1.  Field of the Invention
The present invention relates to diodes, and more particularly, to a vertical structure light emitting diode (LED).  Although the present invention is discussed with reference to a GaN-based light emitting diode, the present invention can be used for various types of light emitting diodes, and can be further used in a wide range of applications including, for example, other types of diodes such as laser diodes. 
2.  Discussion of the Related Art
Light emitting diodes, commonly referred to as “LEDs,” are semiconductor devices which convert electrical power into emitted light.  It is well known in the art that when electrons transition between their allowed energy levels in atoms or molecules, these transitions are always accompanied by a gain or loss of specific quanta of energy.  In a light emitting diode, the generation or injection of a current of either electrons or holes across the diode junction encourages such electronic transitions, which in turn result in either vibrational energy or light, or both.  As is further known in the art, the color of light that can be produced by a light emitting diode is generally limited by the characteristics of the semiconductor material, most significantly by the band-gap which represents the energy level differences between the valence band and conduction band of individual atoms. 
Gallium-Nitride (GaN) has recently gained much attention from researchers in the field of LEDs, since its material characteristics of a wide and direct band gap are suitable for making a blue LED, which has been considered the most difficult one to fabricate among other red and green LEDs. 
Accordingly, GaN-based opto-electronic device technology has rapidly evolved from the realm of device research and development to commercial reality, since these devices have been introduced in the market in 1994.  The efficiency of GaN light emitting diodes, for example, has surpassed that of incandescent lighting, and is now comparable with that of fluorescent lighting. 
The market growth for GaN-based devices has been far exceeding than the industrial market prediction every year.  Despite of such rapid speed of the development, it is still too expensive to realize a full color display with GaN-based devices.  This is because the manufacturing cost of blue LEDs, which are essential to realizing a full color display, is high compared with the other visible LEDs.  The wafer size for making blue LEDs is limited to 2 inches, and the process of growing a GaN epitaxial layer is more difficult than other semiconductor materials.  Therefore, it is crucial that developments of mass production technology, without sacrificing performance, are the main issue in reducing the manufacturing costs of blue LEDs, thereby to utilize full color displays using GaN LEDs at an inexpensive price, the efficiency of which is far better than currently available. 
In general, GaN-based LEDs are fabricated with a lateral structure using a sapphire substrate, since sapphire is the material that makes the GaN epitaxial layer grow with fewer defects than other materials as a substrate.  Since sapphire is an electrical insulator, the lateral type LEDs having both n and p metal contacts on the topside is inevitable to inject current flows in the MQW layer. 
FIG.  1 schematically illustrates conventional lateral type LED device.  Referring to FIG.  1, the convention lateral type LED includes a substrate 100, such as sapphire.  A buffer layer 120, which is optional and is made of, for example, gallium nitride (GaN), is formed on the substrate 100.  An n-type GaN layer 140 is formed on the buffer layer 120.  An active layer such as a multiple quantum well (MQW) layer 160 of aluminum-indium-gallium-nitride (AlInGaN), for example, is formed on the n-type GaN layer 140.  A p-type GaN layer 180 is formed on the active layer 160.  A transparent conductive layer 220 is formed on the p-GaN layer 180.  The transparent conductive layer 220 may be made of any suitable material including, for example, Ni/Au or indium-tin-oxide (ITO).  A p-type electrode 240 is then formed on one side of the transparent conductive layer 220.  The p-type electrode 240 may be made of any suitable material including, for example, Ni/Au, Pd/Au, Pd/Ni and Pt.  A pad 260 is formed on the p-type electrode 240.  The pad may be made of any suitable material including, for example, Au.  The transparent conductive layer 220, the p-GaN layer 180, the active layer 160 and the n-GaN layer 140 are all etched at one portion to form an n-electrode 250 and pad 270. 
Since sapphire is an insulator, the n-GaN layer should be exposed to form an n-metal contact.  A dry-etching method is generally used, since GaN is not etched by a chemical etching method.  This is a significant disadvantage since additional lithography and stripping processes are required.  In addition, plasma damages on the GaN surface are often sustained during a dry-etch process.  Moreover, the lateral device structure requires a large device dimension since two metal contacts need to be formed on top of the device.  Furthermore, the lateral structure device is vulnerable to static electricity because two metal electrodes are positioned close each other.  Thus, the lateral structure GaN based LEDs may not be suitable for high voltage applications, such as traffic indicators and signal lights. 
Currently, a vertical structure of GaN-based LEDs is fabricated by Cree Research Inc. using a silicon carbide (SiC) substrate.  Due to the high manufacturing cost, however, the LEDs with SiC substrate are not suitable for mass production.  In addition, SiC is known in the art to be very sensitive to hydrogen atoms, which exist during the epitaxial growth of GaN layer by metal organic chemical vapor deposition (MOCVD) method, which is the most common way of growing GaN epitaxial, layers considering the epitaxial film quality.  An additional process called “surface treatment” is necessary in order to grow high quality GaN-based epitaxial films.  Furthermore, the GaN based LEDs with a SiC substrate requires an additional conductive buffer layer on the SiC substrate before growing the GaN epitaxial layer, which is not necessary for lateral structure devices.