The importance to study and characterize semiconductors (bulk or thin film), semiconductor heterostructures (superlattices, quantum wells, heterojunctions) and semiconductor interfaces (Schottky barriers, metal-insulator-semiconductors, semiconductor-electrolyte, semiconductor-vacuum, etc.) assumes ever-greater significance, particularly as many of these semiconductors and semiconductor microstructures are fabricated by modern thin-film techniques such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), etc.
The materials and interfaces grown by MBE and MOCVD as well as other methods can be characterized by a variety of optical, electronic and structural methods including photoluminescence, photoluminescence excitation spectroscopy, absorption spectroscopy, modulation spectroscopy, Raman and resonant Raman scattering, cyclotron resonance, Hall effect, transmission electron microscopy, etc. Each of these tools provides specific information about the material of interest. For characterization purposes the experimental tools should be as simple and informative as possible. Many of the methods mentioned above are specialized and sometimes difficult to employ. For example, a number thereof, such as photoluminescence, photoluminescence excitation spectroscopy, absorption, cyclotron resonance, generally require cryogenic temperatures. Because of its simplicity and proven utility, photoreflectance has recently gained importance for the evaluation of semiconductor thin films and heterostructures.
The basic idea of modulation spectroscopy is a very general principle of experimental physics. Instead of directly measuring an optical spectrum, the derivative with respect to some parameter is evaluated. This can easily be accomplished by modulating some parameter of the sample or measuring system in a periodic fashion and measuring the corresponding normalized change in the optical properties. The former perturbation is termed "external" modulation and includes such parameters as electric fields (electromodulation), temperature (thermomodulation), stress (piezomodulation), etc. Changes in the measuring system itself, e.g., the wavelength or polarization conditions can be modulated or the sample reflectance can be compared to a reference sample, are termed "internal" modulation.
In modulation spectroscopy uninteresting background structure is eliminated in favor of sharp lines corresponding to specific transitions between energy levels in the semiconductors and semiconductor microstructures. Also, weak features that may not have been seen in the absolute spectra are enhanced. While it is difficult to calculate a full reflectance (or transmittance) spectrum, it is possible to account for the lineshape of localized spectral features of modulation spectroscopy. The ability to fit the lineshape is an important advantage of modulation spectroscopy. Lineshape fits yield accurate values of the semiconductor energy gap as well as broadening parameter. In addition, since "external" modulation spectroscopy is the a.c. response of the system to the modulating parameter, photoreflectance also provides information in the other modulation variables such as phase, modulation frequency, modulation amplitude, modulation wavelength as will be discussed more fully hereinafter.
In photoreflectance, the built-in electric field of the materials is modulated by the photo-injection of electron-hole pairs created by a pump beam of wavelength .lambda..sub.p which is chopped at frequency-.OMEGA..sub.m. Despite its utility, the mechanism of photoreflectance is not fully understood although several experiments have indicated that photoreflectance is due to the modulation of the built-in electric field through a recombination of the minority species with charge in traps. Thus, by measuring the dependence of the photoreflectance signal on .OMEGA..sub.m it is possible to gain information about trap times with the use of photoreflectance.
Though the use of photoreflectance has been known for more than twenty years, experimental difficulties experienced with the photoreflectance method in relation to other modulation methods lessened interest in the photoreflectance. These difficulties included scattered light from the pump beam and photoluminescence from the sample. A report published in 1985 on the photoreflectance results on semiconductor heterostructures by Glembocki et al., Appl. Phys. Letts. 46, 970 (1985); Proceedings of the SPIE (SPIE, Bellingham, 1985) 524, 86 (1985) produced renewed interest in the use of photoreflectance to study not only these semiconductor structures but also to study bulk (thin film) material. An improved apparatus involving a new normalization procedure which was published in 1987, will be described hereinafter by reference to FIG. 1. The new normalization procedure involved in the apparatus of FIG. 1 helped to solve some of the aforementioned photoreflectance problems, i.e., scattered light from the pump beam and photoluminescence from the sample.
The present invention is concerned with further improving the prior art apparatus to achieve improved signal-to-noise ratios, to further eliminate problems encountered in the prior art apparatus and in particular to utilize novel computerized procedures to gain additional information on the characteristics of the materials examined.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for determining the characteristics of certain materials by photoreflectance which avoid by simple means the shortcomings and drawbacks encountered with the prior art apparatus and methods in the use thereof.
Another object of the present invention resides in a novel apparatus which assures improved signal-to-noise ratio.
A further object of the present invention resides in an apparatus utilizing photoreflectance for determining characteristics of certain materials which is simple to use, provides great versatility in the information which can be obtained and assures high reliability and accuracy.
Still another object of the present invention resides in an apparatus utilizing conventional computer technologies to obtain information on additional characteristics of the materials.
Another object of the present invention resides in a method for determining characteristics of certain materials, such as semiconductor materials and semiconductor heterostructures, which is simple to use, reliable in its operation and accurate in the results obtained therewith.
Another object of the present invention resides in a method of utilizing photoreflectance which permits to obtain such additional information as temperature, uniformity of the sample tested and composition of the alloy.
Still another object of the present invention resides in a method based on photoreflectance which permits continuous monitoring in the manufacture of materials, such as semiconductor materials, that eliminates the shortcomings and drawbacks encountered with the prior art systems.
A further object of the present invention resides in a method based on photoreflectance which permits accurate quality control in the manufacture of semiconductor materials.
A still further object of the present invention resides in a servo computer program which minimizes problems due to system saturation resulting from light intensity changes in the spectrum of the light source as the latter's wavelength is changed.
Still another object of the present invention resides in a servo computer program which reduces noises due to d.c. saturation and/or fluctuations of the light intensity at a selected wavelength while at the same time increasing the speed of operation of the apparatus for obtaining data at different data points.