Info

KNbO,

GaAs:Cr

bisA-NPDA:DEH (4/91)

PMMA-MSAB:BDK:TTA (6/92)

PVK:FDEANST:TNF (1/93)

PVK:TPY:DEANST (9/93)

bisA-NAS:DEH (10/93)

PVK:C60:DEANST (3/94)

10"2 10° 102 10" Response speed, 1/r (s-1)

Fig. 20.7. Comparison of photorefractive device performance parameters for many materials and composites including a C^-polymer composite. All diffraction efficiencies are scaled to a sample thickness of 100 /urn. Polymer composites are referenced in the figure by the date of the first report. The spectral sensitizers are borondiketonate (BDK), trinitrofluorenone (TNF), and thiapyrylium (TPY). The optically active ingredient to assist charge transport is tri-p-tolylamine (TTA) and diethylaminonitrostyrene (DEANST). The best performance to date is with the polyvinylcarbazole (PVK) polymerQo composite [20.21], bisA-NAS:DEH2 [20.23], PMMA-MSAB:BDK:TTA3 [20.33,34,39]) and for PVK:C6û:DEANST, a C60-polymer composite with outstanding photorefractive performance. Although the C60-polymer composite PVKiC^rDEANST exhibits a slower response time than several of the fast inorganic materials, the diffraction efficiency of this C60-polymer composite is more than one order of magnitude larger than that of the best inorganic photorefractive materials.

20.2. Electronics Applications

To date, a variety of M/C60/M (M = metal) rectifying diodes, field effect transistors (FETs), as well as photovoltaic and photorefractive devices have

Polymethylmethacrylate (PMMA) and 4'-[(6-(methacroyloxy)hexyl]methylamino]-4-(methyl-sulfonyl) azobenzene (MSAB).

been proposed and measurements have been carried out on a number of actual electronic devices. A few research groups have concentrated on applications based on the strong bonding at the C60-Si interface (see §17.9.3) [20.40-42] and the incorporation of C60 into existing microfabrication processes. It has been demonstrated that C60 films can be patterned on silicon using conventional photoresist and liftoff techniques [20.43] achieving linewidths between 3 and 5 /¿m. Moreover, the C60 film itself can serve as a negative photoresist (decreasing solubility upon exposure to light) [20.44], yielding line resolution of better than 1 /un as shown below.

While a number of devices based on pristine C60 films have been investigated, the most promising electronic applications proposed to date utilize C60 in conjunction with selected conducting polymer composites (see §20.1). As discussed in §13.5, experiments on C60-polymer composites [20.12-16] have shown that a very fast (subpicosecond) photoinduced electron transfer from the polymer to a nearby C60 molecule occurs, thereby forming a mobile hole on the polymer backbone and a metastable C6() anion. A polymer-Qo heterojunction, therefore, would be expected to exhibit a photovoltaic response, and such devices have been investigated (see §20.2.4). In this section, the structure and properties of C60-based transistors (§20.2.1) and rectifying diodes (§20.2.2) are discussed, and C60-semiconducting polymer heterojunctions are described for both rectifying diodes (§20.2.3) and photovoltaic (§20.2.4) applications. This is followed by a variety of other electronics applications based on fullerenes.

It should be noted that most C60 devices are unstable in air due to the diffusion and photodiffusion of dioxygen [20.45,46] into the large interstitial sites of the fullerene solid. Therefore, if C60-polymer devices are to be commercialized, they need to be packaged in oxygen-resistant coatings. While such packaging is done routinely in the semiconductor industry, fullerenes are unique in that they themselves can be used to make oxygen-resistant seals [20.47].

20.2.1. C60 Transistors

In this subsection, field effect transistors based on C60 are described and are compared to their conventional silicon-based counterparts. The conventional silicon-based metal-oxide-semiconductor field effect transistor [MOSFET; see Fig. 20.8(a)] consists of two p-n junctions placed immediately adjacent to the region of the semiconductor controlled by the MOS gate. The carriers enter the structure through the source (S), leave through the drain (D), and are subject to the control by the bias voltage on the gate (G). The voltage applied to the gate relative to ground is VG, while the drain voltage relative to ground is VD. In this particular device geom-

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