Optically Coupled Electrically Isolated, Monolithically Integrated

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Optically coupled electrically isolated, monolithically integrated switch using AlxGa1-xN/GaN high electron mobility transistor structures on Si (111) sandeep kumar, Anamika Singh Pratiyush, Surani Bin Dolmanan, Hui Ru Tan, Sudhiranjan Tripathy, Rangarajan Muralidharan, and Digbijoy Neelim Nath ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00084 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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ACS Applied Electronic Materials

Optically coupled electrically isolated, monolithically integrated switch using AlxGa1xN/GaN

high electron mobility transistor structures on Si (111)

Sandeep Kumar,1* Anamika Singh Pratiyush,1 Surani Bin Dolmanan,2 Hui Ru Tan,2 Sudhiranjan Tripathy,2 Rangarajan Muralidharan,1 and Digbijoy Neelim Nath1

1Centre

for Nano Science and Engineering (CeNSE), Indian Institute of Science (IISc),

Bangalore 560012, India 2Institute

of Materials Research and Engineering (IMRE), Agency for Science, Technology,

and Research (A*STAR), Innovis 08-03, 2 Fusionopolisway, 138634 Singapore KEYWORDS: UV detector, HEMT, spectral responsivity, broadband, switch

ABSTRACT:

In this letter, we report on an optically coupled, electrically isolated switch based on the IIInitride heterostructures on a silicon substrate. All the circuit elements, including the ultraviolet (UV) detector and a normally-on transistor, were monolithically integrated on a single AlxGa1-xN/GaN high electron mobility transistor (HEMT) stack grown on a 200 mm Si (111) substrate, and the process was fully compatible with conventional III-N HEMT *Corresponding

author email: [email protected] ACS Paragon Plus Environment

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fabrication flow. On-wafer UV detector was realized using a gate-recessed architecture which exhibited excellent spectral responsivity and broadband nature, while the discrete HEMT was found to exhibit ON current of 500 mA/mm. The integrated system was found to switch from an OFF-state-current of 5 A (UV-Off) to an ON-state-current of 50 mA (UV-ON) in ~4 ms and this delay may be further reduced with optimization of such a monolithic device design and improvement of crystal quality of nitrides on the large area Si platforms.

2 ACS Paragon Plus Environment

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INTRODUCTION: The nitrides-based high electron mobility transistors (HEMTs) for power switching and RF amplification are finding rapid market penetration while rivalling contemporary silicon and III-V based technologies1–3. Recently, GaN-based technologies for monolithic optoelectronic integrated circuits have attracted attention for new applications45. Integration of HEMT, light emitting diodes (LED) and detectors for voltage modulatable blue light source and optical interconnect/visible light communication are being reported678. In these reports, blue LED was used as an excitation light source and InGaN/GaN based multiple quantum well detector was used to detect the visible blue light. While III-nitrides are also maturing fast as a promising alternative to silicon for high-efficiency and filter-free UV detection, to the best of our knowledge, there exists no report on monolithic integration of a GaN-based ultraviolet (UV) detector device and a HEMT for possible application toward an optically coupled electrically isolated switch (OCEIS). In this study, we have used a recess etched UV detector (visible blind photo detector) realized on the AlxGa1-xN/GaN HEMT stack, which is easier to implement in the fabrication process as it does not require the additional selective area growth steps. The cost-effective AlxGa1-xN/GaN HEMT structures on large area Si substrates have attracted attention of numerous research groups from academia and industries. For the practical use of such AlxGa1-xN/GaN heterostructures for OCEIS, the abruptness of the interfaces of nitride epilayers are crucial to avoid deterioration in the electrical properties and unexpected electric field concentration. In this letter, we report on OCEIS all realized on an AlGaN/GaN HEMT stack grown on a 200mm diameter Si (111). The fabrication of OCEIS is fully compatible with normally-ON or normally-OFF HEMT process flows, and it can both be scaled up as well as be extended for high current/voltage rated devices.



Experimental details: MATERIAL GROWTH The ~4.1 µm thick HEMT stack on a 1.0 mm p-type Si (111) was grown by a metal organic chemical vapor deposition (MOCVD) technique. The MOCVD growth on the 200 mm Si (111) started with a low-temperature ~30 nm AlN nucleation layer followed by a 375 nm thick high-temperature AlN. About four step-graded AlGaN intermediate layers (Al contents in the layers were tuned from 58 to 4%), with an unintentional C-doped layer were used as buffer structures. The AlGaN layer with an Al content of 4% serves here as a moderately background C-doped layer for blocking voltage control. Such a layering avoids the need to overgrow a rather defective intentionally C-doped GaN which is detrimental to optical quality of the buffer and channel layers. The top two-dimension electron gas (2DEG) heterostructures comprise of a 300 nm undoped GaN channel layer, a thin ~1.0 nm AlN spacer, ~20 nm AlxGa1-xN barrier layer with x = 23%, and a ~2 nm thin GaN cap. The details of the MOCVD growth conditions and the thickness of the epitaxial layers are shown in the supporting information. Figures 1(A) and 1(B) show the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the starting HEMT epilayers grown on a 200 mm p-Si(111). The surface morphology of this HEMT stack measured by atomic force microscopy (AFM) shows a root mean square surface roughness of ~0.2 nm for 5.0 µm × 5.0 µm scans. Figures 1(C) and 1(D) represent the AFM images of the HEMT structure, where smooth surface steps are clearly seen, and the dislocation pits confined only at the step-edges. The high-resolution x-ray diffraction (HRXRD) measurements show an average rocking curve linewidth of about 1170, 473, and 1260 arcsecs for the AlN (002), GaN (002), and GaN (102) peaks, respectively. The broadened (102) GaN rocking curve linewidth is due to overlapping x-ray peaks from both GaN buffer and the Al0.04Ga0.96N intermediate layer. Hall measurements on the 200 mm epiwafer center to edge


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show an average sheet resistance of 372 ohm/sq with a sheet carrier concentration of about 1.1 × 1013 cm-2. Experimental details: DEVICE FABRICATION The schematic of OEICS system with its working principle is shown in the Fig. 2(A). The circuit diagram of the switch with its constituent elements is shown in the Fig. 2(B). The optical microscope image of the fabricated system (a detector (dashed red box), a HEMT (dashed blue box) and a 2DEG resistor (dashed white box) is shown in the Fig. 2(C). The cross-section of HEMT and detector are shown in Fig. 2(D) and Fig. 2(E) respectively. The device fabrication processes started with depositing Ti/Al/Ni/Au and then annealing it at 850 °C (30 s) for Ohmic contacts. Mesa etching of ~300 nm was done by Cl-based reactive ion etching (RIE). Recess etching (for UV-detector) of ~40 nm was done by RIE (Cl-chemistry) at low RF power (10 W). To clean the sample surface, sample was first exposed to O2 plasma and then dipped in a solution of BHF-HCl. 20 nm of-Al2O3 was deposited using atomic layer deposition (ALD) for gate dielectric in the HEMT. The oxide was selectively etched above the contact pads in BHF solution. Gate metal layer (Ni/Au) was e-beam evaporated and this metal layer was also used to connect the different components (detector, HEMT and resistance, bond pads). The sheet resistance of the HEMT epitaxial stack was utilized to fabricate the resistor with width of 2.5 m and length of 400 m. The detector width and recess length were 4×50 m and 3 m respectively. The access region (the detector region excluding the recess and contact) of UV detector was 3 m on the both sides of the recess. RESULTS and DISCUSSION: The UV-illuminated I-V measurement setup used in this study is reported elsewhere91011. The diameter of the UV light spot was ~2 mm which resulted in the illumination of the entire



circuit system including HEMTs and the resistors. The fabricated devices were first characterized as discrete elements under dark and 365 nm UV illuminated condition. Fig. 3(A) shows the spectral response (SR) of the detector at different applied bias. Although metal semiconductor metal (MSM), Schottky, p-i-n and avalanche diodes are the most widely studied photodetector configurations12, in this work, a different/unconventional type of photo detector is used. The fabricated detector can be considered as a conventional AlGaN/GaN high electron mobility transistor (HEMT) where the highly conducting sheet charge (i.e. twodimensional electron gas, 2DEG) is etched away between the Ohmic contacts of source and drain. The AlGaN intermediate layer with ~4% Al content was unintentionally doped (moderate background C-doping ~1017 cm-3) which resulted in an insulating nature of the film. The dark current of the detector constitutes of GaN buffer leakage current and current through the undoped GaN channel layer. As the contacts are Ohmic, the maximum fraction of the applied bias reaches to GaN buffer region. Under UV illumination, the generated e-h pair constitutes the photo current. Due to the above discussed reasons, a reasonably good photo to dark current ratio was achieved in these detectors even though the contacts are Ohmic. When normalized with the device area, the detector in this case exhibited an ultra-high responsivity >105 A/W at 365 nm while maintaining a photo to dark current ratio (IP/ID) of ~ 4000 (5 V) [Inset to Fig. 3(A)]. The ideal responsivity and measured responsivity can be expressed as q/h and Gq/h respectively where all the terms have usual meaning and G represents the gain of the device. Using these expressions, the estimated gain of the detector at 5 V was ~ 4×105. These detectors exhibit photoconductive gain as the contacts are Ohmic and material is intrinsic. The estimated gain of this detector was significantly high due to increased field in the recess region. Also, by employing the multi-finger architecture, the redundant area (the access region) was reduced compared to a similar UV detector reported previously9. Detailed operation (SR, gain mechanism, and intensity dependent characteristics) of a recess-etched


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UV detector on an InAlN/GaN HEMT stack was previously reported9. It is worth noting that the SR characteristics remained nearly flat in the range of 300 nm to 365 nm and responsivity drops to ~104 at 230 nm. The absorption of UV ( Switch ON Light-OFF=> Switch OFF

GaN Buffer C-doped Buffer

AlN

Si

-12

-8 -6

100µ

-4

10µ

-2 UV On

1µ 10m

20m 30m Time (s)

ACS Paragon Plus Environment

0 40m

Vg (V)

1m

Gate Voltage

-10

UV Off

10m

500 nm

Drain current Detector current

100m

I (A)

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Optically Coupled Electrically Isolated, Monolithically Integrated

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