タイトルイメージ 本文へジャンプ
Study on semiconductor interfaces

   Semiconductor interfaces strongly affect the electrical characteristics of semiconductor produces such as solar cells, TFT, and LSI.  Interface states, i.e., defect states at SiO2/Si interfaces, seriously degrade electrical characteristics in spite of the low state density.  In Kobayashi laboratory, we have succeeded in observation of interface states directly using spectroscopic method, i.e., XPS measurements under bias, for the first time.

Figure 1
  Diagram of XPS measurements under bias for observation of energy distribution of interface states at oxide/semiconductor interfaces.


   Figure 1 shows the diagram of measurements of XPS spectra under bias.  An oxide layer with thickness in the range between 1 and 2.5 nm is formed on semiconductors such as Si, followed by deposition of a ~3 nm Pt layer.  The Pt layer is earthed and a bias voltage is applied to the rear semiconductor surface during X-ray irradiation, and emitted photo-electrons are detected in the surface-normal direction.


Figure 2  Principle of observation of interface states by means of XPS measurements under bias: (a) band diagram of MOS structure at zero bias; (b) that under negative bias V applied to the semiconductor with respect to the metal layer.


   Figure 2 shows the principle for determination of energy distribution of interface states.  At zero bias (Fig. 2a), the metal Fermi level and semiconductor Fermi level are located at the same energy, and interface states below the Fermi level is occupied by electrons, while those above it are unoccupied.  Under bias, V, applied to the semiconductor with respect to the metal layer (Fig. 2b), the semiconductor Fermi level deviates from the metal Fermi level.  Consequently, interface states located between the metal Fermi level and the semiconductor Fermi level are newly occupied by electrons.  These charges in the interface states, ΔQi, change the potential gradient across the oxide layer by the magnitude given by
ΔVox= ΔQit / Cox
where  Cox is the capacitance of the oxide layer.  Since the energy difference between the semiconductor valence band maximum and the semiconductor core levels (e.g., Si 2p level) is constant, the semiconductor core levels shift by the magnitude the same as ΔVox, which is detectable by means of XPS.  Therefore, the energy distribution of interface states can be obtained from observation of the energy shift of the semiconductor core level as a function of the bias voltage.
   The energy distribution of interface states is usually elucidated from electrical measurements such as capacitance-voltage measurements.  Determination of the energy distribution of interface states from electrical measurements requires various assumptions: 1) equivalent circuit, 2) assumption that all interface states follow (or do not follow) the low frequency (or high frequency) AC signal, 3) ignorance of edge effect and interface roughness, 4) uniform distribution of dopants, etc.  Moreover, electrical measurements cannot be performed on an ultrathin oxide layer through which a high density leakage current flows.  XPS measurements under bias, on the other hand, can determine the energy distribution of interface states without these assumptions even for an oxide layer with a high leakage current density.


Figure 3   Energy distribution of interface states obtained from XPS measurements under bias for the ultrathin thermal SiO2/Si structures.

   Figure 3 shows the energy distribution of interface states for the ~1.5 nm thermal oxide layer formed on Si at various temperatures.  The energy distribution of interface states strongly depends on the temperature of thermal oxidation.  It is found that the atomic density of the oxide layer increases with the thermal oxidation temperature, leading to the observed variation of interface state spectra.