Strong Recombination Centers Research Articles

Covalent coupling promoting charge transport of CdSeTe/UiO-66 for boosting photocatalytic CO2 reduction

Quantum dots (QDs) based heterojunction is a candidate for the photocatalytic CO2 reduction, owing to the large extinction coefficient and easy modification of band structures. However, the van der Waals interaction causes the large charge resistance and strong recombination centers between QDs and host materials, which makes the poor photocatalytic performance. Herein, a covalent bonded CdSeTe QDs and NH2-UiO-66 heterojunction (NUC-x) is constructed through an acylamino (-CONH-). The results indicate that the acylamino between NH2-UiO-66 and CdSeTe QDs can serve as the transfer channels for the photogenerated charges and stabilize the QDs. The optimized NUC-1200 achieved a CO generation rate of 228.68 µmol/g, which is 13 and 4 times higher than that of NH2-UiO-66 and CdSeTe QDs, respectively. This work provides a new avenue for efficient and stable photocatalysis of QDs.

Control of dislocation clusters by artificially-introduced micro-twins in cast-mono silicon

Cast-mono silicon (CM-Si) is a promising method to fabricate cost-effective photovoltaic silicon wafers, and dislocation clusters are currently one of the most detrimental defects in CM-Si. Once formed, they propagate rapidly and unlimitedly in the single crystalline lattice and then serve as strong recombination centers and shunts in solar cells, which severely influence the crystal quality and cell performance. This work innovatively introduces a method to restrain and modulate the propagation of dislocation clusters during CM-Si growth. The constitutional supercooling effect was induced in CM-Si by performing boron-indium co-doping to generate harmless micro-twins. Related characterizations are mainly focused on the microstructure as well as electrical properties of wafers. In addition, an interaction model that suggests micro-twins’ formation and the interaction with dislocations was proposed. The results show that these micro-twins grow in preferred 〈112〉 orientations so they can partially block the growth and propagation of dislocation clusters within the 〈110〉 directions, and the recombination-active defects at the upper crystal were remarkably redistributed. The density and area of the detrimental defects were decreased. We think these conclusions could offer strategies for silicon wafer manufacturers, and we hope the physical mechanism might intrigue the insights for other crystalline growth.

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.

Recombination and Charge Collection at Nickel Silicide Precipitates in Silicon Studied by Electron Beam‐Induced Current

Nickel di-silicide layers have attracted scientific interest since the 1990’s as lattice-matched Schottky barriers on silicon and as strong carrier recombination centers when embedded in the silicon. High-resolution electron microscopy and electron beam-induced current are used by Philipp Saring and Michael Seibt (article number 2100142) to show that atomically thin silicide precipitates have properties similar to bulk nickel di-silicide in terms of the Schottky barrier height and electric conductivity. The cover image is an artistic view on processes of recombination and charge collection at thin silicide platelets. Experimental data are shown in front of the silicide’s atomic structure in silicon (image courtesy by L. Kroll).

Defects in Oxidized p‐Type Si Wafers Observed by Surface Photovoltage Spectroscopy

Surface photovoltage spectroscopy (SPV) is widely used for semiconductor characterization in modern microelectronics. Herein, the results of a comprehensive SPV analysis of oxidized n‐ and p‐type Czochralski (Cz) Si wafers with an oxygen concentration of about 5–7 × 1017 cm−3 are reported. It is demonstrated that the oxidation of the wafers with different H2/O2 ratios can significantly influence the diffusion length of minority carriers in p‐type Si. This effect is explained by the formation of oxygen–hydrogen‐related defects which are strong recombination centers in such wafers. In addition, a significant degradation of the diffusion length of minority carriers is observed in oxidized p‐type Si wafers after annealing at 90 °C and a subsequent illumination at room temperature. This degradation is found to be independent of oxide growth conditions, and it is not observed in n‐type wafers. This degradation is correlated with the presence of low concentrations of interstitial Fe (<1011 cm−3) in some wafers and with boron–oxygen‐related defects in some other wafers.

Determination of the structure and properties of an edge dislocation in rutile TiO2

A global optimization procedure is used to predict the structure and electronic properties of the b = c[001] edge dislocation in rutile TiO2. Over 1000 different atomic configurations have been generated using both semi-empirical and density functional theory estimates of the energy of the system to identify the most stable structure. Both stoichiometric and oxygen deficient dislocation core structures are predicted to be stable depending on the oxygen chemical potential. The latter is associated with Ti3+ species in the dislocation core. The dislocation is predicted to act as a trap for electrons but not for holes suggesting they are not strong recombination centers. The predicted structures and properties are found to be consistent with experimental results obtained using scanning transmission electron microscopy and electron energy loss spectroscopy on samples produced using the bicrystal approach.

Stable Defects in Semiconductor Nanowires

Semiconductor nanowires are commonly described as being defect-free due to their ability to expel mobile defects with long-range strain fields. Here, we describe previously undiscovered topologically protected line defects with null Burgers vector that, unlike dislocations, are stable in nanoscale crystals. We analyze the defects present in semiconductor nanowires in regions of imperfect crystal growth, i.e., at the nanowire tip formed during consumption of the droplet in self-catalyzed vapor-liquid-solid growth and subsequent vapor-solid shell growth. We use a form of the Burgers circuit method that can be applied to multiply twinned material without difficulty. Our observations show that the nanowire microstructure is very different from bulk material, with line defects either (a) trapped by locks or other defects, (b) arranged as dipoles or groups with a zero total Burgers vector, or (c) have a zero Burgers vector. We find two new line defects with a null Burgers vector, formed from the combination of partial dislocations in twinned material. The most common defect is the three-monolayer high twin facet with a zero Burgers vector. Studies of individual nanowires using cathodoluminescence show that optical emission is quenched in defective regions, showing that they act as strong nonradiative recombination centers.

Carbon-related defects in microelectronics

In the present study electrically active carbon and hydrogen-related (CH) defects, which can act as strong recombination centers in high power devices and CMOS photodetectors, are investigated in n-type Si. Several different CH-related defects are observed by using the deep level transient spectroscopy (DLTS) technique on hydrogenated Si samples with different oxygen content. The concentration of these defects is determined as low as 1012–1013cm−3. By comparing samples with different O, C, and H concentrations the origin of the CH-related defects is derived. We show that the concentration of the electrically inactive substitutional C can be estimated by a comparison of the depth profiles of the electrically active CH-related defects in a sample with those in a reference sample which has an identical oxygen and known carbon content. This approach is applicable even for concentrations of substitutional C lower than 1015cm−3.

Identification of Lifetime-limiting Defects in As-received and Heat Treated Seed-end Czochralski Wafers

Major impurity-induced defects in Czochralski silicon are known to be related to oxygen and metallic elements. In this study we focus on seed-end wafers, and submit them to different solar cell process-related annealings. We find that in the as-received state, these seed-end wafers contain a central low carrier lifetime core. We demonstrate that the effective carrier lifetime in the as-received state is simultaneously governed by several defects including oxygen-related thermal donors and a defect deactivated at low temperature. After high temperature steps, a strong recombination center is formed in the central core. Preliminary results of effective carrier lifetime versus temperature measurements directly support oxygen precipitates to be limiting the carrier lifetime. In the light of all results, we propose a qualitative model for the high temperature behavior of these seed-end wafers.

Influence of copper on the diffusion length of the minority carriers in devices based on n-type Si/SiO2

In the present study we demonstrate that the introduction of Cu leads to a significant reduction of the diffusion length in n-type Si/SiO2 structures. Using capacitance versus time measurements we show that the quasi-neutral generation mechanism is not dominant in Cu-contaminated samples at 90°C. We correlate this behaviour with Cu-related defects which act as strong recombination centres in Si. Their electrical properties are determined and their origin will be discussed.

Growth mode control of InGaN on GaN by using SiH4 post-treatment

We report on a way to control the growth mode of an In0.43Ga0.57N layer on GaN by using SiH4 post-treatment. In0.43Ga0.57N islands were formed on the GaN layers by a using Stranski-Krastanov growth mode without SiH4 post-treatment. When the SiH4 flow rate was increased from 0 to 10 sccm, the InGaN island density decreased from 3.3 × 109 to 1.9 × 109 then to zero, resulting in two-dimensional growth. However, a further increase in the SiH4 flow rate to 15 sccm again led to the formation of InGaN islands. Temperature- and excitation-power-dependent photoluminescence spectra showed that the InGaN islands provided strong localized recombination centers for electrical charge carriers and that the InGaN layers were composed of InGaN islands with different numbers of monolayers. This abnormal growth behavior with SiH4 post-treatment was explained by the combined effects of surfactant-mediated growth modification and the formation of Si x N y with the injection of SiH4.

(Invited) The Impact of Oxide Precipitates on Minority Carrier Lifetime in Czochralski Silicon

Oxide precipitates form in silicon for microelectronic and photovoltaic applications, and act as strong recombination centres. We have measured the injection-dependence of minority carrier lifetime in ~50 samples from p-type and n-type Czochralski silicon wafers with a wide range of precipitate densities. We find that all the data can be parameterized in terms of two independent Shockley-Read-Hall centres. The first is at EV + 0.22eV, and has a capture coefficient for electrons 157 greater than that for holes. The second is at EC - 0.08eV and has a capture coefficient for holes ~1,200 greater than that for electrons. The density of the centres is approximately dependent on the precipitate concentration. The existence of dislocations and stacking faults around the precipitates increases the density of both centres.

Contamination of silicon by iron at temperatures below 800 °C

AbstractIron‐related defects are deleterious in silicon‐based integrated circuits and photovoltaics, ruining devices and acting as strong recombination centres. Unless great care is taken, iron contamination will result from high temperature processing and so it is essential to understand the degree to which this can occur. Iron solubility data above ∼800 °C have been summarised by Istratov et al. (Appl. Phys. A 69, 13 (1999)), but many processes are performed at lower temperatures for which solubility data are scarce. We have studied iron contamination below ∼800 °C. Iron concentrations in intention‐ ally contaminated air‐annealed Czochralski silicon samples were determined from the change in minority carrier lifetime due to photodissociation of FeB pairs measured by quasi‐steady‐state photoconductance. In the ∼600 to 800 °C temperature range the iron concentration was found to vary according to $ 1.3 \times 10^{21} \; {\rm exp} \;\left (‐ { {1.8\;{\rm eV} } \over {kT} } \right)\;{\rm cm}^{‐3}. It is therefore the case that significantly more iron can dissolve in silicon at these temperatures than extrapolation of higher temperature data suggests, with the enhancement being by a factor of >20 at 600 °C. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of oxide precipitates on minority carrier lifetime in p-type silicon

Transient and quasi-steady-state photoconductance methods were used to measure minority carrier lifetime in ∼10 Ω cm p-type Czochralski silicon processed in very clean conditions to contain oxide precipitates. The nucleation and growth times for precipitation were varied to produce 35 samples, which were then characterised by chemical etching and transmission electron microscopy to determine the density and morphology of the precipitates. The effects of other known recombination mechanisms (band-to-band, Coulomb-enhanced Auger, iron-related, and boron-oxygen related) were factored out to isolate the lifetime component associated with the precipitates as accurately as possible. In the samples processed to contain mainly unstrained precipitates, it was shown that the lifetime component due to the precipitates could be extremely high (up to ∼4.5 ms). Recombination at unstrained precipitates is weak and it is estimated that the capture coefficient lies between 3 × 10−8 cm3 s−1 and 1.3 × 10−7 cm3 s−1 at an injection level corresponding to half the doping level. Strained precipitates act as strong recombination centres with a capture coefficient of ∼1 × 10−6 cm3 s−1 at the same level of injection. For the samples investigated, the effective capture coefficient is increased by a factor of ∼3 to 4 when other extended defects (such as dislocations and stacking faults) accompany the strained precipitates. The shape of the injection level dependence of lifetime was similar for all the specimens studied, with the magnitude of the lifetime being dependent on the precipitate density and strain state but approximately independent of precipitate size.

Iron Gettering at Slip Dislocations in Czochralski Silicon

The impact of slip dislocations on the interstitial iron distribution in as-grown CZ silicon wafers is investigated by calibrated MWPCD excess charge carrier lifetime measurements, DLTS measurements and measurements of the dislocation density. In regions of high dislocation density low interstitial iron content as well as low lifetime is observed. A linear correlation between dislocation density and interstitial iron content is found. We explain this linear correlation by the thesis that slip dislocations are 60° dislocations, which have adsorbed one iron atom at each dangling bond along the dislocation axis. Interstitial iron is gettered by slip dislocations but iron silicide, which forms along the dislocation axis, is a very strong recombination center for excess charge carriers as well. Hence, gettering of interstitial iron at slip dislocations does not increase the electrical quality of silicon.

Chemical etching to dissolve dislocation cores in multicrystalline silicon

Multicrystalline silicon wafers are used for approximately half of all solar cells produced at present. These wafers typically have dislocation densities of up to ∼106cm−2. Dislocations and associated impurities act as strong recombination centres for electron–hole pairs and are one of the major limiting factors in multicrystalline silicon substrate performance. In this work we have explored the possibility of using chemical methods to etch out the cores of dislocations from mc-Si wafers. We aim to maximise the aspect ratio of the depth of the etched structure to its diameter. We first investigate the Secco etch (1K2Cr2O7 (0.15M): 2HF (49%)) as a function of time and temperature. This etch removes material from dislocation cores much faster than grain boundaries or the bulk, and produces tubular holes at dislocations. Aspect ratios of up to ∼7:1 are achieved for ∼15μm deep tubes. The aspect ratio decreases with tube depth and for ∼40μm deep tubes is just ∼2:1, which is not suitable for use in bulk multicrystalline silicon photovoltaics. We have also investigated a range of etches based on weaker oxidising agents. An etch comprising 1I2 (0.01M): 2HF (49%) attacked dislocation cores, but its etching behaviour was extremely slow (<0.1μm/h) and the pits produced had a low aspect ratio (<2:1).

Accelerated Light-Induced Degradation (ALID) for Monitoring of Defects in PV Silicon Wafers and Solar Cells

In crystalline silicon, above-bandgap illumination can transform defects into strong recombination centers, degrading minority-carrier lifetime and solar cell efficiency. This light-induced degradation (LID) is due primarily to boron–oxygen and iron–boron defects, and can be reversed using thermal treatments that are distinctly different for each type of defect. Combining illumination and thermal treatment, we have designed an accelerated light-induced degradation (ALID) cycle that, within minutes, transforms defects into the distinct states needed to isolate individual contributions from boron–oxygen dimers (BO2i) and interstitial iron (Fei). In this cycle, the concentrations of BO2i and Fei are determined using surface photovoltage (SPV) diffusion length measurement. The ALID cycle uses reversible defect reactions and gives very good repeatability of wafer-scale mapping of BO2i and Fei in photovoltaic (PV) wafers and final solar cells.

Crystalline and electrical characteristics of C 60-doped GaAs films

C 60 uniformly doped and δ-doped GaAs layers are grown by migration enhanced epitaxy method. Crystalline and electrical characteristics of the layers are investigated by reflection high energy electron diffraction, X-ray diffraction and capacitance–voltage ( C– V) measurements. C 60 high-concentration doping is found to introduce 2D defects, and X-ray diffraction pole-figure measurements show that the rotational domains appear predominantly on {1 1 1}A plane. This indicates that C 60 molecules are mainly incorporated on the dangling bonds of Ga atoms due to the high binding energy between C 60 molecules and Ga atoms. C 60 uniformly doped GaAs layers show highly resistive characteristics, suggesting that C 60 molecules cannot be decomposed into isolated C atoms in the GaAs lattice, and they behave as if they were electron traps or strong recombination centers. C– V profiles of Cr–Au/C 60 δ-doped GaAs Schottky diode suggest that C 60 molecules in GaAs lattice produce electron traps which can be charged or discharged by applied electrical field.

Correlation between residual strain and electrically active grain boundaries in multicrystalline silicon

We report the correlation between residual strain and electrically active grain boundaries (GBs) in multicrystalline silicon. The former concerns the process yield, and the latter affects the solar cell efficiency. The distribution of strain was imaged by scanning infrared polariscope, and the electrically active GBs were characterized by electron-beam-induced current. Large strain was detected near multitwin boundaries and small-angle GBs. The multitwin boundaries are electrically inactive, while small-angle GBs act as strong recombination centers. It indicates that the electrical activities of GBs are not directly related to the residual strain.

Schottky rectifiers fabricated on bulk GaN substrate analyzed by electron-beam induced current technique

We report the fabrication of specially-designed Schottky rectifiers with thin Schottky metal on bulk GaN substrate through a homoepitaxial process. The Schottky rectifiers show consistent easy turn-on behaviors but with a wide distribution of reverse leakage current levels. A turn-on voltage as low as 0.9 V with a low on-resistance of 2.56 mΩ cm 2 was obtained. The easy turn-on properties can be attributed to the low series resistance achieved by fabricating vertical geometry devices on highly conductive GaN substrate. Electron-beam induced current analysis indicates that although the dislocation density in the homo-epilayer is generally low and on the order of ∼1 × 10 7 cm −2, mesoscopic defects such as grain boundaries and “dislocation walls” are present in the epilayer and serve as strong recombination centers. By comparing the reverse leakage current levels and the EBIC images of selected devices, we found that the “dislocation walls” observed are most likely to be the major source of high reverse leakage current. This study not only shows the promise of making high-performance GaN power devices based on homoepitaxial growth approach, but also indicates the importance of successful substrate polishing to the development of bulk-GaN-based devices.