Deep Elastic Strain Engineering of Free-Standing GaN Microbridge

Gallium nitride (GaN) as a direct band gap semiconductor played a central role in the historical development of efficient blue light-emitting diode and laser diode, in which its intrinsic wide band gap shrinks through the addition of indium (In), thus tuning the color of the light by changing the chemistry. This alloying approach, however, does not allow for dynamic and reversible modulation of the band gap and emission color. Here we show that ultralarge tensile elastic strain can be introduced in microfabricated single crystal GaN microbridges and offer an unprecedented opportunity to modulate its band structure and optoelectronic properties continuously through deep elastic strain engineering. The elastic strain-induced energy band gap modulation was characterized and quantified by cathodoluminescence (CL) as well as via strained GaN devices inside a scanning electron microscope. The CL emission of deeply strained ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mo form="prefix">&gt;</a:mo> <a:mn>5</a:mn> <a:mi>%</a:mi> </a:mrow> </a:math> ) GaN microbridges has shown substantial band gap reduction from <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mrow> <d:mo>∼</d:mo> <d:mn>3.4</d:mn> </d:mrow> </d:math> to 2.96 eV ( <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mrow> <f:mo>∼</f:mo> <f:mn>365</f:mn> </f:mrow> </f:math> to 420 nm in wavelength), making its optical emission shift from invisible or UV to the blue visible regime. Experimental results agree well with calculation of the change in band gap with increased strain. This dynamic, reversible, and well-controllable strain engineering would facilitate novel device applications in power electronics and optoelectronics.