Conference-ISSS-7-A Study of Surface Reaction for Molecular-Layer Controlled Epitaxy of GaAs

We observed and clarified the elemental process steps in vapor phase and on surface in metal-organic chemical vapor deposition (MOCVD) reactor by in-situ infrared absorption spectroscopy, and showed schematic diagram of the reaction steps of trimethylgalliun (TMG) and AsH3 (arsine) as precursors. The surface reactions of TMG and AsH3 on the orientation dependence of the substrates were just the opposite each other. In discussion, the dominant reactions and the inhibitive factors for atomic layer epitaxy (ALE) of GaAs were focused. The results lead to the ALE process which dominates the surface reaction of Ga-compound followed by the surface reaction of As-compound on (100), however no reaction of Ga-compound occurs on (111)A surface, and no reaction of AsH3 occurs on (111)B surface. [DOI: 10.1380/ejssnt.2015.357]


I. INTRODUCTION
Ultra-thin film growth technology has many advantages for the production of electronic devices.In a conventional metal-organic chemical vapor deposition (MOCVD), the growth rate has been calibrated by a mass transportation of precursors under an adequate growth condition.On the other hand, atomic layer epitaxy (ALE) has great potential as a growth method with ultimate control on the atomic scale over thickness for semiconductor films [1].
The first ALE mode is molecular beam epitaxy (MBE)like [2], that is, the reactants are delivered to the substrate as elemental atomic or molecular beams, generated thermally in effusion cells, as it is in conventional solid source MBE.For the growth of ZnS, a heated glass substrate was exposed to Zn vapor, the Zn vapor supply was then stopped and re-evaporation of weakly adsorbed Zn occurred.The process was then repeated with sulfur, the first layer of which chemisorbed on the initial Zn layer.Any subsequent physisorbed sulfur would gradually reevaporate from the heated substrate when the sulfur flux was cut off, leaving a monolayer of ZnS.It was shown that ZnS film growth from ZnCl 2 and H 2 S could also be operated in CVD-like ALE mode [3].In this method, chemisorbed ZnCl 2 monolayer loses its chlorine to the hydrogen of the later-arriving H 2 S molecules, forming HCl, again resulting in a monolayer of ZnS.
Most important characteristics in ALE should be selflimiting growth.
We consider that CVD-like ALE has an advantage than the one of MBE-like, because the surface reactions in CVD-like ALE can be controlled by choosing the precursor gases and the reaction steps, on the other hand, substrate temperature and a flux-density is controllable for MBE-like ALE.
This paper contains a review concerning experimental measurement, and is an overview of complicated elemental process steps of TMG and AsH 3 in the vapor phase and on the substrate surface.And we discuss which processes are dominant reactions for GaAs ALE, and what is the factors we should inhibit for the reaction of ALE.

II. EXPERIMENTAL
In order to clarify the reaction steps of TMG and AsH 3 on various-oriented GaAs substrates, infrared absorption spectroscopy (IR) was used in MOCVD system [9].The reactions were measured at various conditions by a sampling method as shown in Fig. 1.In this system, a quartz capillary was set in a quartz reactor to draw the gas of reacting region into a gas cell with KBr windows in infrared spectrometer.The furnace was modulated at a uniform temperature.As gas sources, TMG in H 2 or in N 2 , AsH 3 in H 2 or in N 2 , and H 2 or in N 2 were introduced individually or simultaneously by varying from case to case.The substrate was undoped, or Si doped n-type GaAs, and the orientation dependence of the surface reaction were examined on (100), (111)A, and (111)B surface, respectively.

A. Vapor phase reaction
The vapor phase species in the MOCVD reactor were estimated by IR spectroscopy.In the (TMG + H 2 ) mixture, the reaction proceeded at a temperature above 450 • C, and the TMG reacted with H 2 to produce CH 4 , and Ga was deposited as byproduct.As shown in (TMG+ H 2 ) at 481 • C in Fig. 2(a), TMG react with hydrogen to produce methane, then the reaction is expresses as ( On the other hand, we found that a pyrolysis of TMG occurred at the temperature above 600 • C in the case of (TMG + N 2 ) mixture, in other experiment.The dominant products observed were C 2 H 4 and other hydrocarbons by IR spectra, in addition to the deposition of Ga and C [10].The reaction could be expressed as Ga(CH 3 ) 3 → Ga + C + hydrocarbons. ( As for the decomposition of AsH 3 , a pyrolysis proceeded at a temperature above 600 In the (TMG+AsH 3 +H 2 ) mixture, an unidentified species with 2080 cm −1 absorption spectrum emerged, and disappeared when TMG supply was stopped as shown in Fig. 2 For the purpose to determine the unknown material, the gas was sampled and the composition was analyzed by a mass analyzer, in other experiment [11].The materials detected in amu 92, and 106 correspond to AsH 2 CH 3 , and AsH(CH 3 ) 2 , respectively.This fact shows that the species which has a 2080 cm −1 absorption peak may correspond to AsH 2 CH 3 , or AsH(CH 3 ) 2 which is produced by TMG and AsH 3 at the suitable temperature range [10].The reaction could be expressed as When the temperature was higher than 550 • C, the direct reaction of TMG and AsH 3 occurred, and CH 4 was mainly detected with GaAs deposition in TMG-AsH 3 -N 2 mixture, therefore the direct reaction should be Ga(CH 3 ) 3 + AsH 3 → GaAs + 3CH 4 . (

B. Surface reaction
To investigate the surface reaction of TMG on GaAs surface, Ga deposition by TMG decomposition was studied in a vacuum chamber to prevent the vapor phase reaction, as discussed in other paper [12,13].When TMG alone was supplied at a certain pressure for a few minute on each oriented GaAs surface, a notable difference emerged for Ga deposition on the orientations.At the lowest temperature of 440 • C, Ga deposition was observed on (111)B which was the As-stable surface, and it was observed on (100) at above 520 • C.However, no deposition of Ga was observed even at above 600 • C on (111)A which was the Ga-stable surface.If Ga compounds exist on (111)A, Ga deposition should occur by a related reaction as shown in (2).Therefore, Ga compound desorbs from the (111)A surface before the decomposition reaction of TMG during TMG supply.This result fit with that the growth rate per cycle on (111)A is scarcely observed by alternate supply of TMG and AsH 3 by ALE growth experiment [13].Then the reaction of TMG on GaAs surface should be expressed as and the reaction proceeds on (111)B > (100) ≫ (111)A.( 6) On the other hand, on (111)B and (100), the Ga compounds exist during evacuation followed by TMG supply, and the growth rate per cycle on these surface occurs by alternate supply of TMG and AsH 3 by ALE growth experiment [13].Namely, the Ga compounds should adsorb on arsenic bonds, but not on Ga bonds on the surface, and the Ga deposition temperature on the surface was much lower than the pyrolytic reaction in vapor phase as shown in reaction (2).
The surface reaction of AsH 3 on the surface of GaAs was analyzed by the sampling method of IR absorption spectroscopy.The absorbance of AsH 3 at 2122 cm −1 was estimated with and without GaAs.In Fig. 4, temperature dependence of the deviation of absorbance of AsH 3 with GaAs from the one without GaAs was shown.The deviation was determined as (D − D 0 ), here D is the absorbance of AsH 3 with GaAs, and D 0 is the one without GaAs.In the case with GaAs, the surface of (111)A, (100) and (111)B about 9 cm 2 were placed in a uniform high-temperature region in the reactor, respectively.The results indicate that the decomposition of AsH 3 was affected sensitively by the existence of GaAs, and on the orientation of the surface.Especially, the decomposition was enhanced strongly on (111)A, the next on (100), but not on (111)B.The decomposition on (111)A and (100) proceeded at the temperature above 500 • C, however the decomposition in the vapor phase occurred at above 600 • C as shown in reaction (3).In addition, the rate of decomposition on (111)A was higher than that on (100), then the reaction can be described as and the reaction proceeds on (111)A > (100) ≫ (111)B.(7) In other word, AsH 3 decomposition occurred easily on the Ga stable surface, and the activation energy might be reduced on the Ga atable surface.Actually, the activation energy was estimated by us as 19 kcal/mol for (111)A, 24 kcal/mol for (100), and 34 kcal/mol for (111)B from the decomposition rate of AsH 3 in reciprocal temperature plot [10].The value for (111)B was almost equal to the one with no GaAs, namely almost no effect is expected for AsH 3 decomposition on (111)B.

IV. DISCUSSION
The full picture of the reaction steps of TMG and AsH 3 clarified by this research were shown in Fig. 5. First, TMG reacts with H 2 in vapor phase, therefore TMG in vacuum or with an inert gas should be use when TMG associated compound is assumed as a role of adsorption species in ALE process.However, some processes of GaAs ALE employed TMG and H 2 mixture at sufficient hightemperature to cause the reaction.Under such condition, ALE process might obey mass transfer limited process, but not self-limiting monolayer process.
The direct reaction of TMG and AsH 3 occurs through several reaction paths in vapor phase.One is the reac-tion to produce GaAs deposition and CH 4 desorption, the other is the reaction to produce AsH 2 CH 3 or AsH(CH 3 ) 2 .The direct reaction may be important for MOCVD of GaAs, however the direct reaction should be avoided for ALE which is restricted only surface reactions.
As for the orientation dependence of the substrates, the characteristics of the reaction of TMG and AsH 3 are just the opposite each other.The surface reaction of TMG proceeds on (111)B > (100) ≫ (111)A, however the reaction of AsH 3 proceeds on (111)A > (100) ≫ (111)B.The orientation dependence of TMG reaction predicts that the adsorption or deposition of Ga related species occurs scarcely on (111)A.Actually, the growth of GaAs ALE on (111)A fit the prediction ; no growth occurs on (111)A, but not on (111)B and (100).On the other hand, AsH 3 decomposition occurs on (111)A and (100), but not on (111)B as inverse of the case of TMG.Therefore, GaAs ALE occurs on (100) surface, because of its amphoteric characteristics for the reaction of TMG and AsH 3 .Namely, the characteristics of (100) surface is intermediate for the both reaction, and fit for the alternative reaction of TMG and AsH 3 in ALE process.Even in the case of MOCVD, AsH 3 decomposition will be enhanced as (111)A > (100) ≫ (111)B, the surface stoichiometry during growth can make difference on each surface.The decomposition of AsH 3 produces As 4 on (111)A and (100), and the released As 4 from the surface can react with TMG or Ga related species to produce GaAs, as shown in righthand edge of Fig. 5.This reaction corresponds to the vapor phase transfer in MOCVD.

V. CONCLUSIONS
We observed and clarified the elemental process steps in vapor phase and on surface in MOCVD by in-situ infrared absorption spectroscopy, and showed schematic diagram of the reaction steps of TMG and AsH 3 .The surface reactions of TMG and AsH 3 on the orientation dependence of the substrates were just the opposite each other.In discussion, the dominant reactions for GaAs ALE were pointed out, and the inhibitive factors for it were suggested.To create a leading-edge of growth technology, elucidation of growth mechanism at the level of elemental process steps is indispensable, and we should make an attempt to control each process step individually, as much as possible.
(b).The unknown material was determined quantity by D = log(T a /T b ).Here, D is the absorbance of the unknown material, T a and T b are transmittance of the peak of the unknown material and the value of AsH 3 itself at 2080 cm −1 .The unknown material with 2080 cm −1 absorption spectrum was produced dominantly in (TMG + AsH 3 + N 2 ) system, because the TMG reaction with H 2 is suppressed in N 2 atmosphere.As shown in Fig. 3(a), the absorption peak at 2080 cm −1 emerged at relatively-narrow region of temperature region.And the temperature characteristics of the peak intensity at 2080 cm −1 was different from the one of AsH 3 .This means that the unknown material may by produced by the direct reaction of TMG and AsH 3 , and the peak was observed over the temperature range of about 450-550 • C synchronized with the decomposition with TMG and AsH 3 as shown in Fig. 3(b).

FIG. 4 .
FIG. 4. Temperature dependence of the deviation of absorbance of AsH3 with GaAs from the one without GaAs.The area of GaAs wafer is about 9 cm 2 .

FIG. 5 .
FIG. 5. Schematic diagram of the full picture of the reaction steps of TMG and AsH3 clarified by the spectroscopic experiments.
• C, and there was no difference in decomposition characteristics between the AsH 3 -H 2 and AsH 3 -N 2 mixture in other experiment.Then, the reaction should be AsH 3 → 1/4As 4 + 3/2H 2 .