Development of Novel Ternary Alloy of CSiSn for Low-Cost Next Generation Photovoltaics
Authors: Banishashemian Seyedeh , Mahmoudi Mohammad , Mosleh Aboozar , Naseem Hameed 
Published: December 2024

Multijunction solar cells exhibit enhanced efficiencies by harnessing the energy of various segments of the solar spectrum. Presently, the most efficient cell on the market comprises three layers of InGaP/InGaAs/Ge, denoted as blue, green, and red cells based on their energy absorption levels [1]. However, increasing the number of layers to enhance efficiency escalates production costs. The high costs are mainly due to the availability of the elements and their limited abundance of III-V alloys. To mitigate these expenses associated with multijunction solar cells, it is imperative to develop these cells with a material compatible with silicon substrates, thus facilitating industrial scalability. The best choices of materials are group IV alloys such as C, Si, and Sn. However, achieving this goal necessitates the development of a direct wide bandgap Group IV alloy that seamlessly matches with the silicon lattice. Recent research suggests that alloying Ge and Sn results in a true direct bandgap alloy and incorporating Si into GeSn enables engineering the bandgap and lattice constant [2]. However, this approach lacks sufficient coverage of the blue and green spectrum due to limited bandgap adjustments and a mismatch in lattice size with Si. Introducing a new group IV ternary alloy that resolves this issue by enabling lattice-matched growth to silicon, ensuring full spectrum coverage and compatibility with silicon substrates is the key to revolutionize this industry.

The C-Si-Sn (CST) alloy is the unique answer to this problem, however, developing group IV alloys does not follow the same path for all elements as the lattice size and crystal structures vary from one to another. Diamond cubic crystal structure is shared between the three elements but it is not a universally stable crystal under all conditions; carbon tends to form diamond structures under high temperature and pressure, whereas tin's stability is limited to temperatures below 13°C. However, recent advancements in alloy growth under non-equilibrium conditions showed successful incorporation of Sn in Ge exceeding thermal equilibrium limits up to 25%. Also, our team reported the growth of Sn incorporation in Si as high as 3% using plasma-enhanced chemical vapor deposition at 300°C. Moreover, we have reported on low-temperature diamond deposition and silicon growth at 250°C [3]. Therefore, the path to grow ternary CST alloys is feasible.

In this paper we present the bandgap predictions of CST alloy and its stability using density functional theroy as well as the growth pathways that allow deposition of the alloy using ultra-high vacuum plasma enhanced chemical vapor deposition (UHV-PECVD).

The electronic band structure of CST alloys relies significantly on band bowing, necessitating adjustments in compositions to attain a direct bandgap. Moreover, CST alloys don't achieve full miscibility among constituents, requiring simulation via Density Functional Theory (DFT) to forecast the electronic band gap of both binary alloys of C-Si-Sn and the ternary alloy of CST. DFT calculations were done using generalized gradient approximation- Perdew–Burke–Ernzerhof (GGA-PBE) with Vienna Ab-initio Simulation Package (VASP). The simulations show that a balance between C and Sn is needed to make the crystal stable.

The band structure of CST is estimated using Vegard’s law with proposed bowing parameters reported in an earlier publication [4]. This relationship is shown below:

Upon examination of the direct transitions observed in CSn and Si, it is anticipated that the ternary alloy of CST would exhibit a direct bandgap transition region. The vertices of this triangular configuration represent the maximum bandgap at 2.48 eV for C0.55Sn0.45, the minimum bandgap for Sn at -0.41 eV, and the third point at 0.539 eV for Si0.50Sn0.50.

Considering the DFT simulation results for both bandgap and crystal stability of CST, the growth pathways were designed. Growth was achieved utilizing commercially available precursors: CH4 for carbon, SiH4 for silicon, SnCl4 for tin, and hydrogen (H2) as both the carrier and diluent gas. The growth process was facilitated by a capacitively coupled plasma system powered by a 13.56 MHz radio-frequency (RF) power supply.

Raman spectroscopy (Fig.1)was employed to characterize the CST samples, to investigate the impact of flow ratio and plasma power on material quality and the incorporation of carbon and tin. The observation of the Si-C, Si-Sn, and C-Sn peaks, associated with the transverse optical (TO) and longitudinal optical (LO) modes for each bond. Moreover, the samples were investigated using in-depth Xray Photoelectron Spectroscopy that show uniform incorporation of the C-Si-Sn element. The results will be shown in the conference.

Figure 1