JP2005340384A - Method for manufacturing compound semiconductor element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a compound semiconductor element in which an MgZnO based oxide element layer can be grown with high efficiency and high quality on a substrate of a different material, e.g. sapphire or SiC, by MOVPE. <P>SOLUTION: When a buffer layer 11 composed of MgZnO is formed on the major surface of a substrate 10 for growth comprising a sapphire single crystal substrate having a crystal a-axis normal to the major surface or an SiC single crystal substrate having a crystal c-axis normal to the major surface, a preliminary buffer layer 11S composed of an Mg<SB>x</SB>Zn<SB>1-x</SB>O microcrystal being dispersed to a substrate for growth is grown using O<SB>2</SB>gas as a source gas of O. Subsequently, a buffer layer body 11M composed of an MgZnO single crystal covering the entire surface of the substrate for growth is grown using an MgZnO microcrystal as a seed crystal and a nitrogen oxide as a source gas of O. The buffer layer body 11M is formed by depositing a first unit layer 11A and a second unit layer 11B different in growth temperatures alternately such that the first unit layer 11A touches the preliminary buffer layer 11S wherein the first unit layer 11A is grown at a lower temperature than that of the second unit layer 11B so that the degree of crystallization becomes lower than that of the second unit layer 11B. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing a compound semiconductor device.

JP-A-2001-68485 A II-VI group compound semiconductor element made of a mixed crystal of ZnO or MgO and ZnO (hereinafter collectively referred to as “MgZnO-based oxide”) is a wide alternative to an AlGaInN-based material. Widely applied to gap type light emitting elements (light emitting diodes or semiconductor laser elements), X-ray fluorescent elements such as light receiving elements, piezoelectric elements, and scintillators are being studied.

  Although ZnO-based oxides can be obtained by vapor phase growth in a vacuum atmosphere, it is difficult to obtain inexpensive and high-quality bulk single crystals, so heteroepitaxial growth using a different material substrate such as sapphire or SiC is adopted. It will be. However, although sapphire and SiC have a crystal system close to that of ZnO, their lattice constants are greatly different. Therefore, in order to epitaxially grow an element layer with good crystallinity, an appropriate buffer layer is provided between the substrate and the element layer. It is necessary to insert. In Patent Document 1, a ZnO buffer layer is grown on a sapphire substrate at a low temperature by a molecular beam epitaxy (MBE) method, then heat-treated at a high temperature, and an MgZnO single crystal layer serving as an element layer is formed thereon. A method of growing thicker than the buffer layer is disclosed. According to this document, it is stated that the entire surface of the buffer layer can be grown at a low temperature to flatten the layer surface, and the crystal quality of the MgZnO single crystal layer grown on the buffer layer can be improved.

  In Patent Document 1, in order to improve the quality of the buffer layer such as planarization, as described in paragraph 0034, a Zn-rich preliminary layer is grown by the MBE method, and then irradiated with an O radical beam. A final low temperature ZnO buffer layer is obtained by heat treatment. In other words, in Patent Document 1, a process of growing a preliminary layer with a non-stoichiometric composition is essential, and a crystal growth method that can be adopted is an MBE method that can cope with non-stoichiometric growth. Is essential. However, the MBE method has a drawback that the layer growth rate is low, and it is difficult to realize it with a uniform film thickness when dealing with an increase in area or a plurality of sheets, resulting in poor mass productivity.

  On the other hand, there is a metal-oxide vapor phase (MOVPE) method as a crystal growth method having a relatively high layer growth rate and excellent mass productivity. However, the MOVPE method is basically a method suitable for the growth of a layer having a stoichiometric composition, and a ZnO buffer layer having a stoichiometric composition has a MgZnO single crystal layer grown after the low temperature growth even if low temperature growth is adopted. Therefore, it is difficult to realize a sufficiently good crystal state in terms of flatness or crystal uniformity as the underlayer, and as a result, a MgZnO single crystal layer with good quality cannot be obtained. On the other hand, as described in Patent Document 1, even if the MOVPE method is intentionally slid in a condition that is effective only in the MBE method of growing a preliminary layer having a non-stoichiometric composition at a low temperature, crystals are formed on the substrate. It becomes easy to agglomerate as microcrystals, a substrate covering state is uniform and a flat ZnO buffer layer cannot be formed, and it is similarly impossible to obtain a MgZnO single crystal layer of good quality. About the above problem, when this inventor evaluated the ZnO single-crystal layer obtained by the said method by the photoluminescence (PL) characteristic, the blue fluorescence resulting from a defect is often found rather than the fluorescence resulting from a band edge. This is supported by the fact that stronger results can be obtained, and that when the electrical characteristics are evaluated by the hole measurement method, the hole mobility can be obtained only at the same level as that of the amorphous ZnO layer.

  The subject of this invention is providing the manufacturing method of the compound semiconductor element which can grow an MgZnO type oxide element layer on a dissimilar-material board | substrate, such as sapphire and SiC, by the MOVPE method with high efficiency and high quality.

Means for Solving the Problems and Effects of the Invention

In order to solve the above problems, a method for producing a compound semiconductor device of the present invention includes:
Mg _x Zn _1-x O (provided that, on the main surface of a growth substrate composed of a sapphire single crystal substrate with the crystal a axis as the main surface normal or an SiC single crystal substrate with the crystal c axis as the main surface normal) A buffer layer growth step of growing a buffer layer consisting of 0 ≦ x ≦ 1) by the MOVPE method;
An element layer growth step of epitaxially growing an element layer made of Mg _x Zn _1-x O (where 0 ≦ x ≦ 1) on the buffer layer by the MOVPE method;
The buffer layer growth step includes a preliminary buffer layer growth step of growing a preliminary buffer layer made of Mg _x Zn _1-x O microcrystals dispersed on a growth substrate using O ₂ gas as an O source gas, and the Mg layer A buffer layer body that is grown by using a nitric oxide as an O source gas, with a buffer layer body made of Mg _x Zn _1-x O single crystal covering the entire surface of the growth substrate using _x Zn _1-x O microcrystals as a seed crystal And a growth step.

In the manufacturing method of the present invention, an MgZnO-based buffer layer for growing an MgZnO-based single crystal element layer on a growth substrate made of sapphire or SiC single crystal is grown as a single layer as in Patent Document 1. First, prior to growing the buffer layer body, a preliminary buffer layer made of MgZnO microcrystals is grown using O ₂ gas as an O source gas, and MgZnO microcrystals contained in the preliminary buffer layer are grown. As a seed crystal, a buffer layer body made of an MgZnO single crystal covering the entire surface of the growth substrate is grown using nitrogen oxide gas as an O source gas. When O ₂ gas is used as the O source, a reaction precursor is easily formed in the gas phase by a gas phase reaction between the MO gas of the group II metal (eg, diethyl Zn) and O ₂ gas, and it is deposited on the substrate surface. Therefore, although the in-plane continuity of the layer is easily impaired, since it is microcrystallized in the gas phase, it can be formed as a high quality seed crystal on the main surface of the growth substrate. That is, the continuity of the grown MgZnO tends to be impaired, but high-quality microcrystals with good alignment with the substrate crystal and few crystal defects are easily obtained. Further, rotation within the crystal plane of the microcrystal group grown on the main surface of the growth substrate (a-axis in the case of sapphire, c-axis in the case of SiC) is less likely to occur, and the rotation direction in the crystal plane is less likely to occur. The lattice mismatch is suppressed, and the crystal orientation in the crystal axis direction can be strongly maintained.

  However, if the growth conditions of the preliminary buffer layer are applied to the growth conditions of the buffer layer body as they are, MgZnO crystals exist separately in the main surface of the substrate, and seed crystals of different sizes grow at different growth rates. As a result, a flat film cannot be obtained. Therefore, the preliminary buffer layer is limited to the formation of microcrystals serving as seed crystals, and the subsequent growth of the buffer layer body is switched to an O source gas suitable for maintaining the continuity of the layer in the in-plane direction. And grow. Specifically, nitrogen oxide gas is used as an O source gas for growing the buffer layer body, and at least the first part of the buffer layer body is grown. As a result, the entire main surface of the growth substrate can be covered with a single crystal buffer layer having good crystallinity, and the MOVPE method can be used as a base layer for a single crystal element layer to be grown later. Since a sufficiently good crystalline state can be realized, a high quality single crystal element layer can be obtained.

Also, in the present invention, the buffer layer body is not grown as a single layer at a time, but the first unit layer of low temperature growth and the second unit layer of high temperature growth are alternately stacked. It is characterized by growing points. In the present invention, a microcrystal group serving as a seed crystal is formed non-uniformly on the main surface of the growth substrate (sapphire or SiC) as a preliminary buffer layer, and this is viewed from the viewpoint of the base on which the buffer layer body is formed. As a result, unevenness occurs on the base surface as much as the microcrystal group is dispersed. However, if the buffer layer body to be grown thereafter is formed by alternating growth at low temperature / high temperature as described above, the flatness of the surface is greatly improved while maintaining the crystal state of the finally obtained buffer layer in good condition. Thus, a high-quality single crystal element layer with less unevenness (roughness) can be realized. Naturally, since the entire growth process of the buffer layer is performed by the MOVPE method having a high layer growth rate, it is excellent in mass productivity and extremely industrially effective.

  In the buffer layer body, the main surface of the substrate that is not covered with the seed crystal and the first unit layer are in direct contact with each other. By reducing the crystallinity, the ZnO-based oxide that forms the buffer layer and the growth layer are used. Even if there is a considerable difference in lattice constant between the substrate and the substrate, the effect of lattice mismatch can be absorbed. The first unit layer is crystallized by subsequent heat treatment (described later) or by thermal history when growing the second unit layer which is a high temperature layer. As the growth of the layer proceeds, the atoms constituting the first unit layer grow without intervening the process of heterogeneous nucleation and growth of crystals on the substrate as seen in the initial stage of layer growth. This is thought to be easy to rearrange while maintaining the alignment with the atomic arrangement of the industrial substrate. In addition, when the entire buffer layer is grown as a single low-temperature growth layer, mismatch stress with the substrate increases in proportion to the layer thickness, and misfit dislocations and the like are easily introduced in the crystallization process in heat treatment or the like. However, if the thickness of the first unit layer is sufficiently small, mismatch strain with the substrate does not increase so much, so that it is difficult to introduce misfit dislocations or the like in the crystallization process.

  However, the first unit layer is a thin layer that forms the base portion of the buffer layer body, and conversely, the thin unit layer is the thin buffer layer as a single low-temperature growth layer. This does not cause any problems. However, it alone does not have a thickness that can sufficiently reduce the effect of lattice mismatch with the lower growth substrate and that can flatten the unevenness of the spare buffer layer. On the other hand, since the first unit layer is crystallized by the thermal history applied later, if the thickness is increased forcibly to eliminate the unevenness, the influence of the matching stress field with the substrate is diluted near the layer surface. Thus, since recrystallization from the surface side of the layer easily proceeds, there arises a problem that crystals forming the layer rotate in the plane, and it becomes difficult to make the first unit layer into a single crystal. Therefore, by growing the second unit layer at a high temperature on the first unit layer (which has been crystallized due to the subsequent thermal history), the properties favorable as the layer growth base by the first unit layer are inherited. However, the layer thickness of the buffer layer is increased.

Note that Patent Document 1 discloses the concept that the buffer layer grows as a single layer, and in order to improve the quality of the buffer layer including the improvement of flatness, the stoichiometric ratio of the oxide forming the layer is set. In principle, a very special method of shifting to the Zn-rich side is adopted, and there is a background in which only MBE with low productivity is unsuitable for application to large areas or multiple sheets. In contrast, the present invention forms a buffer layer body using a seed crystal as a preliminary buffer layer after first growing a crystallite with good crystallinity by switching the group VI atom / group II atomic ratio. In addition, since the influence of the lattice mismatch between the buffer layer and the growth substrate is mitigated by utilizing the crystallization process of the first unit layer which is a low temperature growth layer, the buffer layer quality improvement mechanism is In principle, they are completely different, and even the oxides forming the layers have no obstacle to adopting the stoichiometric composition that is the easiest for vapor phase growth. In other words, in the production method of the present invention, the stoichiometric ratio of the oxide forming the layer does not have the necessity of having a troublesome non-stoichiometric composition in principle. Can be grown as a Mg _x Zn _1-x O layer having a stoichiometric composition suitable for the MOVPE method.

It is desirable to use N ₂ O (nitrous oxide) gas as the O source gas for growing the buffer layer body. When N ₂ O is used as the O source, the above gas phase reaction is suppressed, and the reaction on the substrate surface becomes more dominant, which is advantageous for the growth of the buffer layer body with improved continuity. Further, as will be described later, it becomes easy to form the buffer layer body as an amorphous layer.

  The preliminary buffer layer is preferably formed so that the area coverage of the main surface of the growth substrate is 1% or more and 50% or less. Whether the area coverage is less than 1% or more than 50%, the function as a seed crystal is sufficiently secured to obtain the buffer layer body as a single crystal layer continuous in the in-plane direction. I can't do that. The area coverage is more preferably 1% or more and 10% or less.

  Further, the growth temperature of the preliminary buffer layer is desirably set to 200 ° C. or higher and lower than 600 ° C. If the growth temperature is less than 200 ° C., the decomposition of MO gas (particularly diethyl Zn) is difficult to proceed, and the growth of microcrystals is hindered. On the other hand, when the growth temperature is 600 ° C. or higher, the reaction in the gas phase before the arrival of the substrate proceeds too much, and the re-evaporation process of ZnO from the substrate is likely to occur, so the probability of adhesion of microcrystals to the main surface of the substrate Decreases.

When the preliminary buffer layer is grown, the supply ratio of the group II element source gas and the group VI element source gas supplied into the reaction vessel is to promote the formation of microcrystals by the gas phase reaction described above. It is desirable to set it to 500 or more and 10,000 or less in the group VI atom / group II atom ratio. When the ratio is less than 500, the reaction precursor as the seed crystal cannot be sufficiently formed in the gas phase, and the seed crystal is difficult to be chemically adsorbed on the crystal surface. The reaction precursor inside becomes crystal grains completely and cannot be adsorbed on the crystal surface. In either case, a good seed crystal effective for the growth of the Mg _x Zn _1-x O layer cannot be obtained.

    The portion of the buffer layer body that is in contact with the spare buffer layer may be grown as a crystalline layer from the beginning, but is grown as an amorphous layer by lowering the growth temperature, and then the amorphous layer is crystallized at the crystallization temperature. When the method of heating and single crystallizing is adopted as described above, the flatness of the finally obtained buffer layer body can be further improved.

  The first unit layer is grown at a low temperature to lower the crystallinity, absorb the lattice mismatch between the layer and the substrate, and improve the consistency between the layer and the substrate. This can be achieved significantly by growing as a stratified layer. In addition, the first unit layer made of an amorphous layer is advantageous from the viewpoint of easily maintaining the in-plane continuity of the layer even if the thickness is small, and easily obtaining a uniform coating that does not aggregate in an island shape.

  The growth temperature of the first unit layer is desirably set to 200 ° C. or higher and lower than 600 ° C. When the temperature is lower than 200 ° C., there is a high possibility that the decomposition reaction of the organic metal that is the raw material gas of MOVPE does not proceed remarkably. Further, when the growth temperature is 600 ° C. or more, the decomposition reaction of the organometallic and the bonding reaction with O in the gas phase before the arrival of the substrate easily proceed, and the re-evaporation process of ZnO from the substrate becomes dominant. The probability of ZnO adhesion to the substrate is lowered, and it becomes difficult to form the first unit layer having a small layer thickness in a uniform state in the plane. In addition, the crystallization of the first unit layer during growth also proceeds easily. The growth temperature is more preferably set to 250 ° C. or higher.

  On the other hand, the growth temperature of the second unit layer is preferably set to 600 ° C. or higher in order to increase the crystallinity of the second unit layer during growth. On the other hand, when the growth temperature exceeds 1,100 ° C., the decomposition of the organometallic in the gas phase proceeds too rapidly, and desorption from the substrate surface becomes dominant, so that the layer growth rate decreases instead. Since the crystallinity of the resulting layer is also impaired, it is preferable to set it within a range of 1,100 ° C. or less. The growth temperature of the second unit layer is more desirably set in the range of 650 ° C. or more and 950 ° C. or less.

  In order to make the effect of the present invention remarkable, it is necessary to set the crystallinity of the first unit layer to be lower than that of the second unit layer at the time of growth. It is more desirable to set the growth temperature to 450 ° C. or lower. In particular, when the first unit layer is to be formed as an amorphous layer, the growth temperature is preferably set to 400 ° C. or lower.

  The thickness of the first unit layer is preferably set to 1 nm or more and 10 nm or less. When the layer thickness is less than 1 nm, in the stage where the first unit layer after growth is heated and crystallized, the aggregation of the layer is likely to proceed, and the continuity in the in-plane direction of the layer is lost. The crystallinity and flatness of the entire buffer layer finally obtained are also easily lost. On the other hand, if the thickness of the first unit layer exceeds 10 nm, recrystallization from the surface side of the layer tends to proceed as described above, causing problems such as rotation of the crystal forming the layer in the plane. It becomes difficult to monocrystallize the unit layer. The layer thickness of the first unit layer is more preferably 1 nm or more and 5 nm or less.

When the first unit layer is grown by the MOVPE method, when O ₂ is used as the O source, a reaction precursor is obtained by a gas phase reaction between a Group II metal organometallic gas (for example, diethyl Zn) and O ₂ gas. Since an object is formed in the gas phase and is deposited on the surface of the substrate, single-crystal microcrystals are formed on the surface, and the in-plane continuity of the layer is likely to be impaired. However, when N ₂ O is used as the O source, the above gas phase reaction is suppressed, and the reaction on the substrate surface becomes more dominant, so that the continuity of the first unit layer can be improved. It is also easy to make the first unit layer an amorphous layer.

  On the other hand, the thickness of the second unit layer is preferably set to 500 nm or less. As the second unit layer is grown at a high temperature, the crystallinity is good, but the in-plane continuity of the crystal tends to be lower than that of the first unit layer, and the layer thickness exceeds 500 nm. Surface irregularities tend to increase. On the other hand, the overall efficiency of the buffer layer forming process can be improved by forming the second unit layer to be thicker than the first unit layer, and preferably by setting the thickness to 10 nm or more. The layer thickness of the second unit layer is more preferably 10 nm or more and 100 nm or less.

  As described above, the second unit layer is more likely to be uneven on the surface of the layer than the first unit layer, and only forms one cycle of a set of the first unit layer and the second unit layer grown thereon. Then, it may be difficult to ensure sufficient flatness of the finally obtained buffer layer surface. In this case, it is possible to further improve the flatness of the surface of the obtained buffer layer, and finally the element layer obtained by forming the set by repeating a plurality of periods.

  The crystallization of the first unit layer can be advanced by the thermal history at the time of forming the second unit layer, but in order to improve the quality of the second unit layer, at the initial stage of its growth. Further, the crystallinity of the first unit layer as a base that is the basis of the crystallization needs to be sufficiently increased. Therefore, after the first unit layer is grown, a heat treatment is performed to increase the crystallinity of the first unit layer, and then a second unit layer is grown on the first unit layer. It is more desirable to do. In order to make this effect more prominent, it is desirable to perform the heat treatment at a temperature equal to or higher than the growth temperature of the second unit layer. Specifically, the heat treatment temperature is set to 600 ° C. or higher. It is desirable to set. When the heat treatment temperature is less than 600 ° C., the crystallinity of the first unit layer may not be sufficiently increased. On the other hand, considering the economy, the upper limit of the temperature of the heat treatment is about 1,100 ° C. More preferably, the heat treatment temperature is set in the range of 650 ° C. or more and 950 ° C. or less.

  As described above, since the second unit layer is grown at a higher temperature than the first unit layer, the probability of desorption of O from the grown layer is increased, and the O ratio (that is, the ratio of the VI group element) is increased. It tends to tilt to the lack of layer composition. Therefore, when the first unit layer and the second unit layer are grown by the MOVPE method, the supply ratio of the group II element source gas and the group VI element source gas supplied into the reaction vessel is set to group VI. When expressed in terms of atomic / group II atomic ratio, the second unit layer may be configured such that the supply ratio when growing the second unit layer is set higher than the supply ratio when growing the first unit layer. This is desirable from the viewpoint of suppressing O separation during growth. Specifically, the supply ratio when growing the first unit layer is preferably 500 or more and 3,000 or less, and the supply ratio when growing the second unit layer is 10,000 or more and 100,000. 000 or less is preferable.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 schematically shows a light-emitting element to which the present invention is applied in a laminated structure. The light-emitting element 1 includes a buffer layer 11, a first cladding layer (n-type in this embodiment) 34, and an active layer 33 on a first main surface of a sapphire substrate (growth substrate: may be a SiC single crystal substrate) 10. The second cladding layer (in this embodiment, p-type) 32 has a light emitting layer portion 24 laminated in this order. The layers 32 to 34 constituting the buffer layer 11 and the light emitting layer portion 24 are all Mg _a Zn _1-a O layers (0 ≦ a ≦ 1: below, also referred to as MgZnO: provided that the mixed crystal ratio a is within the range. As is clear, even though it is described as MgZnO, it includes the concept of each simple oxide of MgO and ZnO), and is formed by heteroepitaxial growth on the sapphire substrate 10. Note that the sapphire substrate 10 is insulative, and the first cladding layer 34 is exposed by cutting out a part of the light emitting layer portion 24, and the counter electrode 125 formed in the exposed portion and the second cladding layer 32 first. The light emitting layer portion 24 is energized to drive with the main electrode 122 covering a part of one main surface.

  For example, one or more of N, Ga, Al, In, Li, As, and P are added to the p-type cladding layer 32 as a p-type dopant. On the other hand, the n-type cladding layer 34 is inherently easy to be n-type due to O-deficiency of MgZnO, and can be non-doped n-type. However, n, such as B, Al, Ga, and In, is used for carrier concentration control. It is desirable to add the type dopant positively.

The active layer 33 has an emission wavelength determined by the mixed crystal ratio a of Mg _a Zn _1-a O constituting the active layer 33. When ultraviolet light emission with a wavelength of 280 nm to 400 nm is performed, it is selected within the range of 0 ≦ a ≦ 0.5. The height of the potential barrier formed between the cladding layers 33 and 34 is preferably about 0.1 eV to 0.3 eV for a light emitting diode and about 0.25 eV to 0.5 eV for a semiconductor laser light source. . This value can be determined by the mixed crystal ratio of both cladding layers.

  FIG. 2 shows the crystal structure of MgZnO, which has a so-called wurtzite structure. In this structure, oxygen atom layers and metal atom (Zn ions or Mg ions) layers are alternately stacked in the c-axis direction, and as shown in FIG. 2, the c-axis is along the layer thickness direction. Formed as follows. An oxide having a corundum structure has a lattice of oxygen (O) atoms having a hexagonal atomic arrangement, and an O atom (ion) layer and a metal atom (ion: Al in the figure) layer in the c-axis direction. Have a structure in which are alternately stacked. Therefore, a sapphire substrate having the substrate main axis as the a axis can be used as a growth substrate for heteroepitaxially growing an MgZnO type oxide having a wurtzite crystal structure in the orthogonal c axis direction.

  When the in-plane projection positions of the Zn atom as a cation and the O atom as an anion coincide, the Coulomb energy is minimized, and the heterointerface is also most energetically stable. Therefore, the epitaxial growth proceeds in a form as close as possible to the energy stable point. In this case, a crystal base plane (so-called A plane) having the a axis forming the main surface of the sapphire substrate as a normal line, and a crystal base plane having a normal axis having the c axis forming the main surface of the ZnO layer (so-called C plane) The lattice correspondence between and is as shown schematically in FIG. 6 (reference: Fons et al., Appl. Phys. Lett., 77 (2000) 1801). According to this, it turns out that a coincidence point exists periodically between the Zn atom in the C-plane closest packing direction of the ZnO layer and the O atom in the sapphire substrate A surface. Specifically, as is apparent from tracking in the horizontal direction of the drawing, an overlapping point of Zn atoms and sapphire O atoms is generated every two cycles of the arrangement unit on the regular hexagon consisting of seven Zn atoms. It can be seen (more precisely, there is a lattice mismatch of about 0.07%). As is apparent from the figure, the lattice correspondence described above is rotationally symmetric at an angle of 180 ° or less because the lattice coincidence points of Zn atoms and sapphire O atoms are asymmetrical in the in-plane orthogonal direction. Does not have sex. As a result, the in-plane rotation of the growing ZnO crystal is effectively prevented by the Coulomb interaction pinning effect at the coincidence point between the Zn atom and the sapphire O atom.

  In view of the above, the sapphire substrate is suitable as a substrate for growing ZnO by forming the buffer layer 11 for absorbing the lattice mismatch between the two prior to the growth of the element layer, as shown in FIG. Available to: In addition, for hexagonal SiC, a single crystal substrate with the crystal c axis as the main axis can be used as a growth substrate for MgZnO type oxide, but the lattice mismatch rate is 5.49%, which is not as high as that of a sapphire substrate. This is quite large, but the situation is almost the same. The buffer layer 11 is formed as an MgZnO-based oxide layer, which is a ZnO layer in this embodiment. Specifically, as shown in Step 7 in FIG. 5, Zn microcrystals serving as seed crystals are dispersedly formed on the first main surface of the sapphire substrate 10 (the size of each microcrystal is, for example, 1 nm to 10 nm). The buffer layer body 11M is formed by covering the entire surface of the first main surface of the sapphire substrate 10 including the spare buffer layer 11S. Further, the buffer layer body 11M is formed by alternately laminating low-temperature grown first unit layers 11A and high-temperature grown second unit layers 11B from the spare buffer layer 11S side. In FIG. 6, Zn atoms and sapphire O atoms other than the coincidence point have a lattice mismatch of up to 75%. Granular growth called is likely to be dominant. Therefore, it is difficult to grow a continuous ZnO-based oxide layer in the plane, but it is desirable from the viewpoint of forming a seed crystal with good lattice matching including in-plane rotation.

  Hereinafter, an example of the manufacturing process of the light-emitting element will be described. First, as shown in step 1 of FIG. 3, the sapphire substrate 10 is set in a reaction vessel. Next, as shown in the process, the buffer layer 11 made of MgZnO is epitaxially grown on the sapphire substrate 10. Then, on the buffer layer 11, as the light emitting layer portion 24 as an element layer, a first cladding layer 34 (layer thickness, for example, 50 nm), an active layer 33 (layer thickness, for example, 30 nm), and a second cladding layer 32 (layer thickness, for example, 50 nm). ) In this order. The epitaxial growth of each of these layers can be performed by the MOVPE method as described above. Since MgZnO-based oxides are generated by a chemical reaction in the gas phase, the obtained MgZnO-based oxides have a stoichiometric composition (however, the macroscopicity of O deficiency inevitably generated in the layer formation process) The effect on composition is considered negligible). The temperature in the reaction vessel is adjusted by a heating source (in this embodiment, an infrared lamp) in order to promote a chemical reaction for layer formation. The following can be used as the main raw material of each layer.

-Oxygen component source gas: Although oxygen gas can be used, it is desirable to supply it in the form of an oxidizing compound gas from the viewpoint of suppressing an excessive reaction with an organic metal described later. Specifically, N ₂ O, NO, NO ₂ , CO and the like. In this embodiment, N ₂ O (nitrous oxide) is used.
Zn source (metal component source) gas: dimethyl zinc (DMZn), diethyl zinc (DEZn), etc. In this embodiment, diethyl zinc (DEZn) is employed.
Mg source (metal component source) gas: biscyclopentadienyl magnesium (Cp ₂ Mg), etc.

Moreover, the following can also be used as a p-type dopant gas;
Li source gas: normal butyl lithium, etc.
-Si source gas: silicon hydride such as monosilane;
C source gas: hydrocarbon (eg, alkyl containing one or more C);
-Se source gas: hydrogen selenide, etc .;
-As source gas: arsine, etc .;
P source gas: phosphine, etc.

Furthermore, one or more group III elements such as Al, Ga, and In can function as a good p-type dopant by co-addition with N, which is a group V element. The following can be used as the dopant gas:
Al source gas; trimethylaluminum (TMAl), triethylaluminum (TEAl), etc .;
Ga source gas; trimethylgallium (TMGa), triethylgallium (TEGa), etc .;
In source gas; trimethylindium (TMIn), triethylindium (TEIn), etc.
When N is used as a p-type dopant together with a metal element (Ga), a gas serving as an N source is supplied together with an organometallic gas serving as a Ga source when performing vapor phase growth of the p-type MgZnO layer. . For example, in the present embodiment, N ₂ O used as an oxygen component source functions as an N source.

On the other hand, group III elements such as B, Al, Ga, and In can function as n-type dopants when used alone. As dopant gas, about Al, Ga, and In, what was demonstrated by the term of the p-type dopant can be used similarly. As for B, for example, diborane (B ₂ H ₆ ) can be used.

Each of the above source gases is appropriately diluted with a carrier gas (for example, nitrogen gas) and supplied into the reaction vessel. Depending on the mixed crystal ratio of each layer, the flow rate ratio of the organometallic gas MO, which is a group II element Mg source and Zn source, is controlled for each layer by a mass flow controller MFC or the like. Further, the flow rates of N ₂ O and p-type dopant source gas, which are oxygen component source gases, are also controlled by the mass flow controller MFC.

The growth of the buffer layer 11 which is the main part of the present invention is performed as follows. First, prior to layer growth, the sapphire substrate 10 is sufficiently annealed in an oxidizing gas atmosphere. The oxidizing gas can be selected from any of O, CO, and N ₂ O, but N ₂ O is used in the present embodiment because it is shared with an oxygen component source gas during layer growth described later. The annealing temperature is preferably 750 ° C. or higher (lower temperature than the melting point of the substrate) and a holding time of 30 minutes or longer in the MOVPE reaction vessel. However, if the substrate surface can be sufficiently cleaned by wet cleaning or the like, the annealing time may be shortened. When the annealing process is completed, the substrate temperature is set to 250 to 350 ° C. (350 ° C. in the present embodiment) in order to suppress the occurrence of defects and the like while maintaining the oxidizing gas atmosphere. To lower.

Next, in order to grow the preliminary buffer layer, O ₂ gas as an oxygen component source gas (Group VI element source) is circulated at a first flow rate in the reaction vessel, and in this state, by lamp heating, sapphire The temperature of the substrate 10 is raised to a first growth temperature set to 200 ° C. or higher and 600 ° C. or lower, desirably 250 ° C. or higher and 400 ° C. or lower.

When the first growth temperature is reached and stabilized, DEZn, which is an organometallic gas (Group II element source), is supplied into the reaction vessel, and as shown in Step 1 of FIG. 5, a preliminary buffer layer made of ZnO microcrystals Grows 11S. The area coverage of the main surface of the sapphire substrate 10 with ZnO microcrystals is 1% to 50%, preferably 1% to 10% (for example, 5%). Further, the supply ratio 1 of the organometallic gas supplied into the reaction vessel and the oxygen component source gas (here O ₂ gas) is 500 or more and 10,000 in terms of the group VI atom / group II atom ratio. Hereinafter, it is desirably set to a value of 2,000 or more and 5,000 or less.

Next, when the first unit layer 11A reaches a specified thickness, the organometallic gas remains DEZn, and the O source gas is switched to N ₂ O gas to grow the buffer layer body 11M. When O ₂ gas is used as the O source when the preliminary buffer layer 11S is grown, a reaction precursor is formed in the gas phase by a gas phase reaction between the MO gas of group II metal (eg, diethyl Zn) and O ₂ gas. 7 and is deposited on the surface of the substrate, the continuity of the growing layer is likely to be impaired as shown in Step A of FIG. 7, but the alignment with the substrate crystal is good and the number of crystal defects is small. Conversely, high-quality microcrystals are easily obtained. Further, as shown in FIG. 8, the crystallite group (11S) grown on the main surface of the growth substrate 10 is less likely to rotate in the c-plane, and there is a lattice mismatch in the rotation direction within the c-plane. In addition to being suppressed, the crystal orientation in the c-axis direction can be strongly maintained. On the other hand, when N ₂ O gas used for the growth of the buffer layer body 11M is used as the O source gas, as shown in FIG. 9, the microcrystal group (11S) is likely to rotate in the c plane, and the buffer layer This is not suitable as a seed crystal for obtaining the main body 11M as a single crystal.

As described above, the microcrystal contained in the spare buffer layer 11S is used as a seed crystal, and the buffer layer main body 11M is switched to N ₂ O gas and grown. Thereafter, according to the sequence and process shown in FIGS. 4 and 5, the buffer layer body 11M is alternately grown with the first unit layer 11A as the low temperature layer and the second unit layer 11B as the high temperature layer. First, N ₂ O gas as an oxygen component source gas (Group VI element source) is circulated in the reaction vessel at a first flow rate, and in this state, the temperature of the sapphire substrate 10 is set to 200 ° C. or higher and 600 ° C. by lamp heating. The temperature is raised to a first growth temperature T1 set to not higher than ° C., desirably not lower than 250 ° C. and not higher than 400 ° C.

When the first growth temperature T1 is reached and stabilized, an organometallic gas (group II element source) is supplied into the reaction vessel, and the first part of the buffer layer 11 is formed as shown in step 1 of FIG. The first unit layer 11A ′ is grown as, for example, an amorphous layer (“′” is given to the reference sign to indicate that it is a layer before crystallization). The layer thickness of the first unit layer 11A ′ is 1 nm or more and 10 nm or less, preferably 1 nm or more and 5 nm or less. Moreover, in FIG. 4, the supply ratio 1 of the organometallic gas supplied into the reaction vessel and the oxygen component source gas (N ₂ O gas in this case) is a value represented by a group VI atom / group II atomic ratio. A relatively small value R1 of 200 to 3,000, preferably 1,000 to 2,000 is set. The reason why the group VI atom ratio is set to be relatively small is that, since the growth temperature is low, the separation of O atoms from the deposited first unit layer 11A is relatively difficult to occur.

  Returning to FIG. 4, when the first unit layer 11A reaches a specified thickness, the supply of the organometallic gas is stopped, and then the temperature of the sapphire substrate 10 is set to 600 ° C. or higher and 1,100 ° C. or lower, preferably 650 ° C. The temperature is raised to a heat treatment temperature T2 set at 900 ° C. or lower and held for a certain period of time to crystallize the amorphous first unit layer 11A ′ as shown in Step B of FIG. The first unit layer is represented by reference numeral 11A without “′”). As shown in the upper diagram of FIG. 7, the first unit layer 11A formed as an amorphous layer does not dynamically crystallize during the layer growth, but is uniform in a state where the layer growth itself is completed. Since crystallization is performed by atomic rearrangement from an amorphous state, the in-plane mismatch stress distribution after crystallization is uniform, and the layer thickness is small, so the mismatch stress level itself is also small. As a result, as shown in the lower diagram of FIG. 7, the first unit layer 11A maintains good alignment with the atomic arrangement in spite of a large lattice constant difference from the substrate 10, and the crystal quality and flatness are improved. Formed as a high layer. Note that the first unit layer 11A ′ before the heat treatment is not completely amorphous, and may be formed as a partially crystallized layer (for example, within a crystallization rate of 50% or less).

  When the above heat treatment is completed, the temperature of the sapphire substrate 10 is set to a second growth temperature T2 set to 600 ° C. or more and 1,100 ° C. or less, desirably 650 ° C. or more and 900 ° C. or less. Gas (Group II element source) is supplied into the reaction vessel, and the first second unit layer 11B ′ forming a part of the buffer layer 11 is grown as shown in Step C of FIG. This layer is formed as a crystallized layer from the beginning because of the high growth temperature, and has a high growth rate, and thus takes the form of the main part of the buffer layer 11. At this time, since the crystal quality of the first unit layer 11A forming the base is high and the flatness is also good, the second unit layer 11B is homogeneous and has few misfit dislocations and the like even when the growth rate is increased. Can be formed as a high quality layer.

Since the second unit layer 11B is grown at a higher temperature than the first unit layer 11A ′, the probability of O desorption from the grown layer is increased, and the O ratio (that is, the VI group element ratio) is deficient. In FIG. 4, the supply ratio of the organometallic gas supplied into the reaction vessel and the oxygen component source gas (N ₂ O gas in this case) is the same as that of the first unit layer 11A ′ described above. The value R2 is set larger than the value R1 when growing. Specifically, it is set to 10,000 or more and 100,000 or less, preferably 20,000 or more and 50,000 or less in terms of the group VI atom / group II atom ratio. By setting the ratio of the group VI atoms large, the separation of O atoms from the deposited second unit layer 11B can be effectively suppressed.

  In FIG. 4, the heat treatment temperature T2 for crystallizing the first unit layer 11A ′ and the second growth temperature T2 for growing the second unit layer 11B are set to the same temperature in the above embodiment. However, it is also possible to set a heat treatment temperature T2 ′ higher than the second growth temperature T2 in the temperature range of 1,000 ° C. or lower, preferably 950 ° C. or lower, as indicated by a one-dot chain line in the figure. is there.

  The second unit layer 11B has good crystallinity because of the high temperature growth, but the in-plane continuity of the crystal tends to be lower than that of the first unit layer 11A. Therefore, as shown in FIG. 8, when the thickness of the second unit layer 11B is excessively increased (for example, more than 500 nm), the unevenness of the layer surface increases, and the flatness of the buffer layer 11 finally obtained is increased. Damaged. From this viewpoint, the layer thickness of the second unit layer 11B is set to 10 nm to 500 nm, preferably 10 nm to 100 nm. In addition, since the second unit layer 11B is more likely to be uneven on the surface of the layer than the first unit layer 11A, the flatness of the finally obtained buffer layer 11 can be improved only by forming one set of both layers. In some cases, it cannot be secured sufficiently. Therefore, as shown in FIG. 9, by forming the set by repeating a plurality of periods, the concave portion of the second unit layer 11B preceding the first unit layer 11A is filled in the formation process of the first unit layer 11A, and thus, The flatness of the buffer layer 11 can be further improved.

  Specifically, as shown in FIG. 4, when the second unit layer 11B reaches the target growth thickness, the supply of the oxygen component source gas is continued and the supply of the organometallic gas is stopped. The temperature is lowered again to the first growth temperature T1, and Steps 1 to 3 described above are repeated as in Steps 4 to 6 in FIG. 5 to obtain the first unit layer 11A and the second unit layer 11B. Form a pair. In order to obtain a sufficient flatness, the number of formation periods of the set is selected from 2 to 10 cycles, more preferably from 2 to 5 cycles.

  When the formation of the buffer layer 11 is completed as described above, the layers 34, 33, and 32 forming the light emitting layer portion 24 are grown on the buffer layer 11 by the MOVPE method as in Step 3 of FIG. By adopting the above steps, the buffer layer 11 has a flatness on the outermost surface and a good crystal quality, so that the flatness and crystal quality of the resulting light emitting layer portion 24 are also improved, and excellent light emission characteristics are exhibited. Become. When the growth of the light emitting layer portion 24 is completed, as shown in FIG. 1, a part of the active layer 33 and the p-type MgZnO layer 32 is partially removed by photolithography or the like to form the main electrode 122 and the counter electrode 122. Thereafter, if the substrate 10 is diced, the light emitting device 1 is obtained. The light extraction is performed mainly from the transparent sapphire substrate 10 side.

  In the above embodiment, the buffer layer 11 is formed as a ZnO layer, but may be formed as a mixed crystal layer with MgO. Further, although the light emitting element is illustrated as an object to be applied to the present invention, the compound semiconductor element to which the present invention can be applied is not limited to the light emitting element, and a light receiving element such as a photodiode or a phototransistor, a piezoelectric element, a scintillator, or the like. It can also be applied to X-ray fluorescent elements.

FIG. 10 is a scanning electron microscope observation image of the surface of the ZnO layer grown by the conventional method (magnification: 7,500 times). Specifically, by using the MOVPE method using DEZn as the organometallic gas and N ₂ O as the O source gas, the group VI atom / group II atomic ratio is set to 2,000, and the thickness is reduced at a low temperature of 350 ° C. A ZnO buffer layer having a thickness of 10 nm was directly formed on the sapphire substrate. Thereafter, the growth temperature was raised to 800 ° C., and a ZnO layer was grown on the buffer layer with a thickness of 0.5 μm without flowing a dopant gas. As is apparent from the image, the unevenness is very large, and the coverage is also extremely deteriorated such that a part of the substrate surface is exposed. In addition, when the layer was evaluated by photoluminescence (PL) characteristics, white fluorescence having a broad peak near a wavelength of 500 nm, which seems to be caused by defects, was strongly generated in addition to fluorescence from 380 nm caused by the band edge of ZnO. It was. Furthermore, the hole mobility measured by the hole measurement method was about 10 to 20 V / cm · s, and only the same characteristics as the amorphous ZnO layer were obtained.

FIG. 11 is an atomic force microscope topography image of the surface of the ZnO layer grown by the method of the present invention. Specifically, by using a MOVPE method using DEZn as an organometallic gas and O ₂ gas as an O source gas, the group VI atom / group II atomic ratio is set to 2,000 and ZnO is cooled at a low temperature of 350 ° C. A preliminary buffer layer consisting of 10% was formed on the sapphire substrate. Subsequently, the O source gas is switched to N ₂ O gas, the group VI atom / group II atomic ratio is set to 3,000, the first unit layer is grown to a thickness of 10 nm at a low temperature of 300 ° C., and then DEZn The second unit layer is formed by supplying DEZn again by raising the temperature to 800 ° C., heat-treating for 30 minutes, setting the group VI atom / group II atomic ratio to 10,000 at the same temperature, The process of growing to a thickness of 100 nm was repeated two cycles to form a buffer layer body (total thickness 220 nm). Thereafter, the growth temperature was raised to 800 ° C., and a non-doped ZnO layer was grown to a thickness of 0.5 μm on the buffer layer. As is apparent from the image, ZnO grows uniformly in a layered manner, and the flatness is very excellent. Further, when the layer was evaluated by photoluminescence (PL) characteristics, only fluorescence from 380 nm due to the band edge of ZnO was seen, and almost no blue fluorescence was generated. Furthermore, the mobility of electrons by the hole measurement method showed a mobility of about 80 V / cm · s, and it was confirmed that a single crystal level thin film could be obtained without any problem by the MOVPE growth method.

The schematic diagram which shows the light emitting element which is an example of the compound semiconductor element used as the application object of this invention in a lamination | stacking state. The figure which shows the crystal structure of ZnO typically. FIG. 2 is a process explanatory diagram for growing each layer of the light emitting element of FIG. 1 by MOVPE. The figure which illustrates and illustrates the growth sequence of a buffer layer. Explanatory drawing which shows an example of the growth process of a buffer layer. The figure which shows typically the crystal mismatch of a MgZnO type | system | group oxide and a sapphire substrate. The figure which shows typically the function of a reserve buffer layer with a crystal | crystallization matching state with a buffer layer main body. Effect illustration of the use of O ₂ gas as O source gas in growing a preliminary buffer layer. Figure problems explained of using the N _{2 O} gas as O source gas in growing a preliminary buffer layer. The observation image by the scanning electron microscope of the ZnO layer grown by the process of the comparative example. The surface topography image by the atomic force microscope of the ZnO layer grown by the process of the Example.

Explanation of symbols

1 Light emitting device (compound semiconductor device)
10 Sapphire substrate (element substrate)
11 buffer layer 11S spare buffer layer 11M buffer layer body 11A first unit layer 11B second unit layer 24 light emitting layer part (element layer)

Claims (13)

Mg _x Zn _1-x O (provided that, on the main surface of a growth substrate composed of a sapphire single crystal substrate with the crystal a axis as the main surface normal or an SiC single crystal substrate with the crystal c axis as the main surface normal) A buffer layer growth step of growing a buffer layer consisting of 0 ≦ x ≦ 1) by the MOVPE method;
An element layer growth step of epitaxially growing an element layer made of Mg _x Zn _1-x O (where 0 ≦ x ≦ 1) on the buffer layer by a MOVPE method;
The buffer layer growth step includes a preliminary buffer layer growth step of growing a preliminary buffer layer made of Mg _x Zn _1-x O microcrystals dispersed on the growth substrate using O ₂ gas as an O source gas; Using the Mg _x Zn _1-x O microcrystal as a seed crystal, a buffer layer body made of Mg _x Zn _1-x O single crystal covering the entire surface of the growth substrate is grown using nitrogen oxide as an O source gas. A buffer layer body growth step, wherein the buffer layer body has a first unit layer and a second unit layer having different growth temperatures such that the first unit layer is in contact with the preliminary buffer layer. The first unit layer is formed by alternately laminating, and the first unit layer has a lower temperature than the second unit layer so that the degree of crystallinity during the crystal growth is lower than that of the second unit layer. Characterized by growing in A method of manufacturing a compound semiconductor device. The method of manufacturing a compound semiconductor device according to claim 1, wherein the buffer layer is grown as a stoichiometric Mg _x Zn _1-x O layer. 3. The method of manufacturing a compound semiconductor device according to claim 1, wherein N ₂ O gas is used as an O source gas for growing the buffer layer body. 4. The said preliminary buffer layer is formed so that the area coverage of the said main surface of the said board | substrate for a growth may be 1% or more and 50% or less, The Claim 1 thru | or 3 characterized by the above-mentioned. A method for manufacturing a compound semiconductor device. 5. The method of manufacturing a compound semiconductor device according to claim 1, wherein a growth temperature of the preliminary buffer layer is set to 200 ° C. or higher and lower than 600 ° C. 6. When the preliminary buffer layer is grown, the supply ratio of the group II element source gas and the group VI element source gas supplied into the reaction vessel is 500 to 10,000 in terms of the group VI atom / group II atom ratio. The manufacturing method of the compound semiconductor element of any one of Claim 1 thru | or 5 set to these. The method of manufacturing a compound semiconductor device according to claim 1, wherein the first unit layer is grown as an amorphous layer. The first unit layer and the second unit layer are grown by metal organic vapor phase epitaxy, and the growth temperature of the first unit layer is set to 200 ° C. or higher and lower than 600 ° C. 8. The method of manufacturing a compound semiconductor device according to claim 1, wherein a growth temperature of the unit layer is set to 600 ° C. or more and 1,100 ° C. or less. 9. The method of manufacturing a compound semiconductor device according to claim 8, wherein a set of the first unit layer and the second unit layer grown thereon is repeated a plurality of periods. After the first unit layer is grown, the second unit layer is grown on the first unit layer after performing a heat treatment for increasing the crystallinity of the first unit layer. The method for producing a compound semiconductor device according to claim 1, wherein: 11. The method of manufacturing a compound semiconductor device according to claim 10, wherein the heat treatment is performed at a temperature equal to or higher than a growth temperature of the second unit layer. The method of manufacturing a compound semiconductor device according to claim 11, wherein the temperature of the heat treatment is set to 600 ° C. or more and 1,100 ° C. or less. When the first unit layer and the second unit layer are grown by the MOVPE method, the supply ratio of the group II element source gas and the group VI element source gas supplied into the reaction vessel is set to group VI. When represented by an atomic / group II atomic ratio, the supply ratio when growing the second unit layer is set higher than the supply ratio when growing the first unit layer. The method for producing a compound semiconductor device according to claim 1, wherein: