David Adolph
Chalmers University of Technology
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Featured researches published by David Adolph.
APL Materials | 2016
David Adolph; Reza Zamani; Kimberly A. Dick; Tommy Ive
We demonstrate crack-free ZnO/GaN distributed Bragg reflectors (DBRs) grown by hybrid plasma-assisted molecular beam epitaxy using the same growth chamber for continuous growth of both ZnO and GaN without exposure to air. This is the first time these ZnO/GaN DBRs have been demonstrated. The Bragg reflectors consisted up to 20 periods as shown with cross-sectional transmission electron microscopy. The maximum achieved reflectance was 77% with a 32 nm wide stopband centered at 500 nm. Growth along both (0001) and (0001) directions was investigated. Low-temperature growth as well as two-step low/high-temperature deposition was carried out where the latter method improved the DBR reflectance. Samples grown along the (0001) direction yielded a better surface morphology as revealed by scanning electron microscopy and atomic force microscopy. Reciprocal space maps showed that ZnO(0001)/GaN reflectors are relaxed whereas the ZnO(0001)/GaN DBRs are strained. The ability to n-type dope ZnO and GaN makes the ZnO(0...
Proceedings of SPIE | 2017
Filip Hjort; Ehsan Hashemi; David Adolph; Tommy Ive; Åsa Haglund
III-nitride-based vertical-cavity surface-emitting lasers have so far used intracavity contacting schemes since electrically conductive distributed Bragg reflectors (DBRs) have been difficult to achieve. A promising material combination for conductive DBRs is ZnO/GaN due to the small conduction band offset and ease of n-type doping. In addition, this combination offers a small lattice mismatch and high refractive index contrast, which could yield a mirror with a broad stopband and a high peak reflectivity using less than 20 DBR-pairs. A crack-free ZnO/GaN DBR was grown by hybrid plasma-assisted molecular beam epitaxy. The ZnO layers were approximately 20 nm thick and had an electron concentration of 1×1019 cm-3, while the GaN layers were 80-110 nm thick with an electron concentration of 1.8×1018 cm-3. In order to measure the resistance, mesa structures were formed by dry etching through the top 3 DBR-pairs and depositing non-annealed Al contacts on the GaN-layers at the top and next to the mesas. The measured specific series resistance was dominated by the lateral and contact contributions and gave an upper limit of ~10-3Ωcm2 for the vertical resistance. Simulations show that the ZnO electron concentration and the cancellation of piezoelectric and spontaneous polarization in strained ZnO have a large impact on the vertical resistance and that it could be orders of magnitudes lower than what was measured. This is the first report on electrically conductive ZnO/GaN DBRs and the upper limit of the resistance reported here is close to the lowest values reported for III-nitride-based DBRs.
Applied Physics Express | 2017
Ehsan Hashemi; Filip Hjort; Martin Stattin; Tommy Ive; Olof Bäcke; A. Lotsari; Mats Halvarsson; David Adolph; Vincent Desmaris; Denis Meledin; Åsa Haglund
We have investigated the effect of strain-compensating interlayers on the vertical electrical conductivity of Si-doped AlN/GaN distributed Bragg reflectors (DBRs). Samples with 10.5 mirror pairs were grown through plasma-assisted molecular beam epitaxy on SiC. Room-temperature current–voltage characteristics were measured vertically in mesas through 8 of the 10.5 pairs. The sample with no interlayers yields a mean specific series resistance of 0.044 Ω cm2 at low current densities, while three samples with 5/5-A-thick, 2/2-nm-thick, and graded interlayers have resistivities between 0.16 and 0.34 Ω cm2. Thus, interlayers impair vertical current transport, and they must be designed carefully when developing conductive DBRs.
Frontiers of Materials Science | 2015
David Adolph; Tobias Tingberg; T. G. Andersson; Tommy Ive
Plasma-assisted molecular beam epitaxy (MBE) was used to grow ZnO (0001) layers on GaN(0001)/4H-SiC buffer layers deposited in the same growth chamber equipped with both N- and O-plasma sources. The GaN buffer layers were grown immediately before initiating the growth of ZnO. Using a substrate temperature of 440°C–445°C and an O2 flow rate of 2.0–2.5 sccm, we obtained ZnO layers with smooth surfaces having a root-mean-square roughness of 0.3 nm and a peak-to-valley distance of 3 nm shown by AFM. The FWHM for X-ray rocking curves recorded across the ZnO(0002) and
Applied Surface Science | 2014
David Adolph; Tommy Ive
Journal of Crystal Growth | 2015
David Adolph; Tobias Tingberg; Tommy Ive
ZnO(10\bar 15)
Physica Status Solidi B-basic Solid State Physics | 2016
David Adolph; Tommy Ive
IEEE Transactions on Electron Devices | 2018
Johan Bergsten; Mattias Thorsell; David Adolph; Jr-Tai Chen; Olof Kordina; Einar Sveinbjörnsson; Niklas Rorsman
reflections were 200 and 950 arcsec, respectively. These values showed that the mosaicity (tilt and twist) of the ZnO film was comparable to corresponding values of the underlying GaN buffer. It was found that a substrate temperature > 450°C and a high Zn-flux always resulted in a rough ZnO surface morphology. Reciprocal space maps showed that the in-plane relaxation of the GaN and ZnO layers was 82.3% and 73.0%, respectively and the relaxation occurred abruptly during the growth. Room-temperature Hall-effect measurements showed that the layers were intrinsically n-type with an electron concentration of 1019 cm−3 and a Hall mobility of 50 cm2·V−1·s−1.
IEEE Journal of Quantum Electronics | 2018
Filip Hjort; Ehsan Hashemi; David Adolph; Tommy Ive; Olof Bäcke; Mats Halvarsson; Åsa Haglund
Stanford Chalmers Workshop on Advancing Materials Innovatively, Gothenburg | 2017
Åsa Haglund; Seyed Ehsan Hashemi; Filip Hjort; Michael Alexander Bergmann; Martin Stattin; David Adolph; Tommy Ive; Jörgen Bengtsson; Johan S. Gustavsson