AlχGa1-χAs single quantum well interface structures

Optics Communications 102 ( 1993 ) 439-446 North-Holland

OPTICS COMMUNICATIONS

Photoreflectance and photoluminescence characterizations of GaAs/AlxGal_xAs single quantum well interface structures Chenjia C h e n ~, Wei G a o ~, Lizhi Mi ~, D e p i n H u a n g 1, Yong Chen m Department of Physics, Peking University, Beijing, 100871, China

Z. W a n 2 a n d D.W. Liu 2,3 Department of Physics, Florida Atlantic University, Boca Raton, FL 33431, USA Received 25 January 1993; revised manuscript received 10 June 1993

Photoreflectance (PR) and photoluminescence (PL) techniques were utilized as complementary tools to characterize G a A s / AlxGa~_xAS single q u a n t u m well structures in the temperature range of 4-300 K. The samples were grown by MBE on GaAs (001) substrates exactly oriented or slightly misoriented 4 ° toward ( 111 ) Ga. Sharp PR features concerning pseudosmooth interfaces were observed for the sample with 4 ° tilt. Temperature dependence of PL peak positions as well as profiles showed different behaviour for sample grown with and without 4 ° misorientation. Tripeak structures of PL spectra corresponding to monolayer fluctuation are analyzed.

1. Introduction

In the recent years, a great effort has been devoted to the research of the optical properties of quantum well (QW) structure due to the prospective application in the fabrication of new electro-optical devices [ 1 ]. The effect of substrate misorientation on the interface quality of GaAs/AlxGa~_xAs QW structure, revealed by photoluminescence (PL), has recently received growing attention as a possible vehicle for thorough understanding of the fundamental nature of QW interfaces. Tanaka and Sakaki [ 2 ] have concluded from PL experiments that terrace edges on the substrates are either randomly spaced or kinked. Recent experimental data on the PL spectra have shown that quantum wells grown on a GaAs (001) substrate with 4 ° misorientation towards the (111 ) Ga plane (steps parallel to the [ 1i0] direcSupported by the National Natural Science Foundation of China. 2 Supported in part by the Florida Atlantic University, Division o f Sponsored Research, under Grant No. 12-1131-041. 3 Present address: Department of Physics, Worcester Polytechnic Institute, Worcester, MA 01609, USA.

tion) display sharp exciton peaks, corresponding to pseudosmooth interfaces [3 ]. The non-destructive contactless method of modulation photoreflectance (PR) has proved to be a powerful technique to study a large number ofquantized states in multiple quantum well (MQW) structure [4-6], however, only a few PR measurements were reported on the quantized states in a single quantum well structure. Such a modulation method produces derivative-like features in the vicinity of the critical point; consequently, even at room temperature, one can observe very sharp differential reflectance lines. Many studies indicate that the spectra are excitonic at low temperatures and band to band at temperatures above 250 K [7 ]. In this paper we report a detailed PR and PL study on GaAs/AlxGa~_xAs single quantum well structure in the temperature range of 4-300 K, and we investigate the interracial roughness in QW structures grown by MBE on GaAs (001) substrates exactly oriented or 4 ° misoriented towards ( 111 ) Ga.

0030-4018/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

439

Volume 102, number 5,6

OPTICS COMMUNICATIONS

15 October 1993

2. Experiment ,~

A series of high quality QW samples used in this investigation were prepared by MBE system. Sample A was grown on GaAs (001) substrate exactly oriented as reference sample. Sample B was grown 4 ° misoriented toward the ( 111 ) Ga plane. For comparison, samples A and B were grown under the same conditions. The substrate temperature was 610°C. Usual standard growth conditions were fulfilled, both sample A and sample B consisted of three uncoupled GaAs single wells: L z ~ 8 0 A (a), ~ 4 0 b, (b), and ~ 2 0 A, (c) and in between 500 it AlxGal xAs barrier layers ( x = 0 . 3 ) with a 0.95 pm GaAs buffer on a semi-insulating GaAs substrate. Sample C was grown on GaAs (001) substrate exactly oriented, which consisted only o f one uncoupled GaAs single quantum well: L~~ 100 A and sandwiched between 300 A GaAs/AlxGa~ _~As barrier ( x = 0 . 3 ) with a 0.5 g m GaAs buffer on a semi-insulating GaAs substrate. The 6328 A line of a helium-neon laser was used as the excitation source in PL measurements. The samples were mounted in a continuous flow liquid helium cryostat to avoid laser thermal heating. For the PR experiments, light from a quartz halogenlamp source (150 W) passing through a H R D - I double grating m o n o c h r o m a t o r was chosen as the probe beam. The probe beam at wavelength 2 was focused onto the sample placed inside a cryostat. Electromodulation of the sample was established by shining a chopped HeNe laser ( ~ 1 m W ) beam at frequency ~ 150 Hz, which serves as the pump source.

3. Results and discussion Typical PR spectra are reported in figs. 1-3, which show the P R spectra of the samples A and B at 300 K, 150 K, and 77 K, respectively. The dotted lines are the experimental PR spectra. The solid lines in these figures are a least-squares fit of a line-shape function [8 ] AR R

-

e ~ Re[Cjexp(i0,) (E-Egj+iFj)-",] j=~

,

where p is the number of spectral features to be fitted, Cj. 0j, Egj, and ~ are the amplitude, phase, en440

"-2. ¢lJ

Ct2
"~l

i~i T=3OOK

\

V i

-t"

..a

i

1.40

1.48

1.56

ENERGY(eV) Fig. 1. Photoreflectance spectra (dotted) of GaAs/AlxGal_xAs of single quantum wells at 300 K with samples A, B. The solid line is a line-shape fit as discussed in the text. ergy, and broadening parameter of the jth structure, where mj is used to specify the critical point type and depends upon the perturbation type (order to derivative); we used the third derivative function from ( T D F F ) of line shape ( m = 2.5, three-dimensional critical point) fit for GaAs substrate and buffer region, the first derivative function form ( F D F F ) of line shape ( m = 2 , exciton) fit at 77 K, and the T D F F of line shape (m = 3, two-dimensional critical point) fit at 150 K and room temperature. The obtained energies of the various transitions are indicated by arrows at the top of figures. The best fitting was further confirmed by the good agreement between the energies of various transitions and the corresponding values obtained by direct calculation. The feature denoted by Eo (GaAs) corresponds to the direct band gap of GaAs and originates from the GaAs buffer substrate region of the sample. The transitions above Eo (GaAs) can clearly be identified as confined 11H (a o r b , c), 11L (a o r b , c) corresponding to well width of ~ 8 0 A, ~ 4 0 A~, and ~ 2 0 A, respectively. The above notations represent the transitions from first conduction band state to the first heavy ( H ) or light (L) hole valence band state, and the transition 11Hex (a, or b, c) associated with excitonic state. As shown in figs. 1 and 2, the modulation signals AR/R in single Q w related to the transition from first conduction band to the first heavy ( H ) - I IH or light

Volume 102, number 5,6

OPTICS COMMUNICATIONS

~ ....

....

T : I 50 K

~

_W_m-

d

B

=i ul

~

a

i

1.48

i

i

1.56

i

i

1.64

ENERGY(eV) Fig. 2. Photoreflectance spectra (dotted) of OaAs/AlxOa~_~As of single quantum wells at 150 K with samples A, B. The solid line is a line-shape fit as discussed in the text.

A i,,u"

~i i=

~ll;i~

r'r" \ rl,"

1.50

1.66

1.82

ENERGY(eV) Fig. 3. Photoreflectance spectra (dotted) of OaAs/AlxOa~_gAS of single quantum wells at 77 K with samples A, B. The solid line is a line-shape fit as discussed in the text. ( L ) - I 1L hole b a n d state, only o f the widest well ( L z ~ 80 A ) can be observed. The signals o f 11H ( a ) and 1 IL ( a ) are weaker than Eo ( G a A s ) , while they are enhanced significantly as the t e m p e r a t u r e is lowered to 77 K a n d much m o r e PR features from different well widths become visible than that at high t e m p e r a t u r e (see fig. 3). As shown in fig. 3, there is a significant difference between exactly oriented (sample A ) and 4 ° m i s o r i e n t e d substrate ( s a m p l e B) in P R spectra at 77 K. The sharp P R features are

15 October 1993

o b t a i n e d when the substrate is 4 ° misoriented toward (111) G a (steps oriented along the [ 1 ] 0 ] direction). The values of the b r o a d e n i n g p a r a m e t e r from the line shape fit for two samples at 77 K are given in table 1. The effects o f the substrate misorientation from the nominal (001) plane on G a A s / A l x G a l _ x A s heterostructures ( H S s ) grown by MBE have recently received growing attention as possible vehicles towards a better understanding o f interface defects. It is well known that in state-of-the-art MBE grown HSs display growth islands at interface with a typical size o f the order o f 40 A [ 9 ] and a similar terrace size, LT = 40 A can impose on both well interfaces by a 4 ° m i s o r i e n t a t i o n o f the (001) G a A s substrate surface towards ( 111 ) Ga plane. Recent experimental d a t a on the PL properties of single q u a n t u m wells grown on misoriented substrate 4 ° towards the ( 111 ) G a plane display sharp exciton peaks ( p s e u d o s m o o t h interfaces) as in the case of a G a A s substrate exactly oriented along the nominal (001) plane ( s m o o t h interfaces) [ 3 ]. We have performed the measurements o f PL spectra and o b t a i n e d similar results as above (fig. 4). In fig. 5 we plot the energy position of the PL peaks as a function of temperature with sample A. One finds a t e m p e r a t u r e d e p e n d e n c e which well follows the Tdependence o f the GaAs energy gap when T > 70-300 K, indicating that the peak in PL spectra can be ascribed as the free-exciton r e c o m b i n a t i o n at high temperature. At lower temperature, the peak energies deviate to the lower energy side from the ext r a p o l a t e d curve. The Stoke shifts o f 7 meV, 7 meV, and 11 meV are o b t a i n e d for the corresponding 80 ,~, 40 A., and 20 A well, respectively, at 4 K. Since the lack o f possible correlation between the shift energy and well width, the Stoke shift energy is ruled out to be the 1 HH exciton binding energy, but rather Table l Broadening parameter of 77 K at the 11Hex(a) and 11H(a) transitions obtained by line shape fit. Sample

A B

Broadening parameter (meV) 11Hex(a)

11H(a)

4.73 0.938

6.22 2.57

441

Volume 102, number 5,6

OPTICS COMMUNICATIONS

15 October 1993

SAMPLE A

SAMPLE B

iIIII

/,'<~\

i,

>-~ ¢¢,.:-

I-z

SAMPLE A

1,53

1,59

1.65

1.71

1.77

1.83

0

15

30

45

ENERGY (eV)

Fig. 4. PL spectra for isolated GaAs/AI~Ga]_~As quantum well (x=0.3) grown onto (A) a nominal GaAs (001) substrate, (B) a substrate with [ 1TO] step at 4 K. is more a p p r o p r i a t e l y interpreted as the exciton trapping on the intrinsic defect due to the in-plane potential fluctuation in the Q W heterointerfaces. The excitons confined in the G a A s are free within the layer, so they have to experience the potential fluctuation due to the interface defects. The photoexcited excitons can be either localized or delocalized depending on the respective size of the exciton as well as the defect, and on the distance between defects and the exciton diffusion length. If the exciton rec o m b i n a t i o n time between the free and b o u n d exciton can be established, then the emission peak is o b t a i n e d by averaging between the energies o f the free and bound-exciton levels. The low t e m p e r a t u r e P L is d o m i n a t e d by the r e c o m b i n a t i o n of excitons b o u n d to the interfacial defects. The energy match b e y o n d 70 K in fig. 5 can be explained by the fact the free exciton states are p o p u l a t e d when the temperature is raised. The analysis above can be further substantiated by the experimental result that the Stoke shift energy is closely related to the PL peak width. The fwhm o f 4 meV, 4 meV and 10 meV has 442

i

i

i

J

i

I

60

75

90

105

120

135

TEMPERATURE

150

(K)

Fig. 5. Temperature dependence of PL peak position from the heavy hole exciton (E~h) for different well width of sample A. The solid lines give the behaviour of the GaAs bulk band gap. the corresponding shift energy of 7 meV, 7 meV and 11 meV, respectively. A typical PL spectrum of SQW sample C ( L z ~ 100 A ) at low t e m p e r a t u r e ( T = 4 K ) is shown in fig. 6. A bipeak feature observed at 1.558 eV and 1.5568 eV was due to m o n o l a y e r fluctuation, which can be further confirmed by a numerical calibration to relate the SQW heavy hole exciton transition energy and the Lz value, as described by the following equations: (

E,~

~ )

,1/2

[-{2m.CA )

tan[~

1/2

~_E~)~]

_(svo- o) 8E =

ALz .

The calculation was p e r f o r m e d by taking the conduction b a n d energy gap discontinuity as 57% o f the gap discontinuity. The m *(A) and n *(B) are the ef-

Volume 102, number 5,6

OPTICS COMMUNICATIONS

SAMPLE

C

\ x3Z,

18k

x3Z.

7~

\

>. IZ

z

\ i

i 1.53

i

J i 1.54 ENERGY

J 1.58 (eV)

(a)

i

l 1.S7

SAMPLE

C,

i

i. $ 8

150K

15 October 1993

fective mass o f carriers in the well and barrier material, respectively. 5 V, are the b a n d offsets o f conduction and valence bands. E , are the lowest subband energy levels in the conduction b a n d and heavy hole band. By taking ALz, the one m o n o l a y e r thickness, as 2.83 ~i,, we have fiE is about 1.8 meV, which is in good agreement with the experimental result. As the t e m p e r a t u r e s increases, the high energy b a n d is more thermally p o p u l a t e d and becomes the p r e d o m i n a n t peak as the t e m p e r a t u r e is above 10 K. As the temperature is further increased to 30 K, a third energy b a n d starts to emerge on the shoulder o f the high energy side. This so called b a n d filling effect contributes to the increased spectral weight o f high energy relative to the low energy peak as the t e m p e r a t u r e is increased [ 10 ]. W h e n the thermal energy o f the exciton exceeds the barrier energy, the excitons can transfer from the narrower well regions. In a d d i t i o n to the exciton line, a weak but well resolved extrinsic structure is observed at E = 1.536 eV (see fig. 6a left). As the t e m p e r a t u r e increases, the peak intensity displays an unexpected behaviour; it increases up to about 20 K and then undergoes a rapid thermal quenching. The variation o f the extrinsic emission intensity as a function o f temperature, shown in fig. 7, leads to the assignment o f this extrinsic peak as

SAMPLE C ---:.

z

>I--

tz

Z lad I-" Z

El h

U Q_ r',, hi ¢Y (..9 hi I'-Z (b) i

1.50

r

1.52

E

~

]

1.54 ENERGY

i

1.56

i

i

1.58

i

1.60

(eV)

Fig. 6. (a) PL spectra of OaAs/AlxOal_xAs of single quantum well (sample C) measured at various temperatures. E~h is due to the heavy hole exciton recombination related to the lowest conduction and valence subbands. (e-A°) denotes the emission due to the conduction band to neutral acceptor recombination. (b) The heavy and light exciton peaks of sample C at 150 K.

o

'

3'o

,:So

' 6b ' ' t~o' TEMPERTURE (K)

Mso

Fig. 7. Temperature dependence of the emission intensity of the (e-A°) and E~h peaks in GaAs/AlxGal_xAS single quantum well (sample C).

443

Volume 102, number 5,6

OPTICSCOMMUNICATIONS

!

15 October 1993

SA.PLE A!

SAMPLE A L=eOA T=5,6K

140K, xl0

105K, x5 f

\

76K, x2

~

~

~/,~

I I

5oK, x2

/l~t'

I.z

#.

#.

30K

18K

10K

~ 1.55

1.60

'

1.65

5.6K '

1.70

~

1.75

T

1.80

ENERGY (eV)

Fig. 8. PL spectrum as a function of temperature for sample A. the conduction band to the neutral acceptor transition (e-A°). The intensity of the extrinsic peak is about one order of magnitude less than that of the exciton line, indicating a very low concentration of carbon impurities, and the line width is comparable to that of the exciton line, reflecting the sharp localization of the impurities centers. The rate of the conduction band to neutral acceptor recombination (e-A°) is proportional to the product of the number of conduction electrons and of neutral acceptors, the increase of the peak intensity in the range from 4 K to 20 K indicates a rapid increase of the electron number by thermal ionization, despite the reduction of the number of neutral acceptors by thermal ionization. At higher temperature, all the shallow donors are ionized, no more electrons are produced with increasing temperature, thermal quenching occurs since the (e-A°) peak intensity is reduced by thermal ionization of the neutral acceptors. Chen et al. [ 11 ] reported the thermal quenching of extrinsic PL in GaAs/AlxGa]_xAs single narrow QW structure (40 A), now we observe the same behaviour in a high quality GaAs/AlxGa~_xAs single QW structure with wider well width (100 A.). At 150 K the light hole 444

1.556

1.562

1.568

1.574

1.580

1.586

ENERGY (eV)

Fig. 9. The peak profile is decomposedinto three peaks separated by about 2 meV and with comparable width, as discussed in the text. exciton peak emerge at the position 14 meV above the heavy hole exciton peak (fig. 6b), which is significantly higher than the shift peak energy caused by monolayer fluctuation. Figure 8 displays the PL spectrum as a function of temperature for sample A. The peak due to 80 ~i, well has a slight drift towards the high energy side as the temperature increases, and eventually mode softening dominates. This anomalous behaviour of blue shift can be attributed to monolayer fluctuation. The peak profiles actually can be decomposed into three peaks separated by about 2 meV and with comparable widths, as shown in fig. 9. The rather large carrier occupation enhancement of the high energy band when the temperature increases, same as shown in PL spectra of sample C, gives a peak drift towards the high energy side. Meanwhile, the peak profiles exhibit quite unsymmetric at low temperatures ( < 70 K) (fig. 10). In contrast with sample A, smooth and symmetric PL peak profiles are shown in sample B (fig. 11 ), which can be explained by considering that bottom and top well interfaces are pseudosmooth

Volume 102, number 5,6

OPTICS COMMUNICATIONS

SAMPLE A

15 October 1993

SAMPLE B

L=80A 140K

L=80A

120K

105K

":'=

~

~

76K

=~_

90K

I-z

I-

z_

~-

60K

~K

18K

10K

I

~

2OK

5.6K

1,550

i

1.558

i

1.~566

1.574

i

i

1.582

i

1.590

1.550

1.5J58

1.566

1.574

1.582

1.590

ENERGY (eV)

ENERGY (eV)

Fig. l 0. The PL peak profiles and positions for sample A as a function of temperature.

Fig. 11. The PL peak profiles and positions for sample B as a function of temperature.

with an average terrace length ( ~ 40 A ) much smaller than the typical size o f the free exciton ( 200 A ) , i m p o s e d by substrate misorientation. Such a " p s e u d o s m o o t h " nature was manifested by symmetric PL peak profiles with no fine structure e m b e d d e d . The lack o f the fine structure for sample B gives the reason that no significant blue shift is observed at low temperatures. It also demonstrates from fig. 4 that several d e f e c t / i m p u r i t y related emission peaks are observed for sample A, but absent for sample B. Tilting the substrate towards ( 111 ) G a m a y reduce surface sites which inherit a high affinity for d e f e c t / i m p u r i t y incorporation. Substrate tilt towards ( I l l ) G a exposes gallium type step edges (gallium dangling b o n d s ) . The sticking coefficients o f impurities to gallium type step edges m a y be less than the sticking coefficient o f the n o m i n a l l y "flat" interface. In s u m m a r y , the t e m p e r a t u r e sensitive m e t h o d o f PL m e a s u r e m e n t provides an effective tool to detect the m o n o l a y e r fluctuation in the interface. It is indicated that the growth on (001) n o m i n a l direction smooth interfaces are obtained in GaAs/AlxGa~_xAs

single QW, and sharp P R and symmetric PL features (i.e. p s e u d o s m o o t h interfaces) are obtained when the substrate is 4 ° tilted towards the ( 111 ) G a plane, generating G a steps oriented along [ 1 i 0 ] direction. These results also d e m o n s t r a t e that the P R m e t h o d which is less t e m p e r a t u r e d e m a n d i n g provides a convenient and consistent way to probe the quality o f interfaces in Q W and superlattice structures.

References

[ 1] D.S. Chemla, D.A.B. Miller and P.W. Smith, Opt. Eng. 24 (1985) 556. [2] M. Tanaka and H. Sakaki, J. Appl. Phys. 64 (1988) 4503. 131J. Massies, C. Deparis, C. Neri, G. Neu, Y. Chen, B. Gil, P. Auvray and A. Regreny, Appl. Phys. Lett. 55 (1989) 2605. [4] O.J. Glembocki, B.V. Shanabrook, N. Bootka, W.T. Beard and J. Comas, Appl. Phys. Lett. 46 (1985) 970. [ 5] H. Shen, P. Parayanthal, F.H. Pollak, M. Tomkiewicz, T.J. Drummond and J.N. Schulman, Appl. Phys. Lett. 48 (1986) 653. [6] B.V. Shanabrook, O.J. Glembocki and W.T. Beard, Phys. Rev. B 35 (1987) 2540. 445

Volume 102, number 5,6

OPTICS COMMUNICATIONS

[ 7 ] A. Kangarlu, H.R. Chandrasekhar, M. Chandrasekhar, Y.M. Kapoor, F.A. Chambers, B.A. Vojak and J.M. Meese, Phys. Rev. B 37 (1988) 1035. [ 8] O.J. Glembocki, SPIE, 946 Spectroscopic Characterization Techniques for Semiconductor Technology III ( 1988 ) 1.

446

15 October 1993

[ 9 ] M. Tanaka and H. Sakaki, J. Cryst. Growth 81 ( 1987 ) 153. [ 10] R.K. Tsui, G.D. Kramer, J.A. Curless and M.S. Pfeffley, Appl. Phys. Lett. 48 (1986) 940. [11] Y. Chen, A. Hameury, J. Massies and C. Neri, Nuovo Cimento 10 (1988) 1243.