Bulk Carrier Lifetime Research Articles

Contactless Measurement of Carrier Lifetime in Silicon Thick Wafers

Contactless methods of measuring bulk carrier lifetime with a reliable accuracy have scarcely been reported thus far. A contactless method has been studied for Si wafers with thickness of up to 10 mm. This method makes use of the eddy current in the wafer induced by the magnetic fluxes (5 MHz) passing through a thick wafer. To confirm the reliability of this method, the decay of photoconductivity Δσ0 and injection level Δσ0/σ measured by this method are compared with those of the dc method. In the eddy current method, the bulk lifetime and diffusion coefficient are obtained as 1.88 ms and 12.4 cm2/s for an n-Si crystal, and 26 ms and 37.3 cm2/s for a p-Si crystal. The latter value, 26 ms, for bulk lifetime is almost in agreement with the value, 29 ms, obtained by the American Society for Testing and Materials (ASTM) dc method. The sensitivity of this method and the desired thickness of wafer for evaluating the bulk lifetime are discussed.

Photothermal rate-window spectrometry for noncontact bulk lifetime measurements in semiconductors

A new noncontact technique for the determination of excess carrier lifetimes in semiconductors is presented. The technique employs a square laser pulse (hν≥Eg) and measures the infrared photothermal radiometric response of the sample. By applying the photothermal rate-window concept, the excess photoexcited carrier bulk lifetime was measured with optimal signal-to-noise (S/N) ratio and simple, unambiguous interpretation from the maximum position of the rate-window signal. The technique has been applied to Au-, Fe-, and Cr-doped Czochralski silicon crystals. The experimental results from boxcar and lock-in rate-window methods were found to agree very well. The results are further mostly in agreement with those from the noncontact laser/microwave detection method.

Simulation of Time Decay for Photoluminescence Emitted from Silicon Crystals Excited by Short Laser Pulse

The time decay for photoluminescence (PL) emitted from silicon crystals excited by a short laser pulse has been simulated for the first time on the basis of two simplified approximations in order to obtain both bulk carrier lifetime (τb) and surface recombination velocity (S). One approximation is that the PL intensity is proportional to the normalized (averaged) excess carrier densities, whereas the other is that it is proportional to the recombination rate. We compared simulated results with experimental ones and obtained better coincidence in the case of the latter rather than the former approximation, which means that the Buczkowski et al.'s algorithm for obtaining both τb and S [J. Appl. Phys. 69 (1991) 6495] cannot be applied to the PL decay method. Therefore, a new algorithm has been developed in this present work.

Carrier Lifetime and Material Study in Intrinsically Gettered Silicon Wafers for High Precipitate Density

Cz-grown Si wafers, subjected to several ramped heat treatment procedures and having high oxygen preciptate density, are tested to determine the gettering efficiency. The methods used are preferential etching, infrared absorption spectroscopy and carrier lifetime measurement at high-injection levels. The wafer warpage is also measured. The investigations show that the number of precipitates is reduced and the width of precipitate free zone is increased in two narrow regions of the intial oxygen concentration. This effect is caused by a reduction of the small-sixe defect concentration peak in the precipitate profile. This phinomenon is attributed to an interaction between native point defects and interstitial oxygen. It is shown that the carrier lifetime in bulk decreases sublinearily with the bulk oxygen precipitate density in the range between 106 and 108 cm−2. the dependence of wafer warpage on oxygen density is increased for an initial oxygen density Oi ≧ 9 × 1017 cm−9. [Russian text Ignored.]

Measurement of bulk lifetime and surface recombination velocity by infrared absorption due to pulsed optical excitation

Knowledge of both carrier bulk lifetime τb and surface recombination velocity S are necessary to analyze and model electron devices. In this paper, the analytical results of Luke and Cheng [J. Appl. Phys. 61, 2282 (1987)] are applied to examine excess carrier decay due to pulsed Gaussian optical excitation to extract values of τb and S under low level injection. The nondestructive, contactless measurement technique utilizes a visible pulsed laser pump beam and a CW infrared laser probe beam. Values for τb and S are obtained for a number of different Si samples. The effect of varying the pump beam frequency is examined. The technique is simple to use in the laboratory and the parameter extraction from the measured data is straightforward.

Effective lifetime measurements in silicon-on-sapphire material by time-resolved reflectometry

We studied the effective lifetime of photo-injected carriers in silicon-on-sapphire films of different thicknesses by analysing the time-dependent reflectivity change sensed by an IR probe pulse at various delays after the application of a pump pulse of green light. Due to interference effects in the transparent film, both negative and positive reflectivity changes were observed. Using the exponential decay concept, the reflectivity results are compared with the middle-injection level lifetimes previously derived for the same films from photoconductivity measurements. It is shown that, before the onset of Auger recombination, the bulk carrier lifetime is constant over a wide range of excited carrier densities, between 10 16 cm −3 and 10 19 cm −3.

Damage of silicon in low-energy Ar discharge studied by photoinduced modulation absorption

The damage of silicon processed by low-energy (70 eV) Ar discharge was investigated by the technique of photoinduced modulation absorption. It was found that low-energy Ar discharge causes damage that enhances the surface recombination velocity at the processed surface and reduces the carrier diffusion constant in the bulk. However, no significant change of the bulk carrier lifetime was found.

A non-destructive measurement on the damage of silicon induced by a 30 eV Ar magnetron discharge

The damage of a Si substrate processed in a 30 eV Ar magnetron discharge has been revealed, for the first time, from the non-destructive measurement of carrier bulk lifetime, carrier diffusion constant and surface recombination velocity by the photoinduced modulation absorption technique. It was found that slight damage occurring at substrate enhances the surface recombination velocity only little, while severe damage in the bulk much reduces the carrier diffusion. However, no significant change of the carrier bulk lifetime due to bulk damage was observed.

Increased photogeneration in thin silicon concentrator solar cells

Recent progress in silicon concentrator solar cells has resulted in several designs capable of 25-percent efficiency with one group reporting 28 percent under 14 W/cm2of incident power at 25°C. It has been shown that further improvement is possible by restricting the sunlight acceptance angle of the cell. In this letter, a practical implementation which is equivalent in its effect is proposed which results in an increased utilization of weakly absorbed near-bandgap light. This increased absorption is obtained by placing the cells in a cavity with a small entrance aperture. An analysis is given based upon work on the acceptance angle enhancements by Campbell and Green. The design is expected to improve the efficiencies of existing solar cells to 30 percent. If used in conjunction with previously proposed cell improvements, the efficiencies will be improved towards 33 percent, very near the limit efficiency of 36 percent. This design also has the effect of decreasing the differences in performance between the leading candidate concentrator cell designs and diminishing the dependence of the efficiencies on the cell texturization and bulk carrier lifetimes.

Spatially resolved lifetime measurements in neutron-transmutation-doped polycrystalline silicon

Spatially resolved carrier lifetimes have been measured at the same grain boundary using a series of adjacent samples doped to different levels by neutron transmutation. The samples had dopant concentration levels between 1013 and 1017 atoms/cm3. The carrier lifetime was measured using a contactless microwave absorption technique to monitor the decay of photoconductivity which allowed us to perform either isothermal or isochronal annealing to remove the damage caused by irradiation. An optimum anneal of 30 min at 700 °C resulted in almost full recovery to the lifetime before irradiation. A short laser pulse of wavelength 900 nm was focused to a 0.3-mm-diam spot on the samples. Lifetimes of two different sets of carriers were measured, the bulk carrier lifetime at the center of a grain and the carrier lifetime at a grain boundary. This latter carrier lifetime involves both fast and slow recombination mechanisms. The slow decay mechanism is due to interface states at the grain boundary. The fast recombination lifetime was found to be of the same order as the bulk recombination lifetime (of the order of 1 μs) and the slow one was of the order of 1 ms. Doping levels up to 1016 atoms/cm3 show no significant effect on the measured bulk carrier lifetime.

Analytical integration of small-signal transport equations in an MIS structure at flat-band condition

The small-signal transport equations are solved analytically in a homogeneously doped MIS structure at flat-band condition. We can establish exact expressions of ac carrier density, electric field, and potential distributions in the MIS structure for any frequency. The plot of these expressions, in the frequency range from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> Hz, proves the existence of majority and minority modes. A complete impedance expression is sufficient to determine all equivalent circuit components of the MIS structure at flat-band condition. The proposed method is valid for any bulk lifetime of carriers and for any semiconductor thickness.

Surface recombination in photoconductors

A general expression for calculating the average number of photogenerated carriers in a photoconductor has been worked out. Based on this work, it is proposed that the effect of surface recombination is to influence the quantum efficiency of the photoconductor rather than the bulk-carrier lifetime. A modified quantum efficiency expression, which includes the surface recombination velocities of both surfaces of photoconductor, is reported.

Theory of deep level trap effects on generation-recombination noise in HgCdTe photoconductors

We present a theory of the effect of deep level centers on the generation-recombination (g-r) noise and responsivity of an intrinsic photoconductor. The deep level centers can influence the g-r noise and responsivity in three main ways: (i) they can shorten the bulk carrier lifetime by Shockley–Read–Hall recombination; (ii) for some values of the capture cross sections, deep level densities, and temperature, the deep levels can trap a significant fraction of the photogenerated minority carriers. This trapping reduces the effective minority carrier mobility and diffusivity and thus reduces the effect of carrier sweep out on both g-r noise and responsivity; (iii) the deep level centers add a new thermal noise source, which results from fluctuations between bound and free carriers. The strength of this new noise source decreases with decreasing temperature at a slower rate than band-to-band thermal g-r noise. Calculations have been performed for a X=0.21, n-type Hg1−xCdxTe photoconductor using the parameters of a commonly occurring deep level center in this material. We find that for typical operating conditions photoconductive detector performance begins to degrade as the deep level density begins to exceed 1016 cm−3.

The V-groove multijunction solar cell

A new type of silicon photovoltaic converter has been developed called the V-Groove Multijunction (VGMJ) solar cell. The VGMJ solar cell consists of an array of many individual diode elements connected in series to produce a high-voltage low-current output. All the elements of the cell are formed simultaneously from a single silicon wafer by V-groove etching. The results of detailed computer simulations predict a conversion efficiency in excess of 24 percent for this cell when it is operated in sunlight concentrated 100 or more times. The advantages of this cell over other silicon cells include the capability for greater than 20-percent conversion efficiency with only modest bulk carrier lifetimes, a higher open-circuit voltage, a very low series resistance, a simple one-mask fabrication procedure, and excellent environmental protection provided by a glass front surface.

Bulk carrier lifetime measurement from transient diffusion photocurrent in semiconductor diodes

The theoretical analysis of the transient diffusion photocurrent i D( t) created in the diode substrate, following weak carrier injection level by a homogeneous photoexcitation of short duration T, shows that the decay i D( t) can be used in two ways to obtain the substrate bulk carrier lifetime τ ν . Depending both on the decay times t considered and the values of the substrate parameters τ ν , D (minority carrier diffusion constant), W (thickness), s (back contact carrier recombination velocity), β (carrier velocity ratio at the p− n junction boundary), then either (1) τ ν may be deduced from the i D( t) decay time constant τ if τ is measured at long enough times t> t 1, or else (2) τ ν is given by the i D (t)× t product decay time constant θ if θ measured at short enough times t. The numerical illustration of the accuracy and validity conditions of each of the above two ways of exploiting the decay of i D( t) is given in the case of a P type silicon substrate with a resistivity of 0.2, 1 or 10 Ω cm, W from 50 to 500 μm, τ ν from less than 0.1 μs to more than 100 μs and various pairs of s (0 or ∞) and β (0 to ∞) values. In practice, the use of a transient photocurrent decay to obtain the diode substrate carrier lifetime also implies a previous analysis of factors such as the device response time constant and transient bias modulation, and the carrier photoexcitation level in the substrate. Experimental results obtained with N +P silicon-cells ( W = 460 μm and 200 μm) are given and discussed as an illustration of the above study.

Profiles of recombination and transport parameters in thin sos films

SOS film properties are investigated by galvanomagnetic effects which are dominated by either recombination or transport phenomena. The magnetodiode effect allows to obtain both the surface recombination velocities and the bulk lifetime profile : the decrease of the carrier bulk lifetime by an order of magnitude near the Sapphire accounts for the relatively moderate recombination velocity obtained at the Si/Sapphire interface (s ~ 20 to 30 ms-1). Bulk Hall measurements are used, besides the determination of mean drift and Hall mobilities, to study the transport inhomogeneities; a linear decrease of the mobility with the distance from the Si/SiO2 interface would lead to a relatively large electron mobility near this interface (|03BCn| ~ 0.1 m2 V-1 s-1).

Determination of the minority carrier bulk lifetime in rod-like germanium samples at temperatures between 5 and 77 K

The dependence of the effective minority carrier lifetime τeff on a longitudinal magnetic field is investigated up to 140 kG in rod-like germanium samples at temperatures between 5 and 20 K and at 77 K. The effective lifetime increases with increasing magnetic field and reaches a saturation value in the high field region. In accordance with the theoretical model of Lautz and Schulz this value is taken as the average bulk lifetime τv of electrons and holes in the samples. Theory and experimental results are in a good agreement at 77 K; from the comparison an estimation for the surface-recombination velocity s can be obtained. Im Temperaturbereich zwischen 5 und 20 K und bei 77 K wird die Abhängigkeit der effektiven Minoritätsträgerlebensdauer τeff von einem longitudinalen Magnetfeld bis zu 140 kG in stäbchenförmigen Germaniumproben untersucht. Die effektive Minoritäts-trägerlebensdauer wächst mit steigendem Magnetfeld an und erreicht einen Sättigungswert. Dieser Wert entspricht, in Übereinstimmung mit dem theoretischen Modell von Lautz und Schulz, einer mittleren Volumenlebensdauer τv der Elektronen und Löcher. Bei 77 K stimmen Theorie und experimentelle Ergebnisse gut überein aus dem Vergleich kann eine Abschätzung für die Oberflächenrekombinationsgeschwindigkeit s gefunden werden.

Determination of the bulk carrier lifetime in the low-doped region of a silicon power diode, by the method of open circuit voltage decay†

The open-circuit voltage decay (OCVD) of excess carriers injected into the low-doped region of a P+LN+ diode during heavy forward bias, can give a direct measure of the effective carrier lifetime in this region. The complete transient solutions for the excess carrier density valid both for d.c. forward bias current flow and for the OCVD after abrupt cessation of the current, show that for a variety of boundary conditions—zero or finite recombination in the end regions (Davies 1963, Schlangenotto and Gerlach 1972), infinite surface recombination velocity at an ohmic contact (Choo and Mazur 1970)—the OCVD after an initial very rapid transient settles down to a simple decay, governed essentially by the bulk lifetime τB of the L-region or an effective lifetime τc respectively. The diodes investigated had π- or v-type L-regiona with their P—N junctions made by diffusion and their L—H junctions by alloying. Apart from an extremly rapid initial transient, the OCVD measured was linear and showed two regions of di...

Single Crystal Photoconductive Detectors in Lead Selenide

Near intrinsic single crystals of grown from the vapor have been used to construct photoconductive detectors for operation at room temperature. Fabrication of the detectors depends on the use of a newly developed chemical etch to reduce the specimens to thicknesses of a few microns, and to provide surfaces with low carrier recombination velocity. The highest detectivity, , achieved was ; Various methods of estimating bulk carrier lifetimes have been used; the value for the best crystal measured was in excess of 4 μsec. The surface recombination velocity of the best etched surface was below 80 cm sec−1. Similar etches can be used for and .

The Effect of Heat Treatment on the Bulk Lifetime of Excess Charge Carriers in Silicon †

In a recent paper G. Bemski (1956) described the decrease of the bulk lifetime of excess charge carriers in silicon found when silicon samples were quenched from temperatures lower than 550°c to room temperature, and the complete or partial restoration of the original lifetime when the samples were annealed in the same range of temperatures. During recent months we have studied the effects of heat treatment of silicon in the range between room temperature and 1250°c. We found that bulk lifetime both in p- and n-type crystals made by the floating zone method and of n-type crystals made by the Czochralski technique might be increased by annealing at temperatures between about 300°c and 700°c. Annealing at temperatures higher than about 700°C decreased lifetime both in p- and n-type crystals. In ail cases the samples were relatively slowly cooled. The mechanism of these changes in lifetime is not well understood.