Method and apparatus of z-scan photoreflectance characterization

Abstract

A method of z-scan photo-reflectance characterization of semiconductor structures and apparatus for same has been developed. The method and apparatus provides the ability to independently measure electro-refractive and electro-absorptive nonlinearities that occur in conventional photo-reflectance signals. By performing a series of photo-reflectance measurements, each containing photo-modulated nonlinear optical signals, with the sample at a multiplicity of positions along the focal length of the probe light column, and with an aperture fixtured in the reflected probe path, precision characterization of both electro-refractive and electro-absorptive nonlinearities is attained. The Z-scan photo-reflectance method and apparatus characterizes spatial distortions of a coherent photo-reflectance probe light beam due to electro-refractive and electro-absorptive effects.

1 . A method of z-scan photo-reflectance characterization of a semiconductor structure, the method comprising the steps of:
 (a) illuminating an area of a surface of the semiconductor structure using an amplitude modulated pump laser beam, wherein
 (i) the pump laser beam comprises at least one wavelength with energy greater than the smallest interband transition energy of a semiconductor material within the semiconductor structure, thereby inducing time periodic changes in electronic charge density within the semiconductor structure such that the electric field profile within the semiconductor structure obtains a time periodic modulation, and 
 (ii) the semiconductor material within the semiconductor structure is subject to a time periodic modulation of interband transition energies; 
   (b) illuminating a portion of said modulated electric field profile of step (a) with a separate probe laser beam, wherein the probe laser beam comprises at least one wavelength
 (i) nearby an interband transition energy of the semiconductor material within the semiconductor structure, and 
 (ii) suitable for recording the induced changes in semiconductor material optical response which occur nearby to interband transition energies; 
   (c) recording reflected alternating current probe light from the illumination of the semiconductor structure, wherein the alternating current probe light comprises a photo-reflectance signal;   (d) performing a series of photo-reflectance measurements of steps (a), (b), and (c), with the semiconductor structure at a multiplicity of positions along the focal length of the probe light column, wherein an aperture is fixtured in the far field of the probe laser beam;   (e) using the information collected in steps (a), (b), (c), and (d) to determine a physical property of the semiconductor structure.   
     
     
         2 - 5 . (canceled) 
     
     
         6 . The method of  claim 1 , wherein the wavelength of the pump laser is selected from the group consisting of 830 nm, 780 nm, 633 nm, 532 nm, 473 nm, and 440 nm. 
     
     
         7 - 8 . (canceled) 
     
     
         9 . The method of  claim 1 , wherein the aperture in the far field of the reflected probe beam is fixtured to provide on-axis light transmission of about 20 percent. 
     
     
         10 . The method of  claim 1 , wherein the aperture in the far field of the reflected probe beam is fixtured to provide on-axis light transmission of about 50 percent. 
     
     
         11 . The method of  claim 1 , wherein the aperture in the far field of the reflected probe beam is fixtured to provide on-axis light transmission of 100 percent. 
     
     
         12 - 14 . (canceled) 
     
     
         15 . The method of claim I, wherein shift of the probe beam phase front is determined from a photo-reflectance probe signal acquired using a restricted aperture z-scan. 
     
     
         16 - 19 . (canceled) 
     
     
         20 . The method of  claim 1  further comprising determining electronic charge density according to an empirically determined calibration curve. 
     
     
         21 . The method of  claim 1  further comprising determining electric field according to an empirically determined calibration curve. 
     
     
         22 . The method of  claim 1  further comprising monitoring electronic charge depth profile according to an empirically determined calibration curve. 
     
     
         23 . (canceled) 
     
     
         24 . The method of  claim 1  further comprising determining a characteristic using z-scan photo-reflectance information and wherein the characteristic is selected from the group consisting of position, amplitude, spectral width, and spectral shape of the semiconductor interband transition energy. 
     
     
         25 . (canceled) 
     
     
         26 . An apparatus for detecting physical properties of a semiconductor structure, comprising:
 (a) a semiconductor structure with a reflecting surface;   (b) a pump laser system, wherein the pump laser system is operable
 (i) to provide an amplitude modulated laser beam with a modulation frequency in the range of 100 kHz to 50 MHz, 
 (ii) to operate at optical powers of at least about approximately 5 mW, and 
 (iii) to contain at least one wavelength with energy greater than the smallest interband transition energy of a semiconductor material within the semiconductor structure; 
   (c) a probe laser system, wherein the probe laser system is operable
 (i) to provide a continuous wave laser beam, 
 (ii) to operate at optical powers of at most about approximately 15 mW, and 
 (iii) to contain at least one wavelength nearby an interband transition energy of a semiconductor material within the semiconductor structure; 
   (d) a photoreceiver operable to generate an electrical current proportional to input intensity;   (e) an optical system, wherein the optical system is operable
 (i) to focus at least one of the amplitude modulated laser beam and the continuous wave laser beam onto a common focal position on a surface of the semiconductor structure of diameter at most 50 microns, 
 (ii) to translate the common focal position through a distance of approximately 10 times the Rayleigh range of at least one of the amplitude modulated laser beam and the continuous wave laser beam, and 
 (iii) to separate and direct probe light reflected from the semiconductor structure through an aperture fixtured in a far field of the continuous wave laser beam and into the photoreceiver; 
   (f) a phase locked signal detection system operable to record output of the photoreceiver; and   (g) a computer comprising measurement and system control software.   
     
     
         27 - 31 . (canceled) 
     
     
         32 . The apparatus of  claim 26  further comprising a 20× objective lens, wherein the 20× objective lens is operable to focus at least one of the amplitude modulated laser beam and the continuous wave laser beam onto an area of a surface of the semiconductor structure. 
     
     
         33 . (canceled) 
     
     
         34 . The apparatus of  claim 26 , wherein the computer is operable to provide automated z-stepping and photo-reflectance data acquisition. 
     
     
         35 . The apparatus of  claim 26 , wherein the computer is operable to provide automated control setting of the aperture. 
     
     
         36 . A method of determining a physical property of a semiconductor structure, the method comprising the steps of:
 (a) performing beam profiling of a photo-reflectance probe light beam using a plurality of photo-reflectance measurements;   (b) utilizing the plurality of photo-reflectance measurements to measure nonlinearities occurring in the optical response of the semiconductor structure, wherein the non-linearities are selected from the group consisting of refractive non-linearities, absorptive non-linearities, and combinations thereof; and   (c) utilizing the measured non-linearities to determine the physical property of the semiconductor structure.   
     
     
         37 . The method of  claim 36 , wherein the step of measuring the non-linearities comprises
 (i) measuring refractive non-linearities occurring in the plurality of the photo-reflectance measurements, and   (ii) independently measuring absorptive non-linearities in the plurality of the photo-reflectance measurements.   
     
     
         38 . The method of  claim 36 , wherein the beam profiling is selected from the group consisting of pump induced electro-modulation profiling, probe band flattening profiling, and combinations thereof. 
     
     
         39 . The method of  claim 36 , wherein the step of performing the beam profiling comprises utilizing an open aperture. 
     
     
         40 . The method of  claim 36 , wherein the step of performing the beam profiling comprises utilizing a restricted aperture. 
     
     
         41 . The method of  claim 36 , wherein the step of performing the beam profiling comprises utilizing a open aperture and utilizing a restricted aperture.