Dialkylzinc compounds ZnR2 (where R is an organic radical such as methyl, ethyl) are widely used as zinc precursors n the MOCVD/MOVPE practice.
The most commonly used are dimethylzinc and diethylzinc. Dialkylzinc compounds are unpolar colorless liquids, thermostable, reacting vigorously with air and water and soluble in organic solvents. Their thermal decomposition mechanism is α and β-Hydrogen elimination.
Despite their advantages as high vapor pressure and thermal stability, dialkylzinc compounds as CVD precursors have also some drawbacks. Thus, dialkylzinc compounds are pyrophoric (igniting spontaneously in air), thus presenting a serious fire hazard. Even minor amounts of oxygen in the CVD chamber cause formation of zinc oxide powder. Therefore air leaks into the CVD chamber may inhibit zinc oxide growth by CVD using dialkylzinc compounds as precursors.
Many Zn –containing films are deposited from dimethyl- or diethylzinc in combination with H2O, O2 , H2S, H2Se, or gaseous Te2.In addition, zinc alkyls have been used to introduce zinc as a p-type dopant into III-V semiconductors [[i], [ii], [iii], [iv], [v], [vi]]
The thermolyses of dimethylzinc and diethylzinc have been studied in order to elucidate their decomposition chemistry. [[vii]]
The comparison of dimethylzinc and diethylzinc as precursors for the growth of ZnO films by MOVPE was studied. ZnO films grown by low-pressure metal-organic chemical vapor deposition using either diethylzinc (DEZn) or dimethylzinc (DMZn) as zinc precursors exhibited the different growth mechanisms and properties. Typical three-dimensional islands covered ZnO film was grown using DMZn, while quasi-two-dimensional lateral growth features of large hexagonal grains and a smooth surface were present in the ZnO film grown with DEZn. High content of carbon and hydrogen was detected in the DMZn-grown films by Raman scattering measurements; these impurities caused consequently degradation of photoluminescence properties. are In contrast, DEZn-grown ZnO layers exhibited superior optical and structural properties, as well as a lower carbon contamination. [[viii]]
[i]Roth, A. P.; Williams, D. F. J. Electrochem. Soc. 1981, 128, 2684-2686.
[ii]Cockyne, B.; Wright, P. J. J. Cryst. Growth 1984,68, 223-230.
[iii]Strong R. L.: Smith, P. B. J. Vac. Sci. Technol. A 1990.8,1544-1548.
Yeh, M. Y.; Hu, C. C.; Lin, G. L.; Lee, M. K. Thin Solid Films 1992, 215, 142
[v] Chu, T. L.; Chu, S. S.; Firzst, F.; Herrington, C. J. Appl. Phys.
[vi]Fujita, S.; Matsuda, Y.; Sasaki, S. J. Cryst. Growth 1984, 68,231-236
[vii]Kuniya, Y.; Deguchi, Y.; Ichida, M. Appl. Organomet. Chem., 1991, 5, 337-347
[viii]Jiandong Ye, Shulin Gu, Shunming Zhu, Songmin Liu, Wei Liu, Xin Zhou, Liqun Hu, Rong Zhang, Yi Shi, Youdou Zheng, Journal of Crystal Growth, Vol 274, Issues 3–4, 1 Feb 2005, Pages 489–494, “Comparative study of diethylzinc and dimethylzinc for the growth of ZnO”
Dimethylzinc ZnMe2 , in combination with tBuOH as oxygen source, was used as precursor for the growth of ZnO films by MOCVD. [4]
ZnMe2 has been applied for the growth of ZnO films by low-pressure MOCVD, using O2 as oxygen source. The as-grown films were strongly (0002) oriented, film morphology displayed small chain-like network grains. Significant oxygen deficiency during growth was indicated by XPS and PL (a well-resolved shoulder emission at 3.03 eV due to interstitial zinc in the crystal). Post-annealing significantly improved the structure, morphology and optical properties; large hexagonal columnar grains were observed by AFM; photoluminescence efficiency was enhanced due to the reduction of the nonradiative defects. [282]
Dimethylzinc (DMZn), in combination with ditertiarybutyl sulphide (DTBS) tBu2S , has been applied as Zn precursor for the growth of ZnS by MOVPE (see Di-tertiarybutylsulphide tBu2S) [[i]]
[i] C Thiandoume , O Ka, A Lusson, O Gorochov, J. Cryst Growth, Volume 197, Issue 4, 1 March 1999, Pages 805–810)
ZnMe2 (+H2Se) for ZnSe MOVPE
Dimethylzinc, in combination with H2Se, was used for the growth of single crystalline layers of undoped ZnSe on GaAs substrates by atmospheric pressure MOVPE. Premature reactions typical for ZnMe2 + H2Se combination was eliminated completely (even at atmospheric pressure) by controlling the flow each source gas and the source gas molar ratio. Excellent mirror surface morphology (by Nomarski interference microscopy) was obtained even for 8.2 µm thick epilayers grown at 300°C. [283]
ZnMe2 (+Et2Se) for ZnSe MOCVD
Dimethylzinc ZnMe2 has been applied as precursor for the MOCVD growth of ZnSe films (with diethylselenide Et2Se as selenium source). Good quality ZnSe films were obtained on GaAs and glass substrates. ZnMe2 and Et2Se were mixed upstream, while H2Se was introduced downstream (in order to form a Me2Zn·SeEt2 adduct to prevent reaction between Me2Zn and H2Se at room temperature. The film, both single crystal films with smooth morphology, and polycrystalline films (only slightly strained with good adhesion) were obtained at growth rates about 1 µm/h. [284]
ZnMe2 , in combination with H2Se as co-reactant, has been applied for the growth of ZnSe layers by atomic layer epitaxial (ALE) growth in a low pressure horizontal metalorganic vapor phase epitaxial (MOVPE) reactor. Growth was carried out by alternately exposing the GaAs substrate to hydrogen selenide (H2Se) and dimethylzinc (DMZn) using flow modulation epitaxy (FME) approach. At a susceptor temperature of >200°C, the adsorption of either of the reactants is very small and hence self-limiting monolayer growth does not take place; significant deposition of ZnSe took place on the reactor wall in front of the susceptor as well. By using a hot wall reactor with colder susceptor configuration the deposition on the reactor walls was prevented, confirming the proposed growth model. [[i] ]
[i] Ishwara Bhat, Salman Akram, J. Cryst. Growth, Vol 138, Iss 1–4, 2 Apr 1994, p 127–130 “Atomic layer epitaxial growth studies of ZnSe using dimethylzinc and hydrogen selenide”
Dimethylzinc ZnMe2 has been applied for the growth of ZnxCd1 − xSe epilayers by MOCVD at low growth temperature (330 °C) and low-pressure (8 Torr). The dependebce of the crystalline quality of the deposited ZnCdSe layers on the mole fraction of dimethylzinc in supplied group II precursors was studied. The photoluminescence of the epilayers with the solid zinc composition (x>0.3) was investigated. [[i]]
[i] Hiroshi Fujimoto, Noboru Kitamura, Takao Wada, Nobuo Mikuriya,
Materials Letters, Volume 24, Issues 1–3, June 1995, Pages 65–67 , “Effect of the mole fraction of dimethylzinc on epilayers of the ZnCdSe system prepared by the low-temperature and low-pressure MOCVD method”
Dimethylzinc ZnMe2 has been applied as Zn precursor for the deposition of (100)-oriented ZnCdTe layers by atmospheric-pressure MOVPE. High crystal quality ZnCdTe layers were grown in a wide range of Zn compositions (x); the best FWHM for double-crystal XRD rocking-curves (<320 arc-sec) were obtained for x<0.3 and x>0.75. The growth rate of ZnCdTe layers was decreasing monotonically with increase of the DMZn supply ratio; the Zn composition of grown layers increased gradually up to x=0.04 with increase of the DMZn supply ratio from 0 to 0.8, however, further increase of the DMZn ratio led to the abrupt transition of Zn composition to ZnTe; such abrupt transition could be suppressed by increasing the VI/II ratio. The growth mechanism of ZnCdTe layers was explained by the differences of the growth characteristics of CdTe and ZnTe, namely a higher desorption rate from the growth surface of Zn vs. Cd and a higher rate of CdTe formation vs. ZnTe.. [[i]]
[i] K. Yasuda, K. Mori, Y. Kubota, K. Kojima, F. Inukai, Y. Asai and T. Nimura
J. Electronic Materials, 1998, Vol.27, Number 8, 948-953, DOI: 10.1007/s11664-998-0126-z “Growth characteristics of CdZnTe layers grown by metalorganic vapor phase epitaxy using dimethylzinc, dimethylcadmium, diethyltelluride, and dimethyltelluride as precursors”
Dimethylzinc has been applied as Zn precursor for the growth of ZnTe/HgTe superlattices on { }B CdTe, { }B GaAs and {100} GaAs substrates by MOVPE. It was found that the presence of the Zn precursor, dimethyl zinc (DMZn) seriously affected the growth of HgTe, requiring to ensure complete flushing of the reactor between ZnTe and HgTe phases of the growth cycle. SEM, EDX, RHEED, XTEM and IR transmission spectroscopy were used for ZnTe/HgTe superlattice characterisation. [[i] ]
[i] P.A. Clifton, J.T. Mullins, P.D. Brown, N. Lovergine, A.W. Brinkman, J. Woods
Journal of Crystal Growth, Volume 99, Issues 1–4, January 1990, Pages 468–472
Growth of HgTe-ZnTe strained layer superlattices by MOVPE
Dimethylzinc ZnMe2, in combination with phosphine PH3 as phosphorus source) was used for the growth of zinc phosphide Zn3P2 (as well as Zn and P) thin films by photoenhanced-CVD at substrate temperatures between RT and 250°C (using photodissociation of precursors with low‐pressure mercury lamp). The deposition rates of the films were strongy dependent on the substrate temperature, the PH3/ZnMe2 molar ratio, UV light intensity and density of gases. Zn3P2 microcrystallites were grown on Si(111) substrates at a 250°C. [285]
Dimethylzinc ZnMe2 is extensively used as p-dopant for the InP layers grown by MOVPE. In has been determined that large dopant deactivation effects occur when p-InP is cooled in ambients containing DMZn. This appears not to be the result of hydrogen passivation, rather it is believed to be caused by high concentrations of interstitial donors. [[i]]
[i]Cole, S.; Duncan, W.J.; Marsh, E.M.; Skevington, P.J.; Spiller, G.D.T.;
Electronics Letters, Issue Date: 15 March 1990, Vol 26 Issue:6, page(s): 391 – 392
Doping level anomalies in p-InP resulting from exposure to dopant precursors during cool-down in MOVPE
Dimethylzinc ZnMe2 has been applied as p-dopant for the growth of Zn-doped AlInAsSb and AlInAsSb/InGaAs MQW structures by MOVPE. The hole concentration of the AlInAsSb bulk layer increased from 2.0×1015 to 5.8×1017 cm−3 and the mobility decreased from 284 to 120 cm2/Vs when the flow rate of DMZn was increased from 20 to 200 sccm. [286]
Diethylzinc was mentioned to be useful for the deposition of crystalline zinc films by means of plasma-assisted CVD.[ll?]
Diethylzinc ZnEt2 , in combination with molecular oxygen or water, is the most commonly used zinc metal-organic precursor for the growth of ZnO thin films by CVD (including MOCVD, PECVD and ALD.[4]
Diethylzinc ZnEt2 is one the most common precursors for p-doping of (Al,Ga)In(As,P) layers grown by MOCVD.
ZnEt2 for Zn-doped InP CVD
Epitaxial layers of Zn-doped InP have been grown by low pressure MOCVD using diethylzinc (DEZn) as the p-dopant source [293]
Diethyl zinc ZnEt2, in combination with H2S as sulfur source, has been applied for the ZnS thin films by ALD at temperatures 200-350°C. The most important parameters were growth temperature, the ZnEt2 dosing and purge times, indicating that the limited stability of the film surface after the DEZn pulse strongly influenced the film growth. The ZnEt2-grown ZnS:Mn phosphor layer TFEL samples demonstrated efficient light emission and improved stability and symmetry with aging, as compared to traditional chloride-based TFEL samples.[[i]
[i]Gert Stuyven, Patrick De Visschere, Andriy Hikavyy, Kristiaan Neyts
Journal of Crystal Growth, Volume 234, Issue 4, February 2002, Pages 690–698 , “Atomic layer deposition of ZnS thin films based on diethyl zinc and hydrogen sulphide”
Higher zinc alkyls (di-iso-propyl- and di-tert-butylzinc) are unstable at room temperature (decompose at 0°C, should be stored at dry ice temperature) and are therefore less suitable for MOCVD applications [see f.e. Zinc: Organometallic Chemistry, JM Grévy - Encyclopedia of Inorganic Chemistry - Wiley Online Library]
Neveretherless , some attempts of their application as MOCVD precursors were reported.
Di-iso-propylzinc ZniPr2,has been reported to be applicable as zinc precursor for the growth of CdZnTe layers by MOVPE
Thus, diisopropylzinc ZniPr2, in combination with dimethylcadmium CdMe2 and diethyltelluride TeEt2 as co-precursors, has been applied for the growth of (100) Cd1 − xZnxTe (CZT) layers by MOVPE on (100) GaAs substrates at temperatures 375-450°C. As Since ZniPr2 has lower vapor pressure than CdMe2, good compositional controllability was achieved for the CZT layers having a Zn content <0.06, allowing to obtain layers with uniform Zn composition and thickness; the crystallinity of the grown layers was checked by XRD. These results demonstrate that ZnEt2 is suitable for growth of CZT layers lattice-matched with the HgCdTe layers. Introduction of a small amount of DiPZn under fixed flows of DMCd and DETe resulted in the enhancement of the CZT growth rate. The abrupt increase of Zn composition happened upon further increase of the DiPZn flow, combined with decrease of the growth rate. [[i] ,[ii]]
[i] K. Yasuda, M. Minamide, K. Kawamoto, T. Maejima
Journal of Crystal Growth, Volume 159, Issues 1–4, 2 February 1996, Pages 121–125 “MOVPE growth of (100) CdZnTe layers using DiPZn”
[ii]K. Yasuda, K. Kawamoto, T. Maejima, M. Minamide, K. Kawaguchi and H. Maeba
Journal of Electronic Materials, 1996, Volume 25, Number 8, 1362-1365, DOI: 10.1007/BF02655034, “Metalorganic vapor phase epitaxy of (100) CdZnTe layers using diisopropylzinc source”
Methyl-tertbutylzinc ZnMetBu
Methyl-isopropylzinc ZnMeiPr
Di-tert-butylzinc ZntBu2
Methyl-tertbutylzinc ZnMetBu methyl-isopropylzinc ZnMeiPr, di-tert-butylzinc ZntBu2 , along with other zinc alkyls, were applied as single source precursors for the growth of ZnS layers as a component of nanosized ZnS-Passivated CdS, as well as (CdS)ZnS, and CdS-modified ZnS particle Films via the MOCVD Process with Co-fed Single Source Precursors. Single source precursors of CdS and ZnS with sufficiently different reactivity, as judged from thermogravimetry analysis, were used, what resulted in the CdS core (grown from more reactive Cd precursor), and ZnS shell grown from less reactive Zn precursor. These films were characterized by the absorption spectrometry, PL spectroscopy, SEM, and powder XRD. The PL efficiency of the resulting ZnS-coated CdS composite particle film was significantly enhanced versus plain CdS film, due to the effective passivation of surface electronic states of CdS by ZnS, a material with a higher conduction band than that of CdS. [[i] ]
[i] Yung-Jung Hsu, Shih-Yuan Lu, Langmuir, 2004, 20 (1), pp 194–201,
DOI: 10.1021/la0347410