Investigation of the interface quality of GaAsAlGaAs heterostructures

Superlattices and Microstructures, Vol. 8, No. 2, 1990

INVESTIGATION

179

OF THE INTERFACE QUALITY OF GaAs/A1GaAs HETEROSTRUCTURES

T. Schweizer, K. K6hler, P. Ganser, D.J. As, and K.H. Bachem

Fraunhofer Institut far Angewandte Festkfrperphysik Tullastr. 72, 7800 Freibur~ FRG (Received 29 July 1990) We report on optical and electrical properties of GaAs/A1GaAs heterostructures prepared by molecular beam epitaxy (MBE). For fixed Ga and As fluxes the d a m p i n g o f the oscillations of the reflection high energy electron diffraction (RHEED) pattern is strongly dependent on substrate temperature. The minimum of the damping of the intensity oscillations has been observed at 75&C. The best electrical properties of high electron mobility transistors (HEMT) have been found at this substrate temperature. The, maximum electron mobility was 120000 cmZ/Vs at 77 K for a spacer of 50 A and an electron concentration of ~12 cm-.Z For the presented quantum well (QW) structures we obtained the 1 * 10 best results at a substrate temperature of 740°C. In photoluminescence we obtained a fullwidth at half maximum ( F W H M ) for example for a 20 nm, 4.5 nm, 2 n m and I n m wide Q W of 0.27 meV, 1.5 meV, 5 m e V and 7 m e V respectively.

1. Introduction

The influence of growth conditions on the properties of heterostructure devices, such as quantum welllasers and high electron mobility transistors, is of great interest. To obtain best performance of these device structures it is very important to grow the structures with optimized growth parameters. Much work has been done to study the influence of growth conditions such as substrate temperature,1 group III to group V flux ratios 2 and growth rates 3. To obtain optimized growth conditions ex situ studies are necessary which test the electrical properties such as mobility and carrier concentration of HEMT structures or the optical properties such as linewidth of the free exciton recombination and intensity of the luminescence of QW structures by low temperature photoluminescence (PL). In this paper we present an m situ evaluation method of RHEED intensity oscillations to optimize the substrate temperature prior to growth of the heterostructures. In order to verify this evaluation method we have grown HEMT structures and QW structures by MBE at different substrate temperatures. 2. Experimental

The GaAs/A1GaAs heterostructures were grown in a Varian Gen II modular twin chamber system with substrate holders for indium free mounting. The (100) GaAs substrates used for epitaxy were semi-insulating LEC grown 2-inch wafers. The substrate preparation has been described elsewhere 4. The growth rate for GaAs derived from RHEED intensity oscillations was 1 #m/h. The A1GaAs growth rate was varied between 1.33 #m/h and 1.43 #m/h corresponding to an A1 0749-6036/90/060179 + 04 $02.00/0

concentration of 25% - 30%. The As-to-Ga beam equivalent pressure ratio was 20. For the specular beam RHEED intensity measurements we used 10 keV electrons. The RHEED intensity measurements were carried out using the <100> direction which is the direction of twofold periodicity in the As stable (2 x 4) reconstructed GaAs surface. The oscillations were detected by employing a CCDcamera focused on the fluorescence screen and the intensities of interest were measured via a photodetector projected onto a video monitor. PL spectra of the samples were obtained with a spectrometer consisting o f a 1 m grating monochromator and a cooled GaAs photomultiplier. Optical excitation was orovided by the chopped emission of an Ar + laser (51~[.8 nm). Low temperature spectra were taken at 10 K by mounting the samples on the cold finger of a closed cycle cryostat. The Hall effect measurements were carried out with a commercial system at a temperapare of 300 K and 77 K using the van der Pauw methodL 3. Results and discussion

The use of RHEED is a very powerful technique to control the growth process in MBE. It has dearly been shown that the RHEED intensity oscillation period corresoondends to the growth of one monolayer6,7. ~aus it is possible to determine the growth rate using the intensity oseillatigns6,7 as well as the A1 concentration of A1GaAs layers~. In addition to these studies, intensity oscillations have also been used to improve the growth of heterostructures. For example for QWs a precise control of the size of the wells and barriers usin~ RHEED was observedL Deparis and coworkers TM reported the correlation © 1990 Academic Press Limited

180

Superlattices and Microstructures, Vol. 8, No. 2, 1990

between the RHEED intensity level during growth and the linewidth broadening of the excitonic QW transitions. Another important application is the RHEED intensity recovery when growth process is interrupted n. It has been shown that growth interruption leads to a smoothening of the growth surface. This was confirmed by PL studies of QWs ~rown with ~rowth interruption at the eterointerfaces9, t~;13. For our evaluation method we used the well known behaviour of the occurence of RHEED intensity oscillations during growth to optimize the substrate temperature: Starting from a smooth surface RHEED intensity decreases when growth is started. After a coverage of 50% of the surface the intensity reaches a minimum. The intensity then increases until full coverage is obtained. This behaviour corresponds to a two-dimensional growth. However a dampmg of the RHEED intensity was observed because the second laver can start to grow before the first layer is finished 1¢. This effect lowers the degree of twodimensional growth and leads to an interface roughness. To obtain atomically fiat interfaces it is important to optimize the g r o w t h conditions to approach ideal two-dimensional growth. We used the damping of the intensity oscillations as a criterium for the two-dimensional growth. We correlate the degree of two-dimensional growth by forming the ratio of the first and tenth amplitude of the RHEED intensity oscillations. 1. Assumming ideal two-dimensional growth, the ratio of the intensities approaches one. This corresponds to no decay of the RHEED intensity oscillations. 2. The value of the ratio will increase if threedimensional growth occurs, this corresponds to a fast decay of the RHEED intensity oscillations.

We measured the damping ot the intensity oscillations as a function of the substrate temperature. Before each measurement we interrupted the growth at an As stabilized GaAs surface for 300 s in order to start from a smooth surface. All other growth parameters were kept constant. Fig. 1 shows the dependence the ratio of the first amplitude of RHEED intensity oscillation to the tenth amplitude as a function of substrate temperature. From Fig. 1 we conclude that the best condition for two-dimensional growth is obtained at a substrate temperature of 750°C readout of thermocouple. At lower substrate temperature we obtained a lower intensity ratio. This behaviour could be explained by the lower Ga mobility on the growth surface. At higher substrate temperatures the ratio increases due to Ga desorption. To verify the optimized substrate temperature for two-dimensional growth as derived from the ratio of RHEED intensity oscillations, the electrical ies of modulation doped GaAs/AlGaAs structures grown at different substrate temperatures were studied. Apart from the growth temperature, the other growth parameters were kept constant. A schematic of the HEMT structu[e is shown in Fig. 2. The struqture qonsists of a 1500 A undoped GaAs buffer~ a 50 A/85 A GaAs/AlGaAs superlattice, and a 6000 A GaAs buffer followed by a 50~k AlGaAs spacer. The suoply layer consists o f 600 A AlGaAs doped with a gi concent[ation of 1"1018 cm 3. The structure is capped by 200 A undoped GaAs.

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Fig. 1 : Ratio of the first amplitude of RHEED intensity oscillation to the tenth as a function of substrate temperature. The dotted lines are only intended as a guide.

In Fig. 3 we show the electron mobility at 77K as a function of substrate temperature derivedfrom Hall effect measurements. Going from 750°C to higher and lower substrate temperatures we obtained lower electron mobilities. The maximum electron mobility was 120000 cm2/Vs at 77K, with an electron concentration of about 1"1012 cm -2 which is one of the

Superlattices and Microstructures, VoL 8, No. 2, 1990

181 40 ~, CAP LAYER 300 ~. AIGaAs

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best electron mobilities found in the literature for this electron concentration15 Besides the electrical properties we also used optical properties to determine the optimized substrate te~mperature derived from RHEED. We have grown 100 A SOW structures as a function of substrate temperature. The principal QW structure is shown in Fig. 4. The QW slructure starts with a bu,ffer layer consistingpf 60Q0 A GaAs followed by 900 ~A1GaAs with a 20 A/30 A supe[lattice. Then tile 100 A SOW is embedded in a 400 A and a 300 A thick A1GaAs barrier followed by a 40 /~ thick cap layer. The interface quality of the QW structures were characterized with PL. The linewidth of the free exciton recombination was used to determine the best substrate temperature for the present QW str~ctures. In Fig. 5 we show the FWHM of the 100 A SOW structure grown with different substrate temperatures. With increasing substrate temperature we obtained a decrease of the FWHM in PL, corresponding to an improvement of the interface quality. The minimum of the FWHM is observed at 740°C. At higher substrate temperatures the FWHM increases, corresponding to a lowering of the interface quality. We explain the variation of FWHM in PL of the QW structures and the mobility of the HEMT structures, with a lower migration of the Ga-atoms on the growth surface at lower substrate temperatures than about 750°C, and with a noticeable Ga-desorption at temperatures above about 750°C. Both result m an increased interface roughness. The optimized substrate temperature derived from the damping of the RHEED intensity oscillations coincide very well with the results of the electron mobility of the HEMT structures and the FWHM in PL of the QW structures. Based on the improved growth conditions we have grown SOWs with different thicknesses. To increase the interface quality we used growth

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interruption at each heterointerface of our QWs for 60 s under As-flux. In Fig. 6 we show the FWHM of our QWs grown at a substrate temperature of 740°C as a function of QW thickness (full circles) compared with data from C.W. Tu et al. 13 (open circles). For samples with a QW width of 20 nm, 4.5 nm, 2 nm and 1 nm we obtained a FWHM of the free exciton in PL of 0.27 meV, 1.5 meV, 5 meV and 7 meV. These are to our knowledge the best results reported so far. The line in Fig. 6 is the theoretically calculated PL

Superlattices and Microstructures, Vol. 8, No. 2, 1990

182

References "10

1. J. Ralston, G.W. Wicks, and L.F. Eastman, J. Vac. Sci. Technol. B 4 594 (1986). 8

2. W.T. Tsang, and V. Swaminathan, Appl. Phys. Lett. 39 486 (1981).

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3. Ch. Maierhofer, S. Munnix, D. Bimberg, R.K. Bauer, D.E. Mars, and J.N. Miller, Appl. Phys. Lett. 55 50 (1989).

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4. BL K6hler, P. Ganser, T. Schweizer, P. Hiesinger, K.H. Bachem, and H.S. Rupprecht, in Workbook of the 5th International, Conference on Molecular Beam Epitaxy, 1987, p. 433.

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Fig.6 : FWHM of QW structures grown with optimized temperature and growth interruption. 11circles are our results while the open circles are results from C.W. Tu and coworkers 13. The line is the calculated PL broadening of a GaAs/A1GaAs QW assuming fluctuation of the well width of one monolayer.

5. L.J. van der Pauw, Phil. Res. Rep. 13 1 (1958). 6. J.H. Neave, B.A. Joyce, P.J. Dobson, and N. Norton, Appl. Phys. A31 1 (1983). 7. J.M. Van Hove, C.S. Lent, P.R. Pukite, and P.I. Cohen, J. Vac. Sci. Technol B 1 741 (1983). 8. T. Sakamoto, H. Funabashi, K. Ohta, T. Nakagawa, N.J. Kawai, and T. Kojima, Jap. Jour. of Appl. Physics 23 L657 (1984). 9. T. Schweizer, Diploma thesis, University of Freiburg (1989).

broadening of a GaAs/A1GaAs QW assuming a fluctuation of the QW thickness of one monolayer. Our observed FWHM of the QWs is well below this line, thus showing that the main broadening effect of the linewidth in PL is not due to interface roughness of one or more monolayers. 4. Conclusion We have shown the dependence of the damping of the RHEED intensity oscillations on substrate temperature. This behavxour was used to optimize growth temperature to approach two-dimensional growth to obtain atomically flat interfaces. Good agreement between the predicted optimum substrate temperature from RHEED and the optimum substrate temperature obtained by the electrical properties of HEMT structures and optical properties of QW structures support our approach. The FWHM of the QWs obtainedin PL are the best results reported so far.

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