Charge Deep-level Transient Spectroscopy Research Articles

Charge spectroscopy of Si nanocrystallites embedded in a SiO2 matrix

In the present work we have determined the electronic levels in systems of Si nanocrystallites (NCs) embedded in the insulating matrix of silicon dioxide, SiO2, by employing the charge deep-level transient spectroscopy (Q-DLTS) technique. We have clearly shown that these levels are associated with the NCs. Correspondingly, we suggest that the levels that we found are associated mainly with two quantum confinement energies, 0.14 and 0.19 eV. These energies are shown to be consistent with the corresponding theoretical estimates for the presently studied Si–NCs/SiO2 systems. The fact that these levels are almost fixed for the various samples studied suggests the importance of the bulk-surface coupling under quantum confinement conditions.

Deep levels, transport and THz emission properties of SiGe/Si quantum-well structures

Recharging of quantum confinement levels in SiGe quantum wells (QW) was studied by charge deep-level transient spectroscopy (Q-DLTS) for Si/SiGe/Si structures with different Ge contents in the SiGe layer. A peak with activation energy varying in the range from 0 to 100 meV in different temperature intervals was observed in Q-DLTS spectra. Activation energies extracted from Q-DLTS measuremens are in good agreement with energies of quantum confinement levels in the QW.

Charge deep level transient spectroscopy of electron traps in MOVPE grown n‐GaN on sapphire

AbstractWe report investigation of electron traps in n‐GaN, grown on sapphire by metal organic vapour phase epitaxy, by using charge deep level transient spectroscopy (Q ‐DLTS). Measurements have been made isothermally by rate window scanning over the temperature range 300–370 K and for rate windows in the range 105 s–1 to 1 s–1. Two traps are observed in this range with (i) activation energies ∼0.58 eV and ∼0.45 eV and (ii) capture cross sections ∼2 × 10–15 cm2 and ∼3 × 10–19 cm2 respectively. The first of these defects has been observed in all of the DLTS investigations reported in the literature. However, the second trap has not been seen in majority of the earlier reports. Possible reasons for this difference are discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Defect states in pentacene thin films prepared by thermal evaporation and Langmuir–Blodgett technique

We investigate defect states in pentacene thin films (PTFs) prepared by two different deposition techniques: standard thermal evaporation in high vacuum and the Langmuir–Blodgett (LB) technique. Time domain capacitance–voltage (C–V) and charge deep-level transient spectroscopy (DLTS) techniques were employed. C–V measurements indicate that Schottky barrier structures on both types of PTFs exhibit a common behavior: the thinner the PTF, the closer the measured C–V curve to the ideal shape for the Schottky barrier. Charge DLTS measurements show a peak with an activation energy of about 0.70eV above Ev and an attempt-to-escape frequency of 3×1014s−1 for PTFs deposited by thermal evaporation. For PTFs prepared by LB technique two deep-levels with activation energies of 0.20eV and 0.21eV, and an attempt-to-escape frequencies of 2×107s−1 and 2×106s−1, respectively, are found. The amplitude of the charge DLTS signals increases with decreasing PTF sample thickness for both deposition techniques. The observed defects can not be removed by annealing in vacuum. The origin of defects responsible for the measured charge DLTS spectra is discussed.

Charge deep-level transient spectroscopy study of high-energy-electron-beam-irradiated hydrogenated amorphous silicon

We present a study of changes in the defect density of states in hydrogenated amorphous silicon (a-Si:H) due to high-energy electron irradiation using charged deep-level transient spectroscopy. It was found that defect states near the conduction band were removed, while in other band gap regions the defect-state density increased. A similar trend is observed for a-Si:H which has been subjected to light soaking, but in that case the majority of defect states are created around midgap, whereas with electron-beam degradation more defect states are created near the valence-band tail.

Defect-state engineering in a-Si:H: An effective tool for studying processes during light-induced degradation

Charge deep-level transient spectroscopy (Q-DLTS) has proved to be a powerful tool for investigating the defect-state distribution in hydrogenated amorphous silicon (a-Si:H) in both annealed and light-soaked states. In this article, we report on Q-DLTS experiments that were designed to further investigate our recently published model for the Staebler–Wronki effect (SWE). By subjecting an a-Si:H based metal/oxide/semiconductor structure to negative (positive) bias voltage during the annealing above the equilibration temperature a programmed p-type (n-type) defect-state distribution can be established in the undoped a-Si:H layer. In this way defect-state engineering in a-Si:H is achieved that enables us to study the role of different components of the defect-state distribution in the light-induced degradation process. Additional possibility to control the role of a particular defect-state component in the degradation process is to apply a bias voltage also during light soaking. The observed changes in the programmed defect-state distributions due to light soaking under various conditions give additional information on the SWE and can be interpreted with our model for the SWE.

Analysis of structure and defects in thin silicon films deposited from hydrogen diluted silane

Thin silicon layers have been deposited from silane diluted with hydrogen. The dilution ratio R ( R = [H 2]/[SiH 4]) has been varied between R = 0 and R = 40. The structural properties of Si:H films have been studied using transmission electron microscopy imaging and Raman spectroscopy. The phase evolution from the amorphous phase into the mixed and eventually microcrystalline phase strongly depends on the hydrogen dilution. The initiation of the microcrystalline growth occurs between R = 20 and R = 25. The phase transition becomes more abrupt with increasing hydrogen dilution. Optoelectronic properties of the layers have been determined. Increasing hydrogen dilution results in films with increasing effective defect density and Urbach energy, which is related to inhomogeneous growth. The charge deep-level transient spectroscopy technique (Q-DLTS) was applied for the first time on hydrogen diluted thin silicon films in order to investigate the energy distribution of the defect states in these layers as a function of the dilution ratio R. The Q-DLTS spectra indicate a difference in defect-state distribution when the films evolve from the amorphous phase into the microcrystalline phase.

Defect States in Excimer-Laser Crystallized Single-Grain TFTs Studied with Isothermal Charge Deep-level Transient Spectroscopy

AbstractDefect states were quantitatively evaluated in single-grain (SG) Si thin-film transistors (TFTs), prepared by micro-Czochralski (grain filter) process with excimer-laser crystallization, by means of isothermal charge deep-level transient spectroscopy with a high sensitive charge/voltage converter. Its sensitivity reaches 10-16 C and it operates in the range of 2 microseconds - 10 ms. Measurements were performed on the SG-Si TFTs with various energy densities of laser crystallization, various channel areas, and positions in the grain. Our results indicate a direct correlation of fabrication parameters, parameters of the TFT determined from its transfer characteristics, and parameters of defect states (energy position in the band gap, concentration) induced by coincidence site lattice boundaries inside the location-controlled grains and by defects in the grain filter.

Radiation-induced defects in MOS structures after irradiation with high-energy Ar, Kr, Bi heavy ions

Radiation damage in the MOS (metal-oxide-semiconductor) structures irradiated with Ar, Kr and Bi heavy ions with energies from 1 to 7.5 MeV/nucl. has been studied using charge deep level transient spectroscopy (Q-DLTS) and feedback charge capacitance voltage ( C – V ) techniques. Four dominant well-known deep levels such as vacancy–oxygen (VO) complex, divacancy (VV 2−) in the double negative charge state and composite VV −+VSb complex were registered versus ion fluence and annealing temperature. It was shown that VO center concentration at the near-surface silicon substrate layer is determined by ion energy loss in elastic collisions and no effect of high-level electronic stopping power was detected. At the same time, the value of the positive radiation-induced defect charge in the oxide layer was decreased under Bi ions irradiation. Thermal treatment in air ambient at 250 °C restores CV characteristics of the MOS samples to that of un-irradiated state.

Effect of Fermi Level Position in Intrinsic a-Si:H on the Evolution of Defect States under Light Exposure.

AbstractThe evolution of the programmed defect-state distributions in intrinsic hydrogenated amorphous silicon (a-Si:H) due to light soaking was qualitatively determined from charge deep-level transient spectroscopy. The defect-state distribution in a-Si:H was programmed by applying a particular bias voltage on the metal-oxide-semiconductor structure while annealing the structure above the equilibration temperature. The programmed distributions simulate defect-state distributions in different parts of an actual a-Si:H solar cell, particularly in the intrinsic regions close to the p/i and i/n interfaces.The defect-state distribution in the bulk of the intrinsic layer is characterized by comparable contributions from the positively charged defect states above midgap, Dh, neutral states, Dz, and negatively charged states below midgap, De. In the programmedp-type (n-type) defect-state distribution there is an excess of the Dh (De) states. Light exposure modifies the p-type distribution that evolves to a broad distribution of states with a maximum around midgap. This distribution is dominated by Dz states with substantial contributions from Dh and De states. In case of n-type distribution light soaking only slightly influences the distribution by removing a part of the Dh states and by a small increase of Dz and De states.

Traps with near-midgap energies at the bonded Si/SiO2 interface in silicon-on-insulator structures

Capture centers (traps) are studied in silicon-on-insulator (SOI) structures obtained by bonding and hydrogen-induced stratification. These centers are located at the Si/SiO2 interface and in the bulk of the split-off Si layer. The parameters of the centers were determined using charge deep-level transient spectroscopy (Q-DLTS) with scanning over the rate window at fixed temperatures. Such a method allows one to study the traps near the Si midgap at temperatures near 295 K. It is shown that the density of traps with a continuous energy spectrum, which are located at the bonded Si/SiO2 interface, decreases by more than four orders of magnitude at the mid-gap compared with the peak density observed at the activation energy E a ≈0.2–0.3 eV. The capture centers are also found in the split-off Si layer of the fabricated SOI structures. Their activation energy at room temperature is E a =0.53 eV, the capture cross section is 10−19 cm2, and the concentration is (0.7–1.7)×1013 cm−3. It is assumed that these capture centers are related to deep bulk levels induced by electrically active impurities (defects) in the split-off Si layer close to the Si/SiO2 interface.

Origin of charged gap states ina-Si:H and their evolution during light soaking

We report on the evolution of three groups of gap states ( Dh ,Dz , and De) in the charge deep-level transient spectroscopy ~Q-DLTS! spectra during light soaking of undoped hydrogenated amorphous silicon (a-Si:H!. Recently, correlation between Q-DLTS and electron-spin resonance showed that neutral (Dz) states correspond to neutral dangling-bond defects. The present Q-DLTS spectra demonstrate the annihilation of positively charged (Dh) states above midgap different from dangling-bonds in the early stage of light soaking. The initial decrease of the Dh states is not accompanied by an increase in the Dz or negatively charged ( De) states below midgap. Further, we do not observe a direct correlation in light induced changes of all three groups of defect states. Combining our results with findings from recent nuclear magnetic resonance experiments at low temperature and computer simulations of a-Si:H network, microscopic configurations introducing charged gap states are proposed. Positively charged states are related to a complex formed by a hydrogen molecule and a Si dangling bond. Negatively charged states are attributed to the floating bonds. A comprehensive model for the Staebler-Wronski effect ~SWE! is presented, which overcomes a weak point of previous models regarding the origin of mobile hydrogen and provides new insight into complexity of the SWE.

The Role of Charged Gap States in Light-Induced Degradation of Single-Junction a-Si:H Solar Cells

ABSTRACTComputer simulations of single-junction hydrogenated amorphous silicon (a-Si:H) solar cells with different thickness of the intrinsic layer were carried out in order to study the role of charge gap states in their light-induced degradation. It is demonstrated that it is the decrease of positively charged states above midgap, Dh, and the increase of neutral states around midgap,Dz, and negatively charged states below midgap, De in the intrinsic layer that result in a drop of performance of the solar cells due to light soaking. These changes in the gap states are in accordance with our recent experimental results from the charge deep-level transient spectroscopy on undoped a-Si:H. The experimentally observed changes in the dark and illuminated J-V curves and spectral response could not be simulated with the same set of input parameters by only increasing the defect-state density in the intrinsic layer.

Comparison of Electrical Properties of Silicon-on-Insulator Structures Fabricated with Use of Hydrogen Slicing and BESOI

The top silicon layers and the bonded Si/SiO 2 interfaces in silicon-on-insulator (SOI) structures fabricated by (i) wafer bonding and hydrogen slicing, and (ii) by wafer bonding and chemical mechanical thinning (back-etch SOI, BESOI) were investigated and compared by charge deep-level transient spectroscopy. The hydrogen slicing was provided by hydrogen implantation into one of the bonded wafers and led to the high hydrogen concentration during SOI fabrication. Hydrogen presented in SOI during the fabrication process partially neutralizes the traps at the Si/thermal SiO 2 interface, and completely neutralizes shallow acceptors and contamination with deep levels in the top silicon layer.

Classification of charge relaxation processes in diamond on silicon based devices

Diamond thin films on p-type Si prepared via a bias enhanced chemical vapor deposition technique and equipped with Al Schottky contacts are studied by charge deep-level transient spectroscopy. Three charge relaxation processes can be resolved in Q-DLTS spectra: (i) a spectrum of relaxation times centered about a thermal energy of 0.33 eV has been assigned to hole traps induced by Al/diamond interface dipoles. These are metastable when heating the diodes to 450 K during a Q-DLTS scan, (ii) quasi discrete energy levels at 0.82 and 0.98 eV are indicative of an amorphous (hydrogenated) Si (a-Si:H) interlayer between the Si substrate and diamond film. This conclusion results from a direct comparison with Q-DLTS spectra of a-Si:H based devices and (iii) a slow charge relaxation with a non-exponential kinetics responsible for a Q-DLTS signal tail independent of the rate window for longer times of observation. This phenomenon is known from frequency-domain observations as ‘nearly constant loss’.

Transient charging of copper phthalocyanine: model and experiment

Thin films of copper phthalocyanine (CuPc) were deposited on indium tin oxide (ITO) substrates in ultra-high vacuum. Without breaking the vacuum, Ag dots were evaporated through a shadow mask. The present contribution is aimed at comparing experimental and simulated data on feedback-charge capacitance/voltage ( C/ V) and isothermal charge deep-level transient spectroscopy (Q-DLTS) of Ag/CuPc/ITO devices kept at ambient temperature. Two types of charge relaxation of different origins in response to a bias step (pulse) Δ U were resolved: (a) dielectric relaxation within the bulk of the CuPc semi-insulating layer; (b) emission from traps of charges injected and captured at the traps during the pulse. The dielectric relaxation is characterized by a well-defined time constant τ D ≈10 −4 s and is observable for both polarities of Δ U. The onset bias for observing the emission from traps corresponds roughly to one of the forward current. A positive Δ U is to be applied to ITO (hole injecting anode) in order to fill at least a fraction of the traps. Realizing that the traps are spatially distributed, the model previously applied to C/ V of a Si:H based devices has been updated to obtain at least a qualitative picture about the spatial distribution of the density of traps N T ( x) in CuPc. N T ( x) seems to rise when approaching the CuPc/ITO interface. These states are classified as acceptor-like ones, a situation analogous to that reported earlier for poly-( p-phenylene-vinylene). Protonation of the CuPc/ITO interface is discussed as the source of the acceptor-like states.

Electron capture kinetics at AlF3/SiO2 interfaces

Charge deep-level transient spectroscopy (Q-DLTS) was applied to study the mechanism of capturing electrons at the AlF3/SiO2 interfaces of AlF3-coated oxidized p-type Si. The process may be viewed as comprising three consecutive steps: (i) formation of an inversion layer in p-Si; (ii) electron tunneling through the ultra-thin SiO2 interlayer; and (iii) capture of electrons at F-vacancies in AlF3 after surmounting a potential barrier Eb. A semi-empirical model of non-exponential kinetics of the charge transients has been developed and compared with experiment. Three discrete electron traps of a quasi-discrete spectrum of traps in AlF3 could be resolved. The thermal activation energy of a Q-DLTS peak (0.6–0.7eV) seems to reflect Eb, while the full-width at half-maximum (FWHM) corresponds to thermal activation of minority carriers over the band gap of p-Si (≈1.1eV). Coulomb-blockade effects may be responsible for the temperature dependence of the capture cross-section σn=σn0exp(−Eb/kT).

Peculiar features in the electrical characteristics of CuPc based diodes

We have investigated electrical properties of copper phthalocyanine (CuPc) thin films by means of current–voltage (I–V), capacitance–voltage (C–V) and charge deep-level transient spectroscopy (Q-DLTS) measurements. CuPc films were deposited on indium tin oxide (ITO) via organic molecular beam deposition (OMBD) at room temperature followed by Ag evaporation through a shadow mask, resulting in contacts of different sizes of dots. The measurements were carried out in situ at a pressure below 8×10−10mbar. No noticeable changes of electrical characteristics have been found after storing the devices for a few days under these conditions. Diodes of 330nm CuPc sandwiched between ITO and Ag show very low reverse currents with hysteresis-like characteristics for short delay times, which are the times between two consecutive voltage steps in an I–V measurement. Especially the bias at which current passes through zero is drastically affected by the sweep voltage direction. Almost identical curves in both directions can only be achieved using an extremely long delay time of 100s. It was found that the offset voltages for delay times of 5 and 100s are −0.85 and −0.1V for up-sweep and 0.14 and 0V for down-sweep voltage directions, respectively. This effect is likely due to the presence of deep traps which require a large time constant for charging and discharging in response to a change of the external voltage. The forward current, on the other hand, is virtually not affected, increasing initially over many orders of magnitude in a narrow voltage range and then increasing at a reduced slope. This behaviour is not stable when a high external voltage is applied. Q-DLTS measurements show presence of traps with time constants exceeding 50ms at ambient temperature.

Evolution of Charged Gap Statesin a- Si:H Under Light Exposure

AbstractThe charge deep-level transient spectroscopy (Q-DLTS) experiments on undoped hydrogenated amorphous silicon (a-Si:H) demonstrate that during light soaking the states in the upper part of the gap disappear, while additional states around and below midgap are created. Since no direct correlation is observed in light-induced changes of the three groups of states that we identify from the Q-DLTS signal, we believe that we deal with three different types of defects. Positively charged states above midgap are related to a complex formed by a hydrogen molecule and a dangling bond. Negatively charged states below midgap are attributed to floating bonds. Various trends in the evolution of dark conductivity due to light soaking indicate that the kinetics of light-induced changes of the three gap-state components depend on their initial energy distributions and on the spectrum and intensity of light during exposure.

Modification of the bonded interface in silicon-on-insulator structures under thermal treatment in hydrogen ambient

Results of a comparative charge deep-level transient spectroscopy study of Si/SiO2 interfaces in silicon-on-insulator structures prepared by wafer bonding and hydrogen slicing, and subsequently annealed in a hydrogen ambient in the temperature range of 430 to 670 °C are described. The trap transformations at the bonded interface are strongly discriminated from passivation of traps at the Si/thermal SiO2 interface. The nature of traps at the bonded interface is found to differ from that of traps at the Si/thermal SiO2 interface. Heat treatments at T>430 °C are shown to induce significant changes in virgin spectra of traps at bonded interfaces due to the generation of traps.