Ali Asghar

GaN∕AlGaN ultraviolet/infrared dual-band detector

Group III-V wide band gap materials are widely used in developing solar blind, radiation-hard, high speed optoelectronic devices. A device detecting both ultraviolet (UV) and infrared (IR) simultaneously will be an important tool in fire fighting and for military and other applications. Here a heterojunction UV/IR dual-band detector, where the UV/IR detection is due to interband/intraband transitions in the Al0.026Ga0.974N barrier and GaN emitter, respectively, is reported. The UV threshold observed at 360nm corresponds to the band gap of the Al0.026Ga0.974N barrier, and the IR response obtained in the range of 8–14μm is in good agreement with the free carrier absorption model.

Characterization and analysis of InGaN photovoltaic devices

The InGaN material system is investigated to achieve high efficiency solar cells, using tandem and quantum-well structures to implement high efficiency concepts. Here InGaN p-i-n and quantum-well solar cells are designed, grown by MOCVD and fabricated into mesa devices. They are electrically characterized by I-V response under dark, white light and UV illumination and internal quantum efficiency (IQE). Material characterization is done by X-ray diffraction, photoluminescence and photoemission. InGaN solar cells with high In compositions are grown in two configurations, one incorporating it into the i-region of a p-i-n solar cells, and the other incorporating as the well-region of a quantum-well device. A QE of 8% was measured from these quantum-wells. Solar cells with In lean In/sub 0.07/Ga/sub 0.93/N p-i-n device structures show an IQE of 19% as well as photoemission at 500 nm, confirming the suitability of the material for photovoltaic applications.

Magnetic and optical properties of Ga1−xMnxN grown by metalorganic chemical vapour deposition

Epitaxial layers of Ga1−xMnxN with concentrations of up to x = 0.015 have been grown on c-sapphire substrates by metalorganic chemical vapour deposition. No ferromagnetic second phases were detected via high-resolution x-ray diffraction. Crystalline quality and surface structure were measured by x-ray diffraction and atomic force microscopy, respectively. No significant deterioration in crystal quality and no increase in surface roughness with the incorporation of Mn were detected. Optical measurements show a broad emission band attributed to a Mn-related transition at 3.0 eV that is not seen in the underlying GaN virtual substrate layers. Room temperature ferromagnetic hysteresis has been observed in these samples, which may be due to either Mn-clustering on the atomic scale or the Ga1−xMnxN bulk alloy.

GaN∕AlGaN heterojunction infrared detector responding in 8–14 and 20–70μm ranges

A GaN∕AlGaN heterojunction interfacial work function internal photoemission infrared detector responding in 8–14 and 20–70μm ranges has been demonstrated. Free carrier absorption based photoresponse shows a wavelength threshold of 14μm with a peak responsivity of 0.6mA∕W at 80K under −0.5V bias. A sharp peak in the 11–13.6μm range is observed superimposed on the free carrier response. In addition, the work demonstrates 54μm (5.5THz) operation of the detector based on 1s–2p± transition of Si donors in GaN. Possible approaches on improving the performance of the detector are also addressed.

Multifunctional III-nitride dilute magnetic semiconductor epilayers and nanostructures as a future platform for spintronic devices

This work focuses on the development of materials and growth techniques suitable for future spintronic device applications. Metal-organic chemical vapor deposition (MOCVD) was used to grow high-quality epitaxial films of varying thickness and manganese doping levels by introducing bis-cyclopentadienyl as the manganese source. High-resolution X-ray diffraction indicates that no macroscopic second phases are formed during growth, and Mn containing films are similar in crystalline quality to undoped films Atomic force microscopy revealed a 2-dimensional MOCVD step-flow growth pattern in the Mn-incorporated samples. The mean surface roughnesses of optimally grown Ga1-xMnxN films were almost identical to that from the as-grown template layers, with no change in growth mechanism or morphology. Various annealing steps were applied to some of the samples to reduce compensating defects and to investigate the effects of post processing on the structural, magnetic and opto-electronic properties. SQUID measurements showed an apparent ferromagnetic hysteresis behavior which persisted to room temperature. An optical absorption band around 1.5 eV was observed via transmission studies. This band is assigned to the internal Mn3+ transition between the 5E and the partially filled 5T2 levels of the 5D state. The broadening of the absorption band is introduced by the high Mn concentration. Recharging of the Mn3+ to Mn2+ was found to effectively suppress these transitions resulting in a reduction of the magnetization. The structural quality, and the presence of Mn2+ ions were confirmed by EPR spectroscopy, meanwhile no Mn-Mn interactions indicative of clustering were observed. The absence of doping-induced strain in Ga1-xMnxN was observed by Raman spectroscopy.

Lattice vibrations in hexagonal Ga1−xMnxN epitaxial films on c-plane sapphire substrates by infrared reflectance spectra

The lattice vibrations of undoped hexagonal Ga1−xMnxN (x from 0.0% to 1.5%) epitaxial films grown on c-plane sapphire substrates by metalorganic chemical vapor deposition have been investigated using infrared reflectance spectra in the frequency region of 200–2000cm−1 (5–50μm) at room temperature. The experimental reflectance spectra were analyzed using the Lorentz oscillator model for infrared-active phonon observed. The E1(LO) phonon frequency slightly decreases with increasing Mn composition. However, the E1(TO) phonon frequency linearly increases with the Mn composition, which can be well expressed by (558.7+350x)cm−1 and the broadening values are found to be larger than that of the GaN film. It indicates that Mn incorporation decreases the peak values (from the E1 phonon) of the infrared dielectric functions due to the local variation in the lattice constants and to the destruction of the crystal translational symmetry.

The Fermi level dependence of the optical and magnetic properties of Ga1−xMnxN grown by metal–organic chemical vapour deposition

The suppression of the ferromagnetic behaviour of metal–organic chemical vapour deposition grown Ga1−x Mnx Ne pilayers by silicon co-doping, and the influence of the Fermi level position on and its correlation with the magnetic and optical properties of Ga1−x Mnx Na re reported. Variation in the position of the Fermi level in the GaN bandgap is achieved by using different Mn concentrations and processing conditions as well as by co-doping with silicon to control the background donor concentration. The effect on Mn incorporation on the formation of defect states and impurity induced energy states within the bandgap of GaN was monitored by means of photoluminescence absorption an de mission spectroscopy. A broad absorption detected around 1.5 eV is attributed to the presence of a subband introduced by Mn induced energy states due to temperature independent transition energies and linewidths. The intensity and the linewidth of the absorption band correlate with the Mn concentration. Similarly, the magnitude of the magnetization decreases as the Fermi level approaches the conduction band, as the Fermi energy is increased above the Mn(0/−) acceptor state. Silicon concentrations >10 19 cm −3 caused the complete loss of ferromagnetic behaviour in the epilayer. The absorption band at 1.5 eV is also not observed upon silicon co-doping. The observed spectroscopic data favour a double-exchange-like mechanism rather than an itinerant free carrier mechanism for causing the ferromagnetism. This behaviour significantly differs from the properties reported for widely studied (Ga, In)MnAs.

Manganese-induced long-range lattice disorder and vacancy formation in metal-organic chemical vapor deposition grown and ion-implanted Ga1−xMnxN

The structural properties and lattice dynamics of Ga1−xMnxN were studied for Mn concentrations from 0.0% to 1.5%. Ga1−xMnxN layers were fabricated by either Mn incorporation during the metal-organic chemical vapor deposition (MOCVD) growth process or by postgrowth ion implantation into MOCVD-grown GaN epilayers. The crystalline integrity and the absence of major second phase contributions were confirmed by high-resolution x-ray diffraction analysis. Raman spectroscopy showed that increased Mn incorporation in the epilayers significantly affected long-range lattice ordering, revealing a disorder-induced mode at 300cm−1 and a local vibrational mode at 669cm−1. The low intensities of both modes were shown to scale with Mn concentration. These observations support the formation of nitrogen vacancies, even under optimized MOCVD growth conditions. The slight excess of metal components in the growth process compared to undoped GaN growth and the incorporation of Mn deep acceptor levels favors the formation of ni...

Review of recent efforts on the growth and characterization of nitride-based diluted magnetic semiconductors

Wide bandgap nitrides and oxides have been heralded as a possible platform for future semiconductor spintronics applications based on the inherent compatibility of these materials with existing semiconductors as well as theoretical predictions of room temperature ferromagnetism. Experimental reports of room temperature ferromagnetism in these materials are complicated by disparate crystalline quality and phase purity in these materials, as well as conflicting theoretical predictions as to the nature of ferromagnetic behavior in this system. A complete understanding of these materials, and ultimately intelligent design of spintronic devices, will require an exploration of the relationship between the processing techniques, resulting transition metal atom configuration, defects, and electronic compensation as related to the structure, magnetic, and magneto-optical properties of this material. This work explores the growth and properties of Ga1-xMnxN films by metalorganic chemical vapor deposition on cplane sapphire substrates with varying thickness, Mn concentration, and alloying elements. Homogenous Mn incorporation throughout the films was verified with Secondary Ion Mass Spectroscopy (SIMS), and no macroscopic second phases were detected using X-ray diffraction (XRD). SQUID and vibrating sample magnetometry measurements showed an apparent room temperature ferromagnetic hysteresis, whose strength can be altered considerably through annealing and introduction of either Si or Mg during the growth process. Three sets of Raman modes appeared to be sensitive to Mn incorporation. The intensities of a broad band around 300cm-1 and sharper modes near 669cm-1 increased with increasing Mn concentration. The rise of the former is attributed to a decrease in long-range lattice ordering for higher Mn concentration. The second mode is due to nitrogen vacancy-related local vibrational modes of the GaN host lattice. Si co-doped Ga1-xMnxN results in shallow donor states in GaN suppress the formation of nitrogen vacancies by compensating the p-type deep level defects introduced by substitutional Mn. The formation of a Mn-related midgap impurity band is observed via optical transmission measurement in Ga1-xMnxN with strong magnetic signatures, but not for Si co-doped samples. Initial studies on light emitting diodes (LEDs) containing a Mn-doped active region have also been produced. Devices were fabricated with different Mn-doped active layer thicknesses, and I-V characteristics show that the devices become highly resistive as thickness of the Mn-doped active layer increases. The electroluminescence of these devices is dominated by a high suppressed band-edge recombination and a midgap defect-related emission, leading to an orange-colored but weakly emitting LED. These results suggest that traditional theoretical and device approaches akin to those realized in Ga1-xMnxN may be difficult to realize in Ga1-xMnxN, and exploitation of these materials will require further novel device approaches taking into account the nature of this material.

Impact of Manganese incorporation on the structural and magnetic properties of MOCVD-grown Ga 1− x Mn x N

This paper reports the impact of the Mn incorporation on the structural and magnetic properties of Ga1-xMnxN on the metal-organic vapor phase deposition (MOCVD). Crystalline quality and phase purity were determined by high-resolution X-ray diffraction and indicated that no macroscopic second phases are formed during growth. Atomic force microscopy revealed a 2dimensional MOCVD step-flow growth pattern in the Mn-incorporated samples. Various annealing steps were applied to some of the samples to reduce compensating defects and to investigate the effects of post processing on the growth. SQUID measurements showed an apparent ferromagnetic hysteresis behavior. However, none of the requirements for room temperature ferromagnetism in the prevailing mean field DMS theories were found. Therefore, different origins of the ferromagnetic signal are discussed.