Effects of annealing temperature on shape transformation and optical properties of germanium quantum dots

The influences of thermal annealing on the structural and optical features of radio frequency（rf） magnetron sputtered self-assembled Ge quantum dots（QDs） on Si（100） are investigated.Preferentially oriented structures of Ge along the（220） and（111） directions together with peak shift and reduced strain（4.9%to 2.7%） due to post-annealing at 650 ℃ are discerned from x-ray differaction（XRD） measurement.Atomic force microscopy（AFM） images for both pre-annealed and post-annealed（650 ℃） samples reveal pyramidal-shaped QDs（density - 0.26×10^11 cm^-2） and dome-shape morphologies with relatively high density - 0.92×10^11 cm^-2,respectively.This shape transformation is attributed to the mechanism of inter-diffusion of Si in Ge interfacial intermixing and strain non-uniformity.The annealing temperature assisted QDs structural evolution is explained using the theory of nucleation and growth kinetics where free energy minimization plays a pivotal role.The observed red-shift - 0.05 eV in addition to the narrowing of the photoluminescence peaks results from thermal annealing,and is related to the effect of quantum confinement.Furthermore,the appearance of a blue-violet emission peak is ascribed to the recombination of the localized electrons in the Ge-QDs/SiO2 or GeOx and holes in the ground state of Ge dots.Raman spectra of both samples exhibit an intense Ge-Ge optical phonon mode which shifts towards higher frequency compared with those of the bulk counterpart.An experimental Raman profile is fitted to the models of phonon confinement and size distribution combined with phonon confinement to estimate the mean dot sizes.A correlation between thermal annealing and modifications of the structural and optical behavior of Ge QDs is established.Tunable growth of Ge QDs with superior properties suitable for optoelectronic applications is demonstrated.

Germanium nanoislands grown by radio frequency magnetron sputtering: Annealing time dependent surface morphology and photoluminescence

Structural and optical properties of ～ 20 nm Ge nanoislands grown on Si（100） by radio frequency （rI） magnetron sputtering under varying annealing conditions are reported. Rapid thermal annealing at a temperature of 600 ℃ for 30 s, 90 s, and 120 s are performed to examine the influence of annealing time on the surface morphology and photoluminescence properties. X-ray diffraction spectra reveal prominent Ge and GeO2 peaks highly sensitive to the annealing time. Atomic force microscope micrographs of the as-grown sample show pyramidal nanoislands with relatively high-density （～ 10^11 cm^-2）. The nanoislands become dome-shaped upon annealing through a coarsening process mediated by Oswald ripening. The room temperature photoluminescence peaks for both as-grown （～ 3.29 eV） and annealed （～ 3.19 eV） samples consist of high intensity and broad emission, attributed to the effect of quantum confinement. The red shift （～ 0.10 eV） of the emission peak is attributed to the change in the size of the Ge nanoislands caused by annealing. Our easy fabrication method may contribute to the development of Ge nanostructure-based optoelectronics.

Growth of Ge/Si(100) Nanostructures by Radio-Frequency Magnetron Sputtering: the Role of Annealing Temperature

Surface morphologies of Ge islands deposited on Si(100) substrates are characterized and their optical properties determined.Samples are prepared by rf magnetron sputtering in a high-vacuum chamber and are annealed at 600℃,700℃ and 800℃ for 2 min at nitrogen ambient pressure.Atomic force microscopy,field emission scanning electron microscopy,visible photoluminescence (PL) and energy dispersive x-ray spectroscopy are employed.The results for the annealing temperature-dependent sample morphology and the optical properties are presented.The density,size and roughness are found to be strongly influenced by the annealing temperature.A red shift of ~0.29 eV in the PL peak is observed with increasing annealing temperature.

Structural and Optical Behavior of Germanium Quantum Dots

Controlled growth,synthesis,and characterization of a high density and large-scale Ge nanostructure by an easy fabrication method are key issues for optoelectronic devices.Ge quantum dots(QDs)having a density of~1011 cm^(-2) and a size as small as~8 nm are grown by radio frequency magnetron sputtering on Si(100)substrates under different heat treatments.The annealing temperature dependent structural and optical properties are measured using AFM,XRD,FESEM,EDX,photoluminescence(PL)and Raman spectroscopy.The effect of annealing is found to coarsen the Ge QDs from pyramidal to dome-shaped structures as they grow larger and transform the nanoislands into relatively stable and steady state configurations.Consequently,the annealing allows the intermixing of Si into the Ge QDs and thereby reduces the strain energy that enhances the formation of larger nanoislands.The room temperature PL spectra exhibits two strong peaks at~2.87 eV and~3.21 eV attributed to the interaction between Ge,GeO_(x) and the possibility of the presence of QDs core-shell structure.No reports so far exist on the red shift~0.05 eV of the strongest PL peak that results from the effect of quantum confinement.Furthermore,the Raman spectra for the pre-annealed QDs that consist of three peaks at around~305.25 cm^(-1),409.19 cm^(-1) and 515.25 cm^(-1) are attributed to Ge-Ge,Ge-Si,and Si-Si vibration modes,respectively.The Ge-Ge optical phonon frequency shift(~3.27 cm^(-1))associated with the annealed samples is assigned to the variation of shape,size distribution,and Ge composition in different QDs.The variation in the annealing dependent surface roughness and the number density is found to be in the range of~0.83 to~2.24 nm and~4.41 to~2.14×10^(11)cm^(-2),respectively.