GaN containing optical devices and method with ESD stability

This application claims priority to U.S. Ser. No. 61/181,515, filed May 27, 2009; which is commonly assigned and hereby incorporated by reference.

The present invention relates generally to lighting techniques. More specifically, embodiments of the invention include techniques for fabricating an improved optical device with improved electrostatic discharge characteristics fabricated on bulk semipolar or nonpolar crystalline semiconductor materials. Merely by way of example, the invention can be applied to applications such as white lighting, multi-colored lighting, general illumination, decorative lighting, automotive and aircraft lamps, street lights, lighting for plant growth, indicator lights, lighting for flat panel displays, other optoelectronic devices, and the like.

In the late 1800's, Thomas Edison invented the light bulb. The conventional light bulb, commonly called the “Edison bulb,” has been used for over one hundred years. The conventional light bulb uses a tungsten filament enclosed in a glass bulb sealed in a base, which is screwed into a socket. The socket is coupled to an AC power or DC power source. The conventional light bulb can be found commonly in houses, buildings, and outdoor lightings, and other areas requiring light. Unfortunately, drawbacks exist with the conventional Edison light bulb. That is, the conventional light bulb dissipates much thermal energy. More than 90% of the energy used for the conventional light bulb dissipates as thermal energy. Additionally, the conventional light bulb routinely fails often due to thermal expansion and contraction of the filament element.

To overcome some of the drawbacks of the conventional light bulb, fluorescent lighting has been developed. Fluorescent lighting uses an optically clear tube structure filled with a halogen gas and, which typically also contains mercury. A pair of electrodes is coupled between the halogen gas and couples to an alternating power source through a ballast. Once the gas has been excited, it discharges to emit light. Typically, the optically clear tube is coated with phosphors, which are excited by the light. Many building structures use fluorescent lighting and, more recently, fluorescent lighting has been fitted onto a base structure, which couples into a standard socket.

Solid state lighting techniques have also been used. Solid state lighting relies upon semiconductor materials to produce light emitting diodes, commonly called LEDs. At first, red LEDs were demonstrated and introduced into commerce. Red LEDs use Aluminum Indium Gallium Phosphide or AlInGaP semiconductor materials. Most recently, Shuji Nakamura pioneered the use of InGaN materials to produce LEDs emitting light in the blue color range for blue LEDs. The blue colored LEDs led to innovations such as solid state white lighting, the blue laser diode, which in turn enabled the BlueRay™ DVD player, and other developments. Other colored LEDs have also been proposed.

High intensity UV, blue, and green LEDs based on GaN have been proposed and even demonstrated with some success. Efficiencies have typically been highest in the UV-violet, dropping off as the emission wavelength increases to blue or green. Unfortunately, achieving high intensity, high-efficiency GaN-based green LEDs has been particularly problematic. The performance of optoelectronic devices fabricated on conventional c-plane GaN suffer from strong internal polarization fields, which spatially separate the electron and hole wave functions and lead to poor radiative recombination efficiency. Since this phenomenon becomes more pronounced in InGaN layers with increased indium content for increased wavelength emission, extending the performance of UV or blue GaN-based LEDs to the blue-green or green regime has been difficult. Furthermore, since increased indium content films often require reduced growth temperature, the crystal quality of the InGaN films is degraded. The difficulty of achieving a high intensity green LED has lead scientists and engineers to the term “green gap” to describe the unavailability of such green LED. In addition, the light emission efficiency of typical GaN-based LEDs drops off significantly at higher current densities, as are required for general illumination applications, a phenomenon known as “roll-over.” Other limitations with blue LEDs using c-plane GaN exist. These limitations include poor yields, low efficiencies, and reliability issues. Although highly successful, solid state lighting techniques must be improved for full exploitation of their potential. These and other limitations may be described throughout the present specification and more particularly below.

From the above, it is seen that techniques for improving optical devices is highly desired.

According to the present invention, techniques for lighting are provided. More specifically, embodiments of the invention include techniques for fabricating an improved optical device with electrostatic discharge characteristics fabricated on bulk semipolar or nonpolar materials. Merely by way of example, the invention can be applied to applications such as optoelectronic devices, and the like. Other applications that desire polarized emission include liquid crystal display (“LCD”) backlighting, liquid crystal on silicon (“LCOS”) lighting, selected applications of home and/or building lighting, medical applications, biological sampling, plant and algae growth, biofuels, microscopes, film and video (e.g., amusement, action, nature, in-door), multi-dimensional display or televisions, micro and/or pico displays, health and wellness, optical and/or data transmission/communication, security and safety, and others.

In a specific embodiment, the present invention the present invention provides a light emitting diode device, commonly called an LED, e.g., single LED device, array of LEDs. The device has a gallium nitride (GaN) substrate structure, which includes a surface region. The device also has an epitaxial layer overlying the surface region and a p-n junction formed within a portion of the epitaxial layer. In a preferred embodiment, the device also has one or more plane or line defects spatially configured in a manner to be free from intersecting the p-n junction. In a specific embodiment, the one or more plane or line defects are present at a surface defect density of 1×10 6 cm −2 .

Benefits are achieved over pre-existing techniques using the present invention. In particular, the present invention uses either a non-polar or semipolar material, which has improved characteristics. In a specific embodiment, the present method and device using non-polar materials reduces polarization fields, which lead to reduced efficiency and gain. That is, the device has improved efficiency and gain as compared to conventional laser devices made of gallium containing materials made on conventional m-plane technology. In a preferred embodiment, the resulting device is smaller in size and has the capability of achieving power of large conventional devices. In a specific embodiment, the device has improved electro-static discharge characteristics. Depending upon the embodiment, the present apparatus and method can be manufactured using conventional materials and/or methods according to one of ordinary skill in the art. Depending upon the embodiment, one or more of these benefits may be achieved. These and other benefits may be described throughout the present specification and more particularly below.

The present invention achieves these benefits and others in the context of known process technology. However, a further understanding of the nature and advantages of the present invention may be realized by reference to the latter portions of the specification and attached drawings.

FIG. 1 shows a simplified diagram of a cross sectional view of a sample LED device structure fabricated on a bulk GaN substrate wafer according to an embodiment of the present invention;

FIG. 2 is a simplified cross-sectional view illustrating an optical device of the prior art;

FIG. 3 is a simplified cross sectional view illustrating an optical device according to an embodiment of the present invention;

FIG. 4 is a simplified plot of ESD voltage rating against defect density according to an embodiment of the present invention;

FIG. 5 is a simplified diagram of an equivalent circuit simulating the electro-static discharge that is used to test the ESD voltage breakdown characteristics of the LED according to an embodiment of the present invention; and

FIG. 6 is a simplified table of the various classifications of a device based upon the electro-static discharge characteristics tested under the human model circuit according to an embodiment of the present invention.

According to the present invention, techniques for lighting are provided. More specifically, embodiments of the invention include techniques for fabricating an improved optical device with electrostatic discharge characteristics fabricated on bulk semipolar or nonpolar materials. Merely by way of example, the invention can be applied to applications such as white lighting, multi-colored lighting, general illumination, decorative lighting, automotive and aircraft lamps, street lights, lighting for plant growth, indicator lights, lighting for flat panel displays, other optoelectronic devices, and the like.

While conventional optical devices have been fabricated on the c-plane of GaN substrates, researchers are exploring the properties of optical devices fabricated on m-plane GaN substrates. Specifically, c-plane bulk GaN is polar, while m-plane bulk GaN is non-polar along the growth direction. LEDs fabricated on the m-plane of bulk GaN substrates can emit highly polarized light. By utilizing non-polar GaN substrate-based LEDs in applications which require polarized light (such as LCD back-lighting), improved system efficiencies can therefore be achieved. Furthermore, optical devices are also fabricated from GaN substrates wherein the largest area surface is angled from the polar c-plane leading to semi-polar bulk GaN substrates. LEDs fabricated on bulk semi-polar GaN substrates can also emit partially polarized light according to other embodiments. The degree of polarization of the emission can be related to the crystallographic orientation of the largest surface area of the bulk GaN substrate, the composition and constitution of the individual layers that make up the LED structure, the electrical current density at which the polarization ratio is measured, how the measurement occurs, among other factors. Regarding the measurement, complex equipment including selected polarizers, photodetectors, and handling techniques are often required to determine the degree of polarization. The use of non-polar or semi-polar GaN in the fabrication of LEDs allows for the creation of optical devices that produce light of various levels of polarization

In order to increase the performance characteristics and reliability of devices fabricated using LEDs fabricated on bulk GaN substrates, it is necessary to improve the electrostatic discharge (ESD) characteristics of such LEDs. Specifically, improving the electrostatic discharge characteristics increases the breakdown voltage of the LED device allowing it to operate at the high currents generated by electro-static discharge. This in turn, leads to reduced device failure frequency and improved reliability. Such improved reliability is extremely noticeable in devices that use multiple arrays of LEDs such as display devices.

FIG. 1 shows a sample LED device fabricated on a bulk GaN substrate wafer. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize other variations, modifications, and alternatives. The substrate of the wafer includes a high quality nitride crystal with a release layer, as disclosed in U.S. Patent application 61/091,591, entitled, “NITRIDE CRYSTAL WITH RELEASE LAYER, METHOD OF MAKING, AND METHOD OF USE,” commonly assigned, and which is hereby incorporated by reference in its entirety. The nitride crystal comprises nitrogen and has a surface dislocation density below 10 5 cm −2 . The nitride crystal or wafer may comprise Al x In y Ga 1-x-y N, where 0≦x, y, x+y≦1. In one specific embodiment, the nitride crystal comprises GaN. In a preferred embodiment, the nitride crystal is substantially free of low-angle grain boundaries, or tilt boundaries, over a length scale of at least 3 millimeters. Optionally, the nitride crystal has a release layer with an optical absorption coefficient greater than 1000 cm-1 at least one wavelength where the base crystal underlying the release layer is substantially transparent, with an optical absorption coefficient less than 50 cm-1, and may further comprise a high quality epitaxial layer, which also has a surface dislocation density below 10 5 cm −2 . The release layer may be etched under conditions where the nitride base crystal and the high quality epitaxial layer are not. Of course, there can be other variations, modifications, and alternatives.

The substrate may have a large-surface orientation within ten degrees, within five degrees, within two degrees, within one degree, within 0.5 degree, or within 0.2 degree of (0 0 0 1), (0 0 0 −1), {1 −1 0 0}, {1 1 −2 0}, {1 −1 0 ±1}, {1 −1 0 ±2}, {1 −1 0 ±3}, or {1 1 −2 ±2}. The substrate may have a dislocation density below 10 4 cm −2 , below 10 3 cm −2 , or below 10 2 cm −2 . The nitride base crystal or wafer may have an optical absorption coefficient below 100 cm −1 , below 50 cm −1 or below 5 cm −1 at wavelengths between about 465 nm and about 700 nm. The nitride base crystal may have an optical absorption coefficient below 100 cm −1 , below 50 cm −1 or below 5 cm −1 at wavelengths between about 700 nm and about 3077 nm and at wavelengths between about 3333 nm and about 6667 nm.

In one or more embodiments, the device can be configured with an optional release layer. The release layer comprises heavily cobalt-doped GaN, has a high crystal quality, and is substantially black, with an optical absorption coefficient greater than 1000 cm −1 or greater than 5000 cm −1 across the visible spectrum, including the range between about 465 nm and about 700 nm. The release layer is between about 0.05 micron and about 50 microns thick and has a temperature stability approximately the same as the underlying base crystal and exhibits minimal strain with respect to the underlying base crystal.

An n-type Al u In v Ga 1-u-v N layer, where 0≦u, v, u+v≦1, is deposited on the substrate. The carrier concentration may lie in the range between about 10 17 cm −3 and 10 20 cm −3 . The deposition may be performed using metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).

Following deposition of the n-type Al u In v Ga 1-u-v N layer for a predetermined period of time, so as to achieve a predetermined thickness, an active layer is deposited. The active layer may comprise a single quantum well or a multiple quantum well, with 2-10 quantum wells. The quantum wells may comprise InGaN wells and GaN barrier layers. In other embodiments, the well layers and barrier layers comprise Al w In x Ga 1-w-x N and Al y In z Ga 1-y-z N, respectively, where 0≦w, x, y, z, w+x, y+z≦1, where w<u, y and/or x>v, z so that the bandgap of the well layer(s) is less than that of the barrier layer(s) and the n-type layer. The well layers and barrier layers may each have a thickness between about 1 nm and about 20 nm. In another embodiment, the active layer comprises a double heterostructure, with an InGaN or Al w In x Ga 1-w-x N layer about 20 nm to about 500 nm thick surrounded by GaN or Al y In z Ga 1-y-z N layers, where w<u, y and/or x>v, z. The composition and structure of the active layer are chosen to provide light emission at a preselected wavelength. The active layer may be left undoped (or unintentionally doped) or may be doped n-type or p-type.

Next, a p-type doped Al q In r Ga 1-q-r N, where 0≦q, r, q+r≦1, layer is deposited above the active layer. The p-type layer may be doped with Mg, to a level between about 10 17 cm −3 and 10 21 cm −3 , and may have a thickness between about 5 nm and about 500 nm. The outermost 1-30 nm of the p-type layer may be doped more heavily than the rest of the layer, so as to enable an improved electrical contact.

A reflective electrical contact, with a reflectivity greater than about 70%, is then deposited on the p-type semiconductor layer or on the second n-type layer above a tunnel junction, if it is present. In another embodiment, the reflective electrical contact is placed on the n-type side of the device structure. In a preferred embodiment, the reflectivity of the reflective electrical contact is greater than 80% or greater than 90%. The reflective electrical contact may comprise at least one of silver, gold, aluminum, nickel, platinum, rhodium, palladium, chromium, or the like. The reflective electrical contact may be deposited by thermal evaporation, electron beam evaporation, sputtering, or another suitable technique. In a preferred embodiment, the reflective electrical contact serves as the p-type electrode for the textured-surface LED. In another embodiment, the reflective electrical contact serves as an n-type electrode for the textured-surface LED. Further details of the present invention are found throughout the present specification and more particularly below. Of course, there can be other variations, modifications, and alternatives.

As is shown in the FIG. 2 , a cross sectional view of conventional optical devices is shown. The optical device is fabricated on c plane GaN substrates, which have defects that extend from the substrate material into and through the active layer of the optical device. Such defects can include both planar and line defects, including threading dislocations. Typically such defects exist with a surface density of at least 1×10 6 cm-2 in bulk GaN substrates. Such defects propagate along the direction of the c plane. As a result, by growing epitaxial layers on top of c plane GaN substrates, these defects extend into the epitaxial layer in which the optical devices are fabricated.

In device operation, the defects are disadvantageous, by dramatically degrading the electro-static discharge characteristics of the device. Specifically, the presence of defects within the p type, n type and active layers of the device provide regions that reduce the breakdown voltage of the device. This occurs as a result of electrostatic events that produce undesirable charge within the device. The defects serve as conduction pathways through which the charge is transmitted. The large amount of current that is generated within these defects, greatly increases device temperature and causes overall damage to the device, as the charge is not uniformly dissipated throughout the entire device. The subsequent damage and currents cause by individual or repeated electro-static discharge events decrease the break down voltage leading to reduced performance characteristics and subsequent failure of the LED device. Thus, in order to improve applications utilizing such LED devices it is often necessary to provide LEDs with improved electro-static discharge characteristics.

FIG. 3 shows a cross sectional view of the optical device of the present invention with improved electrostatic discharge characteristics according to a specific embodiment. The device includes an epitaxial layer grown on top of a bulk m plane GaN substrate. Bulk m plane GaN is created along the surface that is angled ranging from about 80-100 degrees from the polar orientation of the c plane towards an (h k l) plane wherein l=0, and at least one of h and k is non-zero. Bulk m plane GaN is non-polar leading to the emission of polarized light. Likewise, the bulk GaN material substrate of the present invention can be semipolar, by orientating the surface of the GaN material at an angle to the c plane. Specifically such angles can range from 0.1 to 80 degrees or 110-179.9 degrees from the polar c plane orientation described above towards an (h k l) plane wherein l=0, and at least one of h and k is non-zero in one or more embodiments. Optical devices fabricated on semipolar GaN substrates emit electromagnetic radiation of varying degrees of polarization. The degree of polarization is dependent upon the level of polarity of the GaN material from which the electromagnetic radiation is emitted. Such level of polarity of the GaN substrate is dependent upon the orientation of the GaN substrate surface with respect to the polar c plane.

The optical device of the present invention is fabricated within the epitaxial layer according to a specific embodiment. The device has a p type region layered on top of a n type region in order to create a diode structure. The active region is formed at the junction of the p and n type regions and includes another dopant in order to achieve improved light emission characteristics. The present device substantially eliminates the existence of defects extending from the substrate material into the actual device structure, including the active region in the epitaxial layer. As the defects typically are orientated along the c plane of the GaN substrate material, in using m plane GaN substrate material such defects are generally parallel to the surface of the GaN substrate material, and as a result do not extend into the epitaxial layer. Such defects still exist however at a surface density of at least 1×10 6 cm-2 within the bulk substrate material.

By reducing the dislocation density within the epitaxial layer, and more specifically through the active layer of the optical device, the electro-static discharge characteristics of the optical device are improved. This leads to improve reliability of devices using such LED devices, including systems that use multiple LED arrays. Specifically, as shown in FIG. 4 as the surface dislocation density is decreased, the breakdown voltage of the device increases according to one or more embodiments. Devices with higher breakdown voltages are capable of withstanding greater amounts of electro-static discharge before device failure, thereby leading to improved electro-static discharge characteristics.

In testing the electro-static discharge characteristics of an LED device, a model circuit, shown in FIG. 5 , is designed to simulate the effect of electro-static discharge created through the contact of a human to the LED. The circuit includes a high voltage power source, couple in series with a resistor and through a switch to a capacitor. Once the capacitor becomes charged, the switch is activated such that the capacitor is in series with a resistor and the LED. The resistor corresponds to the resistance of the human body that electro-static discharge would encounter. As soon as the switch is activated, the capacitor discharges exposing the LED to a sudden high level of charge simulating an electro-static discharge event. The process can be repeated numerous times in order to expose the LED to multiple discharge events.

After the LED is exposed to single or numerous electro-static discharge events, the current voltage I-V relationship of the LED is characterized, to determine if breakdown of the device has occurred. The highest voltage caused by electrostatic discharge at which the device can still operate is known as the ESD breakdown voltage. As shown previously the highest break down voltages are achieved when the surface defect density is minimized. As the surface defect density increases, the break down voltage decreases. LEDs that have higher ESD breakdown voltages are capable of withstanding either electro-static discharge events that generate greater amounts of charge, or multiple electro-static discharge events. The sensitivity of a device to electro-static discharge simulated by the human body model are categorized into specific classes as shown in FIG. 6 , based upon the ESD breakdown voltage of the LED.

While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. As an example, the packaged device can include any combination of elements described above, as well as outside of the present specification. Therefore, the above description and illustrations should not be taken as limiting the scope of the present invention which is defined by the appended claims.