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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.
Strontium titanate (SrTiO3) is a wide-band-gap semiconductor with a variety of novel properties. In this work, bulk single crystal SrTiO3 samples were heated to 1200°C, resulting in the creation of point defects. These thermally treated samples showed large persistent photoconductivity (PPC) at room temperature. Illumination with sub-gap light (>2.9 eV) caused an increase in free-electron concentration by over two orders of magnitude. After the light is turned off, the conductivity persists at room temperature, with essentially zero decay over several days. The results of electron paramagnetic resonance (EPR) measurements suggest that a point defect is responsible for PPC because the photo-induced response of one of the EPR signals is similar to that seen for the PPC. Due to a large barrier for recapture, the photo-excited electron remains in the conduction band, where it contributes to the conductivity.
Measurement of recombination and minority-carrier lifetimes has become a very common activity in current semiconductor technology. The two primary measurement techniques are based on photoconductive decay (PCD) and time-resolved photoluminescence (TRPL). The measurement of the “true” lifetime depends on the carriers being confined to a given spatial region of a diagnostic device. When internal electric fields exist that separate the charges, the measured value does not represent the real minority-carrier lifetime. In these cases, the measured quantity is a function of the true lifetime and the measurement technique.
The interface or surface recombination velocity is a critical
and important parameter in many device applications. In this
work, we have developed and applied a contactless microwave
technique, which in combination with a continuously tunable
pulsed light source, is able to probe the excess carrier
lifetime in the surface and bulk regions of a semiconductor
wafer. The technique is called resonant coupled
photoconductive decay (RCPCD) and has been described by the
authors in the literature. For strongly absorbed light, the
initial (t = 0) decay time is a strong function of the absorption
coefficient, α, as well as the bulk lifetime. The effective
bulk lifetime is a well-known function of the two surfaces (interfaces)
and the true bulk lifetime. The effective bulk lifetime is measured
by using very weakly absorbed light, or by measuring the asymptotic
decay rate of strongly absorbed light. The latter occurs after
diffusion has produced a quasi-equilibrium condition in the wafer.
For asymmetric surfaces (such as a wafer polished on one surface
only), the measurement with strongly absorbed light is made at
both wafer surfaces. We have solved simultaneuously three nonlinear
equations, and the solutions provide values for the three unknowns
S1, S2 and τ(bulk). Several examples of the technique
will be demonstrated for silicon wafers.
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