Surface Recombination Model Research Articles

Influence of heavy ion irradiation and high temperature on the performance of 4H–SiC Schottky barrier α particle detector

Traditional semiconductor detectors such as silicon and germanium are easily affected by ambient temperature and charge-particle ion's radiation. They can't work stably in harsh environments such as high temperature and strong radiation for a long time. In this paper, 4H-silicon carbide detectors based on a Schottky barrier diode (SBD) were fabricated and the combined influences of high temperatures and heavy ion irradiation on 4H–SiC detectors have been investigated. With no radiation, the 4H–SiC detectors have excellent I–V characteristics performance during a temperature range of 30–200 °C. Even at an ambient temperature of 200 °C, the leakage current of the 4H–SiC detector was only 107 nA at the full depletion voltage. Then 280 KeV C4+ heavy ions were used to irradiate the 4H-silicon carbide detectors, and the performances of the irradiated detectors were studied for different radiation doses from 1 × 1013 to 1 × 1016 ions/cm2 in a certain higher ambient temperature environment. For the fully depleted 4H-silicon carbide detectors in an ambient temperature of 140 °C, both the I–V characteristics and the energy resolution have shown a certain radiation damage at the radiation fluence of 1 × 1015 ions/cm2, while a significant deterioration was observed when the radiation fluence is up to 1 × 1016 ions/cm2. It means that the 4H–SiC detectors can withstand an irradiation dose of 1 × 1015 ions/cm2 at 140 °C. The time-dependent I–V curves were also investigated for the irradiated detectors for a standing time up to 192 h, and it's found that the irradiated detectors had minor changes in the leakage currents during a long time standing. A PN junction model with current and voltage equations was used to explain the phenomenon that the reverse leakage current increases with the increase of temperature. The surface recombination model was also used to explain the deterioration of I–V characteristics and energy resolution after irradiation.

Diffusion-related lifetime and quantum efficiency of excitons in diamond

We investigate the lifetime of indirect excitons in extremely high purity diamond grown by the chemical vapor deposition method. A clear correlation is found between the lifetime and small strain (magnitude $< $10$^{-4}$) assessed using birefringence, thanks to the effective absence of impurity traps. A surface recombination model, extended for nonradiative recombination at dislocations due to exciton diffusion, is proposed to explain the temperature-dependent lifetime. Based on the derived radiative lifetime, our model enables the prediction of lifetimes at any temperature as well as the highest achievable internal quantum efficiency of exciton luminescence in diamond, which can generally be applicable to a wide range of materials with high exciton binding energies.

An Improved SPICE Model of SiC BJT Incorporating Surface Recombination Effect

The SiC bipolar junction transistor (BJT) offers an attractive alternative to the more popular SiC mosfet . It is important to develop an accurate SPICE model for the SiC BJT to enable its use in power electronic applications. The current gain of an SiC BJT may degrade considerably at high current levels and/or at high temperatures largely due to the surface recombination effect. In this paper, an improved SPICE behavioral model that accurately accounts for the current gain depending on the collector current and the junction temperature is proposed for the SiC BJT. In this paper, the conventional Gummel–Poon model is extended to include this important physical effect by adding a diode between the external base and emitter terminals of the BJT. A two-step model parameter extraction method is developed. First, the basic Gummel–Poon model parameters are extracted from low-current measurement data, and then, the new surface recombination model parameters are extracted by observing the difference between the measured high-level base current and the standard Gummel–Poon model prediction. The simulated static and switching characteristics of the new SiC BJT model match the measured data very well. Finally, the new SPICE model is used in the performance assessment of a proportional base driver technique embedded in a 3.6-kW boost converter, demonstrating its validity in helping with the optimum design of power electronic applications based on SiC BJTs.

Computer analysis of thin-film amorphous silicon heterojunction solar cells

A two-dimensional numerical computer analysis for thin-film-based hydrogenated amorphous silicon (i.e. a-Si : H) solar cells is presented. A comparative performance assessment for various absorbing layers such as a-Si, a-SiGe, a-SiC, combined a-Si+a-SiGe, a-Si/a-SiGe-graded layers and tandem (tdm) design is shown. The device performance is evaluated by implementing special surface recombination model, thermionic field emission model for transport at the heterojunctions and interface traps model at top and bottom side of the heterojunction interfaces. Single absorber with a graded design gives an efficiency of 10.1% for 800 nm thick multiband absorption. Similarly, tandem design shows an efficiency of 10.4% with a total absorber of thickness of 800 nm at bandgap of 1.75 eV and 1.0 eV for the top a-Si and bottom a-SiGe component cells. Numerical simulations predict improved efficiency for tandem design over large range of band discontinuity and interface trap density. Although comparable, the efficiency performance for graded design (i.e. absorbing layer of a-Si and a-SiGe graded layers) approach is very sensitive to the limited range of band discontinuity and interface trap density available at the heterojunctions.

Doping Concentration of GaN Nanowires Determined by Opto-Electrical Measurements

The influence of n-doping on the electrical transport properties of GaN nanowires is investigated by photoconductivity measurements on wires with different diameters. The electrical transport in nanowires is extremely sensitive to the wire diameter because of the size dependent barrier for surface recombination. This effect is used to determine the doping level of the nanowires and to complete and consolidate our previously developed surface recombination model for GaN nanowires.

Characterization of ZnO nanoparticles by resonant Raman scattering and cathodoluminescence spectroscopies

We have measured the Raman and cathodoluminescence (CL) spectra of ZnO nanoparticles with different crystallite sizes. The Raman spectra show that the frequency of the E2 (high) phonon in these nanoparticles, which is initially high, decreases with the crystallite size, while its linewidth increases. This result is explained by the size effect. The CL spectra show that the relative intensity ratio of the CL peak at around 500–600nm to the band-edge CL peak at approximately 360nm drastically increases as the crystallite size decreases. We use the surface recombination model to explain this result. According to this model, the relative intensity ratio increases because of an increase in the concentration of deep levels on the surface of the crystallites and/or an increase in the free-carrier concentration.

Model for a-Si:H/c-Si interface recombination based on the amphoteric nature of silicon dangling bonds

The performance of many silicon devices is limited by electronic recombination losses at the crystalline silicon c-Si surface. A proper surface passivation scheme is needed to allow minimizing these losses. The surface passivation properties of amorphous hydrogenated silicon a-Si:H on monocrystalline Si wafers are investigated here. We introduce a simple model for the description of the surface recombination mechanism based on recombination through amphoteric defects, i.e. dangling bonds, already established for bulk a-Si:H. In this model, the injection-dependent recombination at the a-Si:H/c-Si interface is governed by the density and the average state of charge of the amphoteric recombination centers. We show that with our surface recombination model, we can discriminate between the respective contribution of the two main mechanisms leading to improved surface passivation, which is achieved by a the minimization of the density of recombination centers and b the strong reduction of the density of one carrier type near the interface by field effect. We can thereafter reproduce the behaviors experimentally observed for the dependence of the surface recombination on the injection level on different wafers, i.e., of both p and n doping type as well as intrinsic. Finally, we are able to exploit the good surface passivation properties of our a-Si:H layers by fabricating flat heterojunction solar cells with open-circuit voltages exceeding 700 mV.

Surface recombination model of visible luminescence in porous silicon

The most favourite candidate to explain visible photoluminescence (PL) of porous silicon (PS) is quantum confinement. It has been shown theoretically that decreasing size of silicon particles leads to the widening of the gap (physical confinement). According to calculations, red luminescence requires particle sizes that are too small to be directly observed in PS. On the other hand, chemical effects, other than etching, seem to affect the PL frequency. These effects indicate similarities to the PL of siloxene which is due to isolation of Si 6 rings by oxygen atoms (chemical confinement). Using self-consistent semi-empirical calculations, we have shown that the observed correlation between shift of the PL and Raman bands in PS can only be explained by chemical effects. Therefore, we suggest a surface recombination model of PL in PS, where the emission is determined primarily by physical confinement in features with nanometer dimensions on the surface of microcrystallites, while chemical substitutions tune the PL into the red. Empirical tight binding calculations on such particles show quantitative agreement with observed PL energies.

Aerothermal Study of Mars Pathfinder Aeroshell

An aerothermodynamic study of the Mars Pathfinder aeroshell is carried out through a viscous-shock layer (VSL) analysis with and without surface ablation. Stagnation-point results are obtained along a specified trajectory, and detailed calculations along the body are provided at the peak-heating point. The formulation of VSL equations with a new length scale and the implementation of the Vigneron pressure condition predict the extent of the subsonic region and the surface pressure, which compare well with Navier-Stokes calculations. For equilibrium as well as nonequilibrium flow results, a gas composed of 16 species is used to model the shock layer for the Martian atmosphere (97% COi and 3% N2). A physically consistent surface recombination model (which allows for the diffusion limitation of reactants) for COi is obtained. A parametric study at the trajectory peak-heating point suggests that surface catalysis has the largest effect on heating prediction. The maximum stagnation-point heating of 127 W/cm2 is obtained at an altitude of 40.7 km from nonequilibrium chemistry calculations with fully recombined CO2 as the surface boundary condition. Equilibrium chemistry gives a similar value for the nonablation case at this altitude. Calculations have also been carried out to evaluate the reduction in heating because of ablation. A decrease of about 9% in the stagnation-heating value (with equilibrium chemistry) is obtained at the peak-heating point.

An approximate formula for the angular dependence of the residual defect in a silicon surface barrier detector

A new formula for the angular dependence of the amplitude residual defect based on the surface recombination model for charge losses is presented. Experimental results for Xe and Ni ions are compared with calculated ones. An attempt to apply this formula in a computer code to generate the energy distribution of spontaneous fission events of nuclei implanted into the detector was made.

Two-dimensional numerical optimization study of the rear contact geometry of high-efficiency silicon solar cells

Under one-sun illumination, the highest energy conversion efficiencies of silicon solar cells are presently obtained with bifacially contacted n+p cells, where contact to the p-type substrate is made via small openings in the rear passivating oxide. In this work, a state-of-the-art 2-dimensional (2D) semiconductor device simulator is applied to these devices in order to investigate the effects arising from the rear metallization scheme. The impact of various cell parameters [i.e., substrate resistivity, rear surface recombination model (flatband or surface band bending conditions), positive oxide charge density, capture cross section ratio] on the cell’s current-voltage (I-V) characteristics and the optimum rear contact spacing is investigated. The highly nonideal I-V curves of rear point-contacted high-efficiency silicon solar cells made at the University of New South Wales (UNSW) are modeled with a high degree of accuracy. This is achieved by properly accounting for the complex recombination behavior at the rear oxidized surface. The 2D simulations show that the nonideal I-V curves result from the unequal capture cross sections of electrons and holes at the rear Si-SiO2 interface and the surface band bending induced by positive oxide charges and metal/silicon work function differences. For the UNSW cells, optimum one-sun efficiency is obtained on 2 Ω cm substrates and rear contact spacings of 0.2–0.3 mm. The 2D simulations presented in this work clearly confirm the experimental findings and reveal the physical mechanisms which favor this particular contact design.

Surface recombination of nonequilibrium electron-hole plasma in laser-modified semiconductor-doped glasses

Time-resolved luminescence spectra of highly excited bulk-like CdS-microcrystallites are studied at room temperature as a function of microcrystallite size. The luminescence kinetics shows a consistent increase of the electron-hole plasma recombination rate with decreasing microcrystallite average radius from 1000 Å to 54 Å. The obtained size-dependence is consistent with a surface recombination model in a small sphere, the relevant recombination velocity S being equal to 1.3 × 10 3 cm s -1.

A surface recombination model applied to large features in inorganic phosphor efficiency measurements in the soft x-ray region

The photoluminescent quantum efficiencies of the inorganic phosphors Y2O2S:Eu, Y2O3:Eu, La2O2S:Tm, Gd2O2S:Tb, and Sr5Cl(PO4)3:Eu have been measured in the range 17 to 450 eV. The optical properties of these phosphors from 2 to 160 eV have been determined from inelastic electron scattering measurements. Using a model which involves nonradiative recombination at the surface of the material, we relate photoluminescent efficiency to optical absorption properties, and find that surface recombination is the predominant source of efficiency loss for these materials in the soft x-ray range. From the model, we obtain values for the diffusion length, surface recombination velocity, and bulk quantum efficiency of these materials.

Chemistry of Si-SiO2 interface trap annealing

The kinetics and chemistry of Si-SiO2 interface trap annealing are examined in detail. Measurements of interface trap density Dit as a function of anneal time were performed with several process variables as parameters: oxide thickness, anneal ambient, temperature, bulk carrier type, metallization damage, and orientation. Experiments were carried out using rapid thermal processing and capacitance-voltage measurements of aluminum gate metal-oxide-semiconductor capacitors. Anneal temperature and crystal orientation have the strongest effect on the kinetics. 〈100〉 interfaces can be described by a power-law temporal variation; 〈111〉 kinetics are slightly more complicated. In both cases the experimentally observed anneal behavior is in conflict with the commonly used second-order surface recombination model. We propose a two-reaction model involving atomic hydrogen dimerization and hydrogen/interface trap reactions. This model sucessfully predicts anneal kinetics over a temperature range of 170–500 °C, representing a 106 dynamic range in anneal rates. The difference in anneal behavior between 〈111〉 and 〈100〉 interfaces is explained by postulating different trap anneal mechanisms for the Pb0 and Pb1 defect centers. This hypothesis is supported by trap production kinetics induced by extended anneals.

Influence of the reactivity of reducing agents on anodic photocurrents at TiO 2 electrodes

Effects of reducing agents in electrolytes on photocurrent-potential characteristics of TiO 2 electrodes were investigated. It was observed that the reducing agents with a high reactivity caused negative shifts of the onset potential of the photocurrent. The competitive consumption of photogenerated positive holes between surface recombination and charge transfer reactions seems to be responsible for the phenomena observed. The correlation between the current-potential characteristics and the reactivity of the reducing agents are discussed with the use of a simple surface recombination model. It is shown that theoretically derived rate equations using the simple surface recombination model are useful for predicting the current efficiency for competing reactions at the electrode.

Extension of the surface recombination model and unique determination of the recombination energy by means of reverse current measurements

After the influence of a positive charge a translation of the maximum of surface recombination velocity relative to the minimum of surface conductivity was observed which is connected with a broadening of the recombination curve. The phenomenon is explained as an interaction between the surface recombination centres and slow surface states and gives an unique determination of the recombination energy levels and an extension of the surface recombination model. Es wurde nach Influenz einer positiven Ladung eine Verschiebung des Maximums der Oberflächenrekombinationsgeschwindigkeit relativ zum Minimum des Oberflächenleitwertes festgestellt, die mit einer Verbreiterung der Sperrstromkurve verbunden war, und als Wechselwirkung der Oberflächenrekombinationsterme mit den langsamen Termen gedeutet. Diese Erscheinung gestattet die eindeutige Bestimmung des Energieniveaus der Rekombinationsterme und eine Erweiterung des Oberflächenrekombinationsmodells.

The surface recombination model of p-n diode flicker noise

It is shown that the surface model of p-n diode flicker noise, if properly refined, can explain the recent experiments by Hsu et al. and by Gutkov.

Generation-recombination noise in semiconductors—The equivalent circuit approach

Generation-recombination noise in semiconductors in thermal equilibrium is treated from the standpoint of thermal fluctuations in equivalent electrical circuits. For the general volume recombination model, a method based on network reduction is presented which allows one to calculate the spectral density of the electron and hole fluctuations without solving for the spectra of the fluctuations in occupancy of the recombination centers and traps. The method is extended to a surface recombination model, thereby avoiding the ambiguities found in previous formal treatments. It is shown that the concept of ambipolar diffusion, the location and spectral density of the random sources, and the spatial correlation of Fourier coefficients of carrier density fluctuations all have simple significance in electrical terms. Using transmission line techniques, the generation-recombination (GR) spectrum is calculated for a two-level semiconductor where recombination occurs at opposite plane surfaces. This new result is examined in detail for the limiting cases approached when the recombination process is 1) volume-limited, 2) surface-limited, and 3) diffusion-limited. It is shown that in the first two cases, the spectrum is identical with that obtained from a zero-dimensional analysis provided the time constant is properly defined. For the diffusion-limited case, however, the spectrum varies as 1/ω3/2at high frequencies, and at low frequencies the noise is 5/6 that predicted by the simple theory. The new result is shown to compare favorably with measurements reported previously by Hill and van Vliet.