Surface States Spectroscopic Characterization in GaN—From Bare Wafers to GaN HEMT

Surface states significantly affect various device properties. This is especially true for GaN and similar polar semiconductors. Broad luminescence bands at sub-band gap energies are commonly observed in wide gap polar semiconductors. In GaN, the most known emission feature is yellow-luminescence (YL) peaking around 2.2 eV. The origin of this luminescence remains unclear despite the numerous studies. Proposed explanations roughly divide between bulk and surface states. The former is based on ab-initio studies suggesting that Ga-vacancies compounded with O or O-C complexes in bulk are responsible for this emission. However, most of these first principle studies did not take into account surface states at all, and some suggested that the energetic configuration of defects on the surface may be same as in bulk.[1] In contrast, the latter is based mostly on various spectroscopic measurements, in which the surface origin was clearly evident. Several of these studies have further pointed out a correlation to surface adsorption. Experiments have shown that in air, UV illumination promotes adsorption, while in vacuum, the same illumination induces desorption of O₂.[2] In both cases, both surface photovoltage spectroscopy (SPS) and photoluminescence (PL) spectra of the yellow band were affected, which may be an evidence for its surface origin. Apparently, the most likely reason for the lack of conclusive identification of the molecules responsible for the YL lies in limitations of existing characterization methods. Most methods strongly perturb the surface equilibrium during measurement, thus affecting the measured features. For example, in PL, powerful UV laser beam is utilized for excitation. However, in addition to the desired effect this beam simultaneously changes the measured state. Moreover, PL only shows the total distribution of states, and cannot characterize the charge trapped inside them. Here, we propose a new method for characterization of charge trapped in surface states, based on SPS. We illuminate the sample by constantly increasing below-gap photon energies and measure the change in the surface band bending. At this photon energy range, surface band bending is mostly affected by two processes: excitation of surface charge over the surface barrier, and excitation of deep level charge within the depletion region. In most cases, surface charge excitation is the dominant process due to the high density of surface defects, compared with bulk states.<br/>We first applied our method to GaN HEMT. We obtained quantitative charge distribution and the total charge density at the AlGaN barrier surface, as well as the surface Fermi level position. Operating the transistor at different gate voltages, we were also able to characterize the dynamics of this surface charge.[3] On bare wafers, we were able to obtain qualitative surface charge distributions. After 450 K annealing and cooling back to room temperature in vacuum, we observed a major decrease in the YL-related feature. Although prior studies suggested that desorption of O₂ may underlie this effect, we suspected water. To test this hypothesis, we cooled the sample to 77 K in vacuum and gradually heated it up to achieve surface dehydration by sublimation. This was followed by 450 K anneal and cooling down to 300K while still in vacuum. Now, the YL band in the observed spectrum was practically diminished, while the 2.9 eV feature remained unchanged. These findings suggest that surface adsorbed water, likely in complex with other air elements, is the origin of the YL in GaN.<br/><b>References:</b> [1] C. G. Van de Walle, D. Segev, J. Appl. Phys. 101, 081704 (2007); [2] M. Foussekis et. al, Appl. Phys. Lett. 94, 162166 (2009); [3] Y. Turkulets, I. Shalish, Appl. Phys. Lett. 115, 023502 (2019)<br/><b>Acknowledgement: </b>Financial support from the Office of Naval Research Global through a NICOP Research Grant (No. N62909‐18‐1‐2152) is gratefully acknowledged. Approved for public release (DCN# 43-9231-22).