PHYSICAL REVIEW B VOLUME 27, NUMBER 4 15 FEBRUARY 1983

Optical excitation of defects in molecular beam epitaxy grown GaAswith polarized light

P. Blood and A. D. C. Grassie'Philips Research Laboratories, Redhill, Surrey, RK1 5KA, England

(Received 23 August 1982)

The defects responsible for the deep states M1, M3, and M4 in molecular beam epitaxy

grown GaAs are complexes characteristic of this growth process. We have detected no anisotro-

py greater than 10% in the optical emission rates for electrons from these states with respect tothe orientation of the polarizing vector of the exciting radiation (1.8 and 0.92 p,m). We suggest

that if these defects are extended complexes they have no preferential orientation with respectto the growth plane.

Significant advances have been made in recentyears in the study of the electronic properties of deepdefect states in semiconductors using transient capac-tiance methods. By contrast, information on thephysical configuration of the centers responsible forthese states is sparse: in GaAs the chemical andstructural identity of the majority of deep stateswhich have been cataloged is unknown. ' EPR hasproved a valuable probe of the configuration of de-fect states in Si, and more recently in GaAs, but thedifficulty remains of correlating this information with

particular electronic levels. In many cases the defectstates are specific to a thin epitaxial layer and EPRrarely has the sensitivity to study such structures.The problem is further compounded by the absenceof an accurate and reliable theory of deep defectstates relating configuration and electronic proper-ties.

There is therefore a strong incentive to seekmethods to extend existing electrical techniques toprovide configuration information, retaining the ad-

vantages of sensitivity and selectivity which thesetechniques offer. In principle a number of directionalprobes are available. The effect of the electric fieldin a depletion region on the emission rate has beenstudied in GaAs for several crystal orientations, butthe observed effects have since been ascribed to theorientation dependence of the diode leakage current. 'The available magnetic fields for direct Zeeman per-turbations are too small to give observable splittingsof the levels. Uniaxial stress has had limited applica-tion to specific materials such as Si, in which elec-tronic reorientation of the divacancy can be achievedat low temperatures, and atomic reorientation at hightemperatures. 4 Considerable work has been done onthis particular defect, and of particular interest is thereport of an anisotropy of optical excitation with

respect to the polarization vector of the exciting radi-

ation, detected using transient capacitance tech-niques. '

We report results of an investigation using tran-

TABLE I. Electrical properties of deep states in MBEGaAs (from Ref. 1), and concentration measured in thesample used in this work.

Thermalactivation

energy(eV)

Apparentcross section

(cm2)Concentration

(cm 3)

M1M3M4

0.170.30O.S1

1.8x10 15

7.2 x 10-15x 10

—12

4.0 x 1013

2.6 x 102.2 x 10

sient capacitance methods of the optical excitation bypolarized light of three electron traps commonly ob-served in layers of GaAs grown by molecular beamepitaxy (MBE). The deep states of interest, labeledM1, M3, and M4 are believed not to be due to sim-ple chemical impurities or simple lattice defects. 6

Studies of the occurrence of these states as a functionof growth conditions suggest that they are charac-teristic of the MBE growth process. ' Since thesestates are not thought to be simple defects, and sincethe MBE process builds up the crystal with successiveatomic planes, it is reasonable to conjecture thatthese states may be due to complex defects havingdifferent configurations in the plane of growth and aplane including the direction of growth.

Experiments were conducted on a (001) layer ofn-type GaAs, 2.5 p, m thick, grown at 600'C anddoped with Sn to a carrier concentration ofn = 5 x 10' cm . Deep states were observed in thespace-charge region of an evaporated Al Schottkydiode 0.45 mm in diameter. The principal propertiesof MBE growth defects M1, M3, and M4 are sum-marized in Table I, and the concentrations measuredin the sample are given. The exciting radiation wasgenerated by a 150-W quartz iodine lamp and polar-

2548

BRIEF REPORTS 2549

ized by a rotatable HR Polariod plate. To correct forvariations in intensity with the angle of polarizationthe intensity was sampled during all the measure-ments using a fiber optic and monitored using a Gephotodiode. Two wavelengths were employed:X~1.8 p, m (he~0.69 eV & E~/2), selected with theuse of a Ge filter, to excite transitions from occupieddefect states to the conduction band, and A, =0.92p, m (h v =1.35 eV), produced with the use of amonochromator, to excite transitions both from thevalence band to the defect and from the defect to theconduction band.

Two sample configurations were used for experi-ments with the long-wavelength radiation: the diodewas illuminated in a direction normal to a cleavededge so that the polarization vector was rotatedbetween a direction parallel to the growth directionand a direction in the plane of growth, and the diodewas also illuminated through the substrate in a direc-tion normal to the growth plane so that the vectorwas rotated solely in the growth plane. The sequenceof operations was a follows. The sample was firstcooled to 150 K and the traps filled by a short circuitpulse. The sample was then reverse biased to a de-pletion depth of 1.4 p, m and the shallower levels al-lowed to empty by thermal emission. At 150 K thethermal emission time constants of Ml and M3 are—4 x 10 ' and 0.16 s, respectively, compared with—4.5 x 10 s for M4. Thus, at the end of an emis-sion time —5 s, which was kept constant for each setof the measurements, only the deepest level M4remained occupied. Illumination with 1.8-p, m lightthen produced emission from this center alone with atime constant of about 30 s, and the process wasrecorded by direct logging of the voltage transientV (t) of a constant capacitance servo loop monitoringthe diode depletion capacitance. The polarizationdependence of the optical excitation was characterizedby measuring the difference in the time derivative ofthe signal immediately before and after applying il-lumination for 20' intervals in the polarizer settingand correcting for variations in light intensity. Thismeasurement was repeated at 100 K where both M4and M3 levels remained filled for optical excitation,and finally at 77 K where al1 the three levels are opti-cally excited.

Figure 1 shows the variation of the derivative ofthe optically induced transient signal at the instant ofillumination; we will refer to this quantity as Do

Dp= ~ep'(8)N, (r =0),dt, o

where e„'p' is the optical emission rate and N, (t = 0)the population of occupied centers at the instant of il-

lumination. The data shown in the figure are typica1of the results obtained for all three defects and orien-tations investigated, namely, that no anisotropy in

Do

4(arb. units)

0 40 80 i20 i60Polarizer setting

FIG. 1. Observed variation in the initial-time derivativeof the optically induced charge-state transient of the M4 lev-el in GaAs for edge illumination at 150 K with respect to thepolarization vector of ligth of wavelength X & 1.8 p, m. Therate is corrected for intensity changes (4) as a function ofpolarization angle. The solid line is the variation predictedfor a uniaxial defect.

Dp(8) was detected outside the experimental error of+ 10% (half of this error derives from uncertainty inmeasurement of the 1ow-light levels in this part ofthe spectrum).

For a single uniaxia1 complex defect, orientedalong the growth direction, we expect e„'P'(8)= e cos'8, and hence Do should be proportional tocos 8, where 8 is the angle between the dipolar axisof the defect and the E vector of the polarized light.This variation is plotted in Fig. 1. We conclude thatthis situation does not apply to any of the three de-fects investigated.

It is possible that the defects are oriented in thecrystal with equal populations along either the [111]tetrahedral bond directions or along the [100] direc-tions. Consider as an example the latter case appliedto a donorlike defect state. The time dependence ofthe concentration of unoccupied defects for illumina-tion along an [001] direction is

N+(t ) = N, (2 —exp[- ep(cos28) t ]

—exp[- eP(sin28) t ]]

where N, is the concentration of defects in each ofthe [100] and [010] directions. This transient doesnot have a simple form but it is easily shown that thederivative at t = 0 is Do= e X„ independent of 8.While this does accord with our observations, furtheranalysis shows that the amplitude of the transientshould double on going from 8 = 0, where only the(100) defects respond, to 8=45', where both (100)and (010) defects respond with the same emissionrate. Similar conclusions apply to defects along the[111]directions. No changes in amplitude weredetected for either edge or rear illumination.

At the shorter wavelength of X = 0.92 p, m, greater

2550 BRIEF REPORTS 27

intensity was available so that the optical emissionrates were increased. It was therefore possible to per-form a thermal DLTS scan under constant illumina-tion to look for competition between thermal- andoptically-induced emission processes. This is thesame experiment as that reported for the Si divacan-cy.' In such experiments both these emissionprocesses are much slower than the electron-capturerate in neutral material so the traps are all fully occu-pied at the start of the emission period after the shortcircuit pulse. If only transitions to the conductionband are promoted (e„'"), the DLTS spectrum will

have plateaus at temperatures lower than each ther-mal peak due to this process. If hole transitions tothe valence band (e~"') are also promoted then theamplitude of the peaks and the plateaus will be re-duced to a fraction [(e„'"+e„"')/(e„'"+e„'"'+e~'") ] ofthat observed with thermal emission alone. The ex-pected plateaus due to the conduction-band transi-tions were observed but no evidence was found forthe transitions from the valence band at the intensi-ties employed. We detected no variations in theDLTS spectra with polarizer setting.

In all our experiments we detected no anisotropy in

optical emission rate greater than 10%. We believethat the emission processes we observed were in-

duced by the incident polarized radiation for tworeasons. We analyzed only the optically-inducedemission rate, which we confirmed was proportionalto light intensity, and we confirmed that all the lighttransmitted through the sample was polarized withthe same polarization vector as the incident radiation.We are therefore led to conclude that the defects inMBE-grown GaAs have no preferential orientationwith respect to the growth plane.

We have also shown that anisotropy in the ampli-tude of the transient should be observed for equalpopulations of defects oriented along the [111]bonddirections. The failure to observe such amplitude an-isotropy suggests that, since the dipole radiation will

only couple p states of the defects to the s-like con-duction band, it can be deduced that there is no morethan 10% variation in the relative p„, p„, and p, com-ponents in the wave functions of the defects respon-sible for Ml, M3, and M4.

ACKNOWLEDGEMENT

One of us (A.D.C.G.) was supported by a RoyalSociety/Science and Engineering Research CouncilIndustrial Fellowship.

'On leave from the University of Sussex, Brighton, Sussex,BN1 9QH, England.

G. M. Martin, A. Mittoneau, and A. Mircea, Electron. Lett.13, 191 (1977).

A. Mircea and A. Mittoneau, J. Phys. (Paris) Lett. 40, L31(1979).

S. Makram-Edid, in Proceedings of the Symposium of the Ma-terials Research Society, edited by J. Narayan and T. Y. Tan(North-Holland, Amsterdam, 1981), Vol. 2, p. 495.

4G. D. Watkins and J. W. Corbett, Phys. Rev. 138, A543(1965).

5G. L. Miller, D. V. Lang, and L. C. Kimerling, Annu. Rev.Mat. Sci. 7, 445 (1977).

D, V. Lang, A. Y. Cho, A. C. Gossard, M. Illegems, andW. Weigman, J. Appl. Phys. 47, 2558 (1976).

7J. H. Neave, P. Blood, and B. A. Joyce, Appl. Phys. Lett.36, 311 (1980); P. Blood and J. J. Harris (unpublished).