Types Of Recombination Centers Research Articles

Influence of doping time on spatial distribution of luminescence intensity in ZnSe:Fe

For ZnSe crystals doped with Fe by a high-temperature diffusion technique, the influence of annealing time on the positions of strip-like areas with a high intensity of luminescence at wavelengths of 521 and 715 nm parallel to the doping surface is studied. It is found that the distance between the doping surface and the strip-like areas increases with increasing doping time. A good description of the results is obtained under the assumption of simultaneous diffusion of Fe ions and two different defect/impurity centers. The formation of spatial strip-like areas is shown to be a general phenomenon arising in the co-diffusion of several types of recombination centers.

Electrical, structural and optical properties of Cd1-xZnxTe thick polycrystalline films

Thick polycrystalline Cd1-xZnxTe films with different Zn concentration were deposited by the close-spaced vacuum sublimation method on ITO-coated glass substrates. In order to evaluate the usefulness of the films as a base layer in X-ray detector, the study of their electrical properties and photoresponse was carried out. Structural, microstructural properties and phase composition of the films were investigated using X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and Raman spectroscopy. It was found that films have cubic zinc-blende crystal lattice; the microstructural analysis showed that crystalline quality (e.g. coherent scattering domains size and microstrains level) of the films is decreasing with increasing of Zn concentration. Raman measurements showed only CdTe- and ZnTe-related vibrational modes, which indicates the absence of tellurium inclusions at the surface.The temporal photoresponse of the detector was described with the help of the analytical model, taking into account the presence of two types of recombination centers. As a result, the parameters of the recombination centers and their possible influence on the current flow in the films were established.

Superlinear composition-dependent photocurrent in CVD-grown monolayer MoS2(1-x)Se2x alloy devices.

Transition metal dichalcogenides (TMDs) have emerged as a new class of two-dimensional materials that are promising for electronics and photonics. To date, optoelectronic measurements in these materials have shown the conventional behavior expected from photoconductors such as a linear or sublinear dependence of the photocurrent on light intensity. Here, we report the observation of a new regime of operation where the photocurrent depends superlinearly on light intensity. We use spatially resolved photocurrent measurements on devices consisting of CVD-grown monolayers of TMD alloys spanning MoS2 to MoSe2 to show the photoconductive nature of the photoresponse, with the photocurrent dominated by recombination and field-induced carrier separation in the channel. Time-dependent photoconductivity measurements show the presence of persistent photoconductivity for the S-rich alloys, while photocurrent measurements at fixed wavelength for devices of different alloy compositions show a systematic decrease of the responsivity with increasing Se content associated with increased linearity of the current-voltage characteristics. A model based on the presence of different types of recombination centers is presented to explain the origin of the superlinear dependence on light intensity, which emerges when the nonequilibrium occupancy of initially empty fast recombination centers becomes comparable to that of slow recombination centers.

Model of hopping excitons in GaInNAs: simulations of sharp lines in micro-photoluminescencespectra and their dependence on the excitation power and temperature

The model of hopping excitons in semiconductors proposed by Baranovskii et al (1998 Phys. Rev. B58 13081) has been modified and applied to explain sharp lines observed in micro-photoluminescence(μ-PL) spectra of GaInNAs alloys and their changes with excitation power and temperature.Instead of two types of recombination centres (radiative and nonradiative centres)introduced by Baranovskii et alwe have proposed one kind of localization centre withradiative and nonradiative rates. Such a modification is justifiable due to ourrecent experimental observations for GaInNAs alloys and allows us to explain thefast thermal quenching of localized emission from this alloy. Our simulationsclearly show that the individual sharp PL lines observed at low temperaturesappear for this material due to exciton hopping between localization centres.Taking into account saturation effects and the exciton dissociation phenomenon, ithas been shown that the observed changes in power- and temperature dependentμ-PL spectra can be excellently reproduced by the modified model.

Dynamics of polarized photoluminescence in m-plane InGaN/GaN quantum wells

Spectrally-, polarization-, and time-resolved photoluminescence (PL) experiments have been performed on 2.5 nm thick m-plane single InGaN quantum wells. It has been found that PL decay is mainly determined by nonradiative recombination through several types of recombination centers, while PL rise is largely affected by exciton transfer into localization minima. Prolonged PL rise times and time-dependent spectral shift were used to study exciton transfer into the localization centers. Characteristic time of the exciton transfer is 80–100 ps at lower temperatures and about 50 ps at room temperature, which corresponds to the exciton diffusion length of 200–500 nm. Degree of PL linear polarization was found to decrease at a similar rate. Decreased PL polarization for the localized excitons suggests that the localization centers are related to areas with relaxed strain.

Spectrally resolved thermoluminescence of highly irradiated LiF:Mg,Cu,P detectors

Well known, widely applied high sensitive thermoluminescence (TL) detector LiF:Mg,Cu,P (MCP-N) was investigated. This paper analyses changes of the TL emission spectrum of MCP-N after irradiation with ultra high doses (up to 500 kGy). Spectral dependence of TL on dose is very complex especially in the region of very high doses (>1 kGy). As a general trend we found that the number of peaks increases with dose in the long-wavelength region indicating new types of recombination centres (RCs). Wavelength peak positions for increasing doses are quite stable. Only some of them show slight red-shift.

Validity of simplified Shockley-Read-Hall statistics for modeling carrier lifetimes in crystalline silicon

The Shockley-Read-Hall model, in its simplest and most common form, is often used to describe both injection- and temperature-dependent carrier lifetime measurements. Such lifetime modeling has provided the basis for novel, ultra-sensitive spectroscopic techniques for the study of recombination centres in crystalline silicon. However, this approximate model is only valid when the density of recombination centres is small enough to avoid trapping effects, which cause distortions in the excess mobile carrier concentrations. In this work, the simplified Shockley-Read-Hall model is compared with a more general solution of the continuity equations that takes account of carrier trapping. This comparison leads to an expression for the upper limit on the recombination centre density for which the simplified Shockley-Read- Hall model remains accurate. The limit depends not only on the dopant density, but also on the energy level and electron and hole capture cross-sections for a given type of recombination centre. The results allow experimental conditions that do not invalidate the use of the simplified Shockley-Read-Hall model to be determined.

Spin-dependent recombination of photoinduced carriers in phthalocyanine/C60heterojunctions

Spin-dependent recombination of photoinduced carriers in ${\mathrm{H}}_{2}$-phthalocyanine $({\mathrm{H}}_{2}{\mathrm{P}\mathrm{c})/\mathrm{C}}_{60}$ heterojunctions is studied by electrically detected electron-spin resonance (EDESR) spectroscopy. The EDESR spectrum of the ${\mathrm{H}}_{2}{\mathrm{P}\mathrm{c}/\mathrm{C}}_{60}$ consists of two components A and B, the g values of which are 2.0018$\ifmmode\pm\else\textpm\fi{}$0.0002 and 2.0010$\ifmmode\pm\else\textpm\fi{}$0.0002, respectively. The two components are attributed to exchange-coupled localized electron-hole pairs trapped at different types of recombination centers. Component A has spin-flip satellites due to an interaction between the electron (or hole) spin, and its surrounding nuclear spins of protons which belong to the ${\mathrm{H}}_{2}\mathrm{Pc}$ rings. From the satellite intensity, the distance between the electron (or hole) and the protons is estimated to be 4.33$\ifmmode\pm\else\textpm\fi{}$0.25 \AA{}, indicating that the localized pairs for the component A locate close to the ${\mathrm{H}}_{2}\mathrm{Pc}$ rings. The spin dynamics of the localized pairs for the component A is studied by a microwave recovery experiment, in which the time dependence of the EDESR signal intensity is measured after turning the resonant microwave on and off. A theoretical model of the spin-dependent recombination of the exchange-coupled electron-hole pair is proposed with which the experimental results of the microwave recovery are explained. By a theoretical analysis, it is found that $R\ensuremath{\gtrsim}1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and ${D+W}_{\mathrm{sl}}=6.2(\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}{10}^{4}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for component A at room temperature, where R is the recombination rate of the localized pair in the ${S}_{z}=0$ triplet sublevel, and D and ${W}_{\mathrm{sl}}$ are the dissociation and the spin-lattice relaxation rates, respectively, of the pairs in the triplet sublevels. The photocurrent ${I}_{2}$ that is caused by the dissociation of the localized pairs for component A is about 5% of the total photocurrent.

Inverse correlation between the intensity of luminescence excited by electrons and by visible light in chemical-vapor-deposited diamond films.

Diamond films grown by filament-assisted or microwave-plasma-assisted chemical-vapor deposition (CVD) were characterized by photoluminescence (PL), cathodoluminescence (CL), and Raman spectroscopies. The laser-excited PL spectra of these films in and near the carbon Raman region (1100--1800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ from the 514.5-nm laser line, or 2.18--2.28 eV) are broad and featureless; CL spectra measured within a wider spectral range (1.5--3.5 eV) show several distinct components. Because of its correlation with the Raman band of ${\mathit{sp}}^{2}$-bonded carbon, the visible-laser-excited PL in the carbon Raman region is attributed to ${\mathit{sp}}^{2}$-bonded carbon clusters. The spectrally integrated CL intensity is found to vary from specimen to specimen approximately inversely with the intensity of the laser-excited PL. This inverse correlation is especially strong for one component of the CL, a broad band at 2.85 eV. To explain these results, it is proposed that the luminescence centers in these CVD diamond films can be classified into two types with differing excitation and recombination properties. Rate equations are derived for the luminescence kinetics of such a system of two types of recombination centers.

A theoretical description of complex thermoluminescence curves. II. Method of integral equation

Thermoluminescence kinetics is discussed using a model with one type of recombination centre and an arbitrary number of trapping centre types, assuming that electron retrapping is dominant. A distinctive feature of the suggested method is that an integral equation is obtained for the one-time function z(t) through which both the intensity of the thermoluminescence curve (TLC) and the concentration of charge carriers are expressed. Using a series of approximations, which are analysed in detail, analytical solutions of the integral equation are obtained for an arbitrary elementary peak in a complex TLC consisting of a number of slightly overlapping peaks. The comparison of the theoretical results with the numerical ones obtained by the Runge-Kutta method shows that the obtained analytical expression is precise for the first peak and describes the following TLC peaks satisfactorily. The possibility of using the results of the present study for computer division of a complex TLC into elementary components, even in the case of considerably overlapping peaks, is also discussed.

A theoretical description of complex thermoluminescence curves. I

Thermoluminescence kinetics has been discussed within the framework of a band model assuming an extensive retrapping of charge carriers in comparison with their recombination. Assuming that a luminophor has one type of recombination centre and an arbitrary number of trapping centres the authors succeeded in analysing complex thermoluminescence curves with an arbitrary number of peaks. It has been shown for an elementary peak that its form almost coincides with the Randall-Wilkins curve, and its intensity is proportional to the square of the absorbed dose. The effect of both the processes of localised charge carriers accumulation (irradiation and isothermal decay) and their redistribution on trapping centres during the thermoluminescence process have been dealt with taking into account the form and intensity of the thermoluminescence peaks.

Theory of Steady State Photoconductivity in Amorphous Semiconductors—Sensitization Due to the Presence of Different Types Recombination Centres

The present work attempts to connect Simmons and Taylor’s theory of steady state photoconductivity with Rose model of sensitization. A generalized formula of photoconductivity for a system with several recombination centres is given. An iteration procedure is suggested to solve the related rate equations. Using this formula and combining the particular energy band model of a-Si:H, the following result is obtained: when two types of recombination centres in the material exist and the electron capture coefficient for a centre of the second type is much smaller than that of the first type, and when the state density of the former is greater than that of the latter, the free electron lifetime will be greatly increased, therefore, the material will be sensitized. This theoretical result is consistent with that of the Rose model .The dependence of the photocurrent on temperature and incident photon flux are also given. Comparisons with the experimental results on hydrogenated amorphous silicon are made.

Combined photoacoustic and photoconductive spectroscopic investigation of nonradiative recombination and electronic transport phenomena in crystalline n-type CdS. II. Theory.

A theoretical model of the coupled photoconductive (PC) and microphone gas-coupled photoacoustic (PA) signal generation is presented. The model includes numerical solutions to the following: (i) the carrier transport equations leading to PC signal generation in the presence of two different types of recombination centers, (ii) the Poisson equation relating the difference between electron and hole densities to the internal electric field, and (iii) the heat diffusion equation, relating the carrier population density to the heat generation rate and ultimately to the PA signal generation. Spectroscopic simulations with the CdS absorption spectrum as input data essentially show agreement between theory and experiment in the band-gap and sub-band-gap spectral regions. Parameter variation reveals that PA and PC experimental data (preceding paper) can be predicted if, and only if, a wavelength-dependent nonradiative quantum efficiency is assumed with a substantial increase at photon energies below the band gap, coupled with signal contributions at higher photon energies due to band-to-band intrinsic transitions.

The thermal quenching and supralinearity of photoconductivity in hydrogenated amorphous silicon

Abstract The temperature and illumination intensity dependences of photoconductivity (PC) due to the presence of different types of recombination centres in undoped a‒Si: H are investigated in terms of the Rose-Simmons-Taylor theory. The results show pronounced structure in the temperature dependence curves of PC. In the temperature range where this structure appears, slight supralinearity can be observed. Due to the competition between recombination centres and the electron sensitizers, the PC can be either increased or decreased by more than an order of magnitude; also the features of the PC temperature dependence are different. These are compared with the annealed state or light-soaked state of undoped a‒Si: H.

Forming characteristics of recombination properties of InSb MOS structures

Build-up processes of small-signal photo-emf and space-charge layer capacitance following cooling of the structure are investigated as well as the effect on them of fixed charge in the oxide and length of exposure to a voltage applied to the structure. It is shown that there are two types of recombination centers in the semiconductor which differ in the dependences of the concentrations and build-up times on the applied voltage. The centers of the first type determine the carrier recombination velocity in the weak inversion region, while centers of the second type are deep acceptors operating in the region of strong surface inversion. The latter are produced in the structure in the presence of an electric field. It is assumed that the recombinations centers originate from structural defects in the semiconductor and at the interface, whereby centers of the second type include indium vacancy and hydrogen. A mechanism of their emergence is proposed.

On the possibility of observing ambipolar photoconductivity and its optical quenching in photoconductors

Based on a model with two types of recombination centers and one trapping center for electrons ambipolar photoconductivity can appear in crystals, in which the Fermi-level lies deep in the bandgap, in the neighborhood of the sensitizing centers. The effect of a long-wavelength irradiation on the ambipolar intrinsic photoconductivity is studied theoretically and as a result most quenching and enhancement properties, which are described in the first paper and observed experimentally on S-annealed ZnS crystals, are found again.

Intensity dependence of photoconductivity in the case of three types of recombination centres

Intensity dependence of photoconductivity in the case of three types of recombination centres

Effective recombination levels in n- and p-type silicon irradiated by 4·5 MeV electrons

The effective recombination levels created at room temperature by 4·5 MeV electron irradiation are deduced from the variations in lifetime vs carrier injection rate, electron fluence, and temperature. This paper aims to compare the properties of the created recombination energy levels and defect centers in N- and P-type silicon single crystals. The characteristics of the samples used extend over a wide range of resistivities, doping impurities and crystal growth techniques. A pulsed neodymium laser has been employed to carry out these studies, and the carrier lifetime has been measured by the photoconductivity-decay method. Information on the specific centers is deduced from the comparison of the present macroscopic results on energy levels and annealing studies with the known properties of microscopic defects. From the results obtained, several types of recombination centers are simultaneously created in N- and P-type silicon, and crystal impurities other than oxygen and dopants may play a big part in the constitution of such centers. In the P-type silicon case, 3 types of recombination centers are clearly operative: (1) centers with a ∼ E v +0·20 eV energy level, which could be divacancies, and which would cease to act as recombination centers by trapping irradiation induced interstitial carbon atoms, (2) centers with a ∼ E v +0·24 eV level which may involve aluminium interstitial atoms, and finally (3) centers with a ∼ E v +0·27 eV level, which are K centres. These recombination centers are more or less active, depending on the initial characteristics of the sample. In the N-type silicon case, only two groups of effective recombination levels, ∼ E c −0·17 and E v +0·3 eV, appear in the irradiated materials. However, the effects of centers possibly linked to the presence of contaminants, such as carbon and aluminium, must be added to the known effects of the divacancy, doping atom-vacancy and oxygen-vacancy complexes to explain the carrier lifetime degradation and recovery.

On Sensitizing Recombination Centres in GaSe Single Crystals

AbstractAn investigation is made of the stationary photoconductivity and its kinetic, the thermally stimulated conductivity, and the mobility of current carriers of tin‐doped p‐GaSe single crystals. It is shown experimentally that the recombination in such crystals is controlled by two types of recombination centres: one of them being “slow” (r), another “fast” (s). The principal parameters of “slow” recombination centres (r), thermal (E = 0.58 eV) and optical (E = 0.78 eV) energetic depth from C‐band, electron (Sn r = 5 × 10−14 cm2) and hole (Sp r = 3 × 10−20 cm2) capture cross‐sections, are measured. The concentration is found to be equal to Nr = 3 × 1014 cm−3. It is shown that the r‐centre is a single charged donor which may be due to substituting Ga atoms by Sn atoms in the GaSe lattice.

Recombination Centers on Ion-Bombarded and Vacuum Heat-Treated Germanium Surfaces

Germanium surfaces were bombarded with argon ions and then heat-treated under high vacuum. Room-temperature values of surface recombination velocity, surface conductivity, dark field effect, and field effect under illumination were measured after successive heat treatments. In addition, the temperature dependence of these quantities was determined. It was confirmed that after annealing of the bombardment damage, a large number of acceptor type surface states approximately clamped the surface potential. Two types of recombination centers were identified: type 1, located near the middle of the gap and type 2, located near the valence band. The various heat treatments produced changes in the density of the type 1 centers but did not appear to effect the density of the type 2 centers.