GeSn (Sn content up to 4.2%) photodiodes with vertical pin structures were grown on thin Ge virtual substrates on Si by a low temperature (160 °C) molecular beam epitaxy. Vertical detectors were fabricated by a double mesa process with mesa radii between 5 µm and 80 µm. The nominal intrinsic absorber contains carrier densities from below 1·1016 cm−3 to 1·1017 cm−3 for Ge reference detectors and GeSn detectors with 4.2% Sn, respectively. The photodetectors were investigated with electrical and optoelectrical methods from direct current up to high frequencies (40 GHz). For a laser wavelength of 1550 nm an increasing of the optical responsivities (84 mA/W −218 mA/W) for vertical incidence detectors with thin (300 nm) absorbers as function of the Sn content were found. Most important from an application perspective all detectors had bandwidth above 40 GHz at enough reverse voltage which increased from zero to −5 V within the given Sn range. Increasing carrier densities (up to 1·1017 cm−3) with Sn contents caused the depletion of the nominal intrinsic absorber at increasing reverse voltages.
© 2014 Optical Society of America
Silicon photonics has seen a strong increase in research activities in the last decade [1–3]. Meanwhile, a large number of approaches with passive components and active components could be demonstrated [4–7]. Some of them are now offered commercially. A crucial role in the development of the active optoelectronic components, such as detectors and emitters, plays the group IV material Ge. This material can be grown epitaxially on a Si or SOI substrate with well-defined in situ doped structures. The development of a special ultra-thin virtual substrate technology with thicknesses around 100 nm allows integration of this device family in the CMOS platform. Ge is an indirect semiconductor with Eg,L = 1867 nm bandgap at room temperature. However, this material is a strong absorber in the infrared up to the direct bandgap (Eg,Γ = 1550 nm). At longer wavelengths the absorption decreases dramatically. For the C-band optical communication vertical Ge detectors are not suitable. The indirect transition is too weak for the small device dimension. In this wavelength range vertical high speed Ge photodetectors  integrated on Si with bandwidth up to 50 GHz exhibit only a small response.
A straight forward extension of the spectral range is given by a smaller bandgap semiconductor. The group IV offers here the possibility of the incorporation of Sn in the Ge crystal matrix. This mixture forms a GeSn crystal and the indirect and the direct bandgap of this material shift to longer wavelengths. The material GeSn shows a very interesting physical property: the transition from an indirect to a direct semiconductor is predicted for a Sn concentration of 8-10% [8, 9]. However, experimental evidence has not yet been shown. This is due to the very high challenges during the epitaxial growth of GeSn alloys like a large lattice mismatch, an extreme low equilibrium solid solubility, and massive segregation phenomena . As a consequence GeSn crystals with Sn concentrations above the solubility limit can be deposited only at extreme low growth temperatures. A powerful tool for this application is the molecular beam epitaxy (MBE). GeSn films with a Sn concentration up to 25% (on Si) and 20% (on Ge) were demonstrated with a solid source MBE system . The incorporation of GeSn films in device structures with high Sn concentrations is significantly more complicated. GeSn devices are very sensitive to material quality, strain states and device processing issues because defects open competing nonradiative recombination paths. In recent years, several groups demonstrated GeSn pin photodetectors with Sn content up to 4% [12–14]. A photoconductive detector based on GeSn quantum wells with a Sn concentration of 9% was reported . In this paper GeSn high speed photodetectors with bandwidths higher than 40 GHz under vertical illumination are presented.
2. Material growth and device fabrication
The layer stacks for the GeSn pin photodetectors were grown with MBE. A B-doped Si (100) substrate with a very high specific resistance (ρ > 1000 Ω cm) was used for the high frequency devices. Thus the substrate losses can be minimized during the high frequency measurement. The GeSn pin photodetectors were grown identically as previously reported . The epitaxy layer stack consists of a 400 nm-thick B-doped (1020 cm−3) Ge p+ contact, a 300 nm-thick i-GeSn layer, a 10 nm-thick Ge spacer, and a 200 nm thick Sb-doped (1020 cm−3) Ge/Si heterocontact.
A schematic cross section of the MBE layer stack is shown in the left picture of Fig. 1. The Sn concentration in the intrinsic region was adjusted to 0%, 2% and 4.2%. The strain analysis of the complete device structures is summarized in Table 1. The Ge reference sample 1 has a slight tensile strain. Both GeSn samples (sample 2 and 3) show a compressive strain. However the 2% Sn sample (sample 2) was grown with the lattice constant of the underlying p+ Ge contact (pseudomorphic growth). In contrast the sample with 4.2% Sn (sample 3) is partially relaxed to the underlying p+ Ge contact. This means, that at the interface between these two layers a dislocation network was generated.
The photodetectors are fabricated in a quasi-planar technology with a special hetero top contact . A schematic high frequency device structure of the GeSn pin photodetector with the typical ground signal ground (GSG) probe configuration with a pitch of 100 µm is shown in the right picture of Fig. 1. The device processing by using double mesa etching, passivation and metallization is similar to earlier reported GeSn pin photodiodes . The radii of the fabricated devices vary between 5 µm and 80 µm. The mask layout includes also test structures like a short circuit structure and an open contact structure for S-parameter set-up calibrations. All structures for S-parameter measurements use the same coplanar GSG probing configuration.
3. Electrooptical measurement setup
The room temperature current density-voltage (J-V) characteristics were measured with a Keithley 4200 semiconductor parameter analyzer. For optical characterization the responsivity Ropt of the pin photodiodes was determined for zero bias and recorded in a range from 0 V to 5 V reverse bias. The continuous wave (cw) laser light with a wavelength of 1550 nm was coupled into the photodetectors using a glass fiber.
For the measurement of the electro-optical high frequency properties of the photodetectors a vector network analyzer (VNA) 37679D from Anritsu was used. The radio frequency (RF) measurement setup is showing in Fig. 2. The RF signal is generated by the VNA which is led from port 1 to the RF input of the electro-optical Mach-Zehnder modulator (LiNbO3 modulator) which modulates the cw laser light (λ = 1550 nm) with the RF signal frequency (40 MHz – 40 GHz). This modulated light is guided using an optical fiber to the top window of the photodiode. The coplanar GSG electrical input and output of the photodiode is contacted via on-wafer RF probes and contacted to the input (port 2) of the VNA. A source meter is used to set the bias voltage and it measures simultaneously the diode current. The electrical RF response measurements of the diode are also performed with the VNA in a conventional single port scattering parameter (S-parameter S11) configuration. The de-embedding of the terminal structures and the calculation of the capacity from the S-parameter is done with the program ADS (Advanced Design Systems - @ Agilent Advanced Design System).
4. Electrical and optical characterization
The dark current density-voltage characteristics of the investigated GeSn photodetectors are shown in the left picture of Fig. 3. The reverse characteristics of the dark currents of the photodetectors scale with the device areas. The current densities at a reverse bias of −1 V are given in Table 2. A Sn concentration of 2% increases the dark current by a factor of about 2. However in the sample 3 with 4.2% Sn the dark current increases by more than one order of magnitude in comparison with the Ge reference sample. Probably this strong increase is due to the higher Sn concentration and the partial relaxation of the GeSn film. The relaxation process during the MBE crystal growth creates an additional dislocation network at the interface between the bottom contact layer and the intrinsic GeSn absorber.
The optical responsivities Ropt of the GeSn photodetectors as function of bias at a wavelength of 1550 nm are shown in the right plot of Fig. 3. The optical responsivity of the Ge reference sample 1 decreases slightly at higher reverse voltages. Responsible for this are the Franz-Keldysh oscillations . A Sn content of 2% increases the optical responsivity of more than a factor of 2.1. A further increase of the Sn concentration up to 4.2% increases the optical sensitivity to 218 mA/W. The values for Ropt at zero bias operation and a bias of −1 V are listed in Table 2. The higher Sn content shifts the direct bandgap of the GeSn material to the infrared. This bandgap shift increases the absorption at the telecommunication wavelength of 1550 nm.
An important electrical parameter of a pin photodetector is the width of depletion layer w as a function of applied voltage Vbias. One possibility for the determination of w is the measurement of the capacitance C of the pin photodetectors as a function of Vbias. The capacity of a detector can be determined by using S-parameter measurements. The CV measurements are performed in the frequency range between 40 MHz and 40 GHz at different operating points Vbias. An example of a detector of sample 3 is shown in the Smith chart  in the left plot of Fig. 4. In addition, the corresponding Open (blue curve) and Short (green curve) structures for deembedding of the device under test (DUT) are measured. The high-frequency properties of the pin detectors can be described by using an equivalent small-signal circuit. With the help of the simulation tool ADS, the impedance values of the component can be determined. An example of this circuit model is also added in the Smith chart in Fig. 4 (red curve), which gives a good match between the measurement and the simulation. The capacitance C as a function of Vbias results from the simulation. With this and Ref.  the doping concentration in the intrinsic region can be calculated according to:
The carrier concentrations of the nominal intrinsic absorber regions are shown in the right diagram of Fig. 4 for all 3 samples. The Ge reference (sample 1) has a background doping of less than 1·1016 cm−3. The width w of the depletion layer is smaller than the width of the intrinsic region up to voltage of Vbias = −1 V. Above that reverse voltage the absorption layer is fully depleted. The GeSn photodetector with a Sn content of 2% (sample 2) shows a low background doping concentration of about ni = 1·1016 cm−3. In addition, the intrinsic region of this sample is completely depleted with a Vbias of −1 V. In contrast to these results a punch-through of the electric field in sample 3 cannot be found. The resulting background doping is by one order of magnitude higher (ni ~1·1017 cm−3) than in the other two samples. But a comparison with reported literature data present in all three samples a much smaller background doping . Nakatsuka et al. determined in undoped Ge and GeSn layers with Hall measurements hole concentrations as high as 1·1018 cm−3. The reason for the high doping levels is believed to come from a high concentration of existing vacancies created during the low-temperature growth. The rather low background doping in the layers reported in this paper shows the excellent quality of the epitaxial structures despite the low growth temperature of 160 °C.
The RF response of photodetectors with a radius of 20 µm is shown in the left picture of Fig. 5 where the magnitude of the normalized S21 parameter is given as function of the frequency. The 3-dB bandwidth is defined by the corresponding drop of response at high frequencies. Beyond this frequency the −20 dB roll-off per frequency decade is seen as expected for RC-limited pin photodetectors. In samples 1 and 2, the RF response saturates at reverse biases larger than −1 V. This observation agrees with the punchthrough of the depletion region found in CV measurements. In contrast, in the sample with a Sn content of 4.2% (Sample 3) saturation is not seen even for Vbias of −5 V. In Table 3, the 3-dB bandwidths for the different voltages are summarized. All three samples show a maximum cut-off frequency of 5 GHz. This is because the speed of large detectors is limited by the RC band-width .
The normalized frequency responses of photodetectors with a mesa radius of 5 µm are presented in the right picture of Fig. 5. Due to the significantly smaller capacitance, the 3-dB cutoff frequency increases for all three samples compared to the measurements of the detectors with radii of 20 µm shown in the left diagram of Fig. 5. Even for zero bias operation the Ge reference photodetector (sample 1) surpasses a bandwidth of 40 GHz. This bandwidth corresponds to earlier reported values . At zero bias operation the sample 2 with a Sn content of 2% shows a cut-off frequency of f3dB = 25 GHz. At a reverse bias of −1 V the 3-dB bandwidth exceeds to above 40 GHz. At zero bias sample 3 features a 3-dB bandwidth of 13 GHz. This value increases to more than 40 GHz at a reverse bias of −5 V. All these findings are in agreement with the background carrier level extracted from RF-CV measurements which requires higher reverse voltages with increasing Sn content in order to obtain depletion of the absorber region.
In conclusion, GeSn photodetectors with Sn concentrations up to 4.2% under vertical illumination were investigated. The Sn was incorporated in the intrinsic region by an ultra-low temperature MBE growth process in the intrinsic region of a Ge photodetector. Optical measurements at high frequency were successfully performed on GeSn detectors. A 3-dB bandwidth above 40 GHz was measured at the optical telecommunication wavelength of 1550 nm. The incorporation of Sn in the Ge matrix significantly increases the optical response of the detectors under vertical illumination at 1550 nm. At zero bias operation the optical responsivity of the detector could be increased by more than a factor of 2.6.
The background doping in the intrinsic layer was determined on the RF structures by C-V measurements. The sample with 2% Sn content shows a ni = 1·1016 cm−3 which is almost the same value as for the Ge reference sample. For a Sn concentration of 4.2% the doping concentrations ni is increased to about 1·1017 cm−3. However the comparison with data from the literature shows lower background doping by more than one order of magnitude and proves the excellent quality of the fabricated GeSn layers.
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