Capacitance spectroscopy of electro nic states at the SiO2/n-Si interface in MOS structures irradiated with helium ions

Al/SiO 2 / n -Si/Al MOS structures fabricated on (100) wafers of single-crystal n -type silicon grown with the Czochralski method were studied. The electrical resistivity of silicon was 4.5 Ohm · cm at room temperature. The thickness of the SiO 2 layer thermally formed in dry oxygen was 420 nm. The thickness of the aluminum layer deposited on top of SiO 2 was 0.7 µm. The area of aluminum metallization was 1.85 × 1.85 mm 2 . At a room temperature, the structures were irradiated with helium ions (kinetic energy of an ion was 5 MeV). The radiation fluence ranged from 10 10 to 10 13 cm −2 . According to calculations performed in the SRIM program, the average projective range of the helium ion in the structure was ≈ 24 µm. The impedance modulus Z and the phase shift angle φ between the current and voltage were measured in the frequency range from 20 Hz to 2 MHz using an E4980A LCR meter. The signal amplitude was 40 mV. The constant bias voltage U varied from −40 to 40 V. The MOS structures were kept in the dark at a room temperature. The DLTS spectra were recorded using CE-7C capacitance spectrometer in the temperature range of 80–300 K. The voltage pulses for filling traps U p with c -band electrons and the voltage of emission U e of electrons from traps to c -band of n -Si varied in the range from −0.5 to −9 V. The pulse duration for filling traps with electrons was t p = 0.75 ms, and the pulse duration for emission of electrons from traps was t e = 20 ms. It was found that for the Al/SiO 2 / n -Si/Al structures irradiated with helium ions with fluences of ≤10 12 cm −2 , the dependence of the capacitance on the frequency in the depletion mode is determined by the recharging of the surface electron states at the SiO 2 / n -Si interface. It was shown that in the initial structures for fast (recharging time <1 µs) surface states, the dependence of the energy density of states N ss on the potential energy e ψ of an electron in n -Si near the SiO 2 / n -Si interface has its maximum at e ψ ≈ E F – 0.1 eV (here e is the elementary charge, ψ is the electric potential, E F is the Fermi energy level). After irradiation with helium ions with a fluence of 10 12 cm −2 , this maximum shifts towards lower energies down to e ψ ≈ E F – 0.2 eV. In the irradiated structures, a second maximum appears on the N ss ( e ψ) dependence in the region of e ψ > 0. For an irradiation fluence of 10 12 cm −2 , the maximum is located at e ψ ≈ E F + 0.1 eV. The possibility of studying surface states with the DLTS spectroscopy method in the selected energy range by varying the emission voltage U e at a constant value of the filling voltage U p and/or varying the filling voltage U p at a constant value of the emission voltage U e is shown.