Pressure dependence of shallow bound states in gallium arsenide

Solid State Co---unications, Vol.53,No.12, pp.1069-1076, 1985. Printed in Great Britain.

0038-1098/85 $3.00 + .00 Pergamon Press Ltd.

PRESSURE D E P E N D E N C E OF SHALLOW BOUND STATES IN G A L L I U M ARSENIDE D.J. Wolford and J.A. Bradley IBM Thomas J. Watson Research Center Yorktown Heights, NY 10598 USA

Photoluminescence and PL-excitation of high-purity n-type GaAs is reported at H__~etemperatures and hydrostatic pressures of up to 80 kbar in diamond anvil cells. At low pressures intense PL arises from direct-gap free and bound excitons and band-to-acceptor transitions. Band structure becomes indirect at 41.3 kbar and spectra resemble n-type GaP. Weak indirect (Xic)-ga p recombination attributable to donor-bound-excitons and free excitons is identified. All bound states remain shallow and follow their band edges, thus giving the most precise Ft -Ft5 v and Xt -Fls v gap dependences on pressure (10.73 and -1.34 m e V / k b a r , respectively) yet reported. New indirect band gaps are derived for GaAs at atmospheric pressure.

1. Introduction In semiconducting materials, energy band gaps and electronic states associated with them may depend on external sample perturbations.t Foremost among these perturbations is pressure which, if applied uniformly, preserves crystal symmetries while altering semiconducting properties through changes in inter-atomic distances. In this report we present the results of new luminescence studies performed at both liquid-He temperatures and large applied pressures on undoped high-purity n-type GaAs. In carrying out this work we obtain pressures which are uniform or "hydrostatic" at ~ 5°K to better than 1 part in 500 at up to 80 kbar in diamond anvil pressure cells. Using extensive photoluminescence (PL) and PL excitation (PLE), we identify and trace versus pressure the intrinsic excitonic bound states and localized states formed at residual shallow impurities surrounding both the direct and indirect band gaps of GaAsfl "3 Because of the exceptional spectral resolution resulting in our experiments from a combination of high-sample-purity, low measurement temperature, hydrostatic pressures, and narrow recombination transitions, we obtain (1) the most precise F~¢-F,5 ' gap variations with pressure reported thus far (2) the first direct observation of indirect X,¢-gap states and their pressure dependences (3) accurate band and bound-state-level crossings versus pressure (4) new indirect band-gap energies (Xt -F,5 v, Lt -F,5 v) at atmospheric pressure and (5) donor exciton levels remain shallow near the Ft -Xt¢ crossover. 23 Many of these results conflict both qualitatively and in detail with other recent studies in G a A s # 7 Taken

together, results presented here comprise one of the most comprehensive and spectroscopically detailed accounts of the influence of pressure on "shallow" electronic bound states yet reported for a semiconductor. 2. Methods The GaAs samples were prepared by C13-transport VPE on n + (100) substrates. Layers were undoped high-purity n-type (~ 2x10 ~ cm -3 at room temperature) and 10-15 t~m thick. A more compensated comparison high-purity n-type layer was prepared by MOCVD. For pressure measurement substrates were removed by mechanical lapping and and etching to give final sample thicknesses of ~ 40 om. Resulting platelets were cleaved into ~ 100-tzm-square die and loaded into diamond anvil cells, together with a 10-/~m ruby crystal and a transparent pressure transmitting medium, s Measurements were made in an optical cryostat in flowing He at ~ 5°K; actual sample temperature may be somewhat higher due to local laser heating. Pressures of up to ~ 80 kbar were deduced from shift of ruby emission, X using 14421.8 cm -~ as the atmospheric-pressure reference and -0.76 c m " / k b a r as the pressure derivative for R e l i n e emission at He temperature. `) Pressures were hydrostatic to better than 1 part in 500, as estimated by comparison with calibrated stress splitting of donor excitons (discussed below). Photoluminescence was excited by the focussed (40-t~m) 488.0 nm line of an Ar + laser at power densities of up to 103 W / c m z. Photoluminescence excitation (PLE) was induced by a focussed I069

1070

PRESSbqLE DEPENDENCE

OF SHALLOW BOUND STATES LN GALLIUM ARSENIDE

cw dye laser operating on R110 or R6G dyes. Emission was collected in backscattering and analyzed with a 0.85 m double-grating spectrometer, an RCA 3 I034A photomultiplier, and photon counting, Spectral resolution was at least 0.2 rneV.

Vol. 53, No.

12

as the GaAs band gap increases with applied pressure.-" This is summarized in Fig. 3 where data resulting from more than 40 separate pressure measurements over the direct-gap range are presented. Located less than 10 meV below the F,-edge,

3. Results Photoluminescence spectra of the high-purity n-type VPE GaAs studied under pressure are shown in Fig. 1 at atmospheric pressure. To illustrate sample purity and assist identification of observed transitions, data at both 5 and 1.8°K are compared with 1.8-°K-results from a more compensated highpurity n-type MOCVD sample. At 1.8°K the VPE GaAs shows the dominant transitions of excitons bound to neutral (D 0) and ionized (D~') direct donors, together with the intrinsic free excitonpolariton (Ft-) and 2-electron Auger recombination at donors. "J'** In the comparison MOCVD sample, because of greater residual acceptor (C) doping, bound exciton recombination at the neutral acceptors (A 0) is also detected; we present this data for MOCVD-GaAs to later illustrate the basis for influence of acceptors in PLE data. At the slightly higher temperature of 5°K the VPE sample shows predominantly exciton recombination at the D~" donors (indistinguishable from h - D O ) , io with weak free exciton emission, and still weaker band-to-acceptor (BA) emission involving residual Si acceptors.*2 Results from the same VPE GaAs in a diamond cell are shown in Fig. 2 at selected pressures between 4.07 and 60.3 kbar. 2 For comparison the spectra are aligned vertically according to the dominant exciton transition. As in the atmospheric pressure data of Fig. 1, the pressures corresponding to direct gaps yield spectra dominated by a strong, sharp (2.5 meV) D~- exciton line whose intensity is nearly independent of pressure up to ~ 40 kbar (note scales). At all pressures well resolved emission is also observed from both the ground (n = 1) and first excited (n = 2) states of the free exciton located, respectively, ~ 3.2 and 5.7 meV above D~'. Because at atmospheric pressure the n = 2 emission is known to lie 1-2 meV below the band gap, t~ the strong D~- line in Fig. 2 may be considered as lieing within 6-8 meV of the F, edge. The remaining weak BA transition in Fig. 2 peaks ~ 29 meV below D~" at lowest pressures, and reaches ~ 31-32 meV below the donor near 40 kbar. This suggests a pressure-induced increase in Si-acceptor binding energy from ~ 34 meV at atmospheric pressure to ~ 37-38 meV near crossover. This modest increase may be expected within effective mass approximation (E A = 1 3 . 6 m h* / e 20 meV) ~ because with increasing pressure hole mass will remain nearly constant, .3 while static dielectric constant will decrease, *~ thus contributing to a deeper ground state. These principal direct-edge transitions evident in Fig. 2 all shift smoothly to higher energies

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Photoluminescence at atmospheric pressure (P = 0) and He temperatures of undoped n-type VPE-GaAs used in pressure experiments and comparison n-type MOCVD sample. At 5 ° K spectra show upper-polariton-branch F F free excitons (n = 1, 2), D~-donor excitons, and bandto-acceptor (Si) transitions. At 1.8°K the edge shows additional lower-polaritonbranch F r free excitons, D ° donor excitons, and 2-electron Auger transitions. MOCVD-GaAs shows additional excited D 0" r` donor PL and, because of compensation, A ° acceptor excitons absent in the VPE sample.

Vol. 53, No.

12

PRESSURE DEPENDENCE OF SHALLOW BOL."4D STATES IN GALLIS~'M ARSENIDE

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Representative PL of undoped n-type VPE-GaAs under pressure (4.07 - 60.3 kbar) in a diamond cell at 5°K. Spectra are aligned to the donor-bound-exciton lines. Strong D r donor excitons, F I- free excitons (n = 1, 2), and band-toacceptor (Si) transitions are observed for direct-gap pressures (< 41 kbar). For indirect-gap pressures D ° excitons of two donor species and their phonon replicas, and F x - L A X are observed. At F~-X. crossover (41.3 kbar) both D r, and D O donor states are observed simultaneously, for the first time.

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Principal PL energies of undoped n-type VPE- GaAs versus pressure (0 - 80 kbar) in a diamond cell at 5°K. D F excitons and BA (Si) transitions shift at 10.73 m e V / k b a r ; D ° excitons shift at -1.34 m e V / k b a r . Dr. states lie ~ 6 meV below F~, Dycstates lie ~ 40 meV below X~, and the L t edge is that of Ref. The F~-X~ crossover occurs at 41.3 kbar. An X~-gap of ~ 2.010 eV results at 'atmosphertc pressure.

the intense D I- transition plotted here provides an easily identified and unmistakable signature of the direct-gap dependence. Repeatibility of their transition energies at a given ruby-determined (Rt-line) pressure was found to be within + 2 meV. Least squares fits to these data yield a rigorously linear variation with applied pressure of slope 10.73 + 0.05 meV/kbar.'- Similar results are obtained from fitting the BA transitions also plotted in Fig. 3 and from the free exciton peaks plotted in a more spectroscopically detailed pressure diagram presented in Fig. 4. As is clear from Figs. 2-4, a new set of weak, sharp (0.5 - 1.5 meV) lines appear abruptly within the gap above ~ 39 kbar. These lines coexist with the direct-gap emission until ~ 42 kbar and then replace it at all higher pressures. This emission closely resembles the well known exciton recombi-

1072

PRESSURE DEPENDENCE OF SHALLOW BOUND STATES IN GALLIUM ARSENIDE

nation in n-type GaP ~s and, as such, its first appearance near ~ 40 kbar signals the pressure-induced onset of the direct-indirect crossover. We therefore attribute the main lines to recombination of excitons bound to neutral (D ° ) donors attached to the X~ conduction bandedge, which are referred to in GaP as "C lines. ''~s Taken from more than 30 separate pressure measurements, the D 0 symbols in Fig. 3 represent the average energy of the two prominent zero-phonon lines found in Fig. 2. In the expanded energy scales of Fig. 4 both D ° lines are plotted versus pressure. Repeatibility of these transition energies for a given pressure were within + 1 meV, Least squares fits to these data yield linear variations of -1.32 and -1.37 m e V / k b a r or an average pressure derivative of -1.34 + 0.04 meV/kbar. 2

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All observed PL and PLE (solid) transitions versus pressure surrounding crossover in undoped n-type V P E - G a A s at 5°K. PL shows F 1- free excitons (n = 1, 2), D r donor excitons, D ° (two species) donor excitons (circles) and their phonon replicas, and F x - L A X diamonds. Note increase in direct exciton binding energies above ~ 40 kbar and thresholds for phonon-assisted F X absorption in PLE.

Vol, 53, No. 12

These mgh pressure spectra in Fig. 2 represent the first reported for GaAs showing optical transitions involving electronic states attached to indirect conduction band minima. Therefore, they also permit the first direct observation at low temperatures of short-wavevector phonons from near zone boundary. For example, indicated in Fig. 2 at 49.5 kbar and in Fig. 4 as a function of pressure are momentum-conserving phonons of D ° attributable LA x (29.5 meV) and LO x (32 meV), together with the L O F (38.3 meV) replica from zone-center. In Fig. 2 we also note a 34.8-meV replica originating from within the TO-phonon branch - most probably also from the F-point. These participating phonons are in good agreement with those prominent in ntype GaP luminescence spectra, ~s and their derived energies agree with Raman scattering (involving F-point phonons) z6 we have performed at the same pressures and with phonons of large wavevector in higher temperature neutron scattering experiments. ~7 In contrast to PL data from GaP, however, relative phonon coupling strengths of these phonons (Fig. 2) are somewhat limited. This may be accounted for by a large "band structure e n h a n c e m e n t " expected for the oscillator strength of the D ° zero-phonon line as a result of its small energy separation from the F~ edge near crossover. ~ This influence of band structure on the probablity for recombination is also reflected in the rapid and progressive weakening (note scale factors) of this exciton emission as pressure is further increased and the direct edge withdraws to higher energies (Figs. 3 and 4). For example, upon increasing pressure from 41.3 to 60.3 kbar in Fig. 2 integrated D ° intensity decreases by a factor of more than 300, with the remaining photo-excited carriers presumably being lost to unobserved nonradiative traps within the gap. This degradation is reversible, however, with D ° intensities becoming fully restored as pressures are again reduced to those near crossover. The intensity degradation cannot therefore result from possible pressure-induced permanent defects, but must arise from dependence of D ° radiative rates on level separation from the F) edge. This effect must also be reflected in a corresponding change in radiative lifetime, a result which will be discussed in more detail in another report. 3.~9 We note that because of the large binding energy of indirect donors at least two residual donor species (unresolved as direct donors) appear as D ° lines in the indirect-gap spectra of Fig. 2. Although chemical identity of these donors is unknown, the amphoteric Si impurity detected as an unintentional As-site acceptor will also likely occupy Ga-sites as one of the observed donor species. The other donor may arise from the common group-VI contaminants (e.g., S, Se, or Te) occupying As-sites. 2°.2z

Vol. 53, No. 12

PRESSURE DEPENDENCE OF SHALLOW BOUND STATES IN GALLIUM ARSENIDE

As in n-type GaP, 15 these D ° exciton states in Figs. 2-4 will be made up of two bound electrons with paired (m i = 1 / 2 ) spins sharing the valleyorbit l s ( r t ) - l i k e donor ground state. The excitonic hole bound in the coulomb field of the electrons retains symmetry (FI5) of the valence bandedge. The result in recombination is the single (per donor specie) DOlines seen at 41.3 kbar in Fig. 2. However, when nonuniform sample stress is present the normal double degeneracy of the topmost part of the valence will be lifted to form two radiative D ° exciton components (J = + 1 / 2 , + 3 / 2 ) 20- whose separation will depend, in principle, on the magnitude and direction of stress. Such stress splittin~ accounts for the doublet structure of the pair of D~ lines found at 49.5 kbar in Fig. 2 and signifies a nonhydrostatic component within the diamond cell. Recent extensive stress dependent studies 22'23 of donor lines in GaP:S show D0X splittings are isotropic and increase with stress at ~ 4 m e V / k b a r . Using this as calibration the maximum splitting of ~ 0.6 meV we observe for D ° lines suggests an inadvertant stress of at most 150 bars, or a nonhydrostatic component of less than 1 part in 500 at our maximum pressures of ~ 80 kbars (Fig. 3). This extremely small degree of inhomogeneity in the pressure medium does not, in our experiments, detectibly broaden the ruby R z line usually used as a measure of nonhydrostatic conditions. 24 Hence, our GaAs sample has itself become a "stress gauge" sensitive to extremely small levels of stress. From these results we conclude pressures induced within our diamond cells may be considered virtually hydrostatic. This simplifies considerably analysis of the pressure data since inadvertant lifting of level- or band-degeneracies 23 by possible nonhydrostatic conditions may be largely ignored. In addition to the impurity-related D ° recombination found at indirect-gap pressures, Fig. 2 also shows evidence of intrinsic emission also originating from the X I edge. Unlike the impurities which break translational symmetry of the crystal and thereby display prominent zero-phonon lines for excitons of large wavevector, free excitons (F x) near the Xpoint may radiatively recombine only with the assistance of momentum-conserving phonons. In the spectrum at 49.5 kbar this phonon is the 29.5-meV LA x mode from near zone boundary which also occurred in D ° emission, t5 It is considerably broadened compared to the sharp D O - L A x replica, however, because of excess kinetic energy which the free excitons may possess. The characteristic asymmetric " B o l t z m a n n " lineshape of this F ° - L A x replica in Fig. 2, and indeed the very occurrance of free exciton recombination in steady state, indicates the lattice temperature is greater than the ~ 5 °K ambient temperature measured outside the pressure cell. This discrepancy presumably occurs from inadvertant heating by

1073

the laser excitation. Assuming a free exciton lineshape similar to that in GaP, 25,26 we may deduce from the LA x width (W = 5.5 meV) a maximum sample temperature of T = 2 W / k ~ 3 5 ° K (k is Boltzmann's constant) for the 49.5 kbar spectrum. ~-5 At 60.3 kbar in Fig. 2, where band structure enhancement is reduced and higher iaser power was needed to obtain a spectrum, the F x - L A X band is correspondingly broader due to additional heating. This heating also explains the apparently larger relative population of free excitons compared to those bound to the donors at this pressure. Energies of these LAx-assisted free exciton lines observed at various pressures are plotted for reference in Fig. 4. Using these d a t a we may conveniently estimate the corresponding F x energy at each pressure. For instance, assuming the low temperature threshold for free excitons with zero kinetic energy is given by E T = (Egx-EF)-EL.4, i the exciton edge at 49.5 kbar in Fig. 2 may be estimated as occurring at E T ~ 1.9184 eV or 646.0 nm. As expected, this threshold lies at an energy just above the donor-bound-excitons, in a region where (in the absence of phonon absorption processes) ~ no additional recombination is detected. Compared to this exciton edge the primary (lower) D ° excitons located at 1.9075 eV are bound by ~ 10-13 meV, while the secondary (upper) D ° excitons near 1.9103 eV are bound by only ~ 8 meV. These are in contrast to the somewhat larger 18-21 meV binding energies observed for D ° excitons in GaP. tS If we assume, however, an internal indirect exciton binding energy equal to that in GaP (Ev = 22 meV), 27 we may then derive the Xi-band-ga p energy E % = E T + E F at each pressure for GaAs. z At 49.5 kbar this gap would then be predicted to occur at ~ 1.94 eV or 639.0 nm. The corresponding indirect-gap energies versus pressure derived on this basis are plotted in Fig. 3. To complete this diagram the linear portion of the Li-ga p dependence derived from analysis of transport data 2x is also plotted. To complement the PL results in Fig. 2-4 the near-edge absorption in GaAs ~° under pressure has also been examined through PL excitation (PLE). 2.3 Figure 5 compares above-gap-excited PL with dyelaser-excited PLE surrounding the fundamental gaps. These data, plotted as solid symbols in Fig. 4, represent our typical results for direct-, indirect-, and near-crossover-pressures. For instance, at 33.7 kbar intensity of BA emission at 1.828 eV was monitored as excitation was scanned toward and above the direct edge. The principal PLE peak corresponds to bound excitons resonantly formed at the neutral (A O) acceptor states lieing just below the D r excitons (e.g6, see M O C V D - G a A s of Fig. 1). Although these A r excitons do not dominate the above-gap-excited PL, their absorption remains (in the presence of compensation) and is apparently preferentially enhanced under selective excitation.

1074

PRESSURE DEPENDENCE OF SHALLOW BOUND STATES IN GALLIUM ARSENIDE

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Vol. 53, No. 12

energy separation between the transitions, noticably increase.-" Secondly, all three transitions substantially broadened compared to those found at lower pressures. These changes occur above ~ 42 kbar because the direct excitons have passed above the X t conduction band edge, thus causing resonant intervalley interaction between these bound states and the indirect continuum. The additonal significant feature at 46.6 kbar is the occurrance of new PLE structure resulting directly from the indirect edges. 2 As indicated in Fig. 2, these are thresholds for TA x (~ 10 meV), LA x (~ 30 meV), and optic (~ 38 meV; probably TOx )29 phonon-assisted creation of indirect free excitons. Plotted as solid symbols at 46.6 kbar these transition energies give an independent measure of the F X energies which agrees with our earlier conclusions based solely on the PL identifications. More extensive PLE data from a wider range of pressures is presented elsewhere. 2.3° 4. Discussion

620

Representative PL and PL-excitation (upper curves) of undoped n-type GaAs at a direct (33.7 kbar), indirect (46.6), and near-crossover (42.2) pressure in a diamond cell at 5°K. At 33.7 kbar BA emission is recorded in PLE; at the indirect pressures D ° PLE is recorded. Strong PLE occurs at FF, Dr, and residual A? acceptor excitons. For indirect-gap pressures thresholds for phonon-assisted F x absorption are found in PLE. Note increase in direct-exciton binding energies with pressure.

At higher energies an additional PLE peak coincides with the n = 1 direct free exciton. No additional structure is detected which may be identified with the actual F~ band gap. For the remaining indirect-gap pressures in Fig. 5 PLE intensity is monitored at the D ° excitons. At 42.9 kbar absorption is again dominated by the strong, sharp direct-edge exciton resonances, as is clear by comparison with the PL spectrum. Here, however, an additonal peak arises which coincides with the D c excitons. Increasing pressure only modestly to 46.6 kbar causes the loss of direct-edge emission (due to carrier thermalization to the indirect gap) and illustrates the significant advantage of combining PL and PLE. Here the strong 3-line signature of the direct excitons established at 42.2 kbar continues to mark the F~ edge, despite its absence in thermalized P L From this we note two significant changes over the previous pressure. First, the direct-exciton binding energies, as evidenced by

Recent studies of GaAs under pressure have found different variations of the FI¢-F~5v gap than reported here. Examples are a nonlinear dependence at 300 o K 4-6 such as E p(P, 300 0 K) = Eg~,(P = 0, 300°K) + 1 2 . 6 x 1 0 3 p - 3.77x10sP ~-eV a n ~ i n c o n trast, a linear dependence of 8.5 m e V / k b a r at 120°K. 6 Neither of these agree with the 10.73 + 0.05 m e V / k b a r shift we find here for 0 - 40 kbar in the undoped GaAs 2.3 or in other recent work in GaAsl. P Xdoped with N (x = 0.0, 0.03, 0.115). 31'32 Instead, our data agree more closely with certain earlier stress and pressure measurement (77 3 0 0 ° K ) giving results of 10.5 - 11.3 m e V / k b a r (e.g., see references cited in Ref. 4). In determining gap variations from the shallow exciton states, we have neglected possible pressure dependences of their binding energies. Increased binding for the shallow excitons does occur, as evidenced by the difference in direct-exciton-separation at 42.2 and 46,6 kbar in Figs. 4 and 5. As will be discussed in a later publication, 3° this important but small increase in exciton binding marks the onset of detectible sublinear dependence on pressure. This arises from an increasing electron effective mass as the band gap increases, 33 and as exciton mixing with the heavier mass X~-states occurs above ~ 43 kbar upon degeneracy with those indirect states, and from a pressure-induced decrease in dielectric constant, t4 In addition, incipient nonlinear dependence of the gap as a result of possible nonlinearity in the pressure-induced decrease in lattice parameter is also expected at sufficiently large pressures, 4 These two separate influences, however, have most effect on the direct edge at pressures above that considered here. We conclude the strictly linear F t-ga p dependence derived from the data in Figs. 3 and 4 is the more reliable and accurate for pressures

Vol. 53, No.

12

PRESSURE DEPENDENCE OF SHALLOW BOUN~ STATES IN GALLIUM ARSENIDE

below ~ 45 kbar. We suggest the discrepancy between our gap dependences in this range and those recently reported may arise from difficulties in these studies in (1) accurately identifying transitions in broad optical absorption or PL of heavily doped materials ~'6 (2) applying gap fits determined largely at high pressures to data below 40 kbar ~ and (3) obtaining sufficiently hydrostatic pressure media at low temperatures a n d / o r high pressures# 6 Our data in Figs. 3 and 4 from the indirect X t edge show linear variation of -1.34 + 0.04 m e V / k b a r , this agrees well with measurements of the readily observed X t edge in the indirect-gap material GaP. Here absorption or PL combined with low pressures (below 10 kbar) or stress have led variously to hydrostatic values of -1.1,34 -1.46, 35, -2.022 and -2.6 m e V / k b a r . 36 Because, as we have shown above, unusual experimental conditions are needed to observe states derived from the X t edges in GaAs, past estimates for the X~ dependences in GaAs have been largely inferred, and all with rather large uncertainties. High temperature transport has given average linear terms ranging from -1 to -2.7 m e V / k b a r , 28.37.38 with one author assuming substantial additional nonlinear contributions. 2s Intensity variation of high temperature PL versus pressure or shift in 120-°K-PL from heavily doped (and undoped) GaAs has yielded scattered values from -1 to -2.7 m e V / k b a r , with uncertainties of + 0.5 - 0.8 m e V / k b a r . 5.6 Our more precise and consistent 32 values are possible because of direct observation of the narrow bound states attached to the X~ edge. Based on the data in Fig. 3, we conclude the accepted indirect band gaps of GaAs at atmospheric pressure should be revised. 2.3 Aspnes reports these energies as 1.981 eV for X~ and 1.815 eV for L~ at 0°K. 2s However, extrapolation of the D ° levels in Fig. 3 to P = 0 leads to the unacceptable result of degeneracy of these bound excitons with this generally accepted X~ energy. Instead, our data provide a new low-temperature X I gap of 2.010 + 0.008 eV. This is in close agreement with earlier conclusions reached by O n t o n et al. 39 from 2 - ° K - a b s o r p t i o n absorption in doped GaAs. Because Aspnes accurately deduces from core-level Schottky-barrier electroreflectance spectroscopy the X~-L t separation as 170 + 30 meV (and not the individual band

t075

energies with respect to the valence band edge), we must also therefore reinterpret the L~-gap as 1.840 + 0.040 eV. These new P = 0 band gaps must necessarily influence existing composition dependences of band structure in GaAs-based alloys. Our data also places in question recent report of pressure-induced deepening of donors in GaAs. Kobayashi et al. 7 assert that in PL the direct D F donor exciton abruptly deepens with respect to F~ band edge above 30 kbar and that such optical data supports the conclusion from Hall measurement in A1Ga~.xAs that direct donor activation energies necessarily increase near the F~-X~ crossover. 38.a° Figures 3 and 4 demonstrate the D r excitons are not detectibly perturbed above 30 kbar but instead smoothly follow the intrinsic states to and beyond crossover. Further, Figs. 3 and 4 prove that even the symmetry-forbidden level crossing t.a~ with the D ° exciton (A~) near 40 kbar does not induce detectible deepening of the D r exciton. We conclude the dominant direct donor-exciton in our GaAs remains shallow as expected by effective-mass theory, t Recombination involving DxA pairs near crossover may, in the more impure doped material used by Kobayashi et al. 7, account for their apparently anomalous results. 5. Conclusions Results .presented demonstrate that detailed optical spectroscopies may now be readily performed on semiconductors at both He temperatures and large hydrostatic pressures (up to ~ 80 kbar). 2.3~.32 This combination of experimental extremes has provided the first detailed, highresolution investigation of the influence of pressure on the "shallow" intrinsic- and impurity-bound states surrounding the fundamental gaps in GaAs. Our results therefore comprise one of the most comprehensive and spectroscopically detailed accounts of the pressure dependence of energy band structure in a semiconductor. Acknowledgement - We thank T. Kuech for supplying the MOCVD-GaAs, M. Cardona and J. Martinsen for helpful discussions, and D.C. Collins and J.N. Miller for alerting us to Ref. 7. This work was supported in part by the Office of Naval Research under contract N00014-80-C-0376.

References

1.

2.

F. Bassani and G. Pastori Parravicini, in Electronic States and Optical Transitions in Solids (Pergamon) 244 (1975). D.J. Wolford and J.A. Bradley, Bull. Am. Phys. Soc. 2_.99,291 (1984).

3.

4.

D.J. Wolford, H. Mariette, and J.A. Bradley, Proc. l l t h Inter. Conf. on GaAs and Related Materials, Biarritz (1984). B. Welber, M. Cardona, and H. Muller, Phys. Rev. B I__22,5729 (1975).

1076 5. 6. 7. 8. 9.

10. 11.

12.

13. 14. 15. 16.

17.

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PRESSURE D E P E N D E N C E OF S H A L L O W BOUND STATES IN G A L L I U M A R S E N I D E

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