Characterisation of doped silicon

There are numerous metrics used to characterise the diffused regions of a solar cell, including sheet resistance, dopant concentration, junction depth and spatial uniformity.

The sheet resistance is one of the easiest and quickest metrics to measure and commonly used to distinguish the diffused regions formed from various diffusion processes. It is measured by a four-point-probe (4PP) which has four equally spaced probes, two pins to measure current and voltage separately. The measurement assumes that the current only flows in the layer of interest (in this case, in the diffused region only). A constant current is injected via two pins and the voltage drop measured across the other inner probes.

The sheet resistivity is defined as:

\rho_{\Box}=\frac{\rho}{t}

Where ρ is the resistivity and t is the thickness and is quoted in units of ohms/square. The uniformity of the diffused region can be mapped by taking a series of measurements across the surface of the wafer. This can also be achieved using surface photovoltage (SPV) measurements. In this measurement, Carriers are generated by laser excitation and the flow of the carriers in the layer is detected by an adjacent voltage sensor. Another commonly used method to measure the sheet resistance is by non-contact eddy current systems which can relatively easily be applied inline in high volume manufacturing. In this measurement, an alternating current (AC) is driven through a coil which is in close proximity to the test sample. The electromagnetic field from the coil induces eddy current and resulting power losses in the sample which strongly depend on the conductance of the sample.

In addition to the overall resistance of the layer, it is also important to have more detailed knowledge about the actual dopant concentration in the silicon wafer. The three main techniques that are typically used to extract the dopant profile are: Secondary Ion Mass Spectroscopy (SIMS), Electrochemical Capacitance-Voltage (ECV) and spreading resistance measurements.

A SIMS measurements involves physical etching (i.e. sputtering) of the sample using high energy ions and subsequently analyzing the atomic mass of the etched fragments. As a result, SIMS is able to detect both electrically active and inactive dopants within a sample. However, it is relatively expensive and time-consuming method and challenging to apply on textured surfaces.

Electrochemical capacitance voltage measurement (ECV) is the most commonly used tool to measure the doping profiles in silicon solar cell research and development as it is relatively inexpensive and fast. A silicon sample is emerged into an electrolyte resulting in the formation of a Schottky contact. Subsequently, a small AC bias is applied to the contact resulting in change in the depletion region, and consequently the capacitance. This change in capacitance is used to calculate the carrier concentration in the near surface region. Subsequently, the silicon sample is etched for a predefined time and the measurement is repeated allowing for the measurement of the depth-dependent carrier concentration. ECV only measures electrically-active dopants. An example dopant profile is shown in Figure 1 below.

Doping Profile.png

Figure 1 Electrically active phosphorous dopant profile of a typical industrial phosphorous diffusion, measured using an ECV measurement tool.