Nghiên cứu các tính chất quang của hợp chất bán dẫn vùng cấm rộng

Trong ph6 phat quang cua cac hgp chat ban din nay tii 4,2K den 300K, ngoai nhiing vach dac trung cho su tai hgp biic xa cua cac exciton tu do, exciton lien ket tren cac donor, acceptor,

DAI HOC QUOC GIA HA NOI TRITOING DAI HOC KHOA HOC TlT NHIEN

DE TAI:

NGHIEN c t r u CAC TINH CHAT QUANG CUA HOP CHA

BAN DAN VUNG CAM RONG

Ma so: QT- 05-08

Chi4 tri de rai : TS Pham Van Ben Cdc cdn bg tham gia : ThS Hoang Chi Hieu

CN Nguyen Thanh Chuong

CN Nguyen Anh Tuan

rJAi H O C Q U O C GIA HA NOI rpuMG TAW THONG TIN THU ViEN

or I b^^

HA NOI - 2005

BAO CAO TOM TAT

^•^

DETAI: NGHIEN CUtJ CAC TINH CHAT QUANG CUA HOP CHAT

BAN D A N V U N G C A M RONG

MA SO : QT- 05 - 08

CHU TRI DE TAI : TS PHAM VAN BEN

CAC CAN B O T H A M GIA: TH.S HOANG CHI HIEU

CN NGUYfiN THANH CHUONG

CN NGUYfiN ANH TUAN

1 Muc tieu va noi dung nghien curu:

+ Xay dung he kfch thich va thu pho huynh quang tren ca so may do

sdc each tu kep GDM - 1000 va ky thuat lock-in

+ Nghien cuu cac tfnh chat quang cua cac hop chat ban dSn viing cam i

2 Cac ket qua dat dupe

+ Tim hieu ly thuyet vl tfnh chat quang cua cac hgp chat ban dSn vimg cam rong

+ Xay dung he kfch thfch va thu pho huynh quang tren ca so may dc sdc each tir kep GDM - 1000 va ky thuat lock- in

+ Thu va nghien ciJu pho huynh quang cua cac hop chat ban dSn vian cam rong ZnSe, ZnS \'a ZnS : Mn

'^' Kei qud dao tao: 03 sinh vien bac cu nhan

* Cdc cong trinh cong bo: 02 bao cao khoa hoc tai Hoi nghi Vat '

toan quoc lan thu \ a Ha Noi tir 23 - 25/11/2005

3 Tinh hinh sur dung kinh phi

+ Tong kinh phi duoc cap

+ Tien dien, nude va xSy dung co scf vat chat

+ v a t tu van phong

+ Hoi nghi, in an bao cao

+ Cong tac phi, tau xe

+ Thue mucfn

+ Chi phi nghiep vu chuyen mon

+ Ho tro dao tao nghien ciiu khoa hoc

20.000.000d SOO.OOOd SOO.OOOd 2.000.000d : l.OOO.OOOd S.OOO.OOOd : V.OOO.OOOd : 400.000d

XAC NHAN CUA BCN KHOA VAT LY

PGS.TS Nguy in The Binh

CHU TRI DE TAI

TS Pham Van Ben

PHO HlEl TRUOHG

GS.TS.'^i**^ ^JC

BRIEF REPORT

TTTLE: INVESTIGATION ON THE OPTICAL PROPERTIES OF

SEMOCONDUCTOR COMPOUNDS WITH THE WIDE BAND GAP

CODE: QT- 05-08

COORDIMATOR : PHAM VAN BEN

KEY IMPLEMENTS: HOANG CHI HIEU

NGUYEN THANH CHUONG NGUYEN ANH TUAN

1 Objective and Matter of this study:

+ To construct the excitation system and obtain luminescence spectra using the double grating spectrometer GDM 1000 and the lock-in amplification technique

+ To investigate the optical properties of semiconductor compounds with the wide band gap

+ Investigating some optical properties of some wideband gap semiconductor compounds such as ZnSe, ZnS and ZnS:Mn

Education results: 3 bachelor theses

Publications: 2 papers on Tlie 6^^ Vietnamese Confference on Physic, Hanoi

November 23-25, 2005

MUC LUC

1, Md dau

H Noi dung

i Cdc tinh chdt quang cua cdc hgp chat bdn ddn vung cdm rpng

a Cau tnic tinh the va cau tnic vung nang lugng

b Su thay doi do rong viing cam theo nhiet do

c Su thay doi do rong vung cam theo nong do tap chat

d Cac ca che hap thu trong ban din vung cam rong

e Cac ccf che tai hgp biic xa trong ban dSn vung c^m rong

2, Xdy dung he kich thich vd thu phd huynh quang, tdn xa Raman tren ca sd mdy dcm sdc cdch tu kep GDM -1000 diing ky thuat lock - in

a Md dau

b He kfch thfch pho huynh quang va tan xa Raman

c He thu pho huynh quang va tan xa Raman

d Cac ket qua do pho huynh quang va tan xa Raman

e Ket luan

3, Nghien citu tinh chdt quang cua bdn ddn viing cdm rong ZnSe

a Ma dau

b MSu nghien ciJu va thiet bi thuc nghiem

c Cac ket qua va bien luan

^ Pho phat quang (huynh quang) ciia tinh the ZnSe

* Su phu thuoc vi trf va cucmg do ciia dam phat quang

\aing da cam-do a 606 nm vao mat do nang lugng kfch thfch

* Su t^t nhiet phat quang cua tinh the' ZnSe

d Ket luan

4, Pho huynh quang cua bdn ddn vung cdm rong ZnS

vd ZnS : Mn

L MO DAU Cac hgp chat ban din vung cam rong loai A^B^ A^B^ v6i do rong vung cam khoang mot vai electron von nhu: ZnSe, ZnS, CdTe, CdS, ZnO.vv dugc ling dung rong rai trong cac dung cu quang dien tu Chiing dung de che tao cac di ot phat quang, quang tra, nhung cua so trong suot trong viing kha kien

va hong ngoai gin [1, 2, 3, 4] Trong ph6 phat quang cua cac hgp chat ban din

nay tii 4,2K den 300K, ngoai nhiing vach dac trung cho su tai hgp biic xa cua cac exciton tu do, exciton lien ket tren cac donor, acceptor, cac electron tu do tir viing din xuong cac miic tap chat, con xu^'t hien cac dam rong dac trung cho su tai hgp biic xa cua cac cap donor - acceptor [5, 6, 7, 8, 9]

Khi pha cac kim loai chuyen tiep nhu Mn, Fe, Ni, Co vdi vo dien tir 3d chua lap day vao cac hgp chat ban din wng c^'m rong se tao thanh cac ban din ban tir Cac ion ciia cac kim loai tren (goi la cac ion tijf) se tao nen trong viing cam ciia cac hgp chat ban din nay nhiing miic nang lugng xac dinh Vl the trong pho phat quang ciia cac hgp chat ban din vung cam rong con xuat hien nhiing \'ach dac tnmg cho su tai hgp biic xa cua cac polaron tii lien ket, cac dam phat quang dac tnmg cho cac ion tii va pho phat quang cua chiing dugc chuydn sang viing hong ngoai gan [10, 11, 12, 13, 14]

Nhim lam sang to mot vai ca che tai hgp biic xa va kha nang iing dung ciia cac hgp chat ban din viing cam rong, chiing toi thuc hien de tai: Nghien ciiu cac tfnh chat quang cua hgp chat ban din \aing cam rong

n NOI DUNG

1 Cac tinh chat quang cua cac hgp chat ban dSn vung cam rong

ạ Cau true tinh the va cau true viing nang lixgng

* Cdu trdc tinh the

Cac tinh thd ÁB^ ÂB' dugc tao thanh tii cac kim tu nhom 2 hoac 3 (A)

va cac nguyen tir a kim nhom 6 hoac 5 (B) lien ket vcd nhau theo dang h6n hgp ion va cong hoa tri (vi du nhu ZnS :77% la lien ket ion va 23% la lien ket cong hoa tri) Trong hen ket ion cac nguyen tu kim loai A cho hai dien tir hoa tri de tra thanh ion hoa tri duong hai Â"^ co c^'u hinh dien tii a Icfp ngoai ciing 3d^pM^°, cac nguyen tir a kim B nhan hai dien tir tra thanh ion hoa tri am hai B^' CO cau hinh ldp dien tu ngoai ciing 3s^p^ Trong lien ket cong hoa tri m6i nguyen tir A hay B deu dong gop vao lien ket chung 4 dien tii de tao thanh cau hinh day dii ciia 16p vo dien tụ Nguyen tu A se tra thanh ion hoa tri am hai Ấ

CO cau hinh dien tu ngoai cung 4s^p^ con nguyen tir B se tra thanh ion hoa tri duong hai B^"" vdi cau hinh Icfp vo ngoai cung 3s^p^

Cac nguyen tii A va B lien ket v6i nhau theo mot cau tnic tuan hoan, tao thanh tinh thd Cd nhieu dang hinh thii khac nhau ciia tinh the ban din viing cam rong, nhung deu xuat phat tir hai dang co ban: đ la cau tnic luc giac (Wurtzite) va cau tnic lap phuang (Sphalerite hay Zincblende) Vide tinh thd nhan cau tnic luc giac hoac lap phuang la tuy thuoc vao nhiet do va ban chat ciia tiing hgp chat ban din Vai tinh the ZnS thuoc nhdm A-^B^ thi a nhiet do nung nho han 950°C tinh thd nay thudng cd cau tnic lap phuang a nhiet do tir 950*^ den 1020^C thi cd khoang 70% ZnS a dang cau true luc giac va a nhiet

do tir 1020°C den 1200°C, ZnS hoan toan o dang cau tnic luc giac Nhu vay cd thd xem 1020^C la nhiet do chuydn pha tir cau tnic lap phuang sang cau tnic luc giac V6i cac tinh thd thuoc nhdm ÁB' thi mang tinh thd ciia chiing thudng cd cau tnic dang lap phuang

Tuy nhien khong thd xac dinh dugc khoang nhiet do 6n dinh cho timg dang ca'u hinh cua tinh the Vdi CdTe, cau tnic dang luc giac chi quan sat thay

d mau mang cdn cau tnic dang lap phuang quan sat thay d miu rdn Du d dang cau tnic luc giac hay cau true lap phuong thi trong lidn kd't trat tu gan, mdi nguyen tir A^ (hay B^) ddu nim d tam ciia hinh tii dien tao bdi bdn nguyen tir B^ (hay A')

* Cdu trdc dang lap phucmg (Sphalerite hay Zincblende)

Cau tnic dang lap phuang dugc xay dung trdn co sd xdp quy luat xep ciu ciia hinh lap phuong vdi cac dinh la nguyen tir B (S) dugc ky hieu la O Cac nguyen tir A (Zn) nam trong mot nira sd hdng bdn mat, cac hdng A (Zn) dugc kyfcidu la dinh hudng song song vdi nhau (hinh 1 a)

Hinh 1: Cau tnic cua ban din vung cam rong

a : Dang lap phuang ( Sphalerite hay Zincblende )

b : Dang luc giac ( hay Wurtzite )

Nguydn tir A dugcky hieu la : ^ Nguydn tir B dugc ky hieu la: O

Nhdm ddi xiing khong gian cua Sphalerite la: T"^ F43m 6 cau tnic Sphalerite, mdi 6 mang ngu\en td cd 4 phan tir A^B^ (ZnS) Mdi nguyen tir A (Zn) dugc bao quanh bdi 4 nguydn tir B (S) dugc dat trdn cac dinh cua tii dien

d cung khoang each ^ / 3 - trong dd a la hing sd mang Ngoai ra bat ky mot

nguyen td nao thuoc ciing mot loai cung dugc bao quanh bdi 12 nguyen tir ciing loai dd d khoang each V 2 - , trong dd 6 nguyen tir dat d dinh cua luc

giac nim trdn ciing mot mat phang, 6 nguyen tii cdn lai tao thanh mot phan lang kfnh tam giac Neu dat cac nguyen tir cua mot nguyen td B (S) d cac niit mang lap phuong, tam mang cd toa dg-cau la (0, 0, 0) thi cae nguydn tii cua nguydn td kia tai cac mit mang ciia tinh the Sphalerite nay, nhung vdi niit mang dau cd toa do (1/4, 1/4, 1/4) Khi dd 4 nguyen tir B (S) d cac vi tri (0, 0, 0); (1, 1/2, 1/2); (1/2, 0, 1/2); (1/2, 1/2, 0) va 4 nguydn tir A (Zn) d cac vi tri (1/4, 1/4, 1/4); (1/4, 3/4, 3/4); (3/4, 1/4, 3/4); (3/4, 3/4, 1/4) [s]

* Cdu true dang luc gidc (hay Wurtzite)

+ Cau tnic dang luc giac (hay Wurtzite) dugc xay dung dua tren co sd quy luat xep cau theo hinh 6 canh cua cac nguydn tir B (S) dugc kyOidu la trong dd mot nira sd hdng bdn mat chiia nguydn tir A dugc ky hieu la # dinh hudng song song vdi nhau ( hinh l b )

Nhdm ddi xiing khong gian cua cau tnic luc giac la: (^^^ - P63wc

O cau tnic Wurtzite, mdi mang nguydn td ciing chiia 4 phan tir A^B"^ (ZnS) Toa do cua mdi nguydn tir A dugc bao quanh bdi 4 nguydn tir B (Z) dat tren cac dinh tii dien d ciing khoang each [aV3 + c^(u - 1/2)"]'^ , trong dd a la hing

sd mang, u la hing sd mang doc theo true Z Ngoai ra bat ky mot nguydn td nao thuoc ciing mot loai cung dugc bao quanh bdi 12 nguyen tir cung loai dd, trong dd cd 6 nguyen tir d dinh ciia mot luc giac nim trong cimg mat phing vdi nguydn tir dau va each nd mot khoang la a, 6 nguydn tir kia d dinh mat

lang tru cd day la mot tam dien d khoang each bang [^^ + —] ~ Cac toa

J 4

do cua cac nguydn tir A (Zn) la (0, 0, 0); (1/3, 2/3, 1/2): va cac toa do ciia cac nguyen tuB(S) la (0, 0, 4); (1/3 2/3, 1/2 + u) [3]

* cdu trdc viing ndng lugng

Trong cac hgp chat ban din, do su tuong tac giiia cac nguydn tir dat gan nhau, ma din de'n cac miic nang lugng cua cac nguyen tir bi dich chuyen, tach miic, va md rong tao thanh viing ggi la viing nang lugng Cd hai loai w n g ; vung hda tri va vimg din Viing choan toan bg cac electron va dugc tao ndn tir cac miic nang lugng cua cac electron bdn trong cua cac nguyen tir tu do goi la viing hda tri Viing choan mot phan cac electron hoac tu do va dugc tao ndn tir cac miic nang lugng cua cac electron ben ngoai ciia cac nguyen tir each ly goi

la vung din Giiia viing hda tri va vung din la viing cam Khoang each nang lugng giiia dinh viing hda tri va day vung din ggi la do rong viing cam Eg Vdi ban din viing cam rong thi Eg cd gia tri tir 1 ev de'n vai ev, ndn bing chuyen dong nhiet, cac electron khong the chuyen tir \aing hda tri Idn viing din Nhung nhd cac kfch thfch ben ngoai nhu nhiet nang, quang nang cac electron nhan them nang lugng chuyen tir vimg hda tri Idn viing din

De xac dinh pho nang lugng ciia cac dld'ctron trong cac hgp chat ban din ta phai giai phuang trinh Shrodinger:

Hy = E(k) \j;

(1) Vide tfnh toan ly thuyet nang lugng E(k) ngay ca ddi vdi cac tinh the don gian ciing rat phiic tap

Vdi dang cau tnic viing nang lugng don gian nhat khi \aing khong bi lap day thap nhat cd cue tieu d tam viing va vdi trudng hgp khong suy bien

(k = 0) thi d gin gia tri cue tidu, nang lugng cd thd phan tfch theo chudi luy thira sau:

E(k) = EQ -h Ak'x + Bk-y + Ck'z + cac sd hang bac cao (2) Trong dd: kx ky, kz la ba thanh phan cua vec ta sdng I

A, B, C la cac hing sd duong

+ Trudng hgp tinh thd cd ca'u tnic lap phuang thi A = B = C, nang lugng

tfnh cho day viing nh^n gia tri sau:

2 2

2m e 2m e

Trong dd ^ * la khdi lugng hieu dung cua electron

Tuong tu d gan gia tri nang lugng cue dai trong vimg k = 0 ta cd nang

lugng

E(k) = E'o-Ak; -Bk; -Ck,?

(4) Vdi tinh the cd ca'u tnic lap phuong A = B = C ta cd

2

2mi^ '^'""

Trong dd m', la khdi lugng hieu dung ciia Id trdng

Dd thi cua E(k) theo k iing vdi viing hoa tri va \aing din deu cd dang parabol

Khi cue tieu cua nang lugng \aing din va cue dai cua nang lugng ciia

viing hoa tri deu nim d k = 0 ta cd chuydn ddi dien tir thang (hinh 2a)

Khi cue tieu cua nang lugng vimg din vh cue dai cua nang lugng ciia \amg hoa

tri nim d k khac nhau ta cd chuyen miic dien tir nghieng (hinh 2b)

Khoang each ve mat nang lugng giua day vimg din va dinh vimg hoa tri

goi la do rong \iing cam

E = E o - E o ' = E , - E , (6)

11

E(k) Efk)

Ec

Ev

•^ k

Hinh 2: Chuyen miie thang (a) va chuydn miic nghieng (b)

trong ban din viing cam rong

b Su thay ddi do rong vdng cdm theo nhiet do

Su phu thuoc cua do rong viing cam Eg cua ban din \aing cam rong vao nhiet do dugc bidu didn bing cong thiic Varshini [11] (hinh 3)

Eg(T) = Eg(0)

-T + fi

(7)

Trong dd: Eg(0) la do rong \aing cam d 0 K

0,a la cac he sd dich chuydn nhiet cua do rong \iing cam iing vdi nhiet

do thap va cao

ZnP

Do rong vung cam Eg (v)

a 4.2 K

2.817 3.700 1.606 1.850 2.390 2.580 2.40 1.520 1.42

o 300 K

2.670 3.640 1.50 1.74 2.28 2.53 2.25 1.430 1.28

13

/*»

c Suthay doi cua do rong vdng cdm theo nong do cua tap chdt,

Bing thuc nghiem, ngudi ta thay ring ddi vdi da sd cac hgp chat ban

din viing cam rong, khi tang ndng do ciia tap chat trong mot khoang nao dd

thi do rong \amg cam cua chiing tang (hinh 4)

Eg (eV) 2.6

Mot sd trudng hgp dac bidt: nhu ban din loai A"B^ pha cac kim loai

chuydn tiep cd ldp vd 3d chua lap diy tao thanh ban din ban tir thi khi tang

ndng do tap chat thi do rong viing cam giam cham den mot gia tri nao dd, sau

dd Eg lai tang theo ndng do x cua tap chat Nguydn nhan cua hien tugng nay

la do tuong tac trao ddi s - d , p - d cua cac dien tir 3d cua cac ion ttr vdi cac

dien tir din

Cd 3 each xac dinh do rong \Tmg cam bing phuang phap quang: do pho

hap thu d gin bd hap thu, pho phan xa, pho exciton Trong dd phuong phap do

pho hap thu d bd vimg cam thudng dugc ap dung nhieu nhat va tuong ddi don

gian Ddi vdi nhiing chuyen miic thing dugc phep he sd hap thu ciia hgp chat

ban din dugc xac dinh bing he thiic [13] :

a(v) = A(hv-Ej'^^ (8)

Trong dd A la mot hing sd

X^y dung dd thi hien didn su phu thuoc cua a^ theo hv ta se dugc dudng thing va keo doan thing dd ta cd diem cit true hv tai hv = Eg Ddi vdi cac chuyen miic thing khong the xay ra hap thu vdi hv < Eg

a^

hv

Eg Hinh 5 : Su phu thuoc ciia he sd hap thu d gan bd vimg cam vao nang lugng hap thu cua ban din \aing cam rong

d, Cdc ca che hdp thu trong bdn ddn viing cdm rong

Khi chieu mot biic xa didn tir vdi tan sd v vao mot hgp chat ban din thi

xay ra su hap thu cac photon cua biic xa didn tir dd He sd hap thu a cua hgp

chat ban din cd thd xem la xac suat hap thu photon Neu trong hgp chat ban

din ton tai mot sd co chd'hap thu dgc lap nhau, dac trung bing cac xac suat a

(v) thi xac suat tdng cong cua cac qua trinh hap thu la:

(9) Qua trinh hap thu biic xa lidn quan den su chuydn doi nang lugng cua photon sang dang nang lugng khac cua tinh the, ndn trong ban din \iing cam

rons cd mot sd ca che hap thu sau: (hinh 6)

15

Hinh 6 : Mdt sd co chd'hap thu trong ban din vimg cam rong

+ Hap thu CO ban (hay hap thu ridng) lidn quan den chuydn miic ciia cae didn tir giira cac vimg nang lugng cho phep: tir viing hoa tri Idn \iing din (chuyen miic 1) Hap thu nay cd dac diem

- Nang lugng hap thu hv > Eg

- Phd hap thu ed mot viing ma he sd hap thu giam nhanh iing vdi bd hap thi

- Ddi \'di chuyen miic nghidng nhat thie't phai cd su tham gia cua phonon

+ Hap thu cua cac hat tir didn tu do lidn quan den chuydn miic ciia didn tir e' hoac Id trdng h trong cac vimg nang lugng cho phep (didn tir trong \amg

din hoac 16 trdng trong vung hoa tri) hoac giua cac xiing con cho phep Hap

thu nay cd dang dudng cong don dieu, khong cd cue dai vi hap thu nay khong

cd tinh Igc lua (cac chuydn miic 2 va 2 a) Vdi hap thu iing vdi chuydn miic giiia cac viing con cho phep trong vimg nang lugng cho phep thi cd cue dai,

cue tidu xen ke vi hap thu nay cd tfnh Igc lua (cac chuyen miic 2b, 2c, 2d)

+ Hap thu ling vdi chuyen miic ciia cac hat tai didn lam cho nguyen tir tap chat tir trung hda chuydn sang idn (chuydn miic 3 va 3a) Chuyen miic nay nim trong viing hdng ngoai xa

+ Hip thu ling vdi chuyen miic cua cac hat tai didn lam cho nguydn tir tap chat tii idn chuyen sang trung hda nim trong viing gin bd hap thu co ban (chuyen miic 3b, 3c) Ne'u cac tam dd la cac tam sau thi pho dich ve phia sdng dai

+ Hap thu ling vdi chuyen miic giiJa cac miic tap chat (chuyen miic 4) cung nim gan bd hap thu Nd'u cac tap chat dd la cac tam sau thi pho hap thu ciing dich chuyen vd phia sdng dai

+ Hap thu exciton lien quan dd'n su hinh thanh cua cap didn tir e" va Id trdng h cd tuang tac culong (chuyen miic 5).Pho nang lugng hap thu exciton nim d gan bd hap thu

-I- Hap thu phonon lidn quan de'n vide chuydn nang lugng anh sang thanh nang lugng dao dgng mang d \amg hdng ngoai xa

+ Hap thu plasma xay ra trong didu kien ndng do hat tai didn tu do du

cao xa thudng mang tfnh cong hudng: cong hudng plasma He sd hap thu xiing

nay rat cao Ngudi ta thudng nghidn ciiu phd phan xa cua miu Cue dai eiia phd phan xa plasma thudng nim d \iing hdng ngoai xa

+ Hap thu xay ra trong mot tam tap: Khi pha cac kim loai chuyen tid'p

nhu Mn, Fe, Ni, Co vdi ldp \'d 3d chua lap day vao cac hgp chat ban din viing

cam rong, thi cac idn cua cac kim loai trdn se tao ndn trong vimg cam nhiing miic nang lugng xac dinh Dudi tac dung ciia trudng tinh thd cac miic nang lugng nay dugc tach thanh cac miic con Khi chieu biic xa vao hop chat han

17

OAI HOC QUOC GIA HA NOI

TRUNG TAM THONG m\ THU VIEN

DT /52>6

din ludn xay ra su hap thu lien quan de'n chuyen ddi giira cac phan miic nhd ciia cac nguyen tir va idn ciia cac kim loai chuyen tid'p Ke't qua ta thu dugc mot dam hap thu tuong ddi rdng [11] (hinh 7)

Loai hap thu nay cung xay ra giiia cac miic nang lugng dac trung cho ldp vd chua lap diy 4f ciia cac idn r-at hid'm khi chiing dugc pha- tap vao hgp chat ban din xiing cam rong Nhung pho hap thu dac trung cho cac idn da't hid'm nay thudng la phd vach, vi ldp vd khong chiia day 4f nim sau bdn trong ciia vd nguyen tu nen ft bi anh hudng cua trudng tinh thd

e, Cdc ca che tdi hop bite xq trong bdn ddn vung cdm rong

Khi hap thu nang lugng kfch thfch, trong cac ban din vimg cam rong,

cd the xay ra cac co che tai hgp biic xa chu yeu sau (hinh 8)

i — i i

D ^ D

Hinh 8: mdt sd loai chuyen ddi biic xa trong ban din xiing cam rong

-i-Tai hgp ciia cac didn tir va Id trdng tu do: tai hgp biic xa xoing -viing (dudng 1),

H- Tai hgp biic xa cua exciton tu do va exciton lidn ket (dudng 2)

+ Tai hgp biic xa cua cac hat didn tu do vdi cac hat tai didn dinh xii tren cac tam tap chat: tai hgp vimg - tap chat (dudng 3)

-I- Tai hgp biic xa cua cac hat tai didn dinh xii trdn cac tam tap chat: tai hgp biic xa ciia cap donor - acceptor (dudng 4)

+ Tai hgp biic xa ndi trong mot tam tap

* Tdi hgp bite xq vung - viing

Xet mot ban din xiing cam thing bi kfch thich bdi biic xa (anh sang) Sau qua trinh hap thu photon, cac hat tai didn tir do (e", h) se bi tan xa, sau mot thdi gian hoi phuc, dien tir se chiem vi trf day viing din, cdn Id trdng se chiem

vi tri dinh xoing hoa tri Trang thai nay dugc hinh thanh trong khoang tir lO'^'s Cudng do ciia dam biic xa dac tnmg cho tai hgp viing - xiing dugc xac dinh bins cons thirc (10) [13]:

19

Trong dd: B : la mdt hing sd

Eg : la do rdng vung cam

K : la hing sd Bolztman

T : la nhiet do tuyet ddi

Vdi ban din cd vung cam khdng thing, thi tai hgp biic xa vimg - viir ;

cua dien tir se cd thd xay ra kem theo su biic xa va hap thu phonon Su tha:n

gia cua phonon la dam bao cho xung lugng dugc bao toan

Ne'u chuyen miic kem theo biic xa phonon thi nang lugng biic xa nh')

hon do rdng xiing cam :

h v _ = E,-Ep (11) Nd'u chuyen miic kem theo hip thu phonon thi nang lugng biic xa k a

hon do rdng viing cam :

hv=E^ + Ep (12) Trong dd: Ep la nang lugng cua phonon

Lugng tir sang nay dd bi hgp chat ban din hap thu lai ndn khdng phat ra

dugc

Cudng do cua dam biic xa dac tnmg cho tai hgp xoing - xiing x^di chuyen

miic nghidng dugc xac dinh bing cong thiic:

I = Io(hv - Eg + Ep)^ exp (- f^^'-^g^EP ) (13

KT

* Tdi hgp bite xq exciton tu do

Exciton la trang thai lien ket ciia cap didn tir Id trdng (e" - h) trong

tinh thd ban din Lien ket cua cap hat tai didn tir nay tdn tai la do tuong tic

culong giiia didn tir va Id trdng Cd hai loai exciton: Exciton Fren! J1 va

Exciton Wannier- Mott Exciton Frenkel dd la exciton cd ban kfnh nhd, trong

dd didn tir va Id trdng tuong tac \'di nhau bdi luc culong, chiing ludn lud \ dinh

xii trong mot niit mang Nang lugng hv < E, ma tinh the ban din hap thu dimg

de tao ra cap e- h" dd

Exciton Wannier - Mott dd la exciton cd ban kfnh Idn: Khoang each

trung binh giira e va h Idn hon hing sd mang a cua tinh thd mot vai lin Nhidu

exciton tu do lien ket vdi nhau tao thanh to hgp exciton tu do

Theo md hinh Wannier-Mott, exciton tu do cd cau tnic tuong tu nhu

nguyen tir hidrd : gdm mot Id trdng cd didn tfch duong e va mot didn tir cd

777 777

dien tich - e Nd cd khdi luong hieu dung // = —-—— coi nhu tap trung tai

trgng tam G va chuyen dgng tinh tid'n tu do trong tinh the Nang lugng toan

phin cua exciton tu do gdm cd :

- Nang lugng tuong tac cua dien tir Id trdng trong exciton dugc xac dinh

Trons dd : P = hk la xung lugng chuydn dgng khdi tam ciia exciton

Nd'u chgn gdc nang lugng la dinh xomg hoa tri thi ta cd nang lugng toan

phin cua exciton la :

£ > ^ g - ^ + ^ (17)

n "^

21

Exciton khdng ton tai d nang lugng cao, chiing se bi phan huy thanJi

electron va Id trdng tu do, xi the' ^]^ co the coi nhu nang lugng idn hoa hay

nang lugng lien ket cua exciton

Vdi chuyen miic thing nang lugng cua exciton tu do tinh tir dinh xxing

hoa tri dugc xac dinh bing cdng thiic :

^:=Eg-^ (18)

n

Tit bieu thuc(18) ta thay ring pho hap thu va biic xa cua exciton gdm

nhiing vach hep iing vdi n= 1, 2, 3 Cac x^ach nay se xit lai va lin x^ao xiing

pho hap thu bidn cua hgp chat ban din.Thudng ta chi quan sat dugc nhung

vach ling vdi n =1, 2 vi cac vach tiep theo cd cudng do giam rat nhanh

Trong chuyen miic thing ciia exciton tu do ciing cd the cd su tham gia

cua phonon Khi dd, trong pho biic xa ta quan sat thay vach exciton tu do va

nhirng vach lap lai phonon cua nd dich ve phfa sdng dai

* Tai hgp biic xa cua exciton hdn kd't: Exciton lidn ke't la cap didn tir -16

trdng dugc lidn ke't vdi nhau va lidn kd't trdn mot tam tap nao dd Tam tap cd

the la donor, acceptor trung hoa hoac donor, acceptor da bi ion hda Nhu vky

exciton lien kd't khong chuyen dong tu do trong tinh the ban din Pho biic xa

cua exciton lien kd't gdm nhung vach hep vdi nang lugng nhd han nang lugng

tai hgp cua exciton tu do \1 no khdng chuyen dong dugc trong tinh the

Nang lugng cua cac vach dac trung cho su biic xa cua exciton hen ket

dugc xac dinh bing he thiic: hv := E„ ~ Ei^^ /^g 1

Trong dd: E]^ la nang luong cua exciton lien ket E;;, thoa man dieu kien:

0.055Ei < E„ < 0.33 E, (20)

6 day E, la nang luong idn hda tap chat

Trong cac ban din cd x^ng ca'm khdng thang, cd chiia tap chat, d^ bao toan xung lugng, exciton khdng nhirng cd the trao doi xung lugng vdi phonon ciia mang tinh the ma cdn cd the truyen xung lugng cho nguydn tir tap chat

* Tdi hgp bite xq vdi chuyen mdc giita vung vd mite tap chdt,

+ Chuyen miic ndng: Trong cac hgp chat ban din tdn tai cac miic tap chat bi ion hda donor hoac acceptor, cd the xay ra cac chuyen miic cua cac hat tai didn de'n cac miic dd nhu sau:

- Dien ttr tur vung din chuyen xudng miic donor

- Ld trdng lii vimg hda tri bi bit len miic acceptor Nhiing chuyen miic

tren cd the la cac chuyen miic kem theo biic xa photon, nhung vdi xac xuit nhd hon biic xa phonon vi tiet didn bit hat tai didn trong trudng hgp biic xa phonon Idn hon nhieu tiet didn bit cac hat tai dien trong trudng hgp biic xa photdn (khoang 10"^ lan) Vi nang lugng idn hda Ei nhd, ndn budc sdng cua photon phat ra la Idn, roi vao viing hdng ngoai

-f- Chuydn miic sau: Cac chuyen miic ciia cac dien tir tir miic donor

xudng xoing hda tri va lis xomg din xudng miic acceptor ggi la cac chuyen miic

sau

Ne'u chuyen muc la thing thi : hv = E^ - Ej

Nd'u chuyen miic la nghidng thi: hv = Eg - Ej - Ep

Trong dd: Ej la nang lugng ion hda donor hoac acceptor tuong iing

Ep la nang lugng cua phonon tham gia vao tai hgp Thuc nghiem cho thay cac chuyen miic sau chi xay ra dd'i vdi cac ban din pha tap ft Khi tang ndng do tap chat, cd thd hinh thanh viing tap chat lam nhde ranh gidi cac x^iing

ca ban

Ddi x'di ban din xiing cam thang j]^ < jjj ndn nang luong idn hda

donor E^ nhd hon nang lugng idn hda acceptor E.v Khi dd dd dang phan bidt duoc hai loai chuydn miic sau

23

- Chuyen miic cua cac dien tir til vung din xudng miic acceptor, biic xa

ra cae photdn vdi nang lugng: hvi = Eg - E^

- Chuydn miic cua cac didn tir miic donor xudng xoing hda tri, biic xa

cac photon vdi nang lugng: hv2 = Eg - E^

Ddi vdi ban din cd E^ = E^, mudn xac dinh ban chat cua cac chuyen

miic phai xet su thay doi cua cudng do biic xa theo ndng do tam tap

Nd'u ban din cd viing cam khdng thing thi chuyen miic donor - vimg

hda tri va chuydn miic viing din - miic acceptor can cd su tham gia cua

phondn

* Tdi hgp bite xq cap donor - acceptor

+ Trong cac ban din tap chat, cac nguyen tir tap chat thay thd' cac

nguyen tir ciia mang tinh thd, khi bi idn hda lam xua't hien cac miic nang lugng

trong \amg cam cua ban din Cd hai loai tap chat donor va acceptor Trong

mot sd chit ban din chi tdn tai tap donor hoac acceptor, nhung cd nhieu chat

ban din ddng thdi cd ca tap donor va acceptor Khi dd trong ban din nay tao

thanh cap donor - acceptor vdi nang lugng tuong tac cu long

2

A£ = - ^ (21)

6 r

Trong dd:

r : la khoang each giiia donor va acceptor

£: la hing sd dien mdi cua mdi trudng

Nang lugng lidn kd't cua cap donor - acceptor la:

Tuong tac culong giua donor \'a acceptor lam giam nang lugng lidn ket di AE,

vi the khi tai hgp D - A, photon phat ra cd nang lugng:

2

hv = E g - ( E - A E ) = E - ( E D + E , ) + - ^ (23)

s r

* Tdi hgp bite xq noi trong mot tdm:

Khi dugc kfch thfch, trong cac hgp chit ban din cd pha tap nhiing nguydn td vdi ldp vd didn tir 3d hoac 4f chua lap day, thudng xay ra su chuyen ddi biic xa ciia cac didn tii d eac ldp vd chua lap day nay Nd dugc ggi la biic

xa ndi trong mot t^m Biic xa nay xay ra d mgi nhiet do: tir nhiet do thap dd'n nhiet do cao nhung d nhiet do thip thi cudng do cua dam va vach kha manh Tuy thudc vao timg loai tam tap cd mat trong hgp chat ban din ma pho biic xa cua dac trung cho nd cd the dudi dang nhiing dam kha rdng hoac nhimg vach hep (hinh 9) Neu t^m tap nao bi anh hudng manh ciia trudng tinh the (cac nguyen td vdi ldp vd 3d chua lap day) thi pho biic xa cua nd gdm nhiing dam rdng Ne'u tam tap nao bi anh hudng ft ciia trudng tinh the (cac nguyen td vdi ldp vd 4f chua lap diy) thi pho biic xa cua nd gdm nhirng vach hep

4 A / E

6T

6A

Hinh 9 : Su chuyen rdi biic xa dac trung cho ion Mn

trong ban din Cdi.^ Mn^ Te

2+

2 Xay dimg he kich thich, thu pho huynh quang, tan xa Raman tren ca

sd may don sac each tur kep GDM-1000 va ky thuat lock - in

a Md dau

Xay dung mot he kfch thfch, thu pho cd do nhay tdt on dinh va nang suit phan giai cao la mong mudn cua nhieu phdng thi nghiem quang pho Nhu

25

chiing ta da biet, cac phuong phap pho hgc laser bao gdm quang ph6 hgc Raman, huynh quang, hap thu da va dang dugc iing dung rdng rai de nghidn ciiu cau tnic phan tir, hoa diu, mdi trudng nude va khf, thudc tfnh vat lieu quang dien tir ke ca vat lieu ca'u tnic nano Gin day, cac hieu iing phi tuyd'n phat tan sd tong va phat hoa ba b^c hai da trd thanh cdng cu quang pho de khao sat be mat vdi dac trung be mat cao Quang phd hoa ba bac hai dugc bie't den nhu mot iing cir vien noi bat nhim hieu ro hon vd tfnh chat quang cua cac cau tnic nano [17 , 18] Ngudi ta rit cin eac he quang pho thu dugc tfn hieu yd'u, xap xi nhidu dn Nhirng he may quang pho nhu vay ndi chung rat dit tien Chiing tdi da nghien ciiu nang cap he may quang pho GDM - 1000 cua hang Cari-Zeiss Jena CHDC Du"c cQ thanh mot he thu quang pho sir dung ky thuat khuech dai Lock - in, cd ghep ndi xir ly sd lieu trdn may tinh May quang pho each tir kep GDM-100 cd nang suat phan giai cao nhung phan didn tir thudc the he cQ sir dung tu ghi vdi nhan quang didn loai head - on M12FC-51 vdi viing nhay pho 400-500nm He may nay thfch hgp vdi viec thu pho Raman kfch thfch bing laser Argon cdng suit co 3-5W lien tuc hoac den thuy ngan Tuy nhien thu pho bing tu ghi la mot bit tien Idn khdng cdn phu hgp Chiing tdi da thay thd' he thu pho tu ghi bing ghep ndi may tfnh cimg x'di sir dung bg khuech dai Lock-in DSP-SP830, Stanford (USA) de tang do nhay va on dinh Mot mdto budc thfch hgp dugc ghep vao de didu khien chfnh xac gdc quay cua each tir He thu pho mdi dugc dieu khien va thu nhap sd lieu thdng qua may tfnh dua trdn phin mem viet bing Visual Basic (VB) Nhan quang didn cQ dugc thay the bing nhan quang R-928 ciia Hamamatsu (Japan) dd md rdng x^img nhay phd Cac phep do ban dau dugc tien hanh nhim muc dich can chinh

va xac dinh che do hoat dgng Sau khi cai tid'n thanh cong x'a thiet dat chuan cho he may quang pho mdi chiing tdi da xay dung thanh cdng mot he thu pho Raman kich thfch bing laser Argon cdng suat 0.5W Nhd thay nhan quang didn mdi chiing tdi da cd the thu dugc phd Raman ciia chiet pham dau md kfch thfch bing laser He-Ne cdng suat 30mW Day la mot thuan Igi Idn x'i tranh dugc phdng huynh quang trong pho Raman x'a khong phai lam lanh laser nhu khi diing laser Argon He thu pho mdi thd hien nhieu tid'n bg trong nghien

ciiu cac pho huynh quang va pho Raman cung nhu cac pho tin hieu yeu ndi chung

b He kich thich pho huynh quang va tan xa Raman

De kfch thfch pho huynh quang ciia cac hgp chat ban din viing cam rong, chiing tdi da dung so dd nhu d hinh 10

Den Thuy ngan

HBO- 502

May dan sic OR

May tinh Khuech dai Lock -in

Nhan quang dien

Hinh 10 : So dd kfch thfch pho huynh quang ciia cac chat ban din

xoing cam rdng dimg den thuy ngan ap suat sidu cao HBO-502

Hoat ddng ciia so dd kfch thfch nay la nhu sau:

Biic xa phat ra tir den thuy ngan ap suit sidu cao HBO-502 gdm nhiing vach cd cudng do rat manh dugc thau kfnh thach anh L, (fj = 75mm) hdi tu vao khe may don sic QR Nhd quay du xfch ciia may nay ma lin lugt dugc tu

Idn miu bing mot thau kfnh hdi tu thach anh Lj (f2 = 73mm) dat trudc khe ra

cua may Anh sang huynh quang phat ra ttr miu dugc thau kfnh thuy tinh L3 (f^ = 94mm) hdi tu vao khe ciia may dan sic each tir kep GDM-1000 Sau khi qua nhan quang didn M12FC-51 (hoac R-928), tfn hieu quang dugc chuyen thanh tfn hieu dien xa dugc khuech dai bing bg khuech dai lock-in SR-830 va cudi ciing, tfn hieu dugc dua vao may tfnh xir ly cho pho biic xa cua miu cin nghidn ciiu

Anh sang tan xa do miu phat ra cd cudng do rat nhd so \'di cudng do cua chiim anh sang kich thfch, vi \'ay he kfch thfch pho tan xa Raman can phai tan dung tdi da nang lugng cua chum anh sang kfch thfch va tap trung hau nhu toan bg nang lugng anh sang tan xa vao khe cua may don sic Xuat phat tir hai

27

yeu ciu tren chiing tdi da thie't ke'va xay dung he kfch thfch phd tan xa Raman bing laser argon va He-Ne ( hinh 11)

Khs-ir4v<K-i}sa£

Hinh 11 :He kfch thfch phd tan xa Raman bing laser argon va He - Ne

Vdi he kfch thfch nay nhd cd thau kinh hdi tu Oj (fj = 7.7cm ) va cac guong phing G^ , G2 ma chum anh sang laser di lai nhieu lan trong cuvet chiia miu, cdn anh sang tan xa Raman do miu phat ra dugc guong ciu 1dm G3 (f3 = 16

cm, dudng kfnh d=10.5 cm ) tap trung vao khe may don sic each tir kep

GDM-1000 , vi the cudng do vach tan xa Raman tang Idn gip ba lin (hinh 12)

Hinh 12 : Su phu thuoc cudng do cua vach tan xa Raman Av= 1004 cm^ ciia C7H8 vao cac dung cu quang hgc cua he kfch thfch (AJ-J =488 nm)

a Khdng guong phing G,, G khdng guong cau G3

b Khdng guong phing Gj, G cd guong cau G,

c Cd guong phing G,, G., khdng guong ciu G3

d Cd guong phing Gj, G., cd guong ciu G3

c He thu pho huynh quang va tan xa Raman

* Cau true ciia he thu pho cai tien

So dd khdi cua he thu pho cai tid'n dugc bieu didn trong hinh 13 May don sic GDM - 1000 bao gdm hai each tir 650 vach/mm, viing phd hoat ddng tir 7500-16676 cm'^ (iing vdi pho bac hai) Hinh 14 bieu didn so dd quang ciia may GDM - 1000 Bd phan tu ghi dugc thao ra thay vao dd sd lieu dugc luu trii va xtr ly bing may tfnh ca nhan (PC) Mdt md to budc 1.8°/budc dugc lip vao vi tri thfch hgp de quay each tii chgn budc song Mdt budc nhay cua mdto nay lam thay doi sd sdng la 0.0832cm'' De dieu khien mdto budc nay, chuong trinh di^u khien se giri mdt day cac sd nhi phan bdn bit qua cong song song (LPT) cua may tfnh theo thii tu L 0001; 0011; 0010; 0110; 0100; 1100; 1000;

1001 tuong ling vdi cac vi tri budc l-> 2 ^ ^ 3 ^ 4 - > 5 - > 6-> 7 -> 8 Cac tfn hieu nay chi la cae miic logic TTL ndn can phai cd mdt bg ddm trudc khi dua dd'n mdto budc [19 , 20]

SR830

lock-in

iinplificr

Chopper Signal

Hinh 14 : So dd quang hgc ciia may dan

sic each tir kep GDM-1000

29