InGaAs for infrared photodetectors. Physics and technology

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InGaAs for infrared photodetectors. Physics and technologylations were performed for different wavelengths and differentvalues of the finesse F (F = FSR/Dl 1/2 , where Dl 1/2is half width of a resonant peak). It indicates that h is themost sensitive to the angle of incidence in the high F cavityat the shorter l and quantum efficiency degrades by 20%above 10°. The simulations show that RCE detectors cannotwork well, even with quite slow optics.The performance of RCE-PD mainly depends on the realizationof a low loss cavity. The mirrors and cavity materialmust be non-absorbing at the detection wavelength andthe mirrors must have high reflectivity. Generally, mirrorscan be fabricated as metallic, dielectric, DBR (distributedBragg reflectors made of semiconducting or dielectric layers)or hybride (DBR + metal).The active layer material must have a smaller bandgapmaterial than the mirror and cavity materials but it shouldnot be so small that large heterojunction band offsets hinderthe extraction of photogenerated carriers. The activelayer absorption coefficient should be moderate, i.e.,10 3 –10 4 cm –1 for the operation wavelength.Different material combinations satisfy all of the aboverequirements. The InGaAs material system for RCE detectionis used in two combinations of semiconductors,AlGaAs/GaAs/InGaAs and InP/InGaAs/InAlAs.Application of InGaAs as the active material allows forextension device operation spectrum to the wavelength longerthan 900 nm. This ternary compound containing smallamount of In can be grown on GaAs substrate. In this caseAlAs and GaAs, having good refractive index contrast, canbe used for construction of the mirrors of the resonant cavitywith nearly unit reflectivity with about twenty periodquarter wave superlattice.In x Ga 1–x As containing higher amount of indium (x »0.5) is usually grown on InP substrate. In 0.53 Ga 0.47 As andIn 0.52 Al 0.48 As lattice matched to InP have rather poor refractiveindex contrast and increased number of periods(» 35) are required to achieve nearly unit reflectivity of themirror. For the detectors operating at 1.3–1.55 µm wavelengthrange, quaternary compounds like InGaAlAs andInGaAsP lattice matched to InP are more suitable for mirrorformation.3.2. Practical implementation of RCE-PDsTo the date, InGaAs ternary compound has been the mostcommonly studied ternary compound for RCE detectionbecause of the easiness with which GaAs and InP basedheterostructures can be grown by molecular beam epitaxy(MBE) and metalorganic chemical vapour deposition(MOCVD) [10]. The capabilities have been demonstratedin many practical photodetectors.3.2.1. RCE-Schottky photodetectorsThe AlGaAs/GaAs/InGaAs based RCE-Schottkyphotodetector, presented in Ref. 11, operated at 895 nm.Fig. 14. Schematic cross-section of the RCE Schottky photodiode(after Ref. 11).A schematic representation of this structure is shown inFig. 14. The device structure was grown on a GaAs substrateby MBE. The cavity is formed by 15 pairsAlAs/GaAs DBR bottom reflector and a semi transparentAu contact on the top. The InGaAs absorption layer has anIn mole fraction less than 10% and both heterojunctions arelinearly graded for 25 nm to avoid carrier trapping. The positionof the absorption layer in the depletion region is optimisedto yield minimum transit time for electrons and holes[11]. The device was fabricated with mesa isolation. To reducethe parasitic capacitance of the photodetector, theSchottky metal was connected to the contact pads with anAu air-bridge. A 200 nm Si 3 N 4 coating on the top wasused.The optimised RCE-Schottky photodiode has a 3 dBbandwidth of »100 GHz and bandwidth-efficiency product> 70 GHz.Using the indium tin oxide (ITO) for the Schottky diodesolves the problems of optical losses and scattering causedby the Schottky metal. For the RCE-ITO-Schottky diodefabricated by Biyikli et al. with an area of 5×5 µm, thebandwidth was 60 GHz while the quantum efficiency was75% for a reverse voltage of 4 V [12]. This results in abandwidth-efficiency product of 45 GHz.3.2.2. RCE avalanche PDThe internal gain of avalanche photodiodes (APDs) providessubstantial improvement in signal-to-noise characteristicscompared to other types of PDs. The APD structurethat has been widely deployed is the separate absorptionand multiplication region (SAM) APD. It consists of awide-bandgap multiplication region and a narrow-bandgapabsorbing layer separated by a transition region that reducesthe accumulation of a charge at the interface betweenthe multiplication and absorption regions which improvesthe low-gain bandwidth.In order to reduce the transit time and thus improve thefrequency response at low gains, the APD layers are incorporatedinto a resonant-cavity structure [13,14]. The device144 Opto-Electron. Rev., 12, no. 1, 2004 © 2004 COSiW SEP, Warsaw
Contributed paperFig. 15. Schematic cross-section of InGaAs-InAlAs SAM APD device (after Ref. 14).structure is shown in Fig. 15. Starting from a semi-insulatingInP substrate, the bottom mirror, which consistedof 30 l/4 pairs of n + -In 0.53 Ga 0.47 As/In 0.52 Al 0.48 As, wasgrown by MBE. After the growth of the thin undopedIn 0.52 Al 0.48 As multiplication region, a p-type In 0.52 Al 0.48 Ascharge layer was inserted to ensure that the electric field inthe adjacent absorption region would be < 1×10 5 V/cm.This was followed by the 60 nm-thick In 0.53 Ga 0.47 As absorptionlayer. Finally, an undoped In 0.52 Al 0.48 As spacerand a p + -In 0.52 Al 0.48 As top layer were grown. To fabricate ahigh-speed device, first a Ti-Pt-Au contact metal ring wasdeposited and a mesa with the diameter of 14 µm wasformed by chemical etching.Due to the resonant-cavity scheme, these APDs operatingat 1.55 µm exhibited high external quantum efficiency70% and a high unity-gain bandwidth of 24 GHz. Utilizingthe excellent noise characteristics of a thin InAlAs multiplicationregion, a record value of a gain-bandwidth productof 290 GHz was achieved.layer as the active medium. The thickness of the spacer layerswere chosen so that the three absorbing layers are locatedat the antinodes of the standing wave optical field inthe cavity [Fig. 16(b)]. The investigated devices, shownschematically in Fig. 17(a), were grown by MBE onn + -GaAs substrate. Thirteen period AlAs/GaAs quarter-wavestacks formed bottom mirror with the reflectivityabout 90% at the centre wavelength 900 nm. The threeIn 0.05 Ga 0.95 As active layers with each layer of the thickness40 nm were sandwitched between GaAs layers. The topmirror with 30% reflectivity was formed by air and semiconductorinterface.The peak quantum efficiency of the triple-layer resonant--periodic-absorber photodiode was 65% [see Fig. 17(b)]. An enhancementof 30% has been achieved by a periodic absorberstructure as compared to a standard RCE photodiode with asingle absorbing layer of the same total thickness.3.2.3. RCE p-i-n PDThe first RCE p-i-n was reported by Dentai et al. who studiedan InGaAs/InGaAsP/InP structure designed to operatenear 1.55 µm [15]. A quantum efficiency of 82% wasachieved using a 200 InGaAs absorbing layer with R 1 =0.7 and R 2 = 0.95.RCE p-i-n PDs with a periodic absorber structure weredemonstrated by Huang et al. to detect 870-nm wavelength[16]. Figure 16(a) depicts the schematic structure of a typicalRC photodiode with a single absorbing layer placed in awave antinode position. In order to improve quantum efficiencyof RCE p-i-n PD, it was proposed to use a tripleFig. 16. Schematic structures of RCE PD with a single absorbinglayer (a) and resonant-periodic-absorber RCE PD with triple layersas the absorbing medium (b) (after Ref. 16).Opto-Electron. Rev., 12, no. 1, 2004 J. Kaniewski 145
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