GaAs strained quantum wells as a function of doping density

Superlattices and Microstructures, Vol. 9, No. 1, 1991

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THlgRMAL INTERDID'USION IN I a G a A ~ G t ~ STRAINED QUANTUM WELLS AS A FUNCTION OF DOPING DENSITY W.P. Gillin, K.P. Homewcod', L.K, Howard and M.T. Emeny" Department of Electronic and Electrical Engineering University of Surrey, Guildford, Surrey, GU2 5XH, UIC •Strained Layer Structures Group "Royal Signals and Radar Establishment, St. Andrews Road, Malvern, UK (Received 13 August 1990 )

We have studied the thermal stability and interdiffusion of InGaAs/GaAs single quantum wells as a function of temperature for both p-type, Be and n-type, Si doping at various doping concentrations. The interdiffusion o f the group III elements is monitored using the photoluminescence from the ground states of the valence and conduction band quantum wells. The intermixing is modelled the using a Green's function method to solve the diffusion equation to describe the evolution of the well shapes during processing; the Schr6dinger equation is solved for this well Shape, to provide the ground state energy levels of the system using the interdiffnsion constant as the only unknown fitting parameter which is therefore uniquely determined. Using this approach we have determined the Ga/In interdiffnsion constants for anneal temperatures upto 1050° C for doping levels upto l0 ts urn-3.

Introduction The extent to which the novel physics observed in heterostructures quantum wells and superlattices can be exploited in real device applications will depend on the ability to process such material without deleterious structura[ changes. This concern is particularly relevant for systems which rely on built in strain for there performance. A major part of most semiconductor processes are high temperature anneals for, for example, contact formation or the activation of implants. It is therefore essential that the behaviour of multilayer structures under such thermal excursions is studied in detail. In particular there will be a tendency, due to the strong compositional gradients, in such structures for intermixing of the construe' nt layers leading to a smearing of the interfaces. This may not necessarily be detrimental to device performance but clearly a quantitative understanding of the diffusion parameters is essential. It has been frequently reported[I,2,3] that intermixing rates depend strongly on the dopant concentration and the dopant type. However, much of this work has been largely qualitative. In this paper we describe now photoluminescence (PL) measurements of the ground state emission from a quantum well, when coupled with a suitable model for the interdiffusion, can be used as a sensitive quantitative probe of the intermixing in multilayer structures. We have reviously presented results on the stability of a high ole mobility transistor structure formed in strained

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GalnAs/GaAs[4] where the active region is an undoped quantum well. Here we present quantitative results obtained on the thermal intermixing of single quantum wells in the strained system GainAs/GaA.s as a function of dopant type and concentration.

Experimental Details All the samples measured in this study were IOOA quantum wells of G%lno~As in GaAs cladding grown off a GaAa substrate. The well was 1000]~ below the surface of the sample. In the case of the doped structures the dopants were incorporated during growth and ~ e whole structure was doped uniformly from 7000A below the well, through the well and up to the surface. The samples were grown by molecular beam epitaxy (MBE) in a vacuum generators V80H reactor. The GaAs was grown at a temperature of 580 ° C and the temperature ramped down to 520 ° C for growing the lnGaAs well. the growth rate for all the samples was ~1 monolayer s-1 for the In(~aAs. After growth the wafers were capped with 300A of Plasma Enhanced Chemical Vapour Deposited silicon nitride deposited at a temperature of 300 °C and then cut into - 1 cm ~ squares. The annealing was performed in a double Phite strip heater in a 1 bar nitrogen atmosphere. s furnace has a rise time of- 2 seconds from 700°C to 1000° C and a fall time over the same range of - 4 seconds. To eliminate any scatter, from the small, <3%, sample non-uniformity across the wafer, anneals © 1991 Academic Press Limited

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Superlattices and Microstructures, Vol. 9, No. 1, 1991

for consecutive time intervals for each temperature were carried out on the same sample. The anneals at different temperatures were carried out on separate samples.

energy confined electron and hole states are then obtained by solving the Schr6dinger equation through the diffused structure at various times. We use hole effective masses for GaAs and InAs of 0.35 and electron effective masses of .0665 and .023 respectively and a linear interpolation of these values for the alloy.

After each anneal the photoluminescence from the samples was measured at 77 K. The PL measurements were performed on a conventional 1 metre grating spectrometer. The luminescence was excited with ~ 100 mW of argon 514nm laser light defocused onto the sample. A liquid nitrogen cooled germanium p-i-n diode was used for detecting the luminescence. The OtOluminescence peak observed is from transitions een the ground state energies of the conduction and valence band quantum wells. As we will describe below this can then be used to provide a quantitative measure of the composition of the well. The shift to higher energies of the peak position as a function of annealing can then be used to follow the intermixing process. To obtain quantitative information from the PL we must first calculate the shape of the conduction and valence band wells as intermixing of the Ga and In on the group III sublattice proceeds. We can then calculate the positions of the lowest confined electron and hole stales in these wells for comparison with the measured PL energies. We assume, in the first instance, that the interdiffusion can be simply described by the solution to Fick's second Law: 1

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Fil~ure 1 shows the evolution of the photoluminescence as a function of time for an undoped well annealed at 950 °C. As can be seen there is some increase in the linewidth during annealing from 9 meV for the as grown sample to 18 meV after annealing at 950 °C for 180 s. This smooth shift in peak energy and linewidth clearly show that intermixing is occurring and the well is been gradually broadened with no evidence for sudden strain relaxation. In Figure 2 we have plotted the time evolution of the shift of the photoluminescence peak energy as a function of anneal time, for anneals of 900, 950, 1000, 1050 °C, for the undoped sample. The solid lines are theoretical fits to the data using the model described in the previous section. As can be seen the fit is excellent confirming that the intermixing in this system obeys Ficks law diffusion. For the

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Superlattices and Microstructures, VoL 9, No. 1, 1991

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Superlattices and Microstructures, Vol. 9, No. 1, 1991

42

this data (Figure 4) yields an activation energy for the process of -3 ± 0.4 eV for all the samples. It is obvious that as the quantum well is mixed the indium concentration at the well centre will asymptotically approach zero. For the undoped sample annealed at 1050 °C the indium concentration at the well centre after annealing for 60 seconds was ~14%, further annealing to 600 seconds would have reduced this to -5% and a further factor of ten increase to 6000 seconds would only have reduced it to ~1.7%.

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Conclusion In conclusion we have made a study of the intermixing of Ga and In in the strained GaAs/InGaAs system as a function of doping introduced during growth. We have shown that the diffusion obeys Fiek's law and obtained diffusion constants as a function of both temperature and doping density. Be doping at levels up to l 0 is cm -~ are shown not to affect the diffusion as is doping with Si at 10 t7 cm% Si doping at 10 l" cm -3 is shown to increase the diffusion constant by a factor of three. An activation energy for the interdiffusion of 3.4 ± 0.4 eV has been obtained.

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Acknowledgement - We are grateful to the SERC and MOD for financial support under the UK joint SERC/MOD scheme, D.J. Dunstan for valuble discussions and R. Gwilliam for cap deposition.

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References Figure 4. An Arrhenius plot of the interdiffusion constants for undoped, 10 .7 cm "3and 10js cm } Si doped and 10 '7 cm -3Be doped samples.

undoped samples the diffusion constant varies fi'om 5 x 10 j7 cm2.s -' at 900 *C to 2 x l 0 ts cmz.s -' at 1050 *C. Figure 3(a-e) show the same time evolution for samples doped with (a) l 0 t' cm 3 Si, (b) 10" cm -3 Si and (c) 10 ° cm-' Be. As can be seen for beth Si and Be doping at 10 '7 cm ~ there is little change in the diffusion constant due to the doping, this was also found for a sample doped to 10 ta cm -3with Be. However, as can be seen in figure 3(b), doping with 10 ta cm -~ Si enhances the mixing raising the diffusion constant by approximately a factor of three. An Arrhenins plot of

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