The study of properties of blue-green InGaN/GaN multiple quantum wells grown at different pressures

Highlights

• The influence of different growth pressures on the structural characteristics of multiple quantum wells is investigated.

• The luminescence properties of localization states of quantum wells are studied.

• The formation of V-shaped pits is suppressed by increasing the growth pressure.

• The incorporation efficiency of indium is improved at higher growth pressure.

Abstract

In this work, blue-green InGaN/GaN multiple quantum wells (MQWs) were grown on SiC substrates by metal-organic chemical vapor deposition. The influence of growth pressure on the structural and optical properties and surface morphology of the MQWs was studied. The photoluminescence measurement results indicate that the indium content in the MQWs nearly doubles when the growth pressure increases from 100 to 400 mbar, and the MQWs grown at different pressures exhibit distinct temperature-dependent optical behaviors. Meanwhile, the growth pressure also influences the structural characteristics of the MQWs significantly. Besides, with changing the growth pressure, the surface root-mean-square roughness, V-shaped pits density, V-shaped pits size, and localization state density of the MQWs were changed accordingly.

Introduction

As a material widely used in the active area of light-emitting devices, InGaN alloy plays an important role in the group-III nitride material system and has received considerable attention thanks to its wide application in solid-state lighting [1,2]. By changing indium (In) content, the bandgap of InGaN alloy can be continuously adjusted from 0.7 to 3.4 eV [3], covering the entire visible spectrum. Although numerous studies have been done in InGaN-based blue light-emitting devices (LEDs) and laser diodes (LDs), the luminescence efficiency of InGaN-based long-wavelength LEDs with high In content is still low [4]. The difficulties mainly lie in two aspects. On the one hand, due to the large lattice mismatch (~11%) between InN and GaN [5], the high nitrogen vapor pressure required for InN growth [6], and the weak In–N bonding force [7], the preparation of high-quality InGaN/GaN multiple quantum wells (MQWs) with high In content is challenging. On the other hand, the built-in polarization electric field is much stronger in high In content MQWs, leading to the reduced radiative recombination efficiency [8,9].

The optimization of growth parameters (for instance, V/III ratio, temperature, and pressure) is essential to achieve a high-quality InGaN-based MQWs structure with high In content. Till now, the influence of V/III ratio and growth temperature on the properties of MQWs has been investigated extensively [[10], [11], [12], [13]], and consensus conclusions are formed. For example, decreasing the growth temperature can significantly increase the incorporation efficiency of In, and a higher V/III ratio is beneficial to suppress the phase separation, and therefore to improve the crystal quality, surface morphology, as well as luminescence characteristics of the MQWs. However, few reports focus on the influence of growth pressure on the performance of InGaN/GaN MQWs. Besides, the results of these reports are controversial. Kim et al. reported that the decrease of growth pressure could give rise to a significant increase in the incorporation efficiency of In in the InGaN films, accompanied by a deterioration of crystal quality [14]. However, the study reported by Strittmatter et al. showed that InGaN layers with higher In content could be achieved under a higher growth pressure, while InGaN layers with low In content did not exhibit localization light-emitting characteristics [15]. Park et al. reported that a higher growth pressure could lead to low growth rate of the InGaN layer, and therefore, InGaN/GaN MQWs with high crystalline quality and high In content could be obtained [16].

Despite all these efforts, the influence of growth pressure of InGaN/GaN MQWs on the peculiarity of the In incorporation and its structural characteristics, as well as on the localization states and surface morphology, remains an issue that requires further study. Among them, the localization effect is particularly important for MQWs. Since localization states can effectively confine the carriers and prevent them from being trapped by around defects, thereby increasing the radiation recombination rate [17]. At the same time, according to the theoretical studies, the localization effect also has a significant impact on Auger recombination and efficiency droop of InGaN-based LEDs [18,19]. In this work, InGaN/GaN MQWs were prepared under different growth pressures varying from 100 to 600 mbar. It is found that In incorporation efficiency in the InGaN/GaN MQWs is greatly increased via increasing the growth pressure. At the same time, increasing the growth pressure will also reduce the density and size of the V-shaped pits, and has a notable impact on the localization effect of the MQWs. It is reasonably believed that these results would facilitate the research progress of high-In content InGaN/GaN MQWs and long-wavelength InGaN-based light-emitting devices.