Display apparatus using oxide diode

Provided may be a display apparatus that uses oxide diodes having a nano rod structure, for example, nano-rod diodes formed of a ZnO group material. The display apparatus may include a substrate, a thin film transistor layer on the substrate, and a light emitting layer on the thin film transistor layer, wherein the light emitting layer may include a plug metal layer on the thin film transistor layer, a plurality of nano-rod diodes vertically formed on the plug metal layer, and a transparent electrode on the nano-rod diodes.

PRIORITY STATEMENT

This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2008-0060228, filed on Jun. 25, 2008, in the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference.

BACKGROUND

Example embodiments relate to a display apparatus that uses an oxide diode, and more particularly, to a display apparatus that uses an oxide diode having a nano-rod structure, e.g., a nano-rod structure formed of zinc oxide.

2. Description of the Related Art

Many different kinds of display apparatuses have been developed. One of the representative display apparatus may be a liquid crystal display (LCD). However, in the case of the LCD, in order to display an image on a screen, a backlight unit must be formed on a rear side of a liquid crystal panel, and be in a ‘turn ON’ state while the display is in operation. Accordingly, the backlight unit continuously consumes power. Thus, an LCD may consume a relatively large amount of power compared to other displays.

Also, in the case of an LCD, there may be a relatively large optical loss of light that may be emitted from the backlight unit in a process of passing through a polarizing plate, a liquid crystal layer, and a color filter. Thus, only a portion of light emitted from the backlight unit may be transmitted to the viewers. For this reason, an amount of light greater than light actually used may be required. In practice, in the case of LCD, light recognized by the viewer's eyes may be about 3% of the initial light emitted from the backlight unit, and the efficiency of light may be relatively low. Due to the relatively low optical efficiency, overall power consumption may be further increased.

As a next generation display apparatus, an organic light emitting diode (OLED) is being studied. Display apparatuses that use the OLED may have a wider viewing angle and a shorter response time. Also, the OLEDs may be formed to be thin, and thus, may be bended. Therefore, the OLEDs may be applied to flexible displays. However, an organic light emitting material used in the OLED may have a limited lifespan due to the characteristics of the organic material. An organic light emitting material that emits blue light having a lifetime of about 15,000 hours may have only a half of the lifetime (about 30,000 hours) required for a conventional flat panel TV.

Also, as yet, OLEDs may have a drawback of low light emission efficiency. In order to be realized as a display, the organic light emitting material must have a brightness of about 250 Cd/m2when emitting light. The organic light emitting material that emits blue light currently may have an efficiency of about 15 Cd/A. Thus, in order to obtain a desired brightness, a relatively large amount of power consumption may be required.

Furthermore, the OLED must be manufactured in a bottom emitting structure that may have a relatively large optical loss. In the case of the bottom emitting structure, a light emission area may be reduced due to a thin film transistor region formed under the light emission region for controlling a light emission operation of the organic light emitting material, and also, light must pass a thick glass substrate on a bottom of the OLED, and thus, an additional optical loss may be caused. In the case of a top emission type OLED, because light may be emitted through a transparent upper electrode formed right above the organic light emitting material, such optical loss may not be initiated. However, if a transparent electrode is formed on the organic light emitting material, the organic light emitting material may be damaged and may be more easily degraded in the course of forming the transparent electrode.

SUMMARY

To address the above and/or other problems, example embodiments provide a display apparatus that may have a longer lifespan and increased light emission efficiency by using an oxide diode having a nano-rod structure.

According to example embodiments, a display apparatus may include a substrate, a thin film transistor layer on the substrate, and a light emitting layer on the thin film transistor layer, wherein the light emitting layer may include a plug metal layer on the thin film transistor layer, a plurality of nano-rod diodes vertically formed on the plug metal layer, and a transparent electrode on the nano-rod diodes.

Each of the nano-rod diodes may include a lower layer portion doped with a first type dopant, an upper layer portion doped with a second type dopant which may be opposite-to the first type, and a non-doped region between the lower layer portion and the upper layer portion.

The lower layer portion of the nano-rod diode may be formed of an n-type ZnO group material, the upper layer portion of the nano-rod diode may be formed of a p-type ZnO group material, and the non-doped region of the nano-rod diode may be formed of a non-doped ZnO group material.

The display apparatus may further include a reflection layer for reflecting light emitted from the nano-rod diodes on an upper surface of the plug metal layer between the nano-rod diodes. The reflection layer may be formed to a single metal layer. The reflection layer may be formed of Al or an alloy containing Al. The reflection layer may have a multilayer structure in which two materials having different refractive indexes from each other are alternately stacked.

The reflection layer may have a multilayer structure in which Al and MgF2are alternately stacked. The display apparatus may further include an insulating layer on an upper surface of the reflection layer between the nano-rod diodes. The nano-rod diodes may emit ultraviolet (UV) ray region light. The display apparatus may further include a transparent electrode on the insulating layer, and a phosphor layer on the transparent electrode to transform the UV ray region light emitted from the nano-rod diodes into visible light region light.

The display apparatus may further include a transparent substrate on an upper surface of the phosphor layer. The display apparatus may include a plurality of pixels for displaying an image, and each of the pixels may include three sub-pixels, wherein each of the sub-pixels may include a single thin film transistor layer and a single light emitting layer.

The phosphor layers in the three sub-pixels may be a red phosphor layer that transforms the UV ray region light into red color, a green phosphor layer that transforms the UV ray region light into green color, and a blue phosphor layer that transforms the UV ray region light into blue color. The plug metal layer may be formed of Au or an alloy containing Au.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1is a schematic cross-sectional view of a display apparatus10that uses an oxide diode according to example embodiments. Referring toFIG. 1, the display apparatus10may include a thin film transistor layer20formed on a substrate11, e.g., glass, and a light emitting layer30having a plurality of diodes34having a nano-rod structure.

The thin film transistor layer20may be formed to control the emission of light from the light emitting layer30, for example, the nano-rod diodes34, and may be a conventional thin film transistor layer used in LCDs or OLEDs. For example, as depicted inFIG. 1, the thin film transistor layer20may include a silicon oxide film21formed on the substrate11, a semiconductor layer23partly formed on the silicon oxide film21, a gate insulating layer22covering the semiconductor layer23and the silicon oxide film21, a gate electrode24formed on the gate insulating layer22to face the semiconductor layer23, a first insulating layer25covering the gate electrode24and the gate insulating layer22, a source electrode27and a drain electrode28formed on the first insulating layer25, and a second insulating layer26formed on the first insulating layer25to cover the source electrode27and the drain electrode28.

The silicon oxide film21may block various impurities that may penetrate into the semiconductor layer23and the gate insulating layer22from the glass substrate11during a manufacturing process. The semiconductor layer23may be formed of polysilicon, amorphous silicon, or various types of oxide semiconductors. The source electrode27and the drain electrode28, which act as bit lines, may be connected to both ends of an upper surface of the semiconductor layer23through a first via hole27aand a second via hole28a. In the above structure, when current is applied to the gate electrode24, a current supplied to the source electrode27may flow into the drain electrode28through the semiconductor layer23, and may be transmitted to the nano-rod diodes34through a plug metal layer31, which will be described later.

The structure of the light emitting layer30formed on the thin film transistor layer20will now be described. As depicted inFIG. 1, the light emitting layer30may include the plug metal layer31formed on the thin film transistor layer20, e.g., on the second insulating layer26, a plurality of nano-rod diodes34vertically grown on the plug metal layer31, a transparent electrode35formed on the nano-rod diodes34, a phosphor layer36formed on the transparent electrode35, and a transparent substrate37formed on the phosphor layer36.

Each of the nano-rod diodes34may include a lower layer portion34adoped with an n-type dopant, an upper layer portion34cdoped with a p-type dopant, and a non-doped region34bbetween the lower layer portion34aand the upper layer portion34c. For example, the nano-rod diodes34may be a p-n diode that uses an oxide diode, e.g., a zinc oxide group material. The zinc oxide group material may be, for example, ZnO or MgZnO. In example embodiments, the lower layer portion34aof the nano-rod diode34may be formed of an n-type zinc oxide group material (n-ZnO or n-MgZnO), the upper layer portion34cof the nano-rod diode34may be formed of a p-type zinc oxide material group ((p-ZnO or p-MgZnO), and the non-doped region34bof the nano-rod diode34may be formed of an non-doped zinc oxide group material (i-ZnO or i-MgZnO). In the above structure, light emission may generally occur from the non-doped zinc oxide group material (i-ZnO or i-MgZnO). If the non-doped zinc oxide group material is formed in a nano-rod shape, crystallinity of the material may be improved, and thus, the defect concentration that affects the optical emission efficiency may be reduced. Thus, the zinc oxide diode having a nano-rod structure may increase the efficiency of the display apparatus.

For example, because ZnO has a larger band gap energy of about 3.4 eV, when ZnO is used to formed the nano-rod diode34, ultraviolet (UV) ray region light having a wavelength of about 410 nm or less may be emitted. The display apparatus10according to example embodiments may require the phosphor layer36that transforms the UV ray region light emitted from the nano-rod diode34to a visible light region. Visible light having a desired color may be obtained according to the type of phosphor layer36. For example, if the display apparatus10according to example embodiments includes red, green, and blue sub-pixels, appropriate phosphor materials for emitting red, green, and blue light may be used as the phosphor layer36in each of the sub-pixels.

This configuration may have an advantage in that if different diodes that emit different color are used in each of the sub-pixels, the process for manufacturing the overall light emitting layers may be relatively complicated. However, in example embodiments, the nano-rod diodes34in the light emitting layer30may be simultaneously formed, and only the phosphor layer36in the sub-pixels may be separately printed. Thus, according to example embodiments, the process of manufacturing the light emitting layer30may be simplified. However, other materials that emit visible light may also be used to form the nano-rod diodes34instead of ZnO group materials.

The plug metal layer31may not only act as a growing catalyst substrate for promoting selective growth of the nano-rod diodes34, but also may act as a lower electrode that transmits signals from the thin film transistor layer20to the nano-rod diodes34. Thus, the plug metal layer31may be formed of a material that has increased electrical conductivity and may selectively grow the nano-rod diodes34. For example, when a ZnO group material is used to form the nano-rod diodes34, the plug metal layer31may be formed of Au or an alloy that contains Au. Because the plug metal layer31acts as a lower electrode, the plug metal layer31may be connected to the thin film transistor layer20, for example, the drain electrode28, through a third via hole31a that passes through the second insulating layer26. According to example embodiments, the plug metal layer31may be formed on the entire region of the thin film transistor layer20to maximize or increase a top-emission characteristic. However, in some cases, due to various process reasons, the plug metal layer31may be patterned not to overlap the transistor under the plug metal layer31.

The transparent electrode35may act as an upper electrode of the nano-rod diodes34, and may be formed of a material having increased electrical conductivity and a higher visible light transmittance. For example, the transparent electrode35may be formed of indium tin oxide (ITO), aluminium zinc oxide (AZO), or indium zinc oxide (IZO). These types of transparent electrodes may be annealed at a temperature of, conventionally, about 300° C. or above in order to increase the electrical conductivity. As described above, in the case of an OLED, the characteristic of an organic light emitting material formed under the transparent electrode may be degraded in a process of annealing the transparent electrode. Due to the above drawback, current OLEDs may adopt a bottom emission type. However, in example embodiments, because the diodes are formed using an inorganic material that does not change its characteristics at a higher temperature, e.g., a metal oxide (for example, a ZnO group material), the above transparent electrode35may be used as an upper electrode. Thus, example embodiments may employ a top-emission type which has a relatively wide light emission area.

In order to further increase the top-emission characteristic, a reflection layer32may further be formed on an upper surface of the plug metal layer31between the nano-rod diodes34. The reflection layer32may reflect light emitted from the nano-rod diodes34towards the transparent electrode35. For example, if the nano-rod diodes34are formed of a ZnO group material, the reflection layer32may be formed to have increased reflectivity with respect to UV ray region light. The reflection layer32may be formed in various ways. A simple method may be that the reflection layer32may be formed as a single metal layer. In example embodiments, the reflection layer32may be formed of, for example, Al or an alloy containing Al. In order to further increase the reflectivity of the reflection layer32, a multi-layer structure in which two materials having different refractive indexes from each other are alternately stacked may be adopted. For example, the reflection layer32may have a multi-layer structure in which Al and MgF2may be alternately stacked.

Also, a third insulating layer33may further be formed on an upper surface of the reflection layer32between the nano-rod diodes34to prevent or reduce physical and chemical impacts on the nano-rod diodes34and the reflection layer32and to prevent or reduce an electrical short between the nano-rod diodes34. The third insulating layer33may have transparency with respect to light, and may be a silicon oxide film (SiO2).

FIGS. 2A-2Eare cross-sectional views illustrating a method of forming the light emitting layer30according to example embodiments on the thin film transistor layer20. Referring toFIG. 2A, after depositing the plug metal layer31on the thin film transistor layer20, the reflection layer32may be formed on the plug metal layer31. InFIG. 2A, the reflection layer32, in which an MgF2layer32aand an Al32bare alternately stacked, may be depicted as an example. Afterwards, holes40, where the nano-rod diodes34will be positioned, may be formed in the reflection layer32by etching the reflection layer32. The holes40may be formed to expose the plug metal layer31under the reflection layer32.

Afterwards, as depicted inFIG. 2B, the nano-rod diodes34may be grown in the holes40. As described above, the plug metal layer31may be formed of a catalyst metal that may selectively grow the nano-rod diodes34according to the material used to form the nano-rod diodes34. For example, if the nano-rod diodes34are formed of a ZnO group material, the plug metal layer31may be formed of Au or an alloy containing Au. Thus, the growth of the nano-rod diodes34may be limited in the hole40through which the plug metal layer31is exposed. Various conventional methods of growing the nano-rod diodes34using the ZnO group material may have been reported in the art. According to the conventional method of growing the nano-rod diodes, after growing n-ZnO (or n-MgZnO)34a, i-ZnO (i-MgZnO)34band p-ZnO (or p-MgZnO)34cmay be grown. Because only the reflection layer32is formed around the ZnO nano-rod diodes34, UV ray region light generated during operation of the OLED after the manufacturing is completed may be reflected upwards by the reflection layer32.

Referring toFIG. 2C, the third insulating layer33may cover the nano-rod diodes34by forming the third insulating layer33on the reflection layer32. As described above, the third insulating layer33may be a transparent silicon oxide (SiO2) film. The third insulating layer33may completely cover the nano-rod diodes34. Accordingly, as depicted inFIG. 2D, upper surfaces of the nano-rod diodes34may be exposed by planarizing the third insulating layer33using a CMP process or an overall etching process.

Referring toFIG. 2E, the transparent electrode35as an upper electrode may be deposited on the third insulating layer33and the nano-rod diodes34, and the transparent electrode35may be patterned. Afterwards, the transparent electrode35may be annealed using a conventional annealing method in order to increase the electrical conductivity of the transparent electrode35. The phosphor layer36may be printed on the transparent electrode35, and a transparent substrate may further be formed on the phosphor layer36. Alternatively, the transparent substrate37, on which the phosphor layer36is printed in advance, may be disposed on the transparent electrode35.

FIG. 3is a schematic cross-sectional view of an overall structure for realizing a color image in a display apparatus10that uses an oxide diode according to example embodiments. Referring toFIG. 3, display apparatus10may have a plurality of pixels12for realizing an image. Each of the pixels12may be formed on the substrate11, and may include, for example, a red sub-pixel12R that emits red color, a green sub-pixel12G that emits green color, and a blue sub-pixel12B that emits blue color. Also, each of the sub-pixels may include one thin film transistor (seeFIG. 1) and one light emitting layer (seeFIG. 1) described above. If the nano-rod diodes34in the light emitting layer30emits UV ray region light like the ZnO group material, the red sub-pixel12R may include a red phosphor layer that transforms the UV ray region light to red light. In the same manner, the green sub-pixel12G may include a green phosphor layer that transforms the UV ray region light to green light, and the blue sub-pixel12B may include a blue phosphor layer that transforms the UV ray region light to blue light. A transparent substrate37may be formed on the plurality of pixels12. When the method described above is adopted, the structures of the sub-pixels may be the same or different from each other in the light emitting layer30, and the method of manufacturing the display apparatus10according to example embodiments may be simplified.

While example embodiments have been particularly shown and described with reference to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of example embodiments as defined by the appended claims. Example embodiments should be considered in descriptive sense only and not for purposes of limitation. Therefore, the scope of example embodiments may be defined not by the detailed description of example embodiments but by the appended claims, and all differences within the scope will be construed as being included in example embodiments.