This suggests that Ge/GeO x layers are observed rather than pure Ge NWs, which should help to obtain good resistive switching memory characteristics. To observe the defects in the Ge/GeO x NWs, we recorded PL spectra of the NWs, as shown in Figure 3a. To understand the temperature dependence of the PL spectra, the peak was normalized with respect to PL at 300 K. No significant shift of the emission peak with temperature learn more was observed. However, the PL intensity gradually increases as the temperature increases from 10 to 300

K, revealing that more defect states are activated as the temperature is raised. To identify the defects inside the Ge/GeO x NWs, the PL spectrum measured at 300 K was decomposed into four component peaks using Gaussian fitting, as shown in Figure 3b. The peaks are centered around 387 nm (3.2 eV), 402 nm (3.1 eV), 433 nm (2.9 eV), and 483

nm (2.6 eV). Violet-blue emission is observed from these Ge/GeO x NWs. Because of their large diameter of approximately 100 nm, the quantum confinement effect is not the origin of this broad emission spectrum [41]. Therefore, selleck compound the PL peaks probably originate from oxygen vacancies (V o), oxygen-germanium vacancy pairs (V Ge, V o), and related defects. The broad violet-blue emission can be explained by a simple mechanism. It is assumed that acceptors will form (V Ge, V o), and the donors will form V o. After the excitation of acceptors/donors, a hole (h o) and electron (e) are created on the acceptor and donor, respectively, forming (V Ge, V o) and (V o) according to the following equation [42]: (1) where h is Plank’s constant and

ν is frequency. The violet-blue emission occurs via the reverse reaction. This suggests that the vacancies exist in the Ge/GeO STK38 x NWs, which may improve their resistive switching memory performance. A schematic diagram of the NW-embedded MOS capacitor in an IrO x /Al2O3/Ge NWs/p-Si structure is shown in Figure 4a. The capacitance (C)-voltage (V) hysteresis characteristics of the Ge/GeO x NW capacitors with different sweeping voltages from ±1 to ±5 V were investigated, as shown in Figure 4b. Memory windows of 1.7 and 3.1 V are observed under small sweeping gate voltages of ±3 and ±5 V, respectively. In contrast, a small memory window of 1.2 V under a sweeping gate voltage of ±7 V was observed for the device without Ge/GeO x NW capacitors because of the degradation of the GeO x film (data not shown here). The larger memory window of the device containing Ge/GeO x NW capacitors compared with those without the capacitors may be caused by effective charge trapping on the surface of the Ge/GeO x NWs. Defects on the surface of the Ge/GeO x NWs will trap holes rather than electrons because the C-V signal shifted towards the negative side, which was also observed in the PL spectrum of the NWs.