High Quality Superlattices for Effective P-Type Doping
Effective p-type doping of wide bandgap III-nitride semiconductors is difficult due to the high activation energy of most acceptors. This problem is more pronounced for AlGaN compounds with increasing Al content.
N-type doping also poses a significant challenge for high Al content AlGaN layers. By careful optimization of the SiH4 flow, and the use of a high-quality AlGaN/AlN superlattice template on AlN buffer we were able to realize Al0.5Ga0.5N:Si with n ~1x1018 cm-3 and μ~40 cm²/V·s. The addition of indium yields a carrier concentration of n~5x1018 cm-3 and mobility of μ~80 cm²/V·s.
World's First Lateral Epitaxial Overgrowth of GaN on Silicon
Because of the lack of commercially available native substrates, obtaining low defect III-Nitride semiconductors has remained one of the most challenging issues in this area for numerous years. The dislocations that are formed when growing III-Nitrides heteroepitaxially, i.e. on non-native substrates, have proved detrimental not only to the material's structural, optical, and electrical quality, but also to the device performance.
Researchers at CQD have successfully and extensively explored innovative approaches to reduce the dislocation density in GaN by several orders of magnitude through the use of lateral epitaxial overgrowth (LEO). In particular, researchers at CQD were the first to demonstrate the LEO of GaN on silicon substrates.
aa f p' p flow, and the use of a high-quality AlGaN/AlN superlattice template on AlN buffer we were able to realize Al0.5Ga0.5N:Si with n ~1x1018 cm-3 and μ~40 cm²/V·s. The addition of indium yields a carrier concentration of n~5x1018 cm-3 and mobility of μ~80 cm²/V·s.World's First Lateral Epitaxial Overgrowth of GaN on Silicon
Because of the lack of commercially available native substrates, obtaining low defect III-Nitride semiconductors has remained one of the most challenging issues in this area for numerous years. The dislocations that are formed when growing III-Nitrides heteroepitaxially, i.e. on non-native substrates, have proved detrimental not only to the material's structural, optical, and electrical quality, but also to the device performance.
World's First Solar Blind Ultraviolet Photodetectors and Focal Plane Arrays
The Center for Quantum Devices (CQD) has demonstrated many record-breaking achievements in the development of UV, visible -blind and solar-blind photodetectors.
Photodetectors are devices that transform electromagnetic radiation (light) into an electrical signal (current or voltage). Several applications demand that these devices detect specific energies of light while ignoring others, e.g. sensitive to UV and not visible or infrared light. Using III-Nitride technology, inexpensive, efficient, highly sensitive and robust devices can be achieved.
These high efficiency Al-GaN based UV photodetectors can be used in a number of military and civilian applications including early missile threat warning, secure space-to-space communications, chemical and biological agent detection, engine/flame detection, furnace monitoring, UV dosimetry, ozone/pollution monitoring, and UV astronomy.
Numerous types of photodetectors have been investigated at CQD, including photoconductors, Metal-Schottky-Metal detectors, Schottky photodiodes, pin photodiodes, and most recently avalanche photodiodes.
World's First Back Illuminated Avalanche Photodiodes and Geiger-Mode Single photon Detectors
Research into avalanche photodiodes (APDs) is motivated by the need for high sensitivity ultraviolet detectors in numerous civilian and military applications. By utilizing low-noise impact ionization based gain, GaN APDs can deliver gains of more than 10000 in linear mode; and operating in Geiger mode, the gain can exceed 1×107 and thus single photon detection becomes possible.
We have experimentally studied impact ionization in GaN and found that the hole ionization coefficient is significantly larger than that for electrons. This means that back-illuminated devices exhibit larger gains; we have also observed excess multiplication noise factors that are more than three orders of magnitude lower.
By using high-quality delta-doped p-GaN a consistently lower breakdown voltage can be realized, without significantly affecting the dark current. This has allowed a more than 50x improvement in the maximum gain realized, as compared to devices grown with traditional bulk p-GaN.
In addition to controlling the material and doping quality it is also possible to carefully design the structure to minimize the external bias necessary to reach the critical electric field necessary for breakdown (typically of 3 ×106 V×cm-1). This can be accomplished by reducing the width of the combination absorption/multiplication layer in a traditional p-i-n device structure; however this also increases the leakage current of the diodes, and thus, the dark count rate in Geiger mode. To help overcome this, we have developed a separate absorption and multiplication APD structure (SAM-APD). By separating the absorption and multiplication regions using a p-i-n-i-n structure it becomes possible to absorb more than 99% of the light in the bottom layers resulting in nearly pure hole-injection into the multiplication region. This maximizes the advantage of the higher hole-ionization coefficient resulting in a device with a maximum linear mode gain of more than 40,000.
Violet, Blue, Green Light Emitting Diodes and Lasers
The Center for Quantum Devices has developed the III-Nitride material growth and processing of visible Light Emitting Diodes LED) and lasers.
Light-Emitting Diodes
Using InxGa1-xN/GaN double hterostructures and multiquantum well structures, researchers at the Center have demonstrated high-brightness blue and green LEDs.
Lasers
High-performance laser diodes fabricated from III-nitride materials are complex, yet highly attractive devices due to their potential use in a number of applic ations. The Center for Quantum Devices has been pursuing the MOCVD growth and development of these devices, which incorporate InxGa1-xN/GaN multi-quantum wells. To achieve high efficiency lasers, the CQD has been investigating lateral epitaxial overgrowth (LEO) as a means to improve III-Nitride materials by reducing defects during the growth on both sapphire and silicon substrates.
Semiconductor lasers are highly efficient, compact devices that emit an intense, coherent, monochromatic beam of light.
Blue laser diodes stand to satisfy a number of application needs including high-density data storage, high capacity DVD, high resolution color printing, and laser displays.
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