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DLTS and MCTS analysis of the influence of growth pressure on trap generation in MOCVD GaN
Solid-State Electronics 78 (2012) 121–126
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DLTS and MCTS analysis of the influence of growth pressure on trap generation in MOCVD GaN P.B. Shah ⇑, R.H. Dedhia, R.P. Tompkins, E.A. Viveiros, K.A. Jones US Army Research Laboratory, Sensors and Electron Devices Directorate, Adelphi, MD 20783, United States
a r t i c l e
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Article history: Available online 30 June 2012 Keywords: HEMT GaN DLTS Carbon MOCVD Trap Amphoteric
a b s t r a c t We demonstrate with DLTS and MCTS how changing growth pressure during MOCVD growth of GaN affects the majority and minority carrier trap signatures. Results indicate which specific traps are most strongly connected with the increased carbon concentration that lower growth pressure has led to. Carbon concentration is also verified with SIMS measurements. DLTS and MCTS measurements, related to carrier thermalization from traps, made up to a temperature of 450 K have found four majority carrier and two minority carrier traps. Using a lower growth pressures, at which greater incorporation of carbon is possible, has lead to larger concentrations of most of the traps; just as others have observed. However, the trap at EC 0.48 eV is the least affected by the growth pressure. We relate all the traps found from DLTS and MCTS to those identified by others and mention the constituent atoms that the research community identifies with these traps. Published by Elsevier Ltd.
1. Introduction During metal organic chemical vapor deposition (MOCVD) of gallium nitride (GaN) epi-layers for devices such as high electron mobility transistors (HEMTs), carbon is naturally introduced into the growth chamber by three most likely mechanisms: (1) it is part of the metal organic compounds, (2) it is a contaminant in the source gases or (3) it is etched off the susceptor that transfers heat to the substrate. The amount of carbon incorporation into the growing GaN epi-layers is determined in part by the growth pressure; the relation being lower growth pressure leads to greater incorporation of carbon into the GaN epi-layer as was observed over the range of MOCVD growth pressures from 65 Torr to 500 Torr [1]. However, one should note that carbon incorporation is also known to be influenced by the growth temperature [2]. Carbon is known to be amphoteric and valence considerations indicate that it can appear as a donor when substituting for gallium (CGa) and as an acceptor when substituting for nitrogen (CN). Carbon plays a beneficial role in making the buffer layer of the HEMT semi-insulating through its role as compensating donors, as is needed to reduce parasitic conduction through the buffer and
⇑ Corresponding author. Address: US Army Research Laboratory, Sensors and Electron Devices Directorate, RDRL-SER-E, Adelphi, MD 20783, United States. Tel.: +1 301 394 2809; fax: +1 301 394 1700. E-mail address: [email protected] (P.B. Shah). 0038-1101/$ - see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.sse.2012.05.057
ensure channel pinch off. However, increasing the concentration of carbon also increases the density of traps in the material and this has other detrimental effects [3,2]. One detrimental effect, current collapse, is a reduction of drain current due to electron trapping in the GaN buffer. This is found to occur in GaN HEMTs fabricated on semi-insulating layers [4]. Klein et al. identified two traps as being responsible for current collapse in MOCVD grown AlGaN/GaN HEMTs. Those two traps are located in the semi-insulating buffer layer and their data indicated that the deepest trap is a carbon related defect [5]. Devices fabricated on conductive buffer layers generally do not exhibit drain lag or current collapse due to either there being fewer traps in the buffer regions or with the filling of these traps by charge carriers provided by shallow donors [6]. In fact, the electrical conductivity of the substrate affects current collapse by exhibiting a field plating effect on the back side of the channel [7], and one can take advantage of this since it is known that hot electrons in AlGaN/GaN HEMTs may also enhance trapping into already present traps [8]. Drain induced current collapse is attributed to hot electron injection and trapping in the buffer layer [9,4]. In this process electrons gain energy from the high applied fields and can surmount a barrier to be subsequently trapped in the GaN buffer layer traps. Another detrimental result of traps in AlGaN/GaN HEMTs is the kink effect, which has been related to deep traps in the GaN buffer layer [10]. This effect result in HEMT output conductance increase, transconductance compression, and dispersion between DC and RF characteristics. III-Nitride materials exhibit piezoelectric and spontaneous polarization, and devices designed assuming a certain polarization
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charge may perform differently due to additional bound charges, introduced by trapped mobile charge carriers, that inadvertently add to or cancel out the charges present due to polarization effects [11,12]. These bound charges can introduce effects such as virtual gating of the HEMT. All of these mechanisms can affect AlGaN/GaN HEMT reliability. To quantify the bulk traps in GaN, deep level transient spectroscopy (DLTS) has emerged as a powerful tool providing information about the trap densities, their location in the energy gap, and the capture cross section which can indicate the charge state of the trap. This technique provides information on the majority carrier traps in GaN. A version of this to study minority carriers is called minority carrier transient spectroscopy (MCTS) [13–15]. During MCTS measurements, electrons are photogenerated by light exceeding the energy gap and leave the depletion region while the concurrently generated holes are captured by the minority carrier traps from which their emission by thermalization is analyzed. Unlike optical DLTS where below bandgap light is used to perturb occupancy of deep states, MCTS can also analyzes states that have no measurable optical cross sections. Analyzing minority carriers has a direct relation to understanding the role of carbon in GaN as it has been demonstrated that carbon incorporation does also lead to the formation of hole traps in GaN [16]. Furthermore, once an electron trap captures an electron it is then possible for it to capture a hole. In this paper we analyzed the traps formed in GaN epilayers grown on sapphire substrates at two different pressures to observe the types of traps that manifest as one varies pressure to control carbon incorporation. 2. Procedure Two unintentionally doped GaN samples MOCVD grown on sapphire at pressures of 300 and 500 Torr, provided by SUNY-Albany, were analyzed. The MOCVD growth took place in a Veeco D180 rotating disk reactor using trimethylgallium (TMGa) and ammonia (NH3) as gallium and nitrogen precursors, respectively, and purified hydrogen as carrier gas. The V/III ratio was 3525 implying a nitrogen rich growth environment. Also, the growth temperature was 1050 °C. The nitrogen rich growth environment is expected to lead according to density functional theory calculations to carbon incorporation in roughly equal numbers of CN and CGa and the Fermi level should be pinned near midgap. [17] Before depositing the thick, unintentionally-doped test epi-layers, a low temperature GaN nucleation layer was deposited followed by an additional 3 lm layer of highly doped n + GaN using silane as the dopant source. The majority carrier concentrations were determined by capacitance voltage (CV) measurements. For CV, DLTS and MCTS analysis, planar Schottky diodes were fabricated by first mesa isolating circular regions with an etch using an inductively coupled plasma reactive ion etch process with a BCl3/Cl2/Ar2 plasma chemistry. Then electrical contacts were deposited on top of the GaN epi-layer. For the ohmic contacts Ti/Al/Ni/Au was electron beam evaporation deposited on the higher doped GaN regions exposed by the etch, followed by rapid thermal annealing at 750 °C for 30 s in nitrogen gas. Then an additional layer of Ni/Au was deposited on top of this layer to improve adhesion of gold wire bonds. For the Schottky contact, Ni/Au was electron beam evaporation deposited on the top surface of the mesas. For both Schottky and ohmic contact deposition steps care was taken to assure good contact interfaces by oxide etching the GaN regions and also storing the sample immersed in methanol until the metal layers were deposited. The Schottky diodes were 600 lm in diameter. For the MCTS measurements the Ni/Au Schottky contact through which light could pass consisted of a mesh with 8 lm line widths separated by 40 lm spacings. Following fabrication, the samples were mounted on
alumina substrates using conductive epoxy and wire-bonded to rectangular gold pads roughly 8 mm on an edge; big enough to allow for the slight movement of the probe tips as the temperature is ramped up from 70 K to 450 K. Fig. 1 shows an image of one of the mounted samples. The rate of temperature ramping was carefully controlled during the measurements to ensure that the sample reached and remained at the proper temperature as each capacitance transient was recorded. A piece of each test wafer was also analyzed with secondary ion mass spectrometry (SIMS) at SUNY Albany to obtain compositional data. The SIMS results were averaged over a depth from 0.2 lm to 2.0 lm. For the DLTS, and MCTS measurements a system manufactured by Semitrol, LLC, was used. This system consisted of a Boonton 7200 capacitance meter, a Cryocon 32B temperature controller, and a CTI Cryogenics Refrigerator. DLTS Measurements were made over the temperature range from 75 K to 450 K. We limited the maximum temperature to 450 K to avoid deteriorating the conductive epoxy; however, this limits the observable energy level depth for traps in the energy bandgap. For the DLTS measurements made in darkness under vacuum, the Schottky diodes were reverse biased at 3 V and at each temperature the fill pulse was 1 V for 10 ms. For these measurements we verified that the filling pulse width saturated the traps. Following this fill pulse the capacitance exhibited a transient as the traps emitted captured electrons. At each temperature 200 capacitance transients are averaged. For each transient 800 points were sampled. During the measurement the entire capacitance transient is captured and saved at every temperature. Rate window analysis is then performed [18] on the data during which different start and end times, t1 and t2 (which define a rate window), are chosen and within each rate window a capacitance difference, DC, is obtained for every measurement temperature. We can assume that the capacitance transient follows the exponential time dependence:
CðtÞ ¼
nT ð0Þ t C 0 exp 2ND se
ð1Þ
where C is the capacitance across the Schottky diode, C0 is the steady state capacitance, t is time, nT(0) is the initial occupation of the trap, and se is the inverse of the emission rate of the trap, en. Then if one plots the difference in capacitance at two different times DC = C(t2) C(t1), for all temperature but keeping the rate window defined by t1 and t2 fixed, peaks related to specific traps would appear at certain temperatures. These peaks are related to the emission rate by:
Fig. 1. GaN on sapphire wafer mounted on alumina and wirebonded for probing under vacuum in the DLTS system.
P.B. Shah et al. / Solid-State Electronics 78 (2012) 121–126
se;max ¼
t2 t1 ln tt21
ð2Þ
We then plot many different rate window’s se,max and the corresponding temperature on a ln(seT2) vs. 1/kT grid. From the emission rate formula, en = rnvthNcexp(ET /kT) converted to the following form:
T2 ln en
!
T2 ¼ ln rmT Nc
! þ
ET kT
ð3Þ
where r is the capture cross section, vT is the thermal velocity, Nc is the density of states of the conduction band, and ET is the trap energy level, we see that the slope of the ln(seT2) vs. 1/kT plot gives ET and the intercept gives the capture cross section. Thus capture cross sections determined from DLTS can exhibit high uncertainties as the determination requires an extrapolation of the Arrhenius plot to infinite temperature. From the equation above we see that if the rate window is chosen such that all traps are filled at the start of the rate window and empty at the end of the rate window, we obtain the concentration of the trap from nT(0) = 2NDDC/C0 where DC is the maximum capacitance change possible. For the MCTS measurements the electrons are excited from the valence band to the conduction band using a 280 nm LED from Sensor Electronic Technology, Inc., that was biased to a 4 mA on current (at 8.5 V) with a pulse width of 20 ms. This LED was mounted in the vacuum chamber and the surface of the LED was roughly 6 mm above the mesh Schottky contact of the fabricated GaN Schottky diode. Throughout the measurement a 2 V reverse bias was applied across the Schottky diode. At each temperature the capacitance transient is measured and averaged 200 times as the trapped holes are thermally released. The DC/C plot for minority carriers has an opposite sign to that for majority carriers due to the opposite sign of the charge involved in the measurement. [13]
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decreasing profile, and that the rest of the low doped GaN region had a uniform silicon level not likely caused by diffusion from the high doped GaN region. Silicon concentration is mostly influenced by pressure and ammonia flow and to a lesser extent temperature and TMGa flow [2]. Typically reported sources of oxygen are the sapphire substrate or the nitrogen source gas. The silicon and oxygen present can act as shallow donors or form complexes with other defects. Fig. 2 presents the DLTS data for the MOCVD GaN sample grown at 300 Torr, and Fig. 3 presents the DLTS data for the MOCVD GaN sample grown at 500 Torr. The broad and overlapping nature of the peaks indicates a distribution of deep levels [19]. The characteristics of the traps are calculated from rate window analysis and the information is given in Table 1. In both samples four majority carrier trap peaks are present; however, we will only give details for the three clearest peaks in each case. The broadness of the TE2 peak is perhaps due to emission from a distribution of energy levels. This distribution of energy levels may be the result of PooleFrenkel emission whereby the ionization energy is enhanced by an electric field that lowers the emission barrier. Rate window analysis of this peak provides an energy and capture cross section value; however, a simulation using the rate window analysis results leads to a DLTS peak that is narrower than that measured. Fitting the peak with more trap energy levels doesn’t work well because of the wide distribution of emission rates rather than just two distinct values. Also, due to the breadth of the peaks, we could not obtain enough information about the TE3 trap for the 300 Torr grown sample, or about the TE1 trap for the 500 Torr grown sample.
3. Results and discussion The Schottky diodes exhibited well behaved CV and current– voltage characteristics. Current–voltage measurements over a reasonable sampling indicated in the 300 Torr sample the average barrier height was 0.941 eV and the average ideality factor was 1.072. In the 500 Torr sample, the average barrier height was 0.940 eV and the average ideality factor was 1.091. CV measurements indicated majority carrier concentrations of 8.6 1015 cm3 for the 300 Torr MOCVD GaN and 1.19 1016 cm3 for the 500 Torr MOCVD GaN. SIMS analysis of the GaN sample MOCVD grown at 300 Torr indicated concentrations of 1.1 1017 cm3 for carbon, 1.25 1016 cm3 for silicon and 2.36 1016 cm3 for oxygen. SIMS analysis of the 500 Torr sample indicated concentrations of 6.45 1016 cm3 for carbon, 1.69 1016 cm3 for silicon and 2.3 1016 cm3 for oxygen. Here we see that low pressure growth did lead to increased incorporation of carbon in the MOCVD GaN samples as well as a more semi-insulating behavior due to the compensating nature of the carbon present. The most likely sources of silicon in the unintentionally doped layer is the SiC coated graphite susceptor [2]. We do measure a slight increased silicon concentration in the higher pressure grown sample which may coincide with a greater reaction rate at higher pressure between the gasses in the chamber and the background of silicon on the growth chamber wall. However, the amount of liquid gallium in the chamber that can react with the reactor wall should be small because we have a nitrogen rich growth environment so any excess Ga that could react with the wall would instead react with NH3 to form GaCNH compounds [2]. SIMS data for the silicon in the low doped GaN region indicates that silicon atoms diffused only about 31% of the way into the low doped GaN region from the high doped GaN region with a logarithmically
Fig. 2. DLTS data for the GaN sample MOCVD grown at 300 Torr chamber pressure, with peaks at (a) 125 K and 220 K, and (b) 424 K. The co-plotted Arrhenius plots obtained from rate window analysis of the DLTS data are associated with the axes to the right and top.
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Fig. 3. DLTS data for the GaN sample MOCVD grown at 500 Torr chamber pressure. The co-plotted Arrhenius plots obtained from rate window analysis of the DLTS data are associated with the axes to the right and top.
Table 1 DLTS data for the two GaN samples. r is the capture cross section. Pressure (Torr)
r (cm2)
Ec Et (eV)
DLTS peak T(K)
300
2.8 1019 2.8 1017 1.3 1017 7.7 1017 4.7 1018 7.4 1018
0.13 0.37 0.68 0.36 0.48 0.66
125 220 424 220 328 425
500
TE1 TE2 TE4 TE2 TE3 TE4
The concentrations of the traps in the sample MOCVD grown at 300 Torr are 1.15 1014 cm3 (TE1), 2.32 1014 cm3 (TE2), and 4.47 1014 cm3 (TE4). The concentrations of the traps in the sample MOCVD grown at 500 Torr are 1.55 1014 cm3 (TE2), 2.26 1014 cm3 (TE3), and 3.33 1014 cm3 (TE4). The peak for trap TE3 increased the least as the MOCVD growth pressure changed from 500 to 300 Torr indicating that this trap may be least connected with carbon incorporation. The trap TE1 is similar to the electron trap that Armstrong et al. associate with CGa, having similar energy level and capture cross section as their EC 0.13 eV trap [16]. This compares to a reported GaN region trap with an activation energy of 0.1 ± 0.02 eV identified by transient channel current measurements on GaN/AlGaN/GaN HEMTs containing a GaN cap layer [20]. Trap TE2 has the same energy level as a trap that was related to surface defects by Look et al. [21] Our earlier statement about the broadness of this peak being related to the Poole-Frenkel effect complies with this since electrostatic field strength is highest at the metal-semiconductor interface and then decreases away from the contact into the semiconductor. In fact Ashraf et al, described with the Poole-Frenkel model a trap around this energy level [22]. Trap TE3 is similar to the EC 0.49 eV trap identified by Lee et al [23] who speculated that this trap level is related to either carbon or hydrogen impurities introduced from methyl radicals. Being that this trap increased the least in concentration going from a pressure of 500 Torr to 300 Torr makes it more likely related to hydrogen impurities. Trap TE4 is similar to the EC 0.67 level identified by Haase et al. who suggested that this could be related to nitrogen interstitials [24]. Overall, these DLTS measurements indicate that the concentration of all the traps are larger for GaN MOCVD grown at lower pressure which could also be due to the fact that at low pressure MOCVD growth is faster which allows less time for adsorbed species to transport to proper sites. Perhaps some of the traps also come about from the physical impact of incorporating carbon into the lattice of GaN. For instance there is a large difference between the atomic radii of C and Ga
atoms and this affects the crystal structure around CGa leading to impurity introduced energy levels. When CGa is present the bond length with one nearest neighbor N atom is reduced by about 18.1% and with the three remaining nearest neighbors by 16.6% compared to having a Ga atom in its place [25]. Fig. 4 presents the MCTS data for the sample MOCVD grown at 300 Torr, and Fig. 5. presents the MCTS data for the sample MOCVD grown at 500 Torr. From these data plots we observe that the minimum minority carrier trap densities are also larger in the lower pressure grown GaN wafer. For the 300 Torr sample, the minimum minority carrier trap densities are 2.06 1014 cm3 (TH1) and 2.92 1014 cm3 (TH2), and for the 500 Torr sample, the minimum minority carrier trap densities are 1.2 1014 cm3 (TH1), and 1.73 1014 cm3 (TH2). We mention that these are minimum values because in the MCTS measurement minority carrier holes generated by light absorbed outside but within a diffusion length of the depletion region are the desired types of holes for the emission transient being analyzed since the simultaneously generated majority carriers are excluded from the depletion region by the electric field. However, if minority carrier holes are generated by light absorption within the depletion region, the simultaneously generated electrons can be captured by deep levels and erroneously reduce the measured trap concentration. Tokuda et al. [15] encountered the same issue with their MCTS measurement. The characteristics of the traps are calculated from rate window analysis and the information is given in Table 2. There are three small amplitude peaks in both Figs. 4 and 5 between the two major peaks that are difficult to fit with rate window analysis due to the high background level. This background level could be from leakage current or measurement conditions such as noise from other electronics nearby in the lab. We can assume that the trap TH1 is the CN defect indicated by Armstrong et al. [26] Wang et al. have carried out effective mass calculations for the CN acceptor in GaN and obtained the value EV + 0.152 eV agreeing reasonably well with the value obtained here. The TH1 trap capture cross section of 1 1015 cm2 for holes indicates that in the 500 Torr sample this trap is attractive to holes and therefore may be negatively charged before capturing the hole. Therefore it appears to be an acceptor trap. Fischer et al., also identified a carbon related acceptor trap (CN) at this energy level using photoluminescence measurements [27]. On the other hand, the TH2 trap’s energy level is similar to that associated either with VGa, VGa–H or VGa–2H, [28,29] or with the complex CN–VGa [30]. This trap has also been described in the literature as being responsible for yellow luminescence band in GaN [31,30].
Fig. 4. MCTS data for the GaN sample MOCVD grown at 300 Torr. The co-plotted Arrhenius plots obtained from rate window analysis of the MCTS data are associated with the axes to the right and top.
P.B. Shah et al. / Solid-State Electronics 78 (2012) 121–126
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behavior we observe of lower trap densities with lower carbon incorporation [3]. 4. Conclusions
Fig. 5. MCTS data for the GaN sample MOCVD grown at 500 Torr. The co-plotted Arrhenius plots obtained from rate window analysis of the MCTS data are associated with the axes to the right and top.
Table 2 MCTS data for the two GaN samples. The data assumes that the energy gap is 3.4 eV. r is the capture cross section. Pressure (Torr) 300 500
TH1 TH2 TH1 TH2
r (cm2)
Ec Et (eV)
DLTS peak T(K)
3.4 1016 4.5 1017 2.0 1015 9.5 1017
3.20 2.69 3.19 2.65
123 412 119 420
If we do a direct comparison of the carbon related data we see an interesting correlation. SIMS data indicated that the 300 Torr sample had 70.5% greater carbon concentration than the 500 Torr sample. For a comparable calculation of the TE1 trap we can make a rough estimate of the location of the TE1 peak in Fig. 3, for the 500 Torr sample and obtain a concentration of 6.8 1013 cm3 for this trap. This indicates that the concentration of this trap which can be related to the CGa defect is 68.9 percent larger in the 300 Torr grown sample compared to that of the 500 Torr grown sample. This percentage increase is comparable to the percentage increase of carbon in the sample. On the other hand the concentration of trap TE2 increased by 49.7%, trap TE4 by 34.2 percent and trap TE3 decreased by 3.5%. Also, the concentration of the TH1 trap which could be related to the CN defect is 71.6% larger in the 300 Torr grown sample compared to that of the 500 Torr grown sample. Interestingly the TH2 trap also increased 68.8% so this supports the view that carbon may also form a deep trap at the energy level of TH2 [32,5]. Comparing our DLTS and MCTS results with other experiments related to MOCVD GaN growth at different pressures, Klein et al. investigated carbon incorporation in MOCVD GaN using four different growth pressures (65, 150, 200, and 300 Torr) and identified a carbon trap related signature at EC 3.07 eV [5]. This is close to the TH1 trap identified here. They related the EC 3.07 eV level to what Ogino et al. identified as a source of yellow luminescence at EV + 0.86 eV that could be due to a CN–VGa complex [30]. Armstrong et al. analyzed MOCVD GaN grown at 76 and 760 Torr and observed with DLTS two deep carbon related levels at EC 1.35 eV and EC 3.28 eV [26]. Their EC 3.28 eV level which they also identify as CN may also be related to the TH1 level we have identified. Fang et al. analyzed MOCVD AlGaN/GaN HEMTs grown at 100 Torr and 500 Torr with DLTS and observed electron traps at energy levels further away from the conduction band than those we have observed. They also observed a hole trap near the channel region of the HEMT. Overall, their results indicate the same general
We have investigated using DLTS and MCTS the trap signatures that result from changing the growth pressure for MOCVD grown GaN to influence the incorporation of carbon. Two samples grown at 300 Torr and 500 Torr with greater carbon concentration in the low pressure grown case, as identified by SIMS, were investigated. Four majority carrier traps and two minority carrier traps were identified by DLTS and MCTS respectively. We observe a general increase in all trap densities as the MOCVD growth pressure is decreased. Of the majority carrier traps we observe with DLTS, the trap at EC 0.13 eV appears directly related to carbon substituted for gallium. Other majority carrier traps appear due to either structural modification or are hydrogen related. Of the minority carrier traps observed with MCTS, the trap at EC 3.2 eV appears to be related to carbon substituted for N, and the trap at EC 2.69 appears to be related to complexes of this defect or gallium vacancies. Acknowledgments Thanks to Dr. Daniel Johnstone of Semetrol, LLC, for DLTS related advice. We also want to acknowledge that the materials were grown in Prof. Shahedipour’s lab at SUNY-Albany and SIMS data was provided by Dr. Steve Novak at SUNY-Albany. References [1] Wickenden AE, Koleske DD, Henry RL, Twigg ME, Fatemi M. J Cryst Growth 2004;260:54–62. [2] Koleske DD, Wickenden AE, Henry RL, Twigg ME. J Cryst Growth 2002;242:55–69. [3] Fang ZQ, Claflin B, Look DC, Green DS, Vetury R. J Appl Phys 2010;108:063706. [4] Binari SC, Klein PB, Kazior TE. Proc IEEE 2002;90:1048–58. [5] Klein PB, Binari SC, Ikossi K, Wickenden AE, Koleske DD, Henry RL. Appl Phys Lett 2001;79:3527–9. [6] Binari SC, Ikossi K, Roussons JA, Kruppa W, Park D, Dietrich HB, et al. IEEE Trans Electron Dev 2001;48:465–71. [7] Ikeda N, Kaya S, Li J, Sato Y, Kato S, Yoshida S. In: 20th international symposium on power semiconductor devices and IC’s, ISPSD ‘0; 2008. pp. 287–90. [8] Meneghesso G, Verzellesi G, Danesin F, Rampazzon F, Zanon F, Tazzoli A, et al. IEEE Trans Dev Mater Reliab 2008;8:332–43. [9] Saito W, Omura I, Tsuda K. CS Mantech conference; 2007. pp. 209–12. [10] Meneghesso G, Rossi F, Salviati G, Uren MJ, Munoz E, Zanoni E. Appl Phys Lett 2010;96:263512. [11] Ambacher O, Smart J, Shealy JR, Weimann NG, Chu K, Murphy M, et al. J Appl Phys 1999;85:3222–33. [12] Meneghesso G, Verzellesi G, Pierobon R, Rampazzo F, Chini A, Mishra UK, et al. IEEE Trans Electron Dev 2004;51:1554–61. [13] Muret P, Phillipe A, Monroy E, Munoz E, Beaumont B, Omnes F, et al. J Appl Phys 2002;91:2998–3001. [14] Auret FD, Meyer WE, Wu L, Hayes M, Legodi MJ, Beaumont B, et al. Phys Status Solidi (a) 2004;201:2271–6. [15] Tokuda Y, Yamada Y, Shibata T, Yamaguchi S, Ueda H, Uesugi T, et al. Phys Status Solidi (c) 2011;8:2239–41. [16] Armstrong A, Arehart AR, Green D, Mishra UK, Speck JS, Ringel SA. J Appl Phys 2005;98:053704. [17] Seager CH, Wright AF, Yu J, Gotz W. J Appl Phys 2002;92:6553–9. [18] Schroder DK. Semiconductor material and device characterization, second edition. New York: Wiley Interscience; 1998. [19] Kwon D, Kaplar RJ, Ringel SA, Allerman AA, Kurtz SR, Jones ED. Appl Phys Lett 1999;74:2830–2. [20] Mitrofanov O, Manfra M. Appl Phys Lett 2004;84:422–4. [21] Look DC, Fang ZQ. Appl Phys Lett 2001;79:84–6. [22] Ashraf H, Imran Arshad M, Faraz SM, Wahab Q, Hageman PR, Asghar M. J Appl Phys 2010;108:103708-1–8-5. [23] Lee WI, Huang TC, Guo JD, Feng MS. Appl Phys Lett 1995;67:1721–3. [24] Haase D, Schmid M, Kurner W, Dornen A, Harle V, et al. Appl Phys Lett 1996;69:2525–7. [25] Boguslawski P, Briggs EL, Bernholc J. Appl Phys Lett 1996;69:233–5. [26] Armstrong A, Arehart AR, Moran B, Denbaars SP, Mishra UK, Speck JS, et al. Appl Phys Lett 2004;84:374–6. [27] Fischer S, Wetzel C, Haller EE, Meyer BK. Appl Phys Lett 1995;67:1298–300. [28] Hierro A, Ringel SA, Hansen M, Speck JS, Mishra UK, DenBaars SP. Appl Phys Lett 2000;77:1499–501.
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DLTS and MCTS analysis of the influence of growth pressure on trap generation in MOCVD GaN P.B. Shah ⇑, R.H. Dedhia, R.P. Tompkins, E.A. Viveiros, K.A. Jones US Army Research Laboratory, Sensors and Electron Devices Directorate, Adelphi, MD 20783, United States
a r t i c l e
i n f o
Article history: Available online 30 June 2012 Keywords: HEMT GaN DLTS Carbon MOCVD Trap Amphoteric
a b s t r a c t We demonstrate with DLTS and MCTS how changing growth pressure during MOCVD growth of GaN affects the majority and minority carrier trap signatures. Results indicate which specific traps are most strongly connected with the increased carbon concentration that lower growth pressure has led to. Carbon concentration is also verified with SIMS measurements. DLTS and MCTS measurements, related to carrier thermalization from traps, made up to a temperature of 450 K have found four majority carrier and two minority carrier traps. Using a lower growth pressures, at which greater incorporation of carbon is possible, has lead to larger concentrations of most of the traps; just as others have observed. However, the trap at EC 0.48 eV is the least affected by the growth pressure. We relate all the traps found from DLTS and MCTS to those identified by others and mention the constituent atoms that the research community identifies with these traps. Published by Elsevier Ltd.
1. Introduction During metal organic chemical vapor deposition (MOCVD) of gallium nitride (GaN) epi-layers for devices such as high electron mobility transistors (HEMTs), carbon is naturally introduced into the growth chamber by three most likely mechanisms: (1) it is part of the metal organic compounds, (2) it is a contaminant in the source gases or (3) it is etched off the susceptor that transfers heat to the substrate. The amount of carbon incorporation into the growing GaN epi-layers is determined in part by the growth pressure; the relation being lower growth pressure leads to greater incorporation of carbon into the GaN epi-layer as was observed over the range of MOCVD growth pressures from 65 Torr to 500 Torr [1]. However, one should note that carbon incorporation is also known to be influenced by the growth temperature [2]. Carbon is known to be amphoteric and valence considerations indicate that it can appear as a donor when substituting for gallium (CGa) and as an acceptor when substituting for nitrogen (CN). Carbon plays a beneficial role in making the buffer layer of the HEMT semi-insulating through its role as compensating donors, as is needed to reduce parasitic conduction through the buffer and
⇑ Corresponding author. Address: US Army Research Laboratory, Sensors and Electron Devices Directorate, RDRL-SER-E, Adelphi, MD 20783, United States. Tel.: +1 301 394 2809; fax: +1 301 394 1700. E-mail address: [email protected] (P.B. Shah). 0038-1101/$ - see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.sse.2012.05.057
ensure channel pinch off. However, increasing the concentration of carbon also increases the density of traps in the material and this has other detrimental effects [3,2]. One detrimental effect, current collapse, is a reduction of drain current due to electron trapping in the GaN buffer. This is found to occur in GaN HEMTs fabricated on semi-insulating layers [4]. Klein et al. identified two traps as being responsible for current collapse in MOCVD grown AlGaN/GaN HEMTs. Those two traps are located in the semi-insulating buffer layer and their data indicated that the deepest trap is a carbon related defect [5]. Devices fabricated on conductive buffer layers generally do not exhibit drain lag or current collapse due to either there being fewer traps in the buffer regions or with the filling of these traps by charge carriers provided by shallow donors [6]. In fact, the electrical conductivity of the substrate affects current collapse by exhibiting a field plating effect on the back side of the channel [7], and one can take advantage of this since it is known that hot electrons in AlGaN/GaN HEMTs may also enhance trapping into already present traps [8]. Drain induced current collapse is attributed to hot electron injection and trapping in the buffer layer [9,4]. In this process electrons gain energy from the high applied fields and can surmount a barrier to be subsequently trapped in the GaN buffer layer traps. Another detrimental result of traps in AlGaN/GaN HEMTs is the kink effect, which has been related to deep traps in the GaN buffer layer [10]. This effect result in HEMT output conductance increase, transconductance compression, and dispersion between DC and RF characteristics. III-Nitride materials exhibit piezoelectric and spontaneous polarization, and devices designed assuming a certain polarization
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charge may perform differently due to additional bound charges, introduced by trapped mobile charge carriers, that inadvertently add to or cancel out the charges present due to polarization effects [11,12]. These bound charges can introduce effects such as virtual gating of the HEMT. All of these mechanisms can affect AlGaN/GaN HEMT reliability. To quantify the bulk traps in GaN, deep level transient spectroscopy (DLTS) has emerged as a powerful tool providing information about the trap densities, their location in the energy gap, and the capture cross section which can indicate the charge state of the trap. This technique provides information on the majority carrier traps in GaN. A version of this to study minority carriers is called minority carrier transient spectroscopy (MCTS) [13–15]. During MCTS measurements, electrons are photogenerated by light exceeding the energy gap and leave the depletion region while the concurrently generated holes are captured by the minority carrier traps from which their emission by thermalization is analyzed. Unlike optical DLTS where below bandgap light is used to perturb occupancy of deep states, MCTS can also analyzes states that have no measurable optical cross sections. Analyzing minority carriers has a direct relation to understanding the role of carbon in GaN as it has been demonstrated that carbon incorporation does also lead to the formation of hole traps in GaN [16]. Furthermore, once an electron trap captures an electron it is then possible for it to capture a hole. In this paper we analyzed the traps formed in GaN epilayers grown on sapphire substrates at two different pressures to observe the types of traps that manifest as one varies pressure to control carbon incorporation. 2. Procedure Two unintentionally doped GaN samples MOCVD grown on sapphire at pressures of 300 and 500 Torr, provided by SUNY-Albany, were analyzed. The MOCVD growth took place in a Veeco D180 rotating disk reactor using trimethylgallium (TMGa) and ammonia (NH3) as gallium and nitrogen precursors, respectively, and purified hydrogen as carrier gas. The V/III ratio was 3525 implying a nitrogen rich growth environment. Also, the growth temperature was 1050 °C. The nitrogen rich growth environment is expected to lead according to density functional theory calculations to carbon incorporation in roughly equal numbers of CN and CGa and the Fermi level should be pinned near midgap. [17] Before depositing the thick, unintentionally-doped test epi-layers, a low temperature GaN nucleation layer was deposited followed by an additional 3 lm layer of highly doped n + GaN using silane as the dopant source. The majority carrier concentrations were determined by capacitance voltage (CV) measurements. For CV, DLTS and MCTS analysis, planar Schottky diodes were fabricated by first mesa isolating circular regions with an etch using an inductively coupled plasma reactive ion etch process with a BCl3/Cl2/Ar2 plasma chemistry. Then electrical contacts were deposited on top of the GaN epi-layer. For the ohmic contacts Ti/Al/Ni/Au was electron beam evaporation deposited on the higher doped GaN regions exposed by the etch, followed by rapid thermal annealing at 750 °C for 30 s in nitrogen gas. Then an additional layer of Ni/Au was deposited on top of this layer to improve adhesion of gold wire bonds. For the Schottky contact, Ni/Au was electron beam evaporation deposited on the top surface of the mesas. For both Schottky and ohmic contact deposition steps care was taken to assure good contact interfaces by oxide etching the GaN regions and also storing the sample immersed in methanol until the metal layers were deposited. The Schottky diodes were 600 lm in diameter. For the MCTS measurements the Ni/Au Schottky contact through which light could pass consisted of a mesh with 8 lm line widths separated by 40 lm spacings. Following fabrication, the samples were mounted on
alumina substrates using conductive epoxy and wire-bonded to rectangular gold pads roughly 8 mm on an edge; big enough to allow for the slight movement of the probe tips as the temperature is ramped up from 70 K to 450 K. Fig. 1 shows an image of one of the mounted samples. The rate of temperature ramping was carefully controlled during the measurements to ensure that the sample reached and remained at the proper temperature as each capacitance transient was recorded. A piece of each test wafer was also analyzed with secondary ion mass spectrometry (SIMS) at SUNY Albany to obtain compositional data. The SIMS results were averaged over a depth from 0.2 lm to 2.0 lm. For the DLTS, and MCTS measurements a system manufactured by Semitrol, LLC, was used. This system consisted of a Boonton 7200 capacitance meter, a Cryocon 32B temperature controller, and a CTI Cryogenics Refrigerator. DLTS Measurements were made over the temperature range from 75 K to 450 K. We limited the maximum temperature to 450 K to avoid deteriorating the conductive epoxy; however, this limits the observable energy level depth for traps in the energy bandgap. For the DLTS measurements made in darkness under vacuum, the Schottky diodes were reverse biased at 3 V and at each temperature the fill pulse was 1 V for 10 ms. For these measurements we verified that the filling pulse width saturated the traps. Following this fill pulse the capacitance exhibited a transient as the traps emitted captured electrons. At each temperature 200 capacitance transients are averaged. For each transient 800 points were sampled. During the measurement the entire capacitance transient is captured and saved at every temperature. Rate window analysis is then performed [18] on the data during which different start and end times, t1 and t2 (which define a rate window), are chosen and within each rate window a capacitance difference, DC, is obtained for every measurement temperature. We can assume that the capacitance transient follows the exponential time dependence:
CðtÞ ¼
nT ð0Þ t C 0 exp 2ND se
ð1Þ
where C is the capacitance across the Schottky diode, C0 is the steady state capacitance, t is time, nT(0) is the initial occupation of the trap, and se is the inverse of the emission rate of the trap, en. Then if one plots the difference in capacitance at two different times DC = C(t2) C(t1), for all temperature but keeping the rate window defined by t1 and t2 fixed, peaks related to specific traps would appear at certain temperatures. These peaks are related to the emission rate by:
Fig. 1. GaN on sapphire wafer mounted on alumina and wirebonded for probing under vacuum in the DLTS system.
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se;max ¼
t2 t1 ln tt21
ð2Þ
We then plot many different rate window’s se,max and the corresponding temperature on a ln(seT2) vs. 1/kT grid. From the emission rate formula, en = rnvthNcexp(ET /kT) converted to the following form:
T2 ln en
!
T2 ¼ ln rmT Nc
! þ
ET kT
ð3Þ
where r is the capture cross section, vT is the thermal velocity, Nc is the density of states of the conduction band, and ET is the trap energy level, we see that the slope of the ln(seT2) vs. 1/kT plot gives ET and the intercept gives the capture cross section. Thus capture cross sections determined from DLTS can exhibit high uncertainties as the determination requires an extrapolation of the Arrhenius plot to infinite temperature. From the equation above we see that if the rate window is chosen such that all traps are filled at the start of the rate window and empty at the end of the rate window, we obtain the concentration of the trap from nT(0) = 2NDDC/C0 where DC is the maximum capacitance change possible. For the MCTS measurements the electrons are excited from the valence band to the conduction band using a 280 nm LED from Sensor Electronic Technology, Inc., that was biased to a 4 mA on current (at 8.5 V) with a pulse width of 20 ms. This LED was mounted in the vacuum chamber and the surface of the LED was roughly 6 mm above the mesh Schottky contact of the fabricated GaN Schottky diode. Throughout the measurement a 2 V reverse bias was applied across the Schottky diode. At each temperature the capacitance transient is measured and averaged 200 times as the trapped holes are thermally released. The DC/C plot for minority carriers has an opposite sign to that for majority carriers due to the opposite sign of the charge involved in the measurement. [13]
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decreasing profile, and that the rest of the low doped GaN region had a uniform silicon level not likely caused by diffusion from the high doped GaN region. Silicon concentration is mostly influenced by pressure and ammonia flow and to a lesser extent temperature and TMGa flow [2]. Typically reported sources of oxygen are the sapphire substrate or the nitrogen source gas. The silicon and oxygen present can act as shallow donors or form complexes with other defects. Fig. 2 presents the DLTS data for the MOCVD GaN sample grown at 300 Torr, and Fig. 3 presents the DLTS data for the MOCVD GaN sample grown at 500 Torr. The broad and overlapping nature of the peaks indicates a distribution of deep levels [19]. The characteristics of the traps are calculated from rate window analysis and the information is given in Table 1. In both samples four majority carrier trap peaks are present; however, we will only give details for the three clearest peaks in each case. The broadness of the TE2 peak is perhaps due to emission from a distribution of energy levels. This distribution of energy levels may be the result of PooleFrenkel emission whereby the ionization energy is enhanced by an electric field that lowers the emission barrier. Rate window analysis of this peak provides an energy and capture cross section value; however, a simulation using the rate window analysis results leads to a DLTS peak that is narrower than that measured. Fitting the peak with more trap energy levels doesn’t work well because of the wide distribution of emission rates rather than just two distinct values. Also, due to the breadth of the peaks, we could not obtain enough information about the TE3 trap for the 300 Torr grown sample, or about the TE1 trap for the 500 Torr grown sample.
3. Results and discussion The Schottky diodes exhibited well behaved CV and current– voltage characteristics. Current–voltage measurements over a reasonable sampling indicated in the 300 Torr sample the average barrier height was 0.941 eV and the average ideality factor was 1.072. In the 500 Torr sample, the average barrier height was 0.940 eV and the average ideality factor was 1.091. CV measurements indicated majority carrier concentrations of 8.6 1015 cm3 for the 300 Torr MOCVD GaN and 1.19 1016 cm3 for the 500 Torr MOCVD GaN. SIMS analysis of the GaN sample MOCVD grown at 300 Torr indicated concentrations of 1.1 1017 cm3 for carbon, 1.25 1016 cm3 for silicon and 2.36 1016 cm3 for oxygen. SIMS analysis of the 500 Torr sample indicated concentrations of 6.45 1016 cm3 for carbon, 1.69 1016 cm3 for silicon and 2.3 1016 cm3 for oxygen. Here we see that low pressure growth did lead to increased incorporation of carbon in the MOCVD GaN samples as well as a more semi-insulating behavior due to the compensating nature of the carbon present. The most likely sources of silicon in the unintentionally doped layer is the SiC coated graphite susceptor [2]. We do measure a slight increased silicon concentration in the higher pressure grown sample which may coincide with a greater reaction rate at higher pressure between the gasses in the chamber and the background of silicon on the growth chamber wall. However, the amount of liquid gallium in the chamber that can react with the reactor wall should be small because we have a nitrogen rich growth environment so any excess Ga that could react with the wall would instead react with NH3 to form GaCNH compounds [2]. SIMS data for the silicon in the low doped GaN region indicates that silicon atoms diffused only about 31% of the way into the low doped GaN region from the high doped GaN region with a logarithmically
Fig. 2. DLTS data for the GaN sample MOCVD grown at 300 Torr chamber pressure, with peaks at (a) 125 K and 220 K, and (b) 424 K. The co-plotted Arrhenius plots obtained from rate window analysis of the DLTS data are associated with the axes to the right and top.
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Fig. 3. DLTS data for the GaN sample MOCVD grown at 500 Torr chamber pressure. The co-plotted Arrhenius plots obtained from rate window analysis of the DLTS data are associated with the axes to the right and top.
Table 1 DLTS data for the two GaN samples. r is the capture cross section. Pressure (Torr)
r (cm2)
Ec Et (eV)
DLTS peak T(K)
300
2.8 1019 2.8 1017 1.3 1017 7.7 1017 4.7 1018 7.4 1018
0.13 0.37 0.68 0.36 0.48 0.66
125 220 424 220 328 425
500
TE1 TE2 TE4 TE2 TE3 TE4
The concentrations of the traps in the sample MOCVD grown at 300 Torr are 1.15 1014 cm3 (TE1), 2.32 1014 cm3 (TE2), and 4.47 1014 cm3 (TE4). The concentrations of the traps in the sample MOCVD grown at 500 Torr are 1.55 1014 cm3 (TE2), 2.26 1014 cm3 (TE3), and 3.33 1014 cm3 (TE4). The peak for trap TE3 increased the least as the MOCVD growth pressure changed from 500 to 300 Torr indicating that this trap may be least connected with carbon incorporation. The trap TE1 is similar to the electron trap that Armstrong et al. associate with CGa, having similar energy level and capture cross section as their EC 0.13 eV trap [16]. This compares to a reported GaN region trap with an activation energy of 0.1 ± 0.02 eV identified by transient channel current measurements on GaN/AlGaN/GaN HEMTs containing a GaN cap layer [20]. Trap TE2 has the same energy level as a trap that was related to surface defects by Look et al. [21] Our earlier statement about the broadness of this peak being related to the Poole-Frenkel effect complies with this since electrostatic field strength is highest at the metal-semiconductor interface and then decreases away from the contact into the semiconductor. In fact Ashraf et al, described with the Poole-Frenkel model a trap around this energy level [22]. Trap TE3 is similar to the EC 0.49 eV trap identified by Lee et al [23] who speculated that this trap level is related to either carbon or hydrogen impurities introduced from methyl radicals. Being that this trap increased the least in concentration going from a pressure of 500 Torr to 300 Torr makes it more likely related to hydrogen impurities. Trap TE4 is similar to the EC 0.67 level identified by Haase et al. who suggested that this could be related to nitrogen interstitials [24]. Overall, these DLTS measurements indicate that the concentration of all the traps are larger for GaN MOCVD grown at lower pressure which could also be due to the fact that at low pressure MOCVD growth is faster which allows less time for adsorbed species to transport to proper sites. Perhaps some of the traps also come about from the physical impact of incorporating carbon into the lattice of GaN. For instance there is a large difference between the atomic radii of C and Ga
atoms and this affects the crystal structure around CGa leading to impurity introduced energy levels. When CGa is present the bond length with one nearest neighbor N atom is reduced by about 18.1% and with the three remaining nearest neighbors by 16.6% compared to having a Ga atom in its place [25]. Fig. 4 presents the MCTS data for the sample MOCVD grown at 300 Torr, and Fig. 5. presents the MCTS data for the sample MOCVD grown at 500 Torr. From these data plots we observe that the minimum minority carrier trap densities are also larger in the lower pressure grown GaN wafer. For the 300 Torr sample, the minimum minority carrier trap densities are 2.06 1014 cm3 (TH1) and 2.92 1014 cm3 (TH2), and for the 500 Torr sample, the minimum minority carrier trap densities are 1.2 1014 cm3 (TH1), and 1.73 1014 cm3 (TH2). We mention that these are minimum values because in the MCTS measurement minority carrier holes generated by light absorbed outside but within a diffusion length of the depletion region are the desired types of holes for the emission transient being analyzed since the simultaneously generated majority carriers are excluded from the depletion region by the electric field. However, if minority carrier holes are generated by light absorption within the depletion region, the simultaneously generated electrons can be captured by deep levels and erroneously reduce the measured trap concentration. Tokuda et al. [15] encountered the same issue with their MCTS measurement. The characteristics of the traps are calculated from rate window analysis and the information is given in Table 2. There are three small amplitude peaks in both Figs. 4 and 5 between the two major peaks that are difficult to fit with rate window analysis due to the high background level. This background level could be from leakage current or measurement conditions such as noise from other electronics nearby in the lab. We can assume that the trap TH1 is the CN defect indicated by Armstrong et al. [26] Wang et al. have carried out effective mass calculations for the CN acceptor in GaN and obtained the value EV + 0.152 eV agreeing reasonably well with the value obtained here. The TH1 trap capture cross section of 1 1015 cm2 for holes indicates that in the 500 Torr sample this trap is attractive to holes and therefore may be negatively charged before capturing the hole. Therefore it appears to be an acceptor trap. Fischer et al., also identified a carbon related acceptor trap (CN) at this energy level using photoluminescence measurements [27]. On the other hand, the TH2 trap’s energy level is similar to that associated either with VGa, VGa–H or VGa–2H, [28,29] or with the complex CN–VGa [30]. This trap has also been described in the literature as being responsible for yellow luminescence band in GaN [31,30].
Fig. 4. MCTS data for the GaN sample MOCVD grown at 300 Torr. The co-plotted Arrhenius plots obtained from rate window analysis of the MCTS data are associated with the axes to the right and top.
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behavior we observe of lower trap densities with lower carbon incorporation [3]. 4. Conclusions
Fig. 5. MCTS data for the GaN sample MOCVD grown at 500 Torr. The co-plotted Arrhenius plots obtained from rate window analysis of the MCTS data are associated with the axes to the right and top.
Table 2 MCTS data for the two GaN samples. The data assumes that the energy gap is 3.4 eV. r is the capture cross section. Pressure (Torr) 300 500
TH1 TH2 TH1 TH2
r (cm2)
Ec Et (eV)
DLTS peak T(K)
3.4 1016 4.5 1017 2.0 1015 9.5 1017
3.20 2.69 3.19 2.65
123 412 119 420
If we do a direct comparison of the carbon related data we see an interesting correlation. SIMS data indicated that the 300 Torr sample had 70.5% greater carbon concentration than the 500 Torr sample. For a comparable calculation of the TE1 trap we can make a rough estimate of the location of the TE1 peak in Fig. 3, for the 500 Torr sample and obtain a concentration of 6.8 1013 cm3 for this trap. This indicates that the concentration of this trap which can be related to the CGa defect is 68.9 percent larger in the 300 Torr grown sample compared to that of the 500 Torr grown sample. This percentage increase is comparable to the percentage increase of carbon in the sample. On the other hand the concentration of trap TE2 increased by 49.7%, trap TE4 by 34.2 percent and trap TE3 decreased by 3.5%. Also, the concentration of the TH1 trap which could be related to the CN defect is 71.6% larger in the 300 Torr grown sample compared to that of the 500 Torr grown sample. Interestingly the TH2 trap also increased 68.8% so this supports the view that carbon may also form a deep trap at the energy level of TH2 [32,5]. Comparing our DLTS and MCTS results with other experiments related to MOCVD GaN growth at different pressures, Klein et al. investigated carbon incorporation in MOCVD GaN using four different growth pressures (65, 150, 200, and 300 Torr) and identified a carbon trap related signature at EC 3.07 eV [5]. This is close to the TH1 trap identified here. They related the EC 3.07 eV level to what Ogino et al. identified as a source of yellow luminescence at EV + 0.86 eV that could be due to a CN–VGa complex [30]. Armstrong et al. analyzed MOCVD GaN grown at 76 and 760 Torr and observed with DLTS two deep carbon related levels at EC 1.35 eV and EC 3.28 eV [26]. Their EC 3.28 eV level which they also identify as CN may also be related to the TH1 level we have identified. Fang et al. analyzed MOCVD AlGaN/GaN HEMTs grown at 100 Torr and 500 Torr with DLTS and observed electron traps at energy levels further away from the conduction band than those we have observed. They also observed a hole trap near the channel region of the HEMT. Overall, their results indicate the same general
We have investigated using DLTS and MCTS the trap signatures that result from changing the growth pressure for MOCVD grown GaN to influence the incorporation of carbon. Two samples grown at 300 Torr and 500 Torr with greater carbon concentration in the low pressure grown case, as identified by SIMS, were investigated. Four majority carrier traps and two minority carrier traps were identified by DLTS and MCTS respectively. We observe a general increase in all trap densities as the MOCVD growth pressure is decreased. Of the majority carrier traps we observe with DLTS, the trap at EC 0.13 eV appears directly related to carbon substituted for gallium. Other majority carrier traps appear due to either structural modification or are hydrogen related. Of the minority carrier traps observed with MCTS, the trap at EC 3.2 eV appears to be related to carbon substituted for N, and the trap at EC 2.69 appears to be related to complexes of this defect or gallium vacancies. Acknowledgments Thanks to Dr. Daniel Johnstone of Semetrol, LLC, for DLTS related advice. We also want to acknowledge that the materials were grown in Prof. Shahedipour’s lab at SUNY-Albany and SIMS data was provided by Dr. Steve Novak at SUNY-Albany. References [1] Wickenden AE, Koleske DD, Henry RL, Twigg ME, Fatemi M. J Cryst Growth 2004;260:54–62. [2] Koleske DD, Wickenden AE, Henry RL, Twigg ME. J Cryst Growth 2002;242:55–69. [3] Fang ZQ, Claflin B, Look DC, Green DS, Vetury R. J Appl Phys 2010;108:063706. [4] Binari SC, Klein PB, Kazior TE. Proc IEEE 2002;90:1048–58. [5] Klein PB, Binari SC, Ikossi K, Wickenden AE, Koleske DD, Henry RL. Appl Phys Lett 2001;79:3527–9. [6] Binari SC, Ikossi K, Roussons JA, Kruppa W, Park D, Dietrich HB, et al. IEEE Trans Electron Dev 2001;48:465–71. [7] Ikeda N, Kaya S, Li J, Sato Y, Kato S, Yoshida S. In: 20th international symposium on power semiconductor devices and IC’s, ISPSD ‘0; 2008. pp. 287–90. [8] Meneghesso G, Verzellesi G, Danesin F, Rampazzon F, Zanon F, Tazzoli A, et al. IEEE Trans Dev Mater Reliab 2008;8:332–43. [9] Saito W, Omura I, Tsuda K. CS Mantech conference; 2007. pp. 209–12. [10] Meneghesso G, Rossi F, Salviati G, Uren MJ, Munoz E, Zanoni E. Appl Phys Lett 2010;96:263512. [11] Ambacher O, Smart J, Shealy JR, Weimann NG, Chu K, Murphy M, et al. J Appl Phys 1999;85:3222–33. [12] Meneghesso G, Verzellesi G, Pierobon R, Rampazzo F, Chini A, Mishra UK, et al. IEEE Trans Electron Dev 2004;51:1554–61. [13] Muret P, Phillipe A, Monroy E, Munoz E, Beaumont B, Omnes F, et al. J Appl Phys 2002;91:2998–3001. [14] Auret FD, Meyer WE, Wu L, Hayes M, Legodi MJ, Beaumont B, et al. Phys Status Solidi (a) 2004;201:2271–6. [15] Tokuda Y, Yamada Y, Shibata T, Yamaguchi S, Ueda H, Uesugi T, et al. Phys Status Solidi (c) 2011;8:2239–41. [16] Armstrong A, Arehart AR, Green D, Mishra UK, Speck JS, Ringel SA. J Appl Phys 2005;98:053704. [17] Seager CH, Wright AF, Yu J, Gotz W. J Appl Phys 2002;92:6553–9. [18] Schroder DK. Semiconductor material and device characterization, second edition. New York: Wiley Interscience; 1998. [19] Kwon D, Kaplar RJ, Ringel SA, Allerman AA, Kurtz SR, Jones ED. Appl Phys Lett 1999;74:2830–2. [20] Mitrofanov O, Manfra M. Appl Phys Lett 2004;84:422–4. [21] Look DC, Fang ZQ. Appl Phys Lett 2001;79:84–6. [22] Ashraf H, Imran Arshad M, Faraz SM, Wahab Q, Hageman PR, Asghar M. J Appl Phys 2010;108:103708-1–8-5. [23] Lee WI, Huang TC, Guo JD, Feng MS. Appl Phys Lett 1995;67:1721–3. [24] Haase D, Schmid M, Kurner W, Dornen A, Harle V, et al. Appl Phys Lett 1996;69:2525–7. [25] Boguslawski P, Briggs EL, Bernholc J. Appl Phys Lett 1996;69:233–5. [26] Armstrong A, Arehart AR, Moran B, Denbaars SP, Mishra UK, Speck JS, et al. Appl Phys Lett 2004;84:374–6. [27] Fischer S, Wetzel C, Haller EE, Meyer BK. Appl Phys Lett 1995;67:1298–300. [28] Hierro A, Ringel SA, Hansen M, Speck JS, Mishra UK, DenBaars SP. Appl Phys Lett 2000;77:1499–501.
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