Ga0.5In0.5P heterostructures: growth kinetics, electrical and optical properties

578

Journal of Crystal Growth 111 (1991) 578—583 North-Holland

Chemical beam epitaxy growth of GaAs/Ga0 5In05P heterostructures: growth kinetics, electrical and optical properties J.C. Garcia, P. Maurel, P. Bove, J.P. Hirtz Laboratoire Central de Recherches, Thomson — CSF, F-91404 Orsay Cedex, France

and A. Barski Instrument S.A. Riber, BP 231, F-92503 Rueil Cedex, France

We report the growth kinetics, together with electrical and optical properties of undoped chemical beam epitaxy GaAs/GaInP structures grown using triethylgallium (TEGa), trimethylindium (TMIn), arsine and phosphine as starting materials for group III and 5 cm3, depending on the independent growth temperature. Undoped GaAs/GaInP group V elements. Undoped GaAs layers were found to be p-type, of the gallium-to-arsenic ratio, heterojunctions with free hole concentration ranging fromelectron lx 1015gas to 9with x iO’ exhibit a two-dimensional T =4 K mobilities ranging from 20,000 to 25,000 cm2/V’ s. The optical characteristics of GaAs/GaInP multiquantum wells as a function of growth interruptions at the interfaces were investigated by photoluminescence measurements. The growth kinetics of GaAs and GaInP were studied by reflection high energy electron diffraction (RHEED) oscillations. The composition of GaInP was found to be controlled mainly by the growth rate dependence of the binaries (GaP, InP) on growth parameters. The indium composition was found to be proportional to the flux of TMIn at a fixed TEGa flow. A lattice matching better than 5 x i0~ can be routinely obtained. However, the composition exhibits a strong dependence on temperature. At temperatures above 510°C indium desorption occurs and the composition tends toward a Ga-rich phase, while below 510°C, the indium composition is enhanced by the decrease of the binary GaP growth rate. The PH 3 flow also affects significantly the ternary growth rate, mainly through indium incorporation modification. At low PH3 flow (1—5 SCCM), the indium concentration decreases. This fact is attributed to the preferential Ga—P compared to In—P bond formation.

1. Introduction In the past few years, chemical beam epitaxy (CBE) has emerged as a promising epitaxial technique which combines the respective advantages of both MBE and MOCVD. Indeed, high quality CBE growth of a wide range of Ill—V materials has already been demonstrated [1,2], in particular for phosphorus-based alloys. GaInP latticematched to GaAs, for example, is a very promising alternative to GaAIAs as a wide band gap barrier to GaAs in heterojunction devices, since GaAIAs suffers from various disadvantages. As an example, aluminium acts as a very efficient getter of both oxygen and carbon impurities. Therefore, the growth of high quality GaAs/GaAIAs struc0022-0248/91/$03.50 © 1991

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tures requires extremely rigorous experimental growth procedures and very low reactor impurities levels. In contrast, this does not occur for GaInP growth. Furthermore, GaInP lattice matched to GaAs has been reported to exhibit low deep level concentrations [4]. Little work has been done on the molecular beam epitaxy (MBE) growth of these structures, due to the operational difficulties associated with the handling and control of MBE phosphorus sources and also to the limited material quality achievable, which is mainly limited by the purity of phosphorus charges available [3]. On the other hand, high quality GaAs/GaInP heterostructures have already been grown successfully by metalorganic chemical vapour deposition (MOCVD) [4],

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

J. C. Garcia et a!.

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CBE growth of GaAS/Ga

but specific difficulties have been encountered, such as good homogeneity over large area substrates or problems of immiscibility [5]. The high level of flux control, stability and uniformity available with CBE coupled with its ability to produce very high quality phosphorus containing materials and extremely sharp heterointerfaces, make it extremely attractive for the Ga~In1_~P material system. However, the basic aspects of the kinetics involved in CBE growth of these alloys of are growth still poorly understood, parallel studies mechanisms andand material properties are necessary. In this paper, we report the CBE growth and characterization of GaAs/GaInP structures. Kinetics aspects as well as optical and electrical properties are presented, giving an overall view of the CBE growth of this system.

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Starting gaseous sources were triethylgallium (TEGa), trunethylmdium (TMIn), pure phosphine and arsine. The details of the CBE growth system have been described elsewhere [61.The growth rate of both GaP and InP was obtained using reflection high energy electron diffraction (RHEED) oscillations. The Ga~In 1~ composition was determined by X-ray diffraction analysis. The ternperature was controlled using an infrared pyrometer calibrated with the melting point of InSb (525°C). 2.1. GaAs bulk layers GaAs bulk materials grown by CBE have been extensively studied by various authors [7.8]. GaAs layers grown in the 500—650°C temperature range showed p-type conductivity. As expected, carbon was the main acceptor. The hole concentration 5 cm3 when the increases from x 1015 tofrom 9 X iO’ temperature is 1increased 500 to 650°C. The apparent activation energy for carbon incorporation is found to be 25 kcal/mol, which is close to the value reported by Benchimol et al. [9]. Surprisingly, no significant dependence on the Ill/V ratio has been observed, as has been previously

Fig. 1. 300 and 77 K mobihties of GaAs bulk layers as a function of free hole concentration.

reported by different authors [8,10]. Fig. 1 summarizes the results of the mobility at 300 and 77 K as a function of hole concentration. Good agreement is obtained with the data of Chiu et al. [7], which have been reported in the same figure. RHEED intensity oscillation investigations of the GaAs growth by CBE have already led to valuable information on the understanding of the growth mechanisms involved in the use of organometallic starting sources [11]. Fig. 2 shows the dependence of the growth rate as a function of both arsine and hydrogen flow. As previously reported by various authors [11,12], the growth rate decreases with increasing arsine [11] or hydrogen [12]efficiently flow. Since does [13], not adsorb on molecular the GaAs hydrogen (001) surface as has been previously shown, it seems unlikely that H 2 molecules cause such effects. Considering the fact that GaAs growth using uncracked AsH3 is extremely slow or even impossible [14], one possibility is that hydrogen interacts with arsenic on the surface, forming thermodynamically stable

J. C. Garcia et a!.

580

/ CBE growth of GaAs / Ga03In05P heterostructures The lattice mismatch is about 4 X io-~and the crystalline perfection of the layer is comparable to

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that of the GaAs substrate, with a full width at half maximum (FHWM) value of 15”. At temperatures below 520°C, the crystal quality is drastically degraded, leading to a broad diffraction peak

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for the ternary alloy. Fig. 3b shows the variation, at a constant H2/TEGa flow rate kept at 5 SCCM,

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H2/TMIn of the indium flowcomposition rate. The indium as a function composition of the is

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Fig. 2. GaAs growth rate as a function of arsine (A) and hydrogen (D) flow rates. The arsine flow rate was kept at 5 SCCM when hydrogen was introduced.

chemisorbed AsH~ molecules [15], which block the gallium deposition sites and consequently induce a greater TEGa desorption. 2.2. Ga~Inj ~P bulk material —

Fig. 3a shows the X-ray double diffraction pattern of a 1 ,.im thick GaInP layer grown at 520°C. a

found to .vary linearly, regardless of the temperature used. However, the composition depends strongly on the temperature. This contrasts with the GaInP composition behaviour previously reported by Ozasa et al. [16], who observed both constant growth rate and indium composition over a large temperature range (400—520°C).In fact, these authors used triethylindium as a starting source and it seems likely that the temperature behaviour of the GaInP growth rate differs from that using TMIn. The temperature behaviour we observed can be explained by considering the growth rate of the two binaries GaP and InP [17]. The growth rate of GaP increases with increasing temperature and reaches a maximum near 550°C. Beyond this temperature, the growth rate slightly decreases. In the case of InP, a constant growth rate is observed over a large temperature range

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J. C. Garcia et aL

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CBE growth of GaAs / Ga

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fraction decreases with decreasing phosphine flow

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rates. The strong enhancement in Ga composition can be related the energy between the Ga—P and to In—P bonds. difference Since it has been

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reported that the GaP bond is stronger than the InP one [5], it seems reasonable to conclude that at low phosphorus coverage, the GaP bonds are preferently formed, thus leading to a desorption of In species.

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P113 FLOW- RATE . (sccm) Fig. 4. Indium mole fraction (0) in GaInP and expected indium composition (U) deduced from the phosphine flow rate dependence of the two binaries as a function of phosphine flow rate. Note the strong decrease of indium composition obtained at low phosphine flow rates.

Hall measurements on undoped GaAs/GaInP heterostructures revealed the existence of a two-dimensional electron gas (2DEG) with the2/V. following ~ and mobilities: ~t(4 K) = 20000—26000 ~t(300 K) = 3000—4000 cm2/V. s with cm N = (2—4) x 10 cm The two-dimensionality of the electron gas has been established by the mobility plateau obtained at low temperature measurements, demonstrating the good quality of the in-

(420—510°C). At higher temperatures, the InP growth rate decreases significantly. Therefore, at temperatures higher than 510°C, the indium incorporation in GaInP is mainly limited by the desorption of the indium species, while at lower temperatures the increase in the indium composition is analogous to the decrease of the GaP binary growth rate. More detailed results of RHEED intensity oscillation measurements of the growth rate of the two binaries will be reported elsewhere [17]. The dependence of the indium composition as a function of the phosphine flow is shown in fig. 4. At low and high phosphine flow rate, the indium composition decreases. At high phosphine flow rates, the measured indium concentration agrees well with that deduced from the phosphine flow dependence of the growth rate of both binaries GaP and InP [17], the InP growth rate being reduced more significantly than the GaP growth rate while increasing the phosphine flow, At low phosphine flow rates, the expected indium mole fraction, deduced from the binaries growth rates is found to be constant. As seen in fig. 4, experimental results indicate that indium mole

terface. Growth interruption at the interface was necessary in order to obtain such 2DEG. Multiquantum well (MQW) structures were grown to further assess the quality of the interfaces. The GaAs growth rate was 1 ~tm/h, while GaInP growth rate was fixed to 1.5 ~tm/h. Growth interruption was necessary at the normal GaAs/ GaInP interface in order to obtain reasonably good confinement energies. The experimental procedure is the following: At the normal interface, the growth of GaAs is stopped and the surface is stabilized under phosphine flow for 15 s, the RHEED pattern showing a well defined 2 x 4 reconstruction. Finally, GaInP growth is initiated simply by switching both the TEGa and TMIn flows from the vent to the growth chamber. It should be noted that for higher durations of exposure of the GaAs surface to a phosphine flow, the surface becomes rough. Similarly, the GaInP surface also becomes rough with arsenic exposure up to 10 s. It seems reasonable to suggest that for long time stabilization under phosphine or arsine, intermediates GaInAsP alloys are formed at the interfaces. These observations can be correlated with the work of Lee et al. [18], who have to include intermediate GaP and GaInAsP layers at the GaAs/GaInP and GaInP/GaAs interfaces,

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J. C. Garcia et a!.

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/ CBE growth of GaAs / Ga0 51n05P heterostructures

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respectively, in order to fit the double crystal X-ray diffraction spectrum of a superlattice structure. On the other hand, if no growth interruption occurs, the preferential incorporation of arsenic, if compared with phosphorus, leads to the formation of an intermediate GaInP(As) layer at the normal interface, which does not happen at the inverted interface. The GaInP/GaAs MQW structures were characterized by low temperature photoluminescence (PL) using an excitation wavelength of 0.63 .tm. Fig. 5a shows a typical spectrum of such a structure. The energy shift due to quantum confinement relative to GaAs band gap is shown in fig. 5b as a function of the well width. Good agreement is obtained between the theoretical calculation, performed using a simple square well model assuming a conduction band valence band discontinuity ratio (~ ~ E~)of 0.4, and the experimental values. Wells as thin as 10 A have been obtained with associated energy shifts up to 310 meV. This demonstrates the high quality of the interfaces achievable by CBE. In summary, high quality GaAs/GaInP structures were grown by CBE with a good reproducibility, although a strong composition dependence of the ternary alloy with growth parameters such

as temperature and phosphine flow was observed. Monolayer growth control can be achieved, as demonstrated by the growth of thin quantum wells.

Acknowledgement This work has been supported by EEC under ESPRIT contract No. 5031.

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[9] J.L. Benchimol, F. Alexandre, Y. Gao and F. Alaoui, J. Crystal Growth 95 (1989) 150. [10] J. Saito, K. Ono, K. Nambu, T. Ishikawa and K. Kono, Japan. J. AppI. Phys. 127 (1988) L114. [11] T.H. Ckiu, J.E. Cunningham and A. Robertson, J. Crystal Growth 95 (1989) 136. [12] S. Maruno, Y. Nomura, H. Ogata, M. Gotoda and Y. Morishita, J. Crystal Growth 97 (1989) 578. [13] R.Z. Bachrach and R.D. Bringans, J. Vacuum Sci. Technol. Bi (1983) 142. [14] N. Vodjani, A. Lemarchand and H. Paradan, J. Physique Colloq. 43 (1982) C5-339.

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[15] D.M. Joseph, R.F. Hicks, L.P. Sadwick and K.L. Wang, Surface Sci. 204 (1988) L721. [16] K. Osaza, M. Yuri, S. Tanaka and H. Matsunami, J. AppI. Phys. 65 (1989) 2711. [17] J.C. Garcia, P. Maurel, P. Bove and J.P. Hirtz, J. Appl. Phys., submitted. [18] H.Y. Lee, M.D. Crook, M.J. Hafich, J.H. Quigley, G.Y. Robinson, D. Li and N. Otsuka, Appl. Phys. Letters 55 (1989) 2322.