Short-Wavelength Intersubband Light Emission from Optically Pumped GaN/AlN Quantum Wells
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1202-I10-08
Short-Wavelength Intersubband Light Emission from Optically Pumped GaN/AlN Quantum Wells Roberto Paiella1, Kristina Driscoll1, Yitao Liao1, Anirban Bhattacharyya1, Lin Zhou2, David J. Smith2, and Theodore D. Moustakas1 1
Department of Electrical and Computer Engineering and Photonics Center, Boston University, Boston, MA 02215, U.S.A. 2
Department of Physics, Arizona State University, Tempe, AZ 85287, U.S.A.
ABSTRACT Due to their large conduction-band offsets, GaN/Al(Ga)N quantum wells are currently the subject of extensive research efforts aimed at extending the spectral range of intersubband optoelectronic devices towards shorter and shorter wavelengths. Here we report our recent measurement of optically pumped intersubband light emission from GaN/AlN quantum wells at the record short wavelength of about 2 µm. Nanosecond-scale optical pulses are used to resonantly pump electrons from the ground states to the second excited subbands, followed by radiative relaxation into the first excited subbands. The intersubband origin of the measured photoluminescence is confirmed via an extensive study of its polarization properties and pump wavelength dependence.
INTRODUCTION Intersubband (ISB) transitions in GaN/Al(Ga)N quantum wells (QWs) have been the subject of extensive research efforts for the past several years [1-8]. These heterostructures feature particularly large conduction-band offsets (up to about 1.75 eV) allowing for ISB transitions at record short wavelengths, well into the near-infrared spectral region. As a result they provide new opportunities for ISB device development, particularly in the area of all-optical switching for ultrafast fiber-optic communications [2,3] as well as short-wavelength ISB photodetectors [4,5], modulators [6], and sources [7,8]. Incidentally, it should be mentioned that nitride semiconductors are also promising for the development of THz ISB devices, owing to their particularly large LO-phonon energies [9]. Here we demonstrate optically pumped ISB light emission from a GaN/AlN multiple-QW structure at the record short wavelength of about 2 µm [8]. In these measurements the QWs are pumped with high-power near-infrared pulses from a tunable optical parametric oscillator (OPO), which allow resolving the QW ISB luminescence without the need for highly sensitive lock-in detection techniques. The output luminescence spectra are peaked at wavelengths near 2.05 µm, which represent a new record for the shortest ISB emission wavelength from any QW materials system.
EXPERIMENT The material used in this work was grown on c-plane sapphire by rf plasma-assisted molecular beam epitaxy. Following nitridation of the substrate, a 1-µm-thick AlN buffer layer was deposited to serve as a template for growth of the subsequent layers. The multiple-QW active region consists of 200 periods of nominally 18-Å-thick GaN wells (doped n-type with Si to the level of ~ 2 x 1019 cm-3) separated by 26-Å-thick AlN barriers. During deposition of the AlN layers a small flux of Ga was used as a surfac
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