Evaluating Charge Carrier Lifetime of Laser Hyperdoped Germanium Using Terahertz Spectroscopy
Authors: Dissanayake S. Senali , Ferdous Naheed , Gandhi Hemi , Mazur Eric , Ertekin Elif , Sher Meng-Ju 
Published: September 2020

Gold-hyperdoped germanium extends photodetection into the infrared wavelengths. Pulsed-laser processing makes single crystal material with ultrahigh dopant concentrations. The laser parameters influence the dopant’s atomic position which leads to different light absorption and carrier transport properties. OCIS codes: 160.6990 Transition-metal-doped materials, 300.6495 Spectroscopy, terahertz, 140.3390 Laser materials processing Short wavelength infrared radiation detection is important in many applications and conventional detector materials are expensive, toxic or require low operation temperatures [1-3]. Developing novel semiconductor materials that overcome these limitations are important. Germanium (Ge) is a good material candidate due to its high absorption coefficients across a broad band in short wavelength infrared region (SWIR) and high carrier mobilities [4]. Bandgap energy of Ge is 0.67 eV (1.8 µm) and hence further engineering methods are of great interest to obtain photodetection beyond this cut-off wavelength of elemental Ge. Laser hyperdoping is a viable method to incorporate dopants in ultra-high non-equilibrium concentrations above the solid solubility limit, which enables the formation of an intermediate band between the conduction and valance bands [5]. Hyperdoped Ge has shown to be a promising candidate for efficient SWIR photodetection [6] . Laser hyperdoping can be achieved with ultrashort laser pulses or with nanosecond laser pulses but the resulting material is polycrystalline and has roughened surfaces. A better method to fabricate a hyperdoped material is the ion implantation followed by nanosecond pulsed laser melting which produces high quality materials that is single crystalline (Figure 1). High flux of dopant atoms implanted in the host semiconductor damages the lattice structure and amorphizes the material. Therefore, the pulsed laser process is carried out to melt the material deeper than the damaged layer. This process is followed by rapid solidification, where the material grows epitaxially from the crystalline substrate underneath, as the material cools down. During this rapid re-solidification, the dopants are trapped at concentrations above the thermodynamic solubility limit and hence ultra-high concentrations are achieved while preserving the material quality. The rapid solidification process must be slow enough to ensure the recrystallization and at the same time it must be fast enough to allow the incorporation of non-equilibrium dopant concentrations into the sample. Thus, the energy density (fluence) of the pulsed laser have significant effect on the material quality. This is a well-established method for chalcogen dopants (S, Se, Te) in silicon, but incorporating transition metals at supersaturating concentrations with better properties has more room for study [7]. Figure 01 – Hyperdoping by ion implantation followed by pulse laser melting. Figure adapted from ref. [5] In this work, we use ion implantation followed by pulsed laser melting to incorporate high concentrations of gold (Au) in Ge. Au hyperdoped Ge (Ge:Au) with different dopant doses and various pulsed laser melting fluences are studied. We evaluate the material quality using time resolved terahertz (THz) spectroscopy to investigate the charge carrier lifetime of the material. THz spectroscopy is a photoconductivity measurement with sub-picosecond time resolution. We use a 400-nm fs-laser pump pulse to excite the charge carriers in the material and probe with a broadband sub-picosecond THz pulse to map the photoconductivity decay dynamics [8]. Apart from the charge carrier lifetime analysis (Figure 2), the dopant concentration profile is analyzed by Rutherford Backscattering Spectrometry (RBS). Moreover, the sub band gap absorptance and DFT modelling of dopant distribution and their effect on band structure are being investigated. Figure 02 – Left: The change in photoconductivity of Ge:Au as a function of time. Charge carrier decay dynamics of different dopant concentrations and PLM fluences are investigated. The samples are LD_HF (low dose-high fluence), LD_LF (low dose-low fluence) HD_HF (high dose-high fluence) and HD_LF (high dose-low fluence) respectively. HD_LF has a significantly shorter lifetime than the rest. Right : Upper panel - The inverse of experimentally obtained half-lives. Triangles and circles represent LF and HF respectively. Sub-band gap absorptances are represented by unfilled triangles and circles. Lower panel - Total and substitutional dose of Au in hyperdoped Ge samples from RBS analysis. The unfilled bars and filled bars represent the total and substitutional doses, respectively. HD_LF has the most substitutional Au incorporation, while substitutional Au incorporation in HD_HF sample is much more similar to the two LD samples. In this study, we found that laser fluence determines the fraction of substitutional dopant (Figure 2). Material characterization shows that lifetime and absorption correlate with substitutional dose. Moreover, DFT modeling indicates that substitutional dopants are deep level defects which lead to high absorption and short lifetime. We found that the charge carrier lifetime is very long even after high concentration of Au is incorporated. We also found laser parameters that lead to highly substitutional dopant incorporation which enhances infrared light absorption. THz lifetime characterization allows optimizing processing parameters and shows hyperdoped germanium is a promising material for SWIR photodetectors.