Photoconductivity Relaxation Time Research Articles

In Ukrainian

An experimental study has beencarried out on the spectral dependence of the relaxation time of surface monopolar photoconductivity in the structures of macroporous silicon, with strong surface absorption of light, while the absorption coefficient varied in the range 2.37∙10 3 –3∙10 5 cm –1 , and the absorption depth from 0.03 μm to 4.22 μm. Visible LEDs 0.38–0.62 µm were used for the studies: ultraviolet, violet, blue, green, yellow, orange and red. Structures of macroporous silicon were formed on n-type silicon with orientation [100] and specific resistivity of 4.5 Ω∙cm using photoelectrochemical etching. The macropores had a diameter of D p = 3.5 μm, a depth of h p = 80 μm and a concentration of N p = 3.5∙10 6 cm –2 . In the structures of macroporous silicon, a "slow" relaxation of monopolar photoconductivity was experimentally observed. It has been found that the relaxation time of monopolar photoconductivity is dependent on the wavelength of the illumination and varieds in the range of 15.5–42 s in the visible range of 0.38–0.62 μm. Moreover, the dependence of the photoconductivity relaxation time on the illumination wavelength had a minimum relaxation time of 15.5 s when illuminated by green light with a wavelength of 0.5215 μm. The reason for the dependence of the relaxation time of the monopolar photoconductivity on the wavelength is the competition of the processes of recombination and trapping of the main charge carriers on the so-called "slow" surface levels. The dominance of the recombination or trapping process is influenced by the characteristics of local centers, the surface conditions, the surface bending of the Y s zones, which determines the surface monopolar photoconductivity and its relaxation. In the long-wavelength region of the spectrum, the decrease in the photoconductivity relaxation time from 42 s to 15.5 s is explained by the dominance of the trapping process of the main charge carriers at "slow" surface levels, which are localized within 1 nm of the silicon surface in the SiO x transition layer. In the short-wavelength region of the spectrum, the slow increase in the photoconductivity relaxation time from 15.5 to 23.7 s is explained by the fact that the trapping of the main charge carriers in the "slow" surface states, which are localized in SiO 2 and have a longer photoconductivity relaxation time, begins to influence the photoconductivity relaxation time. The level of sticking gradually turns into the level of slow recombination with a large absorption coefficient, reaching values of 3∙10 5 cm –1 . The photoconductivity relaxation associated with surface levels changes with a change in the wavelength of illumination, which causes a change in the absorption coefficient, absorption depth, and surface bending of the Y s zones. Due to this, a spectral dependence was observed of the relaxation time of surface photoconductivity in the structure of macroporous silicon.

Photoconductivity Relaxation Time in Macroporous Silicon

Macroporous silicon has found applications in sensors, receivers, integrated microchips and solar cells. In this work, we first showed analytically the photoconductivity relaxation time of in macroporous silicon is determined from a system of two transcendental equations. The analytical description of the photoconductivity relaxation model contains the diffusion equation solution both in single crystal substrate and in an effective medium of macroporous layer, as well as boundary conditions recorded for the surfaces of the layer of macroporous silicon and single crystal substrate. We showed that the photoconductivity relaxation time in macroporous silicon rapidly decreases with increasing macropore depth from 0 to 25 μm and reduced thicknesses of the single crystal substrate from 250 to 0 μm. The photoconductivity recombination time in the sample of macroporous silicon is limited by the diffusion of charge carriers from the substrate to the recombination surfaces in the macroporous layer. The system of transcendental equations that we have found will find application in calculating the relaxation time of photoconductivity in macroporous silicon devices such as sensors, receivers, and solar cells.

Electrical Properties of Indium-Oxide Thin Films Produced by Plasma-Enhanced Reactive Thermal Evaporation

The structure and electrical properties of transparent indium-oxide thin films produced by plasma-enhanced reactive thermal evaporation at different substrate temperatures are studied. It is found that the films have a grained structure. An increase in the substrate temperature yields a considerable increase in the conductivity of the films and a decrease in the photoconductivity-relaxation time. An interpretation of the effect of the substrate temperature on the observed changes in the electrical and photoelectric properties of the indium-oxide films under study is proposed.

FREE CARRIER RECOMBINATION LIFETIME CALCULATION FROM PHOTOCONDUCTIVITY DECAY MEASUREMENT IN NON–PASSIVATED SILICON

In indirect band gap semiconductors, for example, in silicon, the free carrier recombination lifetime is determined by recombination through deep level centers and inversely proportional to their concentration. This parameter is of the utmost importance for characterizing the quality of the material. Contactless methods of free carrier recombination lifetime measurements by protoconductivity decay analysis are most widely used. The measurement results are largely affected by surface recombination. The calculation of the lifetime in the bulk of a sample from the characteristic time of photoconductivity decay remains relevant since there is no ambiguous analytical solution of the continuity equation for this case. In this paper, an analysis of the relaxation of photoconductivity in single−crystal silicon wafers with non−passivated surfaces was carried out with numerical methods. The applicability of the well–known formulas for estimating the contribution of surface recombination to the effective photoconductivity decay time was discussed. We show that the time in which the «fast» exponents disappear depends on the relative thickness of the sample. It is only this part of the relaxation curve that the effective decay time is determined by the maximum value of the surface component of the relaxation time and is described by the well−known formulas. The saturation of the effective relaxation time at the point when the signal intensity reaches 45 % of the peak one (the onset point of effective decay time counting pursuant to the SEMI MF 1535 standard recommendation) only occurs in samples with thicknesses less than 3—5 diffusion lengths. For thick samples the contribution of the «fast» exponentials to the effective photoconductivity relaxation time is observed up to 5 % of the peak signal (i.e., until the noise level of the measured signal is reached). Use of the recommended formulas, including for the «infinite recombination rate» case at which the maximum surface lifetime is d2/π2D, leads to a sufficiently large (up to 20 %) error in free carrier recombination lifetime calculation.

Электрические свойства тонких пленок оксида индия, полученных методом плазменно-термического испарения

AbstractThe structure and electrical properties of transparent indium-oxide thin films produced by plasma-enhanced reactive thermal evaporation at different substrate temperatures are studied. It is found that the films have a grained structure. An increase in the substrate temperature yields a considerable increase in the conductivity of the films and a decrease in the photoconductivity-relaxation time. An interpretation of the effect of the substrate temperature on the observed changes in the electrical and photoelectric properties of the indium-oxide films under study is proposed.

On the Problem of Determining the Bulk Lifetime by Photoconductivity Decay on the Unpassivated Samples of Monocrystalline Silicon

In the indirect band gap semiconductors, and in particular, in silicon, the lifetime of nonequilibrium charge carriers is determined by recombination through impurity centers and it is inversely proportional to their concentration. Therefore, this is the most important parameter for determining the quality of the material. The noncontact methods of its measurement, in particular, the noncontact measurements of the decay of the photoconductivity (PC) constant, are the most important ones. The surface recombination strongly influences the shape of the curve. The calculation of the lifetime in the bulk by the decay constant remains relevant since there is no single-valued analytic solution of the continuity equation for this case. In the samples of monocrystalline silicon with unpassivated surfaces, the relaxation of PC is analyzed by numerical methods. The applicability of the known formulas for estimating the contribution of surface recombination to the effective PC relaxation time is discussed. It is shown that the period of time for which the fast exponents disappear depends on the relative thickness of the sample to be measured. The effective decay time is determined by the maximum value of the surface component of the relaxation time and is described by wellknown formulas only in this piece of the relaxation curve. The effective relaxation time attains saturation by the time when the signal intensity reaches 45% of the maximum value (the origin of the effective decay time according to the recommendation of the SEMI MF 1535 standard) only for the samples with a thickness of less than 3–5 diffusion lengths. For larger thicknesses, the contribution of the fast exponents to the effective PC relaxation time is observed right up to 5% of the maximum signal (i.e., until reaching the noise level of the signal to be measured). In this case using the recommended by SEMI MF 1535 standard formulas leads to a sufficiently large systematic error (up to 20%) in evaluating the free charge carrier’s lifetime.

A device for free-carrier recombination lifetime measurements

The design of a microwave system, the user interface, and the basic characteristics of noncontact equipment for measuring the recombination lifetime of free charge carriers in semiconductor materials by the photoconductivity-decay method are described. The features of the microwave-system design eliminate the need for a microwave circulator and other expensive elements of the systems without a loss in the equipment sensitivity. This substantially simplifies the design and provides automated measurements of the free-carrier lifetime in silicon single crystals in a range of 0.2 μs to tens of milliseconds. The upper measurement limit is determined by the geometrical dimensions of a sample and the surface treatment quality. The results of measuring the photoconductivity relaxation time in reference single-crystal silicon samples are presented.

Microwave photoconductivity of bifacial silicon solar cells of p + nn + type under laser irradiation

The microwave photoconductivity of silicon solar cells under laser irradiation has been investigated. It is shown that shunting of the solar cell region exposed to light by the rest of the cell may reduce significantly the relaxation time of microwave photoconductivity in comparison with the nonequilibrium charge carrier lifetime in the base region.

Effect of the direct capture of holes with the emission of optical phonons on impurity-photoconductivity relaxation in p-Si:B

A theoretical model developed for interpretation of the results of measurements of the impurity-photoconductivity relaxation in p-Si:B under pulsed optical excitation by a narrow-band tunable source of radiation in “heating” (10–500 V/cm) electric fields is presented. The model takes into account the capture of holes at the ground and lower excited states of boron with optical-phonon emission. It is shown that the dependence of the photoconductivity-relaxation time on the electric-field intensity can be unsteady taking into account these processes.

Effect of tungsten coating on the properties of high-resistivity gold-doped silicon

We have studied the effect of tungsten atoms (tungsten coating) on the properties of gold-doped silicon. The coating considerably increased the resistivity of the silicon and markedly reduced the photoconductivity relaxation time. The silicon doped with gold in the presence of tungsten was found to have an energy level with an ionization energy E = E c − (0.23 ± 0.02) eV. In uncoated gold-doped silicon, no energy level with such ionization energy was found. This strongly suggests that the energy level with an ionization energy E = E c − (0.23 ± 0.02) eV is due to tungsten centers.

Microwave photoconductivity relaxation time in a bifacial silicon solar cell base in the vicinity of a p-n junction

Microwave photoconductivity relaxation time depending on light intensity is studied in n +-p-p + silicon solar cells. The results from experiments performed under conditions of open-circuit and short-circuit currents are in agreement with the simulated data. The relaxation times of microwave photoconductivity are found for a part of the base region adjacent to the n +-p junction.

Modeling of Photoconductivity of Porous Silicon

The paper investigates a model of the photoconductivity of macroporous silicon in the conditions of homogeneous generation of photocarriers. By the finite element method, the stationary photoconductivity and the time evolution of photoconductivity after instantaneous shutdown of light are calculated. Dependences of the stationary photoconductivity and relaxation time of photoconductivity on the velocity of recombination of nonequilibrium carriers at the surfaces of pores, radius of pores, and average distance between them are analyzed.

Photoconductivity of InAs/GaAs structures with InAs nanoclusters in the near-infrared region

InAs/GaAs multilayered heterostructures containing dense arrays of the low-defect partially relaxed InAs nanoclusters larger than defect-free quantum dots are fabricated by metal-organic chemical vapor deposition in an atmospheric-pressure reactor. The structures have intense photoconductivity in the wavelength range of 1–2 μm at room temperature. The detectivity of fabricated prototypes of photodetectors is D* = 109 cm Hz1/2 W−1. The relaxation time of photoconductivity at a wavelength of 1.5 μm is less than 10 ns.

Photocarrier transport in 2D macroporous silicon structures

AbstractThe mechanisms of photocarrier transport through a barrier in the surface space-charge region (SCR) of 2D macroporous silicon structures have been studied at photon energies comparable to that of the silicon indirect band-to-band transition. It was found that the photoconductivity relaxation time was determined by the light modulation of barrier on the macropore surface; as a result, the relaxation itself obeyed the logarithmic law. The temperature dependence of the photoconductivity relaxation time was determined by the thermionic emission mechanism of the current transport in the SCR at temperatures T > 180 K, and by the tunnel current flow at T < 100 K, with temperature-independent tunnelling probability. The photo-emf was found to become saturated or reverse its sign to negative at temperatures below 130 K because of light absorption due to optical transitions via surface electronic states close to the silicon conduction band. In this case, the surface band bending increases due to the growth of a negative charge of the semiconductor surface. The equilibrium electrons in the bulk and photoexcited holes on the macropore surface recombine through the channel of multistage tunnel recombination between the conduction and valence bands.

Changes in the state of impurities and the corresponding force fields in silicon after high-temperature cyclic annealings

The influence of high-temperature annealings followed by cooling under different conditions on changes in the specific resistance and photoconductivity relaxation time in silicon and on the formation of the acceptor state of gold, when gold diffused from a layer vacuum-deposited on the surface, was studied in experiments including sequences of processes conducted using various temperatures, cooling conditions, and surface states. The experimental data are analyzed based on the concept of changes in the state of impurities present in silicon and in the corresponding dynamic and static force fields.

Microwave photoconductivity in nanocrystalline porous titanium oxide subjected to pulsed laser excitation

A contactless method of microwave photoconductivity was used to study the relaxation of photoexcited charge carriers in nanocrystalline porous titanium oxide. A decrease in the amplitude and a change in the microwave photoconductivity relaxation time is observed with reduction in the nanocrystal size, which is explained by localization of carriers due to dielectric confinement. Capture of holes by the traps on the TiO2 surface increases the amplitude and prolongs the relaxation time of the microwave photoconductivity.

Non-steady-state photoelectromotive force effect in photorefractive Sn_2P_2S_6 crystals

Results of experimental investigation of the non-steady-state photoelectromotive-force effect in ferroelectric photorefractive Sn2P2S6 crystals at a wavelength of 633 nm are reported. The dominating photoconductivity proved to be of the p type, with a diffusion length of photocarriers of ∼1 µm. This characteristic length proved to be shorter than the Debye-screening length evaluated as ∼3 µm in this crystal. The millisecond characteristic time of the space-charge grating formation was determined by the photoconductivity-relaxation time, which was found to be a little bit longer than the dielectric-relaxation time. Both of them dropped sublinearly with increasing average light intensity in the investigated region I0≃0.5–0.01 mW/mm2.

Effect of tungsten coating on the relaxation time of photoconductivity in annealed Czochralski-Grown Si

The effect of tungsten coating on the degradation of the relaxation time of photoconductivity in Czochralski Si upon annealing was studied. It was found that the concentration of process-induced defects in tungsten-coated Si annealed at 800 and 900°C exceeded that in uncoated samples. Moreover, the annealed tungsten-coated samples showed stronger temperature dependences of the electron capture cross section of these defects, which suggests that the defects produced in these samples differ from those in uncoated Si. No additional recombination channels were found in the tungsten-coated sample annealed at 1000°C as compared to the uncoated samples. The resistivity and conductivity type of tungsten-coated Si remained unchanged upon annealing.

Temperature dependence of the relaxation time of photoconductivity and non-steady-state photocurrents in photorefractive Bi12SiO20 grown in an oxygen-free atmosphere

We present temperature measurements of the non-steady-state photocurrent and photoconductivity relaxation time in photorefractive sillenite crystal Bi12SiO20 grown in an oxygen-free atmosphere. The obtained dependencies are explained using a conventional model of semiconductor crystal with shallow energy level. The activation energy of shallow level obtained from photocurrent measurements was found to be ET=(0.45±0.11) eV. Independent measurements of the temperature dependence of the relaxation time of photoconductivity are in reasonable agreement with this estimation and give the value for ET equal to (0.56±0.01) eV.

Time-resolved measurements of photoconductivity in cerium-doped photorefractive strontium–barium niobate using nanosecond light pulses

Time-resolved measurements of photoconductivity in photorefractive strontium–barium niobate crystals doped with cerium are performed. The crystals are illuminated with nanosecond light pulses of a Q-switched frequency-doubled Nd:YAG laser and an external electric field is applied to the samples. Light-induced drift current is amplified and detected with a fast oscilloscope. The relaxation time of photoconductivity yields the lifetime of electrons in the conduction band. The carrier lifetime decreases and the photoconductivity increases with increasing cerium concentration in the crystals. The observed dependences and previous measurements of integrated conductivity are explained considering the participation of shallow levels in the charge transport.