Vacuum-Ultraviolet Photodetection in Two ... - ACS Publications

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Functional Inorganic Materials and Devices

Vacuum-Ultraviolet Photodetection in Two-Dimensional Oxides Wei Zheng, Richeng Lin, Yanming Zhu, Zhaojun Zhang, Xu Ji, and Feng Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04866 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Applied Materials & Interfaces

Vacuum-Ultraviolet Photodetection in Two-Dimensional Oxides Wei Zheng, Richeng Lin, Yanming Zhu, Zhaojun Zhang, Xu Ji, Feng Huang* State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials, Sun Yat-sen University, Guangzhou 510275, China E-mail: [email protected] KEYWORDS: two-dimensional oxides, vacuum-ultraviolet, photodetector, MgO, conformal anneal synthesis

ABSTRACT: To lower the launch cost and prolong the lifetime of deep space explorer, solarand astro-physicists and photonics scientists have devoted much time and energy in exploring and developing a compact and low power consumption semiconductor-based vacuum-ultraviolet (VUV) photodetector. However, the target has not yet been achieved, due to the lack of high external-quantum-efficiency (EQE) VUV photoconductive materials. Here, we found that twodimensional MgO, obtained via conformal anneal synthesis (CAS) method, had ultra-sensitive photoresponse to VUV light. It can identify extremely weak VUV signal (0.85 picowatt), with a high EQE of 1539 %. Such ultra-sensitive photoresponse is attributed to the high chargecollection-efficiency of excited carriers. Our results provide an idea for developing integrated VUV devices with high responsivity and low power consumption, which will prolong the service-time and lower the launch-cost of space explorer.

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1. INTRODUCTION Vacuum-ultraviolet (VUV) detection technology is extensively applied in the fields of cosmic chemistry and space science, for studying the composition and expansion of nebula elements, and monitoring the formation and evolution of solar storms.1-5 At present, space explorers mainly adopted the combination of violet chromatographs and microchannel plates for VUV photodetection.6 However, these traditional detection systems are usually bulky with several thousand driving voltage, which increase the launch-cost and burden the power-supply of space explorers. Materials and devices scientists have been working hard to find new solutions for cosmologists, trying to develop low power consumption and compact VUV detectors based on ultra-wide bandgap semiconductor films, such as AlN and diamond thin films.1-2, 7-10 Normally, compared to thin film materials, two-dimensional (2D) semiconductors have low-dimensional conductivity channel for rapid carrier-collection and high surface-state-density for photocurrent gain incensement.11 These excellent physical properties of 2D materials contribute to efficient collection of photogenerated-carriers, which is rather beneficial to highlysensitive photoresponse. Here, we designed a conformal anneal synthesis (CAS) method to synthesize 2D MgO with ultra-wide bandgap (7.3 eV).12 Based on this material, we fabricated a 2D photodetector sensitive to extremely weak (0.85 picowatt) VUV light, the photoresponsivity of which is 2 orders of magnitude higher than that of the reported commercial silicon-based sensor for VUV light.

2. THEORIES


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2.1 Expression of Photoresponsivity Rλ. Photoconductive detectors are benchmarked by photoconductive gain (Gph), i.e., the number of detected charge carriers per single incident photon. Gph = (Iph/e)/(ϕinηabsηtrans), where, ηtrans is the charge transfer efficiency, ηabs the light absorption efficiency, e the electron charge, Iph the photocurrent, and ϕin = Pin/Eph the incoming photon flux (Pin is the power of light impinging on a device, and Eph=hc/λ is the photon energy).13 Gph can also be quantified by the ratio of the trapped carriers lifetime τ over the drift transit time τtransit, Gph = τ/τtransit. And τtransit is governed by the applied field V, mobility µ, and electrode separation distance L, τtransit = L2/(µV).13 On the other hand, Rλ can be defined as: Rλ=Iph/Pin. By combining all the above formulas, we get Rλ =

µτ V η e λ L2 hc

,

where η=ηtransηabs. 2.2 Deduction in the I-V relation of 2D photoconductor. In view of the 2D photoconductor case shown in Figure 1, carriers are excited in the 2D crystal. With a form of 2D structure bordered by plane x = 0 and x = L, and field E along x-axis, this insulating sample only has to consider one-dimensional flow (along the x-axis). Here, to simplify the model, we didn’t consider the effect of electrode on current, and made an assumption that the carriers are featured by position-independent lifetime τ. Given the fact that holes and electrons are pulled apart by the field, this hypothesis sounds rather artificial, however, without such assumption, the feasibility of analysis is at stake. The field E remains constant as space-charge effects are absent. Leaving out diffusion currents, the continuity equation for carriers (such as holes) under stable conditions can be expressed as below: G−

ρ ( x) dρ ( x ) − µE =0, τ dx


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where G is the volume excitation rate, ρ the density of carriers and µ their mobility.

Due to blocking characteristics of the contact, carriers on the positive surface (x = 0), are being swept away by the field continuously, but without any replenishment. Hence ρ(0)=0. The equation subjected to boundary conditions is: x −   µτ E ρ ( x) = Gτ 1 − e  ,   

The current density is then given by: −L L  µτ E    qµ E  µτ E = − − J = x dx qG E e ( ) 1 1 ρ µτ     , ∫ L   L 0   

Going forward, we can get the photocurrent through integration. I µτ V I (V )= ∫ ∫ Jdydz = 0 2 L 0 0 W T

 µτ V 1 − 2 L  

−L   1 − e µτ V   2

  ,   

where V = EL , I 0 = q G W T (W and T correspond to the thickness and width of the sample, respectively).

3. METHODS 3.1 Material synthesis. 2D MgO was prepared by a two-step CAS method. At first, 2D Mg(OH)2 nano-precursors were grown on sapphire substrate using hydrothermal autoclave at 250℃. Then the as-grown 2D Mg(OH)2 precursor was put under a high temperature annealing furnace (HF-Kejing, OTF-1200X-HP-85) with 20 atmospheres of oxygen. After a six-hour sintering at 400 oC, high quality 2D MgO was obtained. 3.2 Materials characterization. The structured properties of the synthesized 2D Mg(OH)2 and MgO were identified using a Panalytical X’Pert Pro X-ray di℃ractometer with Cu-Kα radiation (λ = 1.5406 Å), a transmission electron microscope (FEI; Tecnai G2 F30 and 300 kV) equipped


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with HR-TEM, and an atomic force microscope AFM (Bruker, Dimension Fastscan). SEM images were measured using scanning electron microscopy instruments of ZEISS AURIGA, Oxford INCAPent aFET-x3 and Hitachi S-4800.

3.3 Molecular dynamics simulation. We performed Ab initio molecule dynamics (AIMD) simulation on Mg(OH)2 annellation at high temperature via Vienna Ab-initio Simulation Package (VASP). The generalized-gradient-approximation (GGA) functional of Perdew-BurkeEhrenkof (PBE) was used as the exchange correlation interaction function. Kohn-Sham orbitals were superimposed by plane waves until the kinetic energy cutoff reaches 500 eV. Adopting canonical ensemble, the whole simulation process controlled the oscillation frequency of temperature by Nosé mass, meanwhile, the temperature is kept at around 1000K. When the distance between 2 H atoms and O atom is around 1 Å, we deemed that water molecules are produced. Considering it was a high temperature experiment, water molecules were easy to evaporate from the system, therefore, we will remove the water molecules once they are generated during the simulation process.

3.4 4 Devices measurements. After obtaining 2D MgO by CAS method, 80 nm thick Au with a gap of 5 µm was deposited as electrodes using thermal evaporation (Cu grid as physical mask). The photocurrent and temporal photoresponse to VUV light source were measured by the beamline 4B8-Vacuum Ultraviolet (Wavelength Range: 125-360 nm; Bandwidth: 0.8 nm; Beam Size: 2mm×1mm) of Beijing Synchrotron Radiation Facility (BSRF), the typical power of 150 nm light is 15.4 nW), the power of monochromatic light was calibrated using VXUV20A photodetector from PTO DIODE CORP (http://optodiode.com/pdf/AXUV20A.pdf), more


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information on the powder-intensity is given in Figure S1. KEITHLEY 2636b was used as SourceMeter, and the device is under vacuum condition when performing all the photoresponse measurement, and its measured photocurrent is shown in Figure S2. The VUV light source was turned on/off by a pneumatic valve with a switching time of 1 s. Noise current was examined by a low frequency noise measurement system (Platform Design Automation, NC300L). To imitate the environment of deep space exploration, all measurements were conducted in vacuum (~ 1×10-5 Torr) to eliminate the effects of surface adsorbate on device performance.

4. RESULTS AND DISCUSSION It is well known that some hydroxides, such as Mg(OH)2, Ni(OH)2, and Ca(OH)2, have layered structures. Those hydroxide will convert to corresponding oxides after decomposition and dehydration during high-temperature annealing.14-15 Therefore, theoretically, we can obtain 2D MgO crystal by conformal synthesis with 2D van der Waals Mg(OH)2 as precursor. Based on this understanding, as shown in Figure 2a, we designed a two-step CAS method (see the method part for details): Firstly, 2D layered Mg(OH)2 single crystal was synthesized on sapphire substrate via high temperature hydrothermal method. Due to the extremely small temperature difference between the crystal and the solution during liquid phase synthesis, the crystal growth is almost a thermodynamic equilibrium process, which is similar to an orderly stacking process of atoms or molecules and contributes to the growth of high-crystalline crystals.11 Secondly, the synthesized 2D Mg(OH)2 was placed in an oxygen atmosphere (2×106 Pa) for high-temperature (400 oC) annealing to accelerate its complete decomposition and dehydration. Consequently, it transferred to 2D MgO with reformed lattice. The inset in Figure 2b shows the optical photographs of the 2D Mg(OH)2 precursors and MgO on sapphire. The observed smooth


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surfaces and regular symmetrical hexagon shape indicate 2D morphology and potential high crystallinity quality of the sample. The color difference between the two samples also implies their thickness variation before and after annealing. Figure 2b displays the X-ray Diffraction patterns of the synthesized MgO (red) and Mg(OH)2 precursors (black). It is obvious that Mg(OH)2 contains (001) and (002) peaks with standard hexagonal lattice, and 2D MgO product contains the characteristic (001) diffraction peak, which is consistent with the value of MgO single crystal.16 The characteristic peak transfer of the 2D MgO product (corresponding to the diffraction peak of the Mg(OH)2 precursors) straightly demonstrates a complete success of our designed CAS method. In order to confirm the transformation from Mg(OH)2 to MgO more accurately, we carried out high-resolution TEM tests on the 2D MgO and Mg(OH)2 precursors, respectively, as shown in Figure 2 c-e (MgO) and Figure S3 (Mg(OH)2). It can be seen that the Mg(OH)2 precursor has a distinct hexagonal structure, while the annealed 2D MgO has been completely transformed into a tetragonal structure. The distances of (202) and (020) planes are 0.15 and 0.21 nm, which are in good agreement with the standard cubic MgO values.16 Our statistical analysis (Figure 3) shows that the size of the synthesized 2D MgO mainly centers at about 8 microns, and its thickness 17 nm. We used the First-principles simulation to verify the CAS process, as shown in Figure 4, and all the calculations were based on VASP package.17-18 During the CAS process, the lowdimensional hexagonal Mg(OH)2 was observed to transfer into rocksalt structured MgO after dehydration reaction, so in the simulation here, nanocluster was taken as the calculation object rather than supercell.19 We constructed a cubic box with 21.405 angstrom as its simulated area, and a Mg(OH)2 nanocluster which was composed of 8 unit cells (see Figure 4a). All simulation


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processes were carried out using NVT ensembles, and the frequency of temperature oscillations was controlled by Nosé algorithm. The whole CAS process can be divided into two parts: one is dehydration of Mg(OH)2 and the other is formation of ordered MgO structured by disordered clusters after dehydration reaction. Figure 4a to Figure 4b show the dehydration of Mg(OH)2, with only 5 ps annealing at 1000 K, independent water molecules decomposed from the Mg(OH)2 crystal (the distance between oxygen and hydrogen atoms is about 1 angstrom). In order to conform to the real experimental situation, we removed the independent water molecules one by one from the calculation system. After several nanoseconds, as shown in Figure 4c, an unordered cluster consisting of 8 Mg atoms and 8 oxygen atoms were formed. What follows then is a continuous annealing at 800 K high temperature (8 ps), as shown in Figure 4d, where the system gradually became orderly, with cubic structure of rocksalt. Based on the material, we prepared 2D MgO-based photoconductive detector with a metalsemiconductor-metal structure, as shown in the inset of Figure 5a. Then, we tested the VUV photoresponse of the 2D MgO photodetector with synchrotron radiation as VUV source (more details are shown in the method section). The photocurrent was measured under an illumination of 150 nm (Figure 5b). It can be seen that even under extremely weak illumination intensity (0.85 picowatt irradiation on the device), the photocurrent (bias 1-20 V) is still two orders of magnitude higher than the dark current, indicating a large signal to noise ratio of the device. The small dark current of the 2D MgO photodetector indicated a small shot noise

in,s = 2eI d B , where B is bandwidth, e is meta-charge, and Id is dark current.20 The shot noise of the 2D MgO device was calculated to be 0.99 fA Hz-1/2 under a bias of 4 V (dark current of 3.04 ×

10-14 A). In addition to the shot noise, we can also calculate the thermal noise in,t with the


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expression in,t = 4kBTB / Rλ , where kB is Boltzmann constant, T is Kelvin temperature, and Rλ is photoresponsivity. Based on the internal resistance obtained by the dark current curve at 4V, the thermal noise was calculated to be 0.22 fA Hz-1/2. Therefore, the total white noise calculated using the formula in =

i n2, s + i n2t was about 1.01 fA Hz

-1/2

. On the other hand, Figure 5c shows

that noise current (mainly from the equipment noise, averagely noise < 9.2 fA Hz-1/2) is irrelevant to frequency, indicating that the noise of photodetectors is mainly determined by white noise. Photoresponsivity Rλ, one of the most important parameters for photodetector, reveals the gain of input and output in the photodetection system, which can be expressed as Rλ = ∆I/(PA) (∆I is the difference between light and dark current, P is light intensity, and A is the effective area of device channel). As shown in Figure 5d, Rλ of the 2D MgO photodetector varies with changing bias under an illumination of 150 nm VUV light. Typically, Rλ is 1.86 A/W at 4 V, and the corresponding EQE=hcRλ/(eλ) is 1539 %, which is much higher than that of those currently reported wide bandgap semiconductor-based VUV detectors,9, 21 and commercial Si-based VUV detectors (VXUV20A photodetector of PTO DIODE CORP). Moreover, based on the obtained Rλ and noise current, the detectivity of the device can be calculated via the expression:

Dλ∗ =

Rλ A1 2 ∗ . At a bias of 4 V, the corresponding D λ is calculated to be 1.8×1010 Jones which in

is close to the value of most deep-UV photodetectors.22-24 Most importantly, the prepared two-dimensional MgO-based photodetector also has excellent VUV spectral selectivity. Figure 5e shows the spectral response curve of the detector illuminated under 150-320 nm light. The results show that the detector exhibits a significant photocurrent gain when the energy of the incident light exceeds the threshold of 7.3 eV (i.e., the wavelength is less than 170 nm). For example, the light current produced at 150 nm is more than


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two orders of magnitude higher than the one produced at 260 nm, and the switching ratio is about 240. So far, although many groups have reported the corresponding high-performance transitionmetal-sulfide based two-dimensional photodetectors, their response cutoff wavelengths are still in the visible or infrared region, far from meeting the requirements of VUV detection. Here, our reported 2D MgO photodetector with an inter-band transition absorption, has a response wavelength of λ < 170 nm, making it suitable for VUV detection. In addition, our device also exhibits a stable switching light response (Figure 5f). For semiconductor based photoconductive detectors, Rλ is determined by various figure-ofmerits of the materials as follows (as shown in the Theories part) Rλ =

µτ V η e λ

(1)

L2 hc

where µ is carrier mobility, τ carrier recombination lifetime, L gap of electrodes, c velocity of light, V applied bias, and η=ηtransηabs (ηtrans is current transfer efficiency, and ηabs is light absorption coefficient). In fact, the important parameter η is a complex function of λ, which is closely related to the band structure of materials. Here, the specific expression between η and λ lies in VUV range is hard to determine, because of the lack of VUV focus system on beamline of the synchrotron radiation facility. To overcome this problem, we attempt to make an ideal assumption, when the photon energy of incident light outweighs that of MgO bandgap, the photon is completely absorbed and the charge totally transferred, and η equals to 100%, then the figure-of-merit µτ of MgO will be the major contributor of the value of Rλ. The product of µτ is a critical parameter of semiconductor photodetector, which reflects the average diffusion length and collection efficiency of carriers, and ultimately determines the responsivity of the photodetector.


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In this case, the µτ factor of the 2D MgO can be extracted from the I-V relation of 2D photoconductive model (Figure 1) as follows (see the Theories part): 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. As shown in Figure S4, the experimental photocurrent of 2D MgO detector is quite close to the simulative value which is modified by equation µτ =1.97×10−3 cm2 /V (L is 5 µm). The µτ of 2D MgO is much larger than that of conventional high-resistivity semiconductor materials (10-4 -10-6 cm2 / V ),25 which is one of the main causes for the high Rλ of the 2D MgO photodetector.

5. CONCLUSIONS In summary, we reported a CAS method to synthesize high-crystallinity 2D MgO. In fact, this strategy is a universal method and can be also applied to the synthesis of other 2D oxides, such as NiO, GaO, and CoO, as they have similar layered-structure hydroxide precursor as 2D MgO. More importantly, we found 2D MgO have high charge-collection-efficiency of excited carriers, and other extraordinary advantages, all these contribute to its ultra-sensitive photoresponse to VUV light. It can even identify the extremely weak VUV light signal of 0.85×10-12 watt. The 2D MgO based VUV detectors which involve a high sensitivity of traditional micro-channel-plane and an integral, compact and low power-consumption of semiconductor-based-devices, provide a novel alternative for developing new generation semiconductor VUV detectors.


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ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI:************* I-V fitting of 2D MgO photoconductor, characterization of 2D layered Mg(OH)2, powerintensity dependence on wavelength of VUV beamline in BSRF (A=2×1mm2), measured photocurrent in 2D MgO device (A=5×22 µm2).

ACKNOWLEDGMENTS The authors acknowledge financial support from National Natural Science Foundation of China (61604178, 91333207, U1505252 and 61427901). The authors would like to thank the VUV measured support from beamline 4B8-Vacuum Ultraviolet of the Beijing Synchrotron Radiation Facility (BSRF) in Beijing.


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Figure 1. I-V relation of 2D photoconductor. Graphic model of two-dimensional photoconductive detector. Through the continuity equation of charge in the above model, the inserted formula is derived, which demonstrates the relationship between the photocurrent I and the bias voltage V in the photoconductive detector.

Figure 2. CAS method for 2D MgO growth. a) Schematic of the CAS process for 2D MgO synthesis. Firstly, 0.1 g Mg(OH)2 and 30 ml deionized water were placed in a 50 ml titanium hydrothermal autoclave. Meanwhile, a piece of sapphire wafer (1×3×0.045 cm3) was inserted vertically as growth substrate. The titanium hydrothermal autoclave was then placed in an oven at 250 °C to heat up for 5 hours so as to promote the growth of enormous 2D layered Mg(OH)2 on the sapphire substrate. Secondly, the obtained 2D layered Mg(OH)2 precursor was put in a closed oxygen furnace and subjected to a six-hour continuous annealing, finally, 2D MgO was obtained. b) XRD patterns of 2D MgO and 2D Mg(OH)2 precursor. There is a significant shift between the diffraction peaks of 2D Mg(OH)2 and MgO. The former is mainly located at 18.66 degrees, corresponding to the (001) of hexagonal structured Mg(OH)2. While the latter is located at 21.24 degrees, corresponding to the (003) of cubic phase MgO. c) and d) present typical TEM of 2D MgO product, and corresponding HR-TEM image (along the [101] zone axis) (the sample is physically transferred to Cu grid for measurement). e) The corresponding SAED pattern, which clearly indicates a tetragonal atomic arrangement of the synthesized 2D MgO product. The distances of (202) and (020) planes of cubic MgO are approximately 0.15 and 0.21 nm, respectively, which is in consistent with the lattice constant of standard cubic MgO.

Figure 3. SEM images of the 2D MgO. a), b) and c) show the typical SEM images of the 2D MgO at different scales. The observed smooth surface and regular symmetrical shapes indicate that the 2D MgO has inherited the morphological properties of Mg(OH)2 completely. d) and e) respectively display the size and thickness distribution of the sample statistically extracted from

a). The scale distribution of 2D MgO is 3-15 µm; and its thickness distribution lies in 6-28 nm. f). SEM image of a typical hexagonal MgO flake. g) display the elemental mapping image of Mg.


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Figure 4. First-principles simulation of CAS process. a) Mg(OH)2 nanocluster. b) Mg(OH)2 is decomposed and dehydrated under high temperature annealing, c) Disordered MgO nanocluster is obtained after all the water molecules were completely disengaged. d) Continuous annealing makes the MgO nanocluster become orderly.

Figure 5. VUV photoresponse measurements of 2D MgO. a) Schematic presentation of VUV photoresponse measurement on 2D MgO photodetector relying on a synchrotron radiation source (beamline 4B8-Vacuum Ultraviolet of Beijing Synchrotron Radiation Facility). The inset shows the top view of the 2D MgO prototype device. Au electrodes were deposited by thermal evaporation using a standard copper mesh as mask. The electrodes gap was 5 µm and the MgO thickness was about 20 nm. b) I-V curves of the 2D MgO device under illumination (150 nm VUV light with power intensity of 0.85 picowatt irradiation on the device) and dark condition at room temperature. In order to clearly illustrate the fluctuation of the dark current, a corresponding plot of logarithm coordinate is displayed in the inset. c) The measured total noise current from 1 to 100 Hz under 4 V bias, the instrument noise floor, the calculated shot noise limit and thermal noise limit. d) Relationship between photoresponsivity and voltage under an illumination of 150 nm light. e) Spectral-dependent photoresponse curve of the 2D MgO photodetector measured at varying wavelengths (ranging from 150 to 320 nm) at a bias of 4 V. The spectral photoresponse curve shows an ultra-wide bandgap (about 170 nm, corresponding to 7.3 eV) of the 2D MgO, a high rejection ratio of VUV/UV-C (responsivity ratio of 150/260 nm),26 which is surpassing two orders of magnitude. f) Time-dependent photoresponse under an illumination intensity of 0.85 picowatt at a bias of 4 V. The VUV light source switch is controlled by a pneumatic valve, and both the switch on and off time is 1 s.


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Figure 1 28x9mm (300 x 300 DPI)



Figure 2 117x85mm (300 x 300 DPI)


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Figure 3 143x131mm (300 x 300 DPI)



Figure 4 93x55mm (300 x 300 DPI)


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Figure 5 170x184mm (300 x 300 DPI)



TOC Figure 43x26mm (300 x 300 DPI)


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