Terahertz optoelectronic devices based on intersubband transitions in III-nitride semiconductors
Sudradjat, Faisal Firmansyah
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The terahertz (THz) spectral region, commonly defined as the frequency (wavelength) range between 0.3 and 10 THz (1 mm and 30 µm) has many important applications in the industrial, biomedical, and military sectors. However, due to a lack of practical semiconductor materials with adequately small bandgap energy, the development of THz light sources and photodetectors has so far been limited. In recent years, devices based on intersubband transitions between discrete energy states in quantum heterostructures have been under intense research and development to address this issue. Of particular promise in the THz range are quantum cascade lasers (QCLs) and quantum well infrared photodetectors (QWIPs), which utilize intersubband transitions in specially designed quantum well (QW) structures to emit light and generate photocurrent, respectively. This research work has focused on the development of THz light sources and photodetectors using intersubband transitions in GaN/A1GaN QvVs, whose basic materials properties allow for improved spectral coverage and high-temperature operation compared to existing semiconductor devices. To design the active region of QCLs and QWIPs based on inter-conduction-subband transitions in these materials, the necessary numerical tools have first been developed. Sequential tunneling, the key electronic transport mechanism ofintersubband light emitters, has then been demonstrated in GaN/A1GaN QC structures. Furthermore, we have measured promising THz electroluminescence spectra from the same devices through the use of lock-in step-scan Fourier transform infrared spectroscopy. In the area of photodetectors, we have developed a novel double-step QW design in order to overcome the material limitations presented by the intrinsic internal electric fields of GaN/A1GaN QWs. With this design approach, we have experimentally demonstrated the operation of a far infrared QWIP with a peak detection wavelength of 23 µm (13 THz frequency), which is the longest wavelength reported for this materials system.
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