Studies of FAR infrared emission in low-dimensional semiconductor structures for the development of new THz sources

Projektinformationen

ID Finanzhilfevereinbarung: INTAS-94-0842

Projekt abgeschlossen

Startdatum 1 Februar 1996

Enddatum 31 Januar 1998

Finanziert unter

• International Association for the promotion of cooperation with scientists from the independent states of the former Soviet Union (INTAS), 1993-

Gesamtkosten € 0,00

EU-Beitrag € 48 050,00

Koordiniert durch

Technische Universität Wien
Austria

Exploitable results

Studies of FAR infrared emission in low dimensional semiconductor structures for the development of new THz sources

1) Streaming motion in p-type Quantum Wells: For the first time streaming motion in p-type quantum wells has been observed. The current-voltage characteristics of 68, 200 and 400 Angström wide modulation doped GaAs/AlGaAs multiple quantum wells (MQW) grown by Y. Ivanov's group show a clear saturation of the current at strong fields which is connected with a strong interaction between holes and LO-phonons (streaming state). Additionally, far infrared (FIR) emission for all the MQW samples is discovered. A. Andronov's group also realised streaming motion of hot carriers in QWs. Ge/Ge1-xSix strained multiple quantum wells were grown with chemical vapour deposition (CVD) on Ge(111) substrates and subsequently investigated with cyclotron resonance (CR) and photoconductive measurements. Just as with the GaAs/AlGaAs MQWs FIR emission could also be detected. Saturation of the FIR emission intensity at electric fields larger than 250 V/cm confirms the streaming motion of the hot holes. Investigations of the emission intensity dependencies on electric and magnetic fields as well as spectral studies of the emitted radiation showed that the radiation results from different intraband transitions of hot holes. Ivanov's group also showed that by applying an additional magnetic field the intensity of the FIR emission increases and negative magnetoresistance can be observed. 2) New mechanism for stimulated THz emission discovered (real space transfer) A. Andronov's current-voltage experiments performed at delta-doped strained InGaAs/GaAs MQWs (grown by MOCVD) showed not only saturation at higher fields (as in the experiments described in section 1.) but also current oscillations. FIR emission experiments exhibited nonmonotonous behaviour with applied electric field. These features can be explained by an effect called real space transfer (RST) which is the spatial redistribution of heated carriers from the quantum wells into the barriers, i.e. in high electric fields the holes in the QW gather enough energy to leave the QW towards the barriers. Inside the barriers the mobility of the carriers drops dramatically due to strong ionised impurity scattering and the heavier mass. Therefore RST is similar to intervalley transfer in the Gunn effect and results in a negative differential conductivity which is apparently responsible for the observed current instabilities. Population inversion seems to be a natural consequence of the RST. The inversion talked about is the inversion between the lowest barrier and quantum well states, i.e. higher occupation numbers on the bottoms of the barriers than that of the wells. The key to the realisation of population inversion is that the carriers in the barrier are less heated by the lateral electric field than the carriers in the QW because of their larger mass and reduced mobility. First experiments to investigate population inversion and FIR stimulated emission between quasi continuum and bound states in shallow quantum wells have also been performed. Finally, the RST population inversion scheme is universal and not strictly connected to the InGaAs/GaAs MQW system.