Microwave Photoconductivity Decay Research Articles

Studying the Effect of Composition on the Crystal Structure, Optical Properties, and Photogenerated Current Carriers Lifetimes in AgxCu1 – xGaSe2 (0 ≤ x ≤ 1) Solid Solutions

A set of AgxCu1 – xGaSe2 (0 ≤ x ≤ 1) solid solution powders has been prepared by solid-state synthesis. Using a combination of X-ray diffraction analysis and Raman spectroscopy, it has been found that the samples have a single-phase tetragonal structure (space group I-42d). It has been shown that their crystal lattice parameters do not follow Vegard’s law up to x ≈ 0.4. It has been revealed that the band gap of the samples also changes nonlinearly: initially it decreases and then increases. Studies of the low-temperature luminescence and microwave photoconductivity decay spectra have shown that a set of samples with x of 0 to ~0.4 and further in the region with x > 0.4 is characterized by an increase in the photogenerated current carrier lifetime in AgxCu1 – xGaSe2 powders. The observed effect is apparently attributed to the replacement of deep charge carrier traps, such as selenium vacancies, by shallower cation vacancies.

Recombination lifetimes and mechanisms of In0.75Ga0.25As and In0.53Ga0.47As as a function of doping density

The minority carrier lifetimes in InGaAs ternary alloys are investigated both experimentally and theoretically. Two groups of InGaAs/InP photodetector samples are designed for experiments, one with In0.75Ga0.25As and the other with In0.53Ga0.47As as the absorption layers. The lifetimes are obtained to be about 100–200 ns in In0.75Ga0.25As and 600 ns-1 µs in In0.53Ga0.47As by microwave photoconductivity decay technique. The calculated lifetimes are also found to be consistent with the experimental ones. Besides, for both InGaAs materials, the Shockley-Read-Hall (SRH), radiative and Auger recombination mechanisms are identified in calculations to be the dominant mechanisms at low, intermediate and high doping densities at room temperature, respectively. Furthermore, the effects of density, capture cross-sections and energy levels (Ets) of recombination centers, and bandgaps (Egs) on the SRH lifetimes are determined and compared for the two InGaAs materials. For an n-type material, when Et is near the mid-bandgap (Eg/2), the SRH lifetime is only inversely proportional to the product of the density and the capture cross-section of the recombination centers. When Et is not near the mid-bandgap, the magnitude of Eg also plays an important role in the SRH lifetime. By increasing the In-content from 53 % in In0.53Ga0.47As to 75 % in In0.75Ga0.25As, where the bandgap narrows from 0.75 to 0.52 eV, the SRH lifetime in In0.75Ga0.25As is shortened by a factor of about 1–42 for Et < Eg/2 and 1–65 for Et > Eg/2 within the bandgap, respectively.

Analysis of defects dominating carrier recombination in CeO2 single crystal for photocatalytic applications

In photocatalytic water splitting and CO2 reduction, recently, cerium oxide (CeO2) has been widely investigated. Defects that can directly dominate carrier recombination are essential for the photocatalytic performance of CeO2. In this study, several photoluminescence (PL) peaks are observed on the (100) face of an undoped CeO2 single crystal, indicating the presence of defects. Moreover, we characterize carrier recombination using time-resolved photoluminescence (TR-PL) and microwave photoconductivity decay (μ-PCD) measurements. The temperature dependence of decay curves is the result of carrier trapping and emission at deep levels. These decay curves are observed separately using a 565 nm band-pass filter (BPF) based on the PL spectra. The trap energy level ( ET ) and majority carrier capture cross-section ( σT ) of each defect are also analyzed using rate equations to fit the experimental results. The temperature-dependent time constants are well reproduced by a recombination model using hole traps HTinfrared and HTvisible at ET of 0.76 and 0.55 eV from the conduction or valence band, with estimated σT of the order of 10−21 and 10−22 cm2 for without and with the BPF, respectively. These findings regarding the presence of multiple defects in a single crystal indicate the necessity to focus on defect control.

Mitigation of carrier trapping effects on carrier lifetime measurements with continuous-wave laser illumination for Pb-based metal halide perovskite materials

We investigated the impact of carrier trapping on the carrier lifetime of metal halide perovskite materials, which are key to solar cell production. We examined NH3CH3PbI3 (MAPbI3), NH3CH3PbBr3 (MAPbBr3), and CsPbBr3 using continuous-wave (CW) laser illumination during microwave photoconductivity decay (μ-PCD) measurements. Traditional pulsed light excitation falls short of mirroring solar cell operating conditions, owing to carrier trapping. Implementing CW laser illumination provides a more accurate estimation of the carrier lifetimes under operational conditions. With an increased photon flux from the CW laser, the μ-PCD decay curves changed, indicating reduced recombination via traps. The experiments revealed extended carrier lifetimes under continuous light for the MAPbI3 polycrystal. This suggests that CW lasers can mitigate trapping effects on carrier lifetime measurements. For the other samples, carrier trapping had a negligible effect on the measured carrier lifetimes. We believe that these findings will aid in the design of perovskite-based devices.

Effect of dislocations on carrier recombination and photoelectrochemical activity in polished and unpolished TiO2 and SrTiO3 crystals

TiO2 and SrTiO3 are stable and efficient materials for photoelectrochemical (PEC) water splitting. PEC activity is influenced by carrier recombination in crystals of these materials. In this study, we analyzed the effect of dislocations on carrier recombination in TiO2 and SrTiO3 using microwave photoconductive decay measurements on polished and unpolished faces. The apparent slow decay in the unpolished face implies that dislocations trap minority carriers. Based on the different dependences of the injected photon density and temperature, the recombination processes in the polished and unpolished faces differ. A high concentration of oxygen vacancies or hydroxyl in the unpolished face of SrTiO3 was observed by x-ray photoelectron spectroscopy, and dislocations in the unpolished faces of both materials were observed using transmission electron microscopy. Additionally, we found that the photocurrent duration in the unpolished faces was shorter than that in the polished faces for both TiO2 and SrTiO3, confirming that dislocations inhibited photoelectrochemical activity.

Relationship between interstitial oxygen,substitutional carbon, resistivity and minority carrier lifetime in metallurgical multicrystalline silicon

. In this study we try to indentify relation between carrier lifetime, resistivity and two mains impurities concentration in a p-type upgrade metallurgical multicrystalline (UMG) silicon ingot. Thanks to this relation, we could prevent the Light Induced Degradation (LID) phenomenon and the SiC particles formation which are, respectively, at the origin of Voc losses and shunts in solar cells. So these 2 parameters are important for photovoltaic panels’ efficiency. Lifetime measurements are achieved by means of the Microwave Photoconductivity Decay “μw-PCD” technique, and concentration measurements are determined by FTIR. We demonstrate that resistivity variations depend on oxygen’s concentration but carbon analyses must be continued.

Surface recombination velocities for 4H–SiC: Dependence of excited carrier concentration and surface passivation

Carrier lifetime plays a crucial role in optimizing the performance and reliability of SiC devices. Surface recombination is one of the factors affecting carrier lifetime. However, the reported values for surface recombination velocity (S) have been inconsistent. This study aimed to elucidate the discrepancy in S observed in various reports by conducting microwave photoconductivity decay measurements on n-type 4H–SiC freestanding epitaxial layers at different excitation carrier concentrations. Additionally, we investigated the impact of passivation by examining the surfaces subjected to oxidation and post-oxidation annealing. We observed an increase in S with increasing excited carrier concentration for the surfaces, irrespective of the crystal faces (S- and C-faces) and passivation. This suggests the presence of a potential barrier or shallow recombination centers on the surfaces. Furthermore, we observed that S remained nearly constant with increasing temperature, contrary to the behavior observed under low-injection conditions. This difference also suggests the existence of a potential barrier or shallow recombination centers on both Si- and C-faces. The comprehensive insights gained from our study have significant implications for the design of SiC devices with enhanced performance and reliability.

Study of optical and passivation properties of hydrogenated silicon carbide thin films deposited by reactive magnetron sputtering for c-Si solar cell application

In this paper, optical and passivating properties of hydrogenated silicon carbide synthesized by reactive magnetron sputtering for c-Si solar cell application were studied. SiC:H films were synthesized by sputtering a SiC target in an argon-hydrogen ambient at a Radio Frequency (RF) power of 100–250 W. The thickness, density, and roughness of the films were measured by X-ray reflectometry. The optical properties of the films were studied by UV–visible spectroscopy. It is shown that with an increase in RF power, the refractive index increases from 1.53 to 2.32 (at λ = 633 nm), and the extinction coefficient does not exceed 0.009 in most of the visible and near-IR spectrum. Based on the measured optical constants using computer simulation the optimal thicknesses were calculated to maximize antireflection effect. IR spectroscopy showed correlation between the structure and physical properties of the films. It has been established that, at a lower RF power (100–150 W), particles are deposited instead of atoms. This leads to the formation of voids at the particle boundaries, which are subsequently filled with oxygen. Using the non-contact microwave photoconductance decay method, it was shown that the maximum effective lifetime (τeff) is achieved at RF power of 250 W. In addition, influence of preliminary surface cleaning procedure and post-annealing in vacuum on the τeff was considered.

Effect on passivation mechanism and properties of HfO2/crystalline-Si interface under different annealing atmosphere

Hafnium oxide (HfO2) thin film has received tremendous research interests as a passivation layer of crystalline silicon (c-Si) due to its thermal stability and positive fixed charges. HfO2 films were prepared both on P-type and N-type polished c-Si via a remote plasma atomic layer deposition process. Post annealing was carried out in oxygen, atmosphere (Air) and nitrogen (N2) at temperature of 500 °C. Electrical characterization was done through a regular metal-insulator-semiconductor structure and capacitance-voltage measurements. The oxide fixed charge (Qf) and density of interface traps (Dit) between HfO2 and c-Si were extracted. Qf and Dit reveal the potential of HfO2 for field effect and chemical passivation, respectively. Effective lifetime measurements by microwave photoconductivity decay in Si wafers passivated with HfO2 were applied to characterize surface recombination velocity to directly evaluate the overall passivation quality. The impacts of post-annealing gas ambient on passivation qualities on Si by HfO2 thin films are systematically studied. The results show that N2-annealed sample possesses the highest Qf of 1.73 × 1012 cm−2 and therefore provides the best field effect passivation. Air-annealed sample has lowest Dit of 1.0 × 1012 eV−1cm−2 and therefore provides the best chemical passivation. Air-annealed sample possesses the highest effective lifetime of 290 μs at injection level of 1 × 1015 cm−3, which means that good passivation was obtained for N-type Si wafer passivated with HfO2 after annealing in Air.

Carrier Recombination Dynamics of Surface-Passivated Epitaxial (100)Ge, (110)Ge, and (111)Ge Layers by Atomic Layer Deposited Al2O3

Germanium (Ge) and its heterostructures with compound semiconductors offer a unique optoelectronic functionality due to its pseudo-bandgap nature, that can be transformed to a direct bandgap material by providing strain and/or mixing with tin. Moreover, two crystal surfaces, (100)Ge and (110)Ge, that are technologically important for ultralow power fin or nanosheet transistors, could offer unprecedented properties with reduced surface defects after passivating these surfaces by atomic layer deposited (ALD) dielectrics. In this work, the crystallographically oriented epitaxial Ge/AlAs heterostructures were grown and passivated with ALD Al2O3 dielectrics, and the microwave photoconductive decay (μ-PCD) technique was employed to evaluate carrier lifetimes at room temperature. The X-ray photoelectron spectroscopy analysis reveals no role of orientation effect in the quality of the ALD Al2O3 dielectric on oriented Ge layers. The carrier lifetimes measured using the μ-PCD technique were benchmarked against unpassivated Ge/AlAs heterostructures. Excitation wavelengths of 1500 and 1800 nm with an estimated injection level of ∼1013 cm–3 were selected to measure the orientation-specific carrier lifetimes. The carrier lifetime was increased from 390 ns to 565 ns for (100)Ge and from 260 ns to 440 ns for (110)Ge orientations with passivation, whereas the carrier lifetime is almost unchanged for (111)Ge after passivation. This behavior indicates a strong dependence of the measured lifetime on surface orientation and surface passivation. The observed increase (>1.5×) in lifetime with Al2O3-passivated (100)Ge and (110)Ge surfaces is due to the lower surface recombination velocity compared to unpassivated Ge/AlAs heterostructures. The enhancement of carrier lifetime from passivated Ge/AlAs heterostructures with (100)Ge and (110)Ge surface orientations offers a path for the development of nanoscale transistors due to the reduced interface state density.

Relationship between epi-wafer photoluminescence and focal plane array performances of InGaAs detectors

The relationship between photoluminescence properties of InGaAs detector epi-wafers and the performances of focal plane arrays has been established according to the measurements of various samples. The average signal, dark signal and noise of the fabricated focal plane arrays were characterized and correlated to the photoluminescence intensities and nonuniformities of the epi-wafers. The average response signal is increased as the increased photoluminescence. The signal nonuniformity and dark signal are markedly related to nonuniformity of photoluminescence intensity, while the noise voltages are not correlated to photoluminescence results. The microwave photoconductance decay measurements reveal that the epi-wafers with stronger photoluminescence intensity has longer minority carrier lifetime. This work supplies a feasible pathway to evaluate and predict the performances of focal plane arrays fabricated by InGaAs detector epi-wafers before device processing.

Surface recombination velocities for the (100) and (001) crystal faces of bismuth vanadate single crystals

Bismuth vanadate (BiVO4) is widely used as a photocatalyst for water splitting, and its carrier lifetime is the most essential parameter for photocatalysts. In this study, we characterized the carrier lifetime in BiVO4 single crystals on the (100) and (001) crystal faces using the microwave photoconductivity decay (μ-PCD) method. For the (001) face, the observed μ-PCD curves with excitation by a 266 nm laser had no injected photon density dependence. In contrast, those excited using a 355 nm laser had an injected photon density dependence. The decay at a high injected photon density was faster than that at a low injected photon density. For the (100) face, the decay curves at both excitations of 266 and 355 nm were not significantly different, and they depended on the injected photon density. These results indicate that the carrier lifetime is dominated by surface recombination only under the 266 nm excitation condition for the (001) face, whereas under other conditions, this is dominated by the Schokley–Read–Hall recombination. The temperature independence of the μ-PCD curves indicates that the recombination center is sufficiently deep in the bandgap. We estimated the surface recombination velocities and bulk lifetimes of the samples by fitting the experimental results to the calculations. We believe that the estimated surface recombination velocity and bulk lifetime will aid in the design of BiVO4 photocatalysts.

Trapping effects and surface/interface recombination of carrier recombination in single- or poly-crystalline metal halide perovskites

The lifetime of a carrier is a crucial parameter for solar cell materials, and metal halide perovskite materials are promising for solar cell applications. In this study, we observed carrier recombination using time-resolved photoluminescence (TR-PL) and microwave photoconductivity decay (μ-PCD) in metal halide perovskite materials: NH3CH3PbI3 (MAPbI3), NH3CH3PbBr3 (MAPbBr3), and CsPbBr3 with single- and poly-crystalline structures. By comparing the decay curves of TR-PL and μ-PCD, we found trap levels in the band gap for all the materials. We employed two excitation wavelengths for the μ-PCD measurements, and we observed faster μ-PCD signal decays for short wavelength excitation for MAPbBr3 and CsPbBr3. Additionally, we established that the poly-crystals exhibited faster decay compared with the single crystals for MAPbBr3 and CsPbBr3. Therefore, we concluded that there are significant contributions of the interface and surface recombination on carrier recombination for MAPbBr3 and CsPbBr3, but not for MAPbI3.

Time-resolved photoconductivity distribution measurement by a synchronized double-scanning method.

The lifetime and the distribution of photoconductivity generated in laser-illuminated semiconductors are critical to photoconductivity-based applications. We propose a synchronized double-scanning method to measure time-resolved diffusion in the form of an afterglow embedded in the distribution map. The method combines spatial scanning of a coaxial resonator with synchronized laser scanning to map the dynamically excited conductivity on a semiconductor wafer. Thus, the photoconductivity afterglow effects can be mapped and retrieved by images of dynamic photoconductivity distribution. The photoconductivity lifetimes of silicon wafers with different thicknesses and by different lasers were measured and evaluated, which were also validated by the microwave photoconductivity decay (μ-PCD) method. In addition, the behavior of photoconductivity diffusion around a structural defect was exhibited. The method is nondestructive and can be applied in the photoconductivity property diagnostic.

Analysis of carrier lifetime in a drift layer of 1.2-kV class 4H–SiC devices toward complete suppression of bipolar degradation

We analyzed the carrier lifetime in a drift layer of 1.2 kV-class SiC p-n diodes to suppress bipolar degradation. According to the device simulation results, the required carrier lifetime in the drift layer was estimated to be shorter than 10 ns with a current density of 300 A/cm2 if the threshold hole density for the expansion of the stacking fault at the interface of the drift layer and substrate was 2 × 1016 cm−3. Numerical analysis revealed that to ensure a carrier lifetime shorter than 10 ns, the 1/e2 lifetime obtained by microwave photoconductivity decay (μ-PCD) measurements should be shorter than 7.2 ns. Experimental μ-PCD measurements showed that 1/e2 lifetimes obtained from the as-received epiwafer were much longer than 7.2 ns, and even after H+ implantation, high-temperature annealing, or electron irradiation, 1/e2 lifetimes were still long. Therefore, carrier lifetime control in the drift layer is not sufficient to suppress bipolar degradation, and a combination of carrier lifetime control with other methods is necessary for the fabrication of 1.2 kV-class SiC p-n diodes for the complete suppression of bipolar degradation at a current capacity of 300 A/cm2.

Effects of dopant type and concentration on surface recombination velocity in hydrogen-terminated silicon

Isolating the effects of the type and concentration of the dopant in lightly doped regions in the reaction between hydrogen-terminated silicon surface and atmospheric impurities in air is very difficult. However, changes in the surface recombination sites can be analyzed accurately through recombination lifetime measurements performed using the microwave photoconductive decay method. Thus, we investigated variations in the effective recombination lifetime in hydrogen-terminated silicon surfaces over time in air for different dopant types and concentrations. For both p-type and n-type silicon wafers, surface recombination velocity, S, increased with decreasing resistivity, namely, increasing dopant concentration. The time-dependent variations of the S for the p-type wafers decreased, and those for the n-type wafers increased with decreasing resistivity. Thus, it was shown that the time-dependent variation of the S depends on the type and concentration of the dopant used.

Estimation of surface recombination velocities and bulk carrier lifetime for SrTiO3 using angle-lapped structures

Strontium titanate (SrTiO3) is a promising photocatalyst for solar-to-hydrogen conversions. The carrier lifetime τ dominates the energy conversion efficiency in photocatalysts, and the surface recombination velocities Ss have significant effects on τ. To estimate Ss for the (100), (110), and (111) faces of SrTiO3, we employed samples with angle-lapped structures. The microwave photoconductivity decay method was adopted for the measurements, and the thickness dependence of τ was analyzed. Consequently, we obtained a more accurate Ss compared to that obtained in a previous study. We also observed differences in bulk τ among the samples, and this difference reflects the crystalline quality.

Impacts of hydrogen annealing on the carrier lifetimes in p-type 4H-SiC after thermal oxidation

Thermal oxidation and hydrogen annealing were applied on a 100 μm thick Al-doped p-type 4H-SiC epitaxial wafer to modulate the minority carrier lifetime, which was investigated by microwave photoconductive decay (μ-PCD). The minority carrier lifetime decreased after each thermal oxidation. On the contrary, with the hydrogen annealing time increasing to 3 hours, the minority carrier lifetime increased from 1.1 μs (as-grown) to 3.14 μs and then saturated after the annealing time reached 4 hours. The increase of surface roughness from 0.236 nm to 0.316 nm may also be one of the reasons for limiting the further improvement of the minority carrier lifetimes. Moreover, the whole wafer mappings of minority carrier lifetimes before and after hydrogen annealing were measured and discussed. The average minority carrier lifetime was up to 1.94 μs and non-uniformity of carrier lifetime reached 38% after 4-hour hydrogen annealing. The increasing minority carrier lifetimes could be attributed to the double mechanisms of excess carbon atoms diffusion caused by selective etching of Si atoms and passivation of deep-level defects by hydrogen atoms.

Carrier lifetime measurement of perovskite films by differential microwave photoconductivity decay

We measure the minority carrier lifetime of perovskite films by differential microwave photoconductivity decay (μ-PCD). Clear decay curves can be detected from bare and laminated methylammonium lead iodide (MAPbI3) films by the differential μ-PCD. The degradation of the bare and laminated MAPbI3 films under air exposure at room temperature is clearly observed as the continuous change of the decay curves. The differential μ-PCD can thus be a quick and non-destructive method for the characterization of the electrical quality of perovskite films and modules.

Effect of laser energy on the properties of neodymium-doped indium zinc oxide thin films deposited by pulsed laser deposition

The neodymium (Nd) doped indium-zinc-oxide (NIZO) is a material with high mobility and great potential in transparent electronic devices. NIZO thin films were prepared by pulsed laser deposition (PLD) at 250, 350, 450, and 550 mJ/pulse laser energy, respectively. With the increase of laser energy, the films gradually change from an amorphous to an amorphous/crystalline state and the In 2 O 3 crystals have preferential growth in the (123) plane. The average transmittance of the film is higher than 80% in the visible range. When the laser energy is 250 mJ, the carrier mobility has the highest value of 14.43 cm 2 V −1 s −1 , and it decreases with the increase of laser energy. The possible reason for this phenomenon is given by electronic structure and crystallization. Based on the content of defect states and the emitted particle number, the carrier concentration of the films is analyzed. • The relative content of defects in the NIZO thin films was characterized by nondestructive and non-contact microwave photoconductivity decay (μ-PCD). • Based on the phase, electronic structure and defects, the Hall mobility variation with laser energy was studied. • Based on the source of the carriers and the relative content of the defect, the carrier concentration variation with laser energy was studied.