| 1. |
- Hsiung, Te-Chih, et al.
(författare)
-
Enhanced surface mobility and quantum oscillations in topological insulator Bi1.5Sb0.5Te1.7Se1.3 nanoflakes
- 2013
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 103:16
-
Tidskriftsartikel (refereegranskat)abstract
- n this study, a series of Bi1.5Sb0.5Te 1.7Se1.3 (BSTS) flakes 80-nm to 140-μm in thickness was fabricated to investigate their metallic surface states. We report the observation of surface-dominated transport in these topological insulator BSTS nanoflakes. The achievement of surface-dominated transport can be attributed to high surface mobility (∼3000 cm2/V s) and low bulk mobility (12 cm2/V s). Up to 90% of the total conductance, the surface channel was estimated based on the thickness dependence of electrical conductance and the result of the Shubnikov-de Hass oscillations in a 200-nm BSTS. The nature of nontrivial Dirac surface states was also confirmed by the weak anti-localization effect.
|
|
| 2. |
- Bremer, Johan, 1991, et al.
(författare)
-
Electric-Based Thermal Characterization of GaN Technologies Affected by Trapping Effects
- 2020
-
Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 67:5, s. 1952-1958
-
Tidskriftsartikel (refereegranskat)abstract
- This article presents an electric-based methodology for thermal characterization of semiconductor technologies. It is shown that for technologies such as gallium nitride (GaN) high electron mobility transistors, which exhibit several field induced electron trapping effects, the thermal characterization has to be performed under specific conditions. The electric field is limited to low levels to avoid activation of trap states. At the same time, the dissipated power needs to be high enough to change the operating temperature of the device. The method is demonstrated on a test structure implemented as a GaN resistor with large contact separation. It is used to evaluate the thermal properties of samples with different silicon carbide suppliers and buffer thickness.
|
|
| 3. |
- Chen, Ding-Yuan, 1991, et al.
(författare)
-
Structural investigation of ultra-low resistance deeply recessed sidewall ohmic contacts for AlGaN/GaN HEMTs based on Ti/Al/Ti-metallization
- 2023
-
Ingår i: Semiconductor Science and Technology. - : IOP Publishing Ltd. - 1361-6641 .- 0268-1242. ; 38:10
-
Tidskriftsartikel (refereegranskat)abstract
- This study presents a novel approach to forming low-resistance ohmic contacts for AlGaN/GaN HEMTs. The optimized contacts exhibit an outstanding contact resistance of approximately 0.15 & omega;& BULL;mm. This is achieved by firstly recessing the barrier of the heterostructure to a depth beyond the channel. In this way, the channel region is exposed on the sidewall of the recess. The coverage of the Ti/Al/Ti ohmic metalization on the sidewall is ensured through tilting of the sample during evaporation. The annealing process is performed at a low temperature of 550 & DEG;C. The approach does not require precise control of the recess etching. Furthermore, the method is directly applicable to most barrier designs in terms of thickness and Al-concentration. The impact of recessed sidewall angle, thickness and ratio of Ti and Al layers, and the annealing procedure are investigated. Structural and chemical analyses of the interface between the ohmic contacts and epi-structure indicate the formation of ohmic contacts by the extraction of nitrogen from the epi-structure. The approach is demonstrated on HEMT-structures with two different barrier designs in terms of Al-concentration and barrier thickness. The study demonstrate large process window in regard to recess depth and duration of the annealing as well as high uniformity of the contact resistance across the samples, rendering the approach highly suitable for industrial production processes.
|
|
| 4. |
- Ding Yuan, Chen, 1991, et al.
(författare)
-
Impact of in situ NH3 pre-treatment of LPCVD SiN passivation on GaN HEMT performance
- 2022
-
Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 1361-6641 .- 0268-1242. ; 37:3
-
Tidskriftsartikel (refereegranskat)abstract
- The impact on the performance of GaN high electron mobility transistors (HEMTs) of in situ ammonia (NH3) pre-treatment prior to the deposition of silicon nitride (SiN) passivation with low-pressure chemical vapor deposition (LPCVD ) is investigated. Three different NH3 pre-treatment durations (0, 3, and 10 min) were compared in terms of interface properties and device performance. A reduction of oxygen (O) at the interface between SiN and epi-structure is detected by scanning transmission electron microscopy (STEM )-electron energy loss spectroscopy (EELS) measurements in the sample subjected to 10 min of pre-treatment. The samples subjected to NH3 pre-treatment show a reduced surface-related current dispersion of 9% (compared to 16% for the untreated sample), which is attributed to the reduction of O at the SiN/epi interface. Furthermore, NH3 pre-treatment for 10 min significantly improves the current dispersion uniformity from 14.5% to 1.9%. The reduced trapping effects result in a high output power of 3.4 W mm(-1) at 3 GHz (compared to 2.6 W mm(-1) for the untreated sample). These results demonstrate that the in situ NH3 pre-treatment before LPCVD of SiN passivation is critical and can effectively improves the large-signal microwave performance of GaN HEMTs.
|
|
| 5. |
- Ding Yuan, Chen, 1991, et al.
(författare)
-
Thin Al 0.5 Ga 0.5 N/GaN HEMTs on QuanFINE ® Structure
- 2021
-
Ingår i: CS MANTECH 2021 - 2021 International Conference on Compound Semiconductor Manufacturing Technology, Digest of Papers. ; , s. 153-155
-
Konferensbidrag (refereegranskat)abstract
- The performance of HEMTs fabricated on a thin Al0.5Ga0.5N/GaN heterostructure with a total barrier thickness of 6.5 nm is presented and benchmarked to the epi-structure with a 13 nm thick Al0.3Ga0.7N barrier on an identical QuanFINE® structure. DC transfer characteristics on both samples with a gate length of 100 nm demonstrate a high current above 1 A/mm. A higher extrinsic gm of 550 mS/mm is measured on the sample with a thinner high Al content barrier. Moreover, low trapping effects with a 12-14 % buffer-related dispersion at a VDSQ of 25 V are characterized for both samples, which indicate the advantage of the iron-free QuanFINE® heterostructure.
|
|
| 6. |
- Lin, Yen-Ku, et al.
(författare)
-
A versatile low-resistance ohmic contact process with ohmic recess and low-temperature annealing for GaN HEMTs
- 2018
-
Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 1361-6641 .- 0268-1242. ; 33:9
-
Tidskriftsartikel (refereegranskat)abstract
- Deeply recessed ohmic contacts for GaN-based high electron mobility transistors (HEMTs) are demonstrated. It is shown that low-resistance ohmic contacts can be achieved with recessing beyond the AlGaN Schottky barrier where the ohmic contacts are formed on the sidewall of the recess. This makes the process versatile and relatively insensitive to the exact recess depth. The ohmic contact is based on a gold-free metallization scheme consisting of a Ta/Al/Ta metal stack requiring a low-temperature annealing. Important parameters for this type of ohmic contact process include the metal coverage, slope of the etched sidewall, bottom Ta-layer thickness, as well as annealing temperature and duration. The optimized contact resistance is as low as 0.24 Omega mm after annealing at 575 degrees C. Moreover, this sidewall contact approach was successfully implemented on different epitaxial heterostructures with different AlGaN barrier thickness as well as with and without AlN exclusion layer. All the samples exhibited excellent contact resistances in a wide range of recess depths. The Ta-based, sidewall ohmic contact process is a promising method for forming an ohmic contact on a wide range of GaN HEMT epitaxial designs.
|
|
| 7. |
- Desieres, Yohan, et al.
(författare)
-
Strong light extraction enhancement using TiO2 nanoparticles-based microcone arrays embossed on III-Nitride light emitting diodes
- 2018
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 112:23
-
Tidskriftsartikel (refereegranskat)abstract
- Colloidal TiO2 nanoparticles were used for embossing of composite microcone arrays on III-Nitride vertical-thin-film blue light emitting diodes (LEDs) as well as on silicon, glass, gallium arsenide, and gallium nitride surfaces. Ray tracing simulations were performed to optimize the design of microcones for light extraction and to explain the experimental results. An optical power enhancement of ∼2.08 was measured on III-Nitride blue LEDs embossed with a hexagonal array of TiO2 microcones of ∼1.35 μm in height and ∼2.6 μm in base width, without epoxy encapsulation. A voltage increase in ∼70 mV at an operating current density of ∼35 A/cm2 was measured for the embossed LEDs. The TiO2 microcone arrays were embossed on functioning LEDs, using low pressures (∼100 g/cm2) and temperatures ≤100 °C.
|
|
| 8. |
- Ding Yuan, Chen, 1991
(författare)
-
Optimization of Ohmic Contacts and Surface Passivation for ‘Buffer-Free’ GaN HEMT Technologies
- 2020
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gallium nitride high electron mobility transistors (GaN HEMTs) draw attention from high frequency and high power industries due to unique properties including high electron mobility and saturation velocity combined with high breakdown voltage. This makes GaN HEMTs suitable for power devices with high switching speed and high frequency applications with high power density requirements. However, the device performance is still partly limited by problems associated with the formation of low resistivity ohmic contact, trapping effects, and the confinement of the two-dimensional electron gas (2DEG). In this work, reproducible deeply recessed Ta-based ohmic contacts with a low contact resistance of 0.2 - 0.3 Ωmm, a low annealing temperature of 550 - 600 °C, and a large process window were optimized. Low annealing temperature reduces the risk of 2DEG degradation and promotes better morphology of the ohmic contacts. Deeply recessed ohmic contacts beyond the barrier layers make the process less sensitive to the etching depth since the ohmic contacts are formed on the sidewall of the recess. The concept of deeply recessed low resistivity ohmic contacts is also successfully demonstrated on different epi-structures with different barrier designs. Passivation with silicon nitride (SiN) is an effective method to suppress electron trapping effects. Low Pressure Chemical Vapor Deposition (LPCVD) of SiN has shown to result in high quality dielectrics with excellent passivation effect. However, the surface traps are not fully removed after passivation due to dangling-bonds and native oxide layer at the interface of passivation and epi-structure. Therefore, a plasma-free in-situ NH3 pretreatment method before the deposition of the SiN passivation was studied. The samples with the pretreatment present a 38% lower surface-related current collapse and a 50% lower dynamic on-resistance than the samples without the pretreatment. The improved dynamic performance and lower dispersion directly yield a 30% higher output power of (3.4 vs. 2.6 W/mm) and a better power added efficiency (44% vs. 39%) at 3 GHz. Furthermore, it was found that a longer pretreatment duration improves the uniformity of device performance. Traditionally, decreasing leakage currents in the buffer and improving electron confinement to the 2DEG are achieved by intentional acceptor-like dopants (iron and carbon) in the GaN buffer and back-barrier layer made by a ternary III-nitride material. However, electron trapping effects and thermal resistivity increase due to the dopants and the ternary material, respectively. In this thesis, a novel approach, where a unique epitaxial scheme permits a thickness reduction of the unintentional-doped (UID) GaN layer down to 250 nm, as compared to a normal thickness of 2 μm. In this way, the AlN nucleation layer effectively act as a back-barrier. The approached, named QuanFINE is investigated and benchmarked to a conventional epi-structure with a thick Fe-doped-GaN buffer. A 2DEG mobility of 2000 cm^2/V-s and the 2DEG concentration of 1.1∙10^13 cm^-2 on QuanFINE indicate that the 2DEG properties are not sacrificed with a thin UID-GaN layer. Thanks to the thin UID-GaN layer of QuanFINE, trapping effects are reduced. Comparable output power of 4.1 W/mm and a PAE of 40% at 3 GHz of both QuanFINE and conventional Fe-doped thick GaN buffer sample are measured.
|
|
