We have studied the zero magnetic field resistivity, ρ, of unique high-mobility two-dimensional electron systems in silicon. At very low electron density, n s (but higher than some sample-dependent critical value, n cr ∼ 10 11 cm −2 ), conventional weak localization is overpowered by a sharp drop of ρ by an order of magnitude with decreasing temperature below ∼ 1 − 2 K. No further evidence for electron localization is seen down to at least 20 mK. For n s < n cr , the sample is insulating. The resistance is empirically found to scale with temperature both below and above n cr with a single parameter which approaches zero at n s = n cr suggesting a metal/insulator phase transition.

We have studied the temperature dependence of resistivity, p, for a two-dimensional electron system in silicon at low electron densities n, 10 cm, near the metal-insulator transition. The resistivity was empirically found to scale with a single parameter Tp, which approaches zero at some critical electron density n, and increases as a power To oc~n,n,~with P = 1.6 6 0.1 both in metallic (n,) n,) and insulating (n, (n) regions. This dependence was found to be sample independent. We have also studied the diagonal resistivity at Landau-level filling factor v =-,where the system is known to be in a true metallic state at high magnetic field and in an insulating state at low magnetic field. The temperature dependencies of resistivity at B = 0 and at v =were found to be identical. These behaviors suggest a true metal-insulator transition in the two-dimensional electron system in silicon at B = 0, in contrast with the well-known scaling theory.

We report on fabrication and characterization of the organic field effect transistors (OFETs) on the surface of single crystals of rubrene. The parylene polymer film has been used as the gate insulator. At room temperature, these OFETs exhibit the ptype conductivity with the field effect mobility up to 1 cm 2 /Vs and the on/off ratio ~ 10 4 . The temperature dependence of the mobility is discussed.An increase of the carrier mobility in the organic field effect transistors (OFETs) is an outstanding problem (for a review, see e.g., Ref. [1, 2, 3]). Solving this problem might open new opportunities for both physics of two-dimensional systems and applications. Relatively high values of the surface carrier mobility (the so-called field-effect mobility) at the room temperature, µ(300 K) ~ 1 cm 2 /Vs, have been reported for the OFETs on the basis of highly ordered vacuum-deposited organic films 4 . These values are comparable with the mobility µ(300 K) for the optically generated carriers in the volume of organic molecular single crystals, measured by the time-of-flight method 5 . However, at low temperatures, the field-effect mobility in OFETs differs significantly from the mobility of "bulk" carriers in organic single crystals. Indeed, the mobility of the bulk carriers increases with cooling and can be as large as several hundred cm 2 /Vs at T < 100 K 5 . In contrast, the field-effect mobility for the thin-film OFETs, being limited by a large concentration of defects in the vacuum-deposited organic films, usually decreases with cooling 4, 6 . The dependence µ(T) in the best pentacene thin-film OFETs varies from thermally activated to almost temperature-independent with µ(300K) increasing from 0.3 cm 2 /Vs to 1.2 cm 2 /Vs 4 .It has been recognized that ordering of the organic film structure is very important for achieving high field-effect mobilities 3, 5 . Fabrication of the OFETs at the surface of single crystals of high-purity organic compounds might increase the mobility significantly. However, this technological process poses a challenge. In particular, the proper choice of the gate insulator, which would form a trapfree interface with organic crystals, is crucial.In this Letter, we report on fabrication and characterization of OFETs on the surface of single crystals of rubrene. Use of thin parylene films as the gate insulator allowed fabrication of the OFETs with reproducible characteristics. At room temperature, the devices demonstrate transistor characteristics with µ up to ~ 1 cm 2 /Vs and the on/off ratios up to 10 4 . The field-effect mobility has been studied over the temperature range T = 77 -300 K. The observed slow decrease of µ with cooling is similar to the µ(T) dependences reported previously for the best organic thin-film FETs 4 .We have used physical vapor transport in hydrogen 7 for the growth of single crystals of rubrene. The purity of the crystals has been tested by measuring the bulk carrier transport prior to the FET fabrication 8 . The 300 Å-thick source and drain contacts were formed at ...

We report on the fabrication and characterization of single-crystal organic p-type field-effect transistors (OFETs) with the fieldeffect hole mobility µ ~ 8 cm 2 /V⋅s, substantially higher than that observed in thin-film OFETs. The single-crystal devices compare favorably with thin-film OFETs not only in this respect: the mobility for the single-crystal devices is nearly independent of the gate voltage and the field effect onset is very sharp. Subthreshold slope as small as S = 0.85 V/decade has been observed for a gate insulator capacitance C i = 2 ± 0.2 nF/cm 2 . This corresponds to the intrinsic subthreshold slope S i ≡ SC i at least one order of magnitude smaller than that for the best thin-film OFETs and amorphous hydrogenated silicon (α-Si:H) devices. a) Electronic mail: podzorov@physics.rutgers.edu b) Also at P. N. Lebedev Physics Institute, 119991 Moscow, RussiaThe quest for high-performance organic field-effect transistors (OFETs) has resulted in a significant increase of the charge carrier mobility µ. 1 In the best devices based on thin organic films, values of µ up to ~ 1.5 cm 2 /V⋅s have been reported. 2 This performance is already comparable with that of amorphous-silicon FETs. 3 However, there are still several important issues to be resolved, most of them being associated with grain boundaries and interfacial disorder in organic thin films. Indeed, currently these structural defects are the major factor which limits the mobility, 2, 4 causes the dependence of the mobility on the gate voltage, 5,6 and results in the broadening of the on/off transition. 7 Grain boundaries can be eliminated in devices fabricated on single crystals of organic semiconductors, which enables to explore the role of other factors.Recently, we developed a technique for the fabrication of single-crystal OFETs, 8 which allowed us to completely eliminate the inter-crystalline boundaries. In this Letter, we report on the optimization of this technique, which has resulted in a dramatic increase of the field effect hole mobility up to µ ~ 8 cm 2 /V⋅s. The large magnitude of µ is not the only advantage of the single-crystal devices: their mobility is nearly gate-voltage independent, and the onset of conductivity is very sharp. Comparison between single-crystal and thin-film OFETs helps to identify the characteristics of the latter devices, which are associated with structural defects.High-quality rubrene crystals have been grown from the vapor phase in a stream of ultra-high-purity hydrogen in a horizontal reactor. 9 Several key factors affect the crystal quality. One of the important parameters is the difference in temperature between the sublimation zone and the growth zone, ∆T = T sblm -T growth , which is an analog of the supersaturation at thermal equilibrium. The regime of small supersaturation, when T sblm is set close to the sublimation threshold of rubrene, is crucial for the mobility improvement. The crystal growth in this regime proceeds by the flow of steps at a very low rate (≤ 5×10 -7 cm/s in a direction perpendicula...

The anomalous conducting phase that has been shown to exist in zero field in dilute two-dimensional electron systems in silicon MOSFETs is driven into a strongly insulating state by a magnetic field of about 20 kOe applied parallel to the plane. The data suggest that in the limit of T -> 0 the conducting phase is suppressed by an arbitrarily weak magnetic field. We call attention to striking similarities to magnetic field-induced superconductor-insulator transitions