Patent Application
20100230671
The present invention relates to a ZnO-based semiconductor including a nitrogen-doped MgZnO crystalline material and a ZnO-based semiconductor device using the ZnO-based semiconductor.
Studies have been made on application of devices made of a ZnO-based semiconductor, which is a type of oxide, to an ultraviolet LED used as a light source for illumination, backlight or the like, a high-speed electronic device, a surface acoustic wave device, and so forth. ZnO has drawn attention to its versatility, large light emission potential and the like. However, no significant development has been made on ZnO as a semiconductor device material. The largest obstacle is that p-type ZnO cannot be obtained because of difficulty in acceptor doping. Nevertheless, as demonstrated by Non-patent Documents 1 and 2, technological progress of recent years has made it possible to produce p-type ZnO, and has proven that light is emitted from the p-type ZnO. Accordingly, active research on ZnO is underway.
A proposal has been made on use of nitrogen as an acceptor for obtaining p-type ZnO. As disclosed in Non-patent Document 4, when ZnO is doped with nitrogen as an acceptor, the temperature of the substrate needs to be lowered because the efficiency of nitrogen doping heavily depends on a growth temperature. However, the lowering of the substrate temperature degrades crystallinity and forms a carrier compensation center that compensates the acceptor, and thus nitrogen is not activated (self-compensation effect). This makes the formation of a p-type ZnO layer, itself, extremely difficult.
With this taken into consideration, Non-patent Document 2 has disclosed a method of forming a p-type ZnO-based layer with a high carrier density by using a −C plane as a principal surface for growth and also using repeated temperature modulation (RTM) in which a growth temperature is alternately changed between 400° C. and 1000° C., the method thereby taking advantage of the temperature dependency of the efficiency of nitrogen doping.
However, this method involves the following problems. The continuous process of heating and cooling results in the alternate repetition of thermal expansion and contraction of the manufacturing machine. This imposes heavy burden on the manufacturing machine. For this reason, the manufacturing machine requires an extensive configuration, and periodic maintenance service at shorter intervals. Furthermore, the method requires the temperature to be accurately controlled because the doping amount is determined by a part of the process at the lower temperature. However, it is difficult to control the temperature so that the temperature will reach 400° C. and 1000° C. accurately in a short time period, and the reproducibility and stability of the doping thus become inadequate. Further, since the method uses a laser as a heating source, the method is not suitable for heating a large area. In addition, it is difficult to grow multiple semiconductor films, although the growth of multiple semiconductor films is needed to reduce device manufacturing costs.
On the other hand, as described in Non-patent Document 3, for example, it has been known that use of the +C plane of a ZnO substrate as the substrate for growth makes the doping of nitrogen easier. So, it is conceivable to use this method to solve the above-described problems. Use of the +C plane allows the doping of a certain amount of nitrogen to be secured even when the substrate temperature is increased, so that the above-described problems that would occur otherwise at the time of the RTM can be solved. Nevertheless, the self-compensation effect still remains. This allows no complete activation of nitrogen, making the conversion into the p-type still difficult.
An object of the present invention, made to solve the above-described problems, is providing a ZnO-based semiconductor capable of alleviating the self-compensation effect and of achieving easier conversion into p-type, and a ZnO-based semiconductor device.
To achieve the above object, the invention according to claim 1 is a ZnO-based semiconductor including a MgXZn1-XO (0<X<1) crystalline material doped with nitrogen, wherein, in a spectrum distribution curve obtained by a photoluminescence measurement performed on the ZnO-based semiconductor at an absolute temperature of 12 Kelvin, a peak intensity of the distribution curve obtained at 3.3 eV or larger is stronger than a peak intensity of the distribution curve obtained at 2.7 eV or smaller.
The invention according to claim 2 is a ZnO-based semiconductor including a MgXZn1-XO (0<X<1) crystalline material doped with nitrogen, wherein, in a spectrum distribution curve obtained by a photoluminescence measurement performed on the ZnO-based semiconductor at an absolute temperature of 12 Kelvin, an integral intensity of the distribution curve obtained at 3.3 eV or larger is stronger than an integral intensity of the distribution curve obtained at 2.7 eV or smaller.
The invention according to claim 3 is a ZnO-based semiconductor including a MgXZn1-XO (0<X<1) crystalline material doped with nitrogen, wherein, in a spectrum distribution curve obtained by a photoluminescence measurement performed on the ZnO-based semiconductor at an absolute temperature of 12 Kelvin, when an integral intensity of the distribution curve obtained at 3.3 eV or larger is denoted by A and an integral intensity of the distribution curve obtained at 2.7 eV or larger is denoted by B, (A/B)≧0.3 is satisfied.
The invention according to claim 4 is the ZnO-based semiconductor according to claim 3, wherein the (A/B) is equal to or larger than 0.4.
The invention according to claim 5 is a ZnO-based semiconductor including a MgXZn1-XO (0<X<1) crystalline material doped with nitrogen, wherein, in a spectrum distribution curve obtained by a photoluminescence measurement performed on the ZnO-based semiconductor at an absolute temperature of 12 Kelvin, when an integral intensity of the distribution curve obtained at 3.3 eV or larger is denoted by A and an integral intensity of the distribution curve obtained at 2.7 eV or larger is denoted by B, {A/(B−A)}≧1 is satisfied.
The invention according to claim 6 is the ZnO-based semiconductor according to any one of claims 3 to 5, wherein to calculate the integral intensity A, the distribution curve at 3.3 eV or larger is approximated by a Gaussian curve, and then the Gaussian curve is integrated.
The invention according to claim 7 is the ZnO-based semiconductor according to claim 6, wherein, if a plurality of luminescence peaks exist in the distribution curve at 3.3 eV or larger, the luminescence peaks are approximated respectively by Gaussian curves.
The invention according to claim 8 is the ZnO-based semiconductor according to any one of claims 1 to 7, wherein a concentration of the doped nitrogen is equal to or higher than 1×1018 cm−3.
The invention according to claim 9 is the ZnO-based semiconductor according to any one of claims 1 to 7, wherein: the crystalline material is a laminate formed by laminating a plurality of layers of MgXZn1-XO (0≦Xn<1) with Mg composition ratios that are different from one another; and at least one of the MgZnO films is doped with nitrogen at a concentration that is equal to or higher than 1×1018 cm−3.
The invention according to claim 10 is the ZnO-based semiconductor according to any one of claims 1 to 9, wherein: the crystalline material includes a MgZnO substrate in which a principal surface on a crystal-growth-direction side has a C plane, and a MgYZn1-YO (0<Y<1) film which is formed on the MgZnO substrate; and a projection axis, obtained by projecting a normal line to the principal surface onto a m-axis/c-axis plane of substrate crystal axes, is inclined in the m-axis direction within a range of 3°.
The invention according to claim 11 is the ZnO-based semiconductor according to any one of claims 1 to 10, wherein the crystalline material is formed by a crystal growth process performed at a growth temperature of 750° C. or higher.
The invention according to claim 12 is a ZnO-based semiconductor device comprising the ZnO-based semiconductor according to any one of claims 1 to 11.
A ZnO-based thin film of the present invention is made of a nitrogen-doped MgxZn1-xO (0<X<1) crystalline material, and is formed so that a photoluminescence measurement on the crystalline material would show that the DAP luminescence is weaker than the band edge luminescence. In addition, the ZnO-based thin film of the present invention is formed so that the peak in the DAP luminescence is smaller than the peak in the band edge luminescence. With this configuration, the self-compensation effect can be particularly reduced, which in turn activates nitrogen. Accordingly, it is possible to obtain a MgZnO thin film or a MgZnO laminate having a crystal quality that is high enough to use the MgZnO thin film or the MgZnO laminate as a p-type MgZnO. In addition, with the MgZnO thin film or the MgZnO laminate, it is possible to fabricate a high-performance ZnO-based semiconductor device.
The present invention is based on our discovery of the fact that a nitrogen-added MgXZn1-XO (0<X<1) crystalline material has an effect to alleviate the self-compensation effect with compared to a crystalline material made solely of ZnO and is easier to be converted to p-type. In addition, we have found parameters that are needed for the conversion into p-type. Some examples of the above-mentioned MgXZn1-XO (0<X<1) crystalline material are a single layer of a MgZnO film, a multilayer laminate obtained by laminating plural layers of MgZnO films, and a laminate of a MgZnO substrate and a MgZnO film.
Most of the studies thus far made to convert a ZnO-based semiconductor into a p-type one are about the p-type ZnO. Typical examples of ZnO-based semiconductors are CdZnO and MgZnO. CdZnO, which is a narrow-gap material, has been rarely studied because of the poisonous nature of Cd. MgZnO, which is a wide-gap semiconductor, has not been considered as a target for the study of conversion into p-type for the following reasons, for example. Firstly, as a usually observable tendency of a wide-gap material, MgZnO has a larger energy for activating the accepter energy (i.e., it is more difficult to generate holes). In addition, it is difficult to increase the purity of MgZnO as it is often made from sintered bodies.
The inventors have discovered that MgZnO has an effect to reduce the self-compensation effect, which is a fact that has been unknown until then.
As a photoluminescence measurement apparatus, an apparatus whose configuration is shown in
A white-circle (∘) curve represents the measurement results of the nitrogen-doped ZnO whereas the other two curves represent the measurement results of the two different kinds of nitrogen-doped MgZnO. The measurement was performed under the condition that the concentration of the doped nitrogen for ZnO was set at 2×1019 cm−3, and, as to MgZnO, the concentration of doped nitrogen for Mg0.1ZnO was set at 2×1019 cm−3 and the concentration of doped nitrogen for Mg0.11ZnO was set at 7×1018 cm−3. The horizontal axis in
The point P1 in each of
When EDAP is the energy of DAP luminescence, EG is the minimum excitation energy, ED is the donor level, EA is the acceptor level, rDA is the distance between the donor and the acceptor, ε0 is the vacuum permittivity, εr is the relative permittivity, e is the charges of electrons, h is the Planck's constant, and ωLO is the LO (longitudinal-optical) phonon frequency, then
E
DAP
=E
G
−E
D
−E
A+(e2/4πε0εrrDA)−(mhωLO/2π).
Here, m is an integer that is equal to or larger than zero.
The DAP luminescence peak position is determined by the equation above. So, given kinds of the donor and of the acceptor and their respective concentrations, the DAP luminescence peak position is determined.
If a line at 3.3 eV is the border to separate the region of band edge luminescence from the region of DAP luminescence, the region of DAP luminescence appears at the lower-energy side of the 3.3-eV line. In addition, as
It is a well-known fact that as the density of the PL excitation light is raised, a blue shift of the luminescence peak of the DAP luminescence occurs. This phenomenon is means that is principally used for identifying the DAP luminescence. The solid-line curve and the dashed-line curve are of the wide-gap MgZnO. So, along the curves of the MgZnO, similar peaks to the band edge luminescence peak for the ZnO are observable, though slightly, at the same positions as that of the band edge luminescence peak P1 for the ZnO. This observation leads to easy understanding of the fact that in the case of the nitrogen-doped ZnO, the DAP luminescence is stronger than the ZnO band edge luminescence when the photon energy equals 3.3 eV or smaller. In the case of ZnO, the band edge luminescence becomes weaker and the DAP luminescence becomes stronger at the time of acceptor doping. Such a trend can be observed also in the cases of ZnSe and GaN, and is therefore quite normal. The fact is a reason why ZnO has been the commonly used material for the conversion into p-type.
The behavior of MgZnO is totally different as
In addition, strong luminescence was observed when a nitrogen-doped MgZnO that has weak DAP luminescence and a ZnO substrate were bonded together. So, the observation showed that forming a nitrogen-doped MgZnO that has weak DAP luminescence is a parameter to achieve the conversion into p-type.
Next, the luminescence spectrum region of the PL measurement is divided into two regions, and the luminescence intensities of these two regions are compared with each other to quantify the parameter for the conversion into p-type. Firstly, on the basis of
As
The nitrogen-doped MgZnO of different concentrations of doped nitrogen were subjected to a PL measurement to obtain spectrum distribution curves. Concerning each of the spectrum distribution curves, the PL intensity was integrated for an energy region starting from 3.3 eV until no PL luminescence can be observed. The value of integral is denoted by A. In this case, as can be seen from
f(x)={K/(2π)1/2}×exp{−(x−m)2/2σ2}
where m is the average or the median value, σ is the standard deviation, and K is a constant.
Specifically, the values of m, σ, and K in the Gaussian curve are changed to calculate a curve that approximates most to the shape of the band edge luminescence peak, and the curve is used to obtain the value of integral A for a range from 3.3 eV to 3.6 eV. The fitting with a Gaussian curve is convenient particularly if there are plural band edge peaks. For example, if the nitrogen-doped MgZnO layer 2 is made of a laminate of MgZnO films having different concentrations of the doped nitrogen as in the case shown in
To be more specific, as
The value of integral A will be referred to as the band edge integral intensity, meaning the value of integral in the band edge luminescence region. Subsequently, the PL intensity is integrated for the energy region from 2.7 eV, which is the border between the deep-level luminescence region and the DAP luminescence region, to a region where no PL luminescence can be observed. The value of integral thus obtained will be denoted by B. In this case, as
PL measurements are performed on the MgZnO and the ZnO with varied nitrogen concentrations, and the proportion A/B, that is, the proportion of the band edge integral intensity to the total integral intensity (the proportion is represented by the vertical axis) is calculated. The calculated results are plotted on the graph of
To calculate A from the data shown in
X1 shown in
As has been described above, the laminate described in
For conversion into p-type, it is necessary to reduce the self-compensation effect and, in addition to prevent the impurities such as Si serving as the donor from being taken into the MgZnO film. In the fabrication of a MgZnO thin film, a radical generator is used as an apparatus to supply a gas element when oxygen, which is a gas element, is supplied, or when nitrogen, which is a gas element, is doped as an acceptor.
A radical generator (radical cell) includes a hollow discharge tube, a high-frequency coil wound around the outer circumference of the discharge tube, and the like. When a high-frequency voltage is applied to the high-frequency coil, the gas introduced into the discharge tube is turned to plasma and is discharged.
The plasma particles are, however, high-energy particles, so that sputtering phenomenon is caused by the plasma particles. The inner wall of the discharge tube is always sputtered by the plasma particles, and the atoms forming the discharge tube are struck out and mixed into the plasma particles.
In the case of an oxide such as the MgZnO thin film, because the gas component is oxygen, the material often used for the discharge tube in the radical cell is not a material that will be decayed by the oxidation, such as pBN, but is quartz. Quartz is used because, for the time being, it is not easy to obtain a highly insulating material that is as highly pure as quarts. Even in the case of quartz, however, the sputtering by the plasma particles flies Si, Al, B, and the like, which form parts of the discharge tube.
In particular, the amount of flying Si, which is one of the elements included in quartz, is large. The flying Si is supplied directly onto the surface of a growth substrate from a discharging opening of the discharge tube together with the raw-material gas, and is taken into the MgZnO thin film. It is easy to imagine that the Si thus taken into MgZnO occupies the site of Zn. The Si thus occupying the Zn site functions as a donor, and makes it more difficult to achieve the conversion into p-type.
As a solution to this problem, the inventors have found that even if the ZnO-based thin film is formed by crystal growth using a radical cell or the like, a flatter surface of the ZnO-based thin film helps to exclude unintended impurities such as Si. Japanese Patent Application No. 2007-221198, which has been already filed, describes the finding.
In particular, Si is one of the elements included in the discharge tube of the radical cell, and is the substance that is mixed in the most. So, Si is taken as an example for the following description.
In each of
The images superposed in the graphs show that the surface flatness of the MgZnO thin film is better in
As shown above, the mixing of impurities such as Si depends on the surface flatness of the MgZnO thin film. Next, description will be given below as to the fact that the flatness of the MgZnO thin film formed on the ZnO substrate 1 depends on the off angle formed between the direction of the normal line to the crystal-growth-side surface of the ZnO substrate 1 and the c-axis, which is one of the crystal axes of the substrate.
Like GaN, ZnO-based compounds have a hexagonal crystal structure known as Wurtzite. The terms such as the C plane and the a-axis can be expressed by so-called Miller indices. For example, the C plane is expressed as (0001) plane. If a MgZnO thin film is formed on a ZnO substrate by crystal growth, the direction of the normal line to the crystal-growth-side principal surface of the ZnO substrate may coincide with the c-axis of the crystal axes of the substrate. Otherwise, the normal line Z to the principal surface of the substrate is usually inclined as shown in
Now, suppose a case where the normal line Z to the principal surface of the substrate exists on the c-axis/m-axis plane of the crystal axes of the substrate. When a ZnO-based thin film is made to grow on a ZnO-based material layer, the growth is usually performed on the C plane, that is, the (0001) plane. If a C-plane just substrate is used, the direction of the normal line Z to the wafer's principal surface coincides with the c-axis direction. It is a well-known fact that even if a ZnO-based thin film is made to grow on a C-plane just MgZnO substrate, no improvement can be achieved in the flatness of the film. In addition, in a bulk crystal, the direction of the normal line to the wafer's principal surface does not coincide with the c-axis direction unless a cleavage plane that the crystal has is used. In addition, the use of only the C-plane just substrate results in lower productivity.
Accordingly, the direction of the normal line to the principal surface of a MgZnO substrate 10 (wafer) is made not to coincide with the c-axis direction. That is, the direction of the normal line Z is inclined from the c-axis of the principal surface of the wafer within the c-axis/m-axis plane, so that an off angle is formed between the direction of the normal line Z and the c-axis. As
Note that each terrace surface 1a corresponds to the C plane (0001) whereas each step surface 1b corresponds to the M plane (10-10). As
In this way, if the step surfaces are formed as surfaces corresponding to the M planes, a ZnO-based semiconductor layer formed by crystal growth on a principal surface can be made as a flat film. Although level-difference portions are formed in the principal surface by the step surfaces 1b, each of the flying atoms that come to these level-difference portions is bonded to the two surfaces, that is, one of the terrace surfaces 1a and a corresponding one of the step surfaces 1b. Accordingly, such atoms can be bonded more strongly than the flying atoms that come to the terrace surfaces 1a. Consequently, the flying atoms can be trapped stably by the level-difference portions.
In a surface diffusion process, the flying atoms are diffused within each terrace. Such atoms are trapped at the level-difference portions where the bonding force is stronger or at kink positions that are formed in the level-difference portions. The trapped atoms are taken into the crystal. The kind of crystal growth that progresses in this way is known as a lateral growth, and is a stable growth. Accordingly, if a ZnO-based semiconductor layer is laminated on a substrate with the normal line to the principal surface of the substrate inclined at least in the m-axis direction, the crystal of the ZnO-based semiconductor layer grow around the step surfaces 1b. Consequently, a flat film can be formed.
To put it differently, what are necessary for the fabrication of a flat film is the step lines 1e which are arranged regularly in the m-axis direction and which have a perpendicular relationship with the m-axis direction. In contrast, if the intervals and the lines of the step lines 1e are improper, the lateral growth described above cannot progress. Consequently, no flat film can be fabricated.
If the inclination angle (off angle) θ shown in
The flatness of a MgZnO film depends also on the growth temperature. Japanese Patent Application No. 2007-27182, which has been already filed, describes in detail the growth-temperature condition. The points will be described again below. ZnO thin films were formed on MgZnO substrates by crystal growth, and the irregularities in the surface of each ZnO thin film were measured. The crystal growth temperature of the ZnO thin film was changed in a fine pitch, and the flatness of the ZnO surface at each temperature was quantified. The graphs of
To obtain the roughness curve, the irregularities formed in the film surface and observed as shown in the superposed images of
In
With the reference length 1 used in the calculation of the arithmetic mean roughness Ra, the root mean square roughness RMS is expressed as
RMS={(1/1)×∫(f(x))2dx}1/2(integral interval is from 0 to 1)
In
Accordingly, when a ZnO-based thin film is made to grow on a MgZnO substrate, a flatter film can be obtained by an epitaxial growth process performed with the substrate temperature kept at 750° C. or higher. In addition, when a layer of a ZnO-based thin film such as a MgZnO film is laminated repeatedly on top of a MgZnO substrate, keeping the substrate temperature at 750° C. or higher allows all the layers of films to be laminated flatly until the uppermost layer, and also prevents mixing of donor impurities such as Si.
Description of the device shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-251482 | Sep 2007 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/067516 | 9/26/2008 | WO | 00 | 3/26/2010 |
