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LUMINESCENCE AND DEEP LEVEL STUDIES OF LINE DISLOCATIONS IN GALLIUM PHOSPHIDE
B. Hamilton, A. Peaker, D. Wight
To cite this version:
B. Hamilton, A. Peaker, D. Wight. LUMINESCENCE AND DEEP LEVEL STUDIES OF LINE
DISLOCATIONS IN GALLIUM PHOSPHIDE. Journal de Physique Colloques, 1983, 44 (C4), pp.C4-
233-C4-241. �10.1051/jphyscol:1983428�. �jpa-00223047�
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Colloque C4, supplément au n°9, Tome 44, septembre 1983 page C4-233
LUMINESCENCE AND DEEP LEVEL STUDIES OF LINE DISLOCATIONS IN GALLIUM PHOSPHIDE
B. H a m i l t o n , A.E. P e a k e r and D.R. Wight*
Department of Electrical Engineering and Electronics, UMIST, U.K.
*R.S.R.E., Malvern, U.K.
Résumé
Les durées de vie des porteurs et des niveaux profonds ont été mesurées par décroissance de la luminescence sur du GaP de type-n non dopé. Les mesures ont été effectuées sur des régions de densité de dislocation variable en vue de mettre en évidence des signatures de niveaux profonds qui pourraient être associées aux dislocations et aux recombinaisons non radiatives efficaces associées à ces défauts.
Abstract
Luminescence decay measurements of carrier lifetime and deep level measure- ments have been made on undoped n-type GaP. Regions of varying dislocations density have been measured in an attempt to investigate deep level signatures which may be associated with dislocations and with the efficient non-radiative recombination associated with these defects.
Introduction
The technological importance of Gallium Phosphide (GaP) as an optoelectronic semiconductor has led to a rather intensive study of recombination mechanisms in this material. In particular, non-radiative recombination due to the presence of
1 2
dislocations is known to be a powerful mechanism.' Usually recombination at dis- locations is in competition with efficient bulk (point defect) deep levels ; the contribution to the total rate from each mechanism depending on their relative concentrations. In this paper we describe measurements made on isothermally grown LPE GaP (undoped n-type), which exhibits small concentrations of point defect deep levels. Consequently, large areas of material (on a given sample) showed a non- radiative recombination rate which was dominated by dislocations. Luminescence decay measurements were made in order to study local variations of lifetime with dislocation density. Deep level measurements were made in regions of varying dis- location density in order to reach for bound states associated with dislocations.
Finally, a detailed count of the number of dislocations threading each diode used for deep level measurements was made using an SEM in the luminescence mode to
4 image dislocations.
Lifetime and Dislocation Density
The minority carrier lifetime, x, was measured from the cathodoluminescence decay of the green emission. Excitation was with a pulsed 70 KV [y luA) electron beam focussed into a 150 um diameter spot. A beam modulation time of ^ 3ns was
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983428
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available using an electrostatic deflection system. In computing T the exponential tail of the decay was used to assess the true transient lifetime T.
For a uniform and parallel array of dislocations, modelled as cylindrically symmetrical line sinks, the continuity equation for the excess minority density Ap(r) is
This form is, of course, valid only provided that the sink is sufficiently powerful to ensure that the carrier loss is diffusion controlled. A comparison of diffusion and recombination limited mechanisms at various forms of sink 2
,
demonstrates that recombination becomes diffusion controlled when the capture radius ro is greater than the carrier mean free pathA.
On this basis, (1) can be solved subject to the boundary conditions:aAp = O a t r = r
-
ar maxAp = 0 at r = r (the capture radius)
Here r is the maximum radius associated with each dislocation and is related max
to the dislocation density ~~(cm-') by .rrrZmax = l/pD.
The lowest mode solution (corresponding to t-+ m) is an exponential decay with a time constant which can be expressed in terms of the dislocation density and the capture radius as:
where D is the diffusion coefficient for minority carriers (holes).
The measured value of T as a function of pD is shown in Figure 1. All the data points are from the same sample.
It is clear that for pD>105ci2 the lifetime is controlled by the dislocations.
In fact the fit to the theoretical expression, equation (2) is extremely good with D taken as 3.3cm2s-' and ro chosen to be one micron. Although the fit is not especially sensitive to the magnitude of ro is is clear that a characteristic capture radius, large compared with the dislocation core is needed to explain the dat&. At low dislocation densities, the lifetime falls below the dislocation controlled line. This is indicative of a constant background shunt lifetime of T ~ Q J 3 ~ s . Adding the shunt lifetime -rS and the dislocation limited lifetime rD in parallel produces an accurate fit over the entire measurement range.
I
A DATA OBTAtNED ON ONE SAMPLE101 " " " " " " " " " ' ""J4
lo3
104 105 106lo7
DISLOCATION DENSITY, ~ ~ ( c r n - ~ )
Figure 1. Dependence of lifetime on dislocation density.
A dislocation with such a large effective capture radius,ro, is clearly a massive electronic perturbation in Gap, capable of carrying almost all of the bulk recombination. However the model described above subsumes the dynamics of the recombination process into the capture radius ro. The recombination is so efficient that the rate limiting mechanism is diffusion of the minority carrier into the sink. It is reasonable to assume that carrier recombination at or near the dislocation is aided by deep electron states caused either by the dislocation itself or by impurities or defects gettered in a Cottrell atmosphere around the core. We now report attempts to correlate deep level measurements with accurate measurements of dislocation density.
Dislocation Density Measurements
For both the lifetime and deep level measurements, dislocation counts were made using an SEM in the cathodoluminescence mode. Contrast is achieved because of the strong local quenching effect of a dislocation on the green emission. For the lifetime work the count is straightforward; a map is obtained (centred on the lifetime measurement point) and the density determined. For the deep level studieq which involve transient capacitance measurements using Schottky diodes, the total number of dislocations threading through the depletion layer is needed. This was obtained by selectively etching the sample (after completion of the deep level measurements) in Transene Gap etchant. This etch removes the semiconductor wihu.t.
C4-236 JOURNAL DE PHYSIQUE
attacking the metal film. In this way mesa structures were formed. The metal was then removed leaving a free semiconductor mesa on which to count the dislocations in individual barriers. This scheme of events is shown in Fig.2., the low magnification cathodoluminescence map (2A) is grossly non-uniform due to the variable dislocation density. The free mesas (2(B)) clearly define where the barriers were formed. Two examples of dislocation densities in individual barriers are shown in 2(c) and 2(D)
C.L. NAP OF ORIGINAL SLICE LOCATION OF MESAS (EMISSIVE IIODE)
Figure 2. Illustrating the SEM technique for counting dislocations.
Deep Level Measurements
Since the dislocations are efficient recombination centres, they clearly exhibit efficient electron and hole capture rates in the bulk. In the depletion layer the role of the dislocation must convert an electron or hole trapping centre for the transient capacitance experiment to detect the localised states. Two classes of measurement, hole capture (to test for hole trapping) and DLTS (to test for electron trapping) were therefore performed.
Hole Capture Measurements
The hole capture experiments utilised near band edge illumination as an injection mechanism for minority carriers. The light absorbed inside a diffusion length of the barrier produces a pure hole flux as shown in Fig.3. Very deep states (below mid gap) act as hole traps for this excitation condition, communicating with the valence band by hole capture and emission. During
excitation capacitance transients (rises) are induced by the build up of positive
, r\
SEMICONDUCTOR Iv i c edecay)
Y T \& (caoacitance rise) RAPID CAPTURE HOLE S 2
F i g u r e 3. The charge exchange p r o c e s s e s corresponding t o h o l e t r a p p i n g .
space charge due t o h o l e c a p t u r e . When t h e e x c i t a t i o n i s removed t h e capacitance decays due t o h o l e emission. The c a p a c i t a n c e r i s e and decay t i m e s c o n t a i n i n f o r m a t i o n on t h e h o l e c a p t u r e c r o s s - s e c t i o n and t h e h o l e binding energy,
r e s p e c t i v e l y . 3 The amplitude of t h e c a p a c i t a n c e t r a n s i e n t i n t h e (low temperature l i m i t ) can b e used t o e s t i m a t e t h e c o n c e n t r a t i o n of a h o l e t r a p . A marked s i m i l a r i t y o f response u s i n g t h i s form of o p t i c a l e x c i t a t i o n , was observed from diode t o d i o d e . T h i s corresponded t o a s i n g l e w e l l d e f i n e d h o l e t r a p a t an energy = Ev + 0.85eV.
The c o n c e n t r a t i o n of t h i s s t a t e a s a f u n c t i o n of d i s l o c a t i o n count i s shown i n Fig.4.
These d a t a show t h a t t h e h o l e t r a p i s roughly uniformly d i s t r i b u t e d a c r o s s t h e sample, and hence i s not coupled d i r e c t l y t o t h e d i s l o c a t i o n s . The h o l e t r a p may, however, c o n t r i b u t e t o t h e uniform 'background' l i f e t i m e of a 3 v s .
I n t h e s e measurements t h e h o l e t r a p i s assumed t o achieve an ( e l e c t r o n ) occupancy f a c t o r
where e and en a r e t h e thermal e l e c t r o n and h o l e emission r a t e s , c i s t h e h o l e
P P
c a p t u r e c o e f f i c i e n t and Ap i s t h e i n j e c t e d h o l e d e n s i t y which i s r e l a t e d t o t h e s t r e n g t h of t h e p h o t o c u r r e n t . I n t h e l i m i t t h a t e -t o and c Ap >> e t h e t r a p
P P P
f i l l s w i t h h o l e s . The t o t a l amplitude of t h e c a p a c i t a n c e t r a n s i e n t can t h e n be used t o e s t i m a t e NT.
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Figure 4. Variation of hole trap concentration with dislocation density.
Nois
[cPl
105
lo"
DLTS
-
DLTS measurements made on Schottky barriers are essentially majority carrier
5 - 2
measurements, measuring electron capture and emission. In regions of pD < 2xlOcm
5 2
no measurable DLTS signal was observed. However, for pD
'
2x10 cm- a broad feature (peaking at T = 165 K using a 100 s-I rate window) was observed. Measurements were made using a Polaron S4700 system. This feature could not be thermally activated with precision, but we estimate that it is characteristic of electron emission from electron states spanning % 0.3 to 0.5 eV from the conduction band edge. Spectra from three diodes are shown in Fig.5.The dislocation density corresponding to the onset of this DLTS response represents a situation in which the capture radius of the dislocation is a significant fraction of the mean dislocation spacing. This fact can be observed in the downward curvature of the lifetime at high dislocation densities (Fig.l.), and is predicted by equation ( 2 ) for T . Although the electron emission was linked to high dislocation densities, no clear correlation between peak height and p,, was detected. This may have been because the range of pD was small in the diodes which showed the DLTS feature. An alternative explanation is that the signal originates exclusively from the defect atmosphere around the core. At high dis- locationsdensities the atmosphere occupies a more significant volume of the entire depletion layer. The approximate concentration of the defect(s) calculated from the DLTS peak height is % 1 0 l ~ c m - ~ . ~ 0 t h electron and hole trap concentrations are small compared to the dangling bond density expected in the diodes of highest PD.
c
. .
:
:
RATE WINDOW= 100
Figure 5. DLTS spectra from regions of high p D
Discussion
Calculations of electronic states associated with dislocation core structure are still the subject of analysis by several theory groups. For example, recent pseudopotential calculations4 suggest that the electronic structure due to the core is essentially band like. Depending on the details of the core the position of the bands may change, but the narrowest band (AE % 0.5 eV), corresponds to dangling bonds normal to the dislocation line. This band would give a DLTS spectrum considerably more smeared out than that observed.
On this basis, we are forced to conclude that neither the electron nor the hole traps observed have their origin in the dislocation core. From an experimental point of view it is interesting to consider whether there are factors which militate against the detection of core related levels for the type of line dislocation measured in the present samples. Two factors may be of importance.
If we adopt the conventional, qualitative, view of the electrical role of a line dislocation we draw it (Fig.6(a)) as a band of states equilibriated with the bulk. The electron occupancy of the dislocation states in n-type material is assumed to produce a net negative charge (on a macroscopic scale) leading to depletion around the dislocation. Rather like a surface, recombination is enhanced because the depletion field is attractive to the minority carrier. Having acquired some positive (minoFity) charge the "forward biased" dislocation is fed also with a (majority) electron flux. In the depletion layer this behaviour is disturbed because the electron (majority) quasi-Fermi level is deep in the gap and the steady state depletion field around the dislocation, in directions orthogonal to the -
dislocation line, may now be much smaller. The precise change in the dislocation
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1
METAL?BARRIER
. .
DEPLETION
F i g u r e 6 . I l l u s t r a t i n g e f f e c t s which may weaken d i s l o c a t i o n response i n c a p a c i t a n c e measurements.
system w i l l r e f l e c t t h e charge adjustment i n t h e c o r e band due t o t h e s h i f t i n Fermi l e v e l (Ef +
an).
One e f f e c t of such a change could be t o weaken h o l e c a p t u r e r a t e s . E l e c t r o n emission r a t e s might a l s o be i n f l u e n c e d i f t h e y depend on t h e s t r e n g t h of t h e r a d i a l e l e c t r i c a l f i e l d .
The second f a c t o r may be s i g n i f i c a n t f o r any t r a n s i e n t c a p a c i t a n c e experiment.
I f t h e l i n e d i s l o c a t i o n were a f u l l y d e p l e t e d s t r u c t u r e , with a r a d i a l d e p l e t i o n f i e l d around t h e c o r e and f i x e d o r o n l y weakly mobile charge along t h e c o r e , t h e n we should draw t h e t o t a l d e p l e t i o n s t r u c t u r e of a S c h o t t k y diode c o n t a i n i n g l i n e d i s l o c a t i o n s a s shown i n F i g . 6 ( b ) . The d e p l e t i o n l a y e r edge merges with t h e d i s l o c a t i o n a s t h e l a t t e r emerges from t h e b a r r i e r . I n t h e u s u a l one dimensional a n a l y s i s of (say) t h e DLTS s i g n a l , t h e c o n t r i b u t i o n t o t h e t r a n s i e n t 6 C i s o b t a i n e d by s o l v i n g P o i s s o n ' s e q u a t i o n along a l i n e X i n t h e b a r r i e r and t h e n s c a l i n g t h e r e s u l t t o t h e t o t a l diode a r e a A . However, i f t h e charge exchange t a k e s p l a c e r i g o r o u s l y along t h e d i s l o c a t i o n line,X, t h e d e p l e t i o n l a y e r edge a p p r o p r i a t e t o t h i s l i n e i s punched deep i n t o t h e c r y s t a l . I n crude terms t h e c a p a c i t o r p l a t e s e p a r a t i o n measured a l o n g t h e d i s l o c a t i o n l i n e may b e huge compared w i t h t h a t o f t h e r e g u l a r b a r r i e r , being measured t o e s s e n t i a l l y where t h e d i s l o c a t i o n ends.
C h a ~ g e exchange t a k i n g p l a c e on t h e d i s l o c a t i o n b u t i n s i d e t h e b a r r i e r would t h e n produce a very weak e f f e c t i n terms of t h e o v e r a l l b a r r i e r c a p a c i t a n c e .
T h i s i d e a i m p l i e s , i n a v e r y q u a l i t a t i v e way, t h a t l i n e d i s l o c a t i o n s of t h e s o r t d i s c u s s e d i n t h i s paper could be more d i f f i c u l t t o d e t e c t , by c a p a c i t a n c e
t e c h n i q u e s , t h a n p o i n t d e f e c t s d i s t r i b u t e d uniformly throughout che b a r r i e r .
References
(1) C . Werkhoven, C . Van Opdorp and A.T. Vink, I.O.P. Conference S e r i e s 33(a) (1977) 317 (2) D . R . Wight, I . D . Blenkinsop, W. Harding and B. Hamilton
Phys. Rev. B.
3
(1981) 5495(3) B. Hamilton, A . R . Peaker and D.R. Wight, J n l . Appld. Phys.
