Ustun Sunay

Effect of local fields on the Mg acceptor in GaN films and GaN substrates

The electron paramagnetic resonance (EPR) spectrum of the Mg acceptor is studied in a variety of GaN samples, including mm-thick free-standing substrates and sub-micron heteroepitaxial films. The former allows a view of the acceptor unique for EPR—in an environment with less than 107u2009cm−3 dislocations and doping densities ranging from 4u2009×u20091016 to 6u2009×u20091018 cm−3. By probing Mg in a broad range of samples in one study a new feature of the acceptor emerges. The EPR data reveal an anisotropic line shape that reflects a change in the crystal field in the vicinity of the Mg acceptor. This feature must be accommodated by any of the models proposed for the Mg acceptor. Here, we show that one such previously proposed model agrees well with the EPR data obtained from the wide variety of samples studied. The work implies that Mg-doped GaN contains a common Mg-related defect which can be affected by local crystal fields established during growth.

Mg-hydrogen interaction in AlGaN alloys

It is well known that hydrogen passivation of Mg in Mg-doped GaN reduces free hole concentrations. While there are numerous studies of passivation of Mg in GaN, little work has been reported concerning passivation rates in AlGaN alloys. We investigated the hydrogen interaction with Mg in nitrides by measuring the intensity of the electron paramagnetic resonance (EPR) signal associated with the acceptor. The samples were isothermally annealed in sequential steps ranging from 5 min - 6.6 h between 300 and 700 oC in H2:N2 (7%: 92%) or pure N2. The signal intensity decreased during the H2N2 anneal and was revived by the N2 anneal as expected; however, the rate at which the intensity changed was shown to depend on Al concentration. In addition, while all signals were quenched at 700 oC in H2:N2, a 750 oC N2 anneal reactivated only about 30% of the Mg in the alloys and 80% of the intensity in the GaN film. These data suggest that the rate of passivation and activation of Mg by hydrogen is dependent on the concentration of Al in the AlxGa-1xN layer. The EPR annealing data could prove to be beneficial in improving p-type optimization in AlGaN alloys.

Reduction in the Number of Mg Acceptors with Al Concentration in Al x Ga 1 x N

High hole concentrations in AlxGa1−xN become increasingly difficult to obtain as the Al mole fraction increases. The problem is believed to be related to compensation, extended defects, and the band gap of the alloy. Whereas electrical measurements are commonly used to measure hole density, in this work we used electron paramagnetic resonance (EPR) spectroscopy to investigate a defect related to the neutral Mg acceptor. The amount and symmetry of neutral Mg in MOCVD-grown AlxGa1−xN with xxa0=xa00 to 0.28 was monitored for films with different dislocation densities and surface conditions. EPR measurements indicated that the amount of neutral Mg decreased by 60% in 900°C-annealed AlxGa1−xN films for xxa0=xa00.18 and 0.28 as compared with xxa0=xa00.00 and 0.08. A decrease in the angular dependence of the EPR signal accompanied the increased x, suggesting a change in the local environment of the Mg. Neither dislocation density nor annealing conditions contribute to the reduced amount of neutral Mg in samples with the higher Al concentration. Rather, compensation is the simplest explanation of the observations, because a donor could both reduce the number of neutral acceptors and cause the variation in the angular dependence.

Charge transfer process for carbon-related center in semi-insulating carbon-doped GaN

Electron paramagnetic resonance (EPR) spectroscopy was used to study the point defects in 2u2009×u20091017–1019u2009cm−3 C-doped GaN substrates grown by hydride vapor phase epitaxy. The intensity of an isotropic signal with gu2009=u20091.987 ± 0.001 increased monotonically with the carbon concentration, indicating that the EPR signal represents a carbon-related defect. In each sample, the signal intensity increased under illumination with photon energy greater than 2.75u2009eV, and the photo-induced signal decreased with subsequent illumination at 0.95u2009eV. A second signal, well-documented to be a shallow donor, appeared along with the gu2009=u20091.987 signal in the most lightly doped samples. The appearance of the donor confirms that the photo-induced increase is caused by excitation of an electron to the conduction band and implies that a defect level for the carbon-related center is about 1u2009eV above the valence band edge, consistent with temperature-dependent Hall measurements.

Charge transfer in Fe-doped GaN: The role of the donor

Several nitride-based device structures would benefit from the availability of high quality, large-area, freestanding semi-insulating GaN substrates. Due to the intrinsic n-type nature of GaN, however, the incorporation of compensating centers such as Fe is necessary to achieve the high resistivity required. We are using electron paramagnetic resonance (EPR) to explore charge transfer in 450 um thick GaN:Fe plates to understand the basic mechanisms related to compensation so that the material may be optimized for device applications. The results suggest that the simple model based on one shallow donor and a single Fe level is insufficient to describe compensation. Rather, the observation of the neutral donor and Fe3+ indicates that either the two species are spatially segregated or additional compensating and donor defects must be present.

The Mg impurity in nitride alloys

Although several magnetic resonance studies address the Mg acceptor in GaN, there are few reports on Mg doping in the alloys, where hole production depends strongly on the Al or In content. Our electron paramagnetic resonance (EPR) measurements of the p-type alloys suggest that the Mg impurity retains the axial symmetry, characteristic of a p-type dopant in both alloys; however, In and Al produce additional, different characteristics of the acceptor. In InGaN, the behavior is consistent with a lowering of the acceptor level and increasing hole density as In concentration increases. For AlGaN, the amount of neutral Mg decreases with increasing Al content, which is attributed to different kinetics of hydrogen diffusion thought to occur in samples with higher Al mole fraction.