Scanning Probe Microscopy for the Nanoscale Electrical Characterization of Semiconductors for PV Applications

Scanning probe microscopy methods are key tools for nanotechnology research and have as a principal application the determination of surface topography.

However, scanning probe microscopy, and in particular atomic force microscopy (AFM), can be successfully used also to probe the electrical properties of semicon­ductors and semiconductor nanostructures leading to enormous insights into material functionality at the nanoscale.

Several techniques, reviewed in [53], have been developed to allow the determination of electrical properties such as resistivity, surface potential and capacitance simultane­ously with topographic information. This has required the development of new instru­mentation, of novel probes and of advanced sample-preparation techniques and image analyses. Furthermore, in order to understand and quantify the results of AFM-based electrical measurements, it has proved important to consider the interplay of topographic and electrical information, the presence of possible artifacts [54], and the role of surface states in determining the electrical response of a material at the nanoscale. Despite these challenges, AFM-based techniques provide unique insights into the electrical character­istics of ever-shrinking semiconductor devices and also allow the electrical properties of defects and self-assembled nanostructures to be probed. The method has been used since 1998 to test the quality and thickness of SiO2 thin layers [55], and more recently to characterize a blend of two semiconducting polymers [56], and to characterize defect structures in nitrides [57, 58].

In the present section the application of AFM to the investigation of electrical proper­ties of hydrogenated nanocrystalline Si thin film for PV applications will be presented.

The structure of thin-fllm nanocrystalline silicon can be described qualitatively as a collection of nanometer-to-micrometer-scale silicon crystallites embedded in a hydro­genated amorphous silicon (a-Si:H) matrix. As already mentioned in Section 5.2.3, the material is highly complex, due to the coexistence of different phases: crystalline nano­grains, nanocrystalline columns, hydrogenated amorphous Si (a-Si:H), defects and voids.

In addition, there can be several types of disordered silicon tissues between the different phases [59]. All these phases may be involved in the transport mechanism, and charge transfer between them may involve tunneling and thermionic emission, or a connected network of aggregates of such components.

When the crystalline phase is larger in volume than the amorphous one, two scenarios have been considered: transport via the crystallite columns and transport in the disordered tissue that encapsulates them. The choice between these two alternatives is not simple due to conflicting data: on the one hand Rezek et al. [60] by microscopic analyses showed that electronic transport occurs through the columns (i. e., through the crystallites that constitute them) while, on the other hand, the macroscopic and microscopic data of Azulay et al. [59] indicated that transport occurs in the disordered tissue surrounding columns or crystallites. The strong debate between the two groups has also led to different data interpretation [61, 62] based on possible artefacts, which must be always considered when dealing with AFM analyses.

An example of the application of AFM for the electrical characterization of nc-Si:H films is shown in Figures 5.18 and 5.19. AFM analyses have been carried out in a Solver P47H-Pro instrument by NT-MDT. Conductive atomic force microscopy (C-AFM) maps were obtained by applying a constant positive bias to the probe while the sample was grounded by an ohmic contact. The AFM operated in constant force mode (contact mode). nc-Si:H films grown on Si and on glass by LEPECVD (low-energy plasma enhanced chemical vapor deposition) were examined. Further experimental details can be found in [54, 63, 64].

Подпись: 500 nm (a) Подпись: 500 nm (b)

Figure 5.18 shows one example of topography (left) and C-AFM (right) maps of undoped nc-Si:H film grown on Si obtained with a bias voltage of 3 V. The current map, at constant applied bias, represents a conductivity map. The topography map shows the

Figure 5.18 Topographical AFM (left) and conductivity C-AFM (right) maps obtained on intrinsic nc-Sigrown on Si, with d = 50%, T = 280°C, growth rate = 3.6nm/s. The applied bias potential is 3 V, the gray-scale ranges from 0 to 25 nm (left), from 0 (white) to 1.8 (dark) nA, right. The area within the circles domains. Reprinted with permission from [63]. Copyright (2010) Institute of Physics.

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Figure 5.19 Topographical AFM (left) and conductivity C-AFM (right) maps on B-doped nc-Si:H grown on corn ing glass. Si lane di lution 1%, D iborane dilution DR = B2H6/SiH4 = 3.6%, growth temperature T = 250 °C, bias potential 0.5 V, the grayscale ranges from 0 to 37nm (a), from 0 (white) to 10 (dark) nA, (b). The area within the circles indicate the domains. Reprinted with permission from [63]. Copyright (2010) Institute of Physics.

presence of large (50 to 100 nm diameter) domains that likely represent clustering of several Si nanocrystals (NCs), whose diameter, as measured by transmission electron microscopy (TEM) [63] varies between 5 to 15 nm. Within these domains the C-AFM map (Figure 5.18, right) shows a number of conductive grains which are separated from each other by a nonconductive tissue. It is worth noting that the conductive grains are mainly located within the domains, not at the domain borders. The conductance values of the Si nanocrystallites ranges from 0.2 to 0.3 nS. This behavior is typical for all the intrinsic layers examined by C-AFM, irrespective of the substrate on which they were deposited, on the deposition temperature and on the growth rate.

Figure 5.19 shows a typical topography (left) and C-AFM (right) of a B-doped nc – Si film grown on glass. The C-AFM map was obtained by applying a bias voltage of 0.5 V. The different behavior in respect to the intrinsic samples clearly appears from the maps: conductive nanocrystallites surrounded by nonconductive tissue are evident in the C-AFM map, but the conductive dots are mainly located within the boundaries between domains. Moreover, the Si NCs conductance values strongly differ: while in intrinsic layers Si NCs show conductance values around 0.2 nS, in doped layers con­ductance values are spread over a large range, from 2 to 8 nS. Similar results have been obtained in all the films examined, independently of their growth temperature or substrate.

These features could be understood as follows. Due to nonuniform strain distribution during film growth, a strong inhomogeneity of the NC size distribution in the film could occur. Isolated NCs could more easily cluster when they are located within a domain border than if they are located within the domains. In intrinsic layers isolated NCs located in the domains are conductive due to spatial localization of carriers, as explained before, while NC clusters located at the domain borders, can be less conductive.

Conversely, in doped layers, cluster of Si NCs located at the domain boundaries can be more conductive due to larger doping efficiency of B in large NCs. As a matter of fact, computational and experimental studies demonstrated the size dependence of the formation energy of B in Si-NCs [63], leading to the conclusion that large Si NCs can easily sustain the doping. Therefore, it can be concluded that small, isolated Si NCs are conductive in intrinsic layers due to quantum confinement effects, while large, clustered Si NCs are conductive in B-doped layers due to the size effect of B doping efficiency.

In conclusion, the C-AFM analysis of hydrogenated nanocrystalline Si is capable of showing that the electrical conduction occurs via the Si crystallites, in intrinsic as well as in doped layers. Moreover, B doping significantly enhances the conductivity of the nc-Si:H films, but the doping efficiency is strongly dependent on the microstructure of the film.