- Email: [email protected]
Synthesis and optoelectronic properties of p-type nitrogen doped ZnSe nanobelts
Materials Letters 92 (2013) 338–341
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and optoelectronic properties of p-type nitrogen doped ZnSe nanobelts Qing Su a, Lijuan Li b, Shanying Li a,n, Haipeng Zhao a a b
Department of Chemistry and Chemical Engineering, Henan University of Urban Construction, Pingdingshan, Henan 467036, PR China Department of Mechanical Engineering, Min Xi Vocational & Technical College, Longyan, Fujian 364021, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 September 2012 Accepted 30 October 2012 Available online 6 November 2012
Single-crystal nitrogen (N) doped p-type ZnSe nanobelts (NBs) with zinc blende structure were synthesized in ammonia atmosphere via a thermal evaporation method. The p-type conductivity of ZnSe:N NBs was confirmed by field-effect transistors (FETs) based on individual NBs. High-performance photodetectors were constructed based on ZnSe:N NBs, which show high sensitivity and relatively fast response speed to the incident light with a sharp cut-off at 460 nm, corresponding to the band-gap of ZnSe. The high photosensitivity and relatively fast response speed are attributable to the high crystal quality of the ZnTe nanowires. These results reveal that such single-crystalline ZnSe NBs are excellent candidates for optoelectronic applications. & 2012 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline materials Semiconductors ZnSe nanobelts Nitrogen doped Photoresponse
1. Introduction One-dimensional (1D) inorganic nanostructures such as nanowires (NWs), nanobelts (NBs), and nanotubes (NTs) have been studied extensively with respect to fundamental properties and potential applications in nanoscale devices [1–8]. They are expected to play important roles as the key units of nextgeneration nanoscale electronic, optoelectronic devices. Due to large surface-to-volume ratios and Debye length comparable to their small sizes, 1D inorganic nanostructures have already displayed superior characteristics in diverse experiments compared with traditional thin-film and bulk materials [9–14]. As one of the most important II–VI nanostructure semiconductors, ZnSe with a direct band-gap of 2.7 eV have attracted much attention in the past decade owing to their unique optical and electrical properties. They are good candidates for the building blocks of functional nano-devices such as field-effect transistors (FETs), photodetectors (PDs), light-emitting diodes (LEDs) and so on [15–19]. In the past decade, synthesis of ZnSe nanostructures has been intensively studied via various growth methods [20–24]. However, contact resistance caused by interface defects and the work function mismatch is usually observed in ZnSe nano-devices. Large contact resistance will result in the device performance degradation, which is needed to be eliminated for realizing high-performance devices. Besides the use of appropriate metal electrode, doping has been demonstrated to be an efficient way, which is also a feasible method
n
Corresponding author. Tel.: þ86 375 2089043; fax: þ86 375 2089041. E-mail address: [email protected] (S. Li).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.112
to tune their optoelectronic properties and further enhance their device performances [25,26]. So far, As and Bi doping have been utilized to enhance the p-type conductivity of the ZnSe nanowires, representing an important achievement in nanomaterials synthesis and offering the opportunities for future nanodevice applications [15,18]. In spite of these progresses, the efficient synthesis of p-type ZnSe nanostructures with reliable and reproducible is much demanded to promote the applications of ZnSe nanostructures in further study. Moreover, N-doping nanostructures such as ZnO NWs and ZnTe NWs via ammonia atmosphere have been reported [27,28]. A simple method with high-doping efficiency is much desired to realize ZnSe nanostructures with well-controlled p-type conductivity. In this paper, we describe the synthesis of N-doped p-type ZnSe NBs in ammonia atmosphere via a thermal evaporation method. FETs and photodetectors based on single ZnSe:N NBs were constructed to study their transport and optoelectronic properties. Our results demonstrated that the p-type ZnSe NBs may have important applications in nanoelectronics and nano-optoelectronics.
2. Experimental ZnSe NBs synthesis and characterization: The undoped ZnSe nanomaterials are highly insulative semiconductors with very low conduction current of 10 13 A [29], which cannot function as elementary components for nanodevices. Hence, N-doping was employed by using ammonia in this work. Growths of N-doped p-type ZnSe NBs were carried out in an alumina tube furnace via thermal evaporation. High-purity ZnSe powder (99.999%) was loaded into an alumina boat and placed at the center of the tube
Q. Su et al. / Materials Letters 92 (2013) 338–341
furnace, and Si substrates coated with a layer of 10 nm gold catalyst were placed in the downstream 10 cm away from the source. Before heating, the system was evacuated to a base pressure of 10 5 Torr and back filled with Ar/NH3 gas mixture at a constant flow rate of 85/15 sccm to a stable pressure of 150 Torr. Afterward, the ZnSe source was heated up to 1050 1C and maintained at that temperature for 2 h. The system was cooled down to room temperature, and the Si substrates were taken out of the furnace. A layer of yellow wool-like product could be observed on the substrates surface. Morphologies and structures of the as-synthesized ZnSe NBs were characterized by X-ray diffraction with Cu Ka radiation (XRD, D/max-rB), field-emission scanning electron microscopy (FESEM, SIRION200, FEI, at 5 kV), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010, at 200 kV) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250). Device construction: To assess the electrical transport and optoelectronic properties of the ZnSe:N NBs, FETs based on the individual ZnSe:N NBs were constructed as follows: First, the ZnSe:N NBs were dispersed on SiO2 (300 nm)/p-Si substrates. Then, photolithography, and lift-off processes were used to fabricate the source and drain Au (100 nm) electrodes on an individual ZnSe:N NB. The p-Si substrates and SiO2 served as the bottom-gate and dielectric, respectively. The electrical transport measurements were conducted with a semiconductor characterization system (Keithley 4200). The photoresponse characteristics of the ZnSe NB photodetectors were detected by a monochromatic light source.
clean and free of evident impurities and particles. The width and length of the NBs are in the range of 2–5 mm and tens of micrometers, respectively. HRTEM image and the corresponding fast Fourier transform (FFT) pattern recorded from a single NB are shown in Fig. 1(b), indicating that the NB is zinc blende structure with [ 1 11] growth orientation. In the XRD patterns of the ZnSe NBs (Fig. 1c), all the diffraction peaks can be assigned to zinc blende ZnSe (JCPDS 88-2345) and no obvious impurity phases and peak shift are observed, suggesting a single phase of the product. In addition, a peak at 398 eV corresponding to the N 1 s was found in XPS detection performed on the ZnSe:N NBs, confirming the existence of N in the NBs. Electrical characterization of ZnSe:N NB FET: Fig. 2(a) and (c) shows the schematic illustration and SEM of FET based on an individual ZnSe:N NB, respectively. Fig. 2(b) depicts the typical I–V curves of an individual ZnSe:N NB measured before and after annealing at 550 1C for 5 min in Ar atmosphere, respectively. The linear curves indicate the good Ohmic contact between the Au electrodes and the ZnSe:N NB [15]. By applying gate voltage (VG) on the bottom-gate of the FET, the gate-dependent source–drain current (IDS) versus source–drain voltage (VDS) curves are detected. From Fig. 2(c), it is shown that the conductance of the ZnSe:N NB decreases (increases) consistently with the increasing (decreasing) of the gate voltage in step of 15 V, revealing a pronounced p-type conductivity of the ZnSe:N NB. The hole mobility (mh) can be estimated to be 0.98 10 2 cm2 V 1 s 1 based on the following equation: gm ¼
3. Results and discussion Characterization of the p-type ZnSe NBs: Fig. 1(a) shows the typical FESEM image of the ZnSe NBs. It is seen that the product is
339
dIDS Z ¼ mh C 0 V DS L dV G
ð1Þ
where gm ¼0.8 nS represents the transconductance of the ZnSe NB FET deduced from the linear part of the IDS–VG curves (Fig. 2d), Z/L is the width-to-length ratio of the NB channel. The capacitance per
Fig. 1. (a) FESEM image, (b) HRTEM image, (c) XRD patterns and (d) XPS spectrum of the as-synthesized ZnSe:N NBs. Insets of (b) and (d) show the corresponding SAED pattern and the enlarged N peaks, respectively.
340
Q. Su et al. / Materials Letters 92 (2013) 338–341
Fig. 2. (a) Schematic illustration of nanoFET based on an individual ZnSe:N NB. (b) I–V curves of the ZnSe:N NB before and after annealing. (c) IDS–VDS curves of the ZnSe:N NB nanoFET measured at varied VG from 30 to 30 V in a step of 15 V. Inset is a SEM image of nanoFET. The scale bar is 5 mm. (d) IDS–VG curves at VDS ¼3 V.
Fig. 3. (a) I–V curves of an individual ZnSe:N NB exposed to light of varied wavelength at a constant intensity of 1.0 mW cm 2, (b) spectral response of the ZnSe:N NB, (c) time response of ZnSe:N NB under light illumination of 460 nm, and (d) enlarged rising and falling edges.
unit area is given by C0 ¼ ee0/h, where e0 is the dielectric constant (3.9 for SiO2) and h (300 nm) is the thickness of the SiO2 dielectric layer. The threshold voltage (Vth) obtained at the point deviated from the linear region of the curve is 18 V and an Ion/Ioff ratio of 6 is obtained when VG changes from 40 V to 40 V. It is worth noting that relative low device performances may duo to the weak
gate coupling caused by the SiO2 dielectric and the bottom-gate device configuration [30,31]. Optoelectronic characterization of ZnSe:N NB PD: ZnSe is a promising material for blue light detection due to the appropriate band-gap of 2.26 eV and the high crystalline quality. Fig. 3(a) depicts the wavelength-dependent I–V curves of the ZnSe:N NB photodetector
Q. Su et al. / Materials Letters 92 (2013) 338–341
Table 1 Performances of ZnSe PD from previous reports are presented for comparison. n ZnSe:Cl with three different doping levels were compared. Ion/Ioff
ZnSe:N 150 ZnSe:Cl 3 2 2 ZnSe:Bi 3.5 n
Rise time (s)
Fall time (s)
0.4 0.5 2 2 27.6 42.8 120.2 394.2 0.38 0.4
Responsivity Photoconductive Reference (AW 1) gain
2.4 104 4.1 102 1.9 104 7.9 105 n.a.
6.5 104 1.1 103 5.1 104 2.1 106 n.a.
This work [29]
[15]
obtained when the device was exposed to monochromatic light at a constant intensity of 1.0 mW cm 2. It is seen that the NB’s conductance strongly depends on the light wavelength, which increases with decreasing wavelength and reaches the maximum value at approximately 460 nm. From the spectral response (Fig. 3b), it is seen that sensitivity is highest around 460 nm and shows a steep decline in the long wavelength direction. The cut-off wavelength of 460 nm is consistent with the ZnSe band-gap, indicating that the electron–hole pairs excited by light with energy larger than the band-gap of ZnSe. Furthermore, the ZnSe NB photodetector also shows good stability and reproducibility to a pulsed light of 460 nm with ILight/IDark of 150 as shown in Fig. 3(c). The enlarged rise and fall edges reveal a relatively fast rise and fall time of 0.4 s and 0.5 s, respectively, which are much faster than the values of Cl-doped ZnSe NWs [29]. Responsivity (R) and photoconductive gain (G) of a photodetetctor are defined as follow: Ip ql R A=W ¼ ¼Z G P ph hc
ð2Þ
where Ip, Pph, Z, h, c and l are photocurrent, incident light power, quantum efficiency, Planck’s constant, light speed and light wavelength, respectively. Based on this equation, R and G for ZnSe:N NB photodetector are estimated to be 2.4 104 AW 1 and 6.5 104, respectively. The high sensitivity of the ZnSe:N NB photodetectors could be ascribed to the high crystal quality and large surface-tovolume ratio of the NBs and consequently a large photocurrent. Table 1 summarizes the performances of the ZnSe:N NB PD, along with the results for other doped ZnSe NWs for comparison, it is worth noting that the devices in this work show robust performances close or superior to the previous results. Our results demonstrated that the p-type ZnSe:N NBs may have important applications in nanoelectronics and nano-optoelectronics.
4. Conclusion In summary, p-type ZnSe:N NBs were synthesized in ammonia atmosphere via a thermal evaporation method. The electrical and optoelectronic properties of ZnSe:N NBs were studied by FETs and photodetectors. The ZnSe NB FETs have confirmed the p-type conductivity of ZnSe:N NBs. The Ion/Ioff, Vth and mh of FET were measured. The ZnSe:N NB photodetectors show a high responsivity
341
of 2.4 104 AW 1, photoconductive gain of 6.5 104 and fast response speed to the incident light with a sharp cut-off at 460 nm. Our results demonstrate the ZnSe:N NBs will have great potential applications in electrical and optoelectronic nano-devices.
Acknowledgments We are thankful for the financial support from Henan Province Scientific and Technological Department Programs (No. 092102210198), Henan Province Scientific and Technological Department Key Programs for Science and Technology Development (No. 102102210439), and Henan Province Education Department Natural Science Research Programs (No. 2010B150002).
References [1] Zhai T, Li L, Wang X, Fang X, Bando Y, Golberg D. Adv Funct Mater 2010;20:4233–48. [2] Shen GZ, Chen D. Recent Pat Nanotechnol 2010;4:20–31. [3] Fang XS, Zhang LD. J Mater Sci Technol 2006;22:721–36. [4] Shen G, Chen D. NanoScale Res Lett 2009;4:779–88. [5] Jie JS, Zhang WJ, Bello I, Lee CS, Lee ST. Nano Today 2010;5:313–36. [6] Du LZ, Lei Y. Mater Lett 2012;73:95–8. [7] Hu L, Yan J, Liao M, Wu L, Fang X. Small 2011;7:1012–7. [8] Hu L, Wu L, Liao M, Hu X, Fang X. Adv Funct Mater 2012;22:998–1004. [9] Wu PC, Dai Y, Ye Y, Yin Y, Dai L. J Mater Chem 2011;21:2563–7. [10] Wu PC, Dai Y, Sun T, Ye Y, Meng H, Fang XL, et al. ACS Appl Mater Interfaces 2011;3:1859–64. [11] Zhai TY, Li L, Ma Y, Liao MY, Wang X, Fang XS, et al. Chem Soc Rev 2011;40:2986–3004. [12] Wu D, Jiang Y, Li SY, Li FZ, Li JW, Lan XZ, et al. Nanotechnology 2011;22:405201–6. [13] Yu YQ, Jie JS, Jiang P, Wang L, Wu CY, Peng Q, et al. J Mater Chem 2011;21: 12632–8. [14] Li S, Jiang Y, Wu D, Wang B, Li J, Zhang Y, et al. Mater Lett 2011;65:1753–5. [15] Zhang XW, Jie JS, Wang Z, Wu CY, Wang L, Peng Q, et al. J Mater Chem 2011;21:6736–41. [16] Fang XS, Xiong SL, Zhai TY, Bando Y, Liao MY, Gautam UK, et al. Adv Mater 2009;21:5016–21. [17] Salfi J, Philipose U, de Sousa CF, Aouba S, Ruda HE. Appl Phys Lett 2006;89:261112–4. [18] Song HS, Zhang WJ, Yuan GD, He ZB, Zhang WF, Tang YB, et al. Appl Phys Lett 2009;95:033117–9. [19] Xiang B, Zhang HZ, Li GH, Yang FH, Su FH, Wang RM, et al. Appl Phys Lett 2003;82:3330–2. [20] Chan SK, Cai Y, Wang N, Sou IK. Appl Phys Lett 2006;88:013108–10. [21] Cai Y, Chan SK, Soar IK, Chan YT, Su DS, Wang N. Adv Mater 2006;18:109–14. [22] Jia TQ, Chen HX, Huang M, Wu XJ, Zhao FL, Baba M, et al. Appl Phys Lett 2006;89:101116–8. [23] Hu ZD, Duan XF, Gao M, Chen Q, Peng LM. J Phys Chem C 2007;111:2987–91. [24] Wang FF, Zhang ZH, Liu RB, Wang X, Zhu X, Pan AL, et al. Nanotechnology 2007:18. [25] Wu D, Jiang Y, Wang L, Li SY, Wu B, Lan XZ, et al. Appl Phys Lett 2010;96:123118–20. [26] Javey A, Kim H, Brink M, Wang Q, Ural A, Guo J, et al. Nat Mater 2002;1: 241–6. [27] Marzouki A, Falyouni F, Haneche N, Lusson A, Galtier P, Rigutti L, et al. Mater Lett 2010;64:2112–4. [28] Li SY, Jiang Y, Wu D, Wang L, Zhong HH, Wu B, et al. Journal of Physical Chemistry C 2010;114:7980–5. [29] Wang Z, Jie J, Li F, Wang L, Yan T, Luo L, et al. ChemPlusChem 2012;77:470–5. [30] Wu PC, Ma RM, Liu C, Sun T, Ye Y, Dai L. J Mater Chem 2009;19:2125–30. [31] Yang MH, Teo KBK, Gangloff L, Milne WI, Hasko DG, Robert Y, et al. Appl Phys Lett 2006;88:113507.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and optoelectronic properties of p-type nitrogen doped ZnSe nanobelts Qing Su a, Lijuan Li b, Shanying Li a,n, Haipeng Zhao a a b
Department of Chemistry and Chemical Engineering, Henan University of Urban Construction, Pingdingshan, Henan 467036, PR China Department of Mechanical Engineering, Min Xi Vocational & Technical College, Longyan, Fujian 364021, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 September 2012 Accepted 30 October 2012 Available online 6 November 2012
Single-crystal nitrogen (N) doped p-type ZnSe nanobelts (NBs) with zinc blende structure were synthesized in ammonia atmosphere via a thermal evaporation method. The p-type conductivity of ZnSe:N NBs was confirmed by field-effect transistors (FETs) based on individual NBs. High-performance photodetectors were constructed based on ZnSe:N NBs, which show high sensitivity and relatively fast response speed to the incident light with a sharp cut-off at 460 nm, corresponding to the band-gap of ZnSe. The high photosensitivity and relatively fast response speed are attributable to the high crystal quality of the ZnTe nanowires. These results reveal that such single-crystalline ZnSe NBs are excellent candidates for optoelectronic applications. & 2012 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline materials Semiconductors ZnSe nanobelts Nitrogen doped Photoresponse
1. Introduction One-dimensional (1D) inorganic nanostructures such as nanowires (NWs), nanobelts (NBs), and nanotubes (NTs) have been studied extensively with respect to fundamental properties and potential applications in nanoscale devices [1–8]. They are expected to play important roles as the key units of nextgeneration nanoscale electronic, optoelectronic devices. Due to large surface-to-volume ratios and Debye length comparable to their small sizes, 1D inorganic nanostructures have already displayed superior characteristics in diverse experiments compared with traditional thin-film and bulk materials [9–14]. As one of the most important II–VI nanostructure semiconductors, ZnSe with a direct band-gap of 2.7 eV have attracted much attention in the past decade owing to their unique optical and electrical properties. They are good candidates for the building blocks of functional nano-devices such as field-effect transistors (FETs), photodetectors (PDs), light-emitting diodes (LEDs) and so on [15–19]. In the past decade, synthesis of ZnSe nanostructures has been intensively studied via various growth methods [20–24]. However, contact resistance caused by interface defects and the work function mismatch is usually observed in ZnSe nano-devices. Large contact resistance will result in the device performance degradation, which is needed to be eliminated for realizing high-performance devices. Besides the use of appropriate metal electrode, doping has been demonstrated to be an efficient way, which is also a feasible method
n
Corresponding author. Tel.: þ86 375 2089043; fax: þ86 375 2089041. E-mail address: [email protected] (S. Li).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.112
to tune their optoelectronic properties and further enhance their device performances [25,26]. So far, As and Bi doping have been utilized to enhance the p-type conductivity of the ZnSe nanowires, representing an important achievement in nanomaterials synthesis and offering the opportunities for future nanodevice applications [15,18]. In spite of these progresses, the efficient synthesis of p-type ZnSe nanostructures with reliable and reproducible is much demanded to promote the applications of ZnSe nanostructures in further study. Moreover, N-doping nanostructures such as ZnO NWs and ZnTe NWs via ammonia atmosphere have been reported [27,28]. A simple method with high-doping efficiency is much desired to realize ZnSe nanostructures with well-controlled p-type conductivity. In this paper, we describe the synthesis of N-doped p-type ZnSe NBs in ammonia atmosphere via a thermal evaporation method. FETs and photodetectors based on single ZnSe:N NBs were constructed to study their transport and optoelectronic properties. Our results demonstrated that the p-type ZnSe NBs may have important applications in nanoelectronics and nano-optoelectronics.
2. Experimental ZnSe NBs synthesis and characterization: The undoped ZnSe nanomaterials are highly insulative semiconductors with very low conduction current of 10 13 A [29], which cannot function as elementary components for nanodevices. Hence, N-doping was employed by using ammonia in this work. Growths of N-doped p-type ZnSe NBs were carried out in an alumina tube furnace via thermal evaporation. High-purity ZnSe powder (99.999%) was loaded into an alumina boat and placed at the center of the tube
Q. Su et al. / Materials Letters 92 (2013) 338–341
furnace, and Si substrates coated with a layer of 10 nm gold catalyst were placed in the downstream 10 cm away from the source. Before heating, the system was evacuated to a base pressure of 10 5 Torr and back filled with Ar/NH3 gas mixture at a constant flow rate of 85/15 sccm to a stable pressure of 150 Torr. Afterward, the ZnSe source was heated up to 1050 1C and maintained at that temperature for 2 h. The system was cooled down to room temperature, and the Si substrates were taken out of the furnace. A layer of yellow wool-like product could be observed on the substrates surface. Morphologies and structures of the as-synthesized ZnSe NBs were characterized by X-ray diffraction with Cu Ka radiation (XRD, D/max-rB), field-emission scanning electron microscopy (FESEM, SIRION200, FEI, at 5 kV), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010, at 200 kV) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250). Device construction: To assess the electrical transport and optoelectronic properties of the ZnSe:N NBs, FETs based on the individual ZnSe:N NBs were constructed as follows: First, the ZnSe:N NBs were dispersed on SiO2 (300 nm)/p-Si substrates. Then, photolithography, and lift-off processes were used to fabricate the source and drain Au (100 nm) electrodes on an individual ZnSe:N NB. The p-Si substrates and SiO2 served as the bottom-gate and dielectric, respectively. The electrical transport measurements were conducted with a semiconductor characterization system (Keithley 4200). The photoresponse characteristics of the ZnSe NB photodetectors were detected by a monochromatic light source.
clean and free of evident impurities and particles. The width and length of the NBs are in the range of 2–5 mm and tens of micrometers, respectively. HRTEM image and the corresponding fast Fourier transform (FFT) pattern recorded from a single NB are shown in Fig. 1(b), indicating that the NB is zinc blende structure with [ 1 11] growth orientation. In the XRD patterns of the ZnSe NBs (Fig. 1c), all the diffraction peaks can be assigned to zinc blende ZnSe (JCPDS 88-2345) and no obvious impurity phases and peak shift are observed, suggesting a single phase of the product. In addition, a peak at 398 eV corresponding to the N 1 s was found in XPS detection performed on the ZnSe:N NBs, confirming the existence of N in the NBs. Electrical characterization of ZnSe:N NB FET: Fig. 2(a) and (c) shows the schematic illustration and SEM of FET based on an individual ZnSe:N NB, respectively. Fig. 2(b) depicts the typical I–V curves of an individual ZnSe:N NB measured before and after annealing at 550 1C for 5 min in Ar atmosphere, respectively. The linear curves indicate the good Ohmic contact between the Au electrodes and the ZnSe:N NB [15]. By applying gate voltage (VG) on the bottom-gate of the FET, the gate-dependent source–drain current (IDS) versus source–drain voltage (VDS) curves are detected. From Fig. 2(c), it is shown that the conductance of the ZnSe:N NB decreases (increases) consistently with the increasing (decreasing) of the gate voltage in step of 15 V, revealing a pronounced p-type conductivity of the ZnSe:N NB. The hole mobility (mh) can be estimated to be 0.98 10 2 cm2 V 1 s 1 based on the following equation: gm ¼
3. Results and discussion Characterization of the p-type ZnSe NBs: Fig. 1(a) shows the typical FESEM image of the ZnSe NBs. It is seen that the product is
339
dIDS Z ¼ mh C 0 V DS L dV G
ð1Þ
where gm ¼0.8 nS represents the transconductance of the ZnSe NB FET deduced from the linear part of the IDS–VG curves (Fig. 2d), Z/L is the width-to-length ratio of the NB channel. The capacitance per
Fig. 1. (a) FESEM image, (b) HRTEM image, (c) XRD patterns and (d) XPS spectrum of the as-synthesized ZnSe:N NBs. Insets of (b) and (d) show the corresponding SAED pattern and the enlarged N peaks, respectively.
340
Q. Su et al. / Materials Letters 92 (2013) 338–341
Fig. 2. (a) Schematic illustration of nanoFET based on an individual ZnSe:N NB. (b) I–V curves of the ZnSe:N NB before and after annealing. (c) IDS–VDS curves of the ZnSe:N NB nanoFET measured at varied VG from 30 to 30 V in a step of 15 V. Inset is a SEM image of nanoFET. The scale bar is 5 mm. (d) IDS–VG curves at VDS ¼3 V.
Fig. 3. (a) I–V curves of an individual ZnSe:N NB exposed to light of varied wavelength at a constant intensity of 1.0 mW cm 2, (b) spectral response of the ZnSe:N NB, (c) time response of ZnSe:N NB under light illumination of 460 nm, and (d) enlarged rising and falling edges.
unit area is given by C0 ¼ ee0/h, where e0 is the dielectric constant (3.9 for SiO2) and h (300 nm) is the thickness of the SiO2 dielectric layer. The threshold voltage (Vth) obtained at the point deviated from the linear region of the curve is 18 V and an Ion/Ioff ratio of 6 is obtained when VG changes from 40 V to 40 V. It is worth noting that relative low device performances may duo to the weak
gate coupling caused by the SiO2 dielectric and the bottom-gate device configuration [30,31]. Optoelectronic characterization of ZnSe:N NB PD: ZnSe is a promising material for blue light detection due to the appropriate band-gap of 2.26 eV and the high crystalline quality. Fig. 3(a) depicts the wavelength-dependent I–V curves of the ZnSe:N NB photodetector
Q. Su et al. / Materials Letters 92 (2013) 338–341
Table 1 Performances of ZnSe PD from previous reports are presented for comparison. n ZnSe:Cl with three different doping levels were compared. Ion/Ioff
ZnSe:N 150 ZnSe:Cl 3 2 2 ZnSe:Bi 3.5 n
Rise time (s)
Fall time (s)
0.4 0.5 2 2 27.6 42.8 120.2 394.2 0.38 0.4
Responsivity Photoconductive Reference (AW 1) gain
2.4 104 4.1 102 1.9 104 7.9 105 n.a.
6.5 104 1.1 103 5.1 104 2.1 106 n.a.
This work [29]
[15]
obtained when the device was exposed to monochromatic light at a constant intensity of 1.0 mW cm 2. It is seen that the NB’s conductance strongly depends on the light wavelength, which increases with decreasing wavelength and reaches the maximum value at approximately 460 nm. From the spectral response (Fig. 3b), it is seen that sensitivity is highest around 460 nm and shows a steep decline in the long wavelength direction. The cut-off wavelength of 460 nm is consistent with the ZnSe band-gap, indicating that the electron–hole pairs excited by light with energy larger than the band-gap of ZnSe. Furthermore, the ZnSe NB photodetector also shows good stability and reproducibility to a pulsed light of 460 nm with ILight/IDark of 150 as shown in Fig. 3(c). The enlarged rise and fall edges reveal a relatively fast rise and fall time of 0.4 s and 0.5 s, respectively, which are much faster than the values of Cl-doped ZnSe NWs [29]. Responsivity (R) and photoconductive gain (G) of a photodetetctor are defined as follow: Ip ql R A=W ¼ ¼Z G P ph hc
ð2Þ
where Ip, Pph, Z, h, c and l are photocurrent, incident light power, quantum efficiency, Planck’s constant, light speed and light wavelength, respectively. Based on this equation, R and G for ZnSe:N NB photodetector are estimated to be 2.4 104 AW 1 and 6.5 104, respectively. The high sensitivity of the ZnSe:N NB photodetectors could be ascribed to the high crystal quality and large surface-tovolume ratio of the NBs and consequently a large photocurrent. Table 1 summarizes the performances of the ZnSe:N NB PD, along with the results for other doped ZnSe NWs for comparison, it is worth noting that the devices in this work show robust performances close or superior to the previous results. Our results demonstrated that the p-type ZnSe:N NBs may have important applications in nanoelectronics and nano-optoelectronics.
4. Conclusion In summary, p-type ZnSe:N NBs were synthesized in ammonia atmosphere via a thermal evaporation method. The electrical and optoelectronic properties of ZnSe:N NBs were studied by FETs and photodetectors. The ZnSe NB FETs have confirmed the p-type conductivity of ZnSe:N NBs. The Ion/Ioff, Vth and mh of FET were measured. The ZnSe:N NB photodetectors show a high responsivity
341
of 2.4 104 AW 1, photoconductive gain of 6.5 104 and fast response speed to the incident light with a sharp cut-off at 460 nm. Our results demonstrate the ZnSe:N NBs will have great potential applications in electrical and optoelectronic nano-devices.
Acknowledgments We are thankful for the financial support from Henan Province Scientific and Technological Department Programs (No. 092102210198), Henan Province Scientific and Technological Department Key Programs for Science and Technology Development (No. 102102210439), and Henan Province Education Department Natural Science Research Programs (No. 2010B150002).
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