Capítulo 2. El consumismo posmoderno y sus agentes
2.3. Influencers: el consumismo en las redes sociales
intervals II to IV, followed by an extended saturation regime V, in which capacitive coupling to the QW is still maintained, implies an increased density of interface states after the annealing process. This is likely to be caused by material intermixing at the InAlAs/Al2O3interface, creating additional energy states at the interface, in compliance
with the also observed reduced mobility in the gating areas II to VI.
On the bottom line, in terms of gating ability, we find no improvement of the transport characteristics for both surface terminations via the application of an additional PDA process. Since an In out-diffusion and an intermixing of the interface materials cannot be excluded also for the InGaAs-capped samples, we decided to omit this additional annealing step in the fabrication process of our devices for the remainder of this thesis.
Given the clear role of defect states at the interface, an interesting test presents the variation of the utilised dielectric material. In the subsequent section, we test the effect of MBE-grown MgO as dielectric, as well as Al2O3in combination with HfO2as interlayer
on newly fabricated top-gated Hall bar devices. As surface termination, we choose InAlAs since we concluded in subsection 7.2.5 that the extended gating areas II and III of sample 𝐻imply a reduced density of interface states.
7.4 Variation of the dielectric material
We test the gating response of our heterostructure with MBE-grown MgO and HfO2
in combination with Al2O3as alternative dielectric materials. As we consider InAlAs
surface-terminated samples to exhibit a reduced interface density of states as compared to heterostructures with a thin layer of InGaAs as capping, we fabricate two new top-gated Hall bar devices, samples M and N, from wafer C160429A (same as for sample 𝐻), for which the InGaAs cap layer is removed by wet-chemical etching.
Fabrication details
For sample 𝑀, we choose a thin layer of ALD-grown HfO2as a physical separation barrier
between the Al-containing semiconductor surface and Al2O3in an attempt to prohibit
material intermixing through exchange of Al atoms and the generation of additional interface states. Furthermore, ALD-grown HfO2presents a promising high-𝑘 dielectric
material, as it was found to reduce the charge defect formation at semiconductor/dielectric interfaces and additionally controls the fixed charge density and polarity inside a subsequent Al2O3 layer [186]. In addition, a self-cleaning effect of ALD-grown HfO2 is shown
in literature, as native oxides are efficiently removed from III-V surfaces during the deposition process, leading to an unpinning of the Fermi level at the semiconductor surface [177, 187–189].
For the ALD deposition of HfO2, we use tetrakis-ethyl-methyl-amino-hafnium (TEMAH)
as the precursor and H2O as the oxidation source. In contrast to Al2O3, the HfO2
deposition process highly depends on the applied process temperature [189], as well as on the individual pulse times for TEMAH and H2O [190]. Thus, in order to guarantee
7 Gating response of various III-V heterostructures
Figure 7.15:MT measurement sequences of sample 𝐻 with Al2O3, sample 𝑀
with HfO2/Al2O3 and sample 𝑁 with MgO/Al2O3 (wafer C160429A) in the
non-illuminated state for 𝑉𝐶 𝐷 = 0𝑉 at 𝑇 = 4.2𝐾: (a) Charge density 𝑛𝑠 as a
function of 𝑉𝑇 𝐺 of samples 𝐻, 𝑀 and 𝑁. (b - d) Mobility responses of sample
𝐻 (b), 𝑀 (c) and 𝑁 (d). The different symbols indicate the corresponding sweep directions of 𝑉𝑇 𝐺. (e) Exemplary MT measurements of sample 𝑁, displaying the
longitudinal resistivity 𝜌𝑥 𝑥(𝐵)for different 𝑉𝑇 𝐺 ranging from −0.2𝑉 (undermost
curve) to +3.0𝑉 (topmost curve).
electric insulation between the metal gate and the semiconducting layers, we additionally deposit several nanometers of Al2O3 on top of HfO2. This is done in a single ALD
process, whereby we thoroughly purge the reaction chamber with N2between the HfO2
and Al2O3deposition to reduce any intermixing of these two dielectrics.
For sample 𝑁, we use MBE-grown MgO as the dielectric material, which presents itself as an attractive alternative to the ALD-deposited Al2O3 and HfO2. The MBE growth
process does not rely on iterative chemical reactions between two chemical compounds, which is often accompanied by the formation of detrimental by-products, as is the case for ALD. Instead, highly purified source materials are used in an UHV chamber, whereby, in principle, the formation of intrinsic defect states is greatly suppressed. An illustration for the superior quality of the bulk MgO crystal can be found in the application of MgO as a tunnel barrier in spin-injection experiments [191, 192], in which a high-purity barrier material is required. We choose a MgO layer thickness of 50𝑛𝑚, followed by a thin layer of ALD-deposited Al2O3.
Measurement sequences
Figure 7.15(a) shows the gating responses of samples 𝑀 (HfO2/Al2O3as dielectric) and 𝑁
(MgO/Al2O3as dielectric), together with the measurement sequence of sample 𝐻 (Al2O3
as dielectric) as a reference. For sample 𝑀, a 𝑉𝑚 𝑎𝑥
𝑇 𝐺 of +2.0𝑉 is chosen; for sample 𝑁, we
apply a 𝑉𝑚 𝑎𝑥
𝑇 𝐺 of +4.0𝑉. As supplementary information, positively biased cool-down MT
measurement sequences of sample 𝐻, 𝑀 and 𝑁 are shown in the appendix in figure B.5. A positive 𝑉𝐶 𝐷 shifts the gating curves of samples 𝑀 and 𝑁 horizontally into positive
7.4 Variation of the dielectric material
𝑉𝑇 𝐺-direction as we also find for sample 𝐻. The gating responses of samples 𝑀 and 𝑁 in
figure 7.15(a) resemble the measurement sequence of sample 𝐻 and can be divided into the characteristic gating areas, defined in the charge transfer model in subsection 7.2.3. The gating curve of sample 𝑁, however, is shifted horizontally into positive 𝑉𝑇 𝐺-direction
as compared to the gating responses of samples 𝐻 and 𝑀. As can be deduced from the slope of the density curves in gating area I, all three samples exhibit a very similar capacitive coupling 𝑐 = 𝜕 𝑛
𝜕𝑉 to the 2DEG. Since for sample 𝑀, we only deposited a thin
layer of HfO2in addition to the much thicker dielectric layer of Al2O3, the Al2O3layer
dominates the contribution of the dielectric layers to the capacitive coupling between the TG and the 2DEG. Thus, the gating response of sample 𝑀 should indeed be equal to the density response of our reference sample 𝐻 in gating area I. For sample 𝑁, we understand the similarity of the experimentally observed capacitive coupling to samples 𝑀 and 𝐻 by means of the equivalent permittivity 𝜀𝑀 𝑔𝑂 to 𝜀𝐴𝑙2𝑂3 [193]. At the end of gating interval
I, samples 𝑀 and 𝑁 both reach 𝑛𝑝 𝑒 𝑎 𝑘
𝑠 =7.4 · 1011𝑐𝑚
−2just as the reference sample 𝐻,
and then evolve into gating regime II. For sample 𝑀, however, we find a shortened gating interval II and III, before the charge density saturates at 𝑛𝑠𝑎𝑡
𝑠 ≈ 6 · 10
11𝑐𝑚−2(see also in the appendix B in figure B.5(b)). The charge density response of sample 𝑁 follows the gating behavior of the reference sample 𝐻, exhibiting an extended gating area II and a comparable hysteresis between up- and down-sweep of 𝑉𝑇 𝐺. At the same time, the
mobility responses of samples 𝑀 and 𝑁, plotted in figure 7.15(b), (c) and (d), differ significantly from sample 𝐻. During the first down-sweep of 𝑉𝑇 𝐺 from 0𝑉 into negative
𝑉𝑇 𝐺-direction after the cool-down, sample 𝑀 exhibits a similar mobility as the reference sample 𝐻. In the following up-sweep of 𝑉𝑇 𝐺, however, we determine that the mobility is
decreased by almost a factor of two as compared to sample 𝐻. The mobility of sample 𝑁 is generally about half the value of the reference sample 𝐻.
Discussion
In summary, for sample 𝑀, which is equipped with a thin interlayer of HfO2between
InAlAs and Al2O3, we determine an earlier onset of charge migration towards the interface
compared to the reference sample 𝐻. Building on our charge transfer model, we assign this behavior to the presence of an enhanced charge density at the interface. The significant reduction of the electron mobility after the first down-sweep of 𝑉𝑇 𝐺 can be attributed to a
band tilting-induced reorganization of charge carriers in the gate stack, induced by the HfO2interlayer. Owing to the HfO2layer, Coulomb scattering centers are additionally
introduced into the heterostructure, which increase the scattering rate of the conduction electrons. Yet, we want to note that this hysteresis in mobility does not manifest itself in the corresponding gating response of the charge density. For sample 𝑁 with MgO as additional dielectric for the physical separation of the Al-containing semiconductor surface and Al2O3, we find the gating response to be equivalent to our reference sample 𝐻. In contrast
to the density response, the mobility is significantly reduced as compared to sample 𝐻. This indicates a strongly increased large-angle backscattering with MgO as dielectric in contrast to Al2O3. The horizontal shift of the gating curve into positive voltage direction
7 Gating response of various III-V heterostructures
implies an increased negative charge accumulation between the metal gate electrode and the QW, which effectively reduces the electric TG-field at the QW. Accordingly, the reduced mobility for device 𝑁 can be assigned to increased Coulombic disorder due to the additional negative charge. This deduction, however, is rather inconsistent to auxiliary conducted biased cool-down measurements with sample 𝑁 (see appendix B), with which a similar horizontal shift of the gating curve can be deliberately induced with an appropriate 𝑉𝐶 𝐷. This shift is also understood as accumulated charge above the QW. Yet, we observe
no significant reduction in mobility in the respective measurement.
A peculiar experimental finding in the MT measurements of the longitudinal resistivity 𝜌𝑥 𝑥(𝐵)of sample 𝑁 is displayed in figure 7.15(e). Even for small values of 𝑛𝐻 𝑎𝑙 𝑙, we
find a distinct knot in the Shubnikov-de Haas oscillations, for example at 𝐵 = 1.5𝑇 at 𝑉𝑇 𝐺 = +0.4𝑉. Evaluating the FFT spectra of 𝜌𝑥 𝑥(𝐵), however, provides no further
insight into the origin of the B-field-dependent magnetooscillation damping, owing to the limited number of available oscillations in 𝜌𝑥 𝑥(𝐵). Furthermore, with good reason,
we can exclude the population of a second size-quantized subband to be the origin of our observed modulation of the Shubnikov-de Haas amplitude. Since we maintain the capacitive coupling between the TG and the 2DEG even for higher 𝑉𝑇 𝐺, we are able to
also exclude a parasitic conductive layer in the system as the possible cause. The exact origin of this conspicuous effect is still unknown.
Conclusion
In our test of MBE-grown MgO and of HfO2 as interlayer dielectrics, we find both
materials to exhibit adverse effects on the gating response of the heterostructure compared to reference samples with ALD-deposited Al2O3: We determine an enhanced charge
migration and a significant reduction of the electron mobility for samples 𝑀 and 𝑁. It is interesting to note that all of our tested dielectrics are based on oxidic-materials. Possible As-O bond formation creates midgap states at the semiconductor/dielectric interface [156], being likely responsible for enhanced charge migration towards the interface and for charge trapping. We therefore propose to test non-oxide based materials, e.g. h-BN, as dielectric materials.