Atomic Layer Epitaxy : a New Tool for Novel Modulated Semiconductor Structures

A novel growth techn ique f o r 111-V compounds, atomic l a y e r epi taxy(ALE1, i s presented as hav ing severa l advantages over such conven t i ona l growth methods as MBE, MOCVD and VPE. I n ALE, a s e l f l i m i t i n g growth mechanism works q u i t e e f f e c t i v e l y . Th i s mechanism makes i t p o s s i b l e t o grow l a y e r s a c c u r a t e l y i n monolayer u n i t s . Exper imenta l r e s u l t s show t h a t ALE growth u s i n g c h l o r i d e source gases, such as GaCl and InC1, r e t a i n s monolayer growth c h a r a c t e r i s t i c s over a wide range o f growth cond i t i ons . The p resen t method i s s u c c e s s f u l l y a p p l i e d t o s e l e c t i v e growth. Monolayer -un i t growth t akes p lace independent o f t h e r a t i o o f window and masked r e g i o n areas, and exce l l e n t s e l e c t i v i t y i S accomplished. Atomic-p1 ane dop ing u s i n g Se as an i m p u r i t y i s demonstrated. F i n a l l y , de l ta -doped FET f a b r i c a t i o n u s i n g Se a tomic-p lane dop ing i s presented as t h e f i r s t ALE dev i ce a p p l i c a t i o n . The measured transconductance o f t h e de l ta -doped FET w i t h a 0.5 vm ga te i s 215 mS/mm and i s comparable t o t h a t ob ta ined f rom MBE-grown de l t a -doped (S i ) FET. I n t e r e s t i n g r e s u l t s _obta ined w i t h ALE u s i n g me ta lo rgan i c compounds as source gases o f group I 1 1 elements a r e a l s o reviewed. 1. I n t r o d u c t i o n I n o rde r t o grow s u p e r l a t t i c e s and modula t ion s t r u c t u r e s f o r va r i ous novel 111-V compound dev ices , growth methods must s a t i s f y va r i ous requirements, of which p r e c i s e grown t h i c k n e s s c o n t r o l w i t h atomic o r mo lecu la r l a y e r accuracy i s a bas i c requ i rement . Molecu lar beam ep i t a x y (MBE) and me ta lo rgan i c chemical Vapor deposi tion(M0CVD) have been developed r e c e n t l y and a r e v e r y u s e f u l t o o l s f o r t h e above requ i rements . Grown th i ckness can be c o n t r o l l e d p r e c i s e l y i n angstrom o r monolayer u n i t s , w i t h adjustment o f growth r a t e (1,2) and use o f t h e RHEED o s c i l l a t i o n techn ique (3,4) i n t h e vacuum chamber. These methods, however, r e q u i r e Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987503 C5-22 JOURNAL DE PHYSIQUE v e r y s t r i c t c o n t r o l o f growth cond i t i ons , such as temperature, f l o w r a t e s and growth t ime, a l l o f which i n f l u e n c e g r e a t l y t h e growth r a t e . Atomic l a y e r epi taxy(ALE1, which was proposed o r i g i n a l l y by T. Sun to la f o r 11-V1 compounds ( 5 ) and has been developed by severa l o t h e r researchers f o r 111-V compounds (6-11) has g r e a t p o t e n t i a l as a new growth t o o l . Th i s method i s based on a new concept f o r s e p a r a t e l y d e p o s i t i n g each mono-atomic p lane o f t h e compound. A l though ALE resea rch i s i n a v e r y p r e l i m i n a r y s tage i n comparison w i t h o t h e r growth methods, we a r e ab le t o show severa l advantages over conven t i ona l methods. The p resen t paper desc r i bes i t s e x c e l l e n t monolayer c o n t r o l l a b i l i t y , w i t h a s e l f l i m i t i n g growth mechanism, s e l e c t i v e growth, a tomic-p lane doping, and a p p l i c a t i o n t o de l ta -doped f i e l d e f f e c t t r a n s i s t o r s , The r e s u l t s o f these f o u r may show t h e f e a s i b i l i t y o f ALE f o r t h e f a b r i c a t i o n o f va r i ous s o p h i s t i c a t e d dev ices i n t h e f u t u r e . The au tho rs a l s o desc r i be here a c t i v i t i e s r e p o r t e d i n t h i s f i e l d t o date. 2. Growth Procedure i n ALE I n conven t i ona l e p i t a x i a l growth methods, components making up t h e compound a r e u s u a l l y supp l i ed s imu l taneous l y t o t h e s u b s t r a t e reg ion . I n ALE, on t h e o t h e r hand, source gases f o r group I 1 1 and group V e lements a r e t ime -sepa ra te l y brought i n t o t h e s u b s t r a t e reg ion . As an example, t h e ALE growth process f o r GaAs, u s i n g GaCl and As , i s shown i n f i g . 1. There a re f o u r bas i c steps. The f i r s t s t e p ( a ) i s an ad$o rp t i on o f t h e GaCl supp l i ed t o t h e s u b s t r a t e sur face. I n t h e nex t s tep (b ) , t h e remain ing GaCl i n t h e gas phase i s purged o u t of t h e s u b s t r a t e reg ion . A f t e r s u f f i c i e n t pu rg ing has been c a r r i e d out , an a rsen i c source gas, such as AsH3 i s s u p p l i e d i n t o t h e s u b s t r a t e r e g i o n ( c ) , where a r e a c t i o n between t h e adsorbed GaCl and a rsen i c source gas occurs t o f o rm a GaAs l a y e r by r e l e a s i n g HC1. Unreacted a rsen i c compound gas and by-product gas a r e bo th them purged f rom t h e s u b s t r a t e r e g i o n (d ) . Thus, a new GaAs l a y e r i s formed on t h e subs t ra te . We used an atmospher ic mult i -growth-chamber r e a c t o r hav ing a c h l o r i d e gas chamber and a h y d r i d e gas chamber (9 ) . The c h l o r i d e gas chamber c o n t a i n s Ga and/or I n meta l sources upstream, and HC1 gas i s s u p p l i e d over t hese meta ls t o f o rm GaCl and/or I n c l . Hydr ide gases o f ASH and PH a r e i n t roduced i n t o t h e hyd r i de gas chamber. The s u b s t r a t e was t r a n s f e r g e d m e c h h i c a l l y between two d i f f e r e n t chambers. I t was exposed t y p i c a l l y t o GaCl f o r 6 seconds 'and t o a rsen i c f o r 10 seconds. One ALE c y c l e , s tep (a ) t o s t e p ( d ) i n F i g . 1, was completed i n about 45 seconds. ALE growth has been a l s o s t u d i e d w i t h meta lorgan ic (M0) compounds, such as t r i m e t h y l g a l lium(TMG1 o r t r i e t h y l g a l l ium(TEG) i n s t e a d . o f GaCl (6-8,10,11 l . The growth process i s b a s i c a l l y s i m i l a r t o t h a t i n Fig.1, a l t hough t h e decomposi t ion o f MO source gases takes p l a c e i n t h e v i c i n i t y o f t h e s u b s t r a t e and/or on t h e su r face o f t h e subs t ra te , and such Ga spec ies as Ga(CH3Ix (x=0-2) a r e thought t o d e p o s i t and form an adso rp t i on l a y e r . 3. S e l f l i m i t i n g Growth i n ALE I n f i g . 2, we summarize t h e ALE GaAs growth so f a r repo r ted , f r om t h e v iewpo in t o f t h e grown t h i c k n e s s c o n t r o l l a b i l i t y . The v e r t i c a l a x i s i n d i c a t e s t h e grown th i ckness p e r one ALE cyc le , and t h e h o r i z o n t a l a x i s rep resen ts t h e growth temperature. I f t h e adso rp t i on o r d e p o s i t i o n o f Ga spec ies (GaC1, Ga(CH X=0-2) t e rm ina tes comp le te l y i t h one monolayer, t h e grown th i ckness pe r ALE c y s l 6 shou ld then be equal t o 2.83 1 f o r a GaAs(100) subs t ra te . It i s c l e a r l y shown here t h a t t h e growth r a t e i n t h e GaCl -ALE l i e s approx imate ly on t h e t h i s i d e a l growth r a t e l i n e . We have named t h i s growth mode, which has a s e l f l i m i t i n g mechanism over a wide range o f growth c o n d i t i o n s , d i g i t a l e p i t a x y . GaCl p a r b i a l preSsure dependence o f t h e growth r a t e has been i n v e s t i g a t e d i n t h e range o f 10 t o 10 atm, and t h e d i g i t a l e p i t a x y was a l s o conf i rmed i n t h a t exper iment (9 ) . Thus, i n d i g i t a l ep i t axy , growth r a t e can be c o n t r o l l e d s o l e l y by t h e number o f r e p e t i t i o n s o f t h e ALE cyc le , and t h e growth proceeds i n monolayer u n i t s , independent o f analogue parameters as p a r t i a l pressure, growth temperature, and gas exposure t ime. I n MO-ALE, t h e decomposi t ion process o f me ta lo rgan i c compounds must be taken i n t o account. The growth r a t e i s a f u n c t i o n o f t h e growth temperature and p a r t i a l p ressure . As seen i n F i g . 2, t h e r e g i o n hav ing growth o f a d i g i t a l n a t u r e i s g e n e r a l l y n o t w ider t h a n t h a t i n t h e GaC1-ALE. However, i t shou ld be noted t h a t d i g i t a l e p i t a x y was ob ta ined by A. Doi ,e t a l . , over a wide range o f growth c o n d i t i o n s w i t h A r l a s e r i r r a d i a t i o n t o t h e s u b s t r a t e (10) . They exp la ined t h e i r r e s u l t s by say ing t h a t t h e Ga monolayer was formed when t h e A r l a s e r enhanced t h e decomposi t ion o f TMG adsorbed on As atoms o f t h e subs t ra te . K. Mor i , e t al., used diethylgalliumchloride(DEGaC1) f o r t h e GaAs ALE as a new MO source (12) . D i g i t a l e p i t a x y was produced i n t h i s system, as was GaC1-ALE. The DEGaCl system has t h e advantages o f bo th GaC1-ALE and MO-ALE. It i s expected t o be a p p l i e d t o l a r g e d iameter wafer processes i n f u t u r e IC f a b r i c a t i o n . 4. C h a r a c t e r i z a t i o n o f ALE f i l m s A l though t h e e l e c t r i c a l and o p t i c a l p r o p e r t i e s o f GaC1-ALE f i l m s have n o t been s y s t e m a t i c a l l y i n v e s t i g a t e d , q u a l i t a t i v e c h a r a c t e r i s t i c s a r e descr ibed here b r i e f l y . Grown l a y e r s o f GaAs on s e m i i n s u l a t i n g subs t ra tes revea led h i g h r e s i s t i v e c h a r a c t e r i s t i c s . Photoluminescece measurements a t 77 K and 4.2 K showed acceptor r e l a t e d luminescence i s v e r y weak as compared w i t h band edge l u m i ~ e s ~ e n c ~ ~ FurShermore, c a r r i e r c o n c e n t r a t i o n can be c o n t r o l l e d by Se dop ing f rom 10 t o 10 cm , as w i l l be desc r i bed l a t e r . Consequently, i t i s l g s t i m j t e d t h a t t h e background c a r r i e r c o n c e n t r a t i o n f o r GaC1-ALE i s lower t han 10 cm w i t h n t ype conduct ion. I n MO-ALE, as-grown GaAs f i l m s g e n e r a l l y revea l a s t r o n g p t ype conduct ion . The most l i k e l y cand ida te f o r background i m p u r i t y i s t h e carbon f rom t h e MO source gas. J. Nishizawa, 75 a13, have r e p o r t e d t h a t t h e background c a r r i e r c o n c e n t r a t i o n i s g r e a t e r t han 10 cm (p t ype ) i n t h e i r molecu lar l a y e r epi taxy(MLE) i n which TMG and ASH a r e used i n a vacuum chamber (13) . S i m i l a r r e s u l t s have been r e p o r t e d by M. A. ~ i s s h l e r , e t a l . (14) . When TEG i s used i n s t e a d o f TMG, t h e background h o l e c o n c e n t r a t i o n tended t o decrease, p robab l y because o f e a s i e r r e l e a s e o f e t h y l r a d i c a l s f r om t h e source gas. N. Kobayashi, e t a l . , have r e p o r t e d f o r t h e i r f l ow r a t e modul a t i o 4 epi$axy (FME) t h a t conduct ion i s o f n type and t h e e l e c t r o n m o b i l i t y exceeds 4 . 2 ~ 1 0 cm /Vs a t a 500°C growth temperature (8) . I n FME, t h e carbon contaminat ion i s a l s o suppressed by adding a smal l amount o f ASH d u r i n g t h e supp ly o f TEG. K . Mor i has i n v e s t i g a t e d t h e growth temperature depenience o f t h e c a r r i e r c o n c e n t r a t i o n i n DEGaC1-ALF7 (1213 Layers grown a t 450°C had a v e r y h i g h h o l e concen t ra t i on , around 5x10 cm . However, t h e c o n c e n t r a t i o n decreased w i t h i n c r e a s i n g growth temppgaturg, and a t 600 'C, n type l a y e r s w i t h a c a r r i e r c o n c e n t r a t i o n o f 1 . 1 ~ 1 0 cm were obta ined. An i nc rease o f growth temperature would c o n t r i b u t e t o l oosen ing t h e bond s t r e n g t h between t h e Ga and a l k y l r a d i c a l s . 5. S e l e c t i v e growth S e l e c t i v e growth i s now sometimes used f o r c o n t a c t l a y e r f o r m a t i o n i n t h e h i g h speed dev ices and o p t i c a l dev ices . I n t h e n e x t s tage o f t h i s t echno logy ' s development, such techn iques w i l l be impor tan t f o r g rowing a c t i v e l a y e r s i n d e s i r e d reg ions . S e l e c t i v e growth i s u s u a l l y performed on a p a r t i a l l y SiO -masked subs t ra te . A bas i c requ i rement i s t h a t d e p o s i t i o n on t h e SiO must be avo iged f o r t h e dev i ce process. Furthermore, t h e grown th i ckness shoufd be c o n t r o l l e d t o a d e s i r e d th i ckness . S e l e c t i v e growth i n GaC1-ALE o f GaAs has been s t u d i e d u s i n g a masked s u b s t r a t e (91, A t a 450°C growth temperature, i t was found t h a t no extraneous d e p o s i t i o n occur red i n t h e SiO reg ion . T h i s i s p robab l y because t h e GaCl adso rp t i on energy on t h e SiO? i s lower t han t h a t on GaAs, which has 3050 kca l /mol (15,16). We have r e p o r t e d p r e v i o u s l y t h a t growth r a t e enhancement i n t.he s e l e c t i v e growth r e g i o n ( g ) , a v e r y common phenomena w i t h convent iona l VPE methbds, can be avoided u s i n g ALE. However, t h e grown th i ckness p e r ALE c y c l e was n o t p r e c i s e l y equal t o t h e monolayer t h i ckness i n t h e s e l e c t i v e growth reg ion . The f o l l o w i n g exper iment was performed t o i n v e s t i g a t e t h e cause. A GaAs s u b s t r a t e hav ing 200 X 700 um windows was used. A f t e r exposure o f t h e s u b s t r a t e t o a GaCl atmosphere, i t was l e f t i n t h e C5-24 JOURNAL DE PHYSIQUE hydrogen stream f o r some pe r i od . A f t e r t h i s , ASH was s u p p l i e d t o t h e s u b s t r a t e reg ion , and GaAs ALE growth was c a r r i e d ou t . We c?ianged t h e res idence t i m e o f t h e s u b s t r a t e i n t h e hydrogen atmosphere and i n v e s t i g a t e d t h e dependence on i t o f t h e growth r a t e . The r e s u l t s ob ta ined a r e shown i n F i g . 3. When t h e t i m e was l e s s t han 5 seconds, i t was found t h a t t h e growth r a t e was enhanced even f o r ALE growth. One p o s s i b l e reason i s t h a t t h e adsorbed GaCl spec ies on t h e SiO does n o t comp le te l y desorb f r om t h e sur face b e f o r e reach ing t h e a rsen i c spec ies i 6 t h e n e x t step, and i t mig ra tes by su r face d i f f u s i o n i n t o t h e window area d u r i n g t h e p e r i o d o f a r sen i c supply. However, t h i s e f f e c t can be avoided and monolayer growth can be achieved i n ALE s e l e c t i v e growth by l e a v i n g t h e s u b s t r a t e f o r g r e a t e r t han 5 seconds a t t h e GaCl purge s tep. Furthermore, t h e r i d g e , which i s a v e r y h i g h growth r a t e r e g i o n observed i n t h e v i c i n i t y o f t h e window frame i n convent iona l VPE, disappeared i n ALE. I n MO-ALE w i t h A r l ase r b y A. Doi , e t a l . , growth occurs s e l e c t i v e l y a t t h e p o s i t i o n where t h e A r l a s e r i s i r r a d i a t e d (17) . They have r e p o r t e d 1x2 mm e l l i p s e shape growth and emphasized t h a t t h e t h e t o p o f t h e e l l i pse -shape i s v e r y f l a t , a l though t h e l i g h t i n t e n s i t y d i s t r i b u t i o n i s Gaussian. Th i s i s a k i n d o f s e l e c t i v e growth w i t h o u t a mask. I f t h e c ross s e c t i o n o f t h e l i g h t beam were sma l l e r t h a t c u r r e n t l y used, a more p r e c i s e s t r u c t u r e would be grown s e l e c t i v e l y . I n GaC1-ALE, i t would be p o s s i b l e i n t h e f u t u r e t o s e l e c t adso rp t i on s i t e s on t h e s u b s t r a t e w i t h o u t a mask, u s i n g va r i ous beams o f l i g h t , e l e c t r o n s and ions . 6. Atomic-plane Doping Atomic-plane o r d e l t a ( 6 1 dop ing i s conducted f o r t h e conf inement o f dopants t o one atomic p lane two -d imens iona l l y and i s used f o r de l ta -doped FET and modula t ion s t r u c t u r e dev ices (18,19). Th i s techn ique has been s t u d i e d through use o f t h e p resen t ALE techn ique ( 9 ) . Hydrogen se len ide (H Se) was used as a dopant source gas. The growth procedure i S as f o l l o w s : t h e 9 G ~ A $ ALE l a y e r s were f i r s t grown on t h e subs t ra te , f o l l o w e d by t h e GaC1-adsorbed s u b s t r a t e exposure t o H Se i n s t e a d o f t o ASH . On t h e i m p u r i t y plane, 9 GaAs l a y e r s were aga in grown. ~ h e g e processes were repsated a d e s i r e d number o f t imes. We i n v e s t i g a t e d t h e dependence o f i nco rpo ra ted Se q u a n t i t y on growth temperature and H Se p a r t i a l p ressure f o r these sample. These r e s u l t s showed t h a t Se i n c o r p o r a t i B n i nc reased w i t h decreas ing t h e growth temperature, and exper imenta l r e s u l t s i n d i c a t e d t h a t a t around 350°C t h e Ga s u r f ace was e n t i r e l y covered w i t h Se atoms. The c a r r i e r c o n c e n t r a t i o n f o r these samples was measured by t h e van de r Pauw method. I t vgd a-yaximum va lue a t a growth temperature of around 450 ' C and was as h i g h as 10 cm f o r l a y e r s on (100) and (111 )A subs t ra tes . The c a r r i e r c o n c e n t r a t i o n , however, decreased r a p i d l y w i t h f u r t h e r i n c o r p o r a t i o n o f selenium. T h i s behav io r i s n o t y e t f u l l y understood, b u t one p o s s i b l e reason i s t h a t t h e Se i m p u r i t y l e v e l may deepen due t o an i nc rease i n t h e Se-Se bond w i t h i n c r e a s i n g dop ing concen t ra t i on , as has been r e p o r t e d by H. P. Hjalmarson (20) . F i g u r e 4 shows a c a r r i e r c o n c e n t r a t i o n p r o f i l e a long t h e growth d i r e c t i o n . T h i s p r o f i l e was measured w i t h t h e C-V method f r o m a sample wh ich had one i m p u r i t y p lane i n t h e ALE GaAs l a y e r s . Growth temperature was 450°C. A v e r y steep p r o f i l e w i t h 8 nm o f f u l l w i d t h h a l f -maximum(FWHM) was obta ined. A c t i v a t i o n energy f o r t h e d i f f u s i o n and t h e p re -exponen t i a l f a c t o r o f d i f f u s i o n cons tan t Do o f Se i n t h e GaAs a re r e p o r t e d t o be 4.16 eV ap923000 cmz/s, r e s p e c t i v e l y (21) . Using these values, t h e d i f f u i o l e n g t h x ( = ( D t ) D: d i f f u s i o n cons tan t t : t i m e ) i s c a l c u l a t e d t o be 2x10 ' 1 a t 45o0C when t h e t i s 4 hours, which corresponds approx imate ly t o t h e t o t a l g rowth t ime. Accord ing ly , we assume t h a t t h e i m p u r i t y d i f f u s i o n i s n o t r e s p o n s i b l e f o r t h e broadening o f t h e FWHM. Furthermore, we eva lua ted one o f these samples w i t h c ross s e c t i o n t r ansm iss ion e l e c t r o n microscopy. The i m p u r i t y p lanes were c l e a r l y observed w i t h i n a 3 l a y e r f l u c t u a t i o n . Th i s f l u c t u a t i o n i s cons idered t o be r e l a t e d t o s u b s t r a t e su r face roughness because t h e s u b s t r a t e was n o t s p e c i a l l y t r e a t e d f o r making an a t o m i c a l l y f l a t su r face p r i o r t o t h e growth. Consequently, c a r r i e r d i f f u s i o n f r o m t h e i m p u r i t y p lane may determine t h e w i d t h o f t h e above p r o f i l e . 7. Device A p p l i c a t i o n t o de l ta -doped FET It has been r e p o r t e d by K. Ploog and E. F. Shubert. e t a l . t h a t i n t h e i r c a l c u l a t i o n and p r e l i m i n a r y exper imenta l r e s u l t s , a f i e l d e f f e c t t r a n s i s t o r (FET) u s i n g a S -doped l a y e r has r e l a t i v e l y h i g h transconductance (18,19). T h i s can be exp la ined by t h e f a c t t h a t e l e c t r o n s o r i g i n a t i n g i n t h e i m p u r i t y p lane a r e con f i ned t o a v e r y s teep channel around t h e i m p u r i t y p lane and behave as a twodimensional e l e c t r o n gas. These resea rche rs achieved such a s t r u c t u r e b y MBE u s i n g S i as an i m p u r i t y . T h i s dev i ce s t r u c t u r e has been f a b r i c a t e d u s i n g t h e ALE under d i scuss ion here. We f i r s t eva luated de l ta -doped l a y e r s th rough m o b i l i t i e s measured by t h e van de r Pauw method a t 77K. Samples measured have a s t r u c t u r e c o n s i s t i n g o f 35 GaAs l aye rs , 1 dop ing l a y e r , 105 GaAs l a y e r s . Growth temperature was around 450°C. F i g u r e 5 shows t h e r e l a t i o n s h i p between m o b i l i t y and sheet c a r r i e r concen t ra t i on . Data from r e f e r e n c e 18 a re a l s o shown. As seen i n t h i s f i g u r e , d a t a ob ta ined f r o m Se d e l t a doped GaC7-ALE c o i n c i d e w e l l w i t h t hose f rom S i de l ta -doped samples grown b y MBE. T h i s i n d i c a t e s t h a t t h e q u a l i t y o f ALE l a y e r s i s comparable w i t h t h a t o f MBE laye rs . F i g u r e 6 shows t h e FET s t r u c t u r e f a b r i c a t e d here (22 ) . F i r s t , a Fe-doped buffer l a y e r o f 0.8 um was grown a t 530°C on t h e s e m i i n s u l a t i n g GaAs subs t ra te . A f t e r t h e b u f f e r l a y e r was grown, 35 GaAs l aye rs , 1 dop ing l a y e r and 105 GaAs l a y e r s were consecu t i v f$y g2own. Sheet s a r r i e r c o n c e n t r a t i o n and m o b i l i t y a t room temperature were 4x10 cm and 1700 cm /Vs, r e s p e c t i v e l y . On t h e t o p o f t h e sample, AuGe/Ni me ta l s f o r source and d r a i n ohmic c o n t a c t s and A1 metal f o r t h e ga te e l e c t r o d e were evaporated. Gate l e n g t h and ga te w i d t h were 1.0 um and 200 um, r e s p e c t i v e l y . F i g u r e 7 shows t h e c u r r e n t v o l t a g e c h a r a c t e r i s t i c s o f a de l ta -doped deplet ion-mode FET. Th i s FET has e x c e l l e n t p i n c h o f f c h a r a c t e r i s t i c s w i t h v e r y smal l ou tpu t conductance i n t h e s a t u r a t e d reg ion . The e x t r i n s i c t ransconductance was 190 mS/mm. Th i s va lue was kep t n e a r l y cons tan t above t h e p i n c h o f f vo l t age . Th i s shows t h a t e l e c t r o n s a re con f i ned t o t h e V-shaped p o t e n t i a l w e l l by t h e 5 -dop ing s t r u c t u r e . Furthermore, t ransconductance as h i g h as 215 mS/mm was ob ta ined w i t h a ga te l e n g t h o f 0.5 um. If a ga te recess s t r u c t u r e i s adopted, 5-doped FET w i t h t h e p resen t method w i l l o f fe r h i g h e r t ransconductance. As f a r as t h e authors know, bes ides t h e S-doped FET ment ioned above, t h e r e has so f a r been no o t h e r r e p o r t o f dev i ce f a b r i c a t i o n u s i n g ALE. However, N. Kobayashi, e t a l . have r e p o r t e d t h e p o s s i b i L i t y o f dev i ce a p p l i c a t i o n i n a AlGats/G2As 2DEGFET s t r u c t u r e u s i n g FME (8). They ob ta ined 2DEG m o b i l i t y as h i g h as 7x10 cm /Vs a t 6K. It has been r e p o r t e d t h a t t h e 2DEG c h a r a c t e r i s t i c s were main ta ined w i t h o u t any deg rada t i on a t a growth temperature as low as 550°C.