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Enhanced NMR with Optical Pumping (OPNMR) Yields As Signals Selectively from a Buried GaAs Interface 75
Matthew M. Willmering, Zayd L. Ma, Melanie A. Jenkins, John F. Conley, and Sophia E. Hayes J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b08970 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 3, 2017
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Journal of the American Chemical Society
Enhanced NMR with Optical Pumping (OPNMR) Yields Selectively from a Buried GaAs Interface Matthew M. Willmering,
†
†
Zayd L. Ma,
Melanie A. Jenkins,
‡
75
As Signals ‡
John F. Conley, Jr.,
and Sophia
∗, †
E. Hayes
†Department ‡School
of Chemistry, Washington University, St. Louis, MO, 63130
of Electrical Engineering and Computer Science, Oregon State University, Corvallis, OR, 97331 E-mail:
[email protected] and spatial distribution throughout the sample determines Abstract:
the portion of the sample being detected.
of single-crystal semi-insulating GaAs that has been coated
75 The nuclear isotope studied here is quadrupolar, As (nu3 clear spin quantum number, I = ), which couples to any 2 electric eld gradient (EFG) present, resulting in a splitting
We have measured with optically-pumped NMR 75 (OPNMR) the As signals arising from the interface region and passivated with an aluminum oxide lm deposited by
11,12
atomic layer deposition (ALD). Using wavelength-selective op-
of the NMR resonance.
tical
from which
from perfect cubic symmetry through bond distortions, thus,
Here, OPNMR signals were
lattice strain can be detected from the amount a quadrupo-
pumping,
the laser
restricts
(OP)NMR signals are collected.
the volume
1315
Stress perturbs structures away
obtained from the interface region and distinguished from sig-
lar resonance is split.
nals arising from the bulk. The interface region is highlighted
previously by OPNMR (and ODNMR) methods to determine nuclear spin temperatures,
by interactions that disrupt the cubic symmetry of the GaAs
I =
lattice, resulting in quadrupolar satellites for nuclear
Strain in GaAs has been studied
3 2
isotopes, whereas NMR of the bulk lattice is nominally unsplit. Quadrupolar splitting at the interface arises from strain
spatial inhomogeneities of strain, tion mechanisms. A
400 µm
16
15
1820
bandstructure eects,
17
and dominant polariza-
bulk semi-insulating single crystal of GaAs
from lattice mismatch between the GaAs and ALD-deposited
(ITME, grown along [100], lot 2137, polished on one side)
aluminum oxide, due to their dierent coe cients of thermal
was used as a substrate. A thin lm of amorphous alumina,
expansion. Such spectroscopic evidence of strain can be useful
hereafter denoted Al 2 O 3 , was deposited using atomic layer
for measuring lattice distortions at heterojunction boundaries
deposition (ALD) on the polished side of the GaAs . The de◦ position temperature was 300 C. Four pulses of tri-methyl
and interfaces.
aluminum were used to remove the native oxides on the surface of the GaAs substrate Semiconductor heterojunctions are created in solid-state electronic devices when two materials with dissimilar band gap energies are chemically bonded to one another.
1
To cre-
4,5
prior to ALD growth.
nal thickness of the Al 2 O 3 lm was
11.2 nm
The
and is shown
schematically in Figure S1. The experimental procedures for OPNMR have been de-
9,21
ate higher performing electronic devices, a better under-
scribed previously, with critical parameters noted:
standing of the interface at a heterojunction is necessary.
nal magnetic eld ( B0 ) of 4.7 T, B1 excitation strengths were ≈20 kHz, σ + polarization and laser power held constant at
The heterojunction studied here (Al 2 O 3 /GaAs) has been proposed for MOSFETs, sivation layer for GaAs.
2,3
25
MOS capacitors,
3
and as a pas-
By determining the interfacial
100 mW,
sample irradiated for a time period of
exter-
τL (90 s) afτD ) was
ter the saturation sequence, a short period of time (
structure in heterojunctions, manipulation of growth and
inserted where the laser was shuttered (
synthetic procedures can be followed for their aect on the
spectra were then acquired by a quadrupolar echo
electronic performance of devices.
sequence (using a 55 ° pulses) while the laser was shuttered
Solid-state nuclear magnetic resonance (NMR) has been used to study inorganic semiconductors;
6
however, conven-
≈ 1 s), and the NMR 22
pulse
(Figure S2). 55 ° tip angles were used to observe the asymmetry of the satellites and refocus the dipolar and quadrupolar
2325
tional NMR techniques are usually incapable of measuring 16 spectra from interfaces due to the limit of ≈ 10 spins
ences 23,24 results in symmetric satellites, which does not
for detection.
properly depict the spin temperature. The
Hyperpolarization
7
allows a lower detection
threshold for NMR spectroscopy of such samples. Optical pumping in direct gap semiconductors uses laser excitation to form polarized conduction electrons. By using circularly polarized light, optical absorption selection rules
interactions.
The
90° − τ − 64°
pulse sequence of refer-
55° − τ − 55°
se-
quence represents the best (approximate) tradeo between optimal refocusing of the central transition and the satellites 25 3 for a spinnucleus. 2 The spatial regions where the light is absorbed determine
create non-equilibrium electron spin populations within the
the portion of the sample that can be observed. The size of
conduction band; which are subsequently captured at de-
such regions is governed by the beam diameter, the position
fect sites. Hyper ne interactions between captured electrons
of the laser on the sample, and the penetration depth of the
and proximate nuclei result in the transfer of electron spin
light. The size of these regions have been used to previously
polarization to the nuclei.
model the photon energy dependence of OPNMR spectra.
NMR)
8,9
Optically-pumped NMR (OP-
detects the nuclear magnetization with traditional
radio-frequency NMR methods.
The OPNMR signals are
10
In this study, all photon energies used were much smaller than the band gap of the Al 2 O 3 (at
≈ 9 eV 26 ),
allowing the
photon energy dependent, and vary as a function of the laser
laser to pass through the ALD lm to the GaAs substrate
penetration depth since the laser-excited electrons are the
underneath.
origin of the hyperpolarization.
10
The laser intensity then decays exponentially
Thus, the laser intensity
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with depth
9
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into the GaAs. At high optical absorption val-
ues, the laser excites conduction electrons in only the interfacial region, where strain is present. The minimal penetration depth of the laser is estimated as
