JP5507234B2 - ZnO-based semiconductor device and manufacturing method thereof - Google Patents

Description

  The present invention relates to a ZnO-based semiconductor device having an n-type ZnO-based semiconductor layer and a method for manufacturing the same.

  Zinc oxide (ZnO) is a direct transition type semiconductor having a band gap energy of 3.37 eV at room temperature. In recent years, application to light emitting diodes (LED) such as ultraviolet light and white light is expected. In addition, the raw material is inexpensive and has a feature that there are few adverse effects on the environment and the human body, and is highly industrially useful.

  For the formation of an n-type ZnO-based semiconductor required for manufacturing a light-emitting element or the like, a group III element such as Ga is used as an n-type dopant. A variety of n-type doping techniques are desired for ZnO-based semiconductors.

Hiroyuki Kato, Kazuhiro Miyamoto, Michihiro Sano "MBE-ZnO growth on c-plane sapphire and ZnO substrates-Improvement of crystal quality and n-type doping-" Applied Physics Society, Crystal Engineering Subcommittee 120th Meeting Text, 2004 Year, p. 27-34

  An object of the present invention is to provide a ZnO-based semiconductor device and a method for manufacturing the same according to a novel n-type doping technique for a ZnO-based semiconductor.

According to an aspect of the present invention, a step of preparing a substrate, and Zn, O, and N are supplied above the substrate, and N is doped, so that the n-type carrier concentration is higher than when N is not doped. Forming a ZnO-based semiconductor layer exhibiting increased n-type conductivity , and supplying Zn, O, and a p-type dopant above the ZnO-based semiconductor layer exhibiting n-type conductivity, thereby forming p-type conductivity And a step of forming a ZnO-based semiconductor layer showing a ZnO-based semiconductor device. According to another aspect of the present invention, a step of preparing a substrate, and Zn, O, and a p-type dopant are supplied above the substrate to form a ZnO-based semiconductor layer exhibiting p-type conductivity. N, in which Zn, O, and N are supplied above the ZnO-based semiconductor layer exhibiting p-type conductivity and doped with N, thereby increasing the n-type carrier concentration compared with the case where N is not doped. A method of manufacturing a ZnO-based semiconductor device is provided that includes a step of forming a ZnO-based semiconductor layer exhibiting type conductivity.

  By doping N, the n-type carrier concentration of the ZnO-based semiconductor layer can be increased.

FIG. 1 is a schematic cross-sectional view showing an example of an MBE apparatus. FIG. 2 is a schematic cross-sectional view showing a sample structure of the first experiment. FIG. 3A is a graph showing the dependence of the n-type carrier concentration and specific resistance on the flux ratio in the first experiment, and FIG. 3B is a graph showing the dependence of the N concentration on the flux ratio in the first experiment. FIG. 4 is a schematic cross-sectional view showing sample structures of the second and third experiments. 5A to 5C are graphs showing XRD 2θ-θ measurement results for the undoped sample, the first, and the second n-type sample, respectively, in the third experiment. FIG. 6 is a graph showing the XRD 2θ-θ measurement results for the p-type sample of the third experiment. FIG. 7A is a schematic cross-sectional view illustrating a sample structure of a fourth experiment (light-emitting elements of Examples and Comparative Examples), and FIGS. 7B and 7C are schematic cross-sectional views of a modified active layer structure. 8A is a profile in the depth direction of the N concentration of the light emitting element of the example, FIG. 8B is a graph showing the current-voltage characteristics of the light emitting element of the example, and FIG. 8C is an EL of the light emitting element of the example. It is a spectrum. FIG. 9A is a depth direction profile of the Ga concentration and N concentration of the light emitting element of the comparative example, and FIG. 9B is a graph showing the current-voltage characteristics of the light emitting element of the comparative example.

  First, a molecular beam epitaxy (MBE) apparatus used for forming an N-doped ZnO-based semiconductor layer or the like according to an embodiment of the present invention will be described.

  FIG. 1 is a schematic cross-sectional view showing an example of an MBE apparatus. The vacuum chamber 1 includes a zinc (Zn) source gun 2, a magnesium (Mg) source gun 3, a gallium (Ga) source gun 4, an oxygen (O) source gun 5, and a nitrogen (N) source gun 6.

  Each of the Zn source gun 2, the Mg source gun 3, and the Ga source gun 4 includes a Knudsen cell that accommodates solid sources of Zn, Mg, and Ga, and emits a Zn beam, an Mg beam, and a Ga beam.

Each of the O source gun 5 and the N source gun 6 includes an electrodeless discharge tube using a radio frequency (RF) of 13.56 MHz, for example, and emits an O radical beam and an N radical beam. For example, O ₂ is used as the O source. For example, N ₂ is used as the N source. Note that various gases containing N, such as N ₂ O, NO, and NH ₃ , can be used as the N source.

It is also possible to introduce N ₂ and O ₂ into the same source gun and emit an O radical beam and an N radical beam from the same source gun.

  A substrate holder 7 including a substrate heater is disposed in the vacuum chamber 1, and the substrate holder 7 holds the substrate 8. By supplying a desired beam on the substrate 8 at a desired timing, a ZnO-based semiconductor layer having a desired composition can be grown.

  The vacuum chamber 1 also includes a gun 9 for reflection high-energy electron diffraction (RHEED), and a screen 10 that displays an RHEED image.

The ZnO-based semiconductor contains at least Zn and O. By adding Mg that substitutes Zn to ZnO, the band gap can be widened. ZnO and MgZnO to which Mg is added are collectively expressed as Mg _x Zn _1-x O. The Mg composition x is 0 for ZnO to which no Mg is added. Further, the upper limit composition x of Mg to be added is about 0.6 so that the hexagonal system of ZnO is maintained.

N can be added to Mg _x Zn _1-x O as a p-type dopant. The n-type conductivity of Mg _x Zn _1-x O can be obtained without adding an n-type dopant, but Ga can be added to increase the n-type carrier concentration.

  Further, as discovered by the present inventors, N can also be used as an n-type dopant. As will be described later, in the ZnO-based semiconductor light emitting device, when Ga is used as the n-type dopant, problems such as diffusion of Ga into the p-type semiconductor layer occur. However, if N is used as the n-type dopant without using Ga. Such a problem can be solved.

  Next, the first experiment will be described. In the first experiment, the growth conditions under which n-type conductivity was obtained by N doping were investigated.

First, the II / VI flux ratio in Mg _x Zn _1-x O layer growth will be described. For group II elements, the Zn beam flux intensity is _JZn , and the Mg beam flux intensity is _JMg ( _JMg = 0 if no Mg is added). For Group VI elements, the flux intensity of O radical beam and _{J O.}

Mg _x Zn _1-x O crystals Zn to O termination surface, depositing a coefficient indicating the respective attachment easiness Mg coefficient _{k Zn,} and _{k Mg,} adhesion ease of O to Zn and Mg endface Let the coefficient shown be k 2 _O. At this time, k _Zn J _Zn , which is a product of _Zn adhesion coefficient k _Zn and flux strength J _Zn, and k _Mg J _Mg , which is a product of _Mg adhesion coefficient k _Mg and flux strength J _Mg , O adhesion coefficient k K _O J _O, which is the product of _O and flux strength J _O , corresponds to the number of Zn atoms, the number of Mg atoms, and the number of O atoms that adhere to the unit area of the substrate per unit time, respectively.

For flux intensity _k O _{J O} in consideration of the sticking coefficient of the Group VI element is the ratio of the flux intensity _{k Zn J Zn + k Mg J} Mg in consideration of the sticking coefficient of the Group II element _{(k Zn J Zn + k Mg} J Mg) / K _O J _O is defined as the II / VI flux ratio. II / VI flux ratio = 1 is stoichiometric condition, II / VI flux ratio <1 is VI group element rich condition, and II / VI flux ratio> 1 is II group element rich condition. Group VI element-rich conditions are called O-rich conditions, and group II element-rich conditions are called Zn-rich conditions.

The Zn and Mg flux strengths _JZn and _JMg are obtained from the deposition rates _FZn and _FMg . Using a film thickness meter using a crystal resonator, the Zn and Mg deposition rates F _Zn and F _Mg at room temperature (cooling water temperature 15 ° C.) are measured by adjusting the cell temperature. The deposition rates F _Zn and F _Mg can be converted into Zn and Mg flux strengths J _Zn and J _Mg by calculation.

Next, a sample manufacturing method in the first experiment will be described with reference to FIG. FIG. 2 is a schematic cross-sectional view showing a sample structure of the first experiment. The c-plane sapphire substrate 21 was degreased and washed with an organic solvent, and then attached to a substrate holder in the chamber of the MBE apparatus, and a high vacuum of 1 × 10 ⁻⁷ Pa or less was applied. A heat treatment was performed at 800 ° C. for 30 minutes in a high vacuum to clean the substrate surface.

Next, the MgO buffer layer 22 having a thickness of 8 nm was formed by irradiating the sapphire substrate 21 with an Mg beam and an O radical beam at a substrate temperature of 650 ° C. The Mg beam was irradiated at a flux strength J _Mg of 1.0 × 10 ¹⁴ atoms / cm ² s (Mg deposition rate F _Mg of 0.025 nm / s). The irradiation with the O radical beam was performed by introducing O ₂ gas at a flow rate of ₂ sccm and turning it into plasma with an RF power of 300 W.

  With the MgO buffer layer 22, a ZnO layer exposing the + c plane can be grown thereon. Such a polarity control method is described in the section “Best Mode for Carrying Out the Invention” in Japanese Patent Application Laid-Open No. 2005-197410.

Next, the substrate temperature was lowered to 350 ° C., and the ZnO buffer layer 23 having a thickness of about 30 nm was formed on the MgO buffer layer 22 by irradiation with a Zn beam and an O radical beam. The Zn beam was radiated at a flux intensity J _Zn of 1.0 × 10 ¹⁴ atoms / cm ² s (Zn deposition rate F _Zn of 0.016 nm / s). The irradiation with the O radical beam was performed by introducing O ₂ gas at a flow rate of ₂ sccm and turning it into plasma with an RF power of 300 W.

  Then, irradiation with the Zn beam and O radical beam was stopped once, and the substrate temperature was raised to 850 ° C. and annealing was performed for 10 minutes, thereby improving the surface flatness of the ZnO buffer layer 23.

Next, the substrate temperature was lowered to 800 ° C., and an N radical beam was irradiated on the ZnO buffer layer 23 together with a Zn beam and an O radical beam to form an N-doped ZnO layer 24 having a thickness of about 1000 nm. The irradiation with the O radical beam was performed by introducing O ₂ gas at a flow rate of ₂ sccm and turning it into plasma with an RF power of 300 W. The irradiation of the N radical beam was performed by introducing N ₂ gas at a flow rate of 0.5 sccm and turning it into plasma with an RF power of 200 W.

In the formation of the N-doped ZnO layer 24, the Zn beam flux intensity J _{Zn is set} to 1.6 × 10 ¹⁴ atoms / cm ^{2 s to} 2.0 × 10 ¹⁵ atoms under constant irradiation conditions of the O radical beam and the N radical beam. The II / VI flux ratio was changed in the range of 0.26 to 3.24 by changing / cm ² s (Zn deposition rate F _Zn in the range of 0.025 nm / s to 0.31 nm / s).

When the Zn beam flux intensity J _Zn = 6.3 × 10 ¹⁴ atoms / cm ² s (Zn deposition rate F _Zn = 0.095 nm / s), the stoichiometric condition (k _Zn J _Zn = k _O J _O )Met.

  For each sample with a different II / VI flux ratio (hereinafter sometimes referred to simply as a flux ratio), the n-type carrier concentration and specific resistance of the N-doped ZnO layer 24 are measured by Hall measurement, and the N concentration is determined as a secondary ion. It was measured by mass spectrometry (SIMS).

  FIG. 3A is a graph showing the dependence of n-type carrier concentration and specific resistance on the flux ratio, and FIG. 3B is a graph showing the dependence of N concentration on the flux ratio.

It has been found that undoped ZnO layers exhibit n-type carrier concentrations below the order of 10 ¹⁵ cm ⁻³ . The N-doped ZnO layer has a flux ratio of 0.9 or more and an n-type carrier concentration of the order of 10 ¹⁶ cm ⁻³ or more, and exhibits an n-type conductivity significantly higher than that of the undoped ZnO layer. It has been found that the carrier concentration increases as the flux ratio increases. An n-type carrier concentration of 3.4 × 10 ¹⁶ cm ^{−3 to} 5.7 × 10 ¹⁷ cm ⁻³ was obtained at a flux ratio of 0.9 to 3.2.

The specific resistance shows a maximum value on the order of 10 ⁶ Ωcm at a flux ratio of about 0.5, and then rapidly decreases as the flux ratio increases. At a flux ratio of 0.9, the specific resistance decreases to the order of 10 ¹ Ωcm, and a flux ratio of 3 .2 has dropped to the order of 10 ⁻¹ Ωcm.

The N concentration is generally low under the O-rich condition and high under the Zn-rich condition. As the flux ratio decreases on the O-rich condition side, the N concentration decreases rapidly. On the Zn-rich side, as the flux ratio increases, the N concentration increases, but the slope of the increase becomes gradual and shows a saturation tendency. An N concentration of the order of 10 ²⁰ cm ⁻³ is obtained at a flux ratio of 0.9 or more. There is a tendency for the n carrier concentration to increase as the N concentration increases.

From the above results, in order to increase the n-type carrier concentration (for example, 1 × 10 ¹⁶ cm ⁻³ or more) with N doping (more than 1 × 10 ¹⁶ cm ⁻³ ), the II / VI flux ratio in the growth of the N-doped ZnO layer is set to 0. It can be said that it is preferably 9 or more. When the II / VI flux ratio is around 0.9, the n carrier concentration and the N concentration decrease sharply when the II / VI flux ratio is slightly reduced. From the viewpoint of stably obtaining a high n carrier concentration, II / VI The VI flux ratio is preferably 1 or more.

  Note that the upper limit of the II / VI flux ratio is considered as follows. Even at a high flux ratio, it is considered that an N-doped n-type ZnO layer can be formed under Zn-rich conditions. Therefore, it can be said that there is no upper limit of a special flux ratio other than due to restrictions on the apparatus used. The II / VI flux ratio is controlled by the deposition amount of the Zn beam and the Mg beam for the group II element and the O2 flow rate for generating the O radical and the power of the RF radical gun for the group VI element. The upper limit value of the flux ratio is determined by the conditions for determining the maximum value of the Zn beam amount and the minimum value of the O radical amount. For example, it is about 55.

Next, the reason why ZnO in which n-type conductivity is enhanced by N doping will be considered. When an N atom replaces the O site of ZnO, the N atom becomes an acceptor and functions as a p-type dopant. On the other hand, for example, if the N ₂ molecule is substituted with O site ZnO, N ₂ molecule is considered to behave as a double donor, thereby, N-doped ZnO is considered to exhibit n-type conductivity. It is considered that the higher the doped N concentration, the easier it is to generate N ₂ molecular double donors and the higher the n carrier concentration.

Next, a growth temperature range suitable for obtaining an n-doped n-type ZnO layer will be considered. The N ₂ molecule double donor is considered to be easily generated by the thermal energy as the growth temperature is higher, and a high growth temperature of 800 ° C. or higher is preferable.

  Further, under conditions close to Zn-rich or stoichiometry where the flux ratio is 0.9 or more, a low temperature of, for example, 700 ° C. or less is not preferable from the viewpoint of crystallinity because the ZnO layer grows three-dimensionally. In order to obtain two-dimensional growth, a growth temperature of at least 750 ° C. or higher is necessary, and 800 ° C. or higher is preferable.

  From the viewpoint of incorporation of N into the film, it is not preferable that the growth temperature exceeds 1000 ° C., because the adhesion coefficient of N is extremely reduced. From the above, it can be said that a growth temperature range suitable for obtaining an n-type ZnO layer by N doping is 800 ° C. or higher and 1000 ° C. or lower.

  N doping is usually used to obtain a p-type ZnO layer. Suitable conditions for obtaining a p-type ZnO layer with N doping are a flux ratio in the range of 0.5 to 0.9 and a growth temperature in the range of 300 ° C. to 800 ° C.

  Next, the second experiment will be described. In the second experiment, an N-doped ZnO layer grown at a low temperature lower than 800 ° C. was annealed after film formation to examine n-type conductivity.

  With reference to FIG. 4, a method for manufacturing a sample in the second experiment will be described. FIG. 4 is a schematic cross-sectional view showing a sample structure of the second experiment. The + c plane of the cleaned c-plane ZnO substrate 31 was irradiated with a Zn beam and an O radical beam at a substrate temperature of 300 ° C. to form a ZnO buffer layer 32 having a thickness of 30 nm. In order to improve the quality of the ZnO buffer layer 32, annealing was performed at 900 ° C. for 20 minutes.

Next, an N-doped ZnO layer 33 having a thickness of about 500 nm was formed on the ZnO buffer layer 32 by irradiating a Zn beam and an O radical beam as well as an N radical beam on the ZnO buffer layer 32. O radical beam irradiation was performed by introducing O ₂ gas at a flow rate of 0.5 sccm and turning it into plasma with an RF power of 200 W. The N radical beam was irradiated by introducing N ₂ gas at a flow rate of 1 sccm and turning it into plasma with an RF power of 100 W. The Zn beam flux intensity J _Zn is 2.0 × 10 ¹⁴ atoms / cm ² s (Zn deposition rate F _Zn is 0.03 nm / s), and the Zn-rich condition in which the II / VI flux ratio is about 1.3. The film was formed.

Next, the N-doped ZnO layer 33 was annealed under conditions of N ₂ gas atmosphere, 900 ° C., 10 minutes, and 1 atmosphere (101.3 kPa). The N concentration of the N-doped ZnO layer 33 after annealing was 2.8 × 10 ²⁰ cm ⁻³ , and the n-type carrier concentration by CV measurement was 2.0 × 10 ¹⁸ cm ⁻³ .

  Thus, it was found that even with an N-doped ZnO layer whose growth temperature is lower than 800 ° C., a high n-type carrier concentration can be obtained by annealing. The annealing temperature may be 800 ° C. or higher. Also, the upper limit of the annealing temperature will be about 1000 ° C.

  It should be noted that annealing for the N-doped ZnO layer and annealing for the lower ZnO buffer layer could be performed together.

  From the first experiment, it was found that a growth temperature of 800 ° C. or higher and 1000 ° C. or lower is preferable for increasing n-type conductivity by N doping. Further, from the second experiment, it was found that even when the growth temperature is lower than 800 ° C., high n-type conductivity due to N doping can be obtained by annealing at 800 ° C. or higher and 1000 ° C. or lower after film formation. These findings can be summarized as heat treatment at 800 ° C. or more and 1000 ° C. or less is preferable during or after the formation of the N-doped ZnO layer in order to increase n-type conductivity with N doping. .

  Next, a third experiment will be described. In the third experiment, the crystal structure of the N-doped n-type ZnO layer was examined. For comparison, the crystal structure of the N-doped p-type ZnO layer was also examined.

  Referring to FIG. 4 again, a method for preparing a sample in the third experiment will be described. The laminated structure of the sample of the third experiment is the same as that of the second experiment. The + c plane of the cleaned c-plane ZnO substrate 31 was irradiated with a Zn beam and an O radical beam at a substrate temperature of 300 ° C. to form a ZnO buffer layer 32 having a thickness of 30 nm. In order to improve the quality of the ZnO buffer layer 32, annealing was performed at 900 ° C. for 20 minutes.

  Next, the Zn-doped ZnO layer 33 was formed by irradiating the ZnO buffer layer 32 with a Zn beam, an O radical beam, and an N radical beam. A plurality of samples were produced by changing the film forming conditions of the N-doped ZnO layer 33.

  In the experiment for examining the N-doped n-type ZnO layer, two types of samples (first and second n-type samples) were produced. For comparison, an undoped ZnO layer 33 sample (undoped sample) that was not irradiated with an N radical beam was also produced.

The N-doped ZnO layer 33 of the first n-type sample was formed at a growth temperature of 850 ° C. and a thickness of about 500 nm. The Zn beam was irradiated with a flux intensity J _Zn of 9.9 × 10 ¹⁴ atoms / cm ² s (Zn deposition rate F _Zn of 0.15 nm / s). The O radical beam and the N radical beam were irradiated by mixing O ₂ gas (flow rate ₂ sccm) and N ₂ gas (flow rate 0.05 sccm) into the same source gun, and turning it into plasma with an RF power of 300 W. The II / VI flux ratio is about 1.2.

The N-doped ZnO layer 33 of the second n-type sample was formed at a growth temperature of 950 ° C. and a thickness of about 500 nm. The Zn beam was irradiated with a flux intensity J _Zn of 9.9 × 10 ¹⁴ atoms / cm ² s (Zn deposition rate F _Zn of 0.15 nm / s). The O radical beam was irradiated by introducing O ₂ gas into the O source gun at a flow rate of 0.5 sccm and turning it into plasma with an RF power of 200 W. The irradiation of the N radical beam was performed by introducing N ₂ gas into the N source gun at a flow rate of 1 sccm and converting it to plasma with an RF power of 100 W. The II / VI flux ratio is about 6.6.

The ZnO layer 33 of the undoped sample was formed at a growth temperature of 900 ° C. and a thickness of about 500 nm. The Zn beam was irradiated with a flux intensity J _Zn of 9.9 × 10 ¹⁴ atoms / cm ² s (Zn deposition rate F _Zn of 0.15 nm / s). The O radical beam was irradiated by introducing O ₂ gas into the O source gun at a flow rate of ₂ sccm and turning it into plasma with an RF power of 300 W. The II / VI flux ratio is about 1.2.

Then, the N-doped ZnO layer 33 of the sample (p-type sample) produced in the experiment for the N-doped p-type ZnO layer was formed at a growth temperature of 600 ° C. and a thickness of about 500 nm. The Zn beam was irradiated with a flux intensity J _Zn of 5.9 × 10 ¹⁴ atoms / cm ² s (Zn deposition rate F _Zn of 0.09 nm / s). The O radical beam was irradiated by introducing O ₂ gas into the O source gun at a flow rate of ₂ sccm and turning it into plasma with an RF power of 300 W. The irradiation of the N radical beam was performed by introducing N ₂ gas into the N source gun at a flow rate of 1 sccm and converting it to plasma with an RF power of 100 W. The II / VI flux ratio is about 0.8.

  FIGS. 5A to 5C and 6 are graphs showing 2θ-θ measurement results of X-ray diffraction (XRD) for the undoped sample, the first and second n-type samples, and the p-type sample, respectively. The horizontal axis of each graph indicates the diffraction angle 2θ in degrees, and the vertical axis indicates the diffraction intensity in log scale cps (counts / seconds) units.

As shown in FIG. 5A, in the undoped sample, a diffraction peak corresponding to the lattice constant in the c-axis direction of ZnO (hereinafter sometimes simply referred to as a diffraction peak) appears at a diffraction angle of about 72.54 degrees. As a result of performing CV measurement on the undoped sample, the n-type carrier concentration was 7.0 × 10 ¹⁵ cm ⁻³ . The undoped sample exhibits n-type conductivity even when undoped, but the n-type carrier concentration is low on the order of 10 ¹⁵ cm ⁻³ .

As shown in FIGS. 5B and 5C, in the first and second n-type samples, the diffraction peak of the N-doped ZnO layer appears on the lower angle side than the diffraction peak of the undoped ZnO of the substrate. The N concentration of the first n-type sample was 4.3 × 10 ¹⁹ cm ⁻³ , and the n-type carrier concentration by CV measurement was 2.0 × 10 ¹⁶ cm ⁻³ . The N concentration of the second n-type sample was 1.3 × 10 ²⁰ cm ⁻³ , and the n-type carrier concentration by CV measurement was 2.2 × 10 ¹⁸ cm ⁻³ .

  The sample of the third experiment is grown in a state in which the in-plane direction of the substrate is aligned with the underlying c-plane ZnO substrate (the a-axis length and the m-axis length are the same as the underlying ZnO substrate). The fact that the diffraction peak of the N-doped n-type ZnO layer appears on the lower angle side suggests that the lattice constant in the c-axis direction of the N-doped n-type ZnO layer is longer than that of undoped ZnO ( (The c-axis length of undoped ZnO is 0.52066 nm).

The degree of lattice mismatch (Δc / c) of the c-axis length is 0.028% (c-axis length 0.52081 nm) in the first n-type sample having an N concentration of 4.3 × 10 ¹⁹ cm ⁻³ , The second n-type sample having an N concentration of 1.3 × 10 ²⁰ cm ⁻³ is 0.033% (c-axis length 0.52083 nm). From this, it is presumed that the lattice constant in the c-axis direction becomes longer as the N concentration increases.

On the other hand, as shown in FIG. 6, in the p-type sample, the diffraction peak of the N-doped ZnO layer appears on the higher angle side than the diffraction peak of the undoped ZnO of the substrate, and the lattice constant in the c-axis direction is undoped. It is suggested that the length is shorter than that of ZnO. The degree of lattice mismatch (Δc / c) of the c-axis length is −0.010% (c-axis length 0.52061 nm). The p-type sample had an N concentration of 1.0 × 10 ²⁰ cm ⁻³ , and the result of CV measurement had a slope opposite to that of the n-type conductivity sample, indicating p-type conductivity.

The reason why the c-axis length is longer in N-doped n-type ZnO than in undoped ZnO and the c-axis length is shorter in N-doped p-type ZnO will be considered. CH Park et al “Origin of p-type doping difficulty in ZnO: The impurity perspective”, PHYSICAL REVIEW B 66, 073202 (2002) and S. Limpijumnong et al “Substitutional diatomic molecules NO, NC, CO, N ₂ , and According to O ₂ : Their vibrational frequencies and effects on p doping of ZnO ”, APPLIED PHYSICS LETTERS 86, 211910 (2005), the Zn-O bond length is 0.193 nm, the Zn-N bond length is 0.188 nm, and the O site is The bond length of the substituted N ₂ molecule (N ₂ ) _O ²⁺ is 0.114 nm.

When substituted with N ₂ molecules, the bond length from Zn to the center of N ₂ (Zn—N + (N—N) / 2) is 0, with half of the N ₂ bond length added to the Zn—N bond length. 188 nm + 0.114 nm / 2 = 0.245 nm, which is considered to be longer than the Zn—O bond length (0.193 nm).

Due to this bond length relationship, in the case of p-type conductivity in which one N atom is substituted at the O site, the c-axis length is shorter than that of undoped, and the two N atoms (N ₂ molecules) are located at the O site. In the case of substituted n-type conductivity, it is understood that the c-axis length is longer than undoped. Note that the value of lattice mismatch is considered to depend on the substrate used and the growth conditions.

Here, based on the first to third experiments, the N concentration suitable for the N-doped n-type ZnO layer will be considered. As explained in the first experiment, it is considered that the higher the N concentration, the easier it is to generate N ₂ molecular double donors and the higher the n-type carrier concentration. On the other hand, from the viewpoint of maintaining good crystal quality, it is desired to lower the N concentration.

In the first experiment, when the heat treatment temperature (growth temperature) was 800 ° C., an n-type carrier concentration of about 10 ¹⁶ cm ⁻³ or more was obtained with an N concentration of about 10 ²⁰ cm ⁻³ . .

In the second experiment, as compared with the first experiment, when the heat treatment temperature (annealing temperature) was increased to 900 ° C., the N concentration was 2.8 × 10 ²⁰ cm ⁻³ and the n-type carrier concentration was 2.0 × 10 ¹⁸ cm. ^-3 samples were obtained.

Also in the third experiment, compared to the first experiment, the n-type carrier concentration with respect to the N concentration of 4.3 × 10 ¹⁹ cm ^{−3 in} the first n-type sample with the heat treatment temperature (growth temperature) increased to 850 ° C. 2.0 × 10 ¹⁶ cm ⁻³ was obtained, and the n-type carrier concentration with ^respect to the N concentration of 1.3 × 10 ²⁰ cm ^{−3 in} the second n-type sample with the heat treatment temperature (growth temperature) increased to 950 ° C. 2.2 × 10 ¹⁸ cm ⁻³ was obtained.

  In the second and third experiments, the II / VI flux ratio is also set to the Zn rich side to increase the n-type carrier concentration.

Thus, by optimizing the conditions such as increasing the heat treatment temperature, a higher n-type carrier concentration can be obtained even at a lower N concentration than in the first experiment. Based on the result of the first n-type sample of the third experiment, it is considered that the N concentration is 1 × 10 ¹⁹ cm ⁻³ or more and the n-type carrier concentration can be 1 × 10 ¹⁶ cm ⁻³ or more. It is done. On the other hand, from the viewpoint of crystal quality, the upper limit of the N concentration is about 4 × 10 ²⁰ cm ⁻³ .

Thus, from the viewpoint of increasing the n-type carrier concentration while suppressing the deterioration of crystal quality, the N-doped n-type ZnO layer has an N concentration of 1 × 10 ¹⁹ cm ⁻³ or more and 4 × 10 ²⁰ cm ^−3. The following range is preferable.

  As discussed in the first experiment, the N-doped n-type ZnO layer can be obtained at a heat treatment temperature of 800 ° C. or higher, but it is easy to obtain a high n-type carrier concentration at a low N concentration. The heat treatment temperature is more preferably 850 ° C. or higher.

  As described above, in the first to third experiments, conditions for obtaining good n-type conductivity by doping N into ZnO (Mg composition x = 0), crystal structure, and the like were considered. However, the knowledge obtained in the first to third experiments may be effective for N doping of MgZnO in which Zn is replaced with Mg.

  Next, a fourth experiment will be described. In the fourth experiment, a ZnO-based semiconductor light-emitting device (light-emitting device of the example) using an N-doped n-type MgZnO layer as an n-type semiconductor layer was produced. For comparison, a ZnO-based semiconductor light-emitting element (a light-emitting element of a comparative example) using a Ga-doped n-type MgZnO layer as an n-type semiconductor layer was also produced.

  With reference to FIG. 7A, a method for manufacturing a sample in the fourth experiment will be described. FIG. 7A is a schematic cross-sectional view showing a sample structure of a fourth experiment.

  First, a method for manufacturing a light-emitting element of an example will be described. A ZnO buffer layer 42 having a thickness of 30 nm was formed on the + c plane of the cleaned c-plane ZnO substrate 41 (showing n-type conductivity) by irradiating a Zn beam and an O radical beam at a substrate temperature of 300 ° C. . In order to improve the quality of the ZnO buffer layer 42, annealing was performed at 900 ° C. for 20 minutes.

  The growth temperature and thickness of the ZnO buffer layer are not limited to 300 ° C. and 30 nm, but are preferably in the range of 200 ° C. to 400 ° C. and 10 nm to 30 nm. Further, the annealing temperature and annealing time of the ZnO buffer layer are not limited to 900 ° C. and 20 minutes, but are preferably in the range of 500 ° C. to 1000 ° C. and 3 minutes to 30 minutes.

Next, the ZnO buffer layer 42 is irradiated with a Zn beam, an Mg beam, an O radical beam, and an N radical beam at a growth temperature of 950 ° C., and an N-doped n-type Mg _x Zn _1-x having a thickness of about 80 nm. An O (x = 0.2) layer 43 was formed. The N concentration of the N-doped n-type Mg _x Zn _1-x O (x = 0.2) layer 43 was about 9.0 × 10 ¹⁹ cm ⁻³ .

The Zn beam was irradiated at a flux intensity J _Zn of 2.0 × 10 ¹⁵ atoms / cm ² s (Zn deposition rate F _Zn of 0.3 nm / s). The Mg beam was irradiated at a flux intensity J _Mg of 4.3 × 10 ¹³ atoms / cm ² s (Mg deposition rate F _Mg of 0.01 nm / s). The irradiation of the O radical beam was performed by introducing O ₂ gas into the O source gun at a flow rate of 1 sccm and turning it into plasma with an RF power of 200 W. The irradiation of the N radical beam was performed by introducing N ₂ gas into the N source gun at a flow rate of 1 sccm and turning it into plasma with an RF power of 150 W. The II / VI flux ratio is about 10.0.

As described in the first and second experiments, the N-doped n-type Mg _x Zn _1-x O layer has a growth temperature of 800 ° C. or higher and an II / VI flux ratio of 0.9 or higher. Preferred for the formation of Note that the growth temperature may be lower than 800 ° C., and then heat treatment may be performed at 800 ° C. or higher (before forming the upper N-doped p-type Mg _z Zn _1-z O layer 45).

Next, a Zn beam and an O radical beam are irradiated onto the N-doped n-type Mg _x Zn _1-x O (x = 0.2) layer 43 at a growth temperature of 900 ° C. to obtain a ZnO activity having a thickness of 35 nm. Layer 44 was formed.

Note that Mg can also be added to the active layer. The Mg _y Zn _1-y O active layer is sandwiched between the lower n-type Mg _x Zn _1-x O layer and the upper p-type Mg _z Zn _1-z O layer to be formed later. From the viewpoint of carrier confinement in the active layer, it is preferable to satisfy y <x and z so that the band gaps of the upper and lower cladding layers are larger. In addition, the upper limit of Mg composition x, y, z is about 0.6 (hence, 0 ≦ y <x, z ≦ 0.6).

As shown in FIGS. 7B and 7C, as a modification, the active layer 44 has a quantum well structure in which Mg _a Zn _1-a O barrier layers 44b and Mg _b Zn _1-b O well layers 44w are alternately stacked. It can also be. B <a so that the band gap of the barrier layer is larger than the band gap of the well layer.

  The growth temperature of the active layer is not limited to 900 ° C., but is preferably in the range of 500 ° C. to 1000 ° C. When the growth temperature is lower than 500 ° C., it is difficult to increase the crystallinity, and when the growth temperature is higher than 1000 ° C., re-evaporation of Zn increases and the growth rate decreases.

Next, the ZnO active layer 44 is irradiated with a Zn beam, an Mg beam, an O radical beam, and an N radical beam at a growth temperature of 700 ° C. to thereby form an N-doped p-type Mg _z Zn ₁₋ having a thickness of about 80 nm. _{A z} O (z = 0.2) layer 45 was formed. The N concentration of the N-doped p-type Mg _z Zn _1-z O (z = 0.2) layer 45 was about 6.0 × 10 ¹⁹ cm ⁻³ .

The Zn beam was radiated at a flux intensity J _Zn of 6.6 × 10 ¹⁴ atoms / cm ² s (Zn deposition rate F _Zn of 0.1 nm / s). The Mg beam was irradiated at a flux intensity J _Mg of 8.6 × 10 ¹³ atoms / cm ² s (Mg deposition rate F _Mg of 0.02 nm / s). The O radical beam was irradiated by introducing O ₂ gas into the O source gun at a flow rate of ₂ sccm and turning it into plasma with an RF power of 300 W. The irradiation of the N radical beam was performed by introducing N ₂ gas into the N source gun at a flow rate of 1 sccm and converting it to plasma with an RF power of 90 W. The II / VI flux ratio is about 0.8.

The growth temperature and thickness of the N-doped p-type Mg _z Zn _1-z O layer are not limited to 700 ° C. and 80 nm, but are preferably in the range of 300 ° C. or more and less than 800 ° C. and 5 nm to 200 nm.

Note that when a light emitting element such as a light emitting diode is manufactured, at least a hole carrier concentration of 1.0 × 10 ¹⁶ cm ⁻³ or more is required. N doped as an acceptor in the p-type Mg _z Zn _1-z O layer has a low activation rate, and 1.0 × 10 ¹⁸ cm ⁻³ is not doped unless effective hole carriers are obtained. Further, when N is doped more than 5 × 10 ²⁰ cm ⁻³ , many defects are generated in the p-type Mg _z Zn _1-z O layer, which may cause a leakage current. The N concentration of the p-type Mg _z Zn _1-z O layer is preferably 1.0 × 10 ¹⁸ cm ^{−3 to} 5 × 10 ²⁰ cm ⁻³ , and 1.0 × 10 ¹⁹ cm ^{−3 to} 3 × 10 ²⁰ cm. ^-3 is more preferable.

  In addition to N, P or As which is a V group element can also be used as the p-type dopant. Moreover, IA group elements, such as Li, Na, Cu, and Ag, and IB group elements can also be used. Further, by combining these, for example, two elements or more such as N and P can be doped at the same time. Among various group V elements, N has an ionic radius close to O, and stable substitution is performed.

Next, an n-side electrode 48 is formed on the back surface of the ZnO substrate 41, and a p-side translucent electrode 46 is formed on the N-doped p-type Mg _z Zn _1-z O (z = 0.2) layer 45. A p-side bonding pad electrode 47 was formed on the p-side translucent electrode 46.

  The n-side electrode 48 is formed, for example, by laminating an Al layer having a thickness of 300 nm to 500 nm on a Ti layer having a thickness of 2 nm to 10 nm. The p-side translucent electrode 46 is formed, for example, by laminating an Au layer having a thickness of 1 nm to 20 nm on a Ni layer having a thickness of 0.5 nm to 5 nm. The p-side bonding pad electrode 47 is formed, for example, by laminating an Au layer having a thickness of 1000 nm on an Ni layer having a thickness of 100 nm. For example, lift-off using a resist film or the like can be used for electrode formation.

  Thereafter, an electrode alloying process is performed in an oxidizing gas atmosphere of, for example, 300 ° C. or higher and lower than 800 ° C. The alloying treatment time is, for example, about 30 seconds to 10 minutes. As described above, the light emitting device of the example was manufactured.

  Next, a method for manufacturing a light-emitting element of a comparative example will be described. The + c plane of the cleaned c-plane ZnO substrate 41 was irradiated with a Zn beam and an O radical beam at a substrate temperature of 300 ° C. to form a ZnO buffer layer 42 having a thickness of 30 nm. In order to improve the quality of the ZnO buffer layer 42, annealing was performed at 900 ° C. for 20 minutes.

Next, the ZnO buffer layer 42 is irradiated with a Zn beam, an Mg beam, an O radical beam, and a Ga beam at a growth temperature of 900 ° C., so that a Ga-doped n-type Mg _x Zn _1-x O (x = 0. 2) A layer 43 was formed. The Ga concentration of the Ga-doped n-type Mg _x Zn _1-x O (x = 0.2) layer 43 was about 8.0 × 10 ¹⁷ cm ⁻³ .

Next, a ZnO active layer 44 is formed on the Ga-doped n-type Mg _x Zn _1-x O (x = 0.2) layer 43 by irradiation with a Zn beam and an O radical beam at a growth temperature of 900 ° C. did.

Next, the ZnO active layer 44 is irradiated with a Zn beam, an Mg beam, an O radical beam, and an N radical beam at a growth temperature of 650 ° C., and N-doped p-type Mg _z Zn _1-z O (z = 0.2) Layer 45 was formed. The N concentration of the N-doped p-type Mg _z Zn _1-z O (z = 0.2) layer 45 was about 9 × 10 ¹⁹ cm ⁻³ .

  Further, as described above, the p-side translucent electrode 46, the p-side bonding pad electrode 47, and the n-side electrode 48 were formed to produce a light emitting device of a comparative example.

  8A and 9A are a depth direction profile of N concentration of the light emitting element of the example and a depth direction profile of Ga concentration and N concentration of the light emitting element of the comparative example, respectively. The depth profile is measured by SIMS.

  8B and 9B are graphs showing the current-voltage characteristics of the light-emitting elements of the example and the current-voltage characteristics of the light-emitting elements of the comparative example, respectively. The horizontal axis is 1 scale 2 V, and the vertical axis is 1 scale 2 mA.

  FIG. 8C is an electroluminescence (EL) spectrum of the light-emitting element of the example.

  As shown in FIG. 9A, in the light emitting device of the comparative example, Ga doped only in the n-type layer is diffused into the active layer and the p-type layer. Due to Ga diffusion, in the active layer, the crystal quality is lowered, and defects acting as non-radiative recombination centers are introduced, leading to a reduction in luminous efficiency. Further, in the p-type layer, the introduction of defects due to the deterioration of crystal quality and the generation of n-type carriers due to Ga mixing reduce the p-type carrier concentration or change from p-type conductivity to n-type conductivity. Will happen.

  Further, as shown in FIG. 9B, in the light emitting element of the comparative example, the threshold value of the current-voltage characteristic is about 1V. When a pn junction is formed of ZnO, the current-voltage characteristic threshold is expected to be about 3 V, but the threshold of the comparative example is lower than that, suggesting that the element characteristics are Schottky. This is considered to be characteristic deterioration due to diffusion of Ga which is a dopant of the n-type layer into the p-type layer. In the light emitting device of the comparative example, light emission due to current injection was not observed.

  On the other hand, as shown in FIG. 8A, in the light emitting device of the example, diffusion of N, which is a dopant of the n-type layer, into the active layer and the p-type layer is not observed.

  Further, as shown in FIG. 8B, the threshold value of the current-voltage characteristic of the light emitting element of the example is about 3V. Compared with the comparative example, it is considered that the device characteristics were improved by suppressing the diffusion of the dopant of the n-type layer to the outside.

  As shown in FIG. 8C, the light emitting device of the example showed light emission at a wavelength of about 380 nm which is considered to be light emission from the vicinity of the band edge of ZnO.

  As described above, N can be used as the n-type dopant in the ZnO-based semiconductor light emitting device. Group III elements such as Ga are not used as n-type dopants. Therefore, the diffusion seen when Ga is used as the n-type dopant can be suppressed, and the characteristics of the light emitting element can be improved.

The substrate on which the ZnO-based semiconductor element is formed is not limited to the ZnO substrate. Sapphire (Al ₂ O ₃ ) substrate, silicon carbide (SiC) substrate, gallium nitride (GaN) substrate, (* 27) hexagonal Mg _x Zn _1-x O (0 <x ≦ 0.5) substrate, cubic crystal It is also possible to use a system Mg _x Zn _1-x O (0.5 <x ≦ 1) substrate, Si substrate, or the like.

In the Mg _x Zn _1-x O epitaxial growth layer of the above-described example, the upper limit of the Mg composition was 0.6. MBE epitaxial growth is performed under non-equilibrium conditions. Up to an Mg composition of 0.6, the hexagonal wurtzite structure and cubic rock-salt structure are not phase-separated and growth is possible while maintaining the hexagonal crystal structure. I have confirmed that. On the other hand, in the Mg _x Zn _1-x O substrate, it is considered that crystal growth is performed under conditions close to an equilibrium state, and the upper limit of the hexagonal Mg composition is 0.5, which is lower than 0.6.

  In order to obtain a ZnO layer with good crystallinity, a substrate having a smaller lattice mismatch is better, and a ZnO substrate is particularly preferable. In the case of manufacturing a light emitting element, an MgZnO substrate having a band gap larger than that of ZnO in order to suppress a decrease in light extraction efficiency from the element due to the substrate absorbing radiation emitted from the active layer. It is also preferable to use.

  A ZnO-based compound semiconductor layer can be grown on various substrate surfaces such as + c plane, −c plane, a plane, and m plane. Further, for example, for the + c plane, various substrates with off-angles in the m direction and the a direction can be used. A template in which an MgZnO layer, a ZnO layer, a GaN layer, or the like is formed on the substrate with a thickness of 1 μm or more may be used.

  Note that in a light emitting element using an insulating substrate such as a sapphire substrate, an n-side electrode cannot be formed on the back surface of the substrate. Therefore, an n-type semiconductor layer is exposed by etching, and an n-side electrode is formed on the exposed n-type semiconductor layer. Form.

  As a combination of the vertical positional relationship between the n-type semiconductor layer and the p-type semiconductor layer in the light emitting element, a structure in which the p-type semiconductor layer is disposed on the substrate side and the n-type semiconductor layer is disposed above the p-type semiconductor layer is also possible. Conceivable.

As described above, by doping N into Mg _x Zn _1-x O, the n-type carrier concentration can be increased compared to the case where N is not doped. It should be noted that the effect of N increasing the n-type carrier concentration can be expected for ZnO-based semiconductors other than Mg _x Zn _1-x O.

  Mg is added to ZnO as necessary to widen the band gap. For example, S, Se, Te, Cd may be added to ZnO to narrow the band gap, or Be may be added to the band gap. It is possible to add to ZnO in order to widen the thickness. Even for such a ZnO-based semiconductor, n-type conductivity could be increased by N doping.

  In addition, although the technique which forms a ZnO type | system | group semiconductor layer by MBE was demonstrated, even if it uses other film-forming methods, it will be possible to dope a ZnO type | system | group semiconductor layer by using N as an n-type dopant. Other film forming methods include, for example, pulse laser deposition (PLD), metal organic vapor phase epitaxy (MOVPE), metal organic chemical vapor deposition (MOCVD), and the like.

  The technique using N as an n-type dopant for a ZnO-based semiconductor is not limited to a light-emitting element, but includes various products including a ZnO-based semiconductor layer (for example, ZnO-based transistors, transparent conductive films, piezoelectric elements, thermoelectric elements, ultraviolet rays). It can be used for sensors and the like.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

DESCRIPTION OF SYMBOLS 1 Vacuum chamber 2 Zn source gun 3 Mg source gun 4 Ga source gun 5 O source gun 6 N source gun 7 Substrate holder 8 Substrate 9 (for RHEED) Gun 10 (for RHEED) Screen 21 Sapphire substrate 22 MgO buffer layer 23 ZnO buffer Layer 24 N-doped ZnO layer 31 ZnO substrate 32 ZnO buffer layer 33 N-doped ZnO layer 41 ZnO substrate 42 ZnO buffer layer 43 N-doped n-type MgZnO layer 44 active layer 45 N-doped p-type MgZnO layer 46 p-side translucent electrode 47 p-side bonding pad electrode 48 n-side electrode

Claims (9)

Preparing a substrate;
By supplying Zn, O, and N over the substrate and doping N, a ZnO-based semiconductor layer having n-type conductivity with an n-type carrier concentration increased as compared with the case where N is not doped is formed. Process ,
Forming a ZnO-based semiconductor layer exhibiting p-type conductivity by supplying Zn, O and a p-type dopant above the ZnO-based semiconductor layer exhibiting n-type conductivity. A method for manufacturing a semiconductor device. Preparing a substrate;
Supplying Zn, O, and a p-type dopant above the substrate to form a ZnO-based semiconductor layer exhibiting p-type conductivity;
By supplying Zn, O, and N above the ZnO-based semiconductor layer exhibiting p-type conductivity and doping N, the n-type conductivity is increased as compared with the case where N is not doped. Forming a ZnO-based semiconductor layer showing
Of manufacturing a ZnO-based semiconductor device. The step of forming the ZnO-based semiconductor layer exhibiting the n-type conductivity is performed by irradiating a Zn beam, an O radical beam, and an N radical beam by molecular beam epitaxy with an II / VI flux ratio of 0.9 or more. 3. The method of manufacturing a ZnO-based semiconductor device according to claim 1 , wherein a layer is formed and a heat treatment at 800 ° C. or more is performed during the growth or after the formation of the ZnO layer . The step of forming the ZnO-based semiconductor layer exhibiting p-type conductivity includes a growth temperature of less than 800 ° C., an II / VI flux ratio of less than 0.9, and molecular beam epitaxy to form a Zn beam, an O radical beam, and an N radical. The method for manufacturing a ZnO-based semiconductor device according to claim 1 , wherein a p-type ZnO layer is formed by irradiation with a beam. In order to change the band gap in the step of forming the ZnO-based semiconductor layer exhibiting n-type conductivity or the step of forming the ZnO-based semiconductor layer exhibiting p-type conductivity, Mg, S, Se The manufacturing method of the ZnO-type semiconductor device of any one of Claims 1-4 which supplies at least 1 of Te, Cd, and Be. A ZnO-based semiconductor layer containing at least Zn and O, and a ZnO-based semiconductor layer exhibiting n-type conductivity in which an n-type carrier concentration is increased by doping N as compared with a case where N is not doped ;
A ZnO-based semiconductor device formed above the ZnO-based semiconductor layer exhibiting n-type conductivity and doped with a p-type dopant and having a ZnO-based semiconductor layer exhibiting p-type conductivity . The ZnO-based semiconductor device according to claim 6, wherein the ZnO-based semiconductor layer exhibiting n-type conductivity has an N concentration of 1 × 10 ¹⁹ cm ⁻³ or more. The ZnO-based semiconductor device according to claim 6, wherein the ZnO-based semiconductor layer exhibiting n-type conductivity has an n-type carrier concentration of 1 × 10 ¹⁶ cm ⁻³ or more. The ZnO-based semiconductor device according to claim 6, wherein the ZnO-based semiconductor layer exhibiting n-type conductivity has a longer lattice constant when doped with N than when not doped with N.

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