BSTJ 60: 9. November 1981: Transmission Cathodoluminescence. (Chin, A.K.; Temkin, H. ; Mahajan, S.)

23.9 MB·English

Transmission Cathodoluminescence

By AK. CHIN, H. TEMKIN, and S. MAHAJAN
(Monuscrnt resived My 11,1881)

‘The application of transmission cathodoleminescence fre.) in eal
ating the quality of tuminescing materials fs reviewed. Thi sean
ning electron microscope technique ts particulary useful in imnging
localized nonuniformities, such an disioeations and inclusions, in
semiconductors Understaniing and analysing such material defers
‘are of practical importance since they greatly affet device perform
‘ance and lifetime. After @ detailed deseription of ten, the advantages
ofthis technique in comparison to defect etching, cathodatunines-
fence imaging, and eleciron beam-induced current fEnic) te pre
sented. Since 7 te simple to perform, tia technique San be used
evatuate anc moniter material grauth and device processing proce
Alares. hough re may be applted to any luminescing material this
paper demonstrates is usefulness for the GaAs/ GaAlAs and inP?
‘InGaAsP materialesyotems which provide mos sources and detectors
for optical commnnaication.

|. wwrRouerion

Diode lasers, light-emicting dios Wwe), and photodiodes are
iniegcal componencs of the curren! lightwave transmission syns
‘These deviors are mainly based on she Ill-V materials ystems, © 8
GaAs/CaAlas nnd InP InGaAsP. Both muterial and device develop~
‘ment are recent, relative co the highly developed Si-hased technology.
Material defect analysis is required evaluate and monitor material
trowth and deview processing procedures, especially during the early
ages of develope

‘The performance and reinbility of optoelectronic devices are deter
mined by dhe quality of materi wn abriation processes, heading
dlislocations heve been shown lo ineroase the leakage current of bath
TnP nd GaAs photodiodes? thea dialocatious are also eourcen of

(cruplastas in avalanche photndioes" Tbe quantum efficiency oF
GaP and GaAlAsS$i Lupe i strongly senendent on the disiocaion
density" Threading dislocations wn inclusions are known sources of
Sark line and dark spot defecca in degraded tars and slid sale lasers
fabricated from the GsAe/CaaL.As und InP/InGaAaP materials aye
lems!" inaly, stresses from dvloetre eaatings axe known va induce
lefecta and aeotlerate device desrdation."

“Ghazacieriation techniques are required to ascess the effects of
rowth nnd processing provedures on material or device quality. Be
fuse of the vatiety of defechy materials, and devices, a number of
harucleriaation techniques have been developed. ‘These techniques
te listed and compared in Table L

“The purpose of this paper ist review the most recently developed
(echninue of transmission eathodaluminescence (ci) and to compare
ie'vith other defor analy eebniques curently in use." Trane
triasion cathodoluminescence is performed on a seanning electron
tricroccope (ses); the magnification and depch of Fild available in an
Seo are exploived.

14 Detoct analysis teciquas

"The technique with the highest magnification and resolution, trans
miwion eloctton microscopy (TEM), reveals the nature und origin of

‘Table | Comparison af various characterization tachniaues.

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2186 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1951
material defects However, it eanot he une! routinely for aeneening
flefective materials because it is destructive, the sample preparation is
very difficult, and the volume of material probed is small relaive co
device dimensions. X-ray topography is relatively slow, has limited
resolution and is Limited to small suraples because of internal stresses
generally associated with devieo wafer." The evaluntion of wafers
by oleetron beam: induced current (uaic}acan, cathodaluminescence
(cx) seam, photoluminescence (ev) scan, photocurrent sean, and in-
Irared microrenpy have alo heen reported™* "The electron excite
tion techniques and cx acan—are more ueefl in comparison to
‘he conceptually similar photoexcitation cechriques. Le. photocurrent
v1 Pt scan, hecause of the avalabilry of an exacely conemponding
fecondary-electron (au) image to iolete sulaco det, Ryen wht
thin disadvantage ia overcome by careful experimental design, dsl
tations which are responsible for poor device performance and rein
bility cannot be readily imaged weing tho Lechrigues alUhough sme
specific examples may be found. For example, dack spots in C iawes
‘nave been idancitied co be ditlocaciona only fox the care of GaP, GaAs,
and Cae" Additionally, itis dificult vo observe dislestions in
GaAxSe." Moreover, dsdoca\ions have nut been imaged in GaAs.
‘Etching isthe simples and, unui she mest widely wed eth for
revealing defects, Once the identi of etch feature haa beet ata
Tish, evaluation of material qulity hy etching ie relatively fast.
‘Aaditionally, etching haa efficient ranlacon toveraalnafse enety
fe high ax “10 cme’, However, site from being destructive, etching”
is material material orientatin, a dopant dapendent. Por exanpl,
the kom ich readily reveals dislocations on the (100) and (LLL)
snfices of Gada" However, iteannot be used for the (110) surface
‘which in the cleavage plane and, thus, rans the miznor face of soli
ace lasers, Huber etch works well for the (100) stfaee of InP but

tno forthe (L10] surface. The AB rich reveals diloraions or he
{21D} surface of InGaAsP, but no etch has been developed for che
echnoloielly important (100) surface" To eatimate dstnation

dlencities uf InGaAsP layers in @ device seructure, the InP confining
layers had to be etched." Finally, the HUC wd ALS elhunls are
sceesefl for che (111)D fave of GaP but cunnot be used for vker

"The interpretation of etch patteme i often dificult even with known,
Aefeetetehanta" Figure | shome the responae of an InP erystl to four
‘iferant etchants reported to reve dislocations (a) Iluber ote, (b)
fd, () RRE, and (@) motifed A.B ele" The sample be he
{200} eurfice of eryaal dope with Sn to 2 210" em and ie =
(pial substrate for device walere, A bref comparizon abiowe large
differences in these etch patterns. Dislocetion and seucer pits are well
delineated with the Huber eich but are noc discernible with the
‘woditied A-H etch. On tho other hand, growth siriations, which are
periodic variations in impurity oomcotrition, are particularly ell
Actined afer A-B etching. The eich palterns of the nther cwo etches

‘2190. THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1981
are in between thal of the Huber und modified 8.18 ecches, Therefore,
{he ase ofan etchant to reveal deeets i often unreliable

"Traneminsion eathodoluminessenve revenls defects s+ dark spote,
Similar tothe ct and atc veehniquest!" The characturstie images of
Aolocationa have heen identified by’ one-Lovne corsespondence with
‘a known diloration etchant.“ Dislocations are roughly the same size
And have a comma shape. The comma shape due ¢o the dislocation
Inuerecting the surface ofthe swmpte aan able angle, Inclusions
‘whose contrast is due to nongeneration of. radiation have random
shapes and sacs. The improved collection efficiency and measurement
fgeomonry of Tet in eamparison to CL allows daferta to he readily
{Eaged in many more materials. ‘The materials to which Te. has been
applied is sted in Table TL

1 EXPERIMENTAL
2.1 Transmission cathodoluminescence techniaue

A schematic of the Teh technique is shown in Fig. 2 Transmission
ceathodoluminescence is accomplished by placing alight detector un
demeath a sample. The ci radiation generaied om the lop surf
provides illumination which ia cranaited through Ube simple am
detected on the opposite side, Both sidea of the sample must be
optically smooth to reduce murfaceastfaces fa rc. images. The Tet
rmothed has an efficent light collotion goometry and no modification
SH the aeae de requlred. On Use other haw, alin opty are
‘quired inthe cu technique to guide the ct. to an external detector. Tn
Wig. 2 aurface defecta with ther lower CL. efcieney are detected
decrease in cl radiation tranemitied through the sample, Volume
Aefeets, i. defects which lie beneath che electron-hole (e-h) excitation

‘Table —Ditferent substrates and device watera examined by ToL
‘Gel Gullts Tanna
BEAR ara water ee

Devinn GaAlAeSepaalnorixn Midd ii

CATHORGLUMINESCENCE 2181
fea 4 ain

ty tk Zoe decor art ees late aa Ty sae Slt
nsmiatay snc wl he vans Sis Some helo eo the
(etc sacs

solume of the elctzon beam, un detected by their interaction with
the surface-generated radiation paasing through the sample to the
Geveclor low. Avoid in Uhe aample volume wil increase ligh: trans-
thission beeauee of ruluoed absampcion. A volume defert will decrease
light tranemiasion because of incrassd absorption or sealtering. Using
this method, defeets deep within a thick sample can be observed,
‘Although the tewrsparency of @ sample to ite own cx radiation is
Inaterial dependent, in the following we wive TCE. data on samp

‘whose thickness i comparable Tht of substrates and device wars

2.2 Experimental consideration

"The data given wis obtained with an RT'RC Corporation autoacan
svt bat any SEM with similar capabilites may be used Two solid
‘tate detectors were use forthe ToL mensuremons, depending on the
Spectral gion of inerest. For wavelengths less than ~.0 gm, asiicon
Photodiode was use for wavelengths shorter than ~LS ym, a gorma-
rium photodiode wae oolected, Those detectors have lot noise equiv-
tlent power (sem, fase reponse x! rer bias, and output linearity with
Tight input. ‘The performiree specifications of the photodiodes are
ated in Table IL

"For typienl SEM parauelere of 20 keV and 10-"A for the electron

2192 THE BELL SYSTEM TECHNICAL JOURNAL. NOVEMBER 198%
Table Il —Charactritics of photodiodes used in TCL experiments

Temoowiity Bey tel

beam, the laminesecnt power radiced by the sample is wpproxicntly
10-7, assuming 1 c% canversian efficiency of 10” This egnversion
sffcieney is typicul uf Gude hut iow for thar maverals"” Mist of
thia radiation is colloeed by the detectar in the Tet. eupfguration
fines ibe plac 2m from the lamineecing source, The wld wee
“tended by Hh phoxadiods inthe teu goometry is x, where he
tolleetion angle in the usual e1 mode ix W's,

Siave the alicon and germanium diodes have an wee of 1! Wy
‘YH unl 10°" W va, reepovtvely, the electors are capable of cene
ing 10 "Wand 40 “W, respscives, of rex. power at a bandwidth of
TIP Hs required for sue imuuzing, Avprosinainly (wo orders of mag
rile greater power ie required lo easy inaye material defects
‘rithowe the uee of dificult oie nuetion techniques ave 25 lck-
tanpifieation eagpled with beam banking. This power inveadilyeval-
tle from the materials Liste in Tale I

"The resuluLiou af the we technique i limited hy The expe of defect
‘gored. The somteast tom dislocalone xe ly nanradtve eco
Twiratingehenetre eilimited bythe rir erie ilfuion lenge
‘The contrat at inclusions x on minty ta nongeneraton af ci, and
‘the resolution af these fers Tied by the electron beam condi-
tions as in sa imaging, Finally, vuuine defects ean be reeolved to
roughly the ot. emission wavelengeh since the defect ie effectively
‘observed by uplieal nen

‘Bure J shoves an raaimpe ato differen defers appearing inthe
some TnP'S eample, The large dark spots are sve bo islocations
inerserting che murface, whuroas Ue cle smaller dark spots 30e
inchwina neated within an oh excitation volume ofthe surface, The
nication of devel ing 1% wil be dizousted in Section 2.4. The
dislocation img is lacy estan he large minority carrier diffusion
length. By imoging there dislocations, he minonity easter diffusion
Aerts mil estimated in chia aample co be ~8 ura. The mares of
inclusions show (hat defect aa aall as 1-2 um can bo imaged under
the beam conditions of 20 keV ane 10" A,

Figur’ tan example ofa volume defect ie liy nena igure
is the 96 mae of atv wnorualfeonure appearing in aa InP /TaGaaP

CATHODOLUMINESCENCE 2103,
Ko

seb lbigrnod el att aca done nl uso ropettocy,

wafer, A et picture of the aame repion, Fig. 4b, shows the area to be
onlumineseene. It is only the Te kewge, Fg. r,thot explains the
‘rign of thin dank area, One can clearly see righ spot inthe center
Of the dar ragon ansocated witha void in the epilaail layer. Since
the void doesnot appear in the c.g i Ties hoa thee hexcitation
volume,

124 Inerpvstation of TCL images

Tin gener eels sppene aa dark apots in a we, imago. To interprot
mates, rk pole uouet be Sdentifed. ortho techni, dark spot
fnvimages of GaSe: Gal, vl CAT were identified tobe dislocations
by demonstrating an exact one-to-on> correspurdenve between then
dlrk spots and dislocation reveled ne etch pits produced by a known
‘leteer thant 8H We ave followed thie procedure for Ibu TC.
technique and applied it xo GaAlasSi, GaAeTe, InP-Te, and InP:
soe

"Two methods weve used ta ahow that dk pols oservl in Te.

2194 THEBELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1081
to

ga Secnday ero nae te eae a ag le
p/Eikehtiaenee mato tie soni ee kt

‘mage represent grovenindielocations, Inthe fzet method. axe. image
fof a aample waa taken prior to otebing to reveal dislocations. iy
‘comparison ofthe eich pattern with the ToL image of the sume region,
fan exact one-to-one correspondence was established. In the eecond
‘method, a (00) wafer was etched to reveal dislocations as etch pits
Subsequently, the sample were lesved along the (110) cleavage
planes. Ry examining with vu the (110) surface where it intersects an
fetch pil,» dark region immediataly below each pit was found, Thus,
teach dislocation intersecting the {100} eurface erresponds to a dark
spot in a Te image. The technique of zee stching the suave and
then showing dav spots at dislocation pits i aol usefl since ure
Features always appear i either a CL of TCL image.

‘in Fig. Se, # TCL image of a Siduped Ga,ALAS layer grown by
liquid phase epizaxy (Lex) on a {100) oriented GaAs aubstente is
shovvn The aluminum concentration varies from x= 0.00 onthe top
furfuce to x = 0.30 a¢ the subelrala-pitaxal layer interface. "This
tradient accurs over a thickness of ~250 jm. The simple haa been
Trrosiine-ethauel polished to produce emooth, damage-free eurteoss,
Only dint purticlon are observed on the tep GaAlAs surface when

x imaging and no feataree could be seca in normal cL
Using an SL photomultiplier, This obwervaion iain agreement with
previousmesrurements on similar material’ However, when examined
hhy er, a number of randomly distributed dazk apota appeur. ‘The size
tfchece dare spots varies frovs ft 10 jan, At high magntiestion, their
ape closely resemblee commas tha ix Une central dark region in
Ccnmacted with afintar ta, The average density of these dark apota
fs approximately 2% 20" ey”

Figure Sb ehams the sm insye of the region ahown in Hig. 6a. The

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ig tein atin: tam of Ca

rn uty: (Secu ecco ea of men sow fl eer a
So-t:anm erhone. Poe cndes euro the sal et» 2yon Gases) loan
in Tae syur sine Sheage 0 Wa oct ie

2190 THE RELL SYSTEM TFCHNICAL JOURNAL, NOVEMBEH 1981