Wet Chemical Treatment in Hydrazine-Sulfide Solutions for Sulfide and Nitride Monomolecular Surface Films on GaAs(100)

Abstract

GaAs surface chemical passivation by wet chemical treatments in hydrazine-sulfide solutions with different pH values has been studied by X-ray photoelectron spectroscopy and spectroscopic ellipsometry. It is found that highly alkaline ͑pH 12͒ solutions result in a coherent monomolecular film of gallium nitride, while treatment in buffered solutions ͑pH Ͻ 8͒ yields a surface layer of gallium sulfide. Both nitride and sulfide surface films are found to be stable against oxidation in air ambient over months, attesting to the validity of the wet chemical treatment in hydrazine sulfide solutions for chemical passivation of the GaAs͑100͒ surface. The chemical mechanism for formation of the surface nitride and sulfide films is presented and discussed.In recent years, wet chemical treatments of GaAs surfaces by sodium and ammonium sulfide solutions have been investigated. 1-4 These treatments produce sulfide overlayers that provide surface chemical and electronic passivation of GaAs͑100͒ surfaces. The latter effect originates from a decrease of the surface state density in the middle of the gap, but the lifetime of the electronic passivation is too short for wide practical applications. 5 Nevertheless, GaAs͑100͒ substrates freshly treated with sodium sulfide solution can be used for the growth of molecular beam epitaxial ͑MBE͒ layers of ZnSebased structure without the growth of buffer layers as reported in Ref. 6.Recently a new wet chemical treatment of GaAs by hydrazinesulfide solutions has been reported. 7,8 The treatment results in the formation of a monolayer of gallium nitride on the GaAs͑100͒ surface. Such a layer was found to be stable against oxidation, and to yield an increase of the room temperature photoluminescence ͑PL͒ intensity. 8 In the present paper, an analysis of the surface chemistry of overlayers produced on the GaAs͑100͒ surface by wet treatments using hydrazine-sulfide solutions is presented. The impact of the pH value of the solution on the surface chemistry is discussed. It is found that depending on the pH value, the chemical treatment in hydrazine-sulfide solutions results in the formation on the GaAs͑100͒ surface of coherent monomolecular films of gallium nitride ͑GaN͒ or gallium sulfide ͑Ga-S͒ bonds, which chemically and electronically passivate the GaAs surface. A key issue for the surface passivation is the monomolecular thickness of such nitride and sulfide films, because for higher thickness the above-described overlayers lose mechanical stability due to the high value of the lattice mismatch with the GaAs substrate. Therefore, the self-limiting character of surface reactions of GaAs substrate with the hydrazinesulfide solution makes the present treatments promising for preparation of the chemically stable extra-thin crystalline surface layers. ExperimentalUndoped GaAs͑100͒ wafers with a residual impurities concentration of ϳ10 16 cm −3 were used as substrates. The GaAs substrates were degreased and etched by a H 2 SO 4 :H 2 O 2 :H 2 O = 6:1:1 solution and underwent a short ͑ϳ1 min͒ treatm...

Chemistry of Wet Treatment of GaAs(111)B and GaAs(111)A in Hydrazine-Sulfide Solutions

“…This pressure stayed below 10 −8 mbar during annealing. The Auger spectra obtained after annealing to 540°C, which is known to remove the physisorbed overlayer, [29][30][31] are shown in Fig. 2.…”

“…3,5 Therefore, because of the high GaAs/GaN lattice mismatch ͑ϳ20%͒, such nitride films include voids, mismatch defects, and arsenic atoms. 3,5,11,19,28 As discussed in previous reports, [29][30][31][32] nitridation of GaAs͑100͒ can alternatively be performed by a simple wet treatment in a solution containing hydrazine molecules ͑N 2 H 4 ͒, with small amounts of sodium sulfide ͑Na 2 S͒ as a source of nucleophilic SH − anions. The wet nitridation proceeds through a removal of surface As atoms by the latter anions and successive dissociative adsorption of hydrazine molecules on the opened Ga atoms.…”

“…After annealing in vacuum, photoemission and ellipsometry reveal a very thin GaN surface layer ͑with an estimated thickness of 0.8 nm͒, without any traces of arsenic. 29,30 Such a layer protects the surface against oxidation in air ambient. After annealing, a small surface roughness of 0.3 nm was measured using tunneling microscopy.…”

Carrier Recombination Processes in GaAs Wafers Passivated by Wet Nitridation

“…Over the past years, a variety of methods have been developed for passivating the GaAs surface. − One conventional but technologically demanding method is an epitaxial growth of a high band gap layer, such as AlGaAs, GaP, Al 2 O 3 , and GaN on the GaAs surface. − The high band gap layers could reduce the surface trap density and thus reduce photoluminescence (PL) decay rate ,, and improve the performance of GaAs-based devices. − Chemical passivation, such as sulfidation and nitridation by wet chemistry, is another typical and inexpensive passivation approach. ,− Sulfidation of the surface by wet chemistry improves the efficiency of solar cells. , However, the sulfide layer is unstable at ambient conditions, and a protective layer is needed to prevent the sulfide layer’s degradation . Surface nitridation by hydrazine sulfide solution can form a GaN monolayer at the GaAs surface.…”

GaAs yarıiletken yüzeyinde mikro yarıkların üretilmesi ve FLIM tekniği ile yüzey karakterizasyonu

“…Örneğin; rutenyum iyonlarının GaAs yüzeyine kimyasal absorpsiyonu ile yüzeydeki rekombinasyon hızı 10 4 mertebesine düşürülebilmiştir [53]. Aynı zamanda sülfidasyon ya da nitridasyon işlemleri ile de GaAs yüzeyi pasifleştirilebilmekte ve yüzey rekombinasyonları en aza indirgenebilmektedir [54,55]. Bu çalışmamızda ise, HF asit çözeltisi içinde aşındırılan ve yüzeyinde mikro yarıklara oluşturulan GaAs üzerinde rekombinasyon hızı 10 2 𝑐𝑚 𝑠 ⁄ mertebesine kadar düştüğü belirlenmiştir.…”