A. R. Arehart

Impact of carbon on trap states in n-type GaN grown by metalorganic chemical vapor deposition

The effect of excess C incorporation on the deep level spectrum of n-type GaN grown by metalorganic chemical vapor deposition was investigated. Low-pressure (LP) growth conditions were used to intentionally incorporate excess C compared to atmospheric pressure (AP) growth conditions. GaN samples with high C content are found to be highly resistive, and samples codoped with C and Si are heavily compensated. From a comparison of deep level optical spectroscopy and deep level transient spectroscopy measurements of the LP-grown codoped GaN:C:Si sample with the AP-grown unintentionally doped GaN, two deep levels at Ec−Et=1.35 and 3.28 eV are observed to have a direct relation to excess C incorporation. Comparing these activation energies to previous theoretical studies strongly suggests that the levels may be associated with a C interstitial and CN defect, respectively. These results suggest that C forms not only a shallow acceptor level but also a deep acceptor level in GaN, and these levels contribute to the...

Impact of deep levels on the electrical conductivity and luminescence of gallium nitride codoped with carbon and silicon

The impact of C incorporation on the deep level spectrum of n-type and semi-insulating GaN:C:Si films grown by rf plasma-assisted molecular-beam epitaxy (MBE) was investigated by the combination of deep level transient spectroscopy, steady-state photocapacitance, and transient deep level optical spectroscopy. The deep level spectra of the GaN:C:Si samples exhibited several band-gap states. A monotonic relation between systematic doping with C and quantitative trap concentration revealed C-related deep levels. A deep acceptor at Ec−2.05eV and a deep donor at Ec−0.11eV are newly reported states, and the latter is the first directly observed deep level attributed to the CGa defect. A configuration-coordinate model involving localized lattice distortion revealed strong evidence that C-related deep levels at Ec−3.0eV and Eν+0.9eV are likely identical and associated with the yellow luminescence in C-doped GaN films. Of the deep levels whose trap concentration increase with C doping, the band-gap states at Ec−3....

Effect of threading dislocation density on Ni/n-GaN Schottky diode I-V characteristics

The impact of threading dislocation density on Ni∕n-GaN Schottky barrier diode characteristics is investigated using forward biased current-voltage-temperature (I-V-T) and internal photoemission (IPE) measurements. Nominally, identical metal-organic chemical vapor deposition grown GaN layers were grown on two types of GaN templates on sapphire substrates to controllably vary threading dislocation density (TDD) from 3×107to7×108cm−2. I-V-T measurements revealed thermionic emission to be the dominant transport mechanism with ideality factors near 1.01 at room temperature for both sample types. The Schottky barrier heights showed a similar invariance with TDD, with measured values of 1.12–1.13eV obtained from fitting the I-V-T results to a thermionic emission-diffusion model. The I-V-T results were verified by IPE measurements made on the same diodes, confirming that the Ni∕n-GaN barrier heights do not show a measurable TDD dependence for the TDD range measured here. In apparent contrast to this result is th...

Deep level optical and thermal spectroscopy of traps in n-GaN grown by ammonia molecular beam epitaxy

The incorporation of deep level defects in n-type GaN grown by ammonia-based molecular beam epitaxy (MBE) is studied via systematic adjustment of the NH3/Ga flux ratio. Deep level optical and transient spectroscopies, which together enable deep level detection throughout the GaN bandgap, reveal defect states whose individual concentrations vary with the NH3/Ga flux ratio. A general trend of lower concentration for deep levels at EC−3.28, EC−1.28, EC−0.62, and EC−0.25 eV with higher NH3/Ga flux ratio was observed, with the strongest reduction at the EC−0.25 eV level, consistent with expectations for a VN-related defect. The known CN impurity state at EC−3.28 eV and suspected CI-related state at EC−1.28 eV also showed a moderate decrease in concentration at the higher NH3/Ga flux ratio. In contrast, the VGa-related defect at EC−2.62 eV was insensitive to the NH3/Ga flux ratio over the range studied here. Taken together, ammonia-MBE GaN has deep level defects with different sensitivities in flux ratios sugge...

Spatially-resolved spectroscopic measurements of Ec − 0.57 eV traps in AlGaN/GaN high electron mobility transistors

Simultaneous temperature-dependent measurements of resistance transients (RTs) and spatially resolved surface potential transients were made after bias switching on AlGaN/GaN high electron mobility transistors (HEMTs). We find an Ec − 0.57 eV trap, previously correlated with HEMT degradation, located in the GaN buffer and not in the AlGaN barrier or at the AlGaN surface. The amplitude of the Ec − 0.57 eV trap in RTs depends strongly on the Fe-concentration in the GaN buffer. Filling of this trap occurs only under bias conditions where electric fields penetrate into the GaN buffer.

Impact of Ga/N flux ratio on trap states in n-GaN grown by plasma-assisted molecular-beam epitaxy

The effect of growth regime on the deep level spectrum of n-GaN using molecular-beam epitaxy (MBE) was investigated. As the Ga/N flux ratio was decreased towards Ga-lean conditions, the concentration of two acceptor-like levels, at Ec−3.04 and 3.28 eV, increased from 1015 to 1016 cm−3 causing carrier compensation in these films. Thus, these two traps behaved as the dominant compensating centers in MBE n-GaN. Furthermore, the increase in trap concentration also strongly correlated with the degradation of both surface morphology and bulk electron mobility towards Ga-lean conditions, where higher pit densities and lower mobility were observed. These results show that the growth regime directly impacts all morphology, bulk transport, and trap states in n-GaN.

Direct comparison of traps in InAlN/GaN and AlGaN/GaN high electron mobility transistors using constant drain current deep level transient spectroscopy

Traps in InAlN/GaN and AlGaN/GaN high electron mobility transistors (HEMTs) are identified and compared using constant drain-current deep level transient spectroscopy (CID-DLTS). For both structures with different barrier materials, the same drain-access electron trap at EC−0.57 eV dominates the drain-controlled CID-DLTS trap spectrum. This suggests that the physical source of this trap, previously associated with drain-lag, is not present in the barrier but instead is likely to reside in the GaN-buffer. Gate-controlled CID-DLS measurements, which are preferentially sensitive to the barrier under the gate, reveal different trap spectra for the two HEMTs, showing that choice of barrier materials can influence under-gate trap signatures.

Deep level defects throughout the bandgap of (010) β-Ga2O3 detected by optically and thermally stimulated defect spectroscopy

Deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) measurements performed on Ni/β-Ga2O3 Schottky diodes fabricated on unintentionally doped (010) substrates prepared by edge-defined film-fed growth revealed a rich spectrum of defect states throughout the 4.84 eV bandgap of β-Ga2O3. Five distinct defect states were detected at EC − 0.62 eV, 0.82 eV, 1.00 eV, 2.16 eV, and 4.40 eV. The EC − 0.82 eV and 4.40 eV levels are dominant, with concentrations on the order of 1016 cm−3. The three DLTS-detected traps at EC − 0.62 eV, 0.82 eV, and 1.00 eV are similar to traps reported in Czochralski-grown β-Ga2O3, [K. Irmscher et al., J. Appl. Phys. 110, 063720 (2011)], suggesting possibly common sources. The DLOS-detected states at EC − 2.16 eV and 4.40 eV exhibit significant lattice relaxation effects in their optical transitions associated with strongly bound defects. As a consequence of this study, the Ni/β-Ga2O3 (010) Schottky barrier height was determined to be 1.55 eV, with good c...

Capture Kinetics of Electron Traps in MBE-Grown n-GaN

The carrier capture kinetics of the E c -0.59 eV and E c -0.91 eV electron traps found in molecular beam epitaxy (MBE)-grown n-GaN have been determined by means of deep level transient spectroscopy (DLTS). The 0.59 eV trap does not show the behaviour of either ideal point defects or line defects. In contrast, the 0.91 eV trap displays the kinetics of linearly arranged interacting point defects, which generate a time-dependent local Coulombic potential with a characteristic time constant of 8.6 μs.

Electrical characterization of n-type Al0.30Ga0.70N Schottky diodes

The carrier trapping properties and current transport behavior of Ni/n-Al0.30Ga0.70N Schottky diodes were quantitatively characterized by a combination of deep level optical spectroscopy (DLOS), thermally based deep level transient spectroscopy (DLTS), current-voltage-temperature (I-V-T), and internal photoemission (IPE) measurements. High quality Schottky diode behavior was observed with an IPE-determined barrier height of 1.66 eV and the observed temperature-independent reverse leakage current behavior was found to be consistent with field emission in reverse bias and thermionic-field emission in forward bias as the dominant transport mechanisms. The trap spectroscopy measurements revealed the presence of several bandgap states located at EC–0.9 eV (seen by both DLOS and DLTS), EC–1.5, EC–3.11, and EC–3.93 eV—all via DLOS. The EC–3.10 level, which is present in very high concentration, is found to correlate with the energy position expected for the cation vacancy in AlGaN, based on the vacuum referred b...