Absorption Modulation in Field Effect Quantum Well Structures

We present investigations of modulation of the absorption in InGaAs/In& modulation doped single quantum well structures used as conducting channel in field effect transistors. The electron concentration can be tuned continuously from N-0 to N=: 6.5~10" cm " using the gate voltage. The effects give in situ information on the 2D-electron gas. Furthermore the total quenching of the absorption seen at the n, = 1 edge has potential applications to optical interconnects for 111-V electronics and to lightwave optoelectronic devices. Large changes in absorption in quantum wells (QW) have previously been successfully induced in semiconductor QW by photocarrier absorption bleaching and by quantum confined Stark effect (QCSE)~. In the first case the saturation of the excitonic absorption is accompanied by a large band gap renormalization so that a significant band to band absorption edge replaces the exciton resonances when these have completely disappeared l . In the case of the QCSE, the reduction in the wavefunction overlap reduces the height of the exciton peak as it shifts 2. Thus in both cases, it is difficult to extinguish the excitonic absorption completely. In this talk we present a new mechanism for modulation of the absorption in QW structures which appears to produce a complete quenching of the excitonic and above-band-gap absorption over a wide spectral range ( > S(kneV) . This effect is observed in modulation-doped (MD) field effect QW-structures by driving electrically the electrons (or holes) in and out of MD-QWs. When carriers are introduced in a MD-QW they fill the 2D-subbands up to the Fermi energy EF , hence blocking the optical transitions to the states below this energy. This is at the origin of the blue shift of the onset of the absorption with respect to the luminescence seen in MD-QW =. It was not realized until recently that it is very easy to control electrically the quenching of the absorption by the carriers in MD-QW using field effect gating, in fact such a control is commonly used to switch 2D-electron gas field effect transistors (MOD-FET) *. In addition to the blocking of transitions brought about at the lowest optical gap, the introduction of carriers in MD-QWs also produce band gap renormalization (BGR) "' and is accompanied by elect1 xtatic fields in the QW . Both of these processes affect the electronic levels and produce other changes in the optical absorption spectrum. Therefore field effect modulation of absorption in MD-QW structures has potential application to optical readout of MOD-FET and direct lightwave modulation and it can also be used to study in situ two dimensional electron gases in these heterostructures '. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987584 C5-394 JOURNAL DE PHYSIQUE The structure we have investigated are recessed gate InGaAs/InAlAs, MOD-FETs. T ~ P , epilayer structures is shown in Fig. (la), it consists of a contact 2 0 0 A n + ~ n ~ ~ ~ ~ ~ a ~ . ~ ~ ~ s cap layer on three In0.52A10.48A~ layers, 140 A , 250 A and 20 A thick respectively, the central one being doped with Si = 1.2x10'~cm they cover a 110A 1n o .53~ao .47~s QW it self on a last 3000A 1n o . 5 2 ~ ~ o . 4 ~ ~ undoped layer. The whole structure was grown by molecular beam epitaxy on an InP substrate. This material system was chosen because the InP substrate and the InAlAs layers are transparent around the gap of the InGaAs-QW, which itself is in the region of maximum transmission of optical fibers. Furthermore it has been demonstrated that InGaAs/InAUs/InP heterostructures can be used to fabricate high quality electronic devices lo.