Growth and characterisation of ordered indium arsenide quantum dots on cross-hatch virtual substrate

Abstract

Growth of InAs quantum dots (QDs) on In[subscript x]Ga[subscript 1-x]As/GaAs(001) cross-hatch virtual substrates (VS) has been carried out by molecular beam epitaxy (MBE).A model explaining the origin of QD aligment on the VS is developed. Cross-hatch surface morphology of the VS is studied by atomic force microscopy(AFM), and its origin by cross-sectional transmission electron microscopy(TEM). The cross-hatch VS consist of 50-nm thick In[subscript 0.15]Ga[subscript 0.85]As on GaAs(001).cross-hatch morphology on the surface of the VS is revealed by AFM and its origin from the networks of misfit dislocations (MDs) is confirmed by TEM. Fast Fourier transform(FFT) and line scan analyses indicate that the cross-hatch is aperiodic with different average lateral spacings in the two orthogonal([1-10] and [110]) directions. The strain and In composition of the VS are examined by high resolution X-ray diffraction(HRXRD). It is found that the degree of strain relaxation of the InGaAs layer increases with increasing thickness and In composition. Consequently ,QDs grown on the different InGaAs layers result in different arrangements of QDs on the cross-hatch surface: ordered QDs are obtained when they are grown on partially-relaxed In[subscript 0.15]Ga[subscript 0.85]As (50 nm) layer; and groups of QDs are obtained when they are grown on more relaxed In[subscript 0.15]Ga[subscript 0.85]As (100 and 150 nm) layers. Photoluminescence (PL) measurements qualitatively agree with the HRXRD results. Various growth interruption(GI) times are introduced after the formation of QDs in order to improve QD uniformity. It is found that a 30-second GI time is the optimum value for the growth of InAs QDs on In[subscript 0.15]Ga[subscript 0.85]As/GaAs VS. After InAs QD formation, The QDs are capped with a thin layer of GaAs in order to study the surface evolution with an aim to using it as a template for further QD growth. It is found that nano holes are seen in the middle of the QDs directly grown on GaAs (which has been reported in the literature) but none are seen when the QDs are grown on the cross-hatch VS (which has not been reported). Instead QDs which are nucleated along the [110] direction become less prominent while those nucleated along the [1-10] direction become more prominent. This result is attributed to the insufficient and asymmetry of strain energies in the underlying plane on which QDs are grown.