JPH10245550A - Zno ultraviolet emitter and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an ultraviolet emitter which has high luminous efficiency because the rate of ultraviolet rays slightly contained in the fluorescent spectrum of ZnO can be increased by irradiation with electron beams or ultraviolet rays by treating the surface of a ZnO solid with a hydrogen plasma. SOLUTION: A ZnO single crystal is grown by for example the flux method. The surface of this crystal is treated with a hydrogen plasma under condition including for example a hydrogen pressure of 10Torr and a temperature of 400-600 deg.C. When the cathodic luminescences produced when the surface of the treated crystal and the surface of the untreated grown crystal are irradiated with electrode beams are measured, the untreated grown crystal gives a spectrum containing a highly intense green emission and a weak ultraviolet emission, whereas the treated crystal gives a spectrum completely containing quite no green emission and intensified ultraviolet rays of excellent monochromaticity. By forming oscillators 5 and 6 on both ends of the stripe treated ZnO crystal 3, an ultraviolet emitter can be fabricated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an ultraviolet light emitter made of ZnO and a method for producing the same. Zn is a Group 2-6 semiconductor. The ZnO film emits fluorescence when irradiated by a laser having a high energy. Light of various wavelengths is emitted depending on the manufacturing method. Broadly speaking, it emits fluorescence of three different wavelengths. One is red having a peak distribution of about 630 nm to 690 nm. Another one is 480nm-5
Green light emission having a broad distribution of about 10 nm. The last one is ultraviolet light of about 380 nm.

[0002] These different wavelengths of fluorescence are emitted depending on the ZnO production method. 380 nm ultraviolet light is band-edge emission and has a distinct origin. In other words, the laser light raises electrons from the valence band to the conduction band, and emits ultraviolet light when the electrons fall again into the valence band. What other mechanisms generate longer wavelength light? It is not clear yet. Since the light has energy lower than the inter-band energy, it must be fluorescence due to transition between levels in the forbidden band. But what is producing the deep levels in the forbidden zone? I don't know yet.

[0003] Since the level generated between bands differs depending on the ZnO production method, the wavelength of fluorescence differs.
Also, although a semiconductor cannot be used, a p-type semiconductor cannot be formed and a pn junction cannot be formed. One method of light emission is to emit fluorescent light by applying an electron beam. The other is to generate fluorescence by applying a laser. Thus, at present, there is no means for emitting ZnO only by light excitation and electron beam excitation. Therefore, ZnO cannot be said to be a light emitting element and should be called a phosphor. It should be called fluorescence instead of light emission, but here we use the word light emission in a broad sense,
Light generation by fluorescence is also included.

[0004] Further, regarding the wavelength, there are several other light emitting elements for long wavelength light having low energy.
Therefore, the significance of generating long-wavelength light by ZnO is small. Therefore, it is not expected much as a material emitting red orange light. Green fluorescent light is characteristic of ZnO and can be used for display panels and the like. This is the one that is currently in the limelight as a use of ZnO. There are other light-emitting elements that emit blue-green color, but ZnO is also considered as a candidate for the wavelength.

[0005] Ultraviolet light emission is due to an interband transition and has only a distinct origin. However, this is weak, and a practical fluorescence intensity cannot be obtained. Since it is not visible light, it is not conceivable to apply it to display panels. Ultraviolet light is light with high energy and has uses other than display. However, it is considered too useless because it is too weak. Therefore, ZnO is not expected as a material that emits ultraviolet light. Green has been the focus of attention.

[0006] ZnO forms wurtzite-type hexagonal crystals. Now, with the crystal growth method, unlike semiconductors such as Si and GaAs, ZnO cannot grow a large single crystal by the Czochralski method or the Bridgman method. Since it is an oxide and has a high melting point (1980 ° C.), it is difficult to melt it by heating to a liquid. Simply heating will cause decomposition. Melting only when heated under high pressure. Therefore, a crystal growth method in which a melt is formed and solidified is not applicable. Attempts have been made to produce single crystals by the flux method, but it has not been successful. Large single crystals are not currently available.

A thin film of ZnO can be formed by sputtering. However, this results in a defective polycrystalline thin film with much oxygen loss. The most commonly used method for producing ZnO at present is a sintering method. The ZnO polycrystalline powder may be mixed with a binder, or may be solidified and heated to form a lump while applying pressure. The sintering method is a preferred method because it is an oxide. Since it is made by a mold, it is possible to make flat and concave ZnO plates of various sizes. Since a large-sized sintered ZnO plate can be produced, it is expected that it can be used for a display panel or the like.

[0008]

2. Description of the Related Art Since ZnO generates two lights (red orange and green), it is considered to be used as a display panel. Japanese Unexamined Patent Publication No. Hei 6-240250 "ZnO visible light emitter" proposes that an orange light emitting ZnO portion and a green light emitting ZnO portion are formed on the same substrate to form a display panel. Z on quartz substrate
n under vacuum at a rate of 10 ° C./min in an air atmosphere.
Raise the temperature to 40 ° C. It is kept at 540 ° C. for 1 hour and oxidized to form a ZnO thin film. Then, part of ZnO is removed using a mask. When this is irradiated with a He-Cd laser, the portion protected by the mask emits orange fluorescence, and the partially removed portion emits green fluorescence. Therefore, a two-color display panel can be made of ZnO.

When ZnO is formed into a thin film by sputtering, it can emit orange (680 nm) fluorescent light. ZnO powder pressed and sintered at 900 ° C. or higher in an Ar atmosphere emits green (480 nm) fluorescent light. Depart from where you put it out. Give a heating rate of 10 ° C / min.
Oxidation of n with air creates the same thin film as made by sputtering. Only the surface becomes a phosphor of 680 nm, and if the surface is scraped off, it becomes a phosphor of 480 nm. This means that a two-color display board can be made.

Since Zn is heated in the air, oxygen diffuses from the surface and is gradually oxidized. It seems that the surface is sufficiently oxygen to be a 680 nm orange phosphor, but the inside is 480 nm green phosphor due to lack of oxygen. It is said that by cutting off a part and exposing the inside, it will emit green fluorescence. This emphasizes both orange and green emission. But ultraviolet doesn't matter. This is because it is useless for the display panel, not for visible light. Claims that the heating rate of the Zn thin film is 10 ° C./min is an important parameter.

K. Vanheusden, WL Warren, CHseag
er, DrTallant, JAVoigt, and BEGnade, "Mechani
sms behind green photoluminescence in ZnO phosphor
powders ", J. Appl. Phys. 79 (10), 15 May 1996 p7983
(1996) describe the 510 nm green fluorescence of ZnO. He states that there is a strong correlation between free carriers, oxygen vacancies (vacancies), and green fluorescence in ZnO.
There is no oxygen vacancy on the surface of the crystal grain, and it becomes diamagnetic. Inside, electrons are trapped in oxygen vacancies. So it takes on monovalent ions. It is inferred that ionized oxygen deficiency is the cause of green emission. In the paper, the number of oxygen vacancies and the number of free carriers are changed in various ranges by redoxing ZnO powder. Banhowden has developed a hypothesis that there is no oxygen deficiency in the outer periphery of the powder particles but oxygen deficiency in the inside. Oxygen deficiency becomes a donor level. Since the semiconductor is an n-type semiconductor, the Fermi level is higher than half of the forbidden band. Oxygen-deficient donors are higher than the Fermi level. Since the surface of the crystal grain has a high potential, the band bends and the donor becomes empty near the surface, so that the surface becomes positively charged. These electrons become free electrons and exist inside the crystal. The band is bent so that the donor level is lower than the Fermi level inside the crystal. It is hypothesized that electrons fall into the valence band from this lowered donor level and emit green 510 nm light. Therefore, we conclude that free carrier density, oxygen deficiency, and green emission are directly proportional to each other. The mystery of green emission is due to oxygen deficiency, and the electrons fall from this donor into the valence band because the bending of the band lowers the donor level below the Fermi level.

K. Vanheusden, CH Seager, WLWarr
en, DR Tallant and JA Voigt, "Correlation betwe
en photoluminescence and oxygen vacancies in ZnO p
hosphors ", Appl. Phys. Lett. vol. 68, No. 3, 15 Janu
ary 1996, p403 (1996) indicates that there is a strong correlation between oxygen deficiency of ZnO, free carriers, and green emission at 510 nm from measurements of paramagnetic resonance, optical absorption spectrum, and photoluminescence. The purpose is the same as. If there is an oxygen vacancy, this generates free electrons, so that free carriers increase.
It becomes a donor level, but falls below the Fermi level, so electrons fall from this to produce green near 510 nm.
That is.

The physical image behind is the same except for the means of measurement. However, the present inventor believes that Van Howden's inference has various problems. The oxygen deficiency on the surface of the ZnO crystal grains is diamagnetic in the inside and paramagnetic in the inside, because the height relationship between the Fermi level inside and outside is reversed. Does band distortion change the height of such donor levels? If the crystal grains are large, such things should be unlikely, and this may not be the case if the crystal grains are small. And is there a convenient situation where a deep level of oxygen vacancy crosses the Fermi level just inside or outside? According to Banhowden's hypothesis, the more free carriers the more green light (fluorescence) is dominant. Although free carriers are due to oxygen vacancies, a large number of oxygen vacancies means that crystals are incomplete. Therefore, green fluorescence becomes stronger as the number of oxygen vacancies in the incomplete crystal and the size of the crystal grain are smaller. If it is a green light emitting device, it means that polycrystals with many defects should be used. Van Howden, however, did not mention ultraviolet light.

As described above, there are few references concerning ZnO.
There are few documents, such as orange (about 680 nm) emission and green (480
５１０510 nm). There is no mention of ultraviolet fluorescence. It is known that this is an inter-band transition (conduction band to valence band). One reason is that the feature is clear and does not attract scholars. Is it possible that a sample that emits green light and ultraviolet light is considered to be too weak in ultraviolet light and has no practical use?

[0015]

SUMMARY OF THE INVENTION It is a first object of the present invention to provide a method for increasing the percentage of ultraviolet light slightly contained in the fluorescence spectrum of ZnO. It is a second object of the present invention to provide an ultraviolet light emitting device using ZnO.

[0016]

According to the present invention, there is provided a method for treating ZnO, comprising subjecting a surface of a ZnO solid to hydrogen plasma treatment. The ultraviolet luminous body of the present invention is a hydrogen plasma-treated Zn
An electron beam, an ultraviolet laser, and X-rays are applied to O to generate ultraviolet light.

[0017]

DETAILED DESCRIPTION OF THE INVENTION The present invention dramatically increases the efficiency of ultraviolet emission by treating a ZnO plate with hydrogen plasma.
Next, a method for producing hydrogen plasma-treated ZnO, a fluorescence spectrum, and the like will be described.

(A) First, Zn is applied by a flux method.
An O sample was prepared. ZnO powder and PbF ₂ powder were mixed. PbF ₂ is a solvent. This is put in a Pt crucible, heated to a temperature of 1040 ° C., and kept for 2 hours.
Thereafter, the temperature is slowly lowered to 950 ° C. at a rate of 5 ° C./h. Thus, the approximate size is 10 mm x 10 mm x
A 0.3 mm ZnO single crystal was obtained. The grown single crystal was yellowish and transparent.

(B) This was heat-treated in an oxygen stream. The annealing temperature was 1000 ° C. and the annealing time was 6 hours. Oxygen annealing was performed to reduce oxygen vacancies (oxygen vacancies). Then 100 ° C / h up to 400 ° C
Temperature. Then, it was allowed to cool to room temperature. (C) Next, this ZnO sample was accommodated in a plasma processing chamber made of quartz. 10 Torr of hydrogen in plasma processing chamber
It was introduced to become. 2.45 by magnetron
A microwave of GHz and 300 W was generated to excite hydrogen gas into plasma. The sample was heated to 400 ° C. and exposed to hydrogen plasma for 7 minutes. The surface is hydrogenated. The hydrogenated sample turned slightly bluish yellow.

Thus, three types of samples are completed. Grown ZnO (a), oxygen-annealed ZnO
(B) ZnO (c) treated with hydrogen plasma. The cathodoluminescence of these three samples (a), (b) and (c) was measured. Cathodoluminescence refers to luminescence that occurs when an electron beam is applied to a sample. The measurement is to obtain a fluorescence distribution for each wavelength (or each energy) by dispersing the spectrum with a spectroscope. Generally, when a sample is irradiated with light, X-rays, or electron beams, light having a specific wavelength distribution is generated. This is called fluorescence when there is no time delay, and is called phosphorescence when there is a time delay. It is called luminescence (lumminescence) including both.
What causes luminescence is various such as X-rays, light, electric fields, and the like. Here, since the electron beam is applied to the sample, it is called cathode luminescence. Any substance emits light when irradiated with an electron beam. Emission (fluorescence) of light emitted by cathodoluminescence does not mean a light-emitting element.

The acceleration energy of the electron beam is 5 kV,
The beam current was varied in the range from 60 pA to 1.8 nA. The entire wavelength region was detected by a CCD of 1040 channels in increments of 0.8 nm.

FIG. 1 shows (a) ZnO as grown,
(B) shows the results of cathodoluminescence (CL) measurement of oxygen-annealed ZnO and (c). The horizontal axis is photons (photons)
Energy and the vertical axis is CL intensity (cps: count number per second). The electron beam acceleration voltage is 5 kV and the current is 1 nA.

(A) As grown ZnO is 2.2 eV
Has a spectrum spread near. Half width is 0.5e
At V, the peak height is about 750 cps. 2.2 eV
Is 560 nm in terms of wavelength. This is green light emission. Further, light emission as small as 3.2 eV is observed. The half width is narrow and the height is 300 cps, which is weak. This is 370 nm ultraviolet light emission.

In the oxygen-annealed state (b), green luminescence is predominant. 1 peak height
The half width spreads to 0.8 eV at 500 cps. It is considered that the oxygen deficiency is reduced due to the oxygen annealing, but the green luminescence is more than doubled.
So I think Van Howden's inference that oxygen deficiency is the cause of green fluorescence is wrong. However, this is unrelated to the present invention and will not be described further.

(C) Hydrogen plasma treatment is completely different from the former spectrum. The green spectrum disappears completely and only 3.2 eV (370 nm) fluorescence is seen.
Of particular importance is the disappearance of the green spectrum.
The 3.2 eV fluorescence is due to an interband transition (exciton emission). The half width is extremely narrow at about 0.15 eV. The peak height is 4000 cps. Those treated with hydrogen plasma emit ultraviolet light having excellent monochromaticity.

From the measurement results, it was found that light emission of 2.2 eV
It can be seen that the 3.2 eV emission is complementary. As the green emission intensity decreases, the ultraviolet emission intensity increases accordingly. That is, by the hydrogen plasma treatment, carrier recombination at a deep level is suppressed and green light is prevented from being generated. As a result, the transition at the band edge is dominant, and short-wavelength light is generated. It is presumed that the hydrogen plasma has lost many deep levels during the forbidden zone. Therefore, no green light is generated.

Van Howden claims that oxygen deficiency is responsible for the green luminescence. In order to eliminate green light emission, it is concluded that oxygen should be forcibly supplied to eliminate oxygen deficiency (vacancy). However, as mentioned earlier, oxygen anneal does not work to reduce green luminescence. Rather, supplemental oxygen tends to enhance green luminescence. Hydrogen plasma treatment produces hydrogen,
Combined with the defect serving as the emission center, the defect may have become inactive.

Hydrogen plasma treatment is a new processing method of the present invention. What is going on with hydrogen plasma processing?
First, for the sample not subjected to the hydrogen plasma treatment, 3.2.
The difference in behavior of eV and 2.2 eV luminescence with respect to temperature was investigated. Figure 2 shows the oxygen-annealed sample (b)
Is a change in the temperature of CL. The electron acceleration voltage is 5 kV and the current is 1.8 nA. Temperature is 30K, 80
K, 120K, 160K, 220K, and 280K. The green luminescence (2.2 eV) attenuates as the temperature rises, and at 280K becomes 1/5 that at 30K. It goes without saying that the luminescence decreases with increasing temperature. The green luminescence has been reduced to 1/5. However, the luminescence intensity of 3.2 eV only drops to about 1/2. 3.36 eV peak position
(30K) changes to 3.26 eV (280K), which is the temperature fluctuation itself of the band gap.

FIG. 3 shows the temperature dependence of the luminescence of the sample (c) subjected to the hydrogen plasma treatment. There is no green luminescence at any temperature. Only the luminescence of ultraviolet light. The temperature dependence was extremely large, and at 30K, a very large CL approaching 20,000 cps could be observed. However, 285K is about 1/100 of 30K
Also drop. The peak energy at 30K was strictly measured and was 3.350 eV. This is the energy of electron-hole recombination in the bound exciton. In other words, when the hydrogen plasma treatment is performed, the emission intensity is increased by two orders of magnitude as compared with that before the treatment, so that stronger ultraviolet light can be emitted.

The temperature dependence of the peak height of such ultraviolet light was examined. FIG. 4 shows the result. The horizontal axis is the reciprocal of temperature (100 / T), and the vertical axis is the CL intensity (cps).
Is taking. When the temperature is high, the CL of 3.2 eV becomes weak. When the temperature is low, CL becomes strong.

The ZnO single crystal and polycrystal have several uses. Electron beam irradiation or ultraviolet ray
This is because strong ultraviolet light is generated by laser irradiation (for example, He-Cd laser). Therefore, an ultraviolet laser was manufactured using single crystal ZnO.

Both surfaces of the ZnO single crystal were subjected to hydrogen plasma treatment. Hydrogen pressure is 10 Torr, temperature is 400 ° C-600
° C. This was cut into a rectangular shape and used as a starting sample. In order to form a stripe structure, a SiN film 2 was formed on a ZnO single crystal 1 as shown in FIG. Then, the SiN film 2 was selectively removed except for the linear portion. Further, exposed ZnO to be performed by using a hydrochloric acid-based or phosphoric acid-based etchant is removed. This is the state shown in FIG. Thereafter, the SiN film 2 serving as a mask is removed. A stripe 3 of ZnO is generated. This surface is a surface subjected to the hydrogen plasma treatment. Resonators 5 and 6 that reflect ultraviolet light are formed at both ends. The remaining surface portion 4 is a portion that has not been subjected to the hydrogen plasma treatment.

When a stripe-shaped electron beam is applied to the stripe 3, light emission of 3.2 eV from ZnO occurs. It has a raised band-like optical path, and the ultraviolet light is confined in the stripe due to the difference in refractive index. Since the light propagates in the longitudinal direction of the band and is reflected by the resonators 5 and 6, laser oscillation occurs. Strong ultraviolet light is propagated in the direction of the stripe. Stripe 3
Here, one is shown, but there may be a plurality.
In that case, a plurality of laser beams can be obtained. It is necessary to apply a band-shaped electron beam over the entire stripe, but if it is difficult to generate such a beam, it is sufficient to scan the electron beam along the stripe.

[0034]

FIG. 7 is a schematic view of a ZnO ultraviolet laser.
A plurality of stripes are formed on the ZnO single crystal 1 by the method using the SiN mask. Although only four lines are shown here, for example, ten stripes are formed in parallel.
The total width of the ten stripes is, for example, about 100 μm. Only the surface 7 of the portion protruding in a stripe shape is the surface subjected to the hydrogen plasma treatment. A field emitter 8 made of Si, diamond, or the like is provided immediately below. This is a mechanism that generates electrons. Field emitter 8 and Z
A voltage 13 for extracting electrons is applied between nO1. Electrons emerge radially from the field emitter. The longitudinal direction of the stripe 3 is defined as the Z axis. The direction perpendicular to the ZnO plane is the Y axis, and the direction perpendicular to the stripe is the X axis. The electrons spread in the Y-axis direction and are emitted. A pair of magnetic poles 9, 10 extending in a direction parallel to the stripe is further provided. The magnetic pole is a U-shaped or U-shaped core and has a portion extending in the X direction, whereby the magnetic poles 9 and 10 are connected. A coil is wound around an appropriate portion, and an alternating current is passed through the coil. Lines of magnetic force are produced in the X direction, which change periodically due to the alternating current. The charged particles traveling in the Y direction by the magnetic field lines in the X direction bend in the Z direction. The beam oscillates in the Z direction because the bending changes periodically. So the electron beam is swung along the stripe. Effectively, the entire length of the stripe is irradiated with the electron beam. The electron beam generates ultraviolet light on each hydrogen plasma treated surface. Ultraviolet light is confined in the stripe due to the difference in refractive index. The light is reflected and amplified by the resonators at both ends. Some are emitted from the end face. Thereby, a plurality of ultraviolet beams are generated at the same time.

[0035]

According to the present invention, an element for generating ultraviolet light can be produced by ZnO, so that an inexpensive ultraviolet light emitting element can be obtained. ZnO may not be a single crystal. Since it may be a powder, any shape can be produced by sintering. Since it may be a thin film, it is easy to manufacture. That is, the degree of freedom of the shape is large. Since it is an ultraviolet light source, it can be converted to light of a longer wavelength.

[Brief description of the drawings]

FIG. 1 is a graph showing the cathodoluminescence measurement results of as-grown ZnO (a), oxygen-annealed ZnO (b), and hydrogen plasma-treated ZnO (c). The horizontal axis is photon energy, and the vertical axis is counts per second (cp).
s).

FIG. 2 is a graph showing the temperature dependence of cathodoluminescence of ZnO that has not been hydrogenated. The horizontal axis is the photon energy (eV), and the vertical axis is the number of photons incident per second (cp).
s).

FIG. 3 is a graph showing the temperature dependence of cathodoluminescence of ZnO subjected to hydrogen plasma treatment. The horizontal axis is the photon energy (eV), and the vertical axis is the number of photons incident per second (cp).
s).

FIG. 4 is a graph showing a measurement result of a temperature change of cathodoluminescence of ZnO subjected to hydrogen plasma treatment.

FIG. 5 is a perspective view showing a ZnO single crystal whose surface is treated with hydrogen plasma and the surface is covered with SiN.

FIG. 6 is a perspective view showing a state in which the SiN mask is removed while leaving a strip-shaped portion.

FIG. 7 is a schematic configuration diagram of an ultraviolet laser having four stripes.

[Description of Signs] 1 ZnO single crystal 2 SiN film 3 ZnO stripe 4 Remaining surface portion 5 Resonator 6 Resonator 7 Hydrogen plasma treated surface 8 Field emitter 9 Magnetic pole 10 Magnetic pole 11 Magnetic field line 12 Electron beam 13 Voltage

Claims (2)

[Claims] 1. A method for producing a ZnO ultraviolet light emitter, comprising subjecting a surface of a ZnO solid to hydrogen plasma treatment. 2. A ZnO ultraviolet light emitter characterized in that ultraviolet light is generated by irradiating an electron beam or ultraviolet light to ZnO subjected to hydrogen plasma treatment.