Scanning Tunneling Luminescence of Semiconductors
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Scanning Tunneling Luminescence of Semiconductors M.J. Romero National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, Golden, CO 80401-3393
ABSTRACT We review scanning tunneling luminescence (STL) − a photon emission spectroscopy based on scanning tunneling microscopy (STM) − and report on its application to photovoltaics. As part of this exploratory research, we have investigated CuInSe2 thin films, solar cells based on quantum dots, and dilute nitride compounds. STL is very attractive because it is capable of nanometer resolution, which is being demanded by the spectacular advancement of nanoscience and nanotechnology. In addition, STM offers both unipolar and bipolar excitation of the luminescence and, consequently, the transport and recombination of electrons and holes can be investigated independently.
INTRODUCTION In the last decade, photon emission from semiconductors has been observed in STM, a phenomenon commonly referred to as scanning tunneling luminescence [1]. The opportunity to use tunneling effects for a photon emission spectroscopy with nanometer resolution is obviously of great interest. Nanostructures might be successfully applied in next-generation photovoltaics, and STL will contribute decisively to our understanding of electron transport and recombination in the nanoscale. As proof of principle, luminescence from individual InP [2,3] and InAs [4] quantum dots has been reported. Improved resolution is not the only benefit of STM. Below the activation energy for secondary electron-hole excitation, STL is excited by diffusion of tunneling electrons and subsequent recombination with available holes for p-type semiconductors – the reverse is true for n-type. In this case, the polarity of the tunneling current needed for STL is specific to the type of semiconductor. The effect can be exploited to observe local type inversion, which will be established by the need to invert the polarity of the tunneling current in order to detect photon emission. In this contribution, we report on the development of STL at the National Renewable Energy Laboratory and its application to semiconductors relevant to our program in photovoltaics. As part of this exploratory research, and in order to illustrate the capabilities of this method, we have investigated grain boundaries in CuInSe2 (CIS) thin films, InGaAs quantum dots (QDs) for thirdgeneration photovoltaics, and GaPN epilayers grown on silicon substrates for tandem solar cells.
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EXPERIMENTAL DETAILS In STL, and for semiconductors, the quantum efficiency of the radiative recombination is 10 to 10-3 for each tunneling electron. Therefore, the most critical aspects in the development of this STM are achieving excellent photon collection and detection sensitivity. We have adapted an STM to be operated in conjunction with cathodoluminescence (CL). One of the advantages of this approach is that the tip can be accurately positioned into the focal point of the parabolic mirror of the CL detection system, improving dramatically the photosensiti
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