Native Oxide Transport and Removal During Atomic Layer Deposition

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Native Oxide Transport and Removal During the Atomic Layer Deposition of TiO2 Films on GaAs(100) Surfaces Alex J. Henegar, Andrew J. Cook, Phillip Dang, and Theodosia Gougousi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08998 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 9, 2016

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ACS Applied Materials & Interfaces

Native Oxide Transport and Removal During the Atomic Layer Deposition of TiO2 Films on GaAs(100) Surfaces Alex J. Henegar, Andrew J. Cook, Phillip Dang, and Theodosia Gougousi* Department of Physics, UMBC, Baltimore, Maryland 21250, USA

ABSTRACT In this manuscript we studied the evolution and transport of the native oxides during the atomic layer deposition (ALD) of TiO2 on GaAs(100) from tetrakis dimethyl amino titanium and H2O. Arsenic oxide transport through the TiO2 film and removal during the ALD process was investigated using transmission Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Experiments were designed to decouple these processes by utilizing their temperature dependence. A 4 nm TiO2 layer was initially deposited on a native oxide surface at 100 °C. Ex-situ XPS confirmed that this step disturbed the interface minimally. An additional 3 nm TiO2 film was subsequently deposited at 150 to 250 °C with and without an intermediate thermal treatment step at 250 °C. Arsenic and gallium oxide removal was confirmed during this second deposition leading to the inevitable conclusion that these oxides traversed at least 4 nm of film so as to react with the precursor and its surface reaction/decomposition byproducts. XPS measurements confirmed the relocation of both arsenic and gallium oxides from the interface to the bulk of the TiO2 film under normal processing conditions. These results explain the continuous native oxide removal observed for alkyl-amine precursor-based ALD processes on III-V surfaces and provide further insight into the mechanisms of film growth. Keywords: Atomic Layer Deposition, Arsenic Oxide, Gallium Oxide, Interface, Diffusion, Dielectrics, “Clean-up”

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INTRODUCTION Atomic layer deposition (ALD) holds many advantages over other deposition techniques which have made it a desirable tool for thin film growth. It can be used to grow highly conformal thin films with sub-nanometer thickness control. Since its early development for the deposition of thin films for electroluminescent displays,1 the applications of ALD broadened into an expansive field ranging from microelectronics to biocompatible coatings.2–4 One of the most common applications is the formation of gate oxides in metal oxide semiconductor devices.5–7 The first mechanistic models of ALD were based on simple ligand exchange reactions between a functionalized surface and reactive precursors. However, computational and experimental studies have shown that the mechanism is significantly more complex than originally thought and may involve precursor decomposition as a possible reaction pathway.8–11 These additional reaction pathways are required to explain the thinning of the surface native oxides during dielectric formation on III-V semiconductors often called “interface clean-up”.11 Frank et al. and Ye et al. were the first to report such thinning of the native oxide interfacial layer during the ALD of Al2O3 from trimethyl aluminum (TMA) and H2O on GaAs.12,13 Since then, several systems exhibiting this native oxide “clean-up” behavior have been identified.14–19 One unexplained observation to this date is that for amine precursors the “clean-up” reaction proceeds well after the native oxide surface has been covered with the formed film.15,20–25 To explain such observations a mechanism that will transport the surface oxides through the growing film is required. Diffusion of indium atoms have been documented for several high-k based gate stacks for which ALD is used for the dielectric deposition.22–24,26–31 For instance, Ye and Gougousi32 demonstrated indium oxide transport to the surface of a 6.4 nm TiO2 film deposited on InAs(100) from tetrakis dimethyl amino titanium (TDMAT) and H2O at 200 °C. There are but a couple of literature observations that document diffusion of arsenic oxides through dielectric films deposited at temperatures below 350 2


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°C and left without further thermal treatment. Chang et al.14 observed the presence of a small amount of AsOx on the surface of a 7.4 nm HfO2 film deposited on native oxide InGaAs(100) surfaces from tetrakis ethylmethylamido hafnium and H2O at 200 °C. Cabrera et al. found arsenic oxide on the surface of 4 nm HfO2 films after deposition on In0.53Ga0.47As at 250 °C and attributed it to arsenic migration to the surface during the ALD process.23 Gallium oxides have not been observed on the top surface of asdeposited ALD dielectrics during interface cleaning studies. In this work we demonstrate the existence of the transport mechanism needed for continuous oxide removal during ALD at typical processing conditions (150-250 °C). We couple transmission Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) measurements to show both indirectly and directly that the surface arsenic and gallium oxide can migrate through 4 nm thick TiO2 films deposited on native oxide GaAs(100) surfaces.

EXPERIMENTAL METHODS TiO2 films were deposited on GaAs(100) surfaces using TDMAT and H2O in a custom ALD reactor previously described by Hackley et al.33 Most depositions were performed on native oxide-covered, high resistivity, double-polished GaAs(100) wafers suitable for transmission FTIR. Samples were cleaved from the same batch of wafers to minimize variations in the native oxide layer from sample to sample. The GaAs pieces were cleaned prior to deposition in acetone, methanol and deionized (DI) water, for one minute each, and then blown dry with N2. Some samples were etched to remove the native oxides using a two-step process starting with immersions in J. T. Baker 100 solution (5 min) followed by DI water (5 min) and blown dry with N2. Next, the samples were dipped in a buffer oxide etch solution (6:1 hydrofluoric acid) and DI water for 20 s each, blown dry with N2 and immediately mounted onto the FTIR bench for data collection. These samples will be referred to as “HF-etched” samples. All samples were heated in the reactor for 30 min prior to every deposition for thermal equilibration. Films were 3



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Step 0 Starting Surface

GaAs

Step 1 4nm TiO2 100°C

Step 2 Thermal Activation

Native Oxide

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Step 3 3nm TiO2 High Temp

TiO2

Figure 1. Schematic illustrating the experimental steps used to decouple the native oxide transport and removal processes. First a 4 nm TiO2 film was deposited on native oxide GaAs at 100 °C. Select films underwent a thermal activation step to promote native oxide migration into the film. A 3 nm TiO2 film was deposited at a higher temperature to remove any native oxide that may have reached the surface. deposited at temperatures ranging from 100 to 250 °C. Stacks examined in this work were prepared by depositing two layers of the same material at different temperatures separated by a thermal treatment step. A schematic of this process is shown in Figure 1: i.

First, a layer of TiO2 was deposited at 100 °C. The low temperature was used to limit the native oxide mobility and “clean-up” reactions known to be thermally activated.18,20,25,34,35 This layer protected the interfacial layer from precursor and air exposure and created a native oxide concentration gradient in the sample.

ii.

Select films were heated for 2-24 hours at 250 °C in the ALD reactor under N2 flow which will be referred to as “thermal activation” (TA) in this manuscript. This temperature was chosen primarily because it is typical for amine-based ALD processes. For such processes a gradual removal of the surface oxides is well-documented. Arsenic and gallium oxide sublimation is known to occur above 350 °C so the chosen TA temperature avoids this complication.23,36

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iii.

A second layer of TiO2 was deposited on select samples at temperatures between 150 °C and 250 °C. Varying the temperature allowed control over the rate of native oxide transport during deposition as well as the rate of the removal reactions. Samples were examined with ex-situ XPS using a Kratos AXIS 165 spectrometer with an Al

monochromatic X-ray source (1486.6 eV) and a hemispherical analyzer (165 mm radius). High resolution spectra of the As 3d and Ga 3d regions were taken using a 20 eV pass energy and were normalized to the substrate peak to facilitate sample comparison. An 80 eV pass energy was used for the As 2p3/2 region which resulted in increased sensitivity and a loss of peak resolution. A 0.1 eV step size was used for all regions. The energy scale was shifted to place the As-Ga substrate peak at 41.1 eV.37–39 A Shirley type background was used for peak fitting. The As 3d region was fit using four doublets for the As-Ga substrate peak, As3+ (As2O3, 44.35 ± 0.1 eV), As5+ (As2O5, 45.8 ± 0.1 eV) and metallic/sub-oxide AsOx (41.7 ± 0.1 eV). The Ga 3d region was fit with doublets for the Ga-As substrate peak (19.2 eV), Ga1+ (Ga2O, 20.4 ± 0.1 eV) and Ga3+ (Ga2O3, 21.1 ± 0.1 eV). All doublets were composed of two Gaussian-Lorentzian peaks with a spin-orbit splitting of 0.7 eV and 0.44 eV for the As 3d and Ga 3d regions respectively. Most samples were analyzed as received unless otherwise noted. Depth profiling was completed using an Arion sputter gun in the XPS chamber. Transmission FTIR measurements were performed using a Nicolet 4700 FTIR spectrometer. Spectra (4000 – 400 cm-1) of 128 scans with a 4 cm-1 resolution were taken using a deuterated triglycine sulfate detector/KBr beam splitter. All spectra were processed with the OMNIC software provided by the vendor. Integrated peaks areas were extracted using the appropriate software tool and a flat baseline. Films were grown on both surfaces of the GaAs wafers (verified by spectroscopic ellipsometry) and both were probed during transmission FTIR. The built-in atmospheric suppression algorithm was used to remove the major peaks from gaseous CO2 and H2O. Spectra of samples taken after cleaning 5



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were compared to each other to verify every sample started with an identical composition. Due to the overlap of the TiO2 broadband40 with the arsenic oxide peak, cancellation of this band was essential to uncovering the effects of processing on the arsenic oxide signal. Therefore the reference spectrum was chosen to be a film of equal TiO2 thickness to the sample of interest. This enabled us to cancel out the TiO2 contribution to each spectrum down to ~600 cm-1 and allow the detection of changes in the arsenic oxide concentration on the order of monolayers. For each measurement the appropriate reference spectrum was used as indicated in the text and figure captions.

RESULTS In order to assess the suitability of the technique to detect changes in the thickness of the native oxide, an etched sample was initially examined with FTIR. The infrared spectrum was referenced to the spectrum of the sample before etching and is presented in Figure 2. The negative peak at 830 cm-1 is assigned to the arsenic oxide phonon modes25,41 and can be used to gauge the effect of processing conditions on the arsenic oxide concentration. Etching the sample provided a metric of the oxide removal efficiency for the TiO2 depositions discussed throughout this manuscript. The loss peak shown here in the etched sample represented the largest possible change in the arsenic oxide signal. Percentages of arsenic oxide lost mentioned in the text are in reference to the total loss in the etched sample unless otherwise noted. To test further the accuracy of this technique a series of depositions were performed from 150 to 250 °C on native oxide-covered GaAs (Figure 2). These spectra were referenced to a 3 nm TiO2 film deposited at 100 °C on native oxide-covered GaAs. The low deposition temperature was used to keep the native oxide layer as undisturbed as possible while maintaining a stable, reproducible growth rate. We observed a monotonic increase in the arsenic oxide removal with temperature in agreement with previous works.18,20,25,35 At 250 °C, the amount of removal matched that of the etched sample indicating complete oxide removal during the process. 6


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In these spectra, as well as in several others shown throughout this work, there were a number of extraneous peaks unrelated to the scope of this work. There were four peaks located above 3600 cm1

. Peaks from -OH stretching modes are found in this region and may correspond to an OH-terminated

surface.42–44 These four peaks also match very well to the Fermi resonances of two combination modes of gaseous CO2.45,46 It is likely this region was comprised of a mixture of features from these two sources. The atmospheric suppression algorithm utilized in OMNIC only applied to the asymmetric stretching

830

HF Etch

Absorbance (a.u.)

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3nm, 150°C

3nm, 200°C

3nm, 250°C

0.001

4000

3000

2000

1000 -1

Wavenumber (cm ) Figure 2. Transmission FTIR spectra for a plain etched GaAs(100) substrate and for 3 nm TiO2 films deposited at 150-250 °C on native oxide GaAs(100). The amount of arsenic oxide removal increased with deposition temperature. The film deposited at 250 °C showed an equivalent amount oxide loss as the etched substrate which signified complete removal at this temperature. All spectra with TiO2 films were referenced to films of equivalent thickness grown at 100 °C. The HF-etched GaAs sample was referenced to cleaned native oxide GaAs. 7



bands of gaseous CO2 (~2360 and 2340 cm-1) and the gaseous H2O peaks (~1900-1400 cm-1) which were not completely suppressed. The two peaks corresponding to the symmetric (2852 cm-1) and asymmetric (2922 cm-1) CH stretching modes of methylene groups of aliphatic molecules were seen clearly in the etched sample and were likely a product of the etching process and/or exposure to the ambient.47,48 The effect of the duration of the thermal activation step (step 2 of Figure 1) was examined in

830

4nm TiO2 at 100°C

Absorbance (a.u.)

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+ 2 Hr TA

+ 4 Hr TA

+ 12 Hr TA

+ 24 Hr TA

0.001

4000

3000

2000

1000 -1

Wavenumber (cm ) Figure 3. Transmission FTIR spectra for 4 nm TiO2 films deposited at 100 °C after TA at 250 °C for the specified time. TA for 2-4 hours had a small effect on the arsenic oxide peak. After 12 hours of TA a maximum change was observed. All spectra were referenced to spectra taken before the TA step.

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detail for a 4 nm TiO2 film that was deposited on GaAs at 100 °C (step 1 of Figure 1). Transmission FTIR is a bulk sensitive technique, so redistribution of the native oxides cannot be detected. However, sublimation or conversion of arsenic oxide to gallium oxide or another arsenic oxide state is possible and must be examined in detail.49–51 Spectra for films subjected to TA between 2 and 24 hours are shown in Figure 3. Each spectrum was referenced to the spectrum taken before the TA step. The dip at ~830 cm-1 intensified as a function of TA time and was significant for heat treatments in excess of 12 hours. At 2

830

7nm, 150°C

Absorbance (a.u.)

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4nm, 100°C + 3nm, 150°C

4nm, 100°C + 2 Hr TA + 3nm, 150°C

4nm, 100°C + 3nm, 250°C

4nm, 100°C + 4 Hr TA + 3nm, 250°C

0.001

4000

3000

2000

1000 -1

Wavenumber (cm ) Figure 4. Transmission FTIR spectra for several 7 nm TiO2 stacks. Arsenic oxide removal was observed during the 150 °C deposition only when it was preceded by TA. At 250 °C, this additional heat treatment was unnecessary. All spectra were referenced to films of equivalent thickness grown at 100 °C on native oxide-covered GaAs.

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hours only a minute effect to this feature was recorded. After 4 hours some loss was apparent and the peak intensity leveled beyond 12 hours. Based on these findings, the samples used for the FTIR studies were thermally treated for 4 hours or less to minimize changes in the arsenic oxide concentration due to heating. Following the TA step a second deposition at 150 °C and 250 °C was completed (step 3 of Figure 1). The same sequence of depositions was performed on samples that were not subjected to the TA step. The spectra for these films presented in Figure 4 have some very interesting features. The deposition of an additional 3 nm at 150 °C did not cause any significant change in the arsenic oxide as indicated by 830 cm-1 spectral region. However, when the deposition was preceded by a 2-hour TA step then a clear dip at the same region was observed and is indicative of loss for the species associated with this feature. To put the amount of arsenic oxide removal into perspective, a 7 nm deposition was completed at 150 °C directly on the native oxide surface and resulted in about a 50% increase in the oxide lost. For depositions at 250 °C arsenic oxide removal was observed regardless of the thermal history of the sample but a ~20% increase in the peak intensity was detected in the sample that was thermally treated compared to the sample that was not thermally treated. The most important point though is that there was removal of arsenic oxides as a result of the second deposition even though the arsenic oxide has been buried under a 4 nm TiO2 film. This can only happen if the arsenic oxides have relocated though the initial 4 nm TiO2 film during any or all of the subsequent processing steps depending on the thermal budget.

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Films deposited at an intermediate temperature of 200 °C on native oxide GaAs were also examined and their infrared spectra are shown in Figure 5. Deposition of 3 nm of TiO2 at 200 °C on top of the 4 nm film resulted in a moderate amount of arsenic oxide removal. Direct deposition of 3 nm of TiO2 on the native oxide GaAs surface showed about twice as much arsenic oxide removal and when the deposition was allowed to proceed to 7 nm nearly complete removal was observed.

830

4nm, 100°C + 3nm, 200°C

Absorbance (a.u.)

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3nm, 200°C

7nm, 200°C

0.001

4000

3000

2000

1000 -1

Wavenumber (cm ) Figure 5. Transmission FTIR spectra for TiO2 stacks. All spectra were referenced to spectra of films of equivalent thickness grown at 100 °C on native oxide-covered GaAs. The deposition at 200 °C removed arsenic oxide in all cases. The fact that removal occurred when 3 nm were deposited on top of the 4 nm film (black spectrum) indicates native oxide transport occurred during the second deposition. 11



4nm, 100°C

4nm, 100°C + 3nm, 250°C

4nm, 100°C + 3nm, 250°C + Sputter

3nm, 200°C

Ti 3p

As2O3

As-Ga

As 3d

As2O5

Ti 3p

As-Ga As2O3

GaAs Starting Surface

Normalized Intensity (CPS)

b. As 3d

As2O5


a.

7nm, 200°C

7nm, 200°C + Sputter

50 48 46 44 42 40 38 36 34 50 48 46 44 42 40 38 36 34

4nm, 100°C

4nm, 100°C + 3nm, 250°C

4nm, 100°C + 3nm, 250°C + Sputter

28

26

24

22

20

18

16

3nm, 200°C

7nm, 200°C

7nm, 200°C + Sputter

28

Binding Energy (eV)

Ga2O

Ga 3d

Ga-As

Binding Energy (eV) O 2s



Ga-As

Ga2O3 Ga2O

Ga 3d

d.

Ga2O3

Binding Energy (eV)

O 2s

c. Normalized Intensity (CPS)

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26

24

22

20

18

16

Binding Energy (eV)

Figure 6. XPS series of the (a, b) As 3d and (c, d) Ga 3d regions for select samples presented in Figures 4 and 5. The sample with 4 nm TiO2 at 100 °C plus 3 nm TiO2 at 250 °C showed a marginal amount of As-O bonding before sputtering and none after sputtering. Gallium oxides were removed after sputtering. Thickness-dependent arsenic and gallium oxide removal was apparent at 200 °C. 12


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XPS was used to corroborate the surface oxide removal demonstrated by FTIR measurements for some of the samples included in Figures 4 and 5. The results for these samples are shown in Figure 6 (a) and (b) along with the spectra for the base sample (4 nm of TiO2 deposited at 100°C) and the GaAs native oxide starting surface. In these spectra only the arsenic oxide peak deconvolution is shown for clarity. The native arsenic oxides are composed of a mixture of As2O3 (As3+) and As2O5 (As5+). As expected deposition of the 4 nm of TiO2 at 100°C changed only marginally the surface arsenic oxides composition and thickness; the As3+ peak intensity decreased marginally. Deposition of an additional 3 nm of TiO2 at 250 °C led to almost complete removal of the spectral features associated with arsenic oxide in complete agreement with the FTIR spectra presented in Figure 4. This sample was thinned in the analytical chamber to facilitate the characterization of the interface. The remaining As-O bonding was removed and a sharp interface was detected indicating that during the second deposition the surface arsenic oxides traversed the 4 nm film to reach the top of the film where they were subsequently removed after reactions with the precursor and/or the ALD reaction byproducts. As additional evidence for the validity of the FTIR-based arsenic oxide detection we also acquired XP spectra for the 3 and 7 nm TiO2 films deposited at 200°C (Figure 5). The FTIR data indicated partial removal of the arsenic oxide after the deposition of the 3 nm film and complete removal after the deposition of 7 nm. These findings were mirrored in the XPS spectra presented in Figure 6b. After the deposition of 3 nm of TiO2 at 200°C a small amount of As2O3 remained in the film. The XPS data (Figure 6) for the 7 nm film were taken before and after sputtering to remove a significant portion of the TiO2 and confirm the complete removal of the arsenic oxides. The XPS analysis of these stacks also provided information about the surface gallium oxides and the results are presented in Figure 6 (c) and (d). The gallium oxides were composed of a mixture of Ga2O (Ga1+) and Ga2O3 (Ga3+). The O 2s signal originated mainly from the growing TiO2 film. Deposition of the 13



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base 4 nm TiO2 layer at 100°C led to a small decrease in the gallium oxide components which confirms the minimal reactivity of the native oxides at the low temperature of 100°C. The more interesting observation though is that after the additional 3 nm were deposited at 250°C substantial removal of the gallium oxides was detected. Thinning of this sample in the XPS chamber revealed that very little gallium oxide remained close to the interface leading to the conclusion that the surface gallium oxides also transported through the 4 nm TiO2 film as a result of the thermal processing. This gradual removal of the gallium oxides is also confirmed for the 3 and 7 nm films that were deposited at 200°C and is in agreement with earlier observations.21 The effects of the thermal treatment on the location of the arsenic oxides in the stack were also analyzed using XPS. A series of spectra in the As 3d region for films with 4 nm of TiO2 deposited at 100 °C are shown in Figure 7a. Only the arsenic oxide peak deconvolution is shown for clarity. A change in the arsenic oxide composition as a result of thermal treatment was confirmed. Heating the samples 2-12 hours decreased the As5+ peak and monotonically increased the As3+ peak. The TiO2 film thickness was not affected by heating as evidenced by the constant Ti 3p:As(-Ga) 3d peak ratio. The exact location of these oxides in the stack can be identified by depth profiling. For that purpose the film that was subjected to 12 hours TA was sputtered for a short time to remove ~1 nm of TiO2. This procedure resulted in the removal of all of the As2O5 and most of the As2O3 in the film. Sputtering again for an equivalent amount of time removed any remaining arsenic oxide. In both steps, a large portion of the TiO2 film remained intact, as indicated by the intensity of the Ti 3p peak, and ensured that the interface was left undisturbed. The depth profile confirmed that arsenic oxide relocated from the TiO2/GaAs interface to the top half of the film as a result of the 12-hour thermal treatment.

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a.

b.

Figure 7. XPS series of the (a) As 3d and (b) As 2p3/2 region for a series samples with 4 nm of TiO2, deposited on native oxide-covered GaAs, that underwent TA for various times. TA lowered the As2O5 peak and raised the As2O3. Spectra show that arsenic oxide was near the surface of the film after the 12-hour TA. When this sample was sputtered the arsenic oxide peaks were removed. Corroborating evidence for the location of the arsenic oxides in the stack can obtained from the As 2p3/2 region (Figure 7b). No signal was observed in this region after a 4 nm deposition at 100 °C or after the subsequent 2- and 4-hour heat treatments. Considering the kinetic energy of the photoelectrons detected (~160 eV) for this binding energy, the inelastic mean free path (~0.5 nm) limited the photoelectron escape depth to ~2 nm.52 Oxide concentrations near the surface were likely too low to detect using XPS. The detection of arsenic oxide after the 12-hour TA places these species within 2 nm from the surface. Sputtering removed these features and the thinning of the stack permitted detection of the As-Ga substrate peak.

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Ga 3d


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O 2s Ga2O3 Ga2O Ga-As


4nm TiO2 at 100 °C

2 Hr TA

4 Hr TA

12 Hr TA 12 Hr TA: 1st Sputter 12 Hr TA: 2nd Sputter

28 26 24 22 20 18 16

Binding Energy (eV) Figure 8. XPS series of the Ga 3d region for a series samples with 4 nm of TiO2, deposited on native oxide-covered GaAs, that underwent TA for various times. The deposition slightly decreased the gallium oxide peaks. TA up to 12 hours increased the Ga2O due to oxide relocation and/or conversion from arsenic oxide. Sputtering removed the higher oxidation state but the some of the oxides remained unaffected near the interface. Figure 8 shows the Ga 3d region for the same set of the samples. TA up to 12 hours resulted in a noticeable increase in Ga1+ oxidation state but left the Ga3+ state practically unchanged within the uncertainty of the fit. After the two sputtering steps, the Ga3+ intensity was reduced to practically zero and the Ga1+ intensity marginally decreased. The extent of oxide migration during TA and removal during 16


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sputtering was far less pronounced for the gallium oxides compared to the arsenic oxides. However, since the signal has an exponential dependence on the originating depth it is very hard to quantify the effect. Clearly gallium oxides are less mobile and harder to remove than arsenic oxides in agreement with previous observations at 200 °C.21

DISCUSSION It has been proposed that native oxide transport is a critical component for its continuous removal during ALD on III-V native oxide surfaces. Ye and Gougousi observed the accumulation of indium oxide on the film surface after the deposition of 6.4 nm of TiO2 on InAs(100) at 200 °C.32 They conjectured that thickness dependent oxide removal was a product of oxide diffusion followed by subsequent reactions with the precursors and ALD reaction byproducts. Cabrera et al.23 found arsenic oxide on the surface of 4 nm HfO2 films after deposition on In0.53Ga0.47As at 250 °C but this was not linked to arsenic oxide removal. Most of the studies that examined the interfacial oxide behavior during deposition were performed at temperatures in excess of 200 °C where the transport and removal mechanisms were coupled making it impossible to discern the fundamentals of native oxide “clean-up.” In this manuscript we have designed a series of experiments that provide a direct link between native oxide transport and continuous native oxide removal. For that purpose we have used transmission FTIR and Figure 2 shows that the technique is capable of detecting small changes in the arsenic oxide content. In this figure we show temperature dependent arsenic oxide removal after the deposition of 3 nm of TiO2 from 150 to 250 °C. The magnitude of the loss peak at 830 cm-1 for the 250 °C film and the etched sample were very similar and corresponded to maximum oxide removal. Similar results have been observed in systems using other measurement techniques.10,18,20,35,53 In particular, in 17



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recent work Ye and Gougousi used XPS to investigate the interface region of 3 nm TiO2 films deposited on native oxide GaAs at temperatures similar to those used in this work.55 The FTIR results in Figure 2 are in complete agreement with the published work and confirm enhanced oxide removal at higher deposition temperatures up to 250°C. More subtle changes in the arsenic oxides were detected as a result of the TA treatment (Figure 3). These measurements, coupled with the fact that the native oxide layer on GaAs(100) is typically 2-3 nm thick and Ga-rich,21,54 demonstrated the capability of transmission FTIR to detect changes in the arsenic oxides with sub-nanometer resolution. As a starting point we should examine whether the initial deposition of 4 nm of TiO2 at 100 °C produced a dense and continuous film able to protect the interface. The As 2p3/2 data shown in Figure 7b can provide strong support for this assumption. As mentioned earlier the electron mean free path for this region is ~0.5 nm; as a result we can only probe the top ~2 nm of the film. The fact the we detect no As 2p3/2 signal for the film deposited at 100 °C and after it was thermally treated at 250 °C for 2 and 4 hours indicates that the as-deposited film was continuous and that this topography did not change drastically after the 250 °C thermal treatment. If there were large variations in the film thickness, in excess of 2 nm peak-to-valley, then we would have detected some As 2p3/2 signal. The XPS data for the surface sensitive As 2p3/2 region also provided the required information regarding the film uniformity. In fact, since the XPS spot size is of the order of 1 mm2 this technique probes and averages signals from a much larger area than any scanning probe technique such as Atomic Force Microscopy. The FTIR beam also probes an area of similar dimension (~80 mm2). An additional piece of evidence to support the uniformity of the starting 4 nm TiO2 film deposited at 100 °C can be found in the direct comparison of the FTIR signals for the 3 nm TiO2 film deposited at 150 °C (Figure 2) to the one for the 4 nm TiO2 film deposited at 100 °C plus 3 nm at 150 °C (Figure 4). In the absence of the thermal activation step no change was observed in the As-related IR signal at 830 cm-1. This observation provides further evidence 18


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that the starting 4 nm film was homogeneous enough to prevent direct contact of the precursor with the native oxides. Subsequently, the effect of the initial deposition at 100 °C on the interfacial oxide composition must be examined. While we cannot detect these changes directly with FTIR due to the need to cancel the TiO2 phonon contributions we have indirect evidence. A pristine native oxide surface was used as a reference for the etched sample (Figure 2). A 3 nm film deposited at 100 °C served a reference spectrum for the film deposited on native oxide at 250 °C. The two samples showed practically identical features at 830 cm-1. This would only be possible if the difference between in the peak intensity for the sample spectrum and reference spectrum matched in both cases. Therefore the 3 nm deposition completed at 100 °C must have similar contributions to the peak as the native oxide GaAs substrate. Any other possibility would have led to variations in the size and width of the dip for the deposition at 250 °C. Although FTIR did not detect any significant effect on the arsenic oxides by the deposition at 100 °C, XPS results (Figure 7a) showed a marginal decrease in the As3+ state in agreement with recently published work.55 This may be due to the variation of the native oxide composition across the wafer batch used in this work. While measures were taken to minimize the native oxide variations among samples, this cannot be ruled out. XPS is more sensitive to composition variations than FTIR which was used for routine sample screening. Another possibility that cannot be excluded is that the decrease in the As3+ intensity was caused by oxide removal through reactions with the precursors. Depositions using TDMAT and other amine-based precursors have been shown to remove the higher oxidation state (As5+) first even though it is more thermodynamically stable.16,18,20,38 Arguments to explain this included the possibility of oxide conversion from As2O5 to As2O3 followed by removal of As2O3. All of these observations were made for depositions in excess of 200 °C where the arsenic oxide conversion and removal reactions are more efficient. At the low temperature of 100 °C, conversion is less likely to occur 19



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as shown by the unaltered As5+ peak after deposition. Conversion of arsenic oxides to gallium oxides is a well-known thermodynamic pathway but it also requires much higher temperatures.51 Another possibility is the formation of non-stoichiometric TiO2 at this low temperature that may reduce the surface oxides. However, the XP spectra of the Ti 2p and O 1s regions (not shown) for the samples presented in Figure 7 showed no change after heating for up to 12 hours. As a result, we think that surface oxide reduction by non-stoichiometric TiO2 was unlikely. Both the FTIR and the XPS measurements indicate some alteration in the surface oxide composition and/or location as a result of the thermal treatment step. In the FTIR spectra there was a monotonic increase in the dip at 830 cm-1 (Figure 3). The XP spectra showed that an increase in the duration of the TA step resulted in a decrease in the As5+ peak and a monotonic increase in the As3+ peak. FTIR is a bulk technique and cannot detect any relocation of the oxide in the beam path. As a result the dips observed at ~830 cm-1 from the TA step, shown in Figure 3, must have originated from arsenic oxide loss or conversion pathways. Arsenic oxide sublimation is known to occur above 350 °C so the effect should be minimal at the chosen TA temperature.23,36 Conversion of the arsenic oxides is well documented though. For example, Chellappan et al. observed As2O5 reduction on InAs(100) surfaces by vacuum annealing at 200 °C and its complete removal at 300 °C.56 Brennan and Hughes detailed the effects of heating on the interfacial oxides of In0.53Ga0.47As and found As2O5 may convert to As2O3 through oxygen transfer when heated up to 300 °C without conversion to either indium or gallium oxides.38 The XPS results in Figure 7a provided some additional clues. The two-hour TA step caused a decrease in As2O5 accompanied by a proportional increase in As2O3. This observation supports the idea about conversion of the As5+ state to As3+. Longer TA steps though, while they led to an increase in the As2O3 peak intensity, left the As2O5 peak intensity practically constant. As a reminder, the corresponding FTIR (Figure 3) showed a gradual increase in the magnitude of the feature at 830 cm-1 under similar 20


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conditions. These two apparently contradictory events can be explained if one considers that XP signal intensity depends on both the location and the concentration of the species. In this instance, in the XP spectra the loss of the As2O5 from conversion to As2O3 has been balanced by an increased detection ability resulting from the migration of the As2O5 through the TiO2 film. If only conversion took place then the corresponding XPS signal would have decreased. Therefore to reconcile the XPS results with the FTIR results, the peak observed at 830 cm-1 must be predominantly associated with the As5+ state. This is in agreement with FTIR results on pure crystalline arsenic oxide compounds presented by Rei Vilar et al.41 Definitive support of continuous removal aided by native oxide transport was seen when the second deposition was completed at 150 °C (Figure 4). Removal was not observed after the surface was covered unless the sample was subjected to the thermal activation step. This step aided the arsenic oxide redistribution through the bulk of the TiO2 film and the subsequent deposition at 150 °C allowed their removal. The TA step has been shown to result in arsenic oxide loss (Figure 3). However, in the case of the 4 nm step (Figure 4) the magnitude of the loss peak was 10 times higher than the TAoriginated loss peak and the difference should be attributed to arsenic oxide removal. Without TA no change to the arsenic oxide content was detected. The limiting factor in this case appears to be the transport rate during deposition. A 7 nm deposition at 150 °C directly on the native oxide-covered surface resulted in higher oxide loss. At these deposition conditions oxide removal likely proceeded at peak efficiency until the interface was covered. After that, removal gradually slowed down because the arsenic oxide transport rate was lower than the film growth rate. Substantial arsenic oxide removal was observed independent of the TA process when the second deposition was completed at 250 °C (Figures 4 and 6). The stack that was subjected to the TA treatment saw a ~23% increase in the FTIR peak intensity compared to the stack without TA. This was lowered to ~8% when conversion effects from the TA step attained from Figure 3 were factored out. The 21



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fact that the transport caused by the thermal treatment had less than a 10% effect on the outcome seems to indicate that the preparatory heating during step 3 as well as the deposition thermal budget was adequate to provide a supply of arsenic oxide to the film surface until it was exhausted. XP spectra taken before and after a sputter step of the sample without thermal treatment (Figure 6a) confirmed the removal of most of the arsenic oxides. “Clean-up,” then, was limited by the reaction rate of removal. When the second deposition was performed at 200 °C removal was still observed regardless of whether the samples were subjected to the TA step (Figure 5). Comparison to the etched sample shows that in both cases less than half of the arsenic oxide was removed. A 7 nm deposition at 200 °C on the native oxide-covered surface showed nearly complete arsenic oxide removal (Figures 5 and 6); the longer deposition allowed the process just described to go to completion. Behavior at this temperature is similar to the behavior at 250 °C but the processes occur on a longer timescale. Regardless of the removal time scale, the fact that arsenic oxides were removed during the second 3 nm deposition at 150 to 250 °C leads to the inevitable conclusion that these oxides had to relocate from the interface to the top of the 4 nm film so as to react with the precursor and the ALD surface reaction byproducts. The difference in the removal time scale attests to the thermally activated nature of both the relocation and removal processes. These experiments provide unambiguous evidence about the existence of the native oxide transport mechanism that results in the gradual removal of these oxides during the ALD of TiO2 on GaAs surfaces. XPS data in Figure 7 gives direct evidence of native oxide transport. The As 3d spectra taken after sputtering a film heated for 12 hours clearly showed that both arsenic oxide states were redistributed near the film surface. All of the As5+ and most of the As3+ state were removed by the first sputtering step. A second similar sputtering step lowered the arsenic oxide concentration under the detection limit of XPS. We estimate that each sputtering step removed ~1 nm of film. Further 22


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confirmation is provided by the surface sensitive As 2p3/2 region. A signal corresponding to As-O bonding appeared in the spectra (Figure 7b) after TA for 12 hours and was removed by sputtering the top layers of the film. This proves that in the 12 hours of TA the arsenic oxides migrated through ~3 nm of TiO2. Transport of the gallium oxides was also observed but to a lesser extent. This is demonstrated in Figure 6c; XP spectra for the stack comprised of 4 nm of TiO2 at 100°C and 3 nm of TiO2 at 250°C show that during the second deposition the majority of the surface gallium oxides were removed. Again the only obvious explanation for this observation is that the gallium oxides transported through the 4 nm film to react with the precursor and/or the ALD reaction film during the second deposition. Sputtering of this film shows very little Ga2O remaining close to the interface. The 3 and 7 nm films deposited at 200°C confirm the gradual removal of the gallium oxides as the XP spectra show different amounts of gallium oxides present. Although gallium oxides moved through the TiO2 films this was done at a slower pace than the arsenic oxides. This observation is confirmed by the data presented in Figure 8. Thermal treatment led to the alteration of the oxide concentration and composition (Figure 8). This may have been the result of arsenic oxide to gallium oxide conversion and/or gallium oxide transport. After sputtering, the Ga3+ peak was completely removed but the majority of the Ga1+ remained intact. Even 12 hours of heating at 250 °C was incapable of transporting all of the Ga1+ state away from the GaAs/TiO2 interface. Although the mobility of the gallium oxides is much lower than the arsenic oxides, the Ga-O losses after sputtering and more importantly the results presented in Figures 6c and 6d showed that gallium oxides migrated as well. Enhanced gallium oxide removal was observed when higher deposition temperatures were used (Figure 6c) and continuous removal was confirmed at 200°C (Figure 6d).

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CONCLUSIONS In summary, the relocation of GaAs native oxides through a 4 nm TiO2 film deposited at 100 °C was confirmed using both indirect and direct evidence. Indirectly, removal of the arsenic and gallium oxides was observed during a subsequent 3 nm TiO2 deposition at 150-250 °C when a large enough thermal budget was applied before and/or during the deposition. These results revealed the oxide removal was transport-limited at 150 °C but reaction-limited at 250°C. Ex-situ XPS coupled with depth profiling provided direct confirmation for the relocation of both arsenic and gallium oxides. Arsenic oxide transport and removal is found to be more efficient than that of the gallium oxides. This work linked native oxide transport to the “clean-up” process to solve the riddle of the continuous native oxide removal observed in the literature for several alkyl amine-based ALD processes on GaAs.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under grant No. DMR-0846445 and ECCS-1407677. The authors would like to thank Dr. Karen Gaskell from the UMD Surface Analysis Center for taking the XPS data.

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For Table of Contents Only Native Oxide TiO2

Thermal Native Oxide Activation

Ar-ion Sputter

GaAs

As2O5 As2O3 48 47 46 45 44 43 48 47 46 45 44 43 48 47 46 45 44 43 Binding Energy (eV)

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Native Oxide Transport and Removal During Atomic Layer Deposition

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