Resonant tunneling diodes (RTDs) have come full-circle in the past 10 years after their demonstration in the early 1990s
as the fastest room-temperature semiconductor oscillator, displaying experimental results up to 712 GHz and fmax values
exceeding 1.0 THz [1]. Now the RTD is once again the preeminent electronic oscillator above 1.0 THz and is being
implemented as a coherent source [2] and a self-oscillating mixer [3], amongst other applications. This paper concerns
RTD electroluminescence – an effect that has been studied very little in the past 30+ years of RTD development, and not
at room temperature. We present experiments and modeling of an n-type In0.53Ga0.47As/AlAs double-barrier RTD
operating as a cross-gap light emitter at ~300K. The MBE-growth stack is shown in Fig. 1(a). A 15-μm-diam-mesa
device was defined by standard planar processing including a top annular ohmic contact with a 5-μm-diam pinhole in the
center to couple out enough of the internal emission for accurate free-space power measurements [4]. The emission spectra
have the behavior displayed in Fig. 1(b), parameterized by bias voltage (VB). The long wavelength emission edge is at
= 1684 nm - close to the In0.53Ga0.47As bandgap energy of Ug ≈ 0.75 eV at 300 K. The spectral peaks for VB = 2.8 and
3.0 V both occur around = 1550 nm (h = 0.75 eV), so blue-shifted relative to the peak of the “ideal”, bulk InGaAs
emission spectrum shown in Fig. 1(b) [5]. These results are consistent with the model displayed in Fig. 1(c), whereby
the broad emission peak is attributed to the radiative recombination between electrons accumulated on the emitter side,
and holes generated on the emitter side by interband tunneling with current density Jinter. The blue-shifted main peak is
attributed to the quantum-size effect on the emitter side, which creates a radiative recombination rate RN,2 comparable to
the band-edge cross-gap rate RN,1. Further support for this model is provided by the shorter wavelength and weaker
emission peak shown in Fig. 1(b) around = 1148 nm. Our quantum mechanical calculations attribute this to radiative
recombination RR,3 in the RTD quantum well between the electron ground-state level E1,e, and the hole level E1,h.
To further test the model and estimate quantum efficiencies, we conducted optical power measurements using a
large-area Ge photodiode located ≈3 mm away from the RTD pinhole, and having spectral response between 800 and
1800 nm with a peak responsivity of ≈0.85 A/W at =1550 nm. Simultaneous I-V and L-V plots were obtained and
are plotted in Fig. 2(a) with positive bias on the top contact (emitter on the bottom). The I-V curve displays a pronounced
NDR region having a current peak-to-valley current ratio of 10.7 (typical for In0.53Ga0.47As RTDs). The external quantum
efficiency (EQE) was calculated from EQE = e∙IP/(∙IE∙h) where IP is the photodiode dc current and IE the RTD current.
The plot of EQE is shown in Fig. 2(b) where we see a very rapid rise with VB, but a maximum value (at VB= 3.0 V) of
only ≈2×10-5. To extract the internal quantum efficiency (IQE), we use the expression EQE= c ∙i ∙r ≡ c∙IQE where
ci, and r are the optical-coupling, electrical-injection, and radiative recombination efficiencies, respectively [6]. Our
separate optical calculations yield c≈3.4×10-4 (limited primarily by the small pinhole) from which we obtain the curve
of IQE plotted in Fig. 2(b) (right-hand scale). The maximum value of IQE (again at VB = 3.0 V) is 6.0%. From the implicit
definition of IQE in terms of i and r given above, and the fact that the recombination efficiency in In0.53Ga0.47As is
likely limited by Auger scattering, this result for IQE suggests that i might be significantly high.
To estimate i, we have used the experimental total current of Fig. 2(a), the Kane two-band model of interband
tunneling [7] computed in conjunction with a solution to Poisson’s equation across the entire structure, and a rate-equation
model of Auger recombination on the emitter side [6] assuming a free-electron density of 2×1018 cm3. We focus on the
high-bias regime above VB = 2.5 V of Fig. 2(a) where most of the interband tunneling should occur in the depletion region
on the collector side [Jinter,2 in Fig. 1(c)]. And because of the high-quality of the InGaAs/AlAs heterostructure (very few
traps or deep levels), most of the holes should reach the emitter side by some combination of drift, diffusion, and tunneling
through the valence-band double barriers (Type-I offset) between InGaAs and AlAs. The computed interband current
density Jinter is shown in Fig. 3(a) along with the total current density Jtot. At the maximum Jinter (at VB=3.0 V) of 7.4×102
A/cm2, we get i = Jinter/Jtot = 0.18, which is surprisingly high considering there is no p-type doping in the device. When
combined with the Auger-limited r of 0.41 and c ≈ 3.4×10-4, we find a model value of IQE = 7.4% in good agreement
with experiment. This leads to the model values for EQE plotted in Fig. 2(b) - also in good agreement with experiment.
Finally, we address the high Jinter and consider a possible universal nature of the light-emission mechanism. Fig.
3(b) shows the tunneling probability T according to the Kane two-band model in the three materials, In0.53Ga0.47As, GaAs,
and GaN, following our observation of a similar electroluminescence mechanism in GaN/AlN RTDs (due to strong
polarization field of wurtzite structures) [8]. The expression is Tinter = (2/9)∙exp[(-2 ∙Ug
2 ∙me)/(2h∙P∙E)], where Ug is the
bandgap energy, P is the valence-to-conduction-band momentum matrix element, and E is the electric field. Values for
the highest calculated internal E fields for the InGaAs and GaN are also shown, indicating that Tinter in those structures
approaches values of ~10-5. As shown, a GaAs RTD would require an internal field of ~6×105 V/cm, which is rarely
realized in standard GaAs RTDs, perhaps explaining why there have been few if any reports of room-temperature
electroluminescence in the GaAs devices.
[1] E.R. Brown,et al., Appl. Phys. Lett., vol. 58, 2291, 1991. [5] S. Sze, Physics of Semiconductor Devices, 2nd Ed. 12.2.1 (Wiley, 1981).
[2] M. Feiginov et al., Appl. Phys. Lett., 99, 233506, 2011. [6] L. Coldren, Diode Lasers and Photonic Integrated Circuits, (Wiley, 1995).
[3] Y. Nishida et al., Nature Sci. Reports, 9, 18125, 2019. [7] E.O. Kane, J. of Appl. Phy 32, 83 (1961).
[4] P. Fakhimi, et al., 2019 DRC Conference Digest. [8] T. Growden, et al., Nature Light: Science & Applications 7, 17150 (2018).
[5] S. Sze, Physics of Semiconductor Devices, 2nd Ed. 12.2.1 (Wiley, 1981).
[6] L. Coldren, Diode Lasers and Photonic Integrated Circuits, (Wiley, 1995).
[7] E.O. Kane, J. of Appl. Phy 32, 83 (1961).
[8] T. Growden, et al., Nature Light: Science & Applications 7, 17150 (2018).
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Ferroelectric surface photovoltage enhancement in chromium-doped SrTiO 3 nanocrystal photocatalysts for hydrogen evolution
Chromium-doped SrTiO 3 nanocrystals of perovskite structure type and 45 nm (±15 nm) edge lengths were obtained by hydrothermal synthesis in water from titanium oxide, strontium hydroxide, and chromium( iii ) nitrate. According to XPS, the majority of the surface chromium (68.3%) is present in the 3+ state and the remainder (32.2%) in the 6+ state. Optical spectroscopy confirms a broad absorption at 2.3–2.9 eV from Cr(3+) dopant states, in addition to the 3.2 eV band edge of the SrTiO 3 host. After modification with Pt nanoparticles, Cr-doped SrTiO 3 nanocrystals catalyze photochemical H 2 evolution from aqueous methanol under visible light illumination (>400 nm) and with an apparent quantum yield of 0.66% at 435 nm. According to surface photovoltage spectroscopy (SPS), Cr-doped SrTiO 3 nanocrystals deposited onto gold substrates are n-type and have an effective band gap of 1.75 eV. SPS and transient illumination experiments at 2.50 eV reveal an anomalous surface photovoltage that increases with prior light exposure to values of up to −6.3 V. This photovoltage is assigned to ferroelectric polarization of the material in the space charge layer at the Au/SrTiO 3 :Cr interface. The polarization is stable for 24 h in vacuum but disappears after 12 h when samples are stored in air. The electric polarizability of SrTiO 3 :Cr is confirmed when films are exposed to static electric fields (1.20 MV m −1 ) in a fixed capacitor configuration. The discovery of a ferroelectric effect in Cr-doped SrTiO 3 could be significant for the development of improved photocatalysts for the conversion of solar energy into fuel.
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- Award ID(s):
- 1900136
- NSF-PAR ID:
- 10192005
- Date Published:
- Journal Name:
- Materials Advances
- Volume:
- 1
- Issue:
- 5
- ISSN:
- 2633-5409
- Page Range / eLocation ID:
- 1382 to 1389
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract While induced spin polarization of a palladium (Pd) overlayer on antiferromagnetic and magneto-electric Cr2O3(0001) is possible because of the boundary polarization at the Cr2O3(0001), in the single domain state, the Pd thin film appears to be ferromagnetic on its own, likely as a result of strain. In the conduction band, we find the experimental evidence of ferromagnetic spin polarized in Pd thin films on a Cr2O3(0001) single crystal, especially in the thin limit, Pd thickness of around 1–4 nm. Indeed there is significant spin polarization in 10 Å thick Pd films on Cr2O3(0001) at 310 K, i.e. above the Néel temperature of bulk Cr2O3. While Cr2O3(0001) has surface moments that tend to align along the surface normal, for Pd on Cr2O3, the spin polarization contains an in-plane component. Strain in the Pd adlayer on Cr2O3(0001) appears correlated to the spin polarization measured in spin polarized inverse photoemission spectroscopy. Further evidence for magnetization of Pd on Cr2O3is provided by measurement of the exchange bias fields in Cr2O3/Pd(buffer)/[Co/Pd]
n exchange bias systems. The magnitude of the exchange bias field is, over a wide temperature range, virtually unaffected by the Pd thickness variation between 1 and 2 nm. -
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Abstract Ferroelectric films suffer from both aging and degradation under high ac‐field drive conditions due to loss of polarization with time. In this study, the roles of defect chemistry and internal electric fields on the long‐term stability of the properties of piezoelectric films were explored. For this purpose, lead zirconate titanate (PZT) films with a Zr/Ti ratio of 52/48 doped with Mn‐ (PMZT) or Nb‐ (PNZT) were deposited on Pt coated Si substrates by the sol‐gel method. It was demonstrated that the magnitude of the internal field is much higher in PMZT films compared to PNZT films after poling in the temperature range of 25‐200°C under an electric field of −240 kV/cm. The development of the internal field is thermally activated, with activation energies from 0.5 ± 0.06 to 0.8 ± 0.1 eV in Mn doped films and from 0.8 ± 0.1 to 1.2 ± 0.2 eV in Nb doped films. The different activation energies for imprint suggests that the physical mechanism underlying the evolution of the internal field in PMZT and PNZT films differs; the enhanced internal field upon poling is attributed to (a) alignment of oxygen vacancy—acceptor ion defect dipoles (
, ) in PMZT films, and (b) thermionic injection of electron charges and charge trapping in PNZT films. In either case, the internal field reduces back switching, enhances the remanent piezoelectric properties, and dramatically improves the aging behavior. PMZT films exhibited the greatest enhancement, with reduced high temperature (180°C) aging rates of 2%‐3%/decade due to improved stability of the poled state. In contrast, PNZT films showed significantly larger high temperature aging rates (15.5%/decade) in the piezoelectric coefficient, demonstrating that the fully poled state was not retained with time.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0MA00463D
