Amir A. Lakhani

Three‐dimensional integration of resonant tunneling structures for signal processing and three‐state logic

We have developed structures with two well‐defined negative differential resistance (NDR) regions by sequentially growing two resonant tunneling devices separated by an n+ connecting layer. Devices fabricated from these structures exhibited three stable operating points for multilevel logic circuits and were used in circuits which multiplied the input signal frequency by 3 or 5. This approach can be extended to obtain more than two NDR regions by vertical integration of additional resonant tunneling structures.

Combining resonant tunneling diodes for signal processing and multilevel logic

A device with multiple negative differential resistances was obtained by combining two AlInAs/InGaAs based resonant tunneling diodes in series. Equal peak currents and large current peak‐to‐valley ratios were demonstrated at room temperature. Three stable operating points were identified for trilevel logic applications and a multiply‐by‐three circuit was demonstrated.

Observation of electron quantum interference effects due to virtual states in a double‐barrier heterostructure at room temperature

Strong electron quantum interference effects have been observed at room temperature in the current‐voltage characteristics of a double‐barrier, wide‐well, lattice‐matched In0.52Al0.48As /In0.53Ga0.47As heterostructure tunneling device. A total of 22 oscillations was observed in the differential conductance. A model is proposed which attributes ten of the oscillations to the usual resonant tunneling via the subbands and the rest to the presence of virtual states that exist in the well and second barrier regions.

Three and six logic states by the vertical integration of InAlAs/InGaAs resonant tunneling structures

Double‐barrier/single‐well resonant tunneling structures based on InAlAs/InGaAs were vertically integrated on an InP substrate to obtain devices with multiple negative‐differential resistance (NDR) regions. These devices, with either two or five tunneling structures, exhibited uniform current peaks and valleys and also had NDR regions about equally spaced in bias voltage. The devices were used in simple circuits to demonstrate three or six stable logic levels which could be set with current pulses.

A vertically integrated resonant tunneling device with multiple negative differential resistances

It was recently shown that connecting two resonant tunneling diodes (RTDs) in parallel so that the bias across them was offset resulted in a current-voltage (I-V) characteristic with two separate negative differential resistance (NDR) regions. More recently, two discrete InAlAs/InGaAs RTDs were combined in tandem to obtain near-ideal room-temperature I-V characteristics that had two well-defined NDR regions. Using molecular-beam epitaxy, the authors have extended this idea by vertically integrating five InAlAs/InGaAs double-barrier RTDs in sequence to obtain a vertically integrated diode (VID) that had five NDR regions. The five tunneling structures were separated from each other by 500-AA n/sup +/ InGaAs layers that which destroyed electron coherence between the tunneling regions, so that each resonant-tunneling structure switched sequentially with increasing bias. The VID has been used to demonstrate a multilevel memory element that has five distinct voltage states that can be set by using small current pulses. The VID was also used in a circuit to generate the parity of an 11-bit word. >

Application of a resonant tunnelling structure to demonstrate subsurface damage and surface migration on InGaAs during AuGe contact anneal

An InAlAs/InGaAs resonant tunnelling structure was used to investigate the damage induced during thermal treatments of Au:Ge-based Ohmic contacts. The long-range coherence required for the observation of negative differential resistance was affected by all anneals at temperatures greater than 200 degrees C for times up to 120 s. The decrease in the sheet resistivity was correlated with the degradation of the tunnelling diode characteristics. Lateral spreading of the Ohmic metal was observed along the (110) and (100) directions for anneals at temperatures above 370 degrees C.

Magnetotransport studies of charge accumulation in an AlInAs/GaInAs tunneling structure

We report a study of the current‐voltage characteristics of a double barrier, lattice matched, quantum well tunneling structure in a quantizing magnetic field (B∥J). Experiments were conducted at fields up to 23 T at 1.5 K. The heterostructure investigated had 400 A spacer layers in the emitter and collector, a barrier width of 72 A, and a 43‐A‐wide quantum well. This structure showed one negative differential resistance region with a peak‐to‐valley ratio of 23 at 4.2 K. We observed magnetoquantum oscillations, periodic in 1/B, associated with tunneling from a quantized state in the emitter. The overall magnetoconductance dramatically changed with applied bias. We associated these variations with a field‐induced increase of the impedance of the undoped spacer layers. The frequency of these oscillations increased linearly with applied bias. A discontinuity in this dependence is observed around the peak bias voltage which is the direct result of the dynamical storage and release of charge in the well.

Enhancement of current peak‐to‐valley ratio in In0.52Al0.48As/In0.53Ga0.47As ‐based resonant tunneling diodes

The electrical characteristics of In0.52Al0.48As/In0.53 Ga 0.47 As resonant tunneling diodes are reported. These devices exhibit current peak‐to‐valley ratios up to 6.7 at room temperature. Differences in the current‐voltage characteristics for devices made from epilayers grown simultaneously on n+ and semi‐insulating InP substrates demonstrate the importance of minimizing the parasitic series resistance. The enhanced peak‐to‐valley ratio has been attributed to thick In0.52Al0.48 As barriers (50 A) and wide, undoped In0.53Ga0.47 As spacer‐layers (400 A).

Magneto electric states in wide-well double barrier tunneling structures

Abstract We report a study of transport in crossed electric and magnetic fields in a double harrier tunneling structure with a wide (600 A) well. Experiments were conducted on a lattice-matched GaInAs/AlInAs structure at 1.5 K in magnetic fields up to 23 T. At zero tesla we observe 21 resonances, 11 of which correspond to extended state resonances for biases > 0.85 V. Upon the application of a transverse magnetic field, the bound resonances evolve into magneto-electric states and arc shifted to higher biases. In the low bias range, for high magnetic fields, additional resonances from barrier-bound states are observed. Simple semi-classical considerations of electronic motion in crossed fields allow us to determine different regimes (i.e. traversing, skipping, bulk-like) in the magnetic field-bias voltage space of the resonances.