Carrier Capture Cross Sections Research Articles

Theoretical description of H behavior in GaN p-n junctions

The diffusion and reactions of hydrogen in GaN are described by applying differential equations for the concentration profiles of H species, charged dopants, and carriers with simultaneous solution of Poisson’s equation. This approach dispenses with the simplifying assumptions of local equilibrium among states and local charge neutrality that were employed previously by us to treat high-temperature H behavior in uniform layers. The result is a more general modeling capability which encompasses nonequilibrium conditions and space-charge effects such as are encountered in devices. Density-functional theory, previously used by us to treat equilibrium H energies, is employed herein to examine activation barriers and wave-function overlaps affecting the rates of relevant H and carrier reactions, thereby guiding the selection of mechanisms to be included and influencing the evaluation of some rate parameters. The model is applied to H-containing p-n junctions, with detailed consideration of the reversible, metastable electrical activation of H-passivated Mg acceptors that has been observed experimentally under forward bias. The calculations point to interstitial H2 as the state of the H resulting from such activation, and this conclusion is supported by good agreement between the predicted and observed onset temperatures for repassivation under open-circuit annealing. In modeling the more complex activation process, experimentally observed qualitative features are reproduced by choosing relative carrier-capture cross sections in accord with ab initio theoretical considerations. In other model calculations, H is shown to be expelled from the carrier-depleted zone of p-n junctions, causing H redistribution under reverse bias.

The role of oxygen in formation of radiative recombination centers in ZnSe1−xTex crystals

The present study was aimed at revealing the influence of oxygen on formation of alloy ZnSe–ZnTe as well as on development of luminescence centers under thermal treatment of scintillators based on isovalently doped ZnSe crystals. The presence of oxygen in ZnSe(Te) crystals as ZnO phase has been shown to hinder formation of (V′ZnTexSe)′ type luminescence centers responsible for luminescence band peaked at 635–640 nm, which is the principal emission band of this scintillator. In ZnSe(Te) crystals doped by mechanically stimulated ion implantation with active forms of oxygen (O′, Ox, O′2, etc.), luminescence quenching effects are not observed. However, the luminescence band in these crystals is blueshifted towards 600–610 nm. Strong luminescence band peaked at λmax=590–610 nm is observed in Te-free ZnSe crystals doped with oxygen in active forms. The experimental results are interpreted by formation of centers (Zni⋅Oxi)⋅ or (Zni⋅OxSe)⋅. Having larger charge carrier capture cross-section than (Zn⋅iV′ZnTexSe)x, the oxygen-containing centers determine kinetics of luminescence in ZnSe1−xTex crystals (x<0.001) treated by active forms of oxygen during mechanical activation of the initial raw material for crystal growth.

Recombination enhanced defect reactions in 1 MeV electron irradiated p InGaP

Direct recombination enhanced annealing of the radiation-induced defect H2 in p InGaP has been observed by deep level transient spectroscopy (DLTS). Detailed analysis of the annealing data at zero and reverse bias shows that annealing rates are independent of the defect charge state or this defect interacts with the two bands, i.e., is a recombination center trapping alternatively an electron, then a hole. An experiment based on minority carrier capture on a majority trap by the double carrier pulse DLTS technique further supports the evidence that H2 has a large minority carrier capture cross section and is an efficient nonradiative recombination center. Recombination-enhanced defect annealing rates obeys a simple Arrhenius law with an activation enthalpy of 0.51±0.09 eV, in contrast to athermal processes observed in GaP. Detailed analysis of results reveals that the mechanism involved in the minority carrier injection annealing of the H2 defect is energy release mechanism in which enhancement is induced by the energy which is released when a minority carrier is trapped on the defect site. Finally, analysis of the depth profiles data relates that H2 acts as a donor, which partially compensates the acceptors.

A Quantitative Model of the Electrical Activity of Metal Silicide Precipitates in Silicon Based on the Schottky Effect

ABSTRACTA quantitative model of the electrical activity of metallic precipitates in Si is presented. An emphasis is placed on the properties of the Schottky junction at the precipitate-Si interface, as well as the carrier diffusion and drift in the Si space charge region. Carrier recombination rate is found to be primarily determined by the thermionic emission charge transport process across the Schottky junction rather than the surface recombination process. It is shown that the precipitates can have a very large minority carrier capture cross-section.

Irradiation-induced defects in Ge studied by transient spectroscopies

Irradiation-induced impurity-point defect complexes have been investigated in n-type germanium crystals that were doped with either antimony or oxygen. Several majority-carrier traps and one minority-carrier trap are characterized by means of deep-level transient spectroscopy and minority-carrier transient spectroscopy. The antimony-vacancy complex $(E$ center) ${E}_{0.37}$ is found to anneal in a way that is fundamentally different from that in silicon, since it is retarded under reverse bias. Temperature-dependent carrier capture cross sections of the E center are an order of magnitude lower than those of the oxygen-vacancy complex $(A$ center) ${E}_{0.27}$ $({\ensuremath{\sigma}}_{n}\ensuremath{\sim}1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17} {\mathrm{cm}}^{2},$ respectively). A trap ${E}_{0.23}$ which is antimony related grows in at room temperature, seemingly by interstitial capture. A trap ${E}_{0.29}$ is assigned to the divacancy, since it is observed after proton irradiation but not after electron irradiation. A minority-carrier trap ${H}_{0.30},$ displaying a strong Poole-Frenkel effect, is Sb related and possibly related to the E center. In view of the experiments, we comment on a range of diverging results in the literature.

Schottky effect model of electrical activity of metallic precipitates in silicon

A quantitative model of the electrical activity of metallic precipitates in Si is formulated with an emphasis on the Schottky junction effects of the precipitate–Si system. Carrier diffusion and carrier drift in the Si space charge region are accounted for. Carrier recombination is attributed to the thermionic emission mechanism of charge transport across the Schottky junction rather than the surface recombination. It is shown that the precipitates can have a very large minority carrier capture cross-section. Under weak carrier generation conditions, the supply of minority carriers is found to be the limiting factor of the recombination process. The plausibility of the model is demonstrated by a comparison of calculated and available experimental results.

Structural, optical and electronic properties of hydrogenated polymorphous silicon films deposited at 150°C

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to the formation of powder, a new type of material, called hydrogenated polymorphous silicon (pm-Si:H) is obtained. This material has increased transport properties with respect to device-grade hydrogenated amorphous silicon (a-Si:H). To understand the origin of such improved transport properties, we made electrical measurements from which we deduced that the density of states at the Fermi level N( E F) and the carrier capture cross-section, σ c, in pm-Si:H films are at least 10 times lower and 5 times lower, respectively, than in a-Si:H films. The crystallite sizes deduced from Raman spectra confirm high-resolution transmission electron microscopy measurements. The infrared stretching modes of pm-Si:H films have a band at ∼2035 cm −1 which is attributed to hydrogen platelets. The smaller density of states at the Fermi level N( E F) is explained in terms of improved amorphous matrix as confirmed by optical measurements. We suggest that the low capture cross-section, σ c, observed in these films results from a preferential carrier recombination path at grain boundary dangling bonds as predicted by theoretical calculations.

Defect states at silicon surfaces

We demonstrate that surface defects can be characterized in terms of the same parameters that are used for studying bulk defects. We have a model for defect formation on the surface that allows us to estimate formation energies. By combining our contactless surface recombination velocity (SRV) measurement with other surface analysis techniques, we are able to determine the electrical activity of many surface species. We demonstrate observation of surface dangling bonds created by O2 dissolved in dilute hydrofluoric acid (HF). Immersion in a methanol/I2 solution is shown to produce electronically passive Si–OR bonds. Minority carrier capture cross sections for metal defects are calculated based on independent measurement of metal surface coverage. Through the understanding of surface defects, we apply our lifetime measurement to monitor metal contamination from dilute HF solutions. A model relating metal deposition rate to HF dilution is developed. We demonstrate sensitivity of 20 parts per trillion (ppt) Cu in a 500:1 HF solution.

Osmium Related Deep Levels in Indium Phosphide

Deep energy levels arising from doping of n- and p-type InP with the 5d transition metal Os are studied, using electrical and optical deep level transient spectroscopy. Electrically active Os-related deep level concentrations up to 8.2 × 1015 cm—3 are observed representing up to 20% of the chemical concentration. Three prominent levels with low-electric-field activation energies of EV +0.29, EC — 0.32 and EV +0.53 eV, labelled OsA, Os1 and OsB, respectively, are identified. Double-correlation deep level transient spectroscopy measurements show strong dependence of carrier emission rates on the electric field for all the three levels. All three levels exhibit a stronger field effect than that reported for any other deep level in InP. The field effect can be interpreted in terms of a phonon-assisted tunneling mechanism, merging in a direct tunneling process at high field strengths for both Os1 and OsA. Detailed data are obtained on the carrier capture cross-sections and their temperature dependence for all three levels. In the case of OsA the data can be interpreted in terms of a multiphonon capture model.

Time-resolved Photoluminescence and Carrier Dynamics in Vertically-coupled Self-assembled Quantum Dots

The relaxation mechanisms of an array of 10 vertically coupled layers of InGaAs/AlGaAs quantum dots were studied by time-resolved photoluminescence. Both resonant and non-resonant excitation were employed and the photoluminescence (PL) intensity in the non-resonant case is a factor of 200 larger than the intensity with resonant excitation. The results obtained in the non-resonant pumping experiment were analysed with a rate equation model. It was found that the PL decay time increases rapidly with the wavelength of detection. Large carrier capture cross-sections [(2.5±0.9)×10-5 cm3/s] were deduced, resulting in a capture time of 1 ps for a carrier concentration of 4×1016 cm-3. A very fast PL risetime was observed with resonant pumping, ruling out a phonon bottleneck effect in these samples. The decay times at a given wavelength are always shorter for resonant than for non-resonant excitation and their difference increases rapidly with wavelength. This is interpreted in terms of a state filling effect for the non-resonant case.

Temperature Dependence of Carrier Recombination Lifetime in Si Wafers

The temperature dependence of carrier recombination lifetime for n‐ and p‐type Si wafers is measured by the photoconductivity decay method in a temperature range between room temperature and about 90 K. The carrier lifetime of as‐polished wafers has very weak temperature dependence, while the carrier lifetime decreases steeply with decreasing temperature for oxidized wafers. For all samples, a slow component appears in the photoconductivity decay curves after an initial fast decay at temperatures below 150 K. From a numerical simulation, we conclude that the decrease in the carrier lifetime with decreasing temperature is due to recombination through shallow recombination centers with an energy level within 0.15 eV from the bandedge, and that the slow component is due to minority carrier traps with a small majority carrier capture cross section.

Spectral shifts associated with dark line defects in degraded II-VI laser diodes

Spectral shifts associated with 〈100〉 dark line defects of degraded II-VI laser diodes based on ZnCdSe/ZnSSe/MgZnSSe separate confinement heterostructures have been studied by spatially resolved cathodoluminescence at room temperature. Dark line defects were induced by electron-beam bombardment. Peak shifts as large as 2 nm were observed towards the blue or the red depending on the local circumstances. Peak widths usually became narrower after degradation. Redshifts and blueshifts are explained in terms of strain relaxation and Cd out-diffusion associated locally with degradation, as well as the kinetic energy dependence of the degradation-related carrier capture cross section.

Direct observation and measurements of neutron-induced deep levels responsible for Neff changes in high-resistivity silicon detectors using TCT

Neutron-irradiation-induced deep levels responsible for changes of the space charge concentration N eff in high-resistivity silicon detectors have been revealed directly using the Transient Current Technique (TCT). It has been observed by TCT that the absolute value and sign of N eff experience changes due to the trapping of non-equilibrium free carriers generated near the surface (about 5 μm depth into the silicon) by short wavelength laser pulses in fully depleted detectors. Electron trapping causes N eff to change towards negative direction (or more acceptor-like space charges) and hole trapping causes N eff to change towards positive direction (or more donor-like space charges). The specific temperatures associated with these N eff changes are the freeze-out temperatures for free carrier emission from the corresponding deep levels. The carrier capture cross sections of various deep levels have been measured directly using TCT with different free carrier injection schemes.

Photoluminescence Quenching Spectroscopy of Trapmediated Er3+ Excitation Mechanisms in Er-Implanted GaN

AbstractTwo-source optical quenching spectroscopy demonstrates that the four site-selective Er3+ photoluminescence (PL) spectra observed in Er-implanted GaN contribute to the above-gap excited Er3+ PL spectrum, with relative efficiencies determined by the carrier capture cross sections and concentrations of the defects or traps which mediate the excitation of each Er site. The above-gap pumped PL spectrum is dominated by two of the trap-mediated Er3+ PL spectra, while the highest concentration Er site, which is efficiently pumped only by direct 4f absorption, contributes only weakly. These experiments indicate that the same defects and impurities are involved in the trapmediated processes responsible for both the above- and the below-gap excitations of the Er3+ PL.

Electron and hole capture kinetics at gold - hydrogen complexes in n-type silicon

The capture kinetics of both electrons and holes at gold - hydrogen-related deep levels have been studied in n-type silicon using the techniques of deep-level transient spectroscopy (DLTS) and the related technique of minority carrier transient spectroscopy (MCTS). Four deep levels are observed in the bandgap, labelled G1 to G4. G1 and G4 are electron levels, and G2 and G3 are hole levels detected by MCTS that have previously been reported for gold - hydrogen complexes in p-type silicon. The combination of DLTS and MCTS in single-measurement techniques has enabled analysis of the carrier recombination processes occurring at the various deep levels by measuring the majority and minority carrier capture cross sections of each deep level. Based on this, it is proposed that a single type of Au - H defect is responsible for three of the observed levels, G1, G2 and G4, with the defect having a single donor (G2), and both single- (G4) and double-acceptor (G1) charge states. G3 is found to be a combination of two closely spaced levels with similar hole capture cross sections but different values of electron capture cross section.

Kinetics of formation and properties of a barrier oxide film on molybdenum

The electrochemical behaviour of pure molybdenum in 85% H 3PO 4 at high positive overpotentials was studied by d.c. polarization, a.c. impedance and photoelectrochemical techniques. It was found that passivation of the electrode occurred via formation of a barrier-like semiconductor film. Photocurrent spectra point to the identification of this film as MoO 3. A.c. impedance measurements in the passive region were consistent with high-field migration controlled film growth and relaxation of the charge carrier density. The surface charge approach proposed earlier was used to compute the kinetic and structural characteristics of the barrier layer: steady state electric field strength, capture cross-section for positive carriers, half-jump distance for a hopping ion, transference number of negative defects and current efficiency for film formation.

The estimation of size of gigantic ions of chromium in corundum lattice

Abstract Some TL glow peaks appear in corundum crystals subjected to ionizing irradiation. The highest temperature TL peak at about 600 K, is due to chromium impurity. The non-chromium TL peaks are suppressed at an increase of chromium concentration. They disappear completely at a sufficiently low chromium concentration because of an anomalous big capture cross-section of charge carriers by chromium ions. The free charge carriers capture radius was estimated from dependence of TL intensity of non-chromium TL glow-peaks on the chromium concentration. It was equal to 4.2 nm.

Photoconductivity and Thermoelectric Power of ZnxCd1-xSe Films

Znx Cd1-x Se (0≤x≤1) films were deposited by two zone hot wall evaporation technique. X-ray diffraction analysis of the films indicated a predominant wurtzite structure for x<0.5, which changed to zinc blende structure for x>0.7. The films had a mixed structure for 0.5<x<0.7. The films were highly resistive (resistivity varying within 3 Ω· cm to 1.5×105 Ω· cm for 0≤x≤1.0) with partially depleted grains. The optical band gap (E g), determined from the reflectance measurements, indicated a direct transition with a bowing effect in E g versus x plot. The conductivity at low temperature (for x<0.6) could be explained by thermionic emission over the intercrystalline barrier, while for higher x quantum mechanical tunneling through the barrier became predominant. The variation of the barrier height (E bd) with the intensity of illumination was obtained from the photoconductivity measurements. The majority carrier capture cross-section ( 1-4×10-18 cm2) of the grain boundary traps ( 1×1011-1×1012 cm-2) was determined. The effect of the intercrystalline barrier on the thermoelectric power measurements was also considered.

The minority carrier lifetime in doped and undoped p-type Hg0.78Cd0.22Te liquid phase epitaxy films

This paper will describe: (1) the first comparative study of recombination mechanisms between doped and undoped p-type Hg1-xCdxTe liquid phase epitaxy films with an x value of about 0.22, and (2) the first determination of τA7i/τA1i ratio by lifetime’s dependence on both carrier concentration and temperature. The doped films were either copper- or gold-doped with the carrier concentration ranging from 2 x 1015 to 1.5 x 1017 cm-3, and the lifetime varied from 2 μs to 8 ns. The undoped (Hg-vacancy) films had a carrier concentration range between 3 x 1015 and 8 x 1016 cm-3, and the lifetime changed from 150 to 3 ns. It was found that for the same carrier concentration, the doped films had lifetimes several times longer than those of the undoped films, limited mostly by Auger 7 and radiative recombination processes. The ineffectiveness of Shockley-Read-Hall (SRH) recombination process in the doped films was also demonstrated in lifetime vs temperature curves. The important ratio of intrinsic Auger 7 lifetime to intrinsic Auger 1 lifetime, τA7i/τA1i, was determined to be about 20 from fitting both concentration and temperature curves. The reduction of minority carrier lifetime in undoped films can be explained by an effective SRH recombination center associated with the Hg vacancy. Indeed, a donor-like SRH recombination center located at midgap (Ev+60 meV) with a capture cross section for minority carriers much larger than that for majority carriers was deduced from fitting lifetime vs temperature curves of undoped films.

Measurement of the recombination velocity at single crystalline/polycrystalline GaAs interfaces using time-resolved photoluminescence

The recombination velocity at single crystalline/polycrystalline Be-doped GaAs interfaces is measured using time-resolved photoluminescence. Samples with AlGaAs/GaAs double heterostructures are grown by molecular beam epitaxy on GaAs substrates patterned with SiO2. For a Be doping level of 3×1016 cm−3, the interface recombination velocity at 77 K is in the range of (2 to 10)×6 cm/s, and it is independent of the polycrystal grain size. The latter is explained by assuming that the carrier capture cross section is constant on the polycrystal grain surface.