Structural and electronic properties of the Zn0.5Cd0.5Se(100) surface

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surface science

ELSEVIER

Surface Science 373 (1997) 350-356

Structural and electronic properties of the Zno.sCdo.sSe(100) surface D.Y.W. Yu a, A. K a h n b,,, A. Cavus °, M.C. Tamargo ° a Physics Department, Princeton University, Princeton, N J 08544, USA b Department of Electrical Engineering, Princeton University, Princeton, N J 08544, USA c Chemistry Department, City College of New York/CUNY, Convent Ave and 138th St., New York, NY10031, USA

Received 28 May 1996; accepted for publication 3 September 1996

Abstract The chemical composition, atomic structure and electron affinity of Zn0.sCd0.sSe(100) grown by molecular beam epitaxy and prepared by Se-decapping are investigated. The Se-terminated surface exhibits a (2 × 1) reconstruction which corresponds to a complete monolayer of Se dimers. The cation-terminated surface exhibits a (1 x 1) structure tentatively attributed to a vacancy structure similar to that of the c(2 x 2) Zn-terminated ZnSe surface, but randomized by the non-periodic distribution of Zn and Cd. The evolution of the electron affinity of the surface is consistent with the formation of a surface dipole induced by a cation-to-anion charge transfer which fills (empties) all anion (cation) dangling bonds. Finally, preferential desorption of Cd is observed for temperatures in excess of 370°C. © 1997 Elsevier Science B.V. All rights reserved. Keywords: Surface reconstruction; Ultra-violet photoemission spectroscopy; Work function measurements; Zinc-cadmium selenide

1. Introduction Several investigations of surface a n d interface atomic, electronic a n d c h e m i c a l p r o p e r t i e s of I I - V I s e m i c o n d u c t o r s with a p p l i c a t i o n s in blue-green l i g h t - e m i t t i n g devices have b e e n r e p o r t e d over the p a s t few years. M o s t of these studies focused on the b i n a r y c o m p o u n d Z n S e I-1-9]. Surface a n d interface issues r e l e v a n t to the g r o w t h a n d perform a n c e of devices, like the a t o m i c a n d electronic structures of surfaces, the electronic p r o p e r t i e s of a n d defects at h e t e r o j u n c t i o n s , o r the n a t u r e of m e t a l c o n t a c t s to n- a n d p - t y p e I I - V I s , were e m p h a s i z e d . ZnSe, however, is o n l y one of the * Corresponding author. Fax: + 1 609 2586279; e-mail: [email protected]

c o n s t i t u e n t s in devices, which c o n t a i n several different t e r n a r y a n d q u a t e r n a r y alloys for b a n d g a p engineered active layers a n d confinement b a r r i ers. T h e electronic a n d chemical p r o p e r t i e s of these alloys are therefore e x t r e m e l y i m p o r t a n t when c o n s i d e r i n g the p e r f o r m a n c e a n d stability of the I I - V I devices. I n a d d i t i o n to the s t r u c t u r a l a n d electronic issues c o m m o n to all selenide c o m p o u n d s , these m a t e r i a l s are expected to exhibit specific c o m p o s i t i o n a n d stability p r o b l e m s associa t e d with the i n c r e a s e d n u m b e r of different cations o r a n i o n s in the same lattice. This s h o r t p a p e r focuses on the (100) surface of Zn~_xCdxSe, a t e r n a r y alloy extensively used for q u a n t u m wells in I I - V I laser diodes. W e c h o o s e the c o m p o s i t i o n x - - 0 . 5 to m a x i m i z e the sensitivity of the e x p e r i m e n t s to issues of surface stability a n d

0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved P H S0039-6028(96)01159-4

D. Y.W. Yu et al./Surface Science 373 (1997) 350-356

bonding configuration of Cd versus Zn. The roomtemperature band gap of the alloy is approximately Eg =2.09 eV for x =0.5 (2.66 eV for ZnSe [10] and 1.67 eV for CdSe [ 11 ]). We use low-energy electron diffraction (LEED) to investigate the atomic structure of the anion- and cation-terminated surfaces. Because of the expected random distribution of the two cations, the atomic ordering on the cationterminated surface is of particular interest. We use Auger electron spectroscopy (AES) and ultra-violet photoemission spectroscopy (UPS) to study the surface composition as a function of preparation temperature. Finally, we use UPS to determine band bending and ionization energy, and contact potential-difference (CPD) to study the variations in surface work-function and electron affinity (EA) as a function of surface termination. In the case of GaAs [12] and ZnSe [2] surfaces, the variations in EA versus surface termination proved to be an important test of coherence between the structure and the cation-to-anion charge exchange expected from the e l e c t r o n - c o u n t i n g rule at reconstructed polar surfaces. The results presented here show the expected similarities between the evolution of the structure and work function of the binary (ZnSe) and ternary surfaces, thus confirming the applicability of rules such as the electron-counting rule for predicting structural models and charge exchange for the ternary surfaces. They also point out specific stability issues regarding the Zn versus Cd composition at temperatures not too far above standard growth temperatures.

2. Experimental Undoped 5000 A-thick Znl_xCdxSe layers (x = 0.5) are grown by molecular beam epitaxy on InP(100) wafers. Unintentional doping results in slightly n-type materials ( n < 5 x 1015 cm-3), The lattice match between Zno.sCd0.sSe (5.872 A) and InP (5.869 A [13]) is 0.1%. The InP substrate surface is prepared by oxide desorption in an overpressure of the group V element, then transferred under UHV to the II-VI growth chamber. The substrate temperature is kept at 170°C for the first minute of the growth, then at 270°C for the remainder of the growth. The growth rate is 1 #m h-1

351

and the Se:cation ratio is 4"1. Following growth, the layer is capped in situ at 0°C with a thick layer of Se to protect the surface against contamination during ambient transfer to the surface analysis chamber. Samples not immediately used are stored in vacuum (~ 10 .7 Torr). The surface analysis is performed in an ultrahigh vacuum chamber (base pressure 10 -1° Torr) in which the II-VI layers are initially decapped at 150°C. Some of the LEED observations are done at 100K to reduce diffuse scattering in Zno.sCdo.sSe which is believed to have a low Debye temperature (OD(ZnSe)=271K and OD(CdSe)= 181 K [14]). Valence-band photoemission is performed with He I (21.2 eV) to measure the position of the Fermi level and the surface ionization energy. High-resolution Zn 3d and Cd 4d core-level spectra are measured with He II (40.8 eV) for curve-fitting. The experimental resolution for the UPS experiments is + 150 meV. The surface composition is studied using the peak-to-peak amplitudes of the Zn (994eV), Cd (376eV) and Se (1313 eV) Auger peaks. Finally, the CPD measurements of work-function changes are performed with a vibrating Kelvin probe leading to a resolution of __+15meV. The UPS, AES and CPD measurements are done at room temperature.

3. Results and discussion The desorption of the Se cap and annealing below 300°C lead to a diffuse (1 x 1) LEED pattern characteristic o f a disordered surface. A similar behavior is obtained on ZnSe(100) [2], and results from the attenuation of the substrate diffraction features in the residual amorphous Se layer. A (2 x 1) structure appears at 310-320°C and a (1 x 1) structure above 350°C (Fig. 1). The size of the diffraction spots, however, indicates that the surface quality is inferior to that of its binary counterpart. Thermal decomposition of the surface occurs above 380-390°C. This sequence is essentially identical to that obtained on ZnSe(100) [2], except for a compressed temperature scale and the fact that the c(2x2) Zn-terminated structure of the binary surface is replaced on the ternary by a (1 x 1) structure.

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D. Y. W. Yu et al. /Surface Science 373 (1997) 350-356

Table 1 Parameters used in the curve fitting of the Zn 3d and Cd 4d core levels shown in Fig. 2

Lorentzian width (eV) " Gaussian width (eV) Branching ratio Spin-orbit splitting (eV) Surface core-level shift (eV)

Fig. 1. LEED patterns from Zn0.sCdo.sSe(100): (a) the Se-terminated (2x 1) surface (46eV) and (b) the cationterminated (1 x 1) surface (55 eV).

The decomposition of the Zn 3d and Cd 4d core levels into bulk and surface components is done using the parameters given in Table 1. The procedure makes use of a least-square fitting routine which simultaneously subtracts the integrated background and adjusts the core-level line shapes. It requires great care because of the small energy separation of the cation core-levels ( ~ 0.7 eV from

Zn 3d

Cd 4d

0.15 0.6 1.48 0.31 0.75

0.17 0.6 1.4 0.68 0.85

Zn 3d5/2 to Cd 4d5/2). The number of core-level components used in the fit is kept to a strict minimum, and stringent limits are placed on all parameters involved. The Gaussian width is somewhat larger than could be expected from experimental broadening due to the He II line and the resolution of the double-pass CMA, and must be put on the count of the quality of the surface. The Lorentzian width and spin-orbit splitting are consistent with previous work [2], and the branching ratio is close to the statistical value for d-levels. The )~z, which is a measure of the goodness of the fit in the decomposition, is defined here as the sum of squares of the residual divided by the number of points analyzed (i.e. 91). It is consistently low and of the order of 7 x 10 - 4 . Note that the spinorbit splitting of Cd (0.68 eV in Table 1) is smaller t h a n the value given for Cd metal [15]. This phenomenon is observed in other II-VI semiconductors such as CdS, ZnSe or HgS [16,17], and is generally associated with a difference in orbital sizes due to metallic versus ionic or covalent bonding, as spin-orbit splitting is inversely proportional to r 3 [ 1 8 ] . Following the low-temperature decapping, the cation core-levels are adequately described with only one component each (Fig. 2a), indicating that the cations are four-fold coordinated (bulk-like). For the (2 x 1) structure, the core levels remain basically described by one bulk component each, although a small shifted component appears on their high binding-energy side (Fig. 2b). This component grows with the appearance of the (1 x 1) structure (Fig. 2c), indicating that a significant fraction of the cations near the surface are now in a different chemical environment than in the bulk. The electron affinity (EA) of the semiconductor

353

D. Y. W. Yu et al./Surface Science 373 (1997) 350-356

.... ' .... I.... ' .... I.... ' ~ 1 .... ' .... I.... ' .... I....

(a).

Cd4dsa.,~ ~ Annealintegmperature Cd4d3n~~ _ 220°C

Ibl oo i31g0t pe u

22

23

24

25

26

27

Kinetic Energy (eV) Fig. 2. Zn 3d and Cd 4d core-level spectra measured by ultraviolet photoemission spectroscopy (HelI, hv=40.8 eV) from (a) the surface decapped at 220°C, (b) the (2x 1) surface annealed at 300°C, and (c) the (1 x 1) surface annealed at 380°C. The spectra are decomposed in bulk (B) and surface (S) components. Filled dots represent experimental data; each solid curve passing through the dots represents the sum of corresponding bulk and surface components; the residual (experiment- sum of components) is given for each decomposition. The curve-fitting parameters are given in Table 1.

is the energy difference between the vacuum level and the bottom of the conduction band. EA varies with the vacuum level upon formation of a surface dipole. A change in CPD, as measured with the Kelvin probe, corresponds to a change in work function (A~b) which, in turn reflects the sum of changes in band bending and in EA. Changes in band bending are accounted for via UPS and are subtracted from A~b to yield changes in EA. In the present case, the band bending is found to be

essentially independent of the surface termination and to correspond to a position of the Fermi level 1.75 eV above the valence-band maximum (VBM). Kelvin-probe measurements show a 150 meV fiattening of the bands under illumination (and thus during photoemission) due to surface photovoltage. The equilibrium (in the dark) position of the Fermi level is therefore estimated at 1.60 eV above VBM, a position which is somewhat higher in the gap than on ZnSe(100) 1,8]. Through measurements of the onset of photoemission, we also determine the ionization energy (IE) to be 6.30__+0.20eV, leading to an EA of 4.21 __+0.20 eV. This Value of EA is close to that found for ZnSe(100) (Table 2), indicating that the larger fraction o f the difference in band gaps is taken by a rise in VBM with respect to the vacuum level. This result is somewhat unexpected given the current belief that 60-80% of the band offset at the Znl_xCdxSe/ZnSe interface is in the conduction band I-19,20]. The relative value of EA, rather than its absolute value, is the most interesting observable in this discussion of the surface. Fig. 3 shows that EA decreases by about 50 meV a t t h e (2 x 1)-to-(1 x 1) transition. The initial low-temperature increase is due to the removal of the amorphous Se capping layer, and is not representative of significant changes occurring in the surface structure. Similarly, the high-temperature increase corresponds to the beginning of the surface decomposition. The shift at the transition is small but highly reproducible, and consistent with what has been previously observed on binary surfaces: a 170 meV shift at the ( 2 x l ) - t o - c ( 2 x 2 ) transition on ZnSe(100) [ 2 ] and a 400-500meV shift at the c(2 x 8)-to-c(8 x 2) transition on GaAs(100) 1,12,21]. These shifts reflect a change in surface

Table 2 Ionization energies and electron affinities of the ZnSe(100) [-27] and Zno.sCd0.sSe(100) surfaces determined by UPS

Ionization energy (eV) Energy gap (eV) Electron affinity (eV)

ZnSe (2xl)

ZnSe c(2x2)

Zno.sCdo.sSe (2×1)

6.87 2.70 4.17

6.70 2.70 4.00

6.30 2.09 4.21

D. Y. W. Yu et aL /Surface Science 373 (1997) 350-356

354

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i .... 400

1 450

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Fig. 3. Evolution of the electron affinity of Zno.sCd0.sSe(100) with annealing temperature. The low-temperature rise is due to desorption of excess Se. The rise above 380°C corresponds to the onset of surface decomposition.The uncertainty on each data point is _+15 meV. The inset defines the parameters EA and IE mentioned in the text. dipole caused by a change in dangling bond (DB) occupancy in the top layers of the crystal. The LEED, core-level spectroscopy and C P D results are consistent with the structural model of ZnSe(100) prepared under similar conditions [2]. The surface is initially Se-rich, as expected from the preparation procedure. The (2 x 1) reconstruction corresponds to a nominally complete monolayer of Se dimers which keep all second-layer cations four-fold coordinated. All the cations of one species (i.e. Zn or Cd), whether just below the surface or in the bulk, are therefore considered as chemically equivalent within the resolution of the experiment and give rise to a single core-level component. As in the ZnSe case, however, defects in the layer of Se dimers due to inhomogeneous thermal decapping leave some of the second-layer cations three-fold coordinated and exposed to vacuum. These exposed cations give rise to the small surface core-level components on the high binding-energy side of the bulk component m Fig. 2b. The principle of semiconductor surface autocompensation [22], together with the electroncounting rule [23], can be used to predict the occupation of the surface dangling bonds (DB). Autocompensation at the surface of a compound

semiconductor is achieved when all acceptor-like DB states corresponding to the more electronegatire species, i.e. the anion, are filled, and all donorlike DB states corresponding to the cation are empty. Electron-counting accounts for all available electrons in the unit cell, and can therefore be used to predict ratios of cations and anions and bonding configuration compatible with surface autocompensation. In the ( 2 x 1) case, electron counting shows that all the Se DBs pointing away from the dimer bonds are doubly occupied [2,23]. The surface is semiconducting and does not require any cation-to-anion charge transfer. The appearance of the (1 x 1) L E E D pattern marks the transition from an anion- to a cationterminated surface. This new surface chemical envir o n m e n t produces the Zn 3d and Cd 4d surface components shifted by 0.75 and 0.85 eV, respe0tively, toward higher binding energy with respect to their bulk components (Fig. 2c). The surface core-level shifts are similar to that found for Zn 3d on the c(2 x 2) Zn-terminated ZnSe surface [2], and are considerably larger (i.e. by a factor of two) than those found on cation-terminated III-V surfaces. This difference comes in part from the different amounts of charge transferred per surface atom required to fill the anion DBs and empty the cation DBs. The basic reconstruction building blocks on III-V surfaces are dimers, and surface cations have one DB which, according to electron counting, must give up 3/4 electron. The c(2 x 2) Zn-terminated ZnSe surface, however, corresponds to a half-monolayer of non-dimerized Zn atoms in two-fold coordinated configuration with two DBs. Each DB gives up in average 1/2 electron to each Se DB in the second layer, leading to a oneelectron transfer per surface cation, and consequently to a large core-level shift. This charge transfer induces the surface dipole which decreases EA at the (2 x 1)-to-c(2 x 2) transition. Part of the increase in surface core-level shift with respect to the III-Vs is also due to a weaker screening of the core hole in the lower dielectric constant II-VIs. The main structural discrepancy between Zno.sCdo.sSe and ZnSe is the absence of reconstruction on the cation-stabilized surface of the former, The data presented above support a termination of the surface with a half-monolayer of two-fold

355

D. Y. IV. Yu et al. /Surface Science 373 (1997) 350-356

coordinated cations, similar to the c(2 x 2) ZnSe case. As mentioned above, the quality of the present ternary surface does not match that of the binary compound and could explain the absence of reconstruction, although the surface quality is sufficient to produce a clearly visible Se-terminated (2 x 1) reconstruction. Surface roughening at the (2 x 1)to-( 1 x 1) transition around 350°C, which is close to the temperature at which Cd desorption becomes noticeable (Fig. 4), is also possible. Finally, the lack of observable reconstruction may also be attributed to characteristics inherent to the ternary alloy, namely the random distribution of Zn and Cd in the top layer. Although their local structure and immediate chemical environment remain those of the binary surface, these two atoms have very different scattering cross-sections, and their random distribution wipes out the periodicity of the top layer and leads to a pattern characteristic of the second and deeper layers. It is also worth noting that the poorer quality of the ternary surface is likely to reduce the observed average surface dipole and the change in EA with respect to the binary surface [2,21]. Cd depletion at higher annealing temperature is evident from both the core-level decomposition (Figs. 2b and 2c) and from the ratio of the Zn, Cd and Se AES lines (Fig. 4). The Zn/Cd ratio increases dramatically above 370°C, whereas the Zn/Se ratio remains constant throughout the tem0.45

. . . . . . . .

04 ................... "

0.35

perature range. This result is consistent with the relative strengths of the Zn-Se and Cd-Se bonds reflected in the heats of fOrmation of the respective compounds: AHf(ZnSe) = -- 159 kJ mol - 1 and AHf(CdSe)= 144 kJ mol=l. [25]. Preferential Cd desorption is Observed in other I I - ¥ I ternary alloys like Znl_xCd=Te [26]. Although the annealing temperature which causes significant problems in the present case is about 100°C above the growth temperature, it is expected that the differences in bond strengths which are at the origin of the preferential desorption complicate the precise control of composition versus growth temperature required in ternary and quaternary: alloys.

4. Conclusions

We have shown that the (100)surface of Zno.sCdo.sSe grown by molecular beam epitaxy and prepared by thermal removal of a protective Se cap exhibits structural properties and composition very similar to those of znse. The (2x l) reconstruction corresponds to a complete monolayer of Se dimers, whereas the (1 × 1) structure corresponds to the cation-terminated surface. The absence of a c(2 x 2)-like reconstruction on the cation-terminated surface is attributed to the random distribution of Cd and Zn. T h e decrease in electron affinity at the (2x 1)-to-(1 x 1) trans-

i . . . . . . . .

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Zn(994eV)/Se(1313eV

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0.2

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0.15

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r

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~

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Annealing temperature ( C ) Fig. 4. Peak-to-peak Zn/Cd and Zn/Se Auger intensity ratios as a function of annealing temperature.

356

D.Y.W. Yu et al./Surface Science 373 (1997) 350-356

ition is consistent with the f o r m a t i o n of a surface dipole due to a charge transfer which empties the c a t i o n DBs a n d fills the a n i o n DBs. Finally, the preferential C d d e s o r p t i o n a b o v e 370°C is consistent with the lower strength of the C d - S e b o n d as c o m p a r e d to that of the Z n - S e b o n d .

Acknowledgements S u p p o r t of this w o r k b y grants of the N a t i o n a l Science F o u n d a t i o n at P r i n c e t o n U n i v e r s i t y ( D M R - 9 3 - 2 1 8 2 6 ) a n d at City College of N e w York (ECS-93-20415) is gratefully acknowledged.

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