氧化硅层厚度对Si/SiO_(2)界面电子态结构与光学性质的影响

Influence of Silicon Oxide Layer Thickness on Electronic State Structure and Optical Properties of Si/SiO_(2) Interface

在氧化硅上生长纳米硅晶,保持氧化硅的直接带隙结构,降低其能带带隙,以用于发光和光伏。采用基于密度泛函理论的第一性原理研究了块体α-方石英、薄膜α-方石英、Si/SiO_(2)界面的电子态结构和Si/SiO_(2)界面的光学性质。结果显示,其均为直接带隙半导体,当薄膜α-方石英厚度和Si/SiO_(2)界面氧化硅层厚度逐渐减小时,能带带隙均逐渐变大,表现出明显的量子限制效应。光学性质计算结果表明:Si/SiO_(2)界面虚部介电峰和吸收峰的峰值随氧化硅层厚度降低而显著升高,且峰位向高能量方向蓝移。使用脉冲激光沉积制备了氧化硅上硅晶薄膜,测量了Si/SiO_(2)界面样品的PL光谱,在670 nm处存在一个强的发光峰,在波长超过830 nm后,Si/SiO_(2)界面样品的发光强度不断升高。因此,可以通过控制Si/SiO_(2)界面氧化硅层厚度有效地调控Si/SiO_(2)界面的电子态结构和光学性质,引进边缘电子态,调控其带隙进入1~2 eV区间,获取硅基发光材料。

The bulk α-cristobalite, the thin film α-cristobalite with different thicknesses and the Si/SiO_(2)interface with different silicon oxide layer thicknesses are all direct bandgap semiconductors. The thickness of the thin film α-cristobalite and the thickness of the silicon oxide layer at the Si/SiO_(2)interface gradually decrease, and the bottom of the conduction band moves continuously to the direction of the high energy level, and the energy band gap gradually increases, showing an obvious quantum confinement effect. The overlapping hybridization of the total density of states and the density of partial electronic states is weakened, and the valence band and conduction band of the energy band structure are more sparse. As the thickness of α-cristobalite decreases from 2.887 nm to 1.047 nm, the band gap of thin film α-cristobalite increases from 5.233 eV to 5.927 eV. As the thickness of silicon oxide decreases from 2.887 nm to 1.047 nm, the band gap of Si/SiO_(2)interface increases from 1.62 eV to 1.782 eV. With the decrease of the thickness of the silicon oxide layer at the Si/SiO_(2)interface, the quantum confinement effect is prominent and the energy band gap gradually increases. The bottom of the conduction band of the Si/SiO_(2)interface moves to the higher energy level with the decrease of the thickness of the silicon oxide layer. The electronic state structure change of the Si/SiO_(2)interface caused by the thickness change of the silicon oxide layer is similar to the electronic state structure change caused by the thickness change of the thin film α-cristobalite.The density of states and wavelength division states of Si/SiO_(2)interface with a thickness of 1.047 nm are all lower than those of Si/SiO_(2)interface with a thickness of 2.887 nm. As a result, the energy band gap is reduced, and the overlapping hybridization of electrons is weakened. The Si/SiO_(2)interface with a silicon oxide layer thickness of 1.047 nm has a very steep peak at-19.5 eV. It can be seen from the total density of states and partial wave electron density diagram of bulk α-cristobalite that this peak mainly comes from the contribution of 2s electrons of oxygen atoms in silicon oxide layer at Si/SiO_(2)interface, because the energy of 2s electrons of oxygen atoms in silicon oxide layer is higher than that of 3s electrons of silicon.Near fermi surface, the density of electronic states at Si/SiO_(2)interface mainly comes from the contribution of 2p states of oxygen atoms. The calculation results of optical properties show that the imaginary part of the dielectric function at the Si/SiO_(2)interface has a dielectric peak near 4.5 eV. With the decrease of the thickness of the silicon oxide layer at the Si/SiO_(2)interface, the dielectric peak slightly moves to the high energy direction, and the peak value of the dielectric peak keeps rising. This is caused by the decrease of the thickness of the silicon oxide layer in Si/SiO_(2)and the increase of the band gap of the Si/SiO_(2)interface.As the thickness of the silicon oxide layer decreases, the Si/SiO_(2)interface state density decreases near the left side of the Fermi plane, and the overlapping hybridization of electrons is weakened, so the energy consumed when the electric dipole is formed inside increases. There is an absorption peak at the Si/SiO_(2)interface near 6 eV, and the peak of the absorption peak of the absorption coefficient of the Si/SiO_(2)interface also increases significantly with the decrease of the thickness of the silicon oxide layer, and the peak position moves to the high energy direction, resulting in a blue shift. Therefore, the electronic state structure and optical properties of the Si/SiO_(2)interface can be effectively regulated by controlling the thickness of the Si oxide layer at the Si/SiO_(2)interface. In the experimental part, the nanosecond pulsed laser deposition method was used to prepare silicon oxide and its silicon thin film, and the oxygen-blowing annealing was performed at a high temperature of 1 000℃, and the growth thickness of the film was changed by controlling the annealing time. During PLD fabrication, a nanosecond pulsed Nd:YAG laser with a third-harmonic 355 nm laser beam was used to deposit silicon crystalline nanolayers on silicon oxide, thereby constructing edge electronic states. In the photoluminescence measurement, the edge electron state on the Si/SiO_(2)interface sample has a strong luminescence peak at 670 nm;under the excitation of 532 nm laser, the Si/SiO_(2)interface electrons can be excited from the valence band to the conduction band, and enter the edge electron state to form a strong quasi-excited light peak. The experimental result verifies the results of the energy band calculation: growing a silicon thin film on silicon oxide can form edge electronic states to effectively reduce its energy band gap and maintain its direct band gap characteristics. The electrons in the electronic state at the edge of Si/SiO_(2)interface can also be transported to the lower partial wave state, forming a luminescent band in the near infrared band. The detection results of the photoluminescence PL spectrum of the silicon crystalline thin film structure sample on silicon oxide verify the results of the computational study. The edge electronic states on the sample narrow the wide direct bandgap of silicon oxide to 1~2 eV, and the position of the luminescence peak is covering visible light and near-infrared bands, thus, the silicon-on-silicon thin film structure will have good application prospects in the fields of light-emitting and photovoltaics.