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GaN-Based MSM Photodetectors Prepared on Patterned Sapphire Substrates
Abstract
GaN-based metal-semiconductor-metal ultraviolet photodetectors (PDs) prepared on a patterned sapphire substrate (PSS) and a conventional flat sapphire substrate were both fabricated and characterized. It was found that we can reduce dark leakage current and enhance by about two orders of magnitude by using a PSS. The internal gain of the PDs prepared on a PSS was also much smaller.
... In contrast, only few studies have so far been made at GaN-based MSM UV PDs fabricated by using PSS. Chang et al. have investigated that the GaN-based MSM UV PDs prepared on PSS can contribute to a much smaller dark leakage and the internal gain than those achieved from the PD fabricated on the conventional flat sapphire substrate [24]. However, the used structure on substrate pattern of GaN-based MSM UV PDs for their study is the shape of paralle stripe. ...
... As a result, the photocurrent to dark current contrast is higher than ten orders of magnitude for the PD fabricated on PTCPSS with a 5 V bias voltage, whose value is larger than that of the PD fabricated on CFSS. Figure 8 shows the spectral responses of both MSM PDs biased at 5 V. It is found that the cutoff A/W for our PD is also found to be larger than that of around 0.38 A/W reported by ref. [24]. In addition, we can also observe the responsivity drops in a sharp cut-off region of 100 nm for the PD with PTCPSS. ...
... In addition, the experimental results also exhibit that the properties in the dark leakage current and the maximum responsivity for our GaN MSM UV PD with suitably geometrical pattern of substrate are better than those of reported by ref. [24]. ...
GaN ultraviolet photodetector with metal-semiconductor-metal structure is achieved by growing on a periodic trapezoid column-shape patterned sapphire substrate using metalorganic chemical vapor deposition. Under 5-V reverse bias, the photodetector fabricated on such patterned sapphire substrate exhibits a lower dark current, a higher photocurrent, and a 476 % enhancement in the maximum responsivity as compare with those of the photodetector fabricated on conventional flat sapphire substrate. It is also found that the much larger UV-to-visible rejection ratio and the fact that responsivity drops in a smaller cut-off region are observed from photodetector fabricated by using a periodic trapezoid column-shape patterned sapphire substrate. These phenomena may all be attributed to the reduction of threading dislocation density and the improved quality of GaN film, as well as the internal reflection and/or scattering effect on the interface between GaN film and the periodic trapezoid column-shape pattern of the substrate.
... In contrast, only few studies have so far been made at GaN-based MSM UV PDs fabricated by using PSS. Chang et al. have investigated that the GaN-based MSM UV PDs prepared on PSS can contribute to a much smaller dark leakage and the internal gain than those achieved from the PD fabricated on the conventional flat sapphire substrate [26]. However, the used structure on substrate pattern of GaN-based MSM UV PDs for their study is the shape of paralle stripe. ...
... The maximum responsivities at 360 nm are 0.98 and 0.17 A/W for the PDs fabricated on PTCPSS and CFSS, respectively. The measured maximum responsivity of 0.98 A/W for the PD with PTCPSS is also found to be larger than that of around 0.38 A/W reported by ref. [26]. In addition, we can also observe the responsivity drops in a sharp cut-off region of 100 nm for the PD with PTCPSS. ...
... In addition, the experimental result also shows that the properties in the dark leakage current and the maximum responsivity for our GaN MSM UV PD with suitably geometrical pattern of substrate are better than those of reported by ref. [26]. ...
GaN ultraviolet photodetector with metal-semiconductor-metal structure is achieved by growing on a periodic trapezoid column-shape patterned sapphire substrate using metalorganic chemical vapor deposition. Under 5-V reverse bias, the photodetector fabricated on such patterned sapphire substrate exhibits a lower dark current, a higher photocurrent, and a 476 % enhancement in the maximum responsivity as compare with those of the photodetector fabricated on conventional flat sapphire substrate. It is also found that the much larger UV-to-visible rejection ratio and the fact that responsivity drops in a smaller cut-off region are observed from photodetector fabricated by using a periodic trapezoid column-shape patterned sapphire substrate. These phenomena may all be attributed to the reduction of threading dislocation density and the improved quality of GaN film, as well as the internal reflection and/or scattering effect on the interface between GaN film and the periodic trapezoid column-shape pattern of the substrate.
... However, the above-discussed strategy is applied in the fabrication of UV LEDs. Nevertheless, the intrinsic logic of defect termination can also be used to fabricate high-performance photodetectors [52][53][54] . For example, Chang et al. reported two types of GaN-based Schottky barrier photodetectors that are grown on GaN NL/PSS and GaN NL/FSS templates [53] . ...
Solar-blind ultraviolet photodetectors (SBPDs) have attracted tremendous attention in the environmental, industrial, military, and biological fields. Aluminum gallium nitride (AlGaN), a kind of representative III-nitride semiconductor, has promising prospects in solar-blind photodetection owing to its tunable wide bandgap and industrial feasibility. Considering the high defect density in the AlGaN epilayer directly grown on a sapphire substrate, employing an AlN/sapphire template turns out to be an effective method to achieve a high-quality AlGaN epilayer, thereby enhancing the SBPD performances. In recent years, a variety of remarkable breakthroughs have been achieved in the SBPDs. In this paper, the progress on photovoltaic AlGaN-based SBPDs is reviewed. First, the basic physical properties of AlGaN are introduced. Then, fabrication methods and defect annihilation of the AlN/sapphire template are discussed. Various photovoltaic SBPDs are further summarized, including Schottky barrier, metal-semiconductor-metal, p-n/p-i-n and avalanche photodiodes. Furthermore, surface modification and photoelectrochemical cell techniques are introduced. Benefitting from the development of fabrication techniques and optoelectronic devices, photovoltaic AlGaN photodiodes exhibit a promising prospect in solar-blind ultraviolet photodetection.
... The band edge at the Semiconductor surface is set to As the accurate computation of the first energy level is numerically expensive and necessitates the explanation of the Schrödinger equation and estimation is utilized. The offset ∆E g is estimated subsequent an establishment of Van Dort et al. [11][12][13][14], which reads as (4) whereas E semicon surface is the magnitude of the electric field at the semiconductor interface and ϵ is the permittivity of the semiconductor. β = 4.1× 10 -8 eVcm is an empirical constant. ...
The paper presents the transmission and reflection analysis on metal-semiconductor structure for quantum confinement model by FDTD technique. The research problem in this study is to understand the transmission and reflection spectrum from the interface of metal and semiconductor combined devices based on the quantum mechanical effect. The objective of this study is to implement the simulation code for quantum mechanics concepts by MATLAB. The analysis on quantum mechanical analysis of the quantum confinement for semiconductor devices which would be designed based on metal-semiconductor structure is numerically investigated in this study. In this paper a macroscopic model was presented, which embraces an innovative approach to equivalent the vertical carrier profile and unites it with a conventional representation in lateral direction firstly. The numerical results prove a momentous enhancement pertaining to the accuracy of the carrier profile and the physical characteristics of the semiconductor devices. The second stage of this study is mainly focused on the transmission and reflection spectrum on the interface of metal/semiconductor structure based on the finite difference time domain techniques.
... So reducing the electrical dislocation density is crucial for further improving the performance of the GaN-based SB PDs. Currently, one of the effective methods to obtain high quality crystalline is the lateral epitaxial overgrowth technology, which can produce low dislocation density areas on patterned sapphire substrates (PSS) 8 . In addition, to enhance the photoelectric conversion efficiency, it is feasible to design and grow specific GaN epi-layers with graded doping because the resultant built-in electric field could push up the drift velocity of the carriers 9 . ...
In this paper, we demonstrate high performance GaN-based Schottky-barrier ultraviolet (UV) photodetectors with graded doping prepared on patterned sapphire substrates. The fabricated devices exhibit an extremely low dark current density of ∼1.3×10⁻⁸ A/cm² under -5 V bias, a large UV-to-visible light rejection ratio of ∼ 4.2×10³, and a peak external quantum efficiency of ∼50.7% at zero bias. Even in the deeper 250-360 nm range, the average external quantum efficiency still remains ∼40%. From the transient response characteristics, the average rising and falling time constants are estimated ∼115 μs and 120 μs, respectively, showing a good electrical and thermal reliability. The specific detectivities D∗, limited by the thermal equilibrium noise and the low-frequency 1/f noise, are derived ∼5.5×10¹³ cm Hz1/2/W (at 0 V) and ∼2.68×10¹⁰ cm Hz1/2 W⁻¹ (at -5 V), respectively.
... These peaks indicate that a crystalline -thin film was formed [8]. For the fabrication of PD, we deposited a thick Ni/Au film through an interdigitated shadow mask onto the sample to serve as the contact electrodes [9], [10]. The dimensions of the interdigitated shadow mask were 2 mm wide and 2.2 mm long with finger width of 0.1 mm and finger spacing of 0.2 mm. ...
The authors report the fabrication of a β-Ga2O 3/GaN solar-blind and visible-blind dual-band photodetector (PD). It was found that we could control the depletion depth of the device and thus switch the operation mode between solar-blind and visible-blind by simply changing the applied bias. It was also found that we could achieve a deep-UV to near-UV contrast ratio of 4.6 × 103 when the device was biased at 1 V. Furthermore, it was found that near-UV to visible contrast ratio of the fabricated β-Ga2O3/GaN PD was 8.5 × 10 2 when biased at 10 V.
Gallium oxide (Ga2O3) has been studied as one of the most promising wide bandgap semiconductors during the past decade. Here, we prepared high quality β-Ga2O3 films by pulsed laser deposition. β-Ga2O3 films of different thicknesses were achieved and their crystal properties were comprehensively studied. As thickness increases, grain size and surface roughness are both increased. Based on these β-Ga2O3 films, a series of ultraviolet (UV) photodetectors with interdigital electrodes structure were prepared. These devices embrace an ultralow dark current of 100 fA, and high photocurrent on/off ratio of 10E8 under UV light illumination. The photoresponse time is 4 ms which is faster than most of previous works. This work paves the way for the potential application of Ga2O3 in the field of UV detection.
The facile enhancement of the optical photocurrent in gallium nitride (GaN) ultraviolet (UV) photodetectors was realized by applying an
in-situ
curing technique to polymer lenses on a GaN surface. To fabricate plano-convex polymer lenses that focus incident UV light, a prepolymer was dropped directly onto the GaN surface and cured at room temperature to induce UV light refraction. When the incident light passed through the polymer lens, the light concentrated on the sensing area of the GaN photodetector, thus generating more photocurrent than GaN photodetectors without the lens. Consequently, photodetectors fabricated with the plano-convex polymer lenses exhibited higher optical sensitivity (up to ~18.16% increase in photocurrent at room temperature) than those fabricated without polymer lenses. This study supports the use of direct and
in-situ
curing of polymer lenses on GaN surfaces for the facile and reliable enhancement of optical sensitivity of various GaN-based optoelectronics.
InGaN/GaN multi-quantum well (MQW) structures were used as an intrinsic semiconductor in InGaN-based p-i-n ultraviolet photodetectors (PDs). The cut-off wavelengths of the PDs grown on normal sapphire substrates (NSSs) and on patterned sapphire substrates (PSSs) were 445 and 450 nm, respectively, which is consistent with the photoluminescence emission wavelength. From the crystal and optical analyses, we found that the crystallinity and the absorption of the PDs grown on PSSs were superior to those of the PDs grown on NSSs. The maximum photoreactivities of the PDs grown on PSSs and on NSSs were 0.176 A/W and 0.109 A/W, respectively. In addition, the external quantum efficiencies of those PDs were 56.1% and 34.8%, respectively. From these results, we suggest that a PSS can play an important role in achieving a high reactivity and external quantum efficiency for InGaN-based PDs due to improved crystallinity and decreased optical absorption in the sapphire substrate.
In this paper, we present a numerical simulation study of nitride-based p-i-n ultraviolet bandpass photodetectors, which was designed with a high Al composition AlGaN unipolar barrier layer. The AlGaN unipolar blocking layers act as potential barrier that reduce the collection of carriers generated by high energy photons in p-Al 0.1 Ga 0.9 N short-wave filter layer. Compared to the conventional nitride-based p-i-n band-pass photodiodes, this new structure devices are capable high short-wave rejection ration. To validate the model, we compared the simulation predictions with experimental data of conventional p-i-n bandpass photodiodes published in the literature. This concept can be applicable for any p-i-n photodetectors.
We report on two-dimensional (2D) numerical simulations of photoresponse characteristics for GaN based p-i-n ultraviolet (UV) photodetectors. Effects of doping density of absorption layer (ni) on the photoresponse have been investigated. The Poisson and continuity equations are solved along each grid point in the 2D structure with three generation-recombination processes as Auger, Shockley-Read-Hall and optical generation-recombination. In order to accurately simulate the device performance, the theoretical calculation includes doping-dependent mobility degradation by Arora model and high field saturation model. Optimal doping concentration of absorption layers at different wavelength are extracted numerically. Theoretical modeling shows that the doping density of the absorption layer can significantly affect the photoresponse of GaN based p-i-n UV photodetectors.
Al0.45Ga0.55N metal–semiconductor–metal photodetectors were grown by MOVPE on planar and on stripe patterned epitaxial laterally overgrown (ELO) AlN/sapphire templates. By comparing devices on different template types, the influence of the average dislocation density, the distribution of dislocations, and composition modulations due to inhomogeneous gallium incorporation on the external quantum efficiency (EQE) were evaluated. The reduction of the average dislocation density from 6 × 109 cm−2 on the planar template to about 2 × 109 cm−2 by ELO increases the EQE. For electron transport perpendicular to the ELO stripes, this increase is about 70%. Due to the stripe-like dislocation distribution in the absorber layers on ELO templates, the EQE becomes up to a factor of 3 higher than for planar templates, when the electrodes are perpendicular to the ELO stripes and electron transport is along the stripes. Photoconductive gain was found at elevated bias for absorber layers on standard ELO templates with electrodes perpendicular to the ELO stripes. The gain can be attributed to carrier transport in zig-zag shaped Ga-rich channels caused by facetted growth on ELO templates. The gain only appears, if the aluminum mole fraction differences Δx are higher than 0.07. At 30 V bias, the EQE of such detectors is more than 3 orders of magnitude higher compared to planar device.
We study the performance of GaN-based p-i-n ultraviolet (UV) photodetectors (PDs) with a 60 nm thin p-type contact layer grown on patterned sapphire substrate (PSS). The PDs on PSS exhibit a low dark current of ~2 pA under a bias of -5 V, a large UV/visible rejection ratio of ~7×103, and a high-quantum efficiency of ~40% at 365 nm under zero bias. The average quantum efficiency of the PDs still remains above 20% in the deep-UV region from 280 to 360 nm. In addition, the noise characteristics of the PDs are also discussed, and the corresponding specific detectivities limited by the thermal noise and the low-frequency 1/f noise are calculated.
In this paper, we experimentally and numerically study the dynamics of semiconductor ring lasers that are subjected to long delay and moderate self-feedback. Through varying the pump current or the feedback strength or both, we study the appearance and parameter dependence of low frequency fluctuations in these systems. In particular, we observe different routes to the building up of the initial power amplitude.
In this letter, GaN-based p-i-n ultraviolet (UV) photodetectors (PDs) are fabricated on patterned sapphire substrate (PSS) for the first time. Based on cathodoluminescence mapping and x-ray diffraction measurement, the as-grown structure on PSS has considerably lower defect density than that of a similar structure grown on standard sapphire substrate (SSS). The PD on PSS exhibits a low dark current density of ~ 5.1 nA/cm2 under -5 V, a high UV/visible rejection ratio of more than 104, and a zero-bias peak responsivity of ~0.19A/W at 360 nm, which corresponds to a maximum quantum efficiency of 65%. In the photo-sensitive wavelength region between 250 and 365 nm, the quantum efficiency of the PD on PSS is, on average, over 30% higher than that of the control device fabricated on SSS.
We demonstrate a dual-operation-mode ultraviolet (UV) Schottky-barrier photodetector (PD) fabricated on high-resistivity GaN homoepitaxial layer with low defect density. The undoped GaN active layer is grown by metal-organic chemical vapor deposition on a conductive bulk GaN substrate. Under reverse and zero bias, the PD works in depletion mode with low dark current and high UV/visible rejection ratio. Under forward bias, the PD works alternatively in photoconductive mode, which exhibits high photo-responsivity and an attractive narrow detection band around 365 nm. In addition, the PD also shows reasonable response speed in both operation modes.
GaN ultraviolet (UV) Schottky barrier photodetectors (PDs) with a ZrO2 or SiO2 layer were successfully fabricated. With an appropriate ZrO2 layer thickness, the dark current of Schottky barrier PDs could be notably suppressed, and the photogenerated carriers could
still reach the electrodes by tunneling through the thin ZrO2 layer under illumination. A UV-to-visible rejection ratio of more than one order of magnitude can be found in the Schottky
barrier PD with ZrO2 layers. It can be seen clearly that the cut-off occurred at approximately 360 nm while the responses above the band-gap were
flat.
A high quality GaN Schottky barrier photodetector (PD) was prepared on patterned sapphire substrates (PSSs) by metallorganic
chemical vapor deposition. Compared with the PD prepared on a conventional flat sapphire substrate, we can reduce dark current
and enhance responsivity. Under a −2 V applied bias, noise equivalent power and normalized detectivity
were
and
, respectively, for the PD prepared on PSS. These values were also better than those achieved from the PD prepared on a flat
sapphire substrate.
We report on a backilluminated GaN/Al0.18Ga0.82N heterojunction ultraviolet (UV) photodetector with high internal gain based on metal-semiconductor-metal structures. A narrow band pass spectral response between 365 and 343 nm was achieved. When operating in dc mode, the responsivity reaches up to the order of 102 A/W under weak UV illumination, which is due to enormous internal gain up to 103. The linear dependence of photocurrent on bias and its square root dependence on optical power are found and explained by a trapping and recombination model. The high photocurrent gain is attributed to trapping and recombination centers with an acceptor character induced by dislocations in GaN. © 2002 American Institute of Physics.
The characteristics of Ni/indium tin oxide (ITO) ohmic contacts to p-type GaN ( ∼ 2×1017 cm−3) have been studied. The Ni/ITO (10 nm/250 nm) layers were prepared by thermal evaporation and rf magnetron sputtering, respectively. Although the as-deposited Ni/ITO contacts present rectified behavior, the linear current–voltage characteristics can be obtained. The contact resistance can be reduced significantly for the ITO/Ni/p-GaN samples after suitable rapid thermal process. The contact property of ITO/Ni/p-GaN shows lowest specific contact resistivity of 8.6×10−4 Ω cm2 and high transparency (above 80% for 450–550 nm) as the sample annealed at 600 °C in air. Possible mechanisms for the observed low contact resistance and high transparency will be discussed. The present process is compatible with the fabrication for the high-efficient GaN light-emitting devices. © 2001 American Institute of Physics.
A model to explain the behaviour of GaN photoconductive detectors is proposed, and it is based on the idea of a volume modulation rather than a carrier density modulation. Space charge regions inside the semiconductor produce a variation of the conductive volume when carriers are photogenerated. The strong non-exponential photocurrent decays result from carrier capture processes over the barriers associated with space charge regions. By means of computer simulation, this model explains quite well the behaviour of current GaN photoconductor devices and predicts their time response, temperature dependence and responsivity properties.
In the past several years, research in each of the wide‐band‐gap semiconductors, SiC, GaN, and ZnSe, has led to major advances which now make them viable for device applications. The merits of each contender for high‐temperature electronics and short‐wavelength optical applications are compared. The outstanding thermal and chemical stability of SiC and GaN should enable them to operate at high temperatures and in hostile environments, and also make them attractive for high‐power operation. The present advanced stage of development of SiC substrates and metal‐oxide‐semiconductor technology makes SiC the leading contender for high‐temperature and high‐power applications if ohmic contacts and interface‐state densities can be further improved. GaN, despite fundamentally superior electronic properties and better ohmic contact resistances, must overcome the lack of an ideal substrate material and a relatively advanced SiC infrastructure in order to compete in electronics applications. Prototype transistors have been fabricated from both SiC and GaN, and the microwave characteristics and high‐temperature performance of SiC transistors have been studied. For optical emitters and detectors, ZnSe, SiC, and GaN all have demonstrated operation in the green, blue, or ultraviolet (UV) spectra. Blue SiC light‐emitting diodes (LEDs) have been on the market for several years, joined recently by UV and blue GaN‐based LEDs. These products should find wide use in full color display and other technologies. Promising prototype UV photodetectors have been fabricated from both SiC and GaN. In laser development, ZnSe leads the way with more sophisticated designs having further improved performance being rapidly demonstrated. If the low damage threshold of ZnSe continues to limit practical laser applications, GaN appears poised to become the semiconductor of choice for short‐wavele-
ngth lasers in optical memory and other applications. For further development of these materials to be realized, doping densities (especially p type) and ohmic contact technologies have to be improved. Economies of scale need to be realized through the development of larger SiC substrates. Improved substrate materials, ideally GaN itself, need to be aggressively pursued to further develop the GaN‐based material system and enable the fabrication of lasers. ZnSe material quality is already outstanding and now researchers must focus their attention on addressing the short lifetimes of ZnSe‐based lasers to determine whether the material is sufficiently durable for practical laser applications. The problems related to these three wide‐band‐gap semiconductor systems have moved away from materials science toward the device arena, where their technological development can rapidly be brought to maturity.
The authors fabricated GaN-based light-emitting diodes (LEDs) on two different GaN templates with the same LED structure. One on thin GaN template (∼2 μ m ) with high dislocation density [low (10<sup>9</sup> cm <sup>-2</sup>) ] grown by metal-organic vapor-phase epitaxy (sample A) and the other on thick GaN template (∼20 μ m ) with comparatively low dislocation density [high (10<sup>8</sup> cm <sup>-2</sup>) ] by hydride vapor-phase epitaxy (sample B). In order to understand the mechanism of leakage current in LEDs, the correlation between current-voltage characteristics and etch pit density of LEDs was studied.
A unified approach to obtain the characteristics of almost-periodic grating slab waveguides including gain in the waveguide is reported. In this approach the waveguides are divided into short segments, and in each segment the gratings are assumed to be periodic, that is, parameters such as coupling coefficient, grating phase, deviations from the Bragg frequency, and gain in the waveguide are independent of a propagation direction z. Then characteristics of almost-periodic grating slab waveguides can be obtained by multiplying each F matrix of a short segment with the proper grating phase conditions at the interface between two adjacent segments. The appropriateness of this approach is shown for typical aperiodic grating waveguides such as tapered, chirped, and phase-shifted gratings. The results obtained by this method are compared with others and prove to be in good agreement with the results obtained by other methods. In addition to these characteristics, it is shown that the F matrix can be used to obtain the threshold conditions for distributed feedback laser oscillations including reflections from cleaved edges.
Apodised chirped gratings based on InGaAsP/InP deep-ridge waveguides with vertical-groove surface gratings were fabricated. Reflectivity ripple and group delay ripple were reduced from around 4 dB to 1 dB and from around 5 ps to 2 ps, respectively, by apodisation over a wavelength range of around 20 nm.
In this paper, we demonstrate the generation of transform-limited short optical pulses, which display excellent spectral and temporal qualities by employing a novel technology, based on an externally injected gain-switched laser in conjunction with a nonlinearly chirped grating. Using this technique, 3.5-ps optical pulses exhibiting a time-bandwidth product (TBP) of 0.45 are generated, which are suitable for use in high-speed 80 Gb/s optical time-division multiplexing (OTDM) communications systems. The numerical integration of a set of rate equations using suitable parameters for the devices used in the experiments were carried out to further confirm the feasibility of the proposed method for developing an optimized pulse source for high-speed photonic systems.
Describes the analysis, fabrication, and characterization of
chirped grating tunable distributed-feedback (CGT DFB) semiconductor
lasers. Both theoretical and experimental results show that the
wavelength tunability of DFB lasers can be enhanced by introducing a
pitch-chirped grating structure. The tuning mechanism is also
theoretically demonstrated. The proposed CGT DFB laser will be useful in
optical systems using wavelength division multiplexed (WDM) networks and
other applications, due to its broader continuous tunability and very
easy fabrication
Near-ultraviolet nitride-based light-emitting diodes (LEDs) with peak emission wavelengths around 410 nm were fabricated onto c-face patterned sapphire substrates (PSS). It was found that the electroluminescence intensity of the PSS LED shown 63% larger than that of the conventional LED. For a typical lamp-form PSS LED operating at a forward current of 20 mA, the output power and external quantum efficiency were estimated to be 10.4 mW and 14.1%, respectively. The improvement in the light intensity could be attributed to the decrease of threading dislocations and the increase of light extraction efficiency in the horizontal direction using a PSS.
Ultraviolet (UV) light-emitting diodes (LEDs) with an InGaN multi-quantum-well (MQW) structure were fabricated on a patterned sapphire substrate (PSS) using a single growth process of metalorganic vapor phase epitaxy. In this study, the PSS with parallel grooves along the sapphire direction was fabricated by standard photolithography and subsequent reactive ion etching (RIE). The GaN layer grown by lateral epitaxy on a patterned substrate (LEPS) has a dislocation density of 1.5× 108 cm-2. The LEPS-UV-LED chips were mounted on the Si bases in a flip-chip bonding arrangement. When the LEPS-UV-LED was operated at a forward-bias current of 20 mA at room temperature, the emission wavelength, the output power and the external quantum efficiency were estimated to be 382 nm, 15.6 mW and 24%, respectively. With increasing forward-bias current, the output power increased linearly and was estimated to be approximately 38 mW at 50 mA.
In this letter we demonstrate that the anomalously low (002) x-ray rocking curve widths for epitaxial hexagonal GaN films on (001) sapphire are a result of a specific threading dislocation (TD) geometry. Epitaxial GaN films were grown on c-plane sapphire by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) in a horizontal flow reactor. Films were grown with (002) rocking curves (ω-scans) widths as low as 40 arcsec and threading dislocation densities of ∼2×1010 cm−2. The threading dislocations in this film lie parallel to the [001] direction and within the limit of imaging statistics, all are pure edge with Burgers vectors parallel to the film/substrate interface. These TDs will not distort the (002) planes. However, distortion of asymmetric planes, such as (102), is predicted and confirmed in (102) rocking curve widths of 740 arcsec. These results are compared with films with (002) rocking curves of ∼270 arcsec and threading dislocation densities of ∼7×108 cm−2.
Schottky barrier GaN ultraviolet detectors, both in vertical and in lateral configuration, as well as in a metal–semiconductor–metal geometry were implemented. All devices exhibit a high gain at both reverse and forward bias. The photoresponse in the forward bias is in the positive current direction. We attribute the gain to trapping of minority carriers at the semiconductor–metal interface. The excellent agreement between the calculated responsivity and the experiment indicates that the model is valid for all device structures under study, and represents a unified description of gain mechanism in GaN Schottky detectors. © 2001 American Institute of Physics.
InGaN multi-quantum-well-structure laser diodes with Al0.14Ga0.86N/GaN modulation doped strained-layer superlattice cladding layers grown on an epitaxially laterally overgrown GaN (ELOG) substrate was demonstrated to have a lifetime of more than 1150 h under room-temperature continuous-wave operation. After 4 μm etching of the ELOG substrate, the etch pit density was about 2×108 cm2 in the region of the 4-μm-wide stripe window, but almost zero in the region of the 7-μm-wide SiO2 stripe. © 1998 American Institute of Physics.
Spectrally resolved streak camera measurements of picosecond pulses emitted by hybridly colliding pulse mode-locked (CPM) laser diodes are presented in this letter. Depending on the modulation frequency both blue-chirped (upchirped) and red-chirped (downchirped) pulses can be observed. The two different regimes and the transition between them are characterized experimentally and the behavior is explained on the basis of our model for the CPM laser dynamics. (C) 1997 American Institute of Physics.
Epitaxially laterally overgrown GaN on sapphire was used to reduce the number of threading dislocations originating from the interface of the GaN epilayer with the sapphire substrate. The GaN layer above the SiO2 mask area surrounding the window and corresponding to the lateral overgrowth was nearly free of threading dislocations. A high density of threading dislocations was observed in the vicinity of GaN grown in the window regions. InGaN multiquantum well-structure laser diodes (LDs) grown on pure GaN substrates, which were fabricated by removing the sapphire substrate, were demonstrated. The LDs with an output power of 5 mW exhibited a lifetime of more than 290 h and an estimated lifetime of 10,000 h despite a relatively large threshold current density. The far-field pattern of the LDs with a cleaved mirror facet revealed single-mode emission without any interference effects.
Dispersion compensation is a ubiquitous problem for the generation and application of ultrashort optical pulses. Early approaches to compensate for the spectral group-delay dependence in materials used prism and grating sequences for this purpose, but are limited in bandwidth. Microstructuring dielectric optical materials on the scale of the optical wavelength have developed as an alternative for inducing a desired spectral group-delay dependence. With this approach a nearly arbitrary dependence of group delay vs. wavelength can be compensated for. We will discuss different approaches to microstructured dispersion compensation, namely chirped Bragg gratings, different generations of chirped mirrors, quasi-phase-matching gratings, and arrayed-waveguide gratings. We will outline common limitations and discuss ideas to expand further the utility of these approaches.
Nitride-based blue LEDs prepared on both patterned and conventional sapphire substrates were both fabricated. It was found that although the EL peak positions of these two LEDs were about the same, the EL intensity of LED grown on patterned sapphire substrate was about 35% larger. The maximum output power of LED grown on patterned sapphire substrate also occurred at higher injection current. The reliability of LED grown on patterned sapphire substrate was also found to be better. There improvements could all be attributed to the reduced dislocation density in the LEDs grown on patterned sapphire substrates.
We demonstrate continuously tunable compensation of linear chirp on a first-harmonic pump pulse to produce a near-transform-limited second-harmonic output pulse through the use of a chirped, fanned, periodically poled lithium niobate quasi-phase-matching grating. Compensation of positive and negative chirps is possible through reversal of device orientation. The device is simple and monolithic and can be applied to compensation of a higher-order phase with minor modification. (C) 2002 Optical Society of America.
InGaN-GaN multiquantum-well (MQW) blue and green light-emitting
diodes (LEDs) were prepared by organometallic vapor phase epitaxy, and
the properties of these LEDs were evaluated by photoluminescence (PL),
double crystal X-ray diffraction, and electroluminescence (EL)
measurements. It was found that there were only small shifts observed in
PL and EL peak positions of the blue MQW LEDs when the number of quantum
well (QW) increased. However, significant shifts in PL and EL peak
positions were observed in green MQW LEDs when the number of QW
increased. It was also found that there was a large blue shift in EL
peak position under high current injection in blue MQW LEDs. However,
the blue shift in green MQW LEDs was negligibly small when the injection
current was large. These observations could all be attributed to the
rapid relaxation in green MQW LEDs since the In composition ratio in the
InGaN well was high for the green MQW LEDs. The forward voltage V<sub>f
</sub> of green MQW LEDs was also found to be larger than that of blue
MQW LEDs due to the same reason
We have fabricated actively mode-locked lasers integrated with
electroabsorption modulators and chirped gratings. A chirped grating
with a large chirp rate of 1.45 Å/μm can be realized by using
multiphase-shifted patterns. Short pulses of 4-6 ps were generated over
a wide frequency range from 18.9-19.8 GHz. We observed jumps in the
wavelength during detuning. These jumps arise from multiple lobes in the
reflectivity spectrum. It is found that the wavelength jumps cause
increases in the intensity noise. We showed that by reducing the grating
length from 150 to 100 μm the sidelobes were suppressed and the
detuning frequency range of over 1% could be realized. A smaller
pulsewidth was obtained for the negatively-chirped gratings when
compared to the positively chirped gratings
A fully post-growth integrated chirped Bragg grating (CBG) in III-V material is presented based on a tapered waveguide geometry. The deeply etched structure provides the high coupling coefficient required in CBG devices without recourse to overgrowth fabrication techniques and the taper method allows for arbitrary chirp functions to be implemented. A 5-nm stopband with 0.0335 ps/nm dispersion is demonstrated and DFB lasers based on the CBG structure are presented
The novel design and fabrication of monolithically integrated passive waveguide mode converters (WMCs), realized through the utilization of the reactive ion-etch lag (RIE Lag) phenomenon, is reported. The low-loss GaAs-AlGaAs WMCs have been characterized over a wavelength range of 900-940 nm, resulting in TE-TM (TM-TE) mode conversion with efficiencies of greater than 96% and low-loss devices with conversion lengths as short as 150 μm. The properties and characteristics of the fully integrated WMCs, fabricated with a single masking and etch process, are in agreement with theoretical predictions.
We demonstrate the first realization of all-active tapered index coupled 1.55-/spl mu/m InGaAsP buried-heterostructure distributed feedback lasers involving chirped gratings. The variation of the effective refractive index along the tapered active stripe is compensated using an optimized continuously chirped grating. The grating has been formed using a novel direct-write electron-beam lithography technique. Lasers with an antireflection/cleaved cavity show stable single-mode operation and high optical output power up to 60 mW. The yield of lasers with a sidemode suppression ration > 40 dB is more than 70%. The -3-dB farfield angles (full-width at half-maximum) amount to 14/spl deg/ and 20/spl deg/ in lateral and vertical direction, respectively.
Nitride-based p-i-n bandpass photodetectors with semitransparent Ni-Au electrodes were successfully fabricated and characterized. The photodetectors exhibit a 20-V breakdown voltage and a small dark current of 40 pA at 4-V reverse bias. It was found that spectral responsivity shows a narrow bandpass characteristics from 337 to 365 nm. Moreover, the peak responsivity was estimated to be 0.13 A/W at 354 nm, corresponding to a quantum efficiency of 44%. The relatively high response at shorter wavelength is due to the unoptimized thickness of p-Al<sub>0.1</sub>Ga<sub>0.9</sub>N absorption layer. At low frequency, the noise of the photodetector is dominant by the 1/f-type noise. For our 330×330 μm<sup>2</sup> device, given a bias of -3.18 V, the corresponding noise equivalent power and normalized detectivity D<sup>*</sup> are calculated to be 5.6×10<sup>-12</sup> W and 3.34×10<sup>11</sup> cmHz<sup>0.5</sup> W<sup>-1</sup>, respectively.
Both the p-gallium nitride (GaN) on an n-light-emitting diode (LED) structure and the n-GaN on p-LED structure were fabricated by metal-organic vapor phase epitaxy on c axis sapphire substrate. The output spectra with different applied bias voltages were studied on these structures. The small amount of the wavelength shift under a forward bias condition in the proposed p-down LED structure was observed but not in the n-down LED structure. This phenomenon was characterized by the effective electric field in the well created from the built-in electric field and the applied electric field. This proposed p-down LED shows an opportunity in the application of wavelength shift-free optoelectronic devices.
Nitride-based blue light emitting diodes (LEDs) with an n<sup>+</sup>-short period superlattice (SPS) tunnel contact layer and an indium tin oxide (ITO) transparent contact were fabricated. Compared with conventional nitride-based LEDs with Ni/Au upper contacts, it was found that we could achieve a 60% increase in electroluminescence (EL) intensity by using ITO upper contacts. However, it was also found that the lifetime of ITO LEDs were much shorter. Furthermore, it was found that we could achieve a longer lifetime and a smaller reverse leakage current (I<sub>R</sub>) by the deposition of a SiO<sub>2</sub> layer on top of the ITO LEDs.
High output power InGaN ultraviolet light emitting diodes fabricated on patterned substrates using metalorganic chemical vapor deposition
- K Tadatomo
- H Okagawa
- T Tsunekawa
- T Jyouichi
- Y Imada
- M Kato
- H Kudo
- T Taguchi