Metamorphic Buffer Layers for Mid-infrared Semiconductor Lasers 1
L. J. Mawst1*, D. Botez1, and T. F. Kuech2
Department of Electrical and Computer Engineering, University of Wisconsin – Madison, 1415 Engineering Dr., Madison, Wisconsin, 53706 2 Department of Chemical and Biological Engineering, University of Wisconsin – Madison, 1415 Engineering Dr., Madison, Wisconsin, 53706 *Corresponding author: Email
[email protected]
Abstract: We have investigated the use of Metamorphic Buffer Layer (MBL) structures for the realization mid-infrared semiconductor lasers employing either Interband or Inter-subband transitions. The resulting surface morphology of the MBL is generally cross-hatched along the orthogonal directions. This surface morphology may negatively impact device structures grown on top of the MBL. We have employed two different techniques to improve the surface morphology of MBLs while maintaining an epiappropriate surface chemical composition and structure: (1) Introduction of Sb as a surfactant to create novel step-graded MBLs; and (2) Chemical-Mechanical Polishing (CMP) of the MBL followed by MOCVD regrowth of layers atop the MBL. 1 Introduction: Metamorphic buffer layers (MBLs) can be utilized as virtual substrates with a specified lattice constant, opening up the palette of III/V alloys available for new device architectures. For compositionally step-graded MBLs, misfit dislocations are localized at the internal interfaces. The localization of these dislocations to internal interfaces serves to substantially reduce threading dislocations in the overlying MBL and in the subsequently grown device structure. While there has been considerable work and success on the devlopment and implementation of MBLs for electronic devices, much less effort has been directed towards these materials for furthering the development of mid-infrared semiconductor lasers. Design simulation studies indicate that
inter-subband transition devices, such as QCLs, would greatly benefit from the use of an MBL for achieving higher performance devices operating in the mid-IR spectral region. 2 Inter-band lasers employing MBL: Current high-performance inter-band lasers in the 2-4 µm wavelength region generally employ GaSb substrates [1-2]. However, semiconductor lasers on a conventional InP substrate would have a significant advantage over GaSb-based devices in terms of lower cost, mature processing technology, better thermal conductivity, and straightforward regrowth of buried-heterostructure or distributedfeedback (DFB) type devices. To that end, InAsxP1-x and InAsxP1-x(Sb) MBL structures were grown on (100) InP substrates by MOCVD [1,2]. Laser emission at LT (77K) near 2.5 µm is achieved from structures employing an InAs QW active region and InGaAs separate confinement heterostructure (SCH) with InAsP cladding layers grown on top of the InAsxP1-x MBL[1]. An improved heterostructure employing InP0.8Sb0.2 claddings and an InAs0.66P0.34 SCH grown on an InAsxP1x/InP MBL exhibits RT PL emission near 3 µm [2]. However, a high density of hillocks was typically observed from the growth of the InP0.8Sb0.2, which tend to align with the underlying cross-hatched surface morphology. Incorporating few percent Sb into the InAsxP1-x/InP MBLs is found to reduce the surface roughness (from ~15nm RMS to 4-9nm RMS), and allow for a larger process window for the growth of low hillock density InP0.8Sb0.2.
Furthermore, employing an As-free, InPySb1-y MBL leads to the lowest RMS roughness (3.4nm). 3 Inter-subband (Quantum Cascade) Lasers employing MBL: Short emission wavelength (i.e., < 4.0µm) quantum cascade (QC) lasers require deep quantum wells and tall barriers (i.e. higher strain) to prevent excessive activeregion carrier leakage, due to the large transition energy. However, the barrier and well compositions that can be accessed are limited by strain-thickness considerations in order to avoid strain relaxation. As the critical thicknesses for strain relaxation are approached, it is anticipated that device reliability may deteriorate through thermally-activated relaxation processes. Our simulation studies suggest that significant enhancement of performance is possible over conventional InP-substrate devices by employing an MBL design on a GaAs substrate. We achieved designs for 3.0- 3.6 µmemitting Tapered-Active (TA) QCLs on a MBL virtual substrate by employing a lattice constant of 5.77 Angstroms [3] (i.e., larger than for GaAs, but smaller than for InP). Note that since the virtual substrate has a smaller lattice constant compared to InP, we are able to employ large energy-bandgap barriers (i.e., AlAs) without encountering strain-relaxation issues. The maximum strain*thickness product (~0.44 Angstroms), is comparable to values employed in longer wavelength (4.8 µmemitting) deep-well TA QCLs grown directly on InP substrates [4]. Thus, we do not anticipate strain relaxation issues when growing this structure on an MBL virtual substrate. X-ray diffraction (XRD) data confirm that deep-well, straincompensated superlattice (SL) structures, representative of those used in the QCL active region, can be grown atop the MBL, although the surface morphology of the underlying MBL plays an important role in achieving planar SL structures. To improve the planarity of the MBL surface, we employ CMP prior to the growth of the SL structures. Optical mi-
croscope images of an InGaAs MBL before and after CMP are shown in Figure 1. Studies on 20-period InxGa1-xAs (wells)/AlyIn1-yAs (barriers) SL grown on an InGaAs step-graded HVPE-grown MBL [5] which had been subjected to CMP, exhibits significantly improved XRD SL fringes [5]. However, further work is required to improve the regrowth interface after employing the CMP process.
20 μm
20 μm
Fig. 1 Optical microscope images of InGaAs MBL before (left) and after CMP(right).
Acknowledgements This work is supported by the National Science Foundation under Grant No. DMR-1121288, Navy STTR N68335-11-C-0432, and ARO MURI W911NF-05-1-0262
References [1] Jeremy Kirch, Toby Garrod, Sangho Kim, Joo H. Park, Jae C. Shin, L. J. Mawst, T.F. Kuech, X. Song , S. E. Babcock , Igor Vurgaftman , Jerry R. Meyer , Tung-Sheng Kuan, “InAsyP1-y Metamorphic Buffer Layers on InP Substrates for Mid-IR Diode Lasers”, Journal of Crystal Growth, v 312, n 8, p 11659, 1 April 2010. [2] J. Kirch, T.W. Kim, J. Konen, L.J. Mawst, T.F. Kuech, T.-S. Kuan, Effects of antimony (Sb) incorporation on MOVPE grown InAsyP1−y metamorphic buffer layers on InP substrates , Journal of Crystal Growth, v 315, n 1, p 96101, 15 Jan. 2011 , [3] L.J. Mawst, J.D. Kirch C.-C. Chang, T. Kim, T. Garrod, D. Botez, S. Ruder, T.F. Kuech, T. Earles, R. Tatavarti, N. Pan, A. Wibowo, InGaAs/AlInAs Strain-Compensated Superlattices Grown on Metamorphic Buffer Layers for Low-Strain, 3.6µm-Emitting Quantum-Cascade-Laser Active Regions, Journal of Crystal Growth, in press. [4] J. D. Kirch, J. C. Shin, C.-C. Chang, L. J. Mawst , D. Botez and T. Earles, “Tapered Active-Region Quantum Cascade Lasers (λ = 4.8 µm) for Virtual Suppression of CarrierLeakage Currents”, Electron. Lett, 48, 234 (2012) [5] K. Shulte, Toby Garrod, Tae Wan Kim, Jeremy Kirch, Steven Ruder, Luke J. Mawst and Thomas F. Kuech, “ Metalorganic vapor phase growth of quantum well structures on thick metamorphic buffer layers grown by hydride vapor phase epitaxy”, IC-MOVPE 2012 Conference, May 2012, Busan, Korea
