Determination of Energy Distribution of Defect State Density for Ultrathin SiO_2/Si Interfaces by Using Photoelectron Yield Spectroscopy

研制了一套高灵敏度的总光电子产额谱(PYS)系统，用于测定H端c-Si表面和热生长的SiO_2/c-Si界面的带隙态密度的能量分布。Pys系统安装在用于X射线/紫外光激发光电子能谱的超高压室中，与开尔文探测器一起使用，使我们能够以几meV.1的精度测定样品表面的费米能量。在PYS系统中，获得了c-Si和SiO_2/c-Si样品8个数量级的动态范围。直接和间接光激发的阈值能量由样品的Pys光谱的线性和三次根图确定。这使得我们可以确定表面价带边缘的能量位置，而与表面能带弯曲量无关。根据从c-Si的XPS价带谱获得的价带边附近的态密度，可以从一阶导数Pys谱中探测和确定低至10^&lt；10&gt；cm^&lt；-2&gt；ev^&lt；-1&gt；的带隙密度。从SiO_2/c-Si系统的产额折减率估算了5.6 eV激发光电子在SiO_2中的逃逸深度为3.1 nm。在2.7 nm厚的SiO_2/n^+Si界面中，禁带中心附近存在密度高达-10^&lt；12-gt；-2&gt；ev^&lt；-1&gt；的界面态。在H_2+N_2混合气体中进行400ﾟC退火可使不含任何栅金属的样品的界面态密度降低一个数量级。

A system for total photoelectron yield spectroscopy (PYS) with a high sensitivity has been developed to determine the energy distribution of gap state densities for H-terminated c-Si surfaces and thermally-grown SiO_2/c-Si interfaces. The PYS system was installed in an UHV chamber for X-ray/ultraviolet excited photoelectron spectroscopy together with a Kelvin probe, which enables us to determine Fermi energy on a sample surface with an accuracy of a few meV.1. In the PYS system, a dynamic range of eight orders of magnitude was obtained for c-Si and SiO_2/c-Si samples.2. The threshold energies for direct and indirect photoexitations are determined by liner and cubic root plots of PYS spectra for the samples. This leads us to determine the energy position of the valence band edge on the surface independent of the amount of surface band bending.3. On the basis of the density of states near the valence band edge obtained from the XPS valence band spectrum for c-Si, the gap state density as low as 10^<10>cm^<-2>eV^<-1> can be detected and determined from the first derivative PYS spectrum.4. The escape depth for 5.6 eV-excited photoelectrons in SiO_2 was estimated to be 3.1nm from the yield reduction rate for the SiO_2/c-Si system.5. There exist interface states around midgap with densities as high as -10^<12>cm^<-2>eV^<-1> in an as-grown 2.7nm-thick SiO_2/n^+Si interface. 400ﾟC annealing in an H_2 + N_2 gas mixture reduces the interface state density by one order of magnitude for the sample without any gate metal.