Towards THz quantum cascade lasers
Although ISB luminescence has been observed at near-infrared wavelengths, demonstration of lasing action in III-nitride based heterostructures through either electrical injection or optical pumping remains quite challenging in this spectral range, beside some theoretical propositions to make use of Raman processes [Sun 06]. One reason stems from the ultrafast non-radiative ISB scattering via LO-phonon emission, which is expected to impose very large injection- current or pumping-power density to achieve population inversion. It was pointed out in a few theoretical works that the situation would be much more favorable at long infrared wavelengths corresponding to the THz frequency domain [Jova 04, Sun 05, Bell 08, Bell 09]. In GaAs-based materials currently used for THz QCLs, there is a considerable slow-down of non-radiative scattering mechanisms when the ISB transition energy is not sufficient for allowing emission of optical phonons [Kohl 01].
It was also shown that GaN-based materials paves the way for THz QCLs operating at temperatures much above room temperature [Bell 08, Bell 09]. Indeed, one intrinsic reason limiting the operation of GaAs-based THz QCLs to cryogenic temperatures is the small energy of the LO phonon in GaAs (36 meV, 8.2 THz). As the temperature is increased, electrons in the upper lasing subband acquire sufficient thermal energy for activating fast (<1 ps) non-radiative relaxations via LO-phonon emission towards the lower lasing subband, hence ruining the population inversion. As illustrated in Fig. 12.29, GaN-based QCLs should not suffer
Fig. 12.29. Non-radiative scattering of thermal electrons by LO phonons between lasing subbands of THz QCLs in the case of GaAs contrary to GaN.
from these parasitic non-radiative channels because of the large energy of the LO phonons (^90 meV), almost three times higher than in GaAs. In addition, standard III-V semiconductor devices (such as GaAs-based QCLs) cannot be operated in the 4.6-11 THz frequency range because of their Reststrahlen band, i.e. the spectral region where the material is completely opaque due to absorption by optical phonons. The larger energy of optical phonons in GaN offers prospects for THz quantum cascade devices, which can operate in a much broader spectral range from 1 to 15 THz.
The first attempt to fabricate GaN-based QCLs at a target frequency of
7.5 THz was reported by Terashima and Hirayama using GaN/InAlGaN QWs and then GaN/ Al0.2Ga0.sN QWs grown by RF MBE [Tera 09, Tera 10]. The authors claimed the observation of THz spontaneous emission at low temperatures, but the emitted power was too small to allow measurement of the emission spectrum. By growing a similar GaN/Al0 2Ga0.8N QCL structure on a low- dislocation-density free-standing GaN substrate, a strong improvement of the emitted power was reported, as well as the observation of a quite sharp electroluminescent line at 1.37 THz [Tera 11]. All these designs were based on a resonant phonon scheme where a fast depopulation of the lower lasing state is provided by LO-phonon emission. However, it was shown by Yashida et al. that this resonant phonon scheme may not be the most appropriate design for providing sufficient gain for lasing action, because of the broadening of the subband levels caused by the very strong interaction between electrons and LO phonons in GaN [Yasu 12].
Observation of ISB electroluminescence at THz frequencies has been reported using in-plane transport of electrons at room temperature in GaN/AlGaN step QWs [Juli 11]. The device relied on forty periods of Al0.iGa0.gN/GaN/ Al0.05Ga0.s5N step QWs exhibiting ISB absorption at 2.1 THz [Mach 10]. Two ohmic contacts have been processed on the surface of the device to contact the 2D electron gas in the step QWs. Under application of a bias, some electrons in the ground subband with sufficient in-plane kinetic energy could be scattered into the first excited subband, giving rise to an ISB spontaneous emission peaking at 2 THz, as illustrated in Fig. 12.30. It should be noted that although this mechanism opens prospects for fast modulated THz light sources, it cannot provide population inversion.
Fig. 12.30. Left: Scheme of the electroluminescent sample relying on in-plane transport. Right: TM-polarized electroluminescent spectrum at room temperature under 15 V applied bias (bottom curve) and transmission spectrum at 4 K for TM- and TE-polarized light (top curves).
One requirement for GaN-based QCLs is the control of vertical transport, which has proved to be elusive until recently. One main difficulty stems from the large densities of dislocations and other structural defects that are typically found in III-nitride materials, especially when grown on highly lattice-mismatched substrates. Such defects can act as strong scattering centers as well as leakage current paths. Negative differential resistance (NDR) has been observed at room temperature in GaN/AlGaN double-barrier structures and attributed to resonant tunneling [Kiku 02, Bely 04, Golk 06, Bayr 10]. However, the current-voltage characteristics exhibit hysteresis with two levels of current, and the NDR is not reproducible for positive and negative voltage sweeps as well as for multiple scans, and disappears at low temperatures. These anomalies have been related to the presence of activated filling of trap states in double-barrier resonant tunneling diodes [Sakr 11].
More convincing results have been achieved using GaN/AlGaN multiple QWs. In particular, sequential resonant tunneling has been reported in superlattices grown on free-standing GaN substrate containing GaN/AlGaN-coupled QW structures, and THz emission has been observed, though it could not be unambiguously ascribed to ISB emission [Sudr 10]. In a recent work, vertical electron transport has been investigated in a seven-period GaN/AlN MQW structure, leading to the observation at room temperature of five reproducible NDR features assigned to the successive resonant electron tunneling between the ground (ei) and the first excited (e2) states of adjacent QWs in a quantum cascade-like configuration [Sakr 12c].