Development of the Rare Earth-doped GaN Material System

Activities include thin film growth by MBE, surface studies, metallization, doping, etching, and device characterization. Rare-earth- (RE)-based electroluminescence (EL) in a matrix of ZnS (or other II-VI compounds) has been the key ingredient in many display technologies such as electroluminescent devices (ELDs). Recently, significant progress has been made using the wide band-gap III-V semiconductor GaN as the host for rare-earth based photoemission at visible and infrared (IR) wavelengths.  In the past two years it has been shown that RE-doped GaN ELDs can produce green (based on Er doping), red , (Pr, Eu) and blue light (Tm), as well as 1.55 µm infrared , emission (Er). The Nanolab utilized molecular beam epitaxy (MBE) in order to demonstrate the first observation of visible light emission from GaN:RE.  GaN:RE ELDs hold several promising qualities which are attractive for small- and mid-scale display and lighting. The emission from a GaN:RE ELD is spectrally sharp and requires no color filtering since it arises from atomic RE transitions. The red, green and blue GaN:RE ELDs utilize similiar device structure and power requirements which could simplify some critical electrical and material obstacles in multi-color integration of single color devices. GaN is an advantageous host for RE-based ELDs for several reasons: it has excellent high field (break down field ~3 MV/cm) carrier transport can provide hot carriers for impact excitation of the RE ions; it is transparent to visible RE emission; it is thermally and chemically rugged; unlike REs in most II-VI hosts the majority of the REs dopant sits substitutionally on the Ga sublattice preventing some doping defects due to charge neutrality. Fabrication of the GaN:RE ELDs is a relatively simple process in terms of the few processing steps involved. With the present device structures, single ELDs have been operated at power densities on the order of several hundred W/cm2 in a stable manner without the usage of a heat sink or active cooling.  Below, the observed RE atomic transistions in GaN are pictured.
Click to Zoom

Rare earth doped GaN Schottky barrier based light emitting diodes have been demonstrated by the NanoLab at room temperature in green,1.5 um IR and red light. A picture of one of the earliest fabricated diodes, affectionately named 'Big Bertha' is pictured to the left. As part of the LED fabrication the NanoLab is currently growing epitaxial GaN films on Si with the Riber MBE-32 system in the Nanolab. Schottky contacts are are patterned with sputter deposited Indium Tin Oxide optimized for each specific emission wavelength. For Erbium doped GaN strong green light emission is observed under reverse bias due to electron impact excitation of the Er atoms. Weaker emission is present under forward bias. The emission spectrum consists of two narrow green lines at 537 and 558 nm and minor peaks at 413, 461, 665, and 706 nm. The green emission spectrum is pictured bottom right. There is also strong emission at 1000 nm and 1540 nm in the IR. The green emission lines have been identified as Er transitions from the 2H11/2 and 4S3/2 levels to the 4I15/2 ground state. The IR emission lines have been identified as transitions from the 4I11/2 and 4I13/2 levels to the 4I15/2 ground state. Red light has been obtained from the same type of diode structures by incorporating Praseodymium doping instead of Erbium. Several other rare earth dopants have been employed in hopes to complete the red-blue-green color sequence so that the devices are a candidate for full color flat panel display use. Several papers about this work are available in Applied Physics Letters and also in the internet journal for the 1998 Fall MRS Meeting along with the 1999 MRS Fall Bulletin cover photo. A chart detailing the CIE coordinates for the GaN:RE ELDs appears at the bottom left. Note the close proximity of GaN:RE devices to the NSTC standard.