Zn interstitial defects and their contribution as efficient light blue emitters in Zn rich ZnO thin films

Accepted Manuscript Zn interstitial defects and their contribution as efficient light blue emitters in Zn rich ZnO thin films Chandni Kumari, Akhilesh Pandey, Ambesh Dixit PII:

S0925-8388(17)34159-2

DOI:

10.1016/j.jallcom.2017.11.377

Reference:

JALCOM 44070

To appear in:

Journal of Alloys and Compounds

Received Date: 30 September 2017 Revised Date:

25 November 2017

Accepted Date: 29 November 2017

Please cite this article as: C. Kumari, A. Pandey, A. Dixit, Zn interstitial defects and their contribution as efficient light blue emitters in Zn rich ZnO thin films, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.377. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Zn interstitial defects and their contribution as efficient light blue emitters in Zn rich ZnO thin films Chandni Kumari1, Akhilesh Pandey2, and Ambesh Dixit1,3,* 1

Department of Physics, Indian Institute of Technology Jodhpur, 342011, India Materials Characterization Division, Solid State Physics Laboratory, New Delhi, 110054, India 3 Center for Solar Energy, Indian Institute of Technology Jodhpur, Rajasthan, 342011, India *[email protected]

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Zinc oxide (ZnO) and zinc rich zinc oxide (Zn:ZnO) thin films are synthesized using vapor

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phase trapping assisted thermal chemical vapor deposition (CVD) process. X-ray diffraction measurements indicate the highly textured c-axis growth for Zn:ZnO thin films as compared

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to ZnO thin film structures. The observed insignificant change in lattice parameters suggests that excess Zn is in interstitial sites. The optical studies substantiate that the excess Zn in ZnO matrix is contributing to the point defects, with integrated luminescence towards the light blue color with respect to that for the pristine ZnO thin film. The electronic carrier

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concentration is about four orders of magnitude higher for Zn:ZnO thin films, causing

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Burstein-Moss shift of ~0.13 eV in Zn:ZnO thin films.

Keywords: Oxide semiconductor; ZnO; Point defects; Photoluminescence; Burstein-Moss shift.

ACCEPTED MANUSCRIPT Metal oxide semiconducting materials are interesting and getting attention in electronic industries because of their wide applications. Zinc Oxide (ZnO) is one of the most important oxides semiconducting materials because of its unique physical properties. ZnO is an n-type wide band gap semiconductor with band gap ~ 3.37 eV at room temperature.1 It is used in

6-7

4-5

, gas sensors

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numerous applications such as light-emitting diodes2, UV diodes3, solar cells

, transparent electronics8 and piezoelectric devices9 etc. The functional properties, such as

the onset of room temperature magnetism and carrier density are governed by the controlled

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manipulations of both intrinsic and extrinsic defects in ZnO.10-13 Thus; the understanding of different point defects in ZnO is required for its potential in different applications. These

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defects have a direct or indirect impact on properties such as doping compensation, minority carrier lifetime, and luminescence efficiency. They also play an important role in the diffusion

mechanisms

involved

in

growth,

processing,

device

performance

and

degradation.14-17 The presence of native point defects, including vacancies, self-interstitials,

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anti-sites, and substitution, may severally affect the materials properties. There are several intrinsic point defects present in ZnO such as oxygen/zinc vacancies and interstitials, respective anti-sites and unintentional presence of foreign elements such as

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hydrogen.18-19 These defects affect the physical properties of zinc oxide and hence, their

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understanding and controlled manipulation are important to achieve the desired physical properties.

14-16

For example, controlling the nature of electrical conductivity, especially the

p-type, is a major issue in ZnO. The small concentrations of native point defects and impurities (down to 10−14 cm−3 or 0.01 ppm) can significantly affect the electrical and optical properties of ZnO semiconductor.15 Transition metal doping in ZnO has been explored extensively for its potential as a dilute magnetic semiconductor.20-21 The direct and wide band gap of ZnO has attracted attention in optoelectronic devices and ultraviolet sensors.2-5 In spite of such potential and enormous efforts, the realization of ZnO based p-n homojunction is still

ACCEPTED MANUSCRIPT a challenge because of controlled defect limitations, especially the p-type dopants, where the intrinsic point defects (donor type) compensate the acceptor dopants.19 The common point defects are oxygen vacancies and zinc interstitials in ZnO, giving rise to the n-type conductivity.20–24 These defects, especially oxygen vacancies, are pre and post synthesis

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process dependent and controlling such defects is difficult. There are both experimental and theoretical studies on oxygen vacancies, and very little is known about zinc interstitials in zinc oxide system.25-28 The ionic radius of zinc is nearly identical to that of an octahedral void

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in zinc oxide and thus, highly probable that zinc interstitials may reside in such voids or at anti-sites. This will rely on the process energetics and lower energetic processes are more

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favorable over the higher energetic process. The formation energy of zinc interstitial defect is ~ 4 eV, which is relatively smaller than zinc antisite formation energy in octahedral voids.29 Thus, the detailed understanding of such defects and their impact on structural, electronic and optical properties is important for any possible application. In this paper, we report the

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synthesis of zinc rich zinc oxide thin films to understand the effect of excess zinc on physical properties of zinc oxide. The studies substantiate that the excess zinc may prefer octahedral voids for interstitial sites, providing electron charge carriers with an effective ~ 0.13 eV

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Burstein-Moss shift in the conduction band. Additionally, the zinc rich zinc oxide thin films

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showed emission near to lighter blue region as compared to pristine zinc oxide, suggesting the zinc interstitial contribution in the emission spectrum. Zinc oxide (ZnO) and zinc rich zinc oxide (Zn:ZnO) thin films are prepared on a very thin layer gold coated (111) oriented n-type silicon substrates using vapor phase trapping assisted process using thermal chemical vapor deposition (CVD) technique.30-31 The silicon substrates (1 cm x 1 cm) are cleaned using standard RCA-1 & 2 cleaning process, followed by nitrogen gas drying.32-33 The cleaned substrates are subjected to the gold thermal evaporation for the deposition of 10 nm thin gold layer as a catalyst for zinc oxide deposition. The thermal

ACCEPTED MANUSCRIPT assisted chemical vapor deposition process is carried out using one high-temperature zone for source material and another low-temperature zone for the substrate, as shown schematically in Figure 1. A small tube is placed with the closed end in higher temperature zone facing upstream and the open end in lower temperature zone. The growth precursor is placed in a

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ceramic boat near the closed end of the smaller tube in high temperature zone. The distance between these two zones is maintained constant ~20 cm during these experiments, keeping

Figure.1.

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substrate near the open end of the smaller tube. Figure 1.

Schematic diagram of a double tube chemical vapour deposition (CVD) method with respective growth

temperature profile

The source material consists of ZnO (99.99%, Alfa Aeser) and Graphite (99.8%, Alfa Aeser)

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in 1:4 weight ratio for zinc oxide thin film deposition, whereas additional zinc powder (99%

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Alfa Aeser) is used for Zn-rich ZnO thin film in 4:1:4 weight ratio of Zn:ZnO:Graphite. Approximately 0.3 g of these mixtures are used in each experiment. The growth system is evacuated up to 10-2 Torr before initiating the deposition process. The source temperature is ramped at 400 °C/hr upto1050 °C and the substrate temperature is ramped at 152 °C/hr upto 450 °C in conjunction with continuous argon gas flow at 70 sccm, maintaining the gas pressure ~ 35 torr. The films are deposited for 80 minutes and further, the films are allowed to cool down to the room temperature. The gold is deposited to act as a

ACCEPTED MANUSCRIPT catalyst for the formation of nanostructures, where the longer deposition duration has resulted in thin film structures. X-ray diffraction (XRD) measurements are carried out on these samples to understand the

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crystallographic properties. The measured X-ray diffractograms are shown in Figures 2 (a & b) for ZnO/Si and Zn:ZnO/Si thin films, respectively. These diffraction patterns can be completely indexed to the hexagonal wurtzite structure (ICDD no. 036-1451), substantiating the phase pure polycrystalline zinc oxide structures. The lattice constant of the samples are

(1)

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1 4 h + hk + k  l =    + (  ) d 3 a c

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measured as

where d is spacing between planes, h, k and l are the Miller indices of the (h k l) plane, a and c are lattice constants of hexagonal unit cell.34 The calculated lattice constants are a=3.183 A° (3.185A°) and c=5.199 A° (5.202 A°) for synthesized ZnO (Zn:ZnO) thin films. These values

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are in agreement to the reported ZnO lattice constants.17 Zn:ZnO thin films show relatively enhanced (002) orientation as compared to that of ZnO thin film. The dislocation line densities δ are estimated using Williamson and Smallman’s formula35-36  = ; where .∗

!" #

is crystallite size35 and strain is estimated as $ =

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 = 



 %!&# 

; where B is the FWHM

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and θ is Bragg’s angle. Further, the texture coefficient of the thin films is calculated as , (ℎ * +) , (ℎ * +) '((ℎ * +) =  0 , (ℎ * +) - . ∑ , (ℎ * +)

(2)

; where, I and I0 are measured and standard intensity, borrowed from the reference ICDD# 036-1451for hexagonal ZnO structure.37 These various calculated crystallographic parameters are summarized in Table 1. The results suggest that Zn:ZnO thin films are relatively better

ACCEPTED MANUSCRIPT textured along (002) as compared to that of ZnO thin films. The excess zinc can either go to the aniti-site or to the interstitial positions. The formation energy of zinc antisite is much larger as compared to that of zinc interstitials, which discards the formation of zinc anti-sites and relatively favors the formation of zinc interstitials. 38 In addition, the larger ionic radii of

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oxygen (1.4 A°) as compared to that of zinc (0.7A°), also favors the zinc interstitials because the smaller radius of zinc in comparison to oxygen. In contrast, excess zinc at antisites should shrink the ZnO lattice, resulting in the reduced lattice parameters. The observations are

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consistent with the reported literature.39 The nearly identical lattice parameters for both ZnO and Zn:ZnO thin films negate the zinc occupancy at anti-sites and strongly favor the

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interstitial sites for excess zinc. The two interstitial sites are possible in zinc oxide (i) tetrahedral and (ii) octahedral voids, where the tetrahedral site is relatively unstable as compared to the octahedral interstitial positions.29 Thus; the excess zinc may occupy the octahedral interstitial positions.

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The crystallographic properties of ZnO thin films are affected under excess zinc conditions, as can be observed from Table I. The dislocation line density has decreased relatively for

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Zn:ZnO/Si thin films with a simultaneous increase in the respective crystallite size.

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TABLE I: Various crystallographic parameters for ZnO/Si and Zn:ZnO/Si thin film Sample

21

FWHM

(degree)

(degree)

a (A°) c (A°)

Crysta

Dislocati

Strain ($ Texture

llite

on

x 10-3)

coefficient

size

density (δ

(nm)

x 1015) (line m-2)

ZnO

34.42

0.258

3.183

5.199

33.68

8.81

3.6

2.4

Zn:ZnO 34.46

0.247

3.185

5.202

35.16

8.01

3.4

3.0

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The surface morphology and elemental compositions are investigated using scanning electron microscope (SEM) and micrographs are summarized in Figure 2 (c & d). The respective

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energy dispersive X-ray (EDX) spectrum is shown as an inset in SEM micrographs. The surface morphology of these thin films suggests that grains are much smaller ~ 1.2 µm for ZnO thin films as compared to that of Zn:ZnO thin films ~ 3.0 µm. The large hexagonal grains, Figure 2 (d), are clearly distinguishable with hexagons pointing upwards as shown in

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inset of figure 2 (d), substantiating the XRD results, where highly textured c-axis oriented

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growth is observed for Zn:ZnO thin films. In contrast, the smaller grains are randomly oriented in ZnO thin films. The observed surface morphologies are nearly uniform across the

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entire surface for both these thin film structures.

Figure 2: X-ray diffractorgram for ZnO (a) and Zn:ZnO (b); Scanning electron microscopic (SEM) and energy dispersive Xray (EDX) measurements in insets for ZnO (c) and Zn:ZnO (d) thin films, in conjunction with zoomed view makring

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The EDX measurements suggest the presence of zinc and oxygen with nearly similar oxygen atomic fraction for both ZnO and Zn:ZnO thin films. However, the relative atomic

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concentration of zinc in ZnO thin film ~ 45.76% is lower than that of 51.76% in Zn:ZnO thin films. The additional zinc, in case of Zn:ZnO thin films, is attributed to the additional zinc

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precursor used for synthesizing these thin films.

Further, room temperature Raman spectroscopic measurements are carried out for these thin

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film structures to understand the microscopic structural evolution. The hexagonal wurtzite zinc oxide belongs to p63mc space group and according to Factor group analysis, Raman modes are represented at gamma point as Γopt= A1 + 2B1 + E1 + 2E2; where B1 mode is Raman silent.

40

A1 and E1 modes are polar phonon modes and can be split into transverse

optical (TO) and longitudinal optical (LO) phonon modes, which are both Raman and

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infrared active. The non-polar E2 modes are Raman active in ZnO.40 A1 and E1 phonon modes are polarized parallel and perpendicular to the c-axis. The measured room temperature

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Raman spectra are shown in Figure 3 (a & b) for ZnO and Zn:ZnO thin films, respectively. A strong Raman mode at 99 cm-1 is observed for both these thin film structures and is assigned

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as E2 (low) mode, which is the characteristic of zinc sub lattice vibrations.41 The E2 (high) mode at 437 cm-1 corresponds to the oxygen sublattice vibrations and is a characteristic band of hexagonal wurtzite zinc oxide.41 Intensity ratio of E2(low) and E2(high) modes (IE2

(low)/

IE2 (high)) is ~1.38 and 1.80 for ZnO and Zn:ZnO thin films. The relative high IE2 (low)/ IE2 (high) ratio for Zn:ZnO thin film substantiates that additional zinc atoms are contributing into the ZnO phonon spectrum. Additionally, a usually forbidden mode at 333 cm-1 is also observed for Zn:ZnO thin film structures, which is attributed to the second order Raman scattering.42 This mode is referred as the zone boundary phonon, appearing at the difference of E2(high)

ACCEPTED MANUSCRIPT and E2 (low) modes wavenumbers and has been observed under resonance conditions, originating from the crystal disturbances.42 This disturbance may be because of the excess zinc present in Zn:ZnO thin films, as this mode has not been observed for the case of ZnO thin films. The broad and low-intensity peaks at 379 cm-1 and 575 cm-1 correspond to A1

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(TO) and A1 (LO) phonon modes.41 These modes are usually observed in ZnO samples with either oxygen vacancy or zinc interstitials defect.41 Thus, the observation of such modes supports the presence of Zn interstitials in Zn:ZnO thin films, as respective modes are not

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seen in case of ZnO thin films. The other peaks at 300 cm-1, 475 cm-1, and 520 cm-1 correspond to the silicon phonon modes, used as substrates to deposit these thin film

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structures. The reported and observed Raman modes are summarized in Table II. Table II: Reported Raman shift 43 and observed Raman shifts in ZnO and Zn:ZnO thin films Observed Raman shift (cm−1)

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Reported Raman shift Observed Raman modes (cm−1)

Present in ZnO

Present in Zn:ZnO

99

99







333

333






378

379






410

-





438

438







A1(LO)

574

575






E1(LO)

590

-





E2(low)

A1(TO) E1(TO)

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E2(high)

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E2(high)-E2(low)

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Figure 3: Raman spectra for ZnO (a) and Zn:ZnO (b); Photoluminescence spectra for ZnO and Zn:ZnO (c) and Color

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calculator spectra for ZnO and Zn:ZnO (d)

Room temperature photoluminescence (PL) spectra are shown in Figure 3 (c) for both the

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samples. We observed two emission bands, one sharp band in UV region near 375 nm and another broad band in the visible region near 500 nm. ZnO thin film shows a strong peak at 376.66 nm and Zn:ZnO shows a sharp peak at 379.77 nm. These near band edge emissions (NBEs) are due to recombination of free exciton and exciton-exciton collision process.44 The NBE gap values are nearly identical ~ 3.28 eV ± 0.02 eV for ZnO and Zn:ZnO thin films. There is an additional weak emission centered at 500 nm for both samples. This weak emission presumably originates from electron hole recombination mediated by defect levels

ACCEPTED MANUSCRIPT such as oxygen vacancies or zinc interstitials present in zinc oxide.45-49 The relative intensity of visible emission in Zn:ZnO sample is higher in comparison to ZnO peak, suggesting higher intrinsic defect concentrations in Zn:ZnO as compared to that of ZnO. Further, to connect the luminescence properties to fluorescent characteristics, PL spectrum is transposed

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to the CIE- 1931 standard color calculator50, Figure 3 (d). The yielded chromatic coordinates for ZnO thin film are (0.164, 0.200), with dark blue color perception, Figure 3(d). The same for Zn:ZnO thin film is (0.162, 0.282), with respective color perception much closer to the

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light blue color. Thus, a considerable shift in chromatic coordinates is observed, suggesting an active contribution of excess zinc interstitials in fluorescent properties of Zn:ZnO thin

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films.

The current-voltage measurements are carried out in the metal-insulator-metal configuration using 1 mm diameter silver contacts at ~ 1 mm separation. The voltage is swept from -5 V to + 5V across electrodes and measured current - voltage (I-V) characteristics are shown in

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Figure 4 (a & b) for ZnO and Zn:ZnO thin films, respectively. The measurements suggest the ohmic contacts and the current measured values are 80 µA and 20 mA for ZnO and Zn:ZnO

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samples at 5 V operating voltage. The observed large change in current for Zn:ZnO thin film is attributed to the presence of interstitial zinc, acting as a shallow donor, providing electrons

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to the conduction band.28

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Figure 4: Current – voltage (I-V) response for ZnO (a) and Zn:ZnO (b); capacitance – voltage (C-V) response for ZnO (c)

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and Zn:ZnO (d).

Capacitance-voltage (C-V) measurements are carried out in top and bottom configuration,

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where top contact is considered on the thin film and bottom contact is considered from the

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highly conducting n-type silicon substrate at 10 kHz. The collected C-V characteristics are shown in Figure 4 (c & d) for ZnO and Zn:ZnO thin films, respectively. The capacitance values at 5 V operating voltage are ~ 3.4x10-7 F and ~ 9x10-6 F for ZnO and Zn:ZnO thin films, respectively.

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Figure 5: Mott- Schottky plots for ZnO (a) and Zn:ZnO (b); (α.E)2 versus energy E plots for ZnO and Zn:ZnO (c) and

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schematic representation of Burstein-Moss shift in Zn:ZnO thin films (d).

The Mott- Schottky plots (1/C2 and voltage) are extracted from the measured capacitancevoltage data and are shown in Figure 5 (a & b) for ZnO and Zn:ZnO thin films, respectively.

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The slope of the Mott-Schottky plots is used to estimate the effective carrier concentration ; <

3: =

asN3 = 4 56678 9 3(>) ?; where, q is the electronic charge, ε0 and εr (=10 for ZnO46) are

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permittivity of vacuum and medium, respectively, A is the contact area and d (1/c2)/d (v) is the slope of the Mott-Schottky curve.51 The nature of the slope suggests that both films are ntype and the estimated carrier concentrations are ~ 7.5x1015 cm-3 and ~ 6x1019 cm-3 for ZnO and Zn:ZnO thin films, respectively. The carriers are four order of magnitude higher for Zn:ZnO thin film with respect to that of ZnO thin film. This observed increase in carrier concentration is attributed to the zinc shallow donors in Zn:ZnO thin films, providing

ACCEPTED MANUSCRIPT additional conduction band electrons. These excess carriers in conduction band may enhance the optical band gap, causing Burstein-Moss (BM) shift, which is a consequence of shifting quasi Fermi level into the conduction band of the semiconductor. The energy band gap ΔEB = :

(∆Eg) C

DE F∗

is

related

to

carrier

concentration

n

as

= (3Π n) ;; where m* is the electron reduced effective mass and n is the I

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widening

carrier concentration and h is the Planck constant.52 Considering the effective mass of

electron in conduction band as m*= 0.38m0, the change in intrinsic band gap i.e. BM shift i.e.

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∆Eg is ~ 0.13 eV for Zn:ZnO thin films. The observed BM shift suggests that the optical band

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gap is ~ 0.13 eV larger for Zn:ZnO thin films as compared to that of ZnO thin films. The band gap calculated from UV-vis spectra, as plotted in the form of (α.E)2 versus energy E, supports that the band gap values are 3.17 eV and 3.27 eV for ZnO and Zn:ZnO thin films, Figure 5 (c). This substantiates the BM shift calculated using the carrier densities in these thin films. The corresponding schematic illustration of BM shift is shown in Figure 5 (d) for

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Zn:ZnO thin films along with band gap for intrinsic ZnO thin films. In summary, ZnO and Zn:ZnO films have been synthesized by vapor phase transport method

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by using a thermal CVD instrument. XRD measurements revealed that Zn:ZnO films are more crystalline and highly c-axis textured. The higher zinc atomic percentage in Zn:ZnO

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with respect to ZnO substantiates the excess zinc in Zn:ZnO thin films at interstitial sites. CIE 1931 color perception suggests that excess zinc interstitial defect energy levels are shifting the emission towards light blue region for Zn:ZnO thin films. The electrical studies suggest the large contribution of carriers in Zn:ZnO thin films, causing addition Burstein-Moss shift of ~ 0.13 eV. The studies provide the contribution of Zn interstitial defects and their contribution to light emission, useful for manipulating electrical properties of ZnO thin films for probable applications.

ACCEPTED MANUSCRIPT Acknowledgement: Author Ambesh Dixit acknowledges the financial assistance from the Department of Science and Technology (DST), Government of India through the project # DST/INT/ISR/P-12/2014

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1. Evidenced the presence of excess Zn as an interstitials in Zn rich ZnO thin films; 2. Active Zn interstitial induced electronic band gap variation;

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4. Zn rich ZnO thin films as effective light blue emitter.

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3. Burstein-Moss shift in Zn rich ZnO thin films due to Zn interstitials and