C–V studies on metal–ferroelectric bismuth vanadate

(Bi2VO5.5)–semiconductor structure

Neelam Kumari, Jayanta Parui, K.B.R. Varma, S.B. Krupanidhi *

Materials Research Centre, Indian Institute of Science, Bangalore 560012, India

Received 20 October 2005; accepted 21 November 2005 by C.N.R. Rao

Available online 23 January 2006

Abstract

Ferroelectric bismuth vanadate Bi2VO5.5 (BVO) thin films have been successfully grown on p-type Si(100) substrate by using chemical solution

decomposition (CSD) technique followed by rapid thermal annealing (RTA). The crystalline nature of the films has been studied by X-ray

diffraction (XRD). Atomic force microscopy (AFM) was used to study the microstructure of the films. The dielectric properties of the films were

studied. The capacitance–voltage characteristics have been studied in metal–ferroelectric–insulator–semiconductor (MFIS) configuration. The

dielectric constant of BVO thin films formed on Si(100) is about 146 measured at a frequency of 100 kHz at room temperature. The capacitance–

voltage plot of a Bi2VO5.5 MFIS capacitor subjected to a dc polarizing voltages shows a memory window of 1.42 V during a sweep of G5 V gate

bias. The flatband voltage (Vf) shifts towards the positive direction rather than negative direction. This leads to the asymmetric behavior of the C–

V curve and decrease in memory window. The oxide trap density at a ramp rate of 0.2 V/s was estimated to be as high as 1.45!1012 cmK2.

q 2006 Elsevier Ltd. All rights reserved.

PACS: 77.55.Cf; 77o.80.KL

Keywords: A. Bi2VO5.5 thin films; A. MFS structure; D. Chemical solution decomposition; D. Capacitance–voltage characteristics

1. Introduction

Ferroelectric thin films have become increasingly important

as future materials for electronic devices. Ferroelectric random

access memory (FeRAM) has been developed as an ultimate

memory with both nonvolatility and a high-speed read/write

operation cycle, which have been quite difficult to attain in

conventional fast static (SRAM) or electrical erasable

programmable read only memories (EEPROM) [1]. Memory

devices using ferroelectric may be categorized in to ferro-

electric random access memory (FRAM), dynamic random

access memory (DRAM), and metal ferroelectric semiconduc-

tor field effect transistors (MFS–FETs). MFS–FET has been

extensively studied because of its non-volatility and high

switching speed. It is fabricated by using the ferroelectric oxide

as the gate insulator in a metal–insulator–semiconductor

structure [2]. Several ferroelectric materials such as Bi2Ti2O7

[3], SrBi2Ta2O9 [4], Bi3.25La0.75Ti3O12 [5] have been studied

0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ssc.2005.11.043

* Corresponding author. Tel.: C91 80 360 1330; fax: C91 80 2360 0085.

E-mail address: sbk@mrc.iisc.ernet.in (S.B. Krupanidhi).

for MF(I)S structure. Bismuth based layered ferroelectric

compounds are being considered as potential candidates for

MFSFET due to their better fatigue characteristics [6]. Bismuth

vanadate Bi2VO5.5 (BVO) is a vanadium analogue of an nZ1

member of Aurivillius family, [Bi2O2]2C[AnK1BnO3nC1]2K of

oxides [7]. Bi2VO5.5 is one of the most promising ferroelectric

materials, particularly for its application to field effect

transistor (FET)-type FeRAM configured in an MFIS structure

[8,9].

In the present study, Bi2VO5.5 thin films were fabricated on

a p-type Si(100) using chemical solution decomposition

technique followed by rapid thermal annealing. Both structural

and electrical properties of the films have been studied. The

main emphasis has been given on the C–V characteristics of the

MF(I)S structure, to understand the nature of the electrical

behavior.

2. Experimental

Bismuth citrate (99.99% purity Aldrich Chemicals) and

ammonium metavanadate (99.0% purity sd fine-chem) were

used as raw materials to prepare the BVO films. Bismuth citrate

was initially dissolved in hydrochloric acid under constant

stirring. Then, ammonium metavanadate was added to the

Solid State Communications 137 (2006) 566–569

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N. Kumari et al. / Solid State Communications 137 (2006) 566–569 567

mixture. The solution was diluted with 2-methoxyethanol, to

adjust its viscosity and surface tension, and filtered through a

0.2 mm syringe filter to remove dust and other suspended

particles. The resultant solution served as the Bi2VO5.5

precursor. It was spinning coated onto a p-type Si(100)

substrate to form a single layer. Each layer was preheated in

air at 110 8C for 2 min. The deposited films were baked at

350 8C for 3 min to remove solvents and residual organics

followed by annealing in oxygen using rapid thermal annealing

(RTA) at 650–700 8C. The phase of the deposited film was

confirmed from the X-ray diffraction patterns. The studies were

carried out using Scintag 3100 system (Fe Kaw1.9373 A). The

surface microstructure of the films was examined using contact

mode atomic force microscope (AFM) (Veeco CP-II). The

thickness of the film was confirmed to be 0.3 mm from the cross

sectional scanning electron micrograph. For electrical

measurements, gold dots of 1.96!10K3 cm2 areas were

deposited on the top surface of the films through a shadow

mask by thermal evaporation technique. Silver paste was used

as bottom electrode for measurements. The high frequency

capacitance–voltage characteristics of Bi2VO5.5/Si were

measured using a HP4294A impedance analyzer in 1 kHz–

1 MHz frequency range and the sweep speed was 0.1 V/s.

Fig. 2. (a) AFM micrograph showing surface morphology of Bi2VO5.5 thin film,

(b) three-dimensional topography of a Bi2VO5.5 thin film.

3. Results and discussions

Fig. 1 represents the X-ray diffraction spectra of BVO thin

films deposited on p-type Si annealed at 650 8C for 3 min. As

shown in Fig. 1, BVO film deposited on Si shows the typical

XRD pattern of BVO layered perovskite polycrystalline

structure with diffraction peak of strong (200) with a preferred

orientation along c axis. X-ray diffraction pattern confirmed the

existence of single ferroelectric phase. Strong and sharp peaks

indicate the good crystallinity of the film. The surface

morphology and microstructure of BVO thin films has been

studied by AFM and the results has been shown in Fig. 2(a) and

(b) as two-dimensional and three-dimensional micrograph. The

AFM micrograph in Fig. 2(a) shows the surface topography of

Fig. 1. X-ray diffraction pattern of BVO thin film grown on p-type Si.

a BVO thin film. It indicates the homogeneous distribution of

grains in Bi2VO5.5 thin film. The grain-size estimated from the

picture lies in the range of 0.3–0.4 mm. The AFM micrograph

in Fig. 2(b) shows the three-dimensional image. The root mean

square of the surface roughness (Ra) lies between 9.58 and

13.76 nm.

Fig. 3 shows The C–V characteristics of the Au/BVO/Si

structures at various sweep voltages. The measurement was

carried out at room temperature with an ac voltage of 0.2 V and

the frequency of 100 kHz. All the hysteresis loops are in a

clockwise sweep direction, indicating that the memory window

of the MFIS structure is induced by the switched ferroelectric

polarization. The C–V curves show typical high frequency

Fig. 3. C–V characteristics of the BVO/SiO2/Si structure with a sweep voltage.

N. Kumari et al. / Solid State Communications 137 (2006) 566–569568

feature of MFIS structure. At relatively low sweep voltage, the

overall C–V curve undergoes a transition between accumu-

lation and depletion region, along the negative voltage axis for

more than 0.5 V with respect to an ideal MOS structure. With

the increase of sweep voltage ranges to larger magnitudes, the

C–V curves shifts towards the positive voltage axis. Such a

shift reveals the existence of fixed negative charges in BVO/Si

interface.

Fig. 4(a) shows the memory window as a function of sweep

bias. The memory window initially increases with the applied

voltage up to a sweep bias of 5 V to a maximum value of

1.42 V. This is due to the fact that the polarization and the

coercive voltage are being increased with the increase of

applied voltage and thus memory window also increases. With

subsequent increase in sweep voltage range above 5 V, the

Fig. 4. (a) Variation in memory window of the BVO/SiO2/Si structure with a

sweep voltage, (b) variation of the flateband voltages in the C–V curves with

bias voltages.

memory window started decreasing, may be due to the electron

injection from silicon. It may be seen from the Fig. 3 that the

memory window did not increase symmetrically with the

sweep voltage. When the sweep voltage changed from the

negative bias (accumulation) state to the positive bias

(inversion) state, the flat band voltage increased with the

sweep voltage. On the other hand, the flat band voltage

decreases very slightly when the bias voltage sweeps from

positive bias state to negative bias state. Such behavior may be

seen because the memory window is caused by ferroelectric

polarization reversal via polarization screening in a semi-

conductor. Fig. 4(b) shows the flatband voltage as a function of

sweep voltage. When the sweep voltage was low, i.e. up to 5 V,

the flatband voltage (Vf1) increased with the sweep direction

from negative to positive state (or positive sweep) and the

flatband voltage (Vf2) decreased slightly with the sweep from

positive to negative state (or negative sweep). As the sweep

voltage increased up to 6 V, the flatband voltage (Vf2) starts

increasing while flatband voltage (Vf1) starts decreasing. This

indicates that the memory window increased due to ferro-

electric polarization without charge injection. If the charge

(electron) injection occurs from Si under the positive bias state

and the injected charges are trapped in the oxide layer, these

trapped charges will be involved in polarization screening. This

will decrease the amount of screening charge in Si, and leads to

a lesser degree of band banding in Si surface. The flatband

voltage (Vf1) tends to saturate at a bias voltage of 6 V,

reflecting the saturation in ferroelectric polarization. After that

it starts decreasing. The variation in flatband voltage can be

accounted in terms of the effect of the local field, which often

tends to oppose the orientation of dipoles towards the external

field. Hence the shift in flatband voltage (Vf1) during positive

sweep is mainly due to ferroelectric polarization while the shift

in flatband voltage (Vf2) during negative sweep is very much

influenced by charge injection in contribution with ferro-

electric polarization. The near-interface oxide trap density can

be calculated by using the following formulation:

Oit ZCox

qs

� �Vot

Here, Cox is the measured capacitance of ferroelectric oxide

layer; Vot is the difference of flatband-voltage shift between the

two opposite sweep directions caused by ferroelectric oxide

traps. The q and s are electron charge and gate area,

respectively. In the MFS structure the determination of Vot is

different from that in traditional MOS structure, because the

remnant ferroelectric charges in the MFS structure can also

shift the flatband voltage. The oxide trap density for the C–V

memory window of 5 V sweep voltage, was estimated out to be

1.45!1012 cmK2. To reduce the effect of the ferroelectric

oxide traps near the ferroelectric/Si interface, the oxides such

as HfO2 and MgO can be inserted between ferroelectric oxide

and silicon. Fig. 5 shows the frequency dependence of the C–V

characteristics. When a voltage is applied, the C–V curve shifts

towards negative voltage with increase of frequency. This

capacitance stretch-out may be due to interface-trapped

Fig. 5. Variation of the C–V window with the applied frequency at a given

sweep voltage.

Fig. 6. Variation of dielectric constant and dissipation factor of a Bi2VO5.5 thin

film (see arrows) as a function of frequency.

N. Kumari et al. / Solid State Communications 137 (2006) 566–569 569

charges because the interface traps occupancy varies with the

applied sweep voltage.

Fig. 6 shows the variation of dielectric constant and the

dissipation factor as a function of frequency measured at room

temperature. The dispersion observed in the dielectric constant

at lower frequency is quite high as compared to the dispersion

at higher frequency. This lesser dispersion observed at higher

frequency is due to the response of the grains, while at lower

frequencies it comprises of grain boundaries, free charges and

neutral dominates, leading to more dispersion. The dissipation

factor has the minimum value between the frequencies of 100

and 500 kHz, where the dielectric constant lies in between 140

and 146.

4. Conclusions

In conclusion, BVO thin films exhibiting good structure and

morphology were successfully prepared on p-type Si by

chemical solution decomposition technique. The films consist

of homogeneous grains of 0.3–0.5 mm sizes. The C–V

characteristics were evaluated for Au/BVO/Si MFS structure,

which exhibited a ferroelectric polarization induced memory

window. The asymmetric behavior of the C–V characteristics

has been discussed in terms of charge injection from Si along

with the ferroelectric polarization. The memory window of

Bi2VO5.5/Si was about 1.42 V for gate bias of G5 V. The near-

interface oxide tape density has been estimated to be as high as

1.45!1012 cmK2.

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