Patent Application
20090170294
This invention relates to a method for film depositing a group III nitride, such as gallium nitride (GaN), aluminium gallium nitride (AlGaN), aluminium nitride (AlN) or indium nitride (InN) on a substrate.
A group III nitride semiconductor, such as GaN, AlGaN, AlN or InN is expected to be applied not only to an optical emission element but also to a high frequency element. Conventional examples of the method for film depositing a group III nitride are listed below.
1) MOCVD utilizing ammonia
2) MBE utilizing high vacuum plasma
3) MBE under high vacuum utilizing ammonia
4) MOCVD utilizing high vacuum plasma
5) laser abrasion under ultrahigh vacuum
Patent Document 1: Japanese Patent Application Laid-Open No. H10-106958
Patent Document 2: Japanese Patent Application Laid-Open No. H04-164859
Non-Patent Document 2: Advanced Electronics I-21. Group III Nitride Semiconductor, written by Isamu Akazaki, issued by Baifukan (1999)
It is demanded that the group III nitride semiconductor as a substrate can be applied not only to sapphire but also to Si, to high molecular material, or to the like in order to expand the application field of a group III nitride semiconductor such as GaN to electronic devices such as high frequency elements from optical devices such as LED.
On the other hand, a ratio (V/III ratio) between a group III material and a reactive nitrogen source is closely related to the growth of a group III nitride film. The V/III ratio is controlled by increasing the pressure to about several Pa under a vacuum condition using ammonia as a nitrogen source in the above-mentioned film depositing methods 1) and 3) or the like. However, it is necessary to increase the growth temperature to such a high temperature as 1000 degree centigrade or more in order to thermally decompose ammonia. This makes it difficult to use Si and a high molecular material as a substrate, thereby limiting the application range of the group III material. Also, a large-scaled detoxifying facility and a high vacuum device are required for ammonia.
The present invention has been made in view of the above-mentioned situation. It is, therefore, an object of the invention to expand the application range by lowering the substrate temperature and to simplify the facility.
In order to achieve the above object, the inventors of the present invention have made a proposal to film deposit a group III nitride such as GaN using an atmospheric pressure plasma (plasma caused by glow discharge or the like in the vicinity of atmospheric pressure), and carried out extensive search and investigation and accomplished the present invention which will be described hereinafter.
The present invention provides a method for growing a group III nitride on a substrate, the method comprising the steps of:
forming a discharge space by applying an electric field between a pair of electrodes under atmosphere in the vicinity of atmospheric pressure; and
bringing a nitrogen, which is introduced into the discharge space, and a metal compound containing a group III metal into contact with the substrate such that a V/III ratio is within a range of from 10 to 100000.
The V/III ratio used herein refers to a ratio between a feed partial pressure of the group V material and a feed partial pressure of the group III material.
A lattice mismatching ratio of the substrate with respect to the group III nitride is preferably as small as possible, for example, 0 to 20%. However, the a-face sapphire is an exception. An epitaxial growth may in some instance occur depending on atom arrangement in actual crystal growth, even if the lattice mismatching ratio is large. Particularly, a substance having a c-axis orientation, such as GaN or ZnO, exhibits this tendency prominently. The c-face sapphire also possesses this tendency.
The lattice mismatching ratio can be defined by the following expression.
lattice mismatching ratio=(afilm−asub)/asub
wherein afilm is a lattice constant in the a-axis direction of the nitrogen thin film, and asub is a lattice constant in the a-axis direction of substrate crystal.
The V/III ratio may be within the above-mentioned range only when the plasma gas contacts the substrate. In case where the V/III ratio varies depending on position (for example, the group III material such as Ga is consumed as it goes toward the downstream of the gas stream and the V/III ratio is increased), the plasma gas may contact the substrate only at such position where the V/III ratio is within the desired range.
The group III metal is preferably selected from Ga, Al and In, and the group III nitride to be obtained is preferably selected from GaN, AlGaN, AlN and InN.
In case where a gallium containing compound is used as the metal compound, a gallium nitride (GaN) is generated as the group III nitride.
The gallium containing compound is preferably selected from the group consisting of trimethylgallium ((CH3)3Ga, hereinafter sometime referred to as “TMG”), triethylgallium ((C2H5)3Ga, hereinafter sometime referred to as “TEG”), quinuclidinegallane (GaH3:N(C7H13), hereinafter sometime referred to as “QUG”) and 1-methylpyrrolidinegallane (GaH3:N(CH3)(C4H4), hereinafter sometime referred to as “1-MPG”). Besides the above ones, trichlorogallium (GaCl3, hereinafter sometime referred to as “TCG”), gallium dimethylamide (Ga2[N(CH3)2]6, hereinafter sometime referred to as “DMEGA”) and the like may be used as the gallium containing compound. It is also accepted that a mixture containing two or more of those gallium containing compounds is used.
In case where an aluminium containing compound is used as the metal compound, aluminium nitride (AlN) is generated as the group III nitride.
The aluminium containing compound is preferably selected from the group consisting of triethylaluminium ((C2H5)Al, hereinafter sometime referred to as “TEA”), 1-methylpyrrolidinealane (AlH3:N(CH3)(C4H4), hereinafter sometime referred to as “1-MPA”), dimethylaluminium hydride ((CH3)2AlH, hereinafter sometime referred to as “DMAH”), aluminium dimethylamide (Al2[N(CH3)2]6, hereinafter sometime referred to as “DMEAA”) and, quinuclidinealane (AlH3:N(C7H13), hereinafter sometime referred to as “QUA”). It is also accepted that a mixture containing two or more of those aluminium containing compounds is used.
The substrate is preferably selected from c-face sapphire, a-face sapphire, ZnO, GaN, SiC and GaAs.
In case where a c-face or a-face sapphire substrate is used as the substrate and a gallium containing compound is used as the metal compound and the V/III ratio is set within a range of from 10000 to 100000, GaN can be epitaxially grown on the substrate. In case where the V/III ratio is set within a range of from about 10 to 1000, a polycrystal of GaN can be obtained.
In case where the substrate is an aluminium containing substrate such as sapphire (Al2O3), aluminium gallium nitride (AlGaN) can be grown as a III group nitride on the substrate by using a gallium containing compound as the metal compound for the group III material without separately adding an aluminium containing compound. GaN can be grown on the AlGaN layer.
In order to prevent organic compounds caused by organic composition of the group III material from mixing into the film, the substrate temperature is preferably brought into a range of from 500 degree centigrade to 700 degree centigrade, more preferably about 650 degree centigrade, although film deposition reaction itself does not require heating of the substrate much. The upper limit of the substrate temperature may be about 700 degree centigrade and such high temperature as 1000 degree centigrade or more is not required. The reaction rate can sufficiently be obtained even by such degree of substrate temperature or so. Since the organic composition contained in the group III material can be removed from the substrate by evaporation, the organic composition can be prevented from mixing into the film.
The lower limit of the substrate temperature is preferably set in such a manner as to correspond to thermal decomposition temperature of the metal compound of the group III material.
In case where the metal compound is TMG, the lower limit of the substrate temperature can be set to about 300 degree centigrade. A polycrystal GaN or an amorphous GaN can be obtained at a temperature near the lower limit temperature. In order to obtain an epitaxial GaN crystal, the substrate temperature is preferably set to about 400 degree centigrade or more, and more preferably within a range of from 450 degree centigrade to 500 degree centigrade or more.
It is known that TEG, QUG and 1-MPG are thermally decomposable even at temperature about 100 degree centigrade lower than the temperature of TMG. Accordingly, in case where the metal compound is TEG, QUG or 1-MPG, the lower limit of the substrate temperature can be set to about 200 degree centigrade in order to obtain a polycrystal GaN or an amorphous GaN and about 300 degree centigrade in order to obtain an epitaxial crystal of GaN. Preferably, in case of TEG, the substrate temperature is set within a range of from 350 degree centigrade to 450 degree centigrade or more. With respect to QUG, it is already confirmed by means of experiment (atmospheric pressure: 2×10E−8 Torr, supply pressure: 5×10E−5) carried out by the inventors that it can be decomposed to Ga at a temperature within a range of from about 200 degree centigrade to 300 degree centigrade. In case of 1-MPG, in consideration of a printing matter (ULVAC TECHNICAL JOURNAL No. 59 2003 P. 25) relating to 1-MPA which is a similar material as 1-MPG, the lower limit of the substrate temperature can be set within a range of from 150 degree centigrade to 250 degree centigrade.
Film deposition processing is carried out preferably under nitrogen atmosphere and more preferably under pure nitrogen (which, however, may contain inevitable impurities) atmosphere. The nitrogen density as the above-mentioned atmosphere is preferably 99.9 vol % or more.
The pressure of atmosphere can properly be set within a range where atmospheric nitrogen plasma or the like can be obtained, and preferably within a range of from 40 kPa to 100 kPa. It is preferable to apply a voltage between a pair of electrodes under atmosphere of nitrogen or the like in the vicinity of the atmospheric pressure.
The electrode construction is preferably a parallel plate electrode. The plasma irradiation system may be a direct system wherein a substrate is directly arranged within a discharge space between a pair of electrodes or may be a remote system wherein a substrate is disposed outside a discharge space and plasma gas generated in the discharge space is sprayed onto the substrate.
The charging voltage may be of magnitude enough to occur stable discharge between the electrodes by nitrogen or the like. For example, in case where the nitrogen atmosphere pressure is about 40 kPa, about Vpp 300 V through 1000 V is suitable.
The frequency is, for example, within a range of from 10 kHz to 30 kHz. The voltage waveform is, for example, a bipolar pulse but it is not limited to this.
The distance between the pair of electrodes is long enough to form an atmosphere pressure plasma discharge between those electrodes within a range of from about 0 point several millimeters to several millimeters.
In the direct system, the thickness of a gap formed between a surface facing the substrate of one of the two electrodes and the substrate is preferably within a range of from 0.1 mm to 5 mm, and more preferably about 0.5 mm.
According to the present invention, a group III nitride such as GaN can be grown on a substrate of sapphire or the like using nitrogen plasma in the vicinity of atmospheric pressure. The V/III ratio can be set sufficiently large and the reaction rate can be increased. The substrate temperature can be set lower compared with the case where the conventional ammonia is used, and the selection range of the substrate material can be expanded, and thus, the application range of the group III nitride semiconductor can be expanded. A large-scaled detoxifying facility and a high vacuum device are no more required, and the facility can be simplified.
a) is a graph showing the result of measurement of the lower limit application voltage where a stable discharge can be obtained, by varying the substrate temperature and process gas in Reference Experiment 3-1, and
The present invention is applied to techniques where a group III nitride such as GaN, AlGaN, AlN or InN is film deposited on a substrate by CVD method.
In this embodiment, GaN is film deposited on a substrate which is composed of a c-face sapphire or a-face sapphire (Al2O3).
N2 is used as a group V material.
TMG, for example is used as a group III material.
A V/III ratio is selected from within a range of from 10 to 100000.
TMG as a group III material is added to N2 as a group V material in a quantity defined by the V/III ratio. As this adding means, a bubbling method using N2 may be employed. Thus obtained process gas which consists of a mixed gas of N2 and TMG is introduced into a plasma space. By doing so, N2 is decomposed and N radical, etc. are obtained. It is estimated that not only N2 but also TMG is decomposed and as a result, such active species as Ga radical, Ga ion, etc. are generated. Plasma gas containing those active species contacts a sapphire substrate, so that a GaN layer can be grown.
This embodiment will be described in more detail.
The reactor 11 includes a chamber 12, a pair of electrodes 13, 14 and a heater 15. A space 11a within the chamber 12 is filled with pure nitrogen gas (N2). Nitrogen pressure within the chamber 12 is set to about 40 kPa.
The pair of electrodes 13, 14 and the heater 15 are housed in the chamber 12.
The pair of electrodes 13, 14 are arranged in vertically opposite relation and thus, they constitute parallel plate electrodes. The upper electrode 13 is connected to a power source 30 and thus, it constitutes a hot electrode. The lower electrode 14 is electrically earthed and thus, it constitutes an earth electrode. A lower surface of the hot electrode 13 and an upper surface of the earth electrode 14 are provided with solid dielectric layers (not shown), respectively. The thickness of each solid dielectric layer is preferably about 1 mm. At least one of the electrodes may be provided with the solid dielectric layer.
The power source 30 outputs a voltage of bipolar pulse waveform, Vpp=500 V, about 30 kHz frequency. Voltage waveform, voltage, frequency, etc. of the power source 30 are not limited to those mentioned above but they may be changed, where necessary.
By voltage fed to the electrode 13 from the power source 30, an electric field is formed between the pair of electrodes 13, 14, and the interelectrode space 11a serves as an electric discharge space.
A substrate 90 composed of c-face sapphire or a-face sapphire, which substrate 90 is an object to be processed, is arranged at a central part on an upper surface of the earth electrode 14. The earth electrode 14 also serves as a base on which the substrate 90 is placed.
A gap formed between the surfaces of the solid dielectric layers of the upper and lower electrodes 13, 14 is, for example, 1 mm, the thickness of the substrate 90 is, for example, 0.5 mm, and a gap formed between the lower surface of the solid dielectric layer of the hot electrode 13 and the upper surface of the substrate 90 is, for example, 0.5 mm. Those dimensions may be changed, where necessary.
A shallow recess for receiving the substrate 90 may be formed in the upper surface of the earth electrode 14.
The heater 15 is arranged underneath the earth electrode 14. The heater 15 may be embedded within the earth electrode 14. The earth electrode 14 is heated by the heater 15 and the substrate 90 is heated through this earth electrode 14. The substrate 90 is preferably heated to about 650 degree centigrade.
The gas feed system 20 for the rector 11 is constituted in the manner mentioned hereinafter.
An N2 feed path 22 extends from a group V material N2 tank 21. The N2 feed path 22 is provided with a main mass flow controller 23 (hereinafter referred to as “main MFC”) and a stop valve V22 which are arranged in order from the upstream.
A carrier feed path 24 is branched from the N2 feed path 22 on the upstream side of the main MFC 23. The carrier feed path 24 is provided with a carrier mass flow controller 25 (hereinafter referred to as “carrier MFC”) and a stop valve V24 which are arranged in order from the upstream. A downstream end of the carrier feed path 24 is inserted inside a thermostatic bath 26 and open there.
TMG as a group III material is stored in the thermostatic bath 26. The thermostatic bath 26 keeps the temperature of TMG to, for example, 0 degree centigrade. Incidentally, the boiling point of TMG, under the atmospheric pressure, is 55.7 degree centigrade and the melting point is −15.9 degree centigrade. TMG within the thermostatic bath 26, at 0 degree centigrade, is in a liquid phase. A downstream end opening of the carrier feed path 24 is located under the liquid surface of TMG within the thermostatic bath 26.
A TMG adding path 27 extends from above the liquid surface of TMG within the thermostatic bath 26. The TMG adding path 27 is provided with a stop valve V27. A downstream end of the TMG adding path 27 is joined with the N2 feed path 22 located at the downstream of the stop valve V22.
A common feed path 29 extends from a joining part 28 between the N2 feed path 22 and TMG adding path 27. The common feed path 29 is provided with an opening control valve V29. A downstream end of the common feed path 29 is inserted within the chamber 12 of the reactor 11 and open in such a manner as to face with one end of the interelectrode space 11a.
An outlet path 41 extends from the other end of the interelectrode space 11a. The outlet path 41 is provided with an opening control valve V41. A rotary pump 40 is connected to a downstream end of the outlet path 41.
A purge path 28 extends from the joining part 28 and is connected to the rotary pump 40.
An exhaust path 43 extends from the chamber 12 of the reactor 11 and is connected to the rotary pump 40 through a turbo molecular pump 44.
The atmospheric pressure nitrogen plasma CVD apparatus 10 thus constructed is used in the following manner.
The inside of the gas feed system 20 is preliminarily purged by opening the purge path 42. After purging operation, the purge path 42 is closed by a stop valve V42.
Air within the chamber 12 of the reactor 11 is exhausted by the turbo molecular pump 44 and N2 is supplied from the N2 tank 21 into the chamber 12, so that the inside of the chamber 12 is filled with pure nitrogen. The nitrogen pressure within the chamber 12 is kept to 40 kPa which is in the vicinity of atmospheric pressure.
Accordingly, it is no more necessary to create a high vacuum and thus, a large-scaled vacuum facility is no more required.
The sapphire substrate 90 is set to a central part of the earth electrode 14. This substrate 90 is heated to 650 degree centigrade by the heater 15.
Then, N2 is allowed to flow to the N2 feed path 22 from the N2 tank 21. Part of N2 is branched to the carrier feed path 24. The flow rate of N2 of the N2 feed path 22 is controlled by the main MFC 23, while the flow rate of N2 of the carrier feed path 24 is controlled by the carrier MFC 25. The flow rate of N2 of the N2 feed path 22 (excluding the part branched to the carrier feed path 24) is, for example, 200 sccm to 500 sccm, while the flow rate of N2 of the carrier feed path 24 is, for example, 0.5 sccm to 1 sccm.
N2 of the carrier feed path 24 is blown into the liquid-phase TMG of the thermostatic bath 26. As a result, TMG is bubbled and evaporated. The evaporating amount of TMG depends on the N2 flow rate in the carrier feed path 24. Because the TMG is cooled down to 0 degree centigrade in the thermostatic bath 26, the evaporation amount without being evaporated by bubbling is almost negligible and thus, the evaporating amount can correctly be controlled.
The evaporated TMG, together with the carrier N2, is joined with N2 of the N2 feed path 22 via the TMG feed path 27. By this, a process gas, which is obtained by adding a predetermined small amount of TMG to N2, is generated. This process gas is introduced into the interelectrode space 11a of the reactor 11 via the common feed path 29.
In parallel with the gas feeding operation, the power source 30 is driven to apply an electric field between the pair of electrodes 13, 14. This builds up an atmospheric glow discharge between the electrodes 13, 14 and the interelectrode space 11a is turned out to be an electric discharge space. N2 in the process gas is decomposed in this discharge space 11a and a nitrogen plasma is generated. It is estimated that TMG is also decomposed.
The plasma gas in the interelectrode space 11a contacts the sapphire substrate 90, thereby to form a GaN layer on the surface of the sapphire substrate 90.
In addition, a thin layer of AlGaN is formed on an interface between the sapphire substrate 90 and the GaN layer. The GaN layer is laminated on this AlGaN layer. It is estimated that Al contained in AlGaN is provided from the sapphire substrate. Therefore, it is not necessary to separately mix the Al source to the process gas in order to form the AlGaN layer.
By stopping the film deposition process before the film glowing component is turned to GaN from AlGaN, it is possible to form only the AlGaN layer. Thereafter, a film having a component different from that of GaN can be formed on the AlGaN layer by way of a separate process.
Since a nitrogen plasma in the vicinity of atmospheric pressure is used, the V/III ratio can be increased at a reaction site and a reaction rate can be gained.
The sapphire substrate 90 is arranged at the central part of the earth electrode 14 where turbulence of electric field hardly occurs and the plasma state is stable and uniform. Thus, the film quality of GaN can be equalized.
Heating of the substrate 90 by the heater 15 makes it possible to further increase the reaction rate and to evaporate organic compounds attributable to the methyl group of TMG and thus, the organic compounds can be prevented from mixing into the film. The processed gas containing those organic compounds is sucked into the outlet path 41 from the interelectrode space 11a and exhausted.
The temperature required for heating the substrate 90 is only about 650 degree centigrade. This substrate temperature is considerably low when compared with 1000 degree centigrade employed in the conventional film deposition method using ammonia and thus, the high-temperature facility can be simplified. In addition, no detoxifying facility is required. This method can also be applied to such substrates as having a small heat-resisting property and thus, the selective range of the substrates can be expanded. The application capability is expanded to such substrates as being composed of high molecular material such as flexible film.
According to the atmospheric pressure nitrogen plasma CVD apparatus 10, the N2 flow rate of the N2 feed path 22 can be controlled by the main MFC 23, while the N2 flow rate of the carrier feed path 24 and thus, the adding amount of TMG can be controlled by the carrier MFC 25. Therefore, the V/III ratio on the sapphire substrate 90 can be controlled by the two MFCs 23, 25, thus making it possible to select the crystal structures of GaN.
That is to say, by setting such that the V/III ratio on the sapphire substrate 90 is within a range of from about 10000 to 100000, GaN can be epitaxially grown on the substrate 90. By setting such that the V/III ratio is within a range of from about 10 to 1000, a polycrystalline GaN can be obtained.
The substrate temperature, at the reaction time, is preferably set to about 400 degree centigrade or more in order to obtain an epitaxial crystal, and preferably to about 300 degree centigrade or more in order to obtain a polycrystal.
Present invention is not limited to the above embodiment.
Instead of the c-face or a-face sapphire substrate, ZnO, SiC, GaAs or the like may be used as the substrate. A suitable substrate has a small mismatching of lattice to a film to be obtained. The mismatching percentage of lattice is preferably small, for example, about 0 to 20%. Incidentally, the lattice mismatching percentage between the c-face sapphire and GaN is 16%.
Instead of TMG, TEG, QUG or 1-MPG may be used as a Ga material. Moreover, TCG, DMEGA or the like may be used as a Ga material.
In case where TEG, QUG or 1-MPG is used as a Ga material, the lower limit of the substrate temperature range can be made lower by about 100 degree centigrade than the temperature in case where TMG is used. In case where an epitaxial crystal is a substance to be obtained, the substrate temperature can be brought to about 300 degree centigrade or more, and in case where a polycrystal is a substance to be obtained, the substrate temperature can be brought to about 200 degree centigrade or more.
In case where TMG is consumed during the time when the process gas (N2+TMG) flows from one end of the discharge space 11a to the central part where the sapphire substrate 90 is located, the initial V/III ratio may be controlled such that a desired V/III ratio can be obtained just on the substrate 90, by taking into consideration the above-mentioned consumed amount.
It may be arranged such that a desired V/III ratio can be obtained just on the substrate 90 by adjusting the length from the time when the process gas (N2+TMG) is introduced to the discharge space 11a to the time when the process gas arrives on the substrate 90.
It may also be arranged such that the V/III ratio is substantially constant at any position on the substrate 90 by controlling the gas flow in the discharge space 11a.
A mask may be applied onto the substrate 90 at any other position than the position where the V/III ratio is within a desired range.
Instead of preliminarily mixing N2 with TMG and introducing the mixture between the electrodes, N2 and TMG may be introduced between the electrodes via separate routes, respectively.
The so-called remote system, in which a substrate is arranged outside a plasma space, may be employed as a plasma radiation structure. In that case, an arrangement may be made such that only N2 is passed between the electrodes and sprayed onto the substrate and TMG is separately sprayed onto the substrate.
One embodiment will now be described. It should be noted, however, that the present invention is not limited to this embodiment.
A film deposition processing was carried out under the following conditions, using the apparatus 10 of
N2 flow rate of N2 feed path 22: 300 sccm
N2 flow rate of carrier feed path 24: 1 sccm
substrate: c-face sapphire
substrate temperature: 650 degree centigrade
processing pressure: 40 kPa
voltage mode: bipolar pulse wave
charging voltage: Vpp=500 V
frequency: 30 kHz
growth time: 30 min
A sample obtained by the processing of this Embodiment 1 was ω-2θ scan analyzed according to the X-ray diffraction method. As a result, diffraction from the 0002 plane of GaN was confirmed as shown in
In the apparatus 10 of
An insertion Figure encircled with a frame of broken lines in
Optical emission from the interelectrode space 11a was analyzed under the following conditions, using a separate photonic spectral analyzer.
processing pressure (nitrogen atmospheric pressure): 40 kPa plusminus 2 kPa
feeding gas: only nitrogen, 400 sccm
substrate temperature: room temperature
The result is shown in
As Embodiment 2, film deposition processing was carried out under the following conditions, using an apparatus having the same construction as that of
N2 flow rate of N2 feed path 22: 400 sccm
N2 flow rate of carrier feed path 24: 0.5 ccm
substrate: c-face sapphire
substrate temperature: 650 degree centigrade
processing pressure: 40 kPa plusminus 2 kPa
frequency: 30 kHz
growth time: 30 min
Also, a photoluminescence spectrum of a sample obtained in Embodiment 2 was measured. The measurement conditions are as follows.
excitation light source: HeCd laser (325 nm)
filter: 370 nm
laser power: 3 mW
measurement wavelength: 350 nm through 700 nm
measurement temperature: 10 points within a range of from 5 K to 300 K
An optical emission at the band edge of GaN was confirmed as shown in
Moreover, a layer estimated as AlGaN was confirmed at the interface between the c-face sapphire substrate and the GaN layer.
The spectrum on the lower side of
A cathode luminescent measurement was carried out while the measurement point is fixed to a point where the film thickness is small and the measurement temperature was varied. As a result, it was confirmed that as shown in
The cathode luminescence is larger in intrusion depth than the photoluminescence and is suitable for analyzing the AlGaN layer at the depth. On the other hand, the photoluminescence is suitable for analyzing only the GaN layer which is a surface layer and not affected by the AlGaN layer.
The cross section of the above sample was observed by a transmission electron microscope. As a result, a black interface layer, which is supposed to be AlGaN, was observed between the sapphire substrate and the GaN film as shown in
An x-ray diffraction image (
a) is a diffraction picture of the photographing point a,
It was confirmed that at the photographing point c on the side near the interface within the sapphire substrate, an image is slightly more blurred than that at the photographing point d which is deeper than the photographing point c, and the crystallinity is lowered. This is supposedly occurred because the photographing point c was exposed to plasma at the time of forming the GaN film.
For reference, only nitrogen was introduced into the interelectrode space 11a and nitrogen plasma was generated. This nitrogen plasma was irradiated directly to a central part of the sapphire substrate. Thereafter, a ω rocking curve of the sapphire substrate was measured.
The result is shown in
It can be contemplated that at the sacrifice of the lowering of crystallinity, Al contained in the sapphire substrate contributes to the generation of AlGaN layer on the interface.
In Embodiment 3, the substrate temperature was set to 400 degree centigrade which was lower than those (650 degree centigrade) of the Embodiments 1 and 2 and film deposition processing was carried out. Other processing conditions are as follows.
processing pressure: 40 kPa plusminus 2 kPa
N2 flow rate of N2 feed path 22: 400 sccm
N2 flow rate of carrier feed path 24: 0.5 sccm
growth time: 30 min
substrate: c-face sapphire
By this, it was confirmed that GaN can be epitaxially grown also at the substrate temperature of 400 degree centigrade. It was confirmed that in respect of crystallinity, the substrate temperature is preferably set to about 650 degree centigrade.
Also, a film deposition processing was carried out at a substrate temperature of 350 degree centigrade, with all other conditions remained same as in the case of 400 degree centigrade mentioned above. The substrate after processing was ω-2θ scanned. As a result, a diffraction peak appeared, as shown in
By this, it was confirmed that film deposition of GaN can be obtained even if the substrate temperature is about 350 degree centigrade.
For reference, a relation among substrate temperature, application voltage and electric current was measured. The measurement conditions are as follows:
processing pressure (nitrogen atmospheric pressure): 40 kPa plusminus 2 kPa
N2 flow rate of N2 feed path 22: 400 sccm
substrate: c-face sapphire
Two cases of N2 flow rates of the carrier feed path 24 were prepared, 0.5 sccm and 0 sccm. In case where the N2 flow rate is 0.5 sccm, the process gas introduced into the plasma space 11a is a mixing gas (N2+TMG) of nitrogen and TMG. In case where the N2 flow rate is 0 sccm, the process gas is only nitrogen.
A lower limit voltage where discharge between the electrodes 13, 14 is in a stable condition and a feed electric current which is fed to the electrode 13 at the time of voltage application were measured for each substrate temperature.
In case where the process gas is only nitrogen, as shown in
In case where the process gas is a mixing gas (N2+TMG) of nitrogen and TMG, as shown in
Optical emission from the plasma space 11a at the time of discharge was analyzed. The result is shown in
In case where the process gas is a mixing gas (N2+TMG) of nitrogen and TMG and the substrate temperature is 650 degree centigrade, generation of Ga radical was confirmed at the wavelengths 403 nm and 417 nm.
On the other hand, in case where the process gas is a mixing gas (N2+TMG) of nitrogen and TMG and the substrate temperature is a room temperature (@ R.T.), the peaks of the wavelengths 403 nm and 417 nm indicating the generation of Ga radical were hardly confirmed.
Instead, the peaks of the wavelengths 415 nm and 419 nm corresponding to nitrogen carbide (CN) appeared. It is apparent that the nitrogen carbonate is generated by decomposition of TMG, in consideration of the fact that in case where the process gas is only nitrogen, the peaks of the wavelengths 415 nm and 419 nm did not appear.
From the foregoing, it became clear that TMG is decomposed by plasma even under the room temperature but that in order to further radicalizing Ga, a certain degree of temperature is required.
It is contemplated that if the temperature is low, energy of plasma is consumed mostly for the decomposition of TMG, and no energy enough to radicalize Ga is remained. In contrast, if the substrate temperature is increased to a certain degree, decomposition of TMG occurs by that heat and thus, plasma energy can sufficiently be allotted for use of radicalization of Ga.
In case where the process gas is N2+TMG in
TEG, QUG, 1-MPG and the like are thermally decomposable even at a temperature lower by about 100 degree centigrade than TMG. Therefore, in case where they are used as group III material, the lower limit of the substrate temperature can be set in the vicinity of 200 degree centigrade.
The application of voltage between the electrodes 13, 14 was stopped and the process gas (mixing gas of nitrogen and TMG), without being plasmatized, was sprayed onto the substrate. The surface of the substrate was observed. As shown in
As a result, it became clear that if the substrate temperature is brought to a high temperature, TMG can be thermally decomposed but this is not sufficient in order to generate GaN and that in order to generate GaN, activation of Ga such as nitrogen plasma and nitriding means are required.
The present invention can be applied to techniques for manufacturing semiconductor elements such as, for example, optical emission elements and high frequency elements.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-228727 | Aug 2005 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2006/315404 | 8/3/2006 | WO | 00 | 7/24/2008 |
