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Growth and photoluminescence study of several single crystal segments relevant to monolithic semiconductor cascade solar cells
Journal of Crystal Growth 63 (1983) 25—33 North-Holland Publishing Company
25
GROWTH AND PHOTOLUMINESCENCE STUDY OF SEVERAL SINGLE CRYSTAL SEGMENTS RELEVANT TO MONOLITHIC SEMICONDUCTOR CASCADE SOLAR CELLS Roger S. SILLMON and Anton F. SCHREINER Department of Chemistry, North Carolina State University, Raleigh, N rth Carolina 27650, USA
and Michael TIMMONS Research Triangle Institute, Research Triangle Park, North Carolina 27709, USA
Received 26 July 1982; manuscript received in final form 31 March 1983
Several representative single crystal stacked layers of 111—V compound and alloy semiconductors were grown which are spatial regions relevant to a monolithic cascade solar cell, including the substrate. n-GaAs(Si). which was pre-growth heat treated in H 2(g) prior to its use. These structures were then studied by cryogenic laser excited photoluminescence (PL). and the substrate portion was explored in a depth profiling mode. Within the forbidden band gap region up to seven recombinations were observed and identified for undoped GaAs layers or the GaAs(Si) substrate, and several other PL recombinations were observed for undoped Al~Ga — ~As and Al ~.Ga1— ~Sb~As1 .~ layers. In addition to the valence and conduction bands, these optical bands are also associated with the presence of C0a, Sioa, SiA~, Cu0~,V~,V0a and vacancy—impurity complexes involving several of these defect types even in the absence of intentional doping. The findings also relate to problems of self-compensation and type inversion, so that the need for growth modifications is indicated.
1. Introduction Several types of single crystal Ill—V compound and alloy semiconductor heterojunctions are currently being studied for various optoelectronic devices. Among such devices is the monolithic cascade solar cell which shows some promise for achieving high solar energy conversion efficiencies [1—3] and perhaps as a broadband detector. A monolithic cascade solar cell requires multiple layers of homojunctions and heterojunctions of semiconductor crystals to be grown in a single structure. Each layer of this sequence of single crystal growth is ideally required to have the same lattice constant and orientation. Another efficiency requirement is that the band gap combinations of the top and bottom cells as well as the tunnel junction must be chosen so as to split the solar spectrum in order to generate the same photocurrent in each cell. One of such optimized 0022-0248/83/0000—0000/$03 .00
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systems employed consists of crystalline layers of GaSb~As1_5 and Al~,Ga1_~Sb2As1~ alloys. The sequence of layers for the complete cascade solar cell is as follows: n-GaAs substrate n-GaSbAs p-GaSbAs ptAlGaSbAs n~-AlGaSbAsH n-AlGaSbAs H p-AlGaSbAs. In practice the growth of such multiple layers of these alloys on GaAs substrates can introduce considerable lattice mismatch, which in turn can degrade solar cell performance by the presence of recombinations involving several types of defects [4,5]. The growth and observed defects of such layers or portions of a cascade cell are described here. The PL technique was selected in order to probe the electronic structures because of its high sensitivity and since many recombinations within the forbidden band gap are radiative. A number of PL bands of Ill—V alloys are identified from similar activation energies of GaAs, since the alloy stoichiometries differ very little from GaAs. For example, Be activation en-
1983 North-Holland
26
R. S. Silimon et a!.
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Growth and PL study of several single crystal segments
ergies of 27 and 23 meV have been found in this laboratory for, respectively, GaAs and Al0 1Ga0 7As. These growth characterizations suggest the need for crystal growth procedure modifications.
2. Experimental 2.1. Crystal preparations
The crystal layers used in this study (figs. 1 and 2) were grown by liquid phase epitaxy (LPE) using a horizontal slider-boat arrangement. The GaAs substrates (Crystal Specialities) were n-type and had (11 l)A orientation. The Si 3 doping levelanofetch the and had substrates 1 x 1018 cm pit densitywas of n 1500—2000 cm2. The substrates were chemomechanically polished and then etched =
A
B
C
D
for 10 s in HNO3 I HF H20 (3 :2:4. V V V) prior to placing them into the LPE system. The crystal layers were grown from melts prepared with (i) excess undoped polycrystalline GaAs, (ii) high purity (6N) Ga. and (iii) Sb and/or Al for the alloy layers. Any resultant doping of layers was unintentional. For the preparation of crystal layer structure 190, the LPE chamber with the substrate was heated to 800°Cfor 4~h prior to growing the layers. Layer growth resulted from cooling the melt at a rate of 1°C/mm.The cooling ranges were 4°C for the first layer and 3°C for each of the remaining layers. The layer thicknesses were ca. 1.5 ~tm. The substrate of crystal layer structure 200 was subjected to 3 h of pre-growth heating at 780°C followed a growth cooling rate of 0.5°C/mm, and thebycooling range was 20°Cfor each layer. Steps were etched at the substrate surface of crystal layer structure 200 using H 2S04 H205 H20 (10: 1: 1, V V V), and the steps sizes were measured with a Sloan Dektak. 2.2. Photoluminescence measurements
Al,,Ga~~As GaAs
____________________________ GaAs GaAs(Sl)
Substrate
Fig. 1. Crystal layer structure 190: n-GaAs(Si) substrate and stepped layers (1—2 pm thick) GaAs II Aloi5Gao85Sb0o5As5o5 lGaAsIIAlosGao85As and photoluminescence probe posi-
A
~A
I
B
C
I GaAS
GaAs
Q,~m j2.6im
GaAs(Si)
Substrate
Fig. 2. Crystal layer structure 200: n-GaAs(Si) substrate and double layers GaAs 1 GaAs and photoluminescence probe positions A, A1, A2, B, and C.
The facility employed for PL measurements consisted of an argon-ion laser operated at 5145 A (25—75 mW) with the beam focused on the crystals. The laser radiation is optically all absorbed in each individual irradiated layer since the penetration depth at this energy in GaAs is only 0.12 ~tm, and the layers are over ten times thicker. The crystal was mounted in a cryogen-throttling optical dewar such that the crystal was continuously cooled in a stream of cryogen vapor. The temperature (78 K) was measured by means of a thermocouple in the mounted same cryogen so thatvapor. it and The the specimen photolumineswere cence radiation waspass passed through a Spex meter double-pass monochromator and 3/4 detected using etther a cooled (195 K) S-i response photomultiplier tube or a cooled (78 K) lnAs photovoltaic detector. Grating orientations were calibrated with a mercury penlamp. The chopped signal was processed using a current-to-voltage linear preamplifier and selective and lock-in amplifiers. Spectra were not corrected for system response as a function of wavelength because the
R. S. Sitimon et at.
/
primary interest is in the comparison of spectra of different crystals over the same energy range. Spectral resolution was 8 A in the wavelength range 0.80—1.00 ~tm and 16 A in the wavelength range 1.00—1.60 ~.tm.
~
3. Results and discussion
Al0
(‘A..
A
ID
0.15 a085 0.05 s095 15Ga085As (D)
A
~
~a
/
~
S
Fig. 3 shows the 78 K PL spectra of crystal layer structure 190, specifically for layers B (AlGaSbAs) and D (AlGaAs) (fig. 1). The bands at — 1.5 and 1.39 eV are due to PL from the underlying GaAs layers and this emission from these layers is excited from the higher energy (— 1.8 eV) alloy PL and alloy photostimulated carrier diffusion. We believe this to be correct because the alloy layers (E5 1.8 eV; 1.5 /.tm thick) are transparent to these 1.5 and 1.39 eV GaAs luminescence energies and because the spectra in this energy region are virtually identical to those obtained from GaAs layers A and C (see below), Each of these layers B and D has another band in the range 1.46—1.47 eV whose energy is different from the band gap by an amount (0.318 eV for B,
8 .
O.~O
O.A~
O.~8 ‘ O.~2
O.~6
‘
1.00 i.00
/
/~40
LS~IJT~-/
~
7~S:kioc
1
1.5210
~
.055
3.1. Crystal layer structure 190, GaAs(Si) I GaAs(A) r’
147
~ ~2
Al
27
Growth and PL study of several single crystal segments
O.~O
0.75
1.45 1.
0.SO 0.85 WAVELENGTH (~.rG
0.90
0.95
Fig. 3. 78 K photoluminescence spectra of crystal layer structure 190 at layers B (Al 9 5Ga955Sb905As095) (—) and D (Al0 5Ga9 85As) (— —
0.308 eV for D) which is very similar to what is found for the difference between the band gap and the donor-to-gallium vacancy (D —s V0a) transition in the GaAs (0.311 eV) [6—9].For this reason and because the 1.46—1.47 eV band is absent in the GaAs underlayer (figs. 1 and 4), we assign this recombination in these alloy layers, whose cornpositions are nearly that of “parent” GaAs, also to a D V111 transition. The 1.8 eV band of layers B (AlGaSbAs) and D (AlGaAs) of structure 190 is assigned to unresolved conduction band-to-valence band and shallow donor-to-valence band transitions (“nearband-gap luminescence”) of the alloys. Our LPE growth system has always produced n-type Al~
—
8AND~W
i.~o iAo
‘
WAVELENGTH (jim)
Fig. 4. 78 K photoluminescence spectra of crystal layer structure 190 at layers A(GaAs) intensity scales of two spectral regions are not equal.
(
)
and C (GaAs)
(— —
—).
Relative
28
R.S. Sillmon et al.
/
Growth and PL study of several single crystal segments
GaAs, so that this highest PL is believed to contam D VB and CB —s VB recombinations. The shallow red shoulder on each 1.8 eV band is assigned to donor-to-carbon acceptor (D CAS) and conduction band-to-carbon acceptor (CB CAS) transitions unresolved, since an analogous 20 meV red-shift from the near-band-gap peak indicates the involvement of such a shallow acceptor for GaAs. This suggestion for layers B and D of structure 190 also agrees quite well with that reported by Swaminathan et al. [10] for a carbon acceptor in a differently grown crystal, Al() 4 Ga0 86As. —~
—~
—~
—
The presence of the high-energy long tail of the 1.521 eV band resulting from irradiating the A1015Ga055Sb005As095 LPE layer B (spectrum B, fig. 3) grown on LPE GaAs, itself on the heat treated GaAs substrate, is also of interest. We believe that this band must originate from underlayer A of GaAs because our PL spectra of a. range of compositions of AlGaAs (direct gap type) on GaAs produce expected variations of Eg but the 1.51 eV peak remains energy invariant. We believe that the band tailing indicates the presence of a non-abrupt crystal junction between the LPE GaAs (layer A) and the AlGaSbAs layer grown over it. In contrast to this band, an analogous band is very much sharper (spectrum D, fig. 3) for the GaAs layer C grown on the epilayer B of AlGaSbAs rather than the heat treated substrate, This optical comparison of layers B (A1GaSbAs) and D (AlGaAs), with layer B having lattice-enlarging Sb in its composition, then in summary leads to the observations that layer B has (i) fewer near-band-gap recombinations (weaker PL inten—
—
—
________________
Si~—p-
___________
—
CB
-
sity), (ii) a less abrupt crystal junction between itself and the underlying GaAs layer. and (iii) more radiative recombinations involving crystal vacancies than layer D. The 78 K P1 spectra of GaAs layer A (grown on the n-GaAs(Si) substrate) and GaAs layer C (grown on the second of two intervening layers above the substrate) of crystal layer structure 190 are shown in fig. 4; the PL assignments are given in fig. 5 and are as follows. The 1.506 eV near-band-gap luminescence (Band II) of layer A (first GaAs epilayer, on substrate) ts tentatively assigned to a silicon donor-to-valence band transition (Si0. VB). The assignment is based on its activation energy of 5.1 meV being in close agreement with that expected for a hydrogenic donor [11] in GaAs (5.7 meV) and on Si being the commonly present contaminant in LPE type layers. This band must come from layer A and not from the GaAs substrate side of the interface because the PL of the top layer of the substrate itself (fig. does not have this band. Band III (1.478 eV) of layer A is of relatively high intensity and is assigned to a cornbination of silicon donor-to-silicon acceptor (Si(,. SiAS) and conduction band-to-silicon acceptor (CB SiA,) transitions unresolved. By first assuming a CB SiA, transition to be present, an effective activation energy of 36 meV is obtained by subtracting the PL peak energy (1.478 eV) from the band gap energy (1.5111 eV) plus the ~kT correction factor (3.35 meV) for a free electron [12]. This assignment is consistent with the findings by Ashen et al. and Kirkman et al. [12,131 that the activation energy for a silicon acceptor is 35 meV in GaAs crystals of their source. Our value of 36 meV is in agreement with this. The near-band-gap transitions of the other GaAs layer (fig. I), or layer C (third epilayer), is dominated by Band I at 1.513 eV. This Band I of layer C is assigned to the conduction band-to-valence band (CB VB) transition in view of its energy (1.5 13 eV); that band was absent (or very weak) when irradiating GaAs layer A grown on the pre-heated substrate. Band III (CB,D —s SiA,) is nearly absent in GaAs layer C, but prominent in layer A (grown on the substrate), inferring the presence of a much higher SiA, acceptor con—~
7)
—~
—~
—~
—+
HI~
V00~~~ —
—~
Si~ VB
Fig. 5. Schematic of radiative recombination assignments for PL bands observed in crystal layer structure 190.
R.S. Sillmon et a!.
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Growth and PL study of several single crystal segments
centration in the layer A/substrate spatial region than in layer C. This idea is consistent with the accepted finding that the pre-growth heat treatment of the GaAs substrate results in creating a high concentration of VA, near the surface, which in turn is favorable for creating the increased concentration of SiA,. For these reasons we believe that these acceptors are PL detected most likely from near the substrate—epilayer interface but on the substrate side. The region can become photoexcited by excited carrier diffusion and PL emission from upper layer A. Band IV at 1.39 eV, observed in both GaAs layers A and C, is perhaps of greatest interest (fig. 4). The PL band is assigned to an arsenic vacancyto-silicon acceptor transition (VA, SiA,). We believe that the intensities quantitatively indicate that the layer A/substrate spatial region has about five times more recombinations involving this vacancy—impurity pair than epitaxial GaAs layer C grown on the freshly grown intervening layer B (figs. 1 and 4). Since the VA, concentrations would be the same for these two as-grown LPE GaAs layers A and C, VA,, formation and Si indiffusion near the substrate surface is a possible cause for this vastly more prominent defect of the layer A/substrate spatial region compared to layer C. The buildup of this silicon acceptor concentration near the epilayer—substrate interface will also lead to at least partial self-compensation near the bottorn of layer A (fig. 1). Furthermore, the depth --~
29
eluded that the band resulted from the loss of arsenic from the crystal surface during the heating, and the intensity of the band is independent of doping except for heavily doped (> 1018 cm _3) concentrations. Similar results were reported by Lurn et al. and Koschel et al. [15—17] in heat treated GaAs. After heating bulk grown GaAs(Cr) at 750°Cfor 2~h, a 20-fold intensity increase of a shallow carbon acceptor band and the appearance of another band (1.413 eV) associated with a VAS CA, complex were observed. Degradation of the surface by heating their crystals produced As vacancies, which, based on previously known activation energies, were postulated to be occupied by C or Si to form acceptor sites. Their results show that their crystal treatment leading to the luminescent recombinations takes place at the surface layer 500 A) and to such an extent that the original GaAs semi-insulating n-type (n = 2.6 3) surface was converted to p-type (p X i0~cm = 1.8 X i0’~cm3) [16]. This process of VAS formation and Si diffusion into VAS could be the source of the presently observed acceptor related Band III (1.478 eV) of GaAs layer A (crystal layer structure 190). Bands VI and VII at 1.20 and 0.97 eV, respectively, were observed (fig. 4) in GaAs layers A and C (fig. 1). The two layers have these recombinations to nearly the same extent. Band VI at 1.2 eV is assigned to a silicon donor-to-gallium vacancy transition (51Ga —s V 0a) [6—9].A PL band at 1.0 eV has also been observed in GaAs crystals of other sources, there is no final unanimous agreement on its assignment. Some have suggested an arsenic vacancy—copper acceptor complex (VA, CuGS) transition [18—21].However, since Bands IV and VI due to VAS SiA,, and Si04 V0a, respectively, are present in our crystals, the association of the 1 eV Band VII with a transition envolving donor VAS and acceptor V0a is not unreasonable. Furthermore, since a copper related Band V (see below) was observed only in the bulk GaAs substrate and not in any of our LPE layers, we have dismissed for our LPE GaAs crystals a Cu related assignment (VAS —s Cu Ga) and assigned Band VII to a arsenic vacancy-to-gallium vacancy transition (VAS —s V04) [19]. These PL transition assignments for crystal layer structure 190 are summarized in fig. 5. —~
(—
—
—
profiling (described below) into the substrate revealed a VA, concentration in the top substrate region, which if followed by site conversion, SiGU —s SiA,, can cause type inversion (n-type to p-type) within the substrate itself. Broad bands like IV observed here near 1.4 eV have also been reported for GaAs crystals of different origins. For example, Birey et al. [6,14] observed a peak at 1.36 eV in uncapped and AIN capped crystals which had been heated in a H2 atmosphere in the range 600—900°C; the hand was suggested to be due to an arsenic vacancy—silicon acceptor complex transition, In another report, Birey et al, [7] observed that the intensity of this 1.36 eV band varied with depth, peaking close to the surface of the crystal and disappearing 2 p.m below the surface. They conhigh
—
—
—~
—~
—
—~
30
R. S. Sillmon et a!.
Crystal layer structure GaAs (B) II GaAs (C)
/
Growth and PL study of several single crystal segments
This crystal layer combination (fig. 2) was grown in order to obtain additional information about several of the above described observations, especially the effects of the thermally degraded GaAs(Si) substrate surface on epilayers. For exampie, the above discussed PL Bands III and IV which are related to vacancies and/or silicon at arsenic sites are much more intense in GaAs layer A, the one grown on the pre-growth heated GaAs(Si) substrate, than in GaAs layer C, the layer grown on a freshly grown intervening AlGaSbAs layer B. As shown in fig. 2, this new structure 200 consists of the pre-growth heated GaAs(Si) substrate, the adjoining LPE GaAs layer B, and a GaAs layer C grown on the first grown layer B. The 78 K PL spectra of GaAs layers B and C are shown in fig. 6. These spectra are somewhat
member of a LO phonon replica of Band III (Ill—Ill’ = 35 meV) because the phonon energy is well known to be 33 meV, Also consistent with this assignment is the fact that the intensity of Band III’ is only 4% of the intensity of Band III The temperature dependence of III’ in a lower temperature range would contribute further to confirming the assignment. Band III is assigned to a combination of donor~toTcarbonacceptor (D C~,)and conduction band-to-carbon acceptor (CB —s CAS) transitions unresolved. It is not known whether carbon or silicon is the dominant donor impurity. However, carbon is probably the dominant acceptor impurity because of the very good agreement between the presently measured activation energy of meV with that previously reported (26 meV) [12,13]. The change of dominant acceptor impurity from silicon (structure 190) to carbon (structure 200) is believed to be attributable to our growth details, i.e., the presence of a larger amount of H20 and 02 in the LPE system
similar, with respect to types and energies of recombinations, to those obtained for the crystalline GaAs layers of structure 190 discussed first, except for the absence of Bands VI and VII in structure 200. For both GaAs layers B and C the PL is dominated by near-band-gap luminescence eV). However, this has shoulders III at 1.492 eV and III’ at 1.457 eV; III’ is assigned to the first
during the growth of structure 200. Morkoc and Eastman [23] have suggested that 02 transports C from the graphite boat to the melt in the form of CO. the subsequent source of C. In addition, Hicks and Greene [24] have observed a reduction of Si contamination with increasing water vapor content. Bands IV and IV’ at 1.41 and 1,38 eV, respec-
3,2.
II
200,
GaAs(Si) (A,AJ,A,)
(—
1,51
[22].
—~
23
8 BANDI 1.511
14 2
o.Ao
‘
0.~4
0.~8
0.~2 ‘ 0.~6 ‘ 1.00 WAVELENGTH (pm)
Fig. 6. 78 K photoluminescence spectra of crystal layer structure 200 at layers B (GaAs) intensity scales of two spectral regions are not equal.
(
) and C (GaAs)
(— —
—).
Relative
R. S. Sillmon et a!.
BAND1~ 1.39
BAND]JI 1.480
8
/
.,..~
1.27
\
J
/ ~6.
/
.
825 .
~
..,,,
.~‘
~1~
0.~0
.B~D51It
__
0.~4
‘
0.~8
0.02
‘
‘
o.~
WAVELENGTH
1.00 ~
(pm)
‘
i.~O
lAO
‘
tOO
Fig. 7. 78 K photoluminescence spectra of crystal layer structure 200 at positions A (substrate surface) surface) (—
—
—).
and A2 (2.6 pm below surface) (_
.
—).
—~
oc
L
_______—~
L
-
—
(
).
A
1 (0.4 pm below Relative intensity scales of two spectral regions are not equal.
tively, are assigned to an arsenic vacancy-to-carbon acceptor transition (VA,, CA,,) and its LO phonon replica for reasons as given for structure 190 above. As in crystal layer structure 190, the intensity of this defect Band IV is larger for the GaAs layer B closer to the highly defective substrate. The substrate was depth profiled as indicated by the PL spectra (fig. 7) of positions A, A1, and A2 (fig. 2) at the native surface, 0.4 p.m. and 2.6 p.m below the surface, respectively. The transition assignments are shown in fig. 8. The PL spectrum from closest to the surface (position A) is dominated by Band IV (VA., —s SiAS) at 1.39 eV. As mentioned already, loss of arsenic from the crystal surface and concomitant Si dopant diffusion during the substrate pre-growth heating stage is the
—
31
Growth and FL study of several single crystal segments
BANDY
/ \
‘..
/
c~
2
Co~~ ~
most likely mechanism by which the VA,—SiA$ associates form. The pre-growth heating of the substrate, which is a procedure in general use and was used here, degraded the surface to a large extent as shown by present PL results. For different growth procedures and other crystal systems this problem has also been reported [15—17,25]. Also observed at all the profiling positions A, A1, and A2 of the substrate are Bands VI and VII (figs. 7 and 8) which are related to gallium vacancies as discussed for structure 190 above. In fact, if the gallium vacancy concentration were the limiting factor of these radiative recombinations D —÷ V04 and VA,, V04, then one would observe little change in intensity in progressing from the surface to 2.6 p.m below the surface [26]. Intensities of Bands VI and VII in fact behave very nearly in this manner so that it can be concluded that the —~
concentration gallium in heat treated GaAs decreasesofvery littlevacancies in going from surface to bulk compared to the concentration of arsenic vacancies giving rise to dramatically depth influenced bands such as IV. In going from the surface to position (0.4 p.m below the surface), Band IV (VAS —s SiAS) vanishes while Band Ill (D,CB SiA,,) (1.480 eV) and a new Band V (1.27 eV) emerge strongly. In regard to identities, Band V is tentatively assigned to a silicon donor-to-copper acceptor transition —~
___________________________ Fig. 8. Schematic of radiative recombination assignments for PL bands observed in crystal layer structure 200 substrate,
32
R. S. Sil!mon et a!.
/
Growth and FL study of several single crystal segments
(Si~14—s Cu04). This band is shifted from its usual — 1.35 eV peak energy due to its overlap with broad Band VI (peaking at 1.20 eV) [81. The source of copper is believed to be the GaAs substrate. While Cu has the highest known diffusion coefficient in GaAs, its presence in the substrate also implies that some of it would diffuse into the LPE GaAs layers. However, we point out in the above discussion that none of the PL peaks in GaAs epilayer spectra can be assigned to the Cu related defect. We simply feel that the indiffused Cu in layers B and C is PL silent for the following reason. First, the LPE GaAs layers were grown from Ga-rich solution. This will leave the GaAs layers nearly devoid of any Ga vacancies. Second. in order to observe Band V (1.27 eV) involving Cu, one must have CuG4 sites.ofBut appreciable concentration V the paucity of an 0~will minimize the formation of Cu04, in turn accounting for the absence from PL spectra of Si04 —s Cu~24(-‘- 1.27 eV) from the GaAs epilayers. The observation remains that no 1.27 eV PL peak is observed when LPE epilayers are photoexcited. The broad nearband-gap emission having peak energy 1.480 eV is, for reasons given above, assigned to CB —s SiA, and Si~14—s SiA, transitions. This is at least consistent with the suggestions cited above that Si atoms from the bulk GaAs(Si) fill the many available arsenic vacancies forming toward the surface during the pre-growth heat treatment stage. Still further into the bulk at position A2 (2.6 p.m below the surface) Band V is still observed but with less intensity, whereas the near-band-gap luminescence increases concomitantly in intensity and shifts to slightly higher energy. These changes suggest the presence of competitively more bandto-band and donor-to-band recombinations within the bulk compared to near the surface, which in part is expected for n-type crystals.
4. Summary High efficiency monolithic cascade solar cells require the growth of multiple single-crystalline layers of heterojunctions as well as homojunctions of semiconductor. Stepped multilayer structures of portions of such as cascade solar cell were grown
and subseqently studied by laser excited photoluminescence spectroscopy. Several types of these GaAs, AIGaAs, and AlGaSbAs crystal layers as well as different spatial depths into the GaAs substrate were probed. PL bands were observed and identified as recombinations involving the conduction and valence bands and the presence of C(;~1. Si(,1, SiA,. Cuy;5, VA,,, V~,, and vacancy—impurity complexes. The following are among the findings for the five-component structure 190, GaAs(Si) (substrate) II GaAs (A) 1 AlGaSbAs (B) II GaAs (C) H A1GaAs (D). The comparison of GaAs (A) and the highly defective heat pre-growth treated substrate with GaAs (C) shows that the former has dominating PL (in just its own spectrum) involv5~A,, (Band III), and much enhanced Pt associing ated with VA., and S~A.,(Band IV) having compensation and type inversion potential. However. both regions have nearly the same PL intensity for PL bands VI and VII. The salient features of comparing PL data of AIGaAs (D) with the slightly lattice expanded A1GaSbAs (B) are the less abrupt interface between GaAs and AlGaSbAs, and the presence of a — 20 meV red shoulder (D,CB CA.,) on the near-band-gap peak (— 1.8 eV) for both layers B and D. The depth profile of the pre-growth heat treated GaAs(Si) substrate of structure 200 revealed the following. As one probes from the surface toward the bulk the near-band-gap or Band III (Si(,, CB —s SiA,) FL intensity ts nearly zero at the surface but increases toward the interior (figs. 2 and Also. Band IV (V4,,, —* SiA,,) has a vastly dominating presence for the surface spectrum (A. fig. but its intensity is strongly diminished for deeper spectra A1 (0.4 p.m) and A2 p.m) (fig. 7). This infers that for our substrate and its treatment VA., and/or SiA,, formations begin to terminate at 0.4 p.m or less. On the other hand. intensities of Bands VI (Si014 —s Vy;~), and VII (VA. —s V014) change very little for surface (A) and step-etched regions A1 (0.4 p.m) and A2 (2.6 jim). —~
7).
7).
(2.6
Acknowledgements The support of the Department of Energy (DEFGO2-79ER10539). the National Science Founda-
R.S. Sillmon et al.
/
Growth and PL study of several single crystal segments
tion (ENG 76-8 1334), and the Microelectronics Corporation of North Carolina are acknowledged. The authors wish to thank S.T. Edwards for helpful discussions.
33
[13] F.T. Kirkman, R.A. Stradling and P.J. Lin-Chung,J. Phys. C (Solid State Phys.) 11(1978) 419. [14) H. Birey. Sung-Jae Pak and JR. Sites, AppI. Phys. Letters 35 (1979) [IS]W.Y. Lum623. and
H.H. Wieder, J. AppI. Phys 49 (1978)
6187.
1161 WY. Lum, H.H. Wieder, W.H. Koschel. S.G. Bishop and
References [I] S.M. Bedair. M.F. Lamorie and JR. Hauser. AppI. Phys. Letters 34 (1979) 38. [2] M.F. Lamorte and D. Abbott. in: Proc. 13th IEEE Photovoltaic Specialists Conf. Record. Washington, DC, 1978 (IEEE, New York, 1978) p. 874. [3] M.F. Lamorle and D. Abbott, Solid State Electron. 26 (1979) 467.
14]
M. Ettenberg and G.H. Olsen. J. AppI. Phys. 48 (1977) 4275. [5] M. Ettenberg and H. Kressel. J. AppI. Phys. 47 (1976) 1538.
[6] H. Birey and JR. Sites. J. AppI. Phys. 51(1980) 619. [7] H. Birey and J.R. Sites. Report, SF-22, Order No. ADA070809. [8] S.Y. Chiang and G.L. Pearson. J. Luminescence 10 (1975) 313. [9) E.W. Williams, Phys. Rev. 168. (1968) 922. [10] V Swaminathan. M.D. Sturge and J.L. Ziko. J. AppI. Phys. 52 (1981) 6306. [II] G.E. Stillman and C.M. Wolfe, Thin Solid Films 31(1976) 69. [121 D.J. Ashen, P.J. Dean, D.T.J. Hurle. J.B. Mullin and AM. White, J. Phys. Chem. Solids 36 (1975) 1041.
B.D. McCombe. AppI. Phys. Letiers 30 (1977) 1. [17] W.H. Koschel, S.G. Bishop, Br?. McCombe, WY. Lum and H.H. Wieder. in: Proc. 6th Intern. Symp. on GaAs and Related Compounds, Edinburgh, 1976, Inst. Phys. 98. Conf. Ser. 33a (Inst. Phys., London, 1977) p. [18] V.A. Brodovoi and G.P. Peka, Soviet Phys.-Solid State 13 (1972) 2015. 1191 K.D. Glinchuk and Ay. Prokhorovich, Phys. Status Solidi (a) 44(1977) 777. [20] Z.l. Alferov. D.Z. Garbuzov, Y.V. Zhilyaev, E.P. Morozov and E.L. Portnoi, Soviet Phys.-Semicond. 2 (1969) 1204. [21] H.J. Guislain, L. DeWolf and P. Clauws, J. Electron. Mater. 7 (1978) 83. [22] E.W. Williams and H.B. Bebb, Semiconductors and Semimetals. Vol. 8 (Academic Press, New York, 1972) p. 387. [23] H. Morkoc and L.F. Eastman. J. Crystal Growth 36 (1976) 109. [241 H.G.B. Hicks and PD. Greene, in: Proc. 3th Intern. Symps. on GaAs and Related Compounds, Aachen, 1970, Inst. Phys. Conf. Ser. 9 (Inst. Phys., London, 1971) p. 92. [251 L.M.F. Kaufmann, K. Heime and W.G Burchard, J. Crystal Growth 34(1976) 289. [26] S.Y. Chiang and G.L. Pearson, J. Appl. Phys. 46 (1975) 2986.
25
GROWTH AND PHOTOLUMINESCENCE STUDY OF SEVERAL SINGLE CRYSTAL SEGMENTS RELEVANT TO MONOLITHIC SEMICONDUCTOR CASCADE SOLAR CELLS Roger S. SILLMON and Anton F. SCHREINER Department of Chemistry, North Carolina State University, Raleigh, N rth Carolina 27650, USA
and Michael TIMMONS Research Triangle Institute, Research Triangle Park, North Carolina 27709, USA
Received 26 July 1982; manuscript received in final form 31 March 1983
Several representative single crystal stacked layers of 111—V compound and alloy semiconductors were grown which are spatial regions relevant to a monolithic cascade solar cell, including the substrate. n-GaAs(Si). which was pre-growth heat treated in H 2(g) prior to its use. These structures were then studied by cryogenic laser excited photoluminescence (PL). and the substrate portion was explored in a depth profiling mode. Within the forbidden band gap region up to seven recombinations were observed and identified for undoped GaAs layers or the GaAs(Si) substrate, and several other PL recombinations were observed for undoped Al~Ga — ~As and Al ~.Ga1— ~Sb~As1 .~ layers. In addition to the valence and conduction bands, these optical bands are also associated with the presence of C0a, Sioa, SiA~, Cu0~,V~,V0a and vacancy—impurity complexes involving several of these defect types even in the absence of intentional doping. The findings also relate to problems of self-compensation and type inversion, so that the need for growth modifications is indicated.
1. Introduction Several types of single crystal Ill—V compound and alloy semiconductor heterojunctions are currently being studied for various optoelectronic devices. Among such devices is the monolithic cascade solar cell which shows some promise for achieving high solar energy conversion efficiencies [1—3] and perhaps as a broadband detector. A monolithic cascade solar cell requires multiple layers of homojunctions and heterojunctions of semiconductor crystals to be grown in a single structure. Each layer of this sequence of single crystal growth is ideally required to have the same lattice constant and orientation. Another efficiency requirement is that the band gap combinations of the top and bottom cells as well as the tunnel junction must be chosen so as to split the solar spectrum in order to generate the same photocurrent in each cell. One of such optimized 0022-0248/83/0000—0000/$03 .00
©
systems employed consists of crystalline layers of GaSb~As1_5 and Al~,Ga1_~Sb2As1~ alloys. The sequence of layers for the complete cascade solar cell is as follows: n-GaAs substrate n-GaSbAs p-GaSbAs ptAlGaSbAs n~-AlGaSbAsH n-AlGaSbAs H p-AlGaSbAs. In practice the growth of such multiple layers of these alloys on GaAs substrates can introduce considerable lattice mismatch, which in turn can degrade solar cell performance by the presence of recombinations involving several types of defects [4,5]. The growth and observed defects of such layers or portions of a cascade cell are described here. The PL technique was selected in order to probe the electronic structures because of its high sensitivity and since many recombinations within the forbidden band gap are radiative. A number of PL bands of Ill—V alloys are identified from similar activation energies of GaAs, since the alloy stoichiometries differ very little from GaAs. For example, Be activation en-
1983 North-Holland
26
R. S. Silimon et a!.
/
Growth and PL study of several single crystal segments
ergies of 27 and 23 meV have been found in this laboratory for, respectively, GaAs and Al0 1Ga0 7As. These growth characterizations suggest the need for crystal growth procedure modifications.
2. Experimental 2.1. Crystal preparations
The crystal layers used in this study (figs. 1 and 2) were grown by liquid phase epitaxy (LPE) using a horizontal slider-boat arrangement. The GaAs substrates (Crystal Specialities) were n-type and had (11 l)A orientation. The Si 3 doping levelanofetch the and had substrates 1 x 1018 cm pit densitywas of n 1500—2000 cm2. The substrates were chemomechanically polished and then etched =
A
B
C
D
for 10 s in HNO3 I HF H20 (3 :2:4. V V V) prior to placing them into the LPE system. The crystal layers were grown from melts prepared with (i) excess undoped polycrystalline GaAs, (ii) high purity (6N) Ga. and (iii) Sb and/or Al for the alloy layers. Any resultant doping of layers was unintentional. For the preparation of crystal layer structure 190, the LPE chamber with the substrate was heated to 800°Cfor 4~h prior to growing the layers. Layer growth resulted from cooling the melt at a rate of 1°C/mm.The cooling ranges were 4°C for the first layer and 3°C for each of the remaining layers. The layer thicknesses were ca. 1.5 ~tm. The substrate of crystal layer structure 200 was subjected to 3 h of pre-growth heating at 780°C followed a growth cooling rate of 0.5°C/mm, and thebycooling range was 20°Cfor each layer. Steps were etched at the substrate surface of crystal layer structure 200 using H 2S04 H205 H20 (10: 1: 1, V V V), and the steps sizes were measured with a Sloan Dektak. 2.2. Photoluminescence measurements
Al,,Ga~~As GaAs
____________________________ GaAs GaAs(Sl)
Substrate
Fig. 1. Crystal layer structure 190: n-GaAs(Si) substrate and stepped layers (1—2 pm thick) GaAs II Aloi5Gao85Sb0o5As5o5 lGaAsIIAlosGao85As and photoluminescence probe posi-
A
~A
I
B
C
I GaAS
GaAs
Q,~m j2.6im
GaAs(Si)
Substrate
Fig. 2. Crystal layer structure 200: n-GaAs(Si) substrate and double layers GaAs 1 GaAs and photoluminescence probe positions A, A1, A2, B, and C.
The facility employed for PL measurements consisted of an argon-ion laser operated at 5145 A (25—75 mW) with the beam focused on the crystals. The laser radiation is optically all absorbed in each individual irradiated layer since the penetration depth at this energy in GaAs is only 0.12 ~tm, and the layers are over ten times thicker. The crystal was mounted in a cryogen-throttling optical dewar such that the crystal was continuously cooled in a stream of cryogen vapor. The temperature (78 K) was measured by means of a thermocouple in the mounted same cryogen so thatvapor. it and The the specimen photolumineswere cence radiation waspass passed through a Spex meter double-pass monochromator and 3/4 detected using etther a cooled (195 K) S-i response photomultiplier tube or a cooled (78 K) lnAs photovoltaic detector. Grating orientations were calibrated with a mercury penlamp. The chopped signal was processed using a current-to-voltage linear preamplifier and selective and lock-in amplifiers. Spectra were not corrected for system response as a function of wavelength because the
R. S. Sitimon et at.
/
primary interest is in the comparison of spectra of different crystals over the same energy range. Spectral resolution was 8 A in the wavelength range 0.80—1.00 ~tm and 16 A in the wavelength range 1.00—1.60 ~.tm.
~
3. Results and discussion
Al0
(‘A..
A
ID
0.15 a085 0.05 s095 15Ga085As (D)
A
~
~a
/
~
S
Fig. 3 shows the 78 K PL spectra of crystal layer structure 190, specifically for layers B (AlGaSbAs) and D (AlGaAs) (fig. 1). The bands at — 1.5 and 1.39 eV are due to PL from the underlying GaAs layers and this emission from these layers is excited from the higher energy (— 1.8 eV) alloy PL and alloy photostimulated carrier diffusion. We believe this to be correct because the alloy layers (E5 1.8 eV; 1.5 /.tm thick) are transparent to these 1.5 and 1.39 eV GaAs luminescence energies and because the spectra in this energy region are virtually identical to those obtained from GaAs layers A and C (see below), Each of these layers B and D has another band in the range 1.46—1.47 eV whose energy is different from the band gap by an amount (0.318 eV for B,
8 .
O.~O
O.A~
O.~8 ‘ O.~2
O.~6
‘
1.00 i.00
/
/~40
LS~IJT~-/
~
7~S:kioc
1
1.5210
~
.055
3.1. Crystal layer structure 190, GaAs(Si) I GaAs(A) r’
147
~ ~2
Al
27
Growth and PL study of several single crystal segments
O.~O
0.75
1.45 1.
0.SO 0.85 WAVELENGTH (~.rG
0.90
0.95
Fig. 3. 78 K photoluminescence spectra of crystal layer structure 190 at layers B (Al 9 5Ga955Sb905As095) (—) and D (Al0 5Ga9 85As) (— —
0.308 eV for D) which is very similar to what is found for the difference between the band gap and the donor-to-gallium vacancy (D —s V0a) transition in the GaAs (0.311 eV) [6—9].For this reason and because the 1.46—1.47 eV band is absent in the GaAs underlayer (figs. 1 and 4), we assign this recombination in these alloy layers, whose cornpositions are nearly that of “parent” GaAs, also to a D V111 transition. The 1.8 eV band of layers B (AlGaSbAs) and D (AlGaAs) of structure 190 is assigned to unresolved conduction band-to-valence band and shallow donor-to-valence band transitions (“nearband-gap luminescence”) of the alloys. Our LPE growth system has always produced n-type Al~
—
8AND~W
i.~o iAo
‘
WAVELENGTH (jim)
Fig. 4. 78 K photoluminescence spectra of crystal layer structure 190 at layers A(GaAs) intensity scales of two spectral regions are not equal.
(
)
and C (GaAs)
(— —
—).
Relative
28
R.S. Sillmon et al.
/
Growth and PL study of several single crystal segments
GaAs, so that this highest PL is believed to contam D VB and CB —s VB recombinations. The shallow red shoulder on each 1.8 eV band is assigned to donor-to-carbon acceptor (D CAS) and conduction band-to-carbon acceptor (CB CAS) transitions unresolved, since an analogous 20 meV red-shift from the near-band-gap peak indicates the involvement of such a shallow acceptor for GaAs. This suggestion for layers B and D of structure 190 also agrees quite well with that reported by Swaminathan et al. [10] for a carbon acceptor in a differently grown crystal, Al() 4 Ga0 86As. —~
—~
—~
—
The presence of the high-energy long tail of the 1.521 eV band resulting from irradiating the A1015Ga055Sb005As095 LPE layer B (spectrum B, fig. 3) grown on LPE GaAs, itself on the heat treated GaAs substrate, is also of interest. We believe that this band must originate from underlayer A of GaAs because our PL spectra of a. range of compositions of AlGaAs (direct gap type) on GaAs produce expected variations of Eg but the 1.51 eV peak remains energy invariant. We believe that the band tailing indicates the presence of a non-abrupt crystal junction between the LPE GaAs (layer A) and the AlGaSbAs layer grown over it. In contrast to this band, an analogous band is very much sharper (spectrum D, fig. 3) for the GaAs layer C grown on the epilayer B of AlGaSbAs rather than the heat treated substrate, This optical comparison of layers B (A1GaSbAs) and D (AlGaAs), with layer B having lattice-enlarging Sb in its composition, then in summary leads to the observations that layer B has (i) fewer near-band-gap recombinations (weaker PL inten—
—
—
________________
Si~—p-
___________
—
CB
-
sity), (ii) a less abrupt crystal junction between itself and the underlying GaAs layer. and (iii) more radiative recombinations involving crystal vacancies than layer D. The 78 K P1 spectra of GaAs layer A (grown on the n-GaAs(Si) substrate) and GaAs layer C (grown on the second of two intervening layers above the substrate) of crystal layer structure 190 are shown in fig. 4; the PL assignments are given in fig. 5 and are as follows. The 1.506 eV near-band-gap luminescence (Band II) of layer A (first GaAs epilayer, on substrate) ts tentatively assigned to a silicon donor-to-valence band transition (Si0. VB). The assignment is based on its activation energy of 5.1 meV being in close agreement with that expected for a hydrogenic donor [11] in GaAs (5.7 meV) and on Si being the commonly present contaminant in LPE type layers. This band must come from layer A and not from the GaAs substrate side of the interface because the PL of the top layer of the substrate itself (fig. does not have this band. Band III (1.478 eV) of layer A is of relatively high intensity and is assigned to a cornbination of silicon donor-to-silicon acceptor (Si(,. SiAS) and conduction band-to-silicon acceptor (CB SiA,) transitions unresolved. By first assuming a CB SiA, transition to be present, an effective activation energy of 36 meV is obtained by subtracting the PL peak energy (1.478 eV) from the band gap energy (1.5111 eV) plus the ~kT correction factor (3.35 meV) for a free electron [12]. This assignment is consistent with the findings by Ashen et al. and Kirkman et al. [12,131 that the activation energy for a silicon acceptor is 35 meV in GaAs crystals of their source. Our value of 36 meV is in agreement with this. The near-band-gap transitions of the other GaAs layer (fig. I), or layer C (third epilayer), is dominated by Band I at 1.513 eV. This Band I of layer C is assigned to the conduction band-to-valence band (CB VB) transition in view of its energy (1.5 13 eV); that band was absent (or very weak) when irradiating GaAs layer A grown on the pre-heated substrate. Band III (CB,D —s SiA,) is nearly absent in GaAs layer C, but prominent in layer A (grown on the substrate), inferring the presence of a much higher SiA, acceptor con—~
7)
—~
—~
—~
—+
HI~
V00~~~ —
—~
Si~ VB
Fig. 5. Schematic of radiative recombination assignments for PL bands observed in crystal layer structure 190.
R.S. Sillmon et a!.
/
Growth and PL study of several single crystal segments
centration in the layer A/substrate spatial region than in layer C. This idea is consistent with the accepted finding that the pre-growth heat treatment of the GaAs substrate results in creating a high concentration of VA, near the surface, which in turn is favorable for creating the increased concentration of SiA,. For these reasons we believe that these acceptors are PL detected most likely from near the substrate—epilayer interface but on the substrate side. The region can become photoexcited by excited carrier diffusion and PL emission from upper layer A. Band IV at 1.39 eV, observed in both GaAs layers A and C, is perhaps of greatest interest (fig. 4). The PL band is assigned to an arsenic vacancyto-silicon acceptor transition (VA, SiA,). We believe that the intensities quantitatively indicate that the layer A/substrate spatial region has about five times more recombinations involving this vacancy—impurity pair than epitaxial GaAs layer C grown on the freshly grown intervening layer B (figs. 1 and 4). Since the VA, concentrations would be the same for these two as-grown LPE GaAs layers A and C, VA,, formation and Si indiffusion near the substrate surface is a possible cause for this vastly more prominent defect of the layer A/substrate spatial region compared to layer C. The buildup of this silicon acceptor concentration near the epilayer—substrate interface will also lead to at least partial self-compensation near the bottorn of layer A (fig. 1). Furthermore, the depth --~
29
eluded that the band resulted from the loss of arsenic from the crystal surface during the heating, and the intensity of the band is independent of doping except for heavily doped (> 1018 cm _3) concentrations. Similar results were reported by Lurn et al. and Koschel et al. [15—17] in heat treated GaAs. After heating bulk grown GaAs(Cr) at 750°Cfor 2~h, a 20-fold intensity increase of a shallow carbon acceptor band and the appearance of another band (1.413 eV) associated with a VAS CA, complex were observed. Degradation of the surface by heating their crystals produced As vacancies, which, based on previously known activation energies, were postulated to be occupied by C or Si to form acceptor sites. Their results show that their crystal treatment leading to the luminescent recombinations takes place at the surface layer 500 A) and to such an extent that the original GaAs semi-insulating n-type (n = 2.6 3) surface was converted to p-type (p X i0~cm = 1.8 X i0’~cm3) [16]. This process of VAS formation and Si diffusion into VAS could be the source of the presently observed acceptor related Band III (1.478 eV) of GaAs layer A (crystal layer structure 190). Bands VI and VII at 1.20 and 0.97 eV, respectively, were observed (fig. 4) in GaAs layers A and C (fig. 1). The two layers have these recombinations to nearly the same extent. Band VI at 1.2 eV is assigned to a silicon donor-to-gallium vacancy transition (51Ga —s V 0a) [6—9].A PL band at 1.0 eV has also been observed in GaAs crystals of other sources, there is no final unanimous agreement on its assignment. Some have suggested an arsenic vacancy—copper acceptor complex (VA, CuGS) transition [18—21].However, since Bands IV and VI due to VAS SiA,, and Si04 V0a, respectively, are present in our crystals, the association of the 1 eV Band VII with a transition envolving donor VAS and acceptor V0a is not unreasonable. Furthermore, since a copper related Band V (see below) was observed only in the bulk GaAs substrate and not in any of our LPE layers, we have dismissed for our LPE GaAs crystals a Cu related assignment (VAS —s Cu Ga) and assigned Band VII to a arsenic vacancy-to-gallium vacancy transition (VAS —s V04) [19]. These PL transition assignments for crystal layer structure 190 are summarized in fig. 5. —~
(—
—
—
profiling (described below) into the substrate revealed a VA, concentration in the top substrate region, which if followed by site conversion, SiGU —s SiA,, can cause type inversion (n-type to p-type) within the substrate itself. Broad bands like IV observed here near 1.4 eV have also been reported for GaAs crystals of different origins. For example, Birey et al. [6,14] observed a peak at 1.36 eV in uncapped and AIN capped crystals which had been heated in a H2 atmosphere in the range 600—900°C; the hand was suggested to be due to an arsenic vacancy—silicon acceptor complex transition, In another report, Birey et al, [7] observed that the intensity of this 1.36 eV band varied with depth, peaking close to the surface of the crystal and disappearing 2 p.m below the surface. They conhigh
—
—
—~
—~
—
—~
30
R. S. Sillmon et a!.
Crystal layer structure GaAs (B) II GaAs (C)
/
Growth and PL study of several single crystal segments
This crystal layer combination (fig. 2) was grown in order to obtain additional information about several of the above described observations, especially the effects of the thermally degraded GaAs(Si) substrate surface on epilayers. For exampie, the above discussed PL Bands III and IV which are related to vacancies and/or silicon at arsenic sites are much more intense in GaAs layer A, the one grown on the pre-growth heated GaAs(Si) substrate, than in GaAs layer C, the layer grown on a freshly grown intervening AlGaSbAs layer B. As shown in fig. 2, this new structure 200 consists of the pre-growth heated GaAs(Si) substrate, the adjoining LPE GaAs layer B, and a GaAs layer C grown on the first grown layer B. The 78 K PL spectra of GaAs layers B and C are shown in fig. 6. These spectra are somewhat
member of a LO phonon replica of Band III (Ill—Ill’ = 35 meV) because the phonon energy is well known to be 33 meV, Also consistent with this assignment is the fact that the intensity of Band III’ is only 4% of the intensity of Band III The temperature dependence of III’ in a lower temperature range would contribute further to confirming the assignment. Band III is assigned to a combination of donor~toTcarbonacceptor (D C~,)and conduction band-to-carbon acceptor (CB —s CAS) transitions unresolved. It is not known whether carbon or silicon is the dominant donor impurity. However, carbon is probably the dominant acceptor impurity because of the very good agreement between the presently measured activation energy of meV with that previously reported (26 meV) [12,13]. The change of dominant acceptor impurity from silicon (structure 190) to carbon (structure 200) is believed to be attributable to our growth details, i.e., the presence of a larger amount of H20 and 02 in the LPE system
similar, with respect to types and energies of recombinations, to those obtained for the crystalline GaAs layers of structure 190 discussed first, except for the absence of Bands VI and VII in structure 200. For both GaAs layers B and C the PL is dominated by near-band-gap luminescence eV). However, this has shoulders III at 1.492 eV and III’ at 1.457 eV; III’ is assigned to the first
during the growth of structure 200. Morkoc and Eastman [23] have suggested that 02 transports C from the graphite boat to the melt in the form of CO. the subsequent source of C. In addition, Hicks and Greene [24] have observed a reduction of Si contamination with increasing water vapor content. Bands IV and IV’ at 1.41 and 1,38 eV, respec-
3,2.
II
200,
GaAs(Si) (A,AJ,A,)
(—
1,51
[22].
—~
23
8 BANDI 1.511
14 2
o.Ao
‘
0.~4
0.~8
0.~2 ‘ 0.~6 ‘ 1.00 WAVELENGTH (pm)
Fig. 6. 78 K photoluminescence spectra of crystal layer structure 200 at layers B (GaAs) intensity scales of two spectral regions are not equal.
(
) and C (GaAs)
(— —
—).
Relative
R. S. Sillmon et a!.
BAND1~ 1.39
BAND]JI 1.480
8
/
.,..~
1.27
\
J
/ ~6.
/
.
825 .
~
..,,,
.~‘
~1~
0.~0
.B~D51It
__
0.~4
‘
0.~8
0.02
‘
‘
o.~
WAVELENGTH
1.00 ~
(pm)
‘
i.~O
lAO
‘
tOO
Fig. 7. 78 K photoluminescence spectra of crystal layer structure 200 at positions A (substrate surface) surface) (—
—
—).
and A2 (2.6 pm below surface) (_
.
—).
—~
oc
L
_______—~
L
-
—
(
).
A
1 (0.4 pm below Relative intensity scales of two spectral regions are not equal.
tively, are assigned to an arsenic vacancy-to-carbon acceptor transition (VA,, CA,,) and its LO phonon replica for reasons as given for structure 190 above. As in crystal layer structure 190, the intensity of this defect Band IV is larger for the GaAs layer B closer to the highly defective substrate. The substrate was depth profiled as indicated by the PL spectra (fig. 7) of positions A, A1, and A2 (fig. 2) at the native surface, 0.4 p.m. and 2.6 p.m below the surface, respectively. The transition assignments are shown in fig. 8. The PL spectrum from closest to the surface (position A) is dominated by Band IV (VA., —s SiAS) at 1.39 eV. As mentioned already, loss of arsenic from the crystal surface and concomitant Si dopant diffusion during the substrate pre-growth heating stage is the
—
31
Growth and FL study of several single crystal segments
BANDY
/ \
‘..
/
c~
2
Co~~ ~
most likely mechanism by which the VA,—SiA$ associates form. The pre-growth heating of the substrate, which is a procedure in general use and was used here, degraded the surface to a large extent as shown by present PL results. For different growth procedures and other crystal systems this problem has also been reported [15—17,25]. Also observed at all the profiling positions A, A1, and A2 of the substrate are Bands VI and VII (figs. 7 and 8) which are related to gallium vacancies as discussed for structure 190 above. In fact, if the gallium vacancy concentration were the limiting factor of these radiative recombinations D —÷ V04 and VA,, V04, then one would observe little change in intensity in progressing from the surface to 2.6 p.m below the surface [26]. Intensities of Bands VI and VII in fact behave very nearly in this manner so that it can be concluded that the —~
concentration gallium in heat treated GaAs decreasesofvery littlevacancies in going from surface to bulk compared to the concentration of arsenic vacancies giving rise to dramatically depth influenced bands such as IV. In going from the surface to position (0.4 p.m below the surface), Band IV (VAS —s SiAS) vanishes while Band Ill (D,CB SiA,,) (1.480 eV) and a new Band V (1.27 eV) emerge strongly. In regard to identities, Band V is tentatively assigned to a silicon donor-to-copper acceptor transition —~
___________________________ Fig. 8. Schematic of radiative recombination assignments for PL bands observed in crystal layer structure 200 substrate,
32
R. S. Sil!mon et a!.
/
Growth and FL study of several single crystal segments
(Si~14—s Cu04). This band is shifted from its usual — 1.35 eV peak energy due to its overlap with broad Band VI (peaking at 1.20 eV) [81. The source of copper is believed to be the GaAs substrate. While Cu has the highest known diffusion coefficient in GaAs, its presence in the substrate also implies that some of it would diffuse into the LPE GaAs layers. However, we point out in the above discussion that none of the PL peaks in GaAs epilayer spectra can be assigned to the Cu related defect. We simply feel that the indiffused Cu in layers B and C is PL silent for the following reason. First, the LPE GaAs layers were grown from Ga-rich solution. This will leave the GaAs layers nearly devoid of any Ga vacancies. Second. in order to observe Band V (1.27 eV) involving Cu, one must have CuG4 sites.ofBut appreciable concentration V the paucity of an 0~will minimize the formation of Cu04, in turn accounting for the absence from PL spectra of Si04 —s Cu~24(-‘- 1.27 eV) from the GaAs epilayers. The observation remains that no 1.27 eV PL peak is observed when LPE epilayers are photoexcited. The broad nearband-gap emission having peak energy 1.480 eV is, for reasons given above, assigned to CB —s SiA, and Si~14—s SiA, transitions. This is at least consistent with the suggestions cited above that Si atoms from the bulk GaAs(Si) fill the many available arsenic vacancies forming toward the surface during the pre-growth heat treatment stage. Still further into the bulk at position A2 (2.6 p.m below the surface) Band V is still observed but with less intensity, whereas the near-band-gap luminescence increases concomitantly in intensity and shifts to slightly higher energy. These changes suggest the presence of competitively more bandto-band and donor-to-band recombinations within the bulk compared to near the surface, which in part is expected for n-type crystals.
4. Summary High efficiency monolithic cascade solar cells require the growth of multiple single-crystalline layers of heterojunctions as well as homojunctions of semiconductor. Stepped multilayer structures of portions of such as cascade solar cell were grown
and subseqently studied by laser excited photoluminescence spectroscopy. Several types of these GaAs, AIGaAs, and AlGaSbAs crystal layers as well as different spatial depths into the GaAs substrate were probed. PL bands were observed and identified as recombinations involving the conduction and valence bands and the presence of C(;~1. Si(,1, SiA,. Cuy;5, VA,,, V~,, and vacancy—impurity complexes. The following are among the findings for the five-component structure 190, GaAs(Si) (substrate) II GaAs (A) 1 AlGaSbAs (B) II GaAs (C) H A1GaAs (D). The comparison of GaAs (A) and the highly defective heat pre-growth treated substrate with GaAs (C) shows that the former has dominating PL (in just its own spectrum) involv5~A,, (Band III), and much enhanced Pt associing ated with VA., and S~A.,(Band IV) having compensation and type inversion potential. However. both regions have nearly the same PL intensity for PL bands VI and VII. The salient features of comparing PL data of AIGaAs (D) with the slightly lattice expanded A1GaSbAs (B) are the less abrupt interface between GaAs and AlGaSbAs, and the presence of a — 20 meV red shoulder (D,CB CA.,) on the near-band-gap peak (— 1.8 eV) for both layers B and D. The depth profile of the pre-growth heat treated GaAs(Si) substrate of structure 200 revealed the following. As one probes from the surface toward the bulk the near-band-gap or Band III (Si(,, CB —s SiA,) FL intensity ts nearly zero at the surface but increases toward the interior (figs. 2 and Also. Band IV (V4,,, —* SiA,,) has a vastly dominating presence for the surface spectrum (A. fig. but its intensity is strongly diminished for deeper spectra A1 (0.4 p.m) and A2 p.m) (fig. 7). This infers that for our substrate and its treatment VA., and/or SiA,, formations begin to terminate at 0.4 p.m or less. On the other hand. intensities of Bands VI (Si014 —s Vy;~), and VII (VA. —s V014) change very little for surface (A) and step-etched regions A1 (0.4 p.m) and A2 (2.6 jim). —~
7).
7).
(2.6
Acknowledgements The support of the Department of Energy (DEFGO2-79ER10539). the National Science Founda-
R.S. Sillmon et al.
/
Growth and PL study of several single crystal segments
tion (ENG 76-8 1334), and the Microelectronics Corporation of North Carolina are acknowledged. The authors wish to thank S.T. Edwards for helpful discussions.
33
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