Intersubband transitions in low-dimensional nitrides
Maria Tchernycheva and Francois H. Julien
Intersubband transitions (ISB) are resonant optical transitions between confined states of semiconductor heterostructures either in the conduction band or in the valence band. They were first observed in the 1970s at far-infrared wavelengths in Si accumulation layers [Kamg 74]. In 1985, West and Eglash reported the observation of 10-pm wavelength ISB absorption in GaAs/Alo.3Gao.7As quantum wells (QW) [West 85]. This was made possible by the progress of epitaxial growth techniques allowing atomic-layer control of layer thicknesses. In the pioneering work of West and Eglash, it was shown that ISB transitions are characterized by a large optical dipole moment, whose length scales like the QW thickness, and that the resonant wavelength can be tailored in a wide spectral range through a proper choice of the layer thickness. During the ensuing years, extensive investigations led to the demonstration of ISB transitions in various material systems, such as InGaAs/AlInAs, InAs/AlSb, or SiGe/Si in the form of QWs [Paie 06], but also quantum dots (QD) [Sauv 97, Webe 99]. The interest was motivated by fundamental aspects but even more by the development of a new class of control-by-design optoelectronic devices such as, for example, focal-plane arrays of quantum-well infrared photodetectors (QWIP) [Levi 87] or quantum cascade lasers (QCL) [Fais 94].
Using conventional III-V semiconductors, the operation wavelength of ISB devices such as QWIPs or QCLs can be tuned from the mid-infrared to the THz spectral range. Operation at short wavelengths is limited by the material transparency and by the available conduction-band offset (^0.35 eV for direct gap GaAs/AlGaAs and ^0.52 eV for lattice-matched InGaAs/InAlAs). To access the near-infrared spectral domain and in particular the 1.3—1.55-pm wavelength range used for fiber-optics telecommunications, materials with a band offset above 1.5 eV are required. There are only few semiconductor systems capable of producing ISB transitions at telecommunication wavelengths: namely, InGaAs/AlAs(Sb) [Neog 99, Hira 94], ZnSe/BeTe [Akim 01, Akim 02], CdS/ZnSe/BeTe [Li 06a, Cong 07], GaInAsN/AlAs [Ma 07] and GaN/Al(Ga)N [Suzu 99, Gmac 00, Kish 02b, Helm 03b]. Among these materials, nitride semiconductors have shown the highest potential for device applications. This is due to their remarkable properties, such as a high conduction-band offset (m.75 eV for AlN/GaN) [Tche 06b] and their remote lateral valleys, as well as an ultra-short ISB absorption recovery time of the order of 140-400 fs [Iizu 00, Hebe 02, Hama 04, Hama 05, Wang 05, Wang 06], which is ten times faster than in InGaAs QWs [Iizu 00, Hira 94]. However, in order to reach short infrared wavelengths, ultrathin layers are required because of the rather large electron effective mass in nitrides. This explains why the development of nitride ISB devices has been crucially dependent on the progress in the epitaxial growth techniques of these materials with atomic-layer thickness control; namely, molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD). Finally, in addition to short-infrared-wavelength ISB devices, III- nitrides are finding appealing applications in the THz frequency domain because of their large optical phonon energy.
This chapter presents an overview of the physics and device applications of ISB transitions in III-nitrides. It is organized along the following lines. Section 12.2 addresses the theoretical aspects of nitride ISB transitions. Section
12.3 reviews the spectroscopic investigations of ISB transitions in QWs and QDs as well as polar, semipolar, and cubic nitrides. Sections 12.4-12.6 recast the present state-of-the-art of nitride ISB devices for optical modulation, detection, and light-emission.