纤锌矿GaN外延层薄膜热膨胀行为的变温Raman散射研究

本文对纤锌矿结构GaN外延层薄膜的热膨胀行为进行了研究,结合热膨胀系数的物理意义与变温Raman散射时声子频移的变化规律,研究了热膨胀系数与变温Raman散射之间的关系.结果表明:通过测量Raman声子E_2(high),A_1(TO)和E_1(TO)频移与温度之间的线性关系,结合相应声子Gruneisen参数的涵义,可对纤锌矿结构GaN外延层薄膜在一定温度范围内的热膨胀系数进行测量.本文提供了一种表征纤锌矿结构GaN外延层薄膜热膨胀行为的有效方法,为进一步研究III族氮化物外延层薄膜在生长过程中热膨胀系数的匹配、降低外延层薄膜中的位错密度并提高发光二极管的发光效率提供了理论依据.

Thermal expansion behaviors of epitaxial film for wurtzite GaN studied by using temperature-dependent Raman scattering

III-nitride materials have attracted considerable attention in the last decade due to their wide applications in solidstate light devices with their direct wide band-gaps and higher quantum efficiencies. InGaN/GaN multiple quantum well is important active region for light-emitting diode, which can be tuned according to indium composition in the InxGa1-xN alloy system. Owing to difficulty in fabricating bulk materials, GaN thin films are heteroepitaxially grown on latticemismatched and thermal-expansion-mismatched substrates, such as sapphire (Al₂O₃), Si and SiC, which subsequently results in a mass of threading dislocations and higher residual strains. On the one hand, dislocations and defects existing in GaN epifilms trap the carriers as scattering centers in the radiative recombination process between electrons and holes, and play an important role in drooping the internal quantum efficiency. On the other hand, higher built-in electric field induced by residual strains existing in GaN epifilm could make the emission wavelength red-shifted.It is common knowledge that temperature is one of the important factors in the growth process of epitaxial films, as a result, further research on thermal expansion behaviors is needed. Based on the above analysis, an in-depth study of thermal expansion behavior of wurtzite GaN epitaxial film is of vital importance both in theory and in application.In this study, we investigate the thermal expansion behaviors of wurtzite GaN epitaxial films by using temperaturedependent Raman scattering in a temperature range from 83 K to 503 K. According to the physical implication, Gruneisen parameter is almost a constant (Gruneisen parameters of all phonon modes are in a range between 1 to 2 for GaN) that characterizes the relationship between the phonon shift and the volume of a solid-state material. More importantly, Gruneisen parameter is relatively insensitive to temperature and suitable for building the connection between the phonon shift and thermal expansion coefficient. Therefore, the linear relationship between the phonon shift and temperature is built and utilized to calculate the thermal expansion coefficient according to the physical implication of the Gruneisen parameter. Conclusions can be obtained as follows. (1) The thermal expansion coefficient of GaN epifilm can be calculated in a certain temperature range by measuring the phonon modes of E₂ (high), A₁ (TO) and E₁ (TO) through using temperature-dependent Raman scattering when the corresponding Gruneisen parameters are determined. (2) The calculated thermal expansion coefficients of GaN epifilm are consistent with the theoretical values.Conclusions and methods in this paper provide an effective quantitative analysis method to characterize the thermal expansion behaviors of other III-nitride epitaxial thin films, such as AlN, InN, AlGaN, InGaN, InAlN etc., which can be of benefit to reducing the dislocation density and improving the luminescence efficiency of light emitting diode. Therefore, research on thermal expansion behaviors of epifilms using temperature-dependent Raman scattering has a direction for further studying the latter-mismatch and thermal-expansion-mismatch between the epitaxial film and substrate.