Priv. Doz. Dr. Alexander Pawlis, Peter Grünberg Institute PGI-9, Forschungszentrum Jülich GmbH
"Modern Device Concepts based on novel aspects in the MBE of classic wide gap II/VI semiconductors"
The classic II/VI semiconductors (ZnSe, MgSe, CdSe) and their alloys have been intensively investigated in the 1980-2000 focusing on the development of blue and green lasers. But it became clear that these materials are not suited for highcurrent/ high-power applications; mainly due to severe problems in the realization of low-resistive non-degenerating n- and ptype ohmic contacts. Instead, enormous potential of the classic II/VI materials relies in the application of modern quantum devices where other material properties are important: Excellent and homogeneous crystallographic properties of molecular beam epitaxy (MBE) grown heterostructures, high oscillator strength of the optical transitions, the possibility of isotope engineering and the availability of sophisticated in-situ nanofabrication techniques. All these aspects are covered by the classic II/VI semiconductors and make them attractive to implement building blocks for solid-state quantum information devices such as single-photon sources  and optically  or electrically controlled spin qubits. However, utilizing these excellent material properties in an optimal way for the realization of such devices requires the development of novel, unconventional aspects and techniques in the MBE growth of the II/VI materials.
In this talk I will present an insight into three of those novel techniques we developed in order to reshape the II/VI materials for potential applications as modern quantum devices. In the first part I will introduce the MBE growth of heterostructures containing isotopically purified 64Zn80Se  which forms a nuclear spin-zero host matrix for spin qubits. Investigations on the electron spin dynamics in natural and isotopically engineered ZnSe [4,5] revealed the absence of the nuclear spin interaction but only a small enhancement of the spin dephasing time T2* of the electron and hole ensemble bound to shallow impurities. In contrast, much longer T2* and T1 times are revealed for specific deep impurities induced by implantation of Fluorine in isotopically engineered 64Zn80Se.
In the second part I will focus on the development of low-resistive ohmic contacts to n-type ZnSe:Cl with linear current/voltage characteristics at room temperature and also at 4 K which remained one of the most challenging open issues for ZnSe . Solving this issue we obtained full access to the development of all-electrical controlled spin qubits such as electrostatically defined quantum dots. Here ZnSe and related compounds can unite the main advantages of to-date established systems like the group-IV and III/V semiconductors.
Finally I will round up my talk by shortly presenting a novel approach to implement hybrid nanowire structures composed of a III/V semiconductor core (GaAs) surrounded by a II/VI semiconductor shell (ZnSe). Here we established a MBE growth technique on pre-patterned (111)-Si substrates in which the crystal phase of the GaAs core can be preserved to a wurzite phase purity grade of 99%  and directly transferred on and maintained during the II/VI shell growth. Such phase-pure in-situ grown nanowires are an excellent basis for the development of II/VI multi-shell single photon sources with potentially high quantum efficiency due to their waveguide-like nanowire geometry and the in-situ nano-growth technique on pre-patterned substrates.
 K. Sanaka, A. Pawlis, T.D. Ladd, D. J. Sleiter, K. Lischka, Y. Yamamoto, Nano Letters 12, 4611 (2012).
 D. J. Sleiter, K. Sanaka, M. Kim, K. Lischka, A. Pawlis, Y. Yamamoto, Nano Letters 13, 116 (2013).
 A. Pawlis, G. Mussler, C. Krause, B. Bennemann, U. Breuer, D. Grützmacher, ACS Appl. Electron. Mater. 1, 44 (2019).
 F. Heisterkamp, E.A. Zhukov, A. Greilich, D.R. Yakovlev, V.L. Korenev, A. Pawlis, M. Bayer, Phys. Rev B 91, 235432 (2015).
 N. E. Kopteva, E. Kirstein, E.A. Zhukov, M. Hussain, A.S. Bhatti, A. Pawlis, D.R. Yakovlev, M. Bayer, A. Greilich, Phys. Rev B 100, 205415 (2019).
 J. Janßen, F. Hartz, T. Huckemann, C. Kamphausen, M. Neul, L.R. Schreiber, A. Pawlis, ACS Appl. Electron. Mater. 2, 898 (2020).
 M.M. Jansen, P. Perla, M. Kaladzhian, N. von den Driesch, J. Janßen, M. Luysberg, M.I. Lepsa, D. Grützmacher, A. Pawlis, ACS Appl. Nano Mater. 3, 11037 (2020).
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