shape controlled sn doped zno nanostructures for tunable optical emission and transport properties

By proper tuning of the growth temperature, morphology of pure ZnO can be changed from tetrapods to multipods.. On the other hand, by varying the doping concentration of Sn in ZnO, the m

transport properties

T Rakshit, I Manna, , and S K Ray,

Citation: AIP Advances 3, 112112 (2013); doi: 10.1063/1.4832219

View online: http://dx.doi.org/10.1063/1.4832219

View Table of Contents: http://aip.scitation.org/toc/adv/3/11

Published by the American Institute of Physics

Shape controlled Sn doped ZnO nanostructures for tunable optical emission and transport properties

T Rakshit,1I Manna,2,aand S K Ray3,b

1Advanced Technology Development Centre, IIT Kharagpur, 721 302, India

2Department of Metallurgical and Materials Engineering, IIT Kharagpur, 721 302, India

3Department of Physics & Meteorology, IIT Kharagpur, 721 302, India

(Received 5 July 2013; accepted 6 November 2013; published online 13 November 2013)

Pure and Sn doped ZnO nanostructures have been grown on SiO2/Si substrates by vapor-solid technique without using any catalysts It has been found that the mor-phology of the nanostructures depend strongly on the growth temperature and doping concentration By proper tuning of the growth temperature, morphology of pure ZnO can be changed from tetrapods to multipods On the other hand, by varying the doping concentration of Sn in ZnO, the morphology can be tuned from tetrapods to flower-like multipods to nanowires X-ray diffraction pattern reveals that the nanostructures have a preferred (0002) growth orientation, and they are tensile strained with the increase of Sn doping in ZnO Temperature-dependent photoluminescence character-istics of these nanostructures have been investigated in the range from 10 to 300 K Pure ZnO tetrapods exhibited less defect state emissions than that of pure ZnO multi-pods The defect emission is reduced with low concentration of Sn doping, but again increases at higher concentration of doping because of increased defects Transport properties of pure and Sn doped ZnO tetrapods have been studied using complex-plane impedance spectroscopy The contribution from the arms and junctions of a tetrapod could be distinguished Sn doped ZnO samples showed lower conductivity but higher relaxation time than that of pure ZnO tetrapods.C 2013 Author(s) All

article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4832219]

I INTRODUCTION

Zinc oxide (ZnO), with direct band gap of 3.37 eV and high excitonic binding energy of 60 meV

at room temperature, has been used in fabricating field effect transistors,1 field emission devices,2 sensors,3room temperature hydrogen storage,4nanolasers,5etc ZnO has the ability to form many configurations and exhibited wide range of morphologies such as nanowires,4nanorods,6nanotubes,7 nanorings,8 etc., which play an important part in governing the properties and applications of nanomaterials

Doping with selective elements can play an active role in tuning the basic physical properties and shape transformation of ZnO.9 12Doping ZnO with different elements such as Li, Na, etc in different concentrations resulted in varying morphologies.13 Shen et al showed that by varying

the concentration of sulphur in S-doped ZnO nanostructures, the morphology can be changed from nanonails to nanowires.14 This change in morphology affects the optical properties of the nanostructures.13,14 The ionic radius of Zn2+ (0.74 Å) is nearly equal to that of Sn4+ (0.69 Å),

so Zn can be easily substituted by Sn without resulting in much lattice distortion Gao et al.15

and Ding et al.16 studied the growth mechanism of Sn doped ZnO nanobelts, and found that the crystalline orientation of Sn particles determined the growth direction and side faces of the nanobelts

a Present address: IIT Kanpur, 208 016, India.

b Corresponding author’s e-mail address: physkr@phy.iitkgp.ernet.in

2158-3226/2013/3(11)/112112/12 3, 112112-1 Author(s) 2013

Besides, doping ZnO with In, Ga, Sn, etc can increase its conductivity.17SnO2has a larger band gap (3.62 eV) and higher excitonic binding energy (130 meV) at room temperature as compared to ZnO Thus, Sn doped ZnO can be used for shorter wavelength optoelectronic devices and room temperature UV laser applications However, a systematic study on morphology, optical emission, and transport properties of Sn doped ZnO nanostructures have not been reported till date

In this paper, we report on the growth of ZnO nanostructures, doped with different concentrations

of Sn, by a simple vapor-solid technique The effect of doping on the structural properties of ZnO has been investigated by electron microscopy and X-ray diffraction analysis The temperature dependent behavior of different excitonic peaks in the near band edge region has been studied in details The transport properties of pure and Sn doped ZnO tetrapods have been studied by impedance analysis, and to the best of our knowledge are being reported for the first time

II EXPERIMENTAL

Pure and Sn doped ZnO nanostructures were grown on SiO2/Si substrates by catalyst-free vapor-solid (VS) technique Zinc powder of high purity (99.99%) was used as the source material for the growth of pure ZnO nanostructures For the growth of Sn doped ZnO nanostructures, a mixture of zinc and tin powder (both of purity 99.99%), mixed in different weight ratios was used The powder was kept in an alumina crucible with the oxidized Si(100) substrate placed on the top of it The thickness of SiO2layer was about 50 nm The crucible was then inserted in the centre of a horizontal tube furnace For the growth of pure ZnO, the deposition was carried out at temperatures of 800◦C and 900◦C for 30 min under a constant flow of 50 SCCM (represents cubic centimeter per minute

at STP) of Ar gas Sn doped ZnO nanostructures were grown at a constant temperature of 800◦C and keeping other parameters same, but varying the concentration of tin powder in the mixture The morphology of the nanostructures was characterized using a field emission scanning electron microscope (FESEM, ZEISS), equipped with energy dispersive X-ray (EDX) analysis EDX analysis was used to determine the average atomic concentration of tin in the Sn doped ZnO samples The phase of the samples was studied by X-ray diffractometer (XRD) (Philips X-Pert MRD) using Cu

Kα radiation (45 kV, 40 mA) of wavelength 0.15418 nm at a grazing angle of 2.0◦ The optical

characteristics was studied through photoluminescence measurements in the temperature range 10–

300 K, using a He-Cd laser (325 nm) as an excitation source, and an output power of 45 mW, along with a cooled Hamamatsu R928 photomultiplier detector fitted TRIAX 320 monochromator Electrical properties of doped ZnO were studied using impedance spectroscopy data recorded with

an Agilent 4294A impedance analyzer

III RESULTS AND DISCUSSION

Figs.1(a)and1(b)show the FESEM images of pure ZnO nanostructures synthesized at a growth temperature of 800◦C and 900◦C, respectively At a growth temperature of 800◦C, ZnO tetrapods are formed As shown in the inset of Fig.1(a), the arms are smooth with hexagonal cross-section with length varying between 1.5-2 μm and uniform diameter of 500–700 nm When the growth

temperature is increased to 900◦C, multipods are formed The arms of the multipods have length

of 1-3μm and diameter of 300-500 nm At a higher growth temperature, ZnO species with higher

surface migration accumulate to form larger clusters Several molecules in the tip of the cluster act as nucleation centres Due to the presence of multiple nuclei, the growth of arms of the multipods occurs

in different directions from the nucleus It may be noted that our previous study on ZnO showed the growth of mainly nanorods at 650◦C; and tripods were formed when the growth temperature was increased to 750◦C.18That’s why the present study aimed on growing ZnO tetrapods and multipods

at higher growth temperatures

The grazing incidence X-ray diffraction (XRD) pattern of the above nanostructures is shown

in Fig.2(a) All the diffraction peaks correspond to the wurtzite ZnO structure, and no peak from zinc or suboxide phases are found, confirming the formation of pure ZnO The growth exhibited

a preferred (0002) orientation, with tetrapods showing stronger intensity than the multipods The photoluminescence (PL) spectra at 10 K of the ZnO tetrapods and multipods, recorded with an

(a)

(b)

FIG 1 FESEM micrographs of pure ZnO nanostructures grown for 30 min at different growth temperature of (a) 800 ◦C

and (b) 900 ◦C Inset shows the magnified view of the micrographs.

excitation wavelength of 325 nm are shown in Fig.2(b) The broad peak around 2.471 eV in the blue-green region is related to complex defect species, which is usually attributed to zinc interstitial

or oxygen vacancies.19 The peak in the ultraviolet region is related to excitonic emission It can be seen that the tetrapods exhibited much lower defect state emissions than the multipods This may be due to the enhanced surface-to-volume ratio with increasing number of arms, and this may induce more oxygen vacancies in the multipods.20

Since better growth orientation and lower defect state emissions were observed for the nanos-tructures grown at 800◦C (tetrapods), we doped these nanostructures with tin to see its effect on the structural, optical, and electrical properties of ZnO Samples with Sn content of 3, 5, 10, and 16 at.%

in ZnO were synthesized, which are designated as SZO3, SZO5, SZO10, and SZO16, respectively, hereafter

The morphologies of the grown nanostructures have been studied using FESEM images Figs.3(a)–3(d)illustrate the FESEM micrographs of the grown SZO3, SZO5, SZO10, and SZO16, respectively As observed, SZO3 and SZO5 have tetrapod structure with diameter and length of arms similar to those of pure ZnO tetrapods The yields of the samples are also almost same The arms of

30 40 50 60 70 80

(a)

(A) Pure ZnO tetrapods (B) Pure ZnO multipods

(B)

(A)

2 (degree)

0

1 0 1

Energy (eV)

Pure ZnO Multipods

(b)

Pure ZnO Tetrapods

FIG 2 (a) Grazing incidence XRD patterns, and (b) photoluminescence spectra at 10 K, of pure ZnO nanostructures.

SZO3 tetrapods are smooth with hexagonal cross-section, which is clearly seen in the inset of the Fig.3(a) But the arms of SZO5 tetrapods are rough, mainly at the end As a result, the hexagonal cross-section of the arms is not clearly visible (shown in the inset of Fig.3(b)) This may be due to the induced strain as a result of higher concentration of Sn in ZnO Interestingly, any further increase

of Sn content in ZnO completely changes the morphology SZO10 shows a flower-like multipod structure (Fig.3(c)) Thus, in Sn doped ZnO, multipods of different morphology can be grown at

a much lower temperature than that of pure ZnO The arms of these Sn doped ZnO multipods are about 2-3μm long and of uniform diameter of 1 μm upto a certain length, which then decreases

gradually towards the tip The size of the multipods is not same at all places over the substrate and the yield is also low compared to that of pure ZnO When Sn concentration is further increased

to 16 at.%, nanowires of several micrometers long and diameter of about 50-190 nm are formed (Fig.3(d)) Inset of the figure shows the magnified view of a single nanowire

The grazing incidence XRD pattern of the above nanostructures is shown in Fig 4(a) The XRD pattern of pure ZnO tetrapods has also been presented for comparison All the samples

(b)

FIG 3 FESEM micrographs of Sn doped ZnO nanostructures grown at 800 ◦C for 30 min., for Sn doping concentrations of

(a) 3 at.%, (b) 5 at.%, (c) 10 at.%, and (d) 16 at.%.

have a preferred (0002) growth orientation, with the intensity of the peak gradually decreasing with doping The diffraction peaks of SZO3 tetrapods, SZO5 tetrapods, and SZO10 flower-like multipods correspond to the wurtzite ZnO structure and no peak from SnO2 was observed For SZO16 nanowires, in addition to the peaks of ZnO, a peak is observed at 26.53◦ corresponding to the tetragonal (110) SnO2structure Fig.4(b)presents the high resolution XRD data showing (0002) peak of pure ZnO tetrapods and Sn doped ZnO nanostructures It can be seen that with increasing

Sn doping, the (0002) peak shifts towards the higher diffraction angle side This is attributed to the smaller ionic radius of Sn4+(0.69 Å) compared to that of Zn2+(0.74 Å), resulting in in-plane tensile strain The XRD result clearly suggests that Sn (upto 10 at.%) is easily incorporated in ZnO lattice without changing the crystal structure But at high doping concentration of Sn (16 at.%), in addition

to ZnO, separate SnO2 phase is formed The full-width-at-half-maximum (FWHM) of the (0002) peak as a function of Sn content in ZnO is shown in Fig.4(c) The variation of FWHM is not much pronounced upto Sn doping concentration of 5 at.%, but increases significantly thereafter, which may be due to increased lattice strain for high Sn concentration

Low temperature (10 K) photoluminescence spectra of the nanostructures, recorded with an excitation wavelength of 325 nm, are shown in Fig.5(a) A typical spectrum reveals two major peaks: one in the ultraviolet region (due to excitonic emissions) and another in the visible region (due to defect states), same as that of pure ZnO nanostructures It is found that the ratio of the intensity of the defect state emission to excitonic emissions gradually decreases with increase of

Sn doping upto 5 at.% But the defect state emission again gradually increases with further doping This is because at higher doping concentration of tin, when all the zinc lattice sites are occupied, the former begin to occupy the interstitial sites resulting in increased concentration of defects Fig.5(b)shows the PL spectra at 10 K of the excitonic emission bands of the grown nanostructures For all the samples, the peak at 3.375 eV is assigned to free excitonic emissions, denoted by FX The peaks at 3.360 eV and 3.364 eV are attributed to excitons bound to neutral donors, i.e D0 X and

D0 X, respectively.21 , 22A peak at 3.311 eV is observed, which is not related to Sn doping, since it is also observed in pure ZnO The binding energy of this peak has been found to be (3.375-3.311 eV)

30 40 50 60 70 80

(a)

2 (degree)

(E) (D) (C) (B)

(110)

(A)

SnO 2

ZnO

0.0 0.4 0.8 1.2

(b)

2 (degree)

Pure ZnO tetrapods SZO3 tetrapods SZO5 tetrapods SZO10 flower-like multipods SZO16 nanowires

0.2 0.4 0.6 0.8 1.0 1.2 (c)

Sn content in ZnO (at.%)

FIG 4 (a) Grazing incidence XRD patterns of (A) pure ZnO tetrapods, (B) SZO3 tetrapods, (C) SZO5 tetrapods, (D) SZO10 flower-like multipods, and (E) SZO16 nanowires (b) Showing only the (0002) peak of the XRD pattern (c) Variation of full-width-at-half-maximum of the (0002) peak with Sn content in ZnO.

i.e 64 meV, suggesting the peak to be excitonic in nature We have assigned this emission due to excitons bound to defect states (SX).23 , 24The peaks at 3.238 eV, 3.166 eV, and 3.094 eV are close to the integral multiple of longitudinal optical (LO) phonon replica (72 meV)25 of SX, and attributed

to SX-1LO, SX-2LO, and SX-3LO, respectively For Sn doped samples, another peak is observed

at 3.355 eV, which is also attributed to excitons bound to neutral donors, i.e D0 X Doping of Sn

in ZnO may have induced a deep level energy state in ZnO, which may give rise to this emission Fig.5(c)shows clearly the presence of the D0X peaks The intensity of these peaks increases with

Sn doping in tetrapods, but decreases for SZO10 flower-like multipods and SZO16 nanowires At higher doping concentration, Sn occupies the interstitial sites and contributes to defect states as discussed before, and thus the contribution of excitonic emission decreases For SZO5 tetrapods, some additional peaks are observed The peaks at 3.288 eV, 3.216 eV, and 3.144 eV are close to the integral multiple of LO phonon (72 meV) replica of D0 X, and assigned to D0 X-1LO, D0 X-2LO, and D0 X-3LO, respectively

Figs.6(a)–6(e)show the temperature dependent PL spectra of the excitonic emission bands of pure ZnO tetrapods, SZO3 tetrapods, SZO5 tetrapods, SZO10 flower-like multipods, and SZO16 nanowires, respectively For pure ZnO, SZO3, and SZO5 tetrapods, the D0X peaks can be distin-guished separately upto 40 K, 25 K, and 70 K, respectively, and thereafter the peaks merge The D0X emission disappears at T>125 K for these samples For SZO10 flower-like multipods and SZO16

nanowires, the D0 X peak cannot be distinguished separately after 25 K, but the D0 X and D0 X peaks can be distinguished upto 70 K and 55 K, respectively The temperatures above which the D0X peak disappears are 85 K and 100 K, respectively, for flower-like multipods and nanowires With

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

0

1

0

1

0

1

0

1

0

1

(A)

(B)

(C)

(D)

(E)

(a)

Energy (eV)

3.34 3.35 3.36 3.37 3.38 3.39

(c)

(E)

(C) (D)

(B) (A)

FX

D 0 2X

D 0 1X

D 0 3X

Energy (eV)

3.08 3.15 3.22 3.29 3.36 3.43

(b)

(E)

(D)

(C) (B) (A)

Energy (eV)

D 0 1X-3LO

D 0 1X-2LO

D 0 1X-1LO SX-3LO SX-2LO

SX-1LO

SX

D 0 3X

D 0 1X

D 0 2X FX

FIG 5 Photoluminescence spectra at 10 K of (A) pure ZnO tetrapods, (B) SZO3 tetrapods, (C) SZO5 tetrapods, (D) SZO10 flower-like multipods, and (E) SZO16 nanowires, showing (a) in full range of 2-3.55 eV, (b) only the excitonic emission bands, and (c) only the free excitonic and excitons bound to neutral donor peaks.

increase in temperature for all the samples, D0X peak intensity gradually decreases as it thermally dissociates to FX The PL spectrum at 300 K has been fitted using Lorentzian function It is found that the spectrum of all the samples is best fitted with three curves, which corresponds to emission peak due to FX, SX, and SX-1LO Since the binding energy of SX is near to the excitonic binding energy, SX and its phonon replica plays an important role in emission upto 300 K The variation

of FX, SX, and SX-1LO peaks with temperature of all the samples has been investigated Fig.6(f)

shows the results for pure ZnO tetrapods as a representative plot The FX peak of all the samples can be well described by Bose-Einstein-type expression:26

exp



θ E

T



− 1

where E g (T ) is the band gap energy, K is the electron-phonon coupling strength, and θ E is the Einstein temperature The extracted Einstein temperatures obtained from the fitted plots are summa-rized in TableI The values agree quite well with that reported in reference 27

For a detailed investigation of the electrical properties, frequency dependent impedance mea-surement was carried out at room temperature The impedance spectrum provides an interconnection between the structural and electrical properties of a material The resistive and capacitive properties

of different regions of the material become well separable when the individual components are related to different relaxation times,τ This relaxation time (τ = RC) is an intrinsic and unique

property of the material, which is independent of any sample geometry factors Therefore, the

TABLE I Einstein temperature of pure and Sn doped ZnO nanostructures.

3.08 3.15 3.22 3.29 3.36 3.43

(a)

SX-1LO

SX-3LO

Energy (eV)

SX-2LO

SX D 0 1X D 0 2X

FX

10 K

100 K 85 K

55 K

25 K

125 K

175 K

225 K

275 K

(b)

300 K

250 K

225 K

175 K

125 K

85 K

55 K

25 K

Energy (eV)

SX-3LO SX-2LO SX-1LO

0

FX

Energy (eV)

(c)

300 K

250 K

275 K

225 K

175 K

125 K

85 K

40 K

SX-3LO

SX-2LO

FX

10 K

3.08 3.15 3.22 3.29 3.36 3.43

Energy (eV)

(d)

300 K

275 K

250 K

225 K

175 K

125 K

85 K

55 K

25 K SX-3LO

SX-2LO SX-1LO

SX D 0 3X D 0 1X

D 0 X FX

3.08 3.15 3.22 3.29 3.36 3.43

SX-3LO

Energy (eV)

(e)

SX-2LO

SX-1LO

SX D 0 3X D 0 1X

D 0 X FX

300 K

250 K

225 K

175 K

125 K

85 K

55 K

25 K

3.18 3.21 3.24 3.27 3.30 3.33 3.36 3.39

3.18 3.21 3.24 3.27 3.30 3.33 3.36 3.39

3.18 3.21 3.24 3.27 3.30 3.33 3.36

3.39

(f)

Temperature (K)

FX SX SX-1LO

FIG 6 Temperature dependent photoluminescence spectra of (a) pure ZnO tetrapods, (b) SZO3 tetrapods, (c) SZO5 tetrapods, (d) SZO10 flower-like multipods, and (e) SZO16 nanowires 300 K graph (solid line) has been fitted with Lorentzian curves (dash lines) The sums of the three Lorentzian curves are indicated by open circles (f) Variation in free excitonic peak, peak due to excitons bound to defect states and its LO phonon replica, with temperature of pure ZnO tetrapods Free excitonic peak have been fitted with Bose-Einstein-type expression (solid line).

(b) (c) (d)

Si

Sn doped or undoped ZnO tetrapods

Al electrode

SiO2

(a)

FIG 7 (a) Schematic diagram of the fabricated device with Al contact on the top Magnified view of the FESEM micrographs

of (b) pure ZnO tetrapods, (c) SZO3 tetrapods, and (d) SZO5 tetrapods.

relaxation processes occurring within a material can be obtained from the analysis of the impedance data

Impedance measurements were carried out with pure and Sn doped ZnO tetrapods only, since they have almost the same size and yield Fig 7(a) shows the schematic diagram of the fab-ricated structure that has been used for the impedance measurement The possible conduction path through tetrapods between the electrodes is shown by the red line Since there is a layer

of SiO2 between the tetrapods and Si, the contribution of Si substrate in the impedance mea-surement is eliminated Figs.7(b)–7(d) are magnified view of the FESEM images of pure ZnO, SZO3, and SZO5 tetrapods, respectively As observed, the arm of a tetrapod is connected to an arm or junction of another one (shown by red arrows), thus making an interconnection between the tetrapods This interconnection makes conduction possible through the tetrapods between the two electrodes

Figs.8(a)–8(c)show the complex plane plot of imaginary Z versus real Z by applying different

dc bias voltages for pure ZnO, SZO3, and SZO5 tetrapods, respectively Within the measured range,

a typical plot shows two overlapping semicircles, a larger semicircle in the low frequency region (say, semicircle-1) and another one in the high frequency region (say, semicircle-2) Inset of each figure clearly shows the presence of semicircle-2 A tetrapod consists of arm and junction, which are two structurally and electrically different regions It is proposed that the origin of semicircles

is attributed one from the arm and the other from the tetrapod junction The proposed equivalent