Thin Solid Films 486 (2005) 67 – 70 www.elsevier.com/locate/tsf
Electric field tuning of superconductivity in lanthanum-doped strontium titanate field-effect transistors Feng Pan, Charles T. Rogers* Department of Physics, CB390, University of Colorado, Boulder, CO 80309-0390, United States Available online 22 February 2005
Abstract We report on the electric field modulation of superconductivity in thin-film field-effect transistors based on lanthanum-doped strontium titanate (La:STO) channels with n-type channel carrier densities in the range of 3–71019 cm3 and thickness in the range of 75–150 nm and undoped strontium titanate (STO) gate insulation. Electric field tuning of normal state resistance and channel critical current are observed in both enhancement and depletion modes. Gate modulation of superconductivity at finite frequency has been observed and is consistent with a small-signal analysis. D 2005 Published by Elsevier B.V. Keywords: Superconductivity; Field effect
1. Introduction Research on correlated metal oxides has uncovered many new materials where carrier density is intimately connected with unusual properties including high temperature superconductivity and colossal magnetoresistance. The possible electric field modulation of carrier density in these materials is expected to provide a variety of new electronic devices [1–3]. STO is an interesting oxide material for electric field modulation studies. Stoichometric STO is a perovskite structured and optically transparent insulating material with a band gap of 3.2 eV . Insulating STO has a notably large dielectric constant of 300 at room temperature. This dielectric response climbs to nearly 30,000 at 4 K, and becomes a strong function of electric field, making STO one of only two known incipient ferroelectrics, the other being KTaO3. STO can be doped n-type with a variety of chemical substituents including Nb on the Ti site and La on the Sr site. The material then becomes a transparent conductor. At doping densities between 1019 and 1021 cm3 STO becomes a superconductor [5,6] and is further distinguished by
* Corresponding author. Tel.: +1 303 492 7456; fax: +1 303 492 2998. E-mail address: [email protected]
(C.T. Rogers). 0040-6090/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.tsf.2004.10.063
having the lowest carrier density for superconductivity among stoichiometric materials. In this letter we report the construction of thin-film field-effect transistors (FETs) based on high dielectric constant STO gate insulator and La:STO channels, which allow continuous tuning of channel resistance, critical current, and superconductivity via gate induced variation of channel depletion width.
2. Device fabrication We fabricate these La-doped STO FETs from epitaxial heterostructures grown on single crystal lanthanum aluminate (LAO) substrates by laser ablation. Fig. 1 shows the physical layered structure and approximate energy band diagram. Our typical device channel width is 50 Am. Conducting film thickness has been studied between 75 nm and 150 nm. Briefly, a krypton-fluorine pulsed excimer laser (248 nm wavelength, 30 ns pulses, and 250 mJ per pulse), operated at a repetition rate of 4 Hz with 1.8 J/cm2 energy density at a stoichiometric target, is used to deposit films onto a heated substrate. The ablation chamber was pumped down to around 1.5106 Torr (oxygen contributing 108 Torr of partial pressure). The sample is placed 1.5 cm off-axis with respect to the plume center. The semi-
F. Pan, C.T. Rogers / Thin Solid Films 486 (2005) 67–70
function and excellent adhesion properties, Mg(1000 2)/ Au(1000 2) contacts were used as ohmic contact electrodes  of the drain and source. 50 Am by 70 Am Ti (50 2)/Au (1500 2) contacts were used as the gate electrodes.
3. Experimental results and discussion
40 10 30 20
conducting layer is grown in vacuum at 870 8C from a powder target with composition La0.01Sr0.99TiO3, which gives a dopant concentration near 1.71020 cm3 consistent with 1 electron carrier per lanthanum dopant atom. After growth of the semiconducting layer, we perform photolithographic patterning to define source and drain pads, channel, and various Hall and resistance voltage taps by HF etching of the semiconductor layer, followed by cleaning in photoresist remover PG. The gate insulator layer is then grown at a few to hundreds of mTorr oxygen background pressure at 820 8C from a single crystal SrTiO3 target. X-ray diffraction shows epitaxial growth of the undoped STO on the semiconducting material. AFM indicates smooth insulating STO with typical rms roughness of 1.4 nm. The film growth is monitored via a polarized He–Ne laser beam reflecting from the surface of the film and the gate insulator thickness is later verified using optical ellipsometry. After growth of the insulator layer, metal ohmic contact pads are defined via HF etching into the conducting layer through the insulator. The metal contacts are deposited using a thermal evaporation system and lift-off technique. Then essentially identical lithographic steps are followed to define the gate and the gate contacts. Because of magnesium’s small work
Fig. 1. FET device structure. (a) Illustration of the thin-film device structure. Semiconducting and insulating strontium titanate are grown by laser ablation and are typically 150 nm and 75 nm, respectively, in thickness. Source (S), drain (D) and gate metal contacts are thermally evaporated. (b) Energy-band diagram in thermal equilibrium at zero gate bias.
Four point probe resistance measurements, gate modulation measurements, Hall-effect measurements up to 6 Tesla and the current–voltage (I–V) characteristic measurements were carried out on these FET devices inside a dilution refrigerator with a base temperature of 30 mK. The Hall measurements indicate an n-type carrier density of 3– 71019 cm3 for all the device channels, independent of temperature from room temperature to liquid helium temperature. By applying positive or negative voltage to the gate respectively, the carriers inside the channel are attracted to or pushed away from the STO/La:STO interface. Modelling of the observed variation of device resistance and gate-channel capacitance support the band diagram of Fig. 1 and indicate gate oxide dielectric constants between 150 and 300, depending on the growth conditions, down to 4 K. In Fig. 2 we illustrate the typical common-source drainsource current–voltage characteristic along with its measured gate leakage current for various gate voltages, V G, at temperature of 100 K. The results are similar to traditional FET characteristics: as the gate voltage is biased positive, the electron carrier density in the conducting channel is enhanced, resulting a larger drain-source current. The drainsource current exhibits pinch off and saturation as the drainsource voltage increases. The normalized transconductance at 100 K is 0.4 AS Am1. Gate leakage current under a small symmetric bias is below 10 nA. The gate leakage current increases with applied drain-source voltage and reaches as
5 10 0 0
VDS(Volt) Fig. 2. Common-source FET DC characteristics and gate leakage current of device 807 measured at 100 K. The drain-source current clearly exhibits pinch off and saturation as drain-source voltage increases. The gate leakage current under small symmetric bias is less than 10 nA. The gate leakage current increases with applied drain-source voltage. Nevertheless the leakage current is much less than the drain-source current.
F. Pan, C.T. Rogers / Thin Solid Films 486 (2005) 67–70
high as 5 AA at V DS=8 V. Nevertheless the leakage current is much less than the drain-source current in all regions of operation. We observed no electrical break-down with a voltage of 0.5 V applied across a 35 nm thick SrTiO3 parallel plate gate and estimate the breakdown electric field of the gate oxide to be above 105 V/cm. Below roughly 400 mK, our La:STO FETs display superconductivity . We have investigated the electric field modulation of superconductivity via channel IV characteristics. Fig. 3 shows examples of gate voltage effects on the channel IV at 100 mK for a device with a 75 nm conducting channel thickness and a 75 nm gate oxide. The figure shows that gate voltage is effective changing the resistance (R N) at high channel current and the critical current (I C) of the channel. As gate biases negatively, R N increases and I C decreases, which is consistent with carrier depletion in the channel. Under positive gate bias, the carriers are attracted to the oxide-channel interface, decreasing R N and increasing I C. We have performed small-signal gate modulation experiments at finite frequency on these devices. Fig. 4 shows inphase and out-of-phase modulation data for a typical device at a few temperature points near Tc. Here we pass a dc current through the channel and monitor channel voltage while providing a small-signal modulating gate voltage (20 mV). The resulting channel voltage is then detected with a lock-in amplifier. First, we consider the in-phase response to gate modulation. When the channel is at normal state, we observe a straight line which is indicated in data at 417 mK. Below 400 mK, the channel voltage show peaks at different values of channel current through the superconducting transition. These results are consistent with the small-signal analysis, which shows that the expected channel response BVch BRN ch BI C yV ch should go as yVch ¼ BV BI C BVg þ BRN BVg yVg . The sharp peak observed as the channel current sweeps through I C is associated with the gating of I C in the first term. The linear behavior for current above I C is associated with the gating of R N in the second term. The out-of-phase signal arises from currents pulled into the device due to the capacitive coupling between the gate and channel. It peaks at the same channel current value as the in-phase signal and
Idc(µA) Fig. 4. Small-signal gate modulation in-phase data (a) and out-of-phase data (b) taken at 15.5 Hz as temperature is varied through the superconducting transition. The peaks and linear/constant behavior at higher current are well described by the small-signal model.
increases linearly with applied frequency, consistent with the capacitively coupled modulation effect. The signal levels out to a constant value above critical current. We observe modulation of the channel properties for all frequencies out to the limits imposed by cryostat wiring above 1 kHz.
g g g g
V(µV) Fig. 3. Averaged IV characteristics under four gate biases at 100 mK.
These data convincingly demonstrate the continuous tuning of superconductivity in La-doped STO and suggest new styles of superconductive electronics, where critical currents are modulated with gate voltages. We particularly envision applications in parametric converters and parametric amplifiers. Also, high dielectric constant STO is structurally and chemically compatible with many perovskite materials and can transfer electric field efficiently to the FET channel to alter the material properties . This electric field tuning approach should be applicable to other correlated oxide systems for research and device applications.
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