Temperature and electroluminescence spectra are critical for the reliability characterization of gallium nitride (GaN) devices. The traditional method of reliability characterization combines the existing temperature measurement and electroluminescence detection. The temperature measurement method is mainly used to measure the lattice temperature and Joule heating. Electroluminescence is primarily used to measure gate current leakage, thermal electrons, and electric fields, which complement each other. Because the electroluminescence spectrum is correlated with temperature, it is necessary to characterize the temperature simultaneously with the measurement of the electroluminescence spectrum to avoid the influence of lattice temperature on the electroluminescence spectrum. Therefore, the simultaneous characterization of the electroluminescence spectrum and in situ temperature is important for the reliability evaluation of GaN devices. Currently, the measurement of the electroluminescence spectrum mainly uses an electroluminescence spectrometer, and temperature characterization method includes micro-Raman spectroscope, thermoreflectance, and scanning thermal field microscope. However, these methods cannot achieve simultaneous in-situ measurement of the electroluminescence spectrum and temperature in micro-nano regions. The core component of scanning probe microscopy (SPM) technology is the fiber probe, which has the ability to transmit optical signals and collect near-field optical signals. However, the conventional fiber probe cannot be used for temperature measurement, whereas the cadmium selenide quantum dot (QD)-modified fiber probe is verified to be suitable for the nondestructive detection of the temperature of living cells. In this study, we propose a new near-field simultaneous measurement method for the electroluminescence spectrum and in-situ temperature using a cadmium selenide QD-modified fiber probe, which is used to characterize the electroluminescence spectra and near-field temperatures of GaN samples under different voltage excitations.

Based on SPM technology, the QD fiber probe approaches an area of tens of nanometers on the surface of a sample under the control of tuning fork atomic force feedback, and heat is transferred from the sample surface to the QDs at the tip of the probe through the near field. The excitation light is injected through the pigtail of the fiber probe and transmitted through the fiber to the QDs at the tip of the probe, which excites the fluorescence of the QDs. The fluorescence peak of the QDs shifts with increasing temperature. The QD fluorescence signal, which carries temperature information, is collected backward by the fiber probe and then demodulated by the fluorescence spectra as a function of temperature. Simultaneously, the QD fiber probe collects the electroluminescence signal emitted by the surface of the GaN sample through the near field and transmits it to the spectrometer via the probe pigtail to obtain the near-field electroluminescence spectra of GaN. Because the probe tip size is on the order of tens of nanometers, it can achieve high-spatial-resolution near-field simultaneous detection of the electroluminescence spectrum and in-situ temperature. Figure 1(a) shows a schematic of the principle of the electroluminescence spectrum and in-situ temperature simultaneous measurement system based on the QD fiber probe, which is based on an atomic force microscope and Raman spectrometer. The QDs at the tip are excited by a 532 nm laser with an excitation power of 0.2 mW and integration time of 1 s. The calibration relationship between the fluorescence spectrum and temperature is shown in Figs. 1(b) and (c). The temperature sensitivity of the probe is 210 pm/℃, and the temperature measurement error is approximately 0.9 ℃.

A GaN sample is prepared on bulk single-crystal GaN material using vacuum evaporation technology; a schematic is shown in Fig. 2(a). To measure the surface height of the GaN sample, a QD fiber probe is used to scan the GaN surface over a large area with a scanning size of 100 m×50 m, as shown in Fig. 2(c). The height of the electrode region is approximately 358 nm, the GaN tends to be relatively flat, and the heights of a few uneven regions reach 705 nm. According to the scanning height, the average roughness of the electrode is approximately 56 nm. The electrode height and roughness are within the control range of the tuning fork spacing, and the probe can be used for the in-situ measurement of this area. To study the electroluminescence characteristics of GaN characterized by the QD fiber probes, the voltage excitation threshold of GaN samples is measured based on the method proposed in this paper, and the measurement results are shown in Fig. 3. When the excitation voltage reaches 9 V, GaN electroluminescence will cause its own impedance to change from 0.23 Ω to 0.10 Ω. Based on this, it is investigated whether the electroluminescence of GaN samples affects the local temperature rise of GaN samples. The measurement results of the GaN power change and surface temperature rise under different voltage excitations and the same voltage excitation time are shown in Fig. 5(a). The temperature gradient in the range of 9?12 V excitation voltage is significantly greater than that in the range of 0?9 V excitation voltage. Based on the analysis in Fig. 3, it can be concluded that this phenomenon is caused by the electroluminescence of the GaN sample. In addition, the temperature rise of GaN and metal electrodes under the condition of 12 V voltage excitation is monitored in real time, and the 1.9 ℃ temperature difference between GaN and metal electrode is measured at 150 s, as shown in Fig. 5(b).

In this study, we propose a method for measuring the electroluminescence spectrum and in-situ temperature of GaN materials in the micro-nano region based on tuning fork feedback QD fiber probes. The GaN samples are detected under different excitation voltages. The results show that the voltage excitation threshold for the electroluminescence of the GaN sample is 9 V, and the peak intensity of the electroluminescence spectra gradually increases with the increase of the excitation voltage. The central peak remains unchanged, and GaN electroluminescence causes its own impedance to change from 0.23 Ω to 0.10 Ω, which affects the change of GaN power. Moreover, the temperature gradient of the GaN surface is significantly greater than that in the excitation voltage range of 9?12 V, which is caused by the GaN electroluminescence. In addition, the temperature rise of GaN and metal electrodes under the power-on and power-off conditions is monitored in real time under the 12 V voltage excitation. The temperature of GaN samples tends to be stable at 150 s, and there is a temperature difference of 1.9 ℃ between GaN and metal electrode. The results show that this method can be used for the near-field simultaneous measurement of GaN micro-nano temperature and electroluminescence spectrum, which has more advantages than the traditional separate measurement methods of electroluminescence and temperature and has promising application prospects in the performance characterization of GaN high-electron-mobility transistors and other semiconductor materials in the future.