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H2-Diluted Precursors for Gaas Doping in Chemical Beam Epitaxy

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H2-Diluted Precursors for Gaas Doping in Chemical Beam Epitaxy H2-diluted precursors for GaAs doping in chemical beam epitaxy . arXiv:2102.13132v1 [cond-mat.mtrl-sci] 25 Feb 2021 K. Ben Saddik et al.: Preprint submitted to Elsevier Page 1 of 8 H2-diluted precursors for GaAs doping in chemical beam epitaxy K. Ben Saddik<, A. F. Braña, N. López, B. J. García<< andS. Fernández-Garrido << Electronics and Semiconductors Group (ElySe), Applied Physics Department, Universidad Autónoma de Madrid, ES-28049 Madrid, Spain ARTICLEINFO ABSTRACT Keywords: A wide range of n- and p-type doping levels in GaAs layers grown by chemical beam epitaxy is A1. Doping achieved using H2-diluted DTBSi and CBr4 as gas precursors for Si and C. We show that the dop- A3. Chemical beam epitaxy ing level can be varied by modifying either the concentration or the flux of the diluted precursor. B1. Inorganic compounds Specifically, we demonstrate carrier concentrations of 6 × 1017–1:2 × 1019 cm*3 for Si, and 9 × 1016– B2. Semiconducting III-V materials 3:7 × 1020 cm*3 for C, as determined by Hall effect measurements. The incorporation of Si and C B2. Semiconducting gallium arsenide as a function of the flux of the corresponding diluted precursor is found to follow, respectively, a first and a fourth order power law. The dependence of the electron and hole mobility values on the car- rier concentration as well as the analysis of the layers by low-temperature (12 K) photoluminescence spectroscopy indicate that the use of H2 for diluting DTBSi or CBr4 has no effect on the electrical and optical properties of GaAs. 1. Introduction the purity standards of the semiconductor industry [8,9]. In addition, at room temperature, DTBSi has a subatmospheric Solid-state opto- and electronic devices are typically ba- vapor pressure, which is high enough for the direct doping sed on junctions formed by the combination of semiconduct- of III-V compounds [9]. Surprisingly, even though the first ing layers with varying electrical conductivities, i. e., with study on the use of this precursor in metal-organic chemical different doping characteristics. The accurate control of the vapor epitaxy (MOVPE) dates from the late nineties [8], we dopant concentration and the possibility of tuning it over a are not aware of any report on the n-type doping of GaAs lay- wide range are both essential requirements for the fabrica- ers using DTBSi in CBE. With respect to C doping, which tion of certain types of devices, such as heterojunction bipo- has a solubility limit in GaAs of approximately 1×1019 cm*3 lar transistors and multi-junction solar cells. In the particu- [3,4], carbon tetrabromide CBr 4 is one of the common pre- lar case of III-As compound semiconductors, two of the most cursors in CBE as well as in gas- and solid-source molecular commonly used elements for n- and p-type doping are Si and beam epitaxy (MBE) [10, 11, 12, 13]. This precursor pro- C, respectively. These two elements are quite often preferred vides very high hole concentration in GaAs, on the order over other alternatives because of both their comparatively of 3 × 1019–1 × 1020 cm*3 [10, 14, 15]. Nevertheless, the low diffusion coefficients, which facilitate the fabrication of real challenge when using CBr4 is to obtain, not high, but junctions with abrupt doping profiles [1,2], and their high low doping levels. So far, two different approaches were re- solubility limits [3,4]. ported to achieve moderate C concentrations using this pre- In chemical beam epitaxy (CBE), where gas sources - cursor. The most extended one consists in decreasing the such as metalorganic compounds and hydrides - are used as CBr4 flux by lowering the temperature of the liquid reservoir, precursors [5], silane and disilane are the typical gases em- therefore, decreasing its vapor pressure [12]. The second ap- ployed for Si doping. The use of these precursors makes it proach is based on controlling the CBr flux by varying the possible to achieve electron concentrations in GaAs of up to 4 18 *3 conductance of the gas line in its way from the reservoir to 1–5×10 cm [6,7]. These values are close to the Si solu- the growth chamber [13]. These approaches yielded hole bility limit at the typical substrate temperatures used in CBE concentrations down to 1 × 1018 cm*3 [12, 13]. However, for the growth of this compound (500–600 ◦C), particulary, 18 *3 this value is still too high for the fabrication of certain de- about 4×10 cm [3,4]. Both silane and disilane are, how- vices, such as GaAs based solar cells, which demand doping ever, highly hazardous gases with very high vapor pressures concentrations on the order of 1×1017 cm*3 [16, 17]. Hence, at room temperature. Specifically, they are toxic as well as there is a clear need for the development of more suitable so- extremely flammable due to their pyrophoric character. In lutions. this context, ditertiarybutylsilane (DTBSi) is an interesting In this work, we analyze the possibility of controlling the alternative for Si doping since it is a non-toxic and non- Si and C doping levels in CBE-grown GaAs layers by using pyrophoric liquid that can also be properly refined to meet controlled dilutions of DTBSi or CBr4 in H2.H2 is thus not <Corresponding author intended to be used as a simple carrier gas using a bubbler, <<Principal corresponding authors but to control the actual flux of the dopant precursor. Such [email protected] (K. Ben Saddik); [email protected] (A.F. an approach is based on previous studies about the growth Braña); [email protected] (N. López); [email protected] (B.J. of GaAs layers doped with Sn [18] and on the use of H - García); [email protected] (S. Fernández-Garrido) 2 ORCID(s): 0000-0003-0060-5651 (K. Ben Saddik); diluted precursors in chemical vapor deposition (CVD) to 0000-0002-2125-8495 (A.F. Braña); 0000-0001-6510-1329 (N. López); decrease the risk associated to the use of hazardous gases as 0000-0002-6711-2352 (B.J. García); 0000-0002-1246-6073 (S. well as to improve the performance of amorphous-Si solar Fernández-Garrido) cells [19, 20]. The analysis of the doping concentration by K. Ben Saddik et al.: Preprint submitted to Elsevier Page 2 of 8 H2-diluted precursors for GaAs doping in chemical beam epitaxy Hall effect measurements as a function of both the concen- precursors to the substrate surface are given here as beam tration and the flux of the H2-diluted precursor demonstrates equivalent pressures (BEP). The BEP values are measured that, by using this approach, it is possible to modify the Si before the growth using an ion gauge, which is placed at the and C concentrations over at least two and three decades, re- same position as the substrate during growth. spectively. Importantly, we also find that the use of H2 to Hall effect measurements are performed at room tem- dilute the precursors has no detrimental effects on either the perature using In contacts and ù 1 cm2 van der Pauw pat- Hall mobility or the luminescence properties, as shown in terns. For the analysis of the Hall data, we take into ac- the latter case by low-temperature photoluminescence (PL) count the thickness of the surface depletion layer associated spectroscopy. to the Fermi level pinning at the GaAs surface. According to Ref. [22], we assume that for n-type GaAs the Fermi level is pinned at 0.68 eV below the conduction band minimum, and 2. Experiment for p-type 0.5 eV above the valence band maximum. The Epi-ready and semi-insulating GaAs(100) wafers, pur- analysis of the samples by photoluminescence (PL) spec- chased from Wafer Technology LTD, are used as substrates troscopy is carried out at 12 K using a closed-cycle He cryo- for the growth of Si- and C-doped GaAs layers. The carrier stat. The PL is excited with an Ar laser, dispersed by an 50- concentration of the wafers is 7:3×106–2:8×107 cm*3. For cm Spex monochromator, and detected by an a combined our experiments, the as received substrates are In-bonded Si/(In,Ga)As photodiode. The excitation power density is onto a Mo holder before being loaded into a Riber CBE32 about 1 kW/cm2. system. A detailed description of the CBE system can be found elsewhere [21]. In order to desorb the protective ox- 3. Results and discussion ide layer, the substrates are first outgassed at 580 ◦C, as mea- sured by an infrared optical pyrometer, in vacuum until ob- 3.1. GaAs doping using H2-diluted DTBSi serving the characteristic reflection high-energy electron di- The electron concentration, n, in GaAs layers doped with ffraction (RHEED) pattern of GaAs. Subsequently, the sub- H2-diluted DTBSi is investigated first as a function of the strates are further outgassed for about 5 min at 600 ◦C under DTBSi dilution factor, f, as defined by eq.(1). Figure1(a) the As precursor flux, which results in the observation of a shows the dependence of the Hall electron concentration on *6 faint (2 × 4) surface reconstruction. Finally, the substrate f for a H2-diluted DTBSi BEP value of 1.9×10 Torr. The 18 19 *3 temperature is decreased down to 530 ◦C for the growth of value of n increases from about 2×10 to 1.4×10 cm either Si- or C-doped GaAs. The growth rate of the GaAs as f is varied between 0.03 and 2 ~.
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