Session O17 - Heterostructures: General.
MIXED session, Thursday afternoon, March 20
Room 2215, Conv. Center

[O17.01] MBE Growth of PbSe/CaF_2/Si(111) Heterostructures

Despite the large thermal expansion mismatch between PbSe and silicon high quality epitaxial layers of PbSe can be grown on (111)-oriented silicon substrates when fluoride buffer layers are used. Such heteroepitaxial growth offers a number of benefits that can lead to fabrication of improved infrared sensor arrays. In this work, we show that high quality PbSe/CaF_2/Si(111) heterostructures can be grown using just one growth chamber of a molecular beam epitaxy system. The PbSe epilayers show mobilities as high as 23,200 cm^2V^-1s^-1 and electron concentrations of 1- 2x10^17 cm^-3 at 77 K. P-type layers were obtained by controlling selenium vacancy concentrations using a selenium flux provided by an EPI valved-cracker source. In addition, we present reflection high- energy electron diffraction intensity oscillation data that confirm layer-by-layer growth modes for both CaF_2 and PbSe. We also present x-ray photoelectron spectroscopy data which show that the PbSe/CaF_2 interface consists of a thin transition layer dominated by Pb-F and Ca-Se bonds.

[O17.02] A Study of the PbSe/CaF_2 Interface Grown on Si(111) by MBE

Epitaxial growth of high quality PbSe/CaF_2/Si(111) heterostructures is an important materials technology for fabricating infrared detector arrays and tunable diode lasers. While the CaF_2/Si(111) interface has been extensively investigated, little has been reported on the PbSe/CaF_2 interface. In this work, we use in situ\/ XPS to study the initial stages of MBE growth of PbSe on CaF_2/Si(111). The surface reaction between CaF_2 and elemental Se is also studied as a comparison. Ca 2p, F 1s, Pb 4f and Se 3d peaks all shift to lower binding energies (1.2--1.6eV) with increasing PbSe coverage. This indicates a strong interaction between the PbSe overlayer and the underlying CaF_2 and the formation of Pb-F and Ca-Se interfacial bonds. No chemically graded reaction products are detected at the PbSe/CaF_2 interface. A transition layer of about 6ML is observed which can be partially accounted for by the open 3D structure of CaF_2 where three layers of atoms are available for bonding with the deposited material. This structure allows interpenetration of the PbSe and CaF_2 layers which necessarily leads to a thin transition layer. Based on our XPS results, we propose models for the bonding arrangements at the PbSe/CaF_2(111) interface. ~~~ This work is supported by NSF grant no. OSR-9550478.

[O17.03] Alloy clustering and interface roughness in InGaAs/GaAs heterostructures

By using cross-sectional scanning tunneling microscopy, we have investigated factors which influence alloy clustering and interfacial roughness in InGaAs/GaAs heterostructures. Identifying the In atoms on the first and second layers as those demonstrated by Pfister et al [1], we study alloy clustering by analyzing the two-dimensional In-In pair correlation and find that a repulsive interaction exists between neighboring In atoms along the [\bar110] direction. This repulsive interaction is discussed in terms of the strain effect. Furthermore, we find that the roughness of the growth front is a major factor in the interface roughness, but a growth interrupt up to 120 sec appears to have limited effects. A segregation of In atoms is observed similar to those reported by Zheng et al [2]. Finally, if the GaAs overlayer on InGaAs is grown at higher temperatures after the growth interrupt, a nearly atomically abrupt interface is produced and the apparent InGaAs layer thi! ckness is reduced. We attribute this effect to In re-evaporation and a smoothing of the growth front during the temperature ramp before the growth of GaAs at a higher temperature. [1] M. Pfister et al, APL 67, 1459 (1995) [2] J.F. Zheng et al, PRL 72, 2414

[O17.04] Epoxy-Bond-And-Stop-Etch (EBASE) Technique for Close Proximity Submicron Backside Gating of AlGaAs/GaAs Heterostructures

Transport experiments on double quantum well (DQW) structures often require independent ohmic contacts to the individual QWs, acheived by depletion gating on both sides of the sample. Backgating of samples mechanically thinned to order 50 \mum suffers from poor lateral resolution and large gate voltage requirements (100 V), while epitaxial growth over patterned substrates is very costly and suffers from the drawbacks of regrowth. We describe a flip-chip technique for patterning the back surfaces of epitaxial AlGaAs/GaAs layers as thin as ~2000 Åover areas up to 1 cm^2. After conventional patterning, the sample's front side is epoxied to a host substrate. The original substrate is then removed by etching down to a stop etch layer, revealing an exceptionally smooth backside surface which is then pattened conventionally. Advantages include low cost, mechanical robustness, low-voltage depletion gating, and submicron resolution. Submicron e-beam written backside gates can be aligned to within a few 100 Åof e-beam written gates on the frontside, enabling new types of quantum transport experiments.

[O17.05] Extreme dislocation glide and reactions in lattice mismatched epitaxial IV-VI semiconductor layers on Si(111) substrates

Epitaxial narrow gap lead-chalcogenide layers on silicon substrates are of interest for fabrication of infrared focal plane arrays. Dislocations move easily on the primary \langle 110\rangle \100\ glide system in this NaCl type semiconductors due to thermal mismatch strains. It was found in PbSe(111) layers on Si(111) that the threading ends of misfit dislocations can glide over distances of several cm on anneals due to these strains. This can be used to reduce the dislocation density to below the 10E6 cm-2 range in layers a few micrometers thick. The scaling law is the usual 1/d (d thickness) dependence for layers just after growth by MBE, however, a stronger dependence on d results after annealings. This is explained by dislocation reactions in a manner that glissile dislocations (g) react to form a single sessile one (s), g + g \rightarrow s, and that this sessile one can be transformed to one glissile by s + g \rightarrow g. Such reactions easily occur in NaCl-type lead-chalcogenide layers.

[O17.06] Energy and Momentum Relaxation Times of 2D Electrons Due to Near Surface Deformation Potential Scattering

The low temperature energy and momentum relaxation rates of 2D electron gas placed near the free or clamped surface of a semi--infinit sample are calculated. To describe the electron--acoustic phonon interaction with allowance of the surface effect the method of elasticity theory Green functions was used. This method allows to take into account the reflection of acoustic waves from the surface and related mutual conversion of LA and TA waves. It is shown that the strength of the deformation potential scattering at low temperatures substantially depends on the mechanical conditions at the surface: relaxation rates are suppressed for the free surface while for the rigid one the rates are enhanced. The dependence of the conductivity on the distance between the 2D layer and the surface is discussed. The effect is most pronounced in the range of temperatures 2 s_l p_F < T < (2 \hbar s_l)/d, where p_F is the Fermi momentum, s_l is the velocity of LA waves, d is the width of the quantum well.

[O17.07] Transfer Matrix Technique for Interface Optical Phonon Modes in Multiple Interface Heterostructures Systems

Interactions of carriers with interface optical phonons dominate over other carrier-phonon interactions in quantum well structures for the case where well widths are less than about 30 ÅHerein, the transfer matrix technique is used to establish a formalism for determining the electrostatic potentials and dispersion relations of the interface-optical phonons for multiple-interface heterostructure systems within the framework of the the macroscopic dielectric model. This method facilitates these calculations for complex structures where the conventional method is very difficult to implement. Several specific cases are treated to illustrate the general formalism. This work was supported, in part, by ARO and ONR.

[O17.08] Effect of disorder on phonon emission from a 2\,DEG

We perform a temperature (50\,mK \leq T \leq 711\,mK) and current (0.3\,nA \le I \le 3\,\muA) dependent transport measurement at zero magnetic field in a GaAs/AlGaAs heterostructure. We use the persistent photo-conductivity effect to change the electron density, 0.64 \times 10^11 \le n \le 1.1 \times 10^11\,cm^-2. Within this range of n, the electron mobility at T=300\,mK increases from 580\,cm^2/Vs to 25,000\,cm^2/Vs and the sheet conductivity from 0.15\,e^2/h to 10.4\,e^2/h, indicating that the disorder of the sample decreases with increasing n. Using the T dependent resistance (R) as a thermometer, we find that the input power (P=I^2R) scales with the effective electron temperature (T_e) as P\sim T_e^b. Strikingly, the exponent b changes systematically from 3.8\pm 0.02 at the lowest n, to 4.4\pm 0.04 at the highest n. Our result demonstrates explicitly that the electron phonon interaction in a 2\,DEG is characteristically different between dirty and clean samples. (E. Chow et al), Phys.\ Rev.\ Lett.\ 77, 1143 (1996), in which Steve Girvin predicts that P is \sim T^4 in the dirty, and \sim T^5 in the clean limit.

[O17.09] 2D Electron Mobility Dependence on the Position of a Quantum Well Within a Slab

We calculate acoustic modes of a slab with free and clamped boundaries as well as matrix elements of 2D electron--phonon interactions via the deformation potential and the piezoelectric mechanisms of scattering. These were done for an arbitrary relation between the thicknesses of the slab d_s and the quantum well (QW) d_w and for an arbitrary position of the QW within the slab. If d_s \approx d_w then at low temperatures the mobility is higher for the slab with clamped boundaries due to peculiarities of the phonon spectrum of such a slab. For d_s \gg d_w a substantial dependence of the mobility on the position of the QW and mechanical conditions on the boundaries of the slab is found when the QW is situated in the vicinity of the slab surface.

[O17.10] The effect of boundary conditions on the Dyakonov-Shur instability in HEMT structures

The hydrodynamic "shallow-water" instability of a HEMT structure discovered by M. Dyakonov and M. Shur \footnote M. Dyakonov and M. Shur, Phys. Rev. Lett. 71, 2465 (1993). is analyzed for a drifting 2D electron gas with ohmic contacts at its ends. When higher-order plasma effects are taken into account, the plasma normal modes of the system are found to involve multiple forward and backward waves. These waves interact with the metallic gate, giving rise to nonlocality of the system response and to the need for additional boundary conditions at the source and drain. As the gate is brought close to the 2D electron gas and the Dyakonov-Shur structure is recovered, the additional plasma waves become localized at the source and drain of the HEMT, and "boundary layers" form. The "deep-water" limit of the system geometry, i.e., a distant gate, is discussed in detail.

[O17.11] Barrier height at the n-n ZnSe/In_0.04Ga_0.96As heterojunction with Zn-rich interface.

n-n ZnSe/In_0.04Ga_0.96As heterojunctions with Zn-rich interfaces grown by molecular beam epitaxy were studied by current- voltage(J-V) and capacitance-voltage(C-V) measurements. J-V curves indicate that this heterojunction behaves like a Schottky barrier with a barrier height of 0.89 eV and ideality factor of 1.2 if interpreted as thermonic emission. The barrier height is much higher than the band discontinuity of 0.26 eV at Zn-rich ZnSe/GaAs heterojunction by photocurrent measurements(V. Pellegrini etal. Appl.Phys.Lett Nov.18, 1996(in press)). The barrier height difference can be explained by modelling a negative delta sheet charge at the ZnSe/In_0.04Ga_0.96As interface.

[O17.12] Schottky barrier formation at ErAs/GaAs (001) interfaces.

Full-potential linear muffin-tin orbital calculations were used to study the structural relaxation, bonding, and Schottky barrier formation at the ErAs/GaAs (001) interface. The two structures investigated contain a common As sublattice but the GaAs is either terminated in a Ga layer or in an As layer. We find that the former has lower energy and exhibits an outward relaxation of the interplanar distance at the interface. The Fermi level at the interface is pinned by interface states which have GaAs surface dangling bond character. The electronic structure at the interface resembles closely that of the ideal surfaces and is indicative of weak bonding.

[O17.13] Constrained Optimization Applied to Graded Ohmic Contacts to p-type ZnSe

A variational technique is introduced which applies the methods of constrained optimization to a problem where an optimum compositional grading profile is desired. The technique is applied to the optimization of graded ohmic contacts to p-type ZnSe via structures like ZnSe-(ZnSe)_1-x(BeTe) _x-BeTe and ZnSe-ZnSe_1-xTe_x-ZnTe. In this case, once a maximum grading length has been specified, a best possible grade x(z) is sought by attempting to minimize the functional \int(1/q\mup)dz across a graded structure connecting bulk regions, subject to the constraint imposed by electrostatics, namely the Poisson eqn. The importance of earlier work on Zn(Se,Te) is briefly revisited and equilibrium valence band profiles for graded structures are reproduced by solving the nonlinear Poisson eqn. The functional is defended as an appropriate measure of quality, shown to be closely related to the specific contact resistance R_C, producing 1st order approximations on comparison, and clearly consistent with barrier lowering. Euler-Lagrange equations are developed and the problem is specified. Paper available in pdf format.

[O17.14] Effects of interface morphology on the Schottky barrier height at Al/GaAs contacts

The problem of Fermi-level pinning at semiconductor-metal contacts is readdressed starting from first-principles calculations for Al/GaAs. We give quantitative evidence that the Schottky barrier height is very little affected by any structural distortion on the metal side, including elongations of the metal-semiconductor bond (i.e. interface strain), whereas it strongly depends on the interface structure on the semiconductor side. A rationale for these findings is given in terms of the interface dipole generated by the ionic effective charges for lattice dynamics. Upon calculating the effective charges of the different ions across the junction, we monitor the continuous transition between the two bulk materials and we measure the interface thickness in a very meaningful way. We thus find that the interface is very sharp on the metal side, while instead a semiconductor ion ``feels'' the presence of the metal even at a distance of a few layers.

Part O of program listing