Future Work

For further work we will continue to fabricate split gates with top gates on high mobility p-type Ge and In_0.75Ga_0.25As wafers. We will expand the work on fractional quanti-sation observed in this project and carry out experiments to see if we can observe fractional quantisation in other systems such as in holes in GaAs along with electrons.

To confirm that the charge is fractionalised several measurements will be setup such as shot noise measurement to measure the charge [145]. Noise will be measured di-rectly to determine the fractional charge value responsible for the conduction process.

This requires the establishment of weak back-scattering when the shot noise is given by S = 2QI_B where I_B is the backscattered current and Q is the charge.

The possibility that a zig-zag regime, which has been discussed theoretically, but not in the context of fractional charge, raises many possible scenarios of fractional

charges and complex spin states even a zero-spin excitation. For further work we will create interferometers and Aharonov-Bohm structures to study phase changes in the various fractional regimes. These devices can include quantum dots which will be of interest in determining whether fractions survive tunnelling and storage.

This project has laid the ground work needed for fabricating more complex de-vices on both In_0.75Ga_0.25As and p-Ge. We have optimised the fabrication methods that are suitable for fabricating more complex devices. We have fabricated a series of devices that include single electron/hole quantum pumps, spin focusing devices and superconductor-semiconductor-superconductor hybrid devices in In_0.75Ga_0.25As and p-Ge.

8.2.1 Quantum pumps

Quantum pumps on both In_0.75Ga_0.25As and p-Ge wafers have been fabricated as a continuation of this project. These devices will be measured in a low noise measurement system. Figure 8.1 shows an SEM image of the quantum pump fabricated on p-Ge.

This will be used to generate quantised current as a function of the frequency of an oscillating gate. Similar pumps were demonstrated in GaAs [146]. The two rightmost gates can be used as a pump, in our design there is a split gate at the exit. The blue arrow indicates the direction of the pumping. The pump can be operated by fixing one of the gates at a positive voltage to form a barrier, and the other gate will be modulated to pump the holes over that barrier. We will develop quantum pumps on both InGaAs and p-Ge to be used in an electron and hole interferometry experiment.

For this we will incorporate the pump with an Aharonov-Bohm ring (AB-ring). When an electron/hole propagates around an AB-ring in the presence of a magnetic flux, it experiences a phase change. When electrons/holes travel through a narrow wire that splits into two channels; they have equal probability of travelling on either channels and then recombine as well as the wave function that also splits into two coherent partial wave functions that recombine when the wires meet. When there is an enclosed magnetic flux, the phase change experienced by these partial wave functions is di↵erent, and they will either combine constructively or destructively depending on the enclosed magnetic flux. Incorporating an AB-ring with two electrons or holes from a quantum pump, should enable us to detect entanglement schemes.

Figure 8.1: SEM image of quantum pump on p-Ge.

8.2.2 Focusing devices

Spin focussing has been demonstrated in both GaAs devices[147,148,149] and recently in InGaAs [150]. We have implemented this in p-Ge to focus spin polarised current in holes. The first step will be to demonstrate focussing in p-type Ge. Once the geometries and fabrication methods are optimised a more complex arrangement of gates can be utilised for on-chip spin manipulation for applications in spintronics and quantum information technology.

8.2.3 Superconductor-Semiconductor-Superconductor hybrid devices on InGaAs and Ge

There is a renewed interest in hybrid superconductor-semiconductor-superconductor (S-Sm-S) junctions and Andreev devices [151]. This is due to the recent reports of detections of Majorana fermions at the interfaces of these junctions [152]. A prereq-uisite for the Majorana fermion is a transmissive interface between a superconductor and 1-dimensional channel that has a large g-factor and spin-orbit interaction. We have started making these junctions on both In_0.75Ga_0.25As and n and p-type Ge with transmissive Niobium superconducting contacts. The collaborative work has led to de-velopment of topological superconductivity based on hybrid Nb-In_0.75Ga_0.25As-Nb that that resulted in hard superconducting gap detection in symmetric, planar, and ballistic Josephson junctions [153].

Fig 8.2 shows an SEM image of one of the Nb contacted Ge devices. Initial mea-surements on these devices led to observation of an Andreev reflection e↵ect between an s-wave superconductor and a germanium channel (see fig. 8.3). The coherence lengths are sufficiently long in the Ge to observe the e↵ect up to 800 nm. The normal interface resistance contribution varies from 4.5 kOhm to 7 kOhm without a significant increase of the zero bias conductance ratio. We will also implement a split gate within these junctions to control the channel width.

Figure 8.2: SEM image of Nb contacted Ge hall bar.

Figure 8.3: The di↵erential conductance normalised to the normal conductance (G_N) in n-type Ge at 1.6 K as a function of dc bias voltage (V_dc) down to 500 nm gap.