Lifetime Of Nonequilibrium Carriers Research Articles

Temperature dependence of the lifetime of nonequilibrium charge carriers in GaP diodes under condition of recombination current domination

Temperature dependence of the lifetime of nonequilibrium charge carriers limited by recombination process in a p-n junction space-charge region has been obtained from current–voltage, capacitance–voltage and thermometric characteristics of GaP p+-n junctions in the temperature range 150–500 K. The results have been refined using the data of the junction relaxation characteristics. Parameters of the carriers' lifetime sensitivity to the temperature and current have been determined. It has been established that the charge carriers recombine predominantly through deep single-level amphoteric-type centres. The depth of the centres makes approx. (EC-1.25 eV). We suppose that the nature of the centres formation is not connected with the junction fabrication technology. It has rather fundamental origin. The results of the present investigation could be used in the development of devices based on wide bandgap semiconductors and particularly high temperature diode sensors.

Influence of impurities and conditions of growing the silicon single crystals by Czochralski method on the value of lifetime of non-equilibrium charge carriers

В работе исследуется влияние содержащихся в монокристаллическом кремнии таких примесей, как кислород, углерод, бор, а также скорости охлаждения выращиваемого монокристалла на время жизни неравновесных носителей заряда. Установлено, что повышение концентрации кислорода и бора способствует увеличению времени жизни неравновесных носителей заряда, а увеличение концентрации углерода и скорости охлаждения монокристалла кремния ведет к уменьшению времени жизни неравновесных носителей заряда.

Strong covalency-induced recombination centers in perovskite solar cell material CH3NH3PbI3.

Inorganic-organic hybrid perovskites are a new family of solar cell materials, which have recently been used to make solar cells with efficiency approaching 20%. Here, we report the unique defect chemistry of the prototype material, CH3NH3PbI3, based on first-principles calculation. We found that both the Pb cations and I anions in this material exhibit strong covalency as characterized by the formation of Pb dimers and I trimers with strong covalent bonds at some of the intrinsic defects. The Pb dimers and I trimers are only stabilized in a particular charge state with significantly lowered energy, which leads to deep charge-state transition levels within the band gap, in contradiction to a recent proposal that this system has only shallow intrinsic defects. Our results show that, in order to prevent the deep-level defects from being effective recombination centers, the equilibrium carrier concentrations should be controlled so that the Fermi energy is about 0.3 eV away from the band edges. Beyond this range, according to a Shockley-Read-Hall analysis, the non-equilibrium carrier lifetime will be strongly affected by the concentration of I vacancies and the anti-site defects with I occupying a CH3NH3 site.

Experimental observation of exciton splitting and relaxation dynamics from PbS quantum dots in a glass matrix

Multiple optical transitions from PbS quantum dots (QDs) in glass matrix are observed. Energy separations between them amount up to $\ensuremath{\sim}$200 meV. Instead of being due to a size distribution of QDs, they are found to be related to a splitting of the lowest exciton levels. Systematic analysis of the relaxation dynamics reveals the lifetimes of the split states ranging from sub-100-ns to $\ensuremath{\sim}$$\ensuremath{\mu}$s. Moreover, we observe excited and ``intrinsic'' states having a sub-100-ps and $\ensuremath{\sim}$ns lifetime, respectively. The behavior of the split structure can be modeled by a phonon-assisted relaxation mechanism. This investigation offers a conclusive interpretation for the wide distribution of experimentally observed nonequilibrium carrier lifetimes in PbS QDs.

Specific features of the spectra and relaxation kinetics of long-wavelength photoconductivity in narrow-gap HgCdTe epitaxial films and heterostructures with quantum wells

The spectra and relaxation kinetics of interband photoconductivity are investigated in narrow-gap Hg1 − xCdxTe epitaxial films with x = 0.19–0.23 and in structures with HgCdTe-based quantum wells (QWs), having an interband-transition energy in the range of 30–90 meV, grown by molecular-beam epitaxy on GaAs (013) substrates. A long-wavelength sensitivity band caused by impurities or defects is found in the spectra of the structures with quantum wells in addition to the interband photoconductivity. It is shown that the lifetimes of nonequilibrium carriers in the structures with QWs is less than in bulk samples at the same optical-transition energy. From the measured carrier lifetimes, the ampere-watt responsivity and the equivalent noise power for a film with x = 0.19 at a wavelength of 19 μm are estimated. When investigating the relaxation kinetics of the photoconductivity at 4.2 K in high excitation regime, it is revealed that radiative recombination is dominant over other mechanisms of nonequilibrium-carrier recombination.

Doslidzhennya rekombinatsiynykh kharakterystyk Cz-kremniyu, implantovanoho ionamy zaliza

A comparative study of the defect formation and changes in the lifetime of nonequilibrium minority charge carriers in silicon while gettering the iron impurity with the use of a combined getter “porous silicon layer + aluminum film” is carried out. It is shown that, while annealing specimens with no getter layer, iron silicide and defects of the vacancy type are formed in the implanted regions and, as a result, a considerable reduction in the lifetime of nonequilibrium minority charge carriers is observed. The influence of the getter layer on the defect formation and the redistribution of iron atoms implanted into silicon are studied in the case of high iron concentrations in a vicinity of the surface. The presence of a getter layer is shown to reduce the efficiency of the silicide formation in the implanted region and to increase the concentration of interstitial defects in silicon. A model of the gettering process is proposed, which makes allowance for the gettering-induced reduction of the iron atom concentration in the implanted region and a decrease of the vacancy defect concentration, as well as the simultaneous increase of the concentration of interstitial-type defects associated with the formation of complexes of iron and boron atoms. These complexes are recombination-active and do not allow the lifetime of charge carriers to be restored to the initial value.

Effect of annealing on the nonequilibrium carrier lifetime in GaAs grown at low temperatures

GaAs samples grown by molecular-beam epitaxy at low (230°C) temperatures are investigated. One of the samples is subjected to aftergrowth annealing at 600°C. Using an unconventional pump-probe scheme for measuring the dynamic variation in the light refractive index, the nonequilibrium charge-carrier lifetime (275 ± 30 fs before annealing) is determined. Such a short carrier lifetime in the unannealed material is due to the high concentration of point defects, mainly AsGa antisite defects. According to X-ray diffraction and steady-state optical absorption data, the AsGa concentration in the samples is 3 × 1019 cm−3, which corresponds to an arsenic excess of 0.26 at %. Upon annealing at 600°C, the superstoichiometric As defects self-organize and form As nanoinclusions in the GaAs crystal matrix. It is shown that in this case the nonequilibrium charge-carrier lifetime increases to 452 ± 5 fs. This lifetime is apparently ensured by the capture of non-equilibrium charge carriers at metal As nanoinclusions.

Production of high-purity Cd, Zn, and Te by multistage vacuum distillation

The quality of detector single crystals is determined by the lifetime and mobility of nonequilibrium charge carriers, which depend on the purity of the components used for synthesis. An original setup for multistage vacuum distillation of Cd, Zn, and Te is designed, which is characterized by the simplicity of equipment and the minimal loss of the initial material. The purified materials have low contents of oxygen and metal impurities, which allow one to significantly increase the purity of the synthesized crystals.

Getter formation in silicon by implantation of antimony ions

A porous silicon layer as a getter of uncontrolled impurities has been prepared by implantation of Sb+ into silicon and subsequent thermal treatments. The lifetime of nonequilibrium charge carriers in n- and p-type silicon wafers with a getter layer is 3–4 times longer than that without a getter.

Effect of the microstructure on electrical properties of high-purity germanium

The interrelation between the electrical properties and the microstructure of high-purity germanium crystals has been revealed. The electrical conductivity of polycrystalline samples increases and the life-time of nonequilibrium charge carriers in them decreases with a decrease in the crystallite sizes.

Study of the light-induced degradation of tandem α-Si:H/μc-Si:H photovoltaic converters

The photo-induced degradation of tandem α-Si:H/μc-Si:H photovoltaic converters with an initial efficiency of 10.4% under light flux densities of 1 and 10 kW m−2 (AM1.5G) is studied. It is shown that the stabilized state is reached after 500 h of exposure to the standard light-flux density and after 300 min at a flux 10 times higher in density. In both cases, the efficiency decreases by 1.2–1.4 abs. %. The experimentally measured spectral and current-voltage characteristics of the photovoltaic converters are used to determine the nonequilibrium carrier lifetimes and to calculate variation dependences of the dangling-bond concentration in i-α-Si:H and i-μc-Si:H layers. The dependences are approximated in terms of the floating-bond model. The calculated dangling-bond concentrations after various exposure times are used to simulate the dependences of the photovoltaic-converter parameters on light exposure. The results obtained show good coincidence between the simulated degradation rates of the current and efficiency of a tandem photovoltaic cell and the experimental data.

Nonequilibrium Charge Carrier Lifetime Testing Equipment for Semiconductor Materials by a Contactless Microwave Phase Method

The accurate measurement of nonequilibrium charge carrier lifetime is of vital significance in research and manufacture of crystalline silicon solar cells. A testing equipment based on a contactless microwave phase method was implemented by being embedded with GPIB, FPGA and a lock-in analyzer. A friendly operation interface was developed, based on the graphic programming language LabVIEW. The virtue of the equipment is achieved by automatic data acquisition and processing, which improves the automatization, efficiency and accuracy of the measurement.

GaAs Pixel Detectors

ABSTRACTResults of investigation of X-ray sensors on the basis of GaAs compensated with chromium (HR GaAs) are presented in this work. HR GaAs material is shown to have the following physical parameters: the resistivity about 1GOhm*cm, the nonequilibrium charge carrier lifetime – hundreds of nanoseconds. Prototypes of microstrip and array HR GaAs sensors have been manufactured and tested. It is demonstrated that the sensors provide spatial resolution according to the pixel pitch and allow obtaining high quality X-ray images.

The physical principles of terahertz silicon lasers based on intracenter transitions

AbstractThe first silicon laser was reported in the year 2000. It is based on impurity transitions of the hydrogen‐like phosphorus donor in monocrystalline silicon. Several lasers based on other group‐V donors in silicon have been demonstrated since then. These lasers operate at low lattice temperatures under optical pumping by a midinfrared laser and emit light at discrete wavelengths in the range from 50 to 230 µm (between 1.2 and 6.9 THz). Dipole‐allowed optical transitions between particular excited states of group‐V substitutional donors are utilized for donor‐type terahertz (THz) silicon lasers. Population inversion is achieved due to specific electron–phonon interactions of the impurity atom. This results in long‐living and short‐living excited states of the donor centers. Another type of THz laser utilizes stimulated resonant Raman‐type scattering of photons by a Raman‐active intracenter electronic transition. By varying the pump‐laser frequency, the frequency of the Raman intracenter silicon laser can be continuously changed between at least 4.5 and 6.4 THz. The gain of the donor and Raman‐type THz silicon lasers is of the order of 0.5 to 10 cm−1, which is similar to the net gain realized in THz quantum cascade lasers and infrared Raman silicon lasers. In addition, fundamental aspects of the laser process provide new information about the peculiarities of electronic capture by shallow impurity centers in silicon, lifetimes of nonequilibrium carriers in excited impurity states, and electron–phonon interaction.

Increases of the non-equilibrium charge carrier lifetime in silicon crystals

The article describes possibility increase of the non-equilibrium charge carrier lifetime in silicon monocrystals at use of initial raw materials with the increased content of impurity is considered

Experimental evaluation of the carrier lifetime in GaAs grown at low temperature

The relaxation dynamics of nonequilibrium charge carriers in gallium arsenide epitaxial films grown by molecular-beam epitaxy at low temperatures has been studied. The growth conditions of the epitaxial layer provided an excess arsenic content of 1.2% in the layer. In a material of this kind, the carrier lifetime is <1 ps. To examine carrier relaxation in the femtosecond range, an original scheme for measuring the refractive index dynamics was developed on the basis of the pump-probe technique. The lifetime of nonequilibrium charge carriers was evaluated to be (200 ± 35) fs.

Influence of Auger recombination on the lifetime of nonequilibrium carriers in InGaAsSb/AlGaAsSb quantum well structures

Carrier recombination processes, including the Auger recombination, are studied in InGaAsSb/AlGaAsSb quantum well nanostructures. Based on the dynamics of photoluminescence, we estimate the emission time of an optical phonon, determined the recombination rate as a function of optical-excitation intensity, and estimated the coefficient characterizing the rate of the resonance Auger recombination.

Estimation of the ultimate efficiency of a three-pin solar cell based on the GaAs/Si heterostructure

The ultimate efficiency of a three-pin solar cell based on the GaAs/Si structure is calculated by means of numerical simulation in a diffusion-drift approximation. Dependences of the efficiency on the GaAs layer thickness and the density of dislocations threading in this layer, which are known to affect the lifetime of nonequilibrium charge carriers. It is shown that the maximum limit efficiency (27%) of such a structure is reached at the GaAs layer thickness of about 1.4 µm and the density of threading dislocations of less than 106 cm−2.

Cathodoluminescence studies of electron irradiation effects in n-type ZnO

The lifetime of non-equilibrium carriers in n-type unintentionally doped ZnOincreases when the sample is exposed to the electron beam of a scanningelectron microscope. This is observed by studying the ZnO cathodoluminescence(CL) spectra at different irradiation time durations and temperatures. Wefound that the decrease in the CL spectra’s peak intensity is related to athermo-activated energy barrier, determined by the calculated activation energy value of259 ± 30 meV. This energy value comes close to the defect energy level of the zinc interstitial,which is possibly the nature of the energy barrier responsible for this decrease.

Controlled microwave module for the noncontact measurement of nonequilibrium carrier lifetime in semiconductors

An original controlled microwave module for the noncontact measurement of the parameters of semiconductors is briefly described. The module allows automatic control of frequency, amplitude, Q factor, and the shape of a resonance curve shape of a microwave cavity upon its loading by a measured semiconductor. The results of the measurements of nonequilibrium carrier lifetime in single-crystal and multicrystalline silicon wafers using the microwave module are presented.