High-resolution Photoinduced Transient Spectroscopy Research Articles

Application of nonlinear regression in the analysis of relaxation photocurrent waveforms

This article discusses the performance of an algorithm for detection of defect centers in semiconductor materials. It is based on direct parameter approximation with nonlinear regression to determine the parameters of thermal emission rate in the photocurrent waveforms. The methodology of the proposed algorithm was presented and its application procedure was described and the results of its application can be seen in measured photocurrent waveforms of a silicon crystal examined with High-Resolution Photoinduced Transient Spectroscopy (HRPITS). The performance of the presented algorithm was verified using simulated photocurrent waveforms without and with noise at the level of 10 -2. This paper presents for the first time the application of the direct approximation method using modern regression and clustering algorithms for the study of defect centers in semiconductors.

Defect-engineered graphene-on-silicon-carbide platform for magnetic field sensing at greatly elevated temperatures

High-temperature electrical properties of p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating vanadium-compensated on-axis 6H-SiC(0001) and high-purity on-axis 4H-SiC(0001) originate from the double-carrier system of spontaneous-polarization-induced holes in graphene and thermally-activated electrons in the substrate. In this study, we pre-epitaxially modify SiC by implanting hydrogen (H+) and helium (He+) ions with energies ranging from 20 keV to 50 keV to reconstruct its post-epitaxial defect structure and suppress the thermally-developed electron channel. Through a combination of dark current measurements and High-Resolution Photo-Induced Transient Spectroscopy between 300 K and 700 K, we monitor the impact of ion bombardment on the transport properties of SiC and reveal activation energies of the individual deep-level defects. We find that the ion implantation has a negligible effect on 6H-SiC. Yet in 4H-SiC, it shifts the Fermi level from ∼600 meV to ∼800 meV below the minimum of the conduction band and reduces the electron concentration by two orders of magnitude. Specifically, it eliminates deep electron traps related to silicon vacancies in the charge state (2-/-) occupying the h and k sites of the 4H-SiC lattice. Finally, we directly implement the protocol of deep-level defect engineering in the technology of amorphous-aluminum-oxide-passivated Hall effect sensors and introduce a mature sensory platform with record-linear current-mode sensitivity of approximately 80 V/AT with -0.03-%/K stability in a broad temperature range between 300 K and 770 K, and likely far beyond 770 K.

Electron paramagnetic resonance of VN–VGa complex in BGaN

A metastable photoinduced electron paramagnetic resonance (EPR) signal at low temperatures is reported for GaN alloyed with boron (i.e., BxGa1−xN) epitaxial layers grown at temperatures ranging from 840 to 1090° C. An isotropic EPR line with g = 2.004 is observed with an intensity depending on the growth temperature for all the samples with boron content between 0.73% and 2.51%. The temperature dependence of EPR intensities is compared with the results of high-resolution photoinduced transient spectroscopy. This enables a link between particular traps and the EPR signal. The activation energies of these traps are consistent with the theoretical position of the VN–VGa complex. Thermal annihilation of the EPR signal with 30 meV activation energy corresponds to shallow donor ionization. A model explaining the light-induced EPR signal that involves redistribution of electrons between deep and shallow donors, mediated by photoionization to the conduction band, is proposed.

Current status of modelling the semi-insulating 4H–SiC transient photoconductivity for application to photoconductive switches

In this paper we present the current status of modelling the time evolution of the transient conductivity of photoexcited semi-insulating (SI) 4H–SiC taking into account the properties of defect centres. A comprehensive model that includes the presence of six, the most significant, point defects occurring in SI 4H–SiC crystals is presented. The defect centres are attributed to the two kinds of nitrogen-related shallow donors, a boron-related shallow acceptor, deep electron and hole traps, and the Z 1/2 recombination centre. We present the results of the state-of-the-art numerical simulations showing how the photoconductivity transients change in time and how these changes are affected by the properties of defect centres. The properties of defect centres assumed for modelling are compared with the results of experimental studies of deep-level defects in high purity (HP) SI 4H–SiC wafers performed by the high-resolution photoinduced transient spectroscopy (HRPITS). The simulated photoconductivity transients are also compared with the experimental photocurrent transients for the HP SI 4H–SiC wafers with different deep-level defects. It is shown that a high-temperature annealing producing the C-rich material enables the fast photocurrent transients to be achieved. The presented analysis can be useful for technology of SI 4H–SiC high-power photoconductive switches with suitable characteristics.

Effect of Nitrogen-Doping on the Properties of Radiation Defect Centers in FZ Silicon

High-resolution photoinduced transient spectroscopy (HRPITS) has been applied to determining the properties and concentrations of radiation defect centers formed in FZ silicon single crystals subjected to doping with phosphorus in neutron transmutation processes. The studies were performed immediately after the neutron irradiation on the samples with the resistivity of the order of 105 Ωcm. The preset target resistivity was around 2000 Ωcm. The N-doping effect on the material radiation damage has been demonstrated. It is shown that in the N-enriched material, the radiation defect centers with activation energies of 81, 125, 160, 185, and 390 meV are not observed and the concentrations of the defect centers with activation energies of 46, 84, 133, 140, 265, 300, 380, 460, and 546 meV are significantly lower than those in the N-free material. We also show that the majority of defect centers produced by the irradiation of FZ silicon with fast neutrons are also observed after neutron transmutation doping.

Characterization of Radiation Defect Centers in Neutron Irradiated Si Using Inverse Laplace Transformation to Analysis of Photocurrent Relaxation Waveforms

High-resolution photoinduced transient spectroscopy has been applied to investigating the e ect of the 1 MeV neutron uence on the electronic properties of radiation defects in Czochralski grown silicon in magnetic eld. A new approach to the analysis of the photocurrent relaxation waveforms as a function of time and temperature has been presented. It is based on using a two-dimensional numerical procedure with implementation of the inverse Laplace transformation for creating images of the sharp spectral fringes depicting the temperature dependences of the thermal emission rate for detected defect centers. In the material irradiated with the uence of 3× 10 cm−2, the dominant traps with activation energies of 75 meV and 545 meV are tentatively identi ed with an aggregate of three Si interstitials and the trivacancy, respectively. In the material irradiated with the uence by the order of magnitude higher, the activation energies of the main traps are found to be 115, 350, 505, 545, and 590 meV. These traps are tentatively attributed to an aggregate of four Si interstitials, as well as to vacancy related centers such as V3 (2-/-), V2O (-/0), V3 (-/0) and V4 (-/0), respectively.

Electrical properties of deuteron irradiated high resistivity silicon

We have investigated resistivity changes introduced on the high-resistivity p-type silicon wafer by the irradiation with deuteron beam with an energy of 4.4GeV performed in the NUCLOTRON superconducting accelerator. Two contactless techniques were used for the measurements of resistivity changes: namely the microwave split post dielectric resonator (SPDR) technique and capacitance measurements in the frequency domain. The first technique allows resistivity measurements in the plane of the wafer, while the second one in the direction perpendicular to the wafer. The resistivity map obtained with the SPDR technique enabled us to obtain a permanent fingerprint of the accelerator beam intensity profile. It has been shown that after the irradiation, the material resistivity increased to ∼3.9×105Ωcm in the wafer region exposed to the maximum beam intensity. Complementary studies of the properties and concentrations of radiation deep-level defects were performed by the high-resolution photo-induced current transient spectroscopy (HRPITS). These studies have shown that the irradiation of the high resistivity silicon with 4.4-GeV deuterons results in the formation of several types of deep-level defects responsible for the charge compensation.

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.

High-resolution photoinduced transient spectroscopy of defect centers in vanadium-doped semi-insulating SiC

High-resolution photoinduced transient spectroscopy (HRPITS) has been applied to studying deep-level defects controlling the charge compensation in semi-insulating (SI), vanadium-doped, bulk 6H– and 4H–SiC. The photocurrent relaxation waveforms were digitally recorded in the temperature range of 300–750 K and a new approach to extract the parameters of defect centers from the temperature-induced changes in the waveforms’ time constants has been implemented. It is based on a two-dimensional analysis using the numerical inversion of the Laplace transform. As a result, the images of spectral fringes depicting the temperature dependences of the emission rate of charge carriers for defect centers are created. For 6H–SiC:V, 11 deep defect centers with activation energies ranging from 660 to 1405 meV were resolved. For 4H–SiC:V, 13 deep traps with activation energies ranging from 560 to 1530 meV were detected. In the both polytypes, the predominant traps were found to be related to vanadium donors located at quasi-cubic and hexagonal lattice sites.

High-resolution Photoinduced Transient Spectroscopy of Defect Centers in Undoped Semi-Insulating 6H-SiC

ABSTRACTHigh-resolution photoinduced transient spectroscopy (HRPITS) has been applied to studying defect centers controlling the charge compensation in semi-insulating (SI), vanadium-free, bulk 6H- SiC. The photocurrent relaxation waveforms were digitally recorded in the temperature range of 50 − 750 K and a new approach to extract the parameters of defect centers from the temperature-induced changes in the time constants of the waveforms has been implemented. It is based on a two-dimensional analysis using the numerical inversion of the Laplace transform. As a result, the images of spectral fringes depicting the temperature dependences of the emission rate of charge carriers for defect centers are created. Using the new procedure for the analysis of the photocurrent relaxation waveforms and the new way of the visualization of the thermal emission rate dependences, a number of shallow and deep defect levels ranging from 80 to 1900 meV have been detected. The obtained results indicate that defect structure of undoped SI bulk 6H-SiC is very complex and the material properties are affected by various point defects occupying the hexagonal and quasi-cubic lattice sites.

High-resolution photoinduced transient spectroscopy of defect centres in semi-insulating InP obtained by iron diffusion

High-resolution photoinduced transient spectroscopy (HRPITS) has been applied to study electronic properties of point defects in semi-insulating (SI) InP obtained by iron diffusion during the high-temperature annealing in iron-phosphide (IP) ambience. A two-dimensional approach to the spectral analysis of the photocurrent decays recorded in a wide range of temperatures and a neural network method to extracting the parameters of defect centres are used. The defect structure of SI InP wafers annealed under the IP and pure phosphorus atmospheres is compared.

Effect of proton fluence on point defect formation in epitaxial silicon for radiation detectors

High-resolution photoinduced transient spectroscopy (HRPITS) has been used to investigate defect centres in n-type epitaxial silicon after 24 GeV/ c proton irradiation with fluences of 3×10 14 and 9×10 14 cm −2 followed by a long-term annealing at 80 oC. The measurements revealed nine traps with activation energies ranging from 5 to 460 meV. In the material irradiated with the lower fluence the concentration of V 2O −/0 complexes is found to be approximately 4 times lower than that of divacancies V 2 −/0. After the irradiation with the higher fluence the material defect structure is different and the concentration of V 2O −/0 complexes is around 6 times higher compared to that of divacancies V 2 −/0.

High-resolution photoinduced transient spectroscopy as a new tool for quality assessment ofsemi-insulating InP

High-resolution photoinduced transient spectroscopy has been applied to study grown-indefect centres in semi-insulating InP:Fe. The defect structure of crystals characterized byvarious values of Hall mobility has been compared. A number of defect centreswith activation energies ranging from 10 to 640 meV were detected. They includeshallow donor and acceptor impurities, native defects, shallow impurity–nativedefect complexes and iron-related defects. It was found that the Hall mobility ismainly affected by the shallow donor concentration which determines the ratio[Fe2+]/[Fe3+]. The electron lifetime is determined either by the iron concentration or the concentrationof shallow impurities.

High-Resolution Photoinduced Transient Spectroscopy of Electrically Active Iron-Related Defects in Electron Irradiated High-Resistivity Silicon

High-resolution photoinduced transient spectroscopy (HRPITS) has been employed to investigate iron-related point defects produced by electron irradiation in high-resistivity silicon. The measurements revealed 16 traps with activation energies ranging from 14 to 530 meV. From the comparison of results, obtained by the HRPITS and ESR measurements performed on the same samples, a trap positioned at Ev+500 meV was attributed to the iron-vacancy pair (Fei)V and that positioned at EV+530 meV to the vacancy complex with two iron atoms (Fei)2V. The former is found to be stable at 400°C, while the latter anneals out at this temperature similarly to the trap at EV+410 meV related to interstitial iron. As a result of the transformation of the (Fei)2V complex and reactions of the interstitial iron with native defects, new iron-related traps with activation energies of 85, 220 and 380 meV are formed.

High-resolution photoinduced transient spectroscopy as a new tool for quality assessment of semi-insulating GaAs

High-resolution photoinduced transient spectroscopy (HRPITS) has been applied to study the electronic properties of grown-in point defects in semi-insulating (SI) GaAs bulk crystals grown by liquid encapsulated Czochralski (LEC) and vertical gradient freeze (VGF) methods. The novel method, which represents the significant improvement of the earlier PITS technique, relies on the extraction of trap parameters from the digitally recorded photocurrent transients by means of the correlation procedure. The influence of the crystallisation conditions on the defect structure of high resistivity GaAs is compared. In the VGF material the concentrations of traps P6 (0.05 eV), P7 (0.08 eV), P12 (0.18 eV), EL14 (0.22 eV) and EL3 (0.58 eV), tentatively identified as V As −/0, Ga As 0/−, B As −/−−, V Ga–V As and O i–V As, respectively, are found to be much lower than in the LEC material.

High-resolution photoinduced transient spectroscopy of neutron irradiated bulk silicon

High-resolution photoinduced transient spectroscopy has been employed in a study on the formation of defects in bulk silicon due to 1 MeV neutron irradiation. Apart from divacancies in various charge states, complexes involving interstitial carbon and oxygen were revealed. The defect structure of float zone and Czochralski-grown material exposed to fluences of 2×10 14 and 6.75×10 14 cm −2 is compared.

Formation of electrically active defects in neutron irradiated silicon

The high-resolution photoinduced transient spectroscopy (HRPITS) has been applied to study formation of electrically active defects produced by 1 MeV neutrons in phosphorus-doped silicon. A number of traps with the activation energy in the range of 0.01–0.52 eV related to shallow donors, bistable C iC s pairs, divacancies V 2 as well as to VO i, VP, V 2O and C iO i complexes are found. The effect of the oxygen concentration in the starting material on the concentration of VO i complexes is shown. The dependence of the concentration of VO i complexes as well as the concentrations of V 2 − −/− and V 2 0/+ on the neutron fluence is also presented.

Investigation of defect levels in semi-insulating materials by modulated and transient photocurrent: comparison of methods

High-resolution photoinduced transient spectroscopy and the modulated photocurrent technique are compared in terms of possible application to the investigation of defect levels in semi-insulating monocrystalline materials. After a description of the theoretical and experimental aspects, the advantages, drawbacks and limitations of each method are discussed. The two techniques complement each other and their potentialities are exemplified by the measurements of trap parameters in the same samples of semi-insulating Cr-doped and undoped GaAs. From these results we deduce a possible model for the properties of the Cr defect in GaAs.