PR Links Triax 320 Spectrograph Triax 550 Spectrograph Sources Single Channel Detectors

Semiconductor Characterisation by Photoreflectance Spectroscopy In the development of high speed semiconductor devices, photoreflectance can be employed as a non-invasive means of determining the electron mobility characteristics of material. Setup A HeNe pump laser pumps an area of the sample. The laser beam is modulated by a chopper. A monochromator selects a spectral bandpass of light from the source. This illumination beam is imaged onto the sample. A second monochromator scans the detection wavelength. The detector signal is processed via a lock-in amplifier that is referenced to the laser chopper frequency. The spectrometer control, acquisition, and data manipulation are performed by SpectraMax spectroscopic software. Measurement Principle The laser beam is absorbed in the semiconductor material, creating electron-hole pairs that alter the dielectric function. The chopper modulates this effect, and hence the material's reflectance coefficient R. The monochromatic illumination beam is focused on the modulated area of the sample and the reflected beam is recorded by the detector. The detection side monochromator acts as a narrow bandpass filter. A lock-in amplifier referenced to the laser chopper is used to extract the modulated signal. After acquisition the ratio of deltaR/R is calculated and plotted versus the reflected light wavelength. The sharp derivative-like features of the photoreflectance spectra correspond to the interband transitions of the material. Accurate information about bandgaps and broadening parameters may be extracted using line shape fitting. Franz-Keldysh oscillations can be used to determine internal fields and doping levels. Applications Determination of energy band structure Characterisation of quantum well depth Trap effect studies Strain analysis Alloy composition in ternary and quaternary compounds Coverage range options This technique allows very sensitive and non-destructive characterisation at room temperature. A wide range of materials. including silicon, III-V or II-IV compounds can be studied with optional detectors, filters and sources. Range (nm) Source Filter Detector 220-400 Xenon None Silicon 375-730 Xenon WG370 Silicon 505-990 Tungsten OG500 Silicon 1000-1690 Tungsten RG850 InGaAs