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Unintentional High-Density p-Type Modulation Doping of a GaAs/AlAs Core–Multishell Nanowire

We show that introducing a multi-shell structure in GaAs/AlAs nanowires (NWs) can lead to an unintentional high density p-type doping. Doped NWs have a large number of potential applications. However, doping NWs grown by vapour-liquid-solid method (VLS) remains a challenge. The direct application of modulation doping techniques developed for epitaxial GaAs, and crucial for the discovery of the fractional quantum Hall effect, have proved to be spectacularly unsuccessful in the case of NWs. This is due to the very different VLS growth regime developed to grow high quality NWs.

Here, we investigate a very simple structure, where a narrow quantum well is placed in the shell of the NW. Such a narrow quantum well could be used to accommodate dopants i.e. as in delta doping technique. We show that simply placing a narrow quantum well in the shell significantly changes the electronic properties of the nanowire. This quantum well works as a very efficient trap for residual carbon in the growth chamber, which acts as an acceptor, leading to a highdensity p-type modulation doping as the carriers are transferred to the core/shell interface.

A strong quantum confinement of the carriers is observed in photoluminescence studies and confirmed by theoretical calculations which predict a quantum confinement of carriers at the core/shell interface due to the presence of ionizedcarbon acceptors in the 1 nm GaAs layer in the shell. The final “smoking gun”fingerprint of the presence of a high density of confined holes is provided by the micro-photoluminescence measurements in high magnetic field; a clear signatureof avoided crossings of the n = 0 Landau level emission with the n = 2 Landau level TO phonon replica is observed. The coupling is caused by the resonant hole phonon interaction, which points to a large 2D hole density in the structure.Moreover, the observed coupling to the TO phonon is in itself evidence for the 2D nature of the carriers involved since coupling to the TO phonon is not observed in bulk.

Our results bring a new understanding concerning delta doping process in nanowires. Moreover, we show that composition of the shell is crucial and canlead to quantum confinement in typical nanowires which nevertheless have a corediameter sufficiently large to display electronic properties indistinguishable from those of bulk GaAs.

Figure 1: Color plot showing differential μPL spectra of core–multishell GaAs/AlAs NW measured as a function of magnetic field. The excitation power was a few nW and the temperature of the measurements was 1.7 K. The lines show the calculated evolution of the two high energy 2D emission lines together with their LO (dashed lines) and TO (solid lines) phonon replicas as described in the text. The observed avoided crossing is the result of resonant polaron coupling.

Reference: “Unintentional High-Density p-Type Modulation Doping of a GaAs/AlAs Core–Multishell Nanowire”, J. Jadczak et al., Nano Lett., 14, p… (2014).