A New Method To Detect Ballistic Electrons In GaAs

Recently there has been great interest in structures designed to study the transport properties of electrons in n-GaAs. This follows years of theoretical and experimental effort aimed at developing ultra-high speed devices that utilize ballistic (collisionless) motion of electrons over short distances. While there has been little doubt that ballistic or at least quasi-ballistic motion occurs, the question has often arisen whether or not a sufficient fraction of the electron population remains ballistic over the length scale and at the carrier concentrations found in useful devices. Perhaps the most notable experiment to address this issue utilized a structure in which electrons are injected through a thin heterobarrier by tunneling and are collected by the high-pass (in energy) filtering action of a second, thicker heteroban-ier.' This experiment determined that about 50% of the electrons move quasi-ballistically through a 30-nm base region doped 1x10l8 ~rn-~. A similar structure using planar-doped barriers led to the conclusion that the ballistic mean free paths in n+ GaAs are a few hundred angstroms? in basic agreement with the former experiment. More recently the collector has been replaced by a double-barrier, resonanttunneling structure that passes only electrons in a narrow energy range, and spectrometric measurements were made on p-type mate~ial.~ In this paper we present a device in which both the analyzer and collector are double-barrier (DB) structures through which the electrons can tunnel with high probability. Fully ballistic electrons can tunnel through the entire structure and are thus detectable by measuring resonances in the I-V and its derivative curve. As we will see below, dramatic resonant-tunneling effects have been observed even at room temperature. The present structure, shown schematically in Fig. l(a), consists of a pair of GaAs/AlAs DB structures separated by a uniform interaction region of GaAs. Three samples were grown for the present study, with interaction region lengths of 25, 50 and 100 nm. In each sample, the interaction region was doped n-type 2x1017 cm-3 and the AlAs barrier and well regions were nominally undoped. The structures were grown by molecular beam epitaxy at 560 "Cy and individual mesa devices were fabricated by a standard sequence of steps. This included patterning of Ni/Ge/Au layers into dots on the epitaxial side of the wafer, followed by alloying of these layers with the GaAs to make Ohmic contacts. The individual mesas were defined by ion beam etching using the patterned metal as a mask. All of the I-V and dI/dV measurements were made on 8-pm-diameter mesas with a commercial probe station that enabled temperature reduction to about 85 K.