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High performance 365 nm vertical-type ultraviolet light-emitting diodes (LEDs) are demonstrated by the insertion of a self-textured oxide mask (STOM) structure using metal-organic chemical vapor deposition. The dislocation densities were reduced significantly via the STOM by the observation of the transmission electron microcopy image. Under an injection current of 20 mA, a 50% light output power enhancement was achieved, representing an enhancement of 35.4% in light extraction efficiency and injected electron efficiency of the LED with STOM in comparison to that without STOM. At 350 mA, the light output power of the STOM-LEDs was approximately 24.4% higher. Measurements of the optical and electrical properties of the LED showed that the corrugated STOM structure improved the light scattering and reflection which increased the light output, and also enhanced the current spreading to intensify radiative recombination.
© 2014 Optical Society of America
1. Introduction
Recently, ultraviolet light-emitting diodes (UV-LEDs) have received renewed attention due of their potential for many applications, such as a pumping source for Hg-free lamps, water purification and biochemical detection [1–3]. In general, for blue- and green-LEDs, the indium fluctuation in the InGaN/GaN quantum wells (QWs) provides a localized state. This state enhances the LED radiative efficiency. Therefore, the indium-free QWs of UV-LEDs are expected to generate a lower radiative efficiency compared to blue- and green-LEDs. Moreover, a wide bandgap of AlGaN is usually employed in a UV-LED structure to avoid the absorption of the emitted UV light (λ ≦ 365 nm) by itself. The mismatch in the lattice constant and thermal expansion coefficient between the epilayers can easily create a poor quality of AlGaN with large dislocation densities. These dislocations serve as non-radiative recombination centers and hence degraded the UV-LED radiative efficiency. There have been several studies aimed at improving the AlGaN quality of UV-LED, including the use of a low-temperature AlN interlayer, a strain-released high-temperature AlN buffer layer, and AlN/AlGaN superlattices [4–6]. However, these preceding researches only focused on the improvement of the LED epilayer quality, and an increase in the light extraction efficiency (LEE) of LED is rarely discussed. Improving LEE is one of the most promising approaches of enhanced LED light output efficiency.
On the other hand, due to the geometry of horizontal-type sapphire-based LEDs, the finite resistance of n- and p-type materials causes poor lateral current spreading, which results in the current crowding near the metal contacts. As the injected current increases, the non-uniform current distribution cannot effectively contribute to the increase in the LED emission efficiency, and induces a simultaneous self-heating effect that accumulates near the electrodes. Vertical-type LED configurations were proposed to diminish the current crowding effect via a vertical current flow principle [7,8]. Although the vertical current flow from the anode to the cathode shows promise for improving current crowding, the lack of a good lateral current spreading mechanism for the vertical LEDs results in the majority of the light being emitted from the QWs underneath the metal electrodes. The emitted light is shaded and absorbed by the electrodes, and therefore lowers the LED output intensity [9,10]. To enhance the current spreading, the addition of injected electron efficiency (EE) structure, like current blocking layers (CBL) [11–13], is necessary.
In previous studies, the performance of the 450 nm horizontal-type blue-LEDs and 380 nm flip-chip UV-LEDs was improved significantly via the incorporation of a self-textured oxide mask (STOM) structure [14,15]. The STOM prevents dislocation density propagation for the reduction of non-radiative recombination efficiency, in addition to acting as a light scattering center for enhancement of the LED LEE. In this study, a STOM was used to improve the performance of 365 nm vertical-type InGaN-free UV-LEDs. Details of vertical-type UV-LED with and without STOM in electrical and optical properties are presented in the succeeding discussion.
2. Experimental
The fabrication of the STOM template was divided into two steps: A textured Al0.2Ga0.8N layer was first formed via low-temperature (870°C) growth (designated as LT-AlGaN), and secondly, a SiO2 layer with a thickness of 500 nm was then deposited on the LT-AlGaN layer using inductively coupled plasma chemical vapor deposition. The SiO2 hexagonal pattern arrangement with a diameter of 3 μm and a spacing of 3 μm was fabricated using etching and photolithography processes. In general, it is difficult to directly overgrow GaN on the corrugated SiO2 mask. To solve the situation, in our previous studies [14,15], an AlN layer was deposited on the SiO2 to act as new nucleation seeds and to regrow the GaN layer. However, in this study, for the 365 nm of the STOM-LED structure, in order to avoid the absorption of 365 nm light by GaN material, the n-AlGaN layer was used to replace the stack combination of AlN and GaN, and directly grown on the STOM. Prior to the n-AlGaN regrowth, the trimethylaluminum as MOCVD source was pre-introduced into MOCVD reactor for 30 sec to form a distribution of Al droplets on the template surface. And then a new AlGaN nucleation layer formed by the Al droplets promoted a vertical AlGaN growth on the roughened STOM. Similar droplet growth modes have been demonstrated in Ref. 16. This was followed by the deposition of an epitaxial LED structure consisting of ten periods of GaN/Al0.15Ga0.85N MQWs, a 40-nm-thick p-Al0.2Ga0.8N cladding layer, and a 50-nm-thick p-GaN contact layer. A 40 × 40 mil2 chip size of the vertical-type STOM-LED with a high-reflectivity mirror layer (Ni/Ag) was bonded to a Si substrate coated with an Au/Sn/Au metal bonding layer, and the sapphire substrate was removed by laser lift-off technique as shown in Fig. 1(a).In order to reduce the series resistance of the vertical LED, a part of the LT-AlGaN layer was removed by inductively coupled plasma reactive ion etching. Then, the multi-metal layer (Al/Ti/Au) was deposited on the surface of the LT-AlGaN layer as n-pad. A vertical LED with a similar epitaxial structure, thickness, and chip size but without STOM was also fabricated for comparison (designated as C-LED).
Fig. 1 (a) Schematic structure and (b) scanning electron microscopy image of the UVLED with STOM. The inset of TEM image of Fig. 1(b) shows the some voids occurred beneath the STOM.
3. Results and discussion
As shown in the scanning and transmission electron microcopy image (Fig. 1(b)), some voids were formed beneath the STOM due to the decomposition of the LT-AlGaN by H2 carrier gas during the n-AlGaN growth. Although some LT-AlGaN regions were decomposed, the expanding tracks of dislocations were still clearly observed, indicating that the STOM effectively stops dislocation propagation and improves the sequential n-AlGaN quality. In Fig. 2, the crystalline quality of the n-AlGaN layer with and without STOM is shown. The full-width at half-maximum (FWHM) of the n-AlGaN with and without STOM for the (002)-plane was 275 and 604 arcsec, respectively. Similarly, in the case of the (102)-plane, the FWHM of n-AlGaN with and without the STOM was 302 and 1382 arcsec, respectively. The FWHM value for the (002)-plane is related to the screw and mixed dislocations and the FWHM for the (102)-plane is related to the screw, edge, and mixed dislocations. It can be seen from the XRD results that the n-AlGaN with the STOM exhibited superior crystal quality in comparison to that without the STOM, which was attributed that the STOM structure suppresses the dislocation extensions.
Fig. 2 XRD rocking curves of n-AlGaN template with and without STOM for the (002) and (102) reflection plane.
The PL spectra measurements for the STOM-LEDs and C-LEDs at −175 °C and 25 °C are shown in Fig. 3.At −175 °C, a shoulder peak of 380 nm was seen in the C-LED spectra, and its intensity was larger than that of the main peak at 365 nm. In contrast, the STOM-LEDs exhibited a smaller intensity in the shoulder peak when compared to its main peak at 365 nm. The shoulder peak was attributed to the defect-related donor and Mg-doped acceptor transition via the carrier leakages (or overflow) from QWs to p-AlGaN [17–19]. In other words, the smaller intensity of the shoulder peak also implies that the addition of the STOM leads to superior crystal quality of QWs in the LEDs. Moreover, irrespective of the LEDs, the wavelength of emission peak shifted to a longer wavelength as the temperature increased from −175 °C to 25 °C due to the bandgap shrinkage caused from the thermal effect [20,21]. In addition, the internal quantum efficiency (IQE) values for the STOM-LEDs and C-LEDs were estimated to be 22.8% and 20.5%, respectively. The reduction of the PL intensity with increasing temperature in the STOM-LEDs occurred at a slower rate than in the C-LEDs. This is because the influence of non-radiative recombination was greatly diminished by reducing the dislocation densities using the STOM. The IQE of the STOM-LEDs was enhanced by 11.2% compared to that of the C-LEDs.
Fig. 3 PL spectra measurements for the STOM- and C-LEDs at −175 °C and 25 °C. The inset illustrates the defect-related donor and Mg-doped acceptor transition via carrier leakages.
Figure 4(a) shows the output power as a function of the injection current for both the STOM- and C-LEDs. One can see that when the injected current was increased from 20 to 350 mA, the output power increased from 12 to 56 mW for the STOM-LEDs and from 8 to 45 mW for the C-LEDs respectively, corresponding to an increase in output power of 50% at 20 mA and 24.4% at 350 mA compared to the C-LEDs. The current-voltage characteristics of the two LEDs were measured and were shown in the inset of Fig. 4(a). The turn-on voltages for these LEDs were approximately 2.8 V at 20 mA and 3.4 V at 350 mA, respectively. The reverse-bias case with a reverse voltage of 5 V was 0.35 and 0.34 μA for the STOM-LEDs and C-LEDs, respectively. The lower turn-on voltage and the larger leakage current may be attributed to the imperfect LT-AlGaN etching process since it is more difficult to etch the AlGaN than the GaN case. Moreover, the series resistance of the STOM-LEDs was a little larger than that of C-LEDs since the STOM structure led to the reduction of current flow area. The external quantum efficiency (EQE) of the two LEDs at 20 mA was 17.6% for the STOM-LEDs and 11.7% for the C-LEDs, respectively. Generally, the EQE is expressed simply as multiplication of the IQE by LEE. This EQE estimation method involves a premise that the EE is set to 100%. However, in fact the EE value is usually overestimated since the strong dependence of the injected efficiency on the lateral spreading mechanism. The injected efficiency can be included in the EQE express using the following equation:
However, it is also not easy to distinguish the contribution in the enhanced output power by LEE or by EE. In our case, the product between the LEE and EE represented an enhancement of 35.4% for the STOM-LEDs (77.2%) in comparison to the C-LEDs (57%). It was suggested that a high light scattering and reflection for LEE and a good lateral current spreading for EE were induced by the corrugated STOM structure and further enhanced the LED intensity. The radiation patterns of STOM- and C-LEDs at the injection current of 350 mA are shown in Fig. 4(b). The divergence angles (50% light emission intensity of the full one) of 135° and 115° for the STOM-LEDs and C-LEDs were measured, respectively. The C-LEDs exhibited a smaller divergence angle than that of the STOM-LEDs, proving that the embedded STOM structure served as the scattering center to boost LED light extraction. This explained the more extensive integrated light emission intensity that was extracted in the case of the STOM-LEDs.Fig. 4 (a) LED output power as functions of injection current of STOM- and C-LEDs. In the inset, forward I-V characteristics of the STOM-LEDs and C-LEDs. (b) LED emission patterns of the STOM- and C-LEDs at an injection current of 350 mA.
Furthermore, the SpeCLED software was used to simulate the current density distribution of the two LEDs. The chip size and n-pad region was 100 × 100 μm2 and 10 × 80 μm2, respectively. The gap between the two n-pads was 50 and 70 μm for the C-LEDs and the STOM-LEDs, respectively. At the injected current of 350 mA, the results of the current distribution simulation in the LT-AlGaN, n-AlGaN, and MQWs layers of the STOM- and C-LEDs are shown in Fig. 5.Obviously, in Figs. 5(a) and 5(d), the injected current was driven into the n-pad, and then gradually spread to the center of the chip. The effect of the STOM in current spreading is more pronounced when the current diffused in the n-AlGaN and MQWs layer. Compared to the current distribution of C-LEDs in Fig. 5(c), a more uniform current density distribution was seen for the STOM-LEDs, which clearly indicated that the current crowding was suppressed via the STOM structure (see Fig. 5(f)). The STOM-LEDs exhibited a higher average current density (2499 A/cm2) in the MQWs layer, in comparison to the C-LEDs (2274 A/cm2). The simulation results verified that the hexagonal STOM arrangement presented a uniform lateral current spreading as a CBL for improved the current crowding effect.
Fig. 5 (a)–(f) Current density distribution in the LT-AlGaN, n-AlGaN, and MQW layer of the STOM- and C-LEDs at 350 mA injected current
4. Conclusion
In this paper, a high-quality 365 nm vertical-type UV-LEDs was successfully fabricated through the insertion of a STOM structure using an MOCVD system. The cross-sectional TEM observations revealed that dislocations in the n-AlGaN layer were effectively reduced by the incorporation of the STOM. The STOM plays the role of the CBL by enhancing current spreading in addition to the role of a reflector by extracting photons after multiple scattering events. Concurrently, the relative light output power was found to be enhanced by a factor of approximately 24.4% at an injection current of 350 mA. These results suggest that the use of the STOM is effective in at elevating the performance of InGaN-free UV-LEDs.
Acknowledgments
The authors would like to thank the Ministry of Economic Affairs under Grant No.102-E0605, and National Science Council of the Republic of China, Taiwan, NSC 101-2221-E-005-023-MY3, 102-2221-E-005-072-MY3, and 102-2622-E-005-006.
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