Intersubband optical absorption and electron relaxation rates in GaN/AlGaN coupled double quantum wells
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Intersubband optical absorption and electron relaxation rates in GaN/AlGaN coupled double quantum wells J. D. Heber, C. Gmachl, H. M. Ng, A. Y. Cho, S.-N. G. Chu1 Bell Laboratories, Lucent Technologies, 600 Mountain Avenue Murray Hill, NJ 07974, U.S.A. 1 Agere Systems, 600 Mountain Avenue Murray Hill, NJ 07974, U.S.A.
ABSTRACT Recent interest in intersubband (IS) transitions in semiconductor heterostructures with large band offset has been fueled by attempts to extend the wavelength range of IS-based optical devices to the fiber-optics wavelength range around ~ 1.55 µm. GaN/AlGaN-based heterostructures are of particular interest due to their large effective electron mass and large longitudinal optical phonon energy. Both are essential to achieve ultrafast electron relaxation at large transition energies. IS absorption in GaN/AlGaN single and coupled double quantum wells (DQWs) has been measured. The samples were grown by molecular beam epitaxy on sapphire substrate and with a large (0.65 or 0.9) AlN-mole fraction in the barriers. Peak absorption wavelengths as short as 1.35 µm and 1.52 µm were measured for a symmetric DQW of 12 Å wide wells coupled by a 10 Å wide barrier, which also showed evidence of excited-state anticrossing. As expected, asymmetric DQWs displayed no such anti-crossing, and the ground-state anti-crossing energies were found to be much smaller – as a result of the comparatively large effective electron mass – than the energy broadening of individual transitions. The asymmetric DQWs displayed peak absorption wavelengths between 1.5 and 2.9 µm. The electron relaxation time, attributed to longitudinal optical phonon scattering has been measured by pump-probe technique as 240 fs for a coupled DQW sample.
INTRODUCTION Studies of intersubband (IS) transitions in GaN/AlGaN based heterostructures are increasingly generating interest, fueled by the possibility of producing ultrafast optical devices in the communications wavelength range of ~ 1.55 µm [1-4]. IS relaxation times in GaN/AlGaN heterostructures are expected to be very short, given the large longitudinal optical (LO) phonon energy (ELO ~ 90 meV) and effective electron mass of m* ~ 0.2–0.4 m0, with m0 being the electron rest mass. Recently, IS electron relaxation times in this material system have been measured as 150 fs at a wavelength of ~4.5 µm [5] and as 370 fs at λ ~ 1.55 µm [6]. However, studies so far have been mainly limited to stacks of isolated single quantum well (QW) samples and no comparative study of IS relaxation times in a variety of heterostructures has been reported. More complex structures such as three level systems in coupled DQWs will be required, e.g. to achieve population inversion and laser action. For these complex devices, a better understanding of the intersubband relaxation dynamics is essential.
I8.7.1
Here, we report to our knowledge on the first systematic study of room temperature absorption and IS relaxation lifetime measurements of GaN/AlGaN MQW and coupled double quantum (DQW) structures.
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