Vertical light-emitting diode device structure with SixNy layer

A vertical light-emitting diode (VLED) structure fabricated with a SixNy layer responsible for providing increased light extraction out of a roughened n-doped surface of the VLED are provided. Such VLED structures fabricated with a SixNy layer may have increased luminous efficiency when compared to conventional VLED structures fabricated without a SixNy layer. Methods for creating such VLED structures are also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to the field of light-emitting diode (LED) technology and, more particularly, to a vertical light-emitting diode (VLED) structure with increased light extraction.

2. Description of the Related Art

Luminous efficiency of LEDs can be defined as the total apparent power of a light source to its actual total input power (luminous flux divided by input power). Having units of lumens per watt, luminous efficiency measures the fraction of power which is useful for lighting. As a type of light source, light-emitting diodes (LEDs) have been designed and developed over the past few decades to make improvements in luminous efficiency and increase the number of possible applications for these solid state devices.

Beginning with a conventional LED structure whose cross-section is shown inFIG. 1, one can see why the luminous efficiency of these devices is relatively poor. A conventional LED100is formed on a substrate104such as sapphire, silicon carbide, silicon, germanium, ZnO or gallium arsenide depending on the composition of the LED layers to be deposited. An n-doped layer102is disposed above the substrate104, and this layer102may comprise n-doped GaN. GaN may be grown on a sapphire substrate for emitting green to ultraviolet (UV) wavelengths of light. A multiple quantum well (MQW) active layer103is deposited above the n-doped layer112, and this is where photon generation occurs when the diode is properly biased. A p-doped layer106is grown above the active layer103inFIG. 1. After portions of the p-doped layer106and the active layer103are removed to expose a portion of the n-doped layer102, electrodes108and110may be formed on the p-doped and n-doped layers, respectively, for forward biasing the LED.

To improve upon some of the design limitations for luminous efficiency of conventional LEDs, the vertical light-emitting diode (VLED) structure was created. The VLED earned its name because the current flows vertically from p-electrode to n-electrode, and a typical VLED200is shown inFIG. 2. To create the VLED200, an n-doped layer102is deposited on a substrate (not shown), and this may comprise any suitable semiconductor material for emitting the desired wavelength of light, such as n-GaN or a combination of undoped GaN and n-GaN. A multiple quantum well (MQW) active layer103from which the photons are emitted is grown above the n-doped layer102. A p-doped layer106is deposited above the active layer103inFIG. 2. A metal layer202may be deposited above the p-doped layer106for electrical conduction and heat dissipation away from the VLED.

Unwanted dislocations112may form in an LED during the growing of one or more of the layers that make up the LED. In conventional LEDs, current flows along the surface very far from the interface of the substrate104and the n-doped layer102so the effects of dislocations on current are not obvious. Unwanted dislocations may also occur in a VLED, and because the dislocations in a VLED may run in the direction of the current, reductions in the dislocation density may have a more noticeable effect on decreasing the leakage current. Leakage current, as defined herein, generally refers to the current measured when −5V of reversed bias is applied to the LED electrodes.

Accordingly, what is needed is a light-emitting solid state device with reduced dislocation density and increased luminous efficiency.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a method of increasing light extraction from a vertical light-emitting diode (VLED) device. The method generally includes depositing a first n-doped layer above a carrier substrate; depositing a SixNymask above the first n-doped layer, wherein the SixNymask has openings exposing portions of the first n-doped layer; depositing a second n-doped layer above the SixNymask such that the second n-doped layer is also deposited in the openings; depositing a p-doped layer above the active layer; depositing one or more metal layers above the p-doped layer; removing the carrier substrate; and roughening a surface of the first n-doped layer, such that the SixNymask is configured to increase light extraction from the roughened surface of the first n-doped layer.

Embodiments of the present invention provide a method of increasing light extraction from a VLED device. The method generally includes depositing a first n-doped layer above a carrier substrate; depositing a SixNymask above the first n-doped layer, wherein the SixNymask has openings exposing portions of the first n-doped layer; depositing a second n-doped layer above the SixNymask such that the second n-doped layer is also deposited in the openings; depositing an active layer for emitting light above the second n-doped layer; depositing a p-doped layer above the active layer; depositing one or more metal layers above the p-doped layer; removing the carrier substrate; removing the first n-doped layer and the SixNymask to expose a surface of the second n-doped layer; and roughening the exposed surface of the second n-doped layer to increase light extraction from the roughened surface.

DETAILED DESCRIPTION

Embodiments of the present invention provide a vertical light-emitting diode (VLED) structure fabricated with a SixNylayer responsible for providing increased light extraction out of a roughened n-doped surface of the VLED. Embodiments of the present invention also provide for a method of creating such a VLED.

An Exemplary VLED Device Having a SixNyLayer

The depositing of a SixNylayer may reduce the dislocation density in the layers of a VLED and lead to increased light extraction.FIGS. 3A-3Fillustrate various steps in the fabrication of a VLED according to an embodiment of the invention.

InFIG. 3A, a first n-doped layer304may be deposited on a carrier substrate302. The first n-doped layer304may comprise any suitable semiconductor material for LED functionality, such as n-doped GaN (n-GaN) or a combination of undoped GaN and n-GaN. Any suitable material such as sapphire, silicon carbide (SiC), silicon, germanium, zinc oxide (ZnO), or gallium arsenide (GaAs) may be used as the carrier substrate302.

The depositing of the first n-doped layer304may be performed using any suitable thin film deposition techniques, such as electrochemical deposition, electroless chemical deposition, chemical vapor deposition (CVD), metal organic vapor phase epitaxy, metal organic CVD (MOCVD), plasma enhanced CVD (PECVD), atomic layer deposition (ALD), physical vapor deposition (PVD), evaporation, plasma spray, or a combination of these techniques. Dislocations306may form in the first n-doped layer304. The thickness of the first n-doped layer304may be any suitable thickness, such as in a range from 0.1 to 10 microns. When MOCVD is used to deposit the first n-doped layer, a first n-doped layer thickness of at least 0.5 microns may be suitable for increased light extraction.

InFIG. 3B, a SixNymask308may be deposited above the first n-doped layer304. The SixNymask308may partially cover the first n-doped layer leaving a connecting area for the first n-doped layer304and a second n-doped layer310that may be deposited later (seeFIG. 3C). The coverage of the SixNymask may not be continuous and may leave an area of the first n-doped layer exposed for subsequent growth of another n-doped layer.

The SixNymask308may be deposited by any suitable technique such as depositing individual “islands” through sputtering or by depositing a continuous SixNylayer and then removing parts of the continuous layer to form individual islands. Using MOCVD, the SixNymask308may be grown using silane (SiH4), disilane (Si2H6), or any derivative of silane (SinH2n+2) combined with NH3(or any material containing nitrogen that can decompose at growth temperatures and emits nitrogen). The SixNymask may be formed using in situ deposition where the SixNymask is created in the process chamber and used as a mask in the position where it was created. The SixNymask308may be deposited with any suitable growth temperature that is higher than the growth temperature of GaN (higher by 5 to 100° C.) especially with MOCVD. When the growth temperature of the SixNymask is higher than the growth temperature of GaN, the leakage current of the LED may be reduced. The shape of the SixNyislands deposited above the first n-doped layer304may be any suitable shape created by any of the above-mentioned processes of depositing the SixNymask such as rectangular, trapezoidal, or pyramidal.

The SixNymask308may be deposited with any suitable thickness as long as the mask does not cover the whole first n-doped layer. Increased coverage of the SixNymask islands may indicate that a thicker layer may be required to recover a continuous n-doped layer after the SixNymask. In order to create a VLED that is practical for mass production, the coverage of the SixNymask is chosen so a subsequent n-doped layer recovers after a suitable growth thickness such as 4 to 8 microns. Using MOCVD, the growth pressure of the SixNymask may not be as important as the growth temperature. As long as the growth temperature of the SixNymask is higher than the n-doped layer's growth temperature (e.g., 5-100° C. higher), the subsequent n-doped layer (a second n-doped layer described below) may recover with much less leakage current in the resulting VLED

InFIG. 3C, a second n-doped layer310may be deposited above the SixNymask and may be formed using any of the above-described deposition techniques. The second n-doped layer310may contain dislocations306in areas that were not covered by the SixNymask308. Some dislocations may have been prevented from developing above the layer with the SixNymask308in the second n-doped layer310resulting in decreased dislocation density in the second n-doped layer310and thus, lower leakage current in the resulting VLED structure.

InFIG. 3D, an active layer314for emitting light may be deposited above the second n-doped layer, and a p-doped layer312may be deposited above the active layer. Both layers312,314may be formed using any of the above-described deposition techniques. InFIG. 3E, one or more metal layers316may be deposited above the p-doped layer312rather than attached with wafer bonding or gluing. The one or more metal layers316may be deposited using any suitable thin film deposition technique, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), plasma enhanced CVD (PECVD), evaporation, ion beam deposition, electrochemical deposition, electroless chemical deposition, plasma spray, or ink jet deposition. The metal layer may comprise any suitable material for electrical and thermal conduction, such as chromium (Cr), platinum (Pt), nickel (Ni), copper (Cu), Cu on a barrier metal material (e.g., titanium nitride, tungsten, tungsten nitride, tantalum nitride), molybdenum (Mo), tungsten (W) or a metal alloy. One or more of the metal layers may be formed by electrochemical plating or electroless chemical plating. One or more metal layers may be deposited on a seed metal layer. The seed metal layer may be grown via electroless plating and assist in the growth of a single metal layer or of multiple metal layers via electroplating.

InFIG. 3F, the carrier substrate302may be removed. The carrier substrate removal may be done using laser, etching, grinding/lapping, chemical mechanical polishing (CMP), wet etching, or any other suitable technique. After the carrier substrate is removed, the exposed surface317of the first n-doped layer may be roughened in an effort to increase the light extraction according to Snell's law. The roughening may occur through any suitable technique such as through a photoelectrochemical oxidation process as described below, by wet etching, or by dry etching. An electrode318for external connection may be deposited on the surface317after roughening.

Roughening may occur using any suitable process, such as the process described in U.S. Pat. No. 7,186,580 to Tran, issued Mar. 6, 2007 and entitled “LIGHT EMITTING DIODES (LEDS) WITH IMPROVED LIGHT EXTRACTION BY ROUGHENING.” One embodiment teaches the photoelectrochemical (PEC) oxidation and etching of the n-doped layer. PEC oxidation and etching may be performed in a system with an aqueous solution, an illumination system, and an electrically biased system. The aqueous solution may be a combination of oxidizing agent and either acid or alkaline solutions. The oxidizing agent may be any suitable agent such as one or the combination of H2O2and K2S2O8, among others. The acid solution may be any suitable solution, such as one or more of H2SO4, HF, HCl, H3PO4, HNO3, and CH3COOH. The alkaline solution may be any suitable solution, such as one or the mixture of KOH, NaOH, and NH4OH. The illumination may be performed through any suitable method, such as by an Hg or Xe arc lamp system with wavelength ranging from the visible to the ultraviolet spectrum. The illumination may be exposed on the n-type III-nitride semiconductors with an intensity less than 200 mW/cm2. An electrical bias may be applied to the conductive substrate, and the voltage may be controlled between −10 and +10 V. The oxidation-dominant, the etching-dominant, or the combined reactions may be controlled in any suitable method, such as being controlled to optimize the roughness of the n-GaN surface by varying the constitution of the aqueous solution, the electrical bias, and/or the illumination intensity. The non-ordered textured morphology also may be revealed after the roughening process.

FIG. 4Aillustrates a cross-sectional schematic representation of a vertical light-emitting diode (VLED) structure400according to an embodiment of the invention. After the above-described process, the SixNymask308may partially cover the first n-doped layer304, and a surface402of the first n-doped layer304may have been roughened as described above.

The VLED410illustrated inFIG. 4Bresults from the roughening process described inFIG. 4Acontinuing until the SixNymask308has been removed. Part of the second n-doped layer310may be removed, as well. An electrode318may be disposed on the roughened surface412as illustrated inFIG. 4B. The roughened surface412of the VLED may most likely result in greater light extraction than if the SixNymask was not present initially.

An Exemplary Surface Roughness Due to the SixNyLayer

The presence of SixNymask “islands” partially covering the first n-doped layer304may reduce the dislocation density in one or more of the second n-doped layer310, the active layer314and the p-doped layer312. In VLEDs, current flows along the direction of the dislocations and so more dislocations can mean more leakage current in the VLED. The reduction of the dislocation density may most likely reduce the leakage current in the VLED.

Additionally, the presence of the SixNymask may provide for increased light extraction from the surface402, when roughened. The presence of the SixNymask “islands” may alter the roughening process and create a more even roughening with greater surface area on the exposed surface of the first n-doped layer or the second n-doped layer and increased light extraction from the VLED. A VLED with a SixNymask as described above may provide for more even roughening on the first or the second n-doped layer depending on the depth of etching and, hence, more light extraction than a conventional VLED without the SixNymask.

The presence of the SixNymask partially covering the first n-doped layer, once exposed by removing the first n-doped layer, may lead to differently shaped surface structures on the second n-doped layer (e.g., higher density of the pyramid structures at the surface resulting in a greater effective surface area for photons to escape) when compared to other VLED structures fabricated without the SixNymask. Resulting surface structures created on the second n-doped layer may be high density smaller pyramidal or conical structures. These structures may increase the light extraction of the VLED. Therefore, the SixNymask may not only decrease the leakage current, but may also increase the light extraction of the VLED.

FIGS. 5A and 5Billustrate exemplary surfaces that may occur after roughening. The light-emitting surface510of the second n-doped layer in a VLED fabricated with a SixNymask shown inFIG. 5Bhas more even roughening with higher density of pyramidal structures resulting in greater surface area than the light-emitting surface500of the n-doped layer in a VLED fabricated without the SixNymask shown inFIG. 5A. Therefore, the VLED fabricated with a SixNymask and second n-doped layer exposed as shown inFIG. 5Bmay most likely have more light extraction according to Snell's Law and, thus, greater luminous efficiency when compared to VLEDs fabricated without the SixNymask.