Generation-recombination Centers Research Articles

Speed Performance Degradation of Electrooptic Modulator Devices by Neutrons Irradiations at High Temperature Effects

In the present paper, we have deeply investigated the transmission efficiency degradation of electrooptic modulator devices in thermal irradiated hard environments over wide range of the operating parameters. It is well known that the radiation-induced electrooptic modulator defects can modify the initial doping concentrations, creating generation-recombination centres and introducing trapping of carriers. Additionally, rate of the lattice defects is thermally activated and reduces for increasing irradiation temperature as a result of annealing of the damage. Both the ambient temperature and the irradiation dose possess sever effects on the electro-optical characteristics and consequently the performance characteristics of electroptic modulator devices. As well as we have deeply developed the modelling basics of electrooptic modulator devices, which may be used to analyzed the modulator quantum efficiency, dark current, modulating voltage, modulating frequency, 3-dB bandwidth, transmitted signal bandwidth, modulator quality factor, modulator sensitivity, modulator sensitivity bandwidth product, switching voltage, modulator device performance index, operating switching time and speed response of these irradiated electrooptic modulator devices after different irradiation fluences.

On the Variability of the Low-Frequency Noise in UTBOX SOI nMOSFETs

The variability of the low-frequency (LF) noise in n-channel MOSFETs fabricated on an Ultra Thin Buried Oxide (UTBOX) Silicon-on-Insulator (SOI) substrate has been studied and compared with the variability in the threshold voltage and low-field mobility of the same devices. No correlation has been found between the noise magnitude and the DC parameters, suggesting that the traps responsible for the current fluctuations do not affect the latter. A possible explanation is that the LF noise is dominated by Generation-Recombination (GR) centers in the silicon film, which have less impact on the drain current.

Junction Field Effect on the Retention Time for One-Transistor Floating-Body RAM

One-transistor floating-body random access memory retention time distribution is investigated on silicon-on-insulator UTBOX devices. It is shown that the average retention time can be improved by two to three orders of magnitude by reducing the body-junction electric field. However, the retention time distribution, which is mainly caused by the generation-recombination center density variation, remains similar.

Speed Response and Performance Degradation of High Temperature Gamma Irradiated Silicon PIN Photodiodes

In the present paper, we have been investigated deeply and parametrically the speed response of Si PIN photodiodes employed in high temperature-irradiated environment. The radiation-induced photodiodes defects can modify the initial doping concentrations, creating generation-recombination centres and introducing trapping of carriers. Additionally, rate of the lattice defects is thermally activated and reduces for increasing irradiation temperature as a result of annealing of the damage. Nonlinear relations are correlated to investigate the current-voltage and capacitance-voltage dependences of the Si PIN photodiodes, where thermal and gamma irradiation effects are considered over the practical ranges of interest. Both the ambient temperature and the irradiation dose possess sever effects on the electro-optical characteristics and consequently the photo-response time and SNR of Si PIN photodiodes. In this paper, we derive the transient response of a Si PIN photodiode for photogeneration currents, when it is exposed to gamma radiation at high temperature. An exact model is obtained, which may be used to optimize the responsivity and speed of these irradiated devices over wide range of the affecting parameters.

Radiation Effects on a Potential Scintillation-Based Solid-State Spectrometer Prototype for Compact Monitoring of Space Radiation/Weather Satellite Conditions

New and emerging scintillators, such as DPA (Diphenylanthracene) and CLYC (Cs2LiYCl6:Ce), coupled with Solid-State Photomultipliers (SSPM) provide a lightweight, low voltage, potentially radiation hard spectrometer for real-time monitoring of space weather conditions. This paper presents the results from proton radiation-hardness tests conducted for candidate scintillators and for individual cells of 1 cm2 SSPMs. Decreases from original light outputs were observed for samples of DPA and CLYC by 4% and 28% respectively with a dose of 1 kGy . Following 1 kGy exposure to the SSPMs, the dark current increased by a factor of ~ 32 for a reverse bias of 30 V. The effects of room temperature annealing on the dark current were also monitored. Decreases in quantum efficiency (22% for 420 nm and 7% for 535 nm) and light response (33% for 420 nm and 26% for 535 nm) were observed as a possible result of the formation of surface recombination-generation centers. The energy resolution of a LYSO scintillator sample coupled with a SSPM and measured for a 22Na gamma source (E = 511 kev) decreased from 15% pre-irradiation to 34% post 1 kGy irradiation.

Characterization of deep-levels in silicon nanowires by low-frequency noise spectroscopy

We have used low-frequency noise (LFN) spectroscopy to characterize generation-recombination (G-R) centers in silicon nanowires grown using chemical vapor deposition. The LFN spectra showed Lorentzian behavior with well-defined corner-frequency indicative of single G-R center in the bandgap. From the temperature-dependent LFN measurement a single deep level at 0.39 eV from the bandedge is identified, which matches closely with the Au donor level in Si. The trap concentration was estimated at 2.0 × 1012 cm−3 with electron and hole capture cross-sections of 9.5 × 10−17 cm2 and 1.4 × 10−16 cm2, respectively. This study demonstrates the potential of the LFN spectroscopy in characterization of deep-levels in nanowires.

Detailed characterization of deep level defects in InGaN Schottky diodes by optical and thermal deep level spectroscopies

Schottky diode properties of semitransparent Ag(4 nm)/Au(4 nm) metal stack on In0.2Ga0.8N were investigated and defect characterization was performed using capacitance deep level transient (DLTS) and optical spectroscopy (DLOS). DLTS measurements made on the In0.2Ga0.8N Schottky diodes, which displayed a barrier height of 0.66 eV, revealed the presence of two deep levels located at Ec-0.39 eV and Ec-0.89 eV with nearly identical concentrations of ∼1.2 × 1015 cm−3. Three deeper defect levels were observed by DLOS at Ec-1.45 eV, Ec-1.76 eV, and Ec-2.50 eV with concentrations of 1.3 × 1015cm−3, 3.2 × 1015cm−3, and 6.1 × 1016 cm−3, respectively. The latter, with its high trap concentration and energy position lying 0.4 eV above the valance band, suggests a possible role in compensation of carrier concentration, whereas the mid-gap positions of the other two levels imply that they will be important recombination-generation centers

Proton Irradiation of 4H-SiC Ultraviolet Single Photon Avalanche Diodes

The effects of proton irradiation on uv 4H-SiC single photon avalanche photodiodes (SPADs) are reported. The SPADs, grown by chemical vapor deposition, were designed for uv operation with dark count rates (DCR) of about 30 kHz and single photon detection efficiency (SPDE) of 4.89%. The SPADs were irradiated with 2 MeV protons to a fluence of 1012 cm-2. After irradiation, the I-V characteristics show forward voltage (<1.9 V) generation-recombination currents 2 to 3 times higher than before irradiation. Single photon counting measurements imply generation-recombination centers created in the band gap after irradiation. For threshold voltage ranging from 23 to 26 mV, the 4H-SiC SPAD showed low DCR (<54 kHz) and high SPDE (>1%) after irradiation. The SPADs demonstrated proton radiation tolerance for geosynchronous space applications.

Optical and Electrical Noise Characteristics of Side Emitting LEDs

Low frequency noise characteristics of nitride based blue side emitting diodes have been investigated. It is shown that investigated devices distinguish by 1/f-type optical and electrical fluctuations caused by various generation-recombination processes through defects formed generation-recombination centers. At higher frequencies optical shot noise due to random photon emission prevails 1/f-type spectrum. The results have shown that low frequency optical and electrical noises are strongly correlated at small current region, but at higher forward current not correlated noise components dominate. Lenses and secondary optics of the investigated devices do not influence output light.

Effect of built-in electric field in photovoltaic InAs quantum dot embedded GaAs solar cell

In this paper, three p–i–n GaAs solar cells were grown and characterized, one with InAs quantum dot (QD) layers embedded in the depletion region (sample A), one with QD layers embedded in the n− base region (B), and the third without QDs (control sample C). QD-embedded solar cells (samples A and B) show broad photoluminescence spectra due to QD multi-level emissions but have lower open-circuit voltages Voc and lower photovoltaic (PV) efficiencies than sample C. On the other hand, the short-circuit current density Jsc in sample A is increased while it is decreased in sample B. Theoretical analysis shows that in sample B where the built-in electric field in QDs is zero, electrons tend to occupy QDs and strong potential variations exist around QDs which deteriorate the electron mobility in the n− base region so that Jsc in sample B is decreased. Hole trapping and electron–hole recombination in QDs are also enhanced in sample B, resulting in a reduced Voc and thus a worse PV effect. In sample A, a strong built-in field exists in QD layers, which facilitates photo-carrier extraction from QDs and thus Jsc is increased. However, QDs in the depletion region in sample A act also as recombination-generation centers so that the dark saturated current density is drastically increased, which reduces Voc and the total PV effect. In conclusion, a nonzero built-in electric field around QDs is vital for using QDs to increase the PV effect in conventional p–i–n GaAs solar cells.

Dopant-Segregated Schottky Junction Tuning With Fluorine Pre-Silicidation Ion Implant

Dopant-segregated Schottky (DSS) junctions are formed by implant-to-silicide (ITS) processing with NiSi. It is shown that a fluorine pre-silicidation ion implant (F-PSII) can be used to reduce the depth of the doped Si region. This provides a new means for engineering the source/drain extension regions in DSS source/drain MOSFETs for performance optimization. It is also shown that there are two distinct regions of diffusion that can result in a kink in the dopant profile for long post-ITS anneals. F-PSII also reduces DSS diode leakage by reducing the extent of Ni diffusion into Si, which reduces the concentration of generation-recombination centers. Finally, F-PSII reduces the electron Schottky barrier height (SBH) in DSS junctions despite a reduction in interfacial dopant concentration apparently due to the reduction in Ni spatial distribution. The majority-carrier SBH is found to be zero or near-zero for DSS junctions formed by ITS processing, with or without F-PSII.

Investigation of generation-recombination centres of Au/n-GaAs Schottky diodes with InAs self-assembled quantum dots

Capacitance and reverse current–voltage measurements have been performed as a function of temperature on Au/n-GaAs Schottky diodes with InAs quantum dots (QDs) embedded between two GaAs spacers. The spacers were either doped or undoped containing a QD layer equivalent to 3 monolayer (ML) QD coverage. In the investigated temperature range of 77–350 K and for all the diodes measured, the apparent free electron concentration close to the QDs layer is lower compared to the concentration at the interface between GaAs doped buffer layer and GaAs n +-substrate, showing that the QDs induce more traps than those created at a conventional interface. Theoretical fitting of the capacitance–voltage characteristics yields the values of the energy level, the sheet concentration and the dispersion of the distribution of the confined states induced by the InAs quantum dots, which is higher for the diode with doped GaAs spacers than for the diode with undoped spacers. The effective activation energies of the traps, obtained from the effective generation lifetime calculated from reverse voltage current, agree well with the results obtained from the theoretical fitting.

Band Gap States in AlGaN/GaN Hetero-Interface Studied by Deep-Level Optical Spectroscopy

AbstractPlanar Pt/AlGaN/GaN Schottky barrier diodes (SBDs) have been characterized by capacitance-voltage and capacitance deep-level optical spectroscopy measurements, compared to reference Pt/GaN:Si SBDs. Two specific deep levels are found to be located at ∼1.70 and ∼2.08 eV below the conduction band, which are clearly different from deep-level defects (Ec - 1.40, Ec - 2.64, and Ec - 2.90 eV) observed in the Pt/GaN:Si SBDs. From the diode bias dependence of the steady-state photocapacitance, these levels are believed to stem from a two-dimensional electron gas (2DEG) region at the AlGaN/GaN hetero-interface. In particular, the 1.70 eV level is likely to act as an efficient generation-recombination center of 2DEG carriers.

Origin of visible-light sensitivity in N-doped TiO 2 films

We report on visible-light sensitivity in N-doped TiO 2 (TiO 2:N) films that were deposited on n +-GaN/Al 2O 3 substrates by reactive magnetron sputtering and subsequently crystallized by annealing at 550 °C in flowing N 2 gas. The N-doping concentration was ∼8.8%, as determined by X-ray photoelectron spectroscopy measurements. From transmission electron microscopic observations and optical absorption measurements, yellow-colored TiO 2:N samples showed an enhanced granular structure and strong absorption in the visible-light region. Photoelectron spectroscopy in air measurements showed a noticeable decrease in ionization energy of TiO 2 by the N doping. Deep-level optical spectroscopy measurements revealed two characteristic deep levels located at ∼1.18 and ∼2.48 eV below the conduction band. The 1.18 eV level is probably attributable to the O vacancy state and can be active as an efficient generation–recombination center. The pronounced 2.48 eV band is newly introduced by the N doping and contributes to band-gap narrowing of TiO 2 by mixing with the O 2p valence band. Therefore, this localized intraband is probably one origin of visible-light sensitivity in TiO 2:N.

Experimental Study of Two-Dimensional Confinement Effects on Reverse-Biased Current Characteristics of Ultrathin Silicon-on-Insulator Lateral, Unidirectional, Bipolar-Type Insulated-Gate Transistors

In this paper, the low-temperature behavior of reverse-biased lateral, unidirectional, bipolar-type insulated-gate transistors (lubistors) fabricated with a 10-nm-thick silicon-on-insulator (SOI) layer is described. Steplike current dependence on reverse bias is observed even at room temperature as well as at low temperatures, which suggests distinct quantum transport in the thin silicon layer. It is demonstrated that the effective activation energies of generation–recombination centers are shallower than simply expected, which has been examined on the basis of theoretical calculations. The quantum confinement effect on the generation–recombination process is strongly illustrated under the reverse-biased conditions.

Deep-Level Characterization of Tris(8-hydroxyquinoline) Aluminum with and without Quinacridone Doping

We have investigated band gap states in tris(8-hydroxyquinoline) aluminum (Alq3) with and without quinacridone (Qd) doping on fabricated indium–tin-oxide (ITO)/Alq3(:Qd)/LiF/Al devices by a deep-level optical spectroscopy (DLOS) technique. A nondoped Alq3 sample shows a discrete trap level located at ∼1.39 eV below the lowest unoccupied molecular orbital band in addition to near-band-edge transitions at 2.2–3.6 eV. This pronounced 1.39 eV level is attributable to the intrinsic nature of Alq3 and can be activated as an efficient generation-recombination (GR) center that may affect photophysical properties. On the other hand, a Qd-doped Alq3 sample exhibits a new deep level at ∼2.40 eV with increasing double-carrier injection rate, corresponding to the highest occupied molecular orbital band of Qd. Simultaneously, this GR center is subject to charge up positively owing to the presence of holes injected into the Qd doping level of the Qd-doped Alq3 sample.

ITON Schottky contacts for GaN based UV photodetectors

Lateral Schottky ultraviolet detectors were fabricated in GaN using indium-tin-oxynitride (ITON) as a contact metal. The GaN semiconductor material was grown on 2 in. sapphire substrate by metal–organic chemical vapor deposition (MOCVD). The Schottky contact has been realized using ITON that has been deposited using sputter techniques. I– V characteristics have been measured with and without UV illumination. The device shows photo-to-dark current ratios of 10 3 at −1 V bias. The spectral responsivity of the UV detectors has been determined. The high spectral responsivity of more than 30 A/W at 240 nm is explained by a high internal gain caused by generation–recombination centers at the ITON/GaN interface. Persistent photocurrent effect has been observed in UV light (on–off) switching operation, time constant and electron capture coefficient of the transition has been determined.

Trap levels in tris(8-hydroxyquinoline) aluminum studied by deep-level optical spectroscopy

We have investigated band gap states in tris(8-hydroxyquinoline) aluminum on fabricated indium tin oxide∕Alq3∕LiF∕Al devices by using a deep-level optical spectroscopy (DLOS) technique. DLOS measurements after double-carrier injection into the Alq3 layer revealed a discrete trap level located at ∼1.39eV below the lowest unoccupied molecular orbital band. The pronounced 1.39eV level is attributable to an intrinsic nature of Alq3 and can be active as an efficient generation-recombination center that may impact the photophysical properties. Additionally, the effective band gap of the Alq3 layer became narrow from 3.05to2.80eV with increasing double-carrier injection rate.

Behavior of elemental tellurium as surface generation-recombination centers in CdTe∕HgCdTe interface

Calculated chemical equations using thermodynamics suggest that elemental Te0 is easily induced during bromine-based etching process on the HgCdTe surface and the induced elemental Te0 can be removed by chemical reaction with hydrazine, forming volatile H2Te. X-ray photoelectron spectroscopy (XPS) confirmed these chemical reactions. The induced elemental Te0 is believed to play a role as surface generation-recombination centers in CdTe∕HgCdTe interface, which was confirmed by XPS, ideality factor, and energy-dispersive interface trap density (Dit) analyses.

Characterization of radiation-related damage in bulk-grown silicon–germanium detectors

Electrical properties of irradiated p-type, PIN, Si 1− x Ge x ( x = 0.028 ) devices were compared to the similar non-irradiated devices, which were fabricated on the same wafer. Differences in effective doping concentration, current generation mechanisms and the process activation energies were observed. Effective doping decrease with increasing irradiation fluence was attributed to the acceptor removal process. Material resistivity was measured, for both irradiated and non-irradiated samples, showing radiation-induced increase in material resistivity, which correlated with the acceptor removal hypothesis. Changes in current generation mechanism were attributed to the radiation-induced generation-recombination centers creation. Dominance of the Space Charge Region (SCR) current generation mechanism was assumed and confirmed for both irradiated and non-irradiated devices. The activation energy of the generation process was shown to be near the theoretical mid-gap. Deep level traps were examined using a current Deep Level Transient Spectroscopy (DLTS) technique. A trap level, with activation energy of 0.43 eV, was detected in both irradiated and non-irradiated samples. The irradiated samples exhibited two additional trap levels with activation energies of 0.12 and 0.27 eV, the levels were attributed to corresponding dislocation types. The 0.12 eV level attribution to B I 0 / + trap correlated with the radiation-induced boron removal hypothesis.