Monolithic series/parallel led arrays formed on highly resistive substrates

[0002] Conventional light emitting diode (LED) materials, such as GaAs, have allowed the construction of only single junction or multiple parallel junction devices when fabricated monolithically. FIG. 1A illustrates a typical multiple parallel junction LED array 10. Several p-type regions 13 are grown over a common n-type region 18. N-contact 11 connects to n-type region 18 and several p-contacts 14 connect to p-type regions 13. The device is fabricating by forming n-type region 18 on a substrate 12, then forming a continuous p-type layer over the n-type region. The p-type layer is then divided into discrete regions by mechanically sawing or chemically etching trenches 15 between p-type regions 13. FIG. 1B illustrates another multiple parallel junction LED array 16. Instead of mechanical sawing or chemical etching, p-type regions 13 are electrically isolated from each other by diffusion. The monolithic arrays illustrated in FIGS. 1A and 1B are limited to the parallel configuration illustrated in FIG. 2 because the use of contacts on opposite sides of the device requires one common conductive layer, i.e., n- or p-layer.

[0003] In accordance with the present invention, a series or parallel LED array is formed on an insulating or highly resistive substrate, such that both the p- and n-contacts for the array are on the same side of the array. The individual LEDs are electrically isolated from each other by trenches or by ion implantation. Interconnects deposited on the array connect the contacts of the individual LEDs in the arrays. In some embodiments, the LEDs are III-nitride devices formed on sapphire substrates. In one embodiment, the III-nitride devices are formed on high-resistance SiC or III-nitride substrates. In one embodiment, two LEDs formed on a single substrate are connected in antiparallel to form a monolithic electrostatic discharge protection circuit. In one embodiment, multiple LEDs formed on a single substrate are connected in series. The series array can operate at a higher voltage than a single LED of the same area, thus simplifying power supply design. In one embodiment, multiple LEDs formed on a single substrate are connected in parallel. In this embodiment, multiple p-type regions are formed on a single n-type region, such that the n-type region surrounds each of the p-type regions and interposes the p-type regions. In some embodiments, a layer of phosphor covers a portion of the substrate on which one or more individual LEDs are formed.

[0004] FIGS. 1A and 1B illustrate typical multiple parallel junction LED arrays.

[0005] FIG. 2 illustrates a circuit diagram of a parallel LED array.

[0006] FIGS. 3-6 illustrate an embodiment of a series LED array according to the present invention at various stages in fabrication.

[0007] FIGS. 7A and 7B illustrate two embodiments of series LED arrays.

[0008] FIG. 8A illustrates a plan view of an embodiment of a series LED array.

[0009] FIG. 8B illustrates a circuit diagram of a series LED array.

[0010] FIG. 9A illustrates a plan view of a series/parallel LED array.

[0011] FIG. 9B illustrates a circuit diagram of a series/parallel LED array.

[0012] FIG. 10 illustrates a circuit diagram of a pair of antiparallel LEDs.

[0013] FIG. 11 illustrates a plan view of an embodiment of a monolithic ESD protection structure.

[0014] FIG. 12 illustrates a cross section of the structure shown in FIG. 11.

[0015] FIG. 13 illustrates a cross section of a parallel LED array.

[0016] FIG. 14 illustrates a plan view of the structure shown in FIG. 13.

[0017] FIG. 15 illustrates a monolithic LED array with phosphor covering one of the individual LEDs.

[0018] Materials systems currently of interest in the manufacture of high-brightness light emitting diodes (LEDs) capable of operation across the visible spectrum are Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. The III-nitride semiconductor layers referred to herein are compounds represented by the general formula Al_xGa_yIn_1-x-yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1), which may further contain group III elements such as boron and thallium and in which some of the nitrogen may be replaced by phosphorus, arsenic, or antimony. Typically, III-nitride devices are epitaxially grown on sapphire, silicon carbide, or gallium nitride substrates by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. Sapphire substrates are often used because of their wide availability and ease of use. Sapphire is an insulator. application Ser. No. 09/469,657, entitled “III-Nitride Light Emitting Device with Increased Light Generating Capability,” filed Dec. 22, 1999 on an invention of Krames et al., and incorporated herein by reference, discloses III-nitride light-emitting devices grown onto high-refractive-index substrates which have low optical absorption. These substrates may be SiC or III-nitride materials and have high electrical resistance due to the low impurity content. III-nitride devices grown on insulating or highly resistive substrates must have both the positive and the negative polarity electrical contacts to the epitaxially-grown semiconductor on the same side of the device. In contrast, semiconductor devices grown on conducting substrates such as those shown in FIGS. 1A and 1B are typically fabricated such that one electrical contact is formed on the epitaxially grown material and the other electrical contact is formed on the substrate.

[0019] With the use of insulating or highly resistive substrates, III-nitride monolithic LED arrays may be fabricated by forming trenches or ion implantation regions between the individual LEDs to electrically isolate the individual LEDs. FIGS. 3-7 illustrate the fabrication of a III-nitride monolithic LED array in accordance with one embodiment of the present invention. In FIG. 3, an n-type layer 22 of, for example, GaN doped with Si, Ge, or O is formed overlying highly resistive substrate 20. An active layer 23 of, for example, InGaN is then formed overlying n-type layer 22, and finally a p-type layer 24 of, for example, AlGaN doped with Zn, Mg, Be, Ca, or Cd is formed overlying the active layer. Layers 22, 23, and 24 may actually contain several sublayers of different composition and dopant concentration which are omitted for clarity. For example, n-type layer 22 may include, a nucleation layer, a highly resistive GaN layer (e.g. a GaN layer that is non-intentionally doped), and a lightly n-doped layer, then a more heavily doped n-layer. Active layer 23 may be, for example, a multiple quantum well structure.

[0020] In FIG. 4, a portion of the n-type layer 22, the active layer 23 and p-type layer 24 is etched away to form a trench 26. The etch used may be, for example, a reactive ion etch with a chlorine-containing etchant gas such as BCl₃. Trench 26 is wide enough to electrically isolate the semiconductor layers on either side of the trench. Trench 26 is etched down to the substrate or to a highly resistive layer underlying n-type layer 22, such as a non-intentionally doped GaN layer. Similarly, the adjacent LEDs may be electrically isolated by use of an ion implantation process which can render the intervening material to be highly resistive. A portion of p-type layer 24 and active layer 23 of each remaining island of semiconductor material is then etched away as illustrated in FIG. 5 using, for example, a reactive ion etch. The second etch exposes ledges 28 on n-type layer 22, where n-type contacts are eventually formed.

[0021] Turning now to FIG. 6, the ledges for n-contact formation are electrically isolated by depositing a dielectric material 30 over the device. The dielectric is then patterned and removed in places to open contact holes on n-type layer 22 and p-type layer 24, such that dielectric 30 is left in trench 26 between the individual LEDs on the substrate and on the mesa walls between the exposed p-type layer and n-type layer of each LED. Dielectric 30 may be, for example, oxides of silicon, nitrides of silicon, oxynitrides of silicon, aluminum oxide, or any other suitable dielectric material.

[0022] FIGS. 7A and 7B show two examples of completed series LED arrays. FIG. 7A illustrates a device where the LEDs in the array are separated by trenches. FIG. 7B illustrates a device where the LEDs in the array are separated by ion implantation regions 301. Electrode materials are deposited and patterned to form p- and n-contacts 32. Typical contact materials are Al or Ti—Al for the n-contact and Ag, Au, Ni, Pt, or alloys thereof for the p-contact. Contacts 32 may be transparent, such as in devices where light is extracted through the surface of the epitaxial layers, or reflective, such as in flip-chip devices where light is extracted through the substrate. After depositing and patterning the contacts, an array of unconnected light emitting diodes has been formed on a single substrate. Other process flows can be used to develop the same final structure. The LEDs can then be connected in many different configurations.

[0023] Interconnects 34 to connect the individual LEDs on the device are then deposited. Interconnect 34 may be, for example Al, Cu, Au, Ag or alloys such as AlSiCu. P- and n-contacts 32 are materials optimized to form ohmic contact with the semiconductor layers, while interconnect 34 is a thick, highly conductive material optimized for carrying current. If light is extracted from the device through transparent contacts, interconnect 34 is deposited to block as little of the contacts as possible to minimize the amount of light absorbed in the interconnect. The two LEDs illustrated in FIGS. 7A and 7B are connected in series, with the n-contact of LED B connected to the p-contact of LED A. As is apparent, metal interconnects 34 can be deposited to connect the LEDs of a monolithic array in many different configurations.

[0024] In one embodiment, such as that shown in FIGS. 8A and 8B, a series array of four LEDs is formed in a balanced square configuration. FIG. 8B shows a circuit diagram of four LEDs connected in series in a square array. FIG. 8A shows a plan view of one embodiment of FIG. 8B. It is desirable to minimize the largest dimension of the LED array, because after fabrication the array is packaged in an optic to direct light out of the packaged array. The optic typically grows geometrically with the size of the light source.

[0025] The array in FIG. 8A has four serially connected LEDs that are electrically isolated via etching to remove III-nitride material to form a trench 80 between the individual LEDs as described above in FIGS. 3-7A. The etching proceeds to at least a highly resistive III-nitride layer, such as a non-intentionally doped GaN layer. The electrical interconnections are provided by metallic traces 81. The resulting device may be represented by the electronic circuit shown in FIG. 8B. This device thus operates at 4× the voltage, and 4× less current, than a single LED of the same active region area. For example, a 1 mm²III-nitride LED may operate at 3.0 V and 350 mA. This same active junction area, broken up into four series-interconnected LEDs as shown in FIG. 8A, provides a device operating at 12.0 V and 87.5 mA. This higher voltage, lower current operation places less demand on the electronic driver circuit for the LED array. In fact, the electronic driver circuit can run at higher efficiency at higher voltages, improving the overall efficiency of the LED lighting system. A monolithic device, according to this embodiment, is preferred over a conventional approach of attaching individual LED dice in series. In the conventional approach, the total area taken up by the LED die is increased because of the tolerances required by die-attach machines. This undesirably increases the optical source size of the total LED and requires an increase in subsequent optics sizes in the LED system. In the preferred embodiment, the diodes may be spaced as closely together as allowed by the trench etching or ion implantation for electrical isolation. The trench or ion implantation region width may be as small as a few microns, so that the packing density of diodes in the embodiment can be very high.

[0026] In accordance with the invention, monolithic series arrays of LEDs may offer several advantages. First, monolithic arrays cut down on the number of connections to external circuitry, such as a submount. If the device is formed such that light is extracted from the epitaxial side of the device through transparent contacts, a reduction in the number of connections to external circuitry means that light can be extracted from more area of the device. In such devices, the LEDs are typically connected to external circuitry by wire bonds, which partially obscure the light which is usefully extracted from the LED die. Interconnects would typically obscure this extracted light to a much smaller extent. If the device is a flip chip, fewer contacts to the submount means that the device can have more active region to generate light. Second, as described above, monolithic serial arrays operate at a higher voltage than an individual LED. A higher operating voltage can simplify the design of a power supply to drive the LED array.

[0027] FIGS. 9A and 9B illustrate a balanced square series/parallel LED array. FIG. 9B shows a circuit diagram of four LEDs connected as two parallel strings of two LEDs connected in series. FIG. 9A shows a plan view of one embodiment of FIG. 9B. Such a series/parallel array is formed as described above in reference to FIGS. 3-7A.

[0028] FIG. 10 illustrates an electrostatic discharge (ESD) protection circuit, where two diodes are connected in an antiparallel configuration. The first LED clamps reverse breakdown in the second LED. FIGS. 11 and 12 illustrate one embodiment 40 of a monolithic ESD protection circuit. Structures A and B are formed on a highly resistive substrate 20. One structure A is connected as an LED to produce light, while the other structure B is used to clamp reverse breakdown in LED A. P-type layers 41aand 41boverlay active regions 49aand 49b,which are formed on n-type layers 42aand 42b.A trench 43 is formed between devices A and B. Ledges for contact formation on n-type layers 42aand 42bare exposed such that the n-electrodes 45aand 45bare opposite trench 43 from each other. A dielectric layer 47 electrically insulates the metallization layer 46afrom all electrical contact except where openings are made for interlayer interconnects, or for contacts to such areas as the p-contacts or n-contacts. P-electrode 44aand n-electrode 45bare connected by interconnect 46asuch that the p-contact of LED A is connected to the n-contact of clamping device B. In the region where interconnect 46ais deposited, the n-contact of LED A is isolated from interconnect 46aby dielectric layer 47, as illustrated in FIG. 12. As shown in FIG. 11, the interconnection between the p-contact of LED A and the n-contact of clamping device B is formed on one side of the device, and the interconnection between the n-contact of LED A and the p-contact of clamping device B is formed on the other side of the device. The structure can then be connected to a submount or other structure (not shown) by solder bumps or wire bonds 48.

[0029] FIGS. 11 and 12 show a structure where the clamping device is the same size as the LED. Since the clamping device does not emit light in normal operating circumstances, the size of the clamping device can be reduced relative to the LED. In one embodiment, the p-n junction for the clamping device can be formed underneath a solder bump or wire bond 48, such that no useful light emitting area is lost. In another embodiment, the size of the two anti-parallel diodes is approximately equal and the device can be operated with an alternating current source.

[0030] Parallel LED arrays may also be formed on highly resistive substrates. FIGS. 13 and 14 illustrate one embodiment of such an array. Three p-type regions 90, 91, and 92 are isolated from each other by a single continuous n-type region 93 interposing the p-type regions. P-contacts 32bdeposited on p-type layer 24 and n-contacts 32adeposited on n-type layer 22 connect to a submount (not shown) by solder bumps 95. The submount may include control circuitry or appropriate connectivity for independently addressing each p-type region. In such embodiments, each LED may be operated independently of the others.

[0031] A single LED where the n-type region interposes portions of a single p-type region is described in application Ser. No. 09/469,657, filed Dec. 22, 1999 on an invention of Krames et al., titled “III-Nitride Light-Emitting Device with Increased Light Generating Capability,”. The Krames et al. device lacks the top horizontal portion 96 of n-contact 93 and the top two solder bumps on the n-contact, shown in FIG. 14. As a result, current from the two lower n-contact solder bumps does not spread readily into the uppermost portions of the vertical arms 97 of n-contact 93. In fact, electromigration of the n-contact material can cut off all current flow into the vertical portions of the n-contact of such a device. In contrast, the symmetrical parallel junction device shown in FIG. 14 provides more avenues for current flow and increased redundancy. The top horizontal arm 96 of the n-contact eliminates “dead ends” at the tops of vertical arms 97, thus current spreads readily into all portions of the n-contact 93.

[0032] One or more of the individual LEDs in either a series or a parallel monolithic array of LEDs can be covered with a phosphor to change the color of the light generated by the LED, as illustrated in FIG. 15. In a flip chip device, a phosphor coating 100 is printed on the bottom surface of a portion of the substrate on which a single LED in the array is formed. One method of applying phosphor coatings to the substrates of flip chip devices is described in more detail in application Ser. No. 09/688,053, filed on Oct. 13, 2000 on an invention of Lowery, titled “Stenciling Phosphor Layers on Light Emitting Diodes,” and incorporated herein by reference. By applying a phosphor coating over some of the LEDs in a monolithic array, LED arrays can simultaneously generate different colors of light. Such an array may be useful to mix colors in order to form white light. For an LED array which incorporates independently addressable parallel LEDs coupled with selective phosphor placement, one may create color-tunable LED arrays.

[0033] While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.