Charge Deep-level Transient Spectroscopy Research Articles

Improvement in stability and exploring the photovoltaic properties of CsPbI2Br thin films for perovskite solar cells

The phase stability and quality of CsPbI2Br perovskite film directly determines the performance of perovskite solar cells (PSCs). However, there is a lack of research on phase stability of perovskite films of PSCs. Here, by simply controlling precursors with specified ratios and solvents, the phase stability, charge transfer resistance and electron lifetime improve significantly which is useful for high performance in perovskite solar cells. Besides this, the information about trap centres by a novel technique Charge Deep Level Transient Spectroscopy (Q-DLTS) also helps to improve efficiency of perovskite solar cells. In this work, CsPbI2Br perovskite thin films have been synthesized using simple solution processing method, by varying CsBr and PbI2 precursors with specified ratios of 1.05 M:1 M and 1.10 M:1 M in Dimethysulfoxide (DMSO) and Dimethyl formamide (DMF). The CsPbI2Br film with ratio of 1.05:1 has more phase stability in ambient air at room temperature than that of 1.10:1. Drop casting method was adopted for thin films preparation and structurally cubic phase of perovskite films obtained by X-ray diffraction (XRD). Scanning electron microscope (SEM) showed more homogeneity on the surface of the sample with ratio 1.05:1 and energy dispersive x-ray spectroscopy (EDX) confirmed the presence of same amounts in chemical composition. Fourier transform infrared spectroscopy (FTIR) transmittance spectra showed that main peak is generally observed because of stretching vibrations of lead-iodide ions. In UV–visible (UV-Vis) little effect was observed on the energy band gaps i.e. 1.93 eV for 1.05 M:1 M and 1.96 eV for 1.10 M:1 M. The electrochemical impedance spectroscopy (EIS) measurements were performed for electronic charges accumulation on the surface of the films which showed semi-circles Nyquist plots and frequency dependent Bode plots were used to find electron lifetime. The charge transfer resistance (Rct) decreased from 52Ω to 45Ω and electron life time were approximately equal to 0.10 s and 0.09 s for the ratio of 1.10:1 and 1.05:1. I-V measurements showed an Ohmic contact formation providing insights into defect concentrations with a linear response to applied bias. Recombination lifetimes were investigated using transient photovoltage (TPV) at various biases, no correlation between the applied bias and recombination lifetime was found. Q-DLTS indicated presence of 3 trap centers for the charge carriers which were actively responsible for degradation of the perovskite material, these traps serve as recombination centres that reduce the overall power conversion efficiency (PCE) of solar cells. Photoluminescence spectroscopy (PL) analysis was done to verify the energy bandgap value which was found similar in comparison to UV–VIS.

Investigation of AlN/Si based heterogeneous Junction using inter-digitated electrodes for enhanced UV light detection

A 200 nm thick AlN layer is fabricated by Hydride Vapor Phase Epitaxial (HVPE) technique onto the n-type Silicon < 111 > wafer. After epitaxial growth the AlN is masked and 100 nm thick interdigitated electrodes of Silver are deposited by the atomistic scaled Physical Vapor Deposition (PVD) system. These interdigitated electrodes are further examined with five possible circuit configurations of AlN/Silicon under multiple biases and temperatures. Different electrical and electro-optical characterization such as Current-Voltage, Charge Deep Level Transient Spectroscopy and Kinetics of Dark and UV current; have been performed for each configuration. For all such cases the Ultra-Violet (UV) light has been sensed and found that the reversed biased silicon coupled with interdigitated AlN schemes may provide largest responsivity under both photovoltaic and photoconductive modes. Also, the photo-conversion gain (ratio of UV Current / Dark Current) of each case under different physical conditions have also been emulated and found that the same reverse biased configuration leads higher extent of photo-conversion gain. The charge based transient analysis using (Q-DLTS) have been performed for all said configurations and it is found that the same reverse biased scheme offers largest magnitude of capture cross section i.e. ~ 10−18 cm2, which may capture larger extent of free charge carriers in UV mode and could be opted for enhanced UV light detection.

Advanced electrical characterization of AlN/Si based heterogeneous junction for photonic applications

AlN/Si based heterogeneous junction is investigated, herein, for photonic applications. A hyperfine layer of AlN onto the n-type Silicon substrate is fabricated, and a detailed analysis of subject stack has been carried out by variety of techniques such as Scanning Electron Microscope (SEM), Current-Voltage (I–V), Capacitance-Voltage (C–V), Charge Deep Level Transient Spectroscopy (Q-DLTS), Transient of Photo-voltages (TPV) and Kinetics of Dark and Luminous Currents. The occurrence of Si3N4 layer at the interface of AlN and Si has been witnessed and extensively studied by the electrical diagnostic approaches to identify a crucial role of the charges, which may directly affect the properties and operation of whole AlN/Si stack. The optical measurements have also been performed and evidence of Si3N4 layer formation at the AlN and Silicon interface, which plays pivotal role to the overall AlN/Si stack. Furthermore, the defect levels and their respective qualitative as well as quantitative analysis near the valence and conduction band edges of AlN is also reported. The given stack has been investigated by multi-bias and switched voltage (positive/negative) routines at different active areas of subject AlN device matrix for their potential usage in Deep ultra-violet (DUV) and other photonic application.

Defect mapping of active layer of CdTe solar cells using charge deep level transient spectroscopy (Q-DLTS)

Photovoltaic solar cells based on thin-film cadmium telluride (CdTe) have emerged as a potential contender in terms of efficiency in comparison to multi-crystalline silicon solar cell technology. The record efficiency presented in literature is reported to be as high as 22%.However, the theoretical approach suggests that the efficiency of up-to 25% can be achieved through these solar cells. The one of the main factors that affect the efficiency of the solar cells is the open circuit voltage (Voc) and the maximum Voc for CdTe solar reported was about 600 mV. Thus, In order to increase the Voc we have intentionally drifted the external voltage bias of 700 mV, 800 mV, 900 mV and 1000 mV to the active layer of CdTe based solar cell and see the physical impact of native defects in form of trapped charges that are actually responsible for limitation of lower open circuit voltage. For this, we have used the diagnostic technique known as Charge Deep Level Transient Spectroscopy (Q-DLTS) under dark and luminous conditions at different values of ambient temperatures and external bias potential. We have experimentally investigated that there exist high recombination trap centers located at 0.448 eV, 0.383 eV, 0.366 eV and 0.341 eV within the energy band gap of CdTe which hinder us to obtain the theoretical limits of Voc.

Carrier removal and transport in photonic integrated circuit ready InGaAsP/InP substrate: Electrical and transients of charges evaluation

InGaAsP based technologies are more favorable due to its unique monolithic integrated nature. For an efficient electro-optical device structure, there is a keen interest to develop active and passive components to suffice the needs of monolithic platform. Thus, carrier's transport and removal regions are carefully fabricated with low optical and electronic signal loss in order to optimize the signal to noise ratio (SNR) to effectively produce monolithic chip. In this specialized study we have provided an insight about the effective multi-implant ion-assisted carrier removal capability of the highly conductive epitaxially grown InGaAsP/InP device matrix as PIC's substrate. For this multi-implant strategy has been adopted with C+ and Ni+ ions implanted separately. Their dosage's along with ion energies are simulated rigorously before the actual implantations. Post implant measurements of Sheet resistance, mobility, drift driven electric fields, current-voltage, Arrhenius based activation energies, charge based transient analysis and detailed investigations of traps have been performed to study the viability of carrier removal scheme for the stable as well as reversible electrical isolation of this photonic integrated circuit's platform.Trap parameters identified as a result of a specialized charge deep level transient spectroscopy provides an insight about the energy levels and possible evaluation of recombination/generation centers for effective photonic activity within the device regions. The optical loss study has been performed to inspect the optical hindrance levels into the said InGaAsP PIC matrix.

Influence of device deposition techniques on the process optimization of CdZnTe thin-film matrix using charge-based analysis

–Cadmium Zinc Telluride (CZT) is tertiary semiconductor material that has numerous applications in photonics, Opto-electronics, and photovoltaics due to its tunable direct band gap of 1.4eV–2.26eV. This bandgap can be altered by varying the elementary concentrations of Cadmium Telluride (CdTe) and Zinc Telluride (ZnTe). There are several different techniques that are used to grow CZT on different substrates. In this study, we have employed two different techniques, Electron Beam Evaporation (E-Beam) process and Resistive Heating (R-Heating) technique, and investigated the variance in the electrical characteristics of CZT matrixes fabricated by these two techniques. We have grown 1 μm thick Cd1-xZnxTe (x = 0, 0.25, 0.5, 0.75, 1) thin films and performed comparative analysis of each sample with its respective growth technique. After fabrication we have analyzed Current Voltage (I–V), Charge Deep Level Transient Spectroscopy (Q-DLTS), Transient of Photo-voltage (TPV), trap analysis for the determination of overall activation energy, deep level traps and their locations within the energy band picture, trap assisted capture cross sections, deep level trapping densities, their comparative ratios (I R-Heating/I E-beam) and provided their comparative nurture case wise. Our analysis revealed that ZnTe sample are least effected by the deposition technique whereas CdTe samples are most effected when deposited by different techniques. We have found that trap centers are present in each fabricated sample and all are above the fermi level. The trap level placed nearer to the conduction band edge has higher capture cross sections than those placed far from the conduction band edge. Trap level energies are ranging from this to this with capture cross sections of this to this, respectively. The trap centers are also requalified from the TPV measurements i.e. lower trap energies provide higher potential and vice-versa.

Charge transient behaviour and spectroscopic ellipsometry characteristics of TiN/HfSiO MOS capacitors

A combination of two powerful techniques, namely, charge deep level transient spectroscopy and spectroscopic ellipsometry is employed on atomic layer deposited Si-metal oxide semiconductor capacitors (MOSCAPs) to investigate the energy efficiency of the physical process. Ultra-thin TiN/HfSiO acted as gate-dielectric stack on Si substrate was carefully subjected to rapid thermal processing and subsequent spectroscopic measurements to determine the transient behaviour of charges and electro-optical characteristics. Some key parameters such as trap concentration, activation energy required to surmount the traps, capture cross section, refractive index and extinction coefficient are found to play an important role in order to assess the energy efficiency of the devices both in terms of post-process quality of the retained surface and residual efficiency of the process by virtue of dynamics at atomistic scales. The results may provide a useful insight to the Si manufacturing protocols at ever decreasing nodes with desirable energy efficiency.

Capacitance characterization of GaP/n-Si structures grown by PE-ALD

Thin layers of μc-GaP and a-GaP grown on n- type silicon wafers by plasma-enhanced atomic layer deposition at 380 °C are characterized by space charge capacitance techniques, C-V profiling and deep level transient spectroscopy (DLTS). Two defect levels with activation energies of 0.30 eV and 0.80 eV were detected by DLTS in the μc-GaP/n-Si structure. Measurements performed on Schottky barriers formed on n-Si after selective etching of the GaP layer did not reveal any defect level meaning that the observed defects in the μc-GaP/n-Si structure are related to μc-GaP layer.

Optical and electronic properties of the partially fluorinated graphene suspensions and films

Structural and optical properties of films and particles prepared from partially fluorinated graphene suspensions were examined. Photoluminescence (PL) coming from partially fluorinated graphene suspensions (quantum dots with fluorinated edges) and films prepared from such suspensions was observed. The necessary conditions for excitation of photoluminescence and PL emission spectra consisting of one to three PL features with energies 2.65, 2.81, and 2.97 eV have been identified. The PL emission spectra were compared with the spectrum of the size quantization levels in partially fluorinated graphene and few-layer graphene films. The spectrum of levels was revealed from the charge deep-level transient spectroscopy measurements taken during studying the electron capture and the emission processes in such films. The PL emission spectra were interpreted with the assumption that fluorination of graphene suspensions leads to formation of quantum dots within graphene. A qualitative model was proposed to explain the observed correlation between the PL emission spectrum and the energy spectrum of graphene quantum dots embedded in a fluorinated graphene matrix.

Charge-based deep level transient spectroscopy of B-doped and undoped polycrystalline diamond films

The undoped and B-doped polycrystalline diamond thin film was synthesized by hot filament chemical vapor deposition and microwave plasma, respectively. The structural characterization was performed by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The electrical properties of synthesized diamond layer were characterized by dc-conductivity method and charge deep level transient spectroscopy. The B-doped diamond layers show higher sp2/sp3 ratios in comparison with that of undoped layers what can have an essential influence on the localized density of states associated with shallow hydrogen acceptor states what is reflected in the values of activation energies which reached the values of 38 meV for B-doped and 55 meV for undoped diamond layers, respectively. The existence of deep level traps, as, for example, associated with B-related acceptors, was not observed.

Studies of Parylene C Microfibrous Thin Films Electrical Properties

Parylene C is known for its flexibility, transparency, biocompatibility, and dielectric reliability. Parylene C is, hence, researched for use as a gate dielectric in transistors and capacitors for flexible electronics as well as for nano/microelectromechanical system applications [1-3]. In this study, we use metal–insulator–metal (MIM) capacitor structures to examine the electrical characteristics of columnar microfibrous thin films (µFTFs) of Parylene C prepared using oblique-angle physicochemical deposition [4,5], whereby a collimated vapor Parylene C is directed at an angle χv with respect to the exposed face of a planar substrate. The values of χv used were 30°, 60°, and 90°. We found that the low-frequency relative permittivity of the columnar µFTF increases with χv. This is explained in terms of the porosity as a function of χv, the porosity being higher for lower χv. Leakage current and charge deep-level transient spectroscopy (Q-DLTS) measurements at temperatures in the 230–470 K range and at frequencies in the 1–1000 kHz range were used to evaluate the dielectric integrity of the columnar µFTFs. Observed capacitance transients and Q-DLTS peaks in temperature were analyzed to uncover carrier traps located in the Parylene C bulk and at the metal/Parylene C interfaces in the MIM structures. The variations of DC and AC conductivities can be explained in terms of charge transport mechanisms involving thermionic emission and electron hopping. W. T. Wondmagegn, N. T. Satyala, I. Mejia-Silva, D. Mao, S. Gowrisanker, H. N. Alshareef, H. J. Stiegler, M. A. Quevedo–Lopez, and B. E. Gnade, Thin Solid Films, 519(13), 4313–4318 (2011).T. Trantidou, D. J. Payne, V. Tsiligkiridis, Y.-C., Chang, C. Toumazou, and T. Prodromakis, Sensors and Actuators B: Chemical, 186(1), 1–8 (2013).B. Park B., K-J. Im, K. Cho, and S. Kim, Organic Electronics, 9(5), 878–882(2008).C. Chindam, N. M. Wonderling, A. Lakhtakia, O. O. Awadelkarim, and W. Orfali, Applied Surface Science, 345(1), 145–155 (2015).Khawaji, I. H., Chindam, C., Orfali, W., Awadelkarim, O. O., Lakhtakia, A. ECS Transactions, 69(5), 113-119 (2015).

Origin of hole and electron traps in graphene oxide

Charge-carrier capture/emission processes proceeding with the participation of localized states in graphene oxide (GO) in test structures of Au/SiO2/GO/SiO2/Si were examined by charge deep-level transient spectroscopy (Q-DLTS). Two groups of traps capable of capturing both electrons and holes in GO were detected. The energy levels of these groups with reference to the electronic band structure of Si were found to be at EV + 0.75 eV (EC − 0.37 eV) and EV + 0.55 eV (EC − 0.55 eV). Such levels are proposed to be inherent to graphene islands in which charge carriers are emitted from energy levels in the vicinity of the Dirac point. Two groups of levels are suggested to be attributed to graphene islands, with and without p-doping with oxygen.

Correlation between electrical parameters and defect states of polythiophene:fullerene based solar cell

We investigated the degradation of the poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C61-butyric acid methyl ester (PCBM) organic solar cells (OSCs) induced by several agents (ambient air and humid air in dark, and photo-oxidation in ambient air). Most of previous studies have focused on such a high degree of the organic film degradation which was far behind the OSC functionality. This study correlates degradation of OSC electrical parameters in time with defect states measured with charge deep-level transient spectroscopy on the same samples. The basic electrical parameters of the OSC, and the generation rate and the dissociation probability of exciton were determined from dark and light current–voltage measurements. Degradation agents were applied to the complete samples or directly to the active P3HT:PCBM layer before back Ca/Ag contact evaporation with the aim to discriminate contributions of particular solar cell regions to the degradation. The results show that a decrease of the OSC power conversion efficiency is mainly caused by the photocurrent loss in all kinds of degradation treatment. We observed the formation of oxygen-related defect states with the energy around 0.1eV above highest occupied molecular orbital of P3HT, which is in accordance with other published data. The increase of the defect states with the degradation time correlates with the photocurrent loss and enhancing exciton recombination. The OSC performance is substantially influenced by the degradation of the back contact and its interface with the active layer. This effect is even more serious than the P3HT:PCBM layer exposure to the ambient or humid air in dark. The fastest degradation was observed in the case of the active layer photo-oxidation.

Charge deep-level transient spectroscopy of SiO2 and Al2O3 layers with embedded Ge nanocrystals

The present study of deep level transient spectroscopy (DLTS) is focused on a comparison of the trap states in two types of Ge nanocrystallites (NCs)-insulator composites. The investigated systems were the dielectric matrices Al2O3 and SiO2 in which the Ge NCs were embedded. We have found couples of traps with related values of activation energies in both the Ge:Al2O3 and the Ge:SiO2 films. In the films with a relatively low Ge content (where only small NCs sized 3–5 nm could have been detected by means of Raman spectroscopy), we observed traps with an energy level ∼50 meV in the Ge:Al2O3 films and 120 and 50 meV in the Ge:SiO2 films. In both systems, we found that the electron traps have a small carrier capture cross-section (10−21–10−23 cm2). We have identified the levels of the traps to be the quantum confinement levels in the small Ge NCs. For samples of higher Ge contents, where the NC size reaches about 20 nm and where an appreciable portion of the dielectric matrix consists of amorphous Ge (α-Ge), we found traps with an energy of 0.22–0.24 eV in the Ge:Al2O3, and 0.26–0.27 eV in the Ge:SiO2 samples. We suggest that this peak in the charge DLTS (Q-DLTS) spectra is associated with a trap at the Ge-NC/α-Ge interface. We have also identified the energy position of a defect level in the Ge:Al2O3 layers, which lies 0.46–0.49 eV below the conduction-band edge of the Si substrate.

Synthesis of carbon films in a plasma-chemical reactor based on beam-plasma discharge

A modification of the plasma-chemical reactor based on beam-plasma discharge is developed for the deposition of carbon (including diamond-like carbon (DLC)) films. The reactor differs from the existing devices by the simplicity of the procedure that makes it possible to control the energy characteristics of the ion beam incident on the film in the course of deposition. A method for the computer simulation of the parameters of the ion flux on an electrically insulated surface upon the modulation of the plasma potential is proposed. The method allows the prediction of the ion energy and ion flux that acts upon the deposited film. DLC films on metal substrates are produced. The charge deep-level transient spectroscopy is used to reveal the effect of the adsorbed water and alcohol vapors on the electrophysical properties of the films, which indicates that the films can be used as active adsorbing materials in chemical sensors.

Luminescence and deep-level transient spectroscopy of grown dislocation-rich Si layers

The charge deep-level transient spectroscopy (Q-DLTS) is applied to the study of the dislocation-rich Si layers grown on a surface composed of dense arrays of Ge islands prepared on the oxidized Si surface. This provides revealing three deep-level bands located at EV + 0.31 eV, EC – 0.35 eV and EC – 0.43 eV using the stripe-shaped p-i-n diodes fabricated on the basis of these layers. The most interesting observation is the local state recharging process which proceeds with low activation energy (∼50 meV) or without activation. The recharging may occur by carrier tunneling within deep-level bands owing to the high dislocation density ∼ 1011 - 1012 cm-2. This result is in favor of the suggestion on the presence of carrier transport between the deep states, which was previously derived from the excitation dependence of photoluminescence (PL) intensity. Electroluminescence (EL) spectra measured from the stripe edge of the same diodes contain two peaks centered near 1.32 and 1.55 μm. Comparison with PL spectra indicates that the EL peaks are generated from arsenic-contaminated and pure areas of the layers, respectively.

Defects in an Electron-Irradiated 6H-SiC Diode Studied by Alpha Particle Induced Charge Transient Spectroscopy: Their Impact on the Degraded Charge Collection Efficiency

Defects in electron-irradiated 6H-SiC diodes have been studied by single alpha particle induced charge transient spectroscopy and deep level transient spectroscopy (DLTS) in order to identify critical defects responsible for the charge collection efficiency (CCE) decreased by high-energy electron irradiation. The defect X2 detected by the charge transient spectroscopy and the electron trap Ei detected by the DLTS had a similar activation energy of around 0.50 eV. In addition, the annealing at 200oC completely removed defects X2 and Ei, and restored the CCE. The defect X2 is attributed to the electron trap Ei, and responsible for the decreased CCE.

Charge spectroscopy of SiO2 layers with embedded silicon nanocrystals modified by irradiation with high-energy ions

The method of charge deep-level transient spectroscopy (Q-DLTS) is used to study and compare the ejection of charge carriers from silicon nanocrystals (NCs) located in an ordered or random way in the SiO2 matrix. It is shown that, in all cases, this ejection is a thermally activated process. The parameters of energy barriers characterizing the processes of ejection of charge carriers from the levels of nanocrystals in the layers NCs:SiO2 before (random distribution) and after their modification by irradiation with high-energy ions (ordered distribution of nanocrystals) are determined. It is found that the activation energies for ejection of charge carriers from nanocrystals and the size of nanocrystals estimated from the difference between energies of two levels observed by the Q-DLTS method decrease as the ion fluence is increased. The density of nanocrystals observed by the Q-DLTS method decreases by approximately an order of magnitude as a result of irradiation with fluence of 1012–1013 cm−2 in comparison with an initial unirradiated structure; this decrease is due to formation of conducting chains of nanocrystals in tracks.

Deep-level spectroscopy studies of confinement levels in SiGe quantum wells

The recharging of quantum confinement levels in SiGe quantum wells (QWs) was studied by charge deep-level transient spectroscopy (Q-DLTS) for Si/SiGe/Si structures with different Ge contents in the SiGe layer. The set of levels were observed as the different slopes in the Arrhenius plots for the same Q-DLTS peak in different temperature ranges. These activation energies were compared to the energies of quantum confinement levels in the QW calculated in frames of six-band model taking into account spin-orbit interaction and attributed to a thermally activated tunneling of holes from the SiGe QW.

Confinement Levels in Passivated SiGe/Si Quantum Well Structures

The set of quantum confinement levels in SiGe quantum wells (QW) was observed in the temperature range from 80 to 300 K by means of charge deep-level transient spectroscopy (Q-DLTS) and transport measurements. These observations proved possible due to a passivation of structure surface with organic monolayer deposition. Si/SiGe/Si structures with different Ge contents in SiGe layer were studied. The confined levels in passivated structures became apparent through DLTS measurements as various activation energies in temperature dependence of the rate of carrier emission from QW. It was found that the recharging of SiGe QWs and carrier emission accomplish due to thermo-stimulated tunneling. The steps in the current-voltage characteristics originated from direct tunneling via the confined states were found to determine the current flow at high fields.