JP4109014B2 - A seed crystal for growing zinc oxide, a method for growing a zinc oxide seed crystal, and a growing apparatus therefor. - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a seed crystal for growing zinc oxide that can be used for industrial applications and is suitable for preventing generation of a crystal defect region during single crystal growth. Relates to crystals.
[0002]
[Prior art]
Conventionally, in semiconductor devices, semiconductor devices formed using amorphous silicon, polycrystalline silicon, or the like as a thin film material have been widely used. In recent years, zinc oxide (ZnO) has attracted attention as a thin film material. Semiconductor devices formed as thin film materials can be applied to existing semiconductor devices such as UV LEDs (Light Emitting Diodes), laser diodes (LDs), and transparent transistors, or research and development for new applications. It is being advanced.
[0003]
By the way, when manufacturing semiconductor devices, it is known that the quality of the thin film formed on the base substrate has a significant effect on its electrical characteristics, optical characteristics, reliability (lifetime), etc. The better the quality, the better the electrical and optical characteristics and reliability.
[0004]
For this reason, when producing a semiconductor device using ZnO as a thin film material, it is important to form a high-quality ZnO thin film without crystal defects on the base substrate.
Factors that determine the quality of the thin film include the difference in the lattice constant between the thin film material and the base substrate material. It is known that the smaller the difference in the lattice constant, the more the thin film without crystal defects can be formed. Note that the lattice constant indicates an arrangement interval of atoms regularly arranged in the crystal.
[0005]
When producing a semiconductor device having a ZnO thin film formed thereon, a base substrate formed of sapphire having a lattice constant relatively close to that of a ZnO single crystal has been used.
However, since the lattice constant differs between sapphire and a single crystal of ZnO by about 18%, in order to form a high-quality ZnO thin film on a base substrate formed of sapphire, it is necessary to form the thin film considerably thicker than usual. Because of the problem of profitability, there was no choice but to compromise with a certain level of quality industrially.
[0006]
Therefore, for example, in order to form an extremely high quality ZnO thin film on the base substrate at low cost, it is conceivable to form the base substrate with a single crystal of ZnO having the same lattice constant as that of the ZnO thin film. A ZnO single crystal having a size that can be used as a base substrate could not be grown.
In growth of a zinc oxide (ZnO) single crystal by a hydrothermal synthesis method, a seed crystal cut out from the zinc oxide single crystal is used, and the single crystal is grown from this seed crystal. The crystal has an a-axis in a hexagonal crystal structure. There is a tendency for crystals to grow selectively in the direction.
[0007]
In general, in order to form the seed crystal from a zinc oxide single crystal, it is often cut out and used as shown in FIG. FIG. 8 (b) is a seed crystal cut out from a zinc oxide single crystal, and FIG. 8 (c) is a schematic diagram showing an example of a method for holding the seed crystal in the growth vessel.
FIG. 8A shows an outline of a zinc oxide single crystal belonging to the hexagonal system and its crystal axis. The hexagonal crystal axes are a total of four a-axis, three a (1, 2, 3) axes perpendicular to the c-axis and forming an angle of 120 ° with each other.
[0008]
First, a plate-like seed crystal is cut out from a zinc oxide single crystal so that its longitudinal direction is along the c-axis (parallel to the c-axis). Next, the plate of the seed crystal is divided along the c-axis, and a time-tree type seed crystal shown in FIG. 8B is cut out. When the single crystal is grown, it grows almost in the c-axis direction, and therefore, the c-axis is cut out in consideration of the longest length.
A ZnO seed crystal is mounted on the upper frame 61 in the growth vessel of the single crystal growth apparatus. For example, seed crystals are grown by a technique in which a ZnO seed crystal is suspended from a frame 61 using a noble metal wire (platinum wire) 62.
The structure of the growing apparatus used for the hydrothermal synthesis method will be described later.
[0009]
In order to reduce crystal defects during the growth process, the surface of the seed crystal needs to be a sufficiently clean surface, the surface is finished by a polishing or lapping process, and a cleaning process is also carefully performed.
In order to reduce these man-hours, it is desirable that the surface of the seed crystal be configured with as few faces as possible, and in many cases, the seed crystal has a long wooden shape that is long in the c-axis direction as shown in FIG. .
[0010]
The growth of seed crystals in the growth vessel is as follows. First, in the hydrothermal synthesis method, the growth rate in the c-axis direction is very slow, and growth cannot be expected so much.
Next, the state of growth in the direction perpendicular to the c-axis will be described with reference to FIG. The figure shows a cross-section perpendicular to the c-axis of the seed crystal and schematically shows how the crystal is grown along the a-axis direction in the growth apparatus.
As described above, since the hydrothermal synthesis method hardly grows in the c-axis direction, various seed crystals such as bonding a plurality of seed crystals in the c-axis direction or adding a thick Zn0 film to a sapphire substrate to form a seed crystal. Methods have been tried and proposed.
[0011]
In FIG. 8D, as an example, the seed crystal is cut out as shown in FIG. 8B, and the cross-sectional shape of the seed crystal 21 is 2 parallel to the c axis and one a axis (a1 axis in the figure). It is a rectangle formed by the plane (110, 110) and two planes (120, 120) parallel to the c-axis and perpendicular to one a-axis (a1 axis in the figure). Here, only the phenomenon of crystal growth will be described, and the explanation of the crystal lattice form of each face, the state of arrangement of atoms and molecules, etc. will be described later.
Normally, a plane 110 parallel to the c-axis and one a-axis (a1 axis in FIG. 8D) is called an m-plane, and if a single crystal grows ideally, a cross-sectional shape perpendicular to the c-axis. Is a shape close to a regular hexagon surrounded by six m-planes.
[0012]
A crystal grows around the seed crystal 21 in the growth vessel, but the surface 115 that grows parallel to the surface 110 has a high growth rate and has a strong tendency to increase the size of the surface parallel to the surface 110. . A good crystal plane with few defects in the molecular arrangement can be obtained.
In the plane 120, the arrangement of the molecules is irregular and cannot extend along the arrangement of the seed crystal parallel to the plane and parallel to the plane. Then, under the influence of the a2 and a3 axes), the crystal growth direction is not determined, and the crystal defect region 44 including various transitions is likely to be formed.
[0013]
On the other hand, at the end of the surface 115, the crystal grows along the surface of the crystal defect region 44 as if bent by 60 ° from the surface 115 under the influence of the a2 and a3 axes.
Therefore, as the crystal growth proceeds, a surface 112 parallel to the a2 axis and a surface 113 parallel to the a3 axis, which are equivalent to the relationship of the surface 115 with respect to the a1 axis, grow, and the crystal defect region 44 becomes an isosceles triangle and is sealed. It will be stopped.
When the seed crystal is covered with the surface 115, the surface 112, and the surface 113 including the crystal defect region 44, thereafter, a single crystal with few defects grows while forming a hexagon parallel to the a1, a2, and a3 axes.
That is, crystal defect regions 44 and 44 are included in the vicinity of the surface of the seed crystal other than the m-plane, for example, the surfaces 120 and 120 orthogonal to any a-axis (inclined by 30 ° with respect to the other two a-axes). The The occurrence of crystal defect regions is recognized to some extent on the surfaces other than the m-plane.
[0014]
[Problems to be solved by the invention]
As described above, on the surface of the seed crystal other than the m-plane, a crystal defect region is generated in the grown single crystal, and a poor quality portion is often included in the vicinity of the seed crystal at the center of the grown single crystal. Since the poor quality part cannot be used, there is a problem that the yield of the cut wafer is deteriorated, a non-defective product sorting step is added, and the cost increase cannot be ignored.
[0015]
In addition, single crystals for cutting out seed crystals have not been available in the past, and multiple seed crystals were connected in the c-axis direction, or a thick ZnO film was cut out on a sapphire substrate. There were many. It is also desired to obtain a ZnO seed crystal that is sufficiently long in the c-axis direction.
[0016]
An object of the present invention is to supply a seed crystal in which a crystal defect region does not occur in the vicinity of the seed crystal in the grown single crystal, and to provide a seed crystal growing method and a growing container.
[0017]
[Means for Solving the Problems]
In order to solve the above problems, the present invention is a zinc oxide growing seed crystal cut out from a single crystal of zinc oxide,
The plane forming the surface parallel to the c-axis of the seed crystal is It is a hexagon consisting of six m-planes consisting of two planes parallel to each of the three a-axes. A seed crystal for growing zinc oxide is provided.
[0019]
Also The present invention is a seed crystal for growing zinc oxide cut out from a single crystal of zinc oxide, and one surface of the surface shape constituting the seed crystal is a cylindrical surface whose central axis is parallel to the c-axis. A seed crystal for growing zinc oxide is also provided.
[0020]
In the present invention, the zinc oxide seed crystal raw material powder zinc oxide and hydrogen gas are reacted in the front container to reduce to zinc,
Introducing the zinc oxide single crystal for nucleus to be the nucleus of zinc oxide single crystal growth, the reduced zinc is introduced into the rear container,
Provided is a method for growing a zinc oxide seed crystal in which oxygen gas is supplied into the rear container and a zinc oxide seed crystal is grown using the zinc oxide single crystal for nuclei as a nucleus.
[0021]
Furthermore, the present invention includes a front container charged with zinc oxide, which is a zinc oxide seed crystal raw material, and supplied with hydrogen gas to be reduced to zinc.
A zinc oxide single crystal for nuclei as a nucleus of a zinc oxide seed crystal is charged, the zinc is introduced from the front vessel, oxygen gas is supplied, and the zinc oxide single crystal is used as a nucleus for the zinc oxide single crystal. And rear container to cultivate
Provided is a device for growing a zinc oxide seed crystal.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Before describing the seed crystal, the outline of the hydrothermal synthesis method will be described with reference to FIG.
FIG. 2 is a diagram showing an example of a single crystal growth apparatus 51 using the single crystal growth vessel previously proposed by the present applicant.
[0023]
The single crystal growing apparatus 51 shown in FIG. 2 includes an autoclave 52 that can apply the temperature and pressure required for growing a ZnO single crystal by hydrothermal synthesis, and the inside of the autoclave 52. It is comprised from the growth container 60 accommodated and used for.
The autoclave 52 is covered with a lid 54 with a packing 57 sandwiched between a container body of the autoclave 52 made of, for example, high-strength steel mainly composed of iron, and fixed inside by a fixing portion 55. The structure is hermetically sealed.
[0024]
The growth container 60 that is accommodated and used in the autoclave 52 is formed of, for example, platinum (Pt) or the like, and the shape thereof is a substantially cylindrical container. And the bellows 70 which acts as a pressure adjustment part is attached to the upper part in the state which sealed the inside of the growth container 60. As shown in FIG.
[0025]
In such a single crystal growing apparatus 51, the ZnO seed crystal 3 is suspended using the frame 61 and the noble metal wire (platinum wire) 62 on the upper side in the growth vessel 60, and the raw material 66 is disposed on the lower side thereof. The seed crystal 3 is grown.
In this case, an internal baffle plate 64 that controls thermal convection is provided between the ZnO seed crystal 3 and the raw material 66, and the inside of the growth vessel 60 is divided into a melting region and a growth region by the internal baffle plate 64. It is delimited.
A plurality of holes are formed in the internal baffle plate 64, and the counter flow rate from the dissolution region to the growth region is controlled by the opening area of the baffle plate 64 determined by the number of holes. A temperature difference can be obtained.
[0026]
Then, a strong alkali solution such as sodium hydroxide (NaOH), calcium carbonate (Na 2 CO 3), potassium hydroxide (KOH), or the like is injected into the growth container 60 and transmitted between the autoclave 52 and the growth container 60. For example, a heat transfer solution 67 such as pure water is injected for heat, and the autoclave 52 is heated by the heaters 56, 56... A dissolved growth solution 71 is generated.
Then, the growth solution 71 is supplied to the growth region via the internal baffle plate 64 so that the ZnO seed crystal 3 is grown.
[0027]
Further, in the single crystal growing apparatus 51 shown in FIG. 2, an external baffle plate 65 is provided outside the growth vessel 60, and the convection outside the growth vessel 60 is restricted by the external baffle plate 65, so that the growth is performed. A temperature difference necessary for the growth of the ZnO seed crystal 3 is obtained between the regions in the container 60.
[0028]
In addition, a bellows 70 is provided as a pressure adjusting unit on the upper part of the growing container 60, and the internal pressure of the growing container 60 and the external pressure are balanced by the expansion and contraction of the bellows 70.
[0029]
Such a single crystal growing apparatus 51 can grow a single crystal of ZnO from a seed crystal by a hydrothermal synthesis method. A ZnO single crystal having a caliber size that can be used for industrial applications can be grown with almost no impurities mixed in the growth vessel 60.
[0030]
An example of the embodiment of the present invention will be described with reference to FIG. FIG. 1 is a diagram schematically showing an example of the shape of a seed crystal of zinc oxide (ZnO) as the present embodiment. A single crystal of ZnO is a hexagonal crystal, and the three a (1 · 2 · 3) axes 43 that form an angle of 120 ° with each other in a plane perpendicular to the c-axis 42 and the c-axis 42 as crystal axes. Exists. FIG. 1A shows a perspective view of a ZnO single crystal 40 from which a seed crystal is cut.
First, as shown in FIG. 1A, a ZnO single crystal thin plate 46 is cut out from a ZnO single crystal 40 so as to be parallel to a c-axis 42 and one a-axis 43 (a1 axis in the figure).
The seed crystal 21 is formed from the thin plate 46, and the shape of the seed crystal in which the generation of the crystal defect region is small is shown as a cross section perpendicular to the c-axis in FIGS.
[0031]
A plane parallel to the c-axis and an arbitrary a-axis is called an m-plane, and a plane parallel to the c-axis on the surface of the single crystal is formed by the m-plane.
As described above, it is known that a crystal grown based on the m-plane of the seed crystal has few defects and a high growth rate. Therefore, if a plane parallel to the c-axis of the surface of the seed crystal is formed with the m-plane, the crystal grown from the m-plane has good quality and grows quickly.
Since the seed crystal is cut in the longitudinal direction parallel to the c-axis, if a cross section perpendicular to the c-axis of the seed crystal is taken, the outer circumferential line (cross-sectional outline) defining this cross-sectional shape is aligned with the c-axis of the seed crystal. It is a line of intersection between the parallel plane and the cross-section plane. That is, a line parallel to any of the a (1 to 3) axes in the outline of the cross section of the seed crystal indicates that the surface is an m plane.
[0032]
The most ideal shape as a seed crystal is a seed crystal 21b having a regular hexagonal cross section formed so that all six surfaces are m-planes, as shown in FIG. Two parallel m-planes 110 and 110 along the a1 axis, two parallel m-planes 112 and 112 along the a2 axis, and 113 and 113 along the a3 axis These are composed of a total of six m-planes, two m-planes parallel to each other. A single crystal grown from a seed crystal having a regular hexagonal cross section can be expected to grow stably.
Since the length of the outline of the cross section (the width of the m-plane measured at right angles to the c-axis) may be arbitrarily changed, even if it is not a regular hexagon as a similar shape, it is thin as shown in FIG. The plate-shaped seed crystal 21c or the hexagonal shape shown in FIG. Here, these are collectively called a hexagon.
These shapes require many man-hours for the grinding and polishing steps for forming each m-plane, and cost is high, but the quality of the single crystal obtained from the regular hexagonal seed crystal 21b is particularly high.
[0033]
Next, in the case where the two sets of parallel m-planes shown in (d) and (e) of FIG. 3 are used, it is considered that the length of one set of the three sets of m-planes is 0. Good.
These seed crystals 21d and 21e are only m-planes as in the hexagonal shape, and no crystal defect region is generated in principle. That is, (d) is a four-sided structure composed of two m-planes 110 and 110 parallel to the a1 axis and two m-planes 113 and 113 parallel to the a3 axis. (E) is two m-planes of 110 and 110 parallel to each other along the a1 axis and two m-planes of 112 and 112 parallel to each other along the a2 axis. It is a quadrilateral composed of four sides.
[0034]
These seed crystals with a parallelogram cross section can be attached to a large number of seed crystals on a simple mounting jig and can simultaneously perform grinding and polishing for seed crystal formation, reducing the number of polishing steps. It is possible, and the practicality is high in balance between the quality of the obtained single crystal and the cost of the seed crystal.
In addition, a seed crystal 21f shown in FIG. 5 (f), which is composed of a set of parallel m-planes and two separate m-planes, each of which is considered as a variation of this example.
[0035]
In the case of the seed crystal 21c in FIG. 5C, when the distance between two m-planes (110, 110) along one a1 axis (parallel to each other) is reduced, m-planes (112, 113, 112, 113) are gradually shortened. That is, it is very close to FIG. 2H where there are planes 120 perpendicular to the a1 axis at both ends.
In the plane 120, a crystal defect region is generated as shown in FIG. 5D. If the plate thickness of the seed crystal is reduced to some extent, the isosceles triangle of the crystal defect region 44 shown in FIG. 5D becomes very small. Such a crystal defect region does not affect the yield of the grown single crystal and can be ignored in practical use.
Thus, if the seed crystal has a thin plate shape with a rectangular cross section, at least two wide surfaces can be used as a m-plane parallel to each other and other than the m-plane can be used practically. As a thin thin plate shape, it is composed of two planes (120, 120) perpendicular to a pair of parallel m-planes (110, 110) like a seed crystal 21h shown in FIG. Processing costs may be reduced.
[0036]
Here, the reason why a crystal plane with few defects grows on the m-plane will be described with a steel ball model. In the steel ball model, steel balls having a diameter equal to the lattice constant (distance of the crystal lattice) are arranged at lattice points instead of the molecules or atoms constituting the crystal.
First, referring to FIG. 3, a hexagonal single crystal structure will be described using a steel ball model. FIG. 3 (a) shows a method of arranging hexagonal steel balls as a perspective view, and FIG. 3 (b) is a projection of the steel ball model from the + c-axis direction and shows the overlapping state of the A and B layers. Is. Since the lattice constant is indicated by K in FIG. 4B, the steel balls having a diameter K are regarded as crystal atoms or molecules, and are arranged so that there is no gap (the surface is in contact).
[0037]
A hexagonal single crystal is formed by alternately overlapping A layers and B layers.
The lowermost drawing in FIG. 6A shows seven steel balls (A1, A2,... A7) belonging to the A layer that are in contact with each other. These steel balls are in contact with each other and constitute an A layer in an infinite number of lines on one plane.
Seven steel balls are centered on A1, and on the outer circumference, six steel balls A2, ... A7 are in contact with adjacent steel balls, and the length of one side is K if the centers of the steel balls are connected. A regular hexagon is formed.
Usually, a parallelogram connecting the centers of the A-layer steel balls A1 to A4 shown in FIG. The smallest component is an equilateral triangle connecting the centers of the steel balls A1 to A3, and the length of one side of the equilateral triangle is the lattice constant (K).
Crystal axes a1, a2, and a3 are straight lines passing through the centers of three steel balls on one straight line. The axes a1, a2, and a3 are arranged at right angles to the c axis and 120 degrees apart from each other, and are perpendicular to the c axis. The three a-axes are equivalent to each other, and when the crystal is rotated by 60 ° around the c-axis, the a-axes overlap (there is no restriction on the orientation of the axes).
[0038]
When the steel balls are arranged so as to be in contact with three adjacent steel balls of the A layer, the B layer is formed.
For example, the B-layer steel ball B1 is disposed in a recess formed by three A-layer steel balls so as to be in contact with the A-layer steel balls (A1, A2, A3). In FIG. 5B, the center of the steel ball B1 is a point indicated by a symbol OB1. Similarly, the steel ball B2 is in contact with the A-layer steel balls (A1, A4, A5), and the center of the steel ball B2 is a point designated by OB2. The steel ball B3 is in contact with the steel balls (A1, A6, A7), and the center of the steel ball B3 is a point OB3.
The steel balls forming the B layer are also in contact with each other, and the B layer is configured innumerably on one plane. If the center of the steel ball constituting the A layer is moved to OB1, OB2, OB3.
[0039]
A second A layer is disposed on the B layer. As seen from the + c-axis direction, the first A-layer steel ball and the second A-layer steel ball appear to completely overlap each other as shown in FIG. That is, the steel ball A12 overlaps with the steel ball A14 over the steel ball A4.
Further, the second B layer is disposed on the second A layer, and the first B layer and the second B layer are also disposed at positions completely overlapping when viewed from the + c-axis direction. In this way, hexagonal crystals are arranged.
In the case of a steel ball considered to be a rigid body, the lattice constant along the c-axis is 1.633K, but an actual crystal shows a slightly different value.
[0040]
Here, the surface normally seen in a hexagonal single crystal will be described. The main plane for growing a crystal is a plane parallel to the c-axis and one a-axis (referred to as the m-plane), and contrasted with this is parallel to the c-axis and perpendicular to one a-axis. It is a surface to do.
It has already been described that a single crystal grows smoothly on the m-plane, has a high growth rate, and a crystal defect region tends to occur on a plane parallel to the c-axis and perpendicular to one a-axis.
In the plane perpendicular to the c-axis (parallel to the a-axis), the steel balls are arranged most densely. The adjacent steel balls are in contact with each other with the smallest gap. The state is shown in FIG. 3B, and the steel ball density (the number of steel balls in the unit plane) is maximized.
[0041]
The plane parallel to the m-plane and the c-axis and perpendicular to the single a-axis will be described with reference to FIG. The figure is a projection of a steel ball model near the crystal surface. The last one of the subscripts in the figure is a plan view seen from the + c-axis direction, 2 is a side view showing a section taken along line AA, and 3 is a front view showing the crystal surface from the outside.
Although it does not change in the steel ball model, in an actual crystal, the lattice constant of the layer close to the surface is different from that in the inside and the distance between molecules is usually different.
[0042]
FIG. 4A shows a case where the m-plane 110 parallel to the c-axis and one a-axis is the surface.
Here, as indicated by the alternate long and short dash line in FIGS. 1A and 1A, the surface of the crystal (m-plane 110) is a plane that passes through the center of the outermost steel ball.
As shown in the figure (a2), a total of steel balls A21 of the A layer located on the surface, steel balls equivalent to the steel balls A21 of the lower A layer, and steel balls B21 of the B layer disposed slightly inside In the figure, the A-layer steel balls (rows of equivalent steel balls) form the outermost surface of the crystal, and the B-layer steel balls are drawn inside and arranged in a row. A depression is formed.
[0043]
When a steel ball B20 corresponding to ZnO molecules is supplied from outside and is captured by approaching the surface, it falls into a recess between two rows of A layers, and two steel balls of A layer and B at the position indicated by the dashed line Occupies a stable position in contact with two steel balls of the layer. When the steel ball B20 is attached to the surface, a new B layer is formed, and the surface grows outward by about 60% of the diameter of the steel ball, and the crystal grows.
When the steel balls are arranged one after another at the same position as the steel ball B20 and completely fill the recess of the B layer, this time, the B layer becomes the crystal surface, and the A layer becomes the position drawn inside, but the entire surface The shape of is exactly the same as FIG. 4 (a1 to a3) just by switching the A layer and the B layer. Even if steel balls (molecules) adhere to each other one after another, the arrangement of the steel balls remains unchanged, and the lattice constant K and the lattice constant in the c-axis direction (1.633K for steel balls) always appear as they are. It can be seen that the surface is in a state where defects are hardly generated. The fast crystal growth is also thought to be due to the fact that the molecular arrangement and lattice constant do not change, and the molecules adhere stably to the surface.
[0044]
FIG. 5B shows a single plane 120 that is parallel to the c-axis and perpendicular to the a-axis. Similarly to (a), a surface passing through the center of the steel ball arranged on the outermost surface of the crystal is defined as a surface 120, which is indicated by a one-dot chain line.
As shown in the figure (b1), the distance between the two A-layer steel balls located on the surface is considerably different from the lattice constant K (1.732K), and B is in the same plane as the two A-layer steel balls. One steel ball in the layer becomes a wedge type, two steel balls in the A layer and one steel ball in the B layer (shown by a one-dot chain line) in a position drawn between the two wedge types in contact with the surface ) To form the same wedge shape and pull it to the position where it touches the surface to face the outside.
This surface 120 will have zigzag-shaped grooves repeatedly arranged, and in a position where molecules are likely to be trapped, since it differs from the original lattice constant, it becomes an arbitrary arrangement for each molecule, and it is difficult for regular molecular arrangement growth to occur. It is a surface where crystal defects are likely to occur.
[0045]
At the actual molecular level, the surface 120 is not formed as a wide surface as shown in the figure, and it is considered that m surfaces along the a-axis that are cleavage surfaces are irregularly arranged on the surface. Therefore, the crystals grow at right angles to the individual m-planes, and as they grow, they interfere with each other to form crystal defect regions.
[0046]
In addition, at the end of the surface, the crystal growth direction changes as if the influence of the other a-axis is stronger than the a-axis parallel to the current surface. For example, if the rightmost steel ball A20 in FIG. 4 (a1) is the end of the seed crystal surface, the influence of the a3 axis is stronger than the a1 axis, and the surface tends to bend and grow in a direction parallel to the a3 axis.
Accordingly, as shown in FIG. 5D, the surface 115 is bent from the surface 115 to the surface 113, and the m-plane grows in a direction parallel to the a3 axis. Eventually, when the m-plane covers the crystal defect region of the seed crystal surface layer, it grows as a normal m-plane thereafter.
Thus, as the number of m-planes increases from the surface of single crystal growth to the surface parallel to the c-axis of the seed crystal, a crystal with fewer defects grows faster.
[0047]
From this point, when the shape of the seed crystal from FIG. 1 (b to g) is reviewed, the seed crystals 21b to 21g) are all composed of only the m-plane surface parallel to the c-axis.
If the parallelogram of the seed crystal 21 (d, e) is used in order to reduce the seed crystal polishing process, a large number of isomorphous seed crystals can be simultaneously attached to a simple jig to obtain a large number of seed crystals in one polishing process. Can do.
The seed crystal shown in FIG. 1 (h) includes a surface 120 perpendicular to the a axis, but by shortening the length of the surface 120 perpendicular to the a axis relative to the length of the m surface 110 parallel to the a1 axis. In addition, the size of the crystal defect region can be reduced to such an extent that there is no practical problem. As described above, if the length of the surface 120 other than the m-plane is made as thin as possible, the seed crystal 21h can minimize the generation of a crystal defect region.
[0048]
By the way, when a single crystal of ZnO is actually grown by the hydrothermal synthesis method using the seed crystal described so far, the quality of the grown single crystal differs depending on the cross-sectional shape perpendicular to the c-axis of the seed crystal. It is recognized that there is. In particular, it appears remarkably when a wafer is cut out from a grown ZnO single crystal at right angles to the c-axis (referred to as c-cut), further subdivided, and used as it is as a light emitting element.
Although it has already been explained that a crystal defect region occurs in the single crystal after growth on the surface of the seed crystal other than the m-plane, and there are often poor quality portions in the vicinity of the seed crystal at the center of the grown single crystal. The influence of the crystal defect region is quite wide. For example, not only the crystal defect region 44 having an isosceles triangle shape shown in FIG. Is spreading.
[0049]
The distribution of photoluminescence intensity can be used as a quality evaluation method when a ZnO single crystal is used as a light emitting element. To briefly explain photoluminescence, it is a phenomenon in which a substance is stimulated by radiation such as visible light or ultraviolet light and emits light having a longer period (this light is called fluorescence or phosphorescence). FIG. 5A schematically shows the intensity distribution of light emission corresponding to the photon energy on the horizontal axis by selectively irradiating the c-cut ZnO with laser. The ideal photoluminescence characteristic is shown by a curve drawn with a broken line, and has a peak in the vicinity of 3.3 eV. The wavelength of this peak is about 375 nm in the ultraviolet region, and is a wavelength necessary for realizing an ultraviolet light emitting element.
[0050]
FIG. 5B shows a schematic view of the surface cut and polished at right angles to the c-axis of c-cut ZnO as a sample. There is a substantially square seed crystal 21 in the center, and isosceles triangular crystal defect regions 44 are formed on the left and right non-m-planes.
Further, the seed crystal peripheral portion 92 inside the outer peripheral surface 91 partitioned into hexagons by broken lines is a portion having many crystal defects which are bad for a light emitting element, and the outside thereof is recognized as a crystal portion having a good quality. . In FIG. 5A, A shows the characteristic of the peripheral portion considered as a high-quality crystal portion, and B shows the characteristics of the seed crystal peripheral portion.
In the measured ZnO crystal, the cause of impurity contamination and oxygen defects has not been completely removed, and both A and B have low peak values in the vicinity of 3.3 eV, but the decrease in B is particularly large.
[0051]
In the growth of ZnO single crystals belonging to the hexagonal system, a seed crystal peripheral portion with many crystal defects exists in a fairly wide range centering on the seed crystal 21 and the crystal defect region 44. It cannot be used as a light emitting element. This indicates that the yield of the wafer for manufacturing the light emitting device is greatly deteriorated.
Further, it is considered that a high-quality crystal portion 93 is not formed unless the outer peripheral surface 91 of the portion having many crystal defects is formed in a substantially regular hexagon.
[0052]
In summary, the regular hexagonal cross section shown in FIG. 6 (a) (FIG. 1 (b) is also the same shape) is optimal as the cross-sectional shape of the seed crystal in consideration of the yield when using a ZnO single crystal as a light emitting device. Become.
However, as already described, polishing the outer shape of the Zn0 seed crystal having a regular hexagonal cross section has many disadvantages in terms of cost, such as an increase in the number of steps.
As one convenient method, as shown in FIG. 6 (b) schematically depicting a cross section perpendicular to the c-axis of the seed crystal, the central axis is parallel to the c-axis as a surface constituting the surface shape of the seed crystal. When the cylindrical shape is adopted, polishing and other operations can be performed almost by machining. Since machining similar to cylindrical polishing is sufficient until finishing, all seed crystals other than cylindrical shapes require processing methods similar to planar lapping, making it easier to fully automate and reduce processing costs. .
The surface shape of the seed crystal is not particularly limited except for the above cylindrical surface portion. The plane corresponding to the upper and lower bottom surfaces of a normal cylinder may be the crystal plane of the single crystal as it is, but it is preferable in terms of machining that it is processed in advance into a plane perpendicular to the c-axis.
[0053]
At one point on the cylindrical surface, the angle formed by the tangent of the cylindrical surface passing through that point and the m-plane is the error angle between the cylindrical surface and the m-plane. The maximum error angle is 30 ° at the intersection 94 (six locations) between the m planes of the a1 to a3 axes on the cylindrical surface, and the minimum is 0 ° at each midpoint 30 ° apart from the three a axes. It becomes.
At least the hatched portion in the figure becomes the crystal defect region 95, but since the change in the error angle is gentle in the vicinity of the error zero point, the m-plane is easily formed over a wide range at an early stage.
Accordingly, a regular hexagonal outer peripheral surface 91 covering the crystal defect region 95 relatively quickly is obtained, and a high-quality crystal portion is formed outside thereof.
Compared with a seed crystal having a regular hexagonal cross section, the presence of some crystal defect regions may be covered by the low cost of machining the seed crystal.
[0054]
Conventionally, it is very difficult to obtain a large single crystal as a seed crystal, and a plurality of single crystals cut out from a small single crystal are connected in the c-axis direction, a thick ZnO film is grown on the surface of a different substrate, etc. I had to make an effort to obtain a seed crystal as large as possible. As described above, an ideal seed crystal is a ZnO single crystal that is long in the c-axis direction and has a hexagonal prism in cross section perpendicular to the c-axis.
Next, a method for growing a ZnO single crystal that can be used as a seed crystal by a vapor phase growth method will be described.
[0055]
A method for growing a ZnO single crystal by the vapor phase growth method (Chemical Vapor Deposition) described below employs a method that can be called a chemical vapor transport method, and will be described with reference to FIG. FIG. 7 shows a schematic cross-sectional view of an example of a growth vessel 80 for growing a ZnO seed crystal by a chemical vapor transport method.
[0056]
The growth container 80 has a shape in which the front container 80 a and the rear container 80 b are combined, and the internal gas can be communicated through the communication hole 85. In the figure, as an example, these containers have a cylindrical shape, and the rear container 80b is provided with a communication hole 86 to the outside.
A pipe 83 passes through the front container 80a and a pipe 84 passes through the rear container 80b, so that gas can be supplied from the outside.
In advance, sintered ZnO powder 87 as a raw material for the ZnO seed crystal is charged in the front container 80a, and a ZnO single crystal 88 for nucleus as a seed crystal nucleus is charged in the rear container 80b. Yes.
[0057]
The ZnO powder 87 in the front container 80a is placed in a high-temperature atmosphere around about 1300 ° C., and hydrogen gas (H ₂ ) And sprayed onto the ZnO powder. At high temperatures, ZnO is deprived of oxygen and reduced, and Zn and H ₂ O.
[0058]
The liberated Zn is introduced into the rear container 80b through the communication hole 85. In the rear vessel 80b, oxygen gas (O ₂ ) Is supplied. The liberated Zn and oxygen precipitate as a single crystal with the single crystal 88 as a nucleus to grow a single crystal 89 of ZnO.
According to this chemical vapor transport method, the growth direction of the c-axis of the single crystal 89 is induced in the flow direction of the air flow, and a high-purity ZnO single crystal (product seed crystal) that is long in the c-axis direction is obtained.
In addition, according to this chemical vapor transport method, there is a high probability that an ideal seed crystal having a c-cut surface close to a regular hexagon will be obtained. As described above, since reduction and single crystal growth are separated into a dedicated growth vessel, seed crystals are efficiently grown.
In addition, since these reactions can be performed near atmospheric pressure, the pressure in the container 80 can be atmospheric pressure.
Further, if excessive hydrogen gas is discharged after combustion by an igniter connected to an external communication hole 86, there is no danger.
[0059]
If a single crystal grown using such a seed crystal is sliced, a ZnO wafer with a high yield is obtained, and a substrate large enough for industrial use can be obtained as a final product. A single crystal may be mass-produced using a seed crystal cut out from the single crystal.
[0060]
Therefore, if such a ZnO wafer is used as a base material and a ZnO thin film is formed on this base material, an extremely high quality ZnO thin film can be formed.
Therefore, if such a ZnO single crystal is used as a base material and a ZnO thin film is formed on this base material to produce a semiconductor device such as an ultraviolet LED, a laser diode, or a transparent transistor, the electrical / optical A semiconductor device having excellent characteristics and reliability can be realized.
[0061]
【The invention's effect】
As described above, the zinc oxide growth seed crystal cut out from the zinc oxide single crystal of the present invention has a thin plate shape in which at least two wide surfaces are parallel m surfaces. Even if constituted by other than the above, the size of the crystal defect region is practically negligible so that the yield of the seed crystal processing can be minimized.
[0062]
In addition, a seed crystal having a parallelogram section composed of two m-planes and a m-plane along the other a-axis can obtain a seed crystal with relatively low processing cost. . Furthermore, if hexagonal seed crystals composed of six m-planes are used, all hexagonal seed crystals can be composed of m-planes, so that a more stable quality zinc oxide single crystal can be obtained.
[0063]
When using a ZnO single crystal as a light-emitting device, the cross-sectional shape of the seed crystal is a seed crystal having a cross section close to a regular hexagon with the same length of the m-plane, but no crystal defect region is theoretically generated. It is optimal because a high-quality crystal part is formed immediately near the seed crystal surface.
In order to reduce the cost of machining a seed crystal having a regular hexagonal cross section, a seed crystal having a cylindrical core whose axis is parallel to the c-axis is very easy to form the seed crystal by polishing or other means. It is excellent in that the processing cost of the seed crystal is low. The thickness of the crystal defect region is small, and a regular hexagonal outer peripheral surface is acquired relatively quickly to form a high-quality crystal portion. Compared to a seed crystal having a regular hexagonal cross section, some of the poor yields have the advantage of being canceled by low machining costs.
[0064]
Furthermore, since a high-quality ZnO single crystal can be obtained, it is a great effect that improvement in electrical and optical characteristics and reliability of semiconductor devices such as ultraviolet LEDs, laser diodes, and transparent transistors can be expected.
[0065]
By adopting the chemical vapor transport method to make the seed crystal by itself, it is possible to easily obtain a ZnO single crystal with high purity that is long in the c-axis direction from the sintered ZnO powder, and the seed crystal can be easily obtained. The advantages are great.
Furthermore, it is a great effect that the growth container can be divided into a front container for zinc oxide reduction and a rear container for single crystal growth to grow a good quality zinc oxide single crystal for seed crystal.
[Brief description of the drawings]
FIG. 1 is a schematic view showing various cross sections of a seed crystal cut out from a ZnO single crystal.
FIG. 2 is a cross-sectional view of a single crystal growing apparatus used in a hydrothermal synthesis method.
FIG. 3 is a schematic diagram by perspective and orthographic projection for explaining a hexagonal crystal state, a crystal lattice, and a crystal axis by a steel ball model.
FIG. 4 is a schematic diagram showing the state of molecular arrangement on the surface of a seed crystal in two directions parallel and perpendicular to the a-axis as a projection diagram of a steel ball model.
FIG. 5 is a schematic diagram showing a measurement result of a photoluminescence intensity of a ZnO single crystal and a measurement position of a cross section of the single crystal.
FIG. 6 is a diagram schematically showing a regular hexagonal shape and a circular cross-sectional shape of a ZnO seed crystal.
FIG. 7 is a cross-sectional view showing one embodiment of a growth vessel for growing a ZnO seed crystal using a chemical vapor transport method.
FIG. 8 is a schematic diagram for explaining a conventional method of cutting a seed crystal from a zinc oxide single crystal and a crystal defect region generated in the vicinity of the seed crystal.
[Explanation of symbols]
3, 21 seed crystal, 10, 20, 40 ZnO single crystal, 42 c-axis, 43, a-axis (a1 to a3), 44, 95 crystal defect region, 45 crystal lattice (unit lattice), 51 single crystal growth apparatus , 52 Autoclave, 54 Lid, 55 Adhering part, 56 Heater, 57 Packing, 60 Growth container, 61 Frame, 62 Platinum wire, 64 Internal baffle plate, 65 External baffle plate, 66 Raw material, 67 Heat transfer solution, 70 Bellows, 71 growing solution,
A1, A2, ... Steel balls constituting layer A, B1, B2, ... Steel balls constituting layer B, 110 m-plane of seed crystal, 112, 113, 115 Single crystal surface (m-plane), 120 plane perpendicular to the a-axis of the seed crystal, OB1, OB2, OB3 Projected position of the center of the steel ball constituting the B layer, K lattice constant,
80 growth container, 80a front container, 80b rear container, 83 pipe, 84 pipe, 85, 86 communication hole, 87 ZnO powder, 88 ZnO single crystal for nucleus,
89 ZnO single crystal (seed crystal ... product), 91 outer peripheral surface, 92 seed crystal peripheral part, 93 high quality crystal part 94 intersections of 6 m faces (six places),

Claims (2)

A seed crystal for growing zinc oxide cut from a single crystal of zinc oxide,
The zinc oxide characterized in that the plane forming the surface parallel to the c-axis of the seed crystal is a hexagon composed of six m-planes composed of two faces parallel to each of the three a-axes. Seed crystal for growth. A seed crystal for growing zinc oxide cut from a single crystal of zinc oxide,
One surface of the surface shape constituting the seed crystal is a cylindrical surface whose center axis is parallel to the c-axis.

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