Effects of GaN/AlGaN/Sputtered AlN nucleation layers on performance of GaN-based ultraviolet light-emitting diodes

Abstract

We report on the demonstration of GaN-based ultraviolet light-emitting diodes (UV LEDs) emitting at 375 nm grown on patterned sapphire substrate (PSS) with in-situ low temperature GaN/AlGaN nucleation layers (NLs) and ex-situ sputtered AlN NL. The threading dislocation (TD) densities in GaN-based UV LEDs with GaN/AlGaN/sputtered AlN NLs were determined by high-resolution X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM), which revealed that the TD density in UV LED with AlGaN NL was the highest, whereas that in UV LED with sputtered AlN NL was the lowest. The light output power (LOP) of UV LED with AlGaN NL was 18.2% higher than that of UV LED with GaN NL owing to a decrease in the absorption of 375 nm UV light in the AlGaN NL with a larger bandgap. Using a sputtered AlN NL instead of the AlGaN NL, the LOP of UV LED was further enhanced by 11.3%, which is attributed to reduced TD density in InGaN/AlInGaN active region. In the sputtered AlN thickness range of 10-25 nm, the LOP of UV LED with 15-nm-thick sputtered AlN NL was the highest, revealing that optimum thickness of the sputtered AlN NL is around 15 nm.

Figure 1

(a) Schematic illustration of UV LED structure. (b,c), and (d) Cross-sectional TEM images of UV LED structure.

Figure 2. Morphological evolution of GaN grown on PSS with thickness of approximately 30, 300, 700, and 1000 nm, respectively.

(a–d) Top-view SEM images of GaN grown on PSS without NL. (e–h) Top-view SEM images of GaN grown on PSS with GaN NL. (i–l) Top-view SEM images of GaN grown on PSS with AlGaN NL. (m–p) Top-view SEM images of GaN grown on PSS with sputtered AlN NL.

Figure 3

Cross-sectional TEM images of GaN grown on PSS with (a) 25-nm-thick GaN NL, (d) 25-nm-thick AlGaN NL, and (g) 25-nm-thick sputtered AlN NL. 1^st Row: The magnified TEM images of GaN NL on (b) flat c-plane sapphire and (c) inclined sidewall of PSS. 2^nd Row: The magnified TEM images of AlGaN NL on (e) flat c-plane sapphire and (f) inclined sidewall of PSS. 3^rd Row: The magnified TEM images of sputtered AlN NL on (h) flat c-plane sapphire and (i) inclined sidewall of PSS.

Figure 4. Cross-sectional TEM images of UV LEDs grown on PSS with GaN/AlGaN/sputtered AlN NLs.

Bright-field TEM images of UV LED grown on PSS with (a) GaN NL, (d) AlGaN NL, and (g) sputtered AlN NL, zone axis GaN []. Bright-filed TEM images of UV LED with (b) GaN NL, (e) AlGaN NL, and (h) sputtered AlN NL when g = 0002. Bright-filed TEM images of UV LED with (c) GaN NL, (f) AlGaN NL, and (i) sputtered AlN NL when g = .

Figure 5

(a) Symmetric (002) and (b) asymmetric (102) XRD ω-scan rocking curves of UV LEDs grown on PSS with GaN/AlGaN/sputtered AlN NLs.

Figure 6

XRD ω-rocking scans of (a) symmetric (002) and (b) asymmetric (102) for UV LEDs grown on PSS with various sputtered AlN NL thicknesses.

Figure 7. Comparison of optical and electrical characteristics of UV LEDs grown on PSS with different NLs.

(a) L-I curves of UV LEDs with 25-nm-thick GaN/AlGaN/sputtered AlN NLs. (b) I-V curves of UV LEDs with 25-nm-thick GaN/AlGaN/sputtered AlN NLs. (c) L-I curves of UV LEDs with various thicknesses of sputtered AlN NL. (d) I-V curves of UV LEDs with various thicknesses of sputtered AlN NL.