UV-Visible-IR Electroluminescence from Si and Ge Nanocrystals in a Wider Bandgap Matrix
- PDF / 1,777,363 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 7 Downloads / 196 Views
ABSTRACT The demonstration of photoluminescence (PL) and electroluminescence (EL) in nanostructures of Si or Ge, such as those found in porous silicon, has significantly improved the prospects of all Si based photonic devices. While the physical mechanisms at work are still a subject of much study, it is clear that the luminescence is associated with the formation of nanometer or "quantum" sized particles. Further, it is clear that prototype NanoCrystal Displays (NCDs) and communication devices are being fabricated in these material systems. We report here on the electroluminescent properties of nanometer sized particles in an Si0 2 host matrix, which were fabricated by LPCVD techniques. The films have demonstrated reproducible emission from well below 400 nm to well above 800 nm. We believe that dispersion effects of the nanocrystals can account for "white" light emission. The films have been characterized using PL, Raman, XRD, TEM, and SIMS. The nanocrystals are primarily in the 2-7 nm range although larger crystal clusters are also observed. The development of stable and efficient Si or Ge nanocrystalline EL based devices could find applications in lamps/LEDs, photonic integrated circuits, and displays.
1. INTRODUCTION The recent observation of photoluminescence (PL) from porous silicon (PS) created a renewed interest in novel silicon based materials for light emitters and detectors. The quest for electroluminescence (EL) in Si based devices has made great strides in recent years, demonstrating functional EL devices from PS [1] and other Si/Ge nanocrystal based materials [2,3]. Silicon MOSFET structures have also demonstrated EL [4]. Importantly, the emitted light is both above and below the bulk bandgap. The indirect bandgap nature of bulk silicon (or germanium) has led to a controversy over the origin of PL (and EL) from nano-scale Si (or Ge) structures. The luminescence is believed to result from either surface state effects at the interface [5] or quantum size effects associated with the nanocrystal structures[6-1 1]. Zhao et al. recently reported evidence for direct-bandgap-like transitions in PL lifetime measurements[10]. Takagahara and Inkeda argue that quantum confinement causes an increased probability for radiative electron recombination as opposed to the phonon assisted electron recombination in indirect bandgap semiconductors[9]. Hybertson calculates that, for nanocrystals below -1.5nm, nonphonon assisted transitions dominate[12]. Khurgin et al. predicted a dramatic increase in the oscillator strength at very small dimensions[ 11]. Porous Si suffers from several major drawbacks such as poor mechanical strength and lack of long term stability. The inherent nature of high porosity and large internal surface, apt to chemical degradation and electron migration, account for many of these problems. The Si/Ge quantum nanocrystal (QNC)-oxide matrix system is structurally superior to PS since the matrix oxide provides mechanical strength and surface passivation, while supporting large numbers of QNCs. Furthe
Data Loading...
