Engineering Hybrid Nanowire/2D Systems for (Opto)Electronics

The combination of nanowires with two-dimensional (2D) systems is an exciting one enabling new growth approaches, new (opto)electronic device functions and new systems. Here we present two such examples, firstly a nanowire-graphene spectrometer, and secondly an array of single nanowires addressed through a two-dimensional electron gas.<br/><br/>Semiconductor nanowires of graded composition are prime candidates for miniaturised spectrometers [1]. In these devices, the nanowire’s bandgap varies with position along the nanowire’s axis, and photons of sufficient energy will selectively excite particular regions depending on the local bandgap. Multiple electrode pairs placed laterally at intervals along the nanowire axis are used to detect local changes in conductivity induced by photoexcitation, with the graded bandgap giving a means to discriminate incoming wavelengths. This concept is powerful, but complicated by the diffusion of photoexcited charge carriers into lower bandgap regions. To overcome this problem, we instead use graphene channels at intervals in contact with the graded bandgap nanowire, and measure the conduction through the graphene. Efficient transfer of photoexcited carriers from the nanowire into the graphene alters its chemical potential and conductivity, which provides a measure of the wavelength-dependent intensity of incident light.<br/><br/>Multiplexed arrays of single InAs nanowires were achieved using a quantum multiplexer chip [2] with individual InAs nanowires transfer printed onto each of the chip's channels. The quantum multiplexer itself consists of a two-dimensional electron gas (2DEG) patterned into a cascaded structure with arms that can be depleted by the application of a gate bias. Depletion prevents conduction through the associated channels. Using the multiplexer, measurements of conductance and quantum transport through 16 individual nanowire channels were achieved with just 9 electrodes. The multiplexer permits a reduction in the number of electrodes needed, with the number of devices scaling as 2^((n-1)/2) with n being the number of electrodes. This multiplexer is particularly beneficial for experiments at cryogenic temperatures in which the number of leads entering a cryostat is limited, and for large statistical studies of nanowires.<br/><br/>[1] Z Yang <i>et al</i>., <i>Science</i> 365: 1017-1020 (2019)<br/>[2] L Smith <i>et al</i>.,<i> ACS Nano</i> 14: 15293-15305 (2020)