Y. S. Katharria

Improved heat dissipation in gallium nitride light-emitting diodes with embedded graphene oxide pattern

The future of solid-state lighting relies on how the performance parameters will be improved further for developing high-brightness light-emitting diodes. Eventually, heat removal is becoming a crucial issue because the requirement of high brightness necessitates high-operating current densities that would trigger more joule heating. Here we demonstrate that the embedded graphene oxide in a gallium nitride light-emitting diode alleviates the self-heating issues by virtue of its heat-spreading ability and reducing the thermal boundary resistance. The fabrication process involves the generation of scalable graphene oxide microscale patterns on a sapphire substrate, followed by its thermal reduction and epitaxial lateral overgrowth of gallium nitride in a metal-organic chemical vapour deposition system under one-step process. The device with embedded graphene oxide outperforms its conventional counterpart by emitting bright light with relatively low-junction temperature and thermal resistance. This facile strategy may enable integration of large-scale graphene into practical devices for effective heat removal.

Work-function-tuned multilayer graphene as current spreading electrode in blue light-emitting diodes

This letter reports on the implementation of multilayer graphene (MLG) as a current spreading electrode in GaN-based blue light-emitting diodes. We demonstrate two facile strategies to maneuver the electrical coupling between p-GaN layer and MLG. Using a work-function-tuned MLG and a thin gold (Au) metal interlayer, the current spreading and thus the device forward voltage are considerably improved. We attribute these improvements to the diminution in work function difference between p-GaN and MLG, the decrease of specific contact resistance, and the enhancement in the conductivity of MLG film as a result of doping. In addition, rapid thermal annealing at elevated temperature is found to provide additional pathway for enhanced carrier injection.

Synthesis of buried SiC using an energetic ion beam

Buried silicon carbide (SiC) was synthesized at room temperature using implantation of 150 keV C+ ions in n-type Si (100) and Si (111) substrates. The dose of implanted C+ was varied from 4 × 1017 to 8 × 1017 cm−2. Post-implantation annealing at 1000° C for 30 min was carried out in inert ambience. Rutherford backscattering spectroscopy, x-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy were employed to characterize the samples. FTIR and Raman spectroscopic techniques in combination with the XRD analysis confirmed the development of the β-SiC phase in the samples. The average size of the SiC precipitates was estimated to be 9 nm from XRD analysis.

Influence of swift heavy ion irradiation on electrical characteristics of Au/n-Si (1 0 0) Schottky barrier structure

The influence of swift heavy (180 MeV 107Ag14+) ion irradiation on Au/n-Si Schottky diode characteristics has been analysed using in situ current–voltage (I–V) characterization. The values of the Schottky barrier height (SBH), the ideality factor and series resistance Rs for each irradiation fluence have been obtained from the forward bias I–V characteristics. For an unirradiated diode, the SBH and ideality factor were 0.74 ± 0.01 eV and 1.71, respectively. The barrier height decreases to 0.69 ± 0.01 eV as the fluence increases to a value of 1 × 1011 ions cm−2. It is found that after an irradiation fluence of 1 × 1011 ions cm−2 the SBH remains immune to further irradiation up to a fluence of 5 × 1012 ions cm−2. The observed behaviour is interpreted on the basis of energy loss mechanisms of energetic ions at the metal–semiconductor interface and irradiation-induced defects.

Performance evaluation of GaN light-emitting diodes using transferred graphene as current spreading layer

This study elucidates the correlation among conductivity of graphene and interface aspects in GaN light-emitting diodes (LEDs). Using a multilayer graphene of low sheet resistance, it is demonstrated that graphene alone can make ohmic contact with p-GaN without necessitating additional interlayer. Large-area blue LED with relatively low contact resistance in the order of 10−2 ohm-cm2 and improved forward voltage of 3.2 ± 0.1 V was realized irrespective of the use of the interlayer. The results from parallel evaluation experiments performed by varying the layer numbers of graphene with ultrathin NiOx interlayer revealed that the poor lateral conductivity of monolayer or few layer graphene can be well compensated by the interlayer. A combination of three layer graphene and NiOx offered device with enhanced electro-optical performance. But the Schottky barrier associated with the inadequate adhesion of transferred graphene dominates all the benefits and becomes a major bottleneck preventing the formation of ...

Temperature-dependent barrier characteristics of swift heavy ion irradiated Au∕n-Si Schottky structure

The electrical behavior of Au∕n-Si(100) structure, irradiated with 120MeV Ag8+107, has been investigated in a wide temperature range (50–300K). The forward bias current-voltage (I-V) and reverse bias capacitance-voltage (C-V) measurements have been used to extract the diode parameters. The variations in various parameters of the irradiated Schottky structure have been systematically studied as a function of temperature. It is found that the flatband barrier height is almost independent of the change in temperature. The ionized-donor concentration decreases while the ideality factor increases with decreasing temperatures. The behavior of Schottky parameters is explained by taking into account the role of the swift heavy ion irradiation induced defects at metal-semiconductor junction. The results are interpreted on the basis of recent models of Fermi level pinning.

Self-organization of 6H‐SiC (0001) surface under keV ion irradiation

In the present study, we have investigated the temporal evolution of 6H‐SiC (0001) surface under 100keV Ar+ ion irradiation at oblique incidence (θ=60°). The topographical changes introduced by ion beam were examined using scanning force microscopy, and it is demonstrated that while at short time scales, surface morphology is dominated by dots with average diameter of 30nm, periodic height modulations or ripples emerge at the later time scales. Existing theories of ripple formation have been invoked to explain various features of the observed ripples. Ripple structures developed on a physically stable material such as SiC are expected to show very small time degradation and therefore, would be more advantageous for various technological applications as compared to those grown on conventional semiconductors such as Si, GaAs, InP, etc.

Stress-relaxed growth of n-GaN epilayers

A significant stress-relaxation was observed in GaN epilayers by integrating a heavily Si-doped GaN (n+-GaN) sacrificial layer in the undoped GaN templates grown on sapphire substrates by metal-organic chemical vapor deposition. Selective GaN growth and electrochemical etching were exploited to achieve embedded air-gaps. Stress-relaxation and its local variations were probed by Raman mapping of high-frequency transverse-optical E2 (high) phonon mode of GaN. Enhanced In incorporation and improved light emission were observed in InGaN/GaN multi-quantum well visible light emitting diode structures fabricated on stress-relaxed GaN-epilayers with embedded air-gaps. Relevant sources for stress reduction and improved optical emission have been discussed.

High performance of InGaN light-emitting diodes by air-gap/GaN distributed Bragg reflectors.

The effect of air-gap/GaN DBR structure, fabricated by selective lateral wet-etching, on InGaN light-emitting diodes (LEDs) is investigated. The air-gap/GaN DBR structures in LED acts as a light reflector, and thereby improve the light output power due to the redirection of light into escape cones on both front and back sides of the LED. At an injection current of 20 mA, the enhancement in the radiometric power as high as 1.91 times as compared to a conventional LED having no DBR structure and a far-field angle as low as 128.2° are realized with air-gap/GaN DBR structures.

Effects of thermal and athermal processing on the formation of buried SiC layers

In the present study, systematic investigations on 100 keV C ion implanted Si (100) substrates annealed subsequently at a temperature of 1000 °C for 2 h or athermally processed using 110 MeV Ni8+ ion irradiation have been performed. A detailed analysis using the techniques of x-ray diffraction, Fourier transform infrared spectroscopy, and transmission electron microscopy (TEM) at high resolutions is performed. The observations suggest the formation of cubic silicon carbide (β-SiC) crystallites surrounded by an amorphous background in the samples thermally annealed at 1000 °C. However, ion irradiation did not influence the as-implanted layers to any significant extent. Various defects formed after annealing inside C implanted Si such as missing planes, edge dislocations, and grain boundaries during thermal crystallization are visualized through high resolution TEM.