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Optical characterisation of acceptors in doped and undoped VPE InP
142
Journal of (‘r~stalGrov,th 64(1 1~h3) 142 145 North—Holland Publishing (‘ompanv
OPTICAL CHARACTERISATION OF ACCEPTORS IN DOPED AND UNDOPED VPE InP C. PICKERING, P.R. TAPSTER, P.J. DEAN, L.L. TAYLOR, P.L. GILES
*
and P. DAVIES
*
Rosa/ Signals and Radar Establishment, St. Andrews Road, Great Ma/tern. Worc~str’rshirc’It R14 3P5. (K
The compensation ratios (8 = NA/ND) of a large number of InP layers. undoped and doped with S and Si donors, have been determined by analysis of photoluminescence spectra. Good agreement with electrical estimates of 0 was obtained for uniform samples with 8 ~ 0.4, while non-uniform samples and samples where the electrical data indicated high compensation showed large discrepancies. A good correlation was obtained with deep acceptor concentrations from DLTS measurements. indicaiing that the compensation is dominated by deep acceptors except at low carrier densities where the shallow (Zn) acceptors are important. N,
5 ~sas
found to increase with carrier density, independent of dopant, and the increase was dominated by the deep acceptors.
1. Introduction The compensation ratio, 0, is normally obtained from an analysis of Hall measurements as a function of temperature. This method relies on ideal contacts to uniform material and can lead to an overestimate of 0 if unknown scattering processes are important. An independent method for determination of 0 is therefore very useful and in 1977 Kamiya and Wagner [11 used analysis of photoluminescence (PL) lineshapes to estimate 0 in GaAs. Pickering et al. [2] recently extended this technique to InP and this paper reports results on very high purity undoped InP recently grown at RSRE [3] and InP doped with S and Si donors grown at Plessey Research Ltd. [41.These results, together with the earlier data [21, are compared with Hall and DLTS measurements.
2. Experimental The epitaxial layers were grown by vapour-phase epitaxy using the ln—PC1~-H2system. The high purity undoped InP layers were grown at RSRE under constant mole fraction conditions but with different values of PCi5 flux [3]. The layers, which were 7—11 j~m thick, were assessed by electro*
Plessey Research (Caswell)
Ltd..
Allen Clark Research
Centre, Caswell, Towcester. Northants NNI2 8QE, UK.
chemical profiling and Hall measurements at 300 and 77 K. The electrical data are given in table I. The InP layers grown at Plessey were 4 6 p.m thick with carrier densities in the range 3 X iü~~7 x iO’~cm~. S and Si donors were incorporated in the layers by doping with H,S and SiH-,CI, in the gas phase. Photoluminescence measurements were made using the 488 nm line from an Ar ion laser and a Spex 0.75 m spectrometer. with the sample in a continuous flow He cryostat. DLTS measurements were performed on 0.5 mm Ti Au Schottky barrier devtces as described by Tapster et al. [5].
3. Review of photoluminescence technique The PL technique for determination of 0 has been described in detail previously [1] and its application to InP discussed fully [2]. Only the main points will therefore he mentioned here. In PL spectra of InP, peaks are often observed, a few tens of meV below the band edge, due to recombination of electrons with holes hound to shallow acceptors. In VPE InP only one shallow acceptor. Zn, is normally observed, with an activation energy of -~ 46.5 meV. Under low excitation conditions. this band can he resolved into two peaks free-to-hound (FB), and donor~acceptor pair (DAP) bands due to recombination of the free and bound electrons, respectively. Analysis of the
0022-0248/83/0000—0000/$03.00 © 1983 North-Holland
/
C. Pickering ci al.
Optical characterisazion of acceptors in
VPE
Table I Electrical properlies and optical results of undoped epitaxial layers grown with various values of 0H,S
477K
(Torr) P( PCI ~)
(XH~H 1014 cm~3) n
10 10’~ 10 “~ 16 160 16 24 33~ 33 ~ 33 5i
(x1014 cm 3) ne,,,,
(cm2 V’ s
1.6
2.1
115000
5.6 0.5
6.4 1.4
87000
2.5 2.7 0.6 3.7 1.5 0.8 2.1
3.3 3.5 1.4 4.7 2.5 1.6 3.3
118000 97000 130000 76000 62500 104000 76000
0~orr
0,3,
I)
.-
0.3 0.05 —
—
0.05 0.2 0.5 0.3 0.7 0.58 0.5
P(PCI
3)
,\A(opi) (X 10’3cm
0.15
0.08
1.8
0
0.03 0.1
2.0 1.5
—0 0.1 0.1 0.1 0.27 0.2 0.15
—0 3.9 1.6 5.2 9.3 4.0
—
—0 0.08 0.17 0.22 0.55 0.37 0.35
143
InP
5.8
3)(
N~(elec) X 1013cm 3) 3.7 —
0 --
—0 3.0 2.9 13 31 9.4 18
.0 Profile approximately flat.
s~Profile ramped
FB/DAP
(
> factor of 2 change).
spectrum at temperatures where the
P(R) is the Poisson distribution and E(
peaks are of a similar intensity (— 10—20 K) gives information about the distribution of electrons between the conduction band and the donors, which is a function of ND, NA, temperature and the donor ionisation energy, ED. The FB/DAP lineshape is described theoretically the sum of theof intensities to each processas taking account competingdue transitions. The final expression for the lineshape. obtained from the individual expressions [1,21, is
3, EA and EF are the band-gap, acceptor ionisation and Fermi energies, respectively. EF depends on ED, the donor ionisation energy, and the concentration dependence of ED must be taken into account [2]. The theoretical lineshape obtained from the above equation is fitted to experimental data obtained 2 with low ND, laserTexcitation (— 5 mW cm [2]) with and W as energy the adjustable parameters. Thus the total donor concentration is ohtamed from the fit and the total acceptor con-
VE/
centration, ratio,or isC—V obtamed usingand ND hence — NA compensation known from Hall
— —
I L)A~ 1E /
+
~eA\(E
4~rN~R34~7’~()
4
=ER
exp
-
N W+ 3
measurements at room temperature. The value of
___
3
xJ2(R)
r
____
e2
NA obtained from the analysis should therefore
N
—
D
include all acceptors, both shallow and deep, even though the analysis is performed on the peaks due to only one shallow acceptor.
A
WDA(R)
*3/2
+~-
2
4..aAm,
/
Xexp~— ~-
X
E(E_E(;+EA)i/
4. Results and discussion
E-EG+EA—EF _____________
kT
P(R)dR
~ w+ w
A(R)
D
4.1. Comparison with Hall measurements Table 1 shows the results obtained on the high purity RSRE samples grown with various values of
where R is the separation of neutral donors and
PCI
neutral acceptors, J(R) is the overlap integral between donor and acceptor wavefunctions, aA is the effective Bohr radius of the acceptor, WDA( R) is the neutral donor — neutral acceptor transition rate, W is the total competing transition rate,
3 partial pressure, P(PC13). The electrical cornpensation ratios were determined using the calculations of Walukiewicz et al. [6] with the measured carrier density and 77 K Hall mobility. However, the samples had measured carrier densities the 3 and at this inlevel range 5 >< 10~~—5 x 1014 cm
144
C. Pickering et at.
/
Optical characterisation of acceptors in VPE InP
of n and 0 have therefore been corrected for depletion assuming a model with a 0.2 eV harrier at the surface and a 0.6 eV barrier at the interface.
1016 ,t9\5Oi~
x ~
•,‘,/~,‘
~
~ 10~ x
~‘,,/~ç’
)
,~/,‘ ,~/,,NAloptl~NAleIec)
•._
NA (elecl (cm )
,~/,~)
~
x
X
•,•
~
x.271
-
(Within the scatter this model produces good agreement between nc,,rr and the results of C--- V measurements.) From the table this can be seen to have a marked effect on 0 determined from the electrical measurements. A further problem for a comparison of the values of 0 obtained from the electrical and optical measurements is the non-uniformity of the layers. Most of this series of sampies had non-uniform depth profiles and the optical technique measures the top 1 p.m or so while the electrical method averages over the whole depth of the layer. In the more uniform layes. the two
06
techniques in reasonable (see later). The trend are of increasing NA agreement with P(PCI 3) as reported in ref. [3] can be clearly seen in the final
Fig. 1. Comparison of optical and electrical acceptor concentrations: (•) uniform, (x) non-uniform depth profiles.
two columns of the table, with the optical surface measurement showing a somewhat less steep in-
i3
~I0 iS
I
I
NA(opt)(cm I
depletion effects at the surface and layer/substrate interface cannot be ignored [7]. The metallurgical thickness, normally obtained from cleaveand-stain measurements, may be larger than the electrical thickness, leading to an underestimate in the carrier density. Since the mobility is not affected by the thickness measurement, the cornpensation ratio will be overestimated. The values l.unuform, x romPed)
could occur here if the interface region was highly compensated and dominated the electrical measurements. In fig. 2 it can be seen that most of the
0-6 ‘
dec 0-4
/
etch
•
51 dec
crease. The acceptor concentrations and compensation ratios obtained by the optical and electrical techniques are compared for a large number of sampies in figs. 1 and 2. These results include the present RSRE and Plessey samples together with the earlier data from ref. [2]. Fig. I shows good agreement between N~(opt)and NA(elec) for nearly all the uniform layers while all the non-uniform samples show large discrepancies. The effect of removing some of the layer by etching can be seen to improve the agreement in some cases but this does not always occur. A source of discrepancy
.~
opt
non-uniform samples had large 0(elec). which were about a factor of twovalues largerofthan the
X-----e
•
• ,/ 0-2 ~
,‘•
•,~• 0 0-2 0-4 0opt 0-6 0-8 Fig. 2. Comparison of optical and electrical compensation ratios: (•) uniform, (x) non-uniform depth profiles.
corresponding 0(opt) values. Clearly for 0(elec) ~ 0.4 there is good agreement between the two techniques for the many uniform layers studied. Also in this region it can be clearly seen that non-uniformity causes disagreement, as shown by etching experiments. However, as 0(elec) approaches values by 0.7,0(elec) the results appear to be better high represented — 20(opt). Although most of the samples in this region are non-uniform, there are also a few uniform layers,
C. Pickering et a!.
/
Optical characterisation of acceptors in VPE lnP
and the effect of the non-uniformity may not be large enough to fully account for the discrepancy. The discrepancy at large compensation may be due either to the presence of unknown scattering processes or a breakdown in the assumption of a random impurity distribution required for the PL analysis. However, the sharpness of the exciton lines in some samples with high O(elec) indicates that it is unlikely that these samples have high impurity concentrations. Further evidence of the self-consistency of the PL method will be given later by the variation of the ratios of the strengths of various lines in the PL spectra. Several scattering mechanisms have been proposed recently to explain anomalously low mobilities such as scattering by localised potentials [8] which can lead to large reductions in 9. Pressure and temperature experiments are in progress together with iterative calculations of mobility to investigate these effects.
145
NA(opt) increases with ND — NA with a fairly constant compensation ratio of approximately 0.1 within the scatter. This may be increasing towards 0.2 as ND — NA tends towards 1016 cm3. It can be seen that this trend is independent of dopant. A constant compensation ratio of about 0.3 based on electrical data has been previously reported for ND NA> 1015 cm3 for VPE GaAs [9] and LPE InP [10) doped with various donors. The present results indicate the existence of a constant cornpensation ratio in VPE InP with ND — NA 3 although at a somewhat lower 10i4_1016 level. A fewcm MOCVD layers [11] have also been investigated and these appear to be less cornpensated than the VPE layers (fig. 4). This may indicate that the compensating centre is associated with P vacancies since the MOCVD layers are grown under much more P-rich conditions. Fig. 5 shows the variation of n~with ND NA and it can be seen that n 1 also increases with ND NA, independent of dopant, and dominates the increase in NA. This diagram also shows that the few Ples—
—
—
—
4.2. Comparison with DLTS measurements
The optical acceptor concentrations have also been compared with the results of DLTS measurements, a technique which also measures the surface region. In most samples, Q, a trap close to the centre of the gap, is the dominant level but traps R and S, which are probably acceptors, are also present. Fig. 3 shows a comparison of NA(opt) with n1, the sum of traps Q, R and S. It can be seen that for the RSRE samples, both uniform and non-uniform there is a good correlation between n1 and NA(opt) except at low concentrations where n~is lower. This would be expected since the shallow Zn acceptor concentration appears in NA(opt) but not in n 1. These results therefore imply that the compensation in the RSRE layers is dominated by deep acceptors3,with a residual Zn a level consistent concentration of < 10i4 cm samples, however, with other analyses. The Plessey fall further below the line of agreement, indicating lower deep acceptor concentrations with a slightly higher Zn background. This is consistent with the observation of higher acceptor/donor bound exciton ratios (ABE/DBE) in some Plessey samples. Further information about the compensating centres may be obtained from their variation as a function of carrier density. Fig. 4 shows that
o’~nt~NA____,..,7 ,. ‘,‘
,‘ //
.
~t 3
14 0
~I
—
Icm’
/
I I~
,‘s • ,/
•,“
•
-
,“
3 0
-. -
i
,
/~
o
/ I
•
./ 12 ;Ixio’3
~
/
I ‘
,‘ ‘~/•
°
~s~ioi3I.5xIO~cm3 i4
I
I
I
I~
I
NA(OPS) (cm”3) Fig. 3. Comparison of optical acceptor concentrations with deep trap concentrations (Q + R + S). Shallow Zn acceptor concentrations would be included in NA(opt) but not n~as shown by the dashed lines. (S) RSRE sample. (D) Plessey sample.
146
C. Pickering ci at.
/
Optical characterisation of acceptors- in VPE lnP
concentrations. sey layers investigated have lower deep acceptor
(.undoped.oS-doped,~Si-doped. ~MOCVD)
;,~ /
IS 10-
“~ / ~/%‘3•~
NAloPtl~-
lcm~3)L 14
O’O-l 7,
0005p”
‘N ,~/‘
~
“
/
..
/
•
•
,,.•
•
~‘
I
acceptor concentration. It was reported previously [21that the ratio of the donor-bound exciton peak to the DAP peak, DBE/DAP. measured under standard conditions, decreased monotonically with
0~, /
~
L
13 ~
~,
by trends in the PL spectra as a function of
/
/
~
,‘
.“
5/
/
/
/
,
dependence increasing NA(opt) on NA(elec). while there For was the no RSRE consistent layers there wasfig.a 6. marked difference between the unlines in Thelayers, high purity undoped RSRE doped and S-doped as shown by the dashed samples of table I are spread between the two
•
~‘
/ /
/
7
I
/ /
13 /
e
io
1Q14 05 ND~NAlcm~3l
lola
—
Fig. 4. Dependence of NA(opt) on carrier density for VPE loP: (•) undoped.(O) S-doped, Si-doped.(+) MOCvE).
(js)
IRSREs.undoped.oS-doped. Plessey • undoped. oS ‘doped,
1016
Additional evidence of the self-consistency of
the PL method for determination of 9 is provided
~
A “
‘
I
0 ~
/
7
5’0-25
4.3. Donor species-dependent effects
I
A
lines where they converge. This is consistent with
the findings of Dean et al. [12), using a photoluminescence bound exciton satellite technique [13], that both S and Si are present in these l.undoped,oS’doped. 1_’ ~ l00~ • ‘i~~: + + ~_____ s~
“
Si - doped I
L
0’
/ / I./
IS 10-
//
“
DBE
~-
limits of NAIopt)~~—.—_~/
4 10
/
// /
0
/ -
0
o
‘
F
13 10-
,/ /
/ / // / /
/
•
0
/
I
... ‘..,,
.—
5
5
5...
5
0
3
S
5 ~,
undoped
o
SS
0
1
0-I 1013
I
•
/
“
0
S-doped
/
,
/
S5
,/
/
/0 • I / / 5 / II ,/ A
5•
/
S
10
/
b
lQ~R’S) lcm’31
+
5
/
Si-dopedl
\ ,,
I/
/
+
9-
S.
5 5
S
IRSREII
5.
io14 I0i~ NAloptllcm’31
10~
I
l0’°
Fig. 6. DBE/DAP intensity ratio versus NA(opt) for Plessey lnP layers ((•) undoped, (0) S-doped, (+) Si-doped) at 4.2 K with 1 W cm 2 power. The dotted lines show the results
carrier
obtained on RSRE undoped and S-doped samples from ref. 12]
S-doped;
with S-doped line revised by the inclusion of more data (scaler indicates range of table I samples).
I
4 0i3 iolND’ NA tcm3l Fig. 5. Dependence of deep trap density
1Q15
(Q + R + S) on
density for VPE InP. RSRE: (S) undoped, (0) Plessey: (•) undoped, (0) S-doped, (~) Si-doped.
C. Pickering ci a!.
/
147
Optical characterisation of acceptors in VPE InP
samples with ~ dominant at the very low carrier densities. At higher carrier densities Si is the domi-
samples with the same NA, due to a large decrease in the DAP peak for Si doping. A difference
nant donor in the undoped samples [12] and the lines diverge. The formation of Zn—S nearestneighbour pairs was proposed as a possible explanation, since the consequent reduction of [Zn] would increase the ordinate. The Plessey undoped and S-doped layers also show a clear dependence of DBE/DAP on NA(opt) but within the scatter there is no difference for the two types of sample. This is again consistent with the observation [12] that the Plessey undoped samples had a higher S content and do not show the change to Si as dominant donor at higher carrier densities as seen in the RSRE undoped samples. The higher Zn background in the Plessey layers deduced earlier may be the reason that these results lie below the RSRE S-doped line. Fig. 6 also shows the DBE/DAP ratios of the Si-doped samples. Here it can be clearly seen that there is a large difference between the ratios for Plessey Si- and S-doped
would have been expected since Si is the dominant donor but in the opposite sense by analogy with the RSRE undoped samples. This surprising result is not understood. Further evidence for the formation of donor—acceptor complexes may he indicated by the observation of an anomalous broad band in some samples. This band decayed rapidly in RSRE samples (fig. 7a) over a period of a few weeks and therefore may have been present initially in many more samples which were only studied some time after growth. A similar band was also seen in some Plessey samples, studied several months after growth, which may have been more persistent or decayed from a higher level. The energy position of the peak may be due to a large concentration of intermediate range associates, presumably of Zn and Si or S. This explanation is supported by the unusually large shift to lower energies as the excitation power is reduced (fig. 7b). The width and temperature dependence of the band indicate that it is not a distant DAP/FB band due to an
Energy 1eV) 142
140
138
unusually shallow acceptor species.
I
I
Storage lemp Solid C02 ~‘R oom
4.4. Surface band-bending effects
At high illumination levels, the optically generated electron—hole flux should produce flat-band
~ a
(month
o:~00A;
7~tavv5~otvr
2 5mWcm
8800
9000
Wovelength (A) Fig. 7. (a) Time dependence and (b) temperature and excitation power dependence of broad band observed in some InP samples.
Fig. 8. Normalised intensity of FB band versus bias for semitransparent Schottky barriers
((x)
200, (0) 40,
(•)
5 mW
cm2)). The intensity levels used for the compensation measurement lie between the dashed and full curves allowing for attenuation by the Au contact.
148
C~ Pickering ci at.
/
Optical characierisation of acceptors in VPE lnP
conditions at the surface, but at the low levels for the compensation analysis band-bending
used may occur. We have investigated this by varying the bias applied to a semi-transparent Au Schottky contact. Fig. 8 shows the bias dependence of the intensity of the FB band for various levels of illumination. Two samples were studied, one with a barrier height of about 0.3 eV which should approximate the conditions at the free surface, and one with an anomalously large barrier height of 0.8 eV, possibly due to some interfacial oxide layer. At zero bias in the “normal” sample there is little dependence on the illumination level. Under reverse bias the PL intensity is depressed due to the increased band-bending and depletion depth hut this effect can be seen to be counteracted by the high illumination levels reducing the band-
concentrations from DLTS measurements, with the Plessey layers having lower deep acceptor concentrations than the RSRE layers. NA was found to increase with carrier density, independent of dopant, and the increase was dominated by the deep acceptors.
bending. The results indicate that the intensity
[2) C. Pickering, P.R. Tapster, P.J. Dean and D.J. Ashen, in:
levels used for the compensation determination are high enough to produce approximately flat-band conditions at the free surface. Furthermore, the FB/DAP ratio is affected very little, surprisingly decreasing slightly at high reverse bias. This mechanism cannot therefore explain the anomalous increase in the FB peak at very low excitation powers reported in ref. [2].
Proc. 10th Intern. Symp. on GaAs and Related Corn-
Acknowledgement
Copyright Controller © FIMSO. London. 1983.
References [1] T Kamiya and E Wagner, J. AppI. Phys. 48 (1977) 1928
pounds. Albuquerque, NM, 1982. Inst. Phys. Conf. Ser. 65 (Inst. Phys.. London, 1983) p. 469. Taylor and D. Anderson, J. Crystal Growth 64(1983)
[31L.L.
[4] P.L. Giles, P. Davies and NB. Hasdell, J. Crystal (irowih 64 (1983) 60. [5]
P.R. Tapster, MS. Skolnick, RU. Humphreys, P.i. t)ean, B. Cockayne and W.R. MacEwan. J. Phys. C (Solid State Phys.) 14 (1981) 5069.
16] W. Walukiewicz, J. Lagowski. L. Jastrzebski. P. Rava. M. Lichtensteiger, C.H. Gatos and H.C. Gaios, J. Appi. Phys.
5. Conclusions 17]
The photolurninescence technique has been used to determine the compensation in a large number of undoped InP layers and layers doped with S and Si donors. Good agreement with electrical estimates was obtained for uniform layers with 0 ~ 0.4 while non-uniform samples and samples with high 0(elec) showed large discrepancies. There was a good correlation between the optical accep-
tor concentrations and estimates of deep acceptor
51(1980) 2659. A. Chandra, C.E.C. Wood, D.W. Woodward and L.F. Eastman, Solid State Electron. 22 (1979) 645.
18]
GB. Stringfellow and H. Kunzel. J. AppI. Phys. 51(19801 3254 [9] J.B. Mullin. A. Royle and S. Benn. J. Crystal Growth 50 (1980) 625. [10] E. Kuphal, J. Crystal Growth 54(1981)117. [II] Si. Bass, C. Pickering and ML. Young. J. Crystal Growth 64 (1983) 68. [12] P.J Dean, M.S Skolnick and L L Taylor. J AppI Phys, to he published. [13] P.i. Dean and MS. Skolnick, J. AppI. Phys. 54(1983) 346.
Journal of (‘r~stalGrov,th 64(1 1~h3) 142 145 North—Holland Publishing (‘ompanv
OPTICAL CHARACTERISATION OF ACCEPTORS IN DOPED AND UNDOPED VPE InP C. PICKERING, P.R. TAPSTER, P.J. DEAN, L.L. TAYLOR, P.L. GILES
*
and P. DAVIES
*
Rosa/ Signals and Radar Establishment, St. Andrews Road, Great Ma/tern. Worc~str’rshirc’It R14 3P5. (K
The compensation ratios (8 = NA/ND) of a large number of InP layers. undoped and doped with S and Si donors, have been determined by analysis of photoluminescence spectra. Good agreement with electrical estimates of 0 was obtained for uniform samples with 8 ~ 0.4, while non-uniform samples and samples where the electrical data indicated high compensation showed large discrepancies. A good correlation was obtained with deep acceptor concentrations from DLTS measurements. indicaiing that the compensation is dominated by deep acceptors except at low carrier densities where the shallow (Zn) acceptors are important. N,
5 ~sas
found to increase with carrier density, independent of dopant, and the increase was dominated by the deep acceptors.
1. Introduction The compensation ratio, 0, is normally obtained from an analysis of Hall measurements as a function of temperature. This method relies on ideal contacts to uniform material and can lead to an overestimate of 0 if unknown scattering processes are important. An independent method for determination of 0 is therefore very useful and in 1977 Kamiya and Wagner [11 used analysis of photoluminescence (PL) lineshapes to estimate 0 in GaAs. Pickering et al. [2] recently extended this technique to InP and this paper reports results on very high purity undoped InP recently grown at RSRE [3] and InP doped with S and Si donors grown at Plessey Research Ltd. [41.These results, together with the earlier data [21, are compared with Hall and DLTS measurements.
2. Experimental The epitaxial layers were grown by vapour-phase epitaxy using the ln—PC1~-H2system. The high purity undoped InP layers were grown at RSRE under constant mole fraction conditions but with different values of PCi5 flux [3]. The layers, which were 7—11 j~m thick, were assessed by electro*
Plessey Research (Caswell)
Ltd..
Allen Clark Research
Centre, Caswell, Towcester. Northants NNI2 8QE, UK.
chemical profiling and Hall measurements at 300 and 77 K. The electrical data are given in table I. The InP layers grown at Plessey were 4 6 p.m thick with carrier densities in the range 3 X iü~~7 x iO’~cm~. S and Si donors were incorporated in the layers by doping with H,S and SiH-,CI, in the gas phase. Photoluminescence measurements were made using the 488 nm line from an Ar ion laser and a Spex 0.75 m spectrometer. with the sample in a continuous flow He cryostat. DLTS measurements were performed on 0.5 mm Ti Au Schottky barrier devtces as described by Tapster et al. [5].
3. Review of photoluminescence technique The PL technique for determination of 0 has been described in detail previously [1] and its application to InP discussed fully [2]. Only the main points will therefore he mentioned here. In PL spectra of InP, peaks are often observed, a few tens of meV below the band edge, due to recombination of electrons with holes hound to shallow acceptors. In VPE InP only one shallow acceptor. Zn, is normally observed, with an activation energy of -~ 46.5 meV. Under low excitation conditions. this band can he resolved into two peaks free-to-hound (FB), and donor~acceptor pair (DAP) bands due to recombination of the free and bound electrons, respectively. Analysis of the
0022-0248/83/0000—0000/$03.00 © 1983 North-Holland
/
C. Pickering ci al.
Optical characterisazion of acceptors in
VPE
Table I Electrical properlies and optical results of undoped epitaxial layers grown with various values of 0H,S
477K
(Torr) P( PCI ~)
(XH~H 1014 cm~3) n
10 10’~ 10 “~ 16 160 16 24 33~ 33 ~ 33 5i
(x1014 cm 3) ne,,,,
(cm2 V’ s
1.6
2.1
115000
5.6 0.5
6.4 1.4
87000
2.5 2.7 0.6 3.7 1.5 0.8 2.1
3.3 3.5 1.4 4.7 2.5 1.6 3.3
118000 97000 130000 76000 62500 104000 76000
0~orr
0,3,
I)
.-
0.3 0.05 —
—
0.05 0.2 0.5 0.3 0.7 0.58 0.5
P(PCI
3)
,\A(opi) (X 10’3cm
0.15
0.08
1.8
0
0.03 0.1
2.0 1.5
—0 0.1 0.1 0.1 0.27 0.2 0.15
—0 3.9 1.6 5.2 9.3 4.0
—
—0 0.08 0.17 0.22 0.55 0.37 0.35
143
InP
5.8
3)(
N~(elec) X 1013cm 3) 3.7 —
0 --
—0 3.0 2.9 13 31 9.4 18
.0 Profile approximately flat.
s~Profile ramped
FB/DAP
(
> factor of 2 change).
spectrum at temperatures where the
P(R) is the Poisson distribution and E(
peaks are of a similar intensity (— 10—20 K) gives information about the distribution of electrons between the conduction band and the donors, which is a function of ND, NA, temperature and the donor ionisation energy, ED. The FB/DAP lineshape is described theoretically the sum of theof intensities to each processas taking account competingdue transitions. The final expression for the lineshape. obtained from the individual expressions [1,21, is
3, EA and EF are the band-gap, acceptor ionisation and Fermi energies, respectively. EF depends on ED, the donor ionisation energy, and the concentration dependence of ED must be taken into account [2]. The theoretical lineshape obtained from the above equation is fitted to experimental data obtained 2 with low ND, laserTexcitation (— 5 mW cm [2]) with and W as energy the adjustable parameters. Thus the total donor concentration is ohtamed from the fit and the total acceptor con-
VE/
centration, ratio,or isC—V obtamed usingand ND hence — NA compensation known from Hall
— —
I L)A~ 1E /
+
~eA\(E
4~rN~R34~7’~()
4
=ER
exp
-
N W+ 3
measurements at room temperature. The value of
___
3
xJ2(R)
r
____
e2
NA obtained from the analysis should therefore
N
—
D
include all acceptors, both shallow and deep, even though the analysis is performed on the peaks due to only one shallow acceptor.
A
WDA(R)
*3/2
+~-
2
4..aAm,
/
Xexp~— ~-
X
E(E_E(;+EA)i/
4. Results and discussion
E-EG+EA—EF _____________
kT
P(R)dR
~ w+ w
A(R)
D
4.1. Comparison with Hall measurements Table 1 shows the results obtained on the high purity RSRE samples grown with various values of
where R is the separation of neutral donors and
PCI
neutral acceptors, J(R) is the overlap integral between donor and acceptor wavefunctions, aA is the effective Bohr radius of the acceptor, WDA( R) is the neutral donor — neutral acceptor transition rate, W is the total competing transition rate,
3 partial pressure, P(PC13). The electrical cornpensation ratios were determined using the calculations of Walukiewicz et al. [6] with the measured carrier density and 77 K Hall mobility. However, the samples had measured carrier densities the 3 and at this inlevel range 5 >< 10~~—5 x 1014 cm
144
C. Pickering et at.
/
Optical characterisation of acceptors in VPE InP
of n and 0 have therefore been corrected for depletion assuming a model with a 0.2 eV harrier at the surface and a 0.6 eV barrier at the interface.
1016 ,t9\5Oi~
x ~
•,‘,/~,‘
~
~ 10~ x
~‘,,/~ç’
)
,~/,‘ ,~/,,NAloptl~NAleIec)
•._
NA (elecl (cm )
,~/,~)
~
x
X
•,•
~
x.271
-
(Within the scatter this model produces good agreement between nc,,rr and the results of C--- V measurements.) From the table this can be seen to have a marked effect on 0 determined from the electrical measurements. A further problem for a comparison of the values of 0 obtained from the electrical and optical measurements is the non-uniformity of the layers. Most of this series of sampies had non-uniform depth profiles and the optical technique measures the top 1 p.m or so while the electrical method averages over the whole depth of the layer. In the more uniform layes. the two
06
techniques in reasonable (see later). The trend are of increasing NA agreement with P(PCI 3) as reported in ref. [3] can be clearly seen in the final
Fig. 1. Comparison of optical and electrical acceptor concentrations: (•) uniform, (x) non-uniform depth profiles.
two columns of the table, with the optical surface measurement showing a somewhat less steep in-
i3
~I0 iS
I
I
NA(opt)(cm I
depletion effects at the surface and layer/substrate interface cannot be ignored [7]. The metallurgical thickness, normally obtained from cleaveand-stain measurements, may be larger than the electrical thickness, leading to an underestimate in the carrier density. Since the mobility is not affected by the thickness measurement, the cornpensation ratio will be overestimated. The values l.unuform, x romPed)
could occur here if the interface region was highly compensated and dominated the electrical measurements. In fig. 2 it can be seen that most of the
0-6 ‘
dec 0-4
/
etch
•
51 dec
crease. The acceptor concentrations and compensation ratios obtained by the optical and electrical techniques are compared for a large number of sampies in figs. 1 and 2. These results include the present RSRE and Plessey samples together with the earlier data from ref. [2]. Fig. I shows good agreement between N~(opt)and NA(elec) for nearly all the uniform layers while all the non-uniform samples show large discrepancies. The effect of removing some of the layer by etching can be seen to improve the agreement in some cases but this does not always occur. A source of discrepancy
.~
opt
non-uniform samples had large 0(elec). which were about a factor of twovalues largerofthan the
X-----e
•
• ,/ 0-2 ~
,‘•
•,~• 0 0-2 0-4 0opt 0-6 0-8 Fig. 2. Comparison of optical and electrical compensation ratios: (•) uniform, (x) non-uniform depth profiles.
corresponding 0(opt) values. Clearly for 0(elec) ~ 0.4 there is good agreement between the two techniques for the many uniform layers studied. Also in this region it can be clearly seen that non-uniformity causes disagreement, as shown by etching experiments. However, as 0(elec) approaches values by 0.7,0(elec) the results appear to be better high represented — 20(opt). Although most of the samples in this region are non-uniform, there are also a few uniform layers,
C. Pickering et a!.
/
Optical characterisation of acceptors in VPE lnP
and the effect of the non-uniformity may not be large enough to fully account for the discrepancy. The discrepancy at large compensation may be due either to the presence of unknown scattering processes or a breakdown in the assumption of a random impurity distribution required for the PL analysis. However, the sharpness of the exciton lines in some samples with high O(elec) indicates that it is unlikely that these samples have high impurity concentrations. Further evidence of the self-consistency of the PL method will be given later by the variation of the ratios of the strengths of various lines in the PL spectra. Several scattering mechanisms have been proposed recently to explain anomalously low mobilities such as scattering by localised potentials [8] which can lead to large reductions in 9. Pressure and temperature experiments are in progress together with iterative calculations of mobility to investigate these effects.
145
NA(opt) increases with ND — NA with a fairly constant compensation ratio of approximately 0.1 within the scatter. This may be increasing towards 0.2 as ND — NA tends towards 1016 cm3. It can be seen that this trend is independent of dopant. A constant compensation ratio of about 0.3 based on electrical data has been previously reported for ND NA> 1015 cm3 for VPE GaAs [9] and LPE InP [10) doped with various donors. The present results indicate the existence of a constant cornpensation ratio in VPE InP with ND — NA 3 although at a somewhat lower 10i4_1016 level. A fewcm MOCVD layers [11] have also been investigated and these appear to be less cornpensated than the VPE layers (fig. 4). This may indicate that the compensating centre is associated with P vacancies since the MOCVD layers are grown under much more P-rich conditions. Fig. 5 shows the variation of n~with ND NA and it can be seen that n 1 also increases with ND NA, independent of dopant, and dominates the increase in NA. This diagram also shows that the few Ples—
—
—
—
4.2. Comparison with DLTS measurements
The optical acceptor concentrations have also been compared with the results of DLTS measurements, a technique which also measures the surface region. In most samples, Q, a trap close to the centre of the gap, is the dominant level but traps R and S, which are probably acceptors, are also present. Fig. 3 shows a comparison of NA(opt) with n1, the sum of traps Q, R and S. It can be seen that for the RSRE samples, both uniform and non-uniform there is a good correlation between n1 and NA(opt) except at low concentrations where n~is lower. This would be expected since the shallow Zn acceptor concentration appears in NA(opt) but not in n 1. These results therefore imply that the compensation in the RSRE layers is dominated by deep acceptors3,with a residual Zn a level consistent concentration of < 10i4 cm samples, however, with other analyses. The Plessey fall further below the line of agreement, indicating lower deep acceptor concentrations with a slightly higher Zn background. This is consistent with the observation of higher acceptor/donor bound exciton ratios (ABE/DBE) in some Plessey samples. Further information about the compensating centres may be obtained from their variation as a function of carrier density. Fig. 4 shows that
o’~nt~NA____,..,7 ,. ‘,‘
,‘ //
.
~t 3
14 0
~I
—
Icm’
/
I I~
,‘s • ,/
•,“
•
-
,“
3 0
-. -
i
,
/~
o
/ I
•
./ 12 ;Ixio’3
~
/
I ‘
,‘ ‘~/•
°
~s~ioi3I.5xIO~cm3 i4
I
I
I
I~
I
NA(OPS) (cm”3) Fig. 3. Comparison of optical acceptor concentrations with deep trap concentrations (Q + R + S). Shallow Zn acceptor concentrations would be included in NA(opt) but not n~as shown by the dashed lines. (S) RSRE sample. (D) Plessey sample.
146
C. Pickering ci at.
/
Optical characterisation of acceptors- in VPE lnP
concentrations. sey layers investigated have lower deep acceptor
(.undoped.oS-doped,~Si-doped. ~MOCVD)
;,~ /
IS 10-
“~ / ~/%‘3•~
NAloPtl~-
lcm~3)L 14
O’O-l 7,
0005p”
‘N ,~/‘
~
“
/
..
/
•
•
,,.•
•
~‘
I
acceptor concentration. It was reported previously [21that the ratio of the donor-bound exciton peak to the DAP peak, DBE/DAP. measured under standard conditions, decreased monotonically with
0~, /
~
L
13 ~
~,
by trends in the PL spectra as a function of
/
/
~
,‘
.“
5/
/
/
/
,
dependence increasing NA(opt) on NA(elec). while there For was the no RSRE consistent layers there wasfig.a 6. marked difference between the unlines in Thelayers, high purity undoped RSRE doped and S-doped as shown by the dashed samples of table I are spread between the two
•
~‘
/ /
/
7
I
/ /
13 /
e
io
1Q14 05 ND~NAlcm~3l
lola
—
Fig. 4. Dependence of NA(opt) on carrier density for VPE loP: (•) undoped.(O) S-doped, Si-doped.(+) MOCvE).
(js)
IRSREs.undoped.oS-doped. Plessey • undoped. oS ‘doped,
1016
Additional evidence of the self-consistency of
the PL method for determination of 9 is provided
~
A “
‘
I
0 ~
/
7
5’0-25
4.3. Donor species-dependent effects
I
A
lines where they converge. This is consistent with
the findings of Dean et al. [12), using a photoluminescence bound exciton satellite technique [13], that both S and Si are present in these l.undoped,oS’doped. 1_’ ~ l00~ • ‘i~~: + + ~_____ s~
“
Si - doped I
L
0’
/ / I./
IS 10-
//
“
DBE
~-
limits of NAIopt)~~—.—_~/
4 10
/
// /
0
/ -
0
o
‘
F
13 10-
,/ /
/ / // / /
/
•
0
/
I
... ‘..,,
.—
5
5
5...
5
0
3
S
5 ~,
undoped
o
SS
0
1
0-I 1013
I
•
/
“
0
S-doped
/
,
/
S5
,/
/
/0 • I / / 5 / II ,/ A
5•
/
S
10
/
b
lQ~R’S) lcm’31
+
5
/
Si-dopedl
\ ,,
I/
/
+
9-
S.
5 5
S
IRSREII
5.
io14 I0i~ NAloptllcm’31
10~
I
l0’°
Fig. 6. DBE/DAP intensity ratio versus NA(opt) for Plessey lnP layers ((•) undoped, (0) S-doped, (+) Si-doped) at 4.2 K with 1 W cm 2 power. The dotted lines show the results
carrier
obtained on RSRE undoped and S-doped samples from ref. 12]
S-doped;
with S-doped line revised by the inclusion of more data (scaler indicates range of table I samples).
I
4 0i3 iolND’ NA tcm3l Fig. 5. Dependence of deep trap density
1Q15
(Q + R + S) on
density for VPE InP. RSRE: (S) undoped, (0) Plessey: (•) undoped, (0) S-doped, (~) Si-doped.
C. Pickering ci a!.
/
147
Optical characterisation of acceptors in VPE InP
samples with ~ dominant at the very low carrier densities. At higher carrier densities Si is the domi-
samples with the same NA, due to a large decrease in the DAP peak for Si doping. A difference
nant donor in the undoped samples [12] and the lines diverge. The formation of Zn—S nearestneighbour pairs was proposed as a possible explanation, since the consequent reduction of [Zn] would increase the ordinate. The Plessey undoped and S-doped layers also show a clear dependence of DBE/DAP on NA(opt) but within the scatter there is no difference for the two types of sample. This is again consistent with the observation [12] that the Plessey undoped samples had a higher S content and do not show the change to Si as dominant donor at higher carrier densities as seen in the RSRE undoped samples. The higher Zn background in the Plessey layers deduced earlier may be the reason that these results lie below the RSRE S-doped line. Fig. 6 also shows the DBE/DAP ratios of the Si-doped samples. Here it can be clearly seen that there is a large difference between the ratios for Plessey Si- and S-doped
would have been expected since Si is the dominant donor but in the opposite sense by analogy with the RSRE undoped samples. This surprising result is not understood. Further evidence for the formation of donor—acceptor complexes may he indicated by the observation of an anomalous broad band in some samples. This band decayed rapidly in RSRE samples (fig. 7a) over a period of a few weeks and therefore may have been present initially in many more samples which were only studied some time after growth. A similar band was also seen in some Plessey samples, studied several months after growth, which may have been more persistent or decayed from a higher level. The energy position of the peak may be due to a large concentration of intermediate range associates, presumably of Zn and Si or S. This explanation is supported by the unusually large shift to lower energies as the excitation power is reduced (fig. 7b). The width and temperature dependence of the band indicate that it is not a distant DAP/FB band due to an
Energy 1eV) 142
140
138
unusually shallow acceptor species.
I
I
Storage lemp Solid C02 ~‘R oom
4.4. Surface band-bending effects
At high illumination levels, the optically generated electron—hole flux should produce flat-band
~ a
(month
o:~00A;
7~tavv5~otvr
2 5mWcm
8800
9000
Wovelength (A) Fig. 7. (a) Time dependence and (b) temperature and excitation power dependence of broad band observed in some InP samples.
Fig. 8. Normalised intensity of FB band versus bias for semitransparent Schottky barriers
((x)
200, (0) 40,
(•)
5 mW
cm2)). The intensity levels used for the compensation measurement lie between the dashed and full curves allowing for attenuation by the Au contact.
148
C~ Pickering ci at.
/
Optical characierisation of acceptors in VPE lnP
conditions at the surface, but at the low levels for the compensation analysis band-bending
used may occur. We have investigated this by varying the bias applied to a semi-transparent Au Schottky contact. Fig. 8 shows the bias dependence of the intensity of the FB band for various levels of illumination. Two samples were studied, one with a barrier height of about 0.3 eV which should approximate the conditions at the free surface, and one with an anomalously large barrier height of 0.8 eV, possibly due to some interfacial oxide layer. At zero bias in the “normal” sample there is little dependence on the illumination level. Under reverse bias the PL intensity is depressed due to the increased band-bending and depletion depth hut this effect can be seen to be counteracted by the high illumination levels reducing the band-
concentrations from DLTS measurements, with the Plessey layers having lower deep acceptor concentrations than the RSRE layers. NA was found to increase with carrier density, independent of dopant, and the increase was dominated by the deep acceptors.
bending. The results indicate that the intensity
[2) C. Pickering, P.R. Tapster, P.J. Dean and D.J. Ashen, in:
levels used for the compensation determination are high enough to produce approximately flat-band conditions at the free surface. Furthermore, the FB/DAP ratio is affected very little, surprisingly decreasing slightly at high reverse bias. This mechanism cannot therefore explain the anomalous increase in the FB peak at very low excitation powers reported in ref. [2].
Proc. 10th Intern. Symp. on GaAs and Related Corn-
Acknowledgement
Copyright Controller © FIMSO. London. 1983.
References [1] T Kamiya and E Wagner, J. AppI. Phys. 48 (1977) 1928
pounds. Albuquerque, NM, 1982. Inst. Phys. Conf. Ser. 65 (Inst. Phys.. London, 1983) p. 469. Taylor and D. Anderson, J. Crystal Growth 64(1983)
[31L.L.
[4] P.L. Giles, P. Davies and NB. Hasdell, J. Crystal (irowih 64 (1983) 60. [5]
P.R. Tapster, MS. Skolnick, RU. Humphreys, P.i. t)ean, B. Cockayne and W.R. MacEwan. J. Phys. C (Solid State Phys.) 14 (1981) 5069.
16] W. Walukiewicz, J. Lagowski. L. Jastrzebski. P. Rava. M. Lichtensteiger, C.H. Gatos and H.C. Gaios, J. Appi. Phys.
5. Conclusions 17]
The photolurninescence technique has been used to determine the compensation in a large number of undoped InP layers and layers doped with S and Si donors. Good agreement with electrical estimates was obtained for uniform layers with 0 ~ 0.4 while non-uniform samples and samples with high 0(elec) showed large discrepancies. There was a good correlation between the optical accep-
tor concentrations and estimates of deep acceptor
51(1980) 2659. A. Chandra, C.E.C. Wood, D.W. Woodward and L.F. Eastman, Solid State Electron. 22 (1979) 645.
18]
GB. Stringfellow and H. Kunzel. J. AppI. Phys. 51(19801 3254 [9] J.B. Mullin. A. Royle and S. Benn. J. Crystal Growth 50 (1980) 625. [10] E. Kuphal, J. Crystal Growth 54(1981)117. [II] Si. Bass, C. Pickering and ML. Young. J. Crystal Growth 64 (1983) 68. [12] P.J Dean, M.S Skolnick and L L Taylor. J AppI Phys, to he published. [13] P.i. Dean and MS. Skolnick, J. AppI. Phys. 54(1983) 346.