Optoelectronic properties and depth profile of charge transport in nanocrystal films

We investigate the charge transport in nanocrystal (NC) films using field effect transistors (FETs) of silicon NCs. By studying films with various thicknesses in the dark and under illumination with photons with different penetration depths (UV and red light), we are able to predictably change the spatial distribution of charge carriers across the films' profile. The experimental data are compared with photoinduced charge carrier generation rates computed using finite-difference time-domain (FDTD) simulations complemented with optical measurements. This enables us to understand the optoelectronic properties of NC films and the depth profile dependence of the charge transport properties. From electrical measurements, we extract the total (bulk) photoinduced charge carrier densities ($n_{\text{photo}}$) and the photoinduced charge carrier densities in the FETs channel ($n_{\text{photo}}^{*}$). We observe that the values of $n_{\text{photo}}$ and their dependence on film thickness are similar for UV and red light illumination, whereas a significant difference is observed for the values of $n_{\text{photo}}^{*}$. The dependencies of $n_{\text{photo}}$ and $n_{\text{photo}}^{*}$ on film thickness and illumination wavelength are compared with data from FDTD simulations. Combining experimental data and simulation results, we find that charge carriers in the top rough surface of the films cannot contribute to the macroscopic charge transport. Moreover, we conclude that below the top rough surface of NC films, the efficiency of charge transport, including the charge carrier mobility, is homogeneous across the film thickness. Our work shows that the use of NC films as photoactive layers in applications requiring harvesting of strongly absorbed photons such as photodetectors and photovoltaics demands a very rigorous control over the films' roughness.

Figure 1

AFM micrographs of Si NC films with (a) $L=700\mathrm{nm}$ and (b) $L=60\mathrm{nm}$. The AFM micrographs are used to build up a simulation model of Si NC films on 130 nm Au and 300 nm silicon oxide on (c), (d) a silicon wafer and on (e), (f) quartz glass. (g) Optical absorption data of Si NC films on quartz glass with $L_{\text{av}}=830\mathrm{nm}$ (open squares) and $L_{\text{av}}=150\mathrm{nm}$ (full squares) and simulated spectra (red lines) using the models in (e) and (f).

Figure 2

(a) $I-U$ characteristics and (b) transfer curves of Si NC FETs with $L=60\pm 15\mathrm{nm}$ (triangles) and $L=680\pm 90\mathrm{nm}$ (diamonds) taken in darkness (black) and under illumination with $\lambda =390\mathrm{nm}$ (blue) and $\lambda =623\mathrm{nm}$ (red).

Figure 3

(a), (b) Conductance $G$, (c), (d) conductivity $\sigma$, and (e), (f) mobility $\mu$ of Si NC FETs measured in darkness (black) and under illumination with $\lambda =390\mathrm{nm}$ (blue) and $\lambda =623\mathrm{nm}$ (red). The illumination intensities are determined to be $7\mathrm{mW}/{\mathrm{cm}}^2$ ($\lambda =390\mathrm{nm}$) and $13\mathrm{mW}/{\mathrm{cm}}^2$ ($\lambda =623\mathrm{nm}$) for sample series I (square symbols) and $14\mathrm{mW}/{\mathrm{cm}}^2$ ($\lambda =390\mathrm{nm}$) and $11\mathrm{mW}/{\mathrm{cm}}^2$ ($\lambda =623\mathrm{nm}$) for sample series II (circle symbols).

Figure 4

(a), (b) Charge carrier density in darkness $n_{\text{dark}}$ calculated using Eq. (5). (c), (d) Photoinduced charge carrier density $n_{\text{photo}}$ for $\lambda =390\mathrm{nm}$ (blue) and $\lambda =623\mathrm{nm}$ (red).

Figure 5

(a), (b) Threshold voltage $U_{\text{T}}$ taken at $U_{\text{DS}}=3\mathrm{V}$ measured in darkness (black) and under illumination with $\lambda =390\mathrm{nm}$ (blue) and $\lambda =623\mathrm{nm}$ (red). (c), (d) Photoinduced charge carrier density calculated using Eq. (8) and the data in (a) and (b).

Figure 6

(a) Current taken at $U_{\text{DS}}=10\mathrm{V}$ as a function of time for a thin Si NC film ($L=130\pm 30\mathrm{nm}$) and a thick Si NC film ($L=1140\pm 80\mathrm{nm}$). Blue-shaded areas represent illumination with UV light ($6\pm 2{\mathrm{mWcm}}^{-2}$). (b) Photocurrent of the thin and thick Si NC films in (a) with different illumination intensities for $\lambda =390\mathrm{nm}$ (blue) and $\lambda =623\mathrm{nm}$ (red). Dashed lines are a fit to the data using Eq. (9).

Figure 7

Optical absorption profile of Si NC films on FET structures illustrated in Fig. 1 with (a), (c) film thicknesses $L=700\mathrm{nm}$ and (b), (d) $L=60\mathrm{nm}$ for (a), (b) $\lambda =390\mathrm{nm}$ and (c), (d) $\lambda =623\mathrm{nm}$.

Figure 8

Calculated charge carrier generation rates in Si NC films (see Figs. 1 and 7) for $\lambda =390\mathrm{nm}$ (blue circles) and $\lambda =623\mathrm{nm}$ (red circles). (a), (b) Generation rates in the 5 percentile thickness $L$ of the Si NC films, (c), (d) generation rates in the lower 50nm of the Si NC films (FET channel). Dashed lines are calculated using the Lambert-Beer law.