Deep Impurity Levels Research Articles

Band engineering of silicon for light emission: First-principles approach to the effect of co-doping with nitrogen and fluorine

A Si-based light emitter has long been the final key component for electronic and photonic integrated circuits on Si, because Si has an indirect band gap. Atomistic and electronic structures and energy gains of formation of possible nitrogen (N) and fluorine (F) complexes in Si have been researched from first-principles, in order to engineer the band structure of Si for light emission. The calculated results show that the substitutional nitrogen N S and bond center fluorine F BC pair complex has large stabilization energy, and that the pair-complex-doped Si has direct band gap, which is reduced with respect to that of Si. These results lead to the possibilities of doping-based engineering of Si optical properties with introduction of deep-level impurity and charge compensation.

Electronic structure of diluted magnetic semiconductors Pb1−x−ySnxCryTe

Abstract Galvanomagnetic effects in diluted magnetic semiconductors Pb 1− x − y Sn x Cr y Te at temperatures 4.2≤ T ≤300 K under variation of matrix composition ( x =0.06–0.235) and impurity content ( y =0.002–0.045) have been investigated. The p–n-conversion of the conductivity type, induced by the chromium doping, was observed. It is shown that in the heavily doped alloys temperature dependencies of resistivity and of the Hall coefficient have a “metallic” character, indicating pinning of the Fermi level by the chromium deep impurity level, situated in the conduction band. Free electron concentration and Fermi energy were calculated as functions of impurity concentration and of matrix composition by analyzing experimental data in the framework of the two-band Kane dispersion law. The impurity content coefficients of the free charge carrier concentration were determined and the diagram of electronic structure rearrangement under varying the alloy composition was obtained for the alloys under study.

Vanadium-induced deep impurity level in Pb1−xSnxTe

The galvanomagnetic properties (T=4.2―300 K, B≤0.08T) of Pb 1―x―y Sn x V y Te alloys (x=0.06-0.26, y=0.002-0.066) have been investigated. Low temperature activation range of the impurity conductivity on the temperature dependencies of resistivity p and of the Hall coefficient R H in the heavily doped samples has been revealed and attributed to the appearance of vanadium-induced deep level in the gap of the alloys. It was found that in insulating phase alloys possess a high photosensitivity to IR excitation at temperatures up to T c ≈ 40 K. In the alloy with x ≈ 0.2 the metal-insulator transition due to the shift of the Fermi level from the valence band to the gap with the increase of the vanadium and tin content and the pinning of the Fermi level by vanadium level were observed. The diagram of electronic structure reconstruction for Pb 1 ―x―y Sn x V y Te alloys was proposed.

NTC and electrical properties of nickel and gold doped n-type silicon material

Silicon materials compensated by deep level impurities such as nickel and gold have negative temperature coefficient (NTC) characteristics. In this work, n-type silicon wafers are smeared by nickel chloride ethanol solution and gold chloric acid ethanol solution, and subsequently put in the opening environment to heat. The electrical resistance and B-value of the thermistors made by this silicon material are measured and analyzed. When the silicon surface concentration of gold atoms is 2 × 10−6 mol/cm2, the uniformity of the single-crystal silicon material is optimal. When the diffusion temperature is between 900 and 1000 °C, a material with high B-value and low electrical resistivity is obtained. The B–T and R–T change laws calculated by the theory of semiconductor deep level energy are basically consistent with the experimental results.

Photoluminescence properties of erbium doped InGaN epilayers

We report on the photoluminescence properties of erbium (Er) doped InxGa1−xNa epilayers synthesized by metal organic chemical vapor deposition. The crystalline quality and surface morphology of Er doped In0.05Ga0.95N were nearly identical to those of Er doped GaN. The photoluminescence intensity of the 1.54 μm emission in Er doped In0.05Ga0.95N was an order of magnitude lower than in Er doped GaN and decreased with the increase of the In content. The reduction in 1.54 μm emission intensity was accompanied by enhanced emission intensities of deep level impurity transition lines.

Optical discrimination of deep trap contribution to carrier recombination in semi-insulating crystals

We demonstrate a novel application of light-induced transient grating technique for discrimination of deep trap contribution to carrier recombination in compensated semiconductors. This application is based on photoexcitation of deep impurity levels by light interference pattern and subsequent optical monitoring of the recharged state dynamics. The spatially modulated deep trap occupation leads to changes in a probe beam absorption and formation of a transient diffraction grating. Employment of light diffraction on a short-period reflection grating allowed to realize conditions when the absorption modulation in deep traps dominates over the coexisting refractive index modulation. Selectivity of a transient reflection grating configuration solely to processes in deep traps was proven experimentally and confirmed by numerical modeling. In this way, we discriminated the deep vanadium impurity governed carrier recombination rate from the other coexisting recombination channels in as-grown and annealed semi-insulating CdTe:V crystals.

Excitation relaxation in copper selenide nanowires

AbstractUltrafast nonequilibrium charge carrier relaxation in highly ordered Cu2–x Se nanowires of 8 nm, 13 nm and 25 nm diameter was investigated by means of femtosecond pump–probe absorption spectroscopy. Transient absorption bleaching was observed in the region of the near infrared absorption band, whereas an induced absorption dominated at higher energies. The transient absorption kinetics is almost independent of the excitation and probe wavelength and shows a biexpoenetial charge carrier recombination with the excitation intensity dependent decay rates. The initial ultrafast relaxation, which gets slower at higher excitation intensities, is followed by the slower decay component emerging at high intensities. These relaxation peculiarities are discussed in terms of recombination enhancement by intragap states, and relaxation kinetics is described by a theoretical model of two concurrent relaxation channels involving deep and shallow impurity levels. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence studies of impurity transitions in Mg-doped AlGaN alloys

Deep ultraviolet photoluminescence spectroscopy was employed to study the impurity transitions in Mg-doped AlGaN alloys. A group of deep level impurity transitions was observed in Mg-doped AlxGa1−xN alloys, which was identified to have the same origin as the previously reported blue line at 2.8eV in Mg-doped GaN and was assigned to the recombination of electrons bound to the nitrogen vacancy with three positive charges (VN3+) and neutral Mg acceptors. Based on the measured activation energies of the Mg acceptors in AlGaN and the observed impurity emission peaks, the VN3+ energy levels in AlxGa1−xN have been deduced for the entire alloy range. It is demonstrated that the presence of high density of VN3+ deep donors translates to the reduced p-type conductivity in AlGaN alloys due to their ability for capturing free holes.

Ultimately Low Schottky Barrier Height at NiSi/Si Junction by Sulfur Implantation after Silicidation for Aggressive Scaling of MOSFETs

AbstractParasitic resistance, particularly source/drain contact resistance becomes one of the most serious problems to extend MOSFET scaling recently. Nickel silicide (NiSi), with advantages of low resistivity and high scalability, has been chosen as the material for source/drain formation. However, its Schottky barrier height (SBH) of 0.65eV for electrons is so high that it would block electrons from tunneling, therefore becomes an obstacle to further reduce the contact resistance, which is necessary to achieve the future scaling. Among several solutions, high concentration impurity-segregation layers have been introduced at NiSi/Si interfaces to reduce SBH of MS-MOSFETs. Sulfur (S) has been considered to be an efficient material for the segregation-layer to reduce SBH owing to Fermi-level pinning effect. Previous studies have investigated segregation by implanting S before NiSi formation. Because of the high diffusivity of S in Si, S profile becomes broad during silicidation process, which leads to loss of S concentration at the interface. Moreover, S ions spread into the substrate and channel region generate deep impurity levels that induce junction leakage and off leakage, resulting in device performance degradation. In this paper, S implantation after Ni silicidation is proposed to suppress S diffusion because NiSi is expected to be an efficient barrier for S diffusion. In addition, NiSi/Si interface can serve as a potential energetic valley which may trap S during thermal treatment after implantation. In this work, S was implanted into NiSi/n-Si diodes at the energy of 10keV with dose of 5�1014 and 1015 cm-3 after NiSi is formed. The projection range of S in NiSi is about 6nm, while thickness of NiSi is 16nm. Some devices were annealed at 300C and 450C. The I-V characteristics show that SBH is sufficiently reduced as the annealing temperature becomes higher, and it reaches as low as 3.4meV for 450C annealing. SBH of 3.4meV is much lower than the previously reported value of 70meV for which S was implanted before silicidation. The SIMS analysis result also proves the S profile is much sharper than having S-implantation before Ni silicidation, which supports our hypothesis that S diffusion is suppressed through our process and avoid the loss of S concentration at the interface. Moreover, despite the worry that S-implantation might damage the NiSi/Si interface morphology, cross sectional TEM images show that the interfacial flatness is completely the same as that of non-implanted NiSi/Si, indicating that no degradation occurs by S implantation. In summary, S-implantation after NiSi formation, which provides ultimately low SBH at NiSi/Si interface, is a promising technique to realize ultra-low parasitic resistance source/drain for future LSI beyond 16nm generation.

Autosolitons in bistable system of silicon with deep impurity levels

Autosolitons in the bistable system of silicon associated with the thermal and electric-field ionization of In deep acceptor levels at a temperature of 77 K are experimentally found and investigated. It is shown that the positive feedback on the activator is related in the considered model of autosolitons to an increasing dependence of the lattice temperature on growth in the hole concentration, while the damping role of the inhibitor is associated with a decrease in the charge-carrier temperature during phonon-induced scattering. This leads to power-loss reduction and lattice-temperature restriction in the vicinity of autosolitons.

Functional possibilities of Ag-N 0AlGaAs-n +GaAs-n 0GaInAs-au structures with an isotype base region

We have considered the spectral characteristics of two-base structures with isotype heterojunctions and with Schottky and Mott barriers to the base regions and explained the features of the impurity and photovoltaic effects in two-base structures containing deep impurity levels of oxygen and intrinsic defects.

Impurity photovoltaic effect in GaAs solar cell with two deep impurity levels

The impurity photovoltaic (IPV) effect has been extensively investigated for silicon solar cells with indium impurities. A small positive effect on the efficiency was theoretically predicted, provided that efficient light trapping schemes are applied. Since IPV effects are predicted to be more pronounced in wider band gap materials and possibly also by introducing more than one impurity level, we carried out a numerical study on GaAs solar cells with copper impurities. Indeed, copper introduces multiple acceptor type mid-gap levels in GaAs, two of which are investigated here: one at 0.14 eV and another at 0.40 eV above the valence band edge. We used a solar cell device simulator (SCAPS) specially adapted to include the IPV effect. We varied the impurity concentration and the light trapping parameters of the device, and calculated the occupation probability of the impurity levels, the generation and recombination through these levels (thermal and optical) and the solar cell characteristics: quantum efficiency, QE( λ), short-circuit current J sc, open-circuit voltage V oc and efficiency η. A significant IPV effect with extended response in the infrared is calculated when efficient light trapping is present. When an internal reflection coefficient R=0.99 can be reached at both the front and the back surface of the solar cell, an increase of the short-circuit current densities 2 mA/cm 2 can be obtained. The consequences of two levels instead of one are calculated and discussed, leading to a trade-off between J sc improvement and a V oc decrease.

The persistent photoconductivity effect in AlGaN/GaN heterostructures grown on sapphire and SiC substrates

In the present study, we reported the results of the investigation of electrical and optical measurements in AlxGa1−xN/GaN heterostructures (x=0.20) that were grown by way of metal-organic chemical vapor deposition on sapphire and SiC substrates with the same buffer structures and similar conditions. We investigated the substrate material effects on the electrical and optical properties of Al0.20Ga0.80N/GaN heterostructures. The related electrical and optical properties of AlxGa1−xN/GaN heterostructures were investigated by variable-temperature Hall effect measurements, photoluminescence (PL), photocurrent, and persistent photoconductivity (PPC) that in turn illuminated the samples with a blue (λ=470 nm) light-emitting diode (LED) and thereby induced a persistent increase in the carrier density and two-dimensional electron gas (2DEG) electron mobility. In sample A (Al0.20Ga0.80N/GaN/sapphire), the carrier density increased from 7.59×1012 to 9.9×1012 cm−2 via illumination at 30 K. On the other hand, in sample B (Al0.20Ga0.80N/GaN/SiC), the increments in the carrier density were larger than those in sample A, in which it increased from 7.62×1012 to 1.23×1013 cm−2 at the same temperature. The 2DEG mobility increased from 1.22×104 to 1.37×104 cm−2/V s for samples A and B, in which 2DEG mobility increments occurred from 3.83×103 to 5.47×103 cm−2/V s at 30 K. The PL results show that the samples possessed a strong near-band-edge exciton luminescence line at around 3.44 and 3.43 eV for samples A and B, respectively. The samples showed a broad yellow band spreading from 1.80 to 2.60 eV with a peak maximum at 2.25 eV with a ratio of a near-band-edge excitation peak intensity up to a deep-level emission peak intensity ratio that were equal to 3 and 1.8 for samples A and B, respectively. Both of the samples that were illuminated with three different energy photon PPC decay behaviors can be well described by a stretched-exponential function and relaxation time constant τ as well as a decay exponent β that changes with the substrate type. The energy barrier for the capture of electrons in the 2DEG channel via the deep-level impurities (DX-like centers) in AlGaN for the Al0.20Ga0.80N/GaN/sapphire and Al0.20Ga0.80N/GaN/SiC heterojunction samples are 343 and 228 meV, respectively. The activation energy for the thermal capture of an electron by the defects ΔE changed with the substrate materials. Our results show that the substrate material strongly affects the electrical and optical properties of Al0.20Ga0.80N/GaN heterostructures. These results can be explained with the differing degrees of the lattice mismatch between the grown layers and substrates.

Deep impurity levels in vanadium-doped Pb1−xMnxTe solid solutions

Measurements of electric and magnetic properties of Pb1−xMnxTe(V) solid solutions have been performed in the temperature range 4.2–300 K. It is demonstrated that the Fermi level position in this semiconductor is defined by formation of an impurity level within the bandgap of the material. Realization of a semi-insulating state at low temperatures is accompanied by absence of the persistent photoconductivity effect and by high homogeneity of electrical parameters in Pb1−xMnxTe(V) which is a unique combination. Measurements of electric properties in the ac field reveal that the main mechanism of the low temperature transport is the hopping conductivity. Pb1−xMnxTe(V) demonstrates a paramagnetic behavior at all temperatures, at least above 10 K.

Evaluation of photoelectrical properties of Bi doped CdTe crystals

Nonequilibrium carrier generation, transport, and recombination features have been investigated in Bi-doped and Yb-codoped bulk CdTe crystals, using time-resolved picosecond free-carrier and photorefractive grating techniques. Peculiarities of deep-impurity dependent carrier generation were studied at below band-gap photoexcitation using 1064 nm wavelength. Carrier transport in light created space charge field provided the instantaneous diffusion coefficient values, dependent on excitation energy, and revealed a sign of the dominant photoexcited carriers in the crystals. The studies provided also a deeper insight into an initial occupation of deep impurity levels, which was found strongly dependent on doping and co-doping. At low Bi density (∼2 × 1017 cm−3), the hole generation was dominant, while at higher doping (8 × 1017 cm−3) the bipolar plasma generation prevailed. Codoping by Yb (∼5 × 1017 cm−3) revealed deep donor features of deep defects and formation of a strong space charge field. Carrier dynamics in the given crystals was compared with CdTe:V ones, where vanadium is known to form the mid-gap donor levels.

Si-doped high Al-content AlGaN epilayers with improved quality and conductivity using indium as a surfactant

Effects of indium as a surfactant for the growth of Si-doped Al0.75Ga0.25N epilayers by metal organic chemical vapor deposition have been studied. It was found that the use of indium as a surfactant improved the overall material quality of these epilayers, as evidenced by decreasing the (a) density of surface pits, (b) screw dislocation density, and (c) intensity of the deep level impurity transition with increasing indium flow rate. Hall effect measurements also yielded increased conductivity and electron concentration with increasing indium flow rate. The results suggested that indium as a surfactant counteracts the incorporation of defects responsible for self-compensation for n-type doping, namely, cation vacancies, in high Al-content AlGaN epilayers. A correlation between the intensity of the deep level impurity transition and screw dislocation density was also established.

Correlation between photoluminescence and magnetic properties of GaMnN films

GaMnN films grown by metal-organic chemical vapor deposition were studied by photoluminescence (PL) spectroscopy and hysteresis measurements. Depending on the growth conditions of these GaMnN films, hysteresis measurements along the easy axis of magnetization show a transformation from magnetic to nonmagnetic behavior. The PL spectra of both magnetic and nonmagnetic GaMnN films exhibited GaN band edge and deep-level impurity transitions at 3.4 and 1.3eV, respectively. The PL emission intensity of the 1.3eV emission peak is stronger considerably for magnetic GaMnN films and is believed to be due to the Mn3+ intraband transition.

Negative photoconductivity in In 0.52Al 0.48As/In 0.7Ga 0.3As heterostructures

We investigated the transport of a two-dimensional electron gas (2DEG) in an In 0.52Al 0.48As/In 0.7Ga 0.3As heterostructure when exposed to light from infrared laser diodes ( λ=0.78, 1.3 μm) by means of both Shubnikov–de Haas and Hall-effect measurements. We observed negative photoconductivity due to a reduction in the number of electrons in the 2DEG when they were illuminated by photons at λ=1.3 μm. We speculate that the negative photoconductivity originates from the diffusion and trapping of photo-induced hot electrons at deep impurity levels in the InAlAs barrier layer near the InAlAs/InGaAs interface.

Characterization of a deep-level compensation ratio through picosecond four-wave mixing on a transient reflection grating

We demonstrate a novel application of a time-resolved four-wave mixing technique for the determination of a deep-level compensation ratio in a semi-insulating crystal. The approach is based on photoexcitation of carriers from deep impurity levels, formation of a space-charge electric field in deep traps, and monitoring dynamics of photorefractive, free- carrier and absorption gratings by light diffraction. The analysis of anisotropic diffraction features on the reflection grating provided requirements for crystal orientation in order to discriminate contribution of amplitude grating from the photorefractive phase grating, both being related to deep-trap occupation. Contributions of these optical nonlinearities were studied experimentally in (0 0 1)-oriented GaAs wafers by using a transient reflection grating configuration with a very small grating period (150 nm). Comparison of the reflection grating picosecond kinetics and its diffraction efficiency with modeling curves allowed us to ascribe the slow decay component to amplitude grating in recharged deep traps and determine their compensation ratio. The proposed technique allowed the determination of the compensation ratio of a deep EL2 donor, equal to 0.6 ± 0.05 in the given GaAs crystal.

The role of dislocations in producing efficient near‐bandgap luminescence from silicon

AbstractCathodoluminescence is used to investigate the phenomenon of relatively efficient band‐to‐band luminescence from silicon specimens processed to contain near surface dislocations. Dislocations are produced either by ion implantation and subsequent high‐temperature annealing or by mechanical abrasion of specimens having a wide range of doping concentrations. It is found that the luminescence is not directly related to the presence of dislocations and, in some cases, can be enhanced by low temperature annealing. This luminescence behaviour is explained by the effect of competing non‐radiative recombination due to impurity deep levels and Auger recombination. The enhancement observed in some specimens is due to gettering of the deep level impurities. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)