JP5506221B2 - Method for manufacturing ZnO-based semiconductor element - Google Patents

Description

The present invention relates to the production how the ZnO-based semiconductor device.

  Zinc oxide (ZnO) is a direct transition type semiconductor having a band gap energy of 3.37 eV at room temperature, and the binding energy of excitons is 60 meV, which is relatively large compared to other semiconductors. In addition, the raw material is inexpensive and has less adverse effects on the environment and the human body. For this reason, it is expected to realize a light-emitting element using ZnO with high efficiency, low power consumption, and excellent environmental performance.

  For example, consider a ZnO-based light-emitting element formed by laminating an n-type semiconductor layer, an active layer, and a p-type semiconductor layer on an insulating substrate such as a sapphire substrate from the substrate side. For example, a p-side electrode is formed on the p-type semiconductor layer, and an n-side electrode is formed on the n-type semiconductor layer exposed by digging the p-type semiconductor layer.

  Thus, in the structure in which the p-side electrode and the n-side electrode are arranged in the same plane, if the film thickness of the n-type semiconductor layer is not sufficiently thick with respect to the current spread, the specific resistance of the n-type semiconductor layer is As the height increases, current concentrates at the end of the mesa structure close to the n-side electrode, and only the outer periphery of the mesa emits light. Such a problem is described in Patent Document 1, for example. The film thickness of the n-type semiconductor layer is preferably 1 μm or more, for example.

  FIG. 7 shows rocking curve measurement results by X-ray diffraction (XRD) of (0002) plane and (10-10) plane for a ZnO layer grown on a sapphire substrate via an MgO buffer layer and a ZnO buffer layer. It is a graph to show. The horizontal axis indicates the film thickness of the ZnO layer, and the vertical axis indicates the full width at half maximum of the rocking curve.

  The full width at half maximum tends to decrease as the thickness of the ZnO layer increases. Since the full width at half maximum of the rocking curve is related to the dislocation density in the film, it can be seen that the dislocation density tends to decrease as the ZnO film thickness increases. For example, when a ZnO layer is grown on a sapphire substrate, the thickness of the ZnO layer is preferably 1 μm or more, and more preferably 1.5 μm or more.

  In order to grow a thick ZnO layer, it is preferable to increase the growth rate. However, at the same time, it is desired that the surface flatness of the ZnO layer is also good.

  Non-Patent Document 1 describes the two-dimensional growth of ZnO under O-rich conditions.

  Patent Document 2 discloses a technique that uses hydrogen as a surfactant in the growth of ZnO.

JP 2007-173404 A JP 2004-221352 A H. Kato, M. Sano, K. Miyamoto, and T. Yao, “High-quality ZnO epilayers grown on Zn-face ZnO substrates by plasma-assisted molecular beam epitaxy”, J. Crystal Growth 265, p375-381 (2004 )

  An object of the present invention is to provide a method for manufacturing a ZnO-based semiconductor element in which a growth rate is improved while suppressing a decrease in surface flatness of a ZnO-based semiconductor layer.

  Another object of the present invention is to provide a novel manufacturing technique of a ZnO-based semiconductor element.

According to one aspect of the present invention, a step of preparing a substrate, introducing a gas containing O ₂ gas and N electrodeless discharge tube, comprising the steps of generating a first beam to discharge, the growth temperature And a step of supplying at least Zn above the substrate and supplying the first beam from the electrodeless discharge tube to grow an n-type ZnO-based semiconductor layer at 600 ° C. or higher. An element manufacturing method is provided.

Irradiation of the first beam from an electrodeless discharge tube into which a gas containing O ₂ gas and N is introduced to grow an n-type ZnO-based semiconductor layer at a growth temperature of 600 ° C. or higher, thereby suppressing surface flatness degradation. However, the growth rate of the n-type ZnO-based semiconductor layer can be improved.

FIG. 1 is a schematic cross-sectional view of a crystal manufacturing apparatus used for manufacturing a ZnO-based semiconductor device according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing sample structures of the first to fourth comparative examples, the first example, the fifth comparative example, and the second example. FIG. 3A is a table summarizing the results of the first to fourth comparative examples. FIG. 3A is a table summarizing the results of the first example, the fifth comparative example, and the second example. FIG. 4 is a schematic cross-sectional view showing ZnO-based semiconductor light emitting devices of the sixth comparative example and the third example. FIG. 5 is a table summarizing the results of the sixth comparative example and the third example. 6A and 6B are graphs showing the relationship between the growth temperature and the growth rate of the ZnO layer. FIG. 7 is a graph showing a rocking curve measurement result by XRD for a ZnO layer on a sapphire substrate.

  First, a crystal manufacturing apparatus used for manufacturing a ZnO-based semiconductor element according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic sectional view of a crystal manufacturing apparatus, in which crystal growth is performed by molecular beam epitaxy (MBE).

  In the ultra-high vacuum vessel 1, a Zn source gun 2, an O source gun 3, an Mg source gun 4, an N source gun 5, and a Ga source gun 6 are provided. The Zn source gun 2, the Mg source gun 4, and the Ga source gun 6 include Knudsen cells that accommodate solid sources of Zn, Mg, and Ga, respectively, and emit a Zn beam, an Mg beam, and a Ga beam, respectively. .

  The O source gun 3 and the N source gun 5 include electrodeless discharge tubes 3a and 5a that use radio frequencies (for example, 13.56 MHz), respectively.

The O electrodeless discharge tube 3a of the source gun 3 via the O ₂ mass flow controller 7 with O ₂ gas is introduced, N ₂ gas is introduced through a N ₂ for mass flow controller 8. A beam generated by discharging to the electrodeless discharge tube 3 a is emitted from the O source gun 3. The beam generated in the electrodeless discharge tube 3a is presumed to contain O radicals and other active species involving N elements.

The electrodeless discharge tube 5a of N source gun 5, N ₂ gas is introduced through a N ₂ mass flow controller 9. The N source gun 5 generates N radicals in the electrodeless discharge tube 5a and emits an N radical beam.

  A stage 10 including a substrate heater is disposed in the ultra-high vacuum container 1, and the stage 10 holds the substrate 11. By supplying a desired beam on the substrate 11 at a desired timing, a ZnO-based semiconductor layer having a desired composition can be grown.

  The ZnO-based semiconductor contains at least Zn and O. By adding Mg supplied from the Mg source gun 4 to make MgZnO, the band gap can be expanded as compared with ZnO. Further, N supplied from the N source gun 5 can be added as a p-type impurity. An n-type ZnO-based semiconductor can be obtained without particularly adding an n-type impurity, but Ga supplied from the Ga source gun 6 can also be added as an n-type impurity.

  The ultra-high vacuum vessel 1 is also provided with a gun 12 for reflection high-energy electron diffraction (RHEED) and a screen 13 for displaying an image of RHEED. From the RHEED image, the flatness of the surface of the crystal layer grown on the substrate 11 can be evaluated. When the crystal grows two-dimensionally and the surface is flat, the RHEED image shows a streak pattern, and when the crystal grows three-dimensionally and the surface is not flat, the RHEED image shows a spot pattern.

An exhaust pump exhausts the inside of the ultra-high vacuum container 1. Note that the ultra-high vacuum means that the degree of vacuum is 1 × 10 ⁻⁷ Torr or less.

  Next, the definition of stoichiometry conditions, Zn rich conditions, and O rich conditions in ZnO crystal growth will be described.

Flux strength Zn and _{J Zn,} a flux intensity of O radical and _{J O.} The coefficient (Zn adhesion coefficient) indicating the easy adhesion of Zn to the O terminal surface of the ZnO crystal is K _Zn, and the coefficient (O adhesion coefficient) indicating the easy adhesion of O to the Zn terminal surface is K. _O.

The product K _Zn J _{Zn of the Zn} adhesion coefficient K _Zn and the flux strength J _Zn corresponds to the number of Zn atoms deposited per unit time on the unit area of the substrate, and the O adhesion coefficient K _O and the flux strength J _O Product K _O J _O corresponds to the number of O atoms adhering to the unit area of the substrate per unit time.

In this specification (and claims), unless otherwise specified, the flux strength multiplied by the adhesion coefficient is simply referred to as flux strength. That is, the product K _Zn J _Zn is called Zn beam flux intensity, and the product K _O J _O is called O radical beam flux intensity.

The ratio of O radical beam flux intensity K _O J _{O to} Zn beam flux intensity K _Zn J _Zn is defined as K _O J _O / K _Zn J _Zn as the flux ratio. A case where the flux ratio is equal to 1 is called a stoichiometry condition, a case where the flux ratio is larger than 1 is called an O-rich condition, and a case where the flux ratio is smaller than 1 is called a Zn-rich condition.

  H. Kato, M. Sano, K. Miyamoto, and T. Yao, “High-quality ZnO epilayers grown on Zn-face ZnO substrates by plasma-assisted molecular beam epitaxy”, J. Crystal Growth 265, p375-381 (2004) discloses that an epitaxial film in which ZnO crystals are two-dimensionally grown can be obtained when the flux ratio is extremely 5.6 and an O-rich condition.

Next, how to obtain the O radical beam flux intensity K _O J _{O and the} like will be described. The growth rate G of the ZnO crystal can be expressed by the following formula (1).
G = [(K _Zn J _Zn ) ⁻¹ + (K _O J _O ) ⁻¹ ] ⁻¹ −R _ZnO (1)
Here, R _ZnO is a term for re-evaporation of ZnO. For example, in the growth of ZnO at + c (Zn polarity), it can be almost ignored when the substrate temperature is 800 ° C. or lower. When the substrate temperature exceeds 900 ° C., R _ZnO is on the order of several tens of nm / h, which affects the growth rate. Further, the Zn adhesion coefficient K _Zn also starts to decrease when the substrate temperature exceeds 800 ° C., which also affects the growth rate.

The growth rate G can be measured by actually growing a ZnO film at a substrate temperature of 800 ° C. or lower where R _ZnO can be ignored and the Zn adhesion coefficient K _Zn is constant. With respect to the Zn beam flux intensity K _Zn J _Zn , the adhesion coefficient K _Zn is constant at a substrate temperature of 800 ° C. or lower, so it is set to 1, and the flux intensity J _Zn can be measured with a film thickness monitor or the like.

By substituting the obtained growth rate G and Zn beam flux intensity K _Zn J _Zn into the above formula (1), the O radical beam flux intensity K under the setting conditions (O ₂ flow rate, RF power, etc.) of the O source gun. _O J _O can be determined. Further, a flux ratio is obtained, and it is determined whether the flux ratio is a stoichiometric condition, an O-rich condition, or a Zn-rich condition.

  Next, the ZnO layer growth method according to the first to fourth comparative examples, the first example, the fifth comparative example, and the second example will be described.

  FIG. 2 is a schematic cross-sectional view showing sample structures of these comparative examples and examples. In these comparative examples and examples, the ZnO layer 23 is grown on the c-plane sapphire substrate 20 via the MgO buffer layer 21 and the ZnO buffer layer 22. In these comparative examples and examples, the growth conditions of the ZnO layer 23 are different. A process for forming the ZnO buffer layer 22 common to these comparative examples and examples will be described.

First, thermal annealing was performed on the c-plane sapphire substrate 20 to clean the substrate surface. Thermal annealing was performed at 900 ° C. for 30 minutes under a high vacuum of 1 × 10 ⁻⁹ Torr.

  Next, the substrate temperature was set to 500 ° C. and the c-plane sapphire substrate 20 was irradiated with an Mg beam and an O radical beam to form an MgO buffer layer 21 having a thickness of 8 nm. When a ZnO layer is grown directly on a c-plane sapphire substrate, a ZnO layer whose surface is an O-polar surface grows. By inserting the MgO buffer layer, a ZnO layer whose surface is a Zn polar face can be grown.

  Next, the substrate temperature was set to 350 ° C., and the ZnO buffer layer 22 was formed by irradiating the MgO buffer layer 21 with a Zn beam and an O radical beam. Next, in order to improve the crystallinity of the ZnO buffer layer 22, the substrate temperature was raised to 800 ° C. and annealing was performed for 20 minutes. The thickness of the ZnO buffer layer 22 was about 10 nm.

Next, a method for growing the ZnO layer 23 of the first comparative example will be described. In the first comparative example, only O ₂ gas is introduced into an O source gun and irradiated with an O radical beam.

A ZnO layer 23 having a thickness of 1 μm was grown by irradiating the ZnO buffer layer 22 with a Zn beam and an O radical beam at a substrate temperature of 700 ° C. The Zn beam was irradiated with Zn having a purity of 7N as a solid source and a Zn beam flux intensity K _Zn J _Zn of 1.6 × 10 ¹⁴ atoms / (cm ² s).

For the O radical beam, pure oxygen gas with a purity of 6N was introduced at 3 sccm and irradiated with RF power of 300 W. This condition corresponds to an O radical beam flux intensity K _o J _O of 1.2 × 10 ¹⁵ atoms / (cm ² s), and the flux ratio is an O-rich condition.

Next, a method for growing the ZnO layer 23 of the second comparative example will be described. Also in the second comparative example, as in the first comparative example, only O ₂ gas is introduced into the O source gun and the O radical beam is irradiated. However, the Zn beam flux intensity K _Zn J _Zn is set higher than that of the first comparative example.

The ZnO layer 23 was grown by irradiating the ZnO buffer layer 22 with a Zn beam and an O radical beam at a substrate temperature of 700 ° C. The Zn beam was irradiated with a Zn beam flux intensity K _Zn J _Zn of 2 × 10 ¹⁵ atoms / (cm ² s).

For the O radical beam, O ₂ gas was introduced at 3 sccm, the RF power was 300 W, and the O radical beam flux intensity K _o J _O was 1.2 × 10 ¹⁵ atoms / (cm ²⁾ as in the first comparative example. Irradiated as s). The flux ratio is a Zn-rich condition.

Next, a method for growing the ZnO layer 23 of the third comparative example will be described. Also in the third comparative example, as in the first comparative example, only O ₂ gas is introduced into the O source gun and the O radical beam is irradiated. However, both the Zn beam flux intensity K _Zn J _Zn and the O radical beam flux intensity K _o J _O are made higher than those in the first comparative example.

The ZnO layer 23 was grown by irradiating the ZnO buffer layer 22 with a Zn beam and an O radical beam at a substrate temperature of 700 ° C. As in the second comparative example, the Zn beam was irradiated at a Zn beam flux intensity K _Zn J _Zn of 2 × 10 ¹⁵ atoms / (cm ² s).

Further, the O radical beam was irradiated with O ₂ gas introduced at 3 sccm, RF power of 400 W, and O radical beam flux intensity K _o J _O of 3 × 10 ¹⁵ atoms / (cm ² s). The flux ratio is an O-rich condition.

  Next, a method for growing the ZnO layer 23 of the fourth comparative example will be described. In the fourth comparative example, the ZnO layer 23 was grown under the same conditions as in the third comparative example except that the substrate temperature was 1000 ° C.

Next, a method for growing the ZnO layer 23 of the first embodiment will be described. In the first embodiment, a mixed gas of O ₂ gas and N ₂ gas is introduced into an O source gun and irradiated with an O radical beam.

Similar to the second comparative example, the substrate temperature was set to 700 ° C., and the Zn beam was irradiated with the Zn beam flux intensity K _Zn J _Zn being 2 × 10 ¹⁵ atoms / (cm ² s).

For the O radical beam, O ₂ gas was introduced at 3 sccm, N ₂ gas was introduced at 0.05 sccm, and irradiation was performed with an RF power of 300 W. That is, at the same O ₂ flow rate and RF power as in the second comparative example, N ₂ was further introduced and the O radical beam was irradiated.

  For the ZnO layers 23 of the first to fourth comparative examples and the first example, the surface flatness is evaluated by RHEED and atomic force microscope (AFM), the surface is visually observed, and secondary ions are further observed. The impurity concentration in the film was measured by mass spectrometry (SIMS). The growth rate was determined from the film thickness and the growth time.

  FIG. 3A is a table summarizing the results of the first to fourth comparative examples, and FIG. 3B is a table summarizing the results of the first example (and the fifth comparative example and the second example). From the top, the AFM image, RHEED image, visual observation evaluation, impurity concentration, and growth rate are shown.

  The AFM image shows a 1 μm square region, and the surface roughness (root mean square roughness) Rms was determined from this observation. The RHEED image is in the [11-20] direction. The impurity concentration was based on that of the first comparative example, and in other samples, elements having a larger amount of contamination than the reference were confirmed.

  The ZnO layer of the first comparative example had a flat surface and a surface roughness Rms of 0.34 nm from AFM observation. The RHEED image was a streak pattern showing two-dimensional growth and was transparent by visual observation. A good ZnO layer is obtained with less contamination of impurities. However, the growth rate is as slow as 125 nm / h. For example, it takes about 8 hours to grow a film thickness of about 1 μm.

In the ZnO layer of the second comparative example, the growth rate is increased by increasing the Zn beam flux intensity K _Zn J _Zn as compared with the first comparative example, and the growth rate is as high as 648 nm / h. However, the AFM image is not flat, the surface roughness Rms is 22.70 nm, and the RHEED image is a spot pattern showing three-dimensional growth, and the surface flatness is remarkably reduced. It is considered that the growth was three-dimensional under Zn-rich conditions. Moreover, it was cloudy by visual observation.

The ZnO layer of the third comparative example has both the Zn beam flux intensity K _Zn J _Zn and the O radical beam flux intensity K _o J _O higher than those of the first comparative example, so that it is more than the second comparative example. Furthermore, the growth rate is increased and the growth rate is increased to 1028 nm / h. However, the AFM image is not flat, the surface roughness Rms is 26.59 nm, and the RHEED image is a spot pattern showing three-dimensional growth, and the surface flatness is remarkably reduced. Moreover, it was cloudy by visual observation.

In the third comparative example, Si is further mixed in at a concentration of 2 × 10 ¹⁹ cm ⁻³ . This is because Si generated by sputtering the inner wall of an electrodeless discharge tube made of quartz is taken into the ZnO layer because the RF power applied to the O source gun is increased in order to increase the supply amount of O radicals. It is thought that

  The third comparative example is an O-rich condition that is close to the stoichiometric condition although it is an O-rich condition. The growth rate is very fast, and Si is mixed into the film. It is thought that it became dimension growth.

  In the fourth comparative example, surface flatness was improved while maintaining a high growth rate by raising the substrate temperature to 1000 ° C. in the third comparative example. The AFM image is flat compared to the third comparative example, the surface roughness Rms is reduced to 2.21 nm, and the RHEED image has a streak pattern indicating two-dimensional growth.

However, although the flatness was improved as compared with the third comparative example, the growth rate was lowered. This can be attributed to the fact that the growth temperature was increased to 1000 ° C., the Zn adhesion coefficient K _Zn decreased, and the amount of ZnO re-evaporation increased.

In addition, the number of pits is so large that it cannot be said to be a good film. It is clouded by visual observation, which is thought to be due to pits. Also in the fourth comparative example, Si caused by sputtering on the inner wall of the electrodeless discharge tube is mixed at a concentration of 2 × 10 ¹⁹ cm ⁻³ , and Si is considered to be a cause of pit generation.

In the first example, the same Zn beam flux intensity K _Zn J _{Zn as} in the second comparative example, and the O ₂ gas introduction amount and RF power of the same O source gun as in the second comparative example (in the O source gun) Nitrogen was also introduced into the O source gun, although the flux ratio was Zn-rich if it was made to correspond to the conditions in the case of introducing only oxygen.

  The ZnO layer of the first example had a flat surface and a surface roughness Rms of 0.33 nm as observed by AFM. For example, when the surface roughness Rms obtained by AFM observation of a 1 μm square region is 1.0 nm or less, it is determined that the surface is flat.

The RHEED image is a streak pattern showing two-dimensional growth, is transparent by visual observation, and a good film is obtained. Furthermore, the growth rate is 1013 nm / h, which is a very high growth rate of 1 μm / h or more. Further, N is added at a concentration of 3 × 10 ¹⁹ cm ⁻³ with the introduction of nitrogen into the O source gun. From the increase in growth rate, it is expected that the O radical beam flux intensity is increased by mixing nitrogen into the O source gun.

  Thus, it was found that by introducing a gas containing oxygen and nitrogen into the O source gun, a very high growth rate can be obtained in addition to good surface flatness. Even though the growth rate has increased significantly, it is surmised that the surface flatness is good because of the surfactant effect of the active species involving N irradiated simultaneously with the O radical. However, this is one guess.

  One of the powerful research techniques for thin film growth in recent years is surfactant-mediated epitaxy. This is a technique for artificially changing the growth mode of a thin film using a surfactant (atom, molecule, etc.) called a surfactant, and is a useful means for controlling epitaxial growth. For example, Japanese Unexamined Patent Application Publication No. 2004-221352 discloses a technique using hydrogen as a surfactant.

  Note that N is usually added as a p-type impurity with respect to ZnO, but the ZnO layer of the example functions as an n-type semiconductor layer even when N is added (see the third example described later).

Next, a method for growing the ZnO layer 23 of the fifth comparative example will be described. In the fifth comparative example, under the conditions of the third comparative example, only the O ₂ gas is introduced into the O source gun and irradiated with the O radical beam, and at the same time, the N radical beam is irradiated from the N source gun.

The ZnO layer 23 was grown by irradiating the ZnO buffer layer 22 with a Zn beam and an O radical beam at a substrate temperature of 700 ° C. The Zn beam was irradiated with a Zn beam flux intensity K _Zn J _Zn of 2 × 10 ¹⁵ atoms / (cm ² s).

The O radical beam was irradiated with O ₂ gas introduced at 3 sccm, RF power of 400 W, and O radical beam flux intensity K _o J _O of 3 × 10 ¹⁵ atoms / (cm ² s). Further, an N radical beam was introduced at ₂ sccm of N ₂ gas and irradiated with an RF power of 90 W.

  Also for the ZnO layer 23 of the fifth comparative example, the surface flatness was evaluated by RHEED and AFM, the surface was visually observed, and the impurity concentration in the film was measured by SIMS. The growth rate was determined from the film thickness and the growth time.

FIG. 3-2 shows the result of the fifth comparative example. From the AFM image, it can be seen that the surface is not flat but grows three-dimensionally. The surface roughness Rms was 30.60 nm. The RHEED image was a spot pattern showing three-dimensional growth. The growth rate was 995 nm / h. Similar to the third comparative example, Si was mixed in at a concentration of 2 × 10 ¹⁹ cm ⁻³ . Moreover, it was cloudy by visual observation.

  In the fifth comparative example, the N radical was irradiated from the N source gun during the O radical irradiation, but the effect of improving the surface flatness was not observed. The growth rate was similar to that of the third comparative example, and no effect of improving the growth rate was observed.

Next, a method for growing the ZnO layer 23 of the second embodiment will be described. In the second embodiment, as in the first embodiment, a mixed gas of O ₂ gas and N ₂ gas is introduced into an O source gun and irradiated with an O radical beam. However, the flow rate of N ₂ gas introduced into the O source gun was set to 3 sccm (volume ratio, O ₂ : N ₂ = 1: 1), which is higher than 0.05 sccm in the first embodiment. Other conditions are the same as in the first embodiment.

  Also for the ZnO layer 23 of the second example, the surface flatness was evaluated by RHEED and AFM, the surface was visually observed, and the impurity concentration in the film was measured by SIMS. The growth rate was determined from the film thickness and the growth time.

FIG. 3-2 shows the result of the second example. It can be seen from the AFM image that the surface is flat. The surface roughness Rms was 0.35 nm. The RHEED image is a streak pattern showing two-dimensional growth, is transparent by visual observation, and a good film is obtained. Furthermore, the growth rate was very fast, 1020 nm / h. N is added at a concentration of 3 × 10 ¹⁹ cm ⁻³ . In addition, mixing of Si is not seen.

Thus, even if the flow rate of the N ₂ gas introduced into the O source gun is increased to, for example, O ₂ : N ₂ = 1: 1 by volume ratio, the surface flatness is improved as in the first embodiment. The effect and the improvement effect of the growth rate were seen.

In order to obtain a high growth rate while maintaining good surface flatness, the mixing ratio (volume ratio) of O ₂ and N ₂ introduced into the O source gun is set to 0 <(N ₂ / O ₂ ) <10. As a result of the examination, it was understood that it is preferable to be in the range of 0.01 (more preferably in the range of 0.01 ≦ (N ₂ / O ₂ ) <10). In the range of N ₂ / O ₂ ≧ 10, the growth rate is hardly increased.

If the total amount of the mixed gas of O ₂ and N ₂ is too large, the degree of vacuum in the crystal growth apparatus cannot be maintained at a high vacuum, the crystallinity of the growth film is deteriorated, and the discharge is not stable in the electrodeless discharge tube. Therefore, it is preferable to appropriately adjust the O ₂ supply amount and the N ₂ supply amount within the range where the high vacuum is maintained and the discharge is stabilized.

Next, ZnO-based semiconductor light emitting devices of the sixth comparative example and the third example will be described. In the sixth comparative example and the example, a ZnO-based semiconductor light emitting element is fabricated on a c-plane sapphire substrate. First, a sixth comparative example will be described. In the sixth comparative example, only O ₂ gas is introduced into the O source gun and the O radical beam is irradiated.

FIG. 4 is a schematic cross-sectional view showing a ZnO-based semiconductor light emitting device of a sixth comparative example (and a third example). First, the c-plane sapphire substrate 30 was subjected to thermal annealing to clean the substrate surface. Thermal annealing was performed at 900 ° C. for 30 minutes under a high vacuum of 1 × 10 ⁻⁹ Torr. The sapphire substrate 30 is insulative.

  Next, the substrate temperature was set to 500 ° C., and the c-plane sapphire substrate 30 was irradiated with an Mg beam and an O radical beam to form the MgO buffer layer 31 with a thickness of 8 nm.

  Next, the substrate temperature was set to 350 ° C., and the ZnO buffer layer 32 was formed by irradiating the MgO buffer layer 31 with a Zn beam and an O radical beam. Next, in order to improve the crystallinity of the ZnO buffer layer 32, the substrate temperature was raised to 800 ° C. and annealing was performed for 20 minutes. The thickness of the ZnO buffer layer 32 was about 10 nm.

Next, the substrate temperature was set to 700 ° C., and a Zn beam and an O radical beam were irradiated onto the ZnO buffer layer 32 to grow an n-type ZnO layer 33 having a thickness of 1.5 μm. The Zn beam was irradiated with Zn having a purity of 7N as a solid source and a Zn beam flux intensity K _Zn J _Zn of 1.6 × 10 ¹⁴ atoms / (cm ² s).

For the O radical beam, pure oxygen gas with a purity of 6N was introduced at 3 sccm and irradiated with RF power of 300 W. This condition corresponds to an O radical beam flux intensity K _o J _O of 1.2 × 10 ¹⁵ atoms / (cm ² s). The irradiation conditions of the Zn beam and the O radical beam on the n-type ZnO layer 33 of the sixth comparative example are the same as those of the first comparative example.

Next, the n-type Zn _x Zn _1-x O (x = 0.25) layer 34 is irradiated by irradiating the n-type ZnO layer 33 with a Zn beam, O radical beam, and Mg beam at a substrate temperature of 700 ° C. Grown up. The Zn beam was irradiated with a Zn beam flux intensity K _Zn J _Zn of 2 × 10 ¹⁵ atoms / (cm ² s).

For the O radical beam, O ₂ gas was introduced at ₂ sccm and irradiated with RF power of 300 W. This condition corresponds to an O radical beam flux intensity K _o J _O of 1 × 10 ¹⁵ atoms / (cm ² s).

The Mg beam was irradiated at a Mg beam flux intensity of 1.7 × 10 ¹⁴ atoms / (cm ² s) using 6N purity Mg as a solid source. The thickness of the n-type Mg _x Zn _1-x O (x = 0.25) layer 34 was 30 nm.

The n-type ZnO layer 33 and the n-type Mg _x Zn _1-x O (x = 0.25) layer 34 are not added with an n-type impurity such as Ga, but an n-type conductivity type can be obtained. .

Next, a Zn beam and an O radical beam were irradiated onto the n-type Mg _x Zn _1-x O (x = 0.25) layer 34 at a substrate temperature of 700 ° C. to grow the ZnO active layer 35. The Zn beam was irradiated with the Zn beam flux intensity K _Zn J _Zn as 1.6 × 10 ¹⁴ atoms / (cm ² s).

As the O radical beam, O ₂ gas was introduced at 3 sccm and irradiated with RF power of 300 W. This condition corresponds to an O radical beam flux intensity K _O J _O of 1.2 × 10 ¹⁵ atoms / (cm ² s). The thickness of the ZnO active layer 35 was 10 nm.

Next, a Zn beam, an O radical beam, an Mg beam, and an N radical beam are irradiated onto the ZnO active layer 35 at a substrate temperature of 700 ° C. to form p-type Mg _x Zn _1-x O (x = 0.25). Layer 36 was grown.

The Zn beam was irradiated at a Zn beam flux intensity of K _Zn J _Zn of 9.5 × 10 ¹⁴ atoms / (cm ² s). The O radical beam was introduced with O ₂ gas at ₂ sccm and irradiated with RF power of 300 W, so that the O radical beam flux intensity K _o J _O was 1 × 10 ¹⁵ atoms / (cm ² s).

The Mg beam was irradiated with an Mg beam flux intensity of 1.7 × 10 ¹⁴ atoms / (cm ² s). For the N radical beam, pure nitrogen gas with a purity of 7N was introduced at 0.5 sccm and irradiated with an RF power of 90 W.

The thickness of the p-type Mg _x Zn _1-x O (x = 0.25) layer 36 was 30 nm. The N concentration in the p-type Mg _x Zn _1-x O (x = 0.25) layer 36 thus grown is 1 × 10 ²⁰ cm ⁻³ or more.

Next, a resist film or a protective film is provided on the sapphire substrate 30 on which the layers up to p-type Mg _x Zn _1-x O (x = 0.25) 36 are stacked, and the shape of the n-side electrode forming portion is set. An etching mask having a notch window is formed. Thereafter, the semiconductor layer in the cutout window is etched by wet etching or reactive ion etching until the n-type ZnO layer 33 is exposed.

  Next, the n-side electrode 40 was formed on the exposed n-type ZnO layer 33. The n-side electrode 40 is formed, for example, by laminating an aluminum layer having a thickness of 500 nm on a titanium layer having a thickness of 10 nm.

Next, the n-side electrode forming etching mask was removed, and the p-side transparent electrode 41 was formed on the p-type Mg _x Zn _1-x O (x = 0.25) layer 36. The p-side transparent electrode 41 is formed, for example, by laminating a 10 nm thick gold layer on a 1 nm thick nickel layer. A p-side bonding electrode 42 was formed on the p-side transparent electrode 41. The p-side bonding electrode 42 is formed of a gold layer having a thickness of 500 nm, for example. Note that a mask such as a resist is appropriately used for forming the p-side electrode.

  Thereafter, an electrode alloying process is performed, for example, for 2 minutes in an oxygen atmosphere at 400 ° C., for example. In this manner, a ZnO-based light emitting diode of the sixth comparative example was produced.

  Next, the ZnO-based semiconductor light emitting device of the third embodiment will be described. The third example is the same as the sixth comparative example except that the growth conditions of the n-type ZnO layer 33 are different from those of the sixth comparative example.

The n-type ZnO layer 33 of the third example was grown in the same manner as in the first example. That is, the substrate temperature was set to 700 ° C., and the Zn beam was irradiated at a Zn beam flux intensity K _Zn J _Zn of 2 × 10 ¹⁵ atoms / (cm ² s). The O radical beam was irradiated with an RF power of 300 W by introducing O ₂ gas into the O source gun at 3 sccm and N ₂ gas at 0.05 sccm. The thickness of the n-type ZnO layer 33 was 1.5 μm. The thickness of the n-type ZnO layer 33 is preferably 1 μm or more, and more preferably 1.5 μm or more.

With respect to the light emitting elements of the sixth comparative example and the third example, the light emission state was observed and the IV characteristics were measured. Further, the surface flatness of the p-type Mg _x Zn _1-x O (x = 0.25) layer 36 was evaluated by AFM before electrode formation. Further, the N concentration of the n-type ZnO layer 33 was measured by SIMS.

  FIG. 5 is a table summarizing the results of the sixth comparative example and the third example. From the upper side, the IV characteristic, the AFM image, and the N density are shown.

  The light emitting characteristics of the light emitting elements of the sixth comparative example and the third example were almost the same, but the IV characteristics (characteristics in forward bias, voltage on the horizontal axis and current on the vertical axis). ) Shows a slightly higher leakage current in the sixth comparative example and better diode characteristics in the third example.

  The AFM image shows a 5 μm square area on the left side and a 1 μm square area on the right side for each of the comparative example and the example. The surface roughness Rms was determined by observing 5 μm square and 1 μm square regions, respectively.

A large number of pits were observed in the p-type MgZnO layer 36 of the sixth comparative example, and the surface roughness Rms was 2.32 nm for 5 μm square and 0.93 nm for 1 μm square. The p-type MgZnO layer 36 of the third example had very few pits, and the surface roughness Rms was 1.04 nm for 5 μm square and 0.45 nm for 1 μm square. The N concentration of the n-type ZnO layer 33 of the third example was 3 × 10 ¹⁹ cm ⁻³ .

  The large leak current in the sixth comparative example seems to correspond to the large amount of pits generated. The reason why the amount of pits generated in the third example is small is considered that the surfactant effect by nitrogen introduced into the O source gun worked.

  Thus, the method of the embodiment in which a gas containing oxygen and nitrogen is introduced into the O source gun can be applied to the growth of an n-type semiconductor layer such as a light emitting element, and a film in which the generation of pits is suppressed is obtained. It was found that there is an effect in reducing the leakage current of the element.

  6A and 6B are graphs showing the results of confirming the relationship between the growth temperature and the growth rate of the ZnO layer.

  As shown in FIG. 6A, when oxygen gas alone is introduced into an electrodeless discharge tube of an O source gun, a region where a two-dimensional growth film excellent in flatness is obtained is a region located on the high temperature, low growth rate side. I (indicated by a diagonal line up to the left). With respect to the region I, on the low temperature and high growth rate side, a large number of pits tend to be generated or three-dimensional growth tends to be facilitated.

As shown in FIG. 6B, a region where a two-dimensionally grown film excellent in flatness is obtained by the method of the embodiment in which a mixed gas of O ₂ and N ₂ is introduced into an electrodeless discharge tube of an O source gun is a region I. It becomes a region II (shown by an oblique line in the upper right) that is lower temperature and higher growth rate (when compared at the same growth rate, when only O ₂ is introduced into the O source gun, even in a region that grows three-dimensionally, When a mixed gas of O ₂ and N ₂ is introduced, there are two-dimensional growth regions).

  In the two-dimensional growth, the lower limit of the substrate temperature at which a predetermined growth rate (for example, 600 nm / h) is obtained is lowered by the method of the embodiment. Further, a high growth rate (for example, 800 nm / h or more) that cannot be realized in the region I can be realized by the method of the embodiment.

  The lower limit substrate temperature at which two-dimensional growth can be obtained increases as the growth rate increases. For example, in order to obtain a high growth rate of 800 nm / h or higher, the growth temperature is preferably 600 ° C. or higher. Note that the upper limit substrate temperature at which two-dimensional growth can be obtained decreases as the growth rate increases. The higher the growth temperature, the greater the effects of re-evaporation and a decrease in the Zn adhesion coefficient, making it difficult to increase the growth rate.

In addition, when the N concentration added to the n-type ZnO layer was also examined by the method of the example, the higher the Zn beam flux intensity K _Zn J _Zn relative to the O radical beam flux intensity K _o J _O (the flux ratio becomes smaller). It was found that the N concentration tends to increase as the value decreases. The N concentration is in the range of 1 × 10 ¹⁸ cm ^{−3 to} 7 × 10 ²⁰ cm ⁻³ . In particular, at an O ₂ flow rate that realizes a high growth rate of 1 μm / h or more, the range is about 1 × 10 ¹⁸ cm ⁻³ to 1 × 10 ²⁰ cm ⁻³ .

  As described above, by supplying a beam generated by introducing a mixed gas containing oxygen and nitrogen into an electrodeless discharge tube of the same source gun and growing a ZnO layer, good surface flatness can be obtained. The n-type ZnO layer can be grown at a high growth rate while maintaining. For example, it becomes easy to grow a thick n-type ZnO layer having a thickness of 1 μm or more.

  In order to obtain a sufficiently high growth rate (for example, 800 nm / h or more), the growth temperature is preferably sufficiently high (for example, 600 ° C. or more). In order to suppress re-evaporation of ZnO, the growth temperature is more preferably 900 ° C. or lower.

For example, an n-type ZnO layer having a high surface flatness with a surface roughness Rms of 1.0 nm or less obtained by AFM observation in a 1 μm square region can be obtained. The n-type ZnO layer thus grown contains N at a concentration of, for example, 1 × 10 ¹⁸ cm ⁻³ or higher.

Incidentally, an example has been described for introducing a gas mixture of O ₂ and N ₂ to O source gun, if a gas containing oxygen and nitrogen, could be used with other gas species. For example, in addition to N ₂ , nitrogen gas such as N ₂ O, NO, NO ₂ , N ₂ O ₃ , N ₂ O ₄ , NH _3, or the like may be used as a gas to be mixed with O ₂ . These may be used alone or in combination.

Alternatively, nitrogen oxides such as N ₂ O, NO, NO ₂ , N ₂ O ₃ and N ₂ O ₄ could be used as the gas containing oxygen and nitrogen. Furthermore, it is also conceivable to use a nitrogen source gas or an oxygen source gas in combination while using nitrogen oxide as a gas containing oxygen and nitrogen.

  In the embodiment, an example in which an n-type ZnO layer is grown has been described. However, the technique of the embodiment will be effective for growing an n-type ZnO-based semiconductor layer. For example, good surface flatness and a high growth rate are expected by growing an n-type MgZnO layer by introducing a gas containing oxygen and nitrogen into an O source gun. Other examples of the ZnO-based semiconductor include BeZnO, ZnCdO, ZnSeO, and ZnSO.

  Further, in the embodiment, an example of growing an undoped n-type ZnO layer has been described. Even when, for example, Ga or Al is added as an n-type impurity, a gas containing oxygen and nitrogen is introduced into the O source gun for growth. As a result, good surface flatness and a high growth rate are expected.

  Note that the substrate is not limited to the sapphire substrate, and for example, a SiC substrate, a GaN substrate, or the like may be used.

  Note that the effect of suppressing the decrease in surface flatness of the ZnO-based semiconductor layer and the effect of improving the growth rate by introducing a gas containing oxygen and nitrogen into an O source gun were confirmed even at a growth temperature lower than 600 ° C. Yes.

  Note that the n-type ZnO-based semiconductor layer obtained by using the technique of the example can be used for various products. For example, it can be used for light emitting diodes (LEDs), laser diodes (LDs), and application products thereof (for example, various indicators, LED displays, LD displays, RGB light sources for projectors, etc.).

  In addition, even if it is other than a light emitting element, it will be able to be used for applications that require a thick ZnO-based semiconductor layer of, for example, 1 μm or more.

  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 Ultra high vacuum container 2 Zn source gun 3 O source gun 3a Electrode discharge tube 4 (of O source gun) 4 Mg source gun 5 N source gun 5a Electrode discharge tube 6 (of N source gun) Ga source gun 7 O ₂ Mass flow controller 8, 9 Mass flow controller 9 for N ₂
10 stage 11 substrate 12 gun for RHEED 13 screen for RHEED 20 sapphire substrate 21 MgO buffer layer 22 ZnO buffer layer 23 (n-type) ZnO layer

Claims (10)

Preparing a substrate;
A step of introducing a gas containing O ₂ gas and N electrodeless discharge tube, generating a first beam to discharge,
The growth temperature is set to 600 ° C. or higher, and at least Zn is supplied above the substrate and the first beam is supplied from the electrodeless discharge tube to grow an n-type ZnO-based semiconductor layer. A method for manufacturing a ZnO-based semiconductor element.   The method for manufacturing a ZnO-based semiconductor element according to claim 1, wherein the substrate is one of a sapphire substrate, a SiC substrate, and a GaN substrate. The gas containing N to be introduced into the electrodeless discharge _{tube, N 2, N 2 O,} NO, NO 2, N 2 O 3, N 2 O 4, and claim 1 comprising any of the NH ₃ Or a method for producing a ZnO-based semiconductor element according to 2; The gas containing N to be introduced into the electrodeless discharge tube, method for manufacturing a ZnO based semiconductor device according to claim 3 which is N _2. Wherein the mixed gas of _{O 2} and _{N 2} to be introduced into the electrodeless discharge tube, the volume ratio of _{O 2} and _{N 2 are, (N 2 / O 2)} <10 ZnO based semiconductor of claim 4 which is in the range of Device manufacturing method.   The method for producing a ZnO-based semiconductor element according to claim 1, wherein the step of growing the n-type ZnO-based semiconductor layer uses molecular beam epitaxy.   The method for manufacturing a ZnO-based semiconductor element according to claim 1, wherein the step of growing the n-type ZnO-based semiconductor layer grows the n-type ZnO-based semiconductor layer to a thickness of 1 μm or more.   The method for producing a ZnO-based semiconductor element according to claim 1, wherein the step of growing the n-type ZnO-based semiconductor layer is performed under a Zn-rich condition. 9. The n-type ZnO-based semiconductor layer grown in the step of growing the n-type ZnO-based semiconductor layer contains N at a concentration of 1 × 10 ¹⁸ cm ⁻³ or more. A method for manufacturing a ZnO-based semiconductor element.   The n-type ZnO based semiconductor grown in the step of growing the n-type ZnO based semiconductor layer is any one of MgZnO, BeZnO, ZnCdO, ZnSeO, and ZnSO. A method for manufacturing a ZnO-based semiconductor element.