Analytic electrostatic model of amorphous-crystalline Ge₂Sb₂Te₅ heterojunction

Abstract

Motivated by its potential properties and applications, the energy band alignment of the amorphous-crystalline Ge2Sb2Te5 heterojunction in thermal equilibrium is explored. An analytic model based on the exact solution to Poisson’s equation is constructed to describe the electrostatics of the heterojunction between the amorphous phase and the face-centred cubic crystalline phase of Ge2Sb2Te5. The model captures the physics of accumulation in the crystalline layer, as well as that of depletion and inversion due to the deep defect distributions in the bulk of the amorphous layer. Without introducing fitting parameters, the model approximates the influence of the density of states parameters of each phase on the electric potential distribution across the heterojunction. It is then validated against the exact solution obtained numerically using Solar Cell Capacitance Simulations. Apart from the small inaccuracy in modelling the electric potential distribution in the depletion region, simulation results reveal that the approximations are successful in modelling the electrostatics of the heterojunction.

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

“…However, considering that ohmic contacts are easily formed between a-GST and metals with different work functions such as Al, W, and Pt [7], and that a-GST has a relatively small bandgap of 0.816 eV [8] (so that the thermal generation of holes is sufficiently high to support the transport of holes through the heterojunction and yield a relatively large hole concentration which is largely independent of the space-charge generated by the injecting contact), it seems reasonable to assume that the current extracted from the device will not be limited by the injecting contact. [7], is placed on the left side of the a-GST layer, in between two electrical contacts.…”

“…The second is tunnelling through the space-charge regions. Given the limited width of the SCR [8] and the significant electric field strength within the SCR (≈10 6 V cm −1 at the interface in equilibrium [8]), tunnelling is expected to play a significant role in hole transport through the a-GST/c-GST heterojunction. In considering tunnelling, the tunnelling probability is usually calculated using the Wentzel-Kramers-Brillouin (WKB) approximation which assumes either a parabolic [28,29] or triangular [35] energy barrier.…”

“…In considering tunnelling, the tunnelling probability is usually calculated using the Wentzel-Kramers-Brillouin (WKB) approximation which assumes either a parabolic [28,29] or triangular [35] energy barrier. But, since the energy barrier to holes within the a-GST SCR is not strictly a parabola [8], the applicability of the WKB approximation and existing expressions for the tunnelling probability to the a-GST/c-GST heterojunction must be addressed before tunnelling can be incorporated. Hence, the effects of external contacts and tunnelling through the spacecharge regions on the hole current density are reserved for future work.…”

“…Yet, despite the range of devices, and materials in contact with GST, considered thus far, a device consisting of the amorphous phase of GST (a-GST) in contact with the crystalline phase of GST (c-GST) has never been explored. This is quite remarkable given that such a device would be based on an amorphous-crystalline isotype heterojunction [8], which could exhibit properties characteristic of amorphous-crystalline or isotype heterojunctions. Inspecting Si-based amorphous-crystalline heterojunctions such as a-Si:H/c-Si [9] or a-SiC: H/c-Si [10], a characteristic of amorphous-crystalline heterojunctions includes the passivation of defects at the surface of the crystalline material by the amorphous material, which makes these heterojunctions suitable for reducing interface recombination and improving the efficiency of solar cells [9,10].…”

“…Recently, the a-GST/c-GST heterojunction was studied for the first time under thermal equilibrium conditions; an approximate analytical model was constructed and validated using Solar Cell Capacitance Simulations (SCAPS) to describe the electrostatics and the resultant energy band alignment of the heterojunction [8]. While this represents an important first step, it should be recalled that the operation of devices depends on how the charge carriers behave under applied electrical bias [16].…”