Nitride semiconductor light emitting device and manufacturing method of the same

There is provided a nitride semiconductor light emitting device including: a light emitting structure including n-type and p-type nitride semiconductor layers and an active layer disposed therebetween; n- and p-electrodes electrically connected to the n-type and p-type nitride semiconductor layers, respectively; and an n-type ohmic contact layer disposed between the n-type nitride semiconductor layer and the n-electrode and including a first layer and a second layer, the first layer formed of an In-containing material, and the second layer disposed on the first layer and formed of a transparent conductive oxide. The nitride semiconductor light emitting device including the n-electrode exhibits high light transmittance and superior electrical characteristics. Further, the nitride semiconductor light emitting device can be manufactured by an optimal method to ensure superb optical and electrical characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Korean Patent Application No. 2007-0134903 filed on Dec. 21, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride light emitting device and a manufacturing method of the same, and more particularly, to a nitride light emitting device including an n-electrode, which exhibits high light transmittance and superb electrical characteristics, and a manufacturing method of the same.

2. Description of the Related Art

A light emitting diode (LED), as one type of a semiconductor light emitting device, generates light of various colors since electrons and holes are recombined at a junction of p-type and n-type semiconductors when a current is supplied. This LED is greatly advantageous over a filament-based light emitting device. That is, the LED has longer useful life, lower voltage, superior initial driving characteristics, high vibration resistance and high tolerance to repetitive power connection/disconnection. This has continually boosted demand for the LED. Notably, of late, a group III nitride semiconductor capable of emitting light at a short wavelength such as blue light has been highlighted.

FIG. 1is a cross-sectional view illustrating a conventional nitride semiconductor light emitting device.

The nitride semiconductor device10includes a conductive substrate14, and an ohmic contact layer17, a p-type nitride semiconductor layer13, an active layer12and an n-type nitride semiconductor layer11sequentially formed on the conductive substrate14. Also, the nitride semiconductor device10includes an n-electrode16formed on a top of the n-type nitride semiconductor layer11.

The nitride semiconductor light emitting device10ofFIG. 1is a vertical light emitting device, and accordingly has electrons and holes recombined in the active layer12to emit light outward mainly through the n-type nitride semiconductor layer11.

Here, the holes are injected uniformly to some extent due to the conductive substrate14serving as a p-electrode. However, the n-electrode16is locally positioned on the top of the n-type nitride semiconductor layer11, thus preventing the injected electrons from being diffused uniformly through the n-type nitride semiconductor layer11. Therefore, current is crowded below the n-electrode16. Here, light generated from the active layer12is considerably absorbed by the n-electrode16. This undermines light emitting characteristics, and decreases an effective area for current flow to thereby degrade electrical characteristics.

Therefore, there has been a demand in the art for a method of manufacturing the LED in which current is distributed uniformly and light generated can be extracted easily.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a nitride semiconductor device including an n-electrode, which exhibits high light transmittance and superior electrical characteristics, and a manufacturing method of the same.

According to an aspect of the present invention, there is provided a nitride semiconductor light emitting device including: a light emitting structure including n-type and p-type nitride semiconductor layers and an active layer disposed therebetween; n- and p-electrodes electrically connected to the n-type and p-type nitride semiconductor layers, respectively; and an n-type ohmic contact layer disposed between the n-type nitride semiconductor layer and the n-electrode and including a first layer and a second layer, the first layer formed of an In-containing material, and the second layer disposed on the first layer and formed of a transparent conductive oxide.

The first layer may be formed of an In alloy.

The In alloy may include at least one element selected from a group consisting of Ti, Al, Cr, Ni, Pd, Pt, Mo, Co and Mg.

The second layer may include at least one material selected from a group consisting of In, Sn, Al, Zn and Ga.

The second layer may include at least one material selected from a group consisting of ITO, CIO, AZO, ZnO, NiO and In2O3.

The first layer may have a thickness ranging from 10 to 300 Å.

The second layer may have a thickness ranging from 500 to 5000 Å.

A surface of the n-type nitride semiconductor layer where the n-type ohmic contact layer is formed may be one of a Ga-polar surface and an N-polar surface.

According to another aspect of the present invention, there is provided a method of manufacturing a nitride light emitting device, the method including: depositing an n-type nitride semiconductor layer, an active layer and a p-type nitride semiconductor layer sequentially to form a light emitting structure; forming an n-type ohmic contact layer by forming a first layer made of an In-containing material on one surface of the n-type nitride semiconductor layer and a second layer made of a transparent conductive oxide on the first layer; forming an n-electrode on the n-type ohmic contact layer; and forming a p-electrode to electrically connect to the p-type nitride semiconductor layer.

The method may further include heat-treating the n-type ohmic contact layer, after the forming an n-type ohmic contact layer.

The heat-treating the n-type ohmic contact layer may be performed at a temperature of 300 to 500° C.

The first layer may be formed by sputtering.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2is a cross-sectional view illustrating a nitride semiconductor light emitting device according to an exemplary embodiment of the invention.

Referring toFIG. 2, the semiconductor light emitting device20of the present embodiment includes a conductive substrate24, and a high reflectivity ohmic contact layer27, a p-type nitride semiconductor layer23, an active layer22, an n-type nitride semiconductor layer21and an n-type ohmic contact layer25sequentially formed on the conductive substrate24. In addition, an n-electrode26is formed on a top of the n-type ohmic contact layer25.

In the present embodiment, the semiconductor light emitting device20is formed of a vertical nitride semiconductor light emitting device. As a known method for manufacturing this semiconductor light emitting device20, the n-type nitride semiconductor layer21, the active layer22and the p-type nitride semiconductor layer23are sequentially grown on a nitride single crystal growth substrate such as a sapphire substrate. Then, a conductive substrate24is formed as a support substrate by plating or bonding, and the sapphire substrate is removed.

Hereinafter, components of the semiconductor light emitting device20will be described in greater detail.

First, the n-type and p-type nitride semiconductor layers21and23and an active layer22constituting a light emitting structure will be described. In this specification, a “nitride semiconductor” denotes a binary, ternary or quaternary compound semiconductor having a composition expressed by AlxInyGa(1−x−y)N, where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1.

That is, the n-type and p-type nitride semiconductor layers21and23may be formed of a semiconductor material doped with n-and p-dopant and having a composition expressed by AlxInyGa(1−x−y)N, where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1. Representative examples of such a semiconductor material include GaN, AlGaN, and InGaN. Also, the n-type dopant employs Si, Ge, Se, Te or C and the p-type dopant utilizes Mg, Zn or Be.

The active layer22is formed of an undoped nitride semiconductor layer having a single or multiple quantum well structure, and emits light with a predetermined energy by recombination of electrons and holes.

The n-type and p-type nitride semiconductor layers21and23, and the active layer22may be grown by a growth process of a semiconductor singe crystal, particularly metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE), which are notably known as processes for growing a nitride single crystal.

The high reflectivity ohmic contact layer27, even though not required essentially, may have a reflectivity of at least 70% and forms an ohmic contact with the p-type nitride semiconductor layer23. This high reflectivity ohmic contact layer27may be formed of at least one layer made of a material selected from a group consisting of Ag, Ni, Al, Rh, Pd, Ir, Ru, Mg, Zn, Pt, Au and a combination thereof. The high reflectivity ohmic contact layer27may be formed of at least one of Ni/Ag, Zn/Ag, Ni/Al, Zn/Al, Pd/Ag, Pd/Al, Ir/Ag. Ir/Au, Pt/Ag, Pt/Al and Ni/Ag/Pt.

The conductive substrate24supports the light emitting structure of a relatively small thickness when the single crystal growth substrate is removed. Also, the conductive substrate24may be bonded to a printed circuit board (PCB) by a conductive bonding layer to act as a p-electrode.

The conductive substrate24may be joined to the light emitting structure by plating or wafer bonding, and is formed of a material such as Si, Cu, Ni, Au, W, and Ti.

The n-electrode26serves as an electrode for electrically connecting the device. Here, the n-electrode26is typically made of an alloy containing Au or Au. This n-electrode26may be formed by deposition or sputtering, which is a general process for growing a metal layer.

The n-type ohmic contact layer25forms an electrical ohmic contact between the n-type nitride semiconductor layer21and the n-electrode26. This ensures injected electrons to be diffused with higher efficiency and current to be distributed uniformly, thereby reducing light absorption in the n-electrode26and increasing emission efficiency of the light emitting device.

To this end, the n-type ohmic contact layer25is structured to include two layers, i.e., first layer25aand second layer25bformed on the first layer25a. Specifically, the first layer25ais made of a material containing Indium (In), and the second layer25bis made of a transparent conductive oxide.

The first layer25acan be formed of any material containing In. Particularly, the first layer25amay be formed of only In or an alloy thereof.

In has a relatively low work function of about 4.12 eV and is suitable as an n-type ohmic contact metal. Particularly, the In layer, when deposited below the transparent conductive oxide layer, exhibits high light transmittance and superior electrical properties. In this case, the In layer may be formed on the n-type nitride semiconductor layer21, particularly, by sputtering in place of e-beam deposition. This is because In has a low melting point of about 157° C. and growth thereof cannot be controlled by general e-beam deposition.

Meanwhile, in a case where the first layer25ais an In alloy, the alloy may contain elements such as Ti, Al, Cr, Ni, Pd, Pt, Mo, Co, and Mg, and the elements will be adequately selected in view of electrical resistance and light transmittance.

The second layer25bmay utilize any material having high transmittance and relatively low electrical conductivity. The most appropriate material is a transparent conductive oxide (TCO).

The second layer25b, when formed of the transparent conductive oxide, can ensure high light transmittance, notably, in the vertical nitride semiconductor light emitting device.

Here, the transparent conductive oxide of the second layer25bis a material containing elements such as In, Sn, Al, Zn, and Ga. For example, the transparent conductive oxide includes ITO, CIO, ZnO, NiO, or In2O3.

Meanwhile, the first layer25aand the second layer25bof the n-type ohmic contact layer25have respective thicknesses t1and t2properly adjusted to control electrical resistance and light transmittance.

Here, to form an ohmic contact, the first layer25ahas a thickness t1ranging from 10 to 300 Å. Also, the second layer25bhas a thickness t2ranging from 500 to 5000 Å to ensure electrical conductivity. These are results obtained by conducting tests on various conditions such as a heat-treatment temperature, and will be described later with reference toFIGS. 4 to 11.

FIG. 3is a cross-sectional view illustrating a nitride semiconductor light emitting device according to a modified embodiment ofFIG. 2.

In the same manner asFIG. 2, the nitride semiconductor light emitting device30ofFIG. 3includes a conductive substrate34, and a high reflectivity ohmic contact layer38, a p-type nitride semiconductor layer33, an active layer32, an n-type nitride semiconductor layer31and an n-type ohmic contact layer35having first and second layers35aand35b.Also, an n-electrode36is formed on a top of the n-type ohmic contact layer35.

Moreover, a GaN substrate37is formed between the n-type nitride semiconductor layer31and the n-type ohmic contact layer35.

The GaN substrate37serves as a substrate for growing a nitride single crystal. The GaN substrate37is electrically conductive and thus can remain in a final light emitting device30without being removed after the light emitting structure is grown. However, the GaN substrate37may be substituted by other substrate made of an electrically conductive material for growing the nitride single crystal. For example, a SiC substrate may be utilized as long as it is easily employed by those skilled in the art.

Besides the above difference, other components ofFIG. 3termed identically are construed to be the same as those ofFIG. 2, and thus will not be described in further detail.

FIGS. 4 to 11are graphs illustrating test examples for deriving an optimal structure and process conditions of the n-type ohmic contact layer applied to the present invention.

First,FIG. 4is a graph illustrating a change in resistance with respect to a thickness of an In layer (first layer) in an n-type ohmic contact layer according to an exemplary embodiment of the invention, and obtained from an I-V curve measured at CTLM spacing of 28 μm. Here, the resistance is equal to a total sum covering contact resistance between the n-type nitride semiconductor layer and the In layer, and resistance of the n-type nitride semiconductor layer.

Also, for test conditions, the second layer is formed of ITO having a thickness of 200 nm. The n-type ohmic contact layer is heat-treated at a temperature of 400° C.

Referring to the graph ofFIG. 4, with increase in thickness of the In layer, resistance is decreased but begins to ascend from a thickness of about 200 Å. This is because too small a thickness of the In layer does not assure easy formation of ohmic contact, thereby increasing resistance.

Therefore, through this test, the first layer, i.e., In layer of the n-type ohmic contact layer has an adequate thickness ranging from 100 to 200 Å. However, the In layer having a thickness outside the above range does not have too big resistance to be used as an ohmic contact layer. Thus, the In layer applicable to the present invention may have a thickness ranging from 10 to 300 Å.

FIG. 5is a graph illustrating a change in resistance with respect to a heat-treatment temperature of an n-type ohmic contact layer.FIG. 6is a graph illustrating a change in surface resistance with respect to a heat-treatment temperature of an n-type ohmic contact layer. For test conditions, the In layer as the first layer has a thickness of 20 nm and the ITO layer as the second layer has a thickness of 200 nm.

Referring to the graph ofFIG. 5, resistance is minimized when the n-type ohmic contact layer is heat-treated at a temperature of 300 to 500° C. Meanwhile, referring to the graph ofFIG. 6, the n-type ohmic contact layer may be heat-treated at a temperature of at least 300° C. to reduce surface resistance.

In consequence, the n-type ohmic contact layer may be heat-treated, particularly, at a temperature of 300 to 500° C.

Meanwhile, the n-type ohmic contact layer is understood to be heat-treated by a known method such as rapid thermal annealing (RTA).

FIG. 7is a graph illustrating a change in resistance with respect to a heat-treatment temperature of an n-type ohmic contact layer, in both cases where a surface of the n-type nitride semiconductor layer where an n-type ohmic contact layer is formed is a Ga-polar surface and is an N-polar surface. Here, the In layer has a thickness of 20 nm, the ITO layer has a thickness of 200 nm, and the result is obtained from an I-V curve measured at CTLM spacing of 120 μm.

FIGS. 4 to 6demonstrate results when the surface of the n-type nitride semiconductor layer where the n-type ohmic contact layer is formed is a Ga-polar surface. In general, when the light emitting device has electrodes arranged in a planar structure, the n-type ohmic contact layer is formed on the Ga-polar surface of the n-type nitride semiconductor layer.

Referring toFIG. 7, the n-type ohmic contact layer exhibits lower resistance when formed on the N-polar surface than on the Ga-polar surface at substantially all heat-treatment temperatures subject to tests.

As described above, the n-type ohmic contact layer of an In/ITO structure ensures better electrical characteristics in a planar light emitting device and a vertical light emitting device as well. Particularly, the n-type ohmic contact layer is more beneficially applicable to the vertical light emitting device since light can be emitted through the n-type ohmic contact layer.

FIGS. 8 to 11are graphs plotted when the transparent conductive oxide layer adopts AZO (Al-doped ZnO).

First,FIGS. 8 to 10are graphs illustrating a change in resistance, contact resistance and a surface resistance with respect to a heat treatment (annealing) temperature of an n-type ohmic contact layer, respectively when AZO is used for a transparent conductive oxide layer (second layer). Here, the contact resistance denotes resistance between the In layer and the n-type nitride semiconductor layer. For test conditions, in the same manner as the previous embodiments, the In layer as the first layer has a thickness of 20 nm, the AZO layer as the second layer has a thickness of 200 nm, and other conditions are identical to those ofFIGS. 5 and 6.

Referring to the graphs ofFIGS. 8 to 10, similar characteristics are plotted when the AZO is utilized as the second layer in a similar manner to when the ITO is adopted as the second layer. That is, when the heat-treatment temperature is 400° C., the contact resistance is approximately 1.6×10−3Ω·cm2. When the n-type ohmic contact layer is heat-treated at a temperature of 300 to 500° C., the lowest electrical resistance is plotted.

FIG. 11is a graph illustrating a change in light transmittance with respect to emission wavelength in an n-type ohmic contact layer of an In/AZO structure according to an exemplary embodiment of the invention.

As shown inFIG. 11, heat-treatment, when performed, leads to greater light transmittance than prior to the heat-treatment, i.e., As-Dep. state. Light transmissivity is most superior when the n-type ohmic contact layer is heat-treated at a temperature of 300° C. to 500° C.

Meanwhile, the n-type ohmic contact layer, i.e., In layer having a thickness of 20 nm and ITO layer having a thickness of 200 nm as shown inFIG. 4exhibits primary function, i.e., linear characteristics based on analysis of an I-V curve measured at CTLM spacing of 28 μm. That is, the n-type ohmic contact layer of the present embodiment has high light transmittance and forms an ohmic contact with the n-type nitride semiconductor layer.

FIG. 12is a cross-sectional view illustrating a light emitting device according to another exemplary embodiment of the invention.

The nitride semiconductor light emitting device50includes a sapphire substrate54, and an n-type nitride semiconductor layer51, an active layer52, a p-type nitride semiconductor layer53, and a p-type ohmic contact layer58formed sequentially on the sapphire substrate54. Also, the n-type nitride semiconductor51is partially etched and an n-type ohmic contact layer55is formed on a portion of the partially etched n-type nitride semiconductor51. The nitride semiconductor light emitting device50also includes n-type and p-type electrodes56aand56b.

In the present embodiment, the n-type and p-type electrodes56aand56bare arranged in a planar configuration. Compared with the vertical light emitting device, the n-type ohmic contact layer55may ensure somewhat lower light transmittance. However, the electrodes of the light emitting device according to the present embodiments may be arranged not only in a vertical but also planar configuration.

The n-type ohmic contact layer55is applicable not only to the vertical but also planar light emitting device as described above. This demonstrates that in any case where the n-type ohmic contact layer is formed on the N-polar surface or the P-polar surface of the n-type nitride semiconductor layer, the n-type ohmic contact layer forms an ohmic contact with the n-type nitride semiconductor layer.

Meanwhile, the p-type ohmic contact layer58is not an essential constituent but may generally utilize an Ni/Au structure to form an ohmic contact with the p-type nitride semiconductor layer53.

Besides this difference, other constituents termed identically are considered as identical to those ofFIG. 2and thus will not be described in further detail.

As set forth above, a nitride semiconductor light emitting device according to exemplary embodiments of the invention includes an n-electrode with high light transmittance and superior electrical properties.

In addition, the nitride semiconductor light emitting device with superior optical and electrical characteristics can be manufactured by an optimal method.