Characterization of as-grown and annealed narrow band gap nitrides grown by molecular beam epitaxy
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The application of narrow band gap nitrides to optoelectronic devices is a rapidly growing field, with the promise to supplant existing material systems used for optical fiber communications. While the field is relatively new, significant device advancements have occurred. These improvements have been driven by experimental advances in growing high quality material. The rush for device improvement has left some gaps in the understanding behind the growth improvements. This work examines several fundamental growth parameters in detail, to improve the understanding of their relationship to optical quality of the films. First, the plasma source operation is examined for nitrogen production and ion production. Samples with and without ion removal are examined. Inconclusive results were found for the as-grown films with and without ion removal. Next, the dependence on growth temperature is examined, with strong improvement in optical quality occurring over the small range of temperatures examined. The RF power level and the growth system pressure is also examined, to look for independent and compounding effects. A critical RF power level was identified, between 175 W and 300 W, where damage to the crystal begins to occur. For the system pressure, a monotonic improvement in optical quality is seen as the system pressure is lowered. To more fully characterize the relationships found herein, annealing was performed on all of the samples examined. The growths with ions removed by deflector plates showed an interdependence between the deflector plate voltage and the gas flow rate: at low gas flow rates, high deflector plate voltage could be used to remove the ions and improve optical quality. At high flow rates, degradation occurred when the deflector voltage was too high. The temperature study samples retained their dependence after anneal, with higher temperature growths being superior optical quality. The critical RF power level was also supported by the post-anneal data, occurring near 300W. Finally, after anneal, the optical quality improvement continued to be seen as the system pressure is lowered. Understanding these observed relationships provide a set of recommendations for further improving the growth of narrow band gap nitride material.