## 1 Introduction  
Recently, technological progress in III-nitride based wide bandgap semiconductors has been greatly encouraged because of a continuous demand for energy saving, long lifetime, and environmentally friendly devices [1; 2; 3]. Since the bandgap of GaInN ternary alloys covers a broad spectral range, from visible to ultra-violet, these materials are suitable for optoelectronic devices such as light-emitting diodes (LEDs), laser diodes, solar-cells, and photo-detectors (PDs) [4; 5; 6; 7]. III-nitride based semiconductors are typically grown on sapphire substrates by using metal-organic chemical vapor deposition (MOCVD) because homo-epitaxial substrates are still expensive due to a difficulty in mass production. However, the direct growth of a GaN film on a sapphire substrate leads to very poor crystal quality due to a large difference in the lattice constant and thermal expansion coefficients of the GaN film and the sapphire substrate. This problem was overcome by [PERSON] et al., who were the first to demonstrate that crystal quality and surface morphology can be remarkably improved by the introduction of a thin buffer layer [1; 8].  
Nowadays, III-nitride based LEDs are commercialized for use in highly-efficient solid-state lighting sources. In particular, an external quantum efficiency (EQE) of more than 80% has been reported for blue LEDs [9], whereas the EQE of green LEDs still remains around 30%-40%. That is, the EQE of GaInN-based LEDs drastically decreases as the emission wavelength increases, which is typically known as the \"green-gap\" problem [10; 11]. Several factors are considered to be the origin of the green-gap, including degradation of crystal quality [12], increase of stress [13], alloy fluctuation [14], and the quantum confined stark effect (QCSE) [15]. All of these are fundamentally related to an increase in the indium composition in the active region. One possible solution to overcome the green-gap could be strain engineering that employs a strain compensation/relaxation layer, e.g., superlattice layers (SLs), an AlGaN interlayer, a GaInN underlayer, and graded indium composition across multiple quantum wells (MQWs) in the active region [16; 17; 18; 19].  
Recently, we reported the improved photoluminescence (PL) efficiency in a GaInN-based green LED by simply replacing a conventional low temperature GaN buffer layer with a sputtered-AlN buffer layer [20]. In spite of the significant improvement in PL efficiency, the optical and electrical properties under electrical injection were not deeply investigated and the physical mechanism behind the observed improvement is not yet fully understood.  
In this study, we compare GaInN-based green LEDs grown on \(c\)-plane sapphire substrates with either a conventional low temperature GaN buffer layer or a sputtered-AlN buffer layer. The electrical and optical properties of both LEDs were compared by means of electro-reflectance (ER) spectroscopy, their current-voltage characteristics (I-V curve), electroluminescence (EL), light output power-current characteristics (L-I curve), and internal quantum efficiency-current characteristics (IQE-I curve) to understand the underlying physical mechanisms of the differences in their behavior.