An edge-emitting ultraviolet n-ZnO:Al/i-ZnO/p-GaN heterojunction light-emitting diode with a rib waveguide is fabricated by filtered cathodic vacuum arc technique at low deposition temperature (~150 °C). Electroluminescence with emission peak at 387 nm is observed. Good correlation between electro- and photo- luminescence spectra suggests that the i-ZnO layer of the heterojunction supports radiative excitonic recombination. Furthermore, it is found that the emission intensity can be enhanced by ~5 times due to the presence of the rib waveguide. Only fundamental TE and TM polarizations are supported inside the rib waveguide and the intensity of TE polarization is ~2.2 time larger than that of TM polarization.
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ZnO (E g = 3.37 eV) is considered to be a promising luminance material for the next generation of ultraviolet (UV) optoelectronic devices due to its large exciton binding energy (~60 meV). Hence, the use of ZnO may lead to the development of high brightness UV light-emitting devices operating at and above room temperature. Recently, n-ZnO/p-GaN heterojunction UV light-emitting diodes (LEDs) using ZnO thin film [1–4] and nanowires [5–7] as the electron injector have been developed. The use of p-GaN is due to its bandgap energy (3.4eV) and lattice constant (lattice mismatch ~1.8%) match with that of the n-ZnO as well as the irreproducibility and low quality of p-type ZnO.
LEDs can be used instead of laser diodes in some applications such as optical storage (i.e., CD and DVD players). Due to the optical feedback from the lenses and storage media inside the optical storage system, the use of laser diode tends to introduce several coherent noises such as optical feedback noise or polarization noise. Hence, LEDs may offer immunity from coherent noise problems. However, conventional surface-emitting LEDs usually have low electrical-to-optical conversion efficiency due to the lack of lateral optical and electrical confinement. In this paper, we report the fabrication of an edge-emitting n-ZnO/i-ZnO/p-GaN heterojunction UV LED with a rib waveguide structure. We would like to show that the proposed p-i-n rib waveguide structure can achieve effective radiative excitonic recombination from the i-ZnO layer. In addition, the electrical-to-optical conversion efficiency of the LEDs can be improved by the presence of rib waveguide structure. Directional and high brightness UV emission can also be obtained from the proposed edge-emitting rib waveguide LEDs. Hence, the optical performance of the proposed edge-emitting rib waveguide LEDs can be better than that of the conventional surface-emitting LEDs [1–4].
2. Fabrication of n-ZnO/i-ZnO/p-GaN heterojunction LED with rib waveguide structure
Figure 1(a) shows a schematic of the proposed edge-emitting n-ZnO:Al/i-ZnO/p-GaN rib waveguide heterojunction LED. A 4×10 mm2 (0001) p-GaN:Mg/sapphire substrate (form Semiconductor Wafer, Inc) with hole concentration as 5×1017 cm−3 was used as p-type substrate. A 150 nm thick i-ZnO film was deposited onto half of the p-GaN:Mg/sapphire substrate by using filtered cathodic vacuum arc (FCVA) deposition technique. During deposition, substrate temperature and oxygen partial pressure were set to ~150 °C and 2×10−5 Torr respectively . Subsequently, a line-mask (with width, thickness and separation equal to 3 µm, 0.8 µm and 500 µm respectively) was coated onto the surface of the i-ZnO film by photolithography technique. The unmasked i-ZnO layer was then completely removed by ion-beam sputtering with an etching rate of ~10nm/min for 15 min . Then, a 120 nm thick of SiO2 cladding layer was deposited onto the sample by E-beam evaporation with substrate temperature set to 50 °C. After the deposition, a lift-off process was carried out in acetone to remove the excess SiO2 layer attached on the surface of the i-ZnO rib waveguides. Finally, a layer of n-ZnO:Al (5%) with thickness of 100 nm was deposited onto the sample by the FCVA technique. The carrier concentration of i-ZnO and n-ZnO:Al (5%) films was found to be about 1018 and 1021 cm−3 respectively. In addition, it is noted that the n-ZnO:Al (5%) film has a bandgap of ~0.12 eV wider than that of the i-ZnO film . In this case, the n-ZnO:Al (5%) layer serves as a transparent injector of electrons. A 100 nm thick Au film was deposited onto the p-GaN:Mg/sapphire substrate as the p-type metal contact by E-beam evaporation. In addition, 100 nm Ni film was used as the metal contact on the n-ZnO:Al(5%) layer. The top view scanning electron microscope image of the heterojunction LED with a rib waveguide of 3 μm wide is shown in Fig. 1(b). For the purpose of comparison, another n-ZnO:Al/i-ZnO/p-GaN heterojunction LED without a rib waveguide structure was also fabricated.
3. Optical and electrical properties of heterojunction with and without rib waveguide
Figure 2 gives the EL spectra of the heterojunction LED with rib waveguide at different bias voltages. A rectangle pulse voltage source (with repetition rate and pulse width of 7.5 Hz and 80 ms) was used to bias the LED. The emitted light was collected from the edge of the LED by an objective lens. The corresponding PL spectra of the i-ZnO and p-GaN:Mg layers are also plotted in the insert of Fig. 2. For the PL measurement, the samples were excited by a 355nm pulsed source (10Hz, 6 ns). It is observed from Fig. 2 that all the EL spectra exhibit only UV emission with peak at 387nm, and this agrees well with that of the PL spectra of the i-ZnO layer. This indicates that radiative excitonic recombination occurs inside the i-ZnO layer of the heterojunction LED. As n-ZnO usually has higher carrier concentration than that of GaN:Mg, it is expected that a large portion of recombination will be taken place inside of GaN region. Therefore, some previously reports have detected emission from p-GaN:Mg layer of the ZnO/GaN heterojunction LEDs . However, no emission was detected from the p-GaN:Mg layer of our proposed n-ZnO:Al/i-ZnO/p-GaN heterojunction LED. This is because the proposed p-i-n heterojunction confines radiative excitonic recombination inside the i-ZnO layer. The heterojunction LED without rib waveguide exhibit similar EL spectra (not shown in the figure) except that the emission intensity is lower than that with a rib waveguide. Figure 3(a) and 3(b) show the photos of the rib waveguide and its corresponding emission image respectively taken by a CCD cameral of electric microscope. It is confirmed that the emission is from the edge of the rib waveguides.
Figure 4(a) shows the current-voltage (I-V) characteristics of the heterojunction LEDs with and without rib waveguide. Both I-V curves exhibit rectifying diode behaviour. It is observed that the turn-on voltage of the heterojunction LED can be reduced from 6 to 3.5 V with the presence of a rib waveguide. The insert of Fig. 4(a) shows the I-V characteristics of the Ni-Ni metal contacts on n-ZnO layer as well as Au-Au metal contacts on p-GaN:Mg/sapphire substrate. It is observed that these two types of metal contacts have a relatively good ohmic property. The light-voltage (L-V) curves of the heterojunction LEDs with and without rib waveguide are also given in Fig. 4(b). It is noted that the presence of a rib waveguide structure enhances the emission intensity by ~5 times and the turn-on voltage is reduced from 6V to 3.5V. This is because the rib waveguide strongly confines injection carriers (i.e., the high resistivity of SiO2 cladding layer confines the carriers injected into the i-ZnO layer) and light (i.e., the refractive index of i-ZnO layer is slightly larger than that of n-ZnO:Al layer and GaN:Mg as well as higher than that of SiO2 cladding layer so that light is confined along the lateral and transverse directions of the rib waveguide through total internal reflection) inside the i-ZnO layer so that the corresponding electrical-to-optical conversion is improved. The near field profiles of the heterojunction LEDs are also shown in the insert of Fig. 4(b). Single light spot is emitted from the edge of i-ZnO rib waveguide. This indicates the emission is strongly confined by the rib waveguide and SiO2 cladding layer, and the emission is directional.
4. Polarization behaviour of heterojunction with and without rib waveguide
Polarization behaviour of the heterojunction LEDs is also studied in order to analyze the optical confinement characteristics of the rib waveguide. Figures 5(a) and (b) show the L-V curves and EL spectra respectively for the TE and TM polarizations of the heterojunction LED without a rib waveguide. It is noted from Fig. 5 that the TE and TM polarizations have similar turn-on voltage, L-V curves and emission spectra. This is expected as the optical confinement is weak in the transverse direction. Figures 6(a) and (b) show the L-V curves and EL spectra respectively for the TE and TM polarizations of the heterojunction LED with a rib waveguide. It is noted that TE polarization has turn-on voltage reduced to 3.5V. Moreover, the EL intensity of TE polarization is improved by ~2.2 times higher than that of the TM polarization. This implies that good optical confinement is achieved by the ZnO rib waveguide with SiO2 cladding layer. From the effective index method calculation of the rib waveguide, it is found that the corresponding confinement factor of fundamental TE and TM polarizations is found to be 0.98 and 0.88 respectively . Hence, the ZnO rib waveguide has better confinement for the TE mode than that of TM mode. Near field emission profiles for the TE and TM polarizations are also shown in the inset of Fig. 6(b). It is observed that both polarizations exhibit a single light spot. This indicates that only fundamental TE and TM modes are supported by the rib waveguide. In addition, light spot of TE polarization is much brighter than that of the TM polarization. This verifies that TE polarization received better electrical-to-optical conversion that that of TM polarization inside the rib waveguide.
In conclusion, an edge-emitting UV n-ZnO:Al/i-ZnO/p-GaN heterojunction LED with a 3 μm wide rib waveguide was fabricated. The heterojunction LED exhibits room temperature UV EL radiation, which is related to radiative excitonic recombination inside the i-ZnO layer, with emission peak at ~387nm. It is found that the turn-on voltage of the heterojunction LED can be reduced from 6V to 3.5V and the corresponding emission intensity can be enhanced by ~5 times with the presence of rib waveguide. In addition, directional emission can be observed from the edge of the rib waveguide. Only fundamental TE and TM polarizations are supported by the rib waveguide. The TE polarization has emission intensity ~2.2 times higher than of TM polarization.
This work was supported by LKY PDF2/08 startup grant.
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