Electron Bombardment Induced Photoconductivity and High Gain in a

Subscriber access provided by University of South Dakota

Article

Electron Bombardment Induced Photoconductivity and High Gain in a Flat Panel Photodetector Based on ZnS Photoconductor and ZnO Nanowire Field Emitters Zhipeng Zhang, Kai Wang, Keshuang Zheng, Shaozhi Deng, Ningsheng Xu, and Jun Chen ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00949 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Electron Bombardment Induced Photoconductivity and High Gain in a Flat Panel Photodetector Based on ZnS Photoconductor and ZnO Nanowire Field Emitters

Zhipeng Zhang, Kai Wang*, Keshuang Zheng, Shaozhi Deng, Ningsheng Xu, Jun Chen* State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China ABSTRACT Flat panel photodetectors are widely studied and implemented in large area imaging. However, developing highly-sensitive flat panel photodetectors with high internal gain is still very challenging due to material limitation where photoelectron multiplication mechanisms have to be established. In this study, we proposed to use electron bombardment induced photoconductivity (EBIPC) mechanism to achieve high internal gain in a flat panel photodetector based on ZnS photoconductor integrated with ZnO nanowire (NW) field emitters. The photoconductivity of the ZnS thin film increased significantly upon bombardment with electrons from the emitters, which led to an internal photoconductive gain of above 104 in a wide wavelength range. The photoresponse behaviors under different device parameters further verified that the high gain depended on the enhanced light responsive performance of the ZnS thin film induced by the EBIPC mechanism. The proposed EBIPC mechanism is promising for use in flat panel photodetectors achieving high gain.

ACS Paragon Plus Environment

1

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

Keywords Flat panel photodetector; ZnS photoconductor; ZnO nanowire field emitters; electron bombardment induced photoconductivity; high internal gain TOC Graphic

INTRODUCTION Optoelectronic devices, especially flat panel photodetectors, are of particular importance for their promising applications in medical and industrial X-ray imaging.1-4 High sensitivity as a result of high internal gain is desirable. So far, commercial high-gain photodetectors such as photomultiplier tubes (PMT) utilize the photoemission and secondary emission to realize photoelectron multiplication. The PMT uses a photocathode to emit photoelectrons into the vacuum, and then these photoelectrons enter the electron multiplier where electrons are multiplied by a secondary emission process.5,6 These electrons are collected by the anode as an output signal. The PMT has been widely investigated and shows the advantages of excellent


2

Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

photoelectron multiplication gain and ultra-fast response time, preferable for low-level light detection.7,8 Nevertheless, those devices are bulky in size and cannot be pixelated, thus their applications in flat panel detectors face great technical challenges. Nowadays, miniaturized avalanche photodiode (APD) is demonstrated.9,10 When light enters a photodiode, electron-hole pairs (EHPs) are generated in the depletion layer if the light energy is higher than the band gap energy. Generated carriers collide with the crystal lattice and produce new EHPs while being accelerated by high electric field applied to the PN junction. This phenomenon is called ionization.11 Newly generated carriers are also accelerated to produce further EHPs, which generated a chain reaction of ionization. The APD has a high internal avalanche multiplication and can be use in a wide range of applications such as analytical instruments, laser ladars, optical rangefinders and fiber communication.12,13 Though the APD can achieve high internal gain, it is still fail to be used in a pixelated flat panel photodetector due to the drawbacks of high reverse voltage, temperature-dependent gain characteristics and gainbandwidth product limit.14,15 Further, pixelated silicon photomultipliers (SiPM) using Geiger-mode APD arrays are fabricated successfully.16,17 In Geiger-mode operation, the APD is biased above breakdown voltage. A saturated output occurs by input of light regardless of whether the light is high or low. The rapid rise in current is limited lastly by a so-called quenching resistor.18 The SiPM is suitable for photon counting since it has the advantages of excellent time resolution, high gain and low noise.19,20 Unfortunately, the effective area is limited due to the silicon-based microfabrication of SiPM. For instance, Piemonte et al. presented a new high-density cell SiPM for ultraviolet and blue light detection featuring a cell pitch of 15 ~ 30 µm, a peak efficiency of


3

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

40% ~ 55% and a coincidence time resolution of 100 ps, but the active area was less than 4 × 4 mm2, unsuitable for large area imaging.21 In order to achieve large area photodetector with high gain, other efforts have been devoted to utilize novel architectures. The highly photosensitive amorphous silicon and indium–gallium– zinc–oxide (IGZO) thin film transistors (TFT) have been designed.21,22 For example, Zhou et al. fabricated a large area polycrystalline silicon readout TFT integrated with an amorphous silicon p-i-n photodiode achieving a high fill factor (﹥70%) and a photoconductive gain (﹥10) at visible wavelengths;21 Yao et al. realized an amorphous IGZO TFT with the largest photocurrent of 50 µA.22 The high gain of those devices can be described as: when the photodiode is irradiated with external light, the charges are generated and accumulated in the photoconductor layer resulting in the gate bias applied to the TFT; then the gate bias leads to a threshold voltage shift of the TFT and the output photocurrent is amplified tremendously in the sub-threshold region as is expected.23,24 Even though high gain is achieved, the gain dependence on wavelength is still strong and the response speed is not high enough for dynamic imaging applications. In addition, early studies about electron beam induced conductivity (EBIC) effect had been widely investigated and the EBIC effect was mainly utilized in probing the electrical properties of nanowires devices at a nanometric scale and for the image intensifier target in camera tubes for low-level light detection.25-28 For example, Hatanaka et al. had obtained the electron multiplication factors of EBIC of 300 ~ 500 in a hydrogenated amorphous silicon film at an accelerating voltage of 7 kV.26 In an image intensifier using the EBIC effect, a photocathode emits electrons in response to a light image. The electrons are electrostatically focused and accelerated to high energy before bombarding the vidicon target having an array of charge


4

Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

storage diodes on its output side. A large number of carriers are generated in the bulk of the target under the bombardment of the accelerated electrons. The charge image is stored and read out. In this case, the photocathode and EBIC target are separated. The fixed amounts of accelerated electrons result in the limited electron multiplication factor. Here, we proposed a novel photoelectron multiplication mechanism called EBIPC to improve the sensitivity of flat panel photodetector. Under this mechanism, the photocurrent was amplified by photo induced EHPs and electron bombardment induced EPHs in a photoconductor, leading to a high sensitivity. Since the photo-absorption of the photoconductor is not a limiting factor for the dynamic range, a wider dynamic range is expected due to electron bombardment. Thus, both high sensitivity and wide dynamic range can be achieved in this device whereas in most cases, there is a tradeoff between them. Photodetectors that use the EBIPC mechanism consist of photoconductors and field emission arrays,29,30 which are also advantageous in large area detection, low dark current and high spatial resolution. RESULTS AND DISCUSSION Device Structure and Light-responsive Performance. In order to obtain the electron bombardment for a photoconductor, a vacuum diode-type cold cathode device29,30 was designed for exploring EBIPC mechanism, which is shown schematically in Figure 1. ZnS photoconductor has been used as a target in the image intensifier of camera tubes for use in low light detection25,26 and exhibit the advantages of high exciton binding energy, large area fabrication, lost cost and ease of integration.2,31 As such makes it a great promise for use in large area and highly sensitive detectors. ZnO NW field emitters have been used in large area flat panel vacuum devices. Applications of ZnO field emitters for flat-panel displays,32 parallel electron-beam lithography33 and flat panel X-ray sources34 have been reported. Therefore, the ZnS


5

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

photoconductor were used for the anode and ZnO NWs was the cathode in the vacuum diodetype cold cathode device. The prepared ZnS photoconductor had a dense structure (Figure 1(b)). The grown ZnO NWs had a height of 2~3 µm and a tip diameter of ~20 nm (Figure 1(c)).

Figure 1. (a) Schematic showing the vacuum diode-type cold cathode device designed for exploring EBIPC mechanism. (b and c) Cross-view SEM images of ZnS photoconductor and ZnO NWs prepared on an ITO-coated glass substrate.

The currents as a function of applied anode electric field (I-E) were measured in the dark and different light intensities with the wavelength of 400 nm (Figure 2(a)). The I-E plots exhibited the classic electron tunneling phenomenon as discussed in Ref.35 Figure 2(b) showed the corresponding light-intensity-dependent photocurrent and spectral responsivity ( ) at a biasing electric field of 5.8 V/µm. The photocurrent increased rapidly with the increasing light intensity. The maximum gain (photo-to-dark current ratio) was approximately 1.2 × 104 at a field of 5.8 V/µm, in which the dark current remained as low as 1.2 × 10-7 A, however, the photocurrent climbed to 1.4 × 10-3 A under the light intensity of 5 mW/cm2. Under the irradiation with light, the corresponding electric field on the ZnO NW surface was elevated from 2.17 V/µm to 5.75


6

Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

V/µm (Supporting Information, Figure S1). In the imaging applications using addressable gated ZnO NW field emitter arrays, the field emission current was 5.6 nA36 for a pixel of 660 × 220 µm2 so that the corresponding gain was 6.8 × 102 estimated from Figure 2(a). The photocurrent and versus light intensity plots showed a roughly linear feature between 1 µW/cm2 and 5 mW/cm2. We can estimate the linear dynamic range (LDR) by using LDR=20log(Phighest/Plowest), where the Phighest and Plowest are the highest and lowest light power within the linear regime, respectively.37 The LDR was estimated to be 74 dB, which is close to that in the perovskite photodetectors (84 dB).37 In addition, our results showed that no obvious changes in morphologies of ZnS thin film and ZnO NWs were observed after the measurements indicating the repeatable properties of photodetectors (Supporting Information, Figure S2 and S3).


7

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

Figure 2. Response of photodetector when irradiated with light. (a) I-E curves in the dark and for different illumination intensities (400 nm). (b) Power-dependent photocurrent and responsivity at a bias of 5.8 V/µm. (c) I-E curves in the dark and for different monochromatic light under a light intensity of 100 nW/cm2. (d) Responsivity and external quantum efficiency as functions of wavelength at a light intensity of 100 nW/cm2 (the inset was the absorption spectrum of a ZnS thin film).

Figure 2(c) showed the I-E curves with different monochromatic lights ranging from the UV to the visible region under a light intensity of 100 nW/cm2. The photocurrent firstly increased and gradually decreased with the increasing wavelength of incident light. The photodetector irradiated with 350 nm light showed a weaker photo-response than those irradiated with 400 nm and 450 nm light. On the one hand, the increase of the photon energy under a constant power was inevitably at the cost of photon number.38 On the other hand, at shorter wavelengths, the absorption coefficient increased and the penetration depth of light became shallower, which eventually reduced the collection efficiency of the photogenerated carriers.39 Those reasons were attributed to the relatively weaker photo-response of 350 nm light. Figure 2(d) showed the corresponding and external quantum efficiency (EQE) versus the wavelength curves. The reached 2.3 × 102 A/W to 4.9 A/W and the EQE reached 7.3 × 104 % to 8.6 × 102 % at wavelengths of 350 nm to 700 nm. A much broader wavelength detection was achieved compared to the reported ZnS photodetector with the detection wavelength less than 450 nm.40 The ZnS thin film exhibited an absorption edge at 457 nm (inset of Figure 2(d)) but the studied device showed a high responsivity under the wavelength between 350 and 700 nm, which indicated the effective utilization of EBIPC mechanism in wide wavelength detection. In Table 1, we summarize the performance metrics of the flat panel photodetectors under different photoelectron multiplication mechanisms. It is evident that the values of responsivity


8

Page 9 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

and EQE of our device are much higher than traditional phototransistors.41 The PMT shows the optimal detection performance but is bulky in size and cannot be pixelated.7,8 Miniaturized APD and pixelated SiPM also can realize the excellent photoelectron multiplication using the avalanche effect, however, those devices are unsuitable for large area imaging due to the siliconbased

microfabrication

and

have

the

drawbacks

of

temperature-dependent

gain

characteristics.10,17,42 For large area detection, even though high gain was achieved by a 3-D TFT in an active pixel format, the gain dependence on wavelength was still strong and the structures were complicated.23 Furthermore, the high-gain avalanche rushing amorphous photoconductor (HARP) was explored by using an a-Se thin film driven by diamond cold cathode, however, the detection performance was still very poor and the diamond cold cathode was difficult to realize large area, uniform and stable emission.43 In addition, the prototype of an X-ray HARP detector using Spindt-type cold cathode with a pixel number of 640 × 480 and a pixel size of 20 × 20 µm2 had been reported, and was expected to be used in detection for high-throughput protein crystallography.44 However, the Spindt-type cold cathodes are not eligible for large area detectors and the amorphous selenium used in the HARP devices is mainly for low energy range of ~20 keV applications due to its relatively low atomic number.45 In order to realize the large area photodetectors with high internal gain, we proposed an EBIPC mechanism using a ZnS photoconductor driven by ZnO NWs field emitters, which are different from the reported photoelectron multiplication mechanisms. Under this mechanism, the device showed the advantages of high internal gain, large area and wide wavelength detection. Such a mechanism is independent on photoconductor material and wavelength. Therefore, it is anticipated that high sensitivity and broadband detection can be achievable in a wide range of material systems using the EBIPC mechanism.


9

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

Table 1. Comparison of performance metrics of the flat panel photodetectors under different photoelectron multiplication mechanisms

Photodetector type

Photoelectron multiplication mechanism

Responsi vity (A/W)

EQE (%) or internal gain

Area (mm2)

Measured spectral response range (nm)

Operation Ref. voltage (V) (in manusc ript)

350 - 950

Gainwavelength dependence (Gainmin/ Gainmax) ~10-5

Traditional phototransistor (Si:H)

None

~0.3

~70

50

1

41

PMT (Multialkali)

Photoemission and secondary emission

7.4×105

2.6×108

8×24

185 - 900

~10-4

1250

7

PMT (InGaAs)

1.6×102

APD (Si)

Photoemission and secondary emission Avalanche effect

2.2×106

3×12

185 - 1010

~10-3

1500

8

21

4.2×103

Φ0.2

320 - 1000

~10-2

Electron Bombardment Induced Photoconductivity and High Gain in a

Recommend Documents