Electrical properties and microstructure formation of V/Al-based n-contacts on high Al mole fraction n-AlGaN layers

Abstract

The electrical and structural properties of V/Al-based n-contacts on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mrow> <mml:mi mathvariant="normal">n</mml:mi> <mml:mo>‐</mml:mo> <mml:msub> <mml:mi>Al</mml:mi> <mml:mi>x</mml:mi> </mml:msub> <mml:msub> <mml:mi>Ga</mml:mi> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi>x</mml:mi> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">N</mml:mi> </mml:mrow> </mml:math> with an Al mole fraction <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m2"> <mml:mrow> <mml:mi>x</mml:mi> </mml:mrow> </mml:math> ranging from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m3"> <mml:mrow> <mml:mi>x</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.75</mml:mn> </mml:mrow> </mml:math> to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m4"> <mml:mrow> <mml:mi>x</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.95</mml:mn> </mml:mrow> </mml:math> are investigated. Ohmic n-contacts are obtained up to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m5"> <mml:mrow> <mml:mi>x</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.75</mml:mn> </mml:mrow> </mml:math> with a contact resistivity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m6"> <mml:mrow> <mml:mn>5.7</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> <mml:mtext> </mml:mtext> <mml:mi mathvariant="normal">Ω</mml:mi> <mml:mo>·</mml:mo> <mml:msup> <mml:mi>cm</mml:mi> <mml:mn>2</mml:mn> </mml:msup> </mml:mrow> </mml:math> whereas for higher Al mole fraction the IV characteristics are rectifying. Transmission electron microscopy reveals a thin crystalline AlN layer formed at the metal/semiconductor interface upon thermal annealing. Compositional analysis confirmed an Al enrichment at the interface. The interfacial nitride-based layer in n-contacts on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m7"> <mml:mrow> <mml:mi mathvariant="normal">n</mml:mi> <mml:mo>‐</mml:mo> <mml:msub> <mml:mi>Al</mml:mi> <mml:mn>0.9</mml:mn> </mml:msub> <mml:msub> <mml:mi>Ga</mml:mi> <mml:mn>0.1</mml:mn> </mml:msub> <mml:mi mathvariant="normal">N</mml:mi> </mml:mrow> </mml:math> is partly amorphous and heavily contaminated by oxygen. The role and resulting limitations of Al in the metal stack for n-contacts on n-AlGaN with very high Al mole fraction are discussed. Finally, ultraviolet C (UVC) LEDs grown on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m8"> <mml:mrow> <mml:mi mathvariant="normal">n</mml:mi> <mml:mo>‐</mml:mo> <mml:msub> <mml:mi>Al</mml:mi> <mml:mn>0.87</mml:mn> </mml:msub> <mml:msub> <mml:mi>Ga</mml:mi> <mml:mn>0.13</mml:mn> </mml:msub> <mml:mi mathvariant="normal">N</mml:mi> </mml:mrow> </mml:math> and emitting at 232 nm are fabricated with an operating voltage of 7.3 V and an emission power of 120 μW at 20 mA in cw operation.

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