Effects of Nanoscale V-Shaped Pits on GaN-Based Light Emitting Diodes

Abstract

This paper reviews the formation of nanoscale V-shaped pits on GaN-based light emitting diodes (LEDs) grown by the metal organic chemical vapor deposition (MOCVD) system and studies the effect of V-shaped pits on quantum efficiency. Since V-pits could provide potential barriers around threading dislocations to lessen non-radiative recombinations in such a high defect environment. In our study, multiple InGaN/GaN quantum well samples with different emission wavelengths of 380, 420, 460, and 500 nm were grown, each with different nanoscale V-shaped pits of three diameters for 150, 200, and 250 nm, respectively. It was found that the multiple quantum well (MQW) sample with larger V-pits had a lower pit density, but a relatively larger total V-pits defected area. The optimum diameter of V-pits showing the highest quantum efficiency from the MQW sample depended on the emission wavelength. MQW samples with wavelengths of 380 and 500 nm exhibited the best internal quantum efficiency (IQE) performance at the smallest V-pits area; however, the best performance for MQW samples with wavelength around 420 and 460 nm occurred when large V-pit areas were presented. Photoluminescence (PL) peak shifts and Raman shifts can provide a relationship between quantum-confined Stark effect (QCSE) and IQE, as well as a comparison between strain and IQE. The results obtained in this phenomenological study shall provide a useful guide line in making high-performance GaN-based LEDs with wide emission spectra.

Keywords: GaN; light emitting diodes (LEDs); multiple quantum wells (MQWs).

Figure 1

Schematic diagram of structure with an ex-situ physical vapor deposition (PVD) AlN nucleation layer. The diameter, space, and height of the cone on the patterned sapphire substrate (PSS) were 2.8, 0.2, and 1.8 μm, respectively. The enlarged image shows the nanoscale V-shaped pit taken by the TEM. MQW: multiple quantum wells; uGaN: unintentionally-doped GaN.

Figure 2

SEM top view images of MQW layers grown on PSS with a pre-strained layer of various thickness grown beneath the MQWs to form V-shaped pits with different sizes. (A) 150 nm V-pits; (B) 200 nm V-pits; and (C) 250 nm V-pits, respectively.

Figure 3

Pits density and pits area ratio as a function of pits diameter.

Figure 4

The SEM image MQWs with 250 nm diameter V-pits at wavelength of (A) 380 nm; (B) 420 nm; (C) 460 nm; and (D) 500 nm. The plane-view monochromatic cathodoluminescence (CL) image of MQWs with 250 nm diameter V-pits at wavelength of (E) 380 nm; (F) 420 nm; (G) 460 nm; and (H) 500 nm.

Figure 5

The normalized internal quantum efficiency (IQE) values of MQWs as a function of power density at 12 and 300 K with varying V-pits diameters of 150, 200, and 250 nm, respectively, at wavelength of (A) 380 nm; (B) 420 nm; (C) 460 nm; and (D) 500 nm.

Figure 5

The normalized internal quantum efficiency (IQE) values of MQWs as a function of power density at 12 and 300 K with varying V-pits diameters of 150, 200, and 250 nm, respectively, at wavelength of (A) 380 nm; (B) 420 nm; (C) 460 nm; and (D) 500 nm.

Figure 6

The droop onset and IQE values as a function of V-shaped pits diameter for samples with different emission wavelength of 380, 420, 460, and 500 nm, respectively.

Figure 7

(A) Peak position and full width at half maximum (FWHM) as a function of pump power (dashed line is the bound of QCSE screening and Burstein–Moss effect); (B) Comparison chart of peak shift and IQE; (C) Raman spectrum of 460 nm sample with 250 nm V-shaped pits. Inset shows a zoomed-in view of the E₂^high peak. (D) Comparison chart of Raman shift and IQE.

Figure 7

(A) Peak position and full width at half maximum (FWHM) as a function of pump power (dashed line is the bound of QCSE screening and Burstein–Moss effect); (B) Comparison chart of peak shift and IQE; (C) Raman spectrum of 460 nm sample with 250 nm V-shaped pits. Inset shows a zoomed-in view of the E₂^high peak. (D) Comparison chart of Raman shift and IQE.