Hole transport through an isotype amorphous-crystalline Ge₂Sb₂Te₅ heterojunction under forward bias

Abstract

The transport of holes through a representative isotype amorphous-crystalline Ge2Sb2Te5 heterojunction under forward bias is explored for the first time. An approximate analytic model, based on the exact solution to Poisson’s Equation, the Continuity Equation, and the Transport Equation, is proposed to describe the forward current-voltage characteristic and hole quasi-Fermi level distribution across the heterojunction with a reduced set of material and device parameters. The proposed model incorporates thermionic emission across the heterojunction interface as well as drift and diffusion across the quasi-neutral regions of the heterojunction layers, but neglects drift and diffusion across the space-charge regions of the heterojunction layers, as well as recombination. Solar cell capacitance simulation results demonstrate general agreement between the approximate and exact solutions. Therefore, the approximate model is effective in capturing the physics of thermionic emission-limited transport at low applied bias and drift-diffusion-limited transport through the quasi-neutral region of the amorphous layer at larger applied biases. However, simulation results also show that an extremely narrow subregion of the space-charge region within the amorphous layer, which has been neglected within the proposed model, limits the transport of holes at very low bias and inhibits transport at all other biases. Nevertheless, the proposed model provides improved accuracy across the entire bias range compared to the individual thermionic emission or drift-diffusion models.

Minority-carrier transport through an isotype amorphous-crystalline Ge2Sb2Te5 heterojunction under forward bias

“…In addition to the drops in E Fn across the c-GST and a-GST layers, E Fn drops also across the c-GST contact and across the a-GST/c-GST interface, though they are not apparent in figure 7. While this indicates thermionic emission across E V respectively, as well as the hole quasi-Fermi level distribution, E Fp (x) [5], are also indicated. The inset magnifies the drop in E Fn (x) across the c-GST layer at 0.15 V. these interfaces, the negligible drops demonstrate that transport across these interfaces does not limit electron transport through the device at either bias.…”

“…By resolving the drift and diffusion components of J e (x), represented by the orange and purple curves in figures 6(a) and (b) respectively, it is found that electron transport through the bulk of the c-GST layer is predominantly diffusive at either applied bias whereas electron transport through the bulk of the a-GST layer is diffusive at low biases (0.15 V) but mixed at larger biases (0.40 V). The reason for the superior contribution of the drift component in this case is the electric field induced within the a-GST QNR by the significant hole current density which passes through the heterojunction at larger applied bias [5].…”

“…The solar cell capacitance simulator (SCAPS-1D) was used to numerically solve the electron transport model in section 2, using the key parameters tabulated in table 1. The electric field distribution, free hole concentration distribution, hole quasi-Fermi level distribution as well as the conduction and valence band edge energy level distributions were obtained in [5] by simulating majority-carrier (hole) transport with SCAPS-1D. The conduction band discontinuity was taken as ∆E C = −0.214 eV [4] and the electron diffusion coefficient was related to the electron mobility presented in table 1 via the well-known Einstein relation [5].…”

“…Though the significance of the minority-carrier component is not expected to affect the dominance of the majority carrier component in this case (i.e. the a-GST/c-GST heterojunction can still be regarded as a majority-carrier device so that the analysis and results presented in [5] remain valid), it is possible that minority-carrier injection will contribute to minority-carrier effects within the a-GST/c-GST heterojunction, such as minority-carrier storage and conductivity modulation [8]. In other words, the study of electron transport facilitates the prediction of minority-carrier effects within the a-GST/c-GST heterojunction.…”

“…As far as the prototypical phase-change material, Ge 2 Sb 2 Te 5 (GST), is concerned, the electrical properties of the heterojunction formed between the amorphous (a-GST) and crystalline (c-GST) phases, i.e. the a-GST/c-GST heterojunction, has only recently gained attention: based on an approximate analytical model describing the electrostatics and resultant energy band alignment of the a-GST/c-GST heterojunction under thermal equilibrium [4], hole transport through the heterojunction was studied and the forward current-voltage characteristic of the heterojunction was approximated under forward bias [5]. However, electron transport through the a-GST/c-GST heterojunction under forward bias has not yet been considered.…”