GaAs heterostructures: interfaces, quantum wells and quantum wires

Journal of Crystal Growth 124 (1992) 199—206 North-Holland

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CRYSTAL GROWTH

LP-MOVPE growth and optical characterization of GaInP/GaAs heterostructures: interfaces, quantum wells and quantum wires F.E.G. Guimarães, B. Eisner, R. Westphaien, B. Spangenberg, H.J. Geelen, P. Balk and K. Heime

1

Inst itut für Haibleitertechnik, RWTHAachen, D-W-5100 Aachen, Germany

Lattice matched Ga

05In05P/GaAs single interfaces, single quantum wells (QW) and finally quantum wires on mesa-like selective GaAs were grown by LP-MOVPE. The photoluminescence (PL) of GaInP/GaAs OW shows an anomalous emission band of high intensity below the GaAs band gap. The effect of the growth temperature, AsH3 partial pressure, gas switching sequence and growth interruption times on these new PL features was investigated. The formation of GainPAs layers at the GaInP-to-GaAs interface, due to the substitution of P to As, is responsible for the anomaly in the luminescence. The stability of the GaInP/GaAs interfaces was checked after growth by rapid thermal annealing (RTA). Intermixing at the GaInP/GaAs heterointerfaces was also considered. The introduction of a 1 nm thick GaP barrier between the GaInP and GaAs layers was enough to suppress the GaInPAs intermediate layers and large area quantum wells and wires were successfully grown.

1. Introduction

Although there are some optical data in the

Low-dimensional heterostructures have recently found increasing interest. Especially the lattice matched Ga05In05P/GaAs system is very attractive for microwave applications and for light emitting devices at energies above the GaAs band gap. Recent work, using metalorganic vapor phase epitaxy (MOVPE), has reported that conventional large area Ga05In05P/GaAs heterostructures with sufficient optical [1] and electrical [21 quality was achieved. Photoluminescence decay times as long as 14 ~s have been observed at undoped Ga0 5In05P/GaAs heterointerfaces grown by MOVPE, which are significantly better than those in A1GaAs/GaAs heterostructures [31. LP-MOVPE systems are also very useful for Selective area growth of this material system because of the perfect lateral and sidewall control

[4,51. Present address: DIMES, Delft University of Technology, 2600 GB Delft, Netherlands. 0022-0248/92/$05.00 © 1992

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literature on GaInP/GaAs heterostructures grown by MOVPE, only few authors have considered the effect of the growth processes on the heterointerfaces in detail. Intermixing of the atom species was observed in GaInP/GaAs superlattices by Raman spectroscopy [6]. A broad PL emission around 1.33 eV has been observed in GaInP/GaAs structures correlated with surface degradation of the GaInP in an AsH3 ambient

[2].Such intermixing processes and structural defects at the GaInP/GaAs interface can play a significant role in determining the ground state energy of thin quantum wells (QWs) or the band offset. Experience also shows that the (100) onented P-rich surfaces are very unstable under purging AsH3 during the switching of the group V supply gases from PH3 to AsH3 [7]. Substitution of P to As has already been observed in GaInAs/InP heterointerfaces [7,81. In this report, lattice matched Ga0 5In05P/ GaAs single interfaces, single quantum wells and quantum wires on mesa-like selective GaAs were

Elsevier Science Publishers B.V. All rights reserved

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/ LP-MOVPE growth and optical characterization of GaInP/ GaAs

grown by low pressure MOVPE. Photoluminescence measurements between 10 and 300 K were used for the study of the optical properties of the heterostructures. First, the effect of the growth temperature, AsH3 partial pressure, gas switch-

nrn thick GaAs well and 100 nm thick GaInP barrier on the top. Double-crystal X-ray diffraction was used to control the lattice match of the studied layers. Subsequently, non-planar

ing sequence and growth interruption on the heterointerfaces was investigated. After that, we considered the effect of introducing a GaP barrier layer (nominal 1 nm thick) between GaInP/GaAs interface materials in order to prevent the group V element substitution during the growth interruption and the intermixing of the atom species at the interfaces. The stability of the

posited on GaAs mesa structures having {100} and {111) facets with submicron dimensions. For the selective growth, a 100 nm thick Si02 masking was deposited on GaAs substrates. This masking layer was structured using electron beam nanolithography technique, followed by reactive ion etching of the patterns. The patterned structures, consisting of 200 ~rn x 0.3 ~sm stripes (repeated 400 times and separated by intervals of 1 /sm), were aligned along the [0111and [0111directions. The PL measurements were carried out in the temperature range between 10 to 300 K. The samples were excited by an argon laser and the luminescence was detected by a GaAs photornultiplier in combination with a 1 rn SPEX monochromator (spectral resolution better than 0.03 nm). Additionally, the stability of the GaInP/GaAs interfaces was checked by rapid thermal annealing (RTA). A Si02 capping layer was deposited on the surface of the heterostructures before annealing, in order to prevent the degradation of the surface. RTA was done in an atmosphere of nitrogen at 700, 750, 800 and 850°Cfor 30 s.

GaInP/GaAs interfaces was also checked, by rapid thermal annealing (RTA) after growth. Furthermore, we present the selective growth of GaInP/GaAs wells with submicron lateral confinement.

2. Experimental procedure The growth experiments were performed in a horizontal MOVPE reactor working at a total pressure of 20 hPa and a gas velocity of 1.2 rn/s. A combined run—vent switching manifold allows a fast exchange of the growth species. The source materials were tnimethylgallium (TMG), trimethylindium (TMI), AsH3 and PH3. The carrier gas was Pd-purified H2. Different gas switching sequences were employed at the GaInP/GaAs interfaces, as discussed below. The substrates were semi-insulating (100) GaAs oriented 2°off to the [0111 direction. The growth temperature was varied between 610 and 670°C.The partial pressure for AsH3 in the gas phase was varied from 4 to 54 Pa while the partial pressures for TMG (115 mPa), TMI (140 mPa) and PH3 (86 Pa) were held constant. Typical growth rates for GaAs and GaInP were around 1.5 j.tm/h. In a first approach, large area structures consisting of lattice matched 0a051n05P/GaAs single interfaces and single quantum wells were grown on a 0.3 ~m GaAs buffer layer. The two single interface structures are 400 nm thick GaAs layer on top of a 500 nm thick GaInP layer and the reversed layer structure. The OW structures consist of a 400 nm thick GaInP barrier layer, 2

GaInP/GaAs QW structures were selectively de-

3. Results and discussion

3.1. Effect of the heterointerfaces on the optical properties Fig. la shows the 10 K PL spectrum of a GaInP/GaAs QW structure in the energy range between the emissions near the band-edges of GaAs (E5 = 1.512 eV) and Ga051n05P (Eg = 1.92 eV for an ordered (Ga, In) distribution on the column III sublattice [101). At the interfaces, growth interruptions of 7 s and purging with the group V element were employed to stabilize the grown surfaces. At the GaInP-to-GaAs interface, PH3 was switched off 2 s and AsH3 was switched

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the GaAs-to-GaInP (upper) single interfaces are shown in figs. lb and ic, respectively. As in the case of the OW structures (fig. la), we observe a similar unusual emission band only for heterostructures with the lower interface (fig. ib). For samples with the upper interface, the expected band-edge emissions for GaAs

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on 3 s before the growth of the GaAs well was started. An inverted switching sequence was used at the GaAs-to-GaInP interface. As shown in fig. la, no OW luminescence was detected between the GaAs and GaInP band-edge emissions. On the contrary, an anomalous emission band of high intensity appeared at energies

((D°,A°),X) and GaInP are detected. The different positions of the unusual band in the spectra of figs. la and lb could be related to confinement effects in QW. In contrast to the sample with the upper interface (fig. ic), the band-edge emissions generated in the 400 nm thick GaAs are not observed in the spectrum corresponding to the sample containing the GaInP-to-GaAs interface (fig. ib). This means that the lower interface contributes more effectively to the radiative decay of the excited minority carriers than the bulk GaAs. In other words, the radiative proof carriers in the OW structure and in samples containing the lower interface (figs. la and ib, respectively) are determined mainly by interface recombinations through the anomalous emission and in the samples containing the upper interface (fig. lc) by bulk recombination processes. A few authors have considered the effect of the MOVPE growth on the GaInP/GaAs heterointerfaces in detail. Wicks et al. [6] reported the intermixing of the atom species in GaInP/ GaP superlattices by Raman spectroscopy. Takikawa et al. [21observed a broad PL emission around 1.33 eV from GaInP/ GaAs structures correlated with surface degradation of the GaInP in an AsH3 ambient. We assume that the lumi-

below the GaAs band gap. Temperature dependent measurements reveal that this anomalous band around 1.44 eV is observed at all temperatures studied (from 10 to 300 K) indicating its intrinsic origin; i.e., this unusual emission is not related to any kind of impurity or defect. In order to investigate the origin of the new luminescence, we studied the lower and upper interfaces of the GaInP/GaAs QWs separately. For this experiment we grew two different single interface structures (see the schematic illustration in the inset of fig. 1). The spectra corresponding to samples with the GaInP-to-GaAs (lower) and

nescence anomaly in fig. 1 is related to the partial substitution of P to As at the lower interface, leading to the formation of GaInPAs intermediate layers. To demonstrate such an effect, we compare in fig. 2 the luminescence of two different Ga05In05As layers grown on GaAs: one layer (fig. 2a) with the normal growth schedule without interruption and the other (fig. 2b) for which the growth was interrupted with AsH3 stabilization (this was repeated 10 times in intervals of 50 nm). The switching sequence during the interruptions is shown in the inset of this figure. The spectrum of fig. 2b shows an additional band between

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Wavelength [~] Fig. 1. 10 K PL spectra of (a) a GaInP/GaAs quantum well (L = 2 nm) and of structures containing (b) the GaInP-toGaAs interface and (c) the GaAs-to-GaInP interface. The inset illustrates the band structure along the growth direction z.

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switching sequences [7], but the emission bands similar to those in figs. la and lb are still observed. P-rich surfaces are not stable under AsH3

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stabilization during the switching of the gas sources of the group V elements. Substitution of P by As has also been observed in GaInAs/InP heterointerfaces [7,8]. Thus, it seems that the substitution of P by As in the MOVPE process is a common effect in 111—V semiconductors con~‘

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Wavelength [nan] — Fig. 2. 10 K PL spectra of GaInP layers without (a) and with (b) growth interruption under AsH3 purge following the switching schedule in the inset.

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3.2. Optimization of the heterointerfaces The variation of growth parameters was not effective to prevent the formation of the GainPAs well layer on the GalnP-to-GaAs heterointerfaces. The reduction of the AsH3 partial pressure by a factor of 15, as well as the reduction of the growth temperature (from 670 to 610°C)and consequently, of the P desorption rate, shift the unusual emission to energies close to the GaAs

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Wavelength [am] Fig. 3. 10without K PL spectra samples having the GaInP-to-GaAs interface (a) andofwith (b) GaP intermediate layer. The inset illustrates the band structure along the growth direction z.

/ LP-MOVPE growth and optical characterization

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On the other hand, the formation of the

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Fig. 4. 10 K PL spectra of GaInP/GaAs single OW structures of 2 nm wide well without (a) and with (b) GaP intermediate layer at the low GaInP-to-GaAs interface. The inset illustrates the band structure along the growth direction z.

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mined mainly by the bulk recombination processes and not by those of the heterointerface, as discussed above in section 3.1. The effect of the intermediate GaP layer on the 2 nm wide OW structures can be seen in fig. 4. The PL spectrum (fig. 4b) shows the desired OW emission between the GaAs and GaInP band-edge emissions. The main reason for the effectiveness of the GaP intermediate layers in protecting the GaInP-to-GaAs interfaces is its small P desorption rate constant under AsH 3 purge as compared to InP and related compounds [9]. Presumably, a substitution of P to As may occur at the GaP-to-GaAs interface with the formation of a GaP~As2—y layer. This nominal 1 nm wide layer seems to be thick enough to prevent an As incorporation into the GaInP layer.

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Wavelan~[nm]— Fig. 5. 10 K PL spectra of GaInP/GaAs single quantum well structures without (a) and with (b) GaP intermediate layer at the lower GaInP-to-GaAs interface before (as grown) and after rapid thermal annealing (RTA) at 850CC for 30 ~•

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the lower interface (see fig. 3a) disappears after RTA at 850°C for 30 s. Band-edge emissions similar to those shown in fig. 3b for GaAs and GaInP were then obtained. Such a result means that the lower interface and/or the GainPAs intermediate layer are not thermally stable. Inter-

shown in fig. 5 for samples without (fig. 5a) and with (fig. Sb) GaP intermediate layer. A broad emission band is now observed in the energy region of the OW emissions after annealing at 850°Cfor 30 s (fig. 5a, bottom), as a result of the compositional change of the Ga~In1..~As~P1 —y

mixing due to diffusion at the interface has probably changed the composition of the Ga~In1—x As~P1-y well layer originated in the P-to-As substitution. Presumably, the effective band-gap of the quaternary layer after RTA is now wider than that of GaAs. The RTA experiments were also carried out for single OW structures. The PL results are

layer at the lower interface. This interface shows the beginning of degradation after RTA, as demonstrated by the broadening of the OW emission. (A similar blue shift is found for OW emissions in samples containing the GaP layer. However, the line shape of the QW emission has negligibly changed after annealing at higher ternperatures. This indicates that the interface with a

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Fig. 6. SEM photographs and schematic diagram of the grown structures of cleaved stripes oriented in the [011] (a) and [011] (b) directions.

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/ LP-MOVPE growth and optical characterization of GaInP/ GaAs

GaP barrier layer is more stable to thermal stress. It is important to state that the blue shift begins at annealing temperatures near the growth ternperature, e.g. around 700°C,suggesting that the interdiffusion processes already take place during the growth. Fig. S shows additionally a shift of the GaInP peaks to higher energies. It is well known that temperature [11], strain [6] and impurities [10] promote intermixing or disordering of the host atoms constituting the GaInP/GaAs heterostructures and the GaInP alloys. Intermixing at the interfaces can change the GaInP or GaAs layers into GaxIn —x P~As1—y layers. The ordered Ga and In sublattices of the GaInP layers grown by MOVPE have shown to be unstable since they disorder when annealed at growth temperatures under conditions that enhance the diffusion [10]. Thus, the blue shift of the GaInP band-edge emission in fig. 5 is a result of a thermal disordering of the group III atoms on the column III sublattice [12]. Disordering due to impurities, such as silicon, which may diffuse from the Si02, might have a small contribution because RTA for 30 s will not cause considerable diffusion of impurities. This is supported by the fact that no emission related to Si impurities was detected, neither in the GaAs nor in the GaInP

[011] directions. On top of the GaAs mesa structures, OWs consisting of a nominal 2 nm GaAs layer embedded in GaInP and containing the GaP barrier layer were grown. Growth of GaAs is not expected on the (11 1}B facets, whereas the growth rate on the (111)A facets is expected to be 15% of that on the (100) ones [4]. Starting the growth with 300 nm wide stripes, perfect lateral confinement below 100 nm has been obtained on top of the GaAs [100] surface by changing the height of the mesa. In fig. 7, a 10 K PL spectrum of the optimized large area OW structure (fig. 7a) is compared with those of wire structures grown on GaAs stripes of different heights oriented in the [011] direction (fig. 7b). The growth time of the stripes was determined in such a way that lateral widths smaller than 100 nm were expected to be produced on the top. The wire structures in fig. 7b show dominant PL emissions that are also observed for all measurement temperatures ranging

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In summary, the shift of the OW emissions to higher energies (fig. 5) can be explained by the increase of the effective well band-gap due to the

quaternary GaxIni_xPyAsiy layer caused by the intermixing at the well interfaces. The intermixing could be promoted by the disordering in the GaInP layer and the strain at the interface caused by the intermediate layers. growth of GaInP/ GaAs wire strucBased on the above results and on the available knowledge of selective growth in the GaIn—As—P system [4], GaInP/GaAs single quan tum well structures with ultrasmall lateral confinement were produced. We used pyramidal shaped GaAs stripes having facets limited by (100), {111}A and (l11)B planes. Figs. 6a and 6b show cleavages of stripes grown in the [011] and

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from 10 to 300 K. The wire recombination splits into two well-resolved peaks and shows a blue shift for increasing lateral confinement, i.e., for increasing mesa height. The full widths at half maximum (FWHM) of such emissions are similar to those of the conventional large area OWs. These results show that an excellent lateral uniformity of the selectively grown GaInP/GaAs stripes with submicron dimensions (smaller than 100 nm) can be achieved.

4. Conclusions Lattice-matched Ga05In05P/GaAs single interfaces (with GaInP both on top and below the GaAs), single quantum wells and quantum wires on mesa-like selective GaAs were grown by LPMOVPE. An anomalous emission band of high intensity, having intrinsic character, appears at energies below the GaAs band gap. The formation of a GaInPAs well at the GaInP-to-GaAs interfaces due to the P-to-As substitution is responsible for the appearance of such emissions. We find that the P-to-As substitution is not the only process occurring at the heterointerfaces; the intermixing of the GaInP/GaAs heterointerfaces during the growth also takes place. This intermixing could be promoted by the disordering in the GaInP layer and the strain at the interface caused by the GaInPAs intermediate layers produced at the interfaces. Based on these results, GaInP/GaAs wire structures with ultrasmall lateral confinement were produced.

Acknowledgements The authors wish to thank A. Kohl for fruitful discussions and M. Stephan for part of the PL measurements. References [1] M.

Razeghi, M. Defour, F. Omnes, M. Dobers, J.P. Vieren and Y. Guldner, AppI. Phys. Letters 55 (1989) 457. [2] M. Takikawa, T. Ohori, M. Takechi, M. Suzuki and J. Komeno, J. Crystal Growth 107 (1991) 942. [3] J.M. Olson, R.K. Ahrenkiel, D.J. Dunlavy, B. Keyes and A.E. Kibbler, AppI. Phys. Letters 55 (1989) 1208. [4] (a) M. Maseen, 0. Keiser, R. Westphalen, F.E.G. Guimarães, J. Geurts, J. Finders and P. Balk, J. Electron. Mater. 21(1992) 257; (b) B. Eisner, R. Westphalen, K. Heime and P. Balk, J. Crystal Growth 124 (1992) 326. [5] A. Usui, H. Sunakawa, FJ. Stiitzler and K. Ishida, Appi. Phys. Letters 56 (1990) 289. [6] G.W. Wicks, D.P. Bour, J.R. Shealy and J.T. Bradshaw, in: Proc. 13th Intern. Symp. on GaAs and Related Compounds, Las Vegas, NV, Inst. Phys. Conf. Ser. 83, Ed. W.T. Lindley (Inst. Phys., London—Bristol, 1987) p. 267. [7] J. Hergeth, D. Griitzmacher, F. Reinhardt and P. Balk, J. Crystal Growth 107 (1991) 537. [8] T.Y. Wang, E.H. Reihlen, H.R. Jen and G.B. Stringfellow, J. Appl. Phys. 66 (1989) 5376. [9] N. Kobayashi and Y. Kobayashi, Japan. J. Appl. Phys. 30 (1991) Li699. [10] F.P. Dabkowski, P. Gavrilovic, K. Meehan, W. Stutius, J.E. Williams, M.A. Shahid and S. Mahajan, AppI. Phys. Letters 52 (1988) 2142. [11] M.C. DeLong, P.C. Taylor and J.M. Olson, AppI. Phys. Letters 57 (1990) 620. [12] A. Gomyo, K. Kobayashi, S. Kawata, I. Hino and T. Susuki, J. Crystal Growth 77 (1986) 367.