INTERFACE RELATED EMISSION FROM AN MBE-GROWN (Al, Ga) As HETEROSTRUCTURE

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INTERFACE RELATED EMISSION FROM AN MBE-GROWN (Al, Ga) As HETEROSTRUCTURE

L. Molenkamp, G.W. T Hooft, W. van der Poel, C. Foxon

To cite this version:

L. Molenkamp, G.W. T Hooft, W. van der Poel, C. Foxon. INTERFACE RELATED EMISSION

FROM AN MBE-GROWN (Al, Ga) As HETEROSTRUCTURE. Journal de Physique Colloques,

1987, 48 (C5), pp.C5-127-C5-130. �10.1051/jphyscol:1987523�. �jpa-00226727�

INTERFACE RELATED EMISSION FROM AN MBE-GROWN (A1,Ga)As HETEROSTRUCTURE

L.W. MOLENKAMP, G.W. 't HOOFT, W.A.J.A. van der POEL and C. T. FOXON*

Philips Research Laboratories, NL-5600 J A Eindhoven, The Netherlands

'philips Research Laboratories, GB-Redhill RHI 5HA, Surrey, Great-Britain

Sommaire. On rapporte I'observation d'un nouveau ditail spectral dans la luminescence d'une hittrostructure ultrapure de GaAs- Ab,,,G~,,As crue par EJM. Cette &mission, centree a 1,510 eV, disparait en dissolvant la couche supirieure de Ab,,,Ga,,,As

.

Ceci indique un rdle important de I'htttrointerface dans la mecanisme de recombination qui cause ce ditail nouveau. De ce point de w e , on discute I'effet de quelques paramltres experimentaux aux proprietts luminescantes.

Abstract. We report the observation of a novel feature in the luminescence of an ultra-pure MBE-grown GaAs- A1,,,GaO,,,As heterostructure.This emission, centered at 1.510 eV, disappears on removing the top cladding A&,,GG,~,AS layer, pointing t o an important role of the heterointerface in the recombination mechanism leading t o this novel band. From this perspec- tive, we discuss the effect of several experimental parameters on the luminescent properties.

The first publication of Kiinzel and Ploog [I] on near-band-gap details in the luminescence of MBE-grown GaAs triggered a strong research activity in this field (see references in [2]). A large number of sharp lines, up to 50, have been detected [3] and they are attributed to defect

-

bound excitons and defect

-

complex acceptors [3,4]. The strong polarization of some of the lines suggests a preferential orientation of these defect pairs. All of the literature cited above deals with single epitaxial GaAs layers. Here we contribute to the discussion concerning the spectral region in question by introducing a novel luminescence band, centered a t 1.509 -1.510 eV. As shown below, this spectral feature originates from the interface of the GaAs active layer and the A1, ,,Ga,,,As top cladding layer.

The sample was grown in a Varian GEN2 molecular beam apparatus at a substrate temper- ature of 63'0°C on an undoped (001) GaAs substrate. For details we refer to 151. To separate the active GaAs layer from the substrate, a 3 pm thick A1, ,Ga,,As layer was grown followed by a 50 period superlattice of 28A GaAs and

IOOA

A1, ,Ga,,As. The superlattice was introduced to improve the quality of the GaAs

-

A1, ,Ga,,As interface. The GaAs layer was 1.5 pm thick and capped with 300A Ab,,Ga,,,As. All the layers are nominally undoped , i.e. the background impurity concentrations are limited to at most 2 x l o u 4 cm-3 in GaAs.

The optical measurements were performed using a dye laser synchronously pumped by a krypton laser. The sample was placed in a temperature-variable optical cryostat. The luminescence was dispersed through a 0.75 m monochromator and detected by a cooled photomultiplier with a GaAs cathode. Photoluminescence decay times were measured using single-photon-counting equipment (time resolution 0.3 ns).

Some 1.6 K photoluminescence spectra are given in Fig. 1, as a function of power density.

At 2.5 W/cm2 incident power (the dash-dotted line), we observe the following features, that are well known for bulk GaAs : line emission from free excitons in the 2s excited state at 1.518 eV.

from free excitons in the i s ground state at '1.5151 eV, excitons bound to neutral donors at 1.5141 eV and excitons bound to neutral acceptors at 1.5124 eV. Additionally, we observe

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987523

JOURNAL DE PHYSIQUE

FIG. I. Phoroluntinescence specrrunt at 1.6 K of our sample for several power densities ( solid line : 100 W/cm2 : dotted line :

25 W/cm2 : rhs11ed linr : 10 W/cm2 arrrl dasl~ed - dotted line : 2.5 W/cm2 1.

The excirarion wavelength is 790 nm.

a relatively broad ( ca. 2 meV), dominant band centered a t 1.509 eV. We found this previously unreported feature in several MBE-grown double heterostructure samples. Note that the photon energy of o u r dye laser is too low t o create band

-

band excitations other than in GaAs. Since, furthermore, single G a A s layers d o not show the new band, it is plausible that this band is as- sociated with the interface of the heterostructure. Therefore, we have removed the Al,33Ga, ,,As top layer by chemical etching. The low

-

temperature photoluminescence spectrum of the uncapped G a A s layer is given in Fig. 2 (excitation density 4.0 W/cm2 ; lower intensities give the same features as in this figure ^). The defect band a t 1 .SO9 eV is now completely absent.

At 1.51 1 eV and 1.505 eV two small defect lines remain, the g and v lines of Kiinzel and Ploog [I]. The free exciton peak has become the dominant recombination h e . Its full width a t half maximum is approximately 0.6 meV. This clearly shows that our G a A s has a very low concentration of defects and that the defect band of Figs. 1 originates from the GaAs

-

Al,,,jGa,,,As interface. T h e large intensity of this band must be explained in terms of a long carrier diffusion length in the ultra pure 1.5 pm G a A s layer, which allows the carriers to diffuse to the levels of lowest energy, in this case near the interface.

Now that the novel emission is identified a s originating from the interface, we can proceed

FIG. 2. Pltotolumine.~cence spectrum or 1.7 K of the some GaAs sarnple as in Figs. I after renloving rhe lop AI, ,,Ga, ,,As dadding Iuj.er. T l ~ e e.rcitoriorr den- sitj is 4 W/cm' or 779 nnr.

P.L.

t

eV -E

1 1 1 1 1 1 1 1 1 1 1 1 r r l l i l b li

I ^x

seems a natural explanation for the line. Fur- thermore. the band lies in the same s ~ e c t r a l range a s ' t h e defect

-

bound exciton lines re- ported previously [1,3].

The luminescence band we have observed has some structure, but is not composed of dis-

h tinct sharp lines as the "Kiinzel and Ploog" lines are. Moreover, in our case n o polarization of the luminescence could be detected. Fig. 3

-

shows that the integrated intensity of the defect band varies almost linearly (slope 0.9) with excitation density, indicating that this is the main recombination path. From Fig. 1 we find that the peak position of the defect band shifts with power density with a rate of ca.

0.5 meV/decade t o t o 1.5100 eV a t a high (100 W/cm2) incident level. F o r defect

-

pair recombination the Coulomb interaction can never exceed the binding energy o f the shallower impurity. Assuming that the feature is related t o a defect pair, the small shift implies that a t

0. least one of the defects is only a few meV deep.

1 10 100 T h e temperature behavior of the

Laser Power (WIcm2 ) luminescence of o u r sample is plotted in Fig. 4.

Below 4 K the intensity o f the new band re- FIG. 3. The integrated intensi?v of the mains almost constant, while between 4 K and free-exciton line (circles) and the 11 K it decreases with a n activation energy of interface band (crosses) in the sample a t least 2.9 meV. This finding is again in ac- of Fig. I, plotted vs. laser power. cordance with a defect

-

pair

=he ,,.a,,elengr,, is again recombination [6], the shallower defect being at least 2.9 meV deep. Time- and frequency 790 nm and the 'e'nperature is K . resolved luminescence data (not shown) reveal that, going from the high (1.510 eV) t o the low energy tail (1.507 eV) o f the band the lifetime increases from 1.1 ns t o 2.1 ns. This observation is again in accordance with the defect-pair recombination model. which predicts a decrease in both radiative recombination rate and re- combination energy with increasing pair separation (less interaction between pair members).

In our opinion, there are two viable candidates for defect pairs associated with interface luminescence. In analogy with the assignment of the g

-

v lines [3,4] o u r defect band could be attributed t o excitons bound t o a n acceptor pair, where one of the acceptors is located in the interface region of the heterojunction. I n favor of this explanation is the agreement in measured lifetimes (1-3 ns) in both cases [4,7]. However, the band we have observed, is not composed of a series of sharp lines and is not polarized. Possibly, the interface allows defect pairs t o develop in less restricted directions. An alternative explanation is in terms of donor

-

acceptor pairs. The defect band has a low energy edge which is only 12 meV smaller than the band gap of GaAs.

D o n o r levels being 6 meV deep, there remains only 6 meV for the depth of the acceptor level.

Such shallow acceptors can be envisaged by locating them in the AI,,,Ga,,,As layer with such a depth that they are almost resonant with the valence band of GaAs. We have not investigated whether such deep acceptor levels are present in our (A1,Ga)As. since the layer is only 300

A

thick and all photo-excited free carriers would diffuse into the GaAs.

Some other assignments for the defect band might be proposed. The extra luminescence could be due t o excitons trapped in the electric field at the heterojunction. However, the electric field needed to shift the free exciton line over 6 meV t o 1.5P9 eV is approximately 5 x l o 4 V/cm 183 , much larger than the lo4 V/cm needed [9] t o dissociate the free exciton in bulk GaAs. An electric-field-induced bound-exciton line is thus highly improbable.

Recently. Altukhov et al. [lo] have reported on luminescence originating from electron

-

hole pair rrtcombina[ior) in a metal - oxidt - semiconductor structure based on silicon. Here a two-

JOURNAL DE PHYSIQUE

dimensional state is formed by carriers a t a surface charge layer. However, this so-called S-band shifts with excitation density [lo] in opposite direction compared to our case, pointing to a dif- ferent physical origin. Also, the background impurity concentration in the 300A (A1,Ga)As is too low ( < 1016 cm-9 to give an appreciable accumulation layer in GaAs. Therefore, it is highly unlikely that our impurity band would be related to luminescence generated by an e

-

h plasma near an.accumulation layer. In GaAs

-

(A1,Ga)As heterojunctions grown with liquid phase epitaxy and having a p

-

doped top cladding layer, Yuan et al. [l 1 ] observed such a band

.

This H

-

band is very broad (9 meV), has a much higher binding energy, and shifts much more with excitation density than our defect band. In view of these differences in sample structures and experimental observations it is improbable that our defect band and the H

-

band of Yuan et al.

correspond to the same recombination mechanism. Note that the blue shift of the H

-

band with excitation [ I I ] can only occur owing to the removal of the depletion layer in the (A1,Ga)As cladding layer caused by the excitation with an argon laser. In our case only the active layer is excited and for luminescence origination from an accumulation layer a red shift is expected [lo]. in contrast to our observations.

In summary, we have observed a defect line attributed to luminescence generated at the GaAs

-

A1,33,Ga,,,,As interface. A possible explanation is donor

-

acceptor pair recombination with the impurities positioned on either side of the interface. An assignment on the basis of excitons bound to defect pairs with one member situated at the interface, however, is equally adequate.

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References.

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H. Kiinzel and K. Ploog, Appl. Phys. Lett. 37, 416 (1980).

G.W. 't Hooft, W.A.J.A. van der Poel, L.W. Molenkamp and C.T. Foxon, Appl. Phys. Lett.

50, 1388 (1987)..

'

M.S. Skolnick, C.W. Tu, and T.D. Harris, Phys. Rev. B33, 8468 (1986).

'

L. Eaves, M.S. Scolnick and D.P. Halliday, J. Phys. C19. L445 (1986).

C.T. Foxon and J.J. Harris, Philips J. Res. 41, 313 (1986).

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'

D.P. Halliday. L. Eaves and P. Dawson, Proc. 13-lh In!. Cot$ ott Defects itt Semicotld.

(Coronudo, 1984). eds. L.C. Kimerling and J.M. Parsey, Jr. (The Metallurgical Society of AIME). p. 1005.

D.A.B. Miller. D.S. Chemla. T.C. Damen, A.C. Gossard. W. Wiegmann, T.H. Wood. and C.A. Burrus. Phys. Rev. B32, 1043 (1985).

Q.H.F. Vrehen. Phys. Rev. 145, 675 (1966).

lo P.D. Altukhov, A.V. Ivanov, Ya.N. Lomasov, and A.A. Rogachev, Sov. Phys. Solid State 27, I016 (1985) [Fiz. Tverd. Tela 27, 1690 (1985)l .

" Y . R . Yuan. M.A.A. Pudenski. G.A. Vawtcr. and J.L. Merz. J.Appl.Phys. 58. 397 (1985).

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Inverse Temperature (K-')

FIG. 4 . Temperature dependence of the inregrated intensitj- of the interface band. The straight line represettts an actir*ation energy of 2.9 meV.