Journal of The Electrochemical Society, 156 6 H434-H442 2009H434
What Do We Know about Hydrogen-Induced Thermal Donorsin Silicon?E. Simoen,a,*,z Y. L. Huang,b Y. Ma,a J. Lauwaert,c P. Clauws,c J. M. Rafí,d
A. Ulyashin,e,g and C. Claeysa,f,**aIMEC, B-3001 Leuven, BelgiumbBaoding TianWei Solar Films Company, Limited, Baoding 071051, ChinacDepartment of Solid State Sciences, Ghent University, B-9000 Gent, BelgiumdCNM (IMB-CSIC), 08193 Bellaterra, SpaineDepartment of Physics, University of Oslo, Blindern N-0316 Oslo, NorwayfElectrical Engineering Department, KU Leuven B-3001 Leuven, Belgium
Manuscript submitted January 13, 2009; revised manuscript received March 4, 2009. Published April 9, 2009. This was Paper1991 presented at the Honolulu, Hawaii Meeting of the Society, October 12–17, 2008.
It is well established now that hydrogen is technologically one ofthe most important neutral impurities in silicon,1 finding many ap-plications today. Because of its high reactivity and mobility, hydro-gen interacts with other impurities, either passivating harmfulgeneration-recombination centers and dopants, such as B and P, oractivating neutral impurities, such as carbon and oxygen. In thelatter case, it is known that H accelerates interstitial oxygendiffusion,2 resulting in a faster oxygen thermal donor TD OTDformation.3-6 At the same time, hydrogen-related shallow TDsSTDHs have also been observed,7-13 with ionization energies inthe range of 35–40 meV and infrared absorption peaks in thesub-300 cm−1 wavenumber range. These centers can be exploitedfor the low temperature formation of deep p-n junctions in p-typeCZ silicon.14-24 An example is shown in Fig. 1, for a 12 h,260–270°C H-plasma-treated 5 cm p-type CZ Si sample, show-ing conversion to n-type in the first 50 m from the exposed sur-face. The fact that high concentrations of donors are formed at tem-peratures well below the usual formation interval of OTDs300–500°C indicates the possible importance of STDHs. Thejunction depth is determined in the first instance by the trap-limiteddiffusion of hydrogen,25-27 so that for longer hydrogenation times orsubsequent anneals at higher temperature a complete p- to n-typeconversion over several hundreds of micrometers is possible.18
The aim of this work is to review what is known about theimpact of hydrogen on OTDs, H-related STDs, and their possibleinteraction. To that end, a systematic study of H-plasma-treated andannealed Si samples with different resistivity and doping type, dif-ferent amounts of interstitial oxygen, and growth method, indicatedin Table I, has been carried out, using a variety of electrical, spec-troscopic, and structural characterization techniques. As will beshown, the formation kinetics of the TDs is a strong function of theH and O concentration, the annealing temperature, but most of all,the doping type and concentration Fermi level. The latter factorhas a major impact on the charge state and, hence, the diffusion of
* Electrochemical Society Active Member.** Electrochemical Society Fellow.
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hydrogen, dictating the formation kinetics of STDHs. Although thenature of these shallow centers is still obscure, a few possibilitiescan be definitively ruled out, such as H-passivated OTDs orvacancy-oxygen centers. Moreover, evidence will be given that theSTDHs studied here are not simply an early stage of OTDs; in otherwords, after prolonged heat-treatments STDHs do not convert intoOTDs, in contradiction with a recently proposed model,28 but ratherbecome passivated by the attachment of further hydrogen atoms.
Experimental
The materials indicated in Table I have been hydrogen-plasmatreated in a plasma-enhanced chemical vapor deposition parallel-plate system at a substrate temperature of typically 260–270°C fortimes ranging from 30 min up to 12 h.29-32 The interstitial oxygenconcentration Oi was measured by Fourier transform infraredFTIR absorption spectroscopy. Prior to the hydrogen-plasma expo-sure, the silicon wafers are ultrasonically cleaned in acetone andmethanol, and rinsed in deionized water to remove the organic re-mains on the silicon surface. Then the wafers are dipped in a 1% HF
1010
1011
1012
1013
1014
1015
1016
104
105
106
107
108
109
0 20 40 60 80 100 120
p-type Cz Si - 12 h H+ plasma
CarrierDensity(cm-3)
SpreadingResistance(Ω)
Depth (µm)
n-type p-type
Figure 1. Profiles of spreading resistance and the free carriers for a p-typeCZ Si sample exposed to hydrogen plasma for 12 h.
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H435Journal of The Electrochemical Society, 156 6 H434-H442 2009 H435
solution to remove the native oxide layer. The detailed parametersused for the hydrogen plasma exposures are given in Ref. 25. Sub-sequent isochronal annealing was performed at temperatures in therange of 275–500°C in air.
Carrier concentration profiles have been derived from spreadingresistance probing SRP on 6 deg bevel-angled samples or from1 MHz capacitance–voltage C-V measurements on reversely bi-ased Au–Schottky barriers n-type Si or p-n junctions p-type Si.The deep and shallow levels have been analyzed by deep-level tran-sient spectroscopy DLTS, FTIR absorption spectroscopy, or pho-tothermal ionization PTI spectroscopy PTIS.
For electrical evaluations, ohmic contacts have been prepared byrubbing InGa p-type or InHg n-type eutectic on the appropriateside of the sample and covering with a piece of In foil. This methodenables the fabrication of contacts without an additional temperaturetreatment influencing the created shallow donors. C-V measure-ments are performed at a fixed frequency of 1 MHz, to extract thefree carrier concentration. DLTS is operated with a 1 ms pulse froma reverse bias VR to a pulse voltage VP with a sampling period tw of51.2 ms, typically. At the same time, a capacitance–temperatureC-T curve at VR is automatically recorded during the DLTS mea-surements.
PTI spectra are obtained using a Bruker IFS66V FTIR spectrom-eter. For these measurements, two parallel ohmic contacts are fabri-cated on the plasma-treated side in the same way as mentionedabove. The sample is mounted in an Oxford Optistat CF cryostat,and the temperature is kept constant using a Lakeshore temperaturecontroller. The specimen is illuminated by a He–Ne laser duringcooling in order to freeze the early OTD centers in their double-donor configuration.33
Results
In this part, the impact of the plasma hydrogenation and subse-quent thermal anneal is reported for the different substrate types ofTable I. First, the C-V results obtained on n-type Si will be brieflysummarized; more detailed results have been published before.29-31
More attention will be given here to the data for p-type material.When comparing the impact of the doping type, a general conclu-sion is that the introduction rate of TDs is significantly faster in p-than in n-type Si. This is thought to be related to the faster diffusionof H+ in p-Si compared to H0 or H− in n-type material.25-27,32 It isalso shown that for prolonged anneals, the H-related enhancementeffect decays due to the outdiffusion or trapping of hydrogen.
Hydrogenation-induced donor formation in n-type silicon.— Thedonor profiles in plasma-hydrogenated and 450°C annealed standard5 cm n-type CZ Si exhibit typically a decay toward the surface.30
At 1 m from the exposed surface, the carrier concentration lev-els off, forming a constant plateau. On the basis of these plateauvalues, one can demonstrate that heat-treatment at 450°C increasesthe free electron concentration from the starting value of 9 1014 cm−3 to 1016 cm−3 after 50 h anneal.30 In addition, onecan derive that plasma hydrogenation speeds up the donor formation
Table I. Relevant material parameters of the substrates investi-gated. The Oi in float zone material was introduced by a hightemperature oxidation and in-diffusion procedure. The substitu-tional carbon concentration was below the detection limit of theFTIR instrument „1016 cm−3
….
Type Resistivity Oi 1017 cm−3 Growth Ref.
n 2 & 5 k cm 1.0 HR FZ:Oi 29n 5 cm 7–8 CZ 30n 500 cm 2–5 MCz 31p 5 cm 8 CZ 32p 2 k cm 5.5 MCzp 5 k cm HR FZ:Oi
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rate, especially for shorter annealing times at 450°C. Longer hydro-genations also yield higher electron concentrations for the same an-nealing time Fig. 2.30
In order to determine more clearly the impact of the plasma-hydrogenation treatment, Fig. 3 depicts the TD enhancement factordefined by30
tHta =
ntHta − n0ta
n0ta1
In Eq. 1, ta is the annealing time at 450°C; ntHand n0 is the carrier
concentration for the n-type CZ material plasma-hydrogenated for atime tH and without plasma treatment tH = 0, respectively. From
Figure 2. Carrier concentration as a function of annealing time for hydro-genated 30, 60, and 120 min and nonhydrogenated n-type CZ material,determined at the plateau at about 1–2 m from the surface after Rafí etal.30.
Figure 3. Donor concentration enhancement factor defined by Eq. 1 as afunction of the annealing time at 450°C for hydrogenated 30, 60, and120 min n-type CZ silicon after Rafí et al.30.
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H436 Journal of The Electrochemical Society, 156 6 H434-H442 2009H436
Fig. 3, one can derive that the maximum enhancement effect comesafter 5 h of annealing at 450°C, irrespective of tH. The factor2 h = 3.5 corresponds with an acceleration rate of 12 betweenthe TD concentration in 2 h of plasma-treated material compared tothe reference without hydrogenation.30 This is in the same range asin previous reports, yielding factors between 4 and 20 at450°C.4,34-36
Interestingly, at longer annealing times, the hydrogen accelera-tion effect dies out according to Fig. 3. This points to an exhaustionof the hydrogen atoms available for TD formation at depths of1 m below the surface either by outdiffusion or trapping. Inaddition, the free-electron concentration tends to the same saturationvalue in Fig. 2, except for the longest tH 2 h, still showing a higherdonor concentration after 50 h of heat-treatment at 450°C. On thebasis of this, the most straightforward explanation for the results ofFig. 2 and 3 is that hydrogen enhances the transport of interstitialoxygen, resulting in an accelerated OTD formation.2-6 Figure 3should then represent the increase of the oxygen diffusion coeffi-cient, provided hydrogen does not interfere with the oxygen cluster-ing process.
Unfortunately, this simple picture is not supported by the DLTSresults on the same material, demonstrating OTD concentrations thatamount to only 20% of the added shallow donors, or given thedouble-donor nature, 40% of the carrier concentration.30 It has beennoted in the past that there can be a difference between the freecarrier concentration in CZ material heat treated around 450°C tocreate OTDs and the resulting concentration derived from DLTS, thelatter method yielding lower values in many cases.5,30,37-40 Espe-cially when the OTD concentration is higher than the backgrounddoping concentration, capacitance DLTS will underestimate the trueOTD concentration for several reasons. One is the fact that theOTDs are attractive deep levels for electrons, so that they exhibit aPoole–Frenkel shift of the peak position to lower temperatures en-ergies with increasing electric field.30,37,38 This shift becomes morepronounced for a higher VR during DLTS and for higher OTD con-centrations. Another reason is that the OTD peak generally consistsof the contributions of different closely spaced donors, so that it isbroader than a single-level peak. This higher width should be ac-counted for when calculating the OTD concentration from the cor-responding DLTS peak amplitude. It can thus be expected that partof the discrepancy between 450°C heat-treatment-induced carrierand OTD concentration is related to measurement artifacts.
However, in the early annealing stages of n-type CZ Si or for thehigh resistivity HR float zone FZ n-type Si material, the correc-tion factors will be limited, as the added OTD concentrations aresmall and a single donor species is dominant, so that one can rely onstandard DLTS analysis for reliable concentration determination. Inaddition, one can derive the free carrier concentration from a C-Vplot at room temperature and at the low temperature plateau50–60 K to calculate the total donor concentration related with thesecond donor level from the difference of the two values, as hasbeen done previously.30 As mentioned above, the contribution to thefree electron concentration is twice the OTD concentration. In prin-ciple, this should allow an estimate of the absolute OTD concentra-tion in n-type Si within 20% in most cases. Keeping these reserva-tions in mind, it will be shown that in most examples studied herethe OTDs represent only a fraction of the created shallow donors, sothat the OTD acceleration model by hydrogen does not completelyexplain the experimental facts.
In order to further investigate the relationship between the carrierconcentration increase n and the OTD concentration, Fig. 4 showsthe carrier profiles for a 2 h plasma-treated sample and a not hydro-genated sample, annealed for 30 min at 450°C. Although no donorshave been introduced within the measurement accuracy of the C-Vtechnique for the no plasma sample see Table II, the free carrierconcentration has tripled for the front side of the plasma-treatedmaterial. Interestingly, the back side of the plasma-doped samplealso shows a non-negligible and accelerated donor increase, whichcould point to the presence of hydrogen throughout the wafer thick-
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ness 400 m. As shown before, prolonged anneals at 450°C leadto a donor profile that penetrates deeper into the material with timeand arrives at the back side after 5 h.41,42 It is thus possible that asmall concentration of H reaches the back side of the wafer alreadyafter a 30 min treatment. Another possibility is that some hydrogenpenetrates from the back surface during the hydrogenation, causingthe formation of TDs. However, the main point of concern here isthat the increase in carrier concentration n is far higher than theobserved increase in OTD concentration NOTD Table II, after a30 min anneal at 450°C at either side of the sample.
A further argument pointing to the importance of additionalSTDs other than OTDs comes from the results obtained on the oxy-genated HR FZ material, indicated by HR FZ:Oi.
29 It has beenshown that while there is a significant increase in the free carrierconcentration for a 30 min 450°C anneal in the plasma-treatedsample, longer anneals lead to a reduction of the dopingconcentration.29 Moreover, from DLTS it was concluded that theOTD concentration was negligibly small in this material; thus,changes in electron concentration must come from other most likelyhydrogen-related shallow donor centers. Perhaps the most strikingresult is the reduction of these centers after 50 h annealing. In ad-dition, the possible role of STDH centers is further supported by thedata on HR magnetic Czochralski MCz silicon,31 showing increas-ing relative contribution of STDs compared to OTDs to the createdshallow donors for lower annealing temperature.
Combined with the data of Fig. 2, we are led to the conclusionthat the STDHs are not transformed in OTDs at some time during
0 1 2 3 4Depth (µm)
1x1015
1x1016
2x1015
3x10154x10155x10156x10157x10158x10159x1015
2x1016
9x10148x10147x10146x1014
5x1014Freeelectron
concentration
(cm-3)
2 h plasma H + 30 min @450°CFront side
2 h plasma H + 30 min @450°CBack side
No plasma H + 30 min @450°CFront side
Figure 4. Free carrier profiles derived from a 1 MHz C-V measurement on5 cm n-type Cz material annealed for 30 min at 450°C.
Table II. Carrier concentration derived from a 1 MHz C-V plot,corresponding to the plateau values in Fig. 4. Also shown is theOTD concentration „NOTD… derived from the singly ionized OTDdonor peak in DLTS.
Hydrogenation + annealingat 450°C
ncm−3
ncm−3
NOTDcm−3
No plasma + 0 min 1.1 1015
No plasma + 30 min 1.1 1015 0.1 1015 1012
2 h H plasma + 30 min 3.3 1015 2.2 1015 1.7 1014
2 h H plasma + 30 minback side
1.4 1015 0.3 1015 1.3 1013
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H437Journal of The Electrochemical Society, 156 6 H434-H442 2009 H437
the 450°C anneal; otherwise, we would expect a higher saturationfree carrier concentration, consisting of the sum of the two speciesin Fig. 2. STDHs cannot be considered here as an intermediate stagein the OTD formation. Their disappearance at longer 450°C annealsin Fig. 2 can then only be explained by a dissociation or a passiva-tion of the STDHs. Dissociation is possible if the local atomic hy-drogen concentration is reduced by outdiffusion or molecular hydro-gen H2 formation, shifting the reaction equilibrium represented byH + X ⇔ HX to the left, X representing the nucleus of the STDH.However, numerous studies have demonstrated the thermal stabilityof different kinds of STDs up to temperatures well above450°C;7-23,43-47 thus, we discard this possibility here.
Passivation corresponds with a further reaction, for examplegiven by H + HX ⇔ H2X, if the STDH center HX is a singledonor. Under this assumption, it follows that the STDHs behavedifferently than the OTDs because the latter cannot be passivated byhydrogen at 450°C.33,48,49 Moreover, both Oi and the STD con-centration are much smaller in the HR FZ material, representing amuch lower concentration of traps for the available atomic hydro-gen. This should yield a faster neutralization of the hydrogen-relatedSTDs.
Comparing the data for standard n-type CZ silicon30 to those forHR FZ:Oi,
29 it is clear that the amount of shallow donors created ismuch higher in the CZ case, for equivalent plasma and heat-treatments. This is further confirmed by the results on high resistiv-ity n-type MCz silicon, where the increase in doping concentrationis not typically higher than 1 1014 cm−3.31 Roughly, a one-decadehigher n is derived for the CZ material, which is of the same orderas the difference in Oi. This emphasizes the impact of the initialinterstitial oxygen concentration on the plasma-induced shallow do-nor formation. Again, however, there is a marked difference withOTD formation in standard CZ material because it is known thatNOTD is proportional with Oi with 4, a much stronger depen-dence than can be derived for the STDH centers. This fourth powerdependence originally led to the conclusion that an OTD consists ofa cluster of four oxygen atoms.50 Combining the data for the differ-ent n-type substrates with Oi ranging from 1 to 8 1017 cm−3
indicates a maximum n in the range 1014 cm−329,31 to 1.5 1015 cm−330 for a 30 min anneal at 450°C. This apparent propor-tionality with Oi suggests that only one or a small number ofoxygen atoms is involved in the STDs. In order to more stronglysupport this hypothesis, it would be worthwhile to subject materialwith a similar range of Oi and perhaps the same initial dopingconcentration to a 2 h plasma treatment, followed by a set of 450°Canneals for times up to 50 h, for example.
Summarizing the data on hydrogenation-induced STD formationin n-type Si, it can be stated that there is a positive correlation withboth H and Oi. The donor formation is shown to consist of twoparallel paths, namely, the creation of STDHs, which most likelybecome neutralized after longer anneals, and an accelerated forma-tion of OTDs by the enhanced diffusion of oxygen. The STDHneutralization process sets in sooner for lower Oi concentration.
Hydrogenation-induced donor formation in p-type silicon.— Inp-type hydrogenated silicon, the introduction of STDs yields a typeof conversion, whereby the p-n junction moves along with the hy-drogen diffusion front.14-27 Consequently, the junction profile is notabrupt but rather linear as illustrated by Fig. 1, which should betaken into account when deriving carrier concentration profiles fromC-V and OTD concentrations from DLTS measurements. Keepingthis in mind, it is found in all samples that the concentration of thedetected OTD centers derived from DLTS is much lower than re-quired for the formation of the p-n junctions. Figure 5 compares theNOTD to the average doping concentration derived from the 1/C2 vsVR slope. The trap densities have been calculated from the DLTSpeak height of the −10 → 0 V spectra, using this carrier concentra-tion and the standard formula, implemented in the software of thedigital DLTS system employed in this work. One clearly sees that
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the OTD concentration is typically one to two orders of magnitudelower than the free carrier concentration. Because the created shal-low donors must also compensate the p-type doping, the extractedOTD concentrations in Fig. 5 are two to three orders of magnitudelower than the shallow donor concentration derived from C-V re-sults.
Note that we are dealing with graded p-n junctions rather thanabrupt ones see Fig. 1. Increasing the reverse bias, the depletionregion expands in both the p- and n-side of the p-n junction. There-fore, using the abrupt junction model to derive the free carrier con-centration will lead to some errors. As shown by the SR profile ofFig. 1, the p-n junctions are rather symmetric. For a given reversebias, the depletion width in the n-type side of the p-n junction de-rived using the abrupt junction model is about twice the real one.Moreover, the graded nature of the doping profile can introduceadditional errors, so that the numbers in Fig. 5 should be consideredas order-of-magnitude estimates. As explained above, in one case wewere able to derive the carrier concentration gradient from SRP.Combined with the depletion depth from the C-V measurements,enables one to estimate a more accurate doping concentration whichwas within 50% of the first-order estimate from the 1/C2-VR analy-sis. A more accurate treatment of the data, which is beyond thescope of the present paper, should systematically combine SRP,C-V, and a numerical solution of the Poisson equation.51 The sameapproach can then be used to derive more precise OTD concentra-tions. Nevertheless, the expected correction by a factor of two 50%in carrier concentration and 50% for the DLTS peak amplitude orrelative trap concentration for the first-order approach used here isnot able to explain the difference of one to two orders of magnitudebetween the free carrier and the OTD concentrations in the presentsamples. It can therefore be concluded that the STDH centers are thedominant donors in the region close to the p-n junction.
This conclusion is supported by the C-T curves measured for theas-hydrogenated material. As shown in Fig. 6, a huge step the in-crease of the capacitance occurs between 25 and 40 K for eachsample. This step is attributed to the freeze-out of the STDH centers.In contrast, no considerable increase of the capacitance can be seenin the C-T plots at a temperature of 60 K and higher, at which theDLTS peak of the OTD+/++ level appears. This result confirms thatin the silicon region close to the p-n junction, the concentration ofthe OTD centers is very small compared with that of the STDHcenters.
It has been reported in previous work25 that a p-n junction iscreated in p-type CZ silicon if the hydrogenation duration, tH, is1 h. It is found that the depth of the p-n junction increases with
1010
1011
1012
1013
1014
1015
300 350 400 450
Concentration(cm- 3)
Anneal Temperature (oC)
p-Cz Sin (C-V)
OTD+/++ (DLTS)
Figure 5. Average free carrier C-V measurements at 1 MHz and OTDconcentration DLTS for the 5 cm p-type CZ Si samples that have beenhydrogenated for 12 h and subsequently annealed at various temperatures for20 min.
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H438 Journal of The Electrochemical Society, 156 6 H434-H442 2009H438
increasing tH, reflecting the in-diffusion of hydrogen. The depths ofthe p-n junctions are 6.8, 11.5, 12.4, 16.4, 18.8, and 27.9 m fortH = 1, 1.5, 2, 3, 4, and 6 h, respectively. It is also observed fromDLTS that the concentration of the OTD centers in these samples aretwo to three orders of magnitude lower than the concentration of theSTDH centers. The STDH centers are the dominant donor speciesused for the compensation of the initial p-type doping in the siliconregion close to the p-n junction. Therefore, the concentration of theSTDH centers at a depth of 6.8, 11.5, 12.4, 16.4, 18.8, and 27.9 mis NS = 1.2 1015 cm−3 at tH = 1, 1.5, 2, 3, 4, and 6 h, respec-tively. In turn, the average formation rate of the STDH centerscan be derived by using
=NS
tH2
Then, a value of of 1.2 1015, 8 1014, 6 1014, 4 1014,3 1014, and 2 1014 cm−3/h is derived for a depth of 6.8, 11.5,12.4, 16.4, 18.8, and 27.9 m at the compensation of the originalp-type doping. Clearly, the formation rate of the STDH centers de-creases with increasing depth. This may result from the diffusion-like profile of hydrogen and points against a catalytic role of hydro-gen only. Indeed, as will be discussed below, a small concentrationof hydrogen should suffice to accelerate the oxygen diffusion andOTD formation and moreover is regenerated if it acts as a catalystonly. However, when hydrogen is an essential part of the STDHcenters, exhaustion of the supply can take place for sufficiently longannealing times, resulting in a smaller at the diffusion front.
In addition, it is found that the formation rate of the STDHs at agiven depth reduces evidently as soon as a p-n junction is created. Itcan be seen in Fig. 1 that the free carrier concentrations at the depthsgiven above are 1.7 1015, 1.5 1015, 1.4 1015, 1.3 1015,1.2 1015, and 0.9 1015 cm−3, respectively. Thus, the total con-centrations of created single donors at these depths are 2.9 1015,2.7 1015, 2.6 1015, 2.5 1015, 2.4 1015, and 2.1 1015 cm−3 as tH = 12 h. Considering the PTIS results to be dis-cussed below, the OTD centers have a significant contribution to thefree-electron concentration at the n-type side in the vicinity of theexposed surface. In turn, the STDH concentration created during theextended hydrogenation at the depths given above can be approxi-mated as 1015 cm−3. This tentative conclusion leads to an averageformation rate of 8 1013 cm−3/h, which means that during theextended hydrogenations reduces by more than one order of magni-tude compared to the value before the formation of the p-n junctionsat a given depth. This is also consistent with the data for n-type Sireported in the previous section Fig. 2.
50 100 150 200 250 3000
50
100
150
200
250
300
12 h H-plasma
Freeze-out of the STDH centersCapacitance(pF)
Temperature (K)
6 h H-plasma
Reverse bias = -10 V
Figure 6. Capacitance-temperature plot measured at VR = −10 V for thesamples hydrogenated for 6 and 12 h.
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Discussion
Before discussing in more detail the presence and possible natureof the STDHs, it is important to better understand the properties ofhydrogen in the plasma-treated samples in the temperature rangefrom 250 to 500°C.
Hydrogen diffusion at low temperatures.— Given the techno-logical relevance, many studies have been devoted in the past twodecades to the diffusion properties of atomic hydrogen in Si in thetemperature range from room temperature up to 500°C.32,52-59 It hasbecome clear that the low temperature diffusivity depends heavilyon the charge state of hydrogen, which also determines the type ofinterstitial site it occupies in the silicon lattice and, hence, the ease atwhich it can move through it. In addition, compared to the hightemperature diffusion behavior,60 one should consider the impact ofhydrogen trapping by different species, such as oxygen or ionizeddopant atoms on the transport, resulting in an effective diffusivity atlow temperatures.
For the temperature range of 350–450°C, it has been shown thatbased on the hydrogenation-induced junction formation, it is pos-sible to extract the diffusivity of H+ in p-type Si,25 yielding32
DH = 122 exp− 1.36
kT cm2/s 3
The activation enthalpy Ha = 1.36 eV is in good agreement withthe values derived by others.4,52,58 It is much lower than the trap-freehydrogen diffusion determined by Van Wieringen and Warmoltz60 athigh temperatures and extrapolated to low temperatures in Fig. 7.This is explained by the fact that the hydrogen diffusion in p-typeCZ silicon is hindered by trapping at oxygen-related centers25 and athydrogen-induced platelets.27,59 From the electric-field dependenceof DH around 450°C, it was concluded that hydrogen drifts as apositive ion in p-type CZ Si and should either be in the positive orneutral charge state to catalyze the OTD formation.26 Interestingly,the trap-limited diffusivity at temperatures in the range of 60–140°Cand mainly due to trapping by B with a concentration of 1.4 1015 cm−3,56 follows closely the dashed line extrapolated fromEq. 3 in Fig. 7. This is probably coincidental because the effectivediffusion coefficient reduces inversely proportional with B.56 Withall other parameters kept the same, this observation implies that thehydrogen trapping radius of B− is much higher than for Oi, whichcan be explained by the different charge state of the trap centers. It
10-16
10-14
10-12
10-10
10-8
10-6
1 1.5 2 2.5 3 3.5 4DiffusionCoefficient(cm2 /s)
1000/T (1/K)
VWW
OTDJ
1015 cm-3 B
Figure 7. Color online Diffusion coefficient of hydrogen in p-type siliconfor different temperature regimes. The VWW data corresponds with trap-freediffusion of monatomic hydrogen derived at high temperatures,60 OTDJ hasbeen determined from hydrogenation-induced junction formation by OTDsin p-type CZ silicon32 and the B data have been derived from B passivationmeasurements in the range from 60 to 140°C in 1.4 1015 cm−3 p-type CZsilicon.56
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H439Journal of The Electrochemical Society, 156 6 H434-H442 2009 H439
should finally be remarked that at room temperature in p-typeSi,53,54 and below room temperature in n-type Si,55 a free hydrogendiffusivity has been determined that is in good agreement with theextrapolated high temperature data.60
On the basis of Eq. 3, a critical hydrogen concentration, neces-sary to enhance the oxygen diffusivity and OTD formation rate inthe 350–450°C range, has been calculated.32 The resulting expres-sion is
Hcrit = 2.53 1019 exp− 1.17
kT cm−3 4
resulting in a minimum concentration of 8.7 109 cm−3 at 350°Cto enhance the OTD formation. This is in reasonable agreement withthe value of 108 cm−3 estimated by Newman et al. for a hydrogen-enhanced oxygen diffusion at 350°C.4 However, as explained be-fore, this catalyst role of hydrogen can only partly explain the ob-servations and is expected to dominate at longer annealing afterhydrogen-plasma exposure in n-type or n-to-p converted Si. In theinitial stages p-n diffusion front; low n-doped Si and/or at annealtemperatures of 300°C, it is believed that hydrogen actively par-ticipates in the STDH formation and another acceleration modelshould be developed, accounting for this parallel shallow-donor-generation process.
Spectroscopy of the OTD and STDH centers.— In order to iden-tify the shallow donors responsible for the free carrier concentrationincrease, a combination of DLTS and PTIS has been applied. Asshown in Fig. 8, the dominant species found in long-time annealedhydrogenated n-type CZ Si are the OTDs. The peak in Fig. 8 corre-sponds with the / level of the double donor, with activationenergy in the range of 0.13–0.15 eV. Combined with infrared ab-sorption spectroscopy, it can be demonstrated that, in this case, thedonor increase can be in good approximation ascribed to the OTDformation.61 At the same time, the hydrogenic series of someH-related STDs, found by Newman et al.12 have been observed aswell, although the concentration is much smaller than for the OTDcenters. This confirms the picture developed above, where it wasconcluded that at longer annealing times at 450°C and correspond-ing with a sufficiently high n-type doping, OTDs are the dominantspecies and a possible hydrogen-related enhancement effect shouldcome from the faster transport of interstitial oxygen.
However, the picture is different for shorter annealing times ofn-type CZ Si or for lower temperature anneals, where the main part
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0.2 0.4 0.6 0.8 1 1.2
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Figure 8. DLT-spectrum and C-T plot corresponding to a 1 h plasma hydro-genated and 30 h annealed n-CZ sample at 450°C. A pulse from −6 to− 2 V was used. The inset shows the corresponding free carrier profile de-rived from a C-V measurement at 1 MHz.
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of the added donors are STDH and not OTD centers.30 This is de-rived from the DLTS concentration of the OTD+/++ peak.31 In orderto identify the missing shallow donors, with activation energy toolow to be resolved in capacitance DLTS, PTIS has been applied,yielding the spectrum of Fig. 9. The absence of the characteristiclines in the region 400–500 cm−1 demonstrates that no OTDs arepresent. However, the hydrogenic series associated with substitu-tional P in Si with lines at 316.6, 342.6, and 350 cm−1 is clearlyobserved. Finally, the presence of STDHs can be derived from thelines at 279.3 and 305.5 cm−1, at 265.0 and 291.4 cm−1, and at291.4, 258.3, and 284.6 cm−1. These correspond with the A-, B-, andC1-type STDs of McQuaid et al.8 Additional features in the regionof 250 cm−1 and the peak at 297 cm−1 appearing well above thenoise level are due to previously unreported lines. They probablybelong to as yet unidentified shallow donors in Si, but the absence ofidentifiable hydrogenic series does not allow drawing firm conclu-sions in this respect. Although the P-related series clearly dominatesthe spectrum and is responsible for 90% of the free carrier concen-tration, it should not be forgotten that the excess STDs occur over alimited thickness of the sample, defined by the diffusion profile ofhydrogen. Therefore, the increase in the local free carrier concentra-tion can be explained based on the STDH donors reported in Fig. 9.
The picture emerging from the combination of Fig. 8 and 9 isthat at a fixed depth from the surface, H-related STDs are formedfirst, defining the excess free carrier concentration at the hydrogen-diffusion front. In parallel, the much slower OTD process takesplace possibly accelerated by the hydrogen-catalyzed oxygen diffu-sivity and, after some time, dominates the introduction of excessfree electrons at this depth. Although we do not completely rule outthat the STDHs convert into classical OTDs at some point in time,all evidence supports the idea that both donor-formation processesare rather independent of each other. The fact that the profile of thedifferent shallow donor species is nonuniform and evolves with an-nealing time makes it hard to separate and quantify the introductionrate of the different shallow-level centers. This is especially true forthe STDHs, which are believed to have a highly nonuniform con-centration, peaking somewhere around the hydrogen diffusion front.Perhaps a combination of SRP and layer-by-layer removal followedby C-V on a new Schottky barrier n-type Si may provide a moredetailed answer to this question.
In p-type CZ material, OTDs are observed already at much lowerannealing temperatures, as shown in Fig. 10, which points to ahydrogen-related acceleration effect, that appears to be much morepronounced than in n-type material. In the latter case, OTD levelshave only been found by DLTS after a 350°C anneal in n-type MCzSi.31 This can result partly from the lower Oi in the MCz siliconTable I, but according to Fig. 5, the evolution of the OTDs and
Figure 9. PTIS of n-type MCz silicon hydrogenated and annealed at 310°Cfor 20 min.
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H440 Journal of The Electrochemical Society, 156 6 H434-H442 2009H440
STDs in p-type CZ Si seems to be remarkably parallel. Bothhydrogen-catalyzed OTD formation and hydrogen-containing STDformation are believed to contribute simultaneously to the p- ton-type conversion. This could be related to the positive charge stateof atomic hydrogen in p-type material.
Further support for this picture follows from the PTI spectra ofFig. 11, obtained on 12 h hydrogenated material and annealed at300°C for 0 as-plasma-treated or 20 min. In the former case, athus far unidentified effective masslike shallow donor is observedwith ionization energy of 35.4 meV. The 2p line remains visibleafter the 20 min 300°C annealing, indicating that this STD center isstill partly present. In addition, the B and C1 hydrogen-related shal-
20 40 60 80 100
DLTSSignal
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Temperature (K)
300 oC
330 oC
350 oC
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450 oC
Figure 10. DLTS spectra measured on p-type CZ silicon samples hydrogen-ated for 12 h and subsequently annealed at various temperatures for 20 min.The spectra are recorded with a VR of −10 V and a VP of 0 V.
Figure 11. PTIS-spectra for the p-type CZ Si samples hydrogenated for12 h, with an additional anneal for a 0 and b 20 min at 300°C. Themeasurement temperature is 23 K.
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low donors of Ref. 8 are visible in the spectrum after annealing.Furthermore, also the first and second donor states of the OTDs havebeen detected in the PTI spectra of the annealed sample.61 The factthat the deeper charge state is visible at 50 K points to a strongcontribution of the OTDs to the donor concentration in this sample.
Nature of STDH centers.— Regarding the nature of the STDHcenters, different interpretations have been advanced in the past. Onthe basis of the similarity of their electron paramagnetic resonancespectra, it has been suggested that STDHs are single donors formedby the attachment of a hydrogen atom to the OTD double-donorcenters.8,10 However, from the arguments presented above, we can-not adopt this model for the present observations. It would indeedimply that the STDHs are formed after the OTDs during the plasmaplus heat-treatment, while evidence has been provided that some ofthe STDHs are already formed during the plasma treatment in p-typeCZ Si, without OTDs being present Fig. 11. This is also confirmedby the results of the n-type HR FZ material.29 Moreover, it was alsofound that the STDHs are much more stable than the partially pas-sivated OTD centers 200°C.62
Hydrogen-related shallow donors can also be formed in annealedparticle-irradiated Si, which led to the suggestion that one or twohydrogen atoms and an oxygen-related radiation-induced defect, ei-ther a vacancy-oxygen complex VO63,64 or an interstitial-carbon-interstitial-oxygen CiOi complex63,65 form the core of the STDHcenter. It was later observed that VOH is a deep acceptor with alevel at EC − 0.31 eV66,67 and that VOH2 is neutral.68 The observa-tion of the STDHs in nonirradiated CZ silicon could also rule outVOH and VOH2 as the core of the STDH centers.11 In comparison,it is known that interstitial carbon Ci can be readily created due tooxygen aggregation during a thermal annealing. Indeed, the shallowdonor character of the CiOiH complex was confirmed by densityfunctional ab initio computation,65 while it was shown at the sametime that CiOiH2 is inactive. Moreover, adding n oxygen atoms tothe CiOi center yields a family of shallow donors, which is in linewith the observation of multiple related STDHs in Fig. 9 and 11 andin the literature.7-13 At the moment, this appears to be the best can-didate to explain the current observations.69-71
At this point, it is necessary to note that the hydrogen-relatedshallow donor centers can be formed also in case of H+ ion implan-tation in Si at room temperature, followed by heat-treatments at400°C. In particular, it was demonstrated that a counter doping bysuch donors of initially p-type CZ Si and in some cases p-type FZ Siand formation of n-type regions around the ion-projected rangeoccurs.72-75 Because in the case of ion implantation, interstitials andvacancies are formed and participate in all postimplantationcomplex-formation processes on postimplantation anneals, for in-stance, it is difficult to make some conclusions about the nature ofshallow donors, which appear at such conditions. It has to be men-tioned in this context that Si interstitials can be involved in theprocess of the STDH formation. One should note that in all the casesdiscussed here, in which a H-enhanced TD formation was observed,an intensive H-initiated structural defect formation in the subsurfaceSi region took place at the same time. This means that the Si bulk issaturated by Si interstitials, which were formed upon such aggres-sive plasma treatment. Details of the defect formation processes onconditions used in this work were investigated in a number of recentpublications.76-82 In particular, it was found that some specific struc-tural defects including voids, dislocation loops, platelets, etc., areformed upon H-plasma treatments and post-treatment anneals,which indicates the presence of interstitials and vacancies during theprocessing.
An indication of the importance of interstitials or vacancies inthe discussed H-related donor formation was given in Ref. 83, wherea strong donor creation as well as counter doping of p-type CZ Siwas observed upon low energy implantation of Ar+ ions at tempera-tures of 400°C. Moreover, formation of H-related donors uponH-plasma treatments was essentially accelerated if a p-type CZ Sisubstrate was subjected to a low energy implantation by Ar+ ions.
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H441Journal of The Electrochemical Society, 156 6 H434-H442 2009 H441
Thus, participation of Si interstitials in the STDH formation processstudied in this work is highly probable and has to be investigated inaddition. The infrared absorption spectra of what are believed to beinterstitial-related STDHs have recently been discussed,84 althoughthey do not match exactly the hydrogenic series reported here in Fig.9 and 11; thus, they belong to another set of H-related STDs.
It is necessary to mention that a direct proof of the oxygen in-volvement into the core of H-related donor complexes was given inRef. 23, 85, and 86, where CZ Si substrates with a denuded zoneDZ were used. It was shown that such donors cannot be formed inthe DZ region. It was therefore proposed86,87 to use the H-relateddonor depth- and lateral-resolved profiles for the characterization ofthe oxygen distribution in Si wafers. Finally, the possibility to makea graded doping by H-related STDs,88 which leads to a drift-fieldformation in the base of Si solar cells and related devices, has to bementioned, demonstrating the potential of the low temperature dop-ing method discussed in this work.
Conclusions
In summary, plasma-hydrogenation-enhanced shallow donor for-mation has been investigated for a wide range of starting materialsand process conditions. From the electrical data and the informationgathered from DLTS and FTIR/PTIS, a consistent picture hasemerged demonstrating that the role of hydrogen is more complexthan acting merely as a catalyst, which accelerates the oxygen trans-port in silicon. It is shown that at least another STD formationavenue exists, where hydrogen is an active participant in the donorcreation. As a consequence, it is also concluded that the STDHsobserved here are not some form of precursor nor a passivated OTDbut rather a different species, which may become passivated inn-type silicon.
Acknowledgments
Discussions with Professor R. Job and Professor W. Fahrner aregratefully acknowledged.
The authors assisted in meeting the publication costs of this article.
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