Systematic investigation of shallow acceptor levels in ZnSe

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C R Y S T A L G R O W T H

ELSEVIER

Journal of Crystal Growth 138 (1994) 310-317

Systematic investigation of shallow acceptor levels in ZnSe Y. Z h a n g a, W. Liu a, B.J. Skromme * 'a, H. Cheng b, S.M. Shibli c,~, M.C. Tamargo c,2 " Department of Electrical Engineering and Center for Solid State Electronics Research, Arizona State University, Tempe, Arizona 85287-5706, USA ~' 3M Company, 201-1N-35, 3M Center, St. Paul Minnesota 55144, USA c Bellcore, 331 Newman Springs Road, Red Bank, New Jersey 07701, USA

Abstract A systematic investigation of shallow acceptor levels in ZnSe grown by molecular beam epitaxy (MBE) has been performed using low temperature photoluminescence (PL) measurements as a function of excitation level, tempcrature, strain, and laser energy (i.e., selectively excited donor-acceptor pair luminescence or SPL). Five of the levels are due to N, Li, As, P, and O, while the chemical origins of two levels, denoted A] and A 2, have not yet been determined. The A 1 level is observed in undoped material after annealing using diamond-like C (DLC) caps, while the A 2 level is observed in nominally Na-doped material. The ionization energies of these levels are accurately determined from the temperature dependence of the band-to-acceptor ( e - A °) peak positions, accounting for strain. Those energies are 114.3 _+ 0.5, 114.2 _+ 0.3, 111.3 + 0.5, 106.1 + 0.6, 95.0 _+ 0.4, 88.4 + 0.5, and 83 _+ 3 meV, respectively, for As, Li, N, A], A2, P, and O in unstrained material. Several excited states have been observed in SPL measurements for As, A2, O, and P for the first time. The excited states of As, O, and A 2 fit well to effective mass theory, while those for P do not. A model for the strain splitting of shallow acceptor-bound excitons has been developed and confirmed using measurements on samples whose substrates have been removed. Haynes's Rule is shown to be inapplicable to shallow acceptors in ZnSe. A strain splitting of the ( e - A °) peak for As or Li acceptors in annealed material is clearly resolved and modeled.

I. Introduction R e m a r k a b l e p r o g r e s s has b e e n m a d e in the last few y e a r s in t h e p - t y p e d o p i n g of Z n S e a n d r e l a t e d m a t e r i a l s , p a r t i c u l a r l y with r e g a r d to t h e use o f N p l a s m a d o p i n g sources d u r i n g M B E g r o w t h [1,2]. H o w e v e r , a f u n d a m e n t a l u n d e r -

* Corresponding author. i Present address: Instituto de Fisica Gleb Wataghin, UNIC A M P 13081, C a m p i n a s / S P , Brazil. 2 Present address: D e p a r t m e n t of Chemistry, City College of the C U N Y , New York, New York 10031, USA.

s t a n d i n g o f the p a s t difficulties in d o p i n g this m a t e r i a l , a n d a d e t a i l e d k n o w l e d g e o f the behavior o f various a c c e p t o r d o p a n t s has not yet b e e n achieved. M o r e o v e r , p - t y p e d o p i n g levels n e e d to be f u r t h e r i m p r o v e d in b o t h m e t a l o r g a n i c chemical v a p o r d e p o s i t i o n ( M O C V D ) a n d m o l e c u l a r b e a m epitaxy ( M B E ) m a t e r i a l . In t h e following, we discuss s o m e p r o g r e s s in t h e s e directions, b a s e d on a study of M B E m a t e r i a l d o p e d with a total of seven d i f f e r e n t a c c e p t o r species. A highly a c c u r a t e a n d r e l i a b l e m e t h o d of d e t e r m i n i n g acc e p t o r b i n d i n g e n e r g i e s in Z n S e is d e s c r i b e d , a n d t h e n a p p l i e d w h e r e v e r possible to t h e s e levels. T h e p r o b l e m s with p r i o r m e a s u r e m e n t tech-

0022-0248/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 0 2 4 8 ( 9 3 ) E 0 7 5 1 - R

Y. Zhang et al. /Journal of Crystal Growth 138 (1994) 310-317

niques are briefly discussed for comparison; in particular, we emphasize the lack of validity of Haynes's Rule type of relationships for shallow acceptors in ZnSe. Two as-yet unidentified acceptor levels are observed and characterized; at least one of these may have significant future potential as a dopant. The excited states of several of these levels are determined and compared to effective mass theory. The effects of strain are carefully examined and shown to have important effects on both acceptor-bound excitons and band-to-acceptor transitions.

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The growth and doping of the N-, Li-, As-, P-, O-, and Na-doped MBE-grown ZnSe/GaAs samples used in this study is described in refs. [2-7], respectively; the capping and annealing procedures used to introduce the "A]", acceptor discussed below are described in ref. [8]. The PL system used in this work is described in ref. [9].

2.1. Accurate determination of acceptor binding energies in ZnSe Our method of determining acceptor ground state binding energies is similar to that which has been used in many III-V materials, and is based on the measurement of conduction band-toacceptor (e-A °) peak positions in low temperature PL spectra at various temperatures. This method has rarely been applied in ZnSe, perhaps because of concerns voiced in ref. [10] concerning the influence of acoustic phonon coupling on the (e-A °) lineshape, or simply because the (e-A °) peaks have never been detected at liquid He temperatures in bulk or homoepitaxial ZnSe, which is usually n-type. A further complication in heteroepitaxial material is the necessity to account for the effects of strain, which we show how to do in the following. Our study of the (e-A °) peaks as a function of temperature, e.g. in Li-doped MBE material [9], has shown that these peaks are in fact frequently observable at low temperature in p-doped MBE samples, even though they have not always been identified as

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Fig. 1. PL spectra under low excitation as a function ot temperature for a high resistivity Na-doped Z n S e / G a A s layer (see ref. [7]) grown at 260°C with a S e / Z n beam equivalent pressure ratio of four and a Na K n u d s e n cell temperature o! 125°C. The elemental Na source was 99.95 + % pure, and the Na concentration in the sample is 3.0 x 1017 cm -3. Instrumental resolution is 0.5 meV.

such in the past. We have also shown that the widths of these peaks agree reasonably well up to about 45 K with the theoretical 1.8kBT dependence (k B is Boltzmann's constant and T the absolute temperature) expected from Eagles' theory of (e-A °) lineshapes, even though this theory ignores phonon coupling [11]. To illustrate the method, we show a set of PI_ data as a function of temperature in Fig. 1 for an MBE sample which was doped with Na [7]. The possible origin of this level (which might not involve Na) is discussed below in section 2.3. In Fig. 1, we clearly observe both the (D°-A °) and the (e-A °) peaks associated with this level at 2.7086 + 0.0004 (1.7 K) and 2.7262 + 0.0004 eV (20.1 K), respectively. The quenching of the (D O_ A°) peak involves the thermal ionization of the shallow donor levels into the conduction band, while the broadening of the (e-A °) peak at high T is due to the kinetic energy distribution of

Y. Zhang et al. /Journal of Crystal Growth 138 (1994) 310-317

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T (K) Fig. 2. Position of the ( e - A °) peak in the sample of Fig. 1 as a function of temperature. The filled triangles are the raw values, and the open circles are corrected for the temperature dependence of the band gap. The solid line is a least squares fit to the corrected data. The error bars (shown only on the corrected data for clarity) are "worst case"; the scatter in the data and results of various fitting procedures used to determine peak positions suggest that the true errors are significantly less than those indicated.

electrons in the conduction band. We note that a trace of the ( e - A °) peak is detectable even at 1.7 K, similar to the previously discussed case of Z n S e : L i [9]. The initial anomalous quenching of this peak from 1.7-10 K is similar to that we observed for ZnSe : Li, and can been explained in terms of temperature-dependent non-radiative recombination rates [9]. The identification of the ( e - A °) peak in this sample has been confirmed using intensity-dependent measurements, and also with magnetospectroscopy in the similar case of ZnSe : Li [9]. The position of the ( e - A °) peak is plotted as a function of lattice temperature (which we assume to equal the electron temperature for the low excitation conditions used in this work) is plotted in Fig. 2 for the data of Fig. 1. Both the raw data and those corrected for the temperature dependence of the band gap determined from the bound exciton peak positions as discussed in ref. [9] are shown. The solid line is a linear fit to the corrected data, which has an intercept of 2.7257 eV and a slope of 0.045 m e V / K , in good agreement with the theoretical value [11] of 0.5k B = 0.043 m e V / K . The use of this linear fitting procedure reduces the random error associated with the

effects of noise on peak position determination by a factor of N 1/2 for the final intercept, where N is the number of data points ( N = 9 in Fig. 2), yielding a conservatively estimated uncertainty of less than +0.00017 eV in the intercept. The uncertainty in Eg(T) introduces some additional error (about _+0.0002 eV, based on Eg(T) data for 24 samples), but this error is systematic rather than random and therefore affects the slope more than the intercept, which varies by no more than + 0.0001 eV for various Eg(T) fits. The intercept is just Eg(T = 0 ) - E A , where E g = 2.8218 eV at T = 0 is the band gap of unstrained ZnSe, and E a is the ground state acceptor binding energy. Uncertainty in Eg(T= 0) (probably +0.0005 eV) affects the E A values obtained from our analysis, but does so equally for all acceptors and does not influence the overall precision, which is about + 0.0002 eV. The above analysis would apply directly to homoepitaxial or unstrained bulk material. However, it does not account for the effects of thermal expansion mismatch strain on this relaxed, heteroepitaxial material. The biaxial tensile strain in this type of sample has the well-known effects of reducing the gap and of splitting the valence band into separate light and heavy hole bands, which is reflected for example in the splitting of the free exciton peak [12]. This shift and splitting are easily resolved and accounted for in the excitonic PL and reflectance spectra of this sample (not shown). In addition, the acceptor level itself is expected to split into light and heavy hole components, which can be estimated in the framework of perturbation theory [13]. Since this expected splitting is not resolved in the data of Fig. 1, it is necessary to determine whether the observed peak positions correspond to the light hole level, the heavy hole level, or an average of the two. We argued in the case of Li [9] and As [14] acceptors that the heavy hole component is usually dominant, especially at about 10 K and above. This is so because the splitting is relatively small, so that the effects of thermalization into the light hole state are generally outweighed by the three times larger oscillator strength of transitions involving the heavy hole ( ] m j ] = 3 / 2 ) states. A splitting similar to the expected strain

Y. Zhang et al. /Journal of Crystal Growth 138 (1994) 310-317

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splitting has even been resolved in the case of the ( e - A °) peak involving P acceptors [15]. However, the possibly axial symmetry of the rather unusual (see below) P-related center makes the origin of the splitting a little ambiguous in that case. Fig. 3 provides data that validate our assumption unambiguously. This figure displays PL spectra as a function of t e m p e r a t u r e for an undoped MBE layer that was annealed under a Zn overpressure for 30 min at 500°C in a "leaky tube" diffusion system designed for Zn diffusion into I I I - V materials [16]. An unintentional result of this particular anneal was the introduction of an acceptor level due to Li or possibly As (which have virtually indistinguishable binding energies, as discussed below). A second effect of the anneal was an increase over the normal amount of thermal tensile strain in the material, due to relaxation of the increased thermally-induced mismatch at the annealing temperature. The increased strain produces a larger than " n o r m a l " splitting in the ( e - A °) peak, which is clearly resolved in the 1.7 K PL spectrum. The observed

splitting is 2.0_+ 0.4 meV. Given the observed free exciton splitting of 3.8 +_ 0.15 m e V in this sample, the ratio of the experimental splittings yields the shear deformation potential of the acceptor-bound hole as b ' = - 0 . 5 6 _+ 0.11 eV. This value is in fair agreement with the theoretical value of b ' = - 0 . 8 1 eV, calculated using the theory of ref. [13] and the parameters in ref. [9]. Even at 1.7 K, the higher energy heavy hole component is dominant (which should be even more true in cases with smaller, unresolved strain splittings). The peak that grows to dominance at high T can be definitely traced to the heavy hole rather than to the light hole component at low T. To our knowledge, this is the first directly resolved observation of the strain splitting of an acceptor level in the PL spectrum of any compound semiconductor. We also note the observation of a lower energy doublet (e.g. at 2.7000 and 2.7020 _+ 0.0004 eV at 20 K), which we tentatively attribute to recombination between donors in their first excited state and holes on the light and heavy hole acceptor levels. This excited-state donor-to-acceptor recombination mechanism has been previously reported in G a A s and InP [17]. Based on the above experimentally determined shear deformation potential, we can return for example to the data of Fig. 1 and estimate a 1.6 _+ 0.3 m e V splitting of the acceptor level from the observed 2.8 m e V X h h / X i h splitting in that sample. The X Lh peak position in this sample is shifted 3.2 m e V to lower energy than the bulk value, implying a light hole band gap E~ h = 2.8186 eV. The intercept observed above then implies that (Eglh --EAhh) = 92.9 _+ 0.2 meV, which is added to the 1.5 m e V (EAh h - E ~ h) splitting to yield the binding energy of the light hole acceptor level in the strained material as 94.4 _+ 0.4 meV. Finally, the binding energy in unstrained material can be calculated following ref. [9], using the experimental b' value given above, to yield 95.0 _+ 0.4 meV. The latter value is the one we quote as characteristic of the level, since the strain varies in different types of material. Using the same procedures, we have determined the binding energies of six different levels, which are listed in Table 1. (The value for O is based on data at only two temperatures, which were fit

Y. Zhang et aL /Journal of Crystal Growth 138 (1994) 310-317

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Fig. 4. Representative PL spectra at ~ 25 K under low level UV excitation for a series of MBE ZnSe/GaAs samples doped with different acceptors (five samples) or annealed after growth (one sample). Both ( D ° - A °) and ( e - A°) peaks and their phonon replicas are visible in each case.

with a fixed slope equal to the theoretical value. A different value of b ' / b = 0.84 was used for the P acceptor, based on the directly resolved splitting of this level.) Where reliable previous estimates are. available for a given level, they are generally in good agreement with these values, and where we obtained good quality data for the same acceptor in several samples, the spread among values obtained from them is within the quoted error limits. To further illustrate the type of data on which these values are based, we show PL spectra in Fig. 4 for samples doped with each of those six levels at a fixed temperature of 25-26 K, where both the (D O - A °) and ( e - A °) peaks are visible in every case. Full temperaturedependent data, however, were always used in the determination of E A. Previous determinations of acceptor binding energies in ZnSe have generally been based on other techniques, such as: (1) estimates based on ( D ° - A °) peak positions at some fixed excitation

315

level, arbitrarily assuming a value for E D and the Coulomb term, (2) estimates based on the localization energy of the neutral acceptor-bound exciton (I l) peak using "Haynes's Rule" [18], (3) fits to the positions of discrete d o n o r - a c c e p t o r pair lines in bulk or homoepitaxial material, with assumed values of the dielectric constant, or (4) estimates based on measurements of the groundstate-excited state separation energies, in conjunction with effective mass theory. The first method is subject to considerable uncertainty in the value of the Coulomb term, which generally depends on the excitation level, the doping level, and the non-radiative recombination rates; the particular donor ionization energy to use in a given sample is also usually unknown. The second method is completely unreliable to distinguish between different shallow acceptors in ZnSe, as we have pointed out previously [9]. The reason is that the exciton localization energy is not a linear function of acceptor binding energy in this (or most other direct gap) materials, contrary to the very early claim by Halsted and Aven [19]. The third method can yield reasonable results in bulk material if the appropriate dielectric constant is well known and a good quality fit to well-resolved data can be obtained. However, the quality of discrete pair line spectra is rarely adequate in modern heteroepitaxial material, and our method is more reliable in that it does not depend on the dielectric constant. The fourth method is subject to several problems, including uncertainties in the valence band parameters, the question of how to incorporate q-dependent screening and central cell "corrections" into effective mass theory, and the accuracy of the theory in general. We conclude that the present method is the best available one to determine binding energies in an accurate and reproducible way. Uncertainties in Eg do affect the results, but do so equally for all acceptors. 2.2. Excited states of shallow acceptor levels in ZnSe We have determined the energy separations between the ground state and several excited states for several acceptors, including four that

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Y.. Zhang et al. /Journal of Crystal Growth 138 (1994) 310-317

have not been studied in this way previously. The experimental method is selectively-excited don o r - a c c e p t o r pair luminescence (SPL) [20]. The experimental data will be presented elsewhere (see ref. [14] for the As case). The results are summarized in Table 1. Strain splittings are generally expected in the data and some splittings were experimentally resolved, but the values in Table 1 are not corrected for strain and represent average values in strained material. The identifications of particular states are based on comparisons with effective mass theory, and on the theoretical expectation that transitions involving even parity states should be the strongest ones in the spectrum. Calculations using effective mass theory [21], with the Luttinger p a r a m e t e r s of ref. [22] and an empirically adjusted dielectric constant, were performed to compare to the data; details will be presented elsewhere. The ls central cell correction of one acceptor was taken as an adjustable parameter, and the correction was assumed to be zero for the p-states and scaled according to the probability density at the origin for the higher s wave functions. However, it was not possible to achieve a good fit to all of the acceptors simultaneously, even by adjusting the Luttinger parameters. Separate adjustment of the dielectric constant and central cell p a r a m e t e r for each acceptor was however quite successful, except in the case of P. The excited states we observed for this acceptor do not seem to follow effective mass theory at all, due perhaps to the possibility that this level may involve a complex.

( D ° - A °) peak similar to the one observed here and in ref. [24], but did not observe the ( e - A °) peak that allows us to evaluate its binding energy accurately. Other alkali metals such as K are common contaminants in Na [25], and could be the origin of this level. Further work is in progress to determine if the level in fact involves Na, K, some other impurity, or a complex. The A 1 acceptor level described in Table 1 was originally observed in ref. [8], although it was mistakenly identified there (on the basis of the I peak position) as Li. The insensitivity of I~ localization energy with respect to E A makes the I~ peak position an unreliable means of identifying acceptors, especially in strained material. This level is observed only in samples that have been subject to a rapid thermal anneal after encapsulation with a diamond-like C film, as described in ref. [8]. Otherwise identical anneals using other capping materials such as SiO 2 and SiN x never produced this level, and it has not been previously reported in the literature, to our knowledge. A 5 s anneal at 500°C produced relatively weak PL features related to this acceptor, while 5 s anneals at 550 and 600°C produced stronger peaks. These results imply that the impurity in question diffuses reasonably rapidly into ZnSe, but not so rapidly at low t e m p e r a t u r e that it is likely to be unstable (like Li) as a dopant. The properties of this impurity suggest that it might be very useful as a new p-type dopant in ZnSe and related materials, once it has been chemically identified. Work is currently in progress toward this objective.

2.3. Observation of new shallow acceptor levels

2.4. Strain splitting of the neutral acceptor-bound exciton emission in ZnSe

The " A 2 " acceptor level observed in the Nadoped M B E sample of Figs. 1 and 2 is not the ~ 124 m e V level usually assigned to Na in bulk material [23], although it may be the same as the one observed in ref. [24] and speculatively attributed there to a Nazn-Vse complex. Since only a donor-to-acceptor ( D ° - A °) peak was observed in ref. [24], whose position can vary considerably with excitation and doping levels, we cannot be certain if it is the same level. Previous studies of Na-doped M B E material [7,25] also reported a

Since we discussed the strain splitting of the ( e - A °) peak above, we mention here that we have developed a detailed model of the strain splitting of the 11 peaks in ZnSe, including the effects of h o l e - h o l e exchange interaction and crystal field splitting (the effects of the e l e c t r o n hole exchange interaction are assumed to be negligible) [9,14]. The model has been confirmed using samples whose substrates have been selectively removed to release the thermal strain [15].

Y. Zhang et al. /Journal of Crystal Growth 138 (1994) 310-317

A more detailed discussion will be given elsewhere.

3. Conclusions Using variable temperature PL measurements and the SPL technique, we have been able to show the existence of seven distinct shallow acceptor levels in ZnSe, not counting the previously investigated ~ 124 meV Na level [23] and the ~ 56 meV "S" level [26]. The binding energies have been accurately determined for all of these levels using temperature-dependent conduction band-to-acceptor PL peak positions. The investigation of the excited states using SPL has shown that As, Li, N, and the "A2" level are all effective mass-like, while the more unusual shallow P-related acceptor is not. Further work is needed to understand the fundamental reasons why As and P have not yet produced p-type conductivity in spite of their shallow levels, and to determine the way in which O incorporates to produce a shallow acceptor level. It is also important to identify the chemical origin of the A 1 and A 2 levels. In particular, there seems to be a significant probability that the A 2 level might be a useful new p-type dopant. Finally, the effects of strain on the acceptor-related PL features in heteroepitaxial material have been observed and successfully modeled.

4. Acknowledgments The ASU portion of this work was supported by the National Science Foundation under Grant No. DMR-9106359. We thank R. Roedel for the use of his leaky-tube diffusion system for the conventional furnace anneals.

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