Investigation of defects in polyhedral oligomeric silsesquioxanes based organic light emitting diodes

We have investigated novel poly(p-phenylene vinylene) (PPV) derivative based organic light emitting diodes (OLEDs). We have used poly(2,3-diphenyl-1,4-phenylenevinylene) (DP-PPV) as an emitter in which an inorganic core of polyhedral oligomeric silsesquioxanes (POSS) have been incorporated. The hybrid structure obtained shows an improvement on the stability and an enhancement of electroluminescence properties. Charge-based deep level transient spectroscopy (Q-DLTS) has been used to study the defect states in indium-tin-oxide (ITO)/polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS)/POSS-DP-PPV/CaAl light emitting devices. Analysis of the Q-DLTS spectra obtained in devices, reveal at least six trap levels. The mean activation energies of traps are distributed in the range 0.3–0.5 eV within the band gap of the hybrid polymer and capture cross sections are of the order of 10¹⁶–10²⁰ cm². The trap densities are in the range of 10¹⁶–10¹⁷ cm^?3. Although the origin of these traps remains not clearly established, we suggest that the trap states with a large capture cross-section would likely to be originated from the inorganic part of hybrid material while those with lower capture cross-section would be related to its organic part.     

A new technique to study transient conductivity under pulsed monochromatic light in Cr-doped GaAs using acoustoelectric voltage measurement

The transient conductivity of high-resistivity, Bridgman grown, Cr-doped GaAs under pulsed monochromatic light is monitored using transverse acoustoelectric voltage (TAV) at 83 K. Keeping the photon flux constant, the height and transient time constant at the TAV are used to calculate the energy dependence of the trap density and its cross section, respectively. Two prominent trap profiles with peak trap densities of approximately 10(17) cm(-3) eV(-1 ) near the valence and the conduction bands are detected. These traps have very small capture cross sections in the range of 10(-23 )-10(-21 )cm(2). A phenomenon similar to the persistent photoconductivity with transient time constants in excess of a few seconds (in some cases, a few hundred seconds) in high-resistivity GaAs at T=83 K is also detected using this technique. These long relaxation times are readily explained by the spatial separation of the photo-excited electron-hole pairs and the small capture cross section and large density of trap distribution near the conduction band. The technique is nondestructive and, because of the dependence of the polarity of the acoustoelectric voltage on the carrier type, it yields information about the charge of the transient carriers and the type of deep traps involved in the release or trapping of these carriers.     

Charge carrier trapping in poly(N-vinylcarbazole)-trinitrofluorenone films

An investigation of hole and electron photo-injection in films of the poly(N-vinylcarbazole)-trinitrofluorenone charge transfer complex by the time-of-flight technique revealed the presence of deep carrier traps. The hole trap density was estimated to be (6–13) × 10¹⁹m^-3 in films where poly(N-vinylcarbazole) is mixed with trinitrofluorenone in a 1:0.2 mole ratio with respect to the monomer unit. In 1:1 mole ratio films, the electron trap density was estimated to be (10–29) × 10¹⁹m^-3. In the film samples typically investigated the number of carriers injected per sample normally exceeded the number of deep traps in the sample. The fraction of injected charge that became trapped was small under these circumstances, and the majority of charge carriers were transported through the film.     

DLTS study of annihilation of oxidation induced deep-level defects in Ni/SiO2/n-Si MOS structures

This paper describes the fabrication of MOS capacitor and DLTS study of annihilation of deeplevel defects upon thermal annealing. Ni/SiO₂/n-Si MOS structures fabricated on n-type Si wafers were investigated for process-induced deep-level defects. The deep-level traps in Si substrates induced during the processing of Ni/SiO₂/n-Si have been investigated using deep-level transient spectroscopy (DLTS). A characteristic deep-level defect at E _C = 0·49 eV which was introduced during high-temperature thermal oxidation process was detected. The trap position was found to shift to different energy levels (E _C = 0·43, 0·46 and 0·34 eV) during thermal annealing process. The deep-level trap completely anneals at 350°C. Significant reduction in trap density with an increase in recombination life time and substrate doping concentration as a function of isochronal annealing were observed.     

Electrical characterization of deep levels existing in fully implanted and rapid thermal annealed p+n InP junctions

In this paper, the deep levels existing in fully implanted and rapid thermal annealed p⁺n junctions obtained by Mg/Si or Mg/P/Si implantations on nominally undoped n-type InP substrates were detected and characterized by the correlation of two electrical techniques: deep level transient spectroscopy (DLTS) and capacitance–voltage transient technique (CVTT). Two ion implantation-induced deep levels (at 0.25 and 0.27 eV below the conduction band) were detected by DLTS. Several characteristics of these traps were derived from CVTT measurements, paying special attention to their physical nature.     

Characterization of deep level defects in Si irradiated with MeV Ar+ ions using constant capacitance time analyzed transient spectroscopy

An isothermal spectroscopic technique called time analyzed transient spectroscopy (TATS) in the constant capacitance (CC) mode has been used to characterize electrically active defects in the MeV Ar⁺ implanted silicon. The problems associated with high defect density and the presence of damaged region in the as-implanted material are overcome by CC-TATS method. The CC-TATS spectra of the as-implanted sample shows two positive peaks and an attendant negative peak. Two distinct traps have also been identified using thermally stimulated capacitance method modified to operate in constant capacitance mode. Variable pulse width measurements using CC-TATS show exponential capture kinetics in contrast to extremely slow capture observed in conventional deep level transient spectroscopy (DLTS) experiment. The results indicate that trapping behaviour is due to point-like defects associated with extended defects such as dislocation and stacking fault.     

The mechanism of persistent photoconductivity induced by minority carrier trapping effect in ultraviolet photo-detector made of polycrystalline diamond film

Performances of long persistent photoconductivity, high responsivity and high photoconductive gain were observed in a metal-semiconductor-metal ultraviolet photo-detector fabricated on a microcrystalline diamond film. Charge-based deep level transient spectroscopy measurement confirmed that a shallow level with activation energy of 0.21 eV and capture cross section of 9.9 × 10⁻²⁰ cm² is presented in the band gap of the diamond film. The shallow level may not act as effective recombination center due to the so small activation energy according to Schockly-Read-Hall statistics. The persistent photoconductivity relaxation fits in with the so called “barrier-limited recombination” model, which may be a minority carrier trapping effect related recombination process. The photo-induced minority carriers (electrons in this paper) may be trapped by the shallow level during light irradiation process and then de-trap slowly via thermal excitation or tunneling effect after removing the light source, which contributes to the persistent photoconductivity. The trapping effect can also reduce the probability of carrier recombination, resulting in the high responsivity and the high gain.     

Effect of electrical operation on the defect states in organic semiconductors

We have investigated the role of the trapping process in degradation mechanisms of poly(9,9-dihexylfluorene-co-N,N-di(9,9-dihexyl-2-fluorenyl)-N-phenylamine) (PF) based diodes, after aging (at half lifetime) by electrical stress. By using the Charge based Deep Level Transient Spectroscopy, we have determined the trap parameters in PF light emitting devices. The mean activation energies of the traps are in the range 0.13–0.60 eV from the band edges with capture cross sections of the order of 10^–18 to 10^–20 cm². The trap densities are in the range of 10^–16 to 10^–17 cm^?3. Upon aging, no new trap levels have been found indicating that the electrical stress did not create additional defect level in the polymer in contrast to previous investigations on other organic materials, which reported that the degradation of devices in humid atmosphere lead to the onset of new traps acting as recombination centers. Furthermore, aging would not affect uniformly the defect levels in the polymer. Shallow trap states (below 0.3 eV) remain stable, whereas the enhancement in trap density of deeper trap levels (above 0.3 eV) have been observed, suggesting that degradation by electrical stress leads to an increase in density of deep levels.     

Defect Engineering for Modulating the Trap States in 2D Photoconductors

Defect‐induced trap states are essential in determining the performance of semiconductor photodetectors. The de‐trap time of carriers from a deep trap can be prolonged by several orders of magnitude as compared to shallow traps, resulting in additional decay/response time of the device. Here, it is demonstrated that the trap states in 2D ReS₂ can be efficiently modulated by defect engineering through molecule decoration. The deep traps that greatly prolong the response time can be mostly filled by protoporphyrin molecules. At the same time, carrier recombination and shallow traps in‐turn play dominant roles in determining the decay time of the device, which can be several orders of magnitude faster than the as‐prepared device. Moreover, the specific detectivity of the device is enhanced (as high as ≈1.89 × 10¹³ Jones) due to the significant reduction of the dark current through charge transfer between ReS₂ and molecules. Defect engineering of trap states therefore provides a solution to achieve photodetectors with both high responsivity and fast response.     

Deep level transient spectroscopy on charge traps in high-k ZrO2

The deep trap properties of high-dielectric-constant (k) ZrO₂ thin films were examined by deep level transient spectroscopy (DLTS). The hole traps of a ZrO₂ dielectric deposited by sputtering were investigated in a MOS structure over the temperature range, 375 K-525 K. The potential depth, cross section and concentration of hole traps were estimated to be ∼ 2.5 eV, ∼ 1.8 × 10^− 16 cm² and ∼ 1.0 × 10¹⁶ cm^− 3, respectively. DLTS of ZrO₂ dielectrics can be used to examine the threshold voltage shift (?V_th) during the operation of SONOS-type flash memory devices, which employ high-k materials.     

Individual interface traps at the Si-SiO2 interface

In submicrometre-sized metal-oxide-semiconductor field-effect transistors, MOSFETs, the alternate capture and emission of carriers at individual Si-SiO₂ interface defects generates discrete switching in the source-drain resistance. The resistance changes are observed in the drain current as random telegraph signals (RTSs) or as stepped transients after a strong perturbation of the trap occupation. The study of individual defects in MOSFETs has provided a powerful means of investigating the capture and emission kinetics of interface traps, it has demonstrated the defect origins of low-frequency (1/f) noise in MOSFETs, and it has provided new insight into the nature of defects at the Si-SiO₂ interface. The analysis of individual interface defects has shown that a Coulomb energy of several hundred millivolts is involved in the transfer and localization of the single charge carrier into the interface trap.     

Individual interface traps at the Si—SiO2 interface

In submicrometre-sized metal-oxide-semiconductor field-effect transistors, MOSFETs, the alternate capture and emission of carriers at individual Si—SiO₂ interface defects generates discrete switching in the source-drain resistance. The resistance changes are observed in the drain current as random telegraph signals (RTSs) or as stepped transients after a strong perturbation of the trap occupation. The study of individual defects in MOSFETs has provided a powerful means of investigating the capture and emission kinetics of interface traps, it has demonstrated the defect origins of low-frequency (1/f) noise in MOSFETs, and it has provided new insight into the nature of defects at the Si—SiO₂ interface. The analysis of individual interface defects has shown that a Coulomb energy of several hundred millivolts is involved in the transfer and localization of the single charge carrier into the interface trap.     

Hot wall epitaxy of II–VI compounds: CdS and CdTe

CdS and CdTe films were grown by hot wall epitaxy on single-crystal CdTe, BaF₂ and SrF₂ substrates. The films grow epitaxially and do not show any misorientations under usual X-ray investigations. N-type films were grown by coevaporation of indium. Typical electron concentrations of up to 2 × 10¹⁷ cm^-3 in CdTe and 3 × 10¹⁸ cm^-3 in CdS were obtained. P-type CdTe layers were also obtained using antimony as a dopant, with hole concentrations up to 10¹⁸–10¹⁹ cm^-3. Room temperature values of the electron mobility up to 600 cm² V^-1 s^-1 for CdTe and 230 cm² V^-1 s^-1 for CdS were obtained. Minority carrier diffusion lengths larger than 1 μm were measured in n-CdTe layers grown on p-CdTe substrates. Deep level transient spectroscopy was used to characterize carrier traps in the CdTe films. The concentration of the traps was a function of the growth conditions and could be drastically reduced with annealing of the samples.     

ESR and TSL study of hole and electron traps in LuAG:Ce,Mg ceramic scintillator

The process of hole and electron localization in LuAG:Ce,Mg ceramics is studied by electron spin resonance (ESR) and thermally stimulated luminescence (TSL). Hole traps, which are created by UV irradiation, are detected in the form of O⁻ centers. Mg-perturbed variants of O⁻ centers are proposed to exist. The thermal stability of such defects is studied, proving that they are stable up to room temperature. The interaction between O⁻ centers and shallow electron traps is studied by thermally stimulated luminescence (TSL) and phosphorescence experiments, which reveal the occurrence of an a-thermal tunneling process between trapped electrons and randomly distributed Ce centers. By correlating the TSL-derived trap parameters and temperature dependent ESR intensities, it is found that O⁻ centers compete with Ce centers in free electron capture.     

Hydrogen effects on deep level defects in proton implanted Cu(In,Ga)Se2 based thin films

Hydrogen effects on deep level defects and a defect generation in proton implanted Cu(In,Ga)Se₂ (CIGS) based thin films for solar cell were investigated. CIGS films with a thickness of 3 μm were grown on a soda-lime glass substrate by a co-evaporation method, and then were implanted with protons. To study deep level defects in the proton implanted CIGS films, deep level transient spectroscopy measurements on the CIGS-based solar cells were carried out, these measurements found 6 traps (including 3 hole traps and 3 electron traps). In the proton implanted CIGS films, the deep level defects, which are attributed to the recombination centers of the CIGS solar cell, were significantly reduced in intensity, while a deep level defect was generated around 0.28 eV above the valence band maximum. Therefore, we suggest that most deep level defects in CIGS films can be controlled by hydrogen effects.     

25th Anniversary Article: Charge Transport and Recombination in Polymer Light‐Emitting Diodes

This article reviews the basic physical processes of charge transport and recombination in organic semiconductors. As a workhorse, LEDs based on a single layer of poly(p‐phenylene vinylene) (PPV) derivatives are used. The hole transport in these PPV derivatives is governed by trap‐free space‐charge‐limited conduction, with the mobility depending on the electric field and charge‐carrier density. These dependencies are generally described in the framework of hopping transport in a Gaussian density of states distribution. The electron transport on the other hand is orders of magnitude lower than the hole transport. The reason is that electron transport is hindered by the presence of a universal electron trap, located at 3.6 eV below vacuum with a typical density of ca. 3 × 10¹⁷ cm^?3. The trapped electrons recombine with free holes via a non‐radiative trap‐assisted recombination process, which is a competing loss process with respect to the emissive bimolecular Langevin recombination. The trap‐assisted recombination in disordered organic semiconductors is governed by the diffusion of the free carrier (hole) towards the trapped carrier (electron), similar to the Langevin recombination of free carriers where both carriers are mobile. As a result, with the charge‐carrier mobilities and amount of trapping centers known from charge‐transport measurements, the radiative recombination as well as loss processes in disordered organic semiconductors can be fully predicted. Evidently, future work should focus on the identification and removing of electron traps. This will not only eliminate the non‐radiative trap‐assisted recombination, but, in addition, will shift the recombination zone towards the center of the device, leading to an efficiency improvement of more than a factor of two in single‐layer polymer LEDs.     

Luminescence properties of hole traps in homojunction gallium nitride diodes grown by metal organic vapour phase epitaxy

A detailed investigation on p–n junction diodes of GaN using deep level transient Fourier spectroscopy (DLTFS) has been carried out. The typical deep level spectra on the various diodes on the same wafer demonstrate three electron levels labelled as E1, E2 and E3 and a hole trap H1 together with a broad band constituting three new hole levels H2, H3 and H4 therein. The electrical parameters like activation energy, trap concentration and capture cross section due to the observed levels have been measured for the comparison with the literature. The hole levels H1–H4 are found to be potentially involved in the radiative recombination and thereby, the luminescence role of the levels in the device is discussed.     

Doping and Switchable Photovoltaic Effect in Lead‐Free Perovskites Enabled by Metal Cation Transmutation

Creating defect tolerant lead‐free halide perovskites is the major challenge for development of high‐performance photovoltaics with nontoxic absorbers. Few compounds of Sn, Sb, or Bi possess ns² electronic configuration similar to lead, but their poor photovoltaic performances inspire us to evaluate other factors influencing defect tolerance properties. The effect of heavy metal cation (Bi) transmutation and ionic migration on the defects and carrier properties in a 2D layered perovskite (NH₄)₃(Sb_(1?x)Bi_x)₂I₉ system is investigated. It is shown, for the first time, the possibility of engineering the carriers in halide perovskites via metal cation transmutation to successfully form intrinsic p‐ and n‐type materials. It is also shown that this material possesses a direct–indirect bandgap enabling high absorption coefficient, extended carrier lifetimes >100 ns, and low trap densities similar to lead halide perovskites. This study also demonstrates the possibility of electrical poling to induce switchable photovoltaic effect without additional electron and hole transport layers.     

Characterization of deep levels in high electron mobility transistor by conductance deep level transient spectroscopy

The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN high electron mobility transistor (HEMT) on silicon (111) substrate is profited to investigate traps that are located between the substrate and the two-dimensional electron gas (2DEG) channel. The transient of the drain current after applying a negative substrate voltage is evaluated in the temperature range from 77 to 600 K. With this method, known as Conductance Deep Level Transient Spectroscopy (CDLTS), majority deep levels with activation energy of 61 meV as well as minority carrier traps at 74 meV and capture cross-section respectively 2.56 × 10^− 15 cm², 2.1 × 10^− 15 cm² are identified. Finally, the correlation between the anomalies observed on the output characteristics and defects is discussed.     

Radiation damage induced in Si photodiodes by high-temperature neutron irradiation

Results are presented of a detailed study of the effects of high-temperature 4-MeV neutron irradiation on the performance degradation of Si pin photodiodes together with the radiation-induced defects, observed by deep level transient spectroscopy (DLTS). It was found that the dark current increases after irradiation, while the photocurrent decreases. After irradiation, two majority electron capture levels with (E _c–0.22 eV) and (E _c–0.40 eV) were induced in the n-Si substrate, while one minority hole capture level with (E _v+0.37 eV) was found. Additionally, the degradation of the device performance and the introduction rate of the lattice defects decreases with increasing irradiation temperature. For a 250 °C irradiation, the reduction of the reverse current is only 20% of the starting value. This result suggests that the creation and recovery of the radiation damage proceeds simultaneously at high temperatures.