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Donor Magnetospectroscopy in high purity epitaxial GaAs
xvi
CONFERENCE ON PHOTOCONDUCTIVITY
Measurement of the dichroism which is ass o c i a t e d with electronic reorientation among J a h n - T e l l e r configurations a l s o allowed the determination of the symmetry of the d e f e c t s which is C3~' : this C3~' symmetry corresponds to the J a h n - T e l l e r distortion of a configuration in which the dopant atom was o r i g i n a l l y in a tetrahedral position. The defect r e s p o n s e to the s t r e s s is determined by the value of the term in the p i e z o s p e c t r o s c o p i c defect tensor which c h a r a c t e r i z e s the r e l a t i v e change in defect energy per unit s t r a i n . This value is ~ - 12 eV/ unit strain. The photoconductivity t r a n s i t i o n which is observed corresponds to a distribution of dipole moments which is an e l l i p s o i d of rotation about the trigonal axis of the defect. This d i s t r i b u t i o n is determined by the numerical v a l u e s of the dichroic ratios. The e l e c t r o n i c reorientation and the transition properties are s a t i s f a c t o r i l y d e s c r i b e d in terms of a simple model using linear combination of atomic o r b i t a l s . R e s u l t s a l s o have been obtained in lithium diffused s i l i c o n irradiated at different temperatures with 1.5 MeV e l e c t r o n s . After irradiation at 77°K the photoconductivity spectrum of such samples has the t y p i c a l shape of the s p e c t r a of n-type s i l i c o n (for instance phosphorus-doped s i l i c o n ) irradiated with e l e c t r o n s . Isochronal annealing r e s u l t s in a very important dhange of the spectrum after a ten minute room temperature anneal; at t h e s e temperatures, lithium is mobile in a r e l a t i v e l y short time. 5-5.
THERMAL AND OPTICAL EMISSION RATES AND CROSS SECTIONS FROM THE IMPURITY PHOTOCURRENT AND PHOTOCAPACITANCE METHODS C,T. Sah, L. F o r b e s , L . L . Rosier and A.F. T a s c h , Jr. (University of I l l i n o i s , Urbana)
Previous methods of measuring the carrier e m i s s i o n rate and capture c r o s s s e c t i o n s at defect centers in semiconductors suffer from two s o u r c e s of error: nonexponential decay law and uncertainty in the defect concentration. T h e s e
Vol. 7, No. 11
can be removed using a reverse b i a s e d p - n junction. The impurity photovoltaic method previously proposed will be d e s c r i b e d in d e t a i l . Other new methods of p h o t o c a p a c i t a n c e and dark junction current t r a n s i e n t s under square wave illumination and/or junction voltage will be described which compliment the impurity photovoltaic method. T h e s e methods are i l l u s t r a t e d using the experimental r e s u l t s of gold a c c e p t o r and donor centers in s i l i c o n . Ten d e c a d e s of electron and hole e m i s s i o n rates at these two gold centers from - 1 0 0 ° C to +100°C are obtained using six s e t s of experiments involving both transient and s t e a d y s t a t e photo and dark junction current and c a p a c i t a n c e measurements. T h e s e r e s u l t s are in e x c e l l e n t agreement with those obtained from generation-recombination noise s p e c t r a and pulsed field-effect c o n d u c t a n c e s of gold-doped field-effect structures. 5-6.
DONOR MAGNETOSPECTROSCOPY IN HIGH PURITY EPITAXIAL GaAs G.E. Stillman, C.M. Wolfe and J.O. Dimrnock (Lincoln Laboratory, Massac h u s e t t s Institute of Technology, Lexington, M a s s a c h u s e t t s , U.S.A.)
We have previously shown that high-purity e p i t a x i a l layers of GaAs provide fast s e n s i t i v e far-infrared photodetectors in the range from 195 to 337 ~ . Here we report s p e c t r a l measurements of the e x t r i n s i c photoconductivity of t h e s e epit a x i a l layers from 10 to 120 crn-' at 4.2 and 1.5°K in magnetic f i e l d s up to 30 kG. The purest sample measured had a donor concentration of 4.8 × 1013cm -3 , a total acceptor content of 2.1 × 1013cm -3 , a 77°K mobility of 210,000 crna/ Vsec, and a peak mobility of 340,000 crna/Vsec at 40°K. At 1.5°K and zero magnetic field the photoconductivity spectrum shows a sharp peak at 35.5 cm- ' c o r r e s p o n d i n g to the ls-to-2p donor level transition, a second weaker peak occurring at 42.2 crn- ~corresponding to the Is-to-3p transition, and a broad continuum at higher frequencies. T h e s e peak energies can be fit very well by the hydrogenic model and yield a donor ionization energy of 47.3 c m - ' o r 5.86 meV. In a magnetic field both of these p e a k s s p l i t and shirt to higher frequencies. From this s p l i t t i n g we
Vol. 7, No. 11
CONFERENCE ON PHOTOCONDUCTIVITY
obtain an electron effective mass of me= (0.0667 + 0.0005) rn. Using this value and a dielectric constant of 12.5 we calculate a hydrogenie impurity ionization energy of 5.80 meV in good agreement with the measured value.
SESSION 6 EFFECTS 6-1.
ORGANIC MATERIALS, PHONON
THE ROLE OF PHONONS IN THE OSCILLATORY PHOTOCONDUCTIVITY SPECTRUM OF SEMICONDUCTING DIAMOND A.T. Collins and E.C. Lightowlers (Wheatstone Laboratory, King's College, Strand, London, England); P.J. Dean (Bell Telephone Laboratories, Murray Hill, New Jersey)
The oscillatory photoconductivity spectrum associated with the acceptor center in semiconducting diamond has been measured over the spectral energy range 0 . 3 5 - 2 . 6 e V (3.5 to 0.47~). The energy separation between the acceptor ground state and the top of the valence band is more than twice the longitudinal optical (LO) phonon energy, and the spectrum is interpreted in terms of hole-capture from the valence band to e x c i t e d s t a t e s of the acceptor center together with the c a s c a d e emission of one or more optical phonons. Transitions involving the emission of up to 14 phonons have been detected. The mean phonon energy in the cascade process is not constant, as has been generally assumed, but decreases gradually as the number of phonons involved increases. This is shown to be consistent with the curw~ture of the valence band and the dispersion curves for LO phonons. In addition, features have been observed in the photoconductivity spectrum which are considered to be due to the phonon-assisted hole-capture to excited states of the acceptor center for which transitions from the ground state are forbidden, and therefore have not previously been observed in absorption measurements on natural semiconducting diamond.
6-2.
xvii
GENERATION AND DETECTION OF 10'2 Hz PHONONS IN n-TYPE GaAs Richard S. Crandall (RCA Laboratories, Princeton, New Jersey)
A beam of the ultra-high frequency (~ 10'2 Hz) acoustic phonons is used to ionize shallow donors in much the same way as light is used to ionize donors. The high frequency phonons are generated by electron-donor recombination following excitation of the donors by impact ionization. The phonons are generated in one n-type layer of a n-i-n structure, propagate through the i region, and are detected in the remaining n layer. Phonon propagation over distances in excess of 1 mm have been observed. The ratio of photon to phonon generation by electron-donor recombination was found to less t h a n 3 × 10 -~. 6-3.
INTRINSIC PHOTOCONDUCTIVITY IN ORGANIC CRYSTALS N.E. Geacintov and M. Pope (Chemistry Department and Radiation and Solid State Laboratory, New York University), New York, New York 10003
The photon energy dependence of the intrinsic photoconductivity of crystalline anthracene, tetracene and perylene exhibits similar characteristic structure. P e a k s and valleys appear which are spaced approximately 0 . 9 - 1 . 3 eV apart and bear no direct relationship to the absorption spectra of these crystals. Evidence is presented that this structure is due to excitations from lowerlying valence levels of the crystals, which in turn are related to the energy sequence of the occupied molecular orbitals. The mechanism of intrinsic photoionization involves mixing of ionized states with neutral upper exciton states, the lifetime of which is very short due to efficient vibrationally induced radiationless transitions. The quantum yield ¢ in anthracene has been studied with blocking electrodes as a function of applied electric field E up to 2 × 105 V - c m - ' . Previous measurements, (up to E = 2.2 x 104 volt-cm - ' ) have shown that in this field range the dependence of qb on E is in accord with the Onsager theory of geminate recombination. There
CONFERENCE ON PHOTOCONDUCTIVITY
Measurement of the dichroism which is ass o c i a t e d with electronic reorientation among J a h n - T e l l e r configurations a l s o allowed the determination of the symmetry of the d e f e c t s which is C3~' : this C3~' symmetry corresponds to the J a h n - T e l l e r distortion of a configuration in which the dopant atom was o r i g i n a l l y in a tetrahedral position. The defect r e s p o n s e to the s t r e s s is determined by the value of the term in the p i e z o s p e c t r o s c o p i c defect tensor which c h a r a c t e r i z e s the r e l a t i v e change in defect energy per unit s t r a i n . This value is ~ - 12 eV/ unit strain. The photoconductivity t r a n s i t i o n which is observed corresponds to a distribution of dipole moments which is an e l l i p s o i d of rotation about the trigonal axis of the defect. This d i s t r i b u t i o n is determined by the numerical v a l u e s of the dichroic ratios. The e l e c t r o n i c reorientation and the transition properties are s a t i s f a c t o r i l y d e s c r i b e d in terms of a simple model using linear combination of atomic o r b i t a l s . R e s u l t s a l s o have been obtained in lithium diffused s i l i c o n irradiated at different temperatures with 1.5 MeV e l e c t r o n s . After irradiation at 77°K the photoconductivity spectrum of such samples has the t y p i c a l shape of the s p e c t r a of n-type s i l i c o n (for instance phosphorus-doped s i l i c o n ) irradiated with e l e c t r o n s . Isochronal annealing r e s u l t s in a very important dhange of the spectrum after a ten minute room temperature anneal; at t h e s e temperatures, lithium is mobile in a r e l a t i v e l y short time. 5-5.
THERMAL AND OPTICAL EMISSION RATES AND CROSS SECTIONS FROM THE IMPURITY PHOTOCURRENT AND PHOTOCAPACITANCE METHODS C,T. Sah, L. F o r b e s , L . L . Rosier and A.F. T a s c h , Jr. (University of I l l i n o i s , Urbana)
Previous methods of measuring the carrier e m i s s i o n rate and capture c r o s s s e c t i o n s at defect centers in semiconductors suffer from two s o u r c e s of error: nonexponential decay law and uncertainty in the defect concentration. T h e s e
Vol. 7, No. 11
can be removed using a reverse b i a s e d p - n junction. The impurity photovoltaic method previously proposed will be d e s c r i b e d in d e t a i l . Other new methods of p h o t o c a p a c i t a n c e and dark junction current t r a n s i e n t s under square wave illumination and/or junction voltage will be described which compliment the impurity photovoltaic method. T h e s e methods are i l l u s t r a t e d using the experimental r e s u l t s of gold a c c e p t o r and donor centers in s i l i c o n . Ten d e c a d e s of electron and hole e m i s s i o n rates at these two gold centers from - 1 0 0 ° C to +100°C are obtained using six s e t s of experiments involving both transient and s t e a d y s t a t e photo and dark junction current and c a p a c i t a n c e measurements. T h e s e r e s u l t s are in e x c e l l e n t agreement with those obtained from generation-recombination noise s p e c t r a and pulsed field-effect c o n d u c t a n c e s of gold-doped field-effect structures. 5-6.
DONOR MAGNETOSPECTROSCOPY IN HIGH PURITY EPITAXIAL GaAs G.E. Stillman, C.M. Wolfe and J.O. Dimrnock (Lincoln Laboratory, Massac h u s e t t s Institute of Technology, Lexington, M a s s a c h u s e t t s , U.S.A.)
We have previously shown that high-purity e p i t a x i a l layers of GaAs provide fast s e n s i t i v e far-infrared photodetectors in the range from 195 to 337 ~ . Here we report s p e c t r a l measurements of the e x t r i n s i c photoconductivity of t h e s e epit a x i a l layers from 10 to 120 crn-' at 4.2 and 1.5°K in magnetic f i e l d s up to 30 kG. The purest sample measured had a donor concentration of 4.8 × 1013cm -3 , a total acceptor content of 2.1 × 1013cm -3 , a 77°K mobility of 210,000 crna/ Vsec, and a peak mobility of 340,000 crna/Vsec at 40°K. At 1.5°K and zero magnetic field the photoconductivity spectrum shows a sharp peak at 35.5 cm- ' c o r r e s p o n d i n g to the ls-to-2p donor level transition, a second weaker peak occurring at 42.2 crn- ~corresponding to the Is-to-3p transition, and a broad continuum at higher frequencies. T h e s e peak energies can be fit very well by the hydrogenic model and yield a donor ionization energy of 47.3 c m - ' o r 5.86 meV. In a magnetic field both of these p e a k s s p l i t and shirt to higher frequencies. From this s p l i t t i n g we
Vol. 7, No. 11
CONFERENCE ON PHOTOCONDUCTIVITY
obtain an electron effective mass of me= (0.0667 + 0.0005) rn. Using this value and a dielectric constant of 12.5 we calculate a hydrogenie impurity ionization energy of 5.80 meV in good agreement with the measured value.
SESSION 6 EFFECTS 6-1.
ORGANIC MATERIALS, PHONON
THE ROLE OF PHONONS IN THE OSCILLATORY PHOTOCONDUCTIVITY SPECTRUM OF SEMICONDUCTING DIAMOND A.T. Collins and E.C. Lightowlers (Wheatstone Laboratory, King's College, Strand, London, England); P.J. Dean (Bell Telephone Laboratories, Murray Hill, New Jersey)
The oscillatory photoconductivity spectrum associated with the acceptor center in semiconducting diamond has been measured over the spectral energy range 0 . 3 5 - 2 . 6 e V (3.5 to 0.47~). The energy separation between the acceptor ground state and the top of the valence band is more than twice the longitudinal optical (LO) phonon energy, and the spectrum is interpreted in terms of hole-capture from the valence band to e x c i t e d s t a t e s of the acceptor center together with the c a s c a d e emission of one or more optical phonons. Transitions involving the emission of up to 14 phonons have been detected. The mean phonon energy in the cascade process is not constant, as has been generally assumed, but decreases gradually as the number of phonons involved increases. This is shown to be consistent with the curw~ture of the valence band and the dispersion curves for LO phonons. In addition, features have been observed in the photoconductivity spectrum which are considered to be due to the phonon-assisted hole-capture to excited states of the acceptor center for which transitions from the ground state are forbidden, and therefore have not previously been observed in absorption measurements on natural semiconducting diamond.
6-2.
xvii
GENERATION AND DETECTION OF 10'2 Hz PHONONS IN n-TYPE GaAs Richard S. Crandall (RCA Laboratories, Princeton, New Jersey)
A beam of the ultra-high frequency (~ 10'2 Hz) acoustic phonons is used to ionize shallow donors in much the same way as light is used to ionize donors. The high frequency phonons are generated by electron-donor recombination following excitation of the donors by impact ionization. The phonons are generated in one n-type layer of a n-i-n structure, propagate through the i region, and are detected in the remaining n layer. Phonon propagation over distances in excess of 1 mm have been observed. The ratio of photon to phonon generation by electron-donor recombination was found to less t h a n 3 × 10 -~. 6-3.
INTRINSIC PHOTOCONDUCTIVITY IN ORGANIC CRYSTALS N.E. Geacintov and M. Pope (Chemistry Department and Radiation and Solid State Laboratory, New York University), New York, New York 10003
The photon energy dependence of the intrinsic photoconductivity of crystalline anthracene, tetracene and perylene exhibits similar characteristic structure. P e a k s and valleys appear which are spaced approximately 0 . 9 - 1 . 3 eV apart and bear no direct relationship to the absorption spectra of these crystals. Evidence is presented that this structure is due to excitations from lowerlying valence levels of the crystals, which in turn are related to the energy sequence of the occupied molecular orbitals. The mechanism of intrinsic photoionization involves mixing of ionized states with neutral upper exciton states, the lifetime of which is very short due to efficient vibrationally induced radiationless transitions. The quantum yield ¢ in anthracene has been studied with blocking electrodes as a function of applied electric field E up to 2 × 105 V - c m - ' . Previous measurements, (up to E = 2.2 x 104 volt-cm - ' ) have shown that in this field range the dependence of qb on E is in accord with the Onsager theory of geminate recombination. There