Gallium Nitride (GaN) based semiconductor to produce blue light
Producing Blue Light from GaN LEDs
This state of the art report will focus on light emitting diodes LEDs made from Gallium Nitride (GaN) based semiconductors to produce blue light. The light ought to be of 475 nm in wavelength for high performance and low-cost energy necessary for energy saving light bulbs. The problem this design seeks to address is the operational cost of lighting, longevity and durability compared to the tungsten incandescent lamp. The scope of the research behind this report will focus on how LEDs work by using Gallium Nitride based semiconductors to emit energy at wavelengths of 475 nm, which is the basic characteristic of blue light, and its application to the Cree LED True White Bulb. The scope of this report, though, does not cover details of how blue light is converted to white light through an epoxy that covers the LED or any other components of the Cree LED True White Light Bulb. That is left and suggested for later research because users may want white light from the use of Gallium Nitride (GaN) LEDs.
An LED is a solid-state diode that is formed at the junction of the two types of semiconductors, n-and p-type semiconductors. The basis of semiconductors and how they operate is crucial in the design of the LEDs. Semiconductors are materials that are neither good conductors nor good insulators. They are largely from group four elements Silicon and Germanium (Fasol 32). The electrical properties that make up semiconductors can be altered by the addition of different impurities into the crystal structure of pure silicon or germanium.
The addition of impurities, also called doping, results in an n-type or p-type semiconductor based on the nature of impurities introduced. Adding impurities helps to achieve the correct dynamic nature and is classified as doping. When the added impurities have more than one electron in its outermost shell compared to the semiconductor material, then extra electrons can conduct electricity, this is called negatively doped semiconductor identified as an n-type (Yoshizawa 19). If the impurity has one electron less than the semiconductor in the crystal structure, there are too few elections and the election can move from atom to atom to fill the hole. Whatever is left behind by a transitioning electron, also called electron-hole, is a positively comparable to a proton. The resultant structure is a p-type semiconductor where holes are more predominate. Therefore, doping pure silicon or germanium with a group five element yields an n-type semiconductor. Doping the same with a group three gives a p-type semiconductor (Szweda 83). It is these semiconductors that form the basis of semiconductor devices such as diode, transistors and integrated circuits.
Diodes are produced through the combination of a p-type and n-type semiconductors, a process that yields a p-n junction where the two contact. This, however, does not yield the light emitting diode. The make an LED a component called the p-n junction must be manufactured where a p-type semiconductor is put in contact with an n-type semiconductor. The common region in between the two, defined as the p-n junction, is depleted of charge carriers. Depletion occurs when the extra electron from the n-type region combines with holes in the p-type region causing neutrality or a removal of charge carriers in the p-n junction, the area with fewer charge carriers is called the depletion region (Szweda 52).
The application of some potential across the p-n junction such that the p-type region has higher potential triggers the drift of electrons from the n-type region whereas holes move from the depleted region to the p-type region. This causes the election to move into the cathode, replenishing the electrons in the n-type region and electrons are removed from the anode replacing the holes in the p-type region. The result is a forward-biasing orientation of the diode as current flows across the diode junction on the application (Szweda 52). The junction is considered as diminished by the application of forward biasing current. Applying the voltage difference such that the potential is higher on the n-type side causes an increase in depletion and the junction widens. Electrons are pulled from the n-type region and into the p-type regions causing a larger depletion region. The result is fewer charge carriers and wider depletion layer that limits the flow of current across the diode, which is considered as connected in the reverse bias (Yoshizawa 23). Electric current must not flow across the diode unless avalanche breakdown occurs.
When electrons and holes have different energy levels these electrons and holes, combine in the depletion region and release energy. The energy difference between the n-type electrons and the p-type holes is small. Therefore, the energy they release is little. But if the energy difference is sufficiently large, then an electron and hole merge and combine neutralizing each other, a process that results in energy release. If the energy release is equivalent to that of a photon of light, then the energy is dispersed in the form of light, the phenomena called electroluminescence (Fasol 34). Different wavelengths or colors of light are associated with different levels of energy. For instance infrared light has less energy than visible light. Red light has the least energy of a visible light spectrum whereas green has the most. The color of light emitted by recombining electrons, therefore, can be predetermined by controlling the amount of energy released from the combination of electrons with holes (Fasol 36). This is achievable since different semiconductors have different energies of electrons and holes, which when controlled from the choice of semiconductor dopers can influence the lighting color. If we fabricate a semiconductor with proper doping, we can modify the wavelength and determine the color of the LED according to our preferences.
Back in 1970, Jacques Pankove placed two probes on the surface of a Zn-doped GaN semiconductor and applied a direct current voltage source across the probes. When the voltage went over 60 Volts, green light emission at 515 nm was observed. From a microscopic assessment, it was noted that the luminescence consisted of some tiny holes located under the probe. This was the first electroluminescence from a GaN film. With consisted studies, research, and experimentations, Pankove fabricated and adjusted the Zn-doping and fabricated a blue light LED emitter that produced light at 475 nm of wavelength (Szweda 85).
Later in 1972, another researcher called Maruska decided to dope GaN films with magnesium. Maruska built a thick non-doped GaN film that that he topped with a thin Mg-doped insulation film. Maruska, further Applied a point contact and provided an electrical current at 150 volts. On the application of the potential difference and concurrent current, Maruska observed a violet light that was emitted at a wavelength of 425 nm, which motivated him to go further and make a functioning metal insulator semiconductor. In the new design, Maruska used a chip of 2 mm by 2 mm cut from the sample that produced violet light. The top Mg-doped film was coated with a thick metal contact. Next, a small metal contact was formed on the non-doped side to serve as another contact (Szweda 85). A wire passed through the head from the back and connected the non-doped region from the side. The device was heated to 400 degrees C to burn off any mercury and provide a solid metal contact point, a process largely known as thermal annealing in fabrication design. A positive bias voltage was applied to the header base giving a light that was emitted out of the top through the sapphire substance. This was the first functioning Mg-doped GaN LED (Szweda 86). Undesirably though, the device had low efficiency and remained dim due to the heavy contamination or oxygen in the form of H20.
In 1994, researchers Akasaki, Amano, and Nagasaki finally demonstrated a truly bright blue GaN-based LED. They used a bunch of advances since 1972 such as a technique from Manasevit of North American Rockwell that used an MOCVD thin film growth technique to develop high-quality nitride films. They also used the early LED structures first used by Pankove with Zn-doped (Nakamura and Fasol 41). They also used magnesium doping approach previously developed by a researcher named Maruska to acquire p-type doping and his annealing process. As well as an electron beam annealing process used to activate the Mg dopants. Finally, a Miller’s buffer layer approach for reducing dislocation densities was used to give the final product.
The production of blue light emissions from LEDs has historically proven challenging. For instance, earlier attempts to use ZnSe and SiC proved ineffective much as the two are known for high indirect band gaps. Gallium Nitride, however, caused relief among researchers and scientists. Being an n-type semiconductor, GaN fall in the II-V category has a similar structure to that of Wirtzite. GaN can grow on sapphire (Al2O3) substrates or SiC much as there are notable differences between their lattice constants (Yoshizawa 62). The doping, however, affects the growth process undesirably making GaN fragile. Generally, GaN crystal defects result in the desired electron conductivity with a 3.4eV direct band gap that corresponds to UV wavelength. A major challenge has been to P-dope GaN controllably. Better p-doping, however, has been facilitated by earlier breakthrough where Zn-doped GaN has been proved to produce more light, a sign of better doping. In addition, experiments to irradiate Mg-doped GaN with low energy electrons have also proved to yield better p-doping properties (Yoshizawa 62).
Acceptors such as Zn and Mg when used in doping combine with hydrogen to form complexes rendering them passive. These complexes dissociate on exposure to electron beams resultantly activating the acceptors. Researchers have revealed that simple thermal annealing or treatment results in effective activation of Mg acceptors (Yoshizawa 63).
The biggest breakthrough and advance that came to blue nitride light-emitting diodes was achieved in 1995-1995 with the development of the critical device structure that made it possible and realistic that blue emission could be produced from nitride semiconductors. The basis of the instrument consists of an alloy of InN and GaN. The formation of ln GaN recombination selection allows them to define exactly the emissions wavelength (Fasol 39). As discussed, the ability to control the wavelength of emission is all that fabricators need to make an LED that produces any color. Regarding this design, this is achievable through the control of the indium mole fraction in the InGaN active layer, by changing the growth temperature, and changing the indium flow rate during InGaN growth stage (Fasol 40). The peak wavelength of the emission from the LEDs, therefore, can be altered to give desired wavelength of 475 nm for blue light.
A p-n junction also called the depletion layer, formed at the contact of respective semiconductor types emits light when electric current is applied. The process is called electroluminescence. In definition, the process involves the emission of light by a semiconductor when an electric field is applied across it. Depending on the biasing, charge carriers may recombine in forward bias or exist in reverse bias. Electrons and holes are in conduction band and valence energy band respectively. Part of the energy dissipated is emitted as light energy in the case of GaN where the energy is at the wavelength of blue light. Sometimes, the energy is produced in the form of heat when the semiconductor is not translucent. The color of light is determined by the choice of dopants such the energy disseminated during electron-hole combinations conforms to the desired color of light by wavelength. In this case, GaN is used to produce energy wavelength of 475 nm giving blue illuminations. The blue light produced using this technique may need conversion into the widely used white light. Future studies, therefore, need to focus on mechanisms to convert this light to white light in a cost effective way. The cost must remain low for viability of this technology in the commercial production of lights. In any case, no one would opt for expensive products when there are cheaper alternatives unless there are underlying advantages.
Fasol, Gerhard. High Brightness Light Emitting Diodes. New York: Academic Press, 2011.
Nakamura, Shuji and Gerhard Fasol. The Blue Laser Diode: GaN Based Light Emitters and Lasers. London: Springer Science & Business Media, 2013.
Szweda, Richard. Gallium Nitride and Related Wide Bandgap Materials & Devices. A Market and Technology Overview 1998-2003. Philadelphia: Elsevier, 2000.
Yoshizawa, Toru. Handbook of Optical Metrology: Principles and Applications. California: CRC Press, 2009.
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