Influence of Quantum well Thickness Fluctuation on Optical Properties of InGaN/GaN Multi Quantum well Structure Grown by PA-MBE

Abstract

I. Introduction

InGaN ternary compounds have excellent properties including a tunable direct bandgap between 0.7 eV (InN) and 3.4 eV (GaN) [1,2,3,4]. In particular, the InGaN/GaN multiple quantum well (MQW) structure plays a key role in the field of optoelectric devices such as light emitting diodes or laser diodes. Such structures have been commercially successful in the blue range, enabling white light generation [5,6,7,8].

Increasing In content for the green and red spectral range is one of the aims of the ongoing development in this field. [9] While it is possible to achieve long wavelength (λ> 530 nm) by simply increasing the indium content in the QWs, a number of problems related to the InGaN ternary alloy system become serious. When the InGaN film was grown over the critical thickness, a rough surface morphology is formed due to the lattice mismatch between InGaN and GaN [10]. This causes the fluctuation of the QW thickness and quantum efficiency droop.

In this paper, we report the effect of QW size fluctuation on the optical properties of an InGaN/GaN MQW structure, which is essential to study MQW structure. The optical properties were investigated by photoluminescence (PL). The degree of disordered InGaN/GaN hetero interface was confirmed by atomic force microscopy (AFM) and transmission electron microscopy (TEM).

II. Experiments

Growth of the InGaN/GaN MQW structure on a 2-inch c-sapphire substrate was carried out in a MBE system, using a standard effusion cell for group III sources. A radio frequency (RF) plasma source was employed for active nitrogen (N^*). Figure 1 shows a schematic diagram illustrating the epitaxial structure. The GaN on the sapphire template consisted of a 50 nm-thick-AlN layer and a 500 nm-thick- GaN buffer layer. Finally, two pairs of In_0.3Ga_0.7N/GaN MQWs structure were grown at 620°C (substrate temperature) between two 50 nm-thick-In_0.05Ga_0.95N optical confinement layers (OCL). During the MQW growth, the source flux of constituting Ga and In atoms was F(Ga)= 1.7 × 10⁻⁷ torr and F(In)=2.1 × 10⁻⁷ torr, respectively. N^* was supplied at a rate of 1.5 sccm with RF plasma power at 280W. These growth conditions correspond to a metal-rich condition for growing a smooth film surface.

The information of the growth front was monitored during the growth in situ by RHEED. The structural properties of the sample were characterized by atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HR-TEM). Photoluminescence (PL) measurement was performed with a 266 nm DPSS laser source at 25 K to study the optical properties of the sample.

III. Results and Discussion

Figure 2 shows the RHEED patterns monitored during the growth of the InGaN/GaN MQW sample. For the growth of the GaN buffer layer, a metallic condition (Ga-rich) was consistently maintained. A smooth surface was indirectly verified from the streaky RHEED pattern (Fig. 2(a)). On the other hand, as the In atoms were incorporated into the InGaN layer, the patterns became increasingly spotty, despite that the growth conditions were unchanged. With increasing In composition of the InGaN film, the RHEED pattern became nearly circular. Figures 2(b) and 2(c) show the RHEED pattern during the In_0.05Ga_0.95N OCL and the In_0.3Ga_0.7N QW, respectively. With increasing In incorporation, the distance between the patterns became narrow due to the lattice constant.

In order to investigate the surface morphologies of each layer, AFM images with an area of 5 μm × 5 μm were taken. A bare GaN template, In_0.05Ga_0.95N on the template, and In_0.3Ga_0.7N/In_0.05Ga_0.95N on the template were separately prepared for characterizing their morphological properties. The underlying GaN template has a flat surface with a root mean square (RMS) roughness of 1.6 nm. As a 50 nm-thick-In_0.05Ga_0.95N layer grows, its surface morphology becomes rough (RMS=2.5 nm). This effect of surface roughening is generated by the 3-dimensional growth mode with strain relaxation, because the thickness exceeds the critical thickness of InGaN alloy. [11] With increasing In composition of the InGaN layer, the surface smoothness was the In_0.3Ga_0.7N layer, despite only 4 nm thickness (Fig. 2(c)).

Figure 4(a) shows a cross-sectional scanning transmission electron microscopy (STEM) image of the sample. The average thickness of each layer composing the MQW structure was measured by distinct contrast between InGaN QW and GaN QB. 30% of In composition for the InGaN QW layer was confirmed using energy dispersive X-ray spectroscope with an attached STEM. On the other hand, a rough InGaN/GaN hetero interface was observed in the HR-TEM image shown in Fig. 4(b). The thickness of the QWs ranges between 2 nm and 6 nm according to the positions, and variation of minimum quantum confined energy level in a well can be inferred. This phenomenon is due to the 3-dimensional growth mode during the growth of the InGaN QW layer.

Figure 5 shows the PL spectrum (open circles) of the InGaN/GaN MQW sample. The dash lines represent the fitted curves. From the fitting, the main peak consists of three PL emissions at 460, 495, and 538 nm. It appears that peak B is associated with quantum confinement emission, and peak C is from the bandgap edge emission in the InGaN QW layer. In terms of our MQW structure, it is calculated that the energy gap between the minimum confined level and the band edge varied from 55 meV (E_g=2.41 eV) to 194 meV (E_g=2.69 eV) as the QW thickness was decrease from 5 nm to 2 nm. It is expected that the broad peak A comes from the fluctuation of the QW thickness. On the other hand, a PL peak was observed at 650 nm and this is presumably due to the phase separation effect. In-rich localization is caused by the miscibility gap between InN and GaN in the InGaN alloy system [1]. More specifically, the strain from the lattice mismatch between InN and GaN plays a key role in the migration of the In atoms. This In-rich phase precipitates formed by the phase separation typically act like quantum dots with a high radiative recombination efficiency, despite their small volume.

IV. Conclusions

The effect of fluctuation of QW thickness on the optical properties of InGaN/GaN MQW structure is studied to gain a better understanding of quantum efficiency droop. As the In composition and thickness of the InGaN layer increased, the surface morphology became rough due to the critical thickness of InGaN layer. This roughening effect leads to inhomogeneity of the minimum confinement energy level in the QWs depending on location, resulting in dispersive PL signals. The formation of a sharp InGaN/GaN hetero interface is one of the most important aspects for preventing quantum efficiency droop in the InGaN/GaN MQW structure.

Acknowledgments

This project was supported by the National Research Foundation (NRF) of Korea (Grant Nos. 2015R1A2 A2A01004782, 2016R1A6A1A03012877 and 2015M2A2 A6A02045252).

Article information

References