HIGH SPEED InAlAs/InGaAs DOUBLE HETEROSTRUCTURE p-i-n s

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1 HIGH SPEED InAlAs/InGaAs DOUBLE HETEROSTRUCTURE p-i-n s J.-C. Bischoff, T. Hollenbeck, R. Nottenburg, M. Tamargo, J. De Miguel, C. Moore, H. Schumacher To cite this version: J.-C. Bischoff, T. Hollenbeck, R. Nottenburg, M. Tamargo, J. De Miguel, et al.. HIGH SPEED InAlAs/InGaAs DOUBLE HETEROSTRUCTURE p-i-n s. Journal de Physique Colloques, 1988, 49 (C4), pp.c4-329-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1988 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 Colloque C4, suppl6ment au n09, Tome 49, septembre 1988 HIGH SPEED InAlAs/InGaAs DOUBLE HETEROSTRUCTURE p-i-n's J.-6. BISCHOFF(~), T.H. HOLLENBECK",('), R.N. NOTTENBURG*-(3), M. C. TAMARGO*, J. L. DE MIGUEL*, C. F. MOORE" and H. SCHUMACHER* Institute for Micro and Optoelectronics, Swiss ~ederal Institute of Technology, CH-1015 Lausanne, Switzerland e ell Communications Research Inc., 331, Newman Spring Rd., Red Bank, NJ U.S.A. Resume - Des photodiodes p-i-n InAlAs/InGaAs a double heterostructure ont Bte produites par MBE et montees sur des guides d'onde coplanaires. Deux methodes: (i) l'incorporation de couches InAlAs non dopees entre la zone d'absorption InGaAs et les couches InAlAs dopees et (ii) l'utilisation d'heterojonctions graduelles ont ete utilisbes pour reduire l'accumulation de porteurs aux interfaces. Bien que les deux methodes ameliorent les performances des diodes p-i-n a double heterojonction, les meilleurs rksultats ont ete obtenus avec les heterojonctions graduelles: une efficacite quantique de 238 % a 1.3 um. un temps de montee de 21 ps et une largeur a mi-hauteur de 40 ps ont ete obtenus pour une diode ayant une surface de 24x24 pmz et une couche d'absoption d'une epaisseur de 0.5 um. Abstract - MBE grown double heterostructure InAlAs/InGaAs p-i-n photodiodes have been flip-chip mounted on coplanar waveguides. Two methods: (i) incorporation of doping setbaclt InAlAs layers between the InGaAs absorption region and the doped InAlAs layers and (ii) compositional grading have been used to reduce carrier pile-up at the heterointerfaces. Although both methods improve the diode performances. the best results were obtained with compositional grading: a quantum efficiency of =38 % at 1.3 urn, a rise time of =21 ps and a FWHM of =40 ps have been obtained for a device with a 24x24 urn2 area and a 0.5 pm thick absorption region. One of the most promising approaches for detection in the urn wavelength range is the use of a p-i-n photodiode coupled to a field effect transistor amplifier. Ino.goCa0.47As based p-i-n photodiodes have already shown low dark current, good quantum efficiency and high speed operation /1//2//3/. Most devices investigated to date are of the homojunction type with a p-type layer formed by diffusion of Zn in thejincaas layer. In contrast, the devices investigated here are MBE grown I~o.~zA~o.~~A~/I~o.~~C~O.~~A~/I~O double heterostructure p-i-n's. Immediate advantages of a double heterostructure are an excellent control of the absorption region thickness and the elimination, ln the photoresponse, of the diffusion tail due to absorption in low field segions. The pulse response of these devices is however, typically characterized by long tails due to carrier pile-up (trapping) at the heterojunction interfaces /4/. We present results of an investigation o'f two possible methods to reduce the deleterious effects of carrier trapping: (i) incorporation of doping setback layers at the heterointerfaces and (ii) compositional grading of the heterointerfaces. (')work done while at Bell Comunications Research Inc., Red Bank, NJ U.S.A.. as a resident visitor (')present address : Princeton University. Princeton. NJ U.S.A. (3)~resent address : AT&T Bell Laboratories. Murray Hill, NJ 07974, U.S.A. Article published online by EDP Sciences and available at

3 The samples were grown by Molecular Beam Epitaxy (MBE) on InP (Fe-doped) semi-insulating substrates. Three types of structures were realized. The first one is a double heterojunction p-i-n structure with abrupt interfaces which consists of an undoped Ino.sctGao.4tAs absorption layer sandwiched by two Ino.szAlo.~eAs layers, one doped with Si (n-type) and the other with Be (p-type). The two others are slightly modified structures. One contains 300 A setback layers of undoped In0.s~Alo.4sAs separating the doped layers from the intrinsic In0.33Ga0.47As and the other one, compositionally grzded (In, Ga, Al) As layers gradually going from the Ino.~zAl0.48As composition to the Ino.ssGao.47As. These graded layers are 300 A thick and undoped. Mesa diodes, designed for back illumination through the semi-insulating InP substrate. were fabricated from the as-grown wafers and mounted, upside down, on coplanar waveguides. With this technique, the losses due to the coupling of the p-i-n diode to the coaxial connector ('Weltron K connector) can be maintained below' 3 db at 26 GHz. Dark currents of respectively 328 pa and 537 pa at 5 V were measured. for devices with compositional grading and areas of respectively 14x14 pm2 and 24x24 pmz. For these devices, whose absorption layer thickness is 0.5 pm, the capacitances at 0 V bias are respectively 50 ff and 145 ff. We present in fig. 1 the reverse I-V characteristics of double heterostructure p-i-n diodes with abrupt heterointerfaces. in the dark (lower curve) and front illuminated with white light through the prober microscope (upper curve). The photoresponse shows a turn-on voltage at = 0.4 V. Dispersion in the characteristics of diodes issued from different processing runs indicates that this turn-on voltage is sensitive to the processing and is related to the quality of the Schottky type Ti-Au contacts. Above the turn-on voltage, the sensitivity of the photodiodes is fairly independent of the applied bias voltage. In fig. 2, we have reported the peak amplitude of the response of double heterostructure p-i-n diodes to optical pulses generated by active mode locking of a laser diode as a function of the reverse bias voltage (characteristics of pulses generated by laser #1: rise time = 44 ps, FWHM = 45~s. repetition rate = 350 MHz and peak power = 14.6 mw.). It appears that these devices have, under pulsed excitation, a behavior which differs significantly from that under continuous illumination. Indeed the characteristics reported in fig. 2 show that our devices do not respond to fast optical pulses when their reverse bias voltage is below a threshold value which depends on the type of heterointerface: =1 V for devices with compositionally graded interfaces, ~2.8 V for devices with doping setback layers and -5 V for devices with abrupt interfaces. Above threshold, the peak current of the three types of devices increases linearly with bias in the voltage range shown in fig.2. The best sensitivity, obtained at 6 V bias for a double heterostructure p-i-n with a 0.5 vm thick absorption layer, is 0.4 A/W (peak amplitude = 291 mv). At 1.3 pm. this corresponds to an external quantum efficiency of 38%. At higher bias voltages, the pulse response peak amplitude levels off (fig. 3, characteristics of pulses generated by laser #2: rise time = 5.3 ps, FWHM = 7.4 ps, repetition rate = 440 MHz and peak power = 6 mw.). Only the devices with compositional grading were measured in the higher voltage range. Indeed for devices with abrupt interfaces or doping setback, the breakdown voltage. = 8 V, was reached before any leveling of the response could be observed. Devices with abrupt interfaces show a 80 ps rise time and a 1.4 ns fall time at 5.5 V bias. This long fall time is strongly temperature dependent and can be reduced to 180 ps by elevating the detector temperature by directing a hot air flow to the diodd Devices which employ doping setback and compositional grading show threshold response at bias levels of 3 V and 1.5 V respectively. Operating the devices, with doping setback, at 5.5 V bias, we observe over an order of magnitude improvement in fall time: 120 ps versus 1.34 ns for the diode with abrupt interfaces. The same observation can be made at 3.3 V bias by comparing the devices having compositional grading with the ones having doping setback: 110 ps versus 1.02 ns.

4 Although the speed of all the devices is increasing with bias voltage the best results are achieved with compositional grading; we obtain a 55 ps rise time, a 70 ps FWHM and a 102 ps fall time at 6 V. This response is limited primarily by our laser diode (laser #I). Indeed measurement of devices with compositional grading on an identical set-up, except for the laser diode (laser #2), have resulted in a 33 ps rise time, a 54 ps FWHM and a 82 ps fall time at 8 V bias for an active area of 24x24 urn2 (fig. 4). Devices of smaller size have not shown any speed improvement over the largest ones. This is.due to their smaller top contact to mesa area ratio resulting in an increased series resistance. To evaluate the intrinsic rise time and FWHM of our devices, we have made the following assumptions: ti) the pulses have a gaussian waveform, (ii) the S4 sampling head rise time is 25 ps and the corresponding FWHM is 35 ps and (iii) the light pulse rise time and FWHM are 5.3 ps and 7.4 ps /5/. Under these assumptions, we obtain for the device whose pulse response appears in fig. 4 an intrinsic rise time of 21 ps and a FWHM of 40 ps. During this work, we made the following observations. The dependence of the diode responsivity on bias voltage is apparent only under pulsed excitation. The responsivity of the diode is larger when the areal density of carriers trapped at the heterointerface under dark conditions is low, for example, when the band discontinuity is small (compositional grading of the heterointerface) or when the band potential minimum is above (for electrons) the quasi-fermi level (structure with doping setback layers under reverse bias). These observations lead us to the assumption that the time spent by a carrier at the heterointerface, and thus the probability to be lost by recombination, decreases when the photogenerated carrier density in the well is large in comparison with the carrier density in the well under dark conditions. However, further work will be necessary to determine exactly the mechanisms involved in the transport of carriers through band discontinuities in double heterostructure p-i-n diodes. In conclusion, we have presented two methods to reduce carrier trapping at the heterointerfaces in InAlAs/InGaAs double heterostructure p-i-n photodiodes: incorporation of doping setback InAlAs layers and of compositionally graded - InAlGaAs layers. The best results have been obtained with devices with compositional grading and are very promising: external quantum efficiency = 38 96, rise time = 21 ps and FWHM 40 ps. The authors wish to acknowledge R. F. Leheny and M. Ilegems for their support and M.-H. Meynadier, C. Caneau, B. W. Meagher, D. A. Humphrey. R. J. Martin and B. Zimmermann for their assistance during the course of this work. We are particularly grateful to H. Temkin for his advise and for providing the laser diodes. References /1/ J. E. Bowers, C. A. Burrus, "Optoelectronic Components and Systems with Bandwidths in Excess of 26 Ghz", RCA Review. Vol. 46, pp , Dec /2/ J. E. Bowers. C. A. Burrus, "InGaAs PIN Photodetectors with Modulation Response to Millimetre Wavelengths". Electronics Letters. Vol. 21. pp , 29'h August /3/ 'H. Temkin, R. E. Frahm, N. A. Olsson, C. A. Burrus, R. J. Mccoy, "Very High Speed Operation of Planar InGaAs/InP Photodiode Detectors", Electronics Letters, Vol. 22, No. 23, pp th Nov /4/ Y. Zebda, P. Bhattacharya, M. S. Tobin and T. B. Simpson, "Design and Performance of Very High-Speed I~o.~~G~o.~~A~/I~o.JzA~o.~~A~ p-i-n Photodiodes Grown by Molecular Beam Epitaxy", IEEE Electron Device Letters. Vol. EDL-8. No. 12. December pp /5/ J. P. van der Ziel, "Active mode locking of double heterostructure lasers in an external cavity", J. Appl. Phys., Vol. 52. No. 7, pp , July 1981.

5 - 300 o abrupt interface8," ',' i : I / 25a -A doping setback.?,. 5: m- E ij " = compositional grsd~ng,:' $, i / A / 50 -?,' " :4 0-, o/,,,, Bias Voltage [VI Pig. 1 I-V characteristics of a 24x24 pm2 double heterostructure p-i-n diode with abrupt interfaces in the dark (lower curve, I = 586 pa for V = 5.0 V) and illuminated with white.light through the prober microscope (upper curve). Fig. 2 Dependence of peak amplitude on bias voltage of 24x24 pm2 diodes with abrupt interfaces, doping setback layers and compositional grading. Optical pulse characteristics: rise time = 44 ps. FWHM = 45 ps, repetition rate = 350 MHz and peak power = 14.6 mw.. -" o amplitude A rke time 116- o FWHM "4- E 112- g,i,. / Fig. 3 Dependence of peak amplitude. rise time and FWHM on bias voltage of a 24x24 prn2 with compositional grading. Optical pulse characteristics: rise time = 5.3 ps, FWHM z 7.4 ps, repetition rate z 440 MHz and peak power = 6 mw. Fig. 4 Pulse response of a 24x24 urnz diode with compositional grading at 8 V bias. Optical pulse characteristics: rise time = 44 ps, FWHM z 45 ps, repetition rate = 350 MHz and peak power = 14.6 mw. Scale x: 50 ps/div. y: 20 mv/div.,