Mechanism of Spintronics and c-HPs

Chiral-induced spin selectivity (CISS) introduces a novel mechanism for spin manipulation. It arises because of the spin-dependent charge carriers through an oriented chiral potential, where the spin orientation aligns with the chiral helicity. This alignment occurs because of the quantum mechanical interaction between the spin of the electron and the chiral potential of the material, which breaks the spatial symmetry.

According to recent research, chiral halide perovskites (c-HPs) have proved to be more effective due to their unique properties. These materials have an exceptional ability to self-organize into highly ordered textured films during solution processing. This self-organization is driven by the interactions between organic and inorganic molecules of the perovskite, which results in layered structures where the chiral molecules are uniformly oriented. Such structural alignment enhances the efficiency of spin injection by ensuring that the chiral potential is aligned with the transport direction of charge carriers. Also, the deterministic enantiomeric purity of c-HPs, which is controlled by selecting specific organic cations such as R-MBA (right-handed) or S-MBA (left-handed), ensures consistent behavior. This means that the material is predictable and reproducible, which eventually allows researchers to reliably determine the spin polarization. This property is very important as it lets the scientist fine-tune the material for specific device applications. Additionally, c-HPs exhibit robust CISS effects at room temperature which is achieved due to their unique hybrid organic-inorganic structure. This structure facilitates strong spin-orbit coupling and suppresses spin relaxation, making them ideal for practical spintronic applications.

Spin-LED Architecture with Multiple Quantum Well and Spin Injection Layer

To explore the potential of c-HPs, researchers developed a spin light emitting diode (spin-LED) by coupling c-HPs with III-V semiconductors. The architecture consists of multiple quantum well (MQW) AlGalnP LED structures integrated with a spin injection layer of (R/S-MBA)2PbI4 c-HPs. The fabrication involves several steps, the base ALGalnP LED structure is fabricated with a GaAs capping layer to protect the surface during storage and transport. Before integrating the c-HP layers, the GaAs layer is chemically etched using a solution of NH4OH, H2O2, and H2O. This etching process removes any contaminants and oxide layers, exposing a pristine AlGalnP surface. The removal of oxides is very important as it can impede carrier injection and reduce spin polarization efficiency. The (R/S-MBA)2PbI4 layers, which serve as spin injectors are deposited via spin-coating.

During spin-coating a solution of c-HP material is dispensed on the LED and then spun at high speeds. This process evenly spreads the material across the surface, hence forming a thin layer. To facilitate charge transport and ensure efficient device operation, a hole transport material (TFB) is spin-coated upon the c-HP layer. Transparent conducting layers, including indium-doped zinc oxide (IZO) and a thin aluminum oxide layer, are deposited to create top contacts. Finally, gold contracts are added via thermal evaporation to complete the structure, these contacts allow for the application of electrical current and collection of emitted light.

Figure 1: Spin LED by Coupling c-HPs with III-V semiconductors

Circularly Polarized Emission Induces Spin Injection                                                                                                  

The spin-LED's ability to accumulate spin-polarized carriers in MQW can be demonstrated by polarized electroluminescence (CP-EL). This phenomenon occurs because the recombination of spin-polarized carriers in the MQW emits light with specific circular polarization, depending on the spin state of the carriers. The degree of circular polarization (DOCP) is a measure of the difference in intensity between left-handed and right-handed circularly polarized light. The researchers achieved DOCP values of up to 15%, indicating significant spin polarization in the emitted light. The direction of circular polarization is correlated with the handedness of the c-HP layer, LEDs fabricated with R-MBA exhibit left-handed polarization and S_MBA exhibits right-handed polarization hence this correlation confirms that the CISS effect is responsible for spin injection.

Figure 2: Metrics of DOCP & EL intensity to Evaluate Spin Injection

Bias-dependent Behavior Affects Spin Injection Efficiency

During the research, it was also found that the DOCP increases with applied current showing a consistent behavior with traditional IIII-V spin-LEDs. At low biases, the depletion region of the c-HP/III-V interface is wide, resulting in low carrier density and increased spin scattering. With higher bias, the depletion region gets depleted hence increasing carrier density and reducing spin scattering, increasing the spin injection efficiency. Also, the p-type layer between the c-HP and MQW introduces a potential barrier through which the electric carriers must traverse. At higher biases the electric field across the layer increases which accelerates the carriers, as the carriers move faster, they reduce the traversing time which eventually minimizes spin relaxation which preserves spin polarization as carriers reach the MQW.

The use of c-HPs offers a lot of advantages over traditional spin injectors. Traditional spin injectors often require tunnel oxide barriers to address conductivity mismatches between the injector and the semiconductor. These barriers often add more complexity and also degrade device performance. The c-HP/III-V interface naturally avoids conductivity mismatches, which enables efficient spin injection without additional barriers. Many spintronic devices rely on cryogenic temperatures to maintain spin polarization, the high spin polarization achieved through CISS operates efficiently at room temperature, making it more practical for real-world applications. Also, the ability to deposit c-HPs via solution-based processes such as spin-coating, simplifies fabrication and also simplifies integration with existing semiconductor technologies.

The c-HPs have transformative potential for spintronic devices. By enabling efficient spin injection across a c-HP/III-V interface, it is possible to integrate spin functionalities in traditional semiconductor platforms. It also paves the way for a new class of spin-based optoelectronic devices by combining semiconductor technology with the unique advantage of chiral materials. The high spin-injection efficiency, room temperature operation, and ease of integration make c-HP spin-LEDs a basis for future spintronic innovations.