Based on hybrid inorganic/organic n-ZnO nanorods/p-GaN thin film/poly(3-hexylthiophene)(P3HT) dual heterojunctions, the light emitting diode (LED) emits ultraviolet (UV) radiation (370 nm – 400 nm) and the whole visible light (400 nm −700 nm) at the low injection current density. Meanwhile, under the high injection current density, the UV radiation overwhelmingly dominates the room-temperature electroluminescence spectra, exponentially increases with the injection current density and possesses a narrow full width at half maximum less than 16 nm. Comparing electroluminescence with photoluminescence spectra, an enormously enhanced transition probability of the UV luminescence in the electroluminescence spectra was found. The P3HT layer plays an essential role in helping the UV emission from p-GaN material because of its hole-conductive characteristic as well as the band alignment with respect to p-GaN. With our new finding, the result shown here may pave a new route for the development of high brightness LEDs derived from hybrid inorganic/organic heterojuctions.
© 2011 OSA
Over the past decades, gallium nitride (GaN) has attracted unprecedented attention due to its direct and wide band gap (3.4 eV) and thus has been comprehensively investigated for the use in light emitting diodes (LEDs). In early age, electroluminescence (EL) has been obtained from schottky barrier diodes and the emission may include blue, green, yellow, orange and red light by the doping of group-I, group-II, and rare-earth elements during the synthesis [1–5]. However, the schottky barrier diode has the extremely high turn-on voltage and thus is highly energy-consuming. An intriguing alternative to overcome the difficulty is to develop p-n heterojunction LEDs.
Zinc oxide (ZnO) has long been thought to be a promising material for optoelectronic applications due to its direct and wide band gap (3.3 eV) and large exciton binding energy (60 meV). However, no matter what method is used to prepare ZnO, ZnO is an intrinsically n-type semiconductor due to the existence of oxygen vacancies. Also, it has been indicated that p-type doping in ZnO is tough task to accomplish [6,7]. Therefore, in order to fabricate ZnO based UV-LEDs, it necessitates a good and reproducible p-type material.
ZnO and GaN have the same wurtzite crystal structure and extremely small in-plane lattice mismatch of 1.8%. Therefore, it is easier to fabricate high quality ZnO on GaN-related substrate. In view of this, much effort has been made to investigate the EL emission from n-type ZnO/p-type GaN heterojunction LEDs [8–17,28]. However, only few of them report a weak ultraviolet (UV) light emission from the LEDs even though both ZnO and GaN are direct and wide band gap semiconductors. In this report, we demonstrate a strongly enhanced UV electrluminescence from hybrid inorganic/organic ZnO/GaN/P3HT dual heterojunctions. P3HT is a well-known polymer for its hole-conductive and optical properties, and has been extensively studied for a wide range of applications [18–20]. We use the organic P3HT rather than inorganic p-type semiconductors to fabricate p-p-n dual heterojunctions because of its low preparation temperature. In such a low temperature, the properties of ZnO/GaN heterojunction can be maintained without being destroyed. Besides, adopting the organic P3HT makes it need no further consideration of the lattice-matching problem that is quite important in the growth of inorganic semiconductors. It is found that the additional poly(3-hexylthiophene)(P3HT) layer plays a crucial role for the enhancement due to its hole-transporting characteristic and the band alignment with respect to p-type GaN. Our approach presented here therefore provides a new route for the creation of highly efficient LEDs based on hybrid inorganic-organic heterojunctions.
In this experiment, the lightly Mg-doped p-type GaN thin film on a sapphire substrate was prepared by standard metal organic chemical vapor deposition (MOCVD) method. According to a four-points Hall-effect measurement, a hole concentration and a mobility of the GaN thin film are respectively 9.0 × 1016 cm−3 and 7 cm2 V−1 s−1 at room temperature. In order to investigate the pure effect of the P3HT layer on the p-GaN/n-ZnO LED and compare the result with our previous work, we use the p-GaN with the same hole concentration. The vertically well-aligned ZnO nanorods around 1.2 micrometers in length were fabricated by the CVD method, which has been oftentimes used to prepare ZnO nanostructures and served as a low-cost technique. During the CVD process, the Zn powder with a high purity of 99.99%, the precursor, was kept in the alumina boat at the center of a tube furnace. Meanwhile, the reaction chamber was evacuated and kept at a pressure of 10 Torr. Subsequently, Argon gas with a high purity of 99.9%, as a carrier gas, was introduced into the reaction chamber at a flow rate of 200 sccm. The growth temperature and dwell time were maintained at 620 °C and one hour, respectively. Additionally, a Si mask was used to shadow one side of the GaN/sapphire substrate and thus ZnO nanorods only grew on the other side.
After the growth, the ZnO sample was spin-coated with the ZEP-520 electron resist, a buffer layer, in order to keep Au/Ti contact from directly touching p-GaN thin film. This method was able to effectively avoid the undesired leakage current. Then, the Au/Ti contact was thermally heated onto the exposed ZnO nanorods. The commercial P3HT was dissolved in chloroform (10 mg mL−1) at a spin speed of 500 rpm for overnight and subsequently dropped onto the GaN part of the sample with one drop of 5 μL. Hereafter, the sample was annealed at 110 °C for 15 minutes in order to remove the residual solvent in P3HT and then the Ag contact was thermally evaporated onto the P3HT sample. The fabrication of P3HT and Ag contact were performed in a nitrogen-filled glove box. The thickness of the P3HT layer was about 3.2 μm according to the SEM image (not shown). It is worth noting that the low preparation temperature of the P3HT layer will not destroy the p-n junction of p-GaN/ZnO and cause unwanted dopant diffusion into p-GaN or ZnO.
The morphology of ZnO nanorods was characterized by using scanning electron microscopy (SEM) (JSM 6500, JEOL). Photoluminescence (PL) spectra were obtained at room temperature with a SPEX Fluorolog-2 instrument equipped with double-grating monochromator and a R928 photomultiplier tube (PMT). The excitation source was provided by a 325 nm He-Cd laser. In addition, room-temperature electroluminescence (EL) spectra were excited electrically by using a Keithley current source and the emission was detected from the sapphire side of the LED by the same PL measurement system.
3. Results and discussions
The schematic of the dual heterojunction structures is shown in Fig. 1a . On one side of the device, ZnO nanorods and Mg-doped p-type GaN thin film form an inorganic-inorganic heterojunction and the band diagram at thermal equilibrium is depicted in Fig. 1b. On the other side, P3HT and Mg-doped p-type GaN thin film construct an organic-inorganic heterojunction. As shown in Fig. 1b, the band diagram is depicted according to the assumption that the Fermi level at the interface is unpinned . The assumption has been extensively used to describe the transport of electrons or holes through the organic/inorganic interface [22,23]. Figures 2a and 2b show the top-view and the cross-sectional SEM images of ZnO nanorods which are vertically well-aligned and have an average diameter of 100 nm and a length of about 1.2 μm.
Before entering to investigate the photoelectric properties of our designed device, we first measure the room temperature photoluminescence (PL) properties of the individual components of the device since it can demonstrate the possible transitions from each material. As shown in Fig. 3 , the band edge emission centered at 3.3 eV dominates the PL spectrum of ZnO nanorods with low defect emission and thus it exhibits high quality of ZnO nanorods. The PL spectrum of P3HT shows two emission peaks occur near 1.76 eV and 1.9 eV. In addition, the typical PL features of GaN include the yellow luminescence (YL) band, the blue luminescence (BL) band, and the ultraviolet luminescence (UVL) band, corresponding to the peaks centered at around 2.2 eV, 2.75 eV and 3.32 eV, respectively. The YL band can be attributed to transitions from shallow donors to deep acceptors while transitions from deep donors to shallow acceptors are responsible for the BL band. Moreover, the UVL band belongs to conduction band to shallow acceptor (e-A) transitions [24–26]. The above mentioned transitions are depicted in the Fig. 1b. According to the early reports about the PL emission from undoped GaN, the BL band and the UVL band have very low quantum efficiency at room temperature [25,27]. However, for the lightly Mg-doped GaN, the increase of shallow acceptor states leads to the enhanced emission from the BL band and the UVL band , which is clearly observed in the PL and EL spectra from our experiment.
As shown in Fig. 4 , the I-V characteristics measured by changing the bias voltage from −10 V to 20 V exhibits the apparent rectifying diode behavior. It is found that the turn-on voltage is around 1 V. Such a low turn-on voltage is mainly attributed to the trap-induced recombination current in the depletion region under a low forward bias. Figures 5a and 5b show room-temperature electroluminescence (EL) spectra of the forward biased LED at various injection current densities. At the low injection current density, as shown in Fig. 5a, the EL spectra consist of the whole visible light (400 nm – 700 nm) and a part of ultraviolet light (370 nm – 400 nm). There exist six pronounced peaks at 1.9 eV, 2.12 eV, 2.2 eV, 2.75 eV, 3.32 eV and 2.91 eV. As depicted in Fig. 1b, the EL peak at 1.9 eV corresponds to the HOMO-LUMO energy gap of P3HT which can also be observed from PL spectrum in Fig. 3a. Moreover, the low drive current EL spectra from GaN consist of the three obvious peaks belonging to transitions from the YL band (2.2 eV), the BL band (2.75 eV), the UVL band (3.32 eV) [24–26]. It is found that the YL band intensity, the BL band intensity, as well as the UVL band intensity are quite distinct in the EL spectrum, which is very different from the PL spectrum of GaN. Furthermore, the EL emission around 2.91 eV is related to the transition from the donor levels of ZnO to the shallow acceptor levels of GaN and has been reported in our previous work . In addition, the peak at 2.12 eV can be related to the defect emission from ZnO. Therefore, according to the band alignment as shown in Fig. 1b, the function of P3HT is not only to form the red-light emitter but also to confine and block the electrons staying in GaN, and enhances the light emission peaks at 2.2 eV, 2.75 eV, 3.32 eV from GaN near P3HT/GaN heterojuction. Without P3HT layer, these emissions cannot be observed in ZnO/GaN LEDs as reported earlier .
According to the Fig. 5b, the LED, at the high injection current density, exhibits extremely strong UVL band emission, which superlinearly increases with the injection current density and with a narrow full width at half maximum (FWHM) less than 0.1 eV (16 nm). It is worth noting that the high enhancement of the UV emission from ZnO/GaN LEDs shown here has not been reported in previous studies [9–11,16]. In addition, we rule out the possibility that the UV light emission at 3.32 eV is from the the band edge emission of ZnO nanorods near GaN/ZnO heterojunction. According to EL spectra from GaN/ZnO heterojunction in our previous work , the recombination probability of 2.91 eV transition is extremely higher than that of 3.3 eV transition (bandedge emission of ZnO) for the EL process. Because the band diagram of GaN/ZnO heterojunction will not be affected by the additional layer of P3HT, the recombination probability of 2.91 eV transition and 3.3 eV transition should remain unchanged. However, according to Fig. 5b, it is found that the emission of 3.32 eV can be higher than that of 2.91 eV at a higher injection current density, which is contradicted to the above mentioned behavior about the recombination probability. Therefore, it is believed that the UV light emission results from UVL band transition in GaN near P3HT/GaN heterojuction.
In the relation, Io is the injection current density, α is the emission efficiency, and the exponent β represents the recombination mechanisms including the recombination of free excitons and free carriers (1 < β < 2), free excitons recombination (β = 1) and free carriers recombination (β = 2). As shown in Fig. 6a , the slope of the fitting line equals to 1.88 in the logarithmic plot, which indicates the transitions of the UVL band can be attributed to the first case as described above . According to Fig. 6b, beyond the UVL band, the other emissions of the LED can be assigned to the second case. In addition, it is found that the intensities of the emissions at 2.75 eV and 2.91 eV start to decrease when the injection current density exceeds 300 mA cm−2. It seems that the UVL band transition is competing with the 2.75 eV and 2.91 eV transitions at high injection current density, because these transitions share the same shallow acceptors. Based on the recombination processes shown in Fig. 1b, it is reasonable to understand that the UVL band transition has the fastest recombination rate and will dominate the recombination when the supply of shallow acceptors is limited.
In summary, we have demonstrated an enhanced UV emission from hybrid inorganic/organic n-type ZnO/p-type GaN/P3HT dual heterojunction LEDs. Beneficial from the P3HT layer, the LED emits light ranging from 370 nm to 700 nm at the low injection current density and a strong UV radiation around 380 nm at the high injection current density. Besides, the UV light exhibits a narrow FWHM less than 16 nm. Comparing the results with our previous study , we find that the dominant emission shifts from 405 nm (or 426 nm) to 380 nm, and thus the UV-LED is achieved because of the additional P3HT layer. It is found that the P3HT layer plays a very important role due to its hole-transporting characteristic as well as the band alignment with respect to p-type GaN. It is believed that our result shown here may pave a new route for the development of highly efficient UV-LEDs based on inorganic/organic heterojunctions.
This work was supported by the National Science Council and the Ministry of Education of the Republic of China.
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