Determination of acceptor binding energies in ZnSe

Journal of Crystal Growth 1.17(1.992)341—347 North-Holland j

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CRYSTAL GROWTH

Determination of acceptor binding energies in ZnSe K. Hingerl, W. Jantsch, P. Juza, M. Lang, H. Sitter Institut für Experimentaiphysik, Unit’ersitdt Linz, A-4040 Linz, Austria

J. Lilja, M. Pessa Department of Physics, Tampere University of Technology, SF-33101 Tampere, Finland

D.J. As and W. Rothemund Fraunhofer-Institut für Angewandte Festkhrperphysik, D-W-7800 Freiburg, Germany

Luminescence and photoconductivity measurements were performed on MBE-grown ZnSe layers with various arsenic concentrations. Two shallow acceptor levels with binding energies of 125 and 260 meV were found. The acceptor binding energies were determined from the photoluminescence of bound excitons using Haynes’ rule, by the evaluation of donor—acceptor pair emission and from the onset of the photoconductivity signal. Increasing the As content in order to increase the number of shallow acceptors resulted in highly compensated samples. In ZnSe epilayers, which are under tensile strain due to the different thermal expansion coefficients, Haynes’ rule can be applied by evaluating the energetic difference between the heavy hole branch of the free exciton and the acceptor bound exciton.

1. Introduction

found to be 60 meV, which is in contradiction to

The preparation of p-type ZnSe is complicated. because of the scarceness of proper dopants. The use of the group V elements P [1] and Sb [2] did not result in p-type conversion, Nitrogen needs to be ionized [3] when doping is done with elementary nitrogen. On the other hand, the group I elements lithium [4—6] and

the values of 110—120 meV found in MBE [8] and LPE [10] grown epilayers. Such a difference in the binding energies would yield a factor of 3 higher hole concentration at the same dopant concentration. The study reported here describes the results of MBE growth of As-doped ZnSe films, using a conventional Knudsen-like effusion cell as an As4

sodium [7] form shallow acceptors and have met some success in doping experiments. However, Li [5] is believed to be an amphoteric dopant. In diffuses readily into ZnSe and has strong chemical affinity to all reactive gases. Doping with As in molecular beam epitaxy (MBE) [81and metalorganic chemical vapour deposition (MOCVD) [9] was more successful, and shallow acceptor levels were observed, Due to a lack of ohmic contacts on p-type ZnSe, the empirical Haynes rule is applied to determine the binding energies of acceptors. In ref. [9] the binding energy of the As acceptor was

beam source. Photoconductivity (PC), cathodoluminescence (CL), and Van der Pauw measurements were applied to characterize these samples in detail. In ZnSe/GaAs epilayers, the free exciton is split into the heavy and light hole branch due to strain in the epilayer. Tensile strain is caused by the difference in the thermal expansion coefficients of ZnSe and GaAs. The difference of the energies of the light (XIh) and heavy (Xhh) hole branches of the free exciton in luminescence spectra shows the magnitude of the tensile strain. The difference, Xhh —XIh, can reach 4—6 meV and increases with increasing growth tempera-

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Elsevier Science Publishers B.V. All rights reserved

342

K. Hingerl et a!.

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Determination of acceptor binding energies in ZnSe

ture, because the temperature difference between growth and measurement procedures is also increased. This energetic difference of 4—6 meV yields differences of 40—60 meV for the binding energies, if either the Xlh or the Xhh emission is taken as reference value in the evaluation of the data by Haynes’ rule. We show that Haynes’ rule can be used if one takes into account the energetic difference between the heavy hole free exciton and the acceptor bound exciton. This energy has to be multiplied by a factor of 10 to obtain the right acceptor binding energy.

2. Experiments procedure 2.1. Experimental setup The epitaxial ZnSe films were grown in an MBE module manufactured by DCA Instruments Ltd. (formerly Kryovac Ltd.). The films were deposited onto semi-insulating (100) GaAs substrates which were misoriented 20 towards the nearest [110] direction. The source materials were elemental Zn, Se and As, all of 6N purity. The substrate temperature was 375 C and the Zn-toSe flux ratio 1:1. The crystalline quality of seven arsenic doped samples, labelled S1,. ..,S7, prepared for the 0

present study was monitored by measuring double-crystal X-ray (Cu Kcr) diffraction rocking curves. The films were 2—3 ~m in thickness except sample Se,, which was 8 ~m thick, to test if the thickness of the depletion zone originating from the GaAs/ZnSe interface has any influence on the electrical properties. The rocking curves exhibited a full width at half maximum (FWHM) of 150 ±20 arc sec for the 2—3 ~.Lmthick layers, independent of the doping level. Sample S6 had a FWHM of 87 arc seconds. Sample Si was undoped, the others were As-doped with increasing concentration in the sequence ~2’’ ~S7~ The electrical properties of ZnSe : As were measured by the van der Pauw technique for the samples of relatively low resistivity ~ and s2), p < 1000 11 cm. For semi-insulating samples (S1— S7), a Hall structure was prepared by pho,

tolithography, and the Hall data were measured in a specially designed cryostat in connection with a constant current source capable of operating between 10 pA and 1 mA. PC was measured at liquid nitrogen temperature. Contacts were made by heating indium dots on ZnSe at 270 °Cfor 1 mm. The contacts were coated with black varnish to prevent photovoltaic effects. A Fourier—Michelson interferometer was employed to measure PC in the mid-infrared region from 100 to 600 meV. PC in the near infrared region from 600 to 1500 meV was studied with a 0.25 m grating monochromator together with a tungsten lamp and a current—voltage converter using the lock-in technique. CL was measured at 17 K by a JEOL JSM-840 scanning electron microscope equipped with a retractable Oxford CL collection and cryosystem CL-301H. The anode currents were 10 nA and the acceleration voltage was set to 15 keV. The electron beam was defocused to a diameter of 50 ~m. The equipment for analyzing and detecting CL consisted of a 0.5 m grating monochromator, a cooled photomultiplier and a phase-sensitive signal detector. A wavelength-to-energy conversion factor of 1239.5 eV nm is used, which includes the refractive index of air. 2.2 EL’aluation of the photoconductitity data .

.

.

.

.

-

The number of iontzed impurities, N which is proportional to the photo current, I, is given by dN/dt=o~N°—RpN (1) ,

with N=N+N°,

(2)

where a- denotes the optical cross section, ~ the photon flux of the lamp, R the recombination rate and p the hole concentration. The steady state solution for eq. (1) for the boundary conditions of weak illumination (o-4 -~azRp) and negligible thermal ionized acceptor density (p = N) is given by 1/2

N

=

(a-4.~N/R)

.

(3)

K Hingerl et a!.

/ Determination

As discussed in ref. [11] the optical cross section, a-, can be described as

{

3~2

(hi.’ E0) 3~’~ a- cx 0 h~

(4)

—

for for

=

hv>E hi.’
where E 0 is the binding energy of the impurity, The exponent, r, depends exact of the impurity potential. For onthethe case of aform s-like potential, r 0, and in the case of a Coulomb =

potential, r 2. The main assumption used for eq. (4) is that the electron—phonon interaction is negligible, e.g., there is no lattice relaxation, By comparing eqs. (3) and (4) one recognizes that the onset of the PC as a function of photon energy is given by the binding energy of the impurity. =

3. Results 3.1. Hall measurements Samples S1 and S2 exhibited low resistivity, For sample S~we obtained3a and net electron concena mobility, ~.i., of tration, 2/V’ n, ofs8 atX 300 1015 K. cmSample S 300 cm 3 and jt 300 cm2/V s 2athad 3 X 300 nK. The i0’~cm mobility of both samples its activation maximum 2/Vs at reached 40 K. The value ofof3000 cm ED, was calculated from the energy donors, slope in the Arrhenius plot of the carrier concen=

‘

=

of acceptor binding energies in ZnSe

343

3.2. Photoconductivity Samples S~and S2 showed no PC response, while samples S 5 and S6 were very photosensitive. The other samples were not investigated. To generate a detectable photosignal in samples S~and S6 from the acceptors the net acceptor concen3 tration must be of the order of I >< 1016 cm because the net donor concentration of the un3. We regard doped sample S1 was 8 x 1015 cm this value as a minimum for the acceptor concentration in samples S 5 and S6. The photo current of sample S5 as a function of incident photon energy is shown in fig. 1. The main observation is that the photo current changed significantly in two narrow energy intervals centered around 125 and 260 meV. These values represent the ionization energies, EA, of As acceptors. For the spectroscopic characterization of impurities it is customary to plot the optical cross section, a-, versus photon energy (see eq. (3)). The spectral dependence of the instrument background photon flux, ~(h~), was measured separately in order to correct the measured photo current for constant çb. Fig. 2 shows the variation of a- as a function of photon energy for samples S5 and S~. Abrupt changes in a- occur at 125 and 260 meV for samples S 5 and ~6’ respectively; the S260 meV structure is also discernible for sample 5. In a separate PC experiment on clean GaAs 6

to be 48 meV. The Hall data obtained for samples S 3, S4 and tration versus inverse temperature and was found S5 were difficult to interpret: Straightforward 3 evaluation of cm2/V’ the datas gave 1 xis 1012 and ~ 1000 at 300n K. It likelycm that electrons from GaAs were transferred to ZnSe because of a small conduction band offset of 30 meV [12] at the GaAs/ZnSe interface. The resulting parallel resistance probably affected these Hall data. Samples S~and S 7 exhibited such high resistivities that no reasonable Hall data could be obtained,

-~

~

3

0

1

%

i~o

200

300

400

500

Photon Energy 1meV] Fig. 1. Photo current as a function of incident photon energy for sample S5.

344

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C

/ Determination

of acceptor binding energies in ZnSe

Ut

sampteS~

0

100

200 300 Photon Energy ]meV]

~0O

sTeS5%P~S6

Photon Energy [eV] Fig. 3. Optical cross section, u, in the near infrared region as a function of photon energy for samples S~and S(.

500

Fig. 2. Optical cross section, a-, in the mid infrared region as a function of photon energy for samples S 5 and Sf,.

tional photo signal with an onset at 980 ±20 meV. Fig. 3 shows the optical cross section for this level as a function of photon energy. For comparison, the photo-current of GaAs was also measured in the same energy region. A threshold energy appeared to be 900 ±20 meV, caused by a

no dopant state could be observed in the energy region of interest, indicating that the structures seen in figs. 1 and 2 originate from ZnSe, The PC measurements on samples S5 and S5 for photon energies from 600 to 1500 meV yielded an addi-

0

/

400

k~L 450

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500

~

SA,’ ~

550

600

650

700

Wavelength Inn] Fig. 4. Cathodoluminescence for samples Si,..., S7.

K Hingerl et a!.

/ Determination

of acceptor binding energies in ZnSe

defect in GaAs. Another structure, related to

345

for the most heavily doped sample S7. Also, the recombination lines, Y0 at 477.3 and S at 497.3 nm become more pronounced, as the arsenic content is increased. Y0 is generally related to extended defects [13],whereas the S-line is correlated with a kind of donor—acceptor pair (DAP) recombination involving shallow donors and acceptors with binding energies of about 270 meV [13]. In the As-doped samples DAP lines with LO phonon replica at 457.1, 462.4 and 467.8 nm have been observed. Fig. 5 shows high resolution exciton luminescence in the range from 440 to 450 nm, Sample S7 exhibits undetectable low exciton luminescence in this wavelength range. The free heavy hole exciton line at 442.1 nm, Xhh, is present in all spectra, the XIh emission is situated at 443.0 nm. This splitting of the free exciton into XIh and Xhh occurs because of the tensile strain in the ZnSe layers caused by different thermal expan-

band-to-band transitions, occurred at 1510 ±20 meV. Because these characteristic features of GaAs were absent for samples S5 and S~we attribute the measured photo currents to ZnSe alone. 3.3. Catbodoluminescence The results of CL measurements are summarized in fig. 4. The spectra are dominated b~ exciton luminescence at 443 nm (except for sample S7). Sample S1 also exhibits a broad band, centered at 600 nm, which is attributed to selfactivated centers (SA) [11]. Another luminescence band arises at 565 nm with increasing intensity for the heavily doped samples. This deep level band most likely arises from arsenic-induced defects, as already suggested by Shibli et al. [8]. In fact, the 565 nm structure becomes dominant

12

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A ~ Xhh ~

ABE

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S5

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53~ /

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S2~

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444 446 Wavelength ]nm]

/

448

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442

—

S

2

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—

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—

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S1 450

Fig. 5. High resolution cathodoluminescence for samples Si,..., S6 in the excitonic region. Sample S7 emitted no light in this energy region.

346

K. Hinger! ci al.

/ Determination of acceptor binding energies in ZnSe

Table 1 Acceptor ionization energies obtained from photoconductivity and cathodoluminescence measurements; A

1, A-, and A5 denote the

different acceptor ionization energies Sample

Ionization energy from PC (meV)

Ionization energy from CL (meV)

Onset of PC

ABE structure

A1 S2 S4 S5

No signal 129 125 —

A2

A5

A1 —

255 260 260

120 120

—

980 980

—

sion coefficients of ZnSe and GaAs [13]. Cooling the sample from growth temperature of 350 C to the measurement temperature of 1.4 K results in a tensile strain and, therefore, in an energetic difference of 4.6 meV between the XIh and Xhh luminescence lines, which is in agreement with the experimentally found splitting of the luminescence lines. Luminescence from samples S1, ~2 and S3 is dominated by two donor bound excitons (DBEs), I~and ‘2’ situated at 443.4 and 443.7 nm, respectively. In sample S2 two acceptor bound excitons (ABEs) appear at 446,1 and 447 nm. An increase in As content causes enhancement and overlapping of these ABE features, while the two DBEs decrease in intensity. For samples S4 and S5. the ABE structures at 444.1 (Ii) and 446—447 nm are very prominent. This energetic position of an As bound exciton (‘i) has already been described in MOCVD-grown As-doped ZnSe on GaAs [141.It is important to note that the reduction in intensity of all the acceptor bound cxcitons at very high As concentrations occurs in concomittance with an enhancement of the deep level luminescence at 565 nm. By comparing our CL data with the photoluminescence data of Shibli et al. [81,one can estimate the order of magnitude of As impurities in ZnSe. It appears that sample S7 has a similar excitonic to deep level luminescence intensity ratio as that 3, of aassample deterhaving an As content of 1 X lOin cm mined by secondary ion mass spectroscopy [81. We deem this value an upper limit of impurity concentration in sample S7.

DAP structure A5

A1

250 255 250 250

115 115 115

—

280 290 285 285

4. Discussion ad conclusions

0

Samples S~and S2 are n-type. Heavily doped samples S5 and S~are p-type, as deduced from PC, but are highly compensated. It is interesting to note in this context that a large lattice relaxation is expected for substitutional As in ZnSe [15], leading to strong self-compensation. Our experiments lend qualitative support to this prediction, because we observe a remarkable enhancement of compensation in the highly doped samples. On the other hand, we are not able to determine the charge state of the As dopant. We can evaluate the acceptor ionization energies both from the PC and CL data. The values for the three acceptor ionization energies A1, A2, A3 are summarized in table 1. PC proved that there exist three acceptor levels which are located at 125 and 260 meV and an additional deep acceptor at 980 meV. It is possible to obtain the ionization energies of acceptors from ABE emission in CL as well, using Haynes’ rule [16] EA/EBX 10. Here EBX is the energy difference between the bound and the heavy hole branch of the free excitons. Applying this rule, the ABE lines at 444.1 and 446.1 nm yield EA of 120 and 250 meV, respectively. EA can be deduced also from the DAP recombination lines, using the formula EA=EG—ED—EDAP+e 2 /41rcE 1r. (5) Here EG refers to the optical band gap of ZnSe at 17 K (EG 2.82 eV); EDAP is the zero phonon =

K Hingerl at a!.

/ Determination

line energy of the DAP structures. The last term on the right-hand side takes into account the Coulomb interaction, which depends on the average distance, r, between the acceptors and donors. At high concentrations ~ 10~~ cm3) this term can be estimated to be 30 meV. In table 1 the different values for the ionization energies are summarized. From the DAP peak at 457.1 nm one obtains, with eq. (5), EA 115 ±5 meV. Applying eq. (5) to the S-line, an activation energy, EA, of 295 ±10 meV is obtained. To conclude, we have shown that Haynes’ rule can be applied also to epilayers. By comparing CL and photoconductivity data the activation energies of As in ZnSe could be determined. =

Acknowledgements This work was supported, in part, by the “Fonds zur Forderung der wissenschaftlichen Forschung in Osterreich” and the Academy of Finland.

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of acceptor binding energies in ZnSe

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[3] T. Mitsuyu, K. Ohkawa and 0. Yamazaki, AppI. Phys. Letters 49 (1986) 1348. [4] M.A. Haase, H. Cheng, J.M. DePuydt and i.E. Potts, J. AppI Phys. 67 (1990) 448 [5] J. Lilja, M. Toivonen, P. Wysocki and M. Pessa, Vacuum 40 (1990) 491. [6] J.M. DePuydt, M.A. Haase, H. Cheng and J.E. Potts, AppI. Phys. Letters 55 (1989) 1103. [7] J.E. Potts, H. Cheng, J.M. DePuydt and M.A. Haase, J. Crystal Growth 101 (1990) 425. [8] S.M. Shibli, M.C. Termago, B.J. Skromme, S.A. Schwarz, CL. Schwartz, R.E. Nahory and R.J. Martin, J. Vacuum Sci. Technol. B 8 (1990) 187. [9] M. Okajima, M. Kawachi, T. Sato, K. Hirahara, A. Kamata and T. Beppu, in: Extended Abstracts 18th Conf. on Solid State Devices and Materials, Tokyo, 1986, p. 647. [10] B.J. Fitzpatrick, C.J. Werkoven, T.F. McGee, P.M. Harnack, S.P. Jerko, RN. Bhargava and P.J. Dean, IEEE Trans. Electron Devices ED-28 (1981) 440. [11] J. Bourgoin and M. Lannoo, Point Defects in Semiconductors II, Series in Solid State Sciences 35 (Springer, Berlin, 1983) p. 113. [12] L. Kassel, H. Abad, J.W. Garland, i.E. Potts, M.A. Haase and H. Cheng, AppI. Phys. Letters 56 (1990) 42. [13] J. Gutowski, N. Presser and G. Kudlek, Phys. Status Solidi 120 (1990) ii. [14] T. Yodo and K. Yamashita, AppI. Phys. Letters 53 (1988) 2403. [15] D.J. Chadi and K.J. Chang, AppI. Phys. Letters 55 (1989) 575. [16] K. Cho, Ed., Excitons, Topics in Current Physics 14 (Springer, Berlin, 1979) pp. 77—82.