锑化镓物理与技术

Recent advances in nonsilica fiber technology have prompted the development of suitable materialsfor devices operating beyond 1.55 um. The Ill-V ternaries and quaternaries (AlGaln)(AsSb) latticematched to GaSb seem to be the obvious choice and have turned out to be promising candidates forhigh speed electronic and long wavelength photonic devices. Consequently, there has beentremendous upthrust in research activities of GaSb-based systems. As a matter of fact, thiscompound has proved to be an interesting material for both basic and applied research, At presentGaSb technology is in its infancy and considerable research has to be carried out before it can beemployed for large scale device fabrication. This article presents an up to date comprehensiveaccount of research carried out hitherto. It explores in detail the material aspects of GaSb startingfrom crystal growth in bulk and epitaxial form, post growth material processing to device feasibilityAn overview of the lattice, electronic, transport, optical and device related properties is presentedSome of the current areas of research and development have been critically reviewed and theilsignificance for both understanding the basic physics as well as for device applications areaddressed. These include the role of defects and impurities on the structural, optical and electricaproperties of the material, various techniques employed for surface and bulk defect passivation andtheir effect on the device characteristics, development of novel device structures, etc. Severalavenues where further work is required in order to upgrade this Ill-V compound for optoelectronicdevices are listed. It is concluded that the present day knowledge in this material svstem is suffcientto understand the basic properties and what should be more vigorously pursued is theirimplementation for device fabrication. 

I.IMPORTANCE OF GALLIUM ANTIMONIDE

Historically, the research and development of variousIIl-V compound semiconductors is associated with thewavelength of the optical fiber loss minima.' The shift in thefiber loss minima towards higher wavelengths from 0.8 umover the past 2 decades has shifted the material of interestfrom time to time.' Even though the present day optical communication systems are tuned to 1.55 um, the next generation systems may have to be operated well above this wavelength. This is because recent developments in the opticalfiber research have shown potentiality for certain classes ofnonsilica fibers for optical communication applicationswhose loss minima fall in the 2-4 um range.2 For examplethe heavy metal fluoride glasses are speculated to have mini-mum attenuation at 2.55 um with a loss, one to two orders ofmagnitude lower than the present day silica fibers. This isalso important since, at longer wavelengths, loss due to Rayleigh scattering is significantly reduced. Consequently, therehas been an upthrust in research activities in new materiasystems for sources and detectors operating in the 2-4 umregime. Among compound Ill-V semiconductors, galliumantimonide (GaSb) is particularly interesting as a substrate material because its lattice parameter matches solid solutionsof various ternary and quaternary Ill-V compounds whoseband gaps cover a wide spectral range from ~0.3 to 1.58eV3 i.e, 0.8-4.3 um, as depicted in Fig. 1. Also, detectionof longer wavelengths, 8-14 um, is possible with intersub-band absorption in antimonide based superlattices.4 Thesehave stimulated a lot of interest in GaSb for basic research aswell as device fabrication. Some of the important materialproperties of GaSb are listed in Table I.

From device point of view, GaSb based structures haveshown potentiality for applications in laser diodes with lowthreshold voltage,6.7photodetectors with high quantumefficiency, high frequency devices, .superlattices with tailored optical and transport characteristics,!l booster cells intandem solar cell arrangements for improved efficiency ofphotovoltaic cells and high efficiency thermophotovoltaic(TPV) cells.12 Interestingly, the spin-orbit splitting of the va-lence band is almost equal to the energy band gap in GaSbleading to high hole ionization coefficients. This results insignificant improvement in the signal-to-noise ratio at 入> 1.3 um in GaAlSb avalanche photodetectors grown onGaSb.8 GaSb is also predicted to have a lattice limited electron mobility greater than GaAs making it of potential inter-est in the fabrication of microwave devices. InGaSb has beenproposed as an ideal material for transferred-electron devicesby Hilsum and Rees'o with a low threshold yield and a largevelocity peak-to-valley ratio, using a Monte Carlo simulationbased on the three-level model.

II.COMPOUND PREPARATION AND CRYSTALGROWTH

A. Phase equilibria

As early as in 1926, Goldschmidt synthesized GaSb anddetermined its lattice constant.21 Since then, it was followedby several workers and the lattice constant was redeterminedmore precisely.22 The phase diagram of this compound hasbeen determined simultaneously by Koster and Thoma23 andGreenfield and Smith.24 Later on, the liquidus has been reevaluated in different regions of the phase diagram by several researchers.25-36 The phase diagram and the calculatedsolidus ofGaSb are shown in Figs. 2 and 3, respectively. Themelting point of GaSb has been reported to lie between 705and 712 ·C.27 A melting point depression of less than 50 °Cis observed for compositions + 30 at. % on either side of thestoichiometric composition.27 Brice and King3% showed thatthe liquid-solid-vapour equilibrium temperature is sensitiveto pressure 712 C being the maximum (refer to Fig. 4).The heat of formation and fusion are - 4.97 + 0.22 and 6.0+ 0.36 kcal/g atom, respectively. The dissociation pressureat the melting point is about 10-2mm Hg. Above 370 CSb starts volatilizing from the melt. Thus, GaSb will decom-pose to yield Sb2) and GaSb dissolved in liquid Ga.38,39 Atthe maximum melting point, the partial vapour pressure ofSb is ~3X 10-6 Torr38 With this partial pressure, up to~2X 1015 Sb atoms per second could be lost from eachsquare centimetre of solid surface. The partial pressure of Gais less than 10-9 Torr at the maximum melting point.