Terahertz Sources Based on Emission from a GaAs /( Al , Ga ) As Heterostructure at Cryogenic Temperatures

A high-electron-mobility GaAs/Al0.36Ga0.64As heterostructure is processed by electron-beam lithography to fabricate samples with a separation of the current-supplying Ohmic contacts of between 28 mu m and 1.3 mm. Voltage pulses of 20-ms duration, with a 4% filling factor and an amplitude up to 30 V, stimulate terahertz emission from samples cooled to 4 K. The radiation spectrum, centered at about 390 GHz (about 1.6 meV), registered with a Fabry-Perot interferometer, shows almost no dependence on the applied voltage or on the shape of the sample. The emission appears in a threshold manner and is accompanied by a jump of current by more than 2 orders of magnitude. The energy of the emitted photons is about three times smaller than in the case of any previously reported impurity-related emission from either bulk GaAs or GaAs-based quantum wells. The emission is interpreted as resulting from a two-step ionization-recombination process involving shallow donors in the (Al,Ga)As barrier, which create bound states of electrons in the GaAs quantum well. Thus, the frequency of emission is argued to be tuned by the width of the spacer layer. We present these as small convenient sources that can be used for calibration and check-up operation of cooled bolometric systems used in astronomical observations.