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Low resistivity AlGaN:Si for deep UV-C light emitters

FBH research: 05.12.2013

Lowest reported resistivities

Fig.1: Comparison of the lowest reported resistivities of AlxGa1-xN:Si

Comparison resistivities

Fig. 2: Resistivities of AlxGa1-xN:Si- vs. SiH4/III ratio in the gas phase – see APL, dx.doi.org/10.1063/1.4833247

One of the key challenges in developing deep ultraviolet (UV) light emitting diodes (LEDs) and laser diodes is the realization of low resistivity n- and p-type AlGaN layers with high aluminum mole fraction. For AlxGa1-xN with x > 0.8, as it is required for cladding layers in UV-C laser diodes or current spreading layers in UV-C LEDs, not only p-type even n-type conductivity is hard to achieve. This is due to the relatively high donor ionization energy and compensation though defects and impurities. AlxGa1-xN epitaxially grown on sapphire usually has a high density of threading dislocations that can trap electrons. Therefore, layers with x > 0.7 are usually highly resistive despite high Si concentrations.

Within the collaboration of TU Berlin and FBH to develop deep UV-C light emitters, the influence of the aluminum mole fraction x and the SiH4/III ratio during MOVPE growth of Si-doped AlxGa1-xN (0.8 < x < 0.96) layers on their electrical properties has been studied [1]. Epitaxially laterally overgrown (ELO) AlN/sapphire templates with low defect density were first fabricated [2] and then overgrown with AlxGa1-xN.

A record low resistivity of 0.026 Ωcm for Al0.81Ga0.19N:Si was obtained with optimized SiH4/III ratio. The resistivity increased almost exponentially with higher aluminum content up to 3.35 Ωcm for Al0.96Ga0.04N:Si, and the optimum SiH4/III ratio shifted towards lower values. Hall effect measurements showed that the increase of the resistivity with growing aluminum mole fraction is mainly caused by a decrease of the carrier density rather than the mobility. Optimized Al0.81Ga0.19N:Si layers exhibit a carrier concentration of 1.5 x 1019 cm-3 and an electron mobility of 16 cm2V-1 s-1. The obtained low resistivity paves the way for UV LEDs with wavelengths down to 230 nm.

This work was partially supported by the Federal Ministry of Education and Research (BMBF) within the Berlin WideBase initiative under contract 03WKBT01C and the German Research Foundation within the Collaborative Research Center 787.

Publications:

[1] F. Mehnke, T. Wernicke, H. Pingel, Ch. Kuhn, Ch. Reich, V. Kueller, A. Knauer, M. Lapeyrade, M. Weyers, M. Kneissl, "Highly conductive n-AlxGa1-xN layers with aluminum mole fractions above 80%", Applied Physics Letters Vol.103, No. 21, 212109 (2013).

[2] V. Kueller, A. Knauer, F. Brunner, U. Zeimer, H. Rodriguez, M. Kneissl, M. Weyers, "Growth of AlGaN and AlN on patterned AlN/sapphire templates", Growth, vol. 315, no. 1, pp. 200-203 (2011).

FBH research: 05.12.2013