More Like this

1. Improving vertical GaN p–n diode performance with room temperature defect mitigation

   https://doi.org/10.1088/1361-6641/ad10c3  

   Sultan Al-Mamun, Nahid ; Gallagher, James ; Jacobs, Alan G. ; Hobart, Karl D. ; Anderson, Travis J. ; Gunning, Brendan P. ; Kaplar, Robert J. ; Wolfe, Douglas E. ; Haque, Aman ( December 2023 , Semiconductor Science and Technology)

   Defect mitigation of electronic devices is conventionally achieved using thermal annealing. To mobilize the defects, very high temperatures are necessary. Since thermal diffusion is random in nature, the process may take a prolonged period of time. In contrast, we demonstrate a room temperature annealing technique that takes only a few seconds. The fundamental mechanism is defect mobilization by atomic scale mechanical force originating from very high current density but low duty cycle electrical pulses. The high-energy electrons lose their momentum upon collision with the defects, yet the low duty cycle suppresses any heat accumulation to keep the temperature ambient. For a 7 × 10⁵A cm⁻²pulsed current, we report an approximately 26% reduction in specific on-resistance, a 50% increase of the rectification ratio with a lower ideality factor, and reverse leakage current for as-fabricated vertical geometry GaN p–n diodes. We characterize the microscopic defect density of the devices before and after the room temperature processing to explain the improvement in the electrical characteristics. Raman analysis reveals an improvement in the crystallinity of the GaN layer and an approximately 40% relaxation of any post-fabrication residual strain compared to the as-received sample. Cross-sectional transmission electron microscopy (TEM) images and geometric phase analysis results of high-resolution TEM images further confirm the effectiveness of the proposed room temperature annealing technique to mitigate defects in the device. No detrimental effect, such as diffusion and/or segregation of elements, is observed as a result of applying a high-density pulsed current, as confirmed by energy dispersive x-ray spectroscopy mapping.

    

   more » « less
2. Effect of High Current Density Pulses on Performance Enhancement of Optoelectronic Devices

   https://doi.org/10.1149/2162-8777/ad28c8  

   Rahman, Md Hafijur ; Glavin, Nicholas ; Haque, Aman ; Ren, Fan ; Pearton, Stephen J. ( February 2024 , ECS Journal of Solid State Science and Technology)

   Thermal annealing is commonly used in fabrication processing and/or performance enhancement of electronic and opto-electronic devices. In this study, we investigate an alternative approach, where high current density pulses are used instead of high temperature. The basic premise is that the electron wind force, resulting from the momentum loss of high-energy electrons at defect sites, is capable of mobilizing internal defects. The proposed technique is demonstrated on commercially available optoelectronic devices with two different initial conditions. The first study involved a thermally degraded edge-emitting laser diode. About 90% of the resulting increase in forward current was mitigated by the proposed annealing technique where very low duty cycle was used to suppress any temperature rise. The second study was more challenging, where a pristine vertical-cavity surface-emitting laser (VCSEL) was subjected to similar processing to see if the technique can enhance performance. Encouragingly, this treatment yielded a notable improvement of over 20% in the forward current. These findings underscore the potential of electropulsing as an efficient in-operando technique for damage recovery and performance enhancement in optoelectronic devices.

    

   more » « less
3. Mitigating Heavy Ion Irradiation‐Induced Degradation in p‐type SnO Thin‐Film Transistors at Room Temperature

   https://doi.org/10.1002/pssa.202300392  

   Al-Mamun, Nahid Sultan ; Rasel, Md Abu Jafar ; Wolfe, Douglas E. ; Haque, Aman ; Schoell, Ryan ; Hattar, Khalid ; Ryu, Seung Ho ; Kim, Seong Keun ( September 2023 , physica status solidi (a))

   The study investigates the mitigation of radiation damage on p‐type SnO thin‐film transistors (TFTs) with a fast, room‐temperature annealing process. Atomic layer deposition is utilized to fabricate bottom‐gate TFTs of high‐quality p‐type SnO layers. After 2.8 MeV Au⁴⁺irradiation at a fluence level of 5.2 × 10¹² ions cm⁻², the output drain current and on/off current ratio (I_on/I_off) decrease by more than one order of magnitude, field‐effect mobility (μ_FE) reduces more than four times, and subthreshold swing (SS) increases more than four times along with a negative shift in threshold voltage. The observed degradation is attributed to increased surface roughness and defect density, as confirmed by scanning electron microscopy (SEM), high‐resolution micro‐Raman, and transmission electron microscopy (TEM) with geometric phase analysis (GPA). A technique is demonstrated to recover the device performance at room temperature and in less than a minute, using the electron wind force (EWF) obtained from low‐duty‐cycle high‐density pulsed current. At a pulsed current density of 4.0 × 10⁵ A cm⁻², approximately four times increase inI_on/I_offis observed, 41% increase inμ_FE, and 20% decrease in the SS of the irradiated TFTs, suggesting effectiveness of the new annealing technique.

    

   more » « less
4. Thermo-mechanical aspects of gamma irradiation effects on GaN HEMTs

   https://doi.org/10.1063/5.0087209  

   Rasel, Md Abu Jafar ; Stepanoff, Sergei P. ; Wetherington, Maxwell ; Haque, Aman ; Wolfe, Douglas E. ; Ren, Fan ; Pearton, Stephen ( March 2022 , Applied Physics Letters)

   We report thermal and mechanical responses accompanying electrical characteristics of depletion mode GaN high electron mobility transistors exposed to gamma radiation up to 10⁷rads. Changes in the lattice strain and temperature were simultaneously characterized by changes in the phonon frequency of E₂(high) and A₁(LO) from the on-state and unpowered/pinched off reference states. Lower doses of radiation improved electrical properties; however, degradation initiated at about 10⁶rads. We observed about 16% decrease in the saturation current and 6% decrease in the transconductance at the highest dose. However, a leakage current increase by three orders of magnitude was the most notable radiation effect. We observed temperature increase by 40% and mechanical stress increase by a factor of three at a dose of 10⁷rads compared to the pristine devices. Spatial mapping of mechanical stress along the channel identifies the gate region as a mechanically affected area, whereas the thermal degradation was mostly uniform. Transmission electron microscopy showed contrast changes reflecting a high vacancy concentration in the gate region. These findings suggest that localized stress (mechanical hotspots) may increase vulnerability to radiation damage by accommodating higher concentration of defects that promote the leakage current.

    

   more » « less
5. Photovoltaic Response of Thin-Film CdTe Solar Cells under Accelerated Neutron Radiation in a TRIGA Reactor

   Powell, Kaden ; Lund, Matthew ; Wang, Meng-Jen ; Qian, Yang ; Magginetti, David ; Reese, Matthew ; Cazalas, Edward ; Sjoden, Glenn ; Yoon, Heayoung ( June 2020 , Electronic Materials Symposium)

   Cadmium telluride (CdTe) solar cells are a promising photovoltaic (PV) technology for producing power in space owing to their high-efficiency (> 22.1 %), potential for specific power, and cost-effective manufacturing processes. In contrast to traditional space PVs, the high-Z (atomic number) CdTe absorbers can be intrinsically robust under extreme space radiation, offering long-term stability. Despite these advantages, the performance assessment of CdTe solar cells under high-energy particle irradiation (e.g., photons, neutrons, charged particles) is limited in the literature, and their stability is not comprehensively studied. In this work, we present the PV response of n-CdS / p-CdTe PVs under accelerated neutron irradiation. We measure PV properties of the devices at different neutron/photon doses. The equivalent dose deposited in the CdTe samples is simulated with deterministic and Monte Carlo radiation transport methods. Thin-film CdTe solar cells were synthesized on a fluorine-doped tin oxide (FTO) coated glass substrate (≈ 4 cm × 4 cm). CdS:O (≈ 100 nm) was reactively RF sputtered in an oxygen/argon ambient followed by a close-spaced sublimation deposition of CdTe (≈ 3.5 μm) in an oxygen/helium ambient. The sample was exposed to a 10 min vapor CdCl2 in oxygen/helium ambient at 430˚C. The samples were exposed to a wet CuCl2 solution prior to anneal 200ºC. A gold back-contact was formed on CdTe via thermal evaporation. The final sample contains 16 CdTe devices. For neutron irradiation, we cleaved the CdTe substrate into four samples and exposed two samples to ≈ 90 kW reactor power neutron radiation for 5.5 hours and 8.2 hours, respectively, in our TRIGA (Training, Research, Isotopes, General Atomics) reactor. We observed a noticeable color change of the glass substrates to brown after the neutron/gamma reactor exposure. Presumably, the injected high-energy neutrons caused the breaking of chemical bonds and the displacement of atoms in the glass substrates, creating point defects and color centers. The I-V characteristics showed noticeable deterioration with over 8 hour radiations. Specifically, the saturation current of the control devices was ≈ 25 nA increasing to 1 μA and 10 μA for the 5.5-hour and 8.2-hour radiated samples, respectively. The turn-on voltage of the control devices (≈ 0.85 V) decreased with the irradiated sample (≈ 0.75 V for 5.5-hour and ≈ 0.5 V for 8.2-hour exposures), implying noticeable radiation damage occurred at the heterojunction. The higher values of the ideality factor for irradiated devices (n > 2.2) compared to that of the control devices (n ≈ 1.3) also support the deterioration of the p-n junction. We observed the notable decrease in shunt resistance (RSH) and the increase in series resistance (Rs) with the neutron dose. It is possible that Cu ions introduced during the CuCl2 treatment may migrate into CdTe grain boundaries (GBs). The presence of Cu ions at GBs can create additional leakage paths for photocarrier transport, deteriorating the overall PV performance. We estimated the radiation dose of CdTe in comparison to Si (conventional PV) using a UUTR model (e.g., MCNP6 2D UTR Reactor simulations). In this model, we simulated Si and CdTe at the center point of the triangular fuel lattice and used an “unperturbed flux” tally in the water. Our simulations yielded a dose rate of 6916 Gy/s of neutrons and 16 Gy/s of photons for CdTe, and 1 Gy/s of neutrons and 21 Gy/s of photons for Si (doses +/- <1%). The large dose rate of neutrons in CdTe is mainly attributed to the large thermal neutron absorption cross-section of 113Cd. Based on this estimation, we calculate that the exposure of our CdTe PVs is equivalent to several million years in LEO (Low-Earth Orbit), or about 10,000 years for Si in LEO. Currently, we are working on a low-dose neutron/photon radiation on CdTe PVs and their light I-Vs and microstructural characterizations to gain better understanding on the degradation of CdTe PVs. 

   more » « less