Activation Energies For Electron Emission Research Articles

The effect of FeGa (0/–) level presence on material properties in dilute AlxGa1−xN layers

AlxGa1−xN epilayers are used as the basis of ultraviolet LEDs and detectors. The trap states produced by defects and impurities can play a key role in the device performance. In this work, conventional deep-level transient spectroscopy, photoluminescence (PL), and secondary ion mass spectrometry have been used to characterize a deep-level trap termed as E3 in dilute AlxGa1−xN (x < 0.063) epilayers grown by metal-organic vapor phase epitaxy (MOVPE) on highly conductive ammono-GaN substrates. The AlxGa1−xN epilayers were doped with silicon to about 3 × 1016 cm−3. The electrical and the optical measurements were conducted on Ni/Au Schottky barrier diodes and virgin samples, respectively. First, we observed a general trend that the E3 (FeGa) electron trap concentration significantly changes along the wafers in AlxGa1−xN layers that is fully consistent with previously reported results for GaN materials grown by the MOVPE technique. Second, we report that the activation energies for electron emission for the E1 and E3 traps in dilute AlxGa1−xN exhibit linear variations with Al content. Moreover, low-temperature PL results show a proportional relation between the intensity of the line with its maximum at 1.299 eV and concentration of residual Fe impurity. Finally, we discuss how the presence of defects resulting from Fe contamination may result in degradation of AlxGa1−xN-based devices.

4H-SiC Schottky barrier diodes as radiation detectors: A role of Schottky contact area

Large-area SiC Schottky barrier diodes can significantly improve the sensitivity in radiation detection due to the increased interacting SiC volume. In this work, we tested a large SiC detector with an area of 1 × 1 cm2. Extensive electrical characterization was performed at temperatures ranging from 150 K to 390 K, demonstrating the impact of barrier inhomogeneities on the electrical performance of the diode. Forward current-voltage (I-V) measurements of the diodes revealed two distinct regions caused by Schottky barrier inhomogeneity present throughout the entire temperature range. The barrier heights in the low- and high-current forward voltage regions were extracted from Richardson plots corrected for the Gaussian distribution of barrier heights, yielding values of 1.52 eV and 1.79 eV, respectively. Deep-level transient spectroscopy (DLTS) revealed only one deep-level defect, Z1/2, with an activation energy for electron emission of 0.67 eV, which was assigned to the known carbon vacancy. The DLTS study showed no correlation between electrically active defects and barrier inhomogeneity. An excellent energy resolution of 3.2 % was measured using a large area 241Am radiation source, consistent with values for small area SiC detectors that exhibited no barrier height inhomogeneities. The impact of temperature on the alpha particle radiation response was determined within a temperature range of 200 K to 390 K.

Deep-level traps in as-grown and electron-irradiated homo-epitaxial n-GaN layers grown by MOVPE

The results of junction spectroscopy measurements on deep-level defects in MOVPE (Metal–Organic Vapor-Phase Epitaxy) n-GaN samples grown on highly doped Ammono-GaN and subjected to 1.5 MeV electron irradiation are compared with the published results for epi-GaN materials grown by other techniques. It is found that in addition to the commonly observed deep-level traps in n-type GaN, such as E1 (0.25 eV) and E3 (0.59 eV), 1.5-MeV electron irradiation introduces two other electron traps, EE1 and EE2, with electronic levels at about 0.14 and 0.98 eV below the conduction band edge (EC), respectively. In the case of the EE1 level, a strong influence of the electric field (E) on the electron emission rate (eem) is observed. This suggests a donor type character of this trap level. Further, we have observed that strong electric field, as high as 2 × 105 V/cm, results in lowering the activation energy of electron emission from the EE1 level down to the value of 0.095 eV. The strong eem(E) dependence for the EE1 trap can explain the wide variation in electronic signatures of this trap reported in previous publications. The analysis of the EE1 trap concentrations in the electron irradiated samples allowed us to estimate the average production rate of this trap by 1.5 MeV electrons as 0.125 cm−1 for n-GaN material grown on Ammono-GaN substrate. A series of DLTS measurements with different filling pulse lengths revealed a complexity of the EE1 trap level structure, since at least three emission signals with different capture and emission rates have been separated within the broad EE1 emission signal. Possible origins of the detected deep-level traps are discussed.

Electric‐Field Enhancement of Electron Emission Rates for Deep‐Level Traps in n‐type GaN

Results of a deep‐level transient spectroscopy (DLTS) study of trapping states in Ni/Au Schottky diodes made on Si‐doped GaN layers grown by metal–organic vapor‐phase epitaxy (MOVPE) on highly conductive n‐type Ammono‐GaN substrates are reported. In all as‐grown samples, the DLTS signals due to traps with the activation energies for electron emission (Eem) of about 0.26 and 0.60 eV (referred to as E1 and E3 traps, respectively) have been detected. It is found that the electric field (E) significantly enhances the electron emission rate (eem) for the E1 trap, while for the E3 trap the eem(E) dependence is not very strong. The eem(E) dependence for the E1 trap is found to be characteristic of attractive traps with spherically symmetric square well potential with a radius of about 2.9 ± 0.25 nm. The eem(E) dependence for the E3 trap is accounted for by a phonon‐assisted tunneling mechanism. The zero‐E Eem values for both traps are determined. Correlations between the concentrations of the E1 and E3 traps and concentrations of impurities (H, C, Si, O, Mg, and Fe) determined by secondary ion mass spectrometry in the samples are investigated. Charge states and origins of the E1 and E3 traps are discussed.

The Role of Si Self‐interstitial Atoms in the Formation of Electrically Active Defects in Reverse‐Biased Silicon n+–p Diodes upon Irradiation with Alpha Particles

Results of a study of changes in electrical characteristics of n+–p diodes on boron‐doped epi‐silicon induced by irradiation with alpha particles under applied reverse bias voltages are presented. It is found that the irradiation results in a significantly lower introduction rate of carrier‐compensating radiation‐induced defects (RIDs) in the space charge region of the reverse‐biased structures compared with those in the neutral base region and in similar diodes irradiated without bias. The dominant hole and electron emission signals in the capacitance transient spectra of the irradiated diodes are characterized and identified with energy levels of some known RIDs in moderately doped p‐type Si:B. Changes in concentration of the defects are monitored after postirradiation minority carrier injection and thermal treatments. It is argued that the observed effect of the reduced concentration of RIDs in the space charge regions of the diodes is related mainly to some specific features of the silicon self‐interstitials (ISi): a very strong dependence of their thermal stability on the charge state and a highly enhanced mobility in p‐type Si under minority carrier injection conditions. The activation energy of electron emission from the doubly positively charged state of ISi is determined.

Influence of neutron radiation on majority and minority carrier traps in n-type 4H-SiC

We report on influence of neutron radiation on majority and minority carrier traps in n-type 4H-SiC. Together with the increase of the well-known carbon vacancy (VC) majority carrier related trap, neutron irradiation has introduced two deep traps, labeled as EH1 and EH3 with the activation energies for electron emission estimated as 0.4 and 0.7 eV bellow the conduction band, respectively. Based on Laplace deep level transient spectroscopy (DLTS) results, we have assigned EH1 trap to silicon vacancy (VSi). Two minority carrier traps labelled as B and D-center were detected by minority transient spectroscopy (MCTS) and assigned to substitutional boron BSi and BC, respectively. Activation energies for hole emission for B and D-center are estimated as 0.27 and 0.60 eV above the valence band, respectively. We have identified two emission lines for D-center by Laplace-MCTS measurements and assigned them to BC sitting at hexagonal (−h) and cubic (−k) lattice sites.

Acceptor levels of the carbon vacancy in 4H-SiC: Combining Laplace deep level transient spectroscopy with density functional modeling

We provide direct evidence that the broad Z1/2 peak, commonly observed by conventional deep level transient spectroscopy in as-grown and at high concentrations in radiation damaged 4H-SiC, has two components, namely, Z1 and Z2, with activation energies for electron emissions of 0.59 and 0.67 eV, respectively. We assign these components to Z1/2=→Z1/2−+e−→Z1/20+2e− transition sequences from negative-U ordered acceptor levels of carbon vacancy (VC) defects at hexagonal/pseudo-cubic sites, respectively. By employing short filling pulses at lower temperatures, we were able to characterize the first acceptor level of VC on both sub-lattice sites. Activation energies for electron emission of 0.48 and 0.41 eV were determined for Z1(−/0) and Z2(−/0) transitions, respectively. Based on trap filling kinetics and capture barrier calculations, we investigated the two-step transitions from neutral to doubly negatively charged Z1 and Z2. Positions of the first and second acceptor levels of VC at both lattice sites, as well as (=/0) occupancy levels, were derived from the analysis of the emission and capture data.

Electron emission and capture by oxygen-related bistable thermal double donors in silicon studied with junction capacitance techniques

It has been recently suggested that oxygen-related bistable thermal double donors (BTDDs) are responsible for the reduction of minority carrier lifetime and conversion efficiency of novel amorphous-crystalline Si heterojunction solar cells with their base formed from n-type Czochralski-grown (Cz) silicon [M. Tomassini et al., J. Appl. Phys. 119, 084508 (2016)]. To test this hypothesis, we have studied processes associated with carrier emission and capture by BTDDs in p+-n and Schottky barrier diodes on n-type Cz-Si materials with the use of junction capacitance techniques. By means of deep level transient spectroscopy, we have detected electron emission signals from the deep donor state of the BTDD-0 and BTDD-1 centers. The values of activation energy for electron emission (Eem) have been determined as 1.01 ± 0.01 and 0.91 ± 0.01 eV for the BTDD-0 and BTDD-1 centers, respectively. Such high Eem values are very unusual for defects in Si. We have carried out measurements of electron capture kinetics and associated shallow donor–deep donor transformations for the BTDD-0 and BTDD-1 defects at different temperatures in the diodes with different doping levels. Energy barriers for the capture-transformation processes have been determined. It is argued that BTDDs are responsible for carrier trapping in n-type Cz-Si crystals but are not effective recombination centers.

Lifetime degradation of n-type Czochralski silicon after hydrogenation

Hydrogen plays an important role in the passivation of interface states in silicon-based metal-oxide semiconductor technologies and passivation of surface and interface states in solar silicon. We have shown recently [Vaqueiro-Contreras et al., Phys. Status Solidi RRL 11, 1700133 (2017)] that hydrogenation of n-type silicon slices containing relatively large concentrations of carbon and oxygen impurity atoms {[Cs] ≥ 1 × 1016 cm−3 and [Oi] ≥ 1017 cm−3} can produce a family of C-O-H defects, which act as powerful recombination centres reducing the minority carrier lifetime. In this work, evidence of the silicon's lifetime deterioration after hydrogen injection from SiNx coating, which is widely used in solar cell manufacturing, has been obtained from microwave photoconductance decay measurements. We have characterised the hydrogenation induced deep level defects in n-type Czochralski-grown Si samples through a series of deep level transient spectroscopy (DLTS), minority carrier transient spectroscopy (MCTS), and high-resolution Laplace DLTS/MCTS measurements. It has been found that along with the hydrogen-related hole traps, H1 and H2, in the lower half of the gap reported by us previously, hydrogenation gives rise to two electron traps, E1 and E2, in the upper half of the gap. The activation energies for electron emission from the E1 and E2 trap levels have been determined as 0.12, and 0.14 eV, respectively. We argue that the E1/H1 and E2/H2 pairs of electron/hole traps are related to two energy levels of two complexes, each incorporating carbon, oxygen, and hydrogen atoms. Our results show that the detrimental effect of the C-O-H defects on the minority carrier lifetime in n-type Si:O + C materials can be very significant, and the carbon concentration in Czochralski-grown silicon is a key parameter in the formation of the recombination centers.

Double negatively charged carbon vacancy at the h- and k-sites in 4H-SiC: Combined Laplace-DLTS and DFT study

We present results from combined Laplace-Deep Level Transient Spectroscopy (Laplace-DLTS) and density functional theory studies of the carbon vacancy (VC) in n-type 4H-SiC. Using Laplace-DLTS, we were able to distinguish two previously unresolved sub-lattice-inequivalent emissions, causing the broad Z1/2 peak at 290 K that is commonly observed by conventional DLTS in n-type 4H-SiC. This peak has two components with activation energies for electron emission of 0.58 eV and 0.65 eV. We compared these results with the acceptor levels of VC obtained by means of hybrid density functional supercell calculations. The calculations support the assignment of the Z1/2 signal to a superposition of emission peaks from double negatively charged VC defects. Taking into account the measured and calculated energy levels, the calculated relative stability of VC in hexagonal (h) and cubic (k) lattice sites, as well as the observed relative amplitude of the Laplace-DLTS peaks, we assign Z1 and Z2 to VC(h) and VC(k), respectively. We also present the preliminary results of DLTS and Laplace-DLTS measurements on deep level defects (ET1 and ET2) introduced by fast neutron irradiation and He ion implantation in 4H-SiC. The origin of ET1 and ET2 is still unclear.

Evidence for room-temperature in-diffusion of nickel into silicon

Interstitial nickel in crystalline Si is shown to be a fast diffuser at room temperature. In this study, Ni is incorporated in Si by wet chemical etching in nickel-contaminated alkaline solutions. Nickel in-diffusion is observed by means of detecting the electrically active NiVO defect, which is formed due to Ni capture to the vacancy–oxygen complex in electron-irradiated Si. The depth profiles of the NiVO concentration measured by the deep-level transient spectroscopy technique extend to ∼15 μm in the samples doped with Ni at 35 °C for 30 min. This allows us to get a lower estimate for the nickel diffusivity at this temperature as 10−9 cm2/s. The activation energy for electron emission from the NiVO level and the apparent capture cross section are equal to 371 meV and 3 × 10−15 cm2, respectively. The NiVO complex dissociates at 300 °C reestablishing the initial concentration of the VO centers.

Vacancy-related defects in n-type Si implanted with a rarefied microbeam of accelerated heavy ions in the MeV range

Deep level transient spectroscopy (DLTS) has been used to study vacancy-related defects formed in bulk n-type Czochralski-grown silicon after implantation of accelerated heavy ions: 6.5MeV O, 10.5MeV Si, 10.5MeV Ge, and 11MeV Er in the single ion regime with fluences from 109cm−2 to 1010cm−2 and a direct comparison made with defects formed in the same material irradiated with 0.7MeV fast neutron fluences up to 1012cm−2. A scanning ion microprobe was used as the ion implantation tool of n-Cz:Si samples prepared as Schottky diodes, while the ion beam induced current (IBIC) technique was utilized for direct ion counting. The single acceptor state of the divacancy V2(−/0) is the most prominent defect state observed in DLTS spectra of n-CZ:Si samples implanted by selected ions and the sample irradiated by neutrons. The complete suppression of the DLTS signal related to the double acceptor state of divacancy, V2(=/−) has been observed in all samples irradiated by ions and neutrons. Moreover, the DLTS peak associated with formation of the vacancy-oxygen complex VO in the neutron irradiated sample was also completely suppressed in DLTS spectra of samples implanted with the raster scanned ion microbeam. The reason for such behaviour is twofold, (i) the local depletion of the carrier concentration in the highly disordered regions, and (ii) the effect of the microprobe-assisted single ion implantation. The activation energy for electron emission for states assigned to the V2(−/0) defect formed in samples implanted by single ions follows the Meyer–Neldel rule. An increase of the activation energy is strongly correlated with increasing ion mass.

Electron-Trap and Hole-Trap Distributions in Metal/Oxide/Nitride/Oxide/Silicon Structures

Distributions of both electron traps and hole traps in metal/oxide/nitride/oxide/silicon (MONOS) structures were studied. These trap distributions were analyzed by using a combination of an avalanche-injection method and capacitance-voltage measurement. The thicknesses of the nitride layers in the MONOS capacitors were varied. It was found that electron traps are mainly located at both the top and bottom oxide/nitride interfaces, whereas hole traps are located at the interfaces as well as in the nitride bulk. This analysis clarified that electron-trap capability decreases anomalously in the range of nitride thickness of less than 4 nm. Moreover, the effective activation energy of electron emission also decreases at nitride thickness less than around 5 nm. These decreases are considered to be accounted for by detrapping due to the Coulomb repulsion between electrons at both interfaces and electron trapping in the interfacial transition layers.

E 1/E2 traps in 6H-SiC studied with Laplace deep level transient spectroscopy

Electrically active defects in n-type 6H-SiC diode structures have been studied by deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS. It is shown that the commonly observed broadened DLTS peak previously ascribed to two traps referenced as E1/E2 has three components with activation energies for electron emission of 0.39, 0.43, and 0.44 eV. Further, defects associated with these emission signals have similar electronic structure, each possessing two energy levels with negative-U ordering in the upper half of the 6H-SiC gap. It is argued that the defects are related to a carbon vacancy at three non-equivalent lattice sites in 6H-SiC.

Local study of the energy spectrum of electrons in CdSe/ZnSe QD structure by current DLTS cooperated with AFM

AbstractCdSe/ZnSe QD structure was investigated by current deep level transient spectroscopy (DLTS) with Laplace transform cooperated with atomic force microscopy (AFM). Cathodoluminescence (CL) measurements were carried out too. Basing on Laplace current DLTS with AFM and CL data an activation energy of electron emission from the quantized levels in a small group of CdSe QDs was measured. We have demonstrated the advanced capabilities of the new method of studying the electrical properties of nano‐objects. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Neutron-irradiation-induced defects in germanium: A Laplace deep level transient spectroscopy study

Deep level transient spectroscopy (DLTS), high-resolution Laplace DLTS (L-DLTS) and L-DLTS combined with uniaxial stress have been used in this work for characterization and identification of electrically active defects induced in Sb-doped germanium crystals by irradiation with fast neutrons. The samples were irradiated with relatively small doses of neutrons (≤5 × 10 11 cm −2) in order to produce uniformly distributed damage and to detect small defect clusters. It is found that for such low neutron doses in many respects the damage produced is similar to that resulting from electron irradiation. Vacancy–antimony (V–Sb) pairs uniformly distributed in the sample bulk are the dominant defects observed in the DLTS spectra. It is inferred from the L-DLTS measurements under application of uniaxial stress that the V–Sb pair has a trigonal symmetry in the doubly negatively charged state. It is argued that an electron trap with the activation energy for electron emission of 0.1 eV is related to an acceptor state of a small vacancy cluster located in highly damaged regions of the neutron-irradiated samples. L-DLTS measurements under application of uniaxial stress indicate that the symmetry of the defect is low, monoclinic-I, C 1h point group, or lower. Environment-induced broadening of the L-DLTS signal due to this centre prevents precise determination of the defect symmetry.

Defects induced by irradiation with fast neutrons in n-type germanium

Electrically active defects induced in n-type Sb-doped Ge crystals by irradiation with fast neutrons have been studied by means of conventional deep level transient spectroscopy (DLTS). The neutron-irradiation-induced DLTS spectra are compared with those recorded after irradiations with 4 MeV electrons. The evolution of the neutron- and electron-radiation-induced defects upon 30-min isochronal annealing in the temperature range 100–300 °C has been investigated. Although the DLTS spectra from electron- and neutron-irradiated materials show some similarities and there are also significant differences. The neutron irradiation resulted in the appearance of a number of peaks related to electron and hole deep level traps in the DLTS spectra. In the upper half of the gap at least six electron traps were observed with the following activation energies for electron emission ( E n): 0.08, 0.12, 0.17, 0.22, 0.29 and 0.34 eV. Some of these traps were detected in the electron-irradiated samples. Particularly the trap with E n=0.34 eV was the dominant one after electron irradiation and was identified as the second acceptor level of the Sb–vacancy pair. In Ge:Sb samples irradiated with high fluences of neutrons the trap with E n=0.29 eV [ E(0.29)] was the dominant electron trap. It is argued that the E(0.29) trap is related mainly to the second acceptor level of Ge divacancy (V 2). It is suggested that upon annealing of neutron-irradiated samples at temperatures higher than 100 °C mobile divacancies and Sb–V pairs can interact with Sb atoms with the formation of Sb–V 2 and Sb 2–V complexes, which have acceptor levels in the lower half of the gap.

High-resolution DLTS of vacancy–donor pairs in P-, As- and Sb-doped silicon

We report results from an experiment designed to characterize, by high-resolution (Laplace) DLTS, the electronic properties of electron radiation induced E-centers in Si associated with P, Sb and As. Four sets of samples that contained (i) only P, (ii) P and Sb, (iii) P and As, and (iv) P, Sb and As were used. In the sample containing only P, the activation energy of electron emission from the single acceptor level of V–P in silicon was found to be (0.458±0.005) eV. For the sample with P and Sb, the Laplace peaks of the V–Sb and V–P were clearly separated and the ratio of their emission rates was always >4. The energy levels extracted were (0.401±0.01) and (0.442±0.01) eV for the V–Sb and V–P, respectively. The levels calculated for V–P in these two samples can be considered to be, within the experimental error, the same. In the sample with As and P, the ratio of the emission rate of V–As to V–P was 1.8 and the result was that, although the V–As and V–P peaks could clearly be split, there is some inaccuracy in their calculated energy level positions of (0.435±0.005) and (0.434±0.01) eV, respectively. In the sample containing all three dopants, the peaks of V–P, V–Sb and V–As could be separated but the DLTS “signatures” of these E-centers differed significantly from those where only one or two E-centers were present.

Donor level of interstitial hydrogen in GaAs

The first data evidencing the existence of the donor level of the interstitial hydrogen in GaAs are presented. The abundant formation of the ( 0 / + ) donor level after in situ low-temperature implantation of hydrogen into the depletion layer of GaAs Schottky diodes has been observed and the activation energy and annealing properties have been determined by Laplace DLTS. The activation energy for electron emission of this donor state is 0.14 eV. Above 100 K the hydrogen deep donor state is unstable, converting to a more stable form when there are electrons available for the capture process. A slightly perturbed form of the hydrogen donor in its neutral charge state can be recovered by illuminating the sample. This process releases twice as many electrons as the ionisation process of the hydrogen donor state itself. This fact, by analogy with the silicon case, evidences the negative -U behaviour of hydrogen in GaAs.

The vacancy–donor pair in unstrained silicon, germanium and SiGe alloys

The thermal stability and electronic properties of the vacancy–donor complexes, oftenreferred to as the E centres, have been studied in silicon, unstrained silicon–germanium andpure germanium. The E centres have been introduced by electron irradiation or gammarays. In silicon, Laplace deep level transient spectroscopy has been used to separate the Ecentre emission from the di-vacancy, thus enabling very reliable data to be obtained for thevacancy complexes with P, As and Sb. In pure Ge only the E centres associatedwith P and Sb are reported and in Ge rich SiGe only V–P. In all the samplesmeasured the thermal stability of V–Sb has been found to be significantly higherthan V–P. With regard to the energy levels, the activation energy of electronemission from the single-acceptor level of the E centre in silicon are for V–Sb0.40 eV and for V–P 0.46 eV. For the pure Ge case, the single acceptor is a holetrap with emission to the valence band having energies for V–P of 0.35 eV andV–Sb of 0.31 eV. Similar values are found for Ge rich SiGe. The double-acceptorstate is not seen in silicon but in germanium produces a state with an activationenergy for electron emission of 0.30 eV for V–P and 0.38 eV for V–Sb. This is alsoreflected in the Ge rich alloys of SiGe:P that have been measured in this work.