domingo, 30 de mayo de 2010

Metalorganic vapor phase epitaxy of high-quality GaAs0.5Sb0.5


Metalorganic vapor phase epitaxy of high-quality GaAs0.5Sb0.5
and its application to heterostructure bipolar transistors

We report the growth and characterization of high-quality InP/GaAs0.5Sb0.5 /InP heterostructures and their application to double-heterojunction bipolar transistors ~DHBT!. The GaAs0.5Sb0.5 layer quality was evaluated by high-resolution x-ray diffraction ~XRD!, low-temperature photoluminescence ~PL!, and atomic force microscopy ~AFM!. The observed 4.2 K PL linewidth was 7.7 meV and XRD rocking curves matched those of dynamical scattering simulations. In contrast to previously reported InP/GaAs0.5Sb0.5 /InP DHBTs, the present devices show nearly ideal base and collector currents, low turn-on and collector offset voltages, and a high current gain.
Self-aligned DHBTs exhibit a cutoff frequency over 75 GHz and common-emitter current gain greater than 100 at 300 K.

GaAs0.5Sb0.5 lattice matched to InP substrates is a promising base material for NpN double heterojunction bipolar
transistors1,2 ~DHBTs! because of the very favorable band alignment at the InP/GaAs0.5Sb0.5 heterojunction. Recent low temperature photoluminescence measurements indicate that the GaAs0.5Sb0.5 conduction band is about ;180 meV above that of InP.3 The valence band offset is about 760 meV between GaAsSb and InP,3 which is two times larger than that of InGaAs/InP ~378 meV!. The large valence band offset effectively blocks hole back injection into the emitter. This band lineup eliminates any possible collector current blocking effect at the base-collector junction,4 and electrons are thus injected ballistically from the GaAs0.5Sb0.5 base to the InP collector. Wide band gap InP collector HBTs can then be grown without compositional grading at the base/collector junction, thus greatly simplifying device design and fabrication.
GaAsSb layers have been grown by metalorganic vapor phase epitaxy ~MOVPE!,1,2,5–8 but only Bhat et al.1 and Mc-
Dermott et al.2 reported the implementation of HBT prototypes based on this material system. These seminal reports of GaAsSb/InP HBTs featured poor junction ideality factors ~1.26–2.4! and a low current gain of about 22,1,2 which was then attributed to acceptor-like deep levels in the GaAs0.5Sb0.5 base layer. In addition, there are few reports concerning the optical properties of GaAsSb grown by MOVPE. In this letter, we report the growth and characterization of high-quality GaAs0.5Sb0.5 layers. Our InP/GaAsSb/ InP DHBTs show significantly improved direct current ~DC! and radio frequency ~RF! device characteristics. Epitaxial layers were grown in a horizontal reactor with a H2 total flow of 6 SLM at a reactor pressure of 100 Torr. Trimethylindium ~TMIn!, triethylgallium ~TEGa!, tertiarybutylarsine ~TBAs!, tertiarybutylphosphine ~TBP!, and trimethylantimony  ~TMSb! were used as precursors. N-type doping was accomplished with H2S ~200 ppm in H2) and p type by CCl4 ~500 ppm in H2). The susceptor was kept at 560 °C by a resistance heater. The growth rates were ;1.0mm/h for InP, and ;1.3mm/h for undoped GaAs0.5Sb0.5 . The substrates were ~001! exactly oriented InP ''Epi-Ready'' Sumitomo wafers. At the GaAsSb/InP interface, both group III and V elements change. In order to facilitate the gasswitching scheme, a thin InGaAs interface layer of less than
100 Å is inserted. At the InGaAs to GaAsSb interface, TEGa was switched to the run line after one second of TBAs and TMSb purging. Detailed studies on the interface layer will be reported elsewhere. With a nominal V/III ratio of 2, the Sb distribution coefficient for undoped GaAsSb layers lattice matched to InP is approximately equal to 0.9. This facilitates composition control for GaAs12xSbx . As the V/III ratio increases, the distribution coefficient decreases.5,6 Under these growth conditions, mirror-like surfaces were obtained for GaAs0.5Sb0.5 grown on InP substrates. A typical atomic force microscopy ~AFM! image of a GaAs0.5Sb0.5 surface is shown in Fig. 1~a! after 700 Å of growth. The root mean square ~RMS! surface roughness is less than 5 Å, but the surface is clearly textured. The reason for the unusual GaAsSb morphology is not clear yet. After the overgrowth of 1000 Å InP on the GaAsSb layer, the surface texture disappears and atomic steps are clearly observed, as shown in Fig. 1~b!. Figure 2 shows the ~004! High resolution x-ray diffraction ~XRD! curve for a 700 Å GaAsSb layer grown on InP: no signs of phase separation can be detected. The XRD peak width is comparable to that simulated by the dynamical x-ray diffraction theory. This clearly shows that high-quality GaAsSb can be grown although a large miscibility gap is predicted at this composition.





By Edgar Alberto Servita 18.856.338
CAF


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