Direct measurement of minority carriers diffusion length using Kelvin probe force microscopy

Abstract

We report on the use of Kelvin force microscopy as a method for measuring very short minority carrier diffusion length in semiconductors. The method is based on measuring the surface photovoltage between the tip of an atomic force microscope and the surface of an illuminated semiconductor junction. The photogenerated carriers diffuse to the junction, and change the contact potential difference between the tip and the sample as a function of the distance from the junction edge. The diffusion length L is then obtained by fitting the measured contact potential difference using the minority carrier continuity equation. The method is applied to measurements of electron diffusion lengths in GaP epilayers.Minority carrier diffusion length ͑L͒ is an important parameter in determining the performance of minority carrier devices such as solar cells, bipolar transistors, optical detectors, and more. In the past many different techniques have been used to determine L. The three most widely used methods are electron-beam induced currents ͑EBICs͒, 1 surface photovoltage ͑SPV͒, 2 and photoluminescence ͑PL͒. 3 In the EBIC method ͑probably the most widely used technique͒ a p-n junction or a Schottky barrier is viewed edge on. With the scanning electron microscope in a line scan mode, the electron beam scans the semiconductor perpendicular to the potential barrier and generates electronhole pairs. The generated charge carriers then diffuse to the junction, where the electrons and holes are separated and a current is generated in the external circuit. This current, referred to as the EBIC current, reflects the amount of excess carriers generated. A theoretical fit to the experimentally measured current allows for the evaluation of L. The main disadvantage of the EBIC technique is that the shape of the EBIC curve depends on several factors, most importantly on the surface recombination velocity of the surface on which the beam impinges. Several theoretical models have been derived to overcome this problem. 4 In the SPV method, a super band-gap energy monochromatic light of a wavelength illuminates the semiconductor. The intensity of the light I is changed so the measured SPV ͑which is proportional to the concentration of the minority carriers available by the surface͒ is constant. Under certain assumptions 2 an I vs curve of the form IϭC͓1/␣() ϩL͔, where C is a constant, is obtained. Combining the I() results with the knowledge of the ␣͑͒ dependence enables the extraction of L. Hence the main disadvantage of the SPV technique is that it requires an accurate knowledge of the ␣͑͒ dependence of the measured semiconductor. The PL technique is based on measuring the minority carrier lifetime 5 and calculating L based on the measured mobility of the sample.Recently, near-field optical imaging 6 and other spatially resolved techniques 7 have been used to measure transport in short-carrier diffusion length semiconductors. In this work we report on a new method for measuring very short minority carrier diffusion lengths in semiconductors. Th...

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