Tem Study of Porous Silicon Fabricated from N- and P-Type Doped Polycrystalline Films
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. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
Table 1: Characteristics of the studied samples and their anodization conditions. Name of sample Poly-Si type
NPI
NP2
NP3
N+P
P+N
n
n
n
0.6
0.6
0.6
3 x 10-3 2-5 60 yes
Substrate type
p
p
P
n+ 2 x 10-3 P
Substrate resistivity (0.cm) Anodization time (s) Illumination during anodization
1-5 90 no
1-5 20 no
1-5 20 es
1-5 90 no
Poly-Si resistivity (0l.cm)
p+ n
RESULTS AND DISCUSSION n POLY-Si /p-Si STRUCTURE: In a first step we describe the porous morphologies in such a structure in the case where the anodization was done in dark. Figure 1 is a XTEM micrograph of the sample NPI (see table 1) showing the porous microstructure of the poly-Si layer and c-Si substrate. The top layer is partially transformed into porous poly-Si (PPS) material but with a low porosity. Large areas in the poly-Si layer remain unattacked. These regions are visible near the interface and around the extended defects of the grains( see arrows in Fig. 1). Although the top n-type layer was not completely transformed, we notice that the pore formation has advanced into the underlying p-type substrate. The porous Si layer (PSL) formed inthe substrate exhibit a very rough interface with the non attacked substrate. This undulation of the anodization front in the substrate is due to an enhanced etch rate at some points located at the original poly-Si/substrate interface. The PSL has a nanoporous structure, with pore orientations radiated from these local points. The bright contrast which appear in the micrograph at these points indicates a high porosity material. We have observed, that these singular points correspond to twins running throughout the grains.
Figure 1. XTEM micrograph of sample NP I anodized in the dark for 90 s.
422
Anodization at reduced etching time (sample NP2 ) may give information about the early stage of the pore formation. Figure 2 shows the result for an anodization time of 20 s. The microstructure of the PPS is quite similar to that described before. It consists of primary pores located within the twinned areas of the grains and secondary tiny pores having slight ramifications. Large unattacked regions are clearly observed inside the poly-Si grains as well as at the poly-Si/substrate interface (see arrow in Fig.2). Figure 3 is a HRXTEM view of a crystalline zone near the interface. We can notice that the unattacked region is bounded by micro-twins. The difference between the microstructures of the pores in the film and substrate is clearly evidenced. Also shown in figure 2 the heteregeneous character of the pores formation in the underlying substrate. Large cavities having various sizes, filled with nanoporous Si, appear. The center of these cavities, from which the PS propagates, corresponds to twins.
Figure 2. XTEM micrograph of sample NP2 anodized in the dark for 20 s. Primary pores are located at twinned areas (TW); unattacked areas (arrowed) around grainboundaries (GB). Notice the thin secondary pores within the grains.
PPs,
Figu
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