Passivation and Optical Emission Modification of Gallium Nitride Surface Quantum Wells

Semiconductors are highly sensitive to surface adsorbents and the bonding of molecules to the surface. Often, surface cleaning and passivation practices are based on empirical evidence or studies conducted in high vacuum environments. An alternative is to use surface quantum wells (SQWs) consisting of a thin layer of lower bandgap material sandwiched by a vacuum or air interface on one side and a higher bandgap material on the other. These structures have been shown previously to have strong emission and to be sensitive to changes in surface recombination velocity that surface passivation treatments can alter. [1,2] Here, we systematically study the luminescence from GaN SQWs as a probe to investigate the interaction of acids and bases with a GaN surface. This is of special interest since III-Nitride devices, such as sensors, can depend on strain-induced spontaneous polarization, which can be strongly influenced by surface charge.<br/>The MOCVD-grown samples studied are similar to GaN capping layers sometimes used on high-electron-mobility transistors (HEMTs). The thin Ga-face terminated GaN SQWs vary in thickness between 2 and 2.8 nm and were formed on Al_0.2Ga_0.8N barriers to create the air or vacuum/GaN/AlGaN/GaN structure. The luminescence experiments were interpreted using self-consistent Schrödinger-Poisson equations to calculate the polarization and band structure of the GaN SQWs.<br/>Using photoluminescence (PL) and cathodoluminescence (CL) spectroscopy, we show that the electronegativity of the passivating agent plays a substantial role in the emission efficiency and wavelength of emitted light. Passivating with more electronegative species like chlorine from hydrochloric acid (HCl) shifts the SQW emission to the blue, while using less electronegative species like sulfur from ammonium sulfide results in a redshift. Additionally, if the sulfide passivation is performed after an HCl passivation, the amount of blueshift can be reduced. This results from the added bonding molecule that passivates the surface defects and changes the charge balance and amount of spontaneous polarization that tilts the SQW energy band.<br/>The changes in the magnitude of the SQW energy band tilt that result in the blue or redshift depend on the electronegativity of the passivating species relative to nitrogen and the number of bonded molecules. For example, when treated with HCl, a large ~8 meV blueshift is seen compared to an untreated sample due to interactions with chlorine anions, which have a greater electronegativity than nitrogen. When treated with ammonium sulfide, a redshift of ~5 meV is observed due to the sulfur anions, which are relatively electropositive compared to nitrogen. If treated with ammonium sulfide after HCl treatment, a smaller blueshift of ~2 meV is observed. We also observe corresponding changes in the intensity of the SQW peak. These changes are attributed to changes in surface recombination velocity and passivation of defects, as well as changes in recombination lifetime associated with the relative positions of the electron and hole wavefunctions as the SQW energy band is tilted.<br/><br/>1. E. Yablonovitch, et al., “Nearly ideal electronic surfaces on naked In0.53Ga0.47As quantum wells,” Applied Physics Letters, vol. 52, no. 12, pp. 1002–1004, Mar. 1988, doi: 10.1063/1.99226.<br/>2. J. F. Muth et al., “Gallium nitride surface quantum wells,” Applied Physics Letters, vol. 87, no. 19, Nov. 2005, doi: 10.1063/1.2123396.