Abstract:
A micro-size LED-like optical element has an n-contact, a p-contact, and an optical active structure connected between the n-contact and the p-contact for generating light when forward biased and for detecting light when reverse biased. The optical active structure has a diameter of about 20 μm or smaller. When the the optical active structure is forward biased, it forms a micro-size LED (μLED). When the optical active structure is reverse biased, it forms a micro-size detector (μdetector). An array of the micro-size optical active structures may be used as a minidisplay, a detector, or a sensor (when each structure is separately wired), or as a hyperbright LED (when the structures are wired to turn on and off simultaneously). Alternatively, a hyperbright LED may be obtained by forming a plurality of micro-size holes extending into an LED wafer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to micro-size light emitting diodes (LEDs) and detectors and their arrays for minidisplay, UV detector arrays, imaging sensors, and hyper-bright LEDs and lighting applications. 
     2. Description of the Prior Art 
     Currently, most conventional broad area LEDs operating in the wavelength region from 530-800 nm are based on the older GaP/GaAs technologies. However, GaP/GaAs based LEDs are not especially bright below 600 nm and their emission wavelengths are too long to excite phosphors for down-conversion to make white light. Current commercially available broad area LEDs based on a new class of semiconductor materials (i.e. III-nitrides) can operate in the spectral region from green, blue to violet. The III-nitride technology may eventually be extended into the red covering the entire visible spectrum. To date, most III-nitride based LEDs are fabricated from quantum well and heterostructures of InGaN/GaN, In x Ga 1-x N/In x′ Ga 1-x′ N (x not equal to x′), and GaN/AlGaN. Blue-green LEDs and laser diodes use InGaN as an active medium, by taking advantage of heterojunctions and quantum wells (QW), and the tunability of the band gap in the alloys from InN (1.9 eV, 652 nm) to GaN (3.4 eV, 365 nm). 
     The conventional LED sizes are typically larger than 200 μm by 200 μm (300 μm by 300 μm for III-nitride LEDs in particular). The efficiencies of these conventional broad area LEDs can be further improved. Moreover, these conventional broad area LEDs are not suited to minidisplays. 
     A need remains in the art for micro-size light emitting diodes (LEDs) for minidisplay, hyper-bright LED, and lighting applications. A need also remains in the art for micro-size detectors for use in detector arrays and imaging sensors with high spatial resolutions and UV sensitivities. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide micro-size light emitting diodes (μLEDs) and μLED arrays for hyper-bright LED and lighting applications, and for minidisplays. Another object is to provide micro-size detector arrays for use in high spatial resolution detector arrays and imaging sensors. 
     A conventional broad area LED is replaced with many micro-size LEDs (μLEDs) connected in a manner that they are turned on and off simultaneously for hyper-bright LED and lighting applications. For example, an array of many of these μLEDs fits into the same area taken up by a conventional broad area LED. The output power of these μLEDs are enhanced over the conventional broad area LEDs. The enhanced quantum efficiency in μLEDs is due to the increased light extraction efficiencies. Additionally, an enhanced quantum efficiency in μLEDs is also an inherent attribute due to micro-size effects as well as a more efficient usage of injected current. Furthermore, strain induced by lattice mismatch between the well and barrier materials (e.g., InGaN and GaN) is partially relieved as the lateral size of the LEDs decreases, which tends to increase the radiative recombination efficiencies in μLEDs. Therefore, replacing a conventional broad-area LED with a micro-size LED array can enhance the total light output for a fixed luminous area. 
     Many micro-size holes (regular or irregular shapes) drilled into a conventional broad area LED (termed as inverted μLEDs hereafter) are also employed to increase the extraction efficiencies of the LEDs. 
     Many μLEDs are connected in a manner that they are addressed individually for minidisplay applications. When these micro-size arrays are reverse biased, they form detector arrays as imaging sensors with high spatial resolutions. Hereafter, the reverse biased micro-sized structures (which are μLEDs when forward biased) are termed μdetectors. 
     The present invention for improving the brightness of LEDs is applicable to semiconductor LEDs, polymer LEDs, as well as others such as organic LEDs. III-nitrides will be used as a specific example throughout the text. For devices based on III-nitrides, III-nitride quantum wells (QWs) or heterostructures are employed as an active media. Such active media include QW and heterostructures of InGaN/GaN, In x Ga 1-x N/In x′ Ga 1-x′ N (x not equal to x′), GaN/AlGaN, Al x Ga 1-x N/Al x′ Ga 1-x′ N, and lattice matched QWs and heterostructures such as In x Al y Ga 1-x-y N/In x′ Al y′ Ga 1-x′-y′ N (x not equal to x′, y not equal to y′) and GaN/In x Al y Ga 1-x-y N. Enhanced efficiency in the UV region will be very important for the realization of commercially viable white LEDs that currently utilize UV light excitation of phosphor materials for down-conversion to make white light. In particular, ultraviolet (UV) LEDs based on III-nitrides is currently used to generate white light by coating the chips with phosphors. Phosphors down-convert part of the shorter wavelength UV light to a longer wavelength visible light. Through color mixing, the eye sees white when two colors are properly balanced. In such an application area, the generation of highly efficient UV photons based on the present invention is particularly beneficial. 
     Minidisplays and imaging sensors based on III-nitrides according to the present invention could be especially useful for full color minidisplays, UV/solar blind detection, medical imaging, etc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 ( a ) is a side perspective showing the structure of a first embodiment of several III-nitride micro-size LEDs (μLEDs) or  5  micro-sized detectors (μdetectors) according to the present invention. FIG.  1 ( b ) is a top view of a fabricated array of the μLEDs/μdetectors of FIG.  1 ( a ). 
     FIGS.  2 ( a )-( d ) are side perspectives showing structures of several μLEDs/μdetectors based on generic LED wafers according to the present invention. 
     FIG.  3 ( a ) shows a top view of one of the working mask patterns for creating a interconnected μLED/μdetector array comprising a few hundred μLEDs/μdetectors by photolithography and etching. 
     FIG.  3 ( b ) shows a top view of a mask pattern for deposition of metallic wires that interconnect a few hundred of the μLEDs/μdetectors in a manner to permit turning all μLEDs/μdetectors on and off simultaneously. 
     FIG.  4 ( a ) shows the top view of a fabricated μLED array comprising a few hundred of the III-nitride μLEDs of FIG. 1 that fit into the same area as a conventional broad area LED. 
     FIG.  4 ( b ) (prior art) shows a top view of a fabricated conventional broad area III-nitride LED that has the same luminous area as the μLED array of FIG.  4 ( a ). 
     FIG. 5 is a side perspective showing one of the structures of a second embodiment of an array of inverted μLEDs based on III-nitrides according to the present invention. 
     FIG. 6 is a plot showing light output power versus input power for a conventional broad area LED, for a μLED array with individual μLEDs of 6 μm in diameter, and for a μLED array with individual μLEDs of 9 μm in diameter. 
     FIGS. 7 a  and  7   b  show a top view of an interconnected array comprising many of the μLEDs/μdetectors of FIG. 1 a  in a manner to permit turning each μLED on and off individually, or sensing the output of each μdetector separately. 
     FIG. 7 c  shows a top view of an actual fabricated array similar to the one shown in FIGS. 7 a  and  7   b.   
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG.  1 ( a ) is a side perspective showing the structure of several III-nitride micro-size LEDs (μLEDs) or micro-size detectors (μdetectors)  102  according to a first embodiment of the present invention. Note that whether structures  102  comprise μLEDs or μdetectors depends upon how the structures are biased: forward biased structures  102  comprise μLEDs, and reverse biased structures  102  are light detectors, here called μdetectors. Each μLED/μdetector is approximately 5-20 μm in diameter. The optical active media are inside structures  102 , and no optical active media is present between structures  102 . Photolithography and dry etching/or chemical wet etching is used to pattern arrays of structures  102  of desired diameters and spacings. The LED wafers are etched into the underneath n-type layer so that no active optical media material is present between structures  102 . 
     As an illustration, a III-nitride semiconductor LED wafer may comprise a III-nitride buffer layer  114  formed on an Al 2 O 3  or Si or SiC substrate  116 . An n-type GaN epilayer  112  is formed on the buffer layer  114 . A multiple quantum wells (MQWs) structure  110  comprising alternating layers of In x Ga 1-x N and GaN; In x Ga 1-x N and In x′ Ga 1-x′ N (x is not equal to x′); In x Ga 1-x N and In x Al y Ga 1-x-y N or a single epitaxial layer of In x Ga 1-x N is formed next as optical active media, followed by a p-type layer  108  of In x Al y Ga 1-x-y N or GaN or AlGaN. Those skilled in the art will appreciate other layer structures based on the same principle, such as adding more III-nitride layers to the structures. 
     N-type Ohmic contact  106  is then deposited on the underneath exposed n-type layer  112  (this is necessary when the substrate is insulating) and p-type Ohmic contact  104  is formed on the top p-type layer  108 . 
     FIG.  1 ( b ) is a top view of a fabricated array of the μLEDs/μdetectors  102  of FIG.  1 ( a ). In this particular example, structures  102  are about 12 μm in diameter and p-type contacts  104  are about 8 μm in diameter. 
     FIGS.  2 ( a )-( d ) are side perspectives showing examples of generic structures of several μLEDs/μdetectors  202  fabricated from various LED wafers according to various embodiments of the present invention. The LED wafers might comprise any of the LED wafers, including those made of semiconductors, polymers and organic materials. Those skilled in the art will be able to apply this structure to any type of LED wafer. 
     Depending on the nature of the substrate, the n-contact can be formed above the exposed underneath n-type layer when the substrate is insulating as shown in FIG.  2 ( a ); the p-contact can be formed above the exposed underneath p-type layer when the substrate is insulating as shown in FIG.  2 ( b ); the n-contact can be formed below the n-type substrate if the substrate is n-type as shown in FIG.  2 ( c ); or the p-contact can be formed below the p-type substrate if the substrate is p-type as shown in FIG.  2 ( d ). 
     FIG.  3 ( a ) shows a top view of a working mask pattern  300   a . This mask is used to pattern an array comprising many of the interconnected μLEDs/μdetectors by photolithography and etching as well as for deposition of p-type Ohmic contacts  104 . The shaded regions  302  are those of μLEDs/μdetectors, while the white regions  304  are etched holes between μLEDs/μdetectors. The mask shown in FIG. 3 a  is approximately 300 μm by 300 μm, so that many μLEDs/μdetectors fit into the same area taken up by a conventional broad area LED. 
     FIG. 3 b  shows a top view of a second mask pattern  300   b  that allows the deposition of metallic wires that interconnect many of the μLEDs/μdetectors patterned by the mask  300   a  through the P-type Ohmic contacts  104 . This array of μLEDs/μdetectors is useful for hyper-bright LEDs and lighting applications, since the efficiency of the μLED array is significantly enhanced over the conventional broad area LEDs. One of the corners  306  (of about 100×100 μm) is to allow the removal of materials above the n-type layer by etching as well as the deposition of n-type Ohmic contact  106  on the underneath exposed n-type layer  112  for current injection (this is needed for the case when the substrate is insulating such as that in FIG. 2 a ). The other corner ( 308 ) (of about 100×100 μm) is to allow the deposition of p-type contact pad on the top, which connects the p-type Ohmic contacts  104  for current injection. 
     FIG.  4 ( a ) shows the top view of a fabricated array  400   a  comprising a plurality of the III-nitride μLEDs that fit into the same area as a conventional broad area LED according to the present invention. FIG.  4 ( b ) (prior art) shows a top view of a fabricated conventional broad area III-nitride LED  400   b  that has the same area as the array of μLEDs  400   a.    
     FIG. 5 is a side perspective showing the structure of a μLED array comprising several inverted μLEDs  500  according to the present invention. This structure comprises a series of micro-size holes drilled into a conventional broad area LED. The holes are drilled all the way to the substrate. The array of inverted μLEDs is useful for hyper-bright LEDs and lighting applications, since the efficiency of the inverted μLED array is significantly enhanced over a conventional broad area LED. Inverted μLEDs  500  cannot be used for minidisplays, however, because they cannot be individually addressed. 
     FIG. 6 is a plot showing light output power versus input power for a conventional broad area LED  400   b  and for two μLED arrays  400   a . The two μLED arrays occupy the same area as the conventional broad area LED and each comprise a few hundred μLEDs. Curve  602  shows the results for an array of 6 μm diameter μLEDs. Curve  604  shows the results for an array of 9 μm diameter μLEDs. Curve  606  shows the results for a single conventional broad area LED having the same luminous area as well as the same optical active media as the array of 6 μm diameter μLEDs of curve  602 , and the array of 9 μm diameter μLEDs of curve  604 . Note that significantly more light is produced by the μLED arrays over the conventional broad area LED. 
     FIGS. 7 a  and  7   b  show a top view of an interconnected array  700  comprising many of the μLEDs  102  of FIG.  1 ( a ) connected in a manner to permit turning each μLED (or pixel in this case) on and off individually (in the case where the elements are forward biased) or to permit detecting light at each μdetector/pixel (in the case where the elements are reversed biased). FIG. 7 a  shows the n-type layer  700   a  of array  700 , and FIG. 7 b  shows the p-type layer  700   b  and the insulating layer  702 . FIG. 7 c  shows a top view of an actual fabricated array  700  similar to the one shown in FIGS. 7 a  and  7   b.    
     Layer  702  is an insulating layer deposited above the exposed underneath n-type layer  114 . Insulating layer  702  is to prevent the current leakage between the n-type and p-type layers. Grid  704  is the n-type Ohmic contact. Conducting wires  706  make the connection between the n-type Ohmic contacts  704  and the contact pads  708  which are used for current injection into n-type Ohmic contact (needed only when the substrate is insulating). 
     Conducting wires  710  make the connections between individual μLEDs through the p-type Ohmic contacts  104  and the μLED control pads  712  which are used for current injection into p-type Ohmic contacts. Each μLED has its own control pad. In this array, the state of the μLEDs  102  is individually controlled. 
     When forward biased, the n-type and p-type ohmic contact and pads are used for current injection into individual pixels. Arrays such as  700  are useful in areas such as head wearing displays, minidisplays, emitters for remote free space functions, short distance optical communication, and optical interconnects. 
     When reverse biased, the n-type and p-type ohmic contacts and pads are used for collecting photocurrent from individual pixels. In this case, array  700  comprises a plurality of μdetectors for detecting light. μdetector array  700  is useful for imaging sensors and detector arrays operating from visible to UV. All these devices have important applications in satellite communications, astronomical imaging, missile detection, medical imaging and minidisplays, etc. 
     The principles discussed above in conjunction with in FIG. 7 apply to arrays comprising many more μLEDs/μdetectors. For an array comprising a vast number of μLEDs/μdetectors (or pixels), pixels are controlled by current injection through a matrix of rows and columns of μLEDs/μdetectors. In this case, each pixel is designated by a two-integer format (i,j). The first integer i designates the row number of the matrix and the second integer j designates the column number of the matrix (equivalently, rows and columns could be interchanged). μLEDs/μdetectors in the same row are connected through the n-type underneath layer, while cross talk between different rows is precluded by employing etching isolation. (For example, for III-nitride μLED/μdetector arrays grown on a sapphire substrate, one can etch into the sapphire substrate so that no III-nitride material is present between different rows). All LEDs/μdetectors in the same column are connected through the top p-type Ohmic contacts, while cross talk between different columns is precluded by depositing an insulating layer  702 . In such a manner, the state of each pixel can be individually controlled. 
     The array can also be bounded to an electronic readout array which is fabricated on crystalline silicon using the complimentary metal oxide semiconductor (CMOS) fabrication methods commonly employed in integrate circuit manufacturing. The CMOS readout array would provide pixel selection, signal amplification, pixel reset, etc. 
     While the exemplary preferred embodiments of the present invention are described herein with particularity, those skilled in the art will appreciate various changes, additions, and applications other than those specifically mentioned, which are within the spirit of this invention.