Larry Wang

An improved mathematical modeling to simulate metallization screen pattern trend for silicon solar cell

The series resistance calculation based on H-type model has been widely used to understand power loss mechanism in semiconductor solar cell, and one of its popular applications is to predict optimal metallization screen printing pattern. The disadvantage of this model is that it is not accurate to estimate the relationship of screen pattern trend vs. cell efficiency, since it is based on an ideal rectangular finger shape, which does not exist in present regular screen printing process. In this work, a revised model is proposed to simulate the contact resistant power loss in multi-crystalline silicon solar cell. Improved accuracy of the new model is found by comparing the simulation results to the I-V testing data of multi-crystalline silicon solar cell printed with Heraeus 96XX front Ag paste.

SurfaceSIMS , Secondary Ion Mass Spectrometry Using Oxygen Flooding: A Powerful Tool for Monitoring Surface Metal Contamination on Silicon Wafers

Secondary ion mass spectroscopy (SIMS) coupled with oxygen flooding of the silicon surface during analysis provides an analytical technique capable of detecting ≤10 10 atoms/cm 2 of many surface elemental contaminants. Of particular importance to meet the future needs of the semiconductor industry is the current ability to detect Al and Fe contamination at a level of 2×10 9 atoms/cm 2 .

Selective and homo emitter junction formation using precise dopant concentration control by ion implantation and microwave, laser or furnace annealing techniques

We investigated phosphorus and boon implanted emitter and selective emitter junction formation comparing; 1) 15keV to 30keV implant energies, 2) implant dopant dose concentration between 3E14/cm2 to 1E16/cm2 and 3) various anneal conditions from high temperature (>;1407°C) laser melt annealing to low temperature (<;500°C) microwave annealing and furnace anneals between 750°C to 1050°C for dopant activation and diffusion. By engineering and optimizing dopant concentration with anneals we could realize homo emitter and selective emitter junctions from 0.25um to 1.5um depth with sheet resistance from 9Ω/□ to 2200Ω/□ and peak surface dopant electrical activation levels from 4E18/cm3 up to 5E20/cm3. Highest dopant activation efficiency was achieved with liquid phase junction diffusion formation method using laser melt annealing and was limited by the dopant source concentration if <;E16/cm2. The POCl3 dopant source concentration of 1E16/cm2 was only 15% efficient with furnace solid phase diffusion activation while laser melt liquid phase diffusion activation was 45% compared to implant which was 35% active with solid phase diffusion and 100% active with liquid phase diffusion.

Accurate CIGS composition measurements using surface analytical techniques

Conversion efficiency for Cu(Inx, Ga1−x)Se2 is dependent on a number of factors including band-gap and defect structures. Material composition affects band gap and it affects the formation of defect structures. A fundamental understanding of the relationship between material composition, band gap and defect structures requires that the composition measurement be accurate. Accuracy is required for really explaining how and why the cells function. Why certain defect levels form as a function of concentration, and how the influence of defects is mitigated by compensation from other defects, also dependent on the composition. [1] In this work we will look at the accuracy of a number of analytical techniques. We evaluate strengths and limitations of each for reporting useful information on thin film CIGS materials and evaluate them for accuracy in reporting CIGS composition.

SIMS Study of C, O and N Impurity Contamination for Multi-Crystalline Si Solar Cells

This paper is a case study of using SIMS to quantitatively measure C, O and N impurity contamination at two sequential commercial process steps: (1) Si feedstock: 7N (modified Siemens) and 5N feedstock (UMG-Si); and (2) multi-crystalline (mc-Si) solar wafers: cut and etched, from directional solidification bricks grown from 7N and 7N/5N (80:20) feedstock. The conclusion of this study is twofold: (a) the primary opportunity to reduce C, O and N contamination in mc-Si solar cells is at the directional solidification process, and (b) the costly specification of highly pure Si feedstock is unnecessary from a C, O and N perspective if a directional solidification process is used.

Quantitative Measurement of Boron and Phosphorus in Solar Grade Silicon Feedstocks by High Resolution Fast-Flow Glow-Discharge Mass Spectrometry

The calibration factors are examined for determination of boron (B) and phosphorus (P) in solar grade silicon samples in the new generation of high resolution fast-flow glow-discharge mass spectrometers (FF-GDMS). It is shown that using the generalized calibration factors from the Standard RSF table, the relative errors observed in the determination of these analytes is not acceptably small for photovoltaic applications. The certified B and P values in NIST SRM 57a Si metal sample do not have the confidence intervals, which would be adequate for refining these calibration factors to the acceptable levels. Thus, well-characterized single crystalline Si wafers with known boron and phosphorus contents traceable to NIST reference materials (SRM 2133 and SRM 2137) were used for accurate calibrations and for establishing good analytical procedures for measurements of wide variety of Si sample forms. The obtained results for these important analytes using the new FF-GDMS procedure are compared to other characterization techniques commonly used in this industry.

SIMS Analysis of Nitrogen in Silicon Carbide Using Raster Change Technique

Today’s state of the art silicon carbide (SiC) growth can produce semi-insulating crystals with a background doping around 5×1015 atoms/cm3 or lower. It is essential to have an accurate measurement technique with low enough detection limit to measure low level nitrogen concentration. Current SIMS detection limit of low E15 atoms/cm3 will provide accurate determination for nitrogen doping level of 5E16 at/cm3 or higher. In order to determine the lower nitrogen concentration, it is necessary to provide a better detection limit and to separate the contribution of background nitrogen properly. The “raster changing” method provides an accurate way to determine and remove contribution of background nitrogen to the signal, because secondary ion intensities and matrix ion intensities can be analyzed at the same location of the sample by changing the primary beam raster size during a profile. In this study we have succeeded in applying the raster changing method to (a) N in the SiC substrate located under an SiC epi layer, and (b) the detection of N as low as 3E14/ cm3 a bulk-doped SiC substrate.