# Patent

## Title
Etch removal of current distribution layer for LED current confinement

## Abstract

A method and structure for forming an array of LED devices is disclosed. The LED devices in accordance with embodiments of the invention may include a confined current injection area in which a current spreading layer protrudes away from a cladding layer in a pillar configuration so that the cladding layer is wider than the current spreading layer pillar.

## Background
BACKGROUND
Field
The present invention relates to light emitting diode (LED) devices. More particularly, embodiments of the invention relate to LED devices with a confined current injection area.
Background Information
Light emitting diodes (LEDs) are increasingly being considered as a replacement technology for existing light sources. For example, LEDs are found in signage, traffic signals, automotive tail lights, mobile electronics displays, and televisions. Various benefits of LEDs compared to traditional lighting sources may include increased efficiency, longer lifespan, variable emission spectra, and the ability to be integrated with various form factors.
One type of LED is an organic light emitting diode (OLED) in which the emissive layer of the diode is formed of an organic compound. One advantage of OLEDs is the ability to print the organic emissive layer on flexible substrates. OLEDs have been integrated into thin, flexible displays and are often used to make the displays for portable electronic devices such as cell phones and digital cameras.
Another type of LED is a semiconductor-based LED in which the emissive layer of the diode includes one or more semiconductor-based quantum well layers sandwiched between thicker semiconductor-based cladding layers. Some advantages of semiconductor-based LEDs compared to OLEDs can include increased efficiency and longer lifespan. High luminous efficacy, expressed in lumens per watt (lm/W), is one of the main advantages of semiconductor-based LED lighting, allowing lower energy or power usage compared to other light sources. Luminance (brightness) is the amount of light emitted per unit area of the light source in a given direction and is measured in candela per square meter (cd/m2) and is also commonly referred to as a Nit (nt). Luminance increases with increasing operating current, yet the luminous efficacy is dependent on the current density (A/cm2), increasing initially as current density increases, reaching a maximum and then decreasing due to a phenomenon known as “efficiency droop.” Many factors contribute to the luminous efficacy of an LED device, including the ability to internally generate photons, known as internal quantum efficiency (IQE). Internal quantum efficiency is a function of the quality and structure of the LED device. External quantum efficiency (EQE) is defined as the number of photons emitted divided by the number of electrons injected. EQE is a function of IQE and the light extraction efficiency of the LED device. At low operating current density (also called injection current density, or forward current density) the IQE and EQE of an LED device initially increases as operating current density is increased, then begins to tail off as the operating current density is increased in the phenomenon known as the efficiency droop. At low current density the efficiency is low due to the strong effect of defects or other processes by which electrons and holes recombine without the generation of light, called non-radiative recombination. As those defects become saturated radiative recombination dominates and efficiency increases. An “efficiency droop” or gradual decrease in efficiency begins as the injection-current density surpasses a low value, typically between 1.0 and 10 A/cm2.
Semiconductor-based LEDs are commonly found in a variety of applications, including low-power LEDs used as indicators and signage, medium-power LEDs such as for light panels and automotive tail lights, and high-power LEDs such as for solid-state lighting and liquid crystal display (LCD) backlighting. In one application, high-powered semiconductor-based LED lighting devices may commonly operate at 400-1,500 mA, and may exhibit a luminance of greater than 1,000,000 cd/m2. High-powered semiconductor-based LED lighting devices typically operate at current densities well to the right of peak efficiency on the efficiency curve characteristic of the LED device. Low-powered semiconductor-based LED indicator and signage applications often exhibit a luminance of approximately 100 cd/m2 at operating currents of approximately 20-100 mA. Low-powered semiconductor-based LED lighting devices typically operate at current densities at or to the right of the peak efficiency on the efficiency curve characteristic of the LED device. To provide increased light emission, LED die sizes have been increased, with a 1 mm2 die becoming a fairly common size. Larger LED die sizes can result in reduced current density, which in turn may allow for use of higher currents from hundreds of mA to more than an ampere, thereby lessening the effect of the efficiency droop associated with the LED die at these higher currents.
Thus, the trend in current state-of-the art semiconductor-based LEDs is to increase both the operating current as well as LED size in order to increase efficiency of LEDs since increasing the LED size results in decreased current density and less efficiency droop. At the moment, commercial semiconductor-based LEDs do not get much smaller than 1 mm2.
SUMMARY
Embodiments of the invention describe LED devices with a confined current injection area. In an embodiment, an LED device includes an active layer between a first current spreading layer pillar and a second current spreading layer. The first current spreading layer pillar is doped with a first dopant type and the second current spreading layer is doped with a second dopant type opposite the first dopant type. A first cladding layer is between the first current spreading layer pillar and the active layer, and a second cladding layer is between the second current spreading layer and the active layer. The first current spreading layer pillar protrudes away from the first cladding layer, and the first cladding layer is wider than the first current spreading layer pillar. In an embodiment, the first current spreading layer pillar is doped with a p-dopant. In an embodiment, the first current spreading layer pillar comprises GaP, and the first cladding layer includes a material such as AlInP, AlGaInP, or AlGaAs. In an embodiment, the active layer includes less than 10 quantum well layers, such as 1-3 quantum well layers. In an embodiment the active layer includes a single quantum wellayer, and does not include multiple quantum well layers. In an embodiment, the active layer of the LED device has a maximum width of 100 μm or less, and the first current spreading layer pillar has a maximum width of 10 μm or less. In an embodiment the active layer of the LED device has a maximum width of 20 μm or less, and the first current spreading layer pillar has a maximum width of 10 μm or less. In an embodiment, the second current spreading layer is wider than the first current spreading layer pillar.
A passivation layer may span along a surface of the first cladding layer and sidewalls of the first current spreading layer pillar. In an embodiment, an opening is formed in the passivation layer on a surface of the first current spreading layer pillar opposite the first cladding layer. A conductive contact can then be formed within the opening in the passivation layer and in electrical contact with the first current spreading layer pillar without being in direct electrical contact with the first cladding layer.
In an embodiment, the LED device is supported by a post, and a surface area of the top surface of the post is less than the surface area of a bottom surface of the first current spreading layer pillar. In such a configuration, the LED device may be on a carrier substrate. In an embodiment, the LED device is bonded to a display substrate within a display area of the display substrate. For example, the LED device may be bonded to the display substrate an in electric connection with working circuitry within the display substrate, or the LED device may be bonded to a display substrate and in electrical connection with a micro chip also bonded to the display substrate within the display area. In an embodiment, the LED device is incorporated within a display area of a portable electronic device.
In an embodiment, a method of forming an LED device includes patterning a p-n diode layer of an LED substrate to form an array of current spreading layer pillars separated by an array of confinement trenches in a current spreading layer of the p-n diode layer, where the confinement trenches extend through the current spreading layer and expose a cladding layer of the p-n diode layer underneath the current spreading layer. A sacrificial release layer is formed over the array of current spreading layer pillars and the cladding layer. The LED substrate is bonded to a carrier substrate, and a handle substrate is removed from the LED substrate. The p-n diode layer is patterned laterally between the array of current spreading layer pillars to form an array of LED devices, with each LED device including a current spreading layer pillar of the array of current spreading layer pillars. Patterning of the p-n diode layer may include etching through a top current spreading layer, a top cladding layer, one or more quantum well layers, and the cladding layer (e.g. bottom cladding layer) to expose the sacrificial release layer.
An array of bottom electrically conductive contacts may be formed on and in electrical contact with the array of current spreading layer pillars prior to forming the sacrificial release layer over the array of current spreading layer pillars and the cladding layer. The sacrificial release layer may additionally be patterned to form an array of openings in the sacrificial release layer over the array of current spreading layer pillars prior to bonding the LED substrate to the carrier substrate. In such an embodiment, the LED substrate is bonded to the carrier substrate with a bonding material that is located within the array of openings in the sacrificial release layer. Upon forming the array of LED devices, the sacrificial release layer may be removed, and a portion of the array of LED devices is transferred from the carrier substrate to a receiving substrate, for example a display substrate, using an electrostatic transfer head assembly.
In an embodiment, a method of operating a display includes sending a control signal to a driving transistor, and driving a current through an LED device including a confined current injection area in response to the control signal, where the LED device includes a current spreading layer pillar that protrudes away from a cladding layer and the cladding layer is wider than the current spreading layer pillar. For example, the display is a portable electronic device. LED devices in accordance with embodiments of the invention may be driven at injection currents and current densities well below the normal or designed operating conditions for standard LEDs. In an embodiment, the current driven through the LED device is from 1 nA-400 nA. In an embodiment the current is from 1 nA-30 nA. In such an embodiment, the current density flowing the LED device may be from 0.001 A/cm2 to 3 A/cm2. In an embodiment the current is from 200 nA-400 nA. In such an embodiment, the current density flowing the LED device may be from 0.2 A/cm2 to 4 A/cm2. In an embodiment the current is from 100 nA-300 nA. In such an embodiment, the current density flowing the LED device may be from 0.01 A/cm2 to 30 A/cm2.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graphical illustration of the relationship of internal quantum efficiency to current density for an LED device in accordance with embodiments of the invention.
FIG. 2 is a cross-sectional side view illustration of a bulk LED substrate in accordance with an embodiment of the invention.
FIG. 3 is a cross-sectional side view illustration of an array of current spreading layer confinement trenches formed through a current spreading layer in accordance with an embodiment of the invention.
FIG. 4 is a cross-sectional side view illustration of a patterned passivation layer formed over an array of current spreading layer pillars in accordance with an embodiment of the invention.
FIG. 5 is a cross-sectional side view illustration of an array of bottom conductive contacts formed over the array of current spreading layer pillars in accordance with an embodiment of the invention.
FIG. 6 is a cross-sectional side view illustration of a patterned sacrificial release layer formed over the array of current spreading layer pillars in accordance with an embodiment of the invention.
FIGS. 7A-7B are cross-sectional side view illustrations of a patterned bulk LED substrate bonded to a carrier substrate with a stabilization layer in accordance with embodiments of the invention.
FIG. 8 is a cross-sectional side view illustration of an LED device layer and carrier substrate after removal of a handle substrate in accordance with an embodiment of the invention.
FIG. 9 is a cross-sectional side view illustration of a top conductive contact layer formed over an LED device layer on a carrier substrate in accordance with an embodiment of the invention.
FIG. 10 is a cross-sectional side view illustration of an array of mesa trenches formed in the LED device layer to form an array of LED devices embedded in a sacrificial release layer in accordance with an embodiment of the invention.
FIG. 11A is a cross-sectional side view illustrations of an array of LED devices supported by an array of stabilization posts after the removal of a sacrificial release layer in accordance with an embodiment of the invention.
FIGS. 11B-11D are top-bottom combination schematic view illustrations of LED devices in accordance with embodiments of the invention.
FIG. 12 is plot of radiative recombination as a function of distance from center of LED devices with different widths in accordance with an embodiment of the invention.
FIG. 13 is a plot of internal quantum efficiency as a function of current density for LED devices with current spreading layer pillars of different widths in accordance with embodiments of the invention.
FIG. 14 is a plot of internal quantum efficiency as a function of current density for LED devices with current spreading layer pillars of different doping in accordance with embodiments of the invention.
FIG. 15A-15E are cross-sectional side view illustrations of an array of electrostatic transfer heads transferring LED devices from carrier substrate to a receiving substrate in accordance with an embodiment of the invention.
FIG. 16A is a top view illustration of a display panel in accordance with an embodiment of the invention.
FIG. 16B is a side-view illustration of the display panel of FIG. 16A taken along lines X-X and Y-Y in accordance with an embodiment of the invention.
FIG. 16C is a side-view illustration of an LED device in electrical connection with a micro chip bonded to a display substrate in accordance with an embodiment of the invention.
FIG. 17 is a schematic illustration of a display system in accordance with an embodiment of the invention.
FIG. 18 is a schematic illustration of a lighting system in accordance with an embodiment of the invention.



DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention describe LED devices and manners of forming LED devices with a confined current injection area. In particular, some embodiments of the present invention may relate to micro LED devices and manners of forming micro LED devices with a confined current injection area.
In various embodiments, description is made with reference to figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions and processes, etc., in order to provide a thorough understanding of the present invention. In other instances, well-known semiconductor processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the present invention. Reference throughout this specification to “one embodiment” means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase “in one embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments.
The terms “spanning”, “over”, “to”, “between” and “on” as used herein may refer to a relative position of one layer with respect to other layers. One layer “spanning,” “over” or “on” another layer or bonded “to” or in “contact” with another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers.
In one aspect, embodiments of the invention describe an LED device integration design in which an LED device is transferred from a carrier substrate and bonded to a receiving substrate using an electrostatic transfer head assembly. In accordance with embodiments of the present invention, a pull-in voltage is applied to an electrostatic transfer head in order to generate a grip pressure on an LED device. It has been observed that it can be difficult to impossible to generate sufficient grip pressure to pick up micro devices with vacuum chucking equipment when micro device sizes are reduced below a specific critical dimension of the vacuum chucking equipment, such as approximately 300 μm or less, or more specifically approximately 100 μm or less. Furthermore, electrostatic transfer heads in accordance with embodiments of the invention can be used to create grip pressures much larger than the 1 atm of pressure associated with vacuum chucking equipment. For example, grip pressures of 2 atm or greater, or even 20 atm or greater may be used in accordance with embodiments of the invention. Accordingly, in one aspect, embodiments of the invention provide the ability to transfer and integrate micro LED devices into applications in which integration is not possible with current vacuum chucking equipment. In some embodiments, the term “micro” LED device or structure as used herein may refer to the descriptive size, e.g. length or width, of certain devices or structures. In some embodiments, “micro” LED devices or structures may be on the scale of 1 μm to approximately 300 μm, or 100 μm or less in many applications. However, it is to be appreciated that embodiments of the present invention are not necessarily so limited, and that certain aspects of the embodiments may be applicable to larger micro LED devices or structures, and possibly smaller size scales.
In one aspect, embodiments of the invention describe LED devices that are poised for pick up and supported by one or more stabilization posts. In accordance with embodiments of the present invention, a pull-in voltage is applied to a transfer head in order to generate a grip pressure on an LED device and pick up the LED device. In accordance with embodiments of the invention, the minimum amount pick up pressure required to pick up an LED device from a stabilization post can be determined by the adhesion strength between the adhesive bonding material from which the stabilization posts are formed and the LED device (or any intermediate layer), as well as the contact area between the top surface of the stabilization post and the LED device. For example, adhesion strength which must be overcome to pick up an LED device is related to the minimum pick up pressure generated by a transfer head as provided in equation (1):

P1A1=P2A2  (1)

where P1 is the minimum grip pressure required to be generated by a transfer head, Al is the contact area between a transfer head contact surface and LED device contact surface, A2 is the contact area on a top surface of a stabilization post, and P2 is the adhesion strength on the top surface of a stabilization post. In an embodiment, a grip pressure of greater than 1 atmosphere is generated by a transfer head. For example, each transfer head may generate a grip pressure of 2 atmospheres or greater, or even 20 atmospheres or greater without shorting due to dielectric breakdown of the transfer heads. Due to the smaller area, a higher pressure is realized at the top surface of the corresponding stabilization post than the grip pressure generate by a transfer head.

In another aspect, embodiments of the invention describe LED devices, which may be micro LED devices, including a confined current injection area. In an embodiment, an LED device includes a first (e.g. bottom) current spreading layer pillar doped with a first dopant type, a first (e.g. bottom) cladding layer on the bottom current spreading layer, an active layer on the bottom cladding layer, a second (e.g top) cladding layer on the active layer, and a second (e.g. top) current spreading layer doped with a second dopant type opposite the first dopant type. The bottom current spreading layer pillar protrudes away from the bottom cladding layer, in which the bottom cladding layer is wider than the bottom current spreading layer pillar. In accordance with embodiments of the invention, the active layer is also wider than the bottom current spreading layer pillar. The top cladding layer, and top current spreading layer may also be wider than the bottom current spreading layer pillar. In this manner, when a potential is applied across the top current spreading layer and bottom current spreading layer pillar, the current injection area within the active layer is modified by the relationship of the areas of the bottom current spreading layer pillar and top current spreading layer. In operation, the current injection area is reduced as the area of the bottom current spreading layer pillar configuration is reduced. In this manner, the current injection area can be confined internally within the active layer. Additionally, embodiments of the invention enable the current to be confined within less than 10 μm from the edge of the current confining feature. Typically, when contact patterning is used to confine current in a standard LED device to prevent injection below optically absorbing metal contacts, current will still spread greater than 10 μm from the edge of the current confining feature. This current spreading distance may be the entire size of a micro LED device and therefore not a feasible approach to current confinement for micro LED devices. Embodiments of this invention describe a method to confine current in a micro LED device where confinement distances of less than 10 μm may be needed.
In this manner, it is possible to design an LED device in which a top surface area of the top surface of the p-n diode layer is larger than a surface area of the bottom current spreading layer pillar. This enables larger LED devices to be fabricated, which may be beneficial for transferring the LED devices using an electrostatic transfer head assembly, while also providing a structure in which the confined current injection area results in an increased current density and increased efficiency of the LED device, particularly when operating at injection currents and injection current densities below or near the pre-droop region of the LED device internal quantum efficiency curve.
In another aspect, it has been observed that non-radiative recombination may occur along exterior surfaces of the active layer (e.g. along sidewalls of the LED devices). It is believed that such non-radiative recombination may be the result of defects, for example, that may be the result of forming mesa trenches through the p-n diode layer to form an array of LED devices or a result of surface states from dangling bonds at the terminated surface that can enable current flow and non-radiative recombination. Such non-radiative recombination may have a significant effect on LED device efficiency, particularly at low current densities in the pre-droop region of the IQE curve where the LED device is driven at currents that are unable to saturate the defects. In accordance with embodiments of the invention, the current injection area can be confined internally within the active layer, so that the current does not spread laterally to the exterior surfaces of the active layer where a larger amount of defects may be present. As a result, the amount of non-radiative recombination near the exterior surfaces of the active layer can be reduced and efficiency of the LED device increased.
The LED devices in accordance with embodiments of the invention are highly efficient at light emission and may consume very little power compared to LCD or OLED display technologies. For example, a conventional display panel may achieve a full white screen luminance of 100-750 cd/m2. It is understood that a luminance of greater than 686 cd/m2 may be required for sunlight readable screens. In accordance with some embodiments of the invention, an LED device may be transferred and bonded to a display backplane such as a thin film substrate backplane used for OLED display panels, where the semiconductor-based LED device replaces the organic LED film of the OLED display. In this manner, a highly efficient semiconductor-based LED device replaces a less efficient organic LED film. Furthermore, the width/length of the semiconductor-based LED device may be much less than the allocated subpixel area of the display panel, which is typically filled with the organic LED film.
LED devices in accordance with embodiments of the invention may operate well below the normal or designed operating conditions for standard LEDs. The LED devices may also be fundamentally different than lasers, and operate at significantly lower currents than lasers. For example, the principle of emission for LED devices in accordance with embodiments of the invention may be spontaneous, non-directional photon emission, compared to stimulated, coherent light that is characteristic of lasers. Lasers typically include distributed Bragg reflector (DBR) layers on opposite sides of the active layer for stimulating coherent light emission, also known as lasing. Lasing is not necessary for operation of LED devices in accordance with embodiments of the invention. As a result, the LED devices may be thinner than typical lasers, and do not require reflector layers on opposite sides of the active layer for stimulating coherent light emission.
For illustrative purposes, in accordance with embodiments of the invention it is contemplated that the LED devices may be driven using a similar driving circuitry as a conventional OLED display panel, for example a thin film transistor (TFT) backplane. However, embodiments are not so limited. For example, in another embodiment the LED devices are driven by micro controller chips that are also electrostatically transferred to a receiving substrate. Assuming subpixel operating characteristics of 25 nA injection current, an exemplary LED device having a 1 μm2 confined current injection area roughly corresponds to a current density of 2.5 A/cm2, an exemplary LED device having a 25 μm2 confined current injection area roughly corresponds to a current density of 0.1 A/cm2, and an exemplary LED device having a 100 μm2 confined current injection area roughly corresponds to a current density of 0.025 A/cm2. Referring to FIG. 1, in accordance with embodiments of the invention these low injection currents and current densities may correspond to a pre-droop region of a characteristic efficiency curve. This is well below the normal or designed operating conditions for standard LEDs. Furthermore, in some embodiments, the low injection currents and current densities may correspond to a portion on the pre-droop region of the characteristic efficiency curve for the LED device in which the slope of the curve is greater than 1:1 such that a small increase in current density results in a greater increase in IQE, and hence EQE, of the LED device. Accordingly, in accordance with embodiments of the invention, significant efficiency increases may be obtained by confining the current injection area of the LED device, resulting in increased luminous efficacy and luminance of the LED device. In some embodiments, LED devices with confined current injection areas are implemented into display panel applications designed for target luminance values of approximately 300 Nit for indoor display applications and up to about 2,000 Nit for outdoor display applications. It is to be appreciated that the above examples, including injection currents and display applications are exemplary in nature in order to provide a context for implementing embodiments of the invention, and that embodiments are not so limited and may be used with other operating conditions, and that embodiments are not limited to display applications or TFT backplanes.
In the following description exemplary processing sequences are described for forming an array of LED devices, which may be micro LED devices. Referring now to FIG. 2, a cross-sectional side view illustration is provided of a bulk LED substrate 100 in accordance with an embodiment of the invention. For example, the bulk LED substrate illustrated in FIG. 2 may be designed for emission of primary red light (e.g. 620-750 nm wavelength), primary green light (e.g. 495-570 nm wavelength), or primary blue light (e.g. 450-495 nm wavelength), though embodiments of the invention are not limited to these exemplary emission spectra. In an embodiment, a bulk LED substrate 100 includes a p-n diode layer 115 formed on a growth substrate 102. The p-n diode layer 115 may be formed of a variety of compound semiconductors having a bandgap corresponding to a specific region in the spectrum. For example, the p-n diode layer 115 can include one or more layers based on II-VI materials (e.g. ZnSe) or III-V materials including III-V nitride materials (e.g. GaN, AlN, InN, InGaN, and their alloys) and III-V phosphide materials (e.g. GaP, AlGaInP, and their alloys). The growth substrate 102 may include any suitable substrate such as, but not limited to, silicon, SiC, GaAs, GaN, and sapphire.
Specifically, exemplary primary processing sequences are described for forming an array of red emitting LED devices. While the primary processing sequences are described for red emitting LED devices, it is to be understood that the exemplary processing sequences can be used for LED devices with different emission spectra, and that certain modifications are contemplated, particularly when processing different materials. Additionally, in different materials the shape of the IQE curve may differ, specifically the peak may occur at current densities other than that shown in FIG. 1. In one embodiment, the bulk LED substrate 100 is designed for emission of red light, and growth substrate 102 is formed of GaAs. Growth substrate 102 may optionally be doped. In the embodiment illustrated growth substrate 102 is n-doped, though in alternative embodiments the growth substrate 102 is p-doped. A current spreading layer 104 is formed on the growth substrate 102 with a first dopant type. In an embodiment, the current spreading layer 104 is n-doped GaAs, though other materials and opposite dopant types may be used. As illustrated, a cladding layer 106 is formed over the current spreading layer 104. Cladding layer 106 may function to confine current within the active layer 108, and possess a larger bandgap energy than the active layer. The cladding layer 106 may be doped or undoped. In an embodiment, the cladding layer 106 is formed of a material such as AlInP, AlGaInP, or AlGaAs. Cladding layer 106 may optionally be doped or undoped. Cladding layer 106 may optionally be doped, for example with the same dopant type as current spreading layer 114. For example, doping of cladding layer 106 may improve vertical current injection into the active layer 108.
An active layer 108 is formed on the cladding layer 106. The active layer 108 may include a multi-quantum-well (MQW) configuration or a single-quantum-well (SQW) configuration. In accordance with embodiments of the invention, a reduced number of quantum wells may offer more resistance to lateral current spreading, higher carrier density, and aid in confining current internally within the completed LED device. In an embodiment, the active layer 108 includes a SQW. In an embodiment, a MQW configuration with a low number of quantum wells may be used, for example, for layer quality. In an embodiment, active layer 108 includes a MQW configuration with less than 10 quantum well layers. In an embodiment, active layer 108 includes 1-3 quantum wells. Additional layers may also be included in the active layer 108, such as one or more barrier layers. In an embodiment, the active layer 108 is formed of a material such as AlGaInP, AlGaAs, or InGaP. In accordance with embodiments of the invention, the materials forming the active layer 108 have a smaller bandgap energy than both the cladding layers 106, 110 on opposite sides of the active layer 108.
Still referring to FIG. 2, a cladding layer 110 is formed on the active layer 108, and a current spreading layer 114 is formed on the cladding layer 110. In accordance with embodiments of the invention, the cladding layer 108 material and thickness may be selected to achieve a desired resistivity at the target operating current so that the cladding layer 110 has a higher resistivity than the current spreading layer 114 from which current spreading layer pillars will be formed. In this manner, the cladding layer 110 resists lateral current spreading to a degree so that current is confined internally within the completed LED device. Similarly as cladding layer 106, cladding layer 110 may function to confine electrons and holes within the active layer 108, and possess a larger bandgap energy than the active layer. In an embodiment, current spreading layer 114 is doped with an opposite dopant type than current spreading layer 104. For example, current spreading layer 114 may be p-doped where current spreading layer 104 is n-doped, and vice versa. In an embodiment, current spreading layer 114 is GaP. In an embodiment, current spreading layer 114 is formed of multiple layers. In an embodiment, the current spreading layer 114 includes a top p-doped GaP layer 112 and underlying InGaP etch stop layer 113 on the cladding layer 110. In an embodiment, the cladding layer 110 is formed of a material such as AlInP, AlGaInP, or AlGaAs. The cladding layer 110 may be doped or undoped. Cladding layer 110 may optionally be doped, for example with the same dopant type as current spreading layer 114. In an embodiment, cladding layer 110 has a lower dopant concentration (including no doping) than cladding layer 106 dopant concentration.
In an embodiment, bulk LED substrate 100 includes a 250-500 μm thick growth substrate 102, a 0.1-1.0 μm thick current spreading layer 104, a 0.05-0.5 μm thick cladding layer 106, an active layer 108, a 0.05-5 μm thick cladding layer 110, and a 0.1-1.5 μm thick current spreading layer 114. These thicknesses are exemplary, and embodiments of the invention are not limited to these exemplary thicknesses.
Referring now to FIG. 3 an array of current spreading layer confinement trenches 116 are formed through a current spreading layer 114 in accordance with an embodiment of the invention. As shown, the current spreading layer confinement trenches may be etched completely through the current spreading layer 114 forming an array of current spreading layer pillars 118. In an embodiment, etching stops on the cladding layer 110. In another embodiment, cladding layer 110 is partially etched to ensure complete removal of the current spreading layer 114. In accordance with embodiments of the invention, etching is stopped before reaching the active layer 108. Etching may be performed using a suitable technique such as wet etching or dry etching techniques. For example, dry etching techniques such as reactive ion etching (RIE), electro-cyclotron resonance (ECR), inductively coupled plasma reactive ion etching (ICP-RIE), and chemically assisted ion-beam etching (CAIBE) may be used. The etching chemistries may be halogen based, containing species such as Cl2, BCl3, or SiCl4. The etching chemistries may also be wet chemistries containing species such as Br2 or HIO4. In an embodiment, the current spreading layer 114 includes a top p-doped GaP layer 112 and underlying InGaP etch stop layer 113 on the cladding layer 110. In such an embodiment, the top p-doped GaP layer 112 is wet etched using a wet etch chemistry containing Br2 or HIO4, stopping on an etch stop layer 113 formed of InGaP. The etch stop layer 113 may then be removed by wet etching in a solution of HCl+H3PO4. Alternatively, both the GaP 112 and InGaP 113 layers can be etched using a timed dry etching technique.
As will become more apparent in the following description, the width of the current spreading layer pillars 118 at least partly determines the ability to increase current density within the LED device as well as the ability to confine current internally within the LED devices and away from the external sidewalls where non-radiative recombination may occur. While some lateral current spreading occurs within the device, embodiments of the invention generally refer to the confined current area as the area of the quantum well directly above the current spreading layer pillars 118. Width of the current spreading layer pillars 118 may also be related to width of the LED devices. In some embodiments, current spreading layer pillars 118 have a width between 1 and 10 μm. In an embodiment, current spreading layer pillars 118 have a width or diameter of approximately 2.5 μm.
FIG. 4 is a cross-sectional side view illustration of a patterned passivation layer 120 formed over an array of current spreading layer pillars 118 in accordance with an embodiment of the invention. In an embodiment, a passivation layer 120 is formed of an electrically insulating material such as an oxide or nitride. In an embodiment, passivation layer is approximately 50 angstroms to 3,000 angstroms thick Al2O3. In an embodiment, passivation layer 120 is formed using a high quality thin film deposition procedure, such as atomic layer deposition (ALD). As will become more apparent in the following description, a high quality thin film deposition procedure may protect the integrity of the passivation layer 120 during the sacrificial release layer etch operation. In an embodiment, passivation layer 120 is approximately 200 angstroms thick Al2O3 deposited by ALD. Openings 122 may then be formed over the current spreading layer pillars 118 to expose the top-most surface of the current spreading layer pillars using a suitable patterning technique such as lithography and etching. In the embodiment illustrated, patterned passivation layer 120 is formed along sidewalls of current spreading layer pillars 118 and on cladding layer 110. In other embodiments, a passivation layer 120 is not formed.
Referring now to FIG. 5, an array of bottom conductive contacts 124 are formed over the array of current spreading layer pillars 118 in accordance with an embodiment of the invention. Conductive contacts 124 may be formed of a variety of conductive materials including metals, conductive oxides, and conductive polymers. In an embodiment, conductive contacts 124 are formed using a suitable technique such as evaporation or sputtering. In an embodiment, conductive contacts 124 may include BeAu metal alloy, or a metal stack of Au/GeAu/Ni/Au layers. In an embodiment, conductive contacts 124 include a first layer to make ohmic contact with current spreading layer pillars 118, and a second bonding-release layer such as gold to control adhesion with a stabilization layer used to bond to a carrier substrate. Following the formation of the bottom conductive contacts 124, or at least the ohmic layer, the substrate stack may be annealed to make ohmic contact, for example, at 510° C. for 10 minutes. In the embodiment illustrated in FIG. 5, conductive contacts 124 do not completely span between adjacent current spreading layer pillars 118. In an embodiment, conductive contacts 124 span along the sidewalls of the current spreading layer pillars 118 covered by passivation layer 120. In an embodiment, conductive contacts 124 do not span along the sidewalls of the current spreading layer pillars 118.
A sacrificial release layer 126 may then be formed over the array of current spreading layer pillars 118 as illustrated in FIG. 6. In the particular embodiment illustrated, the sacrificial release layer 126 is formed within current confinement trenches 116. In an embodiment, the sacrificial release layer 126 is formed of a material which can be readily and selectively removed with vapor (e.g. vapor HF) or plasma etching. In an embodiment, the sacrificial release layer is formed of an oxide (e.g. SiO2) or nitride (e.g. SiNx), with a thickness of 0.2 μm to 2 μm. In an embodiment, the sacrificial release layer is formed using a comparatively low quality film formation technique compared to the passivation layer 120. In an embodiment, the sacrificial release layer 126 is formed by sputtering, low temperature plasma enhanced chemical vapor deposition (PECVD), or electron beam evaporation.
Still referring to FIG. 6, the sacrificial release layer 126 is patterned to from an array of openings 128 over the array of current spreading layer pillars 118. In an embodiment, each opening 128 exposes an underlying conductive contact 124. As will become more apparent in the following description, the dimensions of the openings 128 in the sacrificial release layer 126 correspond to the dimensions and contact area of the stabilization posts to be formed, and resultantly to the adhesion strength that must be overcome to pick up the array of LED devices that is supported by and poised for pick from the array of stabilization posts. In an embodiment, openings 128 are formed using lithographic techniques and have a length and width of approximately 0.5 μm by 0.5 μm, though the openings may be larger or smaller. In an embodiment, openings 128 have a width (or area) that is less than the width (or area) of the current spreading layer pillars 118.
Referring now to FIGS. 7A-7B, in some embodiments a stabilization layer 130 is formed over the patterned sacrificial release layer 126 and the patterned bulk LED substrate 100 is bonded to a carrier substrate 140. In accordance with embodiments of the invention, stabilization layer 130 may be formed of an adhesive bonding material. In an embodiment the adhesive bonding material is a thermosetting material such as benzocyclobutene (BCB) or epoxy. For example, the thermosetting material may be associated with 10% or less volume shrinkage during curing, or more particularly about 6% or less volume shrinkage during curing so as to not delaminate from the conductive contacts 124 on the LED devices to be formed. In order to increase adhesion the underlying structure can be treated with an adhesion promoter such as AP3000, available from The Dow Chemical Company, in the case of a BCB stabilization layer in order to condition the underlying structure. AP3000, for example, can be spin coated onto the underlying structure, and soft-baked (e.g. 100° C.) or spun dry to remove the solvents prior to applying the stabilization layer 130 over the patterned sacrificial release layer 126.
In an embodiment, stabilization layer 130 is spin coated or spray coated over the patterned sacrificial release layer 126, though other application techniques may be used. Following application of the stabilization layer 130, the stabilization layer may be pre-baked to remove the solvents. After pre-baking the stabilization layer 130 the patterned bulk substrate 100 is bonded to the carrier substrate 140 with the stabilization layer 130. In an embodiment, bonding includes curing the stabilization layer 130. Where the stabilization layer 130 is formed of BCB, curing temperatures should not exceed approximately 350° C., which represents the temperature at which BCB begins to degrade. Achieving a 100% full cure of the stabilization layer may not be required in accordance with embodiments of the invention. In an embodiment, stabilization layer 130 is cured to a sufficient curing percentage (e.g. 70% or greater for BCB) at which point the stabilization layer 130 will no longer reflow. Moreover, it has been observed that partially cured BCB may possess sufficient adhesion strengths with carrier substrate 140 and the patterned sacrificial release layer 126. In an embodiment, stabilization layer may be sufficiently cured to sufficiently resist the sacrificial release layer release operation.
In an embodiment, the stabilization layer 130 is thicker than the height of the current spreading layer pillars 118 and openings 128 in the patterned sacrificial release layer 126. In this manner, the thickness of the stabilization layer filling openings 128 will become stabilization posts 132, and the remainder of the thickness of the stabilization layer 130 over the filled openings 128 can function to adhesively bond the patterned bulk LED substrate 100 to a carrier substrate 140.
In the embodiment illustrated in FIG. 7A, after bonding to the carrier substrate 140 a continuous portion of stabilization layer 130 remains over the carrier substrate 140. In an embodiment illustrated in FIG. 7B, the sacrificial release layer 126 (or another intermediate layer) is pressed against the carrier substrate 140 during bonding such that there is not a thickness of the stabilization layer 130 below the stabilization posts 132 to be formed. In such an embodiment, the confinement trenches 116 can function as overflow cavities for the stabilization layer during bonding.
Following bonding of the patterned bulk LED substrate 100 to the carrier substrate 140, the handle substrate 102 is removed as illustrated in FIG. 8. Removal of handle substrate 102 may be accomplished by a variety of methods including laser lift off (LLO), grinding, and etching depending upon the material selection of the growth substrate 102. In the particular embodiment illustrated where handle substrate 102 is a growth substrate formed of GaAs, removal may be accomplished by etching, or a combination of grinding and etching. For example, the GaAs growth substrate 102 can be removed with a H2SO4+H2O2 solution, NH4OH+H2O2 solution, or CH3OH+Br2 chemistry.
Referring now to FIG. 9, following the removal of the growth substrate 102 a top conductive contact layer 152 may be formed. Top conductive contact layer 152 may be formed of a variety of electrically conductive materials including metals, conductive oxides, and conductive polymers. In an embodiment, conductive contact layer 152 is formed using a suitable technique such as evaporation or sputtering. In an embodiment, conductive contact layer 152 is formed of a transparent electrode material. Conductive contact layer 152 may include BeAu metal alloy, or a metal stack of Au/GeAu/Ni/Au layers. Conductive contact layer 152 may also be a transparent conductive oxide (TCO) such as indium-tin-oxide (ITO). Conductive contact layer 152 can also be a combination of one or more metal layers and a conductive oxide. In an embodiment, conductive contact layer 152 is approximately 300 angstroms thick ITO. In an embodiment, after forming the conductive contact layer 152, the substrate stack is annealed to generate an ohmic contact between the conductive contact layer and the current spreading layer 104. Where the stabilization layer 130 is formed of BCB, the annealing temperature may be below approximately 350° C., at which point BCB degrades. In an embodiment, annealing is performed between 200° C. and 350° C., or more particularly at approximately 320° C. for approximately 10 minutes.
In an embodiment, prior to forming the top conductive contact layer 152 an ohmic contact layer 150 can optionally be formed to make ohmic contact with the current spreading layer 104. In an embodiment, ohmic contact layer 150 may be a metallic layer. In an embodiment, ohmic contact layer 150 is a thin GeAu layer. For example, the ohmic contact layer 150 may be 50 angstroms thick. In the particular embodiment illustrated, the ohmic contact layer 150 is not formed directly over the current spreading layer pillars 118, corresponding to the current confinement area within the LED devices, so as to not reflect light back into the LED device and potentially reduce light emission. In some embodiments, ohmic contact layer 150 forms a ring around the current spreading layer pillars 118.
Referring now to FIG. 10, an array of mesa trenches 154 is formed in the LED device layer 115 to form an array of LED devices 156 embedded in the sacrificial release layer in accordance with an embodiment of the invention. In the embodiment illustrated, mesa trenches 154 extend through the top conductive contact layer 152 and LED device layer 115 laterally between the array of current spreading layer pillars 118 stopping on the sacrificial release layer to form an array of LED devices 156. As illustrated, each LED device 156 includes mesa structure with sidewalls 168 formed through the device layer 115 and a current spreading layer pillar 118 of the array of current spreading layer pillars. In an embodiment, current spreading layer pillars 118 are centrally located in the middle of the LED devices 156 so as to confine current equally from the sidewalls 168 of the LED devices 156. At this point, the resultant structure is still robust for handling and cleaning operations to prepare the substrate for subsequent sacrificial layer removal and electrostatic pick up. Etching may be performed using a suitable technique such as dry etching. For example, dry etching techniques such as reactive ion etching (RIE), electro-cyclotron resonance (ECR), inductively coupled plasma reactive ion etching (ICP-RIE), and chemically assisted ion-beam etching (CAIBE) may be used. The etching chemistries may be halogen based, containing species such as Cl2, BCl3, or SiCl4. In an embodiment, etching is continued through passivation layer 120, stopping on the sacrificial release layer 126.
Still referring to FIG. 10, in an embodiment the top conductive contacts 152 on each LED device 156 cover substantially the entire top surface of each LED device 156. In such a configuration, the top conductive contacts 152 cover substantially the maximum available surface area to provide a large, planar surface for contact with the electrostatic transfer head, as described in more detail in FIGS. 15A-15E. This may allow for some alignment tolerance of the electrostatic transfer head assembly.
Following the formation of discrete and laterally separate LED devices 156, the sacrificial release layer 126 may be removed. FIG. 11A is cross-sectional side view illustrations of an array of LED devices 156 supported by an array of stabilization posts 132 after removal of the sacrificial release layer in accordance with an embodiment of the invention. In the embodiment illustrated, sacrificial release layer 126 is completely removed resulting in an open space below each LED device 156. A suitable etching chemistry such as HF vapor, or CF4 or SF6 plasma may used to etch the SiO2 or SiNx sacrificial release layer 126. In an embodiment, the array of LED devices 156 is on the array of stabilization posts 132, and supported only by the array of stabilization posts 132. In the embodiment illustrated, passivation layer 120 is not removed during removal of the sacrificial release layer 126. In an embodiment, passivation layer 120 is formed of Al2O3, and a SiO2 or SiNx sacrificial release layer 126 is selectively removed with vapor HF.
Still referring to FIG. 11A, the LED device includes an active layer 108 between a first current spreading layer pillar 118 and a second current spreading layer 104, where the first current spreading layer pillar 118 is doped with a first dopant type and the second current spreading layer 104 is doped with a second dopant type opposite the first dopant type. A first cladding layer 110 is between the first current spreading layer pillar 118 and the active layer 108. A second cladding layer 106 is between the second current spreading layer 104 and the active layer 108. The first current spreading layer pillar protrudes away from the first cladding layer 110 and the first cladding layer 110 is wider than the first current spreading layer pillar 118. In an embodiment, the first current spreading layer pillar 118 is a bottom current spreading layer pillar, the first cladding layer 110 is a bottom cladding layer, the second cladding layer 106 is a top cladding layer, and the second current spreading layer is a top current spreading layer of the LED device. As shown, the passivation layer 120 may span along a bottom surface of the bottom cladding layer 110 and sidewalls of the bottom current spreading layer pillar 118. An opening is formed in the passivation layer 120 on a bottom surface of the bottom current spreading layer pillar 118. The bottom conductive contact 124 is formed within the opening in the passivation layer and in electrical contact with the bottom current spreading layer pillar 118. In an embodiment, the bottom conductive contact is not in direct electrical contact with the bottom cladding layer 110. In an embodiment, a top surface 162 of the top current spreading layer 104 is wider than a bottom surface of the bottom current spreading layer pillar 118. This may allow for a larger surface area for electrostatic pick up in addition to a structure for confining current. In an embodiment, the LED device 156 is supported by a post 132, and a surface area of a top surface of the post 132 is less than the surface area of the bottom current spreading layer pillar 118.
In accordance with embodiments of the invention the LED devices 156 may be micro LED devices. In an embodiment, an LED device 156 has a maximum width or length at the top surface 162 of top current spreading layer 104 of 300 μm or less, or more specifically approximately 100 μm or less. The active area within the LED device 156 may be smaller than the top surface 162 due to location of the bottom current spreading layer pillars 118. In an embodiment, the top surface 162 has a maximum dimension of 1 to 100 μm, or more specifically 3 to 20 μm. In an embodiment, a pitch of the array of LED devices 156 on the carrier substrate may be (1 to 300 μm) by (1to 300 μm), or more specifically (1 to 100 μm) by (1 to 100 μm), for example, 20 μm by 20 μm, 10 μm by 10 μm, or 5 μm by 5 μm. In an exemplary embodiment, a pitch of the array of LED devices 156 on the carrier substrate is 11 μm by 11 μm. In such an exemplary embodiment, the width/length of the top surface 162 is approximately 9-10μm, and spacing between adjacent LED devices 156 is approximately 1-2 μm. Sizing of the bottom current spreading layer pillars 118 may be dependent upon the width of the LED devices 156 and the desired efficiency of the LED devices 156.
In the above exemplary embodiments, manners for forming LED devices 156 including current spreading layer pillars are described. In the above embodiments, the current spreading layer pillars are formed from current spreading layer 114. In other embodiments, the current spreading layer pillars may be formed from current spreading layer 104. Accordingly, in some embodiments the LED device pillar structure may be inverted. Though an inverted LED device pillar structure may not provide a larger contact area for a transfer operation, such as described with regard to FIGS. 15A-15E.
Referring now to FIGS. 11B-11D, top-bottom combination schematic view illustrations are provided of LED devices with different sidewall configurations in accordance with embodiments of the invention. As illustrated, each LED device may include mesa structure sidewalls 168 and a current spreading layer pillar 118. Sidewalls may include a variety of configurations such as rectangular or square as shown in FIG. 11B, triangular as shown in FIG. 11C, or circular as shown in FIG. 11D, amongst other shapes. Current spreading layer pillars 118 may also assume a variety of shapes including rectangular, square, triangular, circular, etc. In this manner, embodiments of the invention can be used with LED devices of various shapes, which may affect light extraction and EQE of the LED devices. As described above, the current spreading layer pillar 118 may protrude from a bottom of the LED device, or the device may be inverted and the current spreading layer pillar 118 protrudes from a top of the LED device.
FIG. 12 is plot of radiative recombination as a function of distance from center of LED devices with different widths in accordance with an embodiment of the invention. Specifically, FIG. 12 illustrates simulation data for a 10 μm wide LED device and a 100 μm wide LED device, as shown in solid lines, at operating current densities of 300 nA/μm2. The simulation data provided in FIG. 12 is based upon LED devices of constant width, without a pillar formation in the bottom current spreading layer. Referring now specifically to the simulation data for a 100 μm wide LED device, radiative recombination (resulting in light emission) is at a peak value in the center of the LED device indicated by a distance of 0 μm. The peak value is relatively constant moving away from the center until approximately 40 μm from center, where a non-radiative zone begins and the radiative recombination begins to tail off. Thus, this suggests that non-radiative recombination may occur along exterior surfaces of the active layer (e.g. along sidewalls of the LED devices). The simulation data for the 100 μm wide LED device suggests that this non-radiative zone begins to occur at approximately 10 μm from the exterior sidewalls, which may account for 20% of the LED device being affected by the non-radiative recombination zone. The simulation data for the 10 μm wide LED device shows that the peak value of radiative recombination (resulting in light emission) is at a peak value in the center of the LED device and immediately begins to degrade moving away from the center. Furthermore, the peak value of radiative recombination is well below the peak value of the radiative recombination for the 100 μm wide LED device, despite being driven at the same operating current density of 300 nA/μm2. This suggests that non-radiative recombination due to edge effects is dominant within the 10 μm LED device, even within the center of the LED device. Thus, 100% of the LED device may be affected by the non-radiative recombination zone resulting in lower efficiency or EQE.
It is believed that such non-radiative recombination may be the result of defects, for example, that may be the result of forming mesa trenches through the p-n diode layer to form an array of LED devices or a result of surface states from dangling bonds at the terminated surface that can enable current flow and non-radiative recombination. Such non-radiative recombination may have a significant effect on LED device efficiency, particularly at low current densities in the pre-droop region of the IQE curve where the LED device is driven at currents that are unable to saturate the defects. It is expected that as the LED device width (and active layer width) is increased above 10μm the radiative recombination (resulting in light emission) in the center of the device increases as the width increases until the peak value matches that in the 100 μm LED device simulation data. In accordance with embodiments of the invention, the current injection area can be confined internally within the active layer by forming the bottom current spreading layer in a pillar configuration, so that the current does not spread laterally to the exterior surfaces of the active layer where a larger amount of defects may be present. As a result, the amount of non-radiative recombination due to edge effects in the non-radiative zone near the exterior sidewall surfaces of the active layer can be reduced or eliminated and efficiency of the LED device increased.
FIG. 13 is a plot of internal quantum efficiency as a function of current density for exemplary 10 μm wide LED devices (quantum well width) with current spreading layer pillars (p-doped) of different widths (1 μm, 2 μm, 4 μm, 6 μm, 8 μm, and 10 μm) in accordance with embodiments of the invention. As illustrated, IQE for the devices increases as the pillar size is reduced from 10 μm (no pillar) to 1 μm. This suggests that the pillar configuration is successful in confining the injection current internally within the LED devices, over very short distances (less than 10 μm), away from the sidewalls, particularly at low current densities in the pre-droop region of the IQE curve where IQE can be dominated by defects.
FIG. 14 is a plot of internal quantum efficiency as a function of current density for exemplary LED devices with current spreading layer pillars of different doping in accordance with embodiments of the invention. Specifically, the simulation data provided in FIG. 14 is for 10 μm wide LED devices (quantum well width) with 2 μm wide current spreading layer pillars, where n-pillar simulation data is presented along with the 2 μm wide p-pillar data from FIG. 13. The simulation data suggests IQE increases for both p-pillar and n-pillar configurations and that the p-pillar configuration obtains a larger IQE.
FIGS. 15A-15E are cross-sectional side view illustrations of an array of electrostatic transfer heads 204 transferring LED devices 156, which may be micro LED devices, from carrier substrate 140 to a receiving substrate 300 in accordance with an embodiment of the invention. FIG. 15A is a cross-sectional side view illustration of an array of micro device transfer heads 204 supported by substrate 200 and positioned over an array of LED devices 156 stabilized on stabilization posts 132 of stabilization layer 130 on carrier substrate 140. The array of LED devices 156 is then contacted with the array of transfer heads 204 as illustrated in FIG. 15B. As illustrated, the pitch of the array of transfer heads 204 is an integer multiple of the pitch of the array of LED devices 156. A voltage is applied to the array of transfer heads 204. The voltage may be applied from the working circuitry within a transfer head assembly 206 in electrical connection with the array of transfer heads through vias 207. The array of LED devices 156 is then picked up with the array of transfer heads 204 as illustrated in FIG. 15C. The array of LED devices 156 is then placed in contact with contact pads 302 (e.g. gold, indium, tin, etc.) on a receiving substrate 300, as illustrated in FIG. 15D. The array of LED devices 156 is then released onto contact pads 302 on receiving substrate 300 as illustrated in FIG. 15E. For example, the receiving substrate may be, but is not limited to, a display substrate, a lighting substrate, a substrate with functional devices such as transistors or ICs, or a substrate with metal redistribution lines.
In accordance with embodiments of the invention, heat may be applied to the carrier substrate, transfer head assembly, or receiving substrate during the pickup, transfer, and bonding operations. For example, heat can be applied through the transfer head assembly during the pick up and transfer operations, in which the heat may or may not liquefy LED device bonding layers. The transfer head assembly may additionally apply heat during the bonding operation on the receiving substrate that may or may not liquefy one of the bonding layers on the LED device or receiving substrate to cause diffusion between the bonding layers.
The operation of applying the voltage to create a grip pressure on the array of LED devices can be performed in various orders. For example, the voltage can be applied prior to contacting the array of LED devices with the array of transfer heads, while contacting the LED devices with the array of transfer heads, or after contacting the LED devices with the array of transfer heads. The voltage may also be applied prior to, while, or after applying heat to the bonding layers.
Where the transfer heads 204 include bipolar electrodes, an alternating voltage may be applied across a pair of electrodes in each transfer head 204 so that at a particular point in time when a negative voltage is applied to one electrode, a positive voltage is applied to the other electrode in the pair, and vice versa to create the pickup pressure. Releasing the array of LED devices from the transfer heads 204 may be accomplished with a varied of methods including turning off the voltage sources, lowering the voltage across the pair of electrodes, changing a waveform of the AC voltage, and grounding the voltage sources.
Referring now to FIGS. 16A-16B, in an embodiment, an array of LED devices is transferred and bonded to a display substrate. For example, the display substrate 302 may be a thin film transistor (TFT) display substrate (i.e. backplane) similar to those used in active matrix OLED display panels. FIG. 16A is a top view illustration of a display panel 1600 in accordance with an embodiment of the invention. FIG. 16B is a side-view illustration of the display panel 1600 of FIG. 16A taken along lines X-X and Y-Y in accordance with an embodiment of the invention. In such an embodiment, the underlying TFT substrate 300 may include working circuitry (e.g. transistors, capacitors, etc.) to independently drive each subpixel 328. Substrate 300 may include a non-pixel area and a pixel area 304 (e.g. display area) including subpixels 328 arranged into pixels. The non-pixel area may include a data driver circuit 310 connected to a data line of each subpixel to enable data signals (Vdata) to be transmitted to the subpixels, a scan driver circuit 312 connected to scan lines of the subpixels to enable scan signals (Vscan) to be transmitted to the subpixels, a power supply line 314 to transmit a power signal (Vdd) to the TFTs, and a ground ring 316 to transmit a ground signal (Vss) to the array of subpixels. As shown, the data driver circuit, scan driver circuit, power supply line, and ground ring are all connected to a flexible circuit board (FCB) 313 which includes a power source for supplying power to the power supply line 314 and a power source ground line electrically connected to the ground ring 316. It is to be appreciated, that this is one exemplary embodiment for a display panel, and alternative configurations are possible. For example, any of the driver circuits can be located off the display substrate 300, or alternatively on a back surface of the display substrate 300. Likewise, the working circuitry (e.g. transistors, capacitors, etc.) formed within the substrate 300 can be replaced with micro chips 350 bonded to the top surface of the substrate 300 as illustrated in FIG. 16C.
In the particular embodiment illustrated, the TFT substrate 300 includes a switching transistor T1 connected to a data line from the driver circuit 310 and a driving transistor T2 connected to a power line connected to the power supply line 314. The gate of the switching transistor T1 may also be connected to a scan line from the scan driver circuit 312. A patterned bank layer 326 including bank openings 327 is formed over the substrate 300. In an embodiment, bank openings 327 correspond to subpixels 328. Bank layer 326 may be formed by a variety of techniques such as ink jet printing, screen printing, lamination, spin coating, CVD, PVD and may be formed of opaque, transparent, or semitransparent materials. In an embodiment, bank layer 326 is formed of an insulating material. In an embodiment, bank layer is formed of a black matrix material to absorb emitted or ambient light. Thickness of the bank layer 326 and width of the bank openings 327 may depend upon the height of the LED devices 156 transferred to and bonded within the openings, height of the electrostatic transfer heads, and resolution of the display panel. In an embodiment, exemplary thickness of the bank layer 326 is between 1 μm-50 μm.
Electrically conductive bottom electrodes 342, ground tie lines 344 and ground ring 316 may optionally be formed over the display substrate 300. In the embodiments illustrated an arrangement of ground tie lines 344 run between bank openings 327 in the pixel area 304 of the display panel 1600. Ground tie lines 344 may be formed on the bank layer 326 or alternative, openings 332 may be formed in the bank layer 326 to expose ground tie lines 344 beneath bank layer 326. In an embodiment, ground tie liens 344 are formed between the bank openings 327 in the pixel area and are electrically connected to the ground ring 316 or a ground line in the non-display area. In this manner, the Vss signal may be more uniformly applied to the matrix of subpixels resulting in more uniform brightness across the display panel 1600.
A passivation layer 348 formed around the LED devices 156 within the bank openings 327 may perform functions such as preventing electrical shorting between the top and bottom electrode layers 318, 342 and providing for adequate step coverage of top electrode layer 318 between the top conductive contacts 152 and ground tie lines 344. The passivation layer 348 may also cover any portions of the bottom electrode layer 342 to prevent possible shorting with the top electrode layer 318. In accordance with embodiments of the invention, the passivation layer 348 may be formed of a variety of materials such as, but not limited to epoxy, acrylic (polyacrylate) such as poly(methyl methacrylate) (PMMA), benzocyclobutene (BCB), polymide, and polyester. In an embodiment, passivation layer 348 is formed by ink jet printing or screen printing around the LED devices 156 to fill the subpixel areas defined by bank openings 327.
Top electrode layer 318 may be opaque, reflective, transparent, or semi-transparent depending upon the particular application. In top emission display panels the top electrode layer 318 may be a transparent conductive material such as amorphous silicon, transparent conductive polymer, or transparent conductive oxide. Following the formation of top electrode layer 318 an encapsulation layer 346 is formed over substrate 300. For example, encapsulation layer 346 may be a flexible encapsulation layer or rigid layer. In accordance with some embodiments of the invention, a circular polarizer may not be required to suppress ambient light reflection. As a result, display panels 3700 in accordance with embodiments of the invention may be packaged without a circular polarizer, resulting in increased luminance of the display panel.
In an embodiment, one or more LED devices 156 are arranged in a subpixel circuit. A first terminal (e.g. bottom conductive contact) of the LED device 156 is coupled with a driving transistor. For example, the LED device 156 can be bonded to a bonding pad coupled with the driving transistor. In an embodiment, a redundant pair of LED devices 156 are bonded to the bottom electrode 342 that is coupled with the driving transistor T2. The one or more LED devices 156 may be any of the LED devices described herein including a confined current injection area. A ground line is electrically coupled with a second terminal (e.g. top conductive contact) for the one or more LED devices.
A current can be driven through the one or more LED devices, for example, from the driving transistor T2. In a high side drive configuration the one or more LED devices may be on the drain side of a PMOS driver transistor or a source side of an NMOS driver transistor so that the subpixel circuit pushes current through the p-terminal of the LED device. Alternatively, the subpixel circuit can be arranged in a low side drive configuration in which case the ground line becomes the power line and current is pulled through the n-terminal of the LED device.
In accordance with embodiments of the invention, the subpixel circuit may operate at comparatively low currents or current densities in the pre-droop range of the characteristic efficiency curve of the LED devices, or near a maximum efficiency value past the pre-droop range. Thus, rather than increasing the size of the LED devices to increase efficiency, the effective size of the current injection area is confined in order to increase the current density within the LED device. In embodiments where the LED devices are utilized in display applications, as opposed to high-powered applications, the LED devices can operate at comparatively lower current ranges, where a slight increase in current density may result in a significant improvement in IQE and EQE of the LED devices.
In an embodiment, a subpixel circuit comprises a driving transistor, a first terminal (e.g. bottom electrically conductive contact) of an LED device with confined current injection area is coupled with the driving transistor, and a ground line is coupled with a second terminal (e.g. top electrically conductive contact) of the LED device. In an embodiment, the LED device is operated by driving a current through the LED device in response to sending a control signal to the driving transistor. In some embodiments, the current may range from 1 nA-400 nA. In an embodiment, the current ranges from 1 nA-30 nA. In an embodiment, an LED device is operated with a current from 1 nA-30 nA in a display having a 400 pixel per inch (PPI) resolution. In an embodiment, the current ranges from 200 nA-400 nA. In an embodiment, an LED device is operated with a current from 200 nA-400 nA in a display having a 100 PPI resolution. In some embodiments, an LED device is operated with a confined current density from 0.001 A/cm2 to 40 A/cm2. In an embodiment, the current density ranges from 0.001 A/cm2 to 3 A/cm2. In an embodiment, such a current density range may be applicable to a display having a 400 PPI resolution. In an embodiment, the current density ranges from 0.2 A/cm2 to 4 A/cm2. In an embodiment, such a current density range may be applicable to a display having a 100 PPI resolution.
The following examples are provided to illustrate the effect of current confinement, and the relationship of efficiency, current and current density for LED devices in accordance with embodiments of the invention. In accordance with embodiments of the invention, a designer may select a desired efficiency and luminance of an LED device with a characteristic efficiency curve, such as the exemplary efficiency curve illustrated in FIG. 1. Upon selecting the desired efficiency and luminance, the designer may tune the operating current and size of the confined current injection area (e.g. approximate current spreading layer pillar width) within the LED device to achieve the desired efficiency.
EXAMPLE 1
In one embodiment, a display panel is a 5.5 inch full high definition display with 1920×1800 resolution, and 400 pixels per inch (PPI) including a 63.5 μm RGB pixel size. To achieve a 300 Nit output (white) with LED devices having a 10% EQE, the display panel uses approximately 10 nA-30 nA of current per LED, assuming one LED per subpixel. For an LED device with a 10 μm×10 μm confined current injection area this corresponds to a current density of 0.01 A/cm2-0.03 A/cm2. This is well below the normal or designed operating conditions for standard LEDs.
EXAMPLE 2
In an embodiment, the parameters of Example 1 are the same, with a smaller 1 μm×1 μm confined current injection area. With this reduced current injection area the corresponding current density increases to 1 A/cm2-3 A/cm2. Thus, Example 2 illustrates that at operating currents of 10 nA-30 nA, small changes in current injection area from 10 μm×10 μm to 1 μm×1 μm can have a significant effect on current density. In turn, the change in current density may affect efficiency of the LED device.
EXAMPLE 3
In one embodiment, a display panel is a 5.5 inch full high definition display with 1920×1800 resolution, and 400 pixels per inch (PPI) including a 63.5 μm RGB pixel size. Each subpixel includes an LED device with a 10 μm×10 μm confined current injection area. Luminance is maintained at 300 Nit output (white). In this example, it is desired to achieve a 40% EQE. With this increased efficiency, lower operating currents may be used. In an embodiment, an operating current of 3 nA-6 nA per LED is selected. With these parameters an LED device with a 10 μm×10 μm confined current injection area operates at 0.003 A/cm2-0.006 A/cm2, and an LED device with a 1 μm×1 μm confined current injection area operates at 0.3 A/cm2-0.6 A/cm2.
EXAMPLE 4
In one embodiment, a display panel is a 5.5 inch display with a lower resolution of 100 PPI including a 254 μm RGB pixel size. To achieve a 300 Nit output (white) with LED devices having a 10% EQE, the display panel uses a higher operating current of approximately 200 nA-400 nA of current per LED, assuming one LED per subpixel. For an LED device with a 10 μm×10 μm confined current injection area this corresponds to a current density of 0.2 A/cm2-0.4 A/cm2. A 1 μm×1 μm confined current injection area corresponds to a current density of 20 A/cm2-40 A/cm2, and a 3 μm×3 μm confined current injection area corresponds to a current density of 2 A/cm2-4 A/cm2. Thus, Example 4 illustrates that with lower resolution displays, there is a smaller density of LED devices, and higher operating currents are used to achieve a similar brightness (300 Nit) as higher resolution displays.
EXAMPLE 5
In one embodiment, a display panel has 716 PPI including a 35 μm RGB pixel size. To achieve a 300 Nit output (white) with LED devices having a 10% EQE, the display panel uses an operating current of approximately 4-7 nA. With these parameters an LED device with a 10 μm×10 μm confined current injection area operates at 0.004 A/cm2-0.007 A/cm2, and an LED device with a 1 μm×1 μm confined current injection area operates at 0.4 A/cm2-0.7 A/cm2.
EXAMPLE 6
In another embodiment the required brightness of the display is increased to 3000 Nit. In all examples above the required current would increase about 10× if the same EQE is targeted. Subsequently, the current density would also increase 10× for the above examples. In one embodiment the required operating brightness is a range from 300 Nit to 3000 Nit. The current and subsequently the current density would span a range of 1-10× the 300 Nit range. In the case of Examples 1 and 2 (above) where now 300 Nit to 3000 Nit is required, an LED device with a 10 μm×10 μm confined current injection area operates at a current density of 0.01 A/cm2-0.3 A/cm2 and an LED device with a 1 μm×1 μm confined current injection area operates at 1 A/cm2-30 A/cm2.
In each of the above exemplary embodiments, the brightness of the display is such that the LED devices are operating at very low current densities that are not typical of standard LEDs. The typical performance of standard LEDs show low IQEs at current densities below 1 A/cm2. In accordance with embodiments of the invention, the current injection area is confined such that the current density can be increased to allow operation of the LED devices in a current density regime where IQE, and EQE, are optimized.
In an embodiment, the LED devices are bonded to a display substrate in a display area of the display substrate. For example, the display substrate may have a pixel configuration, in which the LED devices described above are incorporated into one or more subpixel arrays. The size of the LED devices may also be scalable with the available area of the subpixels. In some embodiments, the LED devices are bonded to a display substrate having a resolution of 100 PPI or more. In the Examples provided above, exemplary red-green-blue (RGB) pixel sizes of 35 μm were described for a display having 716 PPI, RGB pixels sizes of 63.5 μm were described for a display having 400 PPI, and RGB pixels sizes of 254 μm were described for a display having 100 PPI. In some embodiments, the LED devices have a maximum width of 100 μm or less. As display resolution increases, the available space for LED devices decreases. In some embodiments, the LED devices have a maximum width of 20 μm or less, 10 μm or less, or even 5 μm or less. Referring back to the above discussion with regard to FIG. 12, a non-radiative zone may occur along exterior surfaces of the active layer (e.g. along sidewalls of the LED devices), affecting efficiency of the LED devices. In accordance with embodiments of the invention, a current spreading layer is formed in a pillar configuration, in which the current spreading layer pillar protrudes from a cladding layer, and the width of the current spreading layer pillar may be adjusted relative to the width of the LED device (e.g. width of the active layer) in order to confine current within an interior of the active layer. In some embodiments, current spreading layer pillars have a width between 1 and 10 μm. In an embodiment, current spreading layer pillars have a width or diameter of approximately 2.5 μm.
FIG. 17 illustrates a display system 1700 in accordance with an embodiment. The display system houses a processor 1710, data receiver 1720, a display 1730, and one or more display driver ICs 1740, which may be scan driver ICs and data driver ICs. The data receiver 1720 may be configured to receive data wirelessly or wired. Wireless may be implemented in any of a number of wireless standards or protocols including, but not limited to, Wi-Fi (IEEE 802.11 family), WiMAX (IEEE 802.16 family), IEEE 802.20, long term evolution (LTE), Ev-DO, HSPA+, HSDPA+, HSUPA+, EDGE, GSM, GPRS, CDMA, TDMA, DECT, Bluetooth, derivatives thereof, as well as any other wireless protocols that are designated as 3G, 4G, 5G, and beyond. The one or more display driver ICs 1740 may be physically and electrically coupled to the display 1730.
In some embodiments, the display 1730 includes one or more LED devices 156 that are formed in accordance with embodiments of the invention described above. Depending on its applications, the display system 1700 may include other components. These other components include, but are not limited to, memory, a touch-screen controller, and a battery. In various implementations, the display system 1700 may be a television, tablet, phone, laptop, computer monitor, kiosk, digital camera, handheld game console, media display, ebook display, or large area signage display.
FIG. 18 illustrates a lighting system 1800 in accordance with an embodiment. The lighting system houses a power supply 1810, which may include a receiving interface 1820 for receiving power, and a power control unit 1830 for controlling power to be supplied to the light source 1840. Power may be supplied from outside the lighting system 1800 or from a battery optionally included in the lighting system 1800. In some embodiments, the light source 1840 includes one or more LED devices 156 that are formed in accordance with embodiments of the invention described above. In various implementations, the lighting system 1800 may be interior or exterior lighting applications, such as billboard lighting, building lighting, street lighting, light bulbs, and lamps.
In utilizing the various aspects of this invention, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for forming an LED device including any one of a confined current injection area. Although the present invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as particularly graceful implementations of the claimed invention useful for illustrating the present invention.

## Claims

1. An LED device comprising:
a top current spreading layer;
a top cladding layer below the top current spreading layer;
an active layer below the top cladding layer;
a bottom cladding layer below the active layer, the bottom cladding layer including a bottom surface;
sidewalls spanning the top current spreading layer, the top cladding layer, the active layer, and the bottom cladding layer, wherein the bottom surface of the bottom cladding layer extends between the sidewalls; and
a singular bottom current spreading layer pillar below the bottom cladding layer and in direct contact with the bottom cladding layer, wherein the bottom current spreading layer pillar is centrally located at and protrudes from the bottom cladding layer, and the bottom surface of the bottom cladding layer that extends between the sidewalls is wider than a maximum width of the bottom current spreading layer pillar, and the bottom current spreading layer pillar is doped with a first dopant type and the second current spreading layer is doped with a second dopant type opposite the first dopant type.

2. The LED device of claim 1, further comprising a passivation layer spanning along the bottom surface of the bottom cladding layer and sidewalls of the bottom current spreading layer pillar.

3. The LED device of claim 2, further comprising an opening in the passivation layer on a bottom surface of the bottom current spreading layer pillar opposite the bottom cladding layer.

4. The LED device of claim 3, further comprising a conductive contact within the opening in the passivation layer and in electrical contact with the bottom current spreading layer pillar.

5. The LED device of claim 4, wherein the conductive contact is not in direct electrical contact with the bottom cladding layer.

6. The LED device of claim 1, wherein the top current spreading layer is wider than the bottom current spreading layer pillar.

7. The LED device of claim 1, wherein the bottom current spreading layer pillar is doped with a p-dopant.

8. The LED device of claim 7, wherein the bottom current spreading layer pillar comprises GaP and the bottom cladding layer comprises a material selected from the group consisting of AlInP, AlGaInP, and AlGaAs.

9. The LED device of claim 1, wherein the active layer of the LED device has a maximum width of 100 μm or less, and the bottom current spreading layer pillar maximum width is 10 μm or less.

10. The LED device of claim 1, wherein the active layer of the LED device has a maximum width of 20 μm or less, and the bottom current spreading layer pillar maximum width is 10 μm or less.

11. The LED device of claim 1, wherein the active layer includes 1-3 quantum well layers.

12. A display system comprising:
a display substrate including a display area;
an LED device bonded to the display substrate within the display area and in electrical connection with working circuitry in the display substrate, the LED device comprising:
a top current spreading layer;
a top cladding layer below the top current spreading layer;
an active layer below the top cladding layer;
a bottom cladding layer below the active layer, the bottom cladding layer including a bottom surface;
sidewalls spanning the top current spreading layer, the top cladding layer, the active layer, and the bottom cladding layer, wherein the bottom surface of the bottom cladding layer extends between the sidewalls; and
a singular bottom current spreading layer pillar below the bottom cladding layer and in direct contact with the bottom cladding layer, wherein the bottom current spreading layer pillar is centrally located at and protrudes from the bottom cladding layer, and the bottom surface of the bottom cladding layer that extends between the sidewalls is wider than a maximum width of the bottom current spreading layer pillar, and the bottom current spreading layer pillar is doped with a first dopant type and the second current spreading layer is doped with a second dopant type opposite the first dopant type.

13. The display system of claim 12, wherein the LED device is in electrical connection with a micro chip bonded to the display substrate within the display area.