In this work, we demonstrate a pseudomorphic Al0.87Ga0.13N/Al0.64Ga0.36N metal-oxide-semiconductor HFETs (MOSHFET) on a single crystal bulk AlN substrate with linear ohmic contacts (RC ~ 4.3 Ωmm) enabled by a 30 nm thick reverse composition (x = 87% to 40%) graded contact making AlxGa1-xN layer using MOCVD. In addition to the thermal benefits, AlN substrates allow the growth of pseudomorphic epilayer structures for extreme bandgap (EBG) AlxGa1-xN channel HEMT devices (Al-composition > 60%, bandgap > 4.5 eV). The measured threading dislocation densities in our pseudomorphic epilayers were < 104 cm-2 which should lead to significantly lower gate-leakage thus promoting device stability.[1] These transistor devices showed Baliga Figure of Merit (BFOM) of 460 MW/cm2, which to date represents state-of-the-art performance for AlGaN-channel devices with similar composition. Device fabrication began with mesa isolation using Cl2-based ICP-RIE followed by Zr-stack metal deposition by e-beam evaporation as source/drain contacts, and rapid thermal annealing at 950 °C for 30 s under N2 environment. This was followed by an ICP-RIE etch of the graded layer from the access region, 10 nm SiO2 deposition as the gate oxide, Ni/Au gate, and Ti/Ni/Au probe pads deposition. Finally, the devices were embedded in 400 nm thick SiO2 film by PECVD to relieve the surface electric field for the breakdown voltage measurements. In addition to the bulk AlN substrates, identical devices were also fabricated where the same epilayer structure was deposited on AlN templates. These innovations enabled MOSHFET devices with on-state peak drain current density 610 mA/mm (at +2V gate bias), on-off ratio (107), a three-terminal off-state breakdown field > 3.7 MV/cm, and BFOM of 460 MW/cm2. Further optimization of surface passivation and edge termination could improve the effective off-state critical breakdown field (EC) to > 10 MV/cm, as measured in 2-terminal measurements by our team and others.[2,3] Deep scaling of EBG AlGaN power devices is enabled by this high EC. Compact devices have the additional advantage of reduced capacitance and other parasitics, allowing faster switching. However, the increased power density also leads to thermal limitations in overall system performance, e.g., current droop and premature failure, limiting deep-scaling of III-N power devices, due to poor heat removal through the currently used substrates with poor thermal conductivity (silicon, sapphire, Ga2O3).[4] We demonstrate using a novel thermal titration technique that the device on single crystal bulk AlN compared to identical structures on sapphire substrates shows 70% reduction in thermal impedance to 10 K-mm/W, a value that is comparable to SiC and copper heat-sinks. We will show that the improved electrical performance above is due to this enhanced thermal management from the bulk AlN substrate, arising from its intrinsic excellent thermal conductivity and high-temperature stability.[5] These advantages of EBG III-nitrides are attractive for post-GaN deep-scaled efficient and compact co-designed power electronics with passive thermal management, such as in consumer electronic chargers, and next-generation electric vehicles (EV).[6] [1] S. Rathkanthiwar, J. Houston Dycus, S. Mita, R. Kirste, J. Tweedie, R. Collazo, & Z. Sitar. Applied Physics Letters, 120(20), 202105 (2022). [2] K. Hussain, A. Mamun, R. Floyd, M. D. Alam, M. E. Liao, K. Huynh, ... & A. Khan. Applied Physics Express. (2023). [3] D. Khachariya, S. Mita, P. Reddy, S. Dangi, J. H. Dycus, P. Bagheri, ... & S. Pavlidis. Applied Physics Letters, 120(17), 172106 (2022). [4] R. L. Xu, M. M. Rojo, S. M. Islam, A. Sood, B. Vareskic, A. Katre, ... & E. Pop. Journal of Applied Physics, 126(18), 185105 (2019). [5] N. Yafune, S. Hashimoto, K. Akita, Y. Yamamoto, H. Tokuda, & M. Kuzuhara. Electronics letters, 50(3), 211-212 (2014). [6] T. Kachi. Japanese Journal of Applied Physics, 53(10), 100210 (2014).

Al0.64Ga0.36N Channel MOSHFET on Single Crystal Bulk AlN Substrate for Co-Designed Power Electronics

Extreme bandgap (EBG) semiconductors such as AlxGa1-xN (x>0.6, bandgap energy EG > 4.88 eV) are ideal for high power, high voltage, and high temperature power electronics. Their critical breakdown field (Ec) in lateral devices scales as EG2.5, leads to an estimated value of the Baliga’s Figure of Merit (BFOM) significantly higher than that for GaN[1],[2],[3],[4] For achieving high BFOM, in addition to a large breakdown voltage (VBR), a simultaneous low device specific ON-resistance(RON,SP) is also required. For the EBG AlxGa1-xN channel HEMTs to have a low RON,SP , a 2-dimensional electron gas (2DEG) concentration >1x1013 cm-2 is needed, requiring a barrier AlxGa1-xN layer with x > 0.80. At such high value of “x”, it is challenging to make linear ohmic contacts due to large Schottky barrier height between metal-AlGaN interface. To solve these issues, we demonstrate EBG n-Al0.87Ga0.13N-Al0.64Ga0.36N heterostructures over bulk AlN substrate grown by Low Pressure-MOCVD. To facilitate ohmic contact formation, highly doped reverse-graded AlGaN layer(x=0.87 to 0.40) was grown in-situ. Reciprocal Space Map(105) and AFM analysis revealed pseudomorphic epilayers with step-flow morphology and RMS roughness ~0.18 nm. X-Ray Topography estimation of control sample gave us a TDD ~ 7 x 103 cm-2, indicating the high quality of our pseudomorphic epilayers. A planar Zr-based metal stack was deposited and subsequent removal of the reverse graded n-AlxGa1-xN layer in the access region gave linear ohmic contacts, RC~4.3 Ω-mm. Additionally, planar metal-oxide-semiconductor (MOS) diodes were fabricated and 2-terminal gate-source breakdown measurement was performed. A VBR of 739 V was measured at metal flashover point for 0.65 µm gate-source spacing, giving an electric field value ~ 11.37 MV/cm. From the breakdown data and the RON,SP (~2.4 × 10-4 Ω-cm2) value, we estimate the BFOM for our EBG n-Al0.87Ga0.13N-A0.64Ga0.36N heterostructures to be as high as 2.27 GW/cm2. This, to the best of our knowledge is the highest reported value to date for an extreme bandgap AlxGa1-xN (x>0.6) channel heterostructure.[5] References [1] B. J. Baliga, IEEE Electron Dev. Lett., 10, 455 (1989). [2] R. J. Kaplar, A. A. Allerman, A. M. Armstrong, M. H. Crawford, J. Ray Dickerson, A. J. Fischer, A. G. Baca, and E. A. Douglas, ECS J. Solid State Sci., 6, Q3061 (2016). [3] T. Nanjo, A. Imai, Y. Suzuki, Y. Abe, T. Oishi, M. Suita, E. Yagyu, and Y. Tokuda, IEEE Trans. Electron Devices, 60, 1046 (2013). [4] N. Yafune, S. Hashimoto, K. Akita, Y. Yamamoto, H. Tokuda, and M. Kuzuhara, Electronics Lett., 50, 211 (2014) [5] K. Hussain et al., Appl. Phys. Express (2023)

High Figure of Merit Extreme Bandgap Al0.87Ga0.13N-Al0.64Ga0.36N Heterostructures Over Bulk AlN Substrates

Abstract Body: We demonstrate a technique to spatially map and quickly build the frequency response of optoelectronic devices using optical probes. The transfer function of a linear system is the Fourier transform of its impulse response, which may be obtained from transient photocurrent measurements of devices such as photodetectors and solar cells. We introduce and apply Fourier Transform Impedance Spectroscopy (FTIS) to a PbS colloidal quantum dot (QD)/SiC heterojunction photodiode and corroborate results using intensity-modulated photocurrent spectroscopy. The cutoff frequencies of the QD/SiC devices were as high as ~10kHz, demonstrating their utility in advanced flexible and thin film electronics. The practical frequencies for FTIS lie in the mHz-kHz range, ideal for composite or novel materials such as QD films that are dominated by interfacial trap states. These can lead to characteristic lengths for charge collection ~20-500 μm dominated by transmission line effects, rather than intrinsic diffusion and drift length scales, enabling extraction of interfacial capacitances and series/parallel resistances.

Spatially Resolved Fourier Transform Impedance Spectroscopy—A Technique to Rapidly Characterize Composite Interfaces and a Study of Quantum Dot/Epitaxial Graphene/SiC Optoelectronic Devices

Abstract Body: Ultrawide-bandgap (UWBG) Al-rich (x ≥ 0.4) AlGaN metal oxide heterostructure field-effect-transistors (MOSHFETs) are ideal device candidates for solar-blind detection due to their internal transistor gain and inherent solar blind detection (λ ≤ 300nm) ability. We previously reported the deep-trap energies and physical locations responsible for slower transient time in Al0.65Ga0.35N/Al0.45Ga0.55N MOSHFETs and suggested using back-barrier (BB) to reduce the trapping effects by screening the channel from the growth interface. We, now, report the optical response and current transient behaviors of AlxGa1-xN MOSHFETs with a fixed Al0.65Ga0.35N back-barrier and linearly graded AlxGa1-xN back-barrier. The transfer curves (Id-Vg) shift towards more negative (~0.63V for fixed BB and ~1.05V for graded BB) under light (λ ~ Eg,channel) in both cases indicates the presence of photoelectrons. The MOSHFETs recovered with light turned off, although a small threshold shift remained (Vth,shift), by 0.15 and 0.45V (traps ~ 8×1011 and 6.5×1011 cm-2) in 3 minutes for fixed and graded BB MOSHFETs respectively. Peak responsivities of 3.1×105 and 2.3×105A/W in the saturation mode for fixed and graded BB MOSHFETs were quenched by one order with strong 365nm UV light used to fill the traps. Consequently, the off-transient time of over 10 minutes reduced to ≤ 12s under 365nm light, while the recovery transient became more symmetric, indicating the presence of deep-traps that were filled in the back-barrier MOSHFETs. These results show that the BB does not have a significant impact on the speed of these devices due to the presence of these deep traps, We will present gate length (LG) dependent transient measurements with gated-transmission line model (GTLM) MOSHFET devices without BB to isolate transients arising from the access region and from under the channel. Our initial analysis suggests that the transients are similar for both, strongly indicating that the growth interface where the BB was placed is still responsible for slowing down the device. This result is surprising given that the density of those interface traps is ~5×1011 cm-2, much smaller compared to the polarization charge introduced by the BB ~1013cm-2, suggesting that clustering of point defects near dislocations may play a role as in GaN pn diodes.

A Comparative Study of Slow Current Transients in Al0.4Ga0.6N Channel MOSHFET’s with Back Barriers

A: Group III–Nitrides—Devices

technical paper

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC). This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

EMC 2023

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. 

O: Group III–Nitrides—Novel Nitride Growth

(EMC) I: Silicon Carbide Theory, Characterization and Device Processing 

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC), which will be held during the same week, from June 20-23. This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

![](https://assets.underline.io/uploads/markdown_image/1/image/eb26c24a18d5d1030616afa4f993cf89.png)
	
<a href="https://www.mrs.org/docs/default-source/meetings-events/mrs-conference-services/2021/emc-2021/2021-emc-program-at-a-glance-6-16-21.pdf?sfvrsn=e39f950c_4">Latest Program-at-a-Glance</a>

<a href="https://drive.google.com/file/d/1eiS1ao4p83VNy8xgHs3Rls7gTQjx7Rc4/view?usp=sharing">Full Program with Abstracts</a>

Asif Khan

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Presentations

Al0.64Ga0.36N Channel MOSHFET on Single Crystal Bulk AlN Substrate for Co-Designed Power Electronics

High Figure of Merit Extreme Bandgap Al0.87Ga0.13N-Al0.64Ga0.36N Heterostructures Over Bulk AlN Substrates

Spatially Resolved Fourier Transform Impedance Spectroscopy—A Technique to Rapidly Characterize Composite Interfaces and a Study of Quantum Dot/Epitaxial Graphene/SiC Optoelectronic Devices

A Comparative Study of Slow Current Transients in Al0.4Ga0.6N Channel MOSHFET’s with Back Barriers

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