JP5705656B2 - Method for forming gallium nitride columnar structure and apparatus for forming gallium nitride columnar structure - Google Patents

Description

  The present invention relates to a method for forming a gallium nitride columnar structure using a reactive sputtering method, and an apparatus for forming a gallium nitride columnar structure using the method.

  A gallium nitride based semiconductor, which is one of group III nitride semiconductors, has a direct transition type band gap of energy corresponding to ultraviolet light to visible light. Therefore, as a material for forming optical devices such as light emitting diodes and laser diodes. Widely used. Further, gallium nitride has a high saturation electron velocity in addition to high resistance to voltage breakdown as compared with other semiconductor materials such as silicon and gallium arsenide.

  Therefore, in recent years, attempts have been actively made to apply such nanoscale structures formed of gallium nitride, so-called gallium nitride columnar structures, to electronic devices such as light emitters, field emission elements, and chemical sensors of the above optical devices. It has been broken.

  For forming the gallium nitride columnar structure, for example, as described in Patent Document 1, a vapor-liquid-solid growth method (VLS method) is used. In the VLS method, gaseous raw materials such as trimethyl gallium and ammonia are supplied to a liquid catalyst metal formed on the surface of a solid underlayer such as nickel and platinum. When the raw material dissolved in the catalyst metal becomes supersaturated, gallium nitride aggregates between the metal catalyst and the underlayer, so that the gallium nitride columnar structure is perpendicular to the surface of the underlayer. Is formed.

JP 2010-6670 A

  By the way, in the method of forming a gallium nitride columnar structure by the VLS method, the gallium nitride columnar structure grows in a liquid phase of a metal catalyst. For this reason, the formed gallium nitride columnar structure cannot avoid the mixing of the catalyst metal. When such a gallium nitride columnar structure is incorporated in an electronic device as described above, the catalytic metal in gallium nitride often affects the performance of the electronic device.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for forming a gallium nitride columnar structure capable of suppressing the mixing of a catalytic metal, and an apparatus for forming a gallium nitride columnar structure using the method. It is to provide.

Hereinafter, means for solving the above-described problems and the effects thereof will be described.
The invention according to claim 1 is a method of forming a gallium nitride columnar structure for forming a gallium nitride columnar structure, wherein a bias power is supplied to a gallium target disposed in the vacuum chamber, and the gallium nitride columnar structure is formed in the vacuum chamber. Sputtering the gallium target with the supplied rare gas, the nitrogen-containing gas supplied into the vacuum chamber and activated by plasma, and the sputtered gallium are heated in the vacuum chamber. Forming a gallium nitride columnar structure by reacting on a single crystal substrate, wherein the nitrogen-containing gas is a ratio of the nitrogen-containing gas to the total flow rate of the rare gas and the nitrogen-containing gas. Among the nitrogen concentration, the growth rate of the gallium nitride is supplied into the vacuum chamber so as to be in a nitrogen concentration range determined by the nitrogen supply, The bias power causes gallium to reach the surface of the single crystal substrate in addition to the gallium to be nitrided, and the temperature of the single crystal substrate is such that all of the gallium other than the gallium to be nitrided der those growing gallium nitride at a temperature higher than the temperature to evaporate from the surface is, the a nitrogen-containing gas is nitrogen gas, said nitrogen when the rare gas be argon gas, for forming the gallium nitride The concentration is in a range where the growth rate is 61% or more and 100% or less with respect to the maximum value of the growth rate of the gallium nitride, and the temperature of the substrate when forming the gallium nitride columnar structure is the substrate temperature T (° C. ), When the high frequency power supplied to the gallium target is the bias power P (W / cm ² ), the substrate temperature T and the bias power P However, it is summarized that 600 ≦ T ≦ 1200 and 0 <P ≦ 4.63 are satisfied .

The invention described in claim 3 is a vacuum chamber that houses a single crystal substrate and has a heating unit that heats the single crystal substrate, and an argon gas as a rare gas and nitrogen as a nitrogen- containing gas in the vacuum chamber. and a gas supply unit for supplying a gas, a gallium target disposed in the vacuum chamber, a power supply unit for supplying a bias electric power is high frequency power to the gallium target to sputter the gallium target in the rare gas And a control unit that controls the operation of the heating unit, the gas supply unit, and the power supply unit, wherein the control unit includes the rare gas and the nitrogen Of the nitrogen concentration, which is the ratio of the nitrogen-containing gas to the total flow rate with the gas, the growth rate of gallium nitride is in the range of the nitrogen concentration that is controlled by the nitrogen supply. The driving of the gas supply unit is controlled, and the bias power is output to the power supply unit so that gallium reaches the surface of the single crystal substrate in addition to the gallium to be nitrided, and the nitride is nitrided The substrate is heated so that gallium nitride is grown at a temperature higher than the temperature at which all gallium other than gallium is evaporated from the surface of the substrate, and the nitrogen concentration is a maximum value of the growth rate of the gallium nitride. The driving of the gas supply unit is controlled so that the growth rate is 61% or more and 100% or less, and the temperature of the substrate when the gallium nitride columnar structure is formed is the substrate temperature T ( ° C), when the high frequency power supplied to the gallium target is the bias power P (W / cm ² ), the substrate temperature T and the bias power P However, the gist is to control the heating unit and the power supply unit so as to satisfy 600 ≦ T ≦ 1200 and 0 <P ≦ 4.63 .

  As a result of intensive research by the present inventors, in the gallium nitride process region, if the gallium reaches the surface of the single crystal substrate in addition to the gallium nitride, the gallium that has reached the surface of the single crystal substrate It has been found that due to the fluidity on the surface, the gallium itself diffuses two-dimensionally and the grain growth of gallium nitride is suppressed on the substrate surface. On the other hand, by growing gallium nitride at a temperature higher than the temperature at which all of the gallium other than gallium nitride is evaporated from the surface of the single crystal substrate in such a region, It was found that the reaction proceeded.

  First, the incident flux, which is the flux of gallium reaching the single crystal substrate, increases as the bias power increases. In addition, the emission flux, which is the flux of gallium that evaporates among the gallium that reaches the single crystal substrate, increases as the temperature of the single crystal substrate increases. The difference between the incident flow velocity and the discharge flow velocity is a parameter that gives the degree of supersaturation that governs the growth direction of the gallium nitride crystal.

  Specifically, the growth of gallium nitride at an appropriate degree of supersaturation includes vertical growth, which is growth along the c-axis direction, which is the vertical crystal axis in a single crystal substrate, and a direction parallel to the surface of the single crystal substrate. The growth rate ratio with the lateral growth, which is the growth of Then, layered gallium nitride along the surface of the single crystal substrate is formed. On the other hand, under conditions set so that the degree of supersaturation is sufficiently small, the growth rate in the vertical direction is increased relative to the growth rate in the horizontal direction. It progresses. As a result, gallium nitride grows in the normal direction of the substrate surface away from the substrate surface for each gallium nitride crystal grain on the substrate surface, so that a gallium nitride columnar structure is self-formed. In other words, by controlling the bias power that determines the incident flux of gallium reaching the single crystal substrate and the temperature of the single crystal substrate that determines the emission flux of gallium that evaporates among the gallium reaching the single crystal substrate, respectively. A gallium nitride columnar structure can be formed.

Accordingly, in the method for forming a gallium nitride columnar structure according to claim 1 and the apparatus for forming a gallium nitride columnar structure according to claim 3 , a bias power having a correlation with the amount of gallium supplied from the gallium target to the substrate surface. However, gallium other than gallium to be nitrided is allowed to reach the surface of the substrate. In addition, the temperature of the single crystal substrate having a correlation with the amount of gallium evaporated from the single crystal substrate is higher than the temperature at which all of the gallium other than the gallium to be nitrided is evaporated from the surface of the substrate. As a result, a gallium nitride columnar structure can be formed only by supplying gallium to the substrate surface by sputtering a gallium target and supplying a nitriding source to the substrate surface. In addition, since the gallium nitride columnar structure is formed without using a liquid catalyst metal as in the VLS method, mixing of the catalyst metal into the formed gallium nitride columnar structure is naturally suppressed.

Further, a method of forming a gallium nitride columnar structure according to the claim 1, and according to the forming apparatus of gallium nitride columnar structure according to the claim 3, the nitrogen concentration relative to the maximum value in the growth rate of gallium nitride The growth rate is 61% or more and 100% or less. Then, the temperature of the single crystal substrate, with less than or equal to 1200 ° C. 600 ° C. or higher, a bias power P supplied per unit area of the gallium target, becomes larger 4.63W / cm ² or less than 0 W / cm ^2. Therefore, it is easy to grow gallium nitride as crystal grains and thus form a gallium nitride columnar structure in an environment where the amount of gallium to be evaporated and the amount of gallium supplied onto the single crystal substrate is sufficiently high. It will be a thing.

The invention according to claim 2 is the method for forming a gallium nitride columnar structure according to claim 1 , wherein the nitrogen concentration is 91% or more and 100% or less with respect to the maximum value in the growth rate of the gallium nitride. The bias power is a bias power having a frequency of 13.56 MHz, and the substrate temperature T and the bias power P are P <0.0088T-6.60, P ≧ 0.0116T−. The gist is to satisfy 11.37.

According to a fourth aspect of the present invention, in the apparatus for forming a gallium nitride columnar structure according to the third aspect , the bias power is a bias power having a frequency of 13.56 MHz, and the control unit is configured so that the nitrogen concentration is the nitrogen concentration. The driving of the gas supply unit is controlled so that the growth rate is 91% or more and 100% or less with respect to the maximum value of the growth rate of gallium nitride, and the substrate temperature T and the bias power P are: The gist is to control the heating unit and the power supply unit so as to satisfy P <0.0088T-6.60 and P ≧ 0.0116T-11.37.

In the method for forming a gallium nitride columnar structure according to claim 2 and the apparatus for forming a gallium nitride columnar structure according to claim 4 , the nitrogen concentration is 91% or more to the maximum value in the growth rate of gallium nitride. The growth rate should be within a range that is not more than%. When the bias power P is a bias power having a frequency of 13.56 MHz, the substrate temperature T and the bias power P are P <0.0088T-6.60, P ≧ 0.0116T-11.37. The gallium nitride columnar structure is formed under conditions that satisfy the above conditions. Therefore, it is easy to form gallium nitride crystal grains and to continuously grow such gallium nitride crystal grains away from the substrate surface in the normal direction of the substrate surface, thereby forming a gallium nitride columnar structure. Become.

The figure which shows schematic structure of one Embodiment which embodied the formation apparatus of the gallium nitride columnar structure of this invention as a sputtering device. The timing chart which shows the supply aspect of various gases in one Embodiment of the formation method of the gallium nitride columnar structure of this invention, and the change aspect of a substrate temperature. The graph which shows the relationship between the ratio (%) of the nitrogen gas to the total flow rate of the gas supplied to a vacuum chamber, and the growth rate of gallium nitride. (A)-(d) SEM photograph which imaged the surface of gallium nitride formed on a substrate. The figure which shows the area | region of the growth state of the gallium nitride divided by the substrate temperature and bias electric power. (A)-(f) SEM photograph which imaged the surface of gallium nitride formed on a substrate. (A) (b) The figure which shows the formation method of the gallium nitride columnar structure in the embodiment in order of the process. The SEM photograph which imaged the gallium nitride columnar structure formed on the base layer laminated | stacked on the board | substrate.

  Hereinafter, an embodiment of a method for forming a gallium nitride columnar structure and an apparatus for forming a gallium nitride columnar structure according to the present invention will be described with reference to FIGS. First, an overall configuration of a sputtering apparatus as a gallium nitride columnar structure forming apparatus will be described with reference to FIG.

  As shown in FIG. 1, a cathode housing 12 is disposed in a cylindrical vacuum chamber 11 provided in the sputtering apparatus 10, and a magnet 12 a for generating a magnetron magnetic field is formed inside the cathode housing 12. Is housed. In the cathode housing 12, a backing plate 13 is disposed above the magnet 12 a, and a target 14 connected to the backing plate 13 is disposed above the backing plate 13. The target 14 includes a container 14b disposed on the upper side of the backing plate 13 and gallium 14a accommodated in the container 14b. The gallium 14a accommodated in the container 14b is connected to a high-frequency power source 15 as a power supply unit via a backing plate 13, and is supplied with bias power output from the high-frequency power source 15. The target 14 is arranged so that the magnetron magnetic field generated by the magnet 12a is formed on the surface of the gallium 14a.

A substrate stage 16 that holds a substrate S on which a gallium nitride (GaN) columnar structure is to be formed is provided immediately above the target 14 in the vacuum chamber 11. The substrate S to be formed with a gallium nitride columnar structure is a substrate in which an aluminum nitride (AlN) layer as a single crystal substrate is formed on a single crystal support substrate such as sapphire (Al ₂ O ₃ ). . Further, on the inner surface of the vacuum chamber 11, an adhesion preventing plate 17 for preventing adhesion of gallium nitride is disposed so as to expose the surface of the target 14 facing the substrate S and the substrate stage 16.

  A cylindrical heater chamber 18 is provided at a position facing the substrate stage 16 on the outer surface of the vacuum chamber 11, and a heater unit 19 as a heating unit is installed inside the heater chamber 18. Yes. The heater unit 19 is, for example, a halogen lamp heater, the tip of the heater unit 19 faces the substrate S held by the substrate stage 16, and heats the substrate S in the range of 600 ° C. to 1200 ° C.

  Connected to the vacuum chamber 11 is a mass flow controller MFC1 as a rare gas supply unit for supplying a rare gas such as argon for sputtering the gallium 14a of the target 14. The vacuum chamber 11 is connected to a mass flow controller MFC2 as a nitrogen gas supply unit that supplies nitrogen gas as a nitrogen-containing gas for nitriding the sputtered gallium. Further, the vacuum chamber 11 is connected to an exhaust unit 20 including a vacuum pump for reducing the pressure in the vacuum chamber 11 and a pressure adjusting valve for adjusting the pressure in the vacuum chamber 11 to a predetermined pressure. Has been.

  The sputtering apparatus 10 is equipped with a control unit 30 that is connected to the high-frequency power supply 15, the heater unit 19, and the mass flow controllers MFC 1 and MFC 2 and controls these operations. The control unit 30 stores the bias power per unit area to be supplied to the target 14 as one of the forming conditions, and outputs a drive signal corresponding to the bias power to the high frequency power supply 15. In addition, the control unit 30 stores the temperature of the substrate S during the process as one of the formation conditions, and outputs a drive signal for increasing the temperature of the substrate S to the temperature during the process to the heater unit 19. Further, the control unit 30 stores the flow rate of argon gas during the process as one of the forming conditions, and outputs a drive signal for supplying argon gas at the flow rate to the mass flow controller MFC1. Further, the control unit 30 stores the flow rate of the nitrogen gas at the time of the process as one of the formation conditions, and outputs a drive signal for supplying the nitrogen gas at the flow rate to the mass flow controller MFC2. Then, the control unit 30 sets the flow rate of nitrogen gas in the total flow rate of argon gas and nitrogen gas supplied into the vacuum chamber 11 to a predetermined ratio.

  Of the operation of the sputtering apparatus 10 having the above-described configuration, the operation performed by the sputtering apparatus 10 by the method of forming a gallium nitride columnar structure will be described below with reference to FIGS.

  When the gallium nitride columnar structure is formed by the sputtering apparatus 10, first, the inside of the vacuum chamber 11 is reduced to a predetermined pressure by the exhaust unit 20. Then, when the substrate S is carried into the vacuum chamber 11 from a carry-in / out port (not shown), the substrate S is held by the substrate stage 16. Next, as shown in FIG. 2, when the substrate S is carried into the vacuum chamber 11, the temperature of the substrate S is increased by the heater unit 19 in an argon gas atmosphere (timing t1 to timing t2). Then, when the temperature of the substrate S is raised from room temperature to a predetermined temperature, for example, argon gas and nitrogen gas are supplied so that the flow rate of nitrogen gas in the total flow rate of these gases becomes a predetermined ratio. . Next, supply of bias power at a predetermined bias power P from the high-frequency power supply 15 to the target 14 is started (timing t2). Thereby, the gallium 14a of the target 14 is sputtered by the argon gas continuously supplied from the timing t1. At this time, since the supply of the nitrogen gas is started, the nitrogen gas is activated by the plasma, and the sputtered gallium particles are nitrided on the substrate S by the activated nitrogen gas. Then, a gallium nitride columnar structure is formed on the substrate S (timing t2 to timing t3).

  At this time, nitrogen gas is supplied into the vacuum chamber 11 at a predetermined flow rate Fn1, and argon gas is supplied into the vacuum chamber 11 at a predetermined flow rate Fa2. As a result, the flow rate Fn1 of nitrogen gas is set to a predetermined ratio with respect to the total flow rate Fn1 + Fa2 of gas supplied into the vacuum chamber 11. At this time, by making the argon flow rate Fa1 supplied from the timing t1 to the timing t2 substantially the same as the total flow rate Fn1 + Fa2 from the timing t2, fluctuations in pressure in the vacuum chamber 11 can be suppressed. As a result, even if a nitrogen gas different from the argon gas is added, the plasma state becomes stable at the start of subsequent sputtering regardless of the change in the gas type.

  Then, when a predetermined time elapses from the timing t2, the supply of the argon gas and the supply of the bias power are stopped, thereby terminating the sputtering of the target 14 (timing t3). The control unit 30 stores the reactive sputtering process time corresponding to the length of the gallium nitride columnar structure as one of the forming conditions, and the period from the timing t2 to the timing t3 is set to the process time. Is done.

  Further, at the timing t3, the heating of the substrate S by the heater unit 19 is also finished, whereby the temperature of the substrate S is cooled to, for example, room temperature (timing t4). Nitrogen gas continues to be supplied into the vacuum chamber 11 at a flow rate Fn2 larger than the flow rate Fn1 from timing t3 to timing t4, which is the cooling period of the substrate S. Thereby, it is possible to suppress generation of surface defects due to desorption of nitrogen having a high vapor pressure from the gallium nitride film at the growth temperature of gallium nitride.

  Incidentally, for example, at the time of forming the gallium nitride columnar structure (timing t3 to timing t4), the temperature of the substrate S is set to 1000 ° C., and the nitrogen concentration that is the ratio of the flow rate Fn1 of nitrogen gas to the total flow rate Fn1 + Fa2 is set to 30%. Sometimes, the gallium nitride columnar structure is formed under the following conditions.

  That is, the temperature of the substrate S is raised from room temperature to 1000 ° C. in 20 minutes from timing t1 to timing t2. At this time, the flow rate Fa1 of the argon gas is set to 55 sccm. Then, a bias power of 150 W is supplied to the target 14 having a diameter of 120 mm while maintaining the temperature of the substrate S at 1000 ° C. for 40 minutes from the timing t2 to the timing t3, and the gallium nitride columnar structure is formed on the substrate S. Form. At this time, the flow rate Fa2 of argon gas is set to 38.5 sccm, and the flow rate Fn1 of nitrogen gas is set to 16.5 sccm, whereby the nitrogen concentration is set to 30%. Next, the temperature of the substrate S is cooled from 1000 ° C. to room temperature in 20 minutes from timing t3 to timing t4. At this time, the flow rate Fn2 of nitrogen gas is set to 100 sccm.

  In the method for forming a gallium nitride columnar structure of the present embodiment, the flow rate of nitrogen gas Fn1 occupying the total flow rate Fn1 + Fa2 of the gas supplied into the vacuum chamber 11 when forming the gallium nitride columnar structure from timing t2 to timing t3. The above-mentioned nitrogen concentration, the temperature of the substrate S, and the bias power are controlled within a predetermined range. Thereby, a gallium nitride columnar structure extending in the normal direction of the surface can be formed on the surface of the substrate S.

Details of the nitrogen concentration, substrate temperature, and bias power ranges will be described below with reference to FIGS. FIG. 3 is a graph showing the dependence of the growth rate of gallium nitride formed using the above-described gallium nitride columnar structure forming apparatus on the nitrogen concentration (N ₂ %). In addition, the curve C1 shown with a dashed-dotted line, the curve C2 shown with a continuous line, and the curve C3 shown with a dashed-two dotted line show the growth rate when only the flow volume of nitrogen gas is changed among each formation conditions. ing. Under the conditions of the curves C1, C2, and C3, the substrate temperatures T during the process are equal to each other, and the bias power supplied to the target 14 is set to increase in the order of the curves C1, C2, and C3. ing.

As shown in FIG. 3, the growth rate of gallium nitride varies with the nitrogen concentration (N ₂ %). As is apparent from the three curves C1, C2, and C3, the growth rate has a maximum value with respect to the nitrogen concentration regardless of the conditions of the substrate temperature and the bias power.

  In each of the three curves C1, C2, and C3, the lower concentration side than the nitrogen concentration that gives the maximum value, that is, the nitrogen concentration ranges Na, Nb, and Nc are regions in which the growth rate of gallium nitride increases as the nitrogen concentration increases. It is. Because of such dependence on the nitrogen concentration, most of the low concentration region is a region in which the amount of nitrogen supplied to the surface of the substrate S is smaller than the amount of gallium reaching the surface of the substrate S. Thus, it can be said that the amount of gallium nitrided is limited by the amount of nitrogen. Therefore, in such a nitrogen concentration range Na, Nb, Nc, gallium before being nitrided can be made to flow almost on the surface of the substrate S.

  On the other hand, in each of the three curves C1, C2, and C3, the higher concentration side than the nitrogen concentration that gives the maximum values Ma, Mb, and Mc is a region where the growth rate of gallium nitride decreases as the nitrogen concentration increases. is there. Due to such dependence on the nitrogen concentration, such a high concentration region is a region where the amount of nitrogen supplied to the surface of the substrate S is excessive with respect to the amount of gallium reaching the substrate S. Thus, it can be said that the amount of gallium nitrided is limited by the amount of gallium. Therefore, in such a region, even if the sputtered gallium reaches the surface of the substrate S, the amount of nitrogen present on the surface of the substrate S increases, so the chance of reaction between such gallium and nitrogen increases. However, the diffusion of gallium on the surface of the substrate S is suppressed.

  On the other hand, even when film formation is performed using the above-described nitrogen concentration ranges Na, Nb, and Nc, gallium nitrides having different structures are formed as shown in FIGS. These structures can be determined according to the gallium flux that is the flux of gallium that reaches the surface of the substrate S and the nitrogen flux that is the flux of activated nitrogen that reaches the substrate. This was confirmed by our experiments.

  More specifically, in the nitrogen concentration ranges Na, Nb, and Nc, the range where the gallium flux is excessive with respect to the nitrogen flux, that is, the range where the growth rate is low is as shown in FIG. In addition, it was observed that the gallium simple substance was deposited on the surface of the substrate S in a hemispherical shape. Moreover, when the extent to which the gallium flux becomes excessive with respect to the nitrogen flux in the nitrogen concentration ranges Na, Nb, and Nc becomes smaller than the range shown in FIG. 4A, FIG. 4B shows. Thus, gallium nitride was observed to grow in a layered structure.

  On the other hand, in the above nitrogen concentration range Na, Nb, Nc, in a range where the gallium flux is excessively smaller than the nitrogen flux, that is, in the vicinity of the maximum growth rate, FIG. As shown, the diffusion of gallium is suppressed, and gallium nitride grows three-dimensionally so that the thickness of the stacked layer is greater than that of the layered structure, and the grown surface is formed to be uneven. Was recognized.

  On the other hand, in the nitrogen concentration ranges Na, Nb, and Nc, the extent that the gallium flow rate is excessive with respect to the nitrogen flow rate is smaller than the range shown in FIG. 4A and more than the range shown in FIG. In the temperature range higher than the gallium other than the gallium nitride to evaporate, the gallium nitride is separated from the surface of the substrate S as shown in FIG. In addition, it was recognized that a columnar structure extending in the normal direction of the surface was formed.

  In order to form gallium nitride having a columnar structure as shown in FIG. 4D, the gallium flux reaching the surface of the substrate S and the gallium flux evaporating from the substrate surface are controlled. This indicates that it is necessary to newly define the parameter range in addition to the nitrogen concentration.

  Incidentally, as shown in FIG. 4C, the gallium nitride is thicker than the layered structure as shown in FIG. 4C even on the higher concentration side than the maximum values Ma, Mb, Mc of the growth rate. It was observed that the three-dimensional growth occurred and the grown surface was formed in an uneven shape.

  Here, the larger the bias power supplied to the target 14, the easier the target 14 is sputtered. Therefore, the amount of gallium that reaches the surface of the substrate S has a positive correlation with the magnitude of the bias power supplied to the target 14, and therefore reaches the surface of the substrate S by controlling this bias power. It is possible to independently control the gallium flux.

  On the other hand, even if gallium once reaches the surface of the substrate S, it may evaporate by receiving heat from the substrate S when in a single state. The amount of gallium evaporated from the surface of the substrate S in this way has a positive correlation with the temperature of the substrate S. Therefore, by controlling the temperature of the substrate S, the gallium evaporated from the surface of the substrate S. It is possible to independently control the flux of the.

  In the nitrogen concentration range Na, Nb, Nc, gallium itself diffuses two-dimensionally due to the fluidity of gallium. In addition to this, among the gallium that has reached the surface of the substrate S, the amount of gallium excluding the amount that reacts with nitrogen to become gallium nitride is equal to the amount of gallium evaporated from the surface of the substrate S. That is, if the supersaturation degree of gallium is the optimum condition on the growth surface, the excess gallium diffused as described above evaporates from the surface of the substrate S. Thereby, a gallium nitride film having a layered structure as shown in FIG. 4B is formed.

  On the other hand, the amount of gallium evaporated from the surface of the substrate S is smaller than the amount of gallium except for the amount of gallium nitride, that is, the degree of supersaturation of gallium on the growth surface is larger than the optimum condition. If the conditions are satisfied, excess gallium is deposited as shown in FIG.

  On the other hand, the amount of gallium evaporated from the surface of the substrate S is larger than the amount of gallium nitride except for the amount of gallium nitride, that is, the degree of supersaturation of gallium on the growth surface is higher than the optimum condition. If the conditions are small, a columnar structure of gallium nitride is formed as shown in FIG.

That is, if the formation conditions satisfy the following (Condition 1) to (Condition 3), the gallium evaporated after reaching the substrate S contributes to the formation of the columnar structure. As a result, the gallium nitride as shown in FIG. The columnar structure can be formed.
(Condition 1) The growth rate of gallium nitride is in the range of nitrogen concentration limited by nitrogen supply.
(Condition 2) The bias power is within a range that allows gallium to reach the surface of the substrate S in addition to gallium to be nitrided.
(Condition 3) The temperature of the substrate S is in a range higher than the temperature at which all of the gallium other than the gallium to be nitrided is evaporated from the surface of the substrate S.

  An example of formation conditions that satisfy the above (Condition 1) to (Condition 3) will be described below with reference to FIGS. 5 and 6 show that the nitrogen concentration is within the range in which the growth rate of gallium nitride is determined by the nitrogen supply, and is 91% or more with respect to the maximum value in the growth rate, using the above-described gallium nitride columnar structure forming apparatus. It is the graph and SEM photograph which show the structure of gallium nitride when it is set as the range used as the growth rate of 100% or less.

(Region 1: Precipitation region: Conditions A, B, C, D)
As shown in FIG. 5, under the following conditions A and B where the temperature of the substrate S is relatively low and the bias power supplied to the target 14 is relatively small, as shown in FIG. Precipitation of gallium having a hemispherical shape was observed on the surface of the substrate S.
Condition A: substrate temperature T is 600 ° C. and bias power P is 1.39 W / cm ²
Condition B: substrate temperature T is 700 ° C. and bias power P is 1.39 W / cm ²
Further, even in the following conditions C and D where the substrate temperature is higher than that in the conditions A and B and the bias power supplied to the target 14 is larger, as shown in FIG. Precipitation of hemispherical gallium was observed on the surface of the substrate S although it was small.
Condition C: substrate temperature T is 800 ° C. and bias power P is 4.17 W / cm ²
Condition D: substrate temperature T is 1000 ° C. and bias power P is 4.17 W / cm ²
From these results, the region 1 including the above conditions A to D is that the amount of gallium excluding the nitrided portion of the gallium that has reached the substrate S is more than the amount of gallium evaporated by heating on the substrate S. It can be said that this is an excessive region.

(Region 2: Layered structure region: Conditions E, F, G, H, I)
On the other hand, even if the substrate temperature is the same as that in the condition C, the following conditions E and F in which the bias power supplied to the target 14 is smaller are flat on the surface of the substrate S as shown in FIG. A gallium nitride film was formed.
Condition E: substrate temperature T is 800 ° C. and bias power P is 0.44 W / cm ²
Condition F: substrate temperature T is 800 ° C. and bias power P is 1.39 W / cm ²
Further, even in the following condition G, which is a substrate temperature higher than these conditions E and F, although it is inferior in flatness to the gallium nitride film formed by the conditions E and F, as shown in FIG. Gallium nitride film formation was observed.
Condition G: substrate temperature T is 900 ° C. and bias power P is 1.32 W / cm ²
Furthermore, even under the following conditions where the substrate temperature is higher than the conditions E, F, and G and the bias power is high, the gallium nitride film formed under the conditions E, F, and G is inferior in flatness, but is flat. Gallium nitride film formation was observed.
Condition H: substrate temperature T is 1000 ° C. and bias power P is 2.20 W / cm ²
Condition I: substrate temperature T is 1000 ° C. and bias power P is 2.78 W / cm ²
From these results, the region 2 including the above conditions E to I refers to the amount of gallium existing alone on the substrate S without reacting with nitrogen among the gallium reaching the substrate S, and on the substrate S. It can be said that this is a region where the amount of gallium evaporated by heating is substantially equal.

(Region 3: Columnar structure region: Conditions J, K, L, M, N, O, P, Q)
On the other hand, even if the substrate temperature is the same as the above conditions E and F, under the following condition J where the bias power supplied to the target 14 is smaller, as shown in FIG. The structure was growing.
Condition J: substrate temperature T is 800 ° C. and bias power P is 0.23 W / cm ²
In the following conditions K and L where the substrate temperature is higher than the condition J, as shown in FIG. 6 (f), the direction away from the surface of the substrate S from the concavo-convex structure formed on the substrate S and the surface A linear gallium nitride columnar structure formed so as to extend in the normal direction was observed.
Condition K: substrate temperature T is 900 ° C. and bias power P is 0.44 W / cm ²
Condition L: substrate temperature T is 900 ° C. and bias power P is 0.23 W / cm ²
Even in the following conditions M, N, and O, which are higher substrate temperatures than the conditions K and L, the density of the gallium nitride columnar structures on the surface of the substrate S is lower than that of the gallium nitride columnar structures formed by the conditions K and L. The formation of gallium nitride columnar structures was observed.
Condition M: substrate temperature T is 1000 ° C. and bias power P is 0.23 W / cm ²
Condition N: substrate temperature T is 1000 ° C. and bias power P is 0.93 W / cm ²
Condition O: substrate temperature T is 1000 ° C. and bias power P is 1.39 W / cm ²
Further, the density of the gallium nitride columnar structures on the surface of the substrate S is higher than that of the gallium nitride columnar structures formed by the M, N, and O even under the following conditions P and Q, which are substrate temperatures higher than the above conditions M, N, and O. However, the formation of a gallium nitride columnar structure was observed.
Condition P: substrate temperature T is 1100 ° C. and bias power P is 1.39 W / cm ²
Condition Q: substrate temperature T is 1100 ° C. and bias power P is 2.20 W / cm ²
The region 3 including the above-described conditions K, L, M, N, O, P, and Q is the amount of gallium that reaches the substrate S and the amount of heating on the substrate S rather than the amount obtained by removing the amount reacted with nitrogen. This is a region where a large amount of gallium is evaporated. In addition, this is a region where the substrate temperature is high enough that the amount of gallium that evaporates before reacting with the nitrogen supplied to the surface of the substrate S is larger than the region 2.

(Region 4: Non-deposition region: Conditions R and S)
As shown in FIG. 5, no growth of gallium nitride on the substrate S was observed under the following conditions R and S where the substrate temperature was higher than any of the previous conditions. That is, it can be said that the region 4 including the conditions R and S is a region in which gallium reaching the substrate S evaporates from the substrate S without being nitrided.
Condition R: substrate temperature T is 1150 ° C., bias power is 0.93 W / cm ²
Condition S: substrate temperature T is 1200 ° C. and bias power is 1.39 W / cm ²
Accordingly, the substrate temperature is in the range of 600 ° C. or higher 1200 ° C. or less, and, when the bias power supplied to the target 14 is large 4.63W / cm ² or less than 0 W / cm ² is sandwiched following two linear The substrate temperature T and the bias power P are set so as to be included in the region 4 described above.
-First straight line passing through conditions E, G, H: P = 0.0088T-6.60
Second line passing through conditions M and P: P = 0.116T-11.37
Note that the relationship between the bias power P and the amount of gallium reaching the surface of the substrate S is linearly approximated, and if the substrate temperature T is in the range of 600 ° C. to 1200 ° C., the substrate temperature T And the amount of gallium evaporated from the surface of the substrate is approximately linearly approximated. Therefore, the relationship between the bias power P and the substrate temperature T in which the amount of gallium evaporated from the substrate surface is larger than the amount of surplus gallium on the surface of the substrate S is also linearly approximated.

  A process of forming the gallium nitride columnar structure on the surface of the substrate S will be described with reference to FIG. First, as shown in FIG. 7A, a substrate S is prepared in which a base layer 42 made of, for example, aluminum nitride is laminated on a substrate 41 made of a single crystal such as a sapphire substrate. The underlayer 42 is formed on the surface of the substrate 41 by a film forming method such as sputtering. The underlayer 42 is formed as a collection of crystal grains having a substantially uniform grain size. Thereby, the diameters of the gallium nitride columnar structures 43 grown using the crystal grains of the underlayer 42 as nuclei are substantially equal between the gallium nitride columnar structures formed on the underlayer 42. For this reason, for example, when the gallium nitride columnar structure 43 is used in the electronic device as described above, variation in material characteristics depending on the diameter of the columnar structure is less likely to occur between the gallium nitride columnar structures 43.

  Next, after the substrate S is carried into the sputtering apparatus 10, when reactive sputtering is performed under the above-described conditions, the gallium nitride columnar structure having a diameter substantially equal to the crystal grains forming the base layer 42. 43 is formed on the entire top surface of the substrate S.

As described above, according to the method for forming a gallium nitride columnar structure of the present embodiment, the gallium nitride columnar structure can be formed on the substrate S by reactive sputtering, so that the catalyst metal is nitrided as in the VLS method. Mixing into the gallium columnar structure can be suppressed.
[Example]
As shown in FIG. 8, an aluminum nitride layer 52 made of aluminum nitride having a thickness of 25 nm was formed on a sapphire substrate 51 having a thickness of 0.42 mm and a diameter of 50.8 mm by a reactive sputtering method. Thereafter, a gallium nitride columnar structure 53 extending in the direction away from the surface of the aluminum nitride layer 52 and in the normal direction of the surface was formed by reactive sputtering under the following conditions.

・ Gallium target Diameter 120mm ²
・ Argon gas flow rate 38.5sccm
・ Nitrogen gas flow 16.5sccm
・ Nitrogen concentration 30%
Bias power (13.56 MHz) 1.39 W / cm ² (150 W)
・ Substrate temperature 1000 ℃
Reactive sputtering duration 40 minutes Note that the nitrogen concentration is a growth rate of 91% or more and 100% or less of the maximum value of the growth rate of gallium nitride defined by the substrate temperature and bias power. Nitrogen concentration that gives a speed. When reactive sputtering was performed under these conditions, a gallium nitride columnar structure 53 having a diameter of about 50 nm and a length of about 2000 nm was formed on the aluminum nitride layer 52 at a density of about 1.0 × 10 ¹⁰ pieces / mm ² . Been formed.

According to the embodiment described above, the effects listed below can be obtained.
(1) Bias power is set so that gallium reaches the surface of the substrate S in addition to gallium to be nitrided in a nitrogen concentration range in which the growth rate of gallium nitride is controlled by nitrogen supply. Further, the temperature of the substrate S is set so that all of the gallium other than the gallium nitride is evaporated from the surface of the substrate S. Thus, the gallium nitride columnar structure can be formed only by supplying gallium to the surface of the substrate S by sputtering the target 14 and supplying a nitriding source to the surface of the substrate S.

  In addition, since the gallium nitride columnar structure is formed without using a liquid catalyst metal as in the VLS method, mixing of the catalyst metal into the formed gallium nitride columnar structure is naturally suppressed.

(2) The nitrogen concentration was set to a growth rate of 61% or more and 100% or less with respect to the maximum value in the growth rate of gallium nitride. Then, while a 600 ° C. or higher 1200 ° C. or less the temperature of the substrate S, bias power P was largely 4.63W / cm ² or less than 0 W / cm ² is supplied per unit of the target 14 area. This makes it possible to adjust the balance between the flux of gallium atoms reaching the substrate and the flux of gallium thermally re-evaporated from the substrate, that is, the degree of supersaturation of gallium on the substrate, sufficiently low. As a result, it becomes easy to form a gallium nitride columnar structure.

  (3) The nitrogen concentration is in a range where the growth rate is 91% or more and 100% or less with respect to the maximum value in the growth rate of gallium nitride. When the bias power P is a bias power having a frequency of 13.56 MHz, the substrate temperature T and the bias power P are P <0.0088T-6.60, P ≧ 0.0116T-11.37. The gallium nitride columnar structure was formed under the conditions satisfying the above conditions. Therefore, it is possible to adjust the balance between the flux of gallium atoms reaching the substrate and the flux of gallium thermally re-evaporated from the substrate, that is, the degree of supersaturation of gallium on the substrate, and thus It becomes easy to form a gallium nitride columnar structure.

  The range defined based on the first straight line and the second straight line described above is established only in the nitrogen concentration region where the growth rate is 91% or more and 100% or less with respect to the maximum value of the growth rate of gallium nitride. is there. However, in another nitrogen concentration range, that is, a nitrogen concentration range in which the growth rate of gallium nitride is 61% or more and 100% or less with respect to the maximum value, nitriding is performed by adjusting the bias power and the substrate temperature. Gallium columnar structures can be selectively formed.

In addition, the said embodiment can also be suitably changed and implemented as follows .

  Whether gallium other than the gallium to be nitrided has reached the surface of the substrate S is determined by measuring the flux traveling toward the surface of the substrate S and forming a film with the integrated value of the measured flux. It can also be confirmed by comparing the amount of gallium in the gallium nitride. Therefore, in addition to the range defined based on the first straight line and the second straight line described above, the bias power that causes gallium other than gallium nitride to reach the surface of the substrate is not limited to the flow rate of gallium reaching the substrate and the gallium nitride. It is also possible to set the range based on the measured value with the amount of gallium contained in the gallium. In short, the bias power at the time of film formation is not particularly limited as long as the gallium other than gallium to be nitrided is within a range that reaches the surface of the substrate.

  Whether or not all of the gallium other than the gallium nitride is evaporated from the surface of the substrate can be confirmed based on the amount of gallium other than the gallium nitride and the evaporation rate of gallium. Therefore, the substrate temperature at which all of the gallium other than gallium nitride is evaporated from the surface of the substrate is not limited to the range defined based on the first straight line and the second straight line described above, It can also be set to a range based on the amount of gallium and the evaporation rate of gallium. In short, if the temperature of the substrate during film formation is higher than the range in which all of the gallium other than gallium nitride is evaporated from the surface of the substrate, the method for defining the range is not particularly limited. Absent.

  -The nitrogen-containing gas is in a nitrogen concentration range in which the growth rate of gallium nitride is determined by the nitrogen supply in the nitrogen concentration, which is the ratio of the nitrogen-containing gas to the total flow rate of the rare gas and the nitrogen-containing gas. Any material can be used as long as it is supplied into the vacuum chamber, and the nitrogen concentration is not limited to the above range as long as the nitrogen concentration is such.

The substrate 41 may be a silicon carbide (SiC) substrate, a silicon (Si) substrate, or the like in addition to the sapphire substrate.
The base layer 42 may be a layer made of gallium nitride, for example, in addition to the layer made of aluminum nitride.

· On the substrate S, it may not have the underlying layer is formed.

  DESCRIPTION OF SYMBOLS 10 ... Sputtering device, 11 ... Vacuum chamber, 12 ... Cathode housing, 12a ... Magnet, 13 ... Backing plate, 14 ... Target, 14a ... Gallium, 14b ... Container, 15 ... High frequency power supply, 16 ... Substrate stage, 17 ... Adhesion Plate, 18 ... Heater chamber, 19 ... Heater unit, 20 ... Exhaust unit, 30 ... Control unit, 41 ... Substrate, 42 ... Underlayer, 43, 53 ... Gallium nitride columnar structure, 51 ... Sapphire substrate, 52 ... Aluminum nitride layer , MFC1, MFC2 ... Mass flow controller, S ... Substrate.

Claims (4)

A method for forming a gallium nitride columnar structure for forming a gallium nitride columnar structure,
Supplying a bias power to a gallium target disposed in a vacuum chamber, and sputtering the gallium target with a rare gas supplied in the vacuum chamber;
A gallium nitride columnar structure is prepared by reacting a nitrogen-containing gas activated by plasma and sputtered gallium on a single crystal substrate heated in the vacuum chamber. Forming a step,
The nitrogen concentration range in which the growth rate of the gallium nitride is controlled by the nitrogen supply, of the nitrogen concentration that is the ratio of the nitrogen-containing gas to the total flow rate of the rare gas and the nitrogen-containing gas. To be supplied into the vacuum chamber so that
The bias power causes gallium to reach the surface of the single crystal substrate in addition to the gallium to be nitrided,
The temperature of the single crystal substrate state, and are all gallium except gallium being the nitride that is grown gallium nitride at a temperature higher than the temperature to evaporate from the surface of the single crystalline substrate,
The nitrogen-containing gas is nitrogen gas, the noble gas is argon gas,
The nitrogen concentration when forming the gallium nitride is in a range where the growth rate is not less than 61% and not more than 100% with respect to the maximum value in the growth rate of the gallium nitride,
And, the temperature of the substrate when forming the gallium nitride columnar structure is a substrate temperature T (° C.),
When the high frequency power supplied to the gallium target is a bias power P (W / cm ² ),
The substrate temperature T and the bias power P are
600 ≦ T ≦ 1200
0 <P ≦ 4.63
A method of forming a gallium nitride columnar structure characterized by satisfying The nitrogen concentration is in a range where the growth rate is 91% or more and 100% or less with respect to the maximum value in the growth rate of the gallium nitride,
The bias power is a bias power having a frequency of 13.56 MHz;
The substrate temperature T and the bias power P are
P <0.0088T-6.60
P ≧ 0.0116T-11.37
The method for forming a gallium nitride columnar structure according to claim 1 . A vacuum chamber containing a single crystal substrate and having a heating unit for heating the single crystal substrate;
A gas supply unit for supplying argon gas as a rare gas and nitrogen gas as a nitrogen- containing gas into the vacuum chamber;
A gallium target disposed in the vacuum chamber;
A power supply unit that supplies bias power that is high-frequency power to the gallium target;
A controller for controlling the operation of the heating unit, the gas supply unit, and the power supply unit when the gallium target is sputtered with the rare gas, and a gallium nitride columnar structure forming apparatus comprising:
The controller is
The gas supply so that the growth rate of gallium nitride is within the range of the nitrogen concentration controlled by the nitrogen supply among the nitrogen concentration, which is the ratio of the nitrogen-containing gas to the total flow rate of the rare gas and the nitrogen-containing gas. Control the drive of the unit, and
In addition to the gallium nitride, the bias power is output to the power supply unit so that gallium reaches the surface of the single crystal substrate, and all the gallium other than the gallium nitride is supplied to the single crystal substrate. Heating the single crystal substrate to the heating unit to grow gallium nitride at a temperature higher than the temperature to evaporate from the surface ;
Controlling the driving of the gas supply unit so that the nitrogen concentration falls within a range of 61% to 100% with respect to the maximum value of the growth rate of the gallium nitride; and
The temperature of the substrate when forming the gallium nitride columnar structure is a substrate temperature T (° C.),
When the high frequency power supplied to the gallium target is a bias power P (W / cm ² ),
The substrate temperature T and the bias power P are
600 ≦ T ≦ 1200
0 <P ≦ 4.63
The apparatus for forming a gallium nitride columnar structure is characterized in that the heating unit and the power supply unit are controlled so as to satisfy the above . The bias power is a bias power having a frequency of 13.56 MHz;
The control unit is
Controlling the driving of the gas supply unit so that the nitrogen concentration falls within a range of 91% to 100% with respect to the maximum value of the growth rate of the gallium nitride;
The substrate temperature T and the bias power P are
P <0.0088T-6.60
P ≧ 0.0116T-11.37
The apparatus for forming a gallium nitride columnar structure according to claim 3 , wherein the heating unit and the power supply unit are controlled so as to satisfy the condition.