Insulated gate field effect transistors (IGFET), such as metal insulator semiconductor field effect transistors (MISFET) and, more specifically, metal oxide semiconductor field effect transistors (MOSFET) are utilized in a wide variety of applications. These applications include use in measuring devices and digital and analog applications in integrated circuit technology.
An IGFET includes a semiconductor substrate, such as a monocrystalline silicon (Si) substrate which is typically doped with either an n-type dopant, such as phosphorous, or a p-type dopant, such as boron. An IGFET also includes an insulating layer, typically formed by an oxide such as silicon dioxide (SiO.sub.2), which is grown or deposited on one surface of the semiconductor substrate. In turn, conductive material, such as degeneratively doped polycrystalline (polysilicon) and/or metal, is deposited on the surface of the insulating layer, opposite the semiconductor substrate.
An IGFET also includes laterally spaced source and drain regions formed in the semiconductor substrate on opposite sides of the insulating layer. The source and drain regions are also typically doped with a dopant having the opposite type from that with which the substrate is doped. For example, n-doped source and drain regions are formed within a p-doped substrate of an IGFET.
Electrical contacts are then formed to the metal or degeneratively doped polysilicon layer and to the surface of the semiconductor substrate opposite the insulating layer. The electrode to the metal or degeneratively doped polysilicon layer is generally referred to as the gate electrode and the electrode to the semiconductor layer is generally referred to as the back gate electrode. Source and drain contacts or electrodes are formed to the laterally spaced source and drain regions of the IGFET.
The performance of a semiconductor device, such as an IGFET, is defined, in part, by the energy barrier or barrier height of the interface between the different materials forming the semiconductor device. For example, the performance of an IGFET depends upon the barrier height between the valence band of the semiconductor substrate and the conduction band of the overlying insulating layer. The barrier height depends, in turn, upon the electronegativity difference between the materials. Thus, for an IGFET having a Si substrate and an SiO.sub.2 insulating layer, the SiO.sub.2 insulating layer is more electronegative than the Si substrate such that a dipole layer is produced at the Si-SiO.sub.2 interface. The dipole layer, in turn, determines the relative positions of the band gaps of the two materials and, consequently, the barrier height therebetween. Accordingly, in order to obtain a measure of the expected performance of the IGFET and a measure of the reliability and quality of the fabrication process of the IGFET, the barrier height between the substrate and the insulating layer of an IGFET is measured.
The barrier height of a semiconductor device was initially measured by illuminating a partially transparent electrode of a metal oxide semiconductor (MOS) capacitor with light of varying frequencies and measuring the frequency of the incident light at which electrons surmount the energy barrier and are injected into the oxide layer. See R. Williams, Physics Review, Vol. 140, A569 (1965). Based upon the energy of the incident light at which electrons are injected into the oxide layer, a photoemission threshold or barrier height between the valence band of the semiconductor substrate and the conduction band of the oxide layer was determined.
Another conventional method of measuring the barrier height of a MOSFET is described in a textbook entitled MOS (Metal Oxide Semiconductor) Physics and Technology by coauthors E. H. Nicollian and J. R. Brews, pp. 452-62, John Wiley & Sons, New York (1982). See also C.A. Mead, et al., Applied Physics Letters, Vol. 9, 53 (1966); W. Ludwig, et al., Physics Status Solidi, Vol. 24, K137 (1967); and A. M. Goodman, Physics Review, Vol. 144, 588 (1966). According to this method, a semiconductor device is illuminated and the resulting photocurrent is measured as a function of the wavelength of the incident light with the electric field of the insulating layer as a parameter. The quantum yield, typically defined as the number of injected electrons per absorbed photon, is then determined from the measured photocurrent spectrum with corrections made for light source intensity variations with photon energy and for the effects of optical interference. The quantum yield is then plotted as a function of photon energy and is extrapolated to zero yield to obtain the barrier height corresponding to the chosen electric field of the insulating layer. Finally, the barrier heights thus obtained are plotted as functions of the electric field of the insulating layer and extrapolated to a zero electric field to get the zero field barrier height.
An alternative method of determining the barrier height of a semiconductor service is also described in the textbook entitled MOS (Metal Oxide Semiconductor) Physics and Technology by E.H. Nicollian, et al. See also R.J. Powell, Journal of Applied Physics, Vol. 41, 2424 (1970). According to this alternative method, the electric field of the insulating layer is treated as a variable with the photon energy as the parameter. In order to determine barrier height, the photocurrent V-I characteristics of the semiconductor device are plotted directly for different photon energies. The barrier height and dependence of the barrier height upon the electric field of the insulating layer are then determined by the extrapolation of the V.sup.1/2 - I.sup.1/p characteristics (wherein p=2 or 3) to zero photocurrent. Alternatively, the barrier height alone can be determined by simply examining the second derivative of the V-I characteristics for a sign change.
However, all of these measurements determine the barrier height based upon an extrapolation from values measured upon the application of high electric fields. Further, none of these methods determine a distribution of barrier height caused by disorder in the insulator layer. The degree of disorder of interfaces may be determined by infrared spectroscopy, Raman spectroscopy and index of refraction measurements. However, the prior methods of determining the degree of disorder of an interface and the corresponding barrier height distribution of a semiconductor device typically determine the degree of disorder averaged over a volume which makes these methods relatively insensitive to variations in the degree of disorder or variations in barrier height. Therefore, the measured parameters vary only slightly due to differences in the degree of disorder or variations in the barrier height.
Because, the barrier height in a semiconductor device, such as an IGFET is not generally constant, but is, instead, defined by a distribution of varying barrier heights, a more accurate measurement of the barrier heights of a semiconductor device is desirable. In addition, it is desirable to measure and detect small variations in the barrier height or small changes in the degree of disorder of an interface of a semiconductor device such that minor variations in the performance of the device or slight fluctuations in the fabrication process or the properties of the materials comprising the semiconductor device may be discovered.