Defects in the emitter region of Ga0.51In0.49P/GaAs heterojunction bipolar transistors (HBTs) were investigated by means of deep-level transient spectroscopy. Both annealed (635 degreesC, 5 min) and as grown metalorganic chemical vapor deposition epitaxial wafers were investigated in this study, with an electron trap observed in the HBT emitter space-charge region from both wafers. The deep-level activation energy was determined to be 0.87+/-0.05 eV below the conduction band, the capture cross section 3x10(-14) cm(2) and the defect density of the order of 10(14) cm(-3). This defect was also found to be localized at the emitter-base interface

Bland, S. W.

Cassette, S.

Cherkaoui, Karim

Crean, Gabriel M.

Delage, S. L.

Kelly, P. V.

Murtagh, M. E.

Journal of Applied Physics

Cork Open Research Archive

Title Defect study of GaInP/GaAs based heterojunction bipolar transistoremitter layerAuthor(s) Cherkaoui, Karim; Murtagh, M. E.; Kelly, P. V.; Crean, Gabriel M.;Cassette, S.; Delage, S. L.; Bland, S. W.Publication date 2002-01Original citation Cherkaoui, K., Murtagh, M. E., Kelly, P. V., Crean, G. M., Cassette, S.,Delage, S. L. and Bland, S. W. (2002) 'Defect study of GaInP/GaAsbased heterojunction bipolar transistor emitter layer', Journal of AppliedPhysics, 92(5), pp. 2803-2806.Type of publication Article (peer-reviewed)Link to publisher'sversionhttp://aip.scitation.org/doi/abs/10.1063/1.1500417http://dx.doi.org/10.1063/1.1500417Access to the full text of the published version may require asubscription.Rights © 2002 American Institute of Physics, This article may bedownloaded for personal use only. Any other use requires priorpermission of the author and AIP Publishing. The following articleappeared in Cherkaoui, K., Murtagh, M. E., Kelly, P. V., Crean, G.M., Cassette, S., Delage, S. L. and Bland, S. W. (2002) 'Defect studyof GaInP/GaAs based heterojunction bipolar transistor emitterlayer', Journal of Applied Physics, 92(5), pp. 2803-2806 and may befound at http://aip.scitation.org/doi/abs/10.1063/1.1500417Item downloadedfromhttp://hdl.handle.net/10468/4238Downloaded on 2018-08-23T18:40:54ZDefect study of GaInP/GaAs based heterojunction bipolar transistor emitter layerK. Cherkaoui, M. E. Murtagh, P. V. Kelly, and G. M. CreanS. Cassette and S. L. DelageS. W. BlandCitation: Journal of Applied Physics 92, 2803 (2002); doi: 10.1063/1.1500417View online: http://dx.doi.org/10.1063/1.1500417View Table of Contents: http://aip.scitation.org/toc/jap/92/5Published by the American Institute of PhysicsDefect study of GaInPÕGaAs based heterojunction bipolar transistoremitter layerK. Cherkaoui,a) M. E. Murtagh, P. V. Kelly, and G. M. CreanNMRC, Lee Maltings, Prospect Row, Cork, IrelandS. Cassette and S. L. DelageTHALES Research and Technology-F, Domaine de Corbeville, 91404 Orsay Cedex, FranceS. W. BlandIQE (Europe) Limited, Pascal Close, Cypress Drive, St. Mellons, Cardiff, South Galmorgan CF3 0EG,Wales, United Kingdom~Received 18 March 2002; accepted for publication 25 June 2002!Defects in the emitter region of Ga0.51In0.49P/GaAs heterojunction bipolar transistors ~HBTs! wereinvestigated by means of deep-level transient spectroscopy. Both annealed ~635 °C, 5 min! and asgrown metalorganic chemical vapor deposition epitaxial wafers were investigated in this study, withan electron trap observed in the HBT emitter space-charge region from both wafers. The deep-levelactivation energy was determined to be 0.8760.05 eV below the conduction band, the capture crosssection 3310214 cm2 and the defect density of the order of 1014 cm23. This defect was also foundto be localized at the emitter–base interface. © 2002 American Institute of Physics.@DOI: 10.1063/1.1500417#I. INTRODUCTIONGaAs based heterojunction bipolar transistors ~HBTs!have attracted considerable interest due to the emergingwireless market. The GaInP/GaAs system is considered thetechnology of choice for various applications, indeed theGa0.51In0.49P/GaAs emitter/base heterojunction offers an ex-cellent band-gap lineup compared to other systems ~e.g.,GaAlAs/GaAs! with small conduction band offset, as well asa large valence band discontinuity1 affording high emitterinjection efficiency. Moreover, the very high etch rate selec-tivity between GaInP and GaAs simplifies deviceprocessing.2However, there are still reliability issues that must beaddressed in order to achieve better device performance andlifetime. Hydrogen ~H! incorporation in the epilayers duringmetalorganic chemical vapor deposition ~MOCVD! growthis one of the main reliability issues, acting as a compensatingdonor (H1) for carbon ~C! acceptors in the base layer. Hy-drogen can also form CH and/or C2H complexes passivatingC acceptors. Furthermore, it has been shown that H in thebase is responsible for direct current gain instabilities duringdevice operation.3–6In addition to the effect of hydrogen upon HBT reliabil-ity, deep levels present in the base–emitter depletion regionaffect device operation by increasing base leakage current.Electrons can recombine with holes at near midgap trapslocated in the depletion region, leading to an increase in thebase leakage current. The nonradiative recombination energycan be converted into vibrational ~phonon! energy, inducingpoint defect reactions such as diffusion that may in turn actas recombination centers.7Several studies have investigated deep levels in InGaPmaterial grown by various techniques. Defects were reportedin InGaP layers grown by MOCVD,8–11 vapor phaseepitaxy,12 liquid phase epitaxy,13–15 metalorganic molecular-beam epitaxy,16 as well as gas-source17,18 and solid-sourcemolecular-beam epitaxy.19In this article, we report a deep-level transient spectros-copy ~DLTS! study of the deep levels in the emitter layer ofGa0.51In0.49P/GaAs HBT transistors grown by MOCVD.II. EXPERIMENTAL PROCEDUREThe DLTS experiments were carried out on fully pro-cessed MOCVD HBTs, as summarized in Table I, silicon andcarbon were the n- and p-type dopants, respectively. TheInGaP/GaAs HBTs were grown by MOCVD in an Aixtron2400 multiwafer reactor. The epitaxial layers were grown ata pressure of 200 mbar using trimethygallium ~TMG! andtrimethylindium as the metalorganic sources, arsine andphosphine as the hydride sources, and disilane as the n-typedopant source. Carbon doping in the p-type GaAs base layerwas achieved by intrinsic doping from the TMG source. Twowafers originating from the same batch were processed iden-tically. Prior to the process, however, one of the wafers wasannealed at 635 °C for 5 min, with GaAs proximity capping.Also for the isolation of the individual devices, wafers wereboron implanted instead of proton irradiated to avoid anyfurther H contamination. The contacts were Ti/Pt/Au for thebase and AuGe/Ni/Au alloy for the collector. The emittercontact consisted of refractory metal ~TiWSi/TiPtAu! in or-der to improve the thermal stability as well as to avoid met-allurgical problems; the second layer enhances the conduc-tivity. Large emitter area (1003100 mm2) transistors wereemployed for capacitance measurements.a!Electronic mail: karimc@nmrc.ieJOURNAL OF APPLIED PHYSICS VOLUME 92, NUMBER 5 1 SEPTEMBER 200228030021-8979/2002/92(5)/2803/4/$19.00 © 2002 American Institute of PhysicsThe DLTS setup consists of a Boonton 72 capacitancemeter and computer controlled data acquisition card. Capaci-tance transients were analyzed digitally using the boxcarDLTS method, and recorded over 80–450 K using a continu-ous flow liquid nitrogen cryostat. This DLTS study was car-ried out on the emitter–base junction with the collector po-tential kept floating.III. RESULTS AND DISCUSSIONCapacitance voltage (C – V) measurement and DLTSwere carried out on the p1GaAs/n–InGaP junction of thetransistors. The doping profile shown in Fig. 1, obtained us-ing C – V measurement, reveals dopant reactivation in theannealed emitter layer. We have also noticed similar dopantreactivation for the annealed collector region. As hydrogen isexpected to be in the structures due to the MOCVD growthtechnique, such dopant reactivation, is likely due to hydro-gen outdiffusion, following 635 °C for 5 min.6Figure 2 presents the DLTS spectrum of the emitter–base junction, measured with a reverse bias of 2 V and pulseheight of 1 V with 5 ms duration. A single large peak domi-nates the spectrum at high temperature, above 400 K and isconsistent with a very deep electron trap in the emitter space-charge region. The activation energy (ET) extracted from thecorresponding Arrhenius plot was 0.8760.05 eV, with acapture cross section s53310214 cm2. The amplitude ofthe DLTS signal yielded a defect density of the order of1014 cm23. This defect was detected in all transistors fromboth annealed and nonannealed ~as-grown! wafers, thus it isnot induced by the anneal process.The DLTS peak amplitude was observed to vary stronglywith reverse bias voltage. Figure 3 presents three DLTSspectra obtained for various reverse bias voltages, 3 V to 1 V,at constant filling pulse amplitude, 1 V. The DLTS signalincreased with decreasing reverse bias, that is when probingthe region closer to the emitter/base interface. Conversely,the DLTS signal is totally extinguished when probing farfrom the interface, implying the deep level to thus corre-spond to a defect localized at the emitter–base interface.Note that the small temperature lowering observed for thelow bias conditions is unlikely to be attributed to electric-field enhanced effects, given the constant pulse amplitudeTABLE I. Summary of nominal MOCVD HBT device structure—layerthickness and doping levels. Both nonannealed and 635 °C ~5 min! annealedstructures were examined in this study.LayerThickness~nm!Doping level~cm23!n1 InGaAs including graded layer ’60 231019n1 GaAs contact layer 200 2.131018n1 GaInP interface layer 100 9.231017n GaInP ballast 250 9.431016n GaInP emitter 150 2.831017p1 GaAs base 100 2.531019n GaAs collector 1000 2.231016n1 GaAs subcollector 100 3.131018n1 GaInP etch barrier 25 .131018n1 GaAs subcollector 800 3.531018FIG. 1. Doping profile deduced from C – V measurement for both annealed635 °C for 5 min ~bold line! and nonannealed samples. The dopant reacti-vation observed is consistent with H outdiffusion.FIG. 2. DLTS spectrum ~nonannealed transistor! obtained with reverse biasvoltage Vr52 V and pulse amplitude Vp51 V, emission rate window 20s21, and pulse duration t p55 ms. The corresponding Arrhenius plot isshown in the inset.FIG. 3. DLTS spectra obtained at different reverse bias voltages (Vr) withthe pulse amplitude kept constant, 1 V. Solid line Vr51 V, dashed line Vr52 V, dotted line Vr53 V, with emission rate window 20 s21, and pulseduration tp55 ms.2804 J. Appl. Phys., Vol. 92, No. 5, 1 September 2002 Cherkaoui et al.measurement conditions. A possible origin for such a shiftmay be band gap lowering toward the InGaP/GaAs interface.To investigate more about the 0.87 eV level, severalDLTS spectra were recorded with the same experimentalconditions as presented in Fig. 4, but with varied filling pulseduration, 0.1–100 ms. The dependence of the DLTS peakamplitude upon the pulse duration gives significant informa-tion on the defect capture kinetics. For an ideal point defect,the DLTS signal amplitude varies exponentially with pulseduration.20 However, the DLTS signal for the 0.87 eV levelexhibits a clear logarithmic behavior, saturating after longfilling pulses .10 ms, Fig. 4. The slight DLTS signal in-crease observed following 635 °C for 5 min anneal may arisefrom hydrogen trap depassivation effects.The logarithmic dependence of the DLTS signal withpulse duration has already been observed in plastically de-formed silicon and III–V materials.20,21 Such behavior wasattributed to point defects located in the vicinity of, or adja-cent to, lattice dislocations. In this model, the logarithmicdependence results from a repulsive Coulomb potential, de-pending upon the occupation of the extended defect, itself afunction of the filling pulse duration. Moreover, according tothis model, the defect is neutral before capture and nega-tively charged after, thus implying an acceptor defect.Feng et al.8 observed by DLTS a deep level with anactivation energy of about 0.9 eV in nonintentionally dopedMOCVD In0.49Ga0.51P ~;1 mm! epilayers.8 Interestingly, thislevel was also located close to the n1 GaAs–substrate inter-face. However, the DLTS peak corresponding to the 0.9 eVwas very broad and consisted of a doublet structure. Thedeep level that we have detected in this study shows a broadDLTS peak, however, it was not possible to distinguish anyother components, either by varying the bias or the durationand amplitude of the filling pulse. A DLTS dependence uponpulse duration was also reported by Feng et al.,8 however thenonsaturation of the signal was attributed to a lack of freecarriers. In our case, the free carrier concentration is at leasttwo orders of magnitude higher than the trap density(1014 cm23). Therefore, the logarithmic behavior that weobserved is not due to a lack of free carriers. Considering thesimilarities between the deep level detected in this study andthat reported by Feng et al.8 ~activation energy and interfacelocalization!, we can reasonably, assume, therefore that bothlevels originate from the same defect.Several authors also observed deep levels featuring acti-vation energies within the same energy range as the deeplevel concerned in this study. A deep electron trap at Ec20.85 eV was reported in MOCVD grown InxGa12xP,9 anddetected only with the In composition x.0.532. This defectwas uniformly distributed in the InGaP layer. It was con-cluded that this level was not a lattice-mismatch induced trapbut probably a native defect associated with the stoichiom-etry of In or Ga. Moreover, Paloura et al.12 observed a verybroad DLTS peak corresponding to a deep level in the rangeof 0.7–0.9 eV in undoped metalorganic vapor phase epitaxyGa0.51In0.49P layers. This deep center was also detected inmoderately Se doped layers, but was extinguished at Se dop-ing levels higher than 1017 cm23. This level did not showany bias dependence. The large peak broadening was attrib-uted to microscale variations of the alloy composition.The origin of the single trap observed in this study,therefore, is not clearly identified. However, assuming thesame defect level as observed by Feng et al.,8 we can, there-fore, exclude the possibility that it is induced by the HBTfabrication process. Note that the silicon dopant can also beeliminated as a possible origin of this defect because it wasalso detected in nonintentionally doped epilayers.8 Althoughthese MOCVD structures contain large concentrations of hy-drogen, it is very unlikely that this defect is induced by hy-drogen because it was detected with similar densities in bothannealed and nonannealed samples.Considering this defect localization at the InGaP/GaAsinterface and its logarithmic capture kinetics, we suspect thatit is related to an extendedlike deep level in the InGaP layer.This extendedlike behavior is due to a high localization ofthe deep levels. Indeed, these unusual capture kinetics resultfrom a strong interaction between the emitting centers. Thisonly indicates that the defects are closely spaced in the lat-tice, the exact nature of this defect and the origin of its ex-tendedlike behavior are still uncertain. Therefore, such ex-tendedlike behavior can possibly be associated with bothpoint defects, assuming sufficient localization such that ad-jacent defects interact, or true extended defects.Finally, although it is commonly accepted that the re-combination rate at the emitter–base interface of InGaP/GaAs HBTs is very low compared to AlGaAs/GaAs HBTs,the presence of such deep levels at the InGaP/GaAs interfaceand its near midgap energy position ~In0.49Ga0.51P band gap;1.9 eV!, might lead to an increase in nonradiative recom-bination, thus resulting in an increase in base leakage cur-rent. This may not affect the device operation at the earlystages, however, it could be responsible for long-term HBTdegradation as described by Henderson.7 It should be notedhowever that lifetime results on HBT devices made fromepitaxial layers produced at the same time using the samegrowth process show excellent long-term reliability, with nofailures after more than 6000 h at a current density of 40kAcm2 and junction temperature of 296 °C.22FIG. 4. Logarithmic dependence of the DLTS signal with filling pulse du-ration for both nonannealed ~d! and annealed ~j! HBTs. Note the onset ofsaturation above 10 ms.2805J. Appl. Phys., Vol. 92, No. 5, 1 September 2002 Cherkaoui et al.IV. CONCLUSIONTo conclude, a deep electron trap has been detected inthis DLTS study of MOCVD InGaP/GaAs based HBT de-vices. A deep level with an activation energy around 0.87 eVhas been detected in the emitter layer of fully processedHBTs. This deep level was observed in both annealed andnonannealed samples. We have also shown that this center islocated at the InGaP/GaAs interface and exhibits an unusuallogarithmic capture mechanism.ACKNOWLEDGMENTSThis work was supported by the European Union—HERO’S Project, EU BRITE program-BRPR-CT98-0789.1 M. J. Monday and H. Kroemer, IEEE Electron Device Lett. 6, 175 ~1985!.2 F. Ren, C. R. Abernathy, S. J. Pearton, L. W. Yang, S. T. Fu, Solid-StateElectron. 38, 1635 ~1995!.3 F. Ren, C. R. Abernathy, S. N. G. Chu, J. R. Lothian, and S. J. Pearton,Solid-State Electron. 38, 1137 ~1995!.4 H. Fushimi and K. Wada, IEEE Trans. Electron Devices 44, 1996 ~1997!.5 A. Kawano, K. Imanishi, T. Tomioka, T. Iwai, K. Ishii, and Y. Yamaguchi,Solid-State Electron. 42, 2057 ~1998!.6 S. Cassette, S. L. Delage, E. Chartier, D. Floriot, M. A. Poisson, J. C.Garcia, C. Grattepain, J. Mimila Arroyo, R. Plana, and S. W. Bland, Mater.Sci. Eng., B 80, 279 ~2001!.7 T. Henderson, Microelectron. Reliab. 39, 1033 ~1999!.8 S. L. Feng, J. C. Bourgoin, F. Omnes, and M. Razeghi, Appl. 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Defect study of GaInP/GaAs based heterojunction bipolar transistor emitter layer

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Defect study of GaInP/GaAs based heterojunction bipolar transistor emitter layer

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