Solid State Phenomena Vols. 178-179

Abstract: We report on a new method of external gettering in silicon substrate for semiconductor applications. The proposed method is based on the deposition of a multilayer system formed by introducing a number of thin buried silicon oxide layers into the thick polycrystalline silicon layer deposited on the wafer backside. Oxide films of a few nanometer thicknesses significantly retard both the grain growth and subsequent loss of the gettering capability of the polycrystalline silicon layer during high temperature annealing. The mechanisms of the grain growth and the influence of the embedded oxide layers on the gettering function in the multilayer system are discussed. We used scanning electron microscopy and transmission electron microscopy for the characterization of the multilayer system, and intentional contamination for demonstration of the gettering properties.

Abstract: We have recently shown that Sn impurity atoms are effective traps for vacancies (V) in Ge:Sn crystals irradiated with MeV electrons at room temperature [V.P. Markevich et al., J. Appl. Phys. 109 (2011) 083705]. A hole trap with 0.19 eV activation energy for hole emission to the valence band (E_h) has been assigned to an acceptor level of the Sn-V complex. In the present work electrically active defects introduced into Ge:Sn+P crystals by irradiation with 6 MeV electrons and subsequent isochronal annealing in the temperature range 50-300 °C have been studied by means of transient capacitance techniques and ab-initio density functional modeling. It is found that the Sn-V complex anneals out upon heat-treatments in the temperature range 50-100 °C. Its disappearance is accompanied by the formation of vacancy-phosphorus (VP) centers. The disappearance of the VP defect upon thermal annealing in irradiated Sn-doped Ge crystals is accompanied by the effective formation of a defect which gives rise to a hole trap with E_h = 0.21 eV and is more thermally stable than other secondary radiation-induced defects in Ge:P samples. This defect is identified as tin-vacancy-phosphorus (SnVP) complex. It is suggested that the effective interaction of the VP centers with tin atoms and high thermal stability of the SnVP complex can result in suppression of transient enhanced diffusion of phosphorus atoms in Ge.

Abstract: The isotopic effect of hydrogen and deuterium on hydrogenation of radiation defects introduced in n-type float zone and Czochralski silicon by irradiation with high-energy alphas was investigated. Silicon diodes were first irradiated with 2.4 MeV alphas to a fluence of 1x10¹⁰ cm^-2 and then hydrogen or deuterium was introduced by rf plasma treatment at 250°C. Reactions of hydrogen and deuterium with radiation defects were monitored by deep-level transient spectroscopy during subsequent isochronal annealing at temperatures ranging from 100 to 400°C. Results show that hydrogen rf plasma effectively neutralizes majority of vacancy related radiation defects created by alphas in both materials. In contrast with it, neutralization by deuterium plasma is substantially weaker. Disappearing of vacancy related defect levels due to hydrogen (deuterium) treatment is accompanied by introduction of two dominant deep levels at E_C-0.309 eV and E_C-0.365 eV. While hydrogenation significantly accelerates annealing of radiation defects especially in Czochralski material, deuteration has weaker effect and gives rise to new defect levels during annealing.

Abstract: Accumulation kinetics of vacancy-oxygen (VO) complexes in Czochralski (Cz) n-Si at 360 °C and 1 MeV electron pulse irradiation has been investigated. It is shown that during the irradiation and simultaneous generation and annealing of VO centers, the accumulation kinetics has non-linear dependence with saturation. It is found that there is a maximal concentration of VO centers, which depends on the radiation intensity (J) and temperature as well. It is also established the annealing of VO centers can substantially be stimulated by the intensity of electron irradiation. An increase of J from 1.25×10¹⁵ to 1.25×10¹⁶ electrons/(сm²s) does not influence the generation efficiency of VO, though it accelerates their annealing by more than one order of magnitude.

Abstract: The charge compensation in undoped GaP single crystals is investigated by modeling the Fermi level position for various concentrations of shallow and deep donors and acceptors. The model is based on the numerical solution of the charge neutrality equation and allows for calculating the Fermi energy in the temperature range of 1 –1000 K. The experimental studies of the electronic properties and concentrations of grown-in defect centers are performed by the high-resolution photoinduced transient spectroscopy (HRPITS). We show that at the shallow acceptor concentration below 1x10¹⁵ cm^-3 and the concentration of deep-level defects ~3x10¹⁵ cm^-3 obtaining undoped GaP with the semi-insulating (SI) properties is possible by substantial reducing the residual concentration of shallow donor impurities.

Abstract: Solid phase epitaxial regrowth (SPER) of amorphized layer in Czochralski grown silicon (Cz-Si) created by self-implantation (Si+ dose 2x1016 cm-2, energy 150 keV), subsequently annealed for 5 h at up to 1400 K under Ar pressure up to 1.4 GPa, was investigated by Secondary Ion Mass Spectrometry (SIMS) and X-ray methods. Annealing of Cz-Si:Si resulted in pressure-dependent SPER with a marked carbon and oxygen gettering within regrown region. Depth profiling of carbon and oxygen contaminants provides useful information concerning SPER in implanted single crystalline silicon.

Abstract: Low-temperature diffusion of Cr, Mo, Ni, Pd, Pt, and V in silicon diodes is compared in the range 450 - 800 oC. Before the diffusion, the diodes were implanted with high-energy He2+ to assess, if the radiation defects enhance the concentration of metal atoms at electrically active sites and what is the application potential for carrier lifetime control. The devices were characterized using AES, XPS, DLTS, OCVD carrier lifetime and diode electrical parameters. The metal atoms are divided into two groups. The Pt, Pd and V form deep levels in increased extent at the presence of radiation defects above 600 oC, which reduces the excess carrier lifetime. It is shown as a special case that the co-diffusion of Ni and V from a NiV surface layer results fully in the concentration enhancement of the V atoms. The enhancement of the acceptor level V-/0 (EC 0.203 eV) and donor level V0/+ (EC 0.442 eV) resembles the behavior of substitutional Pts. The second group is represented by the Mo and Cr. They easily form oxides, which can make their diffusion into a bulk more difficult or impossible. Only a slight enhancement of the Cr-related deep levels by the radiation defects has been found above 700 oC.

Abstract: The characteristics of the band-to-band Auger recombination in Czochralski-grown high resistivity Si and Ge single crystals have been studied using a contactless technique to measure excess carrier decay transients based on infrared absorption by free carriers. The measurements are performed using laser light excitation with wavelengths ranging from 1.2 to 2.5 µm to reduce inhomogeneity effects in the extraction of the Auger recombination parameters. A linear approximation of the initial excess carrier decay lifetime yields an approximate value of the Auger recombination coefficient in Ge γ_A,Ge ≈ 2×10^-31 cm⁶/s, which is close to that in Si. These characteristics also indicate that the difference in Auger recombination coefficients for the ehh and eeh processes is small. A more detailed fitting procedure applied simultaneously on a series of experimental transients yields a more accurate value of (8±3)×10^-31 cm⁶/s for the Auger recombination coefficient in Ge.

Abstract: A review of light soaking of solar cells by the use of commercial IV-characterization instruments is presented. The paper addresses the challenges of studying light induced degradation (LID) using a high intensity light source. Issues related to heating of the cell, temporal intensity instability and the impact of the irradiance spectrum are discussed. The main focus of the paper is devoted to the degradation of boron-doped Czochralski silicon (Cz-Si) where boron-oxygen related complexes are responsible for a metastable defect formation. Some advantages and limitations concerning the use of IV characteristics to reveal the degradation properties of boron-doped Cz-Si compared to applying minority carrier lifetime techniques are also presented.

Abstract: Instead of selective emitter technology we investigate an alternative way to optimize contact formation and increased blue responsivity of highly resistive emitter solar cells using screen print technology for the deposition of the frontside metallization grid. We show with the aid of an inline doping/diffusion set-up at Blue Chip Energy that tuning the emitter doping profile is an alternative way to reduce the effect of Auger recombination in the spectral range from 300 nm to 600 nm. By properly choosing the process conditions we were able to minimize the detrimental effect of the low surface concentration of the dopant on the contact resistance. Due to improved blue light responsivity a significant gain in short circuit current Jsc was achieved. This and a reduced reverse saturation current I00E yielded a higher open circuit voltage VOC and an increase of cell efficiency from 17.6 %-avg to more than 17.9 %-avg.