Polarized light emitting diode and method of forming the same

Example embodiments are directed to a polarized light emitting diode and method of forming the same. The polarized light emitting diode may include a support layer, a semiconductor layer structure, and/or a polarization control layer. The semiconductor layer structure may be formed on the support layer and may include a light-emitting layer. The polarization control layer may be formed on the semiconductor layer structure and may include a plurality of metal nanowires. The polarized light emitting diode may be configured to control the polarization of emitted light. The method of forming a polarized light emitting diode may include forming on a substrate a semiconductor layer structure with a light emitting layer. A reflecting layer may be formed on the semiconductor layer structure with an attached support layer. The substrate may be removed from the semiconductor layer structure and a polarization control layer including metal nanowires may be formed on the semiconductor layer structure.

PRIORITY STATEMENT

This application claims the benefit of Korean Patent Application No. 10-2006-0082935, filed on Aug. 30, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

Description of the Related Art

Recently, light emitting diodes have gained attention as a new, increasingly efficient, environmental light source. They have been used in a wide range of applications, including, for example, vehicle light sources, display device light sources, optical communications and general lighting sources.

In a variety of applications, polarization properties of light may be used to take advantage of light emitted from a source. A liquid crystal display device may be formed by injecting a liquid crystal material between substrates on which electrodes may be formed. When a voltage is applied between the two electrodes, an electrical field may be generated to change the liquid crystal molecular alignment. This change of the liquid crystal molecular alignment may vary the light transmittance through the liquid crystal material to form images. Because the liquid crystal display may transmit or block light by manipulating the polarization direction of polarized light, only polarized light in one direction may be used.

Because light from a generic source may not be polarized, polarization plates may be provided on both sides of a liquid crystal display. The polarization plates may transmit light polarized in a given direction and absorb light polarized in other directions. Because the polarization plates may absorb about 50 percent of incident light, light efficiency may be relatively low. In an optical communication device, coupling efficiency with external passive optical devices may vary significantly according to the polarization properties of the light, making light polarizing technologies increasingly important.

SUMMARY

Example embodiments are directed to light emitting diodes and methods of forming the same.

According to example embodiments, a polarized light emitting diode may include a support layer, a semiconductor layer structure with a light-emitting layer, and/or a polarization control layer with a plurality of metal nanowires. The semiconductor layer structure may be formed on the support layer and the polarization control layer may be formed on the semiconductor layer structure. The polarized light emitting diode may be configured to control the polarization direction of emitted light.

The polarization control layer may include an oxide layer in which the metal nanowires may be formed. The metal nanowires may have a polygonal cross-section or circular cross-section, including, for example, a rectangular cross-section. The plurality of metal nanowires may have a thickness ranging from about 50 nm to about 1000 nm inclusive. The metal nanowires may have a width less than or equal to about half the pitch of the plurality of parallel metal nanowires.

According to example embodiments, the polarized light emitting diode may also include a reflecting layer arranged between the support layer and the semiconductor layer structure. The plurality of metal nanowires may have a pitch less than or equal to about half of the wavelength of light emitted from the light-emitting layer. The metal nanowires may be formed of for example Al, Au, Ag, Pd, Pt, an alloy thereof or the like. The plurality of metal nanowires may be used as electrodes to apply a voltage to the semiconductor layer structure.

According to example embodiments, the polarized light emitting diode may also include a depolarizing layer for depolarizing light reflected from the polarization control layer. The depolarizing layer may be formed of an optical anisotropic material having a refractive variable index. The reactive index may vary with the polarization of incident light, and may be a scattering layer.

The depolarizing layer may be arranged between the polarization control layer and the semiconductor layer structure, within the semiconductor layer structure, or between the support layer and the semiconductor layer structure. If a reflecting layer is formed between the support layer and the semiconductor layer structure, the depolarizing layer may be formed between the reflecting layer and the semiconductor layer structure as well.

Example embodiments may further include a reflecting layer formed on side surfaces of the support layer, the semiconductor layer structure, and/or the polarization control layer.

According to example embodiments, a method of forming a polarized light emitting diode may include forming a semiconductor layer structure on a substrate. The semiconductor layer structure may include a first semiconductor layer, a light emitting layer, and a second semiconductor layer. A reflecting layer may be formed on the second semiconductor layer, and a support layer may be formed and attached to the reflecting layer. The substrate may be removed from the semiconductor layer structure, and the polarization control layer may be formed on the first semiconductor layer. The polarization control layer may include metal nanowires.

DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1is a cross-sectional view illustrating a polarized light emitting diode100according to an example embodiment.FIG. 2is a view illustrating an example configuration of a polarization control layer150in the polarized light emitting diode100illustrated inFIG. 1. Referring toFIGS. 1 and 2, the polarized light emitting diode100may include a support layer110, a semiconductor layer structure130formed on the support layer110, and/or a polarization control layer150formed on the semiconductor layer structure130. The semiconductor layer structure130may include one or more semiconductor layers. Although the semiconductor layer structure may be referred to herein as a semiconductor multi-layer for example purposes, it will be understood that the example embodiments are not intended to be limited by the use of this term.

A semiconductor multi-layer130may include a first semiconductor layer132, a second semiconductor layer136, and a light-emitting layer134formed between the first semiconductor layer132and the second semiconductor layer136. The first semiconductor layer132may be P-type and the second semiconductor layer136may be N-type, for example. The light-emitting layer134may be formed in a multi-quantum well structure, for example.

A reflecting layer120may be formed between the support layer110and the semiconductor multi-layer130. The reflecting layer120may reflect light emitted from the light-emitting layer134back towards the polarization control layer150. The reflecting layer120may be formed of, for example, a metal layer or a dielectric mirror. A metal layer may include reflective materials, for example, Al, Au, Ag, Pd, Pt, or an alloy thereof, or the like. A dielectric mirror may have a structure in which dielectric materials having different refractive indexes may be formed in a repeating pattern.

Light emitting diodes may be thin-film type LEDs, which may have upper and lower surfaces that are significantly larger than the side surfaces. Therefore, much of the light emitted from the light-emitting layer134may be emitted through the polarization control layer150. If a large amount of light is lost through the side surfaces, a reflecting layer, for example, a dielectric mirror, may be formed on the side surfaces to reduce the amount of light lost via the sides. The reflecting layer may include an insulating layer and a reflective metal layer, for example, Al, Au, Ag, Pd, Pt, or an alloy thereof, or the like.

The polarization control layer150may be configured to control the polarization of the light emitted from the light-emitting layer134. The polarization control layer150may include a plurality of metal nanowires154. For example, the polarization control layer150may include an oxide layer152in which a plurality of metal nanowires154may be formed. The oxide layer152may include transparent electrode materials, for example, ITO, ZnO, etc . . . , and a transparent oxide, for example, SiO.sub.2 or the like. The plurality of metal nanowires154may be arranged along a first direction (X direction illustrated inFIG. 1), with an associated pitch (p). The longitudinal direction of the plurality of metal nanowires154may be a second direction (Y direction illustrated inFIG. 1). A cross-section of the plurality of metal nanowires154may have a rectangular shape with a predetermined or given width (w) and a predetermined or given thickness (t). The cross-sectional shape of the metal nanowires154is not limited to merely a rectangular shape and may include other shapes as well, for example, any polygonal shape or circular shape, including elliptical and non-circular but rounded shapes.

The plurality of metal nanowires154may reflect incident light whose polarization direction is parallel to the longitudinal direction of the metal nanowires154. The plurality of metal nanowires154may also transmit incident light whose polarization direction is parallel to the width direction of the metal nanowires154. The plurality of metal nanowires154may include a reflective metallic material, for example, Al, Au, Ag, Pd, Pt, an alloy thereof or other materials with similar reflective properties. Because the metal materials may be conductive, the plurality of metal nanowires154may be used as an electrode in order to apply a voltage to the semiconductor multi-layer130.

The width (w), the thickness (t), and the pitch (p) (or alternative dimensional measurements of the polarization control layer150) may be selected by considering the metal nanowires154material composition and the wavelength (λ) of light incident to the polarization control layer150. The thickness (t) of the metal nanowires154should be thick enough to reflect light which may be polarized in the longitudinal direction of the metal nanowires154. For example, the thickness (t) of the metal nanowires154may be approximately 50 nm or more. For an optical absorption, the thickness (t) of the metal nanowires154may be approximately 1000 nm. The width (w) of the metal nanowires154may be sufficiently shorter than the wavelength of light emitted from the light-emitting layer134. In addition, the pitch (p) between the metal nanowires154may be less than or equal to about λ/2 in order to reduce or prevent any diffraction effects that may arise with regular arrangements of optical elements.

FIGS. 3A and 3Bare views illustrating a method of manufacturing a polarized light emitting diode according to an example embodiment.

Referring toFIG. 3A, a semiconductor multi-layer130may be formed on a sapphire substrate (S1). The semiconductor multi-layer130may include a first semiconductor layer136, a light-emitting layer134, and a second semiconductor layer132on the sapphire substrate (S1). The first and second semiconductor layers may be of different conductive types. For example, the first semiconductor layer136and the second semiconductor layer132may be an N-type GaN layer and a P-type GaN layer, respectively. A reflecting layer120may be formed on the second semiconductor layer132, and a support layer110may be attached to the reflecting layer120by bonding or plating. The sapphire substrate (S1) may be detached from the rest of the structure by various means, including, for example, laser lift-off methods, chemical lift-off methods or the like. Secondly, a polarization control layer150having metal nanowires154, may be formed on the first semiconductor layer136.

FIG. 3Bis a view illustrating the use of a SiC substrate (S2) to form a semiconductor multi-layer130, according to an example embodiment. When a SiC substrate (S2) is used as inFIG. 3B, chemical mechanical polishing (CMP), selective etching or the like may be used in order to make the SiC substrate (S2) thinner or remove it entirely. Accordingly, although the SiC substrate (S2) may be shown as completely removed inFIG. 3B, a portion of the SiC substrate (S2) may actually remain. Subsequently, a polarization control layer150having metal nanowires154may be formed on the first semiconductor layer136.

However, it may be possible to manufacture a semiconductor multi-layer and a polarization control layer on the same substrate successively.

When voltage is applied between the second semiconductor layer132and the first semiconductor layer136using electrodes (not shown), carriers of the second semiconductor layer132and carriers of the first semiconductor layer136may be combined in the light-emitting layer134, so that light may be created in and emitted from the light-emitting layer134. This light may be non-polarized light; for example, the polarization of the light may be arbitrary (e.g., even in all directions).

The polarization control layer150may control the polarization direction of the light by using the response characteristics of free electrons in the metal nanowires to the different polarization directions of the light. When light I2, with a second polarization that is parallel to the longitudinal direction of the metal nanowires154, among light I0(non-polarized light) emitted from the light-emitting layer134is incident on the metal nanowires154, the free electrons of the metal nanowires154may vibrate in the longitudinal direction of the metal nanowires154. This vibration of the free electrons generates electromagnetic waves that may interfere with the incident light I2. Therefore, much of the light I2with a second polarization that is parallel to the longitudinal direction of the metal nanowires154may be reflected (the rest may be absorbed) by interference with the electromagnetic waves.

On the other hand, much of light I1, with a first polarization that is parallel to the transverse direction of the metal nanowires154, may be transmitted through the metal nanowires154(the rest may be absorbed) because it may be difficult for the free electrons of the metal nanowires154to vibrate in the transverse direction of the metal nanowires154due to spatial limitations. For example, the metal nanowires154may exhibit a reflective characteristic for light I2with a second polarization, and a lossy dielectric material characteristic for light I1with a first polarization.

FIG. 4is a graph illustrating the transmittance of the example polarization control layer150as a function of the width (w) of the metal nanowires154, expressed as a fraction of the wavelength λ of the incident light.FIG. 5is a graph illustrating the reflectance of the example polarization control layer150as a function of the width (w) of the metal nanowires154, expressed as a fraction of the wavelength λ of the incident light. The graphs ofFIGS. 4 and 5show transmittance and reflectance data, respectively, for the first polarization component I1and the second polarization component I2of light I0emitted from the light-emitting layer134. The example graphs ofFIGS. 4 and 5correspond to the wavelength of light I0being about 460 nm, the width (w) of the metal nanowires154being about 150 nm, and the pitch (p) of the metal nanowires154being about one third of the wavelength of light I0.

Referring toFIG. 4, the transmittance of the first polarization component I1may be larger than that of the second polarization component I2, regardless of the width (w) of the metal nanowires154. As described above, because the free electrons of the metal nanowires154may not vibrate significantly in the first direction of the metal nanowires154due to insufficient space, the incident first polarization component I1may be transmitted effectively. However, as the width (w) of the metal nanowires154is increased, the transmittance of both the first polarization component I1and the second polarization component I2may be decreased. As the width (w) becomes larger, the free electrons may vibrate more in the transverse (width) direction of the metal nanowires154, and both the first polarization component I1and the second polarization component I2, the metal nanowires154may work as reflecting metals.

The graph illustrated inFIG. 5describes the reflectance of the polarization control layer150, and may demonstrate several features in reverse of that ofFIG. 4. For example, regardless of the width (w) of the metal nanowires154, the reflectance of the second polarization component I2may be larger than that of the first polarization component I1. As the width (w) of the metal nanowires154is increased, the reflectance of the first and second components I1and I2may be likewise increased. The width (w) of the metal nanowires154may be sufficiently shorter than the wavelength λ of the light emitted from the light-emitting layer134. The width (w) of the metal nanowires154may be appropriately determined from the pitch (p) of the metal nanowires154. For example, in order to transmit about 40% or more of light I0incident on the polarization control layer150when the pitch (p) of the metal nanowires154is about one third of the wavelength λ, the width (w) of the metal nanowires154may be shorter than about one half of the pitch (p) of the metal nanowires154.

FIG. 6is a graph illustrating example polarization ratio data with respect to the width (w) of the metal nanowires154, expressed as a fraction of the wavelength λ of the incident light, for light transmitted through the polarization control layer150. The polarization ratio may be a useful index for the polarization properties of the light transmitted through the polarization control layer150. The polarization ratio may be defined for the light transmitted through the polarization control layer150as the ratio of the difference between the first polarization component I1and the second polarization component I2of the transmitted light to the sum of the first polarization component I1and the second polarization component I2of the transmitted light. For example, the polarization ratio may be defined mathematically as (T(I2)−T(I2))/(T(I1)+T(I2)).

FIG. 6shows that as the width (w) of the metal nanowires154increases, the polarization ratio may approach a value of 1. The reason for this may be that as the width (w) of the metal nanowires154increases, although both the transmittance of the first polarization component I1and the second polarization component I2may decrease, the transmittance of the second polarization component I2may approach 0 faster than the transmittance of the first polarization component I1. The width (w) of the metal nanowires154may, therefore, be determined according to the pitch (p) of the plurality of parallel metal nanowires154, such that both the polarization ratio and the transmittance of the first polarization component I1may be sufficiently large. For example, for the transmitted amount of the first polarization component (I1) to be more than about half the amount of the incident light, and for the polarization ratio to be about 0.8 or more, the width (w) of the metal nanowires154may range from about 0.1 to about 0.15 of the wavelength λ of the incident light.

FIG. 7is a cross-sectional view illustrating a polarized light emitting diode300according to example embodiments. Referring toFIG. 7, the polarized light emitting diode300may include a support layer310, a semiconductor multi-layer330formed on the support layer310, a depolarizing layer340formed on the semiconductor multi-layer330, and a polarization control layer350formed on the depolarizing layer340. A reflecting layer320may be formed between the support layer310and the semiconductor multi-layer330. The reflecting layer320may reflect light emitted from a light-emitting layer334back towards the polarization control layer350. The reflecting layer320may be formed, for example, from a metal layer or a dielectric mirror. The metal layer may include reflective materials, for example, Ag, Au, Pt, Al Pd, an alloy thereof, or other materials with similar properties. The dielectric mirror may have a structure in which dielectric materials having different refractive indexes may be formed in a repeating pattern.

The semiconductor multi-layer330may include a first semiconductor layer332, a second semiconductor layer336, and a light-emitting layer334formed between the first semiconductor layer332and the second semiconductor layer336. The first and second semiconductor layers may be of different conducting types, for example, N-type, P-type, etc . . . . For example, the light-emitting layer334may be formed in a multi-quantum well structure.

The polarization control layer350may control the polarization of the light emitted from the light-emitting layer334. The polarization control layer350may include a plurality of metal nanowires354. For example, the polarization control layer350may include an oxide layer352in which the plurality of metal nanowires354may be formed. The oxide layer352may include transparent electrode materials, for example, ITO, ZnO, or the like, and/or a transparent oxide, for example, SiO.sub.2 or the like. The plurality of metal nanowires354may be arranged along a first direction (e.g., the x-axis direction inFIG. 1) and may have an associated pitch (p). The longitudinal direction of the plurality of metal nanowires354may be a second direction (e.g., the y-axis direction inFIG. 1). The cross-section of the plurality of metal nanowires354may be a rectangular form with a predetermined or given width (w) and thickness (t). As described above, the cross-section may have different shapes and should not be construed as limited to the rectangular type alone. The plurality of metal nanowires354may reflect the polarization component of the incident light that is parallel to the longitudinal direction of the metal nanowires354.

The plurality of metal nanowires354may transmit the polarization component of the incident light that is parallel to the transverse direction of the metal nanowires354. The plurality of metal nanowires354may include a reflective metal layer, for example, Al, Au, Ag, Pd, Pt, an alloy thereof or the like. The polarization control layer350may have the same configuration as the polarization control layer150inFIG. 1. For example, the first polarization component I1of light I0emitted from the light-emitting layer334may be transmitted through the polarization control layer350, and the second polarization component I2of the light I0may be reflected by the polarization control layer350.

The depolarizing layer340may change the second polarization component I2reflected by the polarization control layer350into non-polarized light (I0′). As illustrated inFIG. 7, the depolarizing layer340may be provided between the semiconductor multi-layer330and the polarization control layer350. The depolarizing layer340may alternately be provided between the reflecting layer320and the semiconductor multi-layer330, or as part of the semiconductor multi-layer330. Accordingly, the location of the depolarizing layer340may vary within the scope of example embodiments.

The depolarizing layer340may be formed of an optical, anisotropic material. Because the refractive index of the optical anisotropic material may vary with the polarization of the incident light, the path of the light reflected or refracted from the optical anisotropic material may also vary with the polarization of the light. Thus, the light may become non-polarized by being reflected or refracted by the optical anisotropic material. The depolarizing layer340may be formed from a scattering layer with diffusive material. Diffusion may occur due to minute changes in the refractive index of the scattering layer, thereby changing the incident light into non-polarized light.

Because much of the light reflected by the polarization control layer350has a second polarization component I2, light having a first polarization component I1may be regenerated by changing the light reflected from the polarization control layer350into non-polarized light using the depolarizing layer340. Thus, regenerated light having the first polarization component I1may be transmitted through the polarization control layer350so that more light having the first polarization component I1may be discharged.

According to example embodiments, a polarized light emitting diode may include a polarization control layer having a plurality of metal nanowires, and may emit polarized light. A polarized light emitting diode may further include a depolarizing layer to increase the emission of polarized light. A polarized light emitting diode according to example embodiments may be useful, for example, as a light source that directly emits polarized light in optical communication devices or display devices which make use of certain polarization properties.