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Structure and optical properties of ternary alloy BeZnO and quaternary alloy BeMgZnO films growth by molecular beam epitaxy
Applied Surface Science 274 (2013) 341–344
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structure and optical properties of ternary alloy BeZnO and quaternary alloy BeMgZnO films growth by molecular beam epitaxy Longxing Su a,∗ , Yuan Zhu a,c,∗ , Quanlin Zhang a , Mingming Chen a , Tianzhun Wu a , Xuchun Gui a , Bicai Pan b , Rong Xiang a , Zikang Tang a,c,∗ a State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yet-Sen University, Guangzhou 510275, People’s Republic of China b Department of Physics and Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China c Physics Department, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
a r t i c l e
i n f o
Article history: Received 14 December 2012 Received in revised form 11 March 2013 Accepted 11 March 2013 Available online 16 March 2013 Keywords: BeZnO BeMgZnO Crystal quality Band-gap MSM detector
a b s t r a c t Ternary alloy BeZnO and quaternary alloy BeMgZnO films were prepared on sapphire (0 0 1) substrate by radio-frequency plasma-assisted molecular beam epitaxy (RF-PAMBE). Based on X-ray diffraction (XRD) analysis, no phase segregation is observed for all the alloys. However, Bex Zn1−x O alloys exhibit a constantly worse crystal quality than Bex Mgy Zn1−x−y O alloys at the similar incorporation contents (i.e. x in BeZnO approximately equals to x + y in BeMgZnO). Optical transmittance spectra were recorded to determine the energy band gap of the films. BeMgZnO was revealed more effective in widening the band gap. Finally, BeZnO and BeMgZnO based MSM structure UV detectors were fabricated. BeMgZnO alloys with better crystal quality showed a favorable optical response and the cutoff wavelength shifted continuously to deep ultraviolet range, while BeZnO based detectors were found no response. This is the first report on BeMgZnO based UV detector, which is a meaningful step forward to the real application. © 2013 Elsevier B.V. All rights reserved.
1. Introduction With wide band-gap (3.37 eV) and large exciton binding energy (60 meV), ZnO is regarded as a promising material in applications of short-wavelength optoelectronic devices. Optically pumped lasing effect in ZnO has been observed at room temperature [1–3]. Nevertheless, full realization of its application potential requires robust control of p-type doping, which is facing two difficulties: (1) high density of donor-like native defects (Vo , Zni ) with low ionization energy; (2) poor solubility and stability of acceptors with high ionization energy [4]. Alloying BeO into ZnO is a new approach showing great potential in solving above problems simultaneously. BeZnO alloys are expected to up-lift the conduction-band minimum (CBM) of ZnO band structure, lower the oxygen vacancies and enhance the nitrogen capture [5]. The CBM of BeO is 5.895 eV higher than ZnO and thus Be incorporation can hoist the ionization energy for the donor-like defects [6]. Moreover, stronger Be N bonding will help capture and maintain more acceptor dopants of N into ZnO host.
∗ Corresponding authors. Tel.: +86 2039943407. E-mail addresses: [email protected] (L. Su), [email protected] (Y. Zhu), [email protected] (Z. Tang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.03.058
On the other hand, the energy band-gap of BeZnO alloys can be continuously modulated from 3.37 eV (ZnO) to 10.6 eV (BeO) while keeping its hexagonal symmetry unchanged. Ryu et al. grown such alloy by a technique of hybrid beam epitaxy [7]. However, their XRD results implied that the crystal quality deteriorated rapidly with the increasing corporation of Be into ZnO host because of the large lattice mismatch between BeO and ZnO [8]. The poor crystal quality is fatal for a high performance UV device. This may be the reason that no further work on BeZnO-based photodetectors has been followed up after their pioneering work. Fan et al. calculate the formation energy of BeZnO with different Be concentration in ZnO host and found that the ternary alloy was unstable in the regime of intermediate Be content (x ∼ 0.5) [9]. In this paper, we prepared Bex Zn1−x O and Bex Mgy Zn1−x−y O alloy films using plasma-assisted molecular beam epitaxy (PAMBE). XRD measurement reveals the quaternary alloy BeMgZnO have a better crystal quality than the ternary alloy BeZnO, in which Mg plays a crucial role in lowering the formation energy of the alloy crystal. The optical investigation also shows BeMgZnO is more effective in adjusting the band gap. MSM structured UV detectors based on BeMgZnO quaternary alloy crystal thin films were fabricated, and they showed a promising optical responsivity with a favorable signal to noise ratio.
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2. Experiments Bex Zn1−x O and Bex Mgy Zn1−x−y O alloy films were grown on cAl2 O3 substrates by PAMBE. Active oxygen radicals were generated by an rf-plasma system (SVTA) using high-purity oxygen gas (6 N). Be (4 N), Zn (6 N) and Mg (6 N) source in Knudsen cells were evaporated by heating. The sapphire substrates were pretreated firstly by ultrasonic cleaning in acetone and ethanol for 15 min and then in the vacuumed growth chamber by thermal cleaning at 750 ◦ C for 20 min. The cleaned substrates were exposed to oxygen radicals to obtain a uniform oxygen-terminated surface. During the deposition process, Zn, Be and Mg fluxes were control by changing temperature of the sources. Generally, a higher source temperature results in a higher flux and thus higher content in the alloys. For Bex Zn1−x O alloys, the substrate and Zn source temperature were kept at 450 and 330 ◦ C respectively, while the Be source were set to 1130, 1140 and 1150 ◦ C for different samples. As to Bex Mgy Zn1−x−y O, we kept the substrate temperature, Mg and Be source temperature steady at 350, 350 and 1160 ◦ C, respectively. The Zn source temperatures were set to 330, 325 and 320 ◦ C for different samples. Typical thickness of the alloy epilayers is about 250 nm, growing for about 2 h. The crystal quality was studied by X-ray diffraction (Rigaku) with Cu K␣ 0.154 nm line as the radiation source. The contents of the Mg and Be in the alloys were estimated by the X-ray Photoelectron Spectroscopy (XPS). The surface morphology of the samples was investigated by atomic force microscope (AFM) technique. The transmission spectra were measured using UV-VIS-NIR scanning spectrophotometer (Shimadzu). The photoluminescence spectra were investigated using the 325 nm line of a continuous wave (cw) He-Cd laser in a backscattering configuration. MSM structured photodetectors were fabricated by depositing Ti/Au inter-digital finger layers onto the Bex Mgy Zn1−x−y O films. The interdigital fingers were fabricated via a photolithography and electron beam evaporation technique. The optical response of the photodetectors was measured using 150 W Xe lamp as the light source and recorded by a semiconductor parameter analyzer (Keithely 2200).
3. Results and discussion Fig. 1a shows XRD spectra of the Bex Zn1−x O films (x = 0, 0.17, 0.28, and 0.4). Besides the strong peak from the sapphire substrate, and trivial K line, a peak at around 35◦ can be observed for all the samples, and this peak can be attributed to (0 0 2) facet direction of wurtzite (hexagonal) ZnO and Bex Zn1−x O alloys. The absence of other characteristic peaks of wurtzite structure implies that the films have preferred (0 0 2) orientation without phase segregation. With increasing Be content, the (0 0 2) peak shifts to large-angle side, indicating the incorporation of Be into the lattice of ZnO. However, when Be content goes up to 0.4, the (0 0 2) peak shifts slightly backward, implies that overdose Be atoms squeeze into the interstitials of the lattice. With increasing Be content, the full width at half maximum (FWHM) of (0 0 2) peak goes broader eventually and the crystal quality deteriorate rapidly when Be content rise to 0.4. The poor crystal quality is attributed to the ˚ c = 4.24 A) ˚ and ZnO large lattice mismatch between BeO (a = 2.67 A, ˚ c = 5.23 A). ˚ With high Be content, a huge lattice relax(a = 3.25 A, ˚ ation is induced and some of the small Be atoms (radius 0.27 A) may get into the interstitials rather than occupy the lattice sites. The situation for BeMgZnO is better, which is identified by calculation based on DFT and experimental results. Theoretically, we substitute little amount of Mg for Be and find that the formation energy of the configuration descends, which indicates the quaternary alloy system becomes more stable (More calculated results will be reported elsewhere). Experimentally, as shown in Fig. 1b, only wurtzite BeMgZnO (0 0 2) and sapphire (0 0 6) peak
Fig. 1. XRD patterns of Bex Zn1−x O and Bex Zny Mg1−x−y O alloy films compared with ZnO films; (a) Bex Zn1−x O: Be0.17 Zn0.83 O, Be0.28 Zn0.72 O, Be0.4 Zn0.6 O; (b) Bex Mgy Zn1−x−y O: Be0.05 Mg0.13 Zn0.82 O, Be0.1 Mg0.23 Zn0.67 O, Be0.11 Mg0.28 Zn0.61 O; and (c) the full wide at half maximum (FWHM) of Bex Zn1−x O and Bex Zny Mg1−x−y O alloy films as a function of the Zn concentration. Bex Mgy Zn1−x−y O alloys have better crystal quality than Bex Zn1−x O at similar incorporation contents (i.e. x in BeZnO approximately equals to x + y in BeMgZnO).
is observed in XRD measurement. With the increase of incorporation elements content (i.e. Be + Zn content x + y), the (0 0 2) peak of W-BeMgZnO shifts less significantly to larger angles than BeZnO does, while without back turning as seen in the case of BeZnO alloys. This may be attributed to the similar radius of Mg (radius ˚ and Zn (radius 0.6 A), ˚ resulting in less lattice relaxation and 0.57 A)
L. Su et al. / Applied Surface Science 274 (2013) 341–344
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Fig. 2. AFM images of MBE deposited Be0.4 Zn0.6 O and Be0.11 Mg0.28 Zn0.61 O films. The root mean square rough of the films were 5.9 and 2.8 nm, respectively.
hence more sites occupation or surly less interstitial occupation. It is also noticeable that the FWHM of BeMgZnO is broadening more slowly than that of BeZnO (Fig. 1c), which evidences that the crystal quality of BeMgZnO is constantly better than that of BeZnO. To confirm this, the surface morphology of Be0.4 Zn0.6 O and Be0.11 Mg0.28 Zn0.61 O thin films was investigated by the AFM technique as shown in Fig. 2. Over a scale of 2 m × 2 m for Be0.4 Zn0.6 O and 3 m × 3 m for Be0.11 Mg0.28 Zn0.61 O, the root-mean-square roughness was 5.9 nm and 2.8 nm, respectively. The quaternary alloy films have a smoother surface than that of ternary alloy film, which support our assertion that Bex Mgy Zn1−x−y O alloys have better crystal quality than Bex Zn1−x O at similar incorporation contents (i.e. x in BeZnO approximately equals to x + y in BeMgZnO). Fig. 3 presents the optical transmission spectra of the as-grown Bex Zn1−x O and Bex Mgy Zn1−x−y O alloy films. All the samples show sharp absorption edge and high transmittance of about 90% in the visible spectrual region. The inset is a plot of (˛hv) [2] versus hv from which the band gaps of the films can be derived. The transmission spectra of the Bex Zn1−x O films (Fig. 3a) indicate: (1) the absorption edge shifts to shorter wavelength with increasing Be content; (2) the absorption edge becomes less steep when more Be atoms is incorporated, suggesting the deteriorated crystal quality; and (3) the optical transparency increases to 30% at 240 nm when x = 0.4. The transmission spectra of the Bex Mgy Zn1−x−y O films (Fig. 3b) seem similar as that of Bex Zn1−x O films, only that the degree of band-gap widening and edge sharpness are different. The energy band gap of Bex Mgy Zn1−x−y O is adjusted from 3.4 eV to 4.1
Ev, while the range of Bex Zn1−x O in Fig. 3a is from 3.4 eV to 3.52 eV. Furthermore, the cut-off edges of BeMgZnO alloys are sharper than that of the BeZnO alloys, which also suggests the better crystal quality of the quaternary alloys. The better crystal quality of BeMgZnO alloy increases their chance in realizing effective UV detectors. Fig. 4 exhibits the photoluminescence spectra of the as-grown Bex Zn1−x O and Bex Mgy Zn1−x−y O alloy films. For Bex Zn1−x O alloys, the near band edge (NBE) emission peak slightly shifts from 380 to 354 nm (x = 0.4) with prominent broadening as can be seen in Fig. 4a. The growing of the green band is also notable. Its intensity is even stronger than that of free exciton for x = 0.4. These finding indicate the crystal quality of Bex Zn1−x O alloys deteriorate rapidly with increasing Be content. Be doping causes severe lattice mismatch and therefore brings tremendous defects to the ZnO host. On contrary, the quaternary Bex Mgy Zn1−x−y O alloy with similar doping concentration (x + y = 0.39) realizes a more effective bandgap widening. As shown in Fig. 4b, its NBE emission peak blue-shifts to as short as 334 nm, while keeping the peak broadening relatively low. In addition, the green band as a sign for defects is hardly seen. This indicates that Bex Mgy Zn1−x−y O alloys are better crystalized than Bex Zn1−x O alloys. It is worth noting that the NBE emission peaks here are located at lower energy side than the cut-off edge in Fig. 3. This can be ascribed to the stokes shift, which is commonly observed in alloy systems [10]. The inset of Fig. 5 shows the schematic illustration of the MSM structured photodetectors made from BeMgZnO crystal films. Interdigital Ti/Au fingers were deposited onto the top surface of
Fig. 3. Optical transmission spectra of Bex Zn1−x O and Bex Mgy Zn1−x−y O alloy films compared with ZnO films. (a) Bex Zn1−x O: Be0.17 Zn0.83 O, Be0.28 Zn0.72 O, Be0.4 Zn0.6 O. (b) Bex Mgy Zn1−x−y O: Be0.05 Mg0.13 Zn0.82 O, Be0.1 Mg0.23 Zn0.67 O, Be0.11 Mg0.28 Zn0.61 O. Insets are the corresponding absorption spectra.
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Fig. 4. Photoluminescence spectra of Bex Zn1−x O (a) and Bex Mgy Zn1−x−y O (b) alloy films.
4. Conclusions
Fig. 5. Normalize spectra responsivity of the BeMgZnO-based MSM structure UV detector measured at 10 V bias; the inset is the schematic illustration of BeMgZnObased MSM UV detector.
the BeMgZnO films as electrodes, where the active area of the device is located in the vicinity of the interdigital Ti/Au fingers. The fingers were 3 m in width and 2 mm in length, and the interelectrode spacing is 8 m. Since the defects will scatter the photoelectrons, good crystal quality is a key prerequisite for the UV detectors. The BeZnO based UV detectors were found no response to UV light. We believe crystal quality is the fatal obstacle for BeZnO based UV detectors, which is so far never reported after several publications on BeZnO alloys and their wide-range bandgap engineering. On contrary, the detectors fabricated based on the quaternary BeMgZnO alloys illustrate a favorable response spectra. Fig. 5 exhibits the response spectral of the BeMgZnO photodetectors at 10 V bias. With increasing Mg + Be content, a shift to shorter wavelength was observed. This result is consistent with the transmission spectra shown in Fig. 3b. This is the first report on BeMgZnO based UV detector, which is a meaningful step toward to the real application.
In summary, single-phase wurtzite BeZnO and BeMgZnO films were grown on c-plane (0 0 1) sapphire substrate by MBE. Their crystal structure properties and energy band gap have been studied. From the XRD result, the crystal qualities of BeMgZnO alloy films are found better than that of BeZnO alloy films. Through optical transmission and photoluminescence spectra, BeMgZnO alloys are found more effective in widening the energy band gap. Finally, BeMgZnObased MSM structure UV detectors have been constructed, and exhibit a favorable response property in UV light range, while the trials on BeZnO-based detectors failed. The advantages of BeMgZnO over BeZnO are primarily ascribed to that the Mg ion radius is very similar to Zn, while the lattice mismatch caused by Be incorporation is large. The co-incorporation of Mg and Be may hopefully keep the crystal quality and widen the band-gap simultaneously. This is the first report on BeZnMgO based UV detector, which is a meaningful step forward to the real application. Acknowledgments This work is partly supported by National Science Foundation of China (No. 10974262) and National Key Basic Research Program (No. 2011CB302000). References [1] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Communications 103 (8) (1997) 459–463. [2] Z.K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Applied Physics Letters 72 (25) (1998) 3270–3272. [3] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Applied Physics Letters 70 (17) (1997) 2230–2232. [4] S.B. Zhang, S.H. Wei, A. Zunger, Physical Review B 63 (7) (2001). [5] M. Chen, et al., Materials Research Bulletin 47 (2012). [6] M. Chen, et al., Journal of Physics D: Applied Physics 45 (45) (2012). [7] Y.R. Ryu, T.S. Lee, J.A. Lubguban, A.B. Corman, H.W. White, J.H. Leem, M.S. Han, Y.S. Park, C.J. Youn, W.J. Kim, Applied Physics Letters 88 (5) (2006). [8] W.J. Kim, J.H. Leem, M.S. Han, Y.R. Ryu, T.S. Lee, Journal of Applied Physics 99 (9) (2006). [9] X.F. Fan, Z. Zhu, Y.S. Ong, Y.M. Lu, Z.X. Shen, J.L. Kuo, Applied Physics Letters 91 (12) (2007). [10] R. Zimmermann, Journal of Crystal Growth 101 (346) (1990).
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structure and optical properties of ternary alloy BeZnO and quaternary alloy BeMgZnO films growth by molecular beam epitaxy Longxing Su a,∗ , Yuan Zhu a,c,∗ , Quanlin Zhang a , Mingming Chen a , Tianzhun Wu a , Xuchun Gui a , Bicai Pan b , Rong Xiang a , Zikang Tang a,c,∗ a State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yet-Sen University, Guangzhou 510275, People’s Republic of China b Department of Physics and Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China c Physics Department, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
a r t i c l e
i n f o
Article history: Received 14 December 2012 Received in revised form 11 March 2013 Accepted 11 March 2013 Available online 16 March 2013 Keywords: BeZnO BeMgZnO Crystal quality Band-gap MSM detector
a b s t r a c t Ternary alloy BeZnO and quaternary alloy BeMgZnO films were prepared on sapphire (0 0 1) substrate by radio-frequency plasma-assisted molecular beam epitaxy (RF-PAMBE). Based on X-ray diffraction (XRD) analysis, no phase segregation is observed for all the alloys. However, Bex Zn1−x O alloys exhibit a constantly worse crystal quality than Bex Mgy Zn1−x−y O alloys at the similar incorporation contents (i.e. x in BeZnO approximately equals to x + y in BeMgZnO). Optical transmittance spectra were recorded to determine the energy band gap of the films. BeMgZnO was revealed more effective in widening the band gap. Finally, BeZnO and BeMgZnO based MSM structure UV detectors were fabricated. BeMgZnO alloys with better crystal quality showed a favorable optical response and the cutoff wavelength shifted continuously to deep ultraviolet range, while BeZnO based detectors were found no response. This is the first report on BeMgZnO based UV detector, which is a meaningful step forward to the real application. © 2013 Elsevier B.V. All rights reserved.
1. Introduction With wide band-gap (3.37 eV) and large exciton binding energy (60 meV), ZnO is regarded as a promising material in applications of short-wavelength optoelectronic devices. Optically pumped lasing effect in ZnO has been observed at room temperature [1–3]. Nevertheless, full realization of its application potential requires robust control of p-type doping, which is facing two difficulties: (1) high density of donor-like native defects (Vo , Zni ) with low ionization energy; (2) poor solubility and stability of acceptors with high ionization energy [4]. Alloying BeO into ZnO is a new approach showing great potential in solving above problems simultaneously. BeZnO alloys are expected to up-lift the conduction-band minimum (CBM) of ZnO band structure, lower the oxygen vacancies and enhance the nitrogen capture [5]. The CBM of BeO is 5.895 eV higher than ZnO and thus Be incorporation can hoist the ionization energy for the donor-like defects [6]. Moreover, stronger Be N bonding will help capture and maintain more acceptor dopants of N into ZnO host.
∗ Corresponding authors. Tel.: +86 2039943407. E-mail addresses: [email protected] (L. Su), [email protected] (Y. Zhu), [email protected] (Z. Tang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.03.058
On the other hand, the energy band-gap of BeZnO alloys can be continuously modulated from 3.37 eV (ZnO) to 10.6 eV (BeO) while keeping its hexagonal symmetry unchanged. Ryu et al. grown such alloy by a technique of hybrid beam epitaxy [7]. However, their XRD results implied that the crystal quality deteriorated rapidly with the increasing corporation of Be into ZnO host because of the large lattice mismatch between BeO and ZnO [8]. The poor crystal quality is fatal for a high performance UV device. This may be the reason that no further work on BeZnO-based photodetectors has been followed up after their pioneering work. Fan et al. calculate the formation energy of BeZnO with different Be concentration in ZnO host and found that the ternary alloy was unstable in the regime of intermediate Be content (x ∼ 0.5) [9]. In this paper, we prepared Bex Zn1−x O and Bex Mgy Zn1−x−y O alloy films using plasma-assisted molecular beam epitaxy (PAMBE). XRD measurement reveals the quaternary alloy BeMgZnO have a better crystal quality than the ternary alloy BeZnO, in which Mg plays a crucial role in lowering the formation energy of the alloy crystal. The optical investigation also shows BeMgZnO is more effective in adjusting the band gap. MSM structured UV detectors based on BeMgZnO quaternary alloy crystal thin films were fabricated, and they showed a promising optical responsivity with a favorable signal to noise ratio.
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2. Experiments Bex Zn1−x O and Bex Mgy Zn1−x−y O alloy films were grown on cAl2 O3 substrates by PAMBE. Active oxygen radicals were generated by an rf-plasma system (SVTA) using high-purity oxygen gas (6 N). Be (4 N), Zn (6 N) and Mg (6 N) source in Knudsen cells were evaporated by heating. The sapphire substrates were pretreated firstly by ultrasonic cleaning in acetone and ethanol for 15 min and then in the vacuumed growth chamber by thermal cleaning at 750 ◦ C for 20 min. The cleaned substrates were exposed to oxygen radicals to obtain a uniform oxygen-terminated surface. During the deposition process, Zn, Be and Mg fluxes were control by changing temperature of the sources. Generally, a higher source temperature results in a higher flux and thus higher content in the alloys. For Bex Zn1−x O alloys, the substrate and Zn source temperature were kept at 450 and 330 ◦ C respectively, while the Be source were set to 1130, 1140 and 1150 ◦ C for different samples. As to Bex Mgy Zn1−x−y O, we kept the substrate temperature, Mg and Be source temperature steady at 350, 350 and 1160 ◦ C, respectively. The Zn source temperatures were set to 330, 325 and 320 ◦ C for different samples. Typical thickness of the alloy epilayers is about 250 nm, growing for about 2 h. The crystal quality was studied by X-ray diffraction (Rigaku) with Cu K␣ 0.154 nm line as the radiation source. The contents of the Mg and Be in the alloys were estimated by the X-ray Photoelectron Spectroscopy (XPS). The surface morphology of the samples was investigated by atomic force microscope (AFM) technique. The transmission spectra were measured using UV-VIS-NIR scanning spectrophotometer (Shimadzu). The photoluminescence spectra were investigated using the 325 nm line of a continuous wave (cw) He-Cd laser in a backscattering configuration. MSM structured photodetectors were fabricated by depositing Ti/Au inter-digital finger layers onto the Bex Mgy Zn1−x−y O films. The interdigital fingers were fabricated via a photolithography and electron beam evaporation technique. The optical response of the photodetectors was measured using 150 W Xe lamp as the light source and recorded by a semiconductor parameter analyzer (Keithely 2200).
3. Results and discussion Fig. 1a shows XRD spectra of the Bex Zn1−x O films (x = 0, 0.17, 0.28, and 0.4). Besides the strong peak from the sapphire substrate, and trivial K line, a peak at around 35◦ can be observed for all the samples, and this peak can be attributed to (0 0 2) facet direction of wurtzite (hexagonal) ZnO and Bex Zn1−x O alloys. The absence of other characteristic peaks of wurtzite structure implies that the films have preferred (0 0 2) orientation without phase segregation. With increasing Be content, the (0 0 2) peak shifts to large-angle side, indicating the incorporation of Be into the lattice of ZnO. However, when Be content goes up to 0.4, the (0 0 2) peak shifts slightly backward, implies that overdose Be atoms squeeze into the interstitials of the lattice. With increasing Be content, the full width at half maximum (FWHM) of (0 0 2) peak goes broader eventually and the crystal quality deteriorate rapidly when Be content rise to 0.4. The poor crystal quality is attributed to the ˚ c = 4.24 A) ˚ and ZnO large lattice mismatch between BeO (a = 2.67 A, ˚ c = 5.23 A). ˚ With high Be content, a huge lattice relax(a = 3.25 A, ˚ ation is induced and some of the small Be atoms (radius 0.27 A) may get into the interstitials rather than occupy the lattice sites. The situation for BeMgZnO is better, which is identified by calculation based on DFT and experimental results. Theoretically, we substitute little amount of Mg for Be and find that the formation energy of the configuration descends, which indicates the quaternary alloy system becomes more stable (More calculated results will be reported elsewhere). Experimentally, as shown in Fig. 1b, only wurtzite BeMgZnO (0 0 2) and sapphire (0 0 6) peak
Fig. 1. XRD patterns of Bex Zn1−x O and Bex Zny Mg1−x−y O alloy films compared with ZnO films; (a) Bex Zn1−x O: Be0.17 Zn0.83 O, Be0.28 Zn0.72 O, Be0.4 Zn0.6 O; (b) Bex Mgy Zn1−x−y O: Be0.05 Mg0.13 Zn0.82 O, Be0.1 Mg0.23 Zn0.67 O, Be0.11 Mg0.28 Zn0.61 O; and (c) the full wide at half maximum (FWHM) of Bex Zn1−x O and Bex Zny Mg1−x−y O alloy films as a function of the Zn concentration. Bex Mgy Zn1−x−y O alloys have better crystal quality than Bex Zn1−x O at similar incorporation contents (i.e. x in BeZnO approximately equals to x + y in BeMgZnO).
is observed in XRD measurement. With the increase of incorporation elements content (i.e. Be + Zn content x + y), the (0 0 2) peak of W-BeMgZnO shifts less significantly to larger angles than BeZnO does, while without back turning as seen in the case of BeZnO alloys. This may be attributed to the similar radius of Mg (radius ˚ and Zn (radius 0.6 A), ˚ resulting in less lattice relaxation and 0.57 A)
L. Su et al. / Applied Surface Science 274 (2013) 341–344
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Fig. 2. AFM images of MBE deposited Be0.4 Zn0.6 O and Be0.11 Mg0.28 Zn0.61 O films. The root mean square rough of the films were 5.9 and 2.8 nm, respectively.
hence more sites occupation or surly less interstitial occupation. It is also noticeable that the FWHM of BeMgZnO is broadening more slowly than that of BeZnO (Fig. 1c), which evidences that the crystal quality of BeMgZnO is constantly better than that of BeZnO. To confirm this, the surface morphology of Be0.4 Zn0.6 O and Be0.11 Mg0.28 Zn0.61 O thin films was investigated by the AFM technique as shown in Fig. 2. Over a scale of 2 m × 2 m for Be0.4 Zn0.6 O and 3 m × 3 m for Be0.11 Mg0.28 Zn0.61 O, the root-mean-square roughness was 5.9 nm and 2.8 nm, respectively. The quaternary alloy films have a smoother surface than that of ternary alloy film, which support our assertion that Bex Mgy Zn1−x−y O alloys have better crystal quality than Bex Zn1−x O at similar incorporation contents (i.e. x in BeZnO approximately equals to x + y in BeMgZnO). Fig. 3 presents the optical transmission spectra of the as-grown Bex Zn1−x O and Bex Mgy Zn1−x−y O alloy films. All the samples show sharp absorption edge and high transmittance of about 90% in the visible spectrual region. The inset is a plot of (˛hv) [2] versus hv from which the band gaps of the films can be derived. The transmission spectra of the Bex Zn1−x O films (Fig. 3a) indicate: (1) the absorption edge shifts to shorter wavelength with increasing Be content; (2) the absorption edge becomes less steep when more Be atoms is incorporated, suggesting the deteriorated crystal quality; and (3) the optical transparency increases to 30% at 240 nm when x = 0.4. The transmission spectra of the Bex Mgy Zn1−x−y O films (Fig. 3b) seem similar as that of Bex Zn1−x O films, only that the degree of band-gap widening and edge sharpness are different. The energy band gap of Bex Mgy Zn1−x−y O is adjusted from 3.4 eV to 4.1
Ev, while the range of Bex Zn1−x O in Fig. 3a is from 3.4 eV to 3.52 eV. Furthermore, the cut-off edges of BeMgZnO alloys are sharper than that of the BeZnO alloys, which also suggests the better crystal quality of the quaternary alloys. The better crystal quality of BeMgZnO alloy increases their chance in realizing effective UV detectors. Fig. 4 exhibits the photoluminescence spectra of the as-grown Bex Zn1−x O and Bex Mgy Zn1−x−y O alloy films. For Bex Zn1−x O alloys, the near band edge (NBE) emission peak slightly shifts from 380 to 354 nm (x = 0.4) with prominent broadening as can be seen in Fig. 4a. The growing of the green band is also notable. Its intensity is even stronger than that of free exciton for x = 0.4. These finding indicate the crystal quality of Bex Zn1−x O alloys deteriorate rapidly with increasing Be content. Be doping causes severe lattice mismatch and therefore brings tremendous defects to the ZnO host. On contrary, the quaternary Bex Mgy Zn1−x−y O alloy with similar doping concentration (x + y = 0.39) realizes a more effective bandgap widening. As shown in Fig. 4b, its NBE emission peak blue-shifts to as short as 334 nm, while keeping the peak broadening relatively low. In addition, the green band as a sign for defects is hardly seen. This indicates that Bex Mgy Zn1−x−y O alloys are better crystalized than Bex Zn1−x O alloys. It is worth noting that the NBE emission peaks here are located at lower energy side than the cut-off edge in Fig. 3. This can be ascribed to the stokes shift, which is commonly observed in alloy systems [10]. The inset of Fig. 5 shows the schematic illustration of the MSM structured photodetectors made from BeMgZnO crystal films. Interdigital Ti/Au fingers were deposited onto the top surface of
Fig. 3. Optical transmission spectra of Bex Zn1−x O and Bex Mgy Zn1−x−y O alloy films compared with ZnO films. (a) Bex Zn1−x O: Be0.17 Zn0.83 O, Be0.28 Zn0.72 O, Be0.4 Zn0.6 O. (b) Bex Mgy Zn1−x−y O: Be0.05 Mg0.13 Zn0.82 O, Be0.1 Mg0.23 Zn0.67 O, Be0.11 Mg0.28 Zn0.61 O. Insets are the corresponding absorption spectra.
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Fig. 4. Photoluminescence spectra of Bex Zn1−x O (a) and Bex Mgy Zn1−x−y O (b) alloy films.
4. Conclusions
Fig. 5. Normalize spectra responsivity of the BeMgZnO-based MSM structure UV detector measured at 10 V bias; the inset is the schematic illustration of BeMgZnObased MSM UV detector.
the BeMgZnO films as electrodes, where the active area of the device is located in the vicinity of the interdigital Ti/Au fingers. The fingers were 3 m in width and 2 mm in length, and the interelectrode spacing is 8 m. Since the defects will scatter the photoelectrons, good crystal quality is a key prerequisite for the UV detectors. The BeZnO based UV detectors were found no response to UV light. We believe crystal quality is the fatal obstacle for BeZnO based UV detectors, which is so far never reported after several publications on BeZnO alloys and their wide-range bandgap engineering. On contrary, the detectors fabricated based on the quaternary BeMgZnO alloys illustrate a favorable response spectra. Fig. 5 exhibits the response spectral of the BeMgZnO photodetectors at 10 V bias. With increasing Mg + Be content, a shift to shorter wavelength was observed. This result is consistent with the transmission spectra shown in Fig. 3b. This is the first report on BeMgZnO based UV detector, which is a meaningful step toward to the real application.
In summary, single-phase wurtzite BeZnO and BeMgZnO films were grown on c-plane (0 0 1) sapphire substrate by MBE. Their crystal structure properties and energy band gap have been studied. From the XRD result, the crystal qualities of BeMgZnO alloy films are found better than that of BeZnO alloy films. Through optical transmission and photoluminescence spectra, BeMgZnO alloys are found more effective in widening the energy band gap. Finally, BeMgZnObased MSM structure UV detectors have been constructed, and exhibit a favorable response property in UV light range, while the trials on BeZnO-based detectors failed. The advantages of BeMgZnO over BeZnO are primarily ascribed to that the Mg ion radius is very similar to Zn, while the lattice mismatch caused by Be incorporation is large. The co-incorporation of Mg and Be may hopefully keep the crystal quality and widen the band-gap simultaneously. This is the first report on BeZnMgO based UV detector, which is a meaningful step forward to the real application. Acknowledgments This work is partly supported by National Science Foundation of China (No. 10974262) and National Key Basic Research Program (No. 2011CB302000). References [1] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Communications 103 (8) (1997) 459–463. [2] Z.K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Applied Physics Letters 72 (25) (1998) 3270–3272. [3] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Applied Physics Letters 70 (17) (1997) 2230–2232. [4] S.B. Zhang, S.H. Wei, A. Zunger, Physical Review B 63 (7) (2001). [5] M. Chen, et al., Materials Research Bulletin 47 (2012). [6] M. Chen, et al., Journal of Physics D: Applied Physics 45 (45) (2012). [7] Y.R. Ryu, T.S. Lee, J.A. Lubguban, A.B. Corman, H.W. White, J.H. Leem, M.S. Han, Y.S. Park, C.J. Youn, W.J. Kim, Applied Physics Letters 88 (5) (2006). [8] W.J. Kim, J.H. Leem, M.S. Han, Y.R. Ryu, T.S. Lee, Journal of Applied Physics 99 (9) (2006). [9] X.F. Fan, Z. Zhu, Y.S. Ong, Y.M. Lu, Z.X. Shen, J.L. Kuo, Applied Physics Letters 91 (12) (2007). [10] R. Zimmermann, Journal of Crystal Growth 101 (346) (1990).