Minority Carrier Lifetime Measurements Research Articles

Electrical Properties of Silver Impurities and their Annealing Behaviour in p-Type Fz Silicon

The electrical activity of silver as well as its annealing properties in 10 Ω cm p-type Fz silicon substrate are studied by means of the four-point probe and minority carrier lifetime measurements. Silver atom concentration in the range 10 14 to 10 15 cm -3 consistently showed a donor type behaviour in the material and its presence led to a reduction of up to two orders of magnitude in the lifetime of minority carriers by the formation of deep-level traps. Isochronal annealing of silver contaminated specimens showed some gettering of the Ag impurities with resulting temperature dependent changes in the resistivity as well as the minority carrier lifetime values. Analysis of our results shows that a large fraction of the silver impurity atoms present forms the deep level defects and both the deep- and donor-levels appear to originate from the same source.

C+-energy-dependent residual ion damage in GaAs:C grown by the low-energy ion-beam doping method

The effects of residual ion damage in low-energy (30 eV–30 keV) C+-doped GaAs were investigated with regard to the electrical and optical activation of C as a function of C+ ion acceleration energy EC+. Systematic variation of EC+ demonstrated that, in the energy range of EC+<170 eV, the net hole concentration (|NA−ND|) slightly increases as EC+ increases and the highest |NA−ND| was obtained at EC+=170 eV under the constant C+ ion-beam current density. For EC+≳170 eV, an increase in EC+ gave rise to an abrupt decrease of |NA−ND| down to two orders of magnitude smaller than that obtained at EC+=170 eV. In low-temperature (2 K) photoluminescence spectra for as-grown samples with EC+=240 and 350 eV, a novel emission ascribable to residual ion damage was observed instead of an essential acceptor–acceptor emission of [g−g]β. However, subsequent annealing at 850 °C made this novel emission disappear and the proper [g−g]β emission was merely observed. An activation process observed for EC+=5, 10, and 30 keV samples was very similar to that by high-energy ion implantation, indicating low activation rate of 10%–15%. Minority-carrier lifetime measurements using scanning tunneling microscope stimulated time-resolved luminescence demonstrated the presence of residual ion damage in as-grown samples at EC+=240 and 350 eV and annealed ones at EC+=5, 10, and 30 keV while no ion damage was observed in as-grown sample at EC+=30 eV. The incorporation and activation behaviors of C atoms that take the form of low-energy ions were found to be considerably affected by changes in ion–surface interaction with increasing EC+ and by the presence of residual ion damage in the layer.

Identification of Cr in p-type silicon using the minority carrier lifetime measurement by the surface photovoltage method

The effect of CrB pair dissociation on the minority carrier lifetime in p-type silicon is studied using the surface photovoltage (SPV) and deep level transient spectroscopy (DLTS) methods. Experiments were conducted using Cr doped samples (0.5×1012–5×1012 cm−3) with resistivity ranging from 1 to 50 Ω cm. The minority carrier lifetime, increased after the dissociation of CrB by a factor ranging, depending upon resistivity, approximately from 2.0 to 10. This is in contrast to a tenfold reduction observed in the minority carrier lifetime following the dissociation of FeB pairs. The resistivity dependence of the minority carrier lifetime in Cr doped sample can be successfully predicted according to the Shockley–Reed–Hall (SRH) model. It is demonstrated that the defects with energy levels of Ev+0.27 eV and Ec −0.22 eV are the main recombination centers for CrB and Cri, respectively.

Noncontact minority carrier lifetime measurement of Si and SiGe epilayers prepared by ultrahigh vacuum electron cyclotron resonance chemical vapor deposition

A 70 MHz inductively coupled rf bridge probe is used to measure the minority carrier lifetime of the Si and SiGe epilayers grown by ultrahigh vacuum electron cyclotron resonance chemical vapor deposition (UHV-ECRCVD) at temperatures below 560 °C. Proper surface treatments before HF immersion are required for the accurate measurement of the bulk minority carrier lifetime. The effects of the process parameters such as the substrate dc bias, the distance of the ECR layer from the substrate, and the substrate temperature, including in situ surface cleaning, on the minority carrier lifetime of the Si and SiGe epilayers are examined by the rf bridge probe. It is confirmed that the rf bridge probe can monitor the epitaxial quality of low temperature Si and SiGe epilayers, making it an indispensable tool for the high quality device fabrication with Si and SiGe epitaxial layers grown by low temperature UHV-ECRCVD.

Sensitivity analysis for the determination of recombination parameters in Si wafers using harmonic carrier generation

The sensitivity of a contactless measurement procedure to determine the minority-carrier recombination parameters in Si wafers is analyzed. The measurement is based on the harmonic generation of excess carriers, whose time dependence is measured either by μ-wave reflection or free-carrier absorption. Both the measurement equipment and the model used to extract the surface recombination velocity and the bulk carrier lifetime are introduced and the sensitivity of the parameter fit procedure is examined. The results of this uncertainty analysis are applied to typical experimental situations, e.g., bulk minority carrier lifetime measurement of a monocrystalline wafer with equal surfaces or a bulk minority carrier lifetime mapping done on an as-grown multicrystalline Si wafer before it obtained saw damage removal. The method proves to be very useful, if the interpretation of the results is carefully done and an appropriate experiment is chosen to produce the best possible results.

Temperature-dependent minority-carrier lifetime measurements of red AlGaAs light emitting diodes

Electroluminescent decay and internal quantum efficiency measurements are made as a function of temperature on two double heterostructure AIGaAs light emitting diodes (LEDs) that emit in the visible (red) portion of the spectrum. The electroluminescent lifetimes increase by more than a factor of ten and the internal quantum efficiency falls by a factor of three as the temperature is raised from 90 to 400K. By analyzing the data with a model that accounts for the transfer with increasing temperature of the minority-carrier electrons from the direct-gap to the indirect-gap minima in the p-type active layer of these near-crossover LEDs, values for the radiative and nonradiative lifetimes as a function of temperature are obtained. A fit to the radiative-lifetime data results in an estimate of 1.3 × 10−10 cm3s−1 for the room-temperature radiative recombination coefficient of Al0.39Ga0.61As, which is very similar to values reported for GaAs. The nonradiative lifetimes are found to be nearly independent of temperature from 220 to 400K and provide upper limits of 940 and 1250 cms−1 for the interface recombination velocities of the two samples. These values are roughly an order of magnitude lower than any previously reported values for high-Al-content (x > 0.3) AlxGa1−xAs heterostructures.

Monitoring and Optimization of Silicon Surface Quality

We have used contactless photoconductance decay measurements to monitor cleaning processes, surface contamination, and surface roughness. Changes in surface chemistry are evidenced by a degradation (increase) in the surface recombination velocity (decrease in measured decay time). We have applied the tool to monitor cleaning effectiveness, surface cleanliness, and roughness during cell processing. Iodine in methanol achieves a superior passivation of Si than H in 48%. The measured minority carrier lifetime in this solution is higher than in 48% . We illustrate that I‐terminated silicon surfaces are more stable than H‐terminated surfaces. We confirm that deionized water is responsible for the roughening of silicon surfaces. We show that is a superior alternate clean to dilute .

Recombination lifetime measurements in AlGaAs/GaAs quantum well structures

We report the results of photoluminescence and minority carrier lifetime measurements on a set of lightly doped AlGaAs/GaAs quantum well structures grown by solid source molecular beam epitaxy. Growth temperatures were varied between 600 and 700 degrees C and the arsenic supplied in dimer or tetramer form. The lifetimes show a marked dependence on growth temperature when As4 is used, peaking sharply in the region of 675 degrees C, but are sensibly independent of growth temperature when using As2. They are nearly an order of magnitude less than the radiative lifetimes expected for the doping levels used, so we interpret our results in terms of Shockley-Read recombination via deep centres in the barrier material. Lifetimes measured on n- and p-type samples are closely similar, which can be explained in terms of a modified version of the standard Shockley-Read process, appropriate to the pseudo two-dimensional structures of interest here, provided the recombination centre is acceptor-like. We propose that it is related to oxygen incorporated in the Al-containing barrier regions.

Nondestructive measurement of minority-carrier lifetime and surface/interface recombination velocity in compound semiconductors

A new optical method is presented for measurement of the bulk lifetime τ and surface recombination velocity S in semiconductors. The method is based on measurement of the photoluminescence decay as a function of the self-absorption under different excitation conditions. Closed-form analytical expressions, accounting for the self-absorption, are derived for the photoluminescence decay for a semi-infinite sample with an arbitrary value of S. A comprehensive method based on the analytical expressions is devised for the nondestructive extraction of the parameters in bulk wafers. The technique is particularly suitable for characterization of the surface passivation of compound semiconductors with a direct band gap, and is demonstrated on a GaAs sample. Application of the technique for characterization of indirect band gap semiconductors is also discussed.

Simplified AC Photovoltaic Measurement of Minority Carrier Lifetime in Czochralski-Grown Silicon Wafers Having Ring-Distributed Stacking Faults

A simplified ac photovoltaic method is proposed for rapid measurement of minority carrier lifetime. It is used for lifetime distribution measurement in a Czochralski-grown silicon wafer having ring-distributed stacking faults. A ring-distributed short-lifetime region was observed which corresponded to the stacking fault ring detected by X-ray topography. In addition, a ring-shaped region where the lifetime is longer than 100 µs was observed for the first time outside the short-lifetime region. This long-lifetime region could not be detected by X-ray topography.

Lifetime measurements on silicon-on-insulator wafers

We report noncontact measurements of the effective minority carrier lifetime in the superficial silicon layer of silicon-on-insulator wafers. The carriers are excited by a pulse of short-wavelength photons (λ≤350 nm), all of which are absorbed in the first 500 Å of the silicon layer. The carriers are detected by the change in microwave reflectance in a resonant circuit to which the wafer is coupled. The results obtained vary from ∼3 μs for a three year old separation by implanted oxygen (SIMOX) wafer to ∼25 μs for current vendor SIMOX and bond-etchback samples. The variation in free surface recombination velocity is eliminated by HF passivating the samples prior to measurement.

Application of a high density inductively coupled plasma reactor to polysilicon etching

An inductively coupled radio-frequency (13.56 MHz) plasma reactor has been applied to polysilicon etching for complementary metal–oxide semiconductor (CMOS) gate fabrication. The system comprises a powered flat coil situated above an upper insulating plate in the reaction chamber. It is found, using Langmuir probe measurements, to be capable of generating a high density plasma (Ni≳1011 cm−3) at pressures of ∼5 mTorr. The intrinsic bias of the system is low and a capacitively coupled substrate bias from an additional 13.56 MHz generator allows independent control of ion energy. The system is capable of etching polysilicon at a high rate (≳3000 Å min−1) in a Cl2 discharge with good selectivity to oxide. Etch results are discussed along with damage studies based on thermal wave, minority carrier lifetime and MOS gate oxide breakdown measurements.

Effect of p- n coupling on reverse recovery transient of a homojunction solar cell

It has been observed that when a p- n junction diode is switched rapidly from forward to reverse direction, an anomalously large current, which often destroys the diode, flows for a short time. Experimental and theoretical investigations have shown that the large current surge is a consequence of the finite decay time of minority carriers stored during forward bias. In the present paper, an attempt has been made to develop a theory of the constant current phase of reverse recovery transient, to include the p- n coupling effect in a homojunction solar cell for measurement of minority carrier life-time.

Minority carrier lifetime and diffusion length in HgTe/CdTe superlattices by molecular beam epitaxy

Transient minority carrier lifetime and steady state diffusion length measurements have been performed on n-type HgTe/CdTe superlattices grown on CdZnTe(100) substrates. The n-type (100) superlattice sample shows a lifetime of 90–100 ns at 77–160 K which then decreases with increasing temperature down to 50 ns near 300 K. p-type HgTe/CdTe superlattices show normal behavior for temperature-dependent lifetimes. The measured lifetime is approximately 20 ns at 77 K and increases continuously to 85 ns at 300 K. An independent measurement of minority carrier diffusion length in a p-type HgTe/CdTe superlattice has led to an excellent agreement with a transient lifetime value of 66 ns at 200 K. The minority carrier diffusion length in a direction parallel to the interfaces indicates that the lateral transport properties are not much different from the transport properties of bulk HgCdTe.

Theory of Constant Current Phase of Reverse Recovery Transient for Measurement of Minority Carrier Lifetime in a Homojunction P+N Solar Cell

It has been observed that when a p+-n junction diode is switched rapidly from forward to reverse direction, anamalously large current, which often destroys the diode, flows for short time. Experimental and theoretical investigations have shown that the large current surge is a consequence of the finite decay time of minority carriers stored during forward bias. Analysis of this result has stimulated development of devices such as charge storage diodes and some special types of varactor diode for frequency multiplication. In the present paper, an attempt has been made to develope theory of the constant current phase of reverse recovery transient, to include the p-n coupling effect in an homojunction solar cell for measurement of minority carrier lifetime.

Measurement of minority-carrier lifetime by time-resolved photoluminescence

Much of the current compound semiconductor device research is based on minority-carrier devices such as heterojunction bipolar transistors and diode lasers. The improvement of minority-carrier-parameters is the focal point of much ongoing materials research. The minority-carrier lifetimes of III-V compound semiconductors are most easily characterized by time-resolved photoluminescence. This is a quick and contactless technique which directly measures the excess minority-carrier carrier density. By using a focused laser beam as the excitation source, the required sample area may be very small. In special diagnostic structures which utilize a confinement or passivating layer, the interface recombination velocity can be determined. Here I will describe the measurement theory and measurement techniques which are used in our laboratory. Recent developments in the III-V materials technology will be reviewed.

Very high gain in carbon-doped base heterojunction bipolar transistor grown by chemical beam epitaxy

Current gains close to 200 are reported in carbon-doped base heterojunction bipolar transistors grown by chemical beam epitaxy, for base sheet resistance of 460Ω/□. This result was made possible by the excellent quality of the carbon-doped GaAs layers, as demonstrated by the hole mobility and the minority carrier lifetime measurements.

Noncontact minority-carrier lifetime measurement at elevated temperatures for metal-doped Czochralski silicon crystals

Minority-carrier recombination lifetimes have been extensively measured with a noncontact laser/microwave method for metal-doped p-type Czochralski silicon crystals in the temperature range from room temperature to 250 °C. The contrasting behavior of the lifetime as a function of temperature is shown for Fe- and Cr-doped silicon crystals. Iron doping greatly shortens the lifetime in the entire temperature range. Although Cr doping also greatly shortens at room temperature, the degradation effect completely disappears at elevated temperatures. Doping the Na, Ni, Cu, and W shows little effect on lifetime; Na doping results in a rather higher lifetime compared with that of undoped silicon. Moreover, lifetime measurement as a function of holding time at an elevated temperature clearly distinguishes uncontaminated silicon from metal-doped silicon.

Measurement of minority carrier lifetime of solar cells using surface voltage and current transients

The voltage decay transients along the emitter surface of the p- n junction solar cell under forward current-induced and nonuniform illuminated cases have been studied experimentally. The open-circuit voltage decay transients in these cases have also been recorded. It is found that the surface voltage decays exponentially with time and is independent of depletion-region effects. The open-circuit voltage decay behaviour, under such situations, shows that the V oc initially increases with time, and after attaining a maximum value, decays usually. The initial increase, in both current- and photo-induced cases, is attributed to the surface voltage. The lifetime measured by surface voltage decay method is more accurate than that measured by open-circuit voltage decay. The open-circuit voltage decay method gives a better estimation of the minority carrier lifetime provided the variations in diode quality factor are considered while analysing the cell using distributed parameters. The lifetime, measured by the surface voltage decay, has been compared with the reverse recovery method and is approximately the same in both cases. However, it is concluded that for the evaluation of minority carrier lifetime, the surface voltage decay in the current-induced case is more convenient and accurate compared to any other methods reported earlier.

Rapid contactless electrical characterization of process-induced damage in HgCdTe

Microwave reflectance measurements of minority carrier lifetime and eddy-current measurements of conductivity are proposed as rapid, contactless electrical characterization methods for the detection and tracking of process-induced damage in HgCdTe. Changes induced in HgCdTe by ion milling are used to illustrate the utility of the proposed methods.