Pulsed Metalorganic Chemical Vapor Deposition ... - Semantic Scholar
Pulsed Metalorganic Chemical Vapor Deposition of High Quality AlN/GaN Superlattices for Intersubband Transitions C. Bayram, B. Fain, N. Péré-laperne, R. McClintock and M. Razeghi* Center for Quantum Devices; Department of Electrical Engineering and Computer Science; Northwestern University; Evanston, Illinois 60208; USA
ABSTRACT A pulsed metalorganic chemical vapor deposition (MOCVD) technique, specifically designed for high quality AlN/GaN superlattices (SLs) is introduced. Optical quality and precise controllability over layer thicknesses are investigated. Indium is shown to improve interface and surface quality. An AlN/GaN SL designed for intersubband transition at a telecommunication wavelength of ~1.5 µm, is grown, and processed for intersubband (ISB) absorption measurements. Room temperature measurements show intersubband absorption centered at 1.49 µm. Minimal (n-type) silicon doping of the well is shown to be crucial for good ISB absorption characteristics. The potential to extend this technology into the far infrared and even the terahertz (THz) region is also discussed. Keywords: metalorganic chemical vapor deposition, AlN/GaN superlattice, intersubband transition, intersubband absorption, terahertz
1. INTRODUCTION 1.1 Applications of Terahertz The terahertz (THz) spectral range offers promising applications in science, industry, and military. The THz range is also important for astronomical research as 98% of the photons emitted since Big Bang are in sub-millimeter and far-infrared regime.1 The difference in THz absorption for different materials could also be used for three dimensional mapping. Absorption of THz frequencies by water can be used to distinguish cells with different water ratios (fat vs. lean cells).2 The body parts with low water contents (like teeth) could be fully mapped for any sign of decay within.1 On the order of millimeter penetration inside the body helps identifying cancer such as basal cell carcinoma (BCC); a most common form of cancer worldwide with over 1 million annual reported incidence in the USA.3 The tumor usually contains more water and less fat, which is resolved in terahertz imaging as darker areas. Based on THz frequency range excitation of intermolecular interactions, NASA used a 2.5THz laser to measure the concentration and distribution of the hydroxyl radical (OH–) in the stratosphere, a critical component in the ozone cycle.1 THz spectroscopy enables characterization of solid materials as well as analysis of different forms of active pharmaceutical ingredients (for example, in a drug).4,5,6 Penetration through nonconductors (fabrics, wood, plastic) enables a more efficient way of performing security checks (for example at airports), as illegal drugs and explosives7 could be detected via this technique. Being a non-ionizing radiation, THz radiation is environment-friendly enabling a safer analysis environment than conventional X-ray based techniques.4 The compact solution to THz application will lead to a continuous monitoring of an environment ensuring a better security than conventional security check-points without effecting privacy. As an industrial impact, quality control of the packed goods could be managed by THz based systems. 1.2 Available Terahertz Sources A gyrotron is a vacuum tube based high power (kW-MW) emitter based on cyclotron resonance maser. Cost, maintenance, and space allocations limit the usage to applications such as heating of nuclear fusion plasmas and industrial thermal treatment of materials. A backward wave oscillator is an electron tube based emitter requiring highly homogeneous magnetic field of ~ 10 kG. The power is ~10 mW and wavelength is limited between 0.1 and 1.5 THz.1 A *
[email protected]; phone 1 (847) 491-7251; fax 1 (847) 476-1817; http://cqd.eecs.northwestern.edu Quantum Sensing and Nanophotonic Devices VI, edited by Manijeh Razeghi, Rengarajan Sudharsanan, Gail J. Brown, Proc. of SPIE Vol. 7222, 722212 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.809829
Proc. of SPIE Vol. 7222 722212-1
more convenient source is the optically pumped terahertz laser (OPTL) which consists of a grating-tuned carbon dioxide pump laser and a far-infrared gas cell mounted in the laser resonator. The stability of an OPTL is affected by slight changes in pumping wavelength, changes in the cavity length, and feedback interaction between the pump laser and the terahertz laser. The usable wavelength is 0.3-10 THz. Other sources such as direct multiplied sources (directly multiplies millimeter-wave sources up to terahertz frequency (~ 1THz)), and frequency mixing have low powers (
ABSTRACT A pulsed metalorganic chemical vapor deposition (MOCVD) technique, specifically designed for high quality AlN/GaN superlattices (SLs) is introduced. Optical quality and precise controllability over layer thicknesses are investigated. Indium is shown to improve interface and surface quality. An AlN/GaN SL designed for intersubband transition at a telecommunication wavelength of ~1.5 µm, is grown, and processed for intersubband (ISB) absorption measurements. Room temperature measurements show intersubband absorption centered at 1.49 µm. Minimal (n-type) silicon doping of the well is shown to be crucial for good ISB absorption characteristics. The potential to extend this technology into the far infrared and even the terahertz (THz) region is also discussed. Keywords: metalorganic chemical vapor deposition, AlN/GaN superlattice, intersubband transition, intersubband absorption, terahertz
1. INTRODUCTION 1.1 Applications of Terahertz The terahertz (THz) spectral range offers promising applications in science, industry, and military. The THz range is also important for astronomical research as 98% of the photons emitted since Big Bang are in sub-millimeter and far-infrared regime.1 The difference in THz absorption for different materials could also be used for three dimensional mapping. Absorption of THz frequencies by water can be used to distinguish cells with different water ratios (fat vs. lean cells).2 The body parts with low water contents (like teeth) could be fully mapped for any sign of decay within.1 On the order of millimeter penetration inside the body helps identifying cancer such as basal cell carcinoma (BCC); a most common form of cancer worldwide with over 1 million annual reported incidence in the USA.3 The tumor usually contains more water and less fat, which is resolved in terahertz imaging as darker areas. Based on THz frequency range excitation of intermolecular interactions, NASA used a 2.5THz laser to measure the concentration and distribution of the hydroxyl radical (OH–) in the stratosphere, a critical component in the ozone cycle.1 THz spectroscopy enables characterization of solid materials as well as analysis of different forms of active pharmaceutical ingredients (for example, in a drug).4,5,6 Penetration through nonconductors (fabrics, wood, plastic) enables a more efficient way of performing security checks (for example at airports), as illegal drugs and explosives7 could be detected via this technique. Being a non-ionizing radiation, THz radiation is environment-friendly enabling a safer analysis environment than conventional X-ray based techniques.4 The compact solution to THz application will lead to a continuous monitoring of an environment ensuring a better security than conventional security check-points without effecting privacy. As an industrial impact, quality control of the packed goods could be managed by THz based systems. 1.2 Available Terahertz Sources A gyrotron is a vacuum tube based high power (kW-MW) emitter based on cyclotron resonance maser. Cost, maintenance, and space allocations limit the usage to applications such as heating of nuclear fusion plasmas and industrial thermal treatment of materials. A backward wave oscillator is an electron tube based emitter requiring highly homogeneous magnetic field of ~ 10 kG. The power is ~10 mW and wavelength is limited between 0.1 and 1.5 THz.1 A *
[email protected]; phone 1 (847) 491-7251; fax 1 (847) 476-1817; http://cqd.eecs.northwestern.edu Quantum Sensing and Nanophotonic Devices VI, edited by Manijeh Razeghi, Rengarajan Sudharsanan, Gail J. Brown, Proc. of SPIE Vol. 7222, 722212 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.809829
Proc. of SPIE Vol. 7222 722212-1
more convenient source is the optically pumped terahertz laser (OPTL) which consists of a grating-tuned carbon dioxide pump laser and a far-infrared gas cell mounted in the laser resonator. The stability of an OPTL is affected by slight changes in pumping wavelength, changes in the cavity length, and feedback interaction between the pump laser and the terahertz laser. The usable wavelength is 0.3-10 THz. Other sources such as direct multiplied sources (directly multiplies millimeter-wave sources up to terahertz frequency (~ 1THz)), and frequency mixing have low powers (
