Light-emitting Thin Films And Devices Based On Silicon-rich Nitride And Nano-silicon Multi-quantum-well Structures

Aiming at monolithic integration of optical components and electronics devices on a silicon chip,silicon photonics is becoming a hot research topic in semiconductor industry.In this scenario,sophisticated microfabrication technology can significantly reduce the cost of optical components.Meanwhile the optical interconnection will provide larger band width for electronics circuit.However,the development of silicon photonics is restricted by lack of efficient and process compatible light source.Silicon material is intrinsically not suitable for active optical applications due to its indirect band-gap structure.Using quantum confinement,silicon nanostructures show promising light emission property,which has been experimental proved by porous silicon and silicon nanocrystals.In this thesis,research results on advanced silicon nanostructures,such as silicon-rich silicon nitride(SiNx) and nano-silicon multi-quantum-well(Nano-Si MQW) thin films and devices are presented:1) SiNx thin films with tunable light emission band in visible range were grown by plasma-enhanced chemical vapor deposition.The flow rate of silane and ammonia determines the silicon content of SiNx,as well as its optical band gap(E_opt) and refractive index.With higher silicon content,the refractive index of SiNx is larger, while the optical band gap is smaller.The PL peak of SiNx blue-shifts when the E_opt becomes larger.The luminescence lifetime of SiNx depends on the emission energy with a characteristic value of nanoseconds.All evidences suggest that PL of SiNx originates from the band-tail states transition.2) PL of annealed SiNx strongly depends on the silicon content.With lower silicon content,PL spectra show band-tail states and defect states emission;with higher silicon content,PL quenches with increasing of the annealing temperature; only with medium silicon content,PL from silicon quantum dot(Si-QD) can be observed with a maxima intensity from 800℃annealed sample.3) The electroluminescence(EL) of SiNx from ITO/SiNx(50nm)/p-Si metal-insulator-semiconductor structure shows an emission band centered at 610～ 670 nm.The EL peak doesn't depend on the Si content of SiNx but on the applied voltage.Under 3.8V 100mA,EL signal starts to be detectable with a power efficiency of 10^-6.The light emission pattern of the structure contains bright spots with a yellow color.By fitting the current-voltage curve with various electrical transport models,we found the transport can be described by Pool-Frenkel emission mechanism. Analyzing the electrical transport and band diagram reveal that EL of SiNx is due to the recombination of the electrons captured by silicon dangling bonds and the holes trapped in the valence-band-tail.4) Nano-Si MQW was grown by alternately depositing silicon-rich silicon oxide (SRSO) and SiO₂ to 5 periods and followed with an 1150℃annealing to grow Si-QD from SRSO layers.Fitting the ellipsometry data with a multilayer model shows the thickness of sub-layers,SRSO and SiO₂ are within 2～4 nm.The growth of Si-QDs is confined inside the SRSO layers,revealed by transmission electron microscopy cross-section image.PL results of Nano-Si MQW show that the size of Si-QD is controlled by the initial thickness of SRSO layer.5) Nano-Si MQW light emitting devices(LED) fabricated with standard CMOS microfabrication processes show power efficiency around 0.04%,17-fold higher than single SRSO layer LED.The driving voltage of Nano-Si MQW LED is within 3～5V, and the EL can be detected at a voltage 1.29V,lower than Si/SiO₂ conduction band or valence band offsets,which is an indication of direct tunneling of electrons and holes involved in the light emission.When the driving voltage increases,the carrier transport is dominated by Fowler-Nordheim hot electron tunneling.In this region, the efficiency of LED decreases rapidly.The EL efficiency of Nano-Si MQW is restricted by the unbalanced electron and hole injection.Due to this reason,the radiative recombination takes place only in the anode side(p-type Si) of Nano-Si MQW,while those Si-QDs closed to the cathode(n-type poly-Si) are not actively involved in the light emission,only to facilitate the direct tunneling current.