Browse Prior Art Database

The impact of annealing steps on minority carrier lifetime of nitrogen co-doped FZ-crystals and correlating DLTS-defect-levels

IP.com Disclosure Number: IPCOM000245241D
Publication Date: 2016-Feb-19
Document File: 8 page(s) / 2M

Publishing Venue

The IP.com Prior Art Database

Abstract

Minority carrier lifetime ("MCL") is a key parameter to characterize the suitability of silicon wafers for solar cell use. High MCLs are a prerequisite to reach high efficiencies, in particular for cell processes aiming for highest efficiencies (PERC, HJT, BJBC etc.). Monocrystalline wafers produced from the crucible-less float zone ("FZ") method reach highest MCL-levels, and are hence a technically excellent base material for high efficiency solar cell processes. Commercially available FZ wafers are typically nitrogen co-doped to suppress the generation of crystal defects during crystal pulling. For any application of FZ wafers with thermal treatments, it must be noted that the minority carrier lifetime (MCL) of nitrogen co-doped FZ wafers shows a strong dependency on annealing processes: 1. annealing at temperatures below ~900°C degrades the "as grown" lifetime, starting from ~300°C, with the strongest effect at ~450-700°C. It does not matter whether this takes place in oxidative or non-oxidative furnace atmosphere 2. annealing at ~900°C or higher does not degrade the lifetime (the minor degradation visible in Graph1 is caused by slip line generation and boat marks during the furnace process) 3. annealing at ~900°C or higher can re-install the originally high lifetime of samples that had been degraded at ~300-900°C before 4. samples that had seen this >~900°C annihilation step remain widely immune against subsequent annealing at lower temperatures <~900°C, means their lifetime does not notably degrade any more Effect 4 shall be taken into account when designing PV cell processes for FZ-material. When applying a higher temperature step >~ 900°C (diffusion for example), the MCL of the FZ wafer will hardly be impacted by subsequent lower temperature treatments. One could also consider to anneal whole ingot blocks before slicing for defect annihilation (in case the annihilation is not based on in/out-diffusion processes), or to use pre-annealed/annihilated wafers as starting material for the PV cell process. Effect 1 shall be taken into account for semiconductor application: FZ wafers with poly backside typically provide a low lifetime. This is because the poly-deposition takes place at temperatures around 650°C. Annealing >~900°C will re-install the originally high lifetime of the as grown crystal. To understand the origin of these defects, DLTS studies have been performed. Depending on annealing temperature, the formation or annihilation of several deep levels can be observed. The appearance and removal of these peaks during annealing well correlates with the MCL behavior. The DLTS-results of n-type crystals with and w/o nitrogen (in the vacancy- as well as in the interstitial-region on the wafer) suggest that both nitrogen and vacancies play a role in the formation of the observed deep levels.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 31% of the total text.

Page 01 of 8

The impact of annealing steps on minority carrier lifetime of nitrogen co-doped FZ-crystals and correlating DLTS-defect-levels

Authors:

Andrej Lenz, now WACKER Chemie AG Burghausen (1996-2009 Siltronic AG) Alois Huber, Siltronic AG Burghausen

Abstract:

Minority carrier lifetime ("MCL") is a key parameter to characterize the suitability of silicon wafers for solar cell use.

High MCLs are a prerequisite to reach high efficiencies, in particular for cell processes aiming for highest efficiencies (PERC, HJT, BJBC etc.).

Monocrystalline wafers produced from the crucible-less float zone ("FZ") method reach highest MCL-levels, and are hence a technically excellent base material for high efficiency solar cell processes.

Commercially available FZ wafers are typically nitrogen co-doped to suppress the generation of crystal defects during crystal pulling.

For any application of FZ wafers with thermal treatments, it must be noted that the minority carrier lifetime (MCL) of nitrogen co-doped FZ wafers shows a strong dependency on annealing processes:

1. annealing at temperatures below ~900°C degrades the "as grown" lifetime, starting from ~300°C, with the strongest effect at ~450-700°C. It does not matter whether this takes place in oxidative or non-oxidative furnace atmosphere

2. annealing at ~900°C or higher does not degrade the lifetime (the minor degradation visible in Graph1 is caused by slip line generation and boat marks during the furnace process)

3. annealing at ~900°C or higher can re-install the originally high lifetime of samples that had been degraded at ~300-900°C before

4. samples that had seen this >~900°C annihilation step remain widely immune against subsequent annealing at lower temperatures <~900°C, means their lifetime does not notably degrade any more

Effect 4 shall be taken into account when designing PV cell processes for FZ-material. When applying a higher temperature step >~ 900°C (diffusion for example), the MCL of the FZ wafer will hardly be impacted by subsequent lower temperature treatments. One could also consider to anneal whole ingot blocks before slicing for defect annihilation (in case the annihilation is not based on in/out-diffusion processes), or to use pre-annealed/annihilated wafers as starting material for the PV cell process.

Effect 1 shall be taken into account for semiconductor application:


FZ wafers with poly backside typically provide a low lifetime. This is because the poly- deposition takes place at temperatures around 650°C. Annealing >~900°C will re-install the originally high lifetime of the as grown crystal.

To understand the origin of these defects, DLTS studies have been performed. Depending on annealing temperature, the formation or annihilation of several deep levels can be observed. The appearance and removal of these peaks during annealing well correlates with the MCL behavior.

The DLTS-results of n-type crystals with and w/o nitrogen (in the vacancy- as well as in the inter...