Dual Beam Photoconductivity Research Articles

The native and metastable defects and their joint density of states in hydrogenated amorphous silicon obtained from the improved dual beam photoconductivity method

In this study, undoped hydrogenated amorphous silicon (a-Si:H) thin films deposited under moderate dilution ratios of silane by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) have been investigated using steady-state photoconductivity and improved dual beam photoconductivity (DBP) methods to identify changes in multiple gap states in annealed and light-soaked states. Four different gap states were identified in annealed state named as A, B, C, and X states. The peak energy positions of these Gaussian distributions are consistent with those recently identified by Fourier transform photocurrent spectroscopy (FTPS). After in situ light soaking, their density increases with different rates as peak energy positions and half-widths remain unaffected. The electron-occupied A and B states located below the dark Fermi level and their density and ratios in the annealed and light-soaked states correlate well with those defects detected by time-domain pulsed electron paramagnetic resonance (EPR) experiments. The A, B, and X states located closer to the middle of the bandgap anneal out at room temperature in dark and define the “fast” states. However, the C states show no sign of room temperature annealing such that they must define the “slow” states in undoped a-Si:H. The results found in this study indicate that the anisotropic disordered network is a more appropriate model than previously proposed defect models based on the continuous random network to define the nanostructure of undoped a-Si:H, where multiple defects, D0 and non-D0 defects, can be identified by using the improved DBP method.

Modulated Dual-Beam Photoconductivity in Hot-Pressured a-As2Se3

Modulated Dual-Beam Photoconductivity in Hot-Pressured a-As₂Se₃

The effects of air, oxygen and water exposure on the sub-bandgap absorption, the electronic conductivity and the ambipolar diffusion length in highly crystalline microcrystalline silicon films for photovoltaic applications

Reversible and irreversible changes due to long term air and short term de-ionized water (DIW) or pure oxygen exposure were investigated in about 1 µm thick hydrogenated microcrystalline silicon (µc-Si:H) films deposited on rough glass substrates, thereby comparing highly crystalline with compact material. Time and temperature dependent dark conductivity, steady-state photoconductivity, the steady-state photocarrier grating and dual-beam photoconductivity methods have been used to study the effects. Standard measurement procedures defined previously have been carefully applied to record the changes after different treatments using the steady-state methods under light. After long term air exposure of highly crystalline µc-Si:H films, a thermal annealing step leads to an increase in dark conductivity (σD) and steady-state photoconductivity (σph) as well as to a significant increase in the sub-bandgap absorption. These effects are likely due to a reversible recovery from surface adsorbents in a porous microstructure after air exposure resulting in surface charge and Fermi level shifts in agreement with earlier results. Compact µc-Si:H films showed only marginal effects upon an annealing after long term air exposure suggesting much reduced susceptibility to surface adsorbent induced by Fermi level shifts. Five hours exposure to de-ionized water at 80 °C caused more than an order of magnitude increase in σD and σph and a substantial decrease in the sub-bandgap absorption spectrum in highly crystalline as well as in compact µc-Si:H films. In addition, minority carrier diffusion lengths measured by the steady-state photocarrier grating method improved significantly. The changes after exposure to water were not reversible upon our standard annealing procedure. Exposure to high purity oxygen gas at 150 °C resulted in similar effects like the exposure to DIW. Also here the changes in material properties were not reversible upon annealing. Results are discussed in terms of adsorption and chemical reactions on surfaces in the porous highly crystalline material versus the materials with more compact structures. Results are compared to earlier observations and consequences for device application will be indicated.

Reversible and irreversible effects after oxygen exposure in thick (>1 μm) silicon films deposited by VHF-PECVD on glass substrates investigated by dual beam photoconductivity

Metastability and instability effects due to oxygen exposure in thick intrinsic hydrogenated microcrystalline silicon films deposited by very high frequency plasma enhanced chemical vapour deposition on smooth glass substrates were investigated using temperature-dependent dark conductivity, steady state photoconductivity, and sub-bandgap absorption measurements obtained using the dual beam photoconductivity (DBP) method. No significant changes in dark conductivity and photoconductivity were detected even after long-term air exposure of samples in room ambient as well as after oxygen exposure when samples were characterized in oxygen ambient. However, characterization of the oxygen-exposed state in high vacuum caused an increase in dark conductivity and photoconductivity as well as a significant decrease in the sub-bandgap absorption coefficient spectra in the low energy region in samples with [Formula: see text]. These changes are partially irreversible for samples [Formula: see text] and mostly reversible for compact materials with significant amorphous fraction. No detectable metastable changes occurred in microcrystalline silicon samples with [Formula: see text] as well as in pure amorphous silicon.

Investigation of meta- and in-stability effects in hydrogenated microcrystalline silicon thin films by the steady-state measurement methods

Metastability effects because of atmospheric exposure, high purity gasses, and deionized water in hydrogenated microcrystalline silicon thin films with different crystalline volume fractions were studied using well accepted steady-state characterization methods of dark conductivity, steady-state photoconductivity, steady-state photocarrier grating (SSPG) and dual beam photoconductivity (DBP) methods. A standard measurement procedure has been established before using the steady state methods, in which a steady state condition of dark conductivity was established by monitoring the time dependence of dark conductivity. Samples deposited on smooth glass and rough glass substrates exhibit similar reversible and irreversible changes in the properties of microcrystalline silicon film. A reliable correlation of reversible and irreversible changes indicate that dark conductivity and photoconductivity values increase, sub-bandgap absorption spectrum obtained from DBP method decrease and correspondingly minority carrier diffusion lengths obtained from the SSPG method increase in the metastable state in various amount for microcrystalline films with crystalline volume fraction, [Formula: see text] > 0.30. Amorphous silicon and microcrystalline silicon films with [Formula: see text] < 0.30 do not show detectable metastable changes as samples exposed to atmospheric condition as well as high purity oxygen gas and deionized water.

Atmospheric aging and light-induced degradation of amorphous and nanostructured silicon using photoconductivity and electron spin resonance

Previous studies indicate that the dark conductivity in amorphous and microcrystalline silicon may increase or decrease with exposure to deionized water (DIW) or pure oxygen at 80 °C but always decreases with light exposure. While the light-induced effect is linked to paramagnetic dangling bonds (Do), the origin of metastability in microcrystalline silicon remains unclear. In this study, we use steady-state photoconductivity (SSPC), dual-beam photoconductivity (DBP), and electron spin resonance (ESR), to study the behaviors under soaking in DIW and (or) pure oxygen at 80 °C and light-exposure of amorphous (a-Si:H) and nanostructured (nc-Si:H) silicon samples deposited in a capacitively coupled plasma-enhanced chemical vapor deposition system. Powders from thick samples of low and high crystallinity (Xc) peeling off large substrates were collected in quartz tubes for ESR measurements. Dark conductivity decreases upon exposure to pure oxygen at 80 °C for nc-Si:H but remains unchanged for a-Si:H. The ESR signal attributed to Do decreases with soaking in DIW for high and low crystallinity nc-Si:H but the effect is more significant for higher Xc. Changes in SSPC, DBP, and ESR are used to compare the degradation mechanisms because of O2 exposure and light for amorphous and nanostructured silicon.

Metastability effects in hydrogenated microcrystalline silicon thin films investigated by the dual beam photoconductivity method

Metastability effects in microcrystalline silicon (μc-Si:H) thin films have been investigated using dark conductivity, σD, photoconductivity, σph, and sub-bandgap absorption methods. Nitrogen and inert gasses can cause reversible aging effect in conductivities but not in the sub-bandgap absorption. However, DI water and O2 gas treatment result in both reversible and nonreversible effects in conductivities as well as in the sub-bandgap absorption. Only oxygen affected the dark conductivity reversibly in amorphous silicon, a-Si:H, films, other results were unaffected from the aging and annealing processes applied.

The effects of native and light induced defects on optoelectronic properties of hydrogenated amorphous silicon–germanium (a-SiGe:H) alloy thin films

Effects of native and light induced defects states in hydrogenated amorphous silicon–germanium alloy thin films with different Ge concentrations have been investigated by using steady-state photoconductivity, dual beam photoconductivity (DBP), transmission spectroscopy and photothermal deflection spectroscopy (PDS) techniques. In the annealed state, sub-bandgap absorption spectra obtained from both PDS and DBP overlap very well at energies above 1.4 eV. However, differences in α (hν) spectrum exist in the lower energy part of absorption spectrum. The α (hν) value measured at 1.0 eV is the lowest for 10% Ge sample and increases gradually as Ge content of the sample increases. In the light soaked state, time dependence of photoconductivity decay obeys to t −x power law, where x changes from 0.30 to 0.60 for samples with low Ge content and 0.05–0.1 for samples with high Ge content. Correspondingly, the increase of the sub-bandgap absorption coefficient at lower energies obeys to t y power law, where y values are lower than the x value of the same sample. It can be inferred that sub-bandgap absorption and photoconductivity measurements are not controlled by the same set of defects created in the bandgap of alloys.

Study of the passivation mechanisms of boron doped diamond using the Amplitude Modulated Step Scan Fourier Transform Photocurrent Spectroscopy

Amplitude Modulated Step Scan Fourier Transform Photocurrent Spectroscopy (AMFTPS) and Dual Beam Photoconductivity are sensitive tools for the measurements of shallow and deep defects related absorption in high resistive semiconductors. Measurements were performed on different homoepitaxial boron doped layers in as-grown and passivated states and will be presented. We will study in details and will discuss the origins of changes in the gap levels due to the passivation of boron by hydrogen in diamond.

The influence of thermal annealing on the electronic defect states in nanocrystalline CVD diamond films

AbstractThe photosensitive, nominally undoped nanocrystalline diamond (NCD) films were grown on insulating substrates at a relatively low temperature below 600 °C. The NCD films were exposed to the hydrogen plasma and then briefly to the oxide plasma. This treatment significantly reduced the surface conductivity and enhanced the bulk photosensitivity. The broad optical spectral range (200–2000 nm) photocurrent and optical absorption spectra were measured by the dual‐beam photoconductivity (DBP), Fourier‐transform photocurrent spectroscopy (FTPS) in amplitude modulated step scan mode, optical transmittance and reflectance and photothermal deflection spectroscopy (PDS). The photoionization cross section spectrum of our NCD films is dominated by the non‐diamond phase with the photo‐ionization threshold energy at about 1 eV. At 350 °C the thermal annealing increased the concentration of defects related to the non‐diamond phase and deteriorated the UV photosensitivity. The significant increase of the optical absorption was observed after annealing at 500 °C. The hydrogenation can reduce to some extend the defects created by the thermal annealing. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photocurrent study of electronic defects in nanocrystalline diamond

The broad optical spectral range (0.2–6 eV) photoionization spectra of the nominally undoped, carefully oxidized and hydrogenated nanocrystalline diamond (NCD) thin films grown on silicon and sapphire substrates were measured using the dual-beam photoconductivity method (under constant UV light bias). The novel amplitude modulated step scan Fourier transform photocurrent spectroscopy (AMFTPS) was applied in IR region. The wet chemistry etching was used to open 6 × 6 mm window in Si substrate and to create NCD membrane in the silicon frame. In photoionization spectra we found localized defect states with the threshold around 1 eV in both hydrogenated and oxidized NCD films. The threshold of the photoionization cross section shifts towards lower photon energies in the hydrogenated samples. The photosensitivity increases by several orders of magnitude with hydrogenation. We suggest that the main deep defect is related to the nanocrystalline grain boundaries and it can be passivated by the hydrogen. In hydrogenated samples we detect at low temperature shallow defect states with photoionization threshold energies well below 1 eV, probably related to the hydrogenated surface.

Comparison of photocurrent spectra measured by FTPS and CPM for amorphous silicon layers and solar cells

Fourier Transform Photocurrent Spectroscopy (FTPS) has been recently introduced as a fast and highly sensitive method for the evaluation of the optical absorption coefficient of photoconductive thin films such as microcrystalline silicon layers. This contribution represents the first study of FTPS utilization for amorphous silicon layers and cells. FTPS spectra are compared with results of Constant Photocurrent Method (CPM) and Dual Beam Photoconductivity (DBP) measured at different chopping frequencies. We will concentrate to highlight the appropriate measuring conditions and evaluation procedures for correct data interpretation. Moreover, we will present our novel approach for the interference free determination of absorption coefficients of thin films grown on transparent substrates, which is mainly important for very thin layers where broad interference fringes do not allow correct evaluation of parameters such as a slope of the Urbach tail and the defect density.

Characterization of Gap Defect States in Hydrogenated Amorphous Silicon Materials

ABSTRACTAn enhanced simulation model based on the carrier recombination through these states was developed to characterize the gap defect states in hydrogenated amorphous silicon materials (a-Si:H). The energy dependent density of electron occupied gap states, kN(E), was derived directly from Dual Beam Photoconductivity (DBP) measurements at different bias currents. Through Gaussian de-convolution of kN(E), the energy peaks of the multiple defect states, including both neutral and charged states, were obtained. These energy levels, together with the information on the capture cross sections, were used as known input parameters to self-consistently fit the subgap absorption spectra, the electron mobility-lifetime products over a wide range of generation rates, as well as the energy dependent density of electron occupied gap state spectra. Accurate gap state information was obtained and the nature of the defect states was studied. Simulation results on light degraded hydrogen diluted, protocrystalline a-Si:H show that the density of charged states is 2.3 times that of neutral states. The two states close to the midgap act as effective recombination centers at low generation rates and play key roles in photoconductivity studies.

Amplitude modulated step scan Fourier transform photocurrent spectroscopy of partly compensated B‐doped CVD diamond thin films

AbstractWe report on the development of amplitude modulated step‐scan Fourier‐transform photocurrent spectroscopy (AMFTPS) as a sensitive tool for detecting shallow and deep electronic states in high resistive semiconductors. We present fundamental aspects of dual beam photoconductivity (DBP) and AMFTPS and apply these methods to detect the defect‐induced localized electronic levels in the energy gap of boron doped epitaxial CVD diamond thin films grown on single crystalline type Ib (111) oriented diamond substrates. The mechanism of acceptor passivation upon oxygen addition during CVD growth in B‐doped films is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Sub-bandgap optical absorption spectroscopy of hydrogenated microcrystalline silicon thin films prepared using hot-wire CVD (Cat-CVD) process

Hydrogenated microcrystalline silicon (μc-Si:H) thin films with different silane concentration (SC) have been prepared using the HW-CVD technique. Dual beam photoconductivity (DBP), photothermal deflection spectroscopy (PDS), and transmission measurements have been used to investigate the optical properties of the μc-Si:H films. Two different sub-bandgap absorption, α( hν), methods have been applied and analyzed to obtain a better insight into the electronic states involved. A good agreement has been obtained in the absorption spectrum obtained from the PDS and DBP measurements at energies above the bandgap. Differences between PDS and DBP spectra exist below the bandgap energy where DBP spectra always give lower α( hν) values and show a dependence on the SC. For some films, differences exist in the α( hν) spectra when the DBP measurements are carried out through the film and substrate side. In addition, for some films, there remains fringe pattern left on the spectrum after the calculation of the fringe-free absorption spectrum, which indicates structural inhomogeneities present throughout the film.

Photoconductivity spectroscopy in hydrogenated microcrystalline silicon thin films

Steady-state photoconductivity and sub-bandgap absorption measurements by the dual-beam photoconductivity (DBP) method were carried out on undoped hydrogenated microcrystalline silicon thin films prepared by VHF-PECVD and hot-wire chemical vapor deposition. The results are compared with those of the constant-photocurrent method (CPM) and photothermal deflection spectroscopy (PDS). It is found that DBP, CPM, and PDS provide complementary data on the optoelectronic processes in microcrystalline silicon.

Bulk and surface defects in a-Si:H films studied by means of the cavity ring down absorption technique

The sub gap absorption at 1.17 eV in hydrogenated amorphous silicon (a-Si:H) has been measured by means of the ex situ cavity ring down (CRD) absorption technique. The evolution of defects has been studied as a function of film thickness and deposition temperatures. A comparison with the dual beam photoconductivity (DBP) technique shows that the CRD results are systematically higher. Furthermore, it is shown that the CRD technique is sensitive to surface defects and typical surface dangling bond coverages of 2×10 −4 up to 6×10 −3 are obtained for air-exposed a-Si:H samples depending on the deposition temperature.

Ex situ and in situ defect density measurements of a-Si:H by means of the cavity ring down absorption technique

ABSTRACTFor the first time it is demonstrated that the surface defect density can be measured using the cavity ring down (CRD) absorption technique and it is shown that CRD is more sensitive for surface defects than dual beam photoconductivity (DBP) technique. Ex situ measurements have shown that the surface defects of the oxidized a-Si:H surface are distributed over a surface region with thickness W(d). The obtained surface defect density is 1.0 × 1012 up to 1.4 × 1012 cm-2. During growth the a-Si:H surface defect density region has at least a thickness of 15 nm. During deposition of 15 nm a-Si:H the surface density increases up to a not yet saturated value of 1×1013 cm-2.

Nanostructures and defects in silicon–hydrogen alloys prepared by argon dilution

Nanostructural heterogeneity of silicon–hydrogen (Si:H) alloy films deposited in a conventional radio frequency plasma-enhanced chemical vapour deposition unit from silane argon mixture has been studied by small-angle X-ray scattering (SAXS). The densities of defect states of Si:H films have been estimated by dual beam photoconductivity (DBP), photothermal deflection spectroscopy (PDS) and the modulated photocurrent (MPC) method. From the structural and defect studies we identify two regions of Ar dilution, where the structure of the films are distinctly different. Up to 90% Ar dilution, the nanostructural as well as the large-scale (>30 nm) structural heterogeneities in the amorphous Si:H (a-Si:H) network decrease. A lowering of the bulk defect density has also been observed in this Ar dilution region. Increasing Ar dilution to greater than 90% of the mixture, the a-Si:H films show some dense regions embedded in the amorphous matrix. The high- and low-density amorphous structures within the films can explain the experimental results obtained from SAXS, DBP and PDS. A negligible conduction band tail, as observed from MPC result, suggests the formation of high degree of crystallinity in the film deposited with 99% Ar dilution and higher rf power density (80 mW/cm 2).

Correlation of Nanostructural Heterogeneity and Light Induced Degradation in a-Si:H Solar Cells

The small angle X-ray scattering (SAXS) method was adopted to estimate the nanostructural heterogeneity in hydrogenated amorphous silicon (a-Si:H) materials deposited by rf plasma enhanced chemical vapor deposition (PECVD) process from silane-argon mixtures at different volume ratios. The performance of the solar cells fabricated by using the same materials as the intrinsic layer has been correlated with the integrated SAXS intensity of the intrinsic layer. The change in the density of states due to light soaking has been measured in solar cell structure by a dual beam photoconductivity method. We have observed a systematic increase in the photoinduced degradation of the photoconductivity, defect density and the solar cell parameters with the increase in the structural heterogeneities in the film. Modification of the growth kinetics due to bombardment of the metastable argon (Ar*) has been found to control the amount of nanostructural heterogeneity in the material.