# Effects of Different InGaN/GaN Electron Emission Layers/Interlayers on Performance of a UV-A LED
## 3 Results and Discussion  
Figure 1b shows the schematic diagrams and TEM cross-sectional image of the layer structures grown on the sapphire substrates by MOCVD. The epi-structures were composed of n-type GaN/un-doped GaN layer, three pairs of Si-doped 2-/6- nm thick In\({}_{0.01}\)Ga\({}_{0.99}\)N/GaN interlayers (IL), and three pairs of 3-/12-nm thick In\({}_{0.01}\)Ga\({}_{0.99}\)N/GaN EELs. The last six pairs of InGaN/GaN MQWs with an 8-nm thick AlGaN barrier are clearly observable in the TEM image (Figure 1c), as is a p-GaN layer with 150-nm thickness. Figure 1d illustrates the schematics of band structures and current/carrier flow of UVLED with large bandgap barriers. The effective barrier heights for both electrons in conduction bands and holes in valence bands are reduced, attributed to the overflowing. Thus, the thermionic carrier escape rate from the MQWs to the barrier layers increases, especially at a higher carrier/current injection. The insertion of EELs/ILs surrounding the MQWs results in an increased effective electron capture rate, which leads to thermionic carrier leakage suppression. To evaluate the surface morphology of EELs before growth of the active layer, atomic force microcopy (AFM) measurements were carried out on a 5 \(\upmu\)m \(\times\) 5 \(\upmu\)m size sample using the GaN barrier layer-terminated EELs/ILs/n-GaN/u-GaN on Al\({}_{2}\)O\({}_{3}\) substrate (named following the UVLED chips as EI-UVLED I, EI-UVLED II, and EI-UVLED III) as shown Figure 2. The AFM images show well-defined step and terrace surface of the GaN barrier layer, which is related to terrace step-kink growth mode during the growth. In addition, a number of V-pits was also observed on the surface, which can affect the formation of pits or defects in the MQWs associated with a threading dislocation (TD) at the apex of a V-shaped pit with facets of {1011} due to the GaN growth process with the assistance of indium (In) atoms [13; 14; 15]. In addition, the observed etch pit density produced by propagation of TDs (yellow point) to the top surface of GaN in EI-UVLED I, EI-UVLED II, and EI-UVLED III is 1.7 \(\times\) 10\({}^{8}\)/cm\({}^{2}\), 1.6 \(\times\) 10\({}^{8}\)/cm\({}^{2}\), and 1.3 \(\times\) 10\({}^{8}\)/cm\({}^{2}\), respectively. The reduction of dislocation density in sample 3 is expected to show improvements in electrical and optical properties of UVLED devices. In addition, the surface roughness of the EELs/ILs/n-GaN/u-GaN/Al\({}_{2}\)O\({}_{3}\) substrate of samples 1, 2, and 3 are 0.202, 0.242, and 0.210 nm, respectively. The varied surface roughness can be attributed to the different diffusion lengths of Ga adatoms with change of the MO source of TMGa and TEGa on the surface of GaN. The shorter diffusion length of Ga atoms from TEGa results in accumulation of Ga atoms at the kink, restricting lateral growth and producing a relatively rough surface morphology [16]. Moreover, the surface line profiles (red line) of the three samples along the terrace and step show the different lengths and heights, as displayed in the right column of Figure 2. The terrace widths and step heights of the GaN surface are clearly distinguishable with the change of the MO source for the growth of GaN barriers. The average terrace length and height were widened from 0.08 \(\upmu\)m and 300 pm to 0.1 \(\upmu\)m and 320 pm, respectively, with change of the MO source from TMGa to TEGa. EI-UVLED II showed intermediate terrace width and height values between those of EI-UVLED I and EI-UVLED III. The terrace width and step height of the GaN surface are important parameters to determine the growth mode of post grown epi-structures [17]. The step and terrace formations are related to the Ehrlich-Schwoebel barrier (ESB) occurring at the step-edge where the energy barrier is higher than the kinetic energy of the Ga adatoms on the terrace [18]. The ESB energies impede movement of Ga adatoms over a step-edge, which creates an asymmetry in the merge of adatoms transported from terraces. The ESB energy strongly affects the surface morphology.