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  • AIP Publishing  (15)
  • Zhao, Yuji  (15)
  • 1
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    Online Resource
    Deep level transient spectroscopy investigation of ultra-wide bandgap (2̄01) and (001) β -Ga2O3
    AIP Publishing ; 2020
    In:  Journal of Applied Physics Vol. 128, No. 20 ( 2020-11-28)
    Details
    In: Journal of Applied Physics, AIP Publishing, Vol. 128, No. 20 ( 2020-11-28)
    Abstract: This work reports on a comprehensive examination of the electrical and thermal properties of vertical Schottky diodes fabricated on (2¯01)- and (001)-oriented samples of β-Ga2O3. The temperature-dependent current–voltage (I–V) and capacitance–voltage (C–V) data were gathered and analyzed down to 60 K. Deep level transient spectroscopy (DLTS) was used to study bulk and interface defects in the two materials from approx. 325 K down to 60 K. In the bulk (2¯01) material, an electron trap was observed at EC−0.46 eV, with a capture cross section of 1.6 × 10−14 cm2 and a lambda-corrected maximum trap density of 9.08 × 1015 cm−3. These results and others indicate that the electron trap is a strong candidate for the well-known E1 defect in β-Ga2O3 based on recent investigations. Additionally, in the (2¯01) material, the smooth modulation typical of interface states is evident at temperatures below 275 K. The (001) samples manifested what is likely the E2* electron trap at EC−0.68 eV, with a capture cross section of 1.64 × 10−15 cm2 and a lambda-corrected maximum trap density of 8.85 × 1015 cm−3. The presence of the E2* defect, in particular, is a contrast to the findings of recent DLTS investigations on β-Ga2O3, which report that E2* emerged only after low-energy proton irradiation. These results help to further map out the defect signatures found in β-Ga2O3 materials, which are of vital importance in the design and fabrication of future β-Ga2O3 devices.
    Type of Medium: Online Resource
    ISSN: 0021-8979 , 1089-7550
    URL: Article
    DOI: 10.1063/5.0021859
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2020
    detail.hit.zdb_id: 220641-9
    detail.hit.zdb_id: 3112-4
    detail.hit.zdb_id: 1476463-5
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  • 2
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    Demonstration of low loss β -Ga2O3 optical waveguides in the UV–NIR spectra
    AIP Publishing ; 2019
    In:  Applied Physics Letters Vol. 115, No. 25 ( 2019-12-16)
    Details
    In: Applied Physics Letters, AIP Publishing, Vol. 115, No. 25 ( 2019-12-16)
    Abstract: In this paper, we report the fabrication of low loss beta-phase gallium oxide (β-Ga2O3) optical waveguides and the propagation loss analysis of the waveguides in the ultraviolet (UV) to near infrared (NIR) spectral region. The β-Ga2O3 thin films were grown on sapphire substrates using metal organic chemical vapor deposition and were fabricated into various waveguide structures using nanofabrication processes. A low propagation loss of 3.7 dB/cm was obtained on the β-Ga2O3 waveguide at the wavelength of 810 nm, which is comparable to the state of the art. Combined with theoretical simulations, various loss mechanisms from two-photon absorption, sidewall scattering, top surface scattering, and bulk scattering were discussed for β-Ga2O3 waveguides, and their contributions to the total optical loss were estimated. These results show that β-Ga2O3 is a promising optical material for the fabrication of various integrated photonic devices in the UV–NIR spectra region.
    Type of Medium: Online Resource
    ISSN: 0003-6951 , 1077-3118
    URL: Article
    DOI: 10.1063/1.5133845
    RVK:
    UA 1000
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2019
    detail.hit.zdb_id: 211245-0
    detail.hit.zdb_id: 1469436-0
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  • 3
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    Demonstration of mechanically exfoliated β -Ga2O3/GaN p-n heterojunction
    AIP Publishing ; 2019
    In:  Applied Physics Letters Vol. 114, No. 16 ( 2019-04-22)
    Details
    In: Applied Physics Letters, AIP Publishing, Vol. 114, No. 16 ( 2019-04-22)
    Abstract: This work demonstrates the construction of p-n heterojunctions between mechanically exfoliated beta-phase gallium oxide (β-Ga2O3) and p-GaN. The detailed mechanical exfoliation process was developed and can be used for further device applications. The atomic force microscopy study showed that the exfoliated β-Ga2O3 flakes had a very smooth surface with a roughness of 0.65 nm. Transmission electron microscopy revealed a clearly defined interface between the exfoliated β-Ga2O3 and p-GaN. The p-n heterojunction exhibited a turn-on voltage of 3.6 V and a rectification ratio of ∼105. The heterojunction also showed good thermal performance up to 200 °C. Ideality factors and turn-on voltages decrease with temperature, tending toward the ideal threshold voltage of 3.2 V as determined by Silvaco simulations. This work provides valuable information on a mechanically exfoliated β-Ga2O3/GaN p-n heterojunction, which opens up the opportunities for a variety of photonic and electronic applications.
    Type of Medium: Online Resource
    ISSN: 0003-6951 , 1077-3118
    URL: Article
    DOI: 10.1063/1.5088516
    RVK:
    UA 1000
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2019
    detail.hit.zdb_id: 211245-0
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  • 4
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    Demonstration of GaN-based metal–insulator–semiconductor junction by hydrogen plasma treatment
    AIP Publishing ; 2020
    In:  Applied Physics Letters Vol. 117, No. 5 ( 2020-08-03)
    Details
    In: Applied Physics Letters, AIP Publishing, Vol. 117, No. 5 ( 2020-08-03)
    Abstract: We demonstrate a nickel/insulating-GaN (i-GaN)/p-type GaN junction and investigate its electrical properties. The i-GaN is formed by exposure to a low-power hydrogen plasma to passivate the p-GaN layer. Cathodoluminescence spectroscopy of the i-GaN is used to understand the passivation effect of the hydrogen plasma on p-GaN. The junction shows very low leakage ( & lt;10−9 A at −50 V), excellent rectifying properties (∼107), high temperature stability, and blue light electroluminescence at forward bias. A bandgap model is proposed to illustrate the electrical properties of hydrogenated p-GaN and to understand the device characteristics.
    Type of Medium: Online Resource
    ISSN: 0003-6951 , 1077-3118
    URL: Article
    DOI: 10.1063/5.0018473
    RVK:
    UA 1000
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2020
    detail.hit.zdb_id: 211245-0
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  • 5
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    Theoretical analysis of modulation doping effects on intersubband transition properties of semipolar AlGaN/GaN quantum well
    AIP Publishing ; 2017
    In:  Journal of Applied Physics Vol. 121, No. 1 ( 2017-01-07)
    Details
    In: Journal of Applied Physics, AIP Publishing, Vol. 121, No. 1 ( 2017-01-07)
    Abstract: The effects of modulation doping on the intersubband transition (ISBT) properties of semipolar AlGaN/GaN quantum well (QW) are investigated theoretically using QW doping, barrier doping, and barrier δ-doping schemes at 150 K. Important ISBT parameters such as intersubband transition energies, dipole matrix elements, and absorption spectra are calculated for QW structures on both semipolar (202¯1) (i.e., with weak polarization) and (101¯3) (i.e., with strong polarization) planes. For (202¯1) QW with weak polarization, it is found that high doping concentrations can cause a significant band bowing to the QW structures, which reduce the absorption coefficients and wavelengths. This band bowing effect will become stronger when doping layers are closer to the QW. For (101¯3) QW with a strong polarization, however, a weak band bowing effect is observed due to the large polarization and large band tilting of (101¯3) QW. The study shows that modulation doping is a promising method to modify the ISBT properties of semipolar AlGaN/GaN QW to achieve an improved performance such as longer ISBT wavelength (e.g., & gt;20 μm).
    Type of Medium: Online Resource
    ISSN: 0021-8979 , 1089-7550
    URL: Article
    DOI: 10.1063/1.4972975
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2017
    detail.hit.zdb_id: 220641-9
    detail.hit.zdb_id: 3112-4
    detail.hit.zdb_id: 1476463-5
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  • 6
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    Analysis of loss mechanisms in InGaN solar cells using a semi-analytical model
    AIP Publishing ; 2016
    In:  Journal of Applied Physics Vol. 119, No. 21 ( 2016-06-07)
    Details
    In: Journal of Applied Physics, AIP Publishing, Vol. 119, No. 21 ( 2016-06-07)
    Abstract: InGaN semiconductors are promising candidates for high-efficiency next-generation thin film solar cells. In this work, we study the photovoltaic performance of single-junction and two-junction InGaN solar cells using a semi-analytical model. We analyze the major loss mechanisms in InGaN solar cell including transmission loss, thermalization loss, spatial relaxation loss, and recombination loss. We find that transmission loss plays a major role for InGaN solar cells due to the large bandgaps of III-nitride materials. Among the recombination losses, Shockley-Read-Hall recombination loss is the dominant process. Compared to other III-V photovoltaic materials, we discovered that the emittance of InGaN solar cells is strongly impacted by Urbach tail energy. For two- and multi-junction InGaN solar cells, we discover that the current matching condition results in a limited range of top-junction bandgaps. This theoretical work provides detailed guidance for the design of high-performance InGaN solar cells.
    Type of Medium: Online Resource
    ISSN: 0021-8979 , 1089-7550
    URL: Article
    DOI: 10.1063/1.4953006
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2016
    detail.hit.zdb_id: 220641-9
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  • 7
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    Energy band engineering of InGaN/GaN multi-quantum-well solar cells via AlGaN electron- and hole-blocking layers
    AIP Publishing ; 2018
    In:  Applied Physics Letters Vol. 113, No. 4 ( 2018-07-23)
    Details
    In: Applied Physics Letters, AIP Publishing, Vol. 113, No. 4 ( 2018-07-23)
    Abstract: In this paper, we perform a comprehensive study on energy band engineering of InGaN multi-quantum-well (MQW) solar cells using AlGaN electron- and hole-blocking layers. InGaN MQW solar cells with AlGaN layers were grown by metalorganic chemical vapor deposition, and high crystal quality was confirmed by high resolution X-ray diffraction measurements. Time-resolved photoluminescence results showed that the carrier lifetime on the solar cells with AlGaN layers increased by more than 40% compared to that on the reference samples, indicating greatly improved carrier collections. The illuminated current-density (J–V) measurements further confirmed that the short-circuit current density (Jsc) of the solar cells also benefited from the AlGaN layer design and increased 46%. At room temperature, the InGaN solar cells with AlGaN layers showed much higher power conversion efficiency (PCE), by up to two-fold, compared to reference devices. At high temperatures, these solar cells with AlGaN layers also delivered superior photovoltaic (PV) performance such as PCE, Jsc, and fill factor than the reference devices. These results indicate that band engineering with AlGaN layers in the InGaN MQW solar cell structures can effectively enhance the carrier collection process and is a promising design for high efficiency InGaN solar cells for both room temperature and high temperature PV applications.
    Type of Medium: Online Resource
    ISSN: 0003-6951 , 1077-3118
    URL: Article
    DOI: 10.1063/1.5028530
    RVK:
    UA 1000
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2018
    detail.hit.zdb_id: 211245-0
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  • 8
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    Nonpolar and semipolar InGaN/GaN multiple-quantum-well solar cells with improved carrier collection efficiency
    AIP Publishing ; 2017
    In:  Applied Physics Letters Vol. 110, No. 16 ( 2017-04-17)
    Details
    In: Applied Physics Letters, AIP Publishing, Vol. 110, No. 16 ( 2017-04-17)
    Abstract: We demonstrate the nonpolar and semipolar InGaN/GaN multiple-quantum-well (MQW) solar cells grown on the nonpolar m-plane and semipolar (202¯1) plane bulk GaN substrates. The optical properties and photovoltaic performance of the nonpolar and semipolar InGaN solar cells were systematically studied, and the results were compared to the conventional polar c-plane devices. The absorption spectra, current density–voltage (J–V) characteristics, external quantum efficiency (EQE), and internal quantum efficiency (IQE) were measured for nonpolar m-plane, semipolar (202¯1) plane, and polar c-plane InGaN/GaN MQW solar cells. Nonpolar m-plane InGaN/GaN MQW solar cells showed the best performance across all devices, with a high open-circuit voltage of 2.32 V, a low bandgap-voltage offset of 0.59 V, and the highest EQE and IQE. In contrast, the polar c-plane device showed the lowest EQE despite the highest absorption spectra. This huge difference is attributed to the better carrier transport and collection on nonpolar m-plane devices due to the reduced polarization effects, which were further confirmed by bias-dependent EQE measurements and energy band diagram simulations. This study demonstrates the high potential of nonpolar and semipolar InGaN solar cells and can serve as guidance for the future design and fabrication of high efficiency III-nitride solar cells.
    Type of Medium: Online Resource
    ISSN: 0003-6951 , 1077-3118
    URL: Article
    DOI: 10.1063/1.4980139
    RVK:
    UA 1000
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2017
    detail.hit.zdb_id: 211245-0
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  • 9
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    Ultra-low turn-on voltage and on-resistance vertical GaN-on-GaN Schottky power diodes with high mobility double drift layers
    AIP Publishing ; 2017
    In:  Applied Physics Letters Vol. 111, No. 15 ( 2017-10-09)
    Details
    In: Applied Physics Letters, AIP Publishing, Vol. 111, No. 15 ( 2017-10-09)
    Abstract: This letter reports the implementation of double-drift-layer (DDL) design into GaN vertical Schottky barrier diodes (SBDs) grown on free-standing GaN substrates. This design balances the trade-off between desirable forward turn-on characteristics and high reverse breakdown capability, providing optimal overall device performances for power switching applications. With a well-controlled metalorganic chemical vapor deposition process, the doping concentration of the top drift layer was reduced, which served to suppress the peak electric field at the metal/GaN interface and increase the breakdown voltages of the SBDs. The bottom drift layer was moderately doped to achieve low on-resistance to reduce power losses. At forward bias, the devices exhibited a record low turn-on voltage of 0.59 V, an ultra-low on-resistance of 1.65 mΩ cm2, a near unity ideality factor of 1.04, a high on/off ratio of ∼1010, and a high electron mobility of 1045.2 cm2/(V s). Detailed comparisons with conventional single-drift-layer (SDL) GaN vertical SBDs indicated that DDL design did not degrade the forward characteristics of the SBDs. At reverse bias, breakdown voltages of the DDL GaN SBDs were considerably enhanced compared to those of the conventional SDL devices. These results showed that GaN vertical SBDs with DDL designs are promising candidates for high efficiency, high voltage, high frequency power switching applications.
    Type of Medium: Online Resource
    ISSN: 0003-6951 , 1077-3118
    URL: Article
    DOI: 10.1063/1.4993201
    RVK:
    UA 1000
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2017
    detail.hit.zdb_id: 211245-0
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  • 10
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    Investigation of GaN-on-GaN vertical p - n diode with regrown p -GaN by metalorganic chemical vapor deposition
    AIP Publishing ; 2018
    In:  Applied Physics Letters Vol. 113, No. 23 ( 2018-12-03)
    Details
    In: Applied Physics Letters, AIP Publishing, Vol. 113, No. 23 ( 2018-12-03)
    Abstract: To mimic selective-area doping, p-GaN was regrown on an etched GaN surface on GaN substrates by metalorganic chemical vapor deposition. Vertical GaN-on-GaN p-n diodes were fabricated to investigate the effects of the etch-then-regrowth process on device performance. The crystal quality of the sample after each epitaxial step was characterized by X-ray diffraction, where the etch-then-regrowth process led to a very slight increase in edge dislocations. A regrowth interfacial layer was clearly shown by transmission electron microscopy. Strong electroluminescence was observed with three emission peaks at 2.2 eV, 2.8 eV, and 3.0 eV. The forward current density increased slightly with increasing temperature, while the reverse current density was almost temperature independent indicating tunneling as the reverse transport mechanism. This result is very similar to the reported Zener tunnel diode comprising a high doping profile at the junction interface. High levels of silicon and oxygen concentrations were observed at the regrowth interface with a distribution width of ∼100 nm. This work provides valuable information on p-GaN regrowth and regrown GaN p-n diodes, which can serve as an important reference for developing selective doping for advanced GaN power electronics for high voltage and high power applications.
    Type of Medium: Online Resource
    ISSN: 0003-6951 , 1077-3118
    URL: Article
    DOI: 10.1063/1.5052479
    RVK:
    UA 1000
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2018
    detail.hit.zdb_id: 211245-0
    detail.hit.zdb_id: 1469436-0
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