Minority Carrier Lifetime Mapping System for High Energy Physics Semiconductor Materials

EPIR Inc. proposes the implementation of a nondestructive characterization system for twodimensional mapping measurements of minority carrier lifetime in silicon, germanium, gallium arsenide, and other semiconductor materials used in high energy physics detectors. The system can accurately map the minority carrier lifetime in 6 inch high purity germanium wafers used in CCD applications and GaAs materials used for scintillation applications. Since these semiconductor devices are extremely sensitive to the contamination that can lead to charge trapping and dark current generation sites, minority carrier lifetime mapping measurements can provide a rapid technique to quantify the quality of the semiconductor substrates, for characterizing initial material quality as well as the impact of processing on the material properties. The minority carrier mapping system developed by EPIR utilizes an optical pumpprobe design. A fast pulse laser with wavelength longer than the bandgap of the material under test serves as excitation source and a long wavelengthinfrared LWIR light beam serves as a probe. The lasergenerated excess carriers lead to increased free carrier absorption related to the excess carrier concentration. Free carrier absorption decay is detected using fast LWIR HgCdTe detectors, specially designed and fabricated at EPIR during Phase 1. The carrier recombination lifetime is extracted based on established material and device physics models. During Phase 1, we finalized the system design, initiated overall system integration and performed preliminary minority lifetime measurements. The measured lifetime values are in good agreement with the ones obtained using the photoconductive decay method. During Phase II, all pumping and probing photons will be delivered and collected using optical fibers. Optical couplers will be employed to ensure flexibility, stability, userfriendliness, and maintenancefree operation. Highspeed analog to digital convertor boards will be used to digitalize the timedependent signal for lifetime extraction. Furthermore, we will fully enable the system’s automatic measurement capability and we will conduct mapping measurements using our two dimensional scanner built in Phase I. This will allow mapping and scanning measurements on areas up to 15 inch × 16 inch, sufficient for any large area semiconductor wafers. The system will be capable of accommodating four 6 inch or 150mmdiameter wafers in a single run. With further maturation of the technology, our proposed system can also be integrated with other semiconductor material evaluation or device fabrication lines, including the ones used for radiation/infrared detectors and light emission devices.