Epitaxially overgrown, stable W–GaAs Schottky contacts with sizes down to 50 nm

Abstract

A processing scheme for the fabrication of embedded W–GaAs contacts has been established and the resulting contact characteristics have been evaluated. The main advantage of these contacts is that they are stable during high-temperature epitaxial overgrowth. The fabrication scheme is based on a liftoff process with electron beam evaporation of tungsten and subsequent epitaxial overgrowth using metalorganic vapor phase epitaxy. Various methods were used to characterize the buried contacts. First, the structural properties of GaAs surrounding embedded W features, with widths down to 50 nm, were characterized by high-resolution transmission electron microscopy. Measurements of the conductivity in individual, buried wires were performed in order to study the influence of the overgrowth process on the properties of the tungsten. We also evaluated the current–voltage characteristics for macroscopic contacts, which revealed a clear dependence on processing parameters. Optimized processing conditions could thus be established under which limited contact degradation occurred during overgrowth. Finally, we used the overgrowth technique to perform a detailed investigation of the electrical and optical properties of floating-potential embedded nano-Schottky contacts by space-charge spectroscopy.

InAs epitaxial lateral overgrowth of W masks

“…The patterning of the wafers was done by electron beam lithography and electron beam evaporation of 20-nm-thick W in a conventional lift-off process [4]. Prior to the overgrowth, oxygen plasma stripping and a short (60 s) etch in HCl:H 2 O (1:1), followed by rinsing in deionized water, were performed to remove traces of the photoresist and the native oxide.…”

Highly functional tunnelling devices integrated in 3D

“…Because the embedded gate consists of thin buried metal wires, the resistance may be high and limit the device performance. From electrical measurements of embedded individual wires, we deduced an experimental value of the metal resistivity to be about 100 cm [13]. In our device model (Figure 3), we used a structure consisting of 25 parallel connected wires 5 m long.…”

“…The growth is interrupted and the sample is patterned with 20-nm-thick W wires placed in a grating with 140 nm wide wires in a period of 350 nm. This covers a total area of 70 m × 100 m. To build the gate, a conventional lift-o process based on electron beam lithography and electron beam evaporation of the W [13] is used. The grating is oriented 60…”

Electrical characterization of thin InAs films grown on patterned W∕GaAs substrates

“…3 The patterns formed consist of lines ͑width of 90-100 nm and periods of 200-700 nm͒ and concentric rings ͑width of 100 nm and periods of 400 nm͒. Metal organic vapor phase epitaxy ͑MOVPE͒ was used to epitaxially overgrow the patterned substrates with InAs at a growth temperature of 500°C and a V/III ratio of 14.…”

“…The mesa geometry was used to deduce the conductivity of the devices from the measured data. Based on reported values for the resistivity for overgrown tungsten lines ͑120 ⍀ cm͒, 3 it was calculated that the contribution from the metal accounted only for about 15% ͑it varied with pattern density from 11% to 18%͒ of the total measured conductance in our devices. It may hence not be responsible for the observed reduction in the resistance as the metal density increased.…”