Strain provides a most useful additional parameter in the design and fabrication of devices based on semiconductor superlattices and multiquantum wells. Thus, compressive strain in III-V semiconductors offers the possibility of making p-channel MODFET's since the strain leads to a reduced hole effective mass. Unwanted non-radiative processes can be supressed in strained laser structures.
The aims of this Action were to:
-investigate and optimise the growth of strained InGaAs/InAlAs and InGaAs/InP structures for HEMT and MIS Like FET devices.
-investigate the basic physics of strained structures: strain, relaxation mechanisms, band structures, transport properties.
-process and test HEMT and MIS Like FET devices, and optimise all processing steps (growth, ohmic and Schottky contacts, channel thickness, doping) in order to evaluate the potential applications of strained structures as devices.
Strained indium gallium arsenic/indium aluminium arsenic and indium gallium arsenic/indium phosphorus structures have been grown and investigated. Strain assessment and strain relaxation mechanisms are presented for tensile and compressive layers. Experimental data of band offset measurements have been compared to theoretical calculations. Processing of high electron mobility transistor (HEMT) and metal insulator semicondutor (MIS) like field effect transistor (FET) have been optimized. I-V good characteristics were obtained in HEMT and indium phosphorus metal insulator semiconductor field effect transistor (MISFET) optimized devices. The maximum value of transconductance was measured in indium 0.6 gallium 0.4 arsenic channels thicker than 120 angstroms.
Photoluminescence (PL) measurements are presented for thin epitaxial layers of indium arsenide (InAs), of thickness between 2.5 and 36 angstroms, grown on indium phosphide by molecular beam epitaxy (MBE). The combination of efficient carrier capture and PL red-shift with increasing InAs thickness clearly indicates for formation of indium arsenide quantum wells on the indium phosphide surface. Data are also presented for indium arsenide/indium phosphide structures capped with strained layers of either gallium arsenide or indium aluminum arsenide and for indium arsenide/indium phosphide layers subjected to oxidation with time. Since radiative recombination within the indium arsenide layers can be distinguished from PL arising from both bulk and surface defects, this system allows the quality of both the indium arsenide/indium phosphide and indium arsenide/air interfaces to be monitored via their influence on the indium arsenide quantum well luminescence.
The excellent electronic and optical properties of indium based compound semiconductors enhanced by strain have made them increasingly important to the realization of long wavelength optical and high speed electronic devices. In particular, the indium arsenide/indium phosphide systems looks promising for the achievement of strained layer quantum well lasers. Single and multiple strained indium arsenide/indium phosphide quantum wells are grown by hydride vapour phase epitaxy which presents advantages such as the control of the surface stabilization or the independence of gaseous species. The ultrathin layers present a regularity of interfaces and a high degree of carrier confinement, revealed by photoluminescence investigations. This work confirms the possibility of the growth of indium phosphide/indium arsenide/indium phosphide quantum wells and multiquantum wells by the hydride vapour phase epitaxy (HVPE) method. The heterostructures are studied by photoluminescence in order to compare them to similar organometallic vapour phase epitaxy (OMVPE) grown structures. The excitonic peak energy values obtained with HVPE grown and OMVPE grown quantum wells are approximately the same. Taking into account the approximations involved in the theoretical approach, the 1.28 eV peak energy can be attributed to 2 +/- 1 monolayers InAs thickness. The HVPE grown single quantum wells exhibit good quality interfaces as evidenced by 6 to 9 meV linewidths of exitonic emissions. Concerning the multiquantum wells grown by HVPE, the interdiffusion of V elements is probably responsible for the broadening of the photoluminescence spectra. Additional bands are present in multiquantum wells grown by HVPE as well as in the single quantum well of about 4 monolayers grown by OMVPE. These bands are attributed to fluctuations of the InAs layer thickness. The HVPE method appears to be comparable to OMVPE method as regards the quality of grown heterostructures. In addition, it is a cheaper and safer method, very promising for heterostructures achievement.
APPROACH AND METHODS
-Strained and unstrained InGaAs/InAlAs structures were grown by MBE and strained and unstrained InGaAs/InP structures by HVPE. Double crystal X-ray diffraction (DCXRD), high-resolution transmission electron microscopy (HRTEM) and photoluminescence spectr oscopy (PL) enable the determination of the composition, structural quality confinement and interface abruptness of deposited quantum wells (QW).
-Strain assessment has been performed by DCXRD and raman spectroscopy and deduced from the PL peak position and from its temperature dependence, which gives the heavy hole-light hole splitting in tensile samples. Relaxation mechanisms are investigated by HRTEM and RHEED oscillations, and alloy clustering in InGaAs layers by phototransmittance and HRTEM.
-Band structures and band offsets are computed in the tight-binding approximation and investigated by DLTS admittance technique, photoluminescence excitation, photoconductivity, photoreflectance, high resolution infra-red absorption, photoluminescence an d raman spectroscopy under electric field.
-Transport properties in device structures are measured by low temperature mobility, parallel transport and vertical transport by resonant tunelling. Schottky and ohmic contacts processing are systematically investigated using XPS, transmission line meas urement and I(V) characteristics. HEMTs are fabricated and optimised devices characterised by extrinsic conductance measurement. Two technologies, Si+ ion implantation and MBE growth of n+ doped InGaAs layers, are used for processing and testing InP MISFET and InP/InGaAs/InP MIS Like FET devices. A quasi-analytical self-consistent model for HEMTs and modelling of charge transfer in MIS-like FET have been developed.
PROGRESS AND RESULTS
-Insulating In0.52Al0.48As layers of highest quality obtained at 530 C. InxGa1-xAs QW and MQW grown in the range 400 C-500 C and in the x range 0.53 - 0.65.
-Homogeneous InAs QW with a PL line at 1.28 eV (FWHM = 6 meV), 1-2 mm thick, and 10 and 20 InAs QW separated by 200 thick InP layers grown by HVPE. Lattice-matched and strained InxGa1-xAs/InP QW 100 grown. The interface abruptness is not yet fully optimised compared to the InxGa1-xAs/InAlAs QW grown by MBE.
-Strain assessment of InxGa1-xAs layers grown by MBE at 515 C performed as a function of thickness. In all samples, composition modulation along <100> directions with wavelength decreasing from 605 nm for thin layers to 240 nm for thick layers found atthe interface. Beyond the critical thickness, misfit dislocations not observed at the interface; relaxation of energy by means of defect nucleation more favourable than a further decrease of the modulation period. Same composition modulation observed in InxAl1-xAs samples. 3-D nucleation onset and strain relaxation determined as a function of composition and growth temperature.
-Alloy disorder in InGaAs is related to spinodal decomposition.
-Good fit obtained between experimental data and calculated results of band offsets. Theoretical investigations focused on the description of bulk and band structure of strained materials; calculation of band alignments in InGaAs/InAlAs system; possibili ty of breaking the transitivity rule, and even tuning band offsets by depositing thin intralayers at the interface; and a study of confinement and parallel conduction in ultrathin InAs/InP QW.
-High Schottky barriers fabricated with n-InP and n-InAlAs and thoroughly probed by XPS as well as transport techniques.
-HEMT structures growth and processing optimised. No deep level found in InAlAs. Mobility of 104cm2/sV measured in In0.65Ga0.35As channels of HEMTs. InxGa1-xAs channels with a thickness of at least 120 and an x value of 0.60 were found to give the besttransconductance which increases from 320 ms/mm at x=0.53 to 360 ms/mm at x=0.60. I-(V) measurements show good characteristics.
-InP MISFETs have been processed using a 800 thick SiO2 layer as insulator and an Si1 ion implantation process carried out through an InGaAs mask (also used as encapsulant). The I-(V) characteristics are among the best static performances reported foran enhancement mode MISFET.
POTENTIAL
This Action provides important knowledge and know-how in: fabrication of strained InAlAs/InGaAs and InGaAs/InP single and multiquantum wells by MBE and HVPE, respectively; strain assessment and relaxation mechanisms, alloys clustering; band structures of strained materials; transport properties measurements; optimisation of HEMT and MISFET devices processing; measurement of device characteristics.
