Research

Our group works at the intersection of materials, electrical engineering and physics to study and engineer next-generation (ultra)wide band gap semiconductors such as Gallium Oxide. We are interested in epitaxial growth, electronic transport, design/modeling, micro/nano fabrication, and characterization of electronic/optoelectronic devices for a wide range of applications such as power electronics, high frequency electronics and ultra-violet optoelectronics. 

Current research projects are focused in the areas of

(1) Epitaxial growth (metal-organic vapor phase epitaxy & MBE ) of Gallium (Aluminum, Indium) Oxide thin films and heterostructures

(2) Lateral and Vertical Power devices 

(3) Analytical modeling/ simulation of novel device concepts/structures

Please take a look at the research group poster showcasing various activities within the research group

Last updated: April 2022

Poster credit: Carl Peterson & Saurav Roy

Research Statement

Last updated: September 2022

OVERVIEW: The Krishnamoorthy research group focuses on exploring and engineering next-generation (ultra)wide band gap semiconductor materials such as Gallium Oxide. Our efforts span epitaxial growth, electronic transport, design/modeling, microfabrication, and characterization of electronic devices, currently focused on power electronic materials and devices. Our research explores and investigates homo-epitaxial single crystal growth of monoclinic β-Ga2O3 (GO) films and heterostructures using metalorganic vapor phase epitaxy (MOVPE) to elucidate the fundamental growth science and realize material quality relevant for power electronic semiconductor devices with high performance metrics.

Power electronics is the technology responsible for the controlled flow of electric power based on solid-state power switches and circuits. Depending on the voltage rating, the applications range from consumer electronics (< 5- 200 V), electric mobility (600-1200 V), all the way to electric grid scale electronics (> 4.5 kV). Field-effect transistors (FETs) based on wide-band-gap (WBG) materials, namely silicon carbide (SiC) and gallium nitride (GaN), have emerged with an appreciably better performance compared to the existing Si-based devices. Looking forward, shifting to ultra-wide bandgap (UWBG) semiconductors can lead to significant enhancements in the system-level size, weight, and power (SWaP) efficiency which comes from the improvements in device-level conduction and switching losses. β-Ga2O3 (GO) is an emerging ultra-wide bandgap (UWBG) semiconductor[1] (Band gap of ~ 4.6 eV) with tremendous promise to enable next-generation power-efficient high voltage power devices and systems. The high projected breakdown field (∼8 MV/cm) of GO translates to BFOM (Baliga’s Figure of Merit) several times larger than that of SiC and GaN. With an added advantage of the availability of high-quality melt-grown bulk substrates, GO offers projected performance advantages over the WBG materials for high-power, high-temperature, and radiation-hard electronics while being potentially cost-effective. In the last decade of research and development, GO-based device performance has witnessed several milestones. Thanks to the rapid strides in crystal growth and devices, critical breakdown fields exceeding the theoretical limits of SiC and GaN have already been demonstrated - establishing GO as a prospective material for next-generation solid-state power-switching applications beyond GaN and SiC.  In the last five years (2017- 2022) our research group at U Utah and UCSB has substantial intellectual contributions in moving the state-of-the-art and realizing high quality materials, translating to record performance lateral and vertical diodes and transistors. We use metal organic vapor phase epitaxy (MOVPE) to realize homoepitaxial films with nanoscale control of electronic properties.

EPITAXIAL GROWTH AND DOPING OF GO/AGO THIN FILMS & HETEROSTRUCTURES

Rapid advances have been made over the last few years in metalorganic vapor phase epitaxy (MOVPE) homoepitaxial growth of GO and AGO, clearly establishing the promise of MOVPE growth technique.

Low temperature (LT) epitaxy: We have demonstrated the growth of high-quality UID (unintentionally doped) and Si-doped GO thin films on Fe-doped (010) bulk substrates at a growth temperature as low as 600ºC, which is more than 200 to 300ºC lower than the conventional MOVPE growth temperatures[2]. High purity films with very low acceptor compensation and excellent transport properties are demonstrated, evident from electronic transport characterization. This demonstration serves as an indirect experimental validation of theoretical predictions of higher diffusion lengths of Ga adatoms on a Ga2O3 surface as compared to GaN - helping in furthering the understanding of the growth window for β-Ga2O3 epitaxy. More recently, we have employed a LT buffer layer followed by HT doped channel layer to achieve highest electron mobility in any doped epitaxial Gallium oxide films, by controlling the surface segregation of compensating Fe species in the intentionally doped channel layers[3],[4]. Using LT epitaxy1, steep doping profiles are achieved with forward Si decay as sharp as 5 nm/dec (10x sharper than in films grown at 810oC). The ability to control the degree of dopant segregation is critical to achieving near- abrupt doping profiles in device structures. The lowered growth temperatures also opened up exciting pathways for heterointegration, enabling the growth of device-quality epitaxial layers on thermally engineered thin Ga2O3/SiC composite substrate platforms[5],[6]. This approach is a promising step towards solving a major challenge of low thermal conductivity in GO-based materials and the need for thermal management. We are also the first group to also demonstrate selective area regrowth of contact layers to channels using LT MOVPE approach[7] and report record low metal/GO specific contact resistance[8]. Extremely low metal/semiconductor specific contact resistance (~ 10-7 Ω.cm2) in an emerging UWBG semiconductor like Ga2O3 is noteworthy. We have leveraged the understanding and control achieved in these efforts to realize record breaking device performance as discussed later.

Delta and Modulation doping: Understanding and engineering doping profiles in highly engineered epitaxial structures is a prerequisite for any compound semiconductor technology. Our group has pioneered delta and modulation doping in MOVPE-grown GO. Spatially separating the donors from electrons in the channel reduces impurity scattering and improves the overall electron mobility of the channel. The primary focus of this work is to realize a modulation-doped two-dimensional electron gas (2DEG) channel at the AGO/GO heterointerface. The 2DEG density at the AGO/GO heterostructure is a function of the conduction band offset, spacer layer thickness and the delta sheet charge doping density in AGO. The band offset at AGO/GO heterostructure is determined by the composition of the AGO alloy layer, limiting the maximum charge that can be quantum confined. To understand the 2DEG formation it is important to study growth and doping of AGO and delta-doping in GO and AGO. MOVPE Growth of pseudomorphic Si-doped AGO was studied and degenerate doping was realized[9], making it one of the most conducting films for a UWBG material (~ 5 eV). Delta-doping in MOVPE-grown GO was realized using a growth interruption-based process. The silicon incorporation and activation studied using secondary-ion mass spectroscopy and capacitance-voltage measurements, revealed surface riding of dopants as a function of growth temperature[10]. By reducing the growth temperature to minimize surface riding, sharp doping profile with a CV measured FWHM of ~3 nm is achieved[11]. By combining the insights from all the previous experiments, GO heterostructure channel with a record low sheet resistance of 5.3 kΩ/square is achieved[12]. Further reduction in the barrier doping resulted in the elimination of parallel conduction in GO, resulting in the formation of a pure 2DEG channel with 1012 cm-3 electron density[13], which is the first demonstration of pure 2DEG in MOCVD-grown GO. In the future, we plan to explore dynamic phonon screening-induced mobility enhancement in higher charge density 2DEG channels. These early experimental reports are expected to serve as the baseline for future developments in high electron mobility channels for potential high frequency device applications.

ELECTRIC FIELD MANAGEMENT APPROACHES

 Electric field management is crucial for realizing the intrinsic potential of UWBG semiconductors. In traditional power semiconductors, complementary doping and guard ring structures are widely used, but with the absence of p-type doping in GO, innovative methods are required. (i) We have designed a GO- based Schottky rectifier with p-type III-Nitride guard rings[14] to minimize electric fields at the electrode edges. We have proposed the hetero-integration of p-type III-Nitrides with β-Ga2O3 around the electrode edges where maximum electric field is expected. We have shown using TCAD simulations that such devices can essentially minimize the corner peak fields and can reach very high breakdown voltages. We plan to pursue experiments in this regard. (ii) Dielectric polarization in extreme permittivity dielectrics can be employed in the absence of p-type GO to reduce the peak electric fields in GO. We have demonstrated GO- based Schottky diode with high BFOM using extreme permittivity dielectric as field plate oxide to minimize edge electric fields[15]. We have used sputtered BaTiO3 ( k ~ 140) and BaTiO3/SrTiO3 superlattice (k ~ 325) as the field plate oxide on (001) oriented halide vapor phase epitaxy-grown (HVPE) β-Ga2O3 drift layer. The use of high permittivity material as field plate oxide redistributes the electric field over wide region and thus minimizes the electric field crowding at the corners. The demonstrated device with specific on-resistance of 0.32 mΩ-cm2 and a breakdown voltage of -687 V has a BFOM value of 1.47 GW/cm2, which is one of the highest reported values for β-Ga2O3 based devices. The high breakdown strength performance is achieved without sacrificing the on-resistance or forward voltage drop. (iii) We have designed[16] and implemented GO- based Schottky diode with extreme permittivity material as a means to mimic doped superjunction devices. The conventional superjunction devices use repeating p/n type junctions to balance the charge in the device and can surpass the theoretical figure of merit of unipolar devices. We have designed and implemented structures where the high permittivity dielectric polarization charge balances the depletion charge in the drift layer. We have successfully demonstrated a lateral β-Ga2O3 based high permittivity dielectric superjunction schottky barrier diode. Considering the current carrying regions, these lateral diodes cross the SiC unipolar figure of merit[17]. This is a very encouraging milestone considering that GO research and development is still in the early stages. (iv) We have demonstrated the growth and characterization of MOCVD in-situ Al2O3 dielectric on β-Ga2O3 where the dielectrics are grown in the same chamber as the underlying semiconducting film to achieve a pristine and contamination free dielectric/semiconductor interface[18]. Study of high-quality dielectrics is very important for UWBG materials as they are limited by the dielectric breakdown. By growing in-situ dielectric using MOCVD, we have demonstrated significant reduction of interface trap densities and also demonstrated promising dielectric high breakdown fields, more recently crossing even 10 MV/cm[19]. This approach of dielectric deposition can be used to deposit high quality gate oxides for metal oxide semiconductor field effect transistor (MOSFET) for high performance applications.

LATERAL AND VERTICAL POWER DEVICES

Our group is equally focused on device engineering and demonstration, which also serves a test vehicle for the high-quality epitaxial materials developed within the group. We reported the first demonstration of MOVPE-regrown ohmic contacts in an all MOVPE-grown β-Ga2O3 metal-semiconductor field-effect transistor (MESFET) with improved ON-state performance. The ON current value of 130 mA/mm achieved here is the highest for a depletion mode β-Ga2O3 MESFET with an ON-OFF ratio of over 1010 simultaneously in a large gate length device7. The selective-area-epitaxy of regrown ohmic contacts has eliminated the need for gate region recessing in devices with non-planar source/drain contacts. This development will be immensely beneficial for lowering ON-state conduction losses and minimizing OFF-state dissipation losses for β-Ga2O3 devices. To date, Ga2O3 lateral devices could not achieve a high lateral figure of merit (LFOM) and kilo-volt (kV) breakdown voltages simultaneously. As the next milestone in this research, by using carefully designed field management techniques, we have demonstrated VBR vs. LGD linearity close to 3 kV in MOVPE-Ga2O3 lateral MESFETs. We have addressed the critical metrics of VBR (breakdown voltage), Ron,sp (specific on-resistance), and EAVG (average breakdown electric field) at the same time -  improving significantly over the state-of-the-art reports. A record-high LFOM of 355 MW/cm2, a VBR of ∼2.5 kV, and EAVG of ∼2.5 MV/cm simultaneously are demonstrated in a β-Ga2O3 MESFET with LGD = 10 μm[20]. This LFOM value is the highest for any β-Ga2O3 lateral device with VBR > 2 kV - setting new state-of-the-art standards in β-Ga2O3 device technology. Multi-kilovolt (up to 4.5 kV) class β-Ga2O3 transistors were demonstrated with state-of-the-art power figures of merit exceeding several times the theoretical maximum of Silicon[21]. High-current and VBR (up to ~ 3kV) epitaxial β-Ga2O3 MOSFETs are realized on an engineered β-Ga2O3/SiC composite substrate – for enhanced bottom-side device cooling. Tri-Gate β-Ga2O3 MESFETs with record high power figure of merit (~0.95 GW/cm2) are also demonstrated3. These device engineering results presents some of the key milestones in Ga2O3 epitaxy/devices and provides the foundational steps towards next-generation energy efficient power electronics. These efforts are summarized in figure 2, where our device performance is benchmarked with literature reports on the Ron,sp – VBR plot. It graphically explains how with improvement in each generation of device, material and structure/design, the device performance was enhanced. UWBG β-Ga2O3 material and device technology is maturing rapidly and offers enormous opportunities for improved energy efficiency by minimizing energy waste. We believe our research can broaden the range of technologies to make electric power usage more efficient and environment-friendly, thus, providing a more sustainable technology space for generations to come.

REFERENCES

[1] Higashiwaki, Masataka, and Gregg H. Jessen. "Guest Editorial: The dawn of gallium oxide microelectronics." Applied Physics Letters 112, no. 6 (2018): 060401.

[2] Bhattacharyya, Arkka, Praneeth Ranga, Saurav Roy, Jonathan Ogle, Luisa Whittaker-Brooks, and Sriram Krishnamoorthy. "Low temperature homoepitaxy of (010) β-Ga2O3 by metalorganic vapor phase epitaxy: Expanding the growth window." Applied Physics Letters 117, no. 14 (2020): 142102.

[3] Arkka Bhattacharyya, Saurav Roy, Praneeth Ranga, Carl Peterson and Sriram Krishnamoorthy, "High-Mobility Tri-Gate β-Ga2O3 MESFETs with a Power Figure of Merit over 0.9 GW/cm2," in IEEE Electron Device Letters, 2022, doi: 10.1109/LED.2022.3196305.

[4] Arkka Bhattacharyya, S. Roy, P. Ranga, S. Krishnamoorthy, "High Electron Mobility Si-doped β-Ga2O3 MESFETs", 5th U.S. Gallium Oxide Workshop (GOX 2022) , August 2022 Washington DC. (Manuscript in Prep).

[5] Yiwen Song, Arkka Bhattacharyya, Anwarul Karim, Daniel Shoemaker, Hsien-Lien Huang, Saurav Roy, Craig McGray, Jacob H. Leach, Jinwoo Hwang, Sriram Krishnamoorthy*,6, Sukwon Choi*,"Ultra-Wide Bandgap Ga2O3-on-SiC MOSFETs", Nature Electronics (Submitted). *co-corresponding authors.

[6] Song, Yiwen, Daniel Shoemaker, Jacob H. Leach, Craig McGray, Hsien-Lien Huang, Arkka Bhattacharyya, Yingying Zhang et al. "Ga2O3-on-SiC composite wafer for thermal management of ultrawide bandgap electronics." ACS Applied Materials & Interfaces 13, no. 34 (2021): 40817-40829.

[7] Bhattacharyya, Arkka, Saurav Roy, Praneeth Ranga, Daniel Shoemaker, Yiwen Song, James Spencer Lundh, Sukwon Choi, and Sriram Krishnamoorthy. "130 mA mm− 1 β-Ga2O3 metal semiconductor field effect transistor with low-temperature metalorganic vapor phase epitaxy-regrown ohmic contacts." Applied Physics Express 14, no. 7 (2021): 076502.

[8] Fikadu Alema*, Carl Peterson*, Arkka Bhattacharyya, Saurav Roy, Sriram Krishnamoorthy and Andrei Osinsky, "Low Resistance Ohmic contact on epitaxial MOVPE grown β-Ga2O3 and β-(AlxGa1-x)2O3 films," in IEEE Electron Device Letters, 2022, doi: 10.1109/LED.2022.3200862. *Equal contribution

[9] Ranga, Praneeth, Ashwin Rishinaramangalam, Joel Varley, Arkka Bhattacharyya, Daniel Feezell, and Sriram Krishnamoorthy. "Si-doped β-(Al0. 26Ga0. 74) 2O3 thin films and heterostructures grown by metalorganic vapor-phase epitaxy." Applied Physics Express 12, no. 11 (2019): 111004.

[10] Ranga, Praneeth, Arkka Bhattacharyya, Ashwin Rishinaramangalam, Yu Kee Ooi, Michael A. Scarpulla, Daniel Feezell, and Sriram Krishnamoorthy. "Delta-doped β-Ga2O3 thin films and β-(Al0. 26Ga0. 74) 2O3/β-Ga2O3 heterostructures grown by metalorganic vapor-phase epitaxy." Applied Physics Express 13, no. 4 (2020): 045501.

[11] Ranga, Praneeth, Arkka Bhattacharyya, Adrian Chmielewski, Saurav Roy, Nasim Alem, and Sriram Krishnamoorthy. "Delta-doped β-Ga2O3 films with narrow FWHM grown by metalorganic vapor-phase epitaxy." Applied Physics Letters 117, no. 17 (2020): 172105.

[12] Ranga, Praneeth, Arkka Bhattacharyya, Adrian Chmielewski, Saurav Roy, Rujun Sun, Michael A. Scarpulla, Nasim Alem, and Sriram Krishnamoorthy. "Growth and characterization of metalorganic vapor-phase epitaxy-grown β-(Al x Ga1− x) 2O3/β-Ga2O3 heterostructure channels." Applied Physics Express 14, no. 2 (2021): 025501.

[13] Praneeth Ranga, PhD Thesis, University of Utah (Embargoed until Jan 2023)- Available upon request.

[14] Roy, Saurav, Arkka Bhattacharyya, and Sriram Krishnamoorthy. "Design of a β-Ga 2 O 3 Schottky Barrier Diode With p-Type III-Nitride Guard Ring for Enhanced Breakdown." IEEE Transactions on Electron Devices 67, no. 11 (2020): 4842-4848.

[15] Roy, Saurav, Arkka Bhattacharyya, Praneeth Ranga, Heather Splawn, Jacob Leach, and Sriram Krishnamoorthy. "High-k oxide field-plated vertical (001) β-Ga 2 O 3 Schottky barrier diode with Baliga’s figure of merit over 1 GW/cm 2." IEEE Electron Device Letters 42, no. 8 (2021): 1140-1143.

[16] Roy, Saurav, Arkka Bhattacharyya, and Sriram Krishnamoorthy. "Analytical Modeling and Design of Gallium Oxide Schottky Barrier Diodes Beyond Unipolar Figure of Merit Using High-k Dielectric Superjunction Structures." arXiv preprint arXiv:2008.00280 (2020).- Modified manuscript in prep.

[17] S. Roy, A. Bhattacharyya, C. Peterson and S. Krishnamoorthy, “High Voltage β-Ga2O3 Lateral Schottky barrier diode with High Permittivity Dielectric RESURF demonstrating > 1 GW/cm2 Power Figure of Merit.” In 80th Device Research Conference (DRC) 2022. (Oral Presentation- Late News)- Submitted to IEEE Electron Device Letters (2022).

[18] Roy, Saurav, Adrian E. Chmielewski, Arkka Bhattacharyya, Praneeth Ranga, Rujun Sun, Michael A. Scarpulla, Nasim Alem, and Sriram Krishnamoorthy. "In Situ Dielectric Al2O3/β‐Ga2O3 Interfaces Grown Using Metal–Organic Chemical Vapor Deposition." Advanced Electronic Materials 7, no. 11 (2021): 2100333.

[19] Saurav Roy, A. Bhattacharyya, C. Peterson, S. Krishnamoorthy, "High Temperature In-situ MOCVD-grown Al2O3 Dielectric on (010) βGa2O3 with 10 MV/cm Breakdown Field", 5th U.S. Gallium Oxide Workshop (GOX 2022) , Washington DC.- Manuscript in prep.

[20] Bhattacharyya, Arkka, Praneeth Ranga, Saurav Roy, Carl Peterson, Fikadu Alema, George Seryogin, Andrei Osinsky, and Sriram Krishnamoorthy. "Multi-kV Class β-Ga₂O₃ MESFETs With a Lateral Figure of Merit Up to 355 MW/cm²." IEEE Electron Device Letters 42, no. 9 (2021): 1272-1275.

[21] Bhattacharyya, Arkka, Shivam Sharma, Fikadu Alema, Praneeth Ranga, Saurav Roy, Carl Peterson, Geroge Seryogin, Andrei Osinsky, Uttam Singisetti, and Sriram Krishnamoorthy. "4.4 kV β-Ga2O3 MESFETs with power figure of merit exceeding 100 MW cm− 2." Applied Physics Express 15, no. 6 (2022): 061001.