Puneet Suvarna

Single Photon Counting UV Solar-Blind Detectors Using Silicon and III-Nitride Materials

Ultraviolet (UV) studies in astronomy, cosmology, planetary studies, biological and medical applications often require precision detection of faint objects and in many cases require photon-counting detection. We present an overview of two approaches for achieving photon counting in the UV. The first approach involves UV enhancement of photon-counting silicon detectors, including electron multiplying charge-coupled devices and avalanche photodiodes. The approach used here employs molecular beam epitaxy for delta doping and superlattice doping for surface passivation and high UV quantum efficiency. Additional UV enhancements include antireflection (AR) and solar-blind UV bandpass coatings prepared by atomic layer deposition. Quantum efficiency (QE) measurements show QE > 50% in the 100–300 nm range for detectors with simple AR coatings, and QE ≅ 80% at ~206 nm has been shown when more complex AR coatings are used. The second approach is based on avalanche photodiodes in III-nitride materials with high QE and intrinsic solar blindness.

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.

GaN Power Schottky Diodes with Drift Layers Grown on Four Substrates

We have examined the performance of gallium nitride (GaN) high-power Schottky diodes fabricated on unintentionally doped (UID) metalorganic chemical vapor deposition (MOCVD) films grown simultaneously on four substrates ranging in threading dislocation density from 5 × 103 cm - 2 to 1010 cm - 2. The substrates were an intentionally doped and a UID freestanding hydride vapor phase epitaxy substrate, an MOCVD GaN template grown on a sapphire wafer, and a bulk GaN substrate grown via an ammonothermal method. Capacitance–voltage (C–V) results showed the carrier concentration was ∼2 × 1016 cm−3 for films grown on each of the four substrates. With that doping level, the theoretical breakdown voltage (Vb) is ∼1600 V. However, measured Vb for the devices tested on each of the four substrates fell short of this value. Also, the breakdown voltages across each of the four substrates were not substantially different. This result was especially surprising for films grown on bulk GaN substrates, because of their superior crystal quality, as determined from their x-ray rocking curve widths. Simple probability calculations showed that most of the diodes tested on the bulk substrate did not cover a single threading dislocation. Although optimization of edge-termination schemes is likely to improve Vb, we believe that point defects, not threading dislocations, are the main reason for the reduced performance of these devices.

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.

Visible-Blind APD Heterostructure Design With Superior Field Confinement and Low Operating Voltage

We report on the polarization engineering of GaN/AlGaN heterostructures for the improvement of III-Nitride photodetectors through physics-based device simulations. Various heterojunction p-i-n and p-i-n-i-n designs are proposed and analyzed in this context. Our analysis shows that the introduction of a higher-bandgap AlGaN layer and n-type doped composition graded interlayers reduce operating voltage of an avalanche photodetector (APD) by almost 40% while enabling backside illumination geometry that is critical for the realization of detector arrays. The results of the simulation studies predict an APD device design that is less susceptible to premature breakdown outside of the multiplication region due to superior electric field confinement.

Ion Implantation-Based Edge Termination to Improve III-N APD Reliability and Performance

We report on the development of ion implantationbased contact-edge termination technique to improve the reliability and performance of p-i-n and p-i-n-i-n GaN ultraviolet avalanche photodiode structures. The GaN photodiode structures were grown on sapphire substrates and implanted along the edge of the p-contact. The implanted devices show an absence of premature breakdown and demonstrate a lower dark-current with reliable ultraviolet photoresponse, compared with the standard unimplanted devices. Device simulations of the implanted structures at the breakdown voltage, show a reduction in crowding and spiking of the electric field along the perimeter of the contact by a factor of ~7, compared with the unimplanted structures.

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 ...

Development of small unit cell avalanche photodiodes for UV imaging applications

High resolution imaging in the UV band has a lot of applications in defense and commercial systems. The shortest wavelength is desired for spatial resolution which allows for small pixels and large formats. UVAPDs have been demonstrated as discrete devices demonstrating gain. The next frontier is to develop UV APD arrays with high gain to demonstrate high resolution imaging. We will discuss model that can predict sensor performance in the UV band using APDs with various gain and other parameters for a desired UV band of interest. SNRs can be modeled from illuminated targets at various distances with high resolution under standard atmospheres in the UV band and the solar-blind region using detector arrays with unity gain and with high-gain APDs. We will present recent data on the GaN based APDs for their gain, detector response, dark current noise and the 1/f noise. We will discuss various approaches and device designs that are being evaluated for developing APDs in wide band gap semiconductors. The paper will also discuss state-of-the-art in UV APDs and the future directions for small unit cell size and gain in the APDs.

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.