氮化物半导体耦合量子阱中非线性光整流特性:压电效应与自发极化效应

Properties of Nonlinear Optical Rectification in Nitride Semiconductor Coupling Quantum Wells: Effects of Piezoelectricity and Spontaneous Polarization

考虑了由于压电与自发极化引起的强内电场效应,基于密度矩阵与久期处理方法,理论考察了纤锌矿氮化物半导体耦合量子阱体系的非线性光整流特性。根据已经成功建立的耦合量子阱的内建电场模型,精确求解了体系的电子本征态。以典型的GaN/InxGa1-xN纤锌矿氮化物耦合量子阱为例进行了数值计算,结果发现共振光整流系数达到了10-6m/V的量级(体系的偶极矩阵元大小超过了2nm),这比同样尺寸的单氮化物量子阱的相应值高一个数量级。而且,计算还发现光整流系数对耦合量子阱的结构与掺杂组分呈现非单调的依赖关系,这一特性被归结为体系的强内建电场与量子尺寸效应对载流子受限特性的强烈竞争。计算结果还表明,通过选择小尺寸阱宽与垒宽的耦合量子阱,适当降低掺杂组分,可在氮化物耦合量子阱中获得较强的光整流效应。

Taking the strong built-in electric field into account, the optical-rectification （OR） coefficient in a nitride semiconductor coupling quantum well （CQW） has been theoretically investigated by using the compact density matrix approach in the present paper. In general, in order to obtain strong even-order optical nonlinearities, one should constitute a quantum confined systems with asymmetrical potential profile, such as compositionally asymmetric quantum well （QW）, and applied-field-based QW. It is well known that there exists very strong built-in electric field in GaN-based heterostructure due to the strong spontaneous macroscopic polarization and large piezoelectric coefficients. For the commonly used [0001]-oriented InGaN/GaN strained QW, the magnitude of the buih-in electric field is estimated to be in the order of MV/cm. The built-in electric field with order of MV/cm can naturally break the inversion symmetry of the nitride quantum confined potential profile. Hence a very large even-order optical nonlinear susceptibility can be expected in such systems. Based on the built-in electric field model already constituted in recent reference, the electronic eigenstates in a nitride CQW are exactly solved. Numerical calculation on a typical GaN/InxGa1-xN CQW was performed. The calculated results reveal that the resonant OR coefficient achieves the magnitude of 10-6 m/V, and the dipole matrix elements of the systems also reach more than 2 nm, which is apparently larger than the corresponding values in single wurtzite quantum wells. Moreover, we find that the OR coefficients of the CQW are not monotonic functions of the well width, barrier width and the doped concentration of the CQW systems, but have complicated dependent relations on them. The reason resulting in this characteristic is mainly attributed to the intense com- petition between the strong built-in electric field effect and the quantum size effect for the carrier in the wurtzite CQW systems. Our calculation also shows that a strong OR effect can be realized in the nitride CQW by choosing optimized structural parameters and relatively low doped fraction.