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1. RTD Light Emission around 1550 nm with IQE up to 6% at 300 K

   https://doi.org/10.1109/DRC50226.2020.9135175  

   Brown, E. R.; Zhang, W-D.; Fakhimi, P.; Growden, T. A.; Berger, P.R. (June 2020, IEEE Device Research Conference (DRC), Columbus, OH (June 22-24, 2020))

   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 ci, 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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2. MOCVD growth and band offsets of κ-phase Ga ₂ O ₃ on c-plane sapphire, GaN- and AlN-on-sapphire, and (100) YSZ substrates

   https://doi.org/10.1116/6.0002106  

   Bhuiyan, A F; Feng, Zixuan; Huang, Hsien-Lien; Meng, Lingyu; Hwang, Jinwoo; Zhao, Hongping (December 2022, Journal of Vacuum Science & Technology A)

   Epitaxial growth of κ-phase Ga 2 O 3 thin films is investigated on c-plane sapphire, GaN- and AlN-on-sapphire, and (100) oriented yttria stabilized zirconia (YSZ) substrates via metalorganic chemical vapor deposition. The structural and surface morphological properties are investigated by comprehensive material characterization. Phase pure κ-Ga 2 O 3 films are successfully grown on GaN-, AlN-on-sapphire, and YSZ substrates through a systematical tuning of growth parameters including the precursor molar flow rates, chamber pressure, and growth temperature, whereas the growth on c-sapphire substrates leads to a mixture of β- and κ-polymorphs of Ga 2 O 3 under the investigated growth conditions. The influence of the crystalline structure, surface morphology, and roughness of κ-Ga 2 O 3 films grown on different substrates are investigated as a function of precursor flow rate. High-resolution scanning transmission electron microscopy imaging of κ-Ga 2 O 3 films reveals abrupt interfaces between the epitaxial film and the sapphire, GaN, and YSZ substrates. The growth of single crystal orthorhombic κ-Ga 2 O 3 films is confirmed by analyzing the scanning transmission electron microscopy nanodiffraction pattern. The chemical composition, surface stoichiometry, and bandgap energies of κ-Ga 2 O 3 thin films grown on different substrates are studied by high-resolution x-ray photoelectron spectroscopy (XPS) measurements. The type-II (staggered) band alignments at three interfaces between κ-Ga 2 O 3 and c-sapphire, AlN, and YSZ substrates are determined by XPS, with an exception of κ-Ga 2 O 3 /GaN interface, which shows type-I (straddling) band alignment. 

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3. First Demonstration of Vertical Superjunction Diode in GaN

   https://doi.org/10.1109/IEDM45625.2022.10019405  

   Xiao, Ming; Ma, Yunwei; Du, Zhonghao; Qin, Yuan; Liu, Kai; Cheng, Kai; Udrea, Florin; Xie, Andy; Beam, Edward; Wang, Boyan; et al (December 2022, First Demonstration of Vertical Superjunction Diode in GaN)

   We report the first experimental demonstration of a vertical superjunction device in GaN. P-type nickel oxide (NiO) is sputtered conformally in 6μm deep n-GaN trenches. Sputter recipe is tuned to enable 1017 cm −3 level acceptor concentration in NiO, easing its charge balance with the 9×1016 cm −3 doped n-GaN. Vertical GaN superjunction p-n diodes (SJ-PNDs) are fabricated on both native GaN and low-cost sapphire substrates. GaN SJ-PNDs on GaN and sapphire both show a breakdown voltage (BV) of 1100 V, being at least 900 V higher than their 1-D PND counterparts. The differential specific on-resistance (RON,SP) of the two SJ-PNDs are both 0.3mΩ⋅ cm 2 , with the drift region resistance (RDR,SP) extracted to be 0.15mΩ⋅ cm 2 . The RON,SP∼BV trade-off is among the best in GaN-on-GaN diodes and sets a new record for vertical GaN devices on foreign substrates. The RDR,SP∼BV trade-off exceeds the 1-D GaN limit, fulfilling the superjunction functionality in GaN. 

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4. Systematic oxygen impurity reduction in smooth N-polar GaN by chemical potential control

   https://doi.org/10.1088/1361-6641/ac3638  

   Szymanski, Dennis; Wang, Ke; Kaess, Felix; Kirste, Ronny; Mita, Seiji; Reddy, Pramod; Sitar, Zlatko; Collazo, Ramon (November 2021, Semiconductor Science and Technology)

   Abstract Process chemical potential control and dislocation reduction were implemented to control oxygen concentration in N-polar GaN layers grown on sapphire substrates via metal organic chemical vapor deposition (MOCVD). As process supersaturation was changed from ∼30 to 3400, the formation energy of the oxygen point defect increased, which resulted in a 25-fold decrease in oxygen incorporation. Reducing dislocations by approximately a factor of 4 (to ∼10 9 cm −3 ) allowed for further reduction of oxygen incorporation to the low-10 17 cm −3 range. Smooth N-polar GaN layers with low oxygen content were achieved by a two-step process, whereas first a 1 µ m thick smooth N-polar layer with high oxygen concentration was grown, followed by low oxygen concentration layer grown at high supersaturation. 

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5. New Device Physics of Cross-Gap Electroluminescence in Unipolar-Doped InGaAs/AlAs RTDs

   https://doi.org/10.1109/DRC46940.2019.9046392  

   P. Fakhimi, W-D. Zhang (June 2019, IEEE Device Research Conference (DRC), Ann Arbor, MI (June 23-26, 2019).)

   Double barrier resonant tunneling diodes (DBRTDs) exhibit a characteristic negative differential resistance (NDR), which allows for high-speed oscillation and switching; e.g. In0.53Ga0.47As/AlAs DBRTDs for high-speed oscillation applications [1, 2]. Recently, a cross-gap electroluminescence (EL) phenomenon from InGaAs DBRTDs at room temperature was discovered despite the absence of p-doped layers [3]. This unipolar-doped EL had not been previously reported in the past 40+ years of RTD history. The indispensable holes for the light emission are thought to be produced by interband tunneling through the narrow bandgap of In0.53Ga0.47As, which is illustrated in Fig. 1. The radiative recombination is thought to occur primarily in the In0.53Ga0.47As emitter region, and thus the emission spectrum is near the bandgap of In0.53Ga0.47As (~1650 nm at 295 K). The EL property combined with high-speed modulation can be utilized for future high-speed optical clocking applications. In this abstract we report a more detailed characterization of unipolar-doped In0.53Ga0.47As/AlAs DBRTDs to gain a better understanding of the new EL phenomenon. The heterostructure was grown by molecular beam epitaxy (MBE) on a semi-insulating InP substrate. The heterostructure stack has a 4.4 nm thick unintentionally doped (UID) In0.53Ga0.47As quantum well and 2.4 nm UID AlAs barriers on either side [Fig. 2 (a)]. The device shown in Fig. 2 (b) was fabricated using four mask levels. The first mask level defined a 15 µm mesa. The mesas were etched, using inductively coupled plasma reactive ion etching (ICP-RIE) with a BCl3 gas mixture, down to the highly doped In0.53Ga0.47As bottom contact layer in the collector region. The second mask layer was used to isolate the individual devices. A conformal passivating SiO2 layer was then deposited using plasma enhanced chemical vapor deposition (PECVD). Contact vias were defined and etched with a CF4 gas mixture using ICP-RIE through the SiO2 layer with the third mask. The last mask was utilized to deposit the contact pads. The annular top (5 µm aperture) contact design allowed for more light to escape from the surface. The characterizations entailed the following measurements: (1) current-voltage (I-V); (2) light emission intensity versus bias voltage (L-V); (3) EL spectrum; (4) shot noise; and (5) EL temperature dependence. Fig. 3 shows the measured I-V and the L-V curves at T ≈ 300 K. The onset of NDR is at 2.2 V with a peak-to-valley ratio of ~8.3. The EL has a threshold at a lower voltage (~1.25 V). Fig. 4 shows both the full shot noise and the total RTD noise power spectral density of the In0.53Ga0.47As/AlAs DBRTD device at T ≈ 300K. The low shot noise at biases >3V into the second positive differential resistance (PDR) region supports our thesis that the holes for the light emission are produced by interband tunneling, not impact ionization [4]. Fig. 5 shows an overlay plot of the normalized EL spectra of an RTD biased at 3.5 V with peaks marked and the theoretically modeled bandgap of In0.53Ga0.47As at various temperatures. The consistency between the theoretical model and measured bandgap corresponding to the spectra peak further supports the theory that describes the light emission as being cross-gap, radiative recombination in the InGaAs emitter [Fig. 5, Fig. 1]. The decreasing trend in full width at half maxima (FWHM) with a decrease in temperature could be due to a reduction in phonon scattering [Fig. 6] [5]. The preliminary external quantum efficiency (EQE) measurements resulted in an EQE of ~0.44% for unipolar-doped In0.53Ga0.47As/AlAs DBRTDs. Balancing the electron resonant and interband tunneling currents via tunneling engineering can result in substantially improved EQEs. Acknowledgements: This work was supported by the U.S. National Science Foundation (under Grants #1711733 & #1711738), Program Director, Dr. Dimitris Pavlidis. References: [1] T. Broekaert et al, Appl. Phys. Lett. 53, 1545 (1988). [2] T. Inata et al, Japanese Journal of Applied Physics 26, L1332-L1334 (1987). [3] E. R. Brown et al, https://arxiv.org/abs/1804.07666. [4] E. R. Brown et al, https://arxiv.org/abs/1806.09270. [5] W. Z. Shen et al, National Lab. of Infrared Phys. 65, 2728 (1994). [6] E. Zielinski et al, J. Appl. Phys. 59, 2196 (1986). 

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