Comparative study of evaporated germanium and silicon films by

Korimoto, Kagoshima 890, Japan. Received: November 11, 1992; In Final Form: February 4, 1993. Vibrational spectra of thin (0.2-1.0 nm) filmsof evapora...
0 downloads 0 Views 743KB Size
Download PDF
J. Phys. Chem. 1993,97, 4491-4496

4491

Comparative Study of Evaporated Germanium and Silicon Films by Inelastic Electron Tunneling Spectroscopy Morihide Higo,' Makoto Isobata,? and Satsuo Kamata Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, Korimoto, Kagoshima 890, Japan Received: November 1 1 , 1992; In Final Form: February 4, 1993

Vibrational spectra of thin (0.2-1 .Onm) films of evaporated germanium on alumina surfaces have been measured by inelastic electron tunneling spectroscopy. The tunneling spectra of the evaporated Ge films were compared with those of the Si films. The tunneling spectra were also compared with the vibrational spectra of surface species on crystalline Ge formed from reactions with atomic hydrogen, water, and oxygen measured by highresolution electron energy loss spectroscopy and multiple internal reflection infrared spectroscopy, and they showed the formation of germanium hydride species. Monohydride is predominantly formed in films prepared both in high vacuum (106 Torr) and in an atmosphere of H 2 0 ( lk5Torr). The tunneling spectra of the heated (125-150 "C)Ge films showed that the monohydride is stable in this temperature range. The vibrational frequencies for the hydride species in the films compare more closely to those on crystalline Ge surfaces than to those in amorphous Ge. The hydride is formed from the reaction with residual water molecules in the vacuum system during the evaporation and is considered to be present mainly on the surfaces and grain boundaries of the films to terminate the dangling bonds as in the case of the Si films. However, the peak intensity of the hydride species in the tunneling spectra of the Ge films is much weaker than that in the tunneling spectra of the Si films, indicating a very small number of the hydride species in the Ge films.

Introduction There is considerableinterest in investigation of thin imbedded semiconductor films. Because such films occurs frequently in many solid-statedevices in the electronics industry, it is important and interesting to analyzethe surfacesand interfacesand to obtain their microscopic information to improve their properties and control their production.l.2 Inelastic electron tunneling spectroscopy (IETS) is a unique technique using the phenomenon of electron tunneling through a metal/insulator/metal tunneling junction at cryogenic temperatures and reveals the vibrational spectrum of the thin film of the in~ulator.~ Recently, we have reported the tunneling spectra of thin (nanometer) imbedded Si, SiO, and Ge films of Al/A1203/X/Pb junctions (X = Si, SiO, Ge) and demonstrated that IETS is a powerful and valuable tool to investigate the very thin imbedded semiconductor films.4-8 The tunneling spectra of thin (1-2 nm) evaporated Si films of the junctions showed the formation of silicon hydride species (SiH,). Monohydride (SiH) is predominantly formed in the films prepared in high vacuum (10-6 Torr), whereas dihydride (SiH2) and trihydride (SiHJ are formed in the films prepared in an atmosphere of H20 (10-5 Monohydrides are preferentially formed in S i 0 and Ge films prepared both in high vacuum and in an atmosphere of H20.738The tunneling spectra of the Si films evaporated on heated (1 5G283 "C) alumina surfaces showed that the SiH is stable,while the SiH2 and SiH3are unstable at high temperature^.^!' The tunneling spectra of the S i 0 films evaporated at 283 "C in an atmosphere of H2O compare closely to those of the corresponding Si films.' The tunneling spectra of the heated (125-150 "C) Ge films showed that GeH is stable in this temperature range.8 The vibrational frequencies for the SiH, and GeH species in the films compare more closely to those on crystalline Si and Ge surfaces than to those in amorphous films."* We have concluded that these hydrides are formed from the reaction with residual water molecules in the vacuum system during theevaporationand they arepresent mainly on thesurfaces and grain boundaries of the films to terminate the dangling bonds.

' Present address: Kisarazu Labour Standards Inspection, Ministry of Labour, 2-4-14 Fujimi, Kisarazu, Chiba 292, Japan. 0022-365419312097-4491%04.00/0

The vibrational spectra of the surface species formed from reactions with atomic h y d r ~ g e n , ~water,10,12-16 -'~ and oxygen17-f8 on crystalline Ge and G e S i alloys were measured by highresolution electron energy loss spectroscopy (HREELS). The vibrational spectra of the surface species on crystalline Ge were also measured by multiple internal reflection (MIR) infrared spectros~opy.'~-~' These vibrational spectra give information on the surface reactions on Ge and Ge-Si alloys and the species formed from the reactions. The infrared and Raman spectra of the thick (micrometer) hydrogenated Ge films made by plasma decomposition of germane and reactive sputtering of Ge provide information on their microscopic structures.22-28A preferential attachment of hydrogen to Si rather than Ge has been reported in hydrogenated G e S i alloy^.^^-^^ These works have been summarized in review paper^.^^,^^ Since Si and Ge are typical elemental semiconductors, the analysis and investigation on the surface and interface properties of the thin films are important in the fields of both materials science and electronics. In this paper, we present the tunneling spectra of evaporated Ge films in various thicknesses (0.2-1.0 nm) together with comparison of those of the Si films. The microscopic structures of both evaporated films are discussed mainly in terms of the hydride formation. The tunneling spectra are also compared with vibrational spectra of surface species on Ge and GeH, species in hydrogenated Ge films. The tunneling spectra of the heated Ge films are given. Detailed analysis of these vibrational spectra provides important information on the reactions occurring on the Ge surfaces and the species formed on them. Comparison between the tunneling spectra of Si and Ge films gives also information on the difference in the properties of both thin films.

Experimental Section The junctions were prepared by an evaporation and oxidation technique with a vacuum evaporator as described before."8 Aluminum (99.999%) was evaporated from a resistively heated molybdenum boat onto a glass slide at a pressure of 10-6 Torr (1 Torr = 133.322Pa), and the surfaces were oxidized in an oxygen dc glow discharge. Germanium (Rare Metallic Co., 99.999%) 0 1993 American Chemical Society

Higo et al.

4492 The Journal of Physical Chemistry, Vol. 97, No. 17, 1993

was evaporated from the molybdenum boat onto the alumina surfaces for 1-5 s with a deposition rate of 0.2 nm/s at room temperature (about 20 "C).The thickness (0.2-1.0nm) of the films was estimated from this deposition rate and the time. Germanium was evaporated onto the alumina surfaces heated by a small ceramic heater in the temperature range 125-150 OC. The Ge films prepared at room temperature were also heated in high vacuum for about 3 min in this temperature range. The temperature was measured with thermocolor labels on the glass slide.69' Thejunctions (1 mm2)were completed with an evaporated Pb (99.999%)strip. Resistances for the measuredjunctions were in the range 20-720 Q. The tunneling spectrum was obtained at liquid helium temperature (4.2K) with the bridgecircuit and a lock-in a m ~ l i f i e r . 3 ~ - ~ ~ A 500-Hz ac modulation signal of 3-4mV,, and a slowly varying dc voltage were applied to the junction. The A1 electrode was negatively biased with respect to the Pb electrode. The peak width and position were independent of the ac modulation signal of 3-6mV,,. The peak positions were corrected by -1 meV (-8 cm-1) owing to the energy gap of the superconducting Pb electrode.3 The resolution of the apparatus and the accuracy of the peak position were estimated to be 2.5 meV (20 cm-I) and fl meV (k8cm-I), respectively. The top Pb electrode is superconducting below 7.2 K. The superconductivitycauses a zero bias anomaly in the tunneling 100 200 300 400 500ElmeV spectrum. Some tunnelingjunctions were kept above the liquid1000 2000 3000 4000 'u/cm-' helium level, and the spectra were measured above 7 K to quench Figure 1. Tunneling spectra of the evaporated Ge films (0.2-1.0 nm) the superconductivityof the Pb electrode. A normal sinusoidal prepared in high vacuum at 2.4-3.5 X lod Torr. Germanium was wave modulation signal at 0 eV and disappearance of the strong evaporated at room temperature (-20 "C).The positions for weak peaks peak due to the superconductivity at about 15 meV (120cm-I) are indicated by markers. The spectrum of the Al/AIzOs/Pb junction showed the quench of the supercond~ctivity.~~ is also shown. The spectra were measured at 4.2 K. I

,

I

,

,

,

I

,

I

Results Ce Films Evaporated in High Vacuum and HzO Atmosphere. The tunneling spectra of the evaporated Ge films in various thicknesses (0.2-1.0 nm) prepared in high vacuum (2.4-3.5 X 10-6Torr) are shown in Figure 1. The tunneling spectrum of the Al/Al203/Pb junction is also shown for comparison. These tunneling spectra, except for that of the thickness of 0.6 nm, are plotted almost on the same scale normalized to the peak at about 940cm-I. The spectrum of the A1/Al2O3/Ge/Pbjunction shows a sloping background which strongly depends on the thickness of the films; the junctions of thicker Ge films gave spectra with rapidly increasingand then decreasing backgrounds. Their peak positions areshowninTable1. These peak positions were obtained by averaging those of two to five spectra. The spectrum of a thin (0.2nm) Ge film has medium peaks at 2103 and 710 cm-I. On the other hand, the spectra of thicker (0.6-1.0 nm) films show weak, medium, and very strong peaks at about 2010,570,and 265 cm-1, respectively. These peak intensities, except for the peak at 265 cm-I, have no significant dependence on the film thickness. The tunneling spectrum of the evaporated Ge film (0.4nm) prepared in the atmosphere of HzO (5.0 X Torr) and that of the Ge film (0.8 nm) prepared in the atmosphere of D20 (2.3 X 10-5 Torr)8 are shown in Figure 2. The tunneling spectrum of the Ge film (0.6nm) prepared in high vacuum (2.4 X 10-6 Torr) is also shown for comparison. These spectra were measured at 4.2 K. The tunneling spectra of the films were measured at about 7 K to quench the superconductivity of the Pb electrode and to discriminate between a zero bias anomaly and the peak at about 265 cm-I. The low-energy region of the spectrum is shown in this figure. Though the peak width in the spectrum measured at about 7 K increases due to thermal smearing of the Fermi surface,a the peak has no effect of the zero bias anomaly. In the spectrum of the Ge film prepared in the atmosphere of HzO, the peaks at about 2010 and 570 cm-I become somewhat stronger. It has a weak peak at 702 cm-1. The peak positions

TABLE I: Vibrational Frequencies (cm-I) of Evaporated Ge Films in Various Thicknesses Prepared in High Vacuum (2.4-3.5 X 10-6 Torr) Measured by IETS (Ce Evaporated at Room Temperature). 0.2 nm

0.6 nm

3622 w, br 2920 vw, br 2103 m

3638 vw, br

944 s 710 m

0.8 nm

1.0 nm 3626 vw,br

2010 w, br 941 s 708 vw 565 m

2006 w, br 940 s 692 vw 568 m

2007 w, br 943 s 716 w 574 m

269 vs

270 vs

265 vs

288 m

vs, very strong; s, strong; m, medium; w, weak; vw, very weak; br, broad.

in the spectra of the Ge films prepared in the atmosphere of HzO and D2O are shown in Table I1 with their assignmentsa8 The peak positions in the spectra of the thin (0.2nm) Ge films and the averaged peak positions in the spectra of the Ge films (0.61 .O nm) prepared in high vacuum are also shown in this table. These assignments were made with reference to those for the surface species on crystalline Ge formed from the reaction with atomic hydrogen," water,l5 and oxygenI7J8studied by highresolution electron energy loss spectroscopy (HREELS). The assignments for the vibrational spectra of the GeH, species on Ge surfaces35and in the hydrogenated amorphous germanium (a-Ge:H) f i l m P were also considered. The two peaks at about 2010 and 570 cm-I in the tunneling spectra of the evaporated Ge films (0.4-1 .O nm) prepared both in high vacuum and in the atmosphere of H20 are assigned to the stretching (v) and the bending ( p ) modes of GeH, respectively, because of the close similarity of their frequenciesand the stability at high temperatures as discussed below. There are no peaks due to GeH2or GeH3 species in the frequency range 660-880 cm-I. The very strong peaks at about 265 cm-I in the spectra of both types of junctions are due to v(Ge-Ge). The medium peak at

Comparative Study of Ge and Si Films by IETS

r

Ge

The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 4493 with the residual water molecules in the vacuum system and that the surfaces covered with the GeH species are inert to these molecules. Ge Films Prepared in High Temperatures. The tunneling spectra of Ge films evaporated on heated A1203 surfaces in the temperature range 125-150 OC in high vacuum were measured. The Ge films evaporated at room temperature were also heated in high vacuum in this temperature range. The typical spectra of the heated (125 and 150 "C) Ge films are shown in Figure 3 with that of the film without the heat treatment. These tunneling spectra show that the peak positions and their relative intensities are independent of the heat treatment in this temperature range. Because monohydride (GeH) is stable in this temperature range and dihydride (GeH2) is unstable at high temperatures?JIJ the presence of GeH seems to be most important in these heated Ge films. The trihydride (GeH3) species are formed especially at very low These tunneling spectra show that the GeH species is predominantly formed in the evaporated Ge films in the temperature range 20-150 OC.

Discussion Surface Reactions of Crystalline Ge. The surface reactions of atomic hydrogen on crystalline Gel] and Ge-Si alloys ( G ~ , S ~ I - , ) ~have J ~ J ~been studied by high-resolution electron energy loss spectroscopy (HREELS). The reaction of atomic hydrogen with Ge( 100) leads to the formation of surface GeH, 0 1000 2000 4000 clcm-' species where the hydrogen atoms are considered to saturate the Figure 2. Tunneling spectra of the evaporated Ge films prepared at room dangling bonds on the surfaces. The surface GeH has the temperature in the atmosphere of HzO at 5.0 X 10-5 Torr (0.4 nm) and D20 at 2.3 X Torr (0.8 nm). The new peaks caused by the GeD vibrational ( v ) frequency at 1980 cm-I and the bending ( p ) mode species are indicated by markers. The spectrum of the Ge film (0.6 nm) at 530 cm-I. GeH2 has the same vibrational frequency and the prepared at room temperature in high vacuum (2.4 X 1W Torr) is also scissoring (y) mode at 850 cm-I. At room temperature and low shown. These spectra were recorded at 4.2 K. The low-energy region surface coverage GeH is formed first, and then GeH2 is formed of the spectrum measured at about 7 K to quench the superconductivity gradually as the coverage increases up to saturation. The thermal of the Pb electrode is shown. evolution of GeH2 into GeH begins at about 150 OC. The surface GeH2 species decomposes completely at about 300 OC, while 2103 cm-1 in the tunneling spectrum of the thin (0.2 nm) Ge film GeH remains at this temperature.Il is assigned to v(0,Ge-H) (GeH units back-bonded to one or The adsorption and reaction of atomic hydrogen on crystalline more oxygen atoms) from a considerationof the higher frequency Ge have also been studied by multiple internal reflection (MIR) and the thickness of the films as discussed below. The tunneling infrared spectroscopy.2°J Chaba120 has measured MIR-IR spectrum of the thin (0.2 nm) film has no peak due to p(GeH). spectra for the hydrogen-saturated Ge( 100) surface at room The weak peaks near 700 cm-I are probably caused by v ( G e temperature and found two vibrational modes of GeH parallel OH). The peaks at about 3630 and 940 cm-I are caused by u(A1OH) and v(Al0) of the A1203 insulator films, r e s p e c t i ~ e l y . ~ ~ - ~ and ~ perpendicular to the surface at 1979 and 1991 cm-I, respectively. The spectra exhibited no GeH2 formation. The However, the former has some contribution of v(Ge0H). In lack of GeH2 formation may suggest that the Ge-Ge dimer on general, the intensity of the peak of u(A1OH) is very weak in this the Ge(100) surface is stable. Crowell and Lu21 have studied type of tunneling junction. The weak intensity of the vibrational adsorption and reactions of atomic hydrogen, digermane, and mode of the OH groups has also been observed in the spectra of disilane on Ge(ll1) by MIR-IR. The adsorption of atomic the AI/A1203/Si/Pb and Al/A1203/SiO/Pb junctions."' The hydrogen on Ge( 1 11) leads to the production of GeH3, GeH2, shoulder at 1907 cm-I in the spectrum of the Ge film prepared and GeH at low temperatures (400 K). AlH in the insulator film.37-39The A1 phonon mode at about 290 Papagno et al.lSJ6 have investigated the interaction of HzO cm-I is superimposed on the very strong peak at about 265 cm-I. with Ge( 100) by HREELS. They found that no dissociation of The spectra of the thin (0.2 nm) film and that prepared in the H20 takes place at room temperature even for high exposures, atmosphere of H2O show weak peaks due to hydrocarbon while H20 adsorbs molecularly together with H and OH species contamination at about 2920 cm-I. The tunneling spectra show even for very low exposures at liquid nitrogen temperature. The that the Ge films prepared both in high vacuum (10-6 Torr) and exposure of Ge( 100) to H 2 0 at liquid nitrogen temperature and in the atmosphere of H2O ( l e 5Torr) at room temperature have then warming to room temperature give only the surface H and predominantly the GeH species. Few GeOH groups are present OH species. This dissociative adsorption of H20 on Ge( 100) has in the evaporated Ge films. also reported by Chabal and Christman using MIR-IR.19 The To ensure the presence of the GeH species and to clarify the HREEL spectra of Ge,Si~-,(100) exposed to H20 have also mechanism for their formation, the Ge films were prepared in an revealed that H20 adsorbs dissociatively, producing surface atmosphere of D20. Though the peak intensity is weak, the hydride and hydroxide~pecies.lOJ~-'~J~ The hydroxyl and hydride tunneling spectrum prepared in the atmosphere of D2O shows group were assumed to form a bond on a different Ge atom to two new peaks at 1474 and 415 cm-1.8 These peaks lie at the saturate the dangling bonds.I3 expected positions (2008/1474 = 1.36, 571/415 = 1.38) and clearly show the formation of the GeD species. The spectra of The GeOH and GeH groups are stable at room temperature; the films doped with H20, CH3OH, and NH3 after completion however, the GeOH group decomposes gradually into germanium showed no change. These findings show that the hydrides are oxide and additional germanium hydride at high temperatures formed mainly during the evaporation of Ge from the reaction (250-350 0C).10*12-14J6 Thedissociationenergy of the 0-H group 100.

200

300

400 3000

500 ElmeV

4494 The Journal of Physical Chemistry, Vol. 97, No. 17, 1993

Higo et al.

TABLE II: Vibrational Frequeocie-8 (cm-I) and Mode Assignments for Evaporated Ge Films Prepared in High Vacuum and H20 (DP)Atmosphere Measured by IETS (Ge Evaporated at Room Temperature)' vacuum (2.4-3.5X 10" Torr) H20 atmosphereb D20 atmosphereb (5.0X lt5Torr) 0.4nm 3635 m, br 2915 w, br

0.6-1 .O nm

0.2 nm 3622 w, br 2920 vw, br 2103 m

3632 vw, br

944 s 710 m

2008 w, br

2006 m, br 1907 sh

941 s 705 vw 569 m

940 s 702 w 567 m

(2.3 X Torr) 0.8nm 3631 w, br

2008 w, br 1474 vw, br 942 s 126 vw 571 m 415 w

288 m 268 vs

262 vs

266 vs

assignment

u(A1OH) and v(Ge0H) v(CH) (contamination) u(0,Ge-H) u(GeH) u(A1H) u(GeD) u(Al0) u(Ge-OH)

PWH) p(GeD) A1 phonon u(Ge4ie)

vs, very strong; s, strong; m, medium; w, weak; vw, very weak; br, broad; sh, shoulder; v, stretch; p, bend or rock. Reference 8.

Ge

150°C

Vacuum

/\

1 -

/

A+-----

,

1000

,

2000

I

,

3000

,

,

4000 'v/cm-'

Figure 3. Tunneling spectra of the heated Ge films (0.6nm) evaporated atroomtemperaturein highvacuumat 2.7-3.7 X 1t6Torr. Thespectrum of the Ge film (0.8nm) prepared at room temperature in high vacuum (3.0X Torr) is also shown. The spectra were measured at 4.2 K.

is too large (4.7 eV) to allow for the dissociation to be explained simply on the basis of thermal dissociation of the 0-H bond at these temperatures.I3 Therefore, it is obvious that the driving force is not 0-H dissociation but rather Ge-O or Ge-H bond formation. The HREEL spectra of heated Ge,Sil,( 100) after chemisorption of H20 showed u(0,Ge-H) (GeH units backbonded to one or more oxygen atoms) peaks. The frequencies of the 0,Ge-H units vary between 2040 and 2 155cm-I, depending on the number of the adjacently bound oxygen atoms.10 This upward shift is caused by a shortening of the Ge-H bond length and increasing force constant due to the increase of the s character in the hydrogen-directed germanium orbital."J~I2~14 The interaction of oxygen with crystalline Ge and Ar+ sputteretched (amorphized) Ge has been investigated by HREELS.I7J8 Exposure of Ge( 1 11) to lo4L of 0 2 at 300 K results in a spectrum with three loss peaks at about 270, 500, and 860 cm-1. These peaks were assigned to the rocking (a), bending (y), and asymmetric stretching (uas) modes of the G e O - G e group, respectively, and suggest that oxygen chemisorbs dissociatively in the bridge configuration.I8 The frequency of the v,,(GeOGe)

mode shifts from 790 cm-I at 10 L to 860 cm-1 at lo4 L. This upward shift with increasing exposure is attributed to an increase in the average bond angle of the Ge-O-Ge bridges.l8 Vibrational Spectra of Hydrogenated Amorphous Ge and GeSi Alloys. The infrared and Raman spectra of a-Ge:H films made by plasma decompositionof germane and reactive sputtering of Ge have three groups of They correspond to stretchingvibrationsofGeH, at 1880-2060~m-~, deformational vibrations of GeH2 and GeH3 at 750-830 cm-I, and bending vibrationsat about 570cm-I. Besides thesemodes, thevibrations of the network of Ge-Ge bonds are observed below 300 cm-I. The u(GeH) band in the a-Ge:H films exists near 1890 cm-I, and the frequency is lower than those of u(GeH) on the crystalline Ge surfaces. This large shift (-100 cm-I) is due to the electrostatic interaction of the G e H bonds with the surrounding high dielectric medium and occurs when the Ge-H bonds are imbedded in the bulk of the solid.35 For the frequencies of u(GeH2) (- 1980 cm-I) and u(GeH3) (2060 cm-I), the shifts are small because GeH2 and GeH3 are considered to exist at the inner surfaces of voids or large cavities.35 The scissoring ( 7 ) mode of GeH2 exists at 830 cm-I, while the split bands at 755820 cm-' would be due to interaction between neighboring GeH2 groups. The bond bending (6, and tias)structure of GeH3 appears at 770 and 830 cm-I, respectively. The vibrational spectra of a-Ge,Si,-l:H films grown by glow discharge decomposition of mixture of germane and silane29-31 and reactive sputtering of Ge and Si targets in a hydrogencontaining ambient32-34have the u(GeH,) and u(SiH,) bands near 1900 and 2000 cm-I, respectively. These bands were deconvoluted to give the relative absorption caused by the GeH, and SiH, groups, assuming the matrix elements are not changed by the local environment and correcting for the relative Ge/Si contents. The obtained relative absorption of these bands shows the preference for H to bond to Si rather than to Ge. Though the preferential ratio depends on the particular deposition technology, it is as large as 10 in the Ge-rich alloys.29JO The formation of the Ge dangling bonds in the alloys has been believed to be enhanced by the preferentialattachment of H to Si. Electron spin resonance (ESR) measurements on a-Ge&-l :H prepared by glow discharge decomposition of GeH4 and SiH4 indicate that the number of Ge dangling bonds is larger than that of Si dangling bonds by a factor of 4-15.3I The poorer photoelectric properties of a-Ge,Si,l:H and a-Ge:H may be attributed to this poorer compensation of dangling-bond defects in these films. A heterogeneous structural model of a - G e S L l : H which supposes the growth of both a-Si:H and a-Ge:H in islands has been proposed to explain these result^.^^-^^ It was supposed that the H attachment is stronger in Si so that Si-H bonds on the surfaces of islands are preferred to reconstructed S i S i bonds, while the H attachment is weaker in Ge so that reconstructed Ge-Ge bonds often win out on the surfaces. The HREELS study

Comparative Study of Ge and Si Films by IETS

The Journal of Physical Chemistry, Vol. 97, No. 17. 1993 4495

of a-Ge,Si,-l:H prepared by glow discharge suggested that the preferential attachment of H to Si exists not only in the bulk but also on the surface of the films.41 Hydride Formation of Evaporated Ge Films. The two peaks at about 2010 and 570 cm-1 in the tunneling spectra of the evaporated Ge films (0.4-1 .O nm) prepared both in high vacuum and in the atmosphere of H20 are assigned to v(GeH) and p(GeH), respectively, because of the close similarity of their frequencies to those of the GeH species on Ge surfaces1 and the stability a t high temperatures. There are no peaks due to GeH2 or GeH3species in the frequency range 660-880 cm-I. The very strong peaks at about 265 cm-I in the spectra of both types of junctions are due to v(Ge-Ge). The medium peak a t 2103 cm-l in the spectrum of the thin (0.2 nm) film is assigned to v(0,Ge-H), because the film is so thin that oxygen atoms on the A1203surface can interact with the GeH units in the film. This frequency shift (-100 cm-I) is considered to be due to the electronegativity of the adjacently bound oxygen atoms. Since the peak position of v((O2Ge)Ge-H) on the Ge surface is 2100 cm-l,Io the contribution of the (02Ge)Ge-H species seems to be most important. This peak suggests that the sensitivity of IETS is high enough to detect the GeH species in the films of the order of monolayer of Ge atoms on A1203. The spectrum of the thin film shows neither the peaks due to p(GeH) nor those due to v(Ge-Ge). The tunneling spectra show that the Ge films prepared both in high vacuum ( Torr) and in the atmosphere of H2O (10-5 Torr) at room temperature have predominantly the GeH species. The tunneling spectra of the evaporated Ge films clearly show the GeH species; however, the presence of the GeOH species is not clear. Some surface reaction similar to that of crystalline Ge with H20 at high temperaturesseems to occur. The transmission infrared spectra of the evaporated Ge films on KBr plates showed the bands of v(GeH) (1960-1980 cm-I) and v(Ge-OH) (675and 770 cm-I) due to dissociation of water on the ~urfaces.~ZThe band of GeH was stable in vacuum; however, it was observed to decline in intensity after exposure of water vapor for few hours. With increasing exposure, the band at 675 cm-I grew steadily for several hours with the appearance of two oxygen-related bands at 825-875 and 730 cm-I. Though the existence of the GeH species in the films just after evaporation is not clear, the band position of v(GeH) is very close to the peak position of the tunneling spectra of the Ge films. The infrared spectra showed that the GeH species is stable with exposure of water vapor for at least a few hours, and therefore, it prevents the rapid hydration and oxidation of the surfaces. We conclude that the GeH species are formed from the reaction with residual H 2 0 molecules in the vacuum system during the evaporation as in the case of the evaporated Si films.67 When Ge is evaporated, it can be assumed that dangling bonds on the surfaces of the hot Ge clusters react readily with the HzO molecules in the vacuum system to give the GeH and GeOH groups before the clusters arrive at the alumina substrate surfaces. However, the GeOH groups on the hot Ge clusters are unstable, and then they dissociate to form additional GeH and GeOGe groups. The oxygen atoms are supposed to be inserted into the Ge-Ge bonds rather than being attached to the surface dangling bonds. The driving force of this reaction is considered to be the Ge-H or Ge-0 bond formation, and the GeH and GeOGe groups form bonds on different Ge atoms as in the case of the crystalline Ge.13 The peaks of the GeOGe group are, however, considered to be too weak to observe in the tunneling spectra of the Ge films.'7 The GeH species exists mainly on the surfaces or grain boundaries of the evaporated germanium films because of the peak position (- 2010cm-I) of the v(GeH) mode. Therefore, their microscopic structures are expected to be similar to those of the hydrogenated microcrystalline Ge (pc-Ge:H) films rather than those of the a-Ge:H films. The GeH species is considered to passivate the

I

IVaCuum

1p20,21

0

0

100

1000

200

300 2000

400

3000

500E/meV 4000 'v/cm-l

Figure 4. Tunneling spectra of the evaporated Ge (0.6 nm) and Si (- 1.O nm) films prepared in high vacuum (2.1-2.4 X 1V Torr). The spectra were obtained at 4.2 K. surfaces against rapid hydration and oxidation due to the covalent character of the Ge-H bond~.~3 The tunneling spectra showed that the surfaces of the Ge films with the GeH species are inert to H 2 0 , CH30H, and NH3. Comparison of Evaporated Ge and Si Films. Comparison between the tunneling spectra of the Ge and Si films (0.6 and 1 .O nm, respectively) prepared in high vacuum (2X 1odTorr) is shown in Figure 4. The strong peak a t 2014 cm-I and the medium peak at 622cm-' in the tunneling spectrum of the Si film have been assigned to v(SiH) and p(SiH), respectively. The medium peak at 474 cm-I is mainly caused by v(Si4i). These peaks have small contribution of the SiH2 species. The y(SiH2) mode appears at 895 cm-I. The peaks due to the SiH2 species become strong when the Si films are prepared in an atmosphere of H20.637The intensity of v(GeH) in the tunneling spectrum of the Ge film is very weak as compared with that in the tunneling spectrum of the Si film. On the other hand, the v(Ge-Ge) peak is sharper and stronger than the v(Si4i) peak. The tunneling spectra of the Ge films have no peaks due to GeH2or GeH3, and also they are almost independent of the atmosphere of H2O. It is assumed that the matrix elements or the cross sections for the vSiH, and vGeH modes in the tunneling spectra of the Si and Ge films are equal as in the case of the infrared spectra of the a-Ge,Sil-,:H f i l m ~ . ~The ~ J very ~ weak intensity of v(GeH) indicates a small number of the GeH species in the Ge films. The number of the GeH species is estimated to be one order of magnitude smaller than that of the SiH, species in the evaporated Si films. These findings suggest the poorer compensation of dangling bonds by hydrogen atoms and predominant formation of Ge-Ge bonds in the evaporated Ge films. The similar structural model of a-Ge,Sil-x:H29J0 is expected to be applied to the very thin ( 1 nm) evaporated Si and Ge films; the H attachment is stronger in Si so that Si-H bonds on the surfacesof the evaporated films are preferred to S i s i bonds, while the H attachment is weaker in Ge so that G f f i e bonds win out on the surfaces. The evaporated Si and Ge films are in an amorphous state and have many deficiencies (dangling bonds) These dangling bonds areclosely related to the properties of the films. From the present study, it is clarified that some of the dangling bonds in the evaporated Ge films are reduced by the reaction with residual water molecules to form the surface GeH species as in the case of the evaporated Si films."' Silicon and germanium are typical elemental semiconductors; their electronegativities and atomic

-

-

4496 The Journal of Physical Chemistry, Vol. 97, No. 17, 1993

radii are very similar.43 However, it is shown that the structures and properties of the evaporated thin films are very different in terms of hydride formation. Conclusion. IETS clarified the surface chemical properties of the very thin films of evaporated Si and Ge. Both films have the hydride species formed from the reaction with HzO molecules in the vacuum system during the evaporation, and the species is considered to be present mainly on the surfaces and grain boundaries of the films to terminate the dangling bonds. However, the number of hydride species in the Ge films is much smaller than that in the Si films. The present study is the first attempt to compare with the tunneling spectra of Ge and Si films and to study semiconductorfilms in detail. This two-pronged approach provides important information on surface chemical properties of the very thin semiconductor films.

Acknowledgment. We thank Professor S.Ikeda of Ryukoku University for his encouragement. The present study was partially supported by Iketani Science and Technology Foundation and the Light Metal Educational Foundation. References and Notes (1) Joannopoulos, J . D.; Lucovsky, G. The Physics of Hydrogenated Amorphous Silicon; Springer: Berlin, 1984. (2) Pankove, J. I. Semiconductors unddemimetals;Academic: Orlando, 1984. (3) Hansma, P. K. TunnelingSpectroscopy; Plenum: New York, 1982. (4) Higo, M.; Hayashi, H.; Kamata, S.Appl. Surf.Sci. 1988, 32, 338. (5) Higo, M.; Nishino, K.; Hayashi, H.; Kamata, S . Chem. Lett. 1988, 1363. (6) Higo, M.; Nishino, K.; Kamata, S.Appl. SurJ Sci. 1991, 51, 61. (7) Higo, M.; Nishino, K.; Kamata, S. J. Phys. Chem. 1992, 96, 1848. (8) Higo, M.; Isobata, M.; Kamata, S.Appl. Surf. Sei. 1992, 59, 219. (9) Schaefer, J. A.; Broughton, J. Q.; Bean, J. C.; Farrell, H. H. Phys. Rev. 1986, 833, 2999. (10) Schaefer, J . A. Surf. Sei. 1986, 178, 90. (11) Papagno, L.; Shen, X.Y.; Anderson, J.; Spagnolo, G. S.;Lapeyre, G. J. Phys. Rev. 1986, 834, 7188. (12) Schaefer, J. A. Surf.Sci. 1987, 1891190, 127. (13) Broughton, J . Q.;Schaefer, J. A,; Bean, J . C.; Farrell, H. H. Phys. Rev. 1986, 833, 6841. (14) Schaefer,J.A.;Broughton,J.Q.;Bean, J.C.;Farrell,H.H.J.Elecrron Spectrosc. Relat. Phenom. 1986, 39, 127. (15) Papagno, L.; Caputi, L. S.;Frankel, D.; Chen, Y.; Lapeyre, G. J. Surf. Sci. 1987, 1891190, 199.

Higo et al. (16) Papagno, L.; Caputi, L. S.;Anderson, J.; Lapeyre, G. J. Phys. Rev. 1989.840. 8443. (17) Shinar, R.; Entringer, A. G.; Shinar, J.; Wu, HA.; Shanks, H. R. J. Vac. Sci. Technol. 1989, A7, 2998. (18) Entringer, A. G.; Shinar, R.; Shanks, H. R. Surf. Sci. 1990,234,221, (19) Chabal, Y. J.; Christman, S. B. Phys. Rev. 1984, 829, 6974. (20) Chabal, Y. J. Surf. Sci. 1986, 168, 594. (21) Crowell, J. E.; Lu, G. J. Electron Specrrosc. Relut. Phenom. 1990, 54/55, 1045. (22) Bermejo, D.; Cardona, M. J. Non-Cryst. Solids 1979, 32, 405. (23) Bermejo, D.; Cardona, M. J. Non-Cryst. Solids 1979, 32, 421. (24) Rudder, R. A.; Cook, J . W.; Lucovsky, G. Appl. Phys. Lett. 1983, 43, 871. (25) Rudder, R. A.; Cook, J. W.; Schetzina, J. F.; Lucovsky, G. J . Vac. Sci. Technol. 1984, AZ, 326. (26) Lucovsky, G.; Chao, S. S.;Yang, J.; Tyler, J . E.; Czubatyj, W. J. Non-Crvsl. Solids 1984, 66. 99. (27).Ross, R. C.; Chao, S. S.;Tyler, J . E.; Czubatyj, W.; Lucovsky, G. J. Vac. Sci. Technol. 1985, A3, 958. (28) Lucovsky,G.;Chao,S.S.;Yang,J.;Tyler,J. E.;Ross,R. C.;Czubatyj, W. Phys. Rev. 1985,831, 2190. (29) Paul, D. K.; von Roedern, B.;Oguz, S.;Blake, J.; Paul, W. J . Phys. SOC.Jon. 1980, 49 (SUDD~. Ah 1261. (30) Paul, W.;Paul,'D.K.;vonRoedern,B.;Blake, J.;Oguz,S.Phys.Rev. Lett. 1981, 46, 1016. (31) Morimoto, A,; Miura, T.;Kumeda. M.; Shimizu, T.Jpn. J . Appl. Phvs. 1981. 20. L833. '(32) Weisz,'S. Z.; Gomez, M.; Muir, J. A.; Resto, 0.;Perez, R.; Goldstein, Y.; Abeles, B. Appl. Phys. Lett. 1984, 44, 634. (33) Rudder, R. A.; Cook, J. W.; Lucovsky, G. Appl. Phys. Lett. 1984, 45, 887. (34) Rudder, R. A.; Cook, J. W.; Lucovsky, G. J. Vac.Sci. Technol. 1985, A3, 567. (35) Cardona, M. Phys. Status Solidi 8 1983, 118, 463. (36) Lucovsky, G. J. Non-Cryst. Solids 1985, 76, 173. (37) Higo, M.; Mizutaru, S.;Kamata, S.Bull. Chem. SOC.Jpn. 1985,58, 2960. (38) Higo, M.; Mizutaru, S.;Kamata, S.Bull. Chem. SOC.Jpn. 1989,62, 1829. (39) Higo, M.; Kamata, S.J. Phys. Chem. 1990, 94, 8709. (40) Hipps, K. W.; Peter, S.L. J . Phys. Chem. 1989, 93, 5717. (41) Sasaki, H.; Deguchi, M.;Sato, K.; Aiga, M. J . Nom-Crysr. Solids 1989, 114, 672. (42) Metcalfe, A.; Shankar, S.U.J . Chem. Soc., Faraduy Trans. 1 1979, 75, 962. (43) Pauline, L. The Nature of the Chemical Bond and the Structure of Molecules and Crystals; Cornell University: Ithaca, NY, 1960; Chapters 3 and 7. (44) Brodsky, M. H.; Kaplan, D.; Ziegler, J. F. Appl. Phys. Lett. 1972, 21, 305.