Mesa Diameter Research Articles

Spin‐polarized surface‐emitting lasers

We investigate the growth, fabrication, and characterization of an electrically-injected spin-polarized laser utilizing an Fe/n+-Al0.1Ga0.9As Schottky tunnel barrier for efficient spin injection into an InGaAs quantum well vertical-cavity surface-emitting laser. A threshold current reduction of 11% and maximum degree of circular polarization of 23% are observed for a 15 µm mesa diameter laser at 50 K. The lack of a threshold current reduction and circularly-polarized electroluminescence in similar nonmagnetic and Fe control lasers provides further evidence of spin injection. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

247 nm solar-blind ultraviolet p-i-n photodetector

We describe solar-blind p-i-n photodetectors based on short period superlattices of AlN∕Al0.08Ga0.92N. The devices are grown using gas source molecular beam epitaxy with ammonia on transparent sapphire substrates. The cutoff wavelength for the device is 247nm. For diodes with 150μm diameter mesas and sidewall surfaces passivated in oxygen plasma, we obtain extremely low dark leakage current of ∼3pA∕cm2 and high zero-bias resistance of ∼6×1014Ω. At 10V reverse bias the observed responsivity is 62mA∕W.

CM-Wave Modulator with High-Voltage 4H SiC pin Diodes

The results of mathematical simulation, development and investigation of a modulator with 4H SiC pin diodes are presented. We simulated the effect of bias modes on isolation and transmission between the modulator input and output in the 1−20 GHz frequency range, for pin diodes with 6 μm long i-region. It was calculated that the isolation in a modulator with three diodes may run into –45 dB, the transmission losses being no more than 2 dB. The modulator was made as an integrated circuit (IC) on the basis of nonsymmetrical strip lines (characteristic impedance of 50 Ω) incorporating chips of high-voltage 4H SiC pin diodes with iregion 6 μm long, mesa diameter of 60 μm and calculated avalanche breakdown voltage of 1000 V. We studied the experimental parameters of this modulator as a function of forward current and reverse voltage in the 2.4−12 GHz frequency range, as well as the microwave signal switching behavior. It was determined that the modulator is characterized by transmission losses of 1.0−2.0 dB and isolation of 27−34 dB (in the 2.4−7 GHz frequency range). The formation of microwave pulses with leading (trailing) edge of 22 (29) ns was also observed.

Scaling of dark count rate with active area in 1.06μm photon-counting InGaAsP∕InP avalanche photodiodes

Reducing the active area of InGaAsP∕InP avalanche photodiodes operated in Geiger mode is investigated for reducing the dark count rate. The dark count rate in Geiger mode is found to scale linearly with the detector’s active area for mesa diameters of 10, 15, and 20μm. Scaling the mesa size from 20to10μm results in a reduction of the room-temperature dark count rate from 290to44kHz at ∼4.5V of overbias, while the photon detection efficiency is ∼45% for all device diameters. The trade-offs associated with shrinking the active area, including reduced optical coupling and higher on-state resistance, are discussed briefly.

GaN ultraviolet avalanche photodiodes with optical gain greater than 1000 grown on GaN substrates by metal-organic chemical vapor deposition

We report the performance of GaN p-i-n ultraviolet avalanche photodiodes grown on bulk GaN substrates by metal-organic chemical vapor deposition. The low dislocation density in the devices enables low reverse-bias dark currents prior to avalanche breakdown for ∼30μm diameter mesa photodetectors. The photoresponse is relatively independent of the bias voltage prior to the onset of avalanche gain which occurs at an electric field of ∼2.8MV∕cm. The magnitude of the reverse-bias breakdown voltage shows a positive temperature coefficient of ∼0.05V∕K, confirming that the avalanche breakdown mechanism dominates. With ultraviolet illumination at λ∼360nm, devices with mesa diameters of ∼50μm achieve stable maximum optical gains greater than 1000. To the best of our knowledge, this is the highest optical gain achieved for GaN-based avalanche photodiodes and the largest area III-N avalance photodetectors yet reported.

Microlens fabrication by selective oxidation of composition-graded digital alloy AlGaAs

We have fabricated refractive microlenses using selective oxidation of composition-graded digital alloy AlGaAs. The chirped short period superlattice of GaAs-AlAs was used to grade the composition of AlGaAs along the growth direction for a reproducible and controllable composition and oxidation profile. The oxidized profile of linearly composition-graded digital alloy AlGaAs shows a circular convex lens shape. The focused spot pattern of the beam at the focal point of the fabricated circular buried microlens reveals the strong focusing function of the microlens. Circular buried microlenses with mesa diameter of 20 μm exhibited focal length of 28-33 μm and a focused spot diameter of 4.0-6.4μm, which are varied by oxidation time.

High performance GaN pin rectifiers grown on free-standing GaN substrates

Gallium nitride pin rectifiers grown on bulk GaN substrate have been fabricated and characterised. The device structures were grown by metal organic chemical vapour deposition (MOCVD) on free-standing GaN substrates. The diode structure consisted of an n+-GaN layer, followed by a 2.5 µm unintentionally doped i-GaN layer, and a p-type GaN layer capped with a p++-GaN cap layer. The mesa-structure pin diodes exhibited a blocking voltage as large as Vr∼−540 V, and a reverse current density of Ir∼0.1 A/cm2 at Vr∼−500 V. The forward voltage drop is only 4.4 V at a forward current density of ∼100A/cm2, with an on-resistance of 3 mΩ · cm2 for a circular device with an 80 µm mesa diameter. The breakdown voltage and on-resistance are believed to be the best values reported for vertical mesa GaN pin rectifiers grown on bulk GaN substrates with comparable i-layer thickness.

Cadmium sulfide passivation of InGaAs/InP mesa p-i-n photodiodes

A cadmium sulfide (CdS) passivation process was demonstrated for the first time on InGaAs/InP p-i-n mesa photodetectors. The passivated devices produced lower reverse bias leakage currents in comparison to devices that received only a thermally deposited SiO2 film. The subsequent deposition of SiO2 on the passivated devices produced virtually no change to the aforementioned leakage currents even after undergoing a 3-h, 300°C thermal treatment. In contrast, similar SiO2 capped devices, fabricated without the CdS passivating layer, show a large increase in leakage current when subjected to the same thermal cycle. Leakage current versus mesa diameter measurements suggest these results are due to reduce surface recombination at the exposed mesa sidewall. X-ray photoelectron spectroscopy (XPS) results indicate the S:Cd ratio of these films to be 0.77.

Fabrication and characterization of 650nm resonant-cavity light-emitting diodes

In this article, we report the fabrication and characterization of 650nm resonant-cavity light-emitting diodes (RCLEDs) with the GaInP∕AlGaInP multi-quantum-well active layer sandwiched between two AlGaAs∕AlAs distributed Bragg reflectors. We compare the performance of RCLEDs with different mesa diameters in the characteristics of forward voltage, light output power, external quantum efficiency, emission spectrum, and dynamic response. All the RCLEDs have a low forward voltage of 1.9–2.0V at 20mA. The RCLED with a window diameter of 120μm exhibits the maximum light output power of 0.79mW at 39mA, the best external quantum efficiency of 2.8% at 1.2mA, a peak wavelength of 647nm, and a full width at half-maximum of 14.5nm at 50mA. The RCLED with a 30μm diameter shows the maximum 3dB frequency bandwidth of 240MHz at a driving current of 40mA.

Very high-speed ultraviolet photodetectors fabricated on GaN

We report on the temporal and the frequency response of both metal-semiconductor-metal (MSM) and p-i-n ultraviolet photodetectors fabricated on single-crystal GaN. The best MSM devices show a fast 10–90% rise-time of ∼28 psec under comparatively low ultraviolet excitation of ∼0.1 W/cm2 averagerirradiance. The fast-Fourier transform (FFT) of the pulse response data indicates a bandwidth, f3dB, of ∼3.8 GHz at a reverse bias of 25 V. This agrees well with the direct frequency response measurement value of ∼3.5 GHz. For the p-i-n devices, we measured a rise-time of ∼43 psec at 15 V reverse bias for a 60 µm diameter mesa with 1 µm thick intrinsic region. The FFT of the p-i-n pulse response obtains f3dB ≈1.4 GHz. Analysis in terms of reverse bias and geometric scaling indicates that the MSM photodetectors are transit-time limited. The p-i-n devices also show evidence of transit-time limited effects based on trends with respect to reverse bias and intrinsic region thickness. However, our larger area p-i-n devices show clear evidence of RC-limited behavior. Modeling of the temporal behavior indicates that a slow component in the time and frequency response data is a consequence of the hole drift velocity. We have also found preliminary evidence of microplasmic effects in the p-i-n devices.

In0.53Ga0.47As/In0.52Al0.48As separate absorption, charge, and multiplication layer long wavelength avalanche photodiode

The authors demonstrate the use of In/sub 0.52/Al/sub 0.48/As as the gain material in a separate absorption, charge, and multiplication layer avalanche photodiode (APD) with sensitivity to 0.9-1.7 /spl mu/m wavelength light. A hole to electron ionisation rate ratio of k=0.23 is observed, representing a significant improvement over the 1/k=0.4 characteristic of InP/In/sub 0.53/Ga/sub 0.47/As avalanche photodiodes. Primary dark currents of /spl sim/10 nA and gains approaching 100 for 100 /spl mu/m diameter mesa devices are observed. A 6 dB sensitivity advantage is measured for an APD receiver over an In/sub 0.53/Ga/sub 0.47/As p-i-n detector receiver at a wavelength of 1.55 /spl mu/m, a bit rate of 1.5 Gbit/s, and 10/sup -9/ bit error rate.

Transport through a buried double-barrier single-electron transistor at low temperatures

We report on transport and magneto-transport properties of an AlGaAs/GaAs double-barrier resonant tunneling transistor with sub-μm lateral dimensions, where lateral confinement causes quasi-zero-dimensional quantization. An MBE-grown structure was designed as a 500 nm diameter mesa on top of the two AlGaAs barriers lying 50 nm below the etched surface. A gate surrounding the mesa causes lateral confinement within the barrier region by means of its accompanying Schottky-type depletion space-charge region. It is shown that the current–voltage characteristics for such structures can be explained by using only the lateral confinement defined by the gate without assuming any defects in the mesa. Quantization effects in the emitter are found close to the first ground-state resonance in strong confinement condition. In addition, on low-doped samples no shift in the onset voltage of the first single electron step could be observed for higher magnetic fields. This effect is explained by the magnetic field dependence of the Fermi-level.

UV, blue and green light emitting diodes based on GaN-InGaN multiple quantum wells over sapphire and (111) spinel substrates

Recently Nakamura et al. have reported on high brightness visible LEDs based on AlGaN-InGaN multiple quantum wells (MQWs) using atmospheric pressure metal-organic chemical vapor deposition (MOCVD) and AlGaN barrier layers around an In x Ga 1 − x N-In y Ga 1 − y N multiple quantum well region. We now report the fabrication of high brightness vertical cavity UV, blue and green light emitting diodes using low pressure MOCVD with GaN-In x Ga 1 − x N multiple quantum wells surrounded by GaN barrier layers. Our device structures over sapphire and cubic (111) spinel substrates consisted of a 10 period GaN-InGaN MQW (25 Å well-50 Å barrier) surrounded by n- and p-GaN layers. Structures with both Mg-doped and undoped quantum wells (active regions) were deposited. Mesa type LED structures were then fabricated using Ti-Al and Ni-Au for the n-and p-ohmic contacts. Light emission was observed in a vertical cavity geometry from the sapphire or the spinel substrate side. For 250 mm diameter mesa devices the series resistances ranged from 10 to 25 Ω. These are some of the lowest reported values. Spectral emission linewidths (FWHM) of 12, 25 and 40 nm were obtained respectively for the UV, blue, and green MQW LEDs. These linewidths are similar to those of Nakamura et al. We also report on optically pumped MQW InGaN-GaN lasers with different quantum well thicknesses. In these devices, we observed the quantum shift related to the subband energy dependence on the well thickness and estimated the effective conduction band discontinuity at the GaN-InGaN heterointerface from these data.

Use of polyimide bonding for hybrid integration of a vertical cavity surface emitting laser on a silicon substrate

The use of polyimide bonding for hybrid integration of a vertical cavity surface emitting laser on an Si substrate was demonstrated. The threshold current was 3.1 mA and the maximum output power was 2.45 mW for a 15 µm diameter mesa. This technology is suitable for integrating photonic devices with an Si-LSI circuit.

Visible vertical cavity surface emitting lasers at λ < 650 nm

The room temperature operation of visible vertical cavity surface emitting lasers at 642 nm is reported. A pulsed threshold current of 45 mA for a 26 μm diameter mesa device (corresponding to 8.5 kA cm −2) was achieved at an operating voltage of 3.5 V. Photoluminescence at 10 K showed that an In 0.48Al 0.3Ga 0.22P direct barrier provides better electron confinement than an In 0.48Al 0.52P indirect barrier with unstrained In 0.48Ga 0.52P quantum wells.

Fabrication of 1.5 μm optically pumped [formula omitted] vertical-cavity surface-emitting lasers

We report the fabrication of Ga 1−x In x As y P 1−y/ InP vertical-cavity surface-emitting lasers for long-wavelength operation and optical pumping. Bottom mirrors and active regions grown by metal-organic vapour-phase epitaxy showed high reflectivities and excellent wavelength tuning. Si/SiO 2 top mirrors were deposited by electron beam evaporation and shaped by lift-off. Laser mesas with various diameters and aspects ratios up to 14 were etched by reactive ion etching and a methane-based plasma. Lasing operation was obtained for mesa diameters down to 5 μm. The average threshold power for the mesa of 5 μm diameter was 18 mW. Devices with diameters of 30 μm or more operated up to 70 °C.

Room temperature continuous wave operation of red vertical cavity surface emitting laser diodes

The first room temperature (23°C) continuous wave visible vertical cavity surface emitting laser diodes are reported. Annular contact devices with a 5 μm optical aperture in a 15 μm diameter mesa emit at 670.4 nm with a threshold current of 5 mA at 2.4V. Devices with 20–60 μm diameter mesas operate continuous wave below −13°C.

Low dark current In 0.53Ga 0.47As/InP SAM avalanche photodiodes grown by gas-source MBE

In 0.53Ga 0.47As/InP avalanche photodiodes (APDs) with separate absorption and multiplication (SAM) regions have been grown by gas-source molecular beam epitaxy (GSMBE). These mesa-structure diodes, with a mesa diameter of 150 μm, exhibit a very low dark current of 17 nA, and primary unmultiplied dark current of less than 1.0 nA at 90% of the breakdown voltage. A gain as high as 100 is observed near the breakdown. Good uniformities of punch-through voltage and breakdown voltage were obtained on a 16 mm×16 mm wafer. Also, a premature breakdown was observed in the In 0.53Ga 0.47As/InP SAM-APDs with a V-shaped defect density of 25,000 cm -2.

Quantum-confined Stark effect in InGaAs/InP multiple quantum wells grown by solid source molecular beam epitaxy

We report the first observation of the quantum-confined Stark effect (QCSE) in In 0.53Ga 0.47As/InP multiple quantum wells (MQWs) grown by solid source molecular beam epitaxy (SSMBE). MQW PIN diodes have been fabricated with reverse bias leakage currents of ≅ 10 nA at -35 V, for 400 μm diameter mesas. Room temperature photocurrent measurements on diodes with nominal 55 Å wells show well resolved excitonic absorption at around 1.55 μm and exhibit shifts of up to 39 meV at -35 V.

Microwave noise of DBRT diode over full bias voltage range

Experimental results on the noise behaviour of a double barrier resonant tunnelling (DBRT) diode are presented. The measurement method has been developed from a standard noise figure meter calibration procedure. Measurements were performed on a stabilised DBRT diode, mounted in a reflection amplifier configuration. The actual circulator in the setup is represented by an ideal circulator and two error networks. Noise figure and noise measure of a DBRT diode at 1 and 1.5 GHz are reported. The bias voltage range includes two positive-conductance regions as well as the intermediate negative-conductance region. In the active region of a 20 μm mesa diameter DBRT diode, the lowest noise measure we observed was 5.5.