The Carrier Transport Properties of B-Doped Si Nanocrystal Films with Various Doping Concentrations

College of Information Engineering, Yangzhou Polytechnic Institute, Yangzhou 225127, China National Laboratory of Solid State Microstructures and School of Electronic Science and Engineering and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China College of Physical Science and Technology, Yangzhou University, Yangzhou 225009, China Tongda College, Nanjing University of Posts and Telecommunications, Nanjing 210003, China


Introduction
Si nanocrystals (Si NCs) have attracted more and more interest in recent years because of their possible applications in many kinds of nanoelectronic and optoelectronic devices such as light emitter devices, thin-film solar cells, photodetectors, and synaptic devices [1][2][3][4][5][6][7][8]. For example, highly efficient electroluminescence devices based on Si NCs have been fabricated by Yao et al. [9]. It was reported that the maximum external quantum efficiency and largest optical power density of the device reached as high as 2.7% and 0.11 mW/cm 2 , respectively. Recently, Si NCs have been utilized to incorporate with organic semiconductors or lead halide perovskites, which can improve the power conversion efficiency of solar cells [10][11][12]. In order to further improve the performance of devices based on Si NCs, active doping is usually required to obtain all kinds of desired properties of the Si NC materials. However, doping in Si NCs is quite different from their bulk counterpart due to the so-called "self-purification effect," and the states of doped atoms in Si NCs such as bonding structures, sites, and electrical activities are usually influenced by the microstructures [13,14]. With the advances in the synthesis of B-and P-doped Si NCs, several researchers have reported successful doping in Si NCs, offering significant freedom in tuning the electrical properties of Si NCs [15][16][17][18]. But a comparable study on the carrier transport properties in doped Si NCs is still lacking. Therefore, further understanding of the fundamental carrier transport properties in doped Si NCs film is necessary as various theoretical aspects and the mechanisms of dopant incorporation are still under debate [19][20][21][22].
In our previous works, undoped, B-doped, and P-doped Si NC films were prepared and their electronic properties were comparably studied. It could be found that very high room temperature conductivities were achieved both in Si NC films after B and P doping, which demonstrates that doping is an effective way to improve the electronic properties of Si NCs for device applications [23]. It was also found that the carrier mobility was gradually improved in P-doped Si NC films compared with the undoped one due to the reduction of barrier height of grain boundaries (GBs) and the passivation of surface defect states after P doping [24]. In the present work, the carrier transport properties in B-doped Si NC films with various doping concentrations were investigated via the temperature-dependent Hall effect measurements. A significant improvement of Hall mobility was still achieved after B doping due to the reduction of barrier height of GBs in the films, which can be well described by the previously proposed model. It was interesting to find that a metal-insulator transition (MIT) took place in the B-doped samples during further increasing the doping concentration. The different carrier transport properties together with scattering mechanisms in the transport process in the B-doped samples with various doping concentrations were investigated and discussed.

Experiment
The B-doped hydrogenated amorphous silicon (a-Si:H) films were prepared by a plasma-enhanced chemical vapor deposition (PECVD) method using a gas mixture of pure silane (SiH 4 ), hydrogen (H 2 ), and diborane (B 2 H 6 , 1% diluted in H 2 ). The flow rate of SiH 4 (F S ) was kept at 5 sccm (standard cubic centimeter per minute). Boron concentrations were then changed by adjusting the flow rate of B 2 H 6 (F B ), which is controlled at 0 sccm (undoped), 0.5 sccm, 1 sccm, and 5 sccm, respectively. The gas-chamber pressure, substrate temperature, and radio frequency power were 10 mTorr, 250°C, and 30 W during the growth process, respectively. After deposition, the samples, all which show the thickness of about 180 nm, were subsequently annealed in a conventional furnace at temperatures of 1000°C for 1 h in nitrogen ambient. Quartz plates and monocrystalline Si wafers were used as substrates for the various measurements.
Raman scattering spectra were detected by a Jobin Yvon Horiba HR800 spectrometer operating with 1800 g/mm grating, where the excitation light source is Ar+ laser with a wavelength of 514 nm. High-resolution transmission electron microscopy (TEM) images were observed by a TECNAI G2F20 FEI high-resolution transmission electron microscopy. The bonding configurations were characterized by PHI 5000 Versa Probe X-ray photoelectron spectroscopy (XPS), and the composition signal was detected after Ar+ etching to 5 nm. Temperature-dependent Hall measurements were measured by the LakeShore 8400 Hall effect measurement system using van der Pauw (VDP) geometry. All the samples were heated at 400 K for 30 min and then measured back to 300 K with the step of 10 K in argon ambient.

Results and Discussion
Raman spectroscopy was utilized to investigate the change of microstructures in the B-doped Si NC films with various doping concentrations. The signals at 480 and 520 cm −1 represent the transverse-optical (TO) vibration mode of amorphous silicon and crystalline silicon, respectively. As shown in Figure 1, all the samples show a sharp and strong peak close to 520 cm −1 , which indicates a pure Si crystalline structure. The crystalline fraction, X c , can be determined based on Raman spectra according to the formula X c = I c / ðI c + 0:88I a Þ, where I c and I a are the crystalline and amorphous parts of the integrated Raman scattering intensity, respectively [25]. It is found that X c , which is about 85% for the undoped sample, decreases to 80% after B doping. Similar results were also reported by our previous work [26]. It can be implied that the reduction of X c is attributed to the introduction of B dopants, which leads to an increase of bond-angle and bond-length fluctuations as the B dopant incorporation in the films and causes the degradation of the short-range order [27]. Furthermore, the size of Si NCs (R) before and after B doping can be roughly estimated from the shift between ω 1 and ω LO−TO using the bending parameters of bulk optical phonon dispersion curve β LO−TO , according to the formula ω 2 where ω 1 and ω LO−TO are the Raman spectra of Si NC films and bulk Si, respectively [28]. We worked out that the average size of Si NCs is about 24 nm for the undoped sample and 13 nm for the samples after B doping. From the above discussion, it can be confirmed that the introduction of B dopants will degrade the Si NCs and generates a reduction of crystallization and grain size in the films.
The microstructures of Si NC films before and after B doping were further investigated using the cross-sectional TEM observations. The sample thickness is clearly observed about 180 nm as shown in Figure 2(a), which is in good agreement with the predesigned value estimated by the deposition rate. The formation of Si NCs can be further identified in the high-resolution TEM image as given in Figure 2(b). A large number of Si NCs with the grain size of more than 20 nm spread over the undoped Si NC film, indicating a good crystallization of the film. However, the grain size of Si NCs is decreased to only about 10 nm after B doping as shown in Figure 2(c), which is in accordance with the estimate from Raman results. Compared with the undoped Si NC film, it can be clearly seen that the B-doped sample has a relatively low density due to the degradation of the film after B doping.
The chemical bonding environments of B-doped Si NC films were then investigated by XPS. The B 1s peaks of all the B-doped Si NC films are shown in Figure 3. For the Bdoped samples, the peak around 186 eV can be attributed to the B-Si signal [29]. It can be clearly identified that the  Journal of Nanomaterials intensity of the B-Si peak is not so weak and increases slightly with B doping concentrations. The B atom contents are estimated to be 0.31%, 0.51%, and 0.57% for the B-doped samples with F B = 0:5 sccm, 1 sccm, and 5 sccm, respectively.
It was reported that the B doping ratio is about 0.5% in Si NCs with F S : F B = 2 : 1 (F S = 31:5 sccm and F B = 15:2 sccm) [15], which is in the same order as in our case. The electrical properties of B-doped Si NC films with various doping concentrations were investigated by Hall effect measurements. Room temperature dark conductivities and carrier concentrations are exhibited in Table 1. The carrier concentration, which is about 1:7 × 10 12 cm -3 for the undoped sample, is significantly increased to 10 20 cm -3 order after B doping, indicating that the Si NC samples are easily   3 Journal of Nanomaterials heavily doped even at a relatively low gas doping ratio. It can be understood that the doping concentration can reach a very high value even if only a few dopants are incorporated into a one-Si dot with a diameter on the nanometer scale. Meanwhile, the dark conductivity, which is strongly determined by the carrier concentration, is increased from 8:6 × 10 −7 S·cm -1 for the undoped sample to the maximum of 456 S·cm -1 for the B-doped sample with the highest doping concentration (F B = 5 sccm).
The carrier mobilities of B-doped Si NC films with various doping concentrations are shown in Figure 4. It is very interesting to find that the Hall mobility is obviously improved after B doping. The Hall mobility, which is only 1.6 cm 2 /V·s for the undoped sample, is enhanced to 10.9 cm 2 /V·s for the B-doped sample with F B = 0:5 sccm. As we know, the carrier mobility usually reduces after introducing the dopants in bulk Si due to the strong scattering effect of impurities. Here, the abnormal behavior of carrier mobility must be caused by other reasons. In our previous works, the similar behavior of carrier mobility was also found in the P-doped Si NC films [24]. In fact, the carrier mobility is usually low in Si NC films due to the strong scattering from the GBs. It was reported that a large amount of GBs exist in the Si NC films, which contain a higher concentration of defects than the grains themselves, leading to carrier trapping at GBs [30]. After trapping the carriers, the traps become electrically charged, producing a potential energy barrier which impedes the carriers moving from one crystallite to another [31,32]. However, the barrier height at GBs can be reduced obviously if the carrier concentration is high enough, which contributes to a decrease of scattering from the GBs [22]. Thus, the significant increase of mobility after B doping can be explained by the reduced barrier height of GBs in the B-doped samples with a high carrier concentration (the carrier concentration reaches 10 20 cm -3 order). Moreover, further increasing the B doping concentration can still increase the Hall mobility, which reaches about 17.8 cm 2 /V·s in the B-doped sample with F B = 5 sccm. According to Seto's theory, the potential barrier height of GBs in the high-doped Si NC films can be estimated by the equation V B = qQ 2 t /8εN, where V B is the potential barrier height at GBs, Q t is the GBs trapping state density, and N is the carrier concentration, respectively [30]. It can be deduced that the potential barrier height V B is decreased with increasing the carrier concentration N. Consequently, the mobility of the B-doped Si NC film can be increased by further increasing the doping concentration, which finally is more than ten times higher than that of the undoped one.
In order to give a deeper understanding of the carrier transport properties in B-doped Si NC films, temperaturedependent dark conductivities were measured for all the samples within the temperature range from 300 K to 400 K. As shown in Figure 5, all the samples can be well fitted by the Arrhenius plots: where σ 0 is the conductivity prefactor, k B is Boltzmann's constant, and E a is the conductivity activation energy. The conductivity activation energy E a can be obtained using the slope of ln σ versus the 1/T curve. For the undoped sample, the value of E a is 0.55 eV which implies the Fermi level locating at the midgap of the Si NCs. After B doping, E a is rapidly decreased to 2 meV for the low-doped sample (F B = 0:5 sccm), which suggests that the Fermi level shifts to near the top of the valence band due to the doping effect. It can be found that σ versus 1000/T exhibits a linear relation in the whole measurement temperature range for the un-and low-doped samples, which means the thermally activated transport dominating the carrier transport processes [33]. However, for the high-doped Si NC films (F B = 1 sccm and 5 sccm), it is interesting to find that the dark conductivity is decreased with temperature increasing, which exhibits metal-like properties. Chen et al. reported that the metalinsulator transition (MIT) usually occurs in semiconductor NC films at some critical carrier concentration. The MIT criterion can be described by the following equation: where n c is the critical carrier concentration, g is the number of equivalent minima in the conduction band of the semiconductor, and ρ is the touching facets of the radius between NCs. Here, the facet radius ρ can be approximated as ρ =  Journal of Nanomaterials , where a is the lattice constant and d is the NC diameter [17]. In the present work, the NC diameter d for the Bdoped sample is about 13 nm from the Raman results. For such value of d, we predicted n c ≃ 1:17 × 1020 cm -3 , which just exceeds the carrier concentration of B-doped samples with F B = 0:5 sccm as shown in Table 1. It can be concluded that the metal-insulator transition took place in the B-doped Si NC films with F B = 1 sccm and 5 sccm, leading to a different behavior of σ − T relationship.
For further surveying the scattering mechanisms in the carrier transport process, the temperature-dependent Hall mobilities of B-doped Si NC films with various doping concentrations were investigated as shown in Figure 6(a). For the undoped sample, the increase of carrier mobility with temperature indicates a strong scattering from the GBs in Si NC film, which was expounded in detail in our previous work [34]. However, the GB scattering will be reduced after B doping according to the above discussion, and the temperaturedependent behavior of mobilities exhibits a different trend in B-doped samples, especially in the high-doped samples (F B = 1 sccm and 5 sccm). It can be found that the mobilities decrease with increasing temperature. Combining the reduction of conductivities with increasing temperature, we can confirm that the band-like transport behaviors take place in the high-doped Si NC films [35,36]. Furthermore, the scattering mechanisms can be investigated by the equation μ H ðTÞ ∝ T n , where n can be estimated by log μ H plotted as a function of log T. Least-squares fits of the data yield n = −0:1, -0.3, and -0.7 for the B-doped Si NC films with F B = 0:5 sccm, 1 sccm, and 5 sccm given in Figure 6(b), respectively. In c-Si, typical scattering mechanisms are the scattering at acoustic phonons and neutral impurities that yield values of −1.5 and ≈0. It can be suggested that a superposition of neutral impurity scattering, acoustic phonon scattering, and weak GB scattering dominates the carrier transport process in B-doped Si NC films. With the increase of B doping concentration, the n-value is gradually decreased to near -1.5, which implies that the acoustic phonon scattering increasingly plays an important role in the carrier transport process.

Conclusion
In summary, B-doped Si NC films with various doping concentrations were prepared and their carrier transport properties together with the microstructures were investigated. The electrical properties of Si NC films after B doping are significantly improved compared with the undoped one. Room temperature Hall mobility and conductivity are increased with increasing the doping concentration and reach 17.8 cm 2 /V·s and 456 S·cm -1 for the B-doped Si NC film with F B = 5 sccm. The enhanced Hall mobility after B doping can be ascribed to the reduced scattering from the GBs. Based on the temperature-dependent Hall effect measurement, it can be found that a MIT occur in the B-doped Si NC films with F B = 1 sccm and 5 sccm. As a consequence, the carrier transport mechanism is correspondingly changed from the thermally activated transport in the B-doped samples at low doping concentration to the band-like carrier transport in the B-doped samples at high doping concentration. Meanwhile, the acoustic phonon scattering gradually dominates the carrier transport process in the B-doped Si NC films with the increase of doping concentration. Metal-insulator transition TEM: Transmission electron microscopy XPS: X-ray photoelectron spectroscopy.

Data Availability
The data used to support the findings of this study are available from the corresponding authors upon request.