Photoluminescence of AlGaAs/GaAs quantum wells ...

Photoluminescence of AlGaAs/GaAs quantum wells grown by metalorganic chemical vapor deposition H. Kawai, K. Kaneko, and N. Watanabe Citation: J. Appl. Phys. 56, 463 (1984); doi: 10.1063/1.333933 View online: http://dx.doi.org/10.1063/1.333933 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v56/i2 Published by the AIP Publishing LLC.

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Photoluminescence of AIGaAs/GaAs quantum weBs grown by metaiorganic chemical vapor deposition H. Kawai, K. Kaneko, and N. Watanabe Sony Corporation Research Center, Fujistuka-cho 174, Hodogayaku. Yokohama, Japan

(Received 19 January 1984; accepted for publication 6 March 1984) Photoluminescence of AI. Gal _ x As/GaAs(x = 0.54) single quantum wells grown by metal organic chemical vapor deposition has been investigated at both 75 and 4.2 K. AIGaAs/GaAs heterojunction abruptness was estimated to be within a few atomic layers by comparing the peak energy of the quantum well photoluminescence with the values calculated on the assumption that the radiative transition takes place between the n = 1 electron subband and the n = 1 heavy-hole subband in the finite square potential well at 75 K. At 4.2 K, however, the peak energy shifts by several meV below the calculated energy, the cause of which may be formation of a twodimensional free exciton. The sharp photoluminescence line of the narrower well indicates that the fluctuation in thickness is less than one half the lattice constant. The emission peak shift to lower energy with the increase of excitation intensity may be the result of exchange interaction among carriers.

I. INTRODUCTION

A few laboratories have succeeded in growing abrupt AIGaAs/GaAs heterojunctions by means of metal organic chemical vapor deposition (MOCVD)I.2 and applying them to the fabrication of novel superlattice structure devices such as two-dimensional electron gas field-effect transistors (2DEG-FETs 3 ), quantum well lasers,4-7 and heterojunction phototransistors. 8 There are few investigations of the optical properties of quantum wells grown by MOCVD compared with those of quantum wells grown by molecular beam epitaxy (MBE.)9.10 Frijlink and Maluenda,2 in their investigation of quantum wells grown by MOCVD, observed no trace ofluminescence in the wavelength region longer than that corresponding to the lowest energy level of the confined particle transitions in single and double quantum well structures at 4 K. They also noted that in therr photoluminescence spectrum the observed emission peak enrgies agreed wen with the energies calculated using the eigenvalues of the quantum states in one-dimensional finite square potential wells in which exciton formation is not taken into account, even at 4 K. In contrast, Vojak. et al. II reported the formation of two-dim ensional (2D) free excitons in photopumped multiple quantum well double hetero (MQW-DH) structure emission spectra. The exciton binding energy was estimated to be 20 and 13 meV, respectively, for electron-heavy-hole and electronlight-hole excitons in nO-A.-wide multiple quantum wells. Miller et al. 12 also estimated the binding energies of 2D electron-free heavy-hole excitons to be 11 and 8 meV for well thicknesses of 42 and 145 A, respectively, from the luminescence excitation spectra of samples grown by MBE. Fluctuation in the thickness of the grown layer (called the "island" of the AlGaAs/GaAs interface) is a crucial problem for superstructure or superlattice devices. The fluctuation in thickness has been investigated for superlattices grown by MBE. 13,15 Little is known, however, about the formation of such islandlike structures grown by MOCVD. Little attention has been paid to the relation between the :luminescence peak wavelength and the excitation inten463

J. Appl. Phys. 56 (2).15 July 1984

sity. Our examination of this relation has allowed new understandings on many-body effects among carriers and thermal relaxation of carriers in the quantum well. The present paper includes: (1) an evaluation of MOCVD grown AlGaAs/GaAs heterojunction abruptness using the photoluminescence spectra of quantum wells at 75 K; (2) a discussion of whether the luminescence state in the quantum well originates from 2D free exciton recombination or recombination of the n = 1 free electron with the n = 1 free hole confined in the well; (3) a plot of the dependence of luminescence line width on well thickness and a discussion of whether or not thickness fluctuates within a layer; and (4) a plot of the dependence of the peak wavelength of luminescence on excitation power density for quantum wells, with thickness as a parameter. II. EXPERIMENT

AIGaAs/GaAs heterostructures in the present work were grown by atmospheric pressure MOCVD in a verticaltype reactor specially designed to quickly exchange the gas composition over the wafer. The source materials used were TMG, TMA, 5% of Arsine/H 2 , and H2 as a carrier gas, total flow rate being l11iter/min. Epitaxial layers were grown on a Cr-doped semi-insulating GaAs substrate. The nominal growth temperature, as measured by a C-A thennocouple inserted half way through the hole in the carbon susceptor, was 780°C. Heterojunctions were successfully grown at a growth rate of250 Aimin for the GaAs layers. Epitaxy operation was regulated by a sequence controller in order to control precisely the growth time in each layer. Emission spectra were measured using a SPEX-1402 double monochrometer equipped with two gratings blazed at 500 nm. The 5145-A line of an argon laser whose beam intensity was about 300 mW was focused onto the sample surface. Luminescence was detected by a cooled RCA-C31034 photomultiplier tube followed by Keithley-427 current amplifier. The sample temperature was controlled. using a continuous-flow cryostat and a temperature controller with either liquid nitrogen or liquid helium as a coolant. The emission

0021-8979/84/140463-05$02.40

© 1984 American Institute of Physics

463


spectra shown in the next section are corrected for the spectral sensitivity of the apparatus.

TABLE I. The equation and the band parameters used in the calculation.

Equation:

III. RESULTS AND DiSCUSSION

FrijIink et al. 2 have grown a series of thin GaAs layers sandwiched between thick AJGaAs barrier layers on a wafer in order to estimate the distribution of AI at the weU/barrier heterointerfaces. We also have grown GaAs quantum wells consisting offour GaAs layers of 30,40, 70, and 100 A. thickness separated by the barriers, 500-A-thick Alo.s4Gao'46As layers, as is shown in the inset to Fig. 1. The A1 mole fraction of 0.54 in the barrier was determined by Auger Electron Spectroscopy (AES). Growth time of each layer was determined from an preliminary experiment on the growth rate of thickly deposited AlGaAs/GaAs layers, the cleaved cross section of which was measured by referring to a scanning electron micrograph.

E:eigenvalue in the ID finite square potential well mb :barrier mass of the particle

m .. :well mass of the particle L, :well width V:barrier height

Band parameters rno = 9.11 X 10- 28 g Vc = 0.85[1.247x + 1.147 (x - 0.45)2] V. = 0.IS[1.247x + 1.147 (x - 0.45)2] Eg(T) = 1.5 I9-S.405 X 10- 4 T2/(204 + T) m~tG.,_,As = (0.48 + O.31x)mo m~"G" ._,As = (0.0665

+ 0.83x)rno

m~~ = (0.48)mo m~ (E) = (0.0665 + 0.0436E + 0.0236E2 - 0.147 E lImo (EineV)

A. Photoluminescence spectra

Figure 1 shows the photoluminescence spectrum of the sample with four quantum wells, measured at 75 K. The excitation density is about 20 W/ cm. 2 The arrows indicate the values calculated using the band parameters from the same source as that adopted by FrijIink et al. It was assumed that the recombination transition takes place between an electron and a heavy hole in the lowest eigenstates in a finite square potential well. The equation and band parameters used in the calculation are listed in Table I. The energydependent effective mass for an electron l6 was taken into account in the calculation. The barrier height was assumed to be independent of temperature. The energy of the confined particle state depended strongly on the well width and depended somewhat less strongly on the mass and the barrier height. The shoulders seen on the high energy side of the peaks assigned to 70- and loo-A.-thick wells were attributed to the transitions between the lowest electron and the lowest I

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464

light-hole eigenstates in the valence band in each well. They disappeared at liquid helium temperature, as is seen in Fig. 3. This disappearance is caused by the depletion of the population in the light-hole band. If the heterointerface of the welVbarrier is compositionally graded, the confined-particle state in the modulated well shifts to higher energy compared to that in the ideal rectangular potential well. The calculated energy shift for a narrow well was larger than that for a wide we11 2 when the same interface grading is assumed. Well size dependence of the emission energy is shown in Fig. 2. Curve (a) is calculated based on the equation in Table Y. Curve (b) in the same figure shows the relation between the emission energy and the wen width in the case of a compositionally graded weillbarrier interface having an exponentially decaying profile with a characteristic length L of 10 A. The lowest eigenvalue of the Shrodinger equation is obtained by a numerical solution of the secular equation deduced from the simultaneous difference equation. The measured peak energies corresponding to each well width, represented as circles in the figure, are

J. Appl. Phys., Vol. 56, No.2, 15 July 1964

650

0

50

100

150

Well Width c A I FIG. 2. Emission wavelength plotted against the well width of the quantum well. Curve (a) denotes the calculated values for the rectangular wells derived by the equation in Table I. Curve (b) shows the values obtained by the direct numerical solution for the Shrodinger equation in the case of a graded wellibarrier interface, as is shown in the inset. Circles on curve (a) are the observed peak wavelengths at 75 K.

Kawai, Kaneko, and Watanabe

464


fairly close to the values in a rectangular well. Compositional gradings defined by the characteristic length L at the interface are estimated to be no more than several Angstroms. Figure 3 shows a photoluminescence spectrum of the same sample at 4.2 K. There is none of the extrinsic luminescence frequently seen in MBE grown samples. 17 Two bands peaking at 8290 A (1.495 eV) and 8350 A (1.485 eV) are due to (C::.,.. - e) and (C::"'s - Sioa ) transitions in the GaAs substrate, respectiVely. When excitation intensity was increased about one order of magnitude, an intrinsic free exciton (1.5156 eV) and donor-bound excitons (1.514 eV) became observable below the band-gap energy, as is shown in the inset in Fig. 3. Variation in spectra from different quantum wells were small when excitation intensity was increased, except that the relative luminescence intensity of the wider wells became stronger than that of the narrow wens. The arrows indicate the calculated energy levels in the rectangular wells at a temperature of 4.2 K. The measured peak energy and the line width in each well at both 75 and 4.2 K from Figs. 1 and 2 are summarized in Table II, which also gives calculated peak energies and their deviations from measured ones, E ca1c - Emeas. The measured peak energy in each wen is lower than the calculated one at liquid helium temperature. The energy difference between the measured peak energy and the calculated value at 4.2 K should have the same value as that at 75 K, provided the radiative transition takes place in the same recombination path as that at 75 K. The last column in Table II compares the difference in energy between the measured peak energy and the calculated value at 75 K with the difference in energy between the measured peak energy and the calculated value at 4.2 K (LiE 4.2 K - LiE 7S K ). The downshift of the emission energy at 4.2 K is significant, and we consider it is the result of the exciton formation already observed in both the optical absorption 10 and luminescence excitation spectra 12 at liquid helium temperature. Estimated exciton binding energy, LiE 4.2 K -.1E 7S K' is about 9 meV

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FIG. 3. Photoluminescence spectrum at4.2 K for the same sample and excitation conditions as is shown in Fig. 1. Calculated emission wavelengths are shown by the arrows near the peaks. The spectrum denoted by (b) designates the GaAs substrate emission which appears when the excitation is raised to about 100 W/ cm 2 • Peaks marked by numbers 1-4 correspond to transitions FE (free exciton), (D, xl, (e-C A.), and (Sioa -CA.), respectively. 465


for a 30-A-thick wen and about 6 meV for a lOO-A-thick well, somewhat smaller than the values of 12 meV for a 30A-thick well and 9 meV for a tOO-A-thick well obtained by luminescence excitation spectra 12 under the weak excitation. In the case of MOS inversion layers, extremely small electron binding energies of two-dimensionally confined impurity states have been observed and it may be mainly due to the screening of the impurity potential by many electrons. 18,19 In the same manner, the relatively small exciton binding energies obtained in our photoluminescence spectra may be the result of the screening effect of two-dimensional carriers on the binding energy of the exciton, B. Photoluminescence line width

Well thickness dependence of the photoluminescence line width at both 75 and 4.2 K are shown in Fig, 4 together with the data from some recent publications. 2 ,\3,14 To evaluate the line width due to the distribution of energy levels brought about by the change in well thickness, the difference in energy was calculated between two eigenstates of two wells whose thickness differs by one-half of the lattice constant (2.8 A). The result is shown by a thin broken line. The photoluminescence line width data of the MBE grown quantum wells l4 in Fig. 4 lies near this broken line, after the thermal energy kT is subtracted from the line width, The variation of the excitation spectrum line width as a function of the thickness of the quantum well grown by MBE also followed this calculated curve. IS It has been suggested that the line width is related to the fluctuation in wen thickness within the layer, namely the formation of an islandlike structure with a thickness of one or two monolayer on the wellibarrier interface. If the island in the interface is larger than the exciton diameter or the mean-free-path length (several hundred Angstroms) for the radiative transition life time of an electron-hole pair, the transition energy in the island is different from that in the adjacent area, Therefore, the line spectrum is broadened by the superposition of such transitions with different energies. The line width data of the quantum well grown by MBE suggest that the fluctuation in thickness is about one half the lattice constant. The data in our experiment monotonically increase with decreasing well layer thickness, as in the previous MBE experiment. The line width in the region of narrow well, however, is much smaller than either the MBE data or the calculated curve, Average fluctuation in well thickness over the interface caused by the islandlike structure is smaller than one half the lattice constant. This may be a characteristic feature of MOCVDgrown quantum wens.

c. Excitation intenSity dependence of the peak wavelength

Figure 5 shows the dependence of the emission peak wavelength on the excitation intensity with the well thickness as a parameter. The wafer was irradiated with an argon Jaser of 300 m Wand the excitation intensity was varied using a set of neutral density filters. The intensity was evaluated by dividing the irradiated power with the area of the spot, which was 200 Jim in diameter. The absolute value of the excitation intensity was not very accurate because of the low KaWai, Kaneko, and Watanabe

465


TABLE II. Peak energy and line width for quantum well photoluminescence. 75 K

.

Well thickness (A)

Emeas

Ecah;

(meV)

(meV)

30 40 70 100

1750.10 1692.06 1589.45 1550.68

1747.39 1683.79 1590.88 1554.96

4.2K

.:lE" K (meV)

b

-2.71 - 8.27 1.43 4.28

Line width (meV)

Em... (meV)

14.08 13.16 9.57 9.89

1751.83 1693.68 1593.54 1555.55

(meV)

.:lE' K (meV)

Line width (meV)

.:lE" K - .:lEu K (meV)

1758.17 1694.60 1601.77 1565.96

6.34 0.92 8.23 10.41

8.41 7.63 5.70 6.40

9.1 9.2 6.8 6.13

EeaJc

denotes the calculated transition energy between the lowest-confined particle state of electron and heavy hole in the GaAs well layer barrie red with Ala 5' Gao 46 As layers. b .:lE, IK equals Ecole Em... at ( ) K.

• Ecole

accuracy of the spot size determination. The time-averaged excitation intensity was kept constant in order to maintain a constant temperature throughout the mesurement by using a rotary chopper with a variable ratio aperture at 200 Hz. While Miller et al. 20 and Xu et al. 21 observed the extra peaks caused by the excited levels of the wells under extremely high photoexcitation conditions, we observed no extra peaks, presumably because maximum excitation intensity in our experimental condition was estimated to be no more than 1000 W / cm. 2 The peak wavelength from each well shifted to longer wavelength with increasing excitation intensity. The peak shift of the wider wells is larger than that of the narrower ones. For example, the shift of the 70-A-thick well was 12 A with an increase in excitation intensity of about two orders of magnitude, whereas the shift of the 30-A-thick well was only

6A. There are two main factors which affected the emission wavelength when the excitation intensity was varied. One is the effective band-gap shrinkage with increasing carrier density due to exchange interaction among carriers22 and the other is the band-filling effect which increases the emission photon energy. These two factors counteract each other.

"Hot" carriers in the well layer injected from the confining layers are immediately thermalized because of the shorter inelastic scattering time with LO phonons, 10- 14 S,23 compared with the recombination lifetime with a hole in the va~ lence band, 10- 9 s. Therefore, there are not many hot electrons that affect the luminescence peak energy. Since, in a quantum well, most of the carriers are distributed near the bottom of the lowest subband, which has a large carrier density due to a staircaselike density of state, the band-filling effect is insignificant compared with that in bulk GaAs with a parabolic density of state. The band-gap shrinkage, then, is more effective in quantum wen photoluminescence under high excitation. The reason for the relatively small peak wavelength shift with excitation intensity in the narrower well compared with that in the wider well may be as follows. If the well thickness is narrower than the electron mean-freepath length for LO phonon scattering, - 60 A, electrons excited in the wall go across the well to the wall on the other side without any energy toss, The narrow wen cannot effectively collect carriers from adja...:::ent barriers, which means that carrier dt:nsity, giving rise to the band-gap shrinkage, in the narrow wen is lower than that in the wide well.

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20

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>Gl E ¥

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16

ref.2:M04K

.I:

DC

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c o

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5

II)

E LU

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Well

&0

Width

BO

100

( A)

FIG. 4. Photoluminescence line width vs quantum well thickness. Broken line denotes the calculated energy variation against well thickness with fluctuation in thickness of one atomic layer at 0 K. Some published data are also shown in the figure. 466


2

10 Excitation

100 Intensity

1000 (w Icm 2 • 5145A)

FIG. 5. Luminescence peak wavelength shift for GaAs quantum wells against the excitation power density. Note that excitation density is not absolutely accurate because of the inaccurate estimation of the spot size.

Kawai, Kaneko. and Watanabe

466


IV. SUMMARY

AIGaAs/GaAs heterojunction abruptness has been evaluated by means of a 75 K photoluminescence spectrum of the sample consisting offour single quantum wells having well thicknesses from 30 to 100 A. on one wafer grown by atmospheric pressure MOCVD. The observed peak energies agree with the values calculated on the assumption that the transition takes place between the n = 1 electron and the n = 1 heavy hole in the finite square potential well. AlGaAs/GaAs heterojunction abruptness is estimated as no more than several Angstroms. At liquid helium temperature, the observed luminescence peak energies shift to lower energy from the calculated energies. A probable cause is 20 free exciton formation, whose binding energies are estimated to be 9.1 and 6.1 meV for 30- and lOO-A.-thick wells, respectively. The photoluminescence line width increases with the decrease in wen thickness, but its rate of increase is not so steep as that derived from a model of the formation of the islandlike structure on the interface nor so steep as that of MBE-grown structures. Emission peak energy shifts to lower energy with the increase of excitation intensity, the reason for which may be that the band-gap shrinkage exceeds the band-filling effect under high excitation conditions in the two-dimensional quantum well. ACKNOWLEDGMENTS

The authors are indebted to Dr. O. Matsuda and Dr. Y. Mori for helpful discussions. This work was performed under the management of the R&D Association for Future Electron Devices as a part of the R&D Project of Basic Technology for Future Industries sponsored by the Agency of Industrial Science and Technology, MITI.

467


'R.D. Dupuis and P.O. Dapkus, Appl. Phys. Lett. 34, 335 (1979). 2P.M. Frijlink and J. Maluenda, Jpn. J. Appl. Phys. Lett. 21, L574 (1982). JJ. Maluenda and P.M. Frijlink, J. Vac. Sci. Technol. B 1334, (1983). "See, for example, N. Holonyak, Jr., R. M. Kolbas, R. D. Dupuis, and P. D. Dapkus, IEEEJ. Quantum. Electron.QE·16, 170 (1980). 'R.D. Burnham, D. R. Scifres, and W. Streifer, Electron, Lett. 18, 507 (1982). "S. Hersee, M. Baldy, P. Assenat, B. DeCremoux, and J. P Duchemin, Electron. Lett. 18,618 (1982). 7H. Kawai, O. Matsuda, and K. Kanedo, Jpn. J. Appl. Phys. Lett. 22, L727 (1983). "R. A. Mirano, P. D. Dapkus, and S. G. Stillman, IEEE Trans. Electron. Devices ED-29, 226 (1982). "R. C. Miller, D. A. Kleinman, W. A. Nordland, Jr., and A. C. Gossard, Phys. Rev. B 22, 863 (1980). 10 R. Dingle, in Festkoerperprobleme, Advances in Solid State Physics, edited by H. J. Queisser (Pergamon/Vieweg, Braunschweig, 1975), Vol. XV, p. 21. "B. A. Vojak, N. Holonyak, Jr., W. D. Laidig, K. Hess, J. J. Colemann, and P. D. Dapkus, Solid State Commun. 35, 477 (1980). '2R. C. Miner, D. A. Kleinman, W. T. Tsang, and A. C. Gossard, Phys. Rev. B 24, 1134 (1981). IJR.C. Miller and w. T. Tsang, Appl. Phys. Lett. 39, 334 (1981). ,4L. Goldstein, Y. Suzuki, S. Tarueha, K. Horikoshi, and H. Okamoto, 30th Spring Meeting of Jpn. Soc. Appl. Phys., 514 (1983) (in Japanese). "C. Weisbueh, R. Dingle, Dingle, A. C. Gossard, and W. Wiegmann, Solid State Commun. 38, 709 (1981). '6B. A. Vojak, W. D. Laidig, H. Holonyak, Jr., M. D. Camras, J. J. Coleman, and P. D. Dapkus, J. AppJ. Phys. 52,621 (1981). '7R. C. Miller, A. C. Gossard, W. T. Tsang, and O. Munteanu, Phys. Rev. B 25,3871 (1982). '"B. Vinter, Phys. Rev. B 26, 6808 (1982). '''Y. Takada, J. Phys. Soc. Jpn. 46,114 (1979). ~. C. Miller, D. A. Kleinman, O. Munteanu, and W. T. Tsang, AppJ. Phys. Lett 39, 1 (1981). 2·Z. Y. Xu, V. G. Kreismanis, and C. L. Tang, Appl. Phys. Lett. 43, 415 (1983). 22H. C. Casey, Jr. and F. Stem, J. AppJ. Phys. 47, 631 (1976). 2JCalcuiated using the mean free path for LO phonon scattering of 63 A, and the thermal velocity of electrons in GaAs of3 X 107 em/so The situation is not so different in the two-dimensional space.

Kawai, Kaneko, and Watanabe

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