SrTiO 3 -based metal–insulator–semiconductor heterostructuresIlaria Pallecchi, Giuseppe Grassano, Daniele Marré, Luca Pellegrino, Marina Putti, and Antonio Sergio Siri

Citation: Applied Physics Letters 78, 2244 (2001); doi: 10.1063/1.1363690 View online: http://dx.doi.org/10.1063/1.1363690 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/78/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Temperature effect on carrier transport characteristics in SrTiO 3 /Si p - n heterojunction Appl. Phys. Lett. 86, 123502 (2005); 10.1063/1.1888039 Sb-doped SrTiO 3 transparent semiconductor thin films Appl. Phys. Lett. 78, 1676 (2001); 10.1063/1.1355992 Pulsed-laser-deposited epitaxial Sr 2 FeMoO 6y thin films: Positive and negative magnetoresistance regimes Appl. Phys. Lett. 74, 3696 (1999); 10.1063/1.123224 Semiconducting epitaxial films of metastable SrRu 0.5 Sn 0.5 O 3 grown by pulsed laser deposition Appl. Phys. Lett. 70, 2147 (1997); 10.1063/1.119082 Effect of oxygen stoichiometry on the electrical properties of La 0.5 Sr 0.5 CoO 3 electrodes J. Appl. Phys. 81, 3543 (1997); 10.1063/1.364991

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APPLIED PHYSICS LETTERS VOLUME 78, NUMBER 15 9 APRIL 2001

This article is

SrTiO3-based metal–insulator–semiconductor heterostructuresIlaria Pallecchi, Giuseppe Grassano, Daniele Marre,a) Luca Pellegrino, Marina Putti,and Antonio Sergio SiriPhysics Department, I.N.F.M./C.N.R., via Dodecaneso 33, 16146 Genova, Italy

~Received 18 October 1999; accepted for publication 16 February 2001!

We explored the feasibility of employing strontium titanate (SrTiO3) as semiconducting material infield-effect metal–insulator–semiconductor epitaxial heterostructures. This idea was suggested bythe observation of a dramatic effect of the oxygen deficiency on SrTiO32d transport properties,which brings about metallic behavior with low-temperature mobility values comparable with thosecommonly found for silicon. By pulsed-laser deposition, we realized patterned field-effect devices,showing a resistance enhancement up to 90%. This promising result could open perspectives forcrystalline-oxide electronics. ©2001 American Institute of Physics.@DOI: 10.1063/1.1363690#

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Transition-metal oxides present a variety of ‘‘exoticproperties, such as colossal magnetoresistance in manites, high-Tc superconductivity in cuprates, ferroelectricitytitanates, which can be accounted for in terms of electrocorrelation. Most of these compounds have a perovsstructure and they can be grown epitaxially one over anoto form heterostructures. Different types of electronic dvices can be fabricated by growing patterned multilayersperovskite oxides; some noteworthy examples can be foin the literature.1–4 Many authors tried to grow crystallinc-oriented perovskites on silicon with a suitable bufflayer.5,6 On the other hand, SrTiO3 seems to be a good isostructural compound with very good semiconducting propties; it has a band gap of nearly 3 eV~Ref. 7! and it becomesmetallic with very low carrier concentration (n;1018carriers/cm3).8

Several attempts were made to accumulate charge cers by field effect in metal–insulator–semiconductor~MIS!geometry, especially in high-Tc superconducting cupratesFar from any transition point, the change of the normal-stresistance of metallic transition-metal oxides seldom excea few parts percent,9 due to the large density of statesthese compounds. Only recently, a 50% change in normstate conductance, as well as a 7 Kshift of the superconducting transition, was observed in ferroelectric field-effect herostructures Pb(ZrxTi12x)O3/GdBa2Cu3O7.

10

In the following, we present results of field-effect mesurements on MIS epitaxial heterostructures made of seconducting SrTiO32d and insulating MgO, deposited bpulsed-laser-deposition on SrTiO3 substrates.

In order to obtain the semiconducting channel, we groxygen-deficient SrTiO3 thin films in high-vacuum~HV!conditions (1029– 1028 Torr), at deposition temperaturebetween 600 and 800 °C, using single crystals as targets.experimental setup is described elsewhere.11 The samplethickness~100–500 Å! was measured by x-ray reflectivitoscillations, which were clearly visible for film depositebelow 650 °C, allowing us to calibrate the growth rate. Tstructural quality of the samples was checked by atomforce microscopy~AFM! and reflection high-energy electro

a!Electronic mail: marre@fisica.unige.it

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diffraction ~RHEED!. As shown in Fig. 1, we found that thdeposited films had atomically flat and particulate-free sfaces, which is crucial for device applications. By RHEEanalysis, a two-dimensional layer-by-layer growthSrTiO32d was observed.

Transport properties were performed by using a standfour-probe technique from 4.2 to 300 K. Depending on tgrowth conditions, we obtained both semiconducting ametallic SrTiO3 samples. At the highest temperatures,observed a reduction of the substrate that becomes metthus participating in the conduction; by decreasing the teperature, this phenomenon can be controlled.12 This isproved in Fig. 2, where room-temperature conductancesamples grown at 650 °C is plotted as a function of ththickness, evaluated by x-ray reflectivity oscillations. Tlinear relationship demonstrates that the charges are conin the thin-film region. Furthermore, the negative interceptthe best-fit straight line suggests that an external layer offilm oxidizes, becoming insulating. The Hall-effect curvturned out to be almost constant with the temperature.sidual resistivity ratios from 10 to 4000 and Hall mobilitieup to 1.53104 cm2 V21 s21 at low temperatures were foundbeing more metallic for the samples deposited at higtemperatures.13 The mobility values are comparable witthose found on bulk single crystals8,14 and, at low tempera-ture, with those usually found on silicon samples; theydetermined by low-impurity scattering and they may be e

FIG. 1. AFM image of an atomically flat SrTiO3 film surface.

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2245Appl. Phys. Lett., Vol. 78, No. 15, 9 April 2001 Pallecchi et al.

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plained in terms of high-structural quality, low-crystal disoder, and, above all, screening of the Coulomb potentiaionized impurities by polarized lattices.

We deposited 500–1000-Å-thick films of MgO fromsingle-crystal targets at substrate temperatures betweenand 800 °C and in oxygen partial pressures up to31026 Torr. We chose MgO as the insulating layer, dueits wide-band gap (Egap57.8 eV! ~Ref. 15! and its possibleepitaxial growth on SrTiO3. Indeed, RHEED sharp spottpatterns revealed a three-dimensional growth of the Mand x-ray measurements showed the good crystallinitythe c orientation of the samples. In order to prevent oxygvacancies from compromising its insulating behavior, Mgmust be grown in oxygen partial pressure. With an oxygpartial pressure of 1.531026 Torr, a resistivity of 3.8531011V cm was obtained.

Following the calibration of the growth parameters, pterned MgO/SrTiO32d multilayers were indeed depositeThe patterns were realized completelyin situ by using fourdifferent shadow masks during the deposition of succeslayers, i.e., the heavily doped metallic source and drain pthe SrTiO3 channel, the insulating MgO layer, and the mtallic gate electrode. Each tantalum mask was placed abthe substrate within the precision of60.1 mm. Thisin situtechnique allowed us to deposit three devices simultaneowith high reproducibility, preservation of HV conditionsand, above all, limited interface contamination. For the mtallic source and drain pads, we tried eitherh-dopedYBa2Cu3O72d or e-doped SrTiO3. The first choice was discarded due to clusters and precipitates of spurious phaseoutgrowths, which caused electrical shorts. With the secchoice, we had to face the problem of the interdependencdeposition parameters for different layers; indeed, the ogen partial pressure required for the growth of the stoichmetric insulating MgO phase compensated the negacharges of any underlyinge-doped layer of SrTiO32d orSrTi0.9Nb0.1O32d , so that source and drain contacts turnout to be highly resistive. For the same reason, also a paneutralization of the SrTiO32d channels underlying MgOfilms was observed.

We performed field-effect conductance measuremeon several MIS heterostructures by an ac synchronous detion technique. In Fig. 3, conductance versus gate voltcurves of a typical device are presented. In this case,SrTiO32d channel had a resistance of 220 kV at room tem-

FIG. 2. Conductance at room temperature as a function of film thicknessthe samples are grown at 650 °C. The straight line is the best fit of the

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perature, and the MgO insulating layer had a thickness oforder of 1000 Å and an estimated resistance of about 10 MV.Gate voltage was varied from –15 to 15 V, below the bredown field of the MgO layer.16 For high-positive and high-negative voltages, the conductance increased due to acclation of charge carriers of either sign at the interface by fieffect. In the measurement shown, curve~a!, the source–drain channel resistance, varied nearly 67%. Actually,minimum observed resistance~67 kV at 9 V of gate voltage!is limited by the source and drain highly resistive SrTiO3

contacts in series to the channel. Indeed, as expectedwelding source and drain electrical wires on more extenareas and closer to the gate plate, we observed a minimchannel resistance of 29 kV @see curve~b!#, corresponding toa relative change of the channel resistance of nearly 9We speculate that the net variation of the channel condtance, after eliminating the contribution of source and drresistances, may be much larger; indeed, as reported in3, at 15 V the charge accumulated in the semiconduccompensated channel is about 1.0mC/cm2. This value, ob-tained considering a MgO dielectric constant of 9«0 ,17

should correspond to an induced charge carrier density1018cm23, if a Debye length of about 200 Å is assumed18

Such a carrier density is enough to bring about the metinsulator transition in SrTiO3.

8

The asymmetry of the conductance versus gate voltcurve mirrors a possible asymmetry of the density of stawith respect to the center of the band gap: the higherslope of the curve is at a certain gate voltage, the lowerdensity of states is at the energy value crossed by the Flevel, which is shifted by the gate voltage itself. Unluckilhighly resistive source and drain contacts in series withchannel make a self-consistent calculation of the densitystates by fitting the experimental conductance d

allta.

FIG. 3. Field-effect conductance measurements on MIS heterostruc~see the sketch in the inset! with a 1-mm-wide compensated SrTiO32d chan-nel, compensated SrTiO32d source~S! and drain~D! pads, and a 1000-Å-thick MgO layer: the two curves were measured on the same samplewith different source and drain contact resistances; in the case of pointcontacts@curve ~a!#, the relative change of the channel resistance was 6at Vgate59 V, while it increased to 89% with more extended contacts@curve~b!#. The top scale (Q/cm2) has been calculated assuming a MgO dielectconstant of 9«0 .

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2246 Appl. Phys. Lett., Vol. 78, No. 15, 9 April 2001 Pallecchi et al.

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unfeasible.17 Nonetheless, the sharp changes of the condtance curve slopes are probably related to features ofband structure.

In conclusion, we proved the possibility of inducincharges in semiconducting SrTiO3 by field effect in MISheterostructures, with remarkable change in conductaDevice performance can be further improved if less-resissource and drain contacts and a more-resistive insulalayer can be obtained in the same heterostructure, eitheoptimizing growth parameters or using a proper hole-doperovskite compound for metallic pads.

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