JP6271390B2 - Nitride semiconductor crystal growth method - Google Patents

Description

  The present invention relates to a nitride semiconductor crystal growth method for forming a crystal layer of AlN, which is a nitride semiconductor, by a molecular beam epitaxy method.

  Nitride semiconductors typified by GaN are widely used as materials for light-emitting diodes and high-power transistors in the short wavelength region. These nitride semiconductor layers are formed by vapor phase growth methods such as metal organic vapor phase (MOVPE) and molecular beam epitaxy (MBE), to form sapphire substrates, Si substrates, It is grown on a SiC substrate. In general, it is known that when a GaN layer is directly grown at a high temperature of about 1000 ° C. on the heterogeneous substrate, the crystal defect density of the GaN layer does not decrease. For this reason, in order to obtain a GaN layer having a low crystal defect density, various methods as described below are usually employed.

  First, there is a method in which a GaN layer or an AlN layer is grown at a low temperature of about 500 ° C., and then a GaN layer is grown at a high temperature of about 1000 ° C. (two-stage growth method).

  Second, there is a method of growing a GaN layer after growing an AlN layer at a high temperature.

  Third, there is a method of depositing an AlON buffer layer in which the composition of oxygen element (O) and nitrogen element (N) is changed on the surface of the substrate, and then growing a GaN layer on the buffer layer ( Patent Document 1).

  When a GaN layer is grown on a dissimilar substrate such as a sapphire substrate, Si substrate, or SiC substrate by the MOVPE method or the MBE method, the first and second methods are mainly employed. However, in general, the crystal defect density of a nitride semiconductor such as a GaN layer or AlN grown on a sapphire substrate by the MBE method using the first method and the second method is the same as that of the nitride semiconductor grown by the MOVPE method. It is inferior to the crystal defect density.

  On the other hand, the MBE method has an advantage that the residual impurity concentration is lower than the MOVPE method. For this reason, also in the MBE method, it is desired to grow a nitride semiconductor having a low crystal defect density on a sapphire substrate or the like. Here, there is a report that the growth of the nitride semiconductor layer by the third method described above is performed by the MBE method (see Non-Patent Document 1). According to the third method, a nitride semiconductor layer having good crystallinity can be obtained even by the MBE method by using an AlON buffer layer.

  Hereinafter, a crystal growth technique by MOVPE using the AlON buffer layer will be described. First, as shown in FIG. 4A, an AlON layer 202 in which the composition of an oxygen element (O) and a nitrogen element (N) is changed on a sapphire substrate 201 by using an electron cyclotron resonance (ECR) sputtering method. , And an AlN layer 203 are deposited at room temperature. The AlON layer 202 and the AlN layer 203 are collectively referred to as an AlON buffer layer. The thickness of this AlON buffer layer is about 20 nm.

  Next, as described above, the sapphire substrate 201 on which the AlON layer 202 and the AlN layer 203 are formed is unloaded from the ECR sputtering apparatus and newly transferred to the MOVPE growth apparatus. Next, a GaN layer 204 is grown at a high temperature of 1000 ° C. on the AlN layer 203 by MOVPE. According to this method, a high quality GaN layer 204 can be obtained.

Japanese Patent No. 4249184

T. Makimoto et al., "RF-MBE Growth of AlN on a Sapphire Substrate Using an AlON Buffer Layer", Materials Research Society Fall Meeting, T6.04, 2013. S. R. Lee et al., "Effect of threading dislocations on the Bragg peakwidths of GaN, AlGaN, and AlN heterolayers", Applied Physics Letters, vol.86, 241904, 2005.

  However, even if the above-described method using the AlON buffer layer is applied to the MBE method, when an AlN layer, which is one of nitride semiconductors, is grown, the crystallinity cannot be improved. There was a problem that a crystal layer could not be formed.

  The present invention has been made to solve the above-described problems, and it is intended to form an AlN crystal layer having better crystallinity using an AlON buffer layer by a molecular beam epitaxy method. Objective.

  The nitride semiconductor crystal growth method according to the present invention is a deposition method other than the molecular beam epitaxy method, and includes a first step of forming a first buffer layer made of AlON on a substrate and a deposition method other than the molecular beam epitaxy method. A second step of forming a second buffer layer made of AlN in contact with the first buffer layer, and supplying Ga or In onto the second buffer layer by a molecular beam epitaxy method, and the surface of the second buffer layer A third step of forming GaO or InO based on oxygen in the natural oxide layer formed in step 4, a fourth step of vaporizing GaO or InO formed on the second buffer layer by heating, After vaporizing GaO or InO, a fifth step of forming a crystal layer made of AlN on the second buffer layer by molecular beam epitaxy, and a third step, 4 process, the fifth step is carried out in the same apparatus. For example, in the first step and the second step, the first buffer layer and the second buffer layer may be formed by sputtering.

  As described above, according to the present invention, it is possible to obtain an excellent effect that an AlN crystal layer having better crystallinity can be formed using an AlON buffer layer by a molecular beam epitaxy method.

FIG. 1 is an explanatory diagram for explaining a nitride semiconductor crystal growth method according to an embodiment of the present invention. FIG. 2 shows a first sample in which an AlN crystal growth layer is formed on an AlON buffer layer via an AlO layer, (a), a second sample in which an AlN crystal growth layer is directly formed on the AlON buffer layer, and (b) Is a characteristic diagram showing an X-ray rocking curve with respect to the AlN (0002) plane. FIG. 3 shows a first sample in which an AlN crystal growth layer is formed on an AlON buffer layer via an AlO layer, (a), a second sample in which an AlN crystal growth layer is directly formed on the AlON buffer layer, and (b) ) Is a characteristic diagram showing an X-ray rocking curve with respect to the (1-102) plane. FIG. 4A is an explanatory diagram for explaining a crystal growth technique by MOVPE using an AlON buffer layer. FIG. 4B is an explanatory diagram for explaining a crystal growth technique by MOVPE using an AlON buffer layer.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory diagram for explaining a nitride semiconductor crystal growth method according to an embodiment of the present invention.

  First, in the first step S101, the first buffer layer 102 made of AlON is formed on the substrate 101. For example, the substrate 101 is a sapphire substrate whose main surface is a c-plane (0001). The first buffer layer 102 changes the composition of oxygen and nitrogen in the layer thickness direction, and the composition of nitrogen is larger than that of oxygen as the distance from the substrate 101 increases. Next, in the second step S102, a second buffer layer 103 made of AlN is formed on and in contact with the first buffer layer 102.

  Here, the deposition (formation) of the first buffer layer 102 and the second buffer layer 103 is performed by a deposition method other than the molecular beam epitaxy method. For example, the first buffer layer 102 and the second buffer layer 103 may be formed by sputtering. The total layer thickness of the first buffer layer and the second buffer layer 103 may be 20 nm.

  Next, in the third step S103, the molecular beam epitaxy method is used to irradiate the Ga or In molecular beam 104 on the second buffer layer 103 to supply Ga or In to form the surface of the second buffer layer 103. An oxide layer 105 made of GaO or InO is formed based on the oxygen of the natural oxide layer thus formed. The oxide layer 105 may be formed, for example, with a layer thickness of about 10 nm.

  Subsequently, in the fourth step S104, GaO or InO formed on the second buffer layer 103 is vaporized by heating, and at least a part of the oxide layer 105 is removed, and at least a part of the second buffer layer 103 is removed. To expose the surface.

  Next, in a fifth step S105, an AlN crystal 107a is grown on at least a part of the surface of the second buffer layer 103 by molecular beam epitaxy. At this time, an AlN layer 106 having poor crystallinity is formed on the remaining oxide layer 105. Subsequently, the growth of the AlN crystal 107a is continued by molecular beam epitaxy, and the crystal layer 107 made of AlN is formed on the second buffer layer 103 in the sixth step S106.

  Here, the crystallinity of the AlN crystal 107 a starting from at least a part of the surface of the second buffer layer 103 is good, and the growth rate is higher than that of the AlN layer 106 having poor crystallinity grown on the remaining oxide layer 105. Is fast. For this reason, the AlN layer 106 grown on the remaining oxide layer 105 is covered with the crystal layer 107 with good crystallinity as the growth of the AlN crystal 107a with good crystallinity started partially. . From the above, finally, a crystal layer 107 with good crystallinity is obtained.

  Here, the third step S103, the fourth step S104, the fifth step S105, and the sixth step S106 are performed in the same apparatus.

  By the growth method described above, the crystal layer 107 is grown on the second buffer layer 103 having no natural oxide layer, so that the crystal layer 107 can be obtained with good crystallinity. Further, since the growth is performed by the molecular beam epitaxy method, a state in which the residual impurity concentration is low can be obtained.

  In the above-described embodiment, GaO or InO formed on the basis of oxygen in the natural oxide layer from Ga or In supplied by the molecular beam epitaxy method is evaporated on the surface to form the second buffer layer 103. It is characterized by removing at least a part of the natural oxide layer (AlO).

  Here, the second buffer layer 103 formed by sputtering is not necessarily an AlN crystal. Since crystalline AlN is a stable substance, it is considered that it is not easily oxidized. On the other hand, the second buffer layer 103 that is not crystalline is not stable and is likely to be oxidized. Therefore, it is considered that an AlO layer formed by natural oxidation is formed on the surface of the second buffer layer 103 in the process of moving to a different apparatus (molecular beam epitaxy apparatus) after forming the second buffer layer 103. It is done.

  Here, it is known that Al—O in which the Al element and the O element are completely combined has a strong bond. However, the naturally oxidized AlO layer formed by oxidizing AlN in the air is in an amorphous state, and not all bonds are completely Al—O bonds. Further, AlN in which Al element and N element are combined also has a strong bond. When such AlN is oxidized, it may be in the form of N—Al—O. In addition, the Al—O bond in this compound is considered to be a weaker bond than a simple Al—O bond.

  In addition to these, it is considered that if a highly reactive metal In or metal Ga is supplied, a part of the Al—O bond can be converted into In—O or Ga—O. Moreover, it can be expected that these InO and GaO are evaporated by increasing the temperature. Thus, if at least a part of the oxide film 105 formed on the second buffer layer 103 can be removed, an AlN raw material is supplied to the removed part, thereby forming an AlN crystal nucleus with good crystallinity. be able to. If an AlN raw material is further supplied from this state, the crystal nuclei are bonded to each other, and finally, an AlN crystal layer 106 having a flat surface can be grown.

  Note that the higher the temperature at which InO and GaO are evaporated, the better. However, in practice, the temperature at which AlN is grown by the MBE method is generally about 700 ° C. to 800 ° C.

  Based on the knowledge of the inventors, it has been found that AlO formed on the surface of the second buffer layer 103 causes a decrease in crystallinity of AlN formed thereon, and based on this finding, By removing the AlO, as described in detail below, the present invention has led to the production of a nitride semiconductor crystal layer with better crystallinity.

  First, as described above, in the MOVPE method, a high-quality nitride semiconductor crystal layer with better crystallinity can be formed by using an AlON buffer layer. On the other hand, in the molecular beam epitaxy method, even if an AlON buffer layer is used, a high-quality nitride semiconductor cannot always be obtained. This is because, after the AlON buffer layer is formed, the AlN layer on the surface of the AlON buffer layer is oxidized in the air from the ECR sputtering device to the growth device by the molecular beam epitaxy method, and a natural oxide layer is formed. It is thought that it is to be done.

  It is considered that this natural oxide layer can be reduced and removed relatively easily by ammonia gas or hydrogen gas used in the MOVPE method. From this, it is considered that the Al—O bond formed by natural oxidation is weaker than the bond of the Al—O crystal. For this reason, in the MOVPE method, the oxide layer formed by being oxidized in the air is changed to an AlN layer that does not contain oxygen atoms, so that a high-quality nitride semiconductor can be grown. On the other hand, in the MBE method in which a nitride semiconductor is grown without using ammonia gas or hydrogen gas, if an oxide layer is formed on the surface of the AlN layer by being oxidized in the air, it cannot be easily removed. . As a result, it is considered that the characteristics of the nitride semiconductor grown on the AlON buffer layer having this oxide layer on the surface deteriorate.

  In Non-Patent Document 1, an AlN layer having a thickness of 10 nm is grown on an AlON buffer layer at a low temperature of 480 ° C., and an AlN crystal layer having a thickness of 900 nm is grown on the AlN buffer layer at a high temperature of 800 ° C. . In this technique, the crystallinity of the AlN crystal layer is improved as compared with the case where the AlN layer is grown directly on the AlON buffer layer. In this technique, since the AlN layer formed at a low temperature exists on the natural oxide layer formed on the surface of the AlON buffer layer, it is considered that the crystallinity of the AlN crystal layer is improved. . However, the characteristics of the AlN crystal layer of Non-Patent Document 1 are inferior to those of the AlN crystal layer grown by the MOVPE method.

  As a result of the various studies described above, the inventors removed the natural oxide layer (AlO) formed on the surface of the AlON buffer layer (second buffer layer 103) by molecular beam epitaxy, and then crystallized AlN. It was considered that an AlN crystal layer having better crystallinity can be formed by using the AlON buffer layer by growing the crystal.

  By the way, AlO cannot be vaporized at a temperature of about 800 ° C. at which the temperature can be increased in an apparatus for performing the molecular beam epitaxy method. On the other hand, if it is GaO or InO, it can be vaporized at a temperature of about 800 ° C. For this reason, in the present invention, the active metal Ga or metal In is supplied onto the second buffer layer 103 by the molecular beam epitaxy method, reacted with AlO to form GaO or InO, and these are vaporized. 2 AlO formed on the buffer layer 103 was removed.

  Hereinafter, the difference in crystallinity of the AlN crystal formed on the AlN buffer depending on the presence or absence of AlO will be described based on the measurement result of the actually produced sample.

[Sample preparation]
First, preparation of a sample will be described. First, using an ECR sputtering method, an AlON buffer layer is deposited at room temperature on a sapphire substrate whose main surface is c-plane (0001). In the ECR sputtering method, a metal aluminum target was used, and ECR plasma was generated with argon. In addition, oxygen gas and nitrogen gas were supplied into the deposition chamber, and an AlON buffer layer was formed on the sapphire substrate. This AlON buffer layer has a laminated structure in which an AlON layer in which the composition of O and N is changed and an AlN layer are laminated in this order from the sapphire substrate side. The AlON layer is a composition gradient layer in which the O composition ratio is gradually reduced from 100% from the sapphire substrate side. The total thickness of the AlON layer and the AlN layer was about 20 nm. Since each layer is formed under room temperature conditions, they are not necessarily crystallized.

  In addition, a sample was formed by further adding an AlO layer on the AlON buffer layer. A sample in which an AlO layer is further formed on the AlON buffer layer is taken as a first sample. A sample in which no AlO layer was formed was used as the second sample.

  Next, as described above, each of the first sample and the second sample is unloaded from the sputtering apparatus, and then loaded into the plasma molecular beam epitaxy apparatus, and AlN is crystal-grown. In a crystal growth method using a plasma molecular beam epitaxy apparatus, an AlN layer is grown by irradiating a heated substrate surface with Al molecules by an Al cell and supplying nitrogen gas activated by plasma to the substrate surface. The nitrogen gas flow rate was 2 ml per minute, and the high frequency power for generating plasma was 500 W.

  By the above, the 1st sample which formed the AlN crystal growth layer on the AlON buffer layer via the AlO layer, and the 2nd sample which formed the AlN crystal growth layer directly on the AlON buffer layer were produced. The thickness of the AlN crystal growth layer of the first sample is 900 nm, and the thickness of the AlN crystal growth layer of the second sample is 2000 nm.

[Characteristic evaluation]
Next, specific comparison was made for each of the first sample and the second sample. FIG. 2 is a characteristic diagram showing an X-ray rocking curve with respect to the AlN (0002) plane for the first sample and the second sample. 2A shows the rocking curve of the first sample, and FIG. 2B shows the rocking curve of the second sample. As shown in FIG. 2, it can be seen that the half width of the X-ray rocking curve is drastically reduced in the second sample (b) compared to the first sample (a).

  Next, FIG. 3 shows an X-ray rocking curve with respect to the (1-102) plane of AlN. FIG. 3 is a characteristic diagram showing an X-ray rocking curve with respect to the AlN (1-102) plane for the second sample and the first sample. FIG. 3 (a) shows the rocking curve of the first sample, and FIG. 3 (b) shows the rocking curve of the second sample. As shown in FIG. 3, the half width of the X-ray rocking curve is drastically reduced in the second sample (b) compared to the first sample (a).

  Thus, the screw dislocation density (NS) and the edge dislocation density (NE) can be calculated by measuring the half width of the X-ray rocking curve with respect to the AlN (0002) plane and the AlN (1-102) plane. Yes (see Non-Patent Document 2). The calculation results are shown in Table 1.

  The X-ray rocking curve of the AlN crystal growth layer in the first sample has a lot of noise because of poor crystallinity. On the other hand, in the second sample, the screw dislocation density is reduced to 1/10 or less and the edge dislocation density is reduced to 1/50 or less. Here, in the second sample, since the thickness of the AlN crystal growth layer grown at 800 ° C. is thick (2000 nm), the dislocation density may be reduced. However, within this film thickness range (900 nm to 2000 nm), the decrease in dislocation density due to the film thickness of the AlN layer is at most about 1/5. In Table 1, since it is confirmed that the dislocation density is further reduced, it is effective to remove the oxide layer (AlN layer) on the AlON buffer layer to grow a high-quality AlN layer. It was revealed experimentally for the first time.

  From the results of the above-described experiment, it has been clarified that a high-quality AlN layer can be crystal-grown by molecular beam epitaxy by removing AlO on the AlON buffer layer.

  As described above, in the present invention, the second buffer layer is formed on the surface of the second buffer layer by supplying Ga or In by irradiating the second buffer layer with Ga or In molecular beam by the molecular beam epitaxy method. Based on the oxygen in the natural oxide layer, GaO or InO is formed and heated to vaporize GaO or InO, so the crystal layer of AlN grows with the natural oxide layer removed. it can. As a result, according to the present invention, an AlN crystal layer with better crystallinity can be formed using the AlON buffer layer by molecular beam epitaxy.

  As a method for removing a natural oxide layer formed by being exposed to the air while being transported to the molecular beam epitaxy apparatus on the surface of the AlN layer constituting the upper layer of the AlON buffer layer, a method using ammonia, A method using hydrogen gas is conceivable. In these methods, it is necessary to use ammonia gas or hydrogen gas, and to perform waste gas treatment, which requires a large production cost.

  On the other hand, according to the present invention, since a molecular beam of Ga or In, which is generally used in a molecular beam epitaxy apparatus of a III-V compound semiconductor, is used, it can be implemented very easily and the manufacturing cost increases. Is not invited.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious. For example, not only a sapphire substrate but also a different substrate such as a Si substrate or a SiC substrate may be used.

  DESCRIPTION OF SYMBOLS 101 ... Substrate, 102 ... First buffer layer, 103 ... Second buffer layer, 104 ... Molecular beam, 105 ... Oxide layer, 106 ... AlN layer, 107 ... Crystal layer, 107a ... AlN crystal.

Claims (2)

A first step of forming a first buffer layer made of AlON on a substrate by a deposition method other than molecular beam epitaxy;
A second step of forming a second buffer layer made of AlN in contact with the first buffer layer by a deposition method other than molecular beam epitaxy;
Third step of supplying Ga or In onto the second buffer layer by molecular beam epitaxy and forming GaO or InO based on the oxygen in the natural oxide layer formed on the surface of the second buffer layer When,
A fourth step of evaporating the GaO or the InO formed on the second buffer layer by heating;
And after vaporizing the GaO or InO, a fifth step of forming a crystal layer made of AlN on the second buffer layer by molecular beam epitaxy,
The third step, the fourth step, and the fifth step are performed in the same apparatus. A nitride semiconductor crystal growth method, wherein: The nitride semiconductor crystal growth method according to claim 1,
In the first step and the second step, the first buffer layer and the second buffer layer are formed by a sputtering method.

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