"Structure of LB-OMCVD Grown V-Groove Quantum Wires:" F. REINHARDT, G. Biasiol, A. Gustafsson, B. Dwir, E. Kapon, Institut de Micro-et Optoélectronique, Département de Physique, École Polytechnique Fédérale de lausanne, CH 1015, Lausanne, Switzerland
Session Chairman: M. Inoue, Department of Electrical Engineering, Osaka Institute of Technology, 5-16-1 Ohmiya, Ashi-ku, Osaka 535, Japan Co-Chairman: Al.L. Efros, Nanostructure Optics Section, Naval Research Laboratory, Washington, D.C. 20375
Low pressure organometallic chemical vapour deposition (OMCVD) on non-planar substrates appears to be a promising method to form arrays of high quality one-dimensional quantum confined structures, as judged structurally by transmission electron microscopy (TEM)1 and optically by photoluminescence or photoluminescence excitation spectroscopy2. Unfortunately TEM is only probing the local structure within ~50 nm and the optical methods are integrating over several um. Although atomic force microscopy (AFM) does not have the same resolution as TEM it can be used to visualize the topography of the surface of a quantum wire with subnanometer resolution up to a total length of 100 um. This characterization would be helpful for interpreting experiments, e.g., of carrier and exciton transport along the quantum wires.
We investigated the structure of the surface of GaAs/AlGaAs and In0.5Ga0.5As/AlGaAs quantum wires grown by low-pressure OMCVD (20 mbar) (T = 650-750¡C, V/III>100, pTGMa = 1.53 ubar, growth rate = 2.5-4.2 Å/s) on periodically corrugated substrates, using post-growth atomic force microscopy (AFM) in air 3-5. The AFM-height profile, obtained at constant force, shows along the [011]-direction (perpendicular to the groove orientation) [311]A-{100}-and nearly {111}A-facets. In the bottom of the groove short (10-40nm) {311}A surfaces separated by a central (100) facet were found to build the upper surface of GaAs or InyGA1-yAs-wires, in agreement with TEM observations1 of the upper interface of buried quantum wires.AFM-height profiles, taken along the groove on each facet separately exhibit different features on a <=100 nm and a 1um scale. A random "long range" height variation (+/-10 nm) in the groove with a typical length of several hundred nanometers were found on all facets. We assign this to inhomogeneities of the grating fabrication by comparing with AFM-profiles of the gratings before growth and to the absence of this effect on an unetched (100) ridge facet. The microscopic surface topography was found to be different for each observed crystal facet. On the {100} facets on the ridges and in the bottom of the groove, terraces separated by monolayer steps are observable. For ridges with widths narrower than ~800 nm the steps are straight and perpendicular to the corrugations. The {311}A-surface show step bunching with a mean periodicity of ~50 nm and a height variation up to 5 nm, the latter being a function of facet width. This effect becomes more pronounced in the case of a pseudomorphic In0.5GA0.5As-layer which can be utilized to grow a GaAs/In0.5Ga0.5As fractional layer superlattices, on the (311) facet, perpendicular to the groove.
In conclusion we found that the (100) surface of the wire at the bottom of the groove is segmented in separated monolayer smooth sections, with a typical length of several hundred nanometers. The length of these sections is limited by etching inhomogeneities. Microscopically the different facets show different topography, which is may be an indication for different growth kinetics. We assume that the transformation of the surface, due to cool-down-process and oxidation after growth is small, because we found that the dimensions of the facets, measured by AFM agreed well with the dimensions obtained from TEM investigations of the buried interface.
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1. A Gustafsson et al., Appl. Phys. Lett. 67 (18 Dec. 1995)
2. F. Vouilloz, D. Oberli, M. A. Dupertuis, C. Fall, E. Kapon, Nuovo Cimento (in print)
3. M. Shinohara, H. Yokohama, Naohisa Inoue, J. Vac. Sci. Technol. B 13(1995), 1773
4. C.C. Hsu, J.B. Xu, I.H. Wilson, Appl. Phys. Lett. 65 (1994), 1394
5. F. Reinhardt, B. Dwir, G. Biasiol, E. Kapon, 134 ICSFI-5 95 (Princeton, NJ), accepted in Appl. Surf. Sci. (1996).
2:10PM, D2
"Fabrication and Characterization of Quasi-One-Dimensional InAs/AlGaSb Quantum Wires:" S. SASA, T. Sugihara, S. Izumiya, Y. Yamamoto, M. Inoue, Department of Electrical Engineering, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku, Osaka 535, Japan
InAs based heterostructures have recently attracted much attention because of their superior transport properties over GaAs or InGaAs based heterostructures. The superior properties arise from the lower electron effective mass of InAs and from the strong electron confinement when AlGaSb or AlSb layers are used as the barrier layer. In an InAs/AlGaSb single quantum well structure, we have observed quantum effects at higher temperatures1). We have demonstrated that one-dimensional subband structures are formed for up to 0.4-um-wide wires by magnetoresistance measurements2). These widths are readily attainable using conventional fabrication technique. Therefore, InAs based heterostructures are very promising for the realization of quantum devices operating at higher temperatures. However, one of the problems to be overcome is the large gate leakage current inherent for this material system. The purpose of this paper is to present the fabrication process in order to eliminate the leakage current for the realization of the quantum devices and to study the electron transport properties in InAs/AlGaSb quantum wire structures (QWRs) for various wire lengths including quasi-ballistic regime.
We have fabricated single and multiple QWRs and quantum wire transistors using InAs/AlGaSb single quantum well heterostructures which were grown on semi-insulating GaAs substrates by molecular beam epitaxy. The two-dimensional electron gas (2DEG) mobility was 4 <-> 105 cm/Vs at 4.2K. This high quality 2DEG allows us to study the transport properties under quasi-ballistic regime at a wire length of about a few um. The QWRs are fabricated conventional photolithography and we themicaletching2). By optimizing the fabrication processes, we succeeded to make narrow wires reaching down 0.1 um. For the systematic study of the electron scattering mechanisms, we have made various samples with the wire lengths ranging from 1 um to 10 um.
We will present our fabrication process which is critical for the elimination of the gate leakage current by using GaAs regrowth and show the device characteristics of the quantum wire transistors. We will also present the transport properties at low electric fields and the current-voltage characteristics of the quantum wire structures measures at 4.2K under the application of magnetic fields up to 8T.
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1) M. Inoue, K. Yoh and A. Nishida, Semicond. Sci. Technol. 9, L966 (1994).
2) S. Sasa, T. Sugihara, K. Tada, S. Izumiya, Y. Yamamoto, and M. Inoue, 3rd Int. Symp. on NPMS, Maui, (1995).
2:30PM, D3+
"Bandgap Modulation and Quantum Wire Formation in Dielectric Strip-Loaded Quantum Wells:" K. ZHANG, K. Kamath, J. Singh, P. Bhattacharya, Department of Electrical Engineering and Computer Science, The University of Michigan, Ann Arbor, MI 48109-2122
Dielectric or metal strip-loaded semiconductors are known to form optical waveguides due to the refractive index changes caused by the induced strain underneath and at the edges of the stripe. The strain induced in different regions of the semiconductor underneath the stripe also changes the bandgap by different amounts. In characterizing the strain field by analyzing photoluminescence(PL) data from a quantum well, located at varying depths under a dielectric stripe, we find that the strain induced bandgap modulation at the quantum well, along the edge of the stripe, is adequate to produce 1D confinement in that direction.
The experimental samples consist of 40-100 Å GaAs/Al0.3Ga0.7As single quantum wells (SQW) grown on (001) GaAs by molecular beam epitaxy (MBE). The quantum wells are grown at different depths from the semiconductor surface to map the strain field induced by a dielectric stripe. SiINY and SiO2 dielectric stripes, of thickness varying in the range of 400 - 800 nm are formed by plasma enhanced chemical vapor deposition (PECVD) and reactive ion etching (RIE). The width and separation of multiple parallel stripes are in the range of 10-40 um.The bandgap modulation and strain field are estimated by comparing 18K PL data from strip-loaded samples with those from strip-free samples. The data show that the single bound exciton peak from a strip-free SQW sample splits into two peaks in the strip-loaded samples. The peak position, intensity and the linewidths of these peaks depend on the location under the stripes and the stripe width and separation. An additional peak is observed at a lower photon energy under the edge of the stripes.
Analysis of the data, using deformation potential theory with the appropriate elastic constants, enables us to map the strain fields and calculate the modulation in bandgap energies spatially and as a function of depth with respect to the stripes. It is found that the PL peak observed at the lowest energy arises from a quantum wire formed by the additional degree of confinement of the well under the stripe edge in the strain field. The stripe dimensions and spacings and their deposition parameters, have been optimized to maximize the 1D confinement. It is seen that with 0.5 um wide stripes separated by 0.1 um, a bandgap change of 120 meV and quantum wires at 40 meV confinement barrier are produced in a quantum well located 200 Å below the stripe. The calculated subband energies and wavefunctions show excellent quantum confinement in the wire. Calculations also indicate that the single stripe of width, it would be possible to create quantum wire with 150 meV confinement barrier.
2:50PM, D4
"AlGaAs/GaAs Narrow Quantum Wires Produced by Schottky In-Plane Gate Technology:" HIROSHI OKADA, Takashi Kudoh, Tamotsu Hashizume and Hideki Hasegawa, Research Center for Interface Quantum Electronics, and Graduate School of Electronics and Information Engineering, Hokkaido University, Sapporo 060, Japan
The key point of the fundamental devices for future quantum electronics is formation of scattering-free one-dimensional electron waveguide with gate control. A promising approach is to confine two-dimensional electron gas (2DEG) into one dimension by utilizing voltage-adjustable depletion layers. The in-plane-gate (IPG) device utilizes electric field parallel to the 2DEG plane and seems to be potentially more effective for realization of quantum devices working at high temperatures due to strong and efficient electron confinement.
The purpose of the present paper is to report transport properties of the novel Schottky-IPG controlled Al0.3Ga0.7As/GaAs quantum well wires (QWWs). IPGs were realized by formation of Schottky contacts to the edges of the 2DEG plane by applying a novel in-situ electrochemical process. The main points are listed below.
(1) Al0.3Ga0.7As/GaAs double-hetero QW wafers were grown by standard molecular beam epitaxy at 600¡C. Hall mobility and sheet carrier density ns at 4.2K were 8x104-2x105 cm2/Vs and 5x1011-1x1012 cm-2, respectively. A QWW was fabricated by electron beam lithography and wet chemical etching. After formation of ohmic contacts, Pt Schottky IPG electrodes were deposited on both sides of the etched channel by the in-situ electrochemical process which consists of anodic etching to remove oxides followed by subsequent cathodic deposition of Pt in the same electrolyte, changing the voltage polarity. Electron beam induced current (EBIC) signal provided evidence for direct Schottky barrier formation at the edges of the channel.
(2) The QWWs showed clear quantized conductance in units of 2e2/h up to 100K in spite of the rather large wire length of 1000-1600nm. The observed maximum temperature highly exceeds that of 42.7K reported for a specially designed split-gate quantum point contact (QPC) device with a channel length of 100nm and a gate separation of as small as 100 nm.
(3) By setting the gate voltage in the plateau region, the drain I-V curves followed the line corresponding to the quantized conductance for small drain voltages. At a certain drain voltage the conductance quantization breaks down, resulting in conductance deviation from the value of the quantized conductance. From the breakdown voltage of conductance quantization, a subband energy separation in the present QWWs is estimated to be 10 meV or higher.
(4) From the results of Shubnikov-de Haas oscillation measurements, it was found that the Schottky IPG gate controls the effective channel width without changing the sheet carrier density. Such behavior, which is very different from that of the split-gate-type QWWs, seems to produce sharper potential profile, resulting in the large subband energy separation.
In summary, the Schottky IPG structures realized large subband energy
separation in AlGaAs/GaAs QWWs, showing that use of smaller device dimensions
and other materials such as InAs/GaSb may eventually lead to higher-temperature
operation of one-dimensional electron waveguides and that the novel IPG
structure is also applicable to single electron devices.
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