Growth and structural characterization of III-V nanowires grown by molecular beam epitaxy

Abstract

Heterostructured semiconductor nanowires (NWs) have attracted considerable attention in recent years because of their potential in future nano-electronic and nanophotonic device applications. NWs are usually grown by vapor-liquid-solid (VLS) growth mechanism using techniques such as metal-organic vapor phase epitaxy, chemical beam epitaxy and molecular beam epitaxy (MBE). Of all the available techniques, MBE is known to be the technique which yields highest purity materials. In this study, the growth of GaAs NWs, GaAsSb NWs, as well as GaAs/GaAsSb axial and GaAs/AlGaAs radial heterostructured NWs on GaAs(111)B substrates by MBE is demonstrated. The structural and optical properties of the NWs grown are characterized by electron microscopy techniques such as scanning and transmission electron microscopy, and micro-photoluminescence, respectively.

Firstly, the optimum growth conditions to obtain rod shaped GaAs NWs on GaAs(111)B substrates by MBE is determined. It has been found that in-addition to the V/III ratio and substrate temperature, buffer growth conditions also play an important role on the orientation of the NWs. The effect of V/III ratio, substrate temperature, and the arsenic species (As2/As4) on the morphology of GaAs NWs has been determined. Transmission electron microscopy (TEM) characterization of NWs revealed that GaAs in NW form exhibit wurtzite (WZ) crystal phase in contrast to zinc blende (ZB) phase adapted in its bulk form. Since WZ crystal phase is a metastable phase of GaAs, the WZ GaAs NWs often exhibit stacking faults. The stacking faults are known to be a detrimental problem, if not properly controlled.

To gain more insight on the growth kinetics of GaAs NWs grown by MBE, several samples such as GaAs NWs grown for different time durations, and GaAs NWs with three GaAsSb inserts, where GaAsSb inserts acts as markers, have been grown. Interestingly, the growth rates of the GaAs segments and GaAsSb inserts were observed to vary with growth time. The variation of growth rate of NW with its time has been attributed to the effect of the inclined molecular beams with respect to the substrate surface in the MBE growth chamber. It has been found that NW growth can be distinguished into three different regimes, in which the growth of NWs depends either on the diffusion of Ga adatoms from the substrate surface to the Au droplet or/and the Ga adatoms hitting the NW sidewall directly and diffusing to the Au droplet. Further, the systematic investigation of GaAs NWs by TEM revealed that stacking faults form during the third regime of NW growth either due to the higher growth rate of NWs or due to the variation in V/III flux ratio.

Interestingly, the GaAsSb NWs grown by MBE exhibited ZB crystal phase with twin defects, in contrast to WZ form exhibited by GaAs NWs. The GaAs/GaAsSb/GaAs axial heterostructured NWs are interesting for various applications such as optical memories and solar cells because of its type-II band alignment. Such NWs grown by MBE were characterized by TEM, and it was found that the transition from WZ GaAs to ZB GaAsSb was abrupt while the transition from ZB GaAsSb to WZ GaAs exhibited twin defects, 4H PT and extended stacking faults. These results confirm that Sb could be a critical ingredient to control the crystalline phase of NWs. It can be due to either an increase of critical supersaturation (Δµc ) or decrease of supersaturation (ΔµLS ) (or both).

It is well known that NWs with high surface area are more prone to surface states which deteriorate the optical properties of NWs. It is possible to passivate these surface states by enclosing the NW with a higher bandgap material, for example by enclosing GaAs NWs in an AlGaAs shell. The growth of GaAs/AlGaAs core-shell NWs on GaAs(111)B substrates is reported in this thesis. It has been found that with increase in Al content, a lower axial and a higher radial growth rate of the AlGaAs shell takes place. It is due to that the Al adatoms has lower diffusion length than that of Ga adatoms. Room temperature and low temperature (4.4 K) micro-photoluminescence measurements show a much higher radiative efficiency from the GaAs core after the NW is overgrown with a radial AlGaAs shell.