Experimental and simulated study of electrical behaviour of ZnO film deposited on Al substrate for device applications

Experimental and simulated study of electrical behaviour of ZnOfilm deposited on Al substrate for device applications

Priyabrata Pattanaik • Sushanta Kumar Kamilla •

Debi Prasad Das • Dilip Kumar Mishra

Received: 15 February 2014 / Accepted: 3 May 2014 / Published online: 10 May 2014

� Springer Science+Business Media New York 2014

Abstract In this paper, zinc oxide (ZnO) film has been

deposited on Al substrate by chemical wet and dry tech-

nique which is just a simple modified version of the dip

coating method. In this method, it is possible to precisely

control the immersion and withdrawal speed, with drying

as well as annealing at the same time. The polycrystalline

nature of ZnO has been confirmed by X-ray diffraction

(XRD) analysis. The XRD analysis clearly indicates that

some percentage of Al diffuses into the ZnO matrix at its

interface region; hence it affects mobility of the sample.

Hall measurement indicates the ZnO semiconductor as

n-type. I–V characteristic of the sample shows that the

contact is Ohmic and it can be used as a sensor at low

potential value. The mobility decreases with increase in

temperature. The simulation study carried out for I–V and

mobility through simulation using ATLAS (SILVACO)

software confirms that the experimental and simulation

results are in close agreement with respect to the I–V

characteristic and the mobility.

1 Introduction

There has been an extensive study on zinc oxide (ZnO) in

recent years due to its potential application in the area of

spintronics, magneto-electronics and optoelectronics [1–7].

Especially, ZnO is a very promising material for the fabri-

cation of semiconductor devices as it is a wide and direct band

gap semiconductor. A wide band gap semiconductor has

many benefits for high temperature and power operations,

reducing electronic noise, making sustenance in large electric

fields possible and raising breakdown voltages [8–10]. These

features make this material attractive for various applications

in power electronics devices, solar cells, semiconductor gas

sensors, optical coating UV semiconductor lasers, spintronics

devices and photocatalytic activities etc. Looking into all

above applications, ZnO films have been grown by several

techniques such as metal–organic chemical vapour deposition

(MOCVD) [11], chemical vapour deposition (CVD) [12], RF

magnetron sputtering [13], DC magnetron sputtering [14] and

pulsed laser deposition (PLD) [15].

The most advantage feature in ZnO is that it can be grown

easily by any techniques. Hence, it is possible to choose a

suitable low cost technology for the fabrication of ZnO

devices. In consideration to the low cost technology, ZnO

film is preferred to be grown by different chemical tech-

niques like sol gel spin and dip coating techniques, which are

versatile and low cost system to prepare ZnO film [16, 17].

Particularly, the sol–gel processes are well adapted to pro-

duce ZnO films in a simple and highly controlled way. A

number of reports highlighted the sol–gel synthesis of

undoped homogeneous ZnO films on different substrate for

various applications including the chemical and physical

parameters influencing their structural properties [18, 19].

The optical and electrical properties of group III metals (Al,

Ga, In) doped in ZnO film prepared by sol–gel process was

reported [20]. However, a few works on ZnO film on Al

substrate have been reported till date [21–23]. The re-

view of electron mobilities, and corresponding carrier con-

centration obtained in nominally undoped bulk and thin-film

ZnO deposited on different substrates by various growth

P. Pattanaik � S. K. Kamilla � D. K. Mishra (&)

Semiconductors Research Laboratory, Institute of Technical

Education and Research, Siksha ‘O’ Anusandhan University,

Khandagiri Square, Bhubaneswar 751030, Odisha, India

e-mail: [email protected]

D. P. Das

CSIR-Institute of Minerals and Materials Technology,

Bhubaneswar 751013, Odisha, India

123

J Mater Sci: Mater Electron (2014) 25:3062–3068

DOI 10.1007/s10854-014-1984-1

techniques was reported earlier. However, no much study on

electrical behaviour, electron mobility and corresponding

carrier concentrations have been carried out on ZnO film

deposited over Al substrate.

Hence, in this study, a uniform thickness of ZnO film is

deposited on Al substrate using the sol–gel and a newly

developed modified chemically wet and dry (CWD) method.

The electrical characterizations of the film were done by

using four probe method and Hall Effect measurement with

the help of data acquisition system (NI PXI-1042) and data

acquisition card NI PXI-4072 including related software.

Experimentally investigated temperature-dependent carrier

mobility and concentration were used to determine the

fundamental material parameters and understand the carrier

scattering mechanism along with an accurate comparison

with theory. In addition to this, these experimentally

obtained physical parameters were optimized by device

simulation software, the Technology Computer Aided

Design (TCAD; Make: ATLAS, SILVACO). It is intended

to determine the optimum condition that would lead to the

manufacturing of well crystallized, semi-conductive and

transparent ZnO films on Al substrate for device fabrication.

2 Experimental

ZnO film on Al substrate was prepared by sol–gel and mod-

ified CWD method. The CWD method used here is simple

and low cost technique for deposition of oxide film. The

technique is just a simple modified version of the dip coating

method. In this method, it is possible to precisely control the

immersion and withdrawal speed, with drying as well as

annealing at the same time. The setup is locally designed and

fabricated. The schematic picture of the CWD setup is shown

in Fig. 1.

The solution for sol–gel was obtained by mixing zinc

acetate dihydrates (extra pure, C98 %, Made: MERCK)

with ethylene glycol (10 ml) and glycerol (0.5 ml

approximately) at room temperature. The solution was

stirred by magnetic stirrer at temperature of 140–160 �C

for 10 min. Then 2-proponal (15 ml) was added while

stirring. Then the stirring is stopped and the solution was

cooled down to room temperature and then slowly trieth-

ylene amine (4 ml) was added, so that no turbidity forms.

The final solution was clear and homogenous to serve as

the coating substance for film preparation. ZnO films were

prepared by taking Al substrate and the precursor solution

with the help of CWD method. In this procedure, the

solution was placed at the bottom of the vertical furnace at

a specific temperature profile and the Al substrate was

attached through a pulley. The pulley was controlled by a

drive motor with the help of a platinum wire. Dipping the

Al substrate in the solution and withdrawing it in upward

direction at a rate of 3 cm/h has been maintained for

achieving uniform thickness of the desired grown sample.

Simultaneously, the grown sample was passed through the

same vertical furnace for drying at 80 �C and at the same

time annealing at 400 �C for 2 h to get the better crystal-

line nature. The constant temperature zone of the vertical

furnace was taken from temperature profile which is shown

Fig. 1 Schematic diagram of CWD set up

J Mater Sci: Mater Electron (2014) 25:3062–3068 3063

123

in Fig. 2. This process was repeated for 5 times in order to

increase the thickness of the film to form a more uniform

layer over the Al substrate.

X-Ray diffraction (XRD) spectrum was recorded at room

temperature with CuKa radiation (k = 1.540 A) within 2hrange from 30� to 70� for structural characterization and

phase identification of ZnO film on Al substrate by using

Phillips diffractometer PW 1715 system. The obtained ZnO

film on Al substrate was first imaged by scanning electron

microscope (SEM) (Model: JSM-6510, Make: JEOL) to

study the surface morphology. To measure the electrical

transport properties like carrier mobility, carrier concentra-

tion and its type and assess the quality of semiconductor film,

the Hall Effect is the most commonly used method. More

specifically, experimental data on Hall measurements by

varying the temperature provide quantitative information on

impurities, imperfections, uniformity, scattering mechanism

etc. So the electrical characterization of the grown sample was

carried out in a temperature controlled furnace (accuracy of

±2 K) by monitoring the variation of electrical resistance

(R) as a function of temperature (T) using four probe tech-

nique as well as Hall Effect measurement. The range of

temperature investigated was 373–473 K for both cases. The

experimental data was simulated by TCAD (Make: ATLAS,

Silvaco) device simulation software. The ZnO film on Al

substrate structure was designed with appropriate contacts

using this TCAD software tool. Different doping concentra-

tions, effect on mobility and I–V curve with different tem-

perature range have been derived and analyzed.

3 Results and discussion

X-Ray diffraction pattern of the ZnO film coated on Al

substrate is shown in Fig. 3. The pattern clearly indicates

the polycrystalline nature of the sample. The XRD pattern

shows the ZnO peaks as well as the peaks of the Al. No

other impurity peaks are observed in the pattern. The major

peaks at 31.85�, 34.44� and 36.27� are observed (inset of

Fig. 3) which proves the Wurtzite structure of ZnO and are

well matched to the referenced JCPDF file number 890510.

The lattice parameters of ZnO film on Al substrate are

estimated using WinCell XRD software and found to be

‘‘a = 3.3638 A and c = 5.3845 A’’ respectively which is

slightly higher than the lattice parameter of ZnO

(a = 3.251 A and c = 5.234 A). However, the c/a ratio

(1.6007) is in agreement with the value of pure ZnO

(1.6099). This variation is due to the substitution of Al

atoms in place of Zn atoms of ZnO at the interface region.

As it is known that the size of Al atom is larger than the

size of Zn atom, the lattice parameter of ZnO film on Al

substrate was slightly varied. In addition to this, the shift-

ing of peaks in ZnO film deposited on Al indicates that that

more number of Al atoms from substrate has been diffused

in ZnO. Similar evidence has been reported in a prior lit-

erature [24].

The SEM examination of the film revealed that the ZnO

film grown on Al substrate exhibited etches pits density

(EPD) (Fig. 4a). From the micrographs, it appears that the

films are not uniform. This nature of morphology is due to

the fact that the material is prepared in an open atmo-

spheric condition which may contain some moisture at the

on its surface. However, it is observed that the film is

grown along a particular axis (shown in Fig. 4b). The

energy dispersive X-ray analysis (Fig. 5) clearly indicates

the presence of Zn and O. The signature of Al coming from

Al substrate is very much prominent and highly intensified.

The signature of C is found due to the carbon coated film

used for good quality micrographs.

Figure 6 shows the variation of resistivity with respect

to temperatures from room temperature (300 K) to 473 K

of ZnO film on Al substrate. The resistivity decreases with

increase in temperature. It is well known that ZnO is a

direct band gap semiconductor and the concentration of

electron increases moving from room temperature to higher

temperature due to the hopping of electrons from valence

band to conduction band [25].

The prepared material was found to be of n-type

semiconductor by using Hall Effect measurement as well

as hot probe method. Nominally undoped ZnO with a

Wurtzite structure naturally becomes an n-type semicon-

ductor. The literature also supports that the n-type

behaviour is due to the presence of intrinsic defects,

which were generally attributed to native defects, such as

the Zn-on-O antisite (ZnO), the Zn interstitial (ZnI), and

the O vacancy (VO) [8]. It has been reported that these

unintentional n-type conductivity, with carrier concentra-

tions are of the order of 1017 cm-3 and carrier mobility of

Fig. 2 The temperature profile of CWD setup

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123

*130 cm2/V s at room temperature which depends upon

the fabrication methods [26–28]. It has also been reported

that after doping in ZnO, the highest carrier concentration

for holes and electrons i.e. 1019 and 1020 cm-3 respec-

tively were obtained [28].

The Hall coefficient and resistivity (q) are experimen-

tally determined and then related to the electrical param-

eters through (for n-type conduction) RH = rH/ne and

lH = RH/q, where RH is the Hall constant or Hall coeffi-

cient, n is the charge carrier concentration, e is the unit

electronic charge, lH is the Hall mobility, and rH is the Hall

scattering factor which is dependent on the particular

scattering mechanism and it assumes that the Hall scat-

tering factor is to be unity. With increase in temperature,

Hall coefficient decreases which means the carrier con-

centration increases as shown in Fig. 7 (modulus of Hall

coefficient given in the curve). At room temperature,

electron concentrations were determined of the order of

1018 cm-3 and from the nature of graph, it shows negative

temperature coefficients.

Fig. 3 XRD peaks of prepared

sample ZnO film deposited on

Al substrate

Fig. 4 A set of SEM micrographs showing the morphological features of the ZnO film on Al substrate by sol–gel CWD process. a 250 lm

width, b 3 lm width

Fig. 5 EDX spectrum of ZnO thin film grown on Al substrate by sol–

gel CWD process

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123

Figure 8 shows the layer structure and doping profile of

ZnO film on Al substrate with mesh from TCAD software

tool using colour code. The net doping of ZnO was taken

from experimental data i.e. 1018/cc. Here the doping is only

applied to ZnO asset based impurity with generic donor.

The n-type doping profile from Al metal substrate interface

to ZnO semiconductor film is varying from 1018/cc to 1013/

cc with the mesh. This indicates that Al atoms are diffused

from the substrate and substituted the Zn atoms at ZnO film

which acts as donors.

Figure 9 shows that I–V characteristic of ZnO film on

Al substrate at temperature 453 K from experimental

observation. The similar type behavior is also observed in

TCAD structure as shown in Fig. 10 at different tempera-

ture ranges. The threshold voltage Vt from the experi-

mental setup was found to be 3.42 V and from the TCAD

extraction Vt was found to be 3.69 V. The experimental as

well as simulated studies showed that the ZnO film on Al

substrate was found to be Ohmic in nature. This indicates

that the contact is perfect. This indicates that ZnO film on

Al substrate can be used as a sensing device for low

potential value.

The variation of mobility as a function of temperature of

the ZnO film on Al substrate is shown in Fig. 11 and it is

found that mobility varies from 76 cm2/V s at 300 K to

38 cm2/V s at 473 K by Hall Effect instrument.

Similarly, variation of mobility with temperature in

simulation study by TCAD which is shown in Fig. 12. It

was observed that trend of the mobility of electrons

declines as the temperature increases where the mobility of

electron is found to be 285 cm2/V s at temperature 250 K

to 220 cm2/V s at temperature 500 K respectively. Albr-

echt et al. [25] predicted the room-temperature electron

Fig. 6 Effect of temperature on resistivity

Fig. 7 Effect of temperature on Hall coefficient

Fig. 8 Layer structure and net doping profile and mesh of ZnO film

deposited on Al substrate from TCAD

Fig. 9 I–V characteristic of ZnO film deposited on Al substrate using

four probe setup at temperature 473 K

3066 J Mater Sci: Mater Electron (2014) 25:3062–3068

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mobility of ZnO is *300 cm2/V s by using Monte Carlo

simulations. That indicates that our simulation study is

much more similar to the reported literature. However the

experimental observation is less than the simulation value.

The decrease in experimental value is due to diffuse of Al

atoms in ZnO matrix at its interface region which is evi-

denced from the XRD peak shift. Hence Al acts as a donor

in the ZnO matrix and decreases the mobility value. In low

temperature region, below 200 K, mobility decreases with

decrease in temperature due to impurity scattering. Similar

trends have been observed from both experimental and

simulation study. The simulation study gives higher

mobility value in comparison to the experimental value

which is obvious due to the non-consideration of Al con-

tribution in ZnO matrix.

4 Conclusion

This paper proposes a new method for ZnO film deposition

on Al substrate. The formation of the ZnO compound and

structure of ZnO film on Al substrate was confirmed

through XRD analysis. The negative sign of Hall coeffi-

cient confirmed that the sample was n-type. The I–V

characteristic shows that the contact is Ohmic. The

mobility decreases with increase in temperature. The the-

oretical study carried out for I–V and mobility through

simulation using ATLAS (SILVACO) software confirms

that the experimental and simulation results are in close

agreement. It suggests that the proposed fabrication method

is a low-cost technique and easy to operate which would

help to synthesize ZnO films on Al substrate. The avail-

ability of the model in technological oriented simulation

tools like ATLAS (SILVACO) also help researchers to

analyze various types of ZnO based devices without using

the expensive fabrication equipments.

Acknowledgments Author Sushanta Kumar Kamilla is thankful to

CSIR, New Delhi for providing financial support from R&D Extra-

mural Fund.

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