J. Senawiratne

The growth and optical properties of large, high-quality AlN single crystals

The effect of impurities and defects on the optical properties of AlN was investigated. High-quality AlN single crystals of more than 20mm2 size were examined. Different crucible materials and growth procedures were applied to the growth of bulk AlN by physical vapor transport method to vary the defect and the impurity concentrations. The crystalline orientation was investigated by Raman spectroscopy. Glow discharge mass spectrometry was used to determine the trace concentration of the incorporated impurities such as oxygen and carbon. The photoluminescence emission and absorption properties of the crystals revealed bands around 3.5 and 4.3eV at room temperature. Absorption edges ranging between 4.1 and 5.95eV were observed. Since no straight correlation of the oxygen concentration was obtained, a major contribution of oxygen or oxygen-related impurities was ruled out to generate the observed emission and absorption bands in the Ultraviolet spectral range. The carbon-related impurities and intrinsic defec...

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.

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.

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.

Optical and Structural Investigations on Mn-Ion States in MOCVD-grown Ga1-xMnxN

The incorporation of Mn into GaMnN epilayers by MOCVD growth was investigated. Samples with high Mn concentrations lead to room temperature ferromagnetism. In addition an absorption band around 1.5 eV was observed. Intensity and linewidth of this band scaled with the Mn concentration and with the room temperature (RT) saturation magnetization. This band is assigned to the internal Mn 3+ transition between the 5 E and the partially filled 5 T2 levels of the 5 D state. The broadening of the absorption band is introduced by the high Mn concentration. Recharging of the Mn 3+ to Mn 2+ was found to effectively suppress these transitions resulting also in a significant reduction of the RT magnetization. The pronounced sensitivity of the relative position of the Fermi level and 1.5 eV absorption band can be used to predict the magnetization behavior of the Ga1-xMnxN epilayers. The absence of doping-induced strain was observed by Raman spectroscopy. The structural quality, the presence of Mn 2+ ions were confirmed by EPR spectroscopy, meanwhile no Mn-Mn interactions were observed.

Superluminescence in Green Emission GaInN/GaN Quantum Well Structures under Pulsed Laser Excitation

We report nonlinear optical investigation of green emission GaInN/GaN multi-quantum structures grown along c- and m-axes on sapphire and bulk GaN substrates, respectively. Under intense pulsed photo excitation, we observed strong superluminescence near the lasing condition in c-plane grown quantum well structures with full width at half maximum of 6 nm. The superluminescence couples out of the edge of the sample in a mode pattern consistent with gain in a high mode of the waveguide. The wavelength of the superluminescence is 474 nm. The threshold intensity of superluminescence was found to be 156 kW/cm 2 . Increasing pump intensity leads to a strong photoluminescence blueshift as large as 380 meV in samples grown along the c-axis on sapphire substrate, while under the same excitation conditions, the blue shift for the m-axis grown structure on bulk GaN substrate is less than 10 meV. The large emission blueshift is hereby attributed to the internal piezoelectric field in the c-axis grown structure. Its absence in the m-axis structure could enable low threshold current visible laser diodes.

Structural Analysis in Low-V-defect Blue and Green GaInN/GaN Light Emitting Diodes

In this study, we characterized the structural defects in blue and green GaInN/GaN LEDs grown on c-plane bulk GaN and sapphire substrates. Low density large V-defects with diameters around 600 nm were found in the blue LEDs on bulk GaN. They were initiated by edge-type threading dislocations (TDs) around the homoepitaxial growth interface. On the other hand, a high density 7×10 cm of smaller V-defects with sidewalls on } 01 1 1 { facets was observed in the active region of green LEDs on sapphire. Their diameter ranges from 150 to 200 nm. Misfit dislocations (MDs) generated in the quantum wells are found to initiate these V-defects. With optimizing the epitaxial growth conditions, the generation of MDs and their smaller V-defects was largely suppressed. As a result, the light output power improved by one order of magnitude. For green LEDs on bulk GaN, another unique type of defect was found in the active region: an inclined dislocation pair (IDP). In it a pair of dislocations propagate at a tilt angle of 18 to 23o from the [0001] growth direction towards > < 00 1 1 . This defect seems to be a path of strain relief in the high indium composition quantum wells.

Green Light Emitting Diodes under Photon Modulation

With an external laser excitation, the electroluminescence (EL) of GaInN/GaN green light emitting diodes (LEDs) grown on sapphire by metal organic vapor phase epitaxy has been investigated. The EL was found significantly enhanced under this bias and the difference can not merely be attributed to additional photoluminescence (PL). Under 325 nm photon bias, the EL enhancement starts at its highest value and decreases along with an increase of the LED current. Under 408 nm and 488 nm bias, it increases first to a value smaller than that of the 325 nm bias and then decreases with a much lower rate along the current increase. The EL enhancement is attributed to the more efficient carrier injection into quantum wells (QWs) resulting from the screening of the QW polarization by photon bias. Therefore, an enhanced balance of majority and minority carriers was obtained resulting in a better radiative recombination rate. Meanwhile, the current-voltage characteristics show a negative current first and then a voltage reduction as forward voltage increases. The reverse photocurrent indicates carrier loss due to the solar cell effect in our LED device while at high current region the carrier loss is attributed to another effect controlled by the external electrical field. At the balance point of those two effects, the EL enhancement is the highest. These findings clarify the transition from highly efficient radiative recombination at low current density to the region of efficiency droop at high current densities.

Loss of Quantum Efficiency in Green Light Emitting Diode Dies at Low Temperature

The electroluminescence, photoluminescence and cathodoluminescence of GaInN/GaN multiple quantum well light emitting diode dies are analyzed at variable low temperature. Three dies of nominally identical structure but strongly different RT performance from 510 nm to 525 nm have been studied. The electroluminescence peak energy exhibits a blue shift from RT to 158 K followed by a red shift for lower temperature. In the same low-temperature range, a secondary emission peak appears near 390 nm (3.18 eV) that resembles a donor-acceptor pair transition from GaN. Depth profiling spectroscopy of this transition at 77 K reveals its location either in the unintentionally doped quantum barriers or within the n-GaN layer, rather than the commonly believed Mg doped p-type GaN layers. The external quantum efficiency of each die increases as the temperature is lowered. A maximum is reached for all near 158 K while for lower temperature as low as 7.7 K, the efficiency continuously drops. The pronounced efficiency maximum is tentatively assigned to a combination of temperature dependent mobility and shift of the actual pn-junction location.

Low-Temperature Cathodoluminescence Mapping of Green, Blue, and UV GaInN/GaN LED Dies

GaInN based light emitting diodes (LEDs) play an important role as energy efficient light sources in solid state lighting. A controversial discussion addresses the origin of lateral light emission variations and their correlation with either of the identified defects, e.g., threading dislocations and V-defects. In order to establish any possible correlation of defects and luminescence centers, we analyze three UV, blue and green LED dies by microscopic mapping of spectroscopic cathodoluminescence and secondary electrons at variable low temperature from 7 K to room temperature. Particular effort is being placed on a quantitative analysis of the luminescence signal. Image intensities are not being scaled and offset for highest contrast as otherwise typical for imaging mode. In standard configuration, we analyze image areas of (0.037 mm) 2 with pixel resolution of 72 nm. Following regions of strong and weak emission we find that remain bright and dark respectively even at low temperature. Those variations increase with the mean emission wavelength of the LEDs and with temperature. The largest peak wavelength variation associated with the intensity contrast was observed in the green LEDs and amounts to 5 nm. Here the peak wavelength is higher in the dark spots than in the bright ones. This finding corresponds to the general trend when comparing the lower efficiency in longer wavelength green emitters to the blue ones.