Apparatus and method to measure semiconductor optical absorption using microwave charge sensing
A time-resolved microwave reflectance apparatus comprises a pulsed or modulated optical source that irradiates a semiconductor sample with an excitation pump beam, a microwave oscillator that irradiates the sample with a continuous beam of microwaves, and a microwave detector that detects the microwaves reflected by the sample. Therefore, charge detection, rather than conventional absorption measurements (that detect the loss of photons), can be used to extract the absorption coefficient and band edge of a semiconductor material.
1. A method to measure semiconductor optical absorption using microwave charge sensing, comprising:
providing a time-resolved microwave reflectance apparatus, the apparatus comprising
a pulsed or modulated wavelength tunable optical source adapted to irradiate a semiconductor sample having an absorber layer with an optical excitation pump beam,
a microwave oscillator adapted to irradiate the semiconductor sample with a continuous beam of microwaves, and
a microwave detector adapted to detect microwaves reflected from the semiconductor sample;
irradiating the semiconductor sample with the optical excitation pump beam at a calibration wavelength at which the optical excitation pump beam is not transmitted through the absorber layer of the semiconductor sample;
measuring a peak response amplitude of a time decay of the microwaves reflected from the absorber layer of the semiconductor sample after irradiation of the semiconductor sample by the optical excitation pump beam at the calibration wavelength;
irradiating the semiconductor sample with the optical excitation pump beam at a measurement wavelength;
measuring a peak response amplitude of a time decay of the microwaves reflected from the absorber layer of the semiconductor sample after irradiation of the semiconductor sample by the optical excitation pump beam at the measurement wavelength; and
comparing the peak response amplitude at the measurement wavelength to the peak response amplitude at the calibration wavelength to obtain an absorption coefficient at the measurement wavelength.
2. The method of claim 1 , wherein the wavelength tunable optical source is tunable in a range less than 1-micron wavelength.
3. The method of claim 1 , wherein the wavelength tunable optical source is tunable in a range between 1 and 100 microns wavelength.
4. The method of claim 3 , wherein the wavelength tunable optical source is tunable in a range from 2 to 12 microns wavelength.
5. The method of claim 1 , wherein the wavelength tunable optical source is tunable in a range greater than 100 microns wavelength.
6. The method of claim 1 , wherein the method further comprises repeating the irradiating and measuring steps for a plurality of measurement wavelengths of the optical excitation pump beam of the wavelength tunable optical source to obtain an absorption coefficient as a function of wavelength.
7. The method of claim 6 , wherein the plurality of wavelengths is tuned over the band gap energy of the semiconductor sample.
8. The method of claim 1 , wherein the pulsed wavelength tunable optical source comprises a pulsed laser or light-emitting diode.
9. The method of claim 1 , wherein the modulated wavelength tunable optical source comprises a modulated continuous wave laser, light-emitting diode, or filtered blackbody source.