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SUPERLATTICE TUNNELING DETECTORS
OPERATING AT λ = 10 µm, BASED ON QUANTUM WELL INTERSUBBAND ABSORPTION
B. Levine, K. Choi, C. Bethea, J. Walker, R. Malik
To cite this version:
B. Levine, K. Choi, C. Bethea, J. Walker, R. Malik. SUPERLATTICE TUNNELING DETECTORS OPERATING AT λ = 10 µm, BASED ON QUANTUM WELL INTERSUBBAND ABSORPTION.
Journal de Physique Colloques, 1987, 48 (C5), pp.C5-611-C5-614. �10.1051/jphyscol:19875131�. �jpa-
00226716�
SUPERLATTICE TUNNELING DETECTORS OPERATING A T X = 10 pm, BASED ON QUANTUM WELL INTERSUBBAND ABSORPTION
B.F. LEVINE, K.K. CHOI, C.G. BETHEA, J. WALKER and R.J. MALIK A T and X Bell Laboratories, Murray Hill, NJ 07974, U.S.A.
We d e m o n s t r a t e a novel 10.3 p m superlattice infrared detector based o n doped q u a n t u m wells of GaAs/AlGaAs. I n t e r s u b b a n d resonance radiation excites a n electron from t h e ground s t a t e i n t o t h e first excited s t a t e , where it rapidly tunnels o u t producing a photocurrent. W e achieve a nar- row bandwidth (10%) photosensitivity with a responsivity a s large a s 1.9 A / W and a n estimated speed of 30 ps.
~ e c e n t l ~ ' we have d e m o n s t r a t e d t h e first 10pm infrared detector based on a superlattice of doped GaAs/AlGaAs q u a n t u m wells. T h e motivation for this work is t h a t t h e fabrication of 10pm infrared d e t e c t o r s from 111-V materials would allow advantageous use of their more highly developed growth a n d processing technologies, as compared with 11-VI ~ o m ~ o u n d s ~ - ~ . F u r t h e r - more, device p a r a m e t e r s (e.g. b a n d gap, o p e r a t i n g temperature, bandwidth
,
and speed) can be tailored in ways t h a t are difficult t o d o with either 11-VI's o r extrinsic S i detectors. W e report here t h e demonstration of a novel high-speed infrared detector based on intersubband absorption and sequential resonant tunneling in doped Gaks/AI,Gal-,As q u a n t u m welt superlattices. W e achieved a responsivity as large a s 1.9 A J W a t A = 10.3 p m , a narrow bandwidth response of A X/X =lo%,
a n d e s t i m a t e t h e speed t o be z 30 ps. From o u r experiments we have d e t e r ~ i n e d t h a t t h e mean free p a t h of t h e photogenerated h o t electrons through t h e superlattice is 4500 A.In order t o understand t h e operation of this detector it is useful t o first discuss tunnel- ing5-' and intersubband absorptiong-'' in doped superlattices T h e infiared radiation is absorbed via t h e q u a n t u m well intersubband resonanceg-'' in a superlattice of doped Ga.4s/AIxGal-,As q u a n t u m wells and t h e photoexcited electrons rapidly t u n n e l o u t of t h e well, thereby producing a photocurrent. W e have studied in detail8 t h e t r a n s p o r t characteristics of these weakly coupled GaAs-GaAIAs multiquantum wells and concluded t h a t t h e stair-like potential profile a s shown in Fig. ( l a )
Fig. 1 (a) Sequential resonant tunneling in high field domain (right-hand side):
tunneling through ground s t a t e (left-hand side).
(b) Photoconductivity produced by absorption of intersubband radiation followed by tunneling o u t of well.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19875131
C5-612 JOURNAL DE PHYSIQUE
is the s t a b l e configuration when t h e applied bia voltage
CV,)
is less t h a n m(E2-El), where m is the n u m b e r of periods of in t h e s t r u c t u r e , and El and E2 are t h e energies of t h e ground and t h e first excited s t a t e s in t h e wells respectively. A t low temperatures w i t h o u t infrared radiation, elec- tric conduction is via sequential resonant tunneling either between t h e ground s t a t e s of each well or between t h e ground s t a t e s and t h e first excited s t a t e s o f , t h e adjacent wells, depending on t h e voltage d r o p across t h e period. When infrared energy equalto$^^-^^
is incident on t h e sample, electrons are excited t o t h e first excited s t a t e , producing hot electrons after tunneling o u t of t h e wells (see Fig. (lb). Sin& t h e mobility of t h e h o t electrons is different from those in t h e wells, a change of conductance is expected. W e named t h i s device a STAIR detector (an acronym for Superlattice T u n n e l i n g and Absorption by Intersubband Resonance detector a n d also because of t h e stair-like band configuration).In order t o t e s t t h e dependence of t h e p h o t e e x c i t e d tunneling on t h e height and thick- ness of t h e A1,Gal-,As tunneling barriers t w o samples were grown and measured. Sample A con- sist$d of a 5 0 period sliperlattice of 65 A GaAs wells (doped 1 . 4 ~ 1 0 ' ~ c r n - ~ an$
95A
Ale
2sGao 75AS barriers, sandwiched between highly doped c o n t a c t layers; sample B had 7 0 A q u a n t u m wells and 140 A A l o , 3 6 G ~ . s 4 A S barriers. These thicknesses and compositions were chosen t o produce only t w o s t a t e s in t h e well with an energy spacing close t o 1 0 p m . In c r d e r t o measure t h e resonance energy and oscillator s t r e n g t h we ~ e r f o r m e d Fourier transform interferome- t e r absorption measurementsg-lo with t h e crystal a t Brewster's angle Ob=73', since t h e polariza- tion selection rule of this transition requires t h e optical electric field t o have a component perpen dicular t o t h e superlattice. T h e absorption of sample A (Fig. 2) is peaked a t 920'cm-' with a full width a t half-maximum of ~ v = 9 7 c m - l corresponding t o an excited s t a t e lifetime of T2=(ii~v)-1=1.1x10-13s(110fs). T h e peak absorbance A = -log (transmission) = 2 . 2 ~ 1 0 - ~ corresponds t o an oscillator s t r e r ~ g t h ~ - ~ ~ of f=0.6 in good agreement with o u r theoretical value f=0.8.
In order t o measure t h e infrared photoconductivity a detector was fabricated by etch- ing a 5 0 p m - d i a m m e s a a n d making O h m i c c o n t a c t t o t h e t o p a n d b o t t o m nt -GaAs layers. In addition, a 45' angle was polished on t h e substrate (see insert in Fig. 2) to allow t h e infrared light t o back illuminate t h e detector a t a 45' angle of incidence.
T h i s allows for a large optical field normal t o the superlattice. T h e strongly resonant c h a r a c t e r of t h e photocurrent (Fig. 2) is in close agreement with t h e measured absorption spectrum: F u r t h e r - more, a s expected, t h e photosignal was determined t o be highly polarized with t h e optical transi- tion dipole moment aligned normal t o t h e superlattice. T h e responsivity R of Sample A increased with bias, reaching a maximum value of R=0.5 A/W a t a bias of 2.6V, corresponding t o a q u a n t u m efficiency of 6%. By fitting this d a t a (shown in Fig. 3)
R(OTUSR(SITIVE LENGTH f (PERIQOSI
PHOTON ENERGY [cm"l
Fig. 3 Solid p o i n t s a r e measured responsivity Fig. 2 Curve is measured absorption spectrum R vs. t h e photosensitive length 1 of t h e high for Sample A; solid points a r e photocurrent field domain for Sample A; t h e curve is vs. photon energy (normalized t o t h e peak theory including t h e h o t electron mean absorbance); insert shows device geometry free p a t h L.
tunneling probability a n a t h e m$an free p a t h L we can determine b o t h of these quantities. T h e result is p=60% a n d L=2500 A. T h i s value for L is much longer t h a n a ballistic mean free path12~13 d u e t o t h e rapid potential drop produced by t h e resonant alignment of El and E2. If we a t t e m p t t o increase t h e responsivity further in Sample A by increasing t h e bias V, t h e d a r k c u r r e n t rapidly increases. However, due t o t h e thicker tunneling banners in Sample B, t h e d a r k c u r r e n t is several o r d e r s of magnitude lower for t h e same bias and hence t h e voltage can t h e n be increased f u r t h e r , thereby increasing the photoexcited tunneling probability and photocurrent. A t Va=9V, we achieve a high responsivity of R=l.SA/W, (which is $early four t i m e s larger t h a n for Sample A) a n d a long hot electron mean free p a t h of L= 4500A, a t Va=3.4V (nearly twice a s large as Sample A). F o r biases larger t h a n 3.4V, t h e mean free p a t h , L , rapidly increases s o t h a t t h e effective photoconductive transport distance, becomes the t o t a l superlattice thickness. W e have developed a detailed theory of t h e intersubband absorption, photoexcited tunneling and h o t electron t r a n s p o r t through these multiquantum well superlattices. A s shown in Fig. 4 o u r calcula- tions a r e in good agreement with experiment over 3 orders of magnitude in responsivity. F r o m this d a t a we can also estimate t h e electron velocity and hence e s t i m a t e t h e response speed t o be very fast (30 psec).
In conclusion we have demonstrated a new concept in long wavelength infrared detec- tors, a n d achieved a high responsivity R=l.SA/W, a n advantageous narrow bandwidth (A = lo%), : and a very high estimated response speed (30 psec).
Fig. 4 T h e voltage dependence of t h e dark c u r r e n t a n d t h e responsivity of Sample B. T h e dashed curve is t h e theoretical f i t t i n g t o t h e responsivity.
R E F E R E N C E S
[I] B. F. Levine, K . K. Choi, C . G. Bethea, J. Walker and R . J. Malik, Appl. P h y s . L e t t 50, 1092(1987).
[2] J. S . Smith, L. C . Chiu, S. Margalit, A. Yariv, and A. Y. Cho, J. Vac, Sci. Technol. B 1, 37 6(1983)
(31 D. D. Coon a n d R. P. G. Karunasiri, Appl. Phys. L e t t . 45, 649(1984).
141 F. Capasso, K. Mohammed, a n d A. Y. Cho, I E E E J. Quantum Electron. QE-22, 1853(1986).
151 L. Esaki and L. L. Chang, P h y s . Rev. L e t t . 33, 495(1974)
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Y. ICawamura, K. W a k i t a , H. Asahi, and I<. K u r u m a d a , J p n . J. Appl. P h y s . 25, L928(1986).
F. Capasso, K. M o h a m m e d , a n d A. Y. Cho, Appl. Phys. L e t t . 48, 478(1986).
I<. I(. Choi, B. F. Levine, R. J . Malik, J. Walker, and C. G. Bethea, P h y s . Rev. B 15, 95, 4172(1987)
L . C. W e s t a n d S. J. Eglash, Appl. Phys.Lett. 46, 1156(1985).
Icleinman, and J. M. Vandenberg, App. Phys. L e t t . 50, 273(1987).
A. Harwit and J . S. Harris, Jr., Appl. Phys. L e t t . 50, 685(1987).
J . R . Hayes a n d A. F. J . Levi, I E E E J . Q u a n t u m Electron. QE-22, 1744(1986).
