Understanding Thermal Evolution and Monolayer Doping of Sulfur

Subscriber access provided by UNIV OF DURHAM

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Understanding Thermal Evolution and Monolayer Doping of Sulfur-Passivated GaAs(100) Eric C. Mattson, and Yves J. Chabal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01316 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on April 3, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 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

The Journal of Physical Chemistry

Understanding Thermal Evolution and Monolayer Doping of Sulfur-Passivated GaAs(100) E.C. Mattson1* and Y.J. Chabal1 1

Dept. of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX 75080 [email protected]

*

Abstract Monolayer doping (MLD) is an attractive method to precisely tailor dopant profiles for nanoelectronic semiconductor devices. The approach has been demonstrated for a number of different dopant/substrate combinations, but the mechanistic understanding of reactions of dopant-containing monolayers, intermediate structures, and the role of capping layers has suffered from lack of in situ characterization. Here, we investigate the thermal evolution Spassivated GaAs(100) without any additional capping layers in the context of MLD using a combination of in situ x-ray photoemission spectroscopy (XPS), low energy ion scattering (LEIS) and infrared (IR) absorption spectroscopy. In the case of (NH4)2S-passivated GaAs(100), the intermediate structure that precedes subsurface diffusion is characterized by a (2×1) reconstruction that has previously been ascribed to several different dimer moieties. These dimer structures are currently unresolved yet could influence subsequent doping processes. Using LEIS, temperature-dependent measurements provide unambiguous information on the chemical origin of the intermediate (2×1) reconstruction, originating from the dimerization of S atoms. Annealing beyond 813K results in a loss of the surface S signal and is accompanied by free-carrier absorption in IR spectra, consistent with doping. The magnitude of this absorption and the carrier densities extracted from these data indicate that peak doping levels exceeding 1020 cm-3 can be achieved, and that the free carrier concentration within a shallow profile can be tuned by adjusting the annealing time.

Introduction Continuous density scaling of electronic devices has underscored a growing need for highprecision semiconductor junction doping.1 In microelectronics, a heavily doped surface region is needed in the active semiconductor layer in transistors to circumvent short channel effects.2 Energy conversion and storage applications such as photovoltaic devices and photocatalysts also require abrupt dopant profiles for p-n junctions and charge separation, often in high aspect ratio structures. Conventional doping methods such as ion implantation and annealing, plasma doping, or dopant spin casting possess drawbacks such as the lack of accuracy and conformity, potential for inducing crystal damage, and inability to obtain and control depth of doping, which limit their effectiveness in producing heavily doped, ultra-shallow junctions. These limitations have driven exploration of new approaches to doping with greater inherent precision.

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

One method that has been recently developed, monolayer doping (MLD),3-5 involves grafting of the semiconductor surface with a dopant containing molecule followed by a high temperature annealing step to drive the dopants into the substrate. Usually, an oxide capping layer, typically Al2O3 or SiO2, is deposited on top of the grafted adlayer to prevent desorption of the dopant atoms. This approach has been successfully demonstrated for a number of n- and p-type dopants on Si3-4, 6-13 and Ge13-14 surfaces. Recently, the method has been extended to III-V semiconductors,15-17 which are becoming increasingly due to their appealing transport characteristics, particularly for applications in optoelectronics,18-19 high-speed nanoelectronics20 and solar energy harvesting.21-22 For III-Vs substrates, most studies have employed existing S-passivation protocols as a precursor to n-type doping processes. To achieve such passivation, the surface oxide is first chemically etched and the clean sample subsequently immersed in a supersaturated (NH4)2S solution to obtain sulfur termination of the surface. A capping layer is then deposited prior to a spike annealing step, which is later removed via acid etching. As for the case of analogous Si MLD processes, this method has been successfully demonstrated in several systems, including InP nanostructures, InAs and InGaAs.15-17, 23-24 However, there is no mechanistic understanding of the thermal evolution of the S atoms and nature of intermediate structures. Moreover, the need for capping layers is not well established. If MLD could be achieved without any capping layer, this would eliminate a major bottleneck for high-volume processing that incorporates both additional deposition steps as well as toxic chemical treatments, usually HF, needed for removal of the cap. Furthermore, the capping layer also has the effect of incorporation of C and O contaminants within the first few monolayers, which can be extremely detrimental to the electronic quality of shallow junctions.25 S-passivation chemistry and subsequent thermal evolution and surface reconstruction has been studied in some detail for III-V substrates, and the S/GaAs(100) surface in particular. It is now well-accepted that, when performed with an immediate water rinsing step,26 (NH4)2S passivation leads to ~1 monolayer (ML) S atom termination, where S atoms bind at Ga bridge sites along the [011] azimuth.27-30 Upon annealing to temperatures ≥ 573K, a (2×1) reconstruction is routinely observed in reflection high energy electron diffraction (RHEED)29 and low energy electron diffraction (LEED)31-32 patterns and scanning tunneling microscopy (STM)28 images. This reconstruction has received considerable attention in the literature, with the underlying atomic structure remaining unresolved. Yet, the nature of S-passivated surfaces is critically important for subsequent S incorporation into the substrate and for electronic devices whose properties are deteriorated by metallic surface states. These reports therefore highlight a number of pertinent questions that must be addressed to fully understand MLD of GaAs and related III-Vs using S-passivation chemistry as a starting point. In this work, we investigate the thermal evolution of S-passivated GaAs(100) surfaces as a model system for exploring the prospects of S-MLD of III-V substrates in the absence of capping layers. The introduction of S on the surface is achieved by standard treatment in ammonium


Page 2 of 22

Page 3 of 22 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


sulfide [(NH4)2S], but detailed characterization is performed in ultra-high vacuum (UHV) conditions so that precise atomic information can be obtained. This information is critical to uncover the nature of intermediate surface structures that have an influence on the transport of S atoms. The spectroscopic studies are performed in situ as a function of temperature on S/GaAs(100) to probe both the surface structural evolution and effectiveness of uncapped Sterminated surfaces as an approach to III-V MLD. Low energy ion scattering (LEIS), x-ray photoemission spectroscopy (XPS) and infrared (IR) absorption measurements offer a complementary, comprehensive picture of the surface structural and chemical evolution and free carrier response upon thermal treatment of S-passivated GaAs(100). Surface sensitive LEIS measurements are performed as a function of temperature to investigate the GaAs(100) (2×1)-S system. The results support a model of the (2×1) reconstruction in which the surface is terminated by S-S dimers, ruling out other proposed models involving Ga-S31 or As-S dimers,32 thus resolving a longstanding controversy surrounding the nature of the intermediate (2×1)-S phase. Continued annealing beyond of the (2×1) temperature window shows a loss of surface S atoms, at which point a Drude absorption band emerges in the IR absorption measurements. The magnitude of this absorption increases with increasing annealing time, suggesting S diffusion and activation. The diffusion of S into GaAs is confirmed by ToF-SIMS measurements that show a shallow, abrupt S profile. Quantification of the freecarrier absorption shows that the carrier density is of the order of 1020 cm-3, thereby demonstrating that heavy doping can be achieved without the use of capping layers.

Experimental Undoped n-type GaAs(100) wafers (International Institute of Electronic Materials Technology, Warsaw, Poland) were prepared using well-established sulfur passivation methods. Briefly, the samples were thoroughly rinsed with dichloromethane, acetone and methanol, and subsequently etched in a 3:1 solution of deionized water : HCl (37%) for 1 minute. The samples were rinsed with deionized water and immersed in a supersaturated solution of ammonium sulfide (0.27 g S added to 20 mL (NH4)2S, 30% aq) for 30 min at room temperature before being loaded into the UHV system. The measurements reported here were performed in a UHV cluster with combined chambers for IR absorption, LEIS and XPS measurements, and two chambers for in situ sample transfer. The base pressure of the cluster system was 573K), there is little change observed in any of the core levels apart from a slight increase in the intensities of the Ga2p3/2 and Ga3s levels. Again, this observation points to a termination characterized by predominantly Ga-S bonding, consistent with previous spectroscopic studies of the (2×1)-S reconstruction.26-27, 35 Annealing to 813K, however, results in a complete loss of the high binding energy shoulder of the Ga2p, which undergoes a substantial increase in its intensity. Similarly, the As3d and Ga3s levels intensities increase, while that of S2p decreases dramatically, remaining just above the detection limit.



Figure 1: Ga2p (A), As3d (B) and S2p (C) core-level photoemission spectra of Spassivated GaAs(100) surfaces as-prepared and following annealing to 673K and 813K. Likewise, further insight into the structural evolution of the surface during annealing is obtained from LEIS measurements. Unlike XPS, the surface peaks in the LEIS spectra are sensitive to only the outermost 2-3 atomic layers and can therefore offer a much more sensitive evaluation of the atomic composition of the surface,36 using He+ and Ne+ to probe light and heavier (Ga and As) elements respectively. To illustrate, Fig. 2A and B show the LEIS spectra of S/GaAs(100) as a function of temperature. The He+ spectra of the sample annealed to 473K shows that the surface is covered with S and O, while the corresponding Ne+ spectra show Ga and As peaks that are of comparable intensity. Upon annealing to 623K, the temperature in which a (2×1) reconstruction is expected to emerge, the Ne+ spectra in Fig. 2B show a preferential enhancement of the Ga peak, while the intensity of the S peak increases slightly and the As peak remains nearly unchanged. A decrease in the intensity of the O accompanies incr ease s in the S, Ga and As pea ks Table 1: Summary of spectroscopic techniques used, and conclusions derived from different upo temperature windows n


Page 6 of 22

Page 7 of 22 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


annealing to 768K, while the small peak that emerges at ≈1475 eV is due to Ne that becomes incorporated into the surface during the spectral acquisition. The O signal falls below the detection limit upon annealing to 788K, with little change in the S, Ga and As peaks. Upon annealing to 813K, which is slightly beyond the temperature at which the surface is reported to revert to a (1×1) termination,29 the S signal falls below the detection limit, while the integrated intensity of the As peak in the Ne+ spectrum increases by a factor of ≈3, indicating an enrichment in surface As. To gain quantitative insight into the relative sensitivity of a 5 keV Ne+ beam to Ga and As, we examined a GaAs wafer that was not subjected to any sulfur passivation chemistry, but rather loaded directly into the UHV system and cleaned with 8 keV Ar+ sputtering to remove the native oxide layer. The resulting Ne+ spectra are shown in Fig. 2C along with the Ne+ spectrum obtained following annealing of the S-passivated surface to 813K. In the case of the sputtered surface, while disordered, an ≈1:1 atomic ratio of Ga:As can be expected due to their very similar masses and therefore similar sputtering yields. Interestingly, the Ga : As area ratio from the sputtered surface is very similar to that of the S-passivated surface after annealing to 813K. Therefore, these data indicate that the composition of the surface following the 813K annealing step is characterized by a Ga:As ratio of ≈1:1, which is consistent with the surface reverting to a (1×1) termination. Therefore, the spectra collected in the 623-788K temperature window, corresponding to the expected window of the (2×1) reconstruction, indicate that the outermost surface region, comprised of the S adlayer and the first atomic layer of the substrate, is Ga-rich, As-deplete relative to the bulk termination. This Ga-rich, As-deplete surface composition is further supported by analysis of the XPS data in Fig. 1, summarized in Fig. 2D where the integrated Ga2p3/2:As3d area ratio is plotted as a function of temperature between 298-813K. Indeed, there is an enhancement in the Ga photoemission signature in the 573-673K temperature window, in reasonable agreement with that for which we observed a preferential enrichment(depletion) in the relative Ga(As) surface concentration (relative to a bulk termination) in the LEIS data (623-788K). As in the case of the LEIS data, the apparent enhancement of the Ga photoemission signal arises due to the preferential shadowing of As atoms near the surface. The small apparent discrepancy in the temperature at which the apparent Ga-enrichment/As-depletion is lost in Fig. 2B vs Fig. 2D falls within the uncertainty of the temperature measurement of the two experiments, as discussed in the experimental section. Having determined that the (2×1) reconstruction of S/GaAs(100) is As-deplete relative to the bulk-terminated surface composition and characterized by the presence of Ga-S bonds, we can now consider structural models put forward in the literature in light of the results presented above. The 3 leading models for the (2×1) reconstruction of the S-passivated GaAs(100) surface are characterized by the presence of dimers on top of a Ga-terminated surface, and the differences in the proposed models lie in the chemical structure in the dimers. The now accepted (1×1) S/GaAs(100) surface formed by (NH4)2S passivation followed by immediate water rinsing is shown Fig. 3A, with the three aforementioned models for the (2×1)



Figure 2: A) 3 keV He+ and B) 5 keV Ne+ LEIS spectra as a function of temperature. C) shows the comparison of the 813K Ne+ spectrum in B) with that of a GaAs sample without any Spassivation subjected to Ar+ ion sputtering to remove the native surface oxide. D) Shows the integrated Ga2p3/2 : As3d peak area ratio as a function of temperature extracted from the data shown in Figs. 1A and B.

reconstructed surface in Figs. 3B-D. As mentioned previously, the (1×1)-S surface (Fig. 3A) is terminated by ≈1 monolayer of S atoms that bridge the 2nd layer Ga atoms. In the model of the (2×1) surface proposed by Berkovits and Paget,33 as well as by Xia et al.27 (shown in Fig. 3B), the terminal S atoms dimerize, maintaining the same total number of surface S atoms as the (1×1) surface in Fig. 3A. Together with the 2nd layer Ga atoms, these dimers form a ring-like structure above half of the third layer As atoms, which would effectively produce a shadowing effect that prevent their detection in an LEIS measurement. The remaining half of the 3rd layer As atoms


Page 8 of 22

Page 9 of 22 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


are detectable by an incident ion beam with minimal shadowing, and therefore the intensity of the As peak would be expected to be surprised by a factor >2 relative to the bulk-terminated (1×1) surface. In the model in Fig. 3C, proposed by Wang and Weinberg,31 half of the S atoms have desorbed and the surface is terminated by Ga-S dimers, again forming rings that shadow the half of the 3rd layer As atoms. The model in Fig. 3D proposed by Ke et. al.32 is similar to the one in 3C but in this case contains As-S dimers rather than Ga-S dimers. Discussion and interpretation: Discrimination between these intermediate surface reconstructions has important implications on many aspects of engineering for both MLD processes and GaAs heterostructures in general. For example, in the case of structures in Figs. 3C-D, half of the initial S monolayer desorbs, thereby limiting the total number of dopants that can be incorporated into the shallow surface region. This would imply a certain degree of control over the final carrier concentration, as the use of a cap to prevent desorption would then lead to a final carrier concentration twice as large as the same process used without a cap. Understanding these intermediate structures is also of importance if performing additional surface reactions or modification is desired, for example to enable binary dopant profiles through stepwise reactions37 or perform subsequent deposition steps. Among the models for the (2×1) reconstruction considered in Fig. 3, it is possible to rule out two of the three in light the experimental data presented in Figs. 1-2. In the structure in Fig. 3D, although half of the third layer As atoms are shadowed by the rings formed by the As-S dimers and 2nd layer Ga atoms, the As atoms within the dimers would be readily detected by LEIS, and would not result in a decreased As binary collision peak relative to the bulk Gaterminated (1×1) surface. This is in contrast with our data, in which the spectra collected within the temperature window corresponding to the (2×1) phase showed that the As peak was decreased by a factor of 3 relative to its intensity in the (1×1) phase, and with our XPS data that showed a preferential Ga enrichment of the surface in the (2×1) temperature window. The model in Fig. 3D can therefore be definitively ruled out as the origin for the (2×1) reconstruction. The structure in Fig. 3C, in which Ga-S dimers and 2nd layer Ga atoms form a ring that shadows half of the 3rd layer As, would result in attenuation of the As peak in the (2×1) temperature window as seen in our data; this model also implies a loss of half of the S initially adsorbed. However, neither our XPS nor LEIS data show such a loss of S in the (2×1) window; in fact, the S2p:Ga2p3/2 ratio is unchanged to within the uncertainty of our measurement between the as-passivated and 673K spectra in Fig. 1. Similarly, no decrease is observed in the S LEIS signal between 473 and 768K. The model in Fig. 3C can therefore be ruled out. On the other hand, the model in Fig. 3B fully accounts for all aspects of the experimental data presented here: the S-S dimers again form a ring with the 2nd layer Ga that blocks half of the 3rd layer As atoms, thereby suppressing the As signal in XPS and LEIS, and no S is lost going from the (1×1)-S to (2×1)-S phases. The S-S dimer termination is also consistent with Ga-S binding through the S backbonds, consistent with the Ga-S component observed in the Ga2p3/2



Page 10 of 22

spectrum. Additionally, the slight increase in the Ga LEIS signal upon annealing to 813K is consistent with a partial shadowing of the 2nd layer Ga atoms by the S-dimers. With these data, we can conclude the S-S dimer-terminated structure in 3B is the most consistent with the experimental data for GaAs(100) (2×1)-S in the 623K-788K temperature window.

Figure 3: Top view structural models of the S-passivated GaAs(100) surface. A) (1×1)-S passivation formed following (NH4)2S etching followed by immediate water rinsing. B) (2×1)-S reconstruction terminated with S-S dimers, C) (2×1)-S reconstruction terminated with Ga-S dimers, and D) (2×1)-S reconstruction terminated by As-S dimers. Electrical Activation and Annealing-dependent Dopant Concentration The data presented above show that, when the surface reconstructs, the complete monolayer of S is preserved; however, they provide no information regarding the fate of the S following annealing to 813K and higher. Previous studies assumed that the S simply desorbs at


Page 11 of 22 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


the point at which the (1×1) periodicity is recovered,29, 35 but we show below that appreciable amounts of S actually diffuse within a shallow surface region to produce a doping effect. Results: While the electrical properties resulting from shallow doping are often characterized ex situ using conventional transport measurements, here we show that IR spectroscopy can be used as a non-invasive, in situ contactless probe of free carriers in an MLD process. Figure 4A shows the differential spectrum obtained upon annealing to 793K referenced to that obtained following annealing to 773K, with the inset showing the Ne+ LEIS spectrum collected immediately after the IR spectral acquisition. Note that here the data were collected from transmission measurements performed at Brewster’s incidence. The data clearly show a strong modulation of the broadband response, with the absorbance increasing rapidly with decreasing wavenumber. The functional form of the absorbance exhibit’s the well-known free carrier response described by the classical Drude model, indicating the incorporation of dopants when the surface reverts to the (1×1) phase. These data therefore prove that at least some fraction of the initial S monolayer penetrate the surface after annealing to 793K and higher and are electrically active. While measurements performed at Brewster’s incidence in general provide enhanced sensitivity due to greater light transmission, quantification of the free carrier response is more straightforward for measurements performed at normal incidence. We therefore performed additional measurements in a different UHV chamber that was modified to accommodate normal incidence measurements to gain a better quantitative handle on the relationships between annealing time, temperature, and carrier density. Fig. 4B shows IR absorption spectra collected from normal incidence transmission measurements of S-passivated GaAs surfaces upon annealing to various temperatures and durations. Interestingly, even after annealing to 823K, at which point the surface S signal falls below the detection limit in LEIS, the magnitude of the broadband absorption is relatively weak. However, when the sample is maintained at 823K for durations of 3, 5 and 7 min, the magnitude of the broadband absorption grows considerably. In order to understand how these changes relate to the spatial distribution of the S in the near surface region, we performed ex situ TOF-SIMS measurements on samples annealed to 793K for 1 min and 823K for 3 min (Fig. S1). Interestingly, the subsurface distribution of S for the two different annealing conditions is nearly identical to within the accuracy of the measurement. Therefore, the increase in the free carrier absorption that accompanies longer annealing times cannot be attributed to a change in the concentration or distribution of the S dopants. Rather, the more likely interpretation of the data is that the prolonged annealing time is instead needed for dopant activation.



Discussion and interpretation: The above observations suggest that, after diffusing into the surface, the S atoms may occupy disordered and interstitial sites, requiring thermal energy to overcome kinetic barriers for exchange into substitutional sites necessary to achieve electrical activation. Based on the S distributions from the TOF-SIMS data, we can determine that the dominant contribution to the free carrier response arises from the outermost 2-3 nm of the near surface region. With this information, we can fit the differential absorbance38 to a classical Drude model characterized by a 2-3 nm thick slab with a variable certain plasma frequency and relaxation time. Given the known value for the effective mass in doped GaAs,39-40 we can then extract from these data the carrier concentration following different annealing times. The differential absorbance arising from the 1 min, 823K annealing treatment is well fit to a 2.5 nm slab with a differential dielectric function whose plasma frequency corresponds a free carrier density of 3.6×1019 cm-3 (Fig. S2A). The spectrum produced following the 7 min annealing step at 823K is more challenging to fit, with the differential absorption deviating from that expected for a single Drude component dielectric function. Several effects could contribute to this discrepancy; first, the gradient in the distribution of activated carriers could become more pronounced as the dopant concentration is increased, increasing the error introduced by approximating the system as a slab with a homogeneous carrier distribution. Second, the true carrier relaxation time is in general not frequency independent as is the case in the classical Drude model. More generally, the relaxation time is treated as a frequency-dependent property that is dependent on the possible relaxation pathways of a given system. In doped GaAs, it has been shown that electron-electron scattering is the dominant relaxation pathway, and therefore the frequencydependence of the electron relaxation time could be expected to become more pronounced as the carrier density increases.41 With these considerations in mind, the best fit to the data is obtained using a model employing a 2.0 nm slab, yielding a carrier concentration of 2.1×1020 cm-3 (as shown in Fig. S2B). Of course, treating the system as a slab is a simplification and the values extracted from the fits should therefore be taken as a lower bound on the peak density of activated carriers, since the actual dopant density is found to decrease by a factor >4 within the first 2 nm from the surface, as observed in the TOF-SIMS profiles. Nevertheless, the data demonstrate that heavy, abrupt doping of GaAs(100) can be achieved for (NH4)2S-passivated surfaces without capping layers. Furthermore, the data in Fig. 4B also suggest a method to tune the free carrier concentration of shallow junctions by simply adjusting the annealing time at a given temperature. It is interesting to compare the results presented here with the dopant concentrations expected from the known S coverage of the S-passivated GaAs(100) surface. Xia et al27 reported a 1.1 monolayer coverage of S, corresponding to ≈6.9×1014 S atoms cm-2. Assuming that the majority of these atoms diffuse into a 2 nm slab, the expected dopant concentration is therefore ≈3.5×1021 cm-2. This is notably higher than the lower bound reported here (2.1×1020


Page 12 of 22

Page 13 of 22 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


cm-2), despite the fact that the peak dopant concentrations within this 2 nm volume are likely considerably higher (by a factor as large as 4) for the reasons discussed above, and that additional S atoms are present to varying extents throughout a 5-10 nm surface region. Two factors could potentially be responsible for this effect: first, it may be a case that only some of the S atoms actually diffuse within the film, and the remainder desorb. This would imply that a sufficient number of S atoms diffuse in to result in very high doping levels. The second possibility is that all or a majority of the S atoms diffuse in, but only a fraction are electrically active and the remainder do not provide any doping effect under the conditions reported here. Such an explanation is actually consistent with the work by D’Costa et al, in which S-MLD was reported on InGaAs films with a capping layer included.24 In their work, the dopant concentrations evaluated from IR spectroscopic ellipsometry performed ex situ after removing the oxide capping layer (800K, even without any capping overlayer as long believed necessary to prevent desorption of S atoms, and that S incorporation is limited to a ≈5 nm shallow surface region. IR absorption spectra reveal that prolonged annealing times are needed for electrical activation of the S atoms, thereby offering a route by which the dopant concentration can be tuned to concentrations exceeding 1020 cm-3. These results have important implications for a wide array of technical applications such as III-V transistors, quantum well devices, and solar energy harvesting.


Page 15 of 22 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


Supporting Information ToF-SIMS profiles and details regarding the fitting of the IR absorption spectra.

Acknowledgements This work was supported by the Semiconductor Research Corporation. The authors are grateful to Dr. Elaine Zhou for carrying out the ToF-SIMS measurements at the Analytical Instrumentation Facility at North Carolina State University.

References 1. Hoefflinger, B., Itrs: The International Technology Roadmap for Semiconductors. In Chips 2020: A Guide to the Future of Nanoelectronics, Hoefflinger, B., Ed. Springer Berlin Heidelberg: Berlin, Heidelberg, 2012; pp 161-174. 2. Frank, D. J.; Dennard, R. H.; Nowak, E.; Solomon, P. M.; Taur, Y.; Wong, H. S. P., Device Scaling Limits of Si Mosfets and their Application Dependencies. Proc. IEEE 2001, 89, 259-288. 3. Ho, J. C.; Yerushalmi, R.; Jacobson, Z. A.; Fan, Z.; Alley, R. L.; Javey, A., Controlled Nanoscale Doping of Semiconductors via Molecular Monolayers. Nat. Mater. 2008, 7, 62-67. 4. Ho, J. C.; Yerushalmi, R.; Smith, G.; Majhi, P.; Bennett, J.; Halim, J.; Faifer, V. N.; Javey, A., WaferScale, Sub-5 Nm Junction Formation by Monolayer Doping and Conventional Spike Annealing. Nano Lett. 2009, 9, 725-730. 5. Ye, L.; de Jong, M. P.; Kudernac, T.; van der Wiel, W. G.; Huskens, J., Doping of Semiconductors by Molecular Monolayers: Monolayer Formation, Dopant Diffusion and Applications. Mater. Sci. Semicond. Process. 2017, 62, 128-134. 6. O’Connell, J.; Collins, G.; McGlacken, G. P.; Duffy, R.; Holmes, J. D., Monolayer Doping of Si with Improved Oxidation Resistance. ACS Appl. Mater. Interfaces 2016, 8, 4101-4108. 7. Alphazan, T.; Florian, P.; Thieuleux, C., Ethoxy and Silsesquioxane Derivatives of Antimony as Dopant Precursors: Unravelling the Structure and Thermal Stability of Surface Species on SiO2. PCCP 2017, 19, 8595-8601. 8. Alphazan, T.; Mathey, L.; Schwarzwalder, M.; Lin, T. H.; Rossini, A. J.; Wischert, R.; Enyedi, V.; Fontaine, H.; Veillerot, M.; Lesage, A., et al., Monolayer Doping of Silicon through Grafting a Tailored Molecular Phosphorus Precursor onto Oxide-Passivated Silicon Surfaces. Chem. Mater. 2016, 28, 36343640. 9. Longo, R. C.; Mattson, E. C.; Vega, A.; Cabrera, W.; Cho, K.; Chabal, Y. J.; Thissen, P., Mechanism of Arsenic Monolayer Doping of Oxide-Free Si(111). Chem. Mater. 2016, 28, 1975-1979. 10. Longo, R. C.; Cho, K.; Schmidt, W. G.; Chabal, Y. J.; Thissen, P., Monolayer Doping Via Phosphonic Acid Grafting on Silicon: Microscopic Insight from Infrared Spectroscopy and Density Functional Theory Calculations. Adv. Funct. Mater. 2013, 23, 3471-3477. 11. Hazut, O.; Agarwala, A.; Amit, I.; Subramani, T.; Zaidiner, S.; Rosenwaks, Y.; Yerushalmi, R., Contact Doping of Silicon Wafers and Nanostructures with Phosphine Oxide Monolayers. ACS Nano 2012, 6, 10311-10318. 12. Hazut, O.; Huang, B. C.; Pantzer, A.; Amit, I.; Rosenwaks, Y.; Kohn, A.; Chang, C. S.; Chiu, Y. P.; Yerushalmis, R., Parallel P-N Junctions across Nanowires by One-Step Ex Situ Doping. ACS Nano 2014, 8, 8357-8362. 13. Taheri, P.; Fahad, H. M.; Tosun, M.; Hettick, M.; Kiriya, D.; Chen, K.; Javey, A., Nanoscale Junction Formation by Gas-Phase Monolayer Doping. ACS Appl. Mater. Interfaces 2017, 9, 20648-20655.



14. Alphazan, T.; Alvarez, A. D.; Martin, F.; Grampeix, H.; Enyedi, V.; Martinez, E.; Rochat, N.; Veillerot, M.; Dewitte, M.; Nys, J. P., et al., Shallow Heavily Doped N Plus Plus Germanium by OrganoAntimony Monolayer Doping. ACS Appl. Mater. Interfaces 2017, 9, 20179-20187. 15. O'Connell, J.; Napolitani, E.; Impellizzeri, G.; Glynn, C.; McGlacken, G. P.; O'Dwyer, C.; Duffy, R.; Holmes, J. D., Liquid-Phase Monolayer Doping of InGaAs with Si-, S-, and Sn-Containing Organic Molecular Layers. ACS Omega 2017, 2, 1750-1759. 16. Cho, K.; Ruebusch, D. J.; Lee, M. H.; Moon, J. H.; Ford, A. C.; Kapadia, R.; Takei, K.; Ergen, O.; Javey, A., Molecular Monolayers for Conformal, Nanoscale Doping of InP Nanopillar Photovoltaics. Appl. Phys. Lett. 2011, 98, 203101. 17. Ho, J. C.; Ford, A. C.; Chueh, Y. L.; Leu, P. W.; Ergen, O.; Takei, K.; Smith, G.; Majhi, P.; Bennett, J.; Javey, A., Nanoscale Doping of InAs via Sulfur Monolayers. Appl. Phys. Lett. 2009, 95, 072108. 18. Davis, R. F., III-V Nitrides for Electronic and Optoelectronic Applications. Proc. IEEE 1991, 79, 702-712. 19. Koch, T. L.; Koren, U., Semiconductor Photonic Integrated-Circuits. IEEE J. Quantum Electron. 1991, 27, 641-653. 20. Chelliah, C.; Swaminathan, R., Current Trends in Changing the Channel in Mosfets by III-V Semiconducting Nanostructures. Nanotech. Rev. 2017, 6, 613-623. 21. Goetzberger, A.; Hebling, C.; Schock, H. W., Photovoltaic Materials, History, Status and Outlook. Mater. Sci. Eng. R. 2003, 40, 1-46. 22. Yan, R. X.; Gargas, D.; Yang, P. D., Nanowire Photonics. Nat. Photonics 2009, 3, 569-576. 23. Subramanian, S.; Kong, E. Y. J.; Li, D. S.; Wicaksono, S.; Yoon, S. F.; Yeo, Y. C., P2S5/(NH4)(2)S-XBased Sulfur Monolayer Doping for Source/Drain Extensions in N-Channel InGaAs FETs. IEEE Trans. Electron Devices 2014, 61, 2767-2773. 24. D'Costa, V. R.; Subramanian, S.; Li, D. S.; Wicaksono, S.; Yoon, S. F.; Tok, E. S.; Yeo, Y. C., Infrared Spectroscopic Ellipsometry Study of Sulfur-Doped In0.53Ga0.47As Ultra-Shallow Junctions. Appl. Phys. Lett. 2014, 104, 232102. 25. Shimizu, Y.; Takamizawa, H.; Inoue, K.; Yano, F.; Nagai, Y.; Lamagna, L.; Mazzeo, G.; Perego, M.; Prati, E., Behavior of Phosphorous and Contaminants from Molecular Doping Combined with a Conventional Spike Annealing Method. Nanoscale 2014, 6, 706-710. 26. Lu, Z. H.; Graham, M. J.; Feng, X. H.; Yang, B. X., Structure of S on Passivated GaAs (100). Appl. Phys. Lett. 1993, 62, 2932-2934. 27. Xia, H.; Lennard, W. N.; Massoumi, G. R.; Vaneck, J. J. J.; Huang, L. J.; Lau, W. M.; Landheer, D., Absolute Coverage Measurements on Sulfur-Passivated GaAs(100). Surf. Sci. 1995, 324, 159-168. 28. Yokoi, N.; Andoh, H.; Takai, M., Surface-Structure of (NH4)2Sx-Treated GaAs (100) in an AtomicResolution. Appl. Phys. Lett. 1994, 64, 2578-2580. 29. Hirayama, H.; Matsumoto, Y.; Oigawa, H.; Nannichi, Y., Reflection High-Energy ElectronDiffraction and X-Ray Photoelectron Spectroscopic Study on (NH4)2Sx-Treated GaAs (100) Surfaces. Appl. Phys. Lett. 1989, 54, 2565-2567. 30. Lu, Z. H.; Graham, M. J., Determination of the Structure of GaAs(100)-S with Chemical-StateSpecific Photoelectron Diffraction. Phys. Rev. B 1993, 48, 4604-4607. 31. Wang, X. S.; Weinberg, W. H., Structural Model of Sulfur on GaAs(100). J. Appl. Phys. 1994, 75, 2715-2717. 32. Ke, Y.; Milano, S.; Wang, X. W.; Tao, N.; Darici, Y., Structural Studies of Sulfur-Passivated GaAs (100) Surfaces with Leed and Afm. Surf. Sci. 1998, 415, 29-36. 33. Berkovits, V. L.; Paget, D., Optical Study of Surface Dimers on Sulfur-Passivated (001)GaAs. Appl. Phys. Lett. 1992, 61, 1835-1837.


Page 16 of 22

Page 17 of 22 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


34. Wang, X. S.; Self, K. W.; Maboudian, R.; Huang, C.; Bresslerhill, V.; Weinberg, W. H., Analysis of GaAs(100) Surfaces Prepared with Various Wet and in-Situ Sample Treatments. J. Vac. Sci. Technol., A 1993, 11, 1089-1093. 35. Moriarty, P.; Murphy, B.; Roberts, L.; Cafolla, A. A.; Hughes, G.; Koenders, L.; Bailey, P., Photoelectron Core-Level Spectroscopy and Scanning-Tunneling-Microscopy Study of the Sulfur-Treated GaAs(100) Surface. Phys. Rev. B 1994, 50, 14237-14245. 36. Brongersma, H. H.; Draxler, M.; de Ridder, M.; Bauer, P., Surface Composition Analysis by LowEnergy Ion Scattering. Surf. Sci. Rep. 2007, 62, 63-109. 37. Thissen, P.; Cho, K.; Longo, R. C., Nanopatterning of Group V Elements for Tailoring the Electronic Properties of Semiconductors by Monolayer Doping. ACS Appl. Mater. Interfaces 2017, 9, 1922-1928. 38. Kuzmenko, A. B., Kramers-Kronig Constrained Variational Analysis of Optical Spectra. Rev. Sci. Instrum. 2005, 76, 083108. 39. Raymond, A.; Robert, J. L.; Bernard, C., Electron Effective Mass in Heavily Doped GaAs. J. Phys. C: Solid State Phys. 1979, 12, 2289-2293. 40. Blakemore, J. S., Semiconducting and Other Major Properties of Gallium-Arsenide. J. Appl. Phys. 1982, 53, R123-R181. 41. Kann, M. J.; Kriman, A. M.; Ferry, D. K., Effect of Electron-Electron Scattering on Intervalley Transition Rates of Photoexcited Carriers in Gaas. Phys. Rev. B 1990, 41, 12659-12665.



TOC Graphic


Page 18 of 22

Page 19 of 22 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


Figure 1: Ga2p (A), As3d (B) and S2p (C) core-level photoemission spectra of S-passivated GaAs(100) surfaces as-prepared and following annealing to 673K and 813K. 179x81mm (300 x 300 DPI)



Figure 2: A) 3 keV He+ and B) 5 keV Ne+ LEIS spectra as a function of temperature. C) shows the comparison of the 813K Ne+ spectrum in B) with that of a GaAs sample without any S-passivation subjected to Ar+ ion sputtering to remove the native surface oxide. D) Shows the integrated Ga2p3/2 : As3d peak area ratio as a function of temperature extracted from the data shown in Figs. 1A and C. 160x142mm (300 x 300 DPI)


Page 20 of 22

Page 21 of 22 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


Figure 3: Top view structural models of the S-passivated GaAs(100) surface. A) (1×1)-S passivation formed following (NH4)2S etching followed by immediate water rinsing. B) (2×1)-S reconstruction terminated with S-S dimers, C) (2×1)-S reconstruction terminated with Ga-S dimers, and D) (2×1)-S reconstruction terminated by As-S dimers. 245x186mm (300 x 300 DPI)



Figure 4: A) Differential IR absorption spectrum of GaAs(100)-S collected after annealing to 793K referenced to the measurement collected at 773K. The data are collected in the Brewster’s angle incidence geometry (≈74° with respect to the surface normal). The inset shows the Ne+ LEIS spectrum collected from the same surface immediately after the IR spectral acquisition. B) Differential IR absorption spectra of GaAs(100)-S collected in the normal incidence transmission geometry for various annealing times/temperatures. 118x153mm (300 x 300 DPI)


Page 22 of 22

Understanding Thermal Evolution and Monolayer Doping of Sulfur

Recommend Documents