JP2009052089A - ZnO-BASED THIN FILM AND SEMICONDUCTOR ELEMENT - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ZnO-based thin film which inhibits unintentional impurities from being doped, and to provide a semiconductor element. <P>SOLUTION: The ZnO-based thin film is formed of Mg<SB>x</SB>Zn<SB>1-x</SB>O(0≤x<1) including p-type impurities, and has a principal surface satisfying at least either of such conditions when the film is observed by an atomic force microscope that a density of observed hexagonal pits is 5×10<SP>6</SP>pieces/cm<SP>2</SP>or less, and that a recess part having a plurality of protruding micro crystallites formed on its bottom part is not observed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a ZnO-based semiconductor element, and more particularly to a ZnO-based thin film and a semiconductor element in which acceptor doping is performed.

Zinc oxide (ZnO) -based semiconductors have high exciton binding energy, can exist stably even at room temperature, and can emit photons with excellent monochromaticity, so that they can be used as light sources for lighting and backlighting. Applications to diodes (LEDs), high-speed electronic devices, surface acoustic wave devices, and the like are underway. However, when a ZnO-based semiconductor containing MgZnO is used as a p-type semiconductor, there is a problem that acceptor doping to the ZnO-based semiconductor is difficult and it is difficult to obtain a p-type ZnO-based semiconductor. Advances in technology have made it possible to obtain p-type ZnO-based semiconductors, and light emission has been confirmed. However, there are restrictions such as using a special substrate called ScAlMgO ₄ (for example, Non-patent documents 1 and 2). Therefore, it is industrially desired to realize a p-type ZnO-based semiconductor film formed on a ZnO substrate.
A. Tsukazaki, et al., “Japanese Journal of Applied Physics vol.44”, 2005, p. 643 A. Tsukazaki et al., “Nature Material 4”, 2005, p. 42

  However, a ZnO-based semiconductor is a substance that is very easy to form a donor and has a deep valence band. Therefore, when holes are generated in the valence band of a ZnO-based semiconductor, the crystal becomes unstable, and the positive A donor that compensates for the holes is likely to be formed. That is, the so-called self-compensation effect is a cause of difficulty in acceptor doping such as nitrogen (N) into the ZnO-based semiconductor. The self-compensation effect is often induced by point defects due to acceptor doping, but in the ZnO film, the self-compensation effect also occurs due to other causes. That is, the self-compensation effect is also caused by impurities such as silicon (Si) mixed in the ZnO film in the manufacturing process. For example, unintentional Si doping occurs in the roughened ZnO film, and as a result, acceptor doping into the ZnO film becomes difficult.

  In view of the above problems, an object of the present invention is to provide a ZnO-based thin film and a semiconductor element in which doping of unintended impurities is suppressed.

According to one embodiment of the present invention, the hexagonal pit density is 5 which is made of Mg _x Zn _1-x O (0 ≦ x <1) containing a p-type impurity and observed with an atomic force microscope. There is provided a ZnO-based thin film having a main surface satisfying at least one of × 10 ⁶ pieces / cm ² or less, or a recess in which a plurality of microcrystalline protrusions are formed on the bottom.

According to another aspect of the present invention, (a) hexagonal pits made of Mg _x Zn _1-x O (0 ≦ x <1) containing p-type impurities and observed in an atomic force microscope A ZnO-based thin film having a main surface satisfying at least one of a density of 5 × 10 ⁶ pieces / cm ² or less, or a recess having a plurality of microcrystalline protrusions formed on the bottom is not observed, and (b) Mg _y Zn _1-y O (0 ≦ y <1) and a substrate having a substrate principal surface in contact with the ZnO-based thin film, and the normal line of the substrate principal surface is projected onto the m-axis c-axis plane of the substrate crystal axis A semiconductor device is provided in which the projection axis is inclined within a range of 3 degrees or less in the m-axis direction.

  According to the present invention, it is possible to provide a ZnO-based thin film and a semiconductor element in which unintended impurity doping is suppressed.

  Next, embodiments of the present invention will be described with reference to the drawings. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals. However, it should be noted that the drawings are schematic, and the relationship between the thickness and the planar dimensions, the ratio of the thickness of each layer, and the like are different from the actual ones. Therefore, specific thicknesses and dimensions should be determined in consideration of the following description. Moreover, it is a matter of course that portions having different dimensional relationships and ratios are included between the drawings.

  Further, the following embodiments exemplify apparatuses and methods for embodying the technical idea of the present invention, and the technical idea of the present invention is the material, shape, structure, The layout is not specified as follows. The technical idea of the present invention can be variously modified within the scope of the claims.

As shown in FIG. 1, the semiconductor element according to the embodiment of the present invention includes a semiconductor layer 2 made of Mg _x Zn _1-x O (0 ≦ x <1) containing p-type impurities. When the main surface 21 of the semiconductor layer 2 is observed by an atomic force microscope (AFM), the density of the observed hexagonal pits is 5 × 10 ⁶ pieces / cm ² or less, or a plurality of microcrystalline protrusions at the bottom Is a ZnO-based thin film that satisfies at least one of the following conditions:

The p-type impurity contained in the semiconductor layer 2 is an impurity doped in the semiconductor layer 2, and for example, nitrogen (N), copper (Cu), phosphorus (P), or the like can be used. The semiconductor layer 2 is disposed on the substrate main surface 11 of the substrate 1. As the substrate 1, for example, Mg _y Zn _1-y O (0 ≦ y <1) can be adopted. The crystal structures of the substrate 1 and the semiconductor layer 2 are hexagonal, and the substrate main surface 11 is a c-plane. Therefore, the main surface 21 of the semiconductor layer 2 formed by growing Mg _x Zn _1-x O on the substrate main surface 11 is a c-plane.

  As already described, the ZnO-based semiconductor film has a strong self-compensation effect. Therefore, in order to make the semiconductor layer 2 a p-type semiconductor by acceptor doping, it is necessary to form a semiconductor element so that a donor that induces a self-compensation effect by unintended acceptor doping is not doped into the semiconductor layer 2. For example, when a molecular beam epitaxy (MBE) method is used to form a ZnO film, unintended acceptor doping of impurities serving as donors may occur in the ZnO film as described below.

Currently, in order to form a ZnO-based semiconductor film including an Mg _x Zn _1-x O film with high purity, it is common to employ the MBE method. Since the MBE method uses an elemental material as a raw material, the purity at the time of the raw material can be increased as compared with a metal organic chemical vapor deposition (MOCVD) method using a compound material.

An example of a thin film forming apparatus used in the MBE method is shown in FIG. A thin film forming apparatus 10 shown in FIG. 2A includes a holder 110 on which a substrate 1 is arranged, a cell 120 for supplying a raw material of a thin film to be formed, a cell 130 and a cell 140. In the example shown in FIG. 2A, zinc (Zn) is supplied from the cell 120 and gallium (Ga) is supplied from the cell 140. The cell 130 is a radical generator and is used when the MBE method is applied to crystal growth of a compound containing a gas element such as a ZnO film or a GaN film. The radical generator has a structure in which a high-frequency coil 132 surrounds the outer periphery of a discharge tube 131 made of PBN (pyrolytic boron initiator) or quartz, and the high-frequency coil 132 is connected to a high-frequency power source (not shown). In the example shown in FIG. 2A, a high frequency voltage (electric field) is applied to oxygen (O ₂ ) supplied into the cell 130 by a high frequency coil 132 to generate plasma, and radical particles (O *) are converted into cells. 130. FIG. 2B shows an example of a quartz discharge tube 131.

  Although plasma is preferable as a method for increasing the chemical activity, a sputtering phenomenon occurs in which plasma particles collide with a member of a surrounding thin film forming apparatus and knock out constituent elements of the member. This sputtering phenomenon increases the possibility of unintentional doping in the thin film formed on the substrate 1. When an oxide such as a ZnO film is grown on the substrate 1, since the source gas is oxygen, quartz is used as the discharge tube 131 instead of PBN that deteriorates due to oxidation. Quartz is used because there is no insulating material with a purity higher than that of quartz. However, Si, aluminum (Al), boron (B), and the like naturally contained in the quartz as impurities from the quartz used as the discharge tube 131 are knocked out by the sputtering phenomenon, and these are grown in the thin film grown on the substrate 1. May be doped.

  FIG. 3A shows an example of the concentration of Si and nitrogen and the secondary ion intensity of MgO in a semiconductor element in which nitrogen (N) is acceptor-doped in the semiconductor layer 2. Here, the semiconductor element having the characteristics shown in FIG. FIG. 3B shows a state of the main surface 21 of the sample A. For comparison, an example of the concentration of Si and nitrogen and the secondary ion intensity of MgO in the semiconductor element (hereinafter referred to as “sample B”) whose main surface 21 is shown in FIG. Shown in a). FIG. 3B and FIG. 4B are diagrams showing the state of the main surface 21 in the range of 20 square μm when observed with an atomic force microscope. The Si concentration is measured by secondary ion mass spectrometry (SIMS) (the same applies hereinafter).

Comparing FIG. 3A and FIG. 4A, it is clear that sample B has a higher Si concentration than sample A. Since the measurement lower limit of the concentration is about 1 × 10 ¹⁷ atoms / cm ³ , Si exists only in the vicinity of the main surface 21 of the semiconductor layer 2 in the sample A as shown in FIG. However, since SIMS is a measurement that is not good at the outermost surface measurement, the Si measurement value on the outermost surface may not be a true value. As shown in FIG. 4A, in the sample B, Si is present up to the middle in the depth direction of the semiconductor layer 2. Further, the nitrogen concentration is higher in FIG.

  Here, as is clear when FIG. 3B is compared with FIG. 4B, the main surface 21 of the sample A is flat, whereas the main surface 21 of the sample B is rough. Although only a few pits 201 are generated on the main surface 21 of the sample A, a plurality of recesses 202 are formed on the main surface 21 of the sample B. A cross-sectional view of the recess 202 shown in FIG. 4B in the II direction is shown in FIG. As shown in FIG. 4 (c), a plurality of microcrystals 203 are formed at the bottom of the recess 202. That is, the main surface 21 of the sample B is characterized in that the flat portion is a plurality of striped islands, and the concave portion 202 of a set of fine microcrystals exists between the islands. On the other hand, on the main surface 21 of the sample A, the flat portion is striped, but there is no set of microcrystals.

3 and the film shown in FIG. 4 are measured by CV measurement using a MOS (Metal Oxide Semiconductor) structure, the donor concentration (N _D ) and acceptor concentration (N _A ) in the film in the state shown in FIG. ) and density difference value of "N _D -N _a" to the range of about 1 × 10 ¹⁶ atoms / cm ^3, the value of the density difference in the film in the state shown in FIG. 4, "N _D -N _a" Was about 5 × 10 ¹⁷ atoms / cm ³ . Therefore, although the activation rate is lower than that of group III elements such as boron (B), Al, and Ga, it is clear that Si acts to increase donors in the semiconductor layer. As shown in FIG. 5, the CV measurement is performed by laminating a ZnO film 101, an MgZnO film 102, and an SOG (Spin on Glass) film 103 in this order, and a columnar electrode 105 disposed on the SOG film 103, The measurement was performed between the electrodes 104 arranged at intervals around the electrodes 105. The electrode 104 and the electrode 105 can employ a structure in which Al and titanium (Ti) are stacked, and the diameter W of the electrode 105 is, for example, 100 μm.

  As described above, from the comparison between the sample A shown in FIG. 3 and the sample B shown in FIG. 4, when the state of the main surface 21 is rough, Si that is unintentionally doped in the semiconductor layer 2 increases. It can be seen that less nitrogen is acceptor doped. Therefore, in order to suppress unintended doping of impurities such as Si into the semiconductor layer 2, the surface state of the main surface 21 is preferably as flat as possible. That is, when the state of the main surface 21 of the semiconductor layer 2 is as shown in FIG. 3B, unintentional doping of Si into the semiconductor layer 2 is suppressed, and p-type impurities such as nitrogen are acceptor doped. By doing so, the semiconductor layer 2 can be easily made into a p-type semiconductor. On the other hand, when the state of the main surface 21 of the semiconductor layer 2 is as shown in FIG. 4B, unintentional doping of Si into the semiconductor layer 2 increases, and acceptor doping with p-type impurities such as nitrogen is performed. This makes it difficult to make the semiconductor layer 2 a p-type semiconductor.

FIG. 6 shows a state of the main surface 21 in which FIG. 4B is enlarged. As shown in FIG. 6, 19 pits 201 exist in a range of 20 square μm. That is, the pit density is 4.75 × 10 ⁶ pieces / cm ² . In this case, the condition that the density of the pits 201 is 5 × 10 ⁶ pieces / cm ² or less is cleared at the last minute, but the condition that the recesses 202 in which the projections of the plurality of microcrystals 203 are formed on the bottom is not observed.

  FIG. 7 shows an example in which a large number of pits 201 exist on the main surface 21. 7A shows the concentration of Si and nitrogen of the semiconductor element and the secondary ion intensity of MgO, and FIG. 7B shows the main surface 21 when the range of 20 square μm is observed with an atomic force microscope. It is the figure which showed the state of. FIG. 7C is a diagram showing a state of the main surface 21 in a partly enlarged 1 square μm range of FIG. 7B.

  FIG. 8 shows an example of a sample (hereinafter referred to as “sample C”) in which the state of the main surface 21 is a limit capable of suppressing unintentional Si doping into the semiconductor layer 2. FIG. 8A shows the Si and nitrogen concentrations of the semiconductor element and the secondary ion intensity of MgO. FIG. 8B is a view showing a state of the main surface 21 when the range of 20 μm is observed with an atomic force microscope, and FIG. 8C is a partially enlarged view of FIG. The figure shows the state of the main surface 21 in the range of 1 square μm. As shown in FIG. 8B and FIG. 8C, the flat portion of the main surface 21 of the sample C is in the form of stripes separated by concave portions as in the case of the sample B. However, as shown in FIG. There are not so many big pits.

  FIG. 9 shows a comparison between sample C in which the semiconductor layer 2 shown in FIG. 8 is not doped with Si and sample B in which the semiconductor layer 2 shown in FIG. 6 is unintentionally doped with Si. . FIG. 9A is a diagram showing the state of the main surface 21 of the sample C in the range of 5 square μm, and FIG. 9B is an enlarged view of FIG. 9A and the sample C in the range of 2 square μm. It is the figure which showed the state of the main surface 21. FIG. 9C is a diagram showing the state of the main surface 21 of the sample B in the range of 5 square μm, and FIG. 9D is an enlarged view of FIG. 9C and the sample B in the range of 2 square μm. It is the figure which showed the state of the main surface 21. From comparison between FIG. 9B and FIG. 9D, the sample B has a concave portion 202 of a collection of microcrystals between the flat portions of the striped island, whereas the flat portion of the sample C. Is a striped island, but there is no collection of microcrystals.

  Note that the sample C shown in FIG. 8 has a higher Si concentration than the sample A shown in FIG. 3 and is most suitable for mass production from the viewpoint of suppressing unintentional Si doping to the semiconductor layer 2. It is not preferable. The state of the main surface 21 of the sample C is a limit that can suppress unintentional Si doping to the semiconductor layer 2, and the sample A is more preferable as the state of the main surface 21.

  FIG. 10 shows an enlarged view of the pit 201. In FIG. 10, it is slightly etched with dilute hydrochloric acid for easy viewing. FIG. 10 is a diagram in which the cross section of the pit 201 is enlarged by about 50,000 times, and the interpolated diagram is a diagram in which the upper surface of the pit 201 is enlarged by 10,000 times. As shown in FIG. 10, the pit 201 has a hexagonal shape when viewed from above, and it is clear that the pit 201 is constituted by a crystal plane. Moreover, the cross section is funnel-shaped, and the shape seen from the upper surface becomes larger as the recess of the pit 201 becomes deeper.

As described above, in order to suppress unintentional doping of Si into the semiconductor layer 2, when the main surface 21 of the semiconductor layer 2 is observed with an atomic force microscope, the density of the observed pits 201 is reduced. It is necessary to satisfy at least one of the conditions of 5 × 10 ⁶ pieces / cm ² or less, or that the concave portion 202 in which the projections of the plurality of microcrystals 203 are formed on the bottom is not observed.

FIG. 11 shows the relationship between the interface Si concentration of the semiconductor layer 2 and the Si concentration in the film. As shown in FIG. 11, the Si concentration in the interface is proportional to the Si concentration in the film, and it can be seen that Si existing at the interface diffuses into the film. Therefore, it must be avoided that the semiconductor layer 2 is doped with Si. The lower limit of measurement of the Si concentration is about 1 × 10 ¹⁷ atoms / cm ³ , and there is no reliability of the measured values shown in FIG.

Next, the substrate 1 will be described. As the substrate 1, a ZnO-based compound such as Mg _y Zn _1-y O (0 ≦ y <1) can be used. Similar to gallium nitride (GaN), the ZnO-based compound has a hexagonal crystal structure called wurtzite shown in FIG. FIG. 12 is a schematic diagram showing a hexagonal unit cell. The expressions c-plane and a-axis can be expressed by a so-called Miller index. For example, the c-plane is expressed as a (0001) plane. In FIG. 12, the hatched surface is the a-plane (11-20), and the m-plane (10-10) indicates a hexagonal crystal column. For example, the {11-20} plane and the {10-10} plane are generic names including the plane equivalent to the (11-20) plane and the (10-10) plane due to the symmetry of the crystal. . The a axis indicates the vertical direction of the a plane, the m axis indicates the vertical direction of the m plane, and the c axis indicates the vertical direction of the c plane.

The substrate _{1 made of} Mg _y Zn _1-y O as a substrate for crystal growth may be a ZnO substrate with y = 0 or a MgZnO substrate mixed with Mg. However, since MgO is a NaCl-type crystal, if Mg of the MgZnO substrate exceeds 50 wt%, it is not preferable because it is difficult to match with the hexagonal ZnO-based compound and phase separation is likely to occur.

  Further, as shown in FIG. 13, the substrate 1 is polished so that the substrate main surface 11 becomes a surface inclined at least in the m-axis direction. FIG. 13 shows the relationship between the surface normal direction of the substrate main surface 11 of the substrate 1 and the c-axis direction, m-axis direction, and a-axis direction of the substrate crystal axis direction of the substrate 1. The angle formed between the surface normal direction of the substrate main surface 11 and the c-axis direction is φ, and the surface normal of the substrate main surface 11 is an m-axis c-axis plane defined by the m-axis and c-axis of the substrate crystal axis. The angle formed by the projected axis and the c-axis direction (hereinafter referred to as “inclination angle component of the surface normal in the m-axis direction”) is φm degrees, and the surface normal of the substrate principal surface 11 is the substrate crystal axis. An angle formed by the projection axis projected on the a-axis c-axis plane defined by the a-axis and the c-axis and the c-axis direction (hereinafter referred to as “a tilt angle component of the surface normal in the a-axis direction”). φa degrees.

  Here, the reason why the surface normal of the substrate main surface 11 is inclined in the m-axis direction will be described. FIG. 15A is a schematic diagram of a substrate in which the surface normal of the substrate main surface 11 is not inclined with respect to either the a-axis or the m-axis. That is, this is a case where the surface normal direction of the substrate main surface 11 coincides with the c-axis direction.

  However, unless the cleavage plane of the bulk crystal is used for the bulk crystal, the surface normal direction of the substrate main surface 11 does not coincide with the c-axis direction as shown in FIG. If you stick to the surface-justified substrate, productivity will also deteriorate. Actually, the surface normal direction of the substrate main surface 11 is inclined from the c-axis and has an off-angle. For example, as shown in FIG. 15B, when the surface normal direction of the substrate main surface 11 is inclined by θ degrees only from the c-axis to the m-axis direction, the substrate main surface 11 (for example, the T1 region) is enlarged. As shown in FIG. 15C, steps having regularity at equal intervals on a terrace surface 1a that is a flat surface and a step portion generated by inclining a surface normal to the c-axis. Surface 1b is formed.

  Here, the terrace surface 1a becomes the c-plane (0001), and the step surface 1b corresponds to the m-plane (10-10). As shown in FIG. 15C, the step surfaces 1b are regularly arranged while maintaining the width of the terrace surface 1a in the m-axis direction. That is, as shown in FIG. 15D, the terrace surface 1a is inclined with respect to the substrate main surface 11, and the inclination angle is the inclination angle θ.

  The state shown in FIG. 15C corresponds to the case of θs = 90 ° in FIGS. The “step edge” shown in FIGS. 3 and 4 is a projection of the stepped portion formed by the step surface 1b on the m-axis a-axis plane defined by the m-axis and the a-axis. A stepped portion is generated on the main surface 11 of the substrate by the step surface 1b. Since the atoms flying to the stepped portion are coupled to the two surfaces of the terrace surface 1a and the step surface 1b, the stepped surface 1b has jumped to the terrace surface 1a. The atoms can bond more strongly than the case, and the incoming atoms can be trapped stably.

  That is, flying atoms diffuse in the terrace surface 1a in the surface diffusion process, but the crystal growth progresses by being trapped at the kink position formed by the stepped portion having a strong bonding force or the stepped portion and being incorporated into the crystal. Stable growth is achieved through growth. Thus, when a ZnO-based semiconductor layer is laminated on a substrate whose surface normal to the substrate main surface 11 is inclined at least in the m-axis direction, the ZnO-based semiconductor layer undergoes crystal growth around the step surface 1b, A flat film can be formed. As described above, when the step surface 1b is a surface corresponding to the m-plane, the semiconductor layer 2 which is a ZnO-based semiconductor grown on the substrate main surface 11 is a film having a flat main surface 21. be able to.

Incidentally, if the inclination angle θ defined in FIG. 15B is too large, the ZnO-based semiconductor layer does not grow flat on the substrate main surface 11. FIG. 16 shows that the flatness of the semiconductor film on the substrate main surface 11 changes depending on the inclination angle θ in the m-axis direction. FIG. 16A shows the state of the main surface 21 of the ZnO-based semiconductor layer 2 grown on the substrate main surface 11 of the substrate 1 made of Mg _y Zn _1-y O with an inclination angle θ of 1.5 °. Show. FIG. 16B shows the state of the main surface 21 of the ZnO-based semiconductor layer 2 grown on the substrate main surface 11 of the substrate 1 made of Mg _y Zn _1-y O having an inclination angle θ of 3.5 °. Show. FIGS. 16A and 16B are images obtained by scanning the main surface 21 with a resolution of 1 μm using an atomic force microscope after crystal growth. The main surface 21 shown in FIG. 16A is formed flat with the step widths uniform, but the main surface 21 shown in FIG. 16B has unevenness on the surface. , Flatness is lost. From the above, the inclination angle component φm degree shown in FIG. 13 is preferably in the range exceeding 0 ° and not more than 3 ° (0 ° <φm ≦ 3 °).

  As described above, it is preferable that the substrate main surface 11 is inclined only in the m-axis direction, and the inclination angle component φm degree is within a range exceeding 0 ° and not more than 3 °. It is difficult to cut out the substrate from the ingot by inclining only in the direction, and as a production technique, it is necessary to allow inclination in the a-axis direction and set the tolerance. For example, as shown in FIG. 13, the surface normal direction of the substrate main surface 11 with respect to the c-axis may have an inclination angle component φm degree in the m-axis direction and an inclination angle component φa degree in the a-axis direction. good. That is, the substrate main surface 11 may be formed so as to be inclined by φm degrees in the m-axis direction and φa degrees in the a-axis direction.

  However, the angle θs formed between the step edge of the step surface 1b and the m-axis direction needs to be within a certain range. In other words, the state in which the step edges are regularly arranged in the m-axis direction is necessary for growing the semiconductor layer 2 having the flat main surface 21. If the step edge interval or the step edge line is disturbed, Therefore, the semiconductor layer 2 having the flat main surface 21 cannot be formed. Below, the range which angle | corner (theta) s can take is demonstrated.

  As shown in FIG. 13, when the surface normal of the substrate main surface 11 is inclined in the m-axis direction and the a-axis direction, it is expressed as shown in FIG. As shown in FIG. 17A, the extending direction of the projection axis obtained by projecting the surface normal of the substrate main surface 11 onto the m-axis a-axis plane is the L direction, and a part of the substrate main surface 11 (for example, T2 region). An enlarged view of FIG. 17 is shown in FIG. A step surface 1d is formed at the stepped portion generated by inclining the flat surface 1c and the substrate main surface 11 with respect to the c-plane. Here, the terrace surface 1c is a c-plane (0001), but the terrace surface 1c is not parallel to the substrate main surface 11, but is an inclined surface, and the c-axis perpendicular to the terrace surface 1c is shown in FIG. Is quoted from the surface normal of the substrate main surface 11 by φ degrees.

  Since the substrate main surface 11 is inclined not only in the m-axis direction but also in the a-axis direction, the step surface 1d is formed obliquely, and the step surface 1d is formed in the L direction as shown in FIG. Will be lined up. This state appears as a step edge arrangement in the m-axis direction as shown in FIGS. Since the m-plane is a thermally and chemically stable plane, depending on the magnitude of the inclination angle component φa degree in the a-axis direction, the oblique step cannot be kept clean, as shown in FIG. The step surface 1d is uneven, the step edge arrangement is disturbed, and a film having a flat main surface cannot be formed on the substrate main surface 11.

In FIG. 18, when the substrate main surface 11 (growth surface) has an off angle (tilt angle component φa) in the a-axis direction in addition to an off angle (tilt angle component φm) in the m-axis direction, It shows how the step width changes. The inclination angle component φm degree in the m-axis direction described in FIG. 13 is fixed to 0.4 °, and the inclination angle component φa degree in the a-axis direction is changed and compared. The change of the inclination angle component φa degree was realized by changing the cut-out surface of the substrate 1 made of Mg _y Zn _1-y O.

  When the inclination angle component φa degree in the a-axis direction is changed so as to increase, the angle θs formed between the step edge and the m-axis direction also changes in the increasing direction. FIG. 18 shows the angle θs. FIG. 18A shows a case where θs = 85 °, but the step edge and the step width are not disturbed. FIG. 18B shows the case of θs = 78 °, but the step edge and step width can be confirmed although there is some disturbance. FIG. 18C shows the case of θs = 65 °, but the disturbance is severe and the step edge and step width cannot be confirmed. When a ZnO-based semiconductor layer is epitaxially grown on the substrate main surface 11 whose surface state is as shown in FIG. 18C, a ZnO-based semiconductor layer with poor surface flatness is formed. In the case of FIG. 18C, this corresponds to 0.15 ° when converted to the inclination angle component φa in the a-axis direction. From the above data, it can be seen that the range of 70 ° ≦ θs ≦ 90 ° is preferable for growing the semiconductor layer 2 on the substrate main surface 11 with good flatness of the main surface 21.

  By the way, with respect to θs, not only when the surface normal of the substrate main surface 11 is inclined by the inclination angle component φa degree in the + a axis direction, but also when it is inclined in the −a axis direction in FIG. Should be considered because they are more equivalent. When this inclination angle component is set to −φa degree and the stepped portion due to the step surface is projected onto the m-axis a-axis plane, it is expressed as shown in FIG. Here, regarding the condition of the angle θi formed by the m-axis direction and the step edge, 70 ° ≦ θi ≦ 90 ° is established as in the case of the angle θs. Since the relationship θs = 180 ° −θi is established, the maximum value of θs is 180 ° −70 ° = 110 °, and finally, a range of 70 ° ≦ θs ≦ 110 ° is formed on the substrate main surface 11. The condition is that the semiconductor layer 2 having a flat main surface 21 can be grown.

The angle α between the projection axis obtained by projecting the surface normal of the substrate principal surface 11 onto the m-axis a-axis plane and the c-axis direction shown in FIG. 13 is expressed by the following equation (1):

α = (180 / π) arctan {tan (πφa / 180) / tan (πφm / 180)} (1)

Here, the unit of the angle α, the inclination angle component φm, and the inclination angle component φa is degrees, and tan represents a tangent and arctan represents an arctangent. Based on FIG. 13, when the angle θs having the unit as degrees (deg) is expressed using the inclination angle component φm and the inclination angle component φa, the following equation (2) is obtained:

θs = 90−α
= 90- (180 / π) arctan {tan (πφa / 180) / tan (πφm / 180)} (2)

From the equation (2), the following equation (3) is obtained as a preferable range of the angle θs for forming the semiconductor layer 2 having the flat principal surface 21 on the substrate principal surface 11:

70 ≦ 90− (180 / π) arctan {tan (πφa / 180) / tan (πφm / 180)} ≦ 110 (3)

The case of θs = 90 ° is a case where the surface normal of the substrate main surface 11 is not inclined in the a-axis direction and is inclined only in the m-axis direction.

  As already described, in order to maintain the flatness of the main surface 21 of the semiconductor layer 2 in order to suppress unintended acceptor doping into the semiconductor layer 2, it is preferable that 0 ° <φm ≦ 3 °. Therefore, a preferable range of the inclination angle component φa degree can be calculated from the equation (3) for determining the inclination angle component φm degree.

  Hereinafter, a method of manufacturing the semiconductor element shown in FIG. 1 by the thin film forming apparatus shown in FIG. 2 will be described. The semiconductor device manufacturing method described below is merely an example, and it is needless to say that the present invention can be realized by various other manufacturing methods including this modification. Note that the substrate 1 in the following description is the angle (inclination angle component φa degree) formed by the projection axis obtained by projecting the surface normal of the substrate principal surface 11 onto the a-axis c-axis plane and the c-axis direction as described above. ) Is 0.1 ° or less, and the angle (tilt angle component φm degrees) between the projection axis obtained by projecting the surface normal of the substrate principal surface 11 onto the m-axis and c-axis plane and the c-axis direction is 3 ° or less. A system substrate is preferred. Or it is preferable that the board | substrate 1 is a board | substrate with which inclination-angle component (phi) a degree and inclination-angle component (phi) m degree satisfy the conditions of Formula (3).

  (A) A substrate 1 having a + c plane as a main surface, for example, made of ZnO is etched with hydrochloric acid, washed with pure water, and then dried with dry nitrogen.

  (B) As shown in FIG. 2, the substrate 1 set in the holder 110 is put into a thin film forming apparatus used for the MBE method from the load lock.

(C) The substrate 1 is heated at 900 ° C. for 30 minutes in a vacuum of about 1 × 10 ⁻⁷ Pa.

(D) The substrate temperature is lowered to 900 ° C., NO gas and O ₂ gas are supplied to the cell 120 to generate plasma, and supplied together with Mg and Zn adjusted to have a desired composition in advance on the substrate 1 A semiconductor layer 2 made of Mg _x Zn _1-x O is grown. Thereafter, acceptor doping such as nitrogen is performed on the semiconductor layer 2.

What is important in the manufacturing conditions of the semiconductor layer 2 is the substrate temperature. As already described, if the main surface 21 of the semiconductor layer 2 is flat like the sample A shown in FIG. 3, Si or the like knocked out from the discharge tube 131 by the sputtering phenomenon is not doped into the semiconductor layer 2. Since nitrogen easily enters the hexagonal + c plane, it is presumed that the + c plane has a mechanism for eliminating cations (cathodic ions) (for example, there is a polarization charge in +). Therefore, the substrate temperature needs to be 750 ° C. or more, and Mg _x Zn _1-x O tends to increase this lower limit temperature. However, if the substrate temperature is 800 ° C., the Mg composition has a Mg composition up to about 20%. The flatness of the main surface 21 can be maintained in the semiconductor layer 2 made of _x Zn _1-x O. Of course, the substrate temperature can be arbitrarily set to a temperature at which the flatness of the main surface 21 can be maintained.

The temperature can be measured, for example, with a pyrometer with Ti / Pt attached to the back surface of the substrate 1 or with a thermoviewer. The temperature shown above is a value measured at ε = 0.18 in the case of a pyrometer and ε = 0.71 in the case of a thermoviewer. When using a thermoviewer, it is necessary to consider the thin film growth equipment. That is, since a glass or quartz viewport normally used in a thin film growth apparatus does not transmit the 8-14 μm wavelength region of the measurement wavelength, a viewport using a barium fluoride (BaF ₂ ) crystal as a window material is used. With this apparatus, long-wavelength infrared can be transmitted through the viewport, and ZnO has a low transmittance in this wavelength region, so there is little risk of measuring the radiation temperature of an object behind the substrate 1, and the substrate temperature It is preferable for the measurement. The substrate temperature (using a heater thermocouple or the like) measured by a method other than the above pyrometer or thermoviewer is not suitable for measuring the substrate temperature because it does not measure the temperature of the substrate itself.

As described above, the present invention shows the conditions of the semiconductor element for suppressing unintentional doping of Si or the like into the semiconductor layer 2, and the semiconductor element shown in FIG. When the main surface 21 of the semiconductor layer 2 is observed by the above, the density of the observed hexagonal pits 201 is 5 × 10 ⁶ pieces / cm ² or less, or the concave portion in which a plurality of microcrystal 203 protrusions are formed at the bottom 202 is not observed. Further, the substrate 1 is used such that the inclination of the substrate main surface 11 in the a-axis direction and the m-axis direction satisfies the condition that the main surface 21 of the semiconductor layer 2 grown on the substrate main surface 11 is flat. The As a result, in the semiconductor element that satisfies the above conditions, unintentional doping of impurities into the semiconductor layer 2 can be suppressed. Therefore, acceptor doping of nitrogen or the like to the semiconductor layer 2 is facilitated, and the semiconductor layer 2 made of Mg _x Zn _1-x O can be made a p-type semiconductor layer.

  As described above, the present invention has been described according to the embodiment. However, it should not be understood that the description and drawings constituting a part of this disclosure limit the present invention. From this disclosure, various alternative embodiments, examples, and operational techniques will be apparent to those skilled in the art. That is, it goes without saying that the present invention includes various embodiments not described herein. Accordingly, the technical scope of the present invention is defined only by the invention specifying matters according to the scope of claims reasonable from the above description.

It is a schematic diagram which shows the structure of the semiconductor element which concerns on embodiment of this invention. It is a schematic diagram which shows an example of a thin film forming apparatus. 3A and 3B are diagrams illustrating characteristics of a semiconductor device according to an embodiment of the present invention, FIG. 3A is a graph illustrating impurity concentration, and FIG. 3B is a diagram illustrating a state of a main surface. FIG. 4A is a graph illustrating the characteristics of a semiconductor element of a comparative example, FIG. 4A is a graph illustrating an impurity concentration, FIG. 4B is a diagram illustrating a state of a main surface, and FIG. It is sectional drawing of 4 (b). FIG. 5A is a schematic diagram for explaining CV measurement, FIG. 5A is a top view of a measurement target sample, and FIG. 5B is a side view of the measurement target sample. It is the figure which expanded FIG.4 (b). FIG. 7A is a graph showing the impurity concentration, FIG. 7B is a diagram showing the state of the main surface, and FIG. These are the figures which expanded FIG.7 (b). It is a figure which shows the other characteristic of the semiconductor element which concerns on embodiment of this invention, FIG.8 (a) is a graph which shows impurity concentration, FIG.8 (b) is a figure which shows the state of a main surface, FIG. 8C is an enlarged view of FIG. FIG. 9A is a diagram illustrating a state of the main surface of the semiconductor element according to the embodiment of the present invention and a comparative example, and FIG. 9A is a diagram illustrating a state of the main surface of the sample of FIG. ) Is an enlarged view of FIG. 9 (a), FIG. 9 (c) is a view showing the state of the main surface of the sample of FIG. 6, and FIG. 9 (d) is an enlarged view of FIG. 9 (c). FIG. It is a figure for demonstrating a pit. It is a graph which shows the relationship between interface Si density | concentration and Si density | concentration in a film | membrane. It is a schematic diagram for demonstrating a hexagonal crystal structure. It is a schematic diagram for demonstrating the inclination with respect to c surface of a ZnO-type board | substrate. FIG. 14A is a schematic diagram showing the relationship between the step edge and the m-axis. FIG. 14A shows a case where the surface normal is inclined in the + a-axis direction, and FIG. 14B shows that the surface normal is inclined in the −a-axis direction. Show the case. FIG. 15A is a schematic diagram for explaining the inclination of the surface normal of the substrate main surface. FIG. 15A shows the case where the surface normal is not inclined, and FIG. 15B shows the case where the surface normal is inclined only in the m-axis direction. FIG. 15C shows the state of the main surface in FIG. 15B, and FIG. 15D shows the relationship between the substrate main surface and the c-plane. It is a figure which shows the state of the main surface of the semiconductor film formed on the board | substrate inclined with respect to c surface, FIG.16 (a) is an inclination angle, when FIG. Shows a case of 3.5 °. FIG. 17A is a schematic diagram for explaining the inclination of the surface normal of the substrate main surface. FIG. 17A shows a case where the surface normal is inclined in the m-axis direction and the a-axis direction, and FIG. The state of the main surface in a) is shown. FIG. 18A is a diagram showing a state of a substrate main surface of a substrate having a different surface normal to the a-axis direction in the a-axis direction by changing the angle θs formed by the step edge and the m-axis direction. When the angle θs is 85 °, FIG. 18B shows the case where the angle θs is 78 °, and FIG. 18C shows the case where the angle θs is 65 °.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Semiconductor layer 10 ... Thin film forming apparatus 11 ... Substrate main surface 21 ... Main surface 110 ... Holder 120 ... Cell 130 ... Cell 131 ... Discharge tube 132 ... High-frequency coil 140 ... Cell 201 ... Pit 202 ... Recess 203 ... Fine crystal

Claims (3)

It consists of Mg _x Zn _1-x O (0 ≦ x <1) containing p-type impurities, and the observed hexagonal pit density is 5 × 10 ⁶ pieces / cm ² or less in observation with an atomic force microscope, Alternatively, a ZnO-based thin film comprising a main surface satisfying at least one of the following: a recess having a plurality of microcrystalline protrusions formed on the bottom is not observed. It consists of Mg _x Zn _1-x O (0 ≦ x <1) containing p-type impurities, and the observed hexagonal pit density is 5 × 10 ⁶ pieces / cm ² or less in observation with an atomic force microscope, Or a ZnO-based thin film having a main surface satisfying at least one of the following: a recess having a plurality of microcrystalline protrusions formed on the bottom is not observed;
A substrate composed of Mg _y Zn _1-y O (0 ≦ y <1) and having a substrate principal surface in contact with the ZnO-based thin film, wherein the normal line of the substrate principal surface is the m-axis c-axis plane of the substrate crystal axis A projection axis projected onto the semiconductor element is inclined within a range of 3 degrees or less in the m-axis direction. The projection axis obtained by projecting the normal line of the substrate main surface onto the a-axis c-axis plane of the substrate crystal axis is φa degrees in the a-axis direction, and the normal line of the substrate main surface is projected onto the m-axis c-axis plane of the substrate crystal axis. The projection axis is inclined φm degrees in the m-axis direction, and φa and φm are
70 ≦ 90− (180 / π) arctan {tan (πφa / 180) / tan (πφm / 180)} ≦ 110
The semiconductor element according to claim 2, wherein the relationship is satisfied.