Mihir Tungare

Design and Growth of Visible-Blind and Solar-Blind III-N APDs on Sapphire Substrates

GaN-based visible-blind and AlGaN-based solar-blind avalanche photodiodes (APDs) have been grown and fabricated on sapphire substrates. The GaN p-i-n APDs show low dark current with high gain. The AlGaN layers for the Al0.55Ga0.45N-based APDs are grown using a newly developed pulsed metalorganic chemical vapor deposition (MOCVD) process, and the material characterization results show excellent material quality. The spectral responsivity of the devices show a bandpass characteristic with cutoffs in the ultraviolet (UV) visible-blind and solar-blind spectrum for GaN- and Al0.55Ga0.45N-based APDs, respectively.

Enhanced performance of an AlGaN/GaN high electron mobility transistor on Si by means of improved adatom diffusion length during MOCVD epitaxy

Four types of AlGaN/GaN high electron mobility transistor (HEMT) structures have been epitaxially grown on Si substrates by metalorganic chemical vapor deposition (MOCVD) and fabricated into devices. To achieve crack-free device structures, various stress-engineering methods have been employed including the use of AlGaN/AlGaN-graded layers, insertion of low-temperature AlN layers and ion implantation of the AlN/Si substrate. To improve material quality, pulsed MOCVD is used to enhance adatom diffusion length during (Al) GaN epitaxy of various layers in the HEMT structure. A comparison between structural and morphological characteristics of the HEMTs shows improvement in the (0 0 0 2) symmetric rocking curve value to 837.9 s−1 and the surface roughness of 0.21 nm for HEMT structures grown using pulsed epitaxy. An OFF-state breakdown voltage of 217 V at a drain current of 1 mA mm−1 at Vg = −8 V was measured for the structure with enhanced material quality.

Modification of dislocation behavior in GaN overgrown on engineered AlN film-on-bulk Si substrate

The changes that the AlN buffer and Si substrate undergo at each stage of our substrate engineering process, previously shown to lead to a simultaneous and substantial reduction in film crack density and dislocation density in overgrown GaN, are presented. Evidence of ion-implantation assisted grain reorientation for AlN islands coupled with physical isolation from the bulk Si substrate prove to be the dominating driving forces. This is further emphasized with x-ray diffraction analysis that demonstrates a reduction in the in-plane lattice constant of AlN from 3.148 A to 3.113 A and a relative change in rotation of AlN islands by 0.135° with regard to the Si substrate after substrate engineering. Misfit dislocations at the AlN-Si interface and disorder that is normally associated with formation of amorphous SiNx at this interface are considered to be two of the major contributors to dislocation nucleation within overgrown GaN. Following our technique, the disappearance of disorder at the AlN-Si interface ...

Density functional calculations of the binding energies and adatom diffusion on strained AlN (0001) and GaN (0001) surfaces

Density functional calculations were carried out to study the binding energies and diffusion barriers of various adatoms on AlN and GaN (0001) surfaces. The binding energies and potential energy surfaces were investigated for Al, Ga, and N adatoms on both Al (Ga) terminated and N terminated (0001) surfaces of AlN (GaN). Calculations were performed to investigate the diffusion paths and obtain diffusion energy barriers of these adatoms. It was found that the N adatom on N terminated AlN and GaN surfaces faces a high diffusion barrier due to strong N-N bond. The Al and Ga adatom on Al (Ga) terminated AlN (GaN) surfaces showed lower diffusion barriers due to the weak metallic bonds. However, the diffusion barrier for an Al adatom was always larger than that of a Ga adatom on any surface. To investigate the effect of strain on diffusion barriers the surfaces were subjected to a hydrostatic compressive and tensile strain in the range of 0 to 5%. The diffusion energy barrier for N adatom on N terminated AlN and GaN surfaces decreased when the strain state was changed from tensile to compressive. In contrast, Al and Ga adatoms show continuous increase in diffusion barriers from tensile to compressively strained Al (Ga) terminated AlN (GaN) surfaces.

GaN Power Schottky Diodes

With its wide bandgap and associated high critical field, GaN is a suitable material for high power electronics. Our group has identified carbon incorporation into the drift region as problematic for the development of GaN power Schottky diodes. The likely source of carbon is from the trimethylgallium source during metalorganic chemical vapor deposition (MOCVD) growth. Diodes grown at low pressure (100 Torr) have a high onresistance, high turn-on voltage and display a snap-on effect. These observations are independent of MOCVD growth system where similar results were obtained for diodes with films grown at 100 Torr at multiple institutions. Similar results were also observed for diodes with films grown on sapphire or freestanding hydride vapor phase epitaxy (HVPE) GaN substrates. Adjusting growth pressure, temperature and V/III ratio can reduce carbon incorporation into the device layers. Diodes with device layers grown at a growth pressure of 500 Torr show low turn-on voltages and a low specific onresistance with a figure of merit (Vb/Ron) of ~ 261 MW/cm. One way of circumventing the carbon issue is to fabricate vertical diodes directly on freestanding HVPE substrates. Ohmic contacts on the N-polar GaN surface consist of a Ti/Al/Ni/Au metal stack annealed at a temperature of 750 oC for 30 s. Circular Schottky metal contacts consisting of a non-annealed Ni/Au metal stack are made to the Ga-polar surface. Metal diameters range from 30 – 300 microns. Carrier concentrations measured via capacitance-voltage techniques range between 10 10 cm. Shown in Figure 1, such diodes were able to achieve breakdown voltages of ~ 900 V; however, the overall figure of merit for power devices is lowered due to a high specific on-resistance. This high specific onresistance stems from the low-doped substrate. This is confirmed by fabricating front-side diodes on UID freestanding HVPE substrates, in which diodes show a reduced specific on-resistance compared to the vertical case. Ideally, GaN power Schottky diodes consist of a low-doped (< 10 cm) epitaxial film for the drift region on a highly conductive substrate, thereby permitting high breakdown voltages and reducing the overall series resistance through the substrate. This talk will focus on such vertical devices. Device layers grown by MOCVD and HVPE on multiple substrates including freestanding HVPE GaN substrates, as well as truly bulk GaN substrates grown by the ammonothermal growth technique, will be discussed. Results will be compared to lateral devices fabricated from GaN device layers grown on sapphire, as well as the previously mentioned diodes fabricated directly on low-doped freestanding HVPE GaN substrates.

GaN power Schottky diodes fabricated on low doped MOCVD layers grown on multiple substrates

With its wide bandgap and high critical field, GaN is a promising material for high power electronics. To date, most GaN films have been grown on foreign substrates such as sapphire or SiC. Lattice mismatch between the film and substrate leads to a large number of threading dislocations (~109 -1010 cm-2). These defects are thought to lead to poor device performance such as premature breakdown. Device properties are generally improved by growth of low-doped (<; 1016 cm-3) GaN layers on high conductivity freestanding hydride vapor phase epitaxy (HVPE) GaN substrates. However, these films still have a large number of dislocations ~ 106 cm-2. Dislocations are randomly oriented in both hetero and homoepitaxial films leading to a wide variation of material quality and thus device performance across the wafer. Recently, a true bulk GaN substrate became available using an ammonothermal growth technique. These substrates have both a low resistivity and low threading dislocation density. Growth of low-doped films on these bulk substrates can potentially address the problems of uniformity and premature breakdown in GaN power Schottky diodes.

III-Nitride devices on Si: Challenges and opportunities

The ability to heteroepitaxially deposit high quality III-Nitride semiconductor layers on Si substrates is of great interest because of the numerous advantages offered by Si substrates: low cost, large-scale availability with high quality, good thermal and electrical conductivities as well as the possibility for integration with Si-based electronics.The results of our testing of HEMT structures along with results of our recent work on green light emitting diodes on Si and comparison with those grown on sapphire substrate will be presented.

Development of Homoepitaxially Grown GaN Thin Film Layers on Freestanding Bulk m-plane Substrates by Metalorganic Chemical Vapor Deposition (MOCVD)

High quality homoepitaxial growth of m -plane GaN films on freestanding m -plane HVPE GaN substrates has been performed using metalorganic chemical vapor deposition. For this a large growth space was studied. Large areas of no-nucleation along with presence of high density of defects were observed when layers were grown under growth conditions for c -plane GaN. It is believed that these structural defects were in large part due to the low lateral growth rates as well as unequal lateral growth rates in a - and c - crystallographic directions. To achieve high quality, fully coalesced epitaxial layers, growth conditions were optimized with respect to growth temperature, V/III ratios and reactor pressure. Higher growth temperatures led to smoother surfaces due to increased surface diffusion of adatoms. Overall, growth at higher temperature and lower V/III ratio decreased the surface roughness and resulted in better optical properties as observed by photoluminescence. Although optimization resulted in highly smooth layers, some macroscopic defects were still observed on the epi-surface as a result of contamination and subsurface damage remaining on bulk substrates possibly due to polishing. Addition of a step involving annealing of the bulk substrate under H2: N2 environment, prior to growth, drastically reduced such macroscopic defects.