Zhang, Y; (2015) Developing Self-Catalysed GaAsP Nanowires on Silicon Substrates for High-Efficiency Solar Cells. Doctoral thesis , UCL (University College London).

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Abstract

GaAsP nanowires (NWs), with a bandgap that can cover wavelengths ranging from green (550 nm) to near infrared (860 nm), are highly promising for photovoltaics (PVs). Moreover, their special nano−scale one−dimensional columnar structure can provide the PV devices with significantly enhanced light−matter interactions, enlarged absorption cross section, good antireflection, superior light trapping, and efficient charge separation and carrier collection. Due to these advantages, GaAsP NW PV devices can achieve high efficiency but only need a small amount of expensive III-V materials compared with the thin film devices. Thus, a self-catalyzed GaAsP NWs growth technique has been developed, with an aim to achieve high-quality core-shell NWs for solar cells. In this thesis, research on self-catalyzed GaAsP NWs was carried out first on un-patterned Si substrates. The nucleation differences between As and P in the NW growth were studied and P showed much stronger nucleation ability compared with that of As. To achieve the GaAsP NW growth, the influence of the V/III flux ratio, growth rate and growth temperature were investigated. Despite the small temperature window, GaAsP NWs with good morphology and crystal quality have been achieved over a large growth rate range by adjusting the V/III flux ratio. The P content of these NWs can be adjusted between 10% and 75%. Moreover, the study of the NW doping indicated that Be can build up the concentration in the droplet, which can change the droplet surface energy and lead to sidewall wetting of the droplets. After the NW growth, a fast temperature decrease during the droplet consumption was found to be beneficial to achieving good tip morphology. In addition, the uniformity control was studied by changing the chemical potential of the vapour phase. A length deviation ranging from 5% to 22% has been demonstrated. The GaAsP shell growth on the core NWs was then investigated. A slow shell growth rate was found to be beneficial to achieving good shell morphology due to sufficient adatom diffusion. In addition, the study of the ternary shell found, for the first time in the world, a quasi-3-fold compositional symmetry in the cross section of the core-shell GaAsP NWs. This phenomenon is believed to be related to the sidewall surface chemical potential and polarity-related adatom bonding energies. To passivate their surface, the core-shell GaAsP NWs were covered with a thin layer of InGaP. A four-time enhancement in PL intensity has been demonstrated, when compared with the un-passivated ones. Based on all the results obtained from the un-patterned growth, research was further carried out on patterned Si substrates. The presence of Si oxide inside the patterned holes was found to be the reason that caused long-term low-yield and low-repeatability issue in the patterned growth. In order to remove the oxide, a high-temperature deoxidization step was introduced, which was shown to be very effective. However, the large nucleation area in the holes caused by the high-temperature cleaning can suppress the formation of the catalyst droplets. Therefore, a Ga pre-deposition step was used to assist the droplet formation. With these two techniques, the patterned NW growth achieved high yield and repeatability. The influence of the droplet size on the NW morphology was also studied. The results suggest that the catalyst droplet size should not be smaller than the patterned hole size, otherwise it will affect the NW growth due to the parasitic VS growth at the bottom of the NWs. Following the core growth, a lattice-matched high-quality shell was grown and demonstrated room-temperature PL emission. The research in this thesis covers some important aspects of NWs. The knowledge of the influence of V/III flux ratio, growth rate and temperature on the NWs is crucial for growing high-quality NWs. Understanding the nucleation differences between As and P is very helpful for the compositional tuning and hence the bandgap control of the GaAsP NWs. The discoveries of novel phenomena about NW doping, droplet consumption, and shell growth are important for the construction of advanced NW device structures. Especially, the theory about the appearance of the quasi-3-fold compositional symmetry in the NW shell growth opens a new way to building a more detailed structure inside the NWs, which is different from the axial and core-shell junction growths. The successful solution to the long-term low-yield and low-repeatability issue of growing NWs on patterned Si substrates can achieve high-level NW growth control and greatly facilitate the device design and fabrication.

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