Influence of low-energy proton irradiation on the effective lifetime in the space charge region of silicon n+-p junctions

The effect of low-energy proton irradiation on the pulse characteristics of silicon n+-p-p+ structures is analyzed. It is shown that irradiation with protons with an energy of 180 keV and a dose of 1015 cm−2 creates a region with an effective lifetime of 5.5·10−8 s in the space charge region of the n+-p junction. Such elements can be used to create high-speed photodiodes with an operating modulation frequency of 18 MHz.


Introduction
Technologies of proton irradiation of semiconductor devices allow selective introduction of recombination centers into silicon structures, reduction of the effective lifetime of charge carriers in hidden layers and creation of layers with hydrogen-containing centers [1]. The disturbed layer formed as a result of implantation of hydrogen ions is heterogeneous. The integral characteristics of the disturbed layer are studied in the work [2]. Reducing the lifetime of charge carriers in the local volume of the structure makes it possible to improve the totality of static and frequency characteristics of devices [3].
The influence of protons with energies Ep = 40 keV and Ep = 180 keV (irradiated samples were kept at temperatures Tp = 83 K and Tp = 300 K) on the parameters of the current-voltage (I-V) characteristics of silicon photovoltaic structures with a diffused n + -p junction with the depth n d = 0,45 m is shown in [4,5]. Protons with an initial energy of 40 keV predominantly change the physical properties of the n + -layer, and protons with an initial energy of 180 keV change the properties of the space charge region (SCR) in the p-layer.
Using the model of the formation of primary radiation defects (PRD) in silicon [6,7], the depth distributions of the average number of interstitial silicon GSi, vacancies GV, divacances GW created by one proton per unit of the projective path length are calculated (figure 1).
Protons with an initial energy of Ep = 40 keV at temperatures Tp = 83 K and Tp = 300 K create primary radiation defects (PRD) in n + -layer at a distance of 0.41 m from the surface. Protons with an initial energy of Ep = 180 keV create PRD in 1.51 m in the entire of SCR n + -p-junction. The number of radiation defects at the maximum of distribution in the n + -layer at Ep = 40 keV, Tp = 83 K is much less than in the n + -layer at Ep = 40 keV, Tp = 300 K and also in the p-layer at Ep = 180 keV, Tp = 83 K. Therefore, irradiation by protons with energy of 180 keV significantly reduces the effective lifetime of charge carriers [8]. Measurements of the photoconductivity decay are used to determine the lifetime [9, 10]. Measurements of the lifetime of minority carriers by the method of recording photoconductivity caused by microwave radiation are used to control the results of technological impacts [11]. Pulsed illumination is used to determine the effect of structural defects on the volume component of the lifetime of minority carriers and the recombination rate in the p-n junction in a two-sided polycrystalline silicon solar cell [12]. In order to exclude the time dependence of the photocurrent and photoconductivity used in the methods [9 -12], the transient voltage in unlit silicon n + -p-p + structures irradiated with low-energy protons was measured in this work.
The aim of the work is to analyze the effect of low-energy proton irradiation on the pulse characteristics and lifetime of silicon structures with n + -p junction.

Research methods
Experimentally investigated the n + -p-p + -structure of silicon grown by the Czochralski method, with a volume resistance of the p-type base  = 10 cm and the concentration of equilibrium holes p0  10 15 cm −3 , the depth of diffusion of the n + -p and p-p + -junctions dn  dp  0,45 m, thickness L  200 m. The surface concentrations of phosphorus and boron were NP  10 20 cm −3 and NB  10 20 cm −3 . Samples with an area of S  1 cm 2 were obtained by laser separation of plates using a solid-state YAG-laser in pulsed mode.
The samples were irradiated from the side of

Analysis of the research results
The functions U(t) (figure 3) are approximated with sufficient accuracy by the one-exponential dependence of the voltage on time (equation 1) for samples no. 2, no. 3, no. 4, and two-exponential dependence (equation 2) for sample no. 1.
where А =  10 mVthe amplitude of the bipolar voltage pulse, t0the beginning of the countdown, , 1, 2, а1, а2,parameters determined as a result of the approximation.

Conclusion
In the silicon structure, protons with energy of 180 keV create PRD in a layer with a thickness of 1.51 m, and protons with energy of 40 keV create PRD in a layer with a thickness of 0.41 m. At the irradiation temperature of the samples of 83 K, the amount of PRD in the distribution peak at the end of the projective path of a proton with Ep = 40 keV is much less than for a proton with Ep = 180 keV, which is due to the difference in the separation processes of SiI, V pairs silicon of n-and p-types of conductivity [4,5]. Irradiation with protons with Ep = 180 keV changes the physical properties of the high-doped n +type layer and the entire SCR of the n + -p junction. Protons with Ep = 40 keV change the properties of the n-type layer without affecting the SCR if the depth of the n + -p junction exceeds the average length of the proton's projective path.
As a result of irradiation with protons with energy of 180 keV and a dose of 10 15 cm −2 , the properties of the SCR n + -p junction changed so that the switching time decreased to 5.510 −8 s. Such elements can be used to create high-speed structures with an operating modulation frequency of 18 MHz.