Improved Tunneling Property of p+Si Nanomembrane/n+GaAs Heterostructures through Ultraviolet/Ozone Interface Treatment

Abstract

Here, heterostructures composed of p+Si nanomembranes (NM)/n+GaAs were fabricated by ultraviolet/ozone (UV/O₃, UVO) treatment, and their tunneling properties were investigated. The hydrogen (H)-terminated Si NM was bonded to the oxygen (O)-terminated GaAs substrate, leading to Si/GaAs tunnel junctions (TJs). The atomic-scale features of the H-O-terminated Si/GaAs TJ were analyzed and compared to those of Si/GaAs heterojunctions with no UVO treatment. The electrical characteristics demonstrated the emergence of negative differential resistance, with an average peak-to-valley current ratio of 3.49, which was examined based on the band-to-band tunneling and thermionic emission theories.

1. Introduction

In recent years, the demands for low-power electronic devices have increased due to the realization of artificial intelligence (AI) technology. A tunnel junction (TJ) diode is a representative source of low-power circuits with a negative differential resistance (NDR) phenomenon via the quantum mechanical tunneling of carriers passing through the energy bands [1,2,3,4,5]. To realize the TJ, the construction of energy bandgaps of highly doped, staggered (II), or broken (III) types is necessary. Several experiments have attempted to acquire crystalline Si/GaAs heterostructures for application in high-speed operation [6,7,8,9]. Since there exist crystal dislocations at the epitaxially grown Si/GaAs interface due to a lattice mismatch, junction bonding techniques were adopted to achieve the heterostructure. The nanomembrane (NM) transfer printing technique allows one to obtain a deterministic assembly of semiconductors with different lattice constants and doping concentrations, leading to the construction of heterojunctions with diverse material combinations and designed bandgap structures [6,10,11,12,13,14]. The main issue that impedes the conduction of heterojunctions is the generation of the interface layer during the bonding process, which deforms the energy bandgap and obstructs the carriers’ injection. The ultraviolet/ozone (UV/O₃, UVO) treatment is a multipurpose process that is proven to be an effective surface passivation method for Ga-based compound semiconductors [15,16,17]. It was proved that the surface energy and interface state density were improved through the UVO treatment.

In this work, TJ diodes comprised of p+Si NM/n+GaAs were fabricated by UVO treatment, and the mechanism for the improved tunneling property was investigated. The hydrogen (H)-terminated Si NM was laminated on the oxygen (O)-terminated GaAs substrate for the formation of the H-O interface. The atomic features of the Si/GaAs interface developed by UVO treatment were inspected by scanning transmission electron microscopy (STEM). The electrical characteristics of the Si/GaAs TJs exhibited an NDR region with an improved tunneling property, which was explained according to the band-to-band tunneling (BTBT) and thermionic emission (TE) principles.

2. Results and Discussion

Figure 1a depicts the vertical crystalline properties of the Si/GaAs TJ interface subjected to UVO treatment. The Si and GaAs comprise of crystal structures of diamond cubic with a lattice constant of 5.43 Å and zinc blende with a lattice constant of 5.65 Å, respectively. Due to the high energy difference due to the lattice mismatch at the TJ interface, it is preferable to develop an interlayer that binds the junction interface, so that this causes the state of the interface energy to be stable [10]. Previous results for Si/GaAs heterojunctions with no UVO treatment demonstrated the growth of the amorphous interlayer, which causes the interface energy to be lower than that of the other structures, such as poly-crystals and single crystals [6]. However, the amorphous layer can work as an energy barrier that impedes the carriers’ flow across the TJ energy bands, obstructing the performance of the TJ. To enhance the tunneling property of the heterostructure, the growth of the interlayer should be minimized. Since the density of the coincident lattice sites at the Si/GaAs interface is low and the spacing of the screw dislocations is short, the position of the interfacial atoms was rearranged into a shape that could enable covalent bonding on the surfaces during the Si/GaAs bonding. The UVO treatment was employed to modify the surface morphology and termination of the GaAs, which affect the growth of the interlayer of the Si/GaAs heterostructure. It was reported that the surface roughness of the GaAs was improved with the UVO treatment due to the absorption of O atoms adjacent to the GaAs surface [18]. The improvement of the surface morphology can lower the density and location of the screw dislocations on the GaAs surface, which can increase the density of the coincident lattice sites at the Si/GaAs interface and lessen the necessity for the energetical generation of an amorphous interlayer.

Figure 1b exhibits the STEM image of the Si/GaAs interface subjected to UVO treatment. Though previous research demonstrated that an approximately 2 nm-thick amorphous layer was formed at the interface with no UVO treatment, here, few atomic layers were observed at the Si/GaAs interface, presumably owing to the increased density of the coincident lattice sites at the Si/GaAs interface due to the UVO treatment. The diffusion of the elements into the interlayer is likely to be negligible at the 375 °C bonding temperature. The twist angle between the atomic lattices of Si and GaAs was generated during the junction bonding, which affected the resolution of the atomic properties at the interface.

Based on the van der Waals (vdW) acoustic mismatch model (AMM), the ITR was associated with phonon transport through the interfaces [12,19,20]. Several results confirmed that the higher operating temperature decreased the tunneling properties of the tunnel diodes, including a decreased peak-to-valley current ratio (PVCR) value [21,22,23]. To improve the tunneling performance of the Si/GaAs TJ, a low ITR value at the junction interface is preferred. It was reported that the bonding energy at the vdW-bonded Si/Si H-O interface (60 mJ/m²) was higher than that of the H-H interface (30 mJ/m²), and the corresponding ITR at the H-O interface (2.8 m²K/GW) was lower than that of the H-H interface (9.2 m²K/GW) [12]. Since the Si/GaAs heterostructure consists of H-terminated Si and O-terminated GaAs, the Si/GaAs TJ features an H-O interface and lower ITR than that of the non-UVO-treated interface.

The electrical characteristics (J-V) of the Si/GaAs TJs were explored in the bias range from −2 V to 2 V at room temperature, in which the different conduction mechanisms for the charge carriers’ flow were observed according to the input bias (Figure 2a). The ohmic contact characteristics of the individual anode and cathode were confirmed prior to the analysis of the tunneling conduction at the TJ interface. Two regions appeared: BTBT under the bias of 1.07 V (A) and TE over the bias of 1.10 V (B). There existed an NDR area between the BTBT and TE regions, where the average PVCR was determined to be 3.49 (Figure 2b). The emergence of the NDR was ascribed to the charge carriers’ tunneling across the energy bands of the Si/GaAs heterostructure. The improvement of 50.4% in the PVCR value compared to the value of the Si/GaAs TJ with no UVO treatment (2.32) in the previous study was confirmed, which can possibly be attributed to the suppressed interlayer growth and the decreased ITR at the Si/GaAs interface due to the UVO treatment. Compared to the PCVR values reported by other groups, with values of 1.30 for Si TJ and 1.45 for GaN/AlN TJ, the PVCR in this work verifies the improvement of the tunneling property through the interface treatment [24,25].

For the further analysis of the charge carriers’ behavior in the individual conduction regions of the p+Si/n+GaAs abrupt TJs, the J-V curve was divided into A and B regions, and fitting was performed using software on the assumption that the same conduction principle was maintained throughout the whole bias range. The BTBT curve in Figure 2c with the red dashed line, plotted by the fitting out of the bias range from the BTBT region, shows the odd function behavior due to the interchangeable direction of the carriers’ injection across the energy bands. The TE curve in Figure 2d displayed with the red dashed line, plotted by fitting, represents the exponential function behavior due to the carriers’ flow along the energy bands. The insets in Figure 2c,d represent the direction of the carriers’ injection across the energy band in the diagrams.

3. Materials and Methods

Si features a high hole mobility (≤450 cm²/V∙s) and quantum phenomenon associated with its thickness, in addition to the low-cost production capability and complementary metal-oxide-semiconductor process compatibility. A GaAs that is a compound semiconductor has a high electron mobility (≤8500 cm²/V∙s) and high-speed applications, such as monolithic microwave integrated circuits and heterostructure-based transistors. Through the formation of p+Si and n+GaAs heterojunctions with high doping concentrations and carrier mobilities, the production of TJs with abrupt energy bands and bipolar junctions, together with high-speed operations, is feasible. The processing steps for p+Si/n+GaAs TJs created by UVO treatment start with the preparation of a Si-on-insulator wafer (SOI, Si/SiO₂ = 100/1000 nm) with a boron doping concentration of 1 × 10¹⁹ cm^–3 (Figure 3a). Here, the undercutting of the sacrificial oxide layer was carried out by dipping NM-patterned SOI into a 49% hydrofluoric acid (HF) solution for 10 min (Figure 3b). The single-crystalline Si NM on the Si substrate was gathered using an elastomeric stamp prior to transfer printing (Figure 3c). The H-terminated surface appeared on the bottom side of Si NM after the HF undercut. The GaAs substrate was grown by metal-organic chemical vapor deposition with a Si doping concentration of 1 × 10¹⁹ cm⁻³ (Figure 3d). The GaAs was loaded into a process chamber to undergo UVO treatment under active wavelengths of 185 nm and 254 nm of the hot cathode and a low-pressure mercury lamp, respectively, with an O₂ dose rate of 7 mg/L for 6 min. The 185 nm UV light dissociates molecular O₂ into triplet atomic O (³P) that forms O₃ by combination with the molecular O₂. The 254 nm UV light then dissociates the O₃ into molecular O₂ and single atomic O (¹D) that reacts with the GaAs surface (Figure 3e). The O-terminated surface was formed on the top side of the GaAs after the UVO treatment. The deterministic transfer printing was performed by laminating the hydrogen-terminated Si NM on the target spot of the oxygen-terminated GaAs (Figure 3f), leading to the formation of crystalline UVO-treated p+Si/n+GaAs heterostructures (Figure 3g). The hydrophilicity induced by the UVO treatment changes the surface energy and junction bonding strength [17]. The Si/GaAs TJs were bonded in the rapid thermal annealing chamber at 375 °C for 5 min. To define the tunneling region, the circular-shaped TJ area of 36 × 10⁻⁴ cm² was patterned on the Si/GaAs heterojunctions using a mask aligner for photolithography, followed by the deposition of Ni/Au on the anode electrode (Figure 3h). The mesa-structure of the cathode was formed by inductively coupled plasma-reactive ion etching (Figure 3i), followed by the patterning of the ground electrode and deposition of the Pd/Ge/Au, with subsequent ohmic annealing at 375 °C for 5 min (Figure 3j).

4. Conclusions

The improved tunneling property of the p+Si NM/n+GaAs heterostructures with UVO-treated interfaces was investigated, and the principles of the conduction were analyzed based on the BTBT and TE. The suppressed growth of the interlayer can possibly be ascribed to the increased density of the coincident lattice sites at the Si/GaAs interface due to the change in the GaAs surface morphology due to the UVO treatment. The atomic-scale properties of the Si/GaAs interface were analyzed using STEM. The Si/GaAs junction interface of the H-O termination may lead to a decreased ITR with respect to that of the non-UVO-treated interface. Compared to the previous research on Si/GaAs TJs with no interface treatment, this work demonstrated the effects of UVO interface treatment on the tunneling property of the Si/GaAs heterostructure through the analysis of the restrained growth of the interlayer and conduction principles. The mechanism of the improved tunneling property was explained by the changes in the density of the dislocations and thermal capability at the interface. The results of the improved tunneling performance through the UVO treatment are applicable to the construction of electronic devices with interfaces comprised of different crystal structures.