We report on the fabrication and characterization of a Schottky diode made using 2D germanane (hydrogenated germanene). When compared to germanium, the 2D structure has higher electron mobility, an optimal band-gap, and exceptional stability making germanane an outstanding candidate for a variety of opto-electronic devices. One-atom-thick sheets of hydrogenated puckered germanium atoms have been synthesized from a CaGe2 framework via intercalation and characterized by XRD, Raman, and FTIR techniques. The material was then used to fabricate Schottky diodes by suspending the germanane in benzonitrile and drop-casting it onto interdigitated metal electrodes. The devices demonstrate significant rectifying behavior and the outstanding potential of this material.

The two-dimensional (2D) nanomaterials have attracted tremendous interest in recent years.1 Although the lamellar structure of graphite had been studied and theorized for over 150 years,2–4 the field exploded in 2004 when Novoselov et al.5 reported the preparation of single layer carbon (C) sheets known as graphene. Since that time, the similar electronic structures of silicon (Si) and germanium (Ge) have led to a number of theoretical6–8 and experimental9–11 studies of their graphene analogues, silicene, and germanene.

Unlike graphene, silicene and germanene have a buckled structure12 resulting in an inherently tunable band gap and high electron mobility. Combined with their potential for doping, these materials have great potential for application in thin electronic devices including photovoltaics, photodetectors, and thermoelectrics.1 

Recently, the first transistor fabricated using silicene was reported.13 Silicene, however, has several inherent drawbacks. It requires specific, typically conductive, substrates making it difficult to synthesize and characterize.14 In addition, it is oxygen sensitive and unstable in air14 because of the amount of energy released during the formation of Si-O bond (8.0 eV). The oxygen affinity of silicene inhibits its applicability in nanoelectronics.

By contrast, the synthesis of the 2D hydrogenated germanene (aka germanane or Ge-H) is thermodynamically favored over silicene. Germanane (Ge-H) has been synthesized from the topochemical deintercalation of CaGe2.11 Ge-H has been reported to be resistant to oxidation and thermally stable up to 75 °C,11 in part because the energy released during the formation of the Ge-O bond (6.6 eV)15 is significantly lower than that of the Si-O bond. In addition, a monolayer of Ge-H is stable when exposed to the ambient atmosphere as the Ge-H bond is slightly stronger (3.2 eV) than Si-H bond (3.0 eV).16 With a predicted direct bandgap of 1.53 eV (vs. 0.67 eV for germanium) and an electron mobility of 18 200 cm2/V·s, five times that of bulk Ge, 2D germanane is an ideal candidate for high efficiency optoelectronic devices, particularly photovoltaic cells.11 In this letter, we report on the fabrication and characterization of a Schottky diode made using 2D germanane (hydrogenated germanene). The properties are compared with a similar device made using bulk 3D germanium powder.

The synthesis of stable multilayers of hydrogenated germanene requires a relatively sophisticated process when compared to the synthesis of other Group IV 2D materials such as graphene and silicene. Although CaGe2 has been known since 1944,17 monolayers of germanium have only recently been developed. Following the work done by Bianco et al.,11 we synthesized germanane through the deintercalation of CaGe2.

The synthesis of germanane took place in a two-step process. The first step was to synthesize a CaGe2 zintl framework in which the Ge atoms are arranged in a plane. This planar arrangement ultimately assists in forming the 2D Ge sheet. For this step, Ca and Ge were loaded in stoichiometric amounts into a quartz tube. As the synthesis of CaGe2 is highly oxygen-sensitive,17 strict air-free procedures were used and a Schlenk line was used to evacuate the quartz tube which was then sealed using a MAP/Pro oxygen torch. The mixture was annealed at 1050 °C for 18–20 h, and cooled over 7 days forming CaGe2 crystals.

The second step was to deintercalate the Ca ions from the CaGe2 system by placing the crystals in a solution of concentrated HCl (35%) and stirring continuously for 8 days at −40 °C. Using this method, the possible mechanism of intercalation for the synthesis of Ge-H can be summarized as follows:

n(CaGe2)+2nHCl(GeH)n+nCaCl2.

Following the 8-N rule of Zintl phases,15 the strong heteropolar bonding character in CaGe2 leads to the formation of puckered (Ge-)n polyanion layers, which are separated from each other by planar monolayers of Ca2+ ions.

Finally, this reaction mixture was washed with Milli-Q H2O and then methanol and dried at room temperature. Both the excess acid and the CaCl2 byproducts of the reaction were removed by the wash, as calcium chloride is a typical ionic halide and is highly soluble in water.

A variety of techniques were used in order to confirm the synthesis of germanane including XRD, Raman, and FTIR. Using FTIR vibrational excitation of the Ge-H system was performed in order to confirm hydrogen termination on the Ge framework. The sample for the FTIR measurements was prepared by the KBr pellet method. A small amount of the Ge-H material was well-mixed into a fine KBr powder and pressed into a pellet. The KBr is transparent in the IR region so the resultant FTIR spectrum is that of the germanane. The FTIR spectrum obtained (Fig. 1) shows evidence of hydrogen terminated germanane bands at ∼2000 cm−1.18 Vibrational fingerprints in the range of ∼800 cm−1 to ∼500 cm−1 were recorded due to stretching and wagging modes of the Ge-H vibrations. A broad and less intense peak is observed in the range of ∼750 cm−1 to ∼850 cm−1 due to the bending vibration mode of the Ge-H2 bond. IR data observed is in complete agreement with the previous reports suggesting the production hydrogen terminated germanane.19 

FIG. 1.

Fourier transform infrared spectra (FTIR) of as-synthesized germanane.

FIG. 1.

Fourier transform infrared spectra (FTIR) of as-synthesized germanane.

Close modal

Raman spectroscopy, in particular, has been shown to be a useful tool in the elucidation and confirmation of the germanane structure. After deposition of germanium and germanane powders onto the interdigitated electrode substrates (see text below), Raman spectra were acquired. As shown in Fig. 2, the peak at 228 cm−1 is present in the germanane curve and is associated with the A1 mode.20 This peak provides clear evidence for the presence of Ge-H. Both spectra show a peak at ∼300 cm−1 corresponding to the Ge-Ge bond vibration. In the Ge-H spectrum, the peak is slightly blue shifted to 301 cm−1. It has been demonstrated previously that three-dimensional confinement of phonons in Ge nanocrystals causes a vibrational red shift,21,22 accordingly the blue shift observed is possibly unique to the sheet-like structure of germanane or the influence of the hydrogen bond termination.

FIG. 2.

Raman spectra of (a) Ge powder, peaks at 160 cm−1 and 440 cm−1, are likely due to GeO2 contamination. (b) Germanane, A1 peak indicates presence of Ge-H bonds, peaks at 200 cm−1 and 405 cm−1 likely a result of a GeO2 with left over Ca impurities.

FIG. 2.

Raman spectra of (a) Ge powder, peaks at 160 cm−1 and 440 cm−1, are likely due to GeO2 contamination. (b) Germanane, A1 peak indicates presence of Ge-H bonds, peaks at 200 cm−1 and 405 cm−1 likely a result of a GeO2 with left over Ca impurities.

Close modal

XRD screening of the sample provides explicit details on the planar 2D structure of the germanane. The XRD patterns of pure germanium, crystal CaGe2, and synthesized Ge-H have been recorded (Figure 3). As seen in Fig. 3, the XRD pattern observed for CaGe2 crystal is markedly different than the pattern recorded for the pure germanium and hydrogenated germanene. At 2θ = 16.5°, a relatively low intensity signature peak for the sample of germanane has been recorded, which is in complete agreement with the previous reports and corresponds to the (002) plane.11 This peak is absent in the XRD pattern of both the samples of germanium powder and CaGe2 crystal. In addition, at 2θ = 26.5°, the germanane shows a small peak corresponding to the (100) plane, again in agreement with the previous reports.20 Although it is clear that there is still some germanium present in our sample, the FTIR Raman, and XRD provide strong evidence for the presence of Ge-H.

FIG. 3.

Powder X-ray diffraction patterns of (a) pure Ge: diffraction planes are indexed in red and are present in both CaGe2 and Ge-H. (b) CaGe2 (with Ge contaminants). (c) Ge-H: inset displays zoomed in section highlighting the (002) and (100). * indicates GeO2 contamination.

FIG. 3.

Powder X-ray diffraction patterns of (a) pure Ge: diffraction planes are indexed in red and are present in both CaGe2 and Ge-H. (b) CaGe2 (with Ge contaminants). (c) Ge-H: inset displays zoomed in section highlighting the (002) and (100). * indicates GeO2 contamination.

Close modal

Having verified the synthesis of Ge-H, Schottky diodes were fabricated from both 2D germanane (Ge-H) and bulk germanium (Ge) powder in order to evaluate the differences between the two. As a first step, colloidal suspensions of Ge and Ge-H in benzonitrile were prepared at a concentration of 20 mg/ml.23 These suspensions were drop cast on top of gold and aluminum interdigitated electrodes which had been evaporated onto SiO2 (Figures 4(a) and 4(b)).24 Equal volumes of the Ge and Ge-H suspensions were drop cast on top of their respective substrates, resulting in devices with equal concentrations of deposited material.

FIG. 4.

Images of the fabricated devices. (a) Drawing showing configuration of interdigitated Au/Al electrode (b) optical microscope image of interdigitated electrodes. (c) and (d) SEM images (200 μm and 50 μm scales) of Ge-H deposited on electrodes (bright lines are gold electrodes; aluminum electrodes appear darker).

FIG. 4.

Images of the fabricated devices. (a) Drawing showing configuration of interdigitated Au/Al electrode (b) optical microscope image of interdigitated electrodes. (c) and (d) SEM images (200 μm and 50 μm scales) of Ge-H deposited on electrodes (bright lines are gold electrodes; aluminum electrodes appear darker).

Close modal

Prior to deposition of the semiconductors, the substrates were tested to ensure that there was no conduction between the gold and aluminum. Each electrode “finger” was 10 μm wide. The separation between the gold and aluminum junctions was measured using an optical microscope and alternated between 1.61 μm and 2.34 μm. The active device surface area (calculated as the area of the separation between the electrodes) was approximately 1.9 mm2.

Figures 4(c) and 4(d) show SEM images of the Ge-H devices. The Ge-H particles are distributed across the device forming many small diodes between the gold and aluminum electrodes. The deposited particles vary in thickness with most being a few layers thick. However, when the particles lay flat across the electrodes, the device gains the benefit of the enhanced mobility of the 2D germanane. In order to measure the thickness of individual layers, atomic force microscopy (AFM) was performed on one larger particle of germanane with clear and accessible steps. For this piece, the measured step size is 0.337 nm (Figure 5). The total thickness of each particle depends on the number of layers.

FIG. 5.

AFM tapping mode (TM) micrographs (top view) of prepared Ge-H. (a) Section analysis chart of height Z vs. horizontal position X. (b) Height sensor image of the 200 × 200 nm area.

FIG. 5.

AFM tapping mode (TM) micrographs (top view) of prepared Ge-H. (a) Section analysis chart of height Z vs. horizontal position X. (b) Height sensor image of the 200 × 200 nm area.

Close modal

The diodes were characterized by measuring I–V curves between the Au and Al electrodes using a Keithley 236 source measure unit. The I–V curves are shown in Fig. 6. As can be seen, the diodes made using Ge and Ge-H behaved differently. The forward and reverse bias directions were opposite for the two materials and both the rectification and the current are higher for the Ge-H device.

FIG. 6.

I–V curves for (a) germanane and (b) germanium Schottky diodes on Au and Al contacts.

FIG. 6.

I–V curves for (a) germanane and (b) germanium Schottky diodes on Au and Al contacts.

Close modal

For the germanium diode, the difference in the forward and reverse currents is not large. This can be attributed to the fact that metal-germanium interfaces exhibit strong fermi level pinning near the germanium valence band due to metal-induced gap states and therefore the Schottky barrier heights for the Al/Ge junction and Au/Ge junction are nearly identical.25 On the other hand, the germanane diode shows a significant rectification (as compared to the germanium). There are no reported experimental measurements of Schottky barrier heights for metal-germanane junctions. However, the different bandgap for Ge-H (a direct gap of 1.53 eV (Ref. 11) vs an indirect gap of 0.67 eV for germanium) and the resulting change in the valence band edge suggests that the germanane will not experience this fermi level pinning and therefore the Al/Ge-H and the Au/Ge-H Schottky barrier heights will be different.

As reported previously by Bianco et al.,11 Ge-H has an electron mobility five times greater than that of Ge. At higher forward biases, the series resistance (Rs) dominates the I-V curve26 and therefore the methods described by Werner26 and Nouchi27 can be used to determine the Rs of the devices. The results are shown in Table I. As expected, the series resistance of the Germanium diode is much higher than that of the Germanane diode.

TABLE I.

Properties of germanium and germanane Schottky diodes.

Germanium (Ge)Germanane (Ge-H)
Series resistance (Ω) 228 478 1513 
Schottky barrier height (eV) 0.28 0.22 
Ideality factor (n) 25 15 
Germanium (Ge)Germanane (Ge-H)
Series resistance (Ω) 228 478 1513 
Schottky barrier height (eV) 0.28 0.22 
Ideality factor (n) 25 15 

In an effort to better understand the diode behavior, an attempt was made to fit the experimental data using the thermionic emission model.25,26 The results obtained for the Schottky Barrier Height and the ideality factor (n), particularly for the germanium diode, were different from those reported in the literature indicating that the model is not ideal for these devices. The ideality factor can become larger than 1 when, for example, mechanisms other than thermionic emission contribute to charge transport27 and when transport losses become large.28 For the reported devices, the departure from ideal is likely due to many factors. For instance, the large size of the devices leads to high series and shunt resistances. The use of drop casting results in high contact resistances. In addition, the use of two different non-ohmic contacts leads to rectification at both electrodes making it difficult to extract the diode parameters using a single I–V curve.26 Finally, the non-homogeneity of the device may contribute significantly to the high ideality factor. Nonetheless, the rectifying behavior of both devices is clearly visible and the differences between the two materials are clear.

In conclusion, a variety of spectral analyses (FTIR, Raman, and XRD) have confirmed the synthesis of germanane via intercalation. The germanane has been utilized to fabricate Schottky diodes. A comparison between the Ge and Ge-H diodes shows that germanane has much lower resistance and greater rectification when compared to germanium. In summary, the work shows the potential of germanane to be utilized in a variety of opto-electronic devices.

The authors gratefully acknowledge the financial support from BASE Fellowship and the Indo-U.S. Science and Technology Forum (IUSSTF). I.U.A and R.J.E. gratefully acknowledge the financial support by the U. S. National Science Foundation (DMR-1506595).

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