Dovas A. Saulys

Study on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatments

The etching of sapphire substrates using H 2 SO 4 , H 3 PO 4 , and a 3:1 H 2 SO 4 :H 3 PO 4 mixture, as a function of temperature and etching time, was systematically studied using atomic force microscopy. The sapphire preparation by liquid-based etchings was compared with H 2 etching at 1100°C and air-annealing at 1400°C. In liquid-based treatments, the smoothest, pit-free surface was obtained by etching in pure H 2 SO 4 at 300°C for 30 min. Sulfuric acid etching at higher temperatures or for longer periods generated an insoluble mixture of Al 2 (SO 4 ) 3 and Al 2 (SO 4 ) 3 .17H 2 O crystalline deposits on the surface. Phosphoric acid and the 3:1 H 2 SO 4 :H 3 PO 4 mixture, which is the routinely employed chemical treatment for sapphire preparation, etched the sapphire preferentially at defect sites and resulted in pit formation on the surface. Sapphire treatment using H 2 at 1100°C did not remove the surface damage. Air annealing the sapphire at 1400°C for 1 h produced an atomically smooth surface consisting of a terrace-and-step structure. The results of this study were described in terms of the chemistry of the sapphire etching process.

A comparative study of GaSb (100) surface passivation by aqueous and nonaqueous solutions

We report a nonaqueous passivation regime consisting of Na2S/benzene/15-crown-5/oxidant. The use of a nonpolar, aprotic organic medium required the addition of a specific chelating agent (15-crown-5) to solubilize sodium sulfide, and organic oxidizing agents (anthraquinone, benzophenone, etc.) to act as electron acceptors. The surface optical and chemical properties of GaSb surfaces after aqueous and nonaqueous sulfide treatments were compared. Nonaqueous passivation resulted in higher photoluminescence (PL) intensity, lower oxide content, and a less amount of elemental Sb than aqueous passivation. The PL intensity from passivated surfaces was correlated with the standard reduction potentials of electron acceptors.

Electrospray mass spectrometry of borane salts: The electrospray needle as an electrochemical cell

Two borane salts ([(Me)4N][B3H8] and Cs[B3H8]) were examined by electrospray mass spectrometry in the positive ion mode. Acetonitrile solutions provided the most informative spectra; the salts exhibited a remarkable degree of clustering under electrospray conditions, and virtually all signals corresponded to cationic cluster ions of the general formula {[cationm+]x[anionn−]y}(mx − ny)+. In contrast, methanol solutions of these salts produced only B(OCH3)4− cluster ions under otherwise identical conditions. 11B NMR analyses corroborate the identities of the methanol solution species that enter the electrospray source and the reaction product generated during the electrospray process.

Growth of oriented lithium niobate on silicon by alternating gas flow chemical beam epitaxy with metalorganic precursors

We present the results of investigations of LiNbO3 film growth on Si by conventional chemical beam epitaxy (CBE) and by alternating gas flow deposition with alkoxide precursors. Both growth methods produced films with an intervening interface amorphous layer, whose thickness depends strongly on growth and annealing temperatures. While films grown by chemical beam epitaxy were always polycrystalline, the LiNbO3 films grown by alternating gas flow deposition were oriented materials. Based on our studies, we hypothesize that the alternating layer deposition technique enhances bulk interdiffusion efficiency of metal ions leading to a more controlled epitaxy of LiNbO3 on Si relative to conventional CBE growth.

Study of the gas phase chemistry in the silicon doping of GaAs grown by metalorganic vapor phase epitaxy using tertiarybutylarsine as the group V source

Abstract Gas phase decomposition studies have been combined with metalorganic vapor phase epitaxy (MOVPE) growth and doping experiments to investigate the mechanism behind the Si doping of GaAs grown using tertiarybutylarsine (t-C 4 H 9 AsH 2 or TBAs) as the Group V source. The use of TBAs leads to an increase in the Si doping efficiency from both SiH 4 and Si 2 H 6 sources and this enhancement was proposed to originate from the homogeneous generation of Si-bearing intermediates, (t-C 4 H 9 )SiH 3 (TBSi) and SiH 3 AsH 2 , from reactions between SiH 4 and the pyrolysis products of TBAs. We have investigated the formation and role of these intermediates in this Si doping chemistry. Results of our decomposition study show that TBSi pyrolyzes in the gas phase at relatively low temperatures to form SiH 4 . Consequently, the use of TBSias a Si dopant source results in a Si doping efficiency and temperature dependence that is identical to SiH 4 . TBSi is therefore not a likely route to increased Si incorporation. Our studies of the co-decomposition of TBAs and SiH 4 or Si 2 H 6 show that SiH 3 AsH 2 is a co-pyrolysis product of TBAs and Si 2 H 6 , but not of TBAs and SiH 4 . Furthermore, there is no indication of gas phase interaction between TBAs and SiH 4 . The results of this study suggest that homogeneous reactions are not as important in this doping chemistry as had previously been considered and that surface processes may instead be responsible for the increase in Si doping observed with TBAs.

Improved characteristics for Au∕n‐GaSb Schottky contacts through the use of a nonaqueous sulfide-based passivation

The influence of nonaqueous sulfide passivation (using Na2S in the inert solvent benzene) on Au∕n‐GaSb Schottky junction behavior was studied. The junction parameters, Schottky barrier height and ideality factor, were derived and compared with those of as-received GaSb surfaces as well as surfaces treated with aqueous sulfide solutions. The Schottky junction made on as-received GaSb is highly nonideal, while S-based passivation treatment of the GaSb surface before contact formation improves the rectifying behavior, and markedly reduces the reverse current. A benzene-based nonaqueous sulfide treatment results in GaSb surfaces with lower oxide and elemental antimony content than does the aqueous sulfide treatment. The produced Schottky barrier height increases to 0.61eV and the Au∕n‐GaSb contact is close to an ideal Schottky junction.

Adsorption and decomposition studies of t-butyl silane on Si(100)-(2 × 1) surfaces using FTIR-ATR spectroscopy

Abstract The adsorption, decomposition and desorption of t-butyl silane (tBSi)(C(CH 3 ) 3 SiH 3 ) on Si(100)-(2 × 1) reconstructed surfaces was studied with Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopy and temperature-programmed desorption (TPD). From the spectra it is postulated that the decomposition pathway is dissociative chemisorption forming t-butylsilylene, with the remaining two hydrogen atoms bonding at neighboring sites. As the sample temperature is increased, the majority of the adsorbed t-butylsilylene undergoes a β-hydride elimination, forming isobutene, which then desorbs. The remaining fraction dehydrogenates and incorporates carbon into the substrate. The carbon incorporation into the substrate was verified ex situ by Auger electron spectroscopy (AES) on a Si(100) wafer after a film was grown with TBSi by ultra-high vacuum chemical vapor deposition (UHV-CVD).

A Mass Spectroscopic Study of the Vapor Phase Thermal Decomposition of Trimethylamine

Trimethylamine has been used in the growth of compound semiconductors either through the formation of an adduct, as in trimethylamine allane, or as a possible growth reactant. The kinetics of the gas-phase decomposition and dominant reaction products relevant to these applications were determined in this study. The multistep thermal decomposition of trimethylamine in hydrogen (H 2 and D 2 ) in a laminar-flow tube reactor was studied by in situ mass spectroscopy and the analysis of isolated decomposition products. A dimethylamino radical and a methyl group are formed in a first-order Arrhenius-type decomposition that begins at ∼525 and is complete at ∼650°C. Additional products of this first step were deuterated methane, CH 3 D 2 (in D 2 ) and ethane, C 2 H 6 . The activation energy is 50.8 kcal/mol. In subsequent steps the dimethvlamino radical degrades through the series N-methylenemethylamine, (CH 3 )N = CH 2 , the imidoyl radical (CH 3 )N = C . H, and hydrogen cyanide, accompanied by loss of H 2 , hydrogen radical, and methyl radical, respectively, as well as by the formation of unidentified solids.

Study of Non-Aqueous Passivation on GaSb (100) Surfaces

Due to the high chemical reactivity of GaSb surfaces, many commonly used aqueous sulfide passivation techniques lead to the growth of surface oxides that degrade device performance. We have developed a non-aqueous passivation regime consisting of Na 2 S/benzene/15-crown-5/oxidant. The use of a non-polar, aprotic organic medium required the addition of a specific chelating agent, i.e. a 15-crown-5 ether, to solubilize sodium sulfide, and organic oxidizing agents, such as anthraquinone and benzophenone, to act as electron acceptors. The surface optical and chemical properties of GaSb surfaces after aqueous and non-aqueous sulfide treatments were compared. Non-aqueous passivation resulted in higher PL intensity, lower oxide content, and a less amount of elemental Sb than aqueous passivation.

Metastable and collision-induced fragmentation studies of all C4H12Si+· isomers; a systematic study of structure-reactivity relations

Metastable ion (MI) and collision-induced dissociation (CID) mass spectra have been recorded and compared for all nine C4H12Si+. isomers. The (Me)4Si+., t-BuSiH3+., s-BuSiH3+, and (Me)2EtSiH+. isomers have unique MI and CID mass spectra. The MI mass spectra, including the kinetic energy release values, of (Me)(i-Pr)SiH2+. and (Me)(n-Pr)SiH2+. are identical, which implies isomerization. MI data also suggest that a fraction of the n-BuSiH3+. ions rearrange into branched (Me)2EtSiH+. ions and a fraction of the n-BuSiH3+. ions rearrange into branched s-BuSiH3+. ions. A comparison with the isomeric C5H12+. pentanes reveals a crucial difference: H2 loss occurs for n-BuSiH3+., i-BuSiH3+., s-BuSiH3+., (Me)(n-Pr)SiH2+., (Me)(i-Pr)SiH2+., and Et2SiH2+., but not for any of the C5Hi12+. isomers. Generation of four- or five-membered silicon containing rings is suggested for H2 loss from the C4H12Si+. silanes.