Room-temperature magnetic p-n junctions for charge-and-spin diodes

Non-magnetic p-n junctions have been fundamental components in the silicon era, serving as the backbone for nearly all Si-based semiconductor devices, including transistors. To tackle challenges such as scaling limitations, excessive latency, and high-power consumption in Si-based electronics, we develop magnetic p-n junctions composed of a p-type amorphous magnetic semiconductor (p-AMS) and n-type Si. These charge-and-spin junctions exhibit typical diode characteristics for charge current, along with distinctive spin diode features. By manipulating spin-polarized space charges, we observed a giant magnetic enhancement of approximately 24.36% at a breakdown current of 5 mA, and an impressive 29-fold increase in magnetic moments for p-AMS. The observed spin behavior is attributed to space charge effects or carrier depletion in the p-AMS with extended hole states.

💡 Research Summary

The authors present a room‑temperature magnetic p‑n junction that simultaneously functions as a conventional charge diode and a spin diode. The device is fabricated by joining a p‑type amorphous magnetic semiconductor (p‑AMS) based on a CoFeTaBOₓ alloy (oxygen content 48.5–56 at %) with an n‑type single‑crystal silicon substrate. The p‑AMS exhibits robust ferromagnetism with a Curie temperature above 600 K and a high hole concentration, while the n‑Si remains non‑magnetic.

Electrical characterization shows typical diode behavior: a forward turn‑on voltage (Vₜ) of ~0.5 V at 300 K that shifts to ~1.0 V at 100 K, and a reverse breakdown voltage (V_br) of about –6.5 V. Importantly, the magnetization (M) of the p‑AMS layer can be modulated by the applied current. In forward bias, a current of 1 mA reduces M by ~4 % (|ΔM|/M₀ ≈ 4 %). In reverse bias, a modest current of –1 mA produces a comparable increase, while a larger reverse breakdown current of –5 mA yields a dramatic 24 % enhancement of M, corresponding to a 29‑fold increase in the effective magnetic moment. These changes occur at an ultra‑low current density of ~2.5 × 10⁻² A cm⁻², orders of magnitude lower than required for spin‑orbit‑torque or spin‑transfer‑torque devices. Thermal analysis shows that Joule heating raises the temperature by only ~0.006 K, ruling out heating as the cause of the magnetization modulation.

Structural analysis using bright‑field STEM reveals a sharp transition from ordered crystalline Si to disordered amorphous p‑AMS, confirming the absence of ferromagnetic precipitates. Differential phase‑contrast (DPC) imaging maps the electrostatic potential across the junction, showing an anomalous potential minimum at the interface and a “triplet” built‑in electric field (positive‑negative‑positive extrema). This unusual field creates a space‑charge region that acts as a spin‑modulating barrier, impeding carrier diffusion and enabling spin‑selective transport.

First‑principles density‑functional calculations of the amorphous p‑AMS model show defect states within the mobility gap and a Fermi level positioned at the top of a delocalized Co‑derived valence band. The Co 3d band provides shallow acceptor states, facilitating hole transport and spin polarization. Electron‑energy‑loss spectroscopy (EELS) combined with HAADF‑STEM demonstrates a redistribution of 3d electrons at the interface: Co loses ~0.41 e⁻ per atom while Fe gains ~0.36 e⁻, indicating transfer of ~10¹⁴ spin‑polarized electrons (predominantly spin‑down) across the junction. This charge transfer accounts for the observed magnetization changes (~10⁻⁶ emu).

The authors interpret the direction‑dependent magnetization amplification as arising from tunnel breakdown of valence electrons from the p‑AMS into n‑Si. The space‑charge region thus functions as an all‑electrical spin modulator, enabling rectification, switching, and amplification of magnetic signals without any external magnetic field. Compared with prior spin‑light‑emitting diodes, magnetic heterojunction diodes, spin‑Esaki diodes, and bipolar magnetic junctions, this device uniquely satisfies four key criteria: room‑temperature operation, low‑power all‑electrical control, bidirectional spin amplification, and simultaneous charge‑diode functionality.

The work opens a pathway toward integrated charge‑and‑spin circuitry, suggesting future directions such as compositional tuning of the p‑AMS, interface engineering to optimize spin injection, and scaling the concept to high‑frequency spin‑tronic components.