injection method or a molecular beam epitaxial (MBE) method. However, this light emission disappears immediately after raising the temperature of the substrate. Therefore, it is difficult to apply this light emission to light emitting devices. Furthermore, the light emission strongly depends on the kind of the substrate (FZ or CZ) and the size of the microcrystals. Accordingly, it is difficult to control the light emission.
An object of the present invention is to provide a light emitting device having a β-FeSi2 film on a Si substrate.
In one aspect, the present invention relates to a light emitting device. This light emitting device comprises: a Si substrate; a β-FeSi2 film which is in contact with the Si substrate; and first and second electrodes which are provided on both sides of the Si substrate. The β-FeSi2 film has an electrical conductivity type different from that of the Si substrate. The first and second electrodes sandwich the Si substrate and β-FeSi2 film therebetween.
In the light emitting device in accordance with the present invention, a pn junction is formed between the Si substrate and the β-FeSi2 film. When an electric current is injected into this light emitting device via the first and second electrodes, the β-FeSi2 film emits light. Since the β-FeSi2 continuously placed on the Si acts as a luminescent layer, the luminescence properties of this light emitting device are not influenced very much by the kind and purity of the substrate.
In another aspect, the present invention relates to a method for manufacturing a light emitting device. This method manufactures a light emitting device comprising a Si substrate, a β-FeSi2 film which is in contact with the Si substrate, and first and second electrodes which are provided on both sides of the Si substrate. The β-FeSi2 film has an electrical conductivity type different from that of the Si substrate. The first and second electrodes sandwich the Si substrate and the β-FeSi2 film therebetween. This method comprises: thermally cleaning the Si. substrate; forming an initial layer made of β-FeSi2 on the Si substrate at a first temperature; growing the initial layer at a second temperature higher than the first temperature to form a β-FeSi2 film; and annealing the β-FeSi2 film at a third temperature higher than the second temperature.
This method can manufacture the above-described light emitting device. By forming an initial layer and then growing the same, the β-FeSi2 film with high crystallinity is formed on the Si substrate.
The present invention will be more fully understood from the following detailed description and the accompanying drawings. The accompanying drawings are only illustrative and are not intended to limit the scope of the present invention.
Further scope of applicability of this invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the description of the drawings, identical symbols are used for identical elements, and these elements will not be explained repeatedly.
The Si substrate 1 is an n-type Si (111) substrate manufactured by Czochralski (CZ) method, that is, a substrate with a principal surface having a plane orientation of (111). The size of the substrate 1 is 2 inches. The substrate 1 has a front side 1A and a back side 1B which are positioned opposite each other.
The β-FeSi2 film 2 is provided on the Si substrate 1 so as to cover the whole of the front side 1A of the Si substrate 1. The β-FeSi2 film 2 has a front surface 2A and a back surface 2B which are positioned opposite each other. The back surface 2B is in contact with the front side 1A of the Si substrate 1. The thickness of the β-FeSi2 film 2 is, preferably, 100-250 nm, and more preferably, 100-200 nm. In the present embodiment, the thickness of the β-FeSi2 film 2 is 200 nm. Different from the Si substrate 1, the electrical conductivity type of the β-FeSi2 film is p-type.
The first electrode 3 is, as shown in
The second electrodes 4 are, as shown in
A method for manufacturing the light emitting device 10 will now be described. First, the temperature of the Si substrate 1 is raised to thermally clean the Si substrate 1. In this cleaning process, the temperature of the substrate 1 is raised to 850° C. under a background pressure of 2×10−7 Torr, and the raised temperature is maintained for 30 minutes.
Next, on the front side 1A of the substrate 1 on which the thermal cleaning has been applied, a thin initial layer of β-FeSi2 is formed. To form the initial layer, a high vacuum sputtering device, more specifically, an RF magnetron sputtering device including a load lock unit, may be used. A known RF magnetron sputtering device may be used. The RF magnetron sputtering device can form a β-FeSi2 film at low temperature and high speed.
The growth temperature is preferably 440 to 550° C., and more preferably, 480-520° C. In this embodiment, the growth temperature is 500° C. Under this temperature, an Fe target with a purity of 99.99% is sputtered to form a β-FeSi2 initial layer. The electrical conductivity type of this initial layer is p-type. The thickness of the initial layer is preferably 5-80 nm. In this embodiment, the thickness of the initial layer is 20 nm. During the formation of the initial layer, the argon pressure is controlled at 3×10−3 Torr.
Subsequently, the temperature of the substrate 1 having the initial layer formed thereon is raised to 730-760° C. in the RF magnetron sputtering device to grow the β-FeSi2 initial layer at a speed of 35 nm/hour to be of a thickness of 200 nm. The film thickness of β-FeSi2 is measured by observing a cross section of the grown film by use of a scanning electron microscope (SEM). The obtained β-FeSi2 film is has a nearly flat front surface. The conductivity type thereof is p-type. The hole concentration of the β-FeSi2 film is on the order of 1018 cm−3 at room temperature, and the hole mobility thereof is approximately 20 cm2/V·s at room temperature.
Next, the β-FeSi2 film is annealed to obtain the β-FeSi2 film 2 of the light emitting device 10 of the present embodiment. The temperature of the heat annealing is preferably 790-850° C. In the present embodiment, the annealing temperature is 800° C. In this heat annealing, the Si substrate 1 on which the β-FeSi2 film has been formed is exposed to an 800° C. nitrogen atmosphere for 20 hours. This heat annealing is carried out in a silica tube. The conductivity type of the β-FeSi2 film remains as p-type. As a result, a pn junction is formed between the n-type Si substrate 1 and the p-type β-FeSi2 film 2. After the β-FeSi2 film 2 is annealed at 800° C., the hole concentration thereof is reduced to the order of 1016 cm−3 at room temperature, and the hole mobility thereof is increased to 100 cm2/V·s at room temperature.
Then, the electrodes are formed on the front side and back side of the Si substrate 1. More specifically, the lower electrode 3 is formed by vacuum depositing Al metal on the back side 1B of the Si substrate 1. Also, the upper electrodes 4 are formed by vacuum depositing Al metal by use of masking on the front surface 2A of the β-FeSi2 film 2. Either the lower electrode 3 or the upper electrodes 4 may be formed first. When these electrodes 3 and 4 have been formed, the light emitting device 10 of the present embodiment is completed.
When a direct current was injected into the heterostructure of p-type β-FeSi2 film 2/n-type CZ-Si substrate 1, which was obtained in the manner described above, via the Al electrodes 3 and 4, a light emission with a wavelength band around 1.5 μm was detected at room temperature.
The light emitting device 10 has the β-FeSi2 film 2 continuously provided on the Si substrate 1 as a luminescent layer. Therefore, the luminescence properties are not influenced very much by the kind and purity of the substrate. Accordingly, it is easy to control the manufacturing processes for the light emitting device 10.
A large-area wafer having a plurality of uniform light emitting devices 10 thereon can be manufactured by use of sputtering. Since sputtering can be simply performed at low cost, the light emitting devices 10 can be mass-produced at low cost.
Hereinafter, a second embodiment of the present invention will be described. The inventors have discovered that when annealing of a β-FeSi2 film is carried out at a higher temperature, the conductivity type of the β-FeSi2 film is changed from p-type to n-type. In the present embodiment, by utilizing this change in the conductivity type, a light emitting device having an n-type β-FeSi2 film on a p-type Si substrate is manufactured.
Similar to the first embodiment, a light emitting device 20 of the present embodiment has the construction shown in
The Si substrate 1 is a p-type Si (111) substrate manufactured by a floating zone (FZ) method. The size of the substrate is 2 inches.
The conductivity type of the β-FeSi2 film 2 is n-type, which is different from the conductivity type of the Si substrate 1. The β-FeSi2 film 2 is provided on the Si substrate 1 so as to cover the whole of the front side 1A of the Si substrate 1.
A method for manufacturing the light emitting device 20 will now be described. Similar to the first embodiment as described above, this method includes a cleaning process, an initial layer forming process, a growth process, and an annealing process.
In the cleaning process, similar to the first embodiment, the temperature of the substrate 1 is raised to 850° C. under a background pressure of 2×10−7 Torr, and the temperature is maintained for 30 minutes.
In the initial layer forming process, an Fe target with a purity of 99.99% is sputtered to form a β-FeSi2 initial layer with a thickness of 5-80 nm. The growth temperature is 450° C. The conductivity type of the initial layer is p-type. For sputtering, an RF magnetron sputtering device is used. During the formation of the initial layer, the argon pressure is regulated at 3×10−3 Torr.
In the growth process, the temperature of the substrate 1 on which the initial layer has been formed is raised to 700-760° C. in the RF magnetron sputtering device to grow the β-FeSi2 initial layer to be of a thickness of 250 nm. The conductivity type of the grown β-FeSi2 film remains as p-type. The hole concentration of the β-FeSi2 film is on the order of 2×1018 cm−3 at room temperature, and the hole mobility thereof is approximately 20 cm2/V·s at room temperature.
Next, the β-FeSi2 film is annealed. The temperature of the heat annealing is preferably 880-900° C. In the present embodiment, the annealing temperature is 890° C. In this heat annealing, the Si substrate 1 on which the β-FeSi2 film has been formed is exposed to an 890° C. nitrogen atmosphere for 20 hours. This heat annealing is carried out in a silica tube. The conductivity type of the β-FeSi2 film is changed to n-type from p-type. As a result, a pn junction is formed between the p-type Si substrate 1 and the n-type β-FeSi2 film 2. Owing to the 890° C. annealing, the carrier concentration decreases and the mobility increases. More specifically, the electron concentration of 3-10×1016 cm−3 and the mobility up to 230 cm2/V·s are obtained.
After the annealing process, the lower electrode 3 and the upper electrodes 4 are formed similarly as in the first embodiment. Thereby, the light emitting device 20 of the present embodiment is completed.
It is possible to cause the light emitting device 20 to emit light at room temperature by injecting a direct current into the heterostructure of n-type β-FeSi2 film 2/p-type FZ-Si substrate 1, which is obtained in the manner described above, via the Al electrodes 3 and 4. As such, in the present embodiment as well, the light emitting device 20 having the β-FeSi2 film 2 continuously provided on the Si substrate as a luminescent layer can be obtained.
The inventors carried out an X-ray diffraction analysis for the β-FeSi2 film, which was grown on the Si substrate 1, before annealing it.
As shown in
The rocking curve (ω scan) of the β-FeSi2 peak has a full width at half maximum (FWHM) of 15 arcmin. This means that the β-FeSi2 peak is quite narrow. Thus, the β-FeSi2 film has high crystallinity.
The inventors examined in-plane epitaxial arrangements for the sample shown in
Furthermore, the inventors examined the relationship between the photon energy and the absorption coefficient of the β-FeSi2 film at room temperature.
It is also possible to directly grow the continuous and highly-oriented β-FeSi2 film on the Si substrate immediately after the thermal cleaning without forming the initial layer. However, according to the results of the X-ray diffraction analysis, the ω scan full width at half maximum in the case where no initial layer is formed is wider by 30% than that of the case where the initial layer is formed. Therefore, forming the initial layer makes it possible to obtain a β-FeSi2 film with higher crystallinity.
In the above-described embodiment, the β-FeSi2 film is formed on a p-type FZ-Si substrate. However, it can be considered that even when a p-type CZ-Si substrate is used, a β-FeSi2 film having high crystallinity can be obtained by forming and growing the initial layer.
A light emitting device in accordance with the present invention has a β-FeSi2 film provided on a Si substrate as a luminescent layer. Since the luminescent layer is not microcrystals inside the substrate but a continuous film on the substrate, the luminescence properties of the light emitting device in accordance with the present invention are not influenced very much by the kind and purity of the substrate. Therefore, the light emitting device in accordance with the present invention can be manufactured by manufacturing processes which are easy to control.
In the above, the present invention has been described in detail based on the embodiments thereof. However, the present invention is not limited to the foregoing embodiments, and may be variously modified without departing from the scope thereof.
For example, in the foregoing embodiments, the electrodes 4 are provided so as to contact the front surface 2A of the β-FeSi2 film. However, it may be also possible to form a Si cap layer on the β-FeSi2 film and provide the electrodes on this cap layer. By providing the cap layer, improvement in light emission efficiency can be expected.
Also, in the foregoing embodiments, the RF magnetron sputtering device is used as a high-vacuum sputtering device to manufacture a β-FeSi2 film on the substrate. However, a magnetron sputtering device by another system may be used. Nevertheless, the RF magnetron sputtering deposition method can be preferably used to provide a continuous and highly-oriented β-FeSi2 film on a Si (111) substrate.
From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| P2002-255007 | Aug 2002 | JP | national |
This application is a continuation-in-part of international application No. PCT/JP03/10961, filed Aug. 28, 2003.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP03/10961 | Aug 2003 | US |
| Child | 11066318 | Feb 2005 | US |
