Category Next Generation of Photovoltaics

Conclusions

From GIXRD, TEM and ED results, it can be concluded that the Ti-implanted Si layer with a dose of 1015 cm-2(average volume concentration of 3-1020 cm-3 in the implanted thickness) and further PLM annealed at the highest energy density (0.8 Jcm-2) presents an excellent reconstruction of the crystal structure. Higher doses have a polycrystalline structure with decreasing grain size. RBS

measurements show that most of the Ti is incorporated at interstitial positions. Sheet resistance and Hall mobility measurements prove the existence of an IB energetically located about 0.36-0.38eV below the conduction band. In this IB, carriers behave as holes with high concentration, in the same order than Ti concentration, and low mobility...

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. Discussion

The fitting obtained with the ATLAS code and with the analytical model are remarkably similar in spite of the different starting hypothesis. In the first case, we assume a heterojunction between the TIL and the substrate and in the second one the limitation is due to a generic temperature-dependent resistance. In this second case,
we use a lumped equivalent circuit where the current limitation between both layers is concentrated at the corners, while ATLAS uses a true tri-dimensional grid. In both cases, we preserve the idea that all the Ti atoms contribute to the sheet conductivity of the implanted layer, which means that the IB is formed...

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Hall Mobility at Low Temperatures

One of the most exciting aspects of this work is the nature (electrons or holes) of the carriers at the IB. The bilayer decoupling is independent on its nature and it is only dependent on the barrier current limitation for the electrons. The impossibility of the IB carriers to go to the substrate is irrespective whether they are electrons or holes. The only difference we can find between electrons or holes is its mobility sign. According to the model, at low temperatures F!0, layers are decoupled and we are measuring only the mobility at the TIL, which is:

—n1an12t1 + p1Mp12t1

Meff = 1 p1 . (13.10)

n1Mn1fi C p1^p1^1

According to our model, holes mobility and its concentration at IB do not depend on the temperature while electrons terms do...

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Model Application to Our Case

An electrical equivalent circuit for our bilayer is depicted in Fig. 13.12. We will assume that both layers are connected with four non-ideal diodes (one on each corner) that represent the electrical TIL-substrate junction. The characteristics of
these diodes have been calculated in reference [26], where we showed that the reverse current depends exponentially with the temperature with an activation energy which is the difference between the IB energetic position and EC. Some experimental data of the rectifying behaviour of these diodes could be found in [21].

This model is, strictly speaking, non linear but we will assume that the reverse characteristics of the non-ideal diode could be represented with a temperature depending resistor Rt...

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An Analytical Model for the Sheet Resistance and for the Hall Mobility

To confirm the ATLAS results and also to simulate the mobility behaviour of our samples, we have developed an analytical model to explain the sheet resistance and mobility characteristics of our bilayers. As far as we know, this model has not been proposed in the literature and for this reason we will explain it from a general point of view below.

General Model for the Sheet Resistance of a Bilayer

We will assume a bilayer composed by layer 1 and layer 2 as in Fig. 13.11. Both layers are connected through resistors Rt/2, which model the current limitation for the current flux from one layer to the other.

Подпись: Ii Подпись: V RC1 image457 Подпись: (13.1)

Currents injected on layer 1 and 2,I1 and I2 could be obtained as:

where V is the voltage drop at the current source terminals. Obviously, I — I1 +12...

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ATLAS Sheet Resistance Simulation

To fully confirm the qualitative explanation of the previous paragraph, we have performed some simulations using the ATLAS code framework [23]. As in this code it is not possible to define a semiconductor with an IB, we have defined the TIL sheet as a “new” semiconductor with the following characteristics:

• Conduction band: Is the same than Si, having the same equivalent density of states, mobility, affinity and so on.

• The gap of this “new” semiconductor is the EC — EIB energy, avoiding the VB at the TIL.

• Density of States (DOS) and hole concentration in the IB: as stated before, RBS measurements showed that Ti is located mostly at interstitial positions...

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Electrical Transport Properties

13.3.2.1 Sheet Resistance and Hall Mobility

Figure 13.7a shows the sheet resistance as a function of measured temperature for a reference Si substrate and for samples with implanted doses of 1014, 1015, 5-1015 and 1016 cm-2. All the implanted samples were PLM annealed with 0.8 Jcm-2 energy density. Hollow symbols are the experimental measurements while lines with symbols correspond to a simulation we will explain later. We also checked that unimplanted substrates have the same electrical characteristics before and after a PLM process. We did this experiment to confirm that the annealing process has not any influence on the substrate sheet resistance or mobility. Figure 13.7b shows the mobility modulus versus temperature for the same samples quoted in the previous figure...

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RBS

RBS profiles have been obtained for doses of 1015, 5-1015 and 5-1016 cm-2 both in random and channeling configuration. The Ti peak is clearly observed at energies close to 1,650 keV, which is far from the maximum backscattered energy due to the

Si surface (1,330 keV). The area of this peak is proportional to the implanted dose and shows the broadening corresponding to the implantation depth that increases with the dose. Ti peaks obtained in random and channeled configurations are almost equal and the comparison of its area for the different doses shows that only a maximum of about 8% of the Ti is located at substitutional positions being therefore the most part of the implanted atoms located interstitially...

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