InGaAs short period superlattices grown by LP-MOVPE

Physica B 273}274 (1999) 835}838

Structural and electronic properties of doped InP/InGaAs short period superlattices grown by LP-MOVPE A.B. Henriques!,*, L.K. Hanamoto!, R.F. Oliveira!, P.L. Souza", L.C.D. Gonc7 alves", B. Yavich" !Instituto de Fn& sica, Universidade de SaJ o Paulo, Caixa Postal 66318, 05389-970 SaJ o Paulo, Brazil "Centro de Estudos em Telecomunicac7 oJ es, Pontifn& cia Universidade Cato& lica, Rua Marques de SaJ o Vicente 225, Ga& vea, 22453-900 Rio de Janeiro, Brazil

Abstract We have studied the structural and electronic properties of lattice-matched InP/InGaAs superlattices with planar doping with Si in the center of the barrier layers, using X-ray spectroscopy, transport and photoluminescence (PL) measurements. The formation of superlattice minibands can be seen in the Shubnikov}de Haas (SdH) spectra. The PL spectra show an emission band at high energies, which is associated with carriers con"ned by the superlattice. As the thickness of the barriers was made smaller, the SdH oscillations decreased in frequency and the PL high-energy emission band narrowed, due to a reduction in the density of free carriers. A possible cause for this is the greater probability of the Si atoms being incorporated into acceptor sites, located within the interface layers, in samples with thinner barriers. ( 1999 Elsevier Science B.V. All rights reserved. Keywords: InGaAs; InP; Interface; Superlattices

1. Introduction Lattice-matched InGaAs/InP-based heterostructures have attracted wide interest due to their important applications in optoelectronic devices. In this work we studied InP/InGaAs superlattices doped with Si in the middle of the InP barriers. To investigate the in#uence of the tunneling probability of carriers through the barriers and of the density of carriers upon the properties of these systems we studied them as a function of the thickness of the barrier layers. The present work is distinguished from previous investigations [1}6] by the high density of free carriers present in our samples. The manifestation of free carriers in the transport and optical properties is

* Corresponding author. Tel.: #55-11-818-7049; fax: #5511-818-6984. E-mail address: [email protected] (A.B. Henriques)

correlated with the structural properties of the system, which are deduced from the X-ray spectra of the samples.

2. Experimental Samples were grown by LP-MOVPE in an AIX200 reactor at 6403C and 20 mbar, using semi-insulating InP substrates, in the (1 0 0) direction. The sources used in the growth were TMIn, TMGa, arsine and phosphine. The growth rate was approximately 5 As s. The InGaAs/InP superlattices consisted of nominally latticematched InGaAs wells separated by InP barriers (15 periods). The well thickness of the InGaAs layers was "xed at 50 As ; the samples di!ered in the thickness of the barrier material. The samples were delta-doped with Si in the middle of the InP layer. To achieve this, growth was interrupted halfway through the deposition of the barriers, by cutting the #ux of TMIn. After 2 s, silane was introduced into the growth chamber, with a #ux of either

0921-4526/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 5 1 6 - 5

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A.B. Henriques et al. / Physica B 273}274 (1999) 835}838

7.4 or 10 (in arbitrary units of #ux). A new delay of 2 s was applied before resuming the growth, by restarting the TMIn #ux. Growth interruption for 2 s was also employed at the interfaces. To start the growth of an InGaAs on top of InP, the TMIn and phosphine #uxes were suspended; after a 2-s delay, #uxes of TMIn, TMGa, and arsine were introduced into the growth chamber. To resume the growth of InP on top of the new InGaAs layer, the TMIn, TMGa and arsine #uxes were suspended, and after a delay of 2 s TMIn and phosphine were reintroduced into the reactor. The X-ray measurements were performed with a commercially available double crystal X-ray di!ractometer around the (4 0 0) di!raction peak of InP. The collection time per data point was 1 min and the density of points was one per 3-arcsec. The period of the superlattices and the alloy composition were determined assuming abrupt interfaces. The conventional measurements were carried out with the detector at a "xed position, while in the h/2h experiments both the detector and the sample were moved simultaneously. Transport measurements were made on etched Hall bars. The sample was inserted inside a cryostat, which contained a superconducting coil, which supplied a magnetic "eld of 0}17 T. The temperature of the sample could be controlled within the 2}300 K interval. Photoluminescence measurements were made using a 0.75 m SPEX monochromator. Excitation of the sample was done using a 640 nm, 10 mW diode laser. Light was conveyed to the sample and collected from the sample using optical "bers. A Ge detector using standard phase-sensitive techniques gathered the PL signal.

3. Results and discussion The alloy composition and the superlattice period were deduced from the X-ray spectra of the samples; the values obtained are shown in Table 1. This shows that the structures with 50 and 40 As barriers are lattice

matched, whereas in the structures with a barrier thickness of 30 As the In content in the ternary layer is smaller than intended, leading to tensile strain in the ternary layer. A simulation of the X-ray spectrum was done for sample 327, using dynamical di!raction theory, which suggested that at each interface a thin strained layer exists, which is 2}3 monolayers wide, in agreement with previous investigations [2]. The strain built up at the interfaces enhances the X-ray satellite peaks [3]. Hall measurements as a function of temperature in the range 4.2}300 K were made for all samples. It is found that the carrier density is constant in the whole temperature range, indicating the absence of electron traps in the structure. The Hall mobility was constant in the range 4.2}100 K, above which temperature the mobility decreased due to the thermal activation of phonons. The Hall density and mobility at 4.2 K are shown in Table 1 for all samples. The Shubnikov}de Haas (SdH) spectra of the samples were measured at 2 K in the range of "elds 0}17 T. The Fourier transform of the SdH curves, plotted against 1/B, are shown in Fig. 1. We can see that for the samples with the largest barrier thickness (50 As ) a single dominant peak characterizes the Fourier transform. As the thickness of the barrier is made smaller (40 As ), this Fourier develops a doublet structure and for the smallest barrier widths (30 As ) two peaks are completely resolved. This is an evidence of the formation of minibands. The Fermi surface evolves from a simple cylindrical shape (in the samples with thick barriers), which has a single extremal cross-sectional area, into a cylinder with a strongly modulated cross-section, which has two extremal crosssectional areas (&belly' and &neck') [7]. According to the semiclassical theory, the magnetoresistance oscillations are periodic in inverse "eld, displaying a set of periods that are associated with each one of the extremal crosssectional areas of the Fermi surface. We denote the periodicities of the magnetoresistance oscillations in our superlattices by D(1/B) and D(1/B) . The N%#, B%--: sheet density of carriers can be estimated by using the

Table 1 Parameters of the samples studied. d and x are the period of the superlattice and the indium content in the ternary layer, respectively, as deduced from the X-ray spectra. n and k are the Hall density per superlattice period and mobility, respectively. n is the sheet H!-H!-S$H density of carriers obtained from the Shubnikov}de Haas spectra by the use of Eq. (1) Sample

Silane #ux (arb. units)

d (As )

X

n H!-(cm~2)

k H!-(cm2/Vs)

n S$H (cm~2)

327 326 331 332 334 333

10 7.4 10 7.4 10 7.4

97 95 86 86 75 77

0.54 0.54 0.53 0.52 0.46 0.44

4.2]1012 3.3]1012 3.4]1012 2.6]1012 1.8]1012 1.6]1012

4320 6440 5460 5120 3360 3330

4.7]1012 3.5]1012 3.4]1012 3.0]1012 1.9]1012 1.7]1012

A.B. Henriques et al. / Physica B 273}274 (1999) 835}838

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Fig. 1. Fourier transforms of the SdH spectra of the samples at ¹"2 K. The number of the sample corresponding to each curve is indicated. The shaded area is the region of the spectrum associated with carriers occupying states in the fundamental electronic miniband.

Fig. 2. Photoluminescence spectra of the samples at ¹"2 K. The number of the sample corresponding to each curve is indicated. The shaded area is associated with the recombination of electrons con"ned by the superlattice potential and photoexcited holes. The dips in the spectra at the energies indicated by the dashed lines are due to absorption in the optical "ber used for light collection.

tight-binding approximation for the miniband dispersion, in which case the density of carriers will be given by

ized all spectra. At the high-energy side of this emission band, a tail is seen which we associate with the recombination of electrons con"ned by the superlattice potential and photoexcited holes. (This association is supported by the observation that when a magnetic "eld is applied parallel to the axis of the superlattice, the continuous high-energy emission tail splits into a fan of Landau levels.) The width of the high-energy tail in the PL spectrum narrows dramatically when the thickness of the barriers is reduced. This is in agreement with the results of the SdH measurements, since a decrease in the density of free carriers will lower the Fermi energy and hence narrow the PL spectrum. In conclusion, we have studied the properties of InP/InGaAs superlattices doped with Si in the center of the InP barriers. X-ray measurements indicate that a strained layer of 2}3 monolayers exists at each interface of the structure. Transport and optical measurements show that for the same density of donor atoms the density of free carriers decreases when the thickness of the barrier layers is narrowed. While other explanations

CAB

1 e n " D S$H h B

AB D

#D B%--:

1 B

~1 .

(1)

N%#,

The carrier density estimated from the Shubnikov}de Haas spectra, obtained by the use of Eq. (1) is shown in Table 1. Fair agreement is obtained between the Shubnikov}de Haas and the Hall density of carriers, indicating that most of the free carriers in our samples occupy quantum states belonging to the fundamental electronic miniband. Samples 326, 331 and 332 also display a lowfrequency oscillatory magnetoresistance, which might be due to a small population of electrons in the excited electronic miniband. The most striking feature, however, is the shift of the whole of the spectrum to lower frequencies as the thickness of the barriers is reduced, indicating a decrease in the density of free carriers. Fig. 2 shows the photoluminescence of the samples at 2 K. A wide emission band centered on 0.9 eV character-

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A.B. Henriques et al. / Physica B 273}274 (1999) 835}838

cannot be ruled out, we propose that the smaller the density of free carriers in the samples with narrower barrier layers be due to the spread out of the Si atoms around the doping plane and to the presence of defects at the interfaces. As it is well known, the Si atoms will spread out around the delta-layer due to di!usion and segregation, and the density of doping atoms perpendicular to the doping plane can be described by a statistical distribution of Gaussian shape [8]. Assuming the statistical distribution of doping atoms to have the same width in all samples, in the samples with narrower barriers the Si doping atoms will have a greater probability of being incorporated into the interface layer. If the interface layer is of a lower crystalline quality than the rest of the structure, in these layers the Si atoms will have a greater chance of moving from donor to acceptor sites, hence the density of free carriers will be smaller.

Acknowledgements A.B.H. thanks Dr. P.M. Koenraad, of the Eindhoven University of Technology, for help with the preparation of the samples and useful discussions, to Dr. M.T. Furtado and Dr. W. Carvalho Jr., of the LaboratoH rio de

Optoeletro( nica/CPqD for the simulation of the X-ray spectrum, and to Dr. E. Abramo! of the Instituto Nacional de Pesquisas Espaciais for pro"table discussions on the X-ray spectra. This work was supported in part by the government agencies FAPESP (grant 97/0635-7) and CNPq (grant 306335/88-3).

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