T. M. Al tahtamouni

Correlation between biaxial stress and free exciton transition in AlN epilayers

Photoluminescence (PL) spectroscopy and x-ray diffraction measurements were employed to study biaxial strain in AlN epilayers grown on different substrates. X-ray diffraction revealed that AlN epilayers grown on AlN bulk substrates (or homoepilayers) have the same lattice parameters as AlN bulk crystals and are almost strain-free. Compared to the free exciton (FX) transition in an AlN homoepilayer, the FX line was 31meV higher in AlN/sapphire due to a compressive strain and 55 (69)meV lower in AlN∕SiC (AlN∕Si) due to a tensile strain. A linear relationship between the FX transition energy peak position and in-plane stress was obtained, and a value of 45meV∕GPa for the linear coefficient of the stress-induced bandgap shift in AlN epilayers was deduced. The work here establishes PL as another simple and effective method for monitoring the biaxial stress in AlN epilayers.

Growth and photoluminescence studies of Al-rich AlN∕AlxGa1−xN quantum wells

Both a-plane and c-plane AlN∕Al0.65Ga0.35N quantum wells (QWs) have been grown by metal organic chemical vapor deposition and their photoluminescence (PL) emission properties were studied and compared. It was found that the low temperature PL characteristics of a-plane QWs are primarily governed by the quantum size effect, whereas those of c-plane QWs are significantly affected by the polarization fields. The PL decay time was found to be only weakly dependent on the well width Lw for a-plane QWs, whereas a strong dependence of the PL decay time on Lw was observed for c-plane QWs. Moreover, Lw dependence studies also revealed that structures with Lw>2nm and Lw≈2nm provide highest emission efficiency in a-plane and c-plane AlN∕Al0.65Ga0.35N QWs, respectively.

Photoluminescence properties of AlN homoepilayers with different orientations

AlN homoepilayers and heteroepilayers were grown on polar c-plane and nonpolar a-plane and m-plane orientations of AlN bulk and sapphire substrates by metal organic chemical vapor deposition. A systematic comparative study of photoluminescence properties of these samples revealed that all AlN homoepilayers (c, a and m planes) were strain free with an identical band gap of about 6.099 (6.035)eV at 10 (300)K, which is about 42meV below the band gap of c-plane AlN heteroepilayers grown on sapphire. Also, nonpolar a-plane homoepilayers have the highest emission intensity over all other types of epilayers. We believe that a-plane AlN homoepilayers have the potential to provide orders of magnitude improvement in the performance of new generation deep UV photonic devices.

Optical polarization in c-plane Al-rich AlN/AlxGa1-xN single quantum wells

The optical polarization of AlN/AlxGa1-xN single quantum wells (x = 0.65) has been studied by means of photoluminescence (PL) spectroscopy. The predominant polarization component of the band-edge PL switched from E ∥ c to E ⊥ c at a well width around 2 nm. The emission intensity with polarization of E ⊥ c and the degree of polarization were found to decrease with increasing well width. The emission intensity with polarization of E ∥ c was found to increase with increasing well width.

Hybrid AlN-SiC Deep Ultraviolet Schottky Barrier Photodetectors

Deep ultraviolet (DUV) Schottky barrier photodetectors have been demonstrated by exploiting the epitaxial growth of high quality AlN epilayer on n-type SiC substrate. The fabricated AlN∕n-SiC hybrid Schottky barrier detectors exhibited a peak responsivity at 200nm with very sharp cutoff wavelength at 210nm, very high reverse breakdown voltages (>200V), very low dark currents (about 10fA at a reverse bias of 50V), and high responsivity and DUV to UV/visible rejection ratio. These outstanding features are direct attributes of the fundamental material properties and high quality of AlN epilayers. The fabricated photodetectors also have a thermal energy limited detectivity at zero bias of about 1.0×1015cmHz1∕2W−1. These results demonstrated that AlN epilayers are an excellent candidate as an active material for DUV optoelectronic device applications.

AlN avalanche photodetectors

Deep ultraviolet (DUV) avalanche photodetectors (APDs) based on an AlN∕n-SiC Schottky diode structure have been demonstrated. The device with a mesa diameter of ∼100μm exhibits a gain of 1200 at a reverse bias voltage of −250V or a field of about 3MV∕cm. The cut-off and peak responsivity wavelengths of these APDs were 210 and 200nm, respectively. This is the highest optical gain and shortest cut-off wavelength achieved for III-nitride based DUV APDs. It was also observed that the reverse breakdown voltage increases with decreasing device size, which suggests that the device performance is limited by the presence of dislocations. The breakdown voltage for dislocation-free AlN was deduced to be about 4.1MV∕cm. The present results further demonstrate the potential of AlN as an active DUV material for future optoelectronic device applications.

Si-Doped High Al-Content AlGaN Epilayers with Improved Quality and Conductivity Using Indium as a Surfactant

Effects of indium as a surfactant for the growth of Si-doped Al0.75Ga0.25N epilayers by metal organic chemical vapor deposition have been studied. It was found that the use of indium as a surfactant improved the overall material quality of these epilayers, as evidenced by decreasing the (a) density of surface pits, (b) screw dislocation density, and (c) intensity of the deep level impurity transition with increasing indium flow rate. Hall effect measurements also yielded increased conductivity and electron concentration with increasing indium flow rate. The results suggested that indium as a surfactant counteracts the incorporation of defects responsible for self-compensation for n-type doping, namely, cation vacancies, in high Al-content AlGaN epilayers. A correlation between the intensity of the deep level impurity transition and screw dislocation density was also established.

Beryllium acceptor binding energy in AlN

The acceptor binding energy of an alternative dopant, Be, in AlN epilayers has been probed by time-resolved photoluminescence (PL) spectroscopy. The binding energy of excitons bound to Be acceptors in AlN is determined to be about 33meV, which implies that the Be acceptor binding energy in AlN is about 0.33eV in accordance with Haynes’ rule. The measured PL decay lifetimes of the acceptor-bound exciton transitions in Be- and Mg-doped AlN (93 and 119ps, respectively) also indicate that the binding energy of Be acceptor is smaller than that of the most common acceptor dopant in AlN, namely, Mg. The smaller activation energy of Be in AlN has the potential to partly address the critical p-type doping issue in AlN- and Al-rich AlGaN by increasing the room temperature free hole concentration by ∼103 compared to the case of Mg doping.

Effects of Mg-doped AlN/AlGaN superlattices on properties of p-GaN contact layer and performance of deep ultraviolet light emitting diodes

Mg-doped AlN/AlGaN superlattice (Mg-SL) and Mg-doped AlGaN epilayers have been investigated in the 284 nm deep ultraviolet (DUV) light emitting diodes (LEDs) as electron blocking layers. It was found that the use of Mg-SL improved the material quality of the p-GaN contact layer, as evidenced in the decreased density of surface pits and improved surface morphology and crystalline quality. The performance of the DUV LEDs fabricated using Mg-SL was significantly improved, as manifested by enhanced light intensity and output power, and reduced turn-on voltage. The improved performance is attributed to the enhanced blocking of electron overflow, and enhanced hole injection.

High quality AlN grown on double layer AlN buffers on SiC substrate for deep ultraviolet photodetectors

High quality AlN epilayers were grown on SiC substrates using double layer AlN buffers growth method by metal organic chemical vapor deposition and exploited as active deep ultraviolet optoelectronic materials through the demonstration of AlN Schottky barrier photodetectors. The grown AlN epilayers have smooth surfaces, low etch-pit density, narrow width of x-ray rocking curves, and strong band edge photoluminescence emission with low impurity emissions. AlN Schottky photodetectors are shown to possess outstanding features including extremely low dark current and high breakdown voltage.