Excitation spectroscopy of thin-film amorphoussemiconductors using a free-electron laser
J. Ristein, B. Hooper,* and P. C. Taylor
Department of Physics, University of Utah, Salt Lake City, Utah 84112
Received November 28, 1988; accepted February 10, 1989
An infrared free-electron laser has been used to measure the first photoluminescence excitation spectrum well belowthe optical absorption edge in a thin-film amorphous semiconductor. This method should be useful for probingbelow-gap absorption mechanisms that contribute to photoluminescence in a wide class of amorphous semiconduct-ing films.
INTRODUCTION
Amorphous semiconductors are often placed in two differentcategories, depending on their local structural order. In onecategory, which contains primarily materials based on groupIV atoms, the local structural order is essentially tetrahe-dral. In this category the average nearest-neighbor coordi-nation number is close to four. In the second category,which contains primarily materials based on group VI at-oms, the average coordination number is usually less thanthree. A division into these two categories has no theoreti-cal justification, and in fact recent developments haveshown convincingly that both categories can be understoodwithin the framework of the same general model.",2 None-theless, there are enough empirical and practical differencesbetween these two categories of amorphous semiconductorsthat the artificial separation is often useful. -
In this paper we shall discuss only the first category, whichis by far the most commercially important at present. Ofthe tetrahedrally coordinated amorphous semiconductorsthe prototype material is amorphous silicon (a-Si). Thismaterial, which can be made only in thin-film form, is oftenalloyed with hydrogen to remove defects such as silicondangling bonds. The resulting material, called hydrogenat-ed amorphous silicon, or a-Si:H, is currently the basis formost electronic devices that employ amorphous semicon-ductors. The most important of these devices are large-areaphotovoltaic cells for use in terrestrial power generation andlarge-area arrays of thin-film transistors for flat-panel dis-plays and photocopying applications.
Electronic devices based on a-Si:H are technically feasiblebecause these films can be doped either n-type or p-type byincorporating group V or group III impurities, respective-ly.3 4 The doping efficiencies are very low (generally lessthan about 1%) but are sufficient to permit the movement ofthe Fermi level to within -0.2 eV of either band edge. Forseveral reasons, but primarily because the mobilities of thecharge carriers are low (of the order of 1 to 10 cm2 V-l sec'-),devices based on a-Si:H are slow. The major advantage ofthese devices is that they can be conveniently and inexpen-sively manufactured over large areas.
In this paper we confine our attention to undoped a-Si:H.In this material the most important defects that are deleteri-ous to device performance are probably silicon dangling
bonds, which are three fold coordinated silicon atoms atwhich the fourth unsatisfied bond contains a single electron.The energy levels of these defects occur well within theenergy gap. The optical energy gap is approximately 1.9 eVin a-Si:H, and the ionization of the singly occupied, danglingbond level (D0 ) is thought to occur 5 near 1.1-1.3 eV. Theseneutral defects can be detected by using electron-spin-reso-nance techniques, and they occur typically at densities lessthan 1016 cm-3 in device-quality films.
There exists a characteristic photoluminescence (PL)peak at 0.8 eV in a-Si:H whose intensity scales with thesilicon dangling bond density as measured by electron spinresonance. For this reason, among others, the PL at 0.8 eVhas been attributed to the D' defect, and the most commoninterpretation of the radiative transition is between an occu-pied localized state at the edge of the conduction band and asingly occupied silicon dangling bond. A schematic diagramof the electronic density of states in a-Si:H is shown in Fig. 1.The energy E (Ec) represents the demarcation below(above) which the electronic states are extended and above(below) which the states are localized. The approximatevalues for the mobility gap (EC - Ev) and the approximateposition of the dangling-bond defect (D°) within the gap areshown in this figure. The doubly occupied dangling-bondlevel (D-), which is also shown, lies above the D° level be-cause of electronic correlation effects.
Many of the technologically important electronic proper-ties of a-Si:H are dominated by the presence of these D°defects with electronic levels near the middle of the energygap. Although as described above the most common inter-pretation of these defects is in terms of the silicon danglingbonds,6 at least one other interpretation has been proposed.7
According to this interpretation the D° defect is in fact anunpaired electron located around a fivefold coordinated sili-con. This defect has been called a floating bond. Becausethe experiments to be discussed in this paper do not distin-guish between these two interpretations, we shall use theterms D° or dangling bond to represent the generic deep gapdefect in a-Si:H.
In addition to the PL band at 0.8 eV, which we havealready described, there also exists a PL band that peaks at1.2-1.4 eV. In good, device-quality films of a-Si:H it is thelatter peak that dominates the PL processes at low tempera-tures. This higher-energy peak is usually interpreted as due
Fig. 1. Schematic diagram of the density of electronic states in a-Si:H. E and E, represent the valence and conduction band mobil-ity edges, respectively. The defect states D+, Do, and D- representdifferent charge states of the silicon dangling-bond defects as de-scribed in the text.
to recombination of electrons in localized conduction bandtail states with holes in localized valence band tail states.
For the experiments described in this paper, the higher-energy PL process is efficiently filtered out so that only thePL at 0.8 eV is measured. Because both PL bands in a-Si:Hare broad, one must be careful to ensure that the peak cen-tered at 0.8 eV dominates over any low-energy tail of the 1.4-eV PL that may exist at 0.8 eV. Since the evidence is atpresent inconclusive,8 we shall examine both possibilities inthe Discussion section.
In PL excitation (PLE) spectroscopy the PL intensityobserved at a specific energy (in our case -0.8 eV) or range ofenergies is plotted as a function of the energy of the excita-tion source. For the 1.4-eV PL in a-Si:H, excitation spectrawere measured previously 9 -"1 for energies above approxi-mately 1.3 eV. Typical PLE spectra for this higher-energyPL process in a-Si:H are shown in Fig. 2. Note that the PLEfalls off exponentially below -1.7 eV owing to the exponen-tially decreasing value of the absorption coefficient in thisregion. 9-"1
EXPERIMENTAL DETAILS AND RESULTS
Samples of a-Si:H were made by the standard glow-dis-charge deposition technique.'2 For this experiment a sam-ple made at the University of Marburg on a roughened-quartz substrate was employed. This particular sample,which was 8 Am thick, was used in order to increase theabsorption of light at energies well below the absorptionedge.
The PLE experiments were performed using the StanfordMark III free-electron laser'3 (FEL), which is currently tun-able in the fundamental wavelength from -2.0 to -5.0 gm.The pulse structure of the Mark III FEL is shown schemati-cally in Fig. 3. Micropulses of -1-psec duration are repeat-ed every 350 psec for approximately 2 jtsec. (For technicalreasons the pulse structure is slightly more complicated thanthat indicated in Fig. 3.) This macropulse structure is re-peated at approximately every 67 msec (15-Hz repetitionrate). Peak powers in the micropulses ranged from -0.15 to
-1.5 MW over the wavelengths used in these experiments(2.0-2.46 Asm), and average powers in the macropulsesranged from -1 to -13 kW.
Our measurements, which covered the range from 1.0 to1.23 um (1.01 to 1.24 eV), were performed using a lithiumniobate crystal to generate second harmonics. For the fre-quency-doubled light the peak powers in the micropulsesranged from -20 to -250 kW, and the average power in themacropulses ranged from -0.2 to 2 kW. By using thissystem, micropulse peak-power densities in the frequency-doubled light at the a-Si:H sample of -1 MW/cm2 weregenerated. These high powers were necessary because theabsorption in the thin film is very low (ad < 10-2, where a isthe absorption coefficient and d the sample thickness) atthese wavelengths and the PL efficiency is probably also low(- < 0.1) at the measurement temperature (T 100 K).Hence these experiments take advantage of the tunabilityand high power of the Mark III FEL.
The PLE experiments were performed using the experi-mental arrangement shown schematically in Fig. 4. A heli-
V-
Uz
UCf,LU1
9
100
I0
1.0
0.11.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
EXCITATION ENERGY (eV)
Fig. 2. PLE spectrum for PL centered at 1.4 eV in powderedsamples of a-Si:H at 77 K. Two of the samples are made by theglow-discharge technique (circles and squares); the third sample(triangles) was made by sputtering. (After Ref. 11.)
(a) FEL Micropulse
1 psec 350psec
(b) FEL Macropulse
2ec 67,js msec -1
Fig. 3. Schematic diagram of micropulse and macropulse structureof Mark III FEL.
1
a- Si: H77K
Wooer
Ristein et al.
Vol. 6, No. 5/May 1989/J. Opt. Soc. Am. B 1005
Fig. 4. Block diagram of the experimental arrangement for measuring PLE spectra using the Mark III FEL. Symbols: HeNe, 4-mW cwhelium-neon laser; Ch, mechanical chopper; (M) movable mirror; DC, lithium niobate doubling crystal; P, polarizer; CF, color filters; BS, beamsplitter; PyD, pyroelectronic detector; S, sample of a-Si:H; Cr, cryostat; L1, L2, collimating lenses; Mono, monochromator; Ge D, germaniumdetector; P S D, lock-in amplifier; Rec, chart recorder; Av, signal-averaging oscilloscope; FEL Trig, FEL klystron. See text for detailsconcerning specific elements.
um-neon alignment beam was employed to set up the opticalpath using visible light. This light was also employed toproduce PL in the a-Si:H film and to optimize the detectionoptics. A mechanical chopper was employed to facilitatephase-sensitive detection of the resulting PL signal.
A beam splitter was employed to reflect -10% of the inci-dent infrared light onto a pyroelectric detector. This detec-tor served as a monitor of the incident laser power. Anaveraging storage oscilloscope was employed to measure theincident power. This arrangement allowed for correctionsdue to long-term (>30-sec) drift in the FEL output.
The PL was collected using an ISA model HR-320 1/4-mmonochromator with a thin (-1-mm) germanium filter overthe entrance slit. The combination of germanium filter anda germanium p-i-n detector (North Coast Model 817P) re-sulted in a response only at -0.79 eV with a half-width of-0.02 eV. The monochromator slits were set such that thelight energies collected during the experiment were 0.79 i0.002 eV.
Phase-sensitive detection was employed, using standardlock-in techniques with the reference channel of the lock-intriggered by the trigger for the FEL klystron. Two filterswere employed to ensure that higher-order harmonics werenot incident upon the a-Si:H film. These two filters hadshort-wavelength cutoffs near 850 nm. Several tests wereperformed to ensure that no higher-order light reached thesample. Output from the lock-in amplifier was recorded ona chart recorder. This output was correlated with any driftsin the input FEL power as monitored on the averaging oscil-loscope.
The sample was mounted on a cold finger in a liquid-nitrogen-cooled Dewar. The estimated temperature at thesample was -80 K, but it was certainly <100 K. A thermo-couple on the cold finger next to the sample suggested atemperature of 80 K, but no precise temperature measure-ments were made. The small absorption coefficients for theinfrared light suggest that there was little heating due to theincident light.
PLE spectra were obtained by measuring the PL intensi-ties at 0.79 eV as a function of the wavelength of the FELexcitation light. At each wavelength the PL intensity wasmeasured as a function of excitation power to ensure that theresulting excitation spectrum was not distorted by differentlaser powers at different wavelengths. The results of theseexperiments are shown in Fig. 5. At all wavelengths the PLintensity scaled with incident laser power I as IJO8, indepen-dent of wavelength. The PLE spectrum was compiled bycorrecting for incident power, using the data of Fig. 5.
The resulting excitation spectrum for PL at 0.79 eV in a-
1.0
0.5
'Zi
'U
Q
U,C.)
Ca,
-j
0.2
0.1
0.05
0.02
0.01
10 20 50 100 200
FEL Micropulse Peak Power (kW)
Fig. 5. Intensity of PL as a function of incident, frequency-dou-bled FEL power at various laser frequencies. Straight lines are aidsto the eye. A peak power of 10 kW corresponds to an average powerdensity on the sample of approximately 1 W.
Ristein et al.
1006 J. Opt. Soc. Am. BNol. 6, No. 5/May 1989
1.0
4-
Q)
.q)
U,
QC.)
U,c
E
0.5
0.2
0.1
0.05
0.02
0.01 V I ' I I I I
1.0 1. 1.2
Excitation Energy (eV)
Fig. 6. PLE spectrum for a-Si:H at -80 K. Experimental pointsare indicated by error bars. The dashed error bar indicates errorsdue to long-term drift between the first and last points taken. Thesolid curve is an aid to the eye. The dashed curve shows the energydependence of the optical absorption (arbitrary units).
Si:H is shown in Fig. 6. Error bars indicate estimated uncer-tainties in the data. The dashed-line error bar at 1.2 eVindicates errors due to long-term drift in the system duringthe course of the experiment (16 h). Both the first and thelast data points in the experiment were taken at 1.2 eV; thesolid curve represents the first point taken and the dashedcurve the last point taken. The dashed curve in Fig. 6represents the energy dependence of the absorption coeffi-cient in this film. Actual values of the absorption coeffi-cient a over the range are of the order of 1 cm-' in filmsgrown on smooth substrates. Films grown on roughenedsubstrates have effective values of a of at most an order ofmagnitude larger.1 4
DISCUSSION
The excitation spectrum in Fig. 6 has two distinguishingfeatures, an exponential rise at lower energies followed by aleveling off above -1.15 eV. Although the precise function-al form of the data at low energies is not uniquely deter-mined, the data are consistent with an exponential risewhose slope is the same as that measured in optical absorp-tion at higher energies [i.e., PLE cc exp(E/Ea), with E0 - 0.05eV].
The PLE spectrum for the PL peak at 1.4 eV, as shown inFig. 2, also has a slope identical with that of the opticalabsorption edge. The slope of the optical absorption isdetermined by a combination of the empty conduction bandtail states and the filled valence band tail states, which areshown schematically in Fig. 1. Because both densities ofstates are exponential and because the valence band tail isconsiderably broader than the conduction band tail (see Fig.1), the slope of the optical absorption edge is in large part ameasure of the valence band tail.15
Thus the 1.4-eV PL in a-Si:H exhibits a PLE spectrumover the range from -1.3 to -1.7 eV that reflects the influ-
ence of the valence band tail (Fig. 2). Over this energy rangethe PLE spectrum for the 0.8-eV PL probably also exhibits asimilar shape, although it was not measured in the datashown in Fig. 1. In both cases the reason that the PLEspectra track the absorption coefficient over this range isthat the absorption of a single photon creates both an elec-tron and a hole and the PL is proportional to the number ofelectrons and holes. The absorption process is the usualphotoexcitation of an electron from the valence band intothe conduction band. For the PL at 1.4 eV the recombina-tion process involves electrons in conduction band tail statesand holes in valence band tail states. For the PL at 0.8 eVthe recombination process6 is thought to be the radiativecapture of an electron into a DI state (see Fig. 1).
For optical excitation at energies below the band gap adifferent absorption process is required. Because the ener-gies are too small to excite directly across the gap with anyefficiency, excitation through the manifold of defect states(D+, Do, D-) is necessary. These absorption processes, ingeneral, involve two steps and as such should be expected toshow a superlinear dependence on excitation intensity. Infact, in experiments performed with a cw YAG laser at 1.17eV the PL above 1.2 eV is observed with a superlinear powerdependence that demonstrates the two-step excitation pro-cess in this case.
Because the modified D states have a finite density('10'5 cm-3), the PL that results from two-step excitationprocesses that proceed through these states should be ex-pected to become linear in excitation intensity at high pow-ers. This dependence at high powers is expected becausethe excitation into the existing defect states saturates. Thissaturation should affect both the 1.2- and the 0.8-eV PLprocesses.
The most logical explanation for the exponential decay ofthe excitation spectrum in Fig. 6 below -1.15 eV is the cutoffof excitation of holes into the valence band tails from the D+level for energies below 1.1 eV. (The D+ level is shown asbetween 0.9 and 1.1 eV above the valence band mobility edgein Fig. 1. Our argument will work if the level is really at-1.15 eV in Fig. 1.) This rapid decrease in the production ofholes in the valence band for energies below -1.15 eV ex-plains quite naturally why the 1.2-eV PL process should falloff since the process involves the recombination of electronsin the conduction band tail with holes in the valence bandtail. Although the argument is more complicated,8 the cut-off of the production of holes in the valence band tail can alsoexplain a decrease in the 0.8-eV PL if that process is theresult of the recombination of electrons in the conductionband tail with Do states.
Without specifying the absorption processes in detail, sev-eral general comments can be made. First, if the 0.8-eV PLis the capture of an electron into a Do state, then one musthave both the production of electrons in the conductionband tail in order to initiate the 0.8-eV PL transition and theproduction of holes in the valence band tail in order toprovide the holes necessary for the higher-energy PL (-1.2eV), which is also observed with below-gap light (in the cw,low-power experiments). Thus on these grounds one musthave both excitation of electrons from the valence band tailinto DI (or D+) states and excitation of electrons from DO (orD-) into the conduction band tail. The transitions involv-ing D+ or D- states may be enhanced owing to increases in
Ristein et al.
Vol. 6, No. 5/May 1989/J. Opt. Soc. Am. B 1007
absorption cross section that usually occur for charged cen-ters.'6 As mentioned above, to the extent that the exponen-tial portion of the PLE spectrum of Fig. 6 is representative ofthe density of states in the valence band tail, transitions ofelectrons from the valence band tail into D+ states are likelycandidates to explain the observed excitation spectrum.
The linear absorption spectrum, on the other hand, con-tinues down well below 1.15 eV. This behavior is consistentwith the description above for the PL excitation mecha-nisms because the absorption is dominated by the excitationof electrons into the conduction band. From Fig. 1 one cansee that this process should decrease only below -0.8 eV.
SUMMARY
An infrared FEL has been employed for the first time to ourknowledge to measure the excitation spectrum for PL in thinamorphous films at energies where the optical absorption isvery weak (ad = 10-4). The measured PLE spectrum in a-Si:H exhibits an exponential falloff below -1.15 eV. ThisPLE spectrum is consistent with existing models for the PLprocess that attribute this radiative recombination to transi-tions between electrons in the conduction band tail statesand silicon dangling-bond defects.
ACKNOWLEDGMENTS
The authors are grateful to the Stanford Photon ResearchLaboratory for use of the Mark III free-electron laser facilityat which these measurements were taken. We particularlyacknowledge experimental guidance and support from S.Benson. We thank W. Fuhs and F. Finger of the Universityof Marburg for supplying the sample of a-Si:H employed inthis study. The authors are also grateful to S. Gu for helpwith preliminary experiments. This research was support-
ed by the Solar Energy Research Institute under subcon-tract XM-5-05009-2, by the U.S. Office of Naval Researchunder contract N00014-86-K-0710, and by the Laser Insti-tute at the University of Utah.
* Present address, Department of Physics, Duke Univer-sity, Durham, North Carolina 27706.
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