Method of forming ohmic contact layer and method of fabricating light emitting device having ohmic contact layer

A method of manufacturing an ohmic contact layer and a method of manufacturing a top emission type nitride-based light emitting device having the ohmic contact layer are provided. The method of manufacturing an ohmic contact layer includes: forming a first conductive material layer on a semiconductor layer; forming a mask layer having a plurality of nano-sized islands on the first conductive material layer; forming a second conductive material layer on the first conductive material layer and the mask layer; and removing the portion of the second conductive material on the islands and the islands through a lift-off process using a solvent. The method ensures the maintenance of good electrical characteristics and an increase of the light extraction efficiency.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2005-0041771, filed on May 18, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The present disclosure relates to an ohmic contact layer and a light emitting device having the same, and more particularly, to an ohmic contact layer which can enhance the light extraction efficiency of a top emission type light emitting device and a light emitting device having the same.

2. Description of the Related Art

The effective supply of current to a semiconductor light emitting device is significantly influenced by an ohmic contact between a semiconductor layer and an electrode. A light emitting device made of a nitride semiconductor, for example, a gallium nitride semiconductor, needs a particularly good ohmic contact. Ohmic contacts of nitride semiconductor light emitting devices are still unsatisfactory in spite of continuous research thereon because of low carrier concentration, high sheet resistance, and low electric conductivity of the p-type gallium nitride. In addition, since internally generated light is emitted through an ohmic electrode layer in a top light emission type light emitting device, the ohmic contact layer must have good electrical characteristics and high light transmittance.

A conventional ohmic contact layer of a top light emission type light emitting device includes a nickel (Ni) layer and a gold (Au) layer sequentially stacked on a second cladding layer. The ohmic contact layer formed of the stacked metal layers has a low specific contact resistance of about 10−3to 10−4Ωcm2after being heat-treated under an oxygen or air atmosphere. However, such a conventional Ni/Au ohmic contact layer has low light extraction efficiency due to its low light transmittance, i.e., about 75% at λ=450 nm. Accordingly, although the ohmic contact layer has low specific contact resistance, the ohmic contact layer cannot be employed in a next generation top light emission type light emitting device having high-output power and high luminance because of its low light transmittance.

To overcome the output power limitations of top emission type light emitting devices, the use of a transparent conductive oxide, for example, ITO, having excellent light transmittance has been suggested [T. Margalith et al., Appl. Phys. Lett. Vol. 74, p 3930 (1999).]

Y. C. Lin et al. discloses a Ni/ITO ohmic contact layer having an 86.6% greater transmittance than a conventional Ni/Au ohmic contact layer used to form a top emission type device having 1.3 times the light output power [Y. C. Lin et al., Solid-State Electronics Vol. 47, p 849 (2003)].

Recently, an electrode surface texturing method has been suggested to maximize the light extraction efficiency of devices. In the electrode surface texturing method, a NiO/ITO ohmic contact layer, for example, is formed on GaN, and then, the ITO electrode is patterned using a plasma etching photolithography method to form holes having diameters of several micrometers in the ITO electrode, thereby increasing the light output power by about 16% [by S.-M. Pan et al., in IEEE Photon. Technol. Left. Vol. 15 p 649 (2003)]. However, since the holes formed in the surface of the electrodes using the electrode surface texturing method have diameters of several micrometers, the light extraction efficiency thereof cannot be maximized. In particular, the device performance may be degraded due to plasma damage to the device during etching.

SUMMARY OF THE DISCLOSURE

The present invention may provide a method of manufacturing an ohmic contact structure having high light transmittance and good electrical characteristics and a method of manufacturing a top light emission type light emitting device having the ohmic contact structure.

According to an aspect of the present invention, there is provided a method of manufacturing an ohmic contact layer which is used for a light emitting device and includes a first conductive material layer and a second conductive material layer having through-holes corresponding to islands, the method including: forming a first conductive material layer on a semiconductor layer; forming a mask layer including a plurality of nano-sized islands on the first conductive material layer; forming a second conductive material layer on the first conductive material layer and the islands in the mask layer; and removing the portion of the second conductive material on the islands and the islands through a lift-off process using a solvent.

According to another aspect of the present invention, there is provided a method of manufacturing an ohmic contact layer which is used for a light emitting device and includes a first conductive material layer and a second conductive material layer having through-holes corresponding to islands, the method including: forming a stacked structure including a first cladding semiconductor layer, an activation layer, and a second cladding semiconductor layer on a substrate; forming a first conductive material layer on the second cladding semiconductor layer; forming a mask layer including a plurality of islands on the first conductive material layer; forming a second conductive material layer on the first conductive material layer and the islands in the mask layer; and removing the portion of the second conductive material on the islands and the islands through a lift-off process using a solvent.

The first and second conductive material layers may be formed of a transparent conductive oxide or transparent conductive nitride.

The transparent conductive oxide may include at least one metal oxide and the metal may be selected from the group consisting of In, Sn, Zn, Ga, Ce, Cd, Mg, Be, Ag, Mo, V, Cu, Ir, Rh, Ru, W, Co, Ni, Mn, Al, and lanthanide metals.

The transparent conductive nitride may be titanium nitride or tantalum nitride.

The method may further include, before the forming of the first conductive material layer, forming a metal thin film layer on the semiconductor layer, wherein the first conductive material layer is formed on the metal thin film layer. The metal thin film layer may have a thickness of approximately 0.1 to 50 nm.

The metal thin film layer may be formed of a material selected from the group consisting of Ni, Co, Cu, Pd, Pt, Ru, Ir, Au, Ag, Cr, Rh, In, Sn, Mg, Zn, Be, Sr, Ba, an alloy thereof, and a solid solution thereof.

The forming of the mask layer may further include: coating a photoresist on the first conductive material layer; exposing the photoresist to form a latent image corresponding to the islands in the photoresist; and forming the islands on the first conductive material layer by developing the photoresist using an solvent.

The exposing of the photoresist may be performed through laser holography using laser interference or electron beam lithography.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.

In an embodiment of the present invention, a contact layer that is not damaged by the impact of, for example, ions, and has a 3-dimensional pattern is formed. The contact layer includes at least a first conductive material layer and a second conductive material layer formed on the first conductive material layer. The second conductive material layer has a plurality of hole arrays. The holes may be circular or rectangular.

The second conductive material layer having the plurality of hole arrays is formed using a lift-off method, and a photoresist used as a sacrificial layer in the lift-off method is exposed by scanning electron beams or a laser hologram which can perform fine patterning. In addition, the sacrificial layer can be patterned using a well known imprint method. The method of patterning the sacrificial layer can be one of various methods, but does not limit the present invention. The laser holography is one of the best methods for exposure based on experimental results.

A method of forming an ohmic contact layer can be understood through the description of a method of manufacturing a nitride-based light emitting device according to embodiments of the present invention provided herein.

FIG. 1is a cross-sectional view of a top emission type nitride-based light emitting device including electrodes manufactured according to an embodiment of the present invention.

Referring toFIG. 1, the nitride-based light emitting device includes a substrate10, a buffer layer20disposed on the substrate10, a n-type lower semiconductor layer, for example, a first cladding layer30, disposed on the buffer layer20, an activation layer40disposed on the first cladding first cladding layer30, an p-type upper semiconductor layer, for example, a second cladding second cladding layer50, disposed on the activation layer40, and a p-ohmic contact layer60which is formed of a transparent conductive thin film, has a 3-dimensional pattern, and is disposed on the second cladding layer50. A first conductive pad90is electrically connected to the first cladding layer30, and a second conductive pad80is connected to the p-ohmic contact layer60.

The substrate10may be formed of sapphire (Al2O3), silicon carbide (SiC), silicon (Si), or gallium arsenide (GaAs). The buffer layer20is optional, but is required when the substrate10is formed of an insulating material.

Each layer from the buffer layer20to the second cladding layer50is formed of a III group nitride-based compound represented by AlxInyGazN (0≦x≦1, 0≦y≦1, 0≦z≦1, and 0≦x+y+z≦1), and the first cladding layer30and the second cladding layer50further include dopants.

In addition, the activation layer40has a single quantum wall (SQW) structure or a multiple quantum wall (MQW) structure.

As an example, in a gallium nitride (GaN) compound semiconductor, the buffer layer20is formed of GaN, the first cladding layer30is formed of GaN with n-type dopants, for example, Si, Ge, Se, or Te, the activation layer40includes an InGaN/GaN MQW or AlGaN/GaN MQW, and the second cladding layer50is formed of GaN with p-type dopants, for example, Mg, Zn, Ca, Sr, or Be. Upper and lower guide layers may be formed on and below the activation layer40.

An n-type ohmic contact layer (not illustrated) may be disposed between the first cladding layer30and the first conductive pad90and the n-type ohmic contact layer may have various commonly known structures, for example, a sequentially stacked structure of Ti and Al.

The second conductive pad80may have a sequentially stacked structure of Ni and Au or a stacked structure of W/Au, Pt/Au, Pd/Au, or Ag/Au.

Each of the layers can be formed using a commonly known deposition method such as electron-beam evaporation, physical vapor deposition (PVD), chemical vapor deposition (CVD), plasma laser deposition (PLD), dual-type thermal evaporator, or sputtering.

The p-ohmic contact layer60having a 3-dimensional texture shaped pattern, which characterizes an embodiment of the present invention, includes an at least two layer stacked structure including a first conductive material layer and a second conductive material layer on the first conductive material layer, and the second conductive material layer is patterned by lift-off using a sacrificial layer. In the lift-off, the semiconductor layer is not damaged by the impact of ion, which occurs in conventional dry etching. The second conductive material layer has a plurality of hole arrays. The holes may be circular or rectangular. A photoresist used as the sacrificial layer is exposed by the scanning electron beams or the laser hologram which can perform fine patterning. In addition, the sacrificial layer can be patterned using a well-known imprint method. The photoresist can be exposed to have a predetermined pattern using the laser hologram, which uses interference characteristics of light.

The ohmic contact layer60may have a thickness of approximately 20 to 1000 nm, and may be formed of a transparent conductive oxide or a transparent conductive nitride.

In addition, the transparent conductive nitride can include at least one of titanium nitride and tantalum nitride.

The electrical characteristics of the ohmic contact layer60can be improved by doping dopants into the transparent conductive oxide or the transparent conductive nitride. The dopants can be metal to a concentration of approximately 0.001 to 20 wt %.

In a process of forming the ohmic contact layer60, the deposition of the transparent conductive oxide or the transparent conductive nitride is performed at, for example, 20 to 1000° C. under vacuum or in a gaseous atmosphere.

The ohmic contact layer60formed by depositing the transparent conductive oxide or the transparent conductive nitride is heat-treated in a temperature ranging from room temperature to approximately 900° C. for approximately 10 seconds to 3 hours. The heat-treatment is performed in an atmosphere having at least one gas selected from nitrogen, argon, helium, oxygen, hydrogen, and air, or under a vacuum.

FIG. 2is a cross-sectional view of a top emission type nitride-based light emitting device manufactured according to another second embodiment of the present invention.

Referring toFIG. 2, the light emitting device includes a metal thin film layer70interposed between the ohmic contact layer60and the second cladding semiconductor layer50to improve ohmic contact characteristics between the ohmic contact layer60and the second cladding semiconductor layer50.

The metal thin film layer70may have a thickness of approximately 0.1 to 50 nm. The metal thin film layer70may have high conductivity, have a high work function, and be made of a material which easily forms a gallium (Ga)-based compound. For example, the metal thin film layer70may be composed of a material selected from the group consisting of Ni, Co, Cu, Pd, Pt, Ru, Ir, Au, Ag, Cr, Rh, In, Sn, Mg, Zn, Be, Sr, Ba, and an alloy or solid solution thereof.

Since the metal thin film layer70has high conductivity, when contacting the upper patterned transparent conductive thin film layer such as the ohmic contact layer60, the metal thin film layer70improves current distribution, thereby decreasing the specific contact resistance in the p-type ohmic electrode structure.

FIGS. 3A through 3Fillustrate a method of forming the ohmic contact layer60(the conductive thin film layer) on the second cladding semiconductor material layer such as the second cladding layer50.

The above-described regular stacked structure of a light emitting device including the buffer layer20, the first cladding semiconductor layer such as the first cladding layer30, the activation layer40, and the second cladding layer50is formed on the substrate using typical processes before forming the ohmic contact layer60.

Referring toFIG. 3A, a first transparent conductive material layer100having a thickness of approximately 0.1 to 300 nm is formed on the second cladding layer50.

Referring toFIG. 3B, a photoresist is coated to a thickness ranging from approximately 10 nm to 2 μm on the first conductive material layer100using, for example, a spin coater to form a sacrificial layer110.

Referring toFIG. 3C, the photoresist is pre-baked, and then exposed. The exposure is performed using electron beam exposure or laser hologram exposure, as described above. After the exposure, a latent image having exposed regions and unexposed regions is formed.

In an embodiment of the present invention, the laser hologram exposure is used. A laser holography apparatus is used for the laser hologram exposure. In the laser hologram exposure, an interference pattern corresponding to a desired pattern is irradiated onto the portion of the sacrificial layer110which will not be unexposed. The laser is radiated for several seconds to several minutes to perform a first exposure on the sacrificial layer110, the structure is properly rotated 0 to 90°, and then a second exposure is performed.

Referring toFIG. 3D, the sacrificial layer110is developed using an etching developer to obtain a nano-sized island array110awhich is 2-dimensionally arranged with a predetermined size and period. After forming the island array110a, the upper surface of the first conductive material layer100is divided into regions covered by the islands and regions not covered by the islands. The size and period of the island array110acan be appropriately controlled. When using the laser hologram, the size and period of the island array110acan be controlled by changing the wavelength or the incident angle of the laser.

Referring toFIG. 3E, a second conductive material layer120having a thickness of approximately 20 to 700 nm is formed on the island array110ausing a commonly known deposition method, for example, electron beam deposition, sputtering, or PLD. The second conductive material layer120is formed of the above-described transparent conductive oxide or transparent conductive nitride.

Immediately after being deposited, the second conductive material layer120is divided into nano-sized portions which are formed on the island array110aof the sacrificial layer110and will be removed, and portions which are formed on the first conductive material layer110will form the ohmic contact layer60with the first conductive material110.

Referring toFIG. 3F, the sacrificial layer110is removed through a lift-off process using a solvent to partially remove the portion of the second conductive material layer120disposed on the island array110a, thereby forming a nano-sized second conductive material layer120in which holes having a well shape are arranged in an array. Thus, the ohmic contact layer60is formed on the second cladding semiconductor material layer50of the light emitting device. When the lift-off is performed, an ultrasonic cleaning process or an ultrasonic spraying process may be employed to effectively remove the photoresist.

FIG. 4is a schematic diagram of a laser holography apparatus used for forming an ohmic contact according to an embodiment of the present invention.

Referring toFIG. 4, a laser generated by a He—Cd gas laser apparatus is transmitted to a beam expander via a laser transmitting pathway having an electric shutter and a plurality of mirrors. The laser is diverged by the beam expander and collimated by a collimating lens. A reflector and a supporter having a predetermined inclined angle, for example, 45°, with respect to the laser transmitting axis, and a rotational stage supporting the reflector and the supporter are disposed after the collimating lens along the laser path. The laser passes through the collimating lens and is radiated onto the reflector and a wafer. The laser radiated onto the reflector is reflected by the reflector, and then radiated to the wafer. Accordingly, the wafer is irradiated with a first laser directly radiated from the collimating lens and a second laser indirectly radiated through the reflector such that an interference pattern caused by the first laser and the second laser is formed on the wafer. A grating pitch (Λ) for forming the interference pattern is determined by the angle (θ) between the first laser and the second laser radiated onto the wafer as follows:
Λ=λ/(2 sin θ)  (1)

where λ is a wavelength of the laser.

The above-described latent image can be formed on the photoresist, i.e., the sacrificial layer, formed on the surface of the wafer using the laser holography apparatus.

FIGS. 5A through 5Care scanning electron microscopy (SEM) images showing a light emitting device manufactured using a method according to an embodiment of the present invention.

Referring toFIG. 5A, the sacrificial layer110having the island array110ais formed by depositing ITO to a thickness of 30 nm as the first conductive material layer100on the second cladding layer50, exposing a photoresist layer for forming the sacrificial layer110, the photoresist layer formed on the first conductive material layer using a 363 nm argon ion (Ar+) laser hologram, and developing the sacrificial layer110using a subsequent develop process.

Each of the islands110ain the sacrificial layer110has a diameter of 200 nm, and a height of 600 nm, and a pitch of 750 nm exists between the islands110a.

The effect of the periodic island pattern is strong when the grating pitch (A) is at a multiple of m*wavelength/4*n, where m is an integer and n is a refractive index of the island.

As m increases, the resonance effect decreases. Accordingly, considering a wavelength of 400 to 500 nm and a refractive index of 2.5 which are currently used ranges, the appropriate range of the grating pitch (Λ) is from at least 40 nm at m=1 to at most 1000 nm (1 μm) at m=20. When m is greater than 20, the effect is insignificant.

FIG. 5Bis an SEM image taken after ITO has been deposited to a thickness of 170 nm as the second conductive material layer120on the structure shown inFIG. 5A.FIG. 5Cis an SEM image showing the second conductive material layer120after removing the sacrificial layer ofFIG. 5B, i.e., after a lift-off process has been performed. Referring toFIG. 5C, holes which have a well shape are formed with a periodic arrangement on the second conductive material layer120through the lift-off process using the sacrificial layer.

FIG. 6is an SEM image showing an upper ohmic contact layer of a blue light emitting device manufactured using a method according to an embodiment of the present invention. Referring toFIG. 6, the ohmic contact layer60formed of the ITO transparent conductive material is formed on the upper portion of the second cladding semiconductor layer, for example, the second cladding layer50, in a blue InGaN/GaN MQW light emitting device. InFIG. 6, the right portion is the ohmic contact layer60and the left dark portion is an n-type electrode.

FIG. 7is graph of electro-luminescence (EL) intensity versus wavelength at a current of 20 mA for a light emitting device manufactured using a method according to an embodiment of the present invention. The light emitting device had a structure in which the first ITO conductive material layer100having a thickness of 30 nm was formed on the second cladding layer50in the blue InGaN/GaN MQW light emitting device shown inFIG. 6and then the second ITO conductive material layer120having a thickness of 170 nm was formed on the first conductive material layer100using the above-described hologram lithography.

In addition, as a reference sample, light emitting devices respectively having an unpatterned ITO electrode structure with a thickness of 200 nm and a conventional Ni/Au ohmic electrode structure were manufactured. In particular, to improve the ohmic contact characteristics between the second cladding layer and the ITO transparent conductive thin film layer, a metal thin film layer70which was made of Ag and had a thickness of 1 nm was formed therebetween, as illustrated inFIG. 2. After forming an electrode structure in the device, heat-treatment was performed at 500° C. for 2 minutes under an air atmosphere. Referring toFIG. 7, the transparent conductive ohmic electrode structure which is 2 dimensionally patterned according to an embodiment of the present invention shows 21% greater EL intensity than the unpatterned sample and, 54% greater EL intensity than the conventional Ni/Au electrode structure. In particular, the 2-dimensional electrode patterning according to embodiments of the present invention has an advantage in that an increase in the specific contact resistance between the second cladding layer and the ohmic electrode structure is prevented by the presence of the thin first conductive material layer and the 2-dimensional transparent conductive thin film layer having the nano-sized holes.

FIG. 8is a graph of light output power versus input current for a light emitting device with a 2-dimensionally patterned ohmic contact layer and a light emitting device without a 2-dimensionally patterned ohmic contact layer. Referring toFIG. 8, the light emitting device having the 2-dimensionally patterned ohmic contact layer has superior light output power to the light emitting device having none for all input current ranging from 20 to 100 mA.

FIGS. 9A and 9Bare schematic diagrams that illustrate light refraction and reflection in a structure having a second cladding layer, an ITO layer and, air layer with respect to electrode patterning formation, according to an embodiment of the present invention.FIGS. 9A and 9Billustrate a principal by which light output power of an ohmic contact layer which is 2-dimensionally patterned, that is, has a 3-dimensional pattern, is improved according to an embodiment of the present invention. The second cladding layer, the ITO transparent conductive thin film layer, and the air layer respectively have refractive indexes of 2.45, 2.06, and 1.0 at a wavelength of 450 nm. When light generated by a medium having a refractive index n1enters another medium having a refractive index n2, a critical angle is equal to sin−1(n2/n1) based on Snell's law. When the incident angle of light is greater than the critical angle, the light is reflected, and when the incident angle of light is less than the critical angle, the light is refracted. For example, when light generated in the second cladding layer is incident on the ITO layer, the critical angle of the light is 57°. Referring toFIG. 9A, in the case where the ITO layer is unpatterned, when light generated from the second cladding layer has an incident angle less than the critical angle of 57°, the light is refracted into the ITO layer. However, light having an incident angle greater than the critical angle of 57° is reflected. Meanwhile, another critical angle of 29° exists between the ITO layer and air due to the difference between the refractive indexes of the ITO layer and the air layer, and thus, the light transmitted through the ITO layer is mostly reflected back into the ITO layer, thereby decreasing the light extraction efficiency. However, referring toFIG. 9B, if the surface of the ohmic contact layer is 2-dimensionally patterned according to the embodiment of the present invention, the more light which has refracted into the ITO layer is transmitted to the outside along the sides of the electrode pattern, thereby decreasing the ratio of light reflected back to the inside, and increasing the light extraction efficiency of the light emitting device.

As described above, in the present invention, dry etching, which may damage semiconductor materials, is not performed such that degradation in the electrical characteristics of the device caused by damage due to dry etching can be prevented. In particular, the method of the present invention can ensure that a light emitting device having excellent electric and optical characteristics can be manufactured.