Grown-in Defects Research Articles

Crystal Originated Defect Monitoring and Reduction in Production Grade SmartSiC™ Engineered Substrates

Power devices electronics based on Silicon Carbide (SiC) are emerging as a breakthrough technology for various applications. The link between the quality of SiC substrates and device performance has been widely discussed [1]. Smart Cut™ technology offers the opportunity to integrate a high quality SiC layer on a low resistivity handle wafer. Moreover the crystal quality of a single donor wafer can be replicated multiple times to provide an epitaxy-ready substrate in high volume [2]. Nevertheless, some extended grown-in defects of SiC starting material, like micro-pipes or bulk inclusions, may generate surface defects called "Crystal Originated Defects" (COD) on transferred layers. This paper explains how SmartSiC™ defect density can be reduced by limiting the number of extended defects on donor wafers. Specific inspection recipes were developed to monitor the starting material and the replicated engineered substrate: COD root-causes and effects were analyzed. We demonstrated how a well-suited quality control of donor wafers plays a major role to guarantee defect-free SmartSiC™ wafers.

Unveiling the Effect of Cooling Rate on Grown-in Defects Concentration in Polycrystalline Perovskite Films for Solar Cells with Improved Stability.

Numerous efforts are devoted to reducing the defects at perovskite surface and/or grain boundary; however, the grown-in defects inside grain is rarely studied. Here, the influence of cooling rate on the point defects concentration in polycrystalline perovskite film during heat treatment processing is investigated. With the combination of theoretical and experimental studies, this work reveals that the supersaturated point defects in perovskite films generate during the cooling process and its concentration improves as the cooling rate increases. The supersaturated point defects can be minimized through slowing the cooling rate. As a result, the optimized FAPbI3 polycrystalline films achieve a superior carrier lifetime of up to 12.6 µs and improved stability. The champion device delivers a 25.47% PCE (certified 24.7%) and retain 90% of their initial value after >1100 h of operation at the maximum power point. These results provide a fundamental understanding of the mechanisms of grown-in defects formation in polycrystalline perovskite film.

Numerical and experimental investigation of the effect of the solid–liquid interface shape on grown-in defects in a silicon single crystal

The demand for grown-in defect-free wafers for high-performance silicon semiconductor devices is significantly increasing. To obtain defect-free crystals, the ratio v/G of the growth rate v to the temperature gradient G in the growth direction near the solid–liquid interface during crystal growth must be kept at a critical value ξcri. Furthermore, ξcri depends on the interface shape, which is interpreted as the change in the diffusion direction of point defects due to the change in the interface shape. In this study, we present a new interpretation based on simulations and experiments, in which ξcri changes due to the effect of the diffusion direction of point defects when the solid–liquid interface shape is convex, as in the conventional interpretation, as well as the effect of thermal stress when it is concave.

Formation of Grown-In Nitrogen Vacancies and Interstitials in Highly Mg-Doped Ammonothermal GaN.

The formation of intrinsic point defects in the N-sublattice of semi-insulating Mg-doped GaN crystals grown by the ammonothermal method (SI AT GaN:Mg) was investigated for the first time. The grown-in defects produced by the displacement of nitrogen atoms were experimentally observed as deep traps revealed by the Laplace transform photoinduced transient spectroscopy in the compensated p-type crystals with the Mg concentrations of 6 × 1018 and 2 × 1019 cm-3 and resistivities of ~1011 Ωcm and ~106 Ωcm, respectively. In both kinds of materials, three closely located traps with activation energies of 430, 450, and 460 meV were revealed. The traps, whose concentrations in the stronger-doped material were found to be significantly higher, are assigned to the (3+/+) and (2+/+) transition levels of nitrogen vacancies as well as to the (2+/+) level of nitrogen split interstitials, respectively. In the material with the lower Mg concentration, a middle-gap trap with the activation energy of 1870 meV was found to be predominant. The results are confirmed and quantitatively described by temperature-dependent Hall effect measurements. The mechanism of nitrogen atom displacement due to the local strain field arising in SI AT GaN:Mg is proposed and the effect of the Mg concentration on the charge compensation is discussed.

Numerical and experimental investigation of effect of oxygen concentration on grown-in defects in a Czochralski silicon single crystal

Grown-in defect-free wafers are required in silicon semiconductor devices. A point defect concentration simulation was performed along with an experimental investigation, demonstrating a wide range of oxygen concentrations from 1.6 × 1017 to 9.1 × 1017 cm−3 in crystals. Thus, the effect of oxygen atoms in a Czochralski silicon single crystal with grown-in defect behavior was revealed. Consequently, the increasing vacancy concentration trapped by the oxygen atom (oxygen coefficient) was estimated as 4.61 × 10−5 per oxygen atom. Previously, for obtaining the oxygen coefficient, a regression equation assuming thermal equilibrium concentrations of vacancy (V) and interstitial Si (I) was applied to the experimental results. However, the interface shape, thermal stress, and hot-zone structure of the experimental level needed to be arranged; this affected the grown-in defect behavior. In this study, the oxygen coefficient and thermal equilibrium concentration of V and I were determined uniquely without arranging the situations experimental level.

Development of High Quality 8 Inch 4H-SiC Substrates

8 inch 4H-silicon carbide (SiC) development faces challenges first from obtaining high-quality 8 inch SiC seed substrate, then reducing grown-in crystal residual stress and defects in the following crystal growth process. Here we report the diameter expansion process from 6 inch 4H-SiC seed substrate to 8 inch 4H-SiC crystal. Based on simulation and experimental results, it is deduced that an optimized radial temperature gradient (RTG) zone in the range of 0.10-0.12 °C/mm is essential for high-quality and efficient SiC crystal diameter expansion. According to the RTG calculation, diameter expansion process is designed and 8 inch 4H-SiC crystal as well as seed substrate is achieved. With the obtained seed substrate, high-quality 8 inch 4H-SiC crystal is developed and the following polished 4H-SiC substrate quality is characterized.

Pulling thin single crystal silicon wafers from a melt: The new leading-edge solar substrate

The Floating Silicon Method (FSM) has been established as a viable, stable method for growing single crystal ribbons directly from a silicon melt. With intense helium jet cooling to drive the linear progress of a [111] facet, pulled in the 〈110〉 direction, ribbons in the 0.6 – 3.0 mm thickness range can be grown at linear growth rates from 0.3 mm/s to >6 mm/s as reported in the literature. We report on recent progress towards growing (100) oriented ribbons with a net thickness of <200 µm and a ribbon width up to 18 cm using a stable, continuous process in the Leading Edge prototype furnace. The 3D details of the single crystal growth are explained using the mechanics of the Limit Cycle Theory, with novel Internal Side Effect morphology described by a proposed Facet Flow Theory. Grown-in crystalline defect distributions are described as well as values of critical impurities like oxygen, carbon, dopants, and metals that are relevant for use as wafers for solar cells.

Positron lifetime spectroscopy of defect structures in Cd1–x Zn x Te mixed crystals grown by vertical Bridgman–Stockbarger method

Positron annihilation lifetime spectroscopy was used to examine grown-in defects in Cd1–x Zn x Te mixed crystals as a function of Zn content (x = 0, 0.07, 0.11, 0.49, 0.9, 0.95, 1) and measuring temperature. All samples were prepared using the high-pressure modified vertical Bridgman–Stockbarger method. The crystal structure and material phase were characterized by X-ray diffraction. The positron lifetime spectra reveal the presence of both open volumes and shallow traps regardless of the sample composition. In particular, both average and bulk lifetimes are found to be much higher in ternary alloys (CdZnTe) than those in binary systems (CdTe and ZnTe). This originates from distinct differences in average electron densities and the nature of open-volume defects between binary and ternary samples. Competition in positron trapping with increasing Zn content is observed between defects characteristic for both structural systems. Moreover, a clear correlation is shown between defects and the lattice thermal conductivity of studied samples. The applicability of the positron trapping model to CdTe-based materials is discussed.

Characterization of grown-in defects in Si wafers by gas decoration

In this paper, a vapor gas etching method is developed to systematically characterize grown-in defects such as crystal originated particles (COPs), oxygen precipitates (OPs) and dislocations in 300 mm boron-doped (001) Czochralski (CZ) silicon wafers. For a given type of defect, an unique morphology reveals after selective etching by hydrogen chloride (HCl) at 1000 °C. The etching mechanism is discussed and the characteristic morphology of bulk defects is investigated by localized light scattering (LLS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). Analyzing the data obtained by these techniques, it can be found that the defect types could be differentiated by light scattering signal, i.e., latex sphere equivalent (LSE) size, and defect regions could be divided by the etch pit density. Moreover, due to the effect of HCl etching, the OP as small as several nanometers can be measured directly by SEM and AFM. This study provides Si industry with a fast and comprehensive defect delineation method.

Modification of the critical v/G of the Voronkov’s theory on the grown-in defects in Si crystals

Voronkov developed a theory on the grown-in defects in Si crystals. One of the most important properties deduced from the theory is the critical value of v/G, (v/G)crit, where high-quality crystals can be grown. According to Voronkov’s explanation on the concentration gradients, we proposed a simple expression of (v/G)crit, which is proportional to the concentration gradient. We examined pair annihilation of vacancies and interstitials with two physical models, namely, a partial annihilation and a free-energy-minimum. Both were deduced from free energy considerations.

Gallium vacancies—common non-radiative defects in ternary GaAsP and quaternary GaNAsP nanowires

Nanowires (NWs) based on ternary GaAsP and quaternary GaNAsP alloys are considered as very promising materials for optoelectronic applications, including in multi-junction and intermediate band solar cells. The efficiency of such devices is expected to be largely controlled by grown-in defects. In this work we use the optically detected magnetic resonance (ODMR) technique combined with photoluminescence measurements to investigate the origin of point defects in Ga(N)AsP NWs grown by molecular beam epitaxy on Si substrates. We identify gallium vacancies, which act as non-radiative recombination centers, as common defects in ternary and quaternary Ga(N)AsP NWs. Furthermore, we show that the presence of N is not strictly necessary for, but promotes, the formation of gallium vacancies in these NWs.

The Role of Grown-In Defects in Silicon Minority Carrier Lifetime Degradation During Thermal Treatment in Epitaxial Growth Chambers

Severe silicon lifetime degradation was found after its high-temperature treatment in III–V material growth chambers for the fabrication of III–V/Si multijunction solar cells. Further improvement of the cell efficiency requires insights into the root cause of such lifetime degradation and how to protect the silicon lifetime accordingly. While the exact origins of such degradation remain largely unknown, most published work concluded that extrinsic impurities that diffuse into the silicon bulk during the thermal treatment are the sole reason. In this article, we show that while not necessarily present in every float-zone silicon wafer, grown-in defects that can be thermally activated is also a key mechanism behind the observed silicon lifetime degradation during anneal in our molecular beam epitaxy chamber. As such, annealing of the silicon wafer in the furnace at 1000 °C to annihilate the grown-in defects, together with the deposition of a SiNX diffusion barrier to prevent the extrinsic impurities from diffusing into the silicon bulk, are both required to preserve the silicon lifetime throughout the III–V material growth steps.

Secondary Phase Particles in Cesium Lead Bromide Perovskite Crystals: An Insight into the Formation of Matrix-Controlled Inclusion.

The metal halide perovskite CsPbBr3 bulk crystals present electrical and optical performance discrepancy since the grown-in defects. Here, we first report the well-defined secondary phase (SP) particles of CsPb2Br5 with polyhedral morphology in CsPbBr3 crystals grown by the vertical Bridgman method. The resulting polyhedral morphology of CsPb2Br5 particles associated with the trapping of PbBr2-rich droplets have been discussed on the basis of the "matrix-controlled" growth. Two morphological evolution paths are proposed, which result in a regular cube SP particle comprised by {100} facets for the final equilibrium. Furthermore, the wafer with superior optical transmittance exhibits a higher photoelectric response on-off ratio (∼2000) in contrast to ∼80 for the wafer with high density SP particles. The corresponding hole mobility (μh) is calculated with the values 289.99 and 26.91 cm-2·V-1·s-1, respectively. The variation of μh is attributed to the carrier transport trajectory affected by SP induced trapping defects and the weak combination.

Analyzing the ‘non-equilibrium state’ of grain boundaries in additively manufactured high-entropy CoCrFeMnNi alloy using tracer diffusion measurements

Additively manufactured (AM) metallic materials contain typically numerous grown-in defects which limit durability and mechanical properties of workpieces. However, the existence and impact of non-equilibrium vacancies and dislocations and especially the state of grain boundaries in AM materials remained completely unexplored. Here we are presenting an ultimate proof of a ‘non-equilibrium state’ of general high-angle grain boundaries in an as-produced AM high-entropy CoCrFeMnNi alloy. A low-temperature annealing treatment relaxes the ‘non-equilibrium’ grain boundary state without invoking grain growth. The ‘non-equilibrium’ state of grain boundaries in AM materials resembles that in severely plastically deformed ones and needs to be taken into account for technological applications. We further report a strong microstructure-induced anisotropy of grain boundary kinetic properties in the AM high-entropy alloy.

Crystal growth, finite element analysis and oxygen annealing of ZnO bulk crystal grown by low-temperature-gradient chemical vapor transport

The growth of high-quality ZnO bulk crystals is an outstanding challenge, and the nature of grown-in defects in crystals is far from well understood. In this work, a specially designed low-temperature-gradient CVT growth system was developed based on finite element analysis. High-quality ZnO single crystals were successfully grown. The supersaturation front in the growth system was controlled, and internal stress in the crystal was reduced by introducing a radiation baffle and a special seed pedestal. The structural, electrical, and optical properties of ZnO crystals were studied before and after oxygen annealing. The scanning electron microscope (SEM) and optical microscope (OM) results indicate the stable growth is achieved under the Zn-rich growth condition. The X-ray rocking curve measurements reveal the high crystalline quality of the grown crystal with the full-width at half-maximum (FWHM) value of 17 arcsec for the (004) reflection. The time-of-flight secondary ion mass spectroscopy (ToF-SIMS) results prove the high purity of grown crystals and reveal the variations of trace impurity concentrations in crystals. When annealed in oxygen at 1000 °C, the optical band gap (Eg) changes from 2.50 eV to 3.15 eV. The Raman analysis indicates that oxygen vacancies (Vo) in the as-grown crystal disappear after annealing. The Hall measurements show that the room temperature (RT) mobilities before and after annealing are 197 cm2V−1c−1 and 207 cm2V−1c−1, respectively. The RT carrier concentration decreases by about two orders of magnitude to 5.45 × 1015 cm−3, while the RT resistivity increases by about one order of magnitude to 5.57 Ω cm after annealing. The positron annihilation lifetimes (PALS) results prove the presence of Zn vacancies (Vzn) after annealing. The origins of as-grown ZnO green luminescence (GL) and the temperature-dependent blue shifts of the GL peak positions are explained based on Zni and Vo defects. The fine structures of annealed ZnO GL are found to be related to Vzn native defects rather than Cu impurity.

Formation of thermal donor enhanced by oxygen precipitation in silicon crystal

The formation of thermal donors in silicon is investigated using Czochralski silicon crystals grown with different grown-in defect regions, such as voids and nuclei of oxidation-induced stacking faults. It was found that the formation rate of thermal donors during annealing at 450 °C increases with an increase in the density of oxide precipitates in the regions containing different grown-in defects. The thermodynamic model for the formation of thermal donors shows that the electrically inactive oxygen trimers as nuclei of thermal donors in an as-grown crystal increase with an increase in the density of oxide precipitates, suggesting that the formation of the nuclei is enhanced due to silicon self-interstitials emitted by oxygen precipitations during the crystal growth.

Effect of thermal stress on point defect behavior during single crystal Si growth

Silicon devices currently require silicon wafers that are free of grown-in defects. Point-defect simulation for silicon crystal growth, which is performed using the advection-diffusion equation considering the pair annihilation of vacancies and self-interstitials, is improved in this study by considering the effect of thermal stress during the growth process. This effect is introduced into the point-defect simulation as the stress term of the formation enthalpy. The stress coefficients in the stress term are determined by analyzing the correlation between the grown-in defect patterns obtained from the actual pulling tests and simulations. The defect patterns obtained from the simulations performed using the improved point-defect simulation method in this study were congruent with the experimental results, particularly for crystals with large diameters and high stress. The improved simulation method proposed in this study shall be useful for designing new hot-zone conditions and enhancing the understanding of point-defect behavior.

Impact of pre-fabrication treatments on n-type UMG wafers for 21% efficient silicon heterojunction solar cells

Silicon heterojunction solar cells achieve high conversion efficiency due to the excellent surface passivation provided by the hydrogenated intrinsic amorphous silicon films. However, they require a high-quality wafer as a starting material because their low-temperature processing does not allow for gettering. Czochralski-grown upgraded metallurgical-grade (UMG-Cz) silicon is a low-cost alternative to electronic-grade silicon for silicon solar cells, but is often limited in lifetime by grown-in defects. We have previously shown that pre-fabrication treatments, namely tabula rasa, phosphorus diffusion gettering, and hydrogenation, can significantly improve the bulk quality of UMG-Cz wafers. These help to mitigate the impact of grown-in oxygen precipitate nuclei and metallic impurities. In this work, we fabricate rear-junction silicon heterojunction solar cells on both as-grown and pre-treated UMG-Cz and electronic-grade wafers. We show that pre-fabrication treatments have a marked impact on solar cell efficiencies. With pre-fabrication treatment, the efficiency improves from 18.0% to 21.2% for the UMG-Cz cells and 21.2%–22.7% for the electronic-grade cells. Comparison of the open-circuit voltages of the as-grown and pre-treated UMG-Cz and electronic-grade cells using Quokka simulations reveals that the bulk lifetime remains the primary limiting factor for the UMG-Cz wafers.

On the structure of defects in the Fe7Mo6 μ-Phase

Topologically close packed phases, among them the μ-phase studied here, are commonly considered as being hard and brittle due to their close packed and complex structure. Nanoindentation enables plastic deformation and therefore investigation of the structure of mobile defects in the μ-phase, which, in contrast to grown-in defects, has not been examined yet. High resolution transmission electron microscopy (HR-TEM) performed on samples deformed by nanoindentation revealed stacking faults which are likely induced by plastic deformation. These defects were compared to theoretically possible stacking faults within the μ-phase building blocks, and in particular Laves phase layers. The experimentally observed stacking faults were found resulting from synchroshear assumed to be associated with deformation in the Laves-phase building blocks.

Ring defects in n-type Czochralski-grown silicon: A high spatial resolution study using Fourier-transform infrared spectroscopy, micro-photoluminescence, and micro-Raman

We investigated ring defects induced by a two-step anneal in n-type Czochralski-grown silicon wafers using a combination of high spatial resolution Fourier Transform Infrared Spectroscopy (FTIR), micro-photoluminescence (PL) mapping, and micro-Raman mapping. Through FTIR measurements, we show the inhomogeneous loss in interstitial oxygen with a positive correlation with the inverse lifetime. Using high-resolution micro-PL mapping, we are able to distinguish individual recombination-active oxygen precipitates within the rings with a decreasing density from the center to the edge of the sample. The radial inhomogeneity of the oxygen precipitates is likely to be related to variations in the distribution of grown-in defects. We also demonstrate that micro-Raman mapping reveals the oxygen precipitates without the smearing effects of carrier diffusion that are present in micro-PL mapping.