Dominant Recombination Center Research Articles

Investigating Wafer Quality in Industrial Czochralski‐Grown Gallium‐Doped p‐Type Silicon Ingots with Melt Recharging

Herein, a systematic study of the electronic quality of gallium‐doped p‐type silicon wafers from Czochralski‐grown ingots with melt recharging is presented. It is found that in the as‐grown state, the ingots contain interstitial iron concentrations in the range of 3 × 109–2 × 1010 cm−3, with a trend of slightly higher concentrations toward the tail end of each ingot, and in subsequently grown ingots. However, analysis of the effective lifetimes indicates that iron–gallium pairs are not the dominant recombination centers in the as‐grown state. Moreover, when these wafers are subjected to a tabula rasa step, an increase in the iron concentration is observed in the range of 1 × 1010–6 × 1010 cm−3, with iron–gallium pairs becoming the dominant recombination centers. This is possibly caused by the dissolution of pre‐existing precipitated iron in the wafers. Nevertheless, the negative impact of iron contamination can be dramatically reduced by subjecting the wafers to a phosphorus diffusion gettering step, as is commonly incorporated in the fabrication of p‐type passivated emitter and rear cells. Therefore, it is concluded that the quality of the ingots is not limited by iron contamination, even after multiple ingots are pulled from the recharged melt.

Simultaneous Suppression of p-Doping and Nonradiative Recombination via Barium Incorporation for Highly Efficient Tin Perovskites.

High p-type self-doping and fast nonradiative recombination severely limit the optoelectronic performance of tin perovskites. Employing first-principles calculations, we show that incorporating barium (Ba) into the prototypical tin perovskite CsSnI3 could enable simultaneous suppression of p-doping and nonradiative recombination. Ba acts as an energetically favorable donor and raises the Fermi level. This not only decreases the background hole concentration but also inhibits the formation of I vacancies that are the dominant nonradiative recombination centers, giving rise to an improvement of more than 2 orders of magnitude in carrier lifetime. Moreover, as the facile migration of I ions is basically a process assisted by I vacancies, Ba incorporation also suppresses the diffusion of I ions, which is beneficial to the enhancement of stability. The present study provides a promising strategy for developing high-performance tin perovskites.

First-principles study of non-radiative carrier capture by defects at amorphous-SiO2/Si(100) interface

Defects have a significant impact on the performance of semiconductor devices. Using the first-principles combined with one-dimensional static coupling theory approach, we have calculated the variation of carrier capture coefficients with temperature for the interfacial defects P b0 and P b1 in amorphous-SiO2/Si(100) interface. It is found that the geometrical shapes of P b0 and P b1 defects undergo large deformations after capturing carriers to form charged defects, especially for the Si atoms containing a dangling bond. The hole capture coefficients of neutral P b0 and P b1 defects are largest than the other capture coefficients, indicating that these defects have a higher probability of forming positively charged centres. Meanwhile, the calculated results of non-radiative recombination coefficient of these defects show that both P b0 and P b1 defects are the dominant non-radiative recombination centers in the interface of a-SiO2/Si(100).

Origins of p-Doping and Nonradiative Recombination in CsSnI3.

It is commonly believed that the spontaneous p-doping in Sn-based perovskites is caused by Sn vacancies. By performing rigorous first-principles calculations for a prototypical Sn-based perovskite CsSnI3 , we reveal that, in fact, the defects dominating p-doping are Cs vacancies. The reason that adding extra Sn2+ could reduce p-doping is that Cs and Sn present the same changing trend in terms of chemical potentials, and thus inhibiting the formation of Sn vacancies will also limit the formation of Cs vacancies. Moreover, we show that I vacancies are the dominant nonradiative recombination centers, and can result in sizable nonradiative losses, which explains why the experimentally measured carrier lifetime is only a few nanoseconds even if p-doping is suppressed. This work provides new insights into the origins of p-doping and nonradiative recombination in CsSnI3 , and suggests that minimizing the formation of Cs and I vacancies is critical to realizing the best device performance.

Origins of p‐Doping and Nonradiative Recombination in CsSnI3

AbstractIt is commonly believed that the spontaneous p‐doping in Sn‐based perovskites is caused by Sn vacancies. By performing rigorous first‐principles calculations for a prototypical Sn‐based perovskite CsSnI3, we reveal that, in fact, the defects dominating p‐doping are Cs vacancies. The reason that adding extra Sn2+ could reduce p‐doping is that Cs and Sn present the same changing trend in terms of chemical potentials, and thus inhibiting the formation of Sn vacancies will also limit the formation of Cs vacancies. Moreover, we show that I vacancies are the dominant nonradiative recombination centers, and can result in sizable nonradiative losses, which explains why the experimentally measured carrier lifetime is only a few nanoseconds even if p‐doping is suppressed. This work provides new insights into the origins of p‐doping and nonradiative recombination in CsSnI3, and suggests that minimizing the formation of Cs and I vacancies is critical to realizing the best device performance.

Minimizing and Controlling Hydrogen for Highly Efficient Formamidinium Lead Triiodide Solar Cells.

Formamidinium lead triiodide (FAPbI3) currently holds the record conversion efficiency in the single-junction perovskite solar cell. Iodine management is known to be essential to suppress defect-induced nonradiative losses in FAPbI3 active layers. However, the origin of nonradiative losses and the underlying mechanism of suppressing such losses by iodine-concentration management remain unknown. Here, through first-principles simulation, we demonstrate that native point defects are not responsible for the nonradiative losses in FAPbI3. Instead, hydrogen ions, which can be abundant under both iodine-rich and iodine-poor conditions in FAPbI3, act as efficient nonradiative recombination centers and are proposed to be responsible for the suppressed power conversion efficiency. Moreover, iodine-moderate synthesis conditions can favor the formation of electrically inactive molecular hydrogen, which can dramatically suppress the detrimental hydrogen ions. This work identifies the dominant nonradiative recombination centers in the widely used FAPbI3 layers and rationalizes how the prevailing iodine management reduces the nonradiative losses. Minimizing the unintentional hydrogen incorporation in the perovskite is critical for achieving high device performance.

All-inorganic halide perovskites as candidates for efficient solar cells

Hybrid perovskites currently exhibit higher efficiencies than their all-inorganic counterparts in photovoltaic applications, which has led to a common belief that the organic cation somehow suppresses defect-assisted nonradiative recombination. Using first-principles calculations, here we show that the dominant nonradiative recombination center in CsPbI 3 is the iodine interstitial, which causes similar nonradiative capture rates as in MAPbI 3 (MA = CH 3 NH 3 ). However, the MA cation can give rise to additional strong nonradiative recombination centers such as hydrogen vacancies, which are absent from CsPbI 3 . One major advantage of MA + over Cs + is a better stability of the perovskite phase. Our study suggests that all-inorganic halide perovskites, if they can be stabilized, hold great promise for high-efficiency optoelectronic applications. These critical insights may prevent all-inorganic halide perovskites from being disregarded as potentially strong candidates for solar cell materials. • CsPbI 3 suffers less from nonradiative recombination than its hybrid counterpart MAPbI 3 • The inferior performance of CsPbI 3 very likely stems from poor stability • All-inorganic halide perovskites hold great promise for efficient solar cells It is commonly believed that hybrid perovskites are superior to their all-inorganic counterparts because of suppressed nonradiative losses. Using rigorous quantum mechanical calculations, Zhang et al. show that, in fact, all-inorganic halide perovskites are also very competitive potential candidates for efficient solar cells.

Optimization of Doping CdTe with Group-V Elements: A First-Principles Study

Group-V substitution at the $\mathrm{Te}$ site, ${X}_{\mathrm{Te}}$ (X = $\mathrm{P}$, $\mathrm{As}$, $\mathrm{Sb}$), under $\mathrm{Cd}$-rich conditions is an effective way to enhance the hole density and, in the meantime, suppresses the dominant nonradiative carrier recombination center in $\mathrm{Cd}\mathrm{Te}$, thus, improving the performance of $\mathrm{Cd}\mathrm{Te}$ thin-film solar cells. However, it is not clear which group-V dopant, X, is the most effective dopant, because it is expected that ${\mathrm{P}}_{\mathrm{Te}}$ will have the shallowest acceptor level due to its high electronegativity, whereas ${\mathrm{Sb}}_{\mathrm{Te}}$ will have the smallest formation energy due to its small size mismatch with $\mathrm{Te}$. Our systematic first-principles study shows that the hole concentration contributed by the acceptor ${X}_{\mathrm{Te}}^{\ensuremath{-}}$ is limited by the related compensating $A{X}^{+}$ center that increases simultaneously with ${X}_{\mathrm{Te}}^{\ensuremath{-}}$ as the chemical potential of dopant X increases. However, the ratio of ${X}_{\mathrm{Te}}^{\ensuremath{-}}$ acceptors to the $A{X}^{+}$ donors can be significantly increased if the sample is grown at high temperature and then annealed to room temperature, achieving a high hole density and low Fermi level $({E}_{F})$. We find that all group-V ($\mathrm{P}$, $\mathrm{As}$, and $\mathrm{Sb}$) dopings can achieve maximum hole densities of about ${10}^{17}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, which are consistent with previous experimental results. Despite the relatively deep acceptor level of 150 meV, $\mathrm{Sb}$ doping can achieve a considerable hole density due to the low formation energy of substituting $\mathrm{Sb}$ for $\mathrm{Te}$ with similar atomic radii. $\mathrm{P}$ doping can achieve a higher hole density than that of $\mathrm{Sb}$ due to its shallow transition energy at \ensuremath{\epsilon}(0/\ensuremath{-}) = 70 meV. However, the highest hole density is achieved through $\mathrm{As}$ doping, which is attributed to its balanced defect level at \ensuremath{\epsilon}(0/\ensuremath{-}) = 80 meV and relatively small formation energy.

Anomalous excitation-power dependence of band-edge emission in Si involving radiation-induced defects

We report on the anomalous excitation-power dependence of the band-edge emission in Si after 2 MeV electron irradiation. It is well-established that the dependence of the emission intensity I on the excitation power L is generally follows the power law, I ∝ Ln with the exponent n between 1 and 2. However, the exponent n increases after the electron irradiation and becomes larger than 2 at temperatures from 60 to 150 K in all the measured samples. The present dependence can be explained by a simple model: the radiation-induced defects act as dominant recombination centers in the low excitation-power range, but their activity becomes saturated in the high excitation-power range.

Iodine interstitials as a cause of nonradiative recombination in hybrid perovskites

The identification of deep-level defects that act as detrimental nonradiative recombination centers is critical for optimizing the optoelectronic performance of hybrid perovskites. Although extensive studies have been devoted to revealing the nature of deep-level defects in hybrid perovskites, it is still unclear what defects are responsible for the experimentally observed nonradiative recombination rates. Employing first-principles approaches, we quantitatively show that the iodine interstitial is a dominant nonradiative recombination center in methylammonium-lead iodide. This important insight points to a target for defect engineering in order to further improve the performance of hybrid perovskites.

Effect of oxygen precipitation through annealing process on lifetime degradation by Czochralski-Si crystal growth conditions

In this study, the effects of oxygen precipitation on the minority carrier lifetime were evaluated in conjunction with crystal growth conditions. It was revealed that oxygen precipitates and dislocations formation are suppressed and lifetime deterioration is suppressed in Czochralski (Cz)-Si, which is maintained at high temperature, followed by rapid cooling during crystal growth. In addition, the interface at the oxygen precipitate/Si matrix and dislocations were the dominant carrier recombination centers. From the above, it was suggested that controlling oxygen precipitation by crystal growth conditions can contribute to high-efficiency solar cells.

Spectroscopic and Microscopic Defect and Carrier-Lifetime Analysis in Cadmium Telluride

Using experiments and theory, we analyze “carrier-lifetime-killer” semiconductor bulk defects in CdTe. We can estimate the concentration of tellurium antisite (TeCd) recombination centers in undoped CdTe from the kinetic modeling of injection-dependent photoluminescence (PL) lifetimes. For the Cd-rich stoichiometry, the concentration of lifetime-limiting [TeCd] is ≥1013 cm–3. For the Te-rich stoichiometry, we find a smaller hole-capture rate constant than predicted by theory, which could be because of TeCd association with the Te interstitials. We also compare PL emission spectra, carrier lifetimes, and radiative efficiencies for Cu, P, and As doping and analyze the microscopic distribution of carrier lifetimes in As-doped single-crystal CdTe. In all the cases examined here, point defects are the dominant bulk recombination centers. Carrier lifetimes are the longest in undoped CdTe, but radiative efficiencies are the highest with group-V doping.

Optimization of SiC Power p-i-n Diode Parameters by Proton Irradiation

Local lifetime reduction by proton irradiation was used to optimize the static and dynamic parameters of a 10 kV SiC p-i-n diode. Carrier lifetime was reduced locally at the anode side by irradiation with 800 keV protons at fluences up to ${1} \times {10}^{{11}}$ cm−2. Results show that proton irradiation followed by annealing at 370 °C can be used for local and controllable reduction of carrier lifetime in SiC devices. The dominant recombination center is the ${Z}_{{1/2}}$ defect, whose distribution can be set by irradiation energy and fluence. Proton irradiation substantially improves diode turn-off while its effect on the forward voltage drop and leakage is not so harmful. Comparison of the technology curve for the unirradiated and proton irradiated p-i-n diode then clearly shows that proton irradiation provides a superior tradeoff between the static and dynamic losses.

Local Lifetime Control in 4H-SiC by Proton Irradiation

The effect of local lifetime control by proton irradiation on the OCVD response of a 10 kV SiC PiN diode was investigated. Carrier lifetime was reduced locally by irradiation with 800 keV protons at fluences up to 1x1011 cm-2. Radiation defects were characterized by DLTS and C-V profiling; excess carrier dynamics were measured by the OCVD and analyzed using the calibrated device simulator ATLAS from Silvaco, Inc. Results show that proton implantation followed by low temperature annealing can be used for controllable local lifetime reduction in SiC devices. The dominant recombination centre is the Z1/2 defect, whose distribution can be set by irradiation energy and fluence. The local lifetime reduction, which improves diode recovery, can be monitored by OCVD response and simulated using the SRH model accounting for the Z1/2 defect.

Evaluation of oxygen precipitation behavior in n-type Czochralski-Si for photovoltaic by infrared tomography: Effects of carbon concentration and annealing process conditions

In this study, we evaluated the effects of carbon and annealing process conditions on the oxygen precipitation in an n-type Czochralski (Cz) wafer for solar cells by infrared light scattering tomography (IR-LST). It was confirmed that precipitates grow larger and denser as carbon concentration increase. Thus, carbon promotes oxygen precipitation. We also evaluated the effect of oxygen precipitation on the minority carrier lifetime by photoluminescence (PL) imaging. It was confirmed that the interface between the precipitate and the Si matrix is a dominant recombination center since the surface area of the precipitate obtained by IR-LST measurement and the PL intensity show good correlation. In addition, it was also confirmed that carbon is involved in the supply of interstitial oxygen to precipitates through the formation and extinction of the thermal donor. We believe that it is important to understand and control the effect of carbon of controlling the oxygen precipitation behavior.

Optimizing GaNP coaxial nanowires for efficient light emission by controlling formation of surface and interfacial defects.

We report on identification and control of important nonradiative recombination centers in GaNP coaxial nanowires (NWs) grown on Si substrates in an effort to significantly increase light emitting efficiency of these novel nanostructures promising for a wide variety of optoelectronic and photonic applications. A point defect complex, labeled as DD1 and consisting of a P atom with a neighboring partner aligned along a crystallographic ⟨ 111 ⟩ axis, is identified by optically detected magnetic resonance as a dominant nonradiative recombination center that resides mainly on the surface of the NWs and partly at the heterointerfaces. The formation of DD1 is found to be promoted by the presence of nitrogen and can be suppressed by reducing the strain between the core and shell layers, as well as by protecting the optically active shell by an outer passivating shell. Growth modes employed during the NW growth are shown to play a role. On the basis of these results, we identify the GaP/GaN(y)P(1-y)/GaN(x)P(1-x) (x < y) core/shell/shell NW structure, where the GaN(y)P(1-y) inner shell with the highest nitrogen content serves as an active light-emitting layer, as the optimized and promising design for efficient light emitters based on GaNP NWs.

Electrical activity of geometrically necessary dislocations in polycrystalline silicon thin films prepared by solid phase crystallization

We combine electron backscatter diffraction (EBSD) with the electron beam induced current (EBIC) technique, by carefully aligning the EBIC images with EBSD grain average misorientation (GAM) maps of selected polycrystalline thin-film regions to correlate intragrain misorientation with the film's electrical properties. Applying this method to large (>3 µm diameter) solid phase crystallization (SPC) poly-Si grains, we find that regions with low EBIC signals coincide with regions that have a high degree of misorientation. EBIC signals from the edge regions of these large grains are about 30% lower than those measured in their central regions. Combination of EBIC and GAM maps suggests that geometrically necessary dislocations (GNDs) are electrically active and present in the regions of the poly-Si grains with misorientations larger than 3° (whereby these regions are typically close to the grain boundaries). On the other hand, grains smaller than 3 µm show a homogenous spatial distribution of the electrical activity of defects. In addition, the electrical performance in the central regions of the large grains is better than that of the smaller grains. These observations suggest that the origins of the dominant recombination centres of the smaller grains are different from those of the larger grains.

Rare Earth Photoluminescence in Bismuth-Germanate Crystals

ABSTRACT In the present work, the photoluminescence (PL) spectra of bismuth germanate (BGO) doped with trivalent rare earth element (REE) ions with different doping concentrations (0.03 wt% Eu, 0.4 wt% Tm, and 1.1 wt% Nd) are reported in the temperature range from 10 to 300 K using different detectors, namely, photomultiplier tube (PMT), InGaAs (IGA), and Si. The luminescence in the NIR region was also measured at room temperature. Two broad emission bands attributed to undoped BGO were found at circa 1350 and 1800 nm, respectively. The broad-band emissions are replaced by narrow-band line emissions defined by the trivalent rare earth dopants. The emission spectra from rare earth ion–doped BGO extend from 500 to 2000 nm. Rare earth ions act as the dominant recombination centers and define the emission spectra. This is interpreted as resulting from direct charge transfer from intrinsic defect traps to rare earth recombination centers. The temperature-dependent luminescence of BGO doped with 0.4 wt% Tm is also presented.

Dependence of the Minority-Carrier Lifetime on the Stoichiometry of CdTe Using Time-Resolved Photoluminescence and First-Principles Calculations

CdTe is one of the most promising materials for thin-film solar cells. However, further improvement of its performance is hindered by its relatively short minority-carrier lifetime. Combining theoretical calculations and experimental measurements, we find that for both intrinsic CdTe and CdTe solar cell devices, longer minority-carrier lifetimes can be achieved under Cd-rich conditions, in contrast to the previous belief that Te-rich conditions are more beneficial. First-principles calculations suggest that the dominant recombination centers limiting the minority-carrier lifetime are the Te antisite and Te interstitial. Therefore, we propose that to optimize the solar cell performance, extrinsic p-type doping (e.g., N, P, or As substitution on Te sites) in CdTe under Cd-rich conditions should be a good approach to simultaneously increase both the minority-carrier lifetime and hole concentration.

Intrinsic point defects in CuInSe2and CuGaSe2as seen via screened-exchange hybrid density functional theory

A fully self-contained study of the thermodynamic and electronic properties of intrinsic point defects in the solar absorber materials CuInSe2 and CuGaSe2 based on screened-exchange hybrid density functional theory is presented. The results are partly at odds with data obtained within local density functional theory in former studies. GaCu electron traps as well as CuIn and CuGa hole traps are found to be the dominant intrinsic recombination centers. In contrast to the accepted view, complex formation of antisites with copper vacancies is not decisive for explaining the favorable properties of CuInSe2, since InCu is already a shallow defect by itself. The localization of holes is observed on CuIn and CuGa as well as on VIn and VGa when supercells of 216 atoms are used. Furthermore, the results raise doubts about the relevance of selenium vacancies and DX centers for experimentally observed metastabilities. Finally, a guide to the optimal preparation conditions in terms of the point defect physics of CuInSe2 and CuGaSe2 for their application as solar cell absorbers is provided.