Room temperature negative differential resistance in terahertz quantum cascade laser structures

Abstract

The mechanisms that limit the temperature performance of GaAs/Al0.15GaAs-based terahertz quantum cascade lasers (THz-QCLs) have been identified as thermally activated LO-phonon scattering and leakage of charge carriers into the continuum. Consequently, the combination of highly diagonal optical transition and higher barriers should significantly reduce the adverse effects of both mechanisms and lead to improved temperature performance. Here, we study the temperature performance of highly diagonal THz-QCLs with high barriers. Our analysis uncovers an additional leakage channel which is the thermal excitation of carriers into bounded higher energy levels, rather than the escape into the continuum. Based on this understanding, we have designed a structure with an increased intersubband spacing between the upper lasing level and excited states in a highly diagonal THz-QCL, which exhibits negative differential resistance even at room temperature. This result is a strong evidence for the effective suppression of the aforementioned leakage channel.

Extraction-controlled terahertz frequency quantum cascade lasers with a diagonal LO-phonon extraction and injection stage

“…This indicates that the current keeps flowing after the alignment of the subbands 1−3 is broken. Considering the applied voltages are quite high, it is possible the carriers could leak into the higher states or continuum states [25,26], which could be another reason for the high current and the low operating temperatures. The effects of the oscillator strength on the maximum operating temperature are shown in the inset of Fig.…”

“…This indicates that continuum states or higher energy states may assist carrier transport from bound states. Recently, thermally activated leakage into the continuum was found to be an important limiting factor for the temperature performance [25,26]. This mechanism may contribute to the rapid rapid degradation of the performance of our designs with increasing temperature.…”

Температурная Зависимость Порогового Тока И Выходной Мощности Квантово-Каскадного Лазера С Частотой Генерации 3.3 ТГц

“…Данный механизм депопуляции " нижнего" лазерного уровня E 3 позволял частично компенсировать депопуляцию " верхних" лазерных уровней E 5 и E 6 за счет температурной активации испускания LO-фононов. Учет утечки электронов в континуум позволил существенно улучшить корреляцию расчетных и экспериментальных зависимостей P out (T ) в работах [18,29].…”

Split-well direct-phonon terahertz quantum cascade lasers

“…3,4 Besides the LO-phonon scattering channel, THz-QCLs also suffer from thermally activated leakage of charge carriers into the continuum 5 or excited bound states. 6,7 Additional potential limiting mechanisms for the performance of THz-QCLs are thermal backfilling 8 and line broadening. 9 In both resonant-phonon (RP) 10,11 and direct-phonon two-well (TW) schemes, 12,13 the phonon well is the widest in the structure.…”

“…As such, a higher level in this well is not far above the upper laser level (ULL), so it provides a leakage channel at high temperatures. In order to suppress this leakage channel, we used a narrower phonon well, 6,14 which resulted in an energy separation of the lower laser level (LLL) and the ground state larger than the LO-phonon energy. This deviation from the resonance slows down the depopulation of the LLL.…”