Deep Level Centers Research Articles

Iron-induced deep-level acceptor center in GaN/AlGaN high electron mobility transistors: Energy level and cross section

Dynamic transconductance dispersion measurements coupled with device physics simulations were used to study the deep level acceptor center in iron-doped AlGaN/GaN high electron mobility transistors (HEMT). From the extracted frequency dependent trap-conductance, an energy level 0.7 eV below the conduction band and a capture cross section of 10−13 cm2 were obtained. The approach presented in this work avoids the non-equilibrium electrical or optical techniques that have been used to date and extracts the device relevant trap characteristics in short channel AlGaN/GaN HEMTs. Quantitative prediction of the trap induced transconductance dispersion in HEMTs is demonstrated.

The impact of trapping centers on AlGaN/GaN resonant tunneling diode

We report on a simulation for aluminum gallium nitride (AlGaN)/gallium nitride (GaN) resonant tunneling diode (RTD) at room temperature by introducing deep-level trapping centers into the polarized AlGaN/GaN/AlGaN quantum well. Theoretical analysis reveals that the degradations of NDR characteristics in GaN based RTDs are actually caused by the combined actions of the activation energy and the trap density. Furthermore, the trapping centers with high activation energy play a dominant role in the degradation of negative differential resistance (NDR) characteristics.

Theoretical study on degradation phenomenon on AlGaN/GaN resonant tunneling diode

In this paper we study theoretically the degradation phenomenon of GaN-based resonant tunneling diode (RTD). The effects of trapping centers on GaN-based RTD are calculated and studied by self-consistently solving the Poisson-Schrödinger aligns when three experimentally obtained deep-level trapping centers are introduced into the AlGaN/GaN/AlGaN quantum well. Results show that the degradations of negative differential resistance (NDR) characteristic in GaN-based RTDs are actually caused by the combined action of the activation energy and the defect density. The deep-level trapping center with high activation energy plays a dominating role in the degradation of NDR characteristics because the probability of ionization is exponentially proportional to the activation energy.

Characterization of 4H-SiC Epitaxial Layers and High-Resistivity Bulk Crystals for Radiation Detectors

Defect and electrical characterization of bulk semi-insulating (SI) 4H-SiC crystals and SI and n-type 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD) on highly doped (0001) 4H-SiC substrates is reported. Optical microscopy, electron beam induced current (EBIC) imaging, current-voltage ( $I$ – $V$ ) measurements, thermally stimulated current (TSC) spectroscopy (94 K–620 K), Hall effect, and van der Pauw measurements have been conducted for characterization and defect correlation studies. Both epitaxial layers exhibited relatively shallow levels related to Al, B, $L$ - and D-centers. Deep level centers in the n-type epitaxial layer peaked at ${\sim} 400$ K ( $E_{a} \sim 1.1$ eV), and ${\sim} 470$ K were correlated with $IL_{2}$ defect and 1.1 eV center in high-purity bulk SI 4H-SiC. The SI epitaxial layer exhibited peak at ${\sim} 290$ K ( $E_{a} = 0.82\hbox{--}0.87$ eV) that was attributed to $IL_{1}$ and HK2 centers, and at ${\sim} 525$ K that was related to intrinsic defects and their complexes with energy levels close to the middle of the band-gap. Results of EBIC and optical microscopy showed segregation of threading dislocations around comet tail defects in the n-type epitaxial layer. The $I$ – $V$ characteristics of the devices on SI epitaxial layer exhibited steps corresponding to the ultimate trap filling of deep centers. The high-temperature resistivity measurements of bulk SI 4H-SiC sample revealed resistivity hysteresis that was attributed to the filling of the deep-level electron trap centers. The responsivity of the n-type epitaxial 4H-SiC detector in the soft X-ray energy range is reported for the first time.

Fabrication and Characterization of ${\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te}$ Schottky Diodes for High Resolution Nuclear Radiation Detectors

Cadmium zinc telluride (CZT) Schottky diodes with very low reverse leakage current have been fabricated and characterized for high resolution gamma ray detectors. The diodes were made using <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te}$</tex></formula> detector grade crystals grown from zone refined Cd, Zn, and Te (7N) precursor materials using low temperature tellurium solvent method. Various crystallographic defects including Te-inclusions/precipitates were identified and characterized using electron beam induced current (EBIC) measurement and thermally stimulated current (TSC) spectroscopy. The EBIC images were correlated with transmission infrared (TIR) images of CZT crystals and the EBIC contrast was attributed to the nonuniformities in spatial distribution of Te inclusions. Characterization by TSC revealed shallow and deep level centers with activation energies 0.25–0.4 eV and 0.65–0.8 eV respectively, which were attributed to intrinsic defects associated with Te inclusions. Pulse height measurements were carried out using <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$^{137}{\rm Cs}$</tex></formula> (662 keV) radiation source and energy resolution of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim 1.51}\%$</tex></formula> full width at half maximum (FWHM) was obtained from the as-grown boule.

Formation and annealing of radiation defects in tin-doped p-type germanium crystals

The effect of tin on the formation and annealing of radiation defects in p-type germanium crystals irradiated with 6-MeV electrons at a temperature of 80 K is studied. It is shown that acceptor complexes SnV with a hole ionization enthalpy of 0.16 eV are dominant in irradiated Ge:(Sn, Ga) crystals after their heating to a temperature of 300 K. These complexes disappeared as a result of the annealing of irradiated crystals in the temperature range 30–75°C. Annealing of irradiated crystals at temperatures in the range 110–150°C brings about the formation of deep-level centers with a donor level at Ev + 0.29 eV; this center is presumably related to a complex consisting of a tin atom and an interstitial gallium atom.

Design, Fabrication, Characterization, and Evaluation of X-ray Detectors Based on n-type 4H-SiC Epitaxial Layer

The Schottky barrier diode (SBD) radiation detectors on n-type 4H-SiC epitaxial layer have been designed, fabricated, and evaluated for low energy x-rays detection. The detectors were found to be highly sensitive to x-ray flux in the 50 eV to few keV range and showed significantly improved response compared to the commercial of-the-shelf (COTS) SiC UV photodiodes. Thermally stimulated current (TSC) measurements performed in wide temperature range (94 - 550 K) revealed low density of deep level centers in the epitaxial layer. The high quality of the epitaxial layer was confirmed by XRD rocking curve measurements and defect delineating chemical etching. The detectors fabricated on 4H-SiC epitaxial layers exhibited low leakage current at 475 K (< 1 nA at 200 V) revealing great possibility of high temperature operation.

Temperature dependence of current conduction in semi-insulating 4H-SiC epitaxial layer

We have investigated temperature dependence (94 K–650 K) of current conduction in semi-insulating 4H-SiC epitaxial layer. The epitaxial layer was grown on highly doped n-type (0001) 4H-SiC substrate using chemical vapor deposition with dichlorosilane precursor. The current—voltage (I-V) characteristics exhibited steps at ∼1 V and ∼70 V that were attributed to the filling of deep level centers by injected electrons. Correlation of the I-V characteristics with the results of thermally stimulated current measurements showed that deep centers peaked at 242 K, 285 K, and 500 K, were responsible for the steps in the I-V characteristics. Slow processes of the injected carrier capture on traps resulted in the I-V characteristic with negative differential resistance.

Characterization of deep levels in n-type and semi-insulating 4H-SiC epitaxial layers by thermally stimulated current spectroscopy

We have investigated deep level centers in n-type and semi-insulating (SI) 4H-SiC epitaxial layers by thermally stimulated current (TSC) spectroscopy. The epitaxial layers were grown using chemical vapor deposition utilizing a dichlorosilane precursor. Both epitaxial layers exhibited relatively shallow levels related to Al, B, L- and D-centers. A deep level center with an activation energy of 1.1 eV, peaked at ∼400 K, was detected in the n-type epitaxial layer and correlated with the IL2 level and the 1.1 eV center in a high purity bulk SI 4H-SiC. The TSC spectra of the SI epitaxial layer was dominated by the peaks at 525–585 K that we attributed to intrinsic defects and their complexes with energy levels close to the middle of the bandgap. The TSC spectra of SI epitaxial layer exhibited peaks with different current polarity which is explained by thermoelectric effect and the built-in electric field reversal. The results of the transfer length method measurements of the SI epitaxial layer and the room temperature current-voltage (I-V) characteristics of both epitaxial layers are also reported.

Formation of Copper-Related Deep-Level Centers in Irradiated P-Type Silicon

Results of a DLTS study on the deep-level centers in the copper-contaminated electron-irradiated FZ-Si are presented. Copper was diffused from the Cu-contaminated surface into previously irradiated samples at 350 K. The most abundant Cu-related complexes formed due to this procedure are the same as in the case of previously studied Cu-diffused Cz-Si.

Characterization of Semi-Insulating 4H Silicon Carbide for Radiation Detectors

Radiation detectors have been fabricated on 8 mm × 8 mm substrates, ~390 μm in thickness, diced from a (0001) 4H-SiC semi-insulating (SI) wafer (≥ 1012 Ohm-cm). The crystals used for detector fabrication have been characterized by x-ray diffraction (XRD), electron beam induced current (EBIC), chemical etching, cross-polarized imaging, and Raman spectroscopy. Current-voltage (I-V) characteristics showed very low leakage current (≤ 50 pA at -800 V) and the capability of detector's operation ≥ 470 K. EBIC investigations revealed that the screw dislocations produce dark EBIC contrast indicating high leakage current in the defective regions. Thermally stimulated current (TSC) measurements and high temperature resistivity measurements revealed deep level centers with activation energies 1.1-1.2 eV, and 1.56 eV. The TSC peak at ~460 K associated with the ~1.2 eV center was much stronger than the other high temperature peaks (e.g., 370 K due to vanadium impurity, 0.95 eV below of conduction band edge), indicating that this level along with the 1.56 eV level should dominate in controlling the resistivity and carrier lifetime in the studied 4H-SiC. Based on the literature data, we associate these centers with intrinsic defects and/or V-related complex. Nuclear detection measurements on the single-element SiC detectors with 241Am X-γ ray source clearly detected 59.6 keV and other low energy x-rays.

Deep electron levels in undoped polycrystalline CdTe annealed in liquid Cd

The method of deep-level transient spectroscopy (DLTS) is used to determine the set of deep electron levels in undoped n-CdTe polycrystals grown by chemical synthesis from vapor phase in highly nonequilibrium conditions and then subjected to annealing in liquid Cd. After annealing, the electron concentration is found to increase from ∼108 to 1015 cm−3. Electron traps with deep levels E1 = 0.84 ± 0.03 eV and E2 = 0.71 ± 0.02 eV with total concentration ∼1014 cm−3 are dominant in the DLTS spectrum. The role of grain boundaries and intrinsic point defects in formation of deep-level centers and their effect on the value of conductivity after annealing are discussed. A relation between the level E1 and complexes of intrinsic point defects and relation of the level E2 to the Cdi point defect are considered.

Copper-related deep-level centers in irradiatedp-type silicon

Deep-level centers are investigated in the $p$-type Si on copper-contaminated samples which were also electron irradiated. Standard and Laplace-transform deep-level transient spectroscopy techniques were employed to characterize the samples. Several Cu-related centers are observed to form either as a result of the low-temperature Cu diffusion into the irradiated crystals or due to irradiation of the Cu-contaminated samples and subsequent annealing up to 400 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C. In all crystals, two Cu-related defects are found to be the most abundant; each of them possesses a pair of levels in the lower half of the gap. The Arrhenius signatures for one pair are measured to be practically identical to those for the donor and acceptor levels of substitutional copper Cu${}_{s}$, respectively, the levels of other defect being only barely different from the Cu${}_{s}$ levels. Analysis of the introduction rates and depth profiles of the deep-level centers points to the vacancy--oxygen complex (VO, the A center) as the precursor of the most abundant Cu-related defects. It is inferred that Cu${}_{s}$ is formed in irradiated silicon due to interaction with the VO centers via the rather stable intermediate CuVO complex.

Thermally stimulated current and high temperature resistivity measurements of 4H semi-insulating silicon carbide

High purity semi-insulating 4H SiC single crystals have potential applications for room temperature radiation detectors because of the wide band gap and radiation hardness. To control carrier lifetime, a key parameter for high performance radiation detectors, it is important to understand the nature of the deep traps in this material. For this purpose, we have successfully applied thermally stimulated current (TSC) and high temperature resistivity measurements to investigate deep level centers in semi-insulating 4H SiC samples grown by physical vapor transport. High temperature resistivity measurements showed that the resistivity at elevated temperatures is controlled by the deep level with an activation energy of 1.56 eV. The dominant traps revealed by TSC measurements were at 1.1–1.2 eV. The deep trap levels in 4H-SiC samples, the impurity and point defect nature of TSC traps peaked at ∼106 K (0.23 eV), ∼126 K (0.32 eV), ∼370 K (0.95 eV), ∼456 K (1.1–1.2 eV) are discussed.

High-Power Ultrafast Current Switching by a Silicon Sharpener Operating at an Electric Field Close to the Threshold of the Zener Breakdown

A new principle of high-power ultrafast current switching by Si sharpener based on a successive breakdown of the series-connected structures has been experimentally implemented and theoretically studied. A voltage pulse with an amplitude of 180 kV and a rise time of 400 ps was applied to a semiconductor device containing 44 series-connected diode structures located in a 50- transmission line. Due to a sharp nonuniformity of the applied voltage distribution across the length of the device, the structures operate in the successive breakdown mode. Each successive structure breaks down with a shorter time interval as the electromagnetic shockwave builds. In the experiments in a 50- transmission line, we have obtained 150-kV output pulses having a 100-ps rise time. The maximum current and voltage rise rates amount to 30 kA/ns and 1.5 MV/ns, respectively. In the numerical simulations, the ionization rate of the process-induced deep-level centers, as well as the band-to-band tunneling, is taken into account. The calculations show that, at a reverse voltage rise rate across the structure of over 10^13 nV/s, the electric fields that are close to the threshold of the Zener breakdown can be achieved even if the structure contains deep-level centers with a concentration of 1011 to 1012 cm-3.

Synthesis and optical properties of purified translucent, orthorhombic boron nitride films

Large-area (>1 cm 2) freestanding translucent orthorhombic boron nitride (oBN) films have been synthesized by magnetron sputtering at a low radio-frequency power of 120 W. The structural characterizations were performed by means of X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. It is demonstrated that oBN is a direct band gap semiconductor ( E g∼3.43 eV). Excited by ultraviolet laser (wavelength at 325 nm), the oBN films emit strong white light, which can be seen by the naked eyes in the dark. In the photoluminescence spectrum, besides the ultraviolet near-band-edge radiative recombination emission, there are three visible emission bands (centered at 400, 538, and 700 nm) arising from the defect-related deep-level centers of oBN, which are mixed to form the white light emission. The hardness and elastic modulus of oBN films are 11.5 and 94 GPa, respectively, examined by nanoindentation measurements.

Unusual Band-Edge Photoluminescence Intensity Emitted by Cu-Diffused Silicon Crystals

Band-edge photoluminescence (BDE-PL) intensities of Czochralski (Cz) and float-zone (FZ)-grown silicon crystals diffused with Cu were observed from room temperature (RT) to 4.2 K. In contrast to the behavior of ordinary transition metals, unexpected enhancements of the BDE-PL intensities of both Cz and FZ samples were observed at RT in an appropriate Cu concentration range. For the observation temperature of 77 K, however, while the BDE-PL intensity of the Cz sample increased following the same trend as that observed at RT, the intensity of the FZ sample abruptly decreased given the same Cu concentration range as that of the Cz sample. The marked difference in the BDE-PL intensities between Cz and FZ samples was also observed with the change in the diffusion temperature of Cu. These unusual BDE-PL intensities were well explained on the basis of two competitive actions of Cu: the intensity enhancement due to surface passivation by Cu and the intensity reduction caused by a deep-level Cu center formed in the bulk.

Luminescence from Zinc Oxide Nanostructures and Polymers and their Hybrid Devices

Zinc oxide (ZnO) is a strong luminescent material, as are several polymers. These two materials have distinct drawbacks and advantages, and they can be combined to form nanostructures with many important applications, e.g., large-area white lighting. This paper discusses the origin of visible emission centers in ZnO nanorods grown with different approaches. White light emitting diodes (LEDs) were fabricated by combining n-ZnO nanorods and hollow nanotubes with different p-type materials to form heterojunctions. The p-type component of the hybrids includes p-SiC, p-GaN, and polymers. We conclude by analyzing the electroluminescence of the different light emitting diodes we fabricated. The observed optical, electrical, and electro-optical characteristics of these LEDs are discussed with an emphasis on the deep level centers that cause the emission.

Deep-Level Defects in Electron Irradiated 6H-SiC

AbstractAn effect of electron irradiation on the concentrations of deep-level centers in C-rich and Si-rich 6H-SiC wafers is investigated. In the former material, the main deep-level centers with activation energies of Ec-0.50, Ec-0.64 and Ec-0.67 eV are found to be related to dicarbon interstitials and CiNC complexes located in hexagonal and quasi-cubic lattice sites, respectively. In the latter material, the dicarbon interstitials are dominant after the irradiation with 1.5-MeV electrons. At the energy of bombarding electrons equal to 0.3 and 0.7 MeV, the activation energies of the dominant deep-level centers are Ec-0.38 and Ec-0.52 eV, respectively. The first center is related to carbon vacancies and the second to silicon interstitials.

Cu deep level center in CdTe solar cell

Cu would be doped and form deep level centers easily in CdTe solar cells. The deep level centers in ZnTe back contact and graphite back contact CdTe solar cells were studied by deep level transient spectroscopy(DLTS). The electronic density of states on zinc blende CdTe，VCd system and CdTe doping Cu were analyzed with density functional theory. The d-orbital splitting of Cu2+ in C3v and Td fields was obtained. The results show that two deep centers Ev+0206 eV and Ev+0122 eV，respectively，are attributed to substitutional Cu，energy of CdTe is reduced after Cu doping，and Cu could replace Cd.