MBE and RE-doped GaN
Achievement of Full-color Emission from GaN:RE ELDs (Don)
We have achieved the first visible color emission using in-situ doped GaN:Er ELDs. Emission of the three primary colors has been accomplished using the following rare earths: Er3+ doping for green light; Eu3+, Pr3+ doping for red light; Tm3+ doping for blue light. Intermediate or mixed colors from GaN:RE ELDs have been also achieved through uniform co-doping during growth, such as turquoise from GaN co-doped with Er and Tm, orange or yellow from GaN co-doped with Er and Eu. Fig. 1 shows those basic and intermediate colors from ELDs. Fig. 2 shows EL spectra exhibiting very narrow peaks of characteristic 4f-4f inner shell transitions of rare earth ions.
All the colors obtained so far have been plotted on CIE diagram as shown in Fig. 3, which shows that the triangle of three basic colors of GaN:Er ELDs is very close to NTSC standard for TV. For some selected rare earth ions Fig. 4 shows observed optical emissions in PL (and EL) and their corresponding energy levels in GaN band gap.
Optimization of GaN:RE ELDs (Don)
Through detailed studies with GaN:RE ELDs we have reached the understanding that the following are the most crucial factors for obtaining and improving EL emissions: RE concentration, growth temperature, and stoichiometry (i.e., V/III ratio) of the host material. Growth and optical properties of GaN:RE thin films, concentrating on the optimization of EL emission as a function of those growth parameters are shown here. Er-doped GaN films were used for this optimization study.
(1) Rare Earth Concentration
We conclude that the optimum Er concentration is ~1 at. % through visible and IR PL intensities plotted in Fig. 5a. Green emission at 537 nm is one of the characteristic emissions due to 4f-4f inner shell transition of Er3+ ion and is attributed to a transition from 2H11/2 to 4I15/2. IR emission at 1.54 m m is a well-known transition from 4I13/2 to 4I15/2 and has special importance in optical communication applications. As shown in Fig. 5a, both of these emissions have maxima at about 1 at.% Er. It is well known in phosphor materials that the optical excitation intensity of rare earth ions exhibits an optimum concentration. The optimum Er concentration is 0.5 - 1 at.% for visible EL emission (Fig. 5b), which is almost as same as that observed in PL. Combining this with the results of visible PL lifetime and XRD previously reported,
(2) Growth Temperature
Experiments designed to determine the optimum growth temperature for GaN:Er films were carried out at temperatures ranging from 100 to 750 ° C. The Er cell temperature was fixed at 860 ° C. Interestingly, all films exhibited EL regardless of growth temperature. The maximum BIV (i. e. the largest current-normalized brightness obtained over the current-voltage range measured in each case) for each device is plotted in Fig. 6. Starting at low growth temperature, BIV increases nearly exponentially with growth temperature, exhibits a maximum at ~ 600 ° C and decreases at higher temperatures. This is the same trend as obtained from the PL data and from structural characterization, reinforcing the conclusion that 600 ° C is the optimum growth temperature for visible emission from GaN:Er.
(3) Effect of V-III Ratio or Ga Flux
RE optical emission from GaN films is a strong function of the ratio of the Ga- and N-bearing fluxes (V/III) during growth. Fig. 7 shows the visible and IR PL intensity as a function of Ga flux. Shaded region in the plot specifies stoichiometric growth region determined by growth rate saturation (Ga flux between 4.5´ 10-7 and 5.0´ 10-7 Torr). Above GaN bandgap excitation with the He-Cd laser (325 nm) was utilized in order to observe emission from Er3+ ions, as well as ultraviolet (UV) band edge emission of the GaN host. Visible and IR data were normalized to the incorporated Er concentration. The GaN band edge emission at 369 nm was much stronger in samples grown under Ga-rich conditions, consistent with the results of other groups. Both visible and IR emission increased with Ga flux up to the stoichiometric growth condition followed by a very abrupt emission reduction, to the detection limit of the measurement. EL emission exhibited the same trend. We conclude that the optimum growth condition of Er optical activity is found under slightly N-rich flux near the stoichiometric region.
Lateral Color Integration (Don, John)
Lateral integration has been successfully achieved through both using shadow mask technique and. Through shadow mask technique we have obtained 2-color integration (red and green) shown in Fig. 8. We have recently realized 3-color integration using SOG (spin on glass) LoL (lift-off layer) technique shown in Fig. 9. We believe that this technique can be used for development of flat panel displays, especially head-mount displays.