Vacuum-Ultraviolet Photodetection in Few-Layered h-BN - ACS

Subscriber access provided by READING UNIV

Functional Inorganic Materials and Devices

Vacuum-Ultraviolet Photodetection in Few-Layered h-BN Wei Zheng, Richeng Lin, Zhaojun Zhang, and Feng Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07189 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 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 20 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 Applied Materials & Interfaces

Vacuum-Ultraviolet Photodetection in Few-Layered h-BN Wei Zheng, Richeng Lin, Zhaojun Zhang, Feng Huang,* State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials, Sun Yat-sen University, Guangzhou 510275, China *Corresponding author: [email protected]

Abstract Over the past twenty years, the astro- and solar-physicists have been working hard to develop a new-generation semiconductor-based vacuum ultraviolet (VUV, 100-200 nm) photodetector with small size and low-power consumption, in order to replace the traditional microchannel detection system which is ponderous and has high energy consumption and finally to reduce the power load and launch costs of explorer satellites. However, this expectation has hardly been achieved due to the relatively low photoresponsivity and external-quantum-efficiency (EQE) of the reported VUV photoconductive detectors based on traditional wide-bandgap materials and structures. Here, based on few-layer h-BN, we fabricated a high-performance two-dimensional (2D) photodetector with selective response to VUV light. Typically, it behaves a high-sensitivity (EQE=2133%, at 20 V) to the extremely weak 160 nm light (3.25 pW). This excellent photoresponsivity can be attributed to the high carriers-collected efficiency and existing surface trap states of few-layer h-BN. In addition, this device can maintain a stable performance in a wide-temperature ranging (80-580 K), which is quite favorable for application in deep space with huge-temperature fluctuation.

Keywords: vacuum ultraviolet, photodetector, h-BN, carriers-collected efficiency, trap states

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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 20

1. INTRODUCTION Vacuum-ultraviolet (VUV, 10-200 nm) detection is essential in the research of space science, which can harvest information on nebula expansion and monitor the formation and evolution of solar storms.1-4 Currently, explorer satellites or solar space mission mainly adopt the combination system of Rowland spectrograph and microchannel plate for VUV detection,2, 5 which needs large space and huge power supply (thousands volts of driving voltage). Scientists have been attempting to utilize wide-bandgap semiconductors (such as AlN and diamond) to fabricate low power consumption and compact VUV detector.6-10 However, due to the absence of high-crystalline and defect-free wide-bandgap semiconductors, the device with desired high responsivity for VUV detection is rarely reported.11 Compared to traditional wide-bandgap materials, 2D h-BN (Eg=5.955 eV), which can be mechanically exfoliated, is relatively easy to achieve high crystallinity.12-13 We believe, with the gradual development of large-scale synthesis of 2D materials,14 the detector based on layered h-BN is expected to open a new path for high-performance VUV detection. In addition

to

high

chemical

and

thermal

stability,15 h-BN

also

has

a

large

surface-area-to-volume ratio, which is beneficial to the increase of responsivity.16-18 In this work, based upon mechanically exfoliated few-layered h-BN, we fabricated a VUV sensitive 2D photodetector. The detector can identify the extremely weak VUV signal with power intensity in picowatt (pW) level. Under a bias of 20 V and illumination of 160 nm light, the device shows an ultra-high EQE of up to 2133%. Additionally, with a fast response speed of several milliseconds and a high stability under a wide temperature ranging from 80 to 580 K, the device is said to be a quite potential candidate for deep space exploration, especially in extreme environment. 2. RESULTS AND DISCUSSION Firstly, high-quality few-layered h-BN single crystal was obtained via mechanical exfoliation (high resolution TEM image, Raman, EDS, and XPS spectra in Figure 1a-e indicate a single crystalline quality and high purity of the h-BN), then it was transferred to silicon dioxide substrate, and finally, an MSM-structured h-BN-based photoconductive 2


Page 3 of 20 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


prototype device was successfully fabricated (device optical graphic is shown in the inset of Figure 2a). It is known that metal electrodes play important roles in MSM-structured devices, and here we used Au as the electrode to obtain the optimal performance and maximum potential of h-BN photodetector. Au has a large work function (5.1 eV) quite close to h-BN, thus they can form Schottky contact with lower barrier.19 In this work, to simulate the universe environment in space detection and exclude the effect of adsorbent on the device performance, all the measurements were performed under the vacuum of about 5×10-5 Torr. To examine the VUV detection performance of the few-layered h-BN detector, synchrotron radiation was used for measurement, as illustrated in Figure 2a (more details are given in the method part). Firstly, the photocurrent of the device was measured under an illumination of 160 nm light, as marked in black dots in Figure 2b. As it can be seen, even under an extremely weak illumination of 3.25 pW, the photocurrent (under a bias of 1-20 V) is still two orders of magnitude higher than the dark current, indicating the device has high signal noise ratio (SNR). Generally, a low dark current of the h-BN photodetector implies a low noise, and the shot noise in,s is determined by the dark current, expressed as in,s = 2eId B ,20 where B denotes the bandwidth, e the electronic charge, Id the dark current. At the bias of 20 V, the shot noise of the h-BN device is calculated to be 2.04 fA Hz−1/2. In addition to the shot noise, the thermal noise in,t can be calculated by the expression: in,t = 4kBTB / R , where kB denotes Boltzmann constant, T the temperature, and R the resistance of the detector. Based on the differential resistance at 20 V obtained from the dark current curve, the thermal noise is calculated to be 0.99 fA Hz−1/2. Therefore, the total white noise in calculated according to the expression below is 2.27 fA Hz−1/2: i n =

i n2, s + i n2t . On the other hand, the noise current is irrelevant to

the frequency (mainly from the instrument noise floor, and the average noise level is as low as 12.6 fA Hz−1/2 at 20 V) can be seen from Figure 2c, indicating that the photodetector noise is decided mainly by white noise instead of 1/f noise. The photoresponsivity Rλ, as one of the most important parameters for photodetectors, reveals the input-output gain of the photoelectric detection system, which can be expressed as 3



Page 4 of 20

Rλ = ∆I/(PA) (∆I is the difference between the photocurrent and dark current, P is the power intensity of the illuminated light and λ is the wavelength of the incident light). As shown in Figure 2d, the photoresponsivity of current h-BN photodetector varies with changing bias, under an illumination of 160 nm VUV light. Typically, at 20 V bias, the obtained responsivity Rλ is about 2.75 A/W, corresponding to an EQE= hcRλ/(eλ) up to 2133%, which is 24.5 times larger than that of current commercial Si-based VUV detector and 609 times larger than that of AlN detector (see Table 1), indicating the extremely high sensitivity of h-BN photodetector to VUV. Moreover, based on the obtained photoresponsivity Rλ and noise current, the detectivity of the device can be calculated via the expression:

D λ∗ =

R λ A1 2 in

. At a bias of 20 V,

the corresponding D λ∗ was calculated to be 3.2 × 1013 Jones, which approaches to the highest value of most deep UV photodetectors.21-24 Most importantly, the fabricated few-layered h-BN photodetector behaves strong VUV spectral selectivity. Figure 2e displays the spectral photoresponse of the detector at a bias of 2, 5 and 20 V under the illumination of synchrotron radiation with wavelengths ranging from 160 to 340 nm. A remarkable increase of the photoresponse is observed when the device is illuminated in VUV light with photon energy above the threshold of 220 nm. The detector has a discrimination ratio of about four orders of magnitude (160 nm vs 250 nm). Through above experimental analyses, it is obvious that few-layered h-BN is more competitive for VUV detection compared with other wide-band-gap semiconductors with traditional structures (films, micro and nanocrystals), especially its high photoresponsivity Rλ. Suggestions for its mechanism are as follows: 1. Contributions by surface trap states：As shown in Figure 3a and 3b, the mechanically exfoliated few-layered h-BN involves obvious edges on the surface, which are actually line defects and also typical carrier-trap centers. The inset of Figure 3c displays the influences of those edges on photoconductivity: when the few-layered h-BN device is under illumination, some photo-excited carriers (such as holes) are restricted to those edges and become surface trap states and the others (such as electrons) are flowing. Driven by the voltage, the trap states contribute to an extra electron transport channel, make an extra non-photogenerated free 4


Page 5 of 20 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


charges transfer, and form a persistent photocurrent. This basic picture of photoconduction can be confirmed by our supplementary experimental data (see Figure S1), that is, the photocurrent density of the few-layered h-BN with line steps is larger than the sample with smooth surface, although the two samples are of similar thickness and size. 2. High-charge-collection efficiency: In mathematical definition, the photoresponsivity (Rλ) is constrained by the carriers mean recombination lifetime τ and mobility µ, which is written as25: Rλ =

µτ V η e λ L2 hc

(1) where, L is the separation distance of electrodes, c the light velocity, h the Planck constant and V the applied bias, and η=ηtransηabs (ηtrans is the charge transfer efficiency and ηabs is the light absorption efficiency). In fact, η, as an important parameter, is a complex function of λ, and it is closely related to the band structure of materials. For the current microscale h-BN, we can hardly obtain that the specific relationship between η and λ through absorption experiment because the employed VUV-beamline of synchrotron radiation facility cannot conduct microscale measurement. Here, we can attempt to assume that when the photon energy of the incident light is larger than that of the bandgap of h-BN, the photon can be completely absorbed and the charge completely transferred, and η is 100%, then the value of R will be mainly determined by figure-of-merit µτ of h-BN, which defines the average diffusion length of an excited carrier and the charge collection efficiency.26 In the model of 2D planar photoconductive detectors, see Fig 3d, photoexcited carriers can be regarded as uniformly distributed in the 2D plane incident photons cover whole surface, as shown in Fig 3b. With an external applied electrical field, the I-V relation of 2D photoconductor can be expressed by 25: I µτ V  µτ V I (V )= 0 2 1 − 2 L L  

−L   1 − e µτ V   2

  ,   

(2) where Io is the saturated current. This formula describes an obvious rule about current fluctuations, that is, when the applied bias V ranges within a relatively low voltage the 5



photocurrent in 2D planar photoconductive detector will increase in line with the former for 2 the carrier transit time t is greater than its lifetime τ, τ

Vacuum-Ultraviolet Photodetection in Few-Layered h-BN - ACS

Recommend Documents