GaN/AlGaN DEVICES
FABRICATION AND CHARACTERIZATION OF GaN BASED LEDs
In brief, the LED is a semiconductor device that consists of a back-to-back sandwich of p-type and n-type semiconductor materials. When an electric current is passed through the diode, electrons in the conduction band flow across the junction from the n-type side, and holes in the valence band flow from the p-type side. The result is that a significant number of electrons and holes recombination in the p-n junction, emission light with an energy hn=Eg (as shown in fig.1).
Packaged LED chip
All commercially available LEDs in the visible to infrared region (about 400-1,500nm) are made of III-V compound semiconductor thin films, consisting of elements belonging to group III (Al, Ga, In) and group V (N, P, As).
In general, the LED structure is the compound semiconductor epitaxial film grown on suitable substrate. Two most modern techniques that have become the workhorses for production of the most advanced devices, including LED, are molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). After getting the LED structure wafer, it is needed to fabricate to the chip and package the chip according to use (as shown in fig.2).
Figure 2 LED vertical supply chain
The visible LED has come a long way since its introduction almost 40 years ago and has yet to show any signs of slowing down. A Blue LED, which has only recently become available in production quantities, will result in an entire generation of new applications. Blue LEDs because of their high photon energies (>2.5eV) and relatively low eye sensitivity have always been difficult to manufacture. In addition the technology necessary to fabricate these LEDs is very different and far less advanced than standard LED materials. The blue LEDs available today consist of GaN (gallium nitride) and SiC (silicon carbide) construction with brightness levels in excess of 1000mcd at 20mA for GaN devices. Since blue is one of the primary colors, (the other two being red and green), full color solid state LED signs, TV’s, very large displays etc. became commercially available. Other applications for blue LEDs include medical diagnostic equipment and photolithography. It is also possible to produce other colors using the same basic GaN technology and growth processes. For example, a high brightness green (approximately 500nm) LED has been developed that is currently being evaluated for use as a replacement to the green bulb in traffic lights. Other colors including purple and white are also possible. It is possible to create every colour with LEDs by combining red, green and blue (RGB), which can create white light and all other colours. RGB technology is an improvement over the old technology of creating white light by combining blue with a coating of yellow.
As of 2001, the major uses of LEDs were for backlighting (30%), automotive purposes, such as headlights and brake lights (26%), signage (26%), electronic equipment such as DVD displays (10%), illumination for functional lighting (4%) and signals (4%).
Figure 3 Packaged High Bright LEDs
In recent years, the III-nitride semiconductors have attracted wide interests and intense research due to their tremendous advantages in application of optoelectronic devices such as green, blue, violet light-emitting diode (LED) and ultra-violet laser diodes. Because the blue LED can tune to any other color with other LEDs or even use a single blue LED in combination with phosphor to make “white LED”, it become capable to achieve solid-state general illumination for humankind.
The III-nitrides are not really new materials; many researchers have made important contribution in this field. Based on these remarkable breakthroughs, Nichia announced the GaN based blue LED is commercially available in 1995, which actually ended the history of non-blue solid-state light source. III-nitride bring a revolution in the optoelectronic technique field, and have become one of the mainstreams in semiconductor industry.
Compared the traditional light source, the LED has many prominent advantages of low energy cost, long life time and little bulk, which is becoming the potential competitor in lighting market. Because lighting accounts for about 20% of total electricity consumption, national programs of promoting LEDs for lighting are underway in the US, Japan, Korea and China, whose main motivation is large-scale energy savings, along with the benefits of reduced oil imports, lower greenhouse gas emissions.
There are two fundamental reasons to choose the III-nitride for the blue light source. Foremost is that AlN, GaN and InN have respective direct band gap energies of 6.2eV, 3.4eV and 0.7eV at room temperature, which cover the spectra from UV to entire visible spectrum. The other main advantage _of III-nitride over other wide-band semiconductors is the stronger chemical bond, which made the nitride very stable and resistant to the degradation _under high electric current and temperature.
Although the GaN based LED has been industrialized for several years, many base issues of the material physics and device (such as the mechanisms of light emission, the role of dislocation and the band gap of InN) are still in debate. So the III-nitride provides wide space for physicist and material researchers.
1. GaN based blue LED structure design
In general, the GaN based LED structure includes the InxGa1-xN/GaN single or multi quantum wells active layers sandwiched between n-type and p- _type nitride materials with larger band gap (as shown in fig.1), which can increase the light emission efficiency by improving the electrons and holes confinement. Holes and electrons are injected on opposite sides of the device, and travel through various layers before recombining in the active region. To the LED structure design, it is a crucial element to locate the active layers into depletion region of PN junction the by adjusting the n and p type doping.
Figure 4 GaN based LED structure
Silicon doping is the almost universally used for achieving n-type GaN typically by flowing SiH4 during metalorganic chemical vapor deposition (MOCVD) growth. The n-type GaN grown on sapphire are typically heavily doped (>1018cm-3) and constitute more than 80% of the entire device structure to achieve both low contact resistance and current spread. In contrast, Mg is most commonly used as the acceptor dopant, and the p-type doping level in GaN is very low due to the Mg impurity form deep acceptors. Thus, films containing hole concentration in the range of 1-10´1017cm-3, have Mg concentration levels more than 1019cm-3. If the LED structure is designed by arranging the p-type GaN next to the active layers, Mg atoms can diffuse into the active layer at the growth temperature, leading to donor-acceptor pair (DAP) recombination, which will depress the light-emitting efficiency. So the undoped GaN or AlGaN spacer is introduced and its thickness should be adjusted to prevent Mg diffusion into the active layer. The spacer layer can also act as the electron-blocking layer.
2. Fabrication
GaN based LED devices are fabricated using standard processing steps that including mesa etching and contact fabrication techniques. First, the Mg acceptor atoms are activated using rapid thermal annealing. The electrical properties of p-GaN can be improved by optimizing annealing temperature _and time. Afterwards, the process includes several main steps as follows:
I) Etching the material down to the n-type region and n-contact fabrication. N-contact fabrication is depositing related metals with a box coater system followed by a lift off of metal and then annealing to improve the n+ contact to the material.
II) Metal deposition and structuring the semi-transparent p-contact. For the p type spread metal we deposit very thin metal to have transparency on the sample surface.
III) Fabrication of a bond pad (touch pad). We deposit thick metals to the corner of the spread layer . The tips biasing current is placed on that thick metal. Then we anneal for contact formation.
IV) The next step is the mesa isolation of devices to get rid of some leak currents. We etch the sample surface protecting the device down to sapphire substrate.
V) As a fifth step we deposit dielectric material for isolation and protection of the device. We make the necessary openings using HF acid solution.
The conductivity of the p-type GaN is not good enough for current spread due to the low active efficiency of Mg dopant. So the semi-transparent metal ohmic contact is introduced to improve the holes current spread. The thickness of this metal layer must be optimized according the contradiction of current spread and light transmission.
3. Characterization
The epitaxial LED wafers and device characteristics of the fabricated samples are measured using several test apparatus.
X-ray diffraction have been widely used to study the crystal quality of these epitaxial films (w scan) on-wafer and analyze the interface quality and structural parameters of InGaN/GaN QWs (2θ-w scan), such as the average In composition in the wells, thickness of the wells and barriers. We will carry out the SEM and AFM to observe the morphology of the surface and the density of the QDs, respectively. The electrical properties of the layers will be investigated using Hall effect measurements, which can also be used to measure the n and p type doping level. Photoluminenace measurement can be carried out to determine the emitting photo energy and the wavelength uniformity of the LED wafer. These measurements are all taken before the fabrication process and the results are the effective references to optimize the growth parameters in reverse.
After the completion of device fabrication, current-voltage measurement will be carried out to evaluate the electrical properties of the LED, such as the forward voltage (at 20 mA) and breakdown characteristics (at –10mA). The spectrum of the light output is taken and using this data the light output power and the degradation characteristic of the LED is also measured.
Figure 5 Blue light from the first LED structure grown in Bilkent MOCVD system
Figure 6 Completed LED devices after first microfabrication in NANO-TR
Metamaterials - Photonic Crystals - GaN/AlGaN Devices - Other Semiconductor Devices and Fabrication Techniques
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