Ring defects in n-type Czochralski-grown silicon: A high spatial resolution study using Fourier-transform infrared spectroscopy, micro-photoluminescence, and micro-Raman
Summary (3 min read)
- J. Appl. Phys. 124, 243101 (2018); https://doi.org/10.1063/1.5057724 124, 243101 © 2018 Author(s).
- The authors also demonstrate that micro-Raman mapping reveals the oxygen precipitates without the smearing effects of carrier diffusion that are present in micro-PL mapping.
- During certain high-temperature treatments, Cz-Si wafers sometimes develop ring defects which influence the uniformity of the electrical properties of the wafer.
- 1,2 Ring defects are usually associated with the inhomogeneous distribution of oxygen precipitates and their associated extended defects.
- PL imaging yields lifetime images with a spatial resolution in the 10-100 μm range, and the relatively low injection level achieved during PL imaging results in significant image smearing due to the lateral diffusion of carriers, which typically extends many tens of micrometers.
A. Sample preparation
- The samples used in this work were quartered sections of 4-in.
- Diameter, 275 μm thick, phosphorus doped,a)E-mail: firstname.lastname@example.org electronic-grade Cz-Si wafers with a resistivity of 1Ω cm. Samples were divided into two different groups; Group I—for the FTIR measurements and Group II—for the micro-PL and micro-Raman mappings.
- Oxygen precipitates were grown intentionally by subjecting the samples from both groups to a low-high, two-step anneal process.
- After annealing, the Group II samples were chemically (HNO3:HF 10:1) etched to remove several micrometers of silicon and any denuded zone,21 before characterization.
B. FTIR scanning
- Radial distributions of [Oi] were measured by line scans before and after annealing, with a step size of 160 μm using a Bruker Vertex 80 FTIR microscope equipped with the liquid-nitrogen-cooled InGaAs detector.
- The FTIR line scans were conducted after removing oxide layers of annealed samples using a laser spot size of 10 μm and a spectral resolution of 4 cm−1.
- 22 During scanning, room temperature transmission measurements were performed in the range from 500 to 2500 cm−1.
- In order to extract the relative [Oi] from the corrected transmission spectra, a baseline value was constructed, being an expected transmission spectrum in the absence of interstitial oxygen.
- The uncertainties in the [Oi] measurements arise mainly from variations in the wafer thickness and overlapping of the interstitial oxygen peak and the oxygen precipitate band and are estimated to be approximately 10%.
C. PL imaging
- The pixel sizes of the PL images were approximately 162 μm and 23 μm for the standard and the magnifying lenses, respectively.
D. Micro-PL and micro-Raman mapping
- Both the micro-PL and the micro-Raman maps were obtained at room temperature using a Horiba LabRam HR Evolution system equipped with a confocal microscope.
- The samples were excited with a 532 nm continuous-wave laser focused onto a spot size of 1.2 μm in diameter with a 50× objective lens.
- At room temperature, the absorption depth of the 532 nm laser light is around 1.2 μm.24.
- The apparent precipitate density calculated from the micro-PL maps was expressed as an average areal density, to avoid the uncertainty of estimating the detection depth that would be required to estimate a volume density.
- Micro-Raman maps were obtained using a silicon charge coupled device (CCD) detector to detect the backscattered Raman light.
A. The loss in [Oi]
- Figure 1 presents the radial distributions of [Oi] in both the as-grown and the Group I (11 h annealed) samples, measured from the center to the edge of the samples.
- It shows that in the as-grown sample, the [Oi] distribution is very uniform along the radial direction.
- Note that the solubility limit of interstitial oxygen in silicon at 1000 °C in an oxygen ambient is around 9 × 1016 cm−3,11 significantly lower than the concentrations measured here, creating a strong driving force for precipitation.
- 26 Based on Voronkov’s theory,27 the type of intrinsic point defect (vacancies and silicon interstitial) in a crystal depends on the so-called V/G criterion (where V is the crystal growth rate and G is the radial temperature gradient).
- 1. Further, the authors observed mm-scale local fluctuations in the loss in [Oi] in the annealed sample.
B. The loss in [Oi] and carrier lifetime
- Figure 2 shows PL-based lifetime images of both the as-grown and the Group I samples, expressed as effective carrier lifetimes via calibration with a photoconductance coil.
- The as-grown sample shows a uniform and high average lifetime (τas-grown) of 1880 μs, as shown in Fig. 2(a).
- Note that the dark spots in this image are surface artifacts.
- Figure 3 shows the radial profiles of the loss in [Oi] and the inverse carrier lifetime for the Group I (11 h annealed) sample.
- 3. This result further supports the conclusion that the lifetime striations are due to the local fluctuations of oxygen precipitates originating from grown-in precipitate nuclei as explained in Sec. III A.
C. PL images, micro-PL maps, and apparent oxygen precipitate density
- Figure 4 shows the PL-based lifetime images of the as-grown sample with a standard lens and the Group II sample with both standard and magnifying lenses expressed as effective carrier lifetimes via calibration with a photoconductance coil.
- The mapped areas in the samples are indicated by red points in Fig. 4, although they are not to scale.
- 5. The recombination sites appear to be sub-micrometer sized and are closely spaced as a result; the conventional PL images were not able to resolve them.
- This observation of precipitate growth accentuates the potential value of the micro-PL technique for studying oxygen precipitate evolution.
- 20,37 Therefore, some of the nano-scale precipitates, which were undetected during the mapping of the 11 h annealed sample, might have grown to a detectable size after the additional 4 h of annealing, leading to the observed increase in the detected precipitate density.
D. The loss in [Oi] and apparent oxygen precipitate density
- Figure 7 presents the apparent areal densities of the oxygen precipitates obtained from the micro-PL maps at several locations in a radial direction and the loss in [Oi] calculated from the FTIR after 11 h of precipitate growth anneal.
- It shows no clear relationship between loss in [Oi] and apparent oxygen precipitate density.
- This could be due to detection limits of the micro-PL, with some of the nanoprecipitates remaining undetected during the mapping.
- Further, the micro-PL mapping is not able to detect the low recombination active oxygen precipitates due to the very high injection level employed here.
- Thus, there could be some smaller sized and unstrained/less-active oxygen precipitates which remained undetected in the micro-PL maps, causing the weak correlation observed in Fig.
E. Micro-Raman map and oxygen precipitates
- Figure 8 shows the micro-PL and the micro-Raman maps at the same location “B,” as shown in the PL image in Fig. 4(b).
- From these two maps, the authors observe two different phenomena—first, they observed several new oxygen precipitates as indicated by “X” in the micro-Raman map and second, oxygen precipitates were much better resolved in the micro-Raman map.
- The concentration of metallic impurities decorating particle “X” could be low or it might be less strained.
- Figure 8(c) presents the line scans of the Raman intensity at 520 cm−1 and the PL intensity at 1130 nm, at the same precipitates as indicated by dotted arrows in Figs. 8(a) and 8(b).
- To obtain a high signal-to-noise ratio and maintain a high spatial resolution (0.5 μm in this work), relatively long acquisition times in the order of 1-5 s per pixel are required.
- The authors have presented a high spatial resolution investigation of ring defects in two-step annealed Cz-Si wafers using a combination of FTIR scanning microscopy, micro-PL mapping at high injection conditions, and micro-Raman mapping.
- The authors observed a direct local correlation between losses in [Oi] obtained from scanning FTIR measurements and the inverse lifetime extracted from PL images in a two-step annealed sample.
- Further, this paper demonstrated the use of micro-PL mapping to investigate ring defects.
- The authors results demonstrate that the ring defects or lifetime striations are indeed due to the cumulative effect of individual oxygen precipitates.
- Furthermore, the micro-Raman maps confirm that besides the precipitates detected in the micro-PL maps, there are in fact smaller and/or less-active precipitates not revealed by the micro-PL technique due to the carrier smearing.
Did you find this useful? Give us your feedback
Cites background or methods from "Ring defects in n-type Czochralski-..."
...The methods featuring a higher spatial resolution, such as micro-Raman, might be capable of detecting an additional share of small precipitates  and, thus, could help to understand the evolution of small precipitates....
... and , and OP density (EPD) on these wafers....
Related Papers (5)
Jonas Haunschild, Isolde E. Reis +2 more
Eddy Simoen, Corneel Claeys +6 more
B. Schuller, R. Carius +1 more