TEMPERATURE AND MAGNETIC FIELD INDUCED BANDSTRUCTURE REVERSAL IN GaSb/AlSb QUANTUM WELLS

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TEMPERATURE AND MAGNETIC FIELD INDUCED BANDSTRUCTURE REVERSAL IN

GaSb/AlSb QUANTUM WELLS

A. Forchel, U. Cebulla, G. Tränkle, W. Ossau, G. Griﬀiths, S. Subbanna, H.

Kroemer

To cite this version:

A. Forchel, U. Cebulla, G. Tränkle, W. Ossau, G. Griﬀiths, et al.. TEMPERATURE AND MAG- NETIC FIELD INDUCED BANDSTRUCTURE REVERSAL IN GaSb/AlSb QUANTUM WELLS.

Journal de Physique Colloques, 1987, 48 (C5), pp.C5-159-C5-162. �10.1051/jphyscol:1987531�. �jpa-

00226735�

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JOURNAL D E PHYSIQUE

Colloque C5, supplement a u noll, Tome 48, novembre 1987

TEMPERATURE AND MAGNETIC FIELD INDUCED BANDSTRUCTURE REVERSAL IN GaSb/AlSb QUANTUM WELLS

A. FORCHEL, U. CEBULLA, G. TRANKLE, W. OSSAU* , G. GRIFFITHS" ( , S. SUBBANNA*' and H. KROEMER**

4. ~hysikalisches Institut, Universitat Stuttgart, Pfaffenwaldring 57, 0-7000 Stuttgart 80, F.R.G.

* ~ h ~ s i k a l i s c h e s Institut, Universitiit Wurzburg, Rentgenring 8 , D-8700 Wiirzburg, F.R.G.

Dep. of Elec. and Cornp. Engineering, University of California, Santa Barbara, CA 93106, U.S.A.

We have investigated temperature and magnetic field induced changes in the occupation of the

r-

and Gpoint minima of the conduction band of GaSb/AlSb quantum wells in the vicinity of the size induced cross-over from direct to indirect bandstructure by luminescence spectroscopy. For indirect gap samples with sufficiently small L-Tenergy splitting a change to effectively direct gap behaviour is observed as the temperature is increased. Direct gap quantum wells with well widths slightly larger than necessary for the cross-over can be converted to indirect gap systems by using the different energy shifts of r-and Lpoint in magnetic fields.

GaSb/AlSb quantum wells show a quantization induced transition from direct to indirect band structure as the well width is reduced below 3.8 nm [I]. The size induced bandstructure change occurs firstly, because the energetic.splitting between the lowest conduction band minimum at the l'-point and the minimum at the L-point is small in bulk GaSb (about 80 meV [2]). Secondly the electron mass at the r-point is smaller by about a factor of 10 than the relevant mass for quantization at the Lpoint [3] and for decreasing well width the different subband level shifts eventually lead to quantum wells in which the lowest minimum of the 2D conduction band is located at the L-point. Because the lowest subband edge of the valence band remains at the r-point, samples with well widths below 3.8nm have indirect bandstructure. As expected e.g.

the carrier lifetime and the quantum efficiency change drastically at the cross-over [I]. GaSb/AlSb have been studied previously predominantly in order to investigate stimulated emission processes 141 and stress related effects ("mass reversal" [5]).

In the present paper we have studied possibilities to reverse the bandstructure from the type observed in low temperature luminescence spectroscopy. This bandstructure dependence on externally controlable parameters is interesting, because it allows to widely vary the physical properties of a quantum well after the epitaxy has been completed. We have used temperature and magnetic field to effectively reverse the bandstructure compared to the one determined at 2K and B=O. The thermal population of the r-point minimum in samples with L <3.5nm (indirect gap samples) is shown to lead to effectively direct gap

z

behaviour. Magnetic fields, on the other hand, can be used to induce indirect bandstructure in the case of the 3.8nm sample, which has a direct band gap at B=O.

For the experiments MBE-grown GaSb/AISb quantum wells [3J with well widths 1.2nm<LZ< lOnm were mounted in a variable temperature dewar (1.8K<T<350K) and excited by a n Ar-ion laser (A =514nm).

( l ) ~ o w a t c . s . I . R . o . . ~ i v . of Radio Phyaico. Epping, NSW 2121. A u s t r a l i a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987531

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C5-I60 JOURNAL DE PHYSIQUE

The luminescence was dispersed by a lm monochromator and detected by a cooled Ge detector. For the study of the magnetic field dependence a cryostat was used which allowed the application of B<9T perpendicular to the wells at variable temperature.

Fig. 1 shows a series of spectra recorded at different temperatures in a sample with indirect band gap (Lz=2.5nm). At 5K the emission of deep impurities (centered at 0.9eV) dominates the spectrum and the indirect (at 1.05eV) and direct gap emissions (maximum at 1.leV) are barely visible. With increasing temperature the spectral shape changes drastically: The L-point related emission vanishes at about 30K whereas the impurity band persists up to about 150K.

The intensity of the direct gap emission, in contrast, shows a strong increase up to 300K after a slight decrease for T<60K.

GaSb/AISb 2.5nm sample

\ T-120K

1.0 1.1 1.2 1.3 1.4 energy (eV)

A similar behaviour is observed for all

&J

Emission spectra of a GaSb/AISb indirect gap samples in the well width multiquantum well sample (LZ = 2.5nm) as a range between 3.7nm and 2.5nm. Samples function of temperature.

with smaller well width show no measu- rable temperature induced increase of the r-point emission intensity.

All direct gap samples show a strikingly different behaviour: Fig. 2 compares the temperature dependence of the Fpoint related emission in a direct gap sample (Lz=5.8nm) with the intensity variation of the

indirect gap (Lz=2.5nm) sample. At low

GoSb/AISb temperature the emission of the direct gap sample is

direct gap sample

L, = 5.8nm stronger by more than two orders of magnitude compared to the sample with an indirect band gap. For

loo

0

As a function of well width we observe for samples

with LZ> 2.5nm at room temperature

r

-point 300 transitionintensitieswhichareapproximately inde- temperature (K) pendent of well width. For smaller well widths down

.- g

^10' a

,

⁸

v

-

.- ; I v

^v ^v^vindirect gap sample ^•^v L, = 2 . 5 ~ 1

Temperature dependence of the r-point to 1.2nm no r-point related emission is observed in transition intensity in samples with direct and the high temperature spectra at all. This contrasts

indirect bandstructure. our results

increasing temperature, however, the simultaneous decrease of the emission of the direct gap sample and the pronounced increase of the r-emission in the indirect sample leads to comparable emission intensities from both samples above about 200K.

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obtained at 2K, where a strong decreases of the luminescence efficiency and a large increase of the carrier lifetime are observed for Lz<3.8nm. In conjunction with the well width dependence of the L a n d r-point related transition energies this is attributed to the size induced change from direct to indirect band structure in the quantum wells.

Low and high temperature results can be understood consistently if we consider the thermal activation of electrons from the L-minima to the I'-minimum of the conduction band. Due to their long lifetime of about loons the L-minima may be regarded as electron reservoirs for the r-point: Depending on the ratio of the r-L-splitting [6] and the thermal energy, electrons can be thermally activated into the I'-minimum where they have a very short radiative lifetime (about 0.5ns) and recombine. At high temperatures quantum wells with sufficiently small r-L-splitting will therefore display the same features as direct gap samples, although the lowest conduction subband is at the L-point.

We have analyzed the temperature dependence of the r-point transition intensities in the indirect gap samples in order to determine the energetic difference between L- and r-point minimum of the conduction band using, a simple Arrhenius model. We obtain well width dependend energy splittings of 24meV, 35meV, and 70meV, which agree qualitatively with the spectroscopic splittingvalues of 35meV, 40meV and 75meV for the 2.5nm, 3.0nm and 3.3nm samples investigated. The thermodynamically determined splittings always are slightly smaller than the spectroscopic values. This may be. related to the nonlinear temperature dependence of the L-r-splitting (in bulk GaSb/AlSb the L-rsplitting increases by about 20 meV between 2K and room temperature [2]).

Using the thermal activation it is possible to effectively induce direct bandstructure properties in samples with indirect bandstructure. By application of a magnetic field perpendicular to the wells on the other hand we are able to change the 3.8nm quantum well situated very closely to the cross-over from direct to indirect band structure. In this sample the r - L splitting at zero field amounts 40 less than 5meV which is smaller than the magnetic field induced energy shifts of the GaSb/ AlSb quantum well emission for BS9T. Because the Landau level shifts are inversly proportional to the carrier mass one expects for sufficiently high fields the conduction band minimum with the lowest energy to be at the L-point.

Fig. 3 displays the magnetic field dependence of GoSb L, = 7.8nrn the I' -point transition intensity for a sample with

800 Lz=7.8nm (dots) in comparison to data recorded

GaSb C, =

from the 3.8nm quantum well (triangles). The 7.8nm sample has a

r

-L splitting of about 70meV and shows for B>2T a monotoneous increase of the emission intensity. The increase is most likely due to changes in the density of states at the band edge and the decreasing influence of interface recombination at higher fields. The 3.8nm sample in contrast shows

0 0

a monotoneous decrease of the emission intensity for 0 2.5 5.0 7.5

,

^O.C increasing magnetic field.

magnetic field B(T) Whether the decrease of the Ftransition intensity Magnetic field dependence of the

r

in the 3.8nm sample is related to a magnetic field -transition intensity in GaSb/NSb samples with induced change from direct to indirect bandstructure

Lz=3.8nm and 7.8nm. can be tested by using the

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C5-162 J O U R N A L DE PHYSIQUE

References

intensity-field relationships at different 1.2,

[I] A. Forchel, U. CebuUa, G. Trankle, H. Kroemer, S. Subbanna, G. Griffiths Surf. Sci. 174,143 (1986).

A. Forchel, U. Cebulla, G. Trankle, E. Lach, T.L. Reinecke, H.Kroemer, S. Subbanna, G. Griffiths, Phys. Rev. Lett. 57,3217 (1986).

[2] Landolt Bornstein, Group 111 Vol. 17, Subvol. a (1982).

[3] G. Griffiths, G. Mohammed, S. Subbanna, H. Kroemer, J. Merz, Appl. Phys. Let.

43,

1059 (1983).

[4] M. Naganuma, Y. Suzuki, H. Okamoto, Inst. Phys. Conf. Ser.

63,

125 (1981).

[5] P. Voisin, C. Delalande, G. Bastard, M. Voos, L.L. Chang, A. Segmuller, C. A. Chang and L. Esaki, Superlattices and Microstructures

1,

155 (1985).

[6] Here the abreviations

"r

-L-splitting" and "L-T -splittingn are used in a way which gives a positive energy difference.

temperatures. Because the

r

-L splitting increases for increasing temperature [2] one

expects to observe a direct gap behaviour

-

_1.0-0

(intensity increase with B) up to higher and

higher fields as the temperature is raised. Fig. 4

- :

>r

shows the experimental data for the _0.8

-

intensitylfield dependence in the 3.8nm sample g for 1.8K (circles), 34K (triangles), and 72K

-

c (squares).

0.6

-

As expected the 34K data start to display a weak increase for B < 4 T which changes into a

m

v t

z

^t 72K

l

l t

GoSb/AISb MQW

'

b 4 . 8 nm _l w 34K

l

1.8K

decrease at higher fields. The data recorded at ₀ ₂ ₄^I ₆ ₈ ₁₀

72K show an intensity increase up to even B(T)

higher fields. In contrast the 7.8nm sample

displays basically a monotoneous increase of Variation of the quantum well emission the emission intensity as the magnetic field is intensity versus B for LZ=3.8nm at different raised (for higher temperatures due to temperatures.

scattering of the carriers in the cyclotron orbits the slope gradually decreases). We therefore conclude that the intensity field dependence

in the 3.8nm sample is due to a magnetic field induced change from direct to indirect bandstructure.

In conclusion we have reported on temperature and magnetic field dependent luminescence studies on GaSbIAISb quantum wells in samples with a small energetic difference between the

I'-

and L-point of the conduction band. Our results demonstrate that by suitable selection of the experimental conditions the quantization induced bandstructure can be effectively reversed compared to the one determined at low temperature and zero magnetic field.

W e acknowledge stimulating discussions with M.H. Pilkuhn. The financial support of the Deutsche Forschungsgemeinschaft (Pi 71/20) and the Office of Naval Research is gratefully appreciated.