Method and apparatus for determining a material's characteristics by photoreflectance

An apparatus for determining characteristics of materials by photoreflectance, comprising monochromatic light source means, means for directing the monochromatic light onto a sample to be examined, pump beam means for directing a beam of energy onto the sample including modulation means for modulating said beam, means for directing at least a part of the non-absorbed monochromatic light and of the non-absorbed modulated beam from the sample onto a detector means operable to produce a d.c. signal and an a.c. signal in its outputs, computer means, means for applying the d.c. signal from the detector means to an input of the computer means including an A/D converter, a two-phases lock-in amplifier means receiving at its input the a.c. signal from the detector means and operatively connected with its output to another input of the computer means, further means for controlling the modulation frequency of said modulation means by said computer means, and control means for keeping substantially constant the operating conditions in a given experiment by maintaining the d.c. signal constant at a predetermined value including variable means for varying the light intensity of the monochromatic light impinging on the sample and actuating means for controlling the variable means by an output from the computer means which is operable to cause the actuating means and therewith the variable means to move so as to achieve said predetermined value.

A method for determining characteristics of a material, especially of semiconductors, semiconductor heterostructures and semiconductor interfaces by photoreflectance, comprising the steps of

• a) directing a probe beam of monochromatic light onto a material sample whose characteristics are to be determined,
• b) electromodulating the sample by directing onto the same a modulated pump beam from a pump source,
• c) collecting the light reflected from or transmitted by the sample in a detector which produces a d.c. signal and an a.c. signal which contains (i) a spurious signal caused by diffuse reflected light from the pump source and/or photoluminescence produced by the pump light reaching the detector and (ii) the true signal,
• d) normalizing the procedure by subtracting the spurious signal from the true signal and
• e) transforming the data to improve the signal-to-noise ratio.

A method for determining characteristics of a material, especially of semiconductors, semiconductor heterostructures and semiconductor interfaces by photoreflectance, comprising the steps of

• a) directing a probe beam of monochromatic light onto a material sample whose characteristics are to be determined,
• b) electromodulating the sample by directing onto the same a modulated pump beam from a pump source,
• c) collecting the light reflected from or transmitted by the sample in a detector which produces a d.c. signal and an a.c. signal which contains (i) a spurious signal caused by diffuse reflected light from the pump source and/or photoluminescence produced by the pump light reaching the detector and (ii) the true signal,
• d) normalizing the procedure by subtracting the spurious signal from the true signal.
• wherein step (d) is accomplished by setting the intensity of monochromatic light to a minimum at a given wavelength of the monochromatic light, analyzing the then-obtained spurious signal to determine the absolute phase of the pump beam appearing at the input of a lock-in amplifier receiving the output of the detector, setting the lock-in amplifier to the correct phase with respect to the optical pump and offsetting the spurious signal by changing the zero setting of the lock-in amplifier.

A method for determining growth condition of semiconductor materials, comprising the steps of

• a) directing a probe beam of monochromatic light onto a material sample whose characteristics are to be determined,
• b) contactlessly electromodulating the sample by directing onto the same a modulated pump beam from a pump source to therewith modulate the built-in electric field,
• c) collecting the light reflected from or transmitted by the sample with a detector which produces a d.c. signal and an a.c. signal which contains (i) a spurious signal caused by diffuse reflected light from the pump source and/or photoluminescence produced by the pump light reaching the detector and (ii) the true signal,
• d) carrying out a normalized procedure to recover the true signal by subtracting the spurious signal from the true signal, and
• e) analyzing the energy band gaps by measuring the position of a respective energy gap as determined under growth condition to obtain information regarding various parameters including temperature, alloy composition and stress.

A method for obtaining information about trap times, especially at interfaces of semiconductors and semiconductor heterostructures, comprising the steps of

• a) directing a probe beam of monochromatic light onto a material sample whose characteristics are to be determined,
• b) electromodulating the sample with a frequency by directing onto the same a modulated pump beam from a pump source,
• c) collecting the light reflected from or transmitted by the sample in a detector which produces a d.c. signal and an a.c. signal containing an in-phase component, and
• d) determining the dependence of the in-phase photoreflectance signal on the pump modulating frequency to obtain information about trap times.