n-ZnO/p-GaN heterojunction light emitting diodes with different interfacial layers were fabricated by pulsed laser deposition. The electroluminescence (EL) spectra of the n-ZnO/p-GaN diodes display a broad blue-violet emission centered at 430 nm, whereas the n-ZnO/ZnS/p-GaN and n-ZnO/AlN/p-GaN devices exhibit ultraviolet (UV) emission. Compared with the AlN interlayer, which is blocking both electron and hole at hetero-interface, the utilization of ZnS as intermediate layer can lower the barrier height for holes and keep an effective blocking for electron. Thus, an improved UV EL intensity and a low turn-on voltage (~5V) were obtained. The results were studied by peak-deconvolution with Gaussian functions and were discussed using the band diagram of heterojunctions.
©2013 Optical Society of America
Due to the wide direct band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature, zinc oxide (ZnO) has been considered to be a promising candidate material for optoelectronic applications, especially for blue to ultraviolet (UV) light emitting diode (LED) and UV detector devices [1,2]. Up to now, in spite of the great potential of ZnO in electron and photonic applications, there are few device applications, since it is difficult to obtain good and reproducible p-type ZnO . As an alternative approach to homojunction, an n-ZnO/p-GaN heterojunction has been put forward as an attractive candidate for device applications, for ZnO and GaN have similar lattice structure and relatively small lattice mismatch [4–9]. Furthermore, according to the Anderson model, the conduction band offset (ΔEC) and valence band offset (ΔEV) of ZnO/GaN are determined to be 0.15 and 0.13 eV, nearly the same barrier heights for electrons and holes, which can lead to radiative recombination in both n-ZnO and p-GaN layers . However, the electroluminescence (EL) of the n-ZnO/p-GaN heterojunction LEDs emits mainly from the p-GaN instead of the n-ZnO, for the electron injection from n-ZnO would win over the hole injection from p-GaN because of the lower carrier concentration in p-GaN . In order to obtain the light emission from n-ZnO, it is required to block the electron injection from n-ZnO or promote the hole injection from p-GaN. To block the electron injection from ZnO, wide band gap materials such as MgO , AlN , HfO2 , have been introduced into n-ZnO/p-GaN LEDs, because of the large conduction-band offsets (ΔEC) for MgO/ZnO (ΔEC = 3.55 eV), AlN/ZnO (ΔEC = 3.29 eV) and HfO2/ZnO (ΔEC = 2.29 eV) interfaces. Though the interfacial layers can effectively increase the barrier heights for electrons, high valence band offsets for MgO/GaN (ΔEV = 0.90 eV), AlN/GaN (ΔEV = 0.94 eV) and HfO2/GaN (ΔEV = 0.30 eV) interfaces would also increase the barrier heights for holes, leading to suppress the EL efficiency of devices.
Alternatively, ZnS, with the modest wide band gap (3.68eV) and large exciton binding energy of 40 meV, has similar fundamental physical properties with ZnO and GaN, including crystal structures, lattice constants, melting points, and so on [14,15]. Therefore, it is possible to take ZnS as the interlayer to improve the performance of n-ZnO/p-GaN LEDs. In this study, the n-ZnO/p-GaN heterojunction LED with a 20 nm ZnS interlayer was fabricated by pulsed laser deposition (PLD). Compared with an n-ZnO/AlN/p-GaN heterojunction LED, significant improvement of ultraviolet EL was observed in n-ZnO/ZnS/p-GaN heterojunction LED.
2. Device fabrication
Figures 1(a), 1(b) and 1(c) show three different structures of n-ZnO/p-GaN based heterojunction LEDs without interlayer and with ZnS and AlN interlayers. Commercial Mg-doped p-GaN epitaxial film produced by metal organic chemical vapor deposition (MOCVD) was used as the substrate. The hole concentration and mobility of p-GaN films were 1.6 × 1017 cm−3 and 14 cm2/Vs, respectively. For the n-ZnO/ZnS/p-GaN and n-ZnO/AlN/p-GaN structure, 20 nm thick ZnS and AlN layer were deposited on the p-GaN substrate by PLD in vacuum environment (<10−6 Pa), followed by the deposition of 200 nm ZnO films in 10 Pa oxygen atmosphere. The substrate temperature was kept at 450°C and a KrF excimer laser (COMPexPro 201, Coherent Inc.) was employed operating with a wavelength of 248 nm. For comparison, another n-ZnO/p-GaN heterojunction LED without any interfacial layer was fabricated, and the growth conditions for the top n-ZnO films were the same as the former. Next, Pt (50nm)/Ti (30nm) and Pt (50nm)/Ni (30nm) contact layers, served as electrodes, were deposited on n-ZnO and p-GaN by PLD, respectively.
The x-ray diffraction (XRD) measurements were performed on Rigaku D/MAX2500V diffractometer with Cu Kα radiation. Photoluminescence (PL) spectra were excited by a 325 nm He-Cd laser (CVI Melles Griot) at room temperature. A monochromator (ARS SP2557) was employed for collecting the emission spectra of PL and EL. The I-V measurements were carried out with a Keithley 2611A source meter. The carrier concentration and Hall mobility of the ZnO films were investigated by Hall measurement (Accent HL5500 PC) with the van der Pauw method.
3. Results and discussion
The undoped ZnO films exhibit n-type conductivity with the electron concentration of 3.0 × 1018 cm−3 and mobility of 20.4 cm2/Vs. The I-V characteristics of all the n-ZnO/p-GaN heterojunction LEDs demonstrate a nonlinear rectifying behavior, as shown in Fig. 2(a). Compared with the n-ZnO/p-GaN heterojunction LED, an additional voltage drop across the intermediate layer is observed from the n-ZnO/ZnS/p-GaN and n-ZnO/AlN/p-GaN device. In addition, it can be found that the forward turn-on voltage of n-ZnO/AlN/p-GaN (~6.5V) heterojunction is higher than that of n-ZnO/ZnS/p-GaN (~5V), for the dielectric constant of the AlN layer is larger than that of the ZnS layer [16,17]. The linear curves in the inset of Fig. 2(a) from both the Pt/Ni on p-GaN and Pt/Ti on n-ZnO reveal good Ohmic contacts at both electrodes, inferring that the rectifying behavior of the LEDs originates from the n-ZnO/p-GaN heterojunction. XRD patterns of the heterojunction LEDs are shown in Fig. 2(b). For all the samples, two diffraction peaks at ~33.9° and ~34.0°, which are corresponding to the ZnO(002) and GaN (002), confirm that the ZnO/GaN heterojunction are strongly c-axis orientation. Furthermore, two additional peaks at ~28.4° and ~36.2° are also found in the XRD patterns of the ZnO/ZnS/p-GaN and ZnO/AlN/p-GaN LEDs, which are corresponding to the wurtzite ZnS(002) (JCPDS card No. 36-1450) and AlN(002) (JCPDS card No. 25-1133).
The PL spectra of the p-GaN and ZnO/Al2O3(0001) are collected at room temperature, as shown in Fig. 3(a). There is a very weak broad band emission centered at around 500nm along with a near-band-edge (NBE) ultraviolet emission at 380 nm in the PL spectrum of the ZnO. The broadened yellow-green luminescence has been reported to be associated with oxygen vacancy related defects [18,19]. The PL spectrum of the p-GaN film reveals an intense deep-level emission at around 440 nm, which can be attributed to the transitions from the conduction band or shallow donors to deep acceptor [20,21].
The EL spectra of n-ZnO/p-GaN heterojunction LED at room temperature are shown in Fig. 3(b). As can be seen, a broad blue-violet emission band centered 430 nm is observed under various currents ranging from 0.4 to 2.0 mA. Compared the EL spectra with the PL spectra in Fig. 3(a), it is considered that the broad blue-violet emission band originates from the p-GaN layer. The results are consistent with the previous works . And the mechanisms of radiative recombination can be understood in terms of the band alignment, as illustrated in the inset of Fig. 3(b). Since ΔEC and ΔEV of ZnO/GaN are determined to be 0.15 and 0.13 eV, the barrier heights for electrons and holes are almost the same . In this case, the EL of the n-ZnO/p-GaN heterojunction LEDs can emerge from both n-ZnO and p-GaN regions. However, the electron injection from n-ZnO could win over the hole injection from p-GaN due to the higher carrier concentration and mobility in n-ZnO. As a result, the EL emission is determined primarily by electron injection from the n-ZnO side of the n-ZnO/p-GaN to the p-GaN side, where radiative recombination occurs .
The EL spectra of the n-ZnO/ZnS/p-GaN and n-ZnO/AlN/p-GaN heterojunction diodes are shown in Figs. 4(a) and 4(b). With the injection currents increasing from 0.4 mA to 2.0 mA, all the EL spectra of the n-ZnO/ZnS/p-GaN and n-ZnO/AlN/p-GaN heterojunction diodes display typical near-UV emission peaks at around 381nm. Although all the peaks of these EL spectra center around the NBE emission of ZnO films, the EL spectra present different profiles. Figure 4(c) plots the full width at half maximum (FWHM) of the EL spectra of the n-ZnO/ZnS/p-GaN and n-ZnO/AlN/p-GaN diodes with different injection currents. As the injection currents increased from 0.4 to 2.0mA, the FWHM of n-ZnO/ZnS/p-GaN narrowed from 68.9 nm to 31.8 nm, while the FWHM of n-ZnO/AlN/p-GaN narrowed from 33.9 nm to 27.5 nm. Compared with the n-ZnO/AlN/p-GaN heterojunction LED, the EL spectra of n-ZnO/ZnS/p-GaN heterojunction LED show a much broader emission band. Figure 4(d) shows the curves of integrated EL intensity versus input power for the n-ZnO/ZnS/p-GaN and n-ZnO/AlN/p-GaN heterojunction diodes. The EL integrated intensities of the n-ZnO/ZnS/p-GaN LED are obviously both higher than that of the n-ZnO/AlN/p-GaN LED at the same input power, which indicates that the ZnS films can effectively improve the EL efficiency of n-ZnO/p-GaN heterojunction diodes.
In order to better understand the origin of the EL emission, a Gaussian function is exploited to simulate the EL spectra of the n-ZnO/ZnS/p-GaN and n-ZnO/AlN/p-GaN heterojunction diodes with injection currents of 2.0 mA, as shown in Fig. 5. The simulated EL spectrum of the n-ZnO/ZnS/p-GaN diode consists of three distinct bands centered at around 380nm, 397nm and 430nm. Compared with PL spectra in Fig. 3(a), the UV emission band centered at around 380 nm is attributed to the NBE of ZnO films, whereas the blue emission band centered at about 430 nm should be ascribed to the transitions from the conduction band or shallow donors to deep Mg acceptor levels in the p-GaN substrates. And the violet emission centered at 397nm, which has been widely investigated in the ZnO-based diodes, is probably attributed to the shallow donors to the valence band or donor-acceptor pair recombination in ZnO [22,23]. According to Fig. 5, the EL spectrum of the n-ZnO/AlN/p-GaN diode exhibits two subbands centered at around 381nm and 397nm. It is worth noting that no blue emission band is observed in the EL spectra of the n-ZnO/AlN/p-GaN diodes.
The radiative recombination processes are illustrated with energy band diagram in the insets of Fig. 5. Electron affinity energies of ZnS and AlN are assumed to be 3.9 and 0.6 eV, respectively [10,15]. In addition, the bandgap energy of ZnS is taken to be about 3.68 eV and that of AlN is assumed to be 6.2 eV. For the n-ZnO/AlN/p-GaN heterojuniction under forward bias, electrons are confined in the ZnO layer by the large electron barrier height at the AlN/ZnO interface (ΔEC = 3.75eV). As a result, the blue emission form GaN is almost undetectable in the n-ZnO/AlN/p-GaN LED. However, as the electron barrier height at ZnS/ZnO interface (ΔEC = 0.45eV) is smaller than that of the AlN/ZnO interface, some of the electrons would inject from n-ZnO to p-GaN and the blue emission has been observed merely in n-ZnO/ZnS/p-GaN diode.
On the other hand, at the interface between dielectric interlayer and GaN, the holes can fluently transfer from GaN to either AlN or ZnS under forward bias because of the formation of potential well, as shown in the insets of Fig. 5. Consequently, holes in the GaN layer can inject to the ZnO layer due to the lower hole barrier height at the dielectric/ZnO interface(ΔEV = 0.92eV for AlN/ZnO interface and 0.14eV for ZnS/ZnO interface). Therefore, the radiative recombination would take place in the ZnO layer. Furthermore, the hole barrier height at the ZnS/ZnO interfaces (0.14 eV) is almost the same as that of the GaN/ZnO interface (0.13 eV), which is far smaller than the hole barrier height of the AlN/ZnO interfaces (0.92 eV). As the much smaller hole barrier height can make hole injection from GaN to ZnO more easily, the possibility of radiative recombination in ZnO has been enhanced in the n-ZnO/ZnS/p-GaN diode, which is the very reason that the EL intensity of n-ZnO/ZnS/p-GaN diode is higher than that of n-ZnO/AlN/p-GaN diode.
In summary, n-ZnO/p-GaN heterojunction LEDs with different interfacial layers have been fabricated by PLD. Having inserted a thin ZnS and AlN intermediate layer into the device, the EL spectra of n-ZnO/ZnS/p-GaN and n-ZnO/AlN/p-GaN heterojunction diodes exhibit a typical near-UV emission peak. Though the EL spectra of n-ZnO/ZnS/p-GaN display a weaker blue emission band due to the smaller electron barrier height in ZnS/ZnO interface, the EL intensity of n-ZnO/ZnS/p-GaN heterojunction LED has been improved due to a much smaller hole barrier height at ZnO/ZnS interfaces. The present work provides a feasible method for improving the EL performance of ZnO/p-GaN heterojunction LEDs.
The authors wish to acknowledge the financial support of the National Natural Science Foundation of China (No. 11144010), Natural Science Foundation of Shandong Province (No.ZR2010AL026) and the project of Shandong Province Higher Educational Science and Technology Program (No. J12LJ03).
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