Charge Carriers diffusion length in boron doped diamond layers by cathodoluminescence

J.C. Piñero1, M. Paz Alegre1, D. Araújo1, M.P. Villar1, M. Kadri2, P.N. Volpe2, A. Fiori2, E. Bustarret2

1Dpto. Ciencias de los Materiales, Universidad de Cádiz, 11510, Puerto Real (Cádiz), Spain.
2Institut Néel, CNRS, 25 Avenue des Martyrs, BP 166, 38042 Grenoble, France.

Despite the exceptionally known electronic properties of diamond, it still suffer from lack of knowledge on some carrier transport parameters. In particular, the influence of doping, point defects and extended defects on the decay of the carrier lifetime and diffusion length has not been, to our knowledge, quantified. This is not only interesting from the point of view of fundamental physics but also for the development of diamond related devices, whose mobility plays an important role. The latter can be obtained by Hall measurements [1], or  recombination lifetimes through time-resolved methods [2]. Here, evaluation of carrier diffusion length, using a CL setup, on boron-doped homoepitaxial layer is presented. 

Fig. 1: Schematic description of the sample preparation by FIB at the edge of the sample
A systematic study changing the boron content is carried out; a cross sectional method (see fig.1) has been developed using a FIB (Focused Ion Beam) to remove material and make accessible the layer from the edge of the sample. Carrier recombination peaks present only in the substrate are used to detect carrier transport from the layer (see fig.2a).  To minimize effect of the extended generation of e-h (electron-holes, first effect: the pear shaped volume of interaction between incident high energetic electron of the e-beam and diamond material, second effect: the size of the e-beam spot) special attention has been dedicated to the e-beam voltage and current to keep the lateral extension of e-h pair generation at full width half maximum (FWHM) below 0.2 µm.


Fig. 2: Monochromatic CL micrograph at 503nm (a) that allows to extract the minority carrier diffraction length (see Fig. 2B) from the CL intensity linescan (b).
Diffusion length is shown to decrease with the boron content from LD = 1.6µm at [B] = 1·1015at/cm3 to  LD = 1.0µm at [B] = 1·1020at/cm3. A good agreement is obtained comparing these results to that obtained by the Einstein relation using mobilities previously published [3].



3.                   T. Malinauskas, K.J., E. Ivakin, V. Ralchenko, A. Gontar, S. Ivakhnenko, Optical evaluation of carrier lifetime and diffusion length in synthetic diamonds. Diamond & Related Materials, 2008. 17: p. 1212–1215.