Light emitting diode having carbon nanotubes

A light emitting diode includes a substrate, a first semiconductor layer, an active layer, a second semiconductor layer, a first electrode, a second electrode and a carbon nanotube structure. The first semiconductor layer, the active layer, and the second semiconductor layer are stacked on the substrate. The first semiconductor layer is a stepped structure and has a first surface and a second surface lower than the first surface. The first electrode is located on and electrically connected to the second semiconductor layer. The carbon nanotube structure is located on the second surface of the first semiconductor layer and electrically connected to the first semiconductor layer. The second electrode is located on and electrically connected to the carbon nanotube structure.

BACKGROUND

1. Technical Field

The present disclosure relates to a light emitting diode (LED).

2. Description of the Related Art

LEDs are semiconductors that convert electrical energy into light. Compared to conventional light sources, the LEDs have higher energy conversion efficiency, higher radiance (i.e., they emit a larger quantity of light per unit area), longer lifetime, higher response speed, and better reliability. At the same time, LEDs generate less heat. Therefore, LED modules are widely used in particular as a semiconductor light source in conjunction with imaging optical systems, such as displays, projectors, and so on.

Referring toFIG. 6, a typical LED10, according to the prior art includes a substrate110, a GaN bumper layer120, an N-type GaN layer132, an active layer134, a P-type GaN layer136, and a transparent contact layer140. The GaN bumper layer120, the N-type GaN layer132, the active layer134, the P-type GaN layer136, and the transparent contact layer140are stacked on the substrate110. The LED10further includes a transparent conductive layer150, a first electrode142, and a second electrode144. The first electrode142is disposed on the N-type semiconductor layer132. The transparent conductive layer150and the second electrode144are disposed on the transparent contact layer140. The transparent conductive layer150is made of indium tin oxide (ITO) and the ITO is sputtered on an area of the transparent contact layer140. Due to the net structure of the ITO layer, the lateral distribution of current applied on the transparent conductive layer150is uniform, thereby improving the extraction efficiency of light of the LED. However, the ITO layer has some faults, such as low mechanical strength and resistance distribution. Furthermore, the transparency of the ITO layer may be decreased in humid environments and the ITO layer may absorb some of the light emitted by the active layer134when the ITO fully covers the P-type semiconductor layer136.

What is needed, therefore, is a light emitting diode, which can overcome the above-described shortcomings.

DETAILED DESCRIPTION

Referring toFIG. 1andFIG. 2, a first embodiment of a light emitting diode (LED)20includes a substrate210, a first semiconductor layer232, an active layer234, a second semiconductor layer236, a first electrode242, a second electrode244, a transparent conductive layer250, and a static electrode240. The first semiconductor layer232, the active layer234, the second semiconductor layer234are orderly stacked on the substrate210. The first electrode242is electrically connected to the first semiconductor layer232. The transparent conductive layer250is disposed on the top surface of the second semiconductor layer236and electrically connected to the second semiconductor layer236. The static electrode240is interposed between the second semiconductor layer236and the transparent conductive layer250. The second electrode244is disposed on the top surface of the transparent conductive layer250and electrically connected to the transparent conductive layer250.

The substrate210may have a thickness of about 300 microns (μm) to about 500 μm and a transparent plate for supporting the other elements, such as the first and second semiconductor layers232,236. The substrate210may be made of sapphire, gallium arsenide, indium phosphate, silicon nitride, gallium nitride, zinc oxide, aluminum silicon nitride, silicon carbon, or their combinations. In one embodiment, the substrate210is made of sapphire and has a thickness of 400 μm.

The first semiconductor layer232, the active layer234, and the second semiconductor layer236can be stacked on the substrate210via a process of metal organic chemical vapor deposition (MOCVD).

The first semiconductor layer232has a thickness of about 1 μm to about 5 μm. The second semiconductor layer236has a thickness of about 0.1 μm to about 3 μm. When the first semiconductor layer232is an N-type semiconductor, the second semiconductor layer236is a P-type semiconductor, and vice versa. In one embodiment, the first semiconductor layer232is an N-type semiconductor and the second semiconductor layer236is a P-type semiconductor. The first semiconductor layer232has a step-shaped structure and includes a first surface262and a second surface264located on the same side as the first surface262. The first surface262and the second surface264have different heights and form a step-shaped structure. The active layer234and the second semiconductor layer236are arranged on the first surface262.

The first semiconductor layer232is configured to provide electrons, and the second semiconductor layer236is configured to provide cavities. When a voltage is applied to the first and second semiconductor layers232,236, the electrons can flow into the second semiconductor236and incorporate with the cavities, thereby emitting light. The first semiconductor layer232may be made of N-type gallium nitride, N-type gallium arsenide, or N-type copper phosphate. The second semiconductor layer236may be made of P-type gallium nitride, P-type gallium arsenide, or P-type copper phosphate. In one embodiment, the first semiconductor layer232is made of N-type gallium nitride and has a thickness of 2 μm, and the second semiconductor layer236is made of P-type gallium nitride and has a thickness of 0.3 μm.

The active layer234, in which the electrons fill the holes, has a thickness of about 0.01 μm to about 0.6 μm. The active layer234is a photon exciting layer and can be one of a single quantum well layer or multilayer quantum well films. The active layer140can be made of GaInN, AlGaInN, GaSn, AlGaSn, GaInP, or GaInSn. In one embodiment, the active layer234has a thickness of 0.3 μm and includes one layer of GaInN stacked with a layer of GaN.

The static electrode240is formed on the top surface of the second semiconductor layer236. The static electrode240may be a P-type electrode or an N-type electrode and is a same type as the second semiconductor layer236. Therefore, in one embodiment, the static electrode240is a P-type electrode. Understandably, the static electrode240can function as a reflection layer. The static electrode240can have one or more layers of metal and may be made of titanium, aluminum, nickel, gold, or any combinations thereof. In one embodiment, the static electrode240has two layers. One layer is made of titanium and has a thickness of 15 nanometers (nm). The other layer is made of gold and has a thickness of 100 nm. The static electrode240is formed on the second semiconductor layer236via a process of physical vapor deposition, such as electron evaporation, vacuum evaporation, ion sputtering, or the like.

Further, a functioning layer may be formed between the substrate210and the first semiconductor layer232. The functioning layer may be one or more of a buffer layers, a reflective layer, and a photon crystal structure. The buffer layer is configured to improve epitaxial growth and decrease lattice mismatch. The buffer layer may be made of GaN, AlN, or the like. The reflective layer is configured to change the transmission route of the light to improve extraction efficiency of light in the LED. The reflective layer may be made of silver, aluminum, rhodium, or the like. The photon crystal structure is configured to improve extraction efficiency of light and may be made of silicon, indium tin oxide, carbon nanotube, or the like. In one embodiment, only the buffer layer220is formed on the substrate210and is made of GaN. The buffer layer220has a thickness of about 20 nm to about 50 nm.

The transparent conductive layer250includes a carbon nanotube structure. The transparent conductive layer250can be directly applied to the top surface of the second semiconductor layer236and the static electrode240. The transparent conductive layer250may only cover the exposed surface of the second semiconductor layer236and fully or partly cover both the top surface of the static electrode240and the second semiconductor layer236. In one embodiment, the transparent conductive layer250fully covers both the second semiconductor layer236and the static electrode240. The carbon nanotube structure may include at least one carbon nanotube film and/or a number of carbon nanotube wires. The use of all types of carbon nanotube films and/or carbon nanotube wires is envisioned to be employed by the transparent conductive layer250. There is no particular restriction on the thickness of the carbon nanotube structure and it may be appropriately selected depending on the purpose, and may be, for example, greater than 0.5 nm, and more specifically from about 0.5 μm to 200 μm.

The carbon nanotube structure can include one or more layers of carbon nanotube films. When the carbon nanotube structure includes a number of carbon nanotube films, the carbon nanotube films are stacked on top of each other. The carbon nanotube structure can employ more carbon nanotube films to increase the tensile strength of the carbon nanotube composite. The carbon nanotube film has a thickness in an approximate range from about 0.5 nm to about 100 mm. The carbon nanotubes films may have a free-standing structure. The film structure being supported by itself and does not require a substrate to maintain its structural integrity. As an example, a corner of the carbon nanotube film can be lifted without resulting in damage to the entire structure.

Referring toFIG. 3, the carbon nanotube films each is formed by the carbon nanotubes, orderly or disorderly, and has substantially a uniform thickness. Ordered carbon nanotube films include films where the carbon nanotubes are arranged along a primary direction. Examples include films wherein the carbon nanotubes are arranged approximately along a same direction or have two or more sections within each of which the carbon nanotubes are arranged approximately along a same direction (different sections can have different directions). In the ordered carbon nanotube films, the carbon nanotubes are oriented along the same preferred orientation and approximately parallel to each other. A film can be drawn from a carbon nanotube array, to form the ordered carbon nanotube film, namely a drawn carbon nanotube film. Examples of drawn carbon nanotube film are taught by U.S. Pat. No. 7,045,108 to Jiang et al., and WO 2007015710 to Zhang et al. Referring toFIG. 7, the drawn carbon nanotube film143includes a plurality of successive and oriented carbon nanotubes145joined end-to-end by van der Waals attractive force therebetween. The drawn carbon nanotube film143is a free-standing film. The carbon nanotube film143can be treated with an organic solvent to increase the mechanical strength and toughness of the carbon nanotube film143and reduce the coefficient of friction of the carbon nanotube film143. A thickness of the carbon nanotube film143can range from about 0.5 nanometers to about 100 micrometers.

The ordered carbon nanotube film may be a pressed carbon nanotube film having a number of carbon nanotubes arranged along a same direction. The carbon nanotubes in the pressed carbon nanotube film can rest upon each other. Adjacent carbon nanotubes are attracted to each other and combined by van der Waals attractive force. An angle between a primary alignment direction of the carbon nanotubes and a surface of the pressed carbon nanotube film is 0 degree to approximately 15 degrees. The greater the pressure applied, the smaller the angle formed. The thickness of the pressed carbon nanotube film ranges from about 0.5 nm to about 1 mm. Examples of pressed carbon nanotube film are taught by US application 20080299031A1 to Liu et al.

The disordered carbon nanotube film comprises carbon nanotubes arranged in a disorderly fashion. Disordered carbon nanotube films include randomly aligned carbon nanotubes. When the disordered carbon nanotube film comprises of a film wherein the number of the carbon nanotubes aligned in every direction is substantially equal, the disordered carbon nanotube film can be isotropic. The disordered carbon nanotubes can be entangled with each other and/or are substantially parallel to a surface of the disordered carbon nanotube film. The disordered carbon nanotube film may be a flocculated carbon nanotube film. The flocculated carbon nanotube film can include a plurality of long, curved, disordered carbon nanotubes entangled with each other. The carbon nanotubes can be substantially uniformly dispersed in the flocculated carbon nanotube film. Adjacent carbon nanotubes are attracted by van der Waals attractive force to form an entangled structure with micropores defined therein. It is understood that the flocculated carbon nanotube film is very porous. Sizes of the micropores can be less than 10 μm. Due to the carbon nanotubes in the flocculated carbon nanotube film being entangled with each other, the carbon nanotube structure employing the flocculated carbon nanotube film has excellent durability, and can be fashioned into desired shapes with a low risk to the integrity of the flocculated carbon nanotube film. The thickness of the flocculated carbon nanotube film can range from about 0.5 nm to about 1 millimeter (mm).

The disordered carbon nanotube film may be a pressed carbon nanotube film having a number of carbon nanotubes arranged along different directions. The pressed carbon nanotube film can be a free-standing carbon nanotube film. When the carbon nanotubes in the pressed carbon nanotube film are arranged along different directions, the pressed carbon nanotube film can be isotropic. As described above, the thickness of the pressed carbon nanotube film ranges from about 0.5 nm to about 1 mm. Examples of pressed carbon nanotube film are taught by US application 20080299031A1 to Liu et al.

Length and width of the carbon nanotube film can be arbitrarily set as desired. A thickness of the carbon nanotube film is in a range from about 0.5 nm to about 100 μm. The carbon nanotubes in the carbon nanotube film can be single-walled, double-walled, multi-walled carbon nanotubes, and combinations thereof. Diameters of the single-walled carbon nanotubes, the double-walled carbon nanotubes, and the multi-walled carbon nanotubes can, respectively, be in the approximate range from about 0.5 nm to about 50 nm, about 1 nm to about 50 nm, and about 1.5 nm to about 50 nm.

The carbon nanotube structure include a number of carbon nanotube wires. The carbon nanotube wires may be arranged side by side on the top surface of the second semiconductor layer or may be weaved into a carbon nanotube layer. The weaved carbon nanotube layer is applied to the second semiconductor layer. The carbon nanotube wire includes untwisted carbon nanotube wire and twisted carbon nanotube wire. The untwisted carbon nanotube wire includes a number of carbon nanotubes parallel to each other. The twisted carbon nanotube wire includes a number of carbon nanotube helically twisted along a longitudinal axis of the twist carbon nanotube wire. In other embodiments, the carbon nanotube structure includes a drawn carbon nanotube film, the drawn carbon nanotube film includes a plurality of carbon nanotubes, the carbon nanotubes are substantially parallel to each other. The carbon nanotube structure includes at least one carbon nanotube film, the carbon nanotube film includes a plurality of carbon nanotubes joined by van der Waals force. In one embodiment, the carbon nanotube structure includes two drawn carbon nanotube films, and an angle between aligned directions of the drawn carbon nanotube films is approximately 90 degrees. The drawn carbon nanotube film includes a plurality of carbon nanotube segments joined end to end by van der Waals force along an axial direction of the carbon nanotubes.

The untwisted carbon nanotube wire can be formed by treating the drawn carbon nanotube film with an organic solvent. The drawn carbon nanotube film is treated by applying the organic solvent to the carbon nanotube film while being free to bundle. After being soaked by the organic solvent, the adjacent paralleled carbon nanotubes in the drawn carbon nanotube film will bundle together, due to the surface tension of the organic solvent when the organic solvent volatilizing, and thus, the drawn carbon nanotube film will be shrunk into untwisted carbon nanotube wire. The carbon nanotubes of the untwisted carbon nanotube wires are substantially parallel to each other along the longitudinal axis of the untwisted carbon nanotube wires. Examples of the untwisted carbon nanotube wire are taught by U.S. Pat. No. 7,045,108 to Fan et al. and US publication No. 20070166223 to Fan et al.

The twisted carbon nanotube wire can be formed by twisting a drawn carbon nanotube film by using a mechanical force to turn the two ends of the drawn carbon nanotube film in opposite directions. Further, the twisted carbon nanotube wire can be treated by applying the organic solvent. After applying the organic solvent, the adjacent carbon nanotubes in the twisted carbon nanotube film will bundle together, due to the surface tension of the organic solvent when the organic solvent volatilizing, and thus, the twisted carbon nanotube wire may have less specific surface area, and larger density and strength than an untreated twisted carbon nanotube wire.

The transparent conductive layer250may be made by steps of forming a metal layer (not shown) on the carbon nanotube structure and heating the metal layer in a temperature of about 300 degrees centigrade to about 500 degrees centigrade for about 3 minutes to about 10 minutes. The metal layer may be a single-layer structure or a multi-layered structure. In one embodiment, the metal layer includes a nickel layer stacked with a gold layer. The nickel layer has a thickness of about 2 nm. The gold layer has a thickness of 5 nm. Since the metal layer decreases in thickness because of the heating, the metal molecule of the metal layer can be melted and can aggregate into a number of metal particles by surface tension. The carbon nanotube structure has a plurality of micropores between adjacent carbon nanotubes of the carbon nanotube structure. These metal particles uniformly disperse in the micropores of the carbon nanotube structure to form a composite film. The composite film, which functions as the transparent conductive layer250, has better electrical conductivity than the pure carbon nanotube structure, thereby improving current injection efficiency and electrical contact between the carbon nanotube structure and the static electrode240, the first electrode240, and the second semiconductor layer236.

In one embodiment, two drawn carbon nanotube films are coated on the second semiconductor layer236and the static electrode240. An angle between the primary directions of the two drawn carbon nanotube films ranges from about 0 degrees to about 90 degrees. In one embodiment, the primary directions of the two drawn carbon nanotube films are perpendicular to each other as shown inFIGS. 8 and 9.

The second electrode244can be deposited on the transparent conductive layer250via physical vapor deposition and may have single-layer structure or multi-layered structure. The second electrode244can be made of titanium or gold. In one embodiment, the second electrode244includes two layers, one layer is titanium and has a thickness of 15 nm and another layer is gold and has a thickness of 200 nm. At least a portion of the carbon nanotube structure is located between the static electrode240and the second electrode244. The second electrode244may be P-type or N-type electrode and is the same type as the static electrode240and the second semiconductor layer236. Since the static electrode240is made of P-type material, the second electrode244is a P-type electrode. When the LED20has the static electrode240, the second electrode244should be located above the static electrode240. When the LED has no static electrode240, the second electrode244can be located at any position on the transparent conductive layer250. In one embodiment, since the LED employs the static electrode240, the second electrode244is located above the static electrode242. The second electrode244and the static electrode240function together as the P-type electrode of the LED.

The second electrode244is a same polarity type with the first semiconductor layer236and may be made of N-type material. The second electrode244is deposited on the second surface264of the first semiconductor layer236. The second electrode244has a same structure as the first electrode242and includes a titanium layer and a gold layer stacked on the titanium layer. The titanium layer has a thickness of about 15 nm and the gold layer has a thickness of about 200 nm. The method of depositing the second electrode244can be the same as that of the first electrode242. The first and second electrodes242,244can be deposited at the same time.

Referring toFIG. 4, in one embodiment, an LED30includes a substrate310, a buffer layer320, a first semiconductor layer332, an active layer334, a second semiconductor layer336, a first electrode342, a second electrode344, a transparent conductive layer350, and a static electrode340. The buffer layer320, the first semiconductor layer332, the active layer334, the second semiconductor layer336are orderly stacked on the substrate310.

The first semiconductor layer332includes a first surface362and a second surface364located on the same side as the first surface362. The first surface362and the second surface364have different heights and form a stepped structure. The active layer334and the second semiconductor layer336are disposed on the first surface362. The transparent conductive layer350is disposed on the second surface364of the first semiconductor layer332and electrically connected to the first semiconductor layer332. Further, the static electrode340is interposed between the first semiconductor layer332and the transparent conductive layer350. The first electrode342is disposed on the top surface of the transparent conductive layer350and electrically connected to the transparent conductive layer350. The second electrode344is electrically connected to the second semiconductor layer336.

Referring toFIG. 5, in one embodiment, an LED40includes a substrate410, a buffer layer420, a first semiconductor layer432, an active layer434, a second semiconductor layer436, a first electrode442, a second electrode444, a first transparent conductive layers450, a second transparent conductive layer452, and a first static electrode440, a second static electrode446. The buffer layer420, the first semiconductor layer432, the active layer434, the second semiconductor layer436are orderly stacked on the substrate310.

The first semiconductor layer432includes a first surface462and a second surface464located on the same side a the first surface462. The first surface462and the second surface464have different heights and form a stepped structure. The second transparent conductive layer452is mounted on the second semiconductor layer436, and the first transparent conductive layer450is mounted on the second surface464of the first semiconductor layer432. Further, the first static electrode440is located between the second semiconductor layer436and the second transparent conductive layer452, and the second electrode444is disposed on the top surface of the second transparent conductive layer452. The second static electrode446is interposed between the first semiconductor layer436and the first transparent conductive layer450, and the first electrode442is disposed on the top surface of the first transparent conductive layer450.

Since the carbon nanotubes have better electrical conductivity and mechanical strength than conventional material, such as indium tin oxide, the carbon nanotube structure has better electrical conductivity and mechanical strength, thereby improving power efficiency and life span. Further, the carbon nanotube structure is transparent in varied humid environments. Therefore less of the light emitted by the active layer is absorbed. Thus, the LED has good extraction efficiency in comparison with the typical LED.