QDev Seminar: Claes Thelander

Associate Professor from Solid State Physics/NanoLund, Lund University, Sweden.

Electron and hole transport in GaSb-InAs nanowire junctions and quantum dots

In this presentation I will discuss material development and recent findings on the electrical properties of GaSb-InAs core-shell nanowires with different core-shell configurations. The GaSb-InAs heterojunction is Type II (broken gap), which is of relevance to various device concepts such as tunnel field-effect transistors, but also to fundamental studies of electron-hole interactions, such as hybridization [1], and ground-state excitons [2].
Radial (core-shell) heterojunctions show gate-controlled ambipolar transport along a wire, where a transition from electron-dominated transport to hole-dominated transport is confirmed by studying the sign of a thermovoltage upon applying a heat gradient [3].
From low temperature studies we find that the band alignment in the heterojunction can be tuned by quantum confinement, and that a transition to a staggered junction occurs for an InAs shell thickness below around 5 nm [4]. Short core-shell segments behave as core-shell quantum dots at low temperatures, where sequential filling of core (hole) states, followed by shell (electron) states is possible going from negative to positive gate voltages. Electrostatic interactions between confined electrons and holes have been studied in core-shell quantum dots with an overlap of conduction and valence band states [5]. 
We are also investigating the reverse core-shell system, with an InAs core and a GaSb shell. Here, the radial (shell) growth can be locally supressed by controlling the InAs crystal phase [6]. Such epitaxially designed core-shell quantum dots, with well-defined InAs wurtzite tunnel barriers [7], and a zinc-blende quantum dot, provide new possibilities for controlling the dot properties [8].

[1] I. Knez, R.-R. Du, G. Sullivan, Phys. Rev. Lett. 107, 136603 (2011)
[2] L. He, G. Bester, A. Zunger, Phys. Rev. Lett. 94, 016801 (2005)
[3] J.G. Gluschke, M Leijnse, B Ganjipour, K.A. Dick, H. Linke, C. Thelander ACS Nano 9, 7033 (2015)
[4] B. Ganjipour, M. Ek, B.M. Borg, K.A. Dick, M.-E. Pistol, L.-E. Wernersson, C. Thelander, Appl. Phys. Lett. 101, 103501 (2012)
[5] B. Ganjipour, M. Leijnse, L. Samuelson, H.Q. Xu, C. Thelander, Phys. Rev.  B 91, 161301 (2015)
[6] L. Namazi, M. Nilsson, S. Lehmann, C. Thelander, K.A. Dick, Nanoscale 7, 10472-10481 (2015)
[7] M. Nilsson, L. Namazi, S. Lehmann, M. Leijnse, K.A. Dick, C. Thelander, Phys. Rev. B. 93, 195422 (2016)
[8] M. Nilsson, L. Namazi, S. Lehmann, M. Leijnse, K.A. Dick, C. Thelander, Phys. Rev. B. 94, 115313 (2016)