Study On The Molecular Beam Epitaxy Growth Of InAs/GaSb Type Ii Superlattice Infrared Detection Structure

The type-II InAs/GaSb superlattices (SLs) grown on the GaSb substrate is anovel material system for infrared detection. The detective range of InAs/GaSbsuperlattice can be tuned from3μm to30μm by changing the thickness of each layer.Type II superlattice IR detectors have lower dark current and higher operatingtemperature. Moreover, III-V material growth and device process are much moremature than II-VI, which is competitive to HgCdTe and InSb. Due to the advantagesin infrared detection for type-II InAs/GaSb superlattices, domestic andforeign research institutions have done a lot of researching and analyzing work on thesuperlattice material physical properties, growth techniques and devices fabrication.In the past few years, type-II InAs/GaSb superlattices (SL) photodetectors havedeveloped quickly and made important break-throughs. In early1990s the firstsuperlattice material was grown, whereafter, in about the year of2000the epitaxialsuperlattice materials reaching the device fabrication level were successfully grownand the single pixel device was fabricated, and to21st century the deviceperformances have been significantly improved and large format focal plane arrays(FPA) have successfully been demonstrated.Despite the rapid progress, the performances of SL infrared detectors reportedhave far not reached their theoretical prediction yet and are inferior to that of HgCdTe.More unfortunately, in China the InAs/GaSb type-II superlattices materials systemand detectors currently are only in the early stages, which are even new andunfamiliar for us. To fabricating the high performance SLs detectors with independentintellectual property rights to fulfill the demand of the3rd generation infrareddetective system, we have spent a lot of time studying SLs material system.The dissertation focuses on the growth of InAs/GaSb superlattices detection structure by molecular beam epitaxy (MBE) technique, to obtain the high quality SLsmaterials. The research is listed as following:Before growing InAs/GaSb SLs on the GaSb substrates, it is essential to growhigh quality GaSb buffer layer. We first studied the properties of theoxidation layer on the GaSb (001) substrate surface because the GaSb was very easyto be oxidized. The oxidation layer was successfully removed by the effectiveexperiment methods, and the smooth GaSb buffer layer with regular atomic steps wasgrown. The root mean square (RMS) roughness value on2μm x2μm scan area of thebuffer layer surface was only1. Besides, we also studied the growth properties ofGaSb buffer layer on vicinal ((100)+20(111)) substrates. The results showed that thebuffer layer grown on vicinal substrates had more atomic steps than that grown on flat(001) substrates, therefore, it made the buffer layer surface more rough and the RMSroughness value increased from0.098nm (grown on flat substrates) to0.223nm. Inaddition, the characteristics of GaSb layers on GaSb (001) substrates grown at lowtemperature ranges were researched. With reducing the growth temperatures, thegrown mode tended towards island structures and its amounts gradually increased.Moreover, by the photoluminescence (PL) measurement we have found that a lot ofnative defects existed in the GaSb epitaxy layers. Finally, we studied the growthproperties of the InAs layers and found that it was easier to be grown layer by layerthan GaSb layers.Growth temperatures play an important role in the SLs crystalline quality,surface morphology and optical property. The optimal growth temperature range of400±100C for InAs/GaSb superlattices was determined. The mismatch between InAslayer and GaSb layer in SLs material structure can be well compensated by the InSb.However, it is challenging to grow high quality and repeated InSb interface layers forthe large lattice mismatch (>7%) between InSb and InAs or GaSb layers. We studiedthe optimal growth mode and thickness of InSb interface layers, and well designed itsshutter sequences during the growth process by MBE. What’s more, we calculated theamounts of defects in the epitaxy SLs layers semi-quantitatively by the good model builted by ourself and fitting the X-ray diffraction (XRD) curves.GaSb substrates are highly conductive, which causes transport propertiesmeasurements of high quality InAs/GaSb SLs grown on the GaSb substratestechnically difficult. We reviewed five typical approaches for the electricalmeasurement of SLs materails. By the capacitance-voltage (C-V) measurement, wegot the background carrier density of1015cm-3in the mid-wave SLs material. westudied the transport properties of the15ML InAs/7ML GaSb long-wavelength SLsmaterial by removing the conducting GaSb substrate. The unintentionally doped15ML InAs/7ML GaSb superlattice structure presented n-type conductivity within thewhole temperature region from13K to room temperature measured by Hall Effectmethod. The residual carrier concentration was measured to be n=1.1×1016cm-3at77K. The quantum efficiency of a longwave SLs detector was increased effectively bysimply introducing Be doping in the intrinsic region.The full width at half maximum (FWHM) of the x-ray diffraction satellite peaksof the midwave and longwave SLs detection structure materials grown in our lab wereboth only19arcsec, which was one of the smallest values ever reported. We grew the5μ m thickness SLs material successfully, which made it possible to obtain highquantum efficiency SLs detectors. Moreover,2＂epitaxy SLs materials showed theexcellent uniformity. The dark current density of the processed photodiodes whichhad a50%cutoff wavelength of10μm at77K at-20mV bias was9.8×10-5A/cm2,and the RmaxA (at-20mV bias) and the R0A (at zero-bias) were277.05Ω·cm2and112.31Ω·cm2, respectively. The dark current density of the processed photodiodeswhich had a50%cutoff wavelength of12μm at77K at-40mV bias was5.1×10-4A/cm2, and the RmaxA (at-40mV bias) and the R0A (at zero-bias) were128.54Ω·cm2and23.8Ω·cm2, respectively. The dark currents of the two longwave SLs photodiodesare among the lowest level ever reported.The mid-wave and long-wave SLs detection materials grown by us have beenused to fabricate the nation’s first high performance SLs mid-wave FPA detectorcontaining128×128pixel cells and first high performance SLs long-wave FPA detector containing320×256pixel cells, respectively. The high performance SLsmid-wave FPA detector showed a50%cutoff wavelength of5.4μm at77K. Thequantum efficiency was measured to be25%. The peak wavelength detectivity was3×1012cmHz1/2/W. The percentage of the dead pixels was only1.8%, non-uniformity5.41%and a noise equivalent differential temperature33.4mK. The high performanceSLs long-wave FPA detector showed a100%cutoff wavelength of10.5μm at77K.The peak wavelength detectivity was8.41×109cmHz1/2/W. The percentage of thedead pixels was only2.6%, non-uniformity6.2%.