Terahertz band-gap in InAs/GaSb type II superlattices

Transcription

1 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 Trart bad-gap i IAs/GaSb typ II suprlattics L.L. Li 1, W. Xu 1, 3, Z. Zg 1, ad Y.L. Si 1 Ky Laboratory of Matrials Pysics, Istitut of Solid Stat Pysics, Cis Acadmy of Scics, Hfi 30031, Cia Kumig Istitut of Pysics, Kumig, Cia ad 3 Trart Rsarc Ctr, Uivrsity of Elctroic Scic ad Tcology, Cgdu , Cia Abstract: W dmostrat tortically tat it is possibl to rali trart (TH) fudamtal bad-gap btw t lctro mii-bad i t IAs layr ad t avy-ol mii-bad i t GaSb layr i IAs/GaSb basd typ II suprlattics (SLs). T TH bad-gap ca b tud by varyig t sampl growt paramtrs suc as t wll widts of t IAs ad/or GaSb layrs. T prsc of suc bad-gap ca rsult i a strog cut-off of optical absorptio at TH frqucis. For typical sampl structurs, t TH cut-off of t optical absorptio dpds strogly o tmpratur ad a sarpr cut-off ca b obsrvd at rlativly ig-tmpraturs. Tis study is prtit to t applicatio of IAs/GaSb typ II SLs as TH potodtctors. PACS umbrs: 7.80.Cw, 7.0.Dp, Cw Kywords: TH bad-gap, IAs/GaSb, typⅡsuprlattics(sls), TH potodtctors doi: /TST Itroductio Trart (10 1 H or TH) rgio is t most scitifically ric ara of t lctromagtic (EM) spctrum 1. T TH wav (or T-ray) tcology is of grat pottial to impact may itrdiscipliary filds suc as tlcommuicatio, biological scic, parmacutical tcology, ati-trrorist, ao-tcology, to mtio but a fw. T raliatio of T-ray sourcs ad ssors as b a importat fild of rsarc i optics ad optolctroics sic 1980 s. From a basic pysics poit of viw, for t gratio ad dtctio of TH EM radiatio, it is cssary to rali a matrial systm i wic t fudamtal rgy gap is aroud TH poto rgy. Tus, TH gratio ad dtctio ca b acivd troug lctroic trasitio accompaid by t missio ad absorptio of TH potos. I tis papr, w propos to mploy IAs/GaSb basd typ II suprlattic (SL) systms as TH bad-gap matrials. I cotrast to covtioal smicoductor SL systms i wic t coductig lctros ad ols ar locatd maily i t sam matrial layr, t cofid lctros ad ols i a IAs/GaSb typ II SL ar sparatd spatially i diffrt wll layrs 3. I suc a SL structur, t rgy-gap btw t cofid lctro stats i t IAs layr ad t cofid ol stats i t GaSb layr ca b tud artificially by simply varyig t sampl growt paramtrs suc as t widts of t IAs ad GaSb layrs. Tus, by bad-gap girig, w ca rali a SL systm i wic t fudamtal rgy-gap btw t valc (or ol) mii-bad i t GaSb layr wit t coductio (or lctro) mii-bad i t IAs layr at TH badwidt. As a rsult, TH optical missio ad absorptio ca b acivd via itr-layr trasitio amog lctro ad ol stats i diffrt wll layrs (amly t typ II trasitio). O t basis of suc structurs, TH optolctroic dvics ca b dsigd ad ralid. It sould b otd tat IAs/GaSb typ II SLs av b proposd as ucoold midifrard 07

2 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 (MIR) potodtctors workig at 3-5 μm wavlgt rag for various applicatios 4. It as b foud xprimtally tat for sort-priod IAs/GaSb SLs wit t wll layr widts aroud.1/.4 m, sarp cut-off of t potorspos ca b obsrvd at MIR badwidt ad t cut-off wavlgt ca b tud by varyig IAs/GaSb layr widts. I t prst study, w grali suc SL systms for t applicatio i t TH rag. W bliv tis ca sd ligt o t applicatio of t IAs/GaSb typ II SLs as advacd ad practical TH dvics suc as TH potodtctors. T papr is orgaid as follows. I Sctio II, w prst a simpl modl to calculat t lctroic mii-bad structur i IAs/GaSb typ II SLs. T tortical approac for t calculatio of t optical absorptio cofficit i a typ II SL is dvlopd i Sctio III. T rsults obtaid from tis ivstigatio ar prstd ad discussd i Sctio IV ad t mai coclusios draw from tis study ar summarid i Sctio V. Fig. 1 Illustratio of t bad aligmt for a IAs/GaSb typ II SL alog t growt dirctio (-axis). Hr, L A ad L B ar rspctivly t wll widts for IAs ad GaSb layrs, ad V S ad V P ar rspctivly t coductio ad valc bad offsts. T solid dots ad op dots rfr rspctivly to t boudd lctros ad ols.. Elctroic mii-bad structur I tis study w grali t stadard Kroig-Py modl 5 to t calculatio of t lctroic mii-bad structur of IAs/GaSb typ II SLs. W cosidr a IAs/GaSb SL i wic t growt dirctio is tak alog t -axis. T diagram of t bad aligmt alog t growt dirctio for suc a SL is illustratd i Fig. 1. I a IAs/GaSb typ-ii SL, t lctros ad ols ar sparatd spatially i t IAs ad GaSb layrs rspctivly. Hr w mploy t followig otatios to dscrib suc a SL. L A ad L B ar rspctivly t IAs ad GaSb layr ticksss ad, trfor, t priodicity of t SL is d = L A +L B. m A ad m B ar rspctivly ffctiv masss for a lctro or a ol i t IAs ad GaSb layrs. V S (masurd from t bottom of coductio bad i IAs layr) ad V P (masurd from t top of valc bad i 08

3 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 GaSb layr) ar rspctivly t coductio ad valc bad offsts wic play t rols as t barrirs for lctros ad ols rspctivly. I t absc of scattrig ctrs ad xtral filds ad glctig t xcitoic itractios btw lctros ad ols i t structur, t lctro ad ol mii-bad structurs ca b solvd sparatly. Firstly, w cosidr a lctro cofid witi t IAs layrs. Udr t ffctiv-mass approximatio, t Scrödigr quatio for a lctro i t growt dirctio is d 1 d ( ) V ( ) ( ) ( ), (1) d m( ) d wr t cofiig pottial rgy (s Fig. 1) is 0, LA 0; V() Vs, 0 LB; () ad ma, LA 0 m () mb, 0 LB (3) is t lctro ffctiv-mass i diffrt layrs. T solutio of Eq. (1) must oby t coditio of t Bloc priodicity: ( d) ikd ( ), (4) wit k = [-π/d, π/d] big t suprlattic wavvctor alog t growt dirctio. I t rot priod, t solutio for a boud stat ε < V S ca b writt as c c, L 0; A ika A ika 1 A ( ), B ikb B ikb c1 c, 0 LB ; (5) Wr k A ma ad k m V B B( S ). 1 Usig t cotiuitis of ψ() ad t wigtd drivativ m d d IAs/GaSb itrfac ad t coditio of t Bloc priodicity, o obtais a 4 4 matrix at t A c 1 A A A ib ib c 0, B c 1 B A A ib ib c (6) 09

4 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 wr i kd kala kblb A k m, B k m, ad. For a otrivial solutio of Eq. A A B B (6), t 4 4 dtrmiat of t corrspodig matrix must b ro, wic rsults i t Kroig-Py quatio for a lctro i a boud stat ε < V S 1 cos( kd) 1 si( kala)si( kblb ) cos( k L )cos( k L ), A A B B (7) wr β = B/A. T corrspodig lctro wavfuctio ca b rwritt as cos( ka) F si( ka), LA 0; ( ), cos( kb) F si( kb), 0 LB, (8) wr ikd cos( kala) cos( kblb ) F ikd si( k L ) si( k L ) A A B B ad A is dtrmid by ormaliatio coditio: L B ( ) ( ) d 1. LA Usig t Kroig-Py quatio giv by Eq. (7), t rgy for a lctro i t t mii-bad i t IAs layr, ( k ), ca b dtrmid ad, t, from it t corrspodig wavfuctio, k () ca b obtaid. Usig t similar approac t rgy, ( k ) ad wavfuctio, (), for a ol i t t mii-bad i t GaSb layr ca also b calculatd. k 3. Optical absorptio cofficit I a SL, t wavfuctio ad rgy spctrum for a lctro ( = = ) or a ol ( = = 1) ca b writt, rspctivly, as ( R) ( ), E k m k ikr K k ( K) ( 1) ( ), (9) wr R = (r, ) = (x, y, ), K = (k, k ) = (k x, k y, k ) wit k big t lctro/ol wavvctor alog t xy-pla ad k t wavvctor of t rciprocal spac alog t growt dirctio, ad m is t dsity-of-stats ffctiv-mass for a lctro or a ol. W ow cosidr tat a EM fild, wic is polarid liarly alog t -dirctio, is applid to t SL. Applyig t lctro ad ol wavfuctios ad rgy spctra giv by Eq.(9) ito t Frmi s gold rul, t 10

5 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 lctroic trasitio rat iducd by dirct itractios btw lctros/ols ad t radiatio fild via absorptio scattrig is obtaid as W ( K, K) W ( K) K, K, wr F 0 m W X ( k ) E ( K) E ( K ), (10) wic masurs t probability for scattrig of a lctro or a ol at a stat, K i layr to a stat, K i layr. Hr, F 0 ad ω ar, rspctivly, t lctric fild strgt ad frqucy of t EM fild, ad X ( k) d k ( ) d ( ) k d is t form factor for lctro/ol couplig wit a poto. T trm K, K rflcts t fact tat t dirct lctro/ol-poto couplig i a SL dos ot cag t momtum of a lctro or a ol. I tis papr, w mploy t smi-classic Boltma quatio as t govrig trasport quatio to study t rspos of t carrirs (lctros ad ols) i a SL to t applid radiatio fild. For a lctro or a ol, t Boltma quatio i a dgrat statistics tak a form f ( K, t) g F (,, t) F (,, t). t K K K K (11) s, K, Hr, g s = couts for spi-dgracy, F ( K,, ) (, ) 1 (, ) K t f K t f K t W ( K, K ), ( K, t), is t momtum-distributio fuctios for a carrir at a stat K, i layr, ad f W KK, is t stady-stat lctroic trasitio rat. I Eq. (11), t ffct of t EM fild as b icludd witi t tim-dpdt lctro/ol distributio fuctios ad witi t lctroic trasitio rat. Tus, to avoid doubl coutig, t forc trm iducd by t EM fild dos ot appar o t lft-ad sid of t Boltma quatio. It is kow tat tr is o simpl ad aalytical solutio to Eq. (11) wit t lctroic trasitio rat giv by Eq. (10). I tis work, w apply t usual balac-quatio approac to solv t problm 6. For t first g E K to momt, t rgy-balac quatio 7 ca b drivd by multiplyig S K bot sids of Eq. (11). I doig so, w obtai two rgy-balac quatios rspctivly for a lctro ad a ol ad, from tm, t total lctroic rgy trasfr rat du to lctro/ol itractios wit potos is obtaid as P p p P, (1), wr p gs E f, t K K K t is t lctroic rgy trasfr rat pr cll of t SL for lctros or ols ad 11

6 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 P 4 f ( E ( K)) 1 f ( E ( K)) W ( K ). (13),, K Hr w av usd a statistical rgy distributio suc as t Frmi-Dirac fuctio as lctro/ol distributio fuctio at a stady-stat. Namly, w av tak 1 x kbt f K, t f E K wr f x 1 is t Frmi-Dirac fuctio wit big t Frmi rgy (or cmical pottial) for a lctro or a ol. T optical absorptio co.cit iducd by lctro ad ol itractios wit t EM fild ca b calculatd troug 8 P F, 0 0 (14) wr ( 0C), k ad 0 ar rspctivly t dilctric costats of t matrial ad t fr spac, ad C is t vlocity of t ligt i vacuum. Tus, t optical absorptio cofficits iducd by itra- (i.., ) ad itr-layr (i.., ) trasitio ar obtaid rspctivly as, c f ( E ( K)) 1 f ( E ( K)) X ( k ) 0,, K ( k ) ( k ), (15) ad 0 ( M m ) ( k ) ( k ),, K f ( x ) 1 f ( x ) X ( k ), (16) wr c 16 ( m ), 3 x m ( k ) m ( k ) m ( m m ) 1 M 1 m 1 m, 8 ( m k ) ad. As ca b s from Eqs. (15) ad (16), if w kow t wavfuctios ad rgy spctra for a lctro ad a ol i a SL aalytically t optical absorptio cofficit ca b calculatd asily wit t simpl approac dvlopd r. 4. Numrical rsults ad discussios 4.1. TH miibad structur I tis work, w grali t usual Kroig-Py modl 5 (s Sctio II) to calculat t lctroic mii-bad structur of IAs/GaSb basd typ II SLs. From tis calculatio, w ca obtai t wavfuctio () ad rgy ( k ) for a lctro ( = ) or a avy-ol ( = k 1

7 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 Fig. (a) Disprsio rlatios of t lctro/ol mii-bad rgis, ( k ) ( k ) (solid/dasd curvs), for t fixd IAs/GaSb layr widts L A /L B as idicatd. (b) Corrspodig disprsio rlatios for t lowst lctro mii-bad i t IAs layr, ( ) 0, ad t igst avy-ol mii-bad i t GaSb layr, ( ) 0 k. k Fig.3 T sam as i Fig. but wit diffrt IAs/GaSb layr widts L A /L B as idicatd. 13

8 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 Fig.4 Squar modul of t lctro/ol wavfuctios at k 0, 0( ) 0( ) (solid/dasd curvs), as a fuctio of t distac for t fixd IAs/GaSb layr widts L A /L B as idicatd. Hr = 0 is at t itrfac btw t IAs ad GaSb layrs. ) i t t miibad i t SL, wit k big t SL wavvctor alog t growt dirctio (or t -axis). T ffctiv masss for carrirs (lctros ad ols) i IAs/GaSb SLs ar tak as: m IAs ma 0.038m 0 wit m0 big t rst lctro mass, m IAs ma 0.40 m0, ad m GaSb m m B 0 m GaSb m 0.33m. I t calculatio, w tak t B coductio-ad-valc-bad offsts at t IAs/GaSb itrfacs to bv E 960mV ad VP EV 450mV ad t coductio-ad-valc-bad ovrlap rgy is 150mV. Ts bad paramtrs av ld to a good agrmt btw t xprimtal ad tortical rsults i IAs/GaSb basd typ II quatum wll systms 9,10. I Fig., w sow t dpdc of t mii-bad rgis i a IAs/GaSb SL o k t wavvctor of t rciprocal spac alog t growt dirctio. W fid tat w t IAs/GaSb layr widts ar aroud L A /L B = 8.50/3.05 m, tr ar tr mii-bads ( k ) (=0, 1 ad ) for lctros i t IAs layr ad tr ar t sam umbr of t mii-bads ( k ) (=0, 1 ad ) for avy-ols i t GaSb layr. ( k) dpds ratr strogly o k wras ( k) dpds littl o k, du to t fact tat a avy-ol as a largr ffctiv mass ta a lctro as ad t GaSb layr is rlativly arrow. From Fig. (b), w s tat t rgy-gap btw t bottom of t lowst lctro mii-bad i t IAs layr ad t top of t igst avy-ol mii-bad i t GaSb layr is about 3 mv, wic is locatd i t TH badwidt. T disprsio rlatios for t lctro/ol mii-bad rgis wit aotr IAs/GaSb SL growt paramtrs, L A /L B = 6.30/7.90 m, ar sow i Fig. 3. W s tat wit suc growt paramtrs, t TH bad-gap ca also b ralid btw t bottom of t lowst lctro mii-bad i t IAs layr ad t top of t igst avy-ol mii-bad i t GaSb layr. Howvr, w L A /L B = 6.30/ S C 14

9 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 m, tr ar two mii-bads for lctros i t IAs layr ad six mii-bads for avy-ols i t GaSb layr. For t applicatios of t typ II SLs as lctroic ad optical dvics, it is cssary to kow t strgt of t ovrlap of t lctro ad ol wavfuctios at t itrfacs btw IAs ad GaSb wll layrs. I Fig. 4, w sow t lctro ad ol wavfuctios at k = 0 for two IAs/GaSb SLs wit layr widts L A /L B = 8.50/3.05 m ad 6.30/7.90 m rspctivly. As ca b s, i a IAs/GaSb typ II SL, t lctros ad ols ar cofid prdomiatly i t IAs- ad GaSb- layrs rspctivly. T ol distributio is mor localid ta t lctro distributio, as a rsult of t quatum si ffct ad diffrt ffctiv masss. Comparig t uppr ad lowr pals i Fig. 4, w fid tat t lctroic tulig ffct i t uppr cas is strogr ta tat i t lowr cas, du to t arrowr GaSb layr widt i t uppr cas. Tis rsults i a fact tat t lctro mii-bad rgis ar mor disprsd i Fig. ta tos i Fig. 3. It sould b otd tat for sampls wit TH bad-gap rgis, owig to tir rlativly largr wll widts L A /L B, t ovrlaps of t lctro ad ol wavfuctios at t IAs/GaSb itrfacs ar sigificatly lss ta tos i sampls wit MIR bad-gap wr L A /L B ~.1/.4 m. Fig.5 Bad-gap rgy btw t bottom of t lowst lctro mii-bad i t IAs layr ad t top of t igst avy-ol mii-bad i t GaSb layr as a fuctio of GaSb (IAs) layr widt L B (L A ) at a fixd IAs (GaSb) layr widt, i t uppr (lowr) pal. T rsults ar sow for k = 0. I Fig. 5, w sow t bad-gap rgy btw t bottom of t lowst lctro miibad i t IAs layr ad t top of t igst avy-ol mii-bad i t GaSb layr as a fuctio of t IAs (GaSb) layr widt at a fixd GaSb (IAs) wll tickss. It is foud tat tr ar two sts of growt paramtrs wic ca b usd to rac TH bad-gap i IAs/GaSb typ II SLs. W t IAs/GaSb layr widts ar about 8.50/.80 m or 6.10/7.90 m, t fudamtal 15

10 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 bad-gap Eg 0(0) 0(0) is of t ordr of TH ad ca b tud by adustig t IAs/GaSb layr widts. T bad-gap rgy E g dcrass wit icrasig t IAs ad/or GaSb layr tickss, bcaus t rgy of t lctro miibad i t IAs layr dcrass wit icrasig L A ad tat of t avy-ol mii-bad i t GaSb layr icrass wit L B. Furtrmor, w fid tat i ts sampl structurs, t rgy sparatios amog t lctro ad ol mii-bads i diffrt layrs [i.., m( k) ( k) ] ar largr ta 100 mv, amly ty ar i t mid-ifrard badwidt. Tis suggsts tat itra-layr lctroic trasitio dos ot affct sigificatly t TH rag. 4.. Optical absorptio spctrum I a udopd IAs/GaSb typ II SL, t prsc of t radiatio fild ca pump lctros i t valac bad i t GaSb layr ito t coductio bad i t IAs layr. Suc procss iducs poto-xcitd carrirs wos trasitios cotribut maily to t optical absorptio i t SL. I gral, t dsity of poto-xcitd carrirs dpds o t radiatio itsity ad frqucy ad o otr scattrig ad rlaxatio mcaisms. I tis study, w assum tat t 17 3 poto-xcitd carrir dsity i udopd SLs is about 10 cm pr SL cll, wic satisfis t coditio of t carg-utrality. Bcaus i a IAs/GaSb SL t lctro ad avy-ol av diffrt ffctiv masss, t o-quilibrium cmical pottials (i.., iducd by t prsc of poto-xcitd carrirs) for lctros ad ols ar diffrt. Itroducig t rgy spctrum for a lctro or a ol i a SL ito t corrspodig dsity-of-stats, t o-quilibrium cmical pottial i t prsc of t radiatio fild ca b calculatd, rspctivly, troug ad g f ( E ( K)) s, K k, s, K k, ( k ) l 1 xp, kt B g (1 f E( K( ))) ( k ) l 1 xp, kt B (17) (18) wr is t carrir dsity ad m k T B. I Fig. 6, w sow t tmpratur dpdc of t cmical pottials for lctros ( )i t IAs layr ad avy-ols ( ) i t GaSb layr i a IAs/GaSb typ II SL at a fixd carrir dsity cm As ca b s, ( ) dcrass (icrass) wit icrasig tmpratur. Tis fatur is i li wit tos obsrvd i t covtioal SL systms. 16

11 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 Fig.6 Tmpratur dpdc of t o-quilibrium cmical pottials for lctros ( ) ad ols ( ) i a typ II SL wit t fixd IAs/GaSb layr widts L A /L B as idicatd. T rsults ar sow for cm pr SL cll. From t rsults obtaid from mii-bad structur calculatios, w kow tat i a IAs/GaSb typ II SL, t rgy spacig amog t lctro/ol mii-bads i t IAs/GaSb layrs (i.., m( k) ( k) 100mV ) ar muc largr ta t TH rgy. As a rsult, t TH optical absorptio is acivd maily via typ II trasitio cal, amly troug t itr-layr trasitio btw t lctro mii-bad i t IAs layrs ad t avy-ol mii-bad i t GaSb layrs. T dpdc of t optical absorptio spctrum o t IAs/GaSb layr widts is sow i Fig. 7 at T = 5 K. Bcaus of a strog typ II optical trasitio i a IAs/GaSb SL, a sarp absorptio cut-off ca b obsrvd. T ratr broad absorptio spctrum s r is iducd maily by t prsc of t disprsd lctro mii-bads i a SL. W fid tat a mor fficit TH absorptio ca b obsrvd for sampls wit largr IAs wll widts (compar t uppr ad lowr pals i Fig. 7). For xampl, a strog absorptio (about a factor of ) at 1 TH ca b s for a sampl wit t wll widts 8.50/3.05 m ta tat wit t wll widts 6.30/7.90 m. Mor prooucd cut-off of t TH absorptio ca b acivd for sampls wit smallr bad-gaps. I Fig. 8, t dpdc of t TH optical absorptio o tmpratur is sow for a IAs/GaSb SL wit t fixd wll layr ticksss. As ca b s, t TH optical absorptio via typ II trasitio i a IAs/GaSb SL dpds strogly o tmpratur. I particular, t blu-sift of t cut-off absorptio ca b obsrvd wit dcrasig tmpratur at rlativly low-tmpraturs. From Eq. (16) w ot tat for typ II optical trasitio i a typ II SL, tr ar two kids of cut-offs for carrir-poto itractio troug optical absorptio scattrig. O is iducd by t trm 0( k) 0( k), wic is rquird by t momtum ad rgy cosrvatio laws durig a scattrig vt (Mcaism I). Aotr is causd by t trm 17

12 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 Fig.7 Optical absorptio spctrum at a fixd IAs (GaSb) layr widt L A (L B ) for diffrt GaSb (IAs) layr widts, 17 3 i t uppr (lowr) pal. T rsults ar sow for 10 cm pr SL cll. Fig. 8 Tmpratur dpdc of t TH absorptio i a IAs/GaSb SL at t fixd IAs/GaSb layr widts L A /L B ad t fixd carrir dsitis pr SL cll as idicatd. 18

13 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr 008 Fig.9 Typ II optical trasitio cal i t prsc of poto-xcitd carris i a typ II SL. Hr, E g is t rgy-gap btw t bottom of lctro mii-bad i t IAs layr ad t top of ol mii-bad i t GaSb layr, ad ar o-quilibrium cmical pottials for t poto-xcitd lctros ad ols rspctivly. f( x ) 1 f( x ), wic rflcts a fact tat optical trasitio ca b acivd via xcitig lctros i t occupid stats i t ol mii-bads i t GaSb layr to t mpty stats i t lctro mii-bad i t IAs layr (Mcaism II). W fid tat for typical IAs/GaSb typ II SLs, t Mcaism I rsults i a cut-off i optical absorptio at rlativly ig tmpraturs (i.., T = 77 K i Fig. 8), wras t Mcaism II is rsposibl for t absorptio cut-off at low-tmpraturs (i.., T = 5 K ad 5 K i Fig. 8). As ca b s i Fig. 9, for a udopd sampl i t prsc of potoxcitd carrirs, tr ar mpty stats i t ol mii-bad i t GaSb layr owig to t xcitatio of lctros i t ol mii-bad ito t lctro mii-bad. Tis implis tat at low-tmpraturs, a poto rgy 0 0 E is rquird i acivig optical trasitio to 0 0 g satisfy t rgy cosrvatio law. Bcaus ( ) dcrass (icrass) wit icrasig tmpratur (s Fig. 6), dcrass wit icrasig tmpratur. Tus, a rd-sift of t cut-off absorptio ca b obsrvd wit icrasig tmpratur at rlativly low-tmpraturs. At rlativly ig-tmpraturs, t ffct of t prsc of t o-quilibrium cmical pottials bcoms wak du to t trmal broadig of t distributio fuctios for lctros ad ols. As a rsult, t optical absorptio cut-off is maily acivd via t Mcaism I at rlativly ig-tmpraturs. From Fig. 8, it is itrstig to ot tat for a IAs/GaSb typ II SL, a sarpr cut-off i TH optical absorptio ca b obsrvd at rlativly ig-tmpraturs. Tis suggsts tat IAs/GaSb typ II SLs ca b applid as TH potodtctors workig at rlativly ig-tmpraturs. 19

14 Trart Scic ad Tcology, ISSN Vol.1, No 4, Dcmbr Coclusios I tis study, w av foud tat t fudamtal TH bad-gap ca b ralid i IAs/GaSb basd typ II SLs. Two sts of t growt paramtrs wit t IAs/GaSb layr widts about 8.5/.8 m ad 6.1/7.9 m ca b usd to aciv t TH rgy-gap btw t bottom of t lowst lctro mii-bad i t IAs layr ad t top of t igst avy-ol mii-bad i t GaSb layr. T TH bad-gap dcrass wit icrasig IAs/GaSb layr widts. I suc SLs, TH optical absorptio ca b acivd via typ II trasitio (or itr-layr trasitio cals) ad t cut-off of t absorptio spctrum ca b obsrvd at TH frqucis. T cut-off frqucy is rd-siftd wit icrasig t IAs/GaSb layr widts ad/or tmpratur. W av also foud tat t sarpr cut-off of t TH absorptio i suc SLs ca b obsrvd at rlativly ig-tmpraturs. Ts faturs favor gratly t applicatio of IAs/GaSb basd typ II SLs as practical TH potodtctors workig at rlativly ig-tmpraturs. W op ts tortical prdictios ad fidigs ca b vrifid xprimtally. Ackowldgmt Tis work was supportd by t Cis Acadmy of Scics ad t Natioal Natural Scic Foudatio of Cia. Rfrcs [1] B. Frguso ad X.C. Zag, Natur Matrials 1, 6, (00). [] For a rviw, s,.g., P.H. Sigl, IEEE Tras. o Microwav Tory ad Tc. 50, 910, (00). [3] S,.g., G. Bastard, Wav Mcaics Applid to Smicoductor Htrostructurs, Moograpis d Pysiqu, Paris, (199). [4] H.J. Hauga, F. Smulowic, G.J. Brow, ad K. Maaligam, J. Appl. Pys. 96, 580, (004). [5] S,.g., G. Grosso ad G.P. Parravicii, Solid Stat Pysics, Acadmic Prss, (000). [6] W. Xu, F.M. Ptrs, ad J.T. Dvrs, Pys. Rv. B 46, 7571, (199). [7] W. Xu, Appl. Pys. Ltt. 89, , (006). [8] X.L. Li ad S.Y. Liu, J. Pys.: Cods. Mattr 1, 4655, (000). [9] P.A. Folks, G. Gumbs, W. Xu, ad M.T. Lara, Appl. Pys. Ltt. 89, 0113, (006). [10] W. Xu, P.A. Folks, ad G. Gumbs, J. Appl. Pys. 10, , (007). 0