Capacitance-Voltage Measurements on MBE-Grown Ge-SixGe1-x-ySny Heterojunction pn-Diodes for Material Characterisation
Authors: Schwarz Daniel , Funk Hannes Simon , Guguieva Kateryna , Oehme Michael , Schulze Joerg 
Published: December 2020

In recent years, there has been an increased interest in the ternary Group-IV semiconductor alloy SixGe1-x-ySny [1–4]. Since it allows the decoupling of its bandgap and its lattice-constant, SixGe1-x-ySny is predestined for the realisation of lattice-matched heterojunctions on Ge(Sn) virtual substrates on the Si platform. Furthermore, SixGe1-x-ySny is a predicted direct bandgap semiconductor at higher Sn concentrations, which makes it attractive for optoelectronic applications. An exemplary heterojunction optoelectronic device would be the single confinement heterostructure laser diode, the still missing key component for the monolithic integration of the optical on-chip communication on Si.

However, the design and dimensioning of heterojunction devices requires detailed knowledge of the electrical properties of the alloy semiconductor over composition, where currently much less is known about SixGe1-x-ySny. Although, previous investigations have focused mostly on the growth conditions and the optical properties of SixGe1-x-ySny bulk alloys [1,2,4].

In this work, we will concentrate on the growth and electrical characterisation of Ge-SixGe1-x-ySny heterojunction diodes. In detail, we will highlight the possibilities of current-voltage (I-V) and capacitance-voltage (C-V) measurements on Ge-SixGe1-x-ySny heterojunction pn-diodes as a useful tool for material characterisation. For this purpose, we grew a series of four samples with different composition of the SixGe1-x-ySny layer, but the SixGe1-x-ySny composition fulfilling lattice-matching on Ge (see Fig 1 (a)). Afterwards, we fabricated single mesa diodes to enable the subsequent electrical characterisation of the grown heterostructure diodes.

The growth process was performed in a 6” molecular beam epitaxy (MBE) system, where Si, Ge and Sn are used as matrix materials and B and Sb as dopants, respectively. Since the condition of lattice matched growth of SixGe1-x-ySny requires a fixed ratio of Si/Sn = 3.66, the Si and Sn fluxes were kept constant for all samples. Thus, we varied the Ge flux to achieve different SixGe1-x-ySny compositions, resulting in a varying SixGe1-x-ySny growth rate. This method enables better control of the lattice matching but also requires a highly precise calibration of the evaporation cells of the alloy compound fluxes. The Sn concentration of the SixGe1-x-ySny layers was varied between 5 % and 12.5 %. Since the most critical growth parameter is the substrate temperature, it is therefore measured and controlled in-situ by a thermocouple as well as an infrared pyrometer. Especially, the infrared pyrometer allows us to observe the dynamic processes on the sample surface during growth. For the growth of the SixGe1-x-ySny layer, we chose to keep the substrate temperature at TSub = 200 °C, which is mainly based on our previous SixGe1-x-ySny growth studies.

The heterojunction pn-diode (see Fig 1 (a)) is formed by a p-doped Ge layer with an acceptor concentration of NA = 1018 cm-3 and the n-doped SixGe1-x-ySny layer with a donor concentration of ND = 5·1019 cm-3. In order to achieve a good ohmic contact to the p-Ge, a highly p-doped Ge layer forms the underlying bottom contact of the diode. The whole diode structure is grown on our standard Ge virtual substrate (VS) on Si(100) [5].

A detailed material characterisation was performed to achieve information of the composition as well as the crystal quality using X-ray diffraction (XRD) and Raman spectroscopy.

After MBE growth, the samples were processed to diodes in a single mesa process. For the subsequent I-V and C-V measurements, we used a Keithley 4200 semiconductor characterisation system using the bottom as signal and the top as ground contact (Fig 1 (b)). For the C-V measurements, a parallel capacitance (C) and conductance (G) model was applied. The maximum measurement frequency was f = 100 kHz. Fig. 1 (c) shows a comparison of the current density-voltage (J-V) characteristics of diodes with a mesa radius of 80 µm for all four compositions. As can be seen, good diode behaviour is achieved even for high Sn concentrations. An exemplary capacitance-voltage and conductance-voltage (C-V and G-V) characteristics of the sample with 10 % Sn is shown in Fig 1 (d). The C-V characteristics shows good agreement with the theoretical curve in the range of -0.6 V ≤ vDC ≤ 0 V. Further calculations in this range allowed us the extraction of the built-in-voltage of the heterojunction diode, an important parameter of a pn-heterojunction. We present detailed results of the C-V measurements of our diodes and the applied methods for material characterisation.

Our results show that C-V measurement of SixGe1-x-ySny heterojunction pn-diodes is a useful tool for the determination of electrical parameters of this novel ternary alloy semiconductor SixGe1-x-ySny. These parameters are crucial for the further design of SixGe1-x-ySny heterojunction devices.

Figure 1