Investigation of AlGaN UV emitting tunnel junction LED devices by off-axis electron holography.

Here we use off-axis electron holography combined with advanced transmission electron microscopy techniques to understand the opto-electronic properties of AlGaN Tunnel Junction-LED devices for UV emission. Four identical AlGaN LED devices emitting at 290 nm have been grown by metal-organic chemical vapour deposition. Then Ge doped n-type regions with and without InGaN or GaN interlayers have been grown by molecular beam epitaxy onto the top Mg doped p-type layer to form a tunnel junction and hence a high quality ohmic metal contact. Off-axis electron holography has then been used to demonstrate a reduction in the width of the tunnel junction from 9.5 to 4.1 nm when an InGaN interlayers are used. As such we demonstrate that off-axis electron holography can be used to reproducibly measure nm-scale changes in electrostatic potential in highly defected and challenging materials such as AlGaN and that systematic studies of devices can be performed. The LED devices are then characterized using standard opto-electric techniques and the improvements in the performance of the LEDs are correlated with the electron holography results.