4.1.1 MOVPE growth and HRXRD characterisation of GaAsSb/InP ...

14 Annual Report 2004 - Solid-State Electronics Department4.1.1 MOVPE growth and HRXRD characterisation of GaAsSb/InPsuperlatticeScientist:S. NeumannBackroundThe low bandgap material GaAs x Sb 1-x (x < 0.73) forms a type-II staggered heterojunction with InP[1]. This characteristic and its ultra-high p-type doping capability make this material ideal as thebase layer in InP-based heterostructure bipolar transistor (HBT) structures. The ternary III/Vsemiconductor GaAs x Sb 1-x has a solid phase miscibility gap ranging from x = 0.2 to 0.8 [2]. UsingMBE and MOVPE the metastable growth of GaAsSb lattice matched on InP has been demonstrated[2]. The use as base material in InP based HBT was first demonstrated in 1996 [3]. A staggeredheterojunction line up enables the use of InP as emitter and collector. Basic double heterostructurebipolar transistor (DHBT) structures can be realized with the advantages of low turn-on and highbreak down voltages [4]The most challenging step in the MOVPE growth of InP/GaAsSb/InP heterostructures for DHBTapplication is the optimisation of the most critical GaAs x Sb 1-x /InP heterojunction forming the baseand emitter layer in a HBT structure. Posibel Sb and As segregation and desorption results information of interlayers which may degrade the performance of devices. A growth interruptionbetween the GaAs x Sb 1-x base layer and the following InP emitter layer with group V stabilization ofthe surface leads to Sb contamination of the surface and to strong segregation of the Sb into the InPemitter layer [5]. To prevent this effect, a surface stabilization with Sb is impossible. Thestabilization with As as group V element, only, leads also to a possible GaAs interlayer with the outdiffusion of Sb at the surface region. Results from molecular beam epitaxie (MBE) indicate the Asto Sb exchange resulting in highly strained and degraded interfacial layers [6]. In this work weinvestigated the formation of the GaAsSb/InP interface grown by MOVPE using nitrogen as carriergas. Superlattice structures with different growth interruptions were realized and characterized withhigh resolution x-ray diffraction.Experimental SetupThe experiments were done on semi insulating, exactly oriented (001) InP substrate in a AIX 200reactor with RF heating at a substrate temperature of T g = 550 °C. Purified nitrogen (N 2 ) as carriergas was used to adjust the reactor pressure of p tot = 50 mbar at a total flow of Q tot = 3.4 slm.Hydrogen was connected to the source gas line, only. We used a complete non gaseous sourceconfiguration with tertiarybutylphosphine (TBP) / tertiarybutylarsine (TBAs) / trimethylantimony(TMSb) as group V sources, carbon tetrabromide (CBr 4 ) as group IV doping sources and the metalorganicsources trimethylindium (TMIn) / triethylgallium (TEGa). The group V to group III ratios(V/III) and also the group IV to group III ratios (IV/III) were calculated from the ratio of the partialpressures of the precursors involved. For the determination of the antimony (Sb) concentration inthe partial highly strained GaAsSb interlayers high resolution x-ray diffractometry (HRXRD) wasused. The integrity of the layer structures is proven by HRXRD measurements in the vicinity of the

Annual Report 2004 - Solid-State Electronics Department 15004 and 002-reflection in a coupled Θ-2Θ-mode using a double monochromator set-up. Therecorded reflection curves were compared to simulations using commercial software.Results and DiscussionThe growth of GaAs x Sb 1-x /InP superlattice with nitrogen as carrier gas at a growth temperature ofT g = 550°C was investigated. At this growth temperature the cracking of TBP is incomplete. Weused a V/III ratio of V/III=80 and a growth rate of r=5 nm/min to obtain InP layers with a mirrorlike surface. The GaAsSb layers were grown with a V/III ratio of V/III=0.8. A IV/V ratio ofIV/V=8.4% resulted in a doping level of p=4x10 19 cm –3 . Figure 1 shows the high resolution x-raydiffraction curves in the vicinity of the(004) - and the (002) - reflection of aselected GaAsSb/InP superlattice. Wesimulate both reflections with one set ofparameters. Equal layers, which weregrown under the same growth conditions,are coupled to minimize the number offree parameters. This enables us todetermine the composition and the layerthickness of each layer. The period of thefringes can be associated to single layersof the superlattice structure. Thedetermined values of composition andthickness (layer parameter) are in goodagreement with the intended data. Thesimulation of the x-ray data of this layerstack shows an excellent agreement to themeasured curves. To optimize the emitterbase junction, a TBAs purge between theGaAsSb:C base layer and the followingInP layer was included. To force a changeof the possible interlayer we varied thepurge time from t p = 1 s to t p = 60 s. Weobserved, that after a short purge a stableAs rich GaAs x Sb 1-x interlayer with aaverage composition of x ≈ 90 occurs. Asummary of the experimental results isgiven in Table 1. The composition isoutside the solid phase miscibility gap forthe used growth temperature ofFig. 6 X-ray recorded and simulated reflected T g =550°C. Independent of the purgeintensity in the vicinity of InP (004) and (002)time, we can not observe a trend of thereflextion of a superlattice structure (10 x 6 nm InP /composition or thickness of this31 nm GaAs 0.46 Sb 0.54 grown with 1 s As purge at thegrowth interruption. An optimum agreement is interlayer which has an average thicknessobtained for a 0.9 nm thick GaAs interlayer.of t interlayer = 7 Å.

16 Annual Report 2004 - Solid-State Electronics DepartmentTable 1:Probe Growth interruption Composition Interlayer thicknessM3148 1s GaAs 0.95 Sb 0.05 0.9 nmM2990 5 s GaAs 1.0 Sb 0.0 0.3 nmM3076 10 s GaAs 0.86 Sb 0.14 0.4 nmM3077 30 s GaAs 0.89 Sb 0.11 0.8 nmM3078 60 s GaAs 0.91 Sb 0.09 0.6 nmSimulation results of the interface composition and thickness between GaAsSb andInP heterostructuresThese results were confirmed independent by reflectance anisotropy (RA) spectroscopy and lowenergyelectron diffraction (LEED) [7]. The As terminated surface lead to a typical GaAsreconstructed surface. Growth of GaAsSb with a t p =10 s, TBAs stabilized growth interruption leadto similar results [8]. A fast change in the RA spectrum could be observed resulting in a 0.76 nmthick GaAs interlayer. All grown superlattic shown a mirror like surface and the HRXRDmeasurement prove the high crystal quality. To prevent the critical GaAs x Sb 1-x /InP heterojunctionfor possible contamination a short growth interruptions is preferable.ConclusionA novel growth method for the fabrication of GaAsSb/InP heterostructures using nitrogen as carriergas has been elaborated. It has been demonstrated that the segregations of Sb is controllable. Toprevent a segration of Sb in following layers an As purge after the growth of GaAs x Sb 1-x isnecessary. This forms a thin GaAs like interlayer between GaAs x Sb 1-x and the following layer.References:[1] M. Peter, N. Herres, F. Fuchs, K. Winkler, K.-H. Bachem, J. Wagner;“ “ , Appl. Phys. Lett, Vol. 74(3), 1999[2] M. J. Cherng, R. M. Cohen, G. B. Stringfellow;” ”, J. of Electronic Materials, Vol. 13, No. 5, 1984[3] R. Bhat, W-P. Hong, C. Caneau, M. A. Koza, C-K. Nguyen, S. Goswami;” “, Appl. Phys. Lett, Vol.68 (7), 1996[4] C. R. Bolognesi, N. Matine, X.G. Xu, G. Soerensen, S. Watkins;” “, Microelectronics Reliability, 39(1999) 1833-1838[5] C.X. Wang, O.J. Pitts, S.P. Watkins;” “, J. Crystal Growth,Vol. 248, 2003[6] R. Kaspi;”Compositional abruptness at the InAs-GaSb interface: optimizing growth by using the Sbdesorbtion signatur“,J. Crystal Growth, Vol. 201/202, 1999[7] Z. Kollonitsch_, K. Möller, F. Willig, T. Hannappel ;”Reconstructions of MOVPE-prepared group-VrichGaAsSb(1 0 0) surfaces“,Journal of Crystal Growth, Vol. 272, 2004[8] O.J. Pitts, S.P. Watkins, C.X. Wang;”RDS characterization of GaAsSb and GaSb grown by MOVPE“,J. Crystal Growth,Vol 248, 2003