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Scaling of conventional silicon based metal-oxide-semiconductor (MOS)
transistors requires thinner and thinner SiO2 films. However, the
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Figure 1: Intel 45 nm SRAM chip and IntelrCoreTM2 family processor.
Hafnium-based high-k dielectric materials are used in the fabrication of those
chips.
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increase of leakage current through thinner SiO2 films puts a
fundamental limit on the existing MOS technology. High dielectric constant
(high-k) materials are natural substitutes for
SiO2 as insulators because they can maintain sufficient thickness to
achieve desired capacitance. In fact, hafnium based high-k dielectric materials are already used by Intel in the
fabrication of the current generation of 45 nm SRAM chips and processors
(Figure 1).
The increasing need for higher speed and lower power consumptions has pushed
Si-based transistors to their performance limit. III-V compound semiconductors,
due to their high carrier mobility, are very promising in replacing Si as
substrates of semiconductor chips. Various combinations of
high-k dielectric
films on III-V substrates are being actively evaluated. However, sufficient
valence and conduction band discontinuities between the
high-k insulator and
the semiconductor substrate are necessary to act as barriers for both electron
and hole injection. Therefore, before a particular
high-k insulator is selected
for semiconductor chip it is very important to know the band offsets between it
and the semiconductor substrate.
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Figure 2: TEM image of LaAlO3 film on
In0.53Ga0.47As with the schematic
band diagram. Numbers in parenthesis are measured or calculated in this study.
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In this work, high-k dielectric LaAlO3
films on In0.53Ga0.47As substrates were grown at Intel
and studied at Stanford Synchrotron Radiation Lab (SSRL). As shown in figure 2,
only the band gap of the In0.53Ga0.47As substrate is
already known. To find out how the bands are aligned, we needed to establish
the valence band offset DEv and the band gap of the
amorphous LaAlO3 film, both of which were measured by photoelectron
spectroscopy at beam line 8-1 and beam line 10-1 of SSRL.
Valence band offset can be obtained by comparing the valence band maximum (VBM)
of LaAlO3 and In0.53Ga0.47As, as shown in
figure 3. However, because LaAlO3 is an insulator, positive charge will be
built up on the surface when photoelectrons are emitted. This positive charge
will cause the whole spectrum to shift. To correct this shift, a sample with
thin (1 nm) LaAlO3 film is prepared because we can collect both As
3d and Al 2p core levels from this sample. These two core levels are used as
references to align with the As 3d of the clean
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Figure 3: Valence-band and core level spectra for a clean n-type
In0.53GaAs (001) sample (a and d), a sample with 15 nm-thick
amorphous LaAlO3 film (b and e), and a sample with 1 nm-thick
LaAlO3 film (c
and f). The spectra were measured with a photon energy of 140 eV.
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and the Al 2p of the sample with
thick (15 nm) LaAlO3 film, respectively. After the spectra are
properly aligned and the spectral shift caused by surface charging is
corrected, the valence band offset is determined as 3.1 eV.
The band gap of LaAlO3 is traditionally determined by optical
measurement. However, the band gap of LaAlO3 film is generally
different from that of bulk LaAlO3. In addition, it is found that
band gaps of high-k dielectric films are also
significantly affected by film growth conditions. Therefore, we can not rely on
previously published LaAlO3 band gap data. In this work, we use the
energy loss features of Al 2p and O 1s core levels to measure the band gap, as
shown in figure 4. This energy loss feature creates a slope on the higher
binding energy side of the background. The energy difference between the onset
of the slope and the core level peak position is the band gap. Here, the band
gap of the LaAlO3 film is measured as 6.2 eV. Consequently, the
conduction band offset is calculated to be 2.35 eV. These measured and
calculated numbers are shown in parenthesis in figure 2.
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Figure 4: Energy loss spectra of (a) Al 2p and (b) O 1s core levels for
9.5 and 15 nm-thick amorphous LaAlO3 films. The two curves in each
plot are offset along the intensity axis for clarity. The band gap was
determined by linear extrapolation, shown by the dashed lines.
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The band offset measurements, in theory, can also be done using conventional
UPS and XPS instead of synchrotron sources. In reality, several problems with
conventional sources make it very difficult. For example, UPS can measure the
valence band, but lack of core level information makes it impossible to align
the spectra in order to correct the charging effect. For the band gap
measurement, the energy loss feature is very weak, and therefore, for a
reliable quantification it is necessary to have a very flat background and
narrow core level peak. In a conventional XPS system the photon energy is not
tunable and the spectral resolution is poorer, making it virtually impossible
to identify the onset of the slope caused by the energy loss feature.
Therefore, because of the high collimation and very high-resolution, coupled
with continuous tenability of the photon energy, XPS systems operating at
synchrotron sources are ideally suited to study the band offsets between thin
films and substrates.
Band gap and band offset measurements are not only very important for
addressing the viability of newer generation of semiconductor chip technology,
but they are also of vital importance in the study of materials used in solar
energy technologies, which are actively investigated in SSRL as well. The
techniques highlighted here pave the way for investigations of many
technologically important electronic devices.
Primary Citation
N. Goel, W. Tsai, C.M. Garner, Y. Sun, P. Pianetta, M. Warusawithana, D.G.
Schlom, H. Wen, C. Gaspe, J.C. Keay, et. al. (2007) Band offsets between
amorphous LaAlO3 and In0.53Ga0.47As. Applied
Physics Letters, 91, no.11, p.113515
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