Transient Photoconductive Decay Research Articles

Excellent surface passivation of germanium by a-Si:H/Al2O3 stacks

Surface passivation of germanium is vital for optimal performance of Ge based optoelectronic devices especially considering their rapidly increasing surface-to-volume ratios. In this work, we have investigated the surface passivation of Ge by a stack consisting of a thin layer of hydrogenated amorphous silicon (a-Si:H) and an aluminum oxide (Al2O3) capping layer. Plasma-enhanced chemical vapor deposition was used to deposit the a-Si:H (0–10 nm), while thermal and plasma-enhanced atomic layer deposition (ALD) were employed for the Al2O3 films (0–22 nm). Transient photoconductance decay measurements revealed a recombination velocity as low as 2.7 cm s−1 for an a-Si:H layer as thin as 1.8 nm and an Al2O3 film of only ∼6 nm. In this state-of-the-art passivation scheme, the plasma-enhanced ALD process for the Al2O3 capping layer proved superior to the thermal ALD process since it resulted in an exceptionally high negative fixed charge density (Qf ∼ 1013 cm−2), which proved a key factor for the low surface recombination velocity. Transmission electron microscopy and energy x-ray dispersion revealed that a thin SiOx layer (∼1.4 nm) forms between a-Si:H and Al2O3 during the ALD process, which is thought to be the origin of this high negative fixed charge density. This passivation stack is regarded as highly interesting for applications such as solar cells, nanolasers, and nano-LEDs based on p-type Ge.

Characterization of tunnel oxide passivated contact with n-type poly-Si on p-type c-Si wafer substrate

The junction properties of tunnel silicon oxide (SiOx) passivated contact (TOPCon) with n-type poly-Si on p-type c-Si wafer are characterized using current-voltage (J-V) and capacitance-voltage (C-V) measurements. The dark J-V curves show a standard diode characteristic with a turn-on voltage of ∼0.63 V, indicating a p-n junction is formed. While the C-V curve displays an irregular shape with features of 1) a slow C increase with the decrease of the magnitude of reverse bias voltage, being used to estimate the built-in potential (Vbi), 2) a significant increase at a given positive bias voltage, corresponding to the geometric capacitance crossing the ultrathin SiOx, and 3) a sharp decrease to negative values, resulting from the charge tunneling through the SiOx layer. The C of depleting layer deviates from the normal linear curve in the 1/C2-V plot, which is caused by the diffusion of P dopants from the n-type poly-Si into the p-type c-Si wafer as confirmed by the electrochemical capacitance-voltage measurements. However, the 1/C2+γ-V plots with γ > 0 leads to linear curves with a proper γ and the Vbi can still be estimated. We find that the Vbi is the range of 0.75–0.85 V, increases with the increase of the doping ratio during the poly-Si fabrication process, and correlates with the passivation quality as measured by the reverse saturated current and implied open circuit voltage extracted from transient photoconductivity decay.

Quasi-steady-state photoconductance bulk lifetime measurements on silicon ingots with deeper photogeneration

Quasi-steady-state photoconductance measurements on silicon ingots and blocks with different photo-generation profiles are simulated in this work. The results show that deeper generation profiles, achieved by using a long-pass optical filter with a longer cut-off wavelength, can reduce the impact of the high surface recombination velocity of the ingot surface. This results in higher measured effective lifetimes and reduces the reliance on transfer functions to convert the measured lifetimes into bulk lifetimes. However, there exists a trade-off between generating carriers further from the surface to reduce surface recombination and ensuring that the generated carriers are within the sensitivity range of the photoconductance sensing coil. The simulations are compared with experimental results measured on a monocrystalline silicon block using both quasi-steady-state and transient photoconductance decay, as the transient method is relatively less prone to the impact of surface recombination, and provides a lower bound on the bulk lifetime. The results confirm an increased accuracy in bulk lifetimes extracted from quasi-steady-state measurements when measured using deeper photo-generation profiles.

Temporal Response of Dilute Nitride Multi-Quantum-Well Vertical Cavity Enhanced Photodetector

The temporal response characteristics of a GaInNAs-based vertical resonant cavity enhanced photodetector device are presented for operation at λ ≈ 1.3 μm. The absorption layers of the device are composed of nine 7-nm-thick Ga0.65In0.35N0.02As0.98 quantum wells and are sandwiched between top and bottom AlGaAs/GaAs distributed Bragg reflectors (DBRs). The temperature dependence of the transient photoconductivity (TPC) under different light intensities and bias voltages is reported. Photoluminescence measurements were also performed on structures with and without the top DBR to determine their optical response under continuous illumination. The response time was measured using excitation from a 1047-nm pulsed neodymium-doped yttrium lithium fluoride laser with pulse width of 500 ps and repetition rate of 1 kHz. The rise time of the TPC was 2.27 ns at T = 50 K, decreasing to 1.79 ns at T = 300 K. The TPC decay time was 25.44 ns at T = 50 K, decreasing to 16.58 ns at T = 300 K. With detectivity of $$ 2.28 \times 10^{10} \,{\hbox{cm}}\sqrt {\hbox{Hz}} / {\hbox{W}} $$ and noise-equivalent power of $$ 2.45 \times 10^{ - 11} \,{\hbox{W/}}\sqrt {\hbox{Hz}} $$ , the proposed device is faster and more sensitive with better signal-to-noise ratio compared with other GaInNAs-based resonant cavity enhanced photodetectors (RCEPDs) for operation at 1.3 μm.

MoOx modified ITO/a-Si:H(p) contact for silicon heterojunction solar cell application

Schottky barrier is a fundamental problem when contacting n-type indium tin oxide with p-type hydrogenated amorphous silicon due to their mismatched work functions in silicon heterojunction solar cell. Here, we developed a facile and effective method to modify the electrical properties of ITO/a-Si:H(p) contact by thermal evaporation an ultrathin MoOx buffer layer. The transient photo-conductance decay measurements showed that both effective carrier lifetime and implied open-circuit voltage increased abruptly after inserting an ultrathin MoOx buffer layer, from 1.43ms to 2.28ms and 735mV to 744mV, respectively, which is mainly attributed to modification of MoOx buffer layer on the Schottky barrier at the interface of ITO/a-Si:H(p) contact. By modulating the thickness of MoOx buffer layer, the MoOx based SHJ solar cell illustrated a champion efficiency up to 21.8%, companying with an absolute Voc gain over 8mV, and fill factor gain over 1.5%, respectively.

Dual-sensor technique for characterization of carrier lifetime decay transients in semiconductors

This work addresses the frequent discrepancy between transient photoconductive (PC) decay and transient photoluminescence (PL) decay. With this dual- sensor technique, one measures the transient PC and PL decay simultaneously with the same incident light pulse, removing injection-level uncertainty. Photoconductive decay measures the transient photoconductivity, Δσ(t). PCD senses carriers released from shallow traps as well as the photo-generated electron-hole pairs. In addition, variations in carrier mobility with injection level (and time) contribute to the decay time. PL decay senses only electron-hole recombination via photon emission. Theory and experiment will show that the time dependence of the two techniques can be quite different at high injection.

Evolution of the Charge Carrier Lifetime Characteristics in Crystalline Silicon Wafers During Processing of Heterojunction Solar Cells

All process steps in the fabrication of amorphous / crystalline (a-Si:H/c-Si) silicon heterojunction solar cells affect interface recombination and thus have an impact on the open circuit voltage of the final solar cell. Transient photoconductance decay is a convenient method to assess the charge carrier lifetime which is a direct measure for recombination and thus the interface quality. In this contribution we present a step-by-step analysis and evaluation of the charge carrier lifetime throughout the processing of the solar cells starting from the saw-damage etched Si wafer to the complete solar cell device. Particular emphasis is put on the optimization of the random pyramid texture. For this, surface morphologies and a-Si:H/c-Si interface properties were quantitatively evaluated and optimized with respect to low recombination and large carrier lifetimes. Based on this optimization a-Si:H/c-Si heterojunction solar cells with conversion efficiencies exceeding 20% were prepared on c-Si wafers textured in isopropanol(IPA)-free alkaline solution.

Novel Free-Carrier Pump-Probe Analysis of Carrier Transport in Semiconductors

We have developed a pump-probe configuration to measure the carrier lifetime using the transient free-carrier density. The free-carrier absorption varies as λ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Δn/μ, where λ is 10.6 μm in this paper. We measure the transient photoconductive decay that is proportional to Δn * μ. The data product gives Δα * Δσ~ λ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Δn(t) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The mobility variation is nullified by multiplying the data from the two parallel measurements. From the product data, both Δn(t) and μ(Δn) can be determined. A large increase in Δα and decrease in μ are observed and caused by space-charge effects in regions of high injection. These data show the unexpected and remarkable result that the lifetime is relatively constant up to an injection level of about three times the doping level. However, the mobility decreases by about a factor of six over the same injection range.

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 &amp; Co. KGaA, Weinheim)

Electrical conduction mechanism in nanocrystalline CdTe (nc-CdTe) thin films

The present paper reports the electrical characterization of nc-CdTe thin films in different temperature ranges. Thin films of nc-CdTe are deposited on the glass substrates by Physical Vapor Deposition (PVD) using the Inert Gas Condensation (IGC) method. The Transmission Electron Microscopy (TEM) studies are made on the CdTe nanocrystals. The surface morphology and structure of the thin films are studied by the Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD) measurements. Dark conductivity measurements are made on the nc-CdTe thin films in the temperature range 110–370 K in order to identify the conduction mechanism in this temperature range. The obtained results reveal three distinct regions at high, low, and sufficiently low temperature regions with decreasing activation energies. The analysis of the high temperature conductivity data is based on the Seto’s model of thermionic emission. At very low temperatures, dc conductivity (σ d) obeys the law: lnσT 1/2∝T −1/4, indicating variable-range hopping in localized states near the Fermi level. The density of the localized states N(E F) and various other Mott’s parameters like the degree of disorder (T O), hopping distance (R), and hopping energy (W) near the Fermi level are calculated using dc conductivity measurements at low temperatures. Carrier type, carrier concentration, and mobility are determined from the Hall measurements. The transient photoconductivity decay measurements are performed on the nc-CdTe thin films at different intensities in order to know the nature of the decay process.

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.

Analysis and optimization approach for the doped amorphous layers of silicon heterojunction solar cells

Comparison of the open-circuit voltage (external voltage Voc,ext) determined by Suns-Voc measurements with the implied voltage Voc,impl determined by transient photoconductance decay lifetime measurements can yield a quick and easy analysis of silicon heterojunction (SHJ) solar cells, especially in regard to finding the optimum doping concentration of the emitter layer [or back surface field (BSF)]. A sufficiently high doping concentration of the emitter and BSF is mandatory to extract the internal Fermi-level splitting and thus the internal voltage, at the solar cell contacts. However increasing the concentration of doping gases during the deposition of doped amorphous silicon layers results in a reduction of the interface passivation quality and Voc,impl. The best trade off is realized when the ratio of Voc,ext to Voc,impl (external/internal Voc-ratio ζ) reaches a saturation value near 1 upon increasing the doping concentration. AFORS-HET (Automat FOR Simulation of HETerostructures) simulations resulted in the conclusion that the characteristics of the external/internal Voc-ratio are mainly determined by the active doping concentration (doping minus defect concentration).

Measurement of charge carrier recombination rates in planar hybrid CdS/poly3-octylthiophene solar cells

Hybrid bilayer solar cells have been prepared by chemical bath deposition of cadmium sulfide (CdS) thin films on conductive glass and drop cast poly3-octylthiophene (P3OT) solution on top of CdS free surface. Gold contact on P3OT film was thermally evaporated. Transient photoconductance decay method was implemented to determine the charge carrier recombination times in CdS films. The same parameter in P3OT films was estimated by time-of-flight technique. Charge recombination processes in CdS/P3OT interface were studied by transient photovoltage method. The relation between the photovoltaic performance of hybrid cells and charge recombination times at CdS/P3OT interface was analyzed as a function of P3OT film thickness. It is suggested that the ambient oxygen or humidity may originate the increased charge recombination center density in thinner P3OT films and, as a result, deteriorate the short-circuit current density and open-circuit voltage values of the corresponding hybrid cells.

Charge carrier lifetime in boron carbide thin films

Charge carrier lifetime is a critical parameter to improve the conversion efficiency of radioisotope power sources and the sensitivity of neutron detectors based on boron carbide thin films. The effective charge carrier lifetime in B4C boron carbide films has been investigated by using transient photoconductivity decay. The carrier lifetime depends on the characterization conditions as well as the structure of the films. The measured lifetime could be up to ∼1 ms in B4C film, which is much longer than that in conventional semiconductors. The photoresistance change in B4C films and the photovoltaic response of B4C/n-Si(100) heterojunctions have also been studied.

Contactless measurement of minority carrier lifetime in silicon ingots and bricks

AbstractMeasuring the bulk lifetime of unpassivated blocks and ingots is of great interest to the solar cell industry. The eddy‐current photoconductance method is a common choice for such measurements, employing the quasi‐steady‐state (QSS) mode for lower lifetime samples, and the transient photoconductance decay (PCD) mode for higher lifetime samples. Due to the high surface recombination velocity in unpassivated bulk samples, the lifetime measured with this method consists of components of recombination at both the surface and in the bulk. In order to determine the bulk lifetime from the measurement data, simulations of both transient and QSS mode measurements were conducted. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Chemical surface passivation of silicon nanowires grown by APCVD

Silicon nanowires (SiNWs) are attractive candidate for solar cells and surface passivation has been recognized an important fabrication steps solar cells. The SiNWs were grown on p-type Si (1 0 0) substrate by atmospheric pressure chemical vapour deposition. Field emission scanning electron microscopy, Raman spectroscopy and Fourier transform infrared spectroscopy were used to study the atomic bonding and microstructural aspect of silicon nanowires. Hydrogen and chlorine passivation were carried out by dilute HF and HCl solutions. The transient photoconductance decay and effective lifetime of SiNWs/c-Si were study by microwave photoconductance decay. The effective lifetime of SiNWs/p-Si were observed in between 0.5 and 0.8 μs.

Contactless Measurement of Carrier Lifetime on Silicon Ingots and Bricks

AbstractHigh efficiency silicon solar cells demand the use of high lifetime silicon wafers. Characterization of boules and bricks before wafering allows poor quality material to be rejected before expensive processing steps. This paper extends simulation techniques previously used in quasi-steady-state-photoconductance to transient photoconductance decay measurements of high lifetime bulk samples. Simulated photogenerated carrier density profiles allow estimation of the bulk lifetime of a thick silicon sample with high surface recombination velocity.

Study of the traps at a mercury cadmium telluride–anodic oxide interface using a transient photoconductive decay technique

An equivalent circuit model approach has been used to understand the influence of the DC bias on the physical behaviour of the peaks recently reported in the transient photoconductive (PC) decay curves of mercury cadmium telluride (HgCdTe) samples. This has resulted in an improved model, which provides more insight into the physical process responsible for the appearance of these peaks in the decay curves. Interpretation of the results and the effects of annealing on HgCdTe samples covered with anodic oxide are presented.

Influence of chemical treatments on the photoinduced charge carrier kinetics of nanocrystalline porous TiO 2 films

Effects of Ti(iso-C 3H 7O) 4 and TiCl 4 treatments on the photoinduced charge carrier kinetics of nanocrystalline porous TiO 2 films were studied by time-resolved microwave conductivity measurements. Both treatments greatly influence the transient microwave photoconductivity decay behaviors by increasing the photogenerated charge carrier concentration and diminishing the charge carrier recombination via suppressing the hole-trapping defects. This leads to improve the I– V behaviors of nanocrystalline porous TiO 2 films effectively. The results are discussed in terms of surface modification of anatase nanocrystallites which grow on the film surface by hydrolysis of Ti(iso-C 3H 7O) 4 and TiCl 4 during the treatments.

Enhancing the surface passivation of TiO2 coated silicon wafers

In this letter, we demonstrate good surface passivation of lightly diffused n-type solar cell emitters using titanium dioxide (TiO2) thin films treated with a furnace oxidation process. Transient-photoconductance decay, x-ray photoelectron spectroscopy, and scanning electron microscopy measurements indicate that the silicon dioxide layer formed at the TiO2:Si interface provides excellent surface passivation. Emitter dark saturation current densities of 4.7×10−14 A/cm2 are achieved by this method, demonstrating that TiO2 films are compatible with high-efficiency solar cell structures.