Optical absorption and stimulated emission in ultrathin single quantum wells

Solid State Communications, Vol. 98, No. 7, pp. 64-649, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/96 $12.00 + .OO

0038-1098(95)00805-5

OPTICAL ABSORPTION

AND STIMULATED EMISSION IN ULTRATHIN QUANTUM WELLS

SINGLE

D. Zhang and R.M. Kolbas Department of Electrical & Computer Engineering, North Carolina State University, Raleigh, NC 27695-7911, U.S.A. (Received 9 June 1995; accepted

17 November

1995 by E.E. Mendez)

Stimulated emission from a series of ultrathin AlGaAs-GaAs separate confinement single quantum well structures is demonstrated, under various photo-excitation energies both above and below the confining layer band edge. It is shown tha$ a single ultrathin quantum well as thin as nine monolayers (Lz = 25A) has sufficient absorption to support stimulated emission. Keywords: A. heterojunctions, A. quantum wells, D. optical properties, E. light absorption and reflection, E. luminescence.

1. INTRODUCTION

2. EXPERIMENTAL

QUANTUM WELL heterostructures have attracted significant interest because they offer exceptional flexibility in tailoring the electronic and optical properties of semiconductor materials and devices [l, 21. Rapid advancements in crystal growth technology have made available high quality heterojunction materials and improved device performance [3]. Previous results have shown that undoped ultrathin [- 1 monolayer (ML)] quantum well heterostructures are capable of supporting stimulated emission with the optical feedback parallel [4, 51 or perpendicular [6] to the single quantum well (SQW). A quantum mechanical model [4, 71 based on the spatial extent of the wave function and its overlap with the barrier states, rather than the well width, explains qualitatively the differences between the experimental observations and the laser performance expected from ultrathin quantum wells based on conventional assumptions [8, 91. In this paper, we report the stimulated emission from separate confinement (ultrathin) single quantum well heterostructures (SC-SQWH) by pumping with photon energies less than the confining layer band edge. Also, high intensity and low intensity optical absorption measurements on ultrathin quantum wells are presented to investigate the absorption characteristics of monolayer thick quantum wells.

All of the samples described in this paper were grown by Molecular Beam Epitaxy (MBE) (Varian 360) on Si-doped (10 0) oriented GaAs substrates. The growth conditions for the Al,Ga,_,As-GaAs SQW separate confinement heterostructures were as follows. First, a 0.6 pm Alo,49Gao.s,As cladding layer was deposited with the substrate temperature held at 686°C. The active region, consisting of a GaAs single quantum well (with well width LZ = 6, 9, 12ML 06 LZ = 17, 25, 34A) sandwiched between two 500 A thick Alo,3Gao,7As confining layers, was then grown at a substrate temperature of 640°C. Finally, a second 0.6pm Alo,,9Gao,S1As cladding layer was grown at 686°C to complete the symmetrical structure. All the layers in these samples were undoped. A short pause (2s) in growth at the quantum well interfaces was performed on all but one sample. Sample preparation for photo-pumped laser operation consisted of removing the GaAs substrate from the epilayers by mechanical polishing and selective chemical etching, cleaving the remaining epitaxial film into rectangular platelets 20-6Opm in width, and pressing the platelets into indium under a sapphire window [lo]. The samples were photo-excited with a cavity-dumped dye laser (DCM, 14ns pulses at 0.8 MHz). Luminescence from the samples was

645

OPTICAL ABSORPTION

646

AND STIMULATED

collected and analyzed using a 0.5 m spectrometer and a cooled S- 1 photomultiplier. conditions for the GaAsThe growth In0,27Gac,,sAs SQWHs were as follows. First a 0.5pm GaAs layer was deposited at a substrate temperature of 620°C. During a stop growth (typically 4min under As over pressure), the temperature was lowered to 520°C for the growth of the In,Gat_,As. The In,Gat_,As alloy was deposited followed immediately by the deposition of a 700A GaAs cap layer. All the layers in these structures are undoped. An absorption spectrophotometer (Varian’s Cary 2300) was used to measure the optical transmission versus wavelength of these GaAs-In,Gai_,As SQWH structures. The absorption samples were backsidepolished in order to obtain smooth surfaces and mounted on a 77 K cold finger. 3. RESULTS AND DISCUSSIONS The band diagram separate confinement 1. A dye laser with a used to pump the

of an Al,Gat_,As-GaAs SQW heterostructure is shown in Fig. tunable output wavelength was structure above [h~+~~ > Eg

WO.GJO.FW and b&w W,,,

< Eg WO.G~-W~

the confining layer band edge. Stimulated emission was achieved in both cases as discussed below. The dashed lines are the calculated quantized electron and hole states of the quantum well. The laser spectra from a 9 ML (25 A) sample at

hv ,mmp< Es (*‘,.,Gao,,*s)

n_l /--

-

B --

3

4

Fig. 1. Energy-band diagram of an Al,Gat _,As-GaAs separate confinement single quantum well heterostructure. Quantum size effects give rise to a series of bound states of electrons (n = 1,2, . . .), heavy holes (n = 1,2, . . .), and light holes (n = l’, 2’, . . .).

Vol. 98, No. 7

EMISSION

Lu

““,mn

II

_

I

i

1

.I

-

11.1,

0

I

11.1’1 11.21

I,l,ll,,‘,l’,,“‘,.,“,.,J 8500

8000

7500

6500

7000

WAVELENGTH

6000

(A,

Fig. 2. 77 K stimulated emission spectra from a 9 ML A10,32Ga,,6sAs-GaAs separate confinement SQW sample at three different pumping wavelengths. The dark markers indicate the corresponding calculated e + hh transitions. The hollow marker indicates the calculated e + lh transition. three different pumping energies are shown in Fig. 2. For comparison, the data in curve (a) is obtained using a cavity-dumped argon ion laser (X = 5145 A, 10ns pulses at 3.8 MHz) which generates electronhole pairs primarily in the top A10,49Gas51A~cladding layer. Stimulated emission from the electron (n = 1) to heavy-hole (n = l), the electron (n = 1) to light-hole (n = l’), the electron (n = 1) to heavy hole (n = 2) (a broken transition) and the A10,32Gas6sA~ confining layer band-to-band transition is observed. In curve (b), the pumping wavelength is 6350A which is 46meV above the A10,32Gas,68A~confining layer band edge but below the Als.49GaeslAs cladding layer band edge. The very small bump at 6885 A is probably due to an n = 1 electron to n = 2 heavy-hole transition (there is no n = 2 electron bound state). In curve (c), the pumping wavelength is 6800 A which is 83 meV below the A1,,,,Gae6sAs confining layer band edge. The significance of curve (c) is that sufficient electron-hole pairs are generated through absorption in the nine monolayer thick quantum well to support stimulated emission in the quantum well. Note, in this case, that the threshold increases by approximately 7 and stimulated emission occurs on the high energy side of th: n = 1 electron to heavy-hole transition (X = 724OA, which is three longitudinal optical (LO) phonon energies below the pump energy, N 3 x 36 = 108meV). If you assume that the absorption coefficient in the quantum well and the confining layers is (Y= 1 x lo4 cn-’ = 1 pm-‘, then the fraction of the incident beam absorbed (by single pass calculation) is (1 - e-O’ where L is the thickness

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of the well. Hence one would expect (1 - e-O.‘)/ (1 - e-o.oo25)= 38 times more absorption to occur in the 1000 A confining layers (h~+,~r > Es,conanins)than in the 25 A quantum well (hv,,, < Es,mnanins).If the absorption coefficient were 5 x 104cm-‘, this ratio would only decrease to 32. On the basis of previous work on separate confinement heterostructures [4, 5, 111, most of the carriers in the confining layers contribute to the recombination in the well [note predominance of well emission compared to barrier emission in Fig. 2(b)]. This factor of 30-40 is quite different from the measured difference of 7, which calls into question a number of assumptions, especially how to use bulk absorption coefficients when working with ultrathin quantum wells. We will return to this point after considering low level optical absorption. If the optical absorption coefficient for ultrathin QWs is larger than expected, then it should be possible to observe this through low level optical absorption experiments. For the absorption experiments pseudomorphic GaAs-In,Gai_,As quantum well samples are selected since the expected absorption edges occur below the absorption edges of the GaAs substrates. The absorption coefficients (a) at 77K for three In,,,,Ga,,,,As-GaAs SOWS Absorption of SQWs on GaAs substrates 7x

I

9900

n

1.1

n

9500

0

n

‘a’*

9100

WAVELENGTH

’

n

8700

EMISSION

647

GaAs-In,Gai_,As SQWHs of varying well widths (x x 0.27; Lz = 17,34,51 A) are shown in Fig. 3. The value of (Yis calculated from the spectrophotometer transmission data by using [12] T = (1 - R)2e-aL 1 _ R2e-2d

’

where T is the measured transmission coefficient, R is the surface reflectivity (R x 0.3 in this case), and L is the thickness over which absorption is occuring. The formula is obtained by taking account the multiple internal reflections due to the semiconductor-air interface. The rapid rise of all the curves at shorter wavelengths in Fig. 3 is due to the absorption from the thick GaAs substrate (Eg = 8220 A). At longer wavelengths (hv < Eg,GaAs)features appear in the spectrum that are attributed to absorption in the In,Gat_,As quantum wells. The spectral width of the absorption peaks is in the range of 7-9meV (kT at 77 K is 6.6 meV). The dark marker under each curve indicates the measured photoluminescence peak wavelength corresponding to the n = 1 electron to heavy-hole transition. The corresponding absorption and photoluminescence samples were from different portions of the same wafer. Note that even the 17A quantum well has a distinct absorption feature. As the well width approaches the critical thickness, strain relaxation causes broadening and fla$ening of the absorption characteristics (Lz = 103A, data not shown).

“1 8300

CP\,

Fig. 3. Relative absorption coefficient versus wavelength at 77K for three GaAs-In$Gai-XAs SQWH samples ofvarying well width: (a) 51 A, (b) 34 A, (c) 17 A. The dark marker adjacent to each curve marks the corresponding photoluminescence peak wavelength. Note that there is an observable absorption feature for the 17A quantum well. The weaker absorption feature from the 51 A well is probably due to strain relaxation.

0

9400

9200

9000

8800

8600

8400

Wavelength h Fig, 4. Quantum well absorption coefficient for the 34A GaAS-Ino,2,G%.T4As SQW at 77 K. The total thickness of the sample is approximately 0.2 mm. The conversion between transmission data*and absorption coefficient assumes a thickness of 34A.

648

OPTICAL ABSORPTION

AND STIMULATED

The quantum well absorption coefficient for the 34A GaAs-Ins26Gas74As SQW sample at 77 K is shown in Fig. 4. The absorption length used in the calculation is based on the well width Lz = 34A, which greatly exaggerates the substrate absorption by a factor of (substrate thickness)/(quantum well thickness) = 5.9 x 104. This is what causes the baseline (which should be at 1.25 x 10’/5.9 x lo4 = 2.1 cm-‘) to be displaced so far from the axis. The peak at 9040 A is attributed to the 34 A quantum well absorption since it matches the photoluminescence data for the n = 1 electron to heavy-hole transition. Subtracting the exaggerated base line from the quantum well absorption peak gives a net absorption coefficient of 5.5 x lo4 cm-‘, which is slightly larger than the absorption coefficient for bulk In,Gat_,As near the band edge. The shape of these single quantum well absorption curves are similar to those for multiple quantum wells reported previously [13, 141 except that the continuum contribution in our case is not resolved from the exaggerated subband gap absorption from the substrate. The important aspect of this work is that the ab!orption from a single quantum well as thin as 17 A can be measured and that the absorption coefficient is quite large. If a single quantum well has a high enough absorption coefficient, it should be possible (via absorption only in the quantum well, not in the barriers) to generate a sufficient electron-hole pair population to support stimulated emission. It would be very interesting to see what happens when pumping occurs below the confining layer band edges but above the lowest quantum well energy state. The 77 K emission spectra obtained from three A1,Gal_,AsGaAs SQW separate confinement structures (Lz y 17, 25, 34A; x M 0.3) photo-pumped at 6800A (h~r~~ < Er,conanins) are shown in Fig. 5. The much lower lasing threshold for sample (c) is due to the absence of a stop growth at the quantum well interfaces. The dark marker at the bottom of each curve indicates the calculated position of the corresponding n = 1 electron to heavy-hole (e 4 hh) confined particle transition. The arrows indicate the corresponding pumping wavelengths and the measured (by photoluminescence) positions of the band gaps of the Al,Ga,_,As confining layers. It is a good assumption that absorption does not occur in the undoped bulk Al,Gat_,As confining layers since the pump is below the band edge and subband gap avalanche absorption [12, 151 is not likely to be significant. Also, we have not observed any confining layer photoluminescence when pumping below the confining layer band edge (data not shown). It is interesting that a single well as thin as 17A has

EMISSION

Al, ,Ga, ,As-GaAs Pump 66OOA 77K Pulsed

Vol. 98, No. 7

SQW

(al l2ML 134A) 106kWlcm’ (bl QML

8500

125A1

8000

7500

7000

WAVELENGTH

6500

6000

(li,

Fig. 5.77 K laser spectra from (a) 12 ML, (b) 9 ML, (c) 6ML Al,Ga,_,As-GaAs single quantum well samples. The arrows indicate the corresponding pumping wavelengths and the measured positions of the band gaps of the A1,Gal_,As confining layers. The dark markers indicate the corresponding calculated positions of e -+ hh transitions. Note that all the pumping wavelengths are below the Al,Gat_,As confining layer band edges. sufficient optical absorption to support stimulated emission. experimental The observations in both A1,Gal_,As -GaAs SQW photopumped lasers and GaAs-In,Ga,_,As SQW absorption samples provide clear evidence that photo-excited carriers are effectively collected, and that ultrathin quantum wells have suthcient absorption to support stimulated emission even if pumped below the confining layer [hr+ump< Eg (confining)] band edges. The results also confirm that carrier collection in a thin quantum well is a quantum mechanical process [4, 7, 161. For thick quantum wells, decreasing the well width decreases the spatial extent of the wave function which decreases the probability that an inelastic scattering or capture event will occur within this small region. However for a sufficiently thin quantum well, the wave function of the quantum well spreads outside the well into the confining layers resulting in a large overlap with the barrier states. This means that the interaction distance (or volume) increases as an ultrathin well gets smaller. Carriers that are generated within (or pass through) the physical dimension covered by this wave function can be captured or

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absorbed by the well. Therefore, even if the photon energy of the optical pump is below the confining layer band edge, electron-hole pairs can be generated and collected in the region adjacent to the well because of the extended wave function.

3. 4. 5. 6.

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N. Holonyak, Jr. & K. Hess, in Synthetic Modulated Structures. Academic Press, New York (1985). W. Streifer, P.D. Dapkus & R.D. Dupuis (eds),

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(1990).

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9. 10.

Acknowledgements-The authors wish to thank Dr J.H. Lee and Dr N. Anderson for helpful discussions. This work was supported by the Strategic Defense Initiative Organization/Innovative Science and Technology through Army Research Office DAAL 03-90G-0018 and DAAH 04-93-G-0254.

649

IEEE Journal of Quantum Electronics-Special Issue on Semiconductor Lasers, Vol. QE-23.

4. CONCLUSIONS In conclusion, we have demonstrated the unique absorption characteristics of ultrathin single quantum wells through high level and low level optical absorption measurements. We have shown that the absorption coefficient of an ultrathin quantum well is high enough to support stimulated emission, even if pumped below the confining layer band edges. These results suggest that additional parameters, e.g. the spatial extent of the wave functions, need to be considered for the design and operation of semiconductor thin quantum wells.

EMISSION

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S.M. Ryvkin, Phys. Status Solidi 11,285 (1965). D. Morris, B. Deveaud, A. Regreny 8z P. Auvray, Phys. Rev. B47,6819 (1993).

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