Ultra-High Hall Mobility (1 x 106 cm2V-1S-1) in a Two-Dimensional Hole Gas in a Strained Germanium Quantum Well Grown by Reduced Pressure CVD

Abstract

In this work, we report a hole mobility of one million in germanium. This extremely high value of 1.1 x 106 cm2V-1s-1 at a carrier sheet density of 3.0 x 1011 cm-2 was observed in a strained Ge quantum well structure grown by reduced-pressure chemical vapor deposition (RP-CVD) and is nearly an order of magnitude higher than previously reported values [1]. Compressively strained Ge offers considerable promise as an alternative channel conduction material to Si because of its previously reported high hole mobility of 120,000 cm2V-1s-1 (at 2 K) [1] in two-dimensional hole gases (2DHG) in modulation-doped SiGe heterostructures [2-4]. Previous studies have used growth techniques such as molecular beam epitaxy (MBE) and low energy plasma enhanced chemical vapor deposition (LEPE-CVD), or a combination of these techniques to take advantage of their low growth rates for accurate control of the epitaxial layer properties. However, recent developments in low temperature epitaxy using reduced-pressure CVD (RP-CVD) now makes it possible using an industrially compatible process to control the thickness of strained epitaxial layers to within a few monolayers [5] and achieve very high levels of purity. The heterostructure studied in this work is shown in Fig. 1. All layers were grown by reduced pressure CVD (RP-CVD) using an ASM Epsilon 2000 reactor and employed a reverse-linearly graded Si0.2Ge0.8 relaxed buffer with low surface roughness (rms value ~ 2 nm, crosshatch spatial wavelength ~ 2 m) and threading dislocation density (≤ 4 x 106 cm-2) [6-7]. The active region included a 20 nm biaxial compressively strained Ge QW, that was grown at low temperature to prevent any relaxation [8]. High resolution x-ray diffraction (HR-XRD) confirmed that the Si0.2Ge0.8 buffer was slightly over-relaxed with respect to the Si substrate and that the Ge channel was fully strained with respect to the buffer. The B-doping density was determined as ~ 1 x 1018 cm-3 using ultra low energy secondary ion mass spectrometry (uleSIMS). Hall bars were fabricated using standard lithography and wet chemical etching while Al contacts were deposited by thermal evaporation and annealed at 425 °C for 20 mins in N2 to ensure ohmic behavior. Resistivity and Hall effect measurements were performed in the temperature range 12 K - 300 K with Fig. 2 showing the Hall mobility and carrier sheet density as a function of temperature. The room temperature Hall mobility was 1200 cm2V-1s-1 and is believed to be dominated by parallel conduction through the MOD heterostructure [2]. However, at 12 K the measured Hall mobility and carrier sheet density were 1.0 x 106 cm2V-1s-1 and 3.0 x 1011 cm-2, respectively and represents the highest hole mobility observed for a 2DHG in strained Ge. Fig. 2 shows that whilst the carrier sheet density saturates, the Hall mobility continues to increase for temperatures below 60 K. Additional experiments at 0.4 K found that the Hall mobility saturated at a value of ~ 1.1 x 106 cm2V-1s-1. At low temperatures, we expect the mobility to be limited by ionized impurity scattering [1]. Our results suggest that the RP-CVD growth process has resulted in both a very low impurity and extremely pure Ge layer, so that even at 12 K ionized impurity scattering is very weak [9] and demonstrates that RP-CVD can be used to grow strained Ge QWs with exceptionally high hole mobilities.