InGaAs Quantum Well Grown on High-Index Surfaces for Superluminescent Diode Applications
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The morphological and optical properties of In0.2Ga0.8As/GaAs quantum wells grown on various substrates are investigated for possible application to superluminescent diodes. The In0.2Ga0.8As/GaAs quantum wells are grown by molecular beam epitaxy on GaAs (100), (210), (311), and (731) substrates. A broad photoluminescence emission peak (~950 nm) with a full width at half maximum (FWHM) of 48 nm is obtained from the sample grown on (210) substrate at room temperature, which is over four times wider than the quantum well simultaneously grown on (100) substrate. On the other hand, a very narrow photoluminescence spectrum is observed from the sample grown on (311) with FWHM = 7.8 nm. The results presented in this article demonstrate the potential of high-index GaAs substrates for superluminescent diode applications.
KeywordsPhotoluminescence Quantum well High-index surfaces Superluminescent diode Atomic force microscopy
Superluminescent diodes (SLDs) have been of great interest due to various applications, such as optical coherence tomography (OCT) and optical sensors [1, 2]. Significant research efforts have been focused upon increasing the spectral bandwidth as broadband SLDs are expected to improve depth resolutions for OCT systems [1, 2, 3, 4]. A variety of approaches have been used to broaden the spectral bandwidth by engineering the device active regions through including quantum dots , multiple quantum wells (MQWs) , stacked twin active layers , asymmetric dual quantum wells , and quantum-well intermixing .
Among the many attempts in broadening the spectral width, there have been surprisingly few, if any, efforts made on high-index substrates based SLDs. Due to the difference in growth kinetic, strain, charge or surface polarity, as well as band structures on high-index surfaces [8, 9, 10, 11, 12], semiconductor quantum structures on these surfaces have been showing interesting phenomena [13, 14, 15, 16]. For example, lateral ordering of QDs and QD clusters have been observed on B-type high-index surfaces and (731) surface, respectively [17, 18]. Even though intensive research efforts and significant progresses have been made on the high-index GaAs surfaces, there is a lack of research on high-indexed substrate for superluminescent diodes.
Accordingly, this work focuses on the effects of surface orientation on morphological and optical properties as well as explores the possibility of high-index surfaces for SLD applications. It has been well known that different substrate orientations have different growth reaction kinetics as well as various microscopic patterning of the surface . The latter property, natural roughness, presents on the high-index substrates influence the structure electronic properties and hence providing an additional means of emission spectrum broadening . Therefore, high-index surfaces may help advance the state of the art in SLDs in broadening emission spectrum. In this article, GaAs (100)-, (210)-, (311)-, and (731)-oriented substrates are used in this study.
The In0.2Ga0.8As/GaAs quantum well structures under investigation are grown simultaneously on epi-ready GaAs substrates by a solid-source molecular beam epitaxy (MBE) 32P Riber system. In this work, four substrates (100), (210), (311), and (731) are chosen and mounted side by side on a same molybdenum block. An important point to mention here is that the growth conditions are optimized only for (100)-oriented substrate. After oxide desorption at 600°C, a GaAs buffer layer of 500 nm is grown at substrate temperature of 580°C. The GaAs growth rate is one monolayer (ML) per second. Sequentially, 400-nm GaAs was grown at 0.7 ML/s, followed by 50 nm Al0.3Ga0.7As layer. The active region consists of only one In0.2Ga0.8As quantum well of 7 nm grown at 520°C, sandwiched by 50 nm barriers of GaAs. Another 50-nm Al0.3Ga0.7As layer is grown following the growth of the GaAs barrier. In the end, the entire growth is finished by another layer of 50 nm GaAs, and an additional 7-nm In0.2Ga0.8As is deposited on the surface for morphology study. All samples are not doped to eliminate the doping effects on optical properties of different oriented planes. Morphology study is carried out by an atomic force microscopy (AFM). The optical properties of all samples are studied by photoluminescence (PL) technique from 77 to 295 K. The excitation wavelength and power is 532 nm and 50 mW, respectively.
Results and Discussion
FWHM values of In0.2Ga0.8As/GaAs PL spectra on all orientations, and energy shift ∆E (= E(100) − E(xyz)) for high-index surfaces compared to (100)
In conclusion, the morphology of In0.2Ga0.8As/GaAs grown on GaAs (100), (210), (311), and (731) substrates has been studied by AFM. Three-dimensional nanostructure has been observed for (210)- and (311)-oriented surfaced. Moreover, the optical properties of the samples have been investigated by PL measurement. Spectral broadening has been observed for sample grown on (210) surface. The results presented in this work suggest future potential of high-index surfaces for superluminescent diode applications.
This work was supported by the Arkansas Biosciences Institute and the MRSEC Program of NSF Grant No. (DMR-0520550).
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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