THz Emission from InN
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1202-I11-02
THz Emission from InN Hyeyoung. Ahn,1 Yi-Jou Ye,1 Yu-Liang Hong,2 and Shangjr Gwo2 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan R.O.C. 2 Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan R.O.C. 1
ABSTRACT We report the terahertz (THz) emission from the wurzite indium nitride (InN) films grown by molecular beam epitaxy (MBE). More than two orders of magnitude of THz power enhancement has been achieved from the InN film grown along the a-axis and magnesium (Mg) doped InN with a critical carrier concentration. The primary radiation mechanism of the a-plane InN film is found to be due to the acceleration of photoexcited carriers under the polarizationinduced in-plane electric field perpendicular to the a-axis. Apparent azimuthal angle dependences of THz wave amplitude and the second harmonic generation are observed from aplane InN. In the Mg-doped films, Mg as the acceptors compensate the native donors in the InN films and large band bending over a wider space-charge region causes the enhancement of THz emission power compared to the undoped InN. INTRODUCTION Due to its narrow bandgap and high electrical mobility, indium nitride (InN) has actively been studied for the fundamental material researches and the fruitful prospects in applications as high-frequency electronic devices, near-infrared optoelectronics, and high-efficiency solar cells. With a unique advantage of large energy difference between the conduction band minimum and the next local minimum, InN also inspires potential applications in the THz range [1-5]. Meanwhile, the performance of short-wavelength optoelectronic devices realized by growing IIInitrides along the c-axis is typically limited by the polarization-induced internal electric fields. Piezoelectric and spontaneous polarizations are responsible for the polarization-induced electric field and in particular, the strain-dependent piezoelectric polarization along the c-axis of the wurtzite crystals increases with the lattice mismatch in the nitride layers. For the layers grown along a- or m-axis direction (so called, nonpolar), on the other hand, polarization-induced electric field perpendicular to the layer interface can be minimized and the efficiency of the devices can be increased. Despite there are abundant results reported for other group-III nitrides, researches on InN grown along the a-axis (a-plane InN) are rare mainly due to the technical difficulty in growing high crystalline quality a-plane InN films. Due to its high electron affinity, undoped InN is unintentionally doped n-type. However, in order to realize the optoelectronic devices, it is essential to have the ability to fabricate both nand p-type InN. p-type doping of InN has been one of the main challenges of intense research efforts. In the electrolyte-based capacitance-voltage (ECV) measurement, Jones et al. showed that the surface Fermi level pinning of InN:Mg causes an extremely large band bending compared to that
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