Observation of 1.798 μm intersubband transition in InGaAs/AlAs pseudomorphicquantum well heterostructuresY. Hirayama, J. H. Smet, L. H. Peng, C. G. Fonstad, and E. P. Ippen Citation: Applied Physics Letters 63, 1663 (1993); doi: 10.1063/1.110729 View online: http://dx.doi.org/10.1063/1.110729 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/63/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Short-wavelength intersubband absorption in strain compensated InGaAs/AlAs quantum well structuresgrown on InP Appl. Phys. Lett. 83, 210 (2003); 10.1063/1.1592315 Investigation of short wavelength intersubband transitions in InGaAs/AlAs quantum wells on GaAssubstrate J. Appl. Phys. 82, 3385 (1997); 10.1063/1.365652 Short wavelength (5.36–1.85 μm) nonlinear spectroscopy of coupled InGaAs/AlAs intersubband quantumwells Appl. Phys. Lett. 65, 2630 (1994); 10.1063/1.112585 Electron intersubband transitions to 0.8 eV (1.55 μm) in InGaAs/AlAs single quantum wells Appl. Phys. Lett. 64, 986 (1994); 10.1063/1.111960 Intersubband transitions in pseudomorphic InGaAs/GaAs/AlGaAs multiple step quantum wells J. Vac. Sci. Technol. B 11, 1840 (1993); 10.1116/1.586487

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.248.155.225 On: Tue, 25 Nov 2014 02:41:52

Observation of 1.798 pm intersubband transition in InGaAs/AIAs pseudomorphic quantum well heterostructures

Y. Hirayama,a) J. H. Smet, L. H. Peng,b) C. G. Fonstad, and E. P. lppen Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

(Received 12 April 1993; accepted for publication 12 July 1993)

The well-width dependence of intersubband transition energies in InGaAs/AlAs pseudomorphic quantum well structures has been studied, and the shortest intersubband wavelength reported to date, 1.798 ,um, has been observed for 6 monolayer wells. Both transverse electric and transverse magnetic optical polarizations are absorbed, with an energy splitting of 67 meV between them.

Since intersubband transitions in quantum well struc- tures were first observed,’ there have been many reports on this topic related to large oscillator strength, large optical nonlinearity, and fast carrier relaxation time. A key issue pertaining to the utilization of intersubband transitions in practical devices is operation wavelength. Transition wave- lengths in the range 4-10 pm are covered by GaAs/ AlGaAs,” G~As/A~As,~ and InGaAs/InAlAs”” material systems. Intersubband absorptions in the range 24 ,um appropriate for ultralow-loss tiber communications have also been realized. An absorption wavelength of 2.4 ,um was reported for the InGaAs/AlAsSb system,6 and a 2.1 pm absorption was reported for a 2.8 pm well in the InGaAs/InAlAs system.’ Because of its applicability to current 1.55 pm optical communication systems, realiza- tion of 1.55 ,um intersubband transitions in quantum wells would be interesting. Very fast relaxation times and large optical nonlinear-it&, which are useful for advanced 1.5 pm photonic devices such as high speed photonic switches, are expected. InP-based material systems such as InGaAs/ AlAs are thought to be among the most suitable systems for this purpose because of the large conduction-band off- set of AlAs on Inc53Gac,47A~. However, there has so far been no report on intersubband transitions at wavelengths shorter than 2 pm. Here, we report observation of inter- subband absorption in this material system at 1.798 pm, the shortest wavelength value reported to date.

For our experiments, a number of samples with differ- ent well widths were fabricated by molecular beam epitaxy (MBE) on Fe-doped semi-insulating InP substrates. The band diagram for samples A and B, which had two In0.53Gaa.47As quantum wells is shown in Fig. 1 (a). The well width in sample A is 6 monolayers, or 1.8 mn, and sample B is 10 monolayers, or 2.9 nm. In order to increase the barrier height, pseudomorphic AlAs barrier layers were employed. Each AlAs layer is 9 monolayers thick. The lattice mismatch between In0.53Gac,47As and AlAs is 3.7%. The growth was monitored by in situ, reflection high energy electron diffraction (RHEED) intensity oscilla- tions, as shown for sample B in Fig. 1 (b) . Intensity oscil- lations are clearly observed for the InAlAs upper cladding -- ‘)On leave from Toshiba Co., Research and Development Center,

Kawasaki 210, Japan. “Division of Applied Sciences, Harvard University, Cambridge,

MA 02138.

layer as well as for the quantum well region. The InGaAs well was highly doped at 1019 cme3. Samples C-F have structures similar to samples A and B but have slight dif- ferences. Samples C-F have single quantum wells with thicknesses of 4.3, 5.0, 6.6, and 7.6 nm, respectively. The composition of the wells for samples C-E is Inc,Gao,3As instead of In0.53Gac.47As. Sample F has 9 nm Ina,,A10,3As strain compensation layers between the AlAs barrier layers and the In0.52Alc.48As cladding layers.

A conventional optical absorption measurement sys- tem with a PbS detector* was used for measuring room- temperature absorption in the wavelength region less than 2 pm. A Fourier-transform infrared (FTIR) spectrometer with a glowbar source and a mercury cadmium telluride (MCT) detector was used for the wavelengths longer than 2 ym.7 The back sides of the samples were polished by a bromine-methanol solution. The sample was sandwiched between a pair .of parallel aluminum plates and light was focused upon the cleaved sample edge.’ The light propa- gates in a total-internal-reflection mode between the pol- ished sample surfaces, and a polarizer was used to separate transverse electric (TE) and transverse optical (TM) po- larizations.

Unpolarized absorption spectra for samples A and B and polarization-resolved absorption spectra for sample A are shown in Figs. 2 (a) and 2 (b) , respectively. For sample A, strong absorption for TM polarized light was observed at 1.798 ,um (690 meV). For sample B, the TM signal was observed at 1.942 pm (638 meV). These absorptions are attributed to the intersubband absorptions. The TE signals were observed at 1.990 and 2.140 pm, respectively, and the splitting energy between TE and TM signals was 67 and 60 meV, respectively, for samples A and B. Similar splittings between TE and TM signals’ were observed for samples with wider wells, as indicated below. Additional peaks at around 1.2 ym for sample A and 1.35 pm for sample B may be due either to intersubband transitions from a con- fined subband level to virtual energy levels in the contin- uum or to impurity related transitions. Further study is needed to identify these signals.

Unpolarized spectra for samples C-E with wider well widths are shown in Fig. 3. The FTIR was used for these measurements. For these samples, the well experiences a compressive strain because the well composition is In,,Ga,,As. The TE and TM signals were identified using the polarizer. The energy differences between the TE and

1663 Appl. Phys. Lett. 63 (12), 20 September 1993 0003-6951/93/63(12)/1663/3/$6.00 @ 1993 American Institute of Physics 1663 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.248.155.225 On: Tue, 25 Nov 2014 02:41:52

Wavelenath (urn) AlAs --+

‘%.52A’o.48*s --, ‘no.53Ga.35A’o.,,As --,

Ino,,Ga.,,As - 100 nm -e+~+--+

Si ;xl cms3

(4

160

"0 10 20 30 40 50 60 70 Time (set)

@I

FIG. 1. (a) Band diagram for InGaAs/AlAs double quantum well struc- ture with 6 (10) monolayer well width. (b) RHEED oscillation signals taken during the growth of the second quantum well, barriers, and upper cladding layer of sample B.

TM signals are 70, 69, and 64 meV for samples C-E, re- spectively. The narrower wells produce wider separation between the TE and TM signals.

The well-width dependence of the intersubband

60

_;;50 6

+40

xl -- 1 1.2 1.4 1.6 1.8 2 2.2

Wavelength (pm)

(a)

tran-

80

70

q60 e t-50

40

30 1 1.2 1.4 1.6 1.8 2 2.2

Wavelength (pm)

W

FIG. 2. Absorption spectra for InGaAs/AlAs double quantum wells measured by conventional absorption measurement system. (a) Unpolar- ized spectra for samples A (well width is 6 monolayers) and B (well width is 10 monolayers). Vertical position for samples B is shifted for easy comparison. (b) Polarization-resolved spectra for sample A.

70 60 50

i- 30 20

IO 0 I I I I I I I

4500 4000 3500 3000 2500 2000 Wavenumber (cm-‘)

FIG. 3. Unpolarized spectra for samples with wider well widths. A FTIR was used for these measurements. Sample C (4.3 nm well width), sample D (5.0 nm well width), and sample E (6.6 nm well width). Vertical positions for samples D and E are shifted for easy comparison.

sition energies between levels 1 and 2 is plotted in Fig. 4. The transition ‘energies increase with decreasing well width. The solid lines show calculated transition energies for levels 1-2, 2-3, 1-3 (parity forbidden), and l-4.

Tensile strain in the AlAs layer results in a lowering of the I’ band minimum by 0.166 eV. Taking this into con- sideration, we assume the conduction-band discontinuity between Inc53Gaa.47A~ and AlAs to be 1.1 eV. The effec- tive mass envelope function approach is then used to cal- culate subband energy levels in a rectangular quantum well structure.5 Band nonparabolicity associated with the real wave vector in the conduction band of the well and with the imaginary wave vector in the gap of the barrier is taken into account. The imaginary part of a complex energy value ts, of the form Eomil?a/2, gives an energy broadening due to tunneling through the AlAs barrier. Using the re- lationship t=fi/I’,, it is found that the tunneling time is approximately 0.2 ps for a IO value of 3 meV for sample B. Polarization selection rules are also given in an envelope function approximation framework. The matrix element of

Monolayer 0 IO 20 30 40

0 2 4 6 8 10 12 Well Width (nm)

FIG. 4. Well-width dependence of intersubband transition energies. Solid lines show calculated values for intersubband transitions between levels 1-2, 2-3, 1-3 (parity forbidden), and 14. Both TE (triangle) and TM (circlej are plotted. Open (closed) triangIes and circles are for In,,,Ga,,,As (In,,,Ga,,,,As) wells. Data corresponding to different sam- ples are identified by the capital letters.

1664 Appl. Phys. Lett., Vol. 63, No. 12, 20 September 1993 Hirayama et a/. 1664 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.248.155.225 On: Tue, 25 Nov 2014 02:41:52

an intersubband transition is approximately given as9

Wfl qJl4a=wl q&ll(~,l(Pi~

+(~‘I&ll(~fl Vbi>, (1)

where VP is the interaction potential, +i and $f are the envelope functions of the initial and final states, and U’ and u are the cell periodic functions. For interband transitions, the second term becomes zero because the cell periodic functions in conduction and valence bands are orthogonal. For intersubband transitions, the first term becomes zero because of the fact that the Hamiltonian used to obtain $i and +f is Hermitian. Therefore, the matrix element is non- zero only for the TM polarized light in the case of inter- subband transitions.’ However, our observation of both TE and TM signals with energy splitting contradicts this sim- ple theory. Disagreement in selection rules and energy splittings between TE and TM signals have been explained by the Dzd tetragonal perturbation of the local crystal and strain field effects on the quantum well.” It is noted that sample F which had strain compensation layers showed a smaller splitting energy of 25 meV.

As shown in Fig. 4, the calculated energy values are not in good agreement with our experimental values. We have also not been able to account for the energy differ- ences between theory and experiments simply by modify- ing this theory to include band-bending or electron- electron exchange interactions. More sophisticated band calculations may be needed.

In conclusion, systematic study of intersubband ab- sorption in very narrow InGaAs/AlAs quantum wells has

produced the first reported absorption at wavelengths less than 2 pm. Both TE and TM optical polarizations are absorbed with an energy splitting between them of 67 meV. Work is continuing on trying to explain the observed dif- ference between calculated and experimental absorption energies.

This work was supported in part at MIT by the Joint Services Electronics Program under Contract DAAL 03- 92-C-0001 the National Science Foundation under Grant ECS-900-84-85 and the Army Research Office under Con- tract DAAL03-9 1 -G-005 1. The authors thank Professor R. Reif at MIT for offering us the use of his FTIR facility, T. P. E. Broekaert for preparing samples, and G. Lenz and P. Martin for helpful discussions.

IL. C. West and S. J. Eglash, Appl. Phys. Lett. 46, 1156 (1985). ‘B. F. Levine, K. K. Choi, C. G. Bethea, J. Walker, and R. J. Malik,

Appl. Phys. Lett. 50, 1092 (1987). “H. Schneider, F. Fuchs, B. Dischler, J. D. Ralston, and P. Koidl, Appl.

Phys. Lett. 58, 2234 (1991). 4B. F. Levine, A. Y. Cho, J. Walker, R. J. Malik, D. A. Kleinman, and

D. L. Sivco, Appl. Phys. Lett. 52, 1481 (1988). ‘H. Asai and Y. Kawamura, Phys. Rev. B 43, 4748 ( 1991). 6H. Asai and Y. Kawamura, in Proceedings of the Fourth InternationaI

Conference on InP and Related Materials (IEEE, New York, 1992), paper THF4, p. 493.

‘L. H. Peng, J. H. Smet, T. P. E. Broekaert, and C. G. Fonstad, Appl. Phys. Lett. 61, 2078 (1992).

‘Y. Hirayama, W.-Y. Choi, L. H. Peng, and C. G. Fonstad, J. Appl. Phys. 74, 570 ( 1993).

’ G. Bastard, Wave Mechanics Applied to Semiconductor Heterostructures (Halsted, New York, 1988).

“L H. Peng, J. H. Smet, T. P. E. Broekaert, and C. G. Fonstad, Appl. Phys. Lett. 62, 2413 (1993).

1665 Appl. Phys. Lett., Vol. 63, No. 12, 20 September 1993 Hirayama et a/. 1665 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.248.155.225 On: Tue, 25 Nov 2014 02:41:52