Temperature-dependent ultraviolet (UV) Raman scattering from AlGaN/GaN heterostructure is investigated. Compared to the visible Raman spectrum, four new peaks at 600, 700, 780, and 840 cm−1 are observed in the UV Raman spectrum. The peak at 780 cm−1 is from the AlGaN A1(LO) mode. According to the calculated dispersion relations of the interface phonon modes in the AlGaN/GaN heterostructure, the peaks at 600 and 840 cm−1 correspond to interface phonon modes. Meanwhile, the peak at 700 cm−1 is attributed to the disorder-active mode near the 2DEG interface. Due to the near-resonant enhancement effect, the intensities of the GaN A1(LO) mode, interface phonon modes, disorder active mode and the AlGaN A1(LO) mode exhibit different temperature dependence. Furthermore, the frequencies of the interface phonon modes and the disorder active mode show anomalous temperature dependence, which can be attributed to the strong built-in electric field near the 2DEG interface.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Due to the superior properties such as wide bandgap, high electron saturation velocity and high breakdown electric field, AlGaN/GaN heterostructures have attracted considerable attention for applications in high frequency and high power microwave electronic devices [1–5]. Although significant progress has been made in improving device performance, the reliability of AlGaN/GaN-based electronic devices under high temperature and high electric field is still the main constraint for further commercial application [6–9]. For better improving the device reliability, the electrical and optical properties of the AlGaN/GaN heterostructures and devices should be fully investigated.
Raman scattering is a rapid, contactless and nondestructive technique for the optical characterization of semiconductors. In previous studies, Raman scattering was mainly used to detect the channel temperature and stress distribution of AlGaN/GaN based heterostructures and high electron mobility transistors (HEMTs) by using visible light as excitation source [10–14]. Additionally, there are several studies on III-nitride materials using ultraviolet (UV) Raman [15,16]. In Ref , UV Raman spectroscopy is employed to characterize material properties of surface and interface layers of AlGaN/GaN heterostructure, such as the free carrier concentration at AlGaN/GaN interface or the aluminium composition of AlGaN barrier layer according to the observed A1(LO) phonon frequencies of GaN and AlGaN layers. Although the interface phonon mode near 600 cm−1 of AlGaN/GaN heterostructure is probed in the UV Raman measurement , this report focuses on the study of epitaxial growth, material characterization and device processing of the AlGaN/GaN heterostructure on a silicon substrate. There is no systematic analysis in the previous UV Raman works. Besides, to our knowledge, no report deals with the temperature dependence of the UV Raman scattering of AlGaN/GaN heterostructure so far.
In this work, we report the UV Raman scattering from AlGaN/GaN heterostructure with varying temperature. The interface phonon modes of AlGaN/GaN heterostructure are investigated by experimental measurement and theoretical calculation. Interestingly, the interface phonon modes exhibit anomalous temperature dependence. To explain the anomalous phenomenon, temperature dependent energy band and the influence of built-in electric field of AlGaN/GaN heterostructure on phonon frequencies are investigated.
2. Experimental details
The AlGaN/GaN heterostructure used in this study was grown on a c-plane sapphire substrate by metal-organic chemical vapor deposition. Trimethylgallium (TMGa), trimethylaluminum (TMA) and ammonia were the precursors. Following a thin GaN layer grown at 488 °C, a 2-µm-thick unintentionally doped GaN buffer layer was grown at 1070 °C. Then, a 1-nm-thick AlN interlayer and a 25-nm-thick Al0.27Ga0.73N barrier layer were grown at 1080 °C. In the growth of the heterostructure, the III/V ratios of AlGaN and GaN are 1/1500 and 1/2000, respectively.
High-resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) were used to characterize the crystalline quality and the surface morphology of the heterostructure. Raman scattering measurements were performed in backscattering configuration by using a He-Cd laser (325 nm) and an Ar+ laser (514 nm) as excitation sources, respectively. Here, z means that the incoming laser beam is propagating parallel to the wurtzite c-axis and means backscattering geometry. (x, _) refers to the polarization of the incident and the scattered light. The “_” means unpolarized detection. In this work, the Raman measurements with 325 nm and 514 nm excitation wavelengths are defined as UV and visible Raman, respectively. The temperature of the sample was controlled from 80 to 600 K by using a liquid-nitrogen-cooled cryostat and a Linkham high-temperature stage.
3. Results and discussion
Figure 1 shows the HRXRD ω-2θ scan of (0002) diffraction plane of the AlGaN/GaN heterostructure. The dominant peak and the shoulder peak at higher Bragg angle are from the GaN and AlGaN layers respectively. The presence of Pendellösung fringes indicates flat surface and interfaces. The smooth surface is confirmed by AFM measurement in Fig. 2. The insets of Fig. 1 show the rocking curves of (002) and (102) planes of GaN. The full-width at half-maximum (FWHM) is 263 arcsec for (002) and 417 arcsec for (102), which also indicates high crystalline quality of the heterostructure.
Figure 3(a) shows the UV and visible Raman spectra of the AlGaN/GaN heterostructure at room temperature. It can be seen that both the GaN E2(high) and A1(LO) phonon modes are observed in the two spectra. Compared to the visible spectrum, the UV spectrum exhibits four new peaks near 600, 700, 780 and 840 cm−1, which can be attributed to the different penetration depth of UV and visible light in AlGaN/GaN heterostructure. The UV light (325 nm) can only penetrate several tens of nanometers into AlGaN/GaN heterostructure due to the absorption of the light in GaN material, while the visible light (514 nm) can penetrate the whole heterostructure . Therefore, the UV light mainly probes the phonons from the near-surface region. The visible light detects the phonon information averaged over the thick GaN buffer layer. So it can be concluded that the four new Raman peaks in the UV Raman spectrum should be from the phonons in the near-surface region.
Beside the expected E2(high) and A1(LO) modes of GaN, there contains the E2(high) and A1(LO) modes of AlGaN barrier layer and interface phonon modes in the near-surface region of AlGaN/GaN heterostructure, which may be observed under back scattering Raman configuration. Therefore, the four new peaks should correspond to the E2(high), A1(LO) modes of AlGaN or interface phonon modes. The peak at 780 cm−1 is assigned to the A1 (LO) phonon mode of the AlGaN barrier layer according to the relationship between the phonon frequencies and the Aluminum composition of AlxGa1-xN . To confirm the origins of the other three new peaks, the dispersion relations of the interface phonon modes in AlGaN/GaN heterostructure with a 1-nm-thick AlN spacer layer were calculated based on the dielectric continuum model (DCM) and Loudon’s uniaxial crystal model . According to the relation among the phonon wave vector and the wave vectors of the incident () and scattered light () that , the phonon wave vector involved in the UV Raman measurement under backscattering configuration is about 1*106 cm−1 (1*106 cm−1) . Therefore, we calculated the dispersion relations of interface phonon modes in the wave vector range of 0 to 1*106 cm−1. The calculated results with and without taking into account the vacuum on the AlGaN barrier layer are shown in Fig. 4(a) and (b), respectively. Compared to the result without taking into account vacuum in Fig. 4(b), there is one more interface phonon mode near 750 cm−1 with the wave vector q≈1*106 cm−1 in Fig. 4(a), indicating that this interface phonon mode should be located near the vacuum/AlGaN interface. As shown in both Fig. 4(a) and (b), there are three interface phonon modes near 600, 840 and 850 cm−1. The interface phonon modes near 600 and 840 cm−1 are in good agreement with the UV Raman experiment. Besides, compared to the UV Raman spectrum of the AlGaN/GaN with 2DEG, the Raman peaks near 600 and 840 cm−1 cannot be observed in the AlGaN/GaN heterostructures without 2DEG that possibly resulted from unintentionally residual Mg acceptors in growth chamber, as shown in Fig. 3(b). This phenomenon can be attributed to the 2DEG-related resonance enhanced effect which will be verified in the following discussion. Based on the above results, the new Raman peaks near 600 and 840 cm−1 are mainly attributed to the interface phonon modes located near the AlN/GaN interface. However, the calculated interface phonon modes near 750 and 850 cm−1 are not detected in the UV Raman spectra of AlGaN/GaN heterostructure. The possible reason is that these two interface phonon modes are located near Vacuum/AlGaN and AlGaN/AlN respectively. They cannot be strongly enhanced by 2DEG-related resonance effect. Besides, no solver can be found near 700 cm−1 in Fig. 4(a) and (b). The peak near 700 cm−1 in Fig. 3(a) can be excluded from interface phonon mode. In fact, this peak can be attributed to a disorder-activated mode related to lattice defects at the 2DEG interface .
To further investigate the phonon vibration characteristics in the near-surface region of AlGaN/GaN heterostrecuture, we measured the UV Raman scattering of this structure at different temperatures. Figure 5(a) and (b) show the UV Raman spectra of AlGaN/GaN heterostructure in the temperature ranges of 80-300 K and 300-600 K, respectively. All Raman active modes can be clearly distinguished in the low-temperature spectra. As shown in Fig. 5(a) and (b), the intensities of the interface phonon mode near 600 cm−1, the disorder activate mode and the A1(LO) phonon mode of GaN increase significantly with decreasing temperature, especially in the low-temperature range. Differently, in the high-temperature range, the intensities of the AlGaN A1(LO) phonon mode and the GaN E2(high) mode initially increases with increasing temperature. When the temperature is higher than 500 K, the intensities will decrease with increasing temperature.
The temperature dependence of the Raman intensities of the AlGaN A1(LO) phonon mode in the high-temperature range can be attributed to the near-resonant enhancement effect. By solving the Schrodinger and Poisson equations self-consistently using the Silvaco Atlas software, the band diagram of the AlGaN/GaN heterostructure with varying temperature can be calculated. The corresponding energy band gaps of AlGaN barrier layer, GaN buffer layer and the energy gap between the first subband, second subband and the valence band of the triangular quantum well in the temperature range of 80-600 K are shown in Fig. 6.
It can be seen that, in the temperature range of 80-300 K, the energy gap between the valence band and the second subband in the triangular quantum well is closest to the excitation photon energy. Meanwhile, it becomes much closer to the photon energy with decreasing temperature. The resonant Raman scattering from the triangular quantum well will become much stronger. Therefore, the intensities of the polar optical phonon modes localized near the triangular quantum well (the interface phonon mode near 600 cm−1, the disorder activated mode near the 2DEG interface, and the A1(LO) phonon mode of GaN) increase with decreasing temperature. On the contrary, in the temperature range of 300-600 K, the band gap of AlGaN barrier layer becomes much closer to the photon energy with increasing temperature. The resonant Raman scattering arises from the AlGaN barrier layer. When the temperature reaches 500 K, the band gap of AlGaN is closest to the excitation energy. Thus, in the high temperature range, the intensity of the AlGaN A1(LO) phonon mode initially increases (in the temperature range of 300-500 K) and then decreases (in the range of 500-600 K) with increasing temperature.
The possible reason for that the intensity of the GaN E2(high) peak is lower in the temperature range of 500~600 K is explained as follows. It is well known that Raman intensity is closely related to the scattering efficiency or cross section of the phonon propagation . For the GaN E2(high) mode, the avaerage number of phonons increases with increasing temperature, resulting in the increase of the scattering efficiency . Correspondingly, the intensity of the optical phonon increases with increasing temperature. However, when the temperature reaches a certain value, the phonon will decay due to the strong interaction between phonons, which will induce the decrease of the phonon intensity.
Besides the temperature dependence of the intensities of the phonon modes in the structure, the phonon frequencies also exhibit different temperature dependence. As shown in Fig. 5, the frequencies of GaN E2(high), A1(LO) and AlGaN A1(LO) modes show slightly red-shift with increasing temperature as expected, mainly due to the thermal expansion of the lattice [22,23]. However, the interface phonon modes near 600 cm−1 and the disorder active mode exhibit significant blue shift with increasing temperature, especially in the low temperature range.
To explain this anomalous Raman phenomenon, the built-in electric field distribution and its influence on the phonon frequencies in the AlGaN/AlN/GaN heterostructure are investigated. Due to the difference of the spontaneous and piezoelectric polarization among the AlGaN, AlN and GaN layers, vertical built-in electric field is induced in the heterostructure. By using the Silvaco atlas software, the distribution of the built-in electric field in the heterostructure is simulated. The result is shown in Fig. 7. It can be seen that the built-in electric field in the AlN interlayer is very strong, of the order of MV/cm, compared to that in the AlGaN and GaN layers (~kV/cm). The vertical built-in electric field can shift the phonon frequencies through the inverse piezoelectric effect which results from the changes in the atomic coordinates, especially for the lattice vibrations along the c-axis . The strong positive electric field in the AlN interlayer reduces the distance between the Ga and N atoms along the c-axis, as shown in the Fig. 8. Therefore the lattice vibration frequency along the c-direction near this layer will increase. However, the influence of the weak built-in field in the AlGaN and GaN layers on the lattice vibration frequencies can be ignored. Meanwhile, the strength of the built-in electric field in AlN layer increases with increasing temperature, due to the increasingly thermal expansion mismatch between AlN and GaN materials. Therefore, the frequencies of the interface phonon mode and the disorder active mode near the 2DEG interface increase significantly with increasing temperature. In this work, we just qualitatively analyze the influence of built-in electric field on phonon frequencies in the AlGaN/GaN heterostructure with AlN interlayer. The quantitative analysis will be explored in further study.
In summary, temperature-dependent UV Raman scattering of AlGaN/GaN heterostructure was investigated. Besides the optical phonon modes in AlGaN and GaN layer, two interface phonon modes and a disorder active mode near the hetero-interface were also observed. The interface phonon modes and the disorder active mode exhibit anomalous temperature dependence, which is attributed to the strong built-in electric field in the AlN interlayer of AlGaN/GaN heterostrucutre.
National Natural Science Foundation of China (No. 61804089, 61634002, 61474060); The Natural Science Foundation of Shandong Province (ZR2016FP09); NSAF(U1830109); The project of Shandong Province Higher Educational and Technology Program (J16LN04, J18KA325); the Yantai key R&D Program (2017ZH064, 2017ZH063, 2016ZH063), The PhD Start-up Fund of Shandong Technology and Business University (BS201608).
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