- Email: [email protected]
n-ZnO NWs Schottky diodes white LEDs
Superlattices and Microstructures 62 (2013) 200–206
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Annealing effect on the electrical and optical properties of Au/n-ZnO NWs Schottky diodes white LEDs M.Y. Soomro a,⇑, I. Hussain b, N. Bano c, O. Nur a, M. Willander a a
Department of Science and Technology, Campus Norrköping, Linköping University, SE-60174 Norrköping, Sweden Ecospark AB SE-60221 Norrköping, Sweden c Thinfilm AB SE-58216 Linköping, Sweden b
a r t i c l e
i n f o
Article history: Received 15 July 2013 Accepted 23 July 2013 Available online 3 August 2013 Keywords: ZnO nanowires Schottky diodes Post growth annealing Electroluminescence
a b s t r a c t We report the post-growth heat treatment effect on the electrical and the optical properties of hydrothermally grown zinc oxide (ZnO) nanowires (NWs) Schottky white light emitting diodes (LEDs). It was found that there is a changed in the electroluminescence (EL) spectrum when post growth annealing process was performed at 600 °C under nitrogen, oxygen and argon ambients. The EL spectrum for LEDs based on the as grown NWs show three bands red, green and blue centered at 724, 518 and 450 nm respectively. All devices based on ZnO NWs annealed in oxygen (O2), nitrogen (N2) and argon (Ar) ambient show blue shift in the violet and the red emissions whereas a red shift is observed in the green emission compared to the as grown NWs based device. The color rendering index (CRI) and the correlated color temperature (CCT) of all LEDs were calculated to be in the range 78–91 and 2753– 5122 K, respectively. These results indicate that light from the LEDs can be tuned from cold white light to warm white light by post growth annealing. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a promising II–VI compound semiconductor, due to its characteristics like a wide band gap of 3.37 eV and relatively large exciton binding energy of 60 meV, which makes it attractive
⇑ Corresponding author. E-mail address: [email protected] (M.Y. Soomro). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.07.014
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material for potential applications in optoelectronic devices such as ultraviolet light emitting diodes, and highly transparent electron devices and lasers diode. It is well known that ZnO exhibits a wide emission from the UV near-band-edge emission and covering the whole visible range [1]. Over the past decade, various ZnO nanostructures such as nanowires, nanorods, nanotubes etc. have been intensively studied due to their potential applications in diverse fields due to the fact that at the nanoscale level materials exhibit distinguished surface morphology and size confinement behavior remarkably different from their bulk counterpart. ZnO nanostructures, possesses relatively a large number of radiative intrinsic deep level defects [2,3] particularly, besides the ultraviolet (UV) emission, ZnO emit blue, green, yellow and red colors; which covers the whole visible region ranging from 400 nm to around 700 nm [4]. Therefore, the optical properties of ZnO have been extensively studied. ZnO typically exhibits one sharp UV peak and possibly one or two wide bands or the so called broad deep band emission (DBE) due to deep radiative defects within the band gap [5]. As a result of the quantum size effect, the oscillation strength of excitons is greatly enhanced in ZnO nanowires (NWs), which is favorable to radiative recombination of excitons at room temperature [6]. ZnO NWs have an internal electric field in the direction of the c-axis to facilitate charge transport and will suppress the recombination of injected electrons [7]. In addition, each NW acts as a wave guide, minimizing side scattering of light, thereby enhancing light emission and extraction efficiency [8]. One of the main advantages of ZnO NWs is that they can easily be synthesized using the hydrothermal method [9] but usually hydrothermal grown ZnO NWs have low crystal quality and incorporate considerable lattice and surfacedefects [10,11]. A defect produces a potential well that can trap and affect the movement of carriers, and degrade device performances [12]. The luminescence emission and electronic properties of any semiconductor device depend on its energy band gap, which is extremely sensitive to its crystal perfection and surface defects. Different post-growth methods have been investigated to improve the crystal quality. Post growth heat treatment such as thermal annealing which can play important role to improve crystalline quality and hence controlling the optical emissions of the ZnO NWs by reducing nanoradiative related defects. In this regard various atmospheres such as oxygen, nitrogen, hydrogen can be employed in order to get a better crystalline quality. Thermal treatment plays an important role to increase the performance of a device and to suppress or eliminate detrimental surface defects. As a result, a surface defect is unable to trap carriers again. Till now, various groups have studied and reported annealing effect on the optical and electrical properties of ZnO thin films [13,14], nanorods [11,15–19] and nanotubes [20]. But to the best of our knowledge no report has dealt with Au/n-ZnO nanowires (NWs) based Schottky diodes anneal in different ambient. The production of high quality ZnO nanostructure-based homojunctions has proved elusive because ZnO still suffers from the lack of reproducible and high quality p-type material as unintentionally undoped ZnO shows typically n-type properties, acceptors may be compensated by intrinsic defects such as Zinc interstitials (Zin), oxygen vacancies (VO), or background impurities such as hydrogen. So in order to solve the problem of stable and reproducible p-type doping, an alternative approach such as the fabrication of heterojunctions and Schottky contacts on n-ZnO nanostructures allows the realization of these electronic devices. In this work, we study on the ambient annealing effects on the electrical and optical properties of the Au/n-ZnO NWs Schottky white light emitting diodes (WLEDs). The results showed that the ambient in which the post-annealing takes place strongly influenced the light emission and the color-rendering properties of the ZnO NWs-based LEDs.
2. Experimental The ZnO NWs used in this study were grown on n-SiC epilayer substrate with doping concentration of 1 1017 cm3. The SiC is a good candidate as a substrate for ZnO nanostructure growth and further applications. The SiC and ZnO have the same wurtzite crystal symmetry and relatively small lattice mismatch (5%). In addition, SiC has useful properties which include excellent electron mobility, high transparency, high break down field, and high thermal conductivity. The SiC substrates were sequen-
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tially ultrasonically cleaned in acetone, ethanol, and DI water, respectively, each for 10 min, and then dried in the air. Then Nickel (Ni) evaporated at the bottom of SiC for making a Ni–SiC ohmic contact. Nickel is an ohmic contact metal to n-type SiC due to its reproducibly low contact resistance and good temperature stability. Activation annealing was performed for 30 min at 900 °C. All the chemicals used were purchased from Sigma–Aldrich and were of analytical grade and used without further purification, and all the aqueous solutions were prepared using de-ionized water with a resistivity of 18.0 M X cm. Then a seed layer was coated onto the substrates with the help of a spin coater spun at a rate of 2000 rpm for 1 min. This procedure was repeated four times as the seed layer acts as a preferential growth site during the growth process. Finally, the substrates were heated at a constant temperature of 180 °C for 20 min in order to solidify the seed layer. After that the substrates were cooled down to room temperature. To grow the ZnO NWs we used a low-temperature method. In this method zinc nitrate hexahydrate [Zn(NO3)26H2O] (99.998%) and hexamethylenetetramine (HMT) (C6H12N4) (99.998%) were mixed in equal molar concentration in DI water and kept under constant magnetic stirring for 30 min to get a uniform growth solution. The preheated SiC substrates were then placed in the solution and placed in a regular laboratory oven and heated at 95 °C for 4 h. Finally, the samples were washed with deionized water and dried in air. After the growth, the samples were divided into four groups in which one groups is used as grown and the remaining three were used for post growth annealing treatment in a quartz tube furnace. The annealing was carried out in a horizontal quartz furnace and kept at an annealing temperature of 600 °C for 30 min. When the required temperature was achieved; the samples were pushed into the furnace. All three samples were then annealed at 600 °C for 30 min in argon (Ar), nitrogen (N2) and oxygen (O2) ambient. The next step is fabrication of the Schottky devices on the post growth annealed samples. To avoid short circuits between the NWs, an insulating spin-on-glass (SOG) was filled by spin coater in order to fill the empty gap between the individual NWs. The insulating SOG prevented potential short circuits due to pinholes in the nanowire array and provided mechanical stability for the measurement. Subsequently, the tips of the ZnO NWs were exposed using simple plasma ion-etching with CF4 prior to contact metal deposition. Finally, In order to complete light emitting diodes (LEDs) processing, Au metal contacts were formed on a group of ZnO NWs by thermal evaporation at a pressure of 2 10-7 Torr, and the contacts were formed as circular dots of 1 mm in diameter and 100 nm in thickness. Four devices were fabricated for comparison. The surface morphologies and sizes of the resulting ZnO NWs were observed by field-emission scanning electron microscope (FESEM JSM 6700F) operated at 12 keV. The current–voltage (I–V) characteristics of the fabricated LEDs were measured by a semiconductor diode parameter analyzer (4145B, Hewlett–Packard), electroluminescence (EL) behavior of the fabricated LEDs was recorded by using an Andor Sharmrock 303iB spectrometer supported with Andor–Newton DU-790 N detector and Keithley 2400 as a source meter. The light was collected from the topside of the device. All of these measurements were carried out at room temperature in ambient atmosphere.
3. Results and discussion The schematic diagram of the Au/n-ZnO Schottky white light emitting diode (WLED) is shown in Fig. 1. The top-view image of the as grown ZnO NWs is shown in Fig. 2(a). It clearly shows that ZnO NWs are densely packed; faceted hexagonal shape and most of the NWs are parallel to each other and predominately in a perpendicular orientation to the substrate. The average diameter and length of the ZnO NWs are 150–200 nm and 3 lm, respectively. Fig. 2(b–d) shows ZnO NWs in different annealed in different ambient. Four Au/n-ZnO NWs WLEDs were fabricated based on the as grown NWs and the annealed in different atmosphere such as in O2, N2 and Ar at 600 °C. Fig. 3 shows the I–V characteristics of the all devices. It can be seen that all the fabricated devices exhibit good rectifying behavior. It clearly shows the nonlinear increase of the current under forward bias. Such type of rectification behavior is best described by using the standard thermionic emission theory [3]. Under forward bias the turn-on voltage of device without annealing is about 5 V. However, in case of the thermally treated diodes, the turn on
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Fig. 1. Schematic illustration of Au/n-ZnO NWs Schottky LED.
Fig. 2. (a) SEM image of the top-view of grown ZnO NWs, (b) SEM image of the ZnO NWs after annealed in oxygen ambient, (c) SEM image of the ZnO NWs after annealed in nitrogen ambient and (d) SEM image of the ZnO NWs after annealed in argon ambient.
voltage changes slowly to 4.5, 4.3, and 4 V in Ar, N2, and O2 annealing ambient respectively. The forward current increased slightly for N2 ambient as compared with other diodes. The electroluminescence (EL) spectra of all devices are shown in Fig. 4. The EL spectra of all devices reveal a broad emission band from 400 to 800 nm covering the whole visible region. The EL emission could be clearly noticeable by the naked eye in darkness. It can be clearly seen that the annealing
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4,0x10-3 as grown 3,2x10-3
annealed in Ar
Current (A)
annealed in O2 annealed in N2
2,4x10-3 1,6x10-3 8,0x10-4 0,0 -10
-5
0
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Voltage (V) Fig. 3. Typical room temperature current–voltage (I–V) curve of Au/n-ZnO Schottky LEDs based on as grown and annealed NWs.
Intensity (Watt / nm*counts)
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annealed in N2 annealed in O2 annealed in Ar as grown
150
100
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0 400
500
600
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Wavelength (nm) Fig. 4. RT EL spectra of Au/n-ZnO Schottky LEDs based on as grown and annealed different environment.
ambient influences the EL spectrum of the devices. The spectra exhibit only one emission band that is a deep level emission (DLE) associated with defect related radiated recombination in ZnO [21]. Furthermore, various functional group elements (related to carbon, nitrogen and hydrogen) beside many lattice defects and surface defects were incorporated during the growth [18]. Quang et al. [11] reported that these defects could act as nonradiative centers and reduce light emission from the ZnO but after annealing these defects could be reduced and all those functional group elements were released from the surface of the ZnO NWs this will greatly enhanced the crystal structure and decrease the nonradiative recombination, leading to the increase of luminescence efficiency of LED. The deep level or trapped-state visible emission attributed to several surface defects in the crystal structure, or the recombination via distribution, of different defects states. But there is generally no accepted mechanism responsible for the green emission. According to most of the models, various impurities and structural intrinsic defect centers such as oxygen vacancy (Vo), zinc vacancy (Vzn), zinc
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interstitial (Zni), oxygen interstitial (Oi), or antisite oxygen (Ozn) are responsible for the deep level emission [3]. The as grown NWs based LED shows three sub DLE bands, a broad red, green and small blue emission peaks are centered at 724, 518, and 450 nm, respectively. The EL intensity of all devices based on annealed NWs in increased. All devices exhibited three sub DLE bands one strong red sub band, green band and a relatively weak violet band. The DLE emission was dominated and the NBE emission which is relatively weak in the spectrum that may be ascribed to the low radiative recombination efficiency and self-absorption effect by the DLE traps in ZnO [22–24]. The EL spectrum from Ar annealed NW based device shows three peaks approximately centered at 524 nm (green), 710 nm (red), 404 (violet). The violet peak corresponds to the Zni [2]. Likewise the EL spectrum for device based on annealed NWs in N2 ambient shows the intensity of the all red, violet and green emission is decreased as compared to LED based on as-grown ZnO NWs. This is exactly the same trend is noticed by the Quang et al. [11], they reported that the red emission decrease due to the reduction in the complex defects when annealing in N2 ambient. In ZnO NWs annealed in N2 rich ambient, the following reaction may occur. In N2 rich ambient, the majority of defects are oxygen vacancies, generated by the evaporation of oxygen [11].
ZnO ! V o þ ZnZn þ 1=2O2 Finally the device based on NWs annealed in O2 environment, it is clearly seen that the intensity of the red and violet emission from annealed NWs based devices decrease to a value of 692 and 400 nm relative to that of as grown NWs based device (724 nm) whereas the green emission increased as compared to as grown NWs based LED. Under O2 rich ambient, the antisite oxide Ozn were easily formed from interstitial oxygen and zinc vacancies Vzn [11]. To investigate the color quality of white LEDs color chromaticity coordinates are plotted in Fig. 5 with the RT EL spectra indicates all components
Fig. 5. Chromaticity diagram of the Au/n-ZnO Schottky LEDs.
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of white emission. The figure shows the CIE 1931 color map chromaticity diagram in the (x, y) coordinates system. The color rendering indices (CRI) of the white LEDs based on the as grown, and the samples annealed in O2, N2, and Ar, were calculated to be 78, 84, 91 and 88 respectively. Alvi et al. [19] also noticed the increase in CRI value after annealed in different ambient compared with as-grown ZnO NWs. The possible reason for the increase in CRI is that during the annealing the deep level defects responsible for the orange and red emission become more active as compared to the defects responsible for green and blue emission in the deep levels of ZnO [19]. The correlated color temperature (CCT) of the white LEDs based on as grown NW, annealed in O2, Ar, N2 were calculated 2753 K, 4327 K, 4539 K and 5122 K respectively. It clearly shows that there is an increase in the CCT of the LEDs when annealed in different ambient also it indicate that light from the LEDs can be tune from cold white light to warm white light by post growth annealing. The chromaticity coordinates are very close to the Planckian locus, which is the trace of the chromaticity coordinates of a blackbody. The colors around the Planckian locus can be regarded as white. It is clear that the fabricated LEDs are in fact white LED and the light emission is actually a white color impression. Considering the color quality, i.e., the CRI, the best value was obtained for LED fabricated using NW annealed in N2 environment and reached a value of 91. From this point of view, It shows that N2 annealed NWs based LED had better optical quality with less nonradiative related defects. From this study and although devices fabricated on the NWs annealed in different ambient and the resulting emission from those devices possesses different light quality. It indicates that white light can be tuned from cold white to warm white light. 4. Conclusion We report luminescence from Au/n-ZnO LEDs based on as grown and annealed NWs in different environments. The devices illustrate broad emission band covering the whole visible region arising from the radiative recombination in the ZnO NWs. The CRI and CCT of the white LEDs were calculated in the range of 78–91 and 2753–5122 K respectively. The examination results show that annealing temperature and environment put simultaneously impact the electrical and optical properties on the fabricated LEDs due to the enhancement of the ZnO NWs crystalline properties. References [1] [2] [3] [4]
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Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Annealing effect on the electrical and optical properties of Au/n-ZnO NWs Schottky diodes white LEDs M.Y. Soomro a,⇑, I. Hussain b, N. Bano c, O. Nur a, M. Willander a a
Department of Science and Technology, Campus Norrköping, Linköping University, SE-60174 Norrköping, Sweden Ecospark AB SE-60221 Norrköping, Sweden c Thinfilm AB SE-58216 Linköping, Sweden b
a r t i c l e
i n f o
Article history: Received 15 July 2013 Accepted 23 July 2013 Available online 3 August 2013 Keywords: ZnO nanowires Schottky diodes Post growth annealing Electroluminescence
a b s t r a c t We report the post-growth heat treatment effect on the electrical and the optical properties of hydrothermally grown zinc oxide (ZnO) nanowires (NWs) Schottky white light emitting diodes (LEDs). It was found that there is a changed in the electroluminescence (EL) spectrum when post growth annealing process was performed at 600 °C under nitrogen, oxygen and argon ambients. The EL spectrum for LEDs based on the as grown NWs show three bands red, green and blue centered at 724, 518 and 450 nm respectively. All devices based on ZnO NWs annealed in oxygen (O2), nitrogen (N2) and argon (Ar) ambient show blue shift in the violet and the red emissions whereas a red shift is observed in the green emission compared to the as grown NWs based device. The color rendering index (CRI) and the correlated color temperature (CCT) of all LEDs were calculated to be in the range 78–91 and 2753– 5122 K, respectively. These results indicate that light from the LEDs can be tuned from cold white light to warm white light by post growth annealing. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a promising II–VI compound semiconductor, due to its characteristics like a wide band gap of 3.37 eV and relatively large exciton binding energy of 60 meV, which makes it attractive
⇑ Corresponding author. E-mail address: [email protected] (M.Y. Soomro). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.07.014
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material for potential applications in optoelectronic devices such as ultraviolet light emitting diodes, and highly transparent electron devices and lasers diode. It is well known that ZnO exhibits a wide emission from the UV near-band-edge emission and covering the whole visible range [1]. Over the past decade, various ZnO nanostructures such as nanowires, nanorods, nanotubes etc. have been intensively studied due to their potential applications in diverse fields due to the fact that at the nanoscale level materials exhibit distinguished surface morphology and size confinement behavior remarkably different from their bulk counterpart. ZnO nanostructures, possesses relatively a large number of radiative intrinsic deep level defects [2,3] particularly, besides the ultraviolet (UV) emission, ZnO emit blue, green, yellow and red colors; which covers the whole visible region ranging from 400 nm to around 700 nm [4]. Therefore, the optical properties of ZnO have been extensively studied. ZnO typically exhibits one sharp UV peak and possibly one or two wide bands or the so called broad deep band emission (DBE) due to deep radiative defects within the band gap [5]. As a result of the quantum size effect, the oscillation strength of excitons is greatly enhanced in ZnO nanowires (NWs), which is favorable to radiative recombination of excitons at room temperature [6]. ZnO NWs have an internal electric field in the direction of the c-axis to facilitate charge transport and will suppress the recombination of injected electrons [7]. In addition, each NW acts as a wave guide, minimizing side scattering of light, thereby enhancing light emission and extraction efficiency [8]. One of the main advantages of ZnO NWs is that they can easily be synthesized using the hydrothermal method [9] but usually hydrothermal grown ZnO NWs have low crystal quality and incorporate considerable lattice and surfacedefects [10,11]. A defect produces a potential well that can trap and affect the movement of carriers, and degrade device performances [12]. The luminescence emission and electronic properties of any semiconductor device depend on its energy band gap, which is extremely sensitive to its crystal perfection and surface defects. Different post-growth methods have been investigated to improve the crystal quality. Post growth heat treatment such as thermal annealing which can play important role to improve crystalline quality and hence controlling the optical emissions of the ZnO NWs by reducing nanoradiative related defects. In this regard various atmospheres such as oxygen, nitrogen, hydrogen can be employed in order to get a better crystalline quality. Thermal treatment plays an important role to increase the performance of a device and to suppress or eliminate detrimental surface defects. As a result, a surface defect is unable to trap carriers again. Till now, various groups have studied and reported annealing effect on the optical and electrical properties of ZnO thin films [13,14], nanorods [11,15–19] and nanotubes [20]. But to the best of our knowledge no report has dealt with Au/n-ZnO nanowires (NWs) based Schottky diodes anneal in different ambient. The production of high quality ZnO nanostructure-based homojunctions has proved elusive because ZnO still suffers from the lack of reproducible and high quality p-type material as unintentionally undoped ZnO shows typically n-type properties, acceptors may be compensated by intrinsic defects such as Zinc interstitials (Zin), oxygen vacancies (VO), or background impurities such as hydrogen. So in order to solve the problem of stable and reproducible p-type doping, an alternative approach such as the fabrication of heterojunctions and Schottky contacts on n-ZnO nanostructures allows the realization of these electronic devices. In this work, we study on the ambient annealing effects on the electrical and optical properties of the Au/n-ZnO NWs Schottky white light emitting diodes (WLEDs). The results showed that the ambient in which the post-annealing takes place strongly influenced the light emission and the color-rendering properties of the ZnO NWs-based LEDs.
2. Experimental The ZnO NWs used in this study were grown on n-SiC epilayer substrate with doping concentration of 1 1017 cm3. The SiC is a good candidate as a substrate for ZnO nanostructure growth and further applications. The SiC and ZnO have the same wurtzite crystal symmetry and relatively small lattice mismatch (5%). In addition, SiC has useful properties which include excellent electron mobility, high transparency, high break down field, and high thermal conductivity. The SiC substrates were sequen-
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tially ultrasonically cleaned in acetone, ethanol, and DI water, respectively, each for 10 min, and then dried in the air. Then Nickel (Ni) evaporated at the bottom of SiC for making a Ni–SiC ohmic contact. Nickel is an ohmic contact metal to n-type SiC due to its reproducibly low contact resistance and good temperature stability. Activation annealing was performed for 30 min at 900 °C. All the chemicals used were purchased from Sigma–Aldrich and were of analytical grade and used without further purification, and all the aqueous solutions were prepared using de-ionized water with a resistivity of 18.0 M X cm. Then a seed layer was coated onto the substrates with the help of a spin coater spun at a rate of 2000 rpm for 1 min. This procedure was repeated four times as the seed layer acts as a preferential growth site during the growth process. Finally, the substrates were heated at a constant temperature of 180 °C for 20 min in order to solidify the seed layer. After that the substrates were cooled down to room temperature. To grow the ZnO NWs we used a low-temperature method. In this method zinc nitrate hexahydrate [Zn(NO3)26H2O] (99.998%) and hexamethylenetetramine (HMT) (C6H12N4) (99.998%) were mixed in equal molar concentration in DI water and kept under constant magnetic stirring for 30 min to get a uniform growth solution. The preheated SiC substrates were then placed in the solution and placed in a regular laboratory oven and heated at 95 °C for 4 h. Finally, the samples were washed with deionized water and dried in air. After the growth, the samples were divided into four groups in which one groups is used as grown and the remaining three were used for post growth annealing treatment in a quartz tube furnace. The annealing was carried out in a horizontal quartz furnace and kept at an annealing temperature of 600 °C for 30 min. When the required temperature was achieved; the samples were pushed into the furnace. All three samples were then annealed at 600 °C for 30 min in argon (Ar), nitrogen (N2) and oxygen (O2) ambient. The next step is fabrication of the Schottky devices on the post growth annealed samples. To avoid short circuits between the NWs, an insulating spin-on-glass (SOG) was filled by spin coater in order to fill the empty gap between the individual NWs. The insulating SOG prevented potential short circuits due to pinholes in the nanowire array and provided mechanical stability for the measurement. Subsequently, the tips of the ZnO NWs were exposed using simple plasma ion-etching with CF4 prior to contact metal deposition. Finally, In order to complete light emitting diodes (LEDs) processing, Au metal contacts were formed on a group of ZnO NWs by thermal evaporation at a pressure of 2 10-7 Torr, and the contacts were formed as circular dots of 1 mm in diameter and 100 nm in thickness. Four devices were fabricated for comparison. The surface morphologies and sizes of the resulting ZnO NWs were observed by field-emission scanning electron microscope (FESEM JSM 6700F) operated at 12 keV. The current–voltage (I–V) characteristics of the fabricated LEDs were measured by a semiconductor diode parameter analyzer (4145B, Hewlett–Packard), electroluminescence (EL) behavior of the fabricated LEDs was recorded by using an Andor Sharmrock 303iB spectrometer supported with Andor–Newton DU-790 N detector and Keithley 2400 as a source meter. The light was collected from the topside of the device. All of these measurements were carried out at room temperature in ambient atmosphere.
3. Results and discussion The schematic diagram of the Au/n-ZnO Schottky white light emitting diode (WLED) is shown in Fig. 1. The top-view image of the as grown ZnO NWs is shown in Fig. 2(a). It clearly shows that ZnO NWs are densely packed; faceted hexagonal shape and most of the NWs are parallel to each other and predominately in a perpendicular orientation to the substrate. The average diameter and length of the ZnO NWs are 150–200 nm and 3 lm, respectively. Fig. 2(b–d) shows ZnO NWs in different annealed in different ambient. Four Au/n-ZnO NWs WLEDs were fabricated based on the as grown NWs and the annealed in different atmosphere such as in O2, N2 and Ar at 600 °C. Fig. 3 shows the I–V characteristics of the all devices. It can be seen that all the fabricated devices exhibit good rectifying behavior. It clearly shows the nonlinear increase of the current under forward bias. Such type of rectification behavior is best described by using the standard thermionic emission theory [3]. Under forward bias the turn-on voltage of device without annealing is about 5 V. However, in case of the thermally treated diodes, the turn on
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Fig. 1. Schematic illustration of Au/n-ZnO NWs Schottky LED.
Fig. 2. (a) SEM image of the top-view of grown ZnO NWs, (b) SEM image of the ZnO NWs after annealed in oxygen ambient, (c) SEM image of the ZnO NWs after annealed in nitrogen ambient and (d) SEM image of the ZnO NWs after annealed in argon ambient.
voltage changes slowly to 4.5, 4.3, and 4 V in Ar, N2, and O2 annealing ambient respectively. The forward current increased slightly for N2 ambient as compared with other diodes. The electroluminescence (EL) spectra of all devices are shown in Fig. 4. The EL spectra of all devices reveal a broad emission band from 400 to 800 nm covering the whole visible region. The EL emission could be clearly noticeable by the naked eye in darkness. It can be clearly seen that the annealing
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4,0x10-3 as grown 3,2x10-3
annealed in Ar
Current (A)
annealed in O2 annealed in N2
2,4x10-3 1,6x10-3 8,0x10-4 0,0 -10
-5
0
5
10
15
20
Voltage (V) Fig. 3. Typical room temperature current–voltage (I–V) curve of Au/n-ZnO Schottky LEDs based on as grown and annealed NWs.
Intensity (Watt / nm*counts)
200
annealed in N2 annealed in O2 annealed in Ar as grown
150
100
50
0 400
500
600
700
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Wavelength (nm) Fig. 4. RT EL spectra of Au/n-ZnO Schottky LEDs based on as grown and annealed different environment.
ambient influences the EL spectrum of the devices. The spectra exhibit only one emission band that is a deep level emission (DLE) associated with defect related radiated recombination in ZnO [21]. Furthermore, various functional group elements (related to carbon, nitrogen and hydrogen) beside many lattice defects and surface defects were incorporated during the growth [18]. Quang et al. [11] reported that these defects could act as nonradiative centers and reduce light emission from the ZnO but after annealing these defects could be reduced and all those functional group elements were released from the surface of the ZnO NWs this will greatly enhanced the crystal structure and decrease the nonradiative recombination, leading to the increase of luminescence efficiency of LED. The deep level or trapped-state visible emission attributed to several surface defects in the crystal structure, or the recombination via distribution, of different defects states. But there is generally no accepted mechanism responsible for the green emission. According to most of the models, various impurities and structural intrinsic defect centers such as oxygen vacancy (Vo), zinc vacancy (Vzn), zinc
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interstitial (Zni), oxygen interstitial (Oi), or antisite oxygen (Ozn) are responsible for the deep level emission [3]. The as grown NWs based LED shows three sub DLE bands, a broad red, green and small blue emission peaks are centered at 724, 518, and 450 nm, respectively. The EL intensity of all devices based on annealed NWs in increased. All devices exhibited three sub DLE bands one strong red sub band, green band and a relatively weak violet band. The DLE emission was dominated and the NBE emission which is relatively weak in the spectrum that may be ascribed to the low radiative recombination efficiency and self-absorption effect by the DLE traps in ZnO [22–24]. The EL spectrum from Ar annealed NW based device shows three peaks approximately centered at 524 nm (green), 710 nm (red), 404 (violet). The violet peak corresponds to the Zni [2]. Likewise the EL spectrum for device based on annealed NWs in N2 ambient shows the intensity of the all red, violet and green emission is decreased as compared to LED based on as-grown ZnO NWs. This is exactly the same trend is noticed by the Quang et al. [11], they reported that the red emission decrease due to the reduction in the complex defects when annealing in N2 ambient. In ZnO NWs annealed in N2 rich ambient, the following reaction may occur. In N2 rich ambient, the majority of defects are oxygen vacancies, generated by the evaporation of oxygen [11].
ZnO ! V o þ ZnZn þ 1=2O2 Finally the device based on NWs annealed in O2 environment, it is clearly seen that the intensity of the red and violet emission from annealed NWs based devices decrease to a value of 692 and 400 nm relative to that of as grown NWs based device (724 nm) whereas the green emission increased as compared to as grown NWs based LED. Under O2 rich ambient, the antisite oxide Ozn were easily formed from interstitial oxygen and zinc vacancies Vzn [11]. To investigate the color quality of white LEDs color chromaticity coordinates are plotted in Fig. 5 with the RT EL spectra indicates all components
Fig. 5. Chromaticity diagram of the Au/n-ZnO Schottky LEDs.
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of white emission. The figure shows the CIE 1931 color map chromaticity diagram in the (x, y) coordinates system. The color rendering indices (CRI) of the white LEDs based on the as grown, and the samples annealed in O2, N2, and Ar, were calculated to be 78, 84, 91 and 88 respectively. Alvi et al. [19] also noticed the increase in CRI value after annealed in different ambient compared with as-grown ZnO NWs. The possible reason for the increase in CRI is that during the annealing the deep level defects responsible for the orange and red emission become more active as compared to the defects responsible for green and blue emission in the deep levels of ZnO [19]. The correlated color temperature (CCT) of the white LEDs based on as grown NW, annealed in O2, Ar, N2 were calculated 2753 K, 4327 K, 4539 K and 5122 K respectively. It clearly shows that there is an increase in the CCT of the LEDs when annealed in different ambient also it indicate that light from the LEDs can be tune from cold white light to warm white light by post growth annealing. The chromaticity coordinates are very close to the Planckian locus, which is the trace of the chromaticity coordinates of a blackbody. The colors around the Planckian locus can be regarded as white. It is clear that the fabricated LEDs are in fact white LED and the light emission is actually a white color impression. Considering the color quality, i.e., the CRI, the best value was obtained for LED fabricated using NW annealed in N2 environment and reached a value of 91. From this point of view, It shows that N2 annealed NWs based LED had better optical quality with less nonradiative related defects. From this study and although devices fabricated on the NWs annealed in different ambient and the resulting emission from those devices possesses different light quality. It indicates that white light can be tuned from cold white to warm white light. 4. Conclusion We report luminescence from Au/n-ZnO LEDs based on as grown and annealed NWs in different environments. The devices illustrate broad emission band covering the whole visible region arising from the radiative recombination in the ZnO NWs. The CRI and CCT of the white LEDs were calculated in the range of 78–91 and 2753–5122 K respectively. The examination results show that annealing temperature and environment put simultaneously impact the electrical and optical properties on the fabricated LEDs due to the enhancement of the ZnO NWs crystalline properties. References [1] [2] [3] [4]
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