Light-emitting diodes emitting in the UV-B spectral range usually contain an n-type Al x Ga 1-x N contact layer with x of about 0.4 and require a low specific contact resistivity in the range of 10 −6 cm 2 to operate at their optimal capacities. We compare the Ti/Al/Mo/Au metal system commonly used on GaN with the V/Al//V/Au metal system on n-Al 0.4 Ga 0.6 N:Si and GaN:Si surfaces. On Al 0.4 Ga 0.6 N, the lowest specific contact resistivity, (2.3 ± 0.2) × 10 −6 cm 2 , was reached with V/Al/V/Au whereas for Ti/Al/Mo/Au the lowest resistivity was (1.7 ± 0.7) × 10 −3 cm 2 . Independently of the metal system, lower contact resistivities are obtained on GaN where Ti/Al/Mo/Au is still performing at least one order of magnitude better than V/Al/V/Au. The contact resistivity of V/Al/V/Au on Al 0.4 Ga 0.6 N was found to be sensitive to surface treatments (wet chemical etching or sputtering) applied prior to metal deposition, to the thickness of the first vanadium layer and to that of the aluminum layer above. Structural and compositional investigations showed that a low contact resistivity is accompanied by the formation of an interfacial layer between AlGaN and V/Al/V/Au on the nm-scale, which contains aluminum, in the form of aluminum nitride, and separately vanadium as metal or metal nitride.
The electrical properties of different metal systems for ohmic contacts on the nitrogen-face of c-plane n-type GaN substrates are investigated. The metal contacts are compatible with the fabrication process and the packaging technology for group III-nitride laser diodes. The metal system Ti/Al/Mo/Ti/Ni/Au/Ti/Pt is determined as the best suitable, since it is ohmic already after annealing at a temperature of 450°C for 60 s. This annealing temperature is high enough to make the contact insensitive against later soldering on a heat-sink at 330°C. At the same time, the temperature is low enough that the Pd-based p-contact, previously annealed at 530°C, does not degrade. In addition, the Ti/W/Al and Pd/Ti/Al metal systems form low-resistance ohmic contacts, too, although they require a longer annealing time of several minutes or a higher temperature of 500°C.Index Terms-GaN laser diodes, contacts to n-GaN, N-face GaN, GaN wet-chemical etching, TMAH.
The long-term stability of ultraviolet (UV)-C light-emitting diodes (LEDs) is of major importance for many applications. To improve the understanding in this field, we analyzed the degradation of AlGaN-based UVC LEDs and modeled the variation of electrical characteristics by 2D simulations based on the results of deep-level optical spectroscopy (DLOS). The increase in the forward leakage current observed during ageing was ascribed an increase in trap-assisted tunneling. The analysis of the degradation kinetics suggests the role of a defect diffusion process, possibly involving impurities coming from the p-type layers.
In this work, the growth and conductivity of semipolar AlxGa1-xN:Si with (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) orientation is investigated. AlxGa1-xN:Si (x = 0.60 ± 0.03 and x = 0.80 ± 0.02) layers were grown with different SiH4 partial pressures and the electrical properties were determined using Hall measurements at room temperature. The aluminum mole fraction was measured by wavelength dispersive X-ray spectroscopy and X-ray diffraction and the Si-concentration was measured by wavelength dispersive X-ray spectroscopy and secondary ion mass spectroscopy. Layer resistivities as low as 0.024 Ω cm for x = 0.6 and 0.042 Ω cm for x = 0.8 were achieved. For both aluminum mole fractions the resistivity exhibits a minimum with increasing Si concentration which can be explained by compensation due to formation of cation vacancy complexes at high doping levels. The onset of self-compensation occurs at larger estimated Si concentrations for larger Al contents.
Mg-doped AlGaN short-period superlattices with a high aluminum mole fraction are promising to fabricate highly efficient deep UV light emitting diodes. We present a robust and easy-to-implement experimental method for quantification of the vertical component of the anisotropic short-period superlattice conductivity based on current–voltage characteristics of devices with varying short-period superlattice thicknesses. In particular, the vertical conductivity of Al0.71Ga0.29N/Al0.65Ga0.35N:Mg short-period superlattices is investigated and found to be strongly affected by the temperature and by the applied electric field. At room temperature, the vertical conductivity varies between 5.5 × 10−7 Ω −1 cm−1 at 0.05 MV cm−1 and 6.7 × 10−5 Ω−1 cm−1 at 0.98 MV cm−1 and increases by almost two orders of magnitude when the temperature increases up to 100 °C. This behavior is in very good agreement with simulations based on a 3D-Poole–Frenkel model. In addition, the zero-field ionization energy and the inter-trap distance of the Mg acceptors in the AlGaN short-period superlattices were determined to be 510 ± 20 meV and 5.1 ± 0.3 nm, respectively.
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