ABSORPTION MODULATION IN FIELD EFFECT QUANTUM WELL STRUCTURES

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ABSORPTION MODULATION IN FIELD EFFECT QUANTUM WELL STRUCTURES

D. Chemla

To cite this version:

D. Chemla. ABSORPTION MODULATION IN FIELD EFFECT QUANTUM WELL STRUCTURES. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-393-C5-397.

�10.1051/jphyscol:1987584�. �jpa-00226788�

ABSORPTION MODULATION IN FIELD EFFECT QUANTUM WELL STRUCTURES

D. S. C H E M L A

AT and T Bell Laboratories, Holmdel, NJ 07733, U.S.A.

Resume

Nous presentons des etudes de la modulation d'absorption dans des puits quantiques InGaAs/InA1As a modulation de dopage, utilises comme canal conducteur de transistors a effet de champs. Par application d'un potentiel aux electrodes, la concentration electronique peut etre variee continuement entre N=O et N=: 6.5~10" . Cet effet donne d'irnportantes informztions in situ sur le gas bidimensionnel d'electrons. D e plus l'elirnination complete de l'absorption autour des resonances n,= 1 a des applications interessantes en interconnection optique de l'electronique a base de semiconducteurs 111-V et en optoelectronique.

Abstract

We present investigations of modulation of the absorption in InGaAs/In& modulation doped single quantum well structures used as conducting channel in field effect transistors. The electron concentration can be tuned continuously from N - 0 to N=: 6.5~10" cm " using the gate voltage.

The effects give in situ information on the 2D-electron gas. Furthermore the total quenching of the absorption seen at the n, = 1 edge has potential applications to optical interconnects for 111-V electronics and to lightwave optoelectronic devices.

Large changes in absorption in quantum wells (QW) have previously been successfully induced in semiconductor QW by photocarrier absorption bleaching and by quantum confined Stark effect (QCSE)~. In the first case the saturation of the excitonic absorption is accompanied by a large band gap renormalization so that a significant band to band absorption edge replaces the exciton resonances when these have completely disappeared l . In the case of the QCSE, the reduction in the wavefunction overlap reduces the height of the exciton peak as it shifts 2. Thus in both cases, it is difficult to extinguish the excitonic absorption completely.

In this talk we present a new mechanism for modulation of the absorption in QW structures which appears to produce a complete quenching of the excitonic and above-band-gap absorption over a wide spectral range ( > S(kneV) . This effect is observed in modulation-doped (MD) field effect QW-structures by driving electrically the electrons (or holes) in and out of MD-QWs. When carriers are introduced in a MD-QW they fill the 2D-subbands up to the Fermi energy EF , hence blocking the optical transitions to the states below this energy. This is at the origin of the blue shift of the onset of the absorption with respect to the luminescence seen in MD-QW =. It was not realized until recently that it is very easy to control electrically the quenching of the absorption by the carriers in MD-QW using field effect gating, in fact such a control is commonly used to switch 2D-electron gas field effect transistors (MOD-FET) *. In addition to the blocking of transitions brought about at the lowest optical gap, the introduction of carriers in MD-QWs also produce band gap renormalization (BGR) "' and is accompanied by elect1 xtatic fields in the QW

. Both of these processes affect the electronic levels and produce other changes in the optical absorption spectrum. Therefore field effect modulation of absorption in MD-QW structures has potential application to optical readout of MOD-FET and direct lightwave modulation and it can also be used to study in situ two dimensional electron gases in these heterostructures '.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987584

C5-394 JOURNAL DE PHYSIQUE

The structure we have investigated are recessed gate InGaAs/InAlAs, MOD-FETs. T ~ P , epilayer structures is shown in Fig. (la), it consists of a contact 2 0 0 A n + - ~ n ~ ~ ~ ~ ~ a ~ . ~ ~ ~ s cap layer on three In0.52A10.48A~ layers, 140 A , 250 A and 20 A thick respectively, the central one being doped with

Si = 1.2x10'~cm they cover a 110A 1n o . 5 3 ~ a o . 4 7 ~ s QW it self on a last 3000A 1n o . 5 2 ~ ~ o . 4 ~ ~ undoped layer. The whole structure was grown by molecular beam epitaxy on an InP substrate.

This material system was chosen because the InP substrate and the InAlAs layers are transparent around the gap of the InGaAs-QW, which itself is in the region of maximum transmission of optical fibers. Furthermore it has been demonstrated that InGaAs/InAUs/InP heterostructures can be used to fabricate high quality electronic devices lo.

SOURCE GATE DRAIN

2 0 0

A

140

d

2 5 0

A

AllnAS 2 0

A

r o o

PI MOO^

Figure la- Schematic of the InGaAs/InAlAs MOD-FETs, showing the epilayer structure.

The MOD-FETs were processed by conventional photolithography lift off techniques. The source and drain are ohmic contacts formed by alloying evaporated AuGe/Au. The Schottky gate consists of a 1.6pnx100pn active region centered in a 5pn gap between the source and the drain and extending to one bonding pad on one side and a 100pnx100p~~ active area for optical probing on the other '. The area covered by this pattern was recessed 230A by slow chemical etching.

Example of the source-drain current-voltage characteristics at 300K are presented in Fig. (lb), they show very nice behavior without looping or hysteresis, the peak transconductance was measured to be 116mS /mm .

30

20

-

-

_*

-

10

0 0 0 0.5 1

IS

v,,

Figure lb- Drain-source current/voltage characteristics of the MOD-FETs.

temperature controlled between TL 10K and 3 W . The changes in transmission of the channel were measured, as function of the wavelength and for various modulation of the gate-to-source voltage AVP, through the substrate from underneath and reflecting off the Schottky-gate. The measured signal AT is the change in transmission of the probe beam after two passes through the InGaAs-QW. In the small signal regime ATIT - ^{%aLZ , where L,} is the QW thickness and Aa = do) - AN) is the difference between the absorption coefficient of the empty QW and that of the QW containing N electrons.

An example of the room temperature differential absorption spectra; Aa(X) is shown in Fig. (2) for AV, =2V (-0.5 V-+ + 1.5 V) at a fixed 0 V source-drain voltage. A large ,change of absorption is seen at the n, = 1 resonance. Its lineshape is indicative of a reduction of the absorption over a wide energy range and the amplitude AT-2% corresponds to an almost total quenching of the absorption i.e. ~ a = 1 0 ~ c r n - ' . ^{At the} n,=2 resonance the differential spectrum has a more complicated lineshape that is more indicative of a shift of the exciton. These results already prove that large bandwidth and deep modulation of the absorption is easily realized in these structures even at room temperature.

Figure 2- Difference-absorption spectra, at a lattice temperature TL =300K, showing respectively the quenching and shift of the n, ⁼ 1 & 2 exciton resonances.

However, in order to characterize more precisely the changes that occur upon the introduction of electrons in the QW it is better to cool the samples. A set of differential spectra Aa(Ew) at TL = 10K for modulation of the density between N = 0 and N "1.3~10'~ -, 6.4x10"cm" are shown in Fig. 3.

At the n, = 1 edge Aa is positive and increases with N until the heavy hole (hh) and later the light hole (lh) exciton peaks appear clearly. As mentioned in the previous paragraph, this behavior corresponds to the progressive quenching of the absorption as the electrons fill up the bottom of the first conduction subband up to the point whkre the full absorption of the empty QW is totally recovered. For the largest values of N the height of the hh and lh exciton peaks changes no more but the width of the quenched region continues to increase. The fitting the Aa high energy tail above the resonances to a Fermi distribution determines accurately the eicctron temperature T,.

At room temperature T, is constant and always equal to TL, Conversely at low temperature the fit

evidences significant heating of the 2D electron gas well above TL. This carrier heating is in good

C5-396 JOURNAL DE PHYSIQUE

agreement with measured leakage current across the Schottky diode (especially important when the conditions for direct bias are approached) and the energy loss rate per carrier in our QW- material ". The fit also determines the photon energy corresponding to the transition at the Fermi energy; EwF =E ,,(N) + (l+m,/mh)EF. In this relation E ll(N) is the energy gap between the n, = 1 conduction and valence subbands. If this quantity was known then one could deduce the electron concentration N fromEwF, this point will be discussed further below.

PHOTON ENERGY (eV)

1 1.2

Figure 3- Differential absorption spectra at a lattice temperature TL-1OK for electron density in the quantum well increasing from N - 1.3~10'~cm-~ to N" 6.4~10"cm-~.

At the n, = 2 and 3 transitions the signal has a "differential" lineshape showing positive and negative parts indicative of shift and broadening (curve 1,2 & 3). However, as N increases negative low energy shoulders develop clearly (last 2 curves). We explain this lineshape as follows.

First we have performed self-consistent calculations of the band bending inside the well due both to the applied voltage and to the presence of electrons. This calculation determines the electrostatic shifts of the single particle states. To this shift is superimposed a broadening that was measured by photocurrent spectroscopy at room temperature around these resonances. Adding the two corrections explains quautitatively lineshape close to the n, = 2 and 3 transition as due to a red shift and a broadening of the "allowed An, =O transitions and the appearance of "forbidden"

transitions An,= 2 or 3 which correspond to the low energy shoulders. It seems that no BGR correction is necessary in this region of the spectrum.

On the contrary at the fundamental gap the many body effects are important

The correction

for the BGR was performed using the functional form N ' / ~ which has been shown to describe the

effect correctly in quasi-2D systems 12*13. Using this procedure it is possible to deduced EF from

EwF and thus determine the electron concentration in the QW at low and high temperature and

for all AVP. At room temperature this quantity can be deduced from C(V) measurements only in

a small range of AVg, where the technique is reliable. For these conditions we have found very

good agreement between the optical and the electrical determinations. This is most likely due to

the fact that the BGR correction is small compared to EF and it gives us confidence that the low

temperature optical measurements also are accurate.

of a single QW caused by the progressive introduction of a 2D electron gas. We observe quenching of the absorption at the fundamental gap whereas the higher ones shift and broaden and new transitions become apparent. We show that optical techniques are able to very simply determine the density and temperature of the 2D gas in a single QW within field effect gated structures.

Further more the effects are large enough to have important applications for light wave communication optoelectronic devices and for optical interconnects in 111-V electronics.

Acknowledgements: This work was performed in close collaboration with I. Bar-Joseph, J.M. Kuo, C. Klingshirn, G. Livescu, DAB. Miller, and T.Y. Chang.

REFERENCES

D.S. Chemla, D.A.B. Miller, J. Opt. Soc. Am. B2, 1155 (1985).

DAB. Miller, D.S. Chernla, T.C. Damen, AC. Gossard, W. Weigmann, T.H. Wood, C.A.

Burrus, Phys. Rev. B32,1043 (1985).

A Pinczuk, J. Shah, R.C. Miller, kc. Gossard, W. Weigmann, Solid State Comm. 50, 735 (1984).

R. Sooryakumar, D.S. Chemla, A Pinczuk, AC. Gossard W. Weigmann, L J. Sham, Solid State Comm. 54,859 (1985).

M.H. Meynadier, J. Orgonasi, J.A Bmm, G.Bastard, M. Voos, G. Weimann, W. Schlapp, Phys. Rev. B34,2482 (1986).

C. Delalande, J. Orgonasi, M.H. Meynadier, J.A. B N ~ , G.Bastard, G. Weimann, W.

Schlapp, Sol. State Commun. 59,613 (1986).

D.S. Chemla, I. Bar-Joseph, C. Klingshirn, DAB. Miller, J.M. Kuo, T.Y. Chang, Appl.

Phys. Lett. 50,585 (1987).

T.J. Drummond, W.T. Masselink,'H. Morkoc Proc. IEEE, 74,773 (1986).

I. Bar-Joseph, J.M. Kuo, C. Klingshirn, G. Livescu, D.A.B. Miller, T.Y. Chang, D.S.

Chemla submitted to Phys. Rev. Lett.

J.M. Kuo, B. Lavevic, T.Y. Chang, Technical Digest of the International Electron Device Meeting (IEDM), Los Angeles, Cal Dec 1986, Proc. 1986, pp. 460-463.

J. Shah, IEEE J. Qumt. Elect~on. QE-22, 1728 (1986).

S. Schmitt-Rink, C. Ell, J. b i n . 30,585 (1985).

D.A. Kleinmann, R.C. Miller, Phys. Rev. B32,2266 (1985).