InAlAs modulation-doped heterostructures grown lattice-mismatched on GaAs substrates

Journal of Crystal Growth 111 (1991) 313—317 North-Holland

313

High-quality In~Ga1-x As/ InAlAs modulation-doped heterostructures grown lattice-mismatched on GaAs substrates K. Inoue, J.C. Harmand and T. Matsuno Semiconductor Research Center, Matsushita Electric Industrial Co. Ltd. 3-15, Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan

We report on the lattice-mismatched growth and properties of InGaAs/InAlAs modulation-doped heterostructures on GaAs substrates for a full In composition range, by molecular beam epitaxy using a linearly graded 2/V.InGaAs s were or obtained InGaAlAs at 77buffer K for layer In grown at a relatively composition from 0.3 to low0.8. temperature. The observed Highmonotomcal electron mobilities increase of of mobility 25,000 toin118,000 this In cm composition range agreed well with the theoretical calculation. It has been shown that the use of wider bandgap material, such as InGaAlAs, in the graded buffer layer is very effective in reducing the residual carrier concentration from 1 x 1012 to less than 1 x 1011 cm —2

1. Introduction

device fabrication, and report some preliminary results on the MODFETs fabricated on these

The lattice-mismatched growth of InGaAs/ InAlAs heterostructures on GaAs substrates has attracted much attention, not only because it will improve the performance of GaAs-based devices, but also because it will expand the freedom in the choice of In composition. Although the feasibility of InGaAs/InAlAs HBTs and MODFETs on GaAs substrates has been demonstrated [1,2], the problem of the lattice mismatch between epilayers and the substrate was still left unsolved. We have previously shown that the use of a compositionally graded InGaAs buffer layer grown at a relatively low temperature of about 400 °Cgreatly improves the electron mobility in such a lattice mismatched system [3,4]. In this paper, we first describe the successful growth of InGaAs/ InAlAs modulation-doped heterostructures for a full In composition range by using this growth technology. Then, the experimental dependence of electron mobility in these heterostructures on In composition has been analyzed and compared with that obtained by the theoretical calculation. Finally, we describe the effect of the buffer layer structure on the residual carrier distribution, which is important for the 0022-0248/91/$03.50 © 1991

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lattice-mismatched structures.

2. Layer structure A schematic cross section of the modulationdoped InGaAs/InAlAs heterostructure on GaAs is shown in fig. 1. The growth was performed by solid source MBE on (001) semi-insulating GaAs

Lattice-Matched Tsub :480-500°C

Si doped In~’Alj~’As 300 A Undoped In~’Ali.~’As 30 A Undoped In~Ga 1.~As 1000 A Undoped In~.Al1~.As 2000 A

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Graded In~(Ga,Ali.,)1.~As (y: 0 -* x’, z: 1-~0) Tsub : 580°C

Undoped GaAs

2000 A

semi-Insulating GaAs Substrate

Fig. 1. Schematic cross section of InGaAs/InAlAs modulation-doped heterostructure grown on a GaAs substrate.

Elsevier Science Publishers B.V. (North-Holland)

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K. Inoue et al.

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substrates. growth of the structure wasabout mitiated with The a 2000 A thick GaAs layer at 580°C. Then, an In~Ga 1~As or In~(Ga~ Al1~)1~Aswith a linearly graded In composition (y) from 0.02 to the desired value (x or x’) was grown at a temperature of 350—400°C. The In composition gradient was fixed to about 50%/jtm. The growth was then interrupted for several minutes and the substrate temperature was raised to 480—500°C.After that, a 2000 A thick In~’Al1_~’Aslayer was grown to separate the active layers from the former buffer layer. These active layers consist of a 1000 A thick undoped In~Ga1_~Aschannel layer, a 30 A thick undoped In~’Al1 ~‘As spacer layer and a 300 A thick In~’Al1_~’As 3. Thedlayers onor layer on top doped of thewith graded Si to buffer 2 X 1018 layercm were lattice-matched to each other except for the case of high In composition of more than 80%, where the conduction-band discontinuity between InGaAs and InAlAs becomes quite small. In such a case, a pseudomorphic InAlAs spacer and donor layers were used (In 07Al03As for x 0.8 and In08Al02As for x 1.0). =

3. Electron mobility Fig. 2 shows electron mobilities measured at 300 and 77 K and sheet electron concentration (Ni) at 77 K as a function of In composition in the InGaAs channel layer. In the figure, the data obtained from similar heterostructures grown lattice-mismatched InP substrates are also plotted, for which aongraded In~Al 1 ~As buffer layer was used. As a general tendency, the electron mobility at 300 K is seen to monotonically increase with thes increase of In10,500 composition, 2/V. at x 0.2, cm2/V• from s at cm to 20,000 cm2/V. s at x 1.0. On the x70000.53 other hand, at 77 K, the mobility showed its minimum value at x about 0.3 and increased almost monotonically with the increase of In corn2/V. s at x 0.3, 51,000 position: cm118,000 cm2/V. s at x = cm2/V. s from at x 25,000 0.53 to 0.8. These high mobility values indicate that the dislocations created to relieve the strain accumulated in the graded buffer layer are efficiently =

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0.0 0.1 0.2 0.3INDIUM 0.4 0.5 COMPOSITION 0.6 0.7 0.8 0.9 1.0 Fig. 2. Electron mobility and concentration (IV~)as a function of In composition. PM1 and PM2 indicate the samples with pseudomorphic InAlAs spacer and donor layers.

confined in this layer and the propagation of threading dislocations to the active layer is well suppressed in these heterostructures, as revealed by TEM observation reported previously [4]. In order to confirm the validity of these dependences of electron mobility on the In composition, a theoretical calculation has been performed assuming a single-subband transport and using the Stern—Howard variational wave function [51.For the total mobility and that limited by polar optical phonon scattering (P0), we have used the iteration method developed by Nag [61. The 2calculated is shown rein sults at 77 K with N~= 1 )< 1012 cm fig. 3. From this figure, the In composition dependence of electron mobility is mostly determined by alloy-scattering-limited mobility (AL) and at about The x 0.3, mobility minimum value. solidthe circles in thetakes figuretheindicate the highest mobility obtained in the experiment for each In composition. The agreement between the experimental results and the calculated ones is excellent in spite of the difference in N 1 value between experiment and calculation. Anyway, this result also supports the good crystallinity of the active layer in our heterostructures. It should be noted that if we look at fig. 2 carefully, the mobility value for x> 0.5 tends to —

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and the The residual carrier thesemeasured structures. results are concentration sununarized in table 1, together with those for modulation-doped heterostructures (samples A and C). The residual carrier concentration in thelayer undoped structure a graded InGaAs buffer was found to on be surprisingly high, about 1 x 1012 cm2, while that

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either on a graded InGaAs buffer layer (sample B) or on a graded InGaAIAs buffer layer (sample D)

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layers, the device isolation by mesa-etching and Schottky diode characteristics were found to be very poor. In order to clarify the cause for such high leakage currents, we have grown some undoped In0 53Ga047As/ InAlAs heterostructures

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INDIUM COMPOSITION . Fig. 3. Calculated 77 K mobility as a function of In composition in the channel layer. Each symbol represents electron mobility limited by polar-optical phonon scattering (P0), alloy scattering (AL), ionized impurity scattering (II) acoustic-phonon deformation potential scattering (DP) and acoustic-phonon piezoelectnc scattenng (PZ).

saturate and the structures on InP or those with pseudomorphic InAIAs spacer and donor layers show higher values than the standard samples on GaAs substrates. The cause for such differences is not clear at present, but the contribution of paraltel conduction in the donor layer or in the thick buffer layer is considered to be one of the factors,

4. Residual carrier and its distribution When the MODFETs were fabricated on the epilayers grown on graded In~Ga1 ~As buffer

on a graded InGaAlAs was one order of magrntude lower, about 1 x 1011 cm2 or less. The high residual carrier concentration in sample B is probably due to the narrower bandgap and/or higher background carrier concentration of the undoped InGaAs graded buffer layer than those of the InGaAlAs graded buffer layer. The locations of the residual electron concentration were then studied using a capacitance— voltage carrier profiling method. Two samples were prepared for this experiment using n-type GaAs substrates: one is an undoped In 05Ga05As/ InAlAs heterostructure on a graded InGaAs buffer layer via a 1000 A thick InAlAs (fig. 4a) and the other is a modulation-doped heterostructure on a graded InGaAlAs buffer layer (fig. 4b). As seen in fig. 4a, for the undoped heterostructure on a graded InGaAs buffer layer, a sharp peak of electron concentration is located at the heterointerface between the 1000 A thick InAlAs and the graded InGaAs buffer layer, just like a two-dimensional electron gas accumulated in modulation-doped

Table 1 Electron mobility (~)and concentration (Ns) for modulation-doped (MD) and undoped (U) In0 53Ga047As/InAlAs heterostructures grown on graded InGaAs buffer layer or graded InGaAlAs buffer layer Sample

Buffer layer

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K Inoue et aL

/ High-quality In~Gaj ~As/InAlAs —

heterostructures. This fact suggests that the electrons are flowing through two InGaAs channels in the samples in fig. 2 grown on InGaAs buffer layers. This also explains the good agreement of the theoretical calculation with the experiment in fig. 3. On the other hand, for the modulationdoped heterostructure grown on InGaAlAs buffer layer in fig. 4b, the peak of residual electron concentration is seen inside the InGaAIAs buffer layer, the amount of which is about one order of magnitude lower than that observed in fig. 4a. The 0.5 ~tm gate In0 5Ga05As/InAlAs MOD-

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FETs were fabricated using a heterostructure with a graded InGaAlAs buffer layer on a semi-insulating GaAs substrate, the layer structure of which is I 1016

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apart was less than 1 ~tA at an applied voltage of 10 V. Although a high transconductance of 370 mS/mm was obtained, the pinch-off characteristics are still needed to be improved, as seen in fig. Sb. We believe that these MODFET characteristics will be further improved by optimizing the layer structure for andwhich reducing carrier concentration, a usethe of residual wider bandgap material such as InAlAs as a graded buffer layer will be effective.

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the same as the one shown in fig. 4b. The device isolation and the Schottky diode characteristics were found to be good, as shown in fig. 5a. These improvements are probably due to the use of InGaAlAs for the two graded buffer layer. The100 leakage current between MODFETs about ,sm

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DISTANCE FROM THE SURFACE (pm) . . Fig. 4. Camer profiles in (a) an undoped In05Ga05As/InAlAs heterostructure grown on a graded In~Ga1~As buffer layer and in (b) a modulation-doped In0 5Ga05As/InAlAs heterostructure grown on a graded In~(Ga~, Al1 .J~ ~As buffer layer.

5. Summary In summary, we have successfully grown InGaAs/ InAlAs modulation-doped heterostructures

K Inoue et aL

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High-quality In~Gaj— ~As/InAlAs

lattice mismatched to GaAs substrates for a full In composition range. The dependence of mobility on In composition was theoretically analyzed, and agreed very well with the experimental results. The importance of using wide gap material for the buffer layer has been pointed out, to reduce the residual carrier concentration and to improve the MODFET characteristics.

MD HSs grown lattice-mismatched on GaAs

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References [1] T. Won S. Agarwala and H. Morkoç, Appl. Phys. Letters 53 (1988) 2311. [2] G.W. Wang, Y.K. Chen, WJ. Schaff and L.F. Eastman, IEEE Trans. Electron Devices ED-35 (1988) 818. [3] J.C. Harmand, T. Matsuno and K. Inoue, Japan. J. Appl. 28 (1989) T. 1101. [4] Phys. J.C. Harmand, Matsuno and K. Inoue, in: Proc. 16th Intern. Symp. on GaAs and Related Compounds, Karuizawa, 1989, Inst. Phys. Conf. Ser. 106, Eds. T. Ikoma

Acknowledgments The authors would like to thank Dr. S. Horiuchi and T. Onuma for their continuous encouragement throughout this work.

and H. Watanabe (Inst. Phys., London—Bristol, 1990) p. 177. [5] F. Stem and W.E. Howard, Phys. Rev. 163 (1967) 816. [6] B.R. Nag, J. Phys. C (Solid State Phys.) 7 (1974) 3541.