M a p p i n g o f t h e l o c a l i z e d i n t e H a c e a n d S u r- a c e s t a t e s o f I n G a A s l a t t i c e

m a t c h e d t o F e - d o p e d l n P b y i n f r a r e d s p e c t r o s c o p y

L . H . Pe ng ,a) T . P . E . Broe kae rt , W . Y . C ho i, B . R . Be n ne tt ,b) J . H . S In a . V . D iad it»k .C)S . H . G rov es ,C) S . C . Pa lm atee r,C) N . Pa n ,d) a nd C . G . Fo nsta d ¯ ¯

Departmenf qf H ea r - - f EM Ine- n ng and Comp - fer Sd ene-, M o sc ha - - f ag - - re qf Techno1010%Combn dge, M - - ach- serfs O42f 39

( R ecei ved 10 A pd 1992; accept ed for pub l icat ion 2 Ju ly 1992 )

I nf rar ed absor pt ion and p hot ocu r ren t m easur em ent s have been appl ied to st ud y t he

photoresponse below the band gap of ind iu m gal l ium ar sen ide ( I non G ao-47A s) gr ow n lat t icem atched to F e-doped sem i-insu lat i ng ind ium phosphide ( I nP ) subst rates by var ious epi tax ial

gr owt h techniques, i ncl udi ng m ol ecul ar beam epi taxy , l iqu id phase epi taxy , and m etalor gan ic

chem ical vapor deposi t ion . I t i s found t hat F e at the I nG aA SA g- in ter face is r esponsib le fo rexcit or1-l ike and polar i zat ion sensi t ive abso rpt ion peak s. Bot h el ect ron and hol e em ission in to

the conduct ion an d valence ban ds, rs peCUR ly , w er e obser ved , and a deep Fe level w as ida m ned

0 .37 eV below the conduct ion band edge of bulk F e- - aA s. L ow er ing of t he local crystal

sym m et r y d ue to t he in ter face elÄ t r i c Held is pr oposed to be the m echanism t hat descr ibes t hed ipo le-al low ed inter ba£ absor p t ion of 3d t r ansi t ion m etal i m pur it ies in nar row band gap I I I -Vcom po unds l i k e t he F eZ+ :111G aA s used in th is stu dy . T he am bigui t y in d ist i ngu ish ing I nG aA s

q uant um w el l i n ters -bband absor pt ion si gnals fr om the F e i rHerbartd absor pt ion si gnals i s also

ad dressed .

l . INTRODUCTION

T her e has been a consid erab le eHor t du r ing t he past

decade t o st udy d efects and da p leve1s at het er î unct ionsbetw een I I I -V com pound sem icon du ct or s-u D evel op ing

good cont ro l of t hese int er face da p l evels w i1l govern t he

ul t im ate dev ice per form ance, a w e1l as la d t o a t horough

undema nd ing of fundam en tal dev ice physics. T w o w el l

M ow n i l l ust r at ions of t he im por tan ce of stud y of d eeplevel s ar e the obser vat ions of E L 2 M d of DX centers in t he

G aA s/ A lG aA s m ater i al system . T hough a tunned m odel

t hat can sat isfactor il y explain aH of t he exper i m en tal dat ai s st iU l ack in g, i t i s non e t he less agreed t hat i t i s non a oi-

ch iom et r ic grow t h ( e.g., A SGa) , dopan t i m p u r it ies ( e.g.,

SiGa) , an d thei r associ ated com pl exes t hat give r i se to t heda p level s in t he G aA s/ A l G aA s system -2

I nG aA s lat t ice m at ched t o I nP is another t ech no lo- -

cal l y i m por tant m ater ial system that has gr eat potent i al inh igh speed elect roni c and op tÄ lect r ot- dev ice appl ica-

t ions. A l t ho ugh m an y n ovel device st r uct ur es have been

pr oposed and great p rogress has ba n repor ted , t her e ar e

st i l l unresolved i ssues in t h is system . I n par t icular , det ai l edanal ysÄ ± of secondar y ion m ass spect r oscopy ( SI M S) ,

opt ical d eep l evel t r a ts- n t spect r oscopy ( D L T S) , andcapaci tance-vol tage ( C õ m easurem en t s ind icat e t h e ex -

i stence of deep , locU i - d stat es at t he i n ter face bet w een

I nG aA s epi layer s and F e- - subst r ates and i t has been

sp- t-lated t hat t hs e states are due t o F e r elat ed defect s.

T he under standing of 3d t ransi t ion m etal im pur i t ies in

the w id e band gap I I -V 1 and I I I -V b inar y com pounds± is

now w el l est ab l ish ed . T he obser vat ion of st r ong nngermy*

111t er atom ic absorpt ion and photolum inescence ( PL ) at

low tem per at ur e betw een the cryst al Held spl i t t ing m ult iplet s ¸ E , doubly degener at e and ¯ T p t Hply degenerate) B

thought t o be evidence t hat t ransi t i on m et al im pur i t ies re-

sid e i n subst i t ut ion al si tes w i t h Td sym m et r y . A typ- -val ue of t he cryst al Held spl i t t ing energy is 3d P cm - l

(O.37 eV ) fo r F e in t he w ide band gap I I I -V com poun* ,7

£ e associat ed Hr st or der spi n-orbi tal spl i t t ing w i t h in the¯ T 2 excit ed state m ul t i p let s is 3m cm - l , and th e second

or der spi n-o rbi tal spl i t t ing w i t h in t he 5E ground state mul-t id es is onl y 50 cm - 1. A s a m or e Spa i nc exam ple, for

Fe i n I nP i t is now w e11 accepted t hat t he 5E F e2+ ground

stat e á O.63 eV below t he I nP conduct ion band edge, ther 2 F eZ+ exci ted sw u m -35 eV h igher t han t he ¼ ground

st ate, and t he F e»+ A l state is 0 .7 1 eV below the ­ p ound

st ate and is ncar l y p inned to the I nP valence band edge atr oom t em per at ur e. ,,£ T hese levels are i l lust rated schemat-

ical l y on t he lef t -hand side of F i g. 1.R ecent l y , D artsas et af-3 invest igated a m olecular beam

epi tax ial l y ( M BE ) grow n na to-53G ao-47A s/ F e-d oped semi-

insul at i ng ( SU I t- i nt er face and concl uded t hat t he Fe

i m pur i t ies ar e local ized at t he in ter face as elect r on-l ike

t r aps w i th an act i vat ion energy of 0.32 ev - K ido- -chi er at. also r epor ted t he SI M S and D L T S obser vat i on of da p

levels associated w i t h the segregat ion of F e im pur i t ies at

t he i n ter face of l i quid ph ase ep it ax ial ly ( L PE ) grownI nG aA sP/ Fe- - j unct ion -6eld eHect t ransist ors (JF ET s) .

T he i nt er faci a1 F e con cent rat ion w as found t o be t hra

or der s of m agn i tude l ar ger than t hat con tained in theFe- - subst rate ( 10 ± cm - 3) . L a tu - he er at 5 also st ud-

ied M B E gr ow n Si d oped and uM oped I no.n G ao-47A s ep-i layer s on both F e-doped SI I nP and S-doped n + -I nP sub-

st rates- A 0 .33 eV deep level found at t he su r face and t he

± D ivision of A ppl ied S ter-Cs , f -aw ard U niversity, Cambridge, M A

O2138.b)Depar tment of M ater ials Science and Engma ro-- M ET ., Cambr idge,

M A 02139.C)L incoln L a² ratory- M I T -- Lexington, M A 02173.d)Rap heon R- arch, 131 Spr ing St., b x-n- on. M A O2 173.

3 6 6 4 J . APPI- Phys. 72 (8), 15 October 1992 O02 1 -8979 / 92 / 203664-O6$04 00 @ 1992 Amer¤an lnShu te of Phys¤ s 3 6 6 4

/ , .

/

FIG . 1. Fe crystal nd d spl i t ting states in bulk I nP under r d symmet ry ( left ) , and Fe state at t he I nGaA s/ 1nP inter face (Hght ) , where the symmet r yis rd uced to Du due to the interface elect ric Held. Pl and P2 are t he moma nuf¤1 mat r i x elements used in nght binding calculat ions.

sym m etr y in the p resence of an in t er face elect r ic Held fr om

T d t o D u w Hl be p r esen ted t o expl ain these exper im ent a1

rÄ u1ts.

H. M EA SUR EMENT S

T he sam p le used in th is st udy are l ist ed i n T able I and

th ey incl ud e M B E grow n doped I nG aA s/ Fe- - , w i t h dM

fer en t dop ing concent rat ions and eP layer th ick ness, L PEgrow n Fe:I nG aA s/ F e- - , and m et alorg anic chem ical va-

po r deposi t ion ( M 0 CV D ) gr ow n uM oped I nG aA s/

m M oped I nP/ F e b ¤ - A test set of sam p les of doped and

undoped I nA s/ I nG aA s/ A l A s singl e quan tum w el1( SQ W ) heter om -- h u es on F e- - w er e a1so inch - ed to

faci l i tate t he ida m na t ion of L W I R signals and inter faceFe inter-band absor pt ion signals. T he I nG aA s epi l ayers

used fo r th is study w ere of h igh crystal l i ne qual i ty ; t yp -cal1y t he lat t ice m ism atch w as less t han 5 ¿ 1o - 4 and t he

fu l1 w i dth at hal f m axim um ( F W H M ) of ( 4M ) double-cr ystal x-ray d iHract ion eP l ayer- peaks w as less t han 28

arc s. R oom t em perat ure phot ol um inescence ( PL ) m ea-

sur em ent s ind icated the I nG aA s band gap to be about O.74

TABLE I . InGaAs/ Fe- - samples.

inter face w as suggested to be oxygen rel at ed . O n t he otherhand , f rom sPed ro- opi c m easur em ent s, G U l lot H 44 6 r e-

por ted a m uch w eaker P L int ensi t y at O.34 eV in th eirstudy of L PE grow n F e-doped I nG aA s/ n + ¹ nP and sug-gest ed t hat t he 5T 2 F e2+ excit ed state w as near 1y in r eso-

nance w i th t he I nG aA s cond uct ion band edge.

I n spi t e of the w or k m en t ioned above, i t r em ains t r uethat no d i rect ly m easur ed inf r ar ed ( I R ) absor pt ion be-

( tw eet1 the cr yst al 6eld spUt t i ng m d u plets of F e in I nG aA shas been repo r ted . T her e is indeed a gr eat need t o under -

S a d t he opt ical r espon se below the band gap of I nG aA s,

s pecial l y because of t he cur r ent in ter est in novel longw avelength inters - band in f r ared absorp t ion9 ( L W I R )

quantum w el l detect or s based on t he I nG aA s/ F e- - sys-

tem . T he pu rpose of the presen t w or k ser ves ( i ) t o c lar ify

the opt ical i oni zat ion ener gy of t he deep level at t he

I nG aA s/ F e- - in t er face and to ident i f y i ts or igin , ( i i ) t o

propose a m od el t hat consist ent l y exp lains t he po lar izat ion

sensit i ve absorpt ion for t r ansi t i on m etal im pur it ies at t he

heter Ì unct ion of nar row band gap m ater n 1s such as t h e

I nG aA s:F e consider ed in t he pr esent st udy , and ( h i ) t o

dem onst rate t he am bi gui t y t hat can ar ise i n ident if y ing and

dist ingui sh ing t h e quan tum w el l in ter s - band abso r pt ion

si- a s an d t he in ter face F e int e- - M signals in t he

InG aA s/ F e- - m at er ia1 system .W e repor t h ere w hat w e bel ieve is the 6r st d i r - t ob- -

sen at ion of exci t ort-l i k e infr ared absor pt i on d ue to elec-tron and hole t r ansi t ions f r om the F e2+ 3d deep l evel to t he

con duct ion and valence band ed ges in I nG aA s. T he deep

Fe l evels are presen t because of the m ovem ent of F e in tothe I nG aA s eP layer f r om t he F e- - subst r ate. T h e polar -

izat ion sensi t i v i t y of t he absor pt ion w as found to be

st ron gl y depen dent on t he in ter face elect r ic neld and hence

wa a funct ion of dopant concent r ati on . L ar ge St ar k shi f t s

betw een T E and T M absor pt ion peaks w ere also observed ., -¢ R ehybr i d izat ion of p orbi t al s (A 1, B 1; Å and E ) of d band

. states ( ¼ and 5å ) due t o the low er in g of local cry stal

Fe mterband transit ionsS a m p l e

A MBE grown, n+ ( 10± cm- ¯)

B MBE grown, uM oped

C LPE grown, Fe do* d

elC Hon-like at inter faced a m n- and hole-l ike

at inter faceelect ron- and hole-l ike at

interface. bulk , and sur faceelectron- and hole-like

at interfaceelectron-like at interface

QW intersubband t ransit ion

D M 0 CVD grown, urMoped

electron-l ike at interface

E M BE grown. dopedInA s/ I nGaA s/ A lA s/ I nG aA s SQW

F M BE grown, urMopedI nA SA t-G aA s/ A lA s/ 111GaA s SQW

Peng ef aL 3 6 6 53 6 6 5 J . APPI- Phys ., Vo l. 72, No . 8, 15 October 1992

pinned t o t he F e2+ 5E level . O n t he 1n G aA s sur face, w e

assum e i t is p i nned t o the I nG aA s sur face states, w h ich ar e0 .2 eV below th e I nG aA s conduct ion band edge.15 W e

fur ther poin t out the ex ist ence of in t er face and sur face

d eplet ion r egions w h ich ( 1 ) in t rod uce st rong elect r ic Heldsand d ipol e layer s, and ( 2 ) low er t he local cryst al sym m e-

t ry f r om Td to D u at th e heter î unct ion and at the su r face.

W e pr oceed by app l y ing gr oup t heor y t o st udy t he F e

elect ron ic states associat ed w i t h D u sym m et ry u nder ast r ong i nter face elect r ic Hel d . T he ham m ed F e3+ 3d state

has » 1 sym m et r y and l ies deep w i th i n t he valence band ofI nG aA s and w Hl be neglected . T he f r ee ion F e2+ 3d® m ul -t i p let s ( 5D , w i t h an gul ar m om en tum f = 2 ) w i l l nr st sp l i t

in to a doubl y degen erate ground state, 5E , and a t n pl ydegener ate exci t ed st ate, 5õ 2, under t he global r d sym m e-

tU ¤Since the exper im en ts w er e done at room tem peratu re,t he second or der sp in -or bi tal sp l i t t ing ( ~ 50 cm - l ) of the

degener ate 5E gr ound states can be neglected and they can

st i l l be t r eat ed as d egener ate; f or conven ience t he 5õ 2 ex -

cit ed states can be t em porar i l y t r eated as degener ate. T hent he in ter face elect r ic Oe1d can be t ur ned on and the sym -m et ry l ow er ed dow n t o, D u . T he degener ate 5E gr ound

states t hen sM i t in t o A l + ç states ( a one-di m ensionalrepresent at ion w i t h basis fu nct ion s g 2 and F , w h ich areAs--l i ke ) , and t h e t n p1y d egener ate 5r 2 exci ted states sp l i t

in to ² + E¯ stat es [w i t h basis f unct ion s Z and (Ö ³ ,

w hi ch ar e Pd ike] . T h is sym m et ry analysi s essent ial ly l eads

to t he descr ipt ion of l ocal ized d band states at t he i nt er face

as a near ly degener at e set of (S ,P ) at om ic orbi tal s und erthe hd uet- e of a st rong in ter face el ect r ic Held . T igh t bi nd -

i ng analysis can then be appl ied w i t h i n t his (S ,P ) near ly

degener ate set , and cl ear ly one w oul d expect to see a lar geStar k eHect and the r ehyb r id i zat ion of t hese at om ic orb i t -al s. T h is t hen exp lains t he polar izat ion sensi t i ve´absorp t ion

and th e ener gy spl i t t i ng betw een t he T E and T M abso rp -t i on peaks ( Star k e& ct ) seen exper im en tal ly . T he pow er -

fu l th ing abou t using such a sym m et r y argu m ent , is t hatbased on i t , one w ou ld expect t o see bot h elect r on-l ik e andhole-l i k e F e-in ter-band absor pt ion under sui tabl e ci rcum -

st ances. T h eoret - - l y t h is r esul t is reached by groupingtogether the (A 1, B 2, E ) states as an elect ronic n ear ly de-

generate set ( B 2 and E ar e resonant w i th in t he I nG aA s

cond uct ion band edge) and gr oupi ng (ç , X , Y, Z ) as t hehol e near ly degenerate set (X , Y, Z are the valence band

at om ic orbi t als of I nG aA s) . A gain a l arge Star k sh i ft i sexpected t o occur betw een the T E and T M hole-l i ke in ter -

band ab sor p t ion . Exper i m en tal ly both elect ron- and hole-

l ike Fe in ta bard absor pt ion is obser ved in the uM oped

I nG aA s/ F e- - and F e doped I nG aA s/ F e- - sam ples,and onl y d em on-1i ke in tem - - absor pt ion is found in

heav i ly n d oped I nG aA s/ F en d sam ples, because of t he

phase space m l ing eHa t - T hese d ipol e al low ed int em - -absor pt i on stat es are a1so d r aw n on the r ight -side of F ig. l .

uuz¤pee--zX» *

F1G . 4. T op (a ) unpolarized F n R absorpt ion spa tra for doped, andundo* d 1nA s/ I nG aA s/ A lA s/ n+¹ nGaA s SQW s, and an n +¹ nG aA s

eP layer on Fe- - substrates. Bottom (b ) p lam ed ¸ TI R absorpt ionSM O ra for Fe intem - -d t ransit tons at an M BE grown n - -InGaA s/

Fe- - mted aÄ .

T his Hgure i s i nd u ded to i l l ust r ate t he am bigu i ty t hat ex -

ist s in dist ingu ishin g the L W I R signal f rom t he F e int er -

ba d sign al s and show s th e po1an a t ion sensi t ive absor p-- t ion peak s associated w i t h th e Fe int er-band signals.

¢ £ . T HEORET ICA L CO NS IDERA T IO N

T here is an exper im en tal obser vat ion associated w i t hthe ° m etanu - cal ly ab r up t ,, I nG aA s/ F e- - in ter face

that is very i m por tant to t h is w or k :l* 12 i t i s i m por tan t to

note t hat a th in F e-r ich I n P su- ace segregat ion r egion is

found t o for m on t he subst r ate as i t is ha ted d ur ing thepregrow t h p rocess. A second im por tant exper im enta1 ob-

ser vat ion i s t hat t he ener gy separ at ion of t he cr yst al neld

spl i t t ing l eve1s of a given 3d im pur i t y are l i k ely indepen

dent of the speciHc host cr yst al fo r a large nu m ber of w ideband gap I L V I a d I I I -V bi nar y com pound sem iconduc-tom -7 T hus, w e assum e Ä a zer ot h or der app roxim at ionthat the F e 3d ( F e3+A UF- + 5E , 57±2) bands ar e cont inu ous

acr oss t he I nG aA SA t- in ter face and penetrat e a m utelength i nt o t he I nG aA s (Ä y , a few na mm et er-s ) due t o t he

d u d g- - n of F e im puru s - T h i s assum pt ion la ds to theconcl usion th at t he F e2+ 5å exci t ed stats r eside near ly

r6 0111m t w i th i n th e conduct ion band edge of I nG aA s

(w hich is also consi st ent w i t h t he accep ted est im at e ofh o ax / I nP conduct ion band o& et ,± 0.2 eV ) . T hough

Fer m i level pin ning l4 at sem icond uct or int er faces and Sur -

f- s i s a com pl i cated fu nct ion of char ge r ed ist r ibut ion ,

dopant concen t r at ion , and sur face stats , and ha ba n oneof the topi cs of debate in sem icond uctor physi cs over t he

, e a t dc ades, w e assum e t hat t he F erm i level w i th in th e

m P subst rat e and near t he I n G aA s/ F e- - i n ter face is

lV . D ISC USSIO N

A Or st look at t he st r ong and shar p absor pt ion peaks in

F ig. 2 w ould m isleadi ngly suggest t hat t hey are f r om som e

unk now n opt ica1 inter ference eHem s w h ich al so m yster i -

ousl y i ncor por ate t he pol ar izat ion eHa t . H ow ever , t he

J. App|. Phys., Vol. 72. No. 8 , 15 October 19923 6 6 7 Peng ef aL 3 6 6 7

the T E absor p t ion because of t he do uting H los of Tu

m d e. H ow ever , t he excel len t polar izat ion select iv i t y of th¤pseudo-w aveguid ing geom et r y clear l y m d s t he equal - L

Ä m u on st r engt h for th e d iger en t polar i zat ions and the

ener gy sh in bet w een T M and T E (T M alw ays has a him -

absor pt ion ener gy w hen com pared w i th T E ) , w h ich aý

p roves the val id i t y of t he proposed m od el . F igure 4 (a)

( u pper panel ) d isp lays t he unpolar i zed abso rpt ion data ofa doped SQ W , uM oped SQ W , and n + I nG aA s/ F e- -

T hese 3 sam ples w er e f r om t he sam e gr ow t h per iod M d

h ad exact ly t he sam e st r uctur es, excep t t hat t he nrst oneincluded a doped I nA s/ I nG aA s/ A 1A s SQ W ,l 8 the sm nd

one had an u M oped SQW , and t he la t one d id not have

an y quant um w el l . I t i s c lear t hat t he Fe i rHerbal1delect r on-l ik e absor pt ion i s repr od ucible f rom sam ple to

sam pl e and the L W I R quan tum w el l si - al cou ld be a ny

m 1sideru ined i f one w as not aw ar e of the Fe i rHerba ld

abso rp t ion . A ccess t o special l y ds igned tÄ t heterom - -

h u es i s essen t ial to being able to iden t i fy the var ious peaks.

A l t hough a fu l l scal e analysis of t he m utu al in teract ion

betw een t he cr ystal Held an d inter face elect r ic Held , M d

t hei r eHects on t he in ter face F e stat es is beyond the sÄ *

of t h is ar t icle, i t is none the less possibl e to see the t rends

i n t he r elat ive Star k sh i f t bet w een T E and T M absor pt ion

peaks and the ab ou t e absorpt ion energy sh i ft in dig¤ -o n l y doped sam ples, basical l y fo l low t he pr ed ict ions of the

t i gh t b ind in g analysis. I t can be seen that for heavi l y domn + -I nG aA s/ F e:I nP ( 10 19 Ctn - 3) , on ly t he elect r on -l ike Fe

in ter-band absor pt ion is al low ed ( because of t he phase

space n l l i ng eHa n - T h is t ransi t ion has the lar gest Starksp l i t t ing ( 4m cm - l , 5O m eV ) and h igher in t em - - ab

som u on ener gy because of t he st ronger in ter f ace el - - icneld - F or t he u1M oped I nG aA s/ F e- - ( 10 15 Ctn - 3) , both

t he elect r on - and hol e-l i ke F e inter-band t r ansi t ions are al -low ed b ut t he St ar k sh i f t i s only 2Ä cm - 1. A n est im ate of

t he in ter face elect r ic Held st rengt h f r om the St ar k shiftgi ves a num ber around 105 V / cm , w hi ch has been recent ly

Ven ned by Z hou Cf aZ 19

photÄ ur ren t m easur em ent dau cemain ly ver i fy that t he eare elect ron ic state t ransi t ions. T he r easo n that such ex -

t rem ely sharp absorpt ion peak s ar e seen even at r oom t em -

per at ur e com es not o nly because t hey invo lve atom ic stat e

t ransi t ions, bu t also because of t he low er d im ensional i t y

( 1D ) ; an analogue is found in quant um w el 1 and quantum

, w i re excHon absorpt ion -

T he assi gn m en ts of t hese absor pt ion peak s are also

l i sted i n T ab1e I . T o bd eny sum m ar ize, t he abso rp t i on peakat 25m cm - l seen in the M B E sam pl es is assigned t o an

el ect ron-l ik e in terban d t r ansi t ion f rom F e2+ 5E t o t he

I nG aA s con duct ion band edge at the I nG aA s/ I nP in ter -face, w hi le the peak at 39m cm - 1 is suggested t o be t he

hole-l i ke i n t er-band t ransi t ion f r om the I nG aA s valence

band edge to Fe2+ ¼ . T hi s can be easi1y seen f r om F i gs.

2 (a ) ( n + -I nG aA s) , 2 ( b ) ( uM oped I nG aA s) , and 2 ( c )

( F e-doped I nG aA s) . T he suu n spi kes 1ocat ed at t he

higher ener gy sides of t hese abso rpt ion peaks are con sis-

tent - found in the M B E sam p les and ar e suggested t o be

r elated to the lÄ al vibrat ion m od es. T he ad d it ional feat u reseen in L PEg row n F e-doped h o ax at 17@ cm - 1 and

46M cm - 1 is assign ed to be elect r on- and hole-1i ke F e

int er-band t r ansi t ions, respect ivel y , at t he ai r -I nG aA s sur -

face, w her e i t is assum ed t hat t he Fer m i level is p inned tothe Fe2+ 5E at the sur face, w h i le t he m ain absor p t ion cen -tered at 3d P cm - l is assigned to be F e in t he m idgap of

I nG aA s. T h e ida m na t ion of t h is F e 3d P cm - 1 ab sor p-

t ion peak agr ees w el l w i t h a r ecent low tem per at ur e PLst udy of L PE-grow n F e:I n G aA s.6

F igu r e 3 ( b ) ( low er panel ) show s th e absor pt i on dataof L PE -gr ow n Fe-doped I nG aA s and F ig. 3 ( a ) ( upper

, - pane1) show s it s phot ocu r r en t response ( t he el ect rode

¢ » spacings are 5 and 10 p m ) . T he excel len t agreem ent be-

tw een these absorp t i on and photocu r rent peak s again

pr oves t hat t he F e level i nt e- - 1d t r ansi t ion is responsible

for t he below bandgap absor pt ion of I nG aA s/ F e- - in

t h is st udy .

I n ter face absor pt ion d ue t o F e w as a1so obser ved i nM 0 CV D sam ples in w hi ch a t h in eP l ayer- of u M oped I n P

( 1M nm ) had been inser ted bet w een the F e- - subst rates

and t he I nG aA s eP l ayer - T h is obser vat ion is consisten t

w i t h our m od el because F e is M ow n to d i - use q uick 1y

th r ough I nP eP layer s grow n on Fe- - subst rates, and hasbeen obser ved using SI M S by H oM es er d l l for L PE

grow n sam pl es M d by K ni gh t er a t 12 for low pressu r e

M O C V D gr ow n sam ples.

T w o fact or s aHect t he over al l photocu r ren t r esponse

cu r ve: ( 1) the b lackbody sou rce ( gl ow bar ) in tensi t y is no t

constant over the w ide spect r al range studi ed here, and ( 2 )

t he fa t sur face and in ter face recom bin at ion speeds can

easi l y obscu re the photÄ ur rent r Ä ponse CUR e because of

the ul t ra sh or t f r ee car r ier l i fet im e i n the pr esence of alar ge densi t y of ann n ¤ defects-l6,l7 F igur e 4 ( b ) ( l ow er

panel ) dem onst r ates t he pol ar izat i on sensi t ive absorp t i onat t he in ter face of n + I nG aA s/ F U r¤ an d cl ear l y in di cat es

a lar ge St ar k shi ft ( 5O m eV ) betw een T E and T M absor p-

t ion peaks. D ue to t he unequal pow er loss for T M an d T E

¯ ® l i gh t propagat ing in an im per fect conduct ing par al l el

p lat es, the unpolar ized l ight absor p t ion data seem s t o favo r

V. CO NC L USIO N

B ased upon t he above analysis, and the obser vat ion ofst r ong in ter band elect ron- and hole-l ik e absor pt ion , a d a

lar ge Star k shi f t betw een the T M and T E absorp t ion peaks,

i t can be concl uded t hat t hese absor pt ion peaks or igi nate

f r om t he m u te ou td im s - n of Fe i m p ur i t ies i nt o the

I nG aA s epi layer in t he v icin i t y of I nG aA s/ F e- - i nter -face. O pt ical data fo r th e bul k F e2+ to conduct ion edget ransi t io n energy ( 3d P cm - 1, 0 .37 eV ) agree w el l w i t h the

p rev iousl y pubushed data. T he existence of these absor p

t ion peak s m eans that caut ion m ust be exercised to d ist in-

gu ish quan tum w el l L W I R absor pt ion f r om the F e in ter -

band absorp t ion .

A C K NO W L IEDG M ENT S

T h is resear ch w as suppor ted by t he N at ional Science

F ound at ion , G r an t ECS 9M 8485. T he au thors w ish to

thank Pr ofessor R . R ei f at M I T fo r offer ing the use of h is

3 6 6 8 J . Appl. Phys., Vd . 72 , No. 8, 15 October 1992 Peng ef aZ 3 6 6 8

F TI R f ac i l i t y , an d P r o fesso r R . J o n es a t 1H1aar±vÏ aaafrd .d U n i v er -

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