NONCOMMUTATIVE STRUCTURE OF GaAs QUANTUM WELL INTERFACES AND INEQUIVALENT INTERFACE IMPURITY INCORPORATION

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NONCOMMUTATIVE STRUCTURE OF GaAs QUANTUM WELL INTERFACES AND INEQUIVALENT INTERFACE IMPURITY

INCORPORATION

D. Bimberg, R. Bauer, D. Oertel, D. Mars, J. Miller

To cite this version:

D. Bimberg, R. Bauer, D. Oertel, D. Mars, J. Miller. NONCOMMUTATIVE STRUCTURE OF GaAs QUANTUM WELL INTERFACES AND INEQUIVALENT INTERFACE IMPURITY INCORPO- RATION. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-93-C5-96. �10.1051/jphyscol:1987515�.

�jpa-00226719�

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NONCOMMUTATIVE STRUCTURE OF GaAs QUANTUM WELL INTERFACES ANI) INEQUIVALENT INTERFACE IMPURITY INCORPORATION

D . BIMBERG, R . K . BAUER, D .

OERTEL,

D . MARS* and J . N .

MILLER'

Institut fiir Festkdrperphysik der Technischen Universitdt Berlin, 0-1000 Berlin 12, F.R.G.

' ~ e w l e t t - ~ a c k a r d Laboratories, Palo Alto, C A 94304, U.S.A.

RBsumB

Nous prouvons de manihre directe l a non-commutativitd des interfaces AlGaAsjGaAs e t GaAs/AIGaAs, par une analyse detaillee du p r o f i l des spectres de photoluminescence dans des puits quantiques. Ces puits ont une Bpaisseur L, = 5 nm, i l s sont obtenus par Bpitaxie par jets 1nol6culaires ?I 620°C, avec un taux de croissance de 1 pm/h, avec interruption de l a croissance soit

A

l'une des deux, aux deux ou encore 5 aucune des interfaces. Nous observons une difference dans l a structure cristallographique ainsi que dans I'incorporation des pieges selon l e type dinterface. Ltinterruption de l a croissance sur l'interface AlGaAs diminue l e rendement quantique des puits

.

Durant I'interruption de l a croissance, u n rapide lissage de l a surface ainsi que l a formation d'ilots de croissance de diametre superieur

a

celui de I'exciton bidimensionnel a l i e u pour les surfaces GaAs; au contraire pour les surfaces AlGaAs, aucune formation 8 i l o t s aussi 6tendus n'a pu e t r e observee dans les conditions actuelles de croissance de nos Bchantillons.

Abstract

D i r e c t proof o f the noncommutativity o f GaAsjAlGaAs and AIGaAsjGaAs interfaces o f quantum wells (QW's) o f width L,=5nm is obtained f r o m a careful studv and mathematical analysis o f the structure and s h G e o f photoluminescence (PL) spectra. QW'S were grown by molecular beam epitaxy (MBE) a t 620°C w i t h a r a t e o f l p m j h and the growth was interrupted a t either one, both, o r none o f the interfaces. A large difference i n the microscopic crystallographic and chemical structure o f the t w o types o f interfaces is observed. I n addition we observe inequivalent incorporation o f impurities and traps a t the t w o interfaces. Interrup- t i o n o f growth a t the AlGaAs surface deteriorates the quantum efficiency o f QW's. Analysis o f the P L lineshape gives clear evidence t h a t the AlGaAs growth surfaces are rougher than the GaAs ones. During growth interruption (GRI) a rapid smoothing and the formation o f monolayer growth islands w i t h l a t e r a l sizes larger than the diameter o f the ZD-exciton occurs only f o r GaAs surfaces i n the samples used f o r this study. AlGaAs surfaces also become smoother upon G R I b u t large island formation is not found a t the present set o f experimental and growth parameters.

Crucial properties o f novel two-dimensional devices based on heterostructures are controlled t o a large extent by the structure and chemistry o f their interfaces. F o r example:

-

Availability o f complernentary logic based on 111-V compounds is essential f o r future generations o f computers. Such a family of logic is based on "normalw and "inverted" two- dimensional-electron-gas f i e l d e f f e c t transistors (TEGFET's). Interface roughness scattering was recently discovered by Hong e t al./l/ t o dominate the m o b i l i t y i n inverted InGaAsjAlInAs structures which are amongst the most promising ones f o r such devices.

-

The spontaneous emission p r o f i l e and the gain p r o f i l e o f a quantum w e l l (QW) laser o f QW w i d t h L, = 6-8 n m are dominated by interface roughness induced broadening o f the QW energy levels. F o r a given f i x e d i n t e r f a c e roughness of T 1 monolayer the s t a t i s t i c a l contribution t o the luminescence h a l f w i d t h o f the (e,hh)-recombination increases f r o m

= 10

meV for L, = 6.5 n m t o

> l 0 0 meV f o r LZ

-

0.0 n m i n AlGaAsjGaAs QW's w i t h an A I content o f 41.2 % 121. The quantum efficiency o f spontaneous recombination on the other hand does not seem t o be

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987515

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JOURNAL DE PHYSIQUE

1

LW' GRDYlH INIERRPlIm4 R I R FREE SR=m ^(X

GO fls

^BOTH7 ^G.? ^RIPR

":

strongly influenced by "intrinsic" interface roughness but by trap incorporation a t the interfaces 131.

Growth kinetics and surface structure o f a semiconductor A are i n general n o t expected t o be identical t o that o f a chemically d i f f e r e n t semiconductor B. Thus A/B (A up, B down) and B / A interfaces are also not expected t o be identical. Evidence f o r such inequivalence was recently reported. The reports are based on evaluations o f RHEED /4,5/ and luminescence experiments 121. Zhang e t al. reemphasized recently that RHEED is purely qualitative and very d i f f i c u l t t o interpret 161. On the other hand we have demonstrated /7/ t h a t interface roughness distribution functions can be determined w i t h sub-A precision from an analysis o f QW luminescence lineshapes. O f course, a detailed understanding o f the character o f the luminescence (singlet, doublet) and the physics governing luminescence lineshapes is a prerequisite f o r such determina- tions /8/.

The purpose of the present paper is twofold. F i r s t we would l i k e t o demonstrate clearly the enormous s t r u c t u r a l difference between interfaces o f type A/B and B/A. We w i l l show how the contributions o f both interfaces t o the broadening o f optical spectra o f QW's can be separated by a clever combination o f growth and luminescence experiments. Secondly, detailed results f o r the roughness o f and i m p u r i t y incorporation a t AlGaAs and GaAs growth surfaces grown by MBE as a function o f the interruption t i m e o f the growth are reported.

ioo The combination GaAs/AlGaAs is

presently the most important one f o r

rl semiconductor heterostructure devices

D 75 and can be regarded as a model f o r

0

U

-

⁵⁰ other heterostructures. M u l t i p l e QW's o f AlGaAs/GaAs/-

U

.-

AlGaAs were grown by MBE w i t h 0.28

01 nm/s growth r a t e a t 620°C on undoped

C

25 GaAs substrates (100 orientation). The

C A1 content i n the barrier is 40 %. The

H thickness o f the QW's is nominally 5

D nm, and they are separated b y 18 n m

1 60 i 62 i 62 i 63 AlGaAs layers. Growth was interrup- E n e r g y C e V l ted f o r 100 S a t none, one o r b o t h o f the interfaces. D e t a i l s o f the growth Fig. 1: Comparison o f P L spectra o f AIGaAs/- are published elsewhere /3/.The GaAs QW's w i t h 100s growth interruption at the photoluminescence was excited w i t h AIGaAs, a t the GaAs, a t both and a t neither o f the 647 n m line o f a Kr-laser w i t h a the growth surfaces. The FWHM are 4.3meV, typical excitation density o f 0.9 10.9meV, 3.7meV and 6.3meV, respectively. _. _. W/cm2. The s ~ e c t r a l resolution o f the

0.5 m mono=hromator was 0.06 nm.

o

I

^p--.l-

v

The P L s ~ e c t r a o f four such otherwise

~m ^OROWTHwmnumw identical samples are shown i n Fig. 1.

AT A F= SURFACE OF The spectrum o f t h e sample without

arowth interruption (GRI) is broad, w i t h a F W H M equal t o 7 meV. G R I on the AlGaAs surface changes the spec- t r u m only quantitative1y:it shifts t o lower energies and i t s FWHM i s reduced t o 4 meV. I f g r o w t h is interrupted a t the GaAs surface we observe a qualitative change o f the spectrum:

t w o ( s t i l l broad) maxima appear. These t w o maxima sharpen considerably i f

3.60 (.M the sample is grown w i t h G R I on both

Magi IeVl interfaces. Growth interruption also causes enhanced i m p u r i t y incorpora- Fig. 2: Comparison o f P L spectra o f 5nm t i o n a t the interfaces /3/. These impu- AIGaAs/GaAs QWS w i t h 100s growth interruption rities are responsible for a number of at the AlGaAs and the GaAs surfaces on a extrinsic emission peaks shown in semilogarithmic scale. Fig.2 i n a semilogarithmic p l o t f o r a

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pronounced i m p u r i t y incorporation, whereas during G R I a t the GaAs surface the i m p u r i t y incorporation i s much weaker. Some extrinsic luminescence can be identified i n Fig. 1 already on a linear scale. While f o r the samples without GRI these features are hardly observable they become clearly visible f o r samples w i t h G R I a t the AlGaAs and at both surfaces.

The integrated PL intensities o f the samples w i t h G R I a t the GaAs surface and without G R I are nearly but n o t completely equal, whereas G R I a t the AlGaAs surface reduces these intensities t o 45 % o f the i n i t i a l value. This strong drop o f the PL emission can be explained only b y rapid formation o f nonradiative centers a t the AlGaAs growth surface which reduces the quantum efficiency

11

and decay time. Our earlier observation /3/ o f a strong reduction o f 11 upon GRI at both interfaces is thus shown t o be predominantly related t o the AlGaAs growth surface.

Apparently t h e strong chemical r e a c t i v i t y o f A I gives rise t o a much stronger incorporation o f impurities and traps a t t h e AlGaAs surface than a t the GaAs surface.

The width o f the recombination and absorption bands o f QW1s are strongly affected b y the chemical and crystallographic roughness o f t h e i r interfaces /3,9,10/. We have demonstrated that luminescence lineshapes o f QW's can be described perfectly by a product o f the 2 0 density o f states step function convoluted b y a Gaussian describing the L, (i.e. energy) distribution caused by interface roughness w i t h a carrier distribution function. A f i t o f theoretical t o experimental iineshapes yields therefore; a/ the variance o f the energy distribution function U (E), b/ the mean transition energy E(L,) and c/ the carrier temperature.

The a (E) values thus obtained can be used t o calculate t h e variance o t t h e quantum w e l l width distribution function G Q (LZ). ^(IQis equal t o 0.079 n m f o r the QW1s without GRI. With G R I a t the GaAs surface i t is reduced t o 0.066 _nm. G Q becomes s t i l l smaller upon GRI a t the AlGaAs surface (0.056 nm) and a t both surfaces (0.036 nm). These results shall be f i r s t used t o test numerically our model and the consistency o f our method o f evaluation.

Ib\ GROWTH INTERRU-"^" ^lllTERFP

I

.

^VALUES

Xf..RR>n 1

2 2 DISCRETE

p LUMINESCENCE

i o r . c. ^VEAKS

Fig. 3: Model f o r the interface disorder o f an AIGaAslGaAs QW and the energy states o f the X(e,hh) exciton i n such a w e l l grown (a) without and (b) w i t h interruption o f the growth.

The statistical distribution of QW-width results f r o m s t a t i s t i c a l distribution o f the t w o interface positions (see Fig. 3).

where G G and GA is the variance of the GaAs and the AlGaAs surface distribution functions, respectively. Now

G&OS,OS)- G :(100s,0s)= U;(OS)- G &(100s)=0.19h2

and G; (OS, OS)

-

^G^; ^(OS, 100s) = G: (OS)

- G L

^(100s)⁼^0.51

W2.

The sum o f these

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JOURNAL DE PHYSIQUE

t w o values [ G & (0 S)

-

ob, (100 S)]

+ [

^{G ~ ( O}S)- ~ i ( 1 0 0 S)] = 0.50 must be equal

t o G; (0 S, 0 S)

-

^G^; ⁽¹⁰⁰^S, 100 S). Indeed we get from the results given above also 0.50

i2.

We w i l l use the GQ-values now t o obtain separate estimates of the roughness o f the uninterrupted GaAs and AlGaAs growth surfaces. F o r convenience we assume t h a t a f t e r 100 S

GR1 a t only one growth surface the contribution f r o m the corresponding i n t e r f a c e t o the width distribution function o f the QW given by Equ. 1 can be neglected. This assumption is justified by our observation /7/ t h a t ^Qalmost shows no variation any more a t GRI times larger than 30 S.

We obtain G G (0 S) = 0.043nm and GA (0 S) = 0.056nm. A s t i l l more refined lineshape theory which includes Lorentzian broadening e f f e c t s yields somewhat smaller absolute values and a larger difference between GaAs and AlGaAs growth surface roughness.. These results w i l l be given elsewhere

1111.

The mean transition energy E(L,) o f the recombination o f n = 1 heavy-hole excitons depends f o r a given A I mole f r a c t i o n only on the QW-width L, i n the theoretical framework o f isolated undoped QW's between undoped barriers. The knowledge o f E (L,) allows us t o calculate the actual L,-value. F o r the noninterrupted QW's this value does not correspond t o an integral number o f GaAs monolayers b u t t o the mean value o f the L, distribution function.

A qualitatively d i f f e r e n t situation prevails i f the G R I takes place a t the GaAs surface. The origin o f the

two

exciton peaks which now appear, can be explained, i f one assumes a larger surface mobility o f Ga atoms as compared t o A I atoms. This m o b i l i t y leads t o a more rapid formation of "large" growth islands d i f f e r i n g i n height by roughly one monolayer (ML) on GaAs surfaces. The size o f these islands must be larger than the 2D-exciton diameter ( = l 7 nm) otherwise they escape observation. The difference o f the mean transition energies o f the t w o exciton bands is therefore AE (LZ) = E (L,)

-

E (L,

+

1 ML). I n the present case these t w o energies are 1.619 eV and 1.613 eV f o r samples w i t h G R I a t the GaAs surfaces and 1.614 eV and 1.6092 eV f o r the samples w i t h G R I a t both surfaces. The calculated energies f o r LZ1s equal t o 18 and 19 ML's (5.088 n m and 5.3706 nm) are 1.615 eV and 1.608 eV. Their difference is 7 meV, which compares favourably w i t h the t w o experimental differences o f 6.0 meV and 5.7 meV.

Since QW's w i t h G R I a t the AlGaAs surface have emission spectra w i t h only one maximum, we conclude t h a t growth islands larger than 17 n m do not develop a t this surface during the interruption. The surface diffusion velocity o f the A I and Ga atoms a t the present growth conditions is nevertheless high enough t o e f f i c i e n t l y reduce the i n i t i a l roughness o f the AlGaAs surface.

We would l i k e t o acknowledge helpful discussions w i t h J.Christen. The work a t B e r l i n was founded by D F G i n the framework o f SFB6.

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