JP5062882B2 - Inorganic electroluminescence device - Google Patents

Description

  The present invention relates to an inorganic electroluminescent device, and more particularly to an inorganic electroluminescent device using a polycrystalline inorganic compound formed by a vapor phase method.

An electroluminescence element is a device that emits light by applying an electric field to a solid phosphor substance, and can be classified into an organic electroluminescence element and an inorganic electroluminescence element depending on the phosphor material used. The organic electroluminescence element is a current injection type light emitting device that emits light by injecting holes and electrons, and can emit light with high luminance of about 100 to 100,000 cd / m ² at a low voltage of DC 10 V or less. However, since organic materials are used, the biggest problem is that the lifetime during driving is short.

  On the other hand, an inorganic electroluminescence device is a device using an inorganic phosphor used for CRT, illumination, etc., and electrons accelerated by a high electric field collide with a host lattice to be ionized, or collide with an emission center to be excited. However, it is a so-called AC light emitting device that emits light when they relax. As an example of such an inorganic electroluminescence element, a thin film type element (for example, Patent Document 1) formed by sandwiching a phosphor thin film between dielectric layers and a phosphor powder such as ZnS: Cu, Cl in a dielectric. And a powder type element (for example, Japanese Patent Application Laid-Open No. H10-228688) dispersed in the material. These inorganic electroluminescent elements are characterized by long life and excellent reliability due to the use of inorganic materials as light emitting materials, and have already been used as backlights for clock dials and display parts of portable devices. It has been put into practical use.

In addition, an inorganic electroluminescence element that can be DC driven at a relatively low voltage has been proposed (for example, Non-Patent Document 1). This inorganic electroluminescent element uses a deposited film in which manganese, copper, and chlorine are added to zinc sulfide, which is a II-VI group semiconductor, and a conductive copper sulfide layer is formed by an initial energization process called a forming process. As a result, current flows and light is emitted. However, in this type of light emitter, most of the current flowing in the element flows through the channel of the copper sulfide layer, which causes a significant problem that the light emission efficiency is extremely lowered and the luminance is extremely lowered.
An example of such an inorganic electroluminescence element is shown in FIG. Usually, a light emitting layer 2 is formed on a glass substrate 1 on which a translucent electrode 5 such as ITO is formed, and a metal electrode 3 is formed on the light emitting layer 2.

Japanese Patent No. 2840185 JP-A-9-92467 Japan Journal of Applied Physics, 10, 326 (1971)

From the viewpoint as described above, inorganic electroluminescent elements are easy to manufacture and have a long lifetime, and are therefore notable for light-emitting elements, particularly for lighting applications.
However, since conventional inorganic electroluminescence elements are driven by alternating current, they require a higher voltage than current injection type devices, and it is impossible to obtain sufficient luminous efficiency and luminance. The range is limited.

  For example, there are demands for lighting applications that not only have a long service life but also enable a large area, low voltage drive, high luminous efficiency, and high brightness. However, it has been difficult to put inorganic electroluminescence elements into practical use.

In addition, as described above, an inorganic electroluminescent element that can be driven by a direct current has been proposed, but such an inorganic electroluminescent element cannot obtain a sufficient luminous efficiency and is difficult to increase in luminance. There was a problem.
Furthermore, there has been a demand for a light source that does not require a large facility investment like a semiconductor laser and can have a long life and a large area.

Under such circumstances, a light emitting layer using a II-VI group compound having a polycrystalline structure formed by a vapor phase method for increasing the area has attracted attention.
However, as a result of various experiments, when a II-VI group compound having a polycrystalline structure formed by a vapor phase method is used as the light emitting layer, it is formed so that the grain boundaries are aligned. It was found that when an electric field is applied, a current flows along the grain boundary, and the flowing current increases without contributing to light emission.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an inorganic electroluminescent element having high luminous efficiency and high luminance.
It is another object of the present invention to provide an inorganic electroluminescent element that has a long life, can be driven at a low voltage, and can have a large area.

In order to solve the above-mentioned problem, the present invention is an inorganic electroluminescent device configured by sandwiching a light emitting layer containing a II-VI group compound formed by a vapor phase method between a pair of opposing electrodes,
The pair of electrodes extend from a position outside the side surface of the light emitting layer containing the II-VI group compound in a direction intersecting the film forming direction of the light emitting layer, and a part of the upper surface or the lower surface of the light emitting layer The II-VI group compound includes a crystal grain boundary, and the electrode has a current flowing between the electrodes that is separated from the crystal. It arrange | positions so that a grain boundary may be crossed, and the said crystal grain boundary is comprised with an inorganic compound with higher electroconductivity than another area | region .
That is, an inorganic electroluminescent element having a light emitting layer containing a II-VI group compound formed by a vapor phase method between a pair of opposing electrodes, and a line connecting the pair of electrodes is the II A light emitting layer containing a -VI group compound is formed so as to intersect with the film forming direction.
According to the above configuration, the two II-VI group compounds having a polycrystalline structure composed of grain crystals (grains) selectively oriented in a direction perpendicular to the substrate are separated from each other in the plane direction of the substrate. Since the electrodes are arranged, when a voltage is applied between the two electrodes, it can be energized in the direction of the substrate surface so as to intersect the grain boundary of the II-VI group compound crystal. For this reason, it is possible to reduce the current loss flowing in the grain boundary direction of the selectively oriented crystal of the II-VI group compound, and to improve the light emission efficiency.
Furthermore, according to the above configuration, a region having high conductivity is formed at the grain boundary, and by passing a current so as to cross the grain boundary, the grain boundary functions as an internal electrode. Can be improved. For this reason, it is possible to reduce the current loss flowing in the grain boundary direction of the selectively oriented crystal of the II-VI group compound, and to improve the light emission efficiency.

  Moreover, this invention includes the said inorganic electroluminescent element in which the said highly conductive inorganic compound is a compound of copper.

In the inorganic electroluminescence device according to the present invention, the II-VI group compound is formed between the electrodes of the pair of electrodes, and the distance between the electrodes is 0.5 to 10 μm. Including.
According to the above configuration, the distance between the two electrodes is preferably 0.5 to 10 μm, and light emission can be performed efficiently. If the distance between the electrodes is smaller than 0.5 μm, the light emission efficiency is low, and if it is larger than 10 μm, the driving voltage increases.

In addition, the present invention includes the above inorganic electroluminescent device in which the II-VI group compound is zinc selenide .
According to the said structure, the band gap of the II-VI group compound which is a luminescent substance is 2.6 eV or more, and blue light emission useful as a light source for illumination is possible.

In addition, the present invention includes the inorganic electroluminescent element in which the II-VI group compound is zinc sulfide.
According to the said structure, the band gap of the II-VI group compound which is a luminescent substance is 2.6 eV or more, and blue light emission useful as a light source for illumination is possible.

In addition, the present invention includes the inorganic electroluminescent element, wherein the ZnS contains oxygen (O) and is ZnS _x O _1-x (0.1 <x <0.9).
According to the above configuration, the band gap can be narrowed by the amount of oxygen introduced, and a desired emission wavelength can be obtained.

The present invention includes the inorganic electroluminescent element, wherein the ZnS is ZnS _x O _1-x (0.4 <x <0.6).
According to the said structure, blue light emission can be obtained and it is effective as a light source for illumination.

  Further, the present invention includes the inorganic electroluminescent element, wherein the pair of electrodes are arranged on the surface of the substrate so as to have a predetermined interval.

According to the inorganic electroluminescent element of the present invention, two electrodes are separated in the plane direction of the substrate with the polycrystalline II-VI group compound composed of grains selectively oriented in the direction perpendicular to the substrate interposed therebetween. Therefore, when a voltage is applied between the two electrodes, it can be energized along the substrate surface direction of the II-VI group compound crystal. For this reason, the current flows across the channel formed at the grain boundary, and the current loss flowing in the grain boundary direction of the selectively oriented crystal of the II-VI group compound can be reduced, and the luminous efficiency is improved. It becomes possible.
In particular, when an inorganic compound having higher conductivity than the II-VI group compound is present at the crystal grain boundary of the II-VI group compound, the effect is remarkable, and the driving can be performed at a low voltage.

Embodiments of the present invention will be described below. The present invention is not limited to the following examples, and it goes without saying that various modes are possible for details.
Hereinafter, embodiments of the present invention will be described in more detail with reference to the accompanying drawings.
(Embodiment 1)
FIG. 1 is a cross-sectional view illustrating an embodiment of an inorganic electroluminescence element of the invention. As shown in FIG. 1, this inorganic electroluminescent element is a direct current drive type element, and the light emitting layer 2 is composed of a II-VI group inorganic compound formed on the surface of the substrate 1 by a vapor phase method, and has two electrodes. 3 is spaced apart in the surface direction of the substrate. The light emitting layer 2 has a polycrystalline structure, and has a structure in which II-VI group inorganic compound crystals 21 selectively oriented in a direction perpendicular to the substrate are oriented through the grain boundaries 22. The inorganic electroluminescence element is configured such that electroluminescence emission occurs in the light emitting layer 2 by connecting the drive power supply 4 via the electrode 3 and energizing the inorganic electroluminescence element.

  Next, the light emitting layer 2 will be described in detail below. The light emitting layer 2 is composed of a II-VI group inorganic compound and is composed of a polycrystalline structure formed by a vapor phase method. Here, as a II-VI group inorganic compound, Zn, Cd is used as a group II element, and O, S, Se, Te is used as a group VI element. Specifically, ZnS and ZnSe are most preferable from the viewpoint of having a band gap suitable as a light emitter for visible light emission and global environmental problems.

A compound that has been extensively studied as a group II-VI wide gap semiconductor for blue emission, and includes ZnO, ZnS, ZnSe, and the like. These compounds can obtain conductivity relatively easily by donor doping, group VI element defects, and interstitial group II elements. In this embodiment, ZnS having the widest band gap and n-type conductivity is used. Here, for the purpose of obtaining a desired emission wavelength or adjusting the band offset at the bonding interface in order to increase the light emission efficiency, an oxygen element may be mixed in ZnS to form ZnS _X O _1-X. . The mixing ratio x of O is adjusted to a value of 0.1 to 0.9, particularly preferably 0.4 to 0.6. If the mixing ratio of oxygen is less than 0.1, the effect cannot be exhibited, and if it exceeds 0.9, it becomes difficult to obtain conductivity.

  Hereinafter, ZnS will be described as an example of a typical II-VI group inorganic compound. A ZnS film formed by a vapor phase method such as resistance heating vapor deposition, electron beam (EB) vapor deposition, or high frequency sputtering is different from an epitaxial film formed by, for example, a molecular beam epitaxy method or a metal organic vapor phase deposition method. It is known to take a structure. The polycrystalline structure is not a random orientation but a structure oriented to some extent in a direction perpendicular to the substrate. An inorganic compound having higher conductivity than ZnS is formed at the crystal grain boundary.

More preferably, a metal such as Cu, Fe, Ni, Co, Mo, Ta, W, Ag, or Au is added in advance to the vapor deposition material, and Cu _x S (x = 1) is obtained by conducting an energization treatment after the film formation. And a method of forming a conductive inorganic compound such as ˜2) at the grain boundary of ZnS. Here, the grain size of the ZnS crystal is about several tens to 100 nm. By arranging the two electrodes in the light emitting layer formed in this manner so as to be separated from each other in the surface direction of the substrate, current does not flow only in the conductive Cu _x S channel, but also flows in the ZnS portion. Therefore, the light emission efficiency with respect to the current can be increased. As the II-VI group inorganic compound used for the light emitting layer, one kind may be used alone, or a mixture of plural kinds may be used. In addition, as the luminescent center material, a local luminescent center element such as Mn, Eu, Tb, or Ce, or a donor-acceptor pair element such as Ag—Cl or Cu—Cl can be added.

  Here, the method for forming the compound thin film is not particularly limited, but may be a resistance heating vapor deposition method, an electron beam vapor deposition method, a high frequency sputtering method, an atomic layer epitaxy method, a metal organic vapor phase deposition method, a halogen transport vacuum CVD method, a laser ablation method, or the like. It can be formed by a method. Moreover, after forming the light emitting layer 3, in order to improve crystallinity, thermal annealing may be performed in a vacuum or in an inert gas or a sulfur-based gas. For example, sulfur easily evaporates and therefore easily escapes from the film. Therefore, annealing may be performed in a gas atmosphere containing sulfur, or annealing may be performed while supplying a gas containing sulfur. Conditions such as annealing temperature and annealing time are not particularly limited. In addition, when a compound thin film is formed by resistance heating vapor deposition or an electric beam vapor deposition method, it is also effective to control the film composition by devising multiple evaporation sources so that the composition can be adjusted. It is. It is also effective to supply a gas containing atoms that are easily evaporated, such as sulfur, to prevent evaporation of the atoms.

  Next, as the substrate 1, a glass substrate such as soda lime glass, non-alkali glass, or quartz, a translucent plastic (resin) substrate, or the like can be used. When a metal substrate, silicon substrate, compound semiconductor substrate, or the like is used, an insulating film needs to be formed on the substrate surface in order to prevent a short circuit between the substrate 1 and the electrode 3. In addition, when light is extracted from the electrode 3 side, it is not necessary to use a light-transmitting substrate, and a silicon substrate, a compound semiconductor substrate, a ceramic substrate, or the like can be used.

  The electrode material only needs to be able to apply a voltage to the two electrodes, and a metal material is used. Here, in this invention, the distance between two electrodes is important, and 0.5-10 micrometers is preferable. If it is smaller than 0.5 μm, the light emission efficiency is low, and if it is larger than 10 μm, the driving voltage becomes large. Here, as a method of forming the electrode, a method of patterning by an etching method using a mask formed by a photolithography method and a method of patterning by a printing method using a metal paste can be mentioned. Further, the two electrodes 3 may be formed so as to cover a part of the upper surface from the side surface of the light emitting layer 2 as shown in FIG.

Here, as one of the electrodes 3 provided on the substrate 1, an anode is provided, which serves to inject holes into the light emitting layer. As the anode, it is preferable to use an electrode material made of a metal, an alloy, an electrically conductive compound, or a mixture thereof having a high work function. Here, as these electrode materials, those having a work function of 4 eV or more are preferably used. Specific examples of such electrode materials include metals such as gold, translucent conductive materials such as CuI, ITO (indium tin oxide), SnO ₂ , and ZnO.

On the other hand, the other electrode of the electrode 3 is provided with a cathode and functions to inject electrons into the light emitting layer. As the cathode, it is preferable to use an electrode material made of a metal, an alloy, an electrically conductive compound and a mixture thereof having a low work function. Here, it is preferable to use a material having a work function of 5 eV or less as these electrode materials. Examples of such electrode materials include sodium, sodium-potassium alloy, lithium, magnesium, aluminum, magnesium-silver mixture, magnesium-indium mixture, aluminum-lithium alloy, Al / Al ₂ O ₃ mixture, Al / LiF mixture, and the like. Can be mentioned.

(Embodiment 2)
In the first embodiment, the electrode 3 is formed on the end face and the upper layer of the light emitting layer 2. However, an anode is disposed on the substrate 1 as the electrode 3, and a cathode is disposed on the substrate 1 as the other electrode 3. The light emitting layer 2 may be formed on the anode and the cathode.
That is, in the inorganic electroluminescent element of the present embodiment, as shown in FIG. 2, the electrode 3 is formed by patterning the lower layer than the light emitting layer 2, that is, the substrate side 1, and the light emitting layer 2 is formed thereon. ing. 4 is a power supply.
Other configurations are the same as those in the above embodiment.

(Embodiment 3)
In the first and second embodiments, one inorganic electroluminescent element has been described. However, a light emitting device in which a large number of inorganic electroluminescent elements are arranged on the substrate 1 is also effective.
That is, as shown in FIG. 3, the light emitting layer base is formed with a plurality of upright electrodes 3 by printing or the like, and the light emitting layer 2 is formed on the entire surface of the substrate by EB vapor deposition or the like. Thus, a plurality of inorganic electroluminescent elements are arranged.
With this configuration, it is possible to obtain a light emitting device in which a plurality of inorganic electroluminescent elements having high luminous efficiency and high reliability are assembled.
Other configurations are the same as those in the above embodiment.
The portion of the light emitting layer 2 that covers the electrode 3 may be removed by anisotropic etching to flatten the surface, but it may be left as it is.

Examples of the present invention will be described below.
<Example 1>
As shown in FIG. 1, an alkali-free glass substrate 1 is prepared, and a ZnS: Mn (0.5 wt%), Cu (1.0 wt%), Cl (1.0 wt%) is formed on the substrate 2 as a light emitting layer 2. ) Was formed by an EB vapor deposition method at a substrate temperature of 200 ° C. to a thickness of 500 nm. Next, Al was formed as an electrode on the light emitting layer by an EB vapor deposition method at a thickness of 100 nm at room temperature, followed by patterning so that the electrode spacing was 2 μm through a photolithography process.

When a voltage of 25 V was applied between the two Al electrodes of the manufactured inorganic electroluminescence element, a current of ² mA / cm ² flowed, and orange light emission with a luminance of 40 cd / m ² was observed.
Prior to driving, a step of applying a large voltage (about 100 V) between the electrodes may be added. By adding this step, Cu can be agglomerated at the grain boundary, the conductivity of the grain boundary can be further increased, and the luminous efficiency is improved.

<Comparative Example 1>
As shown in FIG. 4, an alkali-free glass substrate 1 with ITO on the surface is prepared as a transparent electrode 5, and ZnS: Mn (0.5 wt%), Cu (1) is formed on the substrate 2 as a light emitting layer 2. 0.0 wt%) and Cl (1.0 wt%) were formed by EB vapor deposition at a substrate temperature of 200 ° C. to a thickness of 500 nm. Next, Al was formed as the electrode 3 by 100 nm room temperature by EB vapor deposition.

The produced inorganic electroluminescence element was subjected to direct current drive using an ITO electrode as an anode and an Al electrode as a cathode. When a voltage of 13 V was applied, a current of 20 mA / cm ² flowed, and orange light emission with a luminance of 0.1 cd / m ² was observed.

  In this way, an inorganic electroluminescence device having high luminance and easy to manufacture could be formed.

  As described above, according to the present invention, two electrodes are disposed apart from each other in the plane direction of the substrate, with the II-VI group inorganic compound in which an inorganic compound having high conductivity exists in the crystal grain boundary. Therefore, it is possible to obtain a highly efficient inorganic electroluminescent element that is driven at a low voltage, which is effective as a light source for various illuminations.

Sectional drawing which shows the inorganic electroluminescent element of Embodiment 1 of this invention Sectional drawing which shows the inorganic electroluminescent element of Embodiment 2 of this invention Sectional drawing which shows the inorganic electroluminescent light-emitting device of Embodiment 3 of this invention Sectional drawing which shows the inorganic electroluminescent element of the comparative example of this invention

Explanation of symbols

1 Substrate 2 Light emitting layer 3 Electrode 21 II-VI group compound 22 Grain boundary 4 Power supply

Claims (8)

An inorganic electroluminescent device comprising a light emitting layer containing a II-VI group compound formed by a vapor phase method between a pair of opposing electrodes,
The pair of electrodes,
The light emission layer extends from a position outside the side surface of the light emitting layer containing the II-VI group compound in a direction crossing the film forming direction of the light emitting layer, and covers a part of the upper surface or the lower surface of the light emitting layer. Arranged in a direction that intersects the film forming direction of the layers ,
The II-VI group compound includes a grain boundary,
The electrodes are arranged such that a current flowing between the electrodes intersects the crystal grain boundary,
The said crystal grain boundary is an inorganic electroluminescent element comprised with the inorganic compound with higher electroconductivity than another area | region . The inorganic electroluminescence device according to claim 1 ,
The inorganic electroluminescent device, wherein the highly conductive inorganic compound is a copper compound. An inorganic electroluminescent device according to claim 1 or 2 ,
The pair of electrodes is an inorganic electroluminescent element formed on a substrate surface with the II-VI group compound interposed therebetween, and the distance between the electrodes is 0.5 to 10 μm. An inorganic electroluminescence device according to any one of claims 1 to 3 ,
An inorganic electroluminescent device wherein the II-VI group compound is zinc selenide. An inorganic electroluminescence device according to any one of claims 1 to 3 ,
An inorganic electroluminescence device wherein the II-VI group compound is zinc sulfide. The inorganic electroluminescence device according to claim 5 ,
The zinc sulfide containing oxygen _{(O), ZnS x O 1} -x inorganic electroluminescent device is (0.1 <x <0.9). The inorganic electroluminescence device according to claim 6 ,
The zinc _sulfide, ZnS _x O _1-x inorganic electroluminescent device is (0.4 <x <0.6). An inorganic electroluminescence device according to any one of claims 3 to 7 ,
The pair of electrodes is an inorganic electroluminescence element arranged on the surface of the substrate so as to have a predetermined interval.

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