Microwave Photoconductivity Decay Method Research Articles

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.

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.

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.

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.

Effect of Sputtering Oxygen Partial Pressure on the Praseodymium-Doped InZnO Thin Film Transistor Using Microwave Photoconductivity Decay Method.

The praseodymium-doped indium-zinc-oxide (PrIZO) thin film transistor (TFT) shows broad application prospects in the new generation of display technologies due to its high performance and high stability. However, traditional device performance evaluation methods need to be carried out after the end of the entire preparation process, which leads to the high-performance device preparation process that takes a lot of time and costs. Therefore, there is a lack of effective methods to optimize the device preparation process. In this paper, the effect of sputtering oxygen partial pressure on the properties of PrIZO thin film was studied, and the quality of PrIZO thin film was quickly evaluated by the microwave photoconductivity decay (µ-PCD) method. The μ-PCD results show that as the oxygen partial pressure increases, the peak first increases and then decreases, while the D value shows the opposite trend. The quality of PrIZO thin film prepared under 10% oxygen partial pressure is optimal due to its low localized defect states. The electric performance of PrIZO TFTs prepared under different oxygen partial pressures is consistent with the μ-PCD results. The optimal PrIZO TFT prepared under 10% oxygen partial pressure exhibits good electric performance with a threshold voltage (Vth) of 1.9 V, a mobility (µsat) of 24.4 cm2·V−1·s−1, an Ion/Ioff ratio of 2.03 × 107, and a subthreshold swing (SS) of 0.14 V·dec−1.

Microwave photoconductance decay measurements of n- and p-type silicon irradiated with neutrons and protons

N-type and p-type silicon were irradiated by neutrons and protons from low to high fluence range (108 − 1011 cm-2 for neutrons and 1010 − 1013 cm-2 for protons). The carrier recombination lifetime of each irradiated silicon was measured using a microwave photoconductance decay method, and the resistivity of the irradiated silicon was also measured. Both neutron and proton irradiation induced a similar lifetime at the same irradiation fluence. The defect formation rates were determined to vary with irradiation fluence from the inverse lifetime dependence on irradiation fluence. The sheet resistance was increased above a threshold fluence. The result shows that at least three different values of defect formation rate are present in the fluence range of 109 − 1015 cm-2, and it may have potential for practical use as low-dose dosimeters.

Inhomogeneity of Minority Carrier Lifetime in 4H-SiC Substrates

The minority carrier lifetimes in lightly N-doped n-type and V-doped semi-insulating 4H-SiC crystals were measured by microwave photo-conductance decay method. The resistivity mapping of the n-type 4H-SiC wafer lightly doped with nitrogen was examined by contactless resistivity measurement system to reveal the relationship between minority carrier lifetime and resistivity. Combining with the Raman spectroscopy and theoretical analysis of the nonequilibrium carrier recombination mechanism, it was found that the inhomogeneity of minority carrier lifetime in n-type 4H-SiC wafer was caused by inhomogeneous distribution of nitrogen concentration and the minority carrier lifetime was inversely proportional to the majority carrier concentration. It was also found that the minority carrier lifetime of V-doped semi-insulating 4H-SiC crystal was lower by one order of magnitude than that of N-doped n-type wafer. Second ion mass spectroscopy was used to examine the impurity concentration in different regions of V-doped semi-insulating 4H-SiC. It confirmed that the inhomogeneity of minority carrier lifetime originated from the inhomogeneous distribution of electrically active impurity concentration.

Surface recombination velocities for 4H-SiC: Temperature dependence and difference in conductivity type at several crystal faces

In bipolar SiC devices, which are promising under ultra-high voltage operation, the carrier lifetime is a highly influential parameter for the device performance. Surface recombination is one of the limiting factors for the carrier lifetime, and quantitative values of the surface recombination velocities are required for the design and development of fabrication processes of the devices. In this study, we observe carrier recombination at various temperatures for the Si- and C-faces of n- and p-type 4H-SiC samples and the a- and m-faces of n-type 4H-SiC samples with a treatment of chemical mechanical polishing or reactive ion etching by using the microwave photoconductivity decay method. From the experimental results, we estimate surface recombination velocities and bulk carrier lifetimes of the samples by using an analytical model. As a result, we found the smallest surface recombination velocity of 150 cm/s for the chemical mechanical polished surface of the Si-face of the n-type samples at room temperature. Surface recombination velocities increased with temperature for the chemical mechanical polished surfaces. The surfaces treated with reactive ion etching showed relatively large surface recombination velocities with weak temperature dependence. Based on these results, we discuss the origins of the recombination centers at surfaces of 4H-SiC.

Effective minority carrier lifetime as an indicator for potential-induced degradation in p-type single-crystalline silicon photovoltaic modules

In this paper, we report the effective minority carrier lifetime (τeff) in fresh and potential-induced degradation (PID) acceleration tested p-type single-crystalline Si modules. τeff in different regions of solar cells was measured using the microwave photoconductance decay (μPCD) method. Electroluminescence (EL), lock-in-thermography, and dark and light current–voltage (I–V) measurements were carried out as a complementary analysis of μPCD. In addition, τeff in every stage of Si solar cell fabrication (wafer to solar cell) was measured to investigate the change of carrier dynamics. From the obtained results, a great decrease in τeff was observed in the PID-affected regions, confirming the excess non-radiative recombination centers in that region, suggesting that τeff from the μ-PCD method can be an effective indicator to judge whether PID phenomenon has occurred.

Carrier Lifetime Measurements in Semiconductors through the Microwave Photoconductivity Decay Method

Carrier Lifetime Measurements in Semiconductors through the Microwave Photoconductivity Decay Method

Carrier Lifetime Measurements in Semiconductors through the Microwave Photoconductivity Decay Method.

This work presents a protocol employing the microwave photoconductivity decay (μ-PCD) for measurement of the carrier lifetime in semiconductor materials, especially SiC. In principle, excess carriers in the semiconductor generated via excitation recombine with time and, subsequently, return to the equilibrium state. The time constant of this recombination is known as the carrier lifetime, an important parameter in semiconductor materials and devices that requires a noncontact and nondestructive measurement ideally achieved by the μ-PCD. During irradiation of a sample, a part of the microwave is reflected by the semiconductor sample. Microwave reflectance depends on the sample conductivity, which is attributed to the carriers. Therefore, the time decay of excess carriers can be observed through detection of the reflected microwave intensity, whose decay curve can be analyzed for estimation of the carrier lifetime. Results confirm the suitability of the μ-PCD protocol in measuring the carrier lifetime in semiconductor materials and devices.

Deep-level transient spectroscopy characterization of electrical traps in p-type multicrystalline silicon with gettering and hydrogenation process

In p-type multicrystalline silicon solar cells, defects in silicon bulk are well known to act as carrier recombination centers and considerably affect the conversion efficiency of cells. In this study, effective minority carrier lifetime (τeff) mapping and deep-level transient spectroscopy (DLTS) data were utilized to evaluate gettering efficiency and hydrogenation passivation in bulk defects. Microwave photoconductivity decay method was used to illustrate the effect of phosphorus gettering behavior and hydrogenation process on the bulk carrier lifetime of multicrystalline silicon. Results showed that τeff drastically improved after gettering and further increased after hydrogenation. DLTS results further revealed six traps corresponding to interstitial iron (Fei), Fe–O complex, FeB complex, vanadium, Fei complex and divacancy oxygen. The main trap in a nongettered sample was related to Fei, with two deep levels at about EV + 0.373 and EV + 0.470 eV. However, the Fei complex was the main trap in the gettered sample, with two deep traps at about EV+0.390 and EV+0.442 eV. Finally, we concluded that Fei defect and FeB complex can be significantly removed by phosphorus diffusion gettering. Moreover, divacancy-oxygen complexes, vanadium and Fe–O defects were effectively passivated by hydrogenation process. Fei complex, however, was hardly removed by these two process.

Passivation of Surface Recombination at the Si-Face of 4H-SiC by Acidic Solutions

We carried out carrier lifetime measurements for 4H-SiC single crystals in aqueous solutions with various pH by the microwave photoconductivity decay method. For both n- and p-type 4H-SiC, carrier lifetimes measured by Si-face excitation were longer in acidic aqueous solutions compared with carrier lifetimes measured in other solutions. On the other hand, for C-face excitation, carrier lifetimes did not depend on immersion solutions. These results indicate that carrier recombination centers at the surface of the Si-face was passivated by hydrogen ions. We also estimated surface recombination velocities Ss by a numerical analysis. S of the Si-face was reduced from 700 cm/s (in Na2SO4 1 M) to 200 cm/s (in H2SO4 1 M) for the n-type 4H-SiC.

Correlation Between Carrier Lifetime and Photolytic Performance of Nb-Doped TiO2 Single Crystals

The hydrogen generation by the photolytic of water using the semiconductor photocatalyst attracts attention as the eco-friendly technology. Rutile TiO2 is a typical photocatalyst material because of its stability against corrosion in electrolyte.1) Nb doping to TiO2 enhances electrical conductivity and visible light absorption, and thus increases their photocatalytic performance.2, 3) One of the factors affecting the photocatalytic activity is the survival time of electrons and holes released in the photocatalyst material, i.e. the carrier lifetime.4, 5) However, there is a few report on the influence of carrier lifetime on photolytic performance for Nb-doped TiO2 single crystals. Therefore, we measured carrier lifetimes and photocurrents for TiO2single crystals with different Nb doping. The samples were Nb-doped rutile TiO2 single crystals with concentration of 0.01 wt% and 0.05 wt%. We called these samples Nb-0.01 and Nb-0.05. Both samples were polished on the (110) face, and their thickness was 0.5 mm. Carrier lifetimes of both samples were measured by microwave photoconductivity decay (μ-PCD) method. In μ-PCD, the sample was excited by a pulsed light and the conductivity was probed by reflection of 10 GHz microwave. We used a 355 nm pulsed YAG laser with injected photons of 8.5×1013 cm-2. Then we measured chopped light photocurrent-potential (I-V) measurements with the three-electrode system. The three-electrode system consisted of TiO2 electrode, Pt and a saturated calomel electrode (SCE) as working, counter and reference electrodes, respectively. Electrolyte was an aqueous solution of 1 mol/L H2SO4. The light source was a solar simulator with a power density 100 mW/cm2, and the light was mechanically chopped. Figure 1 shows μ-PCD decay curves for Nb-0.01 and Nb-0.05. The decay for Nb-0.01 was slower than that for Nb -0.05. For Nb-0.01, the carrier lifetime estimated from slopes of exponential parts was ~25 ns, which was longer than that observed for Nb-0.05. From these results, we can conclude that with higher Nb doping concentration, the carrier lifetime is shorter. Deep level formed by Nb doping would act as a recombination center of electron-hole pairs.6) Figure 2 shows chopped light I-V curves for Nb-0.01 and Nb-0.05. For Nb-0.05, the onset potential was about -1.4 V vs SCE and the photocurrent was 0.36 mA/cm2 at 1.5 V vs SCE. For Nb-0.01, the onset potential was the same as for Nb-0.05, while the photocurrent was 0.72 mA/cm2at 1.5 V vs SCE. Therefore, the photocurrent is larger for the sample with lower Nb doping concentration. From the results obtained from Fig. 1 and Fig. 2, both the carrier lifetime and the photocurrent were higher in the Nb-doped rutile TiO2single crystal with concentration of 0.01 wt%. These results suggest that carrier lifetime is increased and the photolytic performance is improved by reducing the Nb doping concentration. References 1) A. Fujishima and K. Honda, Nature 238, 37 (1972). 2) J. F. Baumard and E. Tani, J. Phys. 67, 857 (1977). 3) N. Yamada, T. Shibata, K. Taira, Y. Hirose, S. Nakao, N. Lam Huong Hoang, T. Hitosugi, T. Shimada, T. Sasaki and T. Hasegawa, Appl. Phys. 4, 045801 (2011). 4) D. Colombo and R. Bowman, J. Phys. Chem. 3654, 18445 (1996). 5) W.Choi, A. Termin, and M.R. Hoffmann, J.Phys. Chem. 98, 13669 (1994). 6) T. Miyagi, M. Kamei, I. Sakaguchi, T. Mitsuhashi and A. Yamazaki, Appl. Phys. 43, 775 (2004). Figure 1

Measuring the Minority-Carrier Diffusion Length of n-Type In0.53Ga0.47As Epilayers Using Surface Photovoltage

We report measurements of the minority-carrier diffusion length of n-type In0.53Ga0.47As epilayer samples using the surface photovoltage (SPV) method, and the minority-carrier lifetime of the same samples obtained by the microwave photoconductivity decay (μ-PCD) method. The minority-carrier diffusion length was determined from the surface photovoltage and the optical absorption coefficient of the material. By scanning the SPV probe over the sample, the difference in surface photovoltage could be measured, as well as enabling surface photovoltage mapping. Samples having two different doping concentrations were used: sample A with 3 × 1016 cm−3 and sample B with 1 × 1016 cm−3, having minority-carrier diffusion length at room temperature of 5.59 μm and 6.3 μm, respectively. Meanwhile, sample uniformity was investigated using SPV for the first time. Lifetime measurements were performed on the n-type In0.53Ga0.47As epilayer samples using the μ-PCD technique, obtaining the minority-carrier diffusion length indirectly. Comparison of the minority-carrier diffusion length values obtained from SPV versus μ-PCD showed good consistency. Therefore, the presented method could be useful for characterization of the minority-carrier diffusion length of wafers.

Effect of a rapid thermal annealing process on the electrical properties of an aluminum‐doped indium zinc tin oxide thin film transistor

We report the effect of a rapid thermal annealing process (RTP) on the electrical properties of an aluminum‐doped indium zinc tin oxide (Al‐IZTO) thin film transistor (TFT) with a back‐channel etched (BCE) structure. First, the RTP temperatures were varied from 250 to 350 °C, and their effect on Al‐IZTO TFT was investigated. The 250 °C RTP produced the best transfer characteristics (subthreshold swing = 0.11 V/dec, hysteresis = −0.25 V, and mobility = 26.42 cm2 V−1s−1), and as the temperature increased, the TFTs showed slightly degraded properties in hysteresis and subthreshold swing. Through the transmission line method (TLM), the channel shortening effect and contact properties according to the annealing temperature were studied. The reliability against a negative gate bias stress under illumination (NBIS) was also analyzed. At high RTP temperature (>300 °C), stability under NBIS was considerably improved, and the 350 °C RTP device showed turn‐on voltage shift of −1.7 V. The microwave photoconductivity decay (μ‐PCD) method revealed that the RTP effectively reduced shallow localized states, which enhanced the NBIS stability at high temperature. Finally, the RTP was compared with the conventional furnace process at the same temperature in terms of transfer characteristics and uniformity. Even with a much shorter process time for the RTP, Al‐IZTO TFTs exhibited similar or better transfer properties and uniformity to or than the furnace annealing.

Carrier lifetime measurements on various crystal faces of rutile TiO2 single crystals

Carrier lifetimes in rutile TiO2 single crystals were measured by the microwave photoconductivity decay method. The measurement results showed that carrier lifetimes in rutile TiO2 single crystals were dominated by the surfaces recombination but not recombination centers in the bulk. The surface recombination at the (110) face is the slowest among the crystal faces, and thus photocatalytic activity would be relatively high for a crystal with the (110) face.

Uniformity and passivation research of Al2O3 film on silicon substrate prepared by plasma-enhanced atom layer deposition.

Plasma-enhanced atom layer deposition (PEALD) can deposit denser films than those prepared by thermal ALD. But the improvement on thickness uniformity and the decrease of defect density of the films deposited by PEALD need further research. A PEALD process from trimethyl-aluminum (TMA) and oxygen plasma was investigated to study the influence of the conditions with different plasma powers and deposition temperatures on uniformity and growth rate. The thickness and refractive index of films were measured by ellipsometry, and the passivation effect of alumina on n-type silicon before and after annealing was measured by microwave photoconductivity decay method. Also, the effects of deposition temperature and annealing temperature on effective minority carrier lifetime were investigated. Capacitance-voltage and conductance-voltage measurements were used to investigate the interface defect density of state (Dit) of Al2O3/Si. Finally, Al diffusion P+ emitter on n-type silicon was passivated by PEALD Al2O3 films. The conclusion is that the condition of lower substrate temperature accelerates the growth of films and that the condition of lower plasma power controls the films’ uniformity. The annealing temperature is higher for samples prepared at lower substrate temperature in order to get the better surface passivation effects. Heavier doping concentration of Al increased passivation quality after annealing by the effective minority carrier lifetime up to 100 μs.

X-ray photoelectron spectroscopic study of silicon surface passivation in alcoholic iodine and bromine solutions

We report an X-ray photoelectron spectroscopic (XPS) study of silicon surfaces passivation using alcoholic solutions of iodine and bromine where different behavior with two systems is observed. The minority carrier lifetime determined by microwave photoconductive decay method showed better surface passivation for iodine-alcoholic system with HF preconditioning step. The iodine–ethanol (I–E) passivated samples show strong Si–I bonding (two times) in Si core level spectra for the samples without oxide (25.5%) compared with oxide (10.2%) counterpart. However, bromine–ethanol (B–E) passivated samples show higher Si–Br bonding strength in the samples with oxide (24.7%) compared to without oxide specimens (12.0%). This may be the reason of difference in passivation behavior of I–E and B–E systems. Higher O–Br bonding in O core level spectra of B–E passivated samples with oxide (35.8%), compared to without oxide (20.7%), results in comparable lifetime values in both with and without preconditioning. To understand the effect of solvent on the passivation, experiments are performed using iodine–methanol (I–M) and bromine–methanol (B–M) solutions and XPS analysis shows similar Si–I, Si–Br, and O–Br bonding trends.