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In this work, GZO/ZnO/GaN diodes with the light emitting ZnO layer sandwiched between two SiO2 thin films was fabricated and characterized. We observed a strong excitonic emission at the wavelength 377nm with the Mg2+ deep level transition and oxygen vacancy induced recombination significantly suppressed. In comparison, light emission from the GZO/GaN device (without SiO2 barriers) is mainly dominant by defect radiation. Furthermore, the device with confinement layers demonstrated a much higher UV intensity than the blue-green emission of the GZO/GaN p-n device.
©2009 Optical Society of America
1. Introduction
Recently, ultraviolet (UV) light-emitting diodes (LEDs) and laser diodes (LDs) have attracted much attention for applications to efficient solid state lighting, high-density information storage, and medical treatment. In addition to GaN, ZnO has been identified as a promising material for UV light sources due to its large exciton binding energy of 60 meV and the direct bandgap energy of 3.37 eV at room temperature. ZnO also offers several manufacturing advantages such as its compatibility with wet chemical etching, relatively low material cost, and long-term stability. Up to now, despite homojunction ZnO diodes have been reported [1,2], difficulties in realizing reliable p-type ZnO have driven most ZnO based LEDs developed on heterostructures by growing n-ZnO on p-type semiconductors such as GaN, Si, AlGaN, and p-SrCu2O2 [3–6]. Among the p-type materials of choice, GaN is an excellent candidate as it is readily available and has the same wurtzite structure as ZnO. The n-ZnO/p-GaN based diode structures have been reported in the past several years with light emission ranging approximately between 370 and 600nm [3], [7], [8].
The variations of light emission from ZnO/GaN p-n heterostructures are attributed to several factors. First, radiative recombinations due to deep level defect recombination in p-GaN and oxygen vacancies in ZnO account for the emission at around 430nm and 530~550nm, respectively [9–12]. And light emission at the wavelength shorter than 430nm is associated with the ZnO excitonic recombination or band-to-band transition based on the band lineup calculations using the Thomson model [13], which the emission wavelengh is determined by the location where electron and hole accumulate. In spite of these findings, most reported ZnO/GaN diodes employ a simple p-n or p-i-n heterostructures which the UV recombination efficiency is limited and a noticeable defect band radiation is generated.
To suppress light emission in the defect states and meanwhile enhance UV light emission from the ZnO band edge transition, in this work, we fabricate a ZnO light emitting layer sandwiched between two SiO2 confinement barriers of asymmetric thickness. The electrical and optical properties of the devices are characterized.
2. Device fabrication
The device fabrication started from the preparation of a p-type GaN(p-GaN) epiwafer. The 200nm-thick p-GaN layer was grown on top of a sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The effective carrier concentration of p-GaN is approximately 2x1017 cm−3 after 750°C 30min activation. In the next step, the 300x300 μm2 patterned ZnO light emitting layer sandwiched between two SiO2 (2nm at the bottom and 5nm on top) thin films were coated on top of the p-GaN layer by RF magnetron sputtering at room temperature. The design of the asymmetric thickness of SiO2 barrier layers is aiming on improving the balance of electron and hole concentration in the ZnO layer. Next, the 130nm-thick n-type ZnO layer doped with 0.5 wt % Ga (GZO) was sputtered on the 5nm SiO2 barrier. The device was then annealed at 900 °C for 10 minutes in the nitrogen ambient, resulting in electron concentrations in the ZnO and GZO layers 6x1019 cm−3 and 3x1021cm−3, respectively. Finally, Ni/Au and Ti/Au were evaporated as the p-type and n-type contact electrodes, respectively, and were alloyed for optimum contact conditions. For comparison purpose, we also prepared a LED sample without the SiO2/ZnO/SiO2 structure and is named as a GZO/GaN LED. The inset in Fig. 1 shows the schematic diagram of the GZO/ZnO/GaN diode with two SiO2 confinement layers.
Fig. 1 Room temperature PL spectra of p-GaN (dashed line) and ZnO (solid line) thin films. Both samples were prepared on sapphire substrates. Inset: the schematic diagram of the GZO/ZnO/GaN LED with SiO2 confinement layers.
3. Results and discussions
3.1 Photoluminescent spectra of ZnO and p-GaN
We first performed room-temperature PL (photoluminescent) measurement on a sputtered 40nm-thick ZnO thin film and a MOCVD grown 200nm-thick p-GaN sample by a He–Cd laser operating at the wavelength 325 nm. Both ZnO and GaN were separately grown on sapphire substrates and their annealing conditions are the same as the device fabrication described above. As shown in Fig. 1, the PL spectrum of the ZnO shows a near-band-edge (NBE) emission at 377 nm along with a very weak broad band at the wavelength around 530nm. The green luminescence has been reported to be associated with oxygen vacancy related defects [11,12]. In contrast, the PL spectrum of the p-GaN film reveals an intense deep-level emission at around 433 nm, which can be attributed to the transitions from the conduction band or shallow donors to deep acceptor [ 9-10].
3.2 Electrical and optical properties of GZO/GaN and GZO/ZnO/GaN LEDs
The current-voltage curves in Fig. 2 indicate that both samples with and without SiO2 barriers demonstrate a nonlinear rectifying behavior. An additional voltage drop across the confinement layers is observed for the devices with SiO2 thin films.
Fig. 2 Current-voltage curves of a GZO/GaN LED (solid line) and a GZO/ZnO/GaN device with SiO2 confinement layers (dashed line).
The electroluminescent (EL) spectra of the device without confinement layers are demonstrated in Fig. 3(a) . We observe a broad band emission ranging from 400 to 750nm. At the low injection current (1-5mA), the emission peak at 435nm is associated with the following two factors. First, the large difference of effective carrier concentrations between the ZnO and the GaN leads to a great portion of the depletion region falling in the GaN layer. Second, it is relatively easier for electrons to transport across the ZnO/GaN interface than holes. The p-GaN defect induced recombination prevails as holes accumulate in the GaN layer. When the injection current is increased to a level that more and more holes start to move across the heterojunction under the increased electric field, the ZnO vacancy induced green band emission becomes comparable to the p-GaN defect related transition.
Fig. 3 (a) EL spectra of the GZO/GaN LED. The injection current is increased from 1mA to 9mA. (b) EL spectra of the GZO/ZnO/GaN LED with SiO2 barriers
On the other hand, the insertion of SiO2 barriers serves the purpose of effective carrier confinement in the ZnO layer. As shown in Fig. 3(b), while carriers have fewer chances to tunnel through the barriers at 1mA and recombination at GaN prevails, the UV light emission with a peak wavelength 377nm becomes noticeable at the injection currents above 3mA. The 377nm UV emission, as compared with the PL spectra in Fig. 1, is related to the NBE transition in the ZnO layer. It implies that SiO2 barriers can effectively confine the electron-hole pair for excitonic emission. The band diagram in Fig. 4 illustrates carrier transport across the confinement layers. Under the application of the electric field, carriers either tunnel through or hop over the SiO2 barrier. The electron-hole pair (exciton) has thus formed in the narrow ZnO layer. We also note that the defect induced emission is significantly suppressed as compared with the peak intensity of UV light at the current above 5mA. We believe the decrease of the defect radiation is mainly due to the increased number of excitons in ZnO, while on the other hand defects recombination dominates for the GZO/GaN device across the entire ZnO and GaN layers. And the UV intensity of the device with SiO2 barriers is also much higher than the blue-green emission of the GZO/GaN device in Fig. 3(a). Again, it indicates the effectiveness of light emission as electrons and holes are confined. As we further increase the current from 9mA, the large applied electric field suggests a severe energy band tilt, which causes a red shift of EL peak from 377nm to 383nm. The red shift may also be correlated to the thermal effect.
Fig. 4 Illustration of carrier confinement in the ZnO layer with SiO2 barriers of asymmetric thickness. The band diagram is drawn assuming the application of a forward electric field.
4. Summary
In conclusion, we fabricated and characterized GZO/ZnO/GaN LEDs with the ZnO sandwiched between two SiO2 thin films of asymmetric thickness. An excitonic light emission at 377nm is demonstrated with considerable suppression in defect related radiation. In comparison, light emission from the GZO/GaN device (without SiO2 barriers) is mainly dominated by defect radiation. Also, with the confinement layers, the UV intensity of the device with SiO2 barriers is also much higher than the blue-green emission of the device without barriers
Acknowledgments
This work was supported by the National Science Council of Taiwan under the grants NSC 97-2221-E-002-054-MY3.
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