Info

Figure 70. Upper panel showing the calculated temperature dependencies of the free hole concentration pv(T) (solid curve) and the number concentration of charged acceptors in the QW (dotted curve). The dashed curve is the energy separation Ef^ — l The lower panel displays the measured current as function of the temperature. Upper panel showing the calculated temperature dependencies of the free hole concentration pv(T) (solid curve) and the number concentration of charged acceptors in the QW (dotted curve). The dashed curve is the energy separation Ef^ — l The lower panel displays the measured current as function of the temperature. Reprinted with permission from [189], A. Blom et al., Appl. Phys. Lett. 79, 713 (2001). © 2001, American Institute of Physics.

dependence of pv(T) was due to the thermal excitation of holes from the chemical potential into the 2D subbands. This was governed by the energy separation — ¡i, which depends strongly on the temperature, as shown in the upper panel of Figure 70. These theoretical findings and the following experimental data will support their conjectured mechanism of THz lasing in the SiGe/Si QW.

A Si/Ge015Si0.85/Si QW sample with the previously specified structure was fabricated with a MBE machine. High voltage pulses of 0.3 ¡is duration were applied parallel to the QW via the deposited ohmic contacts. Since the 8 layer of boron in the buffer layer also supplies holes to the substrate, a pn junction type of carrier distribution is formed, which prevents the bypass of current through the substrate. When opposite surfaces parallel to the growth direction were polished to serve as an optical resonator, strong THz radiation was detected at liquid helium temperature. The radiation intensity is orders of magnitude stronger than the intensity of spontaneous emission. In Figure 71 the radiation intensity and the corresponding electric pumping current as functions of the pulse voltage are plotted. The abrupt increase of the current around 100 V marks the onset of impact ionization of the boron acceptors in the QW. A threshold current for the radiation is clearly seen. The characteristic features of the observed THz radiation are exactly the same as those of THz lasing reported recently [203]. This was expected, because the only difference between their sample and the sample in [198, 203]

0 0

Post a comment