Physics, growth, and performance of (In,Ga)As–AlP/InP quantum‐cascade lasers emitting at λ < 4 μm

Abstract

PACS 42.55. Px, 73.21.Fg, 81.15.Hi The design strategy and performance for short wavelength (l < ≈ 4 µm) quantum-cascade lasers (QCLs) is discussed. The QCLs are based on strain-compensated AlAs -In 0 73. Ga 0 27 . As heterostructures grown using gas-source molecular-beam epitaxy on InP substrates. Both composite barriers based on AlAs -In 0 55. Al 0 45 . As and composite wells based on In 0 73 . Ga 0 27 . As -In 0 55 . Al 0 45 . As are used to achieve laser emission at wavelengths as short as 3.05 mm by avoiding leakage from the upper laser state into either the higher-lying miniband or into indirect states within the heterostructure. IntroductionThe ability to create artificial materials through the use of epitaxially grown semiconductor heterostructures has been one of the driving mechanisms of modern electronic and optical devices. Today structures derived from such materials engineering are routinely implemented into high-speed transistors, into photodetectors, and into most semiconductor laser diodes. The requirements for creating the structures for such devices are excellent materials quality, a high level of control over the material composition and doping, and the ability to change the material and/or doping over an extremely short distance, on the order of one atomic monolayer.Recently a new class of semiconductor injection laser has been introduced, the quantum cascade laser (QCL) [1,2], that represents one of the most impressive examples of band structure engineering through advanced epitaxial growth for a semiconductor device. The QCL is a unipolar laser, whose operation is based exclusively on the conduction band with no involvement of holes. Thus, the emission wavelength is determined by the conduction band structure of the heterostructure and not on the band gap between conduction and valence bands.The basic idea of the QCL is that an active region of one or several quantum wells is designed so that the optical transition between two subbands has both the intended energy for the laser emission as well as an adequate oscillator strength. Electrons are injected into the upper state from an injector miniband, they cause a photon to be emitted as they relax to the lower state, and then they vacate the lower state into an extraction miniband. By cascading a number of such active regions, the extraction miniband of one active region becomes the injection miniband of the next active region. Thus, each electron can emit An important direction in QCLs for a number of applications is emission within the first atmospheric window (2.9-5.3 µm). Realization of QCLs within this region is especially challenging because of the large conduction band discontinuity ( c E D ) needed for the large intersubband energy differences required. The three leading contenders for short wavelength QCLs, all with very large conduction band discontinuities, are InAs-AlSb on InAs [3,4], InGaAs-AlAsSb on InP [5,6], and InGaAs-AlAs on InP [7]. Emission wavelengths near 3 µm have been demonstrated in all three systems [4,6,8]. The...

Activation energy study of electron transport in high performance short wavelengths quantum cascade lasers

“…Emission wavelengths down to 3 µm have been reported using strained InGaAs/InAlAs and limiting factors like intervalley scattering and band-offsets for very short wavelengths were investigated. [6,7,8] With increasing strain the growth of the layers becomes more demanding and large band-offsets increase interface roughness scattering. [9] At the same time, the band-offset has to be large enough to suppress leakage of carriers into the continuum.…”

“…For high transition energies scattering into indirect states (X-and L-valleys) has also to be considered. These scattering mechanisms in short wavelength QCLs were already investigated [7,8] but it is very difficult to quantify these effects as the exact parameters are, like energetic position of these states, etc. are not exactly known.…”

High-performance continuous-wave room temperature 4.0-μm quantum cascade lasers with single-facet optical emission exceeding 2 W

“…In addition, as discussed in ref. 7, it is the indium percentage in InGaAs quantum wells, approximately the same for AlInAs/InGaAs and AlAsSb/InGaAs-based QCL structures at ∼4.0 μm that defines bottom of the indirect band profile and therefore carrier leakage through indirect states. Thus, for wavelengths close to 4.0 μm, no significant advantage is expected in using less mature AlAsSb/ InGaAs active regions.…”