raman spectroscopy of dopant impurities and defects in gaas layers

8/6/2019 Raman Spectroscopy of Dopant Impurities and Defects in GaAs Layers

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AMAN SPECTROSCOPY OF DOPANT IMPURITIES AND DEFECTS IN GaAsYERS

Joachim WagnerFraunhoferInstitut fUr Angewandle FestkiirperphysikTullastrasse 72, D7800 Freiburg, Federal Republic ofGermany

yrRODUCTIONRaman spectroscopy has found widespread use for the investigation of intrinsic

'bonon modes and electronic excitations of free carriers in bulk semiconductors as wellis in semiconductor heterostructures 1-3, Raman scattering by impurities and defects 4, 5,J contrast, has been used so far only in a limited number of examples. This alsootrasts to other techniques, such as photoluminescence and absorption spectroscopy,'ch are used routinely for the investigation of defects and impurities.This is, at least partially, due to the weakness of Raman signals from defects and

'mpurities because of their small concentrations as compared to the density of hostlltice atoms. In the last few years, the advent of optical multichannel detectors allowedD increase the sensitivity of the Raman technique considerably 6, which opens newJOssibilities for the study of defects and impurities by Raman scattering. This is, in~ t i c u l a r , true for the recent work on Raman spectroscopy of dopant impurities inllanar (5.) doped GaAs 7 which would have been hardly possible using conventional;figle channel detection.

In Raman spectroscopy, impunues and defects can be observed either viattering by internal electronic excitations 4, 5 or via scattering by localized vibrational

nodes 2, 8-n. Both types of scattering probe directly individual impurities and defects.e intensity of the scattered light per unit solid angle as/an can be written as 12

as/an - a n V (I).vbere a denotes the scattering cross section per impurity, n is the impurity concentraion, and V is the scattering volume. If the sample is opaque for the light used to excite~ Raman spectrum, V is proportional to the probing depth 1/(2") where" denotes the!bsorption coefficient. Using light for which the material is fully transparent, V is forlackscattering geometry proportional to the thickness of the sample. Eq. 1 shows that,~ a given scattering cross section per impurity and a given scattering volume, thelleasured Raman intensity is directly proportional to the impurity concentration. There-

t.igmScattering in Semiconductor Structures and Super/altiustdittd by D.J. Lockwood and I.F. Y01mg, Plenum Press, New York, 1991 275


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fore, it is straight forward to calibrate the Raman technique and to make it a verS.lll_tool for quantitative materials characterization. This is in contrast to photoluminescenc;spectroscopy, where the measured signal intensity is not only determined by the impUritconcentration but also by the carrier life time. 1

From Eq. I it is also evident that there are two ways to enhance the Raman signalfrom scattering by impurities and defects. One possibility is to use incident light forwhich the material is transparent. This leads to a fairly large scattering volume limitedonly by the thickness of the sample, which is typically in the range 0.5 - 5 mm. l'bi!approach has been used successfully to study impurities in bulk semiconductors such asSi 4, 5 and GaAs 4, 5, 13, 14 In the case of GaAs, a detection limit for Raman scattering byelectronic excitations of residual shallow acceptors of 5 x 10 14 cm,3 has been reoported t4 which demonstrates the sensitivity of this technique.

Alternatively, the incident photon energy can be chosen to match an electronicinterband transition which is well above the fundamental band gap of the material. Forthis kind of optical excitation, the probing depth 1/(2,,) is, for e.g., GaAs in the range of10 - 100 am 15, 16 The scattering cross section, on the other hand, can be enhanced significantly by this approach as it has been demonstrated for scallering by electronic excitations of acceptors incorporated in quantum well heterostructures 3,17-19 and for scattering by impurity induced local vibrational modes (LVM) in GaAs I l In the latter caseoptical excitation in resonance with the E1band gap has been used, leading to a probingdepth of only 10 nm IS, 16 This makes the above approach particularly well-suited 10study, e.g., dopant impurities in thin highly doped epitaxial layers,ll, 20, 21 including 6doped structures 7

Defects introduced in large concentrations by ion implantation or reactive ionbeam etching can also affect Raman scattering by intrinsic phonon modes. The firslorder phonon Raman spectrum gets modified by the relaxation of the c o n s t r a i n ~imposed by momentum conservation as studied, e.g., in detail for ion implantedGaAs 22, 23 Resonant dipole forbidden first-order and allowed second-order scatteringby longitudinal optical (LO) phonons is sensitive to ion beam induced defects via thebroadening of the corresponding resonances in the Raman efficiency 23-25

In the following we concentrate on GaAs and discuss Raman spectroscopy ofdopant impurities and defects via vibronic excitations. Firstly, we focus on the effect ofion beam damage on resonant Raman scallering by intrinsic LO phonons both in ionimplanted and in reactive ion etched GaAs. Secondly, recent work on the Ramanspectroscopy of impurity induced LVM in highly-doped GaAs is reviewed. It is furtherdemonstrated how this spectroscopic technique can be applied to the analysis of theincorporation and spread of dopant atoms in 6-doped GaAs structures.EFFECf OF ION BEAM INDUCED DEFECfS ON RAMAN SCATTERING BYINTRINSIC PHONON MODES

Fig. 1 displays the effect of ion implantation on the first-order deformation potential scattering 22, 23 A series of Raman spectra is shown for material implanted with 29Siat an ion energy of 100 keV with doses ranging from I x 1013 to I x 1O t6 cm- l . With in'creasing implantation dose, the LO(r) phonon line shifts towards lower energies andbroadens. These effects have been explained by Tiong et al. 26 by a "spatial correlation276


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600

, GoAS:29Si+LOcr) 100 keYhVL ::: 3.00 eV

TOCr) I ll' fl2y: 2lO(r)1x10 13 em-2, III ,I ,,

12 -, jII hl10 em J:\;"I'I I", ,

tas graw,n1,100CIlcr:'.>-UlZWZz..:>:..cr

200 300 '0 0 500RAMAN SHIFT{cm-')

Fig. 2 Room-temperature Ramanspectra of as-grown and 29Si +-implantedGaAs. Implantation doses are given inthe figure. The spectra were excited at3.00 eV and recorded in the x(y,y)i1 scattering geometry with a spectral resolution of 5 cm- I .

GoAs:29Si+100keV

hill::: 2.57 eVh'O'6 em -2

, 00 200 300 "10 500RAMAN SHIFT lem-')

Z::>

z I-c---I--..:E..cr

Fig. 1 Room-temperature Ramanspectra of 29Si +-implanted GaAs. Implantation doses are indicated in thefigure. The spectra were excited at 2.57eV with the incident light polarized alonga (110) crystallographic direction and thescattered light not analyzed for its polarization. Spectral resolution was 6 cm- l

model". This model is based on a phonon confinement concept 27 with the phonon local-I ization length being as small as - 50 Afor the highest damage level for which a LOphonon peak is resolved. N. can be seen from Fig. 1 for doses exceeding 1 x 10 '5 cm2,

the LO phonon line disappears and three broad bands are observed which arise fromdisorder activated first-order scattering in amorphized GaAJ.. These bands developgradually for doses in the range 10 13 - 10 15 cm-2, indicating that in this range thematerial consists of damaged crystalline, as well as amorphized, regions.

The effect of ion implantation induced damage on dipole-forbidden but defectinduced lLO and resonant 2LO phonon scattering is shown in Fig. 2, which displays asequence of Raman spectra excited at 3.00 eV in resonance with the E, band gap energy 28, In as-grown GaN., the defect induced ILO phonon line intensity is weak and theRaman spectrum is dominated by 2LO phonon scattering. Upon ion implantation thedefect induced 1LO phonon line increases and the 2LO phonon peak decreases in inten~ t y . Already for a moderate implantation dose of 1 x 1013 cm-2, where first-order deformalion potential scattering is only modified slightly (Fig. 1), resonant defect inducedILo and 2LO phonon scattering shows a drastic change with the relative intensities ofboth phonon peaks inverted.

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- 0.59

oo

1o l(lOOP4 Imp)/I{250em'ljl6I{210l/I(S'Ocm-') 1o II210)/1(lO Imp) II

o 0o'0,

0 .... - S l o p t -0.73,o ,

GaAs: 29Si+'0 0 k.V

, ,

,_00 , o

.... 0.010=-6 6, ........ 0'0 ''. ,........ ....::-- Stopt

'4 0 ....Slopt ~ O . 6 6

1011 10'2 1013 10" 10'5 10'6IMPLANTATION DOSE I,m-')

Fig. 4 Relative intensities of allowedfirst-order scattering I(LODP + Imp)/I(250cm"), allowed secondorder scaneringI(2LO)/1(540 cm- I ), and allowed secondorder normalized to symmetry-forbiddenbut defect induced first-order scatteringI(2LO)/I(L0 lm ) versus implantationdose. The ami-vs at the vertical scaleindicate. the corresponding relaliveintensities in as-grown GaAs.

1 0 0 -

OJ)ZUJz 1.0-UJ>J

O.H-

- . l l0- .4 . . . 210

" ,, ,, .\ __ 0.25os_grown2 CdTe: ' ll In+

in1 - 101 ......O f - - - - 1 ' - - - - - - - ==" ' - - - - I. , ; ''"c:~ '"

O f-- - '- - - - - - -- j1 5 . " O l l e ~ _

tel /. ..- . . . . ! ... - o ' - ~ . ' -.~ - - " . L : ~ - - , , " o - ~ - '2.5 2.55 2.6INCIDENT PHOTON ENERGV (eV)

Fig. 3 Measured scanering efficienciesas/an for firstorder (lLO) and secondorder (2LO) Raman scanering in (a) asgrown and (b) and (c) 113In+.implantedCdTe. The full (lLO) and dashed (2LO)lines are drawn to guide the eye. In (a)the 2LO resonance curve displayed hasbeen reduced by a factor of 4.

(. )

This slrong effect of implantation damage on the lLO and 2LO phonon scatteringintensities for excitation in resonance with, e.g., the E t band gap of GaAs, is caused by adamage induced broadening of the corresponding resonance in the scattering efficiency 24, 2 5 This has been demonstrated explicitly for 113In implanted CdTe 25, asshown in Fig. 3. Here the Eo +"0 gap resonance in the 2LO phonon scanering efficienCYbroadens and decreases in height continuously with increasing implantation dose (Fig.3a.c). Th e resonance for lLO phonon scattering also broadens wilh increasing dose, bulits height first increases for low doses because this scanering process is defect induced 1J;(Fig. 3a and b). For higher implantation doses the increase in defect concentrationovercompensated by the broadening of the resonance leading eventually also to adecrease in the lLO phonon scattering intensity (Fig. 3c).

Th e damage induced broadening of the above resonances for lLO and 2LOphonon scattering is understood as follows 23 It has been shown for GaAs thatstructures in the spectrum of the dielectric function E, which are related to interbaod

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;taDsitions such as across the E t band gap, get broadened and smeared out upon ion, plantation 29 The resonances in the efficiency for defect induced 1LO and 2LO,banon scattering can be approximated by la2x/aw21, where X denotes the electric

-'1 ~ s c e p t i b i l i t y and wthe incident photon energy 28, Thus any change in E, which is relatedXby E= 1+41TX, strongly affects the resonances for scattenng by LO phonons.The effect of ion implantation damage on various scattering processes by intrinsic

"anon modes in GaAs is summarized in Fig. 4 24. It shows the intensity of resonant\La phonon scattering normalized to that of dipole forbidden but defect induced ILO,.,altering (I(2LO)/I(L01m )) plotted versus implantation dose. This ratio decreases;lfangly with increasing dost which has been exploited to study the lateral homogeneity:[ian implantation in GaAs wafers 24. For comparison also, the 2LO scattering intensity~ r m a l i z e d to the less resonant second-order scattering by two transverse optical (TO)'banons (I(2LO)/I(540 cm-1)), as well as the scattering intensity of first-order scattering

LO phonons normalized to the TO phonon band in amorphized GaAsI(LODP + Imp)/I(250 em-I)), are plotted in Fig. 4 24.

The sensitivity of resonant LO phonon scattering to ion beam induced damage in.lar semiconductors in general, and in GaAs in particular, can he used to assess theed :amage induced by reactive ion etching (RIE) processes. This is demonstrated in Fig. 550 ere the intensity of resonanl 2LO phonon scattering, normalized to that of dipole forng 'dden but defect induced lLO scattering (I(2LO)/I(LOlmp))' is plotted versus the biasd- oltage applied during the RIE process. The material processed was n-type GaAs with aen 'ee carrier concentration of 2.7 x 1017 cm- 3 which was exposed to a CHF3 plasma. Forng 'as voltages exceeding 300 V, the ratio I(2LO)/I(L01mp) decreases, indicating a degra-an ition of the material within the probing depth of 10 nm underneath the surface. Thealeve

O. 6 GoAs:SiCHF,-RIE

o - - _ o ~

o o

H

- , 0 , 5EHo- '

H: : : 0.4o- 'N

ngyaf/i.ascyig,ut28isa 0.3L L - { f ~ L . . L-__.....l---'o 300 350 400

BIAS VOLTAGE Iv)LO iig. 5 Relative intensity of allowed 2LO scattering normalized to symmetry-forbidden~ : III defect induced lLO scattering in n-type GaAs plotted versus bias voltage applied

~ r i n g reactive ion etching.

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degradation of the material detected by Raman spectroscopy correlates with a degrada.tion of the electrical properties of Schottky diodes fabricated on the same RIEprocessed substrates 30. The above results clearly show that resonant Raman scatteringby intrinsic LO phonon modes provides a useful tool to analyze residual defects of RIEprocessing steps. This is of particular interest for the lateral processing of semiconductorheterostructures to fabricate, e.g., quantum wires and quantum dots 31

RAMAN SCATTERING FROM LOCALIZED VIBRATIONAL MODESDopant impurities which are significantly lighter than the host lattice atoms, give

rise to localized vibrational modes (LYM). These modes, which are observed in OaAsdoped, e.g., with Si, Be, or C, are higher in frequency than the intrinsic host latticephonon modes 32. For a given dopant atom, the frequency and fine structure of the LYMindicate the lattice site occupied by the atom 32 Infrared absorption spectroscopy is thecommonly used technique to study LYM in bulk material and in relatively thick(> 1 I'm) epitaxial layers 32 Recently Raman spectroscopy has been used to investigateimpurity induced LYM in heavily doped OaAs layers 7, 10, 11,20,21,3336.

In the following subsection, Raman scattering by LYM is discussed for homogeneously doped OaAs layers, whereas in the subsection following it emphasis is laid ontbe application of this Raman technique to Ihe study of dopant incorporation in


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GoAs:Si'hwl'=3.00 eV

77 K

ISiJ=l.hl0'9 em-)

[5il" 4.1019 em-)(1:5. I; )1110 '8ern-)

[SiJ=4.7 ,,1018 cm-3",,3.8.,0\8 em- 3

,'i GaI "5''A.I III

10)

350 400 450RAMAN SHIFT {em-'}

Fig. 7 Low-temperature Raman spectraof MBE grown GaAs:Si with differentdopant concentrations lSi] given in thefigure. The free carrier concentration n isalso given. For all spectra, which wereexcited at 3.00 eV and recorded with aspectral resolution of 5 cm- I , the secondorder phonon spectrum has been subtracted.

GoAs: 51

LSD 550RAMAN SHIFT(cm"l

350

>- >-'::iii Vl'" '""' W Ib)'" ='" '". ..1: 1:.. ..a: a:

Fig. 6 Low-temperature Raman spectraof Si doped GaAs grown by MBE (lOp)and of an undoped reference sample(bottom). The spectra were exciled at3.00 eV and recorded with a spectral resolution of 5 cm l

dopant is explained by the Si site occupancy in the different samples. For the two lower5i concentrations, only the LVM of 28SiGa is observed acting as a donor (Fig. 7a and b).For the highest Si concentration, the 28SiGa donor gets compensated by the acceptors28SiAs and Si-X, which produce LVM at 399 and 370 cm-I, respectively 32, 33The incorporation of Si dopant atoms into different lattice sites depends very much onthe delails of the growth conditions used for the epilaxy of the GaAs layer 33, 37, 39 Thisapplies similarly to the incorporation of Si into ion implanted and thermally annealedmaterial 40. It is illustrated in Fig. 8, which shows a sequence of LVM Raman spectrataken from a series of samples all implanted with 100 keV 29Si at a dose of 1 x 10 16 cm-I .Then the samples were subjected to different annealing processes. Wafers A and D wererapid thermal annealed with the sample surface protected by a Si02 (wafer A) or a5i3N4 (wafer D) capping layer. Wafers Band C were furnace annealed under arsineoverpressure (wafer B) or with the surface protected by the proximity technique (waferC). It is evident that the Si site distribution within the probing depth of 10 nmunderneath the surface depends strongly on the details of the annealing process. Therelative concentrations of SiGa and SiAs vary considerably and the formation of the Si-Xcomplex is observed exclusively for annealing under a Si02protective cap 40.

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GcAs: 29Si+1x1O'6 cm -2

hWl:JOOeV77 K

350 400 450 500RAMAN SHIFT lem-')

Fig. 8 Low-temperature Raman spectra of GaAs implanted with 1 x IO t6 29Sijcm2 andannealed under various conditions. Wafer A was capped with Si02 followed by rapidthermal annealing. Wafers Band C were furnace annealed with the surface protected byarsine overpressure (B) or the proximity technique (C). Wafer D capped with Si3N4 wasprocessed by rapid thermal annealing. The Raman spectra excited at 3.00 eV wererecorded with a spectral resolution of 5 cm-].

Another interesting point is the dependence of the Si LVM Raman spectrum onthe incident photon energy. Fig. 9a shows a spectrum excited at 3.00 eV in resonancewith the E I band gap, whereas the spectrum displayed in Fig. 9b was excited below thairesonance at 2.71 eV. The LVM of SiAs and Si-X are observed for both incident photonenergies. The LVM of SiGa, in contrast, is the dominant peak for excitation at 3.00 eVbut absent in the spectrum excited at 2.71 eV. The difference in behaviour for the SiGLVM on one hand, and the SiAs and Si-X LVM on the other, migbt be either due to thedifferent lattice sites occupied or to the different electrical activity as a donor and anacceptor, respectively 21

It is interesting to compare the above findings with the resonance behaviour ofRaman scattering by the 9BeGa LVM. Be is an acceptor occupying a Ga site. Tbe corresponding LVM Raman spectra are displayed in Fig. 10. Both for excitation in resonance at 3.00 eV (Fig. lOa) and below resonance at 2.71 eV (Fig. lOb), the LVM of9Be Ga is observed at 482 em-I, whicb indicates a resonance behaviour for BeGa differentto the one for Si Ga but similar to the SiAs and Si-X acceptors 21 This may lead to (heconclusion that the difference in resonance bebaviour for Si Ga and SiAS is due to thedifferent electrical activity. However, more work is necessary in order to understand theunderlying scattering mechanisms.282


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GoAs:Se3.5_10 19 em-)

17K

'"UJ hUlL'::3.00eVzz..:>:..a:

,00 500 600RAMAN SHIFT (em-I)

2BS' GoAs: 51'GoSi-X 5 . _1018 em-)I 17K

2BSiAslo} II,. I

!: I'" I%uJ I

II

% I


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3S0 '00 450 500RAMAN SHIFT (cm-11

Fig. 12 Low-temperature Ramaospectra of (a) a Si and Be .-doped GaAssawtooth superlattice and of (b) anundoped reference sample. The spectrumof the superlattice with the intrinsicsecond-order phonon spectrum subtracted is displayed in (c),

GoAs'Si,8e5-doped

'"wz( ' )

GoAs:Cp.L.L . l D 1 9 ~ m Jl'l.La2.11 ,V11K

500 600 700RAMAN SHIFT (em -' )

Fig. 11 Low-temperature Ramanspectrum of (a) carbon doped GaAsgrown by MOMBE and of (b) anundoped reference sample. The difference spectrum [(a) - (b)] is displayed in(c). The spectra were excited at 2.71 eVand recorded with a spectral resolutionof 5 em-I

for concentrations ranging from 3 x 1018 up to 1 x 1020 cm- 3 45 This proves the validityof the above concept and the derived calibration factors, The detection limit for bothSiGa and BeGa is 2 - 3 x 1018 cm-]

Dopant Incorporation in Doped StructuresThe applicability of Raman scattering by LVM as a quantitative technique, corn'bined wilh its sensitivity to even very thin doped layers due to the small probing depth of

10 nm for optical excitation in resonance with the E t band gap,15 allows us to study theincorporation of dopants in .-doped GaAs 7 This is demonstrated in Fig, 12 for a Si andBe .-doped GaAs sawtooth superlattice 47. This superlattice consists of a first layer5.3 nm underneath the sample surface .-doped wilh 5 x 1012 cm-2 Be followed by analternating sequence of Si and Be .-doped layers with a spacing of 10.5 nm in-betweenand a doping concentration of 1 x 1013 cm-2 each. Due to the small probing depth of theRaman experiment for excitation at 3,00 eV in resonance with the E I band gap,

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essentially only the topmost two layers doped with Be and Si, respectively, are probed.The spectrum (a) shows the 28SiGa LYM at 384 cm- l as well as the 9BeGa LYM at482 em-I superimposed on a background of intrinsic second-order phonon scattering 28.Fig. 12b displays the second-order phonon spectrum of an undoped reference sampleand Fig_ 12c shows the difference spectrum [(a) - (b)] after subtraction of the secondorder phonon spectrum. Both the 28SiGa and the 9BeGa LYM are well resolved in Fig.12c which demonstrates the sensitivity of the Raman technique to even a single .-dopedlayer-

The .-doping technique has recently found considerable interest both for fundaroental studies and for device applications 47_ An important question related to thisdoping technique is the incorporation and the spread of the dopant atoms along thegrowth direction_ A number of experimental techniques has been applied to characterize.-doping, such as capacitance-voltage (C-Y) profiling 48, magnetotransport measureroents 49-51, secondary-ion mass spectroscopy (SIMS) 52-54, and Raman scattering byLVM 7 In the following, a discussion of how Raman spectroscopy of LYM providesinformation on the depth distribution of those dopant atoms incorporated on lattice siteswill be given.

The basic idea is to grow a series of GaAs layers containing a single .-doping spikeof, e.g., Si at a nominal depth Zo underneath the surface with all the samples identicalexcept for the different depths zo- The measured normalized SiGa LYM intensityI(SiGa)/I(540 cm-t), where the intrinsic second-order phonon scattering strength at540 em-I is used as a reference, can be written for each sample with a given Si depthprofile [SiGa(z-zo) as

I(SiGa)/I(540 em-I) = k JSiGa(z-zo)] e-2az dz_o (2)Here k is a calibration factor and e-2az describes the decay of the Raman sensitivityversus z, which is the coordinate normal to the surface. Q is the absorption coefficientwith 1/(2"') = 10 nm for excitation at 3.00 eY and 77 K 15 With the above mentionedseries of samples at hand, I(SiGa )/I(540 em-I) can be measured for a variety of depths Zoand the actual dopant depth profile is, in principle, obtained by SOlving Eq. 2 for[SiGa(z)]-In Fig. 13, the normalized SiGa LYM intensity I(SiGa)/1(540 em-I) is plotted versus thenominal depth Zo for a series of samples grown by MB E at a substrate temperature of580"C with a nominal dopant density of 8 x 1012 cm-l. The LYM intensity is found toincrease up to a depth of Zo = 20 nm followed by a rapid decrease for larger values of zo0These experimental data can neither be explained by a very narrow Gaussian depth distribution with a full width at half maximum (FWHM) of 2 nm (dotted curve in Fig. 13),nor by a strongly broadened symmetric distribution with a FWHM of 23 nm (dashedCUrve in Fig_ 13). The doping profiles, which have been assumed for the calculation [Eq.2! 01 the I(SiGa)/I(540 em-I) versus Zo curves, plotted in the main part of Fig. 13, aredisplayed in the inset.

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286

To fit the experimental data, one has to postulate that, after the ingrowth and deposition of the dopant atoms, these atoms are incorporated ingrown material at a given 3dimensional dopant density 7, 55 Dopant atoms.not incorporated in the material when finishing the growth of the layer, dethe growth interruption just sit on the surface and are not detected by tIIcexperiment 7 This leads to a strongly asymmetric depth profile with aconcentration over a certain distance in growth direction and an abrupltowards the substrate (solid curve in the inset of Fig. 13). Assuming adopant atoms along the growth direction of z 20 nm, a good quantitative W.UlI-the experimental data is obtained 7, as shown by the solid curve in the maID13.

Fig. 13 Normalized 28Siaa LVM scattering intensity I(Siaa)/I(540 em1) vedepth Zo of the doping spike. The samples were grown by MBE attemperature of 580C with an intended doping level of 8 x 1012 Si/cml.displays different depth profiles of the Si concentration [Siaal used to calculalethe corresponding I(Siaa)/I(S40 cm l ) versus Zo curves which are displayed inpart of the figure.

0.025

0.100

5''";:: 0.075"-'J 0.050-;::

0.125

10 20 30 401 0 (nm)

0.150'r:--,-----------

The actual spread of the dopant atoms in adoped structures depend>.on the growth conditions, such as substrate temperature, and on the 2dopant density 4954 A reduction in the 2.dimensional dopant density andthe substrate temperature are both expected to reduce the acmal wi.dth/spike. This is illustrated by the LVM data of Fig. 14 which were obtallle . of Si a-doped samples grown at a nominal substrate temperature of sOO"C "'rlevel of 2.8 x 10 12 cml . The resulting doping profile is much narro"measured width partially determined by the experimental depth resolunOn,


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0.100

Eu 0.075"'""...... 0.050'8ii i"" 0.025

~ ~ o (nm)I

10 20 30 40I o (nm)

Fig. 14 Same as Fig. 13 for a sequence of samples grown at a nominal substrate temperature of 500C with an intended doping level of 2.8 x 1012 cm-2

the order of 1/(2Cl). Therefore the actual width of the doping profile can only be estimated to S5 nm.

In n-type 6-doped st ructures space charge effects induce a potential well in wbichquantized electron subbands are formed 47 The actual shape of this potential well depends on the spread of the dopant atoms. Ideal o-doping with the dopant atoms confinedto one atomic plane leads to a V-shaped potential well whereas a large spread of thedopant atoms results in a parabolic well 49, 50 The shape of lhis well, in turn, determineslbe energy spacings of the eleclron subbands. Therefore Raman spectroscopy of intersubband transitions provides complementary information on the actual width of dopingspikes in nominally o-doped GaAs. For the present samples grown at a substrate temperalure of 580C with a doping density of 8 x 1012 cm-2 spin-density excitation energies of24.8 and 36.7 meV were found for a nominal depth of Zo = 30 nm 57 Self-consistentelectronic subband calculations, using the actual spread of the dopant atoms measuredby the above described LVM Raman experiments (see Fig. 13) as an input parameter,yield spin-density excitation energies of 25.1 and 36.5 meV for transitions between thesecond and third and the third and fourth subband, respectively. This is in good agreement with the experimental values 57 It demonstrates that a consistent picture isobtained for the spread of dopant atoms in Si 6-doped GaAs from the combination ofLVM and subband Raman spectroscopy.CONCLUSIONS

The effect of impurities and defects on the Raman spectroscopy of vibronic excitations has been discussed. Resonant Raman scattering by intrinsic LO phonon modes has

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been shown to be sensitive to ion beam induced damage which allows to assess, e.g.,residual defects in reactive ion etched semiconductor structures. Dopant atoms signifi.cantly lighter than the host lattice atoms are accessible by Raman spectroscopy via lightscattering by localized vibrational modes. This technique has a detection limit of s 2 - 3,lO t8 cm-3 corresponding to an area density of s 2 - 3 x 10 12 cm-2 for Si and Be in GaAs.This opens the possibility to study dopant impurities in quasi-two-dimensional systemssuch as, e.g., planar (5-) doped GaAs structures. For such structures the incorporationand spread of the dopant atoms has been analyzed and correlated with the energyspacings of the electron subbands formed in the space charge induced potential well.

ACKNOWLEDGMENTSThe author wants to thank M. Ramsteiner (IAF Freiburg), R. Murray, and R.c.Newman (both with the Imperial College London) for their cooperation in the course of

this work as well as P. Koidl and H.S. Rupprecht for many helpful discussions andcontinuous support of the work at the IAF. Thanks are further due to F. Eisen (IAFFreiburg), K. Kohler (IAF Freihurg), W. PIetschen (IAF Freiburg), K. Ploog (MPI/FKFStuttgart), and M. Weyers (RWTH Aachen) for providing samples, as well as to AMaier for help with the preparation of this manuscript.

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G. Giintherodt, Springer, Berlin (1982) p. 49 and references therein.2. G. Abstreiter, A. Pinczuk, and M. Cardona, in: "Light Scattering in Solids IV", eds.

M. Cardona and G. Giintherodt, Springer, Berlin (1984) p. 5 and referencestherein.

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raman spectroscopy of dopant impurities and defects in gaas layers

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