Study of Direct-bandgap transition from tensile strained Ge quantum-dots array

• Main Authors: Shen-kai Chou, 周聖凱
• Other Authors: Pei-wen Li
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/92373395112431460325

碩士 === 國立中央大學 === 電機工程學系 === 103 === In this thesis, we formed abacus-bead SiGe pillar array by using SF6/C4F8 Inductively Coupled Plasma etching, and followed by selectivity oxidation transforming the abacus-bead SiGe pillar array into Ge nanocrystallites/SiO2 ¬pillar array. With the help of Si3N4 sidewall layer on movement and segregation of Ge nanocrystallites, germanium quantum dots (Ge QDs) were fabricated at 900℃ thermal annealing. During high-temperature oxidation, germanium would bond with as-formed SiO2 continuously and then be pulled outwards because of volume expanding of SiO2, leading to the tensile strain state in Ge QDs. Raman Spectroscopy measurement confirmed that the tensile strain in Ge QDs was about 0.4~1.6%, leading to a quasi-direct bandgap transition properties of Ge QDs. Therefore, we developed a Microdisk cavity for fabricated Ge QDs to enhance Ge-QD luminousness and explored its optical properties. The corresponding photoluminescence (PL) peak of Ge-QD Microdisk centered at 0.83eV, which corresponded to the energy difference from Γ valleys to valance band in germanium, demonstrating the probability of direct-transition recombination for Ge QDs. Besides, a fitted α approaching to 1 in the power-dependent PL spectra suggested that PL emission was being dominated by exciton recombination in the Ge QDs and furthermore, the activation energy (Ea) extracted from temperature- dependent PL was about 10~17 meV. Time-resolved photoluminescence show the carrier lifetime of ~ 4.7 ns. This study demonstrated the modification of Ge QDs bandgap by tensile strain, and its peak energy located at 0.83 eV, corresponding to wavelength 1500 nm, showed the promising potential for applications in near infra-red (NIR) communication. In capacitance-voltage (C-V) characterization, significant hysteresis curve under a 980 nm laser illumination double confirmed that tensile strained Ge QDs with quasi direct-bandgap possess the ability of light absorption.