Chế tạo và nghiên cứu một số tính chất của các màng mỏng bán dẫn dung dịch rắn A2B6

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BO GIAO DUG VA OAO TAO DAI HOC QUOC GIA HA N O I TRLTCJNG DAI HOC KHOA HOC TlTNHIEN -oOo- Le Thi Thanh Birth Che tao va nghien cihi mot so tinh chat cua cac mang mong ban dSn dung dich ran A B Chuyen nganh: Vat ly Dien tijf J LUAN AN PHO TIEN SI KHOA HOC TOAN - LY NGUdl HU^6NG DAN KHOA HOC l.GSPTSLeKhacBinh 2. PGS PTS Nguyen Ngoc Long O C r O N ' - '-:••:)'• MAf.OI; HA NOI -1996 MUC LUC Mord^u 1 Chirang 1. CAC HOP CHAT BAN DAN ZnS, CdS vA CAC DUNG DICH RAN BAN DAN Zn,Cd,.,S 1.1. Cft'u true dnh thi cua ZnS va CdS 5 5 1.1.1 CS^u tnic mang lap phuong gia kem 1.1.2 C^u tnic mang luc gidc Wurtzite 1.1.3 Mang tinh the ZnS va CdS 5 6 8 1.2. Ca'u tnic vising nSng lupng cua ZnS va CdS 10 1.2.1 Ca'u tnic viing nang lupng ciia mang I^p phuong 1.2.2 Ca'u true virng nang lupng cua mang luc giac Wurtzite 1.2.3 Viing nang lupng cua ZnS va CdS 1.3. Tinh ch^t h^p thu ciia ban dSn 1.3.1 Khai niem chung 1.3.2 Ha'p thu chuyin ddi thang 1.3.3 Ha'p thu chuyin ddi nghieng 1.4. Cac cach^'phat huynh quang trong ban dan 1.4.1 1.4.2 1.4.3 1.4.4 1.4.5 1.4.6 Khai niem chung Mo hinh tpa dp cau hinh Tai hop viing - viing Tai hop buc xa qua cac trang thai exciton Chuyin ddi vung - tap ch^t Chuyin ddi done - axepto 1.5. Mdt s6 phuong phap ch^'tao tinh thi va mang dung dich thi 1.5.1 Cdc phuong phap klo dan tinh the 1.5.2 Cac phuong phap che tao mang dung dich ran 10 12 13 15 15 17 18 20 20 21 25 28 33 35 37 37 38 1.6. T6ng quan v^ dung dich ran Zn^^Cdi.j^S 39 1.7. K6t luan 41 Chirang 2. KY THUAT THl/C NGHIEM 43 2.1. He tao mang 43 2.1.1 L6 2.1.2 Bpgamau 2.1.3 He thdng vdi phun, khi nen 44 44 45 2.2. Ha bdc bay chan kh6ng 45 2.2.1 Bom chan khdng 2.2.2 Buong hoc bay 2.2.3 Ngudn bdc bay 2.2.4 D^bdc bay 45 45 47 47 2.3. H e do phd huynh quang va phd truyen qua 47 2.3.1 H e do phd huynh quang 2.3.2 H e do ph6 truyen qua 2.3.3 Cac phep do khac 47 50 51 2.4. K ^ luan Chuong3. CHE TAO VA NGHIEN 51 CLTU CAU TRUC CUA M A N G Zn^Cd,.,S 53 3.1. Tao mang Zn^^Cdj.^S bang phuong phap phun dung dich len de'ndng 53 3.2. Anh hudng cua nhiet dd d^d^n ch^t lupng m^ng 55 3.3. Ch^d6 u mang 60 3.4. Che^dOphatap 61 3.4.1 Phuong phap nhiing mang vao dung dich mudi tapcha'trdi ii d nhiet dp cao 61 3.4.2 Phuong phap hda dung dich mudi tap chat cung vao dung dich phun nen 63 3.5. Tao dien cue trong sudt Sn02 va l^m dien cue ohmic len mang Zn^^Cdi.^^S 66 3.6. Che'tao mau Zn^Cd^.^S bang phuong phap diing Id vi song 68 3.7. Ph6 nhilu xa tia X ciia mtlng Zn^Cdi.,S 70 3.8. Anh bfe mat cac mang Zn^Cd^^^S chup bang kinh hiln vi dien tu 73 3.9. K a luan 75 Chirang 4. TINH CHAT QUANG vA HUYNH QUANG CUA M A N G Zn.Cdi KHONG PHA TAP ,S 78 4.1. Tfnh cha't quang cua mang Zn^Cd].xS 78 4.1.1 Ph6 hSp thu cua mang Zn.Cdi.^S 4.1.2 X^c dinh d6 r6ng viing ca'm cua cac mang Zn^^Cdi.^S 4.2. Huynh quang ciia cdc mang Zn^Cdi.^S khdng pha tap 78 80 82 4.2.1 Dang phd huynh quang 4.2.2 Su phu thudc ciia dinh ph6 huynh quang theo thanh phan x 4.2.3 Pho huynh quang phan giai thai gian, su phu thupc ciia pho huynh quang v^o cudng dd kich thich. Thdi gian sdng huynh quang 4.2.4 Anh hudng ciia che'dd li nhiet len huynh quang ciia mang Zn^Cdj.^S 4.2.5 Su phu thudc nhiet dd cua ph6 huynh quang cua cac mang Zn^Cdj.^S 83 86 87 91 92 4.2.6 Co che' biic xa cap dono - axepto trong phd huynh quang cua mang Zn^Cdi.^S khdng pha tap 4.2.7 Hien tupng quang hda ciia mang Zn^Cd,,^S 4.3. K^ luan 94 99 105 Chirang 5. HUYNH QUANG CUA CAC M A N G ZnxCd,.xS PHA TAP Cu VA Mn 106 5.1. Phd huynh quang ciia m ^ g Zn^Cdi^^S : Cu 5.1.1 Dang pho huynh quang. Anh hudng cua sir pha lap ddng len pho huynh quang 5.1.2 Ph6 huynh quang cua mang Zn^Cdj ^SiCu vdi cac thdng sd thanh phan x khac nhau....=... 5.1.3 Ph6 huynh quang cua mang Zn^^Cdi.^S : Cu khi kich thfch lien tuc vdi cac cudng dp khac nhau va a cac nhiel do khac nhau 5.1.4 Ca che' biic xa ciia dai sdng dai trong phd huynh quang cua mang Zn^Cd,.^S:Cu 5.1.5 Tach ph6 huynh quang 5.2. Phd huynh quang ciia mang Zn^Cdi^^S : Mn 5.2.1 Dang phd huynh quang. Anh hudng ciia su pha tap len ph6 huynh quang 5.2.2 Pho huynh quang ciia mang Zn^Cdi.^S : Mn vdi cac thong sd thanh phan x khac nhau 5.2.3 Sir phu ihupc cudng dp kich thfch va nhiet dp cua pho huynh quang ciia mang Zn^Cdi.,S : Mn 5.2.4 Ca che'biic xa ciia dai song dai trong pho huynh quang cua mang Zn^Cdi.^SrMn 106 106 109 111 113 114 117 117 120 121 123 5.3. K ^ luan 124 Ket luan 126 Cac cong trinh khoa hoc lien quan den luan an 129 Danh muc tai lieu tham khao 131 DANH SACH CAC K t HIEU StTDLNG TRONG LUAN AN a <7i e X M 11 T d hv k m*^. m*h Hi ^ X D-A E.A ED E,,, Eg Ei Ep R SA SD Td 1 J 'W w^. - he sd'ha'p thu - tie't dien ha'p thu ciia tam i - hang sd dien moi ciia cha't ban dSn - budc sdng anh sang - khdi lupng rat gpn ciia exciton - ndng dp tap cha't - thdi gian sdng huynh quang ciia mang - dd day ciia mang - nang lupng photon - vecta sdng ciia dien tii - khdi luomg hicu dun^ cua dien tix - khdi lupng hieu dung ciia 16 trong - mat dp trang thai i - vecta sdng cua phonon - thanh phan Zn trong mang - dono - axepto - nang lupng ion hda axepto - nang lupng ion hda dono - nang lupng dinh huynh quang - nang lupng viing ca'm - nang lupng dien tir d trang thai i - nang lupng phonon - he sd phan xa - dai tu kich boat - dai sdng dai - nhiet dp de' - nhiet dp do - cudng dp kich thfch - ban dp rpng dai pho - xac sua't chuyen ddi biic xa cap D-A MO DAU Hap chat ban dan A " B ^ la ddi tupng nghien cuti ciia rat nhieu cong trinh. Cac hop chat A B cd nhieu dac tinh ca ban dang quy nhu dp rpng viing cam Idn, chuyen miic thing. Dp rpng viing cam Idn ciia cac hop chat A B cho phep md rpng gidi ban ufng dung cua cac linh kien quang dien tuf sang mien nhin thay va cue tim gan. Viing cam ciia cac hop chat A B deu la thang, ddng thdi cac chuyen miic phat quang gay ra bdi cac tam sau cd xac suat Idn, nen cd the by vpng cac chuyin ddi dien tu vdi hieu suat lupng tu cao. Vi du ddi vdi ZnS, hieu suat lupng tu phat quang c d t h e d a t g a n 100% [15]. Ngay nay, tit cac hop chat A B , ngudi ta da che tao dupe cac bpt huynh quang hieu suat cao, cac quang trd nhay trong viing nhin thay. Trong cac hpp chat A B , ZnS cd dp rpng viing cam Idn ban ca (Eg = 3,7eV). Cung vi the ma ZnS cd limg dung thuc te rpng rai. Bpt huynh quang ZnS dupe sii dung trong cac tu dien huynh quang, cac man Rangen, man cua cac dng phdng dien tu. Ngudi ta da che tao dupe nhieu loai photodiot tren ca sd chuyen tiep p - n cua ZnS. Suat quang dien dpng (photo emf) <5 cua chuyen tiep p - n tren tinh the ZnS thudng dat tdi 2,5 V [15] Ben canh ZnS, CdS (Eg = 2,4 eV) cung la mot trong cac vat lieu quan trpng nhat thudng dupe sir dung trong cac luxmeter va cac he thdng pin mat trdi. Ddi vdi cac he thdng pin mat trdi, mang mong CdS thudng dupe dting lam vat lieu cUa sd trong cac chuyen tiep di the vdi cac ban dSn boat quang nhu Cu^S [47], CdTe [36]. Khong chi cac hop chat A B , ma ca cac dung dich ran ban dfin tren ca sd cac hap chat nay cung dupe nghien ciin mot each rpng rai va cd nhieu ling dung trong thuc te. Dp rpng viing cam cua dung dich ran A B cd the thay ddi dupe tiiy theo thanh phan cua hop chat trong dung dich ran. Dieu dd tao ra kha nang che tao nhiJng ngudn phat quang va cac dau thu quang lam viec trong viing pho trai rpng tir viing hong ngoai tdi viing kha kien. Mang Zn^Cdi^S la mot trong cac mang dung dich ran A B dupe quan tam nghien cuti nhieu [13, 83, 74, 21, 47]. Dp rpng viing cam cua Zn^Cdi.^S cd the thay doi lien tuc th 2,4 eV den 3,7 eV. Mang Zn^Cdi.^S hiia hen nhieu trien vpng suf dung trong cong nghiep che tao cac linh kien quang dien tir nhu cac pin mat trdi, cac diot phat quang, cac quang trd. Nghien cufu cac tinh chsft quang va huynh quang cua mang Zn^Cdi.xS cho den nay vin la de tai hap dan cac nha vat ly ban dan. Cac ket qua nghien ciiti nay cd the cung cap nhuHg thdng tin ve ban chat cua nhiitig tam tap chat hoac ve cau tnic vi md cua cac tam huynh quang, ca che biic xa huynh quang. De nghien ciiru cac tinh chat quang va huynh quang cua ban dan, ngudi ta thudng sir dung cac phuong phap pho hap thu va pho huynh quang. Viec su dung cac phuang phap huynh quang cho phep don gian hda va lam giam gia thanh he thdng do ludng kiem tra trong san xuat, bdi vi viec kiem tra cac dac tnmg huynh quang cd the thay the viec do dac hang loat cac dac tnmg dien, quang dien cua vat lieu. Trong sd cac phuong phap pho huynh quang thi quang huynh quang la phuong phap dupe pho bien rpng rai nhat. Dd la phuong phap nghien culi nhay nhat ddi vdi nhutig vat lieu cd ndng dp tam tap thap. Phuang phap nay con cd iru diem la khdng can tao dien ctrc len ban dan, phep do cd the thuc hien nhanh, dan gian va chfnh xac ma khdng ddi hoi cac thiet bi phiic tap [31]. Mac dii tinh chat quang va huynh quang cua mang Zn^Cdi^S da dupe mot sd tac gia nghien ciiti, nhung ban chat cua cac dai pho, ca che phat huynh quang cua cac tam vSn con la nhihig van de chua dupe lam sang to. Tinh chat quang va huynh quang cua mang Zn^Cdi.xS che tao bang phuang phap phun dung dich len de ndng Ian dau tien dupe nghien ciiti va cong bd trong luan an nay. Luan an dupe hoan thanh trong 5 nam (1992-1996) tai phdng thi nghiem cua bp mon Vat ly Dai cuang, trudng Dai hpc Khoa hoc Tu nhien. Luan an dupe mang ten "Che tao va nghien ciiti mot sd tinh chat cua cac mang mong ban din dung dich ran A"B^". Muc dich cua luan an nham: 1- Che tao mang dung dich ran ban dan bang phuang phap phun dung dich 2- Pha tap vao mang 3- Nghien ciiru tinh chat quang va huynh quang ciia mang ZUxCd^.^S 4- Nghien ciiu tinh chat huynh quang cua mang Zn^Cdi.xS pha tap Cu va pha tap Mn. Phuang phap nghien ciiu cua luan an la phuang phap thuc nghiem: 1- Cac mang Zn^Cd^.^S dupe che tao tren he phun dung dich do chung tdi tu che tao. 2- Cac phuang phap phan tich bang ky thuat Vi phan tich tia X, nhilu xa tia X, kinh hien vi dien tuf da dupe su dung. 3- Cac pho truyen qua va huynh quang cua cac mang Zn^Cdi.^S dupe khao sat vdi cac thanh phan x thay ddi, ndng dp tap chat thay ddi, trong mien nhiet dp 77K 300K. Mot sd pho dupe do tai 2K va 4K. 4- Cac pho phan giai thdi gian cua mang Zn^Cd^.^S dupe do d 77K va 300K. Cac ket qua chinh cua luan an la: 1- Da xay dung va hoan thien cac he tao mau, lam dien cite va cac he do pho truyen qua va huynh quang. 2- Bang phuang phap phun dung dich len de ndng da che tao dupe mang da tinh the dung dich ran ZuxCdi.^S cd dp rpng viing cam thay ddi tiir 2,4 eV den 3,5 eV va cd hang sd mang phu hop vdi hang so mang cua don tinh the hoac mang Zn^Cdi.^S che tao bang cac phuong phap khac. 3- Lan dau tien nghien cuu mot each he thdng pho huynh quang ciia mang Zn^Cdi.xS che tao bang phuang phap phun dung dich, khdng pha tap va cd pha tap Cu, Mn. Tim ra quy luat phu thudc ciia nang lupng dinh huynh quang cac mang vao thanh phan x, nhiet dp T va ndng dp tap chat t] trong mang. 4- Lan dau tien cac pho huynh quang phan giai thdi gian cua mang Zn^Cdi.^S va su phu thupc cua pho huynh quang vao cudng dp kich thfch dupe nghien ciiti. Cac ket qua thuc nghiem dupe giai thich thoa dang, neu gia thiet ca che phat huynh quang cua mang la su tai hpp cua cap dono - axepto. Ngoai phan md dau, ket luan va danh muc tai lieu tham khao, luan an dupe trinh bay trong 5 chuong: - Chuang 1: Cac hop chat ban dan ZnS, CdS va dung dich ran ban dan Zn^Cd^.^S. - Chuang 2: Ky thuat thirc nghiem - Chuang 3: Che tao va nghien ciiu cau tnic cua mang Zn^Cdi.^S - Chuang 4: Tinh chat quang va huynh quang cua mang Zn^Cdi.^S khdng pha tap - Chuang 5: Huynh quang cua mang Zn^Cdi.^S pha tap Cu va Mn Chuang 1. CAC HOP CHAT BAN DAN ZnS, CdS VA CAC DUNG DICH RAN BAN D A N ZoxCdi XS 1.1, Cau true tinh the cua Z n S va C d S 1.1.1 Cau true mang lap phirong gia Rem [6] , Nhom ddi xihig khong gian cua tinh the^ A. HT^VI B ling vdi mang tinh the nay la Ti-F43m(hinhl) Hinh 1. Cau true mang tinh the lap phuang gia kem [80] .VI nguyen tu B (S), w^ nguyen tu A (Zn) IIT^VI O cau tnic nay mdi d nguyen to chiia 4 phan tu A B vdi tpa dp cac nguyen tula: - 4 nguyen tii B,vi a vi tri |a , cd cac tpa dp: (0, 0, 0), (0, 1/2, 1/2), (1/2, 0, 1/2), (1/2, 1/2, 0) - 4 nguyen tir A d vi tri |c|, cd cac tpa dp: (1/4, 1/4, 1/4), (1/4, 3/4, 3/4), (3/4, 1/4, 3/4), (3/4, 3/4, 1/4) Khac vdi cau tnic lap phuang cua kim cuang do mot nguyen tuf tao nen, cau tnic gia kem chiia hai loai nguyen tut khac nhau. Ddi xiing diem d cac vi tri |a| va |c| tuong utng vdi 43m. Mot nguyen tut bat ky dupe bao bpc bdi 4 nguyen tu khac loai: Mdi nguyen tu A"(B^) dupe bao quanh bdi 4 nguyen tu B (A ) nam d dinh ciia tii dien tren khoang each — j — , vdi a la thdng sd cua mang lap phuang. Mdi nguyen tut A (B ) con dupe bao bpc bdi 12 nguyen tu ciing loai, chiing la lan can bac 2, nam tren khoang each fl . Sau nguyen tu trong sd dd nam d dinh cua luc giac, tren ciing mot mat phang vdi nguyen tir ban dau. Sau nguyen tir con lai tao thanh phan lang tru gdm 3 nguyen tut nam d mat phang cao ban va 3 nguyen tu nam d mat phang thap ban mat phang luc giac ke tren. Dac tinh quan trpng ciia cau tnic nay la khdng ton tai tam ddi xiing hay tam dao. Cac Idp A " - B ^ dupe dinh hudng theo true [111]. Do dd tinh the A " B ^ ' lap phuang gia kem cd tinh di hudng. Cac mat ddi xufng nhau (h k 1) va (h k 1 ), cac hudng ngupc nhau [h k I] va [h k 1 ] cd the cd tinh chat vat ly khac nhau. 1.1.2 Cau true mang luc giac Wurtzite [6] Nhdm ddi xiing khdng gian cua mang tinh the nay la C'^gy - Pdamc. Mdi d nguyen to chiia hai phan tu A B . Trong dd hai nguyen tii A " nam d vi tri: 0, 0, 0 va 1/3, 2/3 , 1/3 va hai nguyen tu B ^ nam d vi tri : 0, 0, u va 1/3, 2/3, 1/3 + u. Vdi u ^ 3/5. Mdi nguyen tu A lien ket vdi 4 nguyen tut B , nam d lan can 4 dinh tii dien: mot nguyen tut of khoang each uc, 3 nguyen tut khac d vi tri [l/3a' + cVl/2)Y^' Xung quanh mdi nguyen tuf cd 12 nguyen tii lan can bac hai: Hinh 2. Ca'u true mang tinh the luc giac Wurtzite [6] nguyen tut A° @ nguyen tir BVI - 6 nguyen tut d dinh luc giac, trong ciing mot mat phang vdi nguyen tuf ban dau va each nguyen tut ban dau mot khoang a. - 6 nguyen tii khac d dinh lang tru tam giac, each nguyen tuf ban dau mot khoang [1/3 a^ + 1/4 c^]'^l Neu - = a 2V2 V3 1,6330 va u = 3/8 thi cac nguyen tu lan can bac mot nam chfnh xac d dinh tii dien va 12 nguyen tut lan can bac hai deu nam each nguyen tut ban dau mot khoang bang nhau. IlnVI Tinh the A B cau tnic luc giac khdng cd tam ddi xiing vi the trong mang ton tai true phan cue song song vdi hudng [0 0 0 1]. Ddi vdi mot chat, giiia cau tnic luc giac va cau tnic lap phuong ton tai mot mdi lien he chat che. Cac hang sd mang cua 6 nguyen to luc giac hen he vdi hang sd mang d nguydn to lap phuang bang cong thiic sau ^h = trong do: 2^K Ch = 3-V3a, a^ va c^ la hang sd mang luc giac. a^ la hang sd mang lap phuang. Vi tri tuang ddi cua nguyen tuf trong cac mang lap phuang va luc giac gan gidng nhau. Su bao bpc cac nguyen tuf A (hay B ) bdi nhung nguyen tuf lan can bac hai trong ca 2 loai mang cung nhu nhau. Su khac nhau ve tpa dp cac nguyen tuf the hien d chd cau tnic luc giac dac tnmg bdi phan lang tru. De phat hien dupe su khac nhau trong cac cau tnic nay can phai xet den cac nguyen tir lan can bac 3. 1.1.3 Mang tinh the ZnS va CdS [6] 1.1.3.1 ZnS Tinh the ZnS thudng cd cau tnic lap phuong gia kem va luc giac Wurtzite. Cau tnic luc giac cua ZnS on dinh d nhiet dp cao. Su chuyen pha tijf luc giac sang lap phuang chi xay ra d nhiet dp 1020*^C - 1150*^C. Cac hang sd mang cua ZnS phu thupc vao dp hoan thien ciia mang tinh the. Su ton tai ciia tap chat ciing gay nen sai khac ve hang sd mang so vdi tinh toan ly thuyet. Oxy thudng la tap chat trong ZnS tinh the luc giac. Nhung sai hong trong tinh the luc giac ZnS cd the tao ra mot viing nhd cd cau tnic lap phuang nam trong tinh the luc giac. Tinh the ZnS ket tinh trong cac dieu kien khac nhau cd the tao ra cac dang cau tnic khac nhau. Dd la cac bien the ciia cau tnic lap phuang va cau tnic luc giac. 1.1.3.2 CdS Tinh the CdS ciing ton tai d cau tnic lap phuang gia kem va luc giac Wurtzite. Hang sd mang cua cac tinh thi thuc te phii hpp tot vdi cac tinh toan ly thuyet. Tuy thupc vao dieu kien cong nghe, cd the nhan dupe cac tinh the CdS lap phuang va luc giac mau do hoac mau vang. Cac thdng sd mang cua CdS va ZnS vdi cac cau true lap phuong va luc giac dupe thdng bao trong bang 1. Hang so mang Hop chat Lap phuong gid kem a (A ) ZnS ZnS CdS CdS 5,4145 5,832 Luc giac Wurtzite a(A°) c(A°) 3,8230 6,2565 4,1348 6,7490 Bang I. Cac thong sd mang cua ZnS va CdS [10] 1.1.3,3 ZnjCdi_j^ Trong dung dich ran, su xuat hien sai hong mang cd the la do sir thay the mot loai nguyen tu nay bang mot loai nguyen tuf khac. Neu hai chat ban dau cd cau tnic khac nhau thi cd su chuyen cau tnic tiif trang thai nay sang trang thai khac. Su bien ddi nay thudng xay ra trong mot khoang thanh phan nao dd. Gidi ban cua khoang nay dupe xac dinh bang phuang phap tao tinh the va phu thupc vao ndng dp tap chat. Trong mien chuyen tiep tii mang nay sang mang khac thi mang tinh the dung dich ran cd the d dang hdn hop hai pha hoac cau tnic hdn don vdi cac gia tri sai hong Idn. Tac gia [14] nghien cuti su chuyin pha trong he Zn^Cd^.^S da chi ra rang str chuyen pha trong mang tinh the lap phuang gia kem sang Wurtzite luc giac cd the xay ra theo trinh tu 3C - lOH - 6H - 4H - 2H ddi vdi nhung thanh phan 0 < x < 0,062. 1.2. Cau true vung nang luong cua ZnS va CdS [6] 1.2.1 Cau true vung nang lupng cua mang lap phuong Mang tinh the lap phuang gia kem cd ddi xutig tinh tien cua mang lap phuang tam mat. Cac vecta tinh tien ciia mang la: ^1 = ^ a(l 1 0) ^2 = | a ( 1 0 1) t 3 = | a ( 0 11) trong do: a la canh ciia mang lap phuOng. Mang dao la mang lap phuang tam khdi, cd cac vecta ca sd la: Xi=27ia-\l,l,-l) X2-27ra-Vl,-l, 1) Xj = 2na\-l, 1, 1) Vung Brillouin ddi vdi mang dao nay la 1 khdi bat dien dupe md ta tren hinh 3. Ak Hinh 3. Vung Brillouin ddi vdi mang tinh the lap phuang gia kem [6] 10 Bang mot sd phuang phap nhu phuang phap gia the, phuang phap sdng phang true giao, ngudi ta da tinh toan dupe cac viing nang lupng cua cac hop chat A B Tat ca cac hpp chat A B hpp chat A B . deu cd viing cam thang. Dp rpng viing cam cua cac giam khi nguyen tuf lupng tang. Ddi vdi mang kim cuang trong khdng gian vecta sdng, tai k = 0, trang thai nang lupng cao nhat cua viing hda tri ling vdi r25 suy bien bac 3. Ddi vdi mang lap phuang gia kem, trang thai cua r25- chuyen thanh Ti^y. Neu tinh den tuang tac spin quy dao thi trang thai ri5v tai k = 0 se suy bien bac 6 thanh trang thai Tg suy bien bac 4 va trang thai Fy suy bien bac 2. Su suy bien tai k = 0 duac bieu dien tren hinh 4. (0,0,0) Hinh 4. Sa do vung nang lupng cua mang tinh the lap phuang gia kem tai lan can^ = 0 [6] Do mang lap phuang gia kem khdng cd ddi xiing dao nen cue dai cua viing hda tri hai lech khdi vi tri k = 0 va lam mat di su suy bien viing cac Id trong nang Vj va 16 trong nhe V2. 11 1.2.2 Cau true viing nang lupng cua mang luc giac Wurtzite Mang tinh the Wurtzite cau tao tii hai mang luc giac long vao nhau. Mot mang chiia anion, mang con lai chiia cation. Cac vecta tinh tien ca sd ddi vdi mang Wurtzite la: ti = |a(l,-V3,0) ^2-^a(l,V3,0) ?3 = c ( 0 , 0 , 1) Mang dao cung cd cau tnic luc giac vdi cac vecta tinh tien ca sd la: 2=27ia-Vl,-V3,0) X2 = 2 ^ - ^ 1 , 7 3 , 0 ) X3 = 27ic"V0,0, 1) Viing Brillouin la mot khdi bat dien bieu diln tren hinh 5. Do cau tnic tinh thi ciia mang lap phuang va mang luc giac cd khac nhau nen the nang tac dung len dien tir trong hai loai tinh the khac nhau. Tuy nhien ddi vdi cung mot chat, khoang each giua cac nguyen tit trong hai loai mang tinh the bang nhau. Lien ket hda hpc ciia cac nguyen tut trong hai loai mang tinh the cung nhu nhau. Chi cd su khac nhau cua trudng tinh the va vhng Brillouin gay ra su khac biet trong the nang tac dung len dien tu. Bang phuang phap nhilu loan dien tu cd the tinh dupe vung nang lupng cua mang luc giac tuf viing nang lupng cua mang lap phuang. Sa do vung dfin va viing hda tri cua A B vdi mang tinh the luc giac dupe dan tren hinh 6. So vdi sa dd viing cua mang lap phuang, ta thay rang do anh hudng cua nhilu 12 loan trudng tinh the ma miic Fg (J = 3/2) va F7 (J = 1/2) cua viing hda tri mang lap phuang bi tach lam 3 md'c F9(A), F7(B) va F7(C) trong mang luc giac. K H Hinh 5. Vung Brillouin ddi vdi mang tinh the Wurtzite [6] Hinh 6. Cau true vung nang lupng ciia mang Wurtzite tai lan can k = 0 [6] 1,2.3 Vung nang lupng ciia ZnS va CdS Tinh the ZnS va CdS cd the ton tai dudi dang lap phuong gia kem, luc giac Wurtzite hoac bien the cua hai dang nay. Tiiy vao dang cu the cua mang tinh the ma cau tnic viing cua ZnS va CdS cd tinh ddi xiing khac nhau. Tuy nhien, tat ca cac 13 dang tinh the ZnS va CdS deu cd dac diem chung la cd cau tnic viing thang: cue dai tuyet ddi cua viing hda tri va cue tieu tuyet ddi cua viing dan cung nam tai gia tri k = 0, tiic la d tam ciia viing Brillouin. ZnS E,eV Hinh 7. Sa do viing nang lupng ciia tinh the lap phuang ZnS [14] CdS E,eV h^ 8 v^ J ^ '^ns .^4 ^s 6 -Ll ^; \ 4 n 2 AEo ^1 ^% L3 0 ^75"\^^^ -2 X Lj L ^ 1 5 A T A 3 r Hinh 8. Sa dd vung nang lupng cua tinh the lap phuang CdS [14] Tren hinh 7 va hinh 8 la sa dd viing nang lupng cua ZnS va CdS dudi dang tinh the lap phuang. Khe nang lupng giua cac diem ddi xiing trong viing nang lupng ciia 14 ZnS va CdS cd the tham khao trong [14]. Khdi lupng hieu dung cua dien tu d Ian can day viing din va cua Id trong d lan can dinh vung hda tri ciia tinh thi ZnS va CdS theo tinh toan ly thuyet va thuc nghiem cd the tham khao trong [6]. 1.3. Tinh chat hap thu ciia ban d i n U . l Khai niem chung [24,17] Neu chieu mot chum sang len mot ban mong ban din thi mot phan anh sang bi phan xa lai tren be mat ban dSn, mot phan anh sang se di xuyen qua ban dSn, phan con lai hoac bi tan xa, hoac bi hap thu trong ban dan. Nang lupng hap thu dupe chuyen thanh cac dang nang lupng khac: npi nang, quang nang (huynh quang), dien nang V.V.. Tuang tac cua anh sang vdi ban dSn dupe xac dinh bdi hai yeu to: - Tinh chat cua anh sang: thanh phan pho, su phan cue, cudng dp, hudng truyen. - Tinh chat cua ban dan: cau tnic viing, cau tnic tinh the v.v.. Ngoai ra, cac dieu kien ben ngoai nhiT: nhiet dp, ap suat, dien tnrdng, tii trudng... ciing anh hudng Idn den qua trinh hap thu ciia ban din. Trong qua trinh hap thu, photon vdi mot nang lupng xac dinh, se kich thich dien tur tiit trang thai nang lupng thap ban len trang thai nang lupng cao han. He sd dac trung cho qua trinh hap thu la he sd hap thu a. Neu anh sang cudng dp IQ, fan sd VQ chieu den ban mong ban dSn cd chieu day d, thi anh sang sau khi ra khdi mlu se cd cudng dp I tuan theo dinh luat Bouguer - Lambert: I(Vo) = Io(Vo)exp(-ad) (1) trong dd a la he sd hap thu cua ban dSn d tan sd VQ cd thii nguyen c m ' \ Nd cho biet dp day tuong ling cua chat ban din ma khi anh sang truyen qua cudng dp se bi giam di e Fan. 15
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