Sb基化合物半导体表面：化学清洗的比较研究

We have studied the surface cleaning of Sb-based compound semiconductors using HF, NH₄OH, and HCl cleans and the metal–oxide–semiconductor (MOS) capacitors fabricated subsequently. GaSb, InGaSb, and AlGaSb surfaces are investigated using low-energy radiation from the synchrotron. Capacitance–voltage (C–V) and photoluminescence measurements are carried out on capacitors made with Al₂O₃ from atomic layer deposition and corroborated with the results from synchrotron spectroscopy. Excellent C–V characteristics with a mid-band-gap interface state density of 3x10¹¹/cm² eV are obtained on samples with the HCl clean. This is consistent with the fifinding that only the HCl acid clean is able to remove the native oxides present on GaSb and InGaSb surfaces, and produce clean and stable surfaces suitable for MOSFET development. Complete removal of AlOx on the AlGaSb surface was not possible using chemical cleaning. Termination of AlGaSb with two monolayers of GaSb is proposed as a solution. 

Antimony (Sb)-based compound semiconductors have the highest electron and hole mobilities among all of the III–V materials. The electron saturation velocity in InSb is the highest among all the semiconductor materials. Room-temperature hole mobilities as high as 1500 cm² /Vs in strained In_0.41Ga_0.59Sb and GaSb channels at sheet charge densities near 10¹²/cm² have also been demonstrated recently.The bandgap of Sb channels can be easily tuned from 0.18 eV for InSb to 0.72 eV for GaSb (direct bandgap) and 1.4 eV for AlSb (indirect bandgap). By combining these antimonide binaries in different stoichiometry, the bandgap can easily be tuned for detecting wavelengths in the 2–14 μm range. GaSb in particular has a bandgap that is well matched to the loss minima in optical fifiber lines. In addition to a highly tunable bandgap, Sb-based channels offer high conduction and valence band offsets with lattice matched Al-containing Sb’s making them suitable for confifining carriers in thin 2D quantum wells for heterostructure-FET applications and lasing.

Schottky-gate FET devices with InxGa1- xSb channel have achieved an fT of 305 GHz at 0.5 V VDS (LG ¼ 85 nm) for n-channel and an fT of 140 GHz (LG ¼ 40 nm) for pchannel.Although buried channel HEMT-like devices with excellent electron and hole transport have been demonstrated, realization of an Sb-channel MOSFET has remained elusive due to the highly reactive nature of the Sb surface. This is illustrated in Fig. 1 where we show atomic force microscopy (AFM) scans of a nominally epiready GaSb surface for a sample removed from the package supplied by the vendor, which was sealed under nitrogen and measured immediately. The root mean square (rms) surface roughness is 0.73 nm. The rms roughness increases to 4.0 nm after exposure to atmosphere for a week. Thus, a stable and clean Sb surface suitable for dielectric deposition is the key if MOSFETs utilizing the high mobilities in these materials are to be realized. Surface cleaning and passivation is also important for infrared photodetectors made using Sb materials to improve the ION/IOFF ratio and response time.

The cleaning of InP/In_xGa_1-xAs surfaces using various chemical cleans has been studied extensively, which, in turn, has enabled MOSFETs utilizing the excellent transport properties in these materials. However, very little work has been reported on surface cleaning of Sb-based III–V semiconductors. In this paper we study the effectiveness of wet chemical cleans of HCl, HF, and NH₄OH, acids and bases used routinely in semiconductor processing, to remove the native oxides present on Sb surfaces and produce a stable and stoichiometric Sb surface suitable for device fabrication. Synchrotron radiation photoemission spectroscopy (SRPES) is used to study the top few monolayers of the surface after different chemical cleans and subsequent annealing in vacuum. Capacitance–voltage (C–V) and photoluminescence (PL) measurements are performed on capacitors fabricated after the various chemical cleans and correlated with the SRPES data.

Fig1

The SRPES experiments were performed at beamlines 8-1 and 10-1 of the Stanford Synchrotron Radiation Lightsource. These two beamlines combined provide a tunable range of monochromatic photons from 60 to 1000 eV, yielding high surface sensitivity with a minimum electron escape depth around 5 A˚ . The Ga 3d, In 4d, Sb 3d, Al 2p, and the valence-band spectra can be studied with high accuracy in this low energy range.

PL has been proposed as a tool to compare the passivation achieved at the surface.11 Figure 6 compares the integrated PL peak intensity for GaSb capacitors with the HCl, NH₄OH, and HF cleans. Increased PL intensity was observed in a GaSb sample with an HCl acid compared to samples with HF and NH₄OH clean, providing another independent proof for improved surface characteristics with HCl clean.