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GaAs (100) for epitaxial regrowth and electron-beam patterning
Applied Surface Science 56-58 (1992) 855-860 North-Holland
applied surface
science
Ca..sSr0.sFJGaAs (100) for epitaxial regrowth and electron-beam patterning S. H o r n g , Y. Hirose, A. K a h n Department of Electrical Fnginecring. Princeton Unicersity, Princeton, NJ 08544, USA
C. W r e n n and R. Pfeffcr Army Electronics Tt'chllologr~' and Decices Laboratory. ~)rl Monmouth. NJ 0770.t. USA
Received h May 19~1; accepted fl)r publication 16 May It/91
Layers of GaAs and Catt.sSr.sF~ are grown by molecular beam epitaxy. The morphology,orientation, corr,positiun and eryslallinity of the layers are analyzed as a function of growth temperature and surface treatment. The fluoride layersshow a high degree of cry,aallinity and a smoothsurface morphology.The crystallinity and morphologyof the GaAs overlayer are improvedby electron irradiation of the fluoride surface prior to GaAs growth l~)llowedby a two-stepgrowthprocedure for the GaAs ovcrlayer. A technique of electron-beam patterningof the fluoride layer is developed and the formationof 3-5 #m features is demonstrated.
1. Introduction The growth of epitaxial fluoride-semiconductor heterostructures has potential technological applications in three-dimensional integration, waveguiding for opto-¢lectronic devices, and elect.ton-beam lithography. Particularly exciting is the possibility of regrowth of a crystalline semiconductor on top of the insulator. Alkali earth fluorides are wide bandgap ( l l - 1 2 eV) insulators with a crystal structure similar to that of conventional diamond and zincblende semiconductor~. The lattice constant of mixed fluorides such as C a , S r t _ x F 2 and BaxSrj_sF2 can be selected for 0 < x < 1 to match that of important semiconductors: St, GaAs, or InP. Fina'y, the high dissociation energy ( ~ 8 eV) of the fluoride molecules allows simple evaporation techniques in molecular beam epitaxy (MBE) and leads to stoichiometric films. The large difference between the thermal expansion coefficients of the fluorides ( ~ 1.8 × I 0 ~ K - I ) attd that of Ill-Vs, e.g., GaAs (0.6× 10 -~ K - I ) , is a potentially serious problem for layers
grown at temperatures typically between 500 and 600°C. We have tried to diminish the impact of this thermal mismatch by growing the flt,oride layers with a composition which minimizes the mismatch at both room and growth temperatures, i.e., x --- 0.5 which produces perfect lattice match at ~ 350°C. A second problem relates to the growth orientation elected for this work. We chose the (100) surface because of its importance for device technology. Yet, the (100) fluoride surface is known to relax by forming {lll} microfacets which degrade the morphology, of the overlayer (GaAs) and can induce the formation of antiphase domains Ill. We have addressed this problerr by altering the fluoride surface through electron-beam exposure prior to the GaAs overgrowth. This results in a considerable improvement in the GaAs morphology. Finally, we have used the sensitivity of the fluoride to an electron beam to pattern the insulating layer. The technique is based on the electron-beam induced loss of F followed by oxidation of the remaining Ca and Sr and finally dissolution of the oxide in water or acid. We report
(116q-4332/92/$(15.(11) ~*!)1992 - Elsevier Science PublishersB.V. All rights resen'ed
856
S. Hornl,,et al. / Cao .~Sro ~F.,/ GaAs( 1(101fiJr epitaxial rt'growth anti electrot~-beam patterning
preliminary results o n the formation of 3 - 5 p.m features in a 2080 A thick Ca,..sSro..sF ., layer which could serve as the basis for the regrowth of well-defined G a A s structures.
2. Experimen~a| techniques The GaAs and fluoride layers we,-e grown by molecular beam epitaxy (MBE) in a system which has been described elsewhere [2,3]. (109) GaAs wafers. Si-doped to I - 2 × 10nS/cma, were degreased with trichloroethylene, acetone and methanol; rinsed in ho~ and room temperature sulfuric acid baths; etched in a 5 : | : I soiution of H~SO 4 : H z O 2 : H 2 0 ; and immersed in deionized (Dl) water. The wafers were In-mounted on Mo holders, preheated to ~ 2 5 0 ° C in a preparation chamber, and heated to 5 8 0 ° C in the growth chamber to desorb the native oxide under arsenic pressure. The A s / G a flux ratio during G a A s growth was set at 4, and the surface exhibits the As-rich (2 × 4) reconstruction. T h e G a A s was typically °grown at 5 8 0 - 6 2 0 ° C and at a rate of about 1 A / s . The fluoride films were grown from independently controlled CaF_~ and SrF z cells loaded with 99.995% pure single-crystal chunks. Graphite-coated PBN crucibles were used in the fluoride effugion cells to prevent reaction between the t.ontent and the PBN. T h e temperature of the cells during growth as ~ IYSO°C and the substrate temperature was 5 3 0 ° C . The typical growth rate was ~ 1 ,~./s. The electron-beam exposure was done with the reflection high-energy electron diffraction ( R H E E D ) beam (grazing incidence 5 keV electrons). The R H E E D pattern was monitored throughout the experiment. A two-step growth sequence was also implemeuted for GaAs: following the fluoride growth, the sul:strate was kep.t at 5 3 0 ° C duril~g the growth of the first 300 A of GaAs, then raised to 610 ° C for the remainder of the g~owth. The electron-beam patterning experimer, ts were performed in two different systems. A scanning Auger microp~oi~e (SAM), with a resolution of 20 /zm, was first used to create 50-100 btm features, l h e electron energy was 5 keV and the
beam current was ~ 10 p.A. The experiment was the;~ refined with a scanning electron microscope ($EM) to create 3 - 5 ~ m features. The electron energy was 25 keV and the beam current was ~ 0.5 /xA. After exposure, the patterning was developed in Dl water or 5% HCI solution, and occasionally with ultrasonic treatment.
3. Results and discussions 3.1. Cao ~Sro sF: / (100) GoAs structures
in all out' experiments, the basic structure involved a 3000 A G a A s buffer layer on the substrate followed by a 2000 .~ Caa.sSrt~.sF2 layer. A ?50'1 .~ top GaAs I~yer was grown when the complete hcterostructure was desired. The GaAs buffer layers exllibited a high degree ,of bulk crystallinity (Xrni, = 3.5% in RBS) and very smooth surfaces with high-quality (2 x 4) R H E E D patterns. R H E E D oscillations were visible throughout the growth. The x = 0.5 fluoride composition was chosen to produce lattice match to G a A s at about 350 o C. The actual fluoride composition was always within 10% of the targeted value, producing a maximum 0.6% mismatch with G a A s at room or growth temperatures. The crystalline and morphological characteristics of the fluoride layers were excellent. Ion channeling with 2 MeV He + produced a Xmin as low as 4.6% (fig. la) which is the state-of-the-art for these mixed layers. Electron channeling pattern (ECP) (fig. lb) and scanning electron micrographs (SEM) (not shown here) showed that the fluoride layers had the (100) orientation and smooth and featureless surfaces, although crazes along the direction of the As-d,mer rows of the G a A s substrate were occasionally observed. The R H E E D patterns of the fluoride layer exhibited spots (fig. lc), indicating a three-dimensional surface structure consistent with the presence or {11 l} mi,:ro-facets. 3.2. GaAs / CaSrF2(IO0) GaAs structures
The G a A s films grown on the untreated Ca~sSr0.sF _, layers, optimized for a growth temperature cf 6 1 0 ° C [2), and exhibited medium
S. Horn;; ct aL / (~o sSro sF2/ Gg4s(lO0) ~ r ~it~ial regrowth and electron-beam pattenzing
Energy (MeV) 08 , 4000
~
82000
.
t.O I
1.2
t.4 " L[ = = 2
1
857
15 MeVI
\
~honnel
Fig. 2. SEM micrograph of GaAs grown on untreated Catt.sSrosF,. The morphology is the result of anti-phase domains. crystallinity with Xali, = 30%. The surfaces of these films were found to be rough in the /zm scale (fig. 2) and exhibited a (I x I) R H E E D pattern. For films grown at 6 2 0 ° C or above, the R H E E D p a t t e r n clearly i n d i c a t e d {111} microfacets in a d d i t i o n to the (100) (1 x 1) domains. T h e r a t h e r poor G a A s surface m o r p h o l o g y and m e d i u m bulk crystallinity, r e p o r t e d above presumably result from an a n t i - p h a s e d o m a i n p r o b l e m [1,4]. W h i l e the ideal (100) z i n c b l e n d e p l a n e exhibits a twofold symmetry, the (1139) fluoride p l a n e exhibits a fourfold symmetry. The growth of G a A s on the (100) fluoride surface can therefore proc e e d by n u c l e a t i o n of G a A s p a t c h e s r o t a t e d by 90 o with respect to each other, r e g a r d l e s s of the p r e s e n c e of steps on the fluoride surface. In addition, the nucleation of G a A s on two {111} facets of the fluoride surface r o t a t e d by 90 ° a r o u n d the (100) axis s h o u l d also lead to a~tr p h a s e d o m a i n s [1]. A l t h o u g h a tilted substrate
Fig. 1. (a) RBS spectrum taken from a 2000 A fluoride layer grown at 530°C on (100) GaAs, showing excellent crystallinity ()train= 4.6%). The upper curves correspond to random incidence of the He + ion beam (the smooth curve is a computer simulation of the experiment). The bottom curve corresponds to lhe yield in the channeling mode; (b) high-quality electron channeling pattern from the fluoride layer; (c) RHEED pattern resulting from the three-dimensional structure ({111} facets) of the fluoride surface.
858
S. Horng et aL / Cao ~Sro sF., / G¢L4sllOO)for epit¢vcialregrowth and electron.beam patteruhtg
can be expected to help b~, artificially introducing a twofold symmetry in the fluoride surface (effective in the case of G;,As growth on Si), the micro-facet structure must be altered prior to the growth of GaAs in order to completely eliminate the anti-phase domain problem. A potential solution for these problems is to modify the surface with an electron beam. The energetic incident electrons dissociate the fluoride molecules through an Auger process [5]. T h e relaxation of the F ion excited with a 2p core hole created by the incident electron can proceed through an LVV transition which results in an F ' ion. This ion, surrounded by the positive metal ions of the fluoride late.ice, is ejected. This change in surface composition should, in turn, eliminate the tacetting imposed by the electrostatic energy of the fluoride (lllO) surface [6]. In this experiment, we exposed the surface kept at 530 o C to a 5 keV grazing incidence ~,lectron beam giving a total dose of ~ 100 v,C/cm-'. During exposure, the fluoride R H E E D pattern developed weak streak-like features indicative of an overall decrease in facetting and increase in the fiat area of the surface (fig. 3). The surface morphology of the G a A s layers deposited on such electron-beam exposed fluoride films was found to be substantially improved. The anti-phase domain problem was not elimi-
Fig. 3. RHEED patter~, from an electron-irradiated Can.sSro.:~Fz.~'/~c:: :.J~,_r',~.ingweak streaks indicativeof partial flattening.
Fig. ,l. (a) RtlEED pattern from GaAs grown on electron irradialed Ca~.~SrttsF2 plus a two-st.:p process, showing a weak (2x) structure and an elongation of the streaks; (b) SEM micrograph from the same surface showingan improved surface morphology.
nated, however, as revealed by SEM micrographs, although the average size of the domain appear to be reduced. The R H E E D patterns obtained from these G a A s overlayers showed a (1 × 1) structure similar to that obtained from layers grown on untreated fluoride films, although the atomic order was generally improved. The ECP results also pointed out an improvement in surface crystallinity. Improving further the quality of the GaAs overlayer was attempted by adding an intermediate growth step after the electron irradiation of the fluoride surface, The R H E E D patterns and
s. Horng et aL / Cao sSro.,;F, / GaAs(lO0) for epitaxial regrowth and electron-beam patternhlg
SEM micrographs indicate that most G a A s films grown or. Ca0.hSr0.hF: at lower temperature tend to have smoother surface morphology, whereas the ECP show that G a A s films grown at higher temperature have better surface crystallinity. A two-step growth procedure was therefore implemented: a 300 A G a A s layer was first deposited at 530 ° C followed by a 2700 A G a A s layer deposited at 610 ° C. The resulting R H E E D pattern displayed considerably longer streaks and showed a 2 x structure (fig. 4a), indicative of a substantial improvement in surface atomic order. The surface morpholo~,ny was also greatly improved, as seen in the SEM micrographs (fig. 4b). The ECP
859
also indicated improved near-surface crystallinity. The improvement in the G a A s layer is thought to be related to the reduction of the Ca and Sr interdiffusion into the GaAs overlayer. It is known from photoemission studies that a chemical reaction at the as-grown GaAs-fluoride interface leads to F-deficiency [2], and the accelerated loss of F by electron-beam exposure increases the interface concentration of free Ca and Sr and promotes interdiffusion. The lower growth temperature for the first 300 ,~ of G a A s might reduce this problem. RBS and ion channeling studies on these G a A s films grown on electron-beam exposed fluoride '.ayers are in progress.
5Bm 2000A~
(d)
~'~
L
~
Cao.sSro.hF2 I GaAssubstrate
Fig. 5. (a) Optical microscopeimage of a 150 ,tim hole in the fluoride layer; (b) SAM Ga image of the hole; (c) SEM-drawn pattern on Cao.sSr~l.5F~; (d) schematicof the patterned structure.
86O
S. tlorng ¢'! al. / Ca o sSro ~t", / (;aAs( IOt)) /or t'pittaial rcgron'th t,~u,~eh'clron-beam patWnting
3.3. E l e c t r o n - b e a m p,ttferning
The dissociation of the fluoride by exposure to an electron beam, explained in the previous section, was exploited to pattern CaF 2 and Can~SrnsF, grown on GaAs. The layers were exposed to 5 keV electrons with a total dose sufficient to locally dissociate the fluoride layer over its entire thickness. An approximate dose of I0 C / e r a -~ at 5 keV was necessary for a 2000 A layer. In principle, the energy of the primary electron needs only to be above the binding energy of the first electron involved in the transition, i.e., ~ 25 eV for the 2p electron. McCord et al. have used a scanning tunneling microscope with a ~ 30 eV bias to expose fluoride layers [7]. Yet a more efficient process (one incident electron can create several core holes) and better focusing with conventional e-gun require a higher electron energy. The exposure alone resulted in an alteration of the optical characteristics of the layer, presumably due to metallizati~n (Ca or Sr) resulting from the loss of F, Ca and Sr are very reactive and readily form oxides, i.e., CaO and SrO, which are water soluble. The electron beam pattern was then developed in D! water or in a weak HCI bath (5% in water) with ultrasonic agitation. Fig. 5a shows an optical microscope image o~( a 150/zm feature obtained from an exposure with the SAM and a 5 rain development in HCI. The elemental sensitivity of the SAM was also used to insure that the fluoride layer was completely removed over the exposed and developed region. Fig. 5b represents a SAM Ga Auger image of the entire surface. It demonstrates the existence of a 15(1/.tm hole in the Can sSr()~F_, layer which uncovers the GaAs substrate. Following these preI;minary experiments, finer features were realized with a JEOL SEM. Fig. 5c shows a pattern written with a 20 nm, 25 keV electron beam carrying a 0.5 p.A current. The electron dose was also several C / e m ' - and the development of the pattern was done in the HCI bath. The X-ray dispersion analysis capability of the SEM was also used to verify the complete removal of the insulating layer a~.ong the patterned features. Patterning
these structures down to 1 p,m features appears therefore to be feasible. Further studies will be required to investigate the "cleanliness" of the pattern, i.e., verticality of the walls and homogeneity of the features, and the extension of the technique to submicron dimensions.
4. Summary GaAs layers were grown on high quality Cat).~Sri,.sFJGaAs (100) layers by molecular beam cpitaxy. Films grown on the untreated fluoride surface at 6 1 0 ° C exhibited medium crystalli~lity with ion channeling X,~i, = 30% and a rough surface morphology in the p~m scale. Electron-beam exposure of the fluoride surface prior to the g'rowth of GaAs added to a two-step growth procedure greatly improves the overall quality of the top GaAs layer. Finally, we demonstrated e-beam patterning in the fluoride layer, suggesting exciting applications of semiconductor regrowth in small structures.
Acknowledgement Support by the Army Research Office (DAAL03-89-K-0035) is gratefully acknowledged.
References [I] K. Tsutsui. T. Asano, H. Ishiwaraand S. Furukawa, Int. Syrup. on GaAs and Related Compounds. Heraklion. Greece (1'987). [2] S. Horng. A. Kahn, C. Wrenn and R. Pfeffer. in: Proc. Semiconddc;or Materials for Optoelect.ronicsand OEIC's. E-MRS (19911). [3] S. Horng.A. Kahn, C. Wrenn and R Preffer, in: Proc. 2nd Int. Conf. on ElectRmic Materials, Mater. Res. Soc. 229 (1990).
[4} S. Sinharoy,Thin Solid Films 187 (1991))231. [5] C.L. Strecker. W.E. Moddeman and J,T. Grant. J. Appl. Phys. 52 (1981) 6921. [6] L.J. Schowaher, R.W. Fa:hauer. L.G. Turner a.~d C.D. Robertson, Mater. Res. Soc. Syrup. Proc. 37 (1985) 1.51, [7] M.A. McCordand R.F.W. Pease. J. Vac. Sci. Technol. B 5 ( 198% 431).
applied surface
science
Ca..sSr0.sFJGaAs (100) for epitaxial regrowth and electron-beam patterning S. H o r n g , Y. Hirose, A. K a h n Department of Electrical Fnginecring. Princeton Unicersity, Princeton, NJ 08544, USA
C. W r e n n and R. Pfeffcr Army Electronics Tt'chllologr~' and Decices Laboratory. ~)rl Monmouth. NJ 0770.t. USA
Received h May 19~1; accepted fl)r publication 16 May It/91
Layers of GaAs and Catt.sSr.sF~ are grown by molecular beam epitaxy. The morphology,orientation, corr,positiun and eryslallinity of the layers are analyzed as a function of growth temperature and surface treatment. The fluoride layersshow a high degree of cry,aallinity and a smoothsurface morphology.The crystallinity and morphologyof the GaAs overlayer are improvedby electron irradiation of the fluoride surface prior to GaAs growth l~)llowedby a two-stepgrowthprocedure for the GaAs ovcrlayer. A technique of electron-beam patterningof the fluoride layer is developed and the formationof 3-5 #m features is demonstrated.
1. Introduction The growth of epitaxial fluoride-semiconductor heterostructures has potential technological applications in three-dimensional integration, waveguiding for opto-¢lectronic devices, and elect.ton-beam lithography. Particularly exciting is the possibility of regrowth of a crystalline semiconductor on top of the insulator. Alkali earth fluorides are wide bandgap ( l l - 1 2 eV) insulators with a crystal structure similar to that of conventional diamond and zincblende semiconductor~. The lattice constant of mixed fluorides such as C a , S r t _ x F 2 and BaxSrj_sF2 can be selected for 0 < x < 1 to match that of important semiconductors: St, GaAs, or InP. Fina'y, the high dissociation energy ( ~ 8 eV) of the fluoride molecules allows simple evaporation techniques in molecular beam epitaxy (MBE) and leads to stoichiometric films. The large difference between the thermal expansion coefficients of the fluorides ( ~ 1.8 × I 0 ~ K - I ) attd that of Ill-Vs, e.g., GaAs (0.6× 10 -~ K - I ) , is a potentially serious problem for layers
grown at temperatures typically between 500 and 600°C. We have tried to diminish the impact of this thermal mismatch by growing the flt,oride layers with a composition which minimizes the mismatch at both room and growth temperatures, i.e., x --- 0.5 which produces perfect lattice match at ~ 350°C. A second problem relates to the growth orientation elected for this work. We chose the (100) surface because of its importance for device technology. Yet, the (100) fluoride surface is known to relax by forming {lll} microfacets which degrade the morphology, of the overlayer (GaAs) and can induce the formation of antiphase domains Ill. We have addressed this problerr by altering the fluoride surface through electron-beam exposure prior to the GaAs overgrowth. This results in a considerable improvement in the GaAs morphology. Finally, we have used the sensitivity of the fluoride to an electron beam to pattern the insulating layer. The technique is based on the electron-beam induced loss of F followed by oxidation of the remaining Ca and Sr and finally dissolution of the oxide in water or acid. We report
(116q-4332/92/$(15.(11) ~*!)1992 - Elsevier Science PublishersB.V. All rights resen'ed
856
S. Hornl,,et al. / Cao .~Sro ~F.,/ GaAs( 1(101fiJr epitaxial rt'growth anti electrot~-beam patterning
preliminary results o n the formation of 3 - 5 p.m features in a 2080 A thick Ca,..sSro..sF ., layer which could serve as the basis for the regrowth of well-defined G a A s structures.
2. Experimen~a| techniques The GaAs and fluoride layers we,-e grown by molecular beam epitaxy (MBE) in a system which has been described elsewhere [2,3]. (109) GaAs wafers. Si-doped to I - 2 × 10nS/cma, were degreased with trichloroethylene, acetone and methanol; rinsed in ho~ and room temperature sulfuric acid baths; etched in a 5 : | : I soiution of H~SO 4 : H z O 2 : H 2 0 ; and immersed in deionized (Dl) water. The wafers were In-mounted on Mo holders, preheated to ~ 2 5 0 ° C in a preparation chamber, and heated to 5 8 0 ° C in the growth chamber to desorb the native oxide under arsenic pressure. The A s / G a flux ratio during G a A s growth was set at 4, and the surface exhibits the As-rich (2 × 4) reconstruction. T h e G a A s was typically °grown at 5 8 0 - 6 2 0 ° C and at a rate of about 1 A / s . The fluoride films were grown from independently controlled CaF_~ and SrF z cells loaded with 99.995% pure single-crystal chunks. Graphite-coated PBN crucibles were used in the fluoride effugion cells to prevent reaction between the t.ontent and the PBN. T h e temperature of the cells during growth as ~ IYSO°C and the substrate temperature was 5 3 0 ° C . The typical growth rate was ~ 1 ,~./s. The electron-beam exposure was done with the reflection high-energy electron diffraction ( R H E E D ) beam (grazing incidence 5 keV electrons). The R H E E D pattern was monitored throughout the experiment. A two-step growth sequence was also implemeuted for GaAs: following the fluoride growth, the sul:strate was kep.t at 5 3 0 ° C duril~g the growth of the first 300 A of GaAs, then raised to 610 ° C for the remainder of the g~owth. The electron-beam patterning experimer, ts were performed in two different systems. A scanning Auger microp~oi~e (SAM), with a resolution of 20 /zm, was first used to create 50-100 btm features, l h e electron energy was 5 keV and the
beam current was ~ 10 p.A. The experiment was the;~ refined with a scanning electron microscope ($EM) to create 3 - 5 ~ m features. The electron energy was 25 keV and the beam current was ~ 0.5 /xA. After exposure, the patterning was developed in Dl water or 5% HCI solution, and occasionally with ultrasonic treatment.
3. Results and discussions 3.1. Cao ~Sro sF: / (100) GoAs structures
in all out' experiments, the basic structure involved a 3000 A G a A s buffer layer on the substrate followed by a 2000 .~ Caa.sSrt~.sF2 layer. A ?50'1 .~ top GaAs I~yer was grown when the complete hcterostructure was desired. The GaAs buffer layers exllibited a high degree ,of bulk crystallinity (Xrni, = 3.5% in RBS) and very smooth surfaces with high-quality (2 x 4) R H E E D patterns. R H E E D oscillations were visible throughout the growth. The x = 0.5 fluoride composition was chosen to produce lattice match to G a A s at about 350 o C. The actual fluoride composition was always within 10% of the targeted value, producing a maximum 0.6% mismatch with G a A s at room or growth temperatures. The crystalline and morphological characteristics of the fluoride layers were excellent. Ion channeling with 2 MeV He + produced a Xmin as low as 4.6% (fig. la) which is the state-of-the-art for these mixed layers. Electron channeling pattern (ECP) (fig. lb) and scanning electron micrographs (SEM) (not shown here) showed that the fluoride layers had the (100) orientation and smooth and featureless surfaces, although crazes along the direction of the As-d,mer rows of the G a A s substrate were occasionally observed. The R H E E D patterns of the fluoride layer exhibited spots (fig. lc), indicating a three-dimensional surface structure consistent with the presence or {11 l} mi,:ro-facets. 3.2. GaAs / CaSrF2(IO0) GaAs structures
The G a A s films grown on the untreated Ca~sSr0.sF _, layers, optimized for a growth temperature cf 6 1 0 ° C [2), and exhibited medium
S. Horn;; ct aL / (~o sSro sF2/ Gg4s(lO0) ~ r ~it~ial regrowth and electron-beam pattenzing
Energy (MeV) 08 , 4000
~
82000
.
t.O I
1.2
t.4 " L[ = = 2
1
857
15 MeVI
\
~honnel
Fig. 2. SEM micrograph of GaAs grown on untreated Catt.sSrosF,. The morphology is the result of anti-phase domains. crystallinity with Xali, = 30%. The surfaces of these films were found to be rough in the /zm scale (fig. 2) and exhibited a (I x I) R H E E D pattern. For films grown at 6 2 0 ° C or above, the R H E E D p a t t e r n clearly i n d i c a t e d {111} microfacets in a d d i t i o n to the (100) (1 x 1) domains. T h e r a t h e r poor G a A s surface m o r p h o l o g y and m e d i u m bulk crystallinity, r e p o r t e d above presumably result from an a n t i - p h a s e d o m a i n p r o b l e m [1,4]. W h i l e the ideal (100) z i n c b l e n d e p l a n e exhibits a twofold symmetry, the (1139) fluoride p l a n e exhibits a fourfold symmetry. The growth of G a A s on the (100) fluoride surface can therefore proc e e d by n u c l e a t i o n of G a A s p a t c h e s r o t a t e d by 90 o with respect to each other, r e g a r d l e s s of the p r e s e n c e of steps on the fluoride surface. In addition, the nucleation of G a A s on two {111} facets of the fluoride surface r o t a t e d by 90 ° a r o u n d the (100) axis s h o u l d also lead to a~tr p h a s e d o m a i n s [1]. A l t h o u g h a tilted substrate
Fig. 1. (a) RBS spectrum taken from a 2000 A fluoride layer grown at 530°C on (100) GaAs, showing excellent crystallinity ()train= 4.6%). The upper curves correspond to random incidence of the He + ion beam (the smooth curve is a computer simulation of the experiment). The bottom curve corresponds to lhe yield in the channeling mode; (b) high-quality electron channeling pattern from the fluoride layer; (c) RHEED pattern resulting from the three-dimensional structure ({111} facets) of the fluoride surface.
858
S. Horng et aL / Cao ~Sro sF., / G¢L4sllOO)for epit¢vcialregrowth and electron.beam patteruhtg
can be expected to help b~, artificially introducing a twofold symmetry in the fluoride surface (effective in the case of G;,As growth on Si), the micro-facet structure must be altered prior to the growth of GaAs in order to completely eliminate the anti-phase domain problem. A potential solution for these problems is to modify the surface with an electron beam. The energetic incident electrons dissociate the fluoride molecules through an Auger process [5]. T h e relaxation of the F ion excited with a 2p core hole created by the incident electron can proceed through an LVV transition which results in an F ' ion. This ion, surrounded by the positive metal ions of the fluoride late.ice, is ejected. This change in surface composition should, in turn, eliminate the tacetting imposed by the electrostatic energy of the fluoride (lllO) surface [6]. In this experiment, we exposed the surface kept at 530 o C to a 5 keV grazing incidence ~,lectron beam giving a total dose of ~ 100 v,C/cm-'. During exposure, the fluoride R H E E D pattern developed weak streak-like features indicative of an overall decrease in facetting and increase in the fiat area of the surface (fig. 3). The surface morphology of the G a A s layers deposited on such electron-beam exposed fluoride films was found to be substantially improved. The anti-phase domain problem was not elimi-
Fig. 3. RHEED patter~, from an electron-irradiated Can.sSro.:~Fz.~'/~c:: :.J~,_r',~.ingweak streaks indicativeof partial flattening.
Fig. ,l. (a) RtlEED pattern from GaAs grown on electron irradialed Ca~.~SrttsF2 plus a two-st.:p process, showing a weak (2x) structure and an elongation of the streaks; (b) SEM micrograph from the same surface showingan improved surface morphology.
nated, however, as revealed by SEM micrographs, although the average size of the domain appear to be reduced. The R H E E D patterns obtained from these G a A s overlayers showed a (1 × 1) structure similar to that obtained from layers grown on untreated fluoride films, although the atomic order was generally improved. The ECP results also pointed out an improvement in surface crystallinity. Improving further the quality of the GaAs overlayer was attempted by adding an intermediate growth step after the electron irradiation of the fluoride surface, The R H E E D patterns and
s. Horng et aL / Cao sSro.,;F, / GaAs(lO0) for epitaxial regrowth and electron-beam patternhlg
SEM micrographs indicate that most G a A s films grown or. Ca0.hSr0.hF: at lower temperature tend to have smoother surface morphology, whereas the ECP show that G a A s films grown at higher temperature have better surface crystallinity. A two-step growth procedure was therefore implemented: a 300 A G a A s layer was first deposited at 530 ° C followed by a 2700 A G a A s layer deposited at 610 ° C. The resulting R H E E D pattern displayed considerably longer streaks and showed a 2 x structure (fig. 4a), indicative of a substantial improvement in surface atomic order. The surface morpholo~,ny was also greatly improved, as seen in the SEM micrographs (fig. 4b). The ECP
859
also indicated improved near-surface crystallinity. The improvement in the G a A s layer is thought to be related to the reduction of the Ca and Sr interdiffusion into the GaAs overlayer. It is known from photoemission studies that a chemical reaction at the as-grown GaAs-fluoride interface leads to F-deficiency [2], and the accelerated loss of F by electron-beam exposure increases the interface concentration of free Ca and Sr and promotes interdiffusion. The lower growth temperature for the first 300 ,~ of G a A s might reduce this problem. RBS and ion channeling studies on these G a A s films grown on electron-beam exposed fluoride '.ayers are in progress.
5Bm 2000A~
(d)
~'~
L
~
Cao.sSro.hF2 I GaAssubstrate
Fig. 5. (a) Optical microscopeimage of a 150 ,tim hole in the fluoride layer; (b) SAM Ga image of the hole; (c) SEM-drawn pattern on Cao.sSr~l.5F~; (d) schematicof the patterned structure.
86O
S. tlorng ¢'! al. / Ca o sSro ~t", / (;aAs( IOt)) /or t'pittaial rcgron'th t,~u,~eh'clron-beam patWnting
3.3. E l e c t r o n - b e a m p,ttferning
The dissociation of the fluoride by exposure to an electron beam, explained in the previous section, was exploited to pattern CaF 2 and Can~SrnsF, grown on GaAs. The layers were exposed to 5 keV electrons with a total dose sufficient to locally dissociate the fluoride layer over its entire thickness. An approximate dose of I0 C / e r a -~ at 5 keV was necessary for a 2000 A layer. In principle, the energy of the primary electron needs only to be above the binding energy of the first electron involved in the transition, i.e., ~ 25 eV for the 2p electron. McCord et al. have used a scanning tunneling microscope with a ~ 30 eV bias to expose fluoride layers [7]. Yet a more efficient process (one incident electron can create several core holes) and better focusing with conventional e-gun require a higher electron energy. The exposure alone resulted in an alteration of the optical characteristics of the layer, presumably due to metallizati~n (Ca or Sr) resulting from the loss of F, Ca and Sr are very reactive and readily form oxides, i.e., CaO and SrO, which are water soluble. The electron beam pattern was then developed in D! water or in a weak HCI bath (5% in water) with ultrasonic agitation. Fig. 5a shows an optical microscope image o~( a 150/zm feature obtained from an exposure with the SAM and a 5 rain development in HCI. The elemental sensitivity of the SAM was also used to insure that the fluoride layer was completely removed over the exposed and developed region. Fig. 5b represents a SAM Ga Auger image of the entire surface. It demonstrates the existence of a 15(1/.tm hole in the Can sSr()~F_, layer which uncovers the GaAs substrate. Following these preI;minary experiments, finer features were realized with a JEOL SEM. Fig. 5c shows a pattern written with a 20 nm, 25 keV electron beam carrying a 0.5 p.A current. The electron dose was also several C / e m ' - and the development of the pattern was done in the HCI bath. The X-ray dispersion analysis capability of the SEM was also used to verify the complete removal of the insulating layer a~.ong the patterned features. Patterning
these structures down to 1 p,m features appears therefore to be feasible. Further studies will be required to investigate the "cleanliness" of the pattern, i.e., verticality of the walls and homogeneity of the features, and the extension of the technique to submicron dimensions.
4. Summary GaAs layers were grown on high quality Cat).~Sri,.sFJGaAs (100) layers by molecular beam cpitaxy. Films grown on the untreated fluoride surface at 6 1 0 ° C exhibited medium crystalli~lity with ion channeling X,~i, = 30% and a rough surface morphology in the p~m scale. Electron-beam exposure of the fluoride surface prior to the g'rowth of GaAs added to a two-step growth procedure greatly improves the overall quality of the top GaAs layer. Finally, we demonstrated e-beam patterning in the fluoride layer, suggesting exciting applications of semiconductor regrowth in small structures.
Acknowledgement Support by the Army Research Office (DAAL03-89-K-0035) is gratefully acknowledged.
References [I] K. Tsutsui. T. Asano, H. Ishiwaraand S. Furukawa, Int. Syrup. on GaAs and Related Compounds. Heraklion. Greece (1'987). [2] S. Horng. A. Kahn, C. Wrenn and R. Pfeffer. in: Proc. Semiconddc;or Materials for Optoelect.ronicsand OEIC's. E-MRS (19911). [3] S. Horng.A. Kahn, C. Wrenn and R Preffer, in: Proc. 2nd Int. Conf. on ElectRmic Materials, Mater. Res. Soc. 229 (1990).
[4} S. Sinharoy,Thin Solid Films 187 (1991))231. [5] C.L. Strecker. W.E. Moddeman and J,T. Grant. J. Appl. Phys. 52 (1981) 6921. [6] L.J. Schowaher, R.W. Fa:hauer. L.G. Turner a.~d C.D. Robertson, Mater. Res. Soc. Syrup. Proc. 37 (1985) 1.51, [7] M.A. McCordand R.F.W. Pease. J. Vac. Sci. Technol. B 5 ( 198% 431).