Evaluation of intrinsic charge carrier transport at insulator-semiconductor interfaces probed by a non-contact microwave-based technique.
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We have successfully designed the geometry of the microwave cavity and the thin metal electrode, achieving resonance of the microwave cavity with the metal-insulator-semiconductor (MIS) device structure.By means of the present measurement system named field-induced time-resolved microwave conductivity (FI-TRMC), the pentacene thin film in the MIS device allowed the evaluation of the hole and electron mobility at the insulator-semiconductor interface of 6.3 and 0.34 cm² V⁻¹ s⁻¹, respectively.This is the first report on the direct, intrinsic, non-contact measurement of charge carrier mobility at interfaces that has been fully experimentally verified.
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Affiliation: Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
ABSTRACT
We have successfully designed the geometry of the microwave cavity and the thin metal electrode, achieving resonance of the microwave cavity with the metal-insulator-semiconductor (MIS) device structure. This very simple MIS device operates in the cavity, where charge carriers are injected quantitatively by an applied bias at the insulator-semiconductor interface. The local motion of the charge carriers was clearly probed through the applied external microwave field, also giving the quantitative responses to the injected charge carrier density and charge/discharge characteristics. By means of the present measurement system named field-induced time-resolved microwave conductivity (FI-TRMC), the pentacene thin film in the MIS device allowed the evaluation of the hole and electron mobility at the insulator-semiconductor interface of 6.3 and 0.34 cm² V⁻¹ s⁻¹, respectively. This is the first report on the direct, intrinsic, non-contact measurement of charge carrier mobility at interfaces that has been fully experimentally verified. No MeSH data available. |
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Mentions: Prior to the measurement of the pentacene/PMMA MIS device, we investigated the relation between ΔNμ and ΔPr by loading standard samples into the microwave cavity. Gold (σ = 4.52 × 105 S cm−1), manganese (σ = 6.94 × 103 S cm−1), and p-doped silicon (σ = 10 S cm−1) were selected as the standard samples28. They were vapor deposited as thin films onto quartz substrates with their thicknesses controlled, so that the total deposited mass was known. The mass of these samples was then converted into the value of ΔNμ using where m (g) and d (g/cm3) denote the mass and density of the standard samples, respectively. The value of ΔNμ can be regarded as the change in the pseudo electrical conductivity. By measuring the value of ΔPr for each standard sample, we obtained the relation between ΔNμ and ΔPr shown in Fig. 4. Two straight lines with different slopes were used to fit the data in the lower ΔNμ (1015–1017) and higher ΔNμ regions (over 1018). For lower ΔNμ, the value of ΔPr becomes small (i.e.: the change in the Q value is small), since ΔPr is proportional to the electrical conductivity change (Δσ)2930. Thus, ΔPr is proportional to the pseudo electrical conductivity ΔNμ, and the data in the low ΔNμ region (for Mn and p-doped Si) can be successfully fitted using the equation: On the other hand, equation (5) does not provide a good fit to the data in the high ΔNμ region (for Au). When samples with high electrical conductivity such as Au are used, the microwave amplitude decreases significantly at the surface of the thin film due to skin-depth effects. In such cases where the interaction of microwaves with a material is confined to the near-surface region, it is necessary to use the skin-depth approximation31: where Δω represents the difference in the resonance frequency between a cavity with and without a sample present, and ω0, C, G, Zs, and μ0 are the resonance angular frequency, the metallic shift associated with a perfect conductor, a resonator constant, the surface impedance, and the permeability, respectively. The Q value, representing the microwave power loss in the cavity, can be divided into Qu, Qc, and Qs components associated with microwave interaction with the inner walls of the cavity, the coupling between the cavity and the waveguide, and the sample, respectively29. The Qu and Qc components are constant during the measurement of microwave power loss, so these can be included in Q1. The relation between Q1, Qs, and Q can be expressed as: The relation between Qs, ω0, and the resonance frequency of the empty cavity f0 ( = ω0/2π) is expressed by31: Microwaves passing through the sample give rise to a decrease in the electric field and a phase shift, as indicated by equation (9), and this causes a change in the Q value and resonant frequency. The intensity of the microwaves reflected from the cavity is expressed by equation (10): The value of ΔPr/Pr is defined by the complex conjugation of the reflectance: In the case of Δω/ω0 ~ 0, equation (11) can be approximated by a linear function of (1/Qs). Then, substituting equations (6), (7) and (9) into equation (12) results in: Remarkably, ΔPr is proportional to (σ1)1/2. Therefore, the data for Au can be fitted using the following equation: The values of N and μ for common organic semiconductors are smaller than those for Au, Mn, and Si. In fact, the value of N in MIS devices is known to be around 1012–1013 through calculation of their capacitance. Since the charge-carrier mobility for organic semiconductors is lower than 10 cm2 V−1 s−1, the value of ΔNμ is 1014 at most, which allows us to conclude that the linear approximation shown in equation (5) sufficiently describes the relation between ΔPr and ΔNμ. |
View Article: PubMed Central - PubMed
Affiliation: Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
No MeSH data available.