Shallow and Deep Level Defects in GaN
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ABSTRACT Shallow and deep electronic defects in MOCVD-grown GaN were characterized by variable temperature Hall effect measurements, deep level transient spectroscopy (DLTS) and photoemission capacitance transient spectroscopy (O-DLTS). Unintentionally and Si-doped, ntype and Mg-doped, p-type GaN films were studied. Si introduces a shallow donor level into the band gap of GaN at -Ec - 0.02 eV and was found to be the dominant donor impurity in our unintentionally doped material. Mg is the shallowest acceptor in GaN identified to date with an electronic level at -Ev + 0.2 eV. With DLTS deep levels were detected in n-type and p-type GaN and with O-DLTS we demonstrate several deep levels with optical threshold energies for electron photoemission in the range between 0.87 and 1.59 eV in n-type GaN. INTRODUCTION The III-V nitride semiconductors, GaInA1N, have evolved into the materials of choice for the fabrication of visible light emitting diodes exhibiting external quantum efficiencies of almost 10 %.1,2 They are direct, wide bandgap semiconductors that also show great promise for high temperature / high power devices.3 However, despite intensive efforts injection laser diodes have not been achieved to date with III-V nitrides. Low p-type doping efficiency and the presence of deep levels that act as competing recombination centers contribute to the difficulties in achieving threshold carrier densities for lasing in III-V nitride laser diodes. To enable injection laser diodes with III-V nitrides the electronic properties of shallow n- and p-type dopants and of deep level defects need to be understood and controlled. Little is known about the electronic properties of Si donors and Mg acceptors, which are commonly employed for doping GaN layers n-type 4 and p-type,5 respectively. Also, the nature of the n-type background conductivity observed in most of the epitaxial GaN films is still not fully understood.6 Shalltw donor and acceptor levels in GaN were mainly characterized by variable temperature Hall effect measurements and activation energies of -27 meV for Si donors 7 and -150 meV for Mg 8 are reported in the literature. However, these values were derived from Arrhenius analyses of the measured electron concentration with the assumption of a temperature independent prefactor and, therefore, have a large uncertainty. Mg doping poses an additional problem: as-grown Mg-doped GaN is usually semiinsulating and postgrowth treatments such as low energy electron beam irradiation (LEEBI)5, furnace annealing, or rapid thermal annealing (RTA) 9 are required to activate the Mg dopants. 10-13 Defects that introduce electronic levels deeper into the bandgap of III-V nitrides have mainly been investigated by photoluminescence spectroscopy (PL).14 16 PL spectroscopy has the ability to detect deep level related radiative emission processes over the entire spectral range of the band gap of GaN but lacks the information on the position of individual electronic states which participate in electron-hole pair recombination processes. Subje
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