Carrier Surface Recombination Research Articles

Effective optical generation rate and carrier surface recombination velocities in variband film photodetecting structures of MBE p-MCT

The profile is calculated for photogenerated nonequilibrium electron concentration in thickness of a three-layer film p-MCT structure in crossed electric and magnetic fields, in which the band gap of the central region is constant, but increases in adjacent variband regions. It is shown that a three-layer structure with variband regions can be replaced by a homogeneous film via introducing effective surface generation and surface recombination velocities at boundaries of central and variband regions.

New modes of FFT impedance spectroscopy applied to semiconductor pore etching and materials characterization

AbstractA systematic approach for the application of fast Fourier transform (FFT) impedance spectroscopy to semiconductor photo‐electrochemistry is given. In particular, photo‐impedance spectroscopy in two novel modes is used in conjunction with conventional current–voltage impedance spectroscopy, and applied to some aspects of pore etching in InP and Si as well as to the characterization of multicrystalline Si or other solar cell material. The technique, applicable in situ with high time resolution, employs optimized hardware and software, extensive mathematical modeling of carrier transport, and the numerical implementation of fast parameter extraction algorithms. The complete system with a fully integrated impedance spectrometer delivers a wealth of new data including key parameters of pore etching. Examples given include the in situ tracking of the fast switch‐over between pore growth modes in InP as well as following a pore etching process over long time scales in Si. FFT impedance spectroscopy in the novel back side illumination mode, in combination with the conventional mode, allows not only some active control of macropore etching in n‐type Si by supplying crucial real‐time data like the valence of the dissolution process and the pore depth, but also provides new fundamental insights into the electrochemical processes occurring at the (porous) electrode. FFT photo‐impedance spectroscopy in the front side illumination mode used in conjunction with a scanned laser beam allows fast local measurements of all semiconductor parameters relevant for solar cell applications, like doping concentration, minority carrier lifetime, diffusion constant, or surface recombination velocity with high spatial resolution; this is demonstrated for large multicrystalline Si wafers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interfacial and Electrical Characterization of HfO2-Gated MOSCs and MOSFETs by C-V and Gated-Diode Method

Metal-oxide-semiconductor capacitors (MOSCs) and Metal-oxide-semiconductor field-effect transistors (MOSFETs) incorporating hafnium dioxide (HfO2) dielectrics were fabricated and investi-gated. The electrical and interfacial properties were characterized including C-V, IDS-VGS, the mean density of interface traps per unit area and energy (), energy of interface traps density distribution, interface traps density (Nit), density of near-interface oxide traps per unit area (NNIOT), effective capture cross-section area (σs), mi-nority carrier lifetime (τFIJ) and surface recombination velocity (so) were studied. Furthermore, a comparison with MOSFETs using conventional SiO2 as gate dielectrics was also made.

Nanocrystalline silicon solar cells from excimer laser crystallization of amorphous silicon

Excimer laser crystallized nanocrystalline silicon layers were used to fabricate solar cells. The laser crystallized layers are characterized with Raman spectroscopy for structural investigations and with atomic force microscopy to study surface modifications upon crystallization. The current–voltage characteristics of the devices completed with aluminium back contacts were investigated with air mass 1.5 G solar simulations. The resulting nanocrystalline solar cells show inferior performance compared with amorphous silicon devices. The degradation of open circuit voltages and short circuit currents with the increase of crystallization energy density is explained to be due to thermal modification of the p-type/intrinsic layer interface and band gap enhancement of fine-grained nanocrystalline layers of the devices along with increased surface and grain boundary recombination of carriers.

Studying the effect of material parameters on detectivity in a p–n In 0.53Ga 0.47As photovoltaic detector

In the paper, a relationship of detectivity with material parameters is given by a theoretical calculation. An In 1− x Ga x As photovoltaic detector structure with x = 0.47 is used as an example for this study. The results show that the detectivity of a photodetector can be enhanced by optimizing the carrier concentration, thickness and surface recombination velocity during the structure growth and device fabrication.

Modeling of Photoinduced Deformation in Silicon Microcantilever.

A model for prediction the photostriction effect in silicon microcantilevers is built up based on the fundamentals of mechanics and semiconductor physics. By considering the spatial distribution and surface recombination of photoinduced carriers in silicon, the model interprets the cause of the photoinduced bending. The results from our model much more closely approximate the experimental values than the former model built up by Datskos, Rajic and Datskou [1](APL, Vol.73 (1998) No.16, pp 3219-2321), represented by the reduction of the error between calculation and measurement from 25 times to 0.85 times.

Silicon-on-insulator nanotransistors with two independent gates

The results of numerical simulation of electrical characteristics of silicon-on-insulator MOSFET nanotransistors with two independent gates are reported. The cases of grounded and floating bases with the surface recombination of charge carriers either disregarded or taken into account are considered. It is shown that, at specified design parameters (gate length 50–100 nm, thickness of silicon layer 25–50 nm), one can vary the transistor’s threshold voltage by 0.45 V, reduce the transistor current in the off state by seven orders of magnitude, and decrease the subthreshold slope of the gate characteristics to 60 mV/decade by varying the voltage applied to the second gate. Suppression of the short-channel effects in the transistors under consideration depends on a number of parameters (listed in order of decreasing effect): the gate material, the lifetime of charge carriers (for floating or grounded base), the thickness of the top silicon layer, the voltage applied to the additional gate, and the channel length.

Characteristics of surface recombination in silicon diode isolation grooves

Carrier surface recombination characteristics are investigated in silicon high voltage diodes with glass isolated grooves by microwave probed photoconductivity transients (MW-PCT) combining different excitation depths within the layered device structures. Comparative analysis of recombination parameters in the device isolation grooves and layered wafers passivated by iodinenethanol solutions is performed to evaluate surface recombination rates. It has been revealed that electrochemical etchingnglass melting steps involved within the passivation technological procedures for fabrication of diode isolation grooves induce a decrease of surface recombination velocity from 3¢10 3 to 10 cm=s.

Temperature dependence of the current gain in power 4H-SiC NPN BJTs

For 1-kV 30-A 4H-SiC epitaxial emitter n-p-n bipolar junction transistors, the dependences of the common-emitter current gain /spl beta//sub CE/ on the collector current I/sub C/ were measured at elevated temperatures. The collector-emitter voltage was fixed (at 100 V) to provide an active operation mode at all collector currents varying in a wide range from 150 mA to 40 A (current densities of 24-6350 A/cm/sup 2/). The maximum current gain was measured to be /spl beta//sub CEmax/=40(I/sub C/=7 A) at room temperature and /spl beta//sub CEmax/=32(I/sub C/=10 A) at 250/spl deg/C. The /spl beta//sub CE/-I/sub C/ dependences were simulated in terms of a model that takes into account the main processes affecting the current gain: 1) recombination in the emitter-base space charge region; 2) surface recombination; 3) crowding of the emitter current; 4) decrease in the emitter-injection coefficient at high-level injection; and 5) ionization of deep acceptors. The minority carrier lifetimes and surface recombination velocity were obtained by means of this simulation.

Carrier recombination velocities at the SiO 2/Si interface investigated by a photo-thermal reflection microscopy

Photo-thermal reflection microscopy has been used for investigating semiconductor materials to evaluate carrier diffusivity, lifetime and surface recombination velocity. We developed this technique to obtain carrier recombination velocities at the interface between SiO 2 oxide film and Si substrate. Two samples with different oxide film thickness of 92 and 45 nm were prepared. Since the oxide film layer does not absorb the pump and probe laser light, carrier recombination velocity at the SiO 2/Si interface can be estimated. Curve fitting procedures with the theoretical prediction results in an estimation of the interface recombination velocity of 100 cm/s for thick oxide sample. When the sample was chemically etched, the recombination velocity increases to 2500 cm/s. The chemical etching results in the drastic increase of the recombination velocity. The etching solution may soak through the SiO 2 oxide film layer and attack the Si surface during the chemical etching. Increase of the number of the interface traps induces the increase of the carrier recombination velocity at the interface. We, therefore, found that the present photo-thermal reflection (PTR) microscopy is a useful technique for investigating the carrier dynamics at SiO 2/Si interface.

Silicon Passivated by Insulating Erbium Oxide Films

The recombination properties of silicon passivated by insulating films of erbium oxide (Er2O3) have been studied. After deposition of an oxide film, the effective lifetime of nonequilibrium charge carriers, measured by method of nonstationary photoconductivity relaxation, increased by a factor of two to three as compared to the initial value. The rates of the surface recombination of charge carriers at the silicon-oxide interface have been determined. The insulating films of erbium oxide are promising passivating coatings for siliconbased devices and integrations.

Multi-wavelength all optical measurement for the characterization of recombination process in thin mc-Si samples

A contactless, all-optical and non-destructive technique for simultaneous measurement of minority carrier recombination lifetime and surface recombination velocity, at low injection level, in multi-crystalline silicon samples is presented. Being contactless and non-destructive with respect to the surface to be analyzed, the method does not need any surface treatment to be applied and therefore is suitable for routine lifetime characterization in solar cell fabrication processes.

Silicon passivated by two-layer insulating films of ytterbium oxide and dysprosium oxide

We have studied the recombination properties of silicon passivated by two-layer insulating films of ytterbium oxide and dysprosium oxide. After deposition of such a two-layer coating of rare earth element (REE) oxides, the effective lifetime of nonequilibrium charge carriers, measured by the method of nonstationary photoconductivity relaxation, is two to three times the initial value. The rates of the surface recombination of charge carriers at the silicon-REE oxide interface have been determined. Two-layer insulating films of REE oxides are promising passivating coatings for semiconductor devices and integrations.

Multi-wavelength transverse probe lifetime measurement for the characterization of recombination lifetime in thin mc-Si samples for photovoltaic industry use

A contactless, all-optical and non-destructive technique for simultaneous measurement of minority carrier recombination lifetime and surface recombination velocity, at low injection level, in multi-crystalline silicon samples is presented. Being contactless and non-destructive with respect to the surface to be analyzed, the method does not need any surface treatment to be applied and therefore is suitable for routine lifetime characterization in solar cell fabrication processes.

Lightwave switching in semiconductor microring devices by free carrier injection

Using a conventional microwave photonic technique, we demonstrate lightwave switching in laterally coupled GaAs-AlGaAs microring resonators by free carrier injection. The ring waveguide is optically pumped just above its bandgap energy, which results in a temporal tuning of the microring resonant wavelengths by the refractive index change due to the induced free carriers. Both the transmission and the phase function of the resonators are investigated and used to demonstrate all-optical switching. The switching time, limited by surface recombination of carriers, is 20 ps. A switching energy of a few picojoules shifts the microring resonance by 1.2 nm.

Determination of free carrier bipolar diffusion coefficient and surface recombination velocity of undoped GaN epilayers

Time-resolved nondegenerate four-wave mixing experiments were performed on 2.6-μm-thick GaN epilayers grown by metalorganic chemical-vapor deposition using picosecond pulses at 355 and 1064 nm for grating recording and probing, respectively. Measurements of free carrier grating kinetics at various grating periods Λ in range from 3 to 10 μm allowed determination of nonequilibrium carrier bipolar diffusion coefficient D=1.7 cm2/s at 300 K. Substitution of the D value into two-dimensional carrier transport model allowed fitting of the whole set of grating kinetics at various Λ varying surface recombination velocity S and linear recombination time τR. This procedure provided us a value of S=5×104 cm/s as well as carrier lifetime of ∼1 ns.

The influence of lateral carrier diffusion and surface recombination on the behavior of semiconductor optical amplifier (SOA)-based MMIs

The influence of lateral carrier diffusion and surface recombination on the self-imaging properties of semiconductor-optical-amplifier-based multimode interference couplers has been verified by simulations using a beam propagation method. It shows a significant degradation of the self-imaging properties of these devices. Buried heterostructures or deeply etched waveguide structures can decrease the impact when the degree of surface recombination is sufficiently low.

Temperature dependence of carrier mobility in Si wafers measured by infrared photocarrier radiometry

A recently introduced infrared photocarrier radiometry technique has been used to determine the temperature dependence of carrier mobility in Si wafers. In addition, its potential to determine simultaneously the carrier lifetime, diffusion coefficient, and surface recombination velocity is reported. This noncontact, nonintrusive, and all-optical technique relies on the detection of infrared radiation from harmonically excited free carriers (pure electronic diffusion-wave detection). Using a multiparameter fitting to a complete theory, the results showed that the lifetime increases with temperature, the diffusion coefficient decreases [D(T)∼T−1.5], and the temperature dependence of carrier mobility is μ(T)=(1.06±0.07)×109×T−2.49±0.01 cm2/V s.

All-optical multiwavelength technique for the simultaneous measurement of bulk recombination lifetimes and front/rear surface recombination velocity in single crystal silicon samples

In this article, a contactless, all-optical, nondestructive method for separating the minority carrier recombination lifetime and surface recombination velocities in silicon samples at a low injection level is presented. The technique can be described as a pump–probe method in which the excess carrier density is probed by analyzing the free carrier absorption transient following laser pulse excitation that have several wavelengths. An attractive characteristic of the proposed technique is its capability to measure different values of surface recombination velocity on front and back surfaces of the silicon sample. The theoretical foundation of the method is also analyzed. Moreover, numerical simulations which validate the proposed methodology and preliminary experimental results that prove the applicability of scheme are presented.

Photothermal microscopy of silicon epitaxial layer on silicon substrate with depletion region at the interface

Photothermal microscopy has been used for investigating semiconductor materials to evaluate carrier diffusivity, lifetime, and surface recombination velocity. The effects of the depletion region between epitaxial and substrate Si with different conduction types are studied. Although the observed curves are well explained by the theoretical predictions for surface reflectivity, no drastic change is observed for the different structures. This may be due to the fact that the thickness of the epitaxial layer is too large to reveal clearly the effect of the depletion region at the PN junction. However, the result for low-frequency modulation at 10 kHz may indicate this effect.