J. Klem

Growth and properties of GaAs/AlGaAs on nonpolar substrates using molecular beam epitaxy

Using molecular beam epitaxy, we have successfully grown device quality GaAs/AlGaAs on (100)‐oriented Ge and Si substrates. Modulation doped field effect transistors have been fabricated from these layers which exhibit room‐temperature transconductances as high as 160 and 170 mS/mm for layers on Ge and Si, respectively, and showed no looping in either case. At 77 K, these values rose to 345 and 275 mS/mm for Ge and Si, respectively. Analysis by transmission electron microscopy of layers grown on Ge showed that the antiphase disorder was contained within the 250‐A‐thick initial layer which was grown at a 0.1‐μ/h growth rate at a substrate temperature of 500u2009°C. For both the layers grown on Si and Ge specular surface morphologies were obtained. The photoluminescence of GaAs/AlGaAs quantum wells grown on Si and Ge was similar in intensity to the same quantum well structures grown on GaAs. In quantum wells grown on Ge, the luminescence was dominated by a donor‐acceptor recombination at 1.479 eV, which appears...

Incorporation rates of gallium and aluminum on GaAs during molecular beam epitaxy at high substrate temperatures

Gallium arsenide, aluminum arsenide, and aluminum gallium arsenide epitaxial layers were grown by molecular beam epitaxy in the substrate temperature range 590–720u2009°C. The incorporation rates of Ga and Al in this temperature range were studied by means of thickness measurements. The growth rates of GaAs and AlxGa1−xAs were observed to be dependent on growth temperature above 640u2009°C while the AlAs growth rate was observed to be independent of growth temperature in the range investigated. The reduction of the GaAs growth rate at a growth temperature above 640u2009°C was found to be lessened by the presence of minute amounts of Al and excess As. For the fixed Ga flux and a growth temperature of 700u2009°C the GaAs growth rate and the Ga contribution to the growth rate of Al0.3Ga0.7As were 0.50 and 0.89 times their low temperature values, respectively, while at 680u2009°C these values were 0.88 and 0.99, respectively.

Ordering in GaAs1−xSbx grown by molecular beam epitaxy

Ordering in (100)u2009GaAs1–x Sbx epilayers grown by molecular beam epitaxy has been studied by transmission electron microscopy. Diffraction patterns taken from plan‐view and cross‐sectional samples of epilayers reveal a unique evolution of the ordering with the growth temperature, showing the formation of a short range order at lower temperatures and a long range order at higher temperatures. A distinct anisotropy of the formation of ordered structures between [011] and [011] axes is observed from all epilayers investigated. Based on the analysis of the growth conditions, it is suggested that the anisotropy of the ordering is caused by the surface atomic structure of the growing epilayers.

On the collapse of drain I-V characteristics in modulation-doped FET's at cryogenic temperatures

The collapse of the drain current-voltage characteristics of modulation-doped field-effect transistors (MODFETs) at cryogenic temperatures, previously thought to be unavoidable, has been investigated. The results indicate that the mechanism responsible for the collapse is dependent on both the device fabrication steps and the parameters of crystal growth. Bulk AlxGa1 - xAsFETs fabricated in our laboratory exhibited little or no collapse in the I-V characteristics at 77 K in the dark, demonstrating that the mechanism responsible for this pheonomenon is not related to problems associated with contacting AlxGa1 - xAs. MODFETs with proper fabrication and growth procedures showed no collapse. In those devices exhibiting no collapse, the source resistance exhibited a substantial decrease upon cooling. At 300 K source resistances slightly over 1.0 Ω . mm with a transconductance of 170 mS/mm were obtained. Upon cooling, the source resistance decreased to 0.5 Ω . mm with a transconductance of 280 mS/mm. These results demonstrate that MODFETs will exhibit enhanced performance at 77 K without exposure to light. Specific contact resistivities measured at room temperature ranged from 2 × 10-7to 2 × 10-6Ω cm2depending on the structural parameters.

Optical properties of GaAs on (100) Si using molecular beam epitaxy

Undoped GaAs layers have been grown on (100) oriented Si by molecular beam epitaxy and have been studied using photoluminescence, photographic, and x‐ray measurements. To minimize antiphase disorder, an As primer layer was successfully used to initiate the polar on nonpolar growth. Photoluminescence spectra show the presence of five bands into which the luminescence from each sample falls. The energy variation within any given band was less than 4 meV. X‐ray and photoluminescence analysis appear to indicate some strain in the crystal. Although very preliminary, the results obtained indicate that GaAs on Si can potentially be used for a number of hybrid ciruits.

High transconductance InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

Pseudomorphic In0.15Ga0.85As/Al0.15Ga0.85As modulation-doped field effect transistors (MODFETs) exhibiting extremely good dc characteristics have been successfully fabricated, dc transconductance in these strained-layer structures of 270 mS/mm were measured for 1-µm gate, normally-on devices at 300 K. Maximum drain current levels are 290 mA/mm, with excellent pinch-off and saturation characteristics. The transconductance increased to 360 mS/mm at 77 K while no persistent photoconductivity or drain collapse was observed. Preliminary microwave results indicate a 300-K current gain cutoff frequency of about 20 GHz. These results are equivalent to the best GaAs/AlGaAs MODFET results and are due in part to the improved transport properties and carrier confinement in the InGaAs quantum well.

Use of a superlattice to enhance the interface properties between two bulk heterolayers

Single interface modulation‐doped Alx Ga1−xAs /GaAs heterostructures with the binary on top of ternary were grown by molecular beam epitaxy. By incorporating a 150‐A‐thick Alx Ga1−xAs /GaAs three‐period superlattice in place of an undoped Alx Ga1−xAs spacer layer, 10‐K mobilities of up to 256u2009000 cm2/Vs were obtained. This value is about 6.5 times that of the previous best value. This dramatic improvement is tentatively attributed to the relief of strain caused by the small but significant lattice mismatch although impurity trapping by the superlattice may also play a role. Normal modulation‐doped structures where the ternary is grown on top of binary also showed mobility improvement (about 30%) when the undoped AlGaAs spacer layer is replaced with a three‐period superlattice of the same thickness. This concept should have a significant role in heterojunction bipolar transistors, field‐effect transistors, lasers, and other heterojunction devices.

Determination of the valence‐band discontinuity between GaAs and (Al,Ga)As by the use of p+‐GaAs‐(Al,Ga)As‐p−‐GaAs capacitors

The valence‐band discontinuity between GaAs and AlxGa1−xAs (0.3≪x≪1.0) was determined from measurements on p+‐GaAs‐(Al,Ga)As‐p−‐GaAs capacitors from current‐voltage measurements as a function of temperature. It was found that thermionic emission dominated the conduction process in these structures for temperatures above 140 K and for low bias voltages. The barrier height was determined from the slope of ln(J/T2) vs l/T. From this, a valence‐band discontinuity of 35% of the total direct band‐gap (EgΓ) discontinuity, being independent of the mole fraction x, between GaAs and (Al,Ga)As was calculated. The corresponding value for the conduction‐band discontinuity, 65%, is considerably lower than the commonly accepted value of 85%.

Electrical characterization of GaAs/AlGaAs semiconductor‐insulator‐semiconductor capacitors and application to the measurement of the GaAs/AlGaAs band‐gap discontinuity

Electrical measurements were made on n+GaAs‐(Al,Ga)As‐n−GaAs and p+GaAs‐(Al,Ga)As‐p−GaAs capacitors. Current conduction is due to thermionic emission at low bias voltages and high temperatures. At low temperatures and high fields Fowler‐Nordheim tunneling contributes to the conduction process. An inverted n‐type capacitor analogous to an inverted modulation doped structure shows electrical characteristics comparable to the ‘‘normal’’ structure grown under optimized conditions. Conduction‐ and valence‐band discontinuities were calculated from measurements of thermionic emission barrier heights as a function of Al mole fraction in the (Al,Ga)As. A conduction‐band discontinuity of 65% of the total band‐gap discontinuity between GaAs and (Al,Ga)As, independent of Al mole fraction, was deduced. A classical theory for the capacitance voltage characteristics of p+GaAs‐(Al,Ga)As‐p−GaAs structures is presented.

Double heterojunction GaAs/AlxGa1−xAs bipolar transistors prepared by molecular beam epitaxy

Double heterojunction AlGaAs/GaAs bipolar junction transistors (DHBJT’s) grown by molecular beam epitaxy (MBE) were fabricated and tested. The incorporation of a graded base‐collector junction improved the dc performance of a DHBJT over that obtained with an abrupt base‐collector junction. Maximum current gains of 500, 900, and 1650 were obtained using graded collector and emitter junctions and base widths of 0.05, 0.2, and 0.1 μm, respectively. The reduction in current gain in going from a 0.1‐μm base width to a 0.05‐μm base width is attributed to a lowering of the electron lifetime resulting from the high base doping level used. The value of 1650 is comparable to the current gains obtained from liquid phase epitaxy (LPE)‐grown HBJT’s and is the best obtained from MBE‐grown HBJT’s. Common‐emitter transistor turn‐on voltages were found to correspond to a difference in the collector and emitter junction turn‐on voltages. High collector growth temperature improved dc performance with 700u2009°C as the optimum g...