| 1. |
- Bergmann, Michael Alexander, 1989, et al.
(författare)
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Towards ultraviolet and blue microcavity lasers
- 2018
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Ingår i: Northen Optics and Photonics conference. - 9789163964886 ; 2018
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Konferensbidrag (refereegranskat)abstract
- The development of III-nitride-based (Al,Ga,In(N)) microcavity lasers is a challenging task. Significant progress in recent years has resulted in realizations of electrically pumped devices with optical output power in the mW-range and with threshold current densities below 20 kA/cm2. However, to become practical, the lifetime and power conversion efficiency of these devices must be improved. Among the challenges are achieving transverse optical mode confinement, highreflectivity mirrors and control over the resonator length. We will highlight our theoretical work on transverse optical mode confinement, emphasising the overwhelming risk of ending up with an optically anti-guided cavity, and its consequences such as very high optical losses that easily could double the threshold gain for lasing. We will show some anti-guided cavities with reasonable threshold gain and built-in modal discrimination. However, all anti-guided cavities are very sensitive to temperature effects and small structural changes in the cavity caused by fabrication imperfections. We have explored electrically conductive distributed Bragg reflectors (DBRs) in both AlN/GaN and ZnO/GaN. The AlN/GaN DBRs were grown with different strain-compensating interlayers, and the DBR without interlayers had the lowest vertical resistivity with a specific series resistance of 0.044 cmfor eight DBRpairs. In the ZnO/GaN DBR, the measured resistance was dominated by lateral and contact contributions, setting a lower measurable limit of ~10 for three DBR-pairs. Numerical simulations show the importance of having in-plane strained layers in the ZnO/GaN DBR, since that leads to cancellation of the spontaneous and piezoelectric polarization. This results in a dramatically reduced vertical resistance, potentially three orders of magnitude lower than what could be measured. cm An alternative to an epitaxially grown DBR is a dielectric DBR, which offers high reflectivity over a broader wavelength range, relaxing the requirements on resonator length control. To deposit a dielectric DBR on the bottom side of the cavity, the sample must first be bonded to a carrier wafer before the substrate can be removed. We used thermocompression gold-gold bonding to successfully bond the laser structure to a Si carrier wafer. The subsequent substrate removal is a challenging process due to the chemical inertness of the III-nitride-based materials. A doping-dependent electrochemical etch technique was used, which allows for the selective removal of a sacrificial (n-doped) layer between the cavity and the substrate. This resulted in nm-precise cavity lift-off with a low root-mean-square surface roughness down to 0.3 nm. Thus, the process is suitable for the fabrication of high-quality optical devices such as microcavity lasers. In addition, the technique offers a new alternative to create III-nitridebased optical resonators, mechanical resonators, thin film LEDs and transistors.
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| 2. |
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| 3. |
- Adolph, David, 1971, et al.
(författare)
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Growth of ZnO(0001) on GaN(0001)/4H-SiC buffer layers by plasma-assisted hybrid molecular beam epitaxy
- 2015
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248. ; 426, s. 129-134
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Tidskriftsartikel (refereegranskat)abstract
- Plasma-assisted molecular beam epitaxy 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 445 °C and an O2 flow rate of 2.5 standard cubic centimeters per minute, we obtained ZnO layers with statistically smooth surfaces having a root-mean-square roughness of 0.3 nm and a peak-to-valley distance of 3 nm as revealed by atomic force microscopy. The full-width-at-half-maximum for x-ray rocking curves obtained across the ZnO(0002) and ZnO(10 1¯ 5) reflections was 198 and 948 arcsec, respectively. These values indicated that the mosaicity of the ZnO layer was comparable to the corresponding values of the underlying GaN buffer layer. Reciprocal space maps showed that the in-plane relaxation of the GaN and ZnO layers was 82% and 73%, respectively, and that the relaxation occurred abruptly during the growth. Roomerature Hall-effect measurements revealed that the layers were inherently n-type and had an electron concentration of 1×1019 cm-3 and a Hall mobility of 51 cm2/V s.
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| 4. |
- Adolph, David, 1971, et al.
(författare)
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Hybrid ZnO/GaN distributed Bragg reflectors grown by plasma-assisted molecular beam epitaxy
- 2016
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Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 4:8, s. 086106-
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Tidskriftsartikel (refereegranskat)abstract
- 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 (000-1) directions was investigated. Low temperaturegrowth 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 scanningelectron microscopy and atomic force microscopy. Reciprocal space maps showed that ZnO(000-1)/GaN reflectors are relaxed whereas the ZnO(0001)/GaN DBRs are strained. The ability to n-type dope ZnO and GaN makes the ZnO(0001)/GaNDBRs interesting for various optoelectronic cavity structures.
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| 5. |
- Adolph, David, 1971, et al.
(författare)
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Impact of O2 flow rate on the growth rate of ZnO(0001) and ZnO(000-1) on GaN by plasma-assisted molecular beam epitaxy
- 2016
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Ingår i: Physica Status Solidi (B): Basic Research. - : Wiley. - 1521-3951 .- 0370-1972. ; 253:8, s. 1523-1528
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Tidskriftsartikel (refereegranskat)abstract
- We studied the effects of a varying O$ flow rate on the growth of ZnO(0001) and ZnO(000-1) layers on GaN/Al2O3-templates by plasma-assisted molecular beam epitaxy. The O2 flow rate through the O-plasma source was varied between 0.25--4.5 standard cubic centimeters per minute corresponding to a growth chamber pressure between 3.0 x 10^-6 -- 5.0x10^-5 Torr. We found that the change of the O2 flow rate had a profound effect on the ZnO layer growth rate. A maximum growth rate was reached for an O2 flow rate of 1.0--2.0 standard cubic centimeters per minute. The same growth rate dependence on the O2 flow rate was observed for ZnO(0001) layers that were grown on GaN/4H-SiC buffer layers for verification. To assess the amount of active O contributing to the ZnO-growth, the spectral composition of the plasma was investigated with optical emission spectroscopy. The integrated optical emission line intensity reached a maximum for an O2 flow rate between 1.0--2.0 standard cubic centimeters per minute. Essentially all emission lines exhibited a maximum intensity for an O2 flow rate between 1.0--2.0 standard cubic centimeters per minute thus coinciding with the flow rate yielding the maximum growth rate.
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| 6. |
- Adolph, David, 1971, et al.
(författare)
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Nucleation and epitaxial growth of ZnO on GaN(0 0 0 1)
- 2014
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332. ; 307, s. 438-443
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Tidskriftsartikel (refereegranskat)abstract
- Plasma-assisted molecular beam epitaxy was used to grow ZnO(0001) layers on GaN(0001)/Al2O3 templates and GaN/4H-SiC(0001) layers. The GaN(0001)/Al2O3 template surfaces were subjected to various pre-treatment procedures (Zn, Ga or N pre-exposure or none) prior to the ZnO growth. We studied the impact of these pre-treatment procedures on the initial growth conditions of ZnO(0001). These layers were compared to ZnO layers deposited on 4H-SiC utilizing a GaN( 0001) buffer layer that was grown in situ on the 4H-SiC substrate and immediately before the growth of ZnO. The GaN buffer layers were not pre-treated or exposed to ambient. Atomic force and scanning electron microscopy as well as secondary ion mass spectroscopy revealed that the pre-treatment procedures resulted in a very high density of islands. The islands coalesced into films as the growth progressed. In contrast, no ZnO growth occurred on the untreated GaN( 0001)/Al2O3 template surfaces. Our main finding is that Ga,03, sub-oxides residing on the surface of the as-received GaN-templates, drastically reduced the ZnO nucleation rate and completely inhibited subsequent coalescence and growth. Our various surface pre-treatment procedures aimed at removing the sub-oxides were necessary for achieving ZnO growth on the GaN-templates. No surface pre-treatment was needed to enable ZnO growth on the in situ grown GaN(0001)14H-SiC layers.
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| 7. |
- Adolph, David, 1971, et al.
(författare)
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Plasma-assisted molecular beam epitaxy of ZnO on in-situ grown GaN/4H-SiC buffer layers
- 2015
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Ingår i: Frontiers of Materials Science. - : Springer Science and Business Media LLC. - 2095-025X .- 2095-0268. ; 9:2, s. 185-191
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Tidskriftsartikel (refereegranskat)abstract
- 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 degrees C-445 degrees C and an O-2 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 ZnO(10 (1) over bar5) 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 degrees 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 10(19) cm(-3) and a Hall mobility of 50 cm(2).V-1.s(-1).
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| 8. |
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| 9. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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Effect of compositional interlayers on the vertical electrical conductivity of Si-doped AlN/GaN distributed Bragg reflectors grown on SiC''
- 2017
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Ingår i: Applied Physics Express. - 1882-0786 .- 1882-0778. ; 10:5, s. 055501-
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Tidskriftsartikel (refereegranskat)abstract
- 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-Å-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.
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| 10. |
- Hjort, Filip, 1991, et al.
(författare)
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Electrically conductive ZnO/GaN distributed Bragg reflectors grown by hybrid plasma-assisted molecular beam epitaxy
- 2017
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9781510606494 ; 10104, s. 1010413-1
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Konferensbidrag (refereegranskat)abstract
- 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.
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