Evolution of the conductivity type in germania by varying the stoichiometry

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Evolution of the conductivity type in germania by varying the stoichiometry

D. R. Islamov, V. A. Gritsenko, C. H. Cheng, and A. Chin

Citation: Applied Physics Letters 103, 232904 (2013); doi: 10.1063/1.4838297 View online: http://dx.doi.org/10.1063/1.4838297

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Evolution of the conductivity type in germania by varying the stoichiometry

D. R. Islamov,1,2,a)V. A. Gritsenko,1,b)C. H. Cheng,3and A. Chin4,c) 1

A. V. Rzhanov Institute of Semiconductor Physics, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russian Federation

2

Novosibirsk State University, Novosibirsk 630090, Russian Federation

3

Department of Mechatronic Technology, National Taiwan Normal University, Taipei 106, Taiwan

4

National Chiao Tung University, Hsinchu 300, Taiwan

(Received 16 September 2013; accepted 17 November 2013; published online 2 December 2013) Information regarding the conductivity type of Si/GeOx/Ni structures with various stoichiometry

has been obtained using experiments on injection of minority carriers fromn- and p-type silicon. Results show that non-stoichiometric GeOxfilms exhibit bipolar conductivity, that is, holes as well

as electrons contribute to the charge transport. Stoichiometric GeO2films exhibit unipolar electron

conductivity.VC _{2013 AIP Publishing LLC. [}_{http://dx.doi.org/10.1063/1.4838297}_]

Amorphous GeO2 films are used as gate insulators in

high mobility field effect transistors based on Ge.1Germania films also show resistive switching effect, which is promis-ing for the next generation of high performance non-volatile flash memory.2,3

The energy diagram of the Ge/GeO2structure was

cal-culated in Ref. 4, using quantum chemical simulation. According to these calculations, highly asymmetric electron and hole barriers on the Si-GeO2interface are predicted. The

barrier for the electrons is 1:3 eV and the hole’s barrier is equal to 3:2 eV. The disordered GeOxgap value of 5:6 eV is

less than the 6:1 eV calculated for crystalline germania.5 One of the most critical parameters of metal-oxide-silicon (MOS) structures is the sign of carriers, which contribute to the current in the insulator. The Hall effect and thermoelectric experiments cannot be performed to determine the carrier’s sign in dielectrics. However, two specific methods can be used to determine the carrier’s sign in the insulator. These methods are carrier sign determination in an MOS transistor,6 and current saturation caused by non-equilibrium minority carrier generation in the depletion layer of the silicon.

Insulators can have two-band (or bipolar) conductivity, as in Si3N4, 6,7 HfO2, 8 and TiO2, 9

or monopolar charge trans-port (e.g., by the electrons), as in Al2O3,

10

and MOS with thermal SiO2.

11

The knowledge of the carrier sign in the in-sulator is crucial for the interpretation of charge transport in MOS devices, as well as for the development of correct mod-els for resistive memory switching.

In this letter, we report on the charge carrier sign deter-mination in silicon MOS structures based on germania GeOx.

Samples were cleaved from the wafers of Si with GeOx

film with a thickness ofd¼ 20 nm. The GeOxfilms were

de-posited using physical vapor deposition (PVD) on p- and n-type Si substrates. Samples from the first group “PDA0” were measured as deposited, that is, they were not treated using post-deposition annealing (PDA), and GeOxfilms

rep-resented non-stoichiometric germania. The same GeOxfilms

are used in ReRAM devices.2The second “PDA5” and the third “PDA20” groups of samples were treated by applying PDA in an O2 atmosphere at 300C for 5 min and 20 min,

respectively. The 300C annealing is a very low temperature process for SiO2sublayer growing.

12

Ellipsometry measure-ments were user to control the thickness of GeOx films.

Structural analysis shows that the resulting GeOxfilms were

amorphous.

The samples for transport measurements were equipped with Ni gates of round form and a radius of 70 lm for electri-cal contact. Current-voltage (J–V) and capacitance-voltage (C–V) measurements were recorded at room temperature. C–V measurements were recorded at a frequency of 100 kHz. A tungsten lamp was used for light illumination. The permittivity e was calculated fromC–V measurements, using the standard formula of a flat capacitor e¼ Cd=e0 in the

accumulation mode, where C is specific capacitance and e0

is the electric constant (vacuum permittivity).

Figures1(a)and1(b)showJ–V and C–V dependencies forn-Si/GeOx/Ni MIS structures from the “PDA0” group for

depletion and accumulation modes in the dark (solid line) and under illumination (dashed line). In accumulation mode, when a positive potential is applied to the Ni contact, almost all the applied voltage drops across the dielectric material, and the current increases exponentially with the increasing electric field. When a negative potential is applied to the metal contact, that is, in the depletion mode, the current increases exponentially at low voltages. The current satura-tion appears at a sufficiently large voltage, and the saturasatura-tion current and its increase under illumination indicate, that in the depletion mode minority carriers are injected from Si into GeOx. The minority carriers are holes in this case.

Independent confirmations of the minority carriers injection include: (1) the existence of capacitance transition from inversion mode to non-equilibrium depletion mode, and (2) capacity increasing in non-equilibrium depletion mode under illumination (Fig.1(b)).

Dependencies forp-Si/GeOx/Ni MIS structures from the

“PDA0” group for depletion and accumulation modes in the dark (solid line) and under illumination (dashed line) are shown for experimentalJ–V and C–V in Figs.1(c)and1(d), respectively. In accumulation mode, the current grows

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APPLIED PHYSICS LETTERS 103, 232904 (2013)

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exponentially with increasing voltage. Current saturation appears in depletion mode, and the saturation level increases under illumination. This phenomenon indicates the injection of minority carriers, namely electrons, from the silicon sub-strate into germanium oxide.

The results demonstrate that GeOxconductivity is

bipo-lar, similar to Si3N4, 6,7 HfO2, 8 and TiO2. 9

Energy band diagrams of n; p-Si/GeO2/Ni structures,

based on calculations of GeO2electronic band structure,

2,4

are shown in Fig.2in flat band mode (Figs. 2(a)and2(b)), in accumulation mode (Figs. 2(c) and 2(f)), and depletion mode (Figs.2(d) and 2(e)). It is shown that the barrier for holes in the Si/GeO2interface is much higher than the barrier

for electrons. Thus, it is expected that the transport in GeO2

is unipolar. The difference of theoretical expectations from experimental results might be explained by one of two hypotheses. The first is based on the assumption that GeOx

films are not pure GeO2, and have different energy

struc-tures, that is, the energy barrier for holes in experiments is less than that in calculations. This difference increases injec-tion probability of the holes from the Si substrate into the dielectric medium. The second hypothesis assumes that GeOx films exhibit hopping conductivity of the holes

between hole traps far from the top of the valence band in the middle of the gap. If the trap density is sufficiently high, the holes hop inside of the band gap, as shown in Figs.2(e)

and2(f).13

To investigate evolution of the trap density, J–V and C–V dependencies were measured for Si/GeO2/Ni MIS

struc-tures after low PDA. The samples of Si/GeO2/Ni after 5 min

of PDA present the same behavior as with no PDA. The con-ductivity of the samples from “PDA5” group is bipolar.

Further PDA procedures led to the disappearance of current and capacity changes under illumination in n-Si/GeOx/Ni structures as shown in Figs. 3(a)and3(b): the

capacitance depletion cannot be observed after annealing ger-mania. Despite the dark current is increased, addition photo-current is not observed in depletion mode. This phenomenon shows that injection of the minority carriers (holes) from Si into GeOxis suppressed. However, non-equilibrium depletion

mode can be observed in p-Si/GeOx/Ni structures (Figs. 3(c)

and 3(d)). These phenomena show that after 300C O2

annealing for long periods, the conductivity of GeOxevolves

from bipolar to unipolar, namely, electronic conduction. In addition, the anomalous capacitance hump shown in Fig.1(d), which is caused by the existence of deep interface traps, can be eliminated using oxygen annealing for periods longer than 20 min. The capacitance change in accumulation mode after anneal might be caused by the following reasons: (1) densifi-cation of poor quality GeOx(thickness decrease), (2) growing

of SiO2 sublayer, and (3) GeOx oxidation and changing of

GeOxdielectric constant. Ellipsometry measurements

demon-strated that the thickness of GeOxfilms with an accuracy of

2% has not changed after PDA. Small thickness increasing took place due to longer Ge–O (than Ge–Ge) bonds were formed by extra O in GeOx bulk. Additional SiO2 sublayer

should decrease total capacitance of Si/SiO2/GeOx/Ni

struc-tures. However, the capacitance increase is observed. The higher dielectric constant e0¼ 13–14 (compared with 4–5 for

samples of the “PDA0” and “PDA5” groups) and a low inter-face state indicates that the well-oxidized GeO2 can be

obtained after 300C O2 annealing for 20 min. These facts FIG. 1. ExperimentalJ–V and C–V curves for Si/GeOx/Ni MIS structures of

“PDA0” group for depletion and accumulation modes in the dark (solid line) and under illumination (dashed line). (a)J–V and (b) C–V of n-Si/GeOx/Ni

structures. (c)J–V and (d) C–V of p-Si/GeOx/Ni.

FIG. 2. Energy band diagram of n-Si/GeO2/Ni (a) and p-Si/GeO2/Ni (b)

structures in flat band mode. The same diagrams in accumulation (c), (f), and depletion mode (d), (e).Je/Jhare flows of injected electrons/holes from

Si into GeO2. Curved arrows show jumps of electrons (d) and holes (e), (f)

between hole traps in the bulk of dielectric.

232904-2 Islamov et al. Appl. Phys. Lett. 103, 232904 (2013)

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confirm both hypotheses regarding the difference in the energy structures of deposited GeOxfilms and pure GeO2, and

the high density of hole traps caused by oxygen vacancies in stoichiometric GeOxfilms.

The dielectric constant of pure Ge is 16. The permittiv-ity of tetragonal GeO2is evaluated of 12–15.

14

Thus, e0 of

intermediate germania GeOxis expected to be between 12

and 16. Low e0 value of 4–5 for non-PDA films does not

match the expectations. However, extremely low dielectric constant values of 4.5–11 of germania films were observed earlier.15–18 The reason why the dielectric constant values GeOx do not fall within the expected interval, and e0 is

increased after annealing, is an open question. It is possible

that during PDA treating germania films are recrystallized, and these changes can cause the increase in the dielectric constant.

In summary, this study indicates that the charge carrier sign in GeOxdepends on stoichiometry, that is, on the methods

and conditions of MIS device fabrication. Non-stoichiometric GeOxfilms exhibit bipolar conductivity, whereas

stoichiomet-ric GeO2films after post deposition annealing exhibit unipolar

electronic conductivity. This study provides a simple and clear method for determining the charge carrier sign.

This work was supported by projects No. 1.13 and 4.18 of the Siberian Branch of the Russian Academy of Sciences and the National Science Council, Taiwan, under Grant No. NSC-100-2923-E-009-001-MY3.

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structures after 20 min PDA at 300C (“PDA20” samples group) in the dark (solid line) and under illumination (dashed line). The depletion mode is not observed in these structures. ExperimentalJ–V (c) and C–V (d) curves for p-Si/GeOx/Ni MIS structures annealed at the same conditions.

232904-3 Islamov et al. Appl. Phys. Lett. 103, 232904 (2013)