Fabrication and Characterization of GaAs Tunnel Diode and ErAs Nanoparticles Enhanced GaAs Tunnel Diode for Multijunctio
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Fabrication and Characterization of GaAs Tunnel Diode and ErAs Nanoparticles Enhanced GaAs Tunnel Diode for Multijunction Solar Cell Tomah Sogabe1, Yasushi Shoji1, Mitsuyoshi Ohba1, Naito Shunya1, Naoya Miyashita1, Chao-Yu Hung1, Akio Ogura1 and Yoshitaka Okada1 1 Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan.
ABSTRACT We report here the fabrication and characterization of GaAs tunnel diode (TD) and ErAs nanoparticles (Nps) enhanced GaAs TD. Four GaAs TDs with different contact area were fabricated by using MOCVD. We found extremely high peak current density of ~250A/cm2 for the TD with r=0.25mm contact area. Moreover a hysteresis loop was appeared during sweeping up and sweeping down the external voltage. A ‘vector load line model’ was proposed to explain the origin of the shape of the hysteresis loop and the onset of the bistability occurred at the intersect of the loadline and the current-voltage (I-V) curve of TD. Meanwhile, we have grown ErAs Nps on GaAs(100) surface by using MBE and succeeded in overgrowth of GaAs after ErAs deposition. GaAs(p+)/ErAs(Nps)/GaAs(n+) TDs were fabricated and characterized. We found the GaAs sample containing 70s deposition of ErAs showed the best TD behavior. No TD behavior was observed for the sample without addition of ErAs Nps, clearly indicating the strong tunneling enhancement effect from ErAs Nps.
INTRODUCTION Among various types of solar cell, III-V multi-junction (MJ) solar cell are the most promising candidate for next generation of high efficiency photovoltaic conversion of sunlight. For instance, Ga0.51In0.49P/Ga0.99In0.01As/Ge triple junction solar cell has reached the recorded efficiency of over 30% under 1 sun and of 40% under concentration ratio of 240 suns [1]. MJ solar cell consists of subcells which are usually interconnected by TDs featured with both low electric resistance and high optic transparence. For MJ solar cell to be operated under light concentration, the enhanced short circuit current density requires the TD to have much lower electric resistance to minimize the voltage drop and much higher peak tunneling current density than the short circuit current of MJ cells [2]. In this work, we report the fabrication and characterization of GaAs TD prepared by MOCVD and ErAs nanoparticles (Nps) enhanced GaAs TD fabricated by MBE. An issue frequently confronted in MBE growth of TD is the insufficient doping concentration. We show that the addition of ErAs Nps into the pn junction region is able to compensate this drawback while improving the tunneling probability through the reduction of tunneling distance due to the semimetallic feature of ErAs Nps [3,4].
EXPERIMENT GaAs TDs were fabricated by a multiwafer AIXTRON MOCVD reactor (AIX2800G3) with 8 × 4 in configuration. AsH3, PH3, TMGa, TMIn, and TMAl were used as precursors. In order to investigate the electrode area effect on the tunneling behavior, we prepared four types of TD with circular electrode area ranging from r=
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