Zahra Vashaei | Case Western Reserve University (original) (raw)

Papers by Zahra Vashaei

Altered chondrules covered by fine-grained rims (FGRs) in Cold Bokkeveld CM chondrite were anatom... more Altered chondrules covered by fine-grained rims (FGRs) in Cold Bokkeveld CM chondrite were anatomized by FIB-TEM/STEM to clarify aqueous alteration record. FIB-TEM/STEM analyses could reveal aqueous alteration by Fe- and Mg-rich fluid.

Proceedings of The National Academy of Sciences, 2008

Peace River is one of the few shocked members of the L-chondrites clan that contains both high-pr... more Peace River is one of the few shocked members of the L-chondrites clan that contains both high-pressure polymorphs of olivine, ringwoodite and wadsleyite, in diverse textures and settings in fragments entrained in shock-melt veins. Among these settings are complete olivine porphyritic chondrules. We encountered few squeezed and flattened olivine porphyritic chondrules entrained in shock-melt veins of this meteorite with novel textures and composition. The former chemically unzoned (Fa24-26) olivine porphyritic crystals are heavily flattened and display a concentric intergrowth with Mg-rich wadsleyite of a very narrow compositional range (Fa6-Fa10) in the core. Wadsleyite core is surrounded by a Mg-poor and chemically stark zoned ringwoodite (Fa28-Fa38) belt. The wadsleyite-ringwoodite interface denotes a compositional gap of up to 32 mol % fayalite. A transmission electron microscopy study of focused ion beam slices in both regions indicates that the wadsleyite core and ringwoodite belt consist of granoblastic-like intergrowth of polygonal crystallites of both ringwoodite and wadsleyite, with wadsleyite crystallites dominating in the core and ringwoodite crystallites dominating in the belt. Texture and compositions of both high-pressure polymorphs are strongly suggestive of formation by a fractional crystallization of the olivine melt of a narrow composition (Fa24-26), starting with Mg-rich wadsleyite followed by the Mg-poor ringwoodite from a shock-induced melt of olivine composition (Fa24-26). Our findings could erase the possibility of the resulting unrealistic time scales of the highpressure regime reported recently from other shocked L-6 chondrites.

The assemblages of wadsleyite and ringwoodite transformed from individual olivine contained in a ... more The assemblages of wadsleyite and ringwoodite transformed from individual olivine contained in a deformed chondrule were identified in the shock melt veins of Peace River L6 chondrite. The assemblages of wadsleyite and ringwoodite were extracted by using a FIB system, and analyzed by TEM/STEM-EDS. There was obvious enrichment or depletion in Mg and Fe ratios between ringwoodite (Fo = 66) and wadsleyite (Fo = 89). Using the interdiffusion coefficients in order to check if the contrasting compositions of ringwoodite and wadsleyite, respectively could have resulted from solid-state interdiffusion alone leads to unrealistically long duration of the shock event. Few stacking faults were observed both in ringwoodite and wadsleyite thus suggesting that both minerals didn't form by a shear mechanism. The deformed chondrules are squeezed and flattened implying strong plastic deformation and partial melting. There is no evidence of mixing of melts of olivine and clinopyroxene, the latter not melted. It is considered that wadsleyite and ringwoodite successively crystallized from individual melts of the former olivine crystals alone. Wadsleyite, low in Fe, crystallized first from melt in the interior of the former olivine crystals followed by Fe-rich ringwoodite nucleating on wadsleyite thus separating it from the residual melt enriched in Fe.

Journal of Mineralogical and Petrological Sciences, 2008

Application of FIB system to ultra-high-pressure Earth science

Research into III-Nitride based avalanche photodiodes (APDs) is motivated by the need for high se... more Research into III-Nitride based avalanche photodiodes (APDs) is motivated by the need for high sensitivity ultraviolet (UV) detectors in numerous civilian and military applications. By designing III-Nitride photodetectors that utilize low-noise impact ionization high internal gain can be realized-GaN APDs operating in Geiger mode can achieve gains exceeding 1×107. Thus with careful design, it becomes possible to count photons at the single photon level. In this paper we review the current state of the art in III-Nitride visible-blind APDs and discuss the critical design choices necessary to achieve high performance Geiger mode devices. Other major technical issues associated with the realization of visible-blind Geiger mode APDs are also discussed in detail and future prospects for improving upon the performance of these devices are outlined. The photon detection efficiency, dark count rate, and spectral response of or most recent Geiger-mode GaN APDs on free-standing GaN substrates are studied under low photon fluxes, with single photon detection capabilities being demonstrated. We also present our latest results regarding linear mode gain uniformity: the study of gain uniformity helps reveal the spatial origins of gain so that we can better understand the role of defects.

Journal of Applied Physics, 2010

GaN/AlN resonant tunneling diodes (RTD) were grown by metal-organic chemical vapor deposition (MO... more GaN/AlN resonant tunneling diodes (RTD) were grown by metal-organic chemical vapor deposition (MOCVD) and negative differential resistance with peak-to-valley ratios as high as 2.15 at room temperature was demonstrated. Effect of material quality on RTDs' performance was investigated by growing RTD structures on AlN, GaN, and lateral epitaxial overgrowth GaN templates. Our results reveal that negative differential resistance characteristics of RTDs are very sensitive to material quality (such as surface roughness) and MOCVD is a suitable technique for III-nitride-based quantum devices.

We report the synthesis, fabrication and testing of a 320 × 256 focal plane array (FPA) of back-i... more We report the synthesis, fabrication and testing of a 320 × 256 focal plane array (FPA) of back-illuminated, solarblind, p-i-n, AlxGa1-xN-based detectors, fully realized within our research laboratory. We implemented a novel pulsed atomic layer deposition technique for the metalorganic chemical vapor deposition (MOCVD) growth of crackfree, thick, and high Al composition AlxGa1-xN layers. Following the growth, the wafer was processed into a 320 × 256 array of 25 μm × 25 μm pixels on a 30 μm pixel-pitch and surrounding mini-arrays. A diagnostic mini-array was hybridized to a silicon fan-out chip to allow the study of electrical and optical characteristics of discrete pixels of the FPA. At a reverse bias of 1 V, an average photodetector exhibited a low dark current density of 1.12×10-8 A/cm2. Solar-blind operation is observed throughout the array with peak detection occurring at wavelengths of 256 nm and lower and falling off three orders of magnitude by 285 nm. After indium bump deposition and dicing, the FPA is hybridized to a matching ISC 9809 readout integrated circuit (ROIC). By developing a novel masking technology, we significantly reduced the visible response of the ROIC and thus the need for external filtering to achieve solar- and visible-blind operation is eliminated. This allowed the FPA to achieve high external quantum efficiency (EQE): at 254 nm, average pixels showed unbiased peak responsivity of 75 mA/W, which corresponds to an EQE of ~37%. Finally, the uniformity of the FPA and imaging properties are investigated.

Al(Ga)N/GaN resonant tunneling diodes (RTDs) are grown by metal-organic chemical vapor deposition... more Al(Ga)N/GaN resonant tunneling diodes (RTDs) are grown by metal-organic chemical vapor deposition. The effects of material quality on room temperature negative differential resistance (NDR) behaviour of RTDs are investigated by growing the RTD structure on AlN, GaN, and lateral epitaxial overgrowth GaN templates. This reveals that NDR characteristics of RTDs are very sensitive to material quality (such as surface roughness and dislocations density). The effects of the aluminum content of AlGaN double barriers (DB) and polarization fields on NDR characteristic of AlGaN/GaN RTDs were also investigated by employing low dislocation density c-plane (polar) and m-plane (nonpolar) freestanding GaN substrates. Lower aluminum content in the DB RTD active layer and minimization of dislocations and polarization fields enabled a more reliable and reproducible NDR behaviour at room temperature.

There is a need for semiconductor-based ultraviolet photodetectors to support avalanche gain in o... more There is a need for semiconductor-based ultraviolet photodetectors to support avalanche gain in order to realize better performance andmore effective compete with existing technologies. Wide bandgap III-Nitride semiconductors are the promising material system for the development of avalanche photodiodes (APDs) that could be a viable alternative to current bulky UV detectors such as photomultiplier tubes. In this paper, we review the current state-of-the-art in IIINitride visible-blind APDs, and present our latest results on GaN APDs grown on both conventional sapphire and low dislocation density free-standing c- and m-plane GaN substrates. Leakage current, gain, and single photon detection efficiency (SPDE) of these APDs were compared. The spectral response and Geiger-mode photon counting performance of UV APDs are studied under low photon fluxes, with single photon detection capabilities as much as 30% being demonstrated in smaller devices. Geiger-mode operation conditions are optimized for enhanced SPDE.

Applied Physics Letters, 2010

GaN avalanche photodiodes ͑APDs͒ were grown on both conventional sapphire and low dislocation den... more GaN avalanche photodiodes ͑APDs͒ were grown on both conventional sapphire and low dislocation density free-standing ͑FS͒ c-plane GaN substrates. Leakage current, gain, and single photon detection efficiency ͑SPDE͒ of these APDs were compared. At a reverse-bias of 70 V, APDs grown on sapphire substrates exhibited a dark current density of 2.7ϫ 10 −4 A / cm 2 whereas APDs grown on FS-GaN substrates had a significantly lower dark current density of 2.1ϫ 10 −6 A / cm 2 . Under linear-mode operation, APDs grown on FS-GaN achieved avalanche gain as high as 14 000. Geiger-mode operation conditions were studied for enhanced SPDE. Under front-illumination the 625-m 2 -area APD yielded a SPDE of ϳ13% when grown on sapphire substrates compared to more than 24% when grown on FS-GaN. The SPDE of the same APD on sapphire substrate increased to ϳ30% under back-illumination-the FS-GaN APDs were only tested under front illumination due to the thick absorbing GaN substrate.

Applied Physics Letters, 2010

AlGaN/GaN resonant tunneling diodes (RTDs), consisting of 20% (10%) aluminum-content in double-ba... more AlGaN/GaN resonant tunneling diodes (RTDs), consisting of 20% (10%) aluminum-content in double-barrier (DB) active layer, were grown by metal-organic chemical vapor deposition on freestanding polar (c-plane) and nonpolar (m-plane) GaN substrates. RTDs were fabricated into 35-μm-diameter devices for electrical characterization. Lower aluminum content in the DB active layer and minimization of dislocations and polarization fields increased the reliability and reproducibility of room-temperature negative differential resistance (NDR). Polar RTDs showed decaying NDR behavior, whereas nonpolar ones did not significantly. Averaging over 50 measurements, nonpolar RTDs demonstrated a NDR of 67 Ω, a current-peak-to-valley ratio of 1.08, and an average oscillator output power of 0.52 mW.

Applied Physics Letters, 2010

AlN/GaN double-barrier resonant tunneling diodes (RTDs) were grown by metal-organic chemical vapo... more AlN/GaN double-barrier resonant tunneling diodes (RTDs) were grown by metal-organic chemical vapor deposition on sapphire. RTDs were fabricated via standard processing steps. RTDs demonstrate a clear negative differential resistance (NDR) at room temperature (RT). The NDR was observed around 4.7 V with a peak current density of 59 kA/cm2 and a peak-to-valley ratio of 1.6 at RT. Dislocation-free material is shown to be the key for the performance of GaN RTDs.

Applied Physics Letters, 2010

III-nitride resonant tunneling diodes (RTDs), consisting Al0.2Ga0.8N/GaN double-barrier (DB) acti... more III-nitride resonant tunneling diodes (RTDs), consisting Al0.2Ga0.8N/GaN double-barrier (DB) active layers, were grown on c-plane lateral epitaxial overgrowth (LEO) GaN/sapphire and c-plane freestanding (FS) GaN. RTDs on both templates, fabricated into mesa diameters ranging from 5 to 35 μm, showed negative differential resistance (NDR) at room temperature. NDR characteristics (voltage and current density at NDR onset and current-peak-to-valley ratio) were analyzed and reported as a function of device size and substrate choice. Our results show that LEO RTDs perform as well as FS ones and DB active layer design and quality have been the bottlenecks in III-nitride RTDs.

Applied Physics Letters, 2010

M-plane GaN avalanche p-i-n photodiodes on low dislocation density freestanding m-plane GaN subst... more M-plane GaN avalanche p-i-n photodiodes on low dislocation density freestanding m-plane GaN substrates were realized using metal-organic chemical vapor deposition. High quality homoepitaxial m-plane GaN layers were developed; the root-mean-square surface roughness was less than 1 A˚ and the full-width-at-half-maximum value of the x-ray rocking curve for (1010) diffraction of m-plane GaN epilayer was 32 arcsec. High quality material led to a low reverse-bias dark current of 8.11 pA for 225 μm2 mesa photodetectors prior to avalanche breakdown, with the maximum multiplication gain reaching about 8000.

Applied Physics Letters, 2010

High quality Al 0.2 Ga 0.8 N / GaN superlattices ͑SLs͒ with various ͑GaN͒ well widths ͑1.6 to 6.4... more High quality Al 0.2 Ga 0.8 N / GaN superlattices ͑SLs͒ with various ͑GaN͒ well widths ͑1.6 to 6.4 nm͒ have been grown on polar c-plane and nonpolar m-plane freestanding GaN substrates by metal-organic chemical vapor deposition. Atomic force microscopy, high resolution x-ray diffraction, and photoluminescence ͑PL͒ studies of SLs have been carried out to determine and correlate effects of well width and polarization field on the room-temperature PL characteristics. A theoretical model was applied to explain PL energy-dependency on well width and crystalline orientation taking into account internal electric field for polar substrate. Absence of induced-internal electric field in nonpolar SLs was confirmed by stable PL peak energy and stronger PL intensity as a function of excitation power density than polar ones.

Physica Status Solidi (c), 2006

Much interest has recently been shown in Mg x Zn 1-x O alloys films as a promising candidate for ... more Much interest has recently been shown in Mg x Zn 1-x O alloys films as a promising candidate for ZnO band gap engineering (from 3.37 eV for ZnO to 7.8 eV for MgO). It has also been found to be a suitable material for the barrier layers of Zn/MgZnO super lattices due to its wider band gap. Mg x Zn 1-x O layers can posses two different crystal structure of rock-salt cubic and hexagonal wurtzite as a result of Mg content difference.

Journal of Applied Physics, 2004

We have investigated the characteristic of the dislocations in the layers grown on sapphire by th... more We have investigated the characteristic of the dislocations in the layers grown on sapphire by the plasma-assisted molecular beam epitaxy under the different flux ratios. The layers were characterized by the transmission electron microscopy (TEM) and the high-resolution x-ray diffraction ( ...

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2005

ABSTRACT The polarity of ZnO films grown on MgO/c-Al2O3 templates are successfully controlled by ... more ABSTRACT The polarity of ZnO films grown on MgO/c-Al2O3 templates are successfully controlled by only varying MgO buffer layer thickness. The crystal structure of the MgO buffer on c-Al2O3 is of wurtzite (WZ), when the MgO buffer thickness is less than 2.7 nm. As the MgO layer thickness exceeds, the crystal structure of MgO changes to rock salt (RS) structure. A ZnO layer grown on WZ-MgO results in O-polar ZnO, while the polarity of the ZnO layer grown on RS-MgO changes to Zn polar. Possible atomic configurations of the ZnO/MgO/Al2O3 interface structure are suggested.

Journal of Applied Physics, 2005

We report on the structure study of MgxZn1-xO films and, in particular, we will focus on MgxZn1-x... more We report on the structure study of MgxZn1-xO films and, in particular, we will focus on MgxZn1-xO layers with x=0.28 and 0.41 MgxZn1-xO layers with different crystal structures of cubic and wurtzite that have been grown by plasma-assisted molecular-beam epitaxy on MgO/c-sapphire with Mg/Zn flux ratio control. The MgxZn1-xO films have been characterized by high-resolution transmission electron microscopy (HRTEM) and high-resolution x-ray diffraction. The dependence of the cation-anion bond length to Mg content has been studied. A virtual crystal model of MgZnO has been applied to interpret the bond-length variation. HRTEM results indicate that the initial stage of the MgZnO growth on a MgO buffer layer starts with a cubic structure even in the case of a wurtzite structure at the end of growth.

Current Applied Physics, 2004

In this study, structural properties of epitaxial Ga-doped Mg0.1Zn0.9O layers grown on ZnO/α-Al2O... more In this study, structural properties of epitaxial Ga-doped Mg0.1Zn0.9O layers grown on ZnO/α-Al2O3 templates by plasma-assisted molecular beam epitaxy have been investigated by high-resolution transmission electron microscopy (HRTEM), and high resolution X-ray diffraction (HRXRD). From analysis of the diffraction pattern, the monocrystallinity of the Mg0.1Zn0.9O layer with hexagonal structure is confirmed. The orientation relationship between Mg0.1Zn0.9O and the template is determined as (0001)Mg0.1Zn0.9O∥(0001)ZnO∥(0001)Al2O3 and [21̄1̄0]Mg0.1Zn0.9O ∥[21̄1̄0]ZnO∥[11̄00]Al2O3. The density of dislocations near the top surface layers measured by plan-view TEM is about 3.61010 cm−2, one order of magnitude higher than the value obtained for ZnO layers on α-Al2O3 with a MgO buffer. Cross-sectional observation revealed that the majority of threading dislocations are in the [0001] line direction, i.e. they lie along the surface normal and consist of edge, screw, and mixed dislocations. Cross- sectional TEM and X-ray rocking curve experiments reveal that most of dislocations are edge dislocations. The interface of Mg0.1Zn0.9O and ZnO layers and the effect of excess Ga-doping in these layers have been also studied.

Altered chondrules covered by fine-grained rims (FGRs) in Cold Bokkeveld CM chondrite were anatom... more Altered chondrules covered by fine-grained rims (FGRs) in Cold Bokkeveld CM chondrite were anatomized by FIB-TEM/STEM to clarify aqueous alteration record. FIB-TEM/STEM analyses could reveal aqueous alteration by Fe- and Mg-rich fluid.

Proceedings of The National Academy of Sciences, 2008

Peace River is one of the few shocked members of the L-chondrites clan that contains both high-pr... more Peace River is one of the few shocked members of the L-chondrites clan that contains both high-pressure polymorphs of olivine, ringwoodite and wadsleyite, in diverse textures and settings in fragments entrained in shock-melt veins. Among these settings are complete olivine porphyritic chondrules. We encountered few squeezed and flattened olivine porphyritic chondrules entrained in shock-melt veins of this meteorite with novel textures and composition. The former chemically unzoned (Fa24-26) olivine porphyritic crystals are heavily flattened and display a concentric intergrowth with Mg-rich wadsleyite of a very narrow compositional range (Fa6-Fa10) in the core. Wadsleyite core is surrounded by a Mg-poor and chemically stark zoned ringwoodite (Fa28-Fa38) belt. The wadsleyite-ringwoodite interface denotes a compositional gap of up to 32 mol % fayalite. A transmission electron microscopy study of focused ion beam slices in both regions indicates that the wadsleyite core and ringwoodite belt consist of granoblastic-like intergrowth of polygonal crystallites of both ringwoodite and wadsleyite, with wadsleyite crystallites dominating in the core and ringwoodite crystallites dominating in the belt. Texture and compositions of both high-pressure polymorphs are strongly suggestive of formation by a fractional crystallization of the olivine melt of a narrow composition (Fa24-26), starting with Mg-rich wadsleyite followed by the Mg-poor ringwoodite from a shock-induced melt of olivine composition (Fa24-26). Our findings could erase the possibility of the resulting unrealistic time scales of the highpressure regime reported recently from other shocked L-6 chondrites.

The assemblages of wadsleyite and ringwoodite transformed from individual olivine contained in a ... more The assemblages of wadsleyite and ringwoodite transformed from individual olivine contained in a deformed chondrule were identified in the shock melt veins of Peace River L6 chondrite. The assemblages of wadsleyite and ringwoodite were extracted by using a FIB system, and analyzed by TEM/STEM-EDS. There was obvious enrichment or depletion in Mg and Fe ratios between ringwoodite (Fo = 66) and wadsleyite (Fo = 89). Using the interdiffusion coefficients in order to check if the contrasting compositions of ringwoodite and wadsleyite, respectively could have resulted from solid-state interdiffusion alone leads to unrealistically long duration of the shock event. Few stacking faults were observed both in ringwoodite and wadsleyite thus suggesting that both minerals didn't form by a shear mechanism. The deformed chondrules are squeezed and flattened implying strong plastic deformation and partial melting. There is no evidence of mixing of melts of olivine and clinopyroxene, the latter not melted. It is considered that wadsleyite and ringwoodite successively crystallized from individual melts of the former olivine crystals alone. Wadsleyite, low in Fe, crystallized first from melt in the interior of the former olivine crystals followed by Fe-rich ringwoodite nucleating on wadsleyite thus separating it from the residual melt enriched in Fe.

Journal of Mineralogical and Petrological Sciences, 2008

Application of FIB system to ultra-high-pressure Earth science

Research into III-Nitride based avalanche photodiodes (APDs) is motivated by the need for high se... more Research into III-Nitride based avalanche photodiodes (APDs) is motivated by the need for high sensitivity ultraviolet (UV) detectors in numerous civilian and military applications. By designing III-Nitride photodetectors that utilize low-noise impact ionization high internal gain can be realized-GaN APDs operating in Geiger mode can achieve gains exceeding 1×107. Thus with careful design, it becomes possible to count photons at the single photon level. In this paper we review the current state of the art in III-Nitride visible-blind APDs and discuss the critical design choices necessary to achieve high performance Geiger mode devices. Other major technical issues associated with the realization of visible-blind Geiger mode APDs are also discussed in detail and future prospects for improving upon the performance of these devices are outlined. The photon detection efficiency, dark count rate, and spectral response of or most recent Geiger-mode GaN APDs on free-standing GaN substrates are studied under low photon fluxes, with single photon detection capabilities being demonstrated. We also present our latest results regarding linear mode gain uniformity: the study of gain uniformity helps reveal the spatial origins of gain so that we can better understand the role of defects.

Journal of Applied Physics, 2010

GaN/AlN resonant tunneling diodes (RTD) were grown by metal-organic chemical vapor deposition (MO... more GaN/AlN resonant tunneling diodes (RTD) were grown by metal-organic chemical vapor deposition (MOCVD) and negative differential resistance with peak-to-valley ratios as high as 2.15 at room temperature was demonstrated. Effect of material quality on RTDs' performance was investigated by growing RTD structures on AlN, GaN, and lateral epitaxial overgrowth GaN templates. Our results reveal that negative differential resistance characteristics of RTDs are very sensitive to material quality (such as surface roughness) and MOCVD is a suitable technique for III-nitride-based quantum devices.

We report the synthesis, fabrication and testing of a 320 × 256 focal plane array (FPA) of back-i... more We report the synthesis, fabrication and testing of a 320 × 256 focal plane array (FPA) of back-illuminated, solarblind, p-i-n, AlxGa1-xN-based detectors, fully realized within our research laboratory. We implemented a novel pulsed atomic layer deposition technique for the metalorganic chemical vapor deposition (MOCVD) growth of crackfree, thick, and high Al composition AlxGa1-xN layers. Following the growth, the wafer was processed into a 320 × 256 array of 25 μm × 25 μm pixels on a 30 μm pixel-pitch and surrounding mini-arrays. A diagnostic mini-array was hybridized to a silicon fan-out chip to allow the study of electrical and optical characteristics of discrete pixels of the FPA. At a reverse bias of 1 V, an average photodetector exhibited a low dark current density of 1.12×10-8 A/cm2. Solar-blind operation is observed throughout the array with peak detection occurring at wavelengths of 256 nm and lower and falling off three orders of magnitude by 285 nm. After indium bump deposition and dicing, the FPA is hybridized to a matching ISC 9809 readout integrated circuit (ROIC). By developing a novel masking technology, we significantly reduced the visible response of the ROIC and thus the need for external filtering to achieve solar- and visible-blind operation is eliminated. This allowed the FPA to achieve high external quantum efficiency (EQE): at 254 nm, average pixels showed unbiased peak responsivity of 75 mA/W, which corresponds to an EQE of ~37%. Finally, the uniformity of the FPA and imaging properties are investigated.

Al(Ga)N/GaN resonant tunneling diodes (RTDs) are grown by metal-organic chemical vapor deposition... more Al(Ga)N/GaN resonant tunneling diodes (RTDs) are grown by metal-organic chemical vapor deposition. The effects of material quality on room temperature negative differential resistance (NDR) behaviour of RTDs are investigated by growing the RTD structure on AlN, GaN, and lateral epitaxial overgrowth GaN templates. This reveals that NDR characteristics of RTDs are very sensitive to material quality (such as surface roughness and dislocations density). The effects of the aluminum content of AlGaN double barriers (DB) and polarization fields on NDR characteristic of AlGaN/GaN RTDs were also investigated by employing low dislocation density c-plane (polar) and m-plane (nonpolar) freestanding GaN substrates. Lower aluminum content in the DB RTD active layer and minimization of dislocations and polarization fields enabled a more reliable and reproducible NDR behaviour at room temperature.

There is a need for semiconductor-based ultraviolet photodetectors to support avalanche gain in o... more There is a need for semiconductor-based ultraviolet photodetectors to support avalanche gain in order to realize better performance andmore effective compete with existing technologies. Wide bandgap III-Nitride semiconductors are the promising material system for the development of avalanche photodiodes (APDs) that could be a viable alternative to current bulky UV detectors such as photomultiplier tubes. In this paper, we review the current state-of-the-art in IIINitride visible-blind APDs, and present our latest results on GaN APDs grown on both conventional sapphire and low dislocation density free-standing c- and m-plane GaN substrates. Leakage current, gain, and single photon detection efficiency (SPDE) of these APDs were compared. The spectral response and Geiger-mode photon counting performance of UV APDs are studied under low photon fluxes, with single photon detection capabilities as much as 30% being demonstrated in smaller devices. Geiger-mode operation conditions are optimized for enhanced SPDE.

Applied Physics Letters, 2010

GaN avalanche photodiodes ͑APDs͒ were grown on both conventional sapphire and low dislocation den... more GaN avalanche photodiodes ͑APDs͒ were grown on both conventional sapphire and low dislocation density free-standing ͑FS͒ c-plane GaN substrates. Leakage current, gain, and single photon detection efficiency ͑SPDE͒ of these APDs were compared. At a reverse-bias of 70 V, APDs grown on sapphire substrates exhibited a dark current density of 2.7ϫ 10 −4 A / cm 2 whereas APDs grown on FS-GaN substrates had a significantly lower dark current density of 2.1ϫ 10 −6 A / cm 2 . Under linear-mode operation, APDs grown on FS-GaN achieved avalanche gain as high as 14 000. Geiger-mode operation conditions were studied for enhanced SPDE. Under front-illumination the 625-m 2 -area APD yielded a SPDE of ϳ13% when grown on sapphire substrates compared to more than 24% when grown on FS-GaN. The SPDE of the same APD on sapphire substrate increased to ϳ30% under back-illumination-the FS-GaN APDs were only tested under front illumination due to the thick absorbing GaN substrate.

Applied Physics Letters, 2010

AlGaN/GaN resonant tunneling diodes (RTDs), consisting of 20% (10%) aluminum-content in double-ba... more AlGaN/GaN resonant tunneling diodes (RTDs), consisting of 20% (10%) aluminum-content in double-barrier (DB) active layer, were grown by metal-organic chemical vapor deposition on freestanding polar (c-plane) and nonpolar (m-plane) GaN substrates. RTDs were fabricated into 35-μm-diameter devices for electrical characterization. Lower aluminum content in the DB active layer and minimization of dislocations and polarization fields increased the reliability and reproducibility of room-temperature negative differential resistance (NDR). Polar RTDs showed decaying NDR behavior, whereas nonpolar ones did not significantly. Averaging over 50 measurements, nonpolar RTDs demonstrated a NDR of 67 Ω, a current-peak-to-valley ratio of 1.08, and an average oscillator output power of 0.52 mW.

Applied Physics Letters, 2010

AlN/GaN double-barrier resonant tunneling diodes (RTDs) were grown by metal-organic chemical vapo... more AlN/GaN double-barrier resonant tunneling diodes (RTDs) were grown by metal-organic chemical vapor deposition on sapphire. RTDs were fabricated via standard processing steps. RTDs demonstrate a clear negative differential resistance (NDR) at room temperature (RT). The NDR was observed around 4.7 V with a peak current density of 59 kA/cm2 and a peak-to-valley ratio of 1.6 at RT. Dislocation-free material is shown to be the key for the performance of GaN RTDs.

Applied Physics Letters, 2010

III-nitride resonant tunneling diodes (RTDs), consisting Al0.2Ga0.8N/GaN double-barrier (DB) acti... more III-nitride resonant tunneling diodes (RTDs), consisting Al0.2Ga0.8N/GaN double-barrier (DB) active layers, were grown on c-plane lateral epitaxial overgrowth (LEO) GaN/sapphire and c-plane freestanding (FS) GaN. RTDs on both templates, fabricated into mesa diameters ranging from 5 to 35 μm, showed negative differential resistance (NDR) at room temperature. NDR characteristics (voltage and current density at NDR onset and current-peak-to-valley ratio) were analyzed and reported as a function of device size and substrate choice. Our results show that LEO RTDs perform as well as FS ones and DB active layer design and quality have been the bottlenecks in III-nitride RTDs.

Applied Physics Letters, 2010

M-plane GaN avalanche p-i-n photodiodes on low dislocation density freestanding m-plane GaN subst... more M-plane GaN avalanche p-i-n photodiodes on low dislocation density freestanding m-plane GaN substrates were realized using metal-organic chemical vapor deposition. High quality homoepitaxial m-plane GaN layers were developed; the root-mean-square surface roughness was less than 1 A˚ and the full-width-at-half-maximum value of the x-ray rocking curve for (1010) diffraction of m-plane GaN epilayer was 32 arcsec. High quality material led to a low reverse-bias dark current of 8.11 pA for 225 μm2 mesa photodetectors prior to avalanche breakdown, with the maximum multiplication gain reaching about 8000.

Applied Physics Letters, 2010

High quality Al 0.2 Ga 0.8 N / GaN superlattices ͑SLs͒ with various ͑GaN͒ well widths ͑1.6 to 6.4... more High quality Al 0.2 Ga 0.8 N / GaN superlattices ͑SLs͒ with various ͑GaN͒ well widths ͑1.6 to 6.4 nm͒ have been grown on polar c-plane and nonpolar m-plane freestanding GaN substrates by metal-organic chemical vapor deposition. Atomic force microscopy, high resolution x-ray diffraction, and photoluminescence ͑PL͒ studies of SLs have been carried out to determine and correlate effects of well width and polarization field on the room-temperature PL characteristics. A theoretical model was applied to explain PL energy-dependency on well width and crystalline orientation taking into account internal electric field for polar substrate. Absence of induced-internal electric field in nonpolar SLs was confirmed by stable PL peak energy and stronger PL intensity as a function of excitation power density than polar ones.

Physica Status Solidi (c), 2006

Much interest has recently been shown in Mg x Zn 1-x O alloys films as a promising candidate for ... more Much interest has recently been shown in Mg x Zn 1-x O alloys films as a promising candidate for ZnO band gap engineering (from 3.37 eV for ZnO to 7.8 eV for MgO). It has also been found to be a suitable material for the barrier layers of Zn/MgZnO super lattices due to its wider band gap. Mg x Zn 1-x O layers can posses two different crystal structure of rock-salt cubic and hexagonal wurtzite as a result of Mg content difference.

Journal of Applied Physics, 2004

We have investigated the characteristic of the dislocations in the layers grown on sapphire by th... more We have investigated the characteristic of the dislocations in the layers grown on sapphire by the plasma-assisted molecular beam epitaxy under the different flux ratios. The layers were characterized by the transmission electron microscopy (TEM) and the high-resolution x-ray diffraction ( ...

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2005

ABSTRACT The polarity of ZnO films grown on MgO/c-Al2O3 templates are successfully controlled by ... more ABSTRACT The polarity of ZnO films grown on MgO/c-Al2O3 templates are successfully controlled by only varying MgO buffer layer thickness. The crystal structure of the MgO buffer on c-Al2O3 is of wurtzite (WZ), when the MgO buffer thickness is less than 2.7 nm. As the MgO layer thickness exceeds, the crystal structure of MgO changes to rock salt (RS) structure. A ZnO layer grown on WZ-MgO results in O-polar ZnO, while the polarity of the ZnO layer grown on RS-MgO changes to Zn polar. Possible atomic configurations of the ZnO/MgO/Al2O3 interface structure are suggested.

Journal of Applied Physics, 2005

We report on the structure study of MgxZn1-xO films and, in particular, we will focus on MgxZn1-x... more We report on the structure study of MgxZn1-xO films and, in particular, we will focus on MgxZn1-xO layers with x=0.28 and 0.41 MgxZn1-xO layers with different crystal structures of cubic and wurtzite that have been grown by plasma-assisted molecular-beam epitaxy on MgO/c-sapphire with Mg/Zn flux ratio control. The MgxZn1-xO films have been characterized by high-resolution transmission electron microscopy (HRTEM) and high-resolution x-ray diffraction. The dependence of the cation-anion bond length to Mg content has been studied. A virtual crystal model of MgZnO has been applied to interpret the bond-length variation. HRTEM results indicate that the initial stage of the MgZnO growth on a MgO buffer layer starts with a cubic structure even in the case of a wurtzite structure at the end of growth.

Current Applied Physics, 2004

In this study, structural properties of epitaxial Ga-doped Mg0.1Zn0.9O layers grown on ZnO/α-Al2O... more In this study, structural properties of epitaxial Ga-doped Mg0.1Zn0.9O layers grown on ZnO/α-Al2O3 templates by plasma-assisted molecular beam epitaxy have been investigated by high-resolution transmission electron microscopy (HRTEM), and high resolution X-ray diffraction (HRXRD). From analysis of the diffraction pattern, the monocrystallinity of the Mg0.1Zn0.9O layer with hexagonal structure is confirmed. The orientation relationship between Mg0.1Zn0.9O and the template is determined as (0001)Mg0.1Zn0.9O∥(0001)ZnO∥(0001)Al2O3 and [21̄1̄0]Mg0.1Zn0.9O ∥[21̄1̄0]ZnO∥[11̄00]Al2O3. The density of dislocations near the top surface layers measured by plan-view TEM is about 3.61010 cm−2, one order of magnitude higher than the value obtained for ZnO layers on α-Al2O3 with a MgO buffer. Cross-sectional observation revealed that the majority of threading dislocations are in the [0001] line direction, i.e. they lie along the surface normal and consist of edge, screw, and mixed dislocations. Cross- sectional TEM and X-ray rocking curve experiments reveal that most of dislocations are edge dislocations. The interface of Mg0.1Zn0.9O and ZnO layers and the effect of excess Ga-doping in these layers have been also studied.