Neeraj Tripathi

Mechanism of large area dislocation defect reduction in GaN layers on AlN∕Si (111) by substrate engineering

A reduction of edge dislocations in the GaN layer on Si substrate by almost an order of magnitude to 8.0×107∕cm2 and reduction in screw dislocations by a factor greater than 4 are achieved for the film grown on the Si (111) substrate engineered to have a polycrystalline defective layer at the AlN∕Si interface. The formation of a polycrystalline defective layer at the AlN∕Si interface by N+ ion implantation provides substrate conditions that result in a heteroepitaxial GaN film with much improved surface morphology and better crystal quality as compared to the film grown directly on AlN∕Si. A mechanism of dislocation defect reduction in the epitaxial film is given based on the detailed study of AlN∕Si interfaces as well as the evolution of the AlN buffer layer in the context of this substrate engineering technique, which shows partial decoupling of the III-nitride layers from the substrate to be responsible for the improved characteristics.

Effect of n+GaN cap polarization field on Cs-free GaN photocathode characteristics

We report on a Cs-free GaN photocathode structure in which band engineering at the photocathode surface caused by Si delta doping eliminates the need for use of cesium for photocathode activation. The structure is capped with a highly doped n+GaN layer. We have identified that n+GaN cap thickness plays an important role in limiting the effect of polarization induced charges at the GaN surface on the photocathode emission threshold. Physics based device simulations is used for further analysis of the experimental results. Our findings clearly illustrate the impact of polarization induced surface charges on the device properties including its emission threshold.

AlGaN based tunable hyperspectral detector

The application of III-nitrides to the development of a tunable hyperspectral detector is reported. The device consists of a triangular step barrier provided by a heterostructure of AlN∕AlxGa1−xN∕GaN. The structure is carefully designed to avoid relaxation of strained layers to prevent further introduction of defects and cracking due to the large tensile strain between different layers of the device. This structure is envisioned for tunable detection of ultraviolet through infrared wavelengths. The particular device structure reported here is expected to span detection energies from ∼1to2eV and from 3.4to5.4eV. The adjustable height of the triangular barrier with applied bias voltage provides tunability of the detected wavelength. The results from a first generation device are reported.

AlGaN based III-nitride tunnel barrier hyperspectral detector: Effect of internal polarization

We report on the recent progress made toward development of a III-nitride based tunable hyperspectral detector pixel with the potential advantages of reduced hardware complexity and increased dynamic control on the detection parameters in the context of existing hyperspectral detection systems. We discuss the concept, experiments, and simulation of devices along with the different obstacles to be overcome before this technology can mature into a commercial application.

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.

Exploiting Phosphate Dependent DNA Immobilization on HfO 2 , ZrO 2 and AlGaN for Integrated Biosensors

A significant challenge for high throughput nucleic acid analysis and sequencing is to increase both throughput and sensitivity. Electrical detection methods are advantageous since they can be easily scaled to high density arrays, are highly sensitive, and do not require bulky optical equipment for readout. A focus of most nucleic acid based sensors is the detection of sequence-specific hybridization events between complementary strands of DNA or RNA. These hybridization events can be detected electrically, due to the intrinsic negative charge associated with the phosphate-rich nucleic acid backbone. Field effect transistors (FETs) and high electron mobility transistors (HEMTs) are ideal devices for detecting such hybridization events, due to their high sensitivity to changes in electrical field strength. A key concern for the construction of DNA-based FET and HEMT biosensors is the immobilization of probe oligonucleotides on the active region of the sensor. In previous work, our group has shown that single stranded DNA can be directly immobilized onto semiconductor materials without the need for complex surface chemistry or crosslinking strategies. In the present work, we have shown that the immobilization of single stranded DNA onto these materials is influenced by the terminal phosphate group of the DNA molecule, independent of backbone phosphates. This agrees with previous studies in which phosphates and phosphonates exhibited strong attachment to a variety of metal oxides. We have also shown that surface-immobilized DNA is available for hybridization and that hybridization is sequence specific. Phosphate-dependent immobilization was demonstrated for HfO 2 , AlGaN, and ZrO 2 surfaces using optical detection of DNA-DNA hybridization, as well as x-ray photoelectron spectroscopy (XPS) analysis of DNA-modified surfaces.

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.