Study on energy band of InGaN/GaN self-assembled quantum dots by deep level transient spectroscopy

Abstract

Optoelectronics in the blue-green spectral range is presently based on the group-. 9 nitride compounds. Blue and green high-brightness light-emitting diodes (LEDs) made from InGaN/GaN quantum wells structures is now commercialized with output powers lager than 5mW at 20mA. But it is currently proposed that a fluctuation of indium composition due to InGaN phase separation results in the formation on Isrich clusters which act as quantum dots (QDs). In QDs, the carriers are deeply localized and their migration toward nonradiative defects is hindered. Therefore, high luminescence efficiency is expected if the density of QDs is much higher than that of dislocations. A different and attractive way for realizing QDs is to take advantage of the selEformation of three dimensional islands during the growth of highly strained InGaN on GaN.Recently, photoluminescence (PL) methods are widely used in the side of a characterization. The methods are comparably convenience and produce direct band-to-band transition information, The information is very useful to application of optical devices, However, PL can not determine offset of energy state of QD related to the band edge of the host matrix. The information is important to application of not only optical devices but also electrical devices. It is characterized by several methods for instance electrical methods such as I-V, C-V, admittance spectroscopy, deep level transient spectroscopy (DLTS), and temperature dependent PL, etc. Among those, DLTS is very useful and powerful tool to find out not only energy state of QDs but also deep level of epi-layer. [ 11 The InGaN QD structures used in this study were grown on Ab03 substrates by selfassembled method using low-pressure metaLorganic chemical vapor deposition system. The GaN buffer layer with a thickness of 1 m was grown at 1080-*under the reactor pressure of 76 torr and then the InGaN QDs and GaN of 20 nm capping layers were grown at 640-The density of QDs was 7.0 x lo9 cm-2, which were confirmed by atomic force microscope measurement (Fig. 1). In fig. 1, PL signal with peak of390 nm is responsible to QDs. Figure 2 shows the DLTS spectra obtained from the InGaN QD structure. Origins of the DLTS signals were reported [2] as nitride vacancy, antisite point defect and interface dates. On the other hands, the signal shown in inset of the circle was only appeared at shallow region (measure and pulse bias can determine scan depth). Thus, we suppose that the origin of this signal may be a confined level of the InGaN QDs. From the Arrhenius plot, the apparent activation energy and the emission cross section by the carrier emission process of QD-related levels are extracted 0.37 eV from the conduction band edge and 1.13 x cm' , respectively. In order to construct the energy band structure and clarify the origins of electrical active defects, further discussions are needed and to be presented. References[I] V.