Correlation of vapor pressure equation and film

Journal of Crystal Growth 248 (2003) 91–98

Correlation of vapor pressure equation and film properties with trimethylindium purity for the MOVPE grown III–V compounds Deo V. Shenai*, Michael L. Timmons, Ronald L. DiCarlo, Gregory K. Lemnah, Robert S. Stennick Shipley Metalorganics, Shipley Company L.L.C., 60 Willow Street, North Andover, MA 01845, USA

Abstract The purity of trimethylindium (TMI) has improved significantly during the past few years, as a result of improvements in its synthesis and purification. However, consistent high purity and batch-to-batch variation remain as primary concerns. In the present study, the impurity concentrations in commercial TMI samples were analyzed at part per billion levels using Fourier transform-nuclear magnetic resonance spectroscopy, inductively coupled plasma-optical emission spectrometry, and inductively coupled plasma-mass spectrometry techniques. The impurity profiles of TMI were compared with the electrical characterization of grown InP layers, e.g. electron mobility, carrier concentration, and glow discharge mass spectrometry analyses of grown layers in order to establish a correlation. The vapor pressure equation for TMI was also re-evaluated using (a) dynamic concentration measurements by Episont monitor and (b) the direct measurement of vapor pressures at various temperatures using a solid-state Baratront capacitance manometer. The resultant equations are reported along with a novel delivery system (UNI-FLOt cylinder) that provides consistent, reproducible delivery of TMI in the vapor phase. The influence of deleterious impurities on the optoelectronic properties is discussed along with the synthesis and purification strategies for the consistent manufacture of high-purity TMI. r 2002 Elsevier Science B.V. All rights reserved. PACS: 01.30.Cc Keywords: A3. Metalorganic vapor phase epitaxy; B1. Precursors; B1. Trimethylindium

1. Introduction During the past few years, MOVPE has become the preferred technology for the deposition of III– V semiconductor films and the fabrication of semiconducting devices such as laser diodes and *Corresponding author. Fax: 978-557-1719. E-mail address: [email protected] (D.V. Shenai).

light-emitting diodes (LEDs). With the rapid growth of the optoelectronic devices market along with MOVPE advancing into high volume production, more stringent criteria are becoming imperative with regard to the higher purity and improved consistency of the MOVPE precursors. Higher purity, in particular, has been the main driver for the development of new synthetic strategies and novel purification routes leading to ultrapure

0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 8 5 4 - 7

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D.V. Shenai et al. / Journal of Crystal Growth 248 (2003) 91–98

trimethylindium (TMI) for indium-containing semiconductors. Although the purity of TMI has improved extensively as a result of improvements in its synthesis and purification, achieving consistency in purity and reducing batch-to-batch variation still remain primary concerns [1]. To meet the customer demands of improved consistency, numerous developments have been successfully undertaken during the past few years, e.g. innovative oxygen-free synthetic routes, improved analytical techniques to attain greater sensitivities to impurity detection, and a novel cylinder design approach to provide reproducibility in evaporation rates of TMI. This study is mainly focused on recent improvements in the synthesis, purification, and impurity characterization of TMI, in conjunction with the correlation between InP film properties and the purity levels attained in TMI. We also present, for the first time, a more accurate vapor pressure equation for TMI, derived from (a) the direct vapor pressure measurements and (b) the dynamic concentration measurements using a novel cylinder design that affords reproducible and stable TMI concentrations in the MOVPE reactor.

2. Synthetic strategies and analytical techniques to ultrapure TMI Various methods for the synthesis of TMI are reported in the literature, which are essentially based on the alkylation of indium trihalide. The common alkylating agents employed in the commercial manufacture include the Grignard reagents [2] and organolithium reagents [3]. Since these routes involve diethyl ether, the TMI obtained is often contaminated with the oxygenated impurity—TMI-etherate adduct. Although this adduct is more volatile than TMI itself, its removal at trace levels using physical techniques is virtually impossible, e.g. the vapor pressures of TMI-etherate and TMI at 251C are 6.0 and 1.6 Torr, respectively. ‘‘Adduct purification’’ routes were therefore developed to remove ether from the metalorganic precursors [4–6]. However, TMI obtained from ether-based routes is often

contaminated with other oxygenated impurities such as dimethylindium methoxide, formed as a result of the reaction between TMI and oxygen from the ether solvent. The conventional adduct purification techniques are found to be practically ineffective for the removal of trace dimethylindium methoxide from TMI. In order to eliminate the oxygenated as well as the metallic impurities from TMI, novel synthesis and purification routes have been developed recently [7]. These ether-free routes are found to yield substantially low oxygen content TMI, and significantly reduced metallic impurities, thus eliminating the need for end-of-process steps such as adduct purification. In the present study, TMI lots were synthesized using an oxygen-free process [7] that involves the direct alkylation of indium trihalide by trimethylaluminum in an inert hydrocarbon solvent and subsequent in situ chemical purification. The impurities in TMI are generally quantified by using standard analytical techniques such as (a) Fourier transform nuclear magnetic resonance spectroscopy (FTNMR) for organic and oxygenated impurities, and (b) inductively coupled plasma-optical emission spectrometry (ICP-OES) and/or inductively coupled plasma-mass spectrometry (ICP-MS) for the metallic impurities. In the present study, the products were analyzed by 200and 500-MHz proton FTNMR (Bruker AC-200 and Bruker AMX-500 spectrometers) for organic and oxygenated impurities at the detection limit below 1 ppm as a result of recent developments, and by using a synergistic combination of SpectroMass 2000 ICP-MS and a SpectroFlame Modula ICP-OES for metallic impurities at ppb levels. The typical analyses of TMI lots, for 38 performance-critical impurities are shown in Table 1. Neither metallic nor organic/oxygenated impurities were detected in all TMI lots at subppm levels. These results thus confirm that as the synthetic techniques have advanced to afford impurities in the lower ppb range, analytical techniques currently used for MOVPE precursors are hard-pressed with a challenge in the differentiation, quantification and assessment of impurities at those lowest levels.

D.V. Shenai et al. / Journal of Crystal Growth 248 (2003) 91–98 Table 1 Trace impurities in trimethylindium lots by ICP-OES/ICP-MS Element

Results (ppm)

Element

Results (ppm)

Ag Al As Au B Ba Be Bi Ca Cd Cr Cu Fe Ga Ge Hg La Li Mg

o0.1 o0.5 o0.2 o0.2 o0.2 o0.3 o0.02 o0.3 o0.03 o0.1 o0.1 o0.05 o0.1 o0.5 o0.2 o0.5 o0.05 o0.2 o0.02

Mn Mo Nb Ni Pb Pd Pt Rh Sb Si Sn Sr Tb Te U V W Y Zn

o0.03 o0.1 o0.05 o0.3 o0.6 o0.2 o0.2 o0.1 o0.2 o0.1 o0.3 o0.03 o0.1 o0.2 o0.2 o0.2 o0.2 o0.05 o0.2

3. InP film growth and post-growth analysis In order to meet the growing customer expectations of quality consistency in TMI, the precursor manufacturers are strategically incorporating higher levels of product qualification. These raised analytical standards include (a) InP film growth, qualifying each batch of TMI, and subsequent electrical property measurements such as Hall data and photoluminescence, and (b) post-growth chemical analysis such as secondary ion mass spectrometry (SIMS) with the detection limits of the order of 1015 cm3 and/or glow discharge mass spectrometry (GDMS) with level of detection about 1013 cm3 [8]. The levels of extrinsic dopant impurities in the precursors as quantified by the routine analytical techniques can thus be correlated, in principle, to the electrical properties of the films grown using these precursors. Post-growth analysis by either SIMS or GDMS also provides further information regarding impurities and probable impact on the optoelectronic properties of grown devices. The advantage of GDMS is the low detection limit for most impurity species in III–V material, one to two orders of magnitude lower than standard time-of-flight SIMS [8].

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All InP films referenced in this study were grown by bandwidth semiconductor, L.L.C., USA, using a reactor with a vertical chamber that accommodates one wafer per growth. This reactor is unique in that it was constructed for source (Ga, Al, In, As, and P) qualification only and has never been used for dopants such as Zn or Mg. This feature helps to eliminate one of the uncertainties associated with undesirable doping that often plague studies of this type. The growth conditions were: growth temperature=6501C, pressure=76 Torr, and the V/III ratio=300. Semi-insulating InP substrates, prepared with solvent cleaning and a light etch, were used for all growths. The growth conditions were selected particularly to emphasize Si incorporation, the most deleterious impurity in TMI sources. A single phosphine source was used to grow most of the layers to reduce source-related variation. Substrates from several vendors were used, although this seemed to make little difference in the data. Hall-effect measurements to determine carrier concentration and mobility were performed using the standard van der Pauw geometry at both 300 and 77 K. The film growth results for various TMI lots are summarized in Table 2. The electrical properties of these films were found to be excellent, with InP mobilities (77 K) as high as 226,000 cm2/V s. As shown in Fig. 1, for over 150 kg of TMI manufactured, the average electron mobility at 77 K was found to be 173,700 cm2/V s and the average carrier concentration 2.0E14 cm3. Table 2 also shows the results obtained with the optimization of growth conditions (lower temperatures and greater V/III ratios) leading to mobilities as high as 287,000 cm2/V s. These excellent electrical properties were realized using representative production TMI sources without any additional purification. Shiva Technologies, Inc., USA, carried out the post-growth analysis by GDMS. Discharge conditions, experimental setup, and thickness of the InP layer were selected to allow the collection of multiple readings for each sample. The conversion of measured ion current/ion counts (instrumental output) to concentration was performed using relative sensitivity factors (RSFs). A blank InP sample was always run prior to analysis to


94 Table 2 InP film growth data Growth conditions Tg ¼ 6501C

VIII ratio ¼ 300

TMI lot number

Run no.

300 K Mobility (cm2/V s)

Carrier conc. (cm3)

77 K Mobility (cm2/V s)

Carrier conc. (cm3)

1 2 3 4 5 6 7 8 9 10 11

3935 3937 3965 3969 3971 3985 3987 3996 4019 4021 4023

5500 5600 5600 5250 4900 5450 5500 5250 5100 5300 4500

2E+14 1E+14 1E+14 8E+13 9E+13 2E+14 1E+14 2E+14 2E+14 3E+14 3E+14

149,000 204,000 203,000 226,000 225,000 200,000 203,000 172,000 163,000 158,000 165,000

3E+14 2E+14 2E+14 1E+14 1E+14 2E+14 2E+14 2E+14 3E+14 3E+14 2E+14

5268

2E+14

188,000

2E+14

Rg ¼ 3 mm/h 6E+13 1E+14 8E+13 1E+14 7E+13 8E+13

287,000 197,000 238,000 255,000 242,000 267,000

Layer=9 mm 8E+13 2E+14 1E+14 8E+13 1E+14 1E+14

Mean Tg ¼ 5802600 12 12 12 12 12 12

VIII ratio ¼ 4502800 3993 5400 4013 4500 4014 5900 4015 4600 4016 5200 4017 5150

Rg ¼ 3 mm/h

verify satisfactory operation of the analytical equipment. Fig. 2 shows the impurity concentrations in the grown layers as determined by GDMS and the electron mobilities at 77 K. The incorporations of Si, O and Zn were always found to be in the lower ppb range for all InP samples regardless of their mobilities. The results thus indicate only a weak correlation of electrical performance with the ppb levels of impurities accumulated in the grown layers. Based on the extensive film growth database and GDMS results, the criteria for product acceptance can be established, in principle, to guarantee the consistency of quality in TMI production. These criteria were identified as: (a) TMI lots must meet the purity specifications determined by the routine analytical chemistry, i.e. no impurities detectible by ICP and FTNMR at lower ppb levels, (b) a 77 K mobility of at least 140,000 cm2/V s, and (c) a 77 K carrier concentration o4E14 cm3. The

Layer ¼ 9 mm

adoption of these stringent criteria has resulted in significant improvements in the consistent performance of TMI in the growth of InP, as can be seen from Fig. 1.

4. Reproducibility in evaporation rate of TMI Solid TMI is the most preferred indium precursor because of its higher vapor pressure compared to other indium precursors. However, the variability in the evaporation rate for TMI has always been a major problem, especially when used in a conventional cylinder equipped with a dip tube. The inconsistency in the vapor phase concentration has been attributed to various factors, such as the reduction in total surface area during the depletion, the formation of ‘‘channels’’ or voids that limit the gas/solid contact time and subsequent saturation of vapors, undue sublimation of TMI to regions secluded from the carrier


95

Fig. 1. The consistency of film growth data for 150 kg of TMI.

Fig. 2. Correlation of electrical properties and impurity levels of GDMS.

gas flow, and the precursor surface contamination that renders TMI inaccessible for vapor transport. In order to address these problems, various improvements in TMI delivery have been tried with limited success. These improvements include

depositing TMI on a porous and inert support [9], reversing the direction of carrier gas flow in a cylinder [10], dual bubbler system [11], and also using TMI in a suspension form in a hydrocarbon [12] or a low volatility amine medium [13]. All

96


Fig. 3. Flow profile for TMI in UNI-FLOt cylinder.

these solutions were found to have inherent limitations, and hence the variability in evaporation rate has remained a cause for concern to the crystal growers. Recently, a novel dip tube-less cylinder design (UNI-FLOt1 cylinder) has been developed, which reduces the linear velocity of the carrier gas and thereby provides prolonged gas/solid contact time resulting in the delivery of more than 90% of solid TMI with exceptionally stable vapor phase concentrations [14]. In the present study, the evaporation rates were examined at reduced pressures and high flow rates using the UNI-FLOt cylinder design. An Episont III ultrasonic monitor was used to determine the TMI gas phase concentrations. An MKS model 640A pressure controller and MKS model 1179A mass flow controller monitored by an MKS model 247B readout provided the carrier gas at a constant pressure and flow rate. As shown in Fig. 3, stable delivery of TMI was accomplished at flow rates as high as 800 sccm and at reduced pressures of 260 Torr. These results 1

UNI-FLO is a trademark of Shipley Company, L.L.C.

indicate an excellent reproducible dosimetry and practical elimination of the variability in evaporation rates. The utilization of TMI was found be >90% with consistent concentration in the vapor phase, as shown earlier [14]. The novel UNIFLOt cylinder thus provides an elegant solution for the use of ultrapure TMI and other solid sources without jeopardizing their purities and concentrations in the MOVPE reactor. This feature is particularly important in light of the fact that alternative approaches based on dispersing TMI in organic solvents can conceivably incorporate impurities in the gas phase at ppb levels—which, in turn, can adversely affect device performance.

5. Vapor pressure equation for TMI The vapor pressure of a metalorganic precursor is a crucial parameter used to govern the precise concentrations of metalorganic precursors, and subsequently the rate of deposition in the MOVPE process. Hence, an accurate assessment of vapor pressure is vital to the MOVPE process. The


97

Fig. 4. Vapor pressures of TMI.

literature reported vapor pressure equation for TMI [15], i.e., log P (Torr)=10.523014/T (K), has been found to overestimate TMI vapor pressure by 20–40%, which has been a concern for crystal growers. Also reported in the literature is a second equation based on static pressure measurements, i.e. log P (mm Hg or Torr)= 9.862830/T (K) [16]. This equation was also found to overestimate the TMI vapor pressure at conditions employed in MOVPE. The discrepancy in vapor pressures can be attributed to the etherbased synthetic routes employed in the past, since the measured vapor pressures of TMI can be affected significantly by the presence of trace ether and TMI-etherate. The improved quality of TMI has thus created the need to establish an accurate vapor pressure equation. The vapor pressure of TMI was measured using an MKS model 627B Baratront capacitance manometer and MKS type 660 digital readout and power supply. Prior to any measurement being done, non-condensable gases were removed by using a standard freeze-thaw-degas technique. The degassed cylinder was then immersed in a constant temperature oven set at the desired temperature. The measurements were recorded

once the vapor pressure had attained equilibrium, within the temperature range 293–313 K. The results of direct vapor pressure measurements were found to be extremely close to those obtained under dynamic conditions using Episont, as shown in Fig. 4. The direct measurements led to the vapor pressure equation: log P (mm Hg or Torr)=11.093246/T (K), whereas the dynamic measurements provided the following equation, log P (mm Hg or Torr)=10.983204/T (K). The Antoine constants of these two equations are within 2%, indicating an excellent match between the dynamic and the static data.

6. Conclusions The purity of TMI has improved significantly as a result of the advancements in the synthetic strategies and the purification techniques. The impurities in TMI are reduced to low-ppb levels, which in turn, have challenged the current analytical techniques to quantify the impurities at these low levels. The improvements in analytical techniques have not yet matched the pace of purity enhancements. Precursor manufacturers

98


have proactively incorporated the film growth qualification and post-growth analysis. The present study conclusively shows that source purity has indeed attained the desired levels, and the consistency requirement is confirmed by routine film growth qualification. The reproducibility of the evaporation rate of solid sources using a UNI-FLOt cylinder confirms that stable vapor concentrations can indeed be achieved using novel cylinder designs and highpurity TMI in its unadulterated solid form. The vapor pressure equation for high-purity, ether-free TMI was determined. Finally, the optimization of growth conditions has led to electron mobilities as high as 287,000 cm2/V s in InP, when high-purity TMI obtained by oxygen-free synthesis was employed as the precursor.

References [1] M.L. Timmons, D.V. Shenai, A. Efimov, E.D. Gagnon, Proceedings of the Tenth Biennial Organometallic Vapor Phase Epitaxy Workshop, San Diego, 2001, p. 2. [2] F. Runge, Z. Anorg. Allg. Chem. 267 (1951) 39. [3] F.W. Reier, P. Wolfram, H. Schumann, J. Crystal Growth 93 (1988) 41.

[4] D.J. Cole-Hamilton, D.F. Foster, S.A. Rushworth, Brit. UK Pat. Appl., 1988, GB 2201418 A1 19880901. [5] A.H. Moore, M.D. Scott, J.J. Davies, D.C. Bradley, M.M. Faktor, H. Chudzynska, J. Crystal Growth 77 (1986) 19. [6] J.B. Mullin, D.J. Cole-Hamilton, E.D. Orrell, P.R. Jacobs, D.V. Shenai-Khatkhate, Brit. UK Pat. Appl., 1985, GB 8509055, and US Pat. 4,812,586, 1989. [7] M.B. Power, D.V. Shenai-Khatkhate, US Pat. 5,756,786, 1998. [8] A. Efimov, Proceedings of the 2000 International Conference on Indium Phosphide and Related Materials, Williamsburg, VA, 2000, p. 52. [9] K. Sanoyoshi, T. Yago, Japan Pat. JP 1-265,511, 1989. [10] G.R. Antell, UK Pat. GB 2,223,509, 1990. [11] N.D. Gerrard, L.M. Smith, A.C. Jones, J. Bosnell, J. Crystal Growth 121 (1992) 500. [12] L.W. Fannin, R.H. Pearce, D.W. Webb, J. Electron. Mater. 22 (1993) 93. [13] D.M. Frigo, W.W. van Berkel, W.A.H. Maassen, G.P.M. van Mier, J.H. Wilkie, A.W. Gal, J. Crystal Growth 124 (1992) 99. [14] M. Timmons, P. Rangarajan, R. Stennick, J. Crystal Growth 221 (2000) 635. [15] G.B. Stringfellow, Organometallic Vapor Phase Epitaxy: Theory and Practice, Second Edition, Academic Press, New York, 1999, p. 164. [16] O. Kayser, H. Heinecke, A. Brauers, H. Luth, P. Balk, Chemtronics 3 (1988) 90.