Phosphorus Diffusion Mechanisms and Deep Incorporation in Polycrystalline and Single-Crystalline CdTe

Abstract

A key challenge in cadmium telluride (CdTe) semiconductors is obtaining stable and high hole density. Group I elements substituting Cd can form acceptors but easily self-compensate and diffuse quickly. For example, CdTe photovoltaics have relied on copper as a dopant, but this creates stability problems and hole density that has not exceeded 10 15 cm-3. If hole density can be increased beyond 10 16 cm-3 , CdTe solar technology can exceed multicrystalline silicon performance and provide levelized costs of electricity below conventional energy sources. Group V elements substituting Te offer a solution, but they are very difficult to incorporate. Using time-of-flight secondary-ion mass spectrometry, we examine bulk and grain boundary (GB) diffusion of phosphorous (P) in CdTe in Cd-rich conditions. We find that in addition to slow bulk diffusion and fast GB diffusion, there is a critical fast bulk diffusion component that enables deep P incorporation in CdTe. Detailed first-principles calculations indicate the slow bulk diffusion component is caused by substitutional P diffusion through the Te sublattice, whereas the fast bulk diffusion component is caused by P diffusing through interstitial lattice sites following the combination of a kick-out step and two rotation steps. The latter is limited in magnitude by high formation energy, but is sufficient to manipulate P incorporation. In addition to an increased physical understanding, this result opens up new experimental possibilities for Group V doping in CdTe applications.

Experimental and theoretical comparison of Sb, As, and P diffusion mechanisms and doping in CdTe

“…The dashed lines in figure 2(c) show the Fisher model for the given diffusion conditions are in extremely good agreement with the dSIMS profiles. The absence of any observed second bulk diffusion mechanism in the sX samples, along with low As signal in the grain centers deep in the polycrystalline films is strong evidence that there is not a fast bulk diffusion mechanism for As in CdTe, which is in contrast to P. As diffusion coefficients and Arrhenius fits are plotted vs inverse temperature in figure 3 and contrasted with earlier data for Sb and P; the detailed values are shown in Table I [21,22]. The magnitude of the slow As bulk diffusion coefficients is similar to that of Sb, though the activation energy is slightly less; about 2.2 eV for As compared to about 2.5 eV for Sb.…”

“…The dissociation barrier, step (3), is the lowest for P compared to As or Sb so we would expect a higher ratio of P i to P Te defects compared to the other gV defects. The interstitial diffusion barriers are low for all of the gV defects compared to the dissociation barriers [21,22]. So the combination of a high substitutional formation energy and low dissociation energy result in a measurable concentration of quickly diffusing interstitial P. There may be fast diffusing, As and Sb, but the concentrations are too low to be observed.…”

“…Defect formation energies can provide useful insights into defects likely involved in the atomistic diffusion processes that lead to the macroscopic diffusion profiles observed experimentally. Figure 4(a) shows data compiled from our previous calculations of the formation energy of P, As, and Sb defects in CdTe in Cd-rich conditions as a function of Fermi Energy [20][21][22]. This compilation makes certain observations more obvious.…”

“…The specific methods are described in detail in previous work [20][21][22] and the salient features are described here for convenience. For the ab-initio bandstructure and total-energy calculations we use DFT as implemented in the VASP code [23][24][25][26].…”

Carrier density and lifetime for different dopants in single-crystal and polycrystalline CdTe

“…Combined, the data indicate that attempting to increase hole density above 10 16 cm −3 with Cu can present significant stability and lifetime obstacles. Group V dopants diffuse less readily, 54,57 and here, the Group V dopant, P, appears to be the most stable. Next, we discuss results when transferring the methods that worked on sX and mX materials to pX CdTe grown by CSS.…”