Keywords : semiconductor , electron beam induced current , depletion region modulation

I N T R O D U C T I O N The minority carrier diffusion length is one of main parameters characterizing the quality of semiconductor materials. Therefore development of methods for diffusion length measurements and reconstruction of its spatial distribution is very important for the characterization of as-grown semiconductor materials and structures, characterization of technology processes and failure analysis. Methods based on an electron or light beam injection such for example, as the electron beam induced current (EBIC) [1-3] and surface photovoltage [4] techniques are now widely used for this purpose. For the EBIC mode in SEM approaches allowing one to obtain the diffusion length in local regions, in order to reveal lateral inhomogeneities in diffusion length and to reconstruct its lateral distribution, have been developed [5,6]. To reconstruct the diffusion length depth distribution a few approaches have been proposed [7-11] but their use needs special preparation of cross section or bevelled samples or some additional information about the diffusion length distribution. The more general approach for the nondestructive reconstruction of diffusion length depth distribution is a development of computer tomography methods [12-14]. As shown in [15], the inverse problem for EBICbased reconstruction of diffusion length distribution is ill posed and the regularization technique [16] should be used for its solution. Examples of such reconstruction procedures were presented in [17]. Unfortunately, the regularization procedure needs rather high measurement precision for the successful reconstruction, therefore new, more effective approaches should be developed. An EBIC technique with depletion region modulation (modulated EBIC), proposed in [18,19], could be considered as one such approach. This in-situ differential method was shown not only to improve measurement precision but also to simplify the inverse problem solution. Its application for diffusion length depth reconstruction [20,21], measurements of diffusion length in thin semiconductor layers [22] and mapping both the diffusion length and depletion region width [19] have already been demonstrated. An application of photocurrent measurements with the depletion region modulation for the diffusion length depth reconstruction was discussed in [23]. B A S I C P R I N C I P L E S Let us assume that the current induced under an electron beam or optical excitation in an ntype semiconductor with the minority carrier diffusion length L = (Dt), where D and tare the minority carrier diffusivity and lifetime, respectively, is collected by the Schottky barrier with large enough dimensions in xand ydirections (Fig. 1). L is assumed to change only in one direction z, perpendicular to the collector plane. Under such assumptions the stationary collected current Ic for the both EBIC and photocurrent (current induced by the optical excitation) can be described as [12]