Resonant-coupled Photoconductive Decay Research Articles

Degradation of photovoltaic devices at high concentration by space charge limited currents

High-injection mobility reduction is examined by theory, modeling, and experimental data acquired by resonance-coupled photoconductive decay (RCPCD). The ambipolar mobility is shown to reduce to zero when the constituent injection-dependent carrier mobilities are taken into account. Modeling of the photoconductivity incorporating the transient, injection-dependent, ambipolar mobility confirms experimental reduction in signal at increasing carrier-generation rates. The onset of the reduction of mobility occurs at approximately 10 times the background carrier density; thus devices that utilize lightly doped materials are susceptible to anomalous injection-based behavior. For photovoltaic applications, high-injection device-performance degradation would result from mobility reduction due to reduced diffusion length.

Resonant coupling for contactless measurement of carrier lifetime

Contactless measurements of the recombination lifetime have become the standard in photovoltaic and electronic materials research, as well as in the associated industries. Fast evaluation is a critical need as well as the need to keep the material under test from becoming contaminated by the measurement apparatus. A technique was developed for measuring the transport properties and recombination kinetics of semiconductors and photoconductors [R. K. Ahrenkiel and S. W. Johnston, Mater. Sci. Eng., B 102, 161172 (2003)]. The primary application of this technique is the measurement of carrier lifetime but carrier mobility can also be linked to the data. The author has named the technique resonance-coupled photoconductive decay and it was developed [R. Ahrenkiel, U.S. patent 5,929,652 (27 July 1999); R. Ahrenkiel and S. Johnston, U.S. patent 6,275,060 (14 August 2001); S. Johnston and R. Ahrenkiel, U.S. patent 6,369,603 (9 April 2002)] at the National Renewable Energy Laboratory. The technique provides the rapid measurement of recombination lifetimes that are of vital importance to electronic and photovoltaic materials. These measurements are also of value to a wide range of optoelectronic technologies. The operating frequencies of the measurements here are in the range of 420 to 430 MHz. The detection is based on the coupling of a high-Q resonant antenna to the sample. The operating frequency is chosen to be in the range of the resonant frequency so that the real and imaginary parts of the system impedance, Z, are changing rapidly. Here, these changes in Z about the resonance produce high sensitivity to changes in carrier concentration produced by photoexcitation.

Transient mobility in silicon as seen by a combination of free-carrier absorption and resonance-coupled photoconductive decay

The combination of the resonance-coupled photoconductive decay (RCPCD) apparatus and a pump-probe free carrier absorption experiment results in a method of viewing transient mobility. RCPCD uses an Nd:YAG laser operating at 1064 nm to pump the p-type silicon wafer, and a microwave coil antenna detects the transient excess-carrier concentration. The pump-probe experiment uses the same pump laser and a 10.6 μm CO2 laser with HgCdTe photodetector to measure the transient change in absorption. The change in conductivity detected by RCPCD is directly proportional to the excess-carrier concentration (Δn) and mobility (μ), whereas the pump-probe experiment has an inversely proportional relationship. By mathematically combining these signals at equivalent optical fluxes, a quantity proportional to the mobility emerges. The mobility is shown to vary both temporally and with respect to injection, countering the assumption that mobility is constant for photoconductive decay measurements. Theory and results are discussed within.

A new lifetime diagnostic system for photovoltaic materials

We have developed a new minority-carrier lifetime measurement apparatus for measuring the recombination lifetime in semiconductors. We describe the technique as transmission modulated photoconductive decay (TMPCD). This is a contactless, non-invasive technique that produces transient photoconductive lifetime data. The measurement procedure is very sensitive to small signals and has a superior time response for measurement of short carrier lifetimes. This technology has several advantages over resonant coupled photoconductive decay (RCPCD) and transient microwave reflection photoconductive decay (μPCD). The response time advantage provides a capability to measure very short lifetimes in thin film materials, such as nanocrystalline silicon films and nanowire composites. This is accomplished while maintaining a sensitivity that is at least comparable to RCPCD. The new technique has been successfully applied to silicon wafers, compound semiconductor thin films, nano-crystalline silicon films, and II–VI nanowires.

Photoconductive analysis of ion implantation damage and annealing in silicon wafers

This work describes the use of a new method of detecting residual damage centers, which are produced in ion-implanted silicon. Contactless resonant-coupled photoconductive decay (RCPCD) measures the minority-carrier lifetime in the unimplanted region of the wafer. The analysis and data show that the lifetime is dominated by recombination in the implanted volume. By varying the wavelength of the RCPCD excitation, one can determine that the dominant recombination occurs in the implanted region. The data show that the carrier lifetime increases by orders of magnitude as the recombination mechanism changes from defect to Auger recombination. The latter arises from high-density, electrically activated acceptors in p-type silicon after thermal annealing.

Lifetime analysis of silicon solar cells by microwave reflection

Microwave photoconductive decay ( μPCD) has become a standard technique for measuring the carrier lifetime of silicon used in solar cells. Here, we have used μPCD to examine the carrier lifetimes at common doping levels used in the base region of silicon photovoltaic devices. For the conductivity range used in the p-type base of n +–p structures, the microwave penetration depth is less than the wafer thickness. In this case, the reflectance–conductivity relationship is very nonlinear. We will show that quasi-steady-state photoconductivity (QSSPC) and resonance-coupled photoconductive decay (RCPCD) lifetime measurements track over a wide range of injection level, and generally agree at higher injection levels. Our μPCD data will be compared with the transient RCPCD data over the same range. The data from the latter agree at low-injection levels, but show serious disagreement at higher injection levels. The conclusion is that μPCD must be limited to low-injection levels in the doping range used for solar cells.

Relationship of Band-Edge Luminescence to Recombination Lifetime in Silicon Wafers

In recent years, intrinsic luminescence has been used as a method to characterize the recombination lifetime of crystalline silicon. The assumption is that the steady-state intrinsic photoluminescence at 1.09 eV (1.134 μm) can be related to the recombination lifetime. In this work, we measured the band-edge photoluminescence (PL) intensities of a number of single-crystal wafers. The resistivity in the wafer set ranges from 1 to 11,000 ohm-cm under constant excitation intensity. We then measured the PL spectra and recombination lifetimes of these wafer sets under a variety of conditions. The lifetime was measured using resonant-coupled photoconductive decay (RCPCD) in both air ambient and an iodine/methanol solution. The same procedure was used for the PL spectra. Plots of the measured lifetime versus the PL intensity showed weak correlation between the two quantities in air ambient. However, there is a positive relationship between the near-bandgap PL intensity and the recombination lifetime for passivated surfaces. Some fundamental physical reasons will be used to explain these results.

Microsecond minority carrier lifetimes in HWCVD-grown films and implications for thin film solar cells

We determine the minority carrier lifetimes of nearly intrinsic Si films 1.5–15 μm thick grown by HWCVD at 300 °C on Si (100) and large-grained polycrystalline templates formed by selective nucleation and solid-phase epitaxy (SNSPE) using resonant-coupled photoconductive decay (RCPCD). Although the microstructure of these films is mostly microcrystalline, minority carrier lifetimes for films on Si (100) range from 5.7 to 14.8 μs while those for films on SNSPE templates range from 5.9 to 19.3 μs. Residual nickel present in the SNSPE templates may contribute a recombination center but does not significantly decrease the lifetime of films grown on SNSPE templates, making the growth of epitaxial layers by HWCVD on SNSPE templates a viable design for thin-film photovoltaics.

Analysis of charge separation dynamics in a semiconductor junction

Charge separation has been experimentally observed in a number of junction devices over the last decade, but there has been little theoretical analysis of the phenomena that describes the detailed dynamics of the effect. In this work, we use computer simulations to assess the ability of time-resolved photoluminescence, resonant-coupled photoconductive decay, and other experimental techniques to characterize free-carrier recombination and charge separation after an ultrafast laser pulse excites carriers in a homojunction or a heterojunction. The results indicate the experimental conditions where charge separation is likely to dominate these measurements and several experimental signatures that can be used to distinguish charge separation from free-carrier recombination. Time-resolved photoluminescence and resonant-coupled photoconductive decay measurements on Si, GaAs/ GaxIn1˛xP, and other junctions confirm and illustrate the results.

Efficient removal of UDMH from dilute nitride MOCVD exhaust streams

Unsymmetrical dimethyl hydrazine (UDMH) (CH 3) 2N 2H 2 is often used in the deposition of dilute nitride semiconductors because it provides a source of nitrogen with a low thermal decomposition temperature (Temperature-dependent carrier lifetime in GaNAs using resonant-coupled photoconductive decay, NCPV Program Review Meeting, Lakewood, Colorado, 14–17 October, 2001). The problems with using this material, however, are its significant toxicity (0.01 ppm compared to ammonia's 25 ppm) and also the fact that it blocks the action of conventional dosed wet scrubbers sometimes used on nitride applications, resulting in diminished efficiency in removing arsine (the source of arsenic), and arsine being similarly toxic (TLV of 0.05 ppm). Efficient removal of UDMH, AsH 3 and hydrogen (which, though not toxic poses a potential safety hazard) by means of a combined thermal oxidation reaction and wet scrubber in series is described at input gas flow rates exceeding those typically encountered in practice. The detection technique employed was Fourier transform infra red spectroscopy (FTIR), and the calibration and resolution techniques will be described. For input UDMH flows of up to 445 sccm (i.e. 1.85×10 −2 mol/min), destructive reaction efficiencies (DREs) of >99.9% were demonstrated, corresponding to the background detection resolution of 0.4 ppm.

Interface recombination velocity measurement by a contactless microwave technique

The interface or surface recombination velocity is a critical and important, parameter in many device applications. In this work. we have developed and applied a contactless microwave technique. which in combination with a continuously tunable pulsed light source. is able to probe the excess carrier life-time in the surface and bulk regions of it semiconductor wafer. The technique is called resonant coupled photoconductive decay (RCPCD) and has been described by the authors in the literature. For strongly absorbed light, the initial (t = 0) decay time is a strong function of the absorption coefficient, α, as well as the bulk lifetime. The effective bulk lifetime is a well-known function of the two surfaces (interfaces) and the true bulk lifetime. The effective bulk lifetime is measured by using very weakly absorbed light, or by measuring the asymptotic decay rate of strongly absorbed light. The latter occurs after diffusion has produced a quasi-equilibrium condition in the wafer. For asymmetric surfaces (such as a wafer polished on one surface only), the measurement with strongly absorbed light is made at both wafer surfaces. We have solved simultaneuously three nonlinear equations and the solutions provide values for the three unknowns S 1 , S 2 and τ(bulk). Several examples of the technique will he demonstrated for silicon wafers.

Measurement of charge-separation potentials in GaAs1−xNx

The ternary alloy GaAs1−xNx is interesting as a semiconductor that can be grown epitaxially on GaAs. As is well known, the band gap can be reduced by as much as 0.4 eV by changing the nitrogen concentration from 0% to 3%. We measured the spectral response and photoconductive lifetime of the alloys as a function of temperature. In this work, the films were grown by gas-source molecular-beam epitaxy on semi-insulating GaAs substrates. All measurements were made using the contactless, resonant-coupled photoconductive decay method. Our data show that the spectral response of GaAs1−xNx alloys consists of photoconductive band tails that extend well into the infrared (beyond the nominal band gap). For example, the photoconductive band tails extend to about 1900 nm for GaAs0.967N0.033 at room temperature. The primary photoconductive decay times are in the range of 200–300 ns at room temperature. At temperatures below about 200 K, the decay rate decreases substantially. By plotting the decay rate values on an Arrhenius plot, we calculate activation energies of 70–80 meV for GaAs1−xNx samples with x=1.1%–3.3%. These energies represent potential barriers that inhibit recombination.

An advanced technique for measuring minority-carrier parameters and defect properties of semiconductors

Rapid and contactless defect characterization is a very desirable goal in most current technologies. We will describe the contactless, resonant-coupled photoconductive decay (RCPCD) technique, as applied to a variety of wafer and thin-film materials. Using this technique, we can measure recombination lifetime over many decades of injection level. We can also measure relative values of minority-carrier mobility and diffusion length. By scanning the excitation wavelength, we can measure spectral response and photoconductive excitation spectra. Deep-level impurities can also be detected by several variations of RCPCD. We will show the general versatility of this technique.

Analysis of Ion Implantation Damage in Silicon Wafers by a Contactless Microwave Diagnostic

AbstractRapid thermal annealing (RTA) of lattice damage created by heavy ion implantation damage is required to maintain the integrity of semiconductor material used for submicron-integrated circuit devices. A quick, efficient, and contactless diagnostic of the implantation damage is highly desirable in both research and production environments. A contactless measurement technique has been recently applied to this problem that uses a deeply penetrating low-frequency microwave probe frequency operating at 420 MHz. Here, we will demonstrate the use of this high frequency resonance-coupled photoconductive decay (RCPCD) technique, which, when combined with a tunable optical excitation source, enables us to map the radiation damage in boron and arsenic-implanted silicon wafers. We quantify the damage by mapping the minority-carrier lifetime as a function of optical penetration depth. In this work, we quickly and efficiently compared the effectiveness of various RTA processes by the RCPCD diagnostic.