Corresponding author: Mikhail V. Stepushkin ( cokpoweheu@yandex.ru ) © 2018 Mikhail V. Stepushkin, Vladimir G. Kostishin, Vladimir E. Sizov, Alexei G. Temiryazev.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Stepushkin MV, Kostishin VG, Sizov VE, Temiryazev AG (2018) Use of atomic force microscope for the synthesis of GaAs/AlGaAs heterostructure base one-dimensional structure. Modern Electronic Materials 4(4): 163-166. https://doi.org/10.3897/j.moem.4.4.47093
|
Electron transport in low-dimensional structures is often studied using semiconductor heterostructures with two-dimensional electron gas in which insulating regions separating the conducting channel from the gates are synthesized using one of available methods. These structures are distinguished by the high quality of the initial wafers and the necessity to change the surface topology during the study, this making photolithography ineffective.
In this work we analyze the technology of insulating grooves that uses atomic force microscope, i.e. the pulse force nanolithography, which allows either treating single samples or forming narrow and deep grooves on semiconductor surfaces to provide good insulation. The experimentally measured transport characteristics of the nanostructures produced using this method confirm channel conductance quantization and the absence of large quantities of introduced defects.
GaAs/AlGaAs heterostructre, atomic force microscope, two-dimensional electron gas, nanostructure, pulse force nanolithography, local anodic oxidation method, channel conductance quantization
The synthesis and study of one-dimensional and zero-dimensional nanostructures is a rapidly developing trend of theoretical and experimental solid state physics since as the dimensions of active semiconductor elements approach the nanometer range the quantum properties of electrons become increasingly expressed. This on the one hand impairs the performance of the classic elements which are based on electron’s behavior as a particle while on the other hand shows good promise for the development of new active elements for data processing systems based on quantum-size effects.
Nanostructure samples are used for the experimental study of carrier transport in nanostructures. Nanostructure sample synthesis methods are multiple, e.g. growth of nanowires [
One condition for quantization is the absence of electron scattering in the channel, i.e., the channel length must be less than the electron path length which is several microns in typical structures at operation temperature (~4.2 К). However under specific conditions quantization may occur at up to 50 K [
Scanning probe microscope (SPM) technologies have recently become increasingly widespread [
Another standard surface processing method relying upon atomic force microscopy is surface scratching with a pressed tilted needle. A steel cantilever with a diamond pyramid or a silicon probe with a diamond-like coating is typically used for hard materials. In both variants the tips have a sufficiently large curvature radius (30–50 nm) reducing the space resolution of the lithography process used.
The novel pulse force nanolithography (PFNL) method was proposed [
The test samples were fabricated from GaAs/AlGaAs heterostructures grown by molecular beam epitaxy (MBE) on semiinsulating GaAs substrates. The layer growth sequence was 1 µm undoped GaAs, 100 nm AlGaAs and 35 nm protective undoped GaAs. The AlGaAs layer contained two Si doped delta layers. The spacer thickness (the distance from the heterointerface where 2D electron gas forms in the triangular potential well of GaAs, to the nearest delta layer) was 50 nm. The 4.2 K electron concentration and mobility in the 2D layer were 3.5 × 1011 cm–2 and 3.5 × 105 cm2/(V × s), respectively, the electron free path being 3–5 µm.
At the first stage which is similar to the earlier described one [
At the second stage nanostructures were synthesized in each sample using PFNL with a Smart SPM, AIST-NT7.
The PFNL operation principle can be demonstrated for the following example of forming four grooves in a semiconductor using different numbers of passes. Initially multiple indentations are sequentially made at 1 nm spacing with a 50 nm vertical amplitude (the vertical amplitude measurement did not take into account the cantilever elasticity, so the actual needle penetration depth into the semiconductor was less, about 20 nm). One can make more indentation passes to obtain deeper grooves. Fig.
(a) SPM image and (b) surface profile along the marked line of sample with several grooves formed using (upward) 1, 20, 30 and 100 needle passes.
Thus a nanostructure consisting of a channel and two gates was formed. Fig.
The electrical parameters of the structures were measured at 1.5–4.2 K. For the measurements the samples were placed in a vacuum attachment submerged into a transport liquid helium Dewar flask. The DC CV curves of the gates showed that the leakage currents were within 1 nA even at >1 V voltages.
The differential conductance (G) of the channel as a function of the gate voltage (UG) was measured at DC with the lock-in detection method. The solid curve in Fig.
The results suggest that PFNL can be successfully used in laboratory practice for producing insulating grooves in AlGaAs/GaAs heterostructures with 2D electron gas. Like all atomic force microscope based methods it allows using lithography processes for separate packaged crystal with quick topology change option during analysis. The method allows producing grooves with depth to width ratios of more than 1, also in structures with deep (more than 50 nm) 2D electron gas localization. The breakdown voltages of such grooves may reach several Volts at leakage currents of less than 1 nA.