Microwave Photoconductivity Decay Research Articles

Persistent Photoconductivity in p-Type 4H-SiC Bulk Crystals

We investigated the photoconductivity decay characteristics of p-type 4H-SiC bulk crystals grown by a modified Lely method by differential microwave photoconductance decay (μ-PCD) measurements using a 349-nm laser as an excitation source. We observed persistent photoconductivity (PPC) in the p-type SiC bulk crystals. The decay time at room temperature was 2600 μs. The decay time decreased with increasing temperature, resulting in 120 μs at 250oC, and the activation energy of the decay times was determined to be 140±10 meV. Long decay characteristics were also observed by below-band-gap excitation at 523 or 1047 nm. On the other hand, no PPC was observed in p-type homoepitaxial layers grown by hot-wall chemical vapor deposition.

Effect of Surface and Interface Recombination on Carrier Lifetime in 6H-SiC Layers

Carrier lifetimes in 6H-SiC epilayers were investigated by using numerical simulations and micro-wave photoconductivity decay measurements. The measured carrier lifetimes were significantly increasing with an increased thickness up to 200 μm while it stays almost constant in layers thicker than 200 μm. From a comparison of the simulation and experimental results, we found that if the bulk lifetime in 6H-SiC is around a few microseconds, both the surface recombination and interface recombination influence the carrier lifetime in layers with thickness less than 200 μm while only the surface recombination determines the carrier lifetime in layers with thickness more than 200 μm. In samples with varying thicknesses, a bulk lifetime = 2.93 μs and carrier diffusion coefficient D= 2.87 cm2/s were derived from the linear fitting of reciprocal lifetime vs reciprocal square thickness.

Microsecond Carrier Lifetimes in Bulk-Like 3C-SiC Grown by Sublimation Epitaxy

High quality bulk-like 3C-SiC were grown on on-axis (0001) 6H-SiC substrate by sublimation epitaxy. The microwave photoconductivity decay mapping measurements revealed that this material shows considerable long carrier lifetimes varied from 3.519 to 7.834 μs under the injection level of 3.5×1012 cm-2, which are comparable with the best carrier lifetimes in 4H-SiC layers. The mapping of high resolution x-ray diffraction obtained from the same region shows that smaller carrier lifetimes seem to correspond to the larger FWHM values and vice versa. This shows that long carrier lifetime obtained in 3C-SiC is due to the improvement of the crystal quality.

Lifetime Measurements on Attached Epilayers and Detached Epifoils Grown on Reorganised Porous Silicon Showing a Bulk Lifetime Exceeding 100 μs

This paper discusses on-going efforts towards reliable lifetime measurements on epilayers and the subsequent decoupling of the bulk and surface recombination components, both while it is still attached to the p+ substrate on which it is grown (“attached epilayer”) and after its detachment from the substrate (“detached epifoil”). For the “attached epilayers”, microwave photoconductance decay (^-PCD) and simulation-assisted photoluminescence (sim-PL) were applied together with a variation in the epilayer thickness to evaluate the bulk lifetime (Tbulk) and total effective surface recombination velocity (Stot) of p-type epilayers. By applying linear fits to reciprocal effective lifetime versus reciprocal epilayer thickness data, Stot of all samples were reliably extracted, resulting in Stot of ∼265cm/s in the absence of porous silicon at the epilayer/substrate interface compared to ∼9220cm/s when present and ∼775cm/s when shielded by a thin back surface field. Based on these Stot values, sim-PL was used to estimate Tbulk to be ∼160 |us in the porous silicon area. For the n-type “detached epifoils”, quasi-steady state photoconductance (QSSPC) was used. However, reliable Tbulk could not be extracted, despite the lower Stot because Tbulk was too high. However, a lower limit to Tbulk was estimated to be >138 μs. Such high bulk lifetimes imply bulk diffusion lengths that are an order of magnitude longer than the epilayer thicknesses, potentially leading to high cell efficiencies.

Long Photoconductivity Decay Characteristics in p-Type 4H-SiC Bulk Crystals

We investigated the photoconductivity decay characteristics of p-type 4H-SiC bulk crystals by differential microwave photoconductance decay (µ-PCD) measurements using a 349-nm laser as an excitation source. The decay time at room temperature was 2600 µs, which is much longer than that of n-type 4H-SiC bulk crystals (40 ns). Decay time decreased with increasing temperature, resulting in 120 µs at 250 °C, and the activation energy of decay time was determined to be 140±10 meV. Long decay characteristics were also observed by below-band-gap excitation at 523 nm.

Characterization of laser-induced damage in silicon solar cells during selective ablation processes

Selective laser ablation of silicon nitride layers on crystalline silicon wafers was investigated for solar cell fabrication. Laser processing was performed with a nanosecond UV laser at various energy densities ranging from 0.2 to 1.5 J cm−2. Optical microscopy was used as a simple mean to assess the ablation threshold that was correlated to the temperature at the interface between the silicon nitride coating and the silicon substrate. Minority carrier lifetime measurements were performed using a microwave photo-conductance decay technique. Band to band photoluminescence spectroscopy proved to be a sensitive technique to qualify the laser-induced damage to the silicon substrate. The crystalline structure of silicon seemed to be maintained after silicon nitride ablation as shown by UV reflectivity measurements. Laser parameters corresponding to fluences of around 0.4 J cm−2 were found to achieve selective ablation of SiNx without causing detrimental damage to the surrounding material.

Deep level transient spectroscopy and minority carrier lifetime study on Ga-doped continuous Czochralski silicon

Continuous-Czochralski (c-Cz) crystal growth has been suggested as a viable technique for the fabrication of photovoltaic Si wafers due to its low resistivity variation of any dopant, independent of segregation, compared to conventional Cz. In order to eliminate light induced degradation due to boron-oxygen traps in conventional p-type silicon wafers, gallium doped wafers have been grown by c-Cz method and investigated using four point probe, deep level transient spectroscopy (DLTS), and microwave-photoconductance decay. Iron-gallium related electrically active defects were identified using DLTS as the main lifetime killers responsible for reduced non-uniform lifetimes in radial and axial positions of the c-Cz silicon ingot. A direct correlation between minority carrier lifetime and the concentration of electrically active Fe-Ga pairs was established.

Research on Effects of Annealing and Al Gettering on Electrical Properties of Upgraded Metallurgical Grade Multicrystalline Silicon

Annealing and Al gettering were performed on upgraded metallurgical grade multicrystalline silicon (UMG multi-Si) wafers with a purity of 99.999%. The dislocation and grain boundaries of samples were characterized by optical microscopy and electron back scattering diffraction (EBSD), respectively. The minority carrier lifetime and resistivity of the Si wafers were measured using microwave photoconductance decay and four-point probe techniques, respectively. The results show that the number of dislocations in Si wafers reduced obviously after annealing and Al gettering for 2 hours at 600～1100°C. The proportion of Σ3 grain boundary increases. But the minority carrier lifetime and resistivity of the Si wafers after annealing decreases. However, the minority carrier lifetime and resistivity of the Si wafers after Al gettering increases firstly and then decreases with increasing of the annealing temperature. It is considered that the metal impurities determine electrical properties of UMG multi-Si wafers rather than dislocations and grain boundary. However, Al gettering can enhances the properties of Si wafers effectively and the optimal effect of Al gettering has been achieved at 800°C.

Assessing the role of iron‐acceptor pairs in solar grade multicrystalline silicon wafers from the metallurgical route

AbstractThe recombination parameters of iron‐boron (FeB), iron‐aluminium (FeAl) and iron‐gallium (FeGa) pairs in Fe implanted FZ monocrystalline silicon wafers and Fe contaminated multicrystalline silicon (mc‐Si) wafers of solar grade feedstock from the metallurgical route have been studied by combining the Deep Level Transient Spectroscopy (DLTS) and Microwave Photoconductive Decay (µ‐PCD) techniques. Energy levels associated with FeB, FeAl and FeGa pairs were detected in the monocrystalline samples. The activation energy and capture cross section of these levels were determined. FeGa gave the strongest recombination effect, reducing the minority carrier lifetime of the sample from about 39 µs to 0.7 µs at a concentration of 4x1013 cm‐3. No electrically active iron‐acceptor pairs could be detected in the mc‐Si wafer. However, it was demonstrated that micrometer‐sized clusters, most likely composed of metallic oxides, collect iron from the bulk. This iron collection may reduce the available amount of iron for creating electrically active iron‐acceptor pairs below the detection limit of DLTS. The contamination did, however, degrade the lifetime from 40 µs to less than 1 µs in the wafer. This is likely a result of at least three overlapping energy levels believed to be related to iron (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effectiveness of Iodine Termination for Ultrahigh Efficiency Solar Cells as a Means of Chemical Surface Passivation

The use of iodine as a passivating agent for chemical modification of silicon surface is demonstrated. The measurement of carrier lifetime using microwave photoconductivity decay method shows an effective passivation with iodine treatment which is 5 times greater than hydrogen passivation. Unlike hydrogen termination, the negative charge created by the iodine termination enhances the solar cell performance. For n-type silicon, the charge effect results in electric passivation. For p-type silicon, the charge effect forms a barrier which acts as back surface field. For cells with the same area, open circuit voltage (VOC), short circuit current density (JSC), fill factor (FF), and efficiency (η) of iodine terminated one were 610 mV, 39.5 mA/cm2, 76.1%, and 18.3% while those of hydrogen passivated one were 600 mV, 33.4 mA/cm2, 73.1%, and 14.7%, respectively.

Transient photoconductivity responses in amorphous In-Ga-Zn-O films

We studied the photoconductivity responses in amorphous In-Ga-Zn-O (a-IGZO) films using a time-resolved microwave photoconductivity decay (μ-PCD) technique. The a-IGZO film characteristics are correlated with three components in the photoconductivity response: the peak value and two decay constants. The peak value originated from the density of the photo-generated free carriers through carrier generation and recombination processes during laser pulse irradiation. Power law characteristics indicated that the peak values are attributed to recombination process related to the exponential distribution of the conduction band tail states. After the laser pulse was turned off, the reflectivity signal decreased rapidly, indicating fast recombination of the photo-generated carriers. This fast decay component is suggested to be related to the recombination processes through the deep level states. Following the fast decay, a slow decay with a decay constant on the order of microseconds appeared. This slow decay was attributed to the reemission of trapped carriers with an activation energy of ∼0.2 eV. In addition, both the fast and slow decays for the wet annealed a-IGZO film were longer than those of the as-deposited a-IGZO film. The decay constants are considered to reflect the density of the subgap states that act as trapping or recombination centers. The μ-PCD method provides a useful estimation of the film quality, such as the density of the defect states, and the physical properties of electronic devices using a-IGZO films.

Heterojunction solar cells produced by porous silicon layer transfer technology

In this paper, we present the result of heterojunction solar cells based on porous silicon layer transfer technology. a-Si/c-Si structured solar cells were prepared in which the c-Si was deposited on annealed double-layer porous silicon by low-pressure chemical vapor deposition. The structural properties and the evolvement of the double-layer porous silicon before and after thermal annealing were investigated by scanning electron microscopy. X-ray diffraction, Raman spectroscopy and a microwave photoconductive decay method were used to investigate the properties of the epitaxial silicon thin films deposited at different pressures. And, the influence of the deposition pressure on the properties of the c-silicon thin films was investigated. The spectral responses of the cells were studied by a quantum efficiency test. The results show that the epitaxial silicon thin film deposited at 100 Pa has better carrier lifetime and better spectral response. Furthermore, the Raman peak intensity of the silicon film prepared at 100 Pa is much closer to that of a monocrystalline silicon wafer. A simple solar cell structure without any light-trapping features showed an efficiency of up to 10.1 %.

Characterization of Annealed HPSI 4H-SiC for Photoconductive Semiconductor Switches

Annealing of high purity semi-insulating (HPSI) 4H-SiC is investigated as a method to improve bulk photoconductive semiconductor switches through recombination lifetime modification. Five samples of HPSI 4H-SiC were annealed at 1810 °C for lengths of time ranging from 3 to 300 minutes. The recombination lifetime of the unannealed and annealed samples was measured using a contactless microwave photoconductivity decay (MPCD) system. The MPCD system consists of a 35 GHz continuous microwave probe and a tripled Nd:YAG pulsed laser. The recombination lifetime was increased from 6 ns, as received, up to 185 ns by annealing for 300 minutes. To experimentally verify switch improvements, identical switches from unannealed and annealed material were fabricated and tested at low voltage. The unannealed device generated a 15 ns pulse with a 2 ns rise-time. The annealed device conducted for upwards of 300 ns with a comparable 2 ns rise-time. The increased recombination lifetime resulted in lower on-state resistance and increased energy transfer.

Correlation between Strain and Excess Carrier Lifetime in a 3C-SiC Wafer

We obtained excess carrier lifetime maps by the microwave photoconductivity decay (µ-PCD) method in a free-standing n-type 3C-SiC wafer, and then we compared the lifetime maps with distributions of strains and defects observed by the optical microscopy and the Raman spectroscopy. We found that the excess carrier lifetimes are short in a strained region in 3C-SiC, which indicates that structural defects exist around a strained region.

Correlation of Extended Defects on Carrier Lifetime in Thick SiC Epilayers

The effect of extended defects on carrier lifetime was investigated in 140 um thick 4H-SiC epilayers using whole wafer ultraviolet photoluminescence (UVPL) and microwave photoconductive decay (uPCD) mapping. Half-loop arrays (HLA) seen in the UVPL images showed a corresponding lifetime degradation in the same region, even before expansion of the HLAs to form SFs. Lifetime lowering was also seen for a defect comprising of a small 3C-SiC inclusion with a larger misoriented 4H-SiC region. Additionally, formation of slip planes after high temperature annealing was observed, which consequently shows a lifetime reduction in that region.

Optoelectronic transport property measurements of an amorphous-silicon-passivated c-silicon wafer using non-contacting methodologies

Photocarrier radiometry (PCR), microwave photoconductivity decay (μ-PCD) and transient and quasi-steady-state eddy currents were used for the characterization of a crystalline (c-) Si wafer passivated with intrinsic hydrogenated amorphous silicon films on either front or back side, or on both sides of the wafer. All three techniques are capable of measuring recombination lifetimes, however additional fundamental free photoexcited carrier transport parameters (diffusion coefficient, front- and back-surface recombination velocity) can be evaluated from the PCR frequency dependence owing to the free-carrier-density wave's diffusion-length depth-dependence on frequency. The measured recombination lifetimes are compared and differences in values among the three techniques are discussed in terms of the role of surface and interface defects. μ-PCD and eddy currents yield effective or vertically averaged lifetime measurements, as opposed to PCR which yields bulk recombination lifetimes separately from surface recombination effects.

High temperature annealing of n-type 4H-SiC: Impact on intrinsic defects and carrier lifetime

In this paper, the impact of high-temperature annealing of 4H silicon carbide (SiC) on the formation of intrinsic defects, such as Z1/2 and EH6/7, and on carrier lifetimes was studied. Four nitrogen-doped epitaxial layers with various initial concentrations of the Z1/2- and EH6/7-centers (1011-1014cm-3) were investigated by means of deep level transient spectroscopy and microwave photoconductance decay. It turned out that the high-temperature annealing leads to a monotone increase of the Z1/2- and EH6/7-concentration starting at temperatures between 1600 °C and 1750 °C, depending on the initial defect concentration. In the case of samples with high initial defect concentration (1014cm-3) a distinct decrease in Z1/2- and EH6/7-concentration in the temperature range from 1600 °C to 1750 °C was observed, being consistent with previous reports. For higher annealing temperatures (Tanneal≥1750 ∘C), the defect concentration is independent of the samples’ initial values. As a consequence, beside the growth conditions, such as C/Si ratio, the thermal post-growth processing has a severe impact on carrier lifetimes, which are strongly reduced for samples annealed at high temperatures.

Passivation Properties of Atomic-Layer-Deposited Hafnium and Aluminum Oxides on Si Surfaces

This paper studies the chemical and field effect passivation properties of silicon surfaces by thin hafnium oxide <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\hbox{HfO}_{2})$</tex></formula> and aluminum oxide <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\hbox{Al}_{2}\hbox{O}_{3})$</tex></formula> layers grown by chemical-vapor-based atomic layer deposition method. Using a rigorous metal–oxide–semiconductor model and results from capacitance–voltage measurements, the density of fixed charges <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$(N_{f})$</tex></formula> and the density of interface traps <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(D_{\rm it})$ </tex></formula> at the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{HfO}_{2}\hbox{/}\hbox{Si}$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Al}_{2}\hbox{O}_{3}\hbox{/}\hbox{Si}$</tex></formula> interfaces were obtained. Microwave photoconductivity decay measurements were used to characterize interface recombination velocities at these interfaces.

Excess Carrier Lifetime in p-Type 4H-SiC Epilayers with and without Low-Energy Electron Irradiation

We measured excess carrier lifetimes in as-grown and low-energy electron-irradiated p-type 4H-SiC epitaxial layers by the microwave photoconductivity decay method. The carrier lifetime increases with the excitation density in the epilayers. This dependence suggests that the dominant recombination center in the epilayers has a larger capture cross section for electrons than for holes. The low-energy electron irradiation reduces the carrier lifetime in the epilayers, and 1000 °C annealing increases the carrier lifetime in the electron-irradiated samples.

液相沉积法制备的二氧化硅薄膜及其钝化性能

The SiO2 film has been successfully synthesized by liquid phase deposition (LPD) method. The microstructure and passivation performance of the SiO2 film have been investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and microwave photoconductive decay (μ-PCD). The results show that the deposited SiO2 film contains a little amount of F element and it is very smooth and dense. The reflectance and surface recombination velocity reduced to 10.88% and 2 830 cm/s, respectively.