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201.  Recent Advances in LWIR Type-II InAs/GaSb Superlattice Photodetectors and Focal Plane Arrays at the Center for Quantum Devices
M. Razeghi, D. Hoffman, B.M. Nguyen, P.Y. Delaunay, E.K. Huang, M.Z. Tidrow, and V. Nathan
IEEE Proceedings, Vol. 97, No. 6, p. 1056-1066-- June 1, 2009
In recent years, Type-II InAs/GaSb superlattice photo-detectors have experienced significant improvements in material quality, structural designs, and imaging applications. They now appear to be a possible alternative to the state-of-the-art HgCdTe (MCT) technology in the long and very long wavelength infrared regimes. At the Center for Quantum Devices, we have successfully realized very high quantum efficiency, very high dynamic differential resistance R0A - product LWIR Type – II InAs/GaSb superlattice photodiodes with efficient surface passivation techniques. The demonstration of high quality LWIR Focal Plane Arrays that were 100 % fabricated in - house reaffirms the pioneer position of this university-based laboratory. reprint
202.  Comparison of ZnO nanostructures grown using pulsed laser deposition, metal organic chemical vapor deposition, and physical vapor transport
V.E. Sandana, D.J. Rogers, F. Hosseini Teherani, R. McClintock, C. Bayram, M. Razeghi, H-J Drouhin, M.C. Clochard, V. Sallett, G. Garry, and F. Falyouni
Journal of Vacuum Science and Technology B, Vol. 27, No. 3, May/June, p. 1678-1683-- May 29, 2009
This article compares the forms and properties of ZnO nanostructures grown on Si (111) and c-plane sapphire (c-Al2O3) substrates using three different growth processes: metal organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), and physical vapor transport (PVT). A very wide range of ZnO nanostructures was observed, including nanorods, nanoneedles, nanocombs, and some novel structures resembelling “bevelled” nanowires. PVT gave the widest family of nanostructures. PLD gave dense regular arrays of nanorods with a preferred orientation perpendicular to the substrate plane on both Si and c-Al2O3 substrates, without the use of a catalyst. X-ray diffraction (XRD) studies confirmed that nanostructures grown by PLD were better crystallized and more highly oriented than those grown by PVT and MOCVD. Samples grown on Si showed relatively poor XRD response but lower wavelength emission and narrower linewidths in PL studies. reprint
203.  Fabrication and characterization of novel hybrid green light emitting didoes based on substituting n-type ZnO for n-type GaN in an inverted p-n junction
C. Bayram, D. Rogers, F. H. Teherani, and M. Razeghi
Journal of Vacuum Science and Technology B, Vol. 27, No. 3, May/June, p. 1784-1788-- May 29, 2009
Details of the fabrication and characterization of hybrid green light emitting diodes, composed of n-ZnO/(InGaN/GaN) multi-quantum-wells/p-GaN on AlN/sapphire, are reported. Scanning electron microscope, atomic force microscopy, high resolution x-ray diffraction, and photoluminescence were used to study the hybrid device. The effects of solvents, annealing, and etching on n-ZnO are discussed. Successful hybridization of ZnO and (In)GaN into a green light emitting diode was realized. reprint
204.  Microstructural compositional, and optical characterization of GaN grown by metal organic vapor phase epitaxy on ZnO epilayers
D.J. Rogers, F. Hosseini Teherani, T. Moudakir, S. Gautier, F. Jomard, M. Molinari, M. Troyon, D. McGrouther, J.N. Chapman, M. Razeghi and A. Ougazzaden
Journal of Vacuum Science and Technology B, Vol. 27, No. 3, May/June, p. 1655-1657-- May 29, 2009
This article presents the results of microstructural, compositional, and optical characterization of GaN films grown on ZnO buffered c-sapphire substrates. Transmission electron microscopy showed epitaxy between the GaN and the ZnO, no degradation of the ZnO buffer layer, and no evidence of any interfacial compounds. Secondary ion mass spectroscopy revealed negligible Zn signal in the GaN layer away from the GaN/ZnO interface. After chemical removal of the ZnO, room temperature (RT) cathodoluminescence spectra had a single main peak centered at ~ 368 nm (~3.37 eV), which was indexed as near-band-edge (NBE) emission from the GaN layer. There was no evidence of the ZnO NBE peak, centered at ~379 nm (~3.28 eV), which had been observed in RT photoluminescence spectra prior to removal of the ZnO. reprint
205.  Material and design engineering of (Al)GaN for high-performance avalanche photodiodes and intersubband applications
M. Razeghi and C. Bayram
SPIE Proceedings, Dresden, Germany (May 4-6, 2009), Vol. 7366, p. 73661F-1-- May 20, 2009
Numerous applications in scientific, medical, and military areas demand robust, compact, sensitive, and fast ultraviolet (UV) detection. Our (Al)GaN photodiodes pose high avalanche gain and single-photon detection efficiency that can measure up to these requirements. Inherit advantage of back-illumination in our devices offers an easier integration and layout packaging via flip-chip hybridization for UV focal plane arrays that may find uses from space applications to hostile-agent detection. Thanks to the recent (Al)GaN material optimization, III-Nitrides, known to have fast carrier dynamics and short relaxation times, are employed in (Al)GaN based superlattices that absorb in near-infrared regime. In this work, we explain the origins of our high performance UV APDs, and employ our (Al)GaN material knowledge for intersubband applications. We also discuss the extension of this material engineering into the far infrared, and even the terahertz (THz) region. reprint
206.  A Review of III-Nitride Research at the Center for Quantum Devices
M. Razeghi and R. McClintock
Journal of Crystal Growth, Vol. 311, No. 10-- May 1, 2009
In this paper, we review the history of the Center for Quantum Devices’ (CQD) III-nitride research covering the past 15 years. We review early work developing III-nitride material growth. We then present a review of laser and light-emitting diode (LED) results covering everything from blue lasers to deep UV LEDs emitting at 250 nm. This is followed by a discussion of our UV photodetector research from early photoconductors all the way to current state of the art Geiger-mode UV single photon detectors. reprint
207.  High performance antimony based type-II superlattice photodiodes on GaAs substrates
B.M. Nguyen, D. Hoffman, E.K. Huang, P.Y. Delaunay, and M. Razeghi
SPIE Porceedings, Vol. 7298, Orlando, FL 2009, p. 72981T-- April 13, 2009
In recent years, Type-II InAs/GaSb superlattices grown on GaSb substrate have achieved significant advances in both structural design and material growth, making Type-II superlattice infrared detector a rival competitor to the state-of-the-art MCT technology. However, the limited size and strong infrared absorption of GaSb substrates prevent large format type-II superlattice infrared imagers from being realized. In this work, we demonstrate type-II superlattices grown on GaAs substrates, which is a significant step toward third generation infrared imaging at low cost. The device performances of Type-II superalttice photodetectors grown on these two substrates are compared. reprint
208.  Background limited performance of long wavelength infrared focal plane arrays fabricated from type-II InAs/GaSb M-structure superlattice
P.Y. Delaunay, B.M. Nguyen and M. Razeghi
SPIE Porceedings, Vol. 7298, Orlando, FL 2009, p. 72981Q-- April 13, 2009
Recent advances in growth techniques, structure design and processing have lifted the performance of Type-II InAs/GaSb superlattice photodetectors. The introduction of a M-structure design improved both the dark current and R0A of Type-II photodiodes. This new structure combined with a thick absorbing region demonstrated background limited performance at 77K for a 300K background and a 2-π field of view. A focal plane array with a 9.6 μm 50% cutoff wavelength was fabricated with this design and characterized at 80K. The dark current of individual pixels was measured around 1.3 nA, 7 times lower than previous superlattice FPAs. This led to a higher dynamic range and longer integration times. The quantum efficiency of detectors without anti-reflective coating was 72%. The noise equivalent temperature difference reached 23 mK. The deposition of an anti-reflective coating improved the NEDT to 20 mK and the quantum efficiency to 89%. reprint
209.  Pulsed metal-organic chemical vapor deposition of high quality AlN/GaN superlattices for near-infrared intersubband transitions
C. Bayram, N. Pere-Laperne, R. McClintock, B. Fain and M. Razeghi
Applied Physics Letters, Vol. 94, No. 12, p. 121902-1-- March 23, 2009
A pulsed metal-organic chemical vapor deposition technique is developed for the growth of high-quality AlN/GaN superlattices (SLs) with intersubband (ISB) transitions at optical communications wavelengths. Tunability of the AlN and GaN layers is demonstrated. Indium is shown to improve SL surface and structural quality. Capping thickness is shown to be crucial for ISB transition characteristics. Effects of barrier- and well-doping on the ISB absorption are reported. reprint
210.  Characterization of ZnO thin films grown on c-sapphire by pulsed laser deposition as templates for regrowth of zno by metal organic chemical vapor deposition
D. J. Rogers ; F. Hosseini Teherani ; C. Sartel ; V. Sallet ; F. Jomard ; P. Galtier ; M. Razeghi
Proc. SPIE 7217, Zinc Oxide Materials and Devices IV, 72170F (February 17, 2009)-- February 17, 2009
The use of ZnO template layers grown Pulsed Laser Deposition (PLD) has been seen to produce dramatic improvements in the surface morphology, crystallographic quality and optical properties of ZnO layers grown on c-sapphire substrates by Metal Organic Chemical Vapor Deposition. This paper provides complementary details on the PLD-grown ZnO template properties. reprint
211.  Fabrication of nanostructured heterojunction LEDs using self-forming Moth-Eye Arrays of n-ZnO Nanocones Grown on p-Si (111) by PLD
D.J. Rogers; V.E. Sandana; F. Hosseini Teherani; M. Razeghi; H.-J. Drouhin
Proc. SPIE 7217, Zinc Oxide Materials and Devices IV, 721708 (February 17, 2009)-- February 17, 2009
ZnO nanostructures were grown on Si (111) substrates using Pulsed Laser Deposition. The impact of growth temperature (Ts) and Ar pressure (PAr) on the morphology, crystal structure and photoluminescence was investigated. Various types of ZnO nanostructures were obtained. Self-forming arrays of vertically-aligned nanorods and nanocones with strong c-axis crystallographic orientation and good optical response were obtained at higher Ts. The nanocone, or "moth-eye" type structures were selected for LED development because of their graded effective refractive index, which could facilitate improved light extraction at the LED/air interface. Such moth-eye arrays were grown on p-type Si (111) substrates to form heteroj unction LEDs with the n-type ZnO nanocones acting as an active component of the device. These nanostructured LEDs gave rectifying I/V characteristics with a threshold voltage of about 6V and a blueish-white electroluminescence, which was clearly visible to the naked eye. reprint
212.  Surface leakage reduction in narrow band gap type-II antimonide-based superlattice photodiodes
E.K. Huang, D. Hoffman, B.M. Nguyen, P.Y. Delaunay and M. Razeghi
Applied Physics Letters, Vol. 94, No. 5, p. 053506-1-- February 2, 2009
Inductively coupled plasma (ICP) dry etching rendered structural and electrical enhancements on type-II antimonide-based superlattices compared to those delineated by electron cyclotron resonance (ECR) with a regenerative chemical wet etch. The surface resistivity of 4×105 Ω·cm is evidence of the surface quality achieved with ICP etching and polyimide passivation. By only modifying the etching technique in the fabrication steps, the ICP-etched devices with a 9.3 µm cutoff wavelength revealed a diffusion-limited dark current density of 4.1×10−6 A/cm2 and a maximum differential resistance at zero bias in excess of 5300 Ω·cm2 at 77 K, which are an order of magnitude better in comparison to the ECR-etched devices. reprint
213.  Thermal analysis of buried heterostructure quantum cascade lasers for long-wavelength infrared emission using 2D anisotropic heat-dissipation model
H.K. Lee, K.S. Chung, J.S. Yu and M. Razeghi
Physica Status Solidi (a), Vol. 206, p. 356-362-- February 1, 2009
We have theoretically investigated and compared the thermal characteristics of 10.6 μm InGaAs/InAlAs/InP buried heterostructure (BH) quantum cascade lasers (QCLs) with different heat-sinking configurations by a steady-state heat-transfer analysis. The heat-source densities were obtained from laser threshold power densities measured experimentally under room-temperature continuous-wave mode. The two-dimensional anisotropic heat-dissipation model was used to calculate the temperature distribution, heat flux, and thermal conductance (Gth) inside the device. For good thermal characteristics, the QCLs in the long-wavelength infrared region require the relatively narrow BH structure in combination with epilayer-down bonding due to thick active core/cladding layers and high insulator losses. The single-ridge BH structure results in slightly higher thermal conductance by 2-4% than the double-channel (DC) ridge BH structure. For W = 12 m with 5 μm thick electroplated Au, the single-ridge BH laser with epilayer-down bonding exhibited the highest Gth value of 201.9 W/K cm2, i.e. increased by nearly 36% with respect to the epilayer-up bonded DC ridge waveguide laser. This value is improved by 50% and 62% with respect to the single-ridge BH laser and DC ridge waveguide laser with W = 20 μm in the epilayer-up bonding scheme, respectively. reprint
214.  Background limited performance of long wavelength infrared focal plane arrays fabricated from M-structure InAs-GaSb superlattices
P.Y. Delaunay, B.M. Nguyen, D. Hoffman, E.K. Huang, and M. Razeghi
IEEE Journal of Quantum Electronics, Vol. 45, No. 2, p. 157-162.-- February 1, 2009
The recent introduction of a M-structure design improved both the dark current and R0A performances of Type-II InAs-GaSb photodiodes. A focal plane array fabricated with this design was characterized at 81 K. The dark current of individual pixels was measured between 1.1 and 1.6 nA, 7 times lower than previous superlattice FPAs. This led to a higher dynamic range and longer integration times. The quantum efficiency of detectors without antireflective coating was 74%. The noise equivalent temperature difference reached 23 mK, limited only by the performance of the testing system and the read out integrated circuit. Background limited performances were demonstrated at 81 K for a 300 K background. reprint
215.  III-Nitride avalanche photodiodes
R. McClintock, J.L. Pau, C. Bayram, B. Fain, P. Giedratis, M. Razeghi and M. Ulmer
SPIE Proceedings, San Jose, CA Volume 7222-0U-- January 26, 2009
Research into avalanche photodiodes (APDs) is motivated by the need for high sensitivity ultraviolet (UV) detectors in numerous civilian and military applications. By designing photodetectors to utilize low-noise impact ionization based gain, GaN APDs operating in Geiger mode can deliver 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 present our latest results regarding linear and Geiger mode III-Nitride based APDs. This includes novel device designs such as separate absorption and multiplication APDs (SAM-APDs). We also discuss control of the material quality and the critical issue of p-type doping - demonstrating a novel delta-doping technique for improved material quality and enhanced electric field confinement. The spectral response and Geiger-mode photon counting performance of these devices are then analyzed under low photon fluxes, with single photon detection capabilities being demonstrated. Other major technical issues associated with the realization of high-quality visible-blind Geiger mode APDs are also discussed in detail and future prospects for improving upon the performance of these devices are outlined. reprint
216.  Mid-infrared quantum cascade lasers with high wall plug efficiency
Y. Bai, B. Gokden, S. Slivken, S.R. Darvish, S.A. Pour, and M. Razeghi
SPIE Proceedings, San Jose, CA Volume 7222-0O-- January 26, 2009
We demonstrate optimization of continuous wave (cw) operation of 4.6 µm quantum cascade lasers (QCLs). A 19.7 µm by 5 mm, double channel processed device exhibits 33% cw WPE at 80 K. Room temperature cw WPE as high as 12.5% is obtained from a 10.6 µm by 4.8 mm device, epilayer-down bonded on a diamond submount. With the semi-insulating regrowth in a buried ridge geometry, 15% WPE is obtained with 2.8 W total output power in cw mode at room temperature. This accomplishment is achieved by systematically decreasing the parasitic voltage drop, reducing the waveguide loss and improving the thermal management. reprint
217.  Pulsed metalorganic chemical vapor deposition of high quality AlN/GaN superlattices for intersubband transitions
C. Bayram, B. Fain, N. Pere-Laperne, R. McClintock and M. Razeghi
SPIE Proceedings, San Jose, CA Volume 7222-12-- January 26, 2009
A pulsed metalorganic chemical vapor deposition (MOCVD) technique, specifically designed for high quality AlN/GaN superlattices (SLs) is introduced. Optical quality and precise controllability over layer thicknesses are investigated. Indium is shown to improve interface and surface quality. An AlN/GaN SL designed for intersubband transition at a telecommunication wavelength of ~1.5 µm, is grown, and processed for intersubband (ISB) absorption measurements. Room temperature measurements show intersubband absorption centered at 1.49 µm. Minimal (n-type) silicon doping of the well is shown to be crucial for good ISB absorption characteristics. The potential to extend this technology into the far infrared and even the terahertz (THz) region is also discussed. reprint
218.  GaN-based nanostructured photodetectors
J.L. Pau, C. Bayram, P. Giedraitis, R. McClintock, and M. Razeghi
SPIE Proceedings, San Jose, CA Volume 7222-14-- January 26, 2009
The use of nanostructures in semiconductor technology leads to the observation of new phenomena in device physics. Further quantum and non-quantum effects arise from the reduction of device dimension to a nanometric scale. In nanopillars, quantum confinement regime is only revealed when the lateral dimensions are lower than 50 nm. For larger mesoscopic systems, quantum effects are not observable but surface states play a key role and make the properties of nanostructured devices depart from those found in conventional devices. In this work, we present the fabrication of GaN nanostructured metal-semiconductor-metal (MSM) and p-i-n photodiodes (PIN PDs) by e-beam lithography, as well as the investigation of their photoelectrical properties at room temperature. The nanopillar height and diameter are about 520 nm and 200 nm, respectively. MSMs present dark currents densities of 0.4 A/cm2 at ±100 V. A strong increase of the optical response with bias is observed, resulting in responsivities higher than 1 A/W. The relationship between this gain mechanism and surface states is discussed. PIN PDs yield peak responsivities (Rpeak) of 35 mA/W at -4 V and show an abnormal increase of the response (Rpeak > 100 A/W) under forward biases. reprint
219.  The importance of band alignment in VLWIR type-II InAs/GaSb heterodiodes containing the M-structure barrier
D. Hoffman, B.M. Nguyen, E.K. Huang, P.Y. Delaunay, S. Bogdanov, P. Manukar, M. Razeghi, and V. Nathan
SPIE Proceedings, San Jose, CA Volume 7222-15-- January 26, 2009
The Type-II InAs/GaSb superlattice photon detector is an attractive alternative to HgCdTe photodiodes and QWIPS. The use of p+ - pi - M - N+ heterodiode allows for greater flexibility in enhancing the device performance. The utilization of the Empirical Tight Binding method gives the band structure of the InAs/GaSb superlattice and the new M- structure (InAs/GaSb/AlSb/GaSb) superlattice allowing for the band alignment between the binary superlattice and the M- superlattice to be determined and see how it affects the optical performance. Then by modifying the doping level of the M- superlattice an optimal level can be determined to achieve high detectivity, by simultaneously improving both photo-response and reducing dark current for devices with cutoffs greater than 14.5 µm. reprint
220.  Background limited performance of long wavelength infrared focal plane arrays fabricated from M-structure InAs/GaSb superlattices
P.Y. Delaunay, B.M. Nguyen, D. Hoffman, E.K. Huang, P. Manurkar, S. Bogdanov and M. Razeghi
SPIE Proceedings, San Jose, CA Volume 7222-0W-- January 26, 2009
Recent advances in the design and fabrication of Type-II InAs/GaSb superlattices allowed the realization of high performance long wavelength infrared focal plane arrays. The introduction of an Mstructure barrier between the n-type contact and the pi active region reduced the tunneling component of the dark current. The M-structure design improved the noise performance and the dynamic range of FPAs at low temperatures. At 81K, the NEDT of the focal plane array was 23 mK. The noise of the camera was dominated by the noise component due to the read out integrated circuit. At 8 µm, the median quantum efficiency of the detectors was 71%, mainly limited by the reflections on the backside of the array. reprint
221.  Inductively coupled plasma etching and processing techniques for type-II InAs/GaSb superlattices infrared detectors toward high fill factor focal plane arrays
E.K. Huang, B.M. Nguyen, D. Hoffman, P.Y. Delaunay and M. Razeghi
SPIE Proceedings, San Jose, CA Volume 7222-0Z-- January 26, 2009
A challenge for Type-II InAs/GaSb superlattice (T2SL) photodetectors is to achieve high fill factor, high aspect ratio etching for third generation focal plane arrays (FPAs). Initially, we compare the morphological and electrical results of single element T2SL photodiodes after BCl3/Ar inductively coupled plasma (ICP) and electron cyclotron resonance (ECR) dry etching. Using a Si3N4 hard mask, ICP-etched structures exemplify greater sidewall verticality and smoothness, which are essential toward the realization of high fill factor FPAs. ICP-etched single element devices with SiO2 passivation that are 9.3 µm in cutoff wavelength achieved vertical sidewalls of 7.7 µm in depth with a resistance area product at zero bias of greater than 1,000 Ω·cm2 and maximum differential resistance in excess of 10,000 Ω·cm2 at 77 K. By only modifying the etching technique in the fabrication steps, the ICP-etched photodiodes showed an order of magnitude decrease in their dark current densities in comparison to the ECR-etched devices. Finally, high aspect ratio etching is demonstrated on mutli-element arrays with 3 µm-wide trenches that are 11 µm deep. reprint
222.  Hybrid green LEDs based on n-ZnO/(InGaN/GaN) multi-quantum-wells/p-GaN
C. Bayram, F. Hosseini Teherani, D.J. Rogers and M. Razeghi
SPIE Proceedings, San Jose, CA Volume 7217-0P-- January 26, 2009
Hybrid green light-emitting diodes (LEDs) comprised of n-ZnO/(InGaN/GaN) multi-quantum-wells/p-GaN were grown on semi-insulating AlN/sapphire using pulsed laser deposition for the n-ZnO and metal organic chemical vapor deposition for the other layers. X-ray diffraction revealed that high crystallographic quality was preserved after the n- ZnO growth. LEDs showed a turn-on voltage of 2.5 V and a room temperature electroluminescence (EL) centered at 510 nm. A blueshift and narrowing of the EL peak with increasing current was attributed to bandgap renormalization. The results indicate that hybrid LED structures could hold the prospect for the development of green LEDs with superior performance. reprint
223.  High-power high-wall plug efficiency mid-infrared quantum cascade lasers based on InP/GaInAs/InAlAs material system
M. Razeghi
SPIE Proceedings, San Jose, CA Volume 7230-11-- January 26, 2009
The latest result at the Center for Quantum Devices about high power, high wall plug efficiency, mid-infrared quantum cascade lasers (QCLs) is presented. At an emitting wavelength of 4.8 µm, an output power of 3.4 W and a wall plug efficiency of 16.5% are demonstrated from a single device operating in continuous wave at room temperature. At a longer wavelength of 10.2 µm, average power as high as 2.2 W is demonstrated at room temperature. Gas-source molecular beam epitaxy is used to grow the QCL core in an InP/GaInAs/InAlAs material system. Fe-doped semiinsulating regrowth is performed by metal organic chemical vapor deposition for efficient heat removal and low waveguide loss. This accomplishment marks an important milestone in the development of high performance midinfrared QCLs. reprint
224.  Quantum dot in a well infrared photodetectors for high operating temperature focal plane arrays
S. Tsao, T. Yamanaka, S. Abdollahi Pour, I-K Park, B. Movaghar and M. Razeghi
SPIE Proceedings, San Jose, CA Volume 7234-0V-- January 25, 2009
InAs quantum dots embedded in InGaAs quantum wells with InAlAs barriers on InP substrate grown by metalorganic chemical vapor deposition are utilized for high operating temperature detectors and focal plane arrays in the middle wavelength infrared. This dot-well combination is unique because the small band offset between the InAs dots and the InGaAs well leads to weak dot confinement of carriers. As a result, the device behavior differs significantly from that in the more common dot systems that have stronger confinement. Here, we present energy level modeling of our QD-QW system and apply these results to interpret the detector behavior. Detectors showed high performance with D* over 1010 cm·Hz1/2W-1 at 150 K operating temperature and with high quantum efficiency over 50%. Focal plane arrays have been demonstrated operating at high temperature due to the low dark current observed in these devices. reprint
225.  GaN nanostructured p-i-n photodiodes
J.L. Pau, C. Bayram, P. Giedraitis, R. McClintock, and M. Razeghi
Applied Physics Letters, Vol. 93, No. 22, p. 221104-1-- December 1, 2008
We report the fabrication of nanostructured p-i-n photodiodes based on GaN. Each device comprises arrays of ~200 nm diameter and 520 nm tall nanopillars on a 1 µm period, fabricated by e-beam lithography. Strong rectifying behavior was obtained with an average reverse current per nanopillar of 5 fA at −5 V. In contrast to conventional GaN diodes, nanostructured devices reproducibly show ideality factors lower than 2. Enhanced tunneling through sidewall surface states is proposed as the responsible mechanism for this behavior. Under backillumination, the quantum efficiency in nanostructured devices is partly limited by the collection efficiency of holes into the nanopillars. reprint

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