Characteristics of electron-beam-gun-evaporated Er2O3 thin films as ...

JOURNAL OF APPLIED PHYSICS

VOLUME 90, NUMBER 10

15 NOVEMBER 2001

Characteristics of electron-beam-gun-evaporated Er2 O3 thin films as gate dielectrics for silicon V. Mikhelashvili and G. Eisenstein Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel

F. Edelmanna) Department of Material Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel

共Received 19 July 2001; accepted for publication 31 August 2001兲 Structural properties of an ultrathin, 4.5 nm, erbium-oxide film and electrical properties of metal– oxide–semiconductor structure based on it are described. The evolution of the dielectric constant, total charge density, breakdown electric field, and leakage current density with annealing temperature in an oxygen environment are reported. The dielectric constant in the as-deposited state is relatively low, ⬃7, possibly because the initial deposition forms ErO 共with low polarizibility兲 rather than Er2O3 . Annealing causes a transformation of ErO to Er2O3 but at the same time it initiates the growth of an interfacial SiO2 layer so that the effective dielectric constant is reduced to 5.5. Using the 4.5 nm film following annealing at up to 750 °C, we demonstrate an effective oxide thickness in the range 2.4 –3.2 nm, with a leakage current density as low as 1 – 2⫻10⫺8 A/cm2 at an electric field of 106 V/cm and a breakdown electric field of 0.8– 1.7⫻107 V/cm. A shift of the flat band voltage to the positive side and lowering of the total positive charge density down to 1012 cm⫺2 with annealing temperature are observed and can be explained by a charge compensation mechanism between the charges accumulated at the SiO2 /Er2O3 and Si/SiO2 interface. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1413239兴

The substrates we used were p-type 共100兲 silicon 共␳ ⫽7–10 ⍀ cm兲, which were treated by standard cleaning processes. The EBG deposition was carried out on an unheated substrate without additional oxygen. The deposition pressure was 3⫻10⫺6 Torr and the growth rate was 0.02 nm/s. Following deposition, the films were annealed in an oxygen environment for 60 min at temperatures ranging from 350 °C to 750 °C. Gold contact with an area of 5⫻10⫺4 cm2 was used as a top electrode, while Al served as a back contact to the Si substrate. The deposited film thickness (d Er2O3 ) was determined by ellipsometry. The surface features were examined by an atomic force microscope operating in tapping mode and the film structure was characterized by x-ray diffraction 共XRD兲. High frequency 共1 MHz兲 capacitance–voltage (C – V) and dc current density–voltage (J – V) measurement were performed using standard techniques and instruments. All electrical measurements were carried out at room temperature. As-deposited Er2O3 layers were found to have a smooth surface with a root-mean-square (R q ) roughness equal to 0.1 nm and the maximum difference between the highest and lowest points in the scan range (R max) on the order of 1 nm. These values are comparable to those of the underlying Si substrate. The XRD spectra of the Er2O3 films revealed, that in the as-deposited state, the film is cubic (a o ⫽10.5 Å) and microcrystalline with the 具111典 orientation dominating absolutely and a grain size of 20 nm. Neither the values of R q and R max nor the microcrystalline structure changed with annealing temperature. Details of the extensive structural study we have performed will be reported separately.

The SiO2 thickness in modern Si devices is approaching the quantum tunneling limit of ⬃1.5–2.5 nm. Potential substitute insulators with large dielectric constants 共high-k dielectrics兲 based on metal oxides have been considered in the last five years.1–3 Among these, there has been a recent interest in rare-earth metal–oxide film such as Y2O3 and Gd2O3 . 4,5 The rare-earth metal oxides are attractive because they offer a combination of high dielectric constants (k ⫽12– 18), large band gaps 共⬃5.4 eV兲, and conduction band offsets on Si 共higher than 2 eV兲. In addition, thin rare-earth metal oxide films are thermodynamically stable with silicon substrates, what prevents the formation of silicides and rough surfaces. Another rare-earth metal oxide—Er2O3 —has been studied extensively as an optical film but its electrical properties as a gate dielectric have only been addressed in a few published investigations.6 It was recently shown7 that Er2O3 reacts very poorly with Si 共compared to other rare-earth metal oxide films such as La2O3 and Gd2O3 兲 during annealing, even at 900 °C. This makes Er2O3 attractive as a gate dielectric and its properties very interesting for study. We have studied electron-bean-gun 共EBG兲-evaporated Er2O3 layers and report here the characteristics of ultrathin, 4.5 nm films. The crystalline structure and surface morphology of the Er2O3 films, as well as the electrical characteristics of metal–oxide–semiconductor 共MOS兲 capacitors were compared for as deposited layers and post annealed in an O2 environment. This study is a report characterizing ultrathin Er2O3 layers. a兲

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© 2001 American Institute of Physics

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J. Appl. Phys., Vol. 90, No. 10, 15 November 2001

FIG. 1. C – V characteristics of MOS structure with 4.5 nm thick Er2O3 insulator film. Annealing time—t ann⫽60 min.

The (C – V) characteristics of as-deposited and postannealed MOS structures are shown in Fig. 1. Figure 2 illustrates the annealing temperature dependencies of the effective relative dielectric constant (k eff) and the effective oxide thickness 共EOT兲 of the Er2O3 . The value of k eff was extracted from the accumulation capacitance 共see Fig. 1兲 and EOT was determined according to the simple relation: t ox ⫽k SiO2 d Er2O3 /k eff , where k SiO2 ⫽3.82. It is seen in Fig. 2, that k eff decrease from 7 to 5.5 and, therefore, the EOT increases with annealing temperature from 2.4 to 3.2 nm. The formation of a 1–2.5 nm thick interfacial SiO2 layer on the Si substrate during annealing in an oxygen environment was observed with Ta2O5 , TiO2 , and Y2O3 layers and was suggested as the main reason of the decrease of the k eff .2– 4 While an interfacial layer reduces k eff , it can not fully explain the behavior described in Fig. 2. We postulate that an additional important factor is the lattice buildup in the initial steps of the deposition. It is possible that the initially grown layers of the film contain the ErO 共erbium monoxide兲 phase,8 which has a lower polarizability than that of Er2O3

FIG. 2. The change of the effective dielectric constant 共Curve 1兲 and EOT 共Curve 2兲 of 4.5 nm thick Er2O3 film with annealing temperature. Annealing time—t ann⫽60 min.

Mikhelashvili, Eisenstein, and Edelmann

FIG. 3. The change of FB voltage 共Curve 1兲 and total charge density 共Curve 2兲 of 4.5 nm thick Er2O3 film with annealing temperature. Annealing time— t ann⫽60 min.

and this in turn may result in a lower overall effective dielectric constant compared to thick films.6,9 The possibility that a measurable fraction of very thin films may contain a different phase with lower polarizability may explain several published observations of low dielectric constants in very thin films.2,3,5 In particular, a very low value of k eff 共less than 9兲 was reported5 for an ultrathin Gd2O3 film deposited using an ultrahigh vacuum system, which avoids the formation of any interfacial SiO2 layer. The annealing process causes in our films a transformation of ErO to Er2O3 and at the same time it enhances the interfacial SiO2 thickness. These two processes effect k eff in an opposite manner yielding the annealing temperature dependence of k eff observed in Fig. 2. For thicker films, the influence of the interfacial layer and the small amount of the transition phase become negligible. Figure 3 shows annealing temperature dependencies of flat band 共FB兲 voltage, V FB and total charge density Q t . A monotonic shift in the value of the FB voltage towards positive values and the reduction of the positive charge density are observed with an increase in the annealing temperature. The values of V FB and Q t change from ⫺1 to ⫺0.55 V and Q t ⫽6⫻1012 to 1012 cm⫺2 , respectively for as-deposited and annealed at 750 °C films. Measured J – V characteristics at different annealing temperatures are shown in Figs. 4 and 5 summarizes the important properties as a function annealing temperature. The leakage current density 共curve 1 in Fig. 4兲 was measured at a total electric field of E t ⫽106 V/cm, where E t ⫽V g /d Er2O3 with V g being the applied voltage. Curve 2 in Fig. 5 describes the breakdown electric field, which are the field values where an abrupt rise in current density is observed 共see the circled regions in Fig. 5兲. It can be seen from Figs. 4 and 5 that an increased annealing temperature causes a large reduction of the leakage current density, in particular beyond 450 °C–550 °C. The lowest leakage current density we measured was 10⫺8 A/cm2 . The electric field causing an irreversible change in the current density through the MOS structure 共circled in Fig. 4兲 is found to be dependent on an annealing temperature and change in the range of 0.8– 1.7 ⫻107 V/cm.


J. Appl. Phys., Vol. 90, No. 10, 15 November 2001

FIG. 4. J – V characteristics of MOS structure with 4.5 nm thick insulator film annealed at different temperature. Curve 1—as-deposited, Curve 2— T ann⫽350 °C, Curve 3—T ann⫽450 °C, Curve 4—T ann⫽550 °C, and Curve 5—T ann⫽750 °C. Annealing time—t ann⫽60 min.

The breakdown electric fields described in Curve 2 of Fig. 5 suggest that the interfacial layer withstands very large fields. Based on a simple calculation considering a series capacitance model, we can estimate that across the 1–2.5 nm SiO2 film, the electric field reaches 107 V/cm, a field value

Mikhelashvili, Eisenstein, and Edelmann

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under which that film would exhibit direct or Fowler– Nordheim tunneling, which would result in currents larger than the observed values of Figs. 4 and 5. We estimate therefore that the breakdown voltage and leakage current characteristics are limited by a barrier formed at the Er2O3 and SiO2 boundary due to the different leakages in the Er2O3 ultrathin and SiO2 films, which cause a charge accumulation at their interface. A similar effect has been demonstrated in Ta2O5 /SiO2 , Y2O3 /SiO2 , and TiO2 /SiO2 stacks.3,4,10,11 In summary, we have described the characteristics of MOS structures which use EBG evaporated Er2O3 as the gate dielectric. The effects of the postdeposition annealing temperature on morphological, structural, and electrical characteristics of a 4.5 nm layer were investigated. The films have a smooth morphology and a microcrystalline cubic structure independent of the annealing temperature. The relative dielectric constant depends on annealing temperature and varies in the range 5.5–7. The minimum value of EOT we achieved was on order of 2.4 –2.8 nm. The electrical characteristics were strongly influenced by the annealing conditions. A shift of the FB voltage to the positive side and lowering of the total positive charge density to 1012 cm⫺2 was observed. A reduction of the leakage current density with annealing temperature was established in contrary to an increase of the total breakdown field. A low leakage current density, on the order of 1 – 2⫻10⫺8 A/cm2 at E t ⫽106 V/cm and a total breakdown electric field of 107 V/cm were demonstrated. 1

FIG. 5. The dependencies of the leakage current density 共at E t ⫽106 V/cm兲共Curve 1兲 and total breakdown electric field 共Curve 2兲 on the annealing temperature. Annealing time—t ann⫽60 min.

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