We report a synthesis procedure for dodecanethiol capped wurtzite ZnO nanocrystals with an average diameter of 4 nm that are monodisperse, highly soluble, and shelf-stable for many months. Compared to previous ZnO ink recipes, we demonstrate improved particle solubility and excellent ink stability, resulting in ZnO nanocrystal inks that are optimized for printed electronics applications. The ZnO nanocrystal solution exhibits an absorption peak at 341 nm (3.63 eV), which represents a blue-shift of approximately 0.3 eV from the bulk ZnO bandgap (∼3.3 eV). This blue shift is consistent with previously reported models for an increased bandgap due to quantum confinement. We used variable-angle spectroscopic ellipsometry (VASE) to determine the optical properties of solution-processed thin films of ZnO nanocrystals, which provides valuable insight into the changes in film composition and morphology that occur during thermal annealing treatments ranging from 150–300 °C. The ZnO nanocrystals maintain their quantum confinement when deposited into a thin film, and the degree of quantum confinement is gradually reduced as the thermal annealing temperature increases. Using infrared absorption measurements (FTIR) and X-ray photoelectron spectroscopy (XPS), we show that the dodecanethiol ligands are removed from the ZnO films during annealing, resulting in a high-purity semiconductor film with very low carbon contamination. Furthermore, we show that annealing at 300 °C results in complete ligand removal with only a slight increase in grain size. Thin-film transistors (TFT) using ZnO nanocrystals as the channel material annealed at 300 °C show moderate mobility (∼0.002 cm 2 V −1 s −1 ) and good on/off ratio >10 4 . These results demonstrate the distinct advantages of colloidal nanocrystals for printed electronics applications: the composition and morphology of the solution-processed film can be carefully tuned by controlling the size and surface coating of the nanocrystals in the ink.
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Optical constants of single-crystalline Ni(100) from 77 to 770 K from ellipsometry measurements
Ellipsometry measurements were taken on single-crystalline Ni(100) at various temperatures between 77 and 770 K. DC conductivity and resistivity are extracted from the model optical constants and their temperature dependence is discussed. The authors find only qualitative agreement in the general trend of the resistivity measured by ellipsometry and electrical measurements. The temperature dependence of the main absorption peak at 4.8 eV indicates that the interband transitions are scattered by magnons with an effective energy of about 53 meV. The width of the main absorption peak reduces by 0.38 eV as the temperature rises, which is interpreted as the ferromagnetic exchange energy at the L-point. The small absorption peak at 1.5 eV is prominent only in the ferromagnetic phase and almost disappears in the paramagnetic phase. This peculiarity is explained by assigning the peak to [Formula: see text] transitions, which accounts for the decrease of the magnitude of the peak and its constant energy.
more » « less- NSF-PAR ID:
- 10364178
- Publisher / Repository:
- American Vacuum Society
- Date Published:
- Journal Name:
- Journal of Vacuum Science & Technology A
- Volume:
- 40
- Issue:
- 3
- ISSN:
- 0734-2101
- Page Range / eLocation ID:
- Article No. 033202
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Rocksalt structure nitrides emerge as a promising class of semiconductors for high-temperature thermoelectric and plasmonic applications. Controlling the bandgap and strain is essential for the development of a wide variety of electronic devices. Here we use (Ti 0.5 Mg 0.5 ) 1−x Al x N as a model system to explore and demonstrate the tunability of both the bandgap and the strain state in rocksalt structure nitrides, employing a combined experimental and computational approach. (Ti 0.5 Mg 0.5 ) 1−x Al x N layers with x ≤ 0.44 deposited on MgO(001) substrates by reactive co-sputtering at 700 °C are epitaxial single crystals with a solid-solution B1 rocksalt structure. The lattice mismatch with the substrate decreases with increasing x , leading to a transition in the strain-state from partially relaxed (74% and 38% for x = 0 and 0.09) to fully strained for x ≥ 0.22. First-principles calculations employing 64-atom Special Quasirandom Structures (SQS) indicate that the lattice constant decreases linearly with x according to a = (4.308 − 0.234 x ) Å for 0 ≤ x ≤ 1. In contrast, the measured relaxed lattice parameter a o = (4.269 − 0.131 x ) Å is linear only for x ≤ 0.33, its composition dependence is less pronounced, and x > 0.44 leads to the nucleation of secondary phases. The fundamental (indirect) bandgap predicted using the same SQS supercells and the HSE06 functional increases from 1.0 to 2.6 eV for x = 0–0.75. In contrast, the onset of the measured optical absorption due to interband transitions increases only from 2.3 to 2.6 eV for x = 0–0.44, suggesting that the addition of Al in the solid solution relaxes the electron momentum conservation and causes a shift from direct to indirect gap transitions. The resistivity increases from 9.0 to 708 μΩ m at 77 K and from 6.8 to 89 μΩ m at 295 K with increasing x = 0–0.44, indicating an increasing carrier localization associated with a randomization of cation site occupation and the increasing bandgap which also causes a 33% reduction in the optical carrier concentration. The overall results demonstrate bandgap and strain engineering in rocksalt nitride semiconductors and show that, in contrast to conventional covalent semiconductors, the random cation site occupation strongly affects optical transitions.more » « less
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Abstract We study helical structures in spin-spiral single crystals. In the continuum approach for the helicity potential energy the simple electronic band splits into two non-parabolic bands. For low exchange integrals, the lower band is described by a surface with a saddle shape in the direction of the helicity axis. Using the Boltzmann equation with the relaxation due to acoustic phonons, we discover the dependence of the current on the angle between the electric field and helicity axis leading to the both parallel and perpendicular to the electric field components in the electroconductivity. The latter can be interpreted as a planar Hall effect. In addition, we find that the transition rates depend on an electron spin allowing the transition between the bands. The electric conductivities exhibit nonlinear behaviors with respect to chemical potential µ . We explain this effect as the interference of the band anisotropy, spin conservation, and interband transitions. The proposed theory with the spherical model in the effective mass approximation for conduction electrons can elucidate nonlinear dependencies that can be identified in experiments. We find the excellent agreement between the theoretical and experimental data for parallel resistivity depending on temperature at the phase transition from helical to ferromagnetic state in a M n P single crystal. In addition, we predict that the perpendicular resistivity abruptly drops to zero in the ferromagnetic phase.more » « less
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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).more » « less
