strain-modulation and service behavior of au-mgo-zno uv ... · nano res.doi 1 strain-modulation and...

Nano Res

1

Strain-modulation and service behavior of

Au-MgO-ZnO UV photodetector by piezo-phototronic

effect

Qingliang Liao1, Mengyuan Liang1, Zheng Zhang1, Guangjie Zhang1, and Yue Zhang1, 2 ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0876-x

http://www.thenanoresearch.com on August 7, 2015

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Au-MgO-ZnO UV photodetectors by piezo-phototronic

effect

Qingliang Liao1, §, Mengyuan Liang1, §, Zheng Zhang1,

Guangjie Zhang1, and Yue Zhang1, 2,*

1 Department of Materials Physics and Chemistry, State

Key Laboratory for Advanced Metals and Materials,

University of Science and Technology Beijing, Beijing

100083, China

2 Key Laboratory of New Energy Materials and

Technologies, University of Science and Technology

Beijing, Beijing 100083, China

§These two authors contributed equally to this work.

The sensitivity of the UV photodetectors was enhanced by inserting an

ultrathin insulating layer and the service behavior the UV

photodetectors modulated by external strain were investigated.

Provide the authors’ webside if possible.

Author 1, webside 1

Author 2, webside 2

1 Nano Res.


Au-MgO-ZnO UV photodetector by piezo-phototronic

effect

Qingliang Liao1, §, Mengyuan Liang1, §, Zheng Zhang1, Guangjie Zhang1, and Yue Zhang1, 2 ()

§

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

photodetectors,

ZnO,

the ultrathin insulating

layer,

sensitivity,

piezo-phototronic effect

ABSTRACT

Au-MgO-ZnO (AMZ) ultraviolet (UV) photodetectors were fabricated to

enhance its sensitivities by an inserting ultrathin insulating MgO layer. With the

insulating layer, the sensitivities of the UV photodetectors were improved by

the reducing of the dark current. Furthermore, the strain-modulation was used

to enhance the sensitivities of AMZ UV photodetectors. The sensitivities of the

photodetectors were enhanced by the piezo-phototronic effect. But there was a

limiting value of the applied strains to enhance the sensitivity of the

photodetector. When the external strains exceed the limiting value, the

sensitivity will decrease due to the tunneling dark current. The external stains

loaded on the photodetectors will result in the degradation of photodetectors

and the applied bias may accelerate the process. This work demonstrates a

prospective approach to engineer the performance of a UV photodetector. In

addition, the study on service behavior of the photodetectors may offer a strain

range and theoretical support for safely using and studying

metal-insulator-semiconductor UV photodetectors.

1 Introduction

The development of ultraviolet (UV) photodetectors

for imaging and switching devices applying in the

invisible wavelength region is an important research

issue for medical and military applications [1-3]. The

key requirements in designing these UV

photodetectors are selectivity, high sensitivity,

responsivity, speed, and low-noise. Recently, this

Nano Research

DOI (automatically inserted by the publisher)

Research Article

Address correspondence to [email protected].

mailto:[email protected]

| www.editorialmanager.com/nare/default.asp

2 Nano Res.

research field is drawing more and more attentions

to improve these properties through using different

device structures or materials systems [4-6]. When

these photodetectors work, the environmental factors

would affect their properties. The environmental

factors such as temperature, strain, pH value may

have effect on the performance of photodetectors. As

an effective modulation method for UV

photodetector, the external strains have been

investigated extensively and the performance of the

UV photodetectors can be enhanced by the

piezo-phototronic effect [7-10]. Most of the

environmental factors were used to enhance the

properties of UV photodetectors. However, some

factors may have a valid values range to achieve the

purpose. And there were few reports on the service

behavior about the values in a particular type of UV

photodetectors like mental-insulator-semiconductor

(MIS) UV photodetectors. The significance of the

investigation on the service behavior is that it can

provide the designers of a particular type of

photodetectors with operating parameters and

theoretical support. This can be used to evaluate the

mechanical reliability of photodetectors and predict

the safe working conditions of photodetectors.

ZnO is recognized as promising building blocks for

future electronic, optoelectronic and

electromechanical devices because of its coupled

piezoelectric and semiconducting properties [11-14].

Since ZnO has a noncentral symmetric wurtzite

structure, a strain along c-axis on the basic unit can

cause a polarization. It results in the piezopotential

inside the crystal [15-17], which would affect its

properties. ZnO has diverse nanostructural

morphologies [18, 19], among which one dimensional

semiconducting nanostructure becomes a vital

building block in UV photodetectors. This is because

it has high surface to volume ratio and quantum size

effect.

In this study, an Au-MgO-ZnO (AMZ) UV

photodetector by inserting an ultrathin insulating

MgO layer between the Au electrode and ZnO

nanowire (NW) arrays is reported. The sensitivity of

the photodetector is improved by the insulating layer

[20, 21]. A detailed explanation for the improvement

will be given. Furthermore, the value range of

applied strains to improve the sensitivity of the

photodetector is investigated. The minimum limiting

value of strain to reduce the sensitivity will be given.

The reason for it is that the dark current of the

photodetectors can be modulated by the strains.

These studies will be helpful for promoting the

development of the MIS UV photodetectors made of

ZnO NW arrays and predicting their reliability.

2 Experimental

Fig. 1 schematically illustrates the key steps for

fabricating the AMZ UV photodetectors. Firstly,

100nm thick Au films were deposited on the FTO

glass substrates (15mm×10mm) using DC sputtering.

The Au films served as electrodes. Secondly, the

bottom electrodes were protected with thermal tape

and about 10 nm thick MgO layers were deposited

on Au films by atom layer deposition (ALD)

(PEALD-200A) with Mg(Cp)2 and H2O under 150℃.

Thirdly, the top electrodes were protected by thermal

tape. Then ZnO NW arrays were synthesized by

using the hydrothermal method on the AZO glass

substrates (15mm×7mm) [22]. At last, the AZO glass

substrates with ZnO NW arrays were inverted on the

FTO glass substrates with Au electrodes and MgO

layers. Simultaneously, the top electrodes and the

bottom electrodes were staggering placed. To

illustrate the effect of the insulating MgO layer, an

Au-ZnO (AZ) structured junction was fabricated to

compare with the MIS one. To maintain a stable and

reliable contact between the FTO glass part and ZnO

NW arrays, all the fabricated devices were wrapped

by tapes and fully packaged with

Polydimethylsiloxane (PDMS). The morphology and

structure of the ZnO NWs were characterized by

scanning electron microscope (FESEM, LEO1530) and

X-ray diffraction (XRD, Rigaku DMAX-RB, Japan).

The fabricated photodetectors were fixed on a

designed cantilever system. The system could apply

axial strains to the ZnO NRAs with accurately

controlled amplitudes. The electrical transport

properties of photodetectors were measured by a

semiconductor characterization system

(Keithley4200-SCS). To characterize their

photosensitive properties, a portable UV lamp

(365nm, 1mW/cm2) was used as the light source.


3 Nano Res.

Figure 1 Schematic diagram of the steps for the

fabrication of a MIS UV photodetector.

3 Results and discussion

The top-view and side-view FESEM images of the

as-synthesized ZnO NW arrays are presented in Fig

2a and 2b, respectively. It can be seen that the ZnO

NW arrays are highly dense and grow

vertically-aligned. The length of the ZnO NW arrays

is about 2µ m in average. Fig 2c shows the XRD

pattern of ZnO NW arrays. The peaks center at 31.6°

and 34.3° correspond to the diffraction plane (100)

and the diffraction plane (002) in ZnO with the

hexagonal wurtzite, respectively. However, (100)

diffraction peak can be almost negligible compared

with (002) diffraction peak. The high intensity (002)

diffraction peak indicates that the ZnO NW arrays

were highly oriented and had good crystallinity. The

fabricated ZnO NW arrays are good for fabricating

the MIS UV photodetectors.

Figure 2 Morphologies and structure characterization

of the highly dense ZnO NW arrays. (a) The top-view

SEM image. (b) The side-view SEM image. (c) The

XRD pattern.

The response properties of the photodetectors are

tested. In Fig 3a, the blue and red lines are the I-V

curves of the AZ and AMZ photodetectors under

dark, respectively. The I-V curves show that Schottky

contacts are formed between the Au electrodes and

ZnO NW arrays. At an applied bias of -1 V, the dark

current of the AMZ photodetector is 0.41µA and the

dark current of the AZ photodetector is 85µA. The

insulating MgO layer decreases the dark current by 2

orders of magnitude compared to the AZ

photodetector. The insert of Fig 3a is the I-V curve of

the AMZ UV photodetector under dark. The device

exhibits an excellent rectifying behavior. Fig 3b and

3c show I-V curves of AZ and AMZ photodetectors.

The black and red lines are I-V curves under dark

and illumination, respectively. At bias of -1V, the

sensitivity (photo-to-dark current ratio) of AZ and

AMZ photodetectors are 4.25 and 197, respectively.

Fig 3d represents the I-t curves of the AZ and AMZ

photodetectors at bias of -3V. The red and dark lines

are the curves of the AZ photodetector and the AMZ

photodetector, respectively. Compared with the AZ

photodetector, a relatively low dark current of 0.41

µA is obtained and the photocurrent is about 92 µA

on the AMZ photodetector .The sensitivity of the

AMZ photodetector is found to be about 224 while

the AZ photodetector with a sensitivity of 3. So the

insulating MgO layer reduces the dark current and

enhances the sensitivity of the device.

4 Nano Res.

Figure 3 (a) I-V characteristics contrast under dark of the MS UV photodetector and the MIS photodetector. The

inset in (a) is I-V curve of the MIS photodetector. (b) I-V curves of the MS photodetector under dark and

illumination. (c)I-V curves of the MIS photodetector under dark and illumination. (d) I-t characteristics contrast

of the MS UV photodetector and the MIS photodetector.

The schematic illustration of the UV photodetectors

with sensitivities is shown in Fig 4. Here the

mechanism for the reduction of the dark current by

inserting the MgO layer is explained. Fig 4a

represents the energy band diagram of the junction

with a forward bias (a positive bias is applied on the

Au electrode) under dark. In Fig 4a, when electrons

flow into the Au electrode through ZnO, they store

enough energy due to the applied electric field. Then

the electrons can easily tunnel through the ultrathin

insulating MgO layer and results in a large dark

current. But because of the blocking of the insulator,

the dark current decreased compared with AZ UV

photodetector. Fig 4b shows the energy band

diagram of the junction with a reverse bias (a

positive bias is applied on the AZO electrode) under

dark. In Fig 4b, electrons flow into the AZO electrode

from the Au electrode and encounter the MgO layer

immediately. Without an accelerating process, most

of the electrons have insufficient energy to tunnel

through MgO layer expect a small part (the green

arrow). The small part mainly composes of electrons

by thermionic emission. As a result, most of the

electrons are blocked on the Au/MgO interface. It

greatly reduces the dark current. Thus, the AMZ

photodetector exhibits a good rectifying behavior in

the dark current and the dark current is smaller. On

UV illumination, the photogenerated electrons are

produced in ZnO NW arrays in large amounts. So the

original existing electrons can be neglected. The

photogenerated electrons will go through an

accelerating process under the reverse bias or the

forward bias. Therefore, all of the photogenerated

electrons have sufficient energy to flow to the

corresponding electrode. As a result, the sensitivity

of the AMZ UV photodetector is enhanced under a

reverse bias compared to the AZ photodetector.

Figure 4 The schematic illustration of the MIS UV

photodetectors with sensitivities. (a) The energy

band diagram of the junction with a forward bias

under dark. The blue arrow is the direction of current

and the green one is the tunneling current. The black

dash dot line is the Fermi level at thermal

equilibrium. (b) The energy band diagram of the

junction with a reverse bias under dark. The green

arrow is electrons by thermionic emission.

To further improve UV sensing property and the


5 Nano Res.

investigate the strains effects on it, I−V characteristic

of the photodetect is measured under different

compressive strains. The result is shown in Fig 5a.

Fig 5b represents how the dark current changes with

the increase of the applied compressive strains at bias

of -3V. These figures in Fig 5a and Fig 5b illustrate

that the dark current decreases first and then

increases along with the applied compressive strains

increasing. At bias of -3V, the sensitivity of the device

is 19.9 under 1.6% strains in contrast to 134.7 under

1.0% strains. The data prove the existence of a range

value from 0% to 1.0% of the applied strains.

Otherwise, there is a limiting value of strains

corresponding to 1.0% [23]. When the applied strains

are over the limiting value, the sensitivity of the

photodetector gradually decreases to the failure of

device.

Figure 5 (a) I-V curves of the MIS UV photodetectors with a series of compressive strains under dark. (b)The

curve about how the dark current changes with the increase of the applied compressive strains at bias of -3V.

Firstly, we explain the mechanism for the first

reduction of the dark current by increasing the

applied compressive strains. Fig 6a shows the energy

band diagram in the junction without a bias or

compressive strains. Fig 6b shows the energy band

diagram in the junction with a reverse bias.

Compared Fig6a and 6b, the reverse bias makes the

width of depletion region (marked as W) wider. The

conduction band bends downward and becomes

sharper near the MgO/ZnO interface. Fig 6c shows

the energy band diagram in the junction with both a

reverse bias and smaller compressive strains,

respectively. The dark current of AMZ UV

photodetectors mainly composes of electrons by

thermionic emission. Under compressive strains, the

photodetector can be regarded as that a negative

potential (Em) is produced on the Au/ZnO interface

by the piezoelectric effect of ZnO. The negative

potential makes the Schottky barrier height (SHB)

increase, as shown in Fig 6c. With strains increasing,

the SHB becomes higher and higher. Because of it,

the thermionic emission electrons gradually reduce

and the dark current becomes smaller and smaller

[24]. Secondly, the mechanism for the later increase

of the dark current with sequentially increasing the

applied compressive strains is explained. The

schematic diagram of the mechanism is illustrated in

Fig 6. Fig 6d represents the energy band diagrams in

the junction with both a reverse bias and bigger

compressive strains. Compared Fig 6b and Fig 6a, the

built-in barrier height (marked as Ψi) increases by

△Φbias after applying a negative bias on the Au

electrode. Meanwhile, W becomes wider. The

conduction band near the MgO/ZnO interface

becomes more downward bending and sharper. Ψi

increases by △Φm again with sequentially applied

compressive strains between Fig 6d and 6b. In

consequence, W became narrower. The conduction


6 Nano Res.

band becomes greater downward bending and

sharper. When the strains are large enough to ensure

a very sharp conduction band, a considerable part of

the previous blocked electrons in Au electrode can

tunnel through MgO and barrier. These tunneling

electrons make the dark current increase.

Figure 6 The schematic diagram of the MIS photodetector with compressive strain under dark. They are the

energy band diagrams in the junction (a) at 0V without applied strains, (b) at a reverse bias without applied

strains and (c) at a reverse bias with insufficient compressive strains, (d) at a reverse bias with enough

compressive strains, respectively. The green bent arrow is electrons by thermionic emission and the straight one

is the tunneling current. Ebias is the applied bias. The black dash dot line is the Fermi level.

On the other hand, a calculation is exhibited to

interpret the increase of the dark current as

mentioned above. As a MIS device with an ultrathin

insulating layer, it has a current at a certain bias.

The current is non-equilibrium, that is to say, the

Fermi energy levels of electron and hole are

separate. There are three main different points

between the MIS devices and the MS devices.

Firstly, the dark current of the MIS device is smaller.

The MIS device has a lower barrier height than the

MS device. This is because of the existence of a

certain voltage drop though the insulator. The MIS

device has a higher ideality factor. The tunneling

current of the MIS device can be described as

following [25]:

* exp exp 1qV

J A BkT

(1)

where J is the dark current density at a fixed bias

(marked as V), A* is the effective Richardson

constant, B is a constant related to temperature, ζ is

the effective barrier, δ (it is measured in Å) is the

thickness of the insulating layer, q is the unit

electronic charge, η is the ideality factor, k is the

Boltzmann constant, and T is the temperature. And

η here can be described as following [25]:

1

1

s its

i i itm

W qD

qD

(2)

where εi is dielectric permittivity, εs is

semiconductor permittivity, WD is the width of

depletion region, Dits is the trap density of the

semiconductor, and Ditm is the trap density of the

metal. From the above two formulas, we can see

that J becomes bigger along with the broadening of

W. The total of the built-in potential can be

described as following [26]: 2

2

Di

s

W N q kTV

q

(3)

where Ψi is the total of the built-in potential, and


7 Nano Res.

ND is donor impurity concentration. As shown in

5c, Ψi is defined as:

i m bi bias (4)

where Ψbi is the built-in potential in the thermal

equilibrium state without compressive strains, △Φm

is the increase of built-in barrier induced by

compressive strains, and △Φbias is the increase of

built-in barrier induced by the applied reverse bias.

When the device is under compressive strains, it

can be regarded as a negative potential producing

on the Au/ZnO interface. The negative potential is

due to the piezoelectric effect of ZnO and makes the

built-in barrier increase by △Φm. The build-in

barrier increases as compressive strains increasing.

Similarly, △Φbias increased as the applied reverse

bias being enhanced. Combining the two formulas,

a new formula can be obtained as following: 2

2

Dm bias bi

s

W N q kTV

q

(5)

where Ψbi can be regarded as a constant without

applied reverse bias and compressive strains. When

△Φm or △Φbias increases, W will be wider. This leads

the tunneling current to increase. Thus, both applied

reverse bias and compressive strains can enhance the

dark current when strains are large enough. To prove

this point, Fig 7a shows I−S (compressive strains)

characteristic of the device measured at different

reverse biases. The compressive strains

corresponding to the turning point of the dark

current are 0.8%, 0.6%, 0.4% at bias of -1V, -2V and

-3V, respectively. According to them, the conclusion

is that the corresponding compressive strains became

smaller as the applied reverse bias increasing. The

S−S (sensitivity − compressive strains) characteristic

of the device at bias of -3V is represented in Fig 7b.

The sensitivity of the device becomes very low with

large enough compressive strains. Finally, the AMZ

UV photodetector degrades and fails.

Figure 7 (a)The curves of the AMZ UV

photodetectors about how the dark current changed

with the increase of the applied compressive strains

(I-S curve) at different biases. (b) The curves of the

AMZ UV photodetectors about how the sensitivity

changed with the increase of the applied compressive

strains (S-S curve) at bias of -3V.

Consequently, the dark current decreases first along

with the increase of applied compressive strains and

the sensitivity of the AMZ UV photodetector is

enhanced. But there is a limiting value of strains.

When the compressive strain is over the limiting

value, it will lead to the performance degradation

and even the failure of the photodetector.

4 Conclusions

In summary, an Au-MgO-ZnO UV photodetector is

successfully fabricated. Inserting the ultrathin

insulating MgO layer is to improve the photo sensing

property of the photodetector. To demonstrate this

point, Au- ZnO UV photodetectors are fabricated.

The performances of the fabricated photodetectors

are characterized and compared. Compared to the

AZ UV photodetectors, the AMZ photodetectors

with inserted ultrathin insulating MgO layers exhibit

good rectifying behaviors under dark and have

higher sensitivities to UV light. The ultrathin

insulator reduces the dark current due to the

selective tunneling current transport. In order to

improve its property, compressive strains are applied

on the AMZ UV photodetector. At first, the increase

of applied compressive strains enhances the SBH.

The increase of SBH leads the dark current to

decrease and sensitivity to increase. The property is

really improved by the piezo-phototronic effect. But

the current increases as strains increasing

continuously. This is because that the tunneling

probability of electrons increases. The increase of the

dark current will lead the sensitivities of

photodetectors decrease. The study demonstrates the

existence of the limiting value of applied


8 Nano Res.

compressive strains. Above the limiting value, the

strains finally make their performance degrade and

fail. These results can offer a particular type of

photodetectors for promoting the photonics or

optoelectronic technology. Simultaneously, data and

theoretical supporting for the study on the safe

working conditions of photodetectors are provided.

It will promote the study on the service and failure of

different photodetectors.

Acknowledgements

This work was supported by the National Major

Research Program of China (No. 2013CB932601), the

Major Project of International Cooperation and

Exchanges (No. 2012DFA50990), the Program of

Introducing Talents of Discipline to Universities

(B14003), National Natural Science Foundation of

China (Nos. 51172022, 51232001, and 51372020), the

Fundamental Research Funds for Central

Universities, State Key Lab of Advanced Metals and

Materials (2014Z-11), and Program for New Century

Excellent Talents in Universities (NCET-12-0777).

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