It is well known that impurities and defects in semiconductors are associated with energy levels in the forbidden gap. Similar states occur at the surface of a semiconductor where the crystal lattice and the symmetry are strongly disturbed. These states are called surface states. Owing to the two-dimensional nature of the surface, their density is measured per unit area, in contrast to bulk states, which are measured per unit volume. A third type of states, similar to surface states, occurs at the interface between two adjacent materials. These states are called interface states. Very often they are also simply called surface states.

Interface states on semiconductor/insulator surfaces

The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of metal–insulator–semiconductor (MIS) GaAs Schottky diodes are investigated and compared with metal–semiconductor (MS) diodes. The MIS diode showed nonideal behavior of I–V characteristics with an ideality factor of 1.17 and a barrier height of 0.97 eV. The energy distribution of interface states density was determined from the forward bias I–V characteristics by taking into account the bias dependence of the effective barrier height, though it is small. The reduction in the saturation current in the MIS case is caused by a thin oxide layer and is due to the combination of increased barrier height and a decrease in the Richardson constant. The carrier concentration anomaly observed between the MIS and MS diodes measured from reverse bias C–V measurements is explained via oxide $(\beta-Ga_2O_3)$ traps due to the Ga-vacancy by deep level transient spectroscopy (DLTS) measurement.

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Effects of thin oxide in metal-semiconductor and metal-insulator-semiconductor epi-GaAs Schottky diodes

A new method of measuring the density of band‐gap interface states at the Si‐SiO2 interface has been developed. This method is based on a charge pumping phenomenon and can be performed on a metal‐oxide‐semiconductor transistor structure without the need for Shockley–Hall–Read statistics. Although the method requires knowledge of the gate voltage‐surface potential relationship, the measured density of states is not as sensitive to either errors or fluctuations in this relationship as other more common techniques. Other advantages of this new method are that it can measure the density of interface states over most of the band gap, it is relatively simple to implement, and has a resolution comparable to either the conductance or deep level transient capacitance techniques. Applying the new method to a transistor with a channel length of 50 μm and a width of 25 μm, the density of interface states has been measured to within 80 meV of the conduction band.

A new charge pumping method of measuring Si‐SiO2 interface states

Interface states at the SiO2-Si interface

A method is described for the measurement of quasistatic CV characteristics in semiconductors which offers significant advantages over previous methods. Using a small voltage step across the device under test, a displacement charge is stimulated and measured. A feedback charge measurement circuit is used, reducing the effect of several significant error terms. This technique has the advantages of higher signal‐to‐noise ratio at the frequencies of interest, independence of signal size from measurement time, and ease of distinction of signal charge from error currents at the time of measurement. Results consistent with those of previous techniques have been obtained on silicon MOS capacitors and other devices using steps from 10 to 100 mV and delays between step and measurement of up to 200 s. Furthermore, the method has been extended to provide additional information useful in determining whether an equilibrium measurement is obtained and in diagnosing and correcting for errors due to dc leakage currents. ...

Improved feedback charge method for quasistatic CV measurements in semiconductors

A new static technique for a very accurate measurement of charge, surface potential, low−frequency capacitance, interface charge, and interface state density in MOS structures is presented. A comparison with the conductance method is made. For a 〈111〉−oriented n−type silicon wafer, oxidized in dry oxygen and annealed in hydrogen, the distribution of the interface state density is shown over an energy range of 0.9 eV within the energy gap.

K. Ziegler

Papers

Static technique for precise measurements of surface potential and interface state density in MOS structures