Solid State Phenomena Vols. 108-109

Abstract: The paper describes the elaboration of a method for producing composite Si/SiC wafers and investigation of their properties. The known two-shaping elements (TSE) method was used to produce the material. Pilot tests show that this composite material can be used for production of solar cells. The structure of silicon grains is elongated relative to the growth direction, the dislocation density in grains is of about (5÷8) ×104 cm-2, the average lifetime of minority carriers is 4÷6 µs.

Abstract: An experimental approach for fast crack detection and length determination in fullsize solar-grade crystalline silicon wafers using a Resonance Ultrasonic Vibrations (RUV) technique is presented. The RUV method is based on excitation of the longitudinal ultrasonic vibrations in full-size wafers. Using an external piezoelectric transducer combined with a high sensitivity ultrasonic probe and computer controlled data acquisition system, real-time frequency response analysis can be accomplished. On a set of identical crystalline Si wafers with artificially introduced periphery cracks, it was demonstrated that the crack results in a frequency shift in a selected RUV peak to a lower frequency and increases the resonance peak band width. Both characteristics were found to increase with the length of the crack. The frequency shift and bandwidth serve as reliable indicators of the crack appearance in silicon wafers and are suitable for mechanical quality control and fast wafer inspection.

Abstract: Changes in the concentration of interstitial iron in multicrystalline silicon wafers after high temperature annealing (900°C) have been monitored by carrier lifetime measurements. Two cooling rates were investigated. The first was considered ‘fast’, meaning the interstitial Fe had no time to diffuse to precipitation sites, and should therefore be frozen-in, despite being far above the solubility limit at lower temperatures. A second ‘slow’ cool down to 650°C allowed ample time for the Fe to reach the surfaces or other internal precipitation sites. Surprisingly, in both cases the Fe remained in a supersaturated state. This indicates the precipitation process is not diffusion-limited, and that another energetic barrier to precipitate formation must be present. Since the slow cooling used here is similar to the cooling rate experienced by multicrystalline ingots after crystallisation, this precipitate-impeding mechanism is probably responsible for the surprisingly high interstitial Fe concentrations often found in as-grown multicrystalline silicon wafers.

Abstract: The structural and electrical properties of as grown multicrystalline (mc) solar silicon have been characterized with special emphasis on the ingot's edge regions. For this purpose a vertical cross section of an mc-Si Bridgman ingot was investigated by Fourier transform infrared spectroscopy (FTIR), laser scattering tomography (LST), lateral photovoltage scanning (LPS), infrared microscopy and microwave detected photoconductivity decay (&PCD). Images of the distribution of dislocations, grain boundaries, precipitates, impurities (O, N, C) and the minority charge carrier lifetime were obtained, partly differentiating the defects by their electrical activity. In particular the LPS method displays dopant striations indicating the shape of the phase boundary. Deviations of the phase boundary from a slightly convex shape in the middle of the ingot to a concave one in the vicinity of the side walls could be observed. The existence of an horizontal temperature gradient deduced from this shape is the reason for convection in the melt. The influence on the concentration profiles of interstitial oxygen and the correlation with the minority charge carrier lifetime are discussed.

Abstract: An overview is given of analytical techniques for the characterization of the electrical and transport parameters in thin (<1 µm) semiconductor layers. Some of these methods have been applied to the lifetime and diffusion length study in thin strain-relaxed buffer (SRB) layers of strained silicon (SSi) substrates, while a second group was dedicated to Silicon-on-Insulator (SOI) materials and devices. The employed techniques can be divided into two groups, whether a device structure (junction, MOS capacitor, MOSFET) is required or not. However, the MicroWave Absorption (MWA) technique can be used in both cases, making it a versatile tool to study both grown-in and processing-induced electrically active defects. The transport properties of SSi wafers are strongly determined by the density of threading and misfit dislocations, although the dependence of the recombination lifetime is weaker than expected from simple Shockley-Read-Hall (SRH) theory. This is related to the high injection regime typically employed, enabling the characterization of the 250-350 nm thick Si1-xGex layer only. At longer carrier decay times, multiple trapping events dominate that can be described by a stretched exponent approach, typical of disordered materials. For SOI substrates, transistor-based techniques will be demonstrated that enable to assess the generation or recombination lifetime in the thin silicon film (<100 nm). The lifetime can be severely degraded by irradiation or hot-carrier degradation. Finally, it will be shown that Generation-Recombination (GR) noise spectroscopy as a function of temperature allows identifying residual ion-implantation-damage related deep levels, which are otherwise hard to detect even by Deep Level Transient Spectroscopy (DLTS).

Abstract: P-n junctions are created in p-type Czochralski silicon after a low temperature (270°C) hydrogen plasma exposure. This is attributed to the formation of hydrogen-related shallow donors. A deep level (E1) with an activation energy of about EC-0.12 eV is observed by DLTS measurement and assigned to a metastable state of the hydrogen-related shallow donors. At an annealing temperature of 340°C, the E1 centres disappear and oxygen thermal donors appear. The concentrations of the oxygen thermal donors are found typically to be 2-3 decades lower than that required for over-compensating the initial p-type doping and for contributing the excess free carriers.

Abstract: In this study we have investigated the thermal stability (in the range 100 oC - 900 oC) of defects introduced in p-Si by electron beam deposition (EBD) of Ti and Ti/Mo Schottky contacts. The depletion regions below these contacts were probed by conventional deep level transient spectroscopy (DLTS) as well as Laplace (high-resolution) DLTS (L-DLTS). We have chosen Ti as the Schottky contact because the barrier height of Ti/p-Si (0.53 eV) is close to that of TiSi2/p-Si (0.50 eV) that forms after annealing at 600 – 650 oC. The Mo was added on top of the Ti in order to prevent annealing degradation. These contacts were annealed in Ar at temperatures of up to 900 oC in 100 oC steps for half-hour periods. Current – voltage (I-V) and capacitance – voltage (C-V) measurements were used to monitor the quality of the Schottky contacts. DLTS was performed after each annealing cycle to monitor the presence of the EBD-induced defects and to obtain heir electronic properties. We have found that that the Ti/Mo contacts were superior to the Ti contacts. Their (Ti/Mo) barrier height after EBD was 0.52 eV and it gradually increased to 0.56 eV after annealing at 500 oC - 600oC and then dropped to 0.50 eV annealing at 700 oC. DLTS revealed that the main defects introduced during metallization are hole traps H(0.17), H(0.23), H(0.37) and H(0.49). Annealing at 350 oC introduced an additional hole trap H(0.39). After annealing at 550 oC all defects were removed from the depletion region.