Laser beam induced current characterisation of HgCdTe processed using plasma techniques

Abstract

Laser beam induced current (LBIC) is a nondestructive optical characterization technique which has been used in a qualitative manner for a number of years to investigate HgCdTe material and devices. The technique involves incrementally scanning a low-power focused laser beam across the surface of a wafer and measuring the photoinduced current through two remote contacts as a function of position. This allows a spatial map of the electro-optical properties of the wafer to be created. The p-n junction device has a characteristic bipolar LBIC profile, features of which can be related to junction properties under properly designed measurement geometry and given certain assumptions [1]. Ideally this should allow for extraction of several quantitative p-n junction characteristics.In this paper, LBIC has been applied to the characterization of HgCdTe n-on-p junctions fabricated through plasma induced type conversion using an inductively coupled plasma reactive ion etching (ICPRIE) tool.Exposure of p-HgCdTe to H 2 /CH 4 -based plasma in a parallel plate RIE tool has previously been demonstrated to result in p-to-n type conversion, and high performance photodiodes have been fabricated using this process [2].The more advanced hybrid ICPRIE tool allows for greater flexibility, in being able to separately control the power primarily generating the high density plasma, and the power driving particles to the wafer surface. The nondestructive LBIC technique allows for characteristics of junctions formed under a number of different ICPRIE plasma process conditions to be investigated using only two remote contacts, in order to refine the fabrication procedure aim for controlled doping modification to produce high performance photodiodes. Junction depth has been determined for junctions formed in vacancy doped Hg 0.7 Cd 0.3 Te under various ICPRIE plasma process parameters by performing a series of wet chemical etches and variable temperature LBIC scans. The shape and peak-to-peak height of the extracted LBIC profiles are related to the junction depth and quality [1]. Preliminary results shown in Figure 1 to Figure 4 illustrate the shape and peak-to-peak LBIC of junctions formed through plasma-induced type conversion under the ICPRIE process. Junction depth has been quantitatively determined by continuing the series of wet etch steps and LBIC scans until the characteristic LBIC profile typical of an n-p junction is no longer observed, and has been found to be a positively related to RIE power, and to a lesser degree on ICP power. In particular the LBIC results are consistent with RIE power and ICP power being positively related to both increased doping density of type converted material and preferential etching of the HgTe material component. Hall measurements are also to be presented that extract more information regarding the carrier concentration in the type converted material, in order to more accurately explain the LBIC characteristics. 991 0-7803-9217-5/05/$20.00©2005 IEEE ThDD4 16:30 -16:45