Li-Jen Cheng

Analysis of the interaction of a laser pulse with a silicon wafer: Determination of bulk lifetime and surface recombination velocity

The decay of excess minority carriers produced in a silicon wafer of thickness d by a laser pulse is analyzed. A comprehensive theory based on this analysis is presented for the determination of bulk lifetime τb and surface recombination velocity S. It is shown that, starting with an exponential spatial profile, the carrier profile assumes a spatially symmetrical form after approximately one time constant of the fundamental mode of decay. Expressions for the spatial average of the carrier density as a function of time are derived for three temporal laser pulse shapes: impulse, square, and Gaussian. Particular attention is paid to the time constants of the fundamental and higher modes of decay. The ratios of the time constants of the higher modes to the fundamental mode, as well as the time constant of the fundamental mode, are presented over wide ranges of values of S and d. Two complementary methods applicable for values of the product Sd≳40 cm2/s are proposed to determine τB and S, without having to inv...

Quantification of the effects of generation volume, surface recombination velocity, and diffusion length on the electron‐beam‐induced current and its derivative: Determination of diffusion lengths in the low micron and submicron ranges

A systematic and quantitative analysis is carried out to investigate the effects of the shape (point, cube, Gaussian) and size of the generation volume, the surface recombination velocity, and the diffusion length on the electron‐beam‐induced current (EBIC) and its derivative (DEBIC). Thick homogeneously doped samples exhibiting diffusion lengths in the low micron and submicron range are considered. The results are presented in computed EBIC curves as a function of scanning distance and of the ratio true diffusion length/effective diffusion length. Using these curves, we show (1) a simple and yet rigorous method for the determination of the true diffusion length, taking into consideration all of the factors cited above, (2) a method for the rapid determination of the surface recombination velocity, (3) the condition under which the source shape becomes insignificant, and (4) a new value for the lower limit of the diffusion length which can be determined by the EBIC technique.

Optical correlators with fast updating speed using photorefractive semiconductor materials.

An updatable optical correlator which uses a photorefractive compound semiconductor to generate real-time matched filters is proposed. Using compound semiconductors offers high speed with low optical input intensities. Here we discuss issues affecting the performance of this correlator. This includes an analysis of the Bragg diffraction and a discussion of the speed and power considerations of these materials. Experimental results obtained using photorefractive GaAs are also presented.

A Chemical/Microwave Technique for the Measurement of Bulk Minority Carrier Lifetime in Silicon Wafers

A chemical/microwave technique for the measurement of bulk minority carrier lifetime in silicon wafers is described. This method consists of a wet chemical treatment (surface cleaning, oxidation in solution, and measurement in HF solution) to passivate the silicon surfaces, a laser diode array for carrier excitation, and a microwave bridge measuring system which is more sensitive than the microwave systems used previously for lifetime measurement. Representative experimental data are presented to demonstrate this technique. The result reveals that this method is useful for the determination of bulk lifetime of commercial silicon wafers.

Real-time VanderLugt optical correlator that uses photorefractive GaAs

A real-time GaAs and liquid-crystal television- (LCTV-) based optical correlator is demonstrated. The speed of this correlator (video rate) is limited by the LCTVs; with faster spatial light modulators, the potential speed of a GaAs-based correlator may be 1000 frames/s or higher. Comparisons are made between VanderLugt and joint transform configurations and between degenerate and nondegenerate four-wave mixings. The edge-enhancement effect and the Bragg diffraction effect are also discussed.

Photorefractive gain in GaAs under a dc electric field

We report the first observation of a photorefractive gain coefficient as high as 2.6 cm−1 in the undoped liquid‐encapsulated Czochralski‐grown GaAs crystals at 1.06 μm under a dc electric field of 13 kV/cm without using the moving grating technique. The absorption coefficient of the crystals used is 1.3 cm−1, showing that a net gain has been achieved. This measured gain coefficient is close to the predicted theoretical value.

Image transfer in photorefractive GaAs

Image transfer from one beam to the other using counterpropagation beam coupling in GaAs was demonstrated. Good image quality was achieved. The results also reveal that local birefringence due to the residual stress/strain field in the crystal can degrade the image quality.

Characterization of the heavily doped emitter and junction regions of silicon solar cells using an electron beam

The electron‐beam‐induced‐current (EBIC) technique is applied to the investigation of heavily doped emitter and junction regions of silicon solar cells via the beam specimen configuration in which the junction is parallel to the surface and the beam is perpendicular to it. It is demonstrated that this technique is very well suited to the investigation of the emitter and junction regions. Even though the experimental EBIC data are collected under three‐dimensional conditions, it is shown analytically with the help of two numerical examples that the solutions obtained with one‐dimensional numerical modeling are adequate. EBIC data for bare and oxide‐covered emitter surfaces are measured and compared with theory. Good agreement is obtained when a cell quality factor of 0.89 is assumed, thus suggesting that ∼11% of the collected carriers recombined in the cell depletion region. This result is similar to Possin’s finding that there was evidence for ∼10% of recombination in the junctions of some of his silicon ...

Index grating lifetime in photorefractive GaAs

The index grating lifetime in liquid encapsulated Czochralski-grown undoped semi-insulating GaAs was measured using a beam coupling technique. The largest lifetime measured was ~8 s under a read beam intensity of 0.7 mW/cm(2) with the grating periodicity being 0.63 microm. The measured value decreases to milliseconds as the read beam intensity and the grating periodicity increase to ~10 mW/cm(2) and 4 microm, respectively. This range of grating lifetime in this material is adequate for its use in real-time spatial light modulators, reconfigurable beam steering devices, and dynamic memory elements for optical computing. In addition, the results suggest that, the lifetime is sensitive to residual imperfections in the crystal.

Electron-beam induced current characterization of back-surface field solar cells using a chopped scanning electron microscope beam

A chopped electron beam induced current (EBIC) technique for the characterization of back‐surface field (BSF) solar cells is presented. It is shown that the effective recombination velocity of the low‐high junction forming the back‐surface field of BSF cells, in addition to the diffusion length and the surface recombination velocity of the surface perpendicular to both the p‐n and low‐high junctions, can be determined from the data provided by a single EBIC scan. The method for doing so is described and illustrated. Certain experimental considerations taken to enhance the quality of the EBIC data are also discussed.