High efficiency NiO/ZnO heterojunction UV photodiode by sol–gel processing

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www.rsc.org/materialsC

0959-9428(2010)20:1;1-A

ISSN 2050-7526

Materials for optical and electronic devices

Journal ofMaterials Chemistry Cwww.rsc.org/MaterialsC Volume 1 | Number 1 | January 2013 | Pages 0000–0000

Journal ofMaterials Chemistry C

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This article can be cited before page numbers have been issued, to do this please use: N. Park, K. Sun, Z. Sun, Y. Jing and D.Wang, J. Mater. Chem. C, 2013, DOI: 10.1039/C3TC31444H.

http://dx.doi.org/10.1039/c3tc31444h

http://pubs.rsc.org/en/journals/journal/TC

Park Page 1

Journal of Materials Chemistry C

High Efficiency NiO/ZnO Heterojuction UV Photodiode by Sol-Gel 1

Processing 2

Namseok Park1, Ke Sun

1, Zhelin Sun

1, Yi Jing

1, Deli Wang

1,2,3,* 3

1Department of Electrical and Computer Engineering,

2Material Science Program,

3Qualcomm 4

Institute, University of California - San Diego, 9500 Gilamn Drive, La Jolla, CA 92093, USA 5

6

7

Page 1 of 18 Journal of Materials Chemistry C

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View Article OnlineDOI: 10.1039/C3TC31444H


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Abstract 1

We studied the thin film heterojunction photodiode made of nickel oxide (NiO) and zinc oxide 2

(ZnO) deposited by low cost energy-efficient sol-gel spin coating. The highly visible-transparent 3

heterojuction photodiode with smooth interface gives rise to a good photoresponse and quantum 4

efficiency under the ultra-violet (UV) light illumination. With an applied reverse bias of 5V, very 5

impressive peak photo responsivity of 21.8 A/W and external quantum efficiency (EQE) 88% at 6

an incident light wavelength of 310 nm were accomplished. 7

8

Table of Contents 9

10

We report a highly sensitive NiO/ZnO heterojunction UV photodiode fabricated by a simple and 11

low cost sol-gel process. 12

13

Keywords 14

Transition-metal oxides, Heterojunction, photodiode, sol-gel, NiO, ZnO 15

16

As a new class of materials, the transition-metal oxides have attracted a lot of research 17

attention due to their unique properties and device applications as alternatives to silicon based 18

devices.1,2

As a result of progress in material synthesis and in-depth understanding of material 19

properties, a number of research breakthroughs have been made in this important area of 20

Page 2 of 18Journal of Materials Chemistry C

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research. For example, Tsukazaki et al. created an oxide heterojunction thin film device made of 1

ZnO and MgxZn1-xO which exhibited high carrier mobility and quantum Hall effect.3 It was a 2

ground breaking work that demonstrated heterostructures formed from two different transition-3

metal oxides exhibit surprisingly comparable properties as in Si or compounds of 4

semiconductors. Because of their emerging functionalities, Since then, significant efforts from 5

many research groups around the world have been made in developing thin film devices from 6

different transition-metal oxides and for different emerging functionalities4,5

, however, it is yet to 7

be achieved the reproducible and high-quality heterostructure thin film devices mainly due to the 8

defects at the interface. 9

Among the transition-metal oxides studied, ZnO and NiO thin films are of particular 10

interests. ZnO, an intrinsic n-type wide band gap semiconductor, is a widely studied transition-11

metal oxide material which has a wide range of applications such as chemical sensors6, memory 12

devices7, thin film transistors

8,9, solar cells

10, and piezoelectric actuators

11,12, etc. On the other 13

hand, a naturally acting as a p-type semiconductor with band gap of 3.7 eV, NiO also finds a 14

variety of applications; transparent conducting oxides13

, electrochromic devices14

, chemical 15

sensors15

, and hole transport layers16

. The pn NiO/ZnO heterojunction thin film devices are one 16

of the promising candidates for optoelectronic and photovoltaic device applications. Indeed, a 17

number of p-NiO/n-ZnO heterojunction based optoelectronic devices such as ultraviolet 18

detectors17,18,19

and light-emitting diodes20

have been realized using aforementioned growth 19

technologies. Mostly, ZnO and NiO thin films are grown or deposited via chemical vapor 20

deposition (CVD)21,22

, pulsed laser deposition (PLD)23,24

, or sputter deposition13,25

, which 21

requires high vacuum and is energy intensive and expensive. The device applications of thin film 22

transition-metal oxides have mainly relied on thin film deposition technologies which require 23


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expensive and energy intensive vacuum equipment in order to achieve a high quality interface of 1

the junction. However, there is an increasing need for and high throughput thin film deposition 2

methods and systems. There are recent strong interests in fabricating oxide thin films using non-3

vacuum process such as spray pyrolysis26

and sol-gel methods 27

. In this work, we present a 4

NiO/ZnO pn heterojunction UV photodiode prepared by a cost-effective, large-area deposition 5

technique of sol-gel spin coating. The active junction area is composed of a stack of transparent 6

thin layers of ZnO and NiO on ITO coated glass substrates. The morphology, optical 7

transmission, and electrical properties of the ZnO and NiO layers and ZnO/NiO junction are 8

characterized. The UV photodetector performance was studied and demonstrated high 9

responsivity. 10

11

Figure 1. (a) SEM and (b, c) AFM images of a sol-gel NiO thin film prepared by 3 times spin-12

coatings on glass substrates. (d) SEM and (e, f) AFM images of a sol-gel ZnO thin film by 3 13

times spin-coatings on glass substrates. 14


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Thorough study of thin film coating was performed and the thin film quality and 1

thickness was found to dependent on spin-coating rate, number of coating, and annealing 2

temperature, etc. Film quality remains with multiple coating and almost linear dependence of 3

film thickness vs numbers of spin-coating was observed (not shown). In order to achieve certain 4

thickness and eliminate potential pin-hole defects, ZnO and NiO thin films were coated for three 5

times for the pn junction devices. Figure 1 shows Top view SEM images and AFM images of 6

NiO and ZnO thin films with 3 time spin-coatings. Both NiO and ZnO films exhibit smooth and 7

pinhole free surface in the large scale, and microscopically show a polycrytalline nature with 8

nanoscale grains. However, the grain size of NiO thin film from SEM image is found to be 9

around 25 nm (Figure 1a), which is much smaller than that of ZnO (about 100 nm) (Figure 1d), 10

and moreover, NiO thin film exhibits denser packing of the nanoparticles with less spaces 11

between grains and the surface is smoother. A typical 3 times coating NiO thin film has an 12

average thickness of around 140 nm, while ZnO thin film is about 90 nm thick, measured from 13

cross section SEM (not shown). AFM images with a scanned area of 1 µm2 manifest the 14

relatively surface smoothness of NiO and ZnO thin films. The measured root mean square 15

roughness of NiO and ZnO is about 1.5 nm (Figures 1b, c) and 4.2 nm (Figures 1e, f), 16

respectively. It is imperative that the surface of thin films be smooth as well as pin-hole defect 17

free for abrupt junction interface. Through this prism, the nanoscale surface roughness of both 18

films is favorable in device applications. 19


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1

Figure 2. (a) Optical transmission spectra of NiO and ZnO thin films. Inset shows the estimation 2

of the direct energy band gap (Eg) for sol-gel NiO and ZnO thin films, respectively. (b) and (c) 3

optical images of the NiO and ZnO thin films, respectively. 4

Figure 2 shows the optical properties of sol-gel processed NiO and ZnO thin films. The 5

NiO thin film (3-time spin-coating and with a thickness of 140 nm) has a transmittance of 6

approximately 80% over the wavelength of 380 nm – 1000 nm (onset absorption at 310 nm), 7

while the ZnO thin film (90 nm thick with 3-time coatings) appears to be highly transparent with 8

an average of >90 % in the wavelength of 400 nm – 1000 nm (onset absorption at 365 nm), 9

(Figure 2a). The inset shows the optical band gaps (Eg) of sol-gel thin films extrapolated using 10

Tauc plot, which are ~ 3.7 eV and ~ 3.3 eV for NiO and ZnO, respectively. Figures 2b and c are 11

photographs of the highly transparent NiO and ZnO thin films on a piece of paper where letters 12

underneath the films are very clearly seen. Note that letters underneath the ZnO thin film is 13

slightly clearer than that underneath NiO film, which agrees well with the transmittance 14

measurements in Figure 2a. 15


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The effect of thin film thickness on the resistance of sol-gel NiO and ZnO films is shown 1

in Figure 3. Three co-planar and equidistant ITO and Al contacts were deposited on each film by 2

3

Figure 3. Current-Voltage (I-V) characteristics of NiO (a) and ZnO (b) thin films with different 4

film thickness. Inset in (b) shows magnified I-V characteristics of single-layer ZnO thin film. (c) 5

NiO and (d) ZnO thin films with 3-time coatings in dark and under illumination. 6

sputtering and electron beam evaporation, respectively. Sputtered ITO is proven to form a good 7

ohmic to NiO thin films, as shown by the linear I-V in Figure 3a, while the e-beam Al forms 8

reasonably good ohmic contacts to ZnO films, as shown by the linear I-V characteristic of the 9

ZnO thin films with different thickness (numbers of coating) (Figure 3b). Albeit there exists no 10

consistent linear relationship between resistivity change and thickness change, it is evident that 11

increase in film thickness results in an increase in film conductance. As the thickness of NiO 12

increased from 50 nm (1-layer NiO) to 140 nm (3-layer NiO), the current measured at an applied 13

10V increases four times. Similar increase in conductance was observed for ZnO thin films with 14

2 and 3 times coating, while the 1-time coating thin film shows much more resistive as shown in 15


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Figure 3b with a magnification factor of 100. The reason is not totally clear at this point, which is 1

possibly due to the depletion of carriers when the ZnO film is very thin. The resistivity of thin 2

films increases with the decrease of thin film thickness, which is in agreement with reported 3

results28

. The 90 nm ZnO thin film is about 100 to 125 times more conducting than the 140nm 4

NiO thin film, which is probably due to high carrier concentration and mobility (electrons) in 5

ZnO than those of the holes in NiO, although both films are too resistive to have reliable Hall 6

measurements. The NiO nanoparticles have much smaller grain sizes (Figure 1) and thus much 7

larger surface area and much more surface trap states. Note that both of the I-V characteristics of 8

ZnO and NiO thin films were measured in dark, due to the photoresponses as shown in Figure 9

3c, d. 10

Figure 3c, d shows the I-V characteristics of ZnO and NiO thin film as photoconductors 11

with an metal-semiconductor-metal (MSM) configuration, where two metal pads (ITO on NiO 12

and Al on ZnO) are separated for about 450 um from each other and light is illuminated from the 13

top (metal contact side) using Xenon lamp. Both NiO and ZnO are prepared by 3-time coating. 14

Both oxide thin films show photoconductor behavior, while current increase due to 15

photogenerated carriers is about 5% and 100% for NiO and ZnO thin films at +10V, 16

respectively. The very small photocurrent in NiO thin film maybe due to recombination at the 17

surface trap states whose number is much larger compared to ZnO thin film because of the much 18

smaller NiO nanoparticle grain sizes and larger surface area (Figure 1). 19

20


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1

Figure 4. (a – b) Top-view low and high magnification SEM images of sol-gel ZnO thin films 2

deposited on top of NiO. Cross-sectional SEM images of (c) NiO thin film spin-coated for 3 3

times on ITO bottom contact and (d) a stack of layers of ZnO/NiO heterojunction on ITO, both 4

ZnO and NiO are spin-coated for 3 times. (e) Optical transmission spectrum of ZnO/NiO 5

heterojunction diode on ITO glass. 6

The NiO/ZnO heterojunction coated on ITO glass substrates by multiple spin-coating 7

steps and stacking ZnO on the top of NiO layer showed excellent surface smoothness and large 8

area uniformity, as shown in Figures 4a-b by the top view SEM images of ZnO surface with 9

different, from low to high, magnifications. The grain sizes and morphology of ZnO thin film 10

coated on NiO film (Figure 4a, b) are similar to that of the ZnO thin films directly coated on bare 11

glass (Figure 1d), indicating reproducible, robust, and substrate independent sol-gel process. 12

Figures 4c, d compares the cross section of a NiO thin film (3-time spin-caoting) and NiO/ZnO 13

heterojunction (both 3-time coating). A very smooth interface between NiO and ZnO is clearly 14

identified, which supported the feasibility, capacity, and robustness of low cost large area sol-gel 15


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process for the formation of heterostructure and functional devices. The transmittance spectrum 1

of the NiO/ZnO heterojunction diode shows good transparency of > 80% over the visible to IR 2

range (500 nm -1000 nm). At short wavelength below 365 nm the pn junction absorbs at least 3

80% of incident light, which agree very well with optical absorption and band gap results 4

measured from individual thin films on bare glass (Figure 2a and inset). 5

Figure 5a shows the rectifying I-V characteristics of a typical NiO/ZnO heterojunction 6

device in dark. The pn diode exhibits a rectification ratio of ~50 (at +/- 5V bias) in the dark. 7

Figure 5b shows the photocurrent and dark current under different biasing voltages. The inset 8

shows the schematic representation of the device under top illumination. An ON/OFF (Iphoto/Idark) 9

ratio of about 100 was achieved at a reverse bias of -5V. Note from Figure 5b that there is no 10

photovoltaic effect observed, which is most likely due to no direct light illumination to the 11

junction area during testing, as discussed in the following paragraph. The nanocrystalline nature 12

13

Figure 5. (a) Rectifying I-V characteristics for the Al/n-ZnO/p-NiO/ITO heterojunction on glass. 14

A picture of n-region ZnO and Al contact is included as an inset. (b) I-V characteristics of the 15

photodetector measured in the dark (black dots) and under AM 1.5 illumination (red triangles). 16

Inset illustrates a schematic cross section layout of the heterojunction photodetector on glass 17

substrates with light illumination from the top (ZnO side). (c) Responsivity (left y-axis) versus 18


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wavelength and calculated detectivity (right y-axis) at a reverse bias of -5V. Inset shows EQE of 1

the heterojunction photodiode vs wavelength. 2

of NiO and ZnO layers and lack of sharp junction interface might also have some effects. Note 3

that in our measurements (Figure 5b inset), the incident light was illuminated from the top of the 4

photodiode, which means the incident light was shined to ZnO layer first and more importantly 5

the majority region of pn junction is not illuminated with light, with only small area at the edge 6

are under illumination by diffusion and diffraction of light. The fact that only very small portion 7

of the pn junction is under direct light illumination maybe the reason causing no photovoltaic 8

effect observed in the NiO/ZnO device measurement. Apparently, light illumination from the 9

back-side (NiO side of the junction) was favorable from a perspective of device physics, NiO has 10

wider band gap than ZnO, and more importantly the entire pn junction will be illuminated under 11

light. However, we are limited by the device structure, Al/ZnO/NiO/ITO on glass, where UV 12

light illumination from the glass side is not possible. Current on-going efforts are making devices 13

on quartz substrate, which is transparent to UV light. The illumination from NiO side possibly 14

renders much larger photoresponse and quantum efficiency not only by allowing more efficient 15

light absorption by junction region and within both NiO and ZnO, but also through increased 16

light absorption by taking advantage of top Al contact as a reflection mirror (Al reflects UV very 17

well29

). An alternative approach will be fabricating devices with reserved structure, such as 18

ITO/NiO/ZnO/Al/glass, which, however, is less desirable due to (i) potential Al2O3 formation 19

between Al and ZnO during sol-gel annealing, (ii) possible poorer morphology and film quality 20

because in this case NiO layer is coated on ZnO film and the latter has much larger grain size 21

and porosity (Figure 1). 22

Figure 5c shows the spectral response of a typical NiO/ZnO device under reverse bias of -23


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5V, which measures the photoelectric sensitivity to incident light energy with the wavelength of 1

interest from 300 nm to 450 nm. The responsivity (Rλ in A/W) measures the ratio of 2

photocurrent to incident-light intensity at certain wavelengths, which was estimated by 3

comparing the photocurrent of the NiO/ZnO device to that of a Si photodetector (Newport 818-4

UV photodetector), assuming an identical illumination 5

Rλ = Jph / Llight = Rλ−SiPD ∗ Jph-NiO/ZnO / Jph-SiPD (1) 6

where Jph is the photocurrent and Llight is the incident light intensity, Rλ−SiPD is the 7

photoresponsivity of calibrated 818-UV photodetector, and Jph-NiO/ZnO and Jph-SiPD are the 8

photocurrent densities measured from the NiO/ZnO device and the Si photodetector reference, 9

respectively. A series of specifications for the measurement is available on our previous work30

. 10

NiO/ZnO devices show excellent UV sensitive visible blind photodetection, with the UV-to-11

visible rejection ratio (R310 nm/R450 nm) of 1,600 (Figure 6c). There are two peaks of responsivity 12

at 310 nm and 364 nm, which are associated with absorption edges of p-NiO and n-ZnO thin 13

films, respectively. The photoresponsivity peaks at 310 nm and with a value of 21.8 A/W. Note 14

that the photoresponsivity at 310 nm is about 100 times of that at 364 nm, which is due to the 15

fact that during our measurement the incident light is from the ZnO side, both ZnO and NiO 16

absorb light at 310 nm but only ZnO absorb light at 364 nm which generate carriers and 17

contribute to the photocurrent. The fact that the photoresponsivity at 364 nm is so much smaller 18

compared to that at 310 nm possibly indicates a very thin effective absorption in ZnO 19

considering carrier concentration might be higher in ZnO and thus smaller depletion and 20

minority diffusion length (Figures 3 and 4). 21

The right y-axis in Figure 5c shows the specific detectivity, which was estimated assuming 22

dark current is the major source of shot noise for the photodiode under a reverse bias of -5V and 23


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using equation (2)31

1

D* = Rλ / (2qJdark)1/2

(2) 2

where R� is the responsivity, q is the elementary charge of a single electron, and Jdark is the dark 3

current at a given reverse bias. The specific detectivity at 310 nm is 1.6 x 1012

cmHz1/2

W-1

or 4

Jones. The external quantum efficiency (EQE) of the heterostructure PD can be also estimated 5

using equation (3) 6

EQE = (Rλ/λ) ∗1240 (nm·W/A) ∗ 100 % (3) 7

where R� is the photo responsivity in A/W at a given wavelength of incident light and � is the 8

wavelength in nm. The calculated EQE is shown in Figure 5c inset with a maximum value of 9

88% at 310 nm. 10

Conclusion 11

The thin film p-NiO/n-ZnO heterojunction photodiode which is formed between two 12

wide-band gap transition-metal oxides has been fabricated using a simple and inexpensive 13

method of sol-gel spin-coating. In dark condition, the p-n heterojunction photodiode exhibits a 14

good rectifying behavior at room temperature. Under a reverse bias condition across the junction, 15

the photodiode gives a photoresponse under illumination. The p-n heterostrucutre operated in the 16

wavelength range of 300 nm to 450 nm gives an exceptional performance at UV region. The 17

general characteristics of the heterojunction photodiode reveal a distinct photoresponse with a 18

very impressive maximum responsivity of 21.8 A/W and equivalent external quantum efficiency 19

of 88% at 310 nm. Our results demonstrated that the high responsivity heterojunction 20

photodiodes, and other functional electronic and optoelectronic devices, based on transition 21

metal oxides can be realized by a simple and cost effective sol-gel thin film deposition technique. 22

23


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Experimental Methods 1

Precursor Synthesis and Thin Film Deposition: Zinc acetate as a starting material for sol-gel 2

ZnO was dissolved in ethanol with existence of monoethanolamine (MEA). The precursor 3

solution of sol-gel NiO was prepared by dissolving Nickel (II) acetate tetrahydrate in a mixture 4

of Dimethylaminoethanol (dmaeH) and MEA. The dmaeH was chosen as a Nickel (II) acetate 5

solvent and MEA was added as a stabilizer. Further refluxing at 70 °C for 2 hrs rendered them 6

clear or dark green solution for ZnO and NiO respectively. The precursor solution was filtered 7

through a syringe filter (Acrodisc®

syringe filter with a 0.2um membrane) and then aged for 24 8

hrs for use. 9

For thin film deposition, the prepared precursor solution was spin-coated (i.e. 40 sec at 3000 10

rpm) on clean glass substrates at different spin speeds and durations. In order to evaporate 11

solvents, the coated film was dried at 300 °C after each spin coating. And multiple coatings were 12

repeated to reach a certain thickness of thin films. Finally spin-coated thin films were annealed in 13

N2 and O2 atmosphere (450 °C) to improve crystallization. 14

ZnO/NiO Heterojuction Device Fabrication: The p-NiO/n-ZnO heterojunction was fabricated on 15

ITO coated glass substrates in cleanroom environment. A stack of layers of NiO/ZnO thin films 16

was made by spin-coating multiple layers of NiO first on top of an ITO/glass substrate. 17

Subsequently, a 90 nm thick ZnO top layer was deposited by repeating successive layer on layer 18

spin coatings. This as-coated junction was then annealed in N2/O2 environment at 450 °C for an 19

hour. In order to complete the device fabrication, Al top contacts were thermally deposited 20

through a polymer photomask and then lifted off. 21

ZnO and NiO Thin Films and ZnO/NiO Device Characterization: The study on surface 22

morphology and cross-sectional structure of thin films and thin film junction was performed by 23


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using a high-resolution scanning electron microscope (FEI XL30-SFEG) and atomic force 1

microscopy (Veeco Scanning Probe Microscope). The Agilent B1500 semiconductor analyzer 2

was used to characterize current-voltage (I-V) characteristics of NiO and ZnO thin films and 3

diode. The optical transmittance spectra and device spectral response measurements were made 4

using a setup of the monochromator and lock-in amplifier equipped with a 150W Xenon lamp as 5

a light source. 6

7

Acknowledgements 8

DW acknowledges the financial support from the National Science Foundation (NSF 9

CBET1236155 and ECCS0901113) and Qualcomm Institute (QI). DW acknowledge Drs. R. Rao 10

and B. Fruhberger of QI and NANO3 facility (UCSD) for their generous support. 11

12

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High efficiency NiO/ZnO heterojunction UV photodiode by sol–gel processing

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