Abstract
We have fabricated the heavily Ga-doped layer in Ge single crystal by the implantation and rapid thermal annealing method. The samples show a crossover from the insulating to the superconducting behavior as the annealing temperature increases. Transport measurements suggest that the superconductivity is from the heavily Ga-doped layer in Ge.
1. Introduction
Since the superconductivity was observed in metal in 1911, intensive efforts have been performed to find new superconductor such as high cuprate [1] and pnictide [2]. The application of superconductor on the traditional industry now motivates scientists to find the superconductivity in semiconductor materials, since it can solve the problem of power consumption of integrated circuit and improve the efficiency of the device. As the superconductivity is induced in the diamond doped with boron [3] and the silicon doped with boron [4], the Ga-implanted germanium [5] with a total dose of 2 × 1016 cm−2 Ga was recently found to show superconductivity at the low temperature with the superconducting transition temperature lower than the germanium heavily doped with 4 × 1016 cm−2 Ga [6]. The origin of superconductivity in the heavily doped semiconductors, however, is an open question yet, even after the intensive study [6–8]. In this work, we fabricated the heavily Ga-doped Ge with a 30 nm SiO2 cover layer by implanting Ga into the Ge single crystals and performed the rapid thermal annealing (RTA) on these samples. After performing the scanning electron microscope (SEM) and transport measurement, we find that the annealing temperature can induce an insulator to superconductor transition in Ga-doped Ge samples. Our study of superconductivity in the Ga-implanted Ge combines the superconductor and semiconductor which will contribute to application of superconductor on the traditional semiconductor industry.
2. Experimental Details
We chose the (1, 0, 0) oriented and P doped germanium as the substrate in this experiment. On the top of the germanium we grew a 30 nm SiO2 cover layer through plasma-enhanced chemical vapor deposition (PECVD). After that, we implanted 4 × 1016 Ga cm−2 at an ion energy of 100 keV into it. Here, the SiO2 capping layer can prevent the surface degradation [6] except the implanted region during implantation as shown in Figure 1.
(a)
(b)
We annealed the Ga-doped Ge samples by the rapid thermal annealing (RTA) for 60 s in flowing Ar atmosphere at various temperatures. The RTA was reported to play an important role in recrystallizing the amorphous implanted layer, which leads to the formation of the highly Ga-doped layer at the SiO2/Ge interface [6, 8]. We therefore investigate the surface structure of the samples by the scanning electron microscope (SEM). The surface degradation does not occur on the surface of the sample because of the protection of the SiO2 capping as discussed before. But there are many discrete holes on the surface as a result of high dose implantation. For the as-implanted samples, the holes are discrete islands containing gallium; but for the samples annealed at 800°C, the gallium islands inside the holes percolate which can be regarded as superconducting grains of gallium percolating in the normal state germanium matrix.
To clarify the properties of the different annealed samples, we performed the electronic transport measurements on those annealed samples. We contacted the electrodes by the bonding lines and carried out the measurements in the usual four-terminal geometry at the temperature from 2.5 K to 300 K in a physical property measurement system (PPMS). The resistance was measured by ETO option whose excitation current is 100 μA. The temperature dependence of resistance in the as-implanted state and annealed at various temperatures under Ar flowing atmosphere is presented in Figure 2.
Figure 2(a) is the dependence of the sheet resistance for the unannealed and annealed samples. The s decreases as the annealing temperature increases. The logarithmic scale of here shows how dramatic the electronic transport can be influenced by the annealing temperature. For different annealing temperature, it shows a transition from the insulating to superconducting behavior. Upon cooling, the as-implanted samples show an increasing resistance, which could reflect intrinsic semiconductor property of Ge, while upon increasing the annealing temperature, the sheet resistance decreases, which could be due to the change of the discrete Ga islands to percolating ones as discussed above.
At the annealing temperatures ranging from 600°C to 850°C, the samples show a superconducting transition below 7 K as shown in Figure 2(b). There is a gradual drop of sheet resistance but zero resistance is not possible because of the discontinued superconducting path in the sample. When focusing on the low temperature transport carefully, we observed a detailed dependence of the superconducting properties on the annealing conditions. There is an optimal annealing temperature window (600°C–800°C). Out of that temperature window, the superconductivity is weak. There are two critical temperatures in the samples with annealing temperature at 830°C and 850°C.
It was shown in Figure 2(c) that the samples with superconducting behavior display a thermal hysteresis loop during the thermal cycle and the arrows mark the direction of those thermal cycles. The thermal hysteresis observed here shows the presence of the first order phase transition which could be attributed to the phase transition of the Ga grains from endothermic and exothermic process [9]. The gallium with exothermic process actually shows a similar superconducting transition temperature [10, 11] as the in Figure 2(b). The superconducting gallium grains actually play an important role in the process. Based on [9], the -Ga, -Ga, and -Ga show the similar hysteresis loop in the single-energy X-ray absorption measurements and the reason is their endothermic and exothermic process during thermal cycle. So we attribute the hysteresis loop to the gallium and relate the superconductivity to the hysteresis loop. Particularly the of -Ga is 6 K. It therefore clarifies the reason that the superconducting behavior is related to thermal hysteresis as observed in this experiment.
To obtain the further understanding of the character of superconducting state, we measured the magnetic field dependence of the sheet resistance of the samples annealed from 300°C to 850°C. The normalized magnetoresistance is plotted in Figure 3(a). (The data is shifted for clarity.) Here the is perpendicular to the ab-plane of samples. As shown in Figure 3(a), the samples show the largest magnetoresistance variation at the annealing temperature window between 600°C and 800°C, which might suggest the strongest superconducting character there. We also performed the dependence of sheet resistance measurement for the sample annealed at 800°C with the applied magnetic fields perpendicular to the -plane of the sample (Figure 3(b)). It is shown in Figure 3(b) that the superconducting transition is suppressed by the applied field . We can define the critical temperature as the 2% drop of normal state resistance for the sample measured at different . The inset in Figure 3(b) is the phase diagram of the samples. The (0 K) at zero temperature was estimated as the 10% drop of at 2 K, which is approximately 0.5. Such a criterion is the same as the early work [6]. By using the standard theory, we can calculate the value of the Ginzburg-Landau coherence length . We deduce the nm via , where is the magnetic flux quantum.
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(b)
3. Summary
In summary, we have fabricated heavily Ga-doped Ge with a 30 nm SiO2 cover layer. The ion implantation and subsequent RTA annealing were employed to form a Ga-doped layer at the SiO2/Ge interface. The RTA annealing enables Ga to redistribute in the sample and realize the superconducting circuits. Ga forms metallic precipitates in the Ge matrix. With increasing annealing temperature, the samples show a crossover from the insulator to the superconductor. The is the same with the gallium’s transition temperature and the hysteresis loop is related to gallium. So, the electronic transport measurements indicate that the Ga-doped layer plays a leading role in the superconductivity.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgment
This work is supported by the “Strategic Priority Research Program (B)” of the Chinese Academy of Sciences Grant no. XDB04040300.