Microscopic Understanding of the Carrier Transport Process in Ge Nanocrystals Films

Abstract

Hydrogenated amorphous germanium ( -Ge:H) films were prepared by a plasma enhanced chemical vapor deposition (PECVD) technique. Ge nanocrystals (Ge NCs) films were obtained by thermal annealing of the as-deposited samples at various temperatures. P-type behavior in Ge NCs films without any external doping was attributed to the holes accumulation caused by acceptor-like surface states. It can be found that the dark conductivity and Hall mobility reached as high as 25.6 S/cm and 182 cm 2 /V⋅s in the Ge NCs film annealed at 500 ∘ C, which were increased by over four and three orders of magnitude higher than that of the as-deposited film (1.3 × 10 −3 S/cm and 0.14 cm 2 /V⋅s, resp.). Carrier transport mechanisms of Ge NCs films association with the microstructural characteristics were investigated. Three kinds of temperature-dependent conductivity behaviors, which exhibit the linear relationships of ln versus −1/4 , −1/2 , and −1 , respectively, were observed in the temperature regions from 10 K to 500 K, showing different microscopic mechanisms governing carrier transport in Ge NCs film.

The Electronic Properties of Boron-Doped Germanium Nanocrystals Films

“…Furthermore, the Hall mobilities 𝜇 𝐻𝑎𝑙𝑙 in B-doped Ge NCs films were also investigated at room temperature as shown in Figure 4 (b). The Hall mobility is about 182 cm 2 •V -1 in Ge NCs film before doping, exhibiting a normal electrical performance compared with other reports [29]. After doping, it is very interesting to find that the value of 𝜇 𝐻𝑎𝑙𝑙 is higher than that of the un-doped sample.…”

“…It was interested to find that the carrier concentration of un-doped Ge NCs film was nearly reached to 10 18 cm -3 order with a p-type behavior. It was reported that a high hole concentration in Ge NCs film without intentional doping was attributed to the deep-acceptor-like surface states which are usually related to the dangling bonds, giving rise to numerous of negative charges built up at the surfaces [29]. As a consequence, the energy bands near the surface tend to bend up and attract extra holes in films, leading to a high hole concentration in the un-doped Ge NCs film.…”

Structures, Electronic Properties and Carrier Transport Mechanisms of Si Nano-Crystalline Embedded in the Amorphous SiC Films with Various Si/C Ratios

“…Moreover, as the Si/C ratio is further increased, the value of E g is obviously raised and reached to 2.8 eV for the sample with R = 5, which can be explained as the contribution from the GBs in the annealed films [26]. In our previous works, we observed a large amount of disordered GBs regions existing in the annealed films, which had a higher optical gap compared with the amorphous and nano-crystalline Si and might play an important role in the overall optical gap of the films [25,27]. Therefore, the increase of optical band gap with Si/C ratio can be ascribed to the appearance of large numbers of GBs in the films, likely due to the enhanced crystallization as we mentioned before.…”

“…It was found that nanocrystalline material contained a large number of nanocrystals joined together by grain boundaries with a lot of defects, which could trap the carriers and then formed the potential barriers. Thus, the transport process was considered to be a tunneling of free charges between the neighboring Ge NCs at the relatively low temperature [27,41]. Consequently, the percolation hopping model can be suitable for explaining the carrier transport mechanism in the present sample with R = 5 because Si grains with large size in such quality crystallization film are separated by the grain boundaries as shown in the TEM images.…”

“…It is reasonable since the charge carriers hopping between the neighboring Si NCs are impeded due to the freeze out of acoustic phonons as the temperature goes down to 50 K. At the lower temperature, the charge transport is proposed as the tunneling process of charge carriers between localized states according to Mott's model [35]. The Mott-VRH conduction had been usually reported in un-doped and P-doped Si and Ge NCs films at low temperature as well [27,[36][37][38]. However, once the temperature is above 50 K, the temperature dependence behaviors of conductivity change again and are no longer described by the Mott-VRH process.…”