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Approach to novel crystalline and amorphous oxide materials for optoelectronics by ion implantation
Materials Science and Engineering BJI (1996) 39 ~3.5
ELSEVIER
Approach
to novel crystalline optoelectronics
and amorphous oxide materials for by ion implantation’
Abstract This paper briefly reviews our recent works on the creation of novel oxide materials for optoelectronics b!, ion implantation. The topics covered are transparent crystalline and amorphous oxides with metallic conduction, fast proton-conducting oxide glasses and nonlinear optical material based on nanosized metal clusters. Kc8~~nwtO:Optoelectronics: Ion implantation: Crystalline amorphous oxidc materials: Proton-conducting
2. Creation oxides with
1. Introduction
Ion implantation is a processing technique to significantly modify surface and near-surface properties of materials. Although this technique has been extensively applied to the creation of functional materials in the fields of semiconductors and metals, the primary research efforts on the application of ion implantation to wide band gap oxides appear to have been restricted on account of radiation effects on the oxides. We have examined chemical interaction of implanted ions in amorphous Si02 through elucidation of structural defects produced by implantation [1,2] and have applied the obtained conclusions to the creation of novel oxide materials with a specific optoelectronic function. This paper briefly reviews our recent works on novel crystalline and amorphous oxide materials for optoelectronic applications.
* Corresponding author. Current address: (a) above. Tel.: + 81 fax: + 81 459225169. ’ Submitted to the Proceedings of the 2nd International Sqmposwum of Oxide Electronics. 459145359;
0921-5107 Yh 515.00 8~‘ I996 PII SOYZI-5107(Y6)()1670-0
Elsevier Science S.A. All rights reserved
oxide glasses: Nonlinear optical @assrs
of transparent crystalline a metallic conduction
and amorphous
Control of electrical conduction in IV and III-V group semiconductors such as Si and GaAs by carrier doping is a great successof ion beam modification of materials. Although the yield of conduction carriers is very low in the as-implanted state because of amorphization of the implanted layer. the activation of dopants can be effectively achieved via recrystallization of the amorphous layers during subsequent thermal annealing or pulsed laser annealing [3]. Much interest has been directed to optically transparent and electrically conducting materials, which have various applications, such as transparent electrodes in liquid-crystal displays and solar cells. Most oxides have wide band gaps of > 3 eV, and hence are transparent in the visible wavelength range. but they are also electrical insulators. However. this is not always true when they contain dopants. because the electrical conductivity is expressed as a product of mobility 11and carrier concentration N. The mobility of electrons (or positive holes) is approximately proportional to the curvature (d’E’dli’) of the conduction band bottom (valence band top) in the energy
bond (E- li) diagram. Therefore. ewn insulating oxides can be converted into conductors if they have an appropriate energy band structure and can be succcssfdlq doped [4]. Iridium magnesium oxide (Mgln,O,, IMO) is a crystal Lvith a spincl-type structure :md a b:lnd gap of about 3.5 V. The bottom of the conduction b:lnd of this crystal is primarily composed of \wxnt 5s orbituls of In ions and short In -111 distances originating from the edge shored mode of the InO,, octahcdr;t [S]. This gi\w lilrge dispersion of the conduction band bottom via delocaliz:ition of orbitnls. Therefore. IMO may be regarded as a potentinl semiconductor. Spincl-type oxides ;lrc filvored for the present purposes for the following tH’o reasons. First. they contain many vacant tctrahcdtxl and octithedral cation sites, M,hich could be :l\fail;tblc for implanted ions. and second. they are resistant to radiation damage. MgAI,O, spinel is used as the primary wall of nuclear rextors and is reported to be stable in the crystalline state even after bombordmcnt up to 250 dpa [(1]. Li + [4] and H + [7] were chosen for implantation bccnuse they ba\,e small mass :md hcncc minimize the dmnage produced by nucleklr collision. The effect of implunt;ition \vith He + is examined for comparison. Fig. 1 shows the elcctric:ll conducti\,ity :lnd optical transmission spectra of specimens before and Lifter imn in the plantation of H ’ ions. The conductivity as-deposited spccimcns at rmm temptxlturc is lo\\w than 10 ’ S cm ’ (IO ” Q) \vhich is the lowr limit of the mexuring system used here. The following features ;lrc eGdent from the figure. Conductivitics (0) aftcl implantation at room temperature are higher than those T(K) 250 t-t
I
150 1
100
77 ,
I
.......as-implanted . ... .. . . s...., .
-..
i
L&.
as-deposited I h j .,b J ,I:! I ,4 1000
T (K ')
before impl:lntation by more than eight orders of magnitude and n is :llmost independent of temperature. No significant change in 0 \vas observed for He - -implanted IMO thin films. The color of specimens chwged from colorless before implantation, to broivnish yellow after implantation with any ion. The inset shows optical transmission spectra of specimens before ;lnd Lifter implantation. The emergence of two new absorption bands are seen in specimens implnnted Lvith H + , ;I \veak band at :tbout 500 nm and an intense itbsorption band abovc about 1000 nm cstcnding to the infrared region. Upon wncaling at 300°C for 5 h, the former bnnd fildcd and the Iiltter developed. Only the former band is seen in He + implnnted specimens. No significant change U‘;IS obserxd in X-ray diffraction patterns after illlplalltiiti0ll. The increase in conductivity of specimens implnnted with Li + or H ’ is due to the gcner;ition of Carrie1 electrons by implantation. The possibility of ionic conduction due to implanted ions is definitely discounted from the conduction type (II) determined by Seebeck and Hi111 coefficients. Tbc effect of radiation d:\magc can also be neglected because no significant increase in CT was seen aftor He ’ impl;lntation. It is therefore reasonable to assu~nc that the carriers iire generated by chemical doping effects. i.e.. when :rn implanted ion occupies ;I vacant ciition site in the lattice of IMO, :I carrier clcctron is generated ~1s iI conscqucnce of mi~intaining clectroncutrality. Here, let us estimate the efficiency of carrier generiltion I/. Pro\%ted that 21 single carrier electron is ideally gcneruted par implanted H ‘or Li ’ . /I is defined as /I = II ,“I’ tl, ivhcr-e II and ;V denote ~~~ll~~lltl~ilti~~lls (Cl11 ‘) of citrrier electrons and tluences (cm ‘) of implanted ions, and tl is the film thickness (cm ‘). C;llculilted efficiencies in as-implanted and post-annealed specimens are about 20% in the as-implanted state for both Li + and H + and become double (about 40?1,) lifter annealing at 300°C for 5 h. Recalling th:lt color centers gi\,ing ;I band at about 500 nm disappeared upon annealing. we consider th:tt a fraction of inactive implanted ions, which arc associ;ltcd with the cobcenters, are activated by this process. Existing amorphous (a-) semiconductors are classified into two ciltcgories, a tetrahedral system represented by amorphous silicon and chalcogenide system by a-As,S,. However, neither system meets both high transparency and high conducti\,ity. Recently. we have proposed a \vorking hypothesis for exploring transparent conducting :lmorphous oxides on the basis of simple considerations concerning chemical bondings [8] and have created sevcrxl new m;iterials [9 I?]. Here. implklnted amorphous cadmium gcrmanates [ 10, I?] are shown as all
cxillllple.
Cadmium germanate (Cd,GeO,) thin films prepared by rf sputtering in Ar O2 gas (4: I ) are in an amorphous state as demonstrated by cross-sectional transmission
41
b
T(K) 300
200
100, 90
80
Lit - implanted H+-Implanted
-10
before I
i ’
5
1
I
’
’
10
I
I’
103/T(K-‘)
Fig. 2. (a) Cross-sectional TEM photograph of as-sputtered 2CdO- Ge02 thin film deposited on anodically oxidized Al substrates and selected area electron diffraction pattern showing diffused ring structures. The specimen for observation \vas prepared by ultramicrotomy instead of it conventional ion milling to avoid any structural damage during thinning. (b) D.c. conductivities of amorphous 2CdO-GeO, thin films before and after implantation of H + or Li * to a fluence of 2 x IO’” cm - ‘. Inset shows optical transmission spectra before and after implantation. (c) Photo of specimens before and after implantation.
electron microscopy (TEM) observation and selected area electron diffraction patterns in Fig. 2 (a). Amorphous Cd,GeO, films are transparent and colorless. visually, and the optical band gap determined by the Taut plot was approx. 3.4 eV. Fig. 2 (b) shows the conductivity of Li + or H +-implanted specimen as a function of temperature. Lithium ions of 2 x IO’” cm ~’ were implanted at room temperature by two steps, 80 keV ( 1 x 10’” cm “) + 160 keV ( 1 x 10’” cm ‘), and the range of implanted Li ions calculated with the TRIM code [ 131was 250 nm (for 80 keV) and 470 nm (for 160 keV). The conductivity of the implanted specimenat 300 K was about 10 S cm ‘, which was higher by about 10 orders of magnitude than the value before implantation, and remained almost constant down to 77 K. Almost the same order of increase in the conductivity was observed for the specimenafter H + -implantation, but no significant change was noted upon He+-implantation.
Fig. 2 (c) shows the photo of the specimensbefore and after H +- or Li +-implantation. It is evident that the specimen remained transparent after the implantation (see also the optical transmission spectra shown in the inset of Fig. 2 (b)). The carrier generation efficiency was calculated as about 2% for H d - or Li + -implantation. Although this value is lower by an order of magnitude then c-MgIn,O,. it is worth noticing that carrier generation by implantation occurs at the yield of a few percent even in amorphous oxides.
3. Creation
of fast proton-conducting
glasses
Fast proton-conducting materials are of importance in various applications such as solid electrodes for electrochromic displays and Hz-O, fuel cells. Although the fundamentals of protonic conduction in oxide glasses
TEMPERATURE 400 I
/
RECIPROCAL
300 I
(K ) 200 I
TEMPERATURE
1000
T -’ ( K~’
Fig. 3 (a) sho\vs tcmpcrature depcndencc of d.c. conductivities of 50MgO 50P205 glasses before and after proton implantation (120 keV. I x IO” cm ‘1. In the calculation of conductivities of the implanted glasses, the thickness of implanted layers was assumed to be equal to the FWHM (about 0.9 km) of the distribution of implanted protons, which wus measured by secondary ion mass spectroscopy [l7]. It is obvious that upon implantation the conductivity at 300 K was enhanced from 10 ” to 4 x 10 ’ S cm ’ by 10 orders of magnitude and the acti\.ation energy was reduced to I 6 (I.2 eV 0. I9 eV). No such a drastic conductivity increase was observed fol proton-implanted SiO, glasses. According to experimental \+.ork on protonic conduction in oxide glasses [ 141, conductivity is related to the concentration of protons and peak ivave number of the infrared absorption band due to OH stretching \,ibrations through I7417
K =
A,#
+ 3’.
Log iI,, = ~ O.O097l’,,,, + 17.1.
(I)
(2)
where 0 1,7 Ic is the conducti\.ity at a reference tcmpcrature of 4 I7 K and .A,, is :I constant depending on the glass composition. and is a measure of proton mobility. These equations have been obtained from the experimental data for about X0 types ol oxide glasses containing only X OH groups (X = Si. P and B) (no molecular water is contained). The conductivity (nj,: K) cnlculatcd by Eqs. (I) itnd (2) in the implanted mugnesium phosphate glasses was I0 I2 S cm ‘, u,hich is smaller by eight orders of mi~gnitude thitn the observed value (about IO ’ S \WAVENUt~lBER(cm
’ I
ha\,e been established by Abe and Hosono [ 14 161. no fast proton-conducting glass showing conductivities of more than 10 ’ S cm ’ at 300 K has been realized yet. to our knowledge. We created fast proton-conducting oxide glasses by implanting protons into phosphate glusses [ 171.
cm
’ ).
What is the primary factor required for high protonic conduction? Comparison of infrared absorption spectra of substrate gliisses before and after implantation provides an ;tnSwer to this question. Fig. 3 (b) shows infrared absorption in the MgO P,Oi gli\sscs induced by proton implantation along with the spectrum of the substritte glass. A broad absorption band peaking at about 3700 cm ‘. which is commonly seen in alI the specimens, is due to the bond stretching of POH groups forming ;I strong hydrogen bonding with ;In oxygen attaching to the glass network. Besides this band. ii new band peaking at about 3400 cm ‘. marked by iIn arrow in the figure. becomes prominent in the substrate implanted to 21 tluence of I x lo’x cm ‘. This band. which is not seen in the substrate before implantation. is ascribed to molecular water H20, referring to the literature [18]. Although :I pair of SiOH (I’(,,, s 3690 cm ‘) and Si H (I*~,,, x
43 20 nm
-::&... ;. ; .. .‘. ..
-a---w
~~~~~~~~i~i:il:i:;::.;-::i.l::-: ,., ::/;...:.x.: .,.,..... .....i.,..,.,.,..::::..~.
5
(
:
r
I
“L
0
%T-~_~ 50
100
150
200
250
z 0
,.,f
,
,
540 WAVELENGTH
t
y
,
jo.05”
600 ( nm)
FIN. 4. SO, glass substrate implanted with Cu ’ to a flucnce of 6 x IO’” ions per cm’ at 160 IteV. (a) Cross-sectional TEM micrograph and electron diffraction pattern indexed to metallic Cu. (b) Cu concentrations from RBS and particle diameters from TEM as a function of depth. R,, and AR,, denote mean range and range atragglin.g calculated using the TRIM code. (c) Third-order nonlinear susceptibility z”) measured b) the Z-scan technique wing a cavity-damped tunable dye laser (pulse duration: S ps) pumped with Nd:YAG laser.
2240 cm ‘) were produced. no formation of molecular water was seen for 50, glass implanted to a fluence of I x 10” cm ’ [7]. It is therefore suggestedthat coexistence of molecular water and acidic POH are requisite for fast proton conduction in oxide glasses.An observation that the activation energy of the implanted phosphate glassesis close to that in fast proton-conducting crystals of heteropolyacids [19] such as H,Mo,,PO,.29 H20 and HU0,P0,.4Hz0, all of which contain both POH and H20. substantiates this suggestion. It is considered that protons are transferred as H,O + and hence the mobility is greatly enhanced compared with the system involving no molecular water.
4. Fabrication of nonlinear optical materials based on nanosized metal clusters Nanometer-sized metals and semiconductors embedded in dielectrics such as a glass have attracted much attention becauseof the fundamental interest in zero-di-
mensional features of the electronic state and potential application such as photonic switching based on a large third order optical susceptibility (I”‘). Formation of small particles of metals or semiconductors in buried layers of the substrate glass provides a new fabrication method for nonlinear optical glasses[1.20,21]. A variety of nanosized colloid particles, including materials chemically active in ambient atmosphere such as amorphous P [22] and As [23], can be fabricated by ion implantation without subsequent thermal annealing. Here, fabrication of nanosized copper clusters [24] embedded in a-SiO, by implantation is briefly described as an example. Recently, we have proposed a criterion to predict the colloid formation in SiO, glasseson the basis of an argument on thermodynamic stability [25]. According to this criterion. the colloid formation of Cu is predicted by implantation of Cu into a-SiO, because AG,{CuO,) is much larger than AG,{SiO,) (where AG,(MOJ denotes standard formation free energy of a metal ion). Fig. 4 (a) shows cross-sectional TEM photograph of SiO, glassesimplanted with 160 keV Cu’ to a fluence of
6 x IO” cm ‘, Particles. which are identified iis Cu by selected area electron diffraction patterns. are observed in the I-angc from the implanted si~rL~cc to il depth )X0 nm. It is noted from the photograph that particle diameters are illmost constilnt ut a gi\,cn depth. but \‘ary in the range ? X nm with depth. Fig. 4 (b) shows the relations bctMecn the shape of the distribution of particle dii~meters and the shape of the distribution of copper concentrations as ;I function of depth from the implanted SUI-l;~cc. It is evident from the figure that the distribution of pal-ticlc dinmcters is close to the distribution of Cu concentration. As a first approximation, the diameter of c’u colloids increases linearly bith the CLI concentrations. This relation will be b’ery helpful in designing colloid size b) implantntion. Fig. 4 (c) shop z”’ of SiO, glass implanted with I60 kcV Cu ’ to ;I Aucnce of 6 x IO” cm 2 ;IS a function of wn\~elength of incident laser light. The measurcmcnt ot technique using light %‘I’ was performed by Z-scan from ;I cavity-dumped tunable dye laser pumped by :I mode locked Nd:YAG laser [%I. Since the pulse duration of the laser light is 5 ps. thermal and photo-thermal effects on the value of x(” arc considered to be negligibly small. As :I co~wx~ucncc. the obtained y’:’ \aluc is due primarily to clcctronic Kerr effects. The z”’ has the maximum (4 x IO h mu) at abollt 580 11111, \vhich corresponds to the top of the absorption band due to surface plasmon of nnnosized copper colloids.
5. Summary
( I ) Insulating lhin films of polycrystalline (c-) and amorphous (a-) Cd,GeO,, both of which have band gaps of about 3.5 eV. were con~~tcd into metallic conductors by implantation of Li - OI- H ’ without being accompanied by 21large loss of optical Mgln,O,
transmission
in
the
\.isiblc
range.
Provided
that
a11
implanted ion ideally produces ;I single electron, the carrier generating efficiencies for implantation of Li ’ or H’ (RUCIKC I x IO” cm ‘) in c-MgIn,O, and aCdGeO, were about Wi o ;IIIC! i\bout ?‘%I.respectively. (2) A high protonic conducting oxide glass \vas obtained in Mg( PO,)? glassesimplanted with 120 keV H + to a fluence of I x IO’” cm ‘. The d.c. conductivity at 300 K was 4 x IO ’ S cm ’ and the activation energy was 0.19 eV. It is suggested that the coexistence of molecular water and POH is required to obtain fast protonic conduction in oxide glasses. (3) Nanometer-sized Cu colloid particles embedded in 50, glassesacre fabricated by implantation of Cu ’ to SiO, glassesat room temperature. The distribution of Cu particle diameters was close to that of the concentrations of implanted Cu. The third-order optical susceptibility of SiO, glassesimplanted \+ith Cu -+ ( I60 keV. 6 x IO” cm ‘) measured by the Z-scan techniciuc
using laser light with 5 ps pulse duration has the maximum value of about 3 x 10 ’ esu iit the peak position (about 580 nm) of the surface plasmon band.
Acknowledgements
The authors thank N. Ucda, N. Kikuchi and K. Kawamura of Tokyo Institute of Technology. i\nd Dr N. Matsunami of Nagoya Universit),, Japan. for theil cooperative efforts. This work was supported in part by the Grant-in-aids for Scientific Researchers !rOm the Ministry of Eduction, Science. Sports and Culture. One of the authors (HH) acknowledges the financial support from The Sumitomo Foundation. The Izumi and the Ognsaw~ra Foundation for Promotion of Science and Technology.
References
ELSEVIER
Approach
to novel crystalline optoelectronics
and amorphous oxide materials for by ion implantation’
Abstract This paper briefly reviews our recent works on the creation of novel oxide materials for optoelectronics b!, ion implantation. The topics covered are transparent crystalline and amorphous oxides with metallic conduction, fast proton-conducting oxide glasses and nonlinear optical material based on nanosized metal clusters. Kc8~~nwtO:Optoelectronics: Ion implantation: Crystalline amorphous oxidc materials: Proton-conducting
2. Creation oxides with
1. Introduction
Ion implantation is a processing technique to significantly modify surface and near-surface properties of materials. Although this technique has been extensively applied to the creation of functional materials in the fields of semiconductors and metals, the primary research efforts on the application of ion implantation to wide band gap oxides appear to have been restricted on account of radiation effects on the oxides. We have examined chemical interaction of implanted ions in amorphous Si02 through elucidation of structural defects produced by implantation [1,2] and have applied the obtained conclusions to the creation of novel oxide materials with a specific optoelectronic function. This paper briefly reviews our recent works on novel crystalline and amorphous oxide materials for optoelectronic applications.
* Corresponding author. Current address: (a) above. Tel.: + 81 fax: + 81 459225169. ’ Submitted to the Proceedings of the 2nd International Sqmposwum of Oxide Electronics. 459145359;
0921-5107 Yh 515.00 8~‘ I996 PII SOYZI-5107(Y6)()1670-0
Elsevier Science S.A. All rights reserved
oxide glasses: Nonlinear optical @assrs
of transparent crystalline a metallic conduction
and amorphous
Control of electrical conduction in IV and III-V group semiconductors such as Si and GaAs by carrier doping is a great successof ion beam modification of materials. Although the yield of conduction carriers is very low in the as-implanted state because of amorphization of the implanted layer. the activation of dopants can be effectively achieved via recrystallization of the amorphous layers during subsequent thermal annealing or pulsed laser annealing [3]. Much interest has been directed to optically transparent and electrically conducting materials, which have various applications, such as transparent electrodes in liquid-crystal displays and solar cells. Most oxides have wide band gaps of > 3 eV, and hence are transparent in the visible wavelength range. but they are also electrical insulators. However. this is not always true when they contain dopants. because the electrical conductivity is expressed as a product of mobility 11and carrier concentration N. The mobility of electrons (or positive holes) is approximately proportional to the curvature (d’E’dli’) of the conduction band bottom (valence band top) in the energy
bond (E- li) diagram. Therefore. ewn insulating oxides can be converted into conductors if they have an appropriate energy band structure and can be succcssfdlq doped [4]. Iridium magnesium oxide (Mgln,O,, IMO) is a crystal Lvith a spincl-type structure :md a b:lnd gap of about 3.5 V. The bottom of the conduction b:lnd of this crystal is primarily composed of \wxnt 5s orbituls of In ions and short In -111 distances originating from the edge shored mode of the InO,, octahcdr;t [S]. This gi\w lilrge dispersion of the conduction band bottom via delocaliz:ition of orbitnls. Therefore. IMO may be regarded as a potentinl semiconductor. Spincl-type oxides ;lrc filvored for the present purposes for the following tH’o reasons. First. they contain many vacant tctrahcdtxl and octithedral cation sites, M,hich could be :l\fail;tblc for implanted ions. and second. they are resistant to radiation damage. MgAI,O, spinel is used as the primary wall of nuclear rextors and is reported to be stable in the crystalline state even after bombordmcnt up to 250 dpa [(1]. Li + [4] and H + [7] were chosen for implantation bccnuse they ba\,e small mass :md hcncc minimize the dmnage produced by nucleklr collision. The effect of implunt;ition \vith He + is examined for comparison. Fig. 1 shows the elcctric:ll conducti\,ity :lnd optical transmission spectra of specimens before and Lifter imn in the plantation of H ’ ions. The conductivity as-deposited spccimcns at rmm temptxlturc is lo\\w than 10 ’ S cm ’ (IO ” Q) \vhich is the lowr limit of the mexuring system used here. The following features ;lrc eGdent from the figure. Conductivitics (0) aftcl implantation at room temperature are higher than those T(K) 250 t-t
I
150 1
100
77 ,
I
.......as-implanted . ... .. . . s...., .
-..
i
L&.
as-deposited I h j .,b J ,I:! I ,4 1000
T (K ')
before impl:lntation by more than eight orders of magnitude and n is :llmost independent of temperature. No significant change in 0 \vas observed for He - -implanted IMO thin films. The color of specimens chwged from colorless before implantation, to broivnish yellow after implantation with any ion. The inset shows optical transmission spectra of specimens before ;lnd Lifter implantation. The emergence of two new absorption bands are seen in specimens implnnted Lvith H + , ;I \veak band at :tbout 500 nm and an intense itbsorption band abovc about 1000 nm cstcnding to the infrared region. Upon wncaling at 300°C for 5 h, the former bnnd fildcd and the Iiltter developed. Only the former band is seen in He + implnnted specimens. No significant change U‘;IS obserxd in X-ray diffraction patterns after illlplalltiiti0ll. The increase in conductivity of specimens implnnted with Li + or H ’ is due to the gcner;ition of Carrie1 electrons by implantation. The possibility of ionic conduction due to implanted ions is definitely discounted from the conduction type (II) determined by Seebeck and Hi111 coefficients. Tbc effect of radiation d:\magc can also be neglected because no significant increase in CT was seen aftor He ’ impl;lntation. It is therefore reasonable to assu~nc that the carriers iire generated by chemical doping effects. i.e.. when :rn implanted ion occupies ;I vacant ciition site in the lattice of IMO, :I carrier clcctron is generated ~1s iI conscqucnce of mi~intaining clectroncutrality. Here, let us estimate the efficiency of carrier generiltion I/. Pro\%ted that 21 single carrier electron is ideally gcneruted par implanted H ‘or Li ’ . /I is defined as /I = II ,“I’ tl, ivhcr-e II and ;V denote ~~~ll~~lltl~ilti~~lls (Cl11 ‘) of citrrier electrons and tluences (cm ‘) of implanted ions, and tl is the film thickness (cm ‘). C;llculilted efficiencies in as-implanted and post-annealed specimens are about 20% in the as-implanted state for both Li + and H + and become double (about 40?1,) lifter annealing at 300°C for 5 h. Recalling th:lt color centers gi\,ing ;I band at about 500 nm disappeared upon annealing. we consider th:tt a fraction of inactive implanted ions, which arc associ;ltcd with the cobcenters, are activated by this process. Existing amorphous (a-) semiconductors are classified into two ciltcgories, a tetrahedral system represented by amorphous silicon and chalcogenide system by a-As,S,. However, neither system meets both high transparency and high conducti\,ity. Recently. we have proposed a \vorking hypothesis for exploring transparent conducting :lmorphous oxides on the basis of simple considerations concerning chemical bondings [8] and have created sevcrxl new m;iterials [9 I?]. Here. implklnted amorphous cadmium gcrmanates [ 10, I?] are shown as all
cxillllple.
Cadmium germanate (Cd,GeO,) thin films prepared by rf sputtering in Ar O2 gas (4: I ) are in an amorphous state as demonstrated by cross-sectional transmission
41
b
T(K) 300
200
100, 90
80
Lit - implanted H+-Implanted
-10
before I
i ’
5
1
I
’
’
10
I
I’
103/T(K-‘)
Fig. 2. (a) Cross-sectional TEM photograph of as-sputtered 2CdO- Ge02 thin film deposited on anodically oxidized Al substrates and selected area electron diffraction pattern showing diffused ring structures. The specimen for observation \vas prepared by ultramicrotomy instead of it conventional ion milling to avoid any structural damage during thinning. (b) D.c. conductivities of amorphous 2CdO-GeO, thin films before and after implantation of H + or Li * to a fluence of 2 x IO’” cm - ‘. Inset shows optical transmission spectra before and after implantation. (c) Photo of specimens before and after implantation.
electron microscopy (TEM) observation and selected area electron diffraction patterns in Fig. 2 (a). Amorphous Cd,GeO, films are transparent and colorless. visually, and the optical band gap determined by the Taut plot was approx. 3.4 eV. Fig. 2 (b) shows the conductivity of Li + or H +-implanted specimen as a function of temperature. Lithium ions of 2 x IO’” cm ~’ were implanted at room temperature by two steps, 80 keV ( 1 x 10’” cm “) + 160 keV ( 1 x 10’” cm ‘), and the range of implanted Li ions calculated with the TRIM code [ 131was 250 nm (for 80 keV) and 470 nm (for 160 keV). The conductivity of the implanted specimenat 300 K was about 10 S cm ‘, which was higher by about 10 orders of magnitude than the value before implantation, and remained almost constant down to 77 K. Almost the same order of increase in the conductivity was observed for the specimenafter H + -implantation, but no significant change was noted upon He+-implantation.
Fig. 2 (c) shows the photo of the specimensbefore and after H +- or Li +-implantation. It is evident that the specimen remained transparent after the implantation (see also the optical transmission spectra shown in the inset of Fig. 2 (b)). The carrier generation efficiency was calculated as about 2% for H d - or Li + -implantation. Although this value is lower by an order of magnitude then c-MgIn,O,. it is worth noticing that carrier generation by implantation occurs at the yield of a few percent even in amorphous oxides.
3. Creation
of fast proton-conducting
glasses
Fast proton-conducting materials are of importance in various applications such as solid electrodes for electrochromic displays and Hz-O, fuel cells. Although the fundamentals of protonic conduction in oxide glasses
TEMPERATURE 400 I
/
RECIPROCAL
300 I
(K ) 200 I
TEMPERATURE
1000
T -’ ( K~’
Fig. 3 (a) sho\vs tcmpcrature depcndencc of d.c. conductivities of 50MgO 50P205 glasses before and after proton implantation (120 keV. I x IO” cm ‘1. In the calculation of conductivities of the implanted glasses, the thickness of implanted layers was assumed to be equal to the FWHM (about 0.9 km) of the distribution of implanted protons, which wus measured by secondary ion mass spectroscopy [l7]. It is obvious that upon implantation the conductivity at 300 K was enhanced from 10 ” to 4 x 10 ’ S cm ’ by 10 orders of magnitude and the acti\.ation energy was reduced to I 6 (I.2 eV 0. I9 eV). No such a drastic conductivity increase was observed fol proton-implanted SiO, glasses. According to experimental \+.ork on protonic conduction in oxide glasses [ 141, conductivity is related to the concentration of protons and peak ivave number of the infrared absorption band due to OH stretching \,ibrations through I7417
K =
A,#
+ 3’.
Log iI,, = ~ O.O097l’,,,, + 17.1.
(I)
(2)
where 0 1,7 Ic is the conducti\.ity at a reference tcmpcrature of 4 I7 K and .A,, is :I constant depending on the glass composition. and is a measure of proton mobility. These equations have been obtained from the experimental data for about X0 types ol oxide glasses containing only X OH groups (X = Si. P and B) (no molecular water is contained). The conductivity (nj,: K) cnlculatcd by Eqs. (I) itnd (2) in the implanted mugnesium phosphate glasses was I0 I2 S cm ‘, u,hich is smaller by eight orders of mi~gnitude thitn the observed value (about IO ’ S \WAVENUt~lBER(cm
’ I
ha\,e been established by Abe and Hosono [ 14 161. no fast proton-conducting glass showing conductivities of more than 10 ’ S cm ’ at 300 K has been realized yet. to our knowledge. We created fast proton-conducting oxide glasses by implanting protons into phosphate glusses [ 171.
cm
’ ).
What is the primary factor required for high protonic conduction? Comparison of infrared absorption spectra of substrate gliisses before and after implantation provides an ;tnSwer to this question. Fig. 3 (b) shows infrared absorption in the MgO P,Oi gli\sscs induced by proton implantation along with the spectrum of the substritte glass. A broad absorption band peaking at about 3700 cm ‘. which is commonly seen in alI the specimens, is due to the bond stretching of POH groups forming ;I strong hydrogen bonding with ;In oxygen attaching to the glass network. Besides this band. ii new band peaking at about 3400 cm ‘. marked by iIn arrow in the figure. becomes prominent in the substrate implanted to 21 tluence of I x lo’x cm ‘. This band. which is not seen in the substrate before implantation. is ascribed to molecular water H20, referring to the literature [18]. Although :I pair of SiOH (I’(,,, s 3690 cm ‘) and Si H (I*~,,, x
43 20 nm
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5
(
:
r
I
“L
0
%T-~_~ 50
100
150
200
250
z 0
,.,f
,
,
540 WAVELENGTH
t
y
,
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600 ( nm)
FIN. 4. SO, glass substrate implanted with Cu ’ to a flucnce of 6 x IO’” ions per cm’ at 160 IteV. (a) Cross-sectional TEM micrograph and electron diffraction pattern indexed to metallic Cu. (b) Cu concentrations from RBS and particle diameters from TEM as a function of depth. R,, and AR,, denote mean range and range atragglin.g calculated using the TRIM code. (c) Third-order nonlinear susceptibility z”) measured b) the Z-scan technique wing a cavity-damped tunable dye laser (pulse duration: S ps) pumped with Nd:YAG laser.
2240 cm ‘) were produced. no formation of molecular water was seen for 50, glass implanted to a fluence of I x 10” cm ’ [7]. It is therefore suggestedthat coexistence of molecular water and acidic POH are requisite for fast proton conduction in oxide glasses.An observation that the activation energy of the implanted phosphate glassesis close to that in fast proton-conducting crystals of heteropolyacids [19] such as H,Mo,,PO,.29 H20 and HU0,P0,.4Hz0, all of which contain both POH and H20. substantiates this suggestion. It is considered that protons are transferred as H,O + and hence the mobility is greatly enhanced compared with the system involving no molecular water.
4. Fabrication of nonlinear optical materials based on nanosized metal clusters Nanometer-sized metals and semiconductors embedded in dielectrics such as a glass have attracted much attention becauseof the fundamental interest in zero-di-
mensional features of the electronic state and potential application such as photonic switching based on a large third order optical susceptibility (I”‘). Formation of small particles of metals or semiconductors in buried layers of the substrate glass provides a new fabrication method for nonlinear optical glasses[1.20,21]. A variety of nanosized colloid particles, including materials chemically active in ambient atmosphere such as amorphous P [22] and As [23], can be fabricated by ion implantation without subsequent thermal annealing. Here, fabrication of nanosized copper clusters [24] embedded in a-SiO, by implantation is briefly described as an example. Recently, we have proposed a criterion to predict the colloid formation in SiO, glasseson the basis of an argument on thermodynamic stability [25]. According to this criterion. the colloid formation of Cu is predicted by implantation of Cu into a-SiO, because AG,{CuO,) is much larger than AG,{SiO,) (where AG,(MOJ denotes standard formation free energy of a metal ion). Fig. 4 (a) shows cross-sectional TEM photograph of SiO, glassesimplanted with 160 keV Cu’ to a fluence of
6 x IO” cm ‘, Particles. which are identified iis Cu by selected area electron diffraction patterns. are observed in the I-angc from the implanted si~rL~cc to il depth )X0 nm. It is noted from the photograph that particle diameters are illmost constilnt ut a gi\,cn depth. but \‘ary in the range ? X nm with depth. Fig. 4 (b) shows the relations bctMecn the shape of the distribution of particle dii~meters and the shape of the distribution of copper concentrations as ;I function of depth from the implanted SUI-l;~cc. It is evident from the figure that the distribution of pal-ticlc dinmcters is close to the distribution of Cu concentration. As a first approximation, the diameter of c’u colloids increases linearly bith the CLI concentrations. This relation will be b’ery helpful in designing colloid size b) implantntion. Fig. 4 (c) shop z”’ of SiO, glass implanted with I60 kcV Cu ’ to ;I Aucnce of 6 x IO” cm 2 ;IS a function of wn\~elength of incident laser light. The measurcmcnt ot technique using light %‘I’ was performed by Z-scan from ;I cavity-dumped tunable dye laser pumped by :I mode locked Nd:YAG laser [%I. Since the pulse duration of the laser light is 5 ps. thermal and photo-thermal effects on the value of x(” arc considered to be negligibly small. As :I co~wx~ucncc. the obtained y’:’ \aluc is due primarily to clcctronic Kerr effects. The z”’ has the maximum (4 x IO h mu) at abollt 580 11111, \vhich corresponds to the top of the absorption band due to surface plasmon of nnnosized copper colloids.
5. Summary
( I ) Insulating lhin films of polycrystalline (c-) and amorphous (a-) Cd,GeO,, both of which have band gaps of about 3.5 eV. were con~~tcd into metallic conductors by implantation of Li - OI- H ’ without being accompanied by 21large loss of optical Mgln,O,
transmission
in
the
\.isiblc
range.
Provided
that
a11
implanted ion ideally produces ;I single electron, the carrier generating efficiencies for implantation of Li ’ or H’ (RUCIKC I x IO” cm ‘) in c-MgIn,O, and aCdGeO, were about Wi o ;IIIC! i\bout ?‘%I.respectively. (2) A high protonic conducting oxide glass \vas obtained in Mg( PO,)? glassesimplanted with 120 keV H + to a fluence of I x IO’” cm ‘. The d.c. conductivity at 300 K was 4 x IO ’ S cm ’ and the activation energy was 0.19 eV. It is suggested that the coexistence of molecular water and POH is required to obtain fast protonic conduction in oxide glasses. (3) Nanometer-sized Cu colloid particles embedded in 50, glassesacre fabricated by implantation of Cu ’ to SiO, glassesat room temperature. The distribution of Cu particle diameters was close to that of the concentrations of implanted Cu. The third-order optical susceptibility of SiO, glassesimplanted \+ith Cu -+ ( I60 keV. 6 x IO” cm ‘) measured by the Z-scan techniciuc
using laser light with 5 ps pulse duration has the maximum value of about 3 x 10 ’ esu iit the peak position (about 580 nm) of the surface plasmon band.
Acknowledgements
The authors thank N. Ucda, N. Kikuchi and K. Kawamura of Tokyo Institute of Technology. i\nd Dr N. Matsunami of Nagoya Universit),, Japan. for theil cooperative efforts. This work was supported in part by the Grant-in-aids for Scientific Researchers !rOm the Ministry of Eduction, Science. Sports and Culture. One of the authors (HH) acknowledges the financial support from The Sumitomo Foundation. The Izumi and the Ognsaw~ra Foundation for Promotion of Science and Technology.
References