Reactive Ion Etching (RIE) Induced p- to n-Type Conversion in Extrinsically Doped p-Type HgCdTe
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extrinsically doped p-type epitaxial layers of HgCdTe (x=0.3 1) has been used to reconvert n-type conversion sustained during RIE processing. For the RIE processing conditions used (400mT, CH 4/H2, 90 W) p- to n-type conversion was observed using laser beam induced current (LBIC) measurements. After a sealed tube mercury anneal at 200°C for 17 hours, LBIC measurements clearly indicated no n-type converted region remained. Subsequent Hall measurements confirmed that the material consisted of a p-type layer, with electrical properties equivalent to that of the initial as-grown wafer (NA-ND=2x 1016 cmn3, .=350 cm 2.V-. s-). INTRODUCTION Reactive Ion Etching The development of dry etching techniques is of great importance for the further advancement of HgCdTe infrared device technology. Anisotropic dry etching, as opposed to wet etching methods, provides opportunity for delineation of a higher density of active device elements incorporating smaller features. However, HgCdTe has a relatively low damage threshold resulting from weak Hg-Te bonds in the crystal lattice [1]. Consequently, dry etching techniques such as ion milling and reactive ion etching (RIE) result in the conversion of p-type HgCdTe to ntype [2-6]. The physical mechanism used to explain this type conversion in vacancy doped p-type HgCdTe material is that during the etching process Hg atoms are liberated near the surface with some proportion of these interstitial atoms diffiising into the underlying bulk material. These interstitial Hg atoms annihilate acceptor vacancies, resulting in residual uncompensated donors converting the region to n-type. The rapid diffusion of Hg interstitials in HgCdTe, even at room temperature, has led to large conversion depths (over 100gm) in some experiments [2,5]. However, it has been suggested that cooling the substrate to lOOK during ion milling and RIE can limit the extent of p- to n-type conversion. Published work has also established that etching induced type conversion is evident even in extrinsically doped p-type HgCdTe in which the Hg vacancies have been previously filled using a mercury anneal [3,7]. This suggests that the conversion mechanism extends beyond Hg interstitial atoms annihilating acceptor vacancies and may point to an additional doping mechanism. Mercury annealing is a technique that is commonly used to alter the electrical properties of HgCdTe to suit device requirements [8,9]. In this work 353
Mat. Res. Soc. Symp. Proc. Vol. 484 0 1998 Materials Research Society
results are presented of a process whereby mercury annealing was used to eliminate the n-type converted regions induced by RIE processing of extrinsically doped p-type HgCdTe. Laser Beam Induced Current Laser beam induced current (LBIC) is a high resolution, non-destructive optical characterisation technique that has been used to spatially map and identify electrically active defects and active regions in HgCdTe materials and devices [10]. In the LBIC technique a low power laser is focused on a region of the semiconductor thus generating e
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