On the nature and charge state of the X-Defect, a radiation-induced Silicon defect with field-enhanced charge carrier emission

TL;DR

This study resolves the nature of the X-Defect observed in irradiated low-resistivity $p$-type Silicon by combining TSC, DLTS, and numerical simulations. The X-Defect is identified as the singly positively charged silicon di-vacancy donor state, $\mathrm{V}_2(+/0)$, with field-enhanced emission arising from phonon-assisted tunnelling (PAT) rather than Poole-Frenkel (PF) effects; Difference-DLTS confirms a quadratic $E_A$ versus electric field, giving $E_{A,0} = 0.1962(7)$ eV. The work shows consistency between DLTS-derived defect parameters and TSC observations, supported by PAT-based simulations that reproduce measured spectra and rules out PF as the dominant mechanism. Importantly, identifying the X-Defect as a neutral defect in the space-charge region explains why it does not affect the effective doping concentration $N_{\text{eff}}$, clarifying previous contradictions and informing detector performance in irradiated silicon devices.

Abstract

The elusive X-Defect, a defect found in low-resistivity $p$-type Silicon after irradiation, observed as a low-temperature shoulder of the $\mathrm{B}_\mathrm{i}\mathrm{O}_\mathrm{i}$ defect (Boron-interstitial-Oxygen-interstitial complex) in Thermally Stimulated Current (TSC) measurements, was investigated to determine its properties, matching them with those of a previously identified defect. Through a combination of TSC, Deep-Level Transient Spectroscopy (DLTS), Difference-DLTS (DDLTS), numerical simulations of field-enhanced charge carrier emissions in TSC measurements and a comparison to literature, the X-Defect was identified as the singly positively charged Silicon di-vacancy $\mathrm{V}_2(+/0)$. This assignment is supported by an agreement in activation energy, capture cross-section, trap type and charge emission process, as well as simulations comparing the effects of phonon-assisted tunnelling (PAT) and Poole-Frenkel (PF) mechanisms on TSC spectra. DDTLS measurements revealed a quadratic dependence of the activation energy on the electric field strength, confirming PAT as the prevailing mechanism over PF in the case of the radiation-induced X-Defect. Assigning the X-Defect to an electrically neutral defect in the space charge region resolves previous contradictions regarding its deficiency in impacting on the effective doping concentration.

Figures (13)

• Figure 1: TSC measurements on the electron-irradiated diode. Measurement conditions: a reverse bias (during cooling and heating) of $U_\text{R}=20V$, a filling temperature of $T_\text{fill}=60K$ with either a 0V bias injection for majority carriers only (blue curve) or a -20V forward bias (1mA current) injection for majority and minority carriers (orange curve), with a filling time of $t_\text{fill}=120s$ and a heating rate of 11Kmin. For the applied $U_\text{R}$ the peak electric field strength at the top of the diode is 7,9kVcm and around 5.2µm are depleted at room temperature.
• Figure 2: DLTS measurements for the electron-irradiated diode. Measurement conditions: a reverse bias of $U_\text{R}=10V$, pulse voltages of $U_\text{P}=0.6V$ reverse bias for a majority carrier only filling (blue curve) and -2V forward bias for a majority and minority carrier filling (orange curve), with a pulse duration of $t_\text{p}=1ms$, evaluated for time windows $T_\text{W}$ of 20ms, 200ms and 2s. Shown are the spectra obtained from the b1 correlator and $T_\text{W}=200ms$. For the applied $U_\text{R}$ the peak electric field strength at the top of the diode is 5,7kVcm and around 3.8µm are depleted at room temperature.
• Figure 3: Simulation of a TSC spectrum using defect parameters obtained from DLTS measurements in \ref{['fig:DLTS_electron']} (blue curve). The TSC measurement was obtained for a reverse bias of 20V, a filling temperature of 70K and a filling voltage of 0V applied for 120s.
• Figure 4: Effect of the filing temperature $T_\text{fill}$ in TSC measurements. Obtained for the electron-irradiated sample with a reverse bias of 20V and a filling voltage of 0V for 120s.
• Figure 5: DLTS measurements for different pulse voltages. All measurements were carried out under a reverse bias of 20V.