We show that the bistability effect observed in the current-voltage characteristics of a doublebarrier resonant-tunneling device can be removed by connecting a suitable capacitor across the device. These measurements cast serious doubt on the recent interpretation of the bistability as an intrinsic space-charge effect.Recently, Goldman et al. ' reported the observation of bistability in the current-voltage characteristics I(V) of double-barrier resonant-tunneling semiconductor heterostructures. This bistability (hysteresis) occurs in the region of negative differential conductivity (NDC). They interpreted it as an intrinsic feature arising from the buildup of negative space charge in the quantum well between the two barriers.

Magnetoquantum oscillations in the tunnel current of double-barrier n-GaAs/(AlGa)As/GaAs/(AlGa)As/GaAs resonant tunneling devices reveal evidence of sequential tunneling in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the GaAs quantum well. The sequential tunneling arises from intersubband scattering between two quasi-bound states of the well. Near this resonance, the charge buildup in the well can be estimated from the magnetoquantum oscillations.

The results of optical absorption, photoluminescence, electrical conductivity, and Hall-effect measurements on LEC-grown Gap: Ti are reported. The positiom of the Ti$/T& acceptor level at 4, -0.50 eV and the Ti&/T& donor level at E, + 1.0 eV are established and the photoionization transitions from and to these deep levels are identified in the absorption spectra and from recharging processes. The internal transitions 3A, -3T1(F) and + 8T1(P) of T i 3 are found in absorption and the e lT, transitions of Ti$ are seen in the absorption as well as luminescence spectra. The interpretation of these bands and sharp lines requires to take into account different kinds of electron-phonon coupling. Photoinduced recharging experiments at low temperatures on high-resistance GaP:Ti and GaP:Ti: Zn are consistently explained including the three charge states of Ti&, of the P G~P~ antisite defect, and of the residual F~Q~ impurity.Die Ergebnisse von Messungen der optischen Absorption, Photolumineszenz, elektrischen Leitfiihigkeit und des Hall-Effekts an LEC-geziichtetem GaP:Ti werden mitgeteilt. Die Lagen des Ti$/Tig Akzeptorniveaus bei Ec -0.50 eV und des T a / T i % Donatorniveaus bei E , + 1.0 eV werden gesichert und die Photoionisationsiibergange von und zu diesen tiefen Niveaus werden in den Absorptionsspektren und aus Umladungsprozessen identifiziert. Die inneren ubergiinge 3A, + sT1(F) und 4 3T1(P) des Ti ; : werden in Absorption gefunden und die *E cf lT, Ubergiinge

Zeeman spectroscopy measurements on the sharp line structure observed in the absorption and photoluminescence spectrum of GaP:Ti around 0.61 eV are interpreted a s 2T2 w 2 E transitions of Ti3' ( d l ) . A related system of lines at 0.57 eV in GaAs:Ti is similarly interpreted.A recent paper (Ulrici el a1 1986) described the optical GaP:Ti:Zn sample (upper spectrum) are shown.In the and electrical properties of Ti in GaAs. Broad optical former spectrum the sharp peaks are superimposed upon absorption bands around 0.66 and 1.01 eV were ascribed what appears to be a broad bell-shaped absorption centred to internal transitions of the TiZ+ centre. In addition, very at around 0.606 eV. This is thought to be due to internal sharp line structure at 0.566 and 0.569 eV was observed in transitions of TiZ+. In the high-resistivity GaP:Ti:Zn absorption, with the line at 0.566 eV also observed in sample the Fermi level will presumably be lower down in photoluminescence (PL). These lines. which have also been the band gap away from the TiZ+/Ti3+ acceptor level so reported in GaAs:Ti by Hennel el a1 (1986). are only there will be no Ti present in the TiZ+ charge state. The observed in Ti-doped material and, indeed, the latter lines at 0.6041 and 0.6046 eV are also observed in PL. as authors ascribe them to internal transitions of Ti3+. In shown in inset (a) of the figure. Of these, the line at order to elucidate the electronic nature of the levels 0.6041 eV is the strongest in both PL and absorption. involved in the sharp line transitions, both Martini el U /

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