Neelam Khan

Effects of compressive strain on optical properties of InxGa1−xN∕GaN quantum wells

In0.2Ga0.8N∕GaN multiple quantum well (MQW) blue light emitting diode (LED) structure was grown on a specially designed sapphire substrate (with increasing thickness from the edge to the center within a single wafer). X-ray diffraction revealed that the GaN lattice constant c decreases continuously from the edge to the center, indicating a continuous variation in the compressive strain. The spectral peak positions of the electroluminescence (EL) spectra exhibited a blueshift when probed at the edge as compared to the center, which is a direct consequence of the continuous variation in the compressive strain across the wafer. Based on the experimental results, a ratio of elastic stiffness constants (C33∕C13) for GaN was deduced to be ∼5.0±1.0, which was in agreement with the calculated value of ∼4.0. A linear relation of the EL emission peak position of LEDs with the biaxial strain was observed, and a linear coefficient of 19meV∕GPa characterizing the relationship between the band gap energy and biaxial st...

High mobility InN epilayers grown on AlN epilayer templates

We report on the growth of InN epilayers on AlN/sapphire templates by metal organic chemical vapor deposition. Compared to InN epilayers grown on GaN templates, significant improvements in the electrical and optical properties of InN epilayers on AlN templates were observed. An increase in electron mobility, a decrease in background electron concentration, and a redshift of photoluminescence emission peak position with increasing the growth temperature and V/III ratio were observed and a room temperature Hall mobility of 1400cm2∕Vs with a free electron concentration of about 7×1018cm−3 was obtained. The improvements were partly attributed to the use of AlN templates, which allows for higher growth temperatures leading to an enhanced supply of nitrogen atoms and a possible reduction in the incorporation of unintentional impurities and nitrogen vacancy related defects.

Mg acceptor level in InN epilayers probed by photoluminescence

Mg-doped InN epilayers were grown on sapphire substrates by metal organic chemical vapor deposition. Effects of Mg concentration on the photoluminescence (PL) emission properties have been investigated. An emission line at ∼0.76eV, which was absent in undoped InN epilayers and was about 60meV below the band-to-band emission peak at ∼0.82eV, was observed to be the dominant emission in Mg-doped InN epilayers. The PL spectral peak position and the temperature dependent emission intensity corroborated each other and suggested that the Mg acceptor level in InN is about 60meV above the valance band maximum.

Comparing the use of multimedia animations and written solutions in facilitating problem solving

We compared the use of solutions to a problem in the form of multimedia animations and static worksheets to help students learn how to solve physics problems that required the use of mathematical integration. We administered four tasks related to electricity and magnetism problems. In each task, students individually attempted a pre-test problem followed by a worksheet problem based on the same concept. Then, we provided students the solution to the worksheet problem either as a narrated multimedia animation or in a written format. Finally, all students solved a post-test problem. Results indicate that on all four tasks, there was a statistically significant improvement in problem solving scores for both the animation and written solution treatments. We found no significant differences between the treatments.

Assessing Hexagonal Boron Nitride Crystal Quality by Defect Sensitive Etching

Hexagonal boron nitride (hBN), the graphite-like polymorph, has been employed for more than 55 years as powders, ceramics, amorphous films, and deposited fine-grain polycrystalline pyrolytic forms (pBN) in applications that take advantage of its high thermal stability, chemical inertness, high thermal conductivity, low x-ray absorption, high electrical resistivity, and lubricating properties [1-4]. Recently, new applications have been envisioned for hBN that take advantage of its unique structural, optical and electronic properties. These include nanophotonics [5] exploiting its highly anisotropic optical properties, two dimensional atomically-thin transistors that employ hBN’s ultra-smooth surfaces, high resistivity, and lattice matching with graphene [6, 7], deep ultraviolet emission, made possible by hBN’s large energy band gap (5.8 eV) and high exciton binding energy [8, 9], and solidstate neutron detectors, which rely on the strong interaction of thermal neutrons with the boron-10 isotope [10, 11]. These applications require hBN of high structural perfection; defects such as dislocations create charge traps, scattering centers, and recombination sites that degrade its optical and charge transport properties (mobility and minority carrier lifetimes) [9, 12-14].

Defect sensitive etching of hexagonal boron nitride single crystals

Defect sensitive etching (DSE) was developed to estimate the density of non-basal plane dislocations in hexagonal boron nitride (hBN) single crystals. The crystals employed in this study were precipitated by slowly cooling (2–4 °C/h) a nickel-chromium flux saturated with hBN from 1500 °C under 1 bar of flowing nitrogen. On the (0001) planes, hexagonal-shaped etch pits were formed by etching the crystals in a eutectic mixture of NaOH and KOH between 450 °C and 525 °C for 1–2 min. There were three types of pits: pointed bottom, flat bottom, and mixed shape pits. Cross-sectional transmission electron microscopy revealed that the pointed bottom etch pits examined were associated with threading dislocations. All of these dislocations had an a-type burgers vector (i.e., they were edge dislocations, since the line direction is perpendicular to the [ 211¯0]-type direction). The pit widths were much wider than the pit depths as measured by atomic force microscopy, indicating the lateral etch rate was much faster t...