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Mg, the only effective p-type dopant for nitrides, is
well-studied in thin films due to the important role the impurity plays in light
emitting diodes and high power electronics. However, there are few reports of Mg
in thick free-standing GaN substrates. Here we evaluate the material quality and
point defects in GaN grown by hydride vapor phase epitaxy (HVPE) using metallic
Mg as the doping source. The crystal quality is typical of commercially grown
HVPE substrates, and the photoluminescence measurements reveal distinctively
sharp excitonic and shallow-donor shallow-acceptor features. Secondary ion mass
spectroscopy indicates total Mg concentrations between 7x1016 and
6x1018 cm-3 in the four separate samples studied but,
more significantly, photoluminescence and electron paramagnetic resonance
spectroscopy show that the Mg is incorporated as a shallow acceptor.
The symmetry of the recombining electrons and holes in lightly photo-excited InGaN LEDs revealed through ODMR is related to the physical structure, band structure and defects present. Calculations of the electron-g within the k• p formalism give the average shift from the free-electron value for GaN but are not fully reconciled with the anisotropy. This theory is also extended to InGaN alloys for both pseudomorphic and relaxed layers. The average shift is close to the experimental values for the green LED. The strongly reduced hole anisotropies seen experimentally are explained by a recently published theory for acceptors in GaN.
The symmetry of the recombining electrons and holes in lightly photo-excited InGaN LEDs revealed through ODMR is related to the physical structure, band structure and defects present. Calculations of the electron-g within the k•p formalism give the average shift from the free-electron value for GaN but are not fully reconciled with the anisotropy. This theory is also extended to InGaN alloys for both pseudomorphic and relaxed layers. The average shift is close to the experimental values for the green LED. The strongly reduced hole anisotropies seen experimentally are explained by a recently published theory for acceptors in GaN.
Magnetic resonance techniques are used to study the recombination processes in GaN-based light emitting diodes (LEDs). Electrically-detected magnetic resonance (EDMR) and electroluminescence-detected magnetic resonance (ELDMR) results on InGaN/AlGaN double heterostructures are presented for blue and green LEDs. In either technique our signals are dominated by a broad feature that we ascribe to a deep Zn-related acceptor. Our ELDMR measurements show that this is associated with the blue or green emission. Our EDMR measurements resolve a second center that is tentatively identified as a deep donor trap.
Photoluminescence (PL) and optically-detected magnetic resonance (ODMR) experiments have been performed on undoped GaN epitaxial layers grown on 6H-SiC substrates. The defects observed in these films are compared with those found from previous ODMR studies of undoped GaN layers grown on sapphire substrates. Strong, sharp donor-bound exciton bands at 3.46 -3.47 eV and weak, broad emission bands at 2.2 eV were observed from several 0.7 and 2.6 μm-thick films. In addition, fairly strong shallow donor - shallow acceptor (SD-SA) recombination with a zero-phonon-line at 3.27 eV was found for GaN layers less than 1 μm-thick. The first observation of magnetic resonance on this SD-SA recombination from undoped GaN is reported in this work. Two magnetic resonance features attributed to effective-mass (EM) and deep-donor (DD) states were detected on the 2.2 eV emission bands from all the GaN/6H-SiC films. These resonances were observed previously on similar emission from undoped GaN layers grown on sapphire substrates. The same EM donor resonance, though much weaker, was also found on the SD-SA recombination. However, a resonance associated with shallow acceptor states was not observed on this emission. The weakness of the donor resonance arises from the weak spin-dependence of the recombination mechanism involving spin-thermalized shallow acceptors. The absence of an EM acceptor is due to the broadening of the resonance through the spreading of the acceptor g-values by random strains in these films.
The presence of two species of both cations and anions permits the construction of InAs/AlSb heterostructures with either AlAs- or InSb-like interfaces. InAs/AlSb superlattices with both types of interfaces were grown using migration-enhanced epitaxial techniques. The layer quality and control of interfacial composition were confirmed by x-ray diffraction, Raman spectroscopy, and photoluminescence measurements. We demonstrate that high-quality superlattices with both InSb- and AlAs-bonded interfaces can be achieved with appropriate growth temperatures.
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