Figure shows the images of the top of the ZnO nanotubes before and after annealing, respectively. The figure shows clearly the morphology and size distribution of the as-grown ZnO nanotubes. Hexagonal, well-aligned, vertical ZnO nanotubes were obtained on the p-GaN substrate. The ZnO NTs grown had a uniaxial orientation of
with an epitaxial orientation with respect to the p-GaN substrate, forming n-ZnO-(NTs)/p-GaN p-n heterojunctions. From the SEM images, the mean inner and outer diameters of the as-grown ZnO nanotubes in this study were found to be approximately 360 and 400 nm, respectively. Figure shows the current-voltage, I
, curves of the n-ZnO NTs/p-GaN LEDs developed in this study. All the LEDs have the same I
curves. The I
curves clearly show a rectifying behavior of the LED as expected with a turn on threshold voltage of about 4 V. This indicates clearly that both metal/GaN and metal/n-ZnO interfaces have formed good ohmic contacts. Figure shows the schematic illustration of the fabricated LEDs.
SEM image of ZnO nanotubes on p-GaN substrate. (a) before annealing, (b) after annealing, (c) typical I-V characteristics for the fabricated LEDs, and (d) The schematic illustration of the fabricated LEDs.
Figure shows the EL spectra of the as-grown and annealed LEDs. All the EL measurements were taken under forward bias of 25 V. The EL spectra consist of violet, violet-blue, orange, orange-red, and red peaks. The violet and violet-blue peaks are centered approximately at 400 nm (3.1 eV) and 452 nm (2.74 eV), respectively. The broad green, orange, orange-red, and red peaks are centered approximately at 536 nm (2.31 eV), 597 nm (2.07 eV), 618 nm (2.00 eV), and 705 nm (1.75 eV), respectively. The EL emission in the ultraviolet (UV) region was not detected here since the authors were interested only in the visible emissions; therefore, the lower EL detector limit was set to 400 nm.
Electroluminescence spectra of the LEDs at an injection current of 3 mA for the as grown and annealed ZnO NTs in different ambients under forward bias of 25 V and it shows the shift in emission peak after annealing in different ambient.
The EL intensity of the samples annealed in argon is low compared to the as-grown and all other samples annealed in different ambients. The ZnO nanotubes having low growth temperature (<100°C) possess many intrinsic defects, such as oxygen vacancy (Vo
), zinc vacancy (Vzn
), interstitial zinc (Zni
), interstitial oxygen (Oi
), etc., and these defects are responsible for the DLEs. These defects are reduced after annealing at high temperature (600°C). Such activation or passivation of intrinsic defects would greatly enhance the crystal's deep level defect structure leading to the modification of luminescence spectra efficiency of the LEDs [16
]. This argument is also confirmed by the EL spectra obtained for ZnO nanotubes annealed in argon (see Figure ). The EL intensities of the violet (400 nm) and violet-blue (452 nm) of all the annealed samples are decreased as compared with the as-grown samples. In the literature, it was reported that the violet emission from undoped ZnO nanorods is related to Zinc interstitial (Zni
]. The violet peak is centered at 3.1 eV (400 nm), and this agrees well with the transition energy from Zni
level to the valence band in ZnO (approximately 3.1 eV). The violet-blue peak was centered at 2.74 eV (452 nm) for all the EL measurements in different ambients. It is attributed to recombination between the Zni
energy level to the VZn
energy level, and approximately is in agreement with the transition energy from Zni
energy level to VZn
energy level (approximately 2.84 eV). There is a difference of 0.11 eV. This difference maybe is due to the effect of GaN substrate, as GaN also emits blue light. There are no shifts in violet and violet-blue peaks after annealing in different ambients. The violet and violet-blue emissions decreased after annealing the as-grown ZnO nanotubes in different ambients. The violet and violet-blue are the high energy emissions in the visible region, and the annealing affects the deep level defects that are responsible for low energy emissions from the green-to-red region in the visible spectra (see in Figure ). It increases the transition recombination rate for the deep level defects that are responsible for the green-to-red emissions. Therefore, the EL intensities of the DLEs (the green to red) are increased, while those of the violet and violet-blue emissions are decreased after annealing in different ambients. Only for the case of the argon ambient, all the defects are modified, and owing to this, the El intensities of all the emissions decreased after annealing.
The broad green peak, centered at 536 nm (2.31 eV) in the EL spectra of the as-grown ZnO nanotube-based LEDs and LEDs based on annealed ZnO nanotubes in argon ambient, is attributed to oxygen vacancy (Vo
). It is believed that this phenomenon is due to band transition from zinc interstitial (Zni
) to oxygen vacancy (Vo
) defect levels in ZnO [22
]. This has been explained by the full potential linear muffin-tin orbital method, which posits that the position of the Vo
level is located approximately at 2.47 eV below the conduction band, and the position of the Zni
level is theoretically located at 0.22 eV below the conduction band. Therefore, it is expected that the band transition from Zni
level is approximately 2.25 eV [22
]. This agrees well with the green peak that is centered approximately at 2.31 eV.
The orange-red peaks are centered at 597 nm (2.07 eV) and 618 nm (2.00 eV) for the samples annealed in air and oxygen, respectively. These emissions are attributed to oxygen interstitials Oi
, and believed to be due to band transition from zinc interstitial (Zni
) to oxygen interstitial (Oi
) defect levels in ZnO [22
]. The position of the Oi
level is located approximately at 2.28 eV below the conduction band, and it is expected that the band transition from Zni
level is approximately 2.06 eV [22
]. This agrees well with the orange-red peaks that are centered approximately at 2.00 and 2.07 eV.
The EL spectra of ZnO nanotubes annealed in oxygen and air ambients are nearly similar. The EL intensity of the sample annealed in oxygen is higher compared to that of the sample annealed in air. Its means that air and oxygen produce the same defects, but the ratio of these defects is more in the case of oxygen. As the orange-red emission is attributed to oxygen interstitials Oi
], the annealing in oxygen ambient increases the amount of oxygen-related Oi
defects; therefore, the orange-red emission dominates the EL spectra.
The red emission centered at 705 nm (1.75 eV) can be attributed to oxygen vacancies (Vo). For the ZnO nanotubes annealed in nitrogen ambient, the following oxygen desorption may occur;
The zinc vacancies are filled with zinc during the annealing of the ZnO nanotubes in the nitrogen ambient. The majority of defects are oxygen vacancies (Vo
) that are created by the evaporation of oxygen [21
]. The red emission centering at 706 nm (1.75 eV) may be attributed to the transition from oxygen vacancy (Vo
) level to top of the valance band in ZnO. Using full-potential linear muffin-tin orbital method, the calculated energy level of the Vo
in ZnO is 1.62 eV below the conduction band [20
]. Hence, the energy interval from the Vo
energy level to the top of the valence band is approximately 1.75 eV. It agrees well with that observed for the red emission centered at 1.75 eV.
By comparing the EL spectra of samples annealed in oxygen and nitrogen, it can be concluded that the total red emission ranging from 620 nm (1.99 eV) to 750 nm (1.65 eV) is the combination of emissions related to Oi and Vo defects. The EL spectra of the samples annealed in oxygen show that after annealing, the red emission is enhanced in the range from 620 nm (1.99 eV) to 690 nm (1.79 eV) when compared to the as-grown samples, and the EL spectra of the samples annealed in nitrogen ambient show that, after annealing, the red emission is enhanced in the range from 690 nm (1.79 eV) to 750 nm (1.65 eV). The EL intensities of the green, yellow, orange, and the red emission (from 620 to 690 nm) are decreased, but the EL intensity of the red emission (from 690 to 750 nm) has increased significantly as compared with the as-grown ZnO nanotubes. Therefore, it is clear that the red emissions from 620 to 690 nm and from 690 to 750 nm have different origins. The red emission in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) can be attributed to Oi, and that in the range of 690 nm (1.79 eV) to 750 nm (1.65 eV) can be attributed to Vo.
Figure shows the CIE 1931 color space chromaticity diagram in the (x, y) coordinates system. The chromaticity coordinates are (0.3559, 0.3970), (0.3557, 3934), (0.4300, 0.4348), (0.4800, 0.4486), and (0.4602, 0.3963) with correlated color temperatures (CCTs) of 4802, 4795, 3353, 2713, and 2583 K for the as-grown ZnO nanotubes, annealed in argon, air, oxygen, and nitrogen, in the forward bias, respectively. 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 the white LEDs.
The CIE 1931 x, y chromaticity space of ZnO nanotubes, for (a) as grown, (b) annealed in argon, (c) annealed in air, (d) annealed in oxygen, (e) annealed in nitrogen, and (f) all together.
Figure shows the schematic band diagram of the DLE emissions in ZnO, based on the full-potential linear muffin-tin orbital method and the reported data.
Figure 4 Schematic band diagram of the DLE emissions in ZnO based on the full potential linear muffin-tin orbital method and the reported data as described in references [9-20,22]. Also oxygen vacancies situated 1.65 eV below the conduction band are denoted to (more ...)
In summary, the origin of red emission in chemically obtained ZnO nanotubes has been investigated by EL spectra. The as-grown samples were annealed in different ambient (argon, air, oxygen, and nitrogen). It was observed that the post-growth annealing in nitrogen and oxygen ambients strongly affected the green, yellow, orange, and red emissions of ZnO nanotubes. The EL intensities of the green, the yellow, the orange, and the red emissions were gradually increased after annealing in air, oxygen ambients, and decrease in argon ambient. However, in nitrogen ambient, the EL emission of the red peak in the range of 690--750 nm was increased, and in the range of 620-690 nm, it was decreased as compared with the as-grown samples. It was found that more than one deep level defect are involved in producing the red emission in ZnO.