# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 5 Reflective p- and n-Electrodes  
### Transparent p-Cladding Layer and Reflective p-Contact  
InGaN-based LEDs normally comprise a p-GaN cladding layer with transparency to the wavelength about above 365 nm. Ag-based solder has reflectivity in \(\lambda\) > 365 nm and many silver-colored metals are well known to have significant reflectivity in NUV (\(\lambda\) > 300 nm). The use of indium tin oxide (InSnO: ITO) enables a conductive transparent (\(\lambda\) > 365 nm) p-contact. By integrating such a cladding layer and p-contact, when the metallized sapphire side of InGaN dies is bonded to the packaging, the pair of transparent p-contacts and the Ag-based solder realizes a high light extraction efficiency  
Figure 5: **(a)** AFM images (6 \(\times\) 6 \(\upmu\)m\({}^{2}\)) of AlN templates on sapphire substrates with various miscut angles after optimizing the AlN growth recipe before starting mass-production. Macrosteps unexpectedly appeared for the low-miscut sapphire samples. The LEDs on these AlN templates showed no adverse effects due to the macrosteps in the reliability tests, indicating the robustness of the QWs grown on macrosteps. **(b)** Height profile of AlN on 1.0\({}^{\circ}\)-miscut sapphire along the line in the image. Ra value of AlN template is 2.26. (**(a)** images for the miscut angle of 0.3\({}^{\circ}\) and 1.0\({}^{\circ}\) and (b) are permitted to be reused from Jpn. J. Appl. Phys. [13]. © Japan Society Applied Physics).  
(LEE). In addition, when the reflective p-contact is flip-chip bonded to the mount, the utilization of EL reflected from the p-contact is possible.  
Meanwhile, for DUV-LEDs, neither a transparent conductive p-cladding layer nor a low resistive p-contact on p-AlGaN cladding layer can be practically used. The conventional p-contact metal of Ni does not reflect DUV at \(\lambda<280\) nm. Although as-deposited Al reflects DUV, Al is unsuitable for formation of low-resistivity contacts on p-GaN, owing to its small work function. This explains why many reports of DUV-LEDs with a p-reflective electrode show a high V\({}_{\text{f}}\). V\({}_{\text{f}}\) of 7 V [23] using a short-period superlattice (SPSL) has been reported. In addition, Ni/Au and Ni/Al contacts with an increased forward voltage of 2-4 V [114] when p-GaN contact layer is replaced by p-AlGaN. Furthermore, a Ni/Al contact with V\({}_{\text{f}}\) over 15 V [42], a Rh electrode with V\({}_{\text{f}}\) = 16 V [21], or a Ni/Mg electrode with an embedded photonic crystal (PhC) structure without recording V\({}_{\text{f}}\)[43] were reported. None of the reports [21; 24; 42; 43; 114] mentioned the lifetime. We also attempted to form a contact on a p-AlGaN layer, resulting in LEDs with a very short lifetime. To suppress the increase in V\({}_{\text{f}}\) and reduction in the lifetime, a meshed p-GaN contact layer was used [40], which increased V\({}_{\text{f}}\) to 6-7 V. Although the reduction in the lifetime was suppressed, the problem was not sufficiently solved, as shown in Figure 6. The reasons for the short lifetime have not yet been clarified and the suggestion of the local heat generation due to insufficient contact is possibly among the reasons. Since LEDs must have a lifetime of over 10,000 h to be superior to the UV bulbs, a p-GaN contact layer is currently thought to be necessary. Simple approaches of the reflective p-contact did not lead to a practical solution. Attempts to solve the problem by inserting a thin InGaN layer [115; 116; 117], incorporating quantum dots [118], or fabricating nanorods [119] are ongoing. However, major manufacturers currently incorporate a p-GaN contact layer. Due to the insufficient conductivity of p-AlGaN [120] of about \(10^{2}/\Omega\)cm, the p-AlGaN contact resistivity of about \(10^{-2}\)\(\Omega\)cm\({}^{2}\)[121], and the absence of a practical reflective p-contact in DUV region, the waste EL towards the p-side is not expected to be utilized in near future.