Silicon solar cells containing boron and oxygen are one of the most rapidly growing forms of electricity generation. However, they suffer from significant degradation during the initial stages of use. This problem has been studied for 40 years resulting in over 250 research publications. Despite this, there is no consensus regarding the microscopic nature of the defect reactions responsible. In this paper, we present compelling evidence of the mechanism of degradation. We observe, using deep level transient spectroscopy and photoluminescence, under the action of light or injected carriers, the conversion of a deep boron-di-oxygen-related donor state into a shallow acceptor which correlates with the change in the lifetime of minority carriers in the silicon. Using ab initio modeling, we propose structures of the BsO2 defect which match the experimental findings. We put forward the hypothesis that the dominant recombination process associated with the degradation is trap-assisted Auger recombination. This assignment is supported by the observation of above bandgap luminescence due to hot carriers resulting from the Auger process.

No abstract

Photoluminescence, infrared absorption, positron annihilation, and deep-level transient spectroscopy ͑DLTS͒ have been used to investigate the radiation damage produced by 24 GeV/ c protons in crystalline silicon. The irradiation doses and the concentrations of carbon and oxygen in the samples have been chosen to monitor the mobility of the damage products. Single vacancies ͑and self-interstitials͒ are introduced at the rate of ϳ1 cm −1 , and divacancies at 0.5 cm −1 . Stable di-interstitials are formed when two self-interstitials are displaced in one damage event, and they are mobile at room temperature. In the initial stages of annealing the evolution of the point defects can be understood mainly in terms of trapping at the impurities. However, the positron signal shows that about two orders of magnitude more vacancies are produced by the protons than are detected in the point defects. Damage clusters exist, and are largely removed by annealing at 700 to 800 K, when there is an associated loss of broad band emission between 850 and 1000 meV. The well-known W center is generated by restructuring within clusters, with a range of activation energies of about 1.3 to 1.6 eV, reflecting the disordered nature of the clusters. Comparison of the formation of the X centers in oxygenated and oxygen-lean samples suggests that the J defect may be interstitial related rather than vacancy related. To a large extent, the damage and annealing behavior may be factorized into point defects ͑monitored by sharp-line optical spectra and DLTS͒ and cluster defects ͑monitored by positron annihilation and broadband luminescence͒. Taking this view to the limit, the generation rates for the point defects are as predicted by simply taking the damage generated by the Coulomb interaction of the protons and Si nuclei.

We have recently found that the silicon trivacancy (V3) is a bistable defect that can occur in fourfold coordinated and (110) planar configurations for both the neutral and singly negative charge states [V. P. Markevich et al., Phys. Rev. B 80, 235207 (2009)]. Acceptor levels of V3 in both these configurations have been determined. It has also been shown that at T > 200 °C, the interaction of mobile trivacancies with interstitial oxygen atoms results in the formation of V3O complex with the first and second acceptor levels at Ec −0.46 and −0.34 eV. In the present work we identify donor levels arising from V3 and V3O complexes by means of deep level transient spectroscopy (DLTS) and high‐resolution Laplace DLTS on n+p silicon structures irradiated with 6 MeV electrons, combined with density functional modeling studies. It is found that both defects possess two donor levels in the (110) planar configurations. First donor levels at Ev +0.19 and +0.235 eV, and the second donor levels at Ev +0.105 and +0.12 eV are found for the V3 and V3O complexes, respectively.

Defect reactions associated with the elimination of divacancies (V 2 ) have been studied in n-type Czochralski (Cz) grown and float-zone (FZ) grown Si crystals by means of conventional deep-level transient spectroscopy and highresolution Laplace deep-level transient spectroscopy (LDLTS). Divacancies were introduced into the crystals by irradiation with 4 MeV electrons. Temperature ranges of the divacancy disappearance were found to be 225-275 • C in Cz Si crystals and 300-350 • C in FZ Si crystals upon 30 min isochronal annealing. Simultaneously with the V 2 disappearance in Cz Si crystals a correlated appearance of two electron traps with activation energies for electron emission 0.23 eV {E(0.23)} and 0.47 eV {E(0.47)} was observed.It is argued that the main mechanism of the V 2 disappearance in Cz Si crystals is related to the interaction of mobile divacancies with interstitial oxygen atoms. This interaction results in the formation of V 2 O centres, which are responsible for the E(0.23) and E(0.47) traps. Electronic properties of the V 2 O complex were found to be very similar to those of V 2 but energy levels of the two defects could easily be separated using LDLTS.In FZ Si crystals, a few electron traps appeared simultaneously with the V 2 annihilation. The small concentration of these traps compared with the V 2 concentration before annealing prevented their reliable identification.

No abstract

Results available in the literature on minority carrier trapping and light‐induced degradation (LID) effects in silicon materials containing boron and oxygen atoms are briefly reviewed. Special attention is paid to the phenomena associated with “deep” electron traps (J. A. Hornbeck and J. R. Haynes, Phys. Rev. 1955, 97, 311) and the recently reported results that have linked LID with the transformation of a defect consisting of a substitutional boron atom and an oxygen dimer (BsO2) from a configuration with a deep donor state into a recombination active configuration associated with a shallow acceptor state (M. Vaqueiro‐Contreras et al., J. Appl. Phys. 2019, 125, 185704). It is shown that the BsO2 complex is a defect with negative‐U properties, and it is responsible for minority carrier trapping and persistent photoconductivity in nondegraded Si:B+O samples and solar cells. It is argued that the “deep” electron traps observed by Hornbeck and Haynes are the precursors of the “slow” forming shallow acceptor defects, which are responsible for the dominant LID in boron‐doped Czochralski silicon (Cz‐Si) crystals. Both the deep and shallow defects are BsO2 complexes, transformations between charge states and atomic configurations of which account for the observed electron trapping and LID phenomena.

Articles you may be interested inElectron and hole deep levels related to Sb-mediated Ge quantum dots embedded in n-type Si, studied by deep level transient spectroscopy Appl. Phys. Lett. 102, 232106 (2013); 10.1063/1.4809595 Improved calculation of vacancy properties in Ge using the Heyd-Scuseria-Ernzerhof range-separated hybrid functional Schottky barriers formed by depositing Au on n-type Ge have been used to study the antimony-vacancy complex ͑E center͒. Both hole and electron transitions have been observed because the formation of an inversion layer at the semiconductor surface enables minority carriers to be injected when the Schottky barrier is forward biased. It is argued that the E center in Ge has three charge states: double negative, single negative, and neutral. The free energy of electron ionization for the double acceptor level of the complex has been found to be ⌬G(ϭ/Ϫ)ϭ0.294 Ϫ4.2 kT (eV), where k is Boltzmann's constant. Consequently, the position of the double acceptor level of the E center ͕E(ϭ/Ϫ)ϭE c Ϫ⌬G(ϭ/Ϫ)͖ is temperature dependent. In moderately Sb-doped (N Sb ϭ10 13 -10 15 cm Ϫ3 ) Ge crystals at equilibrium conditions half-occupancy of the double acceptor state of the Sb-V complex occurs when the Fermi level is at about E c Ϫ0.20 eV. The single acceptor level of the E center is in the lower part of the band gap. The activation energy of hole emission from the E(Ϫ/0) level has been determined as 0.307 eV. The introduction of one Sb-V defect results in the removal of three free electrons in Sb-doped Ge. It is proposed that this is one of the main reasons for the fast free carrier removal and n→p conversion of the conductivity type in Ge:Sb upon electron-or gamma-irradiation at room temperature.