Type-II Quantum Wells for InP based mid-IR Devices

T. Gruendl*, C. Grasse, S. Sprengel, K. Vizbaras, G. Boehm, R. Meyer, M.-C. Amann Walter Schottky Institut, Technische Universitaet Muenchen, 85748 Garching, Germany

BIOGRAPHY Tobias Gruendl was born in Tegernsee/Bavaria Germany (1981). He received the Dipl.-phys. degree in semiconductor physics from the Technische Universitaet Muenchen, Germany, in 2007. He is currently working towards his PhD degree in Prof. Amann’s group at the Walter Schottky Institute in Garching. He is mainly engaged in the design and manufacturing of InP-based high-power, high-speed short-cavity vertical-cavity-surface-emitting-lasers (SC-VCSELs) and Micro-Electro-Mechanical-System devices (MEMS VCSELs). He received three times the first price at international workshops/conferences (iNow 2010/2011 and IPC 2011) and has authored and co-authored 11 papers in leading scientific journals and 30 proceedings including 4 invited papers.

TECHNICAL ABSTRACT As many gases like CH4, CO, NO and CO2 are showing strong absorption lines in the mid-IR regime new light

emitting sources around 3 µm emission wavelength are continuously gaining increasing importance. The nowadays most commonly known application might be Tunable-Diode-Laser-Absorption-Spectroscopy (TDLAS) [1]. Despite of many attempts shifting the wavelength of InP based devices into the mid-IR restrictions originated from epitaxial growth, in particular strain induced relaxation have defined a 2.3 µm laser emission as longest accessible wavelength for this substrate over years [2]. Ongoing investigations on the antimony based material systems recently offered laser emissions from 2.0 to 3.6 µm in edge emitting lasers [3]. However, due to higher costs and worse crystalline quality of GaSb substrates and as process technologies on GaSb substrates are not as well-established as the InP ones it proved to be more worthwhile to modify active regions based on InP for entering wavelengths far beyond 2 µm.

In our investigations we mainly focused on type-II active regions based on GaInAs and GaAsSb as due to the band alignment electrons and holes can easily be spatially separated. According to figure 1 and 2 the absolute value of electron wavefunction shows its maximum within the GaInAs layers whereas holes are trapped within GaAsSb. Theoretical analysis of transition matrix elements between electrons and heavy holes on these material systems promised emission wavelengths far below bandgap energies reaching 4 µm at maximum [4]. The basic design of these type-II quantum wells (QWs) has been alternated from a superlattice (SL) design to a “W”-shaped one (see figure 1 and 2). Concerning the SL-design simulations showed a strong coupling of QW states. The resulting minibands both for electrons and holes are energetically smearing out the sharp transition between conduction and valence band. As a result the observed PL linewidth shows a strong broadening (see figure 3a). Details about the growth technique and the charaterisation methods can be found elsewhere [5]. Enhanced segregation of Sb in the MOVPE grown sample due to higher growth temperatures additionally increases the FWHM of the measured PL signal (see figure 3a - MOVPE). By using the second type-II concept showing two GaInAs QWs for electrons embedding one GaAsSb layer as a single QW for the holes the PL linewidth could be dramatically reduced from 72 to 47 meV (MBE) and 99 to 65 meV (MOVPE), respectively (see figure 3b). This can be understood as at the “W” design the electron wavefunction splits into a symmetric and antisymmetric state whereas only the symmetric one shows a non-negligible transition matrix element with the heavy holes (see figure 2). The experimental verification of this model is carried out by figure 3b showing two “W”-shaped PL test structures grown with 6 and 4 periods of GaInAs/GaAsSb/GaInAs in MOVPE and MBE, respectively. The separation of the “W”-shaped QW structures is done with 10 nm tensile strained GaAs0.58Sb0.42. This also prevents coupling among the “W”-structures and reduces the miniband formation. In figure 3b the present world record value of PL of 3 µm on InP based structures is shown. Further device correlated investigations on broadband LEDs with implemented type-II active regions based on the SL-design proved as well the desired 3 µm emission. With respect to other commercial available mid-IR state-of-the-art GaSb LEDs [6] even our preliminary devices are competitive concerning output power at elevated temperatures (over 30 µW @ 60°C). As an outlook, further investigations on the active region shifting the emission wavelength further into the MIR will be presented.

Keywords: Long-Wavelength, Type-II, InP, PL, 3 µm, LED, mid-IR

*gruendl@wsi.tum.de; phone +49-89-289-12789; www.wsi.tum.de

REFERENCES:

[1] A. Hangauer, J. Chen, R. Strzoda, M. Ortsiefer, and M.-C. Amann, Opt. Lett. 33, 1566-1568 (2008)

[2] T. Sato, M. Mitsuhara and Y. Kondo; Electronic Letters 43, 1143-1144 (2007)

[3] K. Vizbaras, M.-C. Amann, Electronic Letters 47 (17), 980-981 (2011)

[4] J Y T Huang, L J Mawst, T F Kuech, X Song, S E Babcock, C S Kim, I Vurgaftman, J R Meyer and A L Holmes Jr, Journal of Physics D: Applied Physics 42, 025108 (2009)

[5] S. Sprengel, C. Grasse, K. Vizbaras, T. Gruendl, and M.-C. Amann, Appl. Phys. Lett. 99, 221109 (2011)

[6] http://www.roithner-laser.com/led_midir.html#irs

Figure 1: Simulation of electron and hole wavefunctions within the GaInAs/GaAsSb type-II superlattice structure.

Figure 2: Simulation of electron and hole wavefunctions within the alternative “W”-shaped type-II active region. The barriers are consisting of tensile strained GaAsSb.

Figure 3: Comparison of PL of “SL” (a) and “W”-shaped (b) active regions grown by MBE and MOVPE at different excitation powers.

Figure 4: Measured spectrum at different temperatures of the micro-cavity LED. The fringes around 2.7 µm are resulting from water absorption lines. The electrically pumped active area of the LED is D = 300µm (AR denotes anti-reflection-coating).