1. Introduction

ZnO and III-nitrides have been widely investigated in the field of optoelectronics for several decades [1–6]. For example, InGaN/GaN multiple quantum wells (MQWs) have been generally adopted for commercial high-efficiency LEDs. By changing the relative composition of indium (In) and gallium (Ga), the MQWs can emit wavelengths covering from ultraviolet to infrared radiation. Also, ZnO nanostructures have been widely investigated and applied in several optoelectronic devices because of their wide bandgap and high exciton binding energy (60 meV) [3, 4]. In addition, the fabrication of ZnO nanostructures has several flexible options in growth methods with low cost. Nowadays, low-dimensional materials have attracted great attention because of the quite different physical behaviors from their bulk structures. One of the unique properties in low-dimensional nanostructured materials is the strongly anisotropic physical properties. For instance, it has been found that the near-band-edge (NBE) emission from aligned ZnO nanorods (NRs) is strongly polarized with the electric field along the c-axis of ZnO crystal [5]. On the other hand, the side emission from the InGaN/GaN MQWs grown on (0001) sapphire substrates is found to be highly polarized with the electric field in the plane of the MQWs [6]. It is known that an optoelectric device consisting of different materials with distinct properties can exhibit quite unusual behaviors through the coupling between the consistent elements [7]. For instance, if a LED can be designed to have dual emissions with different polarizations, one can choose the emitted wavelength by rotating a polarizer or even by modulating the voltage of a liquid crystal cell [7]. It would bring us advantages in many practical applications, such as optical modulators [8], color selectors [9], displayers [7], and optical communications [10]. Here, we introduced an intelligently designed LED based on the composite consisting of ZnO NRs and InGaN/GaN MQWs. In conventional MQW-based LEDs, carrier overflow is usually prohibited, because if it occurs the electrical energy may be converted into undesired emission. However, in this work, although the electron overflow is prohibited by an electron blocking layer, holes are intentionally led to overflow into the ZnO region to arise a second emission. One more point should be mentioned is that in conventional GaN-based LEDs, the n-GaN was grown before MQWs, because the n-GaN can be grown with higher crystallographic and morphological qualities than p-GaN. In this work, we adopted an inverted LED structure employing a n-ZnO NRs layer grown after the MQWs layer, because the growth temperature of n-ZnO is significantly lower than that required for GaN growth in metal-organic chemical vapor deposition (MOCVD), and this inverted structure is easier for us to grow the ZnO NRs layer, which plays an important role in this device. The inverted structure of ZnO/GaN based LED has been demonstrated to perform quite well in the previous report [11]. According to our design, it is found that the composite consisting of ZnO NRs and InGaN/GaN MQWs becomes an intriguing LED with dual emissions, in which the relative intensity between the emissions can be manipulated by their polarizations. The underlying mechanism can be well interpreted by the optical anisotropy due to the inherent nature of geometric structures of nanomaterials.

2. Experiment

The studied InGaN/GaN MQWs were prepared by MOCVD. First, a 2.5 μm p-GaN layer and a 10 nm p-AlGaN electron-blocking layer were deposited on a (0001) sapphire substrate, and then an intrinsic series containing 5 periods of 2 nm In_0.22Ga_0.78N wells and 5 nm GaN barriers were grown. ZnO NRs were grown on the top surface of MQWs via hydrothermal method as described in the previous report [12]. A small area of InGaN/GaN MQWs was etched to the p-GaN layer, and an indium (In) drop was deposited on p-GaN by a heated iron tip to serve as the p-electrode. The diameter of the indium drop is estimated to be 0.5 mm under an optical microscope (OM). Since indium drops are commonly used for quick-tests of commercial GaN-based LED wafers [13], we believe that the indium electrode performs well in this work. On the other hand, because gold (Au) can form a good ohmic contact with ZnO [2], an Au wire was directly used as the n-electrode. The Au wire with a diameter of 0.1 mm was placed onto the top of ZnO NRs and slightly pressed by a glass plate. Next, the sample was inserted into the rapidly thermal annealing (RTA) system, which was set at 400°C for 30 sec. According to our measurements (not shown here), it is found that under the same forward bias, the current could be enhanced by 100 times after the RTA treatment compared with that before RTA process. Therefore, it is expected that the Au electrode will serve as a good contact in our study. Both p- and n-contact areas were estimated to be about 0.2 mm². Scanning electron microscope (SEM) images were recorded using a JEOL JSM 6500 system. X-ray diffraction (XRD) spectra were obtained on a PANalytical X’Pert Pro diffractometer using Cu K_α radiation (λ = 1.5418 Å) at 45 kV and 40 mA. Photoluminescence (PL) and electroluminescence (EL) measurements were performed by a Jobin Yvon SPEC-1403 spectral system working in the double-grating mode. For examining the polarization state of the emission, a polarizer was inserted before the entrance of the spectrometer, and then a depolarizer was used to exclude the influence of the response due to the orientation of the grating. The optical source was provided by a He-Cd laser with the wavelength of 325 nm, and the electric source was provided by a Keithley 236 unit.

3. Results and discussion

As shown in Figs. 1(a) and 1(b), ZnO NRs with around 2.2 μm in length and 100 nm in diameter were well grown on the top surface of InGaN/GaN MQWs. Figures 1(c) and 1(d) show the X-ray diffraction patterns, which reveal well organized lattice via the corresponding peaks of ZnO, GaN, and InGaN [1, 4, 14]. Since ZnO and GaN have a close lattice constant and the same wurtzite structure, it is expected the growth of ZnO NRs will follow the orientation of GaN substrate [15]. Indeed, the X-ray diffraction spectra in Figs. 1(c) and 1(d) reveal this prediction. It means that ZnO and GaN have the same crystalline phase and orientation.

 

Fig. 1 (a) Top view and (b) side view of scanning electron microscope images of InGaN/GaN/ZnO nanocomposite material. (c) X-ray diffraction spectrum and (d) x-ray diffraction spectrum with enlarged scale of 33.6-36.0 degree in (c).

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Figure 2(a) illustrates the experimental configuration of electroluminescence (EL) measurements of the composite LED, where the arrow symbols denote the flowing direction of electric current under forward bias. Figure 2(b) shows the current through the LED as a function of the bias voltage, in which we can clearly see that the current-voltage characteristic curve of the device reveals a diode-like behavior. Figure 2(c) shows the current-dependent EL spectra of the LED under forward bias. There are two main EL peaks located at 380 nm and 440 nm in the EL spectra under different forward bias. The two peaks could be attributed to the emissions arising from ZnO NRs and InGaN/GaN MQWs, which could be supported by the evidences given below.

 

Fig. 2 (a) Illustration of experimental details of electroluminescence measurements for the composited light-emitting device. (b) The current through the light-emitting device as a function of bias voltage. (c) Current dependence of electroluminescence spectra of the composited light-emitting device. (d) Intensities of the 380 nm and 440 nm emissions as functions of the injection current.

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Figure 3 illustrates the band diagram of the device under forward bias according to the previous reports [16, 17]. Due to the quantum confined effect, the injected electrons from n-ZnO and the holes from p-GaN are easily confined in the intrinsic InGaN layers of MQWs, and the confined electron-hole pairs could recombine via radiative process by emitting 440 nm photons. According to the previous reports [18, 19], if the barriers of MQWs are not specially treated, such as effective doping, holes may readily overflow through intrinsic barriers into n-ZnO NRs under forward bias, especially under higher carrier injection [17]. When the hole current overflows in, electron-hole pairs can also recombine in n-ZnO NRs accompanied by the emission of 380 nm photons as shown in our previous work [20], which could be attributed to the NBE emission of ZnO NRs. As shown in Fig. 2(c), it is worth noting that the intensities of both EL peaks were found to increase with increasing forward bias. In addition, we can also see that the intensity ratio between the two emissions changes with increasing bias voltage. Under lower bias, the emission intensity of 380 nm is weaker than that of 440 nm. However, with higher bias, the intensities of the two emissions become comparable. The intensities of both emissions were plotted in Fig. 2(d) as functions of the injection current. The emission intensity of 440 nm shows a trend of saturation, whereas the increasing rate of the intensity of 380 nm gets faster under higher injection current. This is a typical behavior of the hole overflow [19]. As the injection current increases, the carrier concentration in the MQWs region increases and the Fermi energy rises, so that a higher overflow rate could be reasonably expected [19]. As a result, the emission intensity arising from ZnO NRs can catch up with that from InGaN/GaN MQWs when the injection current increases.

 

Fig. 3 Bias voltage dependence of electroluminescence spectra of the composite light emitting device.

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Quite interestingly, when we examine the polarization state of the emission, it is found that the relative intensity between these two EL peaks strongly depends on the polarization as shown in Fig. 4(a) . Here, 0° is defined as the polarization state parallel to the direction lying in the plane of the MQWs, and 90° is defined as the polarization state parallel to the direction along the growth direction of ZnO NRs (c-axis). In order to understand the polarization property of the LED device more precisely, we fixed the monochromator at 380 nm and 440 nm, respectively, and then recorded the intensity of each EL peak with an interval of 15° of the polarizer angle as shown in Figs. 4(b) and 4(c). Clearly, the intensities of the dual emissions are functions of the angle of polarizer. The result shows that we can obtain the maximum intensity of 380 nm when the polarization axis of the polarizer is parallel to the ZnO nanorod orientation (90°), and the minimum intensity at the perpendicular direction (0°). On the other hand, the emission intensity of 440 nm shows a complementary result that the maximum intensity is at 0° and the minimum intensity is at 90°. As shown in Figs. 4(b) and 4(c), both polarizations of the dual emissions can be described by the Malus’s law quite well [21],

 

Fig. 4 (a) Electroluminescence (EL) spectra from the composited light-emitting device through a rotatable polarizer with the angle of 0° and 90°. (b) Polarizer-angle-dependent EL intensities with the monochromator fixed at 380 nm and (c) 440 nm, respectinely. Malus’s-law-fitted lines of 380 nm and 440 nm emissions by Eq. (2) are also shown in (b) and (c).

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In order to provide more evidences to confirm that the origin of these two emissions arises from ZnO NRs and InGaN/GaN MQWs, we have performed PL measurements on these two materials individually as shown in Fig. 5 . Both samples were grown on sapphire substrates under the same condition as we stated above. The edge emission from ZnO NRs shown in Fig. 5(a) has a narrow line located at 380 nm [3]. The polarization state of the NBE emission is strongly polarized along the growth direction of ZnO NRs (c-axis). On the other hand, the emission spectrum from the edge of pure InGaN/GaN MQWs exhibits a narrow line located at 440 nm with Fabry-Perot interference patterns as shown in Fig. 5(b) [22]. The emission was found to be strongly linear-polarized lying on the epitaxial plane (0001). In other words, the polarization states of the emissions of ZnO NRs and InGaN/GaN MQWs are perpendicular to each other. The degrees of polarization for ZnO NRs and InGaN/GaN MQWs are about 56.3% and 62.6%, respectively, via the calculation by the following formula [21],

 

Fig. 5 Illustrations of the experimental details and measured spectra under different polarization for (a) InGaN/GaN quantum wells and (b) ZnO nanorods.

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4. Summary

In summary, we have demonstrated a novel LED based on the composite consisting of ZnO NRs and InGaN/GaN MQWs. The newly designed LED has the capability to emit dual beam radiations. Quite interestingly, the relative intensity between the dual emissions can be manipulated by a polarizer. The underlying mechanism can be well interpreted in terms of the anisotropic optical properties arising from the geometric structures of the constituent nanoscale materials. Our result presented here can be extended to many other nanostructured composites, and it therefore may open a new pathway for the creation of optoelectronic devices with tunable properties.