Callens, Freddy

Clauws, Paul

Lauwaert, Johan

Simoen, Eddy

Vrielinck, Henk

English

Ghent University Academic Bibliography

Defect creation and passivation by hydrogen plasma insilicon and germaniumJ. Lauwaert1, E. Simoen2, F. Callens1, H. Vrielinck1 andP. Clauws11Department of Solid-State Sciences, University of Ghent,Krijgslaan 281 (S1), 9000 Ghent, Belgium2Imec, Kapeldreef 75, 3001 Leuven, BelgiumThe presence of hydrogen in semiconductors can stronglyinfluence the formation of defects in group IV elementalsemiconductors as silicon and germanium, and thus hasan influence on the reliability of devices. Exposure of amaterial to hydrogen plasma is a very common method tointroduce hydrogen.In this work specific topics will be discussed related todefect creation and passivation by means of hydrogenplasma treatments both on Si and Ge.A first example of defect creation in Si is the catalyticeffect of hydrogen on the formation of oxygen thermaldonors (OTDs), and thus an unexpected amount ofadditional donors can be formed during the processing ofSi devices. Recently strong indications were obtained bymeans of Deep Level Transient Spectroscopy (DLTS) incombination with Capacitance Voltage profiling (CV) thathydrogen not only acts as a catalyst for the formation ofOTDs but plays an active role in the formation ofhydrogen related shallow thermal donors (STDs). Inparticular for short annealings at low temperatures, whichare common in semiconductor technology, an unexpectedincrease in shallow donors was observed. Tounambiguously link this observation to the creation of H-STDs samples were examined by means of far-infraredspectroscopy, which will be discussed hereDue to the fact that Si and Ge have similar properties, ingeneral one would not expect qualitative differences inthe behaviour of H between the two semiconductors. Alsothe chemistry of Si-H and Ge-H compounds is verysimilar. However Estreicher and Maric noted thatwhenever identical experiments involving H areperformed on comparable Si and Ge samples, the resultsare very different, and thus one can conclude thatqualitative differences do exist. Since one could expectfrom thermal effusion experiments that the incorporationof hydrogen in Ge seems is more difficult, a moreaggressive parallel-plate direct plasma system has beenbuilt and used to hydrogenate Ge.Structural characterization of a Ge <100> wafer afterhydrogenation allowed to distinguish defects could bedistinguished. As will be shown, close to the surfacewithin the first 10nm, Ge-H bonds and H2 molecules areobserved by means of Raman spectroscopy. Nopolarization sensitive measurements were performed, butthe similar position and shape of the observed Ramanpeak comparing to the work of Hiller et al. allow to assignthe vibrational modes to Ge-H bonds and H2 within<111> platelets. The first micron was probed with TEM.Within this micron two regions could be distinguished.The first region until 70nm is heavily damaged andcontains mostly <111> platelets. A second region (70 -1100nm) contains plate-like cavity clusters oriented in<100> and <111> planes. The cavities inside are 2-5nm indiameter. Slow positron annihilation spectroscopy probesabout the same region as examined by TEM. To firstorder the region with the defects observed can beapproximated as 300nm thick. The positron signaturecorresponds with open volume defects of at least 3vacancies. The S-parameter might be influenced by thepresence of hydrogen, so the positrons possibly annihilatein the cavities observed by TEM. It may be emphasizedthat the depth of these structures is in good agreementwith the one calculated from the size of the blisters.Infrared absorption spectroscopy in general measures theabsorption integrated over the total sample thickness, butthe disappearance of the bands after removal ofapproximately the first 2 micron demonstrates that thesignal originates from this layer. It is shown that both thestretch and wag mode of Ge-H bonds in this region couldbe observed. The position of these bands is close to thefrequencies calculated by Cardona.Another interesting effect of hydrogenation with respectto applications is the possibility to passivate electricallyactive defects through binding with a hydrogen atom. InSi passivation of shallow dopants and of transition metalimpurities by complex formation with hydrogen has beenwell established. As a last topic we will discuss thepassivation of transition-metal impurities in Ge by meansof direct-plasma treatments. The occurrence of defectcomplexes with hydrogen in Ge is known since thedevelopment of high purity Ge for radiation detectorapplications in the 1970’s. This material is grown in ahydrogen containing atmosphere in order to suppresstrapping, suggesting that deep traps become passivated byhydrogen. On the other hand, the neutral impurities Si, Cand O become ’activated’ and form shallow complexessuch as (Si,H) and (C,H) acceptors and (O,H) donors, aswas observed with Photo Thermal IonizationSpectroscopy by Haller et al. Also a deep level inducingsevere trapping is formed by the V2H complex. Partiallypassivated multivalent acceptors in Ge are veryinteresting since they remain experimentally accessibletrough their electronic level spectrum. The doubleacceptors Be and Zn become single acceptors (Be,H) and(Zn,H) with trigonal symmetry, through binding with ahydrogen atom. Probably the most interesting case in thepresent contextis passivation of the triple acceptor copper, which ispresented in the next section. A simple model to explainpassivation of multiple acceptors is that hydrogen is asingle donor which is attracted by the negatively chargedacceptor, forming a donor-acceptor pair. The acceptormultiplicity is then reduced with one unit for everyhydrogen added. To study the passivation effect ofplasma hydrogenation metal impurities, transition metalimplanted Ge samples have been hydrogenated for 4h at200°C. The most interesting observation by means ofDLTS undoubtedly is the appearance of transition metalspecific levels, which may be tentatively assigned totransition metal-hydrogen complexes. No obvious trendin the (metal,H) levels compared with the substitutionalmetals was observed, similar to the well-known case ofCu-passivation. Only for Cr a similar hierarchy of (Cr,H)with respect to Cr is observed as for (Cu,H) with respectto Cu. In both cases we are concerned with a tripleacceptor + a donor level close to the valence band for thesubstitutional metal. Thus the metal specific levels in theCr implanted hydrogenated specimen may be tentativlyassigned to the Cr-H complex.

Defect creation and passivation by hydrogen plasma in silicon and germanium

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Defect creation and passivation by hydrogen plasma in silicon and germanium

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