Double Mesa Research Articles

Determination of the background doping polarity for unintentionally doped AlGaAsSb and AlInAsSb avalanche photodiodes on InP substrates

Background doping polarity is a critical design parameter for the performance of many optoelectronic devices, including avalanche photodiodes. We have applied a technique by using capacitance–voltage (CV) measurements on double mesa structures with a p-i-n or n-i-p homojunction to determine the background polarity type of the unintentionally doped intrinsic region. Because CV measurements scale with the size of the mesa, they support design flexibility in producing variable-sized top and bottom mesa diameters. In this work, we grew, fabricated, and tested AlGaAsSb and AlInAsSb random alloy double mesa p-i-n structures and undertook CV measurements at 295, 150, and 77 K. It was found that the capacitance varied with the top mesa diameter for both material systems, and not the bottom mesa diameter, indicating that the unintentionally doped intrinsic region is n-type in nature.

Contact Geometrical Study for Top Emitting 980 nm InGaAs/GaAsP Vertical-Cavity Surface Emitting Lasers

Geometrical contacts of a double mesa structure with 16 rows ×15 columns arrays of top emitting GaAs based 980 nm vertical cavity surface emitting lasers (VCSELs) are fabricated and characterized. In this paper, 5 strained In0.22Ga0.78As/Ga0.9AsP0.1 quantum wells (QWs) within λ/2 thick cavity have been employed. The top and the bottom epitaxially grown mirrors are based on the linear graded Al0.9Ga0.1As/GaAs distributed Bragg reflectors (DBRs) with 20.5 and 37 periods, respectively. Static parameters including threshold currents, rollover currents, maximum optical output power and wall-plug efficiency are extracted from light out power-current-voltage (LIV) of VCSELs with fixed oxide aperture diameter of ∅~ 6 μm and various mesa2 diameters. In addition, spectral emission for 980 nm VCSELs of oxide aperture between ∅~ 6 and 19 μm and with fixed ∅~ 6 μm and different bias currents are analyzed. The highest optical output power of around 33 dBm is observed at bias current of 0.8 mA for short−reach optical interconnect applications.

Comparison and analysis of Al0.7InAsSb avalanche photodiodes with different background doping polarities

Background doping polarity is a key parameter in the design of numerous electrical and optoelectronic devices. It is especially critical for avalanche photodiodes (APDs). Recently, high-performance APDs have been demonstrated based on the AlInAsSb digital alloy materials system. A critical element of this work was the determination of the background doping polarity of the molecular beam epitaxial grown wafers. In this work, we determine the unintentional background doping polarity of Al0.7InAsSb using the double mesa capacitance-voltage technique. We fabricated two p-i-n Al0.7InAsSb structures: one with p-type background polarity and the other with n-type. The measurements indicate that devices with different background doping polarities show different capacitance relations to the mesa diameters; moreover, the relationship reverses at high voltage in a p-type background device. Subsequent simulations reveal that this reversal is caused by electrical field confinement after the depletion reaches the smaller top mesa. These findings are consistent with reports of reduced surface leakage current in double and triple mesa structures.

Determination of background doping polarity of unintentionally doped semiconductor layers

We present a method of determining the background doping type in semiconductors using capacitance–voltage measurements on overetched double mesa p–i–n or n–i–p structures. Unlike Hall measurements, this method is not limited by the conductivity of the substrate. By measuring the capacitance of devices with varying top and bottom mesa sizes, we were able to conclusively determine which mesa contained the p–n junction, revealing the polarity of the intrinsic layer. This method, when demonstrated on GaSb p–i–n and n–i–p structures, concluded that the material is residually doped p-type, which is well established by other sources. The method was then applied to a 10 monolayer InAs/10 monolayer AlSb superlattice, for which the doping polarity was unknown, and indicated that this material is also p-type.

Light confinement in a 90° double high mesa slot bend waveguide

In this work, we propose a technique to enhance the confinement of light at 90o sharp bend of a double high mesa slot (DHMS) waveguide based on Silicon on Insulator (SOI). These waveguides deliver high electric field and optical power density in low refractive index Nano-metric slot. The slot is displaced to the inner and outer periphery of the bend and explores the deviation in the relative power. The maximum relative power is attained by shifting the slot towards the outer periphery of the bend. This is only conceivable by choosing the precise slot position where the two evanescent tails of the high index waveguide modes have maximum overlap.

Franz-Keldysh Effect in GeSn Detectors

The fundament for silicon based photonics is Ge grown on Si and SOI. A simple photonic integrated circuit (PIC) consist of an integrated light source, a waveguide, a modulator and a detector. Due to the fact that Ge is an indirect semiconductors, the light source is the main challenge for PICs. One concept is to add Sn into the Ge matrix. Theoretical calculations predict that Ge1-xSnx becomes a direct semiconductor for a Sn content of x = 10 % and therefore is an interesting material mixture for photonic devices integrated on Si. Recent publications have shown that detectors using GeSn as active material excess 40 GHz bandwidth. For PICs, also the modulation of the light is needed. Beside Mach-Zehnder modulators, the modulator can be realized using the Franz-Keldysh (FK) effect. Due to the FK effect, the absorption in the material can be changed by varying the applied electric field.In this paper we report the FK effect in GeSn PIN diodes with Sn concentrations up to 4.2 %. In order to analyze the absorption of GeSn, vertical PIN diodes with different Sn content were produced. First, the epitaxial layers were grown with a solid source molecular beam epitaxy system [1] on B-doped Si (100) substrates with a high specific resistance ρ > 1000 Ωcm. Fig. 1(a) shows the vertical layer sequence. The layer stack consists of a vertical PIN structure. Starting with a 400 nm B-doped Ge contact layer (1020 cm-3), the active layer with 300 nm intrinsic Ge1-xSnx and a 10 nm Ge spacer follow. The top contact consist of 100 nm Sb-doped Ge (1020 cm-3) and 100 nm Sb-doped Si (1020 cm-3). The three investigated samples have a Sn content of x = 0 %, 2 % and 4.2 %, respectively. The GeSn PIN heterojunction photodiodes are produced in a quasi-planar technology. The device processing by double mesa etching, passivation and metallization is similar to earlier reported GeSn pin photodiodes [2]. A schematic high frequency device structure of the GeSn pin photodetector with the typical ground signal ground (GSG) probe configuration with a pitch of 100 µm is shown in Fig 1(b).The electrical and optical characterization of the diodes is done with on-wafer measurement of the current-voltage characteristics. As a light source, a super-continuum laser with a tunable acousto-optical wavelength filter is used. The light couples into the diode through a lensed glass fiber. From measuring the dark current and photo current, the responsivity of the GeSn detectors were determined by dividing the photo current by the optical power.Fig. 2 shows the responsivity of the GeSn PIN diodes with Sn concentrations of x = 0 %, 2 % and 4.2 %. With increasing Sn content, the bandgap shifts to higher wavelengths and therefore, the responsivity increases for higher wavelengths. A voltage dependence of the optical responsivity is only shown near the direct bandgap Eg,Γ. This is due to the FK effect.From the measurement of the responsivity and reflectivity, the internal quantum efficiency can be calculated and further the absorption is determined. The electro-optical modulation from the FK effect was clearly observed in GeSn material. The maximum absorption ratio Δα/α0 for the 2 % Sn sample at room temperature was 1.5 at a wavelength of 1806 nm and a voltage swing of 3 V.

GeSn-on-Si normal incidence photodetectors with bandwidths more than 40 GHz

GeSn (Sn content up to 4.2%) photodiodes with vertical pin structures were grown on thin Ge virtual substrates on Si by a low temperature (160 °C) molecular beam epitaxy. Vertical detectors were fabricated by a double mesa process with mesa radii between 5 µm and 80 µm. The nominal intrinsic absorber contains carrier densities from below 1 · 10(16) cm(-3) to 1 · 10(17) cm(-3) for Ge reference detectors and GeSn detectors with 4.2% Sn, respectively. The photodetectors were investigated with electrical and optoelectrical methods from direct current up to high frequencies (40 GHz). For a laser wavelength of 1550 nm an increasing of the optical responsivities (84 mA/W -218 mA/W) for vertical incidence detectors with thin (300 nm) absorbers as function of the Sn content were found. Most important from an application perspective all detectors had bandwidth above 40 GHz at enough reverse voltage which increased from zero to -5 V within the given Sn range. Increasing carrier densities (up to 1 · 10(17) cm(-3)) with Sn contents caused the depletion of the nominal intrinsic absorber at increasing reverse voltages.

Molecular beam epitaxy grown GeSn p-i-n photodetectors integrated on Si

GeSn p-i-n photodetectors with a low Sn mole fraction made by molecular beam epitaxy on Si substrates show higher optical responsivities for wavelength λ>1400nm compared with p-i-n photodetectors made from pure Ge. The Sn incorporation in Ge is done by a low temperature growth step in order to minimize Sn segregation. The Sn incorporation and the alloy content are investigated by μ-Raman spectroscopy and calibrated Secondary Ion Mass Spectrometry. The photodetectors are manufactured with sharp doping transitions and are realized as double mesa structures with diameters from 1.5μm up to 80μm. The optical measurements are carried out with a broadband super continuum laser from λ=1200nm up to λ=1700nm. At a wavelength of λ=1550nm the optical responsivity of these vertical GeSn diodes is 0.1 A/W. In comparison with a pure Ge detector of the same geometrical dimensions the optical responsivity is increased by factor of three as a result of Sn caused band gap reduction.

Germanium-tin p-i-n photodetectors integrated on silicon grown by molecular beam epitaxy

GeSn heterojunction p-i-n diodes with a Sn content of 0.5% are grown with a special low temperature molecular beam epitaxy. The Sn incorporation in Ge is facilitated by a very low temperature growth step in order to suppress Sn surface segregation. Diodes with sharp doping transitions are realized as double mesa structures with a diameter from 1.5 up to 80 μm. An optical responsivity of these GeSn diodes of 0.1 A/W at a wavelength of λ=1.55 μm is measured. In comparison with a pure Ge detector the optical responsivity is increased by factor of 3 as a result of Sn caused band gap reduction.

Double Mesa Sidewall Silicon Carbide Avalanche Photodiode

We report a double mesa 4H-SiC avalanche photodiode (APD) that suppresses premature edge break-down. The motivation for the double mesa structure is to eliminate the optically dead region that extends radially up to 30 mum beyond the active region of conventional beveled mesa SiC APDs, which can significantly restrict the fill-factor of arrays.

Effects of self-heating on the microwave performance of SiGe HBTs

In this study, the effects of self-heating on the f T and f max frequencies have been investigated in NpN SiGe heterojunction bipolar transistors (HBTs) by two-dimensional numerical device modeling using a commercial simulator. The device studied was a single emitter stripe, double mesa configuration with self-aligned base contacts where a Gaussian distribution was assumed for the base’s boron profile. Increases in the base transit time and the collector delay time due to degradation in the electron mobility and saturation velocity and an increase in the base-collector depletion layer width were found to degrade the f T while increased base resistance also contributed to the reduction in f max. The onset of significant device self-heating and degradation in f T and f max were observed for collector current densities of 5 × 10 4 A/cm 2 for V CE = 2 V.

Effect of leakage current induced by B+H+implantation in the isolation process for self passivated GaAlAs/GaInP/GaAs HBT

The excess leakage current due to the ion implantation isolation process used in the fabrication of double mesa Self Passivated GaAlAs / GaInP / GaAs Heterojunction Bipolar Transistors (SP-HBT) has been investigated. This ion implantation process, used to limit the active emitter length results in a drastic reduction of the current gain. The ideality factor of the recombination current associated with the ion implantation has been found to be 1.8, close to the conventional value of 2.

A 30-Gbit/s demultiplexer IC based on Si/SiGe emitter base heterojunction bipolar transistors

A 30-Gbit/s demultiplexer IC has been fabricated and tested using an improved double mesa Si/SiGe heterojunction bipolar transistor process. This is-to our knowledge-the highest ever reported bit rate for real (as opposed to drift transistor) Si/SiGe HBT circuits. The result was mainly reached by scaling down the transistor sizes to reduce parasitics. The minimum emitter mesa width was 1 /spl mu/m.

Two-dimensional LED arrays for virtual display image sources

The design and fabrication of large-scale two-dimensional (2-D) arrays of visible light emitting diodes (LED's) is described along with their implementation as image sources for prototype virtual displays. The LED pixels were fabricated in the InGaAlP material system using a double mesa etch process. Pixel characteristics are presented and used to predict display luminance properties. The 10-/spl mu/m square LED's produced external quantum efficiencies of 0.5% with an emission spectrum peaked near 650 nm (red). This performance level allowed the target display luminance of 10 fL to be attained with just 2.9 mW of array power consumption. The LED arrays consisted of 240 columns/spl times/144 rows (/spl sim/VGA/8) with pixels on a 20-/spl mu/m pitch and were driven in a column major matrix addressing mode at 60 frames per second for image display. Pixels were driven at a constant current with pulse width modulation to achieve sixteen levels of gray. An analysis of array luminance uniformity is presented. Sample images of text, graphics, and gray scale images demonstrate the capabilities of the LED arrays as monochrome image sources for virtual displays.

Investigation on base surface recombination in Self Passivated GaAlAs/GaInP/GaAs Heterojunction Bipolar Transistor

The dependence of the current gain with the size of the emitter-base junction of double mesa Self Passivated Heterojunction Bipolar Transistors (SP-HBT) has been investigated, the extrinsic base layer being passivated with a n-type GaInP layer. The current gain is widely improved, due to a 18-fold reduction of the surface recombination in the extrinsic base region with respect to unpassivated HBT. The surface recombination current ideality factor has been found to be 1.13.

Base contact resistance limits to lateral scaling of fully self-aligned double mesa SiGe-HBTs

A significant reduction of parasitic resistances and capacitances of the double mesa SiGe Heterojunction Bipolar Transistors (SiGe-HBT) was achieved by using self-aligned processing steps, such as planarization of transistor contacts, outside-spacer-technology for micromasking, contact implantation and low ohmic silicidation. This article presents and analyses the lateral optimization of the double mesa SiGe-HBT by on-wafer measurements and demonstrates lateral scaling limitation resulting from the increasing base metal-semiconductor contact resistance. The increase of the more important base contact resistance is proven by two-dimensional contact simulation. This is shown experimentally by using a fully self-aligned double mesa SiGe-HBT transistor which combines the advantages of superior high frequency characteristics with a simple and low cost fabrication procedure being used mainly in compound semiconductor technology.

Novel oxide planarization for integrated high-speed Si/SiGe heterojunction bipolar transistors

A straightforward oxide planarization for double mesa Si/SiGe heterojunction bipolar transistors (HBTs) is presented. The starting point is a bias-sputtered SiO/sub 2/ film covering a mesa with an auxiliary layer on top. The following planarization is performed only by wet chemical etching. A planarized multiplexer circuit resulted in bit rates up to 18 Gbit/s.

Fully self-aligned double mesa SiGe-HBT with external transistor optimisation

A significant reduction in parasitic resistances and capacitances of the double mesa SiGe-HBT was achieved using several self-aligning processes, such as planarisation for transistor contacts, outside-spacer-technology for micromasking, contact implantations and low ohmic silicides. The authors present and analyse the lateral optimisation by on-wafer measurements and simulations. It is shown that the fully self-aligned transistor combines the advantages of superior high frequency characteristics with a simple and low cost realisation procedure.

24 Gbit/s demultiplexer IC using oxide planarised Si/SiGe heterojunction bipolar transistors

A 24 Gbit/s demultiplexer IC is presented using oxide planarised double mesa Si/SiGe heterojunction bipolar transistors (HBTs). Taking into account an emitter width of 1.5 µm and a high base–collector capacitance due to the large base–collector mesa width of 4.5 µm, this is a very satisfactory result and to the knowledge of the authors is the highest bit rate reported for Si/SiGe circuits.

Formula omitted] heterojunction bipolar transistors for microwave power applications

Si Si 1−x Ge x heterojunction bipolar transistors are considered for microwave power applications. The devices are characterized for collector-base breakdown, small-signal microwave performance, and high-temperature characteristics. We find near-ideal collector-base breakdown voltages for double mesa transistors grown by MBE. Measured f T and f max were 10 and 25 GHz, respectively, for transistors with BV CBO of 40 V. Over a temperature range of 300–500 K the current gain shows a decrease of 34%.