Efficient Terahertz Emission from InGaN/GaN Heterostructure

Abstract

Pulsed terahertz (THz) radiation from semiconductors excited by femtosecond laser has been studied extensively in the recent years. The two main methods [1] are used to generate the broadband THz radiation. The methods are based on THz emission from the biased photoconductive switches and on THz emission from the surface of freestanding semiconductors. Multiple mechanisms are responsible for the THz emission from the freestanding semiconductor surface including nonlinear optical rectification, photocurrent induced by the surface-field, photocurrent induced by the built-in field in p–i–n structures, and by photocurrent induced photo-Dember effect. However, the intensity of THz emission from the surface of freestanding semiconductors still is very low. The analysis [2] of the transient dynamics of photoexcited carriers shows that the surface and contactless p–i–n THz emitters have serious deficiencies that significantly reduce the intensity of THz radiation. First, only a minor part of the excited carriers contributes to the transient current if the inverse absorption coefficient exceeds the surface depletion width or the i-layer thickness of the p–i–n structure. In this case, the major part of carriers is created in the region where the built-in electric field is screened by the extrinsic carriers. Second, the plasma frequency of photocarriers is position-dependent due to the exponential decay of the created carrier density. As a result, the transient response of the created carriers is incoherent in different regions of the structure because the frequency of current oscillations is position-dependent. Therefore, the amplitude of oscillations of the resulting current is significantly reduced by the interference effects. To overcome such deficiencies of THz emitters, the novel δ-doped GaAs/AlGaAs heterostructure is suggested in [2]. It is shown in [2] that THz energy radiated from the freestanding semiconductors is significantly enhanced when the above noted deficiencies are eliminated. It is obtained from Monte Carlo simulations that the efficiency of THz emission from the δ-doped GaAs/AlGaAs heterostructure exceeds the efficiency of THz emission from the homogeneous GaAs by two orders of magnitude. In the present work, the basic ideas suggested in [2] are used to design THz emitters based on InGaN/GaN heterostructures. The pulsed THz emission from the InGaN/GaN heterostructures is studied using Monte Carlo simulations [2, 3] of the transient dynamics of photoexcited electron–hole plasma. The InGaN/GaN heterostructures appear to be very suitable for the development of highly efficient THz emitters. High electron mobility in the optically active InGaN layers provides a large amplitude of the transient current. The offsets of the conduction and valence bands at the InGaN/GaN heterointerfaces are sufficiently high to ensure the perfect confinement of the excited carriers inside the InGaN layers. In addition to the basic ideas of [2], the polarization charges at the InGaN/GaN interfaces are employed in the design of the appropriate potential profile in the InGaN/ GaN heterostructures for the efficient THz emission.