JP2008078186A - Method of growing crystal of nitride compound semiconductor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of growing a crystal of a nitride compound semiconductor wherein the crystal of a p-type nitride compound semiconductor can be made low in resistance as a bulk, and which is in a low level of a defect and high in quality. <P>SOLUTION: Gas containing ammonia and an organic nitrogen compound as a nitrogen material gas is supplied to epitaxially grow a nitride compound semiconductor film. In the film formation process for a single film of the nitride compound semiconductor, the mole flow rate of ammonia and the organic nitrogen compound in the nitrogen material gas is controlled to change a supply mole ratio (R=x/(x+y)) of the organic nitrogen compound (x moles) and ammonia (y moles). Film formation is mainly carried out in an area with a small mole ratio, while hydrogen that enters the film desorbs by outward diffusion in an area with a large mole ratio. A film formation step and an annealing step during film formation are repeated a plurality of times. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a crystal growth method for a nitride compound semiconductor, and more particularly to an epitaxial growth technique for a III-V compound semiconductor thin film crystal containing nitrogen as a V group element.

  A white light source can be formed by a combination of a blue light emitting element and a phosphor. Such a white light source can be a backlight such as a liquid crystal display, a light emitting diode (LED) illumination, an automobile illumination, or a general illumination replacing a fluorescent lamp. Applications such as these have been actively studied, and some of them have already been put into practical use. At present, such blue light emitting devices are mainly produced by growing a thin film of a gallium nitride based semiconductor crystal by a technique such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). .

  FIG. 1 is a diagram for explaining an example of a general crystal growth process for manufacturing a blue LED by stacking and growing a gallium nitride based semiconductor thin film by MOCVD. In the example shown in this figure, sapphire is used as a substrate. First, the sapphire substrate 101 is placed on a susceptor in a reaction furnace for crystal growth, and hydrogen gas is supplied into the furnace to about 1000 ° C. or higher. The substrate surface is kept at the above temperature (1060 ° C. in this figure) for a predetermined time to clean the substrate surface (thermal cleaning) (FIG. 1A).

After this treatment, the substrate temperature is temporarily lowered to a relatively low temperature region of about 550 ° C., and a crystal growth gas is supplied into the furnace while the substrate temperature is sufficiently stabilized to form a so-called low temperature buffer layer 102. To do. The crystal growth gas used here is, for example, trimethylgallium (TMG) which is a gallium supply source and ammonia (NH ₃ ) which is a nitrogen supply source, and these source gases are supplied using hydrogen gas as a carrier gas, A GaN buffer layer 102 is obtained (FIG. 1B).

After the formation of the buffer layer 102, the substrate temperature is raised again to a high temperature region of about 1000 ° C., and after the substrate temperature is sufficiently stabilized, trimethylgallium (TMG) and ammonia (NH ₃ ) are supplied using hydrogen gas as a carrier gas. An i-type GaN layer 103 is formed. Then, monosilane gas (SiH ₄ ), which is a supply source of Si serving as an n-type dopant in the GaN crystal, is mixed into the supply gas at a predetermined flow rate to continue crystal growth, and the top of the i-type GaN layer 103 is Then, an n-type GaN layer 104 is formed (FIG. 1C).

Next, after the substrate temperature is sufficiently stabilized by lowering the substrate temperature to the intermediate region (750 ° C. in this figure), InGaN is formed by alternately laminating a plurality of InGaN quantum well layers and GaN barrier layers at this growth temperature. / GaN multiple quantum well light emitting layer 105 is formed (FIG. 1D). Here, the growth of the GaN barrier layer is performed by supplying trimethylgallium (TMG) and ammonia (NH ₃ ) as hydrogen as a carrier gas, and the growth of the InGaN quantum well layer is further performed by adding trimethylgallium with a predetermined flow rate to the gas. This is performed by mixing indium (TMI).

Subsequently, the substrate temperature is raised again to a high temperature region around 1000 ° C., and trimethylaluminum (TMA), trimethylgallium (TMG), ammonia (NH ₃ ), and a supply source of Mg serving as a p-type dopant using hydrogen as a carrier gas. supply cyclopentadienyl magnesium is (Cp ₂ Mg) by supplying to form a p-type AlGaN layer 106 of Mg-doped (FIG. 1 (E)), further, trimethylaluminum of the feed gas (TMA) Then, the Mg-doped p-type GaN layer 107 is formed (FIG. 1F). The p-type dopant may be Zn or Be instead of Mg.

  By the way, it is known that when hydrogen is bonded to an acceptor (Mg in the above example) in the p-type nitride compound semiconductor crystal, the acceptor is electrically inactivated. Therefore, when a p-type nitride compound semiconductor crystal is grown in an atmosphere containing hydrogen, or is heat-treated in a hydrogen gas or a gas that generates hydrogen, the p-type nitride compound compound is used. The semiconductor crystal has a high resistance. For this reason, in a film formation method such as MOCVD using hydrogen gas for crystal growth, a low-resistance p-type nitride compound semiconductor crystal is in an as-grown state (no special post-film treatment such as heat treatment is performed). It is not easy to get in.

  Therefore, various methods for obtaining a low-resistance p-type nitride compound semiconductor crystal have been studied, and the acceptor can be electrically connected by applying a special treatment to the high-resistance nitride compound semiconductor crystal after film formation. Activation method (first method), a method for reducing the resistance of an as-grown p-type nitride compound semiconductor crystal by devising a crystal growth process (second method), an element structure ( The method can be roughly divided into three methods (third method) in which both the concentration and mobility of p-type carriers (holes) are improved by devising the laminated structure and the resistance value is lowered.

  Specifically, as a first method, a method of electrically activating an acceptor added by applying an electron beam irradiation treatment at an acceleration voltage of about 3 to 30 kV to a gallium nitride compound semiconductor crystal doped with impurities such as Mg (Patent Document 1), a method of performing annealing at a temperature of 400 ° C. or higher in an inert gas atmosphere such as nitrogen pressurized above the decomposition pressure of a gallium nitride compound semiconductor (Patent Document 2), and the like are known. .

Further, as a second method, a nitrogen source gas that does not release hydrogen in a nitrogen releasing process when growing a p-type nitride compound semiconductor by metal organic chemical vapor deposition (that is, an organic nitrogen source instead of NH ₃ ) (Patent Document 3) using a gas), and in p-type gallium nitride grown at a crystal temperature of 700 ° C. or more, focusing on the fact that acceptor passivation by hydrogen occurs at the time of cooling after completion of growth, NH is formed after crystal growth. _A method is known in which the supply of gas is stopped and the crystal is cooled in an inert gas atmosphere containing no hydrogen (Patent Document 4).

  Further, as a third method, one nitride compound semiconductor layer having a high impurity concentration by an InGaN / GaN superlattice structure in which first and second nitride compound semiconductor layers having different compositions are alternately stacked. In the other nitride-based compound semiconductor layer that generates a large amount of carriers and has a low impurity concentration (Patent Document 5), or an acceptor due to donor-induced crystal defects due to an InAlGaN-based superlattice structure There is known a method for reducing the compensation (Patent Document 6) and the like.

  Among these methods, in the electron beam irradiation treatment of the first method, the resistance is reduced only in the vicinity of the sample surface which is the electron beam irradiation surface, and the entire bulk of the p-type gallium nitride compound semiconductor crystal is reduced in resistance. It can not be made. In addition, since it is difficult to irradiate the entire sample surface with an electron beam in a single process, generally, a method of sweeping an electron beam irradiation spot on the sample surface is adopted, so that the activation level of the acceptor There are problems in terms of in-plane uniformity and high-speed process (improvement of throughput).

  In addition, the first method of annealing in an inert gas is that nitrogen desorption from the vicinity of the surface of the sample is complete even when annealing is performed under high pressure conditions that take into account the decomposition pressure of the gallium nitride compound semiconductor crystal. It cannot be suppressed to a low level, and a defect due to nitrogen desorption occurs. This defect functions as an n-type carrier in the gallium nitride compound semiconductor crystal, resulting in a substantial decrease in the p-type carrier concentration in the vicinity of the surface. As a result, the resistance of the gallium nitride compound semiconductor crystal surface cannot be sufficiently lowered due to this phenomenon, and the contact resistance with the electrode formed on the surface increases, so that the entire device becomes highly resistive. There was a problem that.

In the method using the organic nitrogen source gas of the second method, although the incorporation of hydrogen into the crystal is suppressed, carbon contained in the source gas itself is likely to be mixed, and compared with the case where NH ₃ gas is used. As a result, the carbon concentration in the crystal becomes high. Further, since the organic nitrogen raw material does not have the reducing action seen with NH ₃ , the oxygen concentration also increases. Since carbon and oxygen incorporated into these gallium nitride compound semiconductor crystals function as n-type carriers, there is a problem that a desired p-type carrier concentration cannot be obtained.

Further, the method of stopping the gas supply of NH ₃ after growing the p-type crystal of the second method is effective in preventing hydrogen mixing after crystal growth and reducing the hydrogen concentration in the vicinity of the surface. Since the hydrogen already taken into the crystal remains in the bulk as it is, the resistance of the p-type gallium nitride compound semiconductor crystal cannot be sufficiently lowered.

  In the third method for reducing the resistance using the superlattice structure, the band gap and the refractive index also change when the composition of the laminated film changes, so that the degree of freedom in device design is greatly limited. There's a problem. In particular, when InGaN is used, its band gap energy is lower than that of GaN, so that the light absorption loss when this is applied to a light emitting device increases. In addition, since the InAlGaN-based superlattice structure is a structure in which quaternary crystal thin films are stacked, it is not easy to accurately control the composition when each layer is grown.

Furthermore, when forming a superlattice structure, it is necessary to set appropriate growth conditions (growth temperature, carrier gas, growth pressure, etc.) for each layer, so the superlattice structure repeats thin film stacking in a short cycle. In this case, the crystal growth process becomes complicated, and as a result, it becomes difficult to obtain a p-type layer with high quality and high carrier concentration.
JP-A-3-218625 JP-A-5-183189 Japanese Patent Laid-Open No. 10-4211 JP-A-9-199758 JP 11-340509 A JP 2002-319743 A Applied Physics Letters, May 13, 2002, Volume 80, Issue 19, pp. 3554-3556.

  The present invention has been made in view of such problems, and its object is to reduce the resistance of p-type nitride compound semiconductor crystals throughout the bulk without changing various physical properties such as band gaps. An object of the present invention is to provide a high-quality nitride compound semiconductor crystal growth method that is possible and has a low defect level.

In order to solve such problems, the present invention provides a crystal growth method for a nitride compound semiconductor in which at least one nitride compound semiconductor film is vapor-phase grown on a substrate. And supplying a nitrogen source gas containing an organic nitrogen compound (x mol) and ammonia (NH ₃ ) (y mol) when vapor-phase-growing any of the nitride compound semiconductor films, The film forming process is characterized in that the supply molar ratio (R = x / (x + y)) of the organic nitrogen compound (x mol) in the nitrogen source gas is changed.

  According to a second aspect of the present invention, in the crystal growth method for a nitride-based compound semiconductor according to the first aspect, the supply molar ratio (R) is periodically changed at a period T of 1 second to 120 seconds. It is characterized by.

  According to a third aspect of the present invention, in the crystal growth method for a nitride-based compound semiconductor according to the first or second aspect, the supply molar ratio (R) is set to at least two levels (R1, R2: R1 <R2). It is characterized by setting.

  According to a fourth aspect of the present invention, in the nitride compound semiconductor crystal growth method according to any one of the first to third aspects, the supply time of the nitrogen source gas at the two levels (R1 and R2) When T1 and T2 (T = T1 + T2), respectively, T2 ≧ 0.5 × T1 is set.

  According to a fifth aspect of the present invention, in the crystal growth method for a nitride-based compound semiconductor according to the third or fourth aspect, the two levels (R1, R2) are 0 ≦ R1 ≦ 0.6, and 0.4 ≦ R2 ≦ 1.0.

  The invention according to claim 6 is the method for growing a nitride compound semiconductor crystal according to any one of claims 1 to 5, wherein the supply amount of the source gas of the group III element in the film forming process is It is characterized in that it is varied in synchronization with the change in the supply molar ratio (R).

  The invention according to claim 7 is the nitride compound semiconductor crystal growth method according to any one of claims 1 to 6, wherein the organic nitrogen compound in the nitrogen source gas has a minimum supply molar ratio (R = Rmin), the group III element source gas is supplied only when it is supplied.

  The invention according to claim 8 is the nitride compound semiconductor crystal growth method according to any one of claims 1 to 7, wherein the supply amount of the acceptor dopant source gas in the film forming process is the supply amount. Fluctuating in synchronization with the change in the molar ratio (R).

  A ninth aspect of the present invention is the nitride compound semiconductor crystal growth method according to any one of the first to eighth aspects, wherein the organic nitrogen compound in the nitrogen source gas has a minimum supply molar ratio (R = Rmin), the acceptor dopant source gas is supplied only when it is supplied.

  A tenth aspect of the present invention is the crystal growth method for a nitride compound semiconductor according to any one of the first to ninth aspects, wherein the organic nitrogen compound is a hydrazine compound.

  The invention according to claim 11 is the crystal growth method of a nitride compound semiconductor according to claim 10, wherein the hydrazine compound is 1.1-dimethylhydrazine or tertiary butylhydrazine.

  The invention according to claim 12 is the nitride compound semiconductor crystal growth method according to any one of claims 1 to 11, wherein the nitride compound semiconductor film is GaN, AlN, InN, BN, or These are mixed crystals.

A thirteenth aspect of the present invention is a nitride-based compound semiconductor film grown by the method according to any one of the first to twelfth aspects, the oxygen concentration [O] in the film, carbon Concentration [C] and hydrogen concentration [H] are [O] ≦ 1 × 10 ¹⁷ cm ⁻³ , [C] ≦ 1 × 10 ¹⁷ cm ⁻³ , and [H] ≦ 5 × 10 ¹⁸ cm, respectively. ^-3 .

  According to the present invention, the molar ratio between ammonia and the organic nitrogen compound in the nitrogen source gas is changed during the growth of a single nitride-based compound semiconductor film, and the film is formed by deposition in a region where the molar ratio of the organic nitrogen compound is low. The concentration of hydrogen, carbon, and oxygen can be reduced by using the reducing and etching properties of ammonia gas to reduce the concentration of carbon and oxygen impurities in the film while controlling the hydrogen concentration in the film. Therefore, it is possible to reduce the resistance of the p-type nitride compound semiconductor crystal over the entire bulk without changing various physical properties such as the band gap, and the crystal of high quality nitride compound semiconductor having a low defect level. A growth method is provided.

According to the present invention, mixing of hydrogen, oxygen, and carbon into the nitride-based compound semiconductor film is suppressed, the oxygen concentration [O] is 1 × 10 ¹⁷ cm ⁻³ or less, and the carbon concentration [C] is 1 × 10 ¹⁷ cm ⁻³ or less, and the hydrogen concentration [H] can be set to 5 × 10 ¹⁸ cm ⁻³ or less.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

First, referring to FIG. 2, the present inventors examined the dependency of hydrogen and carbon taken into a film from a nitrogen source gas on the source gas mixture ratio when vapor-phase-growing a nitride compound semiconductor. The results will be described. This figure shows a p-type gallium nitride using a mixed gas of ammonia (NH ₃ ) and dimethylhydrazine (DMHy), which is an organic nitrogen compound, as a nitrogen source gas and nitrogen as a carrier gas in crystal growth by a vapor phase method. The hydrogen concentration (left ordinate) and carbon concentration (right ordinate) in the film immediately after forming (p-GaN) are plotted with the supply molar ratio of dimethylhydrazine in the nitrogen source gas as the horizontal axis. is there.

  In this figure, when the molar ratio of dimethylhydrazine is x and the molar ratio of ammonia is y, the horizontal axis is given as x / (x + y), so the molar ratio of dimethylhydrazine is The horizontal axis value is 0 when zero, and the horizontal axis value is 1 when the molar ratio of ammonia is zero.

  As shown in this figure, in the region where the molar ratio of dimethylhydrazine is low (region A), the hydrogen concentration in the film is high, but the carbon concentration is low. On the contrary, in the region where the molar ratio of dimethylhydrazine is high (region B), the carbon concentration in the film is high, but the hydrogen concentration is low. However, it can be seen that no matter how the molar ratio of dimethylhydrazine in the nitrogen source gas is set, the concentration of hydrogen and carbon in the film cannot be sufficiently reduced simultaneously.

  Based on such examination results, the present inventors set the molar ratio of the organic nitrogen compound such as dimethylhydrazine and ammonia in the nitrogen source gas to the single nitride compound semiconductor film formation process. Film formation process in an environment where the carbon concentration in the film is low (region where the molar ratio of the organic nitrogen compound is low), and the subsequent environment where the hydrogen concentration is low in the film (organic) If the annealing process during film formation in a region where the molar ratio of the nitrogen compound is high) is provided in the film formation process of a single film, either the carbon concentration or the hydrogen concentration in the obtained nitride-based compound semiconductor film is determined. As a result, the inventors have obtained the knowledge that it can be reduced, and have come to achieve the present invention.

  According to the study by the present inventors, among the hydrogen and carbon that cause inactivation of the p-type carrier in the nitride-based compound semiconductor crystal, hydrogen is mainly supplied from ammonia, and carbon is mainly from organic nitrogen raw material. Will be supplied. Further, when the supply amount of ammonia, which is a reducing gas, is insufficient or is not supplied, oxygen that is a residual impurity in the film formation environment and is taken into the film is similar to carbon. The amount of incorporation into the film also increases, and this oxygen also causes inactivation of the p-type carrier.

  Therefore, the present inventors changed the molar ratio of ammonia and organic nitrogen compound in the nitrogen source gas during the growth of a single nitride compound semiconductor film, and formed a film in a region where the molar ratio of organic nitrogen compound is low. By using the reducing and etching properties of ammonia gas to reduce the concentration of carbon and oxygen impurities in the film, the concentration of hydrogen, carbon and oxygen can be reduced while controlling the hydrogen concentration in the film to a low level. It was decided to let them.

  More specifically, when a nitride-based compound semiconductor is formed under a condition where the molar ratio of the organic nitrogen compound in the nitrogen source gas is low (the molar ratio of ammonia is high), the hydrogen impurity concentration is high in the film-forming region. High quality crystals with low carbon and low oxygen concentration. Subsequent to this film formation step, when annealing during film formation was performed under a condition where the molar ratio of the organic nitrogen compound in the nitrogen source gas was high (the molar ratio of ammonia was low), it was taken into the film during this annealing. Hydrogen diffuses outward and is desorbed from the surface, and the hydrogen concentration in the film decreases. In addition, since the nitrogen dissociated from the organic nitrogen compound is sufficiently supplied to the atmosphere in the vicinity of the nitride-based compound semiconductor film surface during annealing during the film formation, nitrogen is desorbed from the surface during the annealing. Crystal degradation that occurs is suppressed.

  As the carrier gas used in such annealing during film formation, it is desirable to use nitrogen gas rather than hydrogen gas. When hydrogen gas is used, not only the effect of annealing during film formation, that is, the effect of hydrogen desorption, but also the etching effect is caused, which damages the crystal surface. On the other hand, it is possible to use a hydrogen carrier gas or a mixed gas of hydrogen and nitrogen at the timing when the group III raw material is supplied and crystal growth is performed. However, if the carrier gas is switched many times during crystal growth, not only the film formation process time is lengthened but also pressure fluctuations are likely to occur. Therefore, as the carrier gas, it is desirable to use nitrogen gas during the formation of the Mg doped layer.

  Note that if significant crystal growth occurs during the above-described “annealing during film formation” step, carbon and oxygen as impurities are taken into the film formation region. Therefore, it is preferable to completely stop the group V element source supply during annealing during film formation, or to suppress crystal growth by adjusting the annealing time or controlling the amount of group V element source supply. That is, it is not always necessary to perform this step under conditions that do not cause crystal growth completely, but since hydrogen is desorbed during the step, this is referred to as “annealing during film formation” for convenience. During annealing during film formation, not only hydrogen desorption, but also the effect of planarizing the epitaxial growth surface and the effect of eliminating nitrogen vacancies are generated. When the epitaxial growth film is a mixed crystal, local strain is stabilized. Such effects also occur. These effects will be described below.

  Discontinuing the growth for the purpose of planarizing the epitaxial growth surface is a method commonly used in crystal growth of a III-V compound semiconductor, but when the III-V compound is a nitrogen compound, Because of the etching effect of ammonia, surface flattening cannot be expected even if crystal growth is interrupted in an ammonia atmosphere. However, if the crystal growth atmosphere is an organic nitrogen raw material atmosphere, etching by the atmospheric gas does not occur, so that the surface can be flattened by interrupting the growth.

  In addition, when the nitrogen source is only ammonia, it is difficult to increase the effective value of the group V and group III source supply ratio (V / III). It is formed. However, it is possible to reduce nitrogen vacancy defects in the film by using an organic nitrogen raw material as the group V raw material and stopping the supply of the group III raw material or by reducing the growth rate. Note that stopping the supply of the group III raw material corresponds to setting the raw material supply ratio (value of V / III) to infinity.

  Furthermore, regarding the stabilization of local strain, in the case of mixed crystals containing indium, it is known that the local segregation of indium in the film is largely related to the mechanism of light emission. It is thought that the temperature state during growth is effective. When the growth is interrupted while supplying the organic nitrogen raw material, an etching effect or the like that occurs when ammonia is supplied is unlikely to occur, so that appropriate segregation conditions may be obtained. Note that these effects become more prominent when the annealing is performed during the film formation rather than after the film formation.

  Since the above-described method uses out-diffusion of hydrogen from the vicinity of the surface of the nitride-based compound semiconductor film, nitriding with a desired thickness is performed if the film forming step and the annealing step during film forming are performed only once. In some cases, a physical compound semiconductor film cannot be obtained. In that case, the above film forming process and the annealing process during film forming are repeated a plurality of times until the nitride-based compound semiconductor film has a desired thickness.

FIG. 3 is a diagram for conceptually explaining the supply sequence of the nitrogen source gas in the nitride-based compound semiconductor crystal growth method of the present invention. In this figure, a single nitride-based compound semiconductor film is shown. The supply sequence of the nitrogen source gas at the time of growing is illustrated. A nitride compound semiconductor film is formed on a substrate such as sapphire, GaN, SiC, LiNbO ₃ , LiGaO ₃ , AlN, ScAlMO ₄ , ZnO, etc., using a VPE method, a MOVPE method, an MOCVD method, a CBE (Chemical Beam Epitaxy) method, A gas containing ammonia (NH ₃ ) and an organic nitrogen compound such as 1.1-dimethylhydrazine is supplied as a nitrogen source gas.

  In the present invention, during the growth process of a single nitride compound semiconductor film, the molar flow rate of ammonia and the organic nitrogen compound in the nitrogen source gas is controlled (see FIG. 3A), and the organic nitrogen compound ( x mol) and ammonia (y mol) supply molar ratio (R = x / (x + y)) is changed (see FIG. 3B). In this example, the supply molar ratio (R) is set to at least two levels (R1, R2: R1 <R2). As shown in FIG. 3B, the region (R1) in which the supply molar ratio of the organic nitrogen compound in the nitrogen raw material is small corresponds to the above-described film forming step, and the region (R2) in which the supply molar ratio is large is the above-described component. It corresponds to the in-film annealing process.

  As described above, if the nitride-based compound semiconductor film having a desired thickness cannot be obtained by performing the film-forming process and the annealing process during film-forming only once, the nitride-based compound semiconductor film is desired. The film formation process and the annealing process during the film formation are repeated a plurality of times until the thickness is reached.

  The appropriate range of this repetition cycle is determined by how much hydrogen can be desorbed from the surface layer and depends on the film formation temperature, but the growth rate is 1 μm / hour. When the hydrogen desorption effect can be expected at a standard film formation rate of up to about 30 nm from the surface, it is preferable to repeat at a period T of about 120 seconds at the maximum.

  The shortest repetition period is limited by the driving time of the reaction furnace valve, the sequence program, etc., but excessive opening and closing of the valve may cause turbulent flow of the reaction gas in the furnace. It is desirable to set it above. That is, the period T (= T1 + T2) of change in the supply molar ratio of the nitrogen source gas given by the sum of one film formation step (process time T1) and one film formation annealing step (process time T2) is: It is desirable to set 1 second or more and 120 seconds or less. Further, in order to sufficiently desorb hydrogen in the annealing process during film formation, it is preferable that one annealing process during film formation (process time T2) is set so as to satisfy T2 ≧ 0.5 × T1. For example, it is set so that T2> T1.

  FIG. 3A shows an example in which only the molar flow rate of ammonia is varied while keeping the molar flow rate of the organic nitrogen compound constant, but the molar flow rates of both ammonia and the organic nitrogen compound are varied. It may be. Further, the molar flow rate of ammonia in the nitrogen source gas supplied during the annealing process during film formation is set to zero (see FIG. 3C), or the mole of organic nitrogen compound in the nitrogen source gas supplied during the film forming process. It is possible to adjust the flow rate to zero. However, in order to avoid the deficiency of the nitrogen raw material when the gas molar flow rate is switched, the nitrogen raw material gas in which both ammonia and the organic nitrogen compound are mixed is supplied at least one of the timings (T1, T2). It is preferable to do so.

  Furthermore, switching between the film forming process and the annealing process during film forming (the supply sequence of the nitrogen source gas) does not necessarily have to be stepped. For example, as shown in FIG. 3D, the supply molar ratio can be controlled so as to continuously change from R1 to R2.

  Crystal growth mainly proceeds during the above-described film forming process, but the lower limit of the supply molar ratio of organic nitrogen (R1 in FIG. 3) at this time is 0% (molar flow rate of organic nitrogen compound is zero). The upper limit may be determined from the viewpoint of avoiding incorporation of carbon generated from the organic nitrogen compound into the film. For example, the mixing ratio (3: 2) is such that hydrogen generated from ammonia is combined with, for example, the methyl group of dimethylhydrazine to form stable methane, and specifically about 0.6. For the purpose of further reducing the carbon concentration, it is more desirable to set it to 0.3 or less, which corresponds to half of that.

  The annealing step during film formation (R = R2) is a process whose main purpose is to desorb hydrogen mixed in the film by outward diffusion, so that the molar flow rate of ammonia is zero (R2 = 1. 0) is desirable. However, since ammonia has a lower decomposition efficiency than organic nitrogen compounds, if the supply molar ratio of organic nitrogen compounds is at least about 0.4, the supply of hydrogen from ammonia into the atmosphere will cause hydrogen desorption of the membrane. There is no inhibition. In order to further reduce the hydrogen concentration, a supply molar ratio R that reduces the molar ratio of ammonia by half, that is, 0.7 or more is more desirable.

  That is, when the supply molar ratio (R) is set to, for example, two levels (R1, R2: R1 <R2), the supply molar ratio (R1) of the organic nitrogen raw material in the film forming process is set to 0 ≦ R1 ≦ 0. .6, and the supply molar ratio (R2) of the organic nitrogen raw material in the annealing step during film formation is preferably set in the range of 0.4 ≦ R2 ≦ 1.0.

  While the crystal growth proceeds mainly in the film formation step, an annealing step during film formation is provided mainly for the purpose of desorbing the hydrogen taken in during the crystal growth. The raw material supply of group III elements such as aluminum and indium is changed in synchronization with the change in the supply molar ratio (R) described above. For example, the group III element source gas is supplied only when the organic nitrogen source gas is supplied at the minimum supply molar ratio Rmin (R1 in the example of FIG. 3). If you want to minimize the flow adjustment of the nitrogen source gas in order to suppress the influence on the gas flow in the furnace, the molar flow rate of the organic nitrogen compound under the condition that nitrogen desorption from the film surface does not occur. It is desirable that the molar flow rate of ammonia be linked with the supply of the group III element source gas.

  In addition, when a p-type nitride compound semiconductor is formed, the supply of the acceptor dopant source gas is synchronized with the supply of the group III element source gas. It is varied in synchronization with the change in the supply molar ratio Rmin (R) described above. For example, the acceptor dopant source gas is supplied only when the organic nitrogen source gas is supplied at the minimum supply molar ratio (R1 in the example of FIG. 3).

  Examples of the organic nitrogen compound used in the present invention include hydrazine compounds and amine compounds. However, amine compounds are prone to intermediate reactions, and this intermediate reactant has an adverse effect on crystal growth, making it difficult to obtain a good p-type nitride compound semiconductor film having a high carrier concentration (low resistance). is there. 1.1-Dimethylhydrazine and tertiary butylhydrazine do not have such difficulties, and are not problematic from the viewpoint of decomposition temperature and handling safety, and are desirable as organic nitrogen compounds.

According to such a film formation process of the present invention, mixing of hydrogen, oxygen, and carbon into the nitride-based compound semiconductor film is suppressed, and the oxygen concentration [O] is 1 × 10 ¹⁷ cm ⁻³ or less, The carbon concentration [C] can be 1 × 10 ¹⁷ cm ⁻³ or less, and the hydrogen concentration [H] can be 5 × 10 ¹⁸ cm ⁻³ or less. Further, by appropriately setting the film formation conditions (R1, R2, T1, and T2) according to the type of film to be formed (AlGaN, GaN, etc.) and the film thickness, the oxygen concentration [O] is It can be further reduced to 6 × 10 ¹⁶ cm ⁻³ or less, the carbon concentration [C] to 6 × 10 ¹⁶ cm ⁻³ or less, and the hydrogen concentration [H] to 1 × 10 ¹⁸ cm ⁻³ or less. .

  In addition, by suppressing the incorporation of hydrogen into the film, acceptor activation processing (such as electron beam irradiation and annealing) after the film formation is unnecessary in the p-type crystal, and productivity is improved. In addition to this, the post-deposition annealing, which is a conventional technique, easily causes a nitrogen deficient state on the crystal surface, and the increase in contact resistance induced thereby is a problem. This problem can be avoided and the device resistance of the device can be kept low.

After the p-type GaN film obtained by the film formation process of the present invention is cooled and taken out in a reaction furnace, the hole concentration is measured in the as-grown state, and the carrier concentration is measured. A carrier concentration (1 × 10 ¹⁸ cm ⁻³ ) higher than that of the p-type GaN film annealed in the gas was obtained. In addition, the contact resistance, in light of the data described in Non-Patent Document 1, will be 0.1? Cm ² approximately to 5 × 10 ^-4 Ωcm ² about values are obtained, as compared with the conventional film The contact resistance can be greatly reduced.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  FIG. 4 is a diagram for explaining the crystal growth process of this embodiment in which a gallium nitride based semiconductor thin film is grown by MOCVD, and this embodiment is an example in which a single p-type GaN film is formed. is there. A sapphire substrate 11 with the C-plane as the main surface (growth surface) was placed on the susceptor in the reactor, and the pressure was reduced to 1.3 kPa while flowing hydrogen gas, and a heater was used until the substrate surface temperature reached 1060 ° C. Heat (FIG. 4 (A)).

  After holding the sapphire substrate 11 at 1060 ° C. for 5 minutes, the pressure in the reactor is increased to 0.13 MPa while the temperature is lowered to 550 ° C. Then, at a substrate temperature of 550 ° C., hydrogen is used as a carrier gas, ammonia (NH 3) gas is supplied at a rate of 20 liters / minute, and trimethylgallium (TMG) gas is supplied at a flow rate of 100 μmol / minute for 2 minutes. A low-temperature buffer layer 12 of (GaN) is formed (FIG. 4B).

Thereafter, the supply of TMG is stopped, and heating is performed until only the NH ₃ gas and the hydrogen gas are supplied to the reaction furnace until the substrate temperature reaches 1020 ° C., and at a substrate temperature of 1020 ° C., TMG is added at 300 μmol / min. The high temperature buffer layer 13 is formed by maintaining the supplied state for 1 hour (FIG. 4C). Note that the thickness of the high-temperature buffer layer 13 obtained under these film formation conditions is about 3 μm. Then, a p-type GaN film 14 is formed on the high-temperature buffer layer 13 (FIGS. 4D to 4G).

Next, the carrier gas is switched from hydrogen to nitrogen, and growth of the Mg doped layer is started. FIG. 5 is a diagram for explaining a gas supply sequence in the film formation process of the p-type GaN film 14 of FIGS. First, the supply of NH ₃ gas is stopped, 1.1-dimethylhydrazine (DMHy), which is an organic nitrogen raw material, is vaporized and flowed in the furnace at a flow rate of 500 sccm for 30 seconds to stabilize the DMHy supply (FIG. 4). (D)). At this time, the Ga material is not supplied, so the GaN film does not grow.

Following stabilization of the DMHy supply, 100 μmol / min of TMG as a Ga source, 1.5 μmol / min of cyclopentadienylmagnesium (Cp ₂ Mg) as a p-type dopant source, and a nitrogen source A certain NH ₃ gas is supplied at a rate of 20 liters / minute to form a p-type GaN film 14. Since the flow rate of DMHy (500 sccm) is kept constant, this DMHy is also a nitrogen source. Further, (D) and (E) in FIG. 4 are repeated, and the process of performing this once is referred to as “one cycle” here.

  As shown in FIG. 5, the process time required for film formation in one cycle is 15 seconds, whereby a thin p-type GaN film 14 is obtained (FIG. 4E).

After this film formation step, an annealing step during film formation is provided for the purpose of desorbing hydrogen from the surface of the film. Specifically, the supply of TMG, Cp ₂ Mg, and NH ₃ gas is stopped, and only DMHy (500 sccm) using nitrogen gas as a carrier is supplied for 45 seconds (FIG. 4F).

  Then, the p-type GaN film 14 is made to have a desired thickness by repeating the film formation process for 15 seconds and the annealing process during film formation for 45 seconds (FIG. 4G). In this example, the number of repetitions is 60 (total 60 minutes), and the finally obtained p-type GaN film 14 has a thickness of 0.5 μm.

  FIG. 6 is a diagram for explaining the crystal growth process of this example for producing an LED substrate having a GaN-based multiple quantum well structure by the MOCVD method. First, the C plane is used as the main surface (growth surface) as the substrate. The sapphire substrate 11 is placed on a susceptor in the reaction furnace, depressurized to 1.3 kPa while flowing hydrogen gas, and heated with a heater until the substrate surface temperature reaches 1060 ° C. (FIG. 6A).

After holding the sapphire substrate 11 at 1060 ° C. for 5 minutes, the pressure in the reactor is increased to 0.13 MPa while the temperature is lowered to 550 ° C. Then, at a substrate temperature of 550 ° C., hydrogen is used as a carrier gas, ammonia (NH ₃ ) gas is supplied at a rate of 20 liters / minute, and trimethylgallium (TMG) gas is supplied at a flow rate of 100 μmol / minute for 2 minutes. A low-temperature buffer layer 12 of gallium (GaN) is formed (FIG. 6B).

Thereafter, the supply of TMG is stopped, and the substrate temperature is heated to 1020 ° C. while only NH ₃ gas and hydrogen gas are supplied to the reaction furnace. At a substrate temperature of 1020 ° C., TMG is 300 μm using hydrogen as a carrier gas. Supply is performed at 1 mol / min for 1 hour to form the high-temperature buffer layer 13 (FIG. 6C). Note that the thickness of the high-temperature buffer layer 13 obtained under these film formation conditions is about 3 μm.

Next, monosilane gas (SiH ₄ ), which is a raw material of TMG, NH _3, and silicon (Si) as an n-type dopant, is supplied to form an n-type GaN layer 15 having a thickness of 3 μm (FIG. 6D).

Subsequently, the substrate temperature is lowered to 750 ° C., the carrier gas is switched from hydrogen to nitrogen, and the InGaN / GaN multiple quantum well light emitting layer 16 is formed on the n-type GaN layer 15. Specifically, trimethylindium (TMI), TMG, and NH ₃ are supplied using nitrogen as a carrier gas to form an InGaN quantum well layer 16a (FIG. 6E), and TMG and NH ₃ are supplied to form GaN. A barrier layer 16b is formed (FIG. 6F), and these InGaN quantum well layers 16a and GaN barrier layers 16b are alternately formed and stacked in multiple layers to obtain an InGaN / GaN multiple quantum well light-emitting layer 16. (FIG. 6G).

Subsequently, the substrate is heated again until the substrate temperature reaches 1020 ° C., and the supply of NH ₃ is stopped, while DMHy is vaporized using nitrogen as a carrier gas and supplied at a flow rate of 500 sccm. After stabilizing in this state for 30 seconds, the p-type AlGaN layer 17 is formed while the supply of DMHy (500 sccm) is continued (FIG. 6 (H)). Specifically, the film-forming process for 15 seconds and the annealing process during film-forming for 45 seconds are repeated five times to obtain the p-type AlGaN layer 17 having a thickness of 75 nm. Note that the gas supplied in the film forming step is trimethylaluminum (TMA: 50 μmol / min), TMG (100 μmol / min), Cp using nitrogen as a carrier gas in addition to a constant supply amount of DMHy (500 sccm). ₂ Mg (1.5 μmol / min) and NH ₃ (10 liter / min). In the annealing process during film formation, the supply of source gases other than DMHy (500 sccm) is stopped.

A p-type GaN layer 18 is formed on the p-type AlGaN layer 17 (FIG. 6I). Specifically, a 15-second deposition step and a 45-second annealing step during deposition are repeated 15 cycles to obtain a p-type GaN layer 18 having a thickness of 0.15 μm. In addition to the DMHy (500 sccm) with a constant supply amount, TMG (100 μmol / min), Cp ₂ Mg (1.5 μmol / min), Cp ₂ Mg (nitrogen as a carrier gas), NH ₃ (20 liters / min), and supply of source gases other than DMHy (500 sccm) is stopped in the annealing step during film formation.

  As mentioned above, although the example demonstrated the crystal growth method of the nitride type compound semiconductor of this invention, the said Example is only an example for implementing this invention, and this invention is not limited to these. In the embodiment, the p-type layer made of GaN may be AlN, InN, BN, or a mixed crystal thereof. Further, the growth temperature, the supply amount of each raw material or the film thickness of each layer can be changed according to the purpose. It is apparent from the above description that various modifications of these embodiments are within the scope of the present invention, and that various other embodiments are possible within the scope of the present invention.

  The present invention makes it possible to reduce the resistance of p-type nitride-based compound semiconductor crystals over the entire bulk without changing various physical properties such as band gaps, and to provide high-quality nitride-based compound semiconductor crystals with a low defect level. A growth method is provided.

It is a figure for demonstrating the general crystal growth process which produces a blue LED by carrying out lamination | stacking growth of the gallium nitride type semiconductor thin film by MOCVD method. It is the figure which plotted hydrogen concentration and carbon concentration in the GaN film crystal-grown by the vapor phase method, with the supply molar ratio of dimethylhydrazine in the nitrogen source gas as the horizontal axis. It is a figure for demonstrating notionally the supply sequence of the nitrogen source gas in the crystal growth method of the nitride type compound semiconductor of this invention. It is a figure for demonstrating the crystal growth process of Example 1 which carries out lamination | stacking growth of the gallium nitride semiconductor thin film by MOCVD method. It is a figure for demonstrating the sequence of the gas supply in the film-forming process of a p-type GaN film | membrane. It is a figure for demonstrating the crystal growth process of Example 2 which produces the board | substrate for LED which has a GaN-type multiple quantum well structure by MOCVD method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Sapphire substrate 12 Low temperature buffer layer 13 High temperature buffer layer 14 p-type GaN film 15 n-type GaN layer 16 InGaN / GaN multiple quantum well light emitting layer 16a InGaN quantum well layer 16b GaN barrier layer 17 p-type AlGaN layer 18 p-type GaN layer

Claims (13)

A nitride-based compound semiconductor crystal growth method comprising vapor-phase-growing at least one nitride-based compound semiconductor film on a substrate,
When vapor-phase-growing any of the nitride-based compound semiconductor films, a nitrogen source gas containing an organic nitrogen compound (x mol) and ammonia (NH ₃ ) (y mol) is supplied, and the nitrogen source A crystal growth method for a nitride-based compound semiconductor, comprising a film forming process for changing a supply molar ratio (R = x / (x + y)) of an organic nitrogen compound (x mol) in a gas. 2. The crystal growth method for a nitride-based compound semiconductor according to claim 1, wherein the supply molar ratio (R) is periodically changed with a period T of 1 second or more and 120 seconds or less. The crystal growth method for a nitride-based compound semiconductor according to claim 1 or 2, wherein the supply molar ratio (R) is set to at least two levels (R1, R2: R1 <R2). 2. T2 ≧ 0.5 × T1 is set when T1 and T2 (T = T1 + T2) are set as the supply times of the nitrogen source gas at the two levels (R1 and R2), respectively. 4. The method for growing a nitride compound semiconductor crystal according to any one of items 1 to 3. 5. The nitride compound according to claim 3, wherein the two levels (R 1, R 2) are 0 ≦ R 1 ≦ 0.6 and 0.4 ≦ R 2 ≦ 1.0, respectively. Semiconductor crystal growth method. 6. The nitriding according to claim 1, wherein a supply amount of a group III element source gas in the film forming process is varied in synchronization with a change in the supply molar ratio (R). Crystal growth method for physical compound semiconductor. The group III element source gas is supplied only when the organic nitrogen compound in the nitrogen source gas is supplied at a minimum supply molar ratio (R = Rmin). A method for crystal growth of a nitride-based compound semiconductor as described in 1. The nitride according to any one of claims 1 to 7, wherein a supply amount of an acceptor dopant source gas in the film forming process is varied in synchronization with a change in the supply molar ratio (R). Crystal growth method for a compound semiconductor. 9. The acceptor dopant source gas is supplied only when the organic nitrogen compound in the nitrogen source gas is supplied at a minimum supply molar ratio (R = Rmin). The crystal growth method of the nitride compound semiconductor as described. The method for growing a crystal of a nitride compound semiconductor according to any one of claims 1 to 9, wherein the organic nitrogen compound is a hydrazine compound. The crystal growth method for a nitride-based compound semiconductor according to claim 10, wherein the hydrazine-based compound is 1.1-dimethylhydrazine or tertiary butylhydrazine. 12. The crystal growth method for a nitride compound semiconductor according to claim 1, wherein the nitride compound semiconductor film is GaN, AlN, InN, BN, or a mixed crystal thereof. . A nitride-based compound semiconductor film grown by the method according to claim 1, wherein the oxygen concentration [O], the carbon concentration [C], and the hydrogen concentration in the film [H] is [O] ≦ 1 × 10 ¹⁷ cm ⁻³ , [C] ≦ 1 × 10 ¹⁷ cm ⁻³ , and [H] ≦ 5 × 10 ¹⁸ cm ⁻³ , respectively. Nitride compound semiconductor crystal.

Images