A. Y. Cho

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 590–720 °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 640 °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 640 °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 700 °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 680 °C these values were 0.88 and 0.99, respectively.

Dependence of electron mobility on spatial separation of electrons and donors in AlxGa1−xAs/GaAs heterostructures

Single‐period modulation doped A1xGa1−x As/GaAs heterojunctions have been prepared by molecular beam epitaxy (MBE). Heterojunctions with a Si‐doped A1xGa1−x As layer grown on an unintentionally doped GaAs layer have exhibited enhanced mobility at 78 ° and 300 °K. The A1xGa1−x As, grown with x = 0.25 or x = 0.33, was doped to a level ND?3×1017 cm−3 resulting in a sheet‐charge density of about 8×1011 cm−2. Maximum mobilities of 74 200 cm2/V s at 78 °K and 6930 cm2/V s at 300 °K were observed in different structures. This represents an improvement of more than a factor of 2 over the best previously reported results at 78 °K. These structures, if used for normally off and/or psuedonormally off FET channel layers in high‐speed integrated circuits, can provide improved performance. These extremely high mobilities can lead to a power delay product improvement of about a factor of 2 at room temperature and a factor of more than 12 at liquid‐nitrogen temperature as compared to conventional structures. This increas...

Electron mobility in single and multiple period modulation‐doped (Al,Ga)As/GaAs heterostructures

Single and multiple period modulation‐doped (Al,Ga)As/GaAs heterostructures have been grown by molecular beam epitaxy (MBE) and characterized by Hall measurements. The basic structure consists of alternating layers of undoped GaAs and doped (Al,Ga)As separated by undoped ’’intrinsic’’ layers of (Al,Ga)As. The thickness of each of these layers was varied independently in a one, three, or nine period structure. Mobilities as high as 211 000, 95 500, and 8 360 cm2V−1 s−1 were obtained at 10 K, 78 K, and 300 K, respectively. The total sheet charge concentrations were about 2.25×1012 cm−2 in the highest mobility structures. Single period structures with an AlAs mole fraction of 0.33 typically had higher room‐temperature mobilities while multiple period structures with an AlAs mole fraction of 0.2 gave better results at cryogenic temperatures. The enhanced mobilities are attributed to the separation of electrons and donors by the undoped (Al,Ga)As layers and the high quality interfaces obtainable by MBE. To our...

Photoconductivity effects in extremely high mobility modulation‐doped (Al,Ga)As/GaAs heterostructures

Single period modulation doped Al0.35Ga0.65 As/GaAs heterostructures were grown by molecular beam epitaxy. The mobilities and sheet carrier concentrations were measured as a function of lattice temperature in the dark and in room light. Mobilities as high as 8490, 105 000, and 221 000 cm2/Vs at 300, 78, and 10 K, respectively, were obtained for samples measured in the dark. When measured in light, these mobilities increased to 9090, 136 000, and 286 000 cm2/Vs at the same respective temperatures. In all cases the sheet carrier concentrations were between 4.5×1011 and 9.5×1011 cm−2. These values represent one of the best dark values reported to date and are significant with respect to field effect transistor applications. The 300 K mobility of 9090 is equivalent to the best mobilities obtained in ultrapure GaAs (n⩽1013 cm−3). The change in the mobility and sheet carrier concentration is the result of a persistent photoconductivity effect which is attributed to the ionization of electrons from traps in the ...

Mean Adsorption Lifetimes and Activation Energies of Silver and Gold on Clean, Oxygenated, and Carburized Tungsten Surfaces

Direct measurements of the mean adsorption lifetimes and the activation energies of silver and gold have been made on a polycrystalline tungsten substrate. The lifetime was determined by the time constant of the exponential increase of the surface concentration when an atomic beam impinged on the substrate. The activation energy was determined by the slope of the logarithm of the lifetime as a function of the inverse substrate temperature. The adsorption lifetime of atoms on a metal surface is τ=(1/ν0) exp (E/kT), where ν0 is the vibrational frequency and E is the activation energy. The experiments were conducted in an ultrahigh‐vacuum system free of hydrocarbon contamination. A clean substrate surface was obtained by heating in a partial pressure of oxygen for a long period of time and flashing to 2500°K before each data point was taken. The well‐defined surface condition and the precisely known temperature allow reliable interpretation of the experimental results. The experimental results for silver and...

Experimental and theoretical electron mobility of modulation doped AlxGa1−xAs/GaAs heterostructures grown by molecular beam epitaxy

Single period modulation doped Alx Ga1−x As/GaAs heterostructures have been grown by MBE and characterized. The incorporation of a thin undoped Alx Ga1−x As layer between the doped Alx Ga1−x As and undoped GaAs layers further improves the electron mobility through the reduction of remote donor scattering. Electron mobilities as high as 8165, 80 900, and 115 000 cm2 V−1 s−1 at 300, 78, and 10 K respectively, for an average sheet electron concentration of 5×1011 cm−2 were obtained. The best mobility was obtained when the undoped Alx Ga1−x As layer thickness was 75 A for both x = 0.25 and x = 0.33. These figures represent the best mobilities associated with single period Alx Ga1−x As/GaAs structures reported to date. Good agreement between the experiments and mobility calculations have also been obtained. The theory is based on heavily screened remote impurity scattering and quasi‐two‐dimensional electron‐phonon interaction.

Electron traps created by high temperature annealing in MBE n-GaAs

An electron trap with a thermal activation energy of 0.83 eV from the conduction band is common in the deep level transient spectroscopy (DLTS) spectra of vapor phase epitaxial (VPE) n-GaAs, but is not observed in the DLTS spectra of as-grown molecular beam epitaxial (MBE) n-GaAs. We show here that this trap is created during high temperature annealing of MBE samples with a Si3N4, encapsulant. The trap concentration is correlated with the annealing temperature and time, suggesting the outdiffusion of a constituent atom resulting in the formation of a vacancy or vacancy-complex. Other electron traps observed in the DLTS spectra of asgrown MBE n-GaAs are annealed out for temperatures at or above 800° C.

Effect of background doping on the electron mobility of (Al,Ga)As/GaAs heterostructures

Selectively doped single period (Al,Ga)As/GaAs heterostructures have been grown by molecular beam epitaxy and characterized by Van der Pauw‐Hall measurements. The mobility of the two‐dimensional electron gas localized at the heterojunction interface has been examined as a function of background charge concentration in the GaAs layer. The electron mobility has been found to decrease monotonically with increasing background concentration, as predicted by calculations based on a two‐dimensional screening factor and remote donor‐electron interaction.

Use of a GaAs smoothing layer to improve the heterointerface of GaAs/AlxGa1−xAs field‐effect transistors

The effect of a GaAs smoothing layer on the performance of GaAs/AlxGa1−xAs field‐effect transistors prepared by molecular beam epitaxy was investigated. The GaAs smoothing layer was inserted between the channel layer and the AlxGa1−xAs buffer layer in an attempt to reduce the dependence of interface quality on growth conditions. Current‐voltage characteristics of field‐effect transistors with 1‐μm gate lengths were used to characterize the properties of the heterointerface. Without the GaAs smoothing layer, extremely sharp interfaces, as indicated by the electron velocity, were obtained when the structures were grown at 700 °C. However, the interface sharpness was very sensitive to growth conditions and in particular to the substrate temperature, decreasing from 50 A at 700 °C to 70 A at 640 °C and to 260 A at 580 °C. Incorporation of a 200‐A‐thick undoped GaAs smoothing layer at the heterointerface decreased this sensitivity to growth conditions. With the smoothing layer, interface sharpnesses of 150, 60...

Moderate mobility enhancement in single period AlxGa1−x As/GaAs heterojunctions with GaAs on top

Selectively doped single period AlxGa1−x As/GaAs heterojunctions with the GaAs layer on top of AlxGa1−xAs were prepared by molecular beam epitaxy. Moderate electron mobility enhancement was achieved when the samples were grown near a substrate surface temperature of 700 °C. Samples grown well below and well above 700 °C did not show observable mobility enhancement. The samples grown at 700 °C and with an electron concentration of 1017 cm−3 exhibited 300 K mobilities of about 4 500 cm2/Vs and 78 K mobilities of 8500 cm2/Vs, which is about a factor of 2 higher than that of bulk GaAs at 78 K. To our knowledge, this is the first report of mobility enhancement in these inverted single period AlxGa1−x As/GaAs heterostructures.