Capacitance-voltage profiling

The migration of atomic Si in 5-doped samples was studied by means of capacitance-voltage measurements and rapid thermal annealing. It was shown that these methods could detect diffusion which occurred at length-scales as small as Inm. The capacitance-voltage profile widths broadened, from less than 4nm to 13.7nm, upon annealing (lOOOC, 5s). It was found that the results could be described by ... 

Gordon, B.J., On-line capacitance-voltage doping profile measurement... 

E. Basaran, C. P. Parry, R. A. Kubiak, T. E. Whall, and E. H. C. Parker, Electrochemical capacitance-voltage depth profiling of heavily boron-doped silicon, J. Cryst. Growth 157, 109, 1995. 

Various techniques are used for these measurements. The most popular are capacitance-voltage (C-V) profiling, spreading resistance and secondary ion mass spectroscopy (SIMS). SIMS is not a strictly electrical characterization technique, but is included here because it is routinely used to measure the dopant atom distribution. The basis for these three techniques are very different and I will briefly describe them. 

In the fabrication of hyperabrupt diodes such as varactors, the flexibility in doping profiles that can be produced by ion implantation allows a wide range of capacitance-voltage characteristics to be designed. 

As seen in previous sections, the response to a potential step is a pulse of current, which decreases with time as the electroactive species near the electrode surface is consumed and consists of a faradaic, /f, and a capacitive contribution, Iq. The advantage of most pulse techniques results from the measurement of the current flow near the end of the pulse when the faradaic current has decayed, often to a diffusion-limited value but when the capacitive current is insignificant. Pulse widths, tp, are adjusted to satisfy this condition and the additional condition that time has not been allowed for natural convection effects to influence the response. There is a greatly improved signal-to-noise ratio (sensitivity) compared to steady state techniques and in many cases, greater selectivity. Detection limits are of the order of 10 M. Furthermore, for analytical purposes, most current-voltage profiles from the pulse techniques are faster to interpret than those of dc voltammograms, because they are peak-shaped rather than the typical step curve of conventional voltammet-ric methods. 

Capacitance-voltage methods were used to profile 5-doped layers which had been grown onto Si substrates via metalorganic chemical vapor deposition. It was found that there was a close correlation between dislocation densities in the epitaxial layers and the associated diffusion coefficients. After rapid thermal annealing (800 to lOOOC, 7s), the diffusion data could be described by ... 

The most important application of SCM is two-dimensional dopant concentration profiling in semiconductors. However, the raw capacitance-voltage data obtained from SCM must be converted by a mathematical model into a dopant concentration. Therefore, development and validation of appropriate models represents a large part of SCM methodology. To validate the various models, other experimental techniques must be employed to measure and verify the dopant profiles independently. SCM data are usually compared to secondary ion mass spectrometry (SIMS) measurements of dopant concentrations. Spreading resistance profiling (SRP) and computer simulations are also employed to check model validity. 

This relationship is used in the capacitance-voltage technique for profiling carrier concentrations near diode junctions and is usable for any type of diode in which one side of the junction is much more heavily doped than the other. It can also be used in a transient mode to detect and analyze point defects as they charge and discharge with bias voltage changes. 

For less highly-doped epi films, one can use the C-V method. In this case, use is made of the fact that a Schottky semiconductor diode has a voltage-dependent capacitance. In other words, when such a diode is reverse biased, a depletion layer forms which then has a capacitance determined by the depth of this layer (w) as well as the doping (N) at its edge. The doping profile can be determined from the following relations.9... 

The depletion layer profile contains information about the density of states distribution and the built-in potential. The depletion layer width reduces to zero at a forward bias equal to and increases in reverse bias. The voltage dependence of the jimction capacitance is a common method of measuring W V). Eq. (9.9) applies to a semiconductor with a discrete donor level, and 1 is obtained from the intercept of a plot of 1/C versus voltage. The 1/C plot is not linear for a-Si H because of the continuous distribution of gap states-an example is shown in Fig. 4.16. The alternative expression, Eq. (9.10), is also not an accurate fit, but nevertheless the data can be extrapolated reasonably well to give the built-in potential. The main limitation of the capacitance measurement is that the bulk of the sample must be conducting, so that the measurement is difficult for undoped a-Si H. 

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