Center for Quantum Devices - Journal Articles and Conference Proceedings

Page 11 of 32: Prev << 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 >> Next (778 Items)

251.	Growth and characterization of long wavelength infrared Type-II superlattice Photodiodes on a 3 B.M. Nguyen, G. Chen, M.A. Hoang, and M. Razeghi SPIE Proceedings, San Francisco, CA (January 22-27, 2011), Vol. 7945, p. 79451O-- January 23, 2011 One of the great advantages of Type-II InAs/GaSb superlattice over other competing technologies for the third generation infrared imagers is the potential to have excellent uniformity across a large area as the electronic structure of the material is controlled by the layer thicknesses, not by the composition of the materials. This can economize the material growth, reduce the fabrication cost, and especially allow the realization of large format imagers. In this talk, we report the molecular beam epitaxial growth of Type-II superlattices on a 3-inch GaSb substrate for long wavelength infrared detection. The material exhibits excellent structural, optical and electrical uniformity via AFM, Xray, quantum efficiency and I-V measurements. At 77K, 11μm cutoff photodiodes exhibit more than 45% quantum efficiency, and a dark current density of 1.0x10^-4 A/cm² at 50 mV, resulting in a specific detectivity of 6 x 10¹¹ cm·Hz^1/2/W. reprint
252.	Advances in UV sensitive visible blind GaN-based APDs M. Ulmer, R. McClintock and M. Razeghi SPIE Proceedings, San Francisco, CA (January 22-27, 2011), Vol. 7945, p. 79451G-- January 23, 2011 In this paper, we describe our current state-of-the-art process of making visible-blind APDs based on GaN. We have grown our material 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 are compared. The spectral response and Geiger-mode photon counting performance of UV APDs are studied under low photon fluxes. Single photon detection capabilities with over 30% are demonstrated. We show how with pulse height discrimination the Geiger-mode operation conditions can be optimized for enhanced SPDE versus dark counts. reprint
253.	III-Nitride Optoelectronic Devices: From ultraviolet detectors and visible emitters towards terahertz intersubband devices M. Razeghi, C. Bayram, Z. Vashaei, E. Cicek and R. McClintock IEEE Photonics Society 23rd Annual Meeting, November 7-10, 2010, Denver, CO, Proceedings, p. 351-352-- January 20, 2011 III-nitride optoelectronic devices are discussed. Ultraviolet detectors and visible emitters towards terahertz intersubband devices are reported. Demonstration of single photon detection efficiencies of 33% in the ultraviolet regime, intersubband energy level as low as in the mid-infrared regime, and GaN-based resonant tunneling diodes with negative resistance of 67 Ω are demonstrated. reprint
254.	Highly temperature insensitive quantum cascade lasers Y. Bai, N. Bandyopadhyay, S. Tsao, E. Selcuk, S. Slivken and M. Razeghi Applied Physics Letters, Vol. 97, No. 25-- December 20, 2010 An InP based quantum cascade laser (QCL) heterostructure emitting around 5 μm is grown with gas-source molecular beam epitaxy. The QCL core design takes a shallow-well approach to maximize the characteristic temperatures, T(0) and T(1), for operations above room temperature. A T(0) value of 383 K and a T(1) value of 645 K are obtained within a temperature range of 298–373 K. In room temperature continuous wave operation, this design gives a single facet output power of 3 W and a wall plug efficiency of 16% from a device with a cavity length of 5 mm and a ridge width of 8 μm. reprint
255.	Room-temperature continuous wave operation of distributed feedback quantum cascade lasers with watt-level power output Q.Y. Lu, Y. Bai, N. Bandyopadhyay, Sl Slivken, and M. Razeghi Applied Physics Letters, Vol. 97, No. 23, p. 231119-1-- December 6, 2010 We demonstrate surface-grating distributed feedback quantum cascade lasers (QCLs) with a watt-level power output at 4.75 μm. A device with a 5 mm cavity length exhibits an output power of 1.1 W in room-temperature cw operation. Single-mode operation with a side mode suppression ratio of 30 dB is obtained in the working temperature of 15–105 °C. A double-lobed far field with negligible beam steering is observed. The significance of this demonstration lies in its simplicity and readiness to be applied to standard QCL wafers with the promise of high-power performances. reprint
256.	Photovoltaic MWIR type-II superlattice focal plane array on GaAs substrate E.K. Huang, P.Y. Delaunay, B.M. Nguyen, S. Abdoullahi-Pour, and M. Razeghi IEEE Journal of Quantum Electronics (JQE), Vol. 46, No. 12, p. 1704-1708-- December 1, 2010 Recent improvements in the performance of Type-II superlattice (T2SL) photodetectors has spurred interest in developing low cost and large format focal plane arrays (FPA) on this material system. Due to the limitations of size and cost of native GaSb substrates, GaAs is an attractive alternative with 8” wafers commercially available, but is 7.8% lattice mismatched to T2SL. In this paper, we present a photovoltaic T2SL 320 x 256 focal plane array (FPA) in the MWIR on GaAs substrate. The FPA attained a median noise equivalent temperature difference (NEDT) of 13 mK and 10mK (F#=2.3) with integration times of 10.02 ms and 19.06 ms respectively at 67 K. reprint
257.	Injector doping level dependent continuous-wave operation of InP-based QCLs at λ~ 7.3 µm above room temperature J.S. Yu, S. Slivken, and M. Razeghi Semiconductor Science and Technology (SST), Vol. 25, No. 12, p. 125015-- December 1, 2010 We report the continuous-wave (CW) operation of InGaAs/InAlAs quantum cascade lasers (QCLs) operating at λ ~ 7.3 µm above room temperature. The injector doping level–dependent CW characteristics above room temperature are investigated for doping densities between 7 × 10¹⁶ cm⁻³ and 2 × 10¹⁷ cm⁻³. The device performance, i.e. threshold current density, output power, operating temperature and characteristic temperature, depends strongly on the injector doping density. For a relatively low injector doping density of 7 × 10¹⁶ cm⁻³, a high-reflectivity-coated 10 µm wide and 4 mm long laser exhibits an improved device performance with an output power of 152 mW and a threshold current density of 1.37 kA cm⁻² at 298 K under CW mode, operating up to 343 K. The thermal characteristics are also analyzed by the estimation from the experimentally measured data for the QCLs with different injector doping densities. reprint
258.	High-performance InP-based midinfrared quantum cascade lasers at Northwestern University M. Razeghi, Y. Bai, S. Slivken, and S.R. Darvish SPIE Optical Engineering, Vol. 49, No. 11, November 2010, p. 111103-1-- November 15, 2010 We present recent performance highlights of midinfrared quantum cascade lasers (QCLs) based on an InP material system. At a representative wavelength around 4.7 µm, a number of breakthroughs have been achieved with concentrated effort. These breakthroughs include watt-level continuous wave operation at room temperature, greater than 50% peak wall plug efficiency at low temperatures, 100-W-level pulsed mode operation at room temperature, and 10-W-level pulsed mode operation of photonic crystal distributed feedback quantum cascade lasers at room temperature. Since the QCL technology is wavelength adaptive in nature, these demonstrations promise significant room for improvement across a wide range of mid-IR wavelengths. reprint
259.	High performance long wavelength infrared mega-pixel focal plane array based on type-II superlattices P. Manurkar, S.R. Darvish, B.M. Nguyen, M. Razeghi and J. Hubbs Applied Physics Letters, Vol. 97, No 19, p. 193505-1-- November 8, 2010 A large format 1k × 1k focal plane array (FPA) is realized using type-II superlattice photodiodes for long wavelength infrared detection. Material growth on a 3 in. GaSb substrate exhibits a 50% cutoff wavelength of 11 μm across the entire wafer. The FPA shows excellent imaging. Noise equivalent temperature differences of 23.6 mK at 81 K and 22.5 mK at 68 K are achieved with an integration time of 0.13 ms, a 300 K background and f/4 optics. We report a dark current density of 3.3×10⁻⁴ A·cm⁻² and differential resistance-area product at zero bias R₀A of 166 Ω·cm² at 81 K, and 5.1×10⁻⁵ A·cm⁻² and 1286 Ω·cm², respectively, at 68 K. The quantum efficiency obtained is 78%. reprint
260.	Reliability in room-temperature negative differential resistance characteristics of low-aluminum contact AlGaN/GaN double-barrier resonant tunneling diodes C. Bayram, Z. Vashaei, and M. Razeghi Applied Physics Letters, Vol. 97, No. 18, p. 181109-1-- November 1, 2010 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. reprint
261.	Watt level performance of quantum cascade lasers in room temperature continuous wave operation at λ ∼ 3.76 μm N. Bandyopadhyay, Y. Bai, B. Gokden, A. Myzaferi, S. Tsao, S. Slivken and M. Razeghi Applied Physics Letters, Vol. 97, No. 13-- September 27, 2010 An InP-based quantum cascade laser heterostructure emitting at 3.76 μm is grown with gas-source molecular beam epitaxy. The laser core is composed of strain balanced In_0.76Ga_0.24As/In_0.26Al_0.74As. Pulsed testing at room temperature exhibits a low threshold current density (1.5 kA/cm²) and high wall plug efficiency (10%). Room temperature continuous wave operation gives 6% wall plug efficiency with a maximum output power of 1.1 W. Continuous wave operation persists up to 95 °C. reprint
262.	Broad area photonic crystal distributed feedback quantum cascade lasers emitting 34 W at λ ~ 4.36 μm B. Gokden, Y. Bai, N. Bandyopadhyay, S. Slivken and M. Razeghi Applied Physics Letters, Vol. 97, No. 13, p. 131112-1-- September 27, 2010 We demonstrate room temperature, high power, single mode, and diffraction limited operation of a two dimensional photonic crystal distributed feedback quantum cascade laser emitting at 4.36 μm. Total peak power up to 34 W is observed from a 3 mm long laser with 400 μm cavity width at room temperature. Far-field profiles have M2 figure of merit as low as 2.5. This device represents a significant step toward realization of spatially and spectrally pure broad area high power quantum cascade lasers. reprint
263.	Photoluminescence characteristics of polar and nonpolar AlGaN/GaN superlattices Z. Vashaei, C. Bayram, P. Lavenus, and M. Razeghi Applied Physics Letters, Vol. 97, No. 12, p. 121918-1-- September 20, 2010 High quality Al_0.2Ga_0.8N/GaN superlattices (SLs) with various (GaN) well widths (1.6 to 6.4 nm) 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 (PL) 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. reprint
264.	Room temperature neagtive differential resistance characteristics of polar III-nitride resonant tunneling diodes C. Bayram, Z. Vashaei, and M. Razeghi Applied Physics Letters, Vol. 97, No. 9, p. 092104-1-- August 30, 2010 III-nitride resonant tunneling diodes (RTDs), consisting Al_0.2Ga_0.8N/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. reprint
265.	Comparison of ultraviolet APDs grown on free-standing GaN and sapphire substrates E. Cicek, Z. Vashaei, C. Bayram, R. McClintock, M. Razeghi and M. Ulmer Proceedings, Vol. 7780, p. 77801P, SPIE Optics and Photonics Symposium, Conference on Detectors and Imaging Devices: Infrared, Focal Plane and Single Photon, San Diego, CA -- August 4, 2010 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. reprint
266.	III-nitride based avalanche photo detectors R. McClintock, E. Cicek, Z. Vashaei, C. Bayram, M. Razeghi and M. Ulmer Proceedings, Vol. 7780, p. 77801B, SPIE Optics and Photonics Symposium, Conference on Detectors and Imaging Devices: Infrared, Focal Plane and Single Photon, San Diego, CA -- August 4, 2010 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. reprint
267.	Geiger-mode operation of ultraviolet avalanche photodiodes grown on sapphire and free-standing GaN substrates E. Cicek, Z. Vashaei, R. McClintock, C. Bayram, and M. Razeghi Applied Physics Letters, Vol. 96, No. 26, p. 261107 (2010);-- June 28, 2010 GaN avalanche photodiodes (APDs) were grown on both conventional sapphire and low dislocation density free-standing (FS) c-plane GaN substrates. Leakage current, gain, and single photon detection efficiency (SPDE) of these APDs were compared. At a reverse-bias of 70 V, APDs grown on sapphire substrates exhibited a dark current density of 2.7×10⁻⁴ A/cm² whereas APDs grown on FS-GaN substrates had a significantly lower dark current density of 2.1×10⁻⁶ A/cm². 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 μm² area APD yielded a SPDE of 13% 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 30% under back-illumination—the FS-GaN APDs were only tested under front illumination due to the thick absorbing GaN substrate. reprint
268.	GaN avalanche photodiodes grown on m-plane freestanding GaN substrate Z. Vashaei, E. Cicek, C. Bayram, R. McClintock and M. Razeghi Applied Physics Letters, Vol. 96, No. 20, p. 201908-1-- May 17, 2010 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 Å 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. reprint
269.	Type-II Antimonide-based Superlattices for the Third Generation Infrared Focal Plane Arrays Manijeh Razeghi, Edward Kwei-wei Huang, Binh-Minh Nguyen, Siamak Abdollahi Pour, and Pierre-Yves Delaunay SPIE Proceedings, Infrared Technology and Applications XXXVI, Vol. 7660, pp. 76601F-- May 10, 2010 In recent years, the Type-II superlattice (T2SL) material platform has seen incredible growth in the understanding of its material properties which has lead to unprecedented development in the arena of device design. Its versatility in band-structure engineering is perhaps one of the greatest hallmarks of the T2SL that other material platforms are lacking. In this paper, we discuss advantages of the T2SL, specifically the M-structure T2SL, which incorporates AlSb in the traditional InAs/GaSb superlattice. Using the M-structure, we present a new unipolar minority electron detector coined as the p-M-p, the letters which describe the composition of the device. Demonstration of this device structure with a 14 μm cutoff attained a detectivity of 4x10¹⁰ Jones (-50 mV) at 77 K. As device performance improves year after year with novel design contributions from the many researchers in this field, the natural progression in further enabling the ubiquitous use of this technology is to reduce cost and support the fabrication of large infrared imagers. In this paper, we also discuss the use of GaAs substrates as an enabling technology for third generation imaging on T2SLs. Despite the 7.8% lattice mismatch between the native GaSb and alternative GaAs substrates, T2SL photodiodes grown on GaAs at the MWIR and LWIR have been demonstrated at an operating temperature of 77 K reprint
270.	Novel Green Light Emitting Diodes: Exploring Droop-Free Lighting Solutions for a Sustainable Earth M. Razeghi, C. Bayram, R. McClintock, F. Hosseini Teherani, D.J. Rogers, and V.E. Sandana Journal of Light Emitting Diodes, Vol. 2, No. 1, p. 1-33-- April 30, 2010 The total annual energy consumption in the United States for lighting is approximately 800 Terawatt-hours and costs $80 billion to the public. The energy consumed for lighting throughout the world entails to greenhouse gas emission equivalent to 70% of the emissions from all the cars in the world. Novel solutions to lighting with higher efficiency will drastically reduce the energy consumption and help greenhouse gas emissions to be lowered. Novel green light emitting diodes are the key components of an affordable, durable and environmentally benign lighting solution that can achieve unique spectral quality and promise superior energy conversion efficiency. Light-emitting diodes (LEDs), based on the InGaN alloy, are currently the most promising candidates for realizing solid state lighting (SSL). InGaN is a direct wide bandgap semiconductor with an emission that can span the entire visible spectrum via compositional tuning. However, InGaN LED performance remains wavelength-dependent. Indeed, ultrabright and efficient blue InGaN-based LEDs are readily available but the performance of InGaN-based green LEDs is still far from adequate for use in SSL. Our recent work demonstrated 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.. We have shown that atop grown ZnO layer by Pulsed Laser Deposition can be a good replacement for GaN. The green wavelength emission requires significant indium content in the active layer (growth temperature ~ 700ºC) that makes InGaN quantum wells very susceptible to thermal degradation. With our technology, diffusion and segregation of indium in the green emitting active is inhibited thanks to the lower ZnO deposition temperatures (<600ºC) than is required for GaN (>1000ºC). Our novel technology preserves the integrity of the as-grown active layer and demonstrates superior green spectral quality (as demonstrated for LEDs on c-sapphire). The results indicate that hybrid LED structures could hold prospects for the development of green LEDs with superior performance.
271.	Demonstration of negative differential resistance in GaN/AlN resonant tunneling didoes at room temperature Z. Vashaei, C. Bayram and M. Razeghi Journal of Applied Physics, Vol. 107, No. 8, p. 083505-- April 15, 2010 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. reprint
272.	Spatial Noise and Correctability of Type-II InAs/GaSb Focal Plane Arrays P.Y. Delaunay and M. Razeghi IEEE Journal of Quanutm Electronics, April 2010, Vol. 46, No. 4, p. 584-588-- April 1, 2010 A long wavelength infrared focal plane array based on Type-II InAs/GaSb superlattices was fabricated and characterized at 80 K. The noise equivalent temperature difference of the array was measured as low as 23 mK (f# = 2), for an integration time of 0.129 ms. The spatial noise of the array was dominated by the nonuniformity of the illumination through the circular aperture. A standard two-point nonuniformity correction improved the inhomogeneity equivalent temperature difference to 16 mK. The correctability just after calibration was 0.6. The long-term stability time was superior to 25 hours. reprint
273.	Thin film transistors with wurtzite ZnO channels grown on Si₃N₄/SiO₂/Si (111) substrates by pulsed laser deposition D.J. Rogers; V.E. Sandana; F. Hosseini Teherani; M. Razeghi Proc. SPIE 7603, Oxide-based Materials and Devices, 760318 (March 02, 2010)-- March 7, 2010 Thin Film Transistors (TFT) were made by growing ZnO on Si₃N₄/SiO₂/Si (111) substrates by pulsed laser deposition. X-ray diffraction and scanning electron microscope studies revealed the ZnO to have a polycrystalline wurtzite structure with a smooth surface, good crystallographic quality and a strong preferential c-axis orientation. Transmission studies in similar ZnO layers on glass substrates showed high transmission over the whole visible spectrum. Electrical measurements of a back gate geometry FET showed an enhancement-mode response with hard saturation, mA range Id and a V_ON ∼ 0V. When scaled down, such TFTs may be of interest for high frequency applications. reprint
274.	Quantum cascade lasers that emit more light than heat Y. Bai, S. Slivken, S. Kuboya, S.R. Darvish and M. Razeghi Nature Photonics, February 2010, Vol. 4, p. 99-102-- February 1, 2010 For any semiconductor lasers, the wall plug efficiency, that is, the portion of the injected electrical energy that can be converted into output optical energy, is one of the most important figures of merit. A device with a higher wall plug efficiency has a lower power demand and prolonged device lifetime due to its reduced self-heating. Since its invention, the power performance of the quantum cascade laser has improved tremendously. However, although the internal quantum efficiency can be engineered to be greater than 80% at low temperatures, the wall plug efficiency of a quantum cascade laser has never been demonstrated above 50% at any temperature. The best wall plug efficiency reported to date is 36% at 120 K. Here, we overcome the limiting factors using a single-well injector design and demonstrate 53% wall plug efficiency at 40 K with an emitting wavelength of 5 µm. In other words, we demonstrate a quantum cascade laser that produces more light than heat. reprint
275.	On the interface properties of ZnO/Si electroluminescent diodes J.L. Pau, J. Piqueras, D.J. Rogers, F. Hosseini Teherani, K. Minder, R. McClintock, and M. Razeghi Journal of Applied Physics, Vol. 107, No. 3, p. 033719-1-- February 1, 2010 ZnO layers grown on n⁻–Si(100), n⁺–Si(100), and n^––Si(111) substrates by pulsed-laser deposition were found to give electroluminescence. Light emission was observed in the form of discrete spots for currents over 1 mA with a white appearance to the naked eye. The intensity of these spots showed an erratic behavior over time, appearing and disappearing at random, while showing an associated random telegraph noise in the current signal. Regardless the substrate used, the electroluminescence spectra had a main broadband emission centered at about 600 nm and a relatively small peak at around 380 nm which corresponds to the energy of ZnO near band edge emission. Furthermore, the devices exhibited rectifying characteristics, whose current blocking direction depended on the substrate orientation. Optimization of ZnO conductivity and performing sample growth in N₂ ambient were found to be critical to enhance the emission intensity. Rutherford backscattering characterization revealed the existence of an intermixed region at the interface between ZnO and Si. To study the electronic properties at the interface, frequency dependent capacitance measurements were carried out. The junction capacitance became frequency dependent at the bias voltages at which light emission occurs due to the relatively slow trapping and generation processes at deep centers. These centers are believed to play an important role in the mechanism of light emission. reprint

Page 11 of 32: Prev << 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 >> Next (778 Items)