Fabrication of quantum wires in thermally etched V-grooves by molecular beam epitaxy

Quasi-one-dimensional quantum wires have been formed in V-grooves on GaAs substrates. A new fabrication technique based on in situ thermal etching of masked substrates and subsequent overgrowth by molecular beam epitaxy has been developed. The device geometries enabled formation of low-resistance ohmic contacts. Two-terminal magnetoresistance measurements of single-wire devices show transport qualities previously not witnessed in growth on non-planar substrates. Illumination has been used to modulate the carrier density and lateral potential profile of the wires. Fabrication techniques based on the formation of quantum wells (QW) with additional lateral bandgap confinement are expected to form true one-dimensional (1D) quantum wires. Formation of such wires by direct epitaxial growth on prepatterned substrates is a promising technique. Indeed, fabrication by metal-organic chemical vapour deposition (MOCVD) on substrates with prepatterned V-shaped grooves has lead to very narrow wires with lateral heterojunction confinement [1]. Limitations of material quality, though, have not permitted reliable conductance measurements and experiments have concentrated on the optical properties of such structures. However, the wires’ optical qualities were sufficiently good to make quantum wire laser diodes with much future potential [2]. The most successful realization to date of 1D devices suited for transport measurements, was that made by cleaved edge overgrowth using molecular beam epitaxy (MBE) [3, 6]. Magnetoresistance measurements [4, 5] and quantized conductance [6] effects were measured. Another technique used etching of very high-mobility modulationdoped-heterostructures [7] and evaporated side gates to achieve quantized conductance. In this paper we report on the fabrication and characterization of quasi-one-dimensional (1D) quantum wires grown in (011̄) V-grooves with (111)A slopes, using MBE. Previous reports on MBE-grown wires in grooves were only of their photoluminescence properties [8, 9]. Growth in grooves made in the (011) direction, with (111)B slopes, showed confinement due to the formation of crescent-shaped constrictions at the bottom tip [9]. Our technique is based on an in situ process of thermally etched V-grooves [10], employing a mask † Present address: Nova Measuring Instruments Ltd, Weizmann Science Park, Rehovot 76100, Israel (e-mail address: [email protected]). material compatible with the MBE process, and utilizing novel groove geometries. The technique enables growth on grooves in the (011̄) direction, with (111)A slopes which form a 70◦ corner at the bottom, as well as on unique grooves formed along the (001) diagonal, with (110) slopes which form a 90◦ corner. The kinetics of the MBE growth on the (011̄) grooves’ slopes enables formation of narrow wires of controlled width, on the (001) plane, formed at the bottom of the grooves. Such a structure was not achieved before in a smooth and continuous form because of the unstable growth on (111)A planes. The initial epitaxial growth consists of a 3 μm thick GaAs buffer layer, a 1000 Å thick Al0.3Ga0.7As masking layer and a 1000 Å thick GaAs cap layer. The two top layers are wet etched, after a lithography step, to form an extremely clean and stable mask. After careful cleaning, the patterned wafer is loaded into a thermal processing chamber and heated to a high temperature (∼710 ◦C) under an impinging arsenic flux of about 5× 10−6 Torr, thereby undergoing thermal etching (sublimation) of the GaAs unprotected by the AlGaAs mask. It is surmised that the mask layer remains stable due to the formation of a thin impenetrable AlAs coating after the initial stages of thermal etching. The etch rate of GaAs in the (100) direction in wide areas is about 3000 Å h−1 under these conditions. Extremely smooth and sharp V-shaped grooves are formed in the narrow gaps in the mask, aligned to predetermined crystallographic directions (see figure 1). In narrow grooves the thermal etch rate is somewhat higher due to arsenic flux shadowing. After the V-shaped profile is attained, the effective vertical etch rate drops by almost an order of magnitude. To enhance the purity of the groove surface, the thermal etching is carried out within the thick epitaxial buffer layer and not into the substrate material. After etching, the wafer is transferred in situ into 0268-1242/97/081046+06$19.50 c © 1997 IOP Publishing Ltd MBE quantum wires in V-grooves Figure 1. Thermal etching of a long V-groove in the (011̄) direction. The 1000 Å thick AlGaAs mask is slightly roughened during the thermal etching. Short vertical (011) planes are visible under the mask above the (111)A slopes. The bottom of the V-groove is extremely straight irrespective of roughness of the edge of the mask. the MBE growth chamber and a GaAs–AlGaAs modulation doped structure is grown on it. The growth is problematic because of the instability of the growth on the (111)A slopes and shadowing effects in the groove. Smooth growth on (111)A planes requires high arsenic flux and low substrate temperatures. Additionally, flux coverage of the groove slopes suffers from cyclic shadowing problems due to the rotation of the substrate under the oblique angle of the incoming arsenic flux. This problem is dealt with by maintaining a total V/III beam equivalent flux ratio of over 100 from two widely spaced arsenic sources and using a high substrate rotation frequency (two cycles per nominal monolayer), thus ensuring smooth growth. The nominal total arsenic beam equivalent pressure is about 1.2× 10−5 Torr. Alloying ohmic contacts directly to the narrow wires in the grooves results in a high contact resistance (∼100 k at 4 K after illumination), as is already known [11]. This makes measurements difficult, since four-terminal measurement configurations are not possible. We have utilized the unique qualities of the thermal etching process to integrate the quantum wires in series with the diagonal (001) grooves (see figure 2). This type of groove cannot be formed by anisotropic wet etching techniques because the non-polar (110) slopes do not stabilize during wet etching. The large diffusion length of gallium atoms on (110) slopes inhibits growth on them and a relatively wide epitaxial plane is formed at the bottom of such grooves. For a total nominal growth thickness of 0.4 μm on the (100) plane, the width of the nearly flat bottom is about 1 μm while the width of the wires is about 1000 Å. Additionally, longitudinal surface diffusion of gallium along the grooves, of the order of a micron, causes formation of a smooth funnel-like transition region from the wide-bottom diagonal grooves to the narrow-bottom quantum wire grooves (see figure 2). Figure 2. 7 μm long quantum wire groove integrated with diagonal grooves. Note the extremely smooth surface on the groove slopes and the micron long funnels formed by the epitaxial growth at the meeting of the wire and the diagonal grooves. Figure 3. Differentially stained cross-section of epitaxial growth in a V-groove. Darker layers are AlGaAs, the quantum wire layer is the bright bar about 700 Å below the surface. The thickness of the growth on the slopes is about 60% of that at the bottom. The epitaxial growth consists of a layer sequence similar to that used for conventional modulation doped two-dimensional electron gas (2DEG) structures. The main differences consist of enhanced silicon doping to overcome shadowing effects in the groove and an additional AlGaAs barrier below the electron gas to form a quantum well (QW), thus confining the electrons in both wire and groove slopes. Since the QW width on the slopes is markedly thinner than that on the (100) plane (see figure 3), the energy of the eigenstates there is higher, forming an effective energy barrier. The typical layer sequence from the substrate up is as follows: 50 Å Al0.4Ga0.6As marker, 2500 Å undoped GaAs buffer, 400 Å Al0.4Ga0.6As barrier, 300 Å GaAs QW, 200 Å Al0.4Ga0.6As undoped spacer, 200 Å Al0.4Ga0.6As Si-doped layer (3×1018 cm−3), 100 Å