Electro-Optical Characterization of an Amorphous Germanium-Tin (Ge1-XSnx) Microbolometer

The utilization of amorphous germanium-tin (Ge1-xSnx) semiconducting thin films as temperature-sensing layers in microbolometers was recently presented and patented. The work in this paper was performed as an extension of the latest study to acquire better Sn concentrations % for microbolometer applications. In this work, Ge1-xSnx thin films with various Sn concentrations %, x, where 0.31 ≤ x ≤ 0.48, were sputter-deposited. The elemental composition of each film was evaluated using energy-dispersive X-ray (EDX) spectroscopy, and the surface morphology was evaluated using atomic force microscopy (AFM), showing average roughness values between ~ 0.2 and 0.8 nm. Measurements of the sheet resistance versus temperature were performed and analyzed, revealing temperature coefficients of resistance, TCRs, ranging from –3.11%/K to –2.52%/K for x ranging from 0.31 to 0.40 above which the Ge1-xSnx thin film was found to exhibit metallic behavior at 0.40 < x ≤ 0.48. Empirical relationships relating the resistivity, TCR, and Sn concentration % of the amorphous Ge1-xSnx thin films were derived. One of the films with a 31% Sn concentration (Ge0.69Sn0.31) was used to fabricate 10×10 μm2 microbolometer prototypes using electron-beam lithography and lift-off techniques, and the microbolometer was fabricated on top of oxidized silicon substrates with no air gap between them. The noise behavior and the maximum detected signal of the fabricated microbolometer were measured. The signal-to-noise ratio, voltage responsivity, and noise equivalent power values of the prototypes were calculated. Finally, the expected performance of a proposed air-bridge microbolometer configuration was calculated.


Introduction
Microbolometers are the most commonly used pixel element detectors operating in the long wavelength spectral range of 8-14 μm in uncooled thermal imaging systems [1][2][3]. Two main temperature-sensing mechanisms are employed in commercial microbolometers: silicon-on-insulator (SOI) diode-based [4] and resistive-based [5] sensing. In SOI diode-based sensing, the well-known dependency of the pn-junction diode's saturation current on temperature is utilized for temperature sensing [6]. In resistive-based sensing, the Arrhenius-governed resistive property of semiconducting thin films is exploited [7].
In microbolometers, the main features of thermosensing materials are a high temperature coefficient of resistance (TCR) value (α), moderate resistivity, low noise, and compatibility with the fabrication processes of silicon integrated circuits (ICs) [8,9]. Two main semiconductor materials are used in commercial microbolometers: vanadium oxide and amorphous silicon [10,11]. Vanadium oxide and amorphous silicon films have high temperature coefficients of resistance, and their synthesis processes are CMOS compatible [12,13]. Similarly, other group IV elemental and compound semiconductors possess high TCRs, and their synthesis processes can be CMOS compatible [14]. Due to these attributes, group IV elemental and compound semiconductors are attracting increasing research interest as temperature-sensing layers for microbolometers [15,16].
Recently, amorphous germanium-tin (Ge 1-x Sn x ) has been introduced as a candidate material for temperature-sensing layers in microbolometers [15]. Ge 1-x Sn x thin films were prepared using a low-cost CMOS compatible process with various Sn concentrations %, x, where 0 ≤ x ≤ 0.25. The novel-introduced thin films showed TCRs of −3.96, −3.63, and −3.29%/K with corresponding resistivity values of 164.6, 69.14, and 45.46 Ω•cm.
This work is a complementary study elucidating the temperature-dependent resistive properties of amorphous Ge 1-x Sn x semiconducting thin films with different Sn concentration %, x (0.31 ≤ x ≤ 0.48). The elemental compositions of the synthesized thin films are examined using energy-dispersive X-ray (EDX) spectroscopy, and the temperature-dependent resistive properties of the Ge 1-x Sn x thin films are measured and analyzed. Atomic force microscopy (AFM) measurements are performed to measure the film surface roughness. Empirical relationships are derived relating TCR, resistivity, and Sn concentration %, thus providing a full description of the temperature-dependent resistive properties of the Ge 1-x Sn x thin films. In addition, we evaluated the noise behavior and the maximum detected signal of the fabricated microbolometer with a pixel area of 10×10 μm 2 .

Thin Film Synthesis and Characterization Methods
Ge 1-x Sn x thin films with different Ge:Sn ratios were deposited on Si substrates with 300 nm of thermally grown silicon dioxide (SiO 2 ). The deposition was performed at room temperature with an AJA International sputter coater using high-purity Ge and Sn targets (99.999% purity). Before deposition, the substrates were chemically cleaned and completely dried. The Ge 1-x Sn x thin film samples were deposited using a co-sputtering process. During the deposition processes, the Ge deposition RF power was set at 280 W, and the Sn deposition DC power was set at 4 different values (30, 50, 70, and 100 W) to yield Ge 1-x Sn x thin films with different Ge:Sn concentrations. All samples were deposited at a chamber base pressure of 2×10 −6 torr and argon pressure of 5 m torr. The deposition time for each sample was adjusted to keep the thickness of the deposited thin films at approximately 200 nm, which was confirmed by using a Veeco Dektak 150 surface profiler.
The elemental compositions of the Ge 1-x Sn x thin film samples were identified using EDX spectroscopy (Oxford Inca attached to JEOL 7600F FE-SEM). The AFM measurements were performed using a Veeco Multimode V over a 5 μm × 5 μm scanning area in tapping mode. The temperature-dependent resistive properties of the synthesized Ge 1-x Sn x thin films were assessed by performing measurements of the sheet resistance (R s ) versus temperature using a hot plate with temperature control and a four-point probe system (Jandel RM3000).

Characterization Results and Discussion
The EDX spectra of the four synthesized Ge 1-x Sn x thin film samples are shown in Fig. 1. The energy axis of the displayed EDX spectra was intentionally broken to omit the high-intensity Si Kα peak at 1.74 keV. EDX measurements of each sample were performed multiple times at different locations to confirm the uniformity of the Ge and Sn distributions over the whole Ge 1-x Sn x synthesized film; no clear elemental differences were detected. The EDX spectra of all the Ge 1-x Sn x thin film samples had almost the same trend. All the samples showed peaks at approximately 0.25, 0.51, 1.2, 9.89, and 10.94 keV corresponding to C (Carbon) Kα, O (Oxygen) Kα, Ge L (α, β), Ge Kα, and Ge Kβ ions, respectively. Peaks were detected at 3.45, 3.69, and 3.96 keV corresponding to the Sn Lα1, Sn Lβ1, and Sn Lβ2 lines, respectively. The calculated Sn concentration % of the Ge 1-x Sn x synthesized films with Sn deposited at DC powers of 30, 50, 70, and 100 W were 31, 35, 40, and 48%, respectively. These results are in good agreement with and complement the results reported in Ref. [15], where the Sn concentration % of synthesized films with Sn deposited at DC powers of 10, 15, and 20 W were 17, 22, and 25%, respectively.
AFM analysis was performed on all samples to investigate the surface morphology of the prepared Ge 1-x Sn x thin films, and the results are shown in Fig. 2

3
The sheet resistance (R s ) of the synthesized samples was measured at different temperatures, from 293 to 345 K with a step size of 2 K. Figure 3 shows the measured R s -versus-temperature plots for the Ge 1-x Sn x samples with a Sn concentration % ranging from 0 to 40%. As seen in Fig. 3, R s decreases with increasing Sn concentration %, which introduces more electrons in the conduction band of the Ge 1-x Sn x material, within the synthesized films. The sheet resistances of all the samples, excluding the sample with 48% Sn concentration, were found to be inversely proportional to the temperature, which confirmed the semiconducting behavior of the synthesized films. The sheet resistance of the Ge 0.52 Sn 0.48 sample, which is not plotted in Fig. 3, was found to be directly proportional to the temperature indicating a metal-like behavior. Therefore, it can be deduced that the Ge 1-x Sn x film transitioned from semiconducting to metallic behavior at 0.4 < x ≤ 0.48. Additionally, the dependence of R s on the temperature was more pronounced as the Sn concentration % decreased. The R s -versus-temperature curves were best fitted with an exponential decay function, which confirmed that they exhibited Arrhenius behavior, as shown in Eq. (1). The Arrhenius relationship  where ρ o is the resistivity prefactor, E a is the activation energy, and k is Boltzmann's constant. Furthermore, the TCRs of the synthesized films were calculated using Eq. (2) [18,19]: The extracted TCRs for all studied Sn concentrations % are plotted against temperature in Fig. 4 The room temperature measured R s , and extracted ρ, ρ o , E a , and TCR values of the thin films are shown in Table 1. Ge with zero Sn concentration has the highest TCR, but it also has the highest sheet resistance, which indicates that an increase in the expected noise will occur if it is used as a thermal sensing material. For this reason, by increasing the concentration of Sn in the Ge 1-x Sn x thin films from 17 to 40%, it was found that the sheet resistance of the samples decreased by two orders of magnitude; however, the TCR decreased to 50%.

Fig. 4 |TCR| vs. T plots of the Ge 1-x Sn x thin film samples
For further analysis, the variations in the thin film resistivity and TCR with the Sn concentration % at room temperature are plotted in Fig. 5. It can be seen from the figure that the thin film resistivity decreased exponentially with increasing Sn concentration %. The relationship between resistivity and Sn concentration % was modeled using an exponential decay function, as shown in the same figure, and the extracted empirical formula is shown in Eq. (3), where x represents the Sn concentration %:

3
In addition, the TCR was found to decrease linearly with increasing Sn concentration %. The relationship between the TCR and Sn concentration % was modeled using a linear function, and the extracted empirical formula is given by Moreover, the relationship between the thin film TCR and resistivity at room temperature is shown in Fig. 6. The relationship between the TCR and resistivity is best modeled using an asymptotic exponential function, as given by Furthermore, microbolometers were fabricated to investigate their electrooptical performance. Ge 0 . 69 Sn 0 . 31 microbolometers with pixel areas of 10×10 μm 2 were fabricated on top of oxidized silicon substrates without an air gap using electron-beam lithography and lift-off techniques. Figure 7 shows a scanning electron microscope (SEM) image of the microbolometer prototype.
The fabricated microbolometers were mounted on a ceramic chip carrier and wire bonded to the gold pads of the chip carrier. The fabricated microbolometers showed a resistance of approximately 0.5 MΩ. The chip carrier was mounted on a low-noise readout circuit that was mounted on a 3-axis micropositioning stage. The microbolometer noise measurement was performed using an HP 35670 dynamic signal analyzer. Figure 8 shows the voltage noise density spectrum of the Ge 0 . 69 Sn 0 . 31 microbolometer that was DC-biased with 4 μA, and the spikes that were seen in the measurement were related to 60 Hz power-line harmonics.
The measured signal noise density (V n ) was 141 μV/√Hz at 200 Hz. To measure the microbolometer responsivity, the microbolometer was illuminated through a f/2 zinc selenide lens with a modulated infrared power of 465 mW using a blackbody infrared source with a built-in chopper. The maximum detected signal was 51.4 mV at a 200 Hz modulation frequency, which was measured by using a dual-phase analog lock-in amplifier. The corresponding voltage responsivity (R v ) and signal-to-noise ratio (SNR) were 0.11 V/W and 364 Hz 1/2 , respectively, which resulted in a noise equivalent power (NEP) of 1.2 mW/√Hz. The measured electro-optical properties were used to estimate the performance of a microbolometer with a proposed air-bridge configuration and with the same pixel area and assuming the same noise density (Vn), as shown in Ref. [20]. The expected R v and NEP were 28.5×10 3 V/W and 4.6×10 −6 mW/√Hz, respectively.

Conclusion
The study presented in this paper resulted in a complete description of the temperaturedependent resistive properties of amorphous Ge 1-x Sn x temperature-sensing thin films, with 0.31 ≤ x ≤ 0.48, and an investigation of Ge 0.69 Sn 0.31 microbolometer noise behavior and electro-optical properties. The prepared thin films exhibiting semiconducting behavior showed TCRs of −3.11, −2.73, and −2.52%/K with corresponding resistivity values of 437.9, 403.43, and 380.7 Ω•cm at x = 0.31, 0.35, and 0.40, respectively, whereas the films were found to exhibit metallic behavior at 0.4 < x ≤ 0.48. Empirical relationships were derived relating the TCR, resistivity, and Sn concentration %, x, of the Ge 1-x Sn x thin films with 0 ≤ x ≤ 0.40. Microbolometers made of Ge 0.69 Sn 0.31 alloy with a pixel area of 10×10 μm 2 were fabricated using electron-beam lithography and lift-off techniques on top of oxidized silicon substrates without an air gap between the sensing layer and the substrate. The noise behavior and the maximum detected signal of the fabricated microbolometer were measured. The signal-to-noise ratio and voltage responsivity were found to be 364 Hz 1/2 and 0.11 V/W at 200 Hz modulation frequency, respectively, which resulted in a noise equivalent power of 1.2 mW/√Hz. The expected Ʀv and NEP of the proposed air-bridge microbolometer configuration were 28.5×10 3 V/W and 4.6×10 −6 mW/√Hz, respectively.
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