Shallow Impurity Levels Research Articles

First-principle calculations of the electronic structure and optical properties of β-Ga2O3 with various vacancy defects

In this work, the effects of five types of vacancy defects on the electronic, optical and thermodynamic properties of β-Ga2O3 were investigated using first-principle calculations. The findings revealed that oxygen vacancies were more prevalent than gallium vacancies, with tetrahedral gallium vacancies being easier to form than octahedral ones. Vacancy defects induced a shift in the energy bands towards the lower energy end, with gallium vacancies reducing the bandgap from 4.9 eV to 3.1 eV, while the effect of oxygen vacancies on the bandgap was negligible. Oxygen vacancies introduced deep impurity levels near the Fermi level, while gallium vacancies introduced shallow impurity levels, primarily due to contributions from O-2p, Ga-4s, and Ga-4p orbital electrons. The differential charge density diagrams illustrated that gallium vacancies had a more pronounced impact on electronic distribution compared to oxygen vacancies, with VO3 exhibiting the least influence. As the temperature increases, the β-Ga2O3 with VO1 and VO2 exhibit higher thermodynamic stability compared to the intrinsic β-Ga2O3 and the one with VO3, while the intrinsic β-Ga2O3 has a superior heat capacity value compared to the β-Ga2O3 containing the five vacancy defects. The defect energy levels resulting from vacancies enhanced light absorption and conductivity, particularly oxygen vacancies, which improved the absorption capacity of β-Ga2O3 in the visible light range. These findings provide valuable insights for elucidating optical absorption experimental phenomena and photoluminescence.

Properties of a highly compensated high-purity germanium

The electrical and optical properties of a compensated high-purity germanium (HPGe) single crystal was investigated using various characterization techniques. Aluminium, boron, and phosphorus were the major residual shallow-level impurities identified by photothermal ionization spectroscopy (PTIS). Hall effect measurements performed at low temperatures (77 K) along the growth length reveal that the crystal is p-type at the top and bottom, while n-type in the middle part with a net carrier concentration in the range of 1010–1011 cm−3. The obtained very high resistivity (2.3 ⨯ 108 Ω·cm) at 91 K in the temperature-dependent Hall measurements (TDH) at the bottom part of the crystal indicates a high level of compensation (84%) of charge carriers by deep-level impurities or defects with an activation energy near 0.195 eV. The highest hole mobility(μp=46700cm2/V·s)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$({\\mu _p} = ~46700\\,{\\rm{c}}{{\\rm{m}}^2}\\,/{\\rm{V}}\\cdot{\\rm{s}})$$\\end{document} at 77 K was obtained at the top part, which is moderately compensated. The carrier lifetimes of the grown HPGe crystals were measured using microwave-detected photoconductivity (MDP) at room temperature. The average carrier lifetime decreases from the top to bottom part of the crystal from 130 to 60 µs, which is less than that of the usually observed values in HPGe crystals due to a strong compensation.

Construction of localized state levels and near-infrared light absorption of silicon: B/P doping and first-principles calculations

Theoretically, the light wavelength that a silicon-based detector can detect is less than 1100 nm. After silicon (Si) is singly doped or co-doped with phosphorus (P) and boron (B) impurities, the localized state levels are formed in forbidden band, which makes it possible to absorb long wavelength light for the doped Si. Although the ionization energy of both P and B as shallow level impurities in Si is close to each other, the first-principles study in this paper shows that the near-infrared light absorption of n-Si is weaker than that of p-Si for the same concentration of impurities. After the impurity compensation of p-/n-Si based on P and B co-doping, the localized state levels are constructed by B− and P+ ions in the forbidden band of Si. The near-infrared absorption of n-Si can be significantly enhanced by impurity compensation, whereas the opposite case is true for p-Si. Moreover, even if p-/n-Si is fully compensated by impurities, it still has a strong near-infrared light absorption, which is different from intrinsic Si. Possible reasons on the difference of the above-mentioned light absorption were discussed.

High-pressure synthesis of metastable Bi–S crystalline compounds: Crystal structure and electron transport

A new metastable Bi3S4 single crystal has been obtained on the initial Bi: S = 2 : 3 mixture at a pressure of 8.0–8.5 GPa with slow cooling from 1300 to 800 °C at a rate of ∼3 °C/min followed by quenching. The crystal has a triclinic structure formed by Bi-centered S-polyhedrons and chain-like clusters of Bi atoms. The relationship between the triclinic model based on single-crystal X-ray diffraction data and the orthorhombic model previously proposed for high-pressure Bi–S and Bi–Se phases with a similar structural motif based on X-ray powder diffraction data is discussed. The new Bi3S4 phase is a narrow-gap (band gap of 0.234 eV) intrinsic n-type semiconductor with a shallow impurity level. Metastable Bi3S4 recovers the structure of bismuthinite Bi2S3 after annealing at 200 °C.

Photoluminescence Study of As Doped p-type HgCdTe Absorber for Infrared Detectors Operating in the Range up to 8 µm

A HgCdTe photodiode grown by chemical vapor deposition (MOCVD) on a GaAs substrate operating in the long-wave infrared (LWIR) range was characterized using photoluminescence (PL) measurements. At high temperatures, the PL spectrum originates from a free-carrier emission and might be fitted by a theoretical expression being the product of the density of states and the Fermi–Dirac distribution. At low temperatures, the PL spectrum consists of multiple emission peaks that do not originate solely from the energy gap. Such spectra are not unambiguous to interpret due to the prominence of different optical transitions. Spectral response (SR) measurements were used to determine the energy gap (Eg) and extract the band-to-band transition from the PL spectra. PL peaks visible within the band gap were fitted by a Gaussian distribution. To identify the sources of individual emission peaks, excitation power dependence analysis was conducted. Band-to-band, free-to-bound, acceptor-bound exciton, and defect-bound exciton transitions were identified. At low temperatures, transitions are mainly impurity-related, with shallow impurity levels estimated to be 6 meV and 16 meV for the donor and acceptor, respectively, while deep-level impurities were associated with VHg. The latter transition with an energy of about 78 meV does not vary with temperature. Its relative positions with respect to the energy gap is 0.8 Eg at 18 K and 0.67 Eg at 80 K.

Modulating the valence of In on the thermoelectric properties of Pb0.99In0.01Te1-xSx compounds

In dopant demonstrates a distinct role in PbTe and PbS compounds though they possess the same crystal structure. Therefore, to unravel how the thermoelectric performance evolves in the In-doped PbTe1-xSx solid solution is crucial. Herein, a series of Pb0.99In0.01Te1-xSx samples were synthesized by conventional melting methods. It is found that doping In in PbTe introduces a shallow level impurity state In3+ and a deep level associated with the In+ state simultaneously. Alloying PbS in Pb0.99In0.01Te1-xSx compounds modulates the valence of In dopant, increasing the relative ratio of In3+ content due to the stronger oxidation capacity of S in comparison with that of Te. This augments the room temperature carrier concentration from 3.03 × 1018 cm−3 for Pb0.99In0.01Te compound to 5.76 × 1018 cm−3 for Pb0.99In0.01Te0.91S0.09 compound. Therefore, an improved power factor of 17.4 μW cm−1 K−2 for Pb0.99In0.01Te0.91S0.09 compound is obtained at room temperature. Additionally, alloying PbS in Pb0.99In0.01Te1-xSx compounds enhances the point defect phonon scattering and lowers the lattice thermal conductivity from 1.83 W m−1 K−1 for Pb0.99In0.01Te compound to 0.74 W m−1 K−1 for Pb0.99In0.01Te0.88S0.12 compound at room temperature. As a consequence, a maximum ZT value of 1.12 at 723 K is attained for Pb0.99In0.01Te0.95S0.05 compound, which is improved by 19% in comparison with that of Pb0.99In0.01Te compound. This work provides a new path for modulating the doping behavior of cation site dopant via alloying on anion site to further improve the ZT value in thermoelectric materials.

Improved Simulated-Daylight Photodynamic Therapy and Possible Mechanism of Ag-Modified TiO2 on Melanoma.

Simulated-daylight photodynamic therapy (SD-PDT) may be an efficacious strategy for treating melanoma because it can overcome the severe stinging pain, erythema, and edema experienced during conventional PDT. However, the poor daylight response of existing common photosensitizers leads to unsatisfactory anti-tumor therapeutic effects and limits the development of daylight PDT. Hence, in this study, we utilized Ag nanoparticles to adjust the daylight response of TiO2, acquire efficient photochemical activity, and then enhance the anti-tumor therapeutic effect of SD-PDT on melanoma. The synthesized Ag-doped TiO2 showed an optimal enhanced effect compared to Ag-core TiO2. Doping Ag into TiO2 produced a new shallow acceptor impurity level in the energy band structure, which expanded optical absorption in the range of 400-800 nm, and finally improved the photodamage effect of TiO2 under SD irradiation. Plasmonic near-field distributions were enhanced due to the high refractive index of TiO2 at the Ag-TiO2 interface, and then the amount of light captured by TiO2 was increased to induce the enhanced SD-PDT effect of Ag-core TiO2. Hence, Ag could effectively improve the photochemical activity and SD-PDT effect of TiO2 through the change in the energy band structure. Generally, Ag-doped TiO2 is a promising photosensitizer agent for treating melanoma via SD-PDT.

Experimental and theoretical research on improving the gas sensitivity of ethanol based on Bi-doped ZnSn(OH)6

The wider band gap of ZnSn(OH)6 (ZHS) leads to higher resistance and low sensitivity to gases, and doping is one important way of reducing resistance and improving gas sensitivity. In this paper, cubic ZHS and Bi-doped ZHS were successfully prepared by hydrothermal method and analyzed by XRD, SEM, TEM and XPS, respectively. The results are as following: XRD pattern of the samples are consistent with ZHS standard card, and the XRD characteristic peak shifts to the left after Bi doping; the size of the cubic particle decreases greatly; Zn, Sn, O and Bi elements are evenly distributed. The replacement of Sn with Bi will form a displacement impurity, and insert a shallow donor impurity level at the bottom of the conduction band. As a result, the carrier concentration increases while the band gap width and material resistance decreases. Therefore, the performance of the sensor is greatly improved.

Enhanced Optical and Electrical Interactions at the Pt/MgSe Interfaces Designed for 6G Communication Technology

AbstractMagnesium selenide thin films are coated onto glass and semitransparent Pt substrates (nanosheets) by the vacuum evaporation technique under a vacuum pressure of 10−5 mbar. The effect of Pt nanosheets of thicknesses of 100 nm on the structural, compositional, optical, and electrical properties of MgSe is explored. It is found that platinum nanosheets enhance the light absorbability in the visible and infrared ranges of light. They slightly redshift the energy bandgap and decrease the dielectric constant. The strong interaction between Pt and MgSe increases the electrical conductivity by five orders of magnitude. Pt forms shallow impurity levels in the energy bandgap of MgSe allowing the tunneling process at the Pt/MgSe interfaces. Practically, a Pt/MgSe/Pt tunneling type device is fabricated and tested by the microwave impedance spectroscopy technique. It is found that Pt/MgSe/Pt devices can exhibit resonance–antiresonance and negative capacitance effects which are important in electronic circuits as parasitic capacitance cancelers, signal amplifiers, and noise reducers. In addition, evaluation of the cutoff frequency spectra has shown the ability of using the Pt/MgSe/Pt devices as microwave resonators appropriate for 5G/6G technologies. A cutoff frequency larger than 70 GHz can be achieved by imposing ac signal of amplitude of 0.10 V.

Basal Planes Unlocking and Interlayer Engineering Endows Proton Doped-MoO2.8F0.2 with Fast and Stable Magnesium Storage.

Molybdenum trioxide has served as a promising cathode material of rechargeable magnesium batteries (RMBs), because of its rich valence states and high theoretical capacity; yet, it still suffers from sluggish (de)intercalation kinetics and inreversible structure change for highly polarized Mg2+ in the interlayer and intralayer of structure. Herein, F- substitutional and H+ interstitial doping is proposed for α-MoO3 materials (denoted HMoOF) by the intralayer/interlayer engineering strategy to boost the performance of RMBs. F- substitutional doping generates molybdenum vacancies along the Mo-O-□ or Mo-F-□ configurations (where □ represents the cationic vacancy) for unlocking the inactive basal plane of the layered crystal structure, and it further accelerates Mg2+ diffusion along the b-axis. Interstitial-doped H+ can expand interlayer spacing for reducing Mg2+ energy barrier along the ac plane and serve as a "pillar" to stabilize the interlayer structure. Moreover, anion and cation dual doping trigger shallow impurity levels (acceptors levels and donor levels), which helps to easily acquire the electrons from the valence band and donate the electrons to the conduction band. Consequently, the HMoOF electrode exhibits a high reversible capacity (241 mA h g-1 at 0.1 A g-1), an excellent rate capability (137.4 mAh g-1 at 2 A g-1), and a long cycling stability (capacity retention of 98% after 800 cycles at 1 A g-1) in RMBs. This work affords meaningful insights in layered materials for developing high-kinetics and long-life RMBs.

Modulating the valence of Ga and the deep level impurity for high thermoelectric performance of n-type Pb0.98Ga0.02Te1-xSex compounds

It has been found that the doped Ga has a distinct role in PbTe and PbSe compounds though they possess the same crystal structure. Therefore, to understand how the thermoelectric performance evolves in the Ga-doped PbTe1-xSex solid solution is crucial. Herein, we report that a maximum ZT value of 1.57 at 721 K and a high average ZT value of 1.13 in the temperature range of 300–843 K are attained for Pb0.98Ga0.02Te0.96Se0.04 compound. The experimental studies show that doping Ga into PbTe introduces a shallow level impurity state Ga3+ and a deep level associated with the Ga + state simultaneously. Interestingly, upon Se-alloying all of the Ga + are oxidized into Ga3+ at low temperatures, which increases the room-temperature carrier concentration from 1.16 × 1019 cm−3 to 2.17 × 1019 cm−3. This leads to an improved power factor of 3.04 mW m−1 K−2 for Pb0.98Ga0.02Te0.96Se0.04 sample at 482 K. Additionally, Se-alloying further enhances the point defect phonon scattering and lowers the lattice thermal conductivity to 0.46 W m−1 K−1 for Pb0.98Ga0.02Te0.96Se0.04 sample at 690 K. As a result, the ZT value is increased by about 112% compared with the maximum ZT of 0.74 for pristine PbTe. This work provides a new avenue for tuning the doping behavior of cation site dopant via alloying on anion site to further improve the ZT value in thermoelectric materials.

Optimizing thermoelectric performance of CoSbS0.85Se0.15 by doping 3d transition metal ions M (M = Cr, Mn, Fe and Ni)

Transitional metal compound CoSbS attracted much attention as a thermoelectric material in the past decades. The configuration of the transition metal octahedra has a close relationship with the thermoelectric performance. This study reports the optimized thermoelectric performance of CoSbS through tuning Co octahedra distortion. The substitution of M (M ​= ​Cr, Mn, Fe and Ni) on Co sites could remarkably reduce the lattice thermal conductivity, and typically, Cr substitution endows the lowest lattice thermal conductivity owing to its strongest effect on the octahedra distortion, whereas Ni substitution induces to a relatively weak octahedra distortion at the same substitution level. Additionally, band structure calculations demonstrate that Ni-3d orbitals contribute shallow impurity levels near the conduction band minimum, strikingly increasing the carrier concentration and power factor. As a result, CoSbS0.85Se0.15 with Ni substitution on Co sites exhibits a much higher thermoelectric performance compared with the other counterparts. Furthermore, by optimizing Ni content, a maximum zT value of 0.52 ​at 876 ​K was achieved in Ni0.10Co0.90SbS0.85Se0.15. This study provides an exemplary approach to optimize the thermoelectric performance via polyhedral distortion.

Hydrogen impurities in p-type semiconductors, GeS and GeTe

Hydrogen defects sometimes form shallow impurity levels in semiconductors, and it is an important topic for semiconductor research to investigate their details. One of the experimental methods to determine the state of hydrogen is the muon spin rotation (μSR) experiment. By observing formation of a pseudo-hydrogen atom, called muonium, it is possible to investigate the hydrogen defect levels. In a previous theoretical study, the pinning levels were calculated for various materials as a reference for hydrogen defect levels, and these levels were universally distributed near the hydrogen electrode potential. Based on the prediction, μSR experiments were performed for germanium sulfide (GeS) and germanium telluride (GeTe), where the hydrogen electrode potential is located in the bandgap for GeS, but not for GeTe. As a result, the μSR spectra showed that the muonium forms in GeS, while it does not in GeTe. In GeS, 58% of the muons formed muoniums. The activation energy was obtained as ΔE=26.2±6.9 meV. The hyperfine coupling frequency was ωc(2π)−1=1.95±0.17 GHz, and the Bohr radius of muonium was 1.3 times larger than that in vacuum. These properties indicated that the identified muonium does not form a typical impurity level that affects the electrical properties.

First-principles study of electronic structure and optical properties of monolayer defective tellurene

Monolayer tellurene is a novel two-dimensional semiconductor with excellent intrinsic properties. It is helpful in understanding doping and scattering mechanism to study the electronic structure of defective tellurene, thus it is important for the application of tellurene in electronic and photo-electronic devices. Using first-principles calculation based on the density functional theory, we investigate the effects of commonly seen point defects on the electronic structure and optical properties of monolayer <i>β</i>-Te. Seven kinds of point defects that may be present in <i>β</i>-Te are designed according to the lattice symmetry, including two single vacancies (SV-1, SV-2), two double vacancies (DV-1, DV-2) and three Stone-Wales (SW) defects (SW-1, SW-2, SW-3). It is found that the defect formation energies of these defects are 0.83–2.06 eV, which are lower than that in graphene, silicene, phosphorene and arsenene, suggesting that they are easy to introduce into monolayer <i>β</i>-Te. The two most stable defects are SV-2 and SW-1 where no dangling bond emerges after optimization. The calculated band structures show that all seven defects have little effect on the band gap width of monolayer <i>β</i>-Te, but they can introduce different numbers of impurity energy levels into the forbidden band. Among them, the SV-1, SV-2, DV-1 and SW-2 each act as deep level impurities which can be recombination centers and scattering centers of carriers, SW-1 acts as a shallow level impurity, DV-2 and SW-3 act as both deep level impurity and shallow level impurity. Besides, SW-1, SW-2 and DV-1 can change the band gap of monolayer <i>β</i>-Te from direct band gap to indirect band gap, which may result in the increase of the lifetime of carriers and decrease of photoluminescence of monolayer <i>β</i>-Te. The optical properties of monolayer <i>β</i>-Te, which are sensitive to the change in band structure, are also affected by the presence of defects. New peaks are found in the complex dielectric function and the absorption coefficient of defective monolayer <i>β</i>-Te in an energy range of 0–3 eV, of which the number and the position are dependent on the type of defect. The SV-1, DV-1, DV-2 and SW-2 can enhance the light response, polarization ability and light absorption in the low energy region of monolayer <i>β</i>-Te. This research can provide useful guidance for the applications of <i>β</i>-Te in the electronic and optoelectronic devices.

Hybrid Density Functional Theory Study of Native Defects and Nonmetal (C, N, S, and P) Doping in a Bi2WO6 Photocatalyst.

Native defects and nonmetal doping have been shown to be an effective way to optimize the photocatalytic properties of Bi2WO6. However, a detailed understanding of defect physics in Bi2WO6 has been lacking. Here, using the Heyd–Scuseria–Ernzerhof hybrid functional defect calculations, we study the formation energies, electronic structures, and optical properties of native defects and nonmetal element (C, N, S, and P) doping into Bi2WO6. We find that the Bi vacancy (Bivac), O vacancy (Ovac), S doping on the O site (SO), and N doping on the O site (NO) defects in the Bi2WO6 can be stable depending on the Fermi level and chemical potentials. By contrast, the substitution of an O atom by a C or P atom (CO, PO) has high formation energy and is unlikely to form. The calculated electronic structures of the Bivac, Ovac, SO, and NO defects indicate that the band-gap reduction of Ovac2+, Bivac3–, and SO defects is mainly due to forming shallow impurity levels within the band gap. The calculated absorption coefficients of Ovac2+, Bivac3–, and SO show strong absorption in the visible light region, which is in good agreement with the experimental results. Hence, Ovac2+, Bivac3–, and SO defects can improve the adsorption capacity of Bi2WO6, which helps enhance its photocatalytic performance. Our results provide insights into how to enhance the photocatalytic activity of Bi2WO6 for energy and environmental applications through the rational design of defect-controlled synthesis conditions.

Contributions of chemical potential to the diffusive Seebeck coefficient for bulk semiconductor materials

Predicting the behaviors of thermoelectric material, such as the Seebeck coefficient, using first-principle material properties will accelerate material design and selection to optimize the performance of thermoelectric generator. This paper presents an analytic framework for the Seebeck coefficient. Particularly, it elucidates the important contribution of chemical potential to the overall Seebeck coefficient. This factor, however, is often neglected when Seebeck coefficient is defined based on only electrostatic potential or when methods based on the Boltzmann transportation equation is used to derive the expression of the Seebeck coefficient. (The diffusion component due to chemical potential gradient is generally ignored.) In this paper, methods based on local equilibrium assumption and Fermi–Dirac distribution are utilized to derive the expressions for the Seebeck coefficients using the first-principle parameters. The analytic expression of both the total Seebeck coefficient and the contribution from chemical potential component is derived, which is solved using numerical methods. A case study on a popular thermoelectric material bismuth telluride (both n-type and p-type) is conducted based on the derived expressions. Results of numerical calculations prove that chemical potential contributes significantly to the total Seebeck coefficient of bismuth telluride (76% for n-type and 88% for p-type materials, respectively, at room temperature if the materials are doped with shallow energy level impurities) when working under optimal Seebeck coefficient conditions.

Weak Electron Phonon Coupling and Deep Level Impurity for High Thermoelectric Performance Pb1−xGaxTe

AbstractHigh ZT of 1.34 at 766 K and a record high average ZT above 1 in the temperature range of 300‐864 K are attained in n‐type PbTe by engineering the temperature‐dependent carrier concentration and weakening electron–phonon coupling upon Ga doping. The experimental studies and first principles band structure calculations show that doping with Ga introduces a shallow level impurity contributing extrinsic carriers and imparts a deeper impurity level that ionizes at higher temperatures. This adjusts the carrier concentration closer to the temperature‐dependent optimum and thus maximizes the power factor in a wide temperature range. The maximum power factor of 35 µW cm−1 K−2 is achieved for the Pb0.98Ga0.02Te compound, and is maintained over 20 µWcm−1 K−2 from 300 to 767 K. Band structure calculations and X‐ray photoelectron spectroscopy corroborate the amphoteric role of Ga in PbTe as the origin of shallow and deep levels. Additionally, Ga doping weakens the electron–phonon coupling, leading to high carrier mobilities in excess of 1200 cm2 V−1 s−1. Enhanced point defect phonon scattering yields a reduced lattice thermal conductivity. This work provides a new avenue, beyond the conventional shallow level doping, for further improving the average ZT in thermoelectric materials.

A DFT study on modification mechanism of (N,S) interstitial co-doped rutile TiO2

To obtain a more efficient (N,S) co-doping scheme, we systematically analyze the geometrical parameters, density of states, charge densities, relative dielectric functions and UV–Vis absorption spectra for pure, N/S substitution/interstitial doped and (N,S) substitution/interstitial co-doped TiO2 by using density functional calculations. Compared with (N,S) substitution co-doping, (N,S) interstitial co-doping TiO2 exhibits a more obvious red-shift of absorption edge, because of the band gap is further reduced. Furthermore, there are shallow impurity levels coupling with the top of valence band. The calculated UV–Vis absorption spectra illustrate that (N,S) interstitial co-doping TiO2 has much higher photocatalytic activity in the visible light region.

P-type co-doping effect of (Ga,Mn)P: Magnetic and magneto-transport properties

In this paper, we perform a comparison of magnetic and electrical properties between Mn-doped and (Mn, Zn) co-doped GaP dilute ferromagnetic semiconductors. Due to the shallow Zn impurity level (∼20–40 meV above the top of the III-V compounds valence band), the Zn co-doping leads to the increase of conductivity of (Ga,Mn)P, however both the Curie temperature and magnetization reduce, which is probably due to the suppression of active Mn substitution by Zn co-doping.

A row hammer pattern analysis of DDR2 SDRAM

A DDR2 SDRAM test setup implemented on the Griffin III ATE test system from HILEVEL Technologies is used to analyse the row hammer bug. Row hammer pattern experiments are compared to standard retention tests. The analysis confirms that the row hammer effect is caused by a charge excitation process depending on the number of stress activation cycles. The stress has to occur in the local neighborhood of the cells under test. Shallow impurity levels support the responsible charge carrier transport process in the used DDR2 SDRAM technology.