Germanium thin films: a high-performance resistive anode for micro-pattern gaseous detectors

1 Introduction

The evolving demands of future particle and nuclear physics experiments are driving factors for the development of new detectors and the advancement of existing technologies. Micro-Pattern Gaseous Detectors (MPGDs) have been extensively studied and widely used since the advantages of MPGDs. These include a large effective area at low cost, high-rate capability, radiation resistance, good spatial resolution and time resolution. However, the continuous increase in particle energy and luminosity is creating extremely challenging environments. These are characterized by high rates and intense irradiation backgrounds. As a result, MPGDs are required to meet more stringent performance criteria, such as improved stability, higher gain, and enhanced spatial and time resolution. Among these requirements, stability against discharges is particularly critical. Discharges can damage the detectors and the associated readout electronics. The introduction of resistive electrodes in MPGDs has effectively addressed this issue. This development has significantly enhanced the stability of MPGDs in high-demand experimental environments.

Various innovative methods and materials have been studied to fabricate resistive electrodes, such as resistive paste (RP) (Chefdeville et al. 2021; Alexopoulos et al. 2011), diamond-like carbon (DLC) films (Lv et al. 2020; Bencivenni et al. 2018), and germanium (Ge) films, which were first explored in parallel plate chambers (PPCs) (Bellazzini et al. 1986). Promising results for conventional MPGDs and new structures based on these resistive electrodes have been reported. A comparison of Ge films and DLC films as resistive anodes is shown in Table 1 (RP is excluded as it has not been a mainstream resistive material for MPGDs in recent years). It should be noted that the data in this table reflects the current state of research progress. Advancements in larger-scale or next-generation coating equipment may alter these performance benchmarks in the future. Among these materials, Ge films offer several distinct advantages, such as reliable resistivity controllability, a wide adjustment range, large-area fabrication directly on both soft and rigid substrates, cost efficiency, and simple operation. Notably, prototype detectors, specifically resistive Micromegas detectors (Feng et al. 2021) equipped with Ge anodes have demonstrated high performance, such as high gain and long-term stability. These detectors have been successfully validated in various application scenarios. For example, in the PandaX-III experiment (Wen et al. 2024), the resistive Micromegas made via the thermal bonding method can achieve a high gain in a high-pressure (up to 10 bar) time projection chamber (TPC). In addition, 40 cm × 40 cm Micromegas detectors in muon tomography systems have been successfully operated for more than 4 years (Wang et al. 2025), and a prototype with a double micro-mesh gaseous structure (DMM) operated stably in a high-intensity environment (µA-level current on the Ge anode) for over 8 months.

TABLE 1. Comparison of Ge Films and DLC Films as resistive anodes.

Building on our previous work, which first demonstrated the thermal bonding method for integrating Ge films into Micromegas detectors (Feng et al., 2021) and reported initial performance of Ge film resistive anodes (Feng et al., 2022), this study advances the field through three key, previously unaddressed contributions. First, it for the first time reveals the formation mechanism of Ge films as resistive anodes, providing fundamental insights into the structural and electronic origins of its resistive properties. Second, it offers a refined understanding of the early-stage surface oxidation and passivation of Ge films over extended long-term studies, and establishing its correlation with temperature variations across seasonal cycles. Third, extensive detector fabrication and experimental validation—encompassing long-term stability, gain, and rate capability—have comprehensively verified the feasibility of Ge films. In section 2, the manufacture method using vacuum thermal evaporation is first introduced, followed by a presentation of the Ge film properties, including non-uniformity, stability, and the dependence of surface resistivity on film thickness. Subsequently, the promising results of resistive Micromegas detectors with Ge films are demonstrated in the next section.

2 Manufacturing and characterization of germanium coatings

2.1 Manufacture method

Ge films are deposited on rigid substrates to form a thin resistive electrode by vacuum thermal evaporation via an electromagnetic heater or electron beam. An OFTC-1300 coating system was used for Ge film coating, which is shown in Figure 1. The source material for vacuum thermal evaporation is a pure single crystal of Ge with a purity of 99.9999%. The Ge target is housed in a container made of tungsten. The coating chamber is pumped to approximately 1× 10 − 4 Pa prior to the start of the deposition process. During this process, the temperature of the substrates increases, depending on the rate of deposition and the time. The film deposition rate is tuned to approximately 0.2 nm/s and the thickness of the film is less than 1 µm. Therefore, the temperature will not rise too high and will not exceed 50 °C without additional heating. Previous studies have shown that at such low temperatures, the Ge film will exhibit an amorphous structure, and the surface of the substrates (FR-4 substrates) also contributes to the amorphous nature of the film.

The bulk resistivity of crystalline Ge sources ranges from several tens of Ω cm, which is not sufficient for use as a resistive electrode. However, Ge films deposited on printed circuit board (PCB) substrates exhibit relatively high surface resistivity, ranging from MΩ/sq to GΩ/sq depending on the film thickness. This discrepancy in resistivity is attributed to the amorphous structure of the Ge films. To further understand this phenomenon, the surface resistivities of amorphous and crystalline Ge films of the same thickness were measured and compared. Ge films were coated on ceramic substrates via vacuum thermal evaporation with the substrates temperature not exceeding 50 °C. The crystallinity of the Ge films was characterized via Raman spectroscopy. The red and black curves in Figure 2 show the Raman spectrum of the Ge film and the Ge target, respectively. The film sample exhibits an amorphous wave packet, while the target sample shows higher and sharper convex peaks, corresponding to crystalline Ge. The standard wavenumber of the Ge crystalline state peak is 300.1 cm − 1. To obtain crystalline Ge films, the annealing treatment of the amorphous Ge films was conducted. The sample was heated to 800 °C in a vacuum chamber and quickly cooled to room temperature. The blue curve in Figure 2 shows the Raman spectrum of the annealed Ge film, confirming that crystallization occurs during annealing. Surface resistivity measurements at the same position before and after annealing revealed that the resistivity decreased by approximately three orders of magnitude, from hundreds of MΩ/sq to tens of kΩ/sq. These results indicate that the surface resistivity of amorphous Ge films is 2–3 orders of magnitude higher than that of crystalline Ge films, which is consistent with previous studies (Sharma et al. 1972).

2.2 Characteristics of germanium films

2.2.1 Non-uniformity

Large-area MPGDs are in high demand for future particle physics experiments, highlighting the critical need for large-area, high-performance resistive electrodes. With the large coating system (as shown in Figure 1), the effective area of Ge films has been successfully expanded to 60 × 60 cm². Figure 3a presents a Ge film with a thickness of 400 nm deposited on a PCB substrate. The surface resistivity at different positions was measured using a HIOKI SM7120 high resistance meter, which combined a custom surface resistance measurement electrode (HIOKI SM9001). During the measurement, a circular electrode with a diameter of 30.5 mm and an external ring electrode simultaneously come into contact with the Ge film. The inner and outer diameters of the ring electrode are 57 mm and 63 mm, respectively. Bias is applied to these two electrodes to achieve the measurement of the surface resistivity. This system has good measurement accuracy. The actual test results show that when the resistivity is at the level of several hundred MΩ/sq, the uncertainty is only at a level of 1%–3%, which is also consistent with the technical indicators of the high resistance meter. The distance between each measurement point was approximately 10 cm, with a total of 36 points measured. Figure 3b illustrates the typical surface resistivity distribution of 60 × 60 cm² Ge films. The non-uniformity of the surface resistivity is defined as the root mean square value of all 36 measurement results divided by their average value. The result indicates that a non-uniformity below 20% at an area of 60 × 60 cm² is reproducible with current coating machine. For smaller Ge samples, better non-uniformity results can naturally be achieved. For 40 × 40 cm² samples, the best result was below 10% (Feng et al. 2022).

2.2.2 Variation of surface resistivity with time

A study of the temporal evolution of surface resistivity which is critical for the long-term stability of MPGDs, was also performed. Six Ge films of varying thicknesses (100, 200, 300, 400, 600, and 1000 nm) were coated on substrates from the same batch of PCBs. The surface resistivities of all the samples were measured repeatedly at the same position over several days, with concurrent records of temperature and other relevant parameters. All the samples were stored in a dry air environment with 25% relative humidity between tests.

The test results are presented in Figure 4. The red curve denotes the ambient temperature recorded during testing, while the remaining data points correspond to the different Ge film thicknesses, respectively. As can be seen, the surface resistivity increased to 2-3 times higher than its initial value within the first 2–3 months. This increase is ascribed to the gradual oxidation of the Ge films in air. Subsequently, the formation of a stable oxide passivation layer led to stabilization of the resistivity, which remained within a certain range throughout the approximately 700-day monitoring period. In addition, a clear negative correlation was observed between temperature and surface resistivity. At near-room temperatures, increasing the temperature leads to a continuous rise in carrier concentration within the Ge film, accompanied by a slight decrease in carrier mobility (Jing et al. 2009). Notably, the enhancement in carrier concentration exerts a more dominant influence on conductivity—ultimately resulting in a reduction in the surface resistivity.

2.2.3 Dependence of surface resistivity on thickness

To evaluate the relationship between the surface resistivity and the thickness of Ge films, the mean resistivity values from days 100–700 were averaged. The results are shown in Figure 5, where the x-axis represents the thickness and the y-axis shows the surface resistivity. The error bars represent the root mean square of the measured values. A clear inverse correlation was observed between the surface resistivity and the thickness of Ge films. The results demonstrate that the surface resistivity can be tuned from tens of MΩ/sq to approximately 1 GΩ/sq by varying the Ge film thickness from 1000 nm to 100 nm. This range can meet the requirements for the application of MPGDs.

3 Performance of micromegas detectors with germanium films

High-performance resistive films with large area, uniformity, stability, and controllable resistivity are essential for enhancing the performance of MPGDs. The Micromegas detectors based on the thermal bonding method were developed by the University of Science and Technology of China. Figure 6 shows a schematic view of a resistive Micromegas detector and the thermal bonding method. The Ge film coated on the top layer of an FR-4 substrate is used as the resistive layer. It is separated from the readout layer by an insulating layer. The thickness of the insulating layer is approximately 80 µm. Ge films are connected to an external electrode through some grounding nets for grounding or applying high voltage. The charges produced during the electron avalanche processes are evacuated through these nets.

Using the thermal bonding method with Ge films, we fabricated multiple prototypes and successfully scaled the maximum effective area to 60 × 60 cm². These prototypes incorporate two distinct grounding net designs. The first is border grounding, where the grounding nets are placed along the periphery of the readout PCB. The second design is called grid-dot grounding, which is used in high counting rate applications. The grounding dots are arranged in the sensitive area with a pitch of 10 mm. Each individual grounding dot introduces a dead area with a diameter of 1 mm, and the proportion of the dead area attributed to this grid-dot grounding configuration is approximately 0.8%. The gain performance, rate capability, and stability were studied.

Figure 7 shows the gain results of resistive Micromegas prototypes with different sizes, working gas and gas pressures for 5.9 keV X-rays. All the tests were conducted in a laboratory environment, without making any adjustments for external temperature and atmospheric pressure. The Ge films in these prototypes were all fabricated to a thickness of 400 nm, yielding a surface resistivity of approximately 100 MΩ/sq. The probability of discharge occurrence has been significantly suppressed, thereby allowing the amplification gap voltage to be increased further. As shown in Figure 7a,b, the gain of a 10 × 10 cm² prototype and a 32 × 32 cm² can reach higher than 1 × 10 5 and 6 × 10 4 at 1 bar working gas. The typical energy resolution at 5.9 keV of these detectors is below 20%, with no significant deterioration observed compared to the non-resistive detectors. Furthermore, the resistive Micromegas prototypes equipped with Ge films also demonstrated high gain and stability under high gas pressure and low gas pressure. The gain of the prototypes exceeds 1 × 10 4 under working gas pressures of 10 bar, 0.5 bar and 0.2 bar, which are shown in Figure 7c,d. Adjusting the amplification gap thickness can further optimize performance for specific pressure regimes. In addition, the spark rate of the resistive Micromegas detector was characterized under continuous X-ray exposure at a gain of around 8000. The current from the mesh electrode was measured using a picoammeter (Keithley 6482), and the average and the standard deviation of the current were calculated. A spark event was considered when the deviation of the current from the average value exceeded 10 times the standard deviation. The spark rate is defined as dividing the spark events by the X-ray counts. During the 26 h test, there were only a few sparks, with the rate as low as 5.1 × 10 − 8. The observed high gain and low spark rate collectively validate the excellent performance of the Ge-film resistive anode.

The normalized gain as a function of counting rate was measured to evaluate the rate capability of the resistive Micromegas detectors. The test results are shown in Figure 8, where the Ge films of these detector are all grounded via the grid-dot case. The metal dots distributed at a pitch of 10 mm are used to connect the Ge film to the external electrode. The four curves represent different initial gains of approximately 3000, 6000, 16,000 and 36000 for 8 keV X-rays, respectively. As can be seen, the gain would decrease about 10% at a counting rate exceeding 3 × 10 6 Hz/cm² when the initial gain is 3000 (Fang et al. 2023). When the initial gain at low rate is higher, the lower rate capability will be reached. Due to the higher primary ionization number during X-rays test, the rate capability of the resistive Micromegas detector with Ge films can be further improved under charged particles conditions. Furthermore, the grid-dot grounding method has been shown to significantly improve the rate capability compared with the border grounding method (Feng et al. 2022).

4 Conclusion

In this paper, we present a comprehensive study on the application of Ge films as resistive anodes for MPGDs, detailing the manufacturing process, key characteristics, and successful implementation in high-performance Micromegas detectors. The vacuum thermal evaporation method was demonstrated to be an effective technique for producing large-area Ge films. The scalability of the process was proven by the successful fabrication of uniform films with areas of up to 60 × 60 cm². The surface resistivity can be tuned from tens of MΩ/sq to approximately 1 GΩ/sq, fulfilling the diverse requirements of MPGDs applications. Furthermore, the films exhibited excellent long-term stability after an initial conditioning period, which was attributed to the formation of a passivation oxide layer.

These high-quality Ge films were successfully integrated into Micromegas detectors via the thermal bonding method. The prototype demonstrated good results, including a high gain exceeding 10 5 for 5.9 keV X-rays and a remarkably low spark rate. The grid-dot grounding method has been proven to significantly improve the rate capability, making the detector suitable for high-intensity environments. Using Ge films, various types and sizes of Micromegas detectors have been fabricated and successfully applied in practice, such as neutron detectors (Fang et al. 2023), high gas pressure time projection chamber readout detectors (Wen et al. 2024), tracker detectors (Wang et al. 2021), fast timing detectors (Meng et al. 2025), etc. In conclusion, the good properties of facile fabrication, scalability, and stability of the Ge film demonstrate its substantial application potential as a resistive anode for MPGDs.

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Footnotes

Edited by: Luca Moleri, Weizmann Institute of Science, Israel

Reviewed by: Ronaldo Bellazzini, National Institute of Nuclear Physics of Pisa, Italy

Antonio Serra, University of Salento, Italy