High-specific-detectivity, low-dark-current Ge nanowire metal–semiconductor–metal photodetectors fabricated by Ge condensation method

The whole Ge condensation process as shown in figure 2(a), was carried out through two steps for preparation of pure Ge NW. In the first step, Si_0.85Ge_0.15 on SOI substrate was treated at 1150 °C for two cycles of 100 min oxidation and 30 min annealing. After that, Si_1−xGe_x on insulator (SGOI) with Ge fraction x of 0.37 was obtained, as denoted C₁. In this stage, SiO₂ on the surface was removed by BOE, and stripe patterns with various widths were fabricated with EBL and RIE. These samples were then treated at 900 °C by several cycles of 10 min oxidation and 10 min annealing (referring as 3D Ge condensation) until pure Ge NW were achieved, as denoted C₂. Raman spectroscopy has been employed to identify the crystalline quality and Ge component of Ge NW. Raman spectra of the as-grown, C₁ and C₂ samples during Ge condensation process are illustrated in figure 2(b). Raman spectra of bulk Ge is also given in this figure for comparison. The Raman signal was excited by a 532 nm laser with a spot size of 4 µm and a power of 20 mW. The following signals were detected including Si–Si peak (~520 cm⁻¹) from Si, Si–Si (480–510 cm⁻¹), Si–Ge (~400 cm⁻¹) and Ge–Ge peaks (270–300 cm⁻¹) from SiGe, respectively. The intensity ratio of Ge–Ge peak to Ge–Si peak increased with increase of Ge fraction. What important was that only Ge–Ge mode could be clearly observed in the sample at the final stage, indicating that almost pure Ge NW had been achieved. Compared with Ge–Ge line of bulk Ge, Ge NW should be under tensile strain of 1.2%, extracted from this difference between Ge–Ge peaks from bulk Ge and Ge NW. Ge is an indirect band gap material and the energy difference between direct conduction band (Г) and indirect conduction band (L) is about 136 meV at room temperature. When tensile strain is applied to Ge, the energy band gap is decreased and the energy difference between Г and L is also reduced, even changed to direct bandgap material. The light absorption is enhanced due to the direct band transition at longer wavelength.

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The typical SEM image of Ge NW prepared by Ge condensation technique is shown in figure 2(c). The width of Ge core is about170 nm and SiO₂ shell is about 60 nm, respectively. The thickness of Ge is about 10 nm estimated with 3D condensation model, supposing the conservation of Ge during Ge condensation process, which has been definitely verified [24].

Chemical states of Ge oxides or SiGe oxides on Ge NW on insulator were characterized by XPS spectroscopy. Because what we concern was these chemical states of the Ge oxides near the Ge interface, the oxide layer on the Ge surface was thinned to 5 nm before measuring the XPS spectrum. The corresponding Ge 2p XPS spectra are depicted in figure 2(d), in which the broad Ge 2p electron binding energy peak has been decomposed into three valence states of Ge using a Lorentzian–Gaussian function. As can be seen, peaks around 1217.2 and 1220.6 eV [25] in Ge 2p XPS spectra is corresponding to Ge and GeO₂, respectively. While the other Peaks around 1219.0 eV can be observed clearly, suggesting the existence of Ge suboxides. According to previous research, these following chemical reactions occur during Ge condensation process: Si  +  O  →  SiO₂, Ge  +  O  →  GeO₂, and Si  +  GeO₂  →  Ge  +  SiO₂. Most part of the oxide is SiO₂ [26]. Therefore, when excess oxygen is introduced into oxidation of almost pure Ge, a mixture of SiGeO_x is formed near the Ge surface. Therefore, the XPS analysis confirm the formation of SiGeO_x shell surrounding the Ge core with these existence of oxygen vacancies on the surface of NWs. The sub-stoichiometric oxides shell on the Ge core may provide oxygen vacancies, to be prone to charge trapping [16, 17, 19, 23].

In this part, these Ge NW MSM photodetectors with various widths of 35, 70 and 170 nm, were fabricated and characterized. Current–voltage characteristics of these three Ge NW photodetectors are presented in figure 3(a). Low dark current of 5.1 nA at 0.51 V bias for the 35 nm wide Ge NW photodetector indicates Ge NW with high crystal quality and good passivation. The Ge NW photodetector can be modeled as MSM devices with back-to-back Schottky diodes and thermionic emission (TE) as the dominant mechanism of carrier injection across the metal/semiconductor interface [27, 28]. It is found that this I–V curve can be fitted well by the thermionic emission theory as follows [29]:

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle {{J}_{S}}={{A}^{*}}{{T}^{2}}\exp \left( -\frac{{{\phi }_{B}}}{kT} \right),\nonumber \end{align} \tag{ 1 }$$

where k, T, A^* and ${{\phi }_{B}}$ are Boltzmann's constant, absolute temperature, effective Richardson coefficient and barrier height, respectively. We thus find that schottky barriers (SBs) of Sample1 is 0.41 eV, Sample2 is 0.39 eV, and Sample3 is 0.40 eV for our MSM photodetectors in dark situation. The dark current increases sensitively with Ge NW width suggest that the bulk current, rather than surface leakage current, dominates the total dark current. This result implies that Ge NW surface is well passivated by SiGeO_x formed during Ge condensation process in this case.

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The typical photocurrent of 35 nm wide Ge NW photodetector as a function of bias at fixed wavelengths of 650, 850 and 1100 nm is shown in figure 3(b). The large difference between photocurrent and dark current in one order of magnitude at very low illumination power indicates that the Ge NW photodetector has excellent performance. Otherwise, the almost same photocurrent from a commercial Si photodetector with a collection area of 1 mm² and 35 nm wide, 9 µm long Ge NW photodetector at 1100 nm definitely demonstrates that responsivity of Ge NW photodetector is larger than that of a commercial Si photodetector by several order of magnitudes [30].

In figure 3(c), we plot the responsivity and normalized photocurrent to dark current ratio (NPDR) of the 35 nm wide Ge NW photodetector versus voltage at 650, 850, and 1100 nm wavelengths, respectively. The power incident on the NW has been estimated from the illumination intensity (measured in W cm⁻²) and the NW photosensitive surface area. The effective area of NW exposed for light absorption is approximately the sum area of top and sidewalls of Ge NW. According to structural parameters, these photosensitive surface areas are evaluated about 0.41, 0.72, and 1.62 µm², respectively. The response spectrum of standard Si and InGaAs detector were used to calibrate the optical power corresponding to each wavelength of the light source. The NPDR is defined as [31]:

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle NPDR=\frac{{{I}_{photo}}}{{{I}_{dark}}{{P}_{in}}}=\frac{R}{{{I}_{dark}}},\nonumber \end{align} \tag{ 2 }$$

where I_photo and I_dark are photon and dark current, respectively; P_in is incident optical power; R is responsivity. It indicates that the responsivity and NPDR increases along with bias voltage linearly. Under 0.99 V bias, the responsivity is up to 2.09  ×  10⁵ A W⁻¹ at 650 nm and the NPDR value is about 3.48  ×  10¹² W⁻¹ at 650 nm. The NPDR is significantly higher than that of typical Si (~10 W⁻¹), Ge (~10² W⁻¹) and emerging 2D material (~10² W⁻¹) photodetectors [32] suggesting promising application potential of Ge NW photodetectors.

Figure 3(d) depicts the dependence of responsivity on incident power at 650, 850 and 1100 nm, respectively, for the 35 nm wide Ge NW photodetector under 0.51 V bias. The responsivity decreases with increasing of incident power. A maximum responsivity of 2.55  ×  10⁵ A W⁻¹ was reached under illumination intensity of 0.65 µW cm⁻² at 650 nm and 0.51V bias.

The response spectra of these three samples were measured in a wavelength range of 360 nm to 1160 nm, as shown in figure 4. Figure 4(a) displays response spectra of the 35 nm wide Ge NW photodetector under bias of 0.24, 0.36, and 0.51 V. The Ge NW photodetector shows strong response over the UV, visible and near-infrared wavelength (360–1160 nm). The large photo-responsivity up to 2.02  ×  10⁵ A W⁻¹ at 560 nm under 0.51 V is achieved, which is several orders of magnitude higher compared to those photodetectors made from bulk Ge (R  ⩽  1 A W⁻¹) [33, 34]. The large responsivity can be attributed to large photocurrent gain of the Ge NW photodetector.

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Figure 4(b) displays the dependence of response spectra of Ge NW photodetectors on NW width under 0.51 V bias. It is indicated that responsivity increases with the reduction of Ge NW width in the whole studied wavelength range. Particularly, the responsivity of sample1 of 35 nm wide Ge NW photodetector is larger than that of sample3 of 170 nm wide Ge NW photodetector in beyond two orders of magnitude. This result can be attributed to large specific surface area for small size NW, in which traps at or near the interface between oxides and Ge play an important role in generation of photocurrent gain. The photocurrent gain of NW photodetector can be defined as [16]:

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle G=\frac{{{I}_{photo}}/q}{{{P}_{in}}/h\nu }=\frac{R}{q/h\nu },\nonumber \end{align} \tag{ 3 }$$

where I_photo is photocurrent, P_in is the incident light power, R is responsivity, q is unit charge and hν is photon energy of incident light. Based on equation (3), the photocurrent gain is extracted for these samples under 0.51 V bias, as showed in figure 4(c). The gain increases with the decrease of Ge NW width and the peak value at 560 nm are 4.47  ×  10⁶, 6.51  ×  10⁵ and 6.31  ×  10⁴ for these photodetectors of 35, 70 and 170 nm Ge NW width, respectively. The enormous gain is mainly due to these presence of oxygen vacancies at the interface between oxides and Ge, as reported previously [16–19]. These oxygen vacancies are prone to charge trapping to generate a radial electric field, which drives photo- electrons to the surface and be captured, resulting in an increase in the number of holes; In addition, a large number of holes accumulate at Al/Ge interface, causing the barrier to decrease and increasing the photocurrent.

The specific detectivity (D^*) is another important parameter for characterization of photodetectors. The noise affecting D^* consists of three parts: scattering noise from dark current, Johnson noise from thermal effect, and flicker noise. The main role is the scattering noise, so if we only consider the dark current. The specific detectivities of a photodetector are given by [35]:

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle {{D}^{*}}=R\sqrt{\frac{A}{2q{{I}_{dark}}}}.\nonumber \end{align} \tag{ 4 }$$

Among them, R is responsivity, A is photosensitive surface, q is unit charge, and I_dark is dark current. The unit of D^* is cm·H^1/2·W⁻¹. Based on equation (4), the specific detectivity at 560 nm is extracted and shown in figure 4(d). The high detectivity of 1.26  ×  10¹⁴, 2.54  ×  10¹³ and 1.23  ×  10¹³ cm·Hz^1/2·W⁻¹ for these samples of 35, 70, and 170 nm wide Ge NWs are obtained, respectively. Table 1 displays the summary of parameters of these samples.

Based on above discussions, the large photocurrent gain of these Ge NW photodetectors could be well described in photogating effect [18, 36–38]. Figure 5 depicts the schematic diagram of photon carriers generation, trapping and effect on photocurrent gain in Ge NW photodetectors. The dark current of Ge NW photodetectors is assumed mainly governed by Schottky barrier height. The suboxide SiGeO_x shell surrounding Ge cores provides a large number of oxygen vacancies at or near the surface of Ge NWs. The SiGeO_x can induce a depletion layer on the Ge NW surface with a radial electric field from SiGeO_x to inner NW. Oxygen vacancies serve as electron traps, rendering increase of lifetime of photon generated holes, which has great contribution to the gain. On the other hand, a large number of holes are gathered in the metal and semiconductor contact regions, lowering its Schottky barrier height, further enhancing photocurrent gain.

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We summarize Ge NW photodetectors previewly reported and list their electrical parameters in table 2. The Ge NW photodetectors in this case show excellent properties in terms of low dark current and high specific detectivity.