| 1. |
|
|
| 2. |
- Chen, Jr-Tai, 1982-
(författare)
-
MOCVD growth of GaN-based high electron mobility transistor structures
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present work was to improve the overall quality of GaN-based high electron mobility transistor (HEMT) epitaxial structures grown on semi-insulating (SI) SiC and native GaN substrates, using an approach called bottom-to-top optimization. The bottom-to-top optimization means an entire growth process optimization, from in-situ substrate pretreatment to the epitaxial growth and then the cooling process. Great effort was put to gain the understanding of the influence of growth parameters on material properties and consequently to establish an advanced and reproducible growth process. Many state-of-the-art material properties of GaN-based HEMT structures were achieved in this work, including superior structural integrity of AlN nucleation layers for ultra-low thermal boundary resistance, excellent control of residual impurities, outstanding and nearly-perfect crystalline quality of GaN epilayers grown on SiC and native GaN substrates, respectively, and record-high room temperature 2DEG mobility obtained in simple AlGaN/GaN heterostructures.The epitaxial growth of the wide bandgap III-nitride epilayers like GaN, AlN, AlGaN, and InAlN, as well as various GaN-based HEMT structures was all carried out in a hot-wall metalorganic chemical vapor deposition (MOCVD) system. A variety of structural and electrical characterizations were routinely used to provide fast feedback for adjusting growth parameters and developing improved growth processes.
|
|
| 3. |
- Espenlaub, Andrew C., et al.
(författare)
-
Evidence of trap-assisted Auger recombination in low radiative efficiency MBE-grown III-nitride LEDs
- 2019
-
Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 126:18
-
Tidskriftsartikel (refereegranskat)abstract
- By studying low radiative efficiency blue III-nitride light emitting diodes (LEDs), we find that the ABC model of recombination commonly used for understanding efficiency behavior in LEDs is insufficient and that additional effects should be taken into account. We propose a modification to the standard recombination model by incorporating a bimolecular nonradiative term. The modified model is shown to be in much better agreement with the radiative efficiency data and to be more consistent than the conventional model with very short carrier lifetimes measured by time-resolved photoluminescence in similar, low radiative efficiency material. We present experimental evidence that a hot carrier-generating process is occurring within these devices, in the form of measurements of forward photocurrent under forward bias. The forward photocurrent, due to hot carrier generation in the active region, is present despite the lack of any "efficiency droop"-the usual signature of band-to-band Auger recombination in high-quality III-nitride LEDs. Hot carrier generation in the absence of band-to-band Auger recombination implies that some other source of hot carriers exists within these low radiative efficiency devices, such as trap-assisted Auger recombination.
|
|
| 4. |
- Hilfiker, Matthew, et al.
(författare)
-
Dielectric function tensor (1.5 eV to 9.0 eV), anisotropy, and band to band transitions of monoclinic -(AlxGa1-x)(2)O-3 (x 0.21) films
- 2019
-
Ingår i: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 114:23
-
Tidskriftsartikel (refereegranskat)abstract
- A set of monoclinic -(AlxGa1-x)(2)O-3 films coherently grown by plasma-assisted molecular beam epitaxy onto (010)-oriented -Ga2O3 substrates for compositions x0.21 is investigated by generalized spectroscopic ellipsometry at room temperature in the spectral range of 1.5eV-9.0eV. We present the composition dependence of the excitonic and band to band transition energy parameters using a previously described eigendielectric summation approach for -Ga2O3 from the study by Mock et al. All energies shift to a shorter wavelength with the increasing Al content in accordance with the much larger fundamental band to band transition energies of Al2O3 regardless of crystal symmetry. The observed increase in the lowest band to band transition energy is in excellent agreement with recent theoretical predictions. The most important observation is that charge confinement in heterostructures will strongly depend on the growth condition due to the strongly anisotropic properties of the band to band transitions.
|
|
| 5. |
- Korlacki, Rafal, et al.
(författare)
-
Strain and Composition Dependencies of the Near-Band-Gap Optical Transitions in Monoclinic (AlxGa1-x)2O3 Alloys with Coherent Biaxial In-Plane Strain on Ga2O3(010)
- 2022
-
Ingår i: Physical Review Applied. - : AMER PHYSICAL SOC. - 2331-7019. ; 22:6
-
Tidskriftsartikel (refereegranskat)abstract
- The bowing of the energy of the three lowest band-to-band transitions in beta-(AlXGa1-X)2O3 alloys is resolved using a combined density-functional theory (DFT) and generalized spectroscopic ellipsometry approach. The DFT calculations of the electronic band structure of both beta-Ga2O3 and theta-Al2O3 allow the linear portion of the energy shift in the alloys to be extracted, and provide a method for quantifying the role of coherent strain present in the beta-(AlXGa1-X)2O3 thin films on (010) beta-Ga2O3 substrates. The energies of band-to-band transitions are obtained using the spectroscopic ellipsometry eigenpolarization model approach [A. Mock et al., Phys. Rev. B 95, 165202 (2017)]. After subtracting the effects of strain, which also induces additional bowing and after subtraction of the linear portion of the energy shift due to alloying, the bowing parameters associated with the three lowest band-to-band transitions in monoclinic beta-(AlXGa1-X)2O3 are found.
|
|
| 6. |
- Marcinkevičius, Saulius, et al.
(författare)
-
Dynamics of carrier injection through V-defects in long wavelength GaN LEDs
- 2024
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 124:18
-
Tidskriftsartikel (refereegranskat)abstract
- The efficiency of high-power operation of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on the realization of uniform hole distribution between the QWs. In long wavelength InGaN/GaN QW LEDs, the thermionic interwell hole transport is hindered by high GaN barriers. However, in polar LED structures, these barriers may be circumvented by the lateral hole injection via semipolar 10 1 ¯ 1 QWs that form on the facets of V-defects. The efficiency of such carrier transfer depends on the transport time since transport in the semipolar QWs is competed by recombination. In this work, we study the carrier transfer from the semipolar to polar QWs by time-resolved photoluminescence in long wavelength (green to red) LEDs. We find that the carrier transfer through the semipolar QWs is fast, a few tens of picoseconds with the estimated room temperature ambipolar diffusion coefficient of ∼5.5 cm2/s. With diffusion much faster than recombination, the hole transport from the p-side of the structure to the polar QWs should proceed without a substantial loss, contributing to the high efficiency of long wavelength GaN LEDs.
|
|
| 7. |
- Marcinkevičius, Saulius, et al.
(författare)
-
Effect of Mg doping on carrier recombination in GaN
- 2023
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 134:8
-
Tidskriftsartikel (refereegranskat)abstract
- Time-resolved photoluminescence measurements have been performed on Mg-doped GaN for Mg concentrations in the low- to mid-1019 cm−3. As-grown and annealed (600-675 °C) samples were studied. In the as-grown samples, the nonradiative carrier lifetime was found to be about 200 ps and nearly independent of the Mg concentration. Upon annealing, the carrier lifetimes shorten to ∼150 ps but, again, show little dependence on the annealing temperature. The analysis of possible Shockley-Read-Hall recombination centers and their behavior during doping and annealing suggests that the main nonradiative recombination center is the Mg-nitrogen vacancy complex. The weak dependence of the PL decay times on temperature indicates that carrier capture into this center has a very low potential barrier, and the nonradiative recombination dominates even at low temperatures.
|
|
| 8. |
- Marcinkevicius, Saulius, et al.
(författare)
-
Electron-phonon scattering in beta-Ga2O3 studied by ultrafast transmission spectroscopy
- 2021
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 118:24
-
Tidskriftsartikel (refereegranskat)abstract
- Femtosecond pump-probe experiments in a transmission geometry were performed on Sn-doped n-type beta-Ga2O3. With the pump and probe wavelengths below the bandgap, the differential transmission signal was determined by the free electron dynamics. Differential transmission decay times and their spectral dependence were used to evaluate electron-phonon scattering for polar optical (PO) and intervalley phonons. The obtained average electron-PO phonon scattering time is 4.5 +/- 0.4fs, while the electron scattering to and from the side valley is 80 +/- 5 fs. The energy between the absolute and second lowest conduction band minima is estimated to be 2.6 +/- 0.1eV.
|
|
| 9. |
- Marcinkevičius, Saulius, et al.
(författare)
-
Experimental evidence of hole injection through V-defects in long wavelength GaN-based LEDs
- 2023
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 123:20
-
Tidskriftsartikel (refereegranskat)abstract
- Hole injection through V-defect sidewalls into all quantum wells (QWs) of long wavelength GaN light emitting diodes had previously been proposed as means to increase efficiency of these devices. In this work, we directly tested the viability of this injection mechanism by electroluminescence and time-resolved photoluminescence measurements on a device in which QW furthest away from the p-side of the structure was deeper, thus serving as an optical detector for presence of injected electron-hole pairs. Emission from the detector well confirmed that, indeed, the holes were injected into this QW, which could only take place through the 10 1 ¯ 1 V-defect sidewalls. Unlike direct interwell transport by thermionic emission, this transport mechanism allows populating all QWs of a multiple QW structure despite the high potential barriers in the long wavelength InGaN/GaN QWs.
|
|
| 10. |
- Marcinkevičius, Saulius, et al.
(författare)
-
High internal quantum efficiency of long wavelength InGaN quantum wells
- 2021
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 119:7
-
Tidskriftsartikel (refereegranskat)abstract
- Time-resolved and quasi-cw photoluminescence (PL) spectroscopy was applied to measure the internal quantum efficiency (IQE) of c-plane InGaN single quantum wells (QWs) grown on sapphire substrates using metal-organic chemical vapor deposition. The identical temperature dependence of the PL decay times and radiative recombination times at low temperatures confirmed that the low temperature IQE is 100%, which allowed evaluation of the absolute IQE at elevated temperatures. At 300 K, the IQE for QWs emitting in green and green-yellow spectral regions was more than 60%. The weak nonradiative recombination in QWs with a substantial concentration of threading dislocations and V-defects (similar to 2 x 10(8) cm(-2)) shows that these extended defects do not notably affect the carrier recombination.
|
|
