Concentration Of Deep Level Defects Research Articles

Deep-level defects induced by implantations of Si and Mg ions into undoped epitaxial GaN

The properties and concentrations of deep-level defects induced by implantations of Si and Mg ions into unintentionally doped (UID) epitaxial GaN have been revealed by using the Laplace-transform photoinduced transient spectroscopy (LPITS) and molecular dynamics (MD) calculations. The material lattice damage, produced by the Si ions implanted at room temperature in the single process at the energies of 200 and 340 keV, is compared with that produced by the Mg ions implanted in the similar process at the energies of 150, 210, and 270 keV. The LPITS results indicate that the same deep traps with the activation energies of 396, 512, 531, 587, 635, and 736 meV are present in the tail regions of the semi-insulating Si- and Mg-implanted films. It is argued that the predominant implantation-induced point defects in the tail region of the Si-implanted films are nitrogen vacancies, whose concentration is 7.7 × 1017 cm−3. In the Mg-implanted films, the predominant implantation-induced point defects are gallium interstitials, whose concentration is 1.2 × 1 018 cm−3.

Effect of deep level defects on space charge distribution in CdZnTe crystals

CdZnTe recently emerged as a leading semiconductor crystal for fabricating room-temperature x- and gamma-ray imaging detectors, due to its excellent energy resolution and sensitivity. However, its wide deployment is hampered by the low availability of high-quality CdZnTe crystals. As-grown CdZnTe crystals generally encounter the problems arising from the impurities and defects, especially deep level defects. The presence of impurities and defects leads to severe charge trapping, which significantly affects detector performance. Especially for high counting rate imaging detector used in medical imaging and tomography, the accumulation of space charge at deep levels significantly deforms the electric field distribution and subsequently reduces the charge collection efficiency. Therefore, a considerable interest is focused on the investigation of the space charge accumulation effect in CdZnTe crystal, which is the key factor to improve the performance of high counting rate imaging detector. Thus, the goal of this work is to investigate the effects of deep level defects on space charge distribution and internal electric field in CdZnTe detector. In order to reveal the major problem therein, Silvaco TCAD technique is used to simulate the space charge and electric field distribution profile in CdZnTe detector with considering the typical deep level defects <inline-formula><tex-math id="Z-20201111102325-1">\begin{document}$ \rm Te_{Cd}^{++} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20200553_Z-20201111102325-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20200553_Z-20201111102325-1.png"/></alternatives></inline-formula>in CdZnTe crystals with activation energy of <i>E</i><sub>v</sub> + 0.86 eV and concentration of 1 × 10<sup>12</sup> cm<sup>–3</sup> at room temperature. The simulation results demonstrate that the Au/ CdZnTe /Au energy band tilts intensively with the increase of applied bias, which makes the deep level ionization fraction increase. The space charge concentration also increases in the crystal. Meanwhile, the dead layer of electric field distribution decreases, which is of benefit to the carrier collection of CdZnTe detector. In addition, under the premiseof the high resistivity of CdZnTe crystal, the reduction of deep level defect concentration located at <i>E</i><sub>v</sub> + 0.86 eV can narrow the internal dead layer moderately. The deep level defect located at <i>E</i><sub>v</sub> + 0.8 eV can also reduce the space charge concentration near the cathode, which flattens the electric field distribution with narrower dead layer, thus significantly improving the carrier collection efficiency of CdZnTe detector. These simulation results will provide meaningful theoretical guidance for further optimizing the CdZnTe crystal growth, device design and fabrication.

Impact of deep level defects induced by high energy neutron radiation in β-Ga2O3

The effects of high energy neutron irradiation on the deep level defect concentration profile throughout the bandgap of β-Ga2O3 were investigated by a combination of deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS). For the unintentionally doped edge-defined film-fed growth-grown (010) β-Ga2O3 substrates investigated here, it was found that the dominant effects of neutron irradiation were to produce defects detected by DLOS having energy levels of EC −1.29 eV and EC −2.00 eV, with no discernable impact on traps within ∼1 eV of the conduction band edge. Commensurate with the introduction of these states was a significant amount of net doping reduction, for which lighted capacitance-voltage studies revealed that both of these irradiation-induced deep states are responsible, likely through a compensation mechanism. The sensitivity of the EC −1.29 eV and EC −2.00 eV states on irradiation suggests an intrinsic source, and whereas the EC −2.00 eV state was already present in the as-grown material, the EC −1.29 eV state was not detected prior to irradiation. DLOS and DLTS revealed other defect states at EC −0.63 eV, EC −0.81 eV, and EC −4.48 eV, but none of these responded to neutron irradiation for two different 1 MeV equivalent fluences 8.5 × 1014 cm−2 and 1.7 × 1015 cm−2, which is consistent with the behavior expected for defect states having an extrinsic source.

Thermally activated defects in float zone silicon: Effect of nitrogen on the introduction of deep level states

Float zone silicon (FZ-Si) is typically assumed to be an extremely high quality material, with high minority carrier lifetimes and low concentrations of recombination active defects. However, minority carrier lifetime in FZ-Si has previously been shown to be unstable following thermal treatments between 450 and 700 °C, with a range of unidentified deep level states being linked to reduced carrier lifetime. There are suspicions that nitrogen doping, which occurs from the growth atmosphere, and intrinsic point defects play a role in the degradation. This study aims to address this by using deep level transient spectroscopy (DLTS), minority carrier transient spectroscopy, Laplace DLTS, and photoluminescence lifetime measurements to study recombination active defects in nitrogen-doped and nitrogen-lean n-type FZ-Si samples. We find that nitrogen-doped samples experience increased degradation due to higher concentrations of deep level defects during thermal treatments compared to nitrogen-lean samples. In an attempt to explain this difference, in-diffusion of nickel has been used as a marker to demonstrate the existence of higher vacancy concentrations in the nitrogen-doped samples. The origin of the recombination active defects responsible for the thermally induced lifetime degradation in FZ-Si crystals is discussed.

Defect-induced instability mechanisms of sputtered amorphous indium tin zinc oxide thin-film transistors

We report the device performance and stability of sputtered amorphous indium-tin-zinc-oxide (ITZO) thin-film transistors as a function of oxygen ratio [O2/(Ar + O2)] during growth. Increasing the oxygen ratio enhanced the incorporation of oxygen during ITZO film growth and reduced the concentration of deep-level defects associated with oxygen vacancies. Under illumination with no bias stress, device stability and persistent photocurrent were improved with increased oxygen ratio. Bias stress tests of the devices were also performed with and without illumination. While high oxygen ratio growth conditions resulted in decreased deep-level oxygen vacancies in the ITZO material, the same conditions resulted in degradation of the interfacial layer between the ITZO channel and dielectric due to the migration of energetic oxygen ions to the interface. Therefore, when bias stress was applied, increased carrier trap density at the interface led to a decrease in device stability that offsets any improvement in the material itself. In order to take advantage of the improved ITZO material growth at a high oxygen ratio, the interface-related problems must be solved.

Morphology-controllable growth of vertical ZnO nanorod arrays by a polymer soft template method: Growth mechanism and optical properties

Vertically well-aligned ZnO nanorod arrays were synthesized on silicon substrate by a novel polymer soft template method. The ZnO nanorod arrays have been deposited by immersing a silicon substrate into an aqueous solution containing Zn(CH3COO)2·2H2O, Polyvinylpyrrolidone (PVP) and NH3·H2O at 99 °C and then being annealed in oxygen atmosphere. The effects of reaction conditions on the morphology, structure and optical properties of ZnO nanostructures were investigated. The morphology of ZnO nanostructures was demonstrated to be controllable by adjusting the pH values of PVP-Zn2+ complex solution, zinc acetate concentration and growth time, respectively. The nanorod arrays were hexagonal zincite ZnO and the c-axis orientation growth trend of ZnO became more and more obvious with increasing the pH values of PVP-Zn2+ complex solution from 7.5 to 10.5, the zinc acetate concentration from 5 mM to 200 mM, and the growth time from 1 h to 3 h, respectively. The concentration of deep level defects in ZnO was reducing with increasing the pH values of PVP-Zn2+ complex solution, zinc acetate concentration and growth time, respectively. A growth mechanism of vertical ZnO nanorod arrays based on PVP soft template is proposed.

Deep level defects in dilute GaAsBi alloys grown under intense UV illumination

Dilute GaAs1−xBix alloys exhibiting narrow band edge photoluminescence (PL) were recently grown by molecular beam epitaxy (MBE) with the growth surface illuminated by intense UV radiation. To investigate whether the improved optical quality of these films results from a reduction in the concentration of deep level defects, p+/n and n+/p junction diodes were fabricated on both the illuminated and dark areas of several samples. Deep Level Transient Spectroscopy (DLTS) measurements show that the illuminated and dark areas of both the n- and p-type GaAs1−xBix epi-layers have similar concentrations of near mid-gap electron and hole traps, in the 1015 cm−3 range. Thus the improved PL spectra cannot be explained by a reduction in non-radiative recombination at deep level defects. We note that carrier freeze-out above 35 K is significantly reduced in the illuminated areas of the p-type GaAs1−xBix layers compared to the dark areas, allowing the first DLTS measurements of defect energy levels close to the valence band edge. These defect levels may account for differences in the PL spectra from the illuminated and dark areas of un-doped layers with a similar Bi fraction.

Fabrication of well-aligned ZnO nanorod photoanodes for perovskite solar cells

ZnO nanorods grown on aluminum doped zinc oxide (AZO) substrate and ZnO seed layer coated substrates, including glass slides, fluorine doped tin oxide, and indium tin oxide substrates were studied. ZnO nanorods grown on AZO substrate had the best alignment and highest packing density. The growth rate of ZnO nanorods was regulated by adding polyethyleneimine (PEI) and introducing contact potential. ZnO nanorods co-adjusted by adding PEI and introducing contact-potential-driven growth formed a textured structure that led to a higher transmittance at 600–800 nm. The strong ultraviolet band emission indicated a relatively low concentration of deep level defects in ZnO nanorods. Good crystallization, together with a small amount of aluminum-doping, guaranteed high electron transfer in the ZnO nanorods. The well-aligned ZnO nanorods were incorporated into a perovskite solar cell prepared with carbon counter electrode under completely open-air conditions. A J sc of 15.53 mA cm−2 and an efficiency of 2.71 % were obtained for ZnO nanorods under contact-potential-driven growth. In addition, for ZnO nanorods co-adjusted by adding PEI and introducing contact potential, a J sc of 14.87 mA cm−2 and an efficiency of 3.62 % were obtained. However, fill factors were very low for hybrid solar cells with the structure of AZO/ZnO nanorods-CH3NH3PbIxCl3−x/carbon.

Radiation induced deep level defects in bipolar junction transistors under various bias conditions

Bipolar junction transistor (BJT) is sensitive to ionization and displacement radiation effects in space. In this paper, 35MeV Si ions were used as irradiation source to research the radiation damage on NPN and PNP bipolar transistors. The changing of electrical parameters of transistors was in situ measured with increasing irradiation fluence of 35MeV Si ions. Using deep level transient spectroscopy (DLTS), defects in the bipolar junction transistors under various bias conditions are measured after irradiation. Based on the in situ electrical measurement and DLTS spectra, it is clearly that the bias conditions can affect the concentration of deep level defects, and the radiation damage induced by heavy ions.

Computational prediction of lattice defects in multinary compound semiconductors as photovoltaic materials

In the past 60 years development of photovoltaic semiconductors, the number of component elements has increased steadily, i.e., from silicon in the 1950s, to GaAs and CdTe in the 1960s, to CuInSe2 in the 1970s, to Cu(In, Ga) Se2 in the 1980s, to Cu2ZnSnS4 in the 1990s, and to recent Cu2ZnSn(S, Se)4 and CH3NH3PbI3. Whereas the material properties become more flexible as a result of the increased number of elements, and multinary compound semiconductors feature a dramatic increase of possible point defects in the lattice, which can significantly influence the optical and electrical properties and ultimately the photovoltaic performance. It is challenging to characterize the various point defects and defect pairs experimentally. During the last 20 years, first-principles calculations based on density functional theory (DFT) have offered an alternative method of overcoming the difficulties in experimental study, and widely used in predicting the defect properties of semiconductors. Compared with the available experimental methods, the first-principles calculations are fast, direct and exact since all possible defects can be investigated one by one. This advantage is especially crucial in the study of multinary compound semiconductors which have a large number of possible defects. Through calculating the formation energies, concentration and transition (ionization) energy levels of various possible defects, we can study their influences on the device performance and then identify the dominant defects that are critical for the further optimization of the performance. In this paper, we introduce the first-principles calculation model and procedure for studying the point defects in materials. We focus on the hybrid scheme which combines the advantages of both special k-points and -point-only approaches. The shortcomings of the presentcalculation model are discussed, with the possible solutions proposed. And then, we review the recent progress in the study of the point defects in two types of multinary photovoltaic semiconductors, Cu2ZnSn(S,Se)4 and H3NH3PbI3. The result of the increased number of component elements involves various competing secondary phases, limiting the formation of single-phase multinary compound semiconductors. Unlike ternary CuInSe2, the dominant defect that determines the p-type conductivity in Cu2ZnSnS4 is Cu-on-Zn antisite (CuZn) defect rather than the copper vacancy (VCu). However, the ionization level of CuZn is deeper than that of VCu. The self-compensated defect pairs such as [2CuZn+SnZn] are easy to form in Cu2ZnSnS4, which causes band gap fluctuations and limits the Voc of Cu2ZnSnS4 cells. Additionally the formation energies of deep level defects, SnZn and VS, are not sufficiently high in Cu2ZnSnS4, leading to poor lifetime of minority carriers and hence low Voc. In order to enhance the formation of VCu and suppress the formation of CuZn as well as deep level defects, a Cu-poor/Zn-rich growth condition is required. Compared with Cu2ZnSnS4, the concentration of deep level defects is predicted to be low in Cu2ZnSnSe4, therefore, the devices fabricated based on the Se-rich Cu2ZnSn(S,Se)4 alloys exhibit better performances. Unlike Cu2ZnSnS4 cells, the CH3NH3PbI3 cells exhibit rather high Voc and long minority-carrier life time. The unusually benign defect physics of CH3NH3PbI3 is responsible for the remarkable performance of CH3NH3PbI3 cells. First, CH3NH3PbI3 shows that flexible conductivity is dependent on growth condition. This behavior is distinguished from common p-type photovoltaic semiconductor, in which the n-type doping is generally difficult. Second, in CH3NH3PbI3, defects with low formation energies create only shallow levels. Through controlling the carrier concentration (Fermi level) and growth condition, the formation of deep-level defect can be suppressed in CH3NH3PbI3. We conclude that the predicted results from the first-principles calculations are very useful for guiding the experimental study.

DLTS Studies of bias dependence of defects in silicon NPN bipolar junction transistor irradiated by heavy ions

The characteristic degradation in silicon NPN bipolar junction transistors (BJTs) of 3DG130 type is examined under the irradiation with 35MeV silicon (Si) ions under forward, grounded and reverse bias conditions, respectively. Different electrical parameters were in-situ measured during the exposure under each bias condition. Using deep level transient spectroscopy (DLTS), deep level defects in the base-collector junction of 3DG130 transistors under various bias conditions are measured after irradiation. The activation energy, capture cross section and concentration of observed deep level defects are measured using DLTS technique. Based on the in situ electrical measurement and DLTS spectra, it is clearly that the bias conditions could affect the concentration of deep level defects, and the displacement damage induced by heavy ions.

Tailoring the Electrical Properties of Undoped GaP

The charge compensation in undoped GaP single crystals is investigated by modeling the Fermi level position for various concentrations of shallow and deep donors and acceptors. The model is based on the numerical solution of the charge neutrality equation and allows for calculating the Fermi energy in the temperature range of 1 –1000 K. The experimental studies of the electronic properties and concentrations of grown-in defect centers are performed by the high-resolution photoinduced transient spectroscopy (HRPITS). We show that at the shallow acceptor concentration below 1x1015 cm-3 and the concentration of deep-level defects ~3x1015 cm-3 obtaining undoped GaP with the semi-insulating (SI) properties is possible by substantial reducing the residual concentration of shallow donor impurities.

Device Performance Correlated with Structural Properties of Vertically Aligned Nanorod Arrays in Polymer/ZnO Solar Cells

Polymer-based solar cells consisting of ZnO nanorods and poly(1-methoxy-4-(2-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) are investigated by current−voltage characterization and intensity modulated photovoltage spectroscopy (IMVS). The high quality ZnO nanorod arrays were prepared by electrodeposition, in which the length (Ln) and the concentration of deep level defects of ZnO nanorods were controlled by deposition time (Td). Results show that increasing Td leads to ZnO nanorods with linearly increased Ln but differently increased defect concentration for the MEH-PPV/ZnO solar cells, providing a peak device power conversion efficiency of 0.34% at AM 1.5 illumination (100 mW/cm2) for Td = 10 min. The electron lifetimes in MEH-PPV/ZnO nanorod devices at open circuit were studied by means of IMVS for the first time, and the influences of Ln and defect concentration on the charge recombination kinetics and device performance were revealed. It is found that, in the MEH-PPV/ZnO devices with high quality ZnO nanorods with rather low defect concentration, both photocurrent and recombination rate are mainly dependent on the Ln value as a result of the exponential attenuation of incident light intensity in the device, but the open circuit voltage Voc is more sensitive to the defect concentration. The present study provides new insights into designing the nanostructures for the hybrid photovoltaic devices based on vertically aligned one-dimensional nanoarrays.

Effect of electron irradiation on defect structure of 6H–SiC grown by PVT method

Deep level transient spectroscopy (DLTS) has been applied to study an effect of electron irradiation on the concentrations of deep-level defects in bulk 6H–SiC:N single crystals. Six electron traps labelled as T1A, T1B, T1C, T2, T3 and T4 with activation energies of 0.34, 0.40, 0.50, 0.64, 0.67 and 0.69 eV, respectively, were revealed. It is shown that the irradiation with a dose of ∼2×10 17 cm −2 of 300 keV electrons results in the formation of trap T1C (0.50 eV) attributed to carbon vacancies ( V C ). A significant increase in the concentrations of traps T2 (0.64 eV), T3 (0.67 eV) and T4 (0.69 eV) due to the irradiation has been also found. The results are discussed in terms of generation and annihilation of point defects and give a new insight into microscopic identity of the traps observed.

Deep-Level Defects in Nitrogen-Doped 6H-SiC Grown by PVT Method

ABSTRACTAn effect of the nitrogen concentration on the concentrations of deep-level defects in bulk 6H-SiC single crystals is investigated. Six electron traps labeled as T1A, T1B, T2, T3, T4 and T5 with activation energies of 0.34, 0.40, 0.64, 0.67, 0.69, and 1.53 eV, respectively, were revealed. The traps T1A (0.34 eV) and T1B (0.40 eV), observed in the samples with the nitrogen concentration ranging from ∼2×1017 to 5×1017 cm−3, are attributed to complexes formed by carbon vacancies located at various lattice sites and carbon antisites. The concentrations of traps T2 (0.64 eV) and T3 (0.67 eV) have been found to rise from ∼5×1015 to ∼1×1017 cm−3 with increasing the nitrogen concentration from ∼2×1017 to ∼2.0×1018 cm−3. These traps are assigned to complexes involving silicon vacancies occupying hexagonal and quasi-cubic sites, respectively, and nitrogen atoms. The trap T4 (0.69 eV) concentration also substantially rises with increasing the nitrogen concentration and it is likely to be related to complexes formed by carbon antisites and nitrogen atoms. The midgap trap T5 (1.53 eV) is presumably associated with vanadium contamination. The presented results show that doping with nitrogen involves a significant change in the defect structure of 6H-SiC single crystals.

Substrate effects on ZnO nanostructure growth via nanoparticle-assisted pulsed-laser deposition

The effects of various substrate conditions on the morphology, crystal structure and photoluminescence of ZnO nanostructures synthesized by nanoparticle-assisted pulsed-laser ablation deposition were investigated. It is concluded that the sapphire substrate with a 1 h anneal at 1000 °C is the most favorable to the vertical growth of ZnO nanostructures. SEM analysis indicates that the well-aligned diameter-modulated ZnO nanonails with unique shape were successfully synthesized on the annealed sapphire substrate. The as-synthesized ZnO nanostructures exhibit an ultraviolet emission at around 390 nm and the absent green emission under room temperature, indicating that there is a very low concentration of deep-level defects inside ZnO lattices. The novel ZnO nanostructures could offer novel opportunities for both fundamental research and technological applications.

Influence of the Dislocation Travel Distance on the DLTS Spectra of Dislocations in Cz-Si

We investigated the development of dislocation-related DLTS spectra in n-CZ-Si crystals with small (about 7.104 cm-2) number of long individual dislocations depending on the distance L that dislocations traveled during deformation at 600oC and on the velocity of dislocations. We found that a typical dislocation-related DLTS signal appeared only when dislocations traveled a significant distance that is more than 150-200μm, and it depended strongly on dislocation velocity. The results were interpreted on the assumption that the DLTS signal corresponds to some core defects and atomic impurities accumulated on the dislocations during their slow motion. At high concentration of deep level defects on dislocations a strange “negative DLTS” signal was observed. This can be explained by electron tunneling between deep defects along dislocations.

Structure and photoresponse characteristics of ZnO thin films grown at high oxygen partial pressure

ZnO thin films were deposited by pulsed laser deposition on SiO 2/Si substrates at high oxygen partial pressures of 2, 20 and 200 Pa, respectively. The structure, surface morphology and photoluminescence of the ZnO thin films were investigated. Metal–semiconductor–metal structure prototype detectors were fabricated with ZnO thin films, and their spectral response properties were characterized and discussed. ZnO films deposited at higher oxygen pressure exhibit lower out-of-plane residual compressive stress, larger crystallite size and lower concentration of deep-level defects. Detectors based on ZnO thin films grown at 20 and 200 Pa show no response in visible region. The UV response property of ZnO film grown at 20 Pa oxygen pressure is better than that at 2 and 200 Pa, which may be related to its good crystallinity, smooth surface and less deep-level defects.

Influence of deep level defects on electrical compensation in semi-insulating InP materials

In this paper, we analyze and compare electrical compensation and deep level defects in semi-insulating (SI) materials prepared by Fe-doping and high temperature annealing of undoped InP. Influence of deep level defects in the SI-InP materials on the electrical compensation has been studied thermally stimulated current spectroscopy (TSC). Electrical property of the Fe-doped SI-InP is deteriorated due to involvement of a high concentration of deep level defects in the compensation. In contrast, the concentration of deep defects is very low in high temperature annealed undoped SI-InP in which Fe acceptors formed by diffusion act as the only compensation centre to pin the Fermi level, resulting in excellent electrical performance. A more comprehensive electrical compensation model of SI-InP has been given based on the research results.

Simulated admittance spectroscopy measurements of high concentration deep level defects in CdTe thin-film solar cells

Computer simulations of thin-film p-CdTe∕n-CdS solar cells using the SCAPS software program are examined to offer explanations for a number of experimentally observed admittance spectroscopy results. We show that for a high concentration of deep level defects, the magnitude of the admittance spectroscopy signal which is an indicator of defect concentration is also affected by the cell thickness. For CdTe cells thinner than 3μm, the signal for defects within 0.25eV of the band edge can be weak and may not be detected at all. We also show that Fermi level pinning resulting from high concentrations of deep level defects can distort the measured activation energy and apparent capture cross section. Finally, we show that decreasing capacitance values with increasing temperature can be caused by the interaction between the CdTe cell back contact Schottky barrier, a defect concentration gradient adjacent to the back contact, and a small shallow acceptor concentration relative to the defect concentration.