MATB-435US
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`PATENT
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`LED AND LED DISPLAY AND ILLUMINATION DEVICES
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`FIELD OF INVENTION
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`[0001] The present invention relates to light emitting diodes (LEDs) and, more
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`particularly, to light emitting unit cells and light emitting chips which recycle total internal
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`reflection (TIR) light as a photocurrent source, and methods of forming the same.
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`BACKGROUND OF THE INVENTION
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`[0002] Light emitting diodes (LEDs) generally convert electrical energy to light, and are
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`known to be used as light sources. For example, LEDs may be used in full—color displays,
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`image scanners, optical communication systems and various signal systems. LEDs are
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`generally formed from semiconductor materials and typically include an active layer of
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`semiconductor material located between two oppositely doped layers. When a bias is
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`applied across the doped layers, holes and electrons are injected into the active layer,
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`where they recombine to generate light. The light generated by the active region may be
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`emitted in all directions and may escape from the LED through any exposed surfaces. The
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`material of the active layer may be selected for emission of a particular wavelength of light.
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`For example, gallium nitride (GaN) and zinc selenide (ZnSe) semiconductor materials may
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`be used to emit green or blue light. Other examples of semiconductor materials include
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`gallium phosphide (GaP) for green light, gallium arsenide phosphide (GaAsP) for yellow,
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`orange and red light, and gallium aluminum arsenide (GaAlAs) for red light.
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`[0003] The efficiency of conventional LEDs may be limited by their inability to emit all of
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`the light that is generated by the active layer. When an LED is energized, the light that is
`emitted from the active layer may reach the emitting surfaces/adjacent surfaces at many
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`different angles. LEDs are typically formed from semiconductor materials having relatively
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`high refractive indices (for example, a refractive index of about 2.2-3.8) compared to a
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`refractive index of air (of about 1.0). According to Snell's law, light traveling from a region
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`with a high index of refraction (the semiconductor material) to a region with a low index of
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`refraction (air) that is less than a critical angle (relative to the surface normal direction)
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`may propagate out of the LED. Light that reaches the surface at an angle greater than the
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`critical angle does not pass, but instead experiences total internal reflection (TIR). Because
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`of total internal reflection, much of the light generated by conventional LEDs is not emitted,
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`thereby reducing the external quantum efficiency of the LED.
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`SUMMARY OF THE INVENTION
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`[0004] The present invention relates to light emitting chips and methods of forming light
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`emitting chips. The light emitting chip includes a light emission structure comprising a p-
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`type semiconductor layer, an n—type semiconductor layer and an active layer between the
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`p-type semiconductor layer and the n-type semiconductor layer. The light emitting chip
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`includes at least one light emitting unit comprising a light emitting diode (LED) portion
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`formed from the light emission structure and a plurality of light receiving diode (LRD)
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`portions formed from the light emission structure. The plurality of LRD portions are serially
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`connected and configured to surround the LED portion. The plurality of LRD portions are
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`optically coupled to the LED portion to receive total internal reflection (TIR) light from the
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`LED portion and are configured to convert the TIR light to a photocurrent.
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`[0005] The present invention also relates to a light emitting unit cell comprising a first
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`light emitting diode (LED) electrically connected to a power source, a plurality of light
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`receiving diodes (LRDs) connected in series and a second LED. The plurality of LRDs are
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`optically coupled to the first LED to receive total internal reflection (TIR) light from the first
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`LED and are configured to convert the TIR light to a photocurrent. The second LED is
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`electrically connected in parallel with the plurality of LRDs.
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`[0006] The present invention further relates to a light emitting unit cell comprising a
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`light emitting diode (LED) electrically connected to a power source and a plurality of light
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`receiving diodes (LRDs) connected in series. The plurality of LRDs are optically coupled to
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`the LED to receive total internal reflection (TIR) light from the LED and are configured to
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`convert the TIR light to a photocurrent. The plurality of LRDs feed back the photocurrent to
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`the LED.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`[0007] The invention may be understood from the following detailed description when
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`read in connection with the accompanying drawing.
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`It is emphasized that, according to
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`common practice, the various features of the drawing are not to scale. On the contrary, the
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`dimensions of the various features are arbitrarily expanded or reduced for clarity.
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`Included
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`in the drawing are the following figures:
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`[0008]
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`Fig. 1A (Prior Art) is a top-plan view diagram of a conventional LED;
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`[0009]
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`Fig. 18 (Prior Art) is a cross-section diagram along lines 1B—1B’ of the
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`conventional LED shown in Fig. 1A;
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`[0010]
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`Fig. 2 is a cross-section diagram of a portion of an LED illustrating reflection of
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`light rays within the LED and propagation of light rays out of the LED;
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`[0011]
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`Fig. 3 is a circuit diagram of an exemplary light emitting unit cell, according to an
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`example embodiment of the present invention;
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`[0012]
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`Fig. 4 is a top-plan view diagram of a structure of an exemplary light emitting
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`unit cell shown in Fig. 3, according to an example embodiment of the present invention;
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`[0013]
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`Figs. 5A, SB, 5C, SD, 5E, 5F and 56 are respective cross-section and top-plan
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`views diagrams illustrating an exemplary method of forming a light emitting chip, according
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`to an embodiment of the present invention;
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`[0014]
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`Fig. 6 is a cross—section diagram along lines 6-6’ of the exemplary light emitting
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`chip shown in Fig. SG, according to an embodiment of the present invention;
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`[0015]
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`Figs. 7A and 7B are graphs of theoretical extraction efficiency for an exemplary
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`light emitting chip for different conversion efficiencies, according to an embodiment of the
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`present invention;
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`[0016]
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`Fig. 8 is a circuit diagram of an exemplary light emitting unit cell, according to
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`another embodiment of the present invention;
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`[0017]
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`Fig. 9 is a top-plan view diagram of a structure of the light emitting unit cell
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`shown in Fig. 8, according to another embodiment of the present invention;
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`[0018]
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`Figs. 10A, 103, 10C, 10D, ICE and 10F are top-plan views diagrams illustrating
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`an exemplary method of forming a light emitting chip, according to another embodiment of
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`the present invention;
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`[0019]
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`Fig. 11A is a cross-section diagram along lines 11A—11A’ of the exemplary light
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`emitting chip shown in Fig. 10F, according to another embodiment of the present invention;
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`[0020]
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`Fig. 113 is a cross-section diagram along lines llB-llB’ of the exemplary light
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`emitting chip shown in Fig. 10F, according to another embodiment of the present invention;
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`[0021]
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`Fig. 12 is a graph of extraction efficiency for an exemplary light emitting chip for
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`different conversion efficiencies, according to an exemplary embodiment of the present
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`invention; and
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`[0022]
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`Fig. 13 is a top—plan view diagram of an exemplary micro—pixelated LED,
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`according to another embodiment of the present invention.
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`DETAILED DESCRIPTION OF THE INVENTION
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`[0023] Referring to Figs. 1A and 13, a conventional LED 100 is shown.
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`In particular, Fig.
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`1A is a top-plan view diagram of conventional LED 100 and Fig. IB is a cross—section
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`diagram of LED along lines 1B~1B’.
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`[0024] Conventional LED 100 includes substrate 110, buffer layer 112, n-type GaN layer
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`104, active layer 114 containing a multi-quantum well (MQW) structure, p-type GaN layer
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`116 and transparent electrode 108, which are sequentially laminated on substrate 110.
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`Transparent electrode 108 may be used, for example, to enhance a current spreading
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`effect.
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`[0025] Portions of transparent electrode 108, p-type layer 116 and active layer 114 may
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`be removed by mesa-etching such that a portion of the upper surface of n—type layer 104 is
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`exposed. A negative electrode (n-electrode) 102 is formed on the exposed upper surface of
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`n-type layer 104. A positive electrode (p—electrode) 106 is formed on an upper surface of
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`transparent electrode 108.
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`[0026]
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`In active layer 114, electrons and holes are recombined so as to generate and
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`emit light. The MQW structure of active layer 114 is formed by alternately laminating well
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`layers and barrier layers (not shown). The well layer includes a semiconductor layer with a
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`smaller band gap than n-type layer 104, p-type layer 116, and the barrier layer, thereby
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`providing quantum wells in which electrons and holes may be recombined.
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`[0027] Referring to Fig. 2, a portion of an LED 200 is shown. LED 200 includes n-type
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`layer 208, active layer 210, p—type layer 212 and transparent electrode 214. Fig. 2
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`illustrate the propagation of light generated from active layer 210. Light generated by
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`active layer 210 may propagate out of LED 200 as light ray 202 or light ray 204, for angles
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`less than the critical angle (relative to the surface normal direction). The remainder of light
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`rays 206 have an angle greater than the critical angle, and experience TIR.
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`In general,
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`most of the generated light inside of active layer 210 may be trapped inside LED 200 due to
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`total internal reflection, without escaping outside to be extracted. Accordingly, the
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`extraction efficiency of conventional LEDs tends to be poor (for example, about 40%).
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`[0028] One conventional technique to improve the extraction efficiency is related to ray
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`redirection using, for example, surface roughening, gratings and volume holograms to
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`circumvent TIR. However, these techniques tend to improve the extraction by no more
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`than about 60% from 40% (which is only a 50% increase in efficiency).
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`[0029]
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`In general, LED efficiency may include an internal quantum efficiency, an
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`extraction efficiency and an external quantum efficiency (also referred to herein as power
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`efficiency). The internal quantum efficiency, extraction efficiency and external quantum
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`efficiency may be defined by respective equations (1)-(3), below as:
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`internal quantum efliciency =
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`#of photons created inside LED per unit time
`'
`.
`_
`_
`#of electrons injected per unit time
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`#0
`hotons emitted outside LED er unit time
`extraction efliciency = f—p————_————p———
`#of photons created inSide LED per unit time
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`#0
`hotons emitted outside LED er unit time
`external quantum efliciency = L—‘—lyl—'T‘_— =
`#of electrons injected per unit time
`optical output power emitted outside LED
`'
`_
`.
`injected electrical power
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`power efliciency =
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`(1)
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`(2)
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`(3)
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`Because the external quantum efficiency (power efficiency) is the product of internal
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`quantum efficiency (eq. 1) and extraction efficiency (eq. 2), the external quantum efficiency
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`may be improved by improving either the internal quantum efficiency or the extraction
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`efficiency.
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`[0030] As shown in Fig. 2, light ray 202 is emitted from a top surface of transparent
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`electrode 214 and propagates out of LED 200. Light ray 204 also propagates from a lateral
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`surface of LED 200, such as through active layer 210 and/or p—type layer 212. According to
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`aspects of example embodiments of the present invention, exemplary light emitting unit
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`cells includes a plurality of light receiving diodes (LRDs) optically coupled to the LED to
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`absorb TIR light rays 206 from the LED. The LRDs may convert the absorbed light to a
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`photocurrent. The light emitting unit cell may use the photocurrent to power a further LED
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`or may feed the photocurrent back to the LED. Accordingly, by reusing the TIR light as an
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`applied photocurrent, the current-voltage (I—V) characteristics of the exemplary light
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`emitting unit cell may be improved, such that a lower current may be used to obtain a same
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`light output. Exemplary light emitting unit cells of the present invention may theoretically
`achieve as much as 80% extraction efficiency, resulting in a 100% increase in extraction
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`efficiency relative to a conventional LED.
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`[0031] Referring next to Fig. 3, a circuit diagram of an example light emitting unit cell
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`300 is shown. Light emitting unit cell 300 includes first LED 304 connected in parallel with
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`power source 302. Light emitting unit cell 300 also includes a plurality of light receiving
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`diodes (LRDs) 306 which are optically coupled to first LED 304 and electrically connected to
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`second LED 308.
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`[0032]
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`LRDs 306-1, 306-2, 306-3, 306-4 are connected to each other in series. Anode
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`p4 of LRD 306—4 is electrically connected to anode p5 of second LED 308. Cathode n1 of
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`LRD 306—1 is electrically connected to cathode n5 of second LED 308 and cathode no of first
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`LED 304.
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`[0033]
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`In operation, first LED 304 is powered by power source 302 and second LED 308
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`is powered by LRDs 306 (i.e., LRDs 306 supply current and voltage to second LED 308 for
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`light emission). LRDs 306 may absorb light trapped inside the layers of first LED 304 (i.e.,
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`due to TIR) and convert the absorbed light to a photocurrent. LRDs 306 may be configured
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`to absorb the TIR light without emitting light. Thus, each of LRDs 306 may act as a
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`photodiode. Accordingly, light emitting unit cell 300 may recycle photocurrent that would
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`be lost due to TIR and apply the photocurrent to power second LED 308.
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`[0034] Although four LRDs 306 are shown in Fig. 3, it is understood by the skilled person
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`that light emitting unit cell 300 may include two or more LRDs 306 configured to absorb TIR
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`light from first LED 304, to provide a suitable photocurrent for powering second LED 308.
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`Because second LED 308 may also lose a portion of the respective light emission due to TIR,
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`further LRDs 306’ and a third LED 308’ (shown in phantom) may also be included in light
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`emitting unit cell 300, coupled to second LED 308.
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`[0035] Although one light emitting unit cell 300 is shown in Fig 3, an LED chip (described
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`below with respect to Figs. SA-SG) may include a plurality of unit cells (such as described
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`below with respect to Fig. 13), which is referred to herein as a micro—pixelated LED.
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`[0036] Referring to Fig. 4, a top-plan view diagram of a structure 400 of light emitting
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`unit cell 300 is shown. Structure 400 illustrates the layout and electric connections of unit
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`cell 300 of an example LED chip. First LED 304 is powered by a system power source
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`through anode p0 and cathode n0. Cathode n0 is formed on a fabricated mesa—etched part
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`of an n-type layer (for example of GaN) and is connected to the anode side of the power
`source.
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`[0037] A first LRD, 306-1, is connected to first LED 304 at cathodes n0, n1. Cathode n1
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`is the n-side of LRD 306-1 which is the same as cathode n0. A p—side of LRD 306-1, at
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`anode p1, is connected to the n-side of LRD 306—2, at cathode n2, to form a series
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`connection. LRDs 306—2, 306-3, 306—4 are similarly connected to each other. Because a p—
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`type layer is formed as an upper layer and an n-type laser is formed as a lower layer, non-
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`planar contacts 402 are provided for serial connection of LRDS 306-1, 306-2, 306-3, 306-4.
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`[0038] The p-side of LRD 306-4, at anode p4, is in contact with the p-side of second LED
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`308, at anode p5. The n-side of LEDZ, at cathode n5, is connected to cathodes n0, n1.
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`Because the photocurrent received from LRDs 306 may be a fraction of the power supplied
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`to first LED 304, it may be desirable for second LED 308 to be formed with an area that is
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`smaller than first LED 304.
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`[0039] As shown in Fig. 4, LRDs 306 are formed proximate to first LED 304 such that
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`' LRDs 306 surround first LED 304, in order to receive TIR light from first LED 304.
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`In
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`general, LRDs 306 are formed from a same light emission structure (described below with
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`respect to Figs. 5A-SG) used to form first and second LEDs 304, 308. Because the LRD’s
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`306 are connected in series and the series connected LRD’s 306 are connected in parallel
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`with second LED 308, the potential across any of the LRD’s 306 cannot be sufficient to
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`cause the LRD to emit light. Thus, each of the LRDs acts as a photodiode, converting
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`received light into an electrical current.
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`[0040] Referring to Figs. 5A-SG, an exemplary method of forming light emitting chip 500
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`is shown. As shown in Fig. 5A, buffer layer 504, n-type semiconductor layer 506, active
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`layer 508 and p-type semiconductor layer 510 are sequentially grown on substrate 502, to
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`form light emission structure 501.
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`[0041] Substrate 502 may be formed of a transparent material such as sapphire (for
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`example with a (0001) plane orientation). Substrate 502 may be formed from other
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`materials including, but not limited to, silicon carbide (SiC), GaN or magnesium aluminum
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`oxide (MgAlOz).
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`[0042] Buffer layer 504 may be used to enhance a lattice matching between substrate
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`502 and n—type semiconductor layer 506. Buffer layer 504 may be omitted depending on a
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`process condition and diode characteristic. According to an exemplary embodiment, buffer
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`layer 504 may be formed from about a 10 nm thickness undoped GaN semiconductor layer
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`on a (0001) surface of sapphire substrate 502 for the lattice matching.
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`In addition to GaN,
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`buffer layer 504 may be formed from, but not limited to, (undoped) GaN, aluminum nitride
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`(AIN), aluminum gallium nitride (AlGaN) or aluminum indium nitride (AlInN).
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`[0043] N—type semiconductor layer 506 and p-type semiconductor layer 510 may be
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`formed of semiconductor materials including, but not limited to, AlmGa1-mN (for Osmsl), to
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`emit blue light. According to an example embodiment, n-type semiconductor layer 506 of
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`about 2 pm thickness may be grown from GaN semiconductor material doped with n-type
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`conductive impurities, such as silicon (Si) or germanium (Ge). According to an example
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`embodiment, p-type semiconductor layer 510 of about 200 nm thickness may be grown
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`from GaN semiconductor material doped with p-type conductive impurities, such as
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`magnesium (Mg), zinc (Zn) or beryllium (Be).
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`[0044] To produce light emitting chips for emitting other colors, n—type and p—type
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`semiconductor layers 506, 510 may be formed from different materials. For example, to
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`emit blue light: ZnSe or indium gallium nitride (InGaN) may be used. To emit green light:
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`InGaN, GaP, aluminum gallium indium phosphide (AlGaInP) or aluminum gallium phosphide
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`(AlGaP) may be used. To emit yellow light: gallium arsenide phosphide (GaAsP), AlGaInP or
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`GaP may be used. To emit orange light: GaAsP, AlGaInP or GaP may be used. To emit red
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`light: aluminum gallium arsenide (AlGaAs), GaAsP, AlGaInP or GaP may be used.
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`[0045] Active layer 508 may include a single quantum well (SQW) structure or a MQW
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`structure. According to an exemplary embodiment, active layer 508 of about 50 nm total
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`thickness may be formed from alternating layers of InGaN/GaN. Active layer 508 may be
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`formed of semiconductor material including, but not limited to, InmAlnGa1_m_nN (for O<msl,
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`Osnsl, 0<m+nsl) or Iana1-mN (for O<m<1). Active layer 508 may be omitted depending
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`on a desired process condition and a desired diode characteristic. According to another
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`embodiment, active layer 508 may be omitted for portions of the light emitting structure
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`corresponding to the LEDs or to the LRDs. According to a further embodiment, light
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`emitting structure 501 may be formed with different active layer materials for the portions
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`corresponding to the respective LEDs and LRDs.
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`[0046] Buffer layer 504, n-type semiconductor layer 506, active layer 508 and p-type
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`semiconductor layer 510 may be grown by using any suitable deposition process, including,
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`but not limited to, metal organic chemical deposition (MOCVD) or molecular beam epitaxy
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`(MBE).
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`[0047] As shown in Fig. SB, after forming light emitting structure 501, insulating pattern
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`512 may be formed on p-type semiconductor layer 510, for example, by any suitable
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`photolithographic technique.
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`[0048] According to an exemplary embodiment, a silicon dioxide (SiOz) film (not shown)
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`may be deposited on the p-type semiconductor layer 510 layer, for example, by chemical
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`vapor deposition (CVD). A photoresist (not shown) may be subsequently spun on the SiOz
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`film. A binary chromium(Cr) mask with a desired insulating pattern may be applied for
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`patterning the photoresist. The photoresist may be exposed to ultraviolet (UV) light by a
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`mask aligner or stepper and may be subsequently developed by a developer. The SiOz film
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`may be etched through the photoresist by, for example, reactive ion etching (RIE) with
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`insulating pattern 512.
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`[0049] The patterned SiOz film may be used as a mask for a GaN full etching process.
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`All layers of light emitting structure 501 may be etched via the SiOz mask through to
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`substrate 502. The SiOz film may be subsequently removed after the etching process is
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`completed.
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`[0050] As shown in Fig. 5C, a GaN mesa-etching process is performed. According to an
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`exemplary embodiment, a similar photolithographic method as described for the full etching
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`process (shown in Fig. SB) may be used for n-GaN mesa-etching. For example, a thickness
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`of about 0.55 um may be etched by RIE from a top of n-type semiconductor layer 506 on
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`predetermined portions. The n-GaN mesa-etching may be performed to securely supply
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`free electrons from n-type layer 506 to p-type layer 510 through the interface between the
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`two layers.
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`[0051] The patterns for n-GaN mesa-etching may include: an entire n—electrode portion
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`514, a portion 516 of a first LED portion 520, and a portion 518 of each LRD portion 522.
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`On each portion (514, 516, 518), a top 0.55 pm thickness may be etched away such that
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`the n-type layer 506 is partially exposed.
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`[0052] As shown in Fig. 5D, a transparent electrode layer 528 may be deposited by any
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`suitable process such as sputtering, CVD and evaporation. According to an exemplary
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`embodiment, transparent electrode layer 528 is formed with a thickness of about 200 nm.
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`Transparent electrode layer 528, may be formed from, for example, indium tin oxide (ITO),
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`titanium (Ti), gold (Au), a combination of Ti and Au, tin oxide (SnO) or zinc oxide (ZnO).
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`Transparent electrode layer 528 may be deposited and patterned on predetermined portions
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`by any suitable photolithography process.
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`[0053] Transparent electrode layer 528 may be deposited to cover p-electrode portion
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`526, first LED portion 520 (except for the mesa-etched portion 516), LRD portion 522
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`(except for mesa-etched portion 518), and second LED portion 524. Transparent electrode
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`layer 528 is desirably formed to be substantially transparent to light having a
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`predetermined wavelength. Transparent electrode layer 528 may be formed so that light
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`escaping from the underlying layer (p-type layer 510) may be effectively extracted and so
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`that electrons are spread over transparent electrode layer 528 from p-type layer 510 to p-
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`electrode portion 526. P-electrode portion 526 may also be formed from any suitable
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`PATENT
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`metallic materials, such as Au, copper (Cu), a combination of platinum (Pt) and Au, a
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`combination of nickel (Ni) and Au or a combination of chromium (Cr) and Au, for example
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`by sputtering, CVD and evaporation.
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`[0054] As shown in Fig. 5E, n-electrode portion 514 may be formed, for example, by any
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`suitable photolithography process on the mesa—etched part of n-type layer 506, to provide
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`an ohmic contact. According to an exemplary embodiment, n—electrode portion 514 is
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`formed with a thickness of about 20 nm. The ohmic contact desirably includes a linear I-V
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`curve and low resistance. N-electrode portion 514 may be formed from any suitable
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`metallic materials, such as Au, Cu, a combination of Pt and Au or a combination of Ni and
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`Au, for example by sputtering, CVD and evaporation. Alternatively, ITO may be used as n-
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`electrode portion 514, in which case the patterning of n-electrode portion 514 may be
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`performed together with the transparent electrode patterning (Fig. 5D).
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`[0055] As shown in Fig. 5F, passivation layer 530 may be formed to provide insulation
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`between n-type layer 506 and p-type layer 510 at the series connection of LRD portions
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`522. Passivation layer 530 may help to prevent an electrical short circuit when bridge
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`contact layer 532 (Fig. SG) is formed between p-type layer 510 and an adjacent mesa-
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`etched part of n-type layer 506. Passivation layer 530 may be formed, for example, from
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`SiOz by a sputtering, CVD and/or an evaporation process.
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`[0056] As shown in Fig. 5G, bridge contact layer 532 is then formed to provide an
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`electrical connection between p—type layer 510 on one side of LRD portion 522 and the
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`mesa-etched n—type layer 506 (portion 518) on a side of an adjacent LRD portion 522, thus
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`forming light emitting chip 500. Any metallic material, such as Au, Cu or Ni, may be used
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`for bridge contact layers 532. Bridge contact layer 532 may be formed by any suitable
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`process such as electroless/electro plating or sputtering and CVD.
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`[0057] Referring to Fig. 6, cross—section diagram along lines 6-6’ of light emitting chip
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`500 is shown. Bridge contact layer 532 connects the mesa-etched n-type layer 506 on one
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`side of one LRD 522 to a p-type layer 510 of an adjacent LRD 522. Bridge contact layer 532
`
`is formed over passivation layer 530. Accordingly, photo-electrons created in one LRD 522
`
`travel from n-type layer 506 to a p-type layer 510 of an adjacent LRD 522’ through bridge
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`contact layer 532.
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`MATB-435US
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`PATENT
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`[0058] Next, a theoretical extraction efficiency (new) for light emitting unit cell 300 (Fig.
`
`3) is compared with a conventional extraction efficiency (no) for number m of LEDs 304,
`
`308 (where m is an integer). For m LEDs 304, 308, extraction efficiency next. is shown in
`
`equation (4) as:
`
`nextr = 770 +770a(1_770)+”'770am—1(1_770)
`
`m—l
`
`1—a’”(1—770)"'
`1—a(1—770)
`
`77"
`
`(4)
`
`where the conversion efficiency (on) is the respective efficiencies for converting light to a
`
`photocurrent within LRDs 306.
`
`[0059] A summary of theoretical extraction efficiencies under different conversion
`
`efficiencies (oc=0.8 and 0L=1.0) and a different number of LEDs 304, 308 (m=2, m=3) are
`
`shown in Table 1 below. Referring to Figs. 7A and 7B, graphs are shown which
`
`summarizing the theoretical extraction efficiencies of Table 1.
`
`In particular, Fig. 7A shows
`
`the extraction efficiencies for oc=0.8 and Fig. 7B shows the extraction efficiencies for 0L=1.0.
`
`As shown in Figs. 7A and 7B and Table 1, the extraction efficiencies are improved by a ratio
`
`between about 1.4-2.3 as compared with the conventional extraction efficiency.
`
`In general,
`
`the extraction efficiency increases with an increasing number of LEDs 304, 308.
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`

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`MATB-435US
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`PATENT
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`
`Table 1 Theoretical Extraction Efficienc for Li ht Emittin Unit Cell 300
`no
`Tlextr
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`
`
`
`
`
`
`
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`[0060] Referring next to Fig. 8, a circuit diagram of an exemplary light emitting unit cell
`
`800 is shown. Light emitting unit cell 800 includes LED 804 connected in parallel with
`
`power source 802. Light emitting unit cell 800 also includes a plurality of light receiving
`
`diodes (LRDs) 806 which are optically coupled to LED 804 and are electrically connected in
`
`parallel with LED 804.
`
`[0061]
`
`LRDs 806-1, 806-2, 806-3, 806-4 are connected to each other in series. Anode
`
`p4 of LRD 806-4 is electrically connected to anode p0 of LED 804. Cathode n1 of LRD 806-1
`
`is electrically connected to cathode no of LED 804. Because LRDs 806 are serially
`
`connected, each LRD 806 is powered by a fraction of the received voltage (based on the
`
`number of LRDs 806). Consequently, none of the LRD’s can emit light.
`
`Instead, each of the
`
`LRD’s 806 operates as a photodiode.
`
`[0062]
`
`In operation, LED 804 is powered by power source 802. LRDs 806 may absorb
`
`light trapped inside the layers of LED 804 (i.e., due to TIR) and convert the absorbed light
`
`to a photocurrent. The photocurrent generated in LRDs 806 is fed back to LED 804, to
`
`

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`MATB-435US
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`PATENT
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`supply a photocurrent to light emitting unit cell 800. Because LRDs 806 are serially
`
`connected (and receive a fraction of the voltage), LRDs 806 may be configured to absorb
`
`the TIR light without emitting light. Accordingly, light emitting unit cell 800 may recycle
`
`photocurrent that would be lost due to TIR and apply the photocurrent to further power light
`
`emitting unit cell 800.
`
`[0063] Although four LRDs 806 are shown in Fig. 8, it is understood by the skilled person
`
`that light emitting unit cell 800 may include two or more LRDs 806 configured to absorb TIR
`
`light from LED 804, to feed back a suitable photocurrent to LED 804.
`
`[0064] Although one light emitting unit cell 800 is shown in Fig 8, an LED chip (described
`
`below with respect to Figs. 10A-10F) may include a plurality of unit cells (such as described
`
`below with respect to Fig. 13), which is referred to herein as a micro-pixelated LED.
`
`[0065]
`
`Fig. 9, shows a top-plan view diagram of a structure 900 of light emitting unit cell
`
`800. Structure 900 illustrates the layout and electric connections of unit cell 800 of an
`
`example LED chip. LED 804 is powered by a system power source through anode p0 and
`
`cathode n0. Cathode n0 is formed on a fabricated mesa—etched part of an n-type layer (for
`
`example of GaN) and is connected to the anode side of the power source.
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`[0066] A first LRD, 806-1, is connected to LED 804 at respective cathodes n0, n1. A p-
`
`side of LRD 806—1, at anode p1, is connected to the n-side of LRD 806-2, at cathode n2, to
`
`form a series connection. LRDs 806—2, 806-3, 806-4 are similarly connected to each other.
`
`Because a p-type layer is formed as an upper layer and an n-type laser is formed as a lower
`
`layer, non—planar contact 902 is provided for serial connection of LRDs 806-1, 806-2, 806-3,
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`806-4.
`
`[0067] The p-side of LRD 806-4, at anode p4, is in contact with the p-side of LED 804, at
`
`anode p0, and a cathode of the system power source. The n—side of LRD 806—1, at cathode
`
`n1, is also connected with an anode of the system power source.
`
`[0068] As shown in Fig. 9, LRDs 806 are formed proximate to LED 804 such that LRDs
`
`806 surround LED 804, in order to receive TIR light from LED 804.
`
`In general, LRDs 806
`
`are formed from a same light emission structure (described below with respect to Figs. 10A—
`
`10F) used to form LED 804.
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`MATB-435US
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`PATENT
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`[0069] Referring to Figs. 5A and 10A-10F, an exemplary method of forming light
`
`emitting chip 1000 (Fig. 10F) is shown.
`
`In general, light emitting chip 1000 may be formed
`
`by a process similar to light emitting chip 500, the details of which are described above.
`
`[0070] Light emitting chip 1000 may be formed from the same light emission structure
`
`501 described above with light emitting chip 500. As described with respect to Fig. 5A,
`
`buffer layer 504, n-type semiconductor layer 506, active layer 508 and p—type
`
`semiconductor layer 510 are sequentially grown on substrate 502, to form light emission
`
`structure 501.
`
`[0071] As shown in Fig. 10A, after forming light emitting structure 501, insulating
`
`pattern 1002 may be formed on p-type semiconductor layer 510, for example, by any
`
`suitable photolithographic technique. For example, as described above, a patterned SiOz
`
`film may be used as a mask for a GaN full etching process. The SiOz film may be etched
`
`through the photoresist with insulating pattern 1002. All layers of light emitting structure
`
`501 may be etched via the SiOz mask through to substrate 502. The SiOz film may be
`
`subsequently removed after the etching process is completed.
`
`[0072] As shown in Fig. 10B, a GaN mesa-etching process is performed. According to an
`
`exemplary embodiment, a similar photolithographic method as described for the full etching
`
`process (shown in Fig. 10A) may be used for n—GaN mesa-etching. The n-GaN mesa—
`
`etching may be performed to securely supply free electrons from n-type layer 506 to p-type
`
`layer 510 through the interface between the two layers. The patterns for n-GaN mesa-
`
`etching may include: an entire n-electrode portion 1004, a portion 1006 of LED portion
`
`1010, and a portion 1008 of each LRD portion 1012. On each portion (1004, 1006, 1008),
`
`a top 0.55 pm thickness, for example, may be etched away such that the n-