Microwave-detected Photoconductance Decay Research Articles

Microstructural, electrical, and optoelectronic properties of BaSi2 epitaxial films grown on Si substrates by close-spaced evaporation

BaSi2, an earth-abundant photovoltaic material with a limiting efficiency of 32 %, has the interesting property that grain boundaries do not significantly degrade the film properties. In this study, we investigated the microstructure and its effect on the electrical and optoelectronic properties of the BaSi2 films grown by close-spaced evaporation. Microstructural analysis confirmed the (100)-oriented epitaxial growth on both Si(100) and Si(111) substrates. In addition to conventional epitaxial relationships, lattice matching with the BaSi2(01¯3) plane parallel to Si(110) was observed on Si(100) substrates. The density of the epitaxial domain boundary was found to decrease with increasing film thickness, increasing growth temperature, and using Si(100) substrates instead of Si(111). Hall effect measurements revealed the conductivity type transition at a temperature between 900 and 1000 ∘C and a decreasing tendency of carrier density with film thickness. There was no obvious correlation between mobility and grain boundary density, consistent with previous reports. Carrier recombination kinetics were evaluated by the microwave-detected photoconductivity decay method and the photoconductivity measurement under constant optical irradiation. Both analyses revealed weak negative correlations between excess carrier lifetime and grain boundary density, which clarified the superiority of large-grained BaSi2 films for photovoltaic applications.

Fast Wafer-Level Characterization of Silicon Photodetectors by Photoluminescence Imaging

Photoluminescence imaging (PLI) technique is conventionally used in silicon (Si) photovoltaics (PV) for device characterization and inline quality control, providing substantial assistance for a wafer-level process monitoring from as-cut wafers to fully fabricated devices. Surprisingly, employing this method has not spread outside PV, and thus, its potential remains largely unknown in other fields. In this case study, a fully processed Si photodetector wafer, consisting of photodiodes with various sizes, has been chosen as an example to explore the potential of PLI beyond PV. First, we show that the standard PLI measurement is able to provide a high-resolution full-wafer luminescence image of the complete devices only within a couple of seconds. The image reveals various types of inhomogeneities present in the devices, such as furnace contamination and other processing-induced defects. The measured data are then converted to an effective lifetime image followed by benchmarking with a conventionally measured recombination lifetime map obtained by microwave-detected photoconductance decay (<inline-formula> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>-PCD), demonstrating further superiority of PLI in terms of the spatial resolution and the measurement time. Finally, correlation with diode leakage current and photoresponse measurements show that PLI is able to provide useful information on the final device performance without a need for traditional electrical contact measurements. While this study has focused on Si photodetectors, the results imply that PLI also has potential in other semiconductor devices for fast wafer-level process monitoring purposes as well as for a single device characterization either before or after wafer dicing.

The impact of the dislocation distribution and dislocation type on the charge carrier lifetime in Czochralski germanium single crystals

Germanium crystal grown by the Czochralski method in [211] and [110] direction have been investigated by etching methods and the charge carrier lifetime was analyzed by microwave-detected photoconductance decay. The etch pit density of the crystal increased from nearly free of etch pits to densities in the order of 104 cm−2 throughout the crystal. The etch pits in the [211] crystal were aligned along the [101] and [110] directions, while the etch pits in the [110] crystal were aligned along the [101] and [011‾] directions, which are dislocation glide directions within the glide plane parallel to the growth direction. The charge carrier lifetime was homogeneous throughout most of the crystal in the [211] crystal reaching up to 600 μs and only dropped significantly at a dislocation density of 3 · 104 cm−2. The charge carrier lifetime in the [110] was generally lower, which we address to the dominance of 60° dislocations and it drops significantly at a dislocation density of 2.3 · 104 cm−2.

Atomic hydrogen passivation for photoresponsivity enhancement of boron-doped p-BaSi2 films and performance improvement of boron-doped p-BaSi2/n-Si heterojunction solar cells

Semiconducting barium disilicide (BaSi2) is an emerging material for solar cell applications, and therefore, defect passivation is critical for improving its solar cell performance. Herein, the effect of atomic hydrogen (H) on the photoresponsivity of 500 nm-thick boron (B)-doped p-BaSi2 films was examined. The photoresponsivity reached ∼4 A/W (about twice the highest reported value for H-passivated undoped BaSi2 films) in B-doped p-BaSi2 films exposed to an atomic H supply for 5 − 10 min because of an increased minority-carrier lifetime, as measured by the microwave-detected photoconductivity decay. Furthermore, a ≥15 min atomic H supply was found to degrade photoresponsivity. Ab initio studies were used to interpret and understand experimental observations by analyzing states in the gap region, which can act as traps, in B-doped p-BaSi2 with H incorporation. The effect that atomic H had on the performance of B-doped p-BaSi2/n-Si heterojunction solar cells was also studied. The saturation current density was found to decrease by three orders of magnitude with the atomic H supply, and the conversion efficiency was increased up to 6.2%. Deep-level transient spectroscopy revealed a reduction of defect densities induced by the atomic H supply. Both experimental and theoretical viewpoints show that an atomic H supply is beneficial for BaSi2 solar cells.

Silicon-Based Optoelectronic Tongue for Label-Free and Nonspecific Recognition of Vegetable Oils.

A special electronic tongue system based on photoelectric measurements on Si–Si/SiNX sensitive structures is reported. The sensing approach is based on measuring of minority carrier lifetime in silicon-based substrates using microwave-detected photoconductance decay. This inexpensive and environmentally friendly combinatorial electronic sensing platform is able to create characteristic electronic fingerprints of liquids, detect, and recognize them. In particular, an application of the optoelectronic tongue for recognition of vegetable oils and their mixtures is described.

Electron and proton irradiation effect on the minority carrier lifetime in SiC passivated p-doped Ge wafers for space photovoltaics

We report on the effect of electron and proton irradiation on effective minority carrier lifetimes (τeff) in p-type Ge wafers. Minority carrier lifetimes are assessed using the microwave-detected photoconductance decay (μW-PCD) method. We examine the dependence of τeff on the p-type doping level and on electron and proton radiation fluences at 1 MeV. The measured τeff before and after irradiation are used to estimate the minority carriers’ diffusion lengths, which is an important parameter for solar cell operation. We observe τeff ranging from ≈ 50 to 230 μs for Ge doping levels between 1 × 1017 and 1 × 1016 at.cm−3, corresponding to diffusion lengths of ≈ 500–1400 μm. A separation of τeff in Ge bulk lifetime and surface recombination velocity is conducted by irradiating Ge lifetime samples of different thicknesses. The possible radiation-induced defects are discussed on the basis of literature.

Fabrication of BaSi2 thin films capped with amorphous Si using a single evaporation source

The capping of a BaSi2 film with amorphous Si (a-Si) is a key technology for the solar cell applications of BaSi2. In this study, we have investigated the two-step evaporation process of BaSi2 to fabricate the BaSi2 film capped with an a-Si layer on a Si(100) substrate. The deposition of Si is possible as the second step because a small amount of Si remains after the first BaSi2 evaporation step. Raman and X-ray photoelectron spectroscopies show that a-Si is formed by the two-step evaporation on the sample surface while Ba and carbonate species are not present on the surface, which are detected when the BaSi2 film is fabricated without the second step. Depth composition profiles determined by X-ray photoelectron spectroscopy clearly demonstrate the formation of the layered structure of a-Si and BaSi2. Owing to the a-Si capping, O concentration in the BaSi2 film is lower than that without a-Si, as evidenced by the composition profiles and energy-dispersive X-ray spectroscopy results. In addition, the microwave-detected photoconductivity decay analysis shows that the carrier lifetime in the a-Si/BaSi2 samples is similar to or slightly higher than the BaSi2 film without a-Si layer possibly due to the surface passivation effect and the reduction of oxygen concentration.

Post-annealing effects on the surface structure and carrier lifetime of evaporated BaSi2 films

To improve the surface quality for photovoltaic applications, we have investigated the effects of post-annealing on the surface structure and carrier lifetime of evaporated BaSi2 films. Structural characterizations by Raman spectroscopy and X-ray diffraction analysis show that there is an optimum post-annealing duration for fabricating a homogeneous film up to around the surface. By detailed surface analysis by X-ray photoelectron spectroscopy, the existence of a surface oxidation layer consisting of BaCO3 and barium silicate is revealed, and the thickness of the oxidation layer is found to be smallest for the optimum post-annealing duration. These surface structural changes are discussed from a thermodynamic viewpoint. Carrier lifetime is also investigated by the microwave-detected photoconductivity decay method, which shows that the structural change around the surface by post-annealing has negligible effects on carrier lifetime, possibly because the silicate layer covers the BaSi2 surface irrespective of post-annealing duration.

Surface Voltage and μPCD Mapping of Defect in Epitaxial SiC

Kelvin-probe surface voltage mapping, SVM, on epitaxial SiC, charged with corona into deep depletion, reveals SV defects manifested as spots with decreased surface voltage. For 150μm thick epi-layer, SV defects coincide with low carrier lifetime spots revealed by microwave detected photoconductance decay, μPCD. In the photoluminescence image, these defects are seen as triangular dark spots, described in literature as stacking-fault related triangular defects. For thin epi-layers (2.2μm), defects are visible only in SVM. In this case, high resolution SVM performed with Kelvin Force Microscopy identifies a triangular defect shape. Two mechanisms are proposed, accounting for SV defects. For high intensity defects exhibiting large magnitude fast decreasing voltage, the probable mechanism is defect related leakage; causing neutralization of corona surface ions. Low intensity defects can be explained considering deep level emission. The latter mechanism has been investigated using SV transient and spectral analysis analogous to isothermal DLTS and Laplace DLTS.

Epitaxial Growth of High Quality n-type Silicon Foils in a Quasi-inline APCVD Reactor

Reduction of solar cell fabrication costs is still of importance and can be achieved by going towards thinner silicon wafers. The aim is to achieve thicknesses in the range of100μm and below while avoiding further kerf loss. The approach using porous silicon as a sacrificial detachment layer and as a seed layer for epitaxial growth can accomplish that goal [1]. Several research groups have already shown the high potential of this technique[2]. Most processes have been conducted in microelectronic-grade batch or single-wafer reactors, questioning the industrial feasibility of this concept. In this work n-type silicon wafers, epitaxially grown in a quasi-inline Atmospheric Pressure Chemical Vapour Deposition (APCVD) reactor, will be presented. The wafers were characterized using confocal white light microscopy, quasi steady state photo conductance decay (QSSPC), microwave detected photo conductance decay (MWPCD) and spreading resistance profiling (SRP) measurements. The first batch of n-type wafers showed mean effective carrier lifetimes exceeding 100 μs on 45 x 45 mm2 and locally more than 300 μs. A detailed analysis on the thickness distribution, bulk lifetime and surface recombination velocity, affecting the effective carrier lifetime was conducted. For the second batch of n-type wafers mean effective carrier lifetimes of over 260 μs on 25 x25 mm2 and locally over 500 μs were determined. This corresponds to a local bulk lifetime of over 800 μs and shows the high material quality of epitaxial layers grown in our developed APCVD reactors.

Realization of single-phase BaSi2 films by vacuum evaporation with suitable optical properties and carrier lifetime for solar cell applications

We have realized BaSi2 films by a simple vacuum evaporation technique for solar cell applications. X-ray diffraction analysis shows that single-phase BaSi2 films are formed on alkali-free glass substrates at 500 and 600 °C while impurity phases coexist on quartz or soda-lime glass substrates or at a substrate temperature of 400 °C. The mechanism of film growth is discussed by analyzing the residue on the evaporation boat. An issue on the fabricated films is cracking due to thermal mismatch, as observed by secondary electron microscopy. Optical characterizations by transmittance and reflectance spectroscopy show that the evaporated films have high absorption coefficients, reaching 2 × 104 cm−1 for a photon energy of 1.5 eV, and have indirect absorption edges of 1.14–1.21 eV, which are suitable for solar cells. The microwave-detected photoconductivity decay measurement reveals that the carrier lifetime is approximately 0.027 µs, corresponding to the diffusion length of 0.84 µm, which suggests the potential effective usage of photoexcited carriers.

Influence of the Surfaces Characteristics on the Luminescent Property of Porous Silicon

Photoluminescence of porous silicon (PS) prepared by different etched time was studied. The photoluminescence might originate from the recombination of carriers and surface states, which was proved by FTIR, Raman spectroscopy and SEM. Furthermore, the hydrogen-related groups on the PS surface could eliminate the surface states for the blue emission, but the quantities of surface states for the green emission were depended on the uncovered area on the PS surface. Moreover, the shape of photoconductivity curve was depended on the quantities of the surface states, which also was related to the uncovered area on the PS surface. In addition, the results of the microwave-detected photoconductivity decay measurement indicated thatthe defects on the PS surface increased with the increase of the etched time, which would be related to the increase of the depth of pores.

Influence of grain size and surface condition on minority-carrier lifetime in undoped n-BaSi2 on Si(111)

We have fabricated approximately 0.5-μm-thick undoped n-BaSi2 epitaxial films with various average grain areas ranging from 2.6 to 23.3 μm2 on Si(111) by molecular beam epitaxy, and investigated their minority-carrier lifetime properties by the microwave-detected photoconductivity decay method at room temperature. The measured excess-carrier decay curves were divided into three parts in terms of decay rate. We characterized the BaSi2 films using the decay time of the second decay mode, τSRH, caused by Shockley-Read-Hall recombination without the carrier trapping effect, as a measure of the minority-carrier properties in the BaSi2 films. The measured τSRH was grouped into two, independently of the average grain area of BaSi2. BaSi2 films with cloudy surfaces or capped intentionally with a 3 nm Ba or Si layer, showed large τSRH (ca. 8 μs), whereas those with mirror surfaces much smaller τSRH (ca. 0.4 μs). X-ray photoelectron spectroscopy measurements were performed to discuss the surface region of the BaSi2 films.

Modulating the field-effect passivation at the SiO2/c-Si interface: Analysis and verification of the photoluminescence imaging under applied bias method

In this paper, we study the surface passivation properties of thermally oxidized silicon wafers with controlled surface band bending, using a recently developed characterization technique combining calibrated photoluminescence imaging with the application of an external voltage over the rear side passivation layer. Various aspects of the technique and possible errors in the determination of the effective surface recombination velocity are discussed, including lateral carrier diffusion, leakage currents, and optical effects related to the presence of metal electrodes on the investigated samples. In order to quantitatively describe the recombination activity at the SiO2/c-Si interface and the effect of fixed charges in the oxide layer, the measured effective carrier lifetime vs. voltage curves have been analyzed in the framework of an extended Shockley-Read Hall recombination model. Furthermore, the results have been compared with corresponding results from microwave detected photoconductance decay measurements after depositing corona charges. We find an excellent agreement between the two techniques and between complementary measurements of the oxide charge density. Photoluminescence imaging under applied bias gives fast and repeatable measurements and allows for simultaneous data collection from multiple areas on the sample, and has thus been proven to be powerful tool for quantitative characterization of surface passivation layers.

Influence of Epilayer Thickness and Structural Defects on the Minority Carrier Lifetime in 4H-SiC

4H-SiC homoepitaxial layers with different thicknesses from 12.5 µm up to 50 µm were investigated by microwave-detected photoconductivity decay (µ-PCD), deep level transient spectroscopy (DLTS) and defect selective etching (DSE) to shed light on the influence of the epilayer thickness and structural defects on the effective minority carrier lifetime. It is shown that the effective lifetime, resulting directly from the µ-PCD measurement, is significantly influenced by the surface recombination lifetime. Therefore, an adequate correction of the measured data is necessary to determine the bulk lifetime. The bulk lifetime of these epilayers is in the order of several microseconds. Furthermore, areas with high dislocation density are correlated to areas with locally reduced effective lifetime.

Charge Carrier Lifetime Shift Induced by Temperature Variation during a MWPCD Measurement

In this work the temperature of a silicon sample excited by a laser during a time dependent microwave-detected photoconductance decay (MWPCD) measurement was logged. 2°C temperature increase on the backside of the wafer was observed. Temperature dependent charge carrier lifetime calculations using the determined temperature characteristic are compared to time dependent MWPCD measurements of a boron-doped wafer with high oxygen content. The time duration to reach the final temperature fits well with the time scale of the fast forming recombination center of the boron-oxygen related defect. Charge carrier lifetime variation due to temperature increase is discussed in view of different defect parameters.

Investigation of the recombination mechanism of excess carriers in undoped BaSi2 films on silicon

Excess-carrier recombination mechanisms in undoped BaSi2 epitaxial films grown by molecular beam epitaxy on n-type silicon substrates have been studied by the microwave-detected photoconductivity decay measurement. The measured excess-carrier decay is multiexponential, and we divided it into three parts in terms of the decay rate. Measurement with various excitation laser intensities indicates that initial rapid decay is due to Auger recombination, while the second decay mode with approximately constant decay to Shockley-Read-Hall recombination. Slow decay of the third decay mode is attributed to the carrier trapping effect. To analyze Shockley-Read-Hall recombination, the formulae are developed to calculate the effective lifetime (time constant of decay) from average carrier concentration. The measurement on the films with the thickness of 50–600 nm shows that the decay due to Shockley-Read-Hall recombination is the slower in the thicker films, which is consistent with the formulae. By fitting the calculated effective lifetime to experimental ones, the recombination probability is extracted. The recombination probability is found to be positively correlated with the full width at half-maximum of the X-ray rocking curves, suggesting that dislocations are acting as recombination centers.

Recombination via point defects and their complexes in solar silicon

AbstractElectronic grade Czochralski and float zone silicon in the as grown state have a very low concentration of recombination generation centers (typically &lt;1010 cm−3). Consequently, in integrated circuit technologies using such material, electrically active inadvertent impurities and structural defects are rarely detectable. The quest for cheap photovoltaic cells has led to the use of less pure silicon, multi‐crystalline material, and low cost processing for solar applications. Cells made in this way have significant extrinsic recombination mechanisms. In this paper we review recombination involving defects and impurities in single crystal and in multi‐crystalline solar silicon. Our main techniques for this work are recombination lifetime mapping measurements using microwave detected photoconductivity decay and variants of deep level transient spectroscopy (DLTS). In particular, we use Laplace DLTS to distinguish between isolated point defects, small precipitate complexes and decorated extended defects. We compare the behavior of some common metallic contaminants in solar silicon in relation to their effect on carrier lifetime and cell efficiency. Finally, we consider the role of hydrogen passivation in relation to transition metal contaminants, grain boundaries and dislocations. We conclude that recombination via point defects can be significant but in most multi‐crystalline material the dominant recombination path is via decorated dislocation clusters within grains with little contribution to the overall recombination from grain boundaries.

Determination of the minority carrier lifetime in crystalline silicon thin-film material

The effective minority carrier lifetimes on epitaxial silicon thin-film material have been measured successfully using two independent microwave-detected photoconductivity decay setups. Both measurement setups are found to be equally suited to determine the minority carrier lifetime of crystalline silicon thin-film (cSiTF) material. The different measurement conditions to which the sample under investigation is exposed are critically analyzed by both simulations and measurements on a large number of lifetime samples. No systematic deviation between the lifetime results from different measurement setups could be observed, underlining the accuracy of the determined lifetime value. Subsequently, a method to separate the epitaxial bulk lifetime and the total recombination velocity, consisting of front surface and interface recombination between the epitaxial layer and the substrate, is presented. The method, based on different thicknesses of the epitaxial layer, is applied to all batches of this investigation. Each batch consists of samples with the same material quality but different epitaxial layer thicknesses whereas different batches differ in their material quality. In addition, the same method is also successfully applied on individual cSiTF samples. From the results, it can be concluded that the limiting factor of the effective minority carrier lifetime for the investigated solar-grade cSiTF material is the elevated recombination velocity at the interface between epitaxial layer and the substrate compared with microelectronic-grade material. In contrast, the samples cannot be classified into different material qualities by their epitaxial bulk lifetimes. Even on multicrystalline substrate, solar-grade material can exhibit high epitaxial bulk lifetimes comparable to microelectronic-grade material. Copyright © 2012 John Wiley & Sons, Ltd.

Limitations in the accuracy of photoconductance-based lifetime measurements

The accuracy of photoconductance-based minority carrier lifetime measurement techniques is studied in detail. Regarding their accuracy and comparability, the quasi steady state photoconductance (QSSPC) as well as the novel steady state microwave detected photoconductivity (MDP) method are compared with the traditional microwave detected photoconductance decay (μPCD). We show that differences in measurement conditions and analyzing procedures lead to deviating results. Calculations based on a generalized rate equation system are used to model the photoconductivity and the resulting effective lifetime for different measurement conditions and defect models. The simulation results are compared to measurements on several mono- and multicrystalline silicon samples with and without surface passivation. Our results clearly show, that for low as well as high injection conditions deviations occur for the measurement and analysis techniques investigated. To allow for a comparison of lifetime data, we recommend to report exact measurement conditions and analysis procedures as well as to perform measurements in a certain injection range. Only if no trapping effects are present and the penetration depth of the applied method is significantly larger than the sample width, accurate and comparable lifetime results can be achieved.