JP2008053703A - AlN LAYER AND AlGaN LAYER, AND MANUFACTURING METHOD OF THEM - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high quality AlN crystal layer, and a high quality Al<SB>x</SB>Ga<SB>1-x</SB>N crystal layer of large composition ratio of Al, both formed on a ZnO substrate. <P>SOLUTION: An Al<SB>x</SB>Ga<SB>1-x</SB>N layer, where x satisfies 0.6≤x≤1.0, is grown on the ZnO substrate. In the Al<SB>x</SB>Ga<SB>1-x</SB>N layer, a half-value total width a<SB>1</SB>depending on an angle of X-ray diffraction intensity to a growth surface satisfies 0.001°≤a<SB>1</SB>≤0.5°. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention _relates to Al x _{Ga 1-x} N layer, more particularly of good crystallinity _Al x _{Ga 1-x} N layer that can be used for the light emitting device or the like.

In recent years, research and development of group III-V nitride semiconductors represented by In _x Al _y Ga _(1-xy) N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1) have advanced. The light emission efficiency of light emitting devices such as light emitting diodes and laser diodes has been dramatically improved.

In _x Al _y Ga _(1-xy) N including GaN belongs to the hexagonal system and has been formed mainly by epitaxial growth on the c-plane of a substrate such as sapphire. In a structure in which a quantum well layer made of a mixed crystal of In _x Ga _(1-x) N (0 <x ≦ 1) is stacked as an active layer on a GaN layer, a layer structure for a blue / green LED or a next-generation DVD laser is used. Used or promising.

Group III-V nitride semiconductor In _x Al _y Ga _(1-xy) N has different crystal characteristics such as lattice constant and band gap depending on x and y. A thin film growth method is expected. For example, AlGaN-based compound semiconductor layers having a relatively large x even in AlN and Al _x Ga _1-x N are expected to be useful as materials for ultrashort-wave ultraviolet light-emitting devices due to the size of their band gaps. Similarly, a high quality crystal thin film has been demanded.

Recently, instead of sapphire, an attempt has been made to grow a group III-V nitride layer on a ZnO substrate on which a GaN nitride has a lattice constant close to that of a GaN substrate and is manufactured at a lower cost and a larger diameter. ing. Since a ZnO substrate easily reacts with a III-V nitride at a high temperature, a pulsed laser deposition (PLD method) has been proposed as a growth method capable of lowering the growth temperature. For example, Patent Document 1 (International Publication WO2005 / 006420) describes that GaN or In _x Ga _1-x N (0 ≦ x ≦ 0.4) was grown on a ZnO substrate by a PLD method. ing. However, there is no description about the growth of the AlGaN compound semiconductor layer. Non-Patent Document 1 (Proceedings of the 66th JSAP Academic Lecture Meeting (Autumn 2005) 7a-X-14) uses Al _0.26 Ga _0.74 alloy as a group III target. It has been reported that AlGaN is epitaxially grown on ZnO at room temperature by the PLD method. However, since the film actually grown does not become larger than the Al composition ratio in the Al _0.26 Ga _0.74 alloy used for the target, the Al composition of the AlGaN-based thin film grown in Non-Patent Document 1 is It was only a small one.

Conventionally, the higher the Al composition ratio, the larger the lattice constant difference from ZnO. In addition, Al has a low vapor pressure and a low migration property, so a good quality AlN layer and a large Al composition ratio (that is, a large It was considered difficult to form an Al _x Ga _1-x N layer (with _x ) on the ZnO substrate.
International Publication WO2005 / 006420 Proceedings of the 66th JSAP Academic Lecture Meeting (Autumn 2005) 7a-X-14

As described above, there has been no report that a high-quality crystal layer of Al _x Ga _1-x N having a large AlN and Al composition ratio has been formed on a conventional ZnO substrate, and a method suitable for crystal growth is also known. There wasn't.

An object of the present invention is to provide a high-quality crystal layer of Al _x Ga _1-x N having a large Al composition ratio and AlN formed on a ZnO substrate.

A further object of the present invention is to provide a method for producing AlN having high quality crystallinity and an Al _x Ga _1-x N layer having a large Al composition ratio.

  The present invention relates to the following items.

1. An Al _x Ga _1-x N layer (where x is a number satisfying 0.6 ≦ x ≦ 1.0) grown on a ZnO substrate,
The full width at half maximum a ₁ of the angle dependency of the X-ray diffraction intensity with respect to the growth surface is
An Al _x Ga _1-x N layer satisfying 0.001 ° ≦ a ₁ ≦ 0.5 °.

2. 2. The Al _x Ga _1-x N layer according to ₁ above, which is formed at a growth temperature of less than 500 ° C.

3. 3. The Al _x Ga _1-x N layer according to 1 or 2 above, wherein the surface orientation of the ZnO substrate is a low index surface.

4). 3. The Al _x Ga _1-x N layer according to 1 or 2 above, wherein the plane orientation of the ZnO substrate is any one of {0001} plane, {1-100} plane, and {11-20} plane. .

5. 5. A laminated structure comprising the Al _x Ga _1-x N layer according to any one of 1 to 4 above and another layer formed on the layer.

6). A method of forming an Al _x Ga _1-x N layer (where x is a number satisfying 0.6 ≦ x ≦ 1.0) on a ZnO substrate,
(A) a step of preparing a ZnO substrate; and (b) an Al element, or an Al element and a Ga element are intermittently supplied to the growth surface, an N element is supplied, and the growth temperature is less than 500 ° C. A method for producing an Al _x Ga _1-x N layer, comprising a step of growing an Al _x Ga _1-x N crystal on a ZnO substrate surface.

7). In the step (b), growth is performed by intermittently exciting a target containing Al element (when x = 1) or a target containing Al element and Ga element (when x ≠ 1). 6. The method for producing an Al _x Ga _1-x N layer according to 6.

8). 8. The method according to claim 7, wherein there is substantially no change in the composition of the target during the growth of the Al _x Ga _1-x N crystal.

9. When Al composition x = 1, AlN is used as the target,
7 or 8 above, wherein when Al composition x ≠ 1, a mixture of AlN and GaN or a mixed crystal of AlN and GaN is used as the target, or AlN and GaN are used as separate targets, respectively. The method described.

  10. 10. The method according to 9 above, wherein the target is provided in the form of a sintered body or mixed crystal.

11. When Al composition x = 1, Al metal is used as the target,
9. The method according to 7 or 8 above, wherein when the Al composition x ≠ 1, Al metal and Ga metal are used as separate targets.

  12 The excitation of the target is performed by laser light irradiated intermittently, and the irradiation stop period of the laser light is between 0.001 second and 10 seconds, the method of.

  13. The method according to any one of 6 to 12, wherein the growth temperature is in the range of 20 ° C to 320 ° C.

14 The Al _x Ga _1-x N layer (provided that x is a number satisfying 0.6 ≦ x ≦ 1.0), which is grown by the method described in any one of 6 to 13 above. ).

15. 15. A laminated structure comprising the Al _x Ga _1-x N layer described in 14 above and other layers formed on this layer.

ADVANTAGE OF THE INVENTION According to this invention, the high quality crystal layer of AlxGa1 _{-xN with} a large AlN and Al composition ratio (large _x ) formed on the ZnO substrate can be provided.

Further, according to the present invention, it is possible to provide a method for producing an AlN layer having high quality crystallinity and an Al _x Ga _1-x N layer having a large Al composition ratio (large _x ).

  According to the inventor's study, in order to form a high-quality crystal layer, in addition to adopting a PLD method capable of growing at a low temperature, the Al composition of the growth layer is high and uniform throughout the growth process. It is also important to use a target that can be grown rapidly. According to the present invention, a mixed crystal layer having a desired composition ratio can be stably grown.

(Explanation of symbols such as surface orientation)
First, a method of expressing the crystal plane orientation and the axial direction used in this specification will be described. The crystal plane orientation and axial direction are described by the Miller index. In the hexagonal system, there is a notation method using three exponents, but here, a notation method using four commonly used indexes is adopted. The hexagonal Miller index will be described with reference to FIG. Three (a ₁ , a ₂ , a ₃ directions) in the regular hexagonal plane, and one (c direction) index in a direction perpendicular to the plane called the c-axis. The a ₁ axis, the a ₂ axis, and the a ₃ axis are 120 ° from each other and have the same length. The c-axis orthogonal to these is not equal in length to the a-axis group. The orientation in a completely regular hexagonal plane can be specified by only two of the a ₁ axis, the a ₂ axis, and the a ₃ axis, but another axis is introduced to maintain symmetry. So they are not independent of each other. One parallel plane group is represented as (ijkl (El)), which is the distance from the origin point to cut the first sheet surface counted from the origin a ₁ axis, a ₂ axis, a _3-axis, the c-axis Are a ₁ / i, a ₂ / j, a ₃ / k, and c / l (el), respectively. Since the a ₁ , a ₂ , and a ₃ axes are redundant coordinate systems included in the regular hexagonal plane, i, j, and k are not independent of each other and i + j + k = 0 always holds. Of the four indices, i, j, and k have rotational symmetry, but l (el) is independent.

  In this specification, the plane orientation and the crystal orientation are expressed as follows in accordance with a general notation method in crystallography.

  Individual plane orientations are represented by round brackets (), and curly brackets {} are used to represent a set of equivalent plane orientations. Equivalent plane orientation refers to the plane orientation that can be reached by all symmetry operations that the crystal system allows. For example, {1-100} is an expression that collectively represents all planes equivalent to (1-100), and (1-100) is reached by a rotation operation with the c-axis as a rotation axis (10-10). ), (01-10), (−1100), (−1010), (0-110), and a total of 6 planes, {0001} is expressed as two (0001) and (000-1) Express the face.

  The crystal orientation (crystal axis) is expressed by a set of indices that are the same as the indices of the planes perpendicular to it. Individual crystal orientations are represented by square brackets [], and a set of equivalent orientations uses key brackets <>.

  Further, as generally used, {0001} may be referred to as a c-plane, and <0001> may be referred to as a c-axis. Typical plane orientations of the hexagonal system are expressed as c-plane (0001), a-plane (11-20), m-plane (1-100), r-plane (10-12).

(Explanation of low index surface)
In the present specification, in particular, when the plane orientation is expressed as {ijkl}, the plane orientation satisfying | i | + | j | ≦ 2 and | l (el) | ≦ 2 is excluded ({0000}). ) Is expressed as a low index surface.

(Explanation of X-ray diffraction)
Nitride crystals grown by heteroepitaxial growth with lattice mismatch have a mosaic property, and columnar crystal grains having slightly different crystal orientations gather to form a film. This mosaicism is a reflection of crystal defects such as dislocations and point defects observed at the atomic level. Columnar crystal grains with few dislocations are bonded to each other through a grain boundary, and dislocations exist at the grain boundary. On the other hand, the X-ray diffraction method is widely used as a technique for macroscopically evaluating the mosaic property. A crystal having a mosaic property includes regions having slightly different plane orientations in the crystal. In such a reciprocal space, such a crystal has a reciprocal lattice point spreading in a spherical direction centered on the origin. Therefore, if the extent of this spherical direction is measured by X-ray diffraction, the degree of fluctuation of the plane orientation can be evaluated.

  For this purpose, the angle (2θ) between the incident X-ray and the detector is fixed at the diffraction peak position, and the angle (ω) between the sample crystal and the incident X-ray is scanned near the diffraction condition. In this way, as shown in FIG. 2, the spread of spherical reciprocal lattice points centered at the origin in the reciprocal lattice space can be evaluated as the distribution of diffraction intensity. Such measurement is called X-ray rocking curve (XRC) or ω mode (scan) measurement. The width of the measured diffraction peak reflects the degree of fluctuation of the plane orientation, and the full width at half maximum as a quantitative index. (Half width at half maximum) is used. The full width at half maximum measured in this way indicates that the smaller the value, the higher the quality of the sample crystal and the crystallographically superior crystal.

  The mosaic property of a thin film crystal is related to the growth direction and can be divided into fluctuation of the crystal axis growth direction called tilt and rotation of the crystal about the crystal growth direction called twist. By appropriately selecting the diffractive surface and the rotation axis of the crystal, it is possible to separate tilt and twist in X-ray rocking curve measurement. When a nitride layer is formed on substrates having different crystal structures or lattice constants, the tilt and the twist are not necessarily the same size. Depending on the type of substrate, the growth method and growth conditions of the nitride layer, the magnitude of the tilt is extremely small, but the magnitude of the twist may be extremely large. Such a nitride layer has remarkably poor light emission characteristics and electrical characteristics, and it is difficult to produce a practical light-emitting device using such a nitride layer. When the nitride layer is used for a light emitting device, it is desirable that each of the tilt and the twist is small.

(Explanation of a ₁ and b ₁ as defined herein)
a ₁ represents the full width at half maximum of the angle dependence of the X-ray diffraction intensity with respect to the growth surface when X-ray rocking curve measurement is performed with the growth surface as the diffraction surface. This expresses the tilt quantitatively.

b ₁ is the crystal plane perpendicular to the growth plane when the X-ray rocking curve is measured with the crystal plane perpendicular to the growth plane orientation as the diffraction plane and the growth direction as the rotation axis of the crystal. This represents the full width at half maximum of the angle dependency of X-ray diffraction intensity with respect to X-rays incident from a direction parallel to the growth surface. The obtained full width at half maximum b ₁ of the angle dependency quantitatively represents a twist with the crystal growth direction as the rotation axis in the crystal mosaic property.

(Description of Embodiment of the Present Invention)
As described above, the present invention is an Al _x Ga _1-x N layer grown on a ZnO substrate (where x is a number satisfying 0.6 ≦ x ≦ 1.0).

  When the crystal is grown on the ZnO substrate, the crystal information of the substrate is inherited, and a crystal layer having the same plane orientation as that of the substrate is easily formed. In order to succeed the crystal information of the substrate more accurately, it is desirable that the in-plane dangling bonds are dense, and it is preferable that the nominal main surface of the ZnO substrate has a low index surface. In addition, since GaN-based nitrides are easily oriented in the c-axis direction, it is more preferable to use the c-plane as a nominal main surface in order to grow a crystal layer having excellent crystallinity.

  On the other hand, it is well known that non-polar and semipolar planes can avoid a decrease in light emission efficiency due to the spontaneous polarization occurring in the polar face and the strained electric field of the InGaN quantum well. More preferably, of the low index planes, the nonpolar plane and the semipolar plane are the nominal main plane of the ZnO substrate.

  The nominal main surface is a surface including the case where it is not necessarily completely perpendicular to the target surface orientation, and the axis perpendicular to the main surface is not deviated from the target surface orientation. Good. The range of the deviation is allowed up to 15 °.

The Al _x Ga _1-x N layer grown on this ZnO substrate has a full width at half maximum a ₁ of the angle dependence of the X-ray diffraction intensity with respect to the growth surface,
(Formula 1) It satisfies 0.001 ° ≦ a ₁ ≦ 0.5 °.

As described above, a ₁ represents the tilt component of the mosaic property of the crystal on the growth surface, and Equation 1 indicates that the crystal has a very small mosaic property and is a high-quality crystal.

Preferably, the full width at half maximum a ₁ is further expressed by (Expression 2) 0.001 ° ≦ a ₁ ≦ 0.2 °.
More preferably 0.001 ° ≦ a ₁ ≦ 0.1 °
Satisfied.

In addition, with respect to the crystal mosaic twist component, preferably,
0.01 ≦ b ₁ ≦ 3.0
More preferably 0.01 ≦ b ₁ ≦ 2.0.

_X indicating the Al composition of the Al _x Ga _1-x N layer of the present invention is:
0.6 ≦ x ≦ 1.0
And preferably
0.8 ≦ x ≦ 1.0
Meet.

  One preferred embodiment is when x = 1.0, that is, when the composition is AlN.

The thickness of the Al _x Ga _1-x N layer is preferably 5 nm to 10,000 nm, more preferably 5 nm to 500 nm, and still more preferably 10 nm to 300 nm. In general, a nitride crystal on a different substrate has a threading dislocation density that decreases as the growth film thickness increases, and a high-quality crystal having a narrow full width at half maximum of an X-ray rocking curve can be obtained. The mechanism of this phenomenon is not necessarily clear, but the direction of threading dislocations changes with the film thickness, which is due to the disappearance of dislocations, the formation of dislocation loops, and the arrival of some of the dislocations at the ends of the crystal. It is considered. However, increasing the growth film thickness in order to reduce the dislocation density has a problem that the material cost increases and the manufacturing throughput is limited.

The Al _x Ga _1-x N layer of the present invention is a high-quality nitride crystal having a narrow full width at half maximum of the X-ray rocking curve even if it is an extremely thin crystal film of 300 nm or less. This means that it has extremely high crystallinity from the growth start surface (interface with the substrate). When a nitride layer grown on a conventional heterogeneous substrate shows good crystallinity in the case of a thick film, the crystallinity in the range from the growth start surface to, for example, 50 nm (or 100 nm) may be observed. None of them had good crystallinity. On the other hand, in the present invention, even if the crystallinity in the range of, for example, 50 nm from the growth start surface (this reference distance may be 30 nm or 100 nm, etc., and is thinner than the reference distance, the film thickness) is considered. in it defined as satisfying the _{a 1.}

In the present invention, the Al _x Ga _1-x N layer was formed at a growth temperature of less than 500 ° C., and the growth was started directly on the substrate. The growth temperature is preferably 450 ° C. or lower, more preferably 320 ° C. or lower. The growth temperature can be determined in consideration of the practical growth rate allowed for each actual process. For example, the growth temperature is 0 ° C. or higher, preferably 10 ° C. or higher, more preferably room temperature (20 ° C. to 30 ° C. ) The above temperature is selected. Usually, the preferred growth temperature is in the range of room temperature to 320 ° C, and particularly preferably in the range of room temperature to 100 ° C.

Further, another layer can be further stacked on the Al _x Ga _1-x N layer to form a stacked structure. This “other layer” is a layer having the same composition as the Al _x Ga _1-x N layer of the present invention grown on the substrate (hereinafter referred to as a second Al _x Ga _1-x N layer). In that case, it is also preferable to form the Al _x Ga _1-x N layer of the present invention relatively thin. For example, it can be 200 nm or less. The second Al _x Ga _1-x N layer is preferably grown at a growth temperature higher than the growth temperature of the layer directly grown on the substrate, and is formed at a growth temperature of 400 ° C. to 1300 ° C., for example.

  The “other layer” may be any layer in addition to the above layers, and the material may include an insulator, a semiconductor portion, or a metal portion. Also good. The forming method is hydride vapor phase epitaxy (VPE) method, metal organic vapor phase epitaxy (MOVPE) method, plasma chemical vapor deposition (CVD) method, thermal CVD method, molecular beam epitaxy (MBE) method, sputtering method, vapor deposition method. Any of a widely known film forming method such as PLD method may be used. The formed structure may be a single layer or a multilayer structure, and may be a device structure such as a so-called electronic device or light-emitting device.

However, in order to fully demonstrate the function of the high quality Al _x Ga _1-x N layer of the present invention, the layer to be directly laminated can be improved in quality by using the layer of the present invention as a base. Possible layers are preferred. In particular, at least a part of the layer directly in contact with the layer of the present invention is preferably a lattice-matching layer. For example, a group III-V nitride layer having a different composition or formed by a different growth method is preferable, and examples thereof include crystal layers such as AlN, GaN, AlGaN, and AlInGaN. Particularly preferred are AlN and AlGaN. At this time, the growth method is formed by a film formation method selected from the group consisting of a PLD method, a VPE method, a CVD method, an MBE method, a sputtering method, a vapor deposition method, and a combination of two or more of these methods. Is preferred.

It should be noted that the III-V nitride layer of the invention and the other layers laminated thereon are usually different in film quality when the manufacturing method is different even if the main composition or the target composition is the same. . That is, the amount of impurities contained between the two layers is different, or the interface can be confirmed from the impurity profile of the interface between the two layers, so that the two layers can usually be distinguished and recognized. For example, if the Al _x Ga _1-x N layer of the invention is grown by the PLD method and the other layers are grown by a normal MOVPE method or the like, the level of impurities (for example, hydrogen, carbon, oxygen) to be mixed is Since they are different, it is possible to distinguish the layer of the present invention from the “other layers”.

The Al _x Ga _1-x N layer and the laminated structure of the present invention described above can be used in various applications, and are provided as a substrate provided for forming a device structure thereon, and a device structure thereon. As a part of the device in which the device is formed, as a part of the device in which at least a part of the device structure is formed in a part of the layer, and other forms can be taken. Further, the substrate such as ZnO used for the layer growth may exist as it is, or may be removed at an appropriate stage after the layer growth. That is, the final product or intermediate product provided with the Al _x Ga _1-x N layer of the present invention may or may not have the substrate used during the growth.

In particular, a light emitting diode or semiconductor laser having a high dislocation density, a high quality crystallinity, and a high light emission efficiency by epitaxially growing a light emitting device structure having an active layer on the Al _x Ga _1-x N layer. Can be realized.

〔Production method〕
Next, a manufacturing method will be described.

(Process for preparing the substrate)
First, a ZnO substrate is prepared. The substrate surface orientation is as already described. It is preferable to prepare a sufficiently flattened substrate as the substrate or to flatten the substrate as part of the manufacturing process. The planarization is performed by CMP (Chemical Mechanical Polishing), but it is also preferable to anneal at a high temperature of 800 ° C. to 1600 ° C. Specifically, the polished ZnO substrate is heat-treated in a high temperature oven controlled to a temperature of 800 ° C. or higher and surrounded by a ZnO sintered body in a box shape. In this case, the ZnO substrate only needs to be surrounded by the ZnO sintered body, and it is not essential to enclose the entire ZnO substrate by the surrounding sintered body. Further, for example, a crucible made of a ZnO sintered body may be prepared and a ZnO substrate may be installed therein. Since the purpose of surrounding ZnO is to suppress the escape of Zn having a relatively high vapor pressure, it may be surrounded by a material containing Zn in addition to the ZnO sintered body. As an example of a material containing Zn, for example, a ZnO single crystal may be used, or a Zn plate may be used. By heat-treating the ZnO substrate based on this condition, a flat ZnO substrate can be obtained at the atomic layer level where atomic steps are formed.

(Al _x Ga _1-x N layer growth process)
In the next growth step of the Al _x Ga _1-x N layer, a group III element is intermittently supplied to the growth surface and an N element is also supplied, and the Al _{x is} applied to the surface of the ZnO substrate at a growth temperature of less than 500 ° C. A Ga _1-x N crystal is grown. By growing on the substrate at a temperature of less than 500 ° C., even if the substrate is ZnO, the interface reaction between the substrate and the Al _x Ga _1-x N layer does not occur, so the interface reaction layer is not formed. When the growth temperature is 500 ° C. or higher, good quality crystals are often not obtained.

  The growth temperature is more preferably 450 ° C. or lower, and more preferably 320 ° C. or lower. The growth temperature can be determined in consideration of the practical growth rate allowed for each actual process. For example, the growth temperature is 0 ° C. or higher, preferably 10 ° C. or higher, more preferably room temperature (20 ° C. to 30 ° C. ) The above temperature is selected. Usually, the preferred growth temperature is in the range of room temperature to 320 ° C, and particularly preferably in the range of room temperature to 100 ° C.

  In the present invention, it is presumed that when the raw material is intermittently supplied to the substrate, the migration of atoms that have reached the growth surface is promoted and a high-quality crystal grows.

  It is preferable to grow while maintaining a single crystal structure on the growth surface, and this can be monitored by observing the RHEED image of the growth surface. Accordingly, the growth temperature, growth rate, radical concentration (for example, the output for generating rf plasma is adjusted), supply cycle (adjustment of supply period, supply suspension period), etc. are appropriately selected so that the single crystal structure is maintained. It is preferable to adjust.

  If the growth rate is too high, atoms that have reached the substrate surface cannot migrate sufficiently. In particular, in the case of low-temperature growth, the kinetic energy for migration cannot be supplemented by the thermal energy of the substrate temperature, so that the crystal quality is degraded. Accordingly, the growth rate is preferably less than 300 nm / hour, for example. More preferably, it is 200 nm / hour or less, and most preferably 160 nm / hour or less. Further, when the growth rate is too slow, the amount of impurities taken up increases, and particularly in the case of low-temperature growth, impurities in the growth chamber are more easily adsorbed than in high-temperature growth, so that the crystal quality is lowered. Accordingly, the growth rate is preferably greater than 10 nm / hour, for example, and more preferably 30 nm / hour or more.

As a condition regarding the period when the group III element is intermittently supplied, when the rest period between the supply and the supply is too short, atoms reaching the substrate surface cannot sufficiently migrate and the crystal quality is deteriorated. On the other hand, if the pause period is too long, the growth rate is lowered and the growth is interrupted for a long time, so that the amount of impurities incorporated in the growth film increases and the crystal quality is lowered. Accordingly, the period during which the supply is stopped is preferably in the range of 0.001 second to 10 seconds, more preferably in the range of 0.005 seconds to 1 second, and particularly preferably in the range of 0.01 seconds to 0.5 seconds. A range of 0.02 seconds to 0.1 seconds is most preferred. The supply period is appropriately selected from the range of 1 × 10 ⁻¹³ seconds to 10 seconds, for example, so as to be within the above growth rate range.

  In order to supply the group III element intermittently, typically, a method in which energy is intermittently applied to the target and excited into a plasma state is preferable. A typical method is the PLD method. In the PLD method, energy is given by intermittent pulse laser irradiation to excite material atoms into a plasma state and supply the substrate to the substrate. Therefore, even if the substrate is at a low temperature, the kinematics required for migration on the substrate surface The atom has energy and is suitable for low temperature growth.

  In the present invention, it is particularly preferable to use a target that does not fluctuate while the Al composition of the growth layer remains high during crystal growth. Specifically, a group III element nitride is preferably used as the target. When the Al composition x = 1, that is, when growing AlN, it is preferable to use AlN as a target. When the Al composition x ≠ 1, AlN and GaN can be used as targets (separate targets) (multi-target), but it is preferable to use a mixture or a mixed crystal in terms of process. These nitrides may be in any form such as single crystal, polycrystal, and sintered body, but are usually easy to use in the form of a sintered body, especially when a mixture of AlN and GaN is used. The use of a sintered body is preferable because of its simple process. When nitride is used, the target composition does not change with the progress of growth even in the form of a mixture, and therefore the composition of the growing crystal thin film becomes constant. In addition, the composition in the film can be predicted.

  Further, a group III element metal can also be used as the target. When the Al composition x = 1, that is, when AlN is grown, Al metal can be used as the target. When Al composition x ≠ 1, it is preferable that Al metal and Ga metal are provided as separate targets, that is, multi-targets, and Al and Ga are set independently so as to obtain a predetermined plasma density. Irradiation is performed with two laser beams under the following conditions. When Al composition x ≠ 1, if an alloy of Al metal and Ga metal is used as a target, the Al composition in the growing layer is generally higher than the Al composition in the alloy due to the difference in vapor pressure between Al and Ga. In order to increase the Al composition layer, it is necessary to make the Al composition of the target considerably large in order to grow a high Al composition layer. When such a target is used, a desired high composition layer is initially formed, but Ga that is contained in a small amount in the target is preferentially reduced, and the target Al composition remains unchanged. Will change rapidly. Therefore, the Al composition of the growth layer also varies in the growth direction, and the crystallinity is lowered because the crystal phase in the layer is not single.

  Therefore, particularly when the Al composition x ≠ 1, it is preferable to use a nitride as a target in order to form a high composition AlGaN layer having excellent crystallinity.

  Further, in order to intermittently excite the material atoms, a pulsed particle bundle can be replaced with a laser, and electrons, cluster ions, protons, and neutrons can be used.

  As for the supply of the N element (group V element) to the growth surface, typically, nitrogen gas is excited and supplied as an ionic state and / or a radical state. In particular, it is preferable to supply such that the concentration is high. It is presumed that the active species in the radical state are effective for high-quality crystal growth. When a metal target is used as the target for the group III element, an element in an ionic state and / or a radical state in which nitrogen gas is excited is supplied as the N element. On the other hand, when a nitride such as AlN is used as a target for a group III element as described above, the N element is supplied from the nitride. It is not essential to supply in the radical state. However, supplying active species in an ionic state and / or radical state in which nitrogen gas is excited on the growth surface is also preferable in terms of supplementing the N element which tends to be insufficient. Even when the nitrogen gas is not excited separately, at least the target is preferably excited in the nitrogen gas.

  As another growth method for intermittently supplying the raw material, it is also possible to use a migration enhanced epitaxy (MEE) method or a flow rate modulation method in which a group III raw material and a group V raw material are alternately intermittently supplied.

The PLD method will be described in detail below. Here, for example, a case where an Al _x Ga _(1-x) N layer is formed on a ZnO substrate 11 as a substrate using a PLD apparatus 30 as shown in FIG. 3 will be described as an example.

  The PLD device 30 includes a chamber 31 that forms a sealed space in order to keep the pressure and temperature of the gas filled therein constant. In the chamber 31, the ZnO substrate 11 and the target 32 are arranged to face each other. Here, the target 32 is gallium metal, aluminum metal, GaN (single) crystal, AlN (single) crystal, AlGaN (single) crystal, GaN sintered body, AlN sintered body, AlGaN sintered body, or the like. Depending on the growth conditions, it is possible to appropriately select the target species and single or multi-target film formation.

  The PLD apparatus 30 also includes a KrF excimer laser 33 that emits a high-power pulse laser having a wavelength of 248 nm. The pulse laser beam emitted from the KrF excimer laser 33 is adjusted by the lens 34 so that the focal position is in the vicinity of the target 32, and is provided on the side surface of the chamber 31 and disposed in the chamber 31 through the window 31a. Incident at an angle of about 30 ° to the 32 surface.

  Further, the PLD apparatus 30 includes a gas supply unit 35 for injecting nitrogen gas into the chamber 31 and a radical source 36 for radicalizing the nitrogen gas. The nitrogen radical source 36 excites the nitrogen gas discharged from the gas supply unit 35 once using high frequency to form nitrogen radicals, and supplies the nitrogen radicals into the chamber 31. In addition, between the chamber 31 and the gas supply part 35, in order to control the adsorption | suction state to the ZnO board | substrate 11 in relation to the wavelength of a nitrogen radical gas molecule and a pulse laser beam, the adjustment valve for controlling the density | concentration of gas 36a is provided.

  Further, the PLD device 30 includes a pressure valve 37 and a rotary pump 38 for controlling the pressure in the chamber 31. The pressure in the chamber 31 is controlled by the rotary pump 38 so as to be a predetermined pressure in, for example, a nitrogen atmosphere, taking into account the PLD process for forming a film under reduced pressure.

  In addition, the PLD device 30 includes a rotating shaft 39 that rotates the target 32 in order to move the point irradiated with the pulse laser beam.

  In the PLD apparatus 30 described above, the pulsed laser light is intermittently irradiated while the target 32 is rotationally driven through the rotating shaft 30 while the chamber 31 is filled with nitrogen gas. As a result, the temperature of the surface of the target 32 can be rapidly increased, and ablation plasma containing Ga atoms and Al atoms can be generated. Ga atoms and Al atoms contained in this ablation plasma move to the ZnO substrate 11 while gradually changing the state while repeating collision reaction with nitrogen gas and the like. Then, the particles containing Ga atoms, In atoms, and Al atoms that have reached the ZnO substrate 11 are diffused as they are on the growth surface on the ZnO substrate 11 to form a film in the most stable state of lattice matching.

  In the present invention, the conditions are preferably selected so that the single crystal grows from the initial growth stage while suppressing the interface reaction between the substrate and the growth layer.

  Next, in order to manufacture the laminated structure of the present invention, that is, the laminated structure having the III-V nitride layer and the “other layer” formed on this layer, the material of the “other layer” is used. In addition, it may be appropriately formed on the III-V nitride layer by a known method. Even when the III-V nitride layer of the present invention is formed by the PLD method, it is not necessary to form the “other layers” by the PLD method. For example, according to the material of the layer such as an insulator, a semiconductor, a metal, etc. Well-known film forming methods such as hydride VPE method, MOVPE method, plasma CVD method, thermal CVD method, MBE method, sputtering method, and vapor deposition method can be used. In one embodiment of the present invention, a film forming method selected from the group consisting of a PLD method, a VPE method, a CVD method, an MBE method, a sputtering method, a vapor deposition method, and a combination of two or more of these methods is employed. The formed structure may be a single layer or a multilayer structure, and may be a device structure such as a so-called electronic device or light-emitting device. Even when the “other layer” is a device structure or a part of the device structure, the device structure may be formed by a known method.

In the case where the “other layers” that are in direct contact with the Al _x Ga _1-x N layer of the present invention are formed by the PLD method, the conditions are preferably set at a higher temperature than the formation conditions of the layers of the present invention.

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

(Example 1)
A ZnO substrate having a (0001) plane as a main surface was planarized by annealing at 1250 ° C. for 3.5 hours. Atomic layer steps were observed in the atomic force microscope image after planarization.

Next, the planarized ZnO substrate was introduced into the PLD apparatus, and an AlN layer was formed. The target was composed of an AlN sintered body. The target 32 was disposed so as to be parallel to the growth surface of the ZnO substrate 11 (see FIG. 3). An RF radical source was used as a nitrogen source at 400 W, and the growth pressure was 4.5 × 10 ⁻⁶ Torr.

  The pulse frequency of the pulse laser beam emitted from the KrF excimer laser was 10 Hz, the irradiation time per pulse was 20 ns, and the laser power at the time of target irradiation was 0.125 W. Further, the substrate temperature of the ZnO substrate was kept at room temperature and grown to a thickness of 40 nm. The growth rate was 78 nm / hour.

  In this growth process, it was found that the RHEED image always maintained a streak pattern from the start of growth, and epitaxially grown while maintaining high crystallinity from the initial stage of growth. From the change in RHEED intensity during growth shown in FIG. 4, it is confirmed that AlN has grown by layer-by-layer. FIG. 5 shows an atomic force microscope image of the surface of the AlN layer grown to 40 nm. The AlN surface has a flat step and terrace structure at an atomic level reflecting the surface of the ZnO substrate. In addition, no interface layer was observed between the ZnO and AlN layers.

FIG. 6 shows an X-ray diffraction spectrum. A ₁ representing the mosaic mosaic tilt component of the crystal was 0.02 ° (72 arcsec). Further, b ₁ representing a crystal mosaic twist component was 1.64 ° (5915 arcsec).

(Example 2)
In Example 1, except that the substrate temperature was 320 ° C., Example 1 was repeated to grow an AlN layer. FIG. 7 shows an atomic force microscope image of the surface of the AlN layer grown to 40 nm. The AlN surface has a flat step and terrace structure at an atomic level reflecting the surface of the ZnO substrate. In addition, no interface layer was observed between the ZnO and AlN layers.

Moreover, a ₁ representing the mosaic tilt component of the crystal was 0.08 ° (273 arcsec), and b ₁ representing the twist component was 1.74 ° (6280 arcsec).

(Example 3)
In Example 1, the AlN layer is formed by repeating Example 1 except that the target is Al metal, the pulse frequency of the pulse laser beam emitted from the KrF excimer laser is 30 Hz, and the laser power at the time of target irradiation is 0.495 W. grown.

Moreover, a ₁ representing the mosaic tilt component of the crystal was 0.02 ° (78 arcsec), and b ₁ representing the twist component was 1.83 ° (6590 arcsec).

Example 4
In Example 1, the growth substrate is a ZnO substrate having a (1-100) plane as a main surface, the pulse frequency of pulsed laser light emitted from a KrF excimer laser is 3 Hz, the laser power at the time of target irradiation is 0.058 W, and RF Example 1 was repeated to grow an AlN layer except that the radical source was used at 320 W. The growth rate was 42 nm / hour, and the growth film thickness was about 40 nm.

FIG. 8 shows an RHEED image after growth. The streak pattern indicating the (1-100) plane crystal layer was clearly observed, and it was found that the epitaxial growth occurred while maintaining high crystallinity. FIG. 9 shows an X-ray diffraction spectrum. A ₁ representing the mosaic tilt component of the crystal was 0.11 ° (396 arcsec).

(Example 5)
In Example 1, the growth substrate is a ZnO substrate whose main surface is the (1-100) plane, the pulse frequency of the pulsed laser light emitted from the KrF excimer laser is 10 Hz, the laser power at the time of target irradiation is 0.110 W, the growth pressure Example 1 was repeated to grow an AlN layer except that was set to 4.5 × 10 ⁻⁶ Torr. The growth rate was 100 nm / hour, and the growth film thickness was about 50 nm.

Further, a ₁ representing a mosaic mosaic tilt component of the crystal was 0.16 ° (576 arcsec).

(Example 6)
In Example 1, a growth substrate is a ZnO substrate having a (11-20) plane as a main surface, introduced into a PLD apparatus without annealing, and a substrate temperature of 320 ° C. and a pulsed laser beam emitted from a KrF excimer laser An AlN layer was grown by repeating Example 1 except that the pulse frequency of 20 Hz was used, the laser power at the time of target irradiation was 0.190 W, and the RF radical source was 320 W.

  FIG. 10 shows an RHEED image after growth. A streak pattern was clearly observed, and it was found that epitaxial growth occurred while maintaining high crystallinity.

(Comparative Example 1)
In Example 1, except that the substrate temperature was 500 ° C., Example 1 was repeated to grow an AlN layer. However, a clear (0001) plane peak of AlN for obtaining the full width at half maximum was not observed.

(Comparative Example 2)
In Example 1, the AlN layer was repeated by repeating Example 1 except that the substrate temperature was 1050 ° C., the pulse frequency of the pulsed laser light emitted from the KrF excimer laser was 30 Hz, and the laser power at the time of target irradiation was 0.50 W. Grew up. a ₁ was 1.14 ° (4085 arcsec). In the atomic force microscope image, the step-and-terrace structure was not observed, and it was not a good single crystal.

(Comparative Example 3)
In Example 4, the AlN layer was formed by repeating Example 4 except that the substrate temperature was 590 ° C., the pulse frequency of the pulse laser beam emitted from the KrF excimer laser was 10 Hz, and the laser power at the time of target irradiation was 0.105 W. grown. FIG. 11 shows the RHEED image after growth. The observed RHEED image is not a streak pattern showing a good (1-100) plane single crystal as observed in FIG. 8, but an RHEED pattern showing a (0001) plane, and the pattern is also unclear. From this, it was found that a (0001) plane crystal layer having poor crystallinity was grown.

(Comparative Example 4)
In Example 4, the AlN layer was formed by repeating Example 4 except that the substrate temperature was 850 ° C., the pulse frequency of the pulse laser beam emitted from the KrF excimer laser was 10 Hz, and the laser power at the time of target irradiation was 0.102 W. grown. FIG. 12 shows an RHEED image after growth. The RHEED pattern observed during the growth was not a streak pattern showing a good (1-100) plane single crystal but a ring pattern showing a polycrystal mixed with a (0001) plane.

(Comparative Example 5)
In Example 6, the AlN layer was grown by repeating Example 6 except that the substrate temperature was 950 ° C. and the laser power at the time of target irradiation was 0.202 W. FIG. 13 shows an RHEED image after growth. The RHEED pattern observed during the growth was not a streak pattern showing a good (11-20) plane single crystal but a ring pattern showing a polycrystal mixed with a (0001) plane.

It is a figure for demonstrating the hexagonal system Miller index. It is a figure for demonstrating the measuring method of a X-ray rocking curve (XRC). It is a figure which shows the structure of a PLD apparatus. 2 is an RHEED intensity profile in an AlN growth process of Example 1. FIG. 2 is an atomic force microscope image of the AlN surface of Example 1. FIG. 2 is a diagram showing an X-ray diffraction spectrum of Example 1. FIG. 3 is an atomic force microscope image of the AlN surface of Example 2. FIG. 6 is an RHEED image after AlN growth in Example 4. FIG. 6 is a diagram showing an X-ray diffraction spectrum of Example 4. FIG. 7 is an RHEED image after AlN growth in Example 6. FIG. 7 is an RHEED image after growth of AlN in Comparative Example 3. 7 is an RHEED image after growth of AlN in Comparative Example 4. 6 is an RHEED image after growth of AlN in Comparative Example 5.

Explanation of symbols

30 PLD device 31 Chamber 31a Window 32 Target 33 KrF excimer laser 34 Lens 35 Gas supply unit 36 Nitrogen radical source 36a Adjustment valve 37 Pressure valve 38 Rotary pump 39 Rotating shaft

Claims (15)

An Al _x Ga _1-x N layer (where x is a number satisfying 0.6 ≦ x ≦ 1.0) grown on a ZnO substrate,
The full width at half maximum a ₁ of the angle dependency of the X-ray diffraction intensity with respect to the growth surface is
An Al _x Ga _1-x N layer satisfying 0.001 ° ≦ a ₁ ≦ 0.5 °. The Al _x Ga _1-x N layer according to claim 1, wherein the Al _x Ga _1-x N layer is formed at a growth temperature of less than 500 ° C. 3. The Al _x Ga _1-x N layer according to claim 1, wherein a plane orientation of the ZnO substrate is a low index plane. 3. The Al _x Ga _1-x N according to claim 1, wherein the plane orientation of the ZnO substrate is any one of {0001} plane, {1-100} plane, and {11-20} plane. layer. A laminated structure comprising the Al _x Ga _1-x N layer according to any one of claims 1 to 4 and another layer formed on the layer. A method of forming an Al _x Ga _1-x N layer (where x is a number satisfying 0.6 ≦ x ≦ 1.0) on a ZnO substrate,
(A) a step of preparing a ZnO substrate; and (b) an Al element, or an Al element and a Ga element are intermittently supplied to the growth surface, an N element is supplied, and the growth temperature is less than 500 ° C. A method for producing an Al _x Ga _1-x N layer, comprising a step of growing an Al _x Ga _1-x N crystal on a ZnO substrate surface. In the step (b), growth is performed by intermittently exciting a target containing Al element (when x = 1) or a target containing Al element and Ga element (when x ≠ 1). Item 7. A method for producing an Al _x Ga _1-x N layer according to Item 6. The Al x _Ga 1-x _N The method of claim 7, wherein the change in the composition of the target during the growth of the crystal is substantially free. When Al composition x = 1, AlN is used as the target,
8 or 7, wherein when Al composition x ≠ 1, a mixture of AlN and GaN or a mixed crystal of AlN and GaN is used as the target, or AlN and GaN are used as separate targets, respectively. 8. The method according to 8.   The method according to claim 9, wherein the target is provided in the form of a sintered body or mixed crystal. When Al composition x = 1, Al metal is used as the target,
9. The method according to claim 7, wherein when the Al composition x ≠ 1, Al metal and Ga metal are used as separate targets.   The excitation of the target is performed by laser light irradiated intermittently, and the irradiation pause period of the laser light is between 0.001 second and 10 seconds. The method described.   The method according to any one of claims 6 to 12, wherein the growth temperature is in the range of 20C to 320C. An Al _x Ga _1-x N layer grown by the method according to claim 6, wherein x is a number satisfying 0.6 ≦ x ≦ 1.0. .) A laminated structure comprising the Al _x Ga _1-x N layer according to claim 14 and other layers formed on the layer.