Atomic layer doping of strained Ge-on-insulator thin films with high electron densities W. M. Klesse, G. Scappucci, G. Capellini, J. M. Hartmann, and M. Y. Simmons Citation: Appl. Phys. Lett. 102, 151103 (2013); doi: 10.1063/1.4801981 View online: http://dx.doi.org/10.1063/1.4801981 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v102/i15 Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS 102, 151103 (2013)
Atomic layer doping of strained Ge-on-insulator thin films with high electron densities W. M. Klesse,1,2 G. Scappucci,1,2,a) G. Capellini,3 J. M. Hartmann,4 and M. Y. Simmons1,2 1
School of Physics, University of New South Wales, Sydney NSW 2052, Australia Australian Research Council Center of Excellence for Quantum Computation and Communication Technology, University of New South Wales, Sydney NSW 2052, Australia 3 Dipartimento di Scienze, Universit a degli Studi Roma Tre, Viale Marconi 446, 00146 Roma, Italy 4 CEA, LETI, Minatec Campus, 17, Avenue des Martyrs, 38054 Grenoble Cedex 9, France 2
(Received 20 January 2013; accepted 2 April 2013; published online 15 April 2013) We demonstrate that phosphorous atomic layer doping in ultra-high vacuum is a viable method to obtain n-type doping of strained germanium-on-insulator thin films. By engineering single and multiple, closely-spaced P d-layers, we obtain high active electron concentrations (1 1020 cm3) and low electrical resistivity (120 X/square) whilst keeping control over doping profile, structural integrity, and tensile strain levels (e ¼ 0.35%). Investigation of magnetotransport over a large temperature range (1.7-290 K) allows observation of two-dimensional electrons’ weak localization C 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4801981] up to 30 K. V Germanium-on-insulator (GeOI) is a material that is attracting attention for a variety of applications. In complementary-metal-oxide-semiconductor (CMOS) electronics, GeOI combines the advantageous electrical transport properties of bulk germanium (Ge), such as increased electron and hole mobility compared to silicon (Si),1 with the benefits of on-insulator technology, including lower junction leakage currents and low short channel effects. GeOI is also appealing for the fabrication of Ge optoelectronic devices on the Si photonics platform such as photodetectors and lasers.2 The development of GeOI-based microelectronics or optoelectronic devices, however, must address the same challenges typically associated with Ge. One major hurdle is the difficulty in controlling the diffusion and activation of n-type dopant atoms in Ge, which have hindered the development of high quality n-channel Ge MOS field-effect transistors so far.3,4 Recently we have demonstrated atomic layer doping (ALD) of Ge by fabricating single and multiple phosphorus delta-doped (P-d) layers in ultra-high vacuum (UHV),5 resulting in high electron densities (7.5 1019 cm3). Similar densities were obtained in ALD-based CMOS compatible processes to demonstrate the first electrically pumped Ge laser6 as well as in a reduced-pressure chemical vapor deposition (RP-CVD) cluster tool,7,8 boding well towards scaling this method to real electronic/photonic systems. The ALD process includes three steps: (1) a self-saturating layer of phosphine (PH3) molecules that chemically adsorb to the clean Ge(001) surface, (2) incorporation of the P dopants into the crystal matrix by thermal annealing, and (3) encapsulation of the P doped layer by molecular beam epitaxy (MBE) to embed the donors in a high quality epilayer while minimizing diffusion. The key difference of ALD compared to conventional doping techniques is the separation of the precursor gas adsorption from dopant incorporation.9 This allows full control over the doping process at the atomic level so that each doping step can be optimized separately depending on the specific application, a feature that is critical a)
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in order to dope the nm-scale thin top epilayer of GeOI structures. In this letter we demonstrate a method using atomic layer P-d-doping to obtain high active electron densities (1020 cm3) and low resistivities in thin GeOI films whilst preserving the structural integrity and strain levels of the initial GeOI sample. By using an insulating substrate, GeOI allows to extend the electrical characterization of the two-dimensional electron gas (2DEG) formed to a broader temperature range 1.7-290 K compared to previous studies on Ge d-doping.10 In this paper we use scanning tunneling microscopy (STM) to characterize in situ the surface morphology. Strain characterization and doping profiles of the doped layers were investigated by Raman spectroscopy and Secondary Ions Mass Spectroscopy (SIMS), respectively. The electrical properties of the samples were measured on Hall bars by four-terminal magnetotransport measurements. We shall discuss three different samples (A, B, C) of increasing thickness of the d-layer stack, obtained by changing the separation between stacked d-layers. All samples were cleaved from a single 200 mm GeOI substrate manufactured with the Smart-CutTM approach, using a Ge/Si layer as donor wafer.11 The donor wafer was prepared by depositing on a Si(001) substrate a 2.5 lm thick Ge layer in an Epi Centura RP-CVD using GeH4 in a low/high temperature (400 C/750 C) process. In situ thermal cycling in hydrogen (between 750 C and 890 C) was performed afterwards to improve crystalline quality. We obtained smooth (rootmean-squared (RMS) roughness
