Liquid-Metal-Enabled Synthesis of Aluminum-Containing III-Nitrides by Plasma-Assisted Molecular Beam Epitaxy

Nitride films are promising for advanced optoelectronic and electronic device applications. However, some challenges continue to impede development of high aluminum-containing devices. The two major difficulties are growth of high crystalline quality films with aluminum-rich compositions, and efficiently doping such films p-type. These problems have severely limited use of aluminum-rich nitride films grown by molecular beam epitaxy. A way around these problems is through use of a liquid-metal-enabled approach to molecular beam epitaxy. Although the presence of a liquid metal layer at the growth front is reminiscent of conventional liquid phase epitaxy, this approach is different in its details. Conventional liquid epitaxy is a near-thermodynamic equilibrium process which liquid-metal assisted molecular beam epitaxy is not. Growth of aluminum-rich nitrides is primarily driven by the kinetics of the molecular vapor fluxes, and the surface diffusion of adatoms through a liquid metal layer before incorporation. This paper reports on growth of high crystalline quality and highly doped aluminum-containing nitride films. Measured optical and electrical characterization data show that the approach is viable for growth of atomically smooth aluminum-containing nitride heterostructures. Extremely high p-type doping of up to $6 \times 10$$^{17}$ cm$^{-3}$ and n-type doping of up to $1 \times 10$$^{20}$ cm$^{-3}$ in Al$_{0.7}$Ga$_{0.3}$N films was achieved. Use of these metal-rich conditions is expected to have a significant impact on high efficiency and high power optoelectronic and electronic devices that require both high crystalline quality and highly doped (Al,Ga)N films

The Historical Performance of Real Estate Investment Trusts

This empirical study investigates the performance of Real Estate Investment Trusts (REITs). Although many people have studied REIT performance, their findings on the long-term performance of REITs are generally inconclusive. The objectives of this study are: (1) to evaluate the long-term (1970-1993) performance of REITs; (2) to examine the stability of REIT performance over time; and (3) to investigate the sensitivity of a specific performance measure, the Jensen index, to two general performance benchmarks and two REIT samples. The results indicate that the performance of the REIT portfolios was consistent with the security market line for the 1970-1993 period. However, REIT performance varied over the period. This study also found that the use of the unrepresentative S&P 500 index as a performance benchmark tends to overstate REIT performance. Finally, survivor REITs in general performed better than the overall REIT population.