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

Al/SiO₂/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 SiO₂ layer thermally formed in dry oxygen was 420 nm. The thickness of the aluminum layer deposited on top of SiO₂ was 0.7 µm. The area of aluminum metallization was 1.85 × 1.85 mm². At a room temperature, the structures were irradiated with helium ions (kinetic energy of an ion was 5 MeV). The radiation fluence ranged from 10¹⁰ to 10¹³ 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/SiO₂/n-Si/Al structures irradiated with helium ions with fluences of ≤10¹² 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 SiO₂/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 eψ of an electron in n-Si near the SiO₂/n-Si interface has its maximum at eψ ≈ 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 10¹² cm⁻², this maximum shifts towards lower energies down to eψ ≈ E_F – 0.2 eV. In the irradiated structures, a second maximum appears on the N_ss(eψ) dependence in the region of eψ > 0. For an irradiation fluence of 10¹² cm⁻², the maximum is located at eψ ≈ 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. 

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