Conductivitytype inversion following lowenergy hydrogen implantationTianQun Zhou, Zbigniew Radzimski, Bijoy Patnaik, George A. Rozgonyi, and Bhushan Sopori Citation: Applied Physics Letters 58, 1985 (1991); doi: 10.1063/1.105040 View online: http://dx.doi.org/10.1063/1.105040 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/58/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Hydrogen passivation of silicon carbide by low-energy ion implantation Appl. Phys. Lett. 73, 945 (1998); 10.1063/1.122047 A modified broad beam ion source for low-energy hydrogen implantation Rev. Sci. Instrum. 69, 1499 (1998); 10.1063/1.1148786 Conductivity-type anisotropy in molecular solids J. Appl. Phys. 81, 6804 (1997); 10.1063/1.365238 Surface conductivity of the single crystal aluminum oxide in vacuum and caesium vapors AIP Conf. Proc. 361, 1203 (1996); 10.1063/1.50063 Lowenergy hydrogen ion implantation in Schottky barrier control Appl. Phys. Lett. 47, 426 (1985); 10.1063/1.96133

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Conductivity-type inversion following low-energy hydrogen implantation Tian-Qun Zhou, Zbigniew Radzimski, Bijoy Patnaik, and George A. Rozgonyi Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 2 7695 7916

Bhushan Sopori Solar Energy Research Institute, 1617 Cole Boulevard, Golden, Colorado 80401

(Received 30 November 1990; accepted for publication 5 February 1991)

A surface conductivity-type inversion has been observed following low-energy (400 eV), high-dose, hydrogen implantation of p-type silicon. Detailed structural, chemical, and electrical examination of the surface revealed that the inversion resulted from hydrogen forming n-type complexes with extended defects.

Hydrogenation has been widely studied in photovoltaic materials since it can passivate crystal defectsIT such as dislocations, grain boundaries, and deep level impurities.4+5 Hydrogen atoms are readily trapped at defect sites where electrons are needed for pairing, thereby reducing the num- ber of recombination centers. However, at the same time, hydrogenation can introduce some side effects such as sur- face degradation”* and electrical deactivation of the shal- low impurities. 9-1’ These effects have attracted much inter- est from both a fundamental and practical point of view. In this work it has been discovered that a conductivity-type inversion occurred at the surface of a p-type epitaxial sili- con wafer after exposure to a high dose hydrogen ion beam. Possible mechanisms for this phenomenon will be discussed in this letter.

The samples used in this study were p-type (100) epitaxial silicon layers deposited on a buried Si( Ge) hetero- epitaxial structure which contained two interfacial mis- fit dislocation planes at 4 and 6 pm depth.12 The epi- taxial layers had a resistivity of - 10 n cm. The samples were hydrogenated in a Kaufman ion source with a 400 eV hydrogen ion beam at 350°C for 60 min. Hydrogen concentration depth profiles were obtained using second- ary-ion mass spectroscopy (SIMS) and elastic recoil de- tection (ERD), while structural damage induced during hydrogen ion implantation was investigated using both transmission electron microscopy (TEM) and Rutherford backscattering/channeling (RBS). The effect of hydrogen on the epilayer resistance was measured using spreading resistance profiling (SRP) on an angle-polished sample. The resistance profile measurements were carried out on an ASR-100 Spreading Probe System. The electrical activity of the epilayer before and after hydrogenation was also investigated using scanning electron microscopy in the electron beam induced current mode (EBIWSEM). The sample was bevelled and polished in order to separate depth-dependent electron beam induced current (EBIC) contrast effects. EBIC/SEM Schottky contacts were fabri- cated by thermal evaporation of SIO-A-thick aluminum.

The SIMS depth profile of hydrogen, shown in Fig. 1, reveals that most of the hydrogen is accumulated near the surface within a depth of 250 nm. The hydrogen concen- tration gradually decreases with depth to-the instrumental background level. ERD of hydrogen by 2.2 MeV helium ions also verified the hydrogen accumulation near the sur-

face. Structural analysis of the irradiated surface indicated the presence of a heavily damaged region. A cross-section bright field TEM image, see Fig. 2, shows a uniform heavy damage up to a depth of 100 nm deep, while individual planar defects lying on { 111) habit planes extending as deep as 500 nm were also evident. The planar defects are similar to those identified by Johnson et aL6 and Jeng et al.’ as H-stabilized platelets, induced by the hydrogenation process. Rutherford backscattering of 2 MeV He + ions combined with channeling was also used to quantify the level of surface damage. To obtain the channeling condi- tions the (100) silicon sample was carefully aligned along (100) channeling axis and the spectrum of helium ions backscattered at 165” was recorded, see Fig. 3. Also shown is the random spectrum obtained by a continuous rotation along the azimuthal direction. The presence of an 85-nm- deep near-surface damage layer can be readily determined by comparing the random and channeling spectra. The scattering yield from this layer almost reaches that of the random level, indicating that the distribution of atoms in this layer is essentially random.

The presence of hydrogen and hydrogen-induced de- fects in a surface layer was expected to influence the elec- trical properties of this layer. The first approach in deter- mining the possible alteration was to obtain a survey EBIC/SEM image for a nonhydrogenated reference sam- ple, as shown in Fig. 4(a). Note that a charge collection

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FIG. 1. SIMS profile of hydrogen for the epitaxial (100) p-type silicon sample subjected to hydrogen ion implantation at 350°C with ion beam energy of 400 eV.

1985 Appl. Phys. Lett. 58 (18), 6 May 1991 0003-6951/91/181985-03$02.00 @ 1991 American institute of Physics 1985 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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FIG. 2. Bright field cross-section TEM image of hydrogen ion beam damaged layer. The arrows indicate the top surface.

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signal is only obtained from the space-charge region imme- diately below the Schottky diode. However, for the hydro- genated sample, see Fig. 4(b), a strong EBIC signal ex- tends far beyond the Schottky contact. The extended EBIC signal is intercepted by three - 6 ,um deep parallel trenches which were fabricated prior to hydrogenation. The ex- tended image area is attributed to the existence of an ex- tended electric field, associated with a space-charge region, which collects carriers generated by the electron beam in the same fashion as the Schottky contact. The electric field is attributed to a majority-carrier compensation through the formation of process-induced donor-type levels, and will be discussed in more detail below. In order to more precisely define the extent of this field below the surface, an EBIC image was taken on an angle-polished sample, see Fig. 5. In this case one half of the aluminum Schottky contact was located on the top surface, and the other half on the bevelled surface. Note that on the bevelled surface charge is collected only from the area covered by the Schottky contact, whereas an extended EBIC image is ob- served on the top flat surface. This indicates that a layer with inverted conductivity was removed during polishing. The image taken at 25 keV beam energy reveals the dislo-

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FIG. 3. Rutherford backscattering/channeling spectra for a hydrogenated epitaxial silicon sample.

FIG. 4. SEM/EBIC image of samples (a) before and (b) after surface hydrogenation.

cation networks located at interfaces lying -4 and 6 pm below the top surface. The networks are visible on the angle-polished surface down to the depth where they inter- cept the bevelled surface.

As a result of the surface inversion effect, a region of high resistance was expected. Spreading resistance depth profiles in Fig. 6(a) uere taken on a 1” 9’ bevelled surface with scanning steps of 2.5 pm (corresponding to 50 nm depth increments), and 1 pm in Fig. 6(b) (corresponding to 20 nm depth increments). As expected, a high resistivity layer, about 100 nm thick, was detected at depths less than 300 nm beneath the top surface. The out of range level of resistance indicates the presence of a space-charge region which we believe results from the formation of a thin n-type layer on top of the p-type wafer. This inverted layer has also been verified by using a thermoprobe to check the conductivity type of the surface, which was, in fact, n type. A similar result has also been noticed by Li’” who observed a high resistance layer which corresponded to the presence of hydrogen ion implantation damage.

The EBIC and SRP results indicate the presence of an electrical field at the near-surface region which was di- rectly subjected to the hydrogen ion beam. This field is associated with a space-charge region which exists between the inverted surface region and the p-type epilayer. Two possible mechanisms are likely to be responsible for the inversion of conductivity type at the surface of hydroge- nated sample. First, the inversion of p-type material could result from introducing donor-type impurities at a concen- tration exceeding the density of acceptors. Theoretically there is a possibility that hydrogen atoms would act as a donor, due to its chemical tendency of losing its electron to reach a low-energy state. This will happen when the con- centration of hydrogen is extremely high. However, as re- ported by many investrgators, hydrogen usually deactivates acceptors by forming a boron-hydrogen complex.’ This was concluded from the significant reduction of free-carrier concentration at the top surface which accompanied the

1986 Appl. Phys. Lett., Vol. 58, No. 18, 6 May 1991 i’hou et a/. 1986 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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0.5 mm

FIG. 5. SEM/EBIC image of bevelled sample after hydrogenation. The top surface was subjected to hydrogen ion beam prior to bevelling.

high concentration of hydrogen. In our case, the concen- tration of hydrogen at the top surface layer was more than one order of magnitude higher than that in the bulk. Therefore, we believe that acceptor compensation was an important issue here. However, hydrogen deactivation of acceptor may not be the only mechanism responsible for the inversion of the conductivity type.

It is possible that a large number of donor-type levels can also be introduced into the band gap by the structural defects induced during hydrogen ion implantation. Addi- tional levels could result from structural defect-hydrogen complexes, which are likely to be formed in the present experimental conditions. It is important to notice that the inversion effect due to hydrogenation may not have been found by other investigators because the surface damage was not as heavy as in our case. A similar inversion effect was reported by Chu et all4 for a F + -implanted and ion- damaged silicon sample. However, the authors only sug- gested that the fluorine acted as an acceptor in their n-type Si sample and ignored the important role damage might play in the surface inversion effect.

In conclusion, an unexpected conductivity-type inver- sion occurred during a wafer hydrogenation process which is widely used for passivation of electrically active defects and impurities. The low-energy hydrogen ion implantation tends to create the inversion effect both chemically and structurally. A key role in this inversion effect is played by a complex consisting of a structural defect decorated with hydrogen. Hydrogen deactivation is also a contributing factor. Finally, from a technological point of view it must be pointed out that a less destructive way of introducing hydrogen into silicon should be explored.

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The authors would like to thank Yu-dong Kim at Duke University for assistance with SRP measurements. The support of the Solar Energy Research Institute under grant DOEBERI XL-8-1 8097-2 is also gratefully ac- knowledged.

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1987 Appt. Phys. Lett., Vol. 58, No. 18, 6 May 1991 Zhou et al. 1987 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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