Vertical transport through an InAs/GaSb heterojunction at high pressures and magnetic fields
The conduction band of InAs lies lower in energy than the GaSb valence band. In order to preserve continuity of the Fermi level across the interface, charge transfer takes place resulting in a confined quasi two dimensional electron gas (2DEG) in the In As and a confined quasi two dimensional hole gas (2DHG) in the GaSb. This is the first detailed study into vertical transport in an n-InAs/p-GaSb single heterojunction (SHET). Application of a forward bias (InAs negative with respect to GaSb) increases the 2DEG and 2DHG concentrations and, therefore, their confinement energies. Eventually a critical bias is reached where the electron confinement energy moves above the hole confinement energy (the theoretical voltage induced semimetallsemiconductor transition Vc). Any subsequent increase in voltage is expected to result in a current decrease, and a region of negative differential resistance (NDR) should occur. The SHET can be grown with two distinct interface types, 'InSb-like' and 'GaAslike'. This study shows for the first time that the SHET vertical transport characteristic is very dependent upon this interface monolayer. For example, the temperature dependence of the I/V trace in a SHET with a 'GaAs-like' interface is found to be weak, with similar current peak to valley ratios (PVR) at 300 and 77K. The 'InSblike' SHET, however has a PVR that is very close to 1 at 300K, rising above 2 at 77K. Hydrostatic pressure is used to alter reversibly the InAs conduction/GaSb valence band overlap Δ. Vertical transport measurements taken at pressure confirm that Δ reduces at the same rate for both interface types and that it is larger for the 'InSb-like' interface. Experimental I/V traces at various pressures are compared with the corresponding results from self-consistent band profile calculations. The subsequent discoveries are that NDR occurs after Vc for both interfaces, and that each interface supports a different conduction mechanism - with the 'GaAs-like' interface exhibiting NDR when the band overlap is calculated to be ~ -100 meV. Magnetic fields have been applied both perpendicular and parallel to the SHET interface. The perpendicular field results provide additional evidence that the conduction process must be different at both interfaces and that NDR occurs after Vc. Parallel field I/V traces reveal an entirely different response for the two interface types.