pss solidi status physica M. Trushin *, 1, O. Vyvenko **, 2, T. Mchedlidze 1, 3, M. Reiche 4, and M. Kittler 1, 3

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1 Phys. Status Solidi C 8, No. 4, (2011) / DOI /pss pss Eletrial haraterization of silion wafer bonding interfaes by means of voltage dependent light beam and eletron beam indued urrent and apaitane of Shottky diodes physia M. Trushin *, 1, O. Vyvenko **, 2, T. Mhedlidze 1, 3, M. Reihe 4, and M. Kittler 1, 3 1 Joint Lab IHP/BTU, Konrad-Wahsmann-Allee 1, Cottbus, Germany 2 V.A. Fok Institute of Physis, St. Petersburg State University, Ulyanovskaya 1, St. Petersburg, Russia 3 IHP Miroeletronis, Im Tehnologiepark 25, Frankfurt (Oder), Germany 4 Max Plank Institute of Mirostruture Physis, Weinberg 2, Halle, Germany urrent topis in solid state physis Reeived 5 July 2010, revised 9 August 2010, aepted 8 November 2010 Published online 28 February 2011 Keywords disloation network, Shottky diode, EBIC, oxide preipitates ** Corresponding author: trushmax@tu-ottbus.de, Phone: , Fax: ** vyvenko@gmail.om, Phone: , Fax: Shottky diodes prepared on silion bonded wafers with disloation network (DN) lying parallel to the sample surfae at the depth of ~2 μm were investigated by means of apaitane-voltage (CV), urrent-voltage (IV) measurements at 80 K under optial illumination. A model of the interpretation of CV and IV data is presented and disussed. It is shown that eletron-beam indued urrent (EBIC) mapping under the proper hosen forward bias voltages allows one to trae the inhomogeneities of the potential barrier at DN. The observed non-uniformity of the DN barrier heights was asribed to inhomogeneous distribution of small oxide preipitates lying at the disloation network plane. 1 Introdution Disloation networks (DN) produed by diret bonding of silion wafers, attrat now an inreased interest due to their potential appliation in miroeletronis as effetive light emitter and as perfet ondutor (for review see [1]) as well as a model objet to get a new information about the eletroni properties of disloations and grain boundaries (GB) in silion [2, 3]. The most perspetive strutures for the future appliations are based on a diode (Shottky or p-n juntion) with DN that is positioned parallel to the front sample surfae at the depths of μm, thus enabling an effetive eletron-injetion and/or modulation of potential by appliation of orresponding external bias voltage. One of the important parameters of suh devies is the profile of the eletri potential near the bonded interfae. Commonly used measurements of apaitane voltage (CV) and urrent-voltage (IV) harateristis an provide the information only about the averaged width of the spae harge region (SCR) and the barrier height of the struture. However, these methods are unable to speify the spatial barrier distribution at DN. Previously, a tehnique to define the barrier height along grain boundaries (GB) rossing the sample surfae in m-si, was proposed by Palm et al. [4]. This tehnique assumed to apply a small bias voltage aross the GB and immediate weak exitation with the eletron beam that gives a small loal lowering of the barrier providing predominant path for the majority arriers. This method ould not be used diretly for studying the samples with the DN going parallel to the surfae due to signifiantly more ompliated harater of the struture. Indeed, if the DN is spaed at the depths of μm below the sample surfae, then it will be either fully inside the depletion region of a Shottky diode or the spae harge regions (SCR) of the diode and DN an be spatially separated. In any ase, struture onsisting of two barriers

2 physia p s s 1372 M. Trushin et al.: Eletrial haraterization of silion wafer bonding interfaes arises, making it hallenging to derive of the inner parameters of the DN barrier alone and separately from the ommon Shottky diode barrier. In this work we performed detailed investigation of the Shottky diode - DN struture at low temperatures using CV and IV measurements under optial as well as eletron beam exitation. We will show that by appliation of proper hosen bias voltage a quantitative treatment of the obtained results an be done similar to the Palm s model [4], enabling in this way the lateral EBIC/LBIC mapping of the barrier heights of DN in the future devies. 2 Experimental details 2.1 Sample desription Boron doped Cz-Si wafers with initial resistivity ~15 Ωm and (100) surfae orientation were bonded hydrophobially with the small misorientation tilt and twist angles of 0.34 and 0.68, respetively. Details of the hydrophobi bonding proedure an be found elsewhere (see [5] and referenes therein). After the different thinning proedures the DN was loated at a depth of ~2 μm below the sample surfae. TEM investigations onfirmed the reation of the regular disloation network with higher density of srew disloations [2]. Samples reated by the same hydrophobi bonding proedure are known to ontain the high density of oxide preipitates (OP) lying at the bonding interfae, formed from the thin residual oxide surfae layer [6], as also for the onentri irle strutures (or bubbles ), learly visible both on the EBIC images (as brighter regions) and on photoluminesene (PL) maps (as darker regions) [6, 7]. These irle strutures were related with muh lower OP ontent inside their areas [6]. And it is obvious, that oxide preipitates, being positively harged in silion, an alter signifiantly the own height of DN barrier, thus the inhomogeneity in the oxide preipitates distribution will lead to the appearane of eletrial inhomogeneities at the bonding interfae. 2.2 Measurement equipments EBIC investigations were arried out in Zeiss EVO 40 sanning eletron mirosope using Stanford Researh System SR570 urrent amplifier to evaluate the sample eletrial ativity able to perform the EBIC measurements with different bias voltages applied. Mirosope is equipped with Gatan ooling stage operating at the temperatures down to 70 K. The urrent-/apaitane-voltage harateristis of the bonded sample were measured by a DL8000 deep-level transient spetrosopy (DLTS) system working at the apaitane testing frequeny of 1 MHz. Closed-yled ryostat allows to reah the temperatures well below 20 K. For measurements under illumination, ommon industrial light emitter diode with the wavelength of 630 nm (red light) was mounted inside the ryostat. For suh a wavelength the region of exess eletron-hole pairs generation is well below the depth of DN approximately at 3.5 μm. All the measurements under LED or eletron beam illumination were restrited by the ondition of low injetion level (δp<<p, where δp is the light generated majority arrier density). For the EBIC and IV/CV measurements, Shottky ontats were produed by evaporation of 100 nm-thik titanium dots on the hemially polished oxide free front sample surfae. Ohmi ontat was made by rubbing of InGa euteti on the rear sample side. 3 Experimental results 3.1 Current-voltage and apaitane-voltage measurements CV urves measured at 300 K and at 80 K in darkness and under red LED illumination are shown in Fig. 1a. While no onsiderable effet of illumination was deteted at 300 K, a onsiderable inreasing of sample apaitane under illumination was observed at low temperature 80 K. This result an be understood assuming the presene of depletion region along the bonded interfae indued by DN barrier. At room temperature the over-barrier urrent an but at 80 K annot follow the apaitane testing 1 MHz signal, as it was previously reported for analogous struture with quantum well inside [8]. The presene of the barrier is in aordane with the partiular shape of the dependene of the EBIC olletion effiieny on the aelerated voltage having a loal minimum of the olletion effiieny ourring at the beam energies of kv [7]. Another partiular feature of CV-urves is the kink at -0.8 V in the darkness and at -0.5 V under illumination. We believe that that features reflet the Fermi-level pinning when the Shottky-diode beomes open and the rest of the applied forward bias drops at the DN barrier. The IV harateristis measured at 80K at the same dot of Shottky ontat in darkness and under red LED illumination are shown in Fig. 1b. The drasti inrease of the diode urrent happens at a higher forward bias voltage (~-0.8 V) as it ould be expeted for the ommon Shottky diode (~ -0.5 V). This effet is definitely related with the presene of the DN in the diode ative region and was also observed in the other bonded samples with the smaller depths of DN, but not in the test diodes with the disloation network being ethed off. Under LED illumination, the urrent at reverse bias voltages is higher than the reverse urrent measured in darkness due to the flux of minority arriers eletrons, whih diffuse from the bulk region attrating by the eletri field in the SCR of Shottky diode. Above some forward bias voltage of ~ V the urrent rapidly dereases, exhibits the sign inversion at the threshold voltage U th = V and beomes larger than the forward urrent in the darkness. Just above this threshold voltage a plateau-like segment is learly reognizable whih orresponds to a saturation urrent of a reverse biased diode. One should noted a good orrespondene between IV and CV harateristi voltages

3 Contributed Artile Phys. Status Solidi C 8, No. 4 (2011) 1373 Figure 1 (Color online) CV-urves (upper plot) measured at 300 K and 80 K and IV urves (lower plot) measured at 80 K in darkness and under red LED illumination. Assignments of the urves are shown in the legends. - when the dark urrent starts to grow up, urrent inversion voltage under illumination and the kinks on CV urves. Obviously, both plateau-like urrent segment on the IV urve and the apaitane kink on the CV urve measured under illumination indiate, that the urrent flow for the forward bias voltages in the range of -0.5 V V is defined by the DN-barrier whih is stabilized by the pinning of the Fermi-level with a DN eletroni state. Majority arrier urrent appearing under illumination means the inversion of eletri filed diretion in the layer between the interfae and sample surfae. Evidently, that field inversion and majority arrier urrent as a onsequene, appear at the forward bias of approximately V, where the growing of reverse urrent begins. And at the threshold voltage of U th =-0.53 V the urrent of minority arriers (eletrons) beomes exatly equal to the urrent of majority arriers (holes) resulting in suh a way in the zero total urrent flowing in the struture. Referene diode with the ethed out disloation network did not reveal any inversion of urrent/eletri field diretion. Thus, results of the IV measurements under red LED illumination perfetly onfirm the previous observation [9] onerning the inversion of urrent diretion under eletron beam illumination. More detailed interpretation an be done with the help of the diagram presented in Fig. 2. Figure 2a shows the band diagram for the zero reverse bias (as also for the forward biases not exeeding -0.4 V). From the CV urve the SCR depth for 0 V was alulated as ~1.5 μm. For the typial depletion widths around DN barrier in order of ~1 μm, i. e. 0.5 μm from the right and left sides of DN, respetively, the depletion regions due to Shottky diode and DN barrier will be either spatially separated or slightly overlapped. For suh biases Shottky diode barrier ϕ S is higher than the barrier along the DN ϕ DN. Equivalent iruit of the struture ould be represent as two series apaitors, one due to Shottky diode barrier and the other - DN barrier. Illumination leads to the lowering of the DN barrier height, while leaving the Shottky barrier unhanged (DN barrier in darkness is shown by the dashed lines in Fig. 2a). This is due to the flux of illumination-generated minority eletrons j n whih are attrated to the Ti ontat by the eletri field. Some of them will be aptured by the GB s loalized states (j n ap ) and reombine with the trapped holes, thus reduing the DN barrier height ϕ DN. This will ause the inrease of sample apaitane, just as it ould be seen for the CV urve measured under illumination in Fig. 1a. With the inreasing of the forward bias voltage, Shottky and DN barriers firstly will beome equal, but finally barrier ϕ DN will be higher than ϕ S - this situation is shown in Fig. 2b. As soon as this has happened, the eletri field will hange it diretion in the region between the DN and the sample surfae, leading to the appearane of the majority arriers urrent. Holes from both sides of the GB, whih overome the barrier by thermal exitation, will refill the interfae states (j p ap ), restoring the barrier height. This indues a strong pinning of Fermi level at the defet states, whih is observed as a apaitane kink on the illuminated CV urves in Fig. 1a. As the onentration of the majority holes under low exitation ondition will be onsiderably higher than the onentration of light-indued minority eletrons, the total fration of the aptured majority arriers should not exeed of few perents [4, 10]. Hene, most of majority eletrons overoming the DN barrier ϕ DN, as also those generated inside the sample top layer will be aelerated by the inverse eletri field to give rise to the majority arrier urrent j p. This urrent will at first ompensate the urrent of the minority arriers sueeded to reah the Ti ontat, whih appears as the zero urrent point in Fig. 1b at U th V. At higher forward biases of V V the majority arrier urrent is the dominant one, as it is visible on the IV urve Fig. 1b, where inside this interval of forward bias voltages urrent is larger than the forward urrent in the darkness. Finally, at higher forward biases, the barrier along the disloation network will disappear ompletely and the forward urrent branhes on the IV urves measured in darkness and under illumination will join eah other.

4 physia p s s 1374 M. Trushin et al.: Eletrial haraterization of silion wafer bonding interfaes In general, this implies that by measuring the EBIC signal under proper hosen applied forward bias voltages the inhomogeneities of the DN barrier struture ould easily be traed. 3.2 EBIC measurements EBIC measurements were performed at 80 K with the beam aelerated voltage of 30 kv and mean beam urrent ~100 pa (thus satisfying the low-injetion level ondition) with the different values of applied bias. Examples of the EBIC images in low magnifiation are presented in Fig. 3. At zero bias voltage the EBIC urrent is determined exeptionally by the reombination rate of minority arriers. Hene, the EBIC image in Fig. 3a measured at zero bias reflets the distribution of the reombination ativity of interfaial eletroni states over the ontat area. The variations of the EBIC urrent are rather small, about 2-3 %, arguing the high uniformity of the reombination ativity along the ontat area. Perpendiular lines visible in the upper-right orner resemble the staking fault defets, whih were observed previously in similar bonded samples [5]. Only a faint ontrast at low temperature (80 K), vanishing at higher temperatures, aording to the model of reombination at disloations given in Ref. [11], mathes the Type II ontrast behaviour, whih is onneted with the intrinsi disloation reombination without external ontamination. EBIC images reorded with the forward biases higher or equal to the threshold voltage of ~ -0.5 V reveal the onsiderable differenes from that image obtained with zero bias voltage. First of all, the dark irular region or bubble appeared in the lower half of titanium ontat. It is learly visible on EBIC maps reorded with the forward biases of -0.5 V and -0.6 V and nearly ompletely merges with the bakground at higher forward bias of -0.7V, see Figs. 3b, 3 and 3d. Another remarkable feature is the appearane of highly untypial strutures looking like string of pearls with the bright beads stringed on the bright threads. The magnified EBIC image illustrating suh strings is presented in Fig. 4. No preferable diretion ould be assigned to these bright strings. Moreover, they are distributed non-uniformly over the ontat area: the most of them are spaed inside the bubble but some of them are also visible in the rightupward orner outside the bubble region. Other peuliar features of the EBIC image loated outside the bubble are the bright region in the left-down orner and the dark stripes in the upper part of the ontat. Figure 2 Energy bands diagram of the bonded sample a) at zero bias and b) at forward bias of about V. The higher barrier at the DN in the darkness is shown by the dashed urves. The eletri field inversion ours when the Shottky barrier beomes lower than the barrier along the disloation network. Fermi level is pinned by the deep interfae states. 4 Disussions and summary The dependenes of EBIC urrent on the applied bias voltage are plotted in Fig. 5 for different regions marked by retangles in Fig. 3 as also for the bright pearl region marked by short line in Fig. 4. The data Fig. 5 were alulated for every partiular applied bias voltage as the differene of the EBIC urrent averaged over the area inside the retangles shown in Fig. 3 and the dark urrent. At forward bias voltages between zero and -0,5V no signifiant EBIC hanges our for every seleted region of interest (ROI). The spatial variations of EBIC are also rather small. At forward bias U f = -0.6 V EBIC beomes strongly spatially dependent and dereases steeply down to nearly zero values and further towards the negative urrent values at U f = -0.7 V. Further inrease of forward bias transfers the Shottky-diode to an open state making impossible informative EBIC measurements. These results ould be well explained in the framework of the model suggested in the previous hapter. Due to the somewhat lower minority arrier generation rate under the eletron beam exitation as ompared with LED, it ould be expeted, that the threshold voltage orresponding to the equality of the majority and minority arrier urrents will be slightly higher under eletron beam exitation. So, at forward bias voltage of -0.6 V the EBIC in the dark

5 Contributed Artile Phys. Status Solidi C 8, No. 4 (2011) 1375 a) U f = 0V b) U f = 0,5V ) U f = 0,6V d) U f = 0,7V Figure 3 EBIC images of the bonded sample reorded with the different values of applied bias: a) zero bias voltage, b) forward bias voltage of U f = -0.5 V, ) forward bias voltage of U f = -0.6 V and d) forward bias voltage of U f = -0.7 V. All EBIC images were reorded with the same tuning parameters of the urrent amplifier. Currents in retangles on the image ) were averaged and then plotted against the applied bias voltages in Fig. 5. regions inside the bubble falls up to the nearly zero value, refleting in this way the equality of the majority and the minority urrents. At higher forward bias EBIC signal in all regions beome negative, indiating the prevalene of the majority arrier urrent. For the bright region lying outside the bubble as well as for the bright string of pearls strutures the higher (positive) EBIC at the applied forward bias of -0.6 V (see Fig. 5) implies the higher flux of minority arriers eletrons. Hene, it ould be onluded that the barrier height that defines the majority arrier urrent is also higher in these regions. In opposite, the barrier heights in the darker regions of irular area should be the lowest. An examination of EBIC-voltage dependenes gives the differene of barrier heights outside the bubble region as well as that at the bright pearls about 0.1 ev. This onlusion is in agreement with the previous observation of signifiantly lower OP density (two orders of magnitude!) within the bubbles than in the surrounding parts [6]. And the lower density of positively harged oxide preipitates implies the lower barrier height in p-type Si sample. The fat that in our sample this bubble was not observed on the ommon EBIC image reorded with zero bias (Fig. 3a) ould be explained by the onsiderably lower differene in the preipitates density within the bubble and outside. Moreover, in the previous study of hydrophobially bonded samples [7], in p-type Si sample the EBIC

6 physia p s s 1376 M. Trushin et al.: Eletrial haraterization of silion wafer bonding interfaes 20 µm Figure 4 EBIC piture at higher magnifiation showing in details the string of pearls defets. At the position of short line the dependene on applied bias was derived. Oval indiates the region ontaining the groups of small oxygen preipitates olonies. ontrast of irular areas (bubbles) was onsiderably weaker in omparison with the n-type samples. Obviously, additional TEM investigations are highly needed to larify this question. One also should note that beside the large pearls the bright spots with signifiantly smaller size and ontrast an be distinguished in EBIC map of Fig. 4 - for example in the region marked by oval. Their sizes are less than projeted ranges R of the inident eletrons (about 6 μm at 30 kv). We suggest that they are due to the olonies of the small oxygen preipitates that hinder the flow of majority arriers. This enhaned resolution of EBIC is due to signifiantly smaller diameter of generation volume at the depth of the DN, that in turn, is muh less than R. Interestingly, that these olonies ould be visible in details only inside the bubble region sine outside due to the higher OP density it is impossible to disern the details. In onlusion, we have reported the results of our investigation of Shottky diode - DN double barrier struture at low temperatures using CV and IV measurements under optial and eletron beam exitation. Basis on the obtained experimental results, the energy band diagram explaining the observation of majority arrier urrent under illumination assuming that the urrent through the struture ould be solely ontrolled by the DN barrier for a ertain interval of forward bias voltages was proposed. The forward biased-ebic measurements were able to provide the detailed insights into the partiulars of eletrial inhomogenities of the DN barrier. Observed singularities of DN barrier height were related both with the oxide preipitates at the bonding interfae and with unknown formations - strings of pearls. Referenes [1] M. Kittler, X. Yu, T. Mhedlidze, T. Arguirov, O. Vyvenko, W. Seifert, M. Reihe, T. Wilhelm, M. Seibt, O. Voss, W. Fritzshe, and A. Wolff, Small 3, 964 (2007). [2] T. Mhedlidze, T. Wilhelm, T. Arguirov, M. Trushin, M. Reihe, and M. Kittler, Phys. Status Solidi C 6(8), 1817 (2009). [3] M. Trushin, O. Vyvenko, T. Mhedlidze, O. Kononhuk, and M. Kittler, Solid State Phenom , 283 (2010). [4] J. Palm, D. Steinbah, and H. Alexander, Mater. Si. Eng. B 24, 56 (1994). [5] M. Reihe, Mater. Si. Forum 590, 57 (2008). [6] T. Mhedlidze, T. Wilhelm, X. Yu, T. Arguirov, G. Jia, M. Reihe, and M. Kittler, Solid State Phenom , 503 (2008). [7] G. Jia, W. Seifert, T. Mhedlidze, T. Arguirov, M. Kittler, T. Wilhelm, and M. Reihe, Superlatties Mirostrut. 45, 314 (2009). [8] K. Shmalz, I.N. Yassievih, E.J. Collart, and D.J. Gravesteijn, Phys. Rev. B 54(23), (1996). [9] A. Bondarenko, O. Vyvenko, N. Bazlov, and O. Kononhuk, Physia B 404, 4608 (2009). [10] C.H. Seager, J. Appl. Phys. 52(6), 3960 (1981). [11] V. Kveder, M. Kittler, and W. Shröter, Phys. Rev. B 63, (2001). Figure 5 (Color online) EBIC as a funtion of the applied bias at the positions marked in Fig. 3 and Fig. 4. Assignments of the urves are shown in the legend.