1.3-μm InAs/GaAs quantum-dot lasers monolithically grown on Si substrates using InAlAs/GaAs dislocation filter layers Mingchu Tang,1 Siming Chen,1,* Jiang Wu,1 Qi Jiang,1 Vitaliy G. Dorogan,2 Mourad Benamara,2 Yuriy I. Mazur,2 Gregory J. Salamo,2 Alwyn Seeds,1 and Huiyun Liu1 1

Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, UK 2 Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA * [email protected]

Abstract: We compare InAlAs/GaAs and InGaAs/GaAs strained-layer superlattices (SLSs) as dislocation filter layers for 1.3-μm InAs/GaAs quantum-dot laser structures directly grown on Si substrates. InAlAs/GaAs SLSs are found to be more effective than InGaAs/GaAs SLSs in blocking the propagation of threading dislocations generated at the interface between the GaAs buffer layer and the Si substrate. Room-temperature lasing at ~1.27 μm with a threshold current density of 194 A/cm2 and output power of ~77 mW has been demonstrated for broad-area lasers grown on Si substrates using InAlAs/GaAs dislocation filter layers. ©2014 Optical Society of America OCIS codes: (230.5590) Quantum-well, -wire and -dot devices; (250.5960) Semiconductor lasers; (250.5300) Photonic integrated circuits.

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1. Introduction The need to develop practical laser sources on a Si platform becomes increasingly urgent for silicon photonics [1]. Unfortunately, Si is an indirect bandgap semiconductor and thus an inefficient light-emitting material. Recently, much research effort in both industry and academia has been devoted to searching for the last missing element of Si photonics - an efficient, electrically pumped laser grown directly on a Si substrate. Significant progress in gaining light from Si has been made in the last decade. Novel approaches, such as heterogeneous/monolithic integration of III-V/Si, stimulated Raman scattering, nanostructured Si, and rare-earth-doped Si, have been demonstrated as alternative means to extract light from Si. These efforts have led to successful demonstrations for roomtemperature lasing, including Si Raman lasers, hybrid Si lasers, and III-V and Ge lasers epitaxially grown on silicon [2, 3], that can potentially address the light source issue on Si. However, there are still numerous challenges and practical issues facing these techniques. The application of Raman lasers is constrained by optically pumped operation [4]. The feasibility of using low-dimensional Si materials, such as nanoporous Si and Si nanocrystals, as efficient emitters has not yet been demonstrated [5]. The potential of band-engineered Ge-on-Si lasers

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Received 17 Mar 2014; accepted 27 Apr 2014; published 5 May 2014 19 May 2014 | Vol. 22, No. 10 | DOI:10.1364/OE.22.011528 | OPTICS EXPRESS 11529

is also undermined by high optical loss and a high material gain is required for electrically pumped lasers [6]. In the last a few years, III-V on Si (III-V/Si) photonics via monolithic or heterogeneous integration has attracted much attention. Heterogeneous III-V/Si integration through wafer bonding technology has demonstrated impressive lasers with milliwatt power output and continuous-wave operation to temperatures over 100 °C [7, 8]. However, the yield and reliability for heterogeneous integration has yet to be proved [9, 10]. Monolithic growth of III-V materials on Si is considered as the most desirable approach for III–V/Si integration, but the high dislocation density caused by large lattice mismatch and the difference in thermal expansion coefficient between III-V epilayers and Si substrates make the monolithic III–V/Si integration challenging [11, 12]. Recently, III-V quantum dots (QDs) have been emerging as a promising technique for practical III-V/Si photonics due to their attractive properties, in particular the improved tolerance to defects and delta-function density of states [13, 14]. As a result, high performance QD lasers at optical communications wavelengths with low threshold currents, high power output, and high operation temperature have been demonstrated on Ge and Ge-on-Si substrates [15–20]. For direct epitaxial growth of III-V materials on Si substrates, a buffer between Si and III-V active regions plays a critical role in the performance of laser devices due to the large lattice mismatch [3]. In this paper, the effect of strained-layer superlattices (SLSs) as dislocation filter layers (DFLs) on the density of threading dislocations has been investigated. The density of threading dislocations can be effectively reduced down to ~106 cm−2 by using InAlAs/GaAs SLSs. A QD laser directly grown on a Si substrate with InAlAs/GaAs SLSs by molecular beam epitaxy is demonstrated with a low threshold current density of 194 A/cm2, a peak lasing wavelength at ~1.27 μm, and output power of ~77 mW at room temperature. Operation up to 85 °C has been measured for the as-cleaved broad-area lasers. 2. Effects of SLS on the quality of III-V materials directly grown on Si substrates InAs/GaAs QD samples were grown on n-doped Si (100) substrates with 4° offcut to the [011] plane using a solid-source III-V molecular beam epitaxy system. Oxide desorption of Si substrates was performed at 900 °C for 10 minutes. The substrates were then cooled down to 400 °C for the growth of a GaAs nucleation layer. The nucleation layer consists of an optimized two-step growth scheme [3, 14]. Three repeats of SLS DFLs separated by 400-nm GaAs spacing layers were grown on the top of a 1000-nm GaAs buffer layer. Two types of SLSs, five-period of 10-nm In0.15Ga0.85As/10-nm GaAs and five-period of 10-nm In0.15Al0.85As/10-nm GaAs were investigated in this study. After another 400-nm GaAs spacing layer, a typical ðve-layer InAs/GaAs dot-in-a-well (DWELL) structure was grown at ~510 °C similar to that optimized on GaAs substrates [21–23]. The DWELLs were embedded between two 100-nm GaAs layers grown at 580 °C and 50-nm AlGaAs layers grown at 610 °C. Each DWELL layer consisted of 3-monolayer InAs QD layer sandwiched by 2-nm In0.15Ga0.85As and 6-nm In0.15Ga0.85As. Undoped GaAs spacer layers of 45 nm were used to separate the InAs/InGaAs DWELLs. Atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (TEM) measurements are used to characterize the structural properties of QDs grown on Si substrates with different DFLs. The AFM scans were performed with a Nanoscope Dimension 3100 SPM AFM system in ambient conditions using a tapping mode. The TEM imaging was carried out by using a FEI Titan 80-300 high-resolution TEM. Despite the use of different types of SLSs as DFLs, the InAs/GaAs QDs grown on the Si substrates share similar structural properties. Typical AFM and TEM images of uncapped InAs QDs are shown in Figs. 1(a) and 1(b), respectively. From the AFM and TEM, the dot density is estimated to be ~4 × 1010 cm−2 and typical dot size is about 25 nm in diameter and ~7-8 nm in height. Figure 1(c) shows a cross-sectional TEM image of five-layer DWELL structure grown on Si substrates. No dislocation is observed in a number of similar images, which suggests that, by using the growth techniques we developed to suppress antiphase domains and threading dislocations [15, 18, 24], the active regions with low defect density can be obtained. #208349 - $15.00 USD (C) 2014 OSA

Received 17 Mar 2014; accepted 27 Apr 2014; published 5 May 2014 19 May 2014 | Vol. 22, No. 10 | DOI:10.1364/OE.22.011528 | OPTICS EXPRESS 11530

Fig. 1. (a) AFM image (1 × 1 μm2) of InAs/GaAs QDs grown on a Si substrate. (b) Crosssectional TEM image of an uncapped InAs QD. The scale bar is 10 nm. (c) Cross-sectional TEM bright field image of five layers of DWELL structure grown on Si substrate. The scale bar is 100 nm.

Fig. 2. Cross-sectional TEM dark field multi-beam images showing defect reduction induced by (a) InGaAs/GaAs SLS and (b) InAlAs/GaAs SLS. The scale bars are 1 µm. (c) Reduction of dislocation induced by the SLS layers measured at different position. The defect density is measured in ~20 microns area. After the third SLS, EPD technique is also used to estimate the defect density.

In order to study the effects of InGaAs/GaAs and InAlAs/GaAs SLSs serving as threading dislocation filters, TEM images with low magnification are compared as shown in Figs. 2(a) and 2(b). For both samples, a high density of dislocations is generated at the GaAs/Si interface as a result of the large lattice mismatch. Most of the defects are confined in the first ~200 nm region thanks to the two-step low temperature growth but still a quite high density (~109 cm−2) of threading dislocation is propagating towards the active region. As shown in Figs. 2(a) and 2(b), the SLSs can effectively suppress the propagation of the threading dislocations by bending the threading dislocations into the growth plane [24, 25]. The TEM images in Figs. 2(a) and 2(b) shows that GaAs layers are visually dislocation free after two sets of InAlAs/GaAs SLSs with a few dislocations after the InGaAs/GaAs SLSs. The reduction of dislocation density induced by SLSs is estimated by TEM, as shown in Fig. 2(c). After the first set of SLSs, the dislocation density is reduced over one order of magnitude (

GaAs dislocation filter layers.

We compare InAlAs/GaAs and InGaAs/GaAs strained-layer superlattices (SLSs) as dislocation filter layers for 1.3-μm InAs/GaAs quantum-dot laser structu...
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