 Award ID(s):
 1729487
 NSFPAR ID:
 10195244
 Date Published:
 Journal Name:
 Journal of Materials Chemistry C
 Volume:
 8
 Issue:
 30
 ISSN:
 20507526
 Page Range / eLocation ID:
 10174 to 10184
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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Resonant tunneling diodes (RTDs) have come fullcircle in the past 10 years after their demonstration in the early 1990s as the fastest roomtemperature 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 selfoscillating 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 ntype In0.53Ga0.47As/AlAs doublebarrier RTD operating as a crossgap light emitter at ~300K. The MBEgrowth stack is shown in Fig. 1(a). A 15μmdiammesa device was defined by standard planar processing including a top annular ohmic contact with a 5μmdiam pinhole in the center to couple out enough of the internal emission for accurate freespace 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 blueshifted 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 blueshifted main peak is attributed to the quantumsize effect on the emitter side, which creates a radiative recombination rate RN,2 comparable to the bandedge crossgap 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 groundstate 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 largearea 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 IV and LV plots were obtained and are plotted in Fig. 2(a) with positive bias on the top contact (emitter on the bottom). The IV curve displays a pronounced NDR region having a current peaktovalley 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 ≈2×105. To extract the internal quantum efficiency (IQE), we use the expression EQE= c ∙i ∙r ≡ c∙IQE where ci, and r are the opticalcoupling, electricalinjection, and radiative recombination efficiencies, respectively [6]. Our separate optical calculations yield c≈3.4×104 (limited primarily by the small pinhole) from which we obtain the curve of IQE plotted in Fig. 2(b) (righthand 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 twoband model of interband tunneling [7] computed in conjunction with a solution to Poisson’s equation across the entire structure, and a rateequation model of Auger recombination on the emitter side [6] assuming a freeelectron density of 2×1018 cm3. We focus on the highbias 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 highquality 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 valenceband double barriers (TypeI 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 ptype doping in the device. When combined with the Augerlimited r of 0.41 and c ≈ 3.4×104, 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 lightemission mechanism. Fig. 3(b) shows the tunneling probability T according to the Kane twoband 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 valencetoconductionband 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 ~105. 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 roomtemperature 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

Full Heusler compounds have long been discovered as exceptional ntype thermoelectric materials. However, no ptype compounds could match the high ntype figure of merit ( ZT ). In this work, based on firstprinciples transport theory, we predict the unprecedentedly high ptype ZT = 2.2 at 300 K and 5.3 at 800 K in full Heusler CsK 2 Bi and CsK 2 Sb, respectively. By incorporating the higherorder phonon scattering, we find that the high ZT value primarily stems from the ultralow lattice thermal conductivity ( κ L ) of less than 0.2 W mK −1 at room temperature, decreased by 40% compared to the calculation only considering threephonon scattering. Such ultralow κ L is rooted in the enhanced phonon anharmonicity and scattering channels stemming from the coexistence of antibondinginduced anharmonic rattling of Cs atoms and lowlying optical branches. Moreover, the flat and heavy nature of valence band edges leads to a high Seebeck coefficient and moderate power factor at optimal hole concentration, while the dispersive and light conduction band edges yield much larger electrical conductivity and electronic thermal conductivity ( κ e ), and the predominant role of κ e suppresses the ntype ZT . This study offers a deeper insight into the thermal and electronic transport properties in full Heusler compounds with strong phonon anharmonicity and excellent thermoelectric performance.more » « less

A model is developed that accounts for the effects of thermal disorder (both static and dynamic) in predicting the thermoelectric (TE) performance of weakly bonded semiconductors. With dynamic disorder included, the model is found to fit well with experimental results found in the literature for the densityofstates and the energydependent carrier mobility, which are key for assessing TE properties. The model is then used to analyze the concentrationdependent TE properties of the prototypical small molecular semiconductor rubrene. At low (e.g., intrinsic) carrier concentrations, where Fermi level pinning occurs, dynamic disorder is found to reduce electrical conductivity ([Formula: see text]), Seebeck coefficient ([Formula: see text]), and thermoelectric power factor ([Formula: see text]) to values that are much lower than those traditionally predicted by static disorder models. As carrier concentration ([Formula: see text]) increases, [Formula: see text] exhibits nonlinear behavior, increasing well above the conventional [Formula: see text] vs [Formula: see text] relationship before reaching a peak value ([Formula: see text]). A critical carrier concentration ([Formula: see text] molar ratio) is observed near [Formula: see text] at which thermoelectric transport transitions from traplimited behavior at low concentrations to conventional band behavior at high concentrations. Above this value, [Formula: see text] and [Formula: see text] are reduced compared to the perfect crystal and staticonly conditions, causing a drop in the maximum [Formula: see text] by factors of 3 and 2.3, respectively. This [Formula: see text] reduction, while not as large as the [Formula: see text] reduction that occurs for low carrier concentration, is found to occur in a high concentration regime ([Formula: see text]) that contains the [Formula: see text] maximum and has remained inaccessible to experimentalists due to dopant limitations that are worsened in the presence of dynamic disorder.

Abstract The potential of an environmentally friendly and emerging chalcogenide perovskite CaZrSe_{3}for thermoelectric applications is examined. The orthorhombic phase of CaZrSe_{3}has an optimum band gap (1.35–1.40 eV) for single‐junction photovoltaic applications. The predictions reveal that CaZrSe_{3}possesses an absorption coefficient of ≈4 × 10^{5}cm^{−1}at photon energies of 2.5 eV with an early onset of optical absorption (≈0.2 eV) well below the optimum band gap. Seebeck coefficient,
S , is inversely proportional to the carrier mobility as the calculated average effective mass for electrons is higher than for holes;p ‐type doping enhances the electrical conductivity, σ. The electronic thermal conductivityκ_{e} remains low at all temperatures. The upper limit of the thermoelectric figure of merit (ZT_{e} ) attains ≈1.0 when doped at specific chemical potentials, while a high Seebeck coefficient contributes to the ZT_{e} = 1.95 at 50 K forp ‐type doping with 10^{18}cm^{−3}carrier concentration, demonstrating high thermoelectric efficiency. 
null (Ed.)Orthorhombic BaZrS 3 is a potential optoelectronic material with prospective applications in photovoltaic and thermoelectric devices. While efforts exist on understanding the effects of elemental substitution and material stability, fundamental knowledge on the electronic transport properties are sparse. We employ first principles calculations to examine the electronic band structure and optical band gap and interrogate the effect of electron transport on electrical and thermal conductivities, and Seebeck coefficient, as a function of temperature and chemical potential. Our results reveal that BaZrS 3 has a band gap of 1.79 eV in proximity of the optimal 1.35 eV recommended for single junction photovoltaics. An absorption coefficient of 3 × 10 5 cm −1 at photon energies of 3 eV is coupled with an early onset to optical absorption at 0.5 eV, significantly below the optical band gap. The carrier effective mass being lower for electrons than holes, we find the Seebeck coefficient to be higher for holes than electrons. A notable (≈1.0 at 300 K) upper limit to the thermoelectric figure of merit, obtained due to high Seebeck coefficient (3000 μV K −1 ) and ultralow electron thermal conductivity, builds promise for BaZrS 3 as a thermoelectric.more » « less