Effects of electron irradiation on GaN and AlxGa1-xN doped with acceptor-forming species (Mg, C, ... more Effects of electron irradiation on GaN and AlxGa1-xN doped with acceptor-forming species (Mg, C, Fe, and Mn) were studied by cathodoluminescence and electron beam induced current techniques. Low energy electron beam irradiation was shown to induce a systematic decay of the cathodoluminescence intensity, which is accompanied by increased electronic carrier diffusion length, indicating the increase of carrier lifetime. Temperature-dependent cathodoluminescence
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2013
ABSTRACT ZnO thin films were epitaxially grown on c-plane sapphire substrates by plasma-assisted ... more ABSTRACT ZnO thin films were epitaxially grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. A low temperature homonucleation ZnO layer was found crucial at the interfacial region to absorb the defects formed by the lattice mismatch between the sapphire and ZnO, resulting in a smooth surface that enables smooth 2D epitaxial growth. High quality ZnO films were achieved after careful optimization of critical growth conditions: the sequence of Zn and O source shutters, growth temperature for both the ZnO nucleation and growth layer, and Zn/O ratio. Oxygen plasma pretreatment was not applied prior to the growth, thus shortening the growth time and reducing oxidation of the metallic sources. Resultant epitaxial ZnO films on sapphire demonstrated a root-mean-square surface roughness of 0.373 nm for 1 μm × 1 μm atomic force microscope images with clear hexagonal structure and terrace steps. The x-ray diffraction full width at half maximum (FWHM) for ω and ω-2θ ZnO (0002) triple-crystal rocking curves were measured to be 13 and 26 arc/s, respectively. This FWHM value is lower than any reported to date in the literature, with ω and ω-2θ values indicating excellent coherence of the epitaxial layer along the interface and the growth direction, accordingly. These x-ray diffraction and surface roughness values are lower than those obtained using common nucleation layers such as MgO, indicating that growth with ZnO nucleation layers on sapphire may lead to higher quality electrical and optical devices.
We report on a technique for optimizing transport properties in p- and n-type AlGaN/GaN and GaN/I... more We report on a technique for optimizing transport properties in p- and n-type AlGaN/GaN and GaN/InGaN superlattices. As we show highly conductive heterostructures can be obtained by inserting a graded doped layer, which reduces the barrier height while maintaining high sheet carrier density. For optimized p-type AlGaN/GaN SL, an eight fold reduction of the barrier height and a 1.5 times increase in sheet hole density is obtained compared to typical SL. The optimized structure yields 13 orders of magnitude improvement in vertical conductivity (sigmaV) compared to typical SL, and 35 times improvement in lateral conductivity (sigmaL) compared to bulk p-GaN. For optimized p-type GaN/InGaN SL, an improvement of more than 10 orders of magnitude in sigmaV compared to typical SL is obtained with sigmaL better than that of bulk p-InGaN. We also investigate n-type SLs as current spreading layers. A significant improvement in current distribution is obtained for the optimized SLs.
ABSTRACT In the paper we present band-gap alignment diagrams for type-II/type-I heterostructures ... more ABSTRACT In the paper we present band-gap alignment diagrams for type-II/type-I heterostructures incorporating the strong piezoelectric and spontaneous polarization fields in ZnCdMgO and AlGaN-based materials and interfaces. The paper describes some of the recent progress made in the development of high quality ZnCdO layers and CdZnO/ZnO heterostructures grown epitaxially by RF-plasma molecular beam epitaxy (MBE). We summarize optical and electrical properties of high quality CdxZn1-xO alloys with Cd mole fraction from 0.02 to 0.78 and discuss phase separation phenomenon, which may be present in ternary alloys. A single-crystal wurtzite structure of CdZnO alloys has been confirmed. We achieved a strong optical emission at RT in the 380 nm to 574 nm spectral range from CdxZn1-xO with various compositions. Dependence of the fundamental optical band gap on the composition of CdxZn1- xO alloys, band gap bowing, and the possible effect of composition micro-fluctuations in ternary CdxZn1-xO alloys on the optical bandgap is also discussed. Time resolved photoluminescence shows multi-exponential decay with 21 psec. and 49{plus minus}3 psec. lifetimes, suggesting that composition micro- fluctuations may be present in Cd0.16Zn0.84O film. We also report on crystallographic and optical properties of CdZnO/ZnO multiple quantum wells.
... Due to their low intrinsic carrier concentrations, wide band gap semiconductor sensors based ... more ... Due to their low intrinsic carrier concentrations, wide band gap semiconductor sensors based on GaN or SiC can be operated at lower current levels than conventional Si-based devices and offer the capability of detection to . ...
ABSTRACT We have carried out comprehensive optical studies to evaluate structural and bandgap pro... more ABSTRACT We have carried out comprehensive optical studies to evaluate structural and bandgap properties of Zn1-xCdxO alloys with x {less than or equal to} 0.17 grown by molecular beam epitaxy. High crystalline quality of the alloys was concluded from cathodoluminescence measurements. Based on absorption and reflectance measurements, the compositional dependence of the bandgap energy of ZnCdO, estimated without taking into account excitonic effects, was found to follow the trend Eg(x)=3.28- 2.23x+0.45x2. Degradation in the alloy quality due to possible phase separation was found to cause deviations from this trend, evident from a more rapid red shift of the absorption edge. Effects of Cd incorporation on the variation of the bandgap energies with temperature are also discussed.
Recent advances in the processing of complex-oxide materials has allowed the authors to monolithi... more Recent advances in the processing of complex-oxide materials has allowed the authors to monolithically grow ferroelectrics of lead lanthanum zirconate titanate (PLZT) and barium strontium titanate (BST) systems on a GaN/sapphire structure. High quality films of PLZT and BST were grown on GaN/c-AlO in a thickness range of 0.3--5 m by a sol-gel technique. Field-induced birefringence, as large as 0.02, was measured from a PLZT layer grown on a buffered GaN/sapphire structure. UV illumination was found to result in more symmetrical electrooptic hysteresis loop. BST films on GaN demonstrated a low frequency dielectric constant of up to 800 with leakage current density as low as 5.5 · 10 A/cm².
One of the main issues for high-temperature operation of GaN-based devices is the development of ... more One of the main issues for high-temperature operation of GaN-based devices is the development of reliable metal contacts. We have investigated the thermal stability and device performance of AlGaN/GaN high-electron mobility transistors (HEMTs) with both conventional (Ti/Al/Ni/ ...
Effects of electron irradiation on GaN and AlxGa1-xN doped with acceptor-forming species (Mg, C, ... more Effects of electron irradiation on GaN and AlxGa1-xN doped with acceptor-forming species (Mg, C, Fe, and Mn) were studied by cathodoluminescence and electron beam induced current techniques. Low energy electron beam irradiation was shown to induce a systematic decay of the cathodoluminescence intensity, which is accompanied by increased electronic carrier diffusion length, indicating the increase of carrier lifetime. Temperature-dependent cathodoluminescence
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2013
ABSTRACT ZnO thin films were epitaxially grown on c-plane sapphire substrates by plasma-assisted ... more ABSTRACT ZnO thin films were epitaxially grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. A low temperature homonucleation ZnO layer was found crucial at the interfacial region to absorb the defects formed by the lattice mismatch between the sapphire and ZnO, resulting in a smooth surface that enables smooth 2D epitaxial growth. High quality ZnO films were achieved after careful optimization of critical growth conditions: the sequence of Zn and O source shutters, growth temperature for both the ZnO nucleation and growth layer, and Zn/O ratio. Oxygen plasma pretreatment was not applied prior to the growth, thus shortening the growth time and reducing oxidation of the metallic sources. Resultant epitaxial ZnO films on sapphire demonstrated a root-mean-square surface roughness of 0.373 nm for 1 μm × 1 μm atomic force microscope images with clear hexagonal structure and terrace steps. The x-ray diffraction full width at half maximum (FWHM) for ω and ω-2θ ZnO (0002) triple-crystal rocking curves were measured to be 13 and 26 arc/s, respectively. This FWHM value is lower than any reported to date in the literature, with ω and ω-2θ values indicating excellent coherence of the epitaxial layer along the interface and the growth direction, accordingly. These x-ray diffraction and surface roughness values are lower than those obtained using common nucleation layers such as MgO, indicating that growth with ZnO nucleation layers on sapphire may lead to higher quality electrical and optical devices.
We report on a technique for optimizing transport properties in p- and n-type AlGaN/GaN and GaN/I... more We report on a technique for optimizing transport properties in p- and n-type AlGaN/GaN and GaN/InGaN superlattices. As we show highly conductive heterostructures can be obtained by inserting a graded doped layer, which reduces the barrier height while maintaining high sheet carrier density. For optimized p-type AlGaN/GaN SL, an eight fold reduction of the barrier height and a 1.5 times increase in sheet hole density is obtained compared to typical SL. The optimized structure yields 13 orders of magnitude improvement in vertical conductivity (sigmaV) compared to typical SL, and 35 times improvement in lateral conductivity (sigmaL) compared to bulk p-GaN. For optimized p-type GaN/InGaN SL, an improvement of more than 10 orders of magnitude in sigmaV compared to typical SL is obtained with sigmaL better than that of bulk p-InGaN. We also investigate n-type SLs as current spreading layers. A significant improvement in current distribution is obtained for the optimized SLs.
ABSTRACT In the paper we present band-gap alignment diagrams for type-II/type-I heterostructures ... more ABSTRACT In the paper we present band-gap alignment diagrams for type-II/type-I heterostructures incorporating the strong piezoelectric and spontaneous polarization fields in ZnCdMgO and AlGaN-based materials and interfaces. The paper describes some of the recent progress made in the development of high quality ZnCdO layers and CdZnO/ZnO heterostructures grown epitaxially by RF-plasma molecular beam epitaxy (MBE). We summarize optical and electrical properties of high quality CdxZn1-xO alloys with Cd mole fraction from 0.02 to 0.78 and discuss phase separation phenomenon, which may be present in ternary alloys. A single-crystal wurtzite structure of CdZnO alloys has been confirmed. We achieved a strong optical emission at RT in the 380 nm to 574 nm spectral range from CdxZn1-xO with various compositions. Dependence of the fundamental optical band gap on the composition of CdxZn1- xO alloys, band gap bowing, and the possible effect of composition micro-fluctuations in ternary CdxZn1-xO alloys on the optical bandgap is also discussed. Time resolved photoluminescence shows multi-exponential decay with 21 psec. and 49{plus minus}3 psec. lifetimes, suggesting that composition micro- fluctuations may be present in Cd0.16Zn0.84O film. We also report on crystallographic and optical properties of CdZnO/ZnO multiple quantum wells.
... Due to their low intrinsic carrier concentrations, wide band gap semiconductor sensors based ... more ... Due to their low intrinsic carrier concentrations, wide band gap semiconductor sensors based on GaN or SiC can be operated at lower current levels than conventional Si-based devices and offer the capability of detection to . ...
ABSTRACT We have carried out comprehensive optical studies to evaluate structural and bandgap pro... more ABSTRACT We have carried out comprehensive optical studies to evaluate structural and bandgap properties of Zn1-xCdxO alloys with x {less than or equal to} 0.17 grown by molecular beam epitaxy. High crystalline quality of the alloys was concluded from cathodoluminescence measurements. Based on absorption and reflectance measurements, the compositional dependence of the bandgap energy of ZnCdO, estimated without taking into account excitonic effects, was found to follow the trend Eg(x)=3.28- 2.23x+0.45x2. Degradation in the alloy quality due to possible phase separation was found to cause deviations from this trend, evident from a more rapid red shift of the absorption edge. Effects of Cd incorporation on the variation of the bandgap energies with temperature are also discussed.
Recent advances in the processing of complex-oxide materials has allowed the authors to monolithi... more Recent advances in the processing of complex-oxide materials has allowed the authors to monolithically grow ferroelectrics of lead lanthanum zirconate titanate (PLZT) and barium strontium titanate (BST) systems on a GaN/sapphire structure. High quality films of PLZT and BST were grown on GaN/c-AlO in a thickness range of 0.3--5 m by a sol-gel technique. Field-induced birefringence, as large as 0.02, was measured from a PLZT layer grown on a buffered GaN/sapphire structure. UV illumination was found to result in more symmetrical electrooptic hysteresis loop. BST films on GaN demonstrated a low frequency dielectric constant of up to 800 with leakage current density as low as 5.5 · 10 A/cm².
One of the main issues for high-temperature operation of GaN-based devices is the development of ... more One of the main issues for high-temperature operation of GaN-based devices is the development of reliable metal contacts. We have investigated the thermal stability and device performance of AlGaN/GaN high-electron mobility transistors (HEMTs) with both conventional (Ti/Al/Ni/ ...
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Papers by Andrei Osinsky